Physicochemical Modulations in MXenes for Carbon Dioxide Mitigation and Hydrogen Generation: Tandem Dialogue between Theoretical Anticipations and Experimental Evidences

The dawn of MXenes has fascinated researchers under their intriguing physicochemical attributes that govern their energy and environmental applications. Modifications in the physicochemical properties of MXenes pave the way for efficient energy-driven operations such as carbon capture and hydrogen generation. The physicochemical modulations such as interface engineering through van der Waals coupling with homo/hetero-junctions render the tunability of optoelectronic variables driving the photochemical and electrochemical processes. Herein, we have reviewed the recent achievements in physicochemical properties of MXenes by highlighting the role of intercalants/terminal groups, atomic defects, surface chemistry and few/mono-layer formation. Recent findings of MXenes-based materials are systematically surveyed in a tandem manner with the future outlook for constructing next-generation multi-functional catalytic systems. Theoretical modelling of MXenes surface engineering proffers the mechanistic comprehension of surface phenomena such as termination, interface formation, doping and functionalization, thereby enabling the researchers to exploit them for targeted applications. Therefore, theoretical anticipations and experimental evidences of electrochemical/photochemical carbon dioxide reduction and hydrogen evolution reactions are synergistically discussed.

Theoretical Analysis of Magnetic Coupling in the Ti2C Bare MXene

The nature of the electronic ground state of the Ti₂C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti₂C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields.

Modelling of high-temperature order-disorder phase transitions of non-stoichiometric Mo2C and Ti2C from first principles

High-temperature order-disorder phase transitions play an important role in determining the structure and physical and chemical properties of non-stoichiometric transition metal carbides. Due to the large number of possible carbon vacancy arrangements, it is difficult to study these systems with first-principles calculations. Here, we construct a simple atomistic potential capable of accurately reproducing the energetics of the carbon vacancy arrangements in cubic Mo₂C and Ti₂C obtained from density functional theory calculations. We show that this potential can be applied to correctly predict the transition temperatures between the ordered and disordered states in Monte Carlo simulations on large supercells and reveal the extend of local order in the disordered phases of Mo₂C and Ti₂C that show interesting physical and chemical properties. We find that even the high-temperature disordered phase exhibit a relatively high degree of local order as indicated by the relatively small change in the root mean square number of C atom neighbours of Mo/Ti compared to the ordered phase (from 3.0 to 3.1-3.2). This atomistic potential enables the study of how the structure of these carbides can be tuned through the synthesis temperature to control the properties of carbide materials that are related to the degree of disorder in the system such as catalytic activity and electrical conductivity and play an important role in applications of these carbides. Fundamentally, the successful modelling of these carbides suggests that despite the presence of metallic, covalent and ionic interactions, bonding in carbides can be modelled by simple and physically intuitive interatomic potentials.

High-Throughput Screening of Atomic Defects in MXenes for CO2 Capture, Activation, and Dissociation

The capture, activation, and dissociation of carbon dioxide (CO₂) is of fundamental interest to overcome the ramifications of the greenhouse effect. In this regard, high-throughput screening of two-dimensional MXenes has been examined using well-resolved first-principles simulations through DFT-D3 dispersion correction. We systematically investigated different types of structural defects to understand their influence on the performance of M₂X-type MXenes. Defect calculations demonstrate that the formation of M₂C(V_MC) and M₂N(V_MN) vacancies require higher energy, while M₂C(V_C) and M₂N(V_N) vacancies are favorable to form during the synthesis of M₂X-type MXenes. The M₂X-type MXenes from group III to VII series show remarkable behavior for active capturing of CO₂, especially group IV (Ti₂X and Zr₂X) MXenes exhibit unprecedentedly high adsorption energies and charge transfer (>2e) from M₂X to CO₂. The potential CO₂ capture, activation, and dissociation abilities of MXenes are emanated from Dewar interactions involving hybridization between π orbitals of CO₂ and metal d-orbitals. Our high-throughput screening demonstrates chemisorption of CO₂ on pure and defective MXenes, followed by dissociation into CO and O species.

