(Invited) Controlling the Electronic Energy Structure of ZnSe-AgInSe2 Solid Solution Nanorods for Visible-Light-Driven Photocatalytic H2 Evolution

Abstract

Intense research efforts have been focused on developing high-quality semiconductor nanocrystals (NCs) containing no toxic elements. Especially I-III-VI2 ternary NCs, such as CuInS2 and AgInS2, exhibiting the quantum size effect, have attracted much attention for the application to solar energy conversion systems, because they have large absorption coefficients in the visible wavelength region and their electronic energy structure could be controlled by modifying their size, shape, and chemical composition. Recently we have successfully prepared anisotropic-shaped NCs of ZnS-AgInS2 solid solution. Their photocatalytic H2 evolution activity could be controlled by the chemical composition as well as by the particle size [1], and increased with particle morphology in the order of rice < sphere < rod [2]. To efficiently utilize solar light for photocatalytic reactions, near-IR-responsive NCs have been intensively developed. In general, the Eg of I-III-VI semiconductor decreases with use of Se as group IV element in place of S. Thus, in this study, we prepare rod-shaped NCs composed of less-toxic ZnSe-AgInSe2 solid solution ((AgIn)xZn2(1-x)Se2, ZAISe(x)) and report their photocatalytic activity for H2 evolution.Rod-shaped ZAISe(x) NCs with different x values were synthesized by thermal decomposition of corresponding metal acetates and selenourea in a mixture solution of oleylamine and dodecanethiol at 250 oC. The chemical composition of resulting NCs, that is, x value, could be controlled by varying the fraction of Zn(OAc)2 in metal precursors, in which the NCs had almost stoichiometric composition. The length of nanorods decreased from ca. 116 to 8.2 nm with an increase in the x value, while the width of nanorods increased from ca. 3.7 to 6.9 nm. The absorption onset of ZAISe(x) NCs was blue-shifted from ca. 900 to 500 nm with a decrease in the x value, that is, with an increase in the Zn fraction.The photocatalytic activity of ZAISe(x) NCs was investigated for H2 evolution as a model reaction. The ligands modified on the nanocrystal surface were changed from dodecanethiol to mercaptopropionic acid to improve the dispersibility of NCs in an aqueous solution. The irradiation to ZAISe(x) NCs suspended in an aqueous solution containing Na2S and Na2SO3 as hole scavengers was carried out using Xe lamp light (λ> 350 nm). With elapse of light irradiation, the amount of H2 evolved linearly increased, regardless of kinds of ZAISe(x) NCs. The H2 evolution rate was remarkably dependent on the composition of ZAISe(x) NCs: A volcano-type dependence was observed between the H2 evolution rate and the energy gap (Eg) of ZAISe(x) NCs. The highest photocatalytic activity was obtained for rod-shaped ZAISe(0.5) NCs with Eg of 1.65 eV. This behavior can be explained by the changes in both the electronic energy structure of ZAISe NCs and the amount of photon absorption with the chemical composition. Furthermore, the rod-shaped ZAISe(x) NCs exhibited a higher photocatalytic activity than that of spherical ones with similar Eg.