Hydrogen on hybrid MoS2/graphene nanostructures

Transition metal dichalcogenides are rising candidates for the replacement of Pt catalysts in water splitting. In this theoretical study we focus on the hydrogen evolution reaction part of this process and on how hydrogen (H) interacts with MoS₂ nanostructures, free‐standing or positioned on a graphene substrate. Density functional theory calculations confirm the stability of such nanostructures and our results for H on several configurations, from 2D infinite monolayers to quasi‐1D MoS₂ ribbons and quasi‐0D MoS₂ flakes, are presented. We calculate the adsorption energy of H atoms on various sites of the MoS₂ nanostructures, notably at Mo and S active edges. Comparing free‐standing and MoS₂/graphene hybrid systems we find that the effect of the support on the adsorption of H on MoS₂ nanostructures is quite significant when the substrate induces strain. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Three-dimensional flower-like NiS2/MoS2 assembly of randomly oriented nanoplate for enhanced hydrogen evolution reaction

The continually worsening energy crisis has stimulated research into energy conversion technology to produce pure hydrogen, H₂. Transition metal-based compounds have attracted great attention as electrocatalysts for hydrogen evolution reaction (HER) as alternatives to commercial, high-cost, and scarce noble metal-based catalysts. In this work, a 3D flower-like NiS₂/MoS₂ is synthesized with the advantages of a three-dimensional (3D) morphology and the compositing of different metal compounds, thus leading to enhanced electrocatalytic performance. The structure of 3D flower-like NiS₂/MoS₂ augments the specific surface areas resulting from nanoplate assemblies as well as the heterointerface ascribed to two different phases of NiS₂ and MoS₂. These characteristics are confirmed by electrocatalytic measurements of the lower overpotential of 165 mV at 10 mA/cm² with high charge transfer ability, thus demonstrating the structure's potential for advanced electrocatalysts for the HER.     

电化学析氢反应中单层MoSe2氢吸附机理第一性原理研究

基于密度泛函理论的第一性原理方法,本文计算了单层2H相MoSe₂纳米材料表面及两种边缘（Mo原子边缘、Se原子边缘）不同活性位点、不同氢原子吸附率下的氢吸附吉布斯自由能（Gibbs free energy,用△G_H⁰表示）,并且将对应的微观结构进行了系统分析比较,得出△G_H⁰最接近于0 eV的吸附位点及相应的吸附率.同时,结合差分电荷密度和电负性理论,分析了单层MoSe₂两种边缘氢吸附的电荷转移及成键特性,进一步解释了不同吸附位点呈现的结构与能量趋势.最后,通过基于密度泛函理论的第一性原理分子动力学模拟,研究了高温热运动对两种边缘氢吸附的影响,获得了氢原子发生脱附的临界温度及对应的微观动态过程.该理论研究从原子尺度揭示了单层2H相MoSe₂纳米材料边缘不同位点在不同温度下对氢原子吸附和脱附的微观机理,证实了Mo原子边缘的畸变和重构行为,加深了对实验中单层2H相MoSe₂边缘在不同温度下氢吸附机理的理解,为实验中通过控制MoSe₂边缘设计廉价高效的析氢催化剂提供理论参考.     

A DFT study of Ti3C2O2 MXenes quantum dots supported on single layer graphene: Electronic structure and hydrogen evolution performance

Heterojunction structure has been extensively employed for the design of novel catalysts. In the present study, density functional theory was utilized to investigate the electronic structure and hydrogen evolution performance of Ti₃C₂O₂ MXene quantum dots/graphene (QDs/G) heterostructure. Results show that a slight distortion can be observed in graphene after hybriding with QDs, due to which the electronic structure of QDs have been changed. Associated with such QDs-graphene interaction, the catalytic activity of Ti₃C₂O₂ QDs has been optimized, leading to excellent HER catalytic performance.     

Band renormalization and spin polarization of MoS2 in graphene/MoS2 heterostructures

Transition metal dichalcogenides exhibit spin–orbit split bands at the K‐point that become spin polarized for broken crystal inversion symmetry. This enables simultaneous manipulation of valley and spin degrees of freedom. While the inversion symmetry is broken for monolayers, we show here that spin polarization of the MoS₂ surface may also be obtained by interfacing it with graphene, which induces a space charge region in the surface of MoS₂. Polarization induced symmetry breaking in the potential gradient of the space charge is considered to be responsible for the observed spin polarization. In addition to spin polarization we also observe a renormalization of the valence band maximum (VBM) upon interfacing of MoS₂ with graphene. The energy difference between the VBM at the Γ‐point and K‐point shifts by ～150 meV between the clean and graphene covered surface. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Strain-regulated electronic structure for hydrogen evolution reaction in Fe3S4 monolayer

