CeO 2 Nanostructures Enriched with Oxygen Vacancies for Photocatalytic CO 2 Reduction Abdo Hezam, † Keerthiraj Namratha, ‡ Q. A. Drmosh, ∥ Deepalekshmi Ponnamma, § Jingwei Wang, ⊥ Suchitra Prasad, ∇ Momin Ahamed, ○ Chun Cheng, ⊥ and Kullaiah Byrappa* ,# † Center for Materials Science and Technology, University of Mysore, Vijana Bhavana, Manasagangothiri, Mysuru 570006, India ‡ DOS in Earth Science, University of Mysore, Manasagangothiri, Mysuru 570006, India ∥ Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia § Center for Advanced Materials, Qatar University, P.O box 2713, Doha, Qatar ⊥ Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, People’s Republic of China ∇ Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India ○ Nanomaterials & Catalysis Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India # Adichunchanagiri University, N.H.75, B. G. Nagara, Mandya District 571448, India * S Supporting Information ABSTRACT: Synthesizing nanomaterials at the expense of solar energy and the associated energy generation have utmost significance as far as environmental sustainability is concerned. Here, sunlight-assisted combustion synthesis of a nanoscale metal oxide (CeO 2 ) is reported. The sunlight, as a clean renewable energy source, is used for the first time to initiate the exothermic combustion reaction and to introduce oxygen vacancies into the CeO 2 . The current synthesis setup controls the environmental problems of gas evolution, usually associated with the conventional method, and thus maintains the green pathway. Additionally, for comparison, CeO 2 nanoparticles are also synthesized using the conventional solution combustion method (CeO 2 -CSC). It is found that the CeO 2 synthesized using sunlight-assisted combustion (CeO 2 - SAC) possesses a smaller particle size, a higher concentration of oxygen vacancies, and a narrower band gap than the CeO 2 - CSC. Therefore, CeO 2 -SAC demonstrates higher photocatalytic performance in converting CO 2 to CH 3 OH (0.702 μmol h −1 g −1 ) than the CeO 2 -CSC (0.397 μmol h −1 g −1 ), thus pointing toward environmentally benign photocatalytic CO 2 reduction. KEYWORDS: DFT calculations, green sustainable synthesis, oxygen vacancies, mesoporous CeO 2 , selective CO 2 photoreduction 1. INTRODUCTION Solar-driven photocatalytic CO 2 reduction is a green technology to simultaneously reduce CO 2 emissions associated with fossil fuel combustion and to produce renewable hydrocarbon solar fuels, thus addressing the global challenges of worldwide warming and alternative energy needs. 1−3 Ceria (CeO 2 ) is a cheap, chemically stable, photostable, and nontoxic photocatalyst. 4 Both pristine and modified CeO 2 photocatalysts have been widely used for photocatalytic CO 2 reduction. 5−9 The photocatalytic activity of CeO 2 mostly originates from its capability of switching between Ce 4+ and Ce 3+ oxidation states, which leads to the generation of oxygen vacancies. Extrinsic doping of CeO 2 promotes the creation of oxygen vacancies and thereby enhances its photocatalytic activity. 10 Generation of oxygen vacancies as a kind of self-doping without adding any external impurities is more favorable because it can enhance the photocatalytic activity of CeO 2 without compromising its intrinsic crystal structure. 11 CeO 2 can be synthesized by various chemical and physical methods. 12 However, most of the widely used methods are energy- and/or time-consuming. Moreover, self-doping of CeO 2 by the creation of oxygen vacancies is commonly achieved by calcination, which is again energy-consuming. 11 Solution combustion is a fast, facile, and economic route to synthesize nanostructured metal oxides, alloys, and sulfides in a Received: September 23, 2019 Accepted: December 12, 2019 Published: December 12, 2019 Article www.acsanm.org Cite This: ACS Appl. Nano Mater. 2020, 3, 138-148 © 2019 American Chemical Society 138 DOI: 10.1021/acsanm.9b01833 ACS Appl. Nano Mater. 2020, 3, 138−148 This article is made available for a limited time sponsored by ACS under the ACS Free to Read License, which permits copying and redistribution of the article for non-commercial scholarly purposes. Downloaded via 54.163.42.124 on June 25, 2020 at 22:22:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.