Solution synthesis of high-quality CuInS2 quantum dots as sensitizers for TiO2 photoelectrodes

This study reports the solvothermal synthesis of colloidal CuInS2 quantum dots (QDs) for use as sensitizers for photoelectrochemical cells. The synthesis is conducted in an autoclave containing CuCl, InCl3, and S at a Cu/In/S ratio of 1/1/100. This high sulfur-excess environment leads to burst nucleation of CuInS2 at relatively low temperatures. For synthesis conducted at 110–150 °C for 1 h, the atomic ratio of the CuInS2 products is Cu : In : S = 1.1 : 1.0 : 2.1 and the particle size increases with the temperature from 3.5 to 4.3 nm, with a narrow size distribution within 7–11%. The as-prepared colloidal CuInS2 exhibits the quantum confinement effect in the optical absorption spectra. The photoluminescence emission of the resulting CuInS2 QDs has high energy, which may result from excited electrons falling from quantized levels to the ground states. Under illumination of simulated AM 1.5 G at one sun intensity, the CuInS2-sensitized TiO2 electrodes in aqueous sulfide/sulfite electrolyte show light-to-chemical energy conversion efficiencies of 1.9% at a +0.23 V bias and 1.2% at short-circuit. These encouraging conversion efficiencies are attributed to the high energy state of the photoexcited electrons in the CuInS2 QDs.