Ultrathin 2D Ti3C2 MXene Co-catalyst anchored on porous g-C3N4 for enhanced photocatalytic CO2 reduction under visible-light irradiation

A review on recent advances in g-C3N4-MXene nanocomposites for photocatalytic applications

The solutions for environmental remediation and renewable energy generation have intensified the exploration of efficient photocatalytic materials. Recently, the composites of g-C3N4and MXene have gained considerable interest for their potential applications in photocatalysis. In the g-C3N4-MXene composite, the g-C3N4possesses unique physical, chemical, and optical properties to increase visible light absorption. At the same time, MXene improves conductivity, adsorption of reactant molecules or the active sites, and charge transfer properties. Combining the unique physico-chemical properties of MXene and g-C3N4, the resulting composite exhibits superior photo-responsive behavior and is critical in photocatalytic reactions. Furthermore, the g-C3N4-MXene composite exhibits stability and recyclability, making it a promising candidate for sustainable and scalable photocatalytic material in environmental remediation. This review offers an in-depth analysis of the development and design of g-C3N4-MXene composites through diverse synthesis procedures and a comprehensive analysis of their application in carbon dioxide (CO2) reduction, photocatalytic degradation, water splitting processes, mainly hydrogen (H2) generation, H2O2production, N2fixation, and NOxremoval. The charge transfer mechanism of g-C3N4-MXene composite for photocatalytic application has also been discussed. This review provides insights into the photocatalytic capabilities of g-C3N4-MXene composites, showing their potential to address current environmental challenges and establish a robust foundation for sustainable energy conversion technologies.

Insights on the efficiency and contribution of single active species in photocatalytic degradation of tetracycline: Priority attack active sites, intermediate products and their toxicity evaluation

Photocatalysis has been proven to be an excellent technology for treating antibiotic wastewater, but the impact of each active species involved in the process on antibiotic degradation is still unclear. Therefore, the S-scheme heterojunction photocatalyst Ti3C2/g-C3N4/TiO2 was successfully synthesized using melamine and Ti3C2 as precursors by a one-step calcination method using mechanical stirring and ultrasound assistance. Its formation mechanism was studied in detail through multiple characterizations and work function calculations. The heterojunction photocatalyst not only enabled it to retain active species with strong oxidation and reduction abilities, but also significantly promoted the separation and transfer of photo-generated carriers, exhibiting an excellent degradation efficiency of 94.19 % for tetracycline (TC) within 120 min. Importantly, the priority attack sites, degradation pathways, degradation intermediates and their ecological toxicity of TC under the action of each single active species (·O2-, h+, ·OH) were first positively explored and evaluated through design experiments, Fukui function theory calculations, HPLC-MS, Escherichia coli toxicity experiments, and ECOSAR program. The results indicated that the preferred attack sites of ·O2- on TC were O20, C7, C11, O21, and N25 atoms with high f+ value. The toxicity of intermediates produced by ·O2- was also lower than those produced by h+ and ·OH.

Heterointerface engineering of MXene: Advanced applications in environmental remediation

Contemporary global industrialization, coupled with the relentless growth of the population, has led to a persistent escalation in the emission and accumulation of various toxic and harmful chemicals in the environment, severely disrupting the ecological balance. The development of efficient environmental cleanup materials is a crucial scientific and technological concern. Since the groundbreaking work on Ti3C2Tx in 2011, there has been a huge growing interest in MXene-based composites developed through heterointerface engineering due to its high surface area, hydrophilicity, eco-friendliness, biocompatibility, easy functionalization, excellent thermal/mechanical properties, metal conductivity and rich electronic density. In the area of environmental remediation, MXene-based composites obtained through heterointerface engineering strategies have the ability to effectively remove and systematically monitor contaminants in comparison to virgin MXene, thanks to the synergistic effects and complementary benefits. Heterointerface engineering strategy increases specific surface area, introduces catalytic sites, constructs heterojunctions/Schottky junctions, and facilitates carrier migration and electron-hole separation. These novel MXene-based composites represent significant advances in MXene research and deserve a comprehensive review. Although several excellent reviews and perspectives on the application of MXene-based composites in environmental remediation have been published, there is still a scarcity of comprehensive and systematic assessments on the reliable data and mechanisms of various MXene-based composite materials for pollutant removal and monitoring. In this focused review, the first part briefly introduces the common preparation strategies and characterization methods of single MXene and MXene-based composites, and the second part details the innovative application of MXene-based composites (involving the amalgamation of MXene with metal oxides, metal sulfide, g-C3N4, layered double hydroxides, metal-organic frameworks, single atom/quantum dots, polymers, etc.) in the field of environmental remediation, including carbon dioxide reduction, nitrogen monoxide and volatile organic compounds removal, antibiotic and heavy metal ions degradation, summarizing the relevant performance and mechanisms. Furthermore, the recent advancements in the utilization of MXene-based composites for the sensing of emerging environmental contaminants (antibiotic and antibiotic resistance genes) are summarized. Finally, an outline of the existing challenges and future prospects on this exciting field was narrated for plausible real-world use. This review will help to inspire the diverse design of MXene-based composites and to advance research related to their application in the environmental sector.

