Quantum Tunneling of Photogenerated Charges for Artificial Photosynthesis

Conspectus Photocatalytic conversion of CO 2 and H 2 O to high-value chemicals or fuels provides a crucial pathway for efficiently achieving the conversion and storage of solar-to-chemical energy; however, the overall efficiency is severely restricted by the spatial separation of photogenerated charges. The lifetime of charges in a photoexcited semiconductor particle is mismatched utterly with the rate of interfacial redox reactions, locking kinetically the target reactions. To maximize the availability of photogenerated charges for artificial photosynthesis, constructing various types of heterojunctions including Schottky, p–n, and Z/S-scheme is widely used to manipulate the directional migration of photogenerated charges toward surface active sites, where photoredox catalysis proceeds. Even so, it is unavoidable for the recombination of initially separated charges at the interfacial heterojunctions composed of several atomic layers with less than 1.0 nm thickness, where the Coulomb force remains dominant, leading to the quadratic loss of charges on the surface or interface of catalyst particles. How to eliminate the Coulomb confinement for charges has been a highly interesting topic and yet a formidable challenge in the domain of photocatalysis and solar energy conversion. In this Account, aiming to suppress the recombination of initially separated charges, we introduced a novel strategy of charge tunneling separation to design efficient artificial photosystems for CO 2 conversion. An insulator is inserted between the semiconductor and metal to form the metal–insulator–semiconductor (MIS) structure, where the charge donor and acceptor are spatially separated by the insulating layer with a thickness of a few nanometers, different completely from the conventional Schottky junction with a direct contact M/S interface. Photoexcitation of the semiconductor unit generates a large population of hot electrons and holes, and then they immediately tunnel to the metal catalyst for redox reactions across the two M/I and I/S interfaces and the insulating layer. The tunneling separation proceeds within a few attoseconds at a mean free path. These tunneled electrons or holes are trapped, concentrated, and localized on the catalytic units consisting of metallic single atoms, nanoclusters (NCs), or nanoparticles (NPs), and thus the emphasis of this Account will be put on three aspects: (1) understanding the physical fundamentals of quantum tunneling of photogenerated charges for artificial photosynthesis, (2) smartly designing the chemical components and structures of functional units including photoabsorbers, insulators, catalytic active centers and interfaces to maximize the tunneling probability, and (3) constructing a MIS-type all-solar-driven artificial photosynthetic system, where the functional units responsible for CO 2 reduction and water oxidation are spatially segregated to enable efficient conversion of solar-to-chemical energy. As a result, a solar-to-chemical conversion efficiency (η SCC ) of 13.6% was achieved for the photosynthetic reaction. This work offers guidance for designing novel, high-performance photocatalysts and photoelectrodes and lays the foundation for producing solar fuels at a large scale. Finally, the challenges and outlook for quantum tunneling-based artificial photosynthesis are discussed. Bidirectional tunneling-enhanced artificial leaf technology is expected to facilitate the development of efficient and durable artificial photosystems capable of converting solar energy to fuels and chemicals.