Single-atom photocatalysts have shown their compelling potential and arguably become the most active research direction in photocatalysis due to their fascinating strengths in enhancing light-harvesting, charge transfer dynamics, and surface reactions of a photocatalytic system. While numerous comprehensions about the single-atom photocatalysts have recently been amassed, advanced characterization techniques and vital theoretical studies are strengthening our understanding on these fascinating materials, allowing us to forecast their working mechanisms and applications in photocatalysis. In this review, we begin by describing the general background and definition of the single-atom photocatalysts. A brief discussion of the metal-support interactions on the single-atom photocatalysts is then provided. Thereafter, the current available characterization techniques for single-atom photocatalysts are summarized. After having some fundamental understanding on the single-atom photocatalysts, their advantages and applications in photocatalysis are discussed. Finally, we end this review with a look into the remaining challenges and future perspectives of single-atom photocatalysts. We anticipate that this review will provide some inspiration for the future discovery of the single-atom photocatalysts, manifestly stimulating the development in this emerging research area.

Heterogeneous Single-Atom Photocatalysts: Fundamentals and Applications.

The discovery of improved chemical processes for CO and CO2 hydrogenation to valuable hydrocarbon fuels and alcohols is of paramount importance for the chemical industry. Such technologies have the potential to reduce anthropogenic CO2 emissions by adding value to a waste stream, whilst also reducing our consumption of fossil fuels. Current thermal catalytic technologies available for CO and CO2 hydrogenation are demanding in terms of energy input. Various alternative technologies are now being developed for COx hydrogenation, with solar-driven processes over two-dimensional (2D) and 2D-related composite materials being particularly attractive due to the abundance of solar energy on Earth and also the high selectivity of defect-engineered 2D materials towards specific valuable products under very mild reaction conditions. This review showcases recent advances in the solar-driven COx reduction to hydrocarbons over 2D-based materials. Optimization of 2D catalyst performance demands interdisciplinary research that embraces catalyst electronic structure manipulation and morphology control, surface/interface engineering, reactor engineering and density functional theory modelling studies. Through improved understanding of the structure–performance relationships in 2D-related catalysts which is achievable through the application of modern in situ characterization techniques, practical photo/photothermal/photoelectrochemical technologies for CO and CO2 reduction to high-valuable products such as olefins could be realized in the not-too-distant future.

Two-dimensional-related catalytic materials for solar-driven conversion of COx into valuable chemical feedstocks.

As one of the most critical approaches to resolve the energy crisis and environmental concerns, carbon dioxide (CO2 ) photoreduction into value-added chemicals and solar fuels (for example, CO, HCOOH, CH3 OH, CH4 ) has attracted more and more attention. In nature, photosynthetic organisms effectively convert CO2 and H2 O to carbohydrates and oxygen (O2 ) using sunlight, which has inspired the development of low-cost, stable, and effective artificial photocatalysts for CO2 photoreduction. Due to their low cost, facile synthesis, excellent light harvesting, multiple exciton generation, feasible charge-carrier regulation, and abundant surface sites, semiconductor quantum dots (QDs) have recently been identified as one of the most promising materials for establishing highly efficient artificial photosystems. Recent advances in CO2 photoreduction using semiconductor QDs are highlighted. First, the unique photophysical and structural properties of semiconductor QDs, which enable their versatile applications in solar energy conversion, are analyzed. Recent applications of QDs in photocatalytic CO2 reduction are then introduced in three categories: binary II-VI semiconductor QDs (e.g., CdSe, CdS, and ZnSe), ternary I-III-VI semiconductor QDs (e.g., CuInS2 and CuAlS2 ), and perovskite-type QDs (e.g., CsPbBr3 , CH3 NH3 PbBr3 , and Cs2 AgBiBr6 ). Finally, the challenges and prospects in solar CO2 reduction with QDs in the future are discussed.

Semiconductor Quantum Dots: An Emerging Candidate for CO2 Photoreduction.

