Contrasting transient photocurrent characteristics for thin films of vacuum-doped “grey” TiO^sub 2^ and “grey” Nb^sub 2^O^sub 5

• Published in: Applied catalysis. B, Environmental Vol. 237; p. 339
• Main Authors: de Araújo, Moisés A, Gromboni, Murilo F, Marken, Frank, Parker, Stephen C, Peter, Laurence M, Turner, Josh, Aspinall, Helen C, Black, Kate, Mascaro, Lucia H
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier BV 05-12-2018
• Subjects: Catalysis; Coloration; Electrochemistry; Electrodes; Electromagnetic absorption; Electron mobility; Electron transfer; Heat treatment; Light; Light intensity; Luminous intensity; Niobium oxides; Organic chemistry; Oxide coatings; Oxygen; Photoelectric effect; Photoelectric emission; Photosynthesis; Pyrolysis; Thin films; Titanium dioxide; Vacancies; Vacuum
• ISSN: 0926-3373; 1873-3883

Photo-catalytic performance for oxide films, here for inkjet-printed TiO2 (ca. 1 μm thickness on FTO) and for spray-pyrolysis-coated Nb2O5 (ca. 1 μm thickness on FTO), is affected by oxygen vacancies that form during vacuum-heat treatment at 550 °C. The effects of the oxygen vacancies are associated with formation of Ti(III) and Nb(IV) sites, respectively, and therefore optically visible as “grey” coloration. Photo-electrochemical light-on-off transient experiments are performed in the limit of thin film photoanodes, where front and back illumination result in the same photo-current responses (i.e. with negligible effects from internal light absorption gradients). It is shown that generally the magnitude of photo-currents correlates linearly with light intensity, which is indicative of dominant “photo-capacitive” behaviour. At an applied voltage of 0.4 V vs. SCE (in the plateau region of the photo-current responses) the potential and also the pH (going from 1.0 M KOH to 0.1 M HClO4 in the presence of methanol quencher) have no significant effect on photo-currents; that is, surface chemical/kinetic effects appear to be unimportant and interfacial hole transfer may be rate limiting. Under these conditions (and based on a simplistic mechanistic model) changes in photo-currents introduced by oxygen vacancy doping (detrimental for TiO2 and beneficial for Nb2O5) are assigned primarily to changes in electron mobility.