Mott Insulators: An Early Selection Criterion for Materials for Photoelectrochemical H2 Production

For the last four decades, materials based on oxides have been the primary focus for studies of photo-catalysts for hydrogen production. So far, under visible-light illumination, this approach has not been much successful; only under ultraviolet radiation have oxides shown some limited success. Therefore, it is important to understand the fundamental problem of the photo-conduction properties in oxides at an electronic level. It can be shown that due to their highly localized electronic features, oxides that are Mott insulators are not suitable for photoelectrochemical (PEC) hydrogen production. Even with considerable dopingalloying, improving photo-current in these materials is unlikely. This represents a criterion by which future metal oxide candidates should be screened. Among metal oxides, the 3d elements have greater abundance and are receiving increased attention for application of solar conversion (photovoltaic and PEC), however the issues discussed here would appear to limit their usefulness for this purpose.