The relevance of Cr defects and photoelectrochemical water oxidation activity of monoclinic PbCrO₄ films

Abstract

The presence of Cr defects in monoclinic PbCrO₄ is closely related to its solar water oxidation activity, which has been overlooked in the preparation of PbCrO₄ films and remains unexplored in research on PbCrO₄ photoanodes. Herein, monoclinic PbCrO₄ films with few Cr defects (PbCrO₄-F_VCr) were prepared on FTO substrate by drop-coating Pb²⁺/Cr³⁺ precursor solution to reduce the loss of Cr during thermal treatment. Relative to the monoclinic PbCrO₄ films with rich Cr defects (PbCrO₄-R_VCr), higher solar water oxidation activity was achieved using the PbCrO₄-F_VCr films as photoanodes. At 1.23 V vs. RHE, a higher water oxidation photocurrent of 1.13 mA cm⁻² was produced on the PbCrO₄-F_VCr film photoanodes, which is twice that of the PbCrO₄-R_VCr film photoanodes (0.55 mA cm⁻²). Meanwhile, the PbCrO₄-F_VCr film photoanodes had faster water oxidation kinetics than PbCrO₄-R_VCr film photoanodes. The water oxidation rate constant (k_O2) on the PbCrO₄-F_VCr film photoanodes was 40.6 s⁻¹, while a lower k_O2 of 8.2 s⁻¹ was observed on the PbCrO₄-R_VCr film photoanodes. Experimental and theoretical analyses jointly revealed that PbCrO₄ films with fewer Cr defects have higher solar water oxidation activity for the following reasons: (i) the presence of Cr defects can result in the formation of deep energy levels in PbCrO₄, which are unfavorable for carrier transfer in the bulk of PbCrO₄ films; (ii) the presence of Cr defects leads to the formation of unsaturated O dangling bonds on PbCrO₄, which act as detrimental traps that hinder carrier separation at the surface of PbCrO₄ films; (iii) Cr has a half-filled 3d⁵ orbital to provide active sites for reaction, so the presence of Cr defects weakens the catalytic activity of PbCrO₄ for water oxidation, and the dehydrogenation of *OH into *O becomes the rate-limiting step, which requires a high energy barrier of 1.88 eV. The present work provides insights into monoclinic PbCrO₄ film photoanodes in terms of preparation conditions, Cr defects, water oxidation activity and reaction mechanism.

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1 Introduction

n-Type monoclinic PbCrO₄ has an electronic structure similar to that of monoclinic BiVO₄,¹ and it has been investigated as a photoanode material for photoelectrochemical (PEC) water splitting in recent years.^2–7 Based on its advantageous band gap (∼2.2 eV) and valence band position (∼2.2 V vs. RHE),² monoclinic PbCrO₄ can theoretically harvest ∼20% of solar energy to drive water oxidation.⁸ However, the activity of previously reported PbCrO₄ photoanodes is far below the theoretical expectation, due to the presence of high carrier recombination and sluggish water oxidation kinetics. As is well known, the composition and structure of materials jointly determine their properties and related performance. The presence of defects in semiconductors could impact their electronic structure, carrier separation and migration behavior, thereby obviously changing the photocatalytic and PEC activity.^9,10 For monoclinic PbCrO₄, its Cr⁶⁺ is located at the center of a CrO₄ tetrahedron, and Pb²⁺ is surrounded by 8–9 oxygen atoms due to its lone pair electron effect.^11,12 From the perspectives of electronic structure, one Cr defect carries three effective negative charges in PbCrO₄. In theory, the presence of a high concentration of Cr defects could disrupt the CrO₄ tetrahedron network and form unsaturated O dangling bonds as well as defect-level traps, which are disadvantageous to the delocalized separation and transfer of electrons/holes.

Significantly, drop-/spin-coating combined with thermal treatment is a common approach for the preparation of monoclinic PbCrO₄ film. Typically, Li et al. used a spin-coating/thermal treatment method to prepare PbCrO₄ films on fluorine-doped tin oxide-coated glass (FTO), and invented acetylacetone and poly(ethylene glycol) as dual ligands to regulate the nucleation and crystal growth of monoclinic PbCrO₄ films.^4,6 Cho et al. prepared PbCrO₄ films on FTO using a drop-coating/thermal treatment approach, and found that adding citric acid to the precursor solution can stabilize Pb²⁺ and Cr³⁺ as well as suppress their segregation to form a smooth and fine monoclinic PbCrO₄ film.^5,7 Although these pioneering investigations made significant contributions to the preparation of monoclinic PbCrO₄ films, the underlying influence of thermal treatment on the composition and structure of the resultant PbCrO₄ films has been overlooked. During thermal treatment at high temperature in air, the metal ions in the precursor solution may be lost, along with the pyrolysis of solvents, which could induce the formation of metal defects in the resultant multi-metal oxides. In the preparation of BiVO₄ films using a spray pyrolysis method, Lamers et al. found the loss of V during the VO³⁺/Bi³⁺ precursor solution annealing above 500 °C in air, and observed that the resultant BiVO₄ films contained V defects and low carrier mobility.¹³ It is worthy of special attention that Cr can be thermally evaporated at lower temperature (T > 500 °C) compared to Pb (T > 600 °C). The thermal evaporation of Cr from stainless steels at T > 500 °C has been extensively observed.¹⁴ Thus, it is very possible that Cr defects are formed in monoclinic PbCrO₄ films prepared using drop-/spin-coating combined with thermal treatment for synthesis. Several works have demonstrated that semiconductor photoelectrode materials with low-concentration defects usually have high crystallinity and can drive solar water splitting efficiently.^15–17 To achieve efficient solar water oxidation on PbCrO₄ film photoanodes, suppressing the formation of high concentrations of Cr defects in monoclinic PbCrO₄ films during synthesis is crucial.

