# Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO4 and Mo-doped BiVO4

Xiaofeng Wu, Freddy Oropeza, Shixin Chang, Marcus Einert, Qingyang Wu, Clément Maheu, Julia Gallenberger, Chuanmu Tian, Kangle Lv, Jan Hofmann

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Xiaofeng Wu, Freddy Oropeza, Shixin Chang, Marcus Einert, Qingyang Wu, et al.. Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO4 and Mo-doped BiVO4. Advanced Powder Materials, 2024, 3 (6), pp.100234. 10.1016/j.apmate.2024.100234. hal-04770054

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# Promoting effect of interfacial hole accumulation on photoelectrochemical water oxidation in BiVO₄ and Mo-doped BiVO₄

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Xiaofeng Wu<sup>a, **</sup>, Freddy E. Oropeza<sup>b</sup>, Shixin Chang<sup>c</sup>, Marcus Einert<sup>a</sup>, Qingyang Wu<sup>a</sup>, Clément Maheu<sup>a, d</sup>, Julia Gallenberger<sup>a</sup>, Chuanmu Tian<sup>a</sup>, Kangle Lv<sup>c</sup>, Jan P. Hofmann<sup>a, *</sup>

## ARTICLE INFO

**Keywords:**

Water splitting

Bismuth vanadate

Rate law analysis

Surface holes

Photoelectrochemistry

## ABSTRACT

Hole transfer at the semiconductor-electrolyte interface is a key elementary process in (photo)electrochemical (PEC) water oxidation. However, up to now, a detailed understanding of the hole transfer and the influence of surface hole density on PEC water oxidation kinetics is lacking. In this work, we propose a model for the first time in which the surface accumulated hole density in BiVO₄ and Mo-doped BiVO₄ samples during water oxidation can be acquired via employing illumination-dependent Mott-Schottky measurements. Based on this model, some results are demonstrated as below: (1) Although the surface hole density increases when increasing light intensity and applied potential, the hole transfer rate remains linearly proportional to surface hole density on a log-log scale. (2) Both water oxidation on BiVO₄ and Mo-doped BiVO₄ follow first-order reaction kinetics at low surface hole densities, which is in good agreement with literature. (3) We find that water oxidation active sites in both BiVO₄ and Mo-doped BiVO₄ are very likely to be Bi⁵⁺, which are produced by photoexcited or/and electro- induced surface holes, rather than VOₓ species or Mo⁶⁺ due to their insufficient redox potential for water oxidation. (4) Introduction of Mo doping brings about higher OER activity of BiVO₄, as it suppresses the recombination rate of surface holes and increases formation of Bi⁵⁺. This surface hole model offers a general approach for the quantification of surface hole density in the field of semiconductor photoelectro catalysis.

## 1. Introduction

Photoelectrochemical water splitting offers a promising approach to convert solar energy to chemical energy [1,2]. In comparison with the cathodic hydrogen evolution reaction, photoanodic oxygen evolution is much more sluggish due to slow hole kinetics in the rate limiting step during water oxidation (oxygen evolution reaction, OER) [3–8].

Loading of OER cocatalysts (e.g., RuO₂, MnO₂, CoOₓ, NiOₓ) on a photoabsorber is a general approach to speed up interfacial hole transfer, as electrocatalysts can lower the required overpotential for water oxidation significantly [9–11]. For instance, Sun et al. reported that a Cu@cucurbit [5] uril cocatalyst on a BiVO₄ photoanode can lower the onset potential from 0.50 to 0.15 V vs. RHE and remarkably increase the photocurrent from 2.0 to 4.8 mA cm⁻² at 1.23 V vs. RHE under illumination (AM 1.5G, 100 mW cm⁻²) [12].

Nowadays, significant efforts are dedicated to promoting the efficiency of photoelectrodes in water oxidation by tuning light absorption, separation of charge carriers and interfacial chemical reaction. Studies of charge carrier dynamics are relatively rare, especially the determination of interfacial charge carrier concentration. At present, transient absorption spectroscopy (TAS) is the major technique to probe and determine photo-induced surface hole density in semiconductors [13–16]. Based on this technique, new insight into the carrier dynamics and reaction kinetics at the semiconductor solid-liquid interface has been obtained. For example, Formal et al. demonstrated that the reaction order of water oxidation on Fe₂O₃ hematite and BiVO₄ is closely linked to the surface hole density. First order reaction and third order reaction of water oxidation are revealed at low and high surface accumulated hole density, respectively [17,18].