Recent progression in MXene-based catalysts for emerging photocatalytic applications of CO2 reduction and H2 production: A review

The development of advanced materials for efficient photocatalytic H2 production and CO2 reduction is highly recommended for addressing environmental issues and producing clean energy sources. Specifically, MXenes have emerged as two-dimensional (2D) materials extensively used as high-performance cocatalysts in photocatalyst systems owing to their outstanding features of structure and properties such as high conductivity, large specific surface area, and abundant active sites. Nevertheless, there is a lack of deep and systematic studies concerning the application of these emerging materials for CO2 reduction reaction (CRR) and H2 production (HER). This review first outlines the essential features of MXenes, encompassing the synthesis methods, composition, surface terminations, and electronic properties, which make them highly active as cocatalysts. It then examines the recent progress in MXene-based photocatalysts, emphasizing the synergy achieved by coupling MXenes as co-catalysts with semiconductors, utilizing MXenes as a support for the consistent growth of photocatalysts, leading to finely dispersed nanoparticles, and exploiting MXene as exceptional precursors for creating MXene/metal oxide photocomposite. The roles of engineering surface terminations of MXene cocatalysts, MXene quantum dots (QDs), and distinctive morphologies in MXenes-based photocatalyst systems to enhance photocatalytic activity for both HER and CRR have been explored both experimentally and theoretically using DFT calculations. Challenges and prospects for MXene-based photocatalysts are also addressed. Finally, suggestions for further research and development of effective and economical MXenes/semiconductors strategies are proposed. This comprehensive review article serves as a valuable reference for researchers for applying MXenes in photocatalysis.

Harnessing the Power of Light: The Synergistic Effects of Crystalline Carbon Nitride and Ti3C2Tx MXene in Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution is an environmentally friendly means of energy generation. Although g-C3N4 possesses fascinating features, its inherent shortcomings limit its photocatalytic applications. Therefore, modifying the intrinsic properties of g-C3N4 and introducing cocatalysts are essential to ameliorate the photocatalytic efficiency. To achieve this, metal-like Ti3C2Tx is integrated with crystalline g-C3N4 via a combined salt-assisted and freeze-drying approach to form crystalline g-C3N4/Ti3C2Tx (CCN/TCT) hybrids with different Ti3C2Tx loading amounts (0, 0.2, 0.3, 0.4, 0.5, 1, 5, 10 wt.%). Benefiting from the crystallization of CN, as evidenced by the XRD graph, and the marvelous conductivity of Ti3C2Tx supported by EIS plots, CCN/TCT/Pt loaded with 0.5 wt.% Ti3C2Tx displays an elevated H2 (2) should be subscripted evolution rate of 2651.93 µmol g-1 h-1 and a high apparent quantum efficiency of 7.26% (420 nm), outperforming CN/Pt, CCN/Pt, and other CCN/TCT/Pt hybrids. The enhanced performance is attributed to the synergistic effect of the highly crystalline structure of CCN that enables fleet charge transport and the efficient dual cocatalysts, Ti3C2Tx and Pt, that foster charge separation and provide plentiful active sites. This work demonstrates the potential of CCN/TCT as a promising material for hydrogen production, suggesting a significant advancement in the design of CCN heterostructures for effective photocatalytic systems.

Carbon Nanofiber Membranes Loaded with MXene@g-C3N4: Preparation and Photocatalytic Property

In this study, a Ti3C2 MXene@g-C3N4 composite powder (TM-CN) was prepared by the ultrasonic self-assembly method and then loaded onto a carbon nanofiber membrane by the self-assembly properties of MXene for the treatment of organic pollutants in wastewater. The characterization of the TM-CN and the C-TM-CN was conducted via X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometer (FTIR) to ascertain the successful modification. The organic dye degradation experiments demonstrated that introducing an appropriate amount of Ti3C2 MXene resulted in the complete degradation of RhB within 60 min, three times the photocatalytic efficiency of a pure g-C3N4. The C-TM-CN exhibited the stable and outstanding photocatalytic degradation of the RhB solution over a wide range of pH values, indicating the characteristics of the photodegradation of organic pollutants in a wide range of aqueous environments. Furthermore, the results of the cyclic degradation experiments demonstrated that the C-TM-CN composite film maintained a degradation efficiency of over 85% after five cycles, thereby confirming a notable improvement in its cyclic stability. Consequently, the C-TM-CN composite film exhibits excellent photocatalytic performance and is readily recyclable, making it an auspicious eco-friendly material in water environment remediation.

Construction of bismuth oxide iodide (BiOI)/zinc titanium oxide (Zn2TiO4) p-n heterojunction nanofibers with abundant oxygen vacancies for improving photocatalytic carbon dioxide reduction

