MXenes: Versatile 2D materials with tailored surface chemistry and diverse applications

MXenes,the most recent addition to the 2D material family,have attracted significant attention owing to their distinctive characteristics,including high surface area,conductivity,surface characteristics,mechanical strength,etc.This review begins by presenting MXenes,providing insights into their structural characteristics,synthesis methods,and surface functional groups.The review covers a thorough analysis of MXene surface properties,including surface chemistry and termination group impacts.The properties of MXenes are influenced by their synthesis,which can be fluorine-based or fluorinedependent.Fluorine-based synthesis techniques involve etching with fluorine-based reagents,mainly including HF or LiF/HCl,while fluorine-free methods include electrochemical etching,chemical vapor deposition(CVD),alkaline etching,Lewis acid-based etching,etc.These techniques result in the emergence of functional groups such as-F,-O,-OH,-Cl,etc.on the MXenes surface,depending on the synthesis method used.Properties of MXenes,such as electrical conductivity,electronic properties,catalytic activity,magnetic properties,mechanical strength,and chemical and thermal stability,are examined,and the role of functional groups in determining these properties is explored.The review delves into the diverse applications of MXenes,encompassing supercapacitors,battery materials,hydrogen storage,fuel cells,electromagnetic interference(EMI) shielding,pollutant removal,water purification,flexible electronics,sensors,additive manufacturing,catalysis,biomedical and healthcare fields,etc.Finally,this article outlines the challenges and opportunities in the current and future development of MXenes research,addressing various aspects such as synthesis scalability,etching challenges,and multifunctionality,and exploring novel applications.The review concludes with future prospects and conclusions envisioning the impact of MXenes on future technologies and innovation.

Surface Chemistry and Photochemistry of Cyclohexane on Rutile TiO_(2)(110)

Cyclohexane is a high-valued chemical receivingsignificant interest in liquid hydrogen storage technology.TiO_(2)-based catalysts show high performance in the photocatalytic dehydrogenation of cyclohexane under mild conditions,but the detailed reaction mechanism is not well understood.With the surface science approaches,we have studied the adsorption and surface chemistry of cyclohexane on rutile TiO_(2)(110).The thermal desorption spectroscopy and X-ray photoelectron spectroscopy results both demonstrate the molecular adsorption of cyclohexane on rutile TiO_(2)(110).Upon the UV Hg light irradiation,photodesorption of cyclohexane occurs from both the chemisorbed monolayer and the multilayer.No decomposition nor dehydrogenation of cyclohexane occurs on rutile TiO_(2)(110).These results deepen the fundamental understanding of the surface chemistry of cyclohexane on the TiO_(2)surface.

Modulating the microstructure and surface chemistry of carbocatalysts for oxidative and direct dehydrogenation: A review

The catalytic performance of solid catalysts depends on the properties of the catalytically active sites and their accessibility to reactants, which are significantly affected by the microstructure（morphology, shape, size, texture, and surface structure） and surface chemistry（elemental components and chemical states）. The development of facile and efficient methods for tailoring the microstructure and surface chemistry is a hot topic in catalysis. This contribution reviews the state of the art in modulating the microstructure and surface chemistry of carbocatalysts by both bottom‐up and top‐down strategies and their use in the oxidative dehydrogenation（ODH） and direct dehydrogenation（DDH） of hydrocarbons including light alkanes and ethylbenzene to their corresponding olefins, important building blocks and chemicals like oxygenates. A concept of microstructure and surface chemistry tuning of the carbocatalyst for optimized catalytic performance and also for the fundamental understanding of the structure‐performance relationship is discussed. We also highlight the importance and challenges in modulating the microstructure and surface chemistry of carbocatalysts in ODH and DDH reactions of hydrocarbons for the highly‐efficient, energy‐saving,and clean production of their corresponding olefins.

Textural characteristics,surface chemistry and oxidation of activated carbon

Numerous researches were reviewed and interpreted to depict a comprehensive illustration of activated carbon and its behavior towards oxidation.Activated carbon as one of the most important adsorbents is tried to be described in this review paper by terms of its＂Textural Characteristics＂and＂Surface Chemistry＂.These two terms,coupled with each other,are responsible for behavior of activated carbon in adsorption processes and in catalytic applications.Although as-prepared activated carbons are usually nonselective and their surfaces suffer from lack of enough reactive groups,their different aspects may be improved and developed by diverse types of modifications.Oxidation is one of the most conventional modifications used for activated carbons.It may be used as a final modification or as a pre-modification followed by further treatment.In this paper,methods of oxidation of activated carbon and other graphene-layer carbon materials are introduced and wet oxidation as an extensively-used category of oxidation is discussed in more detail.