<footnote><sup>a</sup> Surface Science Laboratory, Department of Materials- and Geosciences, Technical University of Darmstadt, Peter-Griinberg-Straße 4, 64287, Darmstadt, Germany</footnote>

<footnote><sup>b</sup> Photoactivated Processes Unit, IMDEA Energy Institute, Parque Tecnológico de Móstoles, Avda. Ramón de La Sagra 3, 28935, Móstoles, Madrid, Spain</footnote>

<footnote><sup>c</sup> Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education, College of Resources and Environment, South-Central Minzu University, Wuhan, 430074, China</footnote>

<footnote><sup>d</sup> Nantes Université, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000, Nantes, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: xiaofengwu0501@163.com (X. Wu), hofmann@surface.tu-darmstadt.de (J.P. Hofmann).</footnote>
Apart from TAS, it is worth mentioning that Li et al. recently developed an operando surface potential approach to investigate the relationship between interfacial hole transfer rate and surface potential in the photoelectrochemical water oxidation process. They demonstrated that the surface potential is linearly correlated to the interfacial charge transfer rate of water oxidation and this linear relation is independent of applied bias and light intensity [19].

Despite these efforts, the complex relationship between surface hole density, applied bias, (photo)electrochemically active sites and (photo) current density is not clearly established yet. In this work, we setup a surface hole accumulation model for semiconductor-electrolyte interfaces with which the surface accumulated hole density can be determined. BiVO₄ and Mo-doped BiVO₄ are employed as model systems for studying the interplay between surface hole density and photocurrent. Water oxidation on these model systems follows a first order reaction mechanism, which is in good agreement with results from TAS literature. Besides, we identify the active sites of Mo-doped BiVO₄ being Bi⁵⁺, rather than VOₓ or Mo⁶⁺ species via the analysis of redox potentials. The observed promoting effect of Mo doping for water oxidation mainly originates from a reduced recombination rate.

## 2. Results and discussion

### 2.1. Crystal structure, microstructure, and optical properties

X-ray diffraction (XRD) patterns of as-synthesized BVO and Mo-BVO samples are shown in Fig. S1. The characteristic diffraction peaks showed up at 2θ=18.7°, 18.9°, 28.9°, and 30.4° which are assigned to (110), (011), (121), and (040) crystal facets of monoclinic BiVO₄, respectively (JCPDS No. 14-0688). No impurity phases in BVO and Mo-BVO samples were observed. The phase purity of BiVO₄ can be further evidenced by their Raman spectra (see Fig. S5 and discussion below) since this technique is much more sensitive to detection of secondary phases. The peak of the (121) crystalline facet in BVO shifts from 2θ=29.01°-28.94° after doping, which indicates a change in the lattice parameter, and thus a successful substitution of Mo ions. These data are supported by the Raman as well as X-ray photoelectron spectra (XPS) analysis (see Fig. S4(e) and Fig. S5). Typical wormlike morphologies of bare BVO and Mo-BVO samples are shown in Fig. S2(a) and Fig. S2(c). The widths of these wormlike chains in BVO and Mo-BVO are roughly 150-200 nm and 50-100 nm, respectively. The small porous channel of Mo-BVO implied that Mo doping probably influences the crystal growth of BiVO₄. Cross-sectional analysis evidences a ca. 400 nm thicknesses for BVO and Mo-BVO (Fig. S2(b) and Fig. S2(d)), which is consistent with previous work [20].

