Highly efficient visible-light driven photocatalysts: a case of zinc stannate based nanocrystal assemblies

Abstract

The design and discovery of green and highly efficient visible-light driven catalysts hold significant importance towards efficient harvesting, conversion and utilization of the full-spectrum solar energy. Here in this work, we report several wide band gap (>3.1 eV) semiconducting nanostructures counterintuitively showing excellent catalytic activity under solar light towards organic dye degradation. These nanostructured stannates include amorphous ZnSnO₃ nanocubes and Zn₂SnO₄–SnO₂ nanocrystal assemblies, which possess the merits of high activity, low cost, absence of toxicity, and ease of synthesis. Hydroxyl radicals (˙OH) are confirmed to be the major active species responsible for the dye degradation reactions. The catalysis tests under monochromatic light and optical characterization revealed remarkable activities in the visible light range due to populated defect states in these semiconductor nanostructures. Finally, the Zn₂SnO₄–SnO₂ hetero-junction nanocrystal assemblies produced by slow-ramping thermal decomposition demonstrated very high photocatalytic efficiency under visible light, with dye molecules almost fully degraded in 20 min. Besides more visible-light-active defect states and larger crystallite size, the coherent (hetero-epitaxial) interfaces and strong type II heterojunction interaction between the spinel Zn₂SnO₄ and rutile SnO₂ nano-grains might be the main reasons for the drastically improved photocatalytic performance of the slow-ramp Zn₂SnO₄–SnO₂.