【introduction】

MXenes is a type of transition metal carbide / nitride material with a two-dimensional layered structure. Due to its unique physical and chemical properties and excellent biocompatibility, it has attracted wide attention from researchers. In energy storage / conversion, photovoltaics Many fields such as catalysis, biomedicine, and sensors have great application prospects. By adjusting the surface end and the layer spacing of MXenes material, it can enrich its electrochemical reaction sites and improve the electronic structure, thereby achieving a highly controllable material properties.
Synchrotron radiation light source and advanced characterization technology are important experimental platforms for frontier basic science and national strategic core technology research. X-ray absorption fine structure (XAFS) spectroscopy has been widely used to study the microstructure, energy storage and energy conversion of two-dimensional materials Mechanism, this technology also provides unique insights into the in-depth understanding of the dynamic process of MXenes-based energy materials.

【Achievement Introduction】

Recently, Professor Song Li of the University of Science and Technology of China and Associate Researcher Chen Shuangming (co-corresponding author) and others have focused on the unique advantages of synchrotron radiation technology in the research of MXenes materials, from the structure and preparation strategy of MXenes, surface regulation and interlayer of MXenes materials The four aspects of architecture, MXenes synchrotron radiation characterization, and MXenes-based energy applications are reviewed in recent years, and the future challenges and development directions of MXenes materials engineering and synchrotron radiation characterization research are prospected. Related achievements were published on Advanced Functional Materials as "Tuning 2D MXenes by Surface Controlling and Interlayer Engineering: Methods, Properties, and Synchrotron Radiation Characterizations" . The research group Dr. Wang Changda is the first author of this article .

【Graphic introduction】

Figure 1 Summary of preparation and research direction of MXenes material

Figure 2 The structure of MXenes material with surface ends

(a) Various materials i) TiC, ii) Ti 2 AlC, iii) CrC, iv) Cr 2 AlC and v) TiAl charge density contour map;
(b) Structure of M 2 X and M 2 XT x : I-ii) top and side views of M 2 X, iii-v) top and side views of functionalized MXene model 1, model 2 and model 4;
(c) single layer Ti 3 C 2 and Ti 3 C 2 T x optimized geometry: i) Ti 3 C 2 single-layer side view, ii) I-Ti 3 C 2 F 2 , II-Ti 3 C 2 F 2 and III-Ti 3 C 2 F 2 side view , Iii) I-Ti 3Side view of C 2 (OH) 2 , II-Ti 3 C 2 (OH) 2 and III-Ti 3 C 2 (OH) 2 .

Figure 3: Preparation strategy of MXenes

(a) HF etching preparation: i) Ti 3 C 2 T x synthesis and structure diagram, ii-iii) Ti 3 C 2 T x SEM and HAADF images;
(b) HCl / LiF synthesis method: i) Ti 3 Different synthetic route diagrams of C 2 T x sheets, ii-iii) SEM and TEM of Ti 3 C 2 T x sheets prepared by synthetic route 1 and iv-iv) synthetic route 2 ;
(c) Sintering in molten salt Schematic diagram of the synthesis of Ti 4 N 3 T x from Ti 4 AlN 3 ; (d) Hydrothermal preparation of fluorine-free MXenes: i-iii) Reaction of Ti 3 AlC 2 with NaOH aqueous solution under different conditions; (e) Fluorine-free MXenes Electrochemical preparation: i) Ti 3 AlC 2

A schematic view of the anode material during the etching phase of the body, ii) an electrochemical cell structure, III-IV) of Ti . 3 AlC 2 and of Ti . 3 C 2 T X sectional high resolution the TEM;
(F) in a molten salt freon MXenes Lewis Schematic diagram of acid etching effect;
(g) Preparation of epitaxial MXene thin film by chemical vapor deposition: i) Schematic diagram of synthesis process, ii-iii) Schematic diagram of Ti 3 C 2 T x and STEM.

Figure 4 The effect of functional groups and defects on the surface of MXenes on the electronic structure and properties

Effect of (a) MXenes of the conductive surface of the end: I) of Ti . 3 C 2 T X situ electron energy loss spectrum, II) of Ti . 3 C 2 T X resistivity at room temperature and the concentration of F atoms with annealing temperature change map;
( B) H 2 after annealing of defects of Ti . 3 C 2 T X influence of magnetic properties: I) of Ti . 3 C 2 T X high resolution TEM and H 2 structure after annealing evolution schematic, under ii) 2K, applied with the magnetic moment M Magnetic field H curve;
(c) Effect of F and OH on lithium ion storage capacity of V 2 CT x : i) Side view of V 2 CLi 6 , ii-iv) V 2 CLi 6 , IV 2 CF 2 Li and IV 2Top view of C (OH) 2 Li 1.5 ;
(d) The effect of -F and -O on the performance of Ti 3 C 2 T x pseudocapacitor: i) Schematic diagram of the relationship between capacitance and functional group after n-BuLi treatment, ii-iii) in 1 CV and GCD curves of M-Ti 3 C 2 T x and nM-Ti 3 C 2 T x and LM-Ti 3 C 2 T x modified by n-BuLi or LiOH in MH 2 SO 4 , respectively .

