Over the past few decades, graphitic carbon nitride (g-C3 N4 ) has arisen much attention as a promising candidate for photocatalytic hydrogen evolution reaction (HER) owing to its low cost and visible light response ability. However, the unsatisfied HER performance originated from the strong charge recombination of g-C3 N4 severely inhibits the further large-scale application of g-C3 N4 . In this case, the utilization of cocatalysts is a novel frontline in the g-C3 N4 -based photocatalytic systems due to the positive effects of cocatalysts on supressing charge carrier recombination, reducing the HER overpotential, and improving photocatalytic activity. This review summarizes some recent advances about the high-performance cocatalysts based on g-C3 N4 toward HER. Specifically, the functions, design principle, classification, modification strategies of cocatalysts, as well as their intrinsic mechanism for the enhanced photocatalytic HER activity are discussed here. Finally, the pivotal challenges and future developments of cocatalysts in the field of HER are further proposed.

Emerging Cocatalysts on g-C3N4 for Photocatalytic Hydrogen Evolution

Photocatalytic water splitting is an environmentally friendly technique for hydrogen production. In this work, we report a novel photocatalyst consisting of two-dimensional (2D) titanium carbide (Ti₂C) and graphitic carbon nitride (g-C₃N₄). We observe substantially enhanced water splitting activities due to the efficient synergistic interaction between Ti₂C and g-C₃N₄. Optimal properties are achieved in the g-C₃N₄ with a loading of 0.4 wt% Ti₂C with a hydrogen production rate of 47.5 μmol h⁻¹, which is 14.4 times as much as that in the case using pure g-C₃N₄, and it even outperforms Pt-loaded g-C₃N₄. We further show that the Ti₂C/g-C₃N₄ has high stability and good reproducibility. We expect that the Ti₂C/g-C₃N₄ can be a photocatalyst for large scale applications because both Ti₂C and g-C₃N₄ are low-cost, abundant, and nontoxic.

Synergistic effect of 2D Ti₂C and g-C₃N₄ for efficient photocatalytic hydrogen production

Sparked by natural photosynthesis, solar photocatalysis using metal‐free graphitic carbon nitride (g‐C3N4) with appealing electronic structure has turned up as the most captivating technique to the quest for sustainable energy generation and pollution‐free environment. Nonetheless, low‐dimensional g‐C3N4 is thwarted from sluggish kinetics and rapid recombination of photogenerated carriers upon light irradiation. Among multifarious modification strategies, engineering 2D cocatalysts is anticipated to accelerate redox kinetics, augment active sites and ameliorate electron–hole separation of 2D g‐C3N4 for boosted activity thanks to its face‐to‐face contact surface. It is of timely and technological significance to review the 2D/2D interfaces with state‐of‐the‐art 2D cocatalysts, spanning from carbon‐containing to phosphorus‐containing, metal dichalcogenide, and other cocatalysts. Fundamental principles for each photocatalytic application will be introduced. Thereafter, the recent advances of 2D/2D cocatalyst‐mediated g‐C3N4 systems will be critically evaluated based on their interfacial engineering, emerging roles, and impacts toward stability and catalytic efficiency. Importantly, mechanistic insights into the charge dynamics and structure–performance relationship will be deciphered. Last, noteworthy research directions are prospected to deliver insightful ideas for future development of g‐C3N4. Overall, this review is anticipated to serve as a scaffold and cornerstone in designing dimensionality‐dependent 2D cocatalyst‐assisted g‐C3N4 toward renewable energy and ecologically green environment.