Ultraviolet-visible near infrared (UV-Vis-NIR) absorption spectra were collected to investigate the light-harvesting abilities of as-synthesized BVO samples (Fig. S3(a)). A bare ITO substrate shows an optical absorption onset at the transition energy of 2.5 eV and therefore does not influence the bandgap determinations of BVO and Mo-BVO samples. Intrinsic BiVO₄ is considered as an indirect semiconductor with an optical bandgap of 2.4 eV. However, the direct optical bandgap of BiVO₄ is only 200 meV higher than its indirect optical bandgap, which leads to a high visible light absorption coefficient (~10⁷ m⁻¹) [20,21]. The direct bandgaps of BVO and Mo-BVO samples were determined by Tauc plot analysis to 2.45 eV and 2.56 eV, respectively. The result indicates that the optical bandgap of BVO barely changed after Mo doping.

### 2.2. Analysis of valence states and surface species

X-ray photoemission spectroscopy (XPS) was performed to analyze Mo doping, valence states and surface species of the BVO samples. The emission lines at binding energies 159.0 eV and 164.4 eV are attributed to Bi 4f₇/₂ and Bi 4f₅/₂ (Fig. S4(a)) [22]. The two emission lines of 524.2 eV and 516.6 eV can be ascribed to V 2p₁/₂ and V 2p₃/₂, respectively (Fig. S4(b)) [23]. The spin-orbit splitting energy of the Bi 4f orbitals is

5.4 eV, while that of V 3d orbitals is 7.6 eV [24]. The O1s peaks at 529.7 eV, 530.3 eV, and 532.4 eV are usually assigned to lattice oxygen, surface hydroxyl groups and adsorbed oxygen species, respectively [25,26]. The Mo 3d₃/₂ and 3d₅/₂ states at 235.3 eV and 232.2 eV indicate the successful introduction of Mo⁶⁺ ions into BiVO₄ in Mo-BVO (Fig. S4(e)) [27, 28]. Additionally, N species cannot be observed for both BVO and Mo-BVO samples, which implies that the Bi(NO₃)₃·5H₂O precursor has been completely thermally decomposed. (Fig. S4(d)). The C 1s region can be fitted by three peaks at 288.3 eV, 286.2 eV, and 284.8 eV, corresponding to C=O, C–O–C and C–C (sp³) species, respectively. The carbon signals originate from surface adsorbed hydrocarbons, typically observed at sol-gel derived metal oxide thin films [29].

The phase composition at the surface of BVO and Mo-BVO samples has been additionally confirmed by Raman spectroscopy. As shown in Fig. S5, the bands located at around 210, 328, 367, 712, and 820 cm⁻¹ are assigned to typical characteristic phonon peaks of monoclinic BiVO₄ [30,31]. The band at around 210 cm⁻¹ originates from the external vibration mode of VO₃⁻ group. In comparison with internal modes, the frequencies of the external mode in VO₃⁻ group are usually much lower owning to heavier mass [32]. The Raman bands at 367 cm⁻¹ and 328 cm⁻¹ are the symmetric and asymmetric bending vibration of the VO₃⁻ group, respectively. The peaks at around 820 cm⁻¹ and 712 cm⁻¹ can be assigned to symmetric and asymmetric stretching vibration of the V-O bonds [20]. Symmetric stretching vibration of V-O stemming from the VO₃⁻ group shifts from 824.8 cm⁻¹ to 820.0 cm⁻¹, which implies the substitution of Mo ions on V sites in BiVO₄ [33].

### 2.3. Photoelectrochemical measurements

#### 2.3.1. Surface hole states on BVO and Mo-BVO

In an ideal semiconductor without surface states, holes can only be thermally excited from the valence band in the dark, as no electronic states are available in the band gap [34]. However, in a real semiconductor, various surface states including surface defects, dangling bonds and adsorbed species exist. These surface states forming mid-gap states in the semiconductor can store and release charge carriers and are generally regarded as recombination centers or active sites for surface chemical reactions.

At the surface of photoelectrodes, photo-induced or/and electro-induced holes are mainly localized at specific sites like surface states or states close to the valence band maximum [34]. The chemical speciation of these holes can be expressed as follows, where M denotes a transition metal [35].

$$M(III) - OH + h^+ \rightarrow M(IV) - OH \rightleftharpoons M(III) - O + H^+ \quad (1)$$

Lower valence states of transition metals can be oxidized to higher valence states by photo and/or electro induced holes. Generally, hydroxyl groups exist at the surface of oxide semiconductors when exposed to air or aqueous electrolyte. These adsorbed hydroxyl groups and water can be oxidized when the redox potential of the high valence-state metal ions is high enough to overcome the required energy barriers of the charge transfer steps.

