Exfoliated MoS2 Nanosheets Research Articles

Tris‐Stabilized MoS2 Nanosheets with Robust Dispersibility and Facile Surface Functionalization

AbstractHere, tris(hydroxymethyl)aminomethane (i.e., Tris) is reported to be an effective protecting agent to stabilize 2D nanomaterials in water for various enhanced applications. As demonstrated, molybdenum sulfide (MoS2) nanosheets are first exfoliated from MoS2 bulk under a low‐energy sonication in the assistance of protein bovine serum albumin (BSA), and then BSA molecules are replaced with Tris to stabilize the exfoliated MoS2 nanosheets. The Tris‐stabilized MoS2 nanosheets have a robust dispersibility to resist long‐term storage (e.g., 2.5 years), high temperature (e.g., 220 °C), and high speed centrifugation (e.g., 12 000 rpm). Furthermore, they also exhibit high biocompatibility and promising photothermal effect, which provide a better performance in photothermal therapy for inhibiting the growth of tumor. In addition, the surface Tris molecules readily introduce the grafting of polydopamine on MoS2 nanosheets to form a universal platform for coating of other functional polymers on nanosheets. Overall, this work not only brings more opportunities to exploit the outstanding applications of 2D nanomaterials, but also suggests a new and effective route to prepare the composite of nanosheets and functional polymers for their synergistic properties.

Gas Sensors Based on Mechanically Exfoliated MoS2 Nanosheets for Room-Temperature NO2 Detection.

The unique properties of MoS2 nanosheets make them a promising candidate for high-performance room temperature gas detection. Herein, few-layer MoS2 nanosheets (FLMN) prepared via mechanical exfoliation are coated on a substrate with interdigital electrodes for room-temperature NO2 detection. Interestingly, compared with other NO2 gas sensors based on MoS2, FLMN gas sensors exhibit high responsivity for room-temperature NO2 detection, and NO2 is easily desorbed from the sensor surface with an ultrafast recovery behavior, with recovery times around 2 s. The high responsivity is related to the fact that the adsorbed NO2 can affect the electron states within the entire material, which is attributed to the very small thickness of the MoS2 nanosheets. First-principles calculations were carried out based on the density functional theory (DFT) to verify that the ultrafast recovery behavior arises from the weak van der Waals binding between NO2 and the MoS2 surface. Our work suggests that FLMN prepared via mechanical exfoliation have a great potential for fabricating high-performance NO2 gas sensors.

A novel phosphorus-containing MoS2 hybrid: Towards improving the fire safety of epoxy resin

Layered molybdenum disulfide (MoS2) has received significant attention due to various applications, such as catalysis, supercapacitors, batteries, and polymer nanocomposites. However, developing highly effective MoS2-based polymer nanocomposites with superior dispersion resulting in improved thermal and flame retardant properties still remains a huge challenge. Herein, a kind of MoS2 hybrid was synthesized by the reaction of exfoliated MoS2 nanosheets with (4-carboxybutyl) triphenylphosphoniumbromide (TPP) via a cost-effective method, where TPP serves as a synergistic agent to further improve the dispersion and flame retardant efficiency of MoS2. Then the TPP-MoS2 hybrid was added to epoxy resin (EP) matrix to improve its fire safety properties. The hybrid was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The TPP-MoS2 hybrid was uniformly dispersed in the matrix, leading to significant improvement of fire safety. After the addition of 2 wt% TPP-MoS2, the char residue of the EP nanocomposites was increased by 46% compared with that of pure EP, while the peak heat release rate (PHRR) and total smoke release (TSR) were reduced by 26% and 21%, respectively. The superior dispersion of the hybrid improved the physical barrier effect of MoS2 nanosheets thereby delaying the diffusion of volatile pyrolysis products and the transmission of heat and oxygen to underlying material.

