Significant effects of mixed cations on the morphology and photochemical activities of alkali–metal-antimony (Na,K)Sb3O7

Abstract

Oxide semiconductors with mixed-valence states generally exhibit excellent optoelectronic and photochemical properties due to facile charge transfer in redox reactions. In this work, we investigate the effects of mixed alkali on the optical absorption, luminescence spectra and photocatalytic abilities of (Na_1−xK_x)Sb₃O₇ nanoparticles. All the samples are fabricated using a simple one-step hydrothermal method. The structural studies show that the largest substitution of K⁺ ions in (Na_1−xK_x)Sb₃O₇ is at x = 0.3. In hydrothermal synthesis, the mixed arrangement of K⁺ and Na⁺ in (Na_1−xK_x)Sb₃O₇ has an influence on the crystal shape of particles. NaSb₃O₇ develops into a regular cube shape. With the increase of K⁺ ions in (Na_1−xK_x)Sb₃O₇, the edges and corners of the cube are further ground off, resulting in irregularly spherical particles. This mixed-alkali antimonite belongs to a p-type indirect allowed transition semiconductor, and the optical band gap is 2.71 eV (x = 0.3). The intrinsic luminescence of NaSb₃O₇ is detected at 540 nm, which is nearly quenched in Na_0.7K_0.3Sb₃O₇. It is demonstrated that the substitution of K⁺ in NaSb₃O₇ significantly increases the photodegradation of RhB solutions. There are two types of Sb cations, i.e., Sb⁵⁺ and Sb³⁺ mixed in the structure. The improved photocatalysis is attributed to the charge mediators between Sb⁵⁺/Sb³⁺ couples. The experiment shows that co-doping cations in antimonite oxides may be one of the strategies to improve photochemical properties.