Electrochemical water splitting to generate hydrogen could be an important part of future renewable energy, but faces challenge due to the scarcity of effective earth-abundant electrocatalysts and insufficient understanding of catalytic mechanism. Herein, we predicated strain-induced changes in electronic structure and catalytic performance of low-cost two-dimensional Fe₃S₄ material. The calculations disclose that the half-metallic feature evolves into metallicity under applied external strain, which makes Gibbs adsorption free energy of hydrogen close to zero. Different from traditional doping and defecting strategies, this work demonstrates that excellent catalytic activity for water splitting can be achieved by inducing a small lattice deformation in Fe₃S₄ monolayer. Our findings provide new inspirations for the steering of electronic structure and designing of new-type catalysts.     

梳状波导结构中石墨烯表面等离子体的传播性质

理论上研究了介质/石墨烯/介质梳状波导结构中表面等离子体的传播性质. 波导中表面等离子体模的有效折射率随着石墨烯费米能级的提高而减小, 随着介质折射率的增加而增加. 分析和仿真结果表明, 基于这种梳状波导可以在中红外波段实现新型的纳米等离子体滤波器, 器件的尺度在几百纳米的范围. 通过改变梳状分支的长度, 石墨烯的费米能级, 介质的折射率和波导中石墨烯的层数, 很容易来调节带隙的位置. 另外, 滤波带隙的宽度随着梳状分支数的增加而增加. 这种滤波性质将在可调的高集成光子滤波器件中具有潜在的应用.     

Tunable localized surface plasmon resonances in one-dimensional h-BN/graphene/h-BN quantum-well structure

The graphene/hexagonal boron-nitride(h-BN) hybrid structure has emerged to extend the performance of graphenebased devices. Here, we investigate the tunable plasmon in one-dimensional h-BN/graphene/h-BN quantum-well structures.The analysis of optical response and field enhancement demonstrates that these systems exhibit a distinct quantum confinement effect for the collective oscillations. The intensity and frequency of the plasmon can be controlled by the barrier width and electrical doping. Moreover, the electron doping and the hole doping lead to very different results due to the asymmetric energy band. This graphene/h-BN hybrid structure may pave the way for future optoelectronic devices.     

Laser fragmentation in liquid synthesis of novel palladium-sulfur compound nanoparticles as efficient electrocatalysts for hydrogen evolution reaction

One promising way to tune the physicochemical properties of materials and optimize their performance in various potential applications is to engineer material structures at the atomic level. As is well known, the performance of Pd-based catalysts has long been constrained by surface contamination and their single structure. Here, we employed an unadulterated top-down synthesis method, known as laser fragmentation in liquid (LFL), to modify pristine PdPS crystals and obtained a kind of metastable palladium-sulfur compound nanoparticles (LFL-PdS NPs) as a highly efficient electrocatalyst for hydrogen evolution reaction (HER). Laser fragmentation of the layered PdPS crystal led to a structural reorganization at the atomic level and resulted in the formation of uniform metastable LFL-PdS NPs. Noteworthy, the LFL-PdS NPs show excellent electrocatalytic HER performance and stability in acidic media, with an overpotential of -66 mV at 10 mA· cm^-2, the Tafel slope of 42 mV· dec^-1. The combined catalytic performances of our LFL-PdS NPs are comparable to the Pt/C catalyst for HER. This work provides a top-down synthesis strategy as a promising approach to design highly active metastable metal composite electrocatalysts for sustainable energy applications.     

Intermediate states and structure evolution in the free-falling process of the dislocation in graphene

Abstract

This paper investigated the intermediate states and the structure evolution of the dislocation in graphene when it falls freely from the saddle point of the energy landscape. The O-type dislocation, an unstable equilibrium structure located at the saddle point, is obtained from the lattice theory of the dislocation structure and improved by the ab initio calculation to take the buckling into account. Intermediate states along the kinetics path in the falling process are obtained from the ab initio simulation. Once the dislocation falls from the saddle point to the energy valley, this O-type dislocation transforms into the stable structure that is referred to as the B-type dislocation, and in the meantime, it moves a distance that equals half a Burgers vector. The structure evolution and the energy variation in the free-falling process are revealed explicitly. It is observed that rather than smooth change, a platform manifests itself in the energy curve. The unusual behaviour in the energy curve is mainly originated from symmetry breaking and bond formation in the dislocation core. The results can provide deep insight in the mechanism of the brittle feature of covalent materials.     

