分子力场发展的新趋势

分子模拟中的力场方法是用来精确计算分子结构和能量的计算方法,它通过原子核的位置来计算体系能量。最初的分子力场都是针对某一特定体系的,它们的许多参数要由观测数据拟合得到。当时要建立新的分子力场是十分困难的,因为实验归属振动谱带需要花费大量的时间。所以此后大多数工作者都致力于发展涵盖尽可能多体系的“求全”型分子力场,这种趋势一直延续至今。但是随着各个学科研究的不断深入,所需要研究的体系越来越复杂,要求的精度也越来越高。在保证相当精度的条件下,“求全”型的分子力场要想涵盖所有需要研究的体系常常是十分困难的事情。2003年问世的Direct Force Field软件包能够便捷的建立针对某一特定分子体系,并且有相当精度的分子力场。它的出现为分子力场从“求全”转为向“求精”发展提供了可能。     

Conformational Effect on CNDO Force Field of Furan-2-Aldehyde

The complete sets of force constants for both O, O-cis and O,O-trans furand-2-aldehyde conformers were calculated using the CNDO force method. The force constants were corrected by ten empirical scaling factors. The calculated and observed frequencies agree well in most cases. Comparing the force constants of the two conformers, the largest deviations can be found in the interaction terms belonging to the aldehyde group.     

MoS2‐Based Catalysts for N2 Electroreduction to NH3 – An Overview of MoS2 Optimization Strategies

The nitrogen reduction reaction (NRR) has become an ideal alternative to the Haber‐Bosch process, as NRR possesses, among others, the advantage of operating under ambient conditions and saving energy consumption. The key to efficient NRR is to find a suitable electrocatalyst, which helps to break the strong N≡N bond and improves the reaction selectivity. Molybdenum disulfide (MoS₂) as an emerging layered two‐dimensional material has attracted a mass of attention in various fields. In this minireview, we summarize the optimization strategies of MoS₂‐based catalysts which have been developed to improve the weak NRR activity of primitive MoS₂. Some theoretical predictions have also been summarized, which can provide direction for optimizing NRR activity of future MoS₂‐based materials. Finally, an outlook about the optimization of MoS₂‐based catalysts used in electrochemical N₂ fixation are given.     

氢氧根辅助大面积单层二硫化钼在不同衬底上的生长

<正>二硫化钼是一种典型的二维材料,因为其优异的电学、光学性能和良好的机械柔韧性引起人们的广泛关注~1。单层二硫化钼由于高的载流子迁移率和超薄的厚度,被广泛应用在半导体场效应管和光探测器中~2,生长大面积高质量的单层二硫化钼也一直是大家的研究目标。目前制备大面积单层二硫化钼的主流方法还是化学气相沉积法     

Martini Force Field for Protonated Polyethyleneimine

Polyethyleneimine (PEI), one of the most widely used nonviral gene carriers, was investigated in the presented work at coarse-grained (CG) level. The main focus was on elaborating a realistic CG force field (FF) aimed to reproduce dynamic structural features of protonated PEI chains and, furthermore, to enable massive simulations of DNA–PEI complex formation and condensation. We parametrized CG Martini FF models for PEI in polarizable and nonpolarizable water by applying Boltzmann inversion techniques to all-atom (AA) probability distributions for distances, angles, and dihedrals of entire monomers. The fine-tuning of the FFs was achieved by fitting simulated CG gyration radii and end-to-end distances to their AA counterparts. The developed Martini FF models are shown to be well suited for realistic large-scale simulations of size/protonation-dependent behavior of solvated PEI chains, either individually or as part of DNA–PEI systems. © 2019 Wiley Periodicals, Inc.     

尺寸效应对二硫化钼量子点光学性能的影响

通过油溶法成功地制备出不同粒径大小的二硫化钼,并探究了单层二硫化钼尺寸效应对荧光性质的影响。其中反应时间为3 h制备出的二硫化钼量子点为单层结构,3种尺寸的二硫化钼的荧光光谱和拉曼光谱研究表明,当粒径或激发波长增大时,荧光谱图中峰位发生红移。     

单原子改性二硫化钼电催化析氢

氢能是21世纪最理想的清洁能源之一。相比于天然气和煤炭制氢,电解水制氢具有成本低、效率高、无污染、原料丰富的特点,可以有效缓解CO₂过量排放导致的温室效应。电催化析氢需要活性高、稳定性好、廉价易得的催化剂克服反应能垒并加速动力学过程,对实现分解水制氢的规模化应用具有重要的推动作用。铂基催化剂被公认为性能最优异的析氢电催化剂之一,但由于丰度低、成本高,不适用于大规模产氢。二硫化钼(MoS₂)作为典型的二维材料之一,因其高活性位点暴露和高比表面积在析氢领域展现出一定的应用潜能,并有望取代铂基催化剂。本文基于MoS₂电催化剂在析氢领域的研究现状,对单原子掺杂改性MoS₂以提高其催化活性的研究进行了综述,以析氢过电位(Overpotential)及塔菲尔(Tafel)曲线斜率为依据,总结了贵金属单原子、非贵金属单原子及非金属单原子改性MoS₂催化剂的结构与性能以及它们之间的构效关系,在此基础上,提出MoS₂析氢催化剂目前存在的科学问题并指出了未来的努力方向。     

