Conversion of solar energy into liquid fuel often relies on a redox process that requires the injection of multiple photo-induced charge carriers into an adsorbate. For instance, the water oxidation half-reaction involves the injection of four photo-induced charges, and the photoconversion of CO2 to CH4 involves 8 electrons. The intermediates of these multi-electron processes typically involve highly reactive ions or radicals, which have a high probability of back reaction, causing a major bottleneck in efficient conversion. Now, Technion researchers (in collaboration with the University of Illinois) reveal that these undesirable reaction pathways can be minimized, rendering the photocatalytic reactions more efficient, when charge carriers are harvested from a multi-excitonic state of a semiconductor photocatalyst. Simply put, by transferring multiple charges, essentially simultaneously, to the adsorbate, it is possible to reduce back-reactions, diminish undesirable reaction pathways, and enhance photocatalytic efficiency.
This requires unique enhancement of light absorption by the semiconductor photocatalyst. To that task, the researchers utilize ‘plasmonic’ antenna, which is known to enhance by orders of magnitude various light-matter interactions. Here, a ‘plasmonic’ antenna comprised of Au nanoprism was employed to accomplish feasible levels of multiple carrier excitations in semiconductor nanocrystal-based photocatalytic systems. The antenna’s near-field amplifies the otherwise inherently weak bi-exciton generation in the semiconductor. The two-electron photoreduction of Pt and Pd metal precursors served as model reactions. In the presence of the plasmonic antenna, these photocatalyzed two-electron reactions exhibited enhanced yields and kinetics. This work uniquely relies on a non-linear enhancement that has potential for large amplification of photocatalytic activity in the presence of a plasmonic near-field.
The authors envision that this hitherto unexplored effect of multi-carrier excitations on semiconductor photocatalysis would be beneficial for promoting kinetically challenging reactions of high technological importance, with potential prospects for driving redox chemistry in a highly spatially controlled manner.
Figure. Illustration depicting two electron metal deposition reactions progressing exclusively on semiconductor nanoparticles that are influenced by the plasmonic antenna enchantment.
The research was carried out in the framework of Russell Berrie Nanotechnology Institute (RBNI) and the Nancy and Stephen Grand Technion Energy Program (GTEP), and funded by the I-CORE Program of the Planning and Budgeting Committee, and The Israel Science Foundation (Grant No. 152/11). Dr. Shaik thanks the Schulich and GTEP postdoctoral fellowships for their support.
The study was published in the journal Nano Letters: https://pubs.acs.org/doi/10.1021/acs.nanolett.8b01392