# Elucidating the role of dopants in the photocatalysis of ZnO nanowires for enhanced water remediation: A comprehensive analysis using Al, Ga, Cl, Cu, Sb, Al-Ga, and Al-Cl

Adrien Baillard, Estelle Appert, Hervé Roussel, Jean-Michel Becht, Véronique

Jacob, Vincent Consonni

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Adrien Baillard, Estelle Appert, Hervé Roussel, Jean-Michel Becht, Véronique Jacob, et al. Elucidating the role of dopants in the photocatalysis of ZnO nanowires for enhanced water remediation: A comprehensive analysis using Al, Ga, Cl, Cu, Sb, Al-Ga, and Al-Cl. Journal of Environmental Chemical Engineering, 2025, 13 (2), pp.115865. 10.1016/j.jece.2025.115865. hal-04962066

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# Elucidating the role of dopants in the photocatalysis of ZnO nanowires for enhanced water remediation: A comprehensive analysis using Al, Ga, Cl, Cu, Sb, Al-Ga, and Al-Cl

Adrien Baillard<sup>a,*</sup><sup>ID</sup>, Estelle Appert<sup>a</sup>, Hervé Roussel<sup>a</sup>, Jean-Michel Becht<sup>b</sup>, Véronique Jacob<sup>c</sup>, Vincent Consonni<sup>a,*</sup><sup>ID</sup>

## ARTICLE INFO

Keywords:

ZnO
Chemical bath deposition
Doping
Photocatalysis
Photocorrosion

## ABSTRACT

As a sustainable semiconductor absorbing UV light while exhibiting high photocatalytic activity, ZnO in the form of nanowires has emerged as a promising candidate. Here, its intentional doping with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl is investigated to thoroughly assess its effects on photocatalysis. We show that the photocatalytic processes are governed by ●OH hydroxyl radicals, as the main reactive species degrading methyl orange, and that free h<sup>+</sup><sub>VB</sub> act as the primary active charge carriers on the surfaces of ZnO nanowires, regardless of the nature and concentration of dopants. The main degradation pathway is found to proceed through the oxidation reaction of a phenyl ring bearing the dimethyl amino group. The photocatalytic activity is further revealed to be higher for Sb and Cu acceptors than for Al, Ga and Cl shallow donors. Beyond its ranking, the photocatalytic activity is optimal for a specific concentration of dopants, from ten to several tens of ppm, except for Ga doping. Eventually, a photo-corrosion phenomenon is concomitant with the photocatalytic processes, raising concerns about the chemical stability of ZnO nanowires. These findings revisit the effects of intentional doping of ZnO nanowires for photocatalysis and open perspectives to address chemical and technological challenges.

## 1. Introduction

Water pollution has become an urgent global concern, necessitating the rapid development of efficient and sustainable methods for water remediation [1]. Conventional water treatment approaches often fall short in effectively eliminating diverse pollutants, such as organic dyes, heavy metals, and emerging contaminants, highlighting the need for innovative, efficient, and sustainable approaches [2]. In this context, advanced oxidation processes (AOPs), particularly photocatalysis, have emerged as promising solutions for water remediation [2]. These processes offer the potential for the efficient and environmentally friendly removal of pollutants by activating photocatalysts with photons. The photocatalytic process relies on the ability of a direct band gap semiconductor to generate electron-hole (e<sub>CB</sub>- h<sup>+</sup><sub>VB</sub>) pairs under photon irradiation, when the photon energy (hν) exceeds its bandgap energy as described in Eq. (1) [3]:

$$ZnO + h\nu \rightarrow e_{CB}^{-} + h_{VB}^{+} \quad (1)$$

When the reduction-oxidation potentials of the valence and conduction bands are well-aligned with the potentials of ●OH/OH⁻ and O₂/●O₂⁻, respectively, both reduction and oxidation processes can take place, forming reactive radicals on the surfaces of the semiconductor as described in Eqs. (2) and (3) [3]:

$$e_{CB} + O_{2(ads)} \rightarrow \bullet O_{2}^{-} \quad (2)$$

$$h_{VB}^{+} + OH_{(ads)}^{-} \rightarrow \bullet OH \quad (3)$$

Water molecules are also expected to react with h<sup>+</sup><sub>VB</sub> to form reactive radicals as follows [3]:

$$h_{VB}^{+} + H_{2}O \rightarrow \bullet OH + H^{+} \quad (4)$$

The photocatalytic process thus stems from the ability of charge carriers (i.e., e<sub>CB</sub> and h<sup>+</sup><sub>VB</sub>), generated under photon irradiation, to reduce or oxidize oxygenated species (i.e. O₂(ads), OH(ads)⁻, H₂O) on the

<footnote><sup>a</sup> Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, Grenoble F-38000, France</footnote>

<footnote><sup>b</sup> Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15 rue Jean Starcky, Mulhouse 68057 CEDEX, France</footnote>

<footnote><sup>c</sup> Université Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, Grenoble F-38000, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: adrien.baillard@grenoble-inp.fr (A. Baillard), vincent.consonni@grenoble-inp.fr (V. Consonni).</footnote>
semiconductor surfaces in order to generate ●OH and ●O₂ highly reactive radicals, decomposing the pollutants. Among the family of direct bandgap semiconductors, zinc oxide (ZnO) in the form of nano-wires (NWs) has attracted considerable attention due to its unique properties at nanoscale dimensions and its high photocatalytic activity [4]. ZnO is further made of abundant elements, cost-effective, and relatively easy to grow by chemical deposition techniques [5,6], as well as considered as safe for human health, which boosts its appeal for practical applications in water remediation [4]. However, despite its great promise, it is still necessary to further enhance the photocatalytic activity of ZnO NWs and deepen the understanding of their photocatalytic processes. As a result, various strategies including intentional doping with either donors or acceptors have been explored [4,7]. The intentional doping of ZnO NWs offers several significant advantages for improving their photocatalytic activity. This includes extended spectral absorption of photons by engineering the bandgap energy, and optimized management of free charge carriers through their efficient separation [4]. In principle, following density-of-states (DOS) calculations, dopant states may hybridize with O₂p and Zn₄s states in the valence and conduction energy bands, respectively, to reduce the bandgap energy and enhance the spectral absorption of photons, thereby generating more e⁻·<sub>CB</sub>-h̅<sub>VB</sub> pairs [8]. In addition, the creation of additional energy levels in the bandgap related to dopants may act as trap states, increasing the lifetime of e⁻·<sub>CB</sub> and h̅<sub>VB</sub> and hence limiting their recombination. Nevertheless, the formation of additional energy levels in the bandgap creating e⁻·<sub>CB</sub>-h̅<sub>VB</sub> trap states is not so obvious. In fact, shallow donors such as Ga [9,10] and Al [10,11] do not create additional energy levels in the bandgap, but shift the Fermi level to the conduction band minimum following the Burstein-Moss effect. Conversely, Cu, as an acceptor, creates additional energy levels in the bandgap close to the valence band maximum, [12,13] which may limit the e⁻·<sub>CB</sub>-h̅<sub>VB</sub> pair recombination up to a certain concentration. The role of Sb, as an acceptor, is still open to debate from density-functional theory (DFT) calculations. Interestingly, for Cu, depending on the number of atoms incorporated, various energy levels can be created simultaneously [14]. As expected, anionic dopants like Cl, acting as shallow donors, do not induce additional energy levels in the bandgap, but a hybridization process between Cl₃s and Zn₄s states occurs [15]. In the Al-Ga co-doping case, no additional energy levels are expected to be generated [9], and no information was found for the Al-Cl co-doping case, but no additional energy levels are expected according to both single-doping cases. The present chemical and physical processes are expected to strongly improve the photocatalytic activity, making ZnO NWs doped with either donors or acceptors more efficient at degrading organic pollutants and removing toxic heavy metals from water sources [4,7].

Over the last two decades, the chemical bath deposition (CBD) technique has emerged as a cost-effective and easily implemented chemical route for the growth of ZnO NWs at low-temperature (i.e. in the range of 70–90 °C) [16,17]. It is further compatible with the twelve principles defining the field of green chemistry [18]. Interestingly, the intentional doping of ZnO NWs with either donors or acceptors has thoroughly been developed in an in-situ manner over the past decade, by adding specific chemical additives containing the dopants during the CBD process. It has recently been shown that donors (Al, Ga, Cl) [19–21] or acceptors (Sb, Cu) [22,23] can be incorporated into ZnO NWs at significant concentrations by adjusting the pH value to favor preferential electrostatic and chemical interactions. The intentional co-doping with Al-Ga [24] and Al-Cl [21] dopants has also been demonstrated using the same approach. All these dopants, except Ga [25,26], are supposed to increase the photocatalytic activity of ZnO NWs compared to unintentionally-doped ZnO NWs [27–32]. However, the multitude of photocatalytic characterization setups prevents direct comparisons between the different types of doping achieved in Refs [7,25,26,29]. Also, the effect of different morphologies of non-intentionally-doped and intentionally-doped ZnO NWs on the photocatalytic activity should be distinguished from the effect of the doping itself and thus decoupled by

following a dedicated procedure. For these reasons, it is still unknown how the different dopants affect the photocatalytic processes of ZnO NWs and whether the nature and amount of dopants are of critical importance for enhancing their photocatalytic activity or not. Despite the high potential of the intentional doping of ZnO NWs for water remediation, several challenges still need to be addressed to ensure their successful implementation in water management strategies. These challenges include optimizing the synthesis conditions with the most appropriate doping (i.e. nature and concentration of dopants), characterizing the doped ZnO NWs, ensuring their stability, and understanding the underlying mechanisms governing their enhanced photocatalytic performance.