Development of the CHARMM Force Field for Lipids

The development of the CHARMM additive all-atom lipid force field (FF) is traced from the early 1990's to the most recent version (C36) published in 2010. Though simulations with early versions yielded useful results, they failed to reproduce two important quantities: a zero surface tension at the experimental bilayer surface area, and the signature splitting of the deuterium order parameters in the glycerol and upper chain carbons. Systematic optimization of parameters based on high level quantum mechanical data and free energy simulations have resolved these issues, and bilayers with a wide range of lipids can be simulated in tensionless ensembles using C36. Issues associated with other all-atom lipid FFs, success and limitations in the C36 FF and ongoing developments are also discussed.     

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.     

An Improved Force Field for O2, CO and CN Binding to Metalloporphyrins

Parametrization of a molecular-mechanics program to include terms specific for five- and six-coordinate transition metal complexes is applied to heme complexes. The principal new feature peculiar to five and six coordination is a term that represents the effect of electron-pair repulsion modified by the ligand electronegativity and takes into account the different possible structures of complexes. The model system takes into account the structural differences of the fixing centre in the haemoglobin subunits. The customary proximal histidine is added. The macrocycle heme IX is wholly considered in our model. The calculations show clearly that certain conformations of heme IX–histidine models are much more favourable than others for fixing O₂. From the O₂ binding in haemoglobin and myoglobin and in simple Fe porphyrin models it is concluded that the bent O₂ ligand is best viewed as bound superoxide, O₂ ^?. Rotation of axial ligands are practically free. A small modification of the model in both crystal and protein matrix affects the orientation of the ligands in experimental systems.     

Single Iron-dimer Catalysts on MoS2 Nanosheet for Potential Nitrogen Activation

Electrocatalytic nitrogen reduction reaction(NRR) is a promising way to produce ammonia(NH₃) at ambient temperature and pressure. Herein, we have constructed single Fe dimer catalysts on a molybdenum disulfide monolayer for potential nitrogen activation. By employing ab initio molecular dynamics simulations, it is suggested that a dual iron-single atom site can be dynamically formed, which exhibits the similar Fe-S-Fe structure as the nitrogenase. We further identify an iron dimer with a sulfur vacancy as the active center for realistic nitrogen activation by the free energy calculations since the bridged sulfur is easy to be released in the form of H₂S during the reduction process. It is shown that N₂ mainly adsorbs on the Fe₂ dimer at the sulfur vacancies in the pattern of side-on configuration, and the nitrogen reduction reaction is proceeded by an enzymatic mechanism. Charge analyses further show that the Fe₂ dimer mainly works as an electron reservoir while MoS₂ substrate with one sulfur vacancy acts as an inert carrier to stabilize the Fe₂ dimer. Overall, our work provides important insights into how N₂ molecules were adsorbed and activated on Fe₂-doped MoS₂, and provides new ideas for the transformation of actual reaction sites during electrochemical reactions.     

Single Atoms of Iron on MoS2 Nanosheets for N2 Electroreduction into Ammonia

Efforts have been devoted to achieving a highly efficient artificial synthesis of ammonia (NH₃). Reported herein is a novel Fe-MoS₂ catalyst with Fe atomically dispersed onto MoS₂ nanosheets, imitating natural nitrogenase, to boost N₂ electroreduction into NH₃ at room temperature. The Fe-MoS₂ nanosheets exhibited a faradic efficiency of 18.8 % with a yield rate of 8.63 μg mg_cat.⁻¹ h⁻¹ for NH₃ at −0.3 V versus the reversible hydrogen electrode. The mechanism study revealed that the electroreduction of N₂ was promoted and the competing hydrogen evolution reaction was suppressed by decorating the edge sites of S in MoS₂ with the atomically dispersed Fe, resulting in high catalytic performance for the electroreduction of N₂ into NH₃. This work provides new ideas for the design of catalysts for N₂ electroreduction and strengthens the understanding about N₂ activation over Mo-based catalysts.     

铁基粉末冶金材料中稀土元素与MoS2的交互作用研究

通过实验研究了添加到铁基粉末冶金摩擦材料中的稀土在材料中存在的位置和状态以及稀土对MoS2分解所产生的影响.电子探针检测的结果表明, 稀土在铁基粉末冶金磨擦材料中主要集中分布于孔隙处, 且其分布呈聚集的粒状, 常与Pb等重金属共存; 稀土有强烈的促进MoS2分解的作用, 当加入量超过3倍单位量时即可使加入的4%(质量分数)的MoS2完全分解.     