In situ fabrication of molybdenum disulfide based nanohybrids for reducing fire hazards of epoxy

In this work, magnesium hydroxide (MH) nanoparticles were successfully grown on the surface of molybdenum disulfide (MoS2) nanosheets through hydrothermal method, then the MH-MoS2 hybrids were introduced into epoxy (EP) matrix. X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR) and Transmission electron microscopy (TEM) were performed to observe the structure and morphology of MH-MoS2 hybrids. The MH nanoparticles were tightly attached to exfoliated MoS2 nanosheets, and MH-MoS2 hybrids had well dispersion state in EP matrix. The obtained EP composites displayed lower thermal decomposition rate and higher residual chars compared with those of pristine EP. With the addition of 2 wt% MH-MoS2 hybrids, the peak heat release rate (pHRR) and fire growth rate index (FIGRA) of achieved composites had a substantial decrease at 32% and 39%, respectively, compared with those of pristine EP. Moreover, the peak smoke production rate (pSPR) and the amount of toxic CO yield were reduced by 27% and 38%, respectively. Raman spectra and scanning electron microscopy (SEM) results indicated a successive and compact char layer was formed due to the efficient catalytic charring effect of MH-MoS2 hybrids.

Functional MoS2 nanosheets for precursor and mature microRNA detection in living cells.

Mature microRNAs (miRNAs) are small-sized RNAs cleaved from precursor microRNAs (pre-miRNAs) by the RNase Dicer. Various miRNAs play key regulatory roles in tumorigenesis and metastasis, and are therefore potential diagnostic and prognostic cancer biomarkers. However, detecting miRNAs and pre-miRNAs accurately and selectively in living cells remains a major challenge, as the mature miRNA sequence is also present in its pre-miRNA and current sequence probes exhibit poor gene delivery efficiency. Herein, we report a strategy for selectively and accurately detecting miRNA-21 and pre-miRNA-21 in living cells using functional MoS2 nanosheets (NSs) loaded with rationally engineered molecular beacons (MBs). The exfoliated MoS2 nanosheets (NSs) with a mean lateral diameter of 50-70nm were functionalized with the aptamer AS1411 and polyethylene glycol (MoS2-PEG-AS) to achieve target-cell-specific delivery and to enhance biocompatibility. The large available surface of the MoS2-PEG-AS was loaded with MB probes. The resulting MoS2-PEG-AS/MBs present cancer-cell-targeting ability, good protection properties, good optical stability, and biocompatibility. We demonstrated that the resulting nanoprobes can selectively image miRNA-21 and pre-miRNA-21 in various cell lines by facilitating enhanced fluorescence in the presence of miRNA-21 and pre-miRNA-21. Thus, these MoS2-PEG-AS/MBs are potentially a tool to discriminate between intracellular miRNA and pre-miRNA at different expression levels. Graphical abstract.

New Simultaneous Exfoliation and Doping Process for Generating MX2 Nanosheets for Electrocatalytic Hydrogen Evolution Reaction.

Doping nonmetal atoms into layered transition metal dichalcogenide MX2 structures has emerged as a promising strategy for enhancing their catalytic activities for the hydrogen evolution reaction. In this study, we developed a new and efficient one-step approach that involves simultaneous plasma-induced doping and exfoliating of MX2 bulk into nanosheets-such as MoSe2, WSe2, MoS2, and WS2 nanosheets-within a short time and at a low temperature (ca. 80 °C). Specifically, by utilizing active plasma that is generated with an asymmetric electrical field during the electrochemical reaction at the surface of the submerged cathode tip, we are able to achieve doping of nitrogen atoms, from the electrolytes, into the semiconducting 2H-MX2 structures during their exfoliation process from the bulk states, forming N-doped MX2. We selected N-doped MoS2 nanosheets for demonstrating their catalytic hydrogen evolution potential. We modulated the electronic and transport properties of the MoS2 structure with the synergy of nitrogen doping and exfoliating for enhancing their catalytic activity. We found that the nitrogen concentration of 5.2 atom % at N-doped MoS2 nanosheets have an excellent catalytic hydrogen evolution reaction, where a low overpotential of 164 mV at a current density of 10 mA cm-2 and a small Tafel slope of 71 dec mV-1-much lower than those of exfoliated MoS2 nanosheets (207 mV, 82 dec mV-1) and bulk MoS2 (602 mV, 198 dec mV-1)-as well as an extraordinary long-term stability of >25 h in 0.5 M H2SO4 can be achieved.

