WO3 Photoanodes for Photoelectrochemical Applications

Radová, Barbora; Imrich, Tomáš; Krýsová, Hana; Neumann-Spallart, Michael; Krýsa, Josef

doi:10.70322/prp.2025.10006

Photocatalysis: Research and Potential

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Feature Paper in A Commemorative Issue in Honor of Professor David Ollis: A Pioneer and Mentor

WO₃ Photoanodes for Photoelectrochemical Applications

Barbora Radová ¹ Tomáš Imrich ¹ Hana Krýsová ² Michael Neumann-Spallart ¹ Josef Krýsa ^1,*

Author Information

Department of Inorganic Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague, Czech Republic

J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova 2155/3, 182 23 Prague, Czech Republic

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Photocatalysis: Research and Potential 2025, 2 (2), 10006; https://doi.org/10.70322/prp.2025.10006

Received: 07 January 2025 Accepted: 27 February 2025 Published: 07 March 2025

ABSTRACT: WO₃ layers were prepared by spray pyrolysis of a peroxotungstic acid solution on FTO/glass substrates. Investigated parameters were layer thickness and influence of post-annealing in air. Films deposited at 250 °C were amorphous. Post-annealing at 550 °C for 2 h resulted in the formation of monoclinic crystalline structure. A comprehensive account of electrochemical efficiency in terms of IPCE for WO₃ films as a function of the three parameters (wavelength, thickness and direction of light incidence) fully characterizing the photoelectrodes is presented here for the first time. The highest improvement in crystallinity and also the highest photocurrent response was found for WO₃ layers deposited at 250 °C and post-annealed at 550 °C, namely 1.9 mA/cm² (in 0.1 M HClO₄ at 1.6 V vs. Ag/AgCl) under irradiation with a solar simulator (AM 1.5, 100 mW/cm²) and IPCE = 0.5 at 369 nm (front side irradiation), which is comparable with values obtained by other deposition techniques (e.g., hydrothermal or sol gel). Spray pyrolysis as a method of fabricating WO₃ electrodes has the advantage of being able to produce large electrodes for use in practical applications.

