Syreina Sayegh

Mahmoud Abid

Fida Tanos

Marc Cretin

Geoffroy Lesage

François Zaviska

Eddy Petit

Bruno Navarra

Igor Iatsunskyi

Emerson Coy

Roman Viter

Victoriia Fedorenko

Arunas Ramanavicius

Antonio Razzouk

Juliette Stephan

Mikhael Bechelany
email: mikhael.bechelany@umontpellier.fr

N-doped TiO2 nanotubes synthesized by atomic layer deposition for acetaminophen degradation

Keywords: TiO2 nanotubes, N-doped TiO2, atomic layer deposition, photocatalysis, acetaminophen Graphical

Titanium dioxide (TiO2) is widely used in photocatalysis applications for wastewater treatment. Investigations demonstrated that titanium dioxide structure strongly contributes to increase micropollutant degradation efficiency. Indeed, pollutant degradation rates are enhanced when using photocatalysts with highly ordered structures and high specific surface area, such as TiO2 nanotubes (NTs) synthesized by atomic layer deposition (ALD). Here, TiO2

NTs, fabricated by ALD followed by nitrogen doping via thermal treatment, were compared with TiO2 nanofibers (NFs), prepared by electrospinning. Their morphology and structure were investigated by scanning and transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Photoluminescence measurements showed that TiO2 NT photoluminescence intensity was lower than that of TiO2 NFs, due to lower electron-hole pair recombination, and consequently their degradation efficiency was higher. As the surface to volume ratio was higher in TiO2 NTs than NFs, the rate of non-radiative surface recombination also was higher in TiO2 NTs. Comparison of their performance for photocatalytic degradation of acetaminophen showed higher degradation activity with TiO2 NTs than TiO2 NFs. TiO2 NT doping with nitrogen (N-TiO2 NTs) further enhanced their photocatalytic activity that was 5 times higher than that of TiO2 NFs (degradation rates: 0.05 and 0.01 mg.L -1 .min -1 , respectively).

The N-TiO2 NT photocatalyst was stable after four cycles of acetaminophen degradation. Acute toxicity assays confirmed the release of harmful by-products during the first hours of acetaminophen degradation, but toxicity strongly decreased after 5 hours.

Introduction

The pressing increase in clean water demand has led researchers to focus on developing new technologies for water treatment because the conventional systems cannot efficiently degrade micropollutants (e.g. drugs, pesticides, personal care products, dyes) and eliminate microorganisms [START_REF] Takeuchi | Review of plasma-based water treatment technologies for the decomposition of persistent organic compounds[END_REF][START_REF] Gehrke | Innovations in nanotechnology for water treatment[END_REF] that can resist to the conventional methods and then contaminate drinking water [START_REF]Pharmaceuticals in drinking-water[END_REF][START_REF] Houtman | Emerging contaminants in surface waters and their relevance for the production of drinking water in Europe[END_REF] . Although these contaminants are usually present at very low concentrations, they can harm human health and other living organisms [START_REF] Tran | Occurrence and fate of emerging contaminants in municipal wastewater treatment plants from different geographical regions-a review[END_REF][START_REF] Nam | Occurrence and removal of selected micropollutants in a water treatment plant[END_REF] . For instance, acetaminophen (ACT) is a pharmaceutical product used as painkiller that have been detected in the range of ppm to ppt and has significant negative impacts to the aquatic and ecological systems [START_REF] Vo | Acetaminophen micropollutant: Historical and current occurrences, toxicity, removal strategies and transformation pathways in different environments[END_REF] .

The main challenge is the development of new removal techniques that are cheap and require less energy consumption. Advanced oxidation processes (AOP), particularly photocatalysis, are good candidates at the place or in combination with conventional techniques [START_REF] Stasinakis | Use of selected advanced oxidation processes (AOPs) for wastewater treatment-a mini review[END_REF][START_REF] Rodriguez-Narvaez | Treatment technologies for emerging contaminants in water: A review[END_REF][START_REF] Teodosiu | Emerging pollutants removal through advanced drinking water treatment: A review on processes and environmental performances assessment[END_REF] . Photocatalysis is a green technology in which, pollutants under visible light irradiation are degraded to CO2, H2O, and in some cases small non-toxic molecules , as final degradation products. [START_REF] Friedmann | TiO2 for water treatment: parameters affecting the kinetics and mechanisms of photocatalysis[END_REF][START_REF] Chong | Recent developments in photocatalytic water treatment technology: A review[END_REF] In this system, upon irradiation, the semiconductor is activated by photon energy that is equal to or higher than its bandgap energy. This leads to the formation of electron-hole pairs between the conduction and valence bands that will then generate radicals implicated in the pollutant mineralization [START_REF] Ameta | Chapter 6 -Photocatalysis[END_REF][START_REF] Serpone | Kirk-Othmer Encyclopedia of Chemical Technology[END_REF] . Titanium dioxide (TiO2) is among the most used semiconductors. TiO2 is not expensive and displays good chemical and thermal stability and low toxicity [START_REF] Gaya | Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems[END_REF][START_REF] Hong | Comprehensive study on the morphology control of TiO2 nanorods on foreign substrates by the hydrothermal method[END_REF][START_REF] Ramanavicius | Insights in the application of stoichiometric and nonstoichiometric titanium oxides for the design of sensors for the determination of gases and VOCs (TiO2-x and TinO2n-1 vs. TiO2)[END_REF][START_REF] Ramanavicius | TiO2-x/TiO2-structure based 'selfheated'sensor for the determination of some reducing gases[END_REF] , but its wide bandgap and fast electron-hole pair recombination limit its use under visible light [START_REF] Nakata | TiO2 photocatalysis: Design and applications[END_REF][START_REF] Nasr | Recent Progress on Titanium Dioxide Nanomaterials for Photocatalytic Applications[END_REF][START_REF] Nasr | Photocatalytic degradation of acetaminophen over Ag, Au and Pt loaded TiO2 using solar light[END_REF] . Previous studies on the development of materials and technologies with higher degradation efficiency for water treatment indicate that the catalyst structure and morphology strongly contribute to its catalytic activity under visible light. [START_REF] Bui | Introduction to Recent Advances in Water and Wastewater Treatment Technologies[END_REF][START_REF] Xiang | Interfacial modification of titanium dioxide to enhance photocatalytic efficiency towards H2 production[END_REF][START_REF] Bethi | Nanomaterials-based advanced oxidation processes for wastewater treatment: A review[END_REF] In agreement, TiO2 photocatalytic activity is influenced by its morphology, crystallinity, surface, and textural properties. Many groups have tried to tailor TiO2 morphology to increase its photocatalytic performance [START_REF] Akple | Nitrogen-doped TiO2 microsheets with enhanced visible light photocatalytic activity for CO2 reduction[END_REF][START_REF] Klavarioti | Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes[END_REF][START_REF] Katal | A review on the synthesis of the various types of anatase TiO2 facets and their applications for photocatalysis[END_REF][START_REF] Ghori | A comparative study of photocatalysis on highly active columnar TiO2 nanostructures in-air and in-solution[END_REF] . For instance, 0D, 1D, 2D and 3D TiO2-based materials have been fabricated using various techniques [START_REF] Yang | Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets[END_REF][START_REF] Ramakrishnan | Size controlled synthesis of TiO2 nanoparticles by modified solvothermal method towards effective photo catalytic and photovoltaic applications[END_REF][START_REF] Mamaghani | Role of titanium dioxide (TiO2) structural design/morphology in photocatalytic air purification[END_REF][START_REF] El-Maghrabi | Synthesis of mesoporous core-shell CdS@TiO2 (0D and 1D) photocatalysts for solar-driven hydrogen fuel production[END_REF][START_REF] Ramanavicius | Progress and Insights in the Application of MXenes as New 2D Nano-Materials Suitable for Biosensors and Biofuel Cell Design[END_REF] .