Electronic structure and magnetic behaviors of exfoliated MoS2 nanosheets

The correlation of electronic structure and magnetic behaviors of layered molybdenum disulfide (MoS2) nanosheets, mechanically exfoliated from pristine hexagonal crystal (2H-MoS2) have been studied. Raman spectra show the energy difference (ΔE) between two Raman peaks A1g and was about 20.2 cm−1, indicating the formation of mono-/bi-layered MoS2 nanosheets as obtained after mechanical exfoliation from pristine 2H-MoS2. The absence of the reflection peak (0 0 2) in x-ray diffraction patterns confirms the formation of few-layered and mono-/bi-layered MoS2 nanosheets with reduced thickness. Mo 3d and S 2p XPS core level peaks shifted to higher energy with the reduction of the number of layers in exfoliated MoS2. As the number of layers decreased, valence band maximum position increased from 1.11 eV (pristine MoS2) to 1.57 eV (mono-/bi-layered MoS2 nanosheets), whereas the surface work function (Ф) reduced from 4.85 eV (pristine MoS2) to 4.47 eV (mono-/bi-layered MoS2 nanosheets), as observed from UPS (He-I) measurements. UPS (He-II) spectra, as well as VB-PES spectra of mono-/bi-layered MoS2 nanosheets, exhibits an enhanced valence band density of states (DOS) of S 3p —derived states near Fermi level (Ef). Mo LII-edge and S K-edge x-ray absorption near edge structure spectra of mono-/bi-layered MoS2 nanosheets show the splitting of different peaks that cause a noticeable change in their band structure. Magnetic M–H hysteresis loops measurement clearly demonstrates the increase of room temperature ferromagnetism from pristine to mono-/bi-layer MoS2, due to the existence of defects (‘S’-vacancies or defects at the grain boundaries region) and the increase of DOS.

Liquid Phase Exfoliation of MoS2 Nano-sheets and Observation of Resistive Switching Memory in MoS2 Nano-sheets-PVDF-HFP Composite Films

Abstract To realise the true and versatile application potentials of 2-D materials like MoS2, the preparation of mono-layer or few layer nano-sheets holds the key. In the present study, we report the successful exfoliation of bulk MoS2 into few layer nano-sheets by adopting a two-step process of grinding and then two hours of ultrasonication by using a probe sonicator. The yield of exfoliation was 32 mg/10 ml and the exfoliated MoS2 was very stable without any re-stacking for more than one month. Further, the exfoliated MoS2 nano-sheets are added to Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to prepare the nanocomposite thin films. Two-terminal devices are prepared with ITO and aluminium as bottom and top electrodes, respectively on a plastic substrate. Its electrical properties are investigated to observe the electrical bistability. Nanocomposite based devices showed bipolar resistive switching memory. For composite film with 1wt.% of MoS2, resistance switching is observed with SET and RESET voltages at 2.74 V and -0.9 V respectively. Resistance ratio of the order of 104 is achieved between the ON and OFF states.

Nanofibers with MoS2 nanosheets encapsulated in carbon as a binder-free anode for superior lithium storage

One-dimensional MoS2/carbon nanofibers (MoS2-CNFs) were synthesized by electrospinning using exfoliated MoS2 nanosheets and polyacrylonitrile as the precursors. The exfoliated MoS2 nanosheets about 150 nm across were encapsulated in carbon, and the free-standing MoS2/CNF film was easily cut into a flexible tablet that could be directly used as a binder-free anode for lithium storage. The MoS2/CNFs showed a high reversible capacity of 700 mAh g−1 at 100 mA g−1 after 50 cycles, a high rate capacity of 450 mAh g−1 at 1000 mA g−1 after 200 cycles and good cycling stability.

Temperature Dependent Phononic Response of Few Layered MoS2 Nanosheets

From the temperature dependent phononic studies of few layered liquid phase exfoliated MoS2 nanosheets we find that the E12g (in-plane) and A1g (out-of-plane) Raman modes follow red shift with increase in temperature and exhibits non-linear temperature dependence in the entire temperature range (80 to 600 K). The first-order temperature coefficients for E12g and A1g modes are found to be -0.0133 cm-1K-1 and -0.0092 cm-1K-1, respectively. The physical origin of the non-linear temperature dependence is analyzed using an analytical model that includes contribution of the thermal expansion and an-harmonic effects to the lattice potential. Our analysis suggests that the non-linear temperature dependence of E12g and A1g modes mainly originates from the an-harmonic contributions from three-phonon and four-phonon scattering.

