Keywords: TiO2, hydrothermal, photodegradation, H2 production, graphene, atmospheric pressure plasma jet, Ag nanoparticles

This dissertation is focused on synthesizing Q1D TiO2-based nanocatalysts for degrading aqueous organic pollutants and producing H2. A facile alkaline hydrothermal process was used to synthesize Q1D TiO2 under different hydrothermal synthesis factors (reaction temperature, NaOH concentration and TiO2 precursor concentration). The hydrothermal synthesis factors significantly affected the Q1D TiO2 phase structure, crystal size, specific surface area (SSA), bandgap, photocatalytic activities. A Box-Behnken design (BBD) model was used to optimize the hydrothermal factors for synthesizing Q1D TiO2 with maximum photodegradation rate and H2 production rate.

The optimized Q1D TiO2 with maximum photodegradation rate was further enhanced with partially reduced graphene oxide (RGO) (designated as GT) for degrading aqueous hazardous pollutants. The study also examined the impact of the RGO atomic oxygen-tocarbon (O/C) ratio on GT photocatalytic activities. The highest photocatalytic activity was observed when the RGO atomic O/C ratio was 0.130±0.003. Next, the GT photocatalyst was enhanced with Ag NPs (designated as Ag-GT). The highest photocatalytic activity was observed for a silver content of 10 wt% in the photocatalyst film.

Finally, an atmospheric pressure plasma jet (APPJ) was employed to synthesize micrometer thick Ag nanoparticles modified TiO2 (Ag-TiO2) coatings, presenting a coreshell structure for degrading RhB and trace pharmaceutical compounds using a solar light source. The Ag-TiO2 coatings were characterized having a porous anatase phase, improved charge separation and visible light absorption. The highest photodegradation rate was observed for a silver content of 0.4wt% in the composite.

v DEDICATION I dedicate my dissertation to my parents for their sacrifices, sharing equally my entire emotional burdens, offering me all their love and inspiring me to this goal. I dedicate this dissertation to my wife Zhao Wang. My sincere gratitude to all my loved ones whose support and encouragement has provided me the strength and perseverance needed to complete this dissertation.

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TABLE OF CONTENTS GENERAL INTRODUCTION

Titanium dioxide (TiO2) is widely considered as a potential solution for photodegradation of hazardous pollutant and production of clean energy. TiO2 has been used as photoelectrochemical cell, photovoltaics and photocatalysis, because of its wide bandgap and strong ability to split water for hydrogen production. However, the low efficiency of pure TiO2 photocatalyst is a challenge in practical applications. The factors affecting the use of photocatalysts to degrade hazardous pollutants and produce H2 include phase structure, crystal size, BET specific surface area (SSA), bandgap, charge transportation, charge separation, doping, cocatalyst and coupling other metal oxide semiconductors.

The methods for improving TiO2 overall photocatalytic efficiency include: (1): use of TiO2 nanostructure with increasing SSA, (2): modifying the crystal structure and morphology, (3): narrowing bandgap, (4): suppressing charge recombination, and (5): enhancing charge transfer. In this dissertation, we will focus on synthesis, modification and characterization of quasi-one-dimension (Q1D) TiO2 nanophotocalyst for hazardous wastewater treatment and hydrogen production, as well as the deposition of thin films of TiO2 using the plasma-enhanced chemical vapor deposition (PECVD) for photodegrading organic pollutants. Eight chapters are present in this dissertation.

In Chapter 1, the fundamental photocatalytic processes and the state of the art will be described for synthesis and modification of TiO2. In Chapter 2, the methodology will be described for the synthesis and characterization of Q1D TiO2 nano photocatalysts.

In Chapter 3, the effects of three hydrothermal synthesis factors (reaction temperature, NaOH concentration and TiO2 concentration) on Q1D TiO2 phase structure, crystal size GENERAL INTRODUCTION 2 and SSA, bandgap will be examined using a 3-factor 3-level Box Behnken design (BBD) statistical model. The hydrothermal factors will be optimized for synthesizing Q1D TiO2 with maximum and minimum bandgaps. In Chapter 4, the BBD model will be developed to evaluate the effects of hydrothermal synthesis factors on the photocatalytic activities in terms of: photodegrading aqueous organic pollutants and photocatalytic H2 production using Q1D TiO2. The hydrothermal factors will be optimized for synthesizing Q1D TiO2 with maximum photodegradation rate of pollutants and H2 production rate. The role of phase structure, crystal size and bandgap will be investigated.

In Chapter 5, the optimized Q1D TiO2 with maximum photodegradation rate will be modified with partially reduced graphene oxide (RGO) (designated as GT) and partially RGO plus Ag nanoparticles (NPs) (designated as Ag-GT) for further enhancing the photodegradation. The RGO oxidation degree (atomic O/C ratio) will be investigated to maximize the photodegradation rate for degrading rhodamine b (RhB), methyl orange (MO), methylene blue (MB) and phenol by using ultraviolet (UV) light. Next, the GT with optimum oxidation degree will be deposited with Ag nanoparticles. The surface plasmon resonance (SPR) effect of Ag nanoparticles will enable the photodegradation under visible light. In addition, the impact of Ag content (5-15wt%) on the photodegradation rate will be investigated.

The disadvantages of TiO2 powders include the need for separation or filtration steps, the problematic use in continuous flow systems and the particles agglomeration. Thin film deposition techniques of TiO2 overcame these disadvantages That is why in Chapter 6, micrometer thick Ag nanoparticles (NPs) modified TiO2 (designated as Ag-TiO2) thin films will be deposited by using an atmospheric pressure plasma jet (APPJ) technique

STATE OF THE ART: FABRICATION AND PHOTOCATALYTIC

PROPERTIES OF TIO2 NANOMATERIALS 1.1. Introduction

Titanium dioxide (TiO2) is a widely studied semiconducting metal oxide since the discovery of its photocatalytic splitting of water on TiO2 electrode under ultraviolet irradiation by Fujishima and Honda in 1972 [1][2][3]. Because of its high thermal stability, wide bandgap with suitable band-edge, low cost, non-toxic effect and corrosion-resistant properties [1], TiO2 has been used in many promising applications such as photovoltaics and photocatalysis. The primary applications of photocatalysis include the following two categories: 1. photocatalysis of water into O2 and H2, and 2. photodegradation of aqueous and atmospheric organic pollutant [1][2][3][4]. The photocatalytic efficiency is governed by TiO2 material properties, the irradiance frequency, and irradiance intensity. In particular, the material properties dominating the photocatalytic activity include bandgap, charge transportation, crystal structure, specific surface area (SSA) and particles size [5].

Understanding these properties individually is essential in designing a high performance photocatalyst.

Developing methods to modify TiO2 by increasing the specific surface area (SSA), deceasing particle size, optimizing crystal structure are effective strategies for enhancing the overall photocatalytic reaction efficiency [1,[6][7][8][9]. Nanostructure materials can be classified with sizes less than 100 nm or having low-dimensionality including 0D, 1D and 2D where D represents dimension [10]. Typically, quantum dots with size less than 10 nm is included under the 0D classification. 1D group includes nanotubes, nanowires, CHAPTER 1 5 nanofiber, nanorods and nanobelt. 2D group includes nanoplate and nanosheet. When compared to bulk TiO2, nanostructured TiO2 shows novel properties and can be utilized to improve the photocatalytic activities [11]. TiO2 Q1D nanostructures are particularly interesting due to their high SSA, enhanced charge carrier conductivity, and excellent mechanical strength [1].

The photocatalytic activities of pure TiO2 is able to attain a limited level of photocatalytic efficiency due to the fast recombination of photo-generated electrons (e -) and holes (h + ) pairs [4] and its wide bandgap of approximately 3.2 eV which absorbs only UV light (2-3% of the solar spectrum) [4,5]. A wide variety of different methods have been developed to increase the activity of TiO2 by suppressing charge recombination and extending the wavelength response range in the visible light region [4]. These methods include doping TiO2 nanomaterials with other elements [5,12,13], sensitizing with photosensitizers such as dyes [12], depositing metal nanoparticles [4,14] or coupling with other semiconductors [15] and incorporating carbon materials [16,17].

Principles of photocatalysis

A schematic process shows splitting H2O into H2 and O2 over TiO2 photocatalysts (Figure 1.1) [18]. The three main steps include the following (Figure 1.1a): 1. photogeneration of e -and h + in the semiconductor when excited with photons with energy greater than the TiO2 bandgap, 2. transportation of the charge carriers to the TiO2 surface and 3. surface charge transfer between carriers with reactants (e.g., H2O). In competition, the photo-generated charge carriers may recombine with each other dissipating energy without mediating a reaction with the reactants. Recombination is the major limitation in semiconductor photocatalysis since, it decreases the overall efficiency. In the case of water splitting (Figure 1.1b), the energy position of the lowest level in the conduction band is more negative than the reduction potential of water (0 V versus NHE at pH = 0) (2H + + 2e -= H2) while the energy of highest level in the valence band is more positive than the oxidation potential of water (+1.23 vs NHE at pH = 0) (4OH -+ 4h + = O2 + 2H2O). Photogenerated carriers can also split renewable organic components (e.g. ethanol) to produce H2 [19]. The gas evolution rate in µmol•h -1 units is used to compare the photocatalytic efficiency for different photocatalysts. The efficiency is also based on the quantum yield which is defined as the number of events occurring per photon absorbed [19,20]. Notes: a Valence band is the energy band occupied by the valence electrons and the highest occupied band [21], b conduction band is the lowest unfilled energy band and allows electrons moving freely [21], and c CB and VB represent conduction band and valence band, respectively. CHAPTER 1 7 In addition to reacting with water and ethanol to produce H2 and O2, the photogenerated carries are able to degrade organic pollutants [22]. As shown in Figure 1.2, holes in valence band oxidize water and subsequently generates extremely powerful oxidants (•OH radicals) which functions as oxidants to oxidize organic pollutants [22].

Electrons can be transferred to O2 dissolved in water to subsequently generate a superoxide radical anion (O2 •-). This superoxide radical anion is also a powerful oxidant which reacts with organic pollutants to subsequently produce CO2 [22]. Figure 1.2: Schematic diagram for the reaction steps in photodegradation of aqueous organic pollutants [22]. Note: CB and VB represent conduction band and valence band, respectively.

TiO2 nanomaterials morphologies and crystallographic structures

Thin films, powder, and single crystals are typical forms of bulk TiO2. To increase the SSA, decreasing the TiO2 particle size to the nanometer scale leads to nanomaterials such as nanoparticles, nanorods, nanowires, nanotube, nanofilms, nanosheet and nanoribbons [23]. Nanowires/nanofibers are Q1D structures with diameters less than 100 CHAPTER 1 8 nm and a specific growth direction while nanorods have a similar structure to nanowires, but the length is shorter.

Nanotubes have a hollow interior structure while nanobelt/nanoribbons are similar in structure to nanowires but with well-defined side faces. Models for selected TiO2 Q1D nanostructure are shown in Figure 1.3. TiO2 exists in many different crystal forms under standard temperature and pressure conditions as well other forms have been reported under high pressure. These include anatase (tetragonal), rutile (tetragonal), TiO2-B (monoclinic), brookite (orthorhombic), TiO2-R (ramsdellite-structured), TiO2-H (hollandite-structured), TiO2-II (columbite-type) and TiO2-III (baddeleyite-type) [24]. Anatase, brookite, rutile, and TiO2-B are known to occur naturally [24]. However, the other phases which only exist under high pressure conditions are not stable under ambient conditions [24]. Rutile is considered as the most stable phase in terms of thermodynamics, whereas anatase, TiO2-B and brookite are metastable phases which are transformed into rutile when treated at 600-900 o C [25]. All CHAPTER 1 9 the TiO2 phases are formed by stacking co-coordinated TiO6 octahedral units with the Ti 4+ ion positioned in the center of the octahedron and six oxygen atoms located at the six corners. The TiO6 octahedron stacks via either edge-sharing or corner-sharing of the oxygen atoms (Figure 1.4) [26]. The different phase structure showing differences in the distortion of each octahedron and in the assembly pattern of the octahedral chains is due to the Jahn-Teller effect [27]. The crystallographic and physical properties of anatase, rutile, brookite and TiO2-B are shown in Table 1.1. Anatase, consisting of a tetragonal structure (Figure 1.5) [11], is predominantly configured with edge-sharing octahedron [28]. Each octahedron in anatase has four sharing edges and each O 2-ion is closely surrounded by three Ti 4+ ions. Anatase can be synthesized by employing the wet chemical process [25]. The wet chemical process usually leads to the formation of anatase nanoparticles because anatase is relatively thermodynamically stable when the size is confined to the nanoscale level [25].

CHAPTER 1 Rutile also has a tetragonal structural configuration; however, the c-axis is shorter in comparison to the anatase structure (Figure 1.6) [11]. Rutile is composed predominantly CHAPTER 1 11 of corner-sharing octahedron units [28] with octahedral apices connected to each other by edge-sharing chains parallel to the z-axis and the chains cross-linked in a corner-sharing manner. The {011} and {100} crystal face families in the rutile phase (Figure 1.6) have the lowest energy and thus, is a trunked octahedron that is the mostly observed morphology [30]. Rutile production through solution phase synthesis or transformation of other phases at 600-900 o C is primarily due to its thermodynamic stability when compared to the other phases [25]. [11]. Notes: a red circles correspond to oxygen atoms, and b JCrystal is a Java application for creating, editing, displaying and deploying crystal shapes [11].

TiO2 brookite orthorhombic structure (Figure 1.7) is composed of large unit cells [11].

The structure consists of distorted TiO6 octahedrons which are connected by both edgeand corner-sharing to construct a framework as shown in Figure 1.7. Although brookite is synthesized through the wet chemical process, it normally coexists with the rutile or anatase phases [28]. TiO2-B is constructed with large unit cells (Figure 1.8) having a monoclinic structure with a particularly long a-axis [30,31]. TiO2-B is composed of corrugated sheets with edge-and corner-sharing TiO6 octahedrons. These sheets pile up to construct a 3D framework. TiO2-B can be synthesized by proton exchange and subsequent dehydration of layered titanates at a temperature of approximately 300 o C [31,32]. TiO2-B can be transformed into anatase or rutile at high temperatures because of its thermodynamic instability [31,32].

Figure 1.8: TiO2-B unit cell [30].

TiO2 nanomaterials synthesis

Synthesis of TiO2 nanostructures can be accomplished by employing various methods such as sol-gel, chemical and physical vapor deposition (CVD and PVD), hydro/solvothermal, direct oxidation, surfactant-controlled approach and the electrochemical route. A brief overview of various methods is discussed in the following sections.

Sol-gel method

The sol-gel method is widely applied for synthesizing various ceramic materials [5].

In general, a colloidal suspension is produced from the hydrolysis and polymerization reaction of a precursor molecule [5]. Subsequently, complete polymerization and loss of solvent leads to the formation of a solid gel phase. The solid gel is then easily cast into the desired shape such as nanofilms and nanofibers. Furthermore, the wet gel is transformed into a dense ceramic by drying and heating treatment.

TiO2 nanomaterials usually result from the hydrolysis of a titanium (IV) precursor. In a typical procedure to produce TiO2 nanoparticles [5], a TiO2 sol solution is produced by slowly mixing a precursor such as titanium isopropoxide (TTIP) in an alcoholic solution containing water. The 3D polymeric skeletons with close packing were developed by forming Ti-O-Ti chains. Next, the mixture is heated at 90-100 o C for 6 h to produce the gel. Subsequent heating the gel at 400 o C for 2 h is used to improve the crystallinity of the TiO2 anatase nanoparticles.

CHAPTER 1

The combination of the sol-gel method and an anodic alumina membrane template with long nanopores is useful in producing TiO2 nanowires (Figure 1.9) [33]. A brief description of the process involves dipping porous anodic alumina membrane into a boiling TiO2 sol followed by drying and heating. After heating at 500 o C, the TiO2 sol is converted into anatase TiO2. Next, the anodic alumina membrane template is removed by placing the combined TiO2/alumina membrane template into a H3PO4 aqueous solution.

Figure 1.9: Anatase TiO2 nanotubes SEM image prepared by combining the sol-gel and template methods [33]. Notes: a Synthesized using anodic alumina membrane template and TiO2 sol, and b drying and heating at 60 o C for 12 h, and c anodic alumina membrane template was removed in a 5wt% H3PO4 solution at room temperature for 3 h.

Sol method

The sol method is a non-hydrolytic sol-gel process. The method involves reacting titanium chloride with a metal alkoxide such as titanium alkoxide (Eq. 1.1).

TiCl4 + Ti(OR)4 = 2TiO2 + 4RCl [34] (1.1)

where RO -is an alkoxide. The reaction results in the formation of Ti-O-Ti bridges. The alkoxide groups can be either titanium alkoxides or the products formed by in-situ reacting titanium chloride with alcohols. Zhu et al. [34] reported mixing TiF4 with benzyl alcohol to produce an organic-rich titanium precursor. Next, the mixture is placed in an autoclave at 160 o C for 24 h with the subsequent formation of anatase nanosheets (Figure 1.10).

Figure 1.10: TiO2 anatase nanosheet SEM image [34]. Note: Synthesized using TiF4 and benzyl alcohol at 160 o C for 24 h.

Hydrothermal method

Hydrothermal synthesis is widely employed for producing small particles in the ceramic industry. The method is performed in aqueous solution under controlled temperature and/or pressure conditions. The autoclave reactor consists of steel pressure vessels with or without a Teflon liner (Figure 1.11). The temperature can be set to 300 o C CHAPTER 1

16 by controlling the pressure. The morphology for the prepared TiO2 nanomaterials depends on the type of solution, pH, temperature and the reaction time [5].

Figure 1.11: Commercially available magnetically stirred steel pressure vessel configured with pressure control and an external pump to control the gas flow [35].

Q1D TiO2 nanomaterials have also been produced using the hydrothermal method (Figure 1.12). This widely used method employs treating TiO2 particles in a 5-15 M

NaOH aqueous solution at 110-200 o C for 24-120 h [36][37][38][39][40][41][42]. The transformation of TiO2 powder into nanotubes involves shearing, exfoliation, or delamination of lamellar nanosheets from a crystalline precursor [36]. The lamellar nanosheets edges have many atoms with dangling bonds which are not stable [36]. Next, the lamellar nanosheets bent and scrolled into Q1D nanotubes or nanowire (Figure 1.12) to saturate the dangling bonds [36]. The detailed mechanism is discussed in Section 1.5.4.

Solvothermal method

The solvothermal method is identical to the hydrothermal method except the solvent used is non-aqueous (e.g. toluene). The solvothermal method has better control of the reaction temperature, size, shape and crystallinity of TiO2 products [30]. The temperature can be increased much higher due to employing organic solvents with high boiling points. Solvents with different physical and chemical properties can affect the diffusion behavior, solubility, reactivity of the reactants which in turn influence crystallization and the crystal morphology of the final products [30]. Kim et. al. [38] reported that TiO2 nanorods were produced by mixing titanium isopropoxide with anhydrous toluene and oleic acid at 250 o C for 20 h (Figure 1.13).

Figure 1.13: Nanorods TEM image synthesized using a solvothermal method [38]. Note: TiO2 nanorods were synthesized by mixing titanium isopropoxide, anhydrous toluene and oleic acid at 250 o C for 20 h.

Chemical/physical vapor deposition

Vapor deposition involves producing a vapor which is condensed onto a heated solid.

The process is usually executed in a vacuum chamber. The method is designated as a physical vapor deposition (PVD) if the final product does not involve a chemical reaction, otherwise the process is designated as a chemical vapor deposition (CVD).

Producing TiO2 nanowires using a simple PVD method is illustrated in the SEM image shown in Figure 1.14 [39]. In a typical synthesis process, 1.5 g of source material

(pure Ti metal powder) and a Si (100) substrate are separately placed on 2 graphite boats in a horizontal tube furnace. In this system, the source Ti powder is in the hightemperature zone and the Si (100) substrate is in the low-temperature zone. The furnace is set at 1050 o C in the high-temperature zone and 850 o C in the low-temperature zone.

The PVD process is initiated when an Ar/O2 mixture gas flowing at a pressure of 40,000

Pa (300 Torr) is used to convert Ti into a layer of TiO2 nanowires [39]. Evidence of producing TiO2 anatase nanorods by growing them on silica substrates using the CVD method is shown in Figure 1.15 [40]. By using a N2/O2 mixture gas carrying, Wu and Yu [40] demonstrated the vapors of the titanium acetylacetonate (Ti(C5H7O2)4) precursor were used to produce nanorods at a relative low temperature ranging between 200-230 o C. The precursor reacts and is grown on a relatively high temperature (500-700 o C) silica substrate surface. The phase and morphology of the TiO2 products are tunable by adjusting the substrate temperature and pressure [40]. For Besides the electrodeposition method, electrochemical etching/anodization method is a versatile process which allows the production of a dense and well defined TiO2 nanomaterial onto titanium surface [1,30,42,43]. When titanium metal is exposed to a sufficiently high anodic voltage in a cell, electrochemical oxidation produces Ti 4+ (Ti →Ti 4+ + 4e -). Next, the Ti 4+ ions react with O 2-in the electrolyte (Eq. 1.2) to form a compact TiO2 layer [27,43,44].

Ti + 2H2O = TiO2 + 4H + + 4e - (1.2)
Figure 1.16: SEM images of TiO2 nanowires produced using a electrodeposition method [41].

Direct oxidation method

Oxidizing titanium metal can be employed to produce TiO2 nanomaterials. For example, TiO2 nanorods on a titanium plate were produced by oxidation using 30% H2O2 aqueous solution at 78 o C for 72 h [45]. The TiO2 nanorods phase structure is controllable by adding inorganic salts such as NaF, NaCl or NaSO4 [45]. Pure anatase nanorods are produced by adding NaF or NaSO4, while pure rutile nanorods are produced by adding NaCl.

Surfactant-controlled method

With the assistance of surfactants, anisotropic growth of TiO2 nanostructures is accomplished by the hydrolysis of metal alkoxides. Surfactants have the ability to select the crystal facet and manipulate the final structure of TiO2 nanomaterials, since they modify the surface energy of desired facets [30]. For example, Cozzoli et al. [46] CHAPTER 1 22 reported growing anatase TiO2 nanorods by the hydrolysis of titanium tetraisopropoxide using oleic acid as the surfactant.

Other methods

Sonication has been employed to produce TiO2 nanostructure [5]. In addition, microwaves have been used to prepare TiO2 nanomaterials [5]. The main advantages of using microwaves are rapid heat transfer as well as volumetric and selective heating.

Preparation of quasi-one-dimension TiO2 nanostructure

Why quasi-one-dimension TiO2 nanostructure?

Q1D nanostructures are able to provide high electron mobility, high SSA and excellent mechanical strength [1]. For many applications, maximizing the SSA is crucial in increasing the overall efficiency. Decrease the TiO2 dimension from bulk TiO2 to the Q1D TiO2 nanomaterials results in a large increase in the SSA coupled with enhanced electronic properties [1]. Fast charge transportation along TiO2 Q1D nanostructure is able to improve the overall photocatalytic efficiency [1,47]. In addition, TiO2 Q1D nanostructures allow for a higher control of the chemical and physical behaviors for applications by manipulating the morphology [1]. For example, a free-standing membrane consisting of vertically oriented, both-side-open TiO2 nanotubes was reported and used as micro-photoreactors (Figure 1.17) [48].

Growth of quasi-one-dimension TiO2 nanostructure

Rapid growth in one direction is crucial in obtaining Q1D TiO2 nanostructures during the evolution of nanocrystal [29]. Synthesis of Q1D TiO2 nanostructures have been achieved by various approaches including sol-gel methods, template-assisted methods, hydro/solvothermal approaches, and electrochemical means [29,43]. In the previous section for synthesis of TiO2 nanomaterials, many examples were discussed for producing TiO2 Q1D nanomaterials. Hydrothermal method is a simple approach for scale-up without the use of any templates and surfactants [43]. The hydrothermal process is a soft-chemical technique as the reactions occur at relatively low temperatures. The morphology and crystal structure are controllable by manipulating the synthesis parameters such as reaction temperature and alkaline concentration [37,43]. This protocol has been employed in this study and is discussed in detail below.

Alkaline hydrothermal method and its parameters

The hydrothermal synthesis protocol involves the treatment of a TiO2 precursor with concentrated alkaline solutions sealed in a Teflon-lined stainless steel autoclave at elevated temperatures (110-200 o C) for 24-120 h [44]. Subsequently, the powders were washed with 0.1 M HCl, then distilled water for at least 5 times until pH was close to 7

and calcinated from 300-500 o C to increase the crystallinity of the product.

The TiO2 precursor can be hydrothermal treated with various hydrothermal solutions such as NaOH, KOH and Na2S. However, the precursor incompletely reacts with either KOH or Na2S even after mixing for 2 weeks. Therefore, the NaOH solution is preferred as a hydrothermal solution. Hydrothermal treatment times varies from 24 h to 120 h [37].

For potential application and scale-up in industry, short time synthesis is usually preferred. In this project, we only consider 24 or 48 h as the reaction time.

Hydrothermal temperature, TiO2 to NaOH molar ratio significantly affects the product morphology, SSA, crystal structure, bandgap and photocatalytic activities [37].

Bavykin et. al. [37] demonstrated the average diameter of the nanotubes is dependent on the hydrothermal temperature and on the TiO2 precursor/sodium hydroxide molar ratio.

