Sara El

Tony Chave

Amr A Nada

Stéphanie Roualdes

Sergey I Nikitenko
email: serguei.nikitenko@cea.fr

Tailoring Noble Metal-Free Ti@TiO2 Photocatalyst for Boosting Photothermal Hydrogen Production

Keywords: photocatalysis, hydrogen, nanomaterials, titanium suboxide, sonohydrothermal

In this work, we provide new insights into the design of Ti@TiO2 photocatalyst with enhanced photothermal activity in the process of glycerol reforming. Ti@TiO2 nanoparticles have been obtained by sonohydrothermal treatment of titanium metal nanoparticles in pure water. Variation of sonohydrothermal temperature allows controlling nanocrystalline TiO2 shell on Ti 0 surface. At 100<T<150°C formation of TiO2 NPs occurs mostly by crystallization of Ti(IV) amorphous species and oxidation of titanium suboxide Ti3O presented at the surface of Ti 0 nanoparticles. At T>150°C, TiO2 is also formed by oxidation of Ti 0 with overheated water. Kinetic study highlights the importance of TiO2 nanocrystalline shell for H2 generation. Electrochemical impedance spectroscopy points out more efficient electron transfer for Ti@TiO2 nanoparticles in correlation with photocatalytic data. The apparent activation energy, Ea=(25-31)±5 kJ•mol -1 , assumes that photothermal effect arises from diffusion of glycerol oxidation intermediates or from water dynamics at the surface of catalyst. Under the heating, photocatalytic H2 emission is observed even in pure water.

INTRODUCTION

Hydrogen is a clean fuel that, when consumed in a fuel cell, yields only water. Today, 95% of hydrogen is produced from fossil fuels, such as natural gas and oil [START_REF] Baykara | Hydrogen: A brief overview on its sources, production and environmental impact[END_REF]. Conversion of solar energy into hydrogen via photocatalytic splitting of water is an alternative sustainable process of paramount interest for clean energy storage [START_REF] Ma | Titanium dioxide-based nanomaterials for photocatalytic fuel generations[END_REF][START_REF] Ghosh | Visible-light-active photocatalysis: nanostructured catalyst design, mechanisms and applications[END_REF]. In this view, preparation of stable and nontoxic catalysts from non-precious elements showing high photocatalytic activity under solar light irradiation is of great importance. Among a wide variety of catalysts, titanium oxide, TiO2, has been recognized as one of the most popular photocatalysts. However, TiO2 can absorb only around 6% of the sunlight, owing to a quite large bandgap of 3.2 eV for anatase phase [START_REF] Ghosh | Visible-light-active photocatalysis: nanostructured catalyst design, mechanisms and applications[END_REF]. Another serious bottleneck of TiO2 is a rapid electron-hole recombination leading to the decrease in photocatalytic activity. Therefore, tremendous efforts have been undertaken to narrow bandgap of TiO2 and to improve photogenerated charge separation. The recent advances in photocatalysis with TiO2-based This is a provisional file, not the final typeset article materials pointed out several strategies of TiO2 bandgap engineering: doping of TiO2 with cations or anions, codoping with cations and anions, self-doping of TiO2 with Ti 3+ , and surface sensitization with organic dyes or transition metal complexes [START_REF] Ma | Titanium dioxide-based nanomaterials for photocatalytic fuel generations[END_REF]. On the other hand, fabrication of TiO2 heterojunctions with other semiconductors, anatase-rutile phase junctions, and TiO2 loading with cocatalysts, often noble metal nanoparticles, allows to minimize electron-hole recombination during the photocatalytic process [START_REF] Ghosh | Visible-light-active photocatalysis: nanostructured catalyst design, mechanisms and applications[END_REF].

Design of catalyst morphology is another important strategy to reach maximal photocatalytic activity. Core-shell nanoparticles have attracted a great deal of attention as promising photocatalysts for hydrogen production due to the synergism between the cores and shells and/or new properties providing by the interactions between the cores and shells [START_REF] Gawande | Core-shell nanoparticles: synthesis and applications in catalysis and electrocatalysis[END_REF]. Recently, we reported strong photothermal effect in the process of photocatalytic hydrogen production from the aqueous solutions of methanol and glycerol in the presence of noble metal-free Ti@TiO2 core-shell nanoparticles (NPs) [START_REF] Nikitenko | Photothermal hydrogen production using noble-metal-free Ti@TiO2 core-shell nanoparticles under visible-NIR light irradiation[END_REF][START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF]. It is noteworthy that the combination of photonic and thermal energy could be very beneficial for efficient solar energy harvesting [START_REF] Ma | Review of synergetic photo-thermo-catalysis: mechanisms, materials and applications[END_REF]. However, the mechanism of Ti@TiO2 formation and the influence of TiO2 nanocrystalline shell on the photocatalytic performance have not been examined. In this work, we applied simultaneous ultrasonic and hydrothermal treatment, called sonohydrothermal treatment (SHT), for the preparation of Ti@TiO2 NPs with controlled Ti/TiO2 ratio. SHT is an emerging environmentally benign technique effective for the synthesis of nanocrystalline materials with enhanced properties [START_REF] Nikitenko | Photothermal hydrogen production using noble-metal-free Ti@TiO2 core-shell nanoparticles under visible-NIR light irradiation[END_REF][START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF][START_REF] Cau | Sonohydrothermal synthesis of nanostructured (Ce,Zr)O2 mixed oxides with enhanced catalytic performance[END_REF]. Glycerol reforming was used to evaluate the photothermal performance of prepared Ti@TiO2 NPs. Glycerol is a low-cost biomass derivative suitable for hydrogen production [START_REF] Shimura | Heterogeneous photocatalytic hydrogen production from water and biomass derivatives[END_REF]. Furthermore, quite high boiling point of glycerol (290°C) allows to study the photothermal effect in a large temperature range. In addition, Ti@TiO2 NPs have been studied for the first time using electrochemical impedance spectroscopy (EIS). Combination of characterization techniques and kinetic study provided new insights into the mechanism of hydrogen formation and the origin of photothermal effect in studied system.

MATERIALS AND METHODS

Catalyst preparation

The commercially available titanium nanopowder (Nanostructured & Amorphous Materials,Inc. Ti, 99%) is an air-sensitive material and it was stored in the argon-filled glove box prior use. Stable Ti@TiO2 NPs were prepared by SHT treatment of Ti nanopowder in pure water (Milli-Q 18.2 MΩ•cm at 25 °C). The SHT reactor is shown in Supplementary Figure S1 and described previously [START_REF] Nikitenko | Photothermal hydrogen production using noble-metal-free Ti@TiO2 core-shell nanoparticles under visible-NIR light irradiation[END_REF][START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF][START_REF] Cau | Sonohydrothermal synthesis of nanostructured (Ce,Zr)O2 mixed oxides with enhanced catalytic performance[END_REF]. In a typical experiment, 2 g of air passivated Ti nanopowder was dispersed in 50 mL of water using ultrasonic bath, placed in SHT reactor and heated at selected temperatures in the range of T = 100-214 °C (autogenic pressure P = 1.0-19.0 bar) under simultaneous ultrasonic treatment (f = 20 kHz, Pac = 17 W) for 3 hours. After cooling, the treated NPs were recovered by centrifugation, washed with pure water and dried at room temperature under reduced pressure.

Catalyst characterization

Powder X-Ray Diffraction (XRD) diagrams were recorded with the use of a Bruker D8 Advance Xray diffractometer equipped with a linear Lynx-eye detector (Cu Kα1,2 radiation, λ = 1.54184 Å). XRD patterns were collected between 10° and 90° (θ-2θ mode) at room temperature, with a step size of Δ(2θ) = 0.02° and a counting time of 1.8 s•step -1 . Quantitative phase analysis was performed by Rietveld refinement with the phase detection limit about 5 % [START_REF] León-Reina | Accuracy in Rietveld quantitative phase analysis: a comparative study of strictly monochromatic Mo and Cu radiations[END_REF]. High-resolution transmission electron microscopic (HRTEM) and scanning transmission electron microscopic (STEM) measurements coupled with EDX mapping (SDD Oxford detector) were performed using a Jeol 2200FS (200 kV) microscope. XPS analysis was achieved with an ESCALAB 250 Thermo Electron device operated under ultra-high vacuum. The excitation source was a monochromatic source (Al-K ε = 1486.6 eV) with a ca. 0.4 mm 2 X-ray spot. The pass energy was fixed at 20 eV. The photoelectron spectra were calibrated using the Au 4f 7/2 (83.9 ± 0.1 eV) and Cu 2p 3/2 (932.8 ± 0.1 eV) photoelectron lines. The XPS spectra were treated using AVANTAGE software. The binding energy scale was established by referencing the adventitious C 1s peak at 284.8 eV. The reflectance spectra were recorded in BaSO4 pellets with a Shimadzu UV-3600 spectrophotometer. Thermogravimetric analysis (TGA) was performed by means of TGA-DTA/DSC Setsys Evolution (Setaram Instrumentation) device in air flow (900 °C, 10 °C•min -1 ). The concentration of titanium species in solution after photolysis was measured using SPECTRO ARCOS ICP-OES instrument (detection limit ~ 0.1 ppm).

Electrochemical measurements

Electrochemical Impedance Spectroscopy (EIS) was studied at dark conditions in the frequency range from 0.1 Hz to 100 kHz with an AC amplitude 10 mV. The electrolytic cell was filled with 1 M KOH and bubbled with Ar for 20 minutes prior the measurements. Silver / silver chloride (Ag/AgCl) and platinum (Pt) were used as the reference electrode and counter electrode, respectively. The working electrode was prepared by ultrasonic dispersion of the synthesized photocatalyst (5 mg) in the mixture of 1 mL isopropanol (VWR, ≥99.7%) and 40 µL Nafion (Aldrich, 5 wt% lower aliphatic alcohols, 15-20% water). Then 5 𝜇L of the photocatalyst suspension was deposited onto glassy carbon electrode.

Photothermal hydrogen formation

The photocatalytic study was performed in aqueous glycerol (99 % Sigma-Aldrich) solutions using a thermostated gas-flow cell made from a borosilicate glass and adapted to mass spectrometric analysis of the outlet gases. The image of the photocatalytic cell is shown in Supplementary Figure S2. In a typical run, 7.8 mg of photocatalyst was ultrasonically dispersed in 65 mL of aqueous glycerol solution and placed into the photoreactor. Photolysis was carried out using a white light of ASB-XE-175W xenon lamp equipped with ozone blocking coatings. The lamp was placed at 8 cm away from the reactor and the light power at this distance was measured by X1-1 Optometer (Gigahertz-Optik) using UV-3710-4 (300-420 nm) and RW-3705-4 (400-1100 nm) calibrated detectors. The obtained values of light power were equal to 8.9 W and 0.6 W for vis/NIR and UV spectral ranges respectively, which provides the close spectral match to solar spectra. The solutions inside the reactor were stirred continuously and the temperature was gradually increased up to 95 °C during photolysis. The Ar gas flow through the reactor was kept constant at 58 mL•min -1 and controlled by a volumetric flowmeter. The gaseous products in the outlet gas were analyzed using a Thermo Scientific PRIMA BT mass spectrometer. The H2 formation rate was quantified using external calibration curves prepared with standard gas mixtures in argon (Messer). The water vapors were trapped with molecular sieve (Sigma-Aldrich, 5 Å) prior to mass spectrometric analysis. This is a provisional file, not the final typeset article

RESULTS AND DISCUSSION

Morphological and structural studies

TEM images depicted in Figure 1A reveal a quasi-spherical morphology of air passivated Ti particles with an average size of around 30-150 nm without any crystals at the surface. On the other hand, EDX mapping (Figure 2A) indicates the presence of oxygen-enriched layer at the surface of air-passivated Ti particles. This layer with a thickness about 11 nm is also clearly distinguishable in the HRTEM image of Ti NPs particles in Figure 1A. Sonohydrothermal treatment leads to the formation of nanocrystalline shell composed of 10-20 nm oxygen-enriched particles, however, formed core-shell particles preserve quasi-spherical morphology as it is displayed in Figures 1B,C and2B. We noticed that at the SHT temperature of about 100 °C only few nanocrystals are formed, nevertheless their amounts increase with the increase of SHT temperature. The average particle size increases on ca. 5-10 % after the coating with TiO2 compared to initial Ti particles.

