Photochemical nucleation and growth of palladium on titanium dioxide films studied with electron microscopy and quantitative analytical techniques

Abstract

Transmission electron microscopy (TEM) investigations showed that millisecond UV illumination of thin polycrystalline TiO/sub 2/ films immersed in an aqueous palladium chloride solution caused the deposition of metallic Pd clusters (size range near 1 nm) on the TiO/sub 2/ surface. During prolonged illumination the existing Pd nuclei grew and the nucleation rate decayed to zero. In the steady growth state Pd photodeposition only took place on existing growth centers. These centers grew by repeated three-dimensional nucleation of 2-4-nm Pd clusters. Separate TiO/sub 2/ memory-effect experiments, in which Pd nucleation took place in the dark, showed that at least 10/sup 14/ electrons/cm/sup 2/ can be trapped in states at the TiO/sub 2/ surface. Photodeposited Pd films were found to be porous, with the result that photodeposition continued for long illumination times. The rate of photodeposition decreased in the initial stages and became constant after the deposition of the equivalent of some tens of monolayers. These observations fit the following model of Pd photodeposition. Photogenerated holes are transferred rapidly to the solution causing a negative surface charge. Pd nucleation starts when a critical concentration of electrons is built up at a certain site. During prolonged illumination electrons leave the TiO/sub 2/ surfacemore » at Pd centers while holes react at bare TiO/sub 2/ surface sites. The Pd growth centers serve as potential wells which attract photoelectrons from surrounding areas.« less