Stress-induced bandgap-shifted titania photocatalyst for hydrogen generation

Titania (TiO2) is known to be a photocatalyst, in that when illuminated with ultra-violet (UV) light equivalent in energy to the bandgap of titania, it becomes an electrically active semiconductor and can generate enough surface energy to dissociate hydrogen and oxygen from water. However, there is very little of UV of the required wavelength in solar light, and so hydrogen generation in sunlight is extremely inefficient. We have discovered, however, that we can shift and lower the bandgap of titania into a region where it become photocatalytically active with visible light, by creating and managing tremendous pressures within the titania thin film layer. This is done by a combination of layer thickness, template geometry, and thermal mismatch between the polycarbonate substrate and the titania. Because the layer of titania is so thin, extremely high stresses can be induced, and with them, large bandgap shift. And because of the template geometry, the thin titania stays intact in spite of the large stress because of superior mechanical and chemical adhesion to the substrate. In addition, titania has a high refractive index of 2.4, and so makes for very powerful light concentration within the film.