Herein, monoclinic PbCrO₄ films with few Cr defects (PbCrO₄-F_VCr) were prepared on FTO substrate by drop-coating the Pb²⁺/Cr³⁺ precursor solution to reduce the thermal loss of Cr during annealing treatment. Compared with the monoclinic PbCrO₄ films with rich Cr defects (PbCrO₄-R_VCr) for solar water oxidation, a two-fold increased water oxidation photocurrent was produced on the PbCrO₄-F_VCr film photoanodes (1.13 mA cm⁻² at 1.23 V vs. RHE). Meanwhile, faster water oxidation kinetics were achieved on the PbCrO₄-R_VCr film photoanodes. The PbCrO₄-F_VCr film photoanodes had a water oxidation rate constant (k_O2) of 40.6 s⁻¹, which is ∼5 times the k_O2 of the PbCrO₄-R_VCr film photoanodes (8.2 s⁻¹). Based on experimental and theoretical investigations, PbCrO₄ films with fewer Cr defects having higher activity can be ascribed to the following reasons: (i) the presence of Cr defects can result in the formation of a deep energy level in PbCrO₄, which is unfavorable to carrier transfer in the bulk of PbCrO₄ films; (ii) the presence of Cr defects can cause the formation of unsaturated O dangling bonds on PbCrO₄, which are harmful traps for carrier separation on the surface of PbCrO₄ films; (iii) Cr has a half-filled 3d⁵ orbital to provide active sites for reaction, so the presence of Cr defects weakens the catalytic activity of PbCrO₄ for water oxidation, and the dehydrogenation of *OH into *O becomes the rate-limiting step, which requires a high energy barrier of 1.88 eV.

2 Experimental section

2.1 Preparation of PbCrO₄-F_VCr and PbCrO₄-R_VCr films

The PbCrO₄-F_VCr and PbCrO₄-R_VCr films were prepared by drop-coating combined with thermal treatment (Fig. 1). Typically, Pb(NO₃)₂ (AR, Aladdin) and Cr(NO₃)₃ (AR, Aladdin) were dissolved in 5 mL of distilled water to form 0.2 M Pb²⁺ and 0.2 M Cr³⁺ precursor solutions, respectively. Then, 100 mg of polyethylene glycol (AR, Aladdin) was added to the Pb²⁺ precursor solution to stabilize Pb²⁺, while 70 µL of acetylacetone (Aladdin, 99.8%) was dropped into the Cr³⁺ precursor solution to stabilize Cr³⁺. After that, the Pb²⁺ and Cr³⁺ precursor solutions were treated by ultrasonication at 60 °C for 30 min. Subsequently, the Pb²⁺ and Cr³⁺ precursor solutions were mixed and diluted using ethylene glycol (AR, Kelong) to form a 0.01 M Pb²⁺/Cr³⁺ precursor solution. Finally, 200 µL of Pb²⁺/Cr³⁺ precursor solution was dropped onto a clean FTO glass (10 × 20 × 2 mm, 15 Ω sq⁻¹), and thermally treated in a muffle furnace (chamber: 20 × 30 × 20 cm) at 500 °C (heating rate: 2.5 °C min⁻¹) for 2 h in air. For the preparation of PbCrO₄-F_VCr films, a culture dish (diameter: 6.0 cm; height: 1.6 cm) was used to cover the Pb²⁺/Cr³⁺ precursor solution during thermal treatment. For comparison, PbCrO₄-R_VCr films were prepared without using a culture dish cover during thermal treatment of the Pb²⁺/Cr³⁺ precursor solution.

Fig. 1 Diagram of PbCrO₄-F_VCr and PbCrO₄-R_VCr film preparation.

To check the presence and content difference of O dangling bonds in the as-prepared PbCrO₄ films, PbCrO₄-F_VCr and PbCrO₄-R_VCr films were immersed in 0.01 M 1-octadecanethiol (C₁₈H₃₈S, 97%, Macklin) ethanol solution for 24 h to passivate their O dangling bonds. Then, the two types of PbCrO₄ film were washed with deionized water for photocurrent measurements.

2.2 Characterizations of samples

The crystal structure, morphology and elemental chemical state of the PbCrO₄-F_VCr and PbCrO₄-R_VCr films were characterized using an X-ray diffraction (XRD, SmartLab-9 kW, Rigaku), scanning electron microscopy (SEM, Regulus 8100), transmission electron microscopy (TEM, Tecnai G220) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi), respectively. The UV-vis absorption spectra for the PbCrO₄-F_VCr and PbCrO₄-R_VCr films were measured using a Hitachi spectrophotometer (UH5700). The photoluminescence emission (PL) spectra for the PbCrO₄-F_VCr and PbCrO₄-R_VCr films were recorded using an FLS1000 photoluminescence spectrometer. The electron paramagnetic resonance (EPR) spectra for the PbCrO₄-F_VCr and PbCrO₄-R_VCr films were recorded using a Bruker EPR spectrometer (EMPplus-10/12). The water contact angle of the PbCrO₄-F_VCr and PbCrO₄-R_VCr films was observed using a contact angle meter (SINDIN, SCD-100). The surface photovoltage and transient surface photovoltage spectra for the PbCrO₄-F_VCr and PbCrO₄-R_VCr films were detected using surface photovoltage spectroscopy (Beijing China Education Au-light Co., Ltd, CEL-TPV2000). For the PbCrO₄-F_VCr and PbCrO₄-R_VCr films before and after stability testing, their Raman spectra were detected using a Raman spectrometer (MultiRAM, Bruker).

