Two-dimensional Transition Metal Carbides Research Articles

Surface Plasmons in Two-Dimensional MXenes.

MXenes, a class of layered two-dimensional transition metal carbides and nitrides, exhibit excellent optoelectronic properties and show promise for fields ranging from photonics and communications to energy storage and catalysis. Some members of the MXene family are metallic and exhibit large in-plane conductivity, making them possibly suited for 2D plasmonics. The highly variable chemical structure of MXenes offers a broad chemical space to tune material properties for plasmonic applications, including plasmon-enhanced catalysis, surface-enhanced Raman spectroscopy (SERS), and electromagnetic shielding. However, this synthetic complexity has also presented several roadblocks in the process of moving MXene plasmonics into applications. For example, in the prototypical MXene Ti3C2Tx, there remains disagreement over the bulk plasmon energy and the assignment of a prominent resonance around 1.7 eV. We discuss fundamental models and theories of plasmon physics and apply these models to MXenes in order to clarify some of these problems. We outline the potential for hyperbolic plasmons in MXenes and propose new avenues for MXene photonics research.

Exploring the structural and electronic properties of N-doped Ti2C MXenes for novel applications in advanced materials and devices: A DFT study

MXenes, a category of two-dimensional transition metal carbides and nitrides, have attracted significant interest owing to their distinctive characteristics. This study utilizes Density Functional Theory (DFT) to examine the effects of nitrogen doping on the structural and electrical properties of Ti2C MXenes. N-doping induces significant alterations in the lattice structure and electronic properties, culminating in an elevated density of states at the Fermi level, indicating improved conductivity. Furthermore, optical characteristics such as reflectivity and loss function are affected by N-doping, resulting in a shift in the intensity of the reflectivity peak. The alterations provide N-doped Ti2C MXenes viable candidates for advanced applications in energy storage, catalysis, and electronic devices, facilitating future empirical and theoretical investigations in MXene-based materials.

Impact of PEDOT:PSS/Ti3C2Tx composite hole transport layer on the performance of perovskite light emitting diodes

The two-dimensional transition metal carbide MXenes, often designated by Ti3C2Tx, have garnered significant interest on account of their distinctive optoelectronic characteristics. In this paper, PEDOT:PSS/Ti3C2Tx composite hole transport layer (HTLs) was prepared by adding the MXenes material Ti3C2Tx into PEDOT:PSS, and quasi-two-dimensional perovskite light-emitting diodes (PeLEDs) were fabricated and investigated. While the addition concentration of Ti3C2Tx was 0.25 mg/mL, the optimal device shows the maximum luminance of 6250 cd/m2, and the current efficiency of 6.17 cd/A, which is 133 % and 112 % higher than the reference device without Ti3C2Tx, respectively. It is indicated that the additive incorporation of Ti3C2Tx in PEDOT:PSS can improve the efficiency of hole injection and conductivity of hole transport layer, and passivate the defects of perovskite film, thus enhancing the optoelectronic performance of PeLEDs. The findings of this study indicate that the PEDOT:PSS/Ti3C2Tx composite HTLs has considerable potential for application in PeLEDs.

Spray-on electronic tattoos with MXene and liquid metal nanocomposites

Electronic tattoos present appealing forms of wearable devices operated directly on the skin. Although two-dimensional transition metal carbides (MXenes) are promising material choices, their films are susceptible to fracturing when subjected to tensile deformations. This study presents electronic tattoos comprising MXenes and liquid metal microcapsules fabricated by spray deposition onto the skin. The resulting nanocomposite has a low sheet resistance of approximately 2.2 Ω/sq. and can be repetitively deformed to 50 % strain. The enhanced deformability is attributed to the release of liquid metal from the microcapsules upon stretching, facilitating the repair of cracks in the nanocomposite layer. In addition, the nanocomposite conforms to the skin, sticks well, and moves with the body. It is also resistant to friction and sweat, making it suitable for various applications. A bifunctional electronic tattoo has been further developed, showcasing its capability to acquire biopotential signals and deliver electrical stimulation. Our study effectively improves the deformability of MXene-based nanocomposites for electronic tattoos, achieving seamless device integration in the body.

The Impact of Metal Ions on MXene Membranes: Critical Role of Titanium Vacancies.