Increasing the temperatures from 120 o C to 150 o C caused an increase of the average nanotube diameter [37]. Subsequent increase in the temperatures resulted in a change in crystal form from nanotube to nanowire with an average diameter of 75 nm, a wide distribution in diameter and a length longer than 10 µm [37]. The increase of the TiO2 to NaOH molar ratio resulted in an average diameter increase of the nanotubes and a decrease in the SSA. Non-hollow TiO2 Q1D naonstructure can be formed when the curving of lamellar sheets is inhibited [37,50]. During hydrothermal process, NaOH reacts with TiO2 crystallites and free the TiO6 octahedron. The TiO6 octahedron assembles into lamellar sheets. The concentration of Ti 4+ in the solution impacts the latter process. Wu et al. [51] proposed that the lamellar sheets curve around the b axis to form nanotubes when the Ti 4+ concentration is less than 3.3 molm -3 (Figure 1.20). However, when Ti 4+ concentration exceeds 3.3 molm -3 , curving along the b axis is inhibited. The inhibition of the curving allows the lamellar sheets structures to thicken along a and b axis and lengthen along c axis. Finally, lamellar sheets structures are growing to produce nanorods or nanowires. (CVD), physical vaper deposition (PVD) and plasma-assisted deposition processes [52] has been widely applied to prepare thin film [53].

The plasma-assisted deposition processes generally include two categories: Plasma enhanced chemical vapor deposition (PECVD) and magnetron sputter deposition.

Plasma-assisted deposition processes have increased significant interests in industrial use mainly due to their following:

(1). Low temperature deposition when compared to conventional PVD and CVD processes. For example, silicon nitride coating can be deposited by PECVD at 300 o C.

Whereas the conventional CVD process requires 900 o C [52].

CHAPTER 1 28 (2). A broad range of experimental parameters (e.g. pressure, gas flow, excitation frequency, power) can be adjusted to optimize the films composition and microstructure [54].

(3). High deposition rates (1 nm•s -1 -40 µm•s -1 ). The high deposition rate allows a reliable low-cost fabrication [54].

(4). Plasma-assisted deposition allows uniform coating on different substrate shapes (flat, hemispherical, long fibers, cylindrical shapes, the interior of tubes, etc.) [54].

(5). Plasma-assisted deposition films shows better mechanical properties (e.g. adhesion, stress compensation, abrasion, and wear resistance) when compared to their PVD counterparts [54]. (6). Plasma-assisted deposition allowing doping to optimize the photocatalytic activities of the thin film [54].

Plasma enhanced chemical vapor deposition (PECVD)

PECVD is a method for film preparation using gas phase precursors activated in a glow discharge environment [54]. Indeed, by using a plasma which is a reactive medium, the CVD process which requires fragmentation of the precursor will take place at a much lower temperature because of the collisions with electrons produced in plasma.

The PECVD process can be conducted in both low pressure and atmospheric pressure.

In particular, the atmospheric-pressure plasma processes (e.g. APPJ) (Figure 1.23) [57,58] and dielectric barrier discharges (DBD) plasma [59] have become the most promising candidate systems to replace the thermal CVD or sol-gel processes which are employed for the deposition offunctional coatings [57,58]. This is due to the low cost, high deposition rate, low temperature, one step process, easy implementation, and inlineprocess capabilities. The careful selection of the plasma source, the titanium precursor, precursor carrier gas allows the deposition of well-adherent and dense TiO2 coatings with tunable properties at low temperature [60]. The low temperature condition allows a wide choice of substrates without damaging them (e.g. optical fiber glasses, nickel films and a carbon-polymer composites) [60,61]. . In an open-air APPJ system, the porosity and crystallinity of the TiO2 coating was significantly controlled by varying the deposition parameters such as the pulse frequency, working gas flow rate and the post annealing temperature [58]. A APPJ TiO2 coating deposited at 23 kHz and 2400 L•s -1 was characterized with a high SSA porous structure (Figure 1.23b) and a significant enhancement of photocatalytic efficiency for the degradation of RhB when compared to an N-doped TiO2 coating which was deposited by reactive radio frequency (RF) magnetron sputtering [58,62].

Nie et. al. [59] synthesized nanocrystalline anatase TiO2 using TiCl4 and O2 as precursors by an atmospheric cold DBD plasma (Figure 1.24a). The size of the TiO2 nanocrystal attained was 10-15 nm (Figure 1.24b) when plasma energy density was set at 5.9 kJ•L -1 . They observed particles size decreased with increasing the plasma energy density. The DBD plasma process is promising, due to a single-step, low cost, easy to control and simple equipment for the discharge.

Magnetron sputter deposition

Magnetron sputter deposition is utilized heavily in industry and which provides the possibility to deposit metals, but also oxides, nitrides, alloys etc. [52]. In the case of physical magnetron sputtering [52], the bombarding ions (i.e. argon ions at 500-1000 V) physically sputter the target metallic atoms which then are transferred to and deposited on a substrate. In the case of reactive magnetron sputtering, a reactive gas (i.e. O2 and N2)

whose dissociation products chemically react with the target (Ti) was supplied in addition to the bombarding ions. For example, physical magnetron sputtering of a Ti target with argon as bombarding gas results in the formation of Ti thin film. If O2 gas is supplied as reactive gas, TiO2 thin film can be synthesized [62]. Fakhouri et. al. [62] synthesized N-

Modifications of TiO2 nanomaterials

The majority of pure semiconductors are characterized with a low quantum yield, optical response, utilization rate of visible light. The low photocatalytic efficiency becomes a challenge in designing and developing photocatalysts for practical applications.

Fortunately, the modification of pure semiconductors is a facile strategy to enhance their overall efficiency. The mechanism for the modification includes [4,6,56]: (1).

modifying the crystal structure and morphology, (2). narrowing bandgap, (3). suppressing charge recombination, and (4). enhancing charge transfer.

Modifying TiO2 crystal structure and morphology

The TiO2 crystal structure and morphology can significantly affect the photocatalytic properties [63]. For pure phase, anatase phase is more active than the rutile phase. The poor photocatalytic activities of rutile are due to the intrinsic defects in the rutile structure where the electrons are trapped. Meanwhile, the photocatalytic activity of biphasic anatase-rutile photocatalysts increases significantly when compared to any pure phases of TiO2 [64]. When the two crystal phases are combined, electron migration across the anatase-rutile phase junction facilitates the separation of photogenerated holes and electrons with an increase in the photocatalytic activity (Figure 1.26) [64,65].

The particle size and morphology strongly affect the dynamics of e --h + recombination (e.g. bulk and surface recombination) [66][67][68][69]. Generally, bulk charge recombination is dominant in the well-crystallized large-size semiconductor particles [66]. A decrease in the particle size is closely correlated to a significant decrease in the bulk charge recombination, an increase of the SSA and improved photocatalytic activities [66].

However, according to Okura and Kaneko [70], increasing the SSA beyond a threshold value could lead to enhanced surface defects and surface charge recombination with a subsequent negative effect on photocatalysis. Therefore, a trade-off between bulk charge recombination and surface charge combination must be evaluated before designing a photocatalyst [5].

TiO2 2D nanostructures (nanosheets, nanoplates) and Q1D nanostructures (nanowire, nanotube, nanoribbon, nanorod) also facilitate the charge transfer inside the crystals to the surface and subsequently enhance the charge separation. Atomic scale thickness, smaller diameter and large surface area allow shorter charge transfer distance and time [5,7,71,72]. Thus, the bulk charge recombination will be impeded to some extent.

Figure 1.26: Anatase-rutile phase junction structure [73].

Metal and nonmetal doping

One of the effective solutions to enhance photocatalytic efficiency is to create an impurity in the forbidden band via metal and nonmetal doping [4,6,56]. Doping narrows the wide bandgap of TiO2 and expands TiO2 optical response into a visible-light region. Researchers have reported that metal doping (Sn, Fe, Mo, Ru, Os, Re, V and Rh ion doping) can significantly increase the photocatalytic activities under visible light [12,64]. The mechanism of visible-light-driven photocatalysts using metal ion doping is illustrated in Figure 1.27 [63]. An appropriate doping element creates either a donor level above the original valence band or an acceptor level below the original conduction band [63]. These new levels narrow the original band to expand the photocatalysts optical response into the visible light region [63]. In addition, the doping ions serve as a recombination inhibitor by trapping electrons or holes. This in return promotes the charge separation required for the photocatalytic reaction [63].

Unlike metal-ion dopants, nonmetal-ion dopants shift the valence band edge upward without forming any donor or acceptor levels in the forbidden band. The up-shifting of valence band edge results in a narrowed band gap (Figure 1.28) [63]. Various nonmetal CHAPTER 1

35 ions (e.g. C, N, S) were used to dope TiO2 [63,74]. To date, the N doping is most efficient and most widely investigated [75,76]. In N-doped TiO2, the substitution of N for O leads to the mixing of the 2p states of N with the 2p of O [63]. The narrowing bandgap resulted in N-doped TiO2 having much higher photocatalytic activities in the visible irradiation when compared to pure TiO2.

Depositing noble metals

Deposition of noble metals (Ag, Pt, Au and Pd) on the surface of TiO2 nanostructure can significantly improve the photocatalytic activities [19,77,78]. Noble metals serve as electron trap to delay the recombination of the e --h + pair [20,79]. Figure 1.29 illustrates the electron trapping properties at the Schottky barrier between the metal particle and a semiconductor surface [20,80]. The photoexcited electrons migrate to the metal particles and are trapped in metal particles due to the Schottky barrier, whereas the holes in TiO2 is free to transfer to semiconductor surface and react with reactants [20,80]. The Pt-TiO2 system is significantly efficient to evolve H2 gas, due to electron trapping and catalytic effect of Pt [20,81].

Semiconductor coupling

The semiconductor coupling is another effective method to improve photocatalytic activity due to an enhanced charge separation by forming a heterojunction structure [20,82]. Coupling TiO2 with CdS having a narrow bandgap of 2.5 eV improves its CHAPTER 1 37 photocatalytic activity in the visible light region [15]. The charge separation is shown in

Enhancing TiO2 with carbon materials

Incorporating TiO2 into carbon nanomaterials significantly increases the activity of TiO2 for the photocatalysis of organic pollutants such as phenol and methyl orange [86].

In recent years, increasing interest in using graphene (Figure 1.32), a 2D carbon atoms sheet, is primarily due to its unique physical and chemical properties. Graphene is characterized with a high charge carrier mobility (15000 cm 2 •V -1 •s -1 ), high SSA (up to 2630 m 2 •g -1 ), high thermal conductivity (approximately 5000 W•m -1 • o C -1 ), and a tunable bandgap by chemical doping [87][88][START_REF] Zhu | Carbon-based supercapacitors produced by activation of graphene[END_REF]. These excellent properties are leading reasons why graphene modified TiO2 photocatalysts can find promising applications for air and water purification [16,[START_REF] Yu | Enhanced photocatalytic performance of direct Zscheme g-C3N4-TiO2 photocatalysts for the decomposition of formaldehyde in air[END_REF][START_REF] Lv | One-step synthesis of CdS-TiO2-chemically reduced graphene oxide composites via microwave-assisted reaction for visible-light photocatalytic degradation of methyl orange[END_REF]. A general morphology of graphene modified TiO2 nanotube composites is shown in Figure 1.33 [47]. The TiO2 nanotubes disperse on the graphene sheets maintains a direct contact with the graphene sheets. The graphene sheets functions by improving the TiO2 photocatalytic activities through the following three aspects [16,[START_REF] Zhang | P25-graphene composite as a high performance photocatalyst[END_REF][START_REF] Kim | UV protection of reduced graphene oxide films by TiO2 nanoparticle incorporation[END_REF][START_REF] Fan | A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors[END_REF]:

1. Enhancing photocatalysts adsorptivity to adsorb pollutant molecules. Graphene has indeed a high SSA and adsorbs organic pollutant molecules efficiently through the ππ conjugation [47]. For example, the phenol has the same aromatic compounds as graphene. The π-π conjugation between phenol and graphene facilitates the adsorption of chemicals such as phenol on graphene.

2. Extending light absorption. Graphene renders a red shift in the photo-responding range and improves the utilization of light [16,[START_REF] Zhang | P25-graphene composite as a high performance photocatalyst[END_REF][START_REF] Kim | UV protection of reduced graphene oxide films by TiO2 nanoparticle incorporation[END_REF][START_REF] Fan | A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors[END_REF].

3. Suppressing the charge recombination [16,[START_REF] Zhang | P25-graphene composite as a high performance photocatalyst[END_REF][START_REF] Kim | UV protection of reduced graphene oxide films by TiO2 nanoparticle incorporation[END_REF][START_REF] Fan | A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors[END_REF]. Graphene serves as an electron scavenger that receives electrons from the TiO2 and provides a fast electron transfer route due to its high charge carrier mobility [16,[START_REF] Zhang | P25-graphene composite as a high performance photocatalyst[END_REF][START_REF] Kim | UV protection of reduced graphene oxide films by TiO2 nanoparticle incorporation[END_REF][START_REF] Fan | A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors[END_REF]. The photogenerated e -in TiO2 are transferred to graphene nanosheet, whereas h + remain in TiO2 [16,[START_REF] Zhang | P25-graphene composite as a high performance photocatalyst[END_REF][START_REF] Kim | UV protection of reduced graphene oxide films by TiO2 nanoparticle incorporation[END_REF][START_REF] Fan | A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors[END_REF]. The electrons on graphene are transferred to electron acceptors and the holes remain in TiO2 are transferred to electron donors. In this case, an effective charge separation and transportation are obtained.

Summary of the research objectives

In Chapter 1, the first objective in this thesis is to provide an overview of the principle of photocatalysis, TiO2 morphologies and crystallographic structure, phase structures, synthesis methods and modification of TiO2 for improving photocatalytic activities in the UV light as well as visible light.

In Chapter 2, the second objective is to provide an overview of the chemicals, instruments for characterization and the GO and Q1D TiO2 synthesis process.

The subsequent chapters are organized in manuscript format. In Chapter3, the first part of third objective is to evaluate the impact of the hydrothermal reaction conditions on the phase structure, crystal size, specific surface area (SSA) and bandgap. The second part of the third objective is to optimize the reaction conditions for maximizing and minimizing bandgap using a 3-factor 3-level Box Behnken design (BBD).

In Chapter 4, the first part of fourth objective is to optimize hydrothermal conditions for synthesizing Q1D TiO2 with the maximum photodegradation of aqueous hazardous pollutants using the BBD. The aqueous hazardous pollutants examined in this chapter will include rhodamine b (RhB), methyl orange (MO), methylene blue (MB) and phenol.

Next, the second part of fourth objective is to optimize hydrothermal conditions for synthesizing Q1D TiO2 with the maximum photocatalytic H2 production rate using BBD models.

In In Chapter 6, the first part of sixth objective is to examine micrometer thick Ag nanoparticles (NPs) modified TiO2 (Ag-TiO2) coatings synthesized using a single and facile non-equilibrium atmospheric pressure plasma jet (APPJ) with different concentrations of Ag NPs (0-0.7 wt%). The second part of sixth objective is to examine the effect of the Ag concentration (0-0.7 wt%), air gas flow rate and the discharge pulse frequency on the coating phase structure, crystal size and photocatalytic activity. The third part of sixth objective is to investigate photocatalytic activity efficiency by examining the photodegradation of rhodamine b (RhB) and trace pharmaceutical compounds, namely carbamazepine (CBZ), venlafaxine (VLX) and bezafibrate (BZF), using a solar simulator.

MATERIALS AND EXPERIMENTAL METHODS

Introduction

In this chapter, the chemicals, instruments used to characterize the photocatalysts and the synthesis processes used to synthesize some materials which will be used in whole thesis are detailed in this chapter. This chapter includes three sections: a) Chemicals b) Analytical and characterization instruments c) GO and Q1D TiO2 synthesis processes Note: The chemicals, instruments and the detailed process in this chapter are referenced in subsequent chapters.

Chemicals

Commercial TiO2 nanoparticles (P25) were purchased from Degussa (Evonik Industries, Germany). Graphite flakes (325 mesh, 99.9995%) and KMnO4 (99.0%) were purchased from Alfa Aesar (MA, United States). H3PO4 (85%), H2SO4 (98%), H2O2 were procured from Sigma Aldrich (Saint-Quentin-Fallavier-Vendor location -France, add city). Nitrogen (99.995% purity) and air (99.5% purity) were purchased from Air Liquide (Paris, France). Si (100) wafers were purchased from Siegert Wafer (Aachen, Germany).

Characterization instruments and processes

Electron microscopy

The morphology and nanostructure were investigated using high-resolution transmission electron microscopy (HRTEM) (JEOL 3010, Japan) which allows for direct imaging the atomic structure [1] of the sample. The sample was prepared by ultrasonically dispersing the photocatalyst in ethanol. Next, 5 uL of sample solution was deposited on a carbon film and dried at a temperature of 25 o C. The HRTEM was configured to operate at 300 kV with a high resolution of 0.17 nm.

Field emission scanning electron microscope (FESEM) (Thermo Fisher Scientific, MA., United States) and Energy dispersive X-ray (EDX) (Thermo Fisher Scientific, MA., United States) analysis were employed to examine the surface morphology and sample elemental composition. The photocatalyst sample was dispersed ultrasonically in ethanol and a small amount of the solution (20 uL) was deposited and dried on aluminum film at room temperature. The instrument scanning of the surface of materials using a focused electron beam allows the researcher to gather information related to the sample's surface topography and composition (EDX).

CHAPTER 2 55

Raman spectra

Structural fingerprinting of a sample was examined using an Alpha300 RA argon laser Raman spectrometer (WITec, Germany) equipped with a 532-nm laser. The Raman spectra with a high signal/noise ratio was recorded from 100 to 3000 cm -1 with the accumulation times set as 40 s and integration time as 0.5 s. In this study, the phase of TiO2 sample and the degree of order of reduced graphene oxide (RGO) samples were analyzed using Raman spectroscopy.

X-ray diffraction (XRD) analysis

The phase structure and elemental composition were determined using XRD analysis.

XRD was performed with an X-ray diffractometer (Rigaku, MI, United States) equipped with a Cu Kα radiation source (0.154 nm). The XRD patterns were collected using step scans with a step of 0.01 o 2θ and a count time of 0.2 s per step between 10 o and 80 o 2θ.

Diffuse reflectance UV-visible spectroscopy (DRS) and dye concentration

The sample optical response in the UV and visible region was analyzed using Diffuse reflectance UV-visible spectroscopy (DRS). The DRS was obtained using a Cary 300 UV-visible (Agilent Technologies, United States). Next, the DRS was converted to into an absorption spectra using the Kubelka Munk function [2]. The absorption spectra were used to determine the bandgap for each sample.

CHAPTER 2 [3,4]. Error of electron binding energies of chemical groups was less than 0.2 eV.

Brunauer-Emmett-Teller (BET) and specific surface area (SSA)

Nitrogen adsorption-desorption isotherms were determined at 77K using a

Micromeritics ASAP 2020 Brunauer-Emmett-Teller (BET) analyzer (Micromeritics Instrument Corp. GA, United States). The instrument was used to determine the SSA.

High performance liquid chromatograph (HPLC)

The residual phenol concentration was determined using a high-performance liquid chromatograph (HPLC) (Dionex Ultimate 3000, Sunnyvale, CA) equipped with a UVvisible photodiode array detector set at 274 nm and configured with an Acclaim C18 -3 µm -2.1 mm (ID)× 100 mm (length) column (Dionex, Sunnyvale, CA). The analysis was conducted at 45 o C with an acetonitrile-water mixture (1:4) eluent (Fisher Scientific, Ottawa, ON) at a flow rate set at 0.4 mL min -1 as described in an earlier study [5]. The phenol detection limit was 5.0 µg

Photoelectrochemical measurements

The charge carrier's transportation and separation were measured using photoelectrochemical measurements which were conducted using a CHI 704D (CH Instrument inc, Texas) with a standard three-electrode cell. One side of cell is constructed from quartz for UV-light transmission. Platinum wire was used as the counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode. For the working electrode, 0.25 g photocatalyst solution was mixed with 0.06 g polyethylene glycol (PEG, molecular weight 20 000) and 1 mL deionized water to form a slurry. The slurry afterward was coated onto a 2 cm x 1.2 cm F-doped SnO2-coated (FTO) glass electrode using a doctor blade method [6,7]. The electrode was annealing at 450 o C for 30 minutes. The measurements were performed in a 1 M Na2SO4 aqueous solution as electrolyte. A 300-nm monochromatic UV light with an average intensity of 9 mW•cm -2 was used as the source. In addition, a visible light with a KenKo L41 UV filter (≥410 nm)

with an average intensity of 2 mW•cm -2 was used as visible light source. (Southern New England Ultraviolet Co., Branford, CT). The chamber was installed with sixteen monochromatic (300 nm) UV lamps (Southern New England Ultraviolet Co., Branford, CT) on the outer perimeter of the reactor with a rotating inner carousel as described in a previous study by Ray et al. [5]. The quartz tubes in triplicates were placed on the rotating carousel and the reaction mixture was magnetically stirred to maintain the photocatalyst in suspension, minimize particle agglomeration and mass transfer limitation. The average intensity of irradiance emitted from the lamps was 9 mW•cm -2 . The light intensity was measured using a light intensity meter equipped with a CHAPTER 2 59 300 nm UV sensor (UV Process Supply, Chicago, IL). The reactor temperature was maintained at 37 ± 2 o C.

Optical emission spectroscopy of plasma

Plasma emission were sampled using an optical fiber located downstream at 10 mm from the nozzle exit of the torch and connected to an optical emission spectrometer (OES) (Ocean Optics USB2000 Pro, FL, United States) configured with a 300 gmm -1 grating in the UV-visible spectrum range and a 25 µm slit. The OES spectra were recorded in the range of 200 nm to 1100 nm to identify the excited chemical species in the plasma with and without the presence of the titanium isopropoxide (TTIP) precursor including Ag NPs.

Thin film photocatalytic activity test

The photocatalytic activity was conducted using two methods based on the chemicals under consideration. In the first test, RhB (initial concentration 5 mg•L -1 ) degradation was examined at room temperature.

The TiO2 thin film samples, deposited on 9 × 25 mm 2 glass substrates were placed in In the second procedure, the degradation of VLX, BZF and CBZ was monitored at room temperature. All three compounds are persistent in the environment due to resistance to conventional treatment technologies [8][9][10]. The coated slide samples were immersed in a 90 mm x 50 mm Pyrex glass dish containing 30 mL of DI water spiked with a stock solution (1 mg•L -1 ) containing the three pharmaceutical compounds with each at an initial concentration of 100 μg•L -1 . The three pharmaceutical compounds standards (purity>99%) were purchased from Sigma-Aldrich, and used as received stock solution. Experiments were performed at neutral pH=7 using a 1 mM PBS saline. The pharmaceutical compounds solution was stirred for 60 min under dark conditions to ensure adsorption/desorption equilibrium of the chemicals onto the catalyst. Next, the equilibrated solution was irradiated using a solar simulator for 60 min. During stirring the reaction mixture, 0. Premier, MA, USA) following a method reported by Zucker et al. [11]. The analysis was conducted at 45 o C with a water-methanol (1:9) mixture eluent. The eluent was adjusted to pH 3 using formic acid. The HPLC flowrate was set at 0.5 mL•min -1 . The mass spectrometer was set in positive mode with a detection limit of 0.1 µg•L -1 .

Graphene oxide (GO) and quasi-one-dimension (Q1D) TiO2 synthesis

Synthesis of graphene oxide (GO)

GO was synthesized using an improved Hummer's method [12]. Briefly, a 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 ml), graphite flakes (3.0 g) and KMnO4

(18.0 g) were mixed using a magnetic stirrer in a 1 L beaker for 1h at 20 o C. Next, the black mixture was heated at 50 o C for 12 h. The reaction mixture cooled to room temperature and subsequently mixed with ice flakes (500 mL). 5 mL of 30% H2O2 was added to the dark brown mixture and the mixture was converted into a golden yellow color. The golden yellow solid was collected after 2-h centrifugation (5000 rpm) using Sorvall ST 16 centrifuge (Thermo Scientific, On.). Then, the solid was washed with deionized water (200 mL), followed by washing with 30% HCl (200 mL) and subsequently with 6 washes of ethanol (200 mL). After each washing, the mixture was centrifuged (5000 rpm for 2 h) and the supernatant decanted. Finally, the yellow solid was dialyzed against deionized water using dialysis bags to remove any residual metal ion. The solid in the dialysis bag was collected and vacuum-dried at 60 o C to produce GO powder.

Hydrothermal synthesis

The hydrothermal synthesis liquor containing NaOH and the precursor as TiO2 NPs (commercial P25, a mixture of anatase (75%) and rutile (25%)) were prepared based on the fact that NaOH readily reacts with the TiO2 precursor and the commercial availability of P25 TiO2 NPs.

The Q1D TiO2 was synthesized using hydrothermal method [13][14][15]. To prepare Q1D TiO2 photocatalyst, 2.5 g TiO2 P25 nanoparticles was homogeneously mixed with 70 ml of 5 M NaOH using a magnetic stirrer for 30 min. Then, the mixture was placed in a 100-mL Teflon® capped bottle. The Teflon® bottle was placed into a stainless-steel bottle then capped, heated at 120 o C for 48 h, and subsequently cooled to room temperature. The resulting paste was centrifuged and rinsed with 0.1 M HCl. The white paste was washed several times with deionized water to attain a pH = 7 and calcined at 400 o C for 2 h to produce the Q1D TiO2.

