Oxidative Energy Storage Ability of a TiO₂−Ni(OH)₂ Bilayer Photocatalyst

Abstract

A TiO2-coated indium tin oxide electrode was further coated with Ni(OH)2 by electrodeposition to obtain a TiO2-Ni(OH)2 bilayer film. Upon irradiation of the bilayer film with UV light in a pH 10 buffer, the Ni(OH)2 layer was oxidized, and it turned from colorless to brown; oxidative energy was stored in the layer. The potential of the oxidative energy thus stored was about +0.7 V versus Ag|AgCl. The stored energy could be reversibly taken out of the film by chemical and electrochemical means. The photooxidized Ni(OH)2 was rereduced by alcohols, aldehydes, phenol, I-, H2O2, formate, and acetone.

Metal Oxides and Hydroxides as Rechargeable Materials for Photocatalysts with Oxidative Energy Storage Abilities

“…3(a)]. Although the potential during the discharging (+0.4-+0.5 V) was more negative than that of Ni(OH) 2 (+0.5-+0.6 V), 11,12 Co(OH) 2 can also store oxidative energy. Then photocatalytic remote energy storage (gap = 12.5 µm) in Co(OH) 2 was examined (6 h).…”

“…Then we prepared a NiO film on a TiO 2 film to examine direct photoelectrochemical charging 11 under UV light. Although the NiO film was not charged in air (>80% RH), an increase in absorbance, which suggested oxidation of NiO, was observed in a pH 10 carbonate buffer, although the rate was 10-fold lower than that for Ni(OH) 2 .…”

“…In order to retain its useful effects even after dark, we have developed photocatalysts with energy storage abilities. [3][4][5][6][7][8][9][10][11][12][13][14][15] They can store surplus charges from photocatalysts in rechargeable materials during the day, and the stored charges are used to carry on redox reactions during the night.…”

“…Those photocatalysts with reductive energy storage abilities retain some photocatalytic effects based on cathodic reactions, such as anti-bacterial 6 and anticorrosion 3 effects, even after dark. We also used Ni(OH) 2 , [11][12][13][14] which undergoes redox reactions at positive potentials, as a rechargeable material to develop photocatalysts with oxidative energy storage abilities [ Fig. 1(a)].…”

“…They persist even after dark some effects based on oxidative reactions, such as mineralization of formaldehyde to CO 2 13 and oxidative removal of other harmful organic compounds like phenol. 11 However, their oxidation abilities are limited by the redox potential (ca. +0.7 V vs. Ag«AgCl at pH 10) and stability (pH > 7) of Ni(OH) 2 .…”