Design of hierarchically structured BiOBr-based photocatalysts and photoelectrodes for enhanced solar energy conversion

Abstract

Photocatalysis is deemed as a sustainable approach for environment remediation and solar energy conversion, for which the practical viability is highly dependent on the high-performance photocatalysts and device. The pristine and modified BiOBr materials have recently emerged as promising photocatalysts in solar energy photocatalysis, though the facile constructions of hierarchical structures and the explicit dependence of band structure on metal-dopant have not well developed. This PhD research project aims to fabricate robust BiOBr-based photocatalysts and to unravel the relationship between their photocatalytic performance and photoelectrochemistry properties.

First, the Zn-doped BiOBr (denoted as Zn-BiOBr) were prepared via hydrothermal synthesis under basic solution and tested for dye photodegradation and photocatalytic H2 evolution from water splitting using methanol as hole scavenger. The Zn-BiOBr samples display broader bandgaps (Eg) and higher photocatalytic H₂ evolution but detrimental activity in photodegradation of Rhodamine B (Rh.B) dye in a model wastewater in textile industry. The experimental (UV-Vis spectra, XPS and photoelectrochemistry) and DFT characterisations suggest the Zn-doping uplifts the conduction band minima (CBM) and deepen the valence band maxima (VBM), which provides enhanced driving force for water splitting, though the wider Eg weakens light absorbance and dye sensitisation ability and thus leading to detrimental RhB photodegradation activity.

Second, hierarchically structured Zn-BiOBr and BiOBr samples (annotated as Zn-BiOBr-H and BiOBr-H) composing of nano-flake building blocks were prepared using PEG-assisted hydrothermal synthesis. Comparing with irregular ZnBiOBr and BiOBr, the Zn-BiOBr-H and BiOBr-H exhibit superior performance in RhB photodegradation, photocatalytic H2 evolutions and energy recovery from wastewater. The enhanced performance in these reactions is mainly due to the larger specific surface areas and attenuated light scattering of the hierarchical samples. A well-established mechanism of photocatalytic reaction on the hierarchical architectures was proposed in this work.

Third, the PEG-assisted hydrothermal synthesis was adopted in preparation of hierarchical BiOBr/ZnO heterojunctions (denoted as BiOBr/ZnO-PEG) with varying BiOBr loading. The photocatalysis results suggest the BiOBr/ZnO-PEG heterojunctions possess lower activity in RhB photodegradation under visible light, which is due to the coverages of surface-active sites of BiOBr by ZnO. Such synthesis method is extended to preparation of ZnO and S-doped ZnO, which possess different hierarchical structures compared to those of BiOBr-H and BiOBr/ZnO-PEG, confirming the previous proposed mechanism of construction of hierarchical architectures of BiOBr-H and Zn-BiOBr-H. Moreover, the dye photodegradation and photocatalytic antibacterial results confirm that photocatalysis contributes significantly for antibacterial property on ZnO, while S-doped ZnO possess intrinsic bactericidal capability and less dependence on photo-catalysis.

In order to improve the light harvesting on BiOBr, the I-doped BiOBr (BiOBr_xI_1-x) were prepared using a successive dip-coating method to deposit into CuSCN nanorod arrays grown on ITO substrates. The BiOBr_xI_1-x/CuSCN films with varied Br/I ratios and coating layers were compared in their photocurrent response. The increased photocurrent of the BiOBr_xI_1-x/CuSCN films indicated that coupling with BiOBr_xI_1-x not only increased the light absorption of CuSCN films, but also supressed the charge recombination by the built-in electric field across the interface of BiOBr_xI_1-x/CuSCN heterojunctions. In addition, via electrochemistry impedance characterisations, the fabrication conditions of the BiOBr_xI_1-x/CuSCN heterojunctions are optimised.

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Identifiers

ORCID for Zheng Jiang: orcid.org/0000-0002-7972-6175

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