2.3 Photoelectrochemical measurements

All PEC measurements were conducted on a CHI 760e electrochemical workstation with a three-electrode cell at room temperature. The as-prepared PbCrO₄ films were used directly as working electrodes; the counter electrode was a Pt wire (99%), and the reference electrode was an Ag/AgCl electrode. The light source was a 300 W xenon lamp (Beijing China Education Au-light Co., Ltd) equipped with a 1.5G filter to provide 100 mW cm⁻² illumination. The PEC measurements were carried out using back-side illumination, namely the light first penetrated the FTO, then the PbCrO₄ film. 0.5 M phosphate buffer (PBS, pH 7.0) aqueous solution and 0.1 M potassium hydrogen phthalate (KHP)/0.5 M Na₂SO₃ mixed aqueous solution were used as the electrolytes. The potential applied on the working electrode was converted into RHE potential using the following equation:

E_RHE = E_Ag/AgCl + 0.0592 pH + 0.197 (1)

where E_Ag/AgCl is the applied potential, and pH is the pH of the electrolyte.

The charge separation (η_sep) and injection (η_inj) efficiency testing were performed in 0.5 M PBS/0.2 vol% H₂O₂ electrolyte. The η_sep and η_inj of the PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes were calculated according to the following equations:

where J_abs is the calculated photocurrent density based on the UV-vis absorption data of the films (Fig. 6a), J_H2O2 is the photocurrent density of the film photoanodes in 0.5 M PBS/0.2 vol% H₂O₂ (Fig. S5), and J is the photocurrent density of the film photoanodes in 0.5 M PBS.

The applied bias photon-to-current efficiency (ABPE) of the PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes was calculated using the following equation:

where E_RHE is the applied potential vs. RHE and j is the photocurrent density of the film photoanodes under AM 1.5G illumination.

The Mott–Schottky plots of PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes were recorded in the potential range of 0.3 to 0.5 V vs. RHE under AM 1.5G illumination. The carrier concentration of the PbCrO₄-F_VCr and PbCrO₄-R_VCr films was calculated using the following equation:

1/C² = (2/eεε₀A²N_d)(V_a − V_fb − kT/e) (5)

where C is the capacitance, ε is the relative dielectric constant of PbCrO₄, ε₀ is the permittivity of vacuum (8.854 × 10⁻¹² F cm⁻¹), k is the Boltzmann constant (1.381 × 10⁻²³ J K⁻¹), e is the elemental charge (1.602 × 10⁻¹⁹ C), A is the surface area of the sample, N_d is the carrier concentration, V_a is the applied potential, V_fb is the flat band potential, and T is temperature.

The electrochemical impedance spectroscopy (EIS) measurements were performed at 1.23 V vs. RHE. The testing frequency ranged from 100 000 to 0.01 Hz with an amplitude of 10 mV. The measured EIS data were fitted by Zview software under the proposed equivalent circuit model. Based on the EIS Bode plots, the hole lifetimes (τ) on the two types of PbCrO₄ film photoanode were obtained using the following equation:

where f_m is the maximum phase in the frequency range.

The water oxidation rate constant (k_O2) and charge transfer rate constant (k_tran) of the two types of PbCrO₄ film photoanode were calculated on the basis of the fitted components' values for the equivalent circuit (the values shown in Table 2).^18–21 k_O2 was calculated through the following equation:

k_tran was calculated by the following equation:where R_ct is the water oxidation resistance, R_sc is the charge transfer resistance, CPE_ct is the constant phase element of electrolyte/photoelectrode, and CPE_sc is the constant phase element of the photoelectrode surface.

2.4 Density functional theory calculations

The charge density and the density of states of PbCrO₄ (120) with and without Cr defects were calculated using the Vienna Ab initio Simulation Package (VASP).^22,23 The exchange–correlation potential was described by the Perdew–Burke–Ernzerhof (PBE) generalized gradient approach (GGA).²⁴ The electron–ion interactions were accounted for by the projector augmented wave (PAW) model.²⁵ All density functional theory (DFT) calculations were performed with a cut-off energy of 400 eV, and the Brillouin zone was sampled using a 2 × 1 × 1 Gamma-centered k-point Monkhorst–Pack grid. The energy and force convergence criteria of the self-consistent iteration were set to 10⁻⁴ eV and 0.05 eV Å⁻¹, respectively. The DFT-D3 method was used to describe van der Waals (vdW) interactions.²⁶

The Gibbs free energy changes (ΔG) of water oxidation on PbCrO₄ (120) with and without Cr defects were calculated using the following formula:

ΔG = ΔE + ΔZPE − TΔS + ΔG_U + ΔGpH (9)

where ΔE is the difference in electron energies calculated by DFT; ΔZPE and ΔS are the changes of zero-point energy and entropy, respectively, which are obtained from vibrational frequencies. T is the temperature (298.15 K). ΔG_U = −eU, where U is the applied electrode potential. ΔG_pH = k_BT × ln 10 × pH, where k_B is the Boltzmann constant.