The conversion of carbon dioxide (CO2) into value-added syngas represents an effective approach for addressing both environmental issues and carbon neutrality issue. However, the slow charge dynamics and low CO2 affinity severely limit the photocatalytic CO2 reduction reaction. In this study, bismuth oxide iodide (BiOI)/zinc titanium oxide (Zn2TiO4) composite nanofibers were successfully prepared by immobilizing BiOI nanosheets on Zn2TiO4 electrospun nanofibers through a solvothermal reaction method. The results of photocatalytic research indicate that the BiOI/Zn2TiO4 composite nanofibers exhibit improved photocatalytic activity in CO2 reduction compared to pristine BiOI nanosheets and Zn2TiO4 nanofibers. The highest carbon monoxide (CO) release rate of BiOI/Zn2TiO4 nanofibers could reach 9.10 µmol‧g-1‧h-1, which is 18.6 times and 6.6 times higher than that of pristine BiOI nanosheets and Zn2TiO4 nanofibers, respectively. The enhanced photocatalytic activity can be credited to the formed BiOI/Zn2TiO4 p-n heterojunction, which can boost electron separation, reduce charge recombination at the interface, and promote the reaction process. The presence of oxygen vacancies in BiOI/Zn2TiO4 nanofibers can not only provide active site to facilitate the adsorption and activation of CO2 molecules, but also adjust the energy band structure of the catalyst to accelerate carriers transfer. After four cycles of testing, the CO release rate of BiOI/Zn2TiO4 nanofibers remains nearly constant, demonstrating its excellent stability. This work develops a feasible strategy to improve the efficiency of photoreduction of CO2 through energy band engineering and surface defect technology.

Efficient electron transport by 1D CuZnInS modified 2D Ti3C2 MXene for enhanced photocatalytic hydrogen production

The efficiency of the photocatalytic reactionis mainly determined by the effective separation of photogenerated electron (e-) and hole (h+). As a high electrical conductivity, two-dimensional (2D) Ti3C2 MXene is widely used as an electronic transmission intermediary with a large surface area and active terminal. In this work, 1D CuZnInS are loaded on the surface of 2D Ti3C2 MXene nanosheets to compound 1D/2D CuZnInS/Ti3C2 nanocomposites with effective inhibition of charge-carrier recombination. The H2 production rate of optimized 1D/2D CuZnInS/Ti3C2 composite reached 15.24 mmol h-1 g-1, which is 4.5 times than that of pure CuZnInS (3.38 mmol h-1 g-1), and the apparent quantum efficiencies (AQEs) of composite photocatalysts can reach 0.39% and 0.24% under light irradiation at 365 nm and 420 nm wavelength, respectively. In addition, 1D/2D CuZnInS/Ti3C2 has high stability after 10 cycles. The enhanced photocatalytic performance is attributed to the large specific surface area of 2D Ti3C2 nanosheets, which facilitates the separation and transfer of photogenerated e- and h+ pairs.

Dual Localized Surface Plasmon Resonance effect enhances Nb2AlC/Nb2C MXene thermally coupled photocatalytic reduction of CO2 hydrogenation activity

Nb2AlC/Nb2C MXene (NAC/NC) heterojunction photocatalysts with Schottky junctions were obtained by selective etching of the Al layer, resulting in 146.25 μmol·g-1 electrons and 15.28 μmol·g-1 holes stored in the heterojunction. The average conversion of NAC/NC thermally coupled photocatalytic reduction of CO2 under the simulated solar irradiation reached 110.15 μmol⋅g-1⋅h-1, and the CO selectivity reached over 92%, which was 1.49 and 1.74 times higher than that of pure Nb2AlC and Nb2C MXene, respectively. After light excitation, the localized surface plasmon resonance (LSPR) effect of holes distributed on the surface of Nb2C MXene crystals in the heterojunction will form high-energy thermal holes to dissociate H2 to H+ and reduce CO2 to form H2O at the same time. The high-energy electrons formed by the LSPR effect of Nb2C MXene and the conduction band electrons generated by the photoexcitation of Nb2C MXene can be migrated to Nb2AlC under the action of the interfacial Schottky junction to supplement the electrons needed for the LSPR effect of Nb2AlC, which continuously forms high-energy hot electrons to convert the adsorbed CO2 into *CO2-, b-HCO3, and HCOO. Subsequently, HCOO releases ⋅OH in a cyclic reaction to continuously reduce to form CO. The dual LSPR effect of Nb2AlC and Nb2C MXene is used to enhance the hydrogenation activity of thermally coupled photocatalytic reduction of CO2, which provides a new research idea for the application of MXene in thermally coupled photoreduction of CO2.

Recent developments and perspectives of MXene-Based heterostructures in photocatalysis

Energy crises and environmental degradation are serious in recent years. Inexhaustible solar energy can be used for photocatalytic hydrogen production or CO2 reduction to reduce CO2 emissions. At present, the development of efficient photocatalysts is imminent. MXene as new two-dimensional (2D) layered material, has been used in various fields in recent years. Based on its high conductivity, adjustable band gap structure and sizable specific surface area, the MXene is beneficial to hasten the separation and reduce the combination of photoelectron-hole pairs in photocatalysis. Nevertheless, the re-stacking of layers because of the strong van der Waals force and hydrogen bonding interactions seriously hinder the development of MXene material as photocatalysts. By contrast, the MXene-based heterostructures composed of MXene nanosheets and other materials not only effectively suppress the re-stacking of layers, but also show the superior synergistic effects in photocatalysis. Herein, the recent progress of the MXene-based heterostructures as photocatalysts in energy and environment fields is summarized in this review. Particularly, new synthetic strategies, morphologies, structures, and mechanisms of MXene-based heterostructures are highlighted in hydrogen production, CO2 reduction, and pollutant degradation. In addition, the structure-activity relationship between the synthesis strategy, components, morphology and structure of MXene-based heterostructures, and their photocatalytic properties are elaborated in detail. Finally, a summary and the perspectives on improving the application study of the heterostructures in photocatalysis are presented.

