CuInS₂ quantum dots coated with CdS as high-performance sensitizers for TiO₂ electrodes in photoelectrochemical cells

We report on a high-performance photoelectrode consisting of a nanocrystalline TiO₂ film co-sensitized with CuInS₂ quantum dots (QDs) and CdS layers. In this photoelectrode, solvothermally synthesized CuInS₂ QDs, monodispersed at sizes of 3.5 and 4.3 nm, are attached to a TiO₂ substrate by means of a bifunctional linker, before it is coated with an in situ growth of CdS followed by successive ionic layer adsorption and reaction. The QDs has a high-level conduction band for the efficient injection of electrons into TiO₂. The CdS coating provides high surface coverage to prevent interfacial recombination and releases the quantum confinement of the QDs, resulting in band gap reduction from 2.10-1.80 eV and 1.94-1.76 eV for the 3.5 and 4.3 nm QDs, respectively. With AM 1.5G illumination at 100 mW cm^-2, this heterostructural electrode exhibits a saturated photocurrent as high as 16 mA cm^-2 in a polysulfide solution. Systematic analysis suggests that the photocurrent resulting from the CuInS₂ QDs is increased by more than 100%, thanks to the CdS coating. This coating extends the absorption spectra of the QDs and facilitates charge separation by scavenging photogenerated holes in the valence band of the QDs.