AACVD-fabricated Zn-modified WO₃ photoanodes for enhanced solar water oxidation

Abstract

The photoelectrochemical (PEC) performance of WO₃ photoanodes is limited by poor charge transport and high recombination losses. In this work, Zn-modified WO₃ thin films (0, 2, 10, and 20 wt%) were fabricated via a two-step aerosol-assisted chemical vapour deposition (AACVD) method to investigate the effect of Zn incorporation on material properties and PEC water oxidation performance. The 10 wt% Zn–WO₃ film exhibited the highest photocurrent density (0.68 mA cm⁻² at 1.23 V_RHE under 1-sun illumination), corresponding to an enhancement of over 70% compared to pristine WO₃. This improvement is associated with nanorod morphology, enhanced (002) preferred orientation, bandgap narrowing (2.50 eV), and increased donor density, which promote more efficient charge transport and interfacial charge transfer. Electrochemical impedance and Mott–Schottky analyses further confirm reduced interfacial resistance and increased carrier density for the optimised composition. These results demonstrate that Zn doping improves the PEC water oxidation performance of WO₃ photoanodes.

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1. Introduction

From a sustainability and environmental perspective, hydrogen is promoted as a sustainable and green alternative to fossil fuel-dependent energy systems. However, the current mature production routes are not yet entirely green.^1–3 Photoelectrochemical (PEC) water splitting has gained significant attention as a promising strategy for direct solar-to-hydrogen conversion since the pioneering work of Fujishima and Honda in 1972.⁴ Despite this potential, the overall efficiency of PEC devices is primarily limited by the oxygen evolution reaction (OER) at the photoanode, which is kinetically sluggish and involves complex multi-electron transfer processes.^5–7

Metal oxide semiconductors have been extensively explored as photoanode candidates due to their abundance, stability under oxidative conditions, and tunable electronic properties.^8–11 However, their performance is often constrained by poor charge–carrier mobility, rapid electron–hole recombination, and limited visible light absorption.^11–14 These limitations hinder efficient charge separation and transfer, which are essential for driving water oxidation. To address these challenges, various material engineering strategies have been developed, including co-catalyst loading, heterostructure design, and elemental doping.^15–23 Among these, doping is an effective approach to tailor the electronic structure and defect chemistry of semiconductors, enabling improved carrier concentration, reduced recombination, and enhanced charge transport.^24–26 In addition, doping can influence surface morphology and increase the density of active sites, further facilitating interfacial charge transfer during the OER.²⁷

Tungsten trioxide (WO₃) is a promising n-type semiconductor for PEC water oxidation due to its suitable valence band position, visible light absorption up to ca. 477 nm (bandgap: ∼2.5–2.8 eV), and relatively high chemical stability compared to other photoanodes such as TiO₂ and BiVO₄.^28–32 However, its practical performance is limited by high recombination rates, inefficient charge transport, and an unfavourable conduction band position for hydrogen evolution.^33–35 These limitations prevent WO₃ from achieving its theoretical photocurrent, necessitating further optimisation through material modification.^34–40 Among various dopants, zinc (Zn) has attracted attention due to its ability to modify both the electronic structure and defect landscape of WO₃. The incorporation of Zn²⁺ can induce charge imbalance and promote the formation of oxygen vacancies, which enhance electrical conductivity and charge transport.⁴¹ In addition, Zn-related donor states can increase carrier density and improve charge separation, leading to enhanced photocurrent response compared to pristine WO₃.^42,43

The synthesis method plays a critical role in determining the structural and functional properties of WO₃ thin films. Conventional techniques, such as hydrothermal, sol–gel, sputtering, and pulsed laser deposition, often involve complex procedures or limited scalability.^44–49 In contrast, aerosol-assisted chemical vapour deposition (AACVD) offers a versatile and scalable approach for producing thin films with controlled composition and morphology.⁵⁰ In this process, precursor solutions are atomised into aerosol droplets and transported to a heated substrate, where they decompose to form uniform films with tunable properties.⁵¹

Zn-modified WO₃ thin films with varying Zn contents (0, 2, 10, and 20 wt%) were synthesised using AACVD to examine how Zn incorporation influences the structural and photoelectrochemical properties of WO₃ photoanodes. Zn doping was found to alter the electronic structure and enhance charge transport, resulting in improved photoelectrochemical water oxidation performance. Among the samples, the 10 wt% Zn–WO₃ film showed the highest photocurrent density. These findings highlight the role of Zn in tuning the structural and PEC characteristics of WO₃. To the best of our knowledge, this is the first report of Zn-modified WO₃ films prepared by AACVD for solar-driven water oxidation.

