Enhanced Photocatalytic Activity of Zn-Ag-In-S Semiconductor Nanocrystals with a Dumbbell-Shaped Heterostructure

Nanocrystals with a heterojunction of type-II band alignment have attracted much interest for the construction of solar energy conversion systems because photogenerated electrons and holes can be effectively separated at the heterojunction. Here, we report the preparation of heterostructured multinary nanocrystals composed of ZnS-AgInS2 solid solution (ZAIS), which have tunability of the electronic energy structure depending on their chemical composition. Ellipsoidal ZAIS nanocrystal domains with various compositions were epitaxially grown on both termini of preformed ZAIS nanorods as seeds. The resulting nanocrystals had a dumbbell shape composed of two ellipsoidal nanocrystals (ca. 4-6 nm in width x 7-11 nm in length) connected by a nanorod (ca. 4 nm in width x 16-23 nm in length). Because the Zn fraction in the chemical composition was larger in the rod part than in the two tip parts in the nanodumbbell, heterojunctions of type-II or quasi-type-II were formed at the interface between the rod and tip parts, as confirmed by photoluminescence lifetime measurements. Analysis of the energy structure of ZAIS nanodumbbells indicated that photogenerated electrons and holes were effectively and spatially separated at the heterojunction, resulting in localization of the photogenerated electrons in ellipsoidal ZAIS tips in the nanodumbbell but delocalization of holes over the whole nanocrystal. The photocatalytic H-2 evolution rate of ZAIS nanodumbbells was remarkably improved in comparison to those of single-component counterparts, ZAIS nanorods or free ellipsoidal ZAIS nanocrystals with similar sizes, by tuning the chemical composition of the tip parts to optimize the energy structure of thetype-II heterojunction for H-2 evolution.