Phase diagrams of the terminating oxygen layer on MXenes M2CO sheets (M = Ti, V, Nb)

In this work we have studied the behavior of oxygen adsorbed on the MXenes M2CO (M=Ti, V, Nb) surface by the density functional theory and Monte Carlo methods. An approximation for the lateral interactions potential has been constructed that has the form of a pair potential. We have carried out the quantitative assessment of the errors of the pair potential. Using SuSMoST program we have constructed isotherms of adsorption. The results showed a non-trivial phase behavior of the adlayer: we observed three phases characterized by the 1/3, 1/2 and 2/3 coverage. It has been predicted that complete oxygen removal from the MXenes surface is possible at temperatures above 500 K

Non-Arrhenius Behavior of Surface Diffusion Near a Phase Transition Boundary

We study the non-Arrhenius behavior of surface diffusion near the second-order phase transition boundary of an adsorbate layer. In contrast to expectations based on macroscopic thermodynamic effects, we show that this behavior can be related to the average microscopic jump rate which in turn is determined by the waiting-time distribution W(t) of single-particle jumps at short times. At long times, W(t) yields a barrier that corresponds to the rate-limiting step in diffusion. The microscopic information in W(t) should be accessible by STM measurements.Comment: 4 pages, Latex with RevTeX macro

Relative stability of icosahedral and cuboctahedral metallic nanoparticles

The size and form of metallic nanoparticles (NPs) significantly affects their adsorptive, chemical, and catalytic activity. One of the most interesting nanoscale size effects is the transition from icosahedral to octahedral forms with growth in the NP size. We compared the stability of icosahedral, decahedral and cuboctahedral NPs made from eight metals Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au using the local optimization of total energy, which was computed from the tight-binding second moment approximation and quantum Sutton–Chen potentials. The obtained results predicted that the icosahedral form would be most stable for Ni, and least stable for Au. For Rh, and especially for Ir, a strong dependency of the stability of the different forms on the NP size was revealed

Monte Carlo model of CO adsorption on supported Pt nanoparticle

For molecular simulations with thousands of atoms it is desirable to use a lattice gas model because it is fast and easy-to-use for computations. Unfortunately, simulation of adsorption on heterogeneous surfaces within this model is rather complicated due to a large variety of available adsorption site types. We propose the combined model with lattice representation of adsorbent atoms and arbitrary location of adsorbate atoms. Using this model simulation of CO adsorption on supported Pt nanoparticles has been performed. With the proposed approach the above-mentioned difficulties were successfully overcome

Tensor renormalization group study of hard-disk models on a triangular lattice

High accuracy and performance of the tensor renormalization group (TRG) method have been demonstrated for the model of hard disks on a triangular lattice. We considered a sequence of models with disk diameter ranging from a to 2√3a, where a is the lattice constant. Practically, these models are good for approximate description of thermodynamics properties of molecular layers on crystal surfaces. Theoretically, it is interesting to analyze if and how this sequence converges to the continuous model of hard disks. The dependencies of the density and heat capacity on the chemical potential were calculated with TRG and transfer-matrix (TM) methods. We benchmarked accuracy and performance of the TRG method comparing it with TM method and with exact result for the model with nearest-neighbor exclusions (1NN). The TRG method demonstrates good convergence and turns out to be superior over TM with regard to considered models. Critical values of chemical potential (μc) have been computed for all models. For the model with next-nearest-neighbor exclusions (2NN) the TRG and TM produce consistent results (μc=1.75587 and μc=1.75398 correspondingly) that are also close to earlier Monte Carlo estimation by Zhang and Deng. We found that 3NN and 5NN models shows the first-order phase transition, with close values of μc (μc=4.4488 for 3NN and 4.4<4.5 for 5NN). The 4NN model demonstrates continuous yet rapid phase transition with 2.65<2.7

Generalized lattice-gas model for adsorption of functional organic molecules in terms of pair directional interactions