To investigate the impact of light illumination on the charge carrier concentration of BVO and Mo-BVO samples, Mott-Schottky measurements of as-prepared samples are carried out under dark and different intensities of light irradiation (Fig. 1). Consistently, the slopes of the Mott-Schottky plots in both BVO and Mo-BVO samples decrease with increasing light intensity, which indicates an increased concentration of photo-generated charge carriers within the space charge region. Compared to the bulk charge carrier concentrations in BVO and Mo-BVO samples in the dark, the estimated photogenerated charge carrier concentration in BVO and Mo-BVO samples are 6.5 × 10¹⁸ cm⁻³ and 8.5 × 10¹⁸ cm⁻³, which correspond to 8.8% and 16.6% of the bulk charge carrier concentration measured by Mott-Schottky in the dark, respectively. The concentration of charge carriers and the flat band positions
<figure><img src="image_5.png" /><figcaption>Fig. 1. Mott-Schottky characterization of (a) BVO and (b) Mo-BVO samples, measured in 0.1 M phosphate buffer solution (pH 6.8) in the dark and under illumination. The wavelength of the light source is 435 nm and its intensity is varied from 10 to 200 mW cm⁻² (value given as (Mo-)BVO-XX in legend).</figcaption></figure>

are summarized in Table S1.

To obtain the surface accumulated photoexcited hole density $p_s$ (cm⁻²), a surface hole model is proposed (see derivation and details in ESI). Based on this model, the surface hole density $p_S$ can be expressed by the following equation.

$$p_s = \frac{1}{e A_c} \left( \sqrt{\frac{U - U'_{fb}}{\text{slope}'}} - \sqrt{\frac{U - U'_{fb}}{\text{slope}}} \right) \quad (2)$$

Slope' and slope are the slopes of Mott Schottky plot under illumination and in the dark, respectively. $U'_{fb}$ and $U_{fb}$ is the flat band position under illumination and in the dark, respectively.

The surface hole density vs. potential curves of as-prepared BVO samples is shown in Fig. 2. The surface accumulated photoexcited charge carrier density in BVO and Mo-BVO samples increase when increasing applied potentials and light intensities. The highest surface accumulated hole densities of BVO and Mo-BVO film samples at the potential of 1.23 V (vs. RHE) under illumination (435 nm, 200 mW cm⁻²) can achieve 1.7 × 10¹² cm⁻² and 4.0 × 10¹² cm⁻², respectively.

Linear sweep voltammetry of BVO and Mo-BVO samples was performed for studying the relation between current and surface hole density. The total current density generally consists of faradaic current and capacitive current. Faradaic current is caused by charge transfer in (photo)electrochemical reactions while capacitive current originates from charging or discharging of the electrochemical double layer. Herein, linear sweep voltammetry of BVO and Mo-BVO samples is obtained at sweep rates of 2 mV s⁻¹ (Fig. 3) and 10 mV s⁻¹ (Fig. S6) in the dark and under illumination. Both BVO and Mo-BVO show negligible dark currents at potentials below 1.5 V vs. RHE as higher overpotentials for water oxidation are required. The photocurrents of BVO and Mo-BVO can reach up to 0.009 mA cm⁻² and 0.095 mA cm⁻², respectively, at 1.23 V vs. RHE under illumination (435 nm, 200 mW cm⁻²) at a sweep rate of 2 mV s⁻¹. However, the photocurrents of BVO and Mo-BVO increase to

0.013 mA cm⁻² and 0.21 mA cm⁻², respectively, at a high sweep rate of 10 mV s⁻¹ (Fig. S6). These enhanced photocurrents likely originate from capacitive currents at the interface. So, consequently, the photocurrent densities measured at a low sweep rate of 2 mV s⁻¹ were employed to calculate the real interfacial charge transfer rate $r_h$, derived by the following equation:

$$r_h = e N_A J_{\text{faradaic}} \quad (3)$$

where $N_A$ is Avogadro constant $6.02 \times 10^{23}$ mol⁻¹ and e is the elementary charge $1.6 \times 10^{-19}$ C.