Tailor‐Engineered 2D Cocatalysts: Harnessing Electron–Hole Redox Center of 2D g‐C3N4 Photocatalysts toward Solar‐to‐Chemical Conversion and Environmental Purification

As one of the fascinating visible-light-responsive photocatalysts, the two-dimensional (2D) graphitic carbon nitride (g-C3N4) has drawn broad attention in the field of solar energy conversion and environmental remediation. However, intrinsic drawbacks, such as insufficient surface area, slow electron transfer and unsatisfied visible-light absorption hinder its promising applications. The incorporation of 2D MXenes and their derivatives into g-C3N4 is recognized as an effective approach to totally enhance their performance toward advanced photocatalysts. This work starts with a discussion of the fundamental photocatalytic mechanism over g-C3N4-based photocatalysts incorporated with MXenes and MXene derivatives. Then, we give a comprehensive overview on the main strategies for constructing such hybrid photocatalysts with a solid 2D/2D interfacial contact. Afterward, their typical applications in environmental and energy fields, including H2 generation, CO2 conversion, pollutant degradation, N2 fixation and H2O2 production, are discussed systematically. Finally, some concluding remarks and perspectives in this area are proposed.

Recent advances in g-C3N4-based photocatalysts incorporated by MXenes and their derivatives

Crystalline carbon nitride anchored on MXene as an ordered Schottky heterojunction photocatalyst for enhanced visible-light hydrogen evolution

Recent progress on strategies for the preparation of 2D/2D MXene/g-C3N4 nanocomposites for photocatalytic energy and environmental applications

MXenes have grabbed considerable research attention in photocatalytic field owing to their regular planer structure, elemental composition, excellent metal conductivity, surface termination groups and abundant derivatives. MXene-derived and based materials deliver motivation for fabrication of novel photocatalysts with optimum activity, and long term stability. Meanwhile, TiO 2 being the most extensively studied photocatalyst for solar light driven water splitting and environmental remediation, owing to its noble photocatalytic activity, low cost, nontoxicity, and copious availability. Nevertheless, the major drawbacks of TiO 2 based systems are its ultra-wide band gap and rapid recombination of photoinduced charge carriers. In this context, MXene have been rigorously explored for the modification of TiO 2 which includes the MXene derived TiO 2 (in-situ derived TiO 2 from MXene) and MXene/TiO 2 based nanocomposite (externally added TiO 2 with MXene) for easy channelization of electrons owing to the metal-semiconductor contact where MXene can behave as a co-catalyst or support by enhancing the overall activity. Although a few reviews have been published highlighting the synthesis, inherent properties and applications in various fields, but this particular review highlights the recent advances of the adopted synthetic routes to develop MXene-derived TiO 2 and MXene/TiO 2 based nanocomposite, explaining the mechanism of charge carrier separation due to the formation of schottky junction at the interface of electrically conductive Ti 3 C 2 and optically active TiO 2 for achieving outstanding photocatalytic hydrogen (H 2 ) evolution and pollutant degradation. Furthermore, the future challenges including the materials design process and to improve the overall activity for the promotion of MXenes derivative and allied materials has been proposed with correlating the structural features and activity. This review summarizes the synthetic technique and morphological features of MXene derived TiO 2 and MXene/TiO 2 based nanocomposites towards photocatalytic H 2 production and pollutant degradations. • Recent developments of MXene derived and MXene based photocatalyst is summarized. • Novel MXene/TiO 2 nanocomposites suitability towards energy and environmental application. • Respective role of Ti 3 C 2 and TiO 2 is studied for photocatalytic H 2 generation and dye degradation. • Challenges and future roadmaps regarding these class of nanohybrids are outlined. 

Review on MXene/TiO2 nanohybrids for photocatalytic hydrogen production and pollutant degradations

Rationally designed Ti3C2/N, S-TiO2/g-C3N4 ternary heterostructure with spatial charge separation for enhanced photocatalytic hydrogen evolution.