还原温度对氧化石墨烯结构及室温下H_2敏感性能的影响

以氧化石墨凝胶制备的氧化石墨烯(GO)溶胶为前驱体,在100—350℃温度下还原获得不同还原程度的还原氧化石墨烯(rGO)样品,并采用旋涂工艺制备还原氧化石墨烯气敏薄膜元件.采用X射线衍射、拉曼光谱、傅里叶变换红外光谱和气敏测试等手段研究还原温度对样品结构、官能团和气敏性能的影响.结果表明:经热还原处理的氧化石墨烯结构向较为有序的类石墨结构转变,还原温度为200℃时,样品处于GO向rGO转变的过渡阶段,还原温度达到250℃时,则表现出还原氧化石墨烯特性;无序程度随还原温度的升高先由0.85增大至1.59,随后减小至1.41,总体呈现增加趋势;氧化石墨烯表面含氧官能团随还原温度的升高逐渐热解失去,不同含氧官能团的失去温度范围不同;热还原氧化石墨烯具有优异的室温H_2敏感性能,随着还原温度的升高,元件灵敏度逐渐减小,响应-恢复时间逐渐增大,最佳灵敏度为88.56%,响应时间为30 s.     

Metal substrates-induced phase transformation of monolayer transition metal dichalcogenides for hydrogen evolution catalysis

Monolayer transition metal dichalcogenides can normally exist in several structural polymorphs with distinct electrical, optical, and catalytic properties. Effective control of the relative stability and transformation of different phases in these materials is thus of critical importance for applications. Using density functional theory calculations, we investigate the effects of low-work-function metal substrates including Ti, Zr, and Hf on the structural, electronic, and catalytic properties of monolayer MoS₂ and WS₂. The results indicate that such substrates not only convert the energetically stable structure from the 1H phase to the 1T'/1T phase, but also significantly reduce the kinetic barriers of the phase transformation. Furthermore, our calculations also indicate that the 1T' phase of MoS₂ with Zr or Hf substrate is a potential catalyst for the hydrogen evolution reaction.     

Precisely controlling the twist angle of epitaxial MoS2/graphene heterostructure by AFM tip manipulation

Two-dimensional (2D) moiré materials have attracted a lot of attention and opened a new research frontier of twistronics due to their novel physical properties. Although great progress has been achieved, the inability to precisely and reproducibly manipulate the twist angle hinders the further development of twistronics. Here, we demonstrated an atomic force microscope (AFM) tip manipulation method to control the interlayer twist angle of epitaxial MoS₂/graphene heterostructure with an ultra-high accuracy better than 0.1°. Furthermore, conductive AFM and spectroscopic characterizations were conducted to show the effects of the twist angle on moiré pattern wavelength, phonons and excitons. Our work provides a technique to precisely control the twist angle of 2D moiré materials, enabling the possibility to establish the phase diagrams of moiré physics with twist angle.     

Composite Nanoarchitectonics with iron nickel bimetallic nanoparticles and carbon composite for efficient electrocatalysis in hydrogen evolution reaction

The poor efficiency and stability of cost-effective metal compounds are major hurdles to substitute expensive metal-based nanomaterials for the hydrogen evolution reaction (HER). As a result, new concepts and tactics for developing electrocatalysts based on earth-abundant elements must be developed. We present iron-nickel alloy nanoparticles that are supported with carbon (FeNi@C) to improve HER performance in alkaline conditions. FeNi particle was supported on Trimesic acid (TMA) based carbon. In particular, the high conductivity of the carbon and a large number of catalytically active sites in the FeNi demonstrated a synergistic effect, making the hybrid structure a good choice for HER catalyst. Moreover, the physicochemical interaction between the carbon and FeNi metal enhanced the electrocatalytic performance and resulted in achieving 10 mA/cm² current density at 190 mV overpotential with 15 h chronopotential cycling, proving the possibility for replacing costly Pt-based catalysts.     

锂改性点缺陷石墨烯储氢性能的第一性原理研究

本研究采用基于密度泛函理论的第一性原理方法计算了两种石墨烯点缺陷处原子的分波态密度(PDOS),能带结构和差分电荷密度等,研究了锂掺杂对两种本征石墨烯缺陷C-Bridge和C7557电子结构的改性,以及对其储氢能力的影响.结果表明Li原子能够稳定的掺杂且不易形成团簇,并且Li原子掺杂石墨烯能够对石墨烯能带中的狄拉克锥和费米面起到调控作用,增强了缺陷石墨烯的电子活性.本征缺陷石墨烯的储氢能力较弱,缺陷石墨烯的储氢能力可以通过Li掺杂来改善.     

