Quantum dots (QDs) composed of group I-III-VI semiconductors, such as CuInS₂ and AgInS₂, exhibit unique size- and composition-dependent physicochemical properties different from bulk materials due to the quantum size effect. Furthermore, these QDs have attracted intense attention for developing highly efficient solar energy conversion systems, because of the less toxicity and the large absorption coefficients in visible and near-IR regions. Recently we have successfully prepared alloy QDs composed of ZnAgInS or ZnAgInSe semiconductors, the E_g of which was tunable in the visible or near-IR wavelength regions, respectively [1, 2]. Their photocatalytic H₂ evolution activity was controlled by the chemical composition and size of QDs. On the other hand, the E_g of bulk I-III-VI semiconductors can be controlled by alloying different metal cations of group III elements. Thus, in this study, we prepare spherical QDs composed of less-toxic AgIn_(1-x)Ga_xS₂ (AIGS(x)) alloy semiconductor and report their composition-dependent photocatalytic activity for H₂ evolution under visible light irradiation.

AIGS(x) QDs with different x values were synthesized by thermal decomposition of Ag(OAc), In(acac)₃, Ga(acac)₃, and S powders in a mixture solution of oleylamine and dodecanethiol. The chemical composition of resulting QDs, that is, x value, could be controlled by varying the Ga/(In+Ga) ratio in preparation. Thus-obtained AIGS QDs were surface-coated with ZnS shell of various thickness, resulting in the formation of core-shell-structured QDs, AIGS(x)@ZnS. Finally, the ligands modified on the nanocrystal surface were changed from dodecanethiol to mercaptopropionic acid to improve the dispersibility of QDs in an aqueous solution.

The obtained AIGS QDs had a spherical shape with size of ca. 4 nm. The ZnS thickness of AIGS(x)@ZnS was varied from 0 to ca. 2 nm. The photocatalytic activity of AIGS(x)@ZnS QDs was investigated for H₂ evolution as a model reaction. The irradiation to AIGS(x)@ZnS QDs suspended in an aqueous solution containing Na₂S as a hole scavenger was carried out using Xe lamp light (λ> 350 nm). With the elapse of light irradiation, the amount of H₂ evolved linearly increased, regardless of the composition of AIGS core or the ZnS shell thickness. The H₂ evolution rate was remarkably dependent on the composition of AIGS core: A volcano-type dependence was observed between the H₂ evolution rate and the Ga fraction. The highest photocatalytic activity was obtained for AIGS(x)@ZnS QDs with x=0.6 and the ZnS shell thickness of ca. 2 nm. This behavior can be explained by the changes in the stability of AIGS core in aqueous solutions, the conduction band minimum level of QDs, and the amount of photon absorption.

References

1. Kameyama, T. Takahashi, T. Machida, Y. Kamiya, T. Yamamoto, S. Kuwabata, and T. Torimoto, J. Phys. Chem. C, 2015, 119, 24740-24749.
2. P.-Y. Hsieh, T. Kameyama, T. Takiyama, T. Yamamoto, Y.-J. Hsu and T. Torimoto, J. Mater. Chem. A 2020, 8, 13142-13149.