Metal–organic frameworks (MOFs) have attracted significant research attention in diverse areas due to their unique physical and chemical characteristics that allow their innovative application in ...various research fields. Recently, the application of MOFs in heterogeneous photocatalysis for water splitting, CO2 reduction, and organic transformation have emerged, aiming at providing alternative solutions to address the world‐wide energy and environmental problems by taking advantage of the unique porous structure together with ample physicochemical properties of the metal centers and organic ligands in MOFs. In this review, the latest progress in MOF‐involved solar‐to‐chemical energy conversion reactions are summarized according to their different roles in the photoredox chemical systems, e.g., photocatalysts, co‐catalysts, and hosts. The achieved progress and existing problems are evaluated and proposed, and the opportunities and challenges of MOFs and their related materials for their advanced development in photocatalysis are discussed and anticipated.
Metal–organic frameworks (MOFs) have shown great promise in heterogeneous catalysis for energy and environment applications. This review summarizes the latest development of MOFs as multifunctional materials for photoredox catalysis to operate solar‐to‐chemical‐energy transformations according to their different roles in the photochemical systems, i.e., photocatalysts, co‐catalysts, and hosts.
Imidazolium ionic liquids (ILs), imidazolylidene N‐heterocyclic carbenes (NHCs), and zeolitic imidazolate frameworks (ZIFs) are imidazolate motifs which have been extensively investigated for CO2 ...adsorption and conversion applications. Summarized in this minireview is the recent progress in the capture, activation, and photochemical reduction of CO2 with these three imidazolate building blocks, from homogeneous molecular entities (ILs and NHCs) to heterogeneous crystalline scaffolds (ZIFs). The developments and existing shortcomings of the imidazolate motifs for their use in CO2 utilizations is assessed, with more of focus on CO2 photoredox catalysis. The opportunities and challenges of imidazolate scaffolds for future advancement of CO2 photochemical conversion for artificial photosynthesis are discussed.
Around the block: Imidazolate building blocks can be utilized in three different ways for the adsorption, activation, and photoreduction of CO2: as components of ionic liquids, N‐heterocyclic carbenes, or zeolitic imidazolate frameworks (see picture). The recent developments and existing shortcomings of imidazolate motifs for CO2 utilization, with focus on CO2 photoreduction catalysis, are summarized.
Going flat out: Simultaneous modifications of the textural, surface, and electronic structures of a rigid conjugated carbon nitride polymer has been achieved using direct co‐condensation of urea and ...Ph4BNa. This method gives boron‐doped carbon nitride nanosheets (see picture) that optimize the capture of light, improve the charge‐separation kinetics, and enhance the surface reactivity for hydrogen photosynthesis.
Conjugated carbon nitride (CN) is an emerging and promising semiconductor photocatalyst for water photolysis owing to its unique properties. However, the traditional thermally induced polymerization ...of N‐containing precursors typically produces melon‐based CN solids with amorphous or semi‐crystalline structures with only moderate photocatalytic performance. Many strategies have been developed to prepare crystalline CNs (CCNs), such as high‐temperature and high‐pressure routes, ionothermal synthesis, and microwave‐assisted synthesis. In this Minireview, we summarize the progress that has been made in the synthesis of CCNs and their application in photocatalytic water splitting reactions. Three kinds of CCNs are mainly discussed according to their polymeric subunits. Challenges associated with CCNs and their future development are also included.
Polymeric carbon nitride (CN) typically shows a high recombination rate of photogenerated carriers and moderate photocatalytic performance. These issues can be addressed by increasing the crystallinity of this material. In this Minireview, progress in the synthesis of crystalline CN and its application in photocatalytic water splitting is discussed.
As a promising two‐dimensional conjugated polymer, graphitic carbon nitride (g‐C3N4) has been utilized as a low‐cost, robust, metal‐free, and visible‐light‐active photocatalyst in the field of solar ...energy conversion. This Review mainly describes the latest advances in g‐C3N4 photocatalysts for water splitting. Their application in CO2 conversion, organosynthesis, and environmental purification is also briefly discussed. The methods to modify the electronic structure, nanostructure, crystal structure, and heterostructure of g‐C3N4, together with correlations between its structure and performance are illustrated. Perspectives on the challenges and opportunities for the future exploration of g‐C3N4 photocatalysts are provided. This Review will promote the utilization of g‐C3N4 materials in the fields of photocatalysis, energy conversion, environmental remediation, and sensors.
Metal‐free photocatalysis: Recent progress in g‐C3N4 photocatalysis is highlighted. The four fundamental approaches for modification of g‐C3N4 photocatalysts are discussed: electronic structure modulation, nanostructure design, crystal‐structure engineering, and heterostructure construction. The application of g‐C3N4 photocatalysts are briefly summarized and the opportunities and challenges in this field are discussed.
