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 (QDs) and metal selenide QDs on TiO₂, and proposed a reaction mechanism. Large aggregates of Se particles (∼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/TiO₂). The mean particle size increases with an increase in irradiation time (<i>t</i>_p1) to reach 8.7 nm at <i>t</i>_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(<i>t</i>_p1 = 20 min)/TiO₂ in ethanol and methanol solutions containing Cd²⁺ ions converts the Se QDs into homogeneous CdSe QDs (∼2 nm). The application of this <i>in situ</i> CD-2PD technique to the mesoporous TiO₂ nanocrystalline film enables the uniform incorporation of CdSe QDs into the film (CdSe/mp-TiO₂). QD-sensitized solar cells employing the CdSe/mp-TiO₂ photoanodes afford much higher power conversion efficiencies than that using a photoanode prepared in the aqueous solution