Two-dimensional transition metal carbides and nitrides (MXenes) and MXene-based membranes hold promise for applications including water purification and seawater desalination; however, their environmental behavior and fate in these matrices remain unknown. In this study, we systematically assessed the reaction efficiencies of Ti3C2Tx at varying important environmental conditions. Our experiments revealed that copper and iron ions accelerated the oxidation rate of Ti3C2Tx 55.4 and 33.4 times, respectively. TiO2 and amorphous carbon were identified as the primary solid products. Based on in situ water-phase atomic force microscopy, atomic high-angle annular dark-field scanning transmission electron microscopy, and theoretical results, we postulate that metal ions enhance Ti3C2Tx oxidation by spontaneously migrating and anchoring at Ti vacancies, which then become active sites for this reaction. This process increases the adsorption of H2O and oxygen, making the Ti vacancy-rich surface convex area the most vulnerable site to attack. The findings in this study provide useful information for a comprehensive understanding of the interaction between MXene structural defects and metal ions as well as for the design and modification of MXene membranes resistant to metal ion impact.

Pivotal Role of Surface Terminations in MXene Thermodynamic Stability.

MXenes, i.e., two-dimensional transition metal carbides and nitrides, have been reported as promising materials for various applications, including energy storage, biomedicine, and electronics. The family of MXenes has proliferated, and the chemical space of synthesized MXenes has expanded to 13 transition metals and a dozen elements in surface terminations. The diverse chemistry of MXenes enables systematical tuning of MXene properties to meet the needs of target applications. However, synthesizing new MXene compositions largely relies on a trial-and-error approach. To overcome it, computational predictions of MXene compositions that are thermodynamically stable are crucial to rationalize experimental efforts. Here, we report a comprehensive computational screening for thermodynamically stable MXenes across 29 transition metals and 11 surface terminations. Density functional theory calculations are employed to compute the energy above the convex energy hull as a descriptor of thermodynamic stability. The results are analyzed to explore factors crucial for determining the thermodynamic stability of MXenes, by which the chemistry of surface terminations is found to play a crucial role. The insights on the chemistry of 998 MXene compositions predicted to be (meta)stable are given to systematically guide further research on MXene synthesis and application.

Chalcogen and halogen surface termination coverage in MXenes—structure, stability, and properties

MXenes are a diverse family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides. They can be synthesized through both top-down approaches, such as selective etching of A-layers from MAX phases using acids or molten salts, and bottom-up approaches, such as direct synthesis using chemical vapor deposition. However, the degree of the surface termination coverage depends on the synthesis route and is one key parameter for controlling its properties. This study focuses on halogen- and chalcogen-terminated MXenes, particularly M2CTx where M = Ti, Zr, V, Nb, Ta, and T = S, Se, Te, Cl, Br, I, and with surface termination coverage ranging from 100% (ideal, x = 2) to 50% (x = 1). The incorporation of oxygen on vacant termination sites was also evaluated. Using density functional theory (DFT) calculations, we investigated the structural, electronic, and mechanical properties of these MXenes. Our findings reveal that non-ideal termination coverage (x < 2) is more favorable for MXenes terminated with a larger size of T, such as Ti2CBrx, Nb2CClx, and Ta2CClx, and leads to mixed termination sites and lower binding energies. A reduced binding energy may facilitate delamination into single sheets, however, too low termination coverage may also cause structural collapse. Electronic properties showed an increased number of states at the Fermi level under non-ideal coverage, potentially enhancing the conductivity. Mechanically, we find the moduli of MXenes to be comparable to other 2D materials, such as transition metal chalcogenides and hexagonal boron nitride, indicating their suitability for applications requiring flexibility and durability. This study underscores the potential of tailoring MXene properties through precise control of termination coverage and composition, paving the way for enhanced application-specific performance.

Translocation of Ti2CO2 MXene monolayer through the cell membranes.

Nanoparticle-based therapies represent a cutting-edge direction in medical research. Ti2CO2 MXene is a novel two-dimensional transition metal carbide with a high surface area and reactivity, making it suitable for biomedical applications due to its biocompatibility. In biomedicine, Ti2CO2 MXene is particularly used in photothermal therapy, where its ability to absorb light and convert it into heat can be utilized to target and destroy cancer cells. The study of how temperature influences the interaction between nanoparticles and cell membranes is a critical aspect of this field. Our study conducts a thorough coarse-grained molecular dynamics analysis of a Ti2CO2 MXene nanosheet interacting with a phosphatidylcholine (POPC) membrane under various thermal conditions and nanosheet orientations. We show that the hydrophilic nature of the nanosheet presents a substantial barrier to membrane penetration and an increase in temperature significantly enhances the permeability of the membrane, thereby facilitating the migration of the MXene nanoparticles across it. The peak force required to translocate the nanosheet through the membrane decreases e.g., from 2150 pN at 300 kelvin to 1450 pN at 370 kelvin indicating significant reduction in resistance at higher temperatures. The study also highlights the critical role of the nanosheets' spatial orientation in cellular uptake. Our research underscores the importance of the application of MXenes for nanomedical and photothermal therapy purposes.

