Synthesis, characterization and chemistry of two-dimensional transition metal carbides and nitrides (MXenes)

MXenes represent a relatively new and quickly growing family of two-dimensional (2D) early transition-metal carbides and nitrides first synthesized in 2011 from bulk layered crystalline MAX phases. Because of their 2D structure and unique combination of high conductivity and hydrophilicity, MXenes have raised a significant interest for various applications. However, it has been found that in some cases colloidal MXene flakes are not stable and can spontaneously degrade on a time scale from hours to days. In this work, we investigate the crucial factors for MXene degradation and demonstrate gas analysis as a powerful method to gain further insights into chemical reactivity of MXenes. The degradation rates of MXenes in water were further investigated depending on their monolayer thickness within the same chemical composition, as well as depending on chemical composition of the materials within the same monolayer thickness. We further demonstrate the role of chemical properties and reactivity of MXenes in some of their applications, in particular, related to tribology and adhesion. We investigated the tribological properties of Ti3C2 MXene and MXene/graphene coatings. We have observed that with a careful control of the environment in order to suppress undesirable chemical decomposition of MXenes, the friction of MXene coated silicon substrates can be reduced to superlubric regime. We also studied adhesion of MXenes to silicon using atomic force microscopy. The obtained adhesion evnergies of two types of MXenes to silicon do not depend on the number of MXene monpayers in a stack and are comparable with that of graphene to silicon, showing a great potential of MXenes for (opto)electronic device assembly --Abstract, p. i

Adhesion of two-dimensional titanium carbides (MXenes) and graphene to silicon

Two-dimensional transition metal carbides (MXenes) have attracted a great interest of the research community as a relatively recently discovered large class of materials with unique electronic and optical properties. Understanding of adhesion between MXenes and various substrates is critically important for MXene device fabrication and performance. We report results of direct atomic force microscopy (AFM) measurements of adhesion of two MXenes (Ti3C2Tx and Ti2CTx) with a SiO2 coated Si spherical tip. The Maugis-Dugdale theory was applied to convert the AFM measured adhesion force to adhesion energy, while taking into account surface roughness. The obtained adhesion energies were compared with those for mono-, bi-, and tri-layer graphene, as well as SiO2 substrates. The average adhesion energies for the MXenes are 0.90 ± 0.03 J m-2 and 0.40 ± 0.02 J m-2 for thicker Ti3C2Tx and thinner Ti2CTx, respectively, which is of the same order of magnitude as that between graphene and silica tip

Understanding the Effect of Sodium Polyphosphate on Improving the Chemical Stability of Ti3c2tz Mxene in Water

Degradation of MXenes in aqueous environments severely limits the application and industrialization of this large family of two-dimensional (2D) materials. Hydrolysis and oxidation are now considered as two main degradation mechanisms and while significant efforts have been directed to prolonging the shelf-life of MXenes, separating and studying their degradation mechanisms have lagged behind. Herein, gas analysis via gas chromatography and Raman spectroscopy were used to investigate the effect of sodium polyphosphate, PP, on the degradation of Ti3C2Tz MXene. Transmission and scanning electron microscopies, as well as X-ray photoelectron spectroscopywere also used as complimentary techniques to support conclusions derived from gas analysis and to confirm the extent of degradation via characterization of solid reaction products. Based on these studies we have determined that the addition of PP to an equal mass of Ti3C2Tz solution can effectively suppress hydrolysis and protect Ti3C2Tz from degradation

Towards Holistic Charging Management for Urban Electric Taxi via a Hybrid Deployment of Battery Charging and Swap Stations

While previous studies focused on managing charging demand for private electric vehicles (EVs), we investigate ways of supporting the upgrade of an entire public urban electric taxi (ET) system. Concerning the coexistence of plugin charging stations (CSs) and battery swap stations (BSSs) in practice, it thus requires further efforts to design a holistic charging management especially for ETs. By jointly considering the combination of plug-in charging and battery swapping, a hybrid charging management framework is proposed in this paper. The proposed scheme is capable of guiding ETs to appropriate stations with time-varying requirements depending on how emergent the demand will be. Through the selection of battery charging/swap, the optimization goal is to reduce the trip delay of ET. Results under a Helsinki city scenario with realistic ETs and charging stations show the effectiveness of our enabling technology, in terms of minimized drivers’ trip duration, as well as charging performance gains at the ET and station sides

The first reported case of a patient with small cell lung cancer treated with fan beam computed tomography-guided online adaptive radiotherapy

