Combining Cocatalyst and Oxygen Vacancy to Synergistically Improve Fe2O3 Photoelectrochemical Water Oxidation Performance

Abstract

Considering the poor conductivity of Fe₂O₃ and the weak oxygen evolution reaction associated with it, surface hole accumulation leads to electron hole pair recombination, which inhibits the photoelectrochemical (PEC) performance of the Fe₂O₃ photoanode. Therefore, the key to improving the PEC water oxidation performance of the Fe₂O₃ photoanode is to take measures to improve the conductivity of Fe₂O₃ and accelerate the reaction kinetics of surface oxidation. In this work, the PEC performances of Fe₂O₃ photoanodes are synergistically improved by combining loaded an FeOOH cocatalyst and oxygen vacancy doping. Firstly, amorphous FeOOH layers are successfully prepared on Fe₂O₃ nanostructures through simple photoassisted electrodepositon. Then oxygen vacancies are introduced into FeOOH-Fe₂O₃ through plasma vacuum treatment, which reduces the content of Fe-O (O_L) and Fe-OH (-OH), jointly promoting the generation of oxygen vacancies. Oxygen vacancy can increase the concentration of most carriers in Fe₂O₃ and form photo-induced charge traps, promoting the separation of electron holes and enhancing the conductivity of Fe₂O₃. The other parts of -OH act as oxygen evolution catalysts to reduce the reaction obstacle of water oxidation and promote the transfer of holes to the electrode/electrolyte interface. The performance of FeOOH-Fe₂O₃ after plasma vacuum treatment has been greatly improved, and the photocurrent density is about 1.9 times higher than that of the Fe₂O₃ photoanode. The improvement in the water oxidation performance of PEC is considered to be the synergistic effect of the cocatalyst and oxygen vacancy. All outstanding PEC response characteristics show that the modification of the cocatalyst and oxygen vacancy doping represent a favorable strategy for synergistically improving Fe₂O₃ photoanode performance.

1. Introduction

The extensive use of conventional fossil fuels has resulted in a significant rise in global CO₂ levels, contributing adversely to climate change. Therefore, there is a pressing need to seek out renewable and sustainable energy alternatives to supplant the prevalent use of fossil fuels. Developing hydrogen energy has been regarded as an important path towards global clean energy transformation and carbon neutrality. Photoelectrochemical (PEC) water splitting is one of the most significant ways to convert solar energy into hydrogen, so it has been widely addressed [1,2,3]. In 1972, Fujishima and Honda first achieved photoelectrochemical water splitting using a titanium dioxide photoanode to produce hydrogen [1], and since then, people have explored a large number of semiconductors, including TiO₂ [4], ZnO [5], WO₃ [6], BiVO₄ [7,8], Fe₂O₃ [9,10], SnO₂ [11] and Bi₂O₃ [12,13], which were used as photoanodes for PEC water splitting. However, when used as photoanodes for water splitting, semiconductor materials still show many disadvantages; for example, the water oxidation reaction involves more complicated multi-electron/proton coupling, the kinetic rate is extremely slow, and the chemical structure is not stable enough. Therefore, building an efficient photoanode has become an effective strategy to enhance the PEC water splitting reaction.

Among numerous semiconductor materials, Fe₂O₃ has become one of the ideal PEC water splitting photoanode materials due to its abundance, low cost, nontoxicity, narrow band-gap (about 2.2 eV) [14], low theoretical starting potential (0.4 V_RHE) [15], theoretical photocurrent density of up to 12.6 mA/cm² [16], and good chemical stability in aqueous electrolytes (pH > 3) and alkaline solutions [17]. However, in actual experimental tests, the measured photocurrent density of the Fe₂O₃ photoanode was found to be much lower than 12.6 mA/cm², and the actual opening voltage was higher than the theoretical opening voltage. This was mainly caused by the low conductivity of Fe₂O₃ and insufficient oxygen evolution reaction kinetics [18,19,20]. Therefore, it is particularly necessary to take measures to improve the conductivity and oxygen evolution reaction kinetics of Fe₂O₃.

