Pierre Gaffuri

Tatjana Dedova

Estelle Appert

Mati Danilson

Adrien Baillard

Odette Chaix-Pluchery

Frank Güell

Ilona Oja-Acik

Vincent Consonni
email: vincent.consonni@grenoble-inp.fr

Enhanced photocatalytic activity of chemically deposited ZnO nanowires using doping and annealing strategies for water remediation

Keywords:

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Water pollution is one of the most serious ecological threats humankind faces today. Over the last century, human needs for freshwater were multiplied by a factor of six and they are still increasing significantly owing to the expanding global economy and continuous population growth [1]. However, in the same time, 1.6 billion of people do not have access to freshwater due to poor water management where the demand exceeds the resources [1]. The development of wastewater reclamation and reuse for non-potable and indirect potable purposes has thus received an increasing interest for public institutions [START_REF] Hartley | Public perception and participation in water reuse[END_REF], but is facing challenges through the systematic presence of persistent organic pollutants (POPs) [START_REF] Jones | Persistent organic pollutants (POPs): state of the science[END_REF] and trace contaminants including heavy metals [START_REF] Viers | Chemical composition of suspended sediments in World Rivers: New insights from a new database[END_REF]. Those are severely harmful to human beings and wildlife due to their carcinogenic nature and poor degradability in the environment, respectively. Even at low concentration (<1mg/L for some dyes), major dyes decrease the water transparency, consume oxygen, and elevate biochemical oxygen demand destroying aquatic life [START_REF] Chequer | Textile Dyes: Dyeing Process and Environmental Impact[END_REF]. In the broad family of POPs [START_REF] Jones | Persistent organic pollutants (POPs): state of the science[END_REF], synthetic organic dyes represent a significant fraction as more than 1,000,000 tons are produced each year as coloring agents in the industries and around 20 % are released in the environment, mainly as textile effluents during the manufacturing process [START_REF] Tkaczyk | Synthetic organic dyes as contaminants of the aquatic environment and their implications for ecosystems: A review[END_REF]. The removal of organic dyes from wastewater has therefore been considered as a priority and based on various approaches including adsorption, membrane separation, coagulation, and advanced oxidation processes (AOPs) [START_REF] Oturan | Advanced Oxidation Processes in Water/Wastewater Treatment: Principles and Applications. A Review[END_REF][START_REF] Salimi | Contaminants of emerging concern: a review of new approach in AOP technologies[END_REF]. While the former typically result in the concentration of organic dyes generating a secondary pollution, the latter are known to eliminate organic dyes through a fast, efficient, and non-selective approach. AOPs are based on the in situ generation of highly oxidizing agents, like hydroxyl radicals ( • OH), at room temperature and normal pressure with the assistance of ozone, hydrogen peroxide, Fenton's reagents, ultra-violet light and/or catalysts [START_REF] Oturan | Advanced Oxidation Processes in Water/Wastewater Treatment: Principles and Applications. A Review[END_REF][START_REF] Salimi | Contaminants of emerging concern: a review of new approach in AOP technologies[END_REF].

Heterogeneous photocatalysis, being considered as one of the most efficient and promising AOPs, was developed following the pioneered work of Fuhishima & Honda in 1972, reporting the splitting of water into hydrogen and oxygen by TiO 2 catalyst when irradiated under UV light [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF]. It relies on the formation of electron-hole pairs in direct band gap semiconductors under UV irradiation, hence producing • OH radicals from water molecules and superoxide anion ( • O 2 -) radicals from O 2 molecules on their surfaces [START_REF] Herrmann | Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants[END_REF][START_REF] Herrmann | Heterogeneous photocatalysis: state of the art and present applications[END_REF].

These radicals are able to efficiently decompose adsorbed organic dyes on their surfaces into CO 2 and harmless organic compounds [START_REF] Herrmann | Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants[END_REF][START_REF] Herrmann | Heterogeneous photocatalysis: state of the art and present applications[END_REF]. Amongst many oxide semiconductor photo-catalysts [START_REF] Gusain | Adsorptive removal and photocatalytic degradation of organic pollutants using metal oxides and their composites: A comprehensive review[END_REF] including TiO 2 [START_REF] Han | Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment: A review[END_REF], ZnO [START_REF] Lee | Recent developments of zinc oxide based photocatalyst in water treatment technology: A review[END_REF][START_REF] Qi | Review on the improvement of the photocatalytic and antibacterial activities of ZnO[END_REF], Cu 2 O [START_REF] Mohammed | Review of various strategies to boost the photocatalytic activity of the cuprous oxide-based photocatalyst[END_REF], CeO 2 [START_REF] Kurian | Cerium oxide based materials for water treatment -A review[END_REF], and Fe 2 O 3 [START_REF] Hitam | A review on exploration of Fe 2 O 3 photocatalyst towards degradation of dyes and organic contaminants[END_REF] to name a few, ZnO has emerged as a very promising candidate in the last decade as it is biocompatible and composed of abundant and low-cost elements. Moreover, the energy position of the bottom of the conduction band minimum and of the top of the valence band maximum with respect to the energy position of the normal hydrogen electrode is favorable for its high photocatalytic activity [START_REF] Lee | Recent developments of zinc oxide based photocatalyst in water treatment technology: A review[END_REF][START_REF] Qi | Review on the improvement of the photocatalytic and antibacterial activities of ZnO[END_REF]. Its wide band gap energy of 3.37 eV at room temperature along with its higher absorption coefficient over a broad fraction of the solar spectrum when compared to TiO 2 are of high interest [START_REF] Sakthivel | Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO 2[END_REF]. Eventually, ZnO nanomaterials with a larger surface area including nanoparticles as compared to thin films drastically enhance the photocatalytic activity [START_REF] Mauro | ZnO for application in photocatalysis: From thin films to nanostructures[END_REF][START_REF] Ong | A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications[END_REF]. Alternatively, ZnO nanowires (NWs) have a higher potential for photocatalysis as compared to nanoparticles [START_REF] Baruah | Enhanced visible light photocatalysis through fast crystallization of zinc oxide nanorods[END_REF]. They can be grown by the low-temperature and low-cost chemical bath deposition technique operating in water [START_REF] Vayssieres | Purpose-built anisotropic metal oxide material: 3D highly oriented microrod array of ZnO[END_REF], which is further compatible with the development of green chemistry [START_REF] Anastas | Green Chemistry: Principles and Practice[END_REF]. Different strategies have been developed

to increase the photocatalytic activity of ZnO NWs, such as the formation of type II heterostructures [START_REF] Wang | Visible light driven type II heterostructures and their enhanced photocatalysis properties: a review[END_REF] as well as the management of intrinsic defects including vacancies [START_REF] Chen | Influence of Defects on the Photocatalytic Activity of ZnO[END_REF] and extrinsic doping [START_REF] Samadi | Recent progress on doped ZnO nanostructures for visible-light photocatalysis[END_REF]. In all cases, the main objective has been to maximize the lifetime of electron-hole pairs photo-generated under UV irradiation to increase the photocatalytic activity operating on the surfaces of ZnO NWs.

The extrinsic doping of ZnO NWs creates additional energy levels in the bandgap that are liable to limit the recombination of photo-generated electron-hole pairs by enhancing their separation, but the resulting traps can also act as recombination centers specifically when their concentration is high [START_REF] Lee | Recent developments of zinc oxide based photocatalyst in water treatment technology: A review[END_REF][START_REF] Samadi | Recent progress on doped ZnO nanostructures for visible-light photocatalysis[END_REF]. In this way, the cationic dopants substituting for the Zn site lattice are the main doping elements and include Sb [START_REF] Wu | Photoresponsive and Ultraviolet to Visible-Light Range Photocatalytic Properties of ZnO:Sb Nanowires[END_REF] and Cu [START_REF] Mohan | Enhanced photocatalytic activity of Cu-doped ZnO nanorods[END_REF] as acceptors and Al [START_REF] Khalid | Enhanced photocatalytic activity of Al and Fe co-doped ZnO nanorods for methylene blue degradation[END_REF] and Ga [START_REF] Cardoza-Contreras | Photocatalytic and Antimicrobial Properties of Ga Doped and Ag Doped ZnO Nanorods for Water Treatment[END_REF] as shallow donors. Transition metals including Fe [START_REF] Zhang | Enhanced photocatalytic activity of iron doped zinc oxide nanowires for water decontamination[END_REF][START_REF] Habba | Enhanced Photocatalytic Activity of Iron-Doped ZnO Nanowires for Water Purification[END_REF], Ni [START_REF] Loyola Poul Raj | Significance of Ni doping on structure-morphology-photoluminescence, optical and photocatalytic activity of CBD grown ZnO nanowires for opto-photocatalyst applications[END_REF] or Co [START_REF] Šutka | Co doped ZnO nanowires as visible light photocatalysts[END_REF][START_REF] Li | Enhanced Visible Light Photocatalytic Activity of ZnO Nanowires Doped with Mn 2+ and Co 2+ Ions[END_REF] have also been used to take profit from the different oxidation states of these elements.

