
==== Front
ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c04738
Article
Optimizations of Liquid Phase Deposition Processes for Enhanced Photoelectrocatalytic Activities of Tungsten Oxide Thin Films
Nareejun Watcharapong †
Ponchio Chatchai *†‡
https://orcid.org/0000-0002-4496-2215
Mizuhata Minoru §
https://orcid.org/0000-0002-2360-577X
Minamimoto Hiro *§
† Department of Chemistry, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Khlong 6, Thanyaburi, Pathum Thani 12120, Thailand
‡ Advanced Photochemical and Electrochemical Materials (APEM) Research Unit, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Klong 6, Thanyaburi, Khlong Hok, Pathum Thani 12110, Thailand
§ Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, Rokkodai-cho 1-1, Nada-ku, Kobe 657-8501, Japan
* Email: chatchai@rmutt.ac.th.
* Email: minamimoto@godzilla.kobe-u.ac.jp.
04 09 2024
17 09 2024
9 37 3878838797
19 05 2024
19 08 2024
14 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

This study focuses on the preparation of tungsten oxide (WO3) as the photoanode for water oxidations by the liquid phase deposition (LPD) technique and its optimizations to improve the photoelectrochemical performance. The alternative precursor large stock solution process was achieved to simplify the LPD process for WO3 thin film preparation. The effect of boric acid in the precursor solutions on the physicochemical properties of the deposited WO3 thin films was investigated. As a result, we found that the optimized concentration of boric acid realized the highest photoelectrochemical performance. Through the optimizations of reaction conditions and surface analyses, we concluded that the preparations of a semiconductor film via the LPD technique had the potential to obtain high-performance photoelectrocatalytic applications.

Nippon Sheet Glass Foundation for Materials Science and Engineering 10.13039/100009088 NA Research and Researchers for Industries project NA NA Kansai Research Foundation for technology promotion NA NA Eagle dream Co., Ltd NA N41A650408 Japan Society for the Promotion of Science 10.13039/501100001691 JP22K18315 Japan Society for the Promotion of Science 10.13039/501100001691 JP22K144960 document-id-old-9ao4c04738
document-id-new-14ao4c04738
ccc-price
==== Body
pmc1 Introduction

The photoelectrocatalytic (PEC) technology has gained considerable interest in energy and environmental applications because of its notable efficiency, affordability, simplicity, and ecologically sustainable characteristics.1−3 The development of PEC systems relies heavily on appropriate semiconductor materials, particularly metal oxide semiconductors. Researchers are investigating many metal oxide semiconductors, such as TiO2,4−6 BiVO4,7,8 Fe2O3,9,10 Cr2O3,11 CuYO2,12 Mn2O3,13 or WO3,14,15 as photoanodes for water oxidation because of their exceptional performance and durability. Especially, WO3 would be promising in converting visible-light energy due to its excellent efficiency and stability,16 as well as its narrow band gap energy ranging between 2.4–2.8 eV.17 In addition, the valence band (VB) of WO3 has a more positive potential, which is favorable for promoting high-energy oxidation processes. WO3 is often employed alone or in combination with other semiconductors like BiVO418 or composites like Pt/WO3–C.19 The approach of depositing the WO3 film onto the support material substantially impacts and enhances the photoelectrocatalytic oxidation reactions. Various methods of WO3 thin film fabrication have been employed to improve the PEC efficiency. The spin coating, sputtering, electrodeposition, and liquid phase deposition (LPD) are among the deposition techniques utilized. The structural, morphological, and PEC characteristics of WO3 films are affected differently by each process. Electrodeposition has the ability to produce films that demonstrate outstanding uniformity and adherence.20 Conversely, sputtering allows for the creation of films that have exceptionally high levels of purity and exact control over their thickness.21 The spin coating22 and LPD23,24 methods are widely favored for their cost-effectiveness and ease of usage in achieving crystallographic organization and film formation. Moreover, these two methods provide significant advantages. Therefore, it is essential to develop a method for fabricating the WO3 film that ensures the operations’ simplicity and cost-effectiveness and improves the adhesion between the thin film and the substrate.

The LPD is an attractive technique due to its inherent simplicity, the strong adhesion to the substrate, and lack of need for expensive equipment.25,26 Until now, we have established various types of LPD processes based on the slow hydrolysis of the fluorine metal complex species, leading to high-quality metal oxide films.27−29 Not only for our reports but also for other studies, it is known that the slow hydrolysis of W-fluoro complexes in the presence of boric acid and a fluoride scavenger can achieve WO3 film preparations.30,31 In the LPD reaction solutions, boric acid promotes uniform nucleation and growth of WO3 crystals on the substrate surface while keeping the crystallinity of deposits. The significance of boric acid in controlling the quality and efficiency of WO3 thin film fabricated by the LPD technique was highlighted in the previous report.30 However, there are few reports of in-depth studies about the precursor solution preparation method and the effect of boric acid on the quality of WO3 in the PEC water-splitting process. These insights would greatly influence the performance of the PEC process.

From the above backgrounds, this research focuses on the preparation procedure of the precursor solution and the effect of boric acid on the performance of the WO3 thin film as the photoanode in the PEC cell. As a result, we successfully found the optimized concentration of boric acid in the precursor solution to achieve efficient water oxidation reactions in the PEC process. The outstanding results of the study were that we obtained a well-homogenized precursor solution in a shorter time and were able to significantly increase the efficiency of immobilizing the WO3 film to the substrate. The impact of boric acid on the distinctive features and PEC capabilities of a WO3 photoanode can be comprehensively elucidated in the reaction. The findings would have a significant impact on the advancement of the LPD technique as the fabrication process of the highly efficient electrode for PEC applications.