In this work, we investigate the growth of non-intentionally-doped, single-doped, and co-doped ZnO NWs by CBD using a wide variety of dopants including Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl at several concentrations. The incorporation of dopants and its effect on the morphological, structural, chemical, and optical properties is characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, inductively coupled plasma-mass spectrometry (ICP-MS), and ultraviolet-visible (UV-Vis) absorption. The photocatalytic activity of non-intentionally-doped, single-doped, and co-doped ZnO NWs is evaluated by monitoring the degradation of methyl orange (MO) organic dye under UV photon irradiation. The degradation pathways are further assessed by ultra-performance liquid chromatography (UPLC) to identify the structures of molecules resulting from the degradation of MO organic dye. Eventually, the chemical stability of non-intentionally-doped, single-doped, and co-doped ZnO NWs during the photocatalytic processes is monitored using FESEM and ICP-MS measurements. The present set of experimental data offers an invaluable investigation to unravel the photocatalytic processes in non-intentionally doped and intentionally doped ZnO NWs with a wide variety of dopants acting as shallow donors (e.g. Al, Ga, Cl, Al-Ga, Al-Cl) or acceptors (Cu, Sb) and using several concentrations for each dopant. They further allow us to revisit the effects of the doping processes on the photocatalytic activity and processes of ZnO NWs by using the same photocatalytic characterization set-up in a comparative and comprehensive single study, in which the nature and concentration of dopants have carefully been assessed.

## 2. Experimental section

### 2.1. Deposition techniques

The growth of non-intentionally-doped, single-doped, and co-doped ZnO NWs was achieved by a double-step process using wet chemistry techniques, as depicted in Fig. 1.

The polycrystalline ZnO seed layers were produced by sol-gel process using dip coating. The sol was composed of 375 mM zinc acetate dihydrate [Zn(CH₃COO)₂·2 H₂O, Sigma-Aldrich, ≥ 99.5 %] and 375 mM of monoethanolamine [MEA, Sigma-Aldrich, ≥ 99.5 %] as complexing agent, all in pure ethanol stirred at 60 °C for several hours and then at room temperature. The xerogel was deposited by dip coating under controlled hygrometry (<15 %) at a withdrawal speed of 3.3 cm/s and then heated at 300 °C for 10 min to evaporate the organic compounds and at 500 °C for 1 hour to induce the crystallization process of ZnO. The chemical synthesis of non-intentionally-doped, single-doped, and co-doped ZnO NWs was subsequently performed by CBD. The substrates coated with the polycrystalline ZnO seed layer were immersed in sealed reactors containing a deionized water solution composed of 30 mM zinc nitrate hexahydrate [Zn(NO₃)₂·6 H₂O, Sigma-Aldrich, ≥ 99.0 %] and 30 mM hexamethylenetetramine (HMTA) [C₆H₁₂N₄, Sigma-Aldrich, ≥ 99.0 %]. The pH value was adjusted around 10.8 by adding 725 mM of ammonia [NH₃, Sigma-Aldrich, 28–30 %], in the chemical bath. The sealed reactors were placed for 3 hours in an oven kept at 90 °C. The intentional doping of ZnO NWs with Al, Ga, Cu, Cl, Al-Ga, and Al-Cl was obtained by adding other salts in the chemical bath, including
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Description of the fabrication processes of as-grown and thermally annealed non-intentionally doped and intentionally doped ZnO NWs using dip coating and CBD with a chemical bath containing Zn(NO₃)₂ and HMTA along with an additional dedicated chemical precursor for the *n*-type single doping, *n*-type co-doping, and *p*-type doping, respectively.</figcaption></figure>

aluminium nitrate nonahydrate [Al(NO₃)₃·9 H₂O, Sigma-Aldrich, 99.99 %], gallium nitrate hydrate [Ga(NO₃)₃·xH₂O, Sigma-Aldrich, 99.9 %] with x considered as 8 [33], copper nitrate semi(pentahydrate) [Cu(NO₃)₂·2.5 H₂O, Sigma-Aldrich, ≥ 99.9 %], ammonium chloride [NH₄Cl, Sigma-Aldrich, 99.998 %], and aluminium chloride hexahydrate [AlCl₃·6 H₂O, Sigma-Aldrich, 99.99 %], respectively. The intentional doping of ZnO NWs with Sb was achieved by forming a Sb-glycolate (SbG₂) solution at a 1:12 ratio according to Ref. 22, using Sb (III) acetate [Sb(CH₃CO₂)₃, Sigma-Aldrich, 99.99 %], glycolic acid [C₂H₄O₃, Sigma Aldrich, 99 %] and sodium hydroxide [NaOH, Sigma Aldrich, ≥ 99.99 %]. The concentration of dopants incorporated into ZnO NWs was adjusted by varying the dopant over Zn concentration ratio, as reported in Table 1. The unused volume of the solution was compensated by the addition of deionized water to maintain the total volume constant. Eventually, a thermal annealing of non-intentionally-doped, single-doped, and co-doped ZnO NWs was performed at 400 °C for 1 hour under O₂ atmosphere.

## 2.2. Morphological and structural characterization techniques

During the CBD process of ZnO NWs, the pH and temperature were acquired in an in-situ manner using an InLab Versatile Pro pH electrode from Mettler Toledo. The morphology of ZnO NWs was analyzed by FESEM images using a ZEISS Gemini 300 FESEM instrument equipped with an in-lens detector and operating at an acceleration voltage of 5 kV. The structural properties of ZnO NWs were assessed by XRD using a BRUKER D8 Advance diffractometer equipped with a Cu Kα₁ radiation in the Bragg-Brentano configuration. The θ/2θ XRD patterns were recorded in the 20–80° range (in 2θ scale) and calibrated with the (004) diffraction peak of Si at 2θ = 69.131°.

## 2.3. Chemical characterization techniques

The chemical composition of ZnO NWs was determined using a

<caption>**Table 1**</caption>

Dopant over Zn concentration ratio and dopant concentration used for the intentional doping of ZnO NWs by CBD.

<table><thead><tr><th>Chemical additives containing the dopants</th><th>[Dopant]/[Zn] ratio (%at)</th><th>Dopant Concentration (mM)</th></tr></thead><tbody><tr><td>Al(NO<sub>3</sub>)<sub>3</sub>, Ga(NO<sub>3</sub>)<sub>3</sub>, Cu(NO<sub>3</sub>)<sub>2</sub>, Sb<sub>2</sub>, AlCl<sub>3</sub></td><td>0.3</td><td>0.09</td></tr><tr><td>Al(NO<sub>3</sub>)<sub>3</sub>, Ga(NO<sub>3</sub>)<sub>3</sub>, Cu(NO<sub>3</sub>)<sub>2</sub>, Sb<sub>2</sub>, AlCl<sub>3</sub>, NH<sub>4</sub>Cl</td><td>1</td><td>0.3</td></tr><tr><td>Al(NO<sub>3</sub>)<sub>3</sub>, Ga(NO<sub>3</sub>)<sub>3</sub>, Cu(NO<sub>3</sub>)<sub>2</sub>, AlCl<sub>3</sub>, NH<sub>4</sub>Cl</td><td>3</td><td>0.9</td></tr><tr><td>Cu(NO<sub>3</sub>)<sub>2</sub>, NH<sub>4</sub>Cl</td><td>9</td><td>2.7</td></tr><tr><td>Al(NO<sub>3</sub>)<sub>3</sub> - Ga(NO<sub>3</sub>)<sub>3</sub></td><td>1 - 2</td><td>0.3 - 0.6</td></tr><tr><td>Al(NO<sub>3</sub>)<sub>3</sub> - Ga(NO<sub>3</sub>)<sub>3</sub></td><td>1.5 - 1.5</td><td>0.45 - 0.45</td></tr><tr><td>Al(NO<sub>3</sub>)<sub>3</sub> - Ga(NO<sub>3</sub>)<sub>3</sub></td><td>2 - 1</td><td>0.6 - 0.3</td></tr></tbody></table>

PERKIN-ELMER NexION 1000 ICP-MS instrument. The nebulizer and spray chamber employed consisted of MEINHARD plus Glass Type C and Glass Cyclonic, respectively, both operating at 2 °C. The kinetic energy discrimination (KED) mode was utilized with He (7 % H₂) as the collision gas. Prior to analysis, the ZnO NWs deposited onto glass substrates were dissolved in a 0.6 % nitric acid solution (HNO₃, Carlo Erba) by immersion. The RF power was set to 1450 W, the KED flow rate to 4 mL/min, and the integration time to 10 ms for Zn, Sb and Ga detection, and 100 ms for Al and Cu detection. To generate calibration standards, PERKIN-ELMER reference solutions were diluted to concentrations of 0.01, 0.1, 0.2, 0.5, and 1 mg/L for Zn, Sb, Ga, Al, and Cu. The analyzed solutions were appropriately diluted to fall within the calibration range, and a control solution containing only the glass substrate was examined to ensure the absence of residual contaminants during the sample preparation.

