Unveiling the role of oxygen vacancies in the photoactivity and charge dynamics of Bi 2 WO 6 -based photoanodes for glycerol photoreforming Lucas Leão Nascimento 1,2 , Rafael A. C. Souza 1,3 , Juliane Z. Marinho 1 , Ivo A. Ricardo 1,4 , Adrian M. Gardner 2 , Alexander J. Cowan 2 and Antonio Otavio T. Patrocinio 1,5 * 1 Laboratório de Fotoquímica e Ciência dos Materiais, LAFOT-CM Instituto de Química, Universidade Federal de Uberlândia, 38400–902, Uberlândia, MG, Brazil, 2 Stephenson Institute for Renewable Energy and Department of Chemistry, University of Liverpool, UK, 3 Faculdade De Ciências Exatas e Tecnologia–FACET, Departamento de Ciências Exatas, Universidade Federal de Grande Dourados, 79084–970, Dourados, MS, Brazil, 4 Universidade Save, Faculdade de Ciências Naturais e Exactas, Moçambique, 5 Centro de Excelência em Hidrogênio e Tecnologias Energéticas Sustentáveis – CEHTES, Parque Tecnológico Samambaia, 74690-631, Goiânia, GO, Brazil Photoreforming of biomass-derived waste streams into hydrogen is a promising route toward sustainable fuel production. In this context, Bi 2 WO 6 is semiconductor material with great potential, showing excellent performance for the selective glycerol photoelectroreforming. [1] However, its efficiency remains limited by poor charge separation and sluggish interfacial kinetics. [2] Herein, we report a strategy to enhance the photoactivity of Bi 2 WO 6 by engineering oxygen vacancies through an ease solvothermal synthesis route, Fig 1a, and we systematically investigate their role on the photoactivity, electronic properties and charge carrier dynamics of Bi 2 WO 6 . Structural characterization by X-ray diffraction (XRD), Fig 1b, and Raman spectroscopy confirms the preservation of the orthorhombic Bi 2 WO 6 phase after oxygen vacancies are introduced, while also revealing increased disorder and suppressed vibrational modes, consistent with lattice oxygen removal. Diffuse reflectance spectroscopy (DRS), Fig 1c, and density functional theory (DFT) simulations demonstrate a red-shifted absorption edge and bandgap narrowing from 2.9 to 2.65 eV, attributed to the formation of vacancy-induced midgap states that enhance visible light absorption. Photoelectrochemical tests in crude glycerol electrolyte show a significant improvement in photocurrent density for the oxygen-deficient BiWO(vac) sample compared to pristine Bi 2 WO 6 , Fig 1d. The BiWO (vac) photoanodes showed excellent results for crude glycerol photoreforming, which resulted in a hydrogen evolution rate of 63 µmol cm -2 h -1 with a faradaic efficiency of 91%. Glycerol conversion was found to be 24.5% at pH = 6 with 85% selectivity toward formic acid production. Transient absorption spectroscopy (TAS) under operando conditions reveals that, under applied bias, BiWO(vac) exhibits enhanced signal intensity and prolonged carrier lifetimes, consistent with effective suppression of recombination due to shallow electron trapping at the oxygen vacancy sites. Power-law decay fitting of TAS kinetics yielded α-values of 1.53 for BiWO(vac) and 0.19 for pristine Bi 2 WO 6 , indicating improved transport kinetics. Time-resolved transient photocurrent (TPC) measurements further validate these findings, showing a delayed extraction peak in the millisecond range, corroborating the spectral dynamics observed in TAS. Together, these results confirm that oxygen vacancies serve as functional defects that modulate the charge carrier behaviour, improving both light harvesting and interfacial charge dynamics. This study provides mechanistic insight into defect engineering strategies for Bi 2 WO 6 -based systems and highlights the potential of oxygen vacancy modulation to improve solar-to-fuel conversion efficiency using waste-derived feedstocks.
References 1. L.L. Nascimento, J.Z. Marinho, A.L.R. dos Santos, A.M. de Faria, R.A.C. Souza, C. Wang, A.O.T. Patrocinio. Appl. Catal., A, 646 (2022) 118867. 2. B. Moss, H. Le, S. Corby, K. Morita, S. Selim, C. Sotelo-Vazquez, Y. Chen, A. Borthwick, A. Wilson, C. Blackman, J.R. Durrant, A. Walsh, A. Kafizas.J. Phys. Chem. C, 124 (2020) 18859-18867.
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