XRD diagram of air passivated Ti NPs in Figure 3A displays the patterns of metallic α-phase titanium (JCPDS 00-044-1294) with admixtures of tetragonal nonstoichiometric titanium hydride TiHx (x = 1.53 -1.97, JCPDS 01-079-6209) as it often observed in commercial titanium powders [START_REF] Ageev | X-ray diffraction analysis of titanium hydrides in commercial titanium alloys[END_REF]). However, more thoughtful analysis of XRD data using Rietveld refinement revealed the presence of scarce Ti3O suboxide with 𝑃31𝑐 ̅̅̅̅̅ space group symmetry [START_REF] Yamaguchi | Interstitial order-disorder transformation in the Ti-O solid solution. I. Ordered arrangement of oxygen[END_REF]. To the best of our knowledge, this is the first observation of Ti3O phase at the nanoscale. It is worth noting that the XRD diagram of Ti NPs does not exhibit the presence of crystallized Ti(IV) oxides. The XRD data of the samples SHT treated at 164 °C and 214 °C clearly point out drop of Ti3O content and formation of TiO2 anatase (JCPDS 00-021-1272), which is in line with HRTEM observations (Figure 1B).

High-resolution Ti 2p XPS spectra shown in Figure 4 were fitted using binding energies published in the NIST database [START_REF] Wagner | NIST standard reference database 20[END_REF]. The XPS spectrum of air passivated Ti NPs revealed the presence of Ti 0 and lower oxidation states of titanium at the particle surface in agreement with the XRD analysis revealed the presence of titanium suboxide in Ti NPs. In contrast to XRD data, the XPS spectrum of Ti NPs also exhibits a signal typical for TiO2 (Figure 4A). This discrepancy could be assigned to the formation of amorphous hydrated titanium oxide TiO2•xH2O at the surface of metallic titanium. SHT treatment causes disappearance of Ti 0 , TiO, TixOy and Ti2O3 peaks from the XPS spectrum and the experimental XPS spectrum of Ti@TiO2 NPs can be fitted perfectly well by defect-free TiO2 spectrum indicating effective coating of metallic titanium core (Figure 4B). Table 1 summarizes the phase compositions of initial Ti particles and prepared materials obtained from the Rietveld refinement of powder XRD data. One can conclude that air passivated Ti NPs are stable during the SHT treatment until ca. 100 °C. Further heating until ca. 150 °C causes oxidation of Ti3O to TiO2: Ti3O + 5H2O → 3TiO2 + 5H2

(1)

At higher temperature, metallic titanium is also oxidized yielding TiO2:

Ti + 2H2O → TiO2 + 2H2 (2)
According to the XPS data anatase nanocrystals at the surface of Ti NPs also can be formed by crystallization of TiO2•xH2O species. It is worth noting that SHT oxidation of Ti NPs is more effective than hydrothermal heating without ultrasound (Table 1), which was attributed to better heat and masstransfer in the case of SHT process due to the acoustic cavitation, i.e. formation, nonlinear oscillations, and implosion of microbubbles formed in the hydrothermal water under the effect of ultrasonic waves [START_REF] Cau | Sonohydrothermal synthesis of nanostructured (Ce,Zr)O2 mixed oxides with enhanced catalytic performance[END_REF]. Results of TGA analysis displayed in Supplementary Figure S4 and Table 2 point out the increase of titanium metallic core thermal stability with the increase of SHT temperature, which is explained by the formation of protective TiO2 shell. It should also be emphasized that the hydrothermal oxidation of micrometric titanium powder begins at much higher temperature of about 450 °C [START_REF] Yoshimura | Formation of TiO2 and ZnO powders by hydrothermal oxidation of Ti and Zn metals[END_REF], which is most likely related to the particle size effect.

Light absorption and charge transfer

All Ti-based materials studied in this work have intense black color indicating extented photoresponse with nearly full solar spectrum. The solid state reflectance spectra of both Ti and Ti@TiO2 NPs (Figure 5) exhibit a broad band spanning from UV to NIR spectral range, which was attributed to intraband and interband transitions of metallic Ti [START_REF] Gawande | Core-shell nanoparticles: synthesis and applications in catalysis and electrocatalysis[END_REF][START_REF] Nikitenko | Photothermal hydrogen production using noble-metal-free Ti@TiO2 core-shell nanoparticles under visible-NIR light irradiation[END_REF]. In addition, the spectra of Ti@TiO2 NPs reveal an absorption band centered at 220-350 nm typical for the bandgap transition of crystallized TiO2. Kubelka-Munk treatment of the spectral data using (F(R)hv) 1/2 vs E function [START_REF] López | Band-gap energy estimation from diffuse reflectance measurements on sol-gel and commercial TiO2: a comparative study[END_REF] shown in Supplementary Figure S5 gives a bandgap energy value of 3.52 and 3.51 eV for SHT 164 °C and SHT 214 °C respectively, which is slightly greater than the bandgap energy reported for commercial anatase TiO2 powder (E = 3.26 eV) [START_REF] López | Band-gap energy estimation from diffuse reflectance measurements on sol-gel and commercial TiO2: a comparative study[END_REF]. This difference may originate from the overlap of Ti 0 interband/intraband transitions and TiO2 bandgap in the UV spectral range (Figure 5). Kubelka-Munk treatment could lead to the overestimated value of the bandgap energy in this case.

The electrochemical impedance spectroscopy (EIS) provides a valuable information about the charge transfer and charge recombination processes at the interface of the catalysts and electrolytes (Barsoukov et al. 2005). Figure 6 shows the Nyquist plots and the proposed equivalent circuits of the passivated Ti NPs and Ti@TiO2 NPs obtained after SHT at 101 °C, 164 °C and 214 °C. The reduced size of semicircles indicates a lower charge transfer resistance at the particle/electrolyte interface. Surprisingly, Ti particles coated with TiO2 nanocrystals exhibit more effective electron transfer than air-passivated Ti NPs or Ti NPs treated at 101 °C with very low content of TiO2 NPs at the Ti metal surface. The resistances calculated for equivalent circuits are summarized in the Table 3. The R1 refers to the bulk resistance of electrodes and electrolytes, and R2 represents the resistance at the interface of the particles and electrolytes. A third resistance R3 is only observed for Ti@TiO2 NPs and is most likely related to the second interface formed between TiO2 nanoparticles from the shell and Ti 0 core. Interesting that R3 increases with the temperature of SHT treatment, or, in other words, with the increase of TiO2 anatase content. High R2 value (9.05 kΩ) for Ti 0 NPs most probably is attributed to the passivating layer at the metal surface leading to the hindering of charge transfer. Among Ti@TiO2 NPs, the sample obtained at 214 °C displays the lowest R2 value indicating the fastest charge transfer rate for this material. This conclusion is in an agreement with an effective charge separation in Ti@TiO2 NPs recently demonstrated using photoluminescence spectroscopy [START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF].

Photocatalytic hydrogen production

Typical hydrogen emission profiles and calculated H2 yields for studied photocatalysts shown in Figure 7 highlight the crucial role of TiO2 nanocrystalline shell in the reaction of photocatalytic glycerol reforming. Kinetic data reveal a clear relationship between the temperature of SHT treatment, or, in other words, between anatase content in Ti@TiO2 NPs, and H2 yield (Figure 7B). In addition, photocatalytic activity correlates with EIS data exhibiting more effective charge transfer for SHT 214 °C NPs. The role of TiO2 in the reaction mechanism can be understood in terms of the charge separation between semiconducting TiO2 shell and metallic Ti 0 core. The optical spectra (Figure 5) show that the This is a provisional file, not the final typeset article incident light of the Xe lamp in UV/vis/NIR spectral range is mainly absorbed by Ti 0 core. In nonplasmonic metal, such as titanium, electron-hole pairs can be created by interband transitions via nonradiative Landau damping mechanism [START_REF] Kale | Direct photocatalysis by plasmonic nanostructures[END_REF]. However, in a highly conducting, metallic material, the electron-hole recombination would be extremely rapid in the absence of hole and/or electron scavengers. Therefore, one might suggest that in Ti@TiO2 core-shell NPs TiO2 provides effective charge separation, what was confirmed by fluorescence spectroscopy in previous study [START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF]. In addition, TiO2 can absorb at least part of the UV incident light followed by electron-hole pairs generation in semiconducting particle. In this case, charge separation would be provided by electron migration to Ti 0 core, as it occurs in TiO2 photocatalyst loaded with noble metal nanoparticles [START_REF] Ma | Titanium dioxide-based nanomaterials for photocatalytic fuel generations[END_REF][START_REF] Ghosh | Visible-light-active photocatalysis: nanostructured catalyst design, mechanisms and applications[END_REF].

Furthermore, photocatalytic process with Ti@TiO2 NPs exhibits strong photothermal effect in an agreement with our previous results [START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF]. Figure 7b demonstrates that the yield of H2 increases in ca. 4 times when the bulk temperature increases from 37 °C to 95 °C. On the other hand, in dark conditions formation of H2 is not observed even at 95 °C indicating the photonic origin of studied process. Kinetics of H2 formation obeys an Arrhenius law (Supplementary Figure S6) and the calculated apparent activation energy, Eact, for found to be equal to 31±5 and 25±5 kJ•mol -1 for SHT 164 °C and SHT 214 °C respectively. It is worth noting that these values are much lower than the typical activation energy of chemical bonds and has been previously assigned to the diffusion of intermediates at the catalyst surface [START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF]. Alternatively, the temperature dependence of photocatalytic reactions can be also related to the dynamics of water adsorbed at the surface of the catalyst (Eact = 16-21 kJ•mol -1 , [START_REF] Parrino | Nature of interactions at the interface of two water-saturated commercial TiO2 polymorphs[END_REF].

Figure 8 shows the influence of glycerol concentration on H2 formation with the most active SHT 214 °C Ti@TiO2 photocatalyst. The yield of H2 increases with the glycerol concentration and then levels off at [Glycerol] ≥ 0.5 M in an agreement with Langmuir-type function indicating that the mechanism of photocatalytic process involves glycerol adsorption at the active sites of the catalyst. Similar behavior has been reported for photocatalytic glycerol reforming over Pt/TiO2 [START_REF] Jiang | Photocatalytic reforming of glycerol for H2 evolution on Pt/TiO2: fundamental understanding the effect of co-catalyst Pt and the Pt deposition route[END_REF] and NiO/TiO2 [START_REF] Fujita | Photocatalytic hydrogen production from aqueous glycerol solution using NiO/TiO2 catalysts: effects of preparation and reaction conditions[END_REF] NPs. On the other hand, kinetics with Ti@TiO2 NPs reveals H2 emission even from pure water at elevated temperature (Supplementary Figure S7). This observation is largely in agreement with the above conclusion that both water and glycerol can contribute to the observed photothermal effect.