CHAPTER 2 63

Plasma process used for deposition

Ag-TiO2 coating were prepared by introducing TTIP into an open-air atmospheric pressure plasma jet (APPJ) (Plasmatreater AS400, Plasmatreat GmbH, Steinhagen, Germany) by means of an 'in-house' constructed spraying system (Figure 2.3). High deposition rates (10-20 µm•s -1 ) were achieved by spraying a liquid precursor into the plasma torch. The 100 nm Ag NPs concentration was varied from 0 to 0.7 wt.% and dispersed in the liquid precursor. The plasma torch was created using dry compressed air at flow rates set from 1800 to 3000 L•h -1 . The plasma power was monitored by controlling the pulse frequency of the DC discharge at three level (18 , 21, and 25 kHz) [16]. The detailed parameters were described in work by Fakhouri et al. [16]. After configuring the plasma, the TTIP precursor was sprayed into the plasma jet, near the output of the torch, using dry nitrogen, as a carrier gas, at a flow rate of 12 L•h -1 . The Ag-TiO2 thin film coatings were formed onto the substrate surface which was placed 10 mm away from the plasma nozzle as described in work by Fakhouri et al. [16].

The APPJ is powered using a uni-polar square-wave pulse-width modulation transistor amplifier circuit. The circuit provides electrical power between fully on and fully off at a drive switching frequency from 18 to 25 kHZ. The alternating current (AC) pulse voltage was set at 5 kV. The detailed parameter was described in work by Dowling et al. [17].

Deposition of the TiO2 and Ag-TiO2 thin films were performed on two different substrates.

Microscope glass slides were used for optical and photocatalytic measurements while Si (100) wafers were used for structural and analytical measurements such as X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Prior to deposition, the substrates were ultrasonically cleaned with acetone, ethyl alcohol and then de-ionized water for 20 min each. Pre-treatment with an air plasma at a jet speed of 10 m•min -1 and a frequency of 25 kHz was performed to activate the

THE IMPACT OF HYDROTHERMAL FACTORS ON QUASI-ONE-DIMENSION TIO2 NANOSTRUCTURE, CRYSTAL SIZE AND BANDGAP Synopsis

Quasi-one-dimensional TiO2 nanostructure (Q1D TiO2) synthesized using a hydrothermal method showed significant differences in the crystal phase, crystal size and bandgap properties under different hydrothermal synthesis conditions. The hydrothermal reaction conditions were optimized for maximizing and minimizing bandgap using a 3factor 3-level Box Behnken design (BBD). The factors investigated include temperature, NaOH concentration and TiO2 concentration. A quadratic response surface model predicted the optimized conditions for maximum and minimum bandgap were 3.31 eV and 3.05 eV, respectively. The BBD analysis and use of other statistical tools provided evidence showing the temperature and NaOH concentration significantly affected the crystal phase and Q1D TiO2 bandgap. Pure anatase was associated with a higher bandgap, whereas the biphasic anatase-rutile and anatase-TiO2-B phases were linked with decreasing bandgap. In addition, the impact of crystal size and SSA on the bandgap was due to the size quantization effect. Decreasing the crystal size (increasing SSA) is closely linked with increasing the bandgap. These factors are significant because they can be controlled to engineer Q1D TiO2 based compounds with controllable crystal phase, crystal size, specific surface area (SSA) and bandgap.

Introduction

The discovery of photo-electrochemical splitting of water on n-TiO2 electrodes in 1972 [1] triggered much attention in examining the photoelectrolysis of water to hydrogen (H2) and oxygen (O2). Other applications include employing n-TiO2 in wet solar cells [2] and for the photodegradation of hazardous organics pollutants into carbon dioxide plus water [3][4][5][6]. Quasi-one-dimensional (Q1D) TiO2 nanomaterials have shown significant potential in these applications because of their high electron mobility, large specific surface area (SSA) and high mechanical strength [7][8][9][10][11]. Among the different approaches used to synthesize Q1D TiO2, the alkaline hydrothermal method is a relatively easy process for producing Q1D TiO2 with varied bandgap energies [7][8][9][10][11][12][13][14][15][16][17][18][19].

Bandgap energies are crucial in determining the Q1D TiO2 photocatalytic efficiency [7,19].

The band gap is significantly affected by factors such as phase structure, temperature and crystal size. Phase structure is a major factor affecting the Q1D TiO2 bandgap. TiO2 has an optical bandgap value for different crystal phases. For TiO2, the band gap is 3.2 eV for anatase, 3.0 eV for rutile, 3.1 eV for brookite and 3.2 eV for TiO2-B [10,16,20].

Work by Bavykin et al. [8] has shown that during the alkaline hydrothermal process, TiO2 precursors (anatase, rutile, TiO2-B, brookite and amorphous) were reconstructed into Q1D TiO2 by producing and consecutively wrapping titanite nanosheets. Different crystal phase structures can be produced during hydrothermal processing [7][8][9][10][11][12][13][14][15][16][17][18]. Zhu et al. [9] claimed that the anatase phase was produced when using 15 M NaOH at a temperature between 20 o C and 200 o C for 48 h. In other studies, Kou et al. [16] reported TiO2-B nanotubes were obtained when the conditions were set by employing 10 M NaOH at 120 o C for 96-168 h. In these studies, researchers used selected synthesis conditions to produce Q1D TiO2 structures with varied phase structures. An existing research gap which requires addressing is the assessment of different synthesis conditions on the phase structure and bandgap for Q1D TiO2 nanostructures.

TiO2 band structure changes is also affected by crystal size which is comparable to the electron wavelength [21]. This observation, designated as the size quantization effect [10,21], causes the TiO2 bandgap to shift to larger values or shorter wavelengths for decreasing crystal sizes [10]. Many studies have shown Q1D TiO2 diameters ranging from 5 nm to 200 nm can be controlled using selected hydrothermal temperatures and suitable TiO2 to NaOH molar ratios [8,12,13]. Bavykin et al. [8] reported the average Q1D TiO2 diameter increased with an increase in the temperature from 120 o C to 150 o C and increasing the TiO2 to NaOH weight ratio. Adjusting the temperature and NaOH:TiO2 weight ratio are expected to impact the crystal size and subsequently, the bandgap based on the size quantization effect [10].

The lack of research data describing the role of different hydrothermal synthesis process parameters on the Q1D TiO2 phase structure and its bandgap is the motivating factor for conducting this study. The first part of the third objective is to evaluate the influence of the hydrothermal synthesis factors on the Q1D TiO2 phase structure, crystal size, SSA and bandgap [19]. The second part of the third objective is to employ a BBD to optimize the process conditions leading to maximum and minimum Q1D TiO2 bandgap. The three hydrothermal synthesis factors under consideration will include temperature, NaOH concentration and TiO2 precursor concentration.

Experimental

Preparation of the quasi-one-dimension TiO2 photocatalyst

Q1D TiO2 photocatalyst was prepared using the process described in Chapter 2 (Section 2.4.2). Briefly, homogeneously mix a specified quantity of TiO2 NPs (Aeroxide TiO2 P25, Evonik Industries, Germany) with 70 mL of a NaOH solution. The resulting suspension was transferred into a 100-mL Teflon lined stainless-steel autoclave. The autoclave was maintained at a desired temperature (Table 3.1) for 48 h and subsequently, cooled to room temperature. Next, the suspension was centrifuged to obtain a white precipitate. The precipitate was washed with 1 L of 0.1 M HCl for 24 h at room temperature. The white precipitate was repeatedly washed (5 times) with deionized water.

The washed mixture was centrifuged after each wash. Finally, the collected white precipitate was calcined at 400 o C for 2 h to produce Q1D TiO2 [16]. The selected experimental levels were based on previous studies (Table 3.1) [8].

Table 3.1: Three selected factors and three level.

Level

Characterization studies

Details of all the characterization studies are described in Chapter 2. The Q1D TiO2 morphology was examined using field emission scanning electron microscopy (FESEM) (JEOL, Japan) and high resolution transmission electron microscopy (HRTEM) (300kV, CHAPTER 3 72 JEOL 3010, Japan). X-ray diffraction (XRD) analysis of the samples was conducted using an X-ray diffractometer (Rigaku, MI) equipped with Cu K∂ radiation source. UVvisible spectra analysis was performed using a Cary 300 UV-visible (Agilent Technologies, Santa Clara, CA). The wavelength accuracy and resolution for the Cary 300 UV-visible were ±0.04 nm (at 486 nm) and ≤ 0.193 nm, respectively. The bandgap response was calculated based on diffuse reflectance UV-visible spectroscopy studies.

The diffuse reflectance spectra were converted into a corresponding absorption spectra using the modified Kubelka Munk function (F(R)E) 1/2 [22]. Frequent use of diffuse reflectance UV-visible spectroscopy to determine the bandgap in solid materials is justified based on past research [22]. The bandgap (Eg) for each experiment is shown in Table 3.2. The photoluminescence spectra were conducted at room temperature using a

Varian Cary Eclipse Fluorescence Spectrophotometer (Mississauga, ON). Nitrogen adsorption-desorption isotherms were determined at 77K with a Micromeritics ASAP 2020 Brunauer-Emmett-Teller (BET) analyzer (Micromeritics Instrument Corp. GA, United States).

Experimental design, optimization study and statistical analysis

A three factor three level BBD was constructed with three central points to determine the hydrothermal synthesis factors for evaluating the bandgap. The BBD is unique because it is rotatable and does not comprise an embedded factorial or fractional factorial design. The BBD experimental conditions were selected at the midpoints of edges of the space and at the center.

CHAPTER 3 73

Each experimental factor was varied at a low level (designated as -1), a central level (designated as 0) and a high level (designated as +1) (Table 3.1). The method is constructed using 12 experimental points (#1-#12) plus three central points (#13-#15) and each condition under triplicate conditions (Table 3. where Xi's are input variables which influence the response variable Eg, аo is an offset term, аi is the i th linear coefficient, аii is the quadratic coefficient, and аij is the ij th interaction coefficient. The Xi correspond to the experimental factors (X1 = temperature, X2 = NaOH concentration, X3 = TiO2 concentration) (Table 3.2).

An analysis of variance (ANOVA) was performed using the responses for each experiment (#1-#15) (Table 3.2). The coefficient values for the full quadratic model were calculated using a multiple regression analysis. Only significant terms with p values < 0.05 were included into the final model. A normal distribution plot together with the Anderson-Darling (AD) [23] statistic was used to determining the deviation of the residuals from a normal distribution. The D-optimality analysis [24] was performed to obtain optimal conditions for the three factors under consideration (maximize and minimize the bandgap Eg). During the hydrothermal process, the NaOH reacted with TiO2 crystallites and produced free TiO6 octahedron by cleaving the Ti-O-Ti bonds of the TiO2 precursor [8,27,28]. The TiO6 octahedron was subsequently assembled into lamellar structures via the formation of hydroxyl bridges between the Ti atoms [27,28]. Next, Q1D TiO2 (nanotubes, nanorods and nanowires) was produced by scrolling or growing the lamellar structures [8,29]. The driving force for scrolling lamellar structures into nanotubes or growing to nanorods or nanowires is attributed to the reduction of the high surface energy which was caused by the saturated dangling bonds on the lamellar structures [27].

Results and discussion

Morphology

Bandgap (eV)

Diffuse reflectance UV-visible spectroscopy (DRS) was employed to examine the optical response of the starting material TiO2 nanoparticles and the resulting Q1D TiO2 (Figure 3.2a). A wide optical absorption below a critical value of approximately 410 nm was observed in both samples [30]. This broad band assignment is attributed to the bandband electron transition of the TiO2 nanocrystals based on its band gap energy [30,31].

A red-shift and enhanced light absorption between 300 to 400 nm was observed for the Q1D TiO2 sample #1 when compared to TiO2 nanoparticles. The diffuse reflectance spectra was converted into a corresponding absorption spectra (Figure 3.2b) by plotting (F(R)*E) 1/2 versus E (where F(R) = aKm is the Kubelka Munk function [22] and E is the photon energy). Extrapolating the straight line portion of the UV-visible spectra to (F(R)*E) 1/2 =0 was used to determine the bandgap. A 3.067±0.003eV value for the Q1D TiO2 sample #1 was less than that for the TiO2 nanoparticles bandgap of 3.282±0.006eV.

In this study, the bandgap energies for the 15 BBD samples (ranging from 3.067±0.003eV to 3.298±0.004eV) are shown in Table 3

Analysis of the experimental design

Impacts of factor variables on the bandgap

The bandgap for all the Q1D TiO2 samples are shown in Table 3.

Model fitting using analysis of variance (ANOVA)

An ANOVA was employed to evaluate the significance of the full quadratic model and to determine the significance and adequacy of the model. The ANOVA result (Table 3.3) for the bandgap (Eg) response shows that the model is statistically significant with a p-value less than 0.05. The model F-value of 6.97 which was greater than the F-critical value of 2.01 at an  value = 0.05 indicate the full quadratic fit model was significant [24,[32][33][34]. Note terms with p-values < 0.05 are statistically significant whereas terms with a p-value > 0.05 are insignificant. TiO2 concentration (g•L -1 ), NaOH concentration (M) x NaOH concentration (M) and TiO2 concentration (g•L -1 ) x TiO2 concentration (g•L -1 ) with p-values more than 0.05 are statistically insignificant indicate these the variables did not affect the full quadratic model [24,[32][33][34]. A full quadratic model was simplified using a stepwise procedure such as a backwards elimination method. Other stepwise procedures include forward selection and stepwise regression. The backwards elimination method is advantageous because of considerable predictability. The method is employed to delete statistically insignificant terms with p values > 0.05 in the full model [24]. The refined model (Eq. 3.2) with an Fvalue of 6.97 (> the critical level (6.7, p < 0.00)) is statistical significant.

where Eg is the bandgap (eV), X1, X2 and X3 represent temperature

Response model verification, optimization and validation

The regression coefficient R 2

Phase structure analysis

Evaluating the impact of the crystal phase structure on the bandgap was performed using XRD. The XRD patterns (Figure 3.8a) for selected BBD samples #1, #2 and #6 and the phase structure for all the samples are listed in Table 3.4. The anatase [36], rutile [20,31] and TiO2-B [16,37] phases were detected in selected samples with the anatase phase observed in all the Q1D TiO2 samples. Sample #2, #3, #5, #7, #9, #10, #11, #12, #13, #14 and #15 consisted of pure anatase with bandgap ranging from 3.258±0.003 to A few studies have shown existence of the TiO2-B phase when using temperatures and a NaOH concentration of 150-190 o C and 10 M, respectively [17,41].

Photoluminescence studies (Figure 3.8b) for selected samples and the control (TiO2 NPs) further demonstrated that the phase structure had the fundamental effect on bandgap.

All the samples showing the peak at 3.4 eV was due to the anatase fundamental band-toband transition [42][43][44]. The tail from 3.1 to 3.4 eV in addition to the peak at 3.4 eV is assigned to the exciton trapped at shallow level defects for sample #15 [42,43].

However, the biphasic anatase-rutile (samples #1 and commercial TiO2 NPs) and biphasic anatase-TiO2-B (such as samples #4) showed broader peak with a tail from 2.9 to 3.4 eV in addition to the anatase peak at 3.4 eV when compared to pure anatase (#15).

This long tail is attributed to the lower band-to-band transition of rutile and TiO2-B crystal phase [45]. The band from 2.4 to 2.6 eV is due to the oxygen vacancies which behaved as the centers of indirect recombination [43]. The mean crystal sizes (L, nm) for sample #1-15 (Table 3.5) were calculated using Scherrer equation (Eq. 3.

3) and the XRD data (data not shown).

L=Kλ/β.cosθ [46] (3.3)
where K, λ and β represent the shape factor (0.89), the wavelength of XRD radiation (0.154 nm) and the half maximum of a full peak, respectively.

The main effect of hydrothermal synthesis factors on the mean crystal size are shown in Figure 3.9a. The synthesis temperature and NaOH concentration significantly affected the crystal size, whereas, the TiO2 precursor concentration did not impact the crystal size. Smaller crystal size was closely linked to a larger bandgap according to size quantization effect [10,21]. From the Figure 11, the mean bandgap, E (eV), is:

( (3.4)
Where L (nm) is the mean crystal sizes and T is temperature. In addition, the Q1D TiO2 Where L (nm) is the mean crystal sizes and T is temperature. (quantum efficiency (ε) = 20.2±0.5%). This ε value is 5-fold of the value using Pt-TiO2 photocatalyst (ε=4%) in a pure water substrate. Under a higher temperature (190 o C) and with ≥ 10 M NaOH concentration, the presence of TiO2-B phase resulted in the lowest photocatalytic activity in terms of photocatalysis of organic pollutants and photocatalytic H2 production.

Conclusions

Introduction

Titanium dioxide (TiO2) is a widely studied and efficient photocatalyst for degrading organic pollutants and H2 production [1][2][3][4][5][6]. The TiO2 photocatalytic properties are strongly dependent on properties such as phase structure, crystal size and specific surface area (SSA) [7][8][9]. Hydrothermal synthesis factors such as reaction temperature and the TiO2 to NaOH molar ratio can significantly affect the Q1D TiO2 phase structure, crystal size and SSA as discussed in Chapter 3 [10][11][12]. However, the impact of individual factor and factors in combination on the photocatalytic characteristics of Q1D TiO2 has not been addressed in the past studies [13].

Many studies have reported improved photocatalytic activities for Q1D TiO2 synthesized under different hydrothermal synthesis factors when compared to TiO2 nanoparticles [3,12,[14][15][16][17][18][19]. Recently, Kuo et al. [2] reported that 1% Pt/TiO2 nanotubes (TNTs) synthesized at a relatively low temperature were 20% more effective in producing H2 gas from ethanol when compared to a 1% Pt/TiO2 nanoparticle photocatalyst using UV light. Perera et al. [12] synthesized reduced graphene oxide (RGO)-TNTs composites using a one-step alkaline hydrothermal process. These CHAPTER 4

103 researchers reported Q1D TiO2 such as TNT were homogeneously dispersed between RGO sheets with the composites showing significant improvements in the photocatalysis of malachite green dye under UV light when compared to TiO2 nanoparticles [12].

However, these and other researchers did not address the impact of hydrothermal synthesis factors on Q1D TiO2 photocatalytic activities [2,3,12,[14][15][16][17][18][19].

The work carried out is to execute the fourth objective. The first part of the fourth objective (Section 4.3) was to optimize selected hydrothermal process conditions for the maximum photocatalytic activity of Q1D TiO2 in terms of photolysis organic pollutants.

The second part of the fourth objective (Section 4.4) was to optimize selected hydrothermal process conditions for the maximum photocatalytic H2 production rate.

Photocatalysis of selected organic pollutants

The photodegrading organic pollutants experiments were conducted using the photoreactor configuration described in Chapter 2 (Section 2.3.10).

The reaction mixture consisted of 0.1 g•L -1 of photocatalysts and a specific model pollutant concentration. The model pollutants examined in the study include RhB (purity: 95%), phenol (purity: 99.5%), MO (purity: 85%) and MB (purity: 82%). All aqueous organic pollutants used in the photocatalysis study were purchased from Sigma-Aldrich (ON, Canada). The concentration of RhB, MO, phenol and MB in the reaction mixture was 10 mg•L -1 , 20 mg•L -1 , 20 mg•L -1 , 10 mg•L -1 , respectively. The reaction mixture (50 mL) containing a specific model pollutant and photocatalyst was ultrasonicated for 15 mins and left under dark conditions with magnetic stirring for 1 h to establish an adsorption-desorption equilibrium. The reaction mixture in quartz tubes was thereafter exposed to UV light for a specific duration. The duration of exposure to UV was maintained at 15 minutes for all experiments. A fixed amount of the reaction mixture (5 mL) was withdrawn at specific time intervals (of 3-5 minutes). The sample was centrifuged (5,000 rpm for 10 minutes) and filtered using a 0. where k is the apparent reaction rate constant (min -l ), C is the organic pollutant concentration (mg•L -1 ) and -dC/dt is the pseudo-first order reaction rate.

Photocatalytic hydrogen production

The photocatalytic experiments conducted using the photoreactor are described in Chapter 2 (Section 2.3.10).

The 50-mL reaction mixture in sealed quartz tube contains 0.2 g•L -1 of the photocatalyst and 20% (v/v) ethanol. The reaction mixture was purged with nitrogen (N2) gas to remove oxygen. Next, the reaction mixture was ultrasonicated and magnetically stirred under dark condition for 1 h before initiation of the reaction with UV light. Then, CHAPTER 4 106 the reaction mixture was exposed to UV light and kept at 37±2 o C. At specific time intervals, the H2 concentration was determined using a gas chromatograph (GC) (Varian CP-3800). The GC configuration was described in Chapter 2 (Section 2.3.8). N2 gas was used as a carrier gas at a flowrate of 15 mL min -1 . The photocatalyst reaction mixture was irradiated for 5 h using a 300-nm monochromatic UV light with an average intensity of 9 mW•cm -2 . The irradiation area for each tube was 58 cm 2 .

The quantum efficiency (ε) was calculated using equations (Eq. 4.2) and (Eq. 4.3)

[24].
Where N is the photon number in 1 h; E is the energy rate of incident photons; λ is the wavelength in nm; h is Planck's constant; and c is the photon velocity.

The impact of hydrothermal factors on the photodegradation of aqueous organic pollutants

The morphology, crystal size, crystal phase, bandgap and SSA of the samples were described in Chapter 3 using field emission scanning electron microscope (FESEM CHAPTER 4 107 (JEOL, Japan), high-resolution transmission electron microscopy (HRTEM) (300 kV, JEOL 3010, Japan), X-ray diffraction (XRD) analysis Raman spectra analysis, UVvisible spectra analysis, Nitrogen adsorption-desorption isotherms. The Q1D TiO2 phase structure, crystal size and SSA which were affected by hydrothermal factors strongly impact the photocatalytic activities. In this chapter, the main results from Chapter 3 will be used.

Selecting model chemicals to assess implementing photodegradation as a technology for treating wastewater effluents and drinking water supplies

Industrial development coupled with acceleration in population growth rate has led to the occurrence of toxic organic pollutants in wastewater effluents and drinking water supplies. RhB, MO and MB are produced as aqueous pollutants in textile, leather, paper, plastic and dye industries. Phenol has been classified with carcinogenic, teratogenic, and mutagenic properties for more than 30 years [21]. Exposure to these toxic organic pollutants is linked to serious concerns such as environmental damage and human diseases [25]. Conventional municipal biological wastewater treatment facilities are ineffective in removing toxic organic pollutants [26][27][28]. In addition, operational costs and the generation of secondary toxic pollutants are major problems associated with municipal biological treatment [29,30]. Advanced oxidation processes such as ozone (O3), O3/H2O2, Fenton and photo-Fenton are promising technologies for the degradation of toxic organic pollutants [31]. However, major disadvantages of these processes include incomplete degradation and the production of toxic byproducts. Photocatalysis CHAPTER 4 108 based on TiO2 is an emerging technology which can overcome these problems [26,27,[31][32][33][34][35].

The advantage of using dyes such as RhB, MO and MB is easily measuring their concentration by using UV-visible spectroscopy and subsequently the apparent degradation rate [36]. However, the dye decolorization and decomposition might be caused by dye sensitization [36]. In the case of dye sensitization, the dye is excited by irradiation. Phenol can be excited by irradiation with short wavelength (<270 nm) [37].

In this study, a 300-nm monochrome UV light were employed as irradiation source. The degradation of phenol was closely linked to photoreaction catalyzed by photocatalyst.

Experimental design and statistical analysis

A three factor three level BBD was used to determine the experimental conditions for hydrothermal synthesis of the Q1D TiO2 photocatalyst capable of maximizing the apparent reaction rate constant (min -1 ). Accordingly, the apparent reaction rate constant (min -1 ) was selected as a response variable and RhB was selected as a model pollutant for the optimization study. The hydrothermal conditions for synthesis of Q1D TiO2, namely, temperature ( o C), NaOH concentration (M) and TiO2 concentration (g•L -1 ) were the selected experimental factors for the optimization study. Each experimental factor was varied at a low level (designated as -1), a central level (designated as 0) and a high level (designated as +1) (Table 4.1). The method is defined with three center points and 12 experimental points with three replicates under each condition (Table 4.2). A full quadratic model was evaluated for the response function and the apparent reaction rate constant (min -1 ) was analyzed statistically using Minitab 15 (Minitab Inc., State College, CHAPTER 4 109 PA). Three experiments (triplicates) designated as #13 to #15 under the same conditions were performed at the center points to evaluate the magnitude of error in the experimental analysis. A, R and B correspond to anatase, rutile and TiO2-B, respectively.

Rhodamine b photocatalysis

Under the different hydrothermal factors, the Q1D TiO2 samples showed differences in the photocatalytic efficiency with respect to the discoloration or disappearance of RhB.

Profiles for the residual RhB concentration versus time and the corresponding apparent reaction rate constant, k (kt = -ln(C/Co)), are shown in Figure 4.1 for selected samples (Table 4.3). Control experiments were performed with P25 nanoparticles and without a photocatalyst. The average apparent RhB photocatalysis rate constant value (designated as "apparent RhB k") was 0.003 min -1 without a photocatalyst; however, with the CHAPTER 4 110 addition of P25 nanoparticles, the average value increased to 0.3483±0.0051 min -1 . Only the Q1D TiO2 photocatalyst synthesized under conditions designated as #1 (120 o C, 5 M NaOH and 43 g•L -1 TiO2) showed the apparent RhB k value (0.4227±0.0107 min -1 ) larger than the value for the TiO2 P25 photocatalyst (Table 4.3 and Figure 4.1). The average apparent RhB k value for the sample #4 and sample #9 were 0.0936±0.0007 min -1 and 0.2569±0.0107 min -1 , respectively. Depending on the hydrothermal synthesis conditions, the average apparent reaction rate constant value, k, varied from 0.0936±0.0007 min -1 to 0.4227±0.0039 min -1 (Table 4.2). For sample #1, the enhanced RhB photocatalytic activity was due to the formation of the active biphasic anatase-rutile structure [9,38] with the Q1D nanostructure [15,39]

Modeling and optimization of hydrothermal synthesis

The impact of varying the hydrothermal synthesis factors, namely, temperature, NaOH concentration and the TiO2 concentration (Figure 4.2) is crucial for optimizing the Q1D TiO2 photocatalytic activity. Optimization using a 'one-factor-at-a-time' (OFAT)

approach is tedious and time-consuming. Furthermore, the OFAT method is unable to account for interactions between the various hydrothermal synthesis factors. In this study, response surface methodology (RSM) was utilized to evaluate the impacts of various hydrothermal synthesis factors on the Q1D TiO2 photocatalytic activity.