Ti3 C2 -modified g-C3 N4 /MoSe2 S-Scheme Heterojunction with Full-Spectrum Response for CO2 Photoreduction to CO and CH4

Energy shortage and global warming caused by the extensive use of fossil fuels are urgent problems to be solved at present. Photoreduction of CO2 is considered to be a feasible solution. The ternary composite catalyst g-C3 N4 /Ti3 C2 /MoSe2 was synthesized by hydrothermal method, and its physical and chemical properties were studied by an array of characterization and tests. In addition, the photocatalytic performance of this series of catalysts under full spectrum irradiation was also tested. It is found that the CTM-5 sample has the best photocatalytic activity, and the yields of CO and CH4 are 29.87 and 17.94 μmol g-1  h-1 , respectively. This can be ascribed to the favorable optical absorption performance of the composite catalyst in the full spectrum and the establishment of S-scheme charge transfer channel. The formation of heterojunctions can effectively promote charge transfer. The addition of Ti3 C2 materials provides plentiful active sites for CO2 reaction, and its superior electrical conductivity is also favorable for the migration of photogenerated electrons.

Recent progress of cocatalysts loaded on carbon nitride for selective photoreduction of CO2 to CH4

A photocatalytic system driven by solar light is one of the promising strategies for converting CO₂ into valuable energy. The reduction of CO₂ to CH₄ is widely studied since CH₄ has a high energy density as the main component of nonrenewable natural gas. Therefore, it is necessary to develop semiconductor materials with high photocatalytic activity and CH₄ selectivity. Graphitic carbon nitride (g-C₃N₄/CN) has attracted widespread attention for photocatalytic CO₂ reduction due to its excellent redox ability and visible light response. A hybrid system constructed by loading cocatalysts on g-C₃N₄ can significantly improve the yield of target products, and serve as a general platform to explore the mechanism of the CO₂ reduction reaction. Herein, we briefly introduce the theory of selective CO₂ photoreduction and the basic properties of cocatalysts. Then, several typical configurations and modification strategies of cocatalyst/CN systems for promoting CH₄ selective production are presented in detail. In particular, we systematically summarize the application of cocatalyst/CN composite photocatalysts in the selective reduction of CO₂ to methane, according to the classification of cocatalysts (monometal, bimetal, metal-based compound, and nanocarbon materials). Finally, the challenges and perspectives for developing cocatalyst/g-C₃N₄ systems with high CH₄ selectivity are presented to guide the rational design of catalysts with high performance in the future.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst

Developing efficient and stable photocatalysts is crucial for photocatalytic hydrogen production. Cocatalyst loading is one of the effective strategies for improving photocatalytic efficiency. Here, Ti₃C₂T_x (T_x = F, OH, O) nanosheets have been adopted as promising cocatalysts for photocatalytic hydrogen production due to their metallic conductivity and unique 2D characterization. In particular, surface functionalized Ti₃C₂(OH)_x and Ti₃C₂O_x cocatalysts were synthesized through the alkalization treatment with NaOH and a mild oxidation treatment of Ti₃C₂F_x, respectively. ZnIn₂S₄/Ti₃C₂T_x composites, which were fabricated by the in-situ growth of ZnIn₂S₄ nanosheets on the Ti₃C₂T_x surface, exhibited the promoted photocatalytic performance, compared with the parent ZnIn₂S₄. The enhanced photocatalytic performance can be further optimized through the surface functionalization of Ti₃C₂F_x. As a result, the optimized ZnIn₂S₄/Ti₃C₂O_x composite with oxygen functionalized Ti₃C₂O_x cocatalyst demonstrated excellent photocatalytic hydrogen evolution activity. The characterizations and density functional theory calculation suggested that O-terminated Ti₃C₂O_x could effectively facilitate the transfer and separation of photogenerated electrons and holes due to the formation of a Schottky junction, with the largest difference in work function between ZnIn₂S₄ and Ti₃C₂O_x. This work paves the way for photocatalytic applications of MXene-based photocatalysts by tuning their surface termination groups.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO2 reduction reaction (CRR) is an effective strategy because it affords the conversion of CO2 into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Interface Engineering in 2D/2D Heterogeneous Photocatalysts

Assembling different 2D nanomaterials into heterostructures with strong interfacial interactions presents a promising approach for novel artificial photocatalytic materials. Chemically implementing the 2D nanomaterials' construction/stacking modes to regulate different interfaces can extend their functionalities and achieve good performance. Herein, based on different fundamental principles and photochemical processes, multiple construction modes (e.g., face-to-face, edge-to-face, interface-to-face, edge-to-edge) are overviewed systematically with emphasis on the relationships between their interfacial characteristics (e.g., point, linear, planar), synthetic strategies (e.g., in situ growth, ex situ assembly), and enhanced applications to achieve precise regulation. Meanwhile, recent efforts for enhancing photocatalytic performances of 2D/2D heterostructures are summarized from the critical factors of enhancing visible light absorption, accelerating charge transfer/separation, and introducing novel active sites. Notably, the crucial roles of surface defects, cocatalysts, and surface modification for photocatalytic performance optimization of 2D/2D heterostructures are also discussed based on the synergistic effect of optimization engineering and heterogeneous interfaces. Finally, perspectives and challenges are proposed to emphasize future opportunities for expanding 2D/2D heterostructures for photocatalysis.