2 Experimental Section

2.1 Fabrication of a WO3 Photoanode by the LPD Process

Figure 1a illustrates the preparation procedure of the parent solution of the W-fluorine species. 1.25 g of tungstic acid (H2WO4; SIGMA-Aldrich) powder was added to 15 mL of hydrofluoric acid (HF; SIGMA-Aldrich), and then, the volume was adjusted to 500 mL using distilled water. The homogeneous solution was then stirred and heated at 40 °C. As a comparison, we also heated the solution at 80 °C for 2 h and performed the ultrasonication process for 2 h. The filtrate was used as a precursor solution to obtain W6+ in the subsequent reactions, and then, the solution was stored overnight prior to use.

Figure 1 Schematic diagram of WO3 thin film fabrication by the LPD process. (a) WO3 precursor solution preparation step and (b) LPD deposition step and calcination process.

Boric acid (H3BO3) was added to the W6+ precursor solutions with concentrations of 0.1, 0.2, 0.3, and 0.4 mol/mL, as shown in Figure 1b. A fluorine-doped tin oxide (FTO) (Bangkok Solar Co., Ltd., Thailand) electrode substrate with 2 × 3 cm2 dimension was sonicated in a detergent for 10 min, 3 M NaOH for 30 min, ethanol and deionized water for 10 min, subsequently, and dried before the LPD reactions. The FTO substrate was immersed vertically in a 50 mL precursor solution and kept at 40 °C in a water bath for 6 h. After the deposition, the substrates were subsequently rinsed with distilled water and then calcined at 450 °C for 1 h.

2.2 Characterizations and Photoelectrochemical Measurements

X-ray diffraction (XRD; Rigaku, SmartLab) analyses were performed to examine the crystallinity of the fabricated FTO/WO3 electrode. The morphologies and chemical compositions of the sample were investigated using scanning electron microscopy (SEM; JEOL, JSM −5410LV) and energy-dispersive X-ray spectroscopy (EDX; OXFORD, INCA-350). The oxidation states of deposits were evaluated using X-ray photoelectron spectroscopy (XPS, JEOL JPS-9010MC). The optical characters were examined using a UV–vis spectrophotometer (JASCO, V-7200). The PEC cell for the investigation of water oxidations consisted of FTO/WO3, platinum wire (Pt), and Ag/AgCl (sat. KCl) as the working, counter, and reference electrodes, respectively. The efficiency of the WO3 photoanode for water oxidation was evaluated by applying 1.0 V (vs Ag/AgCl) while subjecting it to 14.4-W light-emitting diode irradiation in a 0.5 M Na2SO4 electrolyte solution. The electrochemical characteristics of the WO3 photoanode were investigated using cyclic voltammetry (CV) with the potential range and scan rate of −0.5 to 1.0 V and 50 mV s–1, respectively. Electrochemical impedance spectroscopy (EIS) measurements were utilized to examine the electrochemical resistance, capacitance, and band state of the fabricated WO3 photoanode using a potentiostat (Princeton Applied Research, Inc., VersaSTAT3).

3 Results and Discussion

3.1 Preparation Process of the WO3 Precursor Solution

As shown in Figure 1a, the precursor solution became more homogeneous as the temperature was raised from 40 to 80 °C, indicating the impact of temperature on promoting the higher solubility of the tungstic precursor in the HF acid solvents. It was discovered that heating the precursor solution at 80 °C results in a clear yellow solution that was derived from [WF8]2– and turns into a colorless [W(OH)8]2– complex solution after the reaction was completed. At higher temperatures, tungstic acid (H2WO4) reacts with hydrofluoric acid to form a tungsten hexafluoride complex (H2WF8) with a faster reaction rate, facilitating the formation of the complex, as in eq 1. The tungsten hexafluoride complex [WF8]2– reacts with H2O to form a modified complex with hydroxide ligands and influences the kinetics of the reaction, as in eq 2. The modified complex reacts with additional H2O, resulting in the formation of the [W(OH)8]2– complex and affects the equilibrium of the reaction (eq 3).1

2

3

The surface of the FTO substrate turned blue after the LPD process using a precursor solution containing a mixture of W6+ and H3BO3. The blue color on FTO glass is a result of a reduction in the oxidation state of tungsten from W6+ to W5+ by the H3BO3 at the FTO surface.24,30−32 The LPD reaction process can be described as the following equations.4

5

After the reaction, the thin WO3 film with a yellow color was obtained by a calcination process in air at 450 °C for 1 h. The material’s color changes from blue (W5+) to yellow (W6+) during the high-temperature treatment. To compare the effects of the precursor solution preparation temperatures, the characteristics of the WO3 thin film obtained from the LPD process in both conditions were investigated, as shown in Figure 2. Figure 2b displays the FTO/WO3 surfaces prepared by the precursor solution at 40 °C. Compared to Figure 2a (bare FTO), the depositions of WO3 were confirmed in both cases. However, it was found that the precursor solution kept at 40 °C led to inhomogeneous depositions (having cracks) and lower adhesions, which can be easily peeled off. The insufficient adhesion of the thin film would be due to the incomplete reaction of H2WO3 and HF (eq 1) at a lower temperature. From this, the complete hydroxylation of the tungsten hexafluoride complex (H2WF8) would lead to the incomplete conversion to [W(OH)8]2– complex solution, as described in eqs 2 and 3. On the contrary, the FTO/WO3 electrodes fabricated from the precursor solution prepared at 80 °C demonstrated a uniform, yellow, and smooth WO3 film with the entire coverage of WO3 (Figure 2c). The SEM image reveals a highly porous and rough surface. It can be expected that these morphologies would facilitate the photoanode activity due to the increased surface area, capacitance, and number of pathways for electron transfer. From this figure, since WO3 covers the entire FTO glass surface, the electrode surface would be appropriate for photoelectrochemical measurements.

