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 °C) were employed and the results show that appropriately elevating the temperature (to approx. 300 °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 °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 °C, which significantly decreases the performance of the co-sensitized electrode. This problem was solved by annealing first a TiO₂ /CdS electrode at 300 °C, followed by CdSe assembly and a second annealing at 150 °C. This electrode appears to have better performance than the others.