One-dimensional (1D) materials, such as 1D TiO2 nanofibers (NFs), nanorods, nanotubes (NTs), nanowhiskers and nanowires [START_REF] Rosales | The influence of the morphology of 1D TiO2 nanostructures on photogeneration of reactive oxygen species and enhanced photocatalytic activity[END_REF][START_REF] Guo | ZnO and TiO2 1D nanostructures for photocatalytic applications[END_REF][START_REF] Erdogan | Alkaline hydrothermal synthesis, characterization, and photocatalytic activity of TiO2 nanostructures: the effect of initial TiO2 phase[END_REF] , are particularly interesting due to their higher light absorption and slower recombination rates, leading to better photocatalytic activity [START_REF] Ge | One-dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting[END_REF][START_REF] Feng | One-dimensional nanostructured TiO2 for photocatalytic degradation of organic pollutants in wastewater[END_REF][START_REF] Shahrezaei | Photocatalytic properties of 1D TiO2 nanostructures prepared from polyacrylamide gel-TiO2 nanopowders by hydrothermal synthesis[END_REF] . Among these structures, tubular nanostructured TiO2 is a versatile material for photocatalysis and photoelectrolysis [START_REF] Chatzitakis | Assessing the photoelectrochemical properties of C, N, F codoped TiO2 nanotubes of different lengths[END_REF][START_REF] Roy | TiO2 nanotubes: synthesis and applications[END_REF][START_REF] Paramasivam | A review of photocatalysis using self-organized TiO2 nanotubes and other ordered oxide nanostructures[END_REF] . Many preparation techniques have been tested for fabricating such 1D nanostructures (e.g. hydrothermal synthesis, template synthesis, anodization, electrospinning and atomic layer deposition, ALD) [START_REF] Zhu | Progress in fabrication of one-dimensional catalytic materials by electrospinning technology[END_REF][START_REF] Kowalski | TiO2 nanotubes, nanochannels and mesosponge: Selforganized formation and applications[END_REF][START_REF] Meng | Controllable atomic layer deposition of onedimensional nanotubular TiO2[END_REF][START_REF] Liang | Photocatalytical properties of TiO2 nanotubes[END_REF] . ALD is a thin film deposition technique based on gas-phase precursors for the deposition of thin-film layers with thickness control at the Ångstrom level [START_REF] Weber | Atomic layer deposition (ALD) on inorganic or polymeric membranes[END_REF][START_REF] George | Atomic Layer Deposition: An Overview[END_REF][START_REF] Elias | Electrochemical growth of ZnO nanowires on atomic layer deposition coated polystyrene sphere templates[END_REF] . ALD has been often combined with electrospinning for the deposition of the desired materials on polymeric fibers that are used as substrate [START_REF] Younes | Elaboration of porous alumina nanofibers by electrospinning and molecular layer deposition for organic pollutant removal[END_REF][START_REF] Najem | Palladium/Carbon Nanofibers by Combining Atomic Layer Deposition and Electrospinning for Organic Pollutant Degradation[END_REF][START_REF] Kim | SnO2 nanotubes fabricated using electrospinning and atomic layer deposition and their gas sensing performance[END_REF][START_REF] Kim | Nanostructured pure anatase titania tubes replicated from electrospun polymer fiber templates by atomic layer deposition[END_REF] . Upon removal of the polymeric core by thermal treatment after the ALD cycles, a tubular structure is formed [START_REF] Szilágyi | Review on one-dimensional nanostructures prepared by electrospinning and atomic layer deposition[END_REF] .

McClure et al. described TiO2 deposition on polymeric nanofibers by ALD, and then compared the precursor effect on the film coating. They found that the precursor choice influenced the final structure of TiO2 NTs [START_REF] Mcclure | Large effect of titanium precursor on surface reactivity and mechanical strength of electrospun nanofibers coated with TiO2 by atomic layer deposition[END_REF] . Su et al. reported the fabrication of TiO2 NTs by ALD using titanium chloride and H2O as precursors and assessed the doping effect on NT photocatalytic and photoelectrochemical performance [START_REF] Su | Photocatalysis and Hydrogen Evolution of Al-and Zn-Doped TiO2 Nanotubes Fabricated by Atomic Layer Deposition[END_REF] . In addition, the number of ALD cycles and the deposition temperature may affect the material crystalline structure [START_REF] Meng | Controllable atomic layer deposition of onedimensional nanotubular TiO2[END_REF] .

