High Catalytic Activity of Nitrogen and Sulfur Co‐Doped Nanoporous Graphene in the Hydrogen Evolution Reaction

Chemical doping has been demonstrated to be an effective way to realize new functions of graphene as metal‐free catalyst in energy‐related electrochemical reactions. Although efficient catalysis for the oxygen reduction reaction (ORR) has been achieved with doped graphene, its performance in the hydrogen evolution reaction (HER) is rather poor. In this study we report that nitrogen and sulfur co‐doping leads to high catalytic activity of nanoporous graphene in HER at low operating potential, comparable to the best Pt‐free HER catalyst, 2D MoS₂. The interplay between the chemical dopants and geometric lattice defects of the nanoporous graphene plays the fundamental role in the superior HER catalysis.     

Revisiting lithium-storage mechanisms of molybdenum disulfide

Molybdenum disulfide（MoS₂）,a typical two-dimensional transition metallic layered material,attracts tremendous attentions in the electrochemical energy storage due to its excellent physicochemical properties.However,with the deepening of the research and exploration of the lithium storage mechanism of these advanced MoS₂-based anode materials,the complex reaction process influenced by internal and external factors hinders the exhaustive understanding of the lithium storage p...     

The study of electronic and optical properties of ZnO/MoS2 and its vacancy heterostructures by first principles

Based on the first principles calculation, the effects of vacancies on the structural, electronic and optical properties of ZnO/MoS₂ heterostructure are investigated in this work. The results show that vacancies could exist stably in the heterojunctions and cause a significant decrease in bandgap. ZnO/MoS₂ with an O vacancy maintains semiconductor property with a bandgap of 0.119 eV, while heterostructure with a Zn vacancy exhibits metallic characteristic. Furthermore, the absorption capability of defective heterojunctions has been extended to infrared light region with obvious redshift. To sum up, vacancy engineering effectively changes the electronic and optical properties of ZnO/MoS₂ heterostructure, which provides a feasible approach for adjusting the optoelectronic properties of two-dimensional heterostructures and broadening their application in functional nanoelectronic and optoelectronic devices.     

Directly Imaging and Regulating the Nanoscale Inhomogeneity of S-Vacancies in Molybdenum Disulfide Monolayer during Electrocatalytic Hydrogen Evolution

The study of electron transfer event on two-dimensional (2D) layered transition metal dichalcogenides has attracted tremendous attentions attributing to their promising applications in electrochemical devices. Herein, we demonstrate an opto-electrochemical strategy to directly map and regulate electron transfer event on molybdenum disulfide (MoS₂) monolayer by combining bright field (BF) imaging technique with electrochemical modulation. The heterogeneity of electrochemical activity on MoS₂ monolayer down to nanoscale is resolved spatiotemporally. The thermodynamics of MoS₂ monolayer is measured during electrocatalytic hydrogen evolution, and the Arrhenius correlations are obtained. We validate that the defect generation engineered by oxygen plasma bombardment dramatically enhances the local electrochemical activity of MoS₂ monolayer, which can be attributed to point defects of S-vacancies as evidenced. Furthermore, by comparing the difference of electron transfer event on MoS₂ with various layers, the interlayer coupling effect is uncovered. This study represents a facile method to image the heterogeneity of electrochemical properties for nanomaterials with atomic thickness and regulate the local activity within the plane by extrinsic factors. It also has potential applications in the design and evaluation of high-performance layered electrochemical systems down to nanoscale.     

Top‐Down and Bottom‐Up Approaches in Engineering 1 T Phase Molybdenum Disulfide (MoS2): Towards Highly Catalytically Active Materials

The metallic 1 T phase of MoS₂ has been widely identified to be responsible for the improved performances of MoS₂ in applications including hydrogen evolution reactions and electrochemical supercapacitors. To this aim, various synthetic methods have been reported to obtain 1 T phase‐rich MoS₂. Here, the aim is to evaluate the efficiencies of the bottom‐up (hydrothermal reaction) and top‐down (chemical exfoliation) approaches in producing 1 T phase MoS₂. It is established in this study that the 1 T phase MoS₂ produced through the bottom‐up approach contains a high proportion of 1 T phase and demonstrates excellent electrochemical and electrical properties. Its performance in the hydrogen evolution reaction and electrochemical supercapacitors also surpassed that of 1 T phase MoS₂ produced through a top‐down approach.     