Oxygen-deficient MoO3−x evoked synergistic photo-thermal catalytic CO2 reduction over g-C3N4

In situ photodeposition of MoO3−x with oxygen vacancies evoked synergistic photo-thermal catalytic CO2 reduction over g-C3N4.

Emergence of MXene and MXene-Polymer Hybrid Membranes as Future- Environmental Remediation Strategies

The continuous deterioration of the environment due to extensive industrialization and urbanization has raised the requirement to devise high-performance environmental remediation technologies. Membrane technologies, primarily based on conventional polymers, are the most commercialized air, water, solid, and radiation-based environmental remediation strategies. Low stability at high temperatures, swelling in organic contaminants, and poor selectivity are the fundamental issues associated with polymeric membranes restricting their scalable viability. Polymer-metal-carbides and nitrides (MXenes) hybrid membranes possess remarkable physicochemical attributes, including strong mechanical endurance, high mechanical flexibility, superior adsorptive behavior, and selective permeability, due to multi-interactions between polymers and MXene's surface functionalities. This review articulates the state-of-the-art MXene-polymer hybrid membranes, emphasizing its fabrication routes, enhanced physicochemical properties, and improved adsorptive behavior. It comprehensively summarizes the utilization of MXene-polymer hybrid membranes for environmental remediation applications, including water purification, desalination, ion-separation, gas separation and detection, containment adsorption, and electromagnetic and nuclear radiation shielding. Furthermore, the review highlights the associated bottlenecks of MXene-Polymer hybrid-membranes and its possible alternate solutions to meet industrial requirements. Discussed are opportunities and prospects related to MXene-polymer membrane to devise intelligent and next-generation environmental remediation strategies with the integration of modern age technologies of internet-of-things, artificial intelligence, machine-learning, 5G-communication and cloud-computing are elucidated.

Self-Assembly 2D Ti3C2/g-C3N4 MXene Heterojunction for Highly Efficient Photocatalytic Degradation of Tetracycline in Visible Wavelength Range

An ultrathin 2D Ti₃C₂/g-C₃N₄ MXene (2D-TC/CN) heterojunction was synthesized, using a facile self-assembly method; the perfect microscopic-morphology and the lattice structure presented in the sample with a 2 wt% content of Ti₃C₂ were observed by the field-emission scanning electron microscopy (SEM) and transmission electron microscope (TEM). The optimized sample (2-TC/CN) exhibited excellent performance in degrading the tetracycline (TC), and the degradation rate reached 93.93% in the conditions of 20 mg/L, 50 mL of tetracycline within 60 min. Except for the increased specific-surface area, investigated by UV-vis diffuse reflectance spectra (UV-vis DRS) and X-ray photoelectron microscopy (XPS) valence spectra, the significantly enhanced photocatalytic activity of the 2-TC/CN could also be ascribed to the formation of Ti-N bonds between Ti₃C₂ and g-C₃N₄ nanosheets, which reduced the width of the band gap through adjusting the position of the valence band, thus resulting in the broadened light-absorption. Furthermore, the facilitated electron transmission was also proved by time-resolved photoluminescence (TRPL) and electrochemical impedance spectroscopy (EIS), which is effective in improving the quantum efficiency of photo-generated electrons. In addition, the resulting radical-capture experiment suggested that superoxide radicals have the greatest influence on photodegradation performance, with the photodegradation rate of TC reducing from 93.16% to 32.08% after the capture of superoxide radicals, which can be attributed to the production of superoxide radicals only, by the 2-TC/CN composites with a high conduction-band value (-0.62 eV). These facilely designed 2D Ti₃C₂/g-C₃N₄ composites possess great application potential for the photodegradation of tetracycline and other antibiotics.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications

Photocatalytic water splitting, CO₂ reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H₂ generation potential, and an excellent ability to capture and activate CO₂ molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H₂ evolution, CO₂ reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

An overview of MXene-Based nanomaterials and their potential applications towards hazardous pollutant adsorption

With the massive development of industrialization, multiple ecological contaminants in gaseous, liquid, and solid forms are vented into habitats, which is currently at the forefront of worldwide attention. Because of the possible damage to public health and eco-diversity, high-efficiency clearance of these environmental contaminants is a serious concern. Improved nanomaterials (NMs) could perform a significant part in the exclusion of contaminants from the atmosphere. MXenes, a class of two-dimensional (2D) compounds that have got tremendous consideration from researchers for a broad array of applications in a variety of industries and are viewed as a potential route for innovative solutions to identify and prevent a variety of obstreperous hazardous pollutants from environmental compartments due to their exceptional innate physicochemical and mechanical features, including high specific surface area, physiological interoperability, sturdy electrodynamics, and elevated wettability. This paper discusses the recent progress in MXene-based nanomaterials' applications such as environmental remediation, with a focus on their adsorption-reduction characteristics. The removal of heavy metals, dyes, and radionuclides by MXenes and MXene-based nanomaterials is depicted in detail, with the adsorption mechanism and regeneration potential highlighted. Finally, suggestions for future research are provided to ensure that MXenes and MXene-based nanomaterials are synthesized and applied more effectively.