A generalized lattice-gas model that takes into account the directional character of pair interactions between the lattice sites is proposed. It is demonstrated that the proposed model can be successfully used to deeply understand the self-assembly process in adsorption monolayers of functional organic molecules driven by specified directional interactions between such molecules (e.g., hydrogen bonding). To illustrate the idea, representative cases of the general model with different numbers of identical functional groups in the chemical structure of the adsorbed molecule are investigated with Monte Carlo and the transfer-matrix methods. The model reveals that the phase behavior of the adsorption systems considered can be characterized as a hierarchical self-assembly process. It is predicted that in real adsorption systems of this type, the energy of hydrogen bonding sufficiently depends on the mutual orientation of the adsorbed molecules

Remnants of the devil's staircase of phase transitions in the model of dimer adsorption at nonzero temperature

The model of dimers adsorption on hexagonal lattice with different orientations to surface and hard-spheres lateral interactions has been studied at nonzero temperature. The transfer-matrix method was used as the main one and the Monte Carlo method was used for checking of some extreme cases. Adsorption isotherms, dependencies of the entropy from the density of the adsorption layer and of the energy from the system temperature at certain points of the phase space, were computed. It was found that at least the first ten phases of the ground state still persist at nonzero temperatures

Model of Fe-Terephthalate ordering on Cu(100)

A lattice model of terephthalic acid (TPA) and iron ordering on the Cu(100) surface is proposed and investigated using Monte Carlo simulation in a grand canonical ensemble. We have an evidence that the emergence of all the experimentally observed metal–organic structures cannot be explained in terms of short-ranged interactions such as hydrogen bonding and metal–carboxylate coordination proposed and discussed in earlier papers. The self-assembly of the “cloverleaf” and “interlocked” structures requires the presence of long-ranged TPA–Fe interaction. The unidentate carboxylate–Fe interaction is demonstrated to be 0.6–0.7 times weaker as compared to the bidentate bond. The phase diagram with all the experimentally observed structures is obtained. It has been established that one type of the ladder structures distinguished on scanning tunneling microscopy images is a metastable state and not a phase in the thermodynamic sense. We have found two new metal–organic structures, which are missed in earlier studies, but apparently formed in the TPA–Fe/Cu(100) adsorption layer. The first one comprises the single −Fe–TPA– rows linked with the TPA molecules in dihapto hydrogen bond motif. This phase is characterized by the lowest density of the monolayer. Another phase is formed at high densities and composed of the alternating rows of “cloverleaves” and TPA molecules linked with a pair of Fe atoms

Metal-organic coordination networks on a titanium carbide MXene: DFT based grand canonical Monte Carlo simulation

The self-assembly of 2D metal–organic networks comprising 1,3,5-tris (pyridyl)benzene (TPyB) molecules and copper atoms on the oxygen-terminated titanium carbide MXene surface was theoretically investigated. We have developed a lattice model of the TPyB-Cu networks on the energetically heterogeneous Ti2CO2(0001) surface. The model based on DFT calculations of the structure and energy of key adsorption complexes and metal–organic structures. Using the grand canonical Monte Carlo method, we have calculated and analyzed adsorption isotherms, structure, potential energy, and heat capacity of the adlayer. Due to steric hindrances in the three-fold Cu-TPyB junction, metal–organic structures consisting of only two-fold Cu-TPyB coordination motifs predominantly emerge on the Ti2CO2(0001) surface: honeycomb (HON), honeycomb filled with Cu3TPyB (HON + Cu3TPyB) and zigzag (ZZ) phases. These phases differ in the local environment of the copper adatoms. Thermal stability of the phases decreases in the following series: ZZ, HON + Cu3TPyB and HON. Self-assembly of these structures offers the opportunity to stabilize and “tune” properties of the single-atom Cu/Ti2CO2(0001) catalyst. We hope that our results will stimulate further experimental studies of hybrid “metal–organic network/MXene” catalysts