Tapping Mode Atomic Force Microscopy Studies of the Photoreduction of Ag⁺ on Individual Submicrometer TiO₂ Particles

In this paper we describe an experimental procedure that can be used to study the photoreduction of metal ions on the surfaces of individual TiO 2 particles. The submicrometer-sized particles are isolated on porous membranes, characterized by atomic force microscopy (AFM), removed from the microscope, exposed to AgNO 3 solution and light, and reimaged. Photoreduction of Ag + produces 5−50 nm particles of Ag 0 distributed over the surfaces of the TiO 2 particle. This procedure allows the spatial distribution of the chemical reactivity of particle surfaces to be mapped at the nanometer scale resolution of the atomic force microscope. Using different excitation wavelengths (366, 400 and ∼600 nm), we demonstrate by AFM that the photoreduction of Ag on single anatase or rutile particles is consistent with the differences in the band gaps of the bulk samples. We also show that the Ag 0 particles are not uniformly distributed over the TiO 2 crystallite surfaces implying that a closer study of the unreacted particle surfaces may reveal insights into the nanometer-scale structures that are responsible for variations in photoactivity of TiO 2 samples of different microstructure or composition.