Shiping Huang

Surface segregation of Ag–Cu–Au trimetallic clusters

Segregation phenomena of Ag–Cu–Au trimetallic clusters with icosahedral structure are investigated by using a Monte Carlo method based on the second-moment approximation of the tight-binding (TB-SMA) potentials. We predict that the Ag atoms segregate to the surface of the Ag–Cu–Au trimetallic icosahedral clusters. The Ag concentrations in the surface layer of the clusters are about 11–29 at.% higher than the overall Ag concentration in all the cases studied. The simulation results also indicate that the Au atoms are mainly distributed in the middle shell and the Cu atoms are located in the center for the 147-, 309-, 561- and 923-atom clusters at 300 K. The segregation phenomena of the Ag, Au and Cu atoms in the Ag–Cu–Au trimetallic clusters are mainly due to the different surface energies of the Ag, Au and Cu atoms. It is found that the size and composition have little effect on the segregation phenomena of Ag, Au and Cu atoms in the Ag–Cu–Au trimetallic cluster.

Two-dimensional GeS with tunable electronic properties via external electric field and strain.

Experimentally, GeS nanosheets have been successfully synthesized using vapor deposition processes and the one-pot strategy. Quite recently, GeS monolayer, the isoelectronic counterpart of phosphorene, has attracted much attention due to promising properties. By means of comprehensive first-principles calculations, we studied the stability and electronic properties of GeS monolayer. Especially, electric field and in-plane strain were used to tailor its electronic band gap. Upon applying electric field, the band gap of GeS monolayer greatly reduces and a semiconductor-metal transition happens under the application of a certain external electric field. Our calculations reveal that the band gaps of GeS monolayer are rather sensitive to the external electric field. On the other hand, for GeS under external strain, quite interestingly, we found that the band gap presents an approximately linear increase not only under compression strain but also under tensile strain from -10% to 10%. For biaxial compressive and tensile strains, the band gap follows the same trend as that of the uniaxial in the zigzag x direction. The present results provide a simple and effective route to tune the electronic properties of GeS monolayer over a wide range and also facilitate the design of GeS-based two-dimensional devices.

Enhanced selectivity and capacity of adsorption of CO2 and CH4 in zeolite-like metal-organic frameworks with different extra-framework cations: a molecular simulation study

In this study, grand canonical Monte Carlo simulation was carried out to systematically study the effects of extra-framework cations on the capacity of storage and separation of carbon dioxide (CO2) and methane (CH4) in cation-exchanged rho-zeolite-like metal-organic framework (rho-ZMOF). Monovalent (Na+, NH4 + and Li+), divalent (Mg2+ and Sr2+) and trivalent (Al3+) cations with different ion radii were adopted as the representative extra-framework cations. The simulations of the single-component adsorption of CO2 molecules indicate that the varieties in extra-framework cations do not bring evident differences in the dispersion interaction contributions to the isosteric adsorption heats of CO2 but rather the electrostatic interaction contributions. Higher the valence a cation has, stronger the electrostatic interaction with CO2 is and, therefore, a higher storage capacity, though the corresponding accommodation number of the extra-framework cations is smaller. For the cations having the same valence, the storage capacity decreases with the increase in the accessible surface area and total free volume of the host structure. The single-component adsorption isotherms of CO2 and CH4 can be described by a dual-site Langmuir–Freundlich equation. Under typical operating conditions (298 K and 1 atm) in a pressure swing adsorption (PSA) process, the simulation results of the adsorption of CO2 and CH4 mixture demonstrate that the Al-exchanged rho-ZMOF exhibits an unprecedented high selectivity of CO2 over CH4 up to 112, compared with other metal-organic frameworks and nanoporous materials reported to date. Our results suggest that the variation of extra-framework cations is an efficient way to improve the adsorption capacity of the rho-ZMOF for the storage and separation of CO2 and CH4 mixtures using the PSA process at ambient temperature and pressure.