2. Experimental

2.1. Materials

All chemicals and reagents utilised in this study were sourced from Sigma-Aldrich and used as received without further purification. Fluorine-modified tin oxide (FTO) glasses were used as conductive substrates for the preparation of the photoanodes. Before the films were deposited, the substrates were subjected to thorough ultrasonic cleaning to ensure their cleanliness. This procedure involved successive 10-minute rinses with acetone and isopropanol to remove organic residues. This was followed by a final rinse with deionised water to eliminate any remaining traces of solvents. The substrates were then dried under ambient conditions to prepare them for the subsequent steps of material deposition and characterisation.

2.2. Preparation of photoanodes

Employing aerosol-assisted chemical vapour deposition (AACVD), unmodified and Zn-modified WO₃ photoanodes were prepared. The detailed description of the AACVD experimental setup has been documented in the literature.^52,53 As depicted in Fig. 1a, AACVD proficiently combines aerosol generation with chemical deposition. In general, AACVD uses an ultrasonic atomiser to convert a liquid precursor solution into a fine aerosol mist. This aerosol is then transported into a reaction chamber through a gas carrier, which directs the aerosol droplets across a heated substrate. Upon contact, the droplets evaporate rapidly, decomposing the precursor and forming a uniform thin film.⁵³

Fig. 1 Schematic illustration of the AACVD reactor setup used for the deposition of WO₃ and Zn-modified WO₃ thin films (a). XRD patterns of WO₃ and Zn-modified WO₃ (2%, 10% and 20%) (b). EDX spectrum of 10% Zn-modified WO₃ with data given below (c). Surface SEM images of pure WO₃ (d) and 2% (e), 10% (f), and 20% (g) Zn-modified WO₃ films.

Initially, unmodified WO₃ films were deposited on FTO substrates by dissolving 0.2 g of tungsten hexacarbonyl [W(CO)₆] in a 45 mL mixture of acetone and methanol (2 : 1 ratio) in a glass bubbler. This solution was then placed on an ultrasonic humidifier equipped with a piezoelectric device (Johnson Matthey Liquifog) to generate the aerosol. A nitrogen gas (N₂, BOC Ltd.) flow of 1 L min⁻¹ was used to transport the aerosol to the chamber, where the FTO substrates were heated at 350 °C. Following about 45 minutes of deposition, during which all the precursors were transferred, the substrates were cooled under a N₂ flow to below 100 °C. The substrates were subsequently heat-treated at 530 °C to enhance the crystallisation and oxidation of WO₃.

For the fabrication of Zn-modified WO₃ films, a second deposition procedure was applied on the pre-deposited WO₃ layer. A similar quantity of W(CO)₆ was dissolved in 30 mL of acetone and mixed with zinc chloride (ZnCl₂) in different molar ratios (2, 10, and 20 wt%) in 15 mL of methanol in three separate experiments to prepare the doping solutions. These solutions, with Zn dopant concentrations of 2%, 10% and 20%, were deposited at 420 °C to incorporate Zn into the WO₃ lattice. The films were then cooled and annealed following the same conditions as those used for pristine WO₃.

