Partially BiVO4-Modified ZnO Porous Nanosheets: Solar‑Driven Photocatalysis with Superior Charge Separation

Introduction

Semiconductor photocatalysis has emerged as a promising, low‑cost route for environmental remediation by harnessing solar energy. Two major challenges remain: expanding light absorption into the visible spectrum and suppressing electron–hole recombination to enhance quantum efficiency. Coupling a wide‑bandgap semiconductor with a visible‑light‑responsive partner, when the band edges are suitably aligned, has become a key strategy to address both issues simultaneously [1–4]. Over the past decade, heterojunctions such as TiO₂/CdS, ZnO/CdS, WO₃/CuO, and BiVO₄-based composites have shown superior photocatalytic performance relative to their individual constituents [5–11]. However, efficient internal electric fields require a composite size exceeding twice the space‑charge width (> 100 nm) [13], which can reduce specific surface area, and the interfacial contact is often imperfect, limiting activity.

Two‑dimensional (2D) ultrathin nanostructures offer a high surface area, a large proportion of low‑coordination surface atoms, and an ultrathin thickness that can be exploited for photocatalysis [14–15]. 2D or monolayer nanosheets have demonstrated excellent activity for organic pollutant degradation, hydrogen evolution, and CO₂ reduction [16–26]. Surface engineering—through doping, quantum dot loading, or chemical modification—has further enhanced carrier separation while preserving the intrinsic advantages of 2D architectures. Nonetheless, surface‑engineered 2D ZnO has been scarcely explored for photocatalytic applications.

ZnO is a widely studied photocatalyst owing to its high photosensitivity, low cost, and environmental benignity. Efforts to improve its visible‑light activity have focused on heterojunctions and elemental doping (C, S, Al, Mg) [27–30], yet these studies primarily involve zero‑dimensional or three‑dimensional ZnO nanostructures, and sunlight‑driven activity remains limited. Therefore, developing a 2D ZnO platform with controlled surface engineering to substantially enhance photocatalytic performance under natural sunlight is highly desirable.

Here, we report a novel route to construct junction and non‑junction domains on ZnO PNSs by anchoring amorphous BiVO₄ onto Zn₅(CO₃)₂(OH)₆ nanosheets and calcining to form BZVO/ZnO composites. By tuning the BiVO₄ content, we achieve an optimal low‑loading configuration (Bi/Zn = 0.01) that maximises photo‑carrier separation through surface potential differences. This configuration delivers remarkable photocatalytic degradation of KE‑7B under weak solar irradiation, surpassing strong visible‑light performance, and demonstrates the critical role of rational band alignment in composite photocatalysts.

Methods

All reagents were analytical grade and used as received. Deionised water served for synthesis and photocatalytic tests.

Synthesis of BiVO₄-modifying ZnO PNSs

Zn₅(CO₃)₂(OH)₆ (ZCH) nanosheets were prepared following the literature [31]. A suspension of ZCH (100 mL) was stirred vigorously, and (NH₄)₃VO₄ was dissolved into it. VO₄³⁻ ions adsorbed onto the ZCH surface via electrostatic interaction. NaHCO₃ was added to maintain pH 6–7. Bi(NO₃)₃·5H₂O in ethylene glycol was dropwise introduced; the Bi/Zn molar ratio was varied (0.005:1 to 0.2:1) to produce a series of ZCH‑BiVO₄ complexes. These were centrifuged, washed, and dried at 65 °C for 12 h.

Calcination at 500 °C for 2 h yielded BiVO₄-modified ZnO PNSs with Bi/Zn ratios 0.005, 0.01, 0.02, 0.05, 0.1, and 0.2, denoted ZB_0.005–ZB_0.2. A colour change from white to yellowish indicated increasing Bi content. Pristine BiVO₄ was synthesized similarly without ZCH.

Sample Characterisation and Measurements

XRD patterns were collected (scan rate 0.02° s⁻¹, 5–80°) on an XD‑6 diffractometer (Cu Kα). TEM, HRTEM, HAADF‑STEM, and EDS mapping were performed on a JEOL JEM‑ARM200F. XPS used a Thermo Fisher Escalab 250Xi with Al Kα. UV‑vis DRS employed an EVOLUTION 220 spectrophotometer with integrating sphere. PL spectra were recorded on a FluoroSENS‑9000 with excitation at 330 nm. Photocurrent measurements used a CHI 660C workstation; samples were deposited on ITO (2 mg cm⁻²) and illuminated with a xenon lamp (0.1 V vs. SCE, pH 6.86).

Photocatalytic Evaluation

Photocatalytic activity was assessed by degrading KE‑7B under outdoor sunlight (11:00–14:00 h, July–October). A 30 mg L⁻¹ KE‑7B solution (80 mL) contained 16 mg of catalyst (0.2 g L⁻¹). After 30 min dark adsorption, the suspension was irradiated; aliquots (5 mL) were sampled at intervals, centrifuged, and analysed by UV‑vis (Shimadzu 1700). Recyclability tests were performed for ZB_0.01.