Molecular Dynamics Study of Bending Deformation of Mo2Ti2C3 and Ti4C3 (MXenes) Nanoribbons

We report a computational study of the bending deformation of two-dimensional nanoribbons by classical molecular dynamics methods. Two-dimensional double transition metal carbides, together with monometallic ones, belong to the family of novel nanomaterials, so-called MXenes. Recently, it was reported that within molecular dynamics simulations, Ti4C3 MXene nanoribbons demonstrated higher resistance to bending deformation than thinner Ti2C MXene and other two-dimensional materials, such as graphene and molybdenum disulfide. Here, we apply a similar method to that used in a previous study to investigate the behavior of Mo2Ti2C3 nanoribbon under bending deformation, in comparison to the Ti4C3 sample that has a similar structure. Our calculations show that Mo2Ti2C3 is characterized by higher bending rigidity at DTi2Mo2C3≈92.15 eV than monometallic Ti4C3 nanoribbon at DTi4C3≈72.01 eV, which has a similar thickness. Moreover, approximately the same magnitude of critical central deflection of the nanoribbon before fracture was observed for both Mo2Ti2C3 and Ti4C3 samples, wc≈1.7 nm, while Mo2Ti2C3 MXene is characterized by almost two times higher critical value of related external force.

Fabrication of Ti3CN quasi-MXene based poly glycidyl methacrylate and poly (3,4-ethylene dioxythiophene): Poly (styrene sulfonate) conducting polymer coated materials for photocatalytic eradicator of organic pollutants

MXene, a two-dimensional (2D) transition metal carbide or nitride, has gained significant attention due to its exceptional properties and wide-ranging applications. MXenes exhibit distinctive physical and chemical characteristics, making them suitable for electrocatalysis, supercapacitors, semiconductors, batteries, sensors, biomedicine, water splitting, and photocatalysis. In environmental photocatalysis, efforts have been made to improve conductivity, structural stability, and morphology. This study synthesized a photocatalyst by coating or doping a conducting polymer onto a quasi-MXene (Ti3CN) substrate. Initially, Ti3AlCN was exfoliated using HF to form 2D Ti3CN quasi-MXene, which was then doped with GMA and PEDOT: PSS conducting polymers for photocatalytic dye degradation. The broad heterogeneous interfaces in this network significantly enhance photocatalytic performance and pollutant removal capacity. The Ti3CN@GMA/PEDOT: PSS photocatalyst was characterized through SEM, FTIR, XPS, XRD, BET, and EDX techniques and tested for the degradation of various pollutants in wastewater under visible light. The degradation rates achieved were 94.1 % for Rose Bengal, 91.6 % for Rhodamine B, and 90.6 % for Methylene Blue, with corresponding rate constants of 0.0366 min−1, 0.0400 min−1, and 0.0388 min−1 over 60 min. The photocatalyst demonstrated excellent performance, highlighting several advantages, including low energy consumption, cost-effectiveness, non-toxic properties, and environmental compatibility, making it a promising solution for wastewater treatment applications.

Exploring the thermal and optical characteristics of Ti3C2 MXene for enhanced photothermal conversion

Ti3C2 MXene, a member of the two-dimensional (2D) transition metal carbides family, has garnered significant attention for its exceptional photothermal properties. To understand the mechanistic underpinnings of this feature, we conducted a comprehensive first-principles density functional theory analysis. This study was aimed at investigating the electronic band structure and vibrational characteristics of Ti3C2 MXene to unravel its microscopic process of photothermal conversion. After obtaining its optical and thermal properties, the process by which the material moves from light absorption to heat release is elucidated. The presence of localized surface plasmon resonance (LSPR) effects in Ti3C2 is proved by Finite-difference time-domain (FDTD) simulations. The synergy between the high thermal conductivity network in the Ti layers and the LSPR phenomenon is believed to be responsible for the superior photothermal conversion capabilities. This investigation enhances the understanding of the intrinsic properties of Ti3C2 MXene, confirming its status as an ultrahigh photothermal conversion material.

2D MXene Biomaterials for Catalytic Medical Applications.