Influence of heating rate on reactivity and surface chemistry of chars derived from pyrolysis of two Chinese low rank coals

A series of char samples were derived from pyrolysis of two typical low-rank coals in China （Shengli lig- nite and Shenmu bituminous coal） at low, medium and fast heating rates, respectively, to the same pyrol- ysis temperature 750℃. Then these chars were characterized by means of thermogravimetric analysis and Fourier transform infrared spectrometer with the aim to investigate the influence of heating rate in pyrolysis process on gasification reactivity and surface chemistry of them. Besides, a homogeneous model was used to quantitatively analyze the activation energy of gasification reaction. The results reveal that Shengli lignite and its derived chars behave higher gasification reactivity and have less content of oxygen functional groups than Shenmu coal and chars. Meanwhile, chars derived from Shengli lignite at 50℃/min and Shenmu coal at 200℃/min have the greatest gasification reactivity, respectively. The oxygen functional groups in Shengli lignite are easily thermo-decomposed, and they are less affected by the heating rate, while that in Shenmu coal have a significant change with the variation of heating rate. In addition, there is no good correlation between the change of oxygen functional groups and that of the gasification reactivity of the derived chars from pyrolysis at different heating rates.

Role of the surface chemistry of activated carbons in dye removal from aqueous solution

Commercial activated carbons were modified by a series of chemical or physical treatments using H202, NH3, and heating under N2 flow without notably changing their pore structures. The resultant carbons were characterized by N2 adsorption and Bohem titration and then used to remove Ponceau 4R, methyl orange and brilliant blue from aqueous solutions. Surface chemistry was found to play a signifi- cantly different role in removing these three compounds. The removal of anionic Ponceau 4R increases with increasing carbon surface ba- sicity due to the predominant dispersive interaction mechanism. In contrast, surface chemistry has little effect on the removal of anionic methyl orange, which can be explained by two parallel mechanisms involving electrostatic and dispersive interactions due to the basic amine group in a dye molecule. The influence of surface chemistry on the removal of amphoteric brilliant blue dye can also be ignored due to a weak interaction between the carbons and dye molecules, which is resulted from strong cohesive energy from electrostatic forces inside amphoteric dye molecules.

Surface chemistry of MXene quantum dots:Virus mechanism-inspired mini-lab for catalysis

Scientific research is currently more interdisciplinary.Researchers have parsed the surface structure of virus,constructed the interaction model of virus-receptors,offering the clues for studying efficient targeted drugs.Likewise,catalysis is also highly relevant to modern human life.Exploring the surface structure and physicochemical properties of catalysts is of great significance for the design of efficient catalysts.Great progresses have been made for endowing specific physicochemical properties of catalysts through controlling the size of materials and coordination chemistry of active sites,particularly at nanometer scale since Sir John Meurig Thomas and Tao Zhang’s early ground-breaking contribution,with casting on a very surface issue.Herein,functional regulation renders the emerging MXene quantum dots(MQDs)excel in contrast to the typical carbon-based quantum dots.In fact,similar to the interaction of virus-receptors model,the surface functional groups decorated MQDs provide a mini-lab to afford a variety of adjustments,involved with the type modification and electronic structure tuning of groups as well as their arrangement,together with the interaction between the groups and active materials/support,ultimately for packaging or designing high-activity catalysts.

Quantum confinement and surface chemistry of 0.8–1.6 nm hydrosilylated silicon nanocrystals

In the framework of density functional theory （DFT）, we have studied the electronic properties of alkene/alkyne- hydrosilylated silicon nanocrystals （Si NCs） in the size range from 0.8 nm to 1.6 nm. Among the alkenes with all kinds of functional groups considered in this work, only those containing -NH2 and -C4H3S lead to significant hydrosilylation- induced changes in the gap between the highest occupied molecular orbital （HOMO） and the lowest unoccupied molecular orbital （LUMO） of an Si NC at the ground state. The quantum confinement effect is dominant for all of the alkene- hydrosilylated Si NCs at the ground state. At the excited state, the prevailing effect of surface chemistry only occurs at the smallest （0.8 nm） Si NCs hydrosilylated with alkenes containing -NH2 and -C4H3S. Although the alkyne hydrosilylation gives rise to a more significant surface chemistry effect than alkene hydrosilylation, the quantum confinement effect remains dominant for alkyne-hydrosilylated Si NCs at the ground state. However, at the excited state, the effect of surface chemistry induced by the hydrosilylation with conjugated alkynes is strong enough to prevail over that of quantum confinement.

