Integration of 2D layered CdS/WO_(3) S-scheme heterojunctions and metallic Ti_(3)C_(2) MXene-based Ohmic junctions for effective photocatalytic H_(2) generation

The rapid recombination of photo-generated electron-hole pairs,insufficient active sites,and strong photocorrosion have considerably restricted the practical application of Cd S in photocatalytic fields.Herein,we designed and constructed a 2D/2D/2D layered heterojunction photocatalyst with cascaded 2D coupling interfaces.Experiments using electron spin resonance spectroscopy,ultraviolet photoelectron spectroscopy,and in-situ irradiation X-ray photoelectron spectroscopy were conducted to confirm the 2D layered CdS/WO_(3) step-scheme(S-scheme)heterojunctions and CdS/MX ohmic junctions.Impressively,it was found that the strong interfacial electric fields in the S-scheme heterojunction photocatalysts could effectively promote spatially directional charge separation and transport between CdS and WO_(3) nanosheets.In addition,2D Ti_(3)C_(2) MXene nanosheets with a smaller work function and excellent metal conductivity when used as a co-catalyst could build ohmic junctions with Cd S nanosheets,thus providing a greater number of electron transfer pathways and hydrogen evolution sites.Results showed that the highest visible-light hydrogen evolution rate of the optimized MX-Cd S/WO_(3) layered multi-heterostructures could reach as high as 27.5 mmol/g/h,which was 11.0 times higher than that of pure CdS nanosheets.Notably,the apparent quantum efficiency reached 12.0% at 450 nm.It is hoped that this study offers a reliable approach for developing multifunctional photocatalysts by integrating S-scheme and ohmic-junction built-in electric fields and rationally designing a 2D/2D interface for efficient light-to-hydrogen fuel production.

MXene-Based Composites as Nanozymes in Biomedicine: A Perspective

MXene-based nanozymes have garnered considerable attention because of their potential environmental and biomedical applications.These materials encompass alluring and manageable catalytic performances and physicochemical features,which make them suitable as(bio)sensors with high selectivity/sensitivity and efficiency.MXene-based structures with suitable electrical conductivity,biocompatibility,large surface area,optical/magnetic properties,and thermal/mechanical features can be applied in designing innovative nanozymes with area-dependent electrocatalytic performances.Despite the advances made,there is still a long way to deploy MXene-based nanozymes,especially in medical and healthcare applications;limitations pertaining the peroxidaselike activity and sensitivity/selectivity may restrict further practical applications of pristine MXenes.Thus,developing an efficient surface engineering tactic is still required to fabricate multifunctional MXene-based nanozymes with excellent activity.To obtain MXene-based nanozymes with unique physicochemical features and high stability,some crucial steps such as hybridization and modification ought to be performed.Notably,(nano)toxicological and long-term biosafety analyses along with clinical translation studies still need to be comprehensively addressed.Although very limited reports exist pertaining to the biomedical potentials of MXene-based nanozymes,the future explorations should transition toward the extensive research and detailed analyses to realize additional potentials of these structures in biomedicine with a focus on clinical and industrial aspects.In this perspective,therapeutic,diagnostic,and theranostic applications of MXene-based nanozymes are deliberated with a focus on future per-spectives toward more successful clinical translational studies.The current state-of-the-art biomedical advances in the use of MXene-based nanozymes,as well as their developmental challenges and future prospects are also highlighted.In view of the fascinating properties of MXene-based nanozymes,these materials can open significant new opportunities in the future of bio-and nanomedicine.

MXene-based fibers:Preparation,applications,and prospects

With the vigorous development and huge demand for portable wearable devices,wearable electronics based on functional fibers continue to emerge in a wide range of energy storage,motion monitoring,disease prevention,electromagnetic interference(EMI)shielding,etc.MXene,as an emerging twodimensional inorganic compound,has shown great potential in functional fiber manufacturing and has attracted much research attention due to its own good mechanical properties,high electrical conductivity,excellent electrochemical properties and favorable processability.Herein,this paper reviews recent advances of MXene-based fibers.Speaking to MXene dispersions,the properties of MXene dispersions including dispersion stability,rheological properties and liquid crystalline properties are highlighted.The preparation techniques used to produce MXene-based fibers and application progress regarding MXene-based fibers into supercapacitors,sensors,EMI shielding and Joule heaters are summarized.Challenges and prospects surrounding the development of MXene-based fibers are proposed in future.This review aims to provide processing guidelines for MXene-based fiber manufacturing,thereby achieving more possibilities of MXene-based fibers in advanced applications with a view to injecting more vitality into the field of smart wearables.

