Defect-rich MoS<Subscript>2</Subscript> nanowall catalyst for efficient hydrogen evolution reaction

Designing efficient electrocatalysts for the hydrogen evolution reaction (HER) has attracted substantial attention owing to the urgent demand for clean energy to face the energy crisis and subsequent environmental issues in the near future.Among the large variety of HER catalysts,molybdenum disulfide (MoS2) has been regarded as the most famous catalyst owing to its abundance,low price,high efficiency,and definite catalytic mechanism.In this study,defect-engineered MoS2 nanowall (NW) catalysts with controllable thickness were fabricated and exhibited a significantly enhanced HER performance.Benefiting from the highly exposed active edge sites and the rough surface accompanied by the robust NW structure,the defect-rich MoS2 NW catalyst with an optimized thickness showed an ultralow onset overpotential of 85 mV,a high current density of 310.6 mA·cm-2 at η =300 mV,and a low potential of 95 mV to drive a 10 mA.cm-2 cathodic current.Additionally,excellent electrochemical stability was realized,making this freestanding NW catalyst a promising candidate for practical water splitting and hydrogen production.     

Hierarchical MoS<Subscript>2</Subscript> intercalated clay hybrid nanosheets with enhanced catalytic activity

Emerging hierarchical MoSR/pillared-montmorillonite (MoS2/PMMT) hybrid nanosheets were successfully prepared through facile in-situ hydrothermal synthesis of MoS2 within the interlayer of cetyltrimethylammonium bromide PMMT,and their catalytic performance was evaluated by the reduction reaction of 4-nitrophenol (4-NP) using NaBH4 as a reductant.Microstructure and morphology characterization indicated that MoS2/PMMT exhibited hybrid-stacked layered structures with an interlayer spacing of 1.29 nm,and the MoS2 nanosheets were intercalated within the montmorillonite (MMT) layers,with most of the edges exposed to the outside.The catalytic activity and stability of MoS2/PMMT were both enhanced by the MMT.With the MoS2/PMMT as the catalyst,the apparent reaction rate constant of the 4-NP reduction was 0.723 min-1 and was maintained at ～0.679 min-1 after five reaction cycles.The structural evolution of MoS2/PMMT and the possible catalysis mechanism for the reduction reaction of 4-NP were investigated.The as-prepared MoS2/PMMT hybrid nanosheets are promising candidates for catalytic application in the water-treatment and biomedical fields.The strategy developed in this study can provide insights for designing hybrid nanosheets with diverse heterogeneous two-dimensional (2D) nanomaterials.     

Ballistic transport in single-layer MoS<Subscript>2</Subscript> piezotronic transistors

Because of the coupling between semiconducting and piezoelectric properties in wurtzite materials, strain-induced piezo-charges can tune the charge transport across the interface or junction, which is referred to as the piezotronic effect. For devices whose dimension is much smaller than the mean free path of carriers (such as a single atomic layer of MoS₂), ballistic transport occurs. In this study, transport in the monolayer MoS₂ piezotronic transistor is studied by presenting analytical solutions for two-dimensional (2D) MoS₂. Furthermore, a numerical simulation for guiding future 2D piezotronic nanodevice design is presented.

Open image in new window

     

Unsaturated-sulfur-rich MoS<Subscript>2</Subscript> nanosheets decorated on free-standing SWNT film: Synthesis,characterization and electrocatalytic application

Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS₂@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS₂ nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm²) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS₂ and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.

Open image in new window

     

MoS<Subscript>2</Subscript> as a long-life host material for potassium ion intercalation

Electrochemical potassium ion intercalation into two-dimensional layered MoS2 was studied for the first time for potential applications in the anode in potassium-based batteries.X-ray diffraction analysis indicated that an intercalated potassium compound,hexagonal K0.4MoS2,formed during the intercalation process.Despite the size of K+,MoS2 was a long-life host for repetitive potassium ion intercalation and de-intercalation with a capacity retention of 97.5％ after 200 cycles.The diffusion coefficient of the K+ ions in KxMoS2 was calculated based on the Randles-Sevcik equation.A higher K+ intercalation ratio not only encountered a much slower K+ diffusion rate in MoS2,but also induced MoS2 reduction.This study shows that metal dichalcogenides are promising potassium anode materials for emerging K-ion,K-O2,and K-S batteries.     

