MXene‐Enabled Wearable Biosensors: A Design Framework for Autonomous Biosensing

Transition metal carbides (TMCs) and transition metal nitrides (TMNs) have attracted attention as promising electrocatalysts that could replace noble metals of high price and limited supply. Relative to parent metals, TMC and TMN behave like noble metals for electrochemical reactions such as oxidation of hydrogen, CO and alcohols, and reduction of oxygen. When TMC and TMN are combined with other metals, the electrocatalytic synergy is often observed in electrochemical reactions. Thus, combinations with a minute amount of Pt or even non-Pt metals give performance comparable to heavily loaded Pt-based electrocatalysts for low temperature fuel cells. It appears that TMC based electrocatalysts are more active as anode catalysts for oxidation of fuels, whereas TMN based catalysts are more active for cathode catalysts for oxygen reduction and more stable.

Recent advances in transition metal carbides and nitrides (MXenes): Characteristics, environmental remediation and challenges

Photoexcited hot carriers in metals can be injected into adjacent semiconductors to collect sub-bandgap photons. In the current work, we experimentally demonstrate that transition metal nitrides and transition metal carbides can act as metals and generate hot electrons by optical illumination similar to metals. Since transition metal nitrides and carbides have broad absorption in the visible spectrum, they have the potential to be used for photocatalytic and photovoltaic applications to harvest solar energy.

Recent advance in two-dimensional MXenes: New horizons in flexible batteries and supercapacitors technologies

We present a systematic investigation of thermodynamic stability, phase-reaction, and chemical activity of Al containing disordered Ti2(Al-Ga)C MAX phases using machine-learning driven high-throughput framework to understand the oxidation resistance behavior with increasing temperature and exposure to static oxygen. The A-site (at Al) disordering in Ti2AlC MAX (M=Ti, A=Al, X=C) with Ga shows significant change in the chemical activity of Al with increasing temperature and exposure to static oxygen, which is expected to enable surface segregation of Al, thereby, the formation of Al2O3 and improved oxidation resistance. We performed in-depth convex hull analysis of ternary Ti–Al–C, Ti–Ga–C, and Ti–Al–Ga–C based MAX phase, and provide detailed contribution arising from electronic, chemical and vibrational entropies. The thermodynamic analysis shows change in the Gibbs formation enthalpy (ΔGform) at higher temperatures, which implies an interplay of temperature-dependent enthalpy and entropic contributions in oxidation resistance Ga doped Ti2AlC MAX phases. A detailed electronic structure and chemical bonding analysis using crystal orbital Hamilton population method reveal the origin of change in phases stability and in oxidation resistance in disorder Ti2(Al1−xGax)C MAX phases. Our electronic structure analysis correlate well with the change in oxidation resistance of Ga doped MAX phases. We believe our study provides a useful guideline to understand to role of alloying on electronic, thermodynamic, and oxidation related mechanisms of bulk MAX phases, which can work as a precursor to understand oxidation behavior of two-dimensional MAX phases, i.e., MXenes (transition metal carbides, carbonitrides and nitrides).

Thermochemistry plays a key role in determining durability of the catalysts and catalytic supports, but is also critical in guiding the rational design of synthesis processes that lead to materials with advanced properties [1, 2]. In the last decade computations based on density functional theory have been used as an important tool in the calculation of thermodynamic properties, such as reaction enthalpies, free energies, and heat capacities. In this work we combine experimental thermochemical data available in the HSC Chemistry software and the data obtained using density functional theory calculations to study the stability of various transition metal carbides and nitrides in aqueous media as a function of pH, cell potential, and temperature. Earth abundant transition metal carbides and nitrides have drawn considerable interest due to their low cost, unique chemical and physical properties, and high catalytic activity for variety of reactions, such as nitrogen electroreduction to ammonia, ammonia decomposition, alcohol electrooxidation, and biomass conversion. We will discuss the thermodynamics directed towards understanding oxidation of transition metal carbides and nitrides at different cell potentials, pH and temperatures. Namely, oxidation of the material will lead to the change in the geometric and electronic structure of the material, which can cause degradation and decrease in the catalytic activity. More specifically, transition metal carbides and nitrides based on molybdenum, iron, vanadium, and nickel will be considered to illustrate the importance of mapping out their stable phase as a function of pH, potential, and temperature. These diagrams can then be used to determine the stability of the materials in the aqueous media and guide the reaction conditions that will ensure highest durability for a targeted application. [1] S. K. Nayak, A. D. Benavidez, I. Matanovic, and F. H. Garzon, Thermochemical analysis of Mo-C-H systems for synthesis of molybdenum carbides, Thermochim. Acta, 676, 27-32 (2019). [2] S. K. Nayak, A. D. Benavidez, F. H. Garzon, Facile synthesis of high surface area molybdenum carbide nanoparticles, J. Am. Ceram. Soc., 00: 1– 7 (2018).