## 2.4. Optical characterization techniques

The Raman scattering measurements of ZnO NWs deposited onto glass substrates were performed using a HORIBA/JOBIN YVON Labram spectrometer equipped with a liquid nitrogen-cooled CCD detector. The Raman spectra were acquired in the 60–800 cm⁻¹ range, employing the 488 nm Ar⁺ laser line with an approximate power of 7 mW on the sample surface. The acquisition time was set to 600 s. The laser beam was focused using a 100 × objective, resulting in a spot size of approximately 1 µm². All Raman spectra were calibrated using a Si reference sample with the theoretical Raman line set to 520.7 cm⁻¹. Total transmittance measurements were conducted using a PERKIN-ELMER Lambda 950 UV-Vis-NIR spectrophotometer equipped with an integrating sphere. The total transmittance spectra covered a wavelength range of 250–800 nm using a step size of 1 nm. The Tauc plot method was employed to determine the optical bandgap energy by identifying the intercept of the linear portion of the (αhv)² vs hv graph with the hv-axis, where α represents the absorption coefficient. The wettability property was assessed through water contact angle measurements using a DSA10 Kruss goniometer, for which three drops were used for each sample.

## 2.5. Photocatalysis characterization techniques

The photocatalytic activity of ZnO NWs was evaluated by monitoring the degradation of MO organic dye as a commonly used model molecule. The experimental set-up consisted of a UV LED with a wavelength of 365 ± 10 nm, an irradiance of 12 W/cm², and an emitting window area measuring 75 x 20 mm² (FireJet™ ONE provided by Phoseon Technology). A stirring plate and a 40 mL crystallizer covered with a watch glass were used to prevent evaporation. The substrates were placed on a UV-polymerized resin support and positioned at a distance of 20 cm below the UV source. The Si substrates coated with the ZnO NW arrays were
cleaved to dimensions of 1.5×1.5 cm². To prevent unwanted side re-
actions, the backside of the Si substrates coated with the ZnO NW arrays
was cleaned with an hydrochloric acid (HCl) solution. The experimental
solution volume was fixed at 30 mL with an initial MO organic dye
concentration of 10 µM/L. To reach adsorption equilibrium, the Si
substrates coated with the ZnO NW arrays along with the solution
containing the MO organic dye were left together in the dark for 1 hour.
The degradation of MO organic dye over time was monitored by
measuring the absorbance at 465 nm using the UV-Vis spectrophotom-
eter. The absorbance measurements were recorded in 3 mL quartz cu-
vettes every 30 minutes for a total duration of 2 hours. After each
absorbance measurement, the solution containing the MO organic dye
was returned to the crystallizer. Throughout the UV-Vis absorbance
measurement period, the UV light was switched off to prevent any
interference. The concentration of Zn(II) species in the solution con-
taining the MO organic dye was quantified after photocatalysis using the
same ICP-MS protocol and equipment as described above. The molecules
resulting from the degradation of MO organic dye (hereafter denoted as
degradation molecules) were identified using an AGILENT UPLC in-
strument coupled to a diode array detector and equipped with a Luna
omega 3 µm PS C-18 100 Å column. The length and interior diameter of
the column were 150 and 3 mm, respectively. The mobile phase was
composed of sodium phosphate monobasic monohydrate [NaH₂-
PO₄·H₂O, Sigma-Aldrich], disodium phosphate dibasic heptahydrate
[Na₂HPO₄·7 H₂O, Sigma-Aldrich], and acetonitrile at ratio 80:20 and
the pH was fixed to 6.8. The wavelengths observed were 360, 430, 460
and 485 nm.

3. Results and discussion

3.1. Morphological and structural properties of non-intentionally and
intentionally doped ZnO nanowires

The intentional doping of ZnO NWs is based on the preferential
adsorption of ionic species containing the dopants on the charged sur-
faces of ZnO NWs [34] through attractive electrostatic interactions in a
high-pH environment, followed by their incorporation. Al(OH)₄⁻ and Ga
(OH)₄ ion complexes predominantly form as the main Al(III) and Ga(III)

species for Al and Ga doping, respectively [24]. Sb-glycolate (SbG₂) ion
complexes and Cl⁻ ions are expected to be the main Sb and Cl(-I) species
for Sb and Cl doping, respectively [21,35]. All these negatively charged
ionic species adsorb on the positively charged sidewalls of ZnO NWs,
which are composed of non-polar *m*-planes [19–21,35]. In contrast, Cu
(NH₃)₄²⁺ ion complexes predominantly form as the main Cu(II) species,
and adsorb on the negatively charged top faces of ZnO NWs, which are
composed of polar *c*-planes [23].

The use of additives containing the dopants in the chemical bath
affects the morphology and dimensions of ZnO NWs since the nature of
the interaction of each ionic species containing the dopants and its
magnitude with their surfaces is highly specific. The top-view and cross-
sectional FESEM images of non-intentionally doped and intentionally
doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl for the highest
additive concentration ratios in the chemical bath and before thermal
annealing are shown in Fig. 2a. ZnO NWs are homoepitaxially grown on
the polycrystalline ZnO seed layer and, to some extent, aligned verti-
cally, regardless of the nature of dopants. They exhibit a pencil-shaped
morphology typically formed in a high-pH environment, when the
radial growth is much slower than the axial growth, except for those
doped with Ga. The use of Ga(NO₃)₃ as a chemical additive is known to
induce a change in the shape of ZnO NWs towards a hexagonal section
owing to the promotion of the radial growth over the axial growth [36].
A FESEM image of Sb-doped ZnO nanostructures grown with a Sb over
Zn concentration ratio of 3 % is presented in Fig. S1 as Supporting In-
formation, for which the morphology switches from a NW to a nanowall
structure. Owing to the shape difference, the Sb over Zn concentration
ratio was limited to 1 %. The mean length and diameter of
non-intentionally doped and intentionally doped ZnO NWs with Al, Ga,
Cu, Sb, Cl, Al-Ga, and Al-Cl were measured from cross-sectional FESEM
images on more than 60 objects, and their apparent density was esti-
mated by counting the number of objects in three areas of 1 × 1 µm² from
top-view FESEM images. The plots of the different dimensions as a
function of dopant over Zn concentration ratio are presented in Fig. 2b.
For the [Al(NO₃)₃]/[Zn(NO₃)₂] ratios ranging from 0 % to 1 %, the
mean length and diameter decrease from 3294 ± 193 nm and 124
± 28 nm to 1844 ± 95 and 70 ± 13 nm, respectively. Subsequently, an
increase to 3224 ± 222 and 119 ± 24 nm occurs for an [Al(NO₃)₃]/[Zn

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) Top-view and cross-sectional view FESEM images of non-intentionally doped, 3 % Al, 3 % Ga, 9 % Cu, 1 % Sb, and 9 % Cl doped ZnO NWs grown by CBD. The scale bar is the same for each top-view and cross-sectional view FESEM image, respectively. (b) Evolutions of the mean length, mean diameter, density, and surface area of non-intentionally doped and intentionally doped ZnO NWs as a function of dopant over Zn concentration ratio in the chemical bath. The mean diameter was measured at half-of-the height of ZnO NWs.</figcaption></figure>
(NO₃)₂] ratio of 3 %. In contrast, for a [Ga(NO₃)₃]/[Zn(NO₃)₂] ratio of 3 %, the mean length decreases to 2298 ± 100 nm, while the mean diameter remains around 130 nm. The shape, however, evolves from a pencil-like morphology to a hexagonal section, indicating the promotion of the radial growth of ZnO NWs. The Sb and Cl doping promotes the axial growth of ZnO NWs with maximum lengths of 6428 ± 315 and 5699 ± 204 nm for a [SbG₂]*/[Zn(NO₃)₂] ratio of 1 % and for a [Cl⁻]/[Zn(NO₃)₂] ratio of 9 %, respectively. A morphology with surface steps is observed following the addition of SbG₂ ions to the chemical bath, which is attributed to a change in growth kinetics due to the SbOₓ species adsorption [35]. In both co-doped cases (i.e. Al-Cl and Al-Ga), it was proposed that Al drives the final morphology, behaving similarly to doped ZnO NWs without the other co-dopant [21,24] with lengths around 2000 nm with Ga and 3000 nm with Cl. The effect of Cu, expected to adsorb on the top faces, is less pronounced. More details on the effect of dopants on the growth of ZnO NWs are discussed in our previous works [19–21,23,24,35,37]. The surface areas of ZnO NWs were estimated by considering the objects as perfect hexagonal prisms. The surface areas of ZnO NWs, expressed per cm² of substrate surface area, are roughly all in the same order of magnitude, lying in the range of 30–170 cm². It should be noted that only the top faces of ZnO NWs, composed of polar c-planes, are mostly active for the photocatalysis of MO organic dye, as shown in Ref [38]. Only the top surface area dedicated to the polar c-plane area and hence related to the mean diameter and density of ZnO NWs affect their photocatalytic activity. Here, the top surface area lies in the range of 3 × 10⁻⁵ – 6 × 10⁻⁴ cm², and its dependence on the nature and concentration of dopants roughly follows the evolution of the surface area of ZnO NWs.

