Plasmon-enhanced photocatalysis: Ag/TiO 2 nanocomposite for the photochemical reduction of bicarbonate to formic acid
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2018.677
Plasmon-enhanced photocatalysis: Ag/TiO2 nanocomposite for the photochemical reduction of bicarbonate to formic acid Hanqing Pan1, Michael D. Heagy1 1 Department of Chemistry, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801
ABSTRACT
Plasmonic metallic nanoparticles can significantly enhance the catalytic efficiency of semiconductors via plasmonic photocatalysis. In this study, a hybrid Ag/TiO2 photocatalyst was synthesized and tested for the photochemical reduction of bicarbonate to value-added formic acid. It was found that under solar irradiation, TiO2 is not very efficient, but formate production is significantly increased with the addition of silver nanoparticles. Under 365 nm irradiation, the photocatalytic efficiency of TiO2 is enhanced, but no effect was observed with the addition of silver nanoparticles. Under solar irradiation, Ag/TiO2 reached an apparent quantum efficiency (AQE) of 7.78 ± 0.04%, the highest AQE observed so far. Enhanced photocatalytic activity is attributed to the synergistic effect between UV photon excitation of TiO2 and surface plasmon resonance enhancement. To elucidate the mechanism of plasmonenhanced photocatalysis, experiments were performed under solar irradiation and 365 nm irradiation. We propose that photo-excited electrons are transferred from above the Fermi level of the metal nanoparticle to the conduction band of the semiconductor through plasmoninduced electron transfer.
INTRODUCTION Even though TiO2 is the benchmark semiconductor for many photocatalytic applications, it is limited by its large band gap of 3.2 eV1 that requires UV photons for photo-excitation. Unfortunately, UV irradiation comprises only 5% of the solar spectrum, leaving the rest of the spectrum unused. Furthermore, TiO2 has a high charge carrier recombination rate that leads to decreased photocatalytic efficiency. To mitigate these issues, many studies have attempted to extend the optical absorption of TiO2 into the visible and to increase the photocatalytic efficiency of TiO2 through surface
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photosensitization,2,3 metal or non-metal doping,4,5 and noble metal loading.6,7,8,9,10 Recently, studies have shown that noble metal nanoparticles (NPs) such as Ag, Au, and Pt can improve the photoresponse of TiO2 in the visible region due to surface plasmon resonance (SPR), which results from the collective oscillations of the surface electrons of the metal nanoparticles when it matches the frequency of the incoming radiation.11 Previous studies have demonstrated tremendous success in enhancing the catalytic efficiency of semiconductors with plasmonic metallic nanoparticles. For example, Liu et al. synthesized Ag/TiO2 nanowires to directly convert CO2 to methanol
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