Construction of MXene–BiVO4–FeOOH composite photoanode with ultra-low onset potential: performance,DFT calculation and mechanism

Synergistic enhancement of carrier separation-transport and acceleration of water oxidation kinetics is an effective way to boost the current density of photoanode. Therefore, in this work, we constructed Ti₃C₂T_x MXene conductive framework by cyclic voltammetric (CV) deposition within BiVO₄ to force carrier separation-transfer while also depositing O-vacancied FeOOH by in situ self-hydrolysis to accelerate water oxidation kinetics. Predictably, the constructed MXene–BiVO₄–FeOOH composite photoanode exhibits an impressive current density of 4.95 mA/cm² (1.23 V vs. RHE) and 4.1 times higher than that of pristine BiVO₄, as well as an ultra-low onset potential and high stability. This fantastic enhancement is attributed to the MXene conductive framework avoiding carrier recombination, tightening the interface of BiVO₄–FeOOH, moreover O-vacancied FeOOH improves the adsorption of H₂O to accelerate the water oxidation kinetics. This work demonstrates a successful method for constructing efficient and stable photoanodes as well as diversifying the application of MXene in photoelectrocatalysis.     

Selective Etching of Silicon from Ti3SiC2 (MAX) To Obtain 2D Titanium Carbide (MXene)

Until now, MXenes could only be produced from MAX phases containing aluminum, such as Ti₃AlC₂. Here, we report on the synthesis of Ti₃C₂ (MXene) through selective etching of silicon from titanium silicon carbide—the most common MAX phase. Liters of colloidal solutions of delaminated Ti₃SiC₂‐derived MXene (0.5–1.3 mg mL^?1) were produced and processed into flexible and electrically conductive films, which show higher oxidation resistance than MXene synthesized from Ti₃AlC₂. This new synthesis method greatly widens the range of precursors for MXene synthesis.     

A Two‐Dimensional Zirconium Carbide by Selective Etching of Al3C3 from Nanolaminated Zr3Al3C5

The room‐temperature synthesis of a new two‐dimensional (2D) zirconium‐containing carbide, Zr₃C₂T_z MXene is presented. In contrast to traditional preparation of MXene, the layered ternary Zr₃Al₃C₅ material instead of MAX phases is used as source under hydrofluoric acid treatment. The structural, mechanical, and electronic properties of the synthesized 2D carbide are investigated, combined with first‐principles density functional calculations. A comparative study on the structrual stability of our obtained 2D Zr₃C₂T_z and Ti₃C₂T_z MXenes at elevated temperatures is performed. The obtained 2D Zr₃C₂T_z exhibits relatively better ability to maintain 2D nature and strucural integrity compared to Ti‐based Mxene. The difference in structural stability under high temperature condition is explained by a theoretical investigation on binding energy.     

Solar Light Driven H2O2 Production and Selective Oxidations Using a Covalent Organic Framework Photocatalyst Prepared by a Multicomponent Reaction

A chemically stable 2D microporous COF ( PMCR-1 ) was synthesized via the multicomponent Povarov reaction. PMCR-1 exhibits a remarkable and long-term stable photocatalytic H₂O₂ production rate (60 h) from pure and sea water under visible light. The H₂O₂ production is markedly enhanced when benzyl alcohol (BA) is added as reductant, which is also due to a strong π–π interaction of BA with dangling phenyl moieties in the COF pores introduced by the multicomponent Povarov reaction. Motivated by the concomitant BA oxidation to benzaldehyde during H₂O₂ formation, the photocatalytic oxidation of various organic substrates such as benzyl amine and methyl sulfide derivatives was investigated. It is shown that the well-defined micropores of PMCR-1 enable size-selective photocatalytic oxidation.     

Tuning the Electrochemical Performance of Titanium Carbide MXene by Controllable In Situ Anodic Oxidation

MXenes are a class of two‐dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high‐rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti₃C₂T_x, T_x refers to ‐F, =O, ‐Cl and ‐OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s^?1 (with respect to the lowest scan rate of 5 mV s^?1) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti₃C₂T_x is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti₃C₂T_x. This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.     

A Two‐Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two‐dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti₃AlC₂, delaminated nanosheets of the composition Ti₃C₂T_x with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m⁻² h⁻¹ bar⁻¹) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long‐time operation also reveals the outstanding stability of the MXene membrane for water purification.     

Direct Observation of Electron-Vibration Coupling at MXene-Solvent Interface?