Figure 2 SEM images of (a) FTO and (b, c) FTO/WO3 photoanodes fabricated by the LPD process. The precursor solutions were prepared at (b) 40 °C and (c) 80 °C, respectively.

3.2 Effect of H3BO3 in the LPD Process for FTO/WO3 Photoanode Fabrication

3.2.1 Chemical Composition, Morphology, and Crystallinity

The present study also evaluated the effect of the H3BO3 concentration in the precursor solution on the fabrication of WO3 films. Figure 3 displays the top-view SEM images of FTO and FTO/WO3 photoanodes prepared by the LPD method with varying concentrations of H3BO3. Figure 3a shows the FTO/WO3 electrode surface prepared with a lower concentration of H3BO3 (0.1 mol/mL). From this result, it was found that a lower concentration of H3BO3 in the LPD process leads to fewer nucleation sites, resulting in insufficient WO3 deposition. The lowest % weight of W was also obtained from the EDX data (Table S1 in the Supporting Information). Notably, the H3BO3 concentration in the precursor solution significantly affected the morphology of the WO3 thin film at the electrode surface, as shown in Figure 3b–d. Especially under 0.2 mol/mL H3BO3 concentration conditions, small WO3 nanoparticles are deposited uniformly on the surface, exhibiting high surface roughness and porosity. The obtained results demonstrate the function of H3BO3 in the production of WO3 films in the LPD method, encompassing the influence on the crystal structure, morphology, and adhesion efficacy of the film to the electrode surface. Especially, Figure 3b exhibits the highest coverage of the WO3 film under the H3BO3 concentration of 0.2 mol/mL. Thus, this finding, consistent with the highest weight % of W in EDX data (Table S1), indicates that the most suitable condition to control WO3 adhesion by the LPD process is determined as the H3BO3 concentration of 0.2 mol/mL. This implies that the LPD technique is most effective for establishing a strong bond between WO3 and a 0.2 mol/mL concentration. The strong adherence is due to enhanced chemical adhesion by the formation of boron–oxygen bonds at the boundary between the WO3 film and the electrode. H3BO3 concentrations of 0.3 and 0.4 mol/mL diminish the adhesion effectiveness and coverage. This is because the abundant boron ions may occupy the reactive sites, reducing the quantity of WO3 deposited and decreasing the film’s homogeneity and adhesion. Conversely, the formation and deposition of WO3 are not effectively supported when the concentration of boron ions is minimal, specifically, at 0.1 mol/mL. Consequently, there is a lack of adhesion and insufficient coverage. Therefore, the structure and bonding of the WO3 thin films are significantly influenced by the concentration of H3BO3. A 0.2 mol/mL concentration is optimal for attaining strong adhesion, high surface roughness, and homogeneous deposition, which leads to improved photoelectrochemical performance.

Figure 3 SEM images of the FTO and FTO/WO3 photoanode prepared by the LPD method with concentrations of H3BO3 of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL.

Figure 4A illustrates the wide-scan XPS spectra of the FTO/WO3 electrode. The nanoparticles are composed of tungsten (W), oxygen (O), and carbon (C), as indicated by these spectra. The presence of these elements suggests that WO3 has been effectively deposited onto the FTO substrate. The narrow scan XPS spectra of the W4f region are depicted in Figure 4B. The W6+ oxidation state of WO3 corresponds with the binding energies of the W4f5/2 and W4f7/2 peaks. The oxidation state of tungsten is unaffected by the varying concentrations of boric acid, as evidenced by the reason that these peaks remain consistent across all concentrations of H3BO3 used in the LPD process.33−35 We employed deconvolutions to assess the oxidation states in the XPS spectra thoroughly. In the WO3 composition, the analyzed spectra of W4f verify the presence of W6+. XPS spectra of the O1s region are illustrated in Figure 4C, which offer insights into the oxidation state of oxygen in the crystalline WO3.36 Further validating the crystalline structure of the deposited WO3, the deconvoluted O1s spectra exhibit peaks associated with lattice oxygen and hydroxyl groups. In the article, the deconvoluted spectra and the observed binding energies agree with the previously reported values for WO3. The chemical composition and oxidation state in the WO3 electrode are carefully evaluated by analyzing high-resolution XPS spectra and including a wide-scan XPS spectrum.

Figure 4 XPS spectra of (A) a wide scan of the WO3 electrode compared with a narrow scan of (B) W4f and (C) O 1s elements on the FTO/WO3 electrode fabricated by the LPD method with concentrations of H3BO3 of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL.

In addition to SEM and XPS analyses, we performed XRD measurements of each substrate as shown in Figure 5. The tetragonal crystalline structure of SnO2 derived from the FTO substrate was assigned to the peak at 2θ of 26.9, 34.1, and 38.8°.37 From XRD patterns of the WO3 film prepared with different H3BO3 concentrations, it was found that the variations in the concentration of H3BO3 affect the crystal structure of WO3 at the surface of the electrodes (Figure 3a–d). Monoclinic WO3 crystalline characteristics were observed as distinct XRD patterns at 23.1, 23.6, 24.4, and 34.1°,38−40 while the boric acid concentration of 0.1 mol/mL exhibits distinct peaks at 2θ of 23.1 and 34.1°. The single broad peak at 23.1° would indicate the reduced crystallite’s size and the crystallographic orientations under lower concentration conditions. When the concentration of H3BO3 increased from 0.2 to 0.4 mol/mL, clear XRD signals, corresponding to the high crystallinity of monoclinic WO3, were obtained. This trend could be understood from eqs 4 and 5. The changes in H3BO3 amounts would lead to different amounts of proton source which accelerates eq 5. In other words, the inappropriate ratio of H3BO3 would induce the reverse reaction in eqs 4 and 5, providing low crystallinity or a small deposition of monoclinic WO3. These findings suggest that an appropriate concentration of H3BO3 can enhance the growth of specific crystallographic phases or orientations, resulting in an enhanced crystalline quality of WO3 on the FTO substrate.

