No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Kinetics of aluminum extraction from Ti3AlC2 in hydrofluoric acid
Abstract
Herein we report on the influence of particle size, time and temperature on the kinetics – quantified by X-ray diffraction – of the selective extraction of Al from the ternary layered transition metal carbide, Ti3AlC2, when powders of the latter are immersed in hydrofluoric acid. Transmission and scanning electron microscopy, energy-dispersive X-ray spectroscopy and thermogravimetric analysis were also used to characterize the resulting powders. Increasing the temperature and immersion times, and decreasing the Ti3AlC2 particle size, led to faster conversion of Ti3AlC2 to its 2-D Ti3C2 counterpart. Arch-shaped edges at the ends of some Ti3C2 layers resembled graphene, corroborating the single-sheet structure of exfoliated Ti3C2. The removal of water and/or OH surface groups from Ti3C2 using drying in vacuum was also attempted.
... and its enlargement compared to the Ti 3 AlC 2 pattern are other evidence of the successful exfoliation of the synthesized Ti 3 C 2 Tx MXene . The morphology investigation of the Ti 3 C 2 Tx MXene attached to the GCE surface by SEM technique in Fig. 1B and B' with different magnifications demonstrates top-view images of well-multilayered accordion-like nanostructures with crumpled shapes by many ridges and rough surfaces [47][48][49][50].This multilayered nanostructure by providing many suitable sites on the surface facilitates attachment of the next substrates on the layers Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
... Rights reserved. [47]. The EDX and map elemental analysis images of the Ti 3 C 2 Tx MXene/GCE surface in Fig. 1C certify the presence of the carbon (C), titanium (Ti) and oxygen (O) elements on the embedded surface .The AuNBs with irregular surface ends composed of a series of high-indexed facets are an angled form of the AuNRs, and because of their cylindrical symmetry, two plasmonic bands are expected from them. ...
For the first time, a novel aptasensing interface based on smart integration of pseudo-gold nanobons (AuNBs) and Ti3C2Tx MXene is introduced for high selective detection of carbamazepine (CBZ). The large specific surface area achieved from the proposed nanocomposite increases the targeted immobilization of the Apt sequence on the surface via AuNBs as the linkage. It embeds a high-performance grafting platform for trapping CBZ with high sensitivity and accuracy in human biofluids and pharmaceutical formulations. The molecular dynamic (MD) simulation method that exhibits how the Apt binds to CBZ in a conformation-switching assay format from a molecular view is a valid certification for the interaction of CBZ on the developed aptasensing interface. The aptasensor measured CBZ from 1 fM to 100 nM with a superior detection limit (LOD) value of 330 aM compared with other reported CBZ sensors. Due to using biocompatible and non-toxic compounds, consuming low energy and chemicals the greenness of the proposed strategy has been certified by the international scoring system.
Graphical Abstract
... Next, 60 mL of 9 mol/L HCl was added, and the mixture was stirred to obtain a uniform solution. Subsequently, 1 g of (Mo 2 / 3 Y 1 / 3 ) 2 AlB 2 powder (400 mesh) was gradually introduced into the solution, which was then heated in a water bath at various temperatures (35,45,50, and 55°C) for 36 and 48 h. After etching, the slurry was transferred to a centrifuge bottle and centrifuged at 3000 rpm for 5 min. ...
... In addition, low-intensity peaks corresponding to YF 3 , i-MAB, and MoB remained in the XRD results, likely due to incomplete reactions between the MAB precursor and the etching solution [29,32]. The etching reaction can be enhanced by increasing the temperature, as indicated by the XRD patterns, which show significant improvement in etching with higher temperatures and reaction time [33][34][35]. The obtained XRD results demonstrated that MBene can be prepared at various etching conditions. ...
Two-dimensional materials attracted significant attention in the development of energy conversion and storage devices due to their unique physical and chemical properties. Among these materials, MBenes as analogous to MXenes, exhibited high surface area, excellent electrical conductivity, and diverse crystalline structures owing to the incorporation of boron. In this study, Mo4/3B2Tx MBene (where Tx represents surface terminations such as O, OH, and F) was synthesized from i-MAB precursor using fluoride salt etching process. The synthesized MBene was employed as an anode material for lithium-ion batteries. The electrochemical performance demonstrated specific capacities of 260.3 and 283 mAh g⁻¹, respectively. In addition, the MBene electrode achieved a coulombic efficiency of 91.96% and retained a specific capacity of approximately 280 mAh g⁻¹ after 100 cycles, indicating excellent cyclic stability. This work broadened the application of Mo4/3B2Tx MBene as a promising anode material for lithium-ion batteries.
Graphical abstract
... The functional groups -OH, -O, and -F in the etching system are easily attached to the surface of this alternating layer structure, eventually forming a 2D layered structure, Ti 3 C 2 T x MXene [31,32]. Ti 3 AlC 2 is most used in the MAX phase, as shown in Figure 1, which presents a schematic diagram of the etching process of Ti 3 AlC 2 by HF, where the Ti 3 AlC 2 structure consists of a single Ti 3 C 2 layer separated by Al atoms. ...
... The functional groups -OH, -O, and -F in the etching system are easily attached to the surface of this alternating layer structure, eventually forming a 2D layered structure, Ti3C2Tx MXene [31,32]. Ti3AlC2 is most used in the MAX phase, as shown in Figure 1, which presents a schematic diagram of the etching process of Ti3AlC2 by HF, where the Ti3AlC2 structure consists of a single Ti3C2 layer separated by Al atoms. ...
Ammonia (NH3) potentially harms human health, the ecosystem, industrial and agricultural production, and other fields. Therefore, the detection of NH3 has broad prospects and important significance. Ti3C2Tx is a common MXene material that is great for detecting NH3 at room temperature because it has a two-dimensional layered structure, a large specific surface area, is easy to functionalize on the surface, is sensitive to gases at room temperature, and is very selective for NH3. This review provides a detailed description of the preparation process as well as recent advances in the development of gas-sensing materials based on Ti3C2Tx MXene for room-temperature NH3 detection. It also analyzes the advantages and disadvantages of various preparation and synthesis methods for Ti3C2Tx MXene’s performance. Since the gas-sensitive performance of pure Ti3C2Tx MXene regarding NH3 can be further improved, this review discusses additional composite materials, including metal oxides, conductive polymers, and two-dimensional materials that can be used to improve the sensitivity of pure Ti3C2Tx MXene to NH3. Furthermore, the present state of research on the NH3 sensitivity mechanism of Ti3C2Tx MXene-based sensors is summarized in this study. Finally, this paper analyzes the challenges and future prospects of Ti3C2Tx MXene-based gas-sensitive materials for room-temperature NH3 detection.
... For instance, HF etching increases the spacing among the MAX layers and thus weakens the van der Waals force between the adjacent MXene sheets, allowing the efficient delamination upon manual shaking or via sonication. MXenes etched by the LiF-HCl mixture typically exhibit a higher structural strength due to the absence of abundant structural defects [13,14]. ...
Two‐dimensional (2D) transition metal carbides, carbonitrides, and nitrides, known as MXenes, have been widely studied at the frontier of 2D materials. The excellent mechanical properties, electrical conductivity, excellent photoelectrical performance, and good thermal stability of MXenes enable wide applications in many fields, including but not limited to energy storage, supercapacitors, EMI shielding, catalysis, optoelectronics, and sensors. In particular, MXene‐based materials exhibit exceptional sensing performance due to their unique tunable surface chemistry, 2D architecture, and exotic electrical/mechanical/electromechanical properties, which are rarely found in other materials. This paper discusses the MXene sensing properties and their mechanisms in different types of sensors, including piezoresistive sensors, flexible sensors, gas sensors, and biosensors. The unique roles of these MXene‐based sensors toward the future of smart living are also outlined. This article may shed light on the rational design of MXene‐based sensors and provide valuable references for corresponding scenario applications.
... A similar shift of (002) peak from ~9.5 • 2θ to ~6.5-6.8 • was reported by [51], where three Ti 3 C 2 T x samples were prepared according to the same procedure (0.8 g of LiF, 10 mL of 32 wt% HCl) from three different MAX precursors. The obtained lower-angle (002) peaks give evidence for an increased interlayer distance with increased d-spacing, implying the presence of intercalant(s) such as water molecules and possible cations between the hydrophilic and negatively charged Ti 3 C 2 T x nanosheets [34], and, thus, revealing successful Ti 3 C 2 T X sheet delamination as also described in [52,53]. ...