How bulk and surface properties of Ti4SiC3, V4SiC3, Nb4SiC3 and Zr4SiC3 tune reactivity: a computational study

We present several in silico insights into the MAX-phase of early transition metal silicon carbides and explore how these affect carbon dioxide hydrogenation. Periodic density functional methodology is applied to models of Ti4SiC3, V4SiC3, Nb4SiC3 and Zr4SiC3. We find that silicon and carbon terminations are unstable, with sintering occurring in vacuum and significant reconstruction taking place under an oxidising environment. In contrast, the metal terminated surfaces are highly stable and very active towards CO2 reduction. However, we predict that under reaction conditions these surfaces are likely to be oxidised. These results are compared to studies on comparable materials and we predict optimal values for hydrogen evolution and CO2 reduction.

Potential environmental applications of MXenes: A critical review

Various environmental pollutants (e.g., air, water and solid pollutants) are discharged into environments with the rapid development of industrializations, which is presently at the forefront of global attention. The high efficient removal of these environmental pollutants is of important concern due to their potential threat to human health and eco-diversity. Advanced nanomaterials may play an important role in the elimination of pollutants from environmental media. MXenes as the new intriguing class of graphene-like 2D transition metal carbides and/or carbonitrides have been widely used in energy storage, environmental remediation benefitting from exceptional structural properties such as highly active sites, high chemical stability, hydrophilicity, large interlayer spacing, huge specific surface area, superior sorption-reduction capacity. However, the comprehensive investigation concerning the removal of various environmental pollutants on MXenes is yet not available up to date. In this review, we summarized the synthesis and properties of MXenes to demonstrate the key roles in ameliorating their adsorption performance; then the recent advances and achievements in environmental application of MXenes on the removal of gases, organics, heavy metals and radionuclides were comprehensively reviewed in details; Finally, the formidable challenges and further perspectives regarding utilizing MXene in environmental remediation were proposed. Hopefully, this review can provide the useful information for environmental scientists and material engineers on designing versatile MXenes in actual environmental applications.

Formation of Dimethyl Carbonate from CO2 and Methanol Catalyzed by Me2SnO: A Density Functional Theory Approach

The conversion of CO₂ into dimethyl carbonate (DMC) is an environmental and industrial appealing topic because it contributes to reduce the emissions of CO₂ and to increase its use as raw material. In the present study we employed the CAM-B3LYP/def2-SVP DFT approach to evaluate the thermodynamic and kinetic parameters for the catalytic conversion of CO₂ and methanol into DMC. Starting with the activation of four methanol molecules by the [Me₂SnO]₂ dimer, we computed all the stationary points along the pathway to convert CO₂ and methanol into the DMC. The capture of two CO₂ molecules is promoted by an alkoxitin intermediate, in an exothermic process, with low activation energy. Formation of a first DMC occurs after an intramolecular rerrangement involving a tetrahedral intermediate. The formation of a second DMC may occur either in a process similar to the first one or by dimerization of the hemicarbonate formed after releasing the first DMC. In this pathway, the [Me₂(OH)SnO(OMe)SnMe₂]₂ complex is formed. This complex is less reactive than [Me₂Sn(OMe)₂]₂ but still conserves the catalytic activity. Identification of this mechanism suggests that the catalytic action of Me₂SnO can be improved by modulating the formation of the final [Me₂(OH)SnO(OMe)SnMe₂]₂ complex.

Interfacial superassembly of MoSe2@Ti2N MXene hybrids enabling promising lithium-ion storage

Our work presents an interfacial superassembly by engineering MoSe₂ nanoflowers coupled with ribbon-like Ti₂N MXene frameworks. It can provide a novel synthesis strategy to improve the performance of LIBs.