Figure 5 XRD patterns of FTO/WO3 electrode fabrication using the LPD process at various H3BO3 concentrations of H3BO3 of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL.

3.2.2 Optical Properties Effect

The effect of H3BO3 concentration on the optical properties of the prepared films was evaluated, as shown in Figure 6. The highest visible light absorption properties of the FTO/WO3 electrode were obtained when fabricated at an H3BO3 concentration of 0.2 mol/mL, as indicated by the highest WO3 content and in good relation to the darkest yellow color of the WO3 semiconductor,41−43 as shown in the insets of Figure 6. When the concentration of H3BO3 in the LPD process was increased to 0.3 and 0.4 mol/mL, the prepared WO3 electrode exhibited decreased visible light absorption and a light-yellow color. The high concentration of H3BO3 could cause a decrease in the amount of WO3 on the electrode surface. This would be attributed to the saturation of active sites by the presence of boron ions, leading to inadequate adhesion and the changes in the electronic state. In the opposite direction, under the lowest H3BO3 concentration of 0.1 mol/mL, the WO3 film is colorless and lacks visible light absorption properties, indicating a very low WO3 level at the electrode surface. This effect occurs because the lowest concentration of H3BO3 is insufficient to promote the formation of the WO3 film through the LPD process. The band gap energy (Eg) was determined using Tauc’s eq (eq 6), which was calculated using the following equation6

where α, hν, A, and Eg are the absorption coefficient, photon energy, a constant, and the calculated bad gap energy, respectively. Figure 6 shows the spectra of (αhν)1/2 against the photon energy. From this figure, it was found that the band gap energy for the prepared film ranged between 2.1 and 2.8 eV. Particularly, the optimal condition for WO3 photoanode fabrication demonstrates the narrowest band gap energy at 2.1 eV due to an appropriate H3BO3 concentration that can facilitate nucleation and well-adhesion on the substrate. This corresponds with the highest weight percent of W in the EDX data of FTO/WO3 electrodes, which causes the electrodes to display the darkest yellow color, resulting in the highest visible light absorption properties. The different band gap energies would be derived from the different oxygen contents or the defect sites, which provide the additional energy states. Thus, it was found that the concentration of H3BO3 affected not only the surface morphology and deposited amounts but also the electronic structure of WO3 through the changes in the boron species amounts. To summarize, the concentration of H3BO3 has a significant impact on the properties of a WO3 thin film, including how it starts, grows, shapes, crystallizes, has flaws, and looks. These variables collectively influence band gap energy changes. An optimal concentration of H3BO3 improves these characteristics, leading to the highest photoelectrochemical efficiency and band gap energy.

Figure 6 Absorption spectra of the FTO substrate and FTO/WO3 photoanode fabricated by the LPD process with various concentrations of H3BO3 in precursor solution as (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL.

3.2.3 Photoelectrocatalytic Activities

For the investigations of photoelectrochemical properties of prepared WO3 films, the CV measurements were conducted in an aqueous solution under various applied electrochemical potentials and visible light illuminations (λex = 420–680 nm). Figure 7 shows the oxidation and reduction currents obtained with the potential range between −0.5 and 1.0 V under dark or light illumination conditions. The current values exhibited a linear relationship with the amount of WO3 deposited on the FTO substrate in both light and dark conditions, related to the effect of H3BO3 concentration during the FTO/WO3 electrode fabrication processes. The current values within this potential range can quantitatively confirm the formation of the WO3 film under each condition and support the findings of the EDX analyses.44 Due to the lack of any sacrificial agent in the aqueous solution, Figure 7B confirms the occurrence of the photoelectrocatalytic water oxidations at the FTO/WO3 photoanode even under the neutral solution condition (as in the inset of Figure 7B). The FTO/WO3 photoanode prepared from H3BO3 at a concentration of 0.2 mol/mL exhibits the highest photoelectrocatalytic activity for water oxidation. Under this condition, the onset potential for water oxidation begins at 0.1 V and is more negative than under other conditions, indicating that the electrocatalytic activity of the electrode was promoted. Under the light illuminations, the light-stimulating WO3 semiconductor causes the separation of electron (e–) and hole (h+) excited from the VB to the conduction band (CB). The generated e– at CB is transferred to the counter electrode, while the remaining h+ at VB oxidizes the H2O at the electrode surface to produce oxygen. While under dark conditions, as shown in Figure 7A, no evidence was found that the water oxidation peak was caused by the absence of light illuminations.

Figure 7 Cyclic voltammograms of the FTO/WO3 electrode fabricated by the LPD method with varying H3BO3 concentrations of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL under (A) dark and (B) visible light irradiation conditions. The potential scan rate and supporting electrolyte were 50 mV s–1 and 0.5 M Na2SO4..

