A critical analysis of the X-ray photoelectron spectra of Ti3C2Tz MXenes

Interest in MXenes is quite high because of their potential a wide range applications. Although several have been discovered to date, most studies so far focused mainly on Ti3C2Tz. Given 2D nature and rich surface chemistry, XPS the preferred technique for analyzing compositions. To only four understanding fitting spectra Ti3C2Tx flakes. While there are commonalities between various protocols, also some inconsistencies that sown confusion. In this review, we summarize models proposed especially Ti3C2Tz propose new model based critical analysis previously published studies. We point out areas where more work needed better understand spectra. feel article will help researchers analyze data using other materials. Since discovery 2011, garnered worldwide interest. structure, surface, or termination, chemistries play vital role X-ray photoelectron spectroscopy (XPS) one common characterization tools quantifying terminations overall chemistry. Herein, critically review MXene literature make case they at best incomplete worst contradictory. algorithm all obtained from method our approach, assign Ti 2p peak 455.1 eV C–Ti–O\O\O, peaks 456.0, 457.0, 457.9, 459.6 assigned C–Ti–O\O\F, C–Ti–O\F\F, C–Ti–F\F\F, TiO2-xF2x, respectively. The first represent possible atom terminations; last an oxyfluoride. C 1s 282 eV, ascribed atoms surrounded by 6 atoms, universal it can almost be used as reference. After titanium carbide (Ti3C2Tz), MXene, nearly 30 decade, others predicted stable according computational calculations awaiting experimental realization.1Naguib M. Kurtoglu Presser V. Lu J. Niu Heon Hultman L. Gogotsi Y. Barsoum M.W. Two-dimensional nanocrystals produced exfoliation Ti3AlC2.Adv. Mater. 2011; 23: 4248-4253Crossref PubMed Scopus (4431) Google Scholar,2Verger Xu C. Natu Cheng H.-M. Ren W. Overview synthesis ultrathin transition metal carbides nitrides.Curr. Opin. 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Kalra Dispersion stabilization alkylated nonpolar solvents pseudocapacitive behavior.Cell Rep. Phys. 1: 100042Abstract (15) electromagnetic interference (EMI) shielding,8Shahzad F. Alhabeb Hatter C.B. Anasori Man Hong Koo C.M. Electromagnetic shielding with (MXenes).Science. 2016; 353: 1137-1140Crossref (2233) among many others. labeled selective etching A atomic layers parent MAX phases. phases general formula Mn+1AXn M stands early metal, X and/or N, mostly group 13 14 element.9Sokol Kota On chemical diversity phases.Trends 210Abstract (210) Scholar, 10Sun Z.M. Progress research development phases: family layered ternary compounds.Int. Rev. 56: 143-166Crossref (729) 11Barsoum Phases. Wiley-VCH Verlag GmbH & Co. KGaA, 2013Crossref (694) When layers, Al, etched, replaced terminations, Mn+1XnTz, Tz represents typically combination –O, –OH, –F.12Halim Cook K.M. Naguib Eklund Rosen select multi-layered (MXenes).Appl. Surf. 362: 406-417Crossref (754) -ene suffix was added signify similarity materials graphene.1Naguib recently molten salt etch layer leads –Cl, –Br, –I, further modifications result –S, –Te, –NH terminations.13Li Luo Chang Chen Zhou al.Element replacement approach reaction Lewis acidic salts synthesize nanolaminated MXenes.J. Am. 141: 4730-4737Crossref (284) Scholar,14Kamysbayev Filatov A.S. Hu Rui X. Lagunas Wang D. Klie R.F. Talapin D.V. Covalent superconductivity MXenes.Science. 