As TiO2 catalysts are often not stable and not much active under visible light irradiation, improving TiO2 morphology might positively influence the photocatalytic behavior compared with unmodified TiO2 (commercial Degussa p25) [START_REF] Lee | TiO2 photocatalyst for water treatment applications[END_REF] . Several methods have been described to extend the semiconductor photo-response to the visible light region, such as doping TiO2 with metals and non-metals and coupling with other semiconductors [START_REF] Fang | Modifications on reduced titanium dioxide photocatalysts: A review[END_REF][START_REF] Fujishima | TiO2 photocatalysis and related surface phenomena[END_REF][START_REF] Khaki | Application of doped photocatalysts for organic pollutant degradation -A review[END_REF] . Doping with non-metals (e.g. carbon, sulfur, phosphorus, nitrogen [START_REF] Mrowetz | Oxidative power of nitrogen-doped TiO2 photocatalysts under visible illumination[END_REF][START_REF] Park | Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting[END_REF][START_REF] Tachikawa | Photocatalytic Oxidation Reactivity of Holes in the Sulfur-and Carbon-Doped TiO2 Powders Studied by Time-Resolved Diffuse Reflectance Spectroscopy[END_REF][START_REF] Takeshita | Transient IR absorption study of charge carriers photogenerated in sulfur-doped TiO2[END_REF][START_REF] Ohno | Preparation of Sdoped TiO2 photocatalysts and their photocatalytic activities under visible light[END_REF] ) reduces the recombination of photogenerated electron-hole pairs in TiO2. Several non-metal dopants were mentioned in the literature such as carbon (C), sulfur (S), phosphorus (P) and nitrogen (N) [START_REF] Mrowetz | Oxidative power of nitrogen-doped TiO2 photocatalysts under visible illumination[END_REF][START_REF] Park | Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting[END_REF][START_REF] Tachikawa | Photocatalytic Oxidation Reactivity of Holes in the Sulfur-and Carbon-Doped TiO2 Powders Studied by Time-Resolved Diffuse Reflectance Spectroscopy[END_REF][START_REF] Takeshita | Transient IR absorption study of charge carriers photogenerated in sulfur-doped TiO2[END_REF][START_REF] Ohno | Preparation of Sdoped TiO2 photocatalysts and their photocatalytic activities under visible light[END_REF] . N-doping is the most studied [START_REF] Liu | Visible-light responsive boron and nitrogen codoped anatase TiO2 with exposed {0 0 1} facet: Calculation and experiment[END_REF] and allows shifting the TiO2 band into the visible light range [START_REF] Wang | Enhanced photoactivity of N-doped TiO2 for Cr(VI) removal: Influencing factors and mechanism[END_REF] . Asahi et al. found that N-doping extends TiO2 optical absorbance towards the visible light region [START_REF] Dong | Marked enhancement of photocatalytic activity and photochemical stability of N-doped TiO2 nanocrystals by Fe3+/Fe2+ surface modification[END_REF] . Moreover, the presence of non-metals in TiO2 generally increases the specific surface area and the anatase phase percentage, while limiting the crystallite size growth [START_REF] Zhang | Nitrogen-doped titanium dioxide visible light photocatalyst: Spectroscopic identification of photoactive centers[END_REF] . Huang et al. fabricated N-doped TiO2 (N-TiO2) by hydrothermal synthesis and calcination under NH3 atmosphere. They found that N-doping promoted the generation of hydroxyl and superoxide radicals that enhanced the photocatalytic activity [START_REF] Huang | Excellent visible light responsive photocatalytic behavior of N-doped TiO2 toward decontamination of organic pollutants[END_REF] . Another study showed that N-doping promotes the formation of oxygen vacancies that contribute to visible light absorption [START_REF] Fujishima | TiO2 photocatalysis and related surface phenomena[END_REF] . These two effects were also observed in hierarchical structures grown by ALD in which N-doping of TiO2 was obtained by incorporating N atoms on the substrate [START_REF] Kertmen | Photoelectrochemically Active N-Adsorbing Ultrathin TiO2 Layers for Water-Splitting Applications Prepared by Pyrolysis of Oleic Acid on Iron Oxide Nanoparticle Surfaces under Nitrogen Environment[END_REF] . Hence, co-doping TiO2 with nitrogen and boron can enhance both the visible light absorption of TiO2 and implement an efficient charge recombination regarding the synthesis technique [START_REF] Chen | Photocatalytic degradation of organic pollutants by co-doped TiO2 under visible light irradiation[END_REF] .

On the other hand, it was widely reported that the morphologies of 1D structures can influence TiO2 photocatalytic properties. Interestingly, the photocatalytic activity can be tuned by several parameters such as particle size, specific surface area, porous structure, crystalline phase etc. [START_REF] Nakata | TiO2 photocatalysis: Design and applications[END_REF] . Rosales et al. compared the photocatalytic activity of different 1D morphologies (NTs, NFs, NWs) on the degradation of methyl orange. The author found that the shape of TiO2 has a major role in determining their photocatalytic activity [START_REF] Rosales | The influence of the morphology of 1D TiO2 nanostructures on photogeneration of reactive oxygen species and enhanced photocatalytic activity[END_REF] , herein the importance of comparing different 1D morphologies such as TiO2 nanofibers and nanotubes for the degradation of organic micropollutant.

Here, we fabricated 1D TiO2 NTs by ALD for photocatalysis. To the best of our knowledge no previous reports have been published on elaboration of TiO2 nanotubes by Atomic Layer Deposition for catalytic activity. Hence 1D TiO2 NTs have shown a great interest for the degradation of micropollutants when compared to NFs and NWs because of their higher efficiency to absorb light. Variation of the number of ALD cycles confirmed that the photocatalytic efficiency was directly influenced by the NT wall thickness. Moreover, TiO2 doping with non-metals (nitrogen) further improved the photocatalytic activity by modifying the NT charge transportation, surface area, reflection and absorption. By comparing the catalytic activity of these TiO2 NTs and of TiO2 NFs prepared by electrospinning we found that ALD allows the formation of a well-structured nanotubular catalyst with higher degradation efficiency than NFs. More than 98% of acetaminophen was degraded in the presence of N-TiO2 NTs after 180 min of visible light irradiation. N-TiO2 NTs recyclability and toxicity tests showed the potential of this catalyst for real applications. We also performed quenching tests to determine the active species responsible for acetaminophen degradation.

Experimental

Materials and chemicals

Titanium(IV) isopropoxide (TTIP, 97%, CAS: 546-68-9), titanium (IV) chloride (TiCl4, 99.9%, CAS: 7550-45-0) , polyacrylonitrile (PAN, Mw=150000, CAS: 25014-41-9), polyvinyl pyrrolidone (PVP, Mw=1300000, CAS: 9003-39-8), acetaminophen (≥99%, CAS: 103-90-2), formalin solution (HCHO, CAS: 50-00-0), boron tribromide (BBr3, 99.9%, CAS :10294-33-4), Nafion™ perfluorinated resin solution (CAS: 31175-20-9), potassium chloride (KCl, ≥99%, CAS:), sodium sulfate (Na2SO4, ≥99%, CAS: 7757-82-6), sodium chloride (NaCl, ≥99%, CAS: 7647-14-5, potassium ferricyanide (K3[Fe(CN)6], ≥99%, CAS 13746-66-2), 2-propanol (99.9%, CAS: 67-63-0), p-benzoquinone (C6H4O2, ≥99.5%, CAS:106-51-4), and ethylenediaminetetraacetic acid (EDTA, 99.995%, CAS: 60-00-4) were purchased from Sigma-Aldrich. Tetrahydrofuran (99.9%, CAS: 109-99-9) was bought from Honeywell. Acetic acid (≥96% CAS: 64-19-7) and ethanol (≥99.8% CAS: 64-17-5) were purchased from VWR Chemicals and used as solvents. All chemicals were used without any additional purification. Indium tin oxide (ITO) deposited on quartz was purchased from Präzisions Glas & Optik. Deionized water (>18.2 MΩ), prepared with the Millipore (Milli-Q® Academic) water purification system, was used for all dilutions and reagent preparation. Argon gas, ammonia and nitrogen were from Linde and were used as received.

