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

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.