The charge transfer between hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) is the rate‐determining step that controls the overall performance of a Z‐scheme ...water‐splitting system. Here, we carefully design reduced graphene oxide (RGO) nanosheets for use as solid‐state mediators to accelerate the charge carrier transfer between HEPs (e.g., polymeric carbon nitride (PCN)) and OEPs (e.g., Fe2O3), thus achieving efficient overall water splitting. The important role of RGO could also be further proven in other PCN‐based Z‐systems (BiVO4/RGO/PCN and WO3/RGO/PCN), illustrating the universality of this strategy.
Improving charge transfer: A ternary heterojunction was designed, in which polymeric carbon nitride (PCN) and Fe2O3 served as the hydrogen evolution photocatalyst and the oxygen evolution photocatalyst, respectively, while reduced graphene oxide (RGO) acted as an electron transfer “freeway”. This all‐solid‐state Z‐scheme system showed promising activity for photocatalytic stoichiometric water splitting under simulated sunlight irradiation.
Metal‐free catalysts have distinct advantages over metal and metal oxide catalysts, such as lower cost as well as higher reliability and sustainability. Among the nonmetal compounds used in ...catalysis, boron‐containing compounds with a few unique properties have been developed. In this Minireview, the recent advances in the field of boron‐containing metal‐free catalysts are presented, including binary and ternary boron‐containing catalytic materials. Additionally, the three main applications in catalysis are considered, namely, electrocatalysis, thermal catalysis, and photocatalysis, with the role of boron discussed in depth for each specific catalytic application. Boron‐containing compounds could have a substantial impact on the field of metal‐free catalysts in the future.
B the cat's whiskers: Metal‐free catalysts have distinct advantages over metal and metal oxide catalysts, such as lower cost as well as higher reliability and sustainability. Among the nonmetal compounds used in catalysis, boron‐containing compounds have a few unique properties. This Minireview presents advances in the field of metal‐free boron‐containing catalysts and considers the three main applications: electro‐, thermal, and photocatalysis.
The delamination of layered crystals that produces single or few‐layered nanosheets while enabling exotic physical and chemical properties, particularly for semiconductor functions in optoelectronic ...applications, remains a challenge. Here, we report a facile and green approach to prepare few‐layered polymeric carbon nitride (PCN) semiconductors by a one‐step carbon/nitrogen steam reforming reaction. Bulky PCN, obtained from typical precursors including urea, melamine, dicyandiamide, and thiourea, are exfoliated into few‐layered nanosheets, while engineering its surface carbon vacancies. The unique sheet structures with strengthened surface properties endow PCNs with more active sites, and an increased charge separation efficiency with a prolonged charge lifetime, drastically promoting their photoredox performance. After an assay of a H2 evolution reaction, an apparent quantum yield of 11.3 % at 405 nm was recorded for the PCN nanosheets, which is much higher than those of PCN nanosheets. This delamination method is expandable to other carbon‐based 2D materials for advanced applications.
Light‐driven hydrogen production: A carbon/nitrogen steam reforming approach was developed to prepare few‐layered polymeric carbon nitride nanosheets with controlled surface carbon vacancies for photocatalytic hydrogen production. The unique sheet structures with strengthened surface properties endow the polymeric carbon nitride material with more active sites.
Polymeric graphitic carbon nitride materials (for simplicity: g‐C3N4) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor ...H content only. In contrast to graphenes, g‐C3N4 is a medium‐bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the “polymer chemistry” of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g‐C3N4 and its modifications have a high thermal and chemical stability and can catalyze a number of “dream reactions”, such as photochemical splitting of water, mild and selective oxidation reactions, and—as a coactive catalytic support—superactive hydrogenation reactions. As carbon nitride is metal‐free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.
Multipurpose catalyst: Graphitic carbon nitride (g‐C3N4; see SEM image) is an effective (photo)catalyst for a whole series of reactions. This Review describes the synthesis of g‐C3N4, how the band positions and bandgaps can be varied by copolymerization and doping and how changes in the solid‐state structure can improve heterogeneous organocatalyst effectiveness.
Conjugated polymers, comprising fully π‐conjugated systems, present a new generation of heterogeneous photocatalysts for solar‐energy utilization. They have three key features, namely robustness, ...nontoxicity, and visible‐light activity, for photocatalytic processes, thus making them appealing candidates for scale‐up. Presented in this Minireview, is a brief summary on the recent development of various promising polymer photocatalysts for hydrogen evolution from aqueous solutions, including linear polymers, planarized polymers, triazine/heptazine polymers, and other related organic conjugated semiconductors, with a particular focus on the rational manipulation in the composition, architectures, and optical and electronic properties that are relevant to photophysical and photochemical properties. Some future trends and prospects for organic conjugated photocatalysts in artificial photosynthesis, by water splitting, are also envisaged.
Split up: Organic conjugated photocatalysts provide an emerging platform for sustainable H2 production, through water splitting, by artificial photosynthesis. The catalysts have unique properties, including light weight, low cost, accessibility, and fine‐tunability of chemical composition, electronic structure, surface properties, and texture.
