Photocatalytic activities of semiconductor particles are well known to be varied depending on the particle morphology as well as their chemical composition. As for semiconductor nanocrystals, most studies have been carried out with binary semiconductors to determine the correlations of crystal size and shape with their physicochemical properties. For example, the photocatalytic H₂ evolution rate became larger for multi-armed CdS rod nanocrystals than spherical or non-branched rod crystals.[1] On the other hand, much interest has recently been shown in I-III-VI-based multinary metal chalcogenide semiconductor crystals as light-absorbing materials for solar cells and photocatalyts because of their tunable physicochemical properties with chemical composition and their low toxicity compared to that of conventional Cd-based binary semiconductors. So far we carried out photocatalytic H₂ evolution with spherical ZnS-AgInS₂ solid solution ((AgIn)_xZn_2(1-x)S₂, ZAIS) nanocrystals as a photocatalyst, the activity being controlled by their size and chemical composition. In this study, we report shape-controlled synthesis of ZAIS nanocrystals via two-step heat treatment process and investigate their photocatalytic H₂evolution activity as a function of chemical composition.[2]

ZAIS nanocrystals with anisotropic shapes were prepared by thermal decomposition of corresponding metal acetates and sulfur compounds of elemental sulfur and 1,3-dibutylthiourea in oleylamine at 250 ^oC. Since thus-obtained crude particles contained both rod- and rice-shaped nanocrystals, the size-selective precipitation was adopted to isolate ZAIS nanocrystals with individual anisotropic shapes. The chemical composition of rod-shaped ZAIS nanocrystals was controlled by changing metal precursor ratio in preparation, while post-preparative Zn²⁺-doping was carried out to increase the Zn²⁺ content in rice-shaped nanocrystals. For comparison, spherical ZAIS nanocrystals (diameter: ca. 5.5 nm) were prepared by the same method reported previously.[3] Thus-obtained ZAIS nanocrystals were suspended in a 2-propanol/water mixture solution containing Na₂S as a hole scavenger and then irradiated with Xe lamp light (λ> 350 nm) to investigate their photocatalytic H₂evolution activity.

The heat treatment at 250 ^oC produced rod-shaped ZAIS nanocrystals with length of ca. 30 nm and width of ca. 5 nm. The chemical composition was controlled from x=0.35 to 0.11, in which the absorption onset was blue-shifted from ca. 470 nm to 370 nm. Rice-shaped ZAIS nanocrystals (size: ca. 9 × ca.16 nm), isolated by size-selective precipitation, had almost constant x value of ca. 0.8~0.9 regardless of the precursor metal ratio in preparation, and then post-preparative Zn²⁺ doping was necessary to vary the chemical composition from 0.92 to 0.25, in which the absorption onset was also blue-shifted from 650 nm to 490 nm with a decrease in x value. TEM measurement revealed that each ZAIS nanocrystal was composed of a single crystal with hexagonal crystal structure, regardless of particle shape. The photocatalytic H₂ evolution rate, R(H₂), was significantly varied depending on the Eg of ZAIS nanocrystals if the comparison was made for the particles with a constant morphology, as shown in Fig. 1, in which the photocatalytic activity became optimal at x of 0.35~0.45 regardless of nanocrystal shapes. In contrast, R(H₂) increased with nanocrystal shape in the order of rice < sphere < rod. The highest apparent quantum yield for photocatalytic H₂evolution was 5.9% for rod-shaped ZAIS nanocrystals, being about two-times larger than that obtained with spherical nanocrystals.

Reference

[1] Liu, et al., Dalton T 2014, 43, 7245-7253.

[2] Torimoto, et al., ACS Appl. Mater. Interfaces 2016, 8, 27151-27161.

[3] Torimoto, et al., J. Phys. Chem. C 2015, 119, 24740-24749.

Figure 1 (a) TEM images of ZAIS nanocrystals with rod, sphere, and rice shapes. (b) Relationship between R(H₂) and Eg of ZAIS nanocrystals. The numbers in panel b represent the x values.