Support and catalyst preparation

Synthesis of TiO2 nanofibers and PAN nanofibers by electrospinning

For TiO2 NF fabrication, a mix of TTIP, PVP, acetic acid and ethanol was stirred for 2h before electrospinning. The solution was loaded into a 22 mL syringe, and the needle was connected to high voltage. The electrospinning conditions were: needle-collector distance of 10cm, tension of 25kV, and solution flow of 1mL.h -1 . For the preparation of PAN NFs, 18mL of tetrahydrofuran was mixed with 2 mg of PAN at 60 °C for 24 h and then electrospinned using a previously described home-built electrospinning system [START_REF] Younes | Elaboration of porous alumina nanofibers by electrospinning and molecular layer deposition for organic pollutant removal[END_REF][START_REF] Najem | Palladium/Carbon Nanofibers by Combining Atomic Layer Deposition and Electrospinning for Organic Pollutant Degradation[END_REF][START_REF] Sayegh | Tunable TiO 2-BN-Pd nanofibers by combining electrospinning and atomic layer deposition to enhance photodegradation of acetaminophen[END_REF] . The collected TiO2 NFs underwent calcination at 750 °C for 4h before utilization. PAN NFs were stabilized at 250 °C for 2h (heating rate of 1 °C.min -1 ) before ALD.

Deposition of TiO2 nanotubes by atomic layer deposition

Stabilized PAN NFs were used as substrates for TiO2 deposition at 100°C, using TiCl4 and H 2O as precursors in a home-built ALD device. After ALD deposition, we removed the PAN u pon annealing under air at high temperatures allowing to transform into nanotubes struc tures.

The typical ALD cycle consisted of 0.2 s TiCl4 pulse, followed by 10 s exposure and 60 s purge. Then, the H2O valve was opened for 2 s, followed by 10 s exposure and 60 s purge with argon gas. The line connected to the reactor was heated at 100°C to avoid condensation. The number of ALD cycles varied between 500 and 1000 to vary TiO2 shell thickness. After TiO2 deposition, PAN-TiO2 NTs were heated at 750°C for 4h with a heating rate of 1 °C/min under air before use.

Nitrogen-doped and boron/nitrogen-co-doped TiO2 NTs

After 500 ALD cycles, PAN-TiO2 NTs were exposed to nitrogen at 750°C for 15 min and the samples were denoted (N-TiO2 NTs) hereafter. For boron/nitrogen co-doped TiO2 NTs (B/N-TiO2 NTs), BN was deposited following a previously described method and five ALD cycles were used in this case [START_REF] Sayegh | Tunable TiO 2-BN-Pd nanofibers by combining electrospinning and atomic layer deposition to enhance photodegradation of acetaminophen[END_REF] .

Characterization of the synthesized nanocomposites

The NT and NF morphology was analyzed by scanning electron microscopy (SEM Hitachi S4800, JAPAN). Samples were placed on aluminum stubs and were sputter-coated with platinum/palladium using a Polaron SC7620 Mini Sputter Coater. X-ray diffraction (XRD) analyses were carried out using a PanAlytical X'pert system with Cu K α radiation ( = 0.15406 nm). The diffraction patterns were measured between 10 and 80° with a step size of 0.0167°.

The tubular morphology NTs was confirmed by transmission electron microscopy (TEM) using the JEOL 2200FS (200 kV) and JEOL ARM-200F (200kV) microscopes. For TEM analysis, samples were dispersed in ethanol, and a drop of the suspension was dried on a carbon support film that covered a standard copper grid. The samples' chemical composition and bandgap were determined by X-ray photoelectron spectroscopy (XPS) measurements, with an Escalab 250 apparatus, and by UV-Visible spectrophotometry (Jasco model V570; equipped with a diffuse reflectance attachment (Shimadzu IRS-2200)), respectively. Optical absorbance was measured by recording the photoluminescence spectra with an optical fiber spectrometer (Ocean Optics usb2000), equipped with excitation wavelength of 266 nm anda nitrogen Nd:YAG laser of 9mW.

Electrochemical measurements

Electrochemical impedance spectroscopy (EIS) and electrochemical active surface (EAS) characterization carried out by cyclic voltammetry (CV) were performed in a three-electrode cell system connected to a Solartron SI 1287 galvanostatic-potentiostat. A halogen lamp (150 w), placed at a distance of 10 cm from the electrode surface, was used as the light source.

Platinum wire and Ag/AgCl were the counter and reference electrode, respectively. The working electrode (2x2.5cm) was a suspension of 5 mg photocatalyst, 1 mL isopropanol, and 40 µL Nafion® aqueous solution. After 30 min in an ultrasonic cleaner, this suspension was dropped on the ITO glass, to obtain the working electrode after isopropanol evaporation. For EIS measurements, the three electrodes were immersed in a Na2SO4 solution (0.1 mol.L -1 ) considered as the electrolyte. For EAS analysis, 10 mM K3[Fe(CN)6] and 1.0 M KCl were used as electrolytes. The samples' electroactive surface area was estimated by CV performed in a voltage range of -0.4 to 0.8 V vs reference at scan rate of 20 mV.s -1 . The EAS was calculated using the Randles-Sevcik equation:

𝐈𝐩 = 𝟐. 𝟔𝟗 × 𝟏𝟎 𝟓 × 𝐀𝐃 𝟏/𝟐 × 𝐧 𝟑/𝟐 × 𝐂𝐯 𝟏/𝟐 ( 1
)
where Ip is the maximum current (A), n is the number of transferred electrons (n = 1), A is the electrode area (cm 2 ), D is the diffusion coefficient of [Fe(CN)6] 3-(7.60 × 10 -6 cm 2 .s -1 ), C is the concentration of [Fe(CN)6] 3-(1 × 10 -5 mol.cm -3 ), and v is the scan rate (0.01 V.s -1 ).

Acetaminophen degradation

The TiO2 samples' photocatalytic activity under visible light (linear halogen lamp; 400W, Avide) was tested by quantifying acetaminophen degradation in an aqueous solution. The solution was maintained at 10 cm from the lamp for all experiments. First, TiO2 NT samples with different wall thicknesses (different ALD cycle number) were compared. Then, the catalytic performances of TiO2 NTs (500 ALD cycles), TiO2 NFs, and doped TiO2 NTs were evaluated.

The photocatalytic degradation experiments were performed in the following conditions:

the photocatalyst with an initial concentration of 0.5 g.L -1 was added to 250 mL of acetaminophen aqueous solution (5 mg.L -1 ) in a 600 mL glass reactor put in a water bath at 30°C to minimize the temperature increase in the solution under light irradiation. After stirring in the dark for 30 min to ensure equilibrium adsorption, the solution was exposed to visible light and at different intervals, 2 mL aliquots were collected and filtered with 0.22 µm filters.

