Zn2GeO4−x/ZnS heterojunctions fabricated via in situ etching sulfurization for Pt-free photocatalytic hydrogen evolution: interface roughness and defect engineering

Abstract

Interface engineering has been regarded as a promising strategy for enhancing the catalytic activities of heterojunction photocatalysts. Herein, we have adopted an in situ etching sulfurization method to construct a Zn₂GeO_4−x/ZnS intimate heterojunction, which exhibited excellent photocatalytic H₂ production in the absence of a Pt co-catalyst. Distinctively, TEM and HRTEM measurements showed that the interface of the Zn₂GeO_4−x/ZnS heterojunction became rough (topologically) due to in situ etching sulfurization, and etching was found to be strongly dependent on the crystal orientation. Moreover, the surface of the Zn₂GeO₄ nanorods from flat (100) planes evolved into an irregular coastline-like structure topologized with (110) and (113) high-index planes. ICP and elemental distribution measurements indicated that during the precipitation of ZnS via in situ etching sulfurization, the migration and dissolution of Zn and Ge ions on the Zn₂GeO₄(100) plane led to the roughening of the interface and the evolution of crystal planes. XPS and EPR analyses showed that Zn₂GeO_4−x/ZnS contained more oxygen vacancies with structural evolution. The theoretical calculations demonstrated that oxygen defects were prone to be generated on the Zn₂GeO₄(113) plane and formed the Ge_3c³⁺–V_O complexes. Compared to the inactive (100) plane, etching caused the Zn₂GeO₄(110) planes to have a higher number of threefold coordinated germanium (Ge_3c⁴⁺) and (113) high-index planes that possessed abundant active sites (Ge_3c³⁺–V_O complexes), which dramatically decreased the barrier and reaction energy of H₂O dissociation. This work not only provides fundamental insights into the topological interface evolution and facet-dependent defect distribution but also offers a strategy for the design of efficient photocatalysts for H₂ production without the use of Pt as a co-catalyst based on a multifunctional interface.