The heat annealing effect on the performance of CdS/CdSe-sensitized TiO2 photoelectrodes in photochemical hydrogen generation

Heat treatment was utilized to anneal the semiconductor sensitizers (CdS, CdSe and CdS/CdSe) assembled on mesoporous TiO2 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 TiO2/CdS and TiO2/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 TiO2/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 TiO2/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.