3 Results and discussion

3.1 Characterization results

Fig. 2a–f show the typical SEM images of PbCrO₄-R_VCr and PbCrO₄-F_VCr films. From the top and sectional SEM images shown in Fig. 2a–c, it can be seen that PbCrO₄-R_VCr films are not dense and flat enough, and obvious gaps are formed and evenly distributed throughout the whole film. By comparison, PbCrO₄-F_VCr films were denser and flatter, with few gaps (Fig. 2d–f). In terms of microscopic morphology, PbCrO₄-R_VCr and PbCrO₄-F_VCr films consist of connected nanoparticles, but the nanoparticle size of PbCrO₄-R_VCr films (average size ∼70 nm) was smaller than that of PbCrO₄-F_VCr films (average size ∼100 nm) (Fig. 2b, e and S1 in SI). In general, PbCrO₄-R_VCr films (∼230 nm, Fig. 2c) were slightly thicker than PbCrO₄-F_VCr films (∼200 nm, Fig. 2f), due to the difference in morphology and evenness, although their preparation used the same amount of Pb²⁺/Cr³⁺ precursor solution. In the HRTEM image of the PbCrO₄-F_VCr film, the crystalline feature was observed and the lattice spacing was ∼0.32 nm, which corresponds to the typical crystal feature of PbCrO₄(120) (Fig. 2g). For the preparation of the two types of PbCrO₄ films, the only condition difference was the presence or absence of a culture dish covering the Pb²⁺/Cr³⁺ precursor solution during annealing treatment. During the evaporation and pyrolysis of the Pb²⁺/Cr³⁺ precursor solution, the use of a culture dish cover enabled the precursor solution to gradually convert into a uniform colloid and film, in contrast to the process without a cover (Fig. S2). The morphology and thickness differences between the PbCrO₄-R_VCr and PbCrO₄-F_VCr films suggested that the formation of the PbCrO₄ film was significantly impacted by the culture dish covering above the Pb²⁺/Cr³⁺ precursor solution during thermal treatment. In further XRD analyses, the typical diffraction peaks of monoclinic PbCrO₄ (PDF#: 08-0209) were clearly observed for both types of PbCrO₄ film, in addition to the signals for the FTO substrate (SnO₂, PDF#:46-1088) (Fig. 2h). Significantly, the (120) diffraction peak intensity of the PbCrO₄-F_VCr film was 1.26 times that of the PbCrO₄-R_VCr film. According to the XRD data, the crystallinity of the PbCrO₄-F_VCr film was found to be 85.58%, which is higher than that of the PbCrO₄-R_VCr film (80.41%) (Table 1). It is generally believed that the crystallinity of metal oxides is closely influenced by the defect content.¹⁵ The crystallinity difference implied that the defect content in the PbCrO₄-R_VCr and PbCrO₄-F_VCr films is different.

Fig. 2 SEM images of (a–c) PbCrO₄-R_VCr and (d–f) PbCrO₄-F_VCr films. (g) HRTEM image of PbCrO₄-F_VCr film. (h) XRD patterns for PbCrO₄-R_VCr and PbCrO₄-F_VCr films.

Table 1 The crystallinity of PbCrO₄-R_VCr and PbCrO₄-F_VCr films

Sample	hkl	2Θ	FWHM	Crystallinity (%)
PbCrO4-FVCr film	(200)	25.636	0.156	85.58
	(120)	27.223	0.133
PbCrO4-RVCr film	(200)	25.624	0.118	80.41
	(120)	27.218	0.107

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To determine the chemical states of the constituent elements, the PbCrO₄-R_VCr and PbCrO₄-F_VCr films were further investigated using XPS. In the survey spectra, the XPS signals of O 1s, Pb 4f and Cr 2p were clearly detected on the PbCrO₄-R_VCr and PbCrO₄-F_VCr films (Fig. S3). In the high-resolution Pb 4f spectrum of both types of PbCrO₄ film, the binding energy peaks at 138.27 and 143.12 eV were in good agreement with the Pb²⁺ in monoclinic PbCrO₄ (Fig. 3a).^27,28 For the high-resolution Cr 2p spectrum, the binding energy peaks of the PbCrO₄-R_VCr and PbCrO₄-F_VCr films generally matched the Cr 2p signals in monoclinic PbCrO₄ (Fig. 3b).^29,30 However, the Cr 2p binding energy peaks of PbCrO₄-R_VCr film (578.67 eV, 587.92 eV) negatively shifted relative to those of the PbCrO₄-F_VCr film (578.87 eV, 588.12 eV), hinting that their Cr have slightly different chemical states. Meanwhile, more obvious oxygen vacancy signals were found in the high-resolution O 1s spectrum of the PbCrO₄-R_VCr film in comparison with the PbCrO₄-F_VCr film, in addition to their similar lattice oxygen and adsorbed oxygen signals (Fig. 3c). Taking into account the Cr 2p and O 1s XPS differences between the PbCrO₄-R_VCr and PbCrO₄-F_VCr films, it can be inferred that the PbCrO₄-R_VCr film contains more defects than the PbCrO₄-F_VCr film. To detect the specific defects in PbCrO₄-R_VCr and PbCrO₄-F_VCr films, their EPR spectra were recorded and compared. As shown in Fig. 3d–f, two signal centers at g = 2.003 and g = 3.883 appeared for both types of PbCrO₄ film, which are the typical EPR signals of oxygen vacancies and Cr defects, respectively.^31–34 In comparison, the EPR signals of Cr defects were much stronger than those of oxygen vacancies, indicating that Cr defects are the predominant defects in PbCrO₄-R_VCr and PbCrO₄-F_VCr films and oxygen vacancies are companion species. Obviously, the PbCrO₄-R_VCr film had stronger Cr defects and oxygen vacancy signals than the PbCrO₄-F_VCr film, indicating a higher content of Cr defects in the PbCrO₄-R_VCr film. For the preparation of both types of PbCrO₄ film, the only condition difference was the presence and absence of a culture dish covering above the Pb²⁺/Cr³⁺ precursor solution during the annealing treatment. The above characterizations show that PbCrO₄ films with few Cr defects can be prepared using a culture dish covering above the Pb²⁺/Cr³⁺ precursor solution to reduce the loss of Cr during thermal treatment.

Fig. 3 High-resolution XPS spectra for (a) Pb 4f, (b) Cr 2p and (c) O 1s of PbCrO₄-R_VCr and PbCrO₄-F_VCr films. (d–f) EPR spectra for PbCrO₄-R_VCr and PbCrO₄-F_VCr films.

