Regulation of the carrier migration path from type II to S-scheme over CdS-loaded CdWO4 polymorphs to boost photocatalytic H2 evolution

Regulation of the carrier migration path in heterojunction photocatalysts is an effective strategy to improve the performance of photocatalytic hydrogen evolution. Herein, CdS/CdWO4 heterojunction photocatalysts based on two crystal forms of CdWO4 were synthesized via an in-situ anion-exchange reaction. It was discovered that crystal structure and surface defect have a significant impact on the charge carrier migration mechanism during photocatalysis. When CdS was loaded onto monoclinic CdWO4 (M-CdWO4), the resulting CdS/M-CdWO4 heterojunction followed the conventional type- II charge transfer mechanism. In contrast, tetragonal CdWO4 (TCdWO4) with a substantial number of oxygen vacancies resulted in the CdS/T-CdWO4 composite adopting an Sscheme transport mechanism. Driven by the distinct carrier migration path, the CdS/T-CdWO4 heterojunction demonstrated superior hydrogen evolution performance compared to the CdS/M-CdWO4. Significantly, the hydrogen evolution rate of CdS/T-CdWO4 was 2.56 mmol h-1 g-1, which is about 14 times higher than CdS/MCdWO4. The excellent photocatalytic activity of CdS/T-CdWO4 could be mainly ascribed to the efficient separation of photogenerated charge carriers and the higher reduction capacity showcasing in S-scheme. Several techniques such as electron paramagnetic resonance (EPR), in-situ X-ray photoelectron spectroscopy (XPS), and selective photodeposition were employed to confirm the regulation of the carrier migration path from type II to S-scheme over CdS-loaded CdWO4 polymorphs. This study provides comprehensive insights into the construction of highly efficient photocatalytic heterojunctions in the viewpoint of polymorph engineering and surface defects from a deeper perspective.