2.3. Physical characterisation

The crystalline structure and phase composition of the pristine and Zn-modified WO₃ films were analysed by X-ray diffraction (XRD) using a modified Bruker AXS D8 diffractometer with Cu Kα radiation (λ = 1.5406 Å), operated at 40 kV and 30 mA. The scans were performed in the 2θ range of 10°–80° in increments of 0.02°. Field emission scanning electron microscopy (FESEM, JEOL JSM-6301F, 5 kV) was employed to observe the surface morphology; the samples were coated with a thin Au layer to reduce charging. Energy dispersive X-ray spectroscopy (EDX) was carried out using the same JEOL JSM-6301F FESEM equipped with an EDX detector to confirm the elemental composition. Surface topography and roughness were assessed using atomic force microscopy (AFM, Bruker Dimension Icon) in tapping mode. Optical absorption spectra were recorded in the 200–800 nm range using an Agilent Cary 60 UV-vis spectrophotometer. Photoluminescence (PL) spectra were measured at room temperature using a HORIBA TCSPC fluorometer at an excitation wavelength of 325 nm, with emission recorded from 380 to 800 nm.

2.4. Photoelectrochemical characterisation

The PEC performance of pristine and Zn-modified WO₃ photoanodes was evaluated in a conventional three-electrode configuration using a Metrohm Autolab potentiostat. The working electrode comprised the WO₃-coated substrate, with a platinum wire and an Ag/AgCl (3 M KCl) electrode serving as the counter and reference electrodes, respectively. All measurements were conducted in 1.0 M KOH aqueous electrolyte (pH ≈ 14) at room temperature. The geometric area exposed to illumination was restricted to 1 cm². Linear sweep voltammetry (LSV) was performed under simulated AM 1.5G solar illumination (100 mW cm⁻²) using a xenon lamp. The scans were recorded at a rate of 10 mV s⁻¹ within the potential window of 0.2 to 1.6 V_RHE. The dark response was also acquired under identical conditions. Potentials measured against the Ag/AgCl electrode were converted to the reversible hydrogen electrode (RHE) scale using the Nernst equation:where E_Ag/AgCl is the potential measured against the Ag/AgCl electrode and under standard conditions.

Mott–Schottky (M–S) measurements were conducted under visible light illumination at a fixed frequency of 1 kHz with a 10 mV AC perturbation. An apparent flat-band potential (E_fb) and donor density (N_D) were determined from the linear region of the 1/C² versus potential plot using the standard Mott–Schottky relation. Electrochemical impedance spectroscopy (EIS) was carried out under illumination at a fixed bias of 1.23 V_RHE over the frequency range of 0.1 Hz to 100 kHz.

3. Results and discussion

This work explored aerosol-assisted chemical vapour deposition (AACVD) as a synthesis-driven strategy for engineering WO₃ thin films with tunable composition and functionality. Introducing Zn during film growth modified the physicochemical properties of WO₃ at the deposition stage, providing a direct route to engineer the photoanode material. The resulting films were then evaluated for photoelectrochemical (PEC) water oxidation to assess their potential as high-performance photoanodes.

Pristine WO₃ and Zn–WO₃ films were successfully synthesised using our home-built AACVD system under optimised conditions (Fig. 1a). A two-step deposition approach was used: an initial base layer of pristine WO₃ was deposited at 350 °C, followed by a Zn-incorporated WO₃ top layer deposited at 420 °C using precursor solutions containing 2, 10 and 20 wt% Zn. The resulting dark blue films were then annealed at 530 °C in air to improve crystallinity. The annealed samples exhibited a uniform pale-yellow colour, which became more intense with increasing Zn content. The increased intensity may suggest a change in the band structure or oxygen vacancy density. This controlled deposition produced smooth, well-adhered films with no visible cracks or delamination.

The crystalline phases of the deposited films were identified by XRD analysis. As shown in Fig. 1b, all films exhibit diffraction peaks that match well with monoclinic WO₃ (ICSD file no. 1620). For pristine WO₃, the main peaks appear at 2θ ≈ 23.9°, 24.5° and 25.2°, corresponding to the (002), (020) and (200) planes, respectively, confirming the formation of monoclinic WO₃.^54,55 Slight deviations from the reference peak positions may be related to minor lattice distortion associated with the deposition process.⁵⁶ In addition, the relatively high intensity of the (020) peak in the unmodified film suggests preferred orientation along this plane.^57–59 The monoclinic structure was retained after Zn addition. A slight shift toward higher 2θ was observed only for the 2% Zn–WO₃ film. According to Bragg's law, this indicates a small decrease in d-spacing, which can be attributed to local lattice strain or defect formation caused by low-level Zn incorporation during film growth. This shift was not clearly observed in the 10% and 20% Zn–WO₃ films. Importantly, all Zn-containing films still showed the characteristic diffraction peaks of monoclinic WO₃, with no new dominant peaks indicating a change in the main crystal phase. A change in relative peak intensity was also observed after Zn addition, with the (002) reflection becoming more prominent relative to (020). Overall, the XRD results indicate that Zn addition affected the peak position and relative intensity only slightly.