Saleh et al. described the phenomena whereby ZnO nanoparticles are doped with Fe 3+ and Fe 2+ in the same time, holes (electrons) are trapped with the transition to Fe 4+ (Fe 2+ ) [START_REF] Saleh | UV light photocatalytic degradation of organic dyes with Fe-doped ZnO nanoparticles[END_REF]. These two oxidation states are less stable and release holes (electrons), going back to Fe 3+ . Other studies have focused on the effect of rare-earth elements such as La [START_REF] Jia | Fabrication, characterization and photocatalytic activity of La-doped ZnO nanowires[END_REF], Ce [START_REF] Yayapao | Sonochemical synthesis, photocatalysis and photonic properties of 3% Ce-doped ZnO nanoneedles[END_REF], and Dy [START_REF] Yayapao | Sonochemical synthesis of Dy-doped ZnO nanostructures and their photocatalytic properties[END_REF] with the idea of benefiting from the specific redox properties of each element, from which the photocatalytic activity of ZnO NWs may be increased. In the vast majority of reports, the extrinsic doping of ZnO NWs has been found to improve their photocatalytic activity.

Nevertheless, the reasons explaining the enhancement that may originate from the effect on the structural morphology (i.e. shape, density), dimensions (i.e. diameter, length), dopant-induced defects, surface properties, and physical properties of ZnO NWs including their optical absorption and electrical conductivity have not been unraveled despite their primary importance and hence are still under debate. Recently, following the pioneered work of Joo et al. aiming at tuning the aspect ratio of ZnO NWs grown by chemical bath deposition (CBD) using the addition of metal cations in the bath [START_REF] Joo | Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis[END_REF], the development of the extrinsic doping of ZnO NWs with Al [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF] and Ga [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF] has thoroughly been achieved. The physicochemical processes at work following the addition of Al nitrate and Ga nitrate in the bath have been studied in detail, along with the CBD conditions required to favor the incorporation of dopants through attractive electrostatic interactions [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF][START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF]. The doping control in ZnO NWs grown by CBD has thus offered recently a greater promise to precisely determine its effect on the performance of devices, in particular in the field of piezoelectric applications [START_REF] Synhaivskyi | Evidence of Piezoelectric Potential and Screening Effect in Single Highly Doped ZnO:Ga and ZnO:Al Nanowires by Advanced Scanning Probe Microscopy[END_REF]. However, no investigation has been reported so far about the effect of the doping with Al and Ga using the approach in Refs. [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF][START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF] on the optical and surface properties of ZnO NWs grown by CBD as well as on the related photocatalytic processes, opening the way for more carefully understanding the specific role of each dopant on their photocatalytic activity.

In this work, we investigate the effect of the Al and Ga doping of ZnO NWs grown by CBD and thermally annealed under oxygen atmosphere on their photocatalytic activity and decouple the different contributions by thoroughly investigating their structural morphology, dimensions, dopant-induced defects, surface properties, and optical absorption and emission, using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman and photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-visible absorption, and contact angle (CA) measurements. The photocatalytic activity of annealed Al-doped ZnO NWs is found to be much larger and stable in time, through the direct action of holes enhanced by the efficient light trapping in the array in addition to the significant action of radicals in the degradation process of three different organic dyes. The present study enables us to report a comprehensive diagram accounting for the precise effect of each Al and Ga dopant as well as of the thermal annealing on the photocatalytic activity of ZnO NWs.

Experimental

Deposition techniques

ZnO NW arrays were grown by chemical bath deposition on a c-axis oriented polycrystalline ZnO seed layer pre-deposited by dip coating on a cleaned (3 × 0.9 cm 2 ) 150 nm-thick ITO / borosilicate glass substrate (Delta Technology LTD) using a double-step process. The sol-gel process using dip coating and leading to the growth of a 40 nm-thick polycrystalline seed layer composed of well crystallized c-axis oriented ZnO nanoparticles is described in Ref. [START_REF] Guillemin | Controlling the Structural Properties of Single Step, Dip Coated ZnO Seed Layers for Growing Perfectly Aligned Nanowire Arrays[END_REF]. The growth of vertically-aligned undoped, Ga-doped, and Al-doped ZnO NWs was performed by CBD as described in Refs. [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF][START_REF] Parize | Effects of Hexamethylenetetramine on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition[END_REF]. a concentration of 0.6 mmol/L corresponding to a dopant to zinc precursor ratio of 2 %. In order to favor the incorporation of Al and Ga into the crystal lattice of ZnO NWs, the pH of the solution was set to 10.9 by the addition of ammonia (NH 3 , VWR Chemicals) to the chemical precursor solution prior to the deposition start.

The CBD of ZnO NWs took place in a sealed glass reactor containing the chemical precursor solution that was further put in an oven kept for 3 hours at 90 °C. Eventually, undoped, Ga-doped and Al-doped ZnO NW arrays were annealed for 1 h at 400 °C under oxygen atmosphere using a dedicated tubular furnace.

Characterization techniques

The morphology of ZnO NWs was investigated by top-view and cross-sectional view FESEM images using a FEI Quanta 250 FEG-SEM operating at 20 keV. XRD patterns were collected with a Bruker D8 Advance diffractometer using Cu Kα 1 radiation according to the Bragg-Brentano configuration. Raman scattering spectra were recorded using a Horiba/Jobin Yvon Labram spectrometer equipped with a liquid-nitrogencooled CCD detector. The 488 nm excitation line of an Ar + laser was used with a power on the sample surface lower than 0.9 mW. The laser beam was focused to a spot size of 1 μm 2 using a 100 times objective.

The spectra were calibrated in wavenumber at room temperature by using a silicon reference sample and considering that the theoretical position of the silicon Raman line is set to 520. The optical properties of ZnO NW array samples, including total transmittance, total reflectance, and diffuse transmittance, were recorded using a JASCO V-670 UV-Vis-NIR spectrophotometer equipped with an integrating sphere. The optical spectra were recorded in the wavelength range of 330-850 nm and used to calculate the optical bandgap energy (E g ) and the Haze factor as defined by the ratio of the diffuse transmittance over the total transmittance. E g was calculated from the intercept of the linear portion of (αhν) 2 vs. hν graph with the hν-axis [START_REF] Viezbicke | Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system[END_REF]. Room-temperature PL measurements were made using a chopped Kimmon IK Series He-Cd laser (325 nm and 40 mW). Fluorescence was dispersed with an Oriel Corner Stone 1/8 74000 monochromator, detected using a Hamamatsu H8259-02 with a socket assembly E717-500 photomultiplier, and amplified through a Stanford Research Systems SR830 DSP. A filter in 360 nm was used to stray light. All spectra were corrected for the response function of the setups.