EFFECT OF HYPERCROSSLINKED RESINS SURFACE CHEMISTRY ON THE ADSORPTION OF PHENOL FROM AQUEOUS SOLUTION

Two hypercrosslinked resins with similar physical characters but different surface chemistry were synthesized and used to remove phenol from aqueous solutions. The FTIR spectra, elemental analysis and the Boehm titration were used to characterize the chemical properties of the resins. The adsorption experiments were carried out using the bottle-point technique, and the effects of the surface chemistry on the adsorption were discussed. The adsorption data fit well with the Freundlich model, indicating the heterogeneity of the resins surface. It could be seen from the experimental results that the adsorption capacity increased with the increase in the total surface concentration of oxygen-containing groups. The pH dependence and the effects of ionic strength were also discussed. The kinetic adsorption data fit well with the pseudo-second order model, and the results showed that the surface oxygen-containing groups have little effect on the adsorption rate.

Tuning microstructure and surface chemistry of reduced graphene oxide by mild reduction

Reduced graphene oxide(RGO) sheets with varied contents and types of oxygenated groups were synthesized by Hummers treatment of natural graphite powders followed by different nontoxic and mild reduction methods, which include thermal and chemical reduction with ethylene glycol, KOH and Fe powder. The changes in microstructure and surface chemistry of RGOs were extensively characterized by SEM, TEM, AFM, XRD, XPS and Raman spectrum. The results show that significant exfoliation occurs during oxidation and is retained in reduction processes, along with the formation of curled wavy morphology. Compared with large d spacing(0.852 nm) of graphene oxide(GO), the(002) plane distance decreases to 0.358-0.384 nm of RGOs, indicating efficient tuning of surface functionalities through mild reduction methods. The ID/IG ratio of RGOs is about 1.0-1.15, indicating that reconstructed sp^2 domains have smaller sizes and larger quantity. The content of sp^2 bonded C in GO(36.93%, molar fraction) increases to 45.48%-72.92%(molar fraction) in RGOs, along with a drastic decrease in hydroxyl and epoxy and minor changes in carbonyl and carboxyl. Thermal reduction or chemical reduction produces RGOs with residual functionalities, which may render different chemical activity and is desirable in various applications.

The Modeling of Grain Surface Chemistry

Astrochemistry has made great progress in recent years.Especially the grain surface chemistry played important roles in the explanation of the formation of the interstellar molecules.In this review,we will discuss the progress,including the di erent numerical methods to simulate the ice mantles in the astrochemical models.We will also introduce the laboratory astrochemical experimental results,and their contributions to the grain surface chemistry in the review.

From blocker to booster:Harnessing garnet surface chemistry for advanced solid-state electrolytes

Garnet electrolytes with high ionic conductivity and electrochemical stability are widely used as fillers to fabricate composite solid electrolytes(CPEs)within polymer matrices.However,the performance of CPEs is significantly influenced by the surface characteristics of the garnet electrolyte.Herein,the impact of garnet surface characteristics on CPEs was systematically investigated and a conversion from a typically unstable and Lewis basic surface to a more stable Lewis acidic surface was realized,which is shown to be more conductive to the improved performance of CPEs.By simultaneously removing the Li_(2)CO_(3)layer and applying a Li-Al-O coating,the influence of surface characteristics on CPEs was investigated.The Lewis acid Li-Al-O surface coating not only promotes lithium salt dissociation,improving the ionic conductivity and ionic transfer number,but also prevents the reformation of the passive Lewis basic Li_(2)CO_(3)layer.Compared to garnet with a Lewis basic Li_(2)CO_(3)surface,the garnet modified with a Lewis acid Li-Al-O coating enhances CPEs,which exhibit an improved critical current density of 1.0 mA·cm^(-2)and highly stable lithium symmetric cell cycling for 400 h at 0.2 mA·cm^(-2).This research highlights the importance of surface chemistry in the design of high-performance solid-state batteries and presents a strategic modification approach for garnetbased CPEs.

