Band Gap Engineering of Semiconductors and Ceramics by Severe Plastic Deformation for Solar Energy Harvesting

The electronic structure of the band gap determines the amount of light and its wavelength that can be absorbed by a semiconductor. Most photocatalysts are semiconductor materials, therefore, the state-of-art band gap engineering plays an important role in the efficiency of the photocatalytic reactions. Metal oxides are the most abundant semiconductors in the Earth's crust, most of which possess large band gaps. In order for oxides to be able to absorb solar energy, the band gap must be reduced. In this review, band gap of high-pressure phases of some well-known metal oxides like TiO2, ZnO, and Y2O3 are studied, which are known to be unstable at ambient pressure while having the advantage of narrow band gaps. High-pressure torsion (HPT) is introduced as an effective method for stabilization of high-pressure phases, and these phases show good activity under visible light for water splitting hydrogen or oxygen production, and/or CO2 reduction reactions. High-entropy oxides and oxynitrides are another group of materials that will be introduced for effective photocatalytic properties, synthesized by the HPT method.