Mass spectrometric measurements indicate the absence of CO2 emissions (Supplementary Figure S8) during the photothermal process with glycerol in consonance with our previous study [START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF]. It is worth noting that photocatalytic glycerol reforming in the presence of nanocrystalline P25 TiO2 (83% of anatase and 17 % of rutile) leads to CO2 and CO formation [START_REF] Liu | Photocatalytic hydrogen production from glycerol and water with NiOx/TiO2 catalysts[END_REF], which implies a certain difference in reaction mechanisms with Ti@TiO2 and TiO2 photocatalysts. We found that in studied system H2 formation is accompanied by acidification of photolyte from pH=6 to pH=4 in a ca. 3 h of photolysis indicating formation of some acidic products without decarboxylation. It has recently been suggested that glycerol is oxidized to glyceric acid in the studied process [START_REF] Nikitenko | Insights into the photothermal hydrogen production from glycerol aqueous solutions over noble metal-free Ti@TiO2 core-shell nanoparticles[END_REF]. In addition, it is known that the wet catalytic oxidation of glycerol can also lead to the formation of other carboxylic acids without decarboxylation, such as tartronic acid, mesooxalic acid, and hydroxypyruvic acid [START_REF] Worz | Platinum-bismuth-catalyzed oxidation of glycerol: kinetics and the origin of selective deactivation[END_REF]).

On the basis of the above data, it can be concluded that the mechanism of H2 formation in studied system involves two reaction pathways illustrated in Figure 9: splitting of water molecules (equations 3-5 presuming four-electron process) and glycerol reforming (equations 6-12):

H2O splitting:

Ti@TiO 2 + H2O ⇄ Ti@TiO2•H2O

(3) Ti@TiO2•H2O + hν → [ e Ti@TiO2 h+ ]•H2O (4) [ e Ti@TiO2 h+ ]•H2O → Ti@TiO 2 + H2 + ½O2

(5) Glycerol reforming:

Ti@TiO 2 + RCH2OH ⇄ Ti@TiO2•RCH2OH (6) Ti@TiO2 where RCOOH in the equation ( 11) could represent a mixture of mentioned above carboxylic acids. It is worth mentioning that the direct water splitting is practically observable at the sufficiently high temperature only. In general, higher yield of hydrogen for glycerol compared to water was attributed to more effective hole scavenging by glycerol than by water [START_REF] Jiang | Photocatalytic reforming of glycerol for H2 evolution on Pt/TiO2: fundamental understanding the effect of co-catalyst Pt and the Pt deposition route[END_REF][START_REF] Tang | Mechanism of photocatalytic water splitting in TiO2. Reaction of water with photoholes, importance of charge carrier dynamics, and evidence for four-hole chemistry[END_REF]. The link between surface charge carrier dynamics and photocatalytic activity of Ti@TiO2 NPs is in line with this conclusion.

Finally, the stability of Ti@TiO2 photocatalysts at studied conditions was tested using HRTEM and ICP-OES techniques. Modification of the particle's morphology was not detected by HRTEM after photothermal experiments as shown in Supplementary Figure S9. ICP-OES analysis has been performed with SPECTRO ARCOS instrument (detection limit of titanium was ca. 0.1 ppm). Particles of catalyst have been removed from the reaction mixture by filtration through 0.2 μm PTFE filter prior analysis. The ICP-OES analysis after 8 hours of photocatalytic experiments in hot 0.5M glycerol solutions revealed a leak of about 0.09 % and 0.29 % of titanium for SHT 214 °C and SHT 164 °C Ti@TiO2 NPs respectively.

CONCLUSIONS

In summary, this work provides some new insights into the structure and thermally-assisted photocatalytic properties of Ti 0 and Ti@TiO2 nanoparticles. Rietveld refinement of XRD data revealed the presence of scarce titanium suboxide Ti3O with a 𝑃31𝑐 ̅̅̅̅̅ space group symmetry in raw α-Ti 0 NPs. According to STEM/EDX study this species is situated at the surface of Ti 0 NPs. Formation of the species with lower oxidation states of titanium was also observed by XPS spectroscopy. Our results confirm previous conclusion [START_REF] Kornilov | Neutron diffraction investigation of ordered structures in the titanium oxygen system[END_REF] about Ti3O stability at room temperature. Sonohydrothermal synthesis allows to control nanocrystalline TiO2 shell in Ti@TiO2 NPs using pure water as a solvent. Mechanism of TiO2 shell formation depends on the SHT temperature: in the temperature range 100-150 °C TiO2 is formed mainly by the oxidation of Ti3O, and oxidation of Ti 0 to TiO2 occurs at higher temperature. Coating of titanium metal nanoparticles with anatase nanocrystals This is a provisional file, not the final typeset article provides their better stability to oxidation and strong photothermal effect of hydrogen production from aqueous glycerol solutions. In a hot water, photocatalytic formation of hydrogen is observed even without glycerol. However, the yield of hydrogen in the presence of glycerol is higher than that in pure water, which can be related to more effective scavenging of photogenerated holes with glycerol. Glycerol reforming leading to hydrogen production is not accompanied by CO2 emission in studied system. The acidification of the photolyte solution implies that glycerol is oxidized to some acidic products, most probably, to the mixture of organic acids derived from glycerol. The apparent activation energy Eact = (25-31)±5 kJ•mol -1 of studied process is much lower than the typical activation energy of chemical bonds indicating that the origin of the photothermal effect is related to the diffusion of intermediates arise from glycerol or from the dynamics of water adsorbed at the surface of the catalyst. This is a provisional file, not the final typeset article

TABLES

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS Figure 1. Typical TEM images of initial Ti particles (A), SHT-treated particles in water at 101 °C (B) and >150 °C (C). The distance of 0.35 nm corresponds to (101) plane of TiO2 anatase.

Figure 2 .

 2 Figure 2. STEM/EDX mapping of air passivated Ti 0 NPs (A) and SHT-treated at 164 °C Ti@TiO2 NPs (B), green dots -Ti, red dots -O.

Figure 3 .

 3 Figure 3. XRD patterns of the initial Ti NPs and Ti@TiO2 NPs treated at different temperatures under SHT conditions (A). Zoom of the figure (A) in 39-41 2θ° range indicating the decrease in Ti3O content with increasing of SHT temperature (B). Example of Rietveld fit of the XRD diagram for Ti NPs in 39-41 2θ° range indicating the presence of Ti3O phase (C), red line -Ti3O, black line -α-Ti.

Figure 4 .

 4 Figure 4. Fitted Ti 2p high-resolution XPS spectra of air-passivated Ti 0 (A) and SHT/214 °C Ti@TiO2 (B) NPs. Red line indicates experimental spectrum.

Figure 5 .

 5 Figure5. Solid-state reflectance spectra of Ti 0 NPs before and after sonohydrothermal treatment at different temperatures. The spectrum of Ti particles SHT treated at 101 °C is very similar to the spectrum of air-passivated Ti0 particles reported recently[START_REF] Nikitenko | Photothermal hydrogen production using noble-metal-free Ti@TiO2 core-shell nanoparticles under visible-NIR light irradiation[END_REF].

Figure 6 .

 6 Figure6. Nyquist plots for Ti 0 NPs and Ti@TiO2 NPs and the equivalent circuit for Ti 0 (A) and Ti@TiO2 (B). Figure7. Typical hydrogen emission profiles with catalysts obtained at 101 °C, 164 °C and 214 °C SHT temperatures in solutions of 0.5 M glycerol under Xe-lamp white light and Ar flow (A), and H2 yields in 0.5 M glycerol solutions and pure water for SHT-214°C catalyst (B). The uncertainty for H2 yields was estimated about 10-15 %.

Figure 7 .

 7 Figure6. Nyquist plots for Ti 0 NPs and Ti@TiO2 NPs and the equivalent circuit for Ti 0 (A) and Ti@TiO2 (B). Figure7. Typical hydrogen emission profiles with catalysts obtained at 101 °C, 164 °C and 214 °C SHT temperatures in solutions of 0.5 M glycerol under Xe-lamp white light and Ar flow (A), and H2 yields in 0.5 M glycerol solutions and pure water for SHT-214°C catalyst (B). The uncertainty for H2 yields was estimated about 10-15 %.

Figure 8 .

 8 Figure 8. Plot of the H2 formation yields against glycerol concentrations over SHT 214 °C Ti@TiO2 photocatalyst at 53 °C and 87 °C.

Figure 9 .

 9 Figure 9. Graphical sketch of suggested mechanism of H2 photocatalytic formation in the presence of Ti@TiO2 core-shell nanoparticles.

Table 1 .

 1 Variation of Ti NPs composition with the temperature of SHT treatment obtained by Rietveld refinement of XRD patterns. Time of treatment was equal to 3 h.

			mol.% (5%)
	Sample	Ti	Ti 3 O	TiO2 anatase
	Air passivated Ti*	86	14	--
	SHT 101 °C	88	12	--
	HT 150 °C **	88	5	7
	SHT 164 °C	≥80.5	≤1.5	18
	SHT 214 °C	≥62	≤3	35
	*The admixture of TiHx and amorphous TiO2•xH2O species were not
	taken into account. **HT stands for hydrothermal treatment without
	ultrasound. XRD and TEM data for these NPs are shown in
	Supplementary Figure S3.

Table 2 .

 2 Thermal stability of Ti and Ti@TiO2 NPs obtained from TGA analysis. Tox indicates the begining of metallic titanium core oxidation calculated as it shown in Supplementary FigureS4.

	Sample	Air passivated Ti	SHT/101 °C	SHT/164 °C	SHT 214°C
	Tox10 °C	230	240	263	304

Table 3 .

 3 EIS parameters for Ti and Ti@TiO2 NPs.

	Samples	R1 (Ω)	R2 (Ω)	R3 (kΩ)
	Ti@TiO2 SHT 214 °C	59.95	7.96	5.44
	Ti@TiO2 SHT 164 °C	48.63	22.85	7 .67
	Ti@TiO2 SHT 101 °C	42.06	34.03	9 .12
	Air passivated Ti NPs	49.84	9.05 (kΩ)	not been observed

ACKNOWLEDGMENTS

The authors kindly acknowledge Dr. Adel Mesbach for Rietveld refinement of XRD data, Dr. Xavier Le Goff for HRTEM measurements, Dr. Cyrielle Rey for TGA measurements and Dr. Valerie Flaud for XPS analysis.

FUNDING

This work has been partially funded by the CNRS Energy unit (Cellule Energie) through the project "PHOTOCAT.Ti@TiO2.H2".

DATA AVAILABILITY STATEMENT

The original contributions presented in the study are included in the article/supplementary materials, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS

All authors contributed to this work. S. I. N. and S. R. conceived the study, S. I. N., S. R., T. C., and S. E. H. prepared the manuscript, T. C. and S. E. H. prepared the samples and performed their characterization, S. E. H. and S. I. N. performed photocatalytic experiments, A. A. N. and S. E. H. performed electrochemical study. All authors approved the submitted version.

Conflict of Interest:

The authors declare that the research was conducted in the absence of any commercial or financial relationships that


Amir Mirzaei
email: amir.mirzaei@inrs.ca

Mustapha Eddah

Stéphanie Roualdès

Dongling Ma

Mohamed Chaker
email: chaker@emt.inrs.ca

Stéphanie Roualdes

Multiple-homojunction gradient nitrogen doped TiO2 for

Keywords: gradient doping, N-doped TiO2, pulsed laser deposition, water treatment, homojunction

Introduction

The presence of antibiotics such as sulfamethoxazole (SMX) in aquatic environment is considered as a rising environmental concern due to their adverse effects on ecosystems and on the development of antibiotic resistant bacteria [START_REF] Mirzaei | Sonocatalytic removal of ampicillin by Zn(OH)F: Effect of operating parameters, toxicological evaluation and by-products identification[END_REF]. Sulfamethoxazole is a widely prescribed antibiotic in both human and veterinary medicine to treat infectious diseases [START_REF] Yang | Degradation of sulfamethoxazole by UV, UV/H2O2 and UV/persulfate (PDS): formation of oxidation products and effect of bicarbonate[END_REF]. Due to its high consumption and high excretion rate, SMX is frequently detected in the aquatic environment such as surface water and underground aquifers, soil and even drinking water [START_REF] Zhou | Sulfate radical-based oxidation of the antibiotics sulfamethoxazole, sulfisoxazole, sulfathiazole, and sulfamethizole: The role of five-membered heterocyclic rings[END_REF]. Accordingly, SMX is listed among the most frequently detected compounds in groundwater by U.S. Geological Survey [START_REF] Barnes | A national reconnaissance of pharmaceuticals and other organic wastewater contaminants in the United States -I) Groundwater[END_REF] and it is usually considered as an antibiotic pollution indicator in environment [START_REF] Jiang | Biodegradation and metabolic pathway of sulfamethoxazole by Pseudomonas psychrophila HA-4, a newly isolated cold-adapted sulfamethoxazoledegrading bacterium[END_REF]. Regarding its antibacterial nature and recalcitrant properties, SMX removal efficiency in conventional biological water treatment systems is low (< 30%) [START_REF] Yang | Degradation of sulfamethoxazole by UV, UV/H2O2 and UV/persulfate (PDS): formation of oxidation products and effect of bicarbonate[END_REF][START_REF] Liu | SMX degradation by ozonation and UV radiation: A kinetic study[END_REF].

Photocatalytic degradation of organic contaminants is a promising process based on the formation of electron and hole pairs under light irradiation with photon energy equal to or higher than the band gap of semiconducting materials [START_REF] Min | Self-assembled encapsulation of graphene oxide/Ag@AgCl as a Zscheme photocatalytic system for pollutant removal[END_REF]. Among various semiconducting materials, TiO2 has attracted unprecedented interest due to its high photocatalytic activity, high stability and nontoxicity [START_REF] Ansari | Nitrogen-doped titanium dioxide (N-doped TiO 2) for visible light photocatalysis[END_REF]. However, the relatively large band gap of TiO2 (3.0-3.2 eV) limits its applications to the ultraviolet (UV) light region, which however represents only ∼4% of sunlight energy [START_REF] Farkas | Optical, compositional and structural properties of pulsed laser deposited nitrogen-doped Titanium-dioxide[END_REF]. In addition, rapid recombination of charge carriers severely restricts the photocatalytic activity of semiconducting materials in general [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF]. Notably, a low removal efficiency of the photocatalysis degradation process may lead to the formation of intermediates and by-products, which might be more toxic than their parent compounds [START_REF] Mirzaei | Sonocatalytic removal of ampicillin by Zn(OH)F: Effect of operating parameters, toxicological evaluation and by-products identification[END_REF][START_REF] Woo | Photocatalytic oxidation of polycyclic aromatic hydrocarbons: Intermediates identification and toxicity testing[END_REF][START_REF] Trovó | Photodegradation of sulfamethoxazole in various aqueous media: persistence, toxicity and photoproducts assessment[END_REF]. Therefore, developing a highly efficient treatment process is needed to degrade recalcitrant contaminants such as antibiotics in aquatic media.

It is proven that the formation of multi-homojunction with gradient-doped concentration can simultaneously enhance light absorption and charge separation through the generation of an extended band bending and an oriented electric field [START_REF] Abdi | Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode[END_REF][START_REF] Wang | Gradient self-doped CuBi2O4 with highly improved charge separation efficiency[END_REF][START_REF] Li | An effective strategy of constructing a multi-junction structure by integrating a heterojunction and a homojunction to promote the charge separation and transfer efficiency of WO3[END_REF]. In gradient-doped homojunctions, the extension of band bending can be tuned by gradually changing the dopant concentration [START_REF] Wu | Gradient Sn-Doped Heteroepitaxial Film of Faceted Rutile TiO2 as an Electron Selective Layer for Efficient Perovskite Solar Cells[END_REF].

Moreover, compared to the heterojunction structure, the lattice mismatch, defects and impurities at the interface can be significantly reduced in the homojunction [START_REF] Abdi | Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode[END_REF][START_REF] Liu | Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity[END_REF][START_REF] Miao | Fabrication of black TiO 2/TiO 2 homojunction for enhanced photocatalytic degradation[END_REF][START_REF] Rasouli | Gradient doping of copper in ZnO nanorod photoanode by electrodeposition for enhanced charge separation in photoelectrochemical water splitting[END_REF]. Notably, not only the dopant concentration but also its location (substitutional or interstitial) can significantly affect the photocatalytic performance [START_REF] Lynch | Substitutional or Interstitial Site-Selective Nitrogen Doping in TiO2 Nanostructures[END_REF]. However, the precise control of dopant concentration and position in the lattice and the fabrication of homojunctions with high quality interfacial contact between layers remain a challenge in terms of synthesis. Liu et al. [START_REF] Liu | TiO2 photoanode structure with gradations in V concentration for dye-sensitized solar cells[END_REF] synthesized gradient vanadium doped TiO2 by layer-by-layer sol-gel method for dye-sensitized solar cell application.

Nevertheless, the fabrication of homojunctions with a precise and controlled concentration of dopant and a high quality interfacial attachment between layers by using wet chemical methods is tedious. Wang et al. [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF] employed ion implantation to introduce nitrogen into a ZnO nanorod structure to form gradient N-doped ZnO structure. Although ion implementation can naturally introduce gradient doping into the host structure, the penetration depth of ions is limited. Abdi et al. [START_REF] Abdi | Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode[END_REF] significantly improved water splitting efficiency by using gradient W doped BiVO4 prepared by spray pyrolysis.

Although using gradient doping to enhance charge carrier separation is a well-established concept, most of the experimental works in this subject have employed wet chemical methods for photocatalysts preparation. While chemical methods are easy to implement and cost-effective, the chemically prepared films generally suffer from the inherent presence of residual precursors, reproducibility problems and difficulty in the precise control of dopant level and position in the lattice [START_REF] Delegan | Bandgap tailoring of in-situ nitrogen-doped TiO2 sputtered films intended for electrophotocatalytic applications under solar light[END_REF][START_REF] Bricchi | Integration of plasmonic Au nanoparticles in TiO2 hierarchical structures in a singlestep pulsed laser co-deposition[END_REF]. Pulsed laser deposition (PLD) is a versatile physical technique offering a wide range of tunable deposition parameters (e.g. energy and flux of species impinging the substrate) that enables the fine control of thin film microstructure, hence properties [START_REF] Suda | Preparation of high quality nitrogen doped TiO2 thin film as a photocatalyst using a pulsed laser deposition method[END_REF]. Moreover, PLD allows the deposition of multilayered films with good adhesion to substrates and high quality at interfaces without requiring complex steps and chemical binders [START_REF] Murcia-López | Tailoring multilayered BiVO4 photoanodes by pulsed laser deposition for water splitting[END_REF]. In addition, PLD provides high surface quality (no contamination) as compared to chemical methods. Finally, the film thickness and doping amountwhich are crucial in the fabrication of multilayered gradient nitrogen doped films-can be precisely controlled by varying the laser pulse number and the ratio between N2 and O2 gases in the deposition chamber, respectively.

To the best of our knowledge, most of the research works on gradient-doped semiconductors have so far been focused on harvesting and converting solar energy to clean and storable chemical [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF][START_REF] Liu | Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity[END_REF][START_REF] Rasouli | Gradient doping of copper in ZnO nanorod photoanode by electrodeposition for enhanced charge separation in photoelectrochemical water splitting[END_REF][START_REF] Antony | Synergistic effect of porosity and gradient doping in efficient solar water oxidation of catalyst-free gradient Mo: BiVO4[END_REF] or electrical energy [START_REF] Wang | Gradient self-doped CuBi2O4 with highly improved charge separation efficiency[END_REF][START_REF] Wu | Gradient Sn-Doped Heteroepitaxial Film of Faceted Rutile TiO2 as an Electron Selective Layer for Efficient Perovskite Solar Cells[END_REF], while their application in environmental purificationwhereas both electrons and holes would participate in the photocatalytic degradation-has not been reported. Herein, we used PLD technique for the fabrication of N-doped TiO2 with the anatase phase at different doping levels of substitutional nitrogen into the TiO2 lattice. Furthermore, a unique, multiple-homojunction structure composed of six layers of gradient doped anatase TiO2 with a columnar-like morphology (g-N-TiO2) was fabricated and its photocatalytic performance was compared with that of pristine TiO2 and N-TiO2 (non-gradient) in terms of photoelectrochemical (PEC) properties and SMX removal in aquatic media.

Experimental procedure 2.1. Photocatalyst preparation

In this study, N-doped anatase TiO2 samples with different amounts of nitrogen were prepared by using the PLD method. A KrF excimer laser (λ=248 nm, 10 Hz repetition rate, pulse width=25 ns)

was used to ablate a rotating Ti target (99.995% purity, Kurt J. Lesker, USA). The pulse energy density (fluence) at the target surface was 4 J/cm 2 . The target was moved and rotated continuously across the laser beam to minimize pitting. The target-to-substrate distance was fixed at 6.5 cm and the synthesis was performed at a substrate temperature of 350 °C in either high purity O2 gas (for TiO2 deposition) or in a mixture of high purity N2 and O2 gas (for nitrogen doped TiO2 deposition).

By changing the N2:O2 ratio from 10:90 to 90:10 at a constant deposition pressure, the N-TiO2

samples were prepared and labelled as 10N to 90N, respectively. The substrates were ultrasonically cleaned in acetone, isopropanol and de-ionized water for 10 min successively, and then dried under N2 stream before deposition. In the next step, gradient doped TiO2 (g-N-TiO2) was fabricated by layer-by-layer deposition of N-TiO2 with different amounts of doped nitrogen on fluorine doped tin oxide (FTO) and polished Si (100). As illustrated in Fig. 1, the first layer deposited on FTO was undoped TiO2, while the nitrogen content of the deposition gas (N2:O2 mixture) was increased for the next layers, successively. For comparison, TiO2 (without doping) and N-TiO2 (prepared with N2:O2=90:10) were also prepared with the similar thickness of g-N-TiO2. The thickness of each layer (∼90 nm) of the multi-layer g-N-TiO2 sample was controlled by varying the pulse number during laser deposition.

Fig. 1. Schematic and energy level diagrams of multi-layer g-N-TiO2.

Characterization instruments

A PANalytical X'Pert MRD, equipped with a Ni-filtered CuKα radiation source operated at 45 kV and 40 mA was used to analyze the crystalline structure of samples in the range of 20-80° and determine their density by X-ray reflectometry (XRR). Accordingly, XRR θ-2θ within the range of 0.1-3° were recorded. The density of films (ρfilm) is calculated using the following equations [START_REF] Drogowska | RBS, XRR and optical reflectivity measurements of Ti-TiO2 thin films deposited by magnetron sputtering[END_REF]:

𝜌 𝑒𝑙 = 𝜃 𝑐 2 𝜋 𝜆 2 𝑟 𝑒𝑙 (1)
𝜌 𝑓𝑖𝑙𝑚 = 𝐴 𝜌 𝑒𝑙 𝑁 𝐴 𝑍 ( 2
)
where ρel is the electron density, θc is the critical angle, λ=1.5406 Å is the wavelength of X-ray radiation source, rel=2.81×10 photocurrent-potential (I-V) measurements and electrochemical impedance spectroscopy.