... Simultaneously, silicon-based anodes have emerged as promising next-generation materials for LIBs due to their significantly higher theoretical capacity of 4200 mAh/g, far surpassing that of graphite [27][28][29][30]. Despite their advantages, silicon anodes face substantial challenges, including mechanical instability and performance degradation caused by volume expansion during charge-discharge cycles [31][32][33][34][35]. Proposed strategies to mitigate these issues include the use of nano-silicon and silicon suboxide (SiOx); however, these approaches are limited by low conductivity and particle aggregation [36][37][38][39]. ...
MXenes, a family of 2D transition metal carbides, nitrides, and carbonitrides, have attracted significant attention due to their exceptional physicochemical properties and electrochemical performance, making them highly promising for diverse applications, particularly in energy storage. Despite notable advancements, MXene synthesis remains a critical challenge, as conventional methods often rely on hazardous hydrofluoric acid-based processes, posing substantial environmental and safety risks. In this study, we present an eco-friendly synthesis approach for MXenes using molten salt processes, which offer a safer, sustainable alternative while enabling scalable production. Additionally, we explore the development of high-performance battery anodes by fabricating nanocomposites of nano-silicon and MXene, followed by a bio-inspired polydopamine coating and carbonization process. This innovative strategy not only enhances the structural stability and electrochemical performance of the anodes but also aligns with environmentally conscious design principles. Our findings demonstrate the potential of eco-friendly MXene synthesis and nanocomposite materials in advancing sustainable energy storage technologies.
... When etching Al from Ti 3 AlC 2 in a solution with 50% HF, Mashtalir et al. looked at how particle size and process parameters affected the etching process. Research has shown that lowering the maximum original particle size, extending the reaction time, and elevating the immersion temperature improves the phase transition from bulky Ti 3 AlC 2 to Ti 3 C 2 T x [43]. An accordion-like particle shape is typically seen when the HF concentration reaches around 10%. ...
MXenes, a groundbreaking class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, have emerged as highly promising materials for photocatalytic applications due to their unique structural, electrical, and surface properties. These materials are synthesized by selectively etching the A layer from MAX phases, yielding compositions with the general formula Mn+1XnTx, where M is a transition metal, X represents carbon or nitrogen, and Tx refers to surface terminations such as –OH, –O, or –F. This review delves into the advanced synthesis techniques of MXenes, including fluoride-free etching and molten salt methods, and explores their potential in photocatalysis for environmental remediation. MXenes exhibit remarkable light absorption capabilities and efficient charge carrier separation, making them highly effective for the photocatalytic degradation of organic pollutants under visible light. Modulating their surface chemistry and bandgap via functional group modifications further enhances their photocatalytic performance. These attributes position MXenes as next-generation materials for sustainable photocatalytic applications, offering significant potential in addressing global environmental challenges.
... A similar shift of (002) peak from ~9.5 • 2θ to ~6.5-6.8 • was reported by [51], where three Ti 3 C 2 T x samples were prepared according to the same procedure (0.8 g of LiF, 10 mL of 32 wt% HCl) from three different MAX precursors. The obtained lower-angle (002) peaks give evidence for an increased interlayer distance with increased d-spacing, implying the presence of intercalant(s) such as water molecules and possible cations between the hydrophilic and negatively charged Ti 3 C 2 T x nanosheets [34], and, thus, revealing successful Ti 3 C 2 T X sheet delamination as also described in [52,53]. ...
... The MAX phase quality, composition and particle size, intercalation agent, etching solution, temperature, and duration are a few examples of the characteristics that might affect MXenes etching parameters. [33] Higher "M" atomic numbers, for example, need longer etching times or higher acid concentrations (for example, M 4 AX 3 MAX phases require more aggressive etching conditions than M 3 AX 2 and M 2 AX), [1,12] The primary benefits of MXenes etching with HF solution are its simplicity and variety since it can produce most MXenes, thus supporting this method for wider applications. [34] The HF-etched MXenes are not monolayer materials, but rather multilayer flakes held together by weak Van der Wal linkages. ...
Nano biosensors based on MXenes have been emerging as a promising tool in the detection of biomarkers, for the discrimination of diseases and in the detection of environmental pollutants. Their potential in sensing applications has also drawn a lot of attention to their unique qualities such as their high conductivity, huge surface area, outstanding hydrophilicity, biocompatibility, and simplicity of surface functionalization. The development of scalable synthesis techniques is essential to the large‐scale manufacturing and broad application of MXene‐based sensors. Furthermore, the stability of the MXene layers in diverse environmental circumstances continues to be a difficulty for their practical application. To increase the dependability and precision of MXene‐based sensors, their selectivity must be increased through functionalization and tuning. With innovative technologies like machine learning, MXene biosensor is now taken advantage of new opportunities. Personalized healthcare solutions, remote data analysis, and real‐time monitoring are all possible when MXene sensors and AI algorithms work together. Herein, the optical properties, synthesis approaches, role of MXene biosensors in machine learning, its significant challenges and future prospects of MXene‐based nano(bio)sensors are deliberated.
... For the reactions confined in the lamellar structure, the diffusion of molecules and/or ions in the interlayer channels is a key step to decide the reaction rate. [23][24][25] Thus, a molecular dynamics (MD) simulation was employed to investigate theoretically the diffusion of HF molecules or dissociated ions (H + and F À ) in the V 2 C MXene interlayers with different interlayer spacings. As shown in Figure 1h, the diffusion coefficients (D) of HF gas molecules at a set reaction temperature of 600°C are calculated to be about 43.0, 93.1 and 139.0 (×10 À 9 m 2 s À 1 ) for interlamellar spacings of 6, 10 and 14 Å, respectively. ...
The rising of MXenes not only enriches the two‐dimensional material family but also brings more opportunities for diverse functional applications. However, the controllable synthesis of MXenes is still unsatisfied via the common liquid‐solid etching route, considering the unsolved problems like safety risk, time cost and easy oxidation. Herein, a facile yet efficient gas‐solid (G‐S) reaction methodology is devised by using hydrogen fluoride gas derived from fluorinated organics as the MAX etchant toward high‐efficiency fabrication of multiple MXenes and their derivatives. The innovative G‐S reaction strategy exhibits superb versatility to achieve different gram‐level MXenes (V2C, Ti3C2, Nb2C, Ti2N, Ti3CN, (Mo2/3Y1/3)2C) in a very short time, and even realizes in situ heteroatom doping or synchronous phase conversion of MXenes directly from MAX phases. The obtained MXenes and their derivatives exhibit excellent structure stability and high electron/ion conductivity, making them promising materials for electrochemical applications. In particular, the N‐doped V2C MXene shows superior adsorption and catalytic activity toward lithium polysulfides for advanced lithium sulfur batteries.
... This acid-etching process encompasses two distinct stages: etching and delamination. Etching is a kinetically controlled reaction that enables the production of various MXenes, conducted at temperatures ranging from ambient to 55 • C by adjusting the reaction time and HF concentration [81,82]. HF is particularly selective for Al and Si, the most common A-elements, making it the preferred etchant. ...
Photocatalytic conversion of solar energy into chemical energy is a prospective solution to the energy crisis and environmental challenges. MXenes, characterized by their unique surface features and physicochemical properties derived from their atomically thin layered structures, are becoming promising candidates for various photocatalytic applications. This review offers a concise analysis of the structure and categorization of MAX phases and MXenes. The discussion covers a succinct overview of different synthesis techniques employed in the preparation of MXenes, encompassing traditional HF etching methods, HF-free alternatives, additive-mediated synthesis, and direct synthesis. This study highlights MXenes and related heterostructures as photocatalysts for H 2 O splitting, CO 2 reduction, N 2 fixation, H 2 O 2 generation, and pollutant degradation. We incorporated two complementary approaches, in-situ characterization methods, and first-principles calculations, in the following section to provide a better understanding. We conclude this review by offering insights into future directions and a concise summary of the potential applications of MXenes and MXene-based heterostructures in photocatalysis. This review could serve as a valuable reference for the design and fabrication of unique and promising MXene-based photocatalysts.