Figure 8A demonstrates that the photoelectrocatalytic activity of the FTO/WO3 photoanode for water oxidation was dependent on the H3BO3 concentrations in the LPD process by using a conventional three-electrode system under on–off light illuminations. This result indicates that a suitable concentration of H3BO3 can facilitate the strong adhesion of the WO3 thin film to the substrate, which improves the photoelectrocatalytic activity of the FTO/WO3 electrodes. The repetitive light illuminations at the constant potential also showed this tendency, as shown in Figure 8A. Moreover, the EIS measurements were utilized to investigate the charge transfer resistance (Rct) of the FTO/WO3 photoanode fabricated by varying concentrations of H3BO3 in the LPD process. Figure 8B demonstrates that FTO/WO3 fabricated at an H3BO3 concentration of 0.2 mol/mL has the smallest semicircle of the Nyquist plot, corresponding to the lowest value of charge transfer resistance (Rct) compared to other conditions. The lowest Rct value under the optimal conditions implies the highest electron transfer rate at the electrode surface, which is in good agreement with the highest photocurrent generations. The research demonstrates that the concentration of H3BO3 in the precursor solution has a substantial influence on the photoelectrocatalytic activity of the FTO/WO3 photoanode produced via the LPD approach. Boric acid is a critical factor in forming and enlarging WO3 crystals, which impacts the films’ adhesion, shape, and arrangement via the LPD method. Our research reveals that a boric acid concentration of 0.2 mol/mL produces WO3 coatings in a highly uniform and highly crystalline state. As a result of their robust adhesion to the FTO substrate, these coatings improve light absorption and charge separation. Conversely, films produced at concentrations that are either higher or lower are less effective, as a result of insufficient nucleation or excessive saturation. Following the reaction with boric acid, it is possible to modify the residual HF in order to generate a fully integrated W-fluoride complex. It is essential to achieve a well-defined crystalline structure and robust adhesion in order to enhance reactivity and optimize the surface area.

Figure 8 (A) Photocurrent from water oxidation and (B) Nyquist plots of the FTO/WO3 photoanode fabricated by the LPD process with concentrations of H3BO3 of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL at the static potential of 1.0 V vs Ag/AgCl under visible light illuminations. The electrolyte was a 0.5 M Na2SO4 aqueous solution. The inset in (B) is the charge transfer rate (Rct) values for each electrode.

3.2.4 Electronic Structure of Deposited Films

Mott–Schottky (MS) curves were utilized to verify the positions of the CB energy (ΦCB) or flat band potentials (Φfb) of the WO3 thin film under the dark condition. Figure 9A demonstrates that all samples showed the n-type semiconductor properties as can be found from the positive slope.45,46 The Φfb values for each electrode were found to be at −0.48, −0.040, 0.20, and −0.040 V vs Ag/AgCl, respectively, when the H3BO3 concentration was varied to 0.1, 0.2, 0.3, and 0.4 mol/mL. From these values, the energy level of the valence band (ΦVB) position of each FTO/WO3 could be determined using the following equation as Φfb = ΦCB + Eg. The ΦVB values for FTO/WO3 under different concentrations of H3BO3 of 0.1, 0.2, 0.3, and 0.4 mol/mL are found to be 2.12, 2.06, 2.80, and 2.66 V (vs Ag/AgCl), respectively. Such a difference would also originate from the different oxygen contents, defect sites at the band gap, or fluorine contents. This is supported by the different slopes of MS plots, which reflect the doping density of the semiconductor.

Figure 9 (A) Mott–Schottky plots and Φfb values for each electrode obtained at a frequency of 10,000 Hz. (B) Energy band diagrams of FTO/WO3 fabricated by the LPD process with various H3BO3 concentrations of (a) 0.1, (b) 0.2, (c) 0.3, and (d) 0.4 mol/mL.

Figure 9B depicts a schematic energy diagram of the electronic structure of fabricated WO3 depending on the H3BO3 concentration. It verifies that the optimal electronic band structure for photoelectrocatalytic water oxidation under visible light irradiation is exhibited by the FTO/WO3 photoanode prepared with a 0.2 mol/mL H3BO3 condition. It displays the narrowest band gap energy value, leading to the highest visible light absorption properties and highest photocurrent values. Furthermore, it was observed that the VB of WO3 was more positive than the water oxidation potential in all situations, verifying its capacity to convert water oxidation into oxygen. The findings validate that WO3 electrodes, prepared with a boric acid concentration of 0.2 mol/mL, exhibit maximum photocurrent. This is attributed to their exceptional light absorption capability and superior h+ production, enhancing the water oxidation efficiency. It is evident from the aforementioned statement that the concentration of H3BO3 in the precursor solution substantially influences the characteristic and photoelectrocatalytic activity of the FTO/WO3 photoanodes. The function of H3BO3 in the precursor solution for WO3 film fabrication in the LPD process is to regulate the amount of HF that remains after the reaction, which promotes the formation of a fully integrated W-fluoride complex compound. This compound production process significantly affects the WO3 thin film’s effective adhesion to the FTO substrate. The effective optimization of H3BO3 concentration in the LPD method facilitates achieving excellent adhesion efficiency for the WO3 film. This is supported by developing an well-defined crystalline structure, distinctive morphology with a substantial surface area for enhanced reactivity, and the capacity to exhibit desirable properties. The exceptional capacity of WO3 to efficiently absorb visible light is attributed to its favorable electrical band structure. The outcome of this study yields a WO3 photoanode with exceptional photoelectrocatalytic water oxidation characteristics. Moreover, it should be emphasized that the present improvements in current density are notable when we compare the photoconversion abilities with other fabrication processes, as can be found in Table 1.47 From the above facts, it is possible to conclude that our current electrode preparation procedure would be a promising candidate for future advancements in energy and environmental management applications.