In recent years, two-dimensional transition metal carbides, nitrides, and carbonitrides, termed as MXenes, have been widely applied in energy storage, photocatalysis and biomedicine owing to their unique physicochemical properties of large specific surface area, high electrical conductivity, excellent optical performance, good stability, etc. Moreover, due to their strong light absorption capacity in the first and second near-infrared bio-window, and their ability of being simply functionalized with multiple organic/inorganic materials, MXene biomaterials have shown great potential in the field of catalytic therapy. This review will summarize the common catalytic mechanism of MXene biomaterials and their latest applications in catalytic medicine such as tumor therapy, antibacterial and anti-inflammatory, and present the current challenges and opportunities in clinical translation for future development to promote the advancement of MXene biomaterials in the field of catalytic medicine.

Catalyst breakthroughs in methane dry reforming: Employing machine learning for future advancements

Rising levels of atmospheric carbon dioxide (CO2) and methane (CH4) have sparked the interest of researchers in resolving this issue. Various technologies have been utilized such as dry reformation of methane (DRM), that turns these greenhouse gases (GHGs) into syngas. Numerous studies have been done in recent years to produce efficient and competitive catalysts for DRM. However, a “state of the art” review is still required due to a lack of literature on improvements and scalability of robust catalytic systems. This study aims to evaluate recent advancements in catalyst formulations, their activities, stability, and selectivity. We have discussed a variety of materials, including Metal-Organic Frameworks (MOF), a type of two-dimensional transition metal carbides and nitrides (MXene), reducible and irreducible metal oxides, transition metal oxides, zeolites, carbon, and biochar-based catalysts, which are responsible for improving the effectiveness of the DRM process. Furthermore, machine learning (ML) approaches are highlighted for their potential in catalyst design and optimization, providing increased accuracy and efficiency. This review highlights major enhancements to the DRM method, which result in higher hydrogen yield and catalyst stability. Novel catalysts have been created to tackle crucial problems including coking and sintering, which are necessary to progress commercialization. To ensure successful commercial application, future research should concentrate on finding catalysts with excellent features that are also economically viable. This study provides a significant resource for future catalyst development and promotes greater cooperation between academic and commercial communities.

Nitrogen-doping engineering in two-dimensional transition-metal carbides for electrochemical hydrogen evolution

Developing electrocatalysts with the trade-off between overall electrocatalytic performance and economic cost towards hydrogen evolution reaction (HER) is crucial for producing green hydrogen fuels. Although theoretical studies have reported the potential of two-dimensional (2D) MXenes for the HER, their electrocatalytic performance is still unsatisfactory for the particular application due to their low intrinsic active and MXene stacking. To address this problem, we herein modify the electronic structure and increase the interlayer spacing between 2D Ti3C2Tx MXenes through ammonia (NH3)-assisted hydrothermal approach. The effect of synthesis temperature on the nitrogen doping mechanism and electrochemical behaviors for the HER is systemically explored. As a result, the nitrogen (N)-doped Ti3C2Tx MXene fabricated at 160 °C exhibits the highest HER activity among all 2D N-doped Ti3C2Tx nanosheets with the overpotential/Tafel slope of 186.91 mVRHE/115.94 mVRHE dec−1 in an acidic solution. Due to the synergistic contribution of nitrogen, the N-doped Ti3C2Tx-160 °C has mass activity of 337.95 mA mg−1 at 0.3 VRHE, being 27.50-fold higher than that of pristine Ti3C2Tx. This work indicated that heteroatom doping is a facile but efficient approach for modulating the electronic structure and intrinsic activity of MXene-based materials for electrochemical reactions.

Strain-tunable electronic and optical absorption in the MXenes nanolayers: A DFT approach

Computational simulations using density functional theory (DFT) were performed to show that biaxial strain engineering within the range of −8% to +8% is an effective method for modifying the fundamental properties of two-dimensional (2D) transition metal carbides (MXenes). In this work, we computationally explored the effect of both compressive (0 to −8%) and tensile (0 to +8%) strain on the structural, electronic, and optical properties of M2CO2 MXenes (M = Ti, Zr, Sc and Hf) nanolayers. When compressive strains are applied, the charge transfer between layers increases because of decreased interlayer coupling. Conversely, tensile strains result in the opposite behavior. The strain-tunable band gap effect in Mo2CO2 nanolayers is revealed by investigating the valence band maximum (VBM) and conduction band minimum (CBM) in the band structure under biaxial strain. It is observed that the indirect-to-direct band gap transition occurs when the tensile strain is applied to Sc2CO2 and Ti2CO2 nanolayers. In most samples, the energy gap increases with an increase in tensile strain and decreases with an increase in compressive strain. The optical absorption coefficient indicates considerable absorption in the visible to ultraviolet (UV) region. Additionally, the absorption can be influenced by biaxial strains. In particular, under compressive strains (−8%) and Z-polarized light, the UV absorption peak in Hf2CO2, Ti2CO2, and Zr2CO2 reaches 68 %, 78 %, and 113 % respectively. The results indicate that these M2CO2 MXenes nanolayers will have significant potential for applications in tunable optoelectronic devices.