A multiple regression analysis was used to develop a quadratic equation (Eq. 4.4). Eq. Anatase is an excellent photocatalyst; however, in combination with the rutile phase, the biphasic anatase-rutile photocatalytic activity is enhanced, such as in the case of TiO2 CHAPTER 4 114 P25 and sample #1. When the two crystal phases are combined, the separation of photogenerated holes and electrons is facilitated with an increase in the photocatalytic activity [9]. Supporting work [38] employing surface enhanced Raman spectroscopy (SERS) has shown that mixed crystal structure TiO2 with an appropriate proportion of anatase and rutile phase is favorable to increasing TiO2-to-molecule charge-transfer as well as interaction between the rutile and anatase phases.

Response surface model development

An analysis of variance (ANOVA) was employed to evaluate the model full quadratic equation (Eq. 4.4) to determine the significance and adequacy of the model (Table 4.5).

Terms with a p-value less than 0.05 are statistically significant, whereas the terms with a p-value greater than 0.05 are insignificant [40][41][42][43]. A p-value of 0 (less than 0.05) and Fvalue (24.46)

Photocatalysis of other pollutants using the optimized Q1D TiO2

The Q1D TiO2 produced under the optimized conditions was also employed to photodegrade phenol, methyl orange and methylene blue ( 4.2. phase negatively affected the photocatalysis of selected aqueous organic pollutants.

Photodegradation conclusions

When using the optimized biphasic anatase-rutile Q1D TiO2, the apparent photocatalysis CHAPTER 4

125 rate constant for phenol, methyl orange and methylene blue under UV light conditions were enhanced by 3-5 fold in comparison to the TiO2-B. The study demonstrated using the hydrothermal synthesis of Q1D TiO2 with controllable phase structure and crystal size was to produce a photocatalyst with optimum photodegrading capabilities.

The impact of hydrothermal synthesis factors on photocatalytic hydrogen

production from water-ethanol mixture

Introduction: Photocatalytic hydrogen production

Hydrogen is considered as a promising fuel for the future. Q1D TiO2 nanomaterials have shown significant potential in producing H2 because of their high electron mobility, large SSA and high mechanical strength [10,16,[46][47][48]. Producing Q1D TiO2 using the alkaline hydrothermal method is a relatively facile process with advantages of large-scale production and varied crystal phase [2,10,11,16,17,[47][48][49][50][51][52]. In studies employing the hydrothermal method, Q1D TiO2 nanomaterials containing anatase [16] or TiO2-B [2,51] were produced under selected conditions. These materials have been utilized to produce hydrogen (H2) from the photocatalytic splitting of water or from other substrates such as ethanol [2,53]. In comparison to TiO2 nanoparticles, Q1D TiO2 has shown improved H2 production from ethanol under UV light conditions [2,53]. Hydrothermal synthesis factors such as reaction temperature and the TiO2-to-NaOH molar ratio can significantly affect the Q1D TiO2 phase structure, crystal size, SSA [10,11,46] and subsequently the H2 production rate. However, the impact of hydrothermal synthesis factors on the Q1D TiO2 photocatalytic H2 production rate and optimization hydrothermal conditions has not been addressed in previous studies.

CHAPTER 4 126

Many factors affecting the use of photocatalysts to produce H2 and other fuels include band gap [54,55], band edge positions [55,56], charge transfer [57], material stability [58,59], cocatalyst [60][61][62], availability [63], complementary metal oxide semiconductor [64,65] and toxicity [63]. Substrates such as water [57,66] and ethanol [61] have been used to produce H2 using photocatalysts. Photocatalyst generates and separates electrons (e -) and holes (h + ) after absorbing photons. Next, the photogenerated electrons reduce H + to generate H2. The water splitting reaction is described as following half-cell reactions:

(photocathode) [63] (4.6) (photoanode) [63] (4.7)

In the presence of sacrificial electron donors [67,68] such as ethanol, the photogenerated holes are consumed by electron donor and subsequently, the recombination of electrons and holes in photocatalyst are suppressed [67,68]. This significantly increases the H2 production rate as well as photocatalytic stability [67,68].

The reaction at the photoanode is described as a water splitting half reaction as follows [61,69]:

[61, 69] (4.8)
Ethanol, a renewable fuel produced from carbon neutral feedstocks, with a specific heating value of approximately 29.7 MJ•kg -1 is less when compared to H2 with a specific heating value of approximately 141.8 MJ•kg -1 [70]. In addition, the decomposition of CHAPTER 4

127 ethanol contributes to a high H2 production rate and quantum efficiency [71]. Hence, H2 with a larger energy content is preferred when compared to ethanol as a fuel source.

Experimental design and statistical analysis

The experimental design was based on a 3-factor and 3-level Box-Behnken design (BBD) [40]. The experimental design was used to optimize key process factors (Table 4.1) to improve the H2 production rate. A total of 15 experimental points (#1 to #15)

were conducted with each condition under triplicate (Table 4. where Xi's are input variables which influence the response variable Y, аo is an offset term, аi is the i th linear coefficient, аii is the quadratic coefficient, and аij is the ij th interaction coefficient. The X1, X2 and X3 represent the synthesis temperature, NaOH concentration and TiO2 concentration, respectively (Table 4 The quadratic model was used to predict reaction conditions which produced Q1D TiO2 with a maximum H2 production rate. A D-optimality analysis [41] was performed to find optimal conditions for the three factors under consideration (maximize the H2 production rate). The coefficient values for fitting the full quadratic model were determined using the experimental response (H2 production rate) and a multiple regression analysis. An analysis of variance (ANOVA) was conducted using the observed responses for samples #1 to #15) (Table 4 ) was approximately 2fold higher than the TiO2 P25 photocatalyst (ε = 10±0.4%)using ethanol as an electron donor. This ε value was 5-fold of ε value using Pt-TiO2 as photocatalyst and pure water as substrate under UV light condition [72]. Photocatalytic H2 production were significantly enhanced using ethanol as electron donar when compared to using water (Figure 4.9b). The H2 production rates are listed in Table 4.6. The effect of ethanol concentration on H2 production rate was evaluated using TiO2 NPs (Figure 4.9b). A 20 v/v% aqueous ethanol level was able to produce the highest H2 production rate.

Impact of hydrothermal conditions on hydrogen production rate

The main effects plot showing the overall impact of three hydrothermal factors for synthesizing Q1D TiO2 on the response H2 yield is illustrated in

Modeling and effects of factors on response variable hydrogen production rate

For the response surface optimization study, three sets (replicates) of 15 experiments were conducted to compute the H2 production rate under 15 conditions (Table 4. Additional experiments were conducted to verify the maximum Y using the Q1D TiO2 synthesized under the predicted conditions. The predicted value was slightly underestimated when compared to the observed H2 production rate of 475±12 µmol•h -1 (ε = 20.2±0.5%). Recall, a pure anatase phase with a mean crystal size of 20.1±0.2 nm and BET SSA of 140±3 m 2 •g -1 was observed in Q1D TiO2 synthesized under optimum conditions. The opitimum H2 production rate was approximately 2-fold larger when compared to commercial P25. This H2 production rate was significantly improved when compared to reported production rates of 348 µmol•h -1 for reported biphasic anatase-CHAPTER 4 135 TiO2-B [2]. Note that in this study, 0.02 g photocatalyst was used to obtain a H2 production rate of 470 µmol•h -1 . The highest specific H2 production rate (per g of photocatalyst) reached 23,500 µmol•h -1 •g -1 which was significantly enhanced when compared to data reported for pure anatase, pure rutile, flame spray pyrolysis-synthesised TiO2 and Au/TiO2 [73].

Figure 4.12: Optimality plot to locate optimum factor levels for maximized hydrogen yield (µmol•h -1 ). Notes: a The D-optimality index ranging from 0 to 1 represets the ideal and worst cases, respectively, and b Y represents H2 production rate.

Response surface model development

An analysis of variance (ANOVA) (Table 4. 4.8 for all the samples. The three phase structures observed in this study included pure anatase [74], biphasic anatase-rutile [9,75,76] and the biphasic anatase-TiO2-B [2] structures (Figure 4.15a). The biphasic anatase-rutile structure, similar to that in P25, was only detected in sample #1 [9,75,76], while pure anatase phase and the biphasic anatase-TiO2-B structures were observed in a majority of the samples. The mean crystal sizes (L, nm) for sample #1-15 (Table 4.6 and Table 4.8)

were calculated using Scherrer equation (Eq. 4.12) and XRD data.

L=Kλ/β.cosθ [77] (4.12)
where K, λ and β represent the shape factor (0.89), the wavelength of XRD radiation (0.154 nm) and the half maximum of a full peak, respectively.

The Kubelka-Munk function was used to transform the diffuse reflectance spectra into the corresponding absorption spectra (Figure 4.15b) [75,78,79]. . The bandgap is closely correlated with the phase structure ( Increasing the H2 production rate was closely correlated with decreasing the anatase mean crystal size from 58.3±0.3 to 20.1±0.2 nm. This H2 production rate increase was due to increasing the SSA [82]. Increasing the SSA was associated with faster rate of eand h+ surface reaction with substrate. The optimum anatase mean crystal size of 20.1±0.2 nm was correlated with a maximum H2 production rate. However, note the lower H2 production rate observed below the optimum anatase mean crystal size was caused by fast e --h + recombination [82]. A decrease in H2 production rate which was correlated with fast e --h + recombination at defective sites on the photocatalyst surface [82]. The significant number of defective site was likely due to decreasing the crystal size below an optimum value. This Q1D TiO2 sample with an optimum mean crystal size was able to maximize the H2 production rate. The Q1D TiO2 with biphasic anatase-rutile and anatase-TiO2-B structures was observed with decreasing photocatalytic H2 production rate. This study demonstrated that Q1D TiO2 containing a controllable phase structure, and crystal size was able to produce a photocatalyst with a H2 production rate greater than reported values.

Conclusions

ENHANCED TIO2 NANORODS PHOTOCATALYSTS WITH PARTIALLY REDUCED GRAPHENE OXIDE AND AG

NANOPARTICLES FOR DEGRADING AQUEOUS HAZARDOUS

POLLUTANTS

Synopsis

Integrating TiO2 nanorods (TNRs) with partially reduced graphene oxide (RGO) (designated as GT) was as a photocatalyst composite for degrading aqueous hazardous pollutants. The degree of RGO oxidation played an important role in affecting the photoelectronic and photocatalytic activities of GT composites. The study examined the impact of the degree of RGO oxidation on the photocatalytic activities. The photocatalytic activity of the materials was investigated for photodegrading rhodamine b (RhB), methyl orange (MO), methylene blue (MB) and phenol by using ultraviolet (UV) light. The highest photocatalytic activity was observed when the atomic oxygen-tocarbon (O/C) ratio of RGO was 0.130±0.003. This study suggested the photocatalytic performance was maximized by preserving a selected amount of the RGO oxygencontaining groups. The work reported in this study on optimizing the RGO based TiO2 photocatalyst could serve as a promising approach for preparing and optimizing other types of carbon based photocatalysts such as "graphene based CdS".

Employing highly active photocatalysts and visible light energy is a promising route for removing organic pollutants from drinking water supplies and wastewater effluents.

Successful commercial installation of TiO2 photocatalytic systems for water and wastewater treatment is dependent on enhanced photocatalytic activity under visible 1. CHAPTER 5 156 irradiation. In this study, a hierarchical three-dimensional (3D) Ag nanoparticle-RGO-TiO2 nanorods (TNRs) (designated as Ag-GT) films with optimum RGO atomic O/C ratio and Ag content was synthesized for degrading toxic organic pollutants under visible irradiation. The Ag-GT photocatalyst film was characterized with a significantly high specific surface area (SSA), improved charge separation and visible light absorption. The photocatalytic activity of the film was investigated for the photocurrent and degradation of organic pollutants (RhB, MB, MO and phenol) using visible light. The maximum photocurrent and photodegradation activities were observed for a silver content of 10% in the film photocatalyst.

Introduction

The development of highly active TiO2 based photocatalysts is an important challenge for remediating polluted water [1][2][3]. Graphene is widely used to modify TiO2 photocatalyst [4][5][6][7][8]. Graphene, a two-dimension carbonaceous material [9][10][11][12], has attracted much attention for various applications such as energy storage devices [12,13], photovoltaics [12], and photocatalysis [14] since its discovery in 2004 [15]. Graphene properties include significantly high mobility of charge carriers [12,16], extremely high thermal conductivity [17], extraordinary elasticity [16,18], large specific surface area (2630 m 2 g -1 ) [19] and mechanic stability [18]. Graphene is a zero-gap semimetal [20] with a strong capability in accepting, storing and subsequently, shuttling photo-induced electrons to enhance the photocatalytic properties of graphene-based semiconductor nanocomposites [4,14,[21][22][23]. The properties of graphene are sensitive to chemical doping and hole doping [24]. The Hummers method [25], involving the oxidization of 1. CHAPTER 5 157 graphite, exfoliation of graphite and reduction of graphene oxide (GO) [25], is a costeffective and widely adopted method to synthesize reduced graphene oxides (RGO). The RGO structure contains lattice defects and residual chemical groups (e.g. oxygencontaining moieties) which degrade the electronic properties when compared to pristine graphene sheets obtained by the mechanical exfoliation method [9,10]. GO and RGO could be treated as chemically doped or functionalized graphene with oxygen-containing groups and structure distortion [26][27][28]. The GO and RGO charge mobility property is significantly dependent on the degree of oxidation (the amount of oxygen-containing group on RGO), species of oxygen containing groups and the degree of structural distortion [27][28][29][30]. In addition, the oxygen-containing groups serving as 'anchor sites' [31] are able to enhance the interaction between RGO and semiconductor oxides [4,30], and thereby increase the electronic [4,14] and photocatalytic properties [4,14].

The discovery of photo-electrochemical water splitting [4,30] using TiO2 electrode in 1972 has triggered more than 45 years of intensive research on developing semiconductor photocatalysts for application in different areas such as H2 production [32], CO2 reduction [33], hazardous pollutant degradation [34,35] and bacterial destruction [34,35].

The photocatalysis reaction involves excitation of a semiconductor such as TiO2 with photons having equal or higher energy than the bandgap to produce electron-hole pairs. The electron-hole pairs either recombine or react with adsorbed species. A highly active photocatalyst is characterized by its reaction efficiency as well as suppressing electronhole combination. The photoactivity of TiO2 can be enhanced by coupling with a noble metal, metal ion doping, anion doping and incorporating electron-accepting materials such as graphene [5,[36][37][38][39][40]. These methods have been used to improve charge 1. CHAPTER 5 158 transportation, to suppress the charge recombination and to extend the optical absorption to visible light region [5,[36][37][38][39][40]. In particular, incorporating graphene into TiO2 provides a facile and effective approach to improve the photocatalytic properties [34]. The role of graphene in photocatalysts includes increasing the adsorbability, expanding the light absorption range, changing TiO2 bandgap and suppressing the electron-hole recombination [4,7]. Many reports [4, 6-8, 41, 42] on employing graphene-based TiO2 composites have mainly focused on the effects of the graphene content, photogenerated carrier transportation, RGO adsorption properties and the chemical moieties on the photocatalytic efficiency. However, these studies did not investigate the degree of RGO oxidization on RGO based TiO2 photocatalyst used in the degradation of aqueous hazardous pollutants. [START_REF] Akhavan | Photocatalytic reduction of graphene oxide nanosheets on TiO2 thin film for photoinactivation of bacteria in solar light irradiation[END_REF][START_REF] Morgan | Alkaline hydrothermal treatment of titanate nanostructures[END_REF] [43,44] demonstrated that bacteria photoinactivation and electron conductivity of RGO based TiO2 photocatalyst were greatly improved by gradually removing the oxygen-containing groups on RGO.

Therefore, the effect of RGO oxidization on photodegrading aqueous hazardous pollutants was assessed in this study.

Furthermore, several researchers have demonstrated improved visible-light photocatalysis using noble-metal-loaded TiO2 photocatalysts [42,[45][46][47][48][49][50]. This improvement in photocatalytic activity, which is associated with the extended light absorption range, is due to the surface plasmon resonance (SPR) effect of noble metal NPs [42,[45][46][47][48][49][50]. The work was carried out to execute the fifth objective in this thesis.

The first part of fifth objective, which is addressed in Section 5.3, is to optimize the RGO atomic oxygen-to-carbon (O/C) ratio [51] for synthesizing RGO-TiO2 nanorods (designated as GT) photocatalyst with maximum photocatalytic activity for 1. CHAPTER 5 159 degradingaqueous hazardous pollutants using UV light. The work outlined in Section 5.4

(second part of the fifth objective) is to synthesize a hierarchical three-dimensional (3D)

Ag nanoparticle-RGO-TiO2 nanorods (TNRs) (designated as Ag-GT) thin films photocatalyst with optimum Ag content for degrading aqueous hazardous pollutants under visible irradiation.

Experimental

Synthesis of reduced graphene oxide-TiO2 nanorods (designated as GT) photocatalyst with varied RGO atomic oxygen-to-carbon (O/C) ratio

The TNRs was synthesized using the hydrothermal method using the optimum parameter conditions described in Chapter 4 [52][53][54][55]. To prepare the TNRs photocatalyst, 1. CHAPTER 5 161

Ag-RGO-TiO2 nanorods (designated as Ag-GT) synthesis and Ag-GT film preparation

The schematic process employed to synthesize the Ag-GT nanocomposites is shown in Figure 5.2. Briefly, GO (3% w/w) (300 mg) and TNRs (0.97 g) were mixed and ultrasonicated (240 W) in 100 mL deionized water for 1 h to ensure re-exfoliating of the GO and inserting TNRs between the GO sheets. The mixture was mixed with 1 mL of hydrazine hydrate, and was heated at 95 o C for 1 h. The RGO-TiO2 (designated as GT) composite photocatalyst was isolated by filtration (PTFE 20 μm pore size) and washed with DI water and ethanol for 3 times, and subsequently dried at 60 o C for 12 h. The silver photocatalysts were synthesized by a deposition-precipitation method (using ethanol as a reducing agent) over the GT photocatalysts. The Ag loading was 5%, 8% 10%, 12% and 15% and referred as 5%Ag-GT, 8%Ag-GT, 10%Ag-GT, 12%Ag-GT and 15%Ag-GT, respectively. Briefly, 1 g of GT was mixed with 600 mL deionized water, 250 mL ethanol and 5 mL 0.1 M NaOH. The mixture was then stirred for 1 h and heated to 50 o C. A specific amount of 9.3 mM AgNO3 were added to the mixture and kept at 50 o C for 3 h. The Ag-GT samples were collected after filtering the mixture and repeated washing with deionized water.

Preparing the Ag-GT film was performed by depositing 0.2 mL of a photocatalyst aqueous solution containing 2 g•L -1 photocatalyst and 0.25 wt% Nafion (C7HF13O5S•C2F4) on a 9 × 9 mm 2 glass substrate and subsequently drying at 50 o C (Figure 5.2 e). Nafion was used to enhance the mechanical properties of the film. The uniformly coated Ag-GT film with a thickness of several micrometers was formed on the glass substrate.

Characterization studies

The morphology and structure were investigated using high-resolution transmission electron microscopy (HRTEM) (JEOL 3010, Japan). The lattice spacing (d) was measured from a digitized HRTEM image by digital image processing and evaluation was conducted using the Digital Micrograph (Gatan, CA). Filed emission scanning

Photoelelctrochemical measurements

Photoelectrochemical measurements were conducted using a CHI 704D (CH Instrument inc, Texas) with a standard three-electrode cell. Platinum wire was used as the counter electrode and a saturated calomel electrode (SCE) was used as a reference electrode. For the GT working electrode, 0.25 g photocatalyst was mixed with 0.06 g polyethylene glycol (PEG, molecular weight 20 000) and 1 mL deionized water to produce a slurry. The slurry was coated onto a 2 cm x 1.2 cm F-doped SnO2-coated 1. CHAPTER 5 164 (FTO) glass electrode using a doctor blade method [56,57]. The electrode was annealed at 450 o C for 30 minutes. The measurements were performed in a 1 M Na2SO4 aqueous solution as electrolyte. A 300-nm monochromatic UV light with an average intensity of 9 mW cm -2 was used as the UV light source.

Photocatalytic activity under ultraviolet (UV) light

The photocatalytic experiments were conducted using instruments and the process described in Chapter 2 (Section 2. where k is the apparent reaction rate constant (min -l ), C is the organic chemical concentration (mg•L -1 ) and -dC/dt is the pseudo-first order reaction rate.

Ag-GT film photocatalytic activity under visible irradiation

The Ag-GT film samples, deposited on glass substrates, were placed in a Pyrex cell

CHAPTER 5 169

The TNRs and its composites XRD patterns are shown in Figure 5.6. All the samples exhibited characteristic anatase and rutile peaks [63]. The intensity pattern indicates the anatase phase was dominant in all the samples with a small amount of the rutile phase.

The GOT and GT, GO(002) samples showed a broad band in a range from ~10 o 2θ [64].

The decreasing GO (002) peak intensity suggests the removal of oxygen-containing group was accomplished when GT was produced from the GOT sample. When compared to GOT, all the GT samples containing a graphene (002) peak located at 26 o 2θ suggest that GO was reduced to RGO [21]. The low intensity of the graphene (002) peak was due to the low RGO content in the photocatalyst. 1. CHAPTER 5 170

Removing oxygen-containing groups and adjusting the the atomic oxygen-tocarbon (O/C) ratio of reduced graphene oxide (RGO)

Investigating changes in chemical structure for RGO was conducted using Raman spectroscopy. Peaks for both TiO2 and carbon nanomaterials were observed for GT (Figure 5.7a) synthesized under different conditions. Peaks at 144 cm -1 (Eg), 399 cm -1 (B1g), 513 cm -1 (A1g), and 638 cm -1 (Eg) indicate the presence of the anatase phase [65].

The D and G peaks located at ~1347 cm -1 and ~1600 cm -1 , respectively, were detected in GOT and GT samples [66]. The D mode, which is observed when sp 3 atoms are present, represents disorder in the crystal structure [67][68][69][70]. The G mode which represents order in the crystal structure corresponds to bond stretching of all pairs of sp 2 atoms [67][68][69][70].

The D band to the G band integrated intensity ratio (ID/IG) was used to establish the degree of defects in the GO and RGO structures [21,66]. The lower integrated ID/IG ratio of RGO showed an increase in the average size of the sp 2 domains (graphitization and order) upon the reduction of GO [67][68][69][70]. The ID/IG values are tabulated in Figure 5.7b and listed in Table 5.1 showed the following decreasing integrated ID/IG ratio trend:

GOT > GT-20 > GT-60 > GT-80 > GT-95 1h > GT-95 2h > GT-95 3h (Figure 5.7b). The decreasing integrated ID/IG ratio suggests that the RGO oxygen containing functional group gradually decreased with an increase in temperature and reaction time [71]. This gradual decrease is caused by increasing the reduction reaction rate as the temperature rises and increasing reaction time [72]. According to Perera et al. [21], reduction of the defective GO is a crucial mechanism which can be employed to enhance the RGO electrical properties. In addition, the layer number of RGO sheets was determined based on the position and the shape of the 2D band. The 2D band of single-layer graphene is located at 2679 cm -1 [44,73]. An increase in graphene layer numbers (2-5 layers) is closely linked to a significant up-shift in the wavenumber by 19 cm -1 for 2D band [44,69,73]. In this study, the 2D bands were observed at 2683 cm -1 for samples (Figure 5.8) that contain GO or RGO. These relatively lower 2D band wavenumbers indicate the samples containing a significant number of single-layer graphene (oxide) sheets.

X-ray photoelectron spectroscopy (XPS) was employed to confirm the trend of oxygen containing group removal. In the deconvoluted C1s spectra (Figure 5.7c), the peaks located at 284.5 and 284.8 eV correspond to the C-C and C=C bonded carbon, respectively, whereas the deconvoluted peaks for C-OH (286.9 eV), C=O (289.0 eV) are attributed to oxygen containing functional groups on the RGO layers [26,72].

Interaction between reduced graphene oxide (RGO) and TiO2 nanorods (TNRs)

The Eg vibrational mode peak for selected samples is shown in Figure 5.9. The Eg peak at 146 cm -1 is associated with the TNR sample. A blue-shift from 146 cm -1 to 155 cm -1 for the GOT Eg peak was observed when compared to TNRs. The Eg peak's blue shift which was caused by the phonon confinement effect and likely due to the strain developed at the interface between TiO2 and GO [21,74] suggests strong bonding interactions between TNRs and GO [4,21,74,75]. The greatest blue shift among the different samples was observed for GOT which had the strongest interactions between TNRs and GO among all the different samples. However, the degree of blue shift decreased with increasing RGO deoxidization was a function of increasing reaction temperature and time. Notice the blue shift decreased with deoxidization or the decreasing O/C ratio from 0.850±0.007 to 0.090±0.002 was observed for a temperature change from 20 o C to 95 o C and reaction time change from 1 h to 3 h. The Eg peak assigned to the GT-95 1h and GT-95 3h samples were located at 152 cm -1 and 150 cm -1 , respectively, and the Eg peaks for GT-20, GT-60 and GT-80 were positioned between the Eg for the GOT and GT-95 1h samples. The decreasing blue shift suggested that the interaction between RGO and TNWs decreased with decreased with RGO deoxidization.