Acetaminophen degradation was quantified by high-performance liquid chromatography coupled to a Quattro-Micro mass spectrometer with an Electrospray ionization probe (Waters Micromass, Wythenshawe, Manchester, UK) as detector (HPLC-MS). The separation was carried out using a C-18 column (RP18 Column, Nucleoshell). The mobile phase was composed of Buffer A (0.1% formic acid in HPLC-grade water) and Buffer B (0.1% formic acid in HPLCgrade acetonitrile) with a 97:3 (A:B) ratio. The flow rate was set at 0.25 mL min -1 and the run time was 3 min.

The recyclability of the catalyst with the highest degradation efficiency was further investigated for 5 cycles of acetaminophen degradation in the same conditions.

The degradation efficiency (D(%)), was calculated according to equation (2):

𝐃(%) = [(𝐂 𝟎 -𝐂)/𝐂 𝟎 ] × 𝟏𝟎𝟎 (2)
where C0 and C are the initial and final acetaminophen concentrations after the test, respectively.

Photocatalytic kinetic model

TiO2 degradation kinetics is usually described using the Langmuir-Hinshelwood model [START_REF] Lin | Adsorption and photocatalytic oxidation of ibuprofen using nanocomposites of TiO2 nanofibers combined with BN nanosheets : Degradation products and mechanisms Chemosphere Adsorption and photocatalytic oxidation of ibuprofen using nanocomposites of TiO 2 n[END_REF][START_REF] Konstantinou | TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: A review[END_REF] .

When the pollutant concentration is low, a pseudo-first-order kinetics is applied [START_REF] Konstantinou | Photocatalytic transformation of pesticides in aqueous titanium dioxide suspensions using artificial and solar light: intermediates and degradation pathways[END_REF] , as described in Equation (3):

𝐥𝐧 (𝐂 𝟎 /𝐂) = 𝐤𝐭 (3)
where C0 (mg/L) is the initial pollutant concentration, C is the pollutant concentration at time t (min), and k (min -1 ) is the pseudo-first-order rate constant.

Eco-toxicity assays

During acetaminophen degradation many by-products may be formed [START_REF] Xuan | Chemosphere Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media[END_REF] . To assess their toxicity, inhibition of Vibrio fischeri bioluminescence was monitored using a Microtox® Model 500 Analyzer (Modern Water Inc.; United Kingdom) as previously described [START_REF] Zhang | Nitrogen-doped titanium dioxide visible light photocatalyst: Spectroscopic identification of photoactive centers[END_REF] .

Quenching tests

Scavenger tests were performed to determine the main active species responsible for acetaminophen degradation. Benzoquinone, isopropanol and EDTA were added to the solution at a concentration of 10 mM before exposure to visible light (same conditions as for the degradation experiments). Aliquot were collected at different intervals, and acetaminophen concentration was measured by HPLC-MS as described in 5.6.

Results and Discussion

Morphological characterization

The morphology of all prepared materials (calcinated TiO2 NFs after electrospinning and TiO2 deposited on PAN NFs by ALD) was verified by scanning electron microscopy (SEM).

Before the analysis, TiO2 was deposited on PAN nanofibers substrate by ALD and TiO2 nanofibers elaborated by electrospinning were subject to thermal treatment at 750°C under air for 4h. A well nanotubular structure was observed after ALD deposition with a TiO2 pulse time of 0.2 s, but not with shorter pulse times (Erreur ! Source du renvoi introuvable.). In addition, it was reported elsewhere that more than 200 ALD cycles are necessary to obtain well-structured NTs [START_REF] Borbón-Nuñez | Fabrication of hollow TiO2 nanotubes through atomic layer deposition and MWCNT templates[END_REF] . Moreover, Figure 1a shows TiO2 nanotubular morphology of several microns in length after removal of the PAN substrate. This indicates that PAN NFs were an efficient 'template' for TiO2 NT synthesis. TiO2 NTs with an inner diameter of approximatively 400 nm and high surface roughness were obtained after 500 ALD cycles (Figure 1b). SEM confirmed the nanofibrous structure of TiO2 NFs (Figure 1(c)). The surface roughness is attributed to the crystalline structure of the nanofibers at 750°C. No clear difference was observed after NT doping (Figure 1 (d-e)), confirming the previous findings that N-and Bdoping do not change TiO2 shape [START_REF] Huang | Excellent visible light responsive photocatalytic behavior of N-doped TiO2 toward decontamination of organic pollutants[END_REF] and that doping with non-metals could occur in the NT bulk [START_REF] Lazar | Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates[END_REF] . The crystalline structure was characterized by X-Ray Diffraction (XRD). TiO2 NT samples with different thickness were composed of an anatase phase at 750 °C, as previously described [START_REF] Wang | Fabrication of TiO2 nanotubes by atomic layer deposition and their photocatalytic and photoelectrochemical applications[END_REF] (Figure 3 81 that corresponded to the anatase phase planes (101), (004), ( 200), ( 105), ( 211), ( 204), ( 116) and (107) (JCPDS-00-071-1167), respectively [START_REF] Thamaphat | Phase characterization of TiO2 powder by XRD and TEM[END_REF] . The anatase phase was more active than the rutile phase due to the presence of an indirect bandgap in the anatase crystal that increased the electron-hole pair separation [START_REF] Soo | Optimal electrospun TiO2 nanofiber photocatalytic performance via synergistic morphology and particle crystallinity with anatase/rutile phase tuning[END_REF] . In N-TiO2 NTs, the anatase peaks were slightly shifted compared with TiO2 NTs, possibly due to N incorporation in TiO2 bulk or crystal lattice, as confirmed by XPS [START_REF] Sinha | Chapter 1 -An overview of synthesis techniques for preparing doped photocatalysts[END_REF] . Unlike B/N co-doping, N-doping inhibits the rutile phase formation. Codoping with B and N (B/N-TiO2 NTs) led to the appearance of additional peaks [START_REF] Lu | Fabrication of Boron-Doped TiO2 Nanotube Array Electrode and Investigation of Its Photoelectrochemical Capability[END_REF][START_REF] Szkoda | Optimization of boron-doping process of titania nanotubes via electrochemical method toward enhanced photoactivity[END_REF] . The peak at 2θ = 28.0° was associated with the rutile phase of the (110) plane [START_REF] Natarajan | Enhanced photocatalytic activity of bismuth-doped TiO 2 nanotubes under direct sunlight irradiation for degradation of Rhodamine B dye[END_REF] . It was reported elsewhere that the rutile phase formation is linked to the percentage of B incorporation into TiO2 [START_REF] Gombac | TiO2 nanopowders doped with boron and nitrogen for photocatalytic applications[END_REF] . Lastly, in TiO2 NFs, both anatase and rutile phases were observed with the rutile phase diffraction peaks (1 1 0), (1 0 1), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (3 1 0) and (1 1 2) (rutile TiO2, JCPDS 21-1276) [START_REF] Doh | Development of photocatalytic TiO2 nanofibers by electrospinning and its application to degradation of dye pollutants[END_REF] . TiO2 NFs did not show the same crystallinity as TiO2 NTs, since the crystalline nature of TiO2 varies with synthesizing techniques and parameters [START_REF] Yu | Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes[END_REF] . To understand the effect of the different morphologies and modifications on TiO2 structure, the percentage of anatase/rutile phase and the crystallite sizes were calculated using the Spurr (4) and Scherrer (5) equations [START_REF] Yang | Photocatalytic activity of magnetically anatase TiO2 with high crystallinity and stability for dyes degradation: Insights into the dual roles of SiO2 interlayer between TiO2 and CoFe2O4[END_REF][START_REF] Jin | Water steam modified crystallization and microstructure of mesoporous TiO2 nanofibers[END_REF] :

