Ferroelastic domains improve photochemical reactivity: a comparative study of monoclinic and tetragonal (Bi1−0.5xNa0.5x)(V1−xMox)O4 ceramics

Abstract

The relative reactivities of monoclinic and tetragonal BiVO₄-based ceramics are described, focusing on the contributions of ferroelastic domains and surface orientations. First, the photo-oxidation of Mn²⁺ on monoclinic BiVO₄ is shown to be domain selective, occurring on the set of domains that is not active for the photo-reduction of Ag⁺. Next, the domain and orientation dependent photochemical reduction of Ag⁺ is investigated for two samples in the co-doped family (Bi_1−0.5xNa_0.5x)(V_1−xMo_x)O₄: a monoclinic sample (x = 0.05) and a tetragonal sample (x = 0.175). Like BiVO₄, the x = 0.05 sample has ferroelastic domains that exhibit contrast in piezoresponse force microscopy (PFM) and exhibit domain specific reactivity. The x = 0.175 sample has no domains, no PFM contrast, and uniform grain reactivity. The orientation dependence of Ag⁺ reduction is similar for the two samples, with surfaces near (001) being more active than others. For any given orientation, however, the monoclinic sample is quantitatively more active than the same orientation in the tetragonal sample. These collected results are consistent with the hypothesis that monoclinic-structured BiVO₄ ceramics have enhanced photochemical activity owing to ferroelastic domain-selective reactivity, which allows for local separation of photogenerated carriers and redox reactions.