XAFS studies of surface structures of TiO₂ nanoparticles and photocatalytic reduction of metal ions

To probe the origin of the unique functions of titanium dioxide (TiO₂) nanoparticles observed in photocatalytic reactions, structures of Ti atom sites in titanium dioxide (TiO₂) nanoparticles with different sizes were studied by Ti K-edge XAFS (X-ray absorption fine structure). Compared to the bulk TiO₂ structure, a shorter Ti-O distance from surface TiO₂ resulting from Ti-OH bonding was observed. The XAFS spectra also revealed an increasing disorder of the lattice with decreasing sizes of the nanoparticles based on a coordination number decrease for the third-shell O atoms as well as changes in relative intensities of preedge peaks A₁, A₂, and A₃. However, the Ti sites largely remain octahedral even in the 30 Å diameter particles. These results imply that the increasing number of surface Ti sites as well as possible corner defects in small nanoparticles may be the main cause of the unique surface chemistry exhibited by nanoparticles of TiO₂. XAFS was also used in monitoring the photoreduction reaction products of Cu²⁺ and Hg²⁺ on TiO₂ nanoparticle surfaces, with or without surface adsorbers, alanine (Ala) and thiolactic acid (TLA). Ala dramatically enhanced photoreduction of Cu²⁺ on TiO₂ nanoparticle surfaces, whereas thiolactic acid did not affect or even hindered Hg²⁺ photoreduction. Although both surface adsorbers chelated with the metal ions in the absence of TiO₂ nanoparticles, this chelation was drastically changed in the Cu-Ala complex but was largely retained in the Hg-TLA complex when TiO₂ was present. This may correlate with the different effects of the adsorbers on the photoreduction of the metal. Our experimental results suggest that a proper balance between the affinities of the adsorber to the metal ions and to the surface Ti atoms of TiO₂ may be one of the keys in selecting a surface adsorber for enhanced photoreduction efficiency.