Two-dimensional ultrathin metal-based nanosheets for photocatalytic CO2 conversion to solar fuels

Artificially simulated photosynthesis has created substantial curiosity as the majority of efforts in this arena have been aimed to upsurge solar fuel efficiencies for commercialization. The layered inorganic 2D nanosheets offer considerably higher tunability of their chemical surface, physicochemical properties and catalytic activity. Despites the intrinsic advantages of such metal-based materials viz., metal oxides, transition metal dichalcogenides, metal oxyhalides, metal organic frameworks, layered double hydroxide, MXene's, boron nitride, black phosphorous and perovskites, studies on such systems are limited for applications in photocatalytic CO₂ reduction. The role of metal-based layers for CO₂ conversion and new strategies such as surface modifications, defect generation and heterojunctions to optimize their functionalities are discussed in this review. Research prospects and technical challenges for future developments of layered 2D metal-based nanomaterials are critically discussed.

Recent Research Progress in the Structure, Fabrication, and Application of MXene-Based Heterostructures

Two-dimensional (2D) materials have received increasing attention in the scientific research community owing to their unique structure, which has endowed them with unparalleled properties and significant application potential. However, the expansion of the applications of an individual 2D material is often limited by some inherent drawbacks. Therefore, many researchers are now turning their attention to combine different 2D materials, making the so-called 2D heterostructures. Heterostructures can integrate the merits of each component and achieve a complementary performance far beyond a single part. MXene, as an emerging family of 2D nanomaterials, exhibits excellent electrochemical, electronic, optical, and mechanical properties. MXene-based heterostructures have already been demonstrated in applications such as supercapacitors, sensors, batteries, and photocatalysts. Nowadays, increasing research attention is attracted onto MXene-based heterostructures, while there is less effort spent to summarize the current research status. In this paper, the recent research progress of MXene-based heterostructures is reviewed, focusing on the structure, common preparation methods, and applications in supercapacitors, sensors, batteries, and photocatalysts. The main challenges and future prospects of MXene-based heterostructures are also discussed to provide valuable information for the researchers involved in the field.

Recent advances and challenges in 2D/2D heterojunction photocatalysts for solar fuels applications

Although photocatalytic technology has emerged as an effective means of alleviating the projected future fuel crisis by converting sunlight directly into chemical energy, no visible-light-driven, low-cost, and highly stable photocatalyst has been developed to date. Due to considerably higher interfacial contact with numerous reactive sites, effective charge transmission and separation ability, and strong redox potentials, the focus has now shifted to 2D/2D heterojunction systems, which have exhibited effective photocatalytic performance. The fundamentals of 2D/2D photocatalysis for different applications and the classification of 2D/2D materials are first explained in this paper, followed by strategies to improve the photocatalytic performance of various 2D/2D heterojunction systems. Following that, current breakthroughs in 2D/2D metal-based and metal-free heterojunction photocatalysts, as well as their applications for H₂ evolution via water splitting, CO₂ reduction, and N₂ fixation, are discussed. Finally, a brief overview of current constraints and predicted results for 2D/2D heterojunction systems is also presented. This paper lays out a strategy for developing efficient 2D/2D heterojunction photocatalysts and sophisticated technology for solar fuel applications in order to address the energy issue.

Carbon vacancy-mediated exciton dissociation in Ti3C2Tx/g-C3N4 Schottky junctions for efficient photoreduction of CO2

Earth-abundant g-C₃N₄ is a promising photocatalyst for CO₂ reduction, but its practical application is severely limited by the excitonic effect of g-C₃N₄ derived from strong binding energy and lack of electron-enriched active sites. Herein, we design a novel 2D/2D Schottky junction photocatalysts comprising of Ti₃C₂T_x-modified defective g-C₃N₄ nanosheets with carbon vacancy (denoted as Ti₃C₂T_x/V_c-CN) by a self-assembly method. The carbon vacancies in g-C₃N₄ promote exciton dissociation into free charge, while the formed Schottky junctions between Ti₃C₂T_x and V_c-CN further enables a directional charge transfer, thus providing an electron-rich catalytic surface for the CO₂ reduction. Thanks to the synergy of promoted exciton dissociation and directional electron transfer, the optimal 20% Ti₃C₂T_x/V_c-CN display a high CO evolution rate of 20.54 µmol·g^-1·h^-1 under visible light irradiation, which is 7.4 times higher than that of bare CN. This work highlights the synergy of the promoted exciton dissociation and directional electron transfer in the activity enhancement of photocatalytic CO₂ reduction.

Emerging architecture titanium carbide (Ti3C2Tx) MXene based photocatalyst toward degradation of hazardous pollutants: Recent progress and perspectives

MXenes family has aroused marvelous consideration as a frontier photoactive candidate for solar energy transformation and environmental remediation. 2D Ti₃C₂ exhibit a unique layered microstructure, large surface functional groups (-F, -OH, -O), substantial sorption selectivity, superior reduction efficiency, and electrical conductivity. Electronically conductive Ti₃C₂T_x with tunable energy band gap (0.92-1.75eV) makes it one of the most potential photoactive materials for photodegradation. The present review paper aims to design cost-effective heterojunctions and Schottky junctions of Ti₃C₂ with transition metal oxides, sulfides, g-C₃N₄, and other organic frameworks. The discussion mainly involves different aspects related to its tunable electronic structure, stability problems, and surface morphology control. In addition, the advantages of Ti₃C₂ in fabricating highly efficient Ti₃C₂ based catalytic junctions exhibiting suppressed charge carrier recombination are discussed with particular emphasis on their adsorption and redox properties for the removal of toxic dyes, heavy metal ions, and various pharmaceuticals. Finally, current challenges and research directions are outlined and prospected for the future development of Ti₃C₂ based photocatalytic systems.