%ʀ = 𝟏 𝟏+𝟎.𝟖[ 𝐈ᴀ(𝟏𝟎𝟏) 𝐈ʀ(𝟏𝟏𝟎) ] (4)
𝐃 = 𝐤𝛌 𝛃𝐜𝐨𝐬𝛉 (5)
where IA and IR are the integrated intensities of the anatase (101) and rutile (110) diffraction peaks, D is the mean crystallite size, K is a shape factor = 0.89, λ is the X-ray wavelength, β is the full width at half maximum of the diffraction peak, and θ is the Bragg angle.

Comparison of the percentage of anatase/rutile phases and their particle sizes in all samples (Table 1) showed that in TiO2 NT samples, doping decreased the crystallite size due to B and N inhibitory effect on the grain growth of TiO2 particles. The anatase particle size of non-doped TiO2 NTs and N-TiO2 NTs were 37.2 nm and 29.9 nm, respectively. This is probably due to the crystal lattice deformation and oxygen vacancies left by the substitution of O atoms by N atoms. The particle size decrease allows the photo-generated electrons to move faster to the TiO2 surface and consequently it improves the catalyst degradation efficiency [START_REF] Wang | Enhanced photoactivity of N-doped TiO2 for Cr(VI) removal: Influencing factors and mechanism[END_REF][START_REF] Vaiano | Heterogeneous photocatalysis: how doping with nitrogen can improve the performance of semiconductor nanoparticles under visible light irradiation[END_REF] . In TiO2 NFs, the crystallite sizes of the anatase and rutile phases were 30.9 and 34.3 nm, respectively. Previous works showed that the rutile phase displays lower photocatalytic activity than the anatase phase [START_REF] Lai | Effects of the Structure of TiO[sub 2] Nanotube Array on Ti Substrate on Its Photocatalytic Activity[END_REF] .

Then, TEM and XPS were used to better describe the morphology of non-doped TiO2 NTs and N-doped TiO2 NTs compared with TiO2-NFs. [START_REF] Sauthier | Laser Synthesis and Characterization of Nitrogen-Doped TiO2 Vertically Aligned Columnar Array Photocatalysts[END_REF][START_REF] Calisir | Nitrogen-doped TiO2 fibers for visiblelight-induced photocatalytic activities[END_REF] . These reports demonstrated the presence of oxygen vacancies due to O substitution by N. Conversely, in our samples, the N peak (Figure 4d) at 399.9 eV was assigned to the presence of interstitial N atoms in the O-Ti-N environment, and not to substitutional doping of TiO2 lattice by N. The N-O interaction in TiO2 NT lattice increases the binding energy of the N 1s level, in agreement with our results [START_REF] Sathishkumar | Contemporary Achievements of Visible Light-Driven Nanocatalysts for the Environmental Applications[END_REF] . Moreover, the existence of Ti-N bonds could be excluded because N1s would be located at lower binding energies (<397 eV) [START_REF] Kertmen | Photoelectrochemically Active N-Adsorbing Ultrathin TiO2 Layers for Water-Splitting Applications Prepared by Pyrolysis of Oleic Acid on Iron Oxide Nanoparticle Surfaces under Nitrogen Environment[END_REF][START_REF] Valentin | N-doped TiO2: Theory and experiment[END_REF] . This result might be interesting for photocatalytic applications because Ndoping in TiO2 NTs without Ti-N bonding causes visible-light sensitization. The total N amount in N-TiO2 NTs was ~0.7%. Conversely, in B/N-TiO2-NTs, B and N amounts were estimated at 6.6 and 1.3%, respectively. The higher N amount in the co-doped samples could be explained by the different doping method used. Only one peak at 192.

Electrochemical measurements

The 2) confirmed that N-TiO2 NTs had the lowest R2 (15.22 kΩ vs 19.11 kΩ for non-doped TiO2 NTs and 16.52 kΩ for TiO2 NFs). This suggests that N-doping decreases TiO2 surface resistance and delays charge recombination at the TiO2-electrolyte interface.

Concerning TiO2 morphology (NTs vs NFs), TiO2 NFs showed better electron-hole pair separation resistance (i.e. lower R2 than non-doped TiO2 NTs), possibly due to their higher crystallinity that favors the charge carrier separation and transport [START_REF] Akple | Nitrogen-doped TiO2 microsheets with enhanced visible light photocatalytic activity for CO2 reduction[END_REF] . Co-doping affected negatively the charging kinetics, indicating the occurrence of trap states and recombination centers 102 . CV with [Fe(CN)6] 3-/[Fe(CN)6] 4-were used to investigate the effect of morphology and doping on the electrochemical response (Figure 5b and Table 2). Concerning the morphology, the current density peak was higher in TiO2 NFs than TiO2 NTs, possibly because of their higher EAS (Table 2), despite their slightly higher Epeak. N-doping led to the highest current density response towards [Fe(CN)6] 3/4 and lower Epeak , suggesting higher electrode reaction rate. The ΔE=Eanode -Ecathode values showed that the mean peak-to-peak separation was shorter in N-TiO2 NTs (0.3711 mV at V= 20 mV/s). When ΔE is shorter, electron transfer is faster in the redox reaction and the electrode surface is more conductive. The EAS values were 1.78 (TiO2 NTs, 500 ALD cycles), 1.85 (TiO2 NFs), 1.90 (B/N-TiO2 NTs), and 2.40 cm 2 (N-TiO2 NTs) 103 . This confirmed that doping plays a major role in enhancing TiO2 photocatalytic activity by improving the electron-hole pair separation, as confirmed by the photoluminescence results.

