Size and surface groups effects in decahedral anatase nanoparticles for photocatalytic applications

A large set of decahedral anatase nanoparticles with the size from 1.1 to 2.7 nm, shape approximately corresponding to maximal photocatalytic activity with S-001/S-101 = 0.55 +/- 0.14 and diverse surface composition and structure have been considered to investigate quantum size effect, positions of photogenerated electrons and holes as well as absorption of light. There was a clear quantum size effect for electronic band gap E-g inside each group of nanoparticles with constant types of surface groups and their location. Photogenerated holes and electrons were distributed over atoms of surfaces (101) and (101) + (001), respectively, in majority of cases. For three cases of dehydroxylated nanoparticles, holes and electrons were distributed in (001) and (101) facets, correspondingly, in agreement to experimental observations on micrometer-sized particles. The presence of surface states caused a decrease in E-g to 2.6 eV for hydroxylated and to 2.3 eV for dehydroxylated nanoparticles in extreme cases. Absorption of visible light was significant for dehydroxylated particles while hydroxylated nanoparticles showed little absorption due to the presence of a smaller number of admixture states inside the band gap. Our results show the way to obtaining stoichiometric anatase nanoparticulate photocatalysts with visible light activity and provide insights into UV-vis spectra for anatase prepared at different calcination temperature and different extent of surface dehydroxylation.