The surface properties were then investigated using SEM and AFM analyses. As presented in Fig. 1(d–g), the SEM images demonstrate that Zn doping modifies the surface morphology of WO₃ films deposited by AACVD. The unmodified WO₃ film (Fig. 1d) displays densely packed, vertical-aligned nanorods with slightly tapered tips. This structure facilitates efficient directional charge transport but may still offer limited active surface area. With the introduction of Zn, the growth of WO₃ nanorods exhibited a noticeable change in morphology. At 10% Zn (Fig. 1f), the film exhibits sharper, spike-like nanorods with increased surface roughness and more exposed active edges. Such features help enhance charge separation and accelerate OER kinetics. However, excess Zn content (20% Zn, Fig. 1g) results in aggregated, cluster-like ends, which in turn impede charge transfer and increase charge recombination losses. These findings confirm that moderate doping optimises the surface morphology, which helps enhance water oxidation activity. In addition, the EDX spectrum of 10% Zn-modified WO₃ presented in Fig. 1c verifies the presence of W, O and Zn elements, confirming that Zn is well incorporated into the WO₃ lattice.

The surface topography, roughness, and surface area of the prepared films were examined using AFM measurements. The two-dimensional AFM images in Fig. 2 highlight noticeable changes in surface texture following Zn doping. As observed in Fig. 2a, the unmodified WO₃ film exhibits relatively regular grains and a smooth, compact surface with minor height variations. In contrast, as Zn concentration increases (Fig. 2b–d), the grain structure becomes less regular, the grain size increases, the surface roughness increases, and the topographical features become more significant with greater height variations. The roughness parameters and surface area values extracted from AFM analysis are summarised in Table 1, reflecting clearly the influence of Zn content on the film's surface texture.

Fig. 2 Two-dimensional AFM surface topography images of pristine WO₃ (a) and 2% (b), 10% (c) and 20% (d) Zn-modified WO₃ films.

Table 1 AFM surface roughness parameters (R_max, R_q and R_a) and surface area of the pristine and Zn-modified WO₃ thin films (measuring range: 10 × 10 µm²)

Samples	Surface roughness (nm)			Surface area (µm^2)
	Rmax	Rq	Ra
20% Zn–WO3	2783	359	282	167
10% Zn–WO3	2407	301	225	166
2% Zn–WO3	1245	99	73	119
Pristine WO3	355	53.3	42.5	29

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The optical absorption behaviour of pristine and Zn-modified WO₃ films was evaluated using UV-vis spectroscopy, as shown in Fig. 3a. All samples exhibit strong absorption in the UV region, with a clear red shift in the absorption edge upon Zn incorporation. The optical bandgap was estimated by plotting (αhν)^1/n against hν using n = 2 for an indirect bandgap transition.³⁹ The extrapolated bandgaps decreased from 2.76 eV for pristine WO₃ to 2.68, 2.50 and 2.58 eV for the 2%, 10% and 20% Zn-modified films, respectively, as shown in the inset of Fig. 3a. The narrowing of the bandgap with Zn incorporation, particularly at 10% doping, suggests the introduction of defect intermediate states and band tailing effects that lower the energy required for electronic excitation. These modifications enhance absorption in the visible region and may contribute to improved photo-response under solar irradiation.

Fig. 3 UV-vis absorbance spectra with bandgap determination (a) and photoluminescence (PL) spectra (b) of unmodified and Zn-modified WO₃ films.