Progress in MXene-based catalysts for oxygen evolution reaction

Electrochemical water splitting for hydrogen generation is considered one of the most promising strategies for reducing the use of fossil fuels and storing renewable electricity in hydrogen fuel.However,the anodic oxygen evolution process remains a bottleneck due to the remarkably high overpotential of about 300 mV to achieve a current density of 10 mA cm^(−2).The key to solving this dilemma is the development of highly efficient catalysts with minimized overpotential,long-term stability,and low cost.As a new 2D material,MXene has emerged as an intriguing material for future energy conversion technology due to its benefits,including superior conductivity,excellent hydrophilic properties,high surface area,versatile chemical composition,and ease of processing,which make it a potential constituent of the oxygen evolution catalyst layer.This review aims to summarize and discuss the recent development of oxygen evolution catalysts using MXene as a component,emphasizing the synthesis and synergistic effect of MXene-based composite catalysts.Based on the discussions summarized in this review,we also provide future research directions regarding electronic interaction,stability,and structural evolution of MXene-based oxygen evolution catalysts.We believe that a broader and deeper research in this area could accelerate the discovery of efficient catalysts for electrochemical oxygen evolution.

MXenes for Bioinspired Soft Actuators:Advancements in Angle-Independent Structural Colors and Beyond

Soft actuators have garnered substantial attention in current years in view of their potential appliances in diverse domains like robotics,biomedical devices,and biomimetic systems.These actuators mimic the natural movements of living organisms,aiming to attain enhanced flexibility,adaptability,and versatility.On the other hand,angle-independent structural color has been achieved through innovative design strategies and engineering approaches.By carefully controlling the size,shape,and arrangement of nanostructures,researchers have been able to create materials exhibiting consistent colors regardless of the viewing angle.One promising class of materials that holds great potential for bioinspired soft actuators is MXenes in view of their exceptional mechanical,electrical,and optical properties.The integration of MXenes for bioinspired soft actuators with angle-independent structural color offers exciting possibilities.Overcoming material compatibility issues,improving color reproducibility,scalability,durability,power supply efficiency,and cost-effectiveness will play vital roles in advancing these technologies.This perspective appraises the development of bioinspired MXene-centered soft actuators with angleindependent structural color in soft robotics.

Rational modulation of emerging MXene materials for zinc-ion storage

Currently,emerging battery technologies are being studied intensively.The lack of ideal electrode materials remains a key hindrance to further development of new batteries.The novel MXene material,due to its unique structural characteristics and physicochemical properties,shows the ability to enhance the electrochemical performance of the electrode.In addition to the adjustable layer spacing,because of excellent electrical conductivity,rich chemical composition,and controllable surface chemistry,MXene has the potential for use in diverse applications as an ideal electrode material in various battery systems.This review covers the recent progress achieved in research of the zinc-ion energy storage of MXenes,including MXene-based cathodes,MXenederived cathodes,MXene-modified zinc anodes,and electrolyte additives.Moreover,further structural design and reaction mechanisms of MXenes in zinc-ion storage are explored.

One-step growth of ultrathin CoSe_(2)nanobelts on N-doped MXene nanosheets for dendrite-inhibited and kinetic-accelerated lithium-sulfur chemistry

The practical application of lithium–sulfur(Li–S)batteries is inhibited by the shuttle effect of lithium polysulfides(LiPSs)and slow polysulfide redox kinetics on the S cathode as well as the uncontrollable growth of dendrites on the Li metal anode.Therefore,both cathode and anode sides must be considered when modifying LiS batteries.Herein,two-dimensional(2D)ultrathin CoSe_(2)nanobelts are in situ grown on 2D N-doped MXene nanosheets(CoSe_(2)@N-MXene)via one-step solvothermal process for the first time.Owing to its unique 2D/2D structure,CoSe_(2)@N-MXene can be processed to crumpled nanosheets by freeze-drying and flexible and freestanding films by vacuum filtration.These crumpled CoSe_(2)@NMXene nanosheets with abundant active sites and inner spaces can act as S hosts to accelerate polysulfide redox kinetics and suppress the shuttle effect of LiPSs owing to their strong adsorption ability and catalytic conversion effect with LiPSs.Meanwhile,the CoSe_(2)@N-MXene film(CoSe_(2)@NMF)can act as a current collector to promote uniform Li deposition because it contains lithiophilic CoSe_(2)and N sites.Under the systematic effect of CoSe_(2)@N-MXene on S cathode and Li metal anode,the electrochemical and safety performance of Li–S batteries are improved.CoSe_(2)@NMF also shows excellent storage performances in flexible energy storage devices.

