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 siz es were studied by Ti K-edge XAFS (X-ray absorption fine structure). C ompared to the bulk TiO2 structure, a shorter Ti-O distance from surfa ce TiO2 resulting from Ti-OH bonding was observed. The XAFS spectra al so revealed an increasing disorder of the lattice with decreasing size s of the nanoparticles based on a coordination number decrease for the third-shell O atoms as well as changes in relative intensities of pre edge peaks A(1), A(2), and A(3). However, the Ti sites largely remain octahedral even in the 30 Angstrom diameter particles. These results i mply that The increasing number of surface Ti sites as well as possibl e corner defects in small nanoparticles may be the main cause of ?he u nique surface chemistry exhibited by nanoparticles of TiO2. XAFS was a lso used in monitoring-the photoreduction reaction products of Cu2+ an d Hg2+ on TiO2 nanoparticle surfaces, with or without surface adsorber s, alanine (Ala) and thiolactic acid (TLA). Ala dramatically enhanced photoreduction of Cu2+ on TiO2 nanoparticle surfaces, whereas thiolact ic 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-A la complex but was largely retained in the Hg-TLA complex when TiO2 wa s present. This may correlate with the different effects of the adsorb ers on the photoreduction of the metal. Our experimental results sugge st 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 efficienc y.