Multidimensional Functional Hybrids

Carbonaceous materials (CMs) such as carbon nanotubes, fullerenes, graphene, carbon quantum dots, graphdiyne, and their derivatives have sparked enormous excitement for solar energy conversion applications. In article number 1900577, Yi-Jun Xu and co-workers focus on the surface/interface engineering issues of diverse CMs. The effects of surface/interface engineering, and the properties and applications of functionalized CMs-based hybrids are highlighted.

Hybrid Photocatalysts

In article number 1900434, Xiaofei Yang, Jingsan Xu, and co-workers demonstrate a new efficient and stable sacrificial agent–free water oxidation photocatalyst by integrating conductive few-layer 2D Ti₃C₂ MXene with a well-documented oxygen-evolving material. The constructed Ag₃PO₄/MXene Schottky heterojunctions exhibit increased light utilization efficiency, accelerated interfacial charge transfer, improved oxygen evolution performance, and highly enhanced photo-stability.

Solar Fuel Production

In article number 1900547, Dan Li, Jinhua Ye, and co-workers summarize the current state-of-the-art progress on metal–organic frameworks (MOFs) for photocatalytic solar fuel production. The strategies for designing stable MOFs and the applications for efficient photocatalytic solar fuel production are comprehensively summarized. The challenges and future perspectives using MOFs for photocatalytic solar fuel production are also discussed.

2D/2D Heterostructured Photocatalysts

In article number 2000132, Wee-Jun Ong and Katrina Pui Yee Shak present the latest insights into the interplay of 2D/2D heterojunctions and charge carrier dynamics toward ameliorated photo-driven water splitting, CO₂ reduction, and N₂ fixation, and discuss challenges and prospects in the field.

Engineering 2D nanomaterials to form 2D/2D heterointerfaces has received tremendous interest due to the large interfacial contact area for excellent charge migration in photocatalysis. Herein, the latest insights into the interplay of 2D/2D heterojunction and charge carrier dynamics toward ameliorated photo-driven water splitting, CO₂ reduction, and N₂ fixation are presented. Challenges and prospects in this field are discussed.

Graphitic carbon nitride (g-C₃N₄) constructs, state-of-the-art low-dimensional heterostructure photocatalysts modulated via doping and defect engineering, band structures tuning, and charge carrier dynamics to realize enhanced visible light absorption, improved photoinduced charge carrier transport/transfer, and spatially separated electron–hole pairs for improved photocatalytic performances are analyzed. The recent progress in g-C₃N₄-based low-dimensional heterostructures for water splitting, CO₂ reduction, and pollutant degradation is presented.

Oxygen vacancy engineering in semiconductive metal oxides is reported to be vital to improve photocatalytic efficiency. This review provides a concise overview of oxygen vacancies in transition metal oxides in photocatalytic systems, including their functions, construction strategies, characterization methods, and applications. Moreover, an outlook on the current challenges and promising opportunities in this research area is provided.

Carbonaceous materials (CMs) are versatile building blocks for constructing multidimensional functional hybrid materials. This Review summarizes the recent progresses in designing efficient CMs-based functional composites for boosted solar energy harvesting through surface/interface atomic, molecular, and defect engineering. The effects of surface/interface engineering as well as the excellent mechanical, electronic, optical properties and applications of the functionalized CMs-based composites are highlighted.

Hydrogen is clearly the most environment-friendly alternative energy source, and its generation by splitting water is the most energy- and environment-friendly approach. A variety of materials are exploited for water splitting of which 2D materials are notable. Herein, a brief review of photocatalytic water splitting carried out using a variety of 2D catalysts is presented.

Stable metal–organic frameworks (MOFs) are one of the best choices for a promising platform for photocatalytic solar fuel production due to its adjustable organic ligands and metal nodes, high void architectures, and tunable chemical structures. Possible strategies, such as organic linkers modulation, metal nodes modulation, and morphology and size modulation are summarized to design high-efficiency and stable MOF photocatalysts.