Boosted CO2 photoreduction performance on Ru-Ti3CN MXene-TiO2 photocatalyst synthesized by non-HF Lewis acidic etching method

Photocatalytic CO₂ reduction to produce value-added products is considered a promising solution to solve the global energy crisis and the greenhouse effect. In this study, Ti₃CN MXene was synthesized using a Lewis acidic etching method without the usage of toxic hydrofluoric acid (HF). Ti₃CN MXene was then used as a support for the in situ hydrothermal growth of TiO₂ and Ru nanoparticles. In the presence of 0.5 wt% Ru, Ru-Ti₃CN-TiO₂ shows CO and CH₄ production rates of 99.58 and 8.97 μmol/g, respectively, in 5 h under Xenon lamp irradiation, more than 20.5 and 9.3 times that of commercial P25. The enhancement in photocatalytic activity was attributed to the synergy between the in-situ growth of TiO₂ on Ti₃CN MXene and Ru nanoparticles. It was proven experimentally that Ti₃CN MXene can provide abundant pathways for electron transfer. The separation and transfer of the photo-induced charge were further increased with the help of Ru and Ti₃CN MXene, leaving more electrons to participate in the subsequent CO₂ reduction reaction. We believe that this work will encourage more attention to designing environment-friendly MXene-based photocatalysts for CO₂ photoreduction using the non-HF method.

The covalent Coordination-driven Bi2S3@NH2-MIL-125(Ti)-SH heterojunction with boosting photocatalytic CO2 reduction and dye degradation performance

Herein, the optional and controllable growth of Bi₂S₃ onto NH₂-MIL-125 via covalent conjunction strategy was reported. The experimental results demonstrate that the obtained heterojunction exhibits boosting photocatalytic reduction CO₂ and organic dye degradation. The 18-Bi₂S₃@NH₂-MIL-125-SH displays the highest yield of 12.46 μmol g^-1h^-1 of CO, >13 times that of pure NH₂-MIL-125. Meanwhile, the reaction kinetic of 18-Bi₂S₃@NH₂-MIL-125-SH in the degradation of methylene blue is uppermost, which is 160 times than that of the commercial P25. The enhancement of photocatalytic performance could be ascribed to the covalent coordination-driven intimate interfacial interaction in n-scheme heterojunction. Meanwhile, the plausible mechanism was also investigated by UV-vis diffuse reflectance (UV-vis), photoluminescence (PL), electrochemical photocurrent, electron spin resonance (ESR) and electrochemical impedance spectroscopy (EIS).

Renewable biomass-derived carbon-supported g-C3N4 doped with Ag for enhanced photocatalytic reduction of CO2

Constructing noble metal-doped g-C₃N₄/carbon composites is a feasible route to overcome the intrinsic drawbacks of pristine g-C₃N₄ for enhanced activity of CO₂ photoreduction. Herein, a novel Ag-doped g-C₃N₄/biomass-derived carbon composite with hollow bird's nest-like (Ag-g-C₃N₄/BN-C) is designed and prepared via a simple yet effective one-step pyrolysis method. In the Ag-g-C₃N₄/BN-C, the highly-dispersed Ag nanoparticles (20-30 nm) with the surface plasmon resonance (SPR) effect act as a significant cocatalyst not only to efficiently trap the photogenerated electrons from g-C₃N₄ to boost the separation of photogenerated electron-hole pairs but also to produce additional active "hot electrons", while the conductive quasi-spherical hollow structure of BN-C doubles the specific surface area with multiple reflections of light, providing abundant active sites and more utilization efficiency of light energy. As a result, the Ag-g-C₃N₄/BN-C exhibits a remarkably enhanced CO evolution rate of 33.3 μmol·g^-1·h^-1 without addition of any sacrificial reagents and photosensitizers, superior to those of both the pristine g-C₃N₄ and many reported g-C₃N₄-based counterparts. The findings of this work demonstrate a good indication for integrating g-C₃N₄ with SPR-dependence noble metal and renewable biomass-derived carbon for enhanced CO₂ photoreduction, which may be extended to modify other semiconductor materials for more photocatalytic applications with enhanced activity.

Bioinspired spike-like double yolk–shell structured TiO2@ZnIn2S4 for efficient photocatalytic CO2 reduction

A spike-like double yolk–shell structured TiO2@ZnIn2S4 (D-Y-TiO2@ZnIn2S4) photocatalyst was designed, which possesses superior photocatalytic CO2 reduction efficiency.

Fast synthesis of porous iron doped CeO2 with oxygen vacancy for effective CO2 photoreduction

The activity of photocatalytic CO₂ conversion to carbon-containing products is determined by the adsorption and activation of CO₂ molecules on the surface of catalyst. Here, iron doped porous CeO₂ with oxygen vacancy (PFeCe) was prepared by one-step combustion method. The amount of CO₂ adsorbed via using the porous structure has been significantly increased in the case of a relatively small specific surface area and CO₂ molecules are more easily captured and undergo a reduction reaction with photoinduced carriers. In addition, oxygen vacancies are formed in the iron doped CeO₂ lattice as the active sites for CO₂ reduction, which can form strong interactions with CO₂ molecules, thereby effectively activating CO₂ molecules. The reduction products of CO₂ over PFeCe composite are CO and CH₄, which is approximately 9.0 and 7.7 folds than that of CeO₂. This work offers insights for the construction of efficient ceria-based photocatalysts to further achieve robust solar CO₂ conversion.

