(Invited) Effect of Cu Doping on the Energy Structure of Dumbbell-Shaped ZnS-AgInS2 Nanocrystals
Authors: Kameyama Tatsuya , Tsuneizumi Shuhei , Uematsu Taro , Kuwabata Susumu , Torimoto Tsukasa 
Published: September 2023

Colloidal semiconductor nanocrystals (NCs) have been intensively developed for applications in photovoltaics, light-emitting diodes, electroluminescent devices, and biological markers, due to their tunable light absorption and excellent light emission properties. Among them, group I-III-VI-based multinary semiconductor NCs, such as CuInS2, CuInSe2 and AgInS2, have received significant attention for the application to solar energy conversion systems because of their large absorption coefficient and low toxicity. Recently, we have successfully prepared ZnS-AgInS2 solid solution (ZAIS) NCs and optimized their photocatalytic activity for hydrogen production by tuning their chemical composition, size and particle morphology.1-3 The photocatalytic activity of ZAIS NCs was further enhanced by the formation of a dumbbell structure that had a type-II heterojunction at the interface between the tip and rod parts of the NCs.4 In this study, to further improve the photocatalytic activity of dumbbell-shaped ZAIS NCs, we attempted to tune the energy structure of the heterojunction by Cu doping in tip parts for effective charge separation.

Rod-shaped ZAIS NCs with sizes of 4.0 × 23 nm were prepared by the previously reported method.3 The crystal growth of tip parts was carried out by heating a mixture of rod-shaped ZAIS NCs, Cu(OAc), Ag(OAc), In(OAc)3, and thiourea in an organic solvent, in which rod-shaped ZAIS NCs acted as a nucleus to form tip parts composed of (Cu
y
Ag(1-y)In)
x
Zn2(1-x)S2 solid solution. Thus-obtained dumbbell-shaped NCs were isolated from the resulting solution via size-selective precipitation. The amount of Cu doping in tip parts was controlled by changing the Cu content in metal precursors.

TEM observations showed that obtained NCs had a dumbbell structure regardless of the Cu content in the metal precursor. Ellipsoidal NCs with sizes of 5.6 × 11 nm were deposited on both tips of rod-shaped ZAIS NCs. The absorption onset of the dumbbell-shaped NCs were red-shifted from 650 nm to 850 nm with an increase in the Cu fraction from y = 0 to 0.75. The photoelectron yield spectroscopy revealed that the conduction band minimum level was constant at ca. -3.2 eV, while the valence band maximum level was gradually shifted from -5.1 eV to -4.8 eV as the Cu fraction in tip parts increased from y = 0 to 0.75. Thus, it is concluded that the absorption property of dumbbell-shaped ZAIS NCs could be successfully extended from the visible to the near-IR region without changing the energy of photogenerated electrons, simply by modulating the degree of Cu doping in the tip parts. This tunability is useful for constructing efficient solar-light-driven photocatalysts for H2 evolution.

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