369: 979-983Crossref (256) morphology MXenes, important determining properties. For example, responsible spontaneous ion intercalation exchange layers.15Ghidiu Halim Bish Ion-exchange cation solvation reactions Ti3C2 MXene.Chem. 28: 3507-3514Crossref (328) They hydrophilicity which renders them easily processable materials.16Natu Effect edge charges stability aggregation colloidal suspensions.J. 2018; 122: 27745-27753Crossref (70) devices supercapacitor batteries, shown participate charge mechanism.3Lin Recent Kamysbayev al.14Kamysbayev showed Nb2CTz dependent type terminations. function tuned modifying its groups, making good electronic contacts circuits.17Schultz T. Frey N.C. Hantanasirisakul Park May S.J. Shenoy V.B. Koch N. Surface termination properties 6590-6597Crossref (144) Apart these applications, influencing optical, mechanical, magnetic, multitude properties.18Jiang Kuklin A.V. Baev Ågren Zhang Prasad P.N. MXenes: morphological electric, magnetic applications.Phys. 848: 1-58Crossref (276) Due properties, characterize properly. There number techniques do so, including (XPS), adsorption (XAS),19Lukatskaya M.R. Bak S.-M. Yu Yang X.-Q. Probing mechanism capacitance situ absorption spectroscopy.Adv. 2015; 5: 1500589Crossref (336) NMR spectroscopy,20Hope M.A. Forse A.C. Griffith K.J. Lukatskaya Ghidiu Grey C.P. reveals functionalisation MXene.Phys. 18: 5099-5102Crossref Scholar,21Harris Bugnet Goward G.R. Direct measurement groups connectivity V2CTx spectroscopy.J. 119: 13713-13720Crossref (107) Raman spectroscopy,22Sarycheva structure chemistry 32: 3480-3488Crossref (169) transmission electron microscopy (TEM) coupled loss (EELS).23Karlsson L.H. Birch Persson P.O.Å. Atomically resolved structural investigation single sheets.Nano Lett. 15: 4955-4960Crossref (232) Scholar,24Magne Mauchamp Chartier Cabioc’h Site-projected MXene: functionalization groups.Phys. 30946-30953Crossref remains widely techniques. advantages sensitive light elements except H He.25Hofmann Auger- X-Ray Photoelectron Spectroscopy Science. Springer, It bonding environments constituent elements, quantify elemental compositions, detect small concentrations if surfaces.25Hofmann Other include non-destructive sample preparation fact restrictions size form low. spectrometers available university settings large laboratories. flakes.12Halim Scholar,17Schultz Scholar,26Persson I. Näslund L.-Å. Darakchieva Palisaitis organization thermal behavior functional surfaces vacuum.2D 2017; 015002Crossref (137) Scholar,27Benchakar Loupias Garnero Bilyk Canaff al.One phase, different guideline crucial conditions chemistry.Appl. 530: 147209Crossref And while similarities sow confusion interpreting results. Ti3C2Tz, model, fit surrounding each rather than oxidation states many, us, done. argue ambiguity higher O F compared those Before going into detailed discussion fits, should noted spectrum has two roughly 455 463 corresponding 2p3/2 2p1/2, split due spin-orbit splitting, both contain information about environment. Therefore, when component deconvolute peaks, components under 2p1/2 (further details given later). Component fitted what species present here onwards regions whenever mentioned refer not envelope peaks. following sections, fits. study dedicated carried al.12Halim 2016. fittings attribute three 455.0 455.8 457.2 Ti+1, Ti+2, Ti+3 states, respectively (Figure 1A, Table 1). separation ΔTi2p, caused splitting (see below) kept fixed 6.2 5.5 5.7 (Table 1).12Halim This ΔTi2p consistent related compounds such carbide, TiC, found ∼6.1–6.2 eV. TiO (Ti+2) ∼5.6–5.7 Ti2O3 (Ti+3) ∼5.7 eV.28Guemmaz Mosser Parlebas J.-C. Electronic changes induced vacancies spectral elastic nitrides.J. Electron. Spectros. Relat. Phenomena. 