Photocatalytic measurements

The photocatalytic activity under near UV illumination of ZnO NW arrays was estimated by following the degradation -i.e. the change in concentration with time -of three different organic dyes including methyl orange (MO), methylene blue (MB), and Rhodamine B (RhB). The organic dye concentrations were monitored every 30 minutes using the JASCO V-670 UV-VIS-NIR spectrophotometer. Prior to the photocatalytic activity measurements, the ZnO NW array samples were put in the dark for one hour into the organic dye solution to reach the equilibrium in the adsorption process. A Philips Mercury lamp (two 15 W lamps, model TL-D, λ max = 365 nm) was used as a UV source. The ZnO NW array samples were mounted in a 5 ml UV-cuvette and filled up with 4 ml of organic dye solution. The initial concentration of the organic dye solution was set to 15 mg/L (15 ppm). The measurement of the MO dye concentration took place at a wavelength of 464 nm, MB at 660 nm and RhB at 554 nm. The surface wettability of ZnO NW arrays was investigated by measuring the static water CA on a DSA 25 KRÜSS instrument at room temperature from three random places on a film surface. CA values were calculated using the sessile drop fitting method. The ZnO NW array samples were cleaned under UV-C irradiation (20 W, λ max = 254 nm) before wettability test and before each photocatalytic activity test using a UV ozone cleaning system (NanoBioAnalytics, UVC-1014). To identify the reactive species actively taken part in the photocatalytic degradation process, one hour long photocatalytic measurements mounting the ZnO NW array samples in the MO solution containing different scavengers were performed. Tert-butanol was used to scavenge hydroxyl radicals ( • OH), pbenzoquinone to detect the superoxide anion radicals ( • O 2-), and ammonium oxalate to scavenge the holes (h + ). The scavenger concentration was set to 1 mmol/L. Finally, the stability of the photocatalytic activity of ZnO NW array was assessed by measuring six repetitive cycles of the MO degradation, each cycle lasting for 3 hours. The repetitive photocatalytic activity test experiments were recorded using the ZnO NW arrays that had undergone the dye degradation and scavenging mechanism tests. In the case of Al doping, the mean diameter of ZnO NWs is similar to that of the undoped ZnO NWs -i.e.

Results and discussion

Structural and optical properties of ZnO nanowires 3.1.1 Morphology and structural properties

about 60 nm with a needle-like tip, which is in agreement with Ref. [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF]. In the case of Ga doping, while the mean diameter of ZnO NWs remains similar, the tip does have a needle-like shape but instead exhibits a flat polar c-plane as already reported in Ref. [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF]. The length of undoped and Ga-doped ZnO NWs is of the same order and equal to about 6 and 5 µm, respectively, whereas Al-doped ZnO NWs are nearly twice shorter with a length of around 2.5 µm. This drastic decrease in the length of Al-doped ZnO NWs is attributed to a higher supersaturation in the chemical bath from the pH shift to higher values caused by the addition of Al(NO 3 ) 3 , in turn favoring the homogeneous growth in the bulk solution at the expense of the heterogeneous growth on the ZnO seed layer [START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF]. The number density of undoped, Al-doped, and Ga-doped ZnO NWs lies in the same range and equals to about 78, 87, and 76 NWs per µm 2 , respectively. The small increase in the density of Al-doped ZnO NWs is again related to the higher supersaturation in the chemical bath, resulting in a higher nucleation rate at the very onset of the growth. The thermal annealing for 1 hour at 400 °C under oxygen atmosphere does not affect the morphology of ZnO NWs, which is stable up to a temperature of around 800 °C as reported in Ref. [START_REF] Villafuerte | Engineering nitrogen-and hydrogen-related defects in ZnO nanowires using thermal annealing[END_REF]. From geometrical considerations, the developed surface areas (S) of undoped, Al-doped, and Ga-doped ZnO NWs are found to be of about 88, 41, and 71 µm 2 per substrate µm 2 . The smaller developed surface area of Al-doped ZnO NWs is mainly due to their shorter length as compared to undoped and Ga-doped ZnO NWs. Beyond the developed surface area value, the surface roughness of ZnO NWs needs to be taken into account for photocatalytic applications. The formation of crystallographic steps induced by the high axial growth rate at that high pH value [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF] and possible erosion phenomena from HO -ions [START_REF] Willander | Zinc oxide nanowires: controlled low temperature growth and some electrochemical and optical nano-devices[END_REF] occurs on the nonpolar m-plane sidewalls of needle-shaped undoped and Al-doped ZnO NWs. In contrast, the presence of Ga in subsurface stabilizes the nonpolar mplanes of Ga-doped ZnO NWs [START_REF] Sohn | Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity[END_REF][START_REF] Sallet | Crystal Facet Engineering in Ga-Doped ZnO Nanowires for Mid-Infrared Plasmonics[END_REF], exhibiting much fewer crystallographic steps on their sidewalls.

The effect of the Al and Ga doping as well as of the thermal annealing on the microstructure, phase, and chemical composition of the bulk of ZnO NWs was assessed by XRD and Raman spectroscopy measurements. The XRD patterns of undoped, Al-doped, and Ga-doped ZnO NW arrays before and after thermal annealing are presented in Fig. 2a. excluding the formation of a residual phase containing the dopants [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF]. More importantly, no shift of the (0002) and (0004) diffraction peaks arises from as-grown and annealed undoped ZnO NW arrays, as shown in Fig. 2b-c. This indicates that they are completely relaxed following the homoepitaxial process on the polycrystalline ZnO seed layer [START_REF] Guillemin | Critical Nucleation Effects on the Structural Relationship Between ZnO Seed Layer and Nanowires[END_REF]. In contrast, the Al and Ga doping of ZnO NWs leads to the alterations of the positions of the (0002) and (0004) diffraction peaks. A significant shift of around 0.1° for the (0002) and (0004) diffraction peaks towards higher angles as compared to undoped ZnO NWs is shown in as-grown Ga-doped ZnO NWs. This corresponds to a decrease in the inter-reticular distance between the c-planes and reveals the massive incorporation of Ga into the crystal lattice of ZnO. Such a shift is a typical feature of asgrown Ga-doped ZnO NWs, as reported in Refs. [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF][START_REF] Park | Effects of In or Ga doping on the growth behavior and optical properties of ZnO nanorods fabricated by hydrothermal process[END_REF][START_REF] Kim | Effects of Doping with Al, Ga, and In on Structural and Optical Properties of ZnO Nanorods Grown by Hydrothermal Method[END_REF][START_REF] Lim | Synergistic effect of Indium and Gallium co-doping on growth behavior and physical properties of hydrothermally grown ZnO nanorods[END_REF] The thermal annealing of Ga-doped ZnO NW arrays results in a slight reduction of the shift of the ( 0002) and (0004) diffraction peaks, owing to a relaxation of internal stresses that is likely associated with a change of the nature and concentration of Ga dopant-induced defects [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF]. Similarly, a small shift of the ( 0002) and ( 0004 Before thermal annealing as presented in Fig. 3a, the E 2 High line in undoped and Al-doped ZnO NW arrays points at a similar position of 438.0 cm -1 , while its full-width-at-half-maximum (FWHM) very slightly increases from 9.6 to 9.7 cm -1 , respectively. In contrast, the E 2 High line in Ga-doped ZnO NW arrays points at 439.9 cm -1 and its FWHM is much larger and of 12.23 cm -1 . The intensity of the E 2 High line is further greater in as-grown undoped ZnO NW arrays than in doped ZnO NW arrays, revealing a better crystallinity before thermal annealing when no dopants disturbing the crystal lattice are used. Importantly, no additional modes (AM) are shown in undoped and Al-doped ZnO NW arrays. In contrast, two additional modes related to Ga (AM Ga ) point at 633.5 and 696.1 cm -1 in as-grown Ga-doped ZnO NW arrays. Those so-called AM Ga modes, only observed in Ga-doped ZnO thin films [START_REF] Bundesmann | Raman scattering in ZnO thin films doped with Fe[END_REF][START_REF] Gabás | High quality ZnO and Ga:ZnO thin films grown onto crystalline Si (100) by RF magnetron sputtering[END_REF][START_REF] Lung | Characterization of the structural and optical properties of ZnO thin films doped with Ga, Al and (Al+Ga)[END_REF], nanoparticles [START_REF] Pineda-Hernández | Morphology evolution of hydrothermally grown ZnO nanostructures on gallium doping and their defect structures[END_REF], and more recently NWs [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF], are additional evidence of a massive incorporation of Ga dopants into the crystal lattice.

After thermal annealing as presented in Fig. 3b, the E 2 High line in undoped, Al-doped, and Ga-doped ZnO NW arrays points at around 438.0 cm -1 . Similarly, its FWHM very slightly increases from 8.20 to 8.29 cm -1 in undoped and Al-doped ZnO NW arrays, and up to 9.4 cm -1 in Ga-doped ZnO NW arrays. Importantly, the classical AMs pointing at around 277 (AM 1 ), 510 (AM 2 ), 581 (AM 3 ), 597 (AM 4 ), and 643 (AM 5 ) cm -1 in doped ZnO [START_REF] Bundesmann | Raman scattering in ZnO thin films doped with Fe[END_REF] occur in undoped, Al-doped, and Ga-doped ZnO NW arrays. They are typically assigned to the B 1 silent modes in ZnO that are activated by the local breakdown of the translational symmetry coming from the incorporation of dopants [START_REF] Manjón | Silent Raman modes in zinc oxide and related nitrides[END_REF]. The presence of these AMs in undoped ZnO NW arrays is likely due to the incorporation of residual impurities coming from the chemical precursors used for the CBD process.

After a normalization of the Raman spectra to the areas under the E 2 High lines, it turns out that the area under the AM lines relative to those in undoped ZnO NW arrays increases by 10 % in Al-doped ZnO NW arrays, and by 372 % in Ga-doped ZnO NW arrays. This confirms the significant and massive incorporations of Al and Ga dopants, respectively, into the crystal lattice of ZnO NWs. The incorporation of Ga dopants into ZnO NWs is however larger than the incorporation of Al dopants, as already reported in ZnO thin films grown by CBD [START_REF] Edinger | Comparison of chemical bath-deposited ZnO films doped with Al, Ga and In[END_REF]. Correlatively, the position of the AM lines is overall shifted by about 2 cm -1 towards higher wavenumbers in Ga-doped ZnO NW arrays.

Eventually, both XRD and Raman scattering measurements demonstrate the high crystalline quality of ZnO NWs, regardless of the nature of extrinsic doping before and after thermal annealing. In the case of the intentional doping, Al and Ga dopants have significantly been incorporated into the crystal lattice.

Spectroscopic properties

In order to proceed with the identification of dopant-induced defects, room temperature PL spectra of undoped, Al-doped and Ga-doped ZnO NWs are presented in Fig. 4. The near-band-edge (NBE) emission of undoped ZnO NWs is centered around 378 nm (i.e. 3.280 eV) and dominated by radiative transitions involving donor-bound A-excitons that include the hydrogen-related defects such as interstitial hydrogen in bond centered sites (H BC ), zinc vacancy-hydrogen defect complexes (V Zn -3H), and hydrogen substituting for the O lattice sites (H O ) through the I 4 labeled line [START_REF] Villafuerte | Zinc Vacancy-Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition[END_REF]. The high concentration of hydrogen-related defects acting as shallow donors is due to the crystallization of ZnO NWs involving a dehydration process of [Zn(H 2 O) 6 ] 2+ and accounts for the high free electron density of undoped ZnO NWs [START_REF] Villafuerte | Zinc Vacancy-Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition[END_REF][START_REF] Lausecker | Modeling the Elongation of Nanowires Grown by Chemical Bath Deposition Using a Predictive Approach[END_REF]. The NBE emission of Al-doped and Ga-doped ZnO NWs is centered at 377.5 nm (i.e. 3.285 eV) and 379 nm (i.e.

3.270 eV), respectively, which is attributed to radiative transitions involving donor-bound A excitons that include Al and Ga substituting for the Zn lattice sites (Al Zn and Ga Zn ) through the respective I 6 and I 8 labeled lines [START_REF] Meyer | Excited state properties of donor bound excitons in ZnO[END_REF]. The high concentration of Al Zn and Ga Zn shallow donors due to their low formation energy [START_REF] T-Thienprasert | Compensation in Al-Doped ZnO by Al-Related Acceptor Complexes: Synchrotron X-Ray Absorption Spectroscopy and Theory[END_REF][START_REF] Demchenko | Impurity complexes and conductivity of Ga-doped ZnO[END_REF] certainly explains their even higher free electron density as reported in Ref. [START_REF] Synhaivskyi | Evidence of Piezoelectric Potential and Screening Effect in Single Highly Doped ZnO:Ga and ZnO:Al Nanowires by Advanced Scanning Probe Microscopy[END_REF]. However, a significant part of Al dopants is expected to lie on the interstitial sites [START_REF] Noriega | Probing the electrical properties of highly-doped Al:ZnO nanowire ensembles[END_REF]. The visible defect band of undoped, Al-doped, and Ga-doped ZnO NWs is centered at around 570 nm (i.e. 2.17 eV), 574 nm (i.e. 2.16 eV), and 588 nm (i.e. 2.11 eV), and is typically attributed to hydrogen-related defect complexes [START_REF] Villafuerte | Zinc Vacancy-Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition[END_REF] that can even include Al and Ga dopants [START_REF] Frodason | Zn vacancy-donor impurity complexes in ZnO[END_REF]. The role of oxygen vacancies (V O ) in luminescence spectroscopy has been ruled out by density-functional theory calculations, showing that the related luminescence peaks lie in an energy range lower than 1 eV [START_REF] Lyons | First-principles characterization of native-defect-related optical transitions in ZnO[END_REF]. Interestingly, the relative intensity of the NBE emission over the visible defect band increases when ZnO NWs are doped with Al dopants and even more significantly with Ga dopants.

Following the thermal annealing under oxygen atmosphere, undoped, Al-doped, and Ga-doped ZnO NWs exhibit a more intense NBE emission centered at around 378 nm (i.e. 3.28 eV). The visible defect band in annealed, undoped ZnO NWs is still pronounced with two main components at around 529 nm (i.e. 2.34 eV)

and 590 nm (i.e. 2.10 eV), although the relative intensity slightly increases showing the relative stability and redistribution of hydrogen-related defects at the annealing temperature of 400 °C [START_REF] Villafuerte | Engineering nitrogen-and hydrogen-related defects in ZnO nanowires using thermal annealing[END_REF]. In contrast, the visible defect band in annealed Al-doped and Ga-doped ZnO NWs almost vanishes [START_REF] Escobedo-Morales | Defect annihilation and morphological improvement of hydrothermally grown ZnO nanorods by Ga doping[END_REF], indicating the drastic decrease in the concentration of hydrogen-related defects that include Al and Ga dopants or not.

Optical absorption properties

To characterize the optical absorption properties of undoped, Al-doped, and Ga-doped ZnO NW arrays, the total transmittance and reflectance along with the diffuse transmittance in the wavelength region of 350-850 nm were recorded by UV-visible spectrophotometry. Haze factors values showing their scattering ability were further calculated as the ratio of the diffuse transmittance over the total transmittance. The total transmittance, reflectance and Haze factor curves versus wavelength are presented in Fig. 5a andb, respectively. from the Tauc plot of (ahv) 2 versus hv as shown in Ref. [START_REF] Viezbicke | Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system[END_REF]: αhν = A(hν -E g ) n , where hν is the photon energy, α is the absorption coefficient, and A is a constant. For direct band gap semiconductors including ZnO, n = 1/2. As an example, the Tauc plot of annealed, undoped ZnO NWs is presented in the inset of Fig. 5b. The E g values of undoped and Al-doped ZnO NWs before and after thermal annealing lie in the range of 3.21-3.25 eV, which is in agreement with the typical values ranging from 3.24 to 3.30 eV reported for ZnO thin films [START_REF] Viezbicke | Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system[END_REF] and nanorods [START_REF] Saenz-Trevizoa | Optical Band Gap Estimation of ZnO Nanorods[END_REF]. In contrast, the E g values of Ga-doped ZnO NWs before and after thermal annealing decreases below 3.20 eV owing to the doping-induced band gap renormalization effect [START_REF] Ye | Fermi-level band filling and bandgap renormalization in Ga-doped ZnO[END_REF], and hence their optical behavior markedly differs.