3.2 PbCrO₄-R_VCr and PbCrO₄-F_VCr films as photoanodes for solar water oxidation

In previous reports, PbCrO₄ films have been demonstrated to work as photoanodes for solar water oxidation in PBS solution, and their activity can be directly reflected in the magnitude of the photocurrent.^4,6 To check the influence of Cr defects on the solar water oxidation activity of PbCrO₄ photoanodes, the photocurrent of the two types of PbCrO₄ film was measured in 0.5 M PBS under AM 1.5G irradiation. As shown in Fig. 4a and S4, a higher photocurrent is produced on the PbCrO₄-F_VCr film than on the PbCrO₄-R_VCr film. At 1.23 V vs. RHE, the PbCrO₄-F_VCr film had a higher photocurrent of 1.13 mA cm⁻² compared to the PbCrO₄-R_VCr film (0.55 mA cm⁻²), indicating its higher solar water oxidation activity. In ABPE measurement, the higher activity was further observed on the PbCrO₄-F_VCr film photoanode; their maximum ABPE (0.073%) was nearly double that of the PbCrO₄-R_VCr film photoanode (0.038%) (Fig. 4b). It is important to note that the PbCrO₄-F_VCr film in the present work has impressive activity compared with recently reported PbCrO₄ films (Table S1). In η_sep detection, PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes were found to have η_sep of 65% and 45% at 1.23 V vs. RHE, respectively (Fig. 4c). Meanwhile, the η_inj of PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes were 29% and 21% at 1.23 V vs. RHE, respectively (Fig. 4d). Taking the η_sep and η_inj differences between the two types of PbCrO₄ film photoanodes as comparisons, it can be found that the PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes exhibit a more significant difference in charge separation than in charge injection. The η_sep and η_inj results suggested that PbCrO₄ films with fewer Cr defects can achieve charge separation more effectively.³⁵ From the morphology observations shown in Fig. 2a–f, the PbCrO₄-R_VCr film appears to have a larger specific surface area than the PbCrO₄-F_VCr film, due to the presence of more gaps. However, electrochemically active surface area (ECSA) testing showed that the PbCrO₄-F_VCr films have a larger ECSA (10.31) than the PbCrO₄-R_VCr film (8.60) (Fig. 4e and S6). In addition, higher electrochemical water oxidation activity was observed on the PbCrO₄-F_VCr film than on the PbCrO₄-R_VCr film under dark conditions (Fig. S7). In terms of the electron configuration of transition metal electrocatalysts,³⁶ Cr would have higher catalytic activity for water splitting than Pb, since Cr has a half-filled 3d orbital (1s²2s²2p⁶3s²3p⁶3d⁵4s¹) to provide active sites for reaction, while Pb has a full-filled 3d and 5d orbitals (1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰6p²).³⁷ Therefore, it can be understood that PbCrO₄ with more Cr defects has lower electrochemical activity for water oxidation. Based on the photocurrent and ECSA data, it was found that the PbCrO₄-F_VCr film has a higher ECSA-normalized photocurrent than the PbCrO₄-R_VCr film (Fig. 4f). The above findings jointly demonstrate that PbCrO₄ films with fewer Cr defects can work as higher activity photoanodes for solar water splitting.

Fig. 4 (a) Chopped LSV curves, (b) ABPE, (c) η_sep, (d) η_inj, (e) ECSA, and (f) ECSA-normalized photocurrent for PbCrO₄-R_VCr and PbCrO₄-F_VCr film photoanodes in 0.5 M PBS.

To get insight into the water oxidation kinetics on the two types of PbCrO₄ film photoanode, their Nyquist and Bode plots were recorded at 1.23 V vs. RHE in 0.5 M PBS under AM 1.5G irradiation. As displayed in Fig. 5a, the Nyquist plot of the PbCrO₄-F_VCr film photoanode had a smaller arc than that of the PbCrO₄-R_VCr film photoanode, suggesting faster kinetics in solar water oxidation. Data fitting indicated that the charge transfer (R_sc) and water oxidation (R_ct) resistance on the PbCrO₄-F_VCr film photoanode were 290 Ω and 186 Ω, respectively (Table 2).^38,39 By comparison, the PbCrO₄-R_VCr film photoanode had higher R_sc (1253 Ω) and R_ct (962 Ω). Based on the fitted components' equivalent circuit values, the PbCrO₄-F_VCr film photoanode was found to have higher k_O2 and k_tran (k_O2: 40.6 s⁻¹; k_tran: 63.3 s⁻¹) than the PbCrO₄-R_VCr film photoanode (k_O2: 8.2 s⁻¹; k_tran: 40.6 s⁻¹) (Fig. 5b), indicating faster water oxidation and charge transfer rates on the PbCrO₄-F_VCr film photoanode. According to the Bode plots, the hole lifetimes (τ) on the PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes were calculated to be 0.13 ms and 0.34 ms, respectively (Fig. 5c). These outcomes confirmed that the holes on the PbCrO₄-F_VCr film photoanode can react with H₂O molecules quickly.^40,41 As previously analyzed, Cr has a half-filled 3d orbital that could provide active sites for water oxidation. Therefore, it is not surprising that the PbCrO₄ film photoanode with fewer Cr defects is kinetically favorable for solar water oxidation.

Fig. 5 (a) Nyquist plots, (b) reaction rate constants, and (c) Bode plots for PbCrO₄O₄-R_VCr and PbCrO₄-F_VCr film photoanodes in 0.5 M PBS at 1.23 V vs. RHE under AM 1.5G irradiation. (d) j–t curves and (e) photocurrent change rate of PbCrO₄O₄-R_VCr and PbCrO₄-F_VCr film photoanodes in 0.1 M KHP/0.05 M Na₂SO₃ at 1.23 V vs. RHE under AM 1.5G irradiation. (f) XRD patterns of PbCrO₄O₄-R_VCr and PbCrO₄-F_VCr films before and after stability testing.