According to the literature, a smaller grain size increases the number of active surface sites and also the surface charge carrier transfer rate in 1D structured photocatalysts, leading to

Photocatalytic experiments

Photocatalytic degradation

Visible light photodegradation of acetaminophen (Figure 6) was compared using different TiO2-based catalysts. This showed that higher number of ALD cycles increased the wall thickness (not presented in this paper) that strongly affected acetaminophen degradation efficiency (Figure 6a), in agreement with the litterature [START_REF] Wang | Fabrication of TiO2 nanotubes by atomic layer deposition and their photocatalytic and photoelectrochemical applications[END_REF] . The highest degradation efficiency was obtained with TiO2 NTs prepared using 500 ALD cycles compared with 800 and 1000 ALD cycles: 95.7% after 90 minutes of visible light irradiation versus 76.6% and 56.0%, respectively.

On the basis of these results, TiO2 NTs fabricated using 500 ALD cycles were used for all the other experiments. As the catalyst morphology plays a major role in the degradation of pharmaceutical pollutants, it was important to understand the effects of the surface area, crystallinity, electron-hole pair recombination, and bandgap on the photocatalytic properties [START_REF] Sauthier | Laser Synthesis and Characterization of Nitrogen-Doped TiO2 Vertically Aligned Columnar Array Photocatalysts[END_REF] . The visible light catalytic properties of TiO2 NFs (fabricated by electrospinning followed by calcination at 750°C for 4h) were compared with those of TiO2 NTs (Figure 6). As N-TiO2 NTs have lower recombination rate and higher surface area than TiO2 NFs (see EIS and EAS data), their photocatalytic activity was higher. It was previously reported that anatase TiO2 displays higher degradation efficiency than a catalyst with both crystalline phases [START_REF] Wang | Fabrication of TiO2 nanotubes by atomic layer deposition and their photocatalytic and photoelectrochemical applications[END_REF][START_REF] Vaiano | Heterogeneous photocatalysis: how doping with nitrogen can improve the performance of semiconductor nanoparticles under visible light irradiation[END_REF] .

Wang et al. observed better photocatalytic degradation of methylene blue by TiO2 nanosheets than TiO2 nanoparticles, highlighting the importance of the catalyst structure. Moreover, they reported that higher surface area further improves degradation under visible light 105 .

Then, different TiO2 NT samples were prepared by varying the dopant nature to explore how to limit the recombination of photogenerated electron-hole pairs for feasible charge separation and transfer, and how to expand the absorption edge to the visible light range.

After 90 minutes of visible light exposure, 98.3%, 95.7% and 31.9% of acetaminophen were degraded in the presence of N-TiO2 NTs, TiO2 NTs, and B/N-TiO2 NTs, respectively (Erreur ! Source du renvoi introuvable.). This indicates the N-doping better increase the degradation efficiency compared with B and D co-doping. This modification step might be affected by many factors, including the type and level of N-doping and the concentration of O2 vacancies, and this will greatly influence the photocatalytic activity 106,107 .

The positive effect of N-doping on TiO2 catalyst properties has been widely reported.

Indeed, N-doping creates O2 vacancies that participate in trapping the photoinduced electrons and act as a reactive center for the photocatalytic process. As the number of articles on TiO2 modification by N-doping is too high, comparing their results is complicated due to the many different experimental conditions (e.g. catalyst structure, concentrations, matrix, lamp source) [START_REF] Zhang | Nitrogen-doped titanium dioxide visible light photocatalyst: Spectroscopic identification of photoactive centers[END_REF][START_REF] Vaiano | Heterogeneous photocatalysis: how doping with nitrogen can improve the performance of semiconductor nanoparticles under visible light irradiation[END_REF][108][109][110] .

Photocatalytic kinetic model

The photocatalytic degradation of acetaminophen under visible light followed a pseudofirst-order kinetics. Indeed, the closely linear relationship between ln(C0/C) and irradiation time (t) fits well with the first-order reaction rate of acetaminophen degradation (Figure 7a).

The degradation rate in the presence of N-TiO2 NTs was 0.045 min -1 , which is 4 times higher than with TiO2 NFs. This result was in agreement with the lower electron-hole pair recombination rate (photoluminescence and EIS measurements). Moreover, the high percentage of rutile phase in TiO2 NFs decreased their degradation efficiency. In conclusion, ALD is a promising technique for fabricating catalysts with well-organized structure and good degradation performance under visible light.

Stability, quenching, and toxicity tests

N-TiO2 NTs (500 ALD cycles) recyclability and stability were tested for five consecutive cycles (Figure 8a). After each cycle, the catalyst was rinsed several times with distilled water and dried at 70 °C before the next cycle. Acetaminophen degradation rates, after 90 min of visible light irradiation, were 98.3% (first cycle), 97.5% (second cycle), 96.0% (third cycle), 93.9% (fourth cycle), and 82.6% (fifth cycle). This indicates a slight loss of degradation efficiency over time, although acetaminophen removal was still >82% after five runs. This activity decrease might be related to N loss in the catalyst after consecutive cycles, or to the accumulation of by-products that are formed during the catalyst surface degradation, thus decreasing the available active sites 111 . Despite this loss of activity (<20%) after five cycles, N-TiO2 NTs can be considered a promising stable catalyst material for water treatment.

Organic pollutant degradation by advanced oxidation processes can lead to the production of by-products 112,113 with cyclic and aromatic structure that are highly toxic, sometimes more than the initial pollutant. To evaluate the acute toxicity of intermediates generated during acetaminophen photodegradation, V. fischeri luminescence intensity variations were quantified after 15-min incubation with aqueous solution aliquots collected during acetaminophen degradation (Figure 8b). Acetaminophen toxicity was very low at the used concentration, as we previously reported 114 . Toxicity strongly increased and luminescence was inhibited by ~90% after 1h of visible light irradiation. This result is consistent with previous studies showing the formation of toxic aromatic by-products (e.g. 1,4-benzoquinone, hydroquinone, benzoic acid, and benzaldehyde) during acetaminophen degradation [115][116][117] .

Then, toxicity progressively decreased and luminescence intensity inhibition was <20% after 3h of irradiation. During this period, further oxidation of the formed molecules would result in the breakdown of their aromatic structures. After 6h of irradiation, when short-chain carboxylic acids are converted into CO2 and H2O, the luminescence inhibition rate was <5%. During photodegradation, OH° radicals, h + and O2 -radicals are generally considered as active species. To determine their contribution to acetaminophen degradation, scavenger tests were performed using EDTA (h + scavenger), isopropanol (OH radicals). and pbenzoquinone (O2 -radicals) and N-TiO2 NTs as catalyst. The results (Figure 8c) suggested that all three active radical species are implicated in acetaminophen degradation because all scavengers decreased its photodegradation, particularly p-benzoquinone (O2 -radical scavenger) that reduced acetaminophen degradation rate from 98.3% to 40.4%.