## 3.2. Chemical properties of non-intentionally and intentionally doped ZnO nanowires

The concentration of dopants incorporated into ZnO NWs was quantitatively investigated by ICP-MS, a powerful tool for detecting the amount of dopants over a broad range from ppm levels. For that purpose, the Si substrates covered with non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Al-Ga, and Al-Cl were dissolved into a 0.6 % nitric acid solution. The concentration of dopants in the nitric acid (HNO₃) solution was normalized to the concentration of Zn to avoid any bias from the size of the Si substrates or the morphology of ZnO NWs. The dopant over Zn atomic ratios before and after thermal annealing as a function of dopant over Zn concentration ratio in the chemical bath are presented in Fig. 3. Interestingly, the dopant over Zn atomic ratios in intentionally doped ZnO NWs are found to linearly increase with respect to the dopant over Zn concentration ratio in the chemical bath, regardless of the nature of dopants. Also, the dopant over Zn atomic ratios in intentionally doped ZnO NWs are roughly the same before and after thermal annealing. However, it is worth mentioning that each dopant exhibits a different incorporation rate, which can be estimated from the slope of the linear portions. The use of Al(NO₃)₃ compared to AlCl₃ is the most efficient way to dope ZnO NWs with Al: the incorporation rate of Al dopants is of 0.15 as compared to 0.05 of Al over Zn concentration ratio in the chemical bath, respectively. The incorporation of Al dopants in the Al-Ga co-doped ZnO NWs is not linear owing to the competitive process with the incorporation of Ga dopants, as discussed in Ref. 24. The incorporation of Ga dopants in Ga-doped and Al-Ga co-doped ZnO NWs follows the same linear behavior, with an incorporation rate of 1/3 of Ga over Zn concentration ratio in the chemical bath. The incorporation of Sb dopants also follows a linear behavior, but with a much higher incorporation rate of 1.2 times the Sb over Zn concentration ratio in the chemical bath. In contrast, the incorporation of Cu dopants follows a linear behavior, but with a much lower incorporation rate of around 0.001 of Cu over Zn concentration ratio in the chemical bath. The competitive process between the incorporation of Cu on the top faces of ZnO NWs and the incorporation of Zn, in addition to the higher defect formation energy required [39], account

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Evolution of the Sb/Zn, Cu/Zn, Ga/Zn, and Al/Zn atomic ratios in intentionally doped ZnO NWs before and after thermal annealing at 400 °C for 1 hour under O₂ atmosphere as a function of dopant over Zn concentration ratio in the chemical bath, as obtained from ICP-MS measurements.</figcaption></figure>

for the lower incorporation of Cu dopants [23]. From this dataset, the facility of the incorporation of dopants in ZnO NWs grown by CBD using the same conditions as a function of the nature of dopants is as follows: Sb > Ga > Al > Cu. All these dopants are expected to be incorporated into the ZnO lattice by substituting for Zn sites as Sb<sub>2</sub>Zn, Ga<sub>2</sub>Zn, Al<sub>2</sub>Zn, and Cu<sub>2</sub>Zn. This ranking is coherent with the magnitude of the defect formation energy, as proposed by DFT calculations in Refs. 39–41. It should further be noted that the incorporation of Cl dopants into the ZnO lattice is expected by substituting for O sites as ClO [42]. Additionally, other types of defects related to the concomitant incorporation of hydrogen and nitrogen from the chemical bath are expected to form in non-intentionally and intentionally doped ZnO NWs [43,44]. The presence of intentional dopants with hydrogen is liable to form additional complexes such as Ga<sub>2</sub>Zn-V<sub>2</sub>Zn-H [45].

## 3.3. Optical properties and phase of non-intentionally and intentionally doped ZnO nanowires

To investigate the incorporation and doping processes, Raman spectra and XRD patterns of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl for the highest additive concentration ratios in the chemical bath are presented in Fig. 4a,b and Fig. 4c,d, respectively. The rest of the Raman spectra and XRD patterns is shown in Fig. S2 and Fig. S3, respectively, as Supporting Information. Before thermal annealing, the Raman lines at 101, 332, 437, and 574 cm⁻¹ corresponding to the E₂Low, E₂High-, E₂Low, E₂High, and A₁(LO) phonon modes of the wurtzite structure of ZnO, respectively, occur in non-intentionally doped and intentionally doped ZnO NWs, regardless of the nature of dopants. The other typical phonon modes of the wurtzite structure of ZnO do not appear owing to the vertically aligned morphology of ZnO NWs. Correlatively, the diffraction peaks pointing at 31.76, 34.42, 36.24, 47.53, 56.57, 62.85, 67.93, 69.06, 72.56, and 76.93° are all assigned to the wurtzite structure of ZnO. The predominance of the (0002) diffraction peak at 34.42° is related to growth of ZnO NWs along the polar c-axis, regardless of the nature and amount of dopants. The other diffraction peaks originate from the fact that ZnO NWs are not perfectly aligned vertically, as seen in FESEM images in Fig. 2. Interestingly, the Ga-doped ZnO NWs show two
<figure><img src="image_6.png" /><figcaption>Fig. 4. (a,b) Raman spectra recorded in the low-wavenumber range and (c,d) XRD patterns of non-intentionally doped, 3 % Al, 3 % Ga, 9 % Cu, 1 % Sb, and 9 % Cl doped, 1.5 % Al – 1.5 % Ga, and 3 % Al – 9 % Cl co-doped ZnO NWs grown by CBD. The data are shown (a,c) before and (b,d) after thermal annealing at 400 °C for 1 hour under O₂ atmosphere.</figcaption></figure>

dedicated additional modes denoted as AM<sub>Ga</sub> and located at 631 and 696 cm<sup>-1</sup>, revealing the incorporation of Ga dopants into the bulk of ZnO NWs [20]. The Sb-doped ZnO NWs also exhibit three dedicated additional modes denoted as AM<sub>Sb</sub> and located at 525, 690, and 715 cm<sup>-1</sup>, revealing the incorporation of Sb dopants into the bulk of ZnO NWs [35]. In contrast, no dedicated additional modes occur in the case of Al-doped ZnO NWs. However, similarly to the Ga-doped ZnO NWs, the intensity of the Raman band corresponding to the A<sub>1</sub>(LO) + AM<sub>3</sub> phonon modes significantly increases in Al-doped ZnO NWs owing to the lattice deformation induced by the incorporation of Al dopants. Eventually, the Cu- and Cl-doped ZnO NWs exhibit no distinct features compared to non-intentionally-doped ZnO NWs. After thermal annealing, five additional modes (AMs) arise at 276, 509, 579, 623, and 643 cm<sup>-1</sup> and are attributed to B<sub>1</sub><sup>low</sup>, 2B<sub>1</sub><sup>low</sup>, B<sub>1</sub><sup>high</sup>, TA + TO, and TA + B<sub>1</sub><sup>high</sup> phonon modes of the wurtzite structure of ZnO, respectively. Most of the AMs correspond to B<sub>1</sub> silent modes, which appear by disorder-activated Raman scattering owing to the local breakdown of the translational symmetry [46,47]. The presence of the five AMs in non-intentionally doped ZnO NWs is attributed to the incorporation of residual impurities from the water medium and chemical precursors, including Zn(NO<sub>3</sub>)<sub>2</sub>, HMTA molecules, and NH<sub>3</sub>. Nevertheless, the significant increase in the intensity of AMs and mainly of AM<sub>2</sub> and AM<sub>3</sub> in

intentionally doped ZnO NWs as compared to non-intentionally doped ZnO NWs indicates the significant incorporation of Ga, Sb, and Al dopants. In contrast, the incorporation of Cu and Cl dopants does not alter the intensity of the Raman lines even after thermal annealing. However, previous reports using similar CBD conditions have demonstrated the incorporation of Cu and Cl dopants, as discussed in Ref [21, 23]. Furthermore, it is also worth noting that the position of the Raman line assigned to the E<sub>2</sub><sup>high</sup> phonon mode shifts significantly towards higher wavenumber, regardless of the nature and amount of dopants, as presented in Fig. S4 as Supporting Information. For instance, the redshift is particularly pronounced in Ga- and Sb-doped ZnO NWs, where the incorporation of Ga and Sb dopants is higher. It is also much more pronounced before than after thermal annealing, indicating the occurrence of efficient strain relaxation processes in doped ZnO NWs following thermal annealing, regardless of the nature of dopants. Additionally, before and after thermal annealing, no additional diffraction peaks and Raman lines coming from other oxide or hydroxide compounds are detected, confirming the high purity of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl. The precipitation of residual phases is either energetically unfavorable (thermodynamic considerations), or occurs very slowly (kinetic considerations), preventing them from competing
with the growth of ZnO NWs.

To investigate the optical properties, UV-vis-NIR absorption measurements of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl were achieved, from which the Tauc plots are presented in Fig. S5 as Supporting Information. The optical bandgap energy plotted in Fig. S6 as Supporting Information was obtained by identifying the intercept of the linear portion of the Tauc plots with the hν-axis. The variations in the optical bandgap energy of intentionally-doped ZnO NWs are generally small and related to renormalization processes and/or the Burstein-Moss effect. The key point to note here is that the wavelength of the lamp used for determining the photocatalytic properties is of 3.4 ± 0.1 eV, which means all the non-intentionally and intentionally doped ZnO NWs, before and after thermal annealing, can efficiently absorb UV light.

### 3.4. Photocatalytic activity of non-intentionally and intentionally doped ZnO nanowires

The photocatalytic activity of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl was assessed through the degradation of MO organic dye under UV-A irradiation with a 12 W/cm² LED.

