Colloidal quantum dots (QDs) are promising building blocks for the realization of future optoelectronic technologies. In particular, they are widely studied for photoelectric conversion, thanks to their low cost and high solar energy conversion efficiency. The majority of the QDs used in devices for photoelectrochemical (PEC) hydrogen generation is based on heavy-metal containing materials, such as Pb and Cd. Unfortunately, the use of these materials hamper their prospective to be commercially developed due to health and environmental concerns. In this work, by using a template assisted cation exchange procedure, we developed eco-friendly CuInSe2/(CuInSexS1-x)5/CuInS2 QDs based on a gradient multi shell architecture, exhibiting tunable Near Infra-Red (NIR) optical absorption and photoluminescence (PL) properties. The NIR QDs are used as sensitizer for mesoporous TiO2 and employed for PEC hydrogen production. The PEC device based on the gradient architecture showed a 200 % improvement compared to the simple core CuInSe2 QDs and 73 % compared to the core/shell structure. The enhancement is ascribed to favorable stepwise electronic band alignment and improved electron transfer rate with the incorporation of interfacial gradient layers. These results could pave the way for the synthesis of heavy-metal-free QDs, exhibiting broad application in emerging optoelectronic technologies.