Advancements in Ti3C2Tx MXene Stability: Synergistic Antioxidant Strategies and Their Impact on Long-Lasting Flexible Sensors.

Two-dimensional (2D) transition metal carbides (Ti3C2Tx MXene) have demonstrated substantial application potential across various fields, owing to their excellent metallic conductivity and solution processability. However, the rapid oxidation of Ti3C2Tx in aqueous environments, leading to a loss of stability within mere days, poses a significant obstacle for its practical applications. Herein, we introduce an antioxidant strategy that combines free radical scavenging with surface passivation, culminating in the design and synthesis of imidazolium-based ionic liquids (ILs) incorporating siloxane groups. By deploying a straightforward hydrolysis-addition reaction, we successfully fabricated IL-modified Ti3C2Tx materials (Ti3C2Tx-IL). The Ti3C2Tx -IL not only displayed exceptional conductivity exceeding 3.85 × 104 S/m and hydrophilic contact angles below 45° but also showcased its superior chemical stability and antioxidation mechanisms through various analyses, including visual color change experiments, spectroscopic and energy spectrum characterization, free radical scavenging tests, and density-functional-theory-based molecular simulations. Furthermore, when utilized as a conductive filler in the fabrication of a poly(vinyl alcohol)/nanocellulose fiber (PVA/CNF) composite hydrogel (PCMIL), the resultant sensors exhibited remarkable mechanical performance with up to 535% strain, 1.59 MPa strength, 4.35 MJ/m3 toughness, and a conductivity of 3.40 mS/cm, as well as a high sensitivity gauge factor of 3.3. Importantly, even after 45 days of storage, the PCMIL retained most of its functionalities, demonstrating superior performance in human-machine interaction applications compared to hydrogels made from unmodified Ti3C2Tx. This research establishes a robust antioxidant protection strategy for Ti3C2Tx, offering substantial technical reinforcement for its prospective applications in the realm of flexible electronics and sensing technologies.

Highly sensitive MXene-based voltammetric sensor for nanomolar clozapine detection

Clozapine is vital for managing schizophrenia and other psychiatric disorders. As a potent medication with risks of severe side effects, accurate detection ensures effective treatment monitoring, patient safety, and optimization of therapeutic outcomes. Herein, niobium carbide two-dimensional transition metal carbides (Nb2CTx MXene) was prepared by etching niobium aluminum carbide (Nb2AlC), which was next partially oxidized by a hydrothermal method to niobium oxide (Nb2O5)/Nb2CTx and ultrasonically bound to carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). It was modified onto a glassy carbon electrode for the detection of the antipsychotic drug clozapine ingested by humans. The morphology and electrochemical properties of MXene-based nanocomposite electrode were verified by scanning electron microscopy, X-ray diffractometry, Raman, cyclic voltammetry and electrochemical impedance characterization methods. The results showed that electrode responded well to clozapine solutions, with good linearity (R2 = 0.9979 and 0.9995) of the peak currents with clozapine concentrations (0.01–1.0 μM and 1.0–10.0 μM) and that this sensor had good immunity, repeatability, reproducibility and stability. The limit of detection was 8.38 nM and the recovery was 92.5 % ~ 106.5 % in real serum and urine samples. The MXene-based sensors offer excellent analytical performance and hold promise for cost-effective generalisation to the detection of nanomolar clozapine in a variety of clinical applications.

MXene Synthesis and Carbon Capture Applications: Mini-Review.

MXenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant attention due to their remarkable potential for energy storage, electrocatalysis, and gas separation applications. The fabrication processes of MXene involve building up the MXene structure from constituent elements and the selective elimination of M-A bonds from the precursor MAX. However, considerable efforts are still required to design and develop efficient MXene-based technologies. This review article aims to briefly analyse the synthesis methods employed for MXene production, ranging from direct synthesis and conventional chemical wet etching approach to the more recent molten salt etching technique. The review highlights the advancements made in achieving precise control over the terminal groups, which is paramount for tailoring the properties of MXenes for specific applications. Furthermore, the potential of MXene-based materials for carbon capture applications, particularly in developing advanced adsorbents, is emphasized. The in-depth examination of MXene synthesis techniques and their implications for carbon capture applications provides a solid foundation for developing and optimizing these promising materials.

