Showing papers by "Yury Gogotsi published in 2021"

The world of two-dimensional carbides and nitrides (MXenes)

Abstract: A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene.

Abstract: One of the primary factors limiting further research and commercial use of the two-dimensional (2D) titanium carbide MXene Ti3C2, as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti3AlC2 MAX phase precursor leads to Ti3AlC2 grains with improved crystallinity and carbon stoichiometry (termed Al-Ti3AlC2). MXene nanosheets (Al-Ti3C2) produced from this precursor are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electronic conductivity up to 20 000 S/cm. Aqueous suspensions of stoichiometric single- to few-layer Al-Ti3C2 flakes produced from the modified Al-Ti3AlC2 have a shelf life of over ten months, compared to 1 to 2 weeks for previously published Ti3C2, even when stored in ambient conditions. Freestanding films made from Al-Ti3C2 suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Furthermore, oxidation of the improved Al-Ti3C2 in air initiates at temperatures that are 100-150 °C higher than that of conventional Ti3C2. The observed improvements in both the shelf life and properties of Al-Ti3C2 will facilitate the widespread use of this material.

Ten Years of Progress in the Synthesis and Development of MXenes

Abstract: Since their discovery in 2011, the number of 2D transition metal carbides and nitrides (MXenes) has steadily increased. Currently more than 40 MXene compositions exist. The ultimate number is far greater and in time they may develop into the largest family of 2D materials known. MXenes' unique properties, such as their metal-like electrical conductivity reaching ≈20 000 S cm-1 , render them quite useful in a large number of applications, including energy storage, optoelectronic, biomedical, communications, and environmental. The number of MXene papers and patents published has been growing quickly. The first MXene generation is synthesized using selective etching of metal layers from the MAX phases, layered transition metal carbides and carbonitrides using hydrofluoric acid. Since then, multiple synthesis approaches have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etchants, halogens, and molten salts, allowing for the synthesis of new MXenes with better control over their surface chemistries. Herein, a brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided. The fact that their production is readily scalable in aqueous environments, with high yields bodes well for their commercialization.

Solution-Processed Ti 3 C 2 T x MXene Antennas for Radio-Frequency Communication

Abstract: Highly integrated, flexible, and ultrathin wireless communication components are in significant demand due to the explosive growth of portable and wearable electronic devices in the fifth-generation (5G) network era, but only conventional metals meet the requirements for emerging radio-frequency (RF) devices so far. Here, it is reported on Ti3 C2 Tx MXene microstrip transmission lines with low-energy attenuation and patch antennas with high-power radiation at frequencies from 5.6 to 16.4 GHz. The radiation efficiency of a 5.5 µm thick MXene patch antenna manufactured by spray-coating from aqueous solution reaches 99% at 16.4 GHz, which is about the same as that of a standard 35 µm thick copper patch antenna at about 15% of its thickness and 7% of the copper weight. MXene outperforms all other materials evaluated for patch antennas to date. Moreover, it is demonstrated that an MXene patch antenna array with integrated feeding circuits on a conformal surface has comparable performance with that of a copper antenna array at 28 GHz, which is a target frequency in practical 5G applications. The versatility of MXene antennas in wide frequency ranges coupled with the flexibility, scalability, and ease of solution processing makes MXene promising for integrated RF components in various flexible electronic devices.

High electrical conductivity and breakdown current density of individual monolayer Ti3C2Tx MXene flakes

Abstract: Summary As the scaling down of integrated circuits continues, there is a growing interest in electrically conductive materials with high current-carrying capacity for next-generation on-chip interconnects. Here, we report very high breakdown current density in Ti3C2Tx MXene, an emerging two-dimensional material. We performed electrical measurements of individual high-quality monolayer Ti3C2Tx flakes, which were prepared by an improved synthesis method, and found that they exhibit electrical conductivities of up to 11,000 S cm−1 and field-effect electron mobilities of up to 6 cm2 V−1 s−1, both representing the best values reported for Ti3C2Tx flakes so far. All flakes exhibited very similar breakdown current densities of about 1.2 × 108 A cm−2, which are comparable with the best two-dimensional materials, including graphene. The remarkable combination of high electrical conductivity and high current-carrying capacity makes Ti3C2Tx promising for nanometer-thin interconnects and warrants investigation of breakdown current densities of other materials from the large MXene family.