Regarding the enhancement of conductivity of Fe₂O₃, it is usually improved by including element doping, such as with Ti [21,22], Ni [23] or Sn [24,25], introducing oxygen vacancies [26,27,28,29,30,31,32], or building heterojunctions [33,34]. Moreover, the performances of photoelectrodes are improved by preparing heterojunctions containing oxygen vacancies [35,36]. Schmuki et al. successfully introduced oxygen vacancies into Fe₂O₃ photoanodes by annealing in a low-oxygen environment, enabling them to enhance the photoelectrochemical water splitting performance [37]. Noh et al. prepared high-density oxygen-deficient α-Fe₂O₃ using the AACVD method. The presence of oxygen vacancies improves light absorption, accelerates charge transport in the film body, and promotes charge separation at the electrolyte/semiconductor interface [38]. As an active site, oxygen vacancy increases the electron transport rate.

Regarding the kinetics of oxygen evolution reactions, which we are seeking to improve, these include developing oxygen evolution cocatalysts and either the formation of hole storage layers or the effective improvement of them through surface treatment. A thin layer of metal oxides such as Al₂O₃, Ga₂O₃, In₂O₃ and ZnO can effectively accelerate the oxidation of solar water splitting by passivating the surface states of Fe₂O₃ [39]. Ye et al. realized the modification of cocatalyst FeOOH on Fe₂O₃ by photoelectrodeposition; as a result, the initial potential of the photocurrent shifted significantly, and the photocurrent density increased by four times [40]. Kim et al. immobilized S-doped FeOOH (S-FeOOH) on the surface of Fe₂O₃ nanorod (NR) arrays via simple chemical bath deposition combined with a thermal sulfuration process. The S-FeOOH layer that was grown not only serves as an efficient catalyst layer to accelerate water oxidation on the surface of the photoelectrode, but also forms a heterojunction with the light-absorbing layer, which promotes the separation and transfer of charge carriers at the interface [41]. This modification usually catalyzes hole transfer and inhibits surface charge recombination, as a result of which the PEC splitting performance of the modified photoanode was significantly improved.

Herein, a simple and efficient two-step approach is proposed to improve the PEC performance of Fe₂O₃ by synergistically combining cocatalysts and oxygen vacancies. In the first step, FeOOH is deposited on Fe₂O₃ to enhance its surface kinetics and reduce the undesirable oxygen precipitation reaction kinetics, which corresponds to the improvement of charge transfer efficiency. In the second step, the introduction of oxygen vacancies in FeOOH-Fe₂O₃ can effectively increase the carrier density and conductivity, thus improving the charge separation efficiency. More importantly, the simultaneous integration of the FeOOH cocatalyst and oxygen vacancies will synergistically improve the charge separation and transfer efficiency, which is why the increase in the photocurrent density of Vo-FeOOH-Fe₂O₃ is greater than that of FeOOH-Fe₂O₃ and Vo-Fe₂O₃. The synergistic effect of FeOOH with oxygen vacancies improves the PEC performance of the Vo-FeOOH-Fe₂O₃ photoanode.

2. Materials and Methods

2.1. Preparation of Fe₂O₃, FeOOH-Fe₂O₃, V_O-Fe₂O₃, and Vo-FeOOH-Fe₂O₃ Photoanodes

SnO₂-coated glass (FTO) was procured from Hefei Kejing Material Technology Co., Ltd. (Hefei, China). Sodium nitrate (NaNO₃), ferric chloride hexahydrate (FeCl₃·6H₂O), ferrous chloride (FeCl₂·4H₂O), and sodium hydroxide (NaOH) were all sourced from Aladdin (Shanghai, China) Reagent Co., Ltd. The FTO glass (1 × 2 cm²) was washed in ethanol and deionized water for 15 min with an ultrasonic cleaner. Firstly, the Fe₂O₃ photoanode was successfully prepared by the Lan method [42], whereby a 30 mL aqueous solution containing 0.09 M ferric chloride and 0.09 M sodium nitrate was placed in a Teflon-lined stainless steel autoclave. Then the autoclave with an FTO glass slide was heated at 95 °C for 4 h to produce FeOOH on FTO, with the FTO facing upwards. Next, the FeOOH-coated substrate was washed and sintered to obtain Fe₂O₃ samples. The rate of temperature increase was 10 °C/min, and the cooling process involved natural cooling to room temperature. The temperature was increased at a rate of 10 °C per minute until it reached the sintering temperature of 550 °C for 2 h and 750 °C for 15 min. After the natural cooling of the muffle furnace, the Fe₂O₃ samples were used to prepare FeOOH-Fe₂O₃ or were treated with air plasma.