Photocatalytic removal experiment

SMX was purchased form Sigma-Aldrich Inc., USA. SMX stock solution (1 mg/L) was prepared by using deionized Milli-Q water (18 μS cm -1 ). A photoreactor was designed which consists of a cell, a sample holder (1 inch × 1 inch), a quartz window (5 cm diameter) and an external reservoir for sampling. The initial volume of solution was 50 mL. The solution in the cell was circulated by a peristaltic pump operated at a constant 250 mL/min flow-rate. Before turning on the solar simulator, the solution was circulated for 30 min to achieve adsorption-desorption equilibrium between the photocatalyst film and SMX. The samples were illuminated by a solar simulator (ABET 2000) during the photocatalytic experiments. During each photocatalytic experiment, about 1.5 mL of the solution was collected at different time intervals for ultra-high performance liquid chromatography (UHPLC) analysis. For comparison, the photolysis experiment was conducted under the similar condition in the absence of the photocatalyst film. Before all experiments, the solar simulator was turned on for 30 min to ensure a stable light intensity. The removal percentage of SMX was calculated by Eq. (3):

𝑅𝑒𝑚𝑜𝑣𝑎𝑙 (%) = (1 - 𝐶 𝑡 𝐶 0 ) × 100 (3)
Pseudo-first-order kinetic model was used to determine the kinetic constants for each photocatalyst [START_REF] Mirzaei | Enhanced photocatalytic degradation of sulfamethoxazole by zinc oxide photocatalyst in the presence of fluoride ions: Optimization of parameters and toxicological evaluation[END_REF]:

-ln

𝐶 𝑡 𝐶 0 = 𝑘𝑡 (4)
where C0 (μg/L) is the initial concentration of SMX, Ct (μg/L) is the concentration at time t, and k is the reaction rate constant (min -1 ). After treatment, the photocatalysts were cleaned for 5 min in deionized water under mild sonication, washed several times, and dried at 50 °C to be used in cyclic experiments.

Analytical measurement and transformation products identification

The SMX concentration measurement and the identification of the transformation products (TPs) were performed by using a Nexera UHPLC system (Shimadzu, Columbia, MD). Accordingly, 1 μL of samples were injected into a Phenomenex Kinetex C18 column (2.1 × 50 mm, 1.7 μm) at the column temperature of 40 °C. The mobile phase was (A) water with 0.1% formic acid and (B) acetonitrile with 0.1% formic acid at the flow rate of 0.3 mL/min. A linear gradient was used starting with 97% (A) for one minute, changing to 50% (A) for 15 minutes and then 15% (A) for with high sensitivity for unknown compound analysis without the need to use standards, which are rarely available [START_REF] Trovó | Photodegradation of sulfamethoxazole in various aqueous media: persistence, toxicity and photoproducts assessment[END_REF].

Toxicity assessment

The performance of synthesized photocatalysts in terms of solution detoxification was evaluated by comparing the toxicity of the antibiotic solutions before and after photocatalytic treatment with TiO2, N-TiO2 and g-N-TiO2 in term of optical density measurement [START_REF] Zhang | Ultra-light and compressible 3D BiOCl/RGO aerogel with enriched synergistic effect of adsorption and photocatalytic degradation of oxytetracycline[END_REF]. Accordingly, five test groups were conducted as follows: (i) the control group (4 mL Lysogeny broth (LB) medium mixed with 4 mL deionized water), (ii) the untreated solution group (4 mL LB medium mixed with 4 mL initial SMX solution without any photocatalytic treatment), (iii) the treated group with TiO2 (4 mL LB medium mixed with 4 mL treated solution with TiO2 for 300 min), (iv) the treated group with N-TiO2 (4 mL LB medium mixed with 4 mL treated solution with N-TiO2 for 300 min), and (v)

the treated group with g-N-TiO2 (4 mL LB medium mixed with 4 mL treated solution with g-N-TiO2 for 300 min). Two parallel experiments were conducted in each group. Afterward, the solutions were kept at 37 °C for 3h and 1 mL (∼1 × 10 8 CFU/mL) JM103 Escherichia coli (E. coli) was added to each solution and incubated at 37 °C. After 3h and 6h the optical density value of culture at 600 nm (OD600) was measured by using a UV-vis spectrophotometry. The growth of bacteria colonies on the agar surface was monitored to visualize the change of toxicity of the solution before and after treatment [START_REF] Courtney | Photoexcited quantum dots for killing multidrug-resistant bacteria[END_REF]. Accordingly, ∼400 μL of solutions, which were incubated for 6h with the previous method, were spread over the surface of the agar that had been prepared by mixing the appropriate amount of agar and nutrient broth. After incubation overnight at 37 °C, the number of grown E. coli colonies on the surface of agars was counted.

Results and discussion

Structural characterization

The composition of the fabricated nitrogen doped TiO2 films was evaluated by X-ray photoelectron spectroscopy (XPS). Fig. 2(a) illustrates the XPS survey spectra of TiO2 and sample 90N before Ar + etching (for easier comparison, only TiO2 and 90N XPS spectra are presented in Fig. 2). From the survey scan, XPS peaks related to Ti, O, N and adventitious C were observed for the as-prepared 90N sample. The Ti2p spectra in Fig. 2(b) show that the as-prepared undoped TiO2 consists of two Ti 4+ located at 459.5 and 465.3 eV, which agrees with the literature [START_REF] Nawaz | Stabilization of Pickering emulsion with surface-modified titanium dioxide for enhanced photocatalytic degradation of Direct Red 80[END_REF], while these two peaks are slightly shifted to 459.2 and 465.0 eV after nitrogen doping [START_REF] Xing | New approaches to prepare nitrogen-doped TiO2 photocatalysts and study on their photocatalytic activities in visible light[END_REF]. This slight shift is due to an increase of the electron density on Ti as a result of the partial replacement of nitrogen with oxygen which shows a higher electronegativity [START_REF] Xing | New approaches to prepare nitrogen-doped TiO2 photocatalysts and study on their photocatalytic activities in visible light[END_REF]. A similar trend is also shown for the O1s XPS spectra as illustrated in Fig. 2(c). The presence of nitrogen into the lattice also shifts the O1s peaks from 532.2 and 530.8 eV to 531.9 and 530.5 eV, which correspond to the bridging hydroxyls and lattice oxygen respectively [START_REF] Nawaz | Stabilization of Pickering emulsion with surface-modified titanium dioxide for enhanced photocatalytic degradation of Direct Red 80[END_REF]. The accurate content of nitrogen was calculated after Ar + etching of the sample surface for 600 s to remove contaminants, if there were any. The high-resolution XPS spectra of N1s peaks of 10N to 40N thin films are presented in Fig. 3(a). As seen, at low N2:O2 ratios, two N1s peaks located above 400 eV are present which are assigned to chemisorbed molecular nitrogen (N-O and N-C)

or interstitially doped nitrogen [START_REF] Farkas | Optical, compositional and structural properties of pulsed laser deposited nitrogen-doped Titanium-dioxide[END_REF]. As illustrated in Fig. 3(b), by increasing the N2:O2 ratio, an additional N1s located at 396 eV appears for 50N-90N samples, which corresponds to substitutional N doping [START_REF] Lynch | Substitutional or Interstitial Site-Selective Nitrogen Doping in TiO2 Nanostructures[END_REF]. Some researchers have concluded that only substitutional doping leads to an increase of photocatalytic activity [START_REF] Asahi | Visible-light photocatalysis in nitrogen-doped titanium oxides[END_REF][START_REF] Morikawa | Band-gap narrowing of titanium dioxide by nitrogen doping[END_REF][START_REF] Hoang | Visible light driven photoelectrochemical water oxidation on nitrogen-modified TiO2 nanowires[END_REF]. Farkas et al. [START_REF] Farkas | Optical, compositional and structural properties of pulsed laser deposited nitrogen-doped Titanium-dioxide[END_REF] reported that substitutional N doping is responsible for shifting the absorption edge to lower photon energies and reducing the band gap of TiO2. [START_REF] Lee | Optimization and an insightful properties-Activity study of electrospun TiO2/CuO composite nanofibers for efficient photocatalytic H2 generation[END_REF]. No additional peaks were detected in the XRD pattern of pristine and N-doped TiO2, confirming the high purity of the synthesized anatase TiO2 films. The lack of common anatase to rutile phase transformation after nitrogen doping in this study might be due to either substitutional doping (rather than interstitial doping) and/or the low amount of N doping in the structure. It is reported that the anatase phase of TiO2 possesses a higher photocatalytic activity compared to the rutile and brookite phases [START_REF] Matsui | Epitaxial growth and characteristics of Ndoped anatase TiO 2 films grown using a free-radical nitrogen oxide source[END_REF]. The higher photocatalytic activity of the anatase phase can be explained by the longer lifetime of charge carriers in this phase [START_REF] Ozawa | Electron-Hole Recombination Time at TiO2 Single-Crystal Surfaces: Influence of Surface Band Bending[END_REF]. The high-temperature nitridation treatment of TiO2 is a widely-used method for the incorporation of nitrogen into the TiO2 lattice although high temperature causes the formation of unwanted side phases, defects, as well as degradation of the FTO substrate and sintering of the nanostructure [START_REF] Hoang | Visible light driven photoelectrochemical water oxidation on nitrogen-modified TiO2 nanowires[END_REF]. The method proposed in this work leads to the substitution of oxygen by nitrogen in the anatase TiO2 in a controlled manner and without phase change. As seen, all peak intensities are decreased by increasing the nitrogen to oxygen ratio during the film deposition. This results from the reduction of the film crystallinity by the incorporation of N into TiO2 [START_REF] Farkas | Optical, compositional and structural properties of pulsed laser deposited nitrogen-doped Titanium-dioxide[END_REF]. Nonetheless, even the 90N sample was crystalline and the (101) diffraction peak could be distinguished. This finding is inconsistent with some research works, where doping nitrogen into TiO2 lattice led to an amorphous structure [START_REF] Farkas | Optical, compositional and structural properties of pulsed laser deposited nitrogen-doped Titanium-dioxide[END_REF][START_REF] Baker | Effect of total gas pressure and O2/N2 flow rate on the nanostructure of N-doped TiO2 thin films deposited by reactive sputtering[END_REF]. Fig. 4(b) shows the enlarged view of the (101) peak of TiO2 and 50N-90N samples. Due to the slightly larger radius of N compared to O, this peak slightly shifts towards a lower angle after nitrogen substitution, confirming that N is successfully incorporated into the anatase TiO2 lattice [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF].

