In-situ growth of TiO2 imbedded Ti3C2TA nanosheets to construct PCN/Ti3C2TA MXenes 2D/3D heterojunction for efficient solar driven photocatalytic CO2 reduction towards CO and CH4 production

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.

Modification engineering of TiO2-based nanoheterojunction photocatalysts

Titanium dioxide (TiO2)-based photocatalysts have gained increasing attention for their versatile applications in organic degradation, hydrogen production, air purification, and CO2 reduction. Various TiO2-based heterojunction structures, including type I, type II, Schottky junction, Z-scheme, and S-scheme, have been extensively studied. The current research frontier is centered on the engineering modifications of TiO2-based nanoheterojunction photocatalysts, such as defect engineering, morphological engineering, crystal phase/facet engineering, and multijunction engineering. These modifications enhance carrier transport, separation, and light absorption, thereby improving the photocatalytic performance. Remarkably, this aspect has been less addressed in existing reviews. This review aims to fill this gap by focusing on the engineering modifications of TiO2-based nanoheterojunction photocatalysts. We delve into specific topics like oxygen vacancies, n-p homojunctions, and double defects. The review also systematically discusses the applications of multidimensional heterojunctions and examines carrier transport pathways in heterophase/facet junctions and their interactions with heterojunctions. A comprehensive summary of multijunction systems, including multi-Schottky junctions, semiconductor-based heterojunction-attached Schottky junctions, and multisemiconductor-based heterojunctions, is presented. Lastly, we outline future perspectives in this promising research field. This paper will assist researchers in constructing more efficient TiO2-based nanoheterojunction photocatalysts.

Synergistic Effect of Nickel Nanoparticles Dispersed on MOF-Derived Defective Co3O4 In Situ Grown over TiO2 Nanowires toward UV and Visible Light Driven Photothermal CO2 Methanation

Catalytic CO2 hydrogenation is an effective approach to producing clean fuels, but this process is expensive, in addition to the low efficiency of catalysts. Thus, photothermal CO2 hydrogenation can effectively utilize solar energy for CH4 production. Metal-organic framework (MOF) derived materials with a controlled structure and morphology are promising to give a high number of active sites and photostability in thermal catalytic reactions. For the first time, a novel heterostructure catalyst was synthesized using a facile approach to in situ grow MOF-derived 0D Co3O4 over 1D TiO2 nanowires (NWs). The original 3D dodecahedral structure of the MOF is engineered into novel 0D Co3O4 nanospheres, which were uniformly embedded over Ni-dispersed 1D TiO2 NWs. In situ prepared 10Ni-7Co3O4@TiO2 NWs-I achieved an excellent photothermal CH4 evolution rate of 8.28 mmol/h at 250 °C under low-intensity visible light, whereas UV light treatment further increased activity by 1.2-fold. UV irradiations promoted high CH4 production while improving the susceptibility of the catalyst to visible light irradiation. The photothermal effect is prominent at lower temperatures, due to the harmonization of both solar and thermal energy. By paralleling with mechanically assembled 10Ni-7Co3O4/TiO2 NWs-M, the catalytic performance of the in situ approach is far superior, attributing to the morphological transformation of 0D Co3O4, which induced intimate interfacial interactions, formation of oxygen vacancies and boosted photo-to-thermal effects. The co-existence of metallic/metal oxide Ni-Co provided beneficial synergies, enhanced photo-to-thermal effects, and improved charge transfer kinetics of the composite. This work uncovers a facile approach to engineering the morphology of MOF derivatives for efficient photothermal CO2 methanation.

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.

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.

Two-dimensional Ti3C2 MXene-derived Ti3C2-Ti2C-TiO2 materials for improved diclofenac sodium adsorption performance

This paper aims to prepare an adsorbent based on MXene material for adsorbing diclofenac sodium (DCF). In this paper, Ti₃C₂-MXene was prepared by etching Ti₃AlC₂ with hydrofluoric acid (HF). Ti₃C₂ was subjected to a convenient and simple solvothermal treatment. TiO₂ and Ti₂C were formed during the solvothermal process. According to the results of FT-IR and XRD, the formation of TiO₂ and Ti₂C will increase the interlayer spacing of the prepared Ti₃C₂-12 h, thereby improving the adsorption performance of MXenes. The main factors affecting the adsorbent, the maximum adsorption capacity, and the interaction between the two factors were analyzed by single-factor experiment, orthogonal experiment, and response surface analysis. The maximum DCF adsorption capacities of Ti₃C₂ and Ti₃C₂-12 h are 201 mg/g and 395 mg/g, respectively. MXene made from HF can absorb DCF under various pH conditions and maintain a high adsorption rate, which has important applications in environmental protection.