Keywords: WO₃; Spray pyrolysis; Crystallinity; Photocurrent; IPCE

Graphical Abstract

1. Introduction

Metal oxide semiconductor materials are attractive for light absorbers, gas sensors, and as catalysts in photoelectrochemical (PEC) processes with applications in (i) light-assisted water electrolysis [1,2,3], (ii) degradation (oxidation) of organic substances [4,5,6] and (iii) photoelectrochemical syntheses of chemicals (e.g., I₂, Br₂ or Cl₂) [7]. Much attention has been paid to photoanodes made of binary transition n-type metal oxide materials such as TiO₂ or WO₃. TiO₂ has a wide bandgap of 3.2 eV, which limits its use (λ_onset = 388 nm). Less than 4% of the incident power (1.5% of all photons) of the solar light, based on published AM1.5 solar spectra (G-173), can be used by TiO₂ [8]. On the other hand, tungsten trioxide is an n-type semiconducting metal oxide, which has a lower bandgap, i.e., 2.7 eV [9], allowing the utilization of a more substantial portion of the solar light spectrum (λ_onset = 459 nm), i.e., approximately 12% of the power (6.2% of all photons) [8]. Dissolution of WO₃ in alkaline media (pH > 6) [10] limits its working range. A possible way of extending it is to cap the WO₃ electrode with a thin protective TiO₂ overlayer. A 20 nm thick TiO₂ layer fabricated by atomic layer deposition on WO₃ decreased the Faradaic efficiency of photocorrosion about 20 times [11]. WO₃ films have been synthesized in many different ways: doctor blading [12], brush painting [13,14], drop casting [5], chemical bath deposition and hydrothermal synthesis [15,16,17,18], spin coating [19], sol gel [20], printing [21,22], thermal evaporation [23], sputtering and High Power Impulse Magnetron Sputtering (HiPIMS) [24,25,26], pulsed laser deposition (PLD) [27], electrodeposition [28,29], anodization [30,31], spray and aerosol pyrolysis [1,32,33,34,35], and electrospray deposition [36]. Spray pyrolysis as a method of fabricating WO₃ electrodes has the advantage of being able to produce large electrodes for use in practical applications in contrary to aerosol pyrolysis where the homogeneous distribution of aerosol on larger area is quite challenging. Crucial parameters influencing the photocurrent of WO₃ films are layer thickness and direction of the incident light (through the electrolyte “EE”, or through the substrate “SE” (if transparent, e.g., FTO/glass)) and should be optimized according to the intended usage. For monochromatic irradiation (365 nm) a layer thickness of around 1 µm was found sufficient to achieve maximum photoresponse, and the reported incident photon to current efficiency (IPCE) value for an applied potential of 1.2 V vs. SCE was 0.45 [13]. The IPCE for the whole wavelength range was measured for a 1 µm thick WO₃ film and was found to correlate well with light absorptance [13]. The integral of the product of IPCE and the solar flux gave the total expected solar photocurrents using the respective electrodes: 0.79 mA/cm² under backside illumination. Recently, using a solar simulator (AM1.5 (1 sun)) and a potential of 0.9 V vs. Ag/AgCl in 0.1 M HClO₄, a photocurrent density around 1 mA/cm² was achieved for a 4 micron thick film obtained by aerosol pyrolysis [1]. In the literature, photocurrents are often reported for various light sources, but comparison with literature data is possible only when wavelength and irradiance are reported or when standardised solar illumination (1.5 AM, 100 mW/cm²) is used or the whole IPCE spectrum is given. This study aimed to synthesize mechanically stable WO₃ films by spray pyrolysis with high photocurrent values in aqueous solution under (simulated) solar AM1.5 front side irradiation by optimizing layer thickness. Besides the use of standardised solar illumination (1.5 AM, 100 mW/cm²) IPCE measurements from both electrolyte/electrode (EE) and substrate/electrode (SE) directions were performed. Such optimized films can be used for photoelectrochemical degradation of water pollutants and/or photoelectrosynthesis of chemicals.

2. Experimental

The chemicals used as received in this work included tungstic acid 99% (Sigma-Aldrich, Burlington, MA, USA), hydrogen peroxide (≥30.0%, Lach-ner, Czech Republic), and perchloric acid (70%, Acros Organics, Geel, Belgium). Triple distilled water was used for the preparation of solutions. Fluorine-doped tin oxide coated 2 mm thick glass (“FTO”, 7 Ω/sq., Sigma-Aldrich), was used as a substrate. Substrates were pre-cleaned ultrasonically by degreasing with trichloroethylene followed by rinsing with acetone, ethanol and water and drying in a stream of argon. WO₃ films were deposited from a solution of peroxotungstic acid prepared by suspending tungstic acid powder in 15 vol.% hydrogen peroxide as described previously in [32]. After stirring for 72 h, a slightly yellowish clear solution was obtained. This solution was further diluted to the concentration of 37.5 mM. Details of the spray pyrolysis (SP) apparatus have been described previously [37,38]. The structural, morphological and optical properties of the deposited WO₃ films were determined by X-ray diffraction (X’pert Philips MPD with a Panalytical X’celerator detector using graphite monochromatized Cu-K_α radiation (wavelength 1.54056 Å)) and profilometric thickness measurements (Dektak XT, Bruker, Billerica, MA, USA). The morphology of the films was also characterized by atomic force microscopy (AFM, Dimension Icon, Bruker) in semicontact (tapping) mode. A silicon cantilever (VTESPA-300) with a resonant frequency, f_res, of approx. 300 kHz, a spring constant, k, of 42 N/m, and a nominal tip radius of 5 nm (Bruker) were employed. The Gwyddion software (v. 2.53) was used for processing AFM image data and for the calculation of the roughness factor (R_f), which represents the ratio between the three-dimensional surface area of the image and its two-dimensional footprint area. Photoelectrochemical measurements used a Voltalab 10 PGZ-100 potentiostat and an Ag/AgCl reference electrode. Voltammetry was carried out under periodical (5 s light/5 s dark) front side illumination of the electrolyte/electrode interface. An aqueous solution of HClO₄ was used as an electrolyte. For irradiation, a solar simulator (150 W Xe arc lamp (Newport) with an AM 1.5G filter, irradiance 1 sun (100 mW/cm²)) was used. For the quantum efficiency measurements (IPCE) an Electrochemical Photocurrent Spectra CIMPS-pcs system (Zahner, Kronach, Germany) with a TLS03 tunable light source was used.

