(Invited) Shape-Dependent Photocatalytic H2 Evolution Activity of ZnS-AgInS2 Solid Solution Nanoparticles

Abstract

Nanoscale materials exhibit size- and shape-dependent physicochemical properties due to both the quantum size effect and the difference in the crystal facets exposed to the solution. So far the preparation for Cd-based II-VI nanoparticles with anisotropic shapes has been intensively investigated via solution phase synthetic methods. Especially rod-shaped nanoparticles have attracted much attention because of unique optical and electronic properties originating from their one-dimensional particle shape, such as polarized emission, improved solar cell performance, and higher photocatalytic activity. On the other hand, recent research efforts have been focused on developing high-quality semiconductor nanoparticles containing no toxic elements such as Cd or Pb. Nanoparticles composed of I, III, and VI group elements, such as AgInS2, CuInS2, and Cu(In,Ga)Se2, have been a promising alternative to Cd-based particles, because their light absorption properties and electronic energy structure could be controlled by modifying the size and shape of nanoparticles as well as by making solid solutions with II-VI group semiconductors. We have reported(1) that nanoparticles of low-toxic ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS), prepared by thermal decomposition of corresponding metal acetates and thiourea in a hot organic solution, exhibited tunable optical properties depending on their chemical composition and particles size. Furthermore these spherical ZAIS nanoparticles worked as an efficient photocatalyst for H2evolution. However little is known about the dependence of photocatalytic activity on the particle shape. In this study, we prepare ZAIS particles with different anisotropic shapes to enhance their photocatalytic activity. ZAIS nanoparticles 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. The obtained crude particle mixture was subjected to size-selective precipitation to isolate ZAIS particles with individual anisotropic shapes. The chemical composition of thus-obtained particles was controlled by changing metal precursor ratio. For comparison, spherical ZAIS particles were prepared as reported previously.(1) The photocatalytic activity of ZAIS nanoparticles was investigated for H2 evolution from aqueous solution as a model reaction. ZAIS particles suspended in a 2-propanol/water mixture solution containing Na2S as a hole scavenger was irradiated with Xe lamp light (λ> 350 nm). The heat treatment at 250 oC for 3 min produced rod-like particles with length of ca. 40 nm and width of ca. 5 nm. With further heating for 10 min, rice-shaped particles were formed as a by-product, the dimensions of which were length of ca.18 nm and width of ca. 9 nm. Rice-shaped particles could be isolated from rod particles by size-selective precipitation. Figure 1a shows TEM images of purified ZAIS particles with rod and rice shapes, as well as spherical particles for comparison. XRD patterns of individual kinds of particles well agreed with wurtzite crystal structure of ZAIS solid solution. The chemical composition of obtained ZAIS particles were almost stoichiometric and the fraction of Zn2+in total metal ions was tunable depending on the ratio of metal precursors in preparation. The irradiation of ZAIS particles suspended in 2-propanol/water solution containing S2- as a sacrificial electron donor induced H2 evolution. Figure 1b shows the time courses of the amount of H2 evolved by irradiation to ZAIS particles with various shapes, in which the x value, representing the solid solution composition of particles, was similar to 0.35-0.45. Regardless of particle shapes, the amount of H2 evolution was linearly increased with elapse of irradiation, indicating that each kind of ZAIS particles efficiently worked as a photocatalyst. However the photocatalytic activity was greatly dependent on the shape of particles, though the Eg of particles was similar to ca.2.3-2.5 eV. ZAIS particles with rod-like and spherical shapes exhibited much higher H2evolution rate than rice-like particles. The highest activity was obtained for rod-like particles, which was ca. 1.5 times larger than that of spherical ones. Since rod-like ZAIS exhibited longer PL lifetime than spherical particles, we concluded that the enhanced photocatalytic activity of rod-like ZAIS particles originated from the change in lifetime of photogenerated charge carriers with the particle size and/or shape. Reference (1) T. Torimoto, et al., J. Phys. Chem. C 119, 24740-24749 (2015). Figure 1