Preparation and Electronic Properties of Monolayer Tungsten Oxide on Silica Particles

We report here a preparation and an examination of the electronic properties of a semiconductor (tungsten oxide) monolayer on silica particles (0.18 pm). This material is similar to the Q-state particles reported by Henglein and co-workers [ 11, Brus and co-workers [ 21, Nozik et al. [ 31, Watzke and Fendler [ 41 and others. Our particles differ from Q-state particles that are small ( =20-100 A) in three dimensions while the layers here are small in only one dimension; thus, these layers are related to semiconductor superlattice structures [ 51. The preparation of a monolayer is possible by a controlled hydrolysis technique, which is different from other methods of preparing a supported catalyst [ 61. Controlled hydrolysis is analogous to the preparation of reverse-phase chromatographic packings [ 71 and modified electrode surface [ 81. This technique is a general method which could be applicable to the preparation of monolayers of other oxides (e.g., iron oxide [ 91) or multilayers of mixed oxides. We used two techniques to characterize the electronic properties of these particles. The bandgap of the suspended particles was measured using UV/VIS absorption spectroscopy with a solution whose refractive index .was closely matched to silica over the wavelength region of interest. In this way, light