The Investigation of Ti3C2Tx MXene Surface Chemistry for Electrochemical Energy Storage

Electrochemical energy storage devices, such as batteries and supercapacitors, play a pivotal role not only for the increasing demand on renewable energy storage but also for the growing electric vehicles industry. In this context, surface redox (pseudocapacitive) active materials have shown a remarkable increase in both energy and power densities. Titanium carbide Ti3C2Tx MXene is an efficient pseudocapacitive 2D material combining high metallic conductivity with hydrophilic surfaces. The Ti3C2Tx reveals large capacitance by using sulfuric acid as an electrolyte owing to the surface redox charging mechanism. The electrochemical performance of Ti3C2Tx MXene is significantly influenced by interlayer spacing between MXene nanosheets which is altered by the amount of the nanoconfined water and/or intercalants. In this work, X-ray-based techniques were used to study the impact of the MXene surface chemistry as well as its interlayer spacing on its overall electrochemical performance. Ti3C2Tx MXene were investigated using synchrotron-based soft X-ray absorption spectroscopy (XAS) and X-ray photoemission electron microscopy (XPEEM). The XAS peaks are very sensitive to changes in the local chemical environment induced by different types of intercalants and the nanoconfined water between the MXene layers. The oxidation state of the surface Ti atoms in Ti3C2Tx has been then extensively investigated as it constitutes a key element in the electrochemical performance. Here we show that the intercalation of organic molecules, like urea, as well as mono- and multi-valent cations such as Li+, Na+, K+, and Mg2+ affects the Ti oxidation state in different environments. We show that a controlled higher oxidation state increases the MXene capacitance. In addition, spatially resolved XA spectra were implemented to study the Ti oxidation state of pristine and intercalated single multi-layered Ti3C2Tx flakes. On the other hand, the interlayer spacing between Ti3C2Tx nanosheets was monitored by X-ray diffraction (XRD). In situ XRD patterns taken at different temperatures revealed for the first time the signature of the nanoconfined water in MXene at low temperatures, which shows the coexistence of hexagonal and cubic ice structures. This work illustrates the significance of the X-ray-based techniques to probe the electronic structure of transition metal oxide surfaces and the nanoconfined water of MXenes in various environments. It paves the way to operando XAS combined with electrochemical performance (cyclic voltammogram) which would help to identify the changes in chemical bonds during a redox reaction.Elektrochemische Energiespeicher, wie Batterien oder Superkondensatoren, spielen eine entscheidende Rolle in Zeiten steigendender Nachfrage an Speichern für erneuerbare Energien, sowie bei der Weiterentwicklung der Elektromobilität. Innerhalb dieses Rahmens konnte die Energie- und Leistungsdichte oberflächenaktiver Materialien, welche fähig sind zu reversiblen Redoxreaktionen (Pseudokapazität), bemerkenswert gesteigert werden. Ein effizientes, pseudokapazitives, zweidimensionales Material stellt Titancarbid in der Form Ti3C2Tx MXene dar. Es kombiniert eine hohe metallische Leitfähigkeit mit hydrophilen Oberflächen. Das Ti3C2Tx offenbart eine hohe elektrische Kapazität unter Verwendung von Schwefelsäure und aufgrund seiner redox-aktiven Oberfläche. Das elektrochemische Verhalten von Ti3C2Tx MXene wird dabei maßgeblich vom Schichtabstand zwischen den einzelnen MXene-Nanosheets beeinflusst. Dieser wird geformt durch Interkalationen und/ oder Wassereinschlüsse. In dieser Arbeit wurde der Einfluss von Oberflächenbeschaffenheit und Schichtabstand auf das allgemeine elektrochemische Verhalten mittels Röntgenspektroskopie studiert. Um das Ti3C2Tx MXene zu untersuchen, wurde die auf Synchrotronstrahlung basierende Röntgenabsorptionsspektroskopie (XAS) sowie die Photoemissionselektronenmikroskopie (XPEEM) verwendet. Die Röntgenabsorption spitze ist sensitiv auf die Änderung der lokalen chemischen Umgebung, hervorgerufen durch verschiedenartige Interkalationen sowie Wassereinschlüsse zwischen den einzelnen MXene-Schichten. Der Oxidationszustand der Ti Atome an der Oberfläche wurde ausgiebig untersucht und stellt ein Schlüsselelement in der elektrochemischen Funktion dar. Es wird gezeigt, dass der Einschluss organischer Moleküle, wie Harnstoff, ebenso wie einfach und mehrfach geladener Kationen, wie Li+, Na+ , K+ und Mg2+ die Oxidationsstufe des Ti beeinflussen können. Dabei wurde festgestellt, dass eine höhere Oxidation die Kapazität steigert. Zusätzlich wurde räumlich aufgelöste Röntgenabsorption angewandt um den Zustand des Ti in unveränderten Ti3C2Tx Spänen mit dem interkalierter und mehrfachgeschichteter Ti3C2Tx Späne zu vergleichen. Die Schichtabstände wurden zusätzlich mittels Röntgenbeugung (XRD) analysiert. Von besonderer Bedeutung ist dabei die Tatsache, in situ Röntgenbeugung dass zum Ersten Mal die Signatur von eingeschlossenem Wasser bei niedrigen Temperaturen aufgezeichnet werden konnte, zeigt die koexistenz von hexagonalen und kubischen Eisstrukturen. Die vorliegende Arbeit zeigt auf, von welcher Bedeutung die Verwendung röntgenbasierter Methoden für die Untersuchung der elektronischen Struktur von übergangsmetalloxidischen Oberflächen und Wassereinschlüssen im MXene in verschiedenen Umgebungen ist. Damit ist der Weg bereitet für operando XAS kombiniert mit elektrochemischer Leistung (cyclic Voltammogram), die helfen würde, die Veränderungen chemischer Bindungen während einer Redoxreaktion zu identifizieren

