Adsorption and gas-sensing characteristics of a stoichiometric α-Fe2O3 (0 0 1) nano thin film for carbon dioxide and carbon monoxide with and without pre-adsorbed O2

Abstract

Herein, for the first time, the adsorption and gas-sensing characteristics of the CO₂ and CO molecules on a stoichiometric α-Fe₂O₃ (0 0 1) nano thin film with and without pre-adsorbed O₂ molecules have been studied using the density functional theory (DFT) method. Without pre-adsorbed O₂ molecules, the CO₂ molecule plays the role of an acceptor and obtains electrons from the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film. For the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film system, the CO₂ molecule also plays the role of an acceptor. However, less number of electrons are transferred to the CO₂ molecule as compared to the pre-adsorbed O₂ molecules. Different from the CO₂ molecule, the CO molecule always plays the role of a donor for the α-Fe₂O₃ (0 0 1) nano thin film system with and without pre-adsorbed O₂. The theoretical results verify that the CO molecule can react with the lattice oxygen or adsorbed oxygen of the α-Fe₂O₃ (0 0 1) nano thin film. The electrons transferred to the stoichiometric α-Fe₂O₃ (0 0 1) nano thin film from the CO molecule or newly formed CO₂ molecule are more than that transferred to the O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film. For the stoichiometric or O₂ pre-adsorption α-Fe₂O₃ (0 0 1) nano thin film, the CO₂ and CO molecules exhibited opposite behaviors of charge transformation. In addition, pre-adsorbed O₂ molecules displayed competitive adsorption with the CO₂ or CO molecule. The pre-adsorbed O₂ molecules hinder electron transfer to the CO₂ molecules from the α-Fe₂O₃ (0 0 1) nano thin film or hinder electron transfer to the α-Fe₂O₃ (0 0 1) nano thin film from the CO molecule. Theoretical results demonstrate that the surface of α-Fe₂O₃ materials (0 0 1) could be prepared for use as adsorbents or gas sensors for CO₂ and CO molecules. Their structures are stable after CO₂ molecules are adsorbed or after the reaction of CO molecules with the lattice oxygen or adsorbed oxygen of the α-Fe₂O₃ (0 0 1) nano thin film.