Table 2 Fitted component values for the equivalent circuit of two types of PbCrO₄ film photoanodes (inset of Fig. 5a)

Sample	Rs/Ω (error)	Rct/Ω (error)	CPEct/F (error)	Rsc/Ω (error)	CPEsc/F (error)
PbCrO4-RVCr	17.67 (1.98%)	962 (6.26%)	9.72 × 10^−5 (5.15%)	1253 (4.39%)	1.50 × 10^−4 (7.20%)
PbCrO4-FVCr	10.67 (1.42%)	186 (2.62%)	8.47 × 10^−5 (5.23%)	290 (7.93%)	2.80 × 10^−4 (7.62%)

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In terms of the stability for solar water oxidation, it was unfortunate that unsatisfactory activity decay was observed on the PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes in 0.5 M PBS during continuous testing at 1.23 V vs. RHE (Fig. S8). Further investigations found that dissolution and photocorrosion issues are the main causes of activity decay on both types of PbCrO₄ film photoanode in 0.5 M PBS (please see the result discussions for Fig. S9 to S11 and Table S2 in the SI). According to these findings, the PEC stability testing for the PbCrO₄-F_VCr and PbCrO₄-R_VCr films was conducted in 0.1 M KHP/0.05 M Na₂SO₃ aqueous solution at 1.23 V vs. RHE. As shown in Fig. 5d and e, the PbCrO₄-F_VCr film has a higher photocurrent than the PbCrO₄-R_VCr film, showing its higher activity for SO₃²⁻ oxidation. After 3000 s of continuous reaction, 90.0% activity was retained on the PbCrO₄-F_VCr film, while the PbCrO₄-R_VCr film was 70.3% (Fig. 5e). Contrastive XRD, Raman and SEM investigations indicated that the crystalline phases, molecular structure and morphology of the two types of PbCrO₄ film photoanode show slight changes after stability testing (Fig. 5f, S10 and S11). However, the PbCrO₄-R_VCr film after stability testing had a more pronounced decrease in the XRD peaks' intensity, relative to the PbCrO₄-F_VCr film (Fig. 5f). From the stability difference between the PbCrO₄-F_VCr and PbCrO₄-R_VCr film photoanodes, it can be seen that PbCrO₄ with fewer Cr defects is more stable to drive PEC reactions.

3.3 The mechanism for the higher activity of PbCrO₄ films with fewer Cr defects

The above investigations demonstrated that PbCrO₄ films with fewer Cr defects have higher solar water oxidation activity. To reveal the mechanism for PbCrO₄ films with fewer Cr defects having higher activity, the physicochemical properties of the PbCrO₄-F_VCr and PbCrO₄-R_VCr films involving solar water oxidation were contrastively checked. First of all, the UV-vis light absorption property of the two types of PbCrO₄ film was investigated. As shown in Fig. 6a, the PbCrO₄-F_VCr and PbCrO₄-R_VCr films have a close absorption edge around 550 nm, which corresponds to the typical band gap of monoclinic PbCrO₄ (∼2.2 eV).^1,42 Meanwhile, the light absorption intensity of the two types of PbCrO₄ film was very close too, and the absorbed photon fluxes of the PbCrO₄-F_VCr and PbCrO₄-R_VCr films were 7.03 mA cm⁻² and 7.08 mA cm⁻², respectively (Fig. 6b). These observations indicated that the obvious activity difference between PbCrO₄-F_VCr and PbCrO₄-R_VCr films is not be caused by a difference in their light absorption properties. In PL spectrum investigations, weaker PL peak intensity was detected on the PbCrO₄-F_VCr film compared with the PbCrO₄-R_VCr film, reflecting lower carrier recombination in/on the PbCrO₄-F_VCr film (Fig. 6c). In addition, stronger surface photovoltage signals were detected on the PbCrO₄-F_VCr films than on the PbCrO₄-R_VCr film, further confirming better carrier separation and transfer in/on the PbCrO₄-F_VCr film (Fig. 6d and S12). As mentioned in the Introduction, the presence of a high concentration of Cr defects in monoclinic PbCrO₄ could disrupt its CrO₄ tetrahedron network and form defect-level traps, which are disadvantageous to the delocalized separation and transfer of electrons/holes in theory. To confirm the effect of Cr defects on carrier separation and transfer, the open-circuit potential (OCP) variations were detected on PbCrO₄-F_VCr and PbCrO₄-R_VCr films under and without AM 1.5G irradiation. As displayed in Fig. 6e, an OCP_light of ∼0.461 V vs. RHE was observed on both types of PbCrO₄ film, while the OCP_dark of the PbCrO₄-F_VCr and PbCrO₄-R_VCr films are 0.582 and 0.552 V vs. RHE, respectively. Accordingly, a more obvious OCP variation was produced on the PbCrO₄-F_VCr film (0.123 V) than the PbCrO₄-R_VCr film (0.098 V), suggesting that PbCrO₄ films with fewer Cr defects have better charge separation and transfer properties.^43,44 In the Mott–Schottky plots (Fig. 6f), a more negative E_fb of 0.12 V vs. RHE was observed on the PbCrO₄-F_VCr film, relative to the PbCrO₄-R_VCr film (0.15 V). Considering the relevance of E_fb in n-type semiconductors to their Fermi level position (E_f), the PbCrO₄-F_VCr film, having a more negative E_fb, indicates that PbCrO₄ with fewer Cr defects has a higher E_f and carrier concentration (PbCrO₄-F_VCr film: 1.98 × 10¹⁹ cm⁻³; PbCrO₄-R_VCr film: 1.51 × 10¹⁹ cm⁻³).

Fig. 6 (a) UV-vis absorption spectra, (b) absorbed photon flux, (c) PL spectra, and (d) transient photovoltage spectra of PbCrO₄-R_VCr and PbCrO₄-F_VCr films in air. (e) OCP variations and (f) Mott–Schottky plots of PbCrO₄-R_VCr and PbCrO₄-F_VCr films in 0.5 M PBS under AM 1.5G irradiation.