The Timoshenko Three-Beams Technique To Estimate The Main Elastic Moduli Of Orthotropic Homogeneous Materials

A New developed technique to estimate the necessary six elastic constants of homogeneous laminate of special orthotropic properties are presented in this paper for the first time. The new approach utilizes the elasto-static deflection behavior of composite cantilever beam employing the famous theory of Timoshenko. Three extracted strips of the composite plate are tested for measuring the bending deflection at two locations. Each strip is associated to a preferred principal axis and the deflection is measured in two orthogonal planes of the beam domain. A total of five trails of testing is accomplished and the numerical results of the stiffness coefficients are evaluated correctly under the contribution of the macromechanics and the approximate bending theory. To insure the validity of the new approach, separate individual tensile tests are performed, and the corresponding results are compared. Excellent agreements are obtained between the different approaches. The ease, simple and accurate predictions are well confident by the new techniqu

Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photo-excited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization, and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy and in-plane momentum. We first show that the rapidly-established quasi-thermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photo-excited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photo-control experiments and optoelectronic device applications.Comment: 23 pages, 12 figure

Enhanced electron-phonon coupling in graphene with periodically distorted lattice

Electron-phonon coupling directly determines the stability of cooperative order in solids, including superconductivity, charge and spin density waves. Therefore, the ability to enhance or reduce electron-phonon coupling by optical driving may open up new possibilities to steer materials' functionalities, potentially at high speeds. Here we explore the response of bilayer graphene to dynamical modulation of the lattice, achieved by driving optically-active in-plane bond stretching vibrations with femtosecond mid-infrared pulses. The driven state is studied by two different ultrafast spectroscopic techniques. Firstly, TeraHertz time-domain spectroscopy reveals that the Drude scattering rate decreases upon driving. Secondly, the relaxation rate of hot quasi-particles, as measured by time- and angle-resolved photoemission spectroscopy, increases. These two independent observations are quantitatively consistent with one another and can be explained by a transient three-fold enhancement of the electron-phonon coupling constant. The findings reported here provide useful perspective for related experiments, which reported the enhancement of superconductivity in alkali-doped fullerites when a similar phonon mode was driven.Comment: 12 pages, 4 figure

Impact of Cation Intercalation on the Electronic Structure of Ti3C2Tx MXenes in Sulfuric Acid

Intercalation in Ti3C2Tx MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the Ti3C2Tx nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the Ti3C2Tx MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li , Na , K , and Mg intercalated Ti3C2Tx MXenes dispersed initially in water and 10 mM sulfuric acid H2SO4 using X ray absorption spectroscopy XAS . The cation intercalation is found to dramatically influence the chemical environment of Ti atoms. The Ti oxidation of the MXene increases progressively upon intercalation of cations of larger sizes after drying in air, while interestingly a low Ti oxidation is observed for all intercalated MXenes after dispersion in diluted H2SO4. In situ XAS at the Ti L edge was conducted during electrochemical oxidation to probe the changes in the Ti oxidation state in the presence of different cations in H2SO4 aqueous electrolyte. By applying the sensitivity of the Ti L edge to probe the oxidation state of Ti atoms, we demonstrate that cation intercalation and H2SO4 environment significantly alter the Ti3C2Tx surface chemistr

