Density functional theory study on the electronic and water adsorption properties of N-, S- and Fe-doped TiO2 anatase (101) nanotubes for photocatalytic water splitting application

• Main Author: Enriquez, John Isaac G.
• Format: text
• Language: English
• Published: Animo Repository 2016
• Subjects: Photocatalysis; Water > Purification > Photocatalysis; Water > Analysis
• Online Access: https://animorepository.dlsu.edu.ph/etd_masteral/5358
• Institution: De La Salle University

Photocatalytic water splitting using titanium dioxide (TiO2) nanotube is a clean and sustainable method of producing hydrogen fuel. One of the possible ways to improve its efficiency is by doping with certain metals and nonmetals. In this study, a theoretical model of pristine, nitrogen-doped, sulfur-doped, and iron-doped TiO2 nanotube based on anatase (101) surface is presented. Spin unrestricted density functional theory calculations were performed to provide a detailed description of the geometries, electronic properties, water adsorption, and water dissociation reaction on pristine and doped TiO2 nanotubes. Analysis of the results suggest that doping can improve the photocatalytic properties of TiO2 nanotube in two ways: (1) lowering the band gap to induce red shift in photoresponse; and (2) lowering the activation energy of water dissociation reaction. Out of the three dopants considered, nitrogen shows the greatest promise. The induced acceptor energy levels above the valence band lowers the band gap and the hybridization of nitrogen 2p states with hydroxyl radical and hydrogen states promote lower activation energy. The optimal nitrogen doping concentration is 1.4%. Doping concentration beyond this value will change the property of the nanotube to a semimetal which will lead to lower photocatalytic efficiency. On the other hand, doping with sulfur lowers the band gap and retains the intrinsic semiconductor property of TiO2 nanotube regardless of doping concentration. However, the sulfur 2p states do not hybridize well with hydroxyl radical states which lead to lower adsorption energy and higher activation energy. Finally, doping with iron induce donor states below the conduction band which changes the property of TiO2 nanotube into a semimetal even for the lowest doping concentration considered (0.67%). However, Fe 3d states hybridized well with both water molecule and hydroxyl radical states which leads to improved adsorption energies and lower activation energy. This study shows that in evaluating potential photocatalysts, analysis of water splitting reaction is as important as the analysis of absorption spectra.