Photocatalytic Degradation of Organic Molecules On Visible-Light-Active Sn(II)-Containing Photocatalysts

Development of visible light active photocatalysts is a challenge we need to overcome to harvest solar energy effectively considering our need for clean fuels. Doping of d0 (TiO2) or d10 (ZnGa2O4) oxides to make them absorb visible light is inefficient because of cation mid-gap dopant states or due to inhomogeneous anion doping. Thus, these doped oxide catalysts generally show lower UV light activity compared with their un-doped counterparts because of this new presence of dopant recombination centers and also due to poor hole mobilities. To offset this, in this presentation, we will explore different ways to surface-modify native oxide and thereby red-shift their absorbance edge while not losing efficiency. Firstly, we focus on the visible light photocatalytic activity of Sn(II)-TiO2. The catalysts are prepared from a simple reaction of titanium butoxide and several tin precursors at 80oC in aqueous solutions. Samples synthesized with SnCl2 have lower band gaps (red-shifted to the visible region) with respect to anatase TiO2 attributed to the presence of surface Sn2+ species comprised of 5s orbitals. The catalysts are isostructural to anatase TiO2 even at the highest loadings of Sn2+. When the precursor is changed to SnCl4, rutile is the predominant phase obtained but no reduction in the band gap is observed further corroborating that the earlier observed red-shift is due to Sn2+-5s orbitals. The majority of surface cation sites are tin, with a fraction of tin also incorporated in the bulk particles. The experiments also indicate the presence of chlorine in the samples, also influencing the optical and catalytic properties as confirmed by comparison to materials prepared using bromide precursors. These catalysts can photocatalytically degrade of organic molecules with rates higher than the standards (P25 TiO2) and also evidenced from the generation of hydroxyl radicals using visible light. The analysis of the degradation of rhodamine B dye by a de-ethylation mechanism using multivariate chemometric methods will also be presented to explain the need to better understand a common test reaction for photocatalytic activity measurements but often incorrectly interpreted. Based on the successful results observed with Sn(II)-titania, this protocol is then extended to incorporate Sn2+ with other oxide semiconductors to prepare photocatalysts with interesting electronic properties. We chose a d10 photocatalyst, ZnGa2O4 previously shown to be active for CO2 reduction to methane and water splitting, albeit in UV light as its band gap is 4.5 eV. In this case, the presence of a surface SnOx species containing Sn2+ enhances both visible light and UV light activity unlike traditional cation of anion doped titania, wherein the new dopant states act as mid-gap electron traps. This improved activity is due to improved hole mobilities observed by running reactions conducted in the presence of scavengers. The results here expand the materials available for efficient visible light photocatalysis and suggest that an array of novel photocatalysts may be obtained from other d0 or d10 oxide precursors upon the presence of surface modified species.