STM Study of Exfoliated Few Layer Black Phosphorus Annealed in Ultrahigh Vacuum

Black Phosphorus (bP) has emerged as an interesting addition to the category of two-dimensional materials. Surface-science studies on this material are of great interest, but they are hampered by bP's high reactivity to oxygen and water, a major challenge to scanning tunneling microscopy (STM) experiments. As a consequence, the large majority of these studies were performed by cleaving a bulk crystal in situ. Here we present a study of surface modifications on exfoliated bP flakes upon consecutive annealing steps, up to 550 C, well above the sublimation temperature of bP. In particular, our attention is focused on the temperature range 375 C - 400 C, when sublimation starts, and a controlled desorption from the surface occurs alongside with the formation of characteristic well-aligned craters. There is an open debate in the literature about the crystallographic orientation of these craters, whether they align along the zigzag or the armchair direction. Thanks to the atomic resolution provided by STM, we are able to identify the orientation of the craters with respect to the bP crystal: the long axis of the craters is aligned along the zigzag direction of bP. This allows us to solve the controversy, and, moreover, to provide insight in the underlying desorption mechanism leading to crater formation

Metal Cation Pre-Intercalated Ti3C2Tx MXene as Ultra-High Areal Capacitance Electrodes for Aqueous Supercapacitors

Two-dimensional transition-metal carbides and nitrides “MXenes” have demonstrated great potential as electrode materials for electrochemical energy storage systems. This is especially true for delaminated Ti3C2Tx, which already shows outstanding gravimetric and volumetric capacitance, with areal capacitance limited by thickness (only a few microns). However, the performance of multilayer Ti3C2Tx has been more modest. Here, we report on using metal cation (viz., Na+, K+, and Mg2+) pre-intercalated multilayer Ti3C2Tx as electrodes for aqueous supercapacitors. These electrodes are scalable and amenable to roll-to-roll manufacturing, with adjustable areal loadings of 5.2 to 20.1 mg/cm2. K–Ti3C2Tx exhibited the highest capacitances at different scan rates. A gravimetric capacitance comparable to that of delaminated MXene of up to 300 F/g was achieved for multilayer K–Ti3C2Tx but with an outstanding ultra-high areal capacitance of up to 5.7 F/cm2, which is 10-fold higher than the 0.5 F/cm2 of delaminated MXene and exceeds the 4.0 F/cm2 of microengineered MXene electrodes

Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene. © 2020 Elsevier Lt

Spatially resolved X ray absorption spectroscopy investigation of individual cation intercalated multi layered Ti3C2Tx MXene particles

Ti3C2Tx MXene is a two dimensional 2D material possessing highly active hydrophilic surfaces coupled with high metallic conductivity. Cations intercalation between the Ti3C2Tx nanosheets has a significant role in many applications such as water purification, desalination, and electrochemical energy storage. The pseudocapacitive charging mechanism involving surface redox reactions at the Ti3C2Tx surface enables higher energy densities than electrical double layer capacitors, and higher power densities than batteries. In this context, the oxidation state of surface Ti atoms involved in redox reactions has a high impact on the capacitance of Ti3C2Tx MXene and this can be impacted by cation intercalation. Thus, the electronic structure of multi layered Ti3C2Tx particles is investigated by X ray absorption XA spectroscopy, while also benefitting from a high spatial resolution of 30 nm from X ray photoemission electron microscopy. In this work, the XA spectra from multi layered intercalated Ti3C2Tx particles of different thicknesses were recorded at the Ti L and O K edges. The Ti oxidation state in pristine, Li , and Mg intercalated Ti3C2Tx was found to be thickness dependent, while Na and K intercalated Ti3C2Tx particles did not reveal differences upon changing thickness. This work demonstrates thickness dependent modification of the MXene surface chemistry upon cation intercalation in different individual Ti3C2Tx particle

Molecular beam growth of graphene nanocrystals on dielectric substrates

We demonstrate the growth of graphene nanocrystals by molecular beam methods that employ a solid carbon source, and that can be used on a diverse class of large area dielectric substrates. Characterization by Raman and Near Edge X-ray Absorption Fine Structure spectroscopies reveal a sp2 hybridized hexagonal carbon lattice in the nanocrystals. Lower growth rates favor the formation of higher quality, larger size multi-layer graphene crystallites on all investigated substrates. The surface morphology is determined by the roughness of the underlying substrate and graphitic monolayer steps are observed by ambient scanning tunneling microscopy.Comment: Accepted in Carbon; Discussion section added; 20 pages, 6 figures (1 updated