(353b) Plasmon-Enhanced Photocatalytic CO2 Reduction on Nanostructured Composite Electrodes

Using inputs of only sunlight, electricity, carbon dioxide, and water, photocatalytic CO₂ reduction could mitigate the effects of climate change caused by the burning of fossil fuels and produce valuable fuels or chemical feedstocks. However, current CO_2 reduction technologies suffer from high overpotentials and low selectivity, producing a mixture of carbon monoxide, methane, ethylene, formate, methanol, and other products.

Plasmon-assisted photocatalytic CO₂ reduction leads to greater selectivity and lower overpotentials by unlocking unique mechanistic pathways. It has been well-documented that the strongly localized near fields at the surface of plasmonic nanoparticles promote electron-hole pair generation in nearby semiconductors.^1â??3 Plasmon decay generates an excited â??hotâ? electron which can be transferred to a surface molecule for direct reduction or injected into an adjacent wide band gap catalyst, effectively limiting carrier recombination through charge separation and expanding the usable portion of the solar spectrum.^4â??7 The electron dynamics in an irradiated plasmonic nanoparticle can alter the electronic coupling with surface adsorbed CO_2 and reaction intermediates, thereby changing the binding energy of these species and the catalytic properties of the plasmonic metals. This plasmon-enhanced effect was demonstrated when optically excited surface plasmons at the roughened surface of an Ag electrode in CO₂-containing solutions produced strong enhancement of the cathodic current.^8â??9 The wide tunability of the plasmon resonance frequency with shape, size, and material confers fine control over these catalytic mechanisms, allowing for optimization of the photocatalytic performance.

To take advantage of plasmonic catalysis, nano-sized spheres, cubes, and wires of gold, silver, and copper have been synthesized as these materials exhibit strong plasmonic and electrocatalytic behavior. For the semiconducting component of the photoelectrodes, nanostructured TiO₂ co-catalysts have also been prepared. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to determine the shape and size distribution of the nanoparticles. X-ray diffraction (XRD) and optical spectroscopy were employed to characterize the electrodes. The CO₂ reduction ability of the plasmonic metal nanoparticles with and without a semiconductor co-catalyst has been tested in a custom glass photoelectrochemical cell with a quartz window that allows for front- and 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. Experiments were run both under isolation of the pressurized cell and with a constant CO­₂ flow rate. The cell was coupled to a light source and a gas chromatograph-mass spectrometer (GC-MS) that allows detection and quantification of gaseous products. Ex-situ high performance liquid chromatography (HPLC) was used to quantify liquid products.

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.

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