MXenes, a new family of two-dimensional (2D) materials, have received extensive interest due to their fascinating physicochemical properties, such as outstanding light-to-heat conversion efficiency. However, the photothermal conversion mechanism of MXenes is still poorly understood. Here, by using femtosecond visible and mid-infrared transient absorption spectroscopy, the electronic energy dissipation dynamics of MXene (Ti₃C₂T_x) nanosheets dispersed in various solvents are carefully studied. Our results indicate that the lifetime of photoexcited MXene is strongly dependent on the surrounding environment. Especially, the interfacial electron-vibration coupling between the MXene nanosheets and the adjacent solvent molecules is directly observed following the ultrafast photoexcitation of MXene. It suggests that the interfacial interactions at the MXene-solvent interface play a critical role in the ultrafast energy transport dynamics of MXene, which offers a potentially feasible route for tailoring the light conversion properties of 2D systems.     

Functional MXene Materials: Progress of Their Applications

Nowadays, two‐dimensional materials have many applications in materials science. As a novel two‐dimensional layered material, MXene possesses distinct structural, electronic, and chemical properties; thus, it has potential applications in many fields, including battery electrodes, energy storage materials, sensors, and catalysts. Up to now, more than 70 MAX phases have been reported. However, in contrast to the variety of MAX phases, the existing MXene family merely includes Ti₂C, Ti₃C₂, (Ti_1/2, Nb_1/2)₂C, (V_1/2, Cr_1/2)₃C₂, Nb₂C, Ti₃CN, Ta₄C₃, V₂C, and Nb₄C₃. Among these materials, the Ti₃C₂T_x MXene exhibits prominently high volumetric capacitance, and the rate at which it transports electron is suitable for electrode materials in batteries and supercapacitors. Hence, Ti₃C₂T_x is commonly utilized as an electrode material in ion batteries such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺ batteries. What is more, Ti₂C has the biggest specific surface area among all of these potential MXene phases, and therefore, Ti₂C has remarkably high gravimetric hydrogen storage capacities. In addition, Ti₂CO₂ materials display extremely high activity for CO oxidation, which makes it possible to design catalysts for CO oxidation at low temperatures. Furthermore, Ti₃C₂T_x with O, OH, and/or F terminations can be used for water purification owing to excellent water permeance, favorable filtration ability, and long‐time operation ability. This review supplies a relatively comprehensive summary of various applications of MXenes over the past few years.     

Reactivity of gaseous sodiated ions derived from benzene dicarboxylate salts toward residual water in the collision gas

The sodium adduct of disodium salts of benzene dicarboxylic acids (m/z 233), when subjected to collision‐induced dissociation (CID), undergoes a facile loss of CO₂ to produce an ion of m/z 189, which retains all the three sodium atoms of the precursor. The CID spectrum of this unusual m/z 189 ion shows significant peaks at m/z 167, 63 and 85. The enigmatic m/z 167 ion, which appeared to represent a loss of a 22‐Da neutral fragment from the precursor ion is in fact a fragment produced by the interaction of the m/z 189 ion with traces of water present in the collision gas. The change of the m/z 167 peak to 168, when D₂O vapor was introduced to the collision gas of a Q‐ToF instrument, proved that such an intervention of water could occur even in collision cells of tandem‐in‐space mass spectrometers. The m/z 189 ion has such high affinity for water; it forms an ion/molecule complex even during the brief residence time of ions in collision cells of triple quadrupole instruments. The complex formed in this way then eliminates elements of NaOH to produce the ion observed at m/z 167. In an ion trap, the relative intensity of the m/z 167 peak increases with longer activation time even at the lowest possible collision energy setting. Similarly, the m/z 145 ion (which represents the sodium adduct of phenelenedisodium, formed by two consecutive losses of CO₂ from the m/z 233 ion of meta‐ and para‐isomers) interacts with water to produce a fragment ion at m/z 123 for the sodium adduct of phenylsodium. Other uncommon ions that originate also from water/ion interactions are observed at m/z 85 and 63 for [Na₃O]⁺ and [Na₂OH]⁺, respectively. Tandem mass spectrometric experiments conducted with appropriately deuterium‐labeled compounds confirmed that the proton required for the formation of the [Na₂OH]⁺ ion originates from traces of water present in the collision gas and not from the ring protons of the aromatic moiety. Copyright © 2010 John Wiley & Sons, Ltd.     

