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.

Cadmium sulfide (CdS) was in situ synthesized onto mesoporous TiO(2) films and used as a sensitizer to fabricate a photoelectrode for hydrogen generation in visible light. The incorporation of CdS extends the optical absorption threshold of a TiO(2) electrode to visible light, enhancing the visible-light-induced photocurrent. A maximum photoconversion efficiency of 3.67% was achieved for this CdS-sensitized TiO(2) electrode under visible light illumination of 100 mW cm(-2). The hydrogen generation rate obtained at conditions of maximum efficiency is 95.5 µmol cm(-2) h(-1). To the best of our knowledge, the hydrogen generation rate is the highest among those reported for a photoelectrochemical cell under the illumination of visible light.

The band-edge levels of CdS-, CdSe-, and CdS/CdSe-sensitized TiO2 electrodes were determined by ultraviolet photoelectron spectroscopy (UPS) to explore the reason leading to the high performance of the TiO2/CdS/CdSe electrode. The obtained UPS results show the stepwise energy level in the TiO2/CdS/CdSe electrode, indicating energy level alignment occurrence between CdS and CdSe in the TiO2/CdS/CdSe. Time-resolved photoluminescence and open-circuit photovoltage decay experiments reveal that the photogenerated electrons in the TiO2/CdS/CdSe have higher injection efficiency, but lower recombination rate to the electrolyte, attributable to the stepwise structure of band-edge levels constructed by the effect of the energy level alignment.

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Heat treatment was utilized to anneal the semiconductor sensitizers (CdS, CdSe and CdS/CdSe) assembled on mesoporous TiO(2) films to enhance the performance of the photoelectrodes in a process of photoelectrochemical hydrogen generation. Various annealing temperatures (150, 300 and 400 degrees C) were employed and the results show that appropriately elevating the temperature (to approx. 300 degrees C) can increase the crystallinity of the CdS and CdSe, improve the charge transport characteristic of a photoelectrode and, therefore, lead to a higher performance of the TiO(2) /CdS and TiO(2) /CdSe electrodes. However, an over-annealing temperature (400 degrees C) may cause serious oxidation and/or decomposition of the sensitizers which is unfavorable to the photoelectrode. For the co-sensitized electrode, counter-diffusion of CdS and CdSe happens at the CdS/CdSe interface when the TiO(2) /CdS/CdSe electrode was co-annealed at 300 degrees C, which significantly decreases the performance of the co-sensitized electrode. This problem was solved by annealing first a TiO(2) /CdS electrode at 300 degrees C, followed by CdSe assembly and a second annealing at 150 degrees C. This electrode appears to have better performance than the others.