CdS/CdSe Co-Sensitized TiO₂ Photoelectrode for Efficient Hydrogen Generation in a Photoelectrochemical Cell

An efficient photoelectrode is prepared by sequentially assembled CdS and CdSe quantum dots (QDs) onto a nanocrystalline TiO 2 film. The CdS/CdSe co-sensitized photoelectrode was found to have a complementary effect in the light absorption. Furthermore, the cascade structure, TiO 2 /CdS/CdSe, exhibits a significant enhancement in the current−voltage response, both in dark conditions and under light illumination. On the contrary, the performance of the reverse structure, TiO 2 /CdSe/CdS, is much less than the electrode using a single sensitizer. The open circuit potentials measured in the dark for these electrodes indicates that a Fermi level alignment occurs between CdS and CdSe after their contact, causing downward and upward shifts of the band edges, respectively, for CdS and CdSe. A stepwise band edge structure is, therefore, constructed in the TiO 2 /CdS/CdSe electrode, which is responsible for the performance enhancement of this photoelectrode. The saturated photocurrent achieved by the TiO 2 /CdS/CdSe electrode under the illumination of UV cutoff AM1.5 (100 mW/cm 2 ) is 14.9 mA/cm 2, which is three times the value obtained by the TiO 2 /CdS and TiO 2 /CdSe electrode. When a ZnS layer is further deposited for passivating the QDs, the corresponding hydrogen evolution rate measured for the TiO 2 /CdS/CdSe/ZnS electrode is 220 μmol/(cm 2 h) (5.4 mL/(cm 2 h)). This performance is presently the highest reported for the QD-sensitized photoelectrochemical cells.