Distinguishing between type II and S-scheme heterojunction materials: A comprehensive review

In the evolving field of photocatalysis, heterojunction photocatalysts, especially Type II and S-scheme, the latter being also known as direct-Z scheme heterojunctions, are gaining increasing recognition for their pivotal role in enhancing photocatalytic efficiency. These heterojunctions, characterized by similar band alignments but distinct charge transfer mechanisms, play a crucial role in facilitating enhanced charge separation and transfer. This comprehensive review delves into the experimental methodologies essential for characterizing these heterojunctions, with a focus on understanding their unique charge transfer mechanisms. Key methods such as Electron Spin Resonance (ESR), radical trapping experiments, Photoluminescence (PL) probing, Nitro Blue Tetrazolium (NBT) transformation, Surface Photovoltage Spectroscopy (SPS), photodeposition of metals, and in-situ X-ray Photoelectron Spectroscopy (in-situ XPS) analysis are discussed in detail. Each technique is presented with necessary guidelines and accompanying information to ensure their appropriate and effective use in pinpointing the specifics of charge transfer processes. The review concludes that the right selection of experimental techniques is crucial in understanding the charge transfer mechanism in staggered type heterojunctions and achieving further advancements in the field of photocatalysis.