As for the FeOOH-Fe₂O₃ sample, photoelectrodeposition was adopted. The electrolyte was 1 M FeCl₂·4H₂O solution. The first step of photoelectrodeposition was conducted under light illumination at 0.6 V, and the total charge deposition was about 0.035 C/cm². The second step of photoelectrodeposition was conducted in a dark environment at 1.2 V for 1 min. The air plasma treatment was performed using a plasma cleaner (Harrick PDC-002, New York, NY, USA) equipped with a radio frequency coil with adjustable power. The plasma cleaner possesses three work models with low power, medium power and high power. Before producing the plasma, the vacuum of the chamber was less than 400 Pa. The model of medium power was utilized, and the treatment time was ten minutes. After air plasma treatment on the Fe₂O₃ surface, the sample produced was Vo-Fe₂O₃. After air plasma treatment on the surface of FeOOH-Fe₂O₃, the sample produced was denoted as Vo-FeOOH-Fe₂O₃.

2.2. Structural Characterization

The morphologies of Fe₂O₃, FeOOH-Fe₂O₃ and Vo-FeOOH-Fe₂O₃ were characterized by use of a scanning electron microscope (SEM, MERLIN compact, Oberkochen, Germany). Ultraviolet–visible (UV-vis) absorbance spectra were recorded with a spectrophotometer (Hitachi U-3310, Tokyo, Japan). X-ray diffraction (XRD) spectra were obtained by Bruker D8 ADVANCE (Karlsruhe, Germany), using Cu Kα radiation. In the Raman experiment, an IHR 550 HORIBA (Kyoto, Japan) Raman spectrometer and a 532 nm single longitudinal-mode solid-state laser with an output power of 100 mW were used to record the Raman scattering spectra. The electron paramagnetic resonance spectrometry (EPR) analysis of the samples was performed on a Bruker A300 system (Ettlingen, Germany). X-ray photoelectron spectroscopy (XPS, ESCALab Xi⁺, Waltham, MA, USA) was used to analyze the surface composition, and the binding energy was calibrated based on the C 1s photoelectron peak of 284.8 eV.

2.3. Photoelectrochemical Measurements

The photoelectrochemical properties of the samples were measured on the three-electrode device of the CHI 660E electrochemical workstation. The simulated sunlight was provided by a xenon lamp light source with a light intensity of 225 mW/cm². All electrochemical tests were carried out in a 1 M NaOH solution (pH 13.6), and the scanning rate of linear sweep voltammogram (LSV) curves was set at 10 mV/s. Electrochemical impedance spectroscopy (EIS) data were collected at −0.1 V vs. Ag/AgCl with a frequency range of 100 kHz to 0.1 Hz under illumination, and the amplitude disturbance was controlled at 100 mV. For the Mott–Schottky measurement, all data were measured at the frequency of 1000 Hz in the dark.

3. Results and Discussion

Ordered Fe₂O₃ nanoparticles were synthesized by the hydrothermal method followed by two-step annealing. As shown in Figure 1a, the pristine Fe₂O₃ shows the morphology of the nanoparticles. After the decoration of FeOOH onto Fe₂O₃, there was no obvious change from the surface morphology as displayed in Figure 1b, which indicates that the FeOOH layer is very thin and amorphous. Furthermore, after plasma treatment, we can infer that the morphology of Vo-FeOOH-Fe₂O₃ remained similar to those of the pristine hematite and FeOOH-Fe₂O₃ samples. Therefore, the plasma vacuum treatment did not destroy the nanostructure of the samples. Figure 1d–f show the cross-sections of FeOOH-Fe₂O₃ and Vo-FeOOH-Fe₂O₃. The thickness of the FeOOH-Fe₂O₃ sample was about 0.52 μm, and that of the Vo-FeOOH-Fe₂O₃ sample was about 0.5 μm.