Performance improvement of MXene-based perovskite solar cells upon property transition from metallic to semiconductive by oxidation of Ti3C2Tx in air

Abstract: The unique properties of MXenes that arise from terminating functional groups and oxidation of MXenes make them attractive for application in photovoltaic devices like perovskite solar cells (PSCs) Here, oxidation of Ti3C2Tx hydrocolloid was carried out to tune its properties desirable for an electron transport layer (ETL) in low-temperature processed PSCs The calculations of the energy levels were carried out using the Vienna ab initio simulation package (VASP) code based on density functional theory (DFT) Oxidation of Ti3C2Tx can generate Ti–O bonds and effectively reduce the macroscopic defects of the film fabricated by spin-coating, while a transition from metallic material to semiconductor occurred after heavy oxidation A better matching of energy levels between perovskite and ETL layer in the case of a hybrid of oxidized and pristine Ti3C2Tx renders a champion power conversion efficiency (PCE) of 1829% The improvement in PCE can be attributed to the increased electron mobility in the ETL, which promotes electron transport and reduces the electron–hole recombination Hence, by presenting a simple method for high performance in PSCs by MXene-derived materials, this work demonstrates the great potential of these materials for applications in low-temperature processed PSCs and other photovoltaic technologies

Titanium Carbide MXene Shows an Electrochemical Anomaly in Water-in-Salt Electrolytes.

Abstract: Identifying and understanding charge storage mechanisms is important for advancing energy storage. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradaic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti3C2Tx MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.

Towards Watt-scale hydroelectric energy harvesting by Ti3C2Tx-based transpiration-driven electrokinetic power generators

Abstract: Nano-hydroelectric technology utilizes hydraulic flow through electronically conducting nanomaterials to generate electricity in a simple, renewable, ubiquitous, and environmentally friendly manner. To date, several designs of nano-hydroelectric devices have been devised to maximize the electrokinetic interactions between water molecules and nanomaterials. However, the reported power generation of the state-of-the-art nano-hydroelectric generators is not sufficient for practical use, as tens of thousands of units were required to operate low-power electronics on a mW scale. Here, we utilize titanium carbide (Ti3C2Tx) MXene nanosheets, which have advantageous properties including metal-like conductivity and hydrophilicity, to facilitate the electrokinetic conversion of the transpiration–driven electrokinetic power generator (TEPG) with a remarkably improved energy generation efficiency compared to that of carbon-based TEPG. The Ti3C2Tx MXene-based TEPG delivered a high pseudo-streaming current of 120 μA by the fast capillary flow promoted by MXene sheets coated on cotton fabric. The strong cationic affinity of Ti3C2Tx enables the generator to achieve an output of 0.68 V and 2.73 mA when NaCl solution is applied. Moreover, incorporation of a conducting polymer (i.e., Ti3C2Tx/polyaniline composite) enhanced the ionic diffusivity while maintaining the electrical network of Ti3C2Tx. The optimized Ti3C2Tx/polyaniline composite TEPG generated a maximum voltage of 0.54 V, a current of 8.2 mA, and a specific power density of 30.9 mW cm−3, which was sufficient to successfully charge a commercial Li-ion battery as well as low-power electronics and devices with a volume of 6.72 cm3.

Can Anions Be Inserted into MXene

Abstract: Despite the continuous progress in the research and development of Ti3C2Tx (MXene) electrodes for high-power batteries and supercapacitor applications, the role of the anions in the electrochemical energy storage and their ability to intercalate between the MXene sheets upon application of positive voltage have not been clarified. A decade after the discovery of MXenes, the information about the possibility of anion insertion into the restacked MXene electrode is still being questioned. Since the positive potential stability range in diluted aqueous electrolytes is severely limited by anodic oxidation of the Ti, the possibility of anion insertion was evaluated in concentrated aqueous electrolyte solutions and aprotic electrolytes as well. To address this issue, we have conducted in situ gravimetric electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurements in highly concentrated LiCl and LiBr electrolytes, which enable a significant extension of the operation range of the MXene electrodes toward positive potentials. Also, halogens are among the smallest anions and should be easier to intercalate between MXene layers, in comparison to multiatomic anions. On the basis of mass change variations in the positive voltage range and complementary density functional theory calculations, it was demonstrated that insertion of anionic species into MXene, within the range of potentials of interest for capacitive energy storage, is not likely to occur. This can be explained by the strong negative charge on Ti3C2Tx sheets terminated by functional groups.

MXene-infused bioelectronic interfaces for multiscale electrophysiology and stimulation.

Abstract: Soft bioelectronic interfaces for mapping and modulating excitable networks at high resolution and at large scale can enable paradigm-shifting diagnostics, monitoring, and treatment strategies. Yet...

Development and Applications of MXene-Based Functional Fibers.