Table 1 Comparison of Current Densities for WO3 Photoanodes by Different Fabrication Methods

photoanode	fabrication process	electrolyte/light source	current density at 1.00 V vs Ag/AgCl	ref	
FTO/WO3	LPD	0.5 M Na2SO4/14.4-W light-emitting diode (LED)	∼120 μAcm–2 at	this work	
FTO/WO3/Fe2O3	sputtering	0.5 M Na2SO4/visible light irradiation	∼50 μAcm–2	(47)	
ITO/WO3	FPD	0.1 M Na2SO4/60-W tungsten lamp	∼80 μAcm–2	(41)	
ITO/WO3	electrodeposition	0.1 M Na2SO4/visible light illuminated	∼90 μAcm–2	(42)	

4 Conclusions

A highly homogeneous precursor solution with a large volume was successfully produced for the fabrication of a WO3 photoanode using the LPD technique. It can reduce inconsistencies, save time, and facilitate the LPD process for fabricating WO3 thin films. The utilization of boric acid in the LPD process has a notable impact on the development of WO3 thin films, particularly in enhancing their adhesion to the electrode substrate. The optimal WO3 thin film production condition is a 0.2 mol/mL concentration of H3BO3 in the precursor solution, which results in various desirable properties. These characteristics include a high current density, low charge transfer resistance, high surface roughness, and good crystallinity. Furthermore, it exhibits a great capacity for absorbing visible light due to its appropriate band energy, hence demonstrating exceptional photoelectrocatalytic characteristics for water oxidation. The findings of this study demonstrate that the WO3 photoanode prepared by the present procedure has remarkable photoelectrocatalytic water oxidation properties, establishing it as a prospective contender for future developments in energy and environmental management applications.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c04738.Energy-dispersive X-ray analyses of the synthesized films shown in Figure 3 (PDF)

Supplementary Material

ao4c04738_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the Research and Researchers for Industries (RRI) project and Eagle dream Co., Ltd. (N41A650408). The JSPS KAKENHI (JP22K144960 and JP22K18315) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, Nippon Sheet Glass Foundation, Kansai Research Foundation for technology promotion is also acknowledged.
==== Refs
References