... However, this approach usually needs a long etching time to ensure complete etching of "A" atoms; for example, the conversion of Ti 3 AlC 2 to Ti 3 C 2 T x MXene typically demands ≈24 h, whereas processing V 4 AlC 3 into V 4 C 3 T x MXene may take up to 200 h. [21,22] The inefficiency of etching and the hazardous nature of HF pose significant challenges to employ this method for large-scale industrial production of MXenes. Recently, the Lewis acidic molten salt etching method has emerged as a promising strategy for the efficient and scalable synthesis of MXenes because of its ability to obtain various MXenes within several hours. ...
Developing green and efficient preparation strategies is a persistent pursuit in the field of 2D transition metal nitrides and/or carbides (MXenes). Traditional etching methods, such as HF‐based or high‐temperature Lewis‐acid‐molten‐salt etching route, require harsher etching conditions and exhibit lower preparation efficiency with limited scalability, severely constraining their commercial production and practical application. Here, an ultrafast low‐temperature molten salt (LTMS) etching method is presented for the large‐scale synthesis of diverse MXenes within minutes by employing NH4HF2 as the etchant. The increased thermal motion and improved diffusion of molten NH4HF2 molecules significantly expedite the etching process of MAX phases, thus achieving the preparation of Ti3C2Tx MXene in just 5 minutes. The universality of the LTMS method renders it a valuable approach for the rapid synthesis of various MXenes, including V4C3Tx, Nb4C3Tx, Mo2TiC2Tx, and Mo2CTx. The LTMS method is easy to scale up and can yield more than 100 g Ti3C2Tx in a single reaction. The obtained LTMS‐MXene exhibits excellent electrochemical performance for supercapacitors, evidently proving the effectiveness of the LTMS method. This work provides an ultrafast, universal, and scalable LTMS etching method for the large‐scale commercial production of MXenes.
... This is also supported by the slight shift in 2θ peak at 9.69° to 9.41° and 9.39° for MXene_5M_24h and MXene_15M_24h samples (Fig. 1d), respectively. Moreover, d-spacing was also observed to slightly increase from 9.10 Å for MAX phase to 9.39 Å for MXene_5M_24h and 9.43 Å for MXene_15M_24h sample, which meant that diffusion of etchant increased with increase in concentration of alkali [22,23]. The two synthesized MXene samples also showed all the corresponding peaks, which indicate only partial etching of aluminium and MXene exfoliation from MAX phase. ...
MXenes are conventionally synthesized by a top-down selective etching process using toxic fluoride-based chemicals. Here, we report a unique one-pot method for fabricating multilayer structures of Ti 3 C 2 OH MXene by etching Ti 3 AlC 2 with alkali. MXene is synthesized by hydrothermal etching of MAX phase (Ti 3 AlC 2 ) using relatively non-toxic alkali (potassium hydroxide) solutions. The quality of synthesized MXenes was studied as a function of alkali concentration, precursor pre-treatment, and total reaction time. Increase in alkali concentration exhibits improved etching capability, yield, and stability of MXene, whereas pre-treatment of precursor at elevated temperature and longer reaction time shows detrimental effects on the quality of synthesized MXene with formation of titanate nanofibers. Moreover, we also fabricated MXene/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate composite aerogels and demonstrated its suitability as active electrode material for supercapacitor applications.
Graphical abstract
... To achieve complete delamination into single-layer MXene, additional procedures are necessary to enhance the layer spacing at the nanometer scale. Consequently, XRD serves as an effective method for monitoring variations in layer spacing, which is essential for optimizing the yield of single-layer MXenes [14]. Naguib et al. demonstrated that the treatment of Ti 3 CNTx with tetrabutylammonium hydroxide (TBAOH) for a duration of two hours resulted in a notable shift in the (002) peak from a 2θ value of 8.26° to 4.57°. ...
Background
The two-dimensional MXene, known as titanium carbide (Ti₃C₂), is characterized by its substantial interlayer spacing, extensive surface area, hydrophilic nature, exceptional thermal stability, and outstanding electrical conductivity. These distinctive attributes render Ti₃C₂ an ideal candidate for detecting target analytes and immobilizing biomolecules. Bismuth oxide (Bi₂O₃), an essential compound of bismuth, frequently acts as a foundational element in bismuth chemistry. Its applications are diverse, from fireworks to oxygen gas sensors and solid oxide fuel cells, with particular emphasis on its behaviour under elevated temperatures and pressures. Notably, phase transitions to various polymorphs, which remain metastable at room temperature, have been documented under these conditions, indicating potential for numerous applications. Integrating MXene with Bi₂O₃ composites holds significant promise for advancements in energy-related electronics, sensing technologies, and photocatalytic processes.
Objective
To optimize the synthesis of titanium carbide-bismuth oxide (Ti₃C₂-Bi₂O₃) nanoparticles to enhance their antimicrobial activity by identifying the best synthesis conditions and assessing their effectiveness against various microbial pathogens.
Materials and methods
The preparation of Ti₃C₂ MXene involves dissolving lithium fluoride in hydrochloric acid, followed by Ti₃AlC₂ and stirring at 40°C for 48 hours. The resulting pellet is then dispersed in ultrapure water and centrifuged to obtain the MXene dispersion. Bi₂O₃ nanoparticles are prepared by preparing bismuth nitrate pentahydrate in nitric acid and adding sodium hydroxide to adjust the pH. The resulting white precipitate is filtered, washed, and dried before being calcined at 400°C for two hours to produce Bi₂O₃ nanoparticles. The Ti₃C₂-Bi₂O₃ composite is synthesized by adding Bi(NO₃)₃ solution to a 5 mg/mL Ti₃C₂Tx MXene solution. The reaction solution is heated to 160°C, and the resulting black powder is labelled as x% Bi₂O₃/MXene. The antimicrobial efficacy of the nanoparticles is assessed using the disk diffusion method. The zones of inhibition are measured and analyzed as indicators of antimicrobial activity.
Results
The scanning electron microscopy (SEM) analysis revealed the presence of Bi₂O₃ particles alongside Ti₃C₂ nanosheets, while the X-ray diffraction (XRD) analysis and energy-dispersive X-ray spectroscopy (EDS) confirmed the high crystallinity of the compound. Furthermore, the compound was determined to be impurity-free and demonstrated antimicrobial properties.
Conclusion
The XRD analysis confirms the effective integration of various materials and the existence of crystalline phases. SEM provides insights into the morphology and organization of particles within sheets, whereas EDS assesses the elemental composition and its uniform distribution. These studies demonstrate the synthesis of Ti₃C₂-Bi₂O₃ composites, suggesting their potential for usage in applications involving antimicrobial action.
... The resulting M n+1 X n T x MXene exhibited a multilayered structure with strong bonding between layers, attributed to van der Waals forces or H-bonding. The duration of HF etching and the temperature varied, influencing the outcome from hours to days and from room temperature to 50 • C [64,65]. Ti 3 C 2 , like other MXenes, demonstrated dispersibility in both organic and aqueous solutions, such as propylene carbonate, ethanol, and dimethylformamide (DMF), enabling its incorporation with polymers and nanomaterials through solvent exchange, mixing, or self-assembly [66]. ...
Recently, a new class of two-dimensional (2D) materials known as MXenes, such as Ti3C2Tx, have received significant attention due to their exceptional structural and physiochemical properties. MXenes are widely used in a variety of applications, including sensors, due to their excellent charge transport, high catalytic, and conducive properties, making them superior materials for sensing applications. Sensing technology has attracted significant interest from the scientific community due to its wide range of applications. In particular, gas sensing technology is essential in today’s world due to its vital role in detecting various gases. Gas sensors have an essential role in real-time environmental monitoring health assessment, and the demand for air quality monitoring is driving the gas sensor market forward. Similarly, optical sensors are a related technology that can rapidly detect toxic substances and biomaterials using optical absorption spectroscopy. MXenes are highly desirable for gas and optical sensing applications due to their abundant active sites, metallic conductivity, optical properties, customizable surface chemistry, and exceptional stability. In this review article, we compile recent advancements in the development of gas sensors and optical sensors using MXenes and their composite materials. This review article would be beneficial for researchers working on the development of MXenes-based gas sensors and optical sensors.