Black phosphorus (BP), a two-dimensional (2D) layered material, has emerged as a highly promising candidate for next-generation electrochemical biosensing platforms. Its unique combination of a layer-tunable direct bandgap, high charge carrier mobility, and pronounced in-plane anisotropy distinguishes it from other 2D materials such as graphene and transition metal dichalcogenides. These intrinsic properties render BP highly responsive to local electronic perturbations, enabling sensitive transduction for low-abundance biological targets. However, practical deployment is severely hampered by rapid degradation under oxygen, water, and light. This review is primarily focused on BP-enabled electrochemical biosensing, emphasizing stability engineering, surface functionalization for biointerfaces, and device integration toward flexible and wearable sensing; imaging or drug-delivery studies are discussed only when they provide transferable stabilization or biocompatibility insights. We analyze BP degradation mechanisms, then compare stabilization strategies with explicit attention to the tradeoffs between physical encapsulation and chemical functionalization (covalent versus non-covalent). We next summarize electrochemical detection across representative analyte classes, spanning small molecules (e.g. neurotransmitters) to macromolecular biomarkers (e.g., nucleic acids and proteins), and we close by outlining wearable integration routes and the remaining barriers for real-world operation in complex biofluids and scalable manufacturing.

The morphologies of the transition metal carbide (TMC) (ZrCx, NbCx, and TaCx), transition metal nitride (TMN) (TiNx), and transition metal diboride (TMD) (NbB2x and TaB2x) particles formed during the self-propagating high-temperature synthesis (SHS) were investigated. The results indicate that the ceramics with wide stoichiometric ranges all show a stoichiometry-induced morphology evolution, i.e., octahedron → truncated-octahedron → spherelike → sphere, for TMCs and TMNs, and hexagonal prism → polyhedron → spherelike, for TMDs. For TMCs and TMNs, the increase in the stoichiometry leads to the increase in the growth rate in the ⟨111⟩ crystalline direction. Hence, their morphologies show an evolution process of gradual exposure of the {100} surfaces and shrinkage of the {111} surfaces. When the exposed {100} surfaces are roughed because of the extremely high combustion temperatures during the SHS and thus turn round, the growth shapes of the TMC and TMN crystals change to spherelike. On the other hand, when...

Transition metal carbides and nitrides as oxygen reduction reaction catalyst or catalyst support in proton exchange membrane fuel cells (PEMFCs)

A new Ni-based alumina-forming alloy, NiCoCrMoAl, has been invented with the high creep strength and excellent high temperature oxidation and corrosion resistance for high temperature applications. The static oxidation tests of the alloy, along with the other commonly used Ni-based high temperature alloys, such as UNS N07214, N06230, N06002, N06625 and N06617, were conducted at the temperatures of 871 °C (1600 °F), 982 °C (1800 °F), and 1149 °C (2100 °F). The oxidation kinetics and cross-sections of the alloys were analyzed to understand their oxidation mechanism and behaviors. At the test temperatures, the alumina-forming alloys, NiCoCrMoAl and N07214, showed excellent oxidation resistance as a result of the formation of a continuous and adherent alumina scale. In addition, the NiCoCrMoAl alloy exhibited similar or superior oxidation resistance when compared to the N07214 alloy, which was attributed to a consistent alumina scale formed on NiCoCrMoAl with less internal oxidation attack. The chromia-forming alloys showed good oxidation resistance, without observed spallation, due to the formation of a protective chromia scale at 871 and 982 °C. At 1149 °C, the chromia-forming alloys showed inferior oxidation resistance, in which N06625 experienced breakaway oxidation after two cycles (336h). Amongst the chromia forming alloys, the N06230 alloy showed the best oxidation resistant performance at the test temperatures.