Mesones S. ; Mena E. ; López-Muñoz M. J. ; Adán C. ; Marugán J. Synergistic and antagonistic effects in the photoelectrocatalytic disinfection of water with TiO2 supported on activated carbon as a bipolar electrode in a novel 3D photoelectrochemical reactor. Sep. Purif. Technol. 2020, 247 , 117002 10.1016/j.seppur.2020.117002.
Zhang J. ; Tang B. ; Zhao G. Selective photoelectrocatalytic removal of dimethyl phthalate on high-quality expressed molecular imprints decorated specific facet of single crystalline TiO2 photoanode. Appl. Catal. B: Environ. 2020, 279 , 119364 10.1016/j.apcatb.2020.119364.
Mu F. ; Dai B. ; Zhao W. ; Zhang L. ; Xu J. ; Guo X. A review on metal-organic frameworks for photoelectrocatalytic applications. Chin. Chem. Lett. 2020, 31 (7 ), 1773–1781. 10.1016/j.cclet.2019.12.015.
Shao Z. ; Zhang Y. ; Yang X. ; Zhong M. Au-Mediated Charge Transfer Process of Ternary Cu2O/Au/TiO2-NAs Nanoheterostructures for Improved Photoelectrochemical Performance. ACS Omega 2020, 5 (13 ), 7503–7518. 10.1021/acsomega.0c00299.32280894
Srevarit W. ; Moonmangmee S. ; Phapugrangkul P. ; Kuboon S. ; Klamchuen A. ; Saito N. ; Ponchio C. Photoelectrocatalytic H2 evolution enhancement over CuO-decorated TiO2 nanocatalysts and promoting E. coli degradation. J. Alloys Compd. 2021, 859 , 157818 10.1016/j.jallcom.2020.157818.
Rojviroon T. ; Laobuthee A. ; Sirivithayapakorn S. Photocatalytic Activity of Toluene under UV-LED Light with TiO2 Thin Films. Int. J. Photoenergy 2012, 2012 (1 ), 1–8. 10.1155/2012/898464.
Tayebi M. ; Lee B.-K. Recent advances in BiVO4 semiconductor materials for hydrogen production using photoelectrochemical water splitting. Renewable Sustainable Energy Rev. 2019, 111 , 332–343. 10.1016/j.rser.2019.05.030.
Zhou S. ; Yue P. ; Huang J. ; Wang L. ; She H. ; Wang Q. High-performance photoelectrochemical water splitting of BiVO4@Co-MIm prepared by a facile in-situ deposition method. Chem. Eng. J. 2019, 371 , 885–892. 10.1016/j.cej.2019.04.124.
Bemana H. ; Rashid-Nadimi S. Incorporation of NiO electrocatalyst with α-Fe2O3 photocatalyst for enhanced and stable photoelectrochemical water splitting. Surf. Interfaces 2019, 14 , 184–191. 10.1016/j.surfin.2018.12.011.
Farooq U. ; Chaudhary P. ; Ingole P. P. ; Kalam A. ; Ahmad T. Development of Cuboidal KNbO3@α-Fe2O3 Hybrid Nanostructures for Improved Photocatalytic and Photoelectrocatalytic Applications. ACS Omega 2020, 5 (32 ), 20491–20505. 10.1021/acsomega.0c02646.32832802
Mansoor M. A. ; Munawar K. ; Naeem R. ; Sarih N. M. ; Asghar M. A. ; Haider A. ; Zubir M. N. M. ; Zaharinie T. Aerosol-assisted facile fabrication of bimetallic Cr2O3-Mn2O3 thin films for photoelectrochemical water splitting. New J. Chem. 2023, 47 (17 ), 8347–8354. 10.1039/D2NJ06274G.
Ahmed S. ; Mansoor M. A. ; Basirun W. J. ; Sookhakian M. ; Huang N. M. ; Mun L. K. ; Söhnel T. ; Arifin Z. ; Mazhar M. The synthesis and characterization of a hexanuclear copper-yttrium complex for deposition of semiconducting CuYO2-0.5Cu2O composite thin films. New J. Chem. 2015, 39 , 1031–1037. 10.1039/c4nj01602e.
Naeem R. ; Mansoor M. A. ; Munawar K. ; Adnan A. ; Zaharinie T. ; Mohd Zubir M. N. Versatile Fabrication of Binary Composite SnO2-Mn2O3 Thin Films by AACVD for Synergistic Photocatalytic Effect. J. Electron. Mater. 2021, 50 (7 ), 3897–3906. 10.1007/s11664-021-08897-6.
Chatchai P. ; Murakami Y. ; Kishioka S.-y. ; Nosaka A. Y. ; Nosaka Y. Efficient photocatalytic activity of water oxidation over WO3/BiVO4 composite under visible light irradiation. Electrochim. Acta 2009, 54 (3 ), 1147–1152. 10.1016/j.electacta.2008.08.058.
Peleyeju G. M. ; Umukoro E. H. ; Babalola J. O. ; Arotiba O. A. Solar-Light-Responsive Titanium-Sheet-Based Carbon Nanoparticles/B-BiVO4/WO3 Photoanode for the Photoelectrocatalytic Degradation of Orange II Dye Water Pollutant. ACS Omega 2020, 5 (10 ), 4743–4750. 10.1021/acsomega.9b02148.32201759
Tayebi M. ; Masoumi Z. ; Lee B.-K. Ultrasonically prepared photocatalyst of W/WO3 nanoplates with WS2 nanosheets as 2D material for improving photoelectrochemical water splitting. Ultrason. Sonochem. 2021, 70 , 105339 10.1016/j.ultsonch.2020.105339.32927250
Seferlis A. K. ; Neophytides S. G. On the kinetics of photoelectrocatalytic water splitting on nanocrystalline TiO2 films. Appl. Catal., B 2013, 132–133 , 543–552. 10.1016/j.apcatb.2012.12.016.
Nomellini C. ; Polo A. ; Mesa C. A. ; Pastor E. ; Marra G. ; Grigioni I. ; Dozzi M. V. ; Giménez S. ; Selli E. Improved Photoelectrochemical Performance of WO3/BiVO4 Heterojunction Photoanodes via WO3 Nanostructuring. ACS Appl. Mater. Interfaces 2023, 15 (45 ), 52436–52447. 10.1021/acsami.3c10869.37921705
Naeem R. ; Afzal S. ; Mansoor M. A. ; Munawar K. ; Sherino B. ; Ahmed R. A composite approach to synthesize a high-performance Pt/WO3-carbon catalyst for optical and electrocatalytic applications. New J. Chem. 2022, 46 (28 ), 13454–13464. 10.1039/D2NJ01497A.
Kangkun N. ; Ponchio C. Photoelectrodeposition of BiVO4 layer on FTO/WO3 photoanodes for highly efficient photoelectrocatalytic chemical oxygen demand sensor applications. Appl. Surf. Sci. 2020, 526 , 146686 10.1016/j.apsusc.2020.146686.
Limwichean S. ; Kasayapanand N. ; Ponchio C. ; Nakajima H. ; Patthanasettakul V. ; Eiamchai P. ; Meng G. ; Horprathum M. Morphology-controlled fabrication of nanostructured WO3 thin films by magnetron sputtering with glancing angle deposition for enhanced efficiency photo-electrochemical water splitting. Ceram. Int. 2021, 47 (24 ), 34455–34462. 10.1016/j.ceramint.2021.08.359.
Chatchai P. ; Nosaka A. Y. ; Nosaka Y. Photoelectrocatalytic performance of WO3/BiVO4 toward the dye degradation. Electrochim. Acta 2013, 94 , 314–319. 10.1016/j.electacta.2013.01.152.
Deki S. ; Yu Yu Ko H. ; Fujita T. ; Akamatsu K. ; Mizuhata M. ; Kajinami A. Synthesis and microstructure of metal oxide thin films containing metal nanoparticles by liquid phase deposition (LPD) method. Eur. Phys. J. D 2001, 16 (1 ), 325–328. 10.1007/s100530170121.