Figure 5. The interlayer structure of MXenes

(a) Theoretical study of ion intercalation M 2 C-based MXenes: i) Schematic diagram of intercalation reaction of Li ⁺ in Ti 2 CO 2 , ii) Global screening of the capacity and voltage of two-ion intercalation;
(b) Electrochemistry Methods for different cationic intercalation;
(c) Cationic intercalation improves the pseudo-capacitance of Ti 3 C 2 T x : i) Synthetic schematic diagram of Ti 3 C 2 T x with controlled interlayer spacing and end group modification , ii-iii ) of Ti . 3 C 2 T X , and-of KOH-of Ti 400 . 3 C 2 the TEM‘s;
(D) synthesis by ion exchange of Co ²⁺ V intercalated 2 C schematic;
(E) S atoms intercalation of Ti . 3 C 2 is Synthetic diagram.

Fig. 6 XAFS characterization for MXenes structure exploration

(a) XANES spectra of Nb 2 AlC, Nb 2 CT x MXene, NbC and Nb 2 O 5 and the Fourier transform spectra of EXAFS of Nb 2 CT x , NbC and Nb 2 O 5 ;
(B) monatomic Pt modification of Ti . 3 C 2 structure MXene analysis of the catalyst: I) Pt . 1 / of Ti . 3-X C 2 T Y prepared schematic, II) Pt . 1 / of Ti . 3-X C 2 T Y of HAADF-STEM, iii) Ti K-edge XANES spectrum and Ti foil, Ti 2 AlC 2 and Ti 3−x C 2 Ty Fourier transform spectrum of EXAFS, iv) XANES spectrum of Pt L 3 edge and Pt foil, PtO 2 and Pt 1 / Ti 3−x C 2 T y Fourier transform spectrum of EXAFS;
(c) Co Structural analysis of ²⁺ intercalated V 2 C MXene: i) SEM and high-resolution TEM of V 2 C @ Co MXenes, ii) Co K-edge XANES spectrum and V 2 C @ Co, CoO, Co 2 O 3 and Co foil The Fourier transform spectrum of EXAFS.

Figure 7. Soft x-ray absorption spectroscopy (sXAS) studies of MXenes structure

(a) Interaction between carbon nitride and Ti 3 C 2 T x nanosheets: i) gC 3 N 4 and Ti 3 C 2 T x nanosheets ( TCCN) Schematic diagram of preparation of hybrid porous membrane, ii) TCCN high-resolution TEM, iii) NK-edge sXAS spectra of TCCN and gC 3 N 4 ;
(b) Surface structure of Sn ⁴⁺ intercalation V 2 C MXene in lithium ion batteries Analysis: i-ii) High-resolution TEM of V 2 C and V 2 C @ Sn MXenes, iii-iv) O and FK side sXAS spectra of V 2 C @ Sn electrode;
(c) Layered V 2 in zinc ion batteries C MXene and CNT film structure study: i) DV 2 C @ CNT film preparation schematic, ii) DV 2 C @ CNT side SEM, iii) DV 2SXAS spectra of VL side and OK side of C @ CNT membrane electrode in H 2 SO 4 electrolyte before and after charging and discharging.

In-situ and ex-situ XAFS studies eight MXenes FIG

V (A) 2 CT X in of Na ⁺ -situ intercalation of XAFS and storage mechanisms: I) V 2 CT X MXenes expansion / contraction schematic, ii) V 2 CT x MXenes ex-situ VK edge XANES spectrum, iii) normalized XANES spectrum at half height corresponding voltage curve and V absorption edge energy change;
(b) V 2 C @ Sn MXenes Li ⁺ dynamics In-situ XAFS study of storage mechanism: i) Schematic diagram of operating XAFS test environment, ii) Normalized XANES spectrum of V 2 C @ Sn electrode in-situ VK during charging and discharging , iii) V 2 C @ Sn at different voltages The average valence of the V atom in the middle compared to V 2 O 3 and VO 2 changes, iv) normalized in situ Sn K-edge XAFS spectrum during the first two charge and discharge processes, v) Sn K after Fourier transform EXAFS spectrum;
(c) Ti 3 C 2 T x in sulfuric acid electrolyteIn-situ XAFS study of kinetic mechanism: i) Schematic diagram of Ti 3 C 2 (OH) 2 structure, ii) CV curve of Ti 3 C 2 T x electrode in 1 MH 2 SO 4 during in-situ XAFS characterization , iii ) The XANES spectrum of the Ti K edge at different voltages, iv) The normalized change in the energy of the Ti absorption edge at the half height of the XANES spectrum at the selected voltage.