Photocatalytic properties and surface composition of ZnO nanowires 3.2.1 Photocatalytic activity

The photocatalytic activity of undoped, Al-doped, and Ga-doped ZnO NW arrays before and after thermal annealing as measured under UV-A irradiation with a 30 W Hg lamp is presented in Fig. 6 through the evolution of the residual over initial concentration ratios of MO, MB, and RhB organic dyes as a function of time along with the photocatalytic degradation kinetics plots. When no ZnO NW-based photocatalysts are used, the residual over initial concentration ratios are around 100 %, indicating that no significant photocatalytic activity occurs. In contrast, the introduction of ZnO NW-based photocatalysts results in a drastic decrease in the residual over initial concentration ratios of all the organic dyes and hence in a strong improvement of the photocatalytic activity. The photocatalytic degradation kinetics was analyzed within the Langmuir-Hinshelwood model typically used when the initial concentration of organic dyes is sufficiently low as follows [START_REF] Turchi | Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack[END_REF][START_REF] Behnajady | Kinetic study on photocatalytic degradation of C.I. Acid Yellow 23 by ZnO photocatalyst[END_REF]:

(1)

where k S is the apparent reaction rate constant, C 0 is the initial concentration of organic dyes, and C is the concentration of organic dyes at reaction time of t, respectively. All the plots of ln(C 0 /C) vs t follow a linear relationship with a high correlation coefficient (R 2 ), showing that the photocatalytic degradation of organic dyes by ZnO NWs obeys the pseudo first-order reaction kinetics. The slope deduced from each fit provides an estimate of k S , as presented in Table 1.

First, it is worth noticing that most of ZnO NWs exhibit a photocatalytic activity that is larger to degrade the MB and RhB organic dyes than to degrade the MO organic dye. This may be due to the fact that the adsorption of organic dyes on the surfaces of ZnO NWs depends on the pH value of the solution lying here in the intermediate range (~5.4). In particular, the anionic MO organic dye latter is known to more efficiently adsorb on the surfaces of ZnO NWs at a lower pH value favoring attractive electrostatic interactions [START_REF] Verma | Photocatalytic degradation kinetics of cationic and anionic dyes using Au-ZnO nanorods: Role of pH for selective and simultaneous degradation of binary dye mixtures[END_REF].

Second, the thermal annealing results in a significant improvement of the photocatalytic activity of all of the annealed ZnO NWs as compared to their as-grown counterparts, regardless of the chemical nature of dopants and of the nature of organic dyes. For example, the photocatalytic activity of undoped ZnO NWs increases by a factor of 1.6 after thermal annealing: the annealed, undoped ZnO NWs have a photocatalytic activity of 58 % for 3h in MO dye solution, whereas the as-grown, undoped ZnO NWs exhibit a photocatalytic activity of only 37 %. The thermal annealing of doped ZnO NWs even shows a more pronounced increase in the photocatalytic activity. For example, the photocatalytic activity of Ga-doped ZnO NWs increases by a factor of 2.7 after thermal annealing: the annealed, Ga-doped ZnO NWs have a photocatalytic activity of 19 % for 3h in MO dye solution, while the as-grown, Ga-doped ZnO NWs exhibit a photocatalytic activity of only 7 %. The photocatalytic activity of Al-doped ZnO NWs doubles after thermal annealing, showing an increase from 33 to 61 %. Therefore, the thermal annealing makes a large contribution to the photocatalytic activity of all of the ZnO NWs, regardless of the chemical nature of dopants including Al and Ga. Third, the chemical nature of dopants has a critical effect on the photocatalytic activity of ZnO NWs. The decrease in the residual over initial concentration ratios of all of the organic dyes is much more pronounced in Al-doped ZnO NWs than in Ga-doped ZnO NWs. The record photocatalytic activity of 61, 63, and 65 % for 3h in MO, MB, and RhB dye solutions, respectively, was achieved using annealed, Al-doped ZnO NWs whereas they exhibit the smallest developed surface area of 41 µm 2 per substrate µm 2 , namely less than half of the developed surface area of annealed, undoped ZnO NWs that is equal to 88 µm 2 per substrate µm 2 . This reveals that the photocatalytic activity of annealed, Al-doped ZnO NWs is much larger than undoped and Ga-doped ZnO NWs before and after thermal annealing. This dependence of the photocatalytic activity on the chemical nature of dopants including Al and Ga is shown regardless of the nature of organic dyes.

Surface composition

In order to understand the differences in the photocatalytic activity, a study of the chemical nature of surfaces in undoped, Al-doped, and Ga-doped ZnO NW arrays before and after thermal annealing was carried out by XPS measurements. The presence of Ga on the surfaces of Ga-doped ZnO NWs is shown in Fig. S1 through the emission peaks of the Ga 2p 1/2 and 2p 3/2 core levels pointing at 1145 and 1118 eV, respectively [START_REF] Wang | Microstructural and optical characteristics of solution-grown Ga-doped ZnO nanorod arrays[END_REF]. In contrast, the emission peaks related to the Al 2p core levels pointing at around 75 eV in Al-doped ZnO NWs does not occur. This does not indicate the absence of Al on the surfaces of ZnO NWs, as the incorporation of Al dopants is weaker than that of Ga dopants, and this difference has already been reported in ZnO:Al and ZnO:Ga thin films grown by CBD [START_REF] Edinger | Comparison of chemical bath-deposited ZnO films doped with Al, Ga and In[END_REF]. In view of all the experimental data, we estimate that the amount of Al dopants incorporated into ZnO NWs is much smaller than the amount of Ga dopants incorporated into ZnO NWs [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF]. A quantitative analysis gives a Ga to zinc atomic ratio of 1.83 % before thermal annealing, and of 3.07 % after thermal annealing. By comparing that quantification with the approximately 2 % atomic ratio obtained by energy-dispersive X-ray spectroscopy on similar Ga-doped ZnO NWs as reported in Ref. [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF], the migration of Ga dopants towards the surfaces of ZnO NWs upon thermal annealing is revealed. This is in good agreement with its presumable stability in the subsurfaces of ZnO [START_REF] Sohn | Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity[END_REF][START_REF] Sallet | Crystal Facet Engineering in Ga-Doped ZnO Nanowires for Mid-Infrared Plasmonics[END_REF]. As regards the nature of surface defects related to zinc and oxygen, the emission peaks attributed to the O 1s and Zn 2p 3/2 core levels are shown in Fig. 7. The emission peaks assigned to the Zn 2p 2/3 core levels are fairly similar, and asymmetric. These peaks are usually fitted with two Gaussian functions located at 1021.5 eV and 1022.5 eV, corresponding to Zn-O bonds, and Zn(OH) 2 groups, respectively [START_REF] Pradhan | Vertical Growth of Two-Dimensional Zinc Oxide Nanostructures on ITO-Coated Glass: Effects of Deposition Temperature and Deposition Time[END_REF]. The emission peaks attributed to the O 1s core levels are also similar to each other. These peaks are usually fitted with three Gaussian functions located at 530 ± 0.1 eV, 531.0 eV, and 532 eV, corresponding to Zn-O bonds [START_REF] Pradhan | Vertical Growth of Two-Dimensional Zinc Oxide Nanostructures on ITO-Coated Glass: Effects of Deposition Temperature and Deposition Time[END_REF], V O [START_REF] Zheng | Synthesis and optical properties of flower-like ZnO nanorods by thermal evaporation method[END_REF], and hydroxyl groups (O-H bonds) [START_REF] Mccafferty | Determination of the concentration of surface hydroxyl groups on metal oxide films by a quantitative XPS method[END_REF],

respectively. The relative ratios of the two/three main components collected on the surfaces of ZnO NWs as calculated from the Zn 2p 3/2 /O 1s core levels in XPS spectra, respectively, are presented in Table 2. Overall, the emission peak corresponding to the Zn-O bonds systematically shifts towards higher energies following the thermal annealing, which indicates a better crystallinity of annealed ZnO NWs over as-grown ZnO NWs.

