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.