Size-controlled MoS<Subscript>2</Subscript> nanodots supported on reduced graphene oxide for hydrogen evolution reaction and sodium-ion batteries

Transition metal dichalcogenide nanodots (NDs) have received considerable interest.We report a facile bottom-up synthetic route for MoS2 NDs by using molybdenum pentachloride and L-cysteine as precursors in oleylamine.The synthesis of NDs with a narrow size distribution ranging from 2.2 to 5.3 nm,was tailored by controlling the reaction time.Because of its coating characteristics,oleyalmine leads to uniformity and monodispersity of the NDs.Moreover,the NDs synthesized have large specific surface areas providing active sites.Graphene possesses outstanding conductivity.Combining the advantages of the two materials,the 0D/2D material exhibits superior electrochemical performance because of the 2D permeable channels for ion adsorption,energy storage,and conversion.The as-prepared MoS2/rGO (～2.2 nm) showed a stable capacity of 220 mAh·g-1 after 10,000 cycles at the current density of 20 A·g-1.Furthermore,a reversible capacity ～140 mAh·g-1 was obtained at a much higher current density of 40 A·g-1.Additionally,this composite exhibited superior catalytic performance evidenced by a small overpotential (222 mV) to afford 10 mA·cm-2,and a small Tafel slope (59.8 mV·decade-1) with good acid-stability.The facile approach may pave the way for the preparation of NDs with these nanostructures for numerous applications.     

A progressive route for tailoring electrical transport in MoS<Subscript>2</Subscript>

Typically, molybdenum disulfide (MoS₂) synthesized by chemical vapor deposition (CVD) is polycrystalline; as a result, the scattering of charge carriers at grain boundaries can lead to performances lower than those observed in exfoliated single-crystal MoS₂. Until now, the electrical properties of grain boundaries have been indirectly studied without accurate knowledge of their location. Here, we present a technique to measure the electrical behavior of individual grain boundaries in CVD-grown MoS₂, imaged with the help of aligned liquid crystals. Unexpectedly, the electrical conductance decreased by three orders of magnitude for the grain boundaries with the lowest on/off ratio. Our study provides a useful technique to fabricate devices on a single-crystal area, using optimized growth conditions and device geometry. The photoresponse, studied within a MoS₂ single grain, showed that the device responsivity was comparable with that of the exfoliated MoS₂-based photodetectors.

Open image in new window

     

High-concentration dispersions of exfoliated MoS<Subscript>2</Subscript> sheets stabilized by freeze-dried silk fibroin powder

Liquid-phase exfoliation (LPE) is an attractive method for the scaling-up of exfoliated MoS₂ sheets compared to chemical vapor deposition and mechanical cleavage. However, the MoS₂ nanosheet yield from LPE is too small for practical applications. We report a facile method for the scaling-up of exfoliated MoS₂ nanosheets using freeze-dried silk fibroin powders. Compared to MoS₂ dispersion in the absence of silk fibroin powder, sonicated MoS₂ dispersions with silk fibroin powder (MoS₂/Silk dispersion) show noticeably higher exfoliated MoS₂ nanosheet yields, with suspended MoS₂ concentrations in MoS₂/Silk dispersions sonicated for 2 and 5 h of 1.03 and 1.39 mg·mL^–1, respectively. The MoS₂ concentration in the MoS₂/Silk dispersion after centrifugation above 10,000 rpm is more than four times that without the silk fibroin. The size of the dispersed silk fibroin is controlled by the change of centrifugation rate, showing the removal of silk fibroin above tens of micrometers in size after centrifugation at 2,000 rpm. Size-controlled silