Enhanced Visible‑Light Photocatalysis via Cu2−xSe‑Modified Monoclinic BiVO4: 15.8‑Fold Activity Boost

Abstract

BiVO₄ is a promising visible‑light photocatalyst, yet its performance is hampered by rapid electron–hole recombination. In this study we engineered a Cu_2−xSe/BiVO₄ heterostructure that leverages band‑structure mismatch to generate an internal electric field, driving efficient charge separation. The Cu_2−xSe component, possessing a larger work function and lower Fermi level than BiVO₄, attracts photogenerated electrons while donating holes in reverse. Photocatalytic tests on Rhodamine B (RhB) under visible light (>420 nm) reveal that a 3 wt% Cu_2−xSe loading boosts activity by 15.8 times relative to pristine BiVO₄. Adding a modest amount of H₂O₂ drives complete RhB degradation within 50 min, demonstrating the composite’s practical potential for environmental remediation.

Introduction

Industrial growth has intensified environmental pollution, prompting the need for sustainable remediation technologies. Photocatalytic decomposition of organic contaminants using solar energy is an attractive, green solution. Bi‑based semiconductors, especially monoclinic BiVO₄ with a 2.4 eV band gap, absorb visible light efficiently and exhibit promising photocatalytic performance. However, fast electron–hole recombination limits their practical use. A proven strategy to mitigate this issue is the construction of heterojunctions that align the band edges of two semiconductors, thereby promoting charge separation.

Cu_2−xSe, a p‑type semiconductor with an indirect band gap of 1.4 eV, is well suited for visible‑light absorption. When combined with BiVO₄, the higher work function of Cu_2−xSe causes electrons to migrate from BiVO₄ to Cu_2−xSe, while holes move in the opposite direction. This charge redistribution establishes an internal electric field that facilitates selective recombination of the less energetic carriers, leaving highly oxidative holes in BiVO₄ capable of generating hydroxyl radicals (•OH) that degrade complex organic molecules.

Here we report, for the first time, the synthesis of Cu_2−xSe/BiVO₄ composites and their application in RhB photodegradation under visible‑light irradiation. The 3 wt% Cu_2−xSe/BiVO₄ sample exhibited a 15.8‑fold increase in activity compared with pure BiVO₄, and complete RhB removal was achieved in 50 min when a small amount of H₂O₂ was introduced. These results establish Cu_2−xSe as an effective cocatalyst for advanced composite photocatalysts.

Methods

Preparation of Cu_2−xSe/BiVO₄ Composites

BiVO₄ was prepared by a standard chemical‑precipitation route. Cu_2−xSe was synthesized following our previously reported protocol. The two powders were dispersed in ethanol, stirred at 60 °C for 4 h, then heated to 80 °C to evaporate the solvent. Finally, the mixture was calcined at 160 °C for 6 h under flowing nitrogen to form the Cu_2−xSe/BiVO₄ composite. The schematic of this process is shown in Figure 1.

The schematic diagram of formation for Cu_2−xSe/BiVO₄ composite

Characterization

X‑ray diffraction (XRD) was performed on a PANalytical X’pert Pro diffractometer using Cu Kα radiation. Morphology was examined by a Hitachi S‑4800 scanning electron microscope (SEM). X‑ray photoelectron spectroscopy (XPS) data were collected on a PHI‑5300 system. Photoluminescence (PL) spectra were recorded on a Cary Eclipse spectrophotometer.

Photocatalytic Reaction

The photocatalytic activity was assessed in an XPA photochemical reactor. A 500 W Xe lamp with a 420 nm cut‑off filter simulated natural sunlight. In a typical experiment, 60 mg of the composite was dispersed in 60 mL of a 5 µM RhB solution. After 2 h of dark stirring to reach adsorption equilibrium, the suspension was illuminated; 6 mL aliquots were withdrawn every 10 min, centrifuged, and analyzed by a Shimadzu UV‑2450 spectrometer.

