Strain Engineering of ZrO2@TiO2 Core@shell Nanoparticle Photocatalysts

Abstract

TiO2 photocatalysts can provide carbon-capture utilization and storage by converting atmospheric CO2 to green hydrogen, but the efficiency of the current photocatalysts is still too low for economical usage. Anatase TiO2 is effective in transferring the electrons and holes produced by the photoelectric effect to reactants because of its oxygen-terminated surfaces. However, the anatase TiO2 bandgap is 3.2 eV, which requires photons with wavelengths of 375 nm or less to produce electron–hole pairs. Therefore, TiO2 is limited to using a small part of the solar spectrum. Strain engineering has been used to design ZrO2@TiO2 core@shell structures with large strains in the TiO2 shell, which reduces its bandgap but maintains octahedral facets for charge separation and oxygen-terminated surfaces for the catalysis of reactants. Finite element analysis shows that shell thicknesses of 4–12 nm are effective at obtaining large strains in a large portion of the shell, with the largest strains occurring next to the ZrO2 surface. The c-axis strains for 4–12 nm shells are up to 7%. The strains reduce the bandgap in anatase TiO2 up to 0.35 eV, which allows for the use of sunlight with wavelengths up to 421 nm. For the AM 1.5 standard spectrum, electron–hole pair creation in 4 nm thick and 10 nm thick TiO2 shells can be increased by a predicted 25% and 23%, respectively. The 10 nm thick shells provide a much larger volume of TiO2 and use proportionally less ZrO2. In addition, surface-plasmon resonators could be added to further extend the usable spectrum and increase the production of electron–hole pairs many-fold.