Figure 2a shows the UV–vis spectra of the samples. The Fe₂O₃ sample exhibits an absorption edge of 600 nm, which corresponds to the characteristic absorption edge of Fe₂O₃ reported in the literature [29]. All four samples show similar optical properties from the ultraviolet to the visible region. The XRD spectra presented in Figure 2b show that the samples of FeOOH-Fe₂O₃ and Vo-FeOOH-Fe₂O₃ contained Fe₂O₃. The diffraction peaks at 35.6, 54.1 and 64.0° are associated with the (110), (116), and (300) crystal planes of Fe₂O₃ (JCPDS No. 33-0664). The additional peaks observed at 26.6, 33.8, 37.8, 51.8, 61.7 and 65.7° in the XRD pattern can be attributed to the (110), (101), (200), (211), (310), and (301) crystal planes of tetragonal-phase SnO₂, originating from the FTO substrate (JCPDS No. 46-1088). The absence of any peaks corresponding to FeOOH suggests that the FeOOH layer is either extremely thin or amorphous. It is also noteworthy that no other compositions were detected during the air plasma treatment process.

The Raman spectroscopy measurements for the samples of FeOOH-Fe₂O₃ and Vo-FeOOH-Fe₂O₃ are shown in Figure 3a. Both samples exhibit peaks at 223 and 497 cm⁻¹, corresponding to A_g, and peaks at 242, 290, 410, and 610 cm⁻¹ for E_g [43,44,45]. The Raman peak at 553 cm⁻¹ corresponds to the background peak of FTO. As Vo-FeOOH-Fe₂O₃ was treated under air plasma conditions, a further red shift and broadening of the Raman spectrum was observed (inset of Figure 3a), indicating that increasing disorder and oxygen vacancy concentration resulted from air plasma treatment [29,43]. To further analyze whether oxygen vacancies were introduced by air plasma treatment onto the surface of the photoanode, EPR tests were performed, and the results are shown in Figure 3b. One can observe that Vo-FeOOH-Fe₂O₃ shows a stronger EPR signal, located at a g value of 2.0, compared to the FeOOH-Fe₂O₃ sample, which indicates that oxygen vacancies have been introduced through the air plasma treatment. Indeed, plasma bombardment removes more oxygen atoms from the surface of the FeOOH-Fe₂O_3. Thus, after plasma treatment, the sample of FeOOH-Fe₂O₃ possesses oxygen vacancies [43,46].

