Relationship between interlayer hydration and photocatalytic water splitting of A′1−xNaxCa2Ta3O10·nH2O (A′=K and Li)

Partial replacement of alkaline metals in anhydrous KCa2Ta3O10 and LiCa2Ta3O10 was studied to control interlayer hydration and photocatalytic activity for water splitting under UV irradiation. A′1−xNaxCa2Ta3O10·nH2O (A′=K and Li) samples were synthesized by ion exchange of CsCa2Ta3O10 in mixed molten nitrates at 400 °C. In K1−xNaxCa2Ta3O10·nH2O, two phases with the orthorhombic (C222) and tetragonal (I4/mmm) structures were formed at x⩽0.7 and x⩾0.5, respectively. Upon replacement by Na+ having a larger enthalpy of hydration (ΔHh0), the interlayer hydration occurred at x⩾0.3 and the hydration number (n) was increased monotonically with an increase of x. Li1−xNaxCa2Ta3O10·nH2O showed a similar hydration behavior, but the phase was changed from I4/mmm (x<0.5, n∼0) via P4/mmm (x∼0.5, n∼1) to I4/mmm (x∼1.0, n∼2). The photocatalytic activities of these systems after loading 0.5 wt% Ni were quite different each other. K1−xNaxCa2Ta3O10·nH2O exhibited the activity increasing in consistent with n, whereas Li1−xNaxCa2Ta3O10·nH2O exhibited the activity maximum at x=0.77, where the rates of H2/O2 evolution were nearly doubled compared with those for end-member compositions (x=0 and 1).