... Consequently, a systematic investigation of a water-free etching technique is of great importance to address the issue. Alternately, a polar organic solvent was employed with ammonium bifluoride (NH 4 HF 2 ) to produce Ti 3 C 2 Tx with a highly fluorinated structure and enhanced water stability [85]. In contrast to the previously described method, this waterless solvent necessitates that the entire synthetic procedure occurs in a protective box. ...
In the present study, the oxidative removal of benzene (model carcinogenic aromatic volatile organic compound (VOC)) from indoor air is investigated using titanium carbide (Ti3C2) MXene/anatase titanium dioxide (TiO2)-supported gold (Au) catalysts under dark and low-temperature (DLT: 30–90 °C) conditions. The reduction pre-treatment (catalyst names with the ‘R’ suffix) has been used to form metallic Au (Au0) nanoparticles and anatase TiO2 in the MXene structure. The relative ordering in the Au catalysts, if assessed in terms of room-temperature (RT) benzene (5 ppm) conversion (XB (%)) at 10,191 h−1 gas hourly space velocity, is found as: 0.5 %-Au/Ti3C2-R (85 ± 5.5 %) > 0.2 %-Au/Ti3C2-R (71 ± 1.8 %) ≈ 0.5 %-Au/Ti3C2 (71 ± 2.8 %) > 1 %-Au/Ti3C2-R (52 ± 5.8 %). The catalytic activity peaks at 0.5 wt% Au loading with reduction pre-treatment and is further enhanced by decreasing the flow rate, benzene concentration, and relative humidity (or by increasing the catalyst mass). The 0.5 %-Au/Ti3C2-R catalyst maintains stable benzene mineralization for 24 h time-on-stream (maximum tested reaction time) at RT without noticeable deactivation. Benzene oxidation on the 0.5 %-Au/Ti3C2-R surface proceeds through diverse reaction intermediates (e.g., phenolate, catecholate, o-, p-benzoquinone, formate, and carbonate). The adsorption of benzene and molecular oxygen (O2) occurs near the Au0 sites. Hydrogen first migrates from benzene to O2, forming an –OOH group attached to Au0. Subsequently, hydrogen transfers from benzene to –OOH, leading to the formation of water as the final product. The benzene ring is then unzipped to yield carbon dioxide through various reaction steps. The present work offers insights into developing Au catalysts for practical DLT control of indoor air pollutants.
The physicochemical properties and application performance of MXenes are fundamentally linked to their surface chemistry. Specifically, two-dimensional (2D) MXenes terminated with the –O group exhibit remarkable potential in a wide range of applications, such as energy storage, catalysis, and electronics. However, conventional synthetic techniques, such as prolonged high-temperature annealing in an electric furnace, usually cause irreversible structural damage or undesirable phase transformations of 2D MXene slabs into bulk 3D structures, resulting in a substantial reduction in surface area and a consequent decline in application performance. Herein, we propose a general method for the precise modification of –O termination on MXenes by employing low-pressure flash annealing (LP-FA) of raw MXenes with complex and diverse terminations, guided by Le Chatelier’s principle. The low-pressure environment facilitates gas-generation solid-solid reactions at relatively lower temperatures, thereby promoting the removal of superficial –F terminations and the formation of –O terminations on MXenes. Additionally, the rapid heating rate (10³ K/s) and short duration (~5 s) of flash annealing, coupled with a lower peak annealing temperature, effectively prevent structural damage or layer-by-layer stacking of MXene slabs and enhance the potential applications of –O terminated MXenes. As an illustration of practical applications, we have demonstrated that the –O terminated Nb2CTx MXene exhibits an exceptional capacity of 420 mAh g−1 at a current density of 50 mA g−1, along with remarkable stability over more than 3000 cycles of Li-ion charge/discharge testing. This performance positions it among the highest-performing MXene anode materials. Consequently, our LP-FA method introduces an additional parameter, beyond conventional temperature and time factors, to modulate general modifications and advance the industrial application of –O terminated MXenes-based advanced functional materials.
This review provides an overview of the fabrication methods for Ti3C2Tx MXene-based hybrid photocatalysts and evaluates their role in degrading organic dye pollutants. Ti3C2Tx MXene has emerged as a promising material for hybrid photocatalysts due to its high metallic conductivity, excellent hydrophilicity, strong molecular adsorption, and efficient charge transfer. These properties facilitate faster charge separation and minimize electron–hole recombination, leading to exceptional photodegradation performance, long-term stability, and significant attention in dye degradation applications. Ti3C2Tx MXene-based hybrid photocatalysts significantly improve dye degradation efficiency, as evidenced by higher percentage degradation and reduced degradation time compared to conventional semiconducting materials. This review also highlights computational techniques employed to assess and enhance the performance of Ti3C2Tx MXene-based hybrid photocatalysts for dye degradation. It identifies the challenges associated with Ti3C2Tx MXene-based hybrid photocatalyst research and proposes potential solutions, outlining future research directions to address these obstacles effectively.
Nanomaterials with electroactive properties have taken a big leap for tissue repair and regeneration due to their unique physiochemical properties and biocompatibility. MXenes, an emerging class of electroactive materials have generated considerable interest for their biomedical applications from bench to bedside. Recently, the application of these two-dimensional wonder materials have been extensively investigated in the areas of biosensors, bioimaging and repair of electroactive organs, owing to their outstanding electromechanical properties, photothermal capabilities, hydrophilicity, and flexibility. The currently available data reports that there is significant potential to employ MXene nanomaterials for repair, regeneration and functioning of electroactive tissues and organs such as brain, spinal cord, heart, bone, skeletal muscle and skin. The current review is the first report that compiles the most recent advances in the application of MXenes in bioelectronics and the development of biomimetic scaffolds for repair, regeneration and functioning of electroactive tissues and organs including heart, nervous system, skin, bone and skeletal muscle. The content in this article focuses on unique features of MXenes, synthesis process, with emphasis on MXene-based electroactive tissue engineering constructs, biosensors and wearable biointerfaces. Additionally, a section on the future of MXenes is presented with a focus on the clinical applications of MXenes.
This review serves as a tutorial on MXene synthesis, outlining laboratory practices and linking them to core scientific concepts. It also examines healthcare applications, computational aspects, and the role of AI technologies in advancing MXene research.
Sodium lactate and conductive carbon are recycled to utilize them as electrolytes of supercapacitors and conductive additives of sodium-ion batteries, respectively.
The rising of MXenes not only enriches the two‐dimensional material family but also brings more opportunities for diverse functional applications. However, the controllable synthesis of MXenes is still unsatisfied via the common liquid‐solid etching route, considering the unsolved problems like safety risk, time cost and easy oxidation. Herein, a facile yet efficient gas‐solid (G‐S) reaction methodology is devised by using hydrogen fluoride gas derived from fluorinated organics as the MAX etchant toward high‐efficiency fabrication of multiple MXenes and their derivatives. The innovative G‐S reaction strategy exhibits superb versatility to achieve different gram‐level MXenes (V2C, Ti3C2, Nb2C, Ti2N, Ti3CN, (Mo2/3Y1/3)2C) in a very short time, and even realizes in‐situ heteroatom doping or synchronous phase conversion of MXenes directly from MAX phases. The obtained MXenes and their derivatives exhibit excellent structure stability and high electron/ion conductivity, making them promising materials for electrochemical applications. In particular, the N‐doped V2C MXene shows superior adsorption and catalytic activity toward lithium polysulfides for advanced lithium sulfur batteries.