To gain insight into the charge carrier dynamics and recombination behaviour, PL measurements were carried out at room temperature. Fig. 3b displays the PL spectra of unmodified WO₃ and Zn-modified WO₃ films with various Zn concentrations, recorded under 325 nm excitation. The unmodified film shows a strong emission centred at ∼435 nm, which is usually associated with radiative recombination through intrinsic defects, mainly oxygen vacancies.^60,61 In contrast, the Zn-modified WO₃ samples exhibit a clear decrease in PL intensity and a slight red shift of the emission peak towards ∼455 nm. This reduction in intensity reflects a lower recombination rate of photogenerated electron–hole pairs, implying improved charge separation in the modified films.⁶² The observed PL quenching becomes most pronounced at 10% and 20% Zn doping, suggesting the formation of additional defect or trap states that act as non-radiative recombination centres. These are likely introduced by the aliovalent substitution of W⁶⁺ with Zn²⁺, which causes local structural distortion and induces oxygen vacancies for charge compensation. The redshift and broadening of the emission further support an increase in structural disorder, consistent with observations from XRD and UV-vis analysis.

XPS analysis confirms the successful incorporation of Zn into WO₃ while largely preserving the WO₃ chemical framework. In the W 4f region (Fig. 4a) for the 10% Zn–WO₃ sample, the spectrum exhibits the characteristic spin–orbit doublet with peaks centred at 35.5 eV (W 4f_7/2) and 37.6 eV (W 4f_5/2), separated by 2.1 eV, which is typical of W⁶⁺ in stoichiometric WO₃. The absence of a pronounced low-binding-energy shoulder suggests that tungsten remains predominantly in the W⁶⁺ oxidation state, indicating that the host lattice is largely maintained after Zn addition. Any reduced W species such as W⁵⁺/W⁴⁺, if present, would be minor or unresolved at this scale. In the Zn 2p region (Fig. 4b–d for 10% Zn–WO₃, 20% Zn–WO₃ and 2% Zn–WO₃ samples, respectively), two well-defined peaks appear at 1022 eV and 1044.7 eV, corresponding to Zn 2p_3/2 and Zn 2p_1/2, respectively, with a spin–orbit splitting of 23 eV, consistent with Zn²⁺ in an oxide environment. Together, these results verify the presence of Zn in the modified sample and indicate that Zn is present in the Zn²⁺ state, while the tungsten environment remains predominantly W⁶⁺–WO₃. Any Zn-induced charge compensation is therefore expected to occur mainly through subtle lattice distortion and/or the creation of oxygen vacancy-related defects, rather than extensive reduction of tungsten. The full survey spectrum of the 10% Zn–WO₃ thin film is presented in Fig. S1.

Fig. 4 Surface X-ray photoelectron spectroscopy (XPS) spectra of (a) W 4f_7/2 and 4f_5/2 for the 10% Zn–WO₃ sample and (b) Zn 2p_3/2 and 2p_1/2 core level scans in the 10% Zn modified WO₃ thin film, (c) Zn 2p_3/2 and 2p_1/2 core level scans in the 20% Zn modified WO₃ thin film and (d) Zn 2p_3/2 and 2p_1/2 core level scans in the 20% Zn modified WO₃ thin film.

The PEC activity of unmodified and Zn-modified WO₃ thin films was examined under simulated 1-sun illumination (AM 1.5G) in a 1.0 M KOH electrolyte via LSV measurements. As illustrated in Fig. 5a, all films exhibited anodic photocurrents with negligible dark currents, confirming that the observed responses originate exclusively from photoinduced charge–carrier interactions. The unmodified WO₃ film delivered a photocurrent density of ∼0.39 mA cm⁻² at 1.23 V_RHE, with an onset potential around 0.70 V_RHE. This relatively reflects sluggish hole transfer at the interface and can be attributed to the compact morphology, low surface area and dominant (020) facet, which have been shown to limit the reaction kinetics and active site density.⁶³ The wide optical bandgap (2.76 eV) and strong PL emission also reflect limited visible-light absorption and high recombination losses, respectively.

Fig. 5 Linear sweep voltammetry (LSV) curves under dark and illuminated conditions (a), the photocurrent density at 1.23 V_RHE for all samples (b), amperometric I–t curves at a potential of 1.08 V_RHE (c), Nyquist plots of electrochemical impedance spectra (d), and Mott–Schottky plots (e) and (f) under AM 1.5G illumination of the pristine WO₃ and Zn-modified WO₃ films.