Highly Conductive MXene Film Actuator Based on Moisture Gradients

MXene (Ti₃C₂T_x) is a new 2D material with both hydrophilicity and high electrical conductivity, and it has shown promise in smart electronic devices. Reported herein is a homogeneous MXene film actuator with high electrical conductivity triggered by moisture gradients. The actuator is highly sensitive to moisture and undergoes deformation, with the maximum bending angle as high as 155° at a relative humidity difference of 65 %. Several analysis methods show that the humidity drive and large deformation of the MXene film occur in situ by asymmetric expansion of the bilayer structure. The combination of deformation and electrical conductivity makes this film applicable to flexible excavators, electrical switches, and other fields, applications that are difficult to achieve directly by using other 2D materials. More importantly, this work further expands the new application range of MXene materials and provides new opportunities for building the next generation of high‐conductivity smart actuators.     

CO oxidation on MXene (Mo2CS2) supported single-atom catalyst: A termolecular Eley-Rideal mechanism

Finding transition metal catalysts for effective catalytic conversion of CO to CO₂ has attracted much attention. MXene as a new 2D layered material of early transition metal carbides, nitrides, and carbo-nitrides is a robust support for achoring metal atoms. In this study, the electronic structure, geometries, thermodynamic stability, and catalytic activity of MXene (Mo₂CS₂) supported single noble metal atoms (NM = Ru, Rh, Pd, Ir, Pt and Au) have been systematically examined using first-principles calculations and ab initio molecular dynamic (AIMD) simulations. First, AIMD simulations and phonon spectra demonstrate the dynamic and thermal stabilities of Mo₂CS₂ monolayer. Three likely reaction pathways, Langmuir-Hinshelwood (LH), Eley-Rideal (ER), and Termolecular Eley–Rideal (TER) for CO oxidation on the Ru₁- and Ir₁@Mo₂CS₂ SACs, have been studied in detail. It is found that CO oxidation mainly proceeds via the TER mechanism under mild reaction conditions. The corresponding rate-determining steps are the dissociation of the intermediate (OCO-Ru₁-OCO) and formation of OCO-Ir₁-OCO intermediate. The downshift d-band center of Ru₁- and Ir₁@Mo₂CS₂ help to enhance activity and improve catalytst stability. Moreover, a microkinetic study predicts a maximum CO oxidation rate of 4.01 × 10² s^-1 and 4.15 × 10³ s^-1 (298.15 K) following the TER pathway for the Ru₁- and Ir₁@Mo₂CS₂ catalysts, respectively. This work provides guideline for fabricating and designing highly efficient SACs with superb catalyts using MXene materials.     

Photoinduced Oxidation of Water with Potassium Persulfate in the Presence of Ruthenium Trinuclear Complex

The kinetics and mechanism of the photoinduced reaction of water oxidation with potassium persulfate in the presence of the tris(bipyridyl) complex bpy₃RuCl₂ ? 6H₂O as a photosensitizer and the trinuclear ruthenium complex (NH₃)₅Ru–O–Ru(NH₃)₄–O–Ru(NH₃)₅Cl₆ have been studied. It has been shown that this complex is converted to a more active water oxidation catalyst when four or five NH₃ ligands are replaced by oxygen atoms.     

MXene: A two-dimensional material in selective water separation via pervaporation

MXene, well-identified as Ti₃C₂T_X, belongs to the family of two-dimensional (2D) materials, which have been currently explored in various applications. Very recently, such materials have been pointed out as potential nanomaterials for advanced solute separations when introduced in membranes, such as ion separation, gas separation, nanofiltration, chiral molecular separation, and solvent separation. This latter separation, generally named Pervaporation (PV), is identified as a highly selective technology for water separations. To date, few pieces of research have been released but providing interesting insights into several solvent (including water) separations. Hence, this brief review aims to analyze and discuss the latest advances for utilizing MXenes for PV membranes. Particular emphasis has been devoted to the relevant outcomes in the field, along with the strategies followed by researchers to tailor membranes. Based on the current findings, the perspectives in the field are also stated.     

MoS2‐on‐MXene Heterostructures as Highly Reversible Anode Materials for Lithium‐Ion Batteries

Two‐dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition‐metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS₂‐on‐MXene heterostructures through in situ sulfidation of Mo₂TiC₂T_x MXene. The computational results show that MoS₂‐on‐MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li₂S adsorption during the intercalation and conversion reactions. These characteristics render the as‐prepared MoS₂‐on‐MXene heterostructures stable Li‐ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications.     