Recent progress on surface chemistryⅠ:Assembly and reaction

Surface chemistry focuses on the investigation of the adsorption,migration,assembly,activation,reaction,and desorption of atoms and molecules at surfaces.Surface chemistry plays the pivotal roles in both fundamental science and applied technology.This review will summarize the recent progresses on surface assembly,synthesis and catalysis investigated mainly by scanning tunneling microscopy and atomic force microscopy.Surface assemblies of water and small biomolecules,construction of Sierpin′ski triangles and surface chirality are summarized.On-surface synthesis of conjugated carbo-and heterocycles and other kinds of carbon nanostructures are surveyed.Surface model catalysis,including single-atom catalysis and electrochemical catalysis,are discussed at the single-atom level.

Why the abnormal phenomena of D-band center theory exist?A new BASED theory for surface catalysis and chemistry

Since the D-band center theory was proposed,it has been widely used in the fields of surface chemistry by almost all researchers,due to its easy understanding,convenient operation and relative accuracy.However,with the continuous development of material systems and modification strategies,researchers have gradually found that D-band center theory is usually effective for large metal particle systems,but for small metal particle systems or semiconductors,such as single atom systems,the opposite conclusion to the D-band center theory is often obtained.To solve the issue above,here we propose a bonding and anti-bonding orbitals stable electron intensity difference(BASED)theory for surface chemistry.The newly-proposed BASED theory can not only successfully explain the abnormal phenomena of D-band center theory,but also exhibits a higher accuracy for prediction of adsorption energy and bond length of intermediates on active sites.Importantly,a new phenomenon of the spin transition state in the adsorption process is observed based on the BASED theory,where the active center atom usually yields an unstable high spin transition state to enhance its adsorption capability in the adsorption process of intermediates when their distance is about 2.5Å.In short,the BASED theory can be considered as a general principle to understand catalytic mechanism of intermediates on surfaces.

Surface chemistry of electrode materials toward improving electrolyte-wettability:A method review

The electrolyte-wettability at electrode material/electrolyte interface is a criticalfactor that governs the fundamental mechanisms of electrochemical reactionefficiency and kinetics of electrode materials in practical electrochemicalenergy storage. Therefore, the design and construction of electrode materialsurfaces with improved electrolyte-wettability has been demonstrated to beimportant to optimize electrochemical energy storage performance of electrodematerial. Here, we comprehensively summarize advanced strategies and keyprogresses in surface chemical modification for enhancing electrolytewettabilityof electrode materials, including polar atom doping by post treatment,introducing functional groups, grafting molecular brushes, and surfacecoating by in situ reaction. Specifically, the basic principles, characteristics,and challenges of these surface chemical strategies for improving electrolytewettabilityof electrode materials are discussed in detail. Finally, the potentialresearch directions regarding the surface chemical strategies and advancedcharacterization techniques for electrolyte-wettability in the future are provided.This review not only insights into the surface chemical strategies forimproving electrolyte-wettability of electrode materials, but also provides strategicguidance for the electrolyte-wettability modification and optimization ofelectrode materials in pursuing high-performance electrochemical energy storagedevices.

Functional carbon materials addressing dendrite problems in metal batteries:surface chemistry,multi-dimensional structure engineering,and defects

Metal batteries that directly use active metals as anodes are considered as one of the most promising solutions to achieve the energy upgrade of battery technologies,while their practical application still suffers from dendrite problems.Functional carbon materials(FCMs)have demonstrated their great potential in suppressing metal dendrites benefitting from the multiple merits such as chemical tunability and capability of multi-dimensional structure assembly.Here,we initiate a review to present the recent progress in employing FCMs to deal with dendrite problems.It focuses on the surface chemistry and multi-dimensional carbon material engineering,which systematically overcomes the problems through diverse methods,such as reinforcing desolvation,improving interface compatibility,homogenizing electric field,buffering volume expansion and lattice mismatch.In addition,we also refine the long-standing debate about whether surface defects in FCMs are beneficial to suppress the metal dendrites or not,especially in the non-aqueous electrolyte regime.Finally,the remaining challenges for utilizing FCMs to suppress metal dendrites and the possible solutions are proposed to guide the future development.