Interface engineering is a vital concern to achieve high efficiency in heterojunction photocatalysts. The judicious design of efficient interfacial electron mediators to accelerate the charge transfer efficiency in Z-scheme heterojunctions with interfacial contact for enhancing the performance of photocatalysts is essential and has been considered an immense challenge. Inspired by nature, multivariate all-solid-state Z-scheme TiO2@Ti3C2/MIS heterojunction composites were fabricated via a simple two-step oxidation strategy for highly promoted multiple photocatalytic applications. The morphological analysis of TiO2@Ti3C2/MIS composites demonstrated that MgIn2S4 (MIS) microflowers were accumulated on the surface of Ti3C2@TiO2 nanosheets, providing dense active sites to the MIS microflowers for efficient photocatalytic applications. The HRTEM and XPS characterization distinctly clarified the close interfacial interaction between MIS with Ti3C2 and TiO2. The optimized TiO2@Ti3C2/MIS-15 photocatalysts exhibited the highest photocatalytic ciprofloxacin degradation (92%) and hydrogen evolution (520.3 μmol h–1) as compared to those of their pristine counterparts. From the mechanistic insights, the charge migration pathway was observed between MIS and TiO2, where Ti3C2 nanosheets served as an electron bridge in constructing the Z-scheme and thus extended the lifetime of the charge carriers photoinduced by MIS and TiO2. The significant participation of •O2– and •OH radicals during photocatalytic CIP degradation was verified by active species trapping experiments, EPR, and liquid chromatography–mass spectrometry (LC-MS) analysis. The current study provides a strategy to design mediator-based Z-scheme heterojunction interfaces for improving the catalytic activity of MXene-derived photocatalysts. 

Interfacial Solid-State Mediator-Based Z-Scheme Heterojunction TiO2@Ti3C2/MgIn2S4 Microflower for Efficient Photocatalytic Pharmaceutical Micropollutant Degradation and Hydrogen Generation: Stability, Kinetics, and Mechanistic Insights

Spatial charge separation and migration are the critical shortcomings dominating the core energy conversion corridors of photocatalytic systems. Here, a biomimetic multi-interfacial architecture providing strong coupled interaction and rapid charge transmission for photostable and competent photocatalytic H2O2 production and H2 evolution is proposed. The triple-hybrid all-solid-state Z-scheme system was formed with the (001) facet exposed TiO2 nanosheets derived from MXene layers and B-g-C3N4 nanosheets (M/(001)TiO2@BCN) through an electrostatic self-assembly strategy with intimate electronic interaction due to Ti orbital modulation and proper stacking among the hybrids. The metallic and highly conductive MXene layers act as solid state electron mediators in the Z-scheme heterojunction that promote electron-hole separation and migration efficiency. Specifically, the MTBCN-12.5 composite provides optimum yield of H2O2 up to 1480.1 μmol h-1 g-1 and a H2 evolution rate of 408.4 μmol h-1 (with ACE 6.7%), which are 4 and 20 fold greater than the pristine BCN, respectively. The enhanced photocatalytic performance is systematically identified by the increased surface area, higher cathodic and anodic current densities of -1.01 and 2.27 mA cm-2, delayed charge recombination as supported by PL and EIS measurement, and excellent photostability. The Z-scheme charge transfer mechanism is validated by time-resolved photoluminescence (TRPL) analysis, cyclic voltametric analysis, and the radical trapping experiment as detected by PL analysis. This research marks a substantial advancement and establishes the foundation for future design ideas in accelerating charge transfer.

Architecture and Kinetic Studies of Photocatalytic H2O2 Generation and H2 Evolution through Regulation of Spatial Charge Transfer via Z-Scheme Path over a (001) Facet Engineered TiO2@MXene/B-g-C3N4 Ternary Hybrid.

Lijarani Biswal

Papers

Recent progress on strategies for the preparation of 2D/2D MXene/g-C3N4 nanocomposites for photocatalytic energy and environmental applications

Review on MXene/TiO2 nanohybrids for photocatalytic hydrogen production and pollutant degradations

Rationally designed Ti3C2/N, S-TiO2/g-C3N4 ternary heterostructure with spatial charge separation for enhanced photocatalytic hydrogen evolution.

Interfacial Solid-State Mediator-Based Z-Scheme Heterojunction TiO2@Ti3C2/MgIn2S4 Microflower for Efficient Photocatalytic Pharmaceutical Micropollutant Degradation and Hydrogen Generation: Stability, Kinetics, and Mechanistic Insights

Architecture and Kinetic Studies of Photocatalytic H2O2 Generation and H2 Evolution through Regulation of Spatial Charge Transfer via Z-Scheme Path over a (001) Facet Engineered TiO2@MXene/B-g-C3N4 Ternary Hybrid.