Dopant-controlled morphology evolution of WO3 polyhedra synthesized by RF thermal plasma and their sensing properties

In this paper, a simple way is developed for the synthesis of Cr-doped WO3 polyhedra controlled by tailoring intrinsic thermodynamic properties in RF thermal plasma. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy are used to characterize the detail structures and surface/near-surface chemical compositions of the as-prepared products. Kinetic factors showed little effects on the equilibrium morphology of Cr-doped WO3 polyhedra, while equilibrium morphologies of WO3 polyhedra can be controlled by the thermodynamic factor (Cr doping). Set crystal growth habits of pure WO3 as an initial condition, coeffects of distortions introduced by Cr into the WO3 matrix, and a chromate layer on the crystal surface could reduce the growth rates along [001], [010], and [100] directions. The morphology evolution was turning out as the following order with increasing Cr dopants: octahedron-truncated octahedron-cuboid. 2.5 at. % Cr-doped WO3 polyhedra exhibit the highest sensing response due to coeffects of exposed crystal facets, activation energy, catalytic effects of Cr, and particle size on the surface reaction and electron transport units. By simply decorating Au on Cr-doped WO3 polyhedra, the sensing responses, detection limit, and response-recovery properties were significantly improved.