Water oxidation and electron extraction kinetics in nanostructured tungsten trioxide photoanodes
File(s)JACS (1).pdf (715.44 KB)
Accepted version
Author(s)
Type
Journal Article
Abstract
A thorough understanding of the kinetic competition between desired water oxidation/electron extraction processes and any detrimental surface recombination is required to achieve high water oxidation efficiencies in transition-metal oxide systems. The kinetics of these processes in high Faradaic efficiency tungsten trioxide (WO3) photoanodes (>85%) are monitored herein by transient diffuse reflectance spectroscopy and correlated with transient photocurrent data for electron extraction. Under anodic bias, efficient hole transfer to the aqueous electrolyte is observed within a millisecond. In contrast, electron extraction is found to be comparatively slow (∼10 ms), increasing in duration with nanoneedle length. The relative rates of these water oxidation and electron extraction kinetics are shown to be reversed in comparison to other commonly examined metal oxides (e.g., TiO2, α-Fe2O3, and BiVO4). Studies conducted as a function of applied bias and film processing to modulate oxygen vacancy density indicate that slow electron extraction kinetics result from electron trapping in shallow WO3 trap states associated with oxygen vacancies. Despite these slow electron extraction kinetics, charge recombination losses on the microsecond to second time scales are observed to be modest compared to other oxides studied. We propose that the relative absence of such recombination losses, and the observation of a photocurrent onset potential close to flat-band, result directly from the faster water oxidation kinetics of WO3. We attribute these fast water oxidation kinetics to the highly oxidizing valence band position of WO3, thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.
Date Issued
2018-11-28
Date Acceptance
2018-11-01
Citation
Journal of the American Chemical Society, 2018, 140 (47), pp.16168-16177
ISSN
1520-5126
Publisher
American Chemical Society
Start Page
16168
End Page
16177
Journal / Book Title
Journal of the American Chemical Society
Volume
140
Issue
47
Copyright Statement
© 2018 American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, after peer review and technical editing by the publisher. To access the final edited and published work see https://dx.doi.org/10.1021/jacs.8b08852
Sponsor
The Royal Society
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/30383367
Grant Number
RSG\R1\180434
Subjects
Science & Technology
Physical Sciences
Chemistry, Multidisciplinary
Chemistry
THIN-FILMS
PHOTOGENERATED HOLES
HEMATITE PHOTOANODES
OPTICAL-PROPERTIES
BIVO4 PHOTOANODES
OXYGEN VACANCIES
WO3 PHOTOANODES
CHARGE-TRANSFER
SURFACE
OXIDE
General Chemistry
03 Chemical Sciences
Publication Status
Published
Coverage Spatial
United States
Date Publish Online
2018-11-01