| Summary: | New composite photocatalysts have been obtained by chemical bath deposition of CdS on top of either nanostructured crystalline ZrO<sub>2</sub> or TiO<sub>2</sub> films previously deposited on conductive glass FTO. Their morphological, photoelectrochemical and photochemical properties have been investigated and compared. Time resolved spectroscopic, techniques show that in FTO/TiO<sub>2</sub>/CdS films the radiative recombination of charges, separated by visible illumination of CdS, is faster than in FTO/ZrO<sub>2</sub>/CdS, evidencing that carrier dynamics in the two systems is different. Photoelectrochemical investigation evidence a suppression of electron collection in ZrO<sub>2</sub>/CdS network, whereas electron injection from CdS to TiO<sub>2</sub> is very efficient since trap states of TiO<sub>2</sub> act as a reservoir for long lived electrons storage. This ability of FTO/TiO<sub>2</sub>/CdS films is used in the reductive cleavage of N=N bonds of some azo-dyes by visible light irradiation, with formation and accumulation of reduced aminic intermediates, identified by ESI-MS analysis. Needed protons are provided by sodium formate, a good hole scavenger that leaves no residue upon oxidation. FTO/TiO<sub>2</sub>/CdS has an approximately 100 meV driving force larger than FTO/ZrO<sub>2</sub>/CdS under illumination for azo-dye reduction and it is always about 10% more active than the seconds. The films showed very high stability and recyclability, ease of handling and recovering.
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