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

Abstract

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.