Prior to the photocatalytic test, water contact angle measurements were conducted, confirming the hydrophilic properties of ZnO NWs, regardless of the nature and amount of dopants, as shown in Fig. S7 as Supporting Information. The evolutions of the residual (C) over initial (C₀) concentration ratios of MO organic dyes as a function of time for non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl before and after annealing are presented in Fig. S8 as Supporting Information. When no ZnO NW-based photocatalysts are used, the C/C₀ value of MO organic dye remains around 100 %, indicating that no significant photo-degradation activity takes place. In contrast, when ZnO NW-based photocatalysts are used, the drastic decrease in the C/C₀ value reveals a strongly enhanced photocatalytic activity, which depends on the nature and amount of dopants. Considering that the C₀ value of MO organic dye is sufficiently low, the Langmuir-Hinshelwood model, describing the photocatalytic degradation kinetics within the framework of surface reaction-limited conditions with a Langmuir-type adsorption, was applied as follows:

$$ \ln\left(\frac{C_0}{C}\right) = k_s t \qquad (5) $$

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) Photocatalytic degradation kinetics plots of MO organic dye for non-intentionally doped, 3 % Al, 3 % Ga, 9 % Cu, 1 % Sb, and 9 % Cl doped, 1.5 % Al-1.5 % Ga and 3 % Al-9 % Cl co-doped ZnO NWs grown by CBD and before thermal annealing. The dashed lines represent the linear fit from the Langmuir-Hinshelwood model in Eq. (5). (b) Evolutions of the kS/area apparent reaction rate constants for non-intentionally doped and intentionally doped ZnO NWs before and after thermal annealing as a function of dopant over Zn concentration ratio in the chemical bath. Evolutions of the kS/area apparent reaction rate constants for non-intentionally doped and intentionally doped ZnO NWs (c) before and (d) after thermal annealing as a function of dopant over Zn atomic ratio obtained by ICP-MS.</figcaption></figure>
where $k_S$ is the apparent reaction rate constant and $t$ is the time. The ln $(C_0/C)$ vs $t$ plots of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl for the highest additive concentration ratios in the chemical bath and before thermal annealing are presented in Fig. 5a. The remaining ln$(C_0/C)$ vs $t$ plots are shown in Fig. S9 as Supporting Information. It appears that all the plots exhibit a linear relationship with a high correlation coefficient close to 1, indicating that the photocatalytic degradation process of MO organic dye follows pseudo-first order reaction kinetics, regardless of the nature and amount of dopants. The reaction rate is hence proportional to the C value of MO organic dye, and the surface reaction sites on the ZnO NWs are not saturated. The slope inferred from each fit provides an estimate of $k_S$ for non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl before and after thermal annealing. To limit bias that might arise from substrate cutting, the $k_S$ value was divided by the substrate surface area. As an example, the $k_S$ value of non-intentionally doped ZnO NWs before thermal annealing is $2.91 \times 10^{-3} \min^{-1}$ and the substrate surface area is of $2.35 \text{ cm}^2$. Therefore, the corrected $k_S$ (i.e. $k_S/\text{area}$) value is $1.24 \times 10^{-3} \min^{-1}\text{cm}^{-2}$. Interestingly, the $k_S/\text{area}$ value of non-intentionally doped ZnO NWs after thermal annealing jumps to $2.59 \times 10^{-3} \min^{-1}\text{cm}^{-2}$. Based on our previous work, one hypothesis explaining the enhanced photocatalytic activity after thermal annealing is the *exo-diffusion* process of hydrogen-related defects in the forms of interstitial hydrogen (HBC) and substitutional hydrogen (HO), which increases the concentration of reactive radicals. [44] Importantly, the photocatalytic activity of non-intentionally doped ZnO NWs before and after thermal annealing is mainly governed by the photocatalytic activity of their top faces, composed of polar c-planes, such that the photocatalytic activity of their sidewalls, composed of non-polar m-planes, is negligible [38]. Accordingly, no correction over the ZnO NW surface area was made here. The $k_S/\text{area}$ mean values of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl before and thermal annealing are provided in Table 2.

Before thermal annealing, it is notable that the effect of doping on the photocatalytic activity of ZnO NWs strongly depends on the nature of dopants, with the following ranking: Cu > Sb >> Cl > undoped ~ Al > Al-Cl >> Al-Ga ~ Ga. The magnitude of the photocatalytic activity has no correlation with the slightly varying surface area, indicating that the slightly varying morphology is not predominant in contrast to the nature and concentration of dopants in the center of intentionally doped ZnO NWs. On the one hand, the incorporation of Cu and Sb acceptors substituting for Zn sites into ZnO NWs leads to a significant increase in their photocatalytic activity, with $k_S/\text{area}$ mean values jumping from $1.24 \times 10^{-3}$ to $1.57 \times 10^{-3}$ and $1.52 \times 10^{-3} \min^{-1}\text{cm}^{-2}$, respectively. This represents a noticeable increase in the photocatalytic activity of around 27 % and 23 %, respectively, with respect to non-intentionally doped ZnO NWs. On the other hand, the incorporation of Al substituting for Zn sites into ZnO NWs does not significantly affect the photocatalytic activity of ZnO NWs. The incorporation of Ga substituting for Zn sites even drastically decreases their photocatalytic activity, with the $k_S/\text{area}$ mean value falling to $0.39 \times 10^{-3} \min^{-1}\text{cm}^{-2}$. A striking feature is that the incorporation of Cl donors substituting for O sites into ZnO NWs slightly increases their photocatalytic activity, with the $k_S/\text{area}$ mean value reaching $1.34 \times 10^{-3} \min^{-1}\text{cm}^{-2}$. This indicates that the nature of dopants with shallow donors, either substituting for Zn (i.e. Al,

Ga) or O (i.e. Cl) sites into ZnO NWs, has a distinct impact on the photocatalytic activity. Eventually, the co-doping of ZnO NWs with Al-Ga significantly decreases their photocatalytic activity and hence is dominated by the negative effect of Ga. The co-doping of ZnO NWs with Al-Cl only decreases very slightly their photocatalytic activity and thus is dominated by the neutral effect of Al. After thermal annealing, the photocatalytic activity of ZnO NWs improves, regardless of the nature of dopants, with the following ranking: undoped >> Cu > Cl > Sb > Al-Cl >> Al >> Al-Ga ~ Ga. The enhancement is particularly marked for Cu- and Cl-doped ZnO NWs after thermal annealing, reaching $k_S/\text{area}$ mean values of $1.85 \times 10^{-3} \min^{-1}\text{cm}^{-2}$, respectively. This represents a significant increase in the photocatalytic activity of around 49 % and 42 %, respectively, with respect to non-intentionally doped ZnO NWs. Similarly, the thermal annealing improves the photocatalytic activity of Ga-doped ZnO NWs, with a $k_S/\text{area}$ mean value of $0.95 \times 10^{-3} \min^{-1}\text{cm}^{-2}$. In contrast, the improvement is much less pronounced for Al- and Sb-doped ZnO NWs after thermal annealing. The evolutions of $k_S/\text{area}$ mean values of co-doped ZnO NWs with Al-Ga and Al-Cl further indicate that the photocatalytic activity remains dominated by the effects of Ga in the case of Al-Ga, and surprisingly by Cl in the case of Al-Cl, contrary to the behavior observed before thermal annealing.

More importantly, the evolution of the $k_S/\text{area}$ value as a function of dopant over Zn concentration ratio in the chemical bath is presented for each dopant in Fig. 5b. Beyond the $k_S/\text{area}$ mean value reported for each dopant in Table 2, the photocatalytic activity of intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl and before thermal annealing is found to be highly dependent upon the dopant over Zn concentration ratio. The photocatalytic activity of Al-doped ZnO NWs remains nearly constant, with a $k_S/\text{area}$ value of $1.24 \times 10^{-3} \min^{-1}\text{cm}^{-2}$, independent upon the Al over Zn concentration ratio ranging from 0.3 % to 3 %, which contrasts with Refs. 25,48,49. The photocatalytic activity of Ga-doped ZnO NWs constantly decreases, reaching a $k_S/\text{area}$ value of $0.13 \times 10^{-3} \min^{-1}\text{cm}^{-2}$ as the Ga over Zn concentration ratio is increased from 0.3 % to 3 %, which agrees with Refs. 25,26. Correlatively, the photocatalytic activity of co-doped ZnO NWs with Al-Ga dominated by the negative effect of Ga follows the same downward trend, reaching a $k_S/\text{area}$ value of $0.13 \times 10^{-3} \min^{-1}\text{cm}^{-2}$. In contrast, the photocatalytic activity of Cl-doped ZnO NWs shows a maximum $k_S/\text{area}$ value of $1.54 \times 10^{-3} \min^{-1}\text{cm}^{-2}$ when the Cl over Zn concentration ratio is 3 %, and then falls to a minimum $k_S/\text{area}$ value of $1.23 \times 10^{-3} \min^{-1}\text{cm}^{-2}$ when the Cl over Zn concentration ratio further reaches 9 %. Correlatively, the photocatalytic activity of co-doped ZnO NWs with Al-Cl follows the same upward trend, reaching a $k_S/\text{area}$ value of $1.69 \times 10^{-3} \min^{-1}\text{cm}^{-2}$ revealing synergetic effects between the Al and Cl doping processes. The photocatalytic activity of Cu- and Sb-doped ZnO NWs also shows a maximum $k_S/\text{area}$ value of $2.11 \times 10^{-3} \min^{-1}\text{cm}^{-2}$ for a Cu over Zn concentration ratio of 0.3 %, which is in agreement with Ref. 50. The photocatalytic activity of Sb-doped ZnO NWs is also greatly enhanced up to a maximum $k_S/\text{area}$ value of $1.66 \times 10^{-3} \min^{-1}\text{cm}^{-2}$ for a Sb over Zn concentration ratio of 1 %, representing the highest concentration to be incorporated into ZnO NWs without affecting the structural morphology, as illustrated in Fig. S1 as Supporting Information [31]. After thermal annealing, the photocatalytic activity of intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl is still found to be highly dependent upon the dopant over Zn concentration ratio, generally following the same trend as before thermal annealing, with some similar or even increased $k_S/\text{area}$.

