Preparation of phosphated Zr-doped TiO2 exhibiting high photocatalytic activity through calcination of ligand-capped nanocrystals

Abstract

We incorporated phosphate and pyrophosphate species into the surface lattice of Zr-doped TiO2 via calcination (>550 degrees C) of triocylphosphine oxide (TOPO)-capped nanocrystals that had been prepared using a non-hydrolytic sol-gel method. The phosphated Zr-doped TiO2 calcined at 550-750 degrees C exhibited 2.88-5.28-fold higher photocatalytic activities for decomposition of bisphenol A (BPA) than did P25. Moreover, the sample prepared through calcination at 950 degrees C performed an extraordinary activity which was 40.3 times higher than that of P25. The high surface reactivities were resulted from co-doing of TiO2 surface with Zr4+ and phosphate/pyrophosphate species. Calcination enhanced the reactivity at elevated temperatures because it changed the microstructures and surface properties of phosphated Zr-doped TiO2. The average crystallite size increased dramatically from 10.2-10.8 to 16.1 nm when the temperature increased from 750 to 950 degrees C. This change was associated with the formation of pyrophosphate species through condensation of the concentrated phosphate species. Dehydration resulted in substantial amounts of oxygen vacancies (O-M/M-S = 1.38) and Ti3+ ions in the surface layers of the phosphated Zr-doped TiO2 calcined at 950 degrees C. The dehydrated TiO2 displayed high affinity toward BPA with a maximum adsorption capacity of 5.35 mg/g. The appearance of signals for Ti4+-O-2(-) in the EPR spectrum of the modified TiO2 in the dark indicated its high capability for the chemisorption of O-2. In addition, the remarkable decrease in the intensities of trapped electrons in the sample under an O-2 atmosphere revealed that efficient charge transfer occurred from the trapped sites to the electron scavenger. Thus, the phosphated Zr-doped TiO2 calcined at high temperatures exhibit improved adsorbability toward reactants and enhanced interfacial charge transfer, resulting in outstanding photocatalytic activity. (C) 2009 Elsevier B.V. All rights reserved.