Precise Tuning of the Charge Transfer Kinetics and Catalytic Properties of MoS2 Materials via Electrochemical Methods

MoS₂ has become particularly popular for its catalytic properties towards the hydrogen evolution reaction (HER). It has been shown that the metallic 1T phase of MoS₂, obtained by chemical exfoliation after lithium intercalation, possesses enhanced catalytic activity over the semiconducting 2H phase due to the improved conductivity properties which facilitate charge‐transfer kinetics. Here we demonstrate a simple electrochemical method to precisely tune the electron‐transfer kinetics as well as the catalytic properties of both exfoliated and bulk MoS₂‐based films. A controlled reductive or oxidative electrochemical treatment can alter the surface properties of the film with consequently improved or hampered electrochemical and catalytic properties compared to the untreated film. Density functional theory calculations were used to explain the electrochemical activation of MoS₂. The electrochemical tuning of electrocatalytic properties of MoS₂ opens the doors to scalable and facile tailoring of MoS₂‐based electrochemical devices.     

Defect Engineering of MoS2 and Its Impacts on Electrocatalytic and Photocatalytic Behavior in Hydrogen Evolution Reactions

Molybdenum disulfide (MoS₂) has been regarded as a favorable photocatalytic co‐catalyst and efficient hydrogen evolution reaction (HER) electrocatalyst alternative to expensive noble‐metals catalysts, owing to earth‐abundance, proper band gap, high surface area, and fast electron transfer ability. In order to achieve a higher catalytic efficiency, defects strategies such as phase engineering and vacancy introduction are considered as promising methods for natural 2H‐MoS₂ to increase its active sites and promote electron transfer rate. In this study, we report a new two‐step defect engineering process to generate vacancies‐rich hybrid‐phase MoS₂ and to introduce Ru particles at the same time, which includes hydrothermal reaction and a subsequent hydrogen reduction. Compositional and structural properties of the synthesized defects‐rich MoS₂ are investigated by XRD, XPS, XAFS and Raman measurements, and the electrochemical hydrogen evolution reaction performance, as well as photocatalytic hydrogen evolution performance in the ammonia borane dehydrogenation are evaluated. Both catalytic activities are boosted with the increase of defects concentrations in MoS₂, which ascertains that the defects engineering is a promising route to promote catalytic performance of MoS₂.     

Fabrication of Large-Area Molybdenum Disulfide Device Arrays Using Graphene/Ti Contacts

Two-dimensional (2D) molybdenum disulfide (MoS₂) is the most mature material in 2D material fields owing to its relatively high mobility and scalability. Such noticeable properties enable it to realize practical electronic and optoelectronic applications. However, contact engineering for large-area MoS₂ films has not yet been established, although contact property is directly associated to the device performance. Herein, we introduce graphene-interlayered Ti contacts (graphene/Ti) into large-area MoS₂ device arrays using a wet-transfer method. We achieve MoS₂ devices with superior electrical and photoelectrical properties using graphene/Ti contacts, with a field-effect mobility of 18.3 cm²/V∙s, on/off current ratio of 3 × 10⁷, responsivity of 850 A/W, and detectivity of 2 × 10¹² Jones. This outstanding performance is attributable to a reduction in the Schottky barrier height of the resultant devices, which arises from the decreased work function of graphene induced by the charge transfer from Ti. Our research offers a direction toward large-scale electronic and optoelectronic applications based on 2D materials.     

类石墨烯二硫化钼及其在光电子器件上的应用

由单层或几层二硫化钼构成的类石墨烯二硫化钼(graphene-like MoS₂)是一种具有类似石墨烯结构和性能的新型二维(2D)层状化合物, 近年来以其独特的物理、化学性质而成为新兴的研究热点. 本文综述了近年来类石墨烯二硫化钼常见的几种制备方法, 包括以微机械力剥离、锂离子插层和液相超声法等为主的“自上而下”的剥离法, 以及以高温热分解、水热法等为主的“自下而上”的合成法; 介绍了其常用的结构表征方法, 包括原子力显微镜(AFM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和拉曼光谱等; 概述了类石墨烯二硫化钼的紫外-可见(UV-Vis)吸收、荧光发射等基本光物理性质及其相关机理; 总结了类石墨烯二硫化钼在二次电池、场效应晶体管、传感器、有机电致发光二极管和电存储等光电子器件领域的应用原理及其研究进展, 展望了这类新型二维层状化合物的研究前景.     

Ultrafast Dynamics of Defect-Assisted Carrier Capture in MoS2 Nanodots Investigated by Transient Absorption Spectroscopy

MoS₂ nanodots are emerging as promising semiconductor materials for optoelectronic devices. However, most of the recent attention is focused on the fabrication of MoS₂ nanodots, and the survey for exciton dynamics of MoS₂ nanodots remains less explored. Herein, we use femtosecond transient absorption spectroscopy to investigate the carrier dynamics of MoS₂ nanodots. Our results show that defect-assisted carrier recombination processes are well consistent with the observed dynamics. The photo-excited carriers are captured by defects with at least two different capture rates via Auger scattering. Four processes are deemed to take part in the carrier relaxation. After photoexcitation, carrier cooling occurs instantly within ~0.5 ps. Then most of carriers are fast captured by the defects, and the corresponding time constant increases from ~4.9 ps to ~9.2 ps with increasing pump fluence, which may be interpreted by saturation of the defect states. Next a small quantity of carriers is captured by the other kinds of defects with a relatively slow carrier capture time within ~65 ps. Finally, the remaining small fraction of carriers relaxes via direct interband electron-hole recombination within ~1 ns. Our results may lead to deep insight into the fundamentals of carrier dynamics in MoS₂ nanodots, paving the way for their further applications.     