In the XPS Ti2p (Figure 5.10) and C1s spectra (Figure 5.7c), no peaks corresponding to Ti-C and Ti-O-C chemical bonds indicate that the interaction is based on physical and not chemical attachment [44,76].

Optical response and bandgap

The atomic O/C ratio exerted a remarkable influence on the optical properties. Lower bandgap energies resulted in more efficient optical absorption. Diffuse reflectance UVvisible spectroscopy (DRS) (Figure 5.11a) was used to examine the optical response for each sample. The bandgap for each sample was calculated by using the Kubelka Munk function to convert the diffuse reflectance spectra into an absorption spectra (Figure 5.11b) [77]. By adding GO or RGO sheets, a significant increase in optical absorption in the visible light region (Figure 5.11a) was observed for GOT when compared to the TNRs. The optical absorption was further increased by reducing GOT to GT which contained a lower RGO atomic O/C ratio. The greatest optical absorption was observed with GT-95 1h containing an RGO atomic O/C ratio of 0.130±0.003. However, further deoxidization by increasing the reaction time to 3 h resulted in a decrease in the optical absorption when compared to GT-95 1h . The decreased optical response for GT-95 2h and GT-95 3h was likely caused by the separation of RGO from TiO2 due to the decreased interaction between RGO and TiO2 when excessive deoxidization occurred.

Decreasing GOT bandgap (3.08 eV) was observed by adding GO to TNRs (Figure 5.11b). Further narrowing of the bandgap occurred by continuing deoxidization of RGO.

With decreasing RGO atomic O/C ratios from 0.850±0.007 to 0.130±0.003, the bandgap decreased trend was as follows: GOT (3.04 eV) > GT-20 (2.91 eV) > GT-60 (2.73 eV) > GT-80 (2.65 eV) > GT-95 1h (2.61 eV) (Figure 5.11b). However, as RGO atomic O/C ratios decreased from 0.130±0.003 to 0.09±0.002, the bandgap increased to 2.75 eV and 2.87 eV for the GT-95 2h and the GT-95 3h samples, respectively. This indicated that an excessive removal of oxygen-containing functional groups caused an increase in the 1. CHAPTER 5 177 bandgap. According to Zhang et al. (2010) [78], the chemical bonds of Ti-O-C were responsible for the interaction between TiO2 and RGO. This interaction narrowed the TiO2 bandgap [78]. For the GT-95 3h sample, an excessive removal of the oxygen containing functional groups weakened the interaction between TiO2 and RGO and this increased the bandgap from 2.61 eV to 2.84 eV.

Charge transportation and separation

The transit photocurrent responses were examined to illustrate the influence of the RGO atomic O/C ratio on the charge transportation and separation for the GT composites.

I-t curves for each sample with an on-off UV-light irradiation source are shown in Figure 5.12. The current for each sample increased when the light source was applied and a rapid decrease to approximately zero was observed after removing the light source.

Incorporation of GO into TNRs resulted in a slight decrease in photocurrent when compared to TNRs. A significant increase in the photocurrent was observed when the GOT sample was reduced to GT. This increase is attributed to enhanced charge carries transfer and separation by incorporating TNR with the highly conductive RGO [78]. The photocurrent continued increasing with a decrease in the RGO atomic O/C ratio from 0.850±0.007 to 0.130±0.003. The highest photocurrent for GT-95 1h was a 12-fold increase when compared to pure TNRs. This photocurrent was approximately 6-fold increase when compared to the UV-assisted TiO2-RGO reported by Williams et al. ( 2008) [5]. The UV-assisted TiO2-RGO which showed approximate 2-fold increase when compared to pure TiO2 was synthesized by photocatalytic reduction of GO under UV light. [START_REF] Akhavan | Photocatalytic reduction of graphene oxide nanosheets on TiO2 thin film for photoinactivation of bacteria in solar light irradiation[END_REF][START_REF] Morgan | Alkaline hydrothermal treatment of titanate nanostructures[END_REF] [43,44] reported that the photocatalytic activity of UV-assisted TiO2-RGO was further improved by gradually removing the oxygencontaining groups. This suggestion of removing the oxygen-containing groups was closely linked to an increase in the photocatalytic activity for the RGO based TiO2 photocatalyst with high atomic O/C ratios. However, the photocurrent decreased sharply with a decrease in the atomic O/C ratio from 0.130±0.003 to 0.090±0.002. This decrease is likely due to the excessive loss of the oxygen containing groups leading to weak interactions between RGO and TNRs. The results indicated that preserving an appropriate amount of oxygen containing groups on RGO sheets will manage and hence, maximize the charge-carries transfer and separation processes.

Photocatalytic activity

The photocatalytic activities for each sample were characterized using MO as a model for no photocatalyst (0.0003±0.0001 min -1 ) and RGO (0.0053±0.0012 min -1 ) were significantly less when compared to 0.2361±0.0021 min -1 for TNRs and 0.2202±0.0025 min -1 for TiO2 P25 based on the Tukey's test [79] (Figure 5.13 and Table 5.2). Coupling TNRs with GO caused a sharp decrease in the apparent MO k to 0.0951±0.0019 min -1 when compared to TNRs. This sharp decrease indicates the GO with a high RGO atomic O/C ratio was unfavorable for the photocatalytic process.

However, the photocatalytic performance was significantly improved though the reduction of GOT to GT. The apparent MO k value increased from 0.0951±0.0019 min -1 to 0.3118±0.0012 min -1 with an increase in reduction temperature up to 95 o C for 1 h corresponded to a decrease in the RGO atomic O/C ratio from 0.850±0.007 to 0.130±0.003. However, at 95 o C for 2 h and 3 h, a further decrease in the O/C ratio caused a decrease in the apparent MO k value to 0.2516±0.0018 and 0.1616±0.0023 min - 1. CHAPTER 5 181 deoxidization in GT-95 3h decreased the photocatalytic performance. Data from the photocatalytic studies are consistent with the photocurrent results (Figure 5.12). For photocatalytic and photocurrent studies, the highest photocatalytic activity and transit photocurrent was observed for the GT-95 1h sample with a moderate atomic O/C ratio of 0.130±0.003.

The bifunctional effect of the RGO atomic O/C ratio on the GT photocatalytic activities was further evaluated by assessing the photocatalysis of RhB, MB and phenol.

RhB and MB are cationic dyes, and phenol, a chemical containing a benzene structure, is difficult to be degraded. The apparent first-order photocatalysis rate constant (apparent k) values for the different organic chemicals are shown Figure 5.14. When compared to MO photodegradation, the photocatalysis of the selected chemicals followed a similar trend based on Tukey's test [79] (Table 5.2). Addition of GO into TNRs without any reduction resulted in a decrease in the apparent k, whereas a large improvement was observed after reduction at 20 o C for a 1 h exposure time. Improving the apparent k, was observed by increasing the reduction temperature. The highest apparent k for each dye was observed when using the GT-95 1h sample with an atomic O/C ratio of 0.130±0.003. However, a sharp decrease in apparent k was observed when the atomic O/C ratio reached 0.110±0.002 and 0.090±0.002 for the GT-95 2h and GT-95 3h samples, respectively. The optimum photocatalytic activity for the photocatalysis of RhB, MB and phenol increased by factors of approximately 1.5, 1.7 and 1.3, respectively, for the GT-95 1h sample. Peak photocatalytic activities suggest that the optimum RGO atomic O/C ratio of 0.130±0.003 was observed in the case of GT-95 1h . This optimized GT-95 1h showed a significantly shorter half-life (t1/2) of approximately 0.44 min for degrading MB when 1. CHAPTER 5 182 compared to the t1/2 of P25-RGO (3-4 min) corroborated by Zhang et al. (2010) [78] and Du et al. (2011) [78]. This is an approximate 8-fold improvement. In addition, the optimized GT-95 1h has an overwhelming advantage over activated carbon based TiO2 which showed t1/2 of approximately 10 min [1]. GT-95 3h 0.1616±0.0023 G 0.7793±0.0067 F 0.9507±0.0127 F 0.0388±0.0007 G Note: A, B, C, D, E, F, G, H, I, J: Means of the terms that do not share a letter in superscript are significantly different in each column according to Tukey's test [80], and A>B>C>D>E>F>G>H>I>J.

1. CHAPTER 5 184

The mechanism of reduced graphene oxide (RGO) atomic oxygen-to-carbon (O/C) ratio

The mechanistic action for GT containing the oxygen containing groups on RGO could be explained using the scheme shown in Figure 5.15 and Figure 5.16. TNRs are attached on the RGO sheets through the interaction between the oxygen containing groups and TNRs (Figure 5.15) [78]. RGO residual oxygen containing groups -OH and =O are able to interact with TNRs' O 2-and Ti 4+ , respectively. Photo-generated electrons are transferred to the highly conductive RGO sheet, and subsequently a reaction is mediated with organic chemicals [21]. ) and subsequently, an increase in the photocatalytic activity [30,58,78].

Increasing the photocatalytic activity was caused by increasing the RGO conductivity [71]. Optimum photocatalytic activity was observed for the GT-95 1h sample with an RGO atomic O/C ratio of 0.130±0.003. Increasing the reducing time to 2 h and 3 h at 95 o C caused an excessive removal of the oxygen containing groups, a decrease in interaction between RGO and TNRs and a significant decrease in the photocatalytic activity. A change in the reaction mixture color from dark to light grey (GT-95 2h and GT-95 3h samples) suggests a significant amount of RGO was separated from the TNRs. Ag NPs with an average diameter of 3 nm were grown on the TNRs surface (Figure 5.17).

Ag + ions introduced after the formation of GT composite were preferably adsorbed on the TiO2 hydrophilic surface instead of on the hydrophobic RGO sheet surface. During reduction, Ag NPs were homogeneously grown on the TNRs.

Structure analysis

X-ray diffraction (XRD) analysis was conducted to evaluate the crystal phase for all the samples (Figure 5.18a). The anatase [81] and rutile [63,82] phases were detected in all the samples. The peaks located at 2θ = 25.4°, 37.9° and 48.1°correspond to the (101), (004) and (200) orientation of the anatase TiO2 phase, respectively. The peak located at 2θ = 25.4° correspond to the (110) orientation of the rutile TiO2 phase. The Ag characteristic peaks (111) and (200) located at 2θ = 38.2° and 44.0°, respectively, were detected for the GT samples containing Ag nanoparticles. The increasing Ag peaks intensity was linked to the increase in the Ag content. Raman spectroscopy was employed to confirm the TiO2 phase and incorporation of RGO into the TiO2 matrix. The peaks [65] at 144 cm -1 (Eg), 399 cm -1 (B1g), 513 cm -1 (A1g), and 638 cm -1 (Eg) (Figure 5.18b) indicate the existence of anatase. Moreover, the Eg peak for GT and Ag-GT shows a blue-shift from 143 cm -1 to 155 cm -1 when compared to TNRs (Figure 5.18c). The Eg peak blue shift is likely caused by the strain at the interface between the TiO2 and RGO sheets [21,74]. This suggests that TNRs has a strong interaction with RGO sheets [4,21,74,75] (Figure 5.18c). Interaction between TNRs and RGO sheets in the hierarchical 3D Ag-GT is an essential element for efficient charge transfer and separation [21]. In the deconvoluted GO X-ray photoelectron spectroscopy (XPS) C1s spectra (Figure 5.18d), the peak located at 284.5 eV and 250.0 eV corresponds to sp 2 or sp 3 bonded carbon, respectively, while the de-convoluted peaks for C-O (286.5 eV), C=O (287.8 eV) and O-C=O (289.0 eV) at higher binding energies are attributed to oxygen-containing function groups on the RGO layers [24]. The significant decrease in the peak intensities of oxygen functionalities for the 10%Ag-GT composite suggests the successful reduction of GO to RGO.

Diffuse reflectance UV-visible spectroscopy (DRS) and bandgap

Diffuse reflectance UV-visible spectroscopy (DRS) (Figure 5.19) was used to further examine the optical response of the samples. The diffuse reflectance spectra were converted into a corresponding absorption spectra using the Kubelka Munk function [77] 1. CHAPTER 5 190 (Figure 5.19b). All the samples showing a wide optical absorption band at approximately 380 nm [83] is based on the TiO2 bandgap of 3.21 eV (Figure 5.19b) [82,83]. The GT with RGO structure in the TiO2 matrix which are associated with increasing visible light absorption and decreasing the bandgap to 2.94 eV, due to the interaction between TiO2

and RGO [84]. The 3D Ag-GT photocatalysts showing the largest visible light absorption between 400 to 700 nm and decreased the bandgap to approximately 2.55 eV (Figure 5.19b) was due to the localized SPR absorption of Ag NPs [39,85,86]. In addition, increasing the Ag content from 5% to 15% caused an increasing visible light absorption between 400 to 700 nm. The narrowing bandgap for the hierarchical 3D Ag-GT photocatalyst suggests that this composite is favorable for visible-light photocatalysis. All photocatalysts films showed approximately 10% adsorption of RhB for 1 h of stir under dark condition. Under visible light condition, only approximately 6% of RhB was degraded without the photocatalyst. The average apparent RhB degradation rate constant value (designated as "apparent RhB k") was 0.016±0.001 h -1 without a photocatalyst (Figure 5.21b). However, with the addition of a film of photocatalyst, the average k value significantly increased. The apparent k of TiO2 NPs film was 0.1203±0.0073 h -1 . The apparent k of TNRs for RhB (0.154±0.0077 h -1 ) was improved by approximately 28.01±0.06% when compared to TiO2 NPs. In addition, adding RGO into TNRs also caused an enhanced k value. This improvement can be attributed to interaction between TiO2 and RGO [22]. The largest k values (0.7374±0.011 h -1 ) were observed for Ag-GT samples 10% Ag (degrading 96% of RhB in 4 h). This largest k value was approximately 2.6-fold increase when compared to the k value of 0.288 h -1 (0.0048 min -1 ) using Ag doped TiO2 reported by Bensouici et al. [43] The decreasing k value caused by Ag concentrations larger than 10% was due to the negative effect of excessive Ag acting as recombination centers between the photogenerated e --h + pairs.

Reaction mechanism

A key factor in achieving a highly active photocatalyst is the unique hierarchical 3D structure of Ag-GT (Figure 5.23) which includes visible-light active, enhanced charge separation and improved SSA. In the unique hierarchical 3-D structure, both RGO [6,78] and Ag NPs were able to extend the absorption band edge of TiO2 into the visible light region. The SPR excited electrons from Ag NPs can be transferred to the conductance band of large SSA TNRs and be subsequently transferred to the RGO sheets to produce superoxide anion radicals (•O2 -) [39]. RGO, an efficient electron sink/transporter, induces electron transfer from the TiO2 conduction band to itself, while the holes trapped in the TNRs and Ag NPs have a longer lifetime [21,22,39,40]. In particular, 3D Ag-RGO-TNRs (Ag-GT) shows significantly improved photocatalytic activities when compared to Ag-RGO-TiO2 NPs, because TNRs, in comparison to TiO2 NPs, have more uniform dispersion on RGO with less agglomeration [57] and have improved charge 1. CHAPTER 5 195 transfer along TiO2 Q1D structure. The electrons are eventually captured by adsorbed molecular oxygen on the RGO sheets surface to produce •O2 -, while the holes accept electrons from H2O on the TiO2 surface to produce hydroxyl radicals (•OH) [88].

Subsequently, these highly reactive radicals oxidize organic molecules such as RhB. The much higher TNRs SSA provided more reactive sites for producing reactive species such as •OH when compared to TiO2 NPs. crystal size. The optimal photodegradation activity was observed for a silver content of 0.4% in the composite coatings.

Introduction

Pharmaceuticals are emerging environmental organic wastewater contaminants (OWCs) due to their potential impact on humans, animals and microorganisms at threshold concentrations [1]. Industrial development coupled with acceleration in population growth rate has led to the occurrence of these chemicals in wastewater effluents and drinking water supplies. Exposure to OWCs is linked to serious concerns such as environmental damage and human diseases [2]. The occurrence of CHAPTER 6 209 pharmaceutical chemicals in municipal wastewaters is due to effluents arising from pharmaceutical manufacturing facilities and human excretions into sewers [3][4][5].

Conventional municipal biological wastewater treatment facilities are ineffective in removing several pharmaceutical chemicals [6][7][8]. In addition, operational costs and the generation of secondary toxic pollutants are major problems associated with municipal biological treatments [9,10]. Advanced oxidation processes such as ozone (O3), O3/H2O2, Fenton and photo-Fenton are promising technologies for the degradation of pharmaceutical chemicals [11]. However, the major disadvantages of these processes include incomplete degradation and the production of toxic byproducts. Photocatalysis based on TiO2 is an emerging technology which can overcome these problems [6,7,[11][12][13][14][15].

The semiconductor TiO2 has widespread applications as a photocatalyst to degrade organic pollutants in water [6,7,13] and for dye-sensitized solar cells [16][17][18][19][20][21]. The valance band of TiO2 is more positive than the O2/H2O redox potential and the conductive band is more negative than the H2/H2O redox potential [9]. These characteristic properties allow TiO2 to be an excellent candidate for many electrochemical applications including water splitting [8,10]. However, the application of TiO2 is limited to UV activation due to its wide band gap (3.2 eV). This limits the use of TiO2 as solar or room-light activated catalysts, because the majority of the sunlight consists of visible light with only a 3-5% UV light content [15]. Developing visible light induced TiO2 photo-catalysts with high efficiency is essential for enhancing the process practicality, feasibility and economics [14]. Another factor restricting the photocatalytic activity of TiO2 is the low quantum yield of excitons due to the fast electron-hole (e -/h + ) recombination [8]. In competition, these electrons and holes can undergo reduction and oxidation before the recombination reaction. However, the small lifetime of e -/h + and absence of suitable scavengers causes dissipation of stored energy within a few nanoseconds by recombination [15].

Many studies have demonstrated that improving the PCA can be accomplished by narrowing the absorption band gap to the visible portion of the spectra, and by reducing the recombination reaction. Incorporating TiO2 with metals has been reported to improve the photocatalytic activities based on improvement in the two factors previously discussed [2,8,14,15,22,23]. Silver is a common element employed to incorporate TiO2. Incorporating TiO2 with Ag is advantageous because of its low cost, inherent antibacterial activity, and facile preparation [2,[24][25][26][27][28][29][30]. In addition, incorporating Ag leads to excellent corrosion resistance and improved hardness [31].

The photocatalytic efficiency of TiO2 thin films is strongly dependent on the preparation methods. In general, the photocatalytic preparatory method can influence the structural and chemical properties such as structure, porosity, stoichiometry, density of defects and recombination rates between the photo-generated carriers [32]. In the case of thin film photocatalysts and other similar photocatalysts, one consideration in developing innovative preparation processes is to maximize the catalytic activity while another is economics by increasing the deposition rate. TiO2 thin films can be prepared using methods such as deposition assisted by plasma [33]. TiO2 coatings deposited by plasma are characterized with excellent physicochemical properties which adhere strongly to different substrates, but with relatively low deposition rates when deposited at low and even at atmospheric pressure [33]. A non-equilibrium atmospheric pressure plasma jet CHAPTER 6 211 (APPJ), characterized by a relatively high gas temperature (1000-1500 K), has been reported to be effective for surface treatments [34][35][36][37] and for coating thin films with high deposition rates [38,39]. However, the physicochemical properties of the coatings are dependent on the operational parameters. In previous studies, Fakhouri et al. [40] reported depositing TiO2 coatings by APPJ and These researchers reported the photocatalytic activity of the TiO2 thin films prepared by APPJ was significantly improved when compared to optimum N-doped TiO2 thin films which was prepared by reactive radio frequency (RF) magnetron sputtering [41]. The APPJ offers the possibility to deposit coatings on 3D substrates as well without the need to use vacuum which is the drawback in magnetron sputtering [41].

In this study, the first part of sixth objective is to use the APPJ system reportedly used by Fakhouri et al. [40] to deposit Ag nanoparticles (NPs) and create a Ag-TiO2 composite coating by means of a PECVD process. The second part of sixth objective is to investigate the impact of the Ag concentration, air gas flow rate and the discharge pulse frequency on the coating phase structure, crystal size and photocatalytic activity. The third part of sixth objective is to investigate the photocatalytic activity of the APPJ Ag-TiO2 coating for degrading RhB and three pharmaceutical compounds, namely carbamazepine (CBZ), venlafaxine (VLX) and bezafibrate (BZF), in water by using a solar simulator light irradiation.

Experimental Methods

The thin film photocatalytic activity tests are included in Chapter 2 (Section 2.3.12).

Ag-TiO2 coating were prepared by introducing TTIP detailed in Chapter 2 (Section 2.4.3).

CHAPTER 6 212

Optical emission spectroscopy of atmospheric pressure plasma jet (APPJ) plasma

Plasma emission was sampled using optical emission spectroscopy described in Chapter 2 (Section 2.3.11). Briefly, plasma emission was sampled using an optical fiber located downstream, at 10 mm from the nozzle exit of the torch, and connected to an optical emission spectrometer (OES) (Ocean Optics USB2000+, FL, USA) which was configured with a grating of 300 gmm -1 in the UV-visible spectrum range and a 25 µm slit. The OES spectra were recorded in the range of 200 nm to 1100 nm to identify the excited chemical species in the plasma with and without the presence of the TTIP precursor including Ag NPs.

Thin film characterization

The thin films crystal structure was characterized by X-ray diffraction (XRD) (X'Pert Pro PW3040-Pro, Panalytical Inc., Nottingham, UK) using a Cu Kα1 (λ = 1.5418 Å) Xray radiation source in the Bragg-Brentano configuration. X'Pert High Score pattern processing was used to collect and process the data. Surface morphology and film thicknesses were determined using field emission scanning electron microscopy (FESEM) (JEOL, Japan).

Results and discussion

Plasma jet characterization

The plasma optical emission spectra for samples with and without the TTIP precursor and with Ag NPs were recorded to understand the formation of the Ag-TiO2 coatings.

During metal deposition, the plasma luminosity was strongly enhanced, and the spectra The highly reactive species (radicals, metastables, vibrationally excited molecules, photons) in the plasma jet is able to enable a very rich chemistry for surface treatment.

The technique used in this study is a blown arc system which is affiliated with high temperatures at the nozzle exit (i.e. Tg ~ 1000-1500 K) and a sharp temperature decrease gradient. These characteristics leads to an interesting heat transfer phenomenon which allows tailoring the physicochemical properties of the coatings as well as the dispersion of Ag in the host material. Rapid quenching of the high temperature further away from the nozzle exit provides the possibility of temperature sensitive materials thermal CHAPTER 6 216 treatment. More interestingly, controlling the heat transfer predominantly impacted the position of the Ag entrapped sites in the host material (TiO2) [40].

Morphology and Ag diffusion in composite Ag-TiO2 thin films

The effect of deposition power (pulse frequency) and air flow rate on the photocatalyst morphology was examined under 3 conditions (18 kHz and1800 L•h -1 , 25 kHz and 1800 L•h -1 , 18 kHz and 3000 L•h -1 ). The surface morphology under the different conditions are shown in Figure 6.2. At low deposition frequencies (low power) and low air flow rates, the preferred composite formation produced core-shell Ag encircled by TiO2 (Figure 6.2a) spheres with diameters ranging between 800 nm and 3000 nm. Ag was partially oxidized between the Ag core and the TiO2 shell as shown in

CHAPTER 6 217

In the case of the APPJ-spray process, the deposition rate is estimated at 10-20 µm•s -1 .

Whereas, in the case of RF reactive sputtering, the deposition rate is typically between 1-10 nm•min -1 for TiO2 in the oxide mode and the films are characterized by a 1D columnar structure [41]. The difference in the porosity between the films deposited by sputtering and APPJ can be explained by the higher deposition rate of the APPJ technique of approximately nine to ten orders of magnitude. Also, the APPJ technique is characterized by a lower kinetic energy of the reactive species in the plasma reaching the surface. This lowering of the kinetic energy which eventually causes reduced mobility of the substrate surface species explains the higher porosity of the coatings. The effect of air flow rate and impulse frequency for samples produced using 0.4% Ag is shown in Figure 6.3b. The data indicate, at 18 kHz, that a low air flow rate (1800 L•h -1 ) was favorable for producing a more crystalline surface when compared to the surface characteristics observed at a higher air flow rate (3000 L•h -1 ). The TiO2 anatase phase coatings were crystalline regardless of the pulse frequency. However, the degree of crystallinity varied with the pulse frequency. The coatings became more crystalline when increasing the power from 18 kHz to 25 kHz. This was coupled with decreasing Ag(111) and Ag(200) peak intensities and changing the morphology from a layer to a 'coral' like structure (see Figure 6.2). In the case of a high air flow rate condition, less crystallinity was detected in the anatase and crystalline Ag phases. The crystal size of anatase crystals was calculated using the Scherer formula (Eq. 6.1)

for the different samples [43][44][45].

Where d is the crystal size (nm), K is a constant called shape factor (typically equal to 0.9), λ is the wavelength of XRD radiation (λ = 0.154 nm), β is the full width at half maximum (FWHM) and θ is the diffraction angle. According to the calculations using equation (Eq. 6.1), higher pulse frequencies are associated with lower crystal sizes. The variation of the anatase crystal size as a function of the Ag concentration is shown in Figure 6.4a. The decrease in anatase crystal size is closely correlated with increasing the Ag concentration. This demonstrates the role of Ag in decreasing TiO2 crystal size. This is in agreement with previous studies which report that the presence of Ag particles on the titanium oxide surface is known to inhibit growth of TiO2 crystals [46][47][48]. Higher deposition power significantly affected the anatase crystal size. For the low air flow rate condition set at 1800 L•h -1 , the crystal sizes were 41 nm, 34 nm and 31 nm for pulse frequencies (Figure 6.4b) of 18 kHz, 21 kHz and 25 kHz, respectively. Achieving enhanced crystallinity using higher pulse frequencies in the jet was likely due to higher energy input into the plasma.