The proliferation of Internet of Things (IoT) applications necessitates the deployment of high-performance ammonia (NH3) sensors. In recent studies, Ti3C2Tx, a novel two dimensional (2D) material, has been extensively investigated for its potential as a room temperature NH3 sensor. However, pristine MXene-based sensors have exhibited deficiencies in long-term stability and reproducibility. In response, an MXene/γ-WO3 composite has been synthesized using an ultra-sonication method to address these challenges. The resulting composite
sensor has demonstrated remarkable performance, attributable to, increased active sites from defects induced by WO3, and the presence of a p-n heterojunction. Notably, electron transfer from WO3 to Ti3C2Tx has been identified as a critical contributing factor to the enhanced characteristics of these nanohybrids, as evidenced by observed alterations in binding energies for the Ti 2p and W 4 f core levels. These findings have expanded our
understanding of the electrical interactions in Ti3C2/WO3 and their potential applications in diverse domains. The pristine MXene sensor exhibited a response of 59.39 % at 300 ppm at room temperature, while the composite MXene/γ-WO3 displayed a response of 92.3 %. Additionally, the response recovery time was recorded at 22 seconds and 9 seconds, respectively. The chemiresistive ammonia sensor’s performance was evaluated across a range of concentrations (25–300 ppm) and relative humidity levels (11–94 % RH). The composite sensor has demonstrated reproducibility and long-term stability, coupled with high sensitivity to NH3. This manuscript aims to highlight the enhanced ammonia sensing properties and selectivity of MXene/γ-WO3 composite materials.
The emerging field of two-dimensional (2D) transition metal carbides and nitrides, collectively known as MXene (Ti 3 C 3 Tx), has garnered significant attention in recent years due to their remarkable properties and multifaceted applications. This work explores the synthesis, characterization and functionalization of MXene with polyethylene glycol (PEG) and its influence on the degradation process of organic dye methylene blue (MB) is investigated. The functionalization of MXene with PEG is detailed, showcasing the diverse chemistries and functionalities these organic compounds bring to the MXene nanosheets. PEG imparts hydrophilicity and stability, and promotes catalytic activity. Further the mechanisms of dye degradation involving MXene-PEG materials are elucidated, highlighting the synergistic effects of MXene and functional groups on the enhanced degradation rates. This work underscores the versatility of MXene as a platform for environmental applications and the significant impact of functionalization with organic molecules on their performance. The findings of this work revealed that when MXene-based nanomaterials suitably functionalized with PEG, they exhibit immense potential in the reduction of methylene blue (MB) dye with a record breaking first order rate constant of 0.16581 ± 0.030 min − 1 .
MXene, regarded as cutting‐edge two‐dimensional (2D) materials, have been widely explored in various applications due to their remarkable flexibility, high specific surface area, good mechanical strength, and interesting electrical conductivity. Recently, 2D MXene has served as a ideal platform for the design and development of electrocatalysts with high activity, selectivity, and stability. This review article provides a detailed description of the structural engineering of MXene‐based electrocatalysts and summarizes the uses of 2D MXene in hydrogen evolution reactions, nitrogen reduction reactions, oxygen evolution reactions, oxygen reduction reactions, and methanol/ethanol oxidation. Then, key issues and prospects for 2D MXene as a next‐generation platform in fundamental research and real‐world electrocatalysis applications are discussed. Emphasis will be given to material design and enhancement techniques. Finally, future research directions are suggested to improve the efficiency of MXene‐based electrocatalysts.
The pressing global issue of organic pollutants, particularly phenolic compounds derived primarily from industrial wastes, poses a significant threat to the environment. Although progress has been made in the development of low-cost materials for phenolic compound removal, their effectiveness remains limited. Thus, there is an urgent need for novel technologies to comprehensively address this issue. In this context, MXenes, known for their exceptional physicochemical properties, have emerged as highly promising candidates for the remediation of phenolic pollutants. This review aims to provide a comprehensive and critical evaluation of MXene-based technologies for the removal of phenolic pollutants, focusing on the following key aspects: (1) The classification and categorization of phenolic pollutants, highlighting their adverse environmental impacts, and emphasizing the crucial need for their removal. (2) An in-depth discussion on the synthesis methods and properties of MXene-based composites, emphasizing their suitability for environmental remediation. (3) A detailed analysis of MXene-based adsorption, catalysis, photocatalysis, and hybrid processes, showcasing current advancements in MXene modification and functionalization to enhance removal efficiency. (4) A thorough examination of the removal mechanisms and stability of MXene-based technologies, elucidating their operating conditions and stability in pollutant removal scenarios. (5) Finally, this review concludes by outlining future challenges and opportunities for MXene-based technologies in water treatment, facilitating their potential applications. This comprehensive review provides valuable insights and innovative ideas for the development of versatile MXene-based technologies tailored to combat water pollution effectively.
MXenes with their wide range of tunability and good surface chemistry provide unique and distinctive characteristics offering potential employment in various aspects of energy management applications. These high-performance materials have attracted considerable attention in recent decades due to their outstanding characteristics. In the literature, most of the work is related to specific methods for the preparation of MXenes. In this Review, we present a detailed discussion on the synthesis of MXenes through different etching routes involving acids, such as hydrochloric acid, hydrofluoric acid, and lithium fluoride, and non-acidic alkaline solution, electrochemical, and molten salt methods. Furthermore, a concise overview of the different structural, optical, electronic, and magnetic properties of MXenes is provided corresponding to their role in supporting high thermal, chemical, mechanical, environmental, and electrochemical stability. Additionally, the role of MXenes in maintaining the thermal management performance of photovoltaic thermal systems (PV/T), wearable light heaters, solar water desalination, batteries, and supercapacitors is also briefly discussed. A techno-economic and life cycle analysis of MXenes is provided to analyze their sustainability, scalability, and commercialization to facilitate a comprehensive array of energy management systems. Lastly, the technology readiness level of MXenes is defined, and future recommendations for MXenes are provided for their further utilization in niche applications. The present work strives to link the chemistry of MXenes to process economics for energy management applications.
The more than 60 ternary carbides and nitrides, with the general formula Mn+1AXn—where n = 1, 2, or 3; M is an early transition metal; A is an A-group element (a subset of groups 13–16); and X is C and/or N—represent a new class of layered solids, where Mn+1Xn layers are interleaved with pure A-group element layers. The growing interest in the Mn+1AXn phases lies in their unusual, and sometimes unique, set of properties that can be traced back to their layered nature and the fact that basal dislocations multiply and are mobile at room temperature. Because of their chemical and structural similarities, the MAX phases and their corresponding MX phases share many physical and chemical properties. In this paper we review our current understanding of the elastic and mechanical properties of bulk MAX phases where they differ significantly from their MX counterparts. Elastically the MAX phases are in general quite stiff and elastically isotropic. The MAX phases are relatively soft (2–8 GPa), are most readily mac...
The MAX phases are a group of layered ternary compounds with the general formula Mn+1AXn (M: early transition metal; A: group A element; X: C and/or N; n = 1‐3), which combine some properties of metals, such as good electrical and thermal conductivity, machinability, low hardness, thermal shock resistance and damage tolerance, with those of ceramics, such as high elastic moduli, high temperature strength, and oxidation and corrosion resistance. The publication of papers on the MAX phases has shown an almost exponential increase in the past decade. The existence of further MAX phases has been reported or proposed. In addition to surveying this activity, the synthesis of MAX phases in the forms of bulk, films and powders is reviewed, together with their physical, mechanical and corrosion/oxidation properties. Recent research and development has revealed potential for the practical application of the MAX phases (particularly using the pressureless sintering and physical vapour deposition coating routes) as well as of MAX based composites. The challenges for the immediate future are to explore further and characterise the MAX phases reported to date and to make further progress in facilitating their industrial application.
The constitution of the titanium-aluminum-carbon ternary system has been investigated combining critical evaluation of literature
data with new experimental results. Three ternary phases occur in this system: Ti3AlC, Ti2AlC, and newly discovered Ti3AlC2. As analyzed by wet chemistry methods, all three phases are carbon deficient with respect to their “ideal≓ stoichiometry,
which is based on the crystal structure formula. Ti2AlC and Ti3AlC melt incongruently at 1625 ± 10 ‡ and 1580 ± 10 ‡, respectively. Ti3AlC2 decomposes in the solid state. The two isothermal sections at 1000 and 1300 ‡ investigated experimentally are corroborated
by thermochemical calculations. A projection of the liquidus surface is given, and a reaction scheme linking this liquidus
projection with the isothermal sections observed is proposed.
A unified approach to the analysis of the mechanisms that lead to the edge reconstruction of graphite and growth of a variety
of non-planar graphitic structures, such as nanotubes, is suggested. Transmission electron microscopy (TEM) shows that nano-arches
are formed on the edge planes of natural and synthetic graphite, as well as graphite polyhedral crystals, which are built
of graphene sheets; this makes the edge reconstruction of graphite different from the surface reconstruction of other crystals.
