Transition metal oxides are amongst the most widely studied materials for solar water oxidation owing to their earth abundance, good aqueous stability and facile syntheses.¹ Understanding the dynamics of the photogenerated charge carriers in these materials is key to highlighting properties that need to be targeted to improve water splitting performance. With a narrower band gap than TiO₂, tungsten trioxide (WO₃) can absorb a larger proportion of the solar spectrum and has a deeper valence band energy to provide a large thermodynamic driving force for water oxidation.^2-3 WO₃ is also often reported to have high electrical conductivity, leading to its frequent implementation as an electron transporting layer in various heterojunction photoanodes.^2-4 This high conductivity is considered to be a result of a large density of charge carriers, caused by intrinsic oxygen vacancies which can act as n-type dopants.⁵ However, deviations from stoichiometry have also been suggested to introduce chemical defects that can result in increased trapping of charges which, rather than boost performance, can often introduce additional recombination pathways.⁶

Using transient diffuse reflectance spectroscopy and transient photocurrent measurements, I will discuss the dynamics of the photogenerated charges in WO₃ photoanodes, synthesised by chemical vapour deposition (CVD). The synthesis method generates heavily doped monoclinic WO_3-x needles, which are annealed in air at elevated temperature to remove most of the oxygen vacancies. These photoanodes exhibit an early photocurrent onset and a high faradaic efficiency (>85%) for water oxidation. Compared to other transition metal oxide photoanodes, we observed rapid water oxidation (>1 ms) but found the rate of electron extraction is significantly slower (>10 ms). We investigated the effect of oxygen vacancy states on electron transport and found that electron trapping in the needles is significant, proposing a trap-mediated mechanism of electron transport to the back contact. We then used ultrafast transient absorption spectroscopy to examine the bias dependence on bulk recombination processes. Finally, we altered the oxygen vacancy content to determine the overall effect of these defects on charge transport and performance.

[1] G. Wang, et al. Energy Environl Sci, 2012, 5, 6180–6187.

[2] Z. Huang, et al. Advanced Materials, 2015, 27 (36), 5309-5327.

[3] X. Liu, et al. PCCP, 2012, 14 (22), 7894-7911.

[4] C.X. Kronawitter, et al. Energy Environl Sci 2011, 4 (10), 3889-3899.

[5] T. Zhu, et al. ChemSusChem, 2014, 7, 2974-2997

[6] M.B. Johansson, et al. J Phys Condens Matter, 2016, 28, 475802.