In order to determine the chemical composition of samples and further assess whether oxygen vacancies were successfully introduced into Vo-FeOOH-Fe₂O₃, the samples were examined by XPS. Figure 4 summarizes the XPS results for FeOOH-Fe₂O₃ and Vo-FeOOH-Fe₂O₃. In Figure 4a, one can observe the peaks of Fe 2p, O 1s, Sn 3d, and C 1s. As shown in Figure 4b, the fine spectrum of Fe 2p shows the classic peaks of Fe³⁺ at 711.1 eV (Fe 2p1/2) and 724.55 eV (Fe 2p3/2), which prove that the prepared sample is Fe₂O₃ [23]. In addition, the peak value of Fe²⁺ was detected at 716.4 eV, and the intensity of Vo-FeOOH-Fe₂O₃ was significantly higher, at 716.4 eV, than that of FeOOH-Fe₂O₃, which indicates that there was more Fe²⁺ in Vo-FeOOH-Fe₂O₃. It was determined that there would be redundant electrons around the position of Fe³⁺ in Vo-FeOOH-Fe₂O₃, accompanied by the removal of oxygen atoms. Therefore, the number of oxygen vacancies is directly proportional to the number of Fe²⁺ sites. It is well known that Fe²⁺ sites can be used as shallow donors to enhance the conductivity of Fe₂O₃ [20,27], which is also consistent with the results of the Mott–Schottky analysis mentioned later. As shown in Figure 4c, the O 1s peak of FeOOH-Fe₂O₃ can be identified through the three peaks at 529.92 eV (O_L), 531.25 eV (O_OH) and 532.56 eV (O_V) [30,47]. Previous reports have proven that the signal peak at 529.92 eV corresponds to the lattice in Fe₂O₃ [43]. Through calculation, we can determine that the oxygen area in -OH accounts for 46% of the total O 1s peak area, which is consistent with the FeOOH spectrum in the literature, thus it can be proved that FeOOH was successfully deposited on Fe₂O₃ samples [47,48]. After air plasma treatment, as shown in Figure 4d, the O 1s peak of Vo-FeOOH-Fe₂O₃ can also be fitted into three peaks of O_L, O_OH and O_V. One can determine the content percentages of each oxygen form by calculating the area under each Gaussian component, and the results are shown in Table 1. One can see in the table that the percentage of O_L decreases after air plasma treatment, which can be attributed to the escape of oxygen atoms of O_L following air plasma treatment, resulting in oxygen vacancies, and the percentage of -OH peaks in Vo-FeOOH-Fe₂O₃ decreases, which may be due to the decomposition of surface -OH groups into oxygen vacancies and H atoms. The decrease in O_L and -OH content contributed to the formation of oxygen vacancies. At the same time, the content of O_V in Vo-FeOOH-Fe₂O₃ increased, and this accelerated the oxygen evolution reaction in the electrolyte [26].

Figure 5 shows the photocurrent density curve of Fe₂O₃, FeOOH-Fe₂O₃, Vo-Fe₂O₃ and Vo-FeOOH-Fe₂O₃ photoanodes. According to the Nernst equation, the Ag/AgCl electrode was transformed into a reversible hydrogen electrode (RHE):

E_RHE = E_Ag/AgCl + 0.059pH + 0.1976V

As can be seen from Figure 5, Fe₂O₃ shows a photocurrent density of 0.19 mA/cm² at 1.23 V_RHE under the light illumination of 225 mW/cm². Compared with the Fe₂O₃ photoanode, the photocurrent density of the FeOOH-Fe₂O₃ photoanode reached 0.22 mA/cm². In addition to loading oxygen evolution cocatalysts, oxygen vacancies also play a certain role in PEC performance. By introducing oxygen vacancy (0.37 mA/cm²) onto the upper surface of the FeOOH-Fe₂O₃ photoanode, the photocurrent density can be further increased. With the help of a single parallel experiment, we analyzed the effects of oxygen cocatalyst and oxygen vacancy doping on PEC performance. As shown in Figure 5, the Vo-Fe₂O₃ photoanode generates a photocurrent density of 0.25 mA/cm² at 1.23 V_RHE. Obviously, the photocurrent density co-modified by FeOOH catalyst and oxygen vacancy doping is obviously higher than that of the photoanode modified alone. The modification of FeOOH can effectively reduce the large PEC water splitting overpotential of Fe₂O₃ due to its poor OER kinetics, and oxygen vacancy doping can reduce the charge transfer resistance and increase the charge carrier density [49,50]. Therefore, the photocurrent density of the Vo-FeOOH-Fe₂O₃ photoanode is obviously improved by combining FeOOH modification with oxygen vacancy doping. In a word, the Vo-FeOOH-Fe₂O₃ photoanode presents a synergistic effect. The improvement of the photocurrent density is greater than that induced by single-factor modification.

As shown in Figure 6a, showing the fit according to the circuit shown in the diagram, the curve represents the fitting data and the point represents the original data. Nyquist diagrams of all samples show different radii of similar semicircles, and the radii represent the charge transfer resistance at the semiconductor/electrolyte interface. Under light conditions, the radius of the FeOOH-Fe₂O₃ photoanode is smaller compared to Fe₂O₃. Similarly, the radius of the Vo-FeOOH-Fe₂O₃ photoanode is smaller than that of Vo-Fe₂O₃, indicating that the charge transfer resistance of FeOOH-Fe₂O₃ (Vo-FeOOH-Fe₂O₃) is smaller, as illustrated by the value of R₃ in Table 2, which is the value of the fitted charge transfer resistance. This indicates that FeOOH has a conductive influence on the charge transfer process and decreases the charge transfer potential barrier at the electrode interface. Due to the decrease in charge transfer barrier, the barrier limiting the oxygen evolution reaction decreases, which improves the OER kinetics of the Fe₂O₃ photoanode and improves the PEC performance of the Fe₂O₃ photoanode [26,40].