3. Results and Discussion

WO₃ films were deposited by SP at 250 °C nominal substrate temperature on FTO/glass using a 37.5 mM peroxotungstic acid solution with a spray rate of 1.6 mL/min. The films were annealed in air at 550 °C for 2 h to form the desired monoclinic WO₃ phase. 3.1. Physical Characterization Spray pyrolysis resulted in well-adherent WO₃films. Post-annealing at 550 °C had no impact on film adhesion. XRD data of as-deposited WO₃ films and of films after post-annealing at 550 °C are shown in Figure 1. The WO₃ films deposited at 250 °C did not contain any crystalline phase, only lines corresponding to the FTO/glass substrate were visible. For identification, Raman spectroscopy was therefore carried out and shown in Figure 2. It proved that the deposited layer consisted of amorphous WO₃. Annealing in air at 550 °C for 2 h resulted in the formation of the WO₃ monoclinic crystalline structure.

Figure 1. XRD patterns of as-deposited WO3 films (250 °C) (lower trace) and post-annealed films in air (550 °C, 2 h) (higher trace). The thickness of the WO3 layers was ~4 µm. XRD reference lines [39]: 04-005-4272 tungsten trioxide (WO3) and 04-003-5833 cassiterite (SnO2).

Figure 2. Raman spectrum of an as-deposited (250 °C) WO3 film. The thickness of the WO3 layers was 4.2 µm.

Figure S1 (in Supplementary Materials) shows AFM images of the FTO substrate (a) and of a WO₃ film deposited on FTO (250 °C) after post-annealing in air (550 °C, 2 h). The WO₃ film of thickness 4.2 µm was very smooth as seen by the decrease of the roughness factor, R_f, by 20% after coverage of the FTO/glass with a WO₃ layer. 3.2. Photoelectrochemical Characterization A typical chopped light polarization curve of a WO₃ film deposited at 250 °C WO₃ and post-annealed in air (550 °C, 2 h) is shown in Figure 3. The photocurrent onset was at ~0.25 V vs. Ag/AgCl. In the range of potentials from 0.25 to 1.6 V vs. Ag/AgCl the dark current was negligible and started to rise at ~1.7 V vs. Ag/AgCl. The maximum photocurrent of WO₃ films was achieved at 1.6 V vs. Ag/AgCl.

Figure 3. Typical chopped light polarization curve of a WO3 film deposited at 250 °C WO3 and post-annealed in air (550 °C, 2 h). Irradiance 1 sun (100 mW/cm2, simulated AM 1.5G). Electrolyte 0.1 M HClO4 (pH 1).

In the next step, we looked in detail at the influence of layer thickness. Values of IPCE for front side (EE) and back side (SE) illumination of WO₃ films of various thicknesses deposited under these conditions are shown in Figure 4a and 4b, respectively.

Figure 4. IPCE of WO3 films deposited at 250 °C WO3 and post-annealed in air (550 °C, 2 h) of thickness 0.4–16.1 μm. (a) front side (EE), (b) back side (SE) illumination in 0.1 M HClO4 at 1.6 V vs. Ag/AgCl.

