Insoo Ro

Plasmon-Enhanced Photoelectrochemical Water Splitting with Size-Controllable Gold Nanodot Arrays

Size-controllable Au nanodot arrays (50, 63, and 83 nm dot size) with a narrow size distribution (± 5%) were prepared by a direct contact printing method on an indium tin oxide (ITO) substrate. Titania was added to the Au nanodots using TiO(2) sols of 2-3 nm in size. This created a precisely controlled Au nanodot with 110 nm of TiO(2) overcoats. Using these precisely controlled nanodot arrays, the effects of Au nanodot size and TiO(2) overcoats were investigated for photoelectrochemical water splitting using a three-electrode system with a fiber-optic visible light source. From UV-vis measurement, the localized surface plasmon resonance (LSPR) peak energy (ELSPR) increased and the LSPR line width (Γ) decreased with decreasing Au nanodot size. The generated plasmonic enhancement for the photoelectrochemical water splitting reaction increased with decreasing Au particle size. The measured plasmonic enhancement for light on/off experiments was 25 times for the 50 nm Au size and 10 times for the 83 nm Au nanodot size. The activity of each catalyst increased by a factor of 6 when TiO2 was added to the Au nanodots for all the samples. The activity of the catalyst was proportional to the quality factor (defined as Q = E(LSPR)/Γ) of the plasmonic metal nanostructure. The enhanced water splitting performance with the decreased Au nanodot size is probably due to more generated charge carriers (electron/hole pair) by local field enhancement as the quality factor increases.

Plasmon-enhanced reverse water gas shift reaction over oxide supported Au catalysts

We show that localized surface plasmon resonance (LSPR) can enhance the catalytic activities of different oxide-supported Au catalysts for the reverse water gas shift (RWGS) reaction. Oxide-supported Au catalysts showed 30 to 1300% higher activity for RWGS under visible light compared to dark conditions. Au/TiO2 catalyst prepared by the deposition–precipitation (DP) method with 3.5 nm average Au particle size showed the highest activity for the RWGS reaction. Visible light is converted into chemical energy for this reaction with up to a 5% overall efficiency. A shift in the apparent activation energy (from 47 kJ mol−1 in dark to 35 kJ mol−1 in light) and apparent reaction order with respect to CO2 (from 0.5 in dark to 1.0 in light) occurs due to the LSPR. Our kinetic results indicate that the LSPR increases the rate of either the hydroxyl hydrogenation or carboxyl decomposition more than any other steps in the reaction network.

Reverse Water–Gas Shift on Interfacial Sites Formed by Deposition of Oxidized Molybdenum Moieties onto Gold Nanoparticles

We show that MoO(x)-promoted Au/SiO2 catalysts are active for reverse water-gas shift (RWGS) at 573 K. Results from reactivity measurements, CO FTIR studies, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) indicate that the deposition of Mo onto Au nanoparticles occurs preferentially on under-coordinated Au sites, forming Au/MoO(x) interfacial sites active for reverse water-gas shift (RWGS). Au and AuMo sites are quantified from FTIR spectra of adsorbed CO collected at subambient temperatures (e.g., 150-270 K). Bands at 2111 and 2122 cm(-1) are attributed to CO adsorbed on under-coordinated Au(0) and Au(δ+) species, respectively. Clausius-Clapeyron analysis of FTIR data yields a heat of CO adsorption (ΔH(ads)) of -31 kJ mol(-1) for Au(0) and -64 kJ mol(-1) for Au(δ+) at 33% surface coverage. Correlations of RWGS reactivity with changes in FTIR spectra for samples containing different amounts of Mo indicate that interfacial sites are an order of magnitude more active than Au sites for RWGS. Raman spectra of Mo/SiO2 show a feature at 975 cm(-1), attributed to a dioxo (O═)2Mo(-O-Si)2 species not observed in spectra of AuMo/SiO2 catalysts, indicating preferential deposition of Mo on Au. XAS results indicate that Mo is in a +6 oxidation state, and therefore Au and Mo exist as a metal-metal oxide combination. Catalyst calcination increases the quantity of under-coordinated Au sites, increasing RWGS activity. This strategy for catalyst synthesis and characterization enables quantification of Au active sites and interfacial sites, and this approach may be extended to describe reactivity changes observed in other reactions on supported gold catalysts.