<caption>**Table 2**</caption>

$k_S/\text{area}$ mean values of non-intentionally doped and intentionally doped ZnO NWs before and after thermal annealing at 400 °C for 1 h under O₂ atmosphere as a function of the nature of dopants.

<table><thead><tr><th rowspan="2"></th><th colspan="7">$k_S/\text{area}$ (10<sup>-3</sup> min<sup>-1</sup>cm<sup>-2</sup>)</th></tr><tr><th>Undoped</th><th>Al</th><th>Ga</th><th>Cu</th><th>Sb</th><th>Cl</th><th>Al-Ga</th><th>Al-Cl</th></tr></thead><tbody><tr><th>As-grown</th><td>1.24</td><td>1.24</td><td>0.39</td><td>1.57</td><td>1.52</td><td>1.34</td><td>0.40</td><td>1.20</td></tr><tr><th>Annealed</th><td>2.59</td><td>1.26</td><td>0.95</td><td>1.85</td><td>1.60</td><td>1.76</td><td>1.03</td><td>1.53</td></tr></tbody></table>
values. The magnitude of the $k_S$/area values and its evolution remains the same for thermally annealed Al- and Sb-doped ZnO NWs as for their as-grown counterparts. In contrast, the magnitude of the $k_S$/area values for thermally annealed Ga-doped and Al-Ga co-doped ZnO NWs is systematically higher than for their as-grown counterparts, with $k_S$/area values strictly following the same trend, again showing the predominance of the effect of Ga. The maximum $k_S$/area value of $2.50 \times 10^{-3}$ min$^{-1}$cm$^{-2}$ for thermally annealed Cu-doped ZnO NWs occurs at a Cu over Zn concentration ratio of 1%, which is higher than the corresponding ratio for as-grown Cu-doped ZnO NWs. In contrast, the maximum $k_S$/area value of $2.40 \times 10^{-3}$ min$^{-1}$cm$^{-2}$ for thermally annealed Cl-doped ZnO NWs occurs at the Cl over Zn concentration ratio of 1%, which is lower than the corresponding ratio for as-grown Cl-doped ZnO NWs. The photocatalytic activity of thermally annealed Al-Cl co-doped ZnO NWs is roughly similar to that of their as-grown counterparts.

To further reveal how the activation of dopants estimated through the dopant/Zn atomic ratios into ZnO NWs as measured by ICP-MS measurements affect the photocatalytic activity, all the $k_S$/area values were re-plotted before and after thermal annealing, as presented in Fig. 5c,d, respectively. Before thermal annealing, the photocatalytic activity of intentionally doped ZnO NWs is found to oscillate around the photocatalytic activity of non-intentionally doped ZnO NWs. It is further shown that the photocatalytic activity of intentionally doped ZnO NWs is initially enhanced and subsequently decreases on average as the dopant/Zn atomic ratio is increased. The range of $k_S$/area values varies from $2.11 \times 10^{-3}$ min$^{-1}$cm$^{-2}$, as a maximum for a Cu/Zn atomic ratio of 12 ppm, down to $0.13 \times 10^{-3}$ min$^{-1}$cm$^{-2}$, as a minimum for a Ga/Zn atomic ratio of 0.89%. The trend in Fig. 5c, thus reveals that the photocatalytic activity of intentionally doped ZnO NWs exhibits an optimum $k_S$/area value in the low range of dopant/Zn atomic ratios of ten to several tens of ppm. A striking exception is represented by the photocatalytic activity of Sb-doped ZnO NWs reaching a $k_S$/area value of $1.66 \times 10^{-3}$ min$^{-1}$cm$^{-2}$ as a maximum for the Sb/Zn atomic ratio of 1.32%, suggesting that the electrical activity of dopants may play an additional role beyond the dopant/Zn atomic ratio. For instance, as a comparison with the Cu doping [51], the Sb doping of ZnO NWs was found to exhibit a high ability to compensate for the free electrons and hence improving the performance of the related piezoelectric devices [35]. After thermal annealing, despite the general improvement as presented in Table 2, the photocatalytic activity of intentionally doped ZnO NWs is found to be systematically lower than the photocatalytic activity of non-intentionally doped ZnO NWs. It is further shown again that the photocatalytic activity of intentionally doped ZnO NWs is initially enhanced and subsequently decreases on average as the dopant/Zn atomic ratio is increased. The trend in Fig. 5d, hence reveals again that the photocatalytic activity of intentionally doped ZnO NWs exhibits another optimum $k_S$/area value in the low range of dopant/Zn atomic ratios of ten to several tens of ppm with the striking exception of Sb dopants. This peculiar behavior after thermal annealing emphasizes the significance of hydrogen- and nitrogen-related defects on the photocatalytic activity of ZnO NWs. As thermal annealing proceeds, associative and dissociative processes of defect complexes along with exo-diffusion processes of hydrogen are expected to occur as documented in Ref. 44 and may depend on the nature of dopants intentionally incorporated into ZnO NWs as suggested in Ref. 45, reporting the formation of passivated $V_{Zn}$-Ga$_{Zn}$ defect complexes with hydrogen.

### 3.5. Photocatalytic processes of non-intentionally and intentionally doped ZnO nanowires

Since the use of scavengers to evaluate the influence of active species, such as electrons, holes, or reactive radicals, can alter the photocatalytic behavior [52], the photocatalytic processes of non-intentionally doped and intentionally doped ZnO NWs with Al, Ga, Cu, Sb, Cl, Al-Ga, and Al-Cl were assessed using UPLC coupled to a

diode array detector. The measurements were collected on the different solutions stored after photocatalysis. The UPLC spectra were acquired at 360, 430, 460, and 485 nm to identify the structures of degradation molecules, as presented in Fig. S10 as Supporting Information. The UPLC parameters regarding the column and eluents were adapted from Ref. 53, which reports the identification of degradation molecules using HPLC coupled to mass spectrometry. Six main peaks were detected on the UPLC spectra, corresponding to the MO organic dye and its degradation molecules. The MO degradation molecules denoted as A, B, C, D, and E are attributed to the following molecules with m/z of 320 (C$_{14}$H$_{14}$N$_3$O$_4$S), 290 (C$_{13}$H$_{12}$N$_3$O$_3$S), 306 (C$_{13}$H$_{12}$N$_3$O$_4$S), 276 (C$_{12}$H$_{10}$N$_3$O$_3$S), and 292 (C$_{12}$H$_{10}$N$_3$O$_4$S), respectively. The nature and presence of these MO degradation molecules were confirmed by UPLC coupled to mass spectrometry collected on the different solutions obtained after photocatalysis from as-grown non-intentionally doped ZnO NWs and annealed intentionally co-doped ZnO NWs with Al-Ga, as presented in Fig. S11 as Supporting Information. The possible pathways for the degradation of MO organic dye are illustrated in Fig. 6a, in accordance with Ref. 54. Other degradation molecules may be expected, as reported in Ref. 55, but they were not detected in the present investigation, as reported in Fig. S11 as Supporting Information. The area under each peak, that is proportional to the concentration of each degradation molecule, was hence plotted as a function of $k_S$ apparent reaction rate constant for non-intentionally doped and intentionally doped ZnO NWs before and after thermal annealing, as presented Fig. 6b. Firstly, it should be noted that the concentration of MO organic dye is well correlated with the $k_S$ values: the concentration of MO organic dye progressively decreases as the $k_S$ value is increased, confirming the reliability of UV-Vis absorbance measurements for assessing the photocatalytic activity of ZnO NWs. This further indicates that the degradation molecules, which may also absorb mainly in the orange part of the electromagnetic spectrum [54], are formed in much lower amounts than the MO organic dye and hence do not influence the UV-Vis absorbance measurements. The evolutions of the concentration of the different degradation molecules identified by UPLC measurements are thus helpful for further discussing the degradation pathways of MO organic dye.