A Tauc plot for an indirect electronic transition derived from the IPCE spectrum of a WO₃ film (deposition at 250 °C, post-annealing at 550 °C, layer thickness 4.2 µm) is shown in Figure S2 (in Supplementary Materials). An indirect bandgap of 2.7 eV was determined in accordance with published values [9]. A comprehensive account of electrochemical efficiency in terms of IPCE for WO₃ films as a function of wavelength, thickness and direction of light incidence (EE and SE) is presented here for the first time. Apart from the applied potential (governing the depletion layer width), which was selected to give maximum photocurrents before dark currents started to rise, these three parameters characterize the photoelectrodes fully. A detailed discussion of the dependence of the quantum efficiency on the above-mentioned parameters was given by Popkirov et al. [40]. For thin to medium thick layers, the observed increase in IPCE with thickness was due to the increase of the penetration depth of light, i.e., the IPCE followed the shape of the absorptance spectrum for all thicknesses as described in [9] and reached a value of 0.5 when all photons were absorbed. Further increase of thickness should keep this value constant but it fell below the optimum. This was probably due to the loss of good electrical contact between grains in the outer layers of the films as often observed in thicker films, impeding the transport of majority carriers towards the back contact (9 and 16 microns for EE irradiation in Figure 4a). An often-used wavelength in practical irradiations is 365 nm as it is a main mercury line, typical for the so called “UVA”. At this wavelength, most photons are absorbed within 1 micron. This has already been shown in [13] together with the saturation of IPCE as a function of layer thickness. This result is reproduced in the present study. The decrease of IPCE below 330 nm for SE illumination is due to light absorption by SnO₂ and glass. It is interesting to note that even the 16 micron thick film which produced only very low IPCE values in the EE mode, produced a maximum IPCE of 0.2 in the SE mode. This can be attributed to a porous film structure. In SE mode, most of the light is absorbed near the back contact so that photogenerated holes can reach the interface to the penetrating electrolyte easily and carry out a charge transfer reaction on the spot. The corresponding conduction band electrons (majority carriers) have to cover only a short distance towards the back contact, in contrast to the difficult transport situation encountered with EE illumination. The example shows again the diagnostic value of conducting EE vs. SE illumination experiments. Figure 5 exemplifies the essence of these observations by plotting IPCE at two selected wavelengths and total photocurrent for white light illumination as a function of layer thickness.

Figure 5. Dependence of photocurrent density and IPCE on layer thickness. Front side simulated solar AM 1.5G irradiation (100 mW/cm2) (open squares, left y axis) and IPCE at 369 nm (filled triangles, right axis) and 454 nm (filled circles, right y axis). Electrode potential 1.6 V vs. Ag/AgCl, electrolyte 0.1 M HClO4.

Table 1 shows a comparison of photocurrents (1.5 AM solar light) and IPCEs (incident photon to current efficiency) of WO₃ electrodes obtained by various deposition techniques found in the literature together with results obtained in the present study. All films had a monoclinic crystalline structure.

Table 1. Photocurrent densities under AM 1.5 solar irradiation (100 mW/cm²) and IPCE at 365−369 nm of WO₃ films prepared by various techniques. Front side illumination (through the electrolyte, EE).

Deposition Technique	Layer Thickness/µm	Electrolyte or pH	Photocurrent Density at 1.2 V vs. RHE/mA/cm²	IPCE at 365–369 nm	Ref.
Spray pyrolysis	4.2	0.1 M HClO₄	1.1	0.5 (1.6 V vs. Ag/AgCl)	this work
Brush painting	1.0–2.7	0.1 M HClO₄	-	0.62 (1.2 V vs. SCE)	[13]
Hydrothermal	2.3	0.5 M Na₂SO₄	1.6	0.75 (1 V vs. SCE)	[18]
Hydrothermal	2.3–3.6	0.9	-	0.53 * (0.95 V vs. Ag/AgCl)	[15]
Hydrothermal	3.5	0.1 M HClO₄	1.25	0.69 (1.4 V vs. Ag/AgCl)	[17]
Sol-gel	1.2	1 M H₂SO₄	2.0	-	[20]
DC-sputtering	0.15	0.1 M Na₂SO₄	0.8	-	[26]
Aerosol pyrolysis	3.8	0.1 M HClO₄	1.2	-	[1]
Aerosol pyrolysis	1.5	0.1 M HClO₄	-	0.4 (1.3 V vs. Ag/AgCl)	[1]
Aerosol pyrolysis	4	0.1 M HClO₄	1.05	-	[4]

* at 375 nm.

