Mesoporous TiO2 as scaffold for Bi-promoted photocatalysts

Photocatalytic degradation of pollutants by nanometric TiO2 is an extremely active research field.Noble metals have consistently been proposed to promote the oxide activity. Here, Bi promotion is investigated as a low cost alternative to enhance TiO2 activity by slowing down the photogenerated charge recombination. Moreover, a template synthesis using block copolymers is employed to obtain mesoporous TiO2 scaffolds with increased surface area and porosity. Three block copolymers with different micelle size (as determined by DLS analysis), are employed as structure directing agents. The copolymers are removed by combining UV and thermal treatments to avoid pore collapse while promoting oxide crystallinity. By changing polymer type and concentration, a fine modulation of pore size and total volume is obtained. Such morphological changes markedly enhance the oxide ability to promote the mineralization of methylene blue stains. The mesoporous oxides are then used as scaffolds to obtain Bi-promoted TiO2. The specificity of the Bi-mesoporous TiO2 composites with respect to traditional sol-gel TiO2 nanomaterials is studied by means of XRD, BET, HRTEM, FTIR, photocurrent measurements and zeta potential determinations. Sample photocatalytic activity is tested for the photodegradation of both methylene blue stains and formic acid. The presence of Bi increases the final mineralization of about 30% with respect to unpromoted TiO2 in both tested reactions. The highest activity is shown by the Bi-promoted mesoporous TiO2, that also shows the largest recombination time of photogenerated charge. This effect can be related to the formation of separated Bi2O3/TiO2 domains promoting an increase in charge separation.