Light-Induced Voltages in Catalysis by Plasmonic Nanostructures

Abstract

Conspectus Plasmonic nanostructures have garnered widescale scientific interest because of their strong light–matter interactions and the tunability of their absorption across the solar spectrum. At the heart of their superlative interaction with light is the resonant excitation of a collective oscillation of electrons in the nanostructure by the incident electromagnetic field. These resonant oscillations are known as localized surface plasmon resonances (LSPRs). In recent years, the community has uncovered intriguing photochemical attributes of noble metal nanostructures arising from their LSPRs. Chemical reactions that are otherwise unfavorable or sluggish in the dark are induced on the nanostructure surface upon photoexcitation of LSPRs. This phenomenon has led to the birth of plasmonic catalysis. The rates of a variety of kinetically challenging reactions are enhanced by plasmon-excited nanostructures. While the potential utility for solar energy harvesting and chemical production is clear, there is a natural curiosity about the precise origin(s) of plasmonic catalysis. One explanation is that the reactions are facilitated by the action of the intensely concentrated and confined electric fields generated on the nanostructure upon LSPR excitation. Another mechanism of activation involves hot carriers transiently produced in the metal nanostructure by damping of LSPRs. In this Account, we visit a phenomenon that has received less attention but has a key role to play in plasmonic catalysis and chemistry. Under common chemical scenarios, plasmonic excitation induces a potential or a voltage on a nanoparticle. This photopotential modifies the energetics of a chemical reaction on noble metal nanoparticles. In a range of cases studied by our laboratory and others, light-induced potentials underlie the plasmonic enhancement of reaction kinetics. The photopotential model does not replace other known mechanisms, but it complements them. There are multiple ways in which an electrostatic photopotential is produced by LSPR excitation, such as optical rectification, but one that is most relevant in chemical media is asymmetric charge transfer to solution-phase acceptors. Electrons and holes produced in a nanostructure by damping of LSPRs are not removed at the same rate. As a result, the slower carrier accumulates on the nanostructure, and a steady-state charge is built up on the nanostructure, leading to a photopotential. Potentials of up to a few hundred millivolts have been measured by our laboratory and others. A photocharged nanoparticle is a source of carriers of a higher potential than an uncharged one. As a result, redox chemical reactions on noble metal nanoparticles exhibit lower activation barriers under photoexcitation. In electrochemical reactions on noble metal nanoparticles, the photopotential supplements the applied potential. In a diverse set of reactions, the photopotential model explains the photoenhancement of rates as well as the trends as a function of light intensity and photon energy. With further g...

The Role of Structural Flexibility in Plasmon‐Driven Coupling Reactions: Kinetic Limitations in the Dimerization of Nitro‐Benzenes

“…In plasmon-driven chemistry, noble-metal nanostructures act as light-antennas and convert photon energy to energetic charges and localized heat. [1][2][3][4][5][6] On the one hand, the high charge densities in metal nanoparticles allow the activation of several authors to occur only in the presence of light. [14,[23][24][25] Hence, the role of electrons was emphasized.…”

“…Recently another possible mechanism that could explain the temperature dependence has come to attention. [4] If the particles act as redox centers, an imbalance in the oxidation and reduction half-reactions can cause a charging of the particles, which is equivalent to a shift in the chemical potential μ. This shift and the broadening of the Fermi-distribution together can be sufficiently large for electrons to thermally overcome the injection barrier.…”

Multimodal Plasmonic Hybrids: Efficient and Selective Photocatalysts

“…As a result, metal-semiconductor nanoarchitectures have been promoted to the forefront of the rapidly developing fields of photovoltaics, photochemistry and photoelectrochemistry. [1][2][3][4][5][6] Along these lines, these hybrid nanosystems can be exploited in order to increase the overall performance of solar cells, [7][8][9] drive different organic transformations, 10,11 or boost water splitting efficiency. 12,13 Plasmonic photosensitization of semiconductors results from the ability of metal NPs to support collective electronic oscillations, also known as localized surface plasmon resonances (LSPRs), excited upon light irradiation.…”

The Role of Structural Flexibility in Plasmon-Driven Coupling Reactions: Kinetic Limitations in the Dimerization of Nitro-Benzenes

“…The emerging field of heterogeneous photocatalysis via plasmon excitations, in short plasmon-chemistry, has shown great potential as platform for efficient light-driven bond formation. In plasmon-driven chemistry, noblemetal nanostructures act as light-antennas and convert photon energy to energetic charges and localized heat [1][2][3][4] . On the one hand, the high charge densities in metal nanoparticles allow the activation of several molecules simultaneously, which facilitates coupling reactions of two or more molecules [5][6][7] .…”

“…Recently another possible mechanism that could explain the temperature dependence has come to attention 4 : If the particles act as redox centers, an imbalance in the oxidation and reduction halfreactions can cause a charging of the particles, which is equivalent to a shift in the chemical potential 𝜇. This shift and the broadening of the Fermi-distribution together can be sufficiently large for electrons to thermally overcome the injection barrier.…”