The influence of C2 dimers on the stability of TimCn metcar clusters

A DFT prediction of two-dimensional MB3 (M = V, Nb, and Ta) monolayers as excellent anode materials for lithium-ion batteries

Transition metal borides (MBenes) have recently drawn great attention due to their excellent electrochemical performance as anode materials for lithium-ion batteries (LIBs). Using the structural search code and first-principles calculations, we identify a group of the MB3 monolayers (M = V, Nb and Ta) consisting of multiple MB4 units interpenetrating with each other. The MB3 monolayers with non-chemically active surfaces are stable and have metal-like conduction. As the anode materials for Li-ion storage, the low diffusion barrier, high theoretical capacity, and suitable average open circuit voltage indicate that the MB3 monolayers have excellent electrochemical performance, due to the B3 chain exposed on the surface improving the Li atoms' direct adsorption. In addition, the adsorbed Li-ions are in an ordered hierarchical arrangement and the substrate structure remains intact at room temperature, which ensures excellent cycling performance. This work provides a novel idea for designing high-performance anode materials for LIBs.

Can calculated harmonic vibrational spectra rationalize the structure of TiC-based nanoparticles?

Nanoscale titanium carbide (TiC) is widely used in composites and energy applications. In order to design and optimize these systems and to gain a fundamental understanding of these nanomaterials, it is important to understand the atomistic structure of nano-TiC. Cluster beam experiments have provided detailed infrared vibrational spectra of numerous Ti_xC_y nanoparticles with well defined masses. However, these spectra have yet to be convincingly assigned to Ti_xC_y nanoparticle structures. Herein, using accurate density functional theory based calculations, we perform a systematic survey of likely candidate nanoparticle structures with masses corresponding to those in experiment. We calculate harmonic infrared vibrational spectra for a range of nanoparticles up to 100 atoms in size, with a focus on systems based on removing either four carbon atoms or a single titanium atom from rocksalt-structured stoichiometric TiC nanoparticles. Our calculations clearly show that Ti-deficient nanoparticles are unlikely candidates to explain the experimental spectra as such structures are highly susceptible to C-C bonding, whose characteristic frequencies are not observed in experiment. However, our calculated infrared spectra for C-deficient nanoparticles have some matching features with the experimental spectra but tend to have more complex infrared spectra with more peaks than those obtained from experiment. We suggest that the discrepancy between experiment and theory may be largely due to thermally induced anharmonicities and broadening in the latter nanoparticles, which are not be accounted for in harmonic vibrational calculations.

Mechanistic Investigation of the Carbon-Iodine Bond Activation on the Niobium-Carbon Cluster

The activation process of carbon-iodine (C-I) bond on neutral and cationic niobium metcars (Nb8C12) is investigated using density functional theory and related computational techniques. Metallocarbohedrenes or metcars are a class of stable metal-carbide clusters of specific stoichiometry and of great interest to cluster chemists since their first discovery. The detailed reaction mechanism along with the overall energy profile of the C-I dissociation reaction on niobium metcar and its cations is presented in this paper. The tunneling-corrected rate constants and their related reaction parameters such as the pre-exponential factor are also included alongside. The major differences between the reaction mechanism of the neutral and cationic metcars are also highlighted as well. Despite the available experimental results, the C-I bond dissociation on metcars has remained an unexplored problem in the theoretical and computational domains. Thus, the present investigation can fill in the gap and may also provide new insights provoking further developments in cluster and materials chemistry in future.

Bonding-restricted structure search for novel 2D materials with dispersed C2 dimers

Currently, the available algorithms for unbiased structure searches are primarily atom-based, where atoms are manipulated as the elementary units, and energy is used as the target function without any restrictions on the bonding of atoms. In fact, in many cases such as nanostructure-assembled materials, the structural units are nanoclusters. We report a study of a bonding-restricted structure search method based on the particle swarm optimization (PSO) for finding the stable structures of two-dimensional (2D) materials containing dispersed C₂ dimers rather than individual C atoms. The C₂ dimer can be considered as a prototype of nanoclusters. Taking Si-C, B-C and Ti-C systems as test cases, our method combined with density functional theory and phonon calculations uncover new ground state geometrical structures for SiC₂, Si₂C₂, BC₂, B₂C₂, TiC₂, and Ti₂C₂ sheets and their low-lying energy allotropes, as well as their electronic structures. Equally important, this method can be applied to other complex systems even containing f elements and other molecular dimers such as S₂, N₂, B₂ and Si₂, where the complex orbital orientations require extensive search for finding the optimal orientations to maximize the bonding with the dimers, predicting new 2D materials beyond MXenes (a family of transition metal carbides or nitrides) and dichalcogenide monolayers.

TiC2: a new two-dimensional sheet beyond MXenes

MXenes are attracting attention due to their rich chemistry and intriguing properties. Here a new type of metal-carbon-based sheet composed of transition metal centers and C2 dimers rather than individual C atom is designed. Taking the Ti system as a test case, density functional theory calculations combined with a thermodynamic analysis uncover the thermal and dynamic stability of the sheet, as well as a metallic band structure, anisotropic Young's modulus and Poisson's ratio, a high heat capacity, and a large Debye stiffness. Moreover, the TiC2 sheet has an excellent Li storage capacity with a small migration barrier, a lower mass density compared with standard MXenes, and better chemical stability as compared to the MXene Ti2C sheet. When Ti is replaced with other transition metal centers, diverse new MC2 sheets containing C=C dimers can be formed, the properties of which merit further investigation.

Threshold photoionization and density functional theory studies of bimetallic-carbide nanocrystals and fragments: Ta3ZrC(y) (y = 0-4)

Gas-phase bimetallic tantalum-zirconium-carbide clusters are generated using a constructed double ablation cluster source. The Ta(3)ZrC(y) (y = 0-4) clusters are examined by photoionization efficiency spectroscopy to extract experimental ionization energies (IEs). The IE trend for the Ta(3)ZrC(y) cluster series is reasonably similar to that of the Ta(4)C(y) cluster series [V. Dryza et al., J. Phys. Chem. A 109, 11180 (2005)], although the IE reductions upon carbon addition are greater for the former. Complementary density functional theory calculations are performed for the various isomers constructed by attaching carbon atoms to the different faces of the tetrahedral Ta(3)Zr cluster. The good agreement between the experimental IE trend and that calculated for these isomers support a 2x2x2 face centered cubic nanocrystal structure for Ta(4)ZrC(4) and nanocrystal fragment structures for the smaller clusters.