A theoretical study of edge zipping in graphite and formation of tubular carbon structures has been performed using an integrated
approach combining molecular dynamics simulation and analytical continual energetics modeling. The suggested theoretical framework
describes the formation of curved surfaces in a wide range of dimensions, which is a general feature of the growth of layered
materials. Layered materials isostructural to graphite, such as hexagonal BN, demonstrate similar edge structures and also
form nanotubes. Thus, the ability of materials to form arches as a result of edge reconstruction points out to their ability
to form nanotubes and vice versa. TEM studies of graphite and hexagonal boron nitride provide experimental verification of
our analytical model.
Herein we report on Li insertion into a new two-dimensional (2-D) layered Ti2C-based material (MXene) with an oxidized surface, formed by etching Al from Ti2AlC in HF at room temperature. Nitrogen sorption of treated powders showed desorption hysteresis consistent with the presence of slit-like pores. At 23 m(2) g(-1), the specific surface area was an order of magnitude higher than untreated Ti2AlC. Cyclic voltammetry exhibited lithiation and delithiation peaks at 1.6 V and 2 V vs. Li+/Li, respectively. At C/25, the steady state capacity was 225 mAh g(-1); at 1C, it was 110 mAh g(-1) after 80 cycles; at 3C, it was 80 mAh g(-1) after 120 cycles: at 10C, it was 70 mAh g(-1) after 200 cycles. Since Ti2C is a member of the MXene family - where M is an early transition metal and X is C and/or N - that to date includes Ti3C2,Ta4C3,TiNbC, and (V-0.5,Cr-0.5)(3)C-2, our results suggest that MXenes are promising as anode materials for Li-ion batteries.
Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane
reactors, permeation barriers, and low-dielectric-constant materials. Synthesis of thin zeolite films using high-aspect-ratio
zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite
nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors
have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation).
We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI.
The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication
of molecular sieve membranes.
Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
Oxidation mechanisms in single- and two-phase Si-Al-O-N ceramics have been studied using scanning and transmission electron microscopy together with energy-dispersive X-ray microanalysis. Silicate layers formed on single-phase (β′) ceramics are non-crystalline, with viscosity and resulting oxidation kinetics controlled by outward diffusion of grain boundary segregated impurities. Aluminium substitution inβ′ is important in compensating for the viscosity reduction imposed by the divalent ion impurities and inhibiting crystallization. Crystallization, induced only on slow furnace cooling, produces mullite and cristobalite phases. Two-phase (β′and matrix) ceramics exhibit comparatively poor oxidation kinetics with formation of a porous crystalline silicate layer due to the continued availability of a high concentration of metallic ions in the matrix phase.
Layered octosilicate immobilized covalently with butylimidazolium groups was fully exfoliated into monolayer nanosheets in water. The thickness of the nanosheets was 1.9 nm (atomic force microscopy (AFM)). The colloidal aggregates of the nanosheets did not show X-ray diffraction (XRD) peaks at lower angles, suggesting the absence of layer stacking. After drying of the colloidal aggregates, a sharp peak at 1.9 nm was observed. This d-value agreed well with the thickness observed by AFM. The in-plane crystal structure of octosilicate was retained after exfoliation because of the presence of the XRD peak at 0.19 nm assignable to the (400) plane of octosilicate. The interlayer surface of Bim-Oct immobilized with butylimidazolium groups is suggested to be easily hydrated, which leads to the swelling and the following exfoliation into nanosheets. The significant change of the silicate surface by the immobilization is novel, which reflects the unique property of butylimidazolium groups. A transparent and colorless film was successfully obtained by spin-coating the colloidal aggregates of nanosheets on a glass substrate. An anionic dye of Orange II was intercalated into the interlayer of restacked nanosheets in the film. The immobilization of imidazolium groups on layered silicates is innovative for the preparation of nanosheets which are designable for various applications.
An infra-red study of the rutile surface has shown that hydroxyl groups and physisorbed water may be removed by thermal activation, and water vapour is readily chemisorbed at room temperature to produce a reversible surface. A model for the surface based on the (110) plane of rutile has enabled interpretation of the spectra indicating two types of hydroxyl groups represented by bands at 3700 and 3670 cm–1 with the latter being the thermally labile species. The oxide shows strong retention of physically adsorbed water molecules at temperatures up to 300°C.
Layered materials with intracrystalline reactivity undergo intercalation and pillaring reactions to produce materials with useful properties for catalysis, electrodes for Li batteries and adsorbents. New possibilities for the use of layered inorganic solids came out from the layered structures capable of delamination. The exfoliated particles are considered a new class of nanomaterial based on single crystal nanosheets. Due to their unique morphological features and properties, these nanosheets can be used as building blocks for nanomaterials with innovative properties. In this feature article we describe the aspects related to layered niobate exfoliation and the new possibilities that arises from the use of niobatenanosheets in the manufacturing of thin films, layer-by-layer (LbL) assemblies, hybrid structures, sensors and other materials.
TiO2 thin films were grown on a glass substrate by sol–gel and dip-coating processes from specially formulated sols, followed by annealing at 460°C. Thermogravimetric analysis (TGA) and differential scanning calorimetric analysis (DSC) of dried sols were performed to explore the thermal events occurring during the annealing process of the sol–gel TiO2 films. The chemical states of some typical elements in the target films were detected by means of X-ray photoelectron spectroscopy (XPS). The morphologies of the original and worn surfaces of the samples were analyzed by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM). The tribological properties of TiO2 thin films sliding against AISI52100 steel and Si3N4 ball were evaluated on a reciprocating friction and wear tester. As the results, the target film was obtained and reaction may have taken place between the film and the glass substrate. TiO2 films are superior in reducing friction and resisting wear compared with the glass substrate. SEM observation of the morphologies of worn surfaces indicates that the wear of glass is characteristic of brittle fracture and severe abrasion. Differently, abrasion, plastic deformation and micro-crack dominate the wear of TiO2 films. The superior friction reduction and wear resistance of TiO2 films are attributed to slight plastic deformation as well as good adhesion of the film to the substrate.
TiO2 films on 304 stainless steel with optimum P-25 loading (PPMSGFs-50) were prepared by the P-25 powder-modified sol–gel method (PPMSGM) at calcination temperatures from 400 to 700°C. The as-prepared PPMSGFs-50 were characterized by differential scanning colorimetry/thermogravimetric analyses (DSC/TGA), scanning electron microscopy (SEM), TEM/high-resolution transmission electron microscope (HR-TEM), X-ray diffraction (XRD), N2 adsorption and X-ray photoelectron spectroscopy (XPS). Their adhesion properties were tested by the scratch test and cross-hatch adhesion test and their photocatalytic activities were evaluated using 4-CBA as a model organic contaminant in water. It was found that decreasing calcination temperature leads to a decrease in the critical loading, which has a detrimental effect on the mechanical stability of the films. On the other hand, decreasing calcination temperature leads to an enhancement of photocatalytic activity. The optimum calcination temperature is 500°C under which both enhanced photocatalytic activity and good adherence on the support could be obtained. The 4-CBA removal efficiency for PPMSGF-50 at 500°C was approximately four times that of PPMSGF-50 at 600°C after 10h of photocatalytic oxidation. Increasing calcination temperature in the range between 500 and 700°C caused a significantly increase in the diffusion of foreign metals (i.e. chromium, iron and manganese) from the stainless steel support to the TiO2 film. It is believed that the enhancement of the photocatalytic activity at lower calcination temperatures (400–500°C) is due to an increase in the amount of two type of crystallites exposed to the solid–liquid interface and decrease in foreign metal ion concentrations (i.e. Cr3+) on the surface of the films.
