Photocatalytic Current Doubling-Induced Generation of Uniform Selenium and Cadmium Selenide Quantum Dots on Titanium(IV) Oxide

We have developed a current doubling-induced two step photodeposition (CD-2PD) technique for forming selenium quantum dots (QIDs) and metal selenide QDs on TiO2, and proposed a reaction mechanism. Large aggregates of Se particles (similar to 100 nm) are generated on TiO2 from aqueous and 2-methyl-2-propanol solutions of H2SeO3 by UV-light irradiation. In contrast, highly dispersed selenium QDs are formed on TiO2 from the H2SeO3 ethanol and methanol solutions (Se/Ti02). The mean particle size increases with an increase in irradiation time (t(p1)) to reach 8.7 nm at t(p1),, = 2 h. The rates of Se photodeposition in the latter solvents are much faster than those in the latter solvents. These striking differences can be attributed to the current doubling effect of ethanol and methanol by photoelectrochemical measurements. Subsequent UV-light irradiation of Se(t(p1), = 20 min)/TiO2 in ethanol and methanol solutions containing Cd2+ ions converts the Se QDs into homogeneous CdSe QDs (similar to 2 nm). The application of this in situ CD-2PD technique to the mesoporous TiO2 nanocrystalline film enables the uniform incorporation of CdSe QDs into the film (CdSeimp-TiO2). QD-sensitized solar cells employing the CdSeimp-TiO2 photoanodes afford much higher power conversion efficiencies than that using a photoanode prepared in the aqueous solution.