Tantalum carbide (TaC) and hafnium carbide (HfC) have some of the highest melting temperatures among the transition metal carbides, borides, and nitrides, making them promising materials for high‐speed flight and high‐temperature structural applications. Solid solutions of TaC and HfC are of particular interest due to their enhanced oxidation resistance compared to pure TaC or HfC. This study looks at the effect of Hf content on the oxidation resistance of TaC–HfC sintered specimens. Five compositions are fabricated into bulk samples using spark plasma sintering (2173 K, 50 MPa, 10 min hold). Oxidation behavior of a subset of the compositions (100 vol% TaC, 80 vol% TaC + 20 vol% HfC, and 50 vol% TaC + 50 vol% HfC) is analyzed using an oxyacetylene torch for 60 s. The TaC–HfC samples exhibit a reduction in the oxide scale thickness and the mass ablation rate with increasing HfC content. The improved oxidation resistance can be attributed to the formation of a Hf6Ta2O17 phase. This phase enhances oxidation resistance by reducing oxygen diffusion and serving as a protective layer for the unoxidized material. The superior oxidation resistance of TaC–HfC samples makes these materials strong contenders for the development of high‐speed flight coatings.

First-principle calculations of the bulk properties of 4 d transition metal carbides and nitrides in the rocksalt, zincblende and wurtzite structures

As the key of hydrogen economy, electrocatalytic hydrogen evolution reactions (HERs) depend on the availability of cost-efficient electrocatalysts. Over the past years, there is a rapid rise in noble-metal-free electrocatalysts. Among them, transition metal carbides (TMCs) are highlighted due to their structural and electronic merits, e.g., high conductivity, metallic band states, tunable surface/bulk architectures, etc. Herein, representative efforts and progress made on TMCs are comprehensively reviewed, focusing on the noble-metal-like electronic configuration and the relevant structural/electronic modulation. Briefly, specific nanostructures and carbon-based hybrids are introduced to increase active-site abundance and to promote mass transportation, and heteroatom doping and heterointerface engineering are encouraged to optimize the chemical configurations of active sites toward intrinsically boosted HER kinetics. Finally, a perspective on the future development of TMC electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials in energy chemistry.