Abstract: The increasing interest toward wearable and portable electronic devices calls for multifunctional materials and fibers/yarns capable of seamless integration with everyday textiles. To date, one particular gap inhibiting the development of such devices is the production of robust functional fibers with improved electronic conductivity and electrochemical energy storage capability. Recent efforts have been made to produce functional fibers with 2D carbides known as MXenes to address these demands. Ti3C2Tx MXene, in particular, is known for its metallic conductivity and high volumetric capacitance, and has shown promise for fibers and textile-based devices when used either as an additive, coating or the main fiber component. In this spotlight article, we highlight the recent exciting developments in our diverse efforts to fabricate MXene functionalized fibers, along with a critical evaluation of the challenges in processing, which directly affect macroscale material properties and the performance of the subsequent prototype devices. We also provide our assessment of observed and foreseen challenges of the current manufacturing methods and the opportunities arising from recent advances in the development of MXene fibers and paving future avenues for textile design and practical use in advanced applications.

Titanium carbide MXene shows an electrochemical anomaly in water-in-salt electrolytes

Abstract: Identifying and understanding charge storage mechanisms is important for advancing energy storage, especially when new materials and electrolytes are explored. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti3C2Tx MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates new opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.

Ti3C2Tx MXene Flakes for Optical Control of Neuronal Electrical Activity.

Abstract: Understanding cellular electrical communications in both health and disease necessitates precise subcellular electrophysiological modulation. Nanomaterial-assisted photothermal stimulation was demonstrated to modulate cellular activity with high spatiotemporal resolution. Ideal candidates for such an application are expected to have high absorbance at the near-infrared window, high photothermal conversion efficiency, and straightforward scale-up of production to allow future translation. Here, we demonstrate two-dimensional Ti3C2Tx (MXene) as an outstanding candidate for remote, nongenetic, optical modulation of neuronal electrical activity with high spatiotemporal resolution. Ti3C2Tx's photothermal response measured at the single-flake level resulted in local temperature rises of 2.31 ± 0.03 and 3.30 ± 0.02 K for 635 and 808 nm laser pulses (1 ms, 10 mW), respectively. Dorsal root ganglion (DRG) neurons incubated with Ti3C2Tx film (25 μg/cm2) or Ti3C2Tx flake dispersion (100 μg/mL) for 6 days did not show a detectable influence on cellular viability, indicating that Ti3C2Tx is noncytotoxic. DRG neurons were photothermally stimulated using Ti3C2Tx films and flakes with as low as tens of microjoules per pulse incident energy (635 nm, 2 μJ for film, 18 μJ for flake) with subcellular targeting resolution. Ti3C2Tx's straightforward and large-scale synthesis allows translation of the reported photothermal stimulation approach in multiple scales, thus presenting a powerful tool for modulating electrophysiology from single-cell to additive manufacturing of engineered tissues.

Multimodal Spectroscopic Study of Surface Termination Evolution in Cr2TiC2Tx MXene

Abstract: DOI: 10.1002/admi.202001789 and O.[9] Modification of the MXene surface termination holds great potential for engineering functional properties. For instance, experiments have demonstrated that altering T can tune the MXene’s metallic conductivity, work function, charge storage properties, and superconductivity.[10–14] Despite a few notable exceptions,[10] termination–property coupling studies have largely focused on Ti3C2Tx, and thus there is a need to explore the rest of the MXene family. Additionally, improved methods must be developed for effective control of the MXene termination. A primary challenge is removal of the initially mixed surface functionalization without damaging the underlying Mn+1Xn material. For example, during vacuum annealing of Ti3C2Tx, secondary Ti oxides form prior to complete F removal and before any loss of O.[14] Another challenge is reliably tracking termination changes, as different techniques tend to report different termination concentrations.[14–18] Ideally, methods capable of directly measuring the surface termination in situ should be developed. A particularly interesting MXene with regards to surface functionalization is Cr2TiC2Tx (shown in Figure 1). Recently, we showed that this MXene undergoes a magnetic transition near ≈30 K, adding an exciting new functionality to the MXene family.[19] However, as opposed to ferromagnetic or Control of surface functionalization of MXenes holds great potential, and in particular, may lead to tuning of magnetic and electronic order in the recently reported magnetic Cr2TiC2Tx. Here, vacuum annealing experiments of Cr2TiC2Tx are reported with in situ electron energy loss spectroscopy and novel in situ Cr K-edge extended energy loss fine structure analysis, which directly tracks the evolution of the MXene surface coordination environment. These in situ probes are accompanied by benchmarking synchrotron X-ray absorption fine structure measurements and density functional theory calculations. With the etching method used here, the MXene has an initial termination chemistry of Cr2TiC2O1.3F0.8. Annealing to 600 °C results in the complete loss of F, but O termination is thermally stable up to (at least) 700 °C. These findings demonstrate thermal control of F termination in Cr2TiC2Tx and offer a first step toward termination engineering this MXene for magnetic applications. Moreover, this work demonstrates high energy electron spectroscopy as a powerful approach for surface characterization in 2D materials.