Oxidation tests of silicon nitride hot pressed with combined Y2O3 and MgO additions in the compatibility field Si3N4-Si2N2O-Y2Si2O7, performed at 1193 to 1658 K under 98 kPa air atmosphere for 30 h, result in parabolic oxidation kinetics and three different oxidation regimes. At T?H=120 kJ mol-1) oxygen diffusion is suggested to be the more probable limiting step for oxidation, whereas at higher temperatures, diffusion of additive and impurities appears rate-controlling (?H=580 kJ mol-1). The very high oxidation rates and ?H values (?H=960 kJ mol-1) at T>1600 K are possibly associated with a softening of the grain-boundary phase. The existance of silicate solid solutions in the grain-boundary phase which are able to accomodate additive and impurity cations in their structure might explain the very good oxidation resistance of (Y2O3+MgO)-doped HPSN at T
CARBONACEOUS materials exist in many forms, with structures varying in the degree of structural order from diamond and hexagonal graphite to the less-ordered chars, soots and coals. Traditionally, graphite is produced from the less-ordered forms of carbon by exposing them to severe conditions of high temperature and pressure. Recently, there has been much interest in preparing highly ordered phases under relatively benign conditions. For example, diamond has been synthesized at low pressures1,2, and carbon deposition on transition-metal catalysts has been shown to produce highly graphitic phases in a variety of morphologies3,4. Here we show that disproportionation of carbon monoxide (2CO C+CO2) catalysed by small iron-containing particles gives rise to a remarkably continuous stacking of fine graphitic layers on flat surfaces of the particles, producing carbon filaments with a ribbon-like morphology. Transmission electron microscopy reveals that stacking of the layers is not only very ordered but also unusual in being orientated perpendicular to the ribbon surface. These filaments may provide unique opportunities for studies of surface adsorption, catalysis and intercalation.
TiO2 rutile of high purity was prepared through direct oxidation of the TiCl3 with atmospheric O2 at room temperature. The rutile phase obtained in this way is microcrystalline and has a high surface area. This phase suffered a crystal growth as a function of temperature (100°C–700°C) with textural properties modifications.
Short-term (<30 h) oxidation of CeO2-doped hot-pressed Si3N4, studied at 773 to 1623 K in flowing dry air, resulted in parabolic-weight gain curves and in oxidation-rate constants nearly independent of the amount of grain-boundary second phase. Exolution of CeO2 crystals occurs from the oxide layer; their morphology depends on oxidation temperature. Either a direct redox reaction between CeO2 and Si3N4 or solution of Si3N4 in the glassy silica-rich oxide layer, followed by its oxidation by dissolved oxygen, have been proposed as possible oxidation mechanisms, both appearing strongly dependent on the low solubility of cerium oxides hi silicate melts. The value of the activation energy for oxidation of 385 kj·mol−1 suggests additive and impurity migration from the bulk to the Si3N4/oxide reaction interface as the most probable rate-limiting step.
Polycrystalline bulk samples of Ti3Al1.1C1.8 have been fabricated by reactively hot isostatically pressing a mixture of titanium, graphite, and Al4C3 powders at a pressure of 70 MPa and temperature of 1400°C for 16 h. The hot isostatically pressed samples are predominantly single phase (containing ∼4 vol% Al2O3), fully dense, and have a grain size of ∼25 μm. This carbide is similar to Ti3SiC2, with which it is isostructural, and has an unusual combination of properties. It is relatively soft (Vickers hardness of ∼3.5 GPa) and elastically stiff (Young's modulus of 297 GPa and shear modulus of 124 GPa); yet, it is lightweight (density of 4.2 g/cm3) and easily machinable. The room-temperature electrical resistivity is 0.35 ± 0.03 μΩ·m and decreases linearly as the temperature decreases. The temperature coefficient of resistivity is 0.0031 K−1. The coefficient of thermal expansion, in the temperature range of 25°—1200°C, is 9.0 (± 0.2) × 10−6 K−1. The room-temperature compressive and flexural strengths are 560 ± 20 and 375 ± 15 MPa, respectively. In contrast to flexure, where the failure is brittle, the failure in compression is noncatastrophic and is accompanied by some plasticity. The origin of that plasticity is believed to be the formation of a “shear” band that is oriented at an angle of ∼45° to the applied load. Ti3Al1.1C1.8 also is a highly damage-tolerant material; a 10-kg-load Vickers indentation made in a bar 1.5 mm thick reduces the post-indentation flexural strength by ∼7%. This material also is quite resistant to thermal shock. At temperatures of >1000°C, the deformation in compression is accompanied by significant plasticity and very respectable ultimate compressive stresses (200 MPa at 1200°C).
The normal impurities in hot-pressed Si3N4 fabricated with MgO as an additive include Ca, Fe, WC, and SiO2. Within the bulk material the conditions are highly reducing, so the Fe and W will be in the reduced state. Thus the equilibrium oxide phases can be predicted from the CaO-MgO-SiO2 phase diagram suitably modified by the solubility of the Si3N4 in the liquid phase. The presence of the Fe and W can lead to enhanced liquid formation on the surface, where the oxygen potential is higher. Only the nitrogen is capable of generating high internal pressures.
The isothermal oxidation of TiC powders was carried out at low temperatures of 350–500 C at oxygen pressures of 3.9, 7.9 and 16 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electro-microbalance. The oxidation kinetics are described by the one-dimensional diffusion equation. It was found that oxidation consists of four steps, I (fast step), II (slow step), III (fast step) and IV (slow step), at all the pressures. Two activation energies were obtained in steps II–IV: 125–150 kJ mol–1 below about 420 C and 42–71 kJ mol–1 above that temperature. The low- and high-temperature oxidation mechanisms are discussed in connection with the formation of oxycarbide/titanium suboxides and the crystallization of anatase, followed by the generation of cracks in the grains.
We describe the synthesis of very thin sheets (between a few and ten atomic layers) of hexagonal boron nitride (h-BN), prepared either on a SiO2 substrate or freely suspended. Optical microscopy, atomic force microscopy, and transmission electron microscopy have been used to characterize the morphology of the samples and to distinguish between regions of different thicknesses. Comparison is made to previous studies on single- and few-layer graphene. This synthesis opens the door to experimentally accessing the two-dimensional phase of boron nitride.
This article is a critical review of the Mn + 1AXn phases (“MAX phases”, where n = 1, 2, or 3) from a materials science perspective. MAX phases are a class of hexagonal-structure ternary carbides and nitrides (“X”) of a transition metal (“M”) and an A-group element. The most well known are Ti2AlC, Ti3SiC2, and Ti4AlN3. There are ~ 60 MAX phases with at least 9 discovered in the last five years alone. What makes the MAX phases fascinating and potentially useful is their remarkable combination of chemical, physical, electrical, and mechanical properties, which in many ways combine the characteristics of metals and ceramics. For example, MAX phases are typically resistant to oxidation and corrosion, elastically stiff, but at the same time they exhibit high thermal and electrical conductivities and are machinable. These properties stem from an inherently nanolaminated crystal structure, with Mn + 1Xn slabs intercalated with pure A-element layers. The research on MAX phases has been accelerated by the introduction of thin-film processing methods. Magnetron sputtering and arc deposition have been employed to synthesize single-crystal material by epitaxial growth, which enables studies of fundamental material properties. However, the surface-initiated decomposition of Mn + 1AXn thin films into MX compounds at temperatures of 1000–1100 °C is much lower than the decomposition temperatures typically reported for the corresponding bulk material. We also review the prospects for low-temperature synthesis, which is essential for deposition of MAX phases onto technologically important substrates. While deposition of MAX phases from the archetypical Ti–Si–C and Ti–Al–N systems typically requires synthesis temperatures of ~ 800 °C, recent results have demonstrated that V2GeC and Cr2AlC can be deposited at ~ 450 °C. Also, thermal spray of Ti2AlC powder has been used to produce thick coatings. We further treat progress in the use of first-principle calculations for predicting hypothetical MAX phases and their properties. Together with advances in processing and materials analysis, this progress has led to recent discoveries of numerous new MAX phases such as Ti4SiC3, Ta4AlC3, and Ti3SnC2. Finally, important future research directions are discussed. These include charting the unknown regions in phase diagrams to discover new equilibrium and metastable phases, as well as research challenges in understanding their physical properties, such as the effects of anisotropy, impurities, and vacancies on the electrical properties, and unexplored properties such as superconductivity, magnetism, and optics.
