The Role of Hot Electrons in Plasmon-Driven Chemical Reactions

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Hot electrons refer to highly energetic electrons generated when metal nanoparticles, particularly those made of noble metals like gold, silver, or copper, are excited by light through a process called localized surface plasmon resonance (LSPR). This process occurs when light interacts with the conduction electrons on the surface of metal nanoparticles, causing these electrons to oscillate collectively. The energy absorbed from light excites the electrons into high-energy, or “hot,” states.

The role of hot electrons is indeed very promising, especially in plasmonic photocatalysis, which relies on the light-induced generation of energetic electrons responsible for the initiation or acceleration of chemical reactions across the surface of nanostructured metals. In this respect, it indeed works effectively in several reactions involving metal nanoparticles such as gold, silver, or copper capable of supporting localized surface plasmon resonance (LSPR). This reaction efficiency depends on the generation, transfer, and use of hot electrons and the nanostructured environment where these processes are occurring. The current article will discuss the role of hot electrons in plasmon-driven chemical reactions, including the mechanism involved and how this process is being harnessed for innovative catalytic applications.

Mechanism of Hot Electron Generation

The absorption of photons by metal nanoparticles generates hot electrons, which results in a so-called LSPR. The incoming light energy excites the electrons of the conduction band of the metal nanoparticles to higher energy levels, or, using other terminology, it creates “hot” carriers. Depending on the specific excitation and the energy distribution within the nanoparticle, these “hot” carriers can either be electrons or holes.

Once excited, hot electrons are usually short-lived. They may be used to drive chemical reactions if this energy is effectively transferred to the adsorbed molecules on the nanoparticle surface. This involves either direct excitation, corresponding to the process when a hot electron jumps to the lowest unoccupied molecular orbital of an adsorbed molecule, or indirect excitation via vibrational or other intermediary states.

These plasmonic nanoparticles act as antennas, which transform light into these highly energetic carriers the latter, in turn, will participate in chemical transformations afterward. At the same time, LSPR is very important due to the local enhancement of the electromagnetic field around the nanoparticle, allowing stronger interactions between the reactants and a metal surface.

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Surface-Enhanced Chemical Reactions

One of the most striking features of plasmonic nanoparticles is their ability to confine light at volumes below the diffraction limit, therefore developing intensive electromagnetic fields, which can create “hot spots” on their surfaces. Such hot spots are defined as those regions where the plasmonic excitation is strongest and followed by the efficient generation of hot electrons and their further participation in chemical reactions.

Considering this fact, the surface properties of nanoparticles become an important issue for high efficiency in electron transfer related to plasmon-driven chemical reactions. Nanostructures such as nanocavities, nanopores, and sharp edges may create localizations of the plasmonic field and multiply the generation of hot electrons at active sites. These locally active areas, in turn, allow the selective activation of chemical bonds by expelling less energy than would be necessary for thermal catalysis.

For example, it has been demonstrated that plasmonic nanoparticles can promote the oxidation of small organic molecules, the reduction of carbon dioxide, and the splitting of water molecules, all of which are very important due to their relevance for environmental and energy-related applications. The possibility of tuning the plasmonic properties by changing the size, shape, and composition of the nanoparticles allows optimization of these reactions toward specific applications.

Applications to Photocatalysis

Photocatalysis driven by plasmons is among the trending areas of research in plasmonic nanomaterials. Being driven by renewable feedstock, plasmonic photocatalysts can drive uphill reactions and hence offer an environmentally benign alternative to many traditional catalytic processes.

A typical application of the catalysis driven by plasmons is the reduction of CO2 into value-added chemicals. The reaction of CO2 reduction itself is energetically demanding and requires the overcoming of sufficiently high energy barriers. This, together with a finite probability of obtaining a certain selectivity of products, makes CO2 reduction quite problematic. However, the role of plasmonic nanoparticles has been envisioned in catalyzing such a reaction by reducing it through an energy supply via hot electron transfer. This would not only reduce the carbon footprint but also produce hydrocarbons and alcohol, useful products that can be used as fuel or as a chemical feedstock.