Flexible perovskite solar cells have attracted plenty of attention in both educational and business communities. Herein, the research background is summarized and technological advancement with regard to flexible substrates is evaluated. In addition, different stability tests with and without encapsulation are briefly examined. Last but not least, the upscaling issues and the material costs are also vividly discussed.

A comprehensive and up-to-date review on metal-free photocatalysis of CO₂ reduction is provided, highlighting surface modification of photocatalysts to enhance CO₂ absorption and activation. Theoretical calculations and operando techniques are also summarized and discussed to gain insight into possible reduction mechanisms and pathways. Furthermore, future prospects and challenges in photocatalytic CO₂ reduction by nonmetallic catalysts are envisioned.

This Review focuses on the recent progress of metal-oxide photoanodes for photoelectrochemical water splitting. In particular, the engineering of charge separation and transportation of metal-oxide photoanodes, including nanostructuring, crystal facet engineering, doping, junctions, and interfacial modifications, is highlighted. Perspectives on the further development of metal-oxide photoanodes for efficient solar energy conversion are also discussed.

The development and recent advances in the design of all-organic photoelectrodes based on conjugated polymers and small molecules for water splitting are reviewed. The effects of the band energy structure, film morphology, and thickness on the photoelectrochemical properties are discussed. In addition, future directions are briefly presented.

The utilization of solar energy to transform biomass feedstock is an important topic that brings to the forefront the concept of solar biorefineries. Herein, the enhancement of O₂-mediated gold-catalyzed oxidation of free sugars using standard illumination conditions is presented. The results highlight that under these conditions, only thermal activation, and not the plasmonic effect, is responsible for the reaction enhancement under illumination.

Phosphorus-doped polymeric carbon nitride (p-PCN) films are achieved through in situ polymerization. p-PCN sheets are vertically aligned on a conductive substrate as photoanodes, which lead to an optimal photocurrent density of ≈120 μA cm⁻². This performance is more than two times the undoped one.

For the first time, the visible light photocatalytic property of Ce-based UiO metal–organic frameworks (MOFs) is studied. Compared with traditional Zr-UiO MOFs, not only the light absorption range extends, but also the carrier separation accelerates. Significantly, a ligand-dependent charge separation efficiency is observed and unveiled via electrochemical tests and photoluminescence spectroscopy, and the possible reaction mechanism is further proposed.

A high-performance photoelectrochemical bioassay system based on the coupled photoelectrochemical and oxidase-catalytic reactions at a triphase interface is fabricated through immobilizing oxidases on a superhydrophobic-aligned TiO₂ nanowire array grown on fluorine-doped tin oxide substrates.

The catalytic mechanism and thermodynamic stability of monolayer NiP₂ (100) are explored as cathode material for solar water splitting. Based on comprehensive theoretical study, surface P atoms with only one single PP bond possess excellent hydrogen evolution reaction (HER) activity. Zn-doping can further enhance the HER activity. Accordingly, the synthesis of these materials for experimental verification is proposed.

Intentional extrinsic doping of Ti⁴⁺ or Sn⁴⁺ into Fe³⁺ sites of the zinc ferrite (ZnFe₂O₄) nanorod photoanode as an electron donor enhances photoelectrochemical water oxidation activity mainly by improving the bulk charge carrier density. When annealed at high temperatures, it also improves the surface charge separation by forming a surface oxide layer of the dopant that passivates surface defect sites.

Metallic MoO₂-modified g-C₃N₄ via an ultrasound-assisted method achieves high efficiency photocatalytic CO₂ reduction. Due to the Schottky junction between metallic MoO₂ and g-C₃N₄, charge transfer and separation greatly accelerate. The simultaneous evolution of CH₄ and CO in metallic MoO₂/g-C₃N₄ is 15 and 5 times greater, respectively, than that in a g-C₃N₄ photocatalyst, realizing an ideal strategy for CO₂ reduction.

A Ag₃PO₄/MXene hybrid is constructed via in situ deposition of Ag₃PO₄ particles on exfoliated MXene nanosheets. The composite shows an enhanced photocatalytic O₂-evolution performance even in the absence of a sacrificial electron acceptor. The improvement of O₂ generation is credited to the electron “pool” effect of 2D MXene and the hydrophilic functionalities on the surface of MXene.

