Capacitive Performance of Two-Dimensional Metal Carbides

Recently a new family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. Unlike graphene, whose chemistry is restricted to carbon, MXenes allow a variety of chemical compositions and are establishing themselves as a large new class of two-dimensional materials. MXenes combine the metallic conductivity of transition metal carbide layers with the hydrophilic nature of their mostly hydroxyl or oxygen terminated surfaces. In essence, they behave as "conductive clays" and have shown much of promise as electrode materials for Li-ion batteries. Prior to the initiation of this study, there have been no reports on the capacitive properties of MXenes. In this work the potential was explored of the new family of the two-dimensional carbides, MXenes, as electrode materials for electrochemical capacitors. This study was focused on Ti3C2Tx. It was established that variety of single- and multiply charged cations (such as Li+, Na+, K+, NH4+, Mg2+) can intercalate MXenes (chemically or electrochemically) and participate in energy storage. Highly reversible electrochemical insertion of the same cations has been demonstrated for Ti3C2Tx in aqueous electrolytes. Perfect capacitive behavior was observed for Ti3C2Tx MXene even at quite high charge and discharge rates, all coupled with excellent cyclability; no drop in capacitance was observed even after 10 000 cycles. Further investigation showed that surface chemistry has significant effect on the resulting capacitance, i.e. by creating predominantly oxygen-containing functionalities the capacitance can be substantially boosted in comparison to the as-received material. It was also demonstrated that Ti3C2Tx clay produced using LiF-HCl mixturewith predominantly oxygen-containing functionalities, showed outstanding capacitance up to 900 F/cm3 and can be manufactured in to electrodes in less than 10 min without need of binder or conductive additive. Electrochemical in-situ XAS measurements detected changes in Ti oxidation state during cycling, which matched closely the observed experimental values of the material's capacitance. Therefore it was concluded that mechanism of electrochemical storage of the Ti3C2Tx MXene clay is predominantly pseudocapacitive. . Also concept of all-solid-state asymmetrical supercapacitor (freestanding and current collector free) based on Ti3C2Tx was developed. Among other applications, using in-situ AFM the potential of the use of MXenes in electrochemical actuators was demonstrated. It was also shown that MXenes other than Ti3C2Tx also demonstrated a lot of promise for electrochemical capacitors: Nb2CTx/CNT paper electrodes showed high volumetric capacitance of 325 F/cm3 when tested in a Li-ion capacitor configuration.Ph.D., Materials Science and Engineering -- Drexel University, 201

Separation and Liquid Chromatography Using a Single Carbon Nanotube

Use of a single template-grown carbon nanotube as a separation column to separate attoliter volumes of binary mixtures of fluorescent dyes has been demonstrated. The cylindrical nanotube walls are used as stationary phase and the surface area is increased by growing smaller multi-walled carbon nanotubes within the larger nanotube column. Liquid-liquid extraction is performed to separate selectively soluble solutes in a solvent, and chromatographic separation is demonstrated using thin, long nanotubes coated inside with iron oxide nanoparticles. The setup is also used to determine the diffusion coefficient of a solute at the sub-micrometer scale. This study opens avenues for analytical chemistry in attoliter volumes of fluids for various applications and cellular analysis at the single cell level

2D metal carbides and nitrides (MXenes) for energy storage

The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research

Effect of Synthesis on Quality, Electronic Properties and Environmental Stability of Individual Monolayer Ti\u3csub\u3e3\u3c/sub\u3eC\u3csub\u3e2\u3c/sub\u3e MXene Flakes

2D transition metal carbide Ti3C2Tx (T stands for surface termination), the most widely studied MXene, has shown outstanding electrochemical properties and promise for a number of bulk applications. However, electronic properties of individual MXene flakes, which are important for understanding the potential of these materials, remain largely unexplored. Herein, a modified synthetic method is reported for producing high-quality monolayer Ti3C2Tx flakes. Field-effect transistors (FETs) based on monolayer Ti3C2Tx flakes are fabricated and their electronic properties are measured. Individual Ti3C2Tx flakes exhibit a high conductivity of 4600 ± 1100 S cm−1 and field-effect electron mobility of 2.6 ± 0.7 cm2 V−1 s−1. The resistivity of multilayer Ti3C2Tx films is only one order of magnitude higher than the resistivity of individual flakes, which indicates a surprisingly good electron transport through the surface terminations of different flakes, unlike in many other 2D materials. Finally, the fabricated FETs are used to investigate the environmental stability and kinetics of oxidation of Ti3C2Tx flakes in humid air. The high-quality Ti3C2Tx flakes are reasonably stable and remain highly conductive even after their exposure to air for more than 24 h. It is demonstrated that after the initial exponential decay the conductivity of Ti3C2Tx flakes linearly decreases with time, which is consistent with their edge oxidation

NMR reveals the surface functionalisation of Ti3C2 MXene.

