(617ea) Hydroxyl Radical Scavenging Activity of Silver Nanoparticles: Enhancing Photocatalytic Reduction and Decreasing Photocatalytic Oxidation Rate

Metallic nanoparticles assisted photocatalysis has been emerging to improve the efficiency of many important processes like water splitting, reduction of CO₂ with H₂O to form hydrocarbon fuels, and degradation of organic molecules.¹ The efficiency of photocatalytic materials can be influenced in the presence of plasmonic metallic nanoparticles due to both radiative energy and electron transfer among them. In this study, the catalytic efficiency of TiO₂ in the presence of silver nanoparticles is evaluated using one reduction and one oxidation reaction. We observed that the presence of Ag nanoparticles (~50nm) with TiO₂(~30nm) significantly enhance the important photocatalytic reduction like hydrogenation of CO₂ derived bicarbonate to produce value-added chemicals such as formate in comparison to same quantity of TiO₂ alone under the solar light (solar simulator, 1.5 AM). A high yield of formate (production rate 160 micromol/gm TiO₂.hr.), was achieved after reducing sodium bicarbonate in glycerolâ??water solution using Silver-TiO₂ nanostrctures. However, the same photocatalytic system combining silver and TiO₂ nanostructures adversely affect the degradation/ oxidation rate of Rhodamine B one of the important industrial pollutants. Further experiments with terephthalic acid (a probe for hydroxyl radical) reveal that presence of silver nanoparticles decrease TiO₂ induced hydroxyl radical, which is required for photocatalytic oxidation of Rhodamine B. On the other hand, scavenging hydroxyl radical may have favorable effect on formate production as strongly oxidizing hydroxyl radicals may oxidize formate to produce â?¢CO2^â??, hence decreasing overall yield.² To understand the mechanism better, we studied the Ag nanoparticle/TiO2-catalyzed photooxidation of Rhodamine B under two different excitation wavelengths, one corresponding to the bandgap of TiO₂ (365 nm) and other corresponding to the plasmonic peak of Ag nanoparticles (450 nm). This enables us to separate processes associated with the plasmon resonance from those associated with electronic transitions in the TiO₂ itself. This study indicates that plasmonic effect may not be important for the scavenging of hydroxyl radical by silver nanoparticles. From this study, we conclude that adding metallic nanoparticles with TiO₂ does not always have unequivocal benefits. In general, understanding of charge transfer among metallic nanoparticles and photocatalysts under different geometric configuration can provide useful mechanistic information that may be used in the improvement of photocatalytic systems for the reduction of CO₂ also stored in the form of carbonates.

1. Hou, W.; Cronin, S. B., A Review of Surface Plasmon Resonance-Enhanced Photocatalysis. Advanced Functional Materials 2013, 23 (13), 1612-1619.

2. Molinari, A.; Samiolo, L.; Amadelli, R., EPR spin trapping evidence of radical intermediates in the photo-reduction of bicarbonate/CO2 in TiO2 aqueous suspensions. Photochemical & Photobiological Sciences 2015, 14 (5), 1039-1046.