Photoelectrochemical Measurements

Photocurrent measurements were carried out on a CHI 660E workstation. The working electrode was prepared by drop‑casting 10 mg of the catalyst with 20 µL Nafion onto a 0.196 cm² ITO glass, followed by 1 h at 200 °C. A Pt foil served as the counter electrode, a Hg/HgCl₂ reference, and 0.5 M Na₂SO₄ as the electrolyte. Illumination matched the photocatalytic conditions (500 W Xe lamp, >420 nm).

Results and Discussion

Figure 2a demonstrates that Cu_2−xSe incorporation markedly improves RhB photodegradation. The optimal loading is 3 wt%, achieving the highest degradation rate constant (k = 0.0174 min⁻¹), which is 15.8 times that of bare BiVO₄ (k = 0.0011 min⁻¹). Figure 2c shows that, with 103 µL H₂O₂ per 100 mL solution, the 3 wt% composite fully degrades RhB within 50 min, and the activity remains stable after three cycles.

a Photocatalytic degradation of RhB over Cu_2−xSe/BiVO₄. b Degradation rate constants for varying Cu_2−xSe contents. c Recycling tests with H₂O₂.

SEM images (Figure 3) reveal that BiVO₄ forms hexagonal bulk particles (0.2–1 µm), while Cu_2−xSe sheets (≈300 nm thick, 4 µm long) are uniformly dispersed on BiVO₄ surfaces. XPS confirms the presence of Cu_2−xSe and the oxidation states of all elements (Figure 5).

The SEM photograph of BiVO₄ (a) and Cu_2−xSe/BiVO₄ (b)

XRD patterns (Figure 4a) confirm the monoclinic phase of BiVO₄ remains unchanged after Cu_2−xSe loading, as the Cu content is too low for distinct diffraction peaks. PL spectra (Figure 4b) show a reduced emission intensity for the composite, indicating enhanced electron–hole separation. Electrochemical impedance spectroscopy (EIS, Figure 6) demonstrates a smaller Nyquist arc for the composite, reflecting lower charge‑transfer resistance and faster interfacial electron transport.

The XRD data for BiVO₄ and 3% Cu_2−xSe/BiVO₄ (a), the PL spectra for BiVO₄ and Cu_2−xSe/BiVO₄ composites (b)

The XPS spectra of Cu_2−xSe/BiVO₄ composite. a Survey, b Bi, c V, d O, e Cu, and f Se

The EIS for BiVO₄ and Cu_2−xSe/BiVO₄ under visible light irradiation in 0.5 M Na₂SO₄ solution

The superior photocatalytic performance arises from an S‑scheme heterojunction formed between Cu_2−xSe and BiVO₄. Band alignment drives electrons from BiVO₄ to Cu_2−xSe and holes in the opposite direction, creating an internal field that selectively recombines the lower‑energy carriers. Consequently, high‑energy holes remain in BiVO₄, efficiently generating •OH radicals that oxidize RhB. This mechanism explains the dramatic activity enhancement and the stability observed in repeated runs.

The schematic diagram of photocatalytic mechanism

Conclusion

We have successfully synthesized Cu_2−xSe/BiVO₄ composites that exhibit markedly enhanced visible‑light photocatalytic activity. The 3 wt% Cu_2−xSe/BiVO₄ sample achieves a 15.8‑fold increase in RhB degradation rate compared to pure BiVO₄, and complete removal of RhB is attained within 50 min when a small amount of H₂O₂ is added. SEM, XPS, PL, and EIS analyses confirm efficient charge separation, reduced recombination, and lower interfacial resistance in the composite. These findings establish Cu_2−xSe as an effective cocatalyst and provide a blueprint for designing high‑performance semiconductor photocatalysts for environmental applications.

Abbreviations

RhB:

Rhodamine B

SEM:

Scanning electron microscope

XRD:

X‑ray diffraction