MoS2 incorporated carbon allotropes (activated carbon,graphene, MWCNT) as electrodes in symmetric supercapacitors

Symmetric supercapacitor devices were fabricated from MoS₂ incorporated carbon allotropes such as activated carbon (AC)/MoS₂, graphene/MoS₂ and MWCNT/MoS₂. The device performance was evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The electrochemical properties of the devices fabricated from carbon allotropes (activated carbon, graphene, MWCNT) were remarkably enhanced to above 50% by the incorporation MoS₂ phases. Out of the three fabricated devices, electrochemical performance of AC/MoS₂ as found to be superior. The specific capacitance and energy density of this device is 216 ​F/g and 6.2 ​Wh/Kg respectively with excellent higher rate capability and longer cyclic durability. The devices fabricated from graphene/MoS₂ and MWCNT/MoS₂ has exhibited a specific capacitance value of 202 ​F/g and 161 ​F/g with an energy density value of 5.68 ​Wh/Kg and 3.95 ​Wh/Kg respectively.     

Antimony anchored in MoS2 nanosheets with N-doped carbon coating to boost potassium storage performance

Potassium ions batteries (PIBs) have been considered as promising energy storage systems owning to potassium rich natural abundances. However, the difficult reaction kinetics and poor cycling of electrode restrict the further development of PIBs. In this work, antimony anchored in MoS₂ nanosheets with N-doped carbon coating (Sb/MoS₂/NCs) are prepared and evaluated as anode for PIBs. In the unique Sb/MoS₂/NCs structure, the volume expansion of Sb particles could be effectively buffered by the around MoS₂ structure. The defects in MoS₂ nanosheets provide more electrochemical reaction sites for sufficient K⁺ insertion/extraction. Furthermore, the N-doped carbon can further accommodate the volume expansion and improve the electronic conductivity of Sb/MoS₂/NCs composites. Due to the above advantages, the Sb/MoS₂/NCs anode delivers a capacity of 235 mAh/g at 50 mA/g after 78 cycles. This work provides a prospective strategy to design advanced anode materials for PIBs using MoS₂ and antimony composites.     

Free‐standing and Flexible MoS2/rGO Paper Electrode for Amperometric Detection of Folic Acid

The development of flexible electrodes is of considerable current interest because of the increasing demand for modern electronics, portable medical products, and compact devices. We report a new type of flexible electrochemical sensor fabricated by integrating graphene and MoS₂ nanosheets. A highly flexible and free‐standing conductive MoS₂ nanosheets/reduced graphene oxide (MoS₂/rGO) paper was prepared by a two‐step process: vacuum filtration and chemical reduction treatment. The MoS₂/graphene oxide (MoS₂/GO) paper obtained by a simple filtration method was transformed into MoS₂/rGO paper after a chemical reduction process. The obtained MoS₂/rGO paper was characterized by scanning electron microscopy, X‐ray diffraction spectroscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, electrochemical impedance spectroscopy. The electrochemical behavior of folic acid (FA) on MoS₂/rGO paper electrode was investigated by cyclic voltammetry and amperometry. Electrochemical experiments indicated that flexible MoS₂/rGO composite paper electrode exhibited excellent electrocatalytic activity toward the FA, which can be attributed to excellent electrical conductivity and high specific surface area of the MoS₂/rGO paper. The resulting biosensor showed highly sensitive amperometric response to FA with a wide linear range.     

Ultrathin MoS2 Nanosheets Supported on N‐doped Carbon Nanoboxes with Enhanced Lithium Storage and Electrocatalytic Properties

Molybdenum disulfide (MoS₂) has received considerable interest for electrochemical energy storage and conversion. In this work, we have designed and synthesized a unique hybrid hollow structure by growing ultrathin MoS₂ nanosheets on N‐doped carbon shells (denoted as C@MoS₂ nanoboxes). The N‐doped carbon shells can greatly improve the conductivity of the hybrid structure and effectively prevent the aggregation of MoS₂ nanosheets. The ultrathin MoS₂ nanosheets could provide more active sites for electrochemical reactions. When evaluated as an anode material for lithium‐ion batteries, these C@MoS₂ nanoboxes show high specific capacity of around 1000 mAh g^?1, excellent cycling stability up to 200 cycles, and superior rate performance. Moreover, they also show enhanced electrocatalytic activity for the electrochemical hydrogen evolution.     