Photocatalytic activity

The photocatalytic activity degradation work was performed by assessing the photodegradation of RhB using the Ag-TiO2 photocatalyst under white solar light conditions. The RhB degradation kinetics in the presence of the Ag-TiO2 samples (with different Ag concentrations at a fixed pulse power of 21 kHz) coated on glass substrates, under white light irradiation is shown in Figure 6.5. The initial concentration of RhB in distilled water was adjusted to 5 mg•L -1 before each experiment. Large photocatalytic activities were observed for samples containing 0.3% and 0.4% Ag. The decreasing photocatalytic activity caused by Ag concentrations larger than 0.4% was due to the negative effect of excessive Ag acting as recombination centers between the photogenerated electron-hole pairs. RhB degradation kinetics was assessed using samples prepared by employing a fixed 0.4% Ag concentration but with different pulse powers and air flow rates (Figure 6.6).

The pseudo first order [49] rate constant was much larger in the case of the Ag-TiO2 coatings when compared to the non-incorporated coatings. Using an optimized pulse frequency at 21 kHz resulted producing a coating which is able to accomplish faster kinetics. Also, the Ag-TiO2 half-life (t1/2) was less than the t1/2 for the TiO2 coatings.

Note that the best t1/2 of 42 min was observed for a sample prepared using a pulse frequency of 18 or 21 kHz and air flow rate equal to 1800 L•h -1 . The data indicate that the major parameters of the deposition parameters affecting the photocatalytic activity of the coatings were the pulse frequency and the air flow rate. Following the preliminary screening test to establish a preforming photocatalytic thin film using RhB as a target contaminant, the photocatalytic activity for 0.4% Ag-TiO2 sample was further examined using CBZ, VLX and BZF. CBZ degradation profiles of photolysis and photo-catalysis using TiO2 and 0.4% Ag-TiO2 composites are shown in Figure 6.7. CBZ is an antiepileptic drug, and one of the most frequently detected compounds in the aquatic environment due to its resistance to biological degradation [50]. At all exposure times, CBZ was consistently not susceptible to photolysis by solar irradiation; however, degradation was observed by employing TiO2-based photocatalysis.

Photocatalysis using the Ag-TiO2 composite sample caused approximately 75% removal of CBZ within 60 mins (Figure 6.7). examined [51] can aid in the selective adsorption of VLX, a hydrophilic and the only positively-charged tested compound (pKa=8.9; logD=0.84, Table 6.1) in this study.

No decrease in CBZ, VLX and BZF concentration in the solution was observed after 60 min of exposure to the TiO2/Ag-TiO2 catalyst under dark conditions. This indicates the adsorption for each pollutant onto the Ag-TiO2 coating is negligible. For photodegradation using solar irradiation, BZF showed only 20.3±5.2% degradation by TiO2; however, the degradation improved significantly to 62.2±4.6% when using 0.4% Ag-TiO2. CBZ has also showed a higher degradation by 0.4% Ag-TiO2 with 73.7±4.6%

as opposed to 34.0±0.5% degradation by TiO2. BZF is negatively charged and relatively hydrophilic at the experimental pH (pKa=3.8; logD=0.97, Table 6.1), while CBZ is neutrally charged and hydrophobic (pKa=13.9; logD=2.77, Table 6.1). The similar behavior of both compounds, possessing different chemical properties, suggests that although a favorable interaction between the target compound and the catalyst is an advantage, it is not prerequisite for photocatalytic oxidation. To summarize, Ag incorporation in the TiO2 clearly provided superior photocatalytic efficiency owing to the improved charge transfer and separation [8,14,15,52] and the porous structure facilitating higher active surface area.

Mechanism of photocatalytic oxidation by Ag-TiO2

Ag NPs at the metal-TiO2 interface region functions as electron traps to facilitate electron-hole separation and eventually promoting the interfacial electron transfer process to enhance the photocatalytic efficiency [53]. In comparison, incorporating Ag into TiO2 host materials is associated with a decrease of the bandgap [14,52]. TiO2 has a bandgap of 3.2 eV and shifting the absorbance towards longer wavelengths by incorporating Ag reduces the band gap to 2.9 eV for Ag-TiO2 [14,52]. The decrease in the band gap extends the light absorption in the visible range and enhanced the photocatalytic activity [15]. However, with extensive concentrations of Ag in the host material, Ag nanoparticles which are able to behave as recombination centers between the photo-generated electron hole pairs within TiO2 drastically reduces the photocatalytic activity of the composite photocatalyst. Hence, 0.4% Ag is the optimum condition for photodegrading organic pollutants. The 0.4% Ag-TiO2 core-shell structure enhanced the photoactivity of TiO2 by increasing the interfacial charge transfer efficiency of the TiO2 thin films. Furthermore, decreasing the anatase grain size caused an increase of the specific surface area, inhibition of bulk recombination between the e -/h + pairs and enhancement of the charge separation.

Conclusions

An arc blown atmospheric pressure plasma jet (APPJ) was used was a simple and effective technique to produce a unique composite Ag-TiO2 coating with controlled porosity and crystal size. The one-step process significantly improved the photocatalytic activity of TiO2 towards the removal of RhB and pharmaceutical compounds (CBZ, VLX and BZF). The coating process was employed to control the Ag NPs distribution and the crystallinity (crystal size) by varying the deposition parameters such as the plasma energy and carrier gas flow rate. The APPJ process produced Ag-TiO2 composite photocatalysts with significant improvement in the homogeneous dispersion of Ag and the photocatalytic activity under solar irradiation conditions when compared to the pure TiO2 coatings deposited by APPJ. The photocatalytic activity of the combined Ag-TiO2 coating was more improved when compared to N-doped TiO2 thin films which were prepared by reactive RF magnetron sputtering. This improvement is due to the Ag incorporation along with the enhanced specific surface area of the APPJ coatings. Finally, the concentration of Ag ions in water exposed to 0.4% Ag-TiO2 slides was below the detection limit. Hence, use of the Ag-TiO2 film synthesized using APPJ can be considered as a potential photocatalyst for drinking water treatment. This novel and simple method to deposit porous TiO2 films at a high deposition rate can be utilized in many applications such as photocatalysis and dye sensitized solar cells.

GENERAL CONCLUSIONS AND RECOMMENDATIONS

The work outlined in this thesis describes the significance of using quasi-onedimension (Q1D) TiO2 as photocatalysts for the photodegrading aqueous hazardous pollutants. In Chapter 3, the effects of three hydrothermal synthesis factors on Q1D TiO2 bandgap were examined using a 3-factor 3-level Box Behnken design (BBD) statistical model. The hydrothermal synthesis factors significantly affected Q1D TiO2 phase structure, crystal size [1] and SSA, bandgap and photocatalytic activities in terms of photodegrading aqueous hazardous pollutants [2] and producing H2 [3] were linked to Q1D TiO2 containing biphasic anatase-rutile phase with smaller mean crystal size (9.5±0.1 nm) and maximum photodegradation rate of aqueous organic pollutants. The RhB photodegradation rate for Q1D TiO2 synthesized using the optimized conditions was approximately 46% greater when compared to anatase-TiO2-B prepared using a surfactant molecular self-assembly process reported by Zhang et al [5].

This result was validated by photodegrading phenol, methyl orange (MO) and methylene blue (MB). The second model (Fourth objective part 2) was developed for photocatalytic H2 production using 20 v/v% ethanol solutions as feedstock. The model demonstrated that lower temperature and higher NaOH concentration were correlated to Q1D TiO2 containing pure anatase phase with a mean crystal size of 20.1±0.2 nm and an enhanced H2 production rate. The optimum conditions for synthesizing Q1D anatase TiO2 with the maximum H2 production rate of 475±12 µmol•h -1 (quantum efficiency (ε) = 20.2±0.5%) was observed at 126 o C, 15 M NaOH and 49 g•L -1 TiO2. The maximum H2 prduction rate was significantly improved when compared to a biphasic anatase-TiO2-B photocatalyst reported by Kuo et al [3]. The model demostrated that higher temperature (190 o C) and higher NaOH concentration (≥10 M) were linked to Q1D TiO2 containing TiO2-B phase with larger crystal size (≥ 46.4±0.3 nm) and low photocatalytic activities.

Table 7.1: The apparent photocatalysis rate constant (min -1 ) for selected photocatalysts using UV irradiation. 7.1). The highest photocurrent increase for GT with optimum RGO atomic O/C ratio was approximately 6-fold when compared to the UV-assisted TiO2-RGO reported by CHAPTER 7 238 Williams et al. [6]. The MB photodegradation rate increase of approximately 8-fold with the optimum GT photocatalyst when compared to Degussa TiO2 P25-RGO is corroborated by Zhang et al. [7] and Du et al. [8]. The 3D Ag-GT photocatalysts with an enhanced photodegradation rate was observed for a silver content of 0.4wt% in the composite coating. The photocatalytic activity of the optimum Ag-TiO2 thin films prepared by APPJ was significantly improved when compared to the pure TiO2 coating obtained by APPJ as well as the low pressure sputtering technique [13,14].

Experiment

The study in this dissertation demonstrated the possibility of using Q1D TiO2 synthesized by the alkaline hydrothermal process as a photocatalyst for wastewater treatment and renewable energy production. Enhancing Q1D TiO2 with partial RGO and Ag nanoparticle improved the photocatalytic efficiency. Immobilizing Ag-GT and Ag-TiO2 simplified the wastewater treatment process when compared to using photocatalyst powder. However, a few obstacles need to be addressed for future application.

Redesigning the existing photocatalytic reactor used to degrade aqueous hazardous pollutants will offer the possibility to transform from batch degradation to a continuous degradation process for practical implementation and scale-up.

Work to reduce the multi-step Ag-GT thin-film synthesis is required to reduce the processing cost. Increasing the H2 production rate using strategies such as coupling with a narrow bandgap semiconductor, doping with ions, co-catalyst loading, surface dyesensitization or noble metal deposition are required to improve the Q1D TiO2 photocatalytic activity [15]. Modifying the Q1D TiO2 catalyst could allow using solar light and significantly enhance the solar energy conversion efficiency [15]. Note, the maximum energy conversion of UV-activated photocatalyst is 2% of the solar energy [15].

Studies on smaller bandgap semiconductors such as CdS are required to enhance visible-light photocatalytic activities. CdS is an alternative solar light photocatalyst. CdS quantum dots (<10 nm) shows a relatively narrow bandgap of 2.42 eV corresponding to a response when using visible light below 510 nm [16]. However, CdS suffers anodic photocorrosion since S 2-is susceptible to chemical corrosion [17][18][19]. Further, modifying CdS with RGO, noble metal (e.g. Pt, Au and Ag), metal-organic framework and other semiconductors will likely improve its photocatalytic activities and chemical stability [17][18][19][20]. Finally, the APPJ used to deposit Ag-TiO2 thin film photocatalyst coatings with improved photocatalytic properties can present a great potential to make other TiO2 modified coatings. Indeed, a wide range of other composites such as functionalized graphene or Fe2O3 (Photo-Fenton system) can be incorporated in TiO2 coating to improve the photocatalytic activity under solar irradiation conditions. Comme perspectives de cette thèse il serait intéressant de réduire la synthèse multiétape des matériaux 3D de Ag-GT pour réduire le coût du traitement. La taux de production H2 peut-être augmenté en utilisant des stratégies comme couplage avec un semi-conducteur à largeur de bande étroite, un dopage avec des ions, l'utilisation de cocatalyseur, l'utilisation des colorants de surface ou le dépôt des métaux nobles sont nécessaires pour améliorer l'activité photocatalytique de TiO2 Q1D [14]. Modifier le catalyseur TiO2 Q1D pourrait permettre d'utiliser la lumière solaire et améliorer significativement l'efficacité de conversion d'énergie solaire [14]. Note, la conversion minimum bandgap and maximum photocatalytic activities for photodegradation and H2 production, respectively.

Another significant outcome from Chapter 3 and 4 is the identification of the optimum phase structure, crystal size (SSA) and bandgap for maximizing pollutants photodegradation rate and H2 production rate. These models are significant because they can aid in engineering Q1D TiO2 based compounds with controllable crystal phase, crystal size, SSA and bandgap and thereby, has been promising to enhance the photocatalytic activities of the existing photocatalytic applications.

In Pt-TiO2 and Ag-ZnO2) and many applications such as photocatalysis and dye sensitized solar cells, among others.

ACKNOWLEDGEMENTSI

  photoluminescence spectra using a Varian Cary Eclipse Fluorescence Spectrophotometer

Figure 1 . 1 :

 11 Figure 1.1: Schematic illustrating water splitting using TiO2 [18, 21]: (a) Charge transfer and recombination during photocatalytic water splitting, (b) The energy position of the valence and conduction bands. Notes: a Valence band is the energy band occupied by the valence electrons and the highest occupied band [21], b conduction band is the lowest unfilled energy band and allows electrons moving freely [21], and c CB and VB represent conduction band and valence band, respectively.

Figure 1 . 3 :

 13 Figure 1.3: Models for selected TiO2 Q1D nanostructures.

Figure 1 . 4 :

 14 Figure 1.4: Model of TiO2 octahedron showing edge-sharing and corner-sharing. Notes: a red circles correspond to oxygen atoms, and b grey circles correspond to titanium atoms.

Figure 1 . 5 :

 15 Figure 1.5: Anatase structure and SEM image: (a) Anatase unit cell and (b) SEM image (up) and the model (below) of truncated octahedron-shaped anatase crystal with exposed {001} and {101} facets [9]. Note: red circles correspond to oxygen atoms.

Figure 1 . 6 :

 16 Figure 1.6: Rutile structure: (a) Rutile unit cell and (b) Rutile JCrystal crystal structure model[11]. Notes: a red circles correspond to oxygen atoms, and b JCrystal is a Java application for creating, editing, displaying and deploying crystal shapes[11].

Figure 1 . 7 :

 17 Figure 1.7: Brookite structure: (a) Brookite unit cell and (b) Brookite JCrystal structure model [11].

Figure 1 . 12 :

 112 Figure 1.12: Quasi-one-dimension TiO2 nanostructure TEM images (a) TiO2 nanotubes synthesized using 10 M NaOH at 140 o C, and (b) Anatase TiO2 nanowire synthesized using 10 M NaOH at 190 o C [37].

Figure 1 . 14 :

 114 Figure 1.14: SEM images of TiO2 nanowires SEM images grown on a Si(100) substrate using the PVD method [39].

Figure 1 . 15 :

 115 Figure 1.15: TiO2 anatase nanorods SEM image [40]. Note: CVD conditions set at 560 o C and 667 Pa (5 Torr) [40].

  560 o C and 630 o C with a pressure at 667 Pa (5 Torr) leads to the production of anatase and rutile TiO2 nanorods, respectively. However, at different conditions of 535 o C and under 480 Pa (3.6 Torr), the results demonstrated the production of TiO2 nanowalls.1.4.6. Electrochemical methodsThe electrochemical methods include electrodeposition and electrochemical etching/anodization. Electrodeposition is commonly used to produce a TiO2 nanocoating on a substrate by the action of reducing titanium ions in an electrolyte at the cathode surface. TiO2 nanowires coating can be synthesized using a template such as an alumina membrane on Al substrate. Typically, titanium deposits into the pores of the template utilizing 0.2 M TiCl3 solution as the electrolyte[41]. Anatase TiO2 nanowires are produced after heating the Ti infused alumina membrane coating at 500 o C for 4 h in the open furnace (Figure1.16). Next, the alumina membranes are partly removed by a chemical etching process in a 3 M NaOH for 5 min.

Figure 1 . 17 :

 117 Figure 1.17: TiO2 nanotube membrane [1]. (a) flow through a membrane consisting of vertically oriented, both-side-open TiO2 nanotubes, (b) image of the membrane, (c) tope view of SEM images of a TiO2 membrane, and (d) bottom view of SEM image of a TiO2 membrane.

1. 5 . 4 .

 54 Quasi-one-dimension TiO2 nanostructure formation mechanisms for alkaline hydrothermal methodThe formation mechanisms for hollow structure (nanotube) and nonhollow structure (nanoribbons and nanowire) are different. The general accepted theory is that TiO2 nanotubes are formed through a 3-step process: 1. the liberation of individual TiO6 octahedron, 2. recrystallization of TiO6 octahedron into lamellar sheet (nanosheets) (Figure1.18) and 3. scrolling of the lamellar sheet into nanotubes (Figure1.19)[36, 49].The onion-like morphologies concurrently produced with the nanotubes. The concentrated NaOH aqueous solution causes the liberation of individual TiO6 octahedron into the solution through the cleavage of Ti-O-Ti bonds of the TiO2 precursor. The lamellar sheet then formed via fusing the TiO6 octahedrons through the formation of hydroxyl bridges between the Ti atoms along both the [001] and [010] axes[49]. The driving force for lamellar sheet scrolling into nanotube is attributed to the reduction of the high surface energy of the nanosheet caused by the saturated dangling bonds on the nanosheets[36].

Figure 1 . 18 :

 118 Figure 1.18: Lamellar sheet formation [49].

Figure 1 . 19 :

 119 Figure 1.19: Proposed mechanism of formation of TiO2 nanotubes [36].

Figure 1 . 20 : 1 . 6 .

 12016 Figure 1.20: Nanowires formation process. (a). TiO2 precursor, (b). lamellar sheets and, (c) non-hollow TiO2 nanowires.

Figure 1 .

 1 Figure 1.21: (a) TiO2 coated glass cylinders using sol-gel method and annealed at 500 o C, and (b) a plug flow system for water purification [55].

Figure 1 . 22 :

 122 Figure 1.22: Design concept for solar parabolic concentrators [56].

Figure 1 .

 1 Figure 1.23: (a) Illustration of an open-air APPJ setups for depositing TiO2 coating on a flat substrate, and (b) SEM image of the cross section of atmospheric pressure plasma jet (APPJ) TiO2 coating with porous structure [58].

Figure 1 .

 1 Figure 1.24: (a) The schematic of dielectric barrier discharges (DBD) reactor for synthesizing anatase TiO2, and (b) transmission electron microscopy (TEM) image of DBD TiO2 nanocrystal [59].

CHAPTER 1 32 doped

 32 TiO2 thin films on Si substrates (Figure1.25) by reactive RF magnetron sputtering using a dual reactive gas mixture of N2 and O2.

Figure 1 . 25 :

 125 Figure 1.25: Scan electron microscope (SEM) images of TiO2 coatings synthesized by reactive radio frequency magnetron sputtering at different pressures (a) 3 mTorr, and (b) 14 mTorr [62]. Note: 1 mTorr represents 0.13 Pa (units for pressure).

Figure 1 . 27 :

 127 Figure 1.27: Donor level and acceptor level in forbidden band by doping metal ion [63].

Figure 1 . 28 :

 128 Figure 1.28: Valence band shifting by doping nonmetal ions [63].

Figure 1 . 29 :

 129 Figure 1.29: Metal modified semiconductor [20].

Figure 1 . 30 .

 130 Figure 1.30. The photogenerated electrons in CdS are transferred to TiO2 conductive

Figure 1 . 30 :

 130 Figure 1.30: Illustrative diagrams of the electron transfer in the CdS/TiO2 system [20].

Figure 1 . 31 :

 131 Figure 1.31: Bandgap edge position for different semiconductors in aqueous solution at pH = 1 [85].

Figure 1 . 32 :

 132 Figure 1.32: Graphene nanosheet composed of one atomic carbon layer.

Figure 1 . 33 :

 133 Figure 1.33: TEM image of graphene modified TiO2 nanotubes (left) and its schematic figure for mechanism of charge transfer and separation (right) [47].

Chapter 5 ,

 5 the first part of the fifth objective is to optimize the RGO atomic O/C ratio for synthesizing RGO-TiO2 nanorods (TNRs) composites photocatalyst (designated as GT) with the maximum photodegradation of aqueous hazardous pollutants under UV light condition. Next, based on the optimum RGO atomic O/C ratio obtained, second part of fifth objective is to synthesize a hierarchical three-dimensional (3D) Ag nanoparticle-RGO-TiO2 nanorods (TNRs) (designated as Ag-GT) thin films photocatalyst with optimum Ag content for degrading aqueous hazardous pollutants under visible irradiation.

Figure 2 . 1 :

 21 Figure 2.1: Schematic diagram of photocatalytic reactor [5].

a

  UV grade reactor cell with a diameter of 35 mm and a height of 20 mm facing the irradiation light source (Figure 2.2). The solution (10 ml) was circulated at a flow rate of 0.5 mL•s -1 using a peristaltic pump (Masterflex, model# 7553-75, Gelsenkirchen, Germany) connected to the radiated photocatalytic cell. The RhB solution was stirred for 60 min. under dark conditions to ensure adsorption/desorption equilibrium of the chemicals onto the catalyst. A 125 W (Philips) white light source was used to irradiate the samples. During the RhB photocatalytic degradation experiments, samples were removed, and the absorbance was monitored using a spectrophotometer (Maya 2000+, Ocean Optics, FL, United States) set at 554 nm.

Figure 2 . 2 :

 22 Figure 2.2: Photodegradation of organic wastewater contaminants (OWCs) using solar simulator.

Figure 2 . 3 :

 23 Figure 2.3: Schematic of the atmospheric pressure plasma jet (APPJ) system coupled with an 'in-house' constructed aluminum spraying system.

  2). The three central point experiments, designated as #13, #14 and #15 under the same conditions, were performed to evaluate the magnitude of error in the experimental analysis. The experimental factors and bandgap (response variable) were modeled using Minitab (Minitab Inc., State College, PA) to fit a full quadratic equation (Eq. 3.1): (3.1)

FESEM

  and HRTEM images for selected Q1D TiO2 samples are shown in Figure 3.1. Q1D TiO2 samples (nanorods and nanowires) synthesized at different hydrothermal conditions which were randomly distributed showed significant differences in the crystal sizes. The FESEM image for sample #1 (120 o C, 5 M and 43 g•L -1 ) (Figure 3.1a) indicate a mixture of nanorods and nanowires with length of approximately 100 nm (Figure 3.1c). Lattice fringes with 0.35 nm spacing (Figure 3.1c) indicate the Q1D TiO2 sample #1 contains anatase [25]. Nanowire morphology with length up to 3 μm was detected in sample #8 (190 o C, 10 M and 100 g•L -1 ) (Figure 3.1b and d). The lattice fringes with 0.62 nm spacing indicate the TiO2 nanowire (sample #8) consisted of the TiO2-B phase [26]. The FESEM image for sample #9 (190 o C, 5 M and 43 g•L -1 ) (Figure 3.1e) indicate a nanowire with length of hundreds of nanometers. The FESEM image for sample #4 (190 o C, 15 M and 14 g•L -1 ) (Figure 3.1f) indicate a nanowire with length of micrometers. Sample #4 shows a wide distribution of diameter from 10 nm to hundreds of nanometers. Lower temperature and NaOH concentration were associated with smaller diameter and crystal size (Sample #1). However, higher temperature and NaOH concentration were linked to larger diameter (Sample # 4 and 6), length (sample #9) and crystal size. The FESEM images indicate the hydrothermal synthesis factors significantly impacted the Q1D TiO2 diameter and crystal size (further discussed in this chapter Section 3.3.6).

Figure 3 . 1 :

 31 Figure 3.1: Q1D TiO2 field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) images (a) FESEM image for sample #1 synthesized at 120 o C, 5 M NaOH and 43 g•L -1 , (b) FESEM image for sample #8 synthesized at 190 o C, 10 M and 100 g•L -1 , (c) HRTEM image sample #1 synthesized at 120 o C, 5 M NaOH and 43 g•L -1 , (d) HRTEM image for sample #8 synthesized at 190 o C, 10 M and 100 g•L -1 , (e) FESEM image for sample #9 synthesized at 150 o C, 5 M NaOH and 14 g•L -1 , and (f) FESEM image for sample #4 synthesized at 190 o C, 15 M NaOH and 14 g•L -1 . Note: d represents lattice interplanar spacing.

. 2 .Figure 3 . 2 :

 232 Photon energy (eV)

  2 and the main effect of 3 hydrothermal synthesis factors on the bandgap is shown in Figure 3.3. Decreasing temperature (120 o C) and NaOH concentration (5M) were linked to the lowest bandgap. A mid-level temperature set at 150 o C as well as the mid-level NaOH concentration of 10 M were closely linked to increasing the bandgap. Either increasing or decreasing temperatures (NaOH concentrations) resulted in decreasing the bandgap.Interaction plots at all level factor categories (Figure3.3b) indicate a similar pattern as that shown in the main effect plot.

Figure 3 . 3 :

 33 Figure 3.3: Three-level Box-Behnken design (BBD) plots for: (a) Main effects for bandgap, and (b) two-factor effects for bandgap. Note: T, NaOH and TiO2 represent temperature, NaOH concentration and TiO2 concentration, respectively.

  : a DF = degrees of freedom, b Seq. SS = sequential sum of square, c MS = mean square, and d X1 = Temperature; X2 = NaOH concentration; X3 = TiO2 concentration.

  ( o C), NaOH concentration (M) and TiO2 concentration (g•L -1 ), respectively.

  Figure 3.8) with the minimum bandgap (3.067±0.004eV) was observed for the Q1D TiO2

Figure 3 . 4 :

 34 Figure 3.4: Bandgap response surface and contour plots for various factors: (a) and (b) hydrothermal temperature and NaOH concentration with a TiO2 concentration at 43 g•L -1 , (c) and (d) hydrothermal temperature and TiO2 concentration with a NaOH concentration at 10 M, and (e) and (f) NaOH concentration and TiO2 concentration with a temperature at 150 o C. Note: A, R and B represent anatase, rutile and TiO2-B, respectively.

