Photoelectrochemical epitaxy of silver oxide clathrate Ag7O8M (M = NO3, HSO4) on rutile-type Nb-doped TiO2 single crystals

Silver oxide clathrate Ag7O8M (M = NO3, HSO4) compounds were synthesized photoelectrochemically on rutile-type Nb-doped TiO2 single-crystal substrates. Epitaxial crystal growth was achieved for some clathrate compositions and substrate surface orientations, where commensurate growth is possible due to lattice matching between the pseudo lattice of the clathrate Ag6O8 cages and the TiO2 surface, similar to the well-known case of epitaxial C60 growth on single-crystal substrates. Particularly for the growth of Ag7O8NO3 on Nb-doped TiO2(110), fully (111)-oriented epitaxial crystallites without any other orientations were obtained. The selectivity for Ag7O8NO3 growth and the suppression of the formation of by-products, such as O2, were found to depend on the electrode potential. The highest selectivity was obtained at +0.2 V vs. Ag in a 0.01 M AgNO3 solution. An investigation of Ag7O8(MM′) (M = NO3, M′ = HSO4) depositions from solutions with different AgNO3 and Ag2SO4 mixing ratios showed that the growth of Ag7O8HSO4 is much faster than that of Ag7O8NO3. The process of incorporating monovalent M anions into the clathrate Ag6O8 cages was identified as the rate-limiting step for the growth of silver oxide clathrate compounds.