2000; 107: 91-101Crossref Scholar,29Biesinger M.C. Lau L.W.M. Gerson A.R. Smart R.S.C. Resolving row metals, oxides hydroxides: Sc, Ti, V, Cu Zn.Appl. 2010; 257: 887-898Crossref (2129) below), 6.1 Ti3C2Tz.Table 1Summary fitsRegionBE (eV)FWHM (eV)FractionAssigned toTi (2p1/2)455.0 (461.2)0.8 (1.5)0.28C–Ti–(O/OH)455.8 (461.3)1.5 (2.2)0.30C–Ti2+–(O/OH)457.2 (462.9)2.1 (2.1)0.32C–Ti3+–(O/OH)458.6 (464.2)0.9 (1.0)0.02TiO2459.3 (465.3)0.9 (1.4)0.03TiO2-xF2x460.2 (466.2)1.6 (2.7)0.05C–Ti–FC 1s282.00.60.54C–Ti–(O/OH/F)284.71.60.38C–C286.31.40.08CHx/C–OO 1s529.91.00.29TiO2531.21.40.18C–Ti–Ox OR532.01.10.18C–Ti–OHx OR532.81.20.19Al2O3 OR533.82.00.17H2O ORF 1s685.01.70.38C–Ti–F685.31.10.29TiO2-xF2x686.42.00.30AlFx688.32.00.02Al(OF)xThe prepared phase 48% HF. BE FWHM values listed columns 2 3, Respective numbers brackets. These fits al.,12Halim Figure 1. Open table tab C–Ti–F set 460.2 fluorinated TiF3.30Mousty-Desbuquoit Riga Verbist J.J. titanium(III) titanium(IV) halides studied solid-phase x-ray spectroscopy.Inorg. 1987; 26: 1212-1217Crossref (29) around 459.3 (ΔTi2p = 5.6 eV) 1) were TiO2 TiO2-xF2x oxides, respectively, formed degradation/oxidation ambient air water. Note assumed originate flakes themselves, but oxyfluorides. same assumption below. Irrespective present, high-resolution associated appear narrow ≈281.9–282.0 1B, signal originates residing octahedra. rest contamination introduced during air. synthesized F-ion-containing acids check calibration As discussed below, exceptions.. 1C, 1), 531.2 532.0 –O –OH Three 529.9 532.8 533.8 to, TiO2, Al2O3, adsorbed interlayer water, Contributions C–O, C=O, –COOH, etc. adventitious tend overlap described earlier, estimation difficult probably leading overestimation C–Ti–F, AlFx, Al(OF)x 685.0, 685.3, 686.4, 688.3 With 38% total photoemission attributed itself, majority aforementioned impurities. Henceforth, referred Fit-I. al.26Persson model. crystal sites occupy.26Persson dealing spectra, differentiated C–Ti–(O,F), C–Ti–F\F\F. notation, henceforth, separated “\” present. C–Ti–O\O\O corresponds octahedra 3 vertices. notation C–Ti–(O,F) mixed terminations.26Persson Their density theory (DFT) Khazaei al.31Khazaei Arai Sasaki Chung C.-Y. Venkataramanan N.S. Estili Sakka Kawazoe Novel nitrides.Adv. Funct. 2013; 2185-2192Crossref (922) who thermodynamically favorable surfaces: occupying positions above central (so-called FCC sites) B right below (Figures 2E ). C–Ti–F\F\F 455.9 456.9 2C, 2). just noted, distinguish C–Ti–O\F\F configurations, constant Also, no oxide included authors scanned after washing minimal exposure air.Table 2Summary (eV)Assigned (2p1/2)455.1 (461.2)0.7 (1.4)C–Ti–O\O\O455.9 (462.0)1.1 (1.9)C–Ti–(O, F)456.9 (463.0)1.1 (1.9)C–Ti–F\F\FC 1s282.00.64Ti–C–Ti284.31.3C–C285.21.3CHx286.41.3C–OHO 1s529.90.89C–Ti–O (bridging)531.31.2C–Ti–O (A site)531.91.6–2.3C–Ti–O/F site)F 1s684.51.0C–Ti–F685.41.0C–Ti–O, FThe 10% brackets.26Persson summary annealing 200°C. 2. bridging sites, third co-absorbed –F sites. respective energies 531.3, 529.9, 531.9 2B, Similarly, observed 684.5 685.4 co-adsorbed 2A, Similar Fit-I, 282.0 flakes; contaminations. henceforth Fit-II. Schultz al.17Schultz measured heating 3). 3G) C–Ti–C, C–Ti–O, 455.2 5.9 eV), 456.2 5.4 457.3, 3G, bonded (i.e., Ti2 3B), second pure 3G). made octahedrally ones closer surfaces. differentiation trivial, say least.Table 3Summary (2p1/2)455.2 (461.1)Ti–C456.2 (461.6)Ti–O457.3 (462.7)Ti–FC 1s281.9Ti–C–Ti284.9C–C286.2C–O288.9C=OO 1s529.8C–Ti–O (bridge)531.6C–Ti–O (A/B site)532.5C–O533.4C=O533.8adsorbed H2O C–Ti–OHF 1s685.2C–Ti–F687.1F contaminationSample Ti3AlC2 LiF + HCl mixture. Numbers brackets column locations outside brackets.17Schultz correspond collected RT. 3. Sample 529.8, 531.6, 3H), A/B 3B 3C), OH claimed