Conversely, this peak shifts towards lower energies in doped ZnO NWs as compared to their undoped counterparts, again showing the better crystallinity of undoped ZnO NWs. The contribution of Zn(OH) 2 species on the surfaces of all of the ZnO NWs is modest and significantly increases in undoped and Ga-doped ZnO NWs after thermal annealing. In contrast, the contribution of Zn(OH) 2 species is fairly constant in Al-doped ZnO NWs after thermal annealing. Correlatively, a significant increase in the contribution of O-H bonds on the surfaces of undoped and Ga-doped ZnO NWs after thermal annealing occurs. For example, the O-H ads /Zn-O ratio in undoped and Ga-doped ZnO NWs increases by a factor of 3.5 and 1.5, from 0.6/1.3 to 2.1/2.0 in as-grown to annealed ZnO NWs, respectively. In contrast, the contribution of O-H bonds on the surfaces of Al-doped ZnO NWs is nearly constant even after thermal annealing. This trend is further supported by wettability measurements, where the CA of all of the ZnO NWs after thermal annealing systematically decreases, before and after an exposition to UV-C irradiation for 10 min as reported in Table S1 [START_REF] Gromyko | Surface properties of sprayed and electrodeposited ZnO rod layers[END_REF]. The enrichment of the surfaces of all of the ZnO NWs by the hydroxyl groups is related to the exo-diffusion of hydrogen in the form of H BC for instance from their bulk upon thermal annealing [START_REF] Villafuerte | Engineering nitrogen-and hydrogen-related defects in ZnO nanowires using thermal annealing[END_REF]. Importantly, the hydroxyl groups are known as some of the main species that contribute to the photocatalytic activity of ZnO NWs through the formation of • OH radicals [START_REF] Gromyko | Surface properties of sprayed and electrodeposited ZnO rod layers[END_REF][START_REF] Guo | Photocatalytic activity of metal oxides-The role of holes and OH radicals[END_REF]. In that sense, the higher photocatalytic activity of annealed ZnO NWs as compared to their as-grown counterparts is, to some extent, attributed to the increase in the concentration of hydroxyl groups on their surfaces.

The contribution of V O on the surfaces of all of the ZnO NWs is fairly weak as compared to the contribution of O-H bonds, suggesting a less than expected impact on their photocatalytic activity [START_REF] Guo | Effect of Native Defects on Photocatalytic Properties of ZnO[END_REF], and strongly increases in undoped and Ga-doped ZnO NWs after thermal annealing. For instance, the V O /Zn-O ratio in undoped and Ga-doped ZnO NWs increases by a factor of 2.5 and 4, from 0.16/0.05 to 0.4/0.2 in asgrown to annealed ZnO NWs, respectively. This is likely due to the exo-diffusion of hydrogen in the form of H O upon thermal annealing, leaving a high concentration of V O on the surfaces of ZnO NWs [START_REF] Villafuerte | Engineering nitrogen-and hydrogen-related defects in ZnO nanowires using thermal annealing[END_REF]. In contrast, the contribution of V O on the surfaces of Al-doped ZnO NWs after thermal annealing is halved, indicating that the presence of Al dopants modifies the surface chemistry. The interplay between hydroxyl groups and V O is characteristic of low-temperature solution-grown ZnO nanostructures [START_REF] Gromyko | Surface properties of sprayed and electrodeposited ZnO rod layers[END_REF].

Eventually, a weak emission peak related to the C 1s core level is revealed in Fig. S2, indicating the minor presence of organic species on the surfaces of all of the ZnO NWs. This probably originates from the adsorption of residues of HMTA molecules on the nonpolar m-plane sidewalls of ZnO NWs [START_REF] Parize | Effects of Hexamethylenetetramine on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition[END_REF].

Photocatalytic processes and stability

The processes responsible for the photocatalytic activity of undoped, Al-doped, and Ga-doped ZnO NWs before and after thermal annealing were investigated by determining the degradation rate of the MO organic dye using the so-called scavenging technique, as presented in Fig. 8. Several selective scavenger solutions were added to the MO organic dye solution to identify the main reactive species. For most of the ZnO NWbased photocatalysts, the degradation rate of the MO organic dye is reduced by the tert-butanol and pbenzoquinone scavengers of radical species (i.e. take part in the decomposition of the MO organic dye, likely explaining the smaller photocatalytic activity of Ga-doped ZnO NWs before thermal annealing. Interestingly, the degradation rate of the MO organic dye is increased by the ammonium oxalate scavenger of holes in undoped and Ga-doped ZnO NWs before and after thermal annealing and is constant in Al-doped ZnO NWs before thermal annealing, indicating that the holes do not significantly contribute to the photocatalytic process in those cases. The increase in the degradation rate induced by the ammonium oxalate scavenger is not due to a variation of pH [START_REF] Verma | Photocatalytic degradation kinetics of cationic and anionic dyes using Au-ZnO nanorods: Role of pH for selective and simultaneous degradation of binary dye mixtures[END_REF], whose the value of ~5.3 is similar to the MO organic dye solution with no scavenger. In contrast, the degradation rate of the organic MO dye is strongly reduced by the ammonium oxalate scavenger of holes in Al-doped ZnO NWs after thermal annealing. The holes thus take part in the decomposition of the MO organic dye when using Aldoped ZnO NWs after thermal annealing, which accounts for their enhanced photocatalytic activity as compared to their counterpart before thermal annealing. The stability of the photocatalytic activity of undoped, Al-doped, and Ga-doped ZnO NWs before and after thermal annealing was assessed by measuring the six repetitive cycles of the degradation of the MO organic dye with a 3 h duration for each cycle, as presented in Fig. 9. For all of the ZnO NW-based photocatalysts, the stability is excellent before and after thermal annealing, regardless of the chemical nature of dopants including Al and Ga. As the stability tests lasted for 3 h without any measurements in between, the photocatalytic activity is even slightly larger as compared to those measured with an interval of 30 minutes. and Ga-doped ZnO NWs before and after thermal annealing for 1 h at 400 °C under oxygen atmosphere.

Effects of the doping and thermal annealing on the photocatalysis of ZnO nanowires

The whole experimental data reported enable us to make a comprehensive analysis of the effect of the thermal annealing and extrinsic doping with Al and Ga on the photocatalytic properties of ZnO NWs grown by CBD, as recapitulated in Fig. 10. It is well-known that the photocatalytic properties of ZnO NWs are dependent upon their morphology driving the developed surface area and the ratio of the nonpolar m-plane sidewalls over top polar c-faces [START_REF] Jang | Fine Tuning of the Face Orientation of ZnO Crystals to Optimize Their Photocatalytic Activity[END_REF][START_REF] Li | Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity[END_REF][START_REF] Mclaren | Shape and Size Effects of ZnO Nanocrystals on Photocatalytic Activity[END_REF][START_REF] Liu | Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations[END_REF][START_REF] Kislov | Photocatalytic Degradation of Methyl Orange over Single Crystalline ZnO: Orientation Dependence of Photoactivity and Photostability of ZnO[END_REF][START_REF] Zhang | Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods[END_REF], the nature and concentration of intrinsic point defects [START_REF] Chen | Influence of Defects on the Photocatalytic Activity of ZnO[END_REF][START_REF] Guo | Effect of Native Defects on Photocatalytic Properties of ZnO[END_REF][START_REF] Li | Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity[END_REF][START_REF] Liu | Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations[END_REF][START_REF] Zhang | Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods[END_REF], as well as their ability to trap the light to improve the optical absorption in the UV or visible part of the electromagnetic spectrum to name a few. ZnO NWs before and after thermal annealing for 1 h at 400 °C under oxygen atmosphere.

