Light absorption in nanoparticles of semiconductors and metals excites electrons from ground states to high-energy levels, generating hot electrons with the addition of kinetic energy, and consequently, complimentary hot holes in the nanoparticles. These hot electrons are capable of injecting themselves into the empty antibonding orbitals of chemical bonds of reactant molecules adsorbed on the surface of the nanoparticles, thereby weakening the chemical bonds to trigger corresponding desirable chemical reactions. Hot-electron chemistry represents a fundamentally different mechanism of solar-to-chemical energy conversion compared to the traditional photochemistry that relies on the direct photo-excitation of electrons in reactant molecules and thermal catalysis. This issue of MRS Bulletin examines the generation and relaxation of hot electrons in typical nanoparticle systems, and the flow of hot electrons across the surfaces of the nanoparticles. The promise of hot-electron chemistry (and the complementary hot-hole chemistry) is supported by its application in many important reactions, including CO2 reduction, water splitting, hydrogenation, and coupling reactions, highlighting its great potential in achieving high energy-conversion efficiency and product selectivity.

Understanding fundamental mechanisms for surface electronic excitation is of great importance in surface chemistry. Charge transport through metal–oxide interfaces plays a significant role in heterogeneous catalysis. Over the last several decades, a number of experimental and theoretical results suggest that this charge flow through metal–support interfaces leads to catalytic enhancement often observed in mixed catalysts. Direct measurement of charge flow on actual catalysts is a rather challenging task because it requires the use of an electronic circuit. This approach has been enabled by a catalytic nanodiode that is mainly composed of a catalytic metal and semiconducting oxides that form a Schottky contact. In this article, we describe the advances in this approach. We show that there is close connection between the phenomena of hot-electron creation and chemical reaction that occur at both gas–solid and liquid–solid interfaces. The intensity of hot-electron flow is well correlated with the turnover rates of corresponding reactions, which opens the possibility for developing new operando methodologies to monitor catalytic reactions as well as a novel scheme for the electronic control of chemical reactions.

Nanoscale materials that contain metallic components can be designed to have excellent light-harvesting capabilities, and can also be used to direct the flow of energy from incident photons into small molecules at or near the surface of metal nanoparticles. One promising route for energy flow is through so-called hot charge carriers, which are optically excited on metal nanoparticles and subsequently transferred to molecules/materials that share an interface with the metal. This article provides an overview of the fundamentals of hot-carrier generation and transfer, discusses both theoretical and experimental means for interrogating these processes, and discusses several potential societally important applications of hot-carrier-driven chemistry to solar fuels and sustainable chemistry.

Plasmonic nanostructures possess broadly tunable optical properties with catalytically active surfaces. They offer new opportunities for achieving efficient solar-to-chemical energy conversion. Plasmonic metal–semiconductor heterostructures have attracted heightened interest due to their capability of generating energetic hot electrons that can be collected to facilitate chemical reactions. In this article, we present a detailed survey of recent examples of plasmonic metal–semiconductor heterostructures for hot-electron-driven photochemistry, including plasmonic metal–oxide, plasmonic metal–two-dimensional materials, and plasmonic metal–metal–organic frameworks. We conclude with a discussion on the remaining challenges in the field and an outlook regarding future opportunities for designing high-performance plasmonic metal–semiconductor heterostructures for photochemistry.

Nanostructures of plasmonic metals naturally combine strong light–matter interactions with catalytic activity, enabling new opportunities for light harvesting, catalytic chemistry, and artificial photosynthesis. Numerous studies have demonstrated that the optical excitation of localized surface plasmons generates hot electrons that can activate adsorbates triggering or facilitating chemical reactions on the surface of the nanoparticle. Going beyond such hot-electron-activated chemistry, a body of studies has shown that electron and hole carriers can be harvested from a plasmonically excited nanoparticle and utilized as redox equivalents for driving chemical reactions involving charge transfer. This article reviews such photoredox chemistry driven by plasmonic excitation of metal nanoparticles. Under certain conditions, a plasmonically excited nanoparticle can catalyze multielectron, multiproton transformations such as the photosynthesis of CO2 to hydrocarbons. We describe how the free energy of plasmonically generated charge carriers can be harvested and utilized for thermodynamically uphill reactions involving the formation of energy-rich chemical bonds or the development of molecular complexity. We end with a discussion of future opportunities in plasmon-excitation-driven photoredox chemistry.

Photocatalytic hydrogen production from water is a facile and clean approach to convert rich solar energy into chemical fuel. Developing efficient and robust catalysts to accelerate water-splitting speed is key. Porphyrins exist widely in green plants and are a key photosensitizer to absorb and transfer light energy to other parts of the photosynthesis system of plants. They are considered an ideal model to construct artificial photocatalysts for hot-carrier-mediated hydrogen production. This article discusses recent achievements in constructing porphyrin-based photocatalysts for hydrogen production, including porphyrin molecules, self-assembled porphyrins, metal–organic frameworks, conjugated porphyrin polymers, and hybrid nanomaterial-based photocatalysts. The design and synthesis principles, structure–property relationships, as well as urgent issues to be solved in the future for every type of photocatalyst are also discussed.

The technology around generating efficient and sustainable energy is rapidly evolving; hydrogen and fuel cells are versatile examples within a portfolio of options. This article provides an overview of the early-stage materials R&D in hydrogen and fuel cells at the US Department of Energy (DOE) Fuel Cell Technologies Office within the Office of Energy Efficiency & Renewable Energy. The article highlights technology status and progress toward achieving DOE targets, discusses R&D needs and challenges, and provides specific examples where advanced materials research is relevant to addressing those challenges. For broader context, materials R&D advances are discussed in the context of DOE’s H2@Scale initiative, which is enabling innovations to generate cost-competitive hydrogen as an energy carrier, enabling renewables, as well as nuclear, fossil fuels, and the grid, to enhance the economics of both baseload power plants and intermittent solar and wind, enhancing resiliency and avoiding curtailment.