In general, significant changes in PEC behaviours were observed in Zn-incorporated films. At low Zn concentration, the photocurrent dropped to ∼0.15 mA cm⁻² at 1.23 V_RHE. This reduction does not follow the expected trend with Zn incorporation and may be associated with local structural distortion, as indicated by the slight shift in the XRD peaks. Such local lattice strain can introduce unfavourable electronic environments or trap sites that hinder effective charge transport and interfacial charge transfer. Despite the reduced bandgap (2.68 eV), these effects appear to limit the photocurrent response. Of all the films, the 10% Zn–WO₃ sample exhibited the highest photocurrent density at 1.23 V_RHE (∼0.68 mA cm⁻²) with an improved onset potential of ∼0.56 V_RHE. This improvement is attributed to enhanced visible-light absorption (E_g = 2.50 eV) and reduced electron–hole recombination as indicated by the quenched PL intensity. Furthermore, XRD analysis revealed a preferential shift in crystal orientation towards the (002) plane, which plays a crucial role in promoting OER kinetics. It is known that the (002) facet exposes a higher density of catalytically active terminal oxygen atoms and facilitates stronger orbital overlap with water molecules, thereby lowering the energy barrier for hole transfer and accelerating the formation of key reaction intermediates such as OH˙ and OOH˙.⁶⁴ In contrast, the (020) facet, which dominates in the unmodified film, is associated with fewer active sites and slower reaction kinetics at the surface.⁶⁵ This transition in crystal orientation coupled with the increased surface roughness and active area observed via AFM improves charge separation and interfacial charge transfer, ultimately leading to a more efficient and earlier PEC response. At a higher Zn content (20%), however, there is a renewed decrease in the photocurrent (∼0.61 mA cm⁻²), which is related to a lower surface roughness and a potential oversaturation of the dopant. Both can reduce the active (002) facet density or promote deep trap states. This suggests that facet exposure is critical but must be balanced by optimal doping to sustain structural integrity and carrier mobility.

This trend is demonstrated in Fig. 5b, which presents a bar chart comparing the photocurrent densities at 1.23 V_RHE for different Zn concentrations. The 10% Zn-modified WO₃ film shows the highest photocurrent density, confirming its superior photoelectrocatalytic activity. This improvement results from the optimisation of several key properties, including enhanced light absorption, improved crystal orientation, and refined surface morphology. These factors promote more efficient charge transfer and reduce the energy barrier for oxygen evolution. Table 2 further contextualises this performance by summarising reported photocurrent values for various modified WO₃ photoanodes and comparing them with the film developed in this study.

Table 2 Comparison of selected WO₃ photoanodes doped with various dopants and their PEC performance for water oxidation reported in the literature

