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

This study reports the solvothermal synthesis of colloidal CuInS ₂ quantum dots (QDs) for use as sensitizers for photoelectrochemical cells. The synthesis is conducted in an autoclave containing CuCl, InCl ₃, 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.