Electron delocalization-enhanced sulfur reduction kinetics on an MXene-derived heterostructured electrocatalyst

Lithium-sulfur(Li-S)batteries mainly rely on the reversible electrochemical reaction of between lithium ions(Li^(+))and sulfur species to achieve energy storage and conversion,therefore,increasing the number of free Li^(+)and improving the Li^(+)diffusion kinetics will effectively enhance the cell performance.Here,Mo-based MXene heterostructure(MoS_(2)@Mo_(2)C)was developed by partial vulcanization of Mo_(2)C MXene,in which the introduction of similar valence S into Mo-based MXene(Mo_(2)C)can create an electron delocalization effect.Through theoretical simulations and electrochemical characterisation,it is demonstrated that the MoS_(2)@Mo_(2)C heterojunction can effectively promote ion desolvation,increase the amount of free Li^(+),and accelerate Li^(+)transport for more efficient polysulfide conversion.In addition,the MoS_(2)@Mo_(2)C material is also capable of accelerating the oxidation and reduction of polysulfides through its sufficient defects and vacancies to further enhance the catalytic efficiency.Consequently,the Li-S battery with the designed MoS_(2)@Mo_(2)C electrocatalyst performed for 500 cycles at 1 C and still maintained the ideal capacity(664.7 mAh·g^(−1)),and excellent rate performance(567.6 mAh·g^(−1)at 5 C).Under the extreme conditions of high loading,the cell maintained an excellent capacity of 775.6 mAh·g^(−1)after 100 cycles.It also retained 838.4 mAh·g^(−1)for 70 cycles at a low temperature of 0℃,and demonstrated a low decay rate(0.063%).These results indicate that the delocalized electrons effectively accelerate the catalytic conversion of lithium polysulfide,which is more practical for enhancing the behaviour of Li-S batteries.

Recent progress in MXene layers materials for supercapacitors:High-performance electrodes

In 2011,Gogotsi et al.discovered a new type of two-dimensional transition metal carbides and nitrides,called MXenes,which have become a dazzling new star in the energy storage industry.MXenes are endowed with a series of fascinating properties due to their unique structures and tunable surface chemical functional groups.The application of MXenes in electrochemical energy storage has attracted special attention,especially showing great potential in supercapacitor applications.Compared with other materials,MXenes have high mechanical flexibility,high energy density,and good electrochemical performance,so they are especially suitable as electrode materials for supercapacitors.However,similar to other 2D materials,due to the strong van der Waals forces,MXene layers inevitably undergo stacking agglomeration,resulting in severe loss of electrochemically active sites.If the self-stacking of MXenes layers can be effectively suppressed,their electrochemical performance will be enhanced.Structural optimization of MXenes and composite doping of MXenes with other materials are two strategies with significant effects.This review summarizes recent advances in MXene synthesis,fundamental properties,and composite materials,focusing on the latest electrochemical performance of MXene-based electrodes/devices,and puts forward the challenges and new opportunities that MXenes face in this emerging energy storage field.

Ultrathin Zn-free anode based on Ti_(3)C_(2)T_(x) and nanocellulose enabling high-durability aqueous hybrid Zn-Na battery with Zn2+/Na+co-intercalation mechanism

With low cost and high safety,aqueous zinc-based batteries have received considerable interest.Nevertheless,the excess utilization of zinc metal in the anodes of these batteries reduces energy density and increases costs.Herein,an ultrathin electrode of approximately 6.2μm thick is constructed by coating Ti_(3)C_(2)T_(x)/nanocellulose hybrid onto a stainless steel foil.This electrode is used as the Zn-free anode for aqueous hybrid Zn-Na battery,in which,a concentrated electrolyte is used to improve electrochemical reversibility.The Ti_(3)C_(2)T_(x)/nanocellulose coating is found to improve the electrolyte wettability,facilitate desolvation process of hydrated Zn^(2+) ions,lower nucleation overpotential,improve zinc plating kinetics,guide horizontal zinc plating along the Zn(002)facet,and inhibit parasitic side reactions.It is also found that the Na_(3)V_(2)(PO_(4))_(3) cathode material adopts a highly reversible Zn^(2+)/Na^(+)co-intercalation charge storage mechanism in this system.Thanks to these benefits,the assembled hybrid Zn-Na battery exhibits excellent rate capability,superior cyclability,and good anti-freezing ability.This work provides a new concept of electrode design for electrochemical energy storage.