### 2.3.2. Water oxidation reaction rate law analysis for BVO and Mo-BVO

According to the model proposed by Durrant et al. [18], the surface accumulated hole density can also be expressed as follows:

$$\frac{dp_s}{dt} = J_{\text{hole}} - k \times p_s^{\beta} \quad (4)$$

where $J_{\text{hole}}$ is the hole flux to the surface, k and β are rate constant and reaction order of the photoelectrochemical reaction, respectively. In steady state, the variation of $\frac{dp_s}{dt}$ is equal to zero, which simplifies eqn. (3) as follows:

$$J_{\text{hole}} = J_{\text{faradaic}} = k \times p_s^{\beta} \quad (5)$$

By combining eqns. (2) and (4), the charge transfer rate $r_h$ can be derived to

$$\log(r_h) = \beta \log(p_s) + \log(eN_Ak) \quad (6)$$

The relationship between charge transfer rate $r_h$ and surface charge density $p_s$ in BVO and Mo-BVO is plotted in Fig. 4. The charge transfer rate is linearly proportional to surface charge density on a log-log scale. It is reported that the reaction order of water oxidation on BiVO₄ depends on surface hole density [18]. Water oxidation follows first-order reaction

<figure><img src="image_6.png" /><figcaption>Fig. 2. Surface hole density vs. applied potential of BVO (a) and Mo-BVO (b). The wavelength of the light source is 435 nm and its intensity is varied from 10 to 200 mW cm⁻² (value given as (Mo-)BVO-XX in legend).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 3. Current-voltage curves of BVO (a) and Mo-BVO (b) films, measured at a sweep rate of 2 mV s⁻¹ in the dark and under illumination. The light intensity varies from 10 to 200 mW cm⁻² as indicated in the sample names (Mo-)BVO-XX.</figcaption></figure>

at the potential of 0.7 V–1.4 V vs. RHE under low surface hole density ($p_s < 0.1$ hole nm⁻²), which is in good agreement with literature [18]. This slightly counterintuitive conclusion implies that the rate-determining step of water oxidation on BVO and Mo-BVO only involves one surface hole [18]. Hydroxyl radicals are considered as the main intermediate during this single hole transfer step [36].

Reaction rate constants of water oxidation on BVO and Mo-BVO and the corresponding rate constants vs. potential variation were determined and plotted in Fig. 5(a–d). For BVO (Fig. 5c), there are two linear regimes, separated by the potential of 1.0 V vs. RHE. The reaction rate constant increases linearly when the potential is below 1.0 V. However, it plateaus off when the potential exceeds 1.0 V vs. RHE. This is possibly caused by oxidation of low valence state vanadium species. The highest apparent rate constant of water oxidation in BVO reaches up to 40 s⁻¹. For Mo-BVO samples, the apparent rate constant increases slowly when increasing the potential from 0.6 to 0.8 V vs. RHE, while it increases pronouncedly when increasing the potential from 0.8 to 1.4 V vs. RHE. In Mo-BVO, this phenomenon can also be explained by oxidation of the low valence state Mo species. To clarify this point further, we need to correlate the emergence of active sites with the redox potentials of the involved species.

### 2.3.3. Photo(electro)chemically active sites and stabilities of BVO and Mo-BVO

Charge transfer occurs via surface active sites at the solid-liquid interface. The highest valence states of metal cations in metal oxides are likely active sites. For example, Chai et al. demonstrated that the Fe component in FeCoCrNi alloy can promote the formation of Ni⁴⁺ species which enhance the intrinsic catalytic activity of OER [37]. Zhang et al. reported that partial substitution of Sr at La sites in LaFeO₃ is favorable to form Fe⁴⁺ species which led to an increase in OER activity [38].

