Characterization of novel Ag on TiO2 films for surface-enhanced Raman scattering

Novel Ag on TiO2 films are generated by semiconductor photocatalysis and characterized by ultraviolet-visible (UV/Vis) spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM), as well as assessed for surface-enhanced Raman scattering (SERS) activity. The nature and thickness of the photodeposited Ag, and thus the degree of SERS activity, is controlled by the time of exposure of the TiO2 film to UV light. All such films exhibit the optical characteristics (lambda(max) congruent with 390 nm) of small (< 20 nm) Ag particles, although this feature becomes less prominent as the film becomes thicker. The films comprise quite large (> 40 nm) Ag islands that grow and merge with increasing levels of Ag photodeposition. Tested with a benzotriazole dye probe, the films are SERS active, exhibiting activity similar to that of 6-nm-thick vapor-deposited films. The Ag/TiO2 films exhibit a lower residual standard deviation (approximately 25%) compared with Ag vapor-deposited films (approximately 45%), which is, however, still unacceptable for quantitative work. The sample-to-sample variance could be reduced significantly (< 7%) by spinning the film during the SERS measurement. The Ag/TiO2 films are mechanically robust and resistant to removal and damage by scratching, unlike the Ag vapor-deposited films. The Ag/TiO2 films also exhibit no obvious loss of SERS activity when stored in the dark under otherwise ambient conditions. The possible extension of this simple, effective method of producing Ag films for SERS, to metals other than Ag and to semiconductors other than TiO2, is briefly discussed.