(1)H and (19)F NMR experiments have identified and quantified the internal surface terminations of Ti3C2Tx MXene. -F and -OH terminations are shown to be intimately mixed and there are found to be significantly fewer -OH terminations than -F and -O, with the proportions highly dependent on the synthesis method.We are grateful for financial support by the Oppenheimer Foundation and EPSRC. Material synthesis and characterization at Drexel University was supported by the US National Science Foundation under grant number DMR-1310245.This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6CP00330

X-ray radio-enhancement by Ti3_{3}C2_{2}Tx_{x} MXenes in soft tissue sarcoma

Radiotherapy is a cornerstone of cancer treatment. However, due to the low tissue specificity of ionizing radiation, damage to the surrounding healthy tissue of the tumor remains a significant challenge. In recent years, radio-enhancers based on inorganic nanomaterials have gained considerable interest. Beyond the widely explored metal and metal oxide nanoparticles, 2D materials, such as MXenes, could present potential benefits because of their inherently large specific surface area. In this study, we highlight the promising radio-enhancement properties of Ti$_{3}$C$_{2}$T$_{x}$ MXenes. We demonstrate that atomically thin layers of titanium carbides (Ti$_{3}$C$_{2}$T$_{x}$ MXenes) are efficiently internalized and well-tolerated by mammalian cells. Contrary to MXenes suspended in aqueous buffers, which fully oxidize within days, yielding rice-grain shaped rutile nanoparticles, the MXenes internalized by cells oxidize at a slower rate. This is consistent with cell-free experiments that have shown slower oxidation rates in cell media and lysosomal buffers compared to dispersants without antioxidants. Importantly, the MXenes exhibit robust radio-enhancement properties, with dose enhancement factors reaching up to 2.5 in human soft tissue sarcoma cells, while showing no toxicity to healthy human fibroblasts. When compared to oxidized MXenes and commercial titanium dioxide nanoparticles, the intact 2D titanium carbide flakes display superior radio-enhancement properties. In summary, our findings offer evidence for the potent radio-enhancement capabilities of Ti$_{3}$C$_{2}$T$_{x}$ MXenes, marking them as a promising candidate for enhancing radiotherapy

Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides

The use of fast surface redox storage (pseudocapacitive) mechanisms can enable devices that store much more energy than electrical double-layer capacitors (EDLCs) and, unlike batteries, can do so quite rapidly. Yet, few pseudocapacitive transition metal oxides can provide a high power capability due to their low intrinsic electronic and ionic conductivity. Here we demonstrate that two-dimensional transition metal carbides (MXenes) can operate at rates exceeding those of conventional EDLCs, but still provide higher volumetric and areal capacitance than carbon, electrically conducting polymers or transition metal oxides.We applied two distinct designs for MXene electrode architectures with improved ion accessibility to redox-active sites. A macroporous Ti3C2Tx MXene film delivered up to 210 F g-1 at scan rates of 10Vs-1, surpassing the best carbon supercapacitors known. In contrast, we show that MXene hydrogels are able to deliver volumetric capacitance of 1,500 F cm-3 reaching the previously unmatched volumetric performance of RuO2

NMR reveals the surface functionalisation of Ti3C2 MXene.

(1)H and (19)F NMR experiments have identified and quantified the internal surface terminations of Ti3C2Tx MXene. -F and -OH terminations are shown to be intimately mixed and there are found to be significantly fewer -OH terminations than -F and -O, with the proportions highly dependent on the synthesis method.We are grateful for financial support by the Oppenheimer Foundation and EPSRC. Material synthesis and characterization at Drexel University was supported by the US National Science Foundation under grant number DMR-1310245.This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6CP00330

Effect of Synthesis on Quality, Electronic Properties and Environmental Stability of Individual Monolayer Ti\u3csub\u3e3\u3c/sub\u3eC\u3csub\u3e2\u3c/sub\u3e MXene Flakes

2D transition metal carbide Ti3C2Tx (T stands for surface termination), the most widely studied MXene, has shown outstanding electrochemical properties and promise for a number of bulk applications. However, electronic properties of individual MXene flakes, which are important for understanding the potential of these materials, remain largely unexplored. Herein, a modified synthetic method is reported for producing high-quality monolayer Ti3C2Tx flakes. Field-effect transistors (FETs) based on monolayer Ti3C2Tx flakes are fabricated and their electronic properties are measured. Individual Ti3C2Tx flakes exhibit a high conductivity of 4600 ± 1100 S cm−1 and field-effect electron mobility of 2.6 ± 0.7 cm2 V−1 s−1. The resistivity of multilayer Ti3C2Tx films is only one order of magnitude higher than the resistivity of individual flakes, which indicates a surprisingly good electron transport through the surface terminations of different flakes, unlike in many other 2D materials. Finally, the fabricated FETs are used to investigate the environmental stability and kinetics of oxidation of Ti3C2Tx flakes in humid air. The high-quality Ti3C2Tx flakes are reasonably stable and remain highly conductive even after their exposure to air for more than 24 h. It is demonstrated that after the initial exponential decay the conductivity of Ti3C2Tx flakes linearly decreases with time, which is consistent with their edge oxidation