Pebax‐Based Membrane Filled with Two‐Dimensional Mxene Nanosheets for Efficient CO2 Capture

Mixed matrix membranes (MMMs) made from inorganic fillers and polymers is a kind of promising candidate for gas separation. In this work, two‐dimensional MXene nanosheets were synthesized and incorporated into a polyether‐polyamide block copolymer (Pebax) matrix to fabricate MMM for CO₂ capture. The physicochemical properties of MXene nanosheets and MXene/Pebax membranes were studied systematically. The introduction of MXene nanosheets provided additional molecular transport channels and meanwhile enhanced the CO₂ adsorption capacity, thereby enhancing both the CO₂ peremance and CO₂/N₂ selectivity of Pebax membrane. The optimized MXene/Pebax membrane with a MXene loading of 0.15 wt % displayed a high separation performance with a CO₂ permeance of 21.6 GPU and a CO₂/N₂ selectivity of 72.5, showing potential application in CO₂ capture.     

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO₂ proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO₂ reduction. Here, by employing findings from operando X-ray absorption spectroscopy at the near and extended Au and Ce L_III energy edges, we focus on the fundamental aspects of highly active Au/CeO₂ catalysts, mainly in the CO oxidation for understanding their complex structure-reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O₂ activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO₂ support, and the dynamics of O₂ activation.     

Density oscillations in a model of water and other similar liquids

It is suggested that the dynamics of liquid water have a component consisting of O^?2z (oxygen) anions and H^+z (hydrogen) cations, where z is a (small) reduced effective electron charge. Such a model may apply to other similar liquids. The eigenmodes of density oscillations are derived for such a two-species ionic plasma, included the sound waves, and the dielectric function is calculated. The plasmons may contribute to the elementary excitations in a model introduced recently for the thermodynamics of liquids. It is shown that the sound anomaly in water can be understood on the basis of this model. The results are generalised to an asymmetric short-range interaction between the ionic species as well as to a multi-component plasma, and the structure factor is calculated.     

Preparation of MXene-Cu2O nanocomposite and effect on thermal decomposition of ammonium perchlorate

MXenes are novel graphene-like 2-D materials. Cu₂O is an effective additive for thermal decomposition of ammonium perchlorate (AP). We reported the synthesis of MXene (Ti₃C₂), Cu₂O and MXene-Cu₂O respectively. The samples were characterized by means of X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Results indicate that the MXene is composed of lots of nano-sheets and the thickness is 30 ± 10 nm, and Cu₂O nanoparticles nucleate and grow heterogeneously directly on the surface of MXene. The effect of these MXene, Cu₂O and MXene-Cu₂O samples on the thermal decomposition of AP were investigated using TG-DSC. The results revealed that MXene-Cu₂O have a great influence on the thermal decomposition of AP than that of pure MXene and Cu₂O.     

Bismuth Nanoparticles Anchored on Ti3C2Tx MXene Nanosheets for High-Performance Sodium-Ion Batteries

Sodium-ion batteries are promising energy-storage systems, but they are facing huge challenges for developing fast-charging anode materials. Bismuth (Bi)-based anode materials are considered as candidates for fast-charging anodes of sodium-ion batteries due to their excellent rate performance. Herein, we designed a two-dimensional Bi/MXene anode material based on a hydrogen thermal reduction strategy. Benefitting from microstructure advantages, Bi/MXene anodes exhibited an excellent rate capability and superior cycle performance in Na//Bi/MXene half-batteries and Na₃V₂(PO₄)₃/C//Bi/MXene full-batteries. Moreover, full-batteries can complete a charge/discharge cycle in 7 min and maintain an excellent cycle life (over 7000 cycles). The electrochemical test results showed that Bi/MXene is a promising anode material with fast charge/discharge capability for sodium-ion batteries.     

Highly Efficient Adsorption of Bilirubin by Ti3C2Tx MXene

We discovered that the 2D Ti₃C₂T_x MXene sheet displays an ultra-high removal capability for bilirubin (BR). In particular, MXene shows 47.6 times higher removal efficiency over traditional activated carbon absorbents. The effect of MXene on the removal rate of BR in BR solution containing different concentrations of bovine serum albumin (BSA) was studied. The adsorption capacity of BSA for BR at high concentration of 5 g L⁻¹ was about 85% of the best adsorption capacity. The MXene before and after adsorption was characterized by SEM, FT-IR and XPS. Furthermore, MXene beads were prepared, and the hemoperfusion simulation experiment was carried out. The results show that the adsorption capacity of MXene for bilirubin can reach 1192.9 mg g⁻¹. This study suggests that MXene may be promising in the treatment of hyperbilirubinemia.