Zhang M. ; Yang C. ; Pu W. ; Tan Y. ; Yang K. ; Zhang J. Liquid phase deposition of WO3/TiO2 heterojunction films with high photoelectrocatalytic activity under visible light irradiation. Electrochim. Acta 2014, 148 , 180–186. 10.1016/j.electacta.2014.10.043.
Yamanaka S. ; Hamaguchi T. ; Muta H. ; Kurosaki K. ; Uno M. Fabrication of oxide nanohole arrays by a liquid phase deposition method. J. Alloys Compd. 2004, 373 (1–2 ), 312–315. 10.1016/j.jallcom.2003.11.019.
Saito Y. ; Sekiguchi Y. ; Mizuhata M. ; Deki S. Continuous Deposition System of SnO2 Thin Film by the Liquid Phase Deposition (LPD) Method. J. Ceram. Soc. Jpn. 2007, 115 , 856–860. 10.2109/jcersj2.115.856.
Mizuhata M. Aqueous solution reaction during liquid-phase deposition and its application in electrochemical materials. J. Ceram. Soc. Jpn. 2022, 130 (9 ), 752–761. 10.2109/jcersj2.22072.
Huang J.-J. ; Lin C.-H. ; Ho Y.-R. ; Chang Y.-H. Aluminium oxide passivation films by liquid phase deposition for TiO2 ultraviolet solid-liquid heterojunction photodetectors. Surf. Coat. Technol. 2020, 391 , 125684 10.1016/j.surfcoat.2020.125684.
Lei P.-H. ; Ding M.-J. ; Lee Y.-C. ; Chung M.-J. Textured zinc oxide prepared by liquid phase deposition (LPD) method and its application in improvement of extraction efficiency for 650nm resonant-cavity light-emitting diode (RCLED). J. Alloys Compd. 2011, 509 (21 ), 6152–6157. 10.1016/j.jallcom.2011.03.039.
Mohammad-Hosseinpour M. ; Yourdkhani A. ; Poursalehi R. Fast-switching electrochromic response of WO3·2H2O of plate-like particles synthesized by liquid phase deposition. J. Alloys Compd. 2021, 879 , 160418 10.1016/j.jallcom.2021.160418.
Deki S. ; Béléké A. B. ; Kotani Y. ; Mizuhata M. Synthesis of tungsten oxide thin film by liquid phase deposition. Mater. Chem. Phys. 2010, 123 (2 ), 614–619. 10.1016/j.matchemphys.2010.05.024.
Darmawi S. ; Burkhardt S. ; Leichtweiss T. ; Weber D. ; Wenzel S. ; Janek J. ; Elm M. ; Klar P. Correlation of electrochromic properties and oxidation states in nanocrystalline tungsten trioxide. Phys. Chem. Chem. Phys. 2015, 17 , 15903–15911. 10.1039/C5CP02482J.26018838
Nareejun W. ; Ponchio C. Novel photoelectrocatalytic/solar cell improvement for organic dye degradation based on simple dip coating WO3/BiVO4 photoanode electrode. Sol. Energy Mater. Sol. Cells 2020, 212 , 110556 10.1016/j.solmat.2020.110556.
Ji R. ; Zheng D. ; Zhou C. ; Cheng J. ; Yu J. ; Li L. Low-Temperature Preparation of Tungsten Oxide Anode Buffer Layer via Ultrasonic Spray Pyrolysis Method for Large-Area Organic Solar Cells. Materials (Basel) 2017, 10 (7 ), 820 10.3390/ma10070820.28773177
Yao J. N. ; Chen P. ; Fujishima A. Electrochromic behavior of electrodeposited tungsten oxide thin films. J. Electroanal. Chem. 1996, 406 (1–2 ), 223–226. 10.1016/0022-0728(96)04552-4.
Leftheriotis G. ; Papaefthimiou S. ; Yianoulis P. ; Siokou A. Effect of the tungsten oxidation states in the thermal coloration and bleaching of amorphous WO3 films. Thin Solid Films 2001, 384 (2 ), 298–306. 10.1016/S0040-6090(00)01828-9.
Yu H. ; Yang T. ; Wang Z. ; Li Z. ; Zhao Q. ; Zhang M. p-N heterostructural sensor with SnO-SnO2 for fast NO2 sensing response properties at room temperature. Sens. Actuators, B 2018, 258 , 517–526. 10.1016/j.snb.2017.11.165.
Kumbhar V. S. ; Lee H. ; Lee J. ; Lee K. Interfacial growth of the optimal BiVO4 nanoparticles onto self-assembled WO3 nanoplates for efficient photoelectrochemical water splitting. J. Colloid Interface Sci. 2019, 557 , 478–487. 10.1016/j.jcis.2019.09.037.31541917
Castillo C. ; Cabello G. ; Chornik B. ; Huentupil Y. ; Buono-Core G. E. Characterization of photochemically grown Pd loaded WO3 thin films and its evaluation as ammonia gas sensor. J. Alloys Compd. 2020, 825 , 154166 10.1016/j.jallcom.2020.154166.
Choi J. ; Song T. ; Kwon J. ; Lee S. ; Han H. ; Roy N. ; Terashima C. ; Fujishima A. ; Paik U. ; Pitchaimuthu S. WO3 nanofibrous backbone scaffolds for enhanced optical absorbance and charge transport in metal oxide (Fe2O3, BiVO4) semiconductor photoanodes towards solar fuel generation. Appl. Surf. Sci. 2018, 447 , 331–337. 10.1016/j.apsusc.2018.03.167.
Kangkun N. ; Kiama N. ; Saito N. ; Ponchio C. Optical properties and photoelectrocatalytic activities improvement of WO3 thin film fabricated by fixed-potential deposition method. Optik 2019, 198 , 163235 10.1016/j.ijleo.2019.163235.
Supanantin F. ; Ponchio C. Improvement ITO/WO3 photo anode electrode fabrication using electrodeposition technique for highly efficient photoelectrocatalytic insecticide degradation. Mater. Sci. Semicond. Process. 2020, 118 , 105212 10.1016/j.mssp.2020.105212.
Bi Q. ; Gao Y. ; Wang Z. ; Dang C. ; Zhang Z. ; Wang L. ; Xue J. Preparation of a direct Z-scheme thin-film electrode based on CdS QD-sensitized BiOI/WO3 and its photoelectrocatalytic performance. Colloids Surf., A 2020, 599 , 124849 10.1016/j.colsurfa.2020.124849.
Xie H. ; Wang Y. ; Liu H. ; Wang H. ; Li Y. ; Qi X. ; Liang T. ; Zeng J. Electrochromic electrode with high optical contrast and long cyclic life using nest-like porous doped-Sm WO3 films. Ceram. Int. 2023, 49 (5 ), 8223–8231. 10.1016/j.ceramint.2022.10.347.
Ba G. ; Liang Z. ; Li H. ; Du N. ; Liu J. ; Hou W. Simultaneous formation of mesopores and homojunctions in graphite carbon nitride with enhanced optical absorption, charge separation and photocatalytic hydrogen evolution. Appl. Catal. B: Environ. 2019, 253 , 359–368. 10.1016/j.apcatb.2019.04.084.
Yan K. ; Liu J. ; Qin J. ; Zhang J. A portable solar light-driven biophotoelectrocatalytic system for pollutant removal powered by photovoltaic cells. J. Hazard. Mater. 2022, 435 , 128989 10.1016/j.jhazmat.2022.128989.35487005
Wiriyachailerd C. ; Horprathum M. ; Eiamchai P. ; Ponchio C. Improved the charge transfer for highly efficient photoelectrochemical water oxidation: the case of WO3 and BiVO4. Mater. Today: Proc. 2018, 5 (6 ), 13874–13878. 10.1016/j.matpr.2018.02.032.