Surface Electron Localization in Cu-MOF-Bonded Double-Heterojunction Cu2O Induces Highly Efficient Photocatalytic CO2 Reduction

Truncated octahedron Cu₂O (TOC) has attracted more attention for its suitable band gap and high carrier separation efficiency due to introduction of the facet heterojunction, but its practical drawback is still the instability caused by the irreversible disproportionation reaction (Cu₂O → Cu + CuO). Here, we design and fabricate the TOC/Cu-MOF (MOF: metal-organic framework) double-heterojunction structures with different Cu-MOF loadings. The introduced heterojunction between TOC and Cu-MOF not only produces a stable interface Cu^x+ bonding structure with the electronic states localized within the average collisional diameter of electrons 1.72 nm for TOC/2.1 wt %Cu-MOF as the active sites, but also promotes the surface energy level difference between the (100) and (111) facet heterojunctions. Meanwhile, the bonded Cu-MOF with a narrow bandgap effectively consumes holes by recombination with the photoexcited electrons from Cu-MOF itself. In our experiments, the TOC/Cu-MOF double heterostructure with a loading amount of 2.1 wt % Cu-MOF shows an optimal photocatalytic CO₂ reduction performance. The CO evolution rate reaches 23.01 μmol g^-1 h^-1, which is about 2.01 and 4.47 times larger than those of octahedral and hexahedral Cu₂O/Cu-MOF, respectively, and an excellent photostability is shown for four cycles with each cycle lasting for 4 h. Such a double heterostructure provides insight into highly efficient electron transfer and photostability in Cu₂O-related composite materials.

Structured clay minerals-based nanomaterials for sustainable photo/thermal carbon dioxide conversion to cleaner fuels: A critical review

In efforts to achieve a sustainable development goal, the utilization of CO₂ to generate renewable fuels is promising, as it is a sustainable technology that provides affordable and clean energy. To realize the production of renewable green fuels, a proficient and low-cost technology is required. Using photo/thermal catalytic process, the goal of sustainable CO₂ hydrogenation can be achieved. There have been several types of catalysts under exploration, however, they are expensive with limited availability. In the current development, green materials such as mineral clays are emerging as cocatalyst/supports for CO₂ hydrogenation. Clays are bestowed with various beneficial properties such as a large surface area, high porosity, abundant basic sites, excellent thermal stability and chemical corrosion resistance. Clays are promising materials that can drastically reduce the cost in catalyst preparation, partially fulfil the energy demand and reduce greenhouse gas emission. This review aims to focus on the various types of clays and their applications in the field of photo/thermal CO₂ hydrogenation to renewable fuels. Firstly, the classifications of clays are provided, whereby they can be differentiated based on their silicate layers, namely 1:1 and 2:1 type clay and their properties are thoroughly discussed to provide advantages and applications. The applications of various clays such as kaolinite, halloysite, montmorillonite, attapulgite, saponite and volkonskoite for CO₂ hydrogenation reactions are systematically discoursed. In addition, various approaches to improve the capability of raw clays as catalyst support are critically discussed, which include thermal treatment, exfoliation, acid-leaching and pillaring approaches. A critical discussion regarding the engineering aspects to further enhance clay-based catalyst for CO₂ hydrogenation are further disclosed. In short, clays are freely available materials that can be found in abundance. However, there are many more different types of natural green clays that have not been studied and explored in various energy applications.

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.

High-temperature stability in air of Ti3C2Tx MXene-based composite with extracted bentonite

Although Ti₃C₂T_x MXene is a promising material for many applications such as catalysis, energy storage, electromagnetic interference shielding due to its metallic conductivity and high processability, it’s poor resistance to oxidation at high temperatures makes its application under harsh environments challenging. Here, we report an air-stable Ti₃C₂T_x based composite with extracted bentonite (EB) nanosheets. In this case, oxygen molecules are shown to be preferentially adsorbed on EB. The saturated adsorption of oxygen on EB further inhibits more oxygen molecules to be adsorbed on the surface of Ti₃C₂T_x due to the weakened p-d orbital hybridization between adsorbed O₂ and Ti₃C₂T_x, which is induced by the Ti₃C₂T_x/EB interface coupling. As a result, the composite is capable of tolerating high annealing temperatures (above 400 °C for several hours) both in air or humid environment, indicating highly improved antioxidation properties in harsh condition. The above finding is shown to be independent on the termination ratio of Ti₃C₂T_x obtained through different synthesis routes. Utilized as terahertz shielding materials, the composite retains its shielding ability after high-temperature treatment even up to 600 °C, while pristine Ti₃C₂T_x is completely oxidized with no terahertz shielding ability. Joule heating and thermal cycling performance are also demonstrated.

A major challenge for the effective use of Ti-based MXenes in applications with harsh environmental conditions is their poor resistance to oxidation. Here, authors report an air-stable Ti₃C₂T_x composite with extracted bentonite able to endure high-temperature annealing in air by an oxygen adsorption competition mechanism.