In order to more deeply understand the changes of the electronic properties of Fe₂O₃ caused by induced oxygen vacancies, Mott–Schottky measurements were conducted, as shown in Figure 6b. The calculation formula is as follows:

${\displaystyle \frac{1}{C^2}}={\displaystyle \frac{2}{\epsilon {\epsilon }_0{A}^{2}e{N}_{\mathrm{D}}}}\left(V-V_{\mathrm{fb}}-{\displaystyle \frac{k_{B}T}{e}}\right)$

where C is the space charge capacitance of the semiconductor, ε is the dielectric constant of Fe₂O₃, ε₀ is the vacuum dielectric constant, A is the active area of the photoanode, e is the electron charge, N_D is the carrier concentration, V is the applied potential, V_fb is the flat potential, k_B is the Boltzmann constant, and T is the absolute temperature. The charge carrier density is calculated based on the following equation:

$N_D={\displaystyle \frac{2}{\epsilon {\epsilon }_0{A}^{2}e}}{[{\displaystyle \frac{d(1/C^2)}{d\left(V\right)}}]}^{-1}$

As can be seen from the Figure 6b, the slopes of the M–S curves are all positive, indicating that the samples are all N-type semiconductors. The carrier concentration in the photoanode can be calculated from the slope of the M–S curve. The donor density of pristine Fe₂O₃ is 2.1 × 10¹⁹ cm⁻³. Through the modification of FeOOH or the introduction of oxygen vacancies, the donor density increases. The donor densities of the samples are 2.6 × 10¹⁹ cm⁻³ and 4.0 × 10¹⁹ cm⁻³ for Vo-Fe₂O₃ and FeOOH-Fe₂O₃, respectively. Moreover, the sample of Vo-FeOOH-Fe₂O₃ exhibits 4.4 × 10¹⁹ cm⁻³, which can be attributed to the synergistic effects of FeOOH cocatalysts and oxygen vacancies. The following reaction shows the defective equilibrium of n-type oxides introducing oxygen vacancies in a reducing environment and the change in Fe ions [51,52]:

${\mathrm{O}}_{\mathrm{O}}^x\to {\mathrm{V}}_{\ddot{\mathrm{O}}}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}$

${\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+}$

It can be concluded that the oxygen vacancy in Vo-FeOOH-Fe₂O₃ makes Fe³⁺ reduced to Fe²⁺, which can be used as a shallow donor to increase the donor density, and the increase in donor density is beneficial to improving the electronic properties of Vo-FeOOH-Fe₂O₃, making the conductivity of Vo-FeOOH-Fe₂O₃ better than that of FeOOH-Fe₂O₃. Therefore, the introduction of oxygen vacancies reduces the charge transport resistance and accelerates the electron transport of the system, thus enhancing the conductivity and further improving its PEC performance.