In terms of chargeability, one Cr⁶⁺ defect carries three effective negative charges in monoclinic PbCrO₄. From a unit cell structural perspective, the presence of Cr defects could cause the formation of unsaturated O dangling bonds on the PbCrO₄ surface. To check the presence and content difference of O dangling bonds on the two types of PbCrO₄ film, their hydrophilicity was detected and compared (Fig. 7a). Interestingly, weaker hydrophilicity was observed on the PbCrO₄-R_VCr film (θ = 35.4°) relative to the PbCrO₄-F_VCr film (θ = 10.2°), although the PbCrO₄-R_VCr film had a rougher surface (SEM images shown in Fig. 2a–f). In essence, the hydrophilicity of the film depends on the interaction intensity between H₂O molecules and the surface compounds.⁴⁵ It has been reported that high concentrations of Al defects in α-Al₂O₃ can result in the reconstitution of unsaturated O dangling bonds and the decrease of surface energy, thereby α-Al₂O₃ with rich Al defects has weaker hydrophilicity than pristine α-Al₂O₃.⁴⁶ The hydrophilicity difference suggested that PbCrO₄-F_VCr and PbCrO₄-R_VCr films have different contents of O dangling bonds on the surface, owing to their different Cr defect contents. The presence of O dangling bonds usually has a negative effect on carrier separation on the surface of semiconductors.^47,48 Through the –SH groups of 1-octadecanethiol passivating the O dangling bonds on PbCrO₄ films,^49–51 the photocurrent of the PbCrO₄-R_VCr film was found to increase by 62%, but there was only a 24% increase on the PbCrO₄-F_VCr film (Fig. 7b and c). The photocurrent enhancement on 1-octadecanethiol-passivated PbCrO₄ films confirmed the presence of O dangling bonds and the negative effect of O dangling bonds on carrier separation on PbCrO₄. Meanwhile, the more obvious photocurrent improvement on the 1-octadecanethiol-passivated PbCrO₄-R_VCr film reflected their higher content of O dangling bonds, due to rich Cr defects.

Fig. 7 (a) Water contact angle images of PbCrO₄-F_VCr and PbCrO₄-R_VCr films. In 0.5 M PBS under AM 1.5G irradiation, LSV curves of (b) PbCrO₄-F_VCr and (c) PbCrO₄-R_VCr films before and after 1-octadecanethiol passivating.

To understand the influence of Cr defects on the band structure and carrier localization of PbCrO₄, the density of states and electron localization function of monoclinic PbCrO₄ with and without Cr defects were simulated by DFT calculations. As shown in Fig. 8a, PbCrO₄ and PbCrO₄-V_Cr had similar total density of state (TDOS), indicating that their theoretical carrier density is relatively close. From the partial density of state (PDOS) (Fig. 8b and c), the valence and conduction bands of PbCrO₄ and PbCrO₄-V_Cr can be observed to be mainly composed of O 2p and Cr 3d orbitals, respectively. However, two small peaks corresponding to O 2p and Cr 3d orbitals were found at the Fermi level of PbCrO₄-V_Cr (Fig. 8c), indicating that a deep energy level is formed in the band structure of PbCrO₄-V_Cr due to the presence of Cr defects. The presence of a deep energy level in PbCrO₄ with Cr defects was actually reflected in the surface photovoltage spectrum of PbCrO₄-R_VCr films (shown in Fig. 6d), in which small photovoltage trailing in the time scale of 10⁻⁵–10⁻⁴ s was displayed. It has been reported that the formation of a deep energy level in semiconductors usually results in high non-radiative carrier recombination.^52,53 In terms of structural theory, the presence of Cr defects in PbCrO₄ could trigger strong non-radiative carrier recombination, as the d–d transition of Cr⁶⁺ is limited by spin selectivity and its radiation rate is low. Electron localization comparisons suggested that the presence of Cr defects increases the degree of carrier localization in PbCrO₄ (Fig. 8d), hinting at the negative impact of Cr defects on carrier transfer in PbCrO₄.^54,55 Combined with the experimental findings of higher carrier recombination in/on PbCrO₄-R_VCr film than PbCrO₄-F_VCr film (Fig. 6c and d), it is certain that the presence of Cr defects in PbCrO₄ can result in the formation of a deep energy level, which is not conducive to carrier transfer in the bulk of PbCrO₄.

Fig. 8 (a) Total density of states, (b and c) partial density of states, and (d) electron localization function of PbCrO₄ with and without Cr defects.

The Gibbs free energy (ΔG) of water oxidation on PbCrO₄ with and without Cr defects was further simulated by DFT calculations based on a typical four-electron pathway (reactions (1)–(4)).^56,57

H₂O(l) + * → *OH + H₂O₂ + H⁺ + e⁻ (1)

*OH + H₂O(l) + H⁺ + e⁻ → *O + H₂O(l) + 2H⁺ + 2e⁻ (2)

*O + 2H₂O(l) + 2H⁺ + 2e⁻ → *OOH + 3H⁺ + 3e⁻ (3)

*OOH + 3H⁺ + 3e⁻ → O₂ + 4H⁺ + 4e⁻ (4)