Reported photocurrent densities for a potential of 1.2 V vs. RHE are in the range of 0.8 to 2.0 mA/cm². The photocurrent densities achieved in the present work (around 1.1 mA/cm²) are similar to those fabricated by hydrothermal techniques [17] and slightly higher than that fabricated by aerosol pyrolysis (1.05 mA/cm²) [1] and DC-sputtering (0.8 mA/cm²) [26]. The IPCE at 369 nm found in this work was 0.5 for the best electrode. This value is similar to that reported for WO₃ films prepared by aerosol spray pyrolysis using peroxotungstic acid as a precursor (0.4) [1]. Other deposition techniques result in higher values of IPCE. For example, IPCE for a layer-by-layer brush coated electrode using WOCl₄ as a precursor [13] was 0.6 and for a hydrothermally grown WO₃ electrode, IPCE values 0.75 [18] and 0.53 [15] was reported. Although WO₃ films prepared by spray pyrolysis (SP) deposition did not achieve the high photocurrent densities and IPCE values as for sol gel or hydrothermal technique, the SP technique is able to fabricate larger WO₃ electrodes needed for practical electrosynthetic, environmentally relevant advanced oxidation processes, and energy conversion applications.

4. Conclusions

As deposited WO₃ layers (temperature 250 °C) were amorphous and the photocurrent density was negligible. Post-annealing at 550 °C resulted in the formation of monoclinic WO₃ and high photocurrent density. Apart from the applied potential (governing the depletion layer width), which was selected to give maximum photocurrents before dark currents started to rise, parameters as wavelength, thickness and direction of light incidence (EE and SE) characterize the photoelectrodes fully. A comprehensive account of electrochemical efficiency in terms of IPCE for WO₃ films as a function of these three parameters was presented here for the first time. Optimized WO₃ layers with a thickness of approximately 4 µm, deposited at 250 °C and annealed at 550 °C, exhibited a photocurrent of ~1.9 mA/cm² (1.6 V vs. Ag/AgCl) and ~1.1 mA/cm² (0.9 V vs. Ag/AgCl) under irradiation with a solar simulator in 0.1 M HClO₄. The IPCE at 369 nm was 0.5 (EE irradiation) for the optimised electrode. In comparison, other deposition techniques as hydrothermal or sol gel technique result in similar values of both photocurrent density and IPCE. On the other hand, spray pyrolysis deposition techniques are able to fabricate larger WO₃ electrodes needed for practical environmental and energy conversion applications.

Supplementary Materials

The following supporting information can be found at: https://www.sciepublish.com/article/pii/445, Figure S1: AFM phase images (left) and 3D height AFM images (right) of (a) FTO/glass substrate, (b) 4.2 µm thick WO₃ layer deposited at 250 °C on FTO/glass substrate and post-annealed at 550 °C for 2 h. The black bar in AFM phase images represents 500 nm; Figure S2: Tauc plot for WO₃ films deposited at 250 °C and post-annealed in air (550 °C, 2 h), layer thickness 4.2 µm.

Acknowledgements

Jiří Olejníček is acknowledged for Raman spectroscopy and Hana Tarábková for AFM analysis.

Author Contributions

B.R.: Investigation, Data curation, T.I.: Investigation, Data curation, Original draft, Writing—review & editing, H.K.: Investigation, M.N.-S.: Conceptualization, Original draft, Writing—review & editing, J.K.: Funding acquisition, Conceptualization, Writing—review & editing.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Funding

This work was supported by the project “Sensors and Detectors for Future Information Society-SENDISO reg. n. CZ.02.01.01/00/22_008/0004596” by the Programme Johannes Amos Comenius, call “Excellent Research”. This work was also supported by the Czech Science Foundation (project number 23-05266S) and by the grant of Specific University Research (UCT Prague)-Grant A2_FCHT_2024_002.

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.

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Photocatalysis: Research and Potential