Optical properties

It is proven that nitrogen doping introduces impurity levels within the forbidden band gap and extends the optical absorption edge of TiO2 to the visible range [START_REF] Liu | Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity[END_REF]. Nitrogen doping has a profound effect on the photocatalytic performance of semiconductors. However the underlying principles behind its mechanism are still not fully understood [START_REF] Lynch | Substitutional or Interstitial Site-Selective Nitrogen Doping in TiO2 Nanostructures[END_REF][START_REF] Zeng | Comparative study on the visible light driven photocatalytic activity between substitutional nitrogen doped and interstitial nitrogen doped TiO2[END_REF][START_REF] He | The controllable synthesis of substitutional and interstitial nitrogen-doped manganese dioxide: the effects of doping sites on enhancing the catalytic activity[END_REF]. Fig. 5(a) illustrates the UV-Vis absorption spectra of pristine and N-TiO2 films. After N doping, the optical absorption edges of TiO2 show a red shift, indicating a better optical absorption capability of N-doped TiO2 in the visible light range. As illustrated in Fig. 5(a), increasing the N2 content in the N2:O2 mixture from 0 to 40% (bare TiO2, 10N-40N) also slightly increases absorption in the visible range, although a notable light absorption enhancement is only achieved above 50% of N2 in the N2:O2 mixture (enlarged in Fig. 5(a)). As confirmed by XPS results, below 50% N2, mainly interstitial nitrogen doping (~400 eV) is obtained and the signal supporting substitutional nitrogen doping (~396 eV) only emerges above 50% N2 content. Comparing two types of N-TiO2, Lynch et al. [START_REF] Lynch | Substitutional or Interstitial Site-Selective Nitrogen Doping in TiO2 Nanostructures[END_REF] reported that samples with a higher percentage of substitutional nitrogen yield a higher light absorption and a lower band gap. The band gap energy of TiO2 can be calculated by the Tauc plot from the intercept of the tangent fitted to the (𝛼ℎ𝜗) 1/2 vs ℎ𝜗 plot [START_REF] Farkas | Optical, compositional and structural properties of pulsed laser deposited nitrogen-doped Titanium-dioxide[END_REF][START_REF] Miao | Fabrication of black TiO 2/TiO 2 homojunction for enhanced photocatalytic degradation[END_REF] as shown in Fig. S1 in the Supporting Information. The band gap of pristine TiO2 was determined as 3.2 eV, which is consistent with the reported value for anatase TiO2 in the literature [START_REF] Ansari | Nitrogen-doped titanium dioxide (N-doped TiO 2) for visible light photocatalysis[END_REF]. As seen, the band gap of N-doped TiO2 is reduced to 3.1, 3.0, 2.9, 2.8 and 2.6 eV for 50N, 60N, 70N, 80N and 90N, respectively. The relationship between the amount of substitutional nitrogen doping and the band gap of TiO2 is depicted in Fig. 5(b). As shown in Fig. S2, bare TiO2 film is highly transparent while by increasing the N2:O2 ratio, the color gradually changes to brownish, which is consistent with UV-Vis absorption results. Since a meaningful enhancement in UV-Vis absorption was obtained by substitutional nitrogen doping, samples 50N-90N were selected for the fabrication of gradient-doped TiO2 in the ensuing experiments.

Morphological structure

The reproducibility as well as the precise film thickness and composition control represent three significant advantages of the PLD compared to the wet chemical methods [START_REF] Bricchi | Integration of plasmonic Au nanoparticles in TiO2 hierarchical structures in a singlestep pulsed laser co-deposition[END_REF][START_REF] Ghosh | Increasing photocurrents in dye sensitized solar cells with tantalum-doped titanium oxide photoanodes obtained by laser ablation[END_REF]. Moreover, the morphology, orientation and porosity of the film can be controlled by tuning the deposition parameters, such as working pressure and substrate temperature [START_REF] Bricchi | Integration of plasmonic Au nanoparticles in TiO2 hierarchical structures in a singlestep pulsed laser co-deposition[END_REF]. In this study at a constant substrate temperature (350°C), the g-N-TiO2 films were deposited at three different pressures (10, 20 and 30 Pa). As seen in Fig. 6(a-b), the g-N-TiO2 sample fabricated at 20 Pa pressure exhibits a vertically aligned columnar-like nanostructure, while the sample prepared at 10 Pa is compact and dense (Fig. S3(a)). By increasing the deposition pressure (30 Pa), the sample changes from a columnar-like to a disordered structure composed of nanoparticles (Fig. S3(b)). This is because the kinetic energy of the ablated species decreases due to their collisions with the nitrogen and oxygen gas molecules. Actually, the kinetic energy of ablated species determines the morphology, orientation and porosity of film [START_REF] Noh | Aligned photoelectrodes with large surface area prepared by pulsed laser deposition[END_REF]. At 10 Pa, the ablated species have a high kinetic leading to the densification of the thin film. In contrast, at 30 Pa, the ablated species experience multiple collisions before reaching the substrate, resulting in the formation of a film with loose interparticle connections (as seen in Fig. S3(b)). The surface roughness (root mean square roughness measured by AFM), was ~3 nm for the sample prepared at 10 Pa (Fig. 6(c-d)) which increased to ~7 nm and ~10 nm by increasing the deposition pressure to 20 Pa and 30 Pa (Fig. S4). By using the XRR method (Fig. S5), the density of the deposited samples at 10, 20 and 30 Pa were calculated to be 3.16, 2.91 and 2.56 g/cm 3 , which corresponded to film porosities of 19.2%, 25.6% and 34.5%, respectively. It is clear that the films are less compact as compared to the anatase TiO2, which has a reported density of 3.91 g/cm 3 [START_REF] Di Fonzo | Hierarchically organized nanostructured TiO2 for photocatalysis applications[END_REF]. Differential reflected X-ray intensity graph (Fig. S6) was used for the precise measurement of the critical angle since the accuracy of density measurements by using XRR method highly depends on the critical angle accuracy. It is reported that increasing the porosity can enhance the specific surface area and the number of active catalytic sites and also, increase the light absorption by the formation of scattering centers inside the porous structures [START_REF] Antony | Synergistic effect of porosity and gradient doping in efficient solar water oxidation of catalyst-free gradient Mo: BiVO4[END_REF]. Highly porous photocatalysts suffer from the lack of robustness and possess high trap states and interfacial recombination centers, whereas one-dimensional columnar-like TiO2 structure reduces charge recombination rate, resulting in enhanced charge-collection efficiency [START_REF] Noh | Aligned photoelectrodes with large surface area prepared by pulsed laser deposition[END_REF].

Compared to the columnar-like structure (prepared at 20 Pa), the dense film (prepared at 10 Pa) as well as the film composed of loosely attached nanoparticles (prepared at 30 Pa) hindered the charge carrier separation by bulk or boundary recombination of charge carriers as was confirmed by the higher PEC results for samples prepared with different density by the PLD process (Fig. S7). More details are provided in Section 3.4.

Photoelectrochemical performance

The effects of morphology on the photoelectrochemical performance of semiconductors were reported in several studies [START_REF] Antony | Synergistic effect of porosity and gradient doping in efficient solar water oxidation of catalyst-free gradient Mo: BiVO4[END_REF][START_REF] Di Fonzo | Hierarchically organized nanostructured TiO2 for photocatalysis applications[END_REF][START_REF] Sauvage | Hierarchical TiO2 photoanode for dye-sensitized solar cells[END_REF]. Noh et al. [START_REF] Noh | Aligned photoelectrodes with large surface area prepared by pulsed laser deposition[END_REF] concluded that vertically-aligned onedimensional structure has a profound effect on the electron dynamics of dye-sensitized solar cells.

One-dimensional columnar-like structures have several advantages, such as increased surfacearea-to-volume ratio and stronger light trapping capability, as compared to compact films. In addition, with an anisotropic morphology, similar to nanowires and nanotubes, the charge transfer along the vertically-aligned direction is facilitated and the mass transfer limitation is reduced by providing readily accessible porosity for diffusion of contaminant molecules and/or hydroxyl ions to generate highly reactive hydroxyl radicals ( • OH) [START_REF] Di Fonzo | Hierarchically organized nanostructured TiO2 for photocatalysis applications[END_REF]. Fabricating one-dimensional TiO2 by PLD, Sauvage et al. [START_REF] Sauvage | Hierarchical TiO2 photoanode for dye-sensitized solar cells[END_REF] reported an increase by more than one order of magnitude in the lifetime of photo-induced electrons compared to a TiO2 film consisting of 20 nm nanocrystalline particles.

In the present work, in order to investigate the effect of film density on the photocatalytic activity, the PEC performance of g-N-TiO2 prepared at 10, 20 and 30 Pa was evaluated. As seen in Fig. S7(a), by increasing the deposition pressure from 10 to 20 Pa and changing the morphology from dense to columnar-like nanostructure, a considerable improvement of the photocurrent density was obtained in cyclic on/off current-time (I-t) experiment. As mentioned before, the sample prepared at 10 Pa possesses a dense structure leading to the bulk recombination of electron and holes.

