WO3 Colloids Characterized through SdFFF and FlFFF.

Tungsten trioxide (WO3) is an indirect band gap semiconductor with interesting photoelectrochemical properties. Nanocrystalline WO3 thin films are used as high performance photoanodes, whose current–voltage behavior is strongly affected by the crystal size. For this reason the nanoparticle formation, evolution and aggregation are aspects which call for a carefully investigation. In this work, colloidal suspensions of about 20-30 nm WO3 particles were obtained through a simple sol-gel. The WO3 colloidal suspensions were subjected to a controlled temperature aging process whose time evolution in terms of particle size distribution was followed by Sedimentation Field-Flow Fractionation (SdFFF) and Flow Field-Flow Fractionation (FlFFF). The experiments performed at 60 °C showed that in a few hours the initially transparent sol of WO3 particles undergo a progressive increase in mass, giving after 5 hours particles and/or aggregates of about 120 nm, expressed in terms of equivalent spherical diameter. The observed shift in particle size distribution SdFFF maxima, the broadening of the FlFFF fractograms and the SEM observations seem to suggest that primary WO3 nanocrystals formed during the initial stage tend to change their shape spontaneously becoming well-defined square particles which aggregate to form large clusters at a rate determined by the temperature set during the aging process. The particle size distribution of the suspensions were related to the photoelectrochemical properties of the WO3 particles, preparing thin films on a transparent conductive glass from the aged suspensions sampled at regular intervals. The current-voltage polarization curves recorded in a biased photoelectrolytic cell in the potential range 0-1.8 V (vs SCE) showed a diminution of the maximum photocurrent from 3.7 mA cm-2 to 2.8 mA cm-2 with aging times of 1h and 5h respectively. This loss of performance was mainly attributed to the reduction of the electroactive surface area.