Cocatalyst-free CdS/g-C₃N₄ 2D-2D step-scheme heterojunctions exhibit significantly enhanced visible-light photocatalytic H₂ evolution.

Herein, doping of foreign Sc ions into the lattice sites of Ta₃N₅ is systematically investigated, which is found to be effective to inhibit the formation of reduced tantalum species, and thereby improve charge separation as well as the O₂ evolution rate of Ta₃N₅ from water under visible light irradiation by one order together with loading of CoO_x cocatalyst.

Nanocages of polymeric carbon nitride (CN) are rationally synthesized via a low-temperature supramolecular preorganization approach with subsequent calcination. The nanocage structure of polymeric CN facilitates light harvesting, charge transfer, and CO₂ adsorption due to the higher specific surface area, and the extended π-delocalization, conjugated aromatic system with more exposed lone-pair electrons, thus improving the performance of photocatalytic CO₂ reduction.

Copolymerized carbon nitride nanosheets, synthesized from urea and 2-aminobenzonitrile, are capable of reducing CO₂ into formate with high selectivity (>90%) under visible light (λ > 400 nm) when combined with a ruthenium(II) complex catalyst. The reinforced visible light absorption of the copolymerized material contributes to an enhanced photocatalytic activity, as compared with the urea-derived pristine carbon nitride.

Ternary g-C₃N₄/ZnNCN@ZIF-8 hybrid photocatalysts with robust interfacial interactions via ZnN bonding are constructed by sequential in situ interfacial reactions, to overcome the two shortcomings of g-C₃N₄ photocatalyst regarding poor charge separation ability and limited CO₂ adsorption ability, and to enhance photocatalytic CO₂ conversion efficiency, due to the synergetic effects of g-C₃N₄/ZnNCN interfacial Z-scheme heterostructuring and surface-passivated ZIF-8 grafting.

Macroporous sponge-like structures of highly crystalline CaFe₂O₄ are synthesized via microwave reaction and subsequent thermal annealing. The optical and electronic properties of the material are thoroughly characterized, and the ability to generate photocurrents under illumination with visible light is demonstrated.

Photoactive carbon nitrides are obtained by thermal condensation of supramolecular assemblies composed of bismuthiol and melamine with different solvents. The modified carbon nitrides show enhanced optical absorption, increased specific surface area, and improved charge separation efficiency, which promotes the hydrogen production rate, nearly 16 times more than that of the bulk counterpart.

In situ-generated Ni⁺² active species offers a new perspective regarding the catalytic behavior of electrodeposited Ni–M (Mo, Co, Fe) alloys onto the micropillar array p-Si photocathode for solar-driven hydrogen evolution reactions. The adaptive species onto the surface of NiMo through the light-induced activation in alkaline solution enhances the photoelectrochemical properties with regard to NiCo and NiFe.

A self-sacrificial strategy is developed to fabricate a ternary nanostructured photocatalyst ZnIn₂S₄/In(OH)₃-NiS with strong interaction with each other. The optimized ZnIn₂S₄/In(OH)₃-NiS shows a high hydrogen evolution rate of 7010 μmol g⁻¹h⁻¹ due to the fast charge separation efficiency.

The constructed 2D/2D heterojunction of Ni(OH)₂/CN exhibits both high hole mobility and more efficient charge separation compared with the precursor of 2D graphitic carbon nitride (2D-CN). The Ni(OH)₂/2D-CN shows a 135.5 times higher hydrogen generation compared with that achieved by 2D-CN.

Photocatalytic air-purifying pavement is fabricated by spraying a graphitic carbon nitride/titania composite hydrosol on the road at room temperature. The composite coating is firmly attached to the substrate, exhibiting nitrogen oxide removal activity and ideal self-cleaning performance. The removal efficiency is influenced by solar irradiation and traffic volume. The latter has more significant influence than former.