Furthermore, the light harvesting might be limited for dense structure compared to porous films [START_REF] Li | Hierarchically structured porous materials for energy conversion and storage[END_REF]. A low photocatalytic activity of dense TiO2 prepared by PLD was also previously reported [START_REF] Di Fonzo | Hierarchically organized nanostructured TiO2 for photocatalysis applications[END_REF]. The highest photocurrent was achieved for the sample prepared at 20 Pa. With further pressure increased to 30 Pa, the photocurrent density dropped considerably due to the high recombination rate of electrons and holes at numerous nanoparticles' boundaries. the light was switched on, which is due to the accumulation of photo-induced holes at the photocatalysts/electrolyte interface without electrolyte injection [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF]. These anodic spikes became less evident at higher potentials where the applied bias forced the photo-induced holes to inject to the electrolyte [START_REF] Dotan | Probing the photoelectrochemical properties of hematite (α-Fe 2 O 3) electrodes using hydrogen peroxide as a hole scavenger[END_REF]. The anodic spikes could be an evidence of facile transport of holes to the interface where the photo-induced holes encounter the injection barrier. As demonstrated in Fig. S8, once the 0.5M H2O2 as hole scavenger added to the electrolyte, the positive current transients vanished. Moreover, as expected, the addition of H2O2 to the electrolyte resulted in an improvement of photocurrent density, which further confirmed the presence of the injection barrier [START_REF] Denisov | Effect of different hole scavengers on the photoelectrochemical properties and photocatalytic hydrogen evolution performance of pristine and Pt-decorated TiO2 nanotubes[END_REF]. In this study, the samples prepared at 20 Pa provided the best morphology and PEC performance. Therefore, TiO2, N-TiO2, and g-N-TiO2 samples were deposited on FTO at 20 Pa to evaluate the effect of gradient doping on PEC and photocatalytic performance. Fig. 7(a) depicts the photocurrent-time (I-t) plot of TiO2, N-TiO2 and g-N-TiO2 samples under intermittent on-off illumination. Under one sun irradiation and by applying 0.2 V vs. Ag/AgCl bias and in 0.2 M Na2SO4 solution, the maximum photocurrent density of ~0.028 mA/cm 2 was achieved for the g-N-TiO2 photoanode which is about 211% and 102% higher than that of the pristine TiO2 and N-TiO2, respectively. The photoanodes revealed an instantaneous decay of the photocurrent in dark condition and a rapid recovery to the initial photocurrent density under illumination in multiple on/off cycles, which is attributed to the fast charge carrier transport in the one-dimensional structure. The dark photocurrent density for all the photoanodes was ~0.1 μA/cm 2 at a bias of 0.2 V vs Ag/AgCl, indicating that the measured photocurrent was exclusively due to the photo-induced charges from the photoanodes. indicating the typical photoelectrochemical property of n-type semiconductors. Similar to I-t results, while the dark current was insignificant, a notable photocurrent density was gained for the g-N-TiO2 sample by applying a positive bias vs. Ag/AgCl and one sun irradiation, which was significantly higher than those achieved by N-TiO2 and TiO2 samples. EIS Nyquist measurements were also conducted under dark and illuminated conditions to observe the electrical resistance. A notable decrease in the diameter of N-TiO2 was observed and the lowest diameter was obtained for the g-N-TiO2 sample, which further confirms a lower resistance of g-N-TiO2 against charge carrier transfer as a result of terraced band bending. As mentioned before, TiO2, N-TiO2 and g-N-TiO2 samples are prepared at a similar deposition condition and possess similar density (2.91 g/cm 3 ). The UV-Vis spectra of these samples are shown in Fig. S9. Interestingly, despite a slight lower light absorption of g-N-TiO2 compared to N-TiO2 sample, the photocatalytic activity of g-N-TiO2 was significantly higher than that of TiO2 and N-TiO2 samples. Therefore, it can be inferred that this improvement is mainly due to the enhanced separation of photo-induced electrons and holes by the formation of homojunction with terraced band bending. It is proven that nitrogen as a non-metal dopant, introduces an acceptor level above the valance band of TiO2 as a n-type semiconductor and lowers the Fermi level (EF) of TiO2 [START_REF] Hoang | Visible light driven photoelectrochemical water oxidation on nitrogen-modified TiO2 nanowires[END_REF]. Therefore, a band bending is induced when different layers (with different EF) are brought into contact [START_REF] Abdi | Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode[END_REF][START_REF] Wang | Gradient self-doped CuBi2O4 with highly improved charge separation efficiency[END_REF]. For non-gradient nitrogen doped sample (N-TiO2), the band bending is limited to a narrow region at the semiconductor/electrolyte interface (depletion region) [START_REF] Luo | Gradient doping of phosphorus in Fe 2 O 3 nanoarray photoanodes for enhanced charge separation[END_REF]. However, in g-N-TiO2 sample, after contacting N-TiO2 layers with different N content, the Fermi energy levels equilibrate by flowing the electron from nitrogen rich layers to layer with lower nitrogen content and finally the pristine TiO2 layer [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF]. More importantly, in gradient-doped TiO2, the terraced valence band level for different layers induces a built-in electric field perpendicular to the substrate, which facilitates the transfer of charge carriers by repelling the electrons and holes in different directions along the vertical direction of columnar-like structure [START_REF] Wang | Gradient self-doped CuBi2O4 with highly improved charge separation efficiency[END_REF][START_REF] Wu | Gradient Sn-Doped Heteroepitaxial Film of Faceted Rutile TiO2 as an Electron Selective Layer for Efficient Perovskite Solar Cells[END_REF][START_REF] Liu | Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity[END_REF]. By measuring the electron conductivity of homogenously doped and gradient-doped hematite (α-Fe2O3), Luo et al. [START_REF] Luo | Gradient doping of phosphorus in Fe 2 O 3 nanoarray photoanodes for enhanced charge separation[END_REF] concluded that the built-in electric field is responsible for the enhanced photocurrent generation. Fabrication of a heterojunction to induce band bending is a well-established strategy to promote the charge separation in photocatalysis even though a higher efficiency of gradient doped homojunction was demonstrated in recent years [START_REF] Wang | N doping to ZnO nanorods for photoelectrochemical water splitting under visible light: engineered impurity distribution and terraced band structure[END_REF][START_REF] Wang | Gradient self-doped CuBi2O4 with highly improved charge separation efficiency[END_REF][START_REF] Wu | Gradient Sn-Doped Heteroepitaxial Film of Faceted Rutile TiO2 as an Electron Selective Layer for Efficient Perovskite Solar Cells[END_REF][START_REF] Liu | Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity[END_REF][START_REF] Rasouli | Gradient doping of copper in ZnO nanorod photoanode by electrodeposition for enhanced charge separation in photoelectrochemical water splitting[END_REF][START_REF] Liu | TiO2 photoanode structure with gradations in V concentration for dye-sensitized solar cells[END_REF][START_REF] Antony | Synergistic effect of porosity and gradient doping in efficient solar water oxidation of catalyst-free gradient Mo: BiVO4[END_REF][START_REF] Cheng | A facile and novel strategy to synthesize reduced TiO2 nanotubes photoelectrode for photoelectrocatalytic degradation of diclofenac[END_REF]. In contrast to heterojunction -where the band bending is limited to the interfaces-the gradient-doped homojunction distributes the band bending over a larger region in the bulk of the sample and thus, promotes the charge separation efficiency [START_REF] Abdi | Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode[END_REF][START_REF] Wang | Gradient self-doped CuBi2O4 with highly improved charge separation efficiency[END_REF]. Moreover, lattice mismatch and interfacial defects are intrinsically reduced for gradient-doped TiO2 homojunction prepared by PLD due to layer-by-layer epitaxial growth of TiO2 with the sample lattice parameters but different EF. In other words, gradient-doped homojunction structure distributes a moderate band bending over the photocatalyst's bulk without interfacial defects between layers. It is reported that the lattice matching in homojunction facilitates the spatial transfer of the photo-induced electron and holes and reduces the electrical resistance in the photocatalyst structure [START_REF] Miao | Fabrication of black TiO 2/TiO 2 homojunction for enhanced photocatalytic degradation[END_REF]. To evaluate the stability of g-N-TiO2 photocatalysts, the long-term I-t experiment under one sun irradiation was carried out. As shown in Fig. S10 in the Supplementary Information, no significant decline in the current density was shown within 8 h, suggesting the high stability of g-N-TiO2 sample.

Photocatalytic performance

Considering the low specific surface area of thin films, the photocatalytic removal efficiency of contaminants in water is usually low in comparison with slurry systems containing nanoparticles.

In order to suppress the recombination of charge carriers and increase the degradation efficiency, in thin-film photocatalytic systems, an external potential bias is applied to the photocatalyst serving as the photoanode in the PEC process [START_REF] Daghrir | Photoelectrocatalytic degradation of carbamazepine using Ti/TiO2 nanostructured electrodes deposited by means of a pulsed laser deposition process[END_REF]. Despite the exciting reported results for the degradation of recalcitrant pollutants with PEC, this process requires applying electrical energy, expensive electrode materials (such as platinum) and also conductive substrate which impose high operating cost to the process. Moreover, PEC needs the addition of a relatively high concentration of salt to the solution (mostly sulfates) as electrolytes, which raises an additional environmental concern. Therefore, increasing the efficiency of thin-film photocatalysts without such costly and/or environmentally unsafe materials, is of great importance. In this study, the photocatalytic performance of photocatalysts was evaluated with the degradation of SMX antibiotic under simulated solar light irradiation. Fig. 8(a) shows the normalized degradation curve of SMX with photolysis, and photocatalysis by using TiO2, N-TiO2 and g-N-TiO2 films. As seen, no significant reduction in the concentration of SMX was achieved by the photolysis process. This is because of the low absorption of light by SMX in the solar spectrum range as previously reported [START_REF] Mestre | Photocatalytic Degradation of Pharmaceuticals Carbamazepine, Diclofenac, and Sulfamethoxazole by Semiconductor and Carbon Materials: A Review[END_REF]. As expected, the removal of SMX by pristine TiO2 is insignificant and only ~19% SMX removal is achieved after 300 min treatment. The low removal efficiency of TiO2 for the photocatalytic degradation is due to the low light absorption of TiO2 in the solar spectrum. Fig. 8(b) shows that photocatalytic degradation of SMX follows a pseudo-first-order kinetics and the photocatalytic kinetic constant (k) by using TiO2 photocatalyst is 0.0006 (min -1 ). In contrast, after 300 min the removal percentage of SMX by using N-TiO2 is ~40 % with a kinetic constant of 0.0017 (min -1 ).

The highest removal efficiency is obtained by using g-N-TiO2 with ~85 % removal after 300 min and the kinetic constant is 0.0061 (min -1 ) which is ten times higher than that of TiO2 photocatalyst.

The improvement in the photocatalytic degradation performance of g-N-TiO2 is consistent with the improvement of its PEC performance. The reason of this dramatic enhancement in photocatalytic degradation efficiency is due to the promotion of charge carriers' separation by the built-in oriented electric field as a result of terraced band bending along with vertical direction of TiO2 as reported before [START_REF] Liu | Vacancy-doped homojunction structural TiO2 nanorod photoelectrodes with greatly enhanced photoelectrochemical activity[END_REF]. As shown in Fig. S8, the transferred and accumulated holes on the surface of g-N-TiO2 can directly degrade SMX molecules by oxidizing them or indirectly by generation of reactive species such as • OH radicals. As a result, a superior enhancement of the photocatalytic performance for g-N-TiO2 was achieved in terms of SMX degradation, kinetics of removal, and solution detoxification. As presented in Table 1, the obtained removal efficiency in this study is among the highest reported values in previous studies of photocatalytic degradation of pharmaceuticals by using thin films. Moreover, the SMX removal efficiency after 300 min, 600 min and 900 treatments by using TiO2, N-TiO2 and g-N-TiO2 in three consecutive runs was compared to evaluate the stability and reusability of photocatalysts. As seen in Fig. 8(c), all tested photocatalysts are very stable and maximum ~12 % loss in the SMX removal efficiency was shown after three cyclic experiments.

Transformation product and mechanism of degradation

In order to identify the TPs during the photocatalytic degradation of SMX, first, LC-MS chromatograms of the samples obtained at different time intervals are compared with those of the untreated sample. The new peaks in the chromatograms of samples are considered as possible transformation products, and according to the "suspect screening" method [START_REF] Schymanski | Non-target screening with high-resolution mass spectrometry: critical review using a collaborative trial on water analysis[END_REF], the MS/MS spectra of the suspected compounds are acquired and compared with the reported TPs in the literature. Accordingly, fifteen TPs were identified during the treatment by TiO2, N-TiO2 and g-N-TiO2. As shown in Fig. 9(a-c), relatively short-chain TPs such as TP 107, TP 114 and TP 149

were not detected with TiO2, while long-chain TPs such as TP 286 and TP 289 (Fig. 9 (n-o The cleavage of benzene ring from sulfur (S) led to the formation of TP 163, while the breakage of isoxazole from nitrogen (N) formed TP 172. TP 163 can proceed to the formation of TP 149 by N-TiO2 and g-N-TiO2 and thereafter, to TP 114 by g-N-TiO2 photocatalyst. Previous studies have also proved the ability of hydroxyl radicals for oxidation of the amino group [START_REF] Hiller | Mechanism of the hydroxyl radical induced oxidation of methionine in aqueous solution[END_REF][START_REF] Wang | Comparative study on sulfamethoxazole degradation by Fenton and Fe (II)activated persulfate process[END_REF] which is shown for TP 195 and TP 284. TPs 163 and 172 were detected for all photocatalysts although the peak intensities of these TPs by using g-N-TiO2 were higher than that of obtained by TiO2 and N-TiO2. Moreover, the successive isoxazole ring cleavage and denitrification (TP 174) and further oxidation of the isoxazole ring (TP 114) were only observed by using the g-N-TiO2 photocatalyst.

This may be due to the improved ability of g-N-TiO2 to generate reactive species and thus, to transform SMX into smaller organic compounds. TP 195 [START_REF] Wang | Comparative study on sulfamethoxazole degradation by Fenton and Fe (II)activated persulfate process[END_REF], TP 270 [START_REF] Du | Hydroxyl radical dominated degradation of aquatic sulfamethoxazole by Fe0/bisulfite/O2: Kinetics, mechanisms, and pathways[END_REF] and TP 289 [START_REF] Ray | Insight into sulfamethoxazole degradation, mechanism, and pathways by AgBr-BaMoO4 composite photocatalyst[END_REF] were only found by using N-TiO2, which have been reported before as transformation products of SMX degradation. Denitrification of the benzene ring (TP 239) and further oxidation of the isoxazole ring (TP 273) were detected for all photocatalysts which have been reported in several studies [START_REF] Ray | Insight into sulfamethoxazole degradation, mechanism, and pathways by AgBr-BaMoO4 composite photocatalyst[END_REF][START_REF] Su | Biodegradation of sulfamethoxazole photo-transformation products in a water/sediment test[END_REF]. TP 199 was also found for all photocatalysts which has been observed in other works [START_REF] Geis-Asteggiante | Structural characterization of product ions by electrospray ionization and quadrupole time-of-flight mass spectrometry to support regulatory analysis of veterinary drug residues in foods[END_REF][START_REF] Długosz | Photocatalytic degradation of sulfamethoxazole in aqueous solution using a floating TiO2-expanded perlite photocatalyst[END_REF].