Visible‑light activity was measured using a 300 W Xenon lamp (420–750 nm). Scavenger experiments (benzoquinone, isopropyl alcohol, triethanolamine) probed reactive species.

Examination of Reactive Species

Scavengers were added to KE‑7B solutions prior to photocatalyst addition; subsequent photodegradation followed the same protocol.

Results and Discussion

XRD analysis confirmed that ZnO retained the wurtzite structure (JCPDS 36‑1451), while BiVO₄ matched the monoclinic phase (JCPDS 14‑0688). In the composites, increasing Bi/Zn ratios reduced the ZnO (100) peak (31.8°) and introduced a new 28.6° peak attributed to tetragonal BZVO (JCPDS 48‑0276). For Bi/Zn > 0.05, additional BiVO₄ peaks appeared. TEM images (Fig. 2) show pristine ZnO as smooth porous sheets; at Bi/Zn = 0.01, dark contrast indicates BZVO formation; higher ratios reveal enlarged dark zones and BiVO₄ particles (Fig. 2c–d). HRTEM of ZB_0.05 (Fig. 3) reveals ZnO lattice (0.245 nm) alongside BZVO domains (0.275 nm and 0.256 nm) and BiVO₄ particles (0.288 nm). EDS and XPS confirm homogeneous BZVO distribution and Zn²⁺ oxidation state shift, indicating electron donation to Bi and V atoms.

UV‑vis DRS (Fig. 5) shows ZnO’s UV cutoff (~415 nm) and BiVO₄ absorption up to 610 nm. The composites progressively harvest visible light with increasing Bi/Zn ratio. BZVO’s band gap (~2.0 eV, calculated by DFT) enables visible‑light absorption, enhancing solar photocatalysis.

Photocurrent responses (Fig. 6) reveal that ZB_0.01 exhibits the highest photocurrent density, followed by ZB_0.05, ZnO, and ZB_0.2, indicating optimal charge separation at low BZVO loading. PL spectra (Additional file 1) confirm reduced radiative recombination for ZB_0.01 and ZB_0.05. The low‑loading BZVO forms p‑n junctions within n‑type ZnO, generating surface potential gradients that drive efficient carrier separation (Fig. 7). High BZVO coverage (ZB_0.2) lacks junctions, leading to reduced photocurrent and increased PL.

Photocatalytic tests (Fig. 8–9) show that ZB_0.01 outperforms all other samples under both visible‑light and sunlight irradiation, achieving an 8× higher degradation rate under weak sunlight compared to strong visible light. Adsorption capacity correlates with Bi content but is not the dominant factor; efficient charge separation governs activity. The superior performance under sunlight, despite lower visible‑light intensity, suggests that UV photons from the solar spectrum are critical for driving the ZnO component, while BZVO extends visible‑light absorption.

Scavenger studies reveal that h⁺ and superoxide radicals (O₂⁻) are the primary active species, whereas hydroxyl radicals play a negligible role. The combination of ZnO (CBM = −0.49 V) and BZVO (CBM = 0.5 V) provides both reducing and oxidising potentials suitable for generating O₂⁻ and oxidising KE‑7B directly.

Recycling experiments confirm that ZB_0.01 retains 96 % of its initial activity after three cycles, demonstrating excellent photostability.

Conclusions

We have developed a surface‑engineering strategy to produce partially BiVO₄-modified ZnO porous nanosheets that exhibit exceptional photocatalytic activity under natural sunlight. By anchoring amorphous BiVO₄ onto ZCH nanosheets and calcining, BZVO domains are formed on the ZnO surface. At an optimal Bi/Zn ratio of 0.01, the system maximises photo‑carrier separation through surface potential gradients between non‑junction and p‑n BZVO/ZnO regions. This configuration delivers an eight‑fold increase in KE‑7B degradation under weak sunlight relative to strong visible light, highlighting the importance of rational band alignment in composite photocatalysts. The method offers a robust platform for designing sunlight‑driven, highly efficient ZnO‑based photocatalysts.

Abbreviations

2D

Two‑dimensional

BZVO

Bi_3.9Zn_0.4V_1.7O_10.5

CBM

Conduction‑band minimum

DOS

Density of states

DRS

Diffuse reflectance spectra

EDS

Energy‑dispersive spectroscopy

HAADF‑STEM

High‑angle annular dark‑field scanning transmission electron microscopy

HRTEM

High‑resolution transmission electron microscopy

ITO

Indium tin oxide

KE‑7B

Reactive brilliant red

PL

Photoluminescence

PNSs

Porous nanosheets

SCE

Saturated calomel electrode

TEM

Transmission electron microscopy

VBM

Valence‑band maximum

XPS

X‑ray photoelectron spectroscopy

XRD

Powder X‑ray diffraction

ZCH

Zn₅(CO₃)₂(OH)₆