Introduction

Pedro H.C. Camargo1 and Emiliano Cortés2

1Department of Chemistry, University of Helsinki, Helsinki, Finland

2Faculty of Physics, Nanoinstitute Munich, University of Munich (LMU), Munich, Germany

Catalysis is central to move toward a more sustainable future and enable our society to transition to a circular economy. For this reason, the possibility of harvesting sunlight to drive, accelerate, and control chemical reactions via photocatalysis has fascinated scientists for years. The unique optical properties of metal nanoparticles in the visible and near-infrared ranges turn them into ideal candidates for sunlight-activated catalysts. In fact, is has been recently established that the excitation of the localized surface plasmon resonance (LSPR) in these systems can be employed to drive and accelerate a variety of chemical reactions. This has led to the rise of plasmonic catalysis as a new frontier in catalysis, photocatalysis, and photoelectrocatalysis.

Plasmonic catalysis is the acceleration of a chemical reaction due to a plasmon excitation. To understand this simple definition is necessary to incorporate concepts from various research fields such as heterogeneous catalysis, nano-optics, physical chemistry, and material science. This book emerged as a necessity for unifying concepts, ideas, techniques, and advances in the rapidly growing field of plasmonic catalysis. To the best of our knowledge, this is the first book dedicated to this emerging area of research, covering its most important concepts and recent developments. To do so, a big, diverse, and heterogeneous group of world leaders in the field prepared exciting contributions for this book. The book comprises 10 chapters encompassing topics such as theoretical considerations of using plasmons for catalysis, optical and catalytic properties in plasmonic nanoparticles and hybrid systems, their synthesis, the fundamentals and mechanisms by which plasmonic excitation leads to the acceleration of reaction rates, examples and discussion of plasmonic catalysis applied to important chemical transformations, plasmonic catalysts based on earth-abundant materials, plasmonic electrocatalysis, and plasmonic metal-semiconductor heterostructures. Here is a quick overview of the main aspects covered in each chapter.

The book starts by describing in Chapter 1 the theoretical framework of plasmon excitation and decay in the context of plasmonic catalysis. Energy conversion from photons to molecules and transfer from the plasmonic catalyst to the environment are the fundamental processes that take place in a plasmon-catalyzed chemical reaction. The chapter provides a detailed theoretical analysis of plasmon excitation, decay mechanisms, energy transfer, and carrier injection across different interfaces, near-field and scattering enhancements, and photoheating. Toward the end of the chapter and in order to exemplify these theoretical concepts, the chapter overviews a series of applications and experiments where these phenomena can be spotted. As such, this chapter sets the ground to understand the physics behind the uses of plasmons for chemistry.

We move next to the characterization and properties of plasmonic catalytic systems in Chapter 2. From conventional heterogeneous catalysis methods to plasmonic techniques, this chapter tackles the integration of conventional methods as well as new methods able to unravel the optical, electronic, and chemical properties of these systems. Different approaches can be followed in order to study chemical reactions mediated by plasmons either at the ensemble level or at the nanoscale, as well as to disentangle the role of light, heat, and carriers in the underlying mechanism. This chapter groups techniques with different temporal, spatial, and chemical resolution in order to gain deeper insight of the behavior of plasmonic catalysts under light illumination.