In the first step, two main degradation molecules emerge from the present dataset. The first one results from the oxidation of the phenyl ring bearing the dimethyl amino group in the presence of ●OH hydroxyl radicals and is named Route 1, producing a phenolic degradation molecule denoted as A. The second one results from a demethylation of the tertiary amine in the presence of ●OH hydroxyl radicals or ●$O_2^-$ superoxide ions and is named Route 2, leading to a secondary amine degradation molecule denoted as B. According to the evolution of the concentration of degradation molecule A, the oxidation reaction of the phenyl ring of the MO organic dye in the presence of ●OH hydroxyl radicals following Route 1 is relatively well correlated with the increase in the $k_S$ value and is thus strongly enhanced. This suggests that Route 1 is the main degradation pathway or that degradation molecule A is a final product. On the other hand, according to the evolution of the concentration of degradation molecule B, the demethylation reaction of the tertiary amine of the MO organic dye in the presence of ●OH hydroxyl radicals or ●$O_2^-$ superoxide ions does not follow a linear increase with the increase in the $k_S$ value, but instead reaches a kind of saturation plateau. This implies that degradation molecules B is only an intermediate step in the degradation pathway or that it is not being generated. However, ZnO NWs doped with Ga and co-doped with Al-Ga instead induce a linear increase in the concentration of degradation molecule B with the $k_S$ value. This suggests that Route 2 is the main degradation pathway for ZnO NWs doped with Ga and co-doped with Al-Ga or that degradation molecule B is not a limiting reactant (i.e. not further degraded) in this case. In contrast, all other dopants, regardless of their nature and amount into ZnO NWs, strongly hinder the formation of degradation molecule B, or promote its further degradation, such that its concentration cannot increase.
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** (a) Illustration of the degradation pathways of the MO organic dye and of the degradation molecules, as identified by UPLC measurements. (b) Evolutions of the concentrations of the MO organic dye and of the degradation molecules as a function of $k_S$ value for non-intentionally and intentionally doped ZnO NWs before and after thermal annealing at 400 °C for 1 hour under O$_2$ atmosphere.</figcaption></figure>

In the second step, degradation molecule C can be obtained from either degradation molecules A or B following Routes 3 and 4, respectively. Again, the evolution of the concentration of degradation molecule C is relatively well correlated with the increase in the $k_S$ value, similar to that of degradation molecule A. Since both the oxidation reaction of the phenyl ring and the demethylation reaction of the tertiary amine are needed to obtain degradation molecule C, it seems that the most favorable pathway is Route 2, followed by Route 4: these consist in the demethylation reaction of the MO organic dye leading to degradation molecule B, followed by the oxidation reaction of the phenyl ring to form degradation molecule C. Indeed, a higher chemical reactivity of degradation molecule B (as compared to A) to form degradation molecule C could explain the saturation plateau seen in Fig. 6b. Route 4, resulting from the reaction of degradation molecule B with •OH hydroxyl radicals to form degradation molecule C, is faster than Route 3, meaning that the majority of degradation molecule B is used to form degradation molecule C, such that its concentration cannot increase. In

other words, degradation molecule B would be the limiting reactant in this case (i.e., further degraded to degradation molecule C). This further accounts for the low concentration of degradation molecule D following Route 5, since most of degradation molecules B is used for the formation of degradation molecule C. The same chemical phenomenon occurs when obtaining degradation molecule E from degradation molecule D along Route 7. It is worth noting that the concentrations of degradation molecules B, D and E are higher in ZnO NWs doped with Ga and co-doped with Al-Ga, while the opposite behavior occurs for degradation molecules A and C when considering with all other dopants, regardless of their nature and amount into ZnO NWs. This strongly suggests that the oxidation reaction of the phenyl ring of the MO organic dye in the presence of •OH hydroxyl radicals is less favorable in ZnO NWs doped with Ga and co-doped with Al-Ga, and that degradation molecule B is not the limiting reactant here. However, the higher amount of degradation molecule C in ZnO NWs doped with Ga and co-doped with Al-Ga after thermal annealing could come from a higher amount of
<figure><img src="image_9.png" /><figcaption>Fig. 7. (a) kS values plotted as a function of the concentration of zinc [Zn] in the different solutions stored after photocatalysis as obtained by ICP-MS measurements. Cross-sectional view FESEM images of (b,c,d,e) Al-doped ZnO NWs grown with [AlCl3]/[Zn(NO3)2] = 3 %, (f, g, h, i) Cu-doped ZnO NWs grown with [Cu(NO3)2] / [Zn(NO3)2] = 0.3 %, (j, k, l, m) Ga-doped ZnO NWs grown with [Ga(NO3)3] / [Zn(NO3)2] = 0.3 %, and (n, o, p, q) non-intentionally doped ZnO NWs. (b, d, f, h, j, l, n, p) before photocatalysis. (c, e, g, i, k, m, o, q) after photocatalysis. (b, c, f, g, j, k, n, o) as-grown ZnO NWs (d, e, h, i, l, m, p, q) thermally annealed ZnO NWs.</figcaption></figure>

degradation molecule B being produced or more reactive radicals being generated. Interestingly, the different behavior of Ga-doped ZnO NWs compared to non-intentionally doped and Al-doped ZnO NWs for the degradation of MO organic dye has been reported using scavengers [25].

To summarize, it appears that the degradation pathway of the MO organic dye with intentionally doped ZnO NWs proceeds mainly through the oxidation reaction of a phenyl ring bearing the dimethyl amino group with ●OH hydroxyl radicals, leading to the formation of degradation molecules A and C. The formation of degradation molecule C is primarily achieved through Route 4 (i.e. B → C) rather than Route 3 (i.e. A → C). A notable exception is observed in the case of ZnO NWs doped with Ga and co-doped with Al-Ga, where a different degradation pathway is followed, consisting of successive Route 2, then Route 5, and finally Route 7 (i.e. B → D → E). This indicates that the reactive species in the photocatalytic processes of intentionally doped ZnO NWs are primarily ●OH hydroxyl radicals, except in the case of ZnO NWs doped

with Ga and co-doped with Al-Ga, and hence that free holes h+VB are the main active charge carriers according to Eq. 3. However, it should be noted that, other degradation pathways may proceed, such as breaking of the N = N bond, for instance [56]. The low intensity of several peaks, compared to the MO organic dye, suggests that part of the MO organic dye may decompose into smaller-sized degradation molecules, which are colorless and not identified by UPLC measurements.

### 3.6. Photo-corrosion phenomenon of non-intentionally and intentionally doped ZnO nanowires

In addition to the classic photocatalytic processes described in Eqs. (1–4) and involving eCB and h+VB to respectively generate reactive radicals such as ●O2 and ●OH that degrade the MO organic dye, [3,57] the chemical stability of ZnO can be affected by the presence of a significant amount of h+VB through a photo-corrosion phenomenon: [58,59]

$$ZnO + 2 h_{VB}^{+} \rightarrow Zn^{2+} + \frac{1}{2}O_2 \quad (5)$$

Since ZnO is highly sensitive to the photo-corrosion phenomenon, the chemical stability of NWs was investigated by analyzing the chemical composition of the different solutions stored after photocatalysis using ICP-MS measurements. The kS value as a function of the concentration of zinc [Zn] in solution is reported in Fig. 7a. Both parameters appear to be well correlated, showing a linear increase in the kS value with the concentration of zinc [Zn] in solution, with a slope value of approximately 5 × 10⁻³ min⁻¹ppm⁻¹ for non-intentionally doped and intentionally doped ZnO NWs (except Ga and Al-Ga). This correlation between the photocatalytic activity and photo-corrosion phenomenon has already been reported in Ref. 58. It is interesting to note that ZnO NWs doped with Ga and co-doped with Al-Ga also show a fairly linear increase in the kS value with the concentration of zinc [Zn] in solution. Nevertheless, a higher slope value of around 11 × 10⁻³ min⁻¹ppm⁻¹ is found, again indicating a significant change of the photocatalytic processes. To further assess the occurrence of a photo-corrosion phenomenon of ZnO NWs, top-view FESEM images were collected before and after photocatalysis. The evolution of the structural morphology of non-intentionally doped and intentionally doped ZnO NWs is illustrated in Fig. 7b-q, with the rest of the dataset available in Fig. S12 and Fig. S13 as Supporting Information. Notably, the pencil-shaped morphology at the top of non-intentionally doped and intentionally doped ZnO NWs before photocatalysis (except Ga and Al-Ga) is lost after two hours in aqueous solution under UV-A irradiation. The evolution of the structural morphology accounts for the detection of a significant concentration of zinc [Zn] in solution as measured by ICP-MS and confirms that the photocatalytic process, with the high MO degradation kinetics, mainly occurs on the top surfaces of ZnO NWs [38]. The evolution of the morphology of ZnO NWs doped with Ga and co-doped with Al-Ga could not be observed due to their initial hexagonal shape [20], as seen in Fig. 2. Moreover, the low MO degradation kinetics might limit the photo-corrosion phenomenon of ZnO NWs doped with Ga and co-doped with Al-Ga. Eventually, the photocatalytic processes in non-intentionally doped and intentionally doped ZnO NWs are found to be concomitant with a photo-corrosion phenomenon, where ZnO acts as a reservoir of Zn²⁺ ions that are progressively released in solution. In other words, the reusability of ZnO NW-based photo-catalysts may not be affected by a few repetitions of photocatalytic measurements, as commonly reported [25,60,61], but their chemical stability over the long-term represents an issue that needs to be addressed.

### 4. Discussion

A schematic diagram summarizing the photocatalytic activity and processes as well as the degradation pathways of the MO organic dye, along with their dependence on the nature of dopants in ZnO NWs grown by CBD is depicted in Fig. 8. As recapitulated in Eqs. (1–4), the
### photocatalytic processes

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Schematic diagram recapitulating the photocatalytic activity and processes as well as the degradation pathways of the MO organic dye, along with their dependence on the nature of dopants in ZnO NWs grown by CBD.</figcaption></figure>

ability of charge carriers (i.e., $e_{CB}^{-}$ and $h_{VB}^{+}$), generated under UV-A irradiation, to reduce or oxidize oxygenated species (i.e. $O_{2(ads)}$, $OH_{(ads)}^{-}$, $H_{2}O$) on the surfaces of ZnO NWs in order to generate ●OH and ●O$_{2}$ highly reactive radicals decomposing the pollutants, governs the photocatalytic activity and processes.