Construction of Frame-Structured Flexible MXene Film Electrode to Achieve High Areal Capacitance Micro-Supercapacitor

Two-dimensional MXene-based film materials as flexible electrodes have been widely studied in wearable microsupercapacitors(MSCs).However,the existence of strong van derWaals interactions leads to serious self-stacking ofMXene layers,resulting in poor ionic dynamics and loss of active sites,which causes MXene film electrodes to exhibit low capacitance and poor rate performance in practical studies.To solve this,a frame-structured hybrid film(labeled as CN-MX hybrid film)is constructed by introducing intercalating agents(nanometer g-C_(3)N_(4))into MXene layers.In this unique hybrid film,the g-C_(3)N_(4)nanoparticles rationally occupy the interspace between MXene layers so as to alleviate layer stacking,thus effectively expanding the electrochemically active surface and promoting proton transfer.Synergistic pseudocapacitance inducted by g-C_(3)N_(4)surface groups,consequently,the CN-MX hybrid film electrode achieves an enhanced capacitive capability.In the three-electrode system,this frame-structured film electrode exhibits an ultra-high areal capacitance of 1932.8 mF cm^(−2).The assembled symmetry flexible MSC device based on CN-MX hybrid film can achieve an energy density of 2.28μWh cm^(−2)at 0.075 mW cm^(−2),as well as a superior cyclic stability with 90.4%retention after 700 cycles in alternating 90o bending and releasing states,revealing its potential in practical applications.

Review of MXenes as new nanomaterials for energy storage/delivery and selected environmental applications

Energy and environmental issues presently attract a great deal of scientific attention. Recently, two-dimensional MXenes and MXene-based nanomaterials have attracted increasing interest because of their unique properties (e.g., remarkable safety, a very large interlayer spacing, environmental flexibility, a large surface area, and thermal conductivity). In 2011, multilayered MXenes (Ti3C2Tx, a new family of two-dimensional (2D) materials) produced by etching an A layer from a MAX phase of Ti3AlC2, were first described by researchers at Drexel University. The term “MXene” was coined to distinguish this new family of 2D materials from graphene, and applies to both the original MAX phases and MXenes fabricated from them. We present a comprehensive review of recent studies on energy and environmental applications of MXene and MXene-based nanomaterials, including energy conversion and storage, adsorption, membrane, photocatalysis, and antimicrobial. Future research needs are discussed briefly with current challenges that must be overcome before we completely understand the extraordinary properties of MXene and MXene-based nanomaterials.

Regulating interfacial morphology and charge-carrier utilization of Ti_(3)C_(2)modified all-sulfide CdS/ZnIn_(2)S_(4)S-scheme heterojunctions for effective photocatalytic H_(2) evolution

The separation efficiency of electrons and holes and the enhancement of the surface reductive reaction in the metal sulfide semiconductor photocatalysts are important factors in boosting photocatalytic H_(2)evolution from water.The control of both interface morphology and the charge-carrier utilization of metal sulfide-based photocatalysts can effectively improve the separation efficiency of electrons and holes and increase the surface reaction active sites,which are considered to be effective methods to improve the photocatalytic activity of semiconductors.Here,the Ti_(3)C_(2)(Mxene)modified all-sulfide 2D/2D Sscheme heterojunction Ti_(3)C_(2)/Zn In_(2)S_(4)(ZIS)/CdS composite material was firstly synthesized by a two-step solvothermal method.The formation of all-sulfide S-scheme heterojunction improves the efficiency of electron-hole separation.The intimate 2D/2D van der Waals structure provides a strong interaction force and a large contact area to enhance charge transfer.The addition of 2D Ti_(3)C_(2)forms the accumulation layer,reducing the recombination of electrons and holes.Under the synergistic promotion,the highest hydrogen production of the prepared Ti_(3)C_(2)/ZIS/CdS composite photocatalyst could reach 8.93 mmol/h/g.This work not only enriches the photocatalytic systems through integrating the ohmic junction and the 2D/2D all-sulfide S-scheme heterojunction,but also provides a satisfactory design strategy for engineering interfacial morphology and charge-carrier utilization.

Synthesis of MXene and its application for zinc-ion storage

Since 2020,some new breakthroughs in the field of MXene synthesis scheme such as water-free etching,HCl-based hydrothermal etching,halogen etching,and other novel synthesis methods have been proposed.Not only that,the application of MXene in zinc-ion storage devices has also made great progress in the past 2 years.The understanding of zinc-ion storage mechanism of MXene has undergone profound changes,and its applications have also become diversified,demonstrating the great potential of MXene for high performance zinc-ion storage devices.In this review,we have summarized the preparation and synthesis of MXene materials and systematically investigated the progress of MXene in aqueous zinc-ion storage devices.In particular,for the synthesis of MXene,we added recent reports of conventional synthesis schemes that have been widely reported to help understand their development and combined with recent novel synthesis schemes to provide a distinct partition framework.In addition,for the application of MXene,we discussed the cognitive change of zinc-ion storage mechanism of MXene and conducted an in-depth discussion about the design philosophy of MXene and their characteristics.Finally,a comprehensive perspective on the future development of MXene in the synthetic strategy and aqueous zinc-ion storage applications have been outlined.