It has been recognized that the optical properties arising from the LSPR excitation are strongly dependent on several physical and chemical parameters that define the plasmonic nanoparticles. These include size, shape, composition, and structure (solid or hollow interiors) of the nanoparticles. Because these properties are related to the performances in plasmonic catalysis, the synthesis of plasmonic nanoparticles where these parameters can be tightly controlled has gained increased attention. In fact, this is important not only to optimize performances, but also to unravel-structure performance relationships that may aid on the rational design of plasmonic catalysts with desired performances for a reaction of interest. In this context, Chapter 3 discusses the fundamentals and important examples on the controlled synthesis of metal nanoparticles that are relevant for plasmonic catalysis. The chapter begins by focusing on several methods for the controllable synthesis of Ag, Au, Cu, and Al nanoparticles. The chapter pays particular attention on shape control, in which morphologies such as quasispheres, nanocubes, nanowires, among others, are described. Then, different assemblies having these nanoparticles are presented. These colloidal assemblies are important as they often outperform their individual counterparts due to the formation of electromagnetic hot spots, which can enhance plasmonic catalytic activities. The chapter then moves to bimetallic nanoparticles. This is attractive because nanoparticles having a plasmonic and a catalytic metal enables one to marry optical and catalytic properties in a single nanoparticle. This way, the plasmonic metal can harvest energy from light through the LSPR excitation, which can then be used to accelerate and control reactions at the sites containing the catalytic metal (which may not display LPPR in the visible or near-infrared ranges). This enables one to extend application of plasmonic catalysis to metals that do not display LSPR resonance in the visible or near-infrared ranges. The chapter ends by discussing applications of controlled nanoparticles in LSPR mediated oxidation, reduction, and dehalogenation reactions as well as electrocatalytic applications (such as alcohol oxidation, H2 evolution, and O2 reduction reactions).

Chapter 4 covers plasmonic catalysis toward hydrogenation reactions. Hydrogenations are important transformations in industry. Thus, the possibility to enhance performances with visible or near-infrared light - enabling milder or greener reaction conditions or controlled selectivity - are highly relevant and expect to lead to high economic and environmental impacts. The chapter begins by discussing the main mechanisms by which the LSPR excitation can lead to higher reaction rates in the context of catalytic hydrogenations. Then, several important examples involving the hydrogenation of alkanes and alkynes are explained. These examples include the use of Ag and Au NPs as well as bimetallic plasmonic-catalytic NPs having Pd and Pt as the catalytic part in plasmonic catalysis. The chapter also discuss examples including the hydrogenation of aldehydes and ketones, which included the use of Cu NPs as plasmonic catalysts. Next, the reduction of nitrocompounds is covered, and examples of bimetallic plasmonic-catalytic NPs are highlighted.

Chapter 5 discusses the harvesting of LSPR-excited charge carriers for driving multielectron redox reactions. The chapter focuses on the fundamentals and applications of this effect toward important energy relevant transformations: the water splitting reaction and the CO2 conversion to hydrocarbon fuels. While water splitting can supply green H2, the CO2 conversion to fuels can help to alleviate its alarming high levels in the atmosphere while producing important molecules. The chapter starts by discussing the fundamentals of the plasmon-induced generation of charge carriers and its subsequent extraction for redox processes. In this case, the mechanisms for charge transfer are explained in detail. Then, the chapter discusses the energetics and kinetics of the carrier harvesting, which is followed by a description of the chemical potential of the plasmonic excitations. Then, several study cases are covered toward the plasmon-excitation-assisted charge transfer reactions. These include the photodriven growth of Ag and Au nanoparticles and the switching of redox states in metal complexes. After establishing these strong foundations, the chapter focuses on the plasmon-excitation-driven processes relevant for fuel generation. In this context, both the H2O splitting reaction and the CO2 reduction are discussed, which includes the possibility of using the LSPR to control selectivity and thus enable the generation of C2+ products from CO2. Interestingly, the chapter includes a discussing on thermodynamic insights into plasmon-excitation-driven CO2 reduction before the future perspectives regarding photoredox reactions and multi-carrier processes that can be addressed by plasmonic catalysis.

Chapter 6 discusses plasmonic catalysis for N2 fixation reactions. In addition to improvements in performances due to LSPR, the chapter focuses on the underlying reaction mechanisms. In this case, the role of near-field enhancement and transfer of LSPR-excited charge carriers (both hot electrons and holes) are presented. The chapter begins by showing the importance of N2 fixation for synthesis of ammonia (NH3) from dinitrogen (N2), and the attractive features of being able to drive this reaction via photocatalysis using sunlight as the energy input. Then, the chapter moves to the discussion of the catalytic enhancements and mechanisms in the plasmonically enhanced NH3 photosynthesis. Three general...