Effects of Concentration and Surface Termination on the Workfunction of Ti3C2X2 MXene (X=F,Cl,O,OH)

Titanium carbide (Ti3C2), belonging to the MXene family, has become a focal point of research due to its exceptional electronic and structural properties. As a two-dimensional transition metal carbide, Ti3C2 exhibits promising potential in a variety of applications, ranging from electronic devices to energy storage. Among the key parameters influencing its electronic behavior, the work function plays a pivotal role. This study aims to elucidate the intricate relationship between concentration, surface termination, and the work function of Ti3C2X2 through first-principles density functional theory (DFT) calculations. First-principles DFT calculations were employed to investigate the electronic structure of Ti3C2X2, focusing on the influence of varying concentrations of surface functional groups. Hydroxyl (-OH), oxygen (-O), chlorine (-Cl), and fluoride (-F) terminations were specifically chosen as representative functional groups for their prevalence in experimental studies and potential technological applications. These calculations provide a quantum-level understanding of the electronic properties and behaviors of Ti3C2 under different surface conditions. Our results indicate a notable sensitivity of the work function of Ti3C2 to changes in both concentration and the type of surface termination. The presence of functional groups induces charge redistribution within the MXene layers, significantly impacting the overall electronic landscape. The variation in charge distribution directly correlates with changes in the work function, illustrating the dynamic nature of Ti3C2 in response to surface modifications. Furthermore, an in-depth analysis of the correlation between concentration, surface termination, and the energetics of charge transfer at the Ti3C2 surface provides valuable insights. The interplay between these factors reveals intricate mechanisms that govern the observed variations in the work function. The concentration-dependent charge transfer at the surface serves as a key determinant, emphasizing the importance of carefully tailoring the composition to achieve desired electronic properties. The understanding gained from this study has significant implications for the tailored design and utilization of Ti3C2 in various technological applications. By manipulating concentration and surface termination, researchers and engineers can fine-tune the work function to meet specific requirements in electronic devices, sensors, and energy storage systems. This level of control over electronic properties enhances the versatility and applicability of Ti3C2 in emerging technologies. In conclusion, our investigation highlights the critical roles of concentration and surface termination in modulating the work function of Ti3C2. The pronounced sensitivity of this parameter to surface modifications underscores the need for a comprehensive understanding of the underlying electronic mechanisms. By elucidating the interplay between concentration, surface termination, and charge transfer, this study provides valuable insights that pave the way for the strategic design and implementation of Ti3C2 in advanced technological applications. The ability to fine-tune electronic properties opens new avenues for exploiting the full potential of Ti3C2 in diverse fields, driving innovation and progress in materials science and technology.

(Invited) Insights into the Electrocatalytic Behavior of Mxenes

The continued rise in global energy consumption, along with the associated environmental hazards, pose a significant risk to our society’s infrastructure unless the main source is changed. Electrocatalysis involving hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and oxygen reduction reaction (ORR), provide a pathway for energy storage and conversion due to their enhanced environmental friendliness and efficient energy input compared to their thermocatalytic counterparts. Currently, the state-of-the-art electrocatalysts suffer from scarcity and high cost. MXenes, a novel family of two-dimensional (2D) transition metal carbide and nitride materials, show potential as cost-efficient and highly abundant electrocatalysts with limited knowledge on their electrocatalytic mechanisms. This is especially true when considering the often-overlooked nitride family of MXenes. Herein, we investigate the electrocatalytic performance of a Ti2N nitride MXene under different electrocatalytic conditions. The effect of surface phenomena is investigated through manipulation of the surface passivation layer, as evidenced by Raman and Fourier-transform infrared (FTIR) spectroscopies, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Successful decoupling of the bulk and surface phenomena is achieved through Raman laser power attenuation. Under acidic medium, the surface reactivity towards HER is poor but the bulk reactivity for NRR is favored, making it an optimal NRR catalyst. Under alkaline medium, the surface reactivity of the pristine Ti2N MXene for ORR is high, but also leads to surface passivation and thus hinders the electrocatalytic activity. Overall, these results provide fundamental insights into future optimization strategies of the Ti2N nitride MXene, along with other MXene electrocatalysts, towards electrocatalytic applications.