The main chemical reactions generating the reactive species during the photocatalysis of organic dyes by undoped ZnO NWs are summarized as follows [START_REF] Guo | Photocatalytic activity of metal oxides-The role of holes and OH radicals[END_REF]:

ZnO + ħν  h VB + + e CB - (2)
h VB + + OH (ads)  • OH (3) h VB + + H 2 O  • OH + H + (4) e CB -+ O 2(ads)  • O 2 - ( 5
) Dye + h VB +  • Dye + (6)
Upon UV irradiation with a photon energy higher than the band gap energy, electron-hole pairs are generated in the bulk of undoped ZnO NWs with the holes h VB + and electrons e CB -being in the valence band (VB) and conduction band (CB), respectively (Eq. 2). The free h VB + migrate from the bulk towards the surfaces of undoped ZnO NWs owing to the upward band bending related to a surface depletion coming from the adsorbed oxygen anions [START_REF] Morrison | Chemical Role of Holes and Electrons in ZnO Photocatalysis[END_REF][START_REF] Soci | ZnO nanowire UV photodetectors with high internal gain[END_REF]. As a result, they can readily react with OH groups and H 2 O molecules to form • OH radicals (Eqs. 3 and 4). Moreover, the free e CB -stay in the bulk of undoped ZnO NWs owing to the upward band bending, but some trapped e CB -on the surfaces of undoped ZnO NWs can efficiently react with O 2 molecules to form • O 2 -radicals (Eq. 5). The photocatalytic activity of undoped ZnO NWs is thus significant and mostly relies on the contribution of • OH and • O 2 -radicals as has been shown by the scavenger experiments. Following the thermal annealing under oxygen atmosphere, the enrichment of the surfaces of undoped ZnO NWs with OH groups as has been shown by XPS spectra is activated by the exodiffusion process of hydrogen-related defects in the form of H BC from their bulk and results in the formation of a higher concentration of • OH radicals. The higher concentration of V O on the surfaces of annealed, undoped ZnO NWs is also due to the exo-diffusion process of hydrogen-related defects in the form of H O and leads to the formation of a higher concentration of • O 2 -radicals. The annealed, undoped ZnO NWs therefore exhibit a higher photocatalytic activity owing to the higher concentration of • OH and • O 2 -radicals.

In the case of Ga-doped ZnO NWs, the photocatalytic activity before thermal annealing is weak and mostly relies on the contribution of • OH radicals. The surfaces of as-grown Ga-doped ZnO NWs are stabilized by the presence of Ga dopants in the subsurface and hence are smooth, drastically reducing the concentration of V O such that nearly no • O 2 -radicals are formed. Following the thermal annealing under oxygen atmosphere, the same exo-diffusion processes of hydrogen-related defects occur, enriching the surfaces of Ga-doped ZnO NWs with OH groups and V O subsequently resulting in a higher concentration of

• OH and • O 2 -radicals. The photocatalytic activity of annealed, Ga-doped ZnO NWs is thus enhanced drastically, but does not reach the one of undoped ZnO NWs owing their smoother surfaces originating from their enrichment with Ga dopants on the subsurface following the thermal annealing.

In the case of Al-doped ZnO NWs, the photocatalytic activity before thermal annealing is high and relies on the contribution of • OH and • O 2 -radicals. Given that the Al dopants can occupy both the substitutional Zn site as Al Zn and interstitial sites as Al i , we expect that they play a more significant role in the incorporation of hydrogen-related defects with a less concentration, in turn producing a higher concentration of OH groups on the surfaces of ZnO NWs even before thermal annealing. Following the thermal annealing under oxygen atmosphere, the photocatalytic activity of Al-doped ZnO NWs is very high and reaches the best value in the present study. However, the reasons for the present improvement after thermal annealing differs from the explanation given in the cases of undoped and Ga-doped ZnO NWs. Here, the enrichment of the surfaces of annealed, Al-doped ZnO NWs with OH groups does not operate because the reservoir of hydrogen-related defects in the form of H BC is less. A reduction of the concentration of V O on the surfaces of Al-doped ZnO NWs is even noticed after thermal annealing, which is a typical behavior under oxygen atmosphere. In that sense, the increase in the photocatalytic activity of annealed, Al-doped ZnO NWs is not mostly attributed to the significant modification of the concentration of • OH and • O 2 -radicals. Instead, the enhancement of the photocatalytic activity of annealed, Al-doped ZnO NWs stems from the additional contribution of h VB + in the process. A direct oxidation path of the organic dye with the free or trapped h VB + on the surfaces of annealed Al-doped ZnO NWs can proceed according to Eq. 6.

Eventually, the needle-shaped morphology of undoped and Al-doped ZnO NWs along with the crystallographic steps on their nonpolar m-planes resulting in the formation of polar c-planes on the sidewalls enhances their photocatalytic activity as compared to Ga-doped ZnO NWs and hence the degradation of organic dyes for the following reasons: i) the surface area of the active polar c-planes is higher in that configuration [START_REF] Jang | Fine Tuning of the Face Orientation of ZnO Crystals to Optimize Their Photocatalytic Activity[END_REF][START_REF] Li | Morphology-Function Relationship of ZnO: Polar Planes, Oxygen Vacancies, and Activity[END_REF][START_REF] Mclaren | Shape and Size Effects of ZnO Nanocrystals on Photocatalytic Activity[END_REF][START_REF] Liu | Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations[END_REF], ii) the proximity of the non-polar m-planes with the polar c-planes on the same sidewalls may form a surface heterojunction separating the photo-generated electron-hole pairs and hence reducing their recombination as reported in TiO 2 nanoparticles [START_REF] Yu | Enhanced Photocatalytic CO 2 -Reduction Activity of Anatase TiO 2 by Coexposed {001} and {101} Facets[END_REF], iii) the density of surface defects including V O is higher to activate a higher concentration of • O 2 -radicals [START_REF] Zhang | Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods[END_REF], iv) the light scattering processes are higher in that configuration as has been shown by transmittance and Haze factor spectra, which may increase the generation rate and amount of electron-hole pairs. Most of these phenomena are expected to lead to the implication of more electron-hole pairs in the photocatalytic processes, which may in turn favor the direct oxidation path using h VB + in the case of Al-doped ZnO NWs. The formation of ZnO NWs grown by CBD at high pH using NH 3 is therefore favorable to incorporate the Al dopants, but also to engineer their structural morphology and surfaces, both of them being highly beneficial to boost their photocatalytic activity for water remediation.

Conclusion

We have investigated the effect of the extrinsic doping of ZnO NWs grown by CBD using Al and Ga and of the thermal annealing under oxygen atmosphere on their photocatalytic activity to degrade three organic dyes including MO, MB, and RhB. The different contributions to the photocatalytic activity originating from the structural morphology, dimensions, dopant-induced defects, surface properties, and optical absorption of

A

  group-III dopant to zinc chemical precursor concentration ratio (i.e. [Ga(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] and [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ]) of 2 % was chosen to keep the morphology of ZnO NWs, as shown in Fig. 1, while ensuring the proper doping of the nanostructures.

Fig. 1 .

 1 Fig. 1. Top-view FESEM images of as-grown, undoped, Al-doped, and Ga-doped ZnO NW arrays by CBD.

Fig. 2 .

 2 Fig. 2. (a) XRD patterns of undoped, Al-doped, and Ga-doped ZnO NWs, before and after thermal annealing for 1 h at 400 °C under oxygen atmosphere. (b)-(c) Zoom-in in the area of interest around the (0002) and (0004) diffraction peaks of ZnO, respectively.

Fig. 2a ,

 2a Fig. 2a, correspond to the ITO layer crystallizing into the bixbyite structure. It is worth mentioning that no Al or Ga containing phases (oxides, hydroxydes, or oxy-hydroxydes) are detected in the XRD patterns,

  ) diffraction peaks towards higher angles is revealed in as-grown Al-doped ZnO NWs, indicating the significant incorporation of Al into the crystal lattice of ZnO. This shift is even reversed towards lower angles corresponding to an increase in the inter-reticular distance between the c-planes in annealed Al-doped ZnO NW arrays, again suggesting a modification of the nature and concentration of Al dopant-induced defects.The Raman scattering spectra of undoped, Al-doped, and Ga-doped ZnO NW arrays before and after thermal annealing are presented in Fig.3a-b, respectively. The typical phonon modes of the wurtzite structure of ZnO NWs occur at 100 (E 2 Low ), 438-440 (E 2 High ), and 574 (A 1 (LO) cm -1 ). A second-order Raman line points are at 333 cm -1 (E 2 High -E 2 Low ). No residual phases containing the dopants are revealed either, which is in agreement with the XRD patterns.

Fig. 3 .