In the case of BVO and Mo-BVO samples, the active sites that are induced by trapping of photogenerated and/or electrochemical holes should be Mo⁶⁺, V⁵⁺ and Bi⁵⁺. However, as displayed in Fig. S7, the Mo and V atoms in Mo-O/V-O tetrahedra are shielded by four O atoms, so they are unlikely to be as active sites according to the OER adsorbate evolution mechanism (AEM). On the other hand, the redox potentials of HV₂O₅/V³⁺ and VO₂⁻/VO²⁺ are only 0.94 V and 1.39 V vs. RHE, respectively, while for Mo⁶⁺/Mo³⁺ and Mo⁶⁺/Mo⁴⁺ couples they are 0.83 V and 0.90 V vs. RHE, respectively, which also implies that these species are thermodynamically unable to oxidize water molecules [39, 40]. The redox potential of VO₂⁻/VO²⁺ thermodynamically meets the requirement of water oxidation, but the lowest overpotential of the four

<figure><img src="image_8.png" /><figcaption>Fig. 4. Relationship between charge transfer rate and surface accumulated hole density under illumination at different potentials with β being the reaction order.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 5. Reaction rate constants (a,b) at different applied potentials and relationship between reaction rate constant vs. applied potential plots of BVO (c) and Mo-BVO (d) samples during photoelectrochemical water oxidation.</figcaption></figure>

elementary steps in water oxidation is 0.37 V according to the OER adsorbate evolution mechanism (AEM) [41,42]. Consequently, $Bi^{5+}$ is the most likely active site in both BVO and Mo-BVO, because the potential of $Bi^{5+}/Bi^{3+}$ redox couples is 1.99 V vs. RHE. This hypothesis is supported by linear sweep voltammetry characterization (Fig. S8). In dark condition, both BVO and Mo-BVO exhibit negligible current densities below the onset potential of 1.94 V vs. RHE due to insufficient oxidation ability. Nevertheless, numerous $Bi^{5+}$ active sites are electrically induced in BVO and Mo-BVO which sharply enhance the current density, when the applied bias exceeds the onset potential of OER. This $Bi^{5+}$ active sites are expected to be verified by the state-of the art in-situ or operando PEC instruments in the future.

To further investigate the water oxidation mechanism on BVO and Mo-BVO samples from the aspect of the interfacial band energetics, the work functions and valence band positions were measured by UPS. The secondary electron cutoff energies of BVO and Mo-BVO samples were determined to be 16.5 eV and 16.7 eV, resulting in work functions of 4.7 eV and 4.5 eV, respectively, while their valence band maxima were measured to be 2.36 and 2.39 eV below the Fermi level, respectively (Fig. S9).

Combined with the optical bandgap values of BVO samples (2.45 eV and 2.56 eV with and without Mo, respectively – see Fig. S3) and the

redox potentials of metal ions, we propose a schematic diagram to explain the reaction mechanism of water oxidation at the surface of BVO and Mo-BVO samples involving $Bi^{5+}$ sites in Fig. 6. The oxidation state of $Mo^{6+}$ ions is larger than that of $V^{5+}$ ions while the electronegativity of Mo atoms (2.16) is greater than that of V atoms (1.63). Therefore, $Mo^{6+}$ ions polarize the Mo-O bond in $MoO_4^{2-}$ tetrahedra more strongly than $V^{5+}$ does in $VO_3^{3-}$ tetrahedrons. This implies Mo doping in BVO can effectively suppress the formation of oxygen vacancies and the emergence of low valence V species during sample preparation at high temperature. According to literature, the surface recombination rate of charge carriers in Mo-BVO is about five times lower than that in BVO [20]. Based on the above analysis, we reckon that low valence V species existing at the surface of BVO are the main factors that lead to a high surface recombination rate and slow water oxidation kinetics. The photocurrent and apparent rate constant of Mo-BVO are 10 times and 4 times higher than that of BVO. This is mainly attributed to the reduced number of low valence state V species at the surface of Mo-BVO induced by Mo doping [20].

To confirm the reliability of our results, the photo-stability of BVO and Mo-BVO samples was measured. As shown in Fig. S10, the photocurrent densities in BVO and Mo-BVO samples slightly decreased after 2 h illumination (435 nm, 100 mW cm$^{-2}$), which implies stable

<figure><img src="image_10.png" /><figcaption>Fig. 6. Proposed schematic diagram of water oxidation at the interfaces of BVO and Mo-BVO samples.</figcaption></figure>
photoelectrochemical performance over the duration of the measure-
ments. Besides, the SEM images (Fig. S17), XRD images (Fig. S18) and
XPS spectra (Fig. S19) of BVO and Mo-BVO samples before and after the
PEC performances are shown; no obvious changes in these character-
izations are observed.