Following the chemical analysis coupled to UPLC measurements, it is unambiguously shown that ●OH hydroxyl radicals are the main reactive species degrading the MO organic dye, and hence that free $h_{VB}^{+}$ are the primary active charge carriers in the photocatalytic processes of non-intentionally doped and intentionally doped ZnO NWs with Al, Cl, Cu, Sb, and Al-Cl. The present statement holds before and after thermal annealing of non-intentionally doped and intentionally doped ZnO NWs with Al, Cl, Cu, Sb, and Al-Cl. The main degradation pathway of MO organic dye is found to operate through the oxidation reaction of a phenyl ring bearing the dimethyl amino group with ●OH hydroxyl radicals. In contrast, a notable exception is revealed in the case of ZnO NWs intentionally doped with Ga and Al-Ga, where the main degradation pathway of MO organic dye involves the demethylation reaction of the tertiary amine and may suggest the significant role of another reactive radical. In addition, the photocatalytic processes are expected to mostly proceed on the top surfaces of non-intentionally doped and intentionally doped ZnO NWs, which are composed of the highly reactive polar c-planes with the Zn polarity, as reported in Ref. [38,62].

Following the chemical analysis coupled to UV-Vis absorbance measurements, the photocatalytic activity of ZnO NWs, which follows a pseudo first-order kinetics, is found to strongly depend on the nature of dopants with the following rankings before thermal annealing - Cu > Sb >> Cl > undoped ~ Al > Al-Cl >> Al-Ga ~Ga - and after thermal annealing - undoped >> Cu > Cl > Sb > Al-Cl >> Al >> Al-Ga ~ Ga. The photocatalytic activity as quantified through the determination of the $k_{S}$/area value is dependent upon the concentration of free $h_{VB}^{+}$ and $OH_{(ads)}$ on the surfaces of ZnO NWs to form the maximum amount of ●OH hydroxyl radicals. Regardless of the nature of dopants, the non-intentionally doped and intentionally doped ZnO NWs all exhibit an n-type doping [43,63], originating from the high concentration of hydrogen-related defects acting as shallow donors [64]. As a consequence, free $e_{CB}$ act as the majority of free charge carriers in the bulk of non-intentionally doped and intentionally doped ZnO NWs. The concentration of free $e_{CB}$ however depends drastically on the nature of dopants and is expected to roughly lie in the range of $5 \times 10^{17} - 10^{20}$ cm$^{-3}$ [65].

The concentration of free $e_{CB}$ is thus lower or in the same range as the concentration of $e_{CB}^{-}$ $h_{VB}^{+}$ pairs generated from UV lighting which lies in the range of $10^{19}$ cm$^{-3}$ or even more with a powerful UV lamp/LED [66]. Interestingly, ZnO NWs grown by CBD are known to exhibit a surface electron depletion region and hence an upward band bending [67,68]. The $e_{CB}^{-}$ $h_{VB}^{+}$ pairs are thus separated such that free $h_{VB}^{+}$ migrate towards the surfaces of ZnO NWs, while free $e_{CB}$ stay in their bulk: this significantly decreases the probability that free $h_{VB}^{+}$ recombine before reaching the surfaces of ZnO NWs. Accordingly, by using the compensating doping with Cu and Sb, a higher number of free $h_{VB}^{+}$ reaches the surfaces of ZnO NWs and participates to the photocatalytic processes. This accounts for the higher photocatalytic activity of ZnO NWs intentionally doped with Cu and Sb. Given that the n-type doping with Al and Ga substituting for Zn sites (i.e. $Al_{Zn}$ and $Ga_{Zn}$) and with Cl substituting for O sites (i.e. $Cl_{O}$) results in the incorporation of more free $e_{CB}$, the photocatalytic activity is much lower. In the case of n-type doping, the degradation of the photocatalytic activity even follows the ability of incorporating and activating Al-, Ga-, and Cl-related dopants. The incorporation of dopants and the resulting concentration of free $e_{CB}$ into ZnO NWs is for instance higher in the case of Ga than in the case of Al, as reported in ref [65]. The fact that thermally annealed and non-intentionally doped ZnO NWs exhibit the highest photocatalytic activity further reveals that the engineering of non-intentionally incorporated defects, such as hydrogen-related defects to some extent, aiming at reducing the concentration of shallow donors (i.e. for instance by exo-diffusing hydrogen and instead forming acceptors) outperforms the activation of dopants into intentionally doped ZnO NWs. An additional effect may also come for the hydroxyl termination of the surfaces of non-intentionally doped and intentionally doped ZnO NWs, for which the concentration of O-H groups likely depends on the nature of dopants. More importantly, beyond the rankings reported here, the photocatalytic activity of intentionally doped ZnO NWs is found to be optimal for a given concentration of dopants roughly lying in the range of ten to several tens of ppm (i.e. $4.2 \times 10^{17}$ to a few $10^{18}$ at cm$^{-3}$), except the Ga single- and Al-Ga co-doping.

Eventually, the presence of a high concentration of free $h_{VB}^{+}$ on the surfaces of ZnO NWs induces a concomitant photo-corrosion phenomenon as described in Eq. (5) regardless of the nature of dopants, whose magnitude is directly correlated with the magnitude of the photocatalytic activity. The production of oxygen molecules from the photo-

### photocatalytic activity

**Before thermal annealing**

Cu > Sb >> Cl > undoped ~ Al > Al-Cl >> Al-Ga ~ Ga
acceptors donors

**After thermal annealing**

undoped >> Cu > Cl > Sb > Al-Cl >> Al >> Al-Ga ~ Ga

### degradation pathway

<figure><img src="image_11.png" /></figure>
corrosion phenomenon is even not helpful for the photocatalytic pro-
cesses owing to the surface electron depletion region. This raises con-
cerns about the chemical stability of ZnO NWs grown by CBD for
photocatalytic devices. This may be addressed for instance by covering
the surfaces of ZnO NWs with a passivating shell, such as TiO₂ [69] or
any more stable compound semiconductors, exhibiting a high photo-
catalytic activity, or by changing the photocatalytic processes at work
such that free e<sub>CB</sub> would become the primary active charge carriers on
the surfaces of ZnO NWs. The present perspectives for the photocatalytic
devices made of ZnO NWs would require a downward band bending on
the surfaces of ZnO NWs as in the case of ZnO single crystals with the
nonpolar *m*-plane orientation [62,70], and hence a more efficient
compensating doping. This would join the perspectives for piezoelectric
devices made of ZnO NWs, for which the screening of the piezoelectric
potential by free e<sub>CB</sub> represents one of the major issues [71,72].

5. Conclusions

In summary, we have investigated the intentional doping of ZnO
NWs using a wide variety of dopants including Al, Ga, Cu, Sb, Cl, Al-Ga,
and Al-Cl and using several concentrations for each dopant to thor-
oughly assess the effects on the photocatalytic activity and processes.
Through the combination of morphological, structural, optical, and
chemical characterization techniques, we have shown that the photo-
catalytic processes are governed by ●OH hydroxyl radicals as the main
reactive species degrading the MO organic dye and that free h<sub>VB</sub> act as
the primary active charge carriers on the surfaces of ZnO NWs due to
upward band bending, regardless of the nature and concentration of
dopants. Except for Ga single- and Al-Ga co-doping, the main degrada-
tion pathway is found to proceed through the oxidation reaction of a
phenyl ring bearing the dimethyl amino group. In the case of Ga single-
and Al-Ga co-doping, the main degradation pathway involves the
demethylation reaction of the tertiary amine The photocatalytic activity,
which follows a pseudo first-order kinetics is further revealed to strongly
depend on the nature of dopants with the following rankings before
thermal annealing - Cu > Sb >> Cl > undoped ~ Al > Al-Cl >> Al-Ga
~Ga - and after thermal annealing - undoped >> Cu > Cl > Sb > Al-
Cl >> Al >> Al-Ga ~ Ga. It is thus higher in the case of intentional
doping with Sb and Cu dopants, acting as acceptors, than with Al, Ga,
and Cl, acting as shallow donors. More importantly, beyond its ranking
before and after thermal annealing, the photocatalytic activity of
intentionally doped ZnO NWs is optimal for a specific concentration of
dopants, roughly in the range of ten to several tens of ppm (i.e. 4.2 × 10¹⁷
to a few 10¹⁸ at cm⁻³), except for Ga single- and Al-Ga co-doping.
Eventually, a photo-corrosion phenomenon is concomitant with the
photocatalytic processes, raising concerns about the chemical stability
of ZnO NWs. These findings shed light on the effects of intentional
doping of ZnO NWs for photocatalytic devices and open perspectives to
address chemical and technological challenges using passivating semi-
conducting shells, or alternative photocatalytic processes involving free
e<sub>CB</sub>.

CRediT authorship contribution statement

**Baillard Adrien:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis, Data curation.
**Consonni Vincent:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Funding acquisition, Data curation, Conceptualization.
**Jacob Véronique:** Writing – review & editing, Investigation, Formal analysis, Data curation.
**Becht Jean-Michel:** Writing – review & editing, Investigation, Formal analysis, Data curation.
**Roussel Hervé:** Writing – review & editing, Investigation, Formal analysis, Data curation.
**Roussel Hervé:** Writing – review & editing, Investigation, Formal analysis, Data curation.
**Appert Estelle:** Writing – review & editing, Validation, Supervision, Resources, Data curation.

**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

Acknowledgements

The authors acknowledge the financial support from the French National Research Agency through the DOSETTE (ANR-17-CE24-0003) and IMINEN (ANR-22-CE09-0032) projects. This research has further benefited from some of the characterization equipments of Grenoble INP-CMTC platform and of IUT 1 Chemical Department platform.

**Appendix A. Supporting information**

Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.jece.2025.115865.

Data availability

Data will be made available on request.

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