g-C3N4 has drawn lots of attention due to its photocatalytic activity, low-cost and facile synthesis, and interesting layered structure. However, to improve some of the properties of g-C3N4, such as photochemical stability, electrical band structure, and to decrease charge recombination rate, and towards effective light-harvesting, g-C3N4–metal oxide-based heterojunctions have been introduced. In this review, we initially discussed the preparation, modification, and physical properties of the g-C3N4 and then, we discussed the combination of g-C3N4 with various metal oxides such as TiO2, ZnO, FeO, Fe2O3, Fe3O4, WO3, SnO, SnO2, etc. We summarized some of their characteristic properties of these heterojunctions, their optical features, photocatalytic performance, and electrical band edge positions. This review covers recent advances, including applications in water splitting, CO2 reduction, and photodegradation of organic pollutants, sensors, bacterial disinfection, and supercapacitors. We show that metal oxides can improve the efficiency of the bare g-C3N4 to make the composites suitable for a wide range of applications. Finally, this review provides some perspectives, limitations, and challenges in investigation of g-C3N4–metal-oxide-based heterojunctions.

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A Comprehensive Review of Graphitic Carbon Nitride (g-C3N4)–Metal Oxide-Based Nanocomposites: Potential for Photocatalysis and Sensing

2D carbides and nitrides of transition metals, also known as MXenes, are an emerging class of 2D nanomaterials that have shown excellent performances and broad application prospects in the fields of energy storage, catalysis, sensing, electromagnetic shielding, electronics and photonics, and life sciences. This unusual diversity of applications is due to their superior hydrophilicity and conductivity, high carrier concentration, ultra-high volumetric capacitance, rich surface chemistry, and large specific surface area. However, it is difficult to make MXenes with the desired surface functional groups that deliver high reactivity and high stability, because most MXenes are extracted from ceramics (MAX phase) by an etching process, where a large number of metal atoms are inevitably exposed on the surface, with other anions and cations embedded uncontrollably. The exposed metal atoms and implanted ions are thermodynamically unstable and readily react with trace oxygen or oxygen-containing groups to form the corresponding metal oxides or degrade chemically, resulting in a sharp decline in activity and loss of excellent physicochemical properties. The addition of certain synergistic additives during the intercalation and chemical modification of surface functional groups under non-hazardous conditions can result in stable and efficient MXene-based materials with exceptional optical, electrical, and magnetic properties. This review discusses several such methods, mainly additive-mediated intercalation and chemical modification of the surface functional groups of MXene-based materials, followed by their potential applications. Finally, perspectives are given to discuss the future challenges and promising opportunities of this exciting field.

Additive-mediated intercalation and surface modification of MXenes.

Improving stability of MXenes

Carbon-based transition metal (TM) single-atom catalysts (SACs) have shown a great potential toward electrochemical water splitting and H2 production. Given that two-dimensional (2D) materials are widely exploited for sustainable energy...

Machine Learning Assisted High-Throughput Screening of Transition Metal Single Atoms Based Superb Hydrogen Evolution Electrocatalysts

Titanium Carbide MXene-Based Hybrid Hydrogel for Chemo-Photothermal Combinational Treatment of Localized Bacterial Infection.

MXenes, an emerging two-dimensional (2D) transition metal carbides, nitrides and carbonitrides, have exhibited great potential as electrocatalysts for hydrogen evolution reaction (HER) due to the excellent characters, including excellent structural and chemical stability, superior electrical conductivity, and large active surface area. In this comprehensive study, firstly, the preparation advances of MXenes are systematically summarized. Then, the representative applications of MXenes-based HER electrocatalysts are introduced, from experimental and theoretical aspects. Thirdly, the strategies for improving HER catalytic activity of MXenes are demonstrated, such as optimizing active sites by termination modification and metal-atom doping, increasing active sites by fabricating various nanostructures. Finally, the existing challenges and new opportunities for MXenes-based electrocatalysts are also elucidated. This paper provides reference for the future development of new and efficient MXenes-based electrocatalysts for hydrogen production through water-splitting technology.

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Recent advances of MXenes as electrocatalysts for hydrogen evolution reaction

Strategic design and fabrication of MXenes-Ti3CNCl2@CoS2 core-shell nanostructure for high-efficiency hydrogen evolution

Excitonic effects induced via Coulomb interactions and an appropriate interfacial contact of electrode modified materials play crucial roles in transfer/separation of interfacial charge carriers during the electrochemical sensing process. Herein, an oxygen vacancy (Vo) mediated step-scheme (S-scheme) heterojunction of WO2.9/g-C3N4 was fabricated by electrostatic self-assembly of high-purity commercial WO2.9 nanoparticles and ultrathin g-C3N4 nanosheets, where g-C3N4 nanosheets were prepared via polymerization followed ultrasonic-assisted exfoliation. The fabricated Vo-S-scheme WO2.9/g-C3N4 heterojunction-based sensor exhibited superior electrochemical performance for detecting environmental hormone 4-nitrophenol (4-NP). Due to the introduction of Vo, an increased free-charge-carriers, a built-in electric field and a high adsorption energy are generated at the heterojunction interface, which all can significantly improve the sensitivity of this proposed sensor. It also showed good reproducibility, high accuracy, good recovery, and strong feasibility for 4-NP detection in real samples. This work not only conduces significantly to understanding the defective structures and S-scheme heterojunctions, but also provides new insights into designing and constructing novel efficient electrochemical sensors.

Saishuai Bai

Papers

Recent advances of MXenes as electrocatalysts for hydrogen evolution reaction

Improving stability of MXenes

Strategic design and fabrication of MXenes-Ti3CNCl2@CoS2 core-shell nanostructure for high-efficiency hydrogen evolution

Oxygen vacancy mediated step-scheme heterojunction of WO2.9/g-C3N4 for efficient electrochemical sensing of 4-nitrophenol

Carbonitride MXene Ti3CN(OH)x@MoS2 hybrids as efficient electrocatalyst for enhanced hydrogen evolution