A Kirkwood-Buff Derived Force Field for Aqueous Alkali Halides

A classical nonpolarizable force field is presented for the simulation of aqueous alkali halide solutions (MX), where M = Li(+), Na(+), K(+), Rb(+), Cs(+) and X = F(-), Cl(-), Br(-), I(-), and their interactions with biomolecules. The models are specifically designed to reproduce the experimental Kirkwood-Buff integrals, and thereby the solution salt activities, as a function of salt concentration. Additionally, we demonstrate that these models reasonably reproduce other experimental properties including ion diffusion constants, dielectric decrements, and the excess heats of mixing. The parameters are developed by considering the properties of aqueous NaX and MCl solutions using a previously established model for NaCl. Transferability of the parameters to other salts is then established by the successful simulation of additional aqueous salt solutions, KI and CsBr, not originally included in the parameterization procedure.     

单个MoS2纳米片氢析出催化特性研究

MoS₂作为高效的电催化氢析出(HER)催化剂已有大量文献报道. 实验和DFT计算结果都表明MoS₂的高氢析出活性来源于边缘,而其基面是催化惰性的。为了进一步验证此结论,本文利用巯基羧酸在恒电位下自组装单层修饰的纳/微米电极固定不同尺寸的单个纳米片状,对MoS₂氢析出催化活性与其尺寸的关系进行研究,发现纳米片状MoS₂具有较高的催化活性,同时较小尺寸的MoS₂氢析出活性更高,说明MoS₂的边缘的增多对其催化活性有巨大提升,即证明了边缘部分具有更高的氢析出催化活性.     

Nanomechanical and nanotribological characterization of multilayer self-lubricating Ti/MoS2/Si/MoS2 nanocoating on aluminium-silicon substrate

Ti/MoS₂/Si/MoS₂ multilayer coating was fabricated by a pulse laser deposition method from a titanium, molybdenum disulphide, and silicon targets, and the coating was deposited in layers on aluminium-silicon substrates, at room temperature. The structural analysis and surface morphology of multilayer Ti/MoS₂/Si/MoS₂ coating were analysed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy integrated with energy dispersive X-ray spectroscopy. Nanomechanical tests were performed on Ti/MoS₂/Si/MoS₂ coating at small loads of 2000 to 6000 μN to study the effect of load on hardness and Young's modulus. Nanoscratch and nanowear tests were conducted on Ti/MoS₂/Si/MoS₂ coating at a low load of 1000 to 5000 μN and 100 to 500 μN, respectively, to study deformation and failure behaviours of coating/substrate system and also their nanotribological properties. The results show that hardness and Young's modulus of Ti/MoS₂/Si/MoS₂ coating decrease with increase in load. A low friction coefficient of 0.09 to 0.16 was observed, which proves that the Ti/MoS₂/Si/MoS₂ coating possesses self-lubricating property. The wear rate of Ti/MoS₂/Si/MoS₂ coating increases 3.3 × 10⁻¹⁰ to 7.8 × 10⁻¹⁰ mm³/Nm with increase in load. Ti/MoS₂/Si/MoS₂ multilayer coating shows a smooth wear track with no cracks and debris on the surface, which attributed plastic flow of softer coating material around the wear track.     

Electrochemical Sensor Based on Biomass Yeast Integrated Sulfur-doped Graphene and Carboxylated Carbon Nanotubes/MoS2 for Highly-sensitive Detection of Pb2+

Glassy carbon electrodes (GCEs) modified with sulfur-doped graphene (SG)/carboxylated carbon nanotube (CNT−COOH)/MoS₂/yeast composite were prepared for electrochemical detection for lead ions by the simple hydrothermal methods and ultrasonic methods. The combination of SG and CNT−COOH could form a double-layer carbon structure, providing more active detection sites for detection for lead, which could also contribute to adherence of yeast and MoS₂. The SG/CNT−COOH/MoS₂/yeast exhibited a high response in detecting low concentrations of lead ions. And then the SG/CNT−COOH/MoS₂/yeast was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Transmission electron microscope (TEM), Scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Compared with traditional detection technology, the linear range of the sensor was 10⁻⁶∼10⁻¹⁴ g/L. And the lower of detection (LOD) down to 2.61×10⁻¹⁵ g/L was achieved. The sensor showed prospective applications in detection of Pb²⁺ in real serum samples.     

Hierarchical MoS2 Shells Supported on Carbon Spheres for Highly Reversible Lithium Storage

Hierarchical MoS₂ shells supported on carbon spheres (denoted as C@MoS₂) have been synthesized through a one‐step hydrothermal method. The obtained hierarchical C@MoS₂ microspheres simultaneously integrate the structural and compositional design rationales for high‐energy electrode materials based on two‐dimensional (2D) nanosheets. When evaluated as an anode material for lithium‐ion batteries (LIBs), the hierarchical C@MoS₂ microspheres manifest high specific capacity, enhanced cycling stability and good rate capability.