Mechanisms for the Growth of Thin Films of WO3 and Bronzes from Suspensions of WO3 Nanoparticles

Abstract

The electrochemical deposition of tungsten oxide thin films from a suspension of nanoparticles is studied with the help of the electrochemical quartz crystal microbalance (EQCM). Aqueous tungsten oxide suspensions are prepared from acidic solutions of sodium tungstate (Na2WO4). Tungsten oxide films are deposited from hydrated tungsten oxide nanoplates formed in the suspension. Two different mechanisms are responsible for the electrochemical deposition of the nanoplates. One is a faradaic process at cathodic potentials due to proton reduction and subsequent transfer to the tungsten oxide particles. The proton transfer mechanism is characterized by high deposition rate and reversibility. A second, non-faradaic, mechanism consists of the physical attachment of WO3 nanoplates on the electrode surface. This process requires electrode potential cycling between proton reduction potentials and more anodic values. It is proposed that potential cycling allows to attain a negative charged surface on the nanoplates and a positive charged surface on the electrode, favoring interaction and deposition. The faradaic process predominates in potential step deposition, whereas the non-faradaic predominates in potential cycling deposition. Both mechanisms give rise to films with different morphology and composition. Aging of the suspension decreases the growth rate due to formation of heavier, larger tungsten oxide particles.