For the adsorption of H₂O on PbCrO₄, it was observed that H₂O molecules preferentially adsorbed on the Cr sites of PbCrO₄ (Fig. 9a), as Cr has a half-filled 3d orbital to provide active sites.³⁶ The adsorption energy of H₂O molecules on PbCrO₄ was −2.034 eV, which is more negative than that on PbCrO₄-V_Cr (−1.564 eV). PbCrO₄ having a more negative H₂O adsorption energy means that PbCrO₄ has better properties for H₂O adsorption than PbCrO₄-V_Cr, which is consistent with the hydrophilicity results shown in Fig. 7a. For water oxidation, strong H₂O adsorption on photoanodes is energetically favorable for the subsequent dissociation.^58–61 In subsequent calculations for the dehydrogenation of H₂O into O₂ on PbCrO₄ and PbCrO₄-V_Cr, different ΔG values were found in each elementary step (Fig. 9b–d). Significantly, the rate-limiting step of water oxidation on PbCrO₄-V_Cr was found to be the dehydrogenation of *OH into *O, and its energy barrier was 1.88 eV. For PbCrO₄, its rate-limiting step of water oxidation was the reaction of *OH with H₂O to form *OOH, and its energy barrier was 1.61 eV. This comparison indicates that the presence of Cr defects makes the rate-limiting step of water oxidation on PbCrO₄ the dehydrogenation of *OH into *O, which has a higher energy barrier of 1.88 eV. Based on the EIS experimental results (Fig. 5a–c and Table 2), it can be further stated that the presence of Cr defects weakens the catalytic activity of PbCrO₄ for water oxidation, and turns the rate-limiting step to the dehydrogenation of *OH into *O, which has a high energy barrier of 1.88 eV. Considering the negative effect of Cr defects and the ease of formation of Cr defects in PbCrO₄ films using the drop-coating/thermal treatment approach, we discovered that PbCrO₄ films with higher water oxidation activity can be prepared using a slightly excess of Cr precursor to compensate for the loss of Cr in thermal treatment, in addition to using a culture dish to cover the precursor solution. As shown in Fig. 9e and f, the highest water oxidation photocurrent was achieved on the PbCrO₄ films that were prepared using the precursor solution with a Pb/Cr atomic ratio of 100.0/100.7. Meanwhile, the PbCrO₄ film prepared with a Pb/Cr atomic ratio of 100.0/100.7 had the highest crystallinity (86.11%) (Fig. S13 and Table S3). The above findings provide a solution for the preparation of high-activity PbCrO₄ films.

Fig. 9 (a) Adsorption configuration of H₂O on PbCrO₄ with and without Cr defects. Intermediate configuration of water oxidation on PbCrO₄ (b) with and (c) without Cr defects. (d) Free energy diagrams for water oxidation on PbCrO₄ with and without Cr defects at U = 0, pH = 0 and T = 298 K. (e) LSV curves and (f) water oxidation photocurrent at 1.23 V vs. RHE for PbCrO₄ films prepared using different Pb/Cr atomic ratios.

Summarizing the findings from experimental observations and DFT calculations, the mechanism for the higher activity of PbCrO₄ films with fewer Cr defects can be explained as follows: (i) the presence of Cr defects results in the formation of a deep energy level in PbCrO₄, which is unfavorable to carrier transfer in the bulk of PbCrO₄ films; (ii) the presence of Cr defects leads to the formation of unsaturated O dangling bonds on PbCrO₄, which are harmful traps for carrier separation on the surface of PbCrO₄ films; (iii) Cr has a half-filled 3d⁵ orbital to provide active sites for reaction, so the presence of Cr defects weakens the catalytic activity of PbCrO₄ for water oxidation, and makes the rate-limiting step the dehydrogenation of *OH into *O, which requires a high energy barrier of 1.88 eV.

4 Conclusions

In summary, monoclinic PbCrO₄ films with few Cr defects (PbCrO₄-F_VCr) were prepared on FTO substrate through drop-coating Pb²⁺/Cr³⁺ precursor solution to reduce the thermal loss of Cr during annealing treatment. Compared with the monoclinic PbCrO₄ film with rich Cr defects (PbCrO₄-R_VCr), the PbCrO₄-F_VCr film as a photoanode had higher solar water oxidation activity. A higher water oxidation photocurrent of 1.13 mA cm⁻² at 1.23 V vs. RHE was produced on the PbCrO₄-F_VCr film photoanode, while the PbCrO₄-R_VCr film photoanode was 0.55 mA cm⁻². Meanwhile, faster water oxidation kinetics were observed on the PbCrO₄-F_VCr film photoanode, relative to the PbCrO₄-R_VCr film photoanode. The PbCrO₄-F_VCr film photoanode had a water oxidation rate constant (k_O2) of 40.6 s⁻¹, and the k_O2 of PbCrO₄-R_VCr film photoanode was 8.2 s⁻¹. The higher solar water oxidation activity of PbCrO₄ films with fewer Cr defects can be attributed to following reasons: (i) the presence of Cr defects can result in the formation of a deep energy level in PbCrO₄, which is unfavorable for carrier transfer in the bulk of PbCrO₄ films; (ii) the presence of Cr defects can cause the formation of unsaturated O dangling bonds on PbCrO₄, which are harmful traps for carrier separation on the surface of PbCrO₄ films; (iii) Cr has a half-filled 3d⁵ orbital to provide active sites for reaction, so the presence of Cr defects weakens the catalytic activity of PbCrO₄ for water oxidation, and makes the dehydrogenation of *OH into *O the rate-limiting step, which requires a high energy barrier of 1.88 eV. The present work provides insight into monoclinic PbCrO₄ film photoanodes in terms of preparation conditions, Cr defects, water oxidation activity and reaction mechanism.

Author contributions

Jiahe Li designed and performed the experiments, and analysed most of the photoelectrochemical data (65%). Gaili Ke analysed partial photoelectrochemical data (35%). Minji Yang assisted with the analysis of DFT calculations data. Guoliang Lv and Lanyi Cao assisted with the XRD and SEM characterizations of samples. Wenjun Li and Wenrong Wang assisted with the XPS and EPR characterizations of samples. Tao Han and Yong Zhou assisted with the preparation and revision of manuscript. Huichao He conceived and directed this work. All authors discussed the results and commented on the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this manuscript are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5sc08541a.

Acknowledgements

This work was supported by National Natural Science Foundation of China (41702037), Natural Science Foundation of Chongqing (no. CSTB2023NSCQ-MSX0765), and Research Foundation of Chongqing University of Science and Technology (ckrc2022003).

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