4. Conclusions

This work realizes the modification of cocatalyst and oxygen vacancy without changing the microstructure of a photoanode via a simple two-step method. Firstly, FeOOH oxygen evolution cocatalysts are loaded on the Fe₂O₃ photoanode by photoelectrodeposition, which decreases the reaction barriers for water oxidation, accelerates the kinetics of water oxidation and facilitates the transfer of holes to the electrode/electrolyte interface. The FeOOH layer on the surface of the photoanode provides an alternative way to avoid the accumulation of holes in the Fe₂O₃ photoanode, such that holes can effectively participate in the process of water oxidation. Then, through air plasma treatment, not only do oxygen atoms in O_L escape to form oxygen vacancies, but also part of the -OH is decomposed into oxygen vacancies, and the surface oxygen vacancies are used as an active site, which increases the electron transmission rate. On the other hand, due to the introduction of oxygen vacancies on the surface, there is more Fe²⁺ on the surface of Fe₂O₃, and the Fe²⁺ site can be used as a shallow donor to improve the carrier concentration, which enhances the conductivity of Fe₂O₃. There exists a significant synergistic effect between the surface modification of FeOOH and the doping of oxygen vacancies, which enhances the PEC water splitting performance more markedly when both are implemented concurrently, rather than applying either FeOOH surface modification or oxygen vacancy doping process in isolation. Notably, following air plasma treatment, the photocurrent density of Vo-FeOOH-Fe₂O₃ has been observed to reach 0.37 mA/cm² at 1.23 V_RHE, which is approximately 1.9 times higher than that of the original sample. These findings underscore that combining cocatalyst and oxygen vacancy strategies is an effective approach to boosting the performance of Fe₂O₃ photoanodes. We believe that this work provides a new method of enhancing the PEC performance of Fe₂O₃ photoanode, and is expected to be further applied to other metal oxide semiconductors.

Author Contributions

C.L.: conceptualization, validation, formal analysis, resources, data curation, writing. J.L.: methodology, investigation, conceptualization. W.Z.: validation, investigation, supervision. C.Z.: conceptualization, supervision, resources, funding acquisition, review and editing. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author.

Conflicts of Interest

There are no conflict of interest to be disclosed.

Footnotes

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Figures and Tables

Figure 1. SEM images of (a) Fe2O3, (b) FeOOH-Fe2O3, and (c) Vo-FeOOH-Fe2O3; cross-sectional SEM images of (d) FeOOH-Fe2O3 and (e,f) Vo-FeOOH-Fe2O3.

Figure 2. (a) UV-vis spectra of Fe2O3, VO-Fe2O3, FeOOH-Fe2O3 and Vo-FeOOH-Fe2O3. (b) XRD spectra of FeOOH-Fe2O3 and Vo-FeOOH-Fe2O3.

Figure 3. (a) Raman spectra of FeOOH-Fe2O3 and Vo-FeOOH-Fe2O3 (inset figure: magnified view of 290 cm−1 peaks of the two samples). (b) EPR spectra of FeOOH-Fe2O3 and Vo-FeOOH-Fe2O3.

Figure 4. XPS results regarding (a) the full spectrum of Vo-FeOOH-Fe2O3, (b) the Fe 2p of FeOOH-Fe2O3 and Vo- FeOOH-Fe2O3, (c) the O 1s of FeOOH-Fe2O3, and (d) the O 1s of Vo-FeOOH-Fe2O3.

Figure 5. LSV cures collected for Fe2O3, Vo-Fe2O3, FeOOH-Fe2O3 and Vo-FeOOH-Fe2O3 under light irradiation and dark conditions.

Figure 6. (a) EIS measured at 0.9 VRHE under light irradiation. (b) Mott–Schottky plots measured in 1 M NaOH solution at 1 kHz frequency.

Table 1

Summary of O 1s peak area ratios of different oxygen species in FeOOH-Fe₂O₃ and Vo-FeOOH-Fe₂O₃.

Sample	OL (Proportion)	-OH (Proportion)	OV (Proportion)
FeOOH-Fe2O3	47%	46%	7%
Vo- FeOOH-Fe2O3	44%	31%	25%

Table 2

R and CPE values of Fe₂O₃, Vo-Fe₂O₃, FeOOH-Fe₂O₃ and Vo- FeOOH-Fe₂O₃

Sample	R1 (Ω)	R2 (Ω)	R3 (Ω)	CPE1 (F)	CPE2 (F)
Fe2O3	12.35	44.35	3732	3.4006 × 10−5	2.9527 × 10−6
Vo-Fe2O3	8.01	61.31	2312	5.4402 × 10−5	6.6265 × 10−6
FeOOH-Fe2O3	12.38	81.98	2035	4.9873 × 10−6	3.3932 × 10−6
Vo- FeOOH-Fe2O3	13.69	7.14	471.9	8.9288 × 10−6	3.3378 × 10−5

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