Size and Morphology of Suspended WO3 Particles Control Photochemical Charge Carrier Extraction and Photocatalytic Water Oxidation Activity

Abstract

Tungsten(VI) oxide (WO3) is a robust n-type semiconductor with a 2.7 eV bandgap and proven activity for photoanodic water oxidation in the context of tandem water splitting photocatalysis. Here we present a systematic investigation of three types of WO3 particles to determine the influence of particle size and morphology on photocatalytic oxygen evolution, optical properties, energetics, and photocurrent generation. Nanodots (32 ± 16 nm), nanoplates (476 ± 98 nm by 58 ± 16 nm), and WO3 microcrystals (~2 μm) for the study were synthesized by calcination of WO3 powders or by hydrolysis of Na2WO4, followed by calcination. All samples crystallize in the monoclinic rhenium trioxide structure type and have band gaps between 2.75 and 2.87 eV. From 0.01 M aqueous NaIO4 under 610 mW cm−2 visible illumination (>400 nm), 30 mg of the WO3 dots, plates, and microcrystals evolve oxygen at 31.6, 16.5, and 2.9 μmol h−1, respectively. Photoelectrochemistry on WO3 particle films in aqueous K2SO4 at pH 3.5 confirms decreasing anodic photocurrents (25, 17.8, and 7.7 μA cm−2, respectively, at +1.0 V NHE) with decreasing particles size, and similar photoonset potentials of +0.25 V vs. NHE for all samples. This suggests that the photocatalytic activity differences among the WO3 series are controlled not by the energetics but by the kinetics of minority and majority carrier transport within the particles. The reactivity trends can be quantitatively described with the one-dimensional continuity model for charge generation, recombination, and transport.