Direct Exfoliation of Natural SiO₂-Containing Molybdenite in Isopropanol: A Cost Efficient Solution for Large-Scale Production of MoS₂ Nanosheetes.

The cost-effective exfoliation of layered materials such as transition metal dichalcogenides into mono- or few- layers is of significant interest for various applications. This paper reports the preparation of few-layered MoS2 from natural SiO2-containing molybdenite by exfoliation in isopropanol (IPA) under mild ultrasonic conditions. One- to six-layer MoS2 nanosheets with dimensions in the range of 50-200 nm are obtained. By contrast, MoS2 quantum dots along with nanosheets are produced using N-methyl-pyrrolidone (NMP) and an aqueous solution of poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (P123) as exfoliation solutions. Compared with molybdenite, commercial bulk MoS2 cannot be exfoliated to nanosheets under the same experimental conditions. In the exfoliation process of the mineral, SiO2 associated in molybdenite plays the role of similar superfine ball milling, which significantly enhances the exfoliation efficiency. This work demonstrates that isopropanol can be used to exfoliate natural molybdenite under mild conditions to produce nanosheets, which facilitates the preparation of highly concentrated MoS2 dispersions or MoS2 in powder form due to the volatility of the solvent. Such exfoliated MoS2 nanosheets exhibit excellent photoconductivity under visible light. Hence, the direct mild exfoliation method of unrefined natural molybdenite provides a solution for low-cost and convenient production of few-layered MoS2 which is appealing for industrial applications.

Highly enhanced hydrogen sensing properties of sericin-induced exfoliated MoS2 nanosheets at room temperature

Herein, we report a simple and inexpensive strategy for the scaling-up of highly stabilized MoS2 nanosheets using a bio-waste sericin-assisted liquid phase exfoliation (LPE) method. MoS2 nanosheets (NSs) was also prepared under the same conditions but without sericin for comparison. The as-prepared samples were then identified as MoS2 NSs and sericin-MoS2 NSs. Each sample was drop casted on Si/SiO2 substrates then fabricated with interdigitated electrodes prior to studying their H2 sensing properties. The as-fabricated sericin-MoS2 NSs exhibited better H2 sensing properties at room temperature compared to those of the MoS2 NSs. An outstanding sensor response of 49% was reached at low ppm with an ultra-fast response time (10 s) and recovery time (6 s). This exceptional H2 sensing performance was influenced by the adsorptive coating of sericin on the edges of the MoS2 NSs. The coating of sericin protein forms an electron depletion layer which can interact with metals (Mo and S) through their functional groups (NH2, COOH, OH and SH). Furthermore, such an interaction leads to a reduction of the electron depletion layer of exfoliated MoS2, and thereby enhances the H2 sensitivity, stability and response/recovery time. We expect that the present strategy of sericin-MoS2 combination will satisfy the demand for exfoliated MoS2 based high performance H2 sensors to-date.

Enhancement of sunlight-induced photocatalytic activity of ZnO nanorods by few-layer MoS2 nanosheets

Few-layer exfoliated MoS2 nanosheets were easily composited with ZnO nanorods. It was found that MoS2 nanosheets could enhance the sunlight-induced photocatalytic activity rate of ZnO by 74%. In addition, we showed that under UV-blocked sunlight irradiation, MoS2 weakens the photocatalytic activity rate of ZnO by 33%. Finally, we discussed the mechanisms behind the enhanced (weakened) photocatalytic activity under sunlight (UV-blocked sunlight) irradiation based on the UV-Vis absorption and photoluminescence spectra as well as the potential band diagrams of the ZnO/MoS2 composite.

Scalable Exfoliation of Bulk MoS₂ to Single- and Few-Layers Using Toroidal Taylor Vortices.