Pyrolyzed biomass-derived nanoparticles:a review of surface chemistry,contaminant mobility,and future research avenues to fill the gaps

Nanoparticles are abundant in the subsurface,soil,streams,and water bodies,and are often a critical control on elemental speciation,transport and cycling in the natural environment.This review provides an overview of pyrolyzed biomass-derived nanoparticles(PBNPs),their surface properties and reactivity towards aqueous species.We focus specifically on biochar-derived nanoparticles and activated carbon-derived nanoparticles which fall under our classification of PBNPs.Activated carbon-iron(nano)composites are included in some instances where there are significant gaps in literature because of their environmental relevance.Increased use of activated carbon,along with a resurgence in the manufacture and application of biochar for water treatment and soil amendment,has generated significant concerns about the mobility and toxicity of PBNPs derived from the bulk material in environmental applications.Recent examples are discussed to highlight current progress in understanding the influence of PBNPs on contaminant transport,followed by a critical discussion of gaps and future research directions.

A preliminary analysis of the surface chemistry of atmospheric aerosol particles in a typical urban area of Beijing

Atmospheric aerosol particle samples were collected using an Ambient Eight Stage（Non-Viable） Cascade Impactor Sampler in a typical urban area of Beijing from 27 th Sep.to 5th Oct.,2009.The surface chemistry of these aerosol particles was analyzed using Static Time of Flight-Secondary Ion Mass Spectrometry（Static TOF-SIMS）.The factors influencing surface compositions were evaluated in conjunction with the air pollution levels,meteorological factors,and air mass transport for the sampling period.The results show that a variety of organic ion groups and inorganic ions/ion groups were accumulated on the surfaces of aerosol particles in urban areas of Beijing;and hydrophobic organic compounds with short-or middle-chain alkyl as well as hydrophilic secondary inorganic compounds were observed.All these compounds have the potential to affect the atmospheric behavior of urban aerosol particles.PM1.1–2.1and PM3.3–4.7had similar elements on their surfaces,but some molecules and ionic groups demonstrated differences in Time of Flight-Secondary Ion Mass Spectrometry spectra.This suggests that the quantities of elements varied between PM1.1–2.1and PM3.3–4.7.In particular,more intense research efforts into fluoride pollution are required,because the fluorides on aerosol surfaces have the potential to harm human health.The levels of air pollution had the most significant influence on the surface compositions of aerosol particles in our study.Hence,heavier air pollution was associated with more complex surface compositions on aerosol particles.In addition,wind,rainfall,and air masses from the south also greatly influenced the surface compositions of these urban aerosol particles.

Surface chemistry of polymer-supported nano-hydrated ferric oxide for arsenic removal: effect of host pore structure

Immobilization of hydrous ferric oxide(HFO) particles inside solid hosts of porous structure is an important approach to improve their applicability in advanced water treatment such as arsenic and heavy metal removal. Here, we fabricated three polystyrene(PS)-based nano-HFOs and explored the effect of host pore structure on the surface chemistry of the immobilized HFOs. Potentiometric titration of the hybrids and surface complexation modeling of their adsorption towards arsenite and arsenate were performed to evaluate the surface chemistry variation of the loaded HFOs. Polymer hosts of higher surface area and narrower pore size would result in smaller particle size of HFOs and lower the value of the point of zero charge. Also, the site density(normalized by Fe mass) and the deprotonation constants of the loaded HFOs increased with the decreasing host pore size. Arsenite adsorption did not change the surface charge of the loaded HFOs, whereas arsenate adsorption accompanied more of the negative surface charges. Adsorption affinity of both arsenic species with three HFO hybrids were compared in terms of the intrinsic surface complexation constants optimized based on the adsorption edges. HFO loaded in polystyrene host of smaller pore size exhibits stronger affinity with arsenic species.

Surface chemistry of nanoscale Fe_3O_4 dispersed in magnetic fluids

The interaction between stabilizers and nanoparticles is one of the important factors to prepare stable magnetic fluids. The magnetic nano-size Fe3O4 core with single domain and the average grain size around 8-12 nm were prepared by chemical precipitation method. The O/Fe molar ratio of the particle surface was measured by X-ray photoelectron spectroscopy (XPS). The heat effects of stabilizers ad- sorption on nanoparticles were measured by solution calorimetry. The excess amount of oxygen was possibly the result of the hydroxygen formed on the surface of the nanoparticles. The heat effects showed that compounds containing carboxyl groups can be adsorbed chemically on magnetite by forming chemical bonds. The other stabilizers involving NH-groups, such as polyethylene-imine, can be adsorbed physically. The exothermic value is about half of the former case.