Photoanode	Dopant, content	Synthesis technique	Morphology	Photocurrent performance	Electrolyte, illumination	Ref.
Zn-doped WO3	Zn, 10 wt%	AACVD	Spike-like nanorods	0.68 mA cm^−2 at 1.23 VRHE	1 M KOH, AM 1.5G	This work
Ti-doped WO3	Ti, 1.16 at%	Single-step hydrothermal	Nanoparticles	1.139 mA cm^−2 at 1.23 VRHE	0.1 M Na2SO4, AM 1.5G	66
In-doped WO3	In, 3 wt%	Hydrothermal	Nanoblocks	2.18 mA cm^−2 at 1.23 VRHE	0.1 M Na2SO4, AM 1.5G	67
Ta-doped WO3	Ta, 3.93 at%	Reactive magnetron co-sputtering	Granular with well-defined boundaries	0.65 mA cm^−2 at 1.0 VAg/AgCl	0.2 M Na2SO4, AM 1.5G	68
Sn-doped WO3	Sn, 0.80 at%	Single-step hydrothermal	Coagulated nanoparticles + plate-like structures	0.427 mA cm^−2 at 1.23 VRHE	0.5 M Na2SO4, AM 1.5G	69
Al-doped WO3	Al, 0.77 at%	RF magnetron co-sputtering + hydrothermal	Nanoplates	1.14 mA cm^−2 at 1.2 VAg/AgCl	0.5 M H2SO4, AM 1.5G	70
Ni-doped WO3	Ni, 3 at%	Two-step hydrothermal	Nanoplate arrays	0.80 mA cm^−2 at 1.0 VAg/AgCl	0.5 M Na2SO4, AM 1.5G	71
Cu-doped WO3	Cu, 1.41 wt%	Hydrothermal	Nanoparticle assembly	0.487 mA cm^−2 at 1.23 VRHE	0.5 M Na2SO4, AM 1.5G	72
Co-doped WO3	Co, 0.36 wt%	Hydrothermal	Nanorods and nanoparticles	0.636 mA cm^−2 at 1.23 VRHE	0.5 M Na2SO4, AM 1.5G	72
Gd-doped WO3	Gd, 4%	Hydrothermal	Nanoplate arrays	2.28 mA cm^−2 at 1.0 VAg/AgCl	0.2 M Na2SO4, AM 1.5G	73
Bi-doped WO3	Bi, 0.17 at%	Modified hydrothermal	Porous nanoparticle film	1.51 mA cm^−2 at 1.23 VRHE	0.1 M Na2SO4, AM 1.5G	35

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Under the same PEC experimental conditions, the stability of the 10% Zn-modified WO₃ film was evaluated and compared with that of pristine WO₃ at approximately 1.08 V_RHE, as shown in Fig. 5c. The Zn-modified film exhibited a higher and more stable photocurrent than the unmodified photoanode. This enhanced stability suggests more efficient charge transfer and suppressed surface recombination during PEC operation.

Post-PEC XRD patterns (Fig. S2) show reduced peak intensity, which may be due to partial dissolution of WO₃ in the highly alkaline electrolyte. The Zn-modified sample shows smaller changes compared to pristine WO₃, consistent with its higher and more stable photocurrent.

The EIS measurements were performed under illumination to assess the charge transfer characteristics at the interface of the photoanodes (Fig. 5d). All samples exhibited characteristic semicircular Nyquist arcs, representing the charge transfer resistance (R_ct) at the photoanode–electrolyte interface. Among all compositions, the 10% Zn–WO₃ sample displayed the smallest arc diameter, indicating the lowest R_ct and thus the most efficient interfacial charge transport. This reflects an enhancement in electrical conductivity and lower recombination losses, both of which contribute to the excellent PEC performance of this sample. In comparison, the pristine and 2% Zn–WO₃ films showed significantly larger arcs, suggesting slower charge transfer dynamics and lower conductivity. The high Zn content sample exhibited an intermediate arc size, suggesting that excessive doping impairs the interfacial transport, likely due to increased structural disorder or trap-state formation. The high-frequency region, magnified in the inset of Fig. 5d, provides valuable insight into the solution resistance (R_s). The 10% Zn-modified WO₃ film exhibited the lowest R_s among the films, showing enhanced electrolyte conductivity within the overall PEC system. This reduction in R_s further contributes to efficient charge extraction and minimised resistive losses.

To further probe the effects of doping on the electronic properties of Zn-modified WO₃ samples, Mott–Schottky analysis was conducted under illumination (Fig. 5e and f). The obtained plots exhibit linear regions with positive gradients for all samples, indicating that the materials behave as n-type semiconductors, where electrons are the dominant charge carriers. The flat band potential was determined by extrapolating the linear region to the potential axis. The estimated values were approximately 1.0 V for 2% Zn–WO₃, 0.49 V for 10% Zn–WO₃, and 1.13 V for 20% Zn–WO₃ (vs. Ag/AgCl). The observed shift in flat band potential with increasing Zn content suggests a modification in the electronic structure and a shift in the Fermi level position. Furthermore, the variation in the slope of the Mott–Schottky plots reflects differences in charge carrier density among the samples. In particular, the 10% Zn–WO₃ sample exhibits the lowest slope, suggesting an increase in effective carrier density or capacitance response compared to the other compositions. This enhancement in charge carrier density is expected to improve electrical conductivity and facilitate charge transport within the material.⁷⁴ From an electrochemical perspective, when a potential more positive than the flat band potential is applied to an n-type semiconductor, electron accumulation occurs at the electrode/electrolyte interface.⁷⁵ This promotes electron transfer towards the surface and enhances electrochemical reactions. Conversely, at potentials more negative than the flat band potential, charge depletion occurs, reducing the availability of electrons at the surface. This behaviour is governed by band bending at the semiconductor/electrolyte interface.^76,77 The lower flat band potential of the 10% Zn–WO₃ sample indicates that its Fermi level is closer to the conduction band, which results in a higher electron density. This increased electron density improves charge separation and facilitates electron transfer at the semiconductor/electrolyte interface.^78,79 Based on these results, the 10% Zn-modified WO₃ sample demonstrates superior electrochemical performance due to its favourable flat band potential and higher charge carrier density, which enhance interfacial charge transfer. These properties make it a promising candidate for applications such as photoelectrochemical systems and photocatalysis.