The production of a large amount of high-quality transition metal dichalcogenides is critical for their use in industrial applications. Here, we demonstrate the scalable exfoliation of bulk molybdenum disulfide (MoS2) powders into single- or few-layer nanosheets using the Taylor-Couette flow. The toroidal Taylor vortices generated in the Taylor-Couette flow provide efficient mixing and high shear stresses on the surfaces of materials, resulting in a more efficient exfoliation of the layered materials. The bulk MoS2 powders dispersed in N-methyl-2-pyrrolidone (NMP) were exfoliated with the Taylor-Couette flow by varying the process parameters, including the initial concentration of MoS2 in the NMP, rotation speed of the reactor, reaction time, and temperature. With a batch process at an optimal condition, half of the exfoliated MoS2 nanosheets were thinner than ~3 nm, corresponding to single to ~4 layers. The spectroscopic and microscopic analysis revealed that the exfoliated MoS2 nanosheets contained the same quality as the bulk powders without any contamination or modification. Furthermore, the continuous exfoliation of MoS2 was demonstrated by the Taylor-Couette flow reactor, which produced an exfoliated MoS2 solution with a concentration of ~0.102 mg/mL. This technique is a promising way for the scalable production of single- or few-layer MoS2 nanosheets without using hazardous intercalation materials.

Hydrophobic and Electronic Properties of the E‐MoS2 Nanosheets Induced by FAS for the CO2 Electroreduction to Syngas with a Wide Range of CO/H2 Ratios

AbstractThe electrochemical CO2 reduction reaction (CO2RR) to produce CO and H2 (syngas) is a promising method for clean energy, but challenges remain, such as controlling the CO/H2 ratios required for the syngas yield. Herein, hydrophobic exfoliated MoS2 (H‐E‐MoS2) nanosheets are fabricated from bulk MoS2 by a cost‐effective ball‐milling method, followed by decoration with fluorosilane (FAS). H‐E‐MoS2 is a cost‐effective electrocatalyst capable of directly reducing CO2 and H2O for tuneable syngas production with a wide range of CO/H2 ratios (from 1:2 to 4:1). In addition, H‐E‐MoS2 shows a high current density, 61 mA cm−2 at −1.1 V, and the highest CO FE of 81.2% at −0.9 V, which are higher than those of unmodified MoS2. According to density functional theory calculations, FAS decoration on the surface of MoS2 electrode can change the electronic properties of the edge Mo atom, which facilitates the rate‐limiting CO‐desorption step, thus promoting CO2RR. Moreover, the hydrophobic surface of H‐E‐MoS2 depressed the H2 evolution reaction and created abundant three‐phase contact points that provided sufficient CO2. The hydrophobization of the electrode may provide an effective strategy for easily tuning the CO/H2 ratio of syngas in a large range for the direct electroreduction CO2 to syngas with an optimized CO/H2 ratio.

Selective modification of two-dimensional MoS2 nanosheets by polymer grafting

As an emerging two-dimensional (2D) nanomaterial, single layer or a few layers of molybdenum disulfide (MoS2) has drawn significant attention in the past decade. In the present work, we report a strategy in direct creating polymer brushes on MoS2 surfaces via SC bond in the presence of UV light. The modification of MoS2 nanosheets can be achieved bilaterally or homolaterally by performing the UV-grafting polymerization on exfoliated MoS2 nanosheets or a single layer of MoS2 deposited on a silicon substrate. A series of vinyl monomers including methyl methacrylate (MMA), styrene (St) and 2-isopropenyl-2-oxazoline (IPOx) could be applied to this approach, leading to the formation of poly(methyl methacrylate) (PMMA), polystyrene (PS) and poly(2-isopropenyl-2-oxazoline) (PIPOx) brushes. AFM, IR, and XPS characterizations indicate the successful formation of homogeneous brush layers on MoS2 surfaces. The polymer brushes modified MoS2 may found potential applications in photo dynamic therapy and sensing technologies.

