Characterization of TiO2 photocatalysis in the gas phase as a photoelectrochemical system: Behavior of salt-modified systems

Photocatalytic degradation rates of 2-propanol to acetone were examined by using TiO2 films modified with Pd stripes whose interval was varied from 25 mum to 5 mm, to characterize their photoelectrochemical behavior in the ga s phase. Even though the ratio of Pd area to bare TiO2 area was kept consta nt (1:1), the photocatalytic activity increased as the stripe interval was decreased, in the case where a certain amount of NaCl had been deposited on the Pd-modified TiO2. This is indicative of the formation of photoelectroc hemical micro-cells in the vicinity of the Pd deposited on the TiO2 thin fi lm in the gas phase. The width of the micro-cell was, for example, estimate d to be less than or equal to 250 mum when 0.2,mug cm(-2) of NaCl had been deposited on the sample film. The micro-cell formation required adsorbed wa ter on the TiO2 surface, which should be essential for the dissociation of NaCl; the adsorbed water layer in which NaCl is dissolved should act as the electrolyte of the micro-cell. The width of the micro-cell varied from the order of mum to mm, depending strongly on the amount of NaCl (0.02-20 mug cm(-2)). On the other hand, the width of the micro-cell at the salt-free Ti O2 surface was found to be less than 25,um. Comparison between the experime ntally observed dependence and the theoretically simulated one suggests tha t ionic conductivity in the adsorbed water layer is suppressed, probably du e to electrostatic and/or chemical interaction between the ions and the TiO 2 surface.