Figure 3 . 5 : 3 85 3 .

 3533 Figure 3.5: Evaluating the accuracy of the response surface model. (a) Scatter plot of the bandgap values versus the experimental order (45 experiments). (b) Anderson-Darling normality plot of residuals. Notes: a N = the number of experiments that was conducted in this study, b P-Value = level of confidence, and c AD = Anderson-Darling statistic.

Figure 3 . 6 :Figure 3 . 7 :

 3637 Figure 3.6: Optimality plot to locate optimum factor levels for (a) maximized response bandgap Eg, and (b) minimized bandgap Eg.

3 .

 3 298±0.004 eV. The biphasic anatase-rutile structure dominated the Q1D TiO2 sample #1 while the biphasic anatase-TiO2-B was dominant in sample #4, #6 and #8. This difference in phase change which is also shown in Figure 3.4 illustrates the impacts of hydrothermal synthesis factors affecting the phase structure and bandgap. Temperature and NaOH concentration were significant factors affecting the Q1D TiO2 phase structure. At a relative lower temperature (120 o C) and a low NaOH concentration (5 M), a biphasic anatase-rutile structure was observed in sample #1. Sample #1 was the only experimental design condition with the same biphasic anatase-rutile structure as TiO2NPs (P25)[31, 38, 39]. The low bandgap (3.067±0.004 eV) for the bi-phasic anataserutile structure is attributed to rutile which has a relative narrow bandgap of 3.0 eV[20, 40]. Increasing the temperature and NaOH concentration resulted in the production of a pure anatase structure with a relative high bandgap value (3.258±0.003-3.298±0.005 eV).When the temperature was set at 190 o C with the NaOH concentration at 10 M or 15 M, the TiO2-B phase (3.20 eV) was predominant in samples #4, #6 and #8 together with a small quantity of the anatase phase. The existence of TiO2-B lowered the bandgap range from 3.203±0.006 to 3.229±0.009 eV, since TiO2-B has a bandgap of 3.20 eV[16, 41].

Figure 3 . 8 :

 38 Photon energy (eV)

  Decreasing crystal size was closely linked with lower temperatures and lower NaOH concentrations. Increasing the hydrothermal temperature from 120 o C to 190 o C or NaOH concentration from 5 M to 15 M resulted in an increase in the mean Q1D TiO2 crystal size from 9.5±0.1 nm to 58.3±0.3 nm. The interaction plots (Figure 3.9b) indicate a similar pattern as shown in the main effect plot. This observation is consistent with work reported by Bavykin et al.[8]

  Expt. # represents one experimental condition, and b T, NaOH and TiO2 represents temperature, NaOH concentration and TiO2 concentration, respectively.

Figure 3 .

 3 Figure 3.9: (a) main effects, and (b) interaction plots for mean crystal size (nm). Note: T = temperature; NaOH = NaOH concentration and TiO2 = TiO2 concentration.

Figure 3 .

 3 Figure 3.10: (a) main effects and, and (b) interactionplots for mean BET SSA (m 2 g -1 ). Note: T = temperature; NaOH = NaOH concentration and TiO2 = TiO2 concentration.

  synthesized at 190 o C further demonstrated that decreasing the crystal size from 57.2±0.4 to 21.5±0.2 nm (increasing the BET SSA) was closely correlated with increasing the bandgap from 3.203±0.005 to 3.274±0.002 eV. The mean bandgap, E (ev), is (3.5)

Figure 3 . 11 :

 311 Figure 3.11: Interaction between temperature and NaOH concentration on (a). bandgap, (b). mean crystal size (L) and (c). BET SSA. Note: L, T and NaOH represent mean crystal size, temperature and NaOH concentration, respectively.

A

  statistical model was developed to evaluate the effects of the hydrothermal synthesis factors on the Q1D TiO2 bandgap. The AD statistic indicated an adequate fit of the statistical model to the experimental data. The model predicted optimized hydrothermal conditions for maximum and minimum bandgap. The temperature and NaOH concentration significantly affected the Q1D TiO2 bandgap, phase, crystal size and SSA, whereas the TiO2 concentraction effect was negilible. The phase structure was the main reason for changes in bandgap. When compared to pure anatase, decreasing bandgap was observed for the biphasic anatase-rutile and anatase-TiO2-B structures. In addition, lower temperature and NaOH concentration were associated with decreasing crystal size and increasing SSA. The correlation between decreasing the crystal size (increasing the SSA) with increasing bandgap was due to the size quantization effect.The statistical model developed in this work can be employed to produce Q1D TiO2 with controllable bandgap, phase, mean crystal size and BET SSA for photocatalytic applications.

  PHOTODEGRADING AQUEOUS ORGANIC POLLUTANTS AND

4. 2 . 4 104Table 4 . 1 :

 2441 Photocatalytic experiments 4.2.1. Preparation of the Q1D nanometric TiO2 photocatalyst Q1D TiO2 photocatalyst was prepared by homogeneously mixing a specified quantity of TiO2 nanoparticles (Aeroxide TiO2 P25, Evonik Corporation, Parsippany, New Jersey) with 70 mL of a NaOH solution. Table4.1 list the reaction temperature, NaOH concentration and TiO2 concentration for hydrothermal process. The mixture was poured into a 100 mL Teflon® capped container. The Teflon® container was placed into a stainless-steel bomb, capped and heated to a desired temperature for 48 h and subsequently, cooled to room temperature. The resulting product was centrifuged and rinsed with 0.1 M HCl solution. The white paste was repeatedly washed (5 times) with deionized water and calcinated at 400 o C for 2 h to produce Q1D TiO2[2].CHAPTER Levels of the selected hydrothermal synthesis factors. , NaOH and TiO2 represent temperature, NaOH and TiO2 concentration.

  1 µm polyvinylidene difluoride (PVDF) filter (Sigma-Aldrich, Oakville, ON) to separate the photocatalystCHAPTER 4 105 from the reaction mixture. The residual aqueous organic pollutants concentration (C) in the filtrate was determined by measuring the optical absorbance at 554 nm, 464 nm and 664 nm for RhB, MO and MB, respectively, using a UV-visible spectrophotometer. The detection limits for RhB, MO and MB were 1 µg•L -1 . The residual phenol concentration was determined using a high-performance liquid chromatograph (HPLC) (Dionex Ultimate 3000, Sunnyvale, CA). The HPLC configuration details were described in Chapter 2 (Section 2.3.7). The residual concentration of the pollutant was used to determine the reaction rate of decoloration or disappearance, termed hereafter as the apparent reaction rate. The apparent reaction rate was modeled using a pseudo-first order rate equation (Eq. 4.1)[20][21][22][23].

  photocatalytic activity.

Figure 4 . 1 :

 41 Figure 4.1: RhB photocatalysis concentration profiles and kinetic plots for selected samples. (a) RhB photocatalysis concentration profiles using 300 nm UV light, and (b) apparent reaction rate constant. Notes: a BBD conditions are shown in Table 4.2, b blank represents no photocatalyst, and c RhB represents rhodamine b.

Figure 4 . 2 :

 42 Figure 4.2: (a) main effects, and (b) interaction plots for the different experimental factors. Notes: a . T = temperature; NaOH = NaOH concentration and TiO2 = TiO2 concentration, and b . apparent RhB k reprents "apparent rhodamine b photocatalysis rate constant value".

4. 4

 4 describes the apparent reaction rate constant (k) for RhB photocatalysis or the apparent RhB k as a function of the hydrothermal process factors.CHAPTER 4 113 where k (min -1 ) is the apparent RhB rate constant, X1, X2 and X3 represents the hydrothermal synthesis temperature ( o C), NaOH concentration (M) and TiO2 concentration (g•L -1 ), respectively. The three-dimensional response surface and the corresponding contour plots of experimental factors and apparent RhB k is presented in Figure 4.3 with the indication of crystal phase results. The main crystal phase and bandgap results are shown in Table 4.4. The response surface plots indicate interaction between the temperature and NaOH concentration (Figure 4.3a and b) with larger k values was observed at lower temperatures and lower NaOH concentrations. In addition, the lower temperatures and lower NaOH concentrations which are associated with smaller crystal size (Figure 4.4) resulted in higher SSA and a subsequent increase in the reaction kinetics. The larger k value was associated with a biphasic anatase-rutile phase structure which showed lowest bandgap. With increasing temperature or NaOH concentration, the rutile phase composition decreased while increasing the anatase phase composition with an enhanced bandgap. The increasing temperature and NaOH concentration which was linked to the increasing crystal size (Figure 4.4) gave rise to decreasing SSA and k values. When the temperature was set at 190 o C with the NaOH concentration at 10 M or 15 M, the TiO2-B phase (large crystal size) was predominant with a low amount of the anatase phase.

  Higher k values were observed at lower temperatures and TiO2 concentrations of approximately 35 g•L -1 (Figure 4.3c and d). The photocatalytic activity of the Q1D TiO2 structure was slightly affected by the TiO2 precursor concentration. Enhancing the k value could be achieved at a TiO2 concentration of 35 g•L -1 . Interactions between the NaOH and TiO2 concentrations (Figure 4.3e and f) confirmed that a low NaOH concentration and an appropriate TiO2 concentration of approximately 35 g•L -1 TiO2 were important factors causing an increase in the k value.

Figure 4 . 3 :

 43 Figure 4.3: Response surface and contour plots for factors affecting the apparent RhB rate constant (min -1 ): (a) and (b) for hydrothermal temperature and NaOH concentration set at 43 g•L -1 TiO2, (c) and (d) forhydrothermal temperature and TiO2 concentration set at 5 M NaOH concentration, and (e) and (f) for NaOH concentration and TiO2 concentration set at 120 o C temperature. Notes: a The crystal phase for different synthesis condition were discussed in Chapter 3 in detail, b apparent RhB k reprents "apparent rhodamine b photocatalysis rate constant value", and c A, B and R represent anatase, TiO2-B and rutile phases, respectively.

Figure 4 . 4 :

 44 Figure 4.4: (a) Main effect of hydrothermal factors on the crystal size of Q1D TiO2, and (b) interaction plots for the effect of different factors on crystal size of Q1D TiO2. Note: The results are from Chapter 3.

Figure 4 . 5 :

 45 Figure 4.5: Optimality plot to locate optimum factor levels for maximized response k (min -1 ).

  greater than F-critical value (2.01 at 5% level of significance) for the model indicates that the quadratic model is statistically significant since most of the terms are significant. The F-value of 24.46 for model was greater than the F-critical value of 2.01 (at 5% level of significance)[41][42][43][44]. However, temperature ( o C) x temperature ( o C) and temperature ( o C) x NaOH concentration (M) with p-value larger than 0.05 are statistically insignificantly, indicating these two variables had no individual effect on the response (Eq. 4.4). Statistically insignificant terms in the full quadratic model were deleted using a backward elimination method[42]. The final response surface model is designated as equation (Eq. 4.5):where k (min -1 ) is the apparent RhB photocatalytic rate constant, X1, X2 and X3 represents temperature ( o C), NaOH concentration (M) and TiO2 concentration (g•L -1 ), respectively. All terms are statistically significant in equation (Eq. 4.5).CHAPTER 4 119

4. 3 . 7 .Figure 4 . 6 : 2 Figure 4 . 7 :

 3746247 Figure 4.6: Evaluating accuracy of the response surface model. (a) Scatter plot of the apparent RhB k versus experiment order (45 experiments), and (b) Anderson-Darling normality plot of residuals. Notes: a N = the number of experiments conducted in this study, b P-Value = level of confidence, and c AD = Anderson-Darling statistic.

Figure 4 . 8 )Figure 4 . 8 :

 4848 photodegrade phenol, methyl orange and methylene blue (Figure4.8). The apparent k

A

  photocatalysis. The AD statistic indicated an adequate fit of the statistical model to the

  6). Experiments, designated as #13, #14 and #15 under the same conditions were conducted as central points to evaluate the magnitude of the error in the study. The experimental factors and H2 production rate (Y) (response variable) were modeled using Minitab 15 (Minitab Inc., State College, PA) to fit a full quadratic equation (Eq. (4.9)):

CHAPTER 4 129 4 . 4 . 3 .

 4443 Photocatalytic hydrogen production Significant differences in H2 production were observed for Q1D TiO2 samples prepared using different reaction conditions. Hydrogen production versus time profiles are shown in Figure 4.9a for BBD sample #3 plus the control experiment. Control experiments were conducted with P25 nanoparticles and without a photocatalyst. The mean H2 production rate (Y value) was 0 µmol•h -1 without a photocatalyst; however, with the addition of P25 nanoparticles, the mean production rate increased to 235±8 μmol•h -1 . In comparison, the production rate of 463±14 μmol•h -1 (ε = 19.3±0.6%) for sample #3 (synthesized at 120 o C, 15 M NaOH and 43 g•L -1 TiO2

Figure 4 . 9 :

 49 Figure 4.9: (a) Hydrogen production for sample #1 and TiO2 precursor using 300-nm UV light, and (b) Effect of ethanol concentration on hydrogen production rate for TiO2 nanoparticles (NPs) (P25).

Figure 4 .

 4 10a. The main effects plots indicate decreasing H2 yield is correlated with relative high temperature (190 o C) and 5 M NaOH concentration. Middle level of TiO2 concentration is correlated with increasing H2 yield value. The interaction plots (Figure 4.10b) provide detailed results. A larger H2 yield value was obtained at lower temperatures as well as higher NaOH concentration and with the TiO2 level set at 43 g•L -1 .

Figure 4 . 10 :

 410 Figure 4.10: Matrix of plots of three factors for hydrogen (H2) yield in a three-factor, three-level Box-Behnken design (BBD): (a) main effects for H2 yield, and (b) two-factor effects for H2 yield. Note: T, NaOH and TiO2 represent temperature, NaOH concentration and TiO2 concentration, respectively.

4 1323D

 4 6). A multiple regression analysis was used to develop a full quadratic Eq. (4.10) which describes the H2 production rate, Y, as a function of the hydrothermal process conditions. where Y is the H2 production rate (µmol•h -1 ), X1, X2 and X3 represent the reaction temperature ( o C), NaOH concentration (M) and TiO2 concentration (g•L -1 ), respectively. CHAPTER Surface plots and the corresponding contour plots (Figure 4.11) shows the relation among the synthesis factors, phase structure and the H2 production rate. Interaction between the temperature and the NaOH concentration (Figure 4.11a and b) suggests that both parameters significantly affected the Q1D TiO2 phase structure and the H2 production rate. Relative lower temperature at 120 o C and a high NaOH concentration at 15 M were closely associate with producing a pure anatase structure with a high H2 production rate. Increasing temperatures or decreasing NaOH concentrations were correlated with decreasing H2 production rates. The biphasic anatase-rutile structure, synthesized at 120 o C and 5 M NaOH, was linked to a lower H2 production rate. The biphasic anatase-TiO2-B structure synthesized at 190 o C and ≥10 M NaOH showed a significant decrease in the H2 production rate. These observations indicate the rutile and TiO2-B phases are closely correlated with decreasing photocatalytic H2 evolution activities. Interaction between temperature and TiO2 concentration (Figure 4.11c and d) and interaction between NaOH concentration and TiO2 concentration (Figure 4.11e and f) confirmed the decrease of the synthesis temperature and increasing NaOH concentration were closely associated with increasing H2 production rate. In addition, the surface and contour plots indicate the optimum H2 production rate was obtained with a TiO2 concentration of approximately 50 g•L -1 .

Figure 4 .

 4 Figure 4.11: 3D Reponse surface and contour plots for: (a) and (b) Interaction between hydrothermal temperature and NaOH concentration set at 43 g•L -1 TiO2, (c) and (d) Interaction between hydrothermal temperature and TiO2 concentration set at 15 M NaOH concentration, and (e) and (f) Interaction between NaOH concentration and TiO2 concentration set at 120 o C temperature. Notes: a A, R and B represent anatase, rutile and TiO2-B, respectively, and b H2 yield represents H2 production rate.

7 )

 7 was employed to evaluate the model full quadratic equation (Eq. 4.10) was employed to determine the significance and adequacy of the model. The quadratic model with an overall p-value ≤ 0.05[40][41][42][43] and F-value ≤ critical value (2.01 at 5% level of significance)[40][41][42][43] indicate the model was statistically significant. However, two variables (X2 2 and X1X3) with p-value ≥ 0.05 are statistically insignificant with no individual effect on the full quadratic model. These two CHAPTER 4 136 terms were deleted from the full quadratic model using a backward elimination method[41]. The revised response surface model is designated as equation (Eq. 4.11):where Y is the H2 production rate (µmol•h -1 ), X1, X2 and X3 represent temperature ( o C), NaOH concentration (M) and TiO2 concentration (g•L -1 ), respectively.

Figure 4 . 13 :Figure 4 . 14 : 4 139 overestimate at 5 M 1 (Figure 4 .14c). 4 . 4 . 8 .

 4134144514448 Figure 4.13: Accuracy evaluation of the response surface model. (a) Scatter plot of the hydrogen (H2) produciton rate (μmol•h -1 ) versus experiment order (45 experiments). (b) Anderson-Darling normality plot of residuals. Notes: a N = the number of experiments conducted in this study, b P-Value = level of confidence, and c AD = Anderson-Darling statistic.

Figure 4 .Figure 4 . 16 :

 4416 Photon energy (eV)

CHAPTER 4 143 4 . 4 . 9 .

 4449 Conclusions: Photocatalytic hydrogen productionMany Q1D TiO2 shapes ranging from nanorods to nanowires were produced using a hydrothermal method with variable reaction conditions. The reaction temperature and NaOH concentration significantly affected the Q1D TiO2 phase structure, crystal size, SSA, bandgap and photocatalytic activity. A statistical model was employed to evaluate the effects of the hydrothermal synthesis factors on the photocatalytic activity of Q1D TiO2 in terms of the H2 production rate. The AD and validation test indicated an adequate fit of the statistical model to the experimental data under the conditions examined. The model revealed that a relatively low temperature (126 o C) and high NaOH concentration (15 M) were closely linked to producing Q1D TiO2 containing pure anatase.

Q1D

  TiO2 photocatalytic activities, in terms of photodegrading organic pollutants and H2 production, were significantly affected by hydrothermal reaction temperature and NaOH concentration. A biphasic anatase-rutile Q1D TiO2 structure produced at 120 o C temperature and a 5 M NaOH solution was associated with the lowest bandgap and the highest photocatalytic activities for photodegrading aqueous organic pollutant. However, a pure anatase Q1D TiO2 structure synthesized at 120 o C and 15 M NaOH was correlated CHAPTER 4 144 with the highest bandgap and photocatalytic H2 production rate from ethanol. The TiO2-B which was predominant at hydrothermal conditions of 190 o C and ≥10 M NaOH showed with lowest photocatalytic activities in terms of photodegrading organic pollutants and producing H2.1. CHAPTER 5 155

2. 5 g

 5 TiO2 P25 nanoparticles was homogeneously mixed with 70 ml of 5 M NaOH by stirring for 30 min. Next, the mixture was placed in a 100-mL Teflon® bottle and capped with a Teflon® cover. The capped Teflon® bottle was placed into a stainless-steel container then capped. The container was heated at 120 o C for 48 h and subsequently, cooled to room temperature. The precipitate was separated by centrifugation and subsequently rinsed with 0.1 M HCl. Next, the precipitate was separated again by centrifugation and subsequently washed with deionized water. The centrifuging and washing processes were repeated several times until pH = 7. Next, the precipitate w calcined at 400 o C for 2 h to produce the TNRs. The procedure for synthesizing the GT composites with varied atomic O/C ratio is outlined in Figure 1. Briefly, GO (3wt% (based on per mass of GT composite)) (15 mg) and TNRs (485 mg) were mixed and ultra-sonicated in 100 mL Milli-Q water for 1 h to 1. CHAPTER 5 160 ensure re-exfoliating of the GO sheets and dispersing the TNRs between the GO sheets. The GO and TNRs mixture was designated as GOT. Next, 1 ml of hydrazine hydrate was added to the yellow gel. The mixture was maintained at a desired temperature (20 o C, 60 o C, 80 o C, and 95 o C) for 1 h to deoxidize (reduce) the GOT to RGO based TNRs (designated as GT). Further deoxidization was conducted by increasing the reaction time to 3 h at 95 o C. The resulting product (GT) was isolated by filtration (PTFE 20 um pore size membrane), washed with DI water and 3 times with methanol then dried at 60 o C for 12h. The GT produced at 20 o C, 60 o C and 80 o C for 1 h were designated as GT-20, GT-60, GT-80, respectively. The GT reduced samples synthesized at 95 o C for 1 h, 2 h and 3 h were designated as GT-95 1h , GT-95 2h and GT-95 3h , respectively.

Figure 5 . 1 :

 51 Figure 5.1: Graphene oxide based TiO2 nanorods (designated as GOT photocatalyst) and reduced graphene oxide based TiO2 nanorods (GT) synthesis process [51].

Figure 5 . 2 :

 52 Figure 5.2: Schematic process showing the synthesis of the Ag-GT photocatalyst film. The GO sheets (a) were exfoliated and (b) homogeneously mixed with TNRs using ultrasonication, (c) next, GO-TNRs mixture were reduced to GT and finally, (d) Ag-GT were formed by depositing Ag NPs on GT, and (e) Ag-GT thin film preparation.

CHAPTER 5 165in Chapter 2 (

 52 3.10). The reaction mixture consisted of a specific concentration of organic chemical and 100 mg•L -1 of photocatalysts. The organic chemicals examined in the study included RhB, phenol, MO and MB. The initial concentration (C0) of MO, RhB, phenol and MB was 20 mg•L -1 , 10 mg•L -1 , 20 mg•L -1 , 10 mg•L -1 , respectively. The 50-mL reaction mixture containing a specific organic chemical plus photocatalyst was ultrasonicated for 15 minutes and subsequently, magnetically stirred under dark conditions for 1 h to attain adsorption-desorption equilibrium. The reaction mixture was exposed to UV light for 10 minutes. A 5-mL reaction mixture was collected at specific time intervals of 2 minutes. The liquid sample was centrifuged (5, 000 rpm for 60 minutes) and filtered using a 0.1 µm polyvinylidene difluoride (PVDF) filter (Sigma-Aldrich, Oakville, ON, Canada). The residual RhB, MO and MB concentration (C) in the filtrate was determined by measuring the optical absorbance at 554 nm, 464 nm and 664 nm, respectively, using a UV-Vis spectrophotometer (Agilent Technologies, United States). The detection limit for RhB, MO and MB were 1 µg•L -1 .The residual phenol concentration was determined using a high performance liquid chromatograph (HPLC) (Dionex Ultimate 3000, Sunnyvale, CA, United States) described 1. Section 2.3.8). The residual concentration of the chemicals was used to determine the reaction rate of discoloration or disappearance, termed hereafter as apparent reaction rate. The apparent reaction rate was modeled using a pseudo-first order rate equation (Eq. 5.1)[58][59][60].

(1. CHAPTER 5 166 5 . 3 .Figure 5 . 3 :Figure 5 . 4 :

 5535354 photodegradation of phenol, MO and MB was monitored using the same procedure as

Figure 5 . 5 :

 55 Figure 5.5: Field emission scanning electron microscopy (FESEM) image and energy dispersive X-ray (EDX) patterns for GT-95 1h samples. (a) GT-95 1h FESEM image, (b) C element EDX mapping, (c) O element EDX mapping, and (d) Ti element EDX mapping. Note: GT-95 1h represents enhanced TiO2 nanorods (TNRs) with reduced graphene oxide (RGO) photocatalyst synthesized at 95 o C for 1 h.

Figure 5 . 6 :

 56 Figure 5.6: X-ray diffraction (XRD) patterns for TiO2 nanorods (TNRs) and its composites. Notes: a A and R represent anatase and rutile, respectively, b GT represents reduced graphene oxide based TiO2 nanorods, c GO and RGO represent graphene oxide and reduced graphene oxide, respectively, and d the number after GT represents the temperature ( o C) condition and time (h) to synthesize the photocatalysts.

Figure 5 . 7 :Figure 5 . 8 :Table 5 . 1 :

 575851 Figure 5.7: Raman spectra and X-ray photoelectron spectroscopy (XPS) for samples. (a) Raman spectra, (b) Integrated intensity ratio ID/IG, (c) C1s X-ray photoelectron spectroscopy (XPS) spectra, and (d) Integrated atomic oxygen-to-carbon (O/C) ratio of reduced graphene oxide (RGO). Notes: a GT represents reduced graphene oxide based TiO2 nanorods, b GOT represents graphene oxide based TiO2 nanorods, and c the number after GT represents the temperature ( o C) condition and time (h) to synthesize the photocatalysts.

Figure 5 . 9 :

 59 Figure 5.9: Raman spectra for the Eg peak blue shift. Notes: a TNRs represents TiO2 nanorods, b GT represents reduced graphene oxide based TiO2 nanorods, c GOT represents graphene oxide based TiO2 nanorods, and d the number after GT represents the temperature ( o C) condition and time (h) to synthesize the photocatalysts.

Figure 5 . 10 :

 510 Figure 5.10: Ti2p X-ray photoelectron spectroscopy (XPS) spectra for TiO2 nanorods (TNRs) and reduced graphene oxide based TiO2 nanorods synthesized at 95 o C for 1 h reaction (GT-95 1h ).