Polyaniline: A novel bridge to reduce the fire hazards of epoxy composites

As a graphene-like two dimensional nanomaterial, MoS2 had been considered as promising nanofillers for fabrication of polymer-based materials with high performances. However, its dispersion and functionalization represented a critical challenge. In present work, polyaniline (PANI) coating was successfully grown on the surface of exfoliated MoS2 nanosheets by in situ polymerization method in order to improve the dispersion quality and interfacial interactions between MoS2 and polymer matrix. The obtained PANI-MoS2 hybrids were characterized through XRD, Raman spectrum, FTIR, TGA, XPS, TEM and further introduced into epoxy matrix. It was clearly observed that the addition of a PANI coating not only improved the dispersion of MoS2 in the matrix and the interfacial bonding between MoS2 and epoxy, but also obviously reduced the fire hazards of epoxy. By adding 2 wt% PANI-MoS2, the peak heat release rate and total heat release values of epoxy composites were remarkably reduced by 49% and 22%, respectively, in comparison with those of neat epoxy. In addition, the amount of smoke produced and toxic CO released was inhibited obviously. The well dispersion, labyrinth barrier effect, catalytic charring effect of PANI-MoS2 hybrids and combination effect between PANI and MoS2 were believed to the primary source for the remarkable improvement of fire safety.

Anchoring of ZnO nanoparticles on exfoliated MoS2 nanosheets for enhanced photocatalytic decolorization of methyl red dye

In this work, a facile method has been employed for the decoration of ultrafine ZnO nanoparticles over the exfoliated MoS2 nanosheets (ZnO-MoS2 composite) for the decolorization of methyl red dye under solar light irradiation. Further, the exfoliation of MoS2 nanosheets from bulk MoS2 powder and synthesis of pure ZnO nanoparticles was also demonstrated. Then, the prepared different photocatalyst materials have been characterized using various analytical techniques. The formation of ZnO and exfoliation of MoS2 in the composite has been evidently confirmed by XRD, FT-IR and Raman spectroscopic analysis. HR-TEM analysis clearly revealed the formation of ultrafine ZnO nanoparticles that are anchored on the exfoliated MoS2 nanosheets. In addition, the size of ZnO nanoparticles in the composite was significantly reduced compared to bare ZnO nanoparticles since the presence of MoS2 nanosheets which hinders the growth of ZnO nanoparticles. Due to the synergistic interaction between ZnO nanoparticles and exfoliated MoS2 nanosheets, the prepared composite exhibits superior photocatalytic activity towards the decolorization of methyl red dye up to 89% after 60 min irradiation time compared to pure ZnO nanoparticles (69%) and exfoliated MoS2 nanosheets (55%).

Hierarchical photocatalyst of In2S3 on exfoliated MoS2 nanosheets for enhanced visible-light-driven Aza-Henry reaction

Exploration of appropriate photocatalysts for visible-light-driven organic synthesis is of great importance. Here we design and construct a two-dimensional hierarchical In2S3/MoS2 composite as an excellent and reusable photocatalyst for Aza-Henry reaction. The dahlia-flower-like In2S3 nanostructures are homogeneously grown on both sides of the two-dimensional MoS2 nanosheets via a hydrothermal reaction. The as-prepared two-dimensional hierarchical In2S3/MoS2 composite exhibits higher photocatalytic performance than pure In2S3. The hierarchical heterostructure can enhance the light absorption in the visible region, facilitate the separation of the photo-induced electron–hole pairs, offer rich active sites for photoredox reactions and promote the generation and migration of the O2‐ and h+. Profiting from these compositional and structural features, the two-dimensional hierarchical In2S3/MoS2 composite shows remarkably enhanced photocatalytic performance and good stability.

Two-Dimensional MoS2 Confined Co(OH)2 Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes.

The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS2 confined Co(OH)2 nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS2 nanosheets followed by Co2+ exchange in alkaline media to form MoS2 intercalated with Co(OH)2 nanoparticles (denoted Co-Ex-MoS2), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER activity and excellent stability, with an onset overpotential and Tafel slope as low as 15 mV and 53 mV dec-1, respectively, which are among the best values reported so far for the Pt-free HER in alkaline media. Furthermore, density functional theory calculations show that the cojoint roles of Co(OH)2 nanoparticles and MoS2 nanosheets result in the excellent activity of the Co-Ex-MoS2 electrocatalyst, and the good stability is attributed to the confinement of the Co(OH)2 nanoparticles. This work provides an imporant strategy for designing HER electrocatalysts in alkaline solutions, and can, in principle, be expanded to other materials besides the Co(OH)2 and MoS2 used here.