Fig. 9 Application of capacitors based on modified MXenes

(a) Nitrogen doping improves the performance of Ti 3 C 2 T x supercapacitors: i) Schematic diagram of Ti 3 C 2 T x MXene doped with nitrogen atoms , ii) Hydrated electrolyte ions in Ti Schematic diagram of charge storage in 3 C 2 T x and N- Ti 3 C 2 T x MXenes, iii) Ti 3 C 2 T x and N- Ti 3 C 2 at different scan rates in 1 MH 2 SO 4 electrolyte Specific capacitance of T x ; (b) Ti 3 C 2 arranged vertically and T x capacitance are independent of thickness: i) Ti 3 arranged vertically
Schematic diagram of ion transport in C 2 T x MXene membrane, ii) SEM of MXLLC membrane, iii) CV curve of vacuum filtered MXene and MXLLC membrane at a scan rate of 100 mV / s, iv) Stability of 200 μm thick MXLLC membrane And GCD curves at different current densities;
(c) Asymmetric microsupercapacitors based on MnO 2 and Ti 3 C 2 MXene: i) Schematic diagram of asymmetric microsupercapacitors, ii) Planar supercapacitors and traditional Schematic diagram of ion transport in sandwich-type supercapacitors, iii-iv) Compared with traditional sandwich-structure supercapacitors, the capacitance and Ragone diagram of asymmetric planar microsupercapacitors ;
(d) KV 2 C MXenes based hybrid capacitors: i) Schematic diagram of the synthesis process of KV 2 C MXene, ii) GCD curve of KV 2 C anode and K x MnFe (CN) 6 cathode, iii) Rate performance of KV 2 C // K x MnFe (CN) 6 battery.

Fig. 10 Battery application based on modified MXenes

(a) CTAB-Sn (IV) @Ti 3 C 2 lithium-ion battery: i) Schematic diagram of CTAB-Sn (IV) @Ti 3 C 2 preparation, ii) Rate performance test;
( b) V 2 C @ Sn MXene lithium ion battery: i-iii) V 2 C @ Sn electrode rate performance, GCD curve and stability test in lithium ion battery;
(c) Ti 2 C-based lithium-sulfur battery: i) Schematic diagram of S-Ti-C bond replacing Ti-OH bond when heated or in contact with polysulfide, ii) GCD curve of 70S / Ti 2 C;
(d) Surface modification of Ti3C2Tx MXene to improve lithium battery performance: i ) Surface atom modification enhances the anchoring ability of Li 2 Sn and S 8 to Ti 3 C 2 T x MXene, ii-iii) S 8 and Li 2 Sn in Ti 3 C 2 and Ti 3 C2 T x adsorption energy;
(e) Li 3 metal electrode based on Ti 3 C 2 T x MXene / graphene frame: i) Schematic diagram of 3D MG-Li anode preparation process and corresponding photos, ii) Based on MG-Li , RGO-Li and metal Li electrode battery constant current cycle.

Figure 11. Catalytic application based on modified MXenes

(a) Hydrogen evolution reaction of Mo 2 TiC 2 T x MXene based on single Pt atom modification : i-ii) Schematic diagram of electrochemical stripping process and Mo 2 TiC 2 T x fixed by single Pt atom Synthesis mechanism of MXene, iii) HER 2 polarization curve of Mo 2 TiC 2 T x with platinum foil as counter electrode , iv) Mo 2 TiC 2 T x -Pt SA and other samples with graphite rod as counter electrode at 0.5 MH 2 HER polarization curve in SO 4 solution;
(b) DFT study of electro-reduced CO 2 based on OH terminated MXenes : i) free energy diagram of the lowest energy path of CH 4 ; ii) volcanic curve of OH terminated MXenes;
( c) DFT study based on functionalized MXenes electrochemical reduction of nitrogen: i) Schematic diagram of nitrogen reduction process on MXenes surface, ii) Calculation of Mo 2 containing different functional groupsPourbaix graph obtained by C, iii) Calculate the free energy graph of the NRR mechanism on Mo 2 C MXene at different ends .

【Summary and Outlook】

The MXenes modified by surface modification and interlayer regulation show great advantages and potential in many fields such as supercapacitors, batteries, catalysts and so on. The surface groups and interlayer distances of MXenes have a significant impact on their structure and application. However, the study of the impact of monofunctional groups is usually limited to theoretical calculations. Therefore, it is necessary to improve the preparation method and process to synthesize MXenes with specific surface ends, while achieving Selective intercalation of specific intercalation agents to achieve improved performance in different areas. In addition, the rational design of MXenes-based composite materials is also crucial for achieving breakthroughs in different fields.
Thanks to the development of synchrotron radiation light sources, synchrotron radiation technology has been successfully applied in the in-depth study of two-dimensional materials, providing a new perspective for the in-depth study of MXenes-based energy materials. Facing the future research needs of MXenes structure and mechanism, other synchrotron radiation characterization such as SXRD, SXPS, SR-FTIR and APXPS should also be developed and applied to promote the further research of MXenes materials.

Literature link: Tuning 2D MXenes by Surface Controlling and Interlayer Engineering: Methods, Properties, and Synchrotron Radiation Characterizations. (Adv. Funct. Mater. 2020, DOI: 10.1002 / adfm.202000869)

Information source: material cattle