Recently, a family of layered ternary transition-metal carbides and/or nitrides, known as the MAX phases, received increasing attention as precursors for two-dimensional (2D) transition-metal carbides and/or nitrides known as MXenes. The MAX phases have the general formula Mn+1AXn, where n= 1 to 3 and M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen.1 - 2 By selectively etching the A-element layers from MAX phases, MXenes have been synthesized. Given that the MXene surfaces are terminated with OH, and F groups resulting from the etching process, it is more accurate to refer to them as Mn+1XnTx, where T represents the surface terminating groups, such as O, F and OH. There are more than 70 compounds known in the MAX family to date (plus dozens of solid solutions),3 most of which offer good electronic conductivity, lamellar structure, and temperature and environmental stability. As-prepared MXenes exist in a multilayered structure, which appears as stacked multilayer flakes. The multilayered structures can be delaminated into single-layer flakes by intercalation and/or sonication.4 - 5 Due to their high specific surface areas, metallic conductivity, and hydrophilic surfaces, MXenes have shown promising performance as electrodes for supercapacitors. MXenes are also promising anode materials for LIBs due to their excellent conductivity and cation intercalation capability.6 - 7 Nb4C3 MXene shows stable capacity, also at high rates (Figure 1). A capacity of 410 mAh g−1 for a Ti3C2 MXene “paper” anode at 1C rate without adding any binder has been reported. This Ti3C2 MXene “paper” was fabricated by intercalating Ti3C2 with dimethyl sulfoxide, bath sonication for 6 h, and filtering the Ti3C2 colloidal solution.8 - 9 The experimental procedure is very simple. MXene-based LIB anodes exhibited Li-ion capacity up to 800 mAh g−1, good rate performance, and excellent cycling stability.9 - 10 These results suggest a potential for MXene-supported hybrid electrodes for energy storage applications. By decreasing the particle size, we successfully demonstrated that not only MXenes but also their precursors - MAX phases, such as Ti2SC and Ti3SiC2, exhibit Li-ion storage capacity and show promise as LIB anode materials. Capacities of 180 and 150 mAh g−1 were achieved by submicrometer size Ti2SC and Ti3SiC2, respectively, accompanied by good cycle life and excellent rate performance.11 It opens the door to exploring a large family of potential anode materials. 1. Wang, X., et al. Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors. Nature communications 2015, 6. 2. Mashtalir, O.; Lukatskaya, M. R.; Zhao, M. Q.; Barsoum, M. W.; Gogotsi, Y., Amine-Assisted Delamination of Nb2C MXene for Li-Ion Energy Storage Devices. Advanced Materials 2015, 27 (23), 3501-3506. 3. Hu, C.; Zhang, H.; Li, F.; Huang, Q.; Bao, Y., New phases’ discovery in MAX family. International Journal of Refractory Metals & Hard Materials 2013, 36, 300-312. 4. Kim, S. J.; Naguib, M.; Zhao, M.; Zhang, C.; Jung, H. T.; Barsoum, M. W.; Gogotsi, Y., High mass loading, binder-free MXene anodes for high areal capacity Li-ion batteries. Electrochimica Acta 2015, 163, 246-251. 5. Ling, Z.; Ren, C. E.; Zhao, M. Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y., Flexible and conductive MXene films and nanocomposites with high capacitance. Proceedings of the National Academy of Sciences 2014, 111 (47), 16676-81. 6. Naguib, M.; Come, J.; Dyatkin, B.; Presser, V.; Taberna, P. L.; Simon, P.; Barsoum, M. W.; Gogotsi, Y., MXene: a promising transition metal carbide anode for lithium-ion batteries. Electrochemistry Communications 2012, 16 (1), 61-64. 7. Sun, D.; Wang, M.; Li, Z.; Fan, G.; Fan, L. Z.; Zhou, A., Two-dimensional Ti3C2 as anode material for Li-ion batteries. Electrochemistry Communications 2014, 47 (10), 80-83. 8. Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall’Agnese, Y.; Min, H.; Barsoum, M. W.; Gogotsi, Y., Intercalation and delamination of layered carbides and carbonitrides. Nature communications 2013, 4 (2), 216-219. 9. Luo, J.; Tao, X.; Zhang, J.; Xia, Y.; Huang, H.; Zhang, L.; Gan, Y.; Liang, C.; Zhang, W., Sn4+ Ions Decorated Highly Conductive Ti3C2 MXene: Promising Lithium-Ion Anodes with Enhanced Volumetric Capacity and Cyclic Performance. ACS Nano 2016, 10 (2). 10. Ren, C. E.; Zhao, M. Q.; Makaryan, T.; Halim, J.; Boota, M.; Kota, S.; Anasori, B.; Barsoum, M. W.; Gogotsi, Y., Porous Two-Dimensional Transition Metal Carbide (MXene) Flakes for High-Performance Li-Ion Storage. ChemElectroChem 2016, 3 (5), 689-693. 11. J. Xu, M.-Q. Zhao, Y. Wang, W. Yao, C. Chen, B. Anasori, A. Sarycheva, C. E. Ren, T. Mathis, L. Gomes, Z. Liang, Y. Gogotsi, Demonstration of Li-ion capacity of MAX phases, ACS Energy Letters, 2016, 1, 1094−1099. Figure 1

We investigate the ability of the local density approximation (LDA) in density functional theory to predict the near-edge structure in electron energy-loss spectroscopy in the dipole approximation. We include screening of the core hole within the LDA using Slater's transition state theory. We find that anion K-edge threshold energies are systematically overestimated by 4.22±0.44 eV in twelve transition metal carbides and nitrides in the rock-salt (B1) structure. When we apply this `universal' many-electron correction to energy-loss spectra calculated within the transition state approximation to LDA, we find quantitative agreement with experiment to within one or two eV for TiC, TiN and VN. We compare our calculations to a simpler approach using a projected Mulliken density which honours the dipole selection rule, in place of the dipole matrix element itself. We find remarkably close agreement between these two approaches. Finally, we show an anomaly in the near-edge structure in CrN to be due to magnetic structure. In particular, we find that the N K edge in fact probes the magnetic moments and alignments of the Cr sublattice.