Z-Scheme Heterostructures Using Band-Gap-Tunable ZnO by Metal Doping and Coupling with Polypyrrole for Enhanced Photocatalytic Water Splitting

Abstract

Solar-to-hydrogen (H-2) conversion is one of the sustainable and renewable approaches to mitigate ever-increasing environmental pollution and the global energy crisis. Despite significant progress, the most critical aspect remains the design of highly efficient and stable photocatalysts. Although ZnO-based photocatalysts exhibit high catalytic activity, they suffer from the intrinsic drawback of the broadband gap with ultraviolet (UV) light absorption and are susceptible to oxidation. Herein, a strategy to extend light harvesting in the visible region by metal doping (M = Bi, Cu, and Al) of ZnO nanocrystals and their functionalization with polypyrrole (PPy) nanofibers to drive water-splitting efficiently has been presented. The interfacial band alignment and charge transport of nanohybrids reveal electron transfer from metal-doped ZnO to PPy through the Z-scheme mechanism. Impedance spectra indicate efficient charge separation of ZnO:Bi/PPy nanohybrids, which exhibit a 10-fold increase in photocurrent density for visible-light-driven water splitting and improved photostability compared to PPy. The ZnO:Bi/PPy nanohybrid shows a H-2 generation of 13.5 mmol/g/h, similar to 9.6 times higher than PPy nanofibers (1.4 mmol/g/h). In contrast, the pure ZnO nanocrystal leads to the formation of a p-n ZnO/PPy junction with moderate catalytic efficiency. This study identifies a viable approach for developing high-performance conducting polymer-based nanohybrid photocatalysts for water splitting to produce green hydrogen.