In general, four degradation mechanism might be involved in the degradation of SMX in this study including: (i) bond cleavage; (ii) oxidation; (iii) denitrification; and (iv) desulfonation or a combination of these pathways as illustrated in Fig. 10. The ToF/MS-MS spectra of detected TPs are provided in the Supplementary Information as Fig. S11.

Toxicity assessment

As seen in Figs. 9 and 10, several TPs were formed during the photocatalytic degradation of SMX.

Some research works have reported equivalent or higher toxicity of the mixture of SMX and its TPs compared to SMX alone [START_REF] Qi | Degradation of sulfamethoxazole by microwaveactivated persulfate: Kinetics, mechanism and acute toxicity[END_REF][START_REF] Gmurek | Phototransformation of sulfamethoxazole under simulated sunlight: Transformation products and their antibacterial activity toward Vibrio fischeri[END_REF]. Therefore, the toxicity of the solution after treatment should be assessed in addition to the SMX removal. Fig. 11 compares the optical density of solutions treated by TiO2, N-TiO2 and g-N-TiO2 with the untreated SMX solution as well as the control sample (without SMX) at two time intervals. As expected, a high optical density (bacteria growth) was noticed for the control sample, whereas the untreated solution significantly inhibited the growth of E. coli bacteria. Due to the high concentration of remaining SMX in the solution, an insignificant detoxification was achieved by using TiO2. A relatively high detoxification occurred by using N-TiO2. Remarkably, a notable, further enhancement in detoxification was achieved for g-N-TiO2 as photocatalyst, with considerable bacteria growth and optical density even comparable to the case without any SMX. It is to say that the effective photocatalysis of SMX by g-N-TiO2 led to a basically non-toxic environment. The lower detoxification efficiency of the SMX solution with TiO2 and N-TiO2 as compared to g-N-TiO2 might either result from the higher concentration of not-degraded SMX, or be due to the presence of some TPs with relatively similar structures to SMX (such as TP 195, TP 270, TP 286 and TP 289) which may still have antibacterial properties.

Fig. 11. Optical density measurements of E. coli. bacteria at a wavelength of 600 nm (OD600) for samples treated by TiO2, N-TiO2 and g-N-TiO2 as well as control and untreated solutions.

Fig. 12 illustrates the growth of bacterial colonies on the agar surface. As shown, the surface of agar corresponding to the control sample was completely covered by E. coli colonies, whereas the untreated SMX solution (C0) obviously hindered bacterial growth. The presence of small colonies of E. coli in the case of untreated SMX solution is due to the relatively low concentration of SMX in this study (initial solution), which allows bacterial growth to some extent (background growth).

As seen in Fig. 12(c) in addition to the background growth, some new colonies appeared on the agar surface as indicated with red circles. However, the number of colonies was limited for the solution treated by TiO2 consistently with the optical density (Fig. 11) and with the SMX degradation results (Fig. 8). Although a noticeable bacterial growth was observed for the treated solution with N-TiO2, the highest detoxification was achieved for the sample treated by g-N-TiO2.

As seen in Fig. 12(e), the E. coli growth on g-N-TiO2 plate was almost similar to the control petri dish, which further confirmed the high efficiency of g-N-TiO2 for detoxification of antibiotic solution.

Conclusions

In summary, by using pulsed laser deposition, we synthesized substitutional nitrogen doped TiO2

  18 minutes. Data were acquired in information-dependent acquisition (IDA) mode, with time-offlight mass spectrometry (TOF-MS) survey acquisition from m/z 80-925, followed by MS/MS on the 3 most intense ions from m/z 200-450 with dynamic background subtraction with a total cycle time of 1.0 s. The MS and MS/MS spectra were collected on a hybrid quadrupole-time-of-flight (QqTOF) TripleTOF 5600 mass spectrometer (SCIEX, Concord, ON, Canada) equipped with a DuoSpray ion source in the positive ion mode set at 5 kV source voltage, 450 °C source temperature, and 50 psi for GS1/GS2 gas flows. QqTOF offers abundant structural information

Fig. 2 .

 2 Fig. 2. (a) XPS survey spectra, (b) XPS spectra of Ti2p and (c) XPS spectra of O1s of TiO2 and sample 90N.

Fig. 3 .

 3 Fig. 3. (a) High-resolution N1s XPS spectra of the N-doped TiO2 films prepared in N2:O2 ratio lower than 50% (10N-40N), and (b) equal to or higher than 50% (50N-90N) after 600s Ar + etching.

Fig. 4 (

 4 Fig. 4(a) shows the XRD patterns of 50N-90N samples as well as pristine TiO2. As mentioned

Fig. 4 .

 4 Fig. 4. (a) XRD patterns of the films deposited at 350 °C substrate temperature at different N2:O2 ratio (50N-90N); (b) enlarged XRD patterns showing the (101) diffractions of the samples changed from 24° to 27°.

Fig. 5 .

 5 Fig. 5. (a) UV-Vis absorption spectra of various samples prepared at different N2:O2 ratios (10N-90N), (b) changing of the N atomic percentage and band gap of TiO2 by N2:O2 ratio during deposition.

Fig. 6 (

 6 Fig. 6 (a) Side view and (b) top view SEM images; (c, d) AFM topographic image of g-N-TiO2 film prepared at 20 Pa.

  Fig.S7(b)shows the linear sweep voltammetry (LSV) photocurrent density under on/off cyclic conditions. All the samples showed negligible dark current in the positive applied bias voltage vs. Ag/AgCl. In accordance with I-t results, the sample prepared at 20 Pa showed the highest current density in LSV experiment under one sun irradiation. The sample prepared at 30 Pa obtained the lowest current density despite of its higher porosity which allows a deeper penetration of electrolyte into the film and perhaps, more absorption due to multiple scattering. As seen in Fig.S3(b), for the film prepared at 30 Pa, the physical contact between the nanoparticles is not good enough for efficient electron transfer, which provides a higher chance of electron trapping at the interface between the TiO2 nanoparticles[START_REF] Noh | Aligned photoelectrodes with large surface area prepared by pulsed laser deposition[END_REF][START_REF] Kang | Nanorod-based dyesensitized solar cells with improved charge collection efficiency[END_REF]. The dark and illuminated Nyquist plots of electrochemical impedance spectroscopy (EIS) are illustrated in Fig.S7(c). A lower diameter of semicircle of EIS plots under dark and illuminated conditions further confirms the lower charge transfer resistance of the g-N-TiO2 sample prepared at 20 Pa. For the sample prepared at 20 Pa, the photocurrent in the I-V curve experienced sharp anodic spikes (positive current transients) after

Fig. 7 .

 7 Fig. 7. (a) On/off cycle of photocurrent density (I-t) performed in 0.2 M Na2SO4 solution, (b) chopping linear sweep voltammetry (LSV) curves and (c) Nyquist plots of EIS measurements of TiO2, N-TiO2 and g-N-TiO2. 0.2 V vs. Ag/AgCl was applied for I-t and EIS experiments.

Fig. 7 (

 7 Fig. 7(b) shows the I-V plot of samples under cyclic dark and illumination conditions. The

Fig. 8 .

 8 Fig. 8. (a) Normalized degradation curves of SMX by TiO2, N-TiO2 and g-N-TiO2; (b) pseudo-first-order kinetics model, and inset: corresponding kinetic constants (k) during the photocatalytic process; (c) recycling tests of photocatalytic degradation of SMX after 300 min, 600 min and 900 min treatments by using TiO2, N-TiO2 and g-N-TiO2.

  )) were not observed during treatment by g-N-TiO2. The presence of short-chain TPs in the samples treated by g-N-TiO2 suggests the capability of this photocatalyst to generate more reactive species such as • OH radicals, which leads to the consecutive breaking of SMX into shorter TPs. In contrast, insufficient reactive radicals with TiO2 and N-TiO2 cause oxidation (but not bond cleavage) of the benzene and isoxazole rings (TP 286) and denitrification and oxidation of benzene ring as well as oxidation of the isoxazole ring (TP 289).

Fig. 9 .Fig. 10 .

 910 Fig. 9. Evolution of (a) TP 107, (b) TP 114, (c) TP 149, (d) TP 163, (e) TP 172, (f) TP 174, (g) TP 190, (h) TP 195, (i) TP 199, (j) TP 239, (k) TP 270, (l) TP 273, (m) TP 284, (n) TP 286, and (o) TP 289, during the photocatalytic degradation of SMX by TiO2, N-TiO2 and g-N-TiO2.

Fig. 12 .

 12 Fig. 12. Growth of E. coli. bacteria on petri dish for (a) control, (b) untreated SMX solutions and samples treated by (c) TiO2, (d) N-TiO2 and (e) g-N-TiO2.

  at low temperature without the common phase transformation of anatase to rutile. The results showed that substitutional doping is more effective than interstitial doping for enhancing light absorption by the photocatalyst. Moreover, compared to dense and highly porous structures, columnar-like structures showed more photocurrent generation and separation efficiency. Precise control of the deposition condition of the PLD allowed us to grow TiO2 layers with different N content at the optimum deposition pressure to fabricate multi-homojunction gradient-doped TiO2 (g-N-TiO2). In comparison with pristine and conventional nitrogen doped TiO2 (non-gradient), g-N-TiO2 demonstrated a superior performance in terms of photocurrent density, SMX removal, kinetic of degradation and detoxification of solution. The obtained photocatalytic degradation efficiency obtained in the present study is as high as with a photoelectrochemical process without applying external bias potential and introducing salt as electrolyte. The markedly enhanced performance of g-N-TiO2 is attributed to (i) substitutional nitrogen doped into the anatase phase with enhanced light absorption, (ii) fabrication of layer-by-layer homojunction with low interfacial defects, (iii) porous, well-aligned and one-dimensional structure favoring charge transport and (iv) presence of terraced band structure over the entire thickness of film, which induces an oriented electrical field and facilitates charge carrier separation and transport.

Table 1 .

 1 Comparison of SMX removal efficiency in this study with the values reported in the literature for the removal of pharmaceuticals by using thin film photocatalysts.

	(a)	(b)	(c)	(d)	(e)
		Photocatalysts	Contaminants	Light source	Remarks	Ref.
		Bi2MoO6-boron-doped diamond hybrid electrode	Ibuprofen C0= 10 mg/L	150 W Xe lamp	 About 30% after 6h ibuprofen removal	[64]
		N-doped TiO2	Chlortetracycline C0= 0.1 mg/L	150 W xenon lamp	 81.3% removal after 3 h	[65]
		BiOBr deposited on FTO	Tetracycline C0= 10 ppm	(λ > 420 nm) 300-W Xe arc lamp	 39% removal after 2.5 h	[66]

Acknowledgements

The authors are grateful for financial support provided by Environment Damages Fund (EDF-CA-2017i001). A.M. is also grateful from the Fonds de recherche du Québec -Nature et technologies (FRQNT) for scholarship (282732). M.C. and D.M. thanks the Canada Research Chairs Program for its support.

Conflicts of interest

There are no conflicts to declare.