COF-5/CoAl-LDH Nanocomposite Heterojunction for Enhanced Visible-Light-Driven CO2 Reduction

Photocatalytic conversion of CO₂ into value-added chemical fuels is an attractive route to mitigate global warming and the energy crisis. Reasonable design of optical properties and electronic behavior of the photocatalyst are essential to improve their catalytic activity. Herein, the 1D/2D heterojunction by direct in-situ synthesis of the covalent organic framework (COF)-5 colloid on the surface of CoAl layered double hydroxide (LDH) was used as the prospective photocatalyst for CO₂ reduction. COF-5/CoAl-LDH nanocomposite achieved 265.4 μmol g^-1 of CO with 94.6 % selectivity over CH₄ evolution in 5 h under visible light irradiation, which was 4.8 and 2.3 times higher than those of COF-5 colloid and CoAl-LDH, respectively. The enhanced catalytic activity was derived from the increased visible-light activity and the construction of type II-2 heterojunction, which greatly optimized visible light harvesting and accelerated the efficient separation of the photoinduced holes and electrons. This work paves the way for rational design of heterojunction catalysts in photocatalytic CO₂ reduction.

MXenes based nano-heterojunctions and composites for advanced photocatalytic environmental detoxification and energy conversion: A review

Extensive research is being done to develop multifunctional advanced new materials for high performance photocatalytic applications in the field of energy production and environmental detoxification, MXenes have emerged as promising materials for enhancing photocatalytic performance owing to their excellent mechanical properties, appropriate Fermi levels, and adjustability of chemical composition. Numerous experimental and theoretical research works implied that the dimensions of MXenes have a significant impact on their performance. For photocatalysis to thrive in the future, we must understand the current state of the art for MXene in different dimensions. Using MXene co-catalysts in widely used in photocatalytic applications such as CO2 reduction, hydrogen production and organic pollutant oxidation, this study focuses on the most recent developments in MXenes based materials, structural modifications, innovations in reaction and material engineering. It has been reported that using 5 mg of CdS-MoS2-MXene researchers were able to generate as high as 9679 μmol/g/h hydrogen under visible light. The MXenes based heterojunction photocatalyst Co3O4/MXene was utilized to degrade 95% bisphenol A micro-pollutant in just 7 min. Numerous novel materials, their preparations and performances have been discussed. Depending upon the nature of MXene-based materials, the synthesis techniques and photocatalytic mechanism of MXenes as co-catalyst are also summarized. Finally, some final thoughts and prospects for developing highly efficient MXene-based photocatalysts are provided which will indeed motivate researchers to design novel hybrid materials based on MXenes for sustainable solutions to energy and pollution issues.

2D metal carbides and nitrides (MXenes) for sensors and biosensors

MXenes are layered two-dimensional (2D) materials discovered in 2011 (Ti₃C₂X) and are otherwise called 2D transition metal carbides, carbonitrides, and nitrides. These 2D layered materials have been in the limelight for a decade due to their interesting properties such as large surface area, high ion transport, biocompatibility, and low diffusion barrier. Therefore, MXenes are widely preferred by researchers for applications in electronics, sensing, biosensing, electrocatalysis, super-capacitors and fuel cells. There are a number of methods available for the bulk synthesis of MXene-based nanomaterials. In addition, the possibility of structural modification as required and its outstanding surface chemistry offer a fascinating interface for the development of novel biosensors. In this review, we specifically discuss important MXene synthesis routes. Moreover, critical parameters such as surface functionalization that can dictate the mechanical, electronic, magnetic, and optical properties of MXenes are also discussed. Following this, methods available for the surface functionalization and modification strategies of MXenes are also discussed. Furthermore, the emergence of gas, electrochemical, and optical biosensors based on MXenes since its first report is discussed in detail. Finally, future directions of MXenes biosensors for critical applications are discussed.

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.

Interface modification by defect engineering for g-C3N4/LaPO4−x nanorods towards efficient CO2 photoreduction

In this work, a series of specific surface defects are introduced at the interface of LaPO4/g-C3N4 composites.

2D/2D Schottky heterojunction of in-situ growth FAPbBr3/Ti3C2 composites for enhancing photocatalytic CO2 reduction

Mimicking the natural photosynthesis process to convert carbon dioxide into value-added chemicals is vital to solving both the climate crisis worldwide and the depletion of fossil fuels. Herein, we explore the synthesis of 2D FAPbBr₃ nanoplate combined with 2D Ti₃C₂ nanosheet to form a 2D/2D FAPbBr₃/Ti₃C₂ Schottky heterojunction using facile hot-injection and in-situ growth approaches. The Schottky heterojunction of FAPbBr₃/Ti₃C₂ over large interfacial contact provides abundant channels for transferring photogenerated carriers from FAPbBr₃ nanoplate to Ti₃C₂ nanosheet. The experimental results showed a CO yield of 93.82 μmol·g^-1·h^-1 with ethyl acetate/deionization water as a sacrificial reagent for FAPbBr₃/Ti₃C₂ composite, which was 1.25-fold enhancement that on pristine FAPbBr₃ nanoplates. The large 2D heterointerface can efficiently accelerate the spatial separation and transfer of photogenerated carriers and result in the superior photocatalytic activity and favorable stability of FAPbBr₃/Ti₃C₂ photocatalysts, which are proved by in-situ X-ray photoelectron spectroscopy, photoluminescence, transient absorption spectra, and Mott-Schottky measurement. Thus, this work unveils that 2D/2D Schottky heterostructures would significantly improve the reaction activities of halide perovskite-based photocatalysts.

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.

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.