Theoretical investigation of growth, stability, and electronic properties of beaded ZnO nanoclusters

A beaded ZnO nanocluster as a novel stand-alone system has been introduced by interconnecting different numbers of highly stable (ZnO)12 basic units. Geometries, stabilities, electronic properties, and vibrational spectra of the beaded ZnO nanoclusters have been systematically studied by using density functional theory. The results indicate that the beaded ZnO nanoclusters with large binding energies have high stabilities during their growth process. The energy gaps of the (ZnO)12×n nanoclusters (n ≥ 3) show a relatively slow decrease, indicating that the energy gaps are insensitive to the cluster size for the large clusters. The low-lying highest occupied molecular orbital and the high-lying lowest unoccupied molecular orbital are observed to shift to the top of the low energy levels and the bottom of the high energy levels, respectively, leading to the reduction of the energy gap. In addition, by calculating its energy gap, vertical ionization potential, adiabatic electron affinity, and chemical hardness, we find that the beaded ZnO nanocluster has a higher chemical reactivity during its growth process. Vibrational frequencies of ZnO clusters and nanoclusters are also discussed in detail.

Experiment and Modeling of Pure and Binary Adsorption of n-Butane and Butene-1 on ZSM-5 Zeolites with Different Si/Al Ratios

Abstract Four ZSM-5 zeolite catalysts with different Si/Al ratios for the catalytic cracking of C 4 fractions to produce ethylene and propylene were prepared in this study. First, the adsorption isotherms of pure n -butane and butene-1 and their mixtures on these catalysts at 300K and p =0–100kPa were measured using the intelligent gravimetric analyzer. The experimental results indicate that the presence of Al can significantly affect the adsorption of butene-1 than that of n -butane on ZSM-5 zeolites. Then, the double Langmuir (DL) model was applied to study the pure gas adsorption on ZSM-5 zeolites for pure n -butane and butene-1. By combining the DL model with the ideal adsorbed solution theory (IAST), the IAST-DL model was applied to model the butene-1 (1)/ n -butane (2) binary mixture adsorption on ZSM-5 zeolites with different Si/Al ratios. The calculated results are in good agreement with the experimental data, indicating that the IAST-DL model is effective for the present systems. Finally, the adsorption over a wide range of variables was predicted at low pressure and 300K by the model proposed. It is found that the selectivity of butene-1 over n -butane increases linearly with the decrease of Si/Al ratio. A correlation between the selectivity and Si/Al ratio of the sample was proposed at 300K and p =0.08MPa.

Structural and electronic properties of atomically thin germanium selenide polymorphs

Using comprehensive density functional theory calculations, we systematically investigate the structure, stability, and electronic properties of five polymorphs of GeSe monolayer, and highlight the differences in their structural and electronic properties. Our calculations show that the five free-standing polymorphs of GeSe are stable semiconductors. β-GeSe, γ-GeSe, δ-GeSe, and ε-GeSe are indirect gap semiconductors, whereas α-GeSe is a direct gap semiconductor. We calculated Raman spectra and scanning tunneling microscopy images for the five polymorphs. Our results show that the β-GeSe monolaye r is a candidate for water splitting.中文摘要本文利用密度泛函理论, 系统研究了五种单层GeSe晶型的结构、稳定性和电子结构特性, 并着重分析了其结构和电子性质差异. 研究结果表明, 五种单层GeSe晶型均表现出稳定的半导体特性. 不同的是β-GeSe、γ-GeSe、δ-GeSe和ε-GeSe晶型结构是间接带隙半导体材料, 而α-GeSe是直接带隙半导体. 计算进一步提供了五种晶型结构的拉曼光谱和扫描隧道显微镜图像. 带边排布分析表明β-GeSe单层材料适用于光催化分解水.

Core?shell-structured bimetallic clusters and nanowires

We report the structures of Ag–Cu and Ag–Ni bimetallic clusters and nanowires (NWs), which are well known as effective Ag-based catalysts, by using an effective semi-grand-canonical ensemble Monte Carlo method. The metal–metal interactions are modeled by the second-moment approximation of the tight-binding potentials. The simulation results show that the Ag–Cu and Ag–Ni bimetallic nanomaterials, including clusters and NWs, possess core–shell structures at different compositions, in which the Ag atoms lie on the surface, while the Cu or Ni atoms occupy the cores of the clusters and NWs. It is found that the pentagonal multi-shell-type structure can be transformed into cylindrical multi-shell-type structures for Ag–Cu and Ag–Ni bimetallic NWs at 100, 300, and 500 K. On the other hand, with the increase of Ag mole fraction in the Ag–Cu and Ag–Ni bimetallic clusters, the Ag atoms occupy the surface shell first, then the interior shell, and finally the central sites of the clusters. It is also found that the initial shape, composition, and temperature have little effect on the core–shell structures of the bimetallic clusters and NWs. The formation of core–shell Ag–Cu and Ag–Ni bimetallic clusters and NWs is due to the fact that a single Ag impurity is favorable to be situated in the core of the Cu or Ni clusters and NWs.

Theoretical investigations of sp-sp2 hybridized zero-dimensional fullerenynes

Widely recognized as the quintessential material, sp(2) hybridized carbon material with low dimensions, such as zero-dimensional fullerene, one-dimensional carbon nanotube and two-dimensional graphene, has already compiled an impressive list of superlatives. Quite recently, one-dimensional sp-sp(2) hybridized carbon tubular arrays with a wall thickness of about 40 nm and two-dimensional carbon films with the average thickness of 970 nm have been synthesized successfully. Thus, we expect that the existence of a sp-sp(2) hybridized zero-dimensional carbon allotrope is possible. A novel and stable zero-dimensional carbon allotrope (fullerenyne) with sp-sp(2) hybridization is introduced by means of density functional theory calculation and molecular dynamics confirmation. Unique porous characteristic C(96) fullernenyne with an O(h) symmetry group exhibits exceptionally high stability. We hope that the present study will lead to a further development of a broad new class of carbon materials.

The effect of electric field on hydrogen storage for B/N-codoped graphyne

Hydrogen adsorption on a B/C/N sheet under different external electric fields is investigated by first-principles calculations. Through the analyses of structural properties of the B/C/N system, we find that NbBf, BbNo, BaNe, and NaBg are more probable to be synthesized. Through molecular dynamics calculations, it was found that the structures for B/N doped graphyne are stable. For NbBf, BbNo, BaNe, and NaBg, the most stable positions for hydrogen adsorption are the H1 sites. For a single H2 adsorbed on a B/C/N sheet, the adsorption energy increases greatly as the electric field increases, and the maximum adsorption energy is 0.506 eV when the electric field is 0.035 a.u. It is also found that the adsorption energy of H2 adsorbed on NbBf under electric field increases faster than H2 adsorbed on other sheets. The interaction between H2 molecule and B/C/N sheet is the Kubas interaction under an external electric field.

Density Functional Theory Study on Electronic and Magnetic Properties of Mn‐Doped (MgO)n (n=2–10) Clusters

We study the geometries, stabilities, electronic and magnetic properties of ( MgO ) n (n=2–10) clusters doped with a single Mn atom using the density functional theory with the generalized gradient approximation. The optimized geometries show that the impurity Mn atom prefers to replace the Mg atom which has low coordination number in all the lowest‐energy MnMg n−1 O n (n=2–10) structures. The stability analysis clearly represents that the average binding energies of the doped clusters are larger than those of the corresponding pure ( MgO ) n clusters. Maximum peaks of the second order energy differences are observed for MnMg n−1 O n clusters at n=6, 9 , implying that these clusters exhibit higher stability than their neighboring clusters. In addition, all the Mn‐doped Mg clusters exhibit high total magnetic moments with the exception of MnMgO 2 which has 3.00 μ B . Their magnetic behavior is attributed to the impurity Mn atom, the charge transfer modes, and the size of MnMg n−1 O n clusters.