3. Conclusions

In this work, BiVO₄ (BVO) and Mo-doped BiVO₄ (Mo-BVO) thin film photoelectrodes were employed as model systems to investigate the impact of surface hole accumulation on the photocurrent density in photoelectrochemical water splitting. Through employing illumination-dependent Mott-Schottky measurements, a semiconductor electrolyte junction model was developed for determining the surface accumulated hole density and investigating the kinetics of water oxidation on BVO and Mo-BVO photoelectrodes. Based on this model, we demonstrated that (1) surface hole density at the interface of the BVO and Mo-BVO model systems depends on the light irradiation intensity and applied potential. (2) Water oxidation on both BVO and Mo-BVO samples follows a first order reaction mechanism under low surface accumulated density. (3) According to the redox potential of the involved metal ions, we assume that the active sites in both BVO and Mo-BVO are very likely to be Bi⁵⁺ which can be generated by photoexcitation and/or electric potential, rather than VOₓ species and Mo⁶⁺. (4) The observed promotional effect of Mo doping in water oxidation mainly originates from a reduced interfacial recombination rate. This work offers a relatively complete physical image for generation and transfer of the photo(electro)-induced holes, as well as their accumulation and distribution within the space charge region (SCR) of a semiconductor photoanode and surface hole-related water oxidation kinetics with possibility to transfer to other materials systems.

4. Experimental section

4.1. Preparation of BiVO₄ and Mo-doped BiVO₄ photoanodes

The synthesis procedure of BiVO₄ and Mo-doped BiVO₄ films follows literature [1]. Briefly, ITO substrates (2×2 cm², 10 Ω per square, Xop glass) were cleaned by ultrasound in distilled water, acetone, and ethanol and UV/ozone treatment for further use. The recipes of BiVO₄ precursor solution was displayed as follows: 1 mmol of Bi(NO₃)₃·5H₂O (99.99%, Sigma-Aldrich) in 4 mL of 2-methoxyethanol (99.5%, Carl Roth) as non-aqueous co-solvent and 1 mmol of vanadyl-acetylacetonate (98%, Sigma-Aldrich) in 4 mL of methanol form a colorless bismuth source solution and a dark blue V source solution, respectively. Subsequently, the dark blue vanadium source solution was dropped into the Bi-containing solution, followed by addition of 0.5 mL of acetylacetone (99%, Sigma-Aldrich) for tuning the viscosity. Finally, 50 µL of the clear combined precursor solution was spread homogeneously on the ITO substrates by spin coating at the speed of 1800 rpm for 1 min. After that, the samples were transferred to a muffle furnace and calcined at 470 °C for 4 h in air with a ramp rate of 2 °C min⁻¹. For the preparation of Mo-doped BiVO₄ samples, a certain amount of bis(acetylacetonato) dioxomolybdenum (VI) (99%, Sigma-Aldrich) was added to the V precursor solution. The stoichiometric molar ratio of V to Mo compounds is 0.97: 0.03. The other procedures are the same as for the bare BiVO₄. The finally collected samples were named BVO and Mo-BVO.

4.2. Materials characterization

XRD patterns were measured from 2θ=5°–80° on a D8 Advance diffractometer (Bruker, Germany) with monochromatic Cu Kα radiation at a scan rate of 0.02° min⁻¹. The morphologies of as-prepared BVO samples were investigated by field emission scanning electron microscopy (FE-SEM, SU8010, Hitachi). UV-Vis-NIR spectra were carried out on a Cary 7000 universal measurement spectrometer (Agilent Technologies, Santa Clara, USA). Baseline correction of air was conducted before