Facile synthesis of Co-doped SnS2 as a pre-catalyst for efficient oxygen evolution reaction

Co-doped flower-like SnS₂ was synthesized using a one-step hydrothermal method. The Co content of Co-doped SnS₂ was facilely tuned by controlling the [Co]/[Sn] molar concentration ratio (SC-x; x = 0.05, 0.5, 1.0 2.0, where x indicates the [Co]/[Sn] ratio). The morphology of the samples did not significantly change despite changes in the Co dopant content. Compared to SC-0 (667 mV), SC-0.05 (400 mV), SC-0.5 (382 mV), and SC-1.0 (374 mV), SC-2.0 showed higher catalytic performance, with an overpotential of 323 mV at a current density of 10 mA/cm² in 1 M KOH solution. Moreover, SC-2.0 exhibited high stability for 12 h during chronopotentiometry. SC-2.0 was unexpectedly transformed to weakly crystallized CoOOH nanoparticles after the stability test. The transformation rate from Co-doped SnS₂ to CoOOH was decreased with an increase in the Co content.     

Theoretical study of K3Sb/graphene heterostructure for electrochemical nitrogen reduction reaction

Instead of the energy-intensive Haber-Bosch process, electrochemical nitrogen reduction reaction (NRR) is an exciting new carbon neutral technique for ammonia synthesis under ambient conditions. In this work, we investigated K-based electrocatalysts theoretically and demonstrated that K₃Sb/graphene performs excellent activity and inhibits hydrogen evolution on alternating reaction pathway. The first hydrogenation step from N₂* to NNH* was found to be the most energetic and limiting step (0.61 eV). Graphene substrate plays the critical role to promote electronic conductivity between K₃Sb and dinitrogen.     

Enhanced lithium-ion storage and hydrogen evolution reaction catalysis of MoS<Subscript>2</Subscript>/graphene nanoribbons hybrids with loose interlaced three-dimension structure

Molybdenum disulfide hybridized with graphene nanoribbon (MoS₂/GNR) was prepared by mild method. MoS₂/GNR hybrids interlace loosely into a three-dimension structure. GNR hybridization can improve the dispersity of MoS₂, reduce the grain size of MoS₂ to 3–6 nm, increase the specific surface area, and broaden the interlamellar spacing of MoS₂ (002) plane to 0.67–0.73 nm, which facilitates the transportation of Li⁺ ions for lithium-ion battery. MoS₂/GNR hybrids have better cyclic durability, higher specific discharge capacity, and superior rate performance than MoS₂. The electrocatalytic activity in hydrogen evolution reaction shows that MoS₂/GNR hybrids have the lower overpotential and the larger current density with a negligible current loss after 2000 cycles. Hybridizing with GNRs enhances both the lithium-ion electrochemical storage and the electrocatalytic activity of MoS₂.

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Graphical abstract MoS₂/GNR hybrid prepared by a mild method is interlaced loosely into a three-dimension structure. Superior electrochemical performances of MoS₂/GNR hybrids than MoS₂ have been highlighted for the potential application for long- term durability energy-storage devices and HER electrocatalytic materials.

     

A facile sonochemical route for the synthesis of MoS2/Pd composites for highly efficient oxygen reduction reaction

For the alkaline fuel cell cathode reaction, it is very essential to develop novel catalysts with superior catalytic properties. Here, we report the synthesis of highly active and stable MoS₂/Pd composites for the oxygen reduction reaction (ORR), via a simple, eco-friendly sonochemical method. The bulk MoS₂ was first transformed into single and few layers MoS₂ nanosheets through ultrasonic exfoliation. Then the exfoliated MoS₂ nanosheets served as supporting materials for the nucleation and further in-situ growth of Pd nanoparticles to form MoS₂/Pd composites via ultrasonic irradiation. Cyclic voltammetry and rotating disk voltammetry measurements demonstrate that as-prepared MoS₂/Pd composites which provides a direct four-electron pathway for the ORR, have better electrocatalytic activity, long-term operation stability than commercial Pt/C catalyst. We expect that the present work would provide a promising strategy for the development of efficient oxygen reduction electrocatalyst. In addition, this study can also be extended to the preparation of other hybrid with desirable morphologies and functions.     

基于差分进化算法的核磁共振T2谱多指数反演

提出了一种基于差分进化(DE)算法的核磁共振弛豫信号多指数反演新方法. 将核磁共振T₂谱反演问题转化为带非负约束的非线性优化问题,不需要预先给定横向弛豫时间T_{2分布,直接利用差分进化算法进行反演计算. 在测量信号低信噪比的情况下,计算机模拟和实验数据反演都表明了该方法在分析处理NMR弛豫信号中的有效性. 关键词： 核磁共振 多指数反演 差分进化 岩心分析}