Another major application lies in the field of water splitting, where plasmonic nanoparticles facilitate the photochemical dissociation of water into hydrogen and oxygen. Highly energetic electrons generated by these plasmonic nanoparticles can reduce water with ease to produce hydrogen, a clean, renewable source of energy. This can mark the beginning of a revolutionary production method for hydrogen fuel, further making it more viable and economical.

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Challenges and Future Directions

Despite the enormous achievements, a range of challenges have yet to be overcome in the field of plasmon-driven chemical reactions. The biggest one, perhaps, is underpinning the very tough interplay among several factors influencing the hot electron generation that transfers, size, shape of the nanoparticles, wavelength, and intensity of incident light, or the nature of the adsorbed molecules.

Another problem is that the lifetime of hot electrons is extremely short. Thus, the transfer of hot electrons to adsorbed molecules is greatly limited in efficiency. Many efforts should be made to lengthen the lifetime or raise the efficiency of hot electron transfer for further improvement of the performance of plasmon-driven catalytic systems.

Heterogeneous systems of metal nanoparticles with any other material, such as semiconductors or molecular catalysts, have recently become of increased interest. Indeed, hybrid systems can enhance the efficiency and selectivity of the plasmon-driven reactions by adding the properties of both kinds of materials involved in the reaction.

Besides, enhanced computational modeling and experimental capabilities, such as ultrafast spectroscopy, provide new insights into the dynamics of both the generation and transfer of hot electrons. These will enable researchers to optimize the design of more efficient plasmonic catalysts and tune their performances toward a reaction of interest.

Conclusion

The involvement of hot electrons in plasmon-driven chemical reactions may be pointed out as a revolutionary way of catalysis, guaranteeing new frontiers toward energy-efficient and sustainable chemical processes. The plasmonic nanoparticles can drive varied chemical transformations, from environmental remediation to the production of clean fuels, by converting light energy into reactive electrons. With further advances, we foresee highly innovative applications of hot electron-driven chemistry in the future.

References

  1. Fang, S. and Hu, Y.H., 2022. Thermo-photo catalysis: a whole greater than the sum of its parts. Chemical Society Reviews, 51(9), pp.3609-3647.
  2. Devasenathipathy, R., Wang, J.Z., Xiao, Y.H., Rani, K.K., Lin, J.D., Zhang, Y.M., Zhan, C., Zhou, J.Z., Wu, D.Y. and Tian, Z.Q., 2022. Plasmonic photoelectrochemical coupling reactions of para-aminobenzoic acid on nanostructured gold electrodes. Journal of the American Chemical Society, 144(9), pp.3821-3832.
  3. Mahapatra, S., Schultz, J.F., Li, L., Zhang, X. and Jiang, N., 2022. Controlling localized plasmons via an atomistic approach: attainment of site-selective activation inside a single molecule. Journal of the American Chemical Society, 144(5), pp.2051-2055.
  4. Askes, S.H. and Garnett, E.C., 2021. Ultrafast thermal imprinting of plasmonic hotspots. Advanced Materials, 33(49), p.2105192.
  5. Zhao, J., Xue, S., Ji, R., Li, B. and Li, J., 2021. Localized surface plasmon resonance for enhanced electrocatalysis. Chemical Society Reviews, 50(21), pp.12070-12097.
  6. Lin, Q., Hu, S., Földes, T., Huang, J., Wright, D., Griffiths, J., Elliott, E., de Nijs, B., Rosta, E. and Baumberg, J.J., 2022. Optical suppression of energy barriers in single molecule-metal binding. Science advances, 8(25), p.eabp9285.
  7. Yang, K., Yao, X., Liu, B. and Ren, B., 2021. Metallic plasmonic array structures: principles, fabrications, properties, and applications. Advanced Materials, 33(50), p.2007988.

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