Sodium doped flaky carbon nitride with nitrogen defects for enhanced photoreduction carbon dioxide activity

Sodium doped flaky carbon nitride (g-C₃N₄) with nitrogen defects (bmw-DCN-x) were synthesized via two steps method to enhance photocatalytic reduction of carbon dioxide (CO₂). After ball milling and calcination, dicyandiamide was evenly dispersed on the sodium chloride (NaCl) template to form a flaky structure. The NaCl not only provided part of sodium (Na) source for Na doped g-C₃N₄, but also introduced a large number of nitrogen (N) defects. Meanwhile, sodium hydroxide (NaOH) significantly enhanced Na doping. The bmw-DCN-30, a proportion of modified g-C₃N₄, showed heightened photo-reduction CO₂ performance, with satisfactory carbon monoxide (CO) and methane (CH₄) productivity at a rate of 30.6 μmol·g^-1·h^-1 and 5.4 μmol·g^-1·h^-1 respectively. This productivity was 15 and 11 times as much as that of bulky g-C₃N₄ (BCN). The related characterizations confirmed that N defects produced more reactive sites and enhanced the adsorption capacity of carbon nitride to CO₂. The accompanying Na doping and flaky structure characteristics improved the optical absorption ability and the effective separation of photogenerated carriers. Accordingly, this work provides further insights into constructing modified materials based on carbon nitride for CO₂ reduction.

Rational construction of Ni(OH)2 nanoparticles on covalent triazine-based framework for artificial CO2 reduction

Artificial photoreduction of CO₂ to chemical fuel is an intriguing and reliable strategy to tackle the issues of energy crisis and climate change simultaneously. In the present study, we rationally constructed a Ni(OH)₂-modified covalent triazine-based framework (CTF-1) composites to serve as cocatalyst ensemble for superior photoreduction of CO₂. In particular, the optimal Ni(OH)₂-CTF-1 composites (loading ratio at 0.5 wt%) exhibited superior photocatalytic activity, which surpassed the bare CTF-1 by 33 times when irradiated by visible light. The mechanism for the enhancement was systematically investigated based on various instrumental analyses. The origin of the superior activity was attributable to the enhanced CO₂ capture, more robust visible-light response, and improved charge carrier separation/transfer. This study offers an innovative pathway for the fabrication of noble-metal-free cocatalysts on CTF semiconductors and deepens the understanding of photocatalytic CO₂ reduction.

Potential environmental applications of MXenes: A critical review

Various environmental pollutants (e.g., air, water and solid pollutants) are discharged into environments with the rapid development of industrializations, which is presently at the forefront of global attention. The high efficient removal of these environmental pollutants is of important concern due to their potential threat to human health and eco-diversity. Advanced nanomaterials may play an important role in the elimination of pollutants from environmental media. MXenes as the new intriguing class of graphene-like 2D transition metal carbides and/or carbonitrides have been widely used in energy storage, environmental remediation benefitting from exceptional structural properties such as highly active sites, high chemical stability, hydrophilicity, large interlayer spacing, huge specific surface area, superior sorption-reduction capacity. However, the comprehensive investigation concerning the removal of various environmental pollutants on MXenes is yet not available up to date. In this review, we summarized the synthesis and properties of MXenes to demonstrate the key roles in ameliorating their adsorption performance; then the recent advances and achievements in environmental application of MXenes on the removal of gases, organics, heavy metals and radionuclides were comprehensively reviewed in details; Finally, the formidable challenges and further perspectives regarding utilizing MXene in environmental remediation were proposed. Hopefully, this review can provide the useful information for environmental scientists and material engineers on designing versatile MXenes in actual environmental applications.

In-situ synthesized porphyrin polymer/TiO2 composites as high-performance Z-scheme photocatalysts for CO2 conversion

The promising photocatalytic conversion of CO₂ into valuable fuel promotes the development of photocatalyst through various methods. In this work, TiO₂ nanoparticle was composited with covalent porphyrin polymers (COP-Ps) to fabricate composite photocatalysts. The resultant COP-Ps/TiO₂ composites by in situ hydrothermal method exhibit much improved photocatalytic activity for the conversion of CO₂ into CO relative to two components, and it is attributable to improved charge transfer between two moieties led by strong interaction. Especially, TiO₂ is composited more evenly with the sulfonated hollow COP-P (sh-COP-P). The resultant composite sh-COP-P/TiO₂ performs best with a CO production rate of 5.70 μmol·g^-1·h^-1, which is approximately 20.4 times as high as that of pure TiO₂ and 2.3 times of sh-COP-P polymer. For comparison, the simple physical mixture of sh-COP-P and TiO₂ (sm-sh-COP-P/TiO₂) was fabricated, and it performs more badly due to poor mixing uniformity. A Z-scheme photocatalytic mechanism was proposed for sh-COP-P/TiO₂ composite on the basis of energy band analysis and hydroxyl radical test. This study provides a new in situ strategy to fabricate organic polymer/metal oxide composites of high photocatalytic activity for CO₂ reduction.