Fig. 6 illustrates the proposed mechanism for PEC water splitting using a Zn modified WO₃ thin film under light illumination. WO₃ is an n-type semiconductor with a relatively narrow bandgap (2.6–2.8 eV), and its valence band edge lies at a highly positive potential (>+3.0 V vs. RHE), well above the water oxidation potential (+1.23 V vs. RHE). This makes WO₃ highly suitable for driving the oxygen evolution reaction (OER). However, WO₃ alone is not capable of overall water splitting because its conduction band position is more positive than the H⁺/H₂ reduction potential, limiting its ability to generate hydrogen. On the other hand, ZnO is an n-type semiconductor with a wider bandgap (3.2 eV) and a more negative conduction band edge, which is thermodynamically favorable for hydrogen evolution. Under illumination, electrons are excited from the valence band to the conduction band in both semiconductors, leaving behind holes in their respective valence bands. In the Zn modified WO₃ films, the band alignment promotes charge separation. The photogenerated electrons in the conduction band of ZnO can be efficiently transferred to the external circuit (via FTO and Pt), where they participate in the reduction of water to produce hydrogen. Meanwhile, the holes accumulated in the valence band of WO₃, due to its more positive potential, drive the oxidation of water to generate oxygen. This doping facilitates effective spatial separation of charge carriers, reduces electron–hole recombination, and enhances overall charge transfer efficiency.

Fig. 6 Schematic representation of the mechanism of PEC water splitting in Zn modified WO₃ films on FTO substrates.

4. Conclusion

To conclude, Zn-modified WO₃ thin films were successfully synthesised via a two-step AACVD method, and the effect of Zn incorporation on their structural, optical, and photoelectrochemical properties was investigated. Zn doping modifies the morphology, crystal orientation, and electronic properties of WO₃, leading to improved PEC performance for water oxidation. The 10 wt% Zn–WO₃ film exhibited the highest photocurrent density (∼0.68 mA cm⁻² at 1.23 V_RHE), together with improved onset potential and reduced interfacial resistance, which are attributed to bandgap narrowing, increased donor density, and enhanced charge transport. Overall, Zn incorporation enhances the PEC behaviour of WO₃ photoanodes under the tested conditions.

Author contributions

A. A. supervised the research, guided the experimental design, reviewed the manuscript, and provided scientific direction throughout the study. S. A. conducted the synthesis of the materials, performed all experiments, data analysis, and discussions, and wrote the manuscript. H. A. contributed to the synthesis of the samples.

Conflicts of interest

The authors declare that they have no conflicts of interest related to this work.

Data availability

The data that support the findings of this study are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ma00092d.

Additional experimental data, characterization results, and electrochemical measurements generated during the current study are available from the corresponding authors upon reasonable request.

Acknowledgements

The authors gratefully acknowledge Mr Mohammed A. Alkhamisah and Mr Abdulrahman A. Alghihab for their technical support and assistance. This work was supported by a research grant from the Research, Development, and Innovation Authority (RDIA), Kingdom of Saudi Arabia (grant no. 12866-KACST-2023-KACST-R-2-1-EI).

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Footnote

† These authors contributed equally.

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