Merging hydrogen (H2) evolution with oxidative organic synthesis in a semiconductor-mediated photoredox reaction is extremely attractive because the clean H2 fuel and high-value chemicals can be coproduced under mild conditions using light as the sole energy input. Following this dual-functional photocatalytic strategy, a dreamlike reaction pathway for constructing C-C/C-X (X = C, N, O, S) bonds from abundant and readily available X-H bond-containing compounds with concomitant release of H2 can be readily fulfilled without the need of external chemical reagents, thus offering a green and fascinating organic synthetic strategy. In this review, we begin by presenting a concise overview on the general background of traditional photocatalytic H2 production and then focus on the fundamental principles of cooperative photoredox coupling of selective organic synthesis and H2 production by simultaneous utilization of photoexcited electrons and holes over semiconductor-based catalysts to meet the economic and sustainability goal. Thereafter, we put dedicated emphasis on recent key progress of cooperative photoredox coupling of H2 production and various selective organic transformations, including selective alcohol oxidation, selective methane conversion, amines oxidative coupling, oxidative cross-coupling, cyclic alkanes dehydrogenation, reforming of lignocellulosic biomass, and so on. Finally, the remaining challenges and future perspectives in this flourishing area have been critically discussed. It is anticipated that this review will provide enlightening guidance on the rational design of such dual-functional photoredox reaction system, thereby stimulating the development of economical and environmentally benign solar fuel generation and organic synthesis of value-added fine chemicals.

Cooperative Coupling of Oxidative Organic Synthesis and Hydrogen Production over Semiconductor-Based Photocatalysts.

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp2 or sp3 C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Photoredox-Catalyzed C-H Functionalization Reactions.

The functionalization of aliphatic C-H bonds is both a major challenge and a desirable goal in organic synthesis. Here, we describe the successful arylation of unactivated alkanes with heteroarenes by using iridium polypyridyl complexes as the photocatalyst and persulfate as the HAT catalyst precursor under visible-light irradiation. This reaction features good functional group tolerance and broad scope with regard to both alkane and heteroarene substrates (37 examples), which allows direct access to alkyl-substituted N-heteroarenes, a key structural motif in natural products and bioactive molecules.

Direct Arylation of Unactivated Alkanes with Heteroarenes by Visible-Light Catalysis

A highly enantioselective conjugate addition of indolizine and its analogues with α,β-unsaturated 2-acyl imidazoles have been developed. In the presence of 1.0 mol % of Δ-Rh1, the corresponding adducts were...

Catalytic Asymmetric Conjugate Addition of Indolizines to Unsatu-rated Ketones Catalyzed by Chiral-at-metal Complexes

CuBi2O4 has received a lot of attention because of its suitable bandgap and positive onset potential. However, the reported photocurrent density based on the CuBi2O4 photocathode is still far less than its theoretical value. Herein, a layer of amorphous ZnO is coated on the surface of CuBi2O4/CuO heterojunction photocathodes by an electrodeposition method. The photocurrent density achieved by CuBi2O4/CuO/ZnO is 2.1 times that of CuBi2O4/CuO, from 0.56 mA cm−2 to 1.18 mA cm−2 at 0.2 VRHE in 0.5 M Na2SO4. Additionally, after 12 h stability testing, the photocurrent retention rate of CuBi2O4/CuO photocathode with a ZnO protective layer can be maintained at 84%, while only 58% photocurrent retention can be obtained with the bare photocathode. The performance improvement can be attributed to the fact that the electrodeposited amorphous ZnO overlayer provides more active sites, reduces the surface charge transfer resistance, and protects the unstable CuBi2O4/CuO photocathode. This work manifests a ZnO overlayer prepared by the electrodeposition approach is one of efficient strategies to fabricate one durable and high-performance CuBi2O4 based photocathodes. 

CuBi2O4/CuO Heterojunction Coated with Electrodeposited ZnO Overlayer for Stable Solar Hydrogen Evolution

Amine catalysis is powerful for C-H bond functionalization of carbonyl compounds in chemical synthesis. For an α-C-H bond activation of ketone, directing groups are also required to guide the reaction selectivity. Represented herein is the first α-alkylation of cyclic ketones in the absence of amine catalyst and directing group.1H NMR, XPS, EPR studies and DFT calculation indicate that an α-carbon radical intermediate is formed through direct and selective activation of inert α-C-H bond of ketones chelating on the surface of colloidal quantum dots (QDs) catalyst, which is essential to weaken the C-H bond. As examplified, the activation platform using CdSe QDs as sole photocatalyst executes α-C-H alkylation of cyclic ketones with alkenes under visible light irradiation. Without amine catalyst and directing group, the high step- and atom-economy transformation under redox-neutral condition opens a new way for α-C-H functionalization of ketone in carbonyl chemistry.

Cheng Huang

Papers

Direct Arylation of Unactivated Alkanes with Heteroarenes by Visible-Light Catalysis

Catalytic Asymmetric Conjugate Addition of Indolizines to Unsatu-rated Ketones Catalyzed by Chiral-at-metal Complexes

CuBi2O4/CuO Heterojunction Coated with Electrodeposited ZnO Overlayer for Stable Solar Hydrogen Evolution

Amine-Free, Directing-Group-Free and Redox-Neutral α-Alkylation of Saturated Cyclic Ketones.