Defect Engineering of Two-Dimensional Molybdenum Disulfide

Two-dimensional (2D) molybdenum disulfide (MoS₂) holds great promise in electronic and optoelectronic applications owing to its unique structure and intriguing properties. The intrinsic defects such as sulfur vacancies (SVs) of MoS₂ nanosheets are found to be detrimental to the device efficiency. To mitigate this problem, functionalization of 2D MoS₂ using thiols has emerged as one of the key strategies for engineering defects. Herein, we demonstrate an approach to controllably engineer the SVs of chemically exfoliated MoS₂ nanosheets using a series of substituted thiophenols in solution. The degree of functionalization can be tuned by varying the electron-withdrawing strength of substituents in thiophenols. We find that the intensity of 2LA(M) peak normalized to A_1g peak strongly correlates to the degree of functionalization. Our results provide a spectroscopic indicator to monitor and quantify the defect engineering process. This method of MoS₂ defect functionalization in solution also benefits the further exploration of defect-free MoS₂ for a wide range of applications.     

High‐Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition‐Metal Dichalcogenide Nanosheets

High‐resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H‐MoS₂ nanosheets, MoS₂, and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.     

High-Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition-Metal Dichalcogenide Nanosheets

High-resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H-MoS₂ nanosheets, MoS₂, and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.     

Nanostructured MoS2 Nanorose/Graphene Nanoplatelet Hybrids for Electrocatalysis

Tailoring and enhancing electrocatalytic activity is of the utmost importance from the viewpoints of sustainable energy and sensing. MoS₂ and graphene show great promise for the electrocatalysis of many reactions. Given that both graphene and MoS₂ are highly anisotropic in nature, with edge planes that are several orders of magnitude more catalytically active than basal planes, a new hybrid material with maximized edge‐plane density to provide efficient electron transfer, high catalytic activity, and conductive cores was engineered. The hybrid material consists of radial MoS₂ nanosheets with a high density of edge planes and unsaturated active sulfur atoms as well as interspersed with conductive graphene nanoplatelets. This hybrid material exhibits excellent activity for the hydrogen evolution reaction and the detection of DNA nucleobases. Such a nanoengineered, nanostructured hybrid material may play a major role in future electrocatalytic devices.     

Water-processable nanocomposite based on polyaniline and 2D molybdenum disulfide for NIR-transparent ambipolar transport layers

Polymer nanocomposite based on stable water-dispersible polyaniline complex with poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PANI–PAMPSA) and 2D molybdenum disulphide (MoS₂) was developed. The nanocomposite layers obtained by drop-casting were characterized by Vis–NIR- and FTIR spectroscopies, as well as by atomic force, transmission electron, and Kelvin-probe microscopies, X-ray diffraction, cyclic voltammetry, Hall effect, and DC-conductivity measurements. It was shown that the preparation procedure allows easy adjusting of MoS₂ content in the nanocomposite resulting in the growth of DC conductivity by up to six times in the case of 20 wt% MoS₂ as compared with the additive-free PANI–PAMPSA complex. FTIR spectroscopy revealed the existence of hydrophobic interactions between PANI–PAMPSA and 2D MoS₂ nanophase, which facilitate interchain electron transfer. Hall effect studies showed that while increasing MoS₂ content in the nanocomposite, a transition occurs from monopolar hole transport, characteristic of PANI–PAMPSA, to ambipolar transport. This feature makes the obtained PANI–PAMPSA/MoS₂ composite a promising material for different optoelectronic devices, in particular tandem solar cells.     

Functional Group-induced p-Doping of MoS2 by Titanium(IV) Bis(ammonium lactato) Dihydroxide Physisorption

P-type doping is of critical importance for the realization of certain high-performance electrical and optoelectronic devices based on molybdenum disulfide (MoS₂). Charge transfer doping is a feasible strategy for tuning the conductance properties via facile treatment. In this work, the electrical properties of few-layer MoS₂ were modulated with titanium(IV) bis(ammonium lactato) dihydroxide molecules (denoted as TALH) via physisorption. The functional groups such as electronegative hydroxyl (−OH) and carboxylate groups (−COO) included in TALH molecules are expected to induce p-doping effect through surface charge transfer when being attached to MoS₂. The p-doping is proved by X-ray photoelectron spectroscopy (XPS) with the downshift of Mo 3d and S 2p peaks. Control experiments and density functional theory calculations validate that the p-type doping mainly originated from the −OH group in TALH, which drew electrons from MoS₂. These results suggest that functional group-mediated p-doping effect show a path to modulate the carrier transition in MoS_2, and enrich the molecule series for device modification.