Field-Enhanced Photocatalysis at Nanoscale Gaps

Modern industry heavily relies on heterogeneous catalysis that often requires thermal energy. This is costly and decrease catalyst lifetime. Instead of using high temperatures to drive chemical reactions, researchers have recently found that irradiating metal nanoparticles with visible light could increase conversion efficiency. Such metal nanoparticles can strongly interact with the incoming light and act as nanoscale antennas. Interestingly, this interaction is strongly dependent on the system geometry (size, shape and interparticle distance). Recently, the light irradiation of rhodium nanocubes has been found to improve the conversion of harmful CO2 emissions into methane, a useful chemical fuel. However, the light intensities required were 100 times higher than the average solar light intensity. It would be highly beneficial if the efficiency of this light induced reaction could be increased. Additionally, the overall mechanism of this enhancement is still under debate and would require more in-depth studies. This proposal targets the design, synthesis and characterization of metal nanoparticles separated with nanoscale gaps. Such nanoscale gaps are known to dramatically enhance the E-field inside and at the metal surface, and increase light absorption, which is a key factor for accelerating chemical reactions. Our simulations show that the proposed structures should enhance light absorption by a factor > 100 compared to the isolated Rh catalyst used in previous studies, which should lead to an increase in reaction rate of at least 100. This could potentially make such a reaction system industrially relevant. This proposal will produce novel structures and test whether very high electric fields can lead to dramatic increase in reaction rate. State-of-the-art X-ray absorption techniques will provide crucial information about the light absorption process within the different part of the catalytic nanoreactors, which is currently missing. In combination with our numerical simulations and the photocatalytic experiments, this proposal will provide a complete picture of the various mechanisms at play that leads to this exciting enhanced reaction rates.

Modern industry heavily relies on heterogeneous catalysis that often requires thermal energy. This is costly and decreases catalyst lifetime. Instead of using high temperatures to drive chemical reactions, researchers have recently found that irradiating metal nanoparticles with visible light could increase conversion efficiency. Such metal nanoparticles act as nanoscale antennas by concentrating the incoming light into nanoscale volumes, which can be used to increase chemical reaction rates. Because light-matter interaction strongly depends on the nanostructure geometry (size, shape and interparticle distance) and composition, combining different materials at the nanoscale is essential to optimize light concentration and catalytic activity. This is synthetically very demanding, especially when attempting to combine different materials, e.g. where, for example, a metal that enhances light, and another that acts as a catalyst. Thus, synthetic limitations have prevented the field from going further. This research project has developed chemical and electrochemical approaches to synthesize a variety of heterometallic and metal/Si nanoscale gap structures with an ultra-high spatial resolution, e.g. down to the sub-2 nanometer regime. We used electromagnetic simulations to screen, select, and optimize nanoscale systems with the best light enhancing properties. The final structures were composed of an optically inactive metal catalyst, which was integrated within an optically active structure, which was either metal-based or silicon-based. In both cases, we could experimentally demonstrate a significant increase in light enhancement at the metal catalyst. A state-of-the-art electron microscopy technique was used to map optical fields, e.g. light enhancement with a nanometer resolution, to confirm our electromagnetic simulations and experimental results, which were done on macro-scale samples. Our most important results so far was the ability to enhance light absorption around small metal nanoparticles located at the surface of different types of nanostructured silicon substrates. Photocatalytic activity of these substrates was investigated and we found a much higher activity when the metal nanoparticles are located within nanostructured silicon compared to a flat silicon substrate. This shows that we can use nanostructured silicon as a light enhancing platform, which is relevant for photocatalytic investigations. Because it is relatively simple to prepare nanostructured silicon in standard chemical laboratories, these advances are expected to accelerate research on light-activated chemical processes.