Materials 2023, 16, 1081. https://doi.org/10.3390/ma16031081 www.mdpi.com/journal/materials Review Advanced Photocatalysts for CO2 Conversion by Severe Plastic Deformation (SPD) Saeid Akrami 1, *, Tatsumi Ishihara 2,3,4 , Masayoshi Fuji 1,5 and Kaveh Edalati 2,3, * 1 Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Tajimi 507-0071, Japan 2 WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan 3 Mitsui Chemicals, Inc.—Carbon Neutral Research Center (MCI-CNRC), Kyushu University, Fukuoka 819-0395, Japan 4 Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan 5 Advanced Ceramics Research Center, Nagoya Institute of Technology, Tajimi 507-0071, Japan * Correspondence: saeidakrami91@gmail.com (S.A.); kaveh.edalati@kyudai.jp (K.E.); Tel.: +81-572-24-8110 (S.A.); +81-82-802-6744 (K.E.) Abstract: Excessive CO2 emission from fossil fuel usage has resulted in global warming and environmental crises. To solve this problem, the photocatalytic conversion of CO2 to CO or useful components is a new strategy that has received significant attention. The main challenge in this regard is exploring photocatalysts with high efficiency for CO2 photoreduction. Severe plastic deformation (SPD) through the high-pressure torsion (HPT) process has been effectively used in recent years to develop novel active catalysts for CO2 conversion. These active photocatalysts have been designed based on four main strategies: (i) oxygen vacancy and strain engineering, (ii) stabilization of high-pressure phases, (iii) synthesis of defective high-entropy oxides, and (iv) synthesis of low-bandgap high-entropy oxynitrides. These strategies can enhance the photocatalytic efficiency compared with conventional and benchmark photocatalysts by improving CO2 adsorption, increasing light absorbance, aligning the band structure, narrowing the bandgap, accelerating the charge carrier migration, suppressing the recombination rate of electrons and holes, and providing active sites for photocatalytic reactions. This article reviews recent progress in the application of SPD to develop functional ceramics for photocatalytic CO2 conversion. Keywords: functional properties; ultrafine-grained (UFG) materials; nanostructured materials; photocatalytic CO2 conversion; high-pressure torsion (HPT); oxygen vacancies; high-pressure phases; high-entropy ceramics 1. Introduction Nowadays, environmental crises, especially global warming caused by CO2 emission from burning fossil fuels and humankind activities, are considered one of the most significant challenges in the world. Reduction of CO2 to reactive CO gas or useful components and fuels, such as CH4 and CH3OH, using photocatalysts is one of the clean and new strategies, which is developing rapidly [1–3]. In photocatalytic CO2 conversion, excited electrons transfer from the valence band to the conduction band of the photocatalysts by solar irradiation and contribute to the reduction of CO2 to form desirable products, as shown in Figure 1a [3]. To perform these reduction reactions, some thermodynamic and kinetic conditions should be provided. From the viewpoint of thermodynamics, the standard potential of the reduction and oxidation reactions should be between the valence band and the conduction band of the photocatalyst [3,4]. On the other hand, from the kinetic viewpoint, the electrons should absorb the light, transfer to the conduction band, migrate to the surface of the photocatalyst, and take part in the Citation: Akrami, S.; Ishihara, T.; Fuji, M.; Edalati, K. Advanced Photocatalysts for CO2 Conversion by Severe Plastic Deformation (SPD). Materials 2023, 16, 1081. https://doi.org/10.3390/ma16031081 Academic Editor: Ovidiu Oprea Received: 31 December 2022 Revised: 22 January 2023 Accepted: 24 January 2023 Published: 26 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).