Figure 5 . 11 :

 511 Photo energy (eV)

Figure 5 . 12 :

 512 Figure 5.12: Transient photocurrent responses for samples in 1 M Na2SO4 aqueous solution under ultraviolet-light irradiation (300 nm) at 0.5 V versus saturated calomel electrode (SCE). Notes: a TNRs represents TiO2 nanorods, b GT represents reduced graphene oxide based TiO2 nanorods, c GOT represents graphene oxide based TiO2 nanorods, and d the number after GT represents the temperature ( o C) condition and time (h) to synthesize the photocatalysts.

  pollutant under UV light (Figure 5.13). Two control experiments were conducted without any photocatalyst and with RGO. Approximate 5% and 2% MO was absorbed onto the GOT and TNRs samples, respectively, after 1-h stirring under dark conditions stirring at 20 o C. This indicates 3% RGO negligibly increased the MO adsorption onto the sample. The apparent first-order MO photocatalysis rate constants (apparent MO k)

Figure 5 . 13 :

 513 Figure 5.13: Methyl orange photocatalysis using selected photocatalysts under UV light. (a) Residual methyl orange concentration versus time for selected samples. (b) Apparent first-order methyl orange photocatalysis rate constants (apparent MO k) (min -1 ) for a control and selected photocatalysts. Notes: a Control contains no photocatalyst, b TNRs represents TiO2 nanorods, c GT represents reduced graphene oxide based TiO2 nanorods, d GOT represents graphene oxide based TiO2 nanorods, e RGO represents reduced graphene oxide, and f the number after GT represents the temperature ( o C) condition and time (h) to synthesize the photocatalysts.

Figure 5 . 14 :

 514 Figure 5.14: Apparent first-order photocatalysis rate constant (k) for the photocatalysis under ultraviolet (UV) light irradiation for (a) RhB, (b) MB, and (c) phenol. Notes: a Blank contains no photocatalyst, b TNRs represents TiO2 nanorods, c GT represents reduced graphene oxide based TiO2 nanorods, d GOT represents graphene oxide based TiO2 nanorods, and e the number after GT represents the temperature ( o C) condition and time (h) to synthesize the photocatalysts.

Figure 5 . 15 :

 515 Figure 5.15: Charge transportation and separation on the partially reduced graphene oxide based TiO2 nanorods (GT) photocatalyst.

  Separated RGO was detected floating on the surface of the reaction mixture or settling to the bottom of glass vial (Sample to the right side of Figure5.16). RGO separation resulted in a sharp decrease in the charge transfer, charge separation and optical response.

Figure 5 . 16 :

 516 Figure 5.16: Color changes observed with a decrease in the atomic oxygen-to-carbon (O/C) ratio. Notes: a GT represents reduced graphene oxide based TiO2 nanorod, and b GOT represents graphene oxide based TiO2 nanorods.

Figure 5 . 17 :

 517 Figure 5.17: Field emission scan electron micrograph (FESEM) and high-resolution transmission electron microscopy (HRTEM) images for GO and 10%Ag-GT. (a) GO FESEM image, (b) 10%Ag-GT FESEM image, (c) 10%Ag-GT HRTEM image, and (d) 10%Ag-GT HRTEM image.

Figure 5 .

 5 Figure 5.18: X-ray diffraction (XRD) and Raman Spectra. (a) XRD patterns for all samples, (b) the Raman spectra for different samples, (c) Eg bands, and (d) C1s X-ray photoelectron spectroscopy (XPS) spectra of 10%Ag-GT and TiO2 nanords (TNRs) (top) and graphene oxide (GO) (bottom).

Figure 5 . 19 :Figure 5 . 20 :

 519520 Photo energy (eV)

1. CHAPTER 5 192 5 . 4 . 5 .

 5545 Photocatalytic activityThe visible light photocatalytic degradation capacity for the different samples was firstly evaluated using RhB as a contaminant target (Figure5.21). Control experiments were performed with TiO2 NPs and without a photocatalyst (referred as photolysis). The initial concentration of RhB in deionized water was 5 mg•L -1 before each experiment.

2 Figure 5 . 21 :Figure 5 . 2 Figure 5 . 22 :

 252152522 Figure 5.21: Photocatalytic degradation of RhB using photocatalysts under visible light. (a) Residual RhB concentration versus time, and (b) Apparent RhB degradation rate constant (Apparent RhB k) (h -1 ).

Figure 5 .

 5 Figure 5.23: A proposed mechanism for the photocatalytic degradation of organic dyes employing the hierarchical three-dimension (3D) Ag-RGO-TiO2 nanorods (Ag-GT) photocatalyst under visible light irradiation.

Figure 6 . 1 :

 61 Figure 6.1: (a) optical emission spectra with TTIP spray injection under two different pulse frequencies and air flow rates, (b) optical emission spectra with TTIP spray using an air flow of 1800 L h -1 and a pulse frequency of 25 kHz without Ag NPs, and (c) optical emission spectra without TTIP spray injection using an air flow of 1800 L h -1 and a pulse frequency of 25 kHz. Notes: a The plasma cycle time is 100%, and b the optical fiber located downstream at 10 mm from the nozzle exit of the torch.

Figure 6 .

 6 Figure 6.2d. At a high frequency of 25 kHz, Ag was present as isolated and small NPs in

Figure 6 . 2 :

 62 Figure 6.2: Field emission scanning electron microscopy (FESEM) images for Ag-TiO2 thin films deposited by atmospheric pressure plasma jet (APPJ) under three conditions (a) 18 kHz and1800 Lh -1 , (b) 25 kHz and 1800 Lh -1 , (c) 18 kHz and 3000 Lh -1 , and (d) The proposed Ag-TiO2 core-shell structure.

3 .

 3 Effect of Ag incorporating on the crystal structureThe XRD patterns for the Ag-TiO2 thin films deposited by APPJ with different Ag concentrations at 21 kHz and an air flow rate of 1800 L•h -1 are depicted in Figure6.3a. The coatings post annealing conditions at 450 o C for 1 h in ambient air produced primarily the anatase phase regardless of the Ag concentration. The peaks located at 2θ = 25.4°, 37.9°, 38.7, 48.1°, 54.1°, 55.2° and 66.8° correspond to the (101), (004), (112), (200), (105), (211) and (204) orientation of the anatase TiO2 phase, respectively, with (101) as the preferential orientation. The peaks located at 2θ = 38.2°, 44.0° and 64.6° correspond to the (111), (200) and (220) surfaces of Ag, respectively.

Figure 6 . 3 :

 63 Figure 6.3: XRD patterns for Ag-TiO2 thin films deposited by APPJ using Ag-TTIP liquid solution sprayed and annealed at 450°C for 1 h in ambient air, (a) with different Ag concentrations at 21 kHz and air flow rate of 1800 L•h -1 , and (b) with 0.4% Ag at different pulse frequencies/ different air flow rates.

Figure 6 . 4 :

 64 Figure 6.4: Anatase crystal size as a function of (a) Ag concentration, (b) and pulse frequency.

Figure 6 . 5 :Figure 6 . 6 :

 6566 Figure 6.5: RhB photocatalytic degradation (initial concentration 5.0 mg•L -1 ) with the Ag-TiO2 coatings deposited by atmospheric pressure plasma jet (APPJ) for different Ag concentrations.

Figure 6 . 7 :

 67 Photolysis TiO2 0.4Ag-TiO2

  photodegrading aqueous hazardous pollutants[2] and producing H2[3]. The factors

  photoelectrochemical current and photodegrading rhodamine b (RhB), methyl orange

Figure 7 . 1 :

 71 Figure 7.1: Photocatalytic degradation rate constant (k) for phenol, methyl orange (MO), methylene blue (MB) and rhodamine B (RhB) under visible light conditions. Note: Optimized Q1D TiO2 and GT-95 1h data are from Chapter 4 and Chapter 5, respectively.

  photocatalyseur contenant 10 % de l'argent. Le taux de dégradation photocatalytique a été

  photocatalysts were characterized with enhanced visible-light activities, charge

10 Table 1 .

 101 1: Properties of anatase, rutile, brookite and TiO2-B[11, 28, 29].

	Property	Anatase	Rutile	Brookite	TiO2-B
	Crystal structure	Tetragonal	Tetragonal	Orthorhombic	Monoclinic
	Space group	l md	P	Pbca	C
	Lattice
	parameters (Å)

  3 mL samples were removed periodically for further analysis.

	CHAPTER 2
	Uniform and continuous irradiation for the photocatalytic reaction was achieved using a
	150 W ozone-free Xe arc lamp solar simulator (Sciencetech Inc, SS150W, Canada) with
	a sharp cut off at 256 nm and an output spectrum equivalent to natural sunlight at 48.2°
	latitudes at sea level. The incident irradiance, as measured by a spectroradiometer
	(International Light, Model ILT 900R, MA, USA), was approximately 500 W•m -2
	integrated in the wavelength range of 280-950 nm. Pharmaceutical compounds were
	monitored and quantified by high performance liquid chromatography (HPLC) (1100
	series HPLC, Agilent, Les Ulis, France) configured with a ACE-RP phenyl column (2.1
	mm×250 mm) (Aberdeen, Scotland) coupled to a mass spectrometer (Q-Tof MS, Waters
	61

Table 3 .

 3 2: Design matrix for experimental factors and responses of bandgap Eg at different factor levels. Expt. # represents one experimental condition, b T, NaOH and TiO2 represents temperature, NaOH concentration and TiO2 concentration, respectively, and c Eg represents bandgap. Eg1, Eg2 and Eg3 are replicates results for Eg.

			Factors b				Response
	Expt.					Bandgap Eg (eV) c
	# a	T ( o C) NaOH (M) TiO2 (g•L -1 )	Eg1	Eg2	Eg3	Mean Eg
	1	120	5	43	3.071 3.063 3.068 3.067±0.004
	2	190	5	43	3.271 3.274 3.277 3.274±0.003
	3	120	15	43	3.254 3.258 3.261 3.258±0.003
	4	190	15	43	3.214 3.202 3.215 3.210±0.007
	5	120	10	14	3.289 3.293 3.298 3.293±0.004
	6	190	10	14	3.208 3.196 3.206 3.203±0.006
	7	120	10	100	3.270 3.262 3.264 3.262±0.004
	8	190	10	100	3.239 3.219 3.228 3.229±0.009
	9	150	5	14	3.303 3.299 3.293 3.298±0.005
	10	150	15	14	3.268 3.268 3.272 3.271±0.002
	11	150	5	100	3.270 3.259 3.277 3.269±0.008
	12	150	15	100	3.268 3.270 3.276 3.271±0.004
	13	150	10	43	3.274 3.278 3.284 3.279±0.005
	14	150	10	43	3.283 3.287 3.284 3.285±0.002
	15	150	10	43	3.293 3.287 3.291 3.290±0.003
	Notes: a

Table 3 .

 3 3: ANOVA results of the experimental response at each factor level.

	Source	DF a	Seq. SS b	MS c	F-value	p-value
	Model	9	0.09347	0.01039	6.97	0
	X1	1	0.00035	0.00035	0.23	0
	X2	1	0.00206	0.00206	1.39	0.001
	X3	1	0.00029	0.00029	0.2	0.620
	X1 2	1	0.03601	0.03601	24.18	0
	X2 2	1	0.00742	0.00742	4.98	0.146
	X3 2	3	0.00365	0.00365	2.45	0.255
	X1X2	1	0.04115	0.04115	27.63	0
	X1X3	1	0.00015	0.00015	0.1	0.046
	X2X3	1	0.00001	0.00001	0	0.007
	Error	35	0.05216	0.00008
	Total	44	0.14559
	Notes

Table 3 .

 3 4. Phase structures and corresponding bandgap energies for all BBD samples

	Sample #	Phase	Bandgap (eV)
	2, 3, 5, 7 and 9-15	Anatase	3.258±0.003 -3.298±0.004
	1	Anatase and rutile	3.067±0.004
	4, 6, 8	Anatase and TiO2-B	3.203±0.006 -3.229±0.009
	3.3.6. Crystal size and specific surface area (SSA)

Table 3 .

 3 5: Design matrix for experimental factors and responses of mean crystal size and BET SSA at different factor levels.

	Expt. # a	T ( o C)	Factors b NaOH (M)	TiO2 (g•L -1 )	Mean crystal size L (nm)	BET SSA (m 2 g -1 )
	1		5	43	9.5±0.1	181±5
	2		5	43	21.5±0.2	123±5
	3		15	43	19.9±0.1	137±3
	4		15	43	57.2±0.4	36±3
	5		10	14	18.2±0.1	145±5
	6		10	14	46.4±0.3	72±3
	7		10	100	17.6±0.2	149±5
	8		10	100	48.2±0.3	39±3
	9		5	14	16.9±0.2	172±5
	10		15	14	58.3±0.3	60±3
	11		5	100	21.2±0.2	139±4
	12		15	100	18.8±0.2	137±5
	13		10	43	19.5±0.2	150±4
	14		10	43	19.8±0.2	151±5
	15		10	43	20.5±0.2	148±5
	Notes: a

Table 4 .

 4 2: Design matrix for the hydrothermal synthesis factors and responses (apparent reaction rate constant) at different factor levels.

	Sample #	T ( o C)	Factors a NaOH (M) (g•L -1 ) TiO2	Response Apparent reaction rate constant (k) (min -1 )	Phase b
	1	120	5	43	0.4227±0.0107	A+R
	2	190	5	43	0.2962±0.0102	A
	3	120	15	43	0.1703±0.0053	A
	4	190	15	43	0.0936±0.0007	A+B
	5	120	10	14	0.2596±0.0039	A
	6	190	10	14	0.1171±0.0041	A+B
	7	120	10	100	0.1832±0.0037	A
	8	190	10	100	0.1268±0.0017	A+B
	9	150	5	14	0.2569±0.0039	A
	10	150	15	14	0.1703±0.0093	A
	11	150	5	100	0.1993±0.0013	A
	12	150	15	100	0.1525±0.0081	A
	13	150	10	43	0.1906±0.0062	A
	14	150	10	43	0.2062±0.0076	A
	15	150	10	43	0.2187±0.0099	A
	Notes: a T, NaOH and TiO2 represent temperature, NaOH and TiO2 concentration, and b

Table 4 .

 4 3: apparent RhB photocatalysis rate constant value (k) for selected samples.

	Experiment	k (min -1 )	Linear regression R 2 value
	Control (no photocatalyst)	0.0030 ± 0.0002	0.9991
	P25	0.3483 ± 0.0051	0.9823
	#1	0.4227 ± 0.0107	0.9729
	#4	0.0936 ± 0.0007	0.9674
	#9	0.2569 ± 0.0039	0.9740

Table 4 .

 4 4: Phase structures and corresponding bandgap energies for all BBD samples

	Sample #	Phase	Bandgap (eV)
	1	Anatase-rutile	3.067±0.004
	2, 3, 5, 7, 9, 10, 11, 12, 13, 14, 15	Anatase	3.258±0.003 -3.298±0.004
	4, 6, 8	TiO2-B-anatase	3.203±0.006 -3.229±0.009

Table 4 .

 4 5: ANOVA results of the experimental response at each factor level. DF = degrees of freedom, b Seq. SS = sequential sum of square, and c MS = mean square.

	Source	DF a	Seq. SS b	MS c	F-value	P-value
	Model	9	0.24459	0.02718	24.46	0
	X1	1	0.04991	0.04991	44.91	0
	X2	1	0.10955	0.10955	98.58	0
	X3	1	0.00632	0.00632	5.69	0.023
	X1 2	1	0.00281	0.00281	2.53	0.121
	X2 2	1	0.01163	0.01163	10.46	0.003
	X3 2	3	0.01389	0.01389	12.50	0.001
	X1X2	1	0.00062	0.00062	0.56	0.46
	X1X3	1	0.00535	0.00535	4.82	0.035
	X2X3	1	0.00617	0.00617	5.55	0.024
	Error	35	0.03889	0.00111
	Total	44	0.28348
	Notes:

a

Table 4 .

 4 6: Design matrix for experimental factors and response (hydrogen production rate, mean crystal size and BET SSA) at different factor levels.

	.6).

Table 4 . 7

 47

	Source	DF a	Seq. SS b	MS c	F-value	P-value
	Model	9	593896	65988	96.78	0
	X1	1	116309	116309	170.58	0
	X2	1	5352	5352	7.58	0.008
	X3	1	8961	8961	13.14	0.001
	X1 2	1	144590	144590	212.05	0
	X2 2	1	501	501	0.73	0.397
	X3 2	3	97085	97085	142.38	0
	X1X2	1	181600	181600	266.33	0
	X1X3	1	973	973	1.43	0.240
	X2X3	1	33226	33226	48.73	0
	Error	35	23865	682
	Total	44	617760
	Notes: a

: ANOVA results of the experimental response at each factor level.

Table 4

 4

	CHAPTER 4

.8)

. The pure anatase (e.g. sample #3)

Table 4 .

 4 8. Phase structures, bandgap energies and hydrogen yield for all BBD samples.

	Sample #	Phase	Bandgap (eV)	Mean L (nm)	BET SSA (m 2 •g -1 )	Y (µmol•h -1 )
	1	Anatase and rutile	3.067±0.006	9.5±0.1	182±5	193±7
	2, 3, 5, 7, 9 and 11-15	Anatase	3.258±0.003 -3.298±0.005	16.9±0.2 -21.5±0.2	125±5 -174±5	204±8-463±14
	10	Anatase	3.271±0.004	57.2±0.4	61±3	284±10
	4, 6, 8	Anatase and TiO2-B	3.210±0.007-3.229±0.009	46.4±0.3 -58.3±0.3	37±3 -73±3	72±4 -103±5
	Note: L and Y represent crystal size and H2 production rate, respectively.
	The H2 production rates, crystal size and BET SSA are summarized in Table 4.8 for
	BBD Q1D TiO2 samples synthesized under different reaction conditions. Depending on
	the hydrothermal synthesis conditions, the H2 production rates varied from 72±4 to
	475±12 μmol•h -1 . The phase structure significantly affected the H2 production rate. Pure
	anatase (sample #2, #3, #5, #7, #9, #11-15 and optimum) was closely linked with
	increasing the H2 production rate while the biphasic anatase-rutile (sample #1) and
	anatase-TiO2-B structures (sample #4, #6 and #8) were associated with decreasing H2
	production rate. The lowest H2 production rate was detected for the biphasic anatase-
	TiO2-B structure (sample #4, 6 and 8).

Table 5 .

 5 2: Apparent photocatalysis rate constants k for selected organic chemicals.

		Apparent photocatalysis rate constant k (min -1 ) for:
	Experiment	MO	RhB	MB	Phenol
	Blank	0.0003±0.0001 J	0.003±0.0008 I	0.0046±0.0004 I 0.0014±0.0001 I
	RGO	0.0053±0.0012 I	-	-	-
	TNRs	0.2361±0.0021 F 0.4470±0.0041 G 0.5891±0.0108 G 0.0431±0.0008 F
	GOT	0.0951±0.0019 H 0.3455±0.0049 H 0.4052±0.0095 H 0.0293±0.0004 H
	GT-20	0.2582±0.0012 CD 0.8560±0.0042 DE 1.1530±0.0142 D 0.0450±0.0005 E
	GT-60	0.2619±0.0011 CD 0.9343±0.0061 C 1.3018±0.0110 C 0.0469±0.0012 D
	GT-80	0.2711±0.0011 B 0.9830±0.0052 B 1.3749±0.0099 B 0.0488±0.0006 BC
	GT-95 1h	0.3118±0.0012 A 1.1345±0.0052 A 1.5758±0.0110 A 0.0548±0.0005 A
	GT-95 2h	0.2516±0.0018 E 0.8632±0.0048 DE 1.0531±0.0101 E 0.0481±0.0006 BC

Table 6 .

 6 1: Physicochemical properties of the examined compounds Source:www.chemicalize.com (last accessed May 1, 2018), and b RhB, CBZ, VLX, and BZF represent rhodamine b, carbamazepine, venlafaxine and bezafibrate, respectively.

	Compound	Class	Molecular formula	pKa 1	Charge (pH=7)	LogD a (pH=7)
	RhB b	Dye	C28H31ClN2O3	4.3	negative	2.34
	CBZ	Antiepileptic	C15H12N2O	13.9	neutral	2.77
	VLX	Antidepressant	C17H27NO2	8.9	positive	0.84
	BZF	Lipid regulator	C19H20ClNO4	3.8	negative	0.97
	Notes: a

, respectively, when compared to GT-95 1h . This observation indicates the excessive

Photolysis

Figure 6.8: Pharmaceutical compounds degradation percentage (C0, CBZ/VLX/BZF =100 μg•L -1 , pH=7) using photolysis (irradiation only), photocatalytic oxidation using TiO2 and 0.4% Ag-TiO2 (deposited at 21 kHz and air flow rate of 1800 L h -1 ) after 60 min irradiation under solar simulated irradiation

The degradation of CBZ, VLX and BZF each at an initial concentration of 100 μg•L -1 and pH=7 was examined using photolysis (irradiation only), TiO2 and 0.4% Ag-TiO2 under solar light irradiation for 60 min (Figure 6.8). The results show that all tested pharmaceutical compounds are not susceptible to photolysis; however, substantial degradation by TiO2 and 0.4% Ag-TiO2 photocatalysis was observed in the following order: VLX > CBZ > BZF. Significantly higher degradation rates of the tested chemicals were observed for the Ag-TiO2 composite catalyst when compared to the unmodified TiO2 samples. The highest removal rates observed for VLX by employing TiO2 and Ag-TiO2 photocatalysts were 69.4±4% and 85.7±1%, respectively. These large removal rates can be explained based on the electrostatic interactions between VLX and the photocatalyst surface. A negatively-charged TiO2 surface under the experimental conditions visible light absorption between 400 to 700 nm and decreasing bandgap to approximately 2.55 eV was due to the localized surface plasmon resonance (SPR) absorption of Ag NPs [10,11]. In addition, the impact of Ag content (5-15wt%) on the photocatalytic activities was investigated. A maximum photocurrent and photodegradation rate were observed for a photocatalyst film containing 10% silver. The photocatalytic degradation rate was significantly enhanced when compared to the optimized Q1D TiO2 photocatalyst in Chapter 4 and optimized GT-95 1h (Figure 7.1). This largest photodegradation rate observed with the optimum Ag-GT catalyst was approximately a 2.6-fold increase when compared to a Ag doped TiO2 film reported by Bensouici et al [12].

In

ABSTRACT

This dissertation is focused on synthesizing Q1D TiO2-based nanocatalysts for degrading aqueous organic pollutants and producing H2. A facile alkaline hydrothermal process was used to synthesize Q1D TiO2 under different hydrothermal synthesis factors (reaction temperature, NaOH concentration and TiO2 precursor concentration). Hydrothermal synthesis factors significantly affected the Q1D TiO2 phase structure, crystal size, specific surface area (SSA), bandgap, photocatalytic activities. A Box-Behnken design (BBD) model was used to optimize the hydrothermal factors for synthesizing Q1D TiO2 with maximum photodegradation rate and H2 production rate. The optimized Q1D TiO2 with maximum photodegradation rate was further enhanced with partially reduced graphene oxide (RGO) (designated as GT) for degrading aqueous hazardous pollutants. The study also examined the impact of the RGO atomic oxygen-to-carbon (O/C) ratio on GT photocatalytic activities. The highest photocatalytic activity was observed when the RGO atomic O/C ratio was 0.130±0.003. Next, the GT photocatalyst was enhanced with Ag NPs (designated as Ag-GT). The highest photocatalytic activity was observed for a silver content of 10 wt% in the photocatalyst film. Finally, an atmospheric pressure plasma jet (APPJ) was employed to synthesize micrometer thick Ag nanoparticles modified TiO2 (Ag-TiO2) coatings, presenting a core-shell structure for degrading RhB and different pharmaceutical compounds using a solar light source. Ag-TiO2 coatings were characterized having a porous anatase phase, improved charge separation and visible light absorption.

The highest photodegradation rate was observed for a silver content of 0.4wt% in the composite. Keywords: TiO2, hydrothermal, photodegradation, H2 production, graphene, atmospheric pressure plasma jet.

RESUME

Cette thèse porte sur la synthèse des nanocatalyseur à base de TiO2 Q1D (quasiunidimensionnel) pour dégrader des polluants organiques aqueux et pour produire de l'hydrogène. Un processus hydrothermal alcalin simple a été utilisé pour synthétiser TiO2 Q1D avec des paramètres de synthèse différents (la température, la concentration de NaOH de précurseur TiO2). Ces paramètres affectent la structure de phase de TiO2 Q1D, la taille du cristal, la surface spécifique (SS), l'énergie du gap et les activités photocatalytiques qui en résultent. Le modèle Boîte-Behnken (BBD) est utilisé pour optimiser les paramètres hydrothermaux permettant d'obtenir un taux de photodégradation maximal des polluants étudiés, et un taux de production d'H2 maximal. Le TiO2 Q1D optimisé a été davantage amélioré à l'aide d'oxyde de gràphène partiellement réduit (RGO) pour obtenir le catalyseur désigné GT. L'impact du rapport atomique O/C du RGO sur l'activité catalytique du GT est étudié. La valeur maximale d'activité photocatalytique est obtenu pour O/C =0.130±0.003. Puis, le GT est amélioré avec les NP d'Ag (Ag-GT). L'activité maximale est obtenue pour 10%d'Ag. Enfin un jet plasma atmosphérique (APPJ) a été employé pour synthétiser des couches minces nanométriques de TiO2 avec inclusion des NP d'Ag de structure coeur-coquille pour la dégradation du colorant RhB et des différents des produits pharmaceutiques à l'aide d'un simulateur solaire. Les couches composites minces (Ag-TiO2) sont très poreuses de phase anatase avec une absorption dans le visible et une amélioration de la séparation des charges. La valeur maximale de la photo-dégradation a été obtenue pour 0.4% d'Ag dans la couche composite. Mots clés: TiO2; hydrothermal; photodégradation; graphène ; production d'hydrogène ; jet plasma pression atmosphérique.