Intense research efforts have been focused on developing high-quality semiconductor nanoparticles containing no toxic elements for the practical applications of biomolecular markers, light-emitting devices, and solar light energy conversion systems. Especially nanoparticles of chalcopyrite-type I-III-VI₂ group semiconductors, such as AgInS₂, CuInS₂ and Cu(In,Ga)Se₂, have attracted much attention, because their light absorption properties and electronic energy structures could be controlled by modifying the size and shape of nanoparticles as well as by making solid solutions with II-VI group semiconductors or I-III-VI₂ group semiconductors. It has been reported in our previous paper⁽¹⁾ that nanoparticles of low-toxic ZnS-AgInS₂ solid solution ((AgIn)_xZn_2(1-x)S₂, ZAIS(x)) exhibiting intense photoluminescence could be prepared by thermal decomposition of metal complex precursors in a hot organic solution, in which the optical properties were tunable by changing the chemical composition of solid solution particles. In this study, we investigate the photocatalytic activity of the colloidally-synthesized of ZAIS particles with well-controlled size and chemical composition for H₂ evolution by visible light irradiation.⁽²⁾

ZAIS(x) nanoparticles with different x value were synthesized by thermal decomposition of corresponding metal acetates and thiourea in a mixture solution of oleylamine and dodecanethiol at 250 ^oC. The chemical composition of resulting particles, that is, x value, could be controlled by varying the fraction of Zn(OAc)₂ in metal precursors. Regardless of x value, the particle size decreased from ca. 9 to 4 nm with an increase in the content of dodecanethiol in the solvent used for preparation from 0 to 8.7 vol%. The precise composition of thus-obtained particles was determined by X-ray fluorescence spectroscopy: The particles had composition slightly deviated from stoichiometry, In-rich and Zn-deficient composition for each xvalue. The absorption onset of thus-obtained particles was blue-shifted from ca. 700 to 500 nm with an increase in the content of ZnS in particles or with a decrease in the particle size.

The photocatalytic activity of ZAIS(x) nanoparticles was investigated for photocatalytic H₂ evolution as a model reaction as a function of the chemical composition and particles size. The irradiation to ZAIS(x) particles suspended in an aqueous solution containing Na₂S as a hole scavenger was carried out using Xe lamp light (λ> 350 nm). With elapse of light irradiation, the amount of H₂ evolved linearly increased regardless of kind of ZAIS(x) particles. The H₂ evolution rate was remarkably dependent on both the composition and size of ZAIS(x) particles. As shown in Fig. 1, a volcano-type dependence was observed between the H₂ evolution rate and the size of ZAIS(x) particles for each x value of ZAIS(x) particles, except for x= 1.0.  The highest photocatalytic activity was obtained for ZAIS(x) nanoparticles having the optimal size of ca. 4.2~5.5 nm, the energy gaps of which were ca. 2.3~2.4 eV. This behavior can be explained by changes in both the electronic energy structure of ZAIS particles and their amount of light absorption with chemical composition and particle size.

 References

1. T. Torimoto, T. Kameyama, and S. Kuwabata, J. Phys. Chem. Lett., 2014, 5, 336-347.
2. T. Kameyama, T. Takahashi, T. Machida, Y. Kamiya, T. Yamamoto, S. Kuwabata, and T. Torimoto, J. Phys. Chem. C, 2015, 119, 24740-24749.