Adaptive radiotherapy (ART) allows control of dosimetric impact of patient anatomical and functional variations over the treatment course, to minimize normal tissue exposure and maximize dose delivery to tumor. We present the first reported case of fan beam computed tomography (FBCT)-guided online ART for the treatment of small cell lung cancer (SCLC). A 62-year-old woman was diagnosed with histologically proven limited-stage SCLC. During definitive radiochemotherapy (50 Gy in daily fractions of 2.5 Gy), the tumor shrinkage resulted in an unexpected dose escalation to organs at risk (OAR). To correct the dose change, she received an online ART treatment session in our center with four-dimensional FBCT before the 12th fraction was delivered. The application of online ART, including imaging, recontouring and replanning, was feasible as the total treatment time was <25 min. Further research is warranted to verify the benefit of online ART in individualized treatment

Mxene Thermal and Chemical Stability and Degradation Mechanism

In recent years, there has been increased attention toward a large family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides known as MXenes. Owing to their distinctive structure and exceptional properties, MXenes not only hold immense potential in various applications but also represent a unique and large model family for fundamental studies of the physics and chemistry of 2D materials. In practice, however, many applications of MXenes are hindered by concerns related to their insufficient thermal and chemical stability, especially when exposed to environment. In this chapter, we provide an overview of ongoing research endeavors aimed at investigating the chemical and thermal stability of MXenes, and proposing strategies to suppress their degradation and broaden applicability. The discussion covers MXene surface modification, degradation mechanisms, and thermally induced transformations. We analyze approaches including optimization of MAX phase synthesis, exploring diverse etchants, refining storage environments, applying antioxidants, acids, bases, and other additives, and employing thermal annealing techniques to produce more stable and robust MXenes

Hydrolysis of 2D Transition-Metal Carbides (MXenes) in Colloidal Solutions

Although oxidation was deemed as the main factor responsible for the instability of MXenes in aqueous colloids, here we put forward and test a hypothesis about the central role of water as the primary factor. We show that water and related processes of MXene hydrolysis play the main role in the phenomena leading to complete transformations of 2D titanium carbide MXenes into titania in aqueous environments. To demonstrate the role of water, the stability of two MXenes, Ti3C2Tx and Ti2CTx, has been systematically studied in aqueous and nonaqueous colloids exposed to oxygen and inert gas atmospheres. The calculated time constant for degradation of Ti3C2Tx dispersed in anhydrous iso-propanol saturated with pure oxygen exceeds 5 years, in striking contrast to the same MXene dispersed in water, where more than a half of it would transform into titania even in an oxygen-less atmosphere over ∼41 days. A thinner Ti2CTx MXene showed similar behavior, albeit with shorter time constants in both solvents, correspondingly. UV-vis and Raman spectroscopy were used to analyze the oxidation kinetics and composition of fresh and aged MXenes. An intense anatase peak was observed in MXenes stored in aqueous solutions under Ar atmosphere, while no signs of oxidation could be found in iso-propanol solutions of the MXenes stored under O2 atmosphere over a similar period of time

Combination of High PH and an Antioxidant Improves Chemical Stability of Two-Dimensional Transition-Metal Carbides and Carbonitrides (MXenes) in Aqueous Colloidal Solutions

MXenes, a large family of two-dimensional (2D) transition-metal carbides/nitrides, have attracted increased attention in recent years because of their excellent electronic, mechanical, thermal, and optical properties. Studying chemical properties of MXenes is important to prolong the shelf life of their colloids and provide robust performance of MXenes in devices and applications. While the role of MXene reactivity with the environment, including water and components of air, is becoming more recognized, less is known about the role of parameters influencing the reactivity. In this work, we investigate the individual and combined effects of the pH and antioxidant on chemical stability of Ti2CTx, Ti3CNTx, and Ti3C2TxMXenes using GC, XPS, UV-vis, and Raman spectroscopy. In contrast to indirect indicators of MXene degradation, such as film conductivity or performance in electrochemical energy storage systems, we focus on detection of reaction products as the most sensitive and direct way of monitoring the chemical transformations of MXenes. Based on our knowledge of MXene chemistry and interactions with the environment, we propose a combination of sodium hydroxide and sodium l-ascorbate to effectively slow down degradation of MXenes in colloidal solutions by suppressing their hydrolysis and oxidation reactions, respectively

Understanding Chemistry of Two-Dimensional Transition Metal Carbides and Carbonitrides (MXenes) with Gas Analysis

MXenes, a large family of two-dimensional materials that are intensely investigated for a broad range of applications, are unstable in water, spontaneously forming TiO2. Several hypotheses have been proposed recently to explain the transformations of MXenes in aqueous environments based on characterization of solid products and measurements of solution pH. However, no studies of the gaseous products of these reactions have been reported. In this work, we demonstrate the use of Raman spectroscopy and gas chromatography techniques to study the gaseous reaction products of Ti2C, Ti3C2, Ti3CN, and Nb2C MXenes in aqueous environments. Based on the analysis of gases, the reactivities of MXenes with different monolayer thickness and chemical composition have been analyzed. We demonstrate the analysis of gases produced during MXene transformations as a powerful technique that can be used for better understanding of their nontrivial chemistry