Conclusion

In this work, we first compared the photocatalytic properties of two different 1D TiO2 structures fabricated using two different approaches. TiO2 NTs, synthesized by ALD, displayed 3 times higher acetaminophen degradation rates (after 90 min of visible light irradiation) than TiO2 NFs, produced by electrospinning. Then, we investigated whether non-metal doping increased TiO2 NT degradation efficiency. N-doping enhanced the catalytic properties, as indicated by the degradation rate of 0.045 min -1 of N-TiO2 NTs. This result could be explained by the lower recombination rate of the photogenerated charges and higher electroactive surface area (2.40 cm 2 in N-TiO2 NTs versus 1.78 cm 2 in TiO2 NTs). The recyclability results suggest that ALD is an interesting technique for preparing photocatalysts on immobilized supports with good degradation efficiency and high stability. Quenching tests indicated that superoxide radicals played a major role in acetaminophen degradation. In this work, we confirmed the higher degradation activity of highly structured 1D materials under visible light. Now, these catalysts should be tested using a real wastewater matrix with different organic pollutant to test the selectivity of the photocatalyst and the ability to degrade different types of pollutants.

Figure 1 .

 1 Figure 1. SEM images of (a-b) TiO2NTs, (c) TiO2 NFs, (d) N-TiO2 NTs, and (e) B/N-TiO2 NTs. TEM images (Figure 2) confirmed TiO2 NT nanotubular structure with a nanotube wall thickness of ~60 nm in doped and non-doped samples. After N-doping, TiO2 nanotubular morphology and crystal lattice values (selected area electron diffraction images (SAED)) were not changed. Moreover, elemental mapping (Figure 2b) indicated the presence of N, T and O.

Figure 2 .

 2 Figure 2. High resolution TEM, elemental mapping, and selected area electron diffraction images SAED of (a) TiO2 NTs and (b) N-TiO2 NTs (500 ALD cycles in both cases).

  and S2). TiO2 NTs and N-TiO2 NTs displayed the anatase crystalline structure peaks at 2θ = 25.3, 36.9, 37.7, 38.5, 48.0, 53.8, 55.0, 62.6, 68.7, 70.2, 74.9 and 75.9°

Figure 3 .

 3 Figure 3. XRD analysis of doped and non-doped TiO2 NTs (500 ALD cycles) and of TiO2 NFs (a) from 15 to 80° and (b) zoom on the peaks from 24 to 29°.

*

  The error on the crystallite size is lower than ± 0.01%The XPS survey of N-TiO2 NTs confirmed the presence of Ti, O, C and N (Figure4a). Carbon detection could be due to the presence of residues after calcination and PAN NF removal. The high-resolution XPS spectra of Ti 2p and of O 1s included two peaks (at 458.4 for Ti 2p3/2 and at 464.1 eV for Ti 2p1/2) and three peaks (at 529.5, 530.6 and 532.3 eV), respectively. The binding energies of Ti 2p and O 1s for TiO2 NTs (Erreur ! Source du renvoi introuvable.) were lower than those of doped TiO2 NTs, unlike previous literature data showing that N-doping decreases the binding energy values

  3 eV was attributed to B 1s (Figure4e), suggesting the presence of B-O-Ti groups[START_REF] Liu | Porous BN/TiO2 hybrid nanosheets as highly efficient visible-light-driven photocatalysts[END_REF] . The absence of B-N linkages indicates that B/N-TiO2-NTs were not doped by BN, but co-doped by B and N. The N 1s peak at 398.2 eV (N 1s A) was explained by the substitutional N atom that replaced one O atom in the TiO2 lattice.The peak at 401.5 (N 1s B) eV corresponded to NH 4 + ions at the TiO2 surface 100 . The N atom location in the TiO2 lattice depended on the synthetic route of N-doped TiO2.

Figure 4 .

 4 Figure 4. XPS spectra of N-TiO2 NTs: (a) Survey spectra, (b) Ti 2p, (c) O 1s, (d) N 1s; and of B/N-TiO2-NTs: (e) B1s and (f) N1s.

  Figure 5 (a) in which R1 is the electrolyte resistance and R2 the charge transfer resistance. N-TiO2 NTs displayed the lowest radius arc (i.e. R2 value) among all samples, indicating higher charge transport and very low recombination rate. Calculation of the charge transfer resistance values (R2) (Table 2) confirmed that N-TiO2 NTs had the lowest R2 (15.22 kΩ vs 19.11

  photocatalytic performance is influenced not only by one factor but different factors in the same time such as particle size, specific surface area, porous structure, crystalline phase, and exposed surface facets. All these parameters should be assessed to understand the catalytic performance of the material.

Figure 5 .

 5 Figure 5. a) Nyquist plots and b) Cyclic voltammograms, obtained in potassium hexacyanoferrate solution with a scan rate of 20 mV/s, for TiO2 NTs, TiO2 NFs, N-TiO2 NTs, and B/N-TiO2 NTs (surface= 1 cm 2 ).

Figure 6 .

 6 Figure 6. Acetaminophen degradation under visible light in function of a) TiO2 wall thickness (number of ALD cycles), b) TiO2 structure, morphology, and doping.

Figure 7 .

 7 Figure 7. Photocatalytic degradation of acetaminophen by the indicated TiO2 catalysts. a) Plot of ln(C0/C) in function of time (fitted to first order rate law), and b) Values the first order rate constant.

Figure 8 .

 8 Figure 8. a) Reusability of N-TiO2-NTs for photocatalytic activity. b) Inhibition of of V. fischeri luminescence emission during acetaminophen photodegradation. c) Effect of radical scavengers on acetaminophen degradation.

Table 1 :

 1 Percentage of anatase and rutile phases and crystallite sizes of the indicated samples.

	Sample	Anatase (A) phase	Rutile (R) phase	Crystallite size (A)	Crystallite size (R)
		(%)	(%)	(nm)*	(nm)*
	TiO2 NTs (500 AD cycles)	100.0	-	37.2	-
	TiO2 NTs (1000 ALD cycles)	100.0	-	42.1	-
	TiO2 NFs	24.5	75.5

Table 2 :

 2 Electrochemical results for TiO2 NTs, TiO2 NFs, B/N-TiO2 NTs, and N TiO2 NTs.

	Sample	R2	Ip	ΔE	EAS
		(kΩ)	(A.cm -2 )	(mV)	(cm 2 )

Acknowledgements

This work was funded by the H2020-MSCA-RISE-2017 project, 'Novel 1D photonic metal oxide nanostructures for early stage cancer detection' (Project number: 778157). This study was supported also by the Research Council of Lithuania (LMTLT), GILIBERT 2021 program agreement No S-LZ-21-4, and was co-funded by Campus France grant No. 46593RA (PHC GILIBERT 2021). I.I. acknowledges partial financial support from the SONATA BIS project 2020/38/E/ST5/00176. .1.