Photocatalytic hot-carrier chemistry
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Introduction A significant component of chemistry is the study of chemical reactions involving the cleavage of chemical bonds in reactants and the formation of chemical bonds in products. The reorganization of chemical bonds (or electron pairs) in a reaction originates from elevating the potential energy of the reactant molecules along their potential energy surface (PES) to form transient intermediates. When the energy of the transient intermediates approximates the conical intersection of the reactant’s PES and the product’s PES, the transient intermediates can relax along the product PES to dissociate old electron pairs and reassemble new electron pairs, yielding the products. The initial electron energization in the reactants is the ratelimiting step of a chemical reaction. Heating the reactants thermalizes the chemical bonds (i.e., electron pairs), enabling the electrons to gain energy. The thermalization strategy usually energizes all electrons of the reactants at elevated temperatures, which may simultaneously activate multiple chemical bonds to form various products with undesirable selectivity. Delivering energy precisely to specific chemical bonds in reactants is the most promising way to drive chemical reactions with high product selectivity and energy utilization efficiency. Photochemistry, in which reactant molecules absorb light of a specific wavelength to activate chemical bonds for reaction, represents a successful example (Figure 1).
The difference in quantum states between one photon and one free electron prevents the direct exchange of energy between individual photons and electrons. An ensemble of multiple electrons in atoms/molecules exhibits quantum states different from those of free electrons by forming unique band structures, within which electron–hole pairs can directly exchange energy with the appropriate light quanta. The excited electron–hole pairs dissociate into holes and energized electrons that can jump into the empty antibonding orbitals of molecules to weaken (or cleave) the corresponding chemical bonds (top, Figure 1). The limited number of electrons and energy states in single molecules limits the population of quantum states that can match the incident photons, restraining the light absorption power in the molecules. On the other hand, the population of quantum states matching photons of the incident light in a nanoparticle of a semiconductor or metal is significantly higher because of the increased number of electrons and energy states. As a result, nanoparticles usually exhibit a strong absorption of light over a broad spectral range, for example, visible light, which is of the greatest interest for research. Light absorption in a nanoparticle excites electrons from the ground states to energy states higher than the Fermi level of the nanoparticle, resulting in the generation of so-called “hot electrons” and complementary “hot holes.” The hot electrons
Yugang Sun, Temple University, USA; [email protected] Zhiyong Tang, National Center for Nanoscience and Technology, China; z
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