 3 Fig. 3. Raman scattering spectra of undoped, Al-doped, and Ga-doped ZnO NWs (a) before and (b) after thermal annealing for 1 h at 400 °C under oxygen atmosphere. The intensity was normalized to the area under each E 2 High line in (b).

Fig. 4 .

 4 Fig. 4. Room temperature PL spectra of undoped, Al-doped, and Ga-doped ZnO NWs (a) before and (b) after thermal annealing for 1 h at 400 °C under oxygen atmosphere. The intensity was normalized to the highest value. The insert represents the zoom-in in the NBE emission.

Fig. 5 .

 5 Fig. 5. (a) Total transmittance (solid lines) and reflectance (dashed lines) spectra and (b) Haze factor of undoped, Al-doped, and Ga-doped ZnO NWs before and after thermal annealing for 1 h at 400 °C under oxygen atmosphere. The inset represents the Tauc plot of annealed, doped ZnO NWs, where the red dashed line denotes the fit of the linear portion to deduce the optical band gap energy from its intercept with the hνaxis.

Fig. 6 .

 6 Fig. 6. Evolution of the residual over initial concentration ratios of MO, MB, and RhB organic dyes (top) as a function of time and photocatalytic degradation kinetics plots (bottom), showing the photocatalytic activity under UV-A irradiation with a 30 W Hg lamp of undoped, Al-doped, and Ga-doped ZnO NW arrays before and after thermal annealing for 1h at 400 °C under oxygen atmosphere. The dashed lines in the photocatalytic degradation kinetics plots represent the linear fit from the Langmuir-Hinshelwood model in Eq.(1).

Fig. 7 .

 7 Fig. 7. XPS spectra centered at the emission peaks corresponding to the O 1s and Zn 2p 3/2 core levels of undoped, Al-doped, and Ga-doped ZnO NW arrays, before and after thermal annealing for 1 h at 400 °C under oxygen atmosphere.

Fig. 8 .

 8 Fig. 8. Degradation rate of the MO organic dye for undoped, Al-doped, and Ga-doped ZnO NWs before and after thermal annealing for 1 h at 400 °C under oxygen atmosphere and in the presence of • OH scavenger (tert-butanol), • O 2 -scavenger (p-benzoquinone), h + scavenger (ammonium oxalate).

Fig. 9 .

 9 Fig. 9. Evolution of the degradation rate as a function of the MO degradation cycles for undoped, Al-doped,

Fig. 10 .

 10 Fig. 10. Schematic diagram summarizing the photocatalytic processes in undoped, Al-doped, and Ga-doped

ZnO

  NWs have been decoupled to cast a light on the dominant photocatalytic processes. We have shown that undoped and doped ZnO NWs exhibit a more efficient photocatalytic activity to degrade the MB and RhB organic dyes than to degrade the MO organic dye and that the photocatalytic processes systematically obey a pseudo-first order reaction kinetic. The photocatalytic activity of as-grown, undoped and Al-doped ZnO NWs involving • OH and • O 2 -radicals with a high concentration is much larger than the photocatalytic activity of Ga-doped ZnO NWs only involving • OH radicals. The thermal annealing under oxygen atmosphere further results in a drastic enhancement of the photocatalytic activity through i) the formation of • OH and • O 2 -radicals with a higher concentration coming from the exo-diffusion process of hydrogenrelated defects enriching the surfaces with OH groups and V O in undoped and Ga-doped ZnO NWs, and i) the activation of the direct oxidation path using h VB + in Al-doped ZnO NWs. The needle-shaped morphology of undoped and Al-doped ZnO NWs along with the crystallographic steps on their nonpolar m-planes leading to the formation of active polar c-planes on the sidewalls additionally play a major role in the improvement of the photocatalytic activity. The annealed, Al-doped ZnO NWs are hence found to exhibit the most efficient photocatalytic activity despite their smallest developed surface area. The present study reports a comprehensive analysis accounting for the precise effect of each Al and Ga dopant as well as of the thermal annealing on the photocatalytic activity of ZnO NWs and shows the interest in working at high pH during the CBD using NH 3 . It eventually emphasizes the critical importance to select the dopant with care and the requirement to decouple the intricate contributions to the photocatalytic processes at work in ZnO NWs for water remediation.

  The chemical precursor solution consisted of the standard equimolar (30 mmol/L) mix of zinc nitrate hexahydrate (Zn(NO 3 ) 2 ꞏ6H 2 O, Sigma-Aldrich) and HMTA (C 6 H 12 N 4 , Sigma-Aldrich) in deionized water. To grow Ga-and Al-doped ZnO NWs,

	gallium nitrate hydrate (Ga(NO 3 ) 3 ꞏxH 2 O, Sigma-Aldrich) and aluminium nitrate nonahydrate
	(Al(NO 3 ) 3 ꞏ9H 2 O, Sigma-Aldrich) were, respectively, incorporated into the chemical precursor solution with

Table 1 .

 1 Fitting parameters k S and correlation coefficients R 2 deduced from the photocatalytic degradation kinetics plots using the Langmuir-Hinshelwood model. Note that the developed surface area (S) of ZnO NWs was calculated over a substrate surface area of one µm 2 .

		MO		MB		RhB		Developped
								surface area
		kS (10 -3 )	R 2	kS (10 -3 )	R 2	kS (10 -3 )	R 2	S (µm 2 )
	ZnO	2.4	0.99	3.6	0.99	3.5	0.99	88
	ZnO annealed	4.5	0.99	4.9	0.99	5.2	0.99	88
	ZnO:Al	2.2	0.99	4.2	0.99	3.4	0.99	41
	ZnO:Al annealed	5.0	0.99	5.7	0.99	6.1	0.99	41
	ZnO:Ga	0.5	0.97	0.3	0.93	0.6	0.99	71
	ZnO:Ga annealed	1.3	0.99	2.1	0.99	3.0	0.99	71

Table 2 .

 2 ZnO surface component ratios calculated from the resolved integrated areas of the emission peaks corresponding to the O 1s, Zn 2p 3/2 and C 1s core levels.

		O 1s		Zn 2p3/2
		at%/at%	at%/at%
	Samples	VO/ZnO OH ads /ZnO Zn(OH) 2 /ZnO
	ZnO	0.16	0.60	0.17
	ZnO annealed	0.40	2.10	0.30
	ZnO:Al	0.14	1.45	0.18
	ZnO:Al annealed	0.07	1.21	0.18
	ZnO:Ga	0.05	1.30	0.16
	ZnO:Ga annealed	0.20	2.00	0.46

  • OH and • O 2 -radicals, respectively). This reveals that the OH and • O 2 -radicals represent the main species taking part in the decomposition of the MO organic dye for most of the ZnO NW-based photocatalysts. A notable exception is shown in the case of Ga-doped ZnO NWs before thermal annealing, for which the degradation rate of the MO organic dye is constant even in the presence of the p-benzoquinone scavenger of • O 2 -radicals. This indicates that the only • OH radical species

•

Acknowledgments

The authors would like to thank Hervé Roussel, Grenoble, France for his assistance in the XRD experiments. P.G. was supported by a doctoral fellowship from the CDP Eco-SESA project (ANR-15-IDEX-02). V.C. gratefully thanks the financial support from the French National Research Agency through the project DOSETTE (ANR-17-CE24-0003). They further acknowledge the facilities, and the scientific and technical assistance of the CMTC characterization platform of Grenoble INP, which is supported by the Centre of Excellence of Multifunctional Architectured Materials (LabEx CEMAM) under the contract ANR-10-LABX-44-01 funded by the "Investments for the Future" Program. Funding by the Carnot Institute Energies du Futur through the project ECOLED is also acknowledged. The authors acknowledge the MAEDI and MENESR French Ministries and the Estonian Research Council for a financial support through the Parrot program (n°42401QH). The study was also funded by the Estonian Research Council project PRG627, the Estonian Centre of Excellence project TK141 (TAR16016EK) "Advanced materials and high-technology devices for energy recuperation systems", and the European Commission's H2020 program under the ERA Chair project 5GSOLAR grant agreement No 952509.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary material

Supplementary data to this article can be found online.