the measurements. X-ray photoelectron spectra (XPS) of samples were
acquired on a Thermo Fisher Escalab 250 with a monochromic Al Kα X-
ray source (hv=1486.6 eV, 13 mA, 15 kV), measured in an analytical
chamber with a base pressure below 5 × 10⁻⁹ mbar. The pass energy,
energy step length and dwell time were set to 10 eV, 0.05 eV, 50 ms,
respectively. Subsequently, the high-resolution XPS spectra of as-
prepared samples were recorded. An electron gun was used to compen-
sate for surface charging and all XPS spectra were binding energy cali-
brated by setting the main peak of the C 1s spectrum to 284.8 eV
(assigned to sp3 C). The XPS spectra were analyzed with CasaXPS
(version 2.3.19PR1.0) [2], using Shirley-type background, the weighted
least-squares method and model curves (70% Gaussian and 30% Lor-
entzian). Work function and valence band edge positions were deter-
mined by He-I (21.22 eV) ultraviolet photoelectron spectroscopy (UPS)
on the same spectrometer. 2.5 × 10⁻⁸ mbar of He was introduced and
pass energy, step length and dwell time were set to 2.5 eV, 0.05 eV, 100
ms, respectively. To estimate the secondary electron cutoff energy, a −6
V bias was applied to impart additional kinetic energy and have a more
resolved portion of this spectrum. Raman spectroscopy was performed on
a Bruker SENTERRA microscope with a green excitation laser (532 nm).
The spectra were collected by averaging 5 scans with a step length of 2
cm⁻¹.

4.3. Photoelectrochemical measurements

All photoelectrochemical characterization was carried out in 0.1 M sodium phosphate buffer solution (KPi, pH 6.8) in a standard three electrode configuration (PECC-2 cell Zahner Elektrik GmbH, Germany). The BVO film samples were used as working electrode with an active irradiation area of 0.283 cm². Pt wire was used as counter electrode (CE) and an Ag/AgCl electrode was used as reference electrode (RE). Linear sweep voltammetry (LSV) curves of as-prepared BVO samples were acquired from +0.2 V<sub>RHE</sub> to +1.5 V<sub>RHE</sub> at a scan rate of 2 mV and 10 mV. Mott Schottky plots were performed from +0.2 V to +1.4 V at a frequency of 1 kHz in the dark or under illumination. The intensity of the LED light source (λ<sub>center</sub>=435 nm; *hv*<sub>center</sub> = 2.85 eV) was varied from 10 to 200 mW cm<sup>-2</sup>. The scan rate and light intensity of chopped light voltammetry (CLV) measurement were set to 50 mV s<sup>-1</sup> and 100 mW cm<sup>-2</sup>, respectively. Incident Photon-Electron Conversion Efficiency (IPCE) of BVO and Mo-BVO samples was measured at 1.23 V vs. RHE.

Data availability statement

Raw and processed data supporting the figures and tables in the
manuscript and the supporting information can be retrieved from
ZENODO.org under DOI:10.5281/zenodo.13788815.

CRediT authorship contribution statement

**Xiaofeng Wu:** Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Funding acquisition. **Freddy E. Oropeza:** Formal analysis, Writing – review & editing. **Shixin Chang:** Investigation, Writing – review & editing. **Marcus Einert:** Formal analysis, Writing – review & editing. **Qingyang Wu:** Investigation, Writing – review & editing. **Clément Maheu:** Formal analysis, Investigation, Writing – review & editing. **Julia Gallenberger:** Investigation, Writing – review & editing. **Chuanmu Tian:** Investigation, Writing – review & editing. **Kangle Lv:** Supervision, Writing – review & editing. **Jan P. Hofmann:** Conceptualization, Project administration, Resources, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
## Acknowledgements

Xiaofeng Wu, Qingyang Wu and Chuanmu Tian acknowledge support of the China Scholarship Council, affiliated to the Ministry of Education of the P.R. of China (Scholarships no. 201708420159, 202208320036 and 202008420222). JG and JPH acknowledge financial support from the German Ministry of Education and Research BMBF under project 03HY105H. Dr. Marcus Einert and Dr. Clément Maheu acknowledge funding from the German Research Foundation (DFG) under projects 469377211 and 423746744, respectively. The authors thank Prof. Dr. Wolfram Jaegermann, Dr. Thomas Mayer, and PD Dr. Bernhard Kaiser for fruitful discussions (TU Darmstadt).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apmate.2024.100234.

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