Plasmon-Enhanced Photocatalytic CO2 Reduction Using Colloidal Nanocrystals

Abstract

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 CO2 reduction technologies suffer from high overpotentials and low selectivity, producing a mixture of carbon monoxide, methane, ethylene, formate, methanol, and others. 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.1–3 Plasmon decay generates an excited “hot” 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.4–7 The electron dynamics in an irradiated plasmonic nanoparticle can alter the electronic coupling with surface adsorbed CO2 and reaction intermediates, thereby changing the binding energy of these species and the catalytic properties of the plasmonic metals. 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 coupled to a light source and 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. (1) Thomann, I.; Pinaud, B. A.; Chen, Z.; Clemens, B. M.; Jaramillo, T. F.; Brongersma, M. L. Nano Lett. 2011, 11, 3440–3446. (2) Ingram, D. B.; Linic, S. J. Am. Chem. Soc. 2011, 133, 5202–5205. (3) Gao, H.; Liu, C.; Jeong, H. E.; Yang, P. ACS Nano 2012, 6, 234–240. (4) Mukherjee, S.; Zhou, L.; Goodman, A. M.; Large, N.; Ayala-Orozco, C.; Zhang, Y.; Nordlander, P.; Halas, N. J. J. Am. Chem. Soc. 2014, 136, 64–67. (5) Mubeen, S.; Lee, J.; Singh, N.; Krämer, S.; Stucky, G. D.; Moskovits, M. Nat. Nanotechnol. 2013, 8, 247–251. (6) Naya, S.; Teranishi, M.; Isobe, T.; Tada, H. Chem. Commun. 2010, 46, 815–817. (7) Lee, J.; Mubeen, S.; Ji, X.; Stucky, G. D.; Moskovits, M. Nano Lett. 2012, 12, 5014–5019.