(351f) Surface Plasmon-Assisted Photoelectrochemical CO2 Reduction on Well-Defined Nanostructured Silver Electrodes

Plasmonic hot-carriers and the strong local electric fields produced by plasmon excitation may open new pathways for CO₂ reduction, resulting in increased product selectivity and lower overpotential. Plasmon decay generates “hot” electrons which can be transferred selectively to a surface molecule for direct reduction. Simultaneously, the enhanced electric fields can alter the electronic coupling with surface adsorbed molecules, thereby changing the binding energy of these species and the catalytic properties of the plasmonic metals.^1-3 Plasmon-assisted photoelectrochemical CO₂ reduction was demonstrated when optically excited surface plasmons at the roughened surface of a Ag electrode in CO₂-containing solutions produced strong enhancement of the cathodic current.^4-5

Here we reveal the mechanism of plasmonic hot-carriers interacting with CO₂ and associated adsorbates on plasmonically-active Ag nanopyramid catalysts. Photocurrent measurements and photoelectrochemical spectroscopy were performed on the plasmonic Ag nanopyramid photocatalysts in electrolyte saturated with CO₂ and Ar. We observed that the photocatalytic reaction is much stronger in the presence of CO₂ and is resonantly enhanced at a specific potential. In Ar-saturated electrolyte, no characteristic feature of photocurrent was observed. These results indicate that the hot-carriers created by the localized surface plasmon resonance effect are reacting specifically with CO₂ molecules and CO₂ reduction intermediates. Further, the photocurrent onset potential was significantly reduced with respect to the dark current. Our findings suggest that the plasmonic hot-carriers behave as an effective co-catalyst with Ag and may play a critical role in discovering a CO₂reduction process that forms readily usable hydrocarbon fuels.

Scanning electron microscopy (SEM) was used to confirm the shape, size, and distribution of the silver nanopyramids. Photocurrent measurements were performed in a custom glass photoelectrochemical cell with a quartz window that allows for front- or back-illumination of the electrode. The two electrode chambers are separated by a membrane and can achieve a gas-tight seal that allows for precise pressure measurements. The cell was coupled to a gas chromatograph (GC) that allows for detection and quantification of gaseous products. Ex-situ proton nuclear magnetic resonance spectroscopy (¹H NMR) and high performance liquid chromatography (HPLC) were used to quantify liquid products.

References
[1] A. O. Govorov, et. al., Nano Today 9, 85 (2014)
[2] R. Sundararaman, et. al., Nature Comm. 5, 5788 (2014)
[3] S. Mukherjee, et. al., J. Am. Chem. Soc. 136, 64 (2014)
[4] R. Kostecki, et. al., J. Chem. Phys. Lett. 194, 386 (1992)
[5] R. Kostecki, et. al., J. Appl. Phys.77, 4701 (1995)

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
The work is supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1106400.
Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.