Titanium dioxide (TiO2) thin films were prepared on silicon substrates by plasma-enhanced chemical vapor deposition (PECVD) using Ti(OiC3H7)4 and oxygen. PECVD of TiO2 films has been evaluated with various process parameters. The characteristics of films were investigated by X-ray diffraction, scanning electron microscopy, TG/DTA, FTIR, UV/visible spectroscopy and Auger electron spectroscopy. Typical as-deposited film was amorphous and transparent with a refractive index of 2.05. As the deposition time increased, surface morphology became coarser, and structure was transformed from amorphous to mixtures of amorphous and crystal. As-deposited amorphous TiO2 films had a dielectric constant of 13.7 and flat-band voltage of −1.3 V. The effects of post-treatment through N2 or O2 plasma on the electrical properties of as-deposited films were evaluated. Electrical properties could be enhanced by O2 plasma treatment.
MoS2 has been exfoliated into monolayers by intercalation with lithium followed by reaction with water. X-ray diffraction analysis has shown that the exfoliated MoS2 in suspension is in the form of one-molecule-thick sheets. X-ray patterns from dried and re-stacked films of exfoliated MoS2 indicate that the layers are randomly stacked. Exfoliated MoS2 has been deposited on alumina particles in aqueous suspension, enabling recovery of dry exfoliated MoS2 supported on alumina.
Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.
2D Ti 3C 2 nanosheets, multilayer structures, and conical scrolls produced by room temperature exfoliation of Ti 3AlC 2 in HF are reported. Since Ti 3AlC 2 is a member of a 60+ group of layered ternary carbides and nitrides, this discovery opens a door to the synthesis of a large number of other 2D crystals.
The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS.
A wide variety of cation-exchangeable layered transition metal oxides and their relatively rare counterparts, anion-exchangeable layered hydroxides, have been exfoliated into individual host layers, i.e., nanosheets. Exfoliation is generally achieved via a high degree of swelling, typically driven either by intercalation of bulky organic ions (quaternary ammonium cations, propylammonium cations, etc.) for the layered oxides or by solvation with organic solvents (formamide, butanol, etc.) for the hydroxides. Ultimate two-dimensional (2D) anisotropy for the nanosheets, with thickness of around one nanometer versus lateral size ranging from submicrometer to several tens of micrometers, allows them to serve either as an ideal quantum system for fundamental study or as a basic building block for functional assembly. The charge-bearing inorganic macromolecule-like nanosheets can be assembled or organized through various solution-based processing techniques (e.g., flocculation, electrostatic sequential deposition, or the Langmuir-Blodgett method) to produce a range of nanocomposites, multilayer nanofilms, and core-shell nanoarchitectures, which have great potential for electronic, magnetic, optical, photochemical, and catalytic applications.
Graphene has changed from being the exclusive domain of condensed-matter physicists to being explored by those in the electron-device community. In particular, graphene-based transistors have developed rapidly and are now considered an option for post-silicon electronics. However, many details about the potential performance of graphene transistors in real applications remain unclear. Here I review the properties of graphene that are relevant to electron devices, discuss the trade-offs among these properties and examine their effects on the performance of graphene transistors in both logic and radiofrequency applications. I conclude that the excellent mobility of graphene may not, as is often assumed, be its most compelling feature from a device perspective. Rather, it may be the possibility of making devices with channels that are extremely thin that will allow graphene field-effect transistors to be scaled to shorter channel lengths and higher speeds without encountering the adverse short-channel effects that restrict the performance of existing devices. Outstanding challenges for graphene transistors include opening a sizeable and well-defined bandgap in graphene, making large-area graphene transistors that operate in the current-saturation regime and fabricating graphene nanoribbons with well-defined widths and clean edges.
The surface area of a single graphene sheet is 2630 m(2)/g, substantially higher than values derived from BET surface area measurements of activated carbons used in current electrochemical double layer capacitors. Our group has pioneered a new carbon material that we call chemically modified graphene (CMG). CMG materials are made from 1-atom thick sheets of carbon, functionalized as needed, and here we demonstrate in an ultracapacitor cell their performance. Specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively, have been measured. In addition, high electrical conductivity gives these materials consistently good performance over a wide range of voltage scan rates. These encouraging results illustrate the exciting potential for high performance, electrical energy storage devices based on this new class of carbon material.
Polyhedral nano- and microstructures with shapes of faceted needles, rods, rings, barrels, and double-tipped pyramids, which we call graphite polyhedral crystals (GPCs), have been discovered. They were found in pores of glassy carbon. They have nanotube cores and graphite faces, and they can exhibit unusual sevenfold, ninefold, or more complex axial symmetry. Although some are giant radially extended nanotubes, Raman spectroscopy and transmission electron microscopy suggest GPCs have a degree of perfection higher than in multiwall nanotubes of similar size. The crystals are up to 1 micrometer in cross section and 5 micrometers in length, and they can probably be grown in much larger sizes. Preliminary results suggest a high electrical conductivity, strength, and chemical stability of GPC.
We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions,
metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between
valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in
concentrations up to 1013 per square centimeter and with room-temperature mobilities of ∼10,000 square centimeters per volt-second can be induced by
applying gate voltage.
High-resolution electron microscopy and lithium-7 nuclear magnetic resonance measurements were carried out for a disordered
carbon material, prepared by heat treatment of polyphenylene, in which lithium was stored electrochemically. The nuclear magnetic
resonance spectrum suggests the existence of Li2 covalent molecules in the carbon material. This extra covalent site of lithium storage promises extraordinarily high energy
density for secondary batteries.
- N V Tzenov
- M W Barsoum
N.V. Tzenov, M.W. Barsoum, J. Am. Ceram. Soc. 83 (2000) 825e832.
- W G Lee
- S I Woo
- J C Kim
- S H Choi
- K H Oh
W.G. Lee, S.I. Woo, J.C. Kim, S.H. Choi, K.H. Oh, Thin Solid Films 237 (1994)
105e111.
- M Naguib
- M Kurtoglu
- V Presser
- J Lu
- J J Niu
- M Heon
- L Hultman
- Y Gogotsi
- M W Barsoum
M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J.J. Niu, M. Heon, L. Hultman,
Y. Gogotsi, M.W. Barsoum, Adv. Mater. 23 (2011) 4248e4253.
- M Naguib
- O Mashtalir
- J Carle
- V Presser
- J Lu
- L Hultman
- Y Gogotsi
- M W Barsoum
M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi,
M.W. Barsoum, ACS Nano 6 (2012) 1322e1331.
- M Naguib
- J Come
- B Dyatkin
- V Presser
- P L Taberna
- P Simon
- M W Barsoum
- Y Gogotsi
M. Naguib, J. Come, B. Dyatkin, V. Presser, P.L. Taberna, P. Simon,
M.W. Barsoum, Y. Gogotsi, Electrochem. Commun. 16 (2012) 61e64.
- F Schwierz
F. Schwierz, Nat. Nanotechnol. 5 (2010) 487e496.
- R Ma
- T Sasaki
R. Ma, T. Sasaki, Adv. Mater. 22 (2010) 5082e5104.
- W G Zhang
- W M Liu
- C T Wang
W.G. Zhang, W.M. Liu, C.T. Wang, Wear 253 (2002) 377e384.
- K Varoon
- X Y Zhang
- B Elyassi
- D D Brewer
- M Gettel
- S Kumar
- J A Lee
- S Maheshwari
- A Mittal
- C Y Sung
- M Cococcioni
- L F Francis
- A V Mccormick
- K A Mkhoyan
- M Tsapatsis
K. Varoon, X.Y. Zhang, B. Elyassi, D.D. Brewer, M. Gettel, S. Kumar, J.A. Lee,
S. Maheshwari, A. Mittal, C.Y. Sung, M. Cococcioni, L.F. Francis,
A.V. McCormick, K.A. Mkhoyan, M. Tsapatsis, Science 333 (2011) 72e75.
Phase effects in SigN 4 containing Y20~ or CeO 2. If. Oxidation
- C R Quackenbush
- J J Smith
C. R. Quackenbush and J. J. Smith, "Phase effects in SigN 4 containing Y20~ or CeO 2. If. Oxidation," Am. Ceram. S.c. Bull., 59, No. 5, 533-537 (1980).
Development and investigation of silicon nitride base constructional materials,” Author's Abstract of Candidate's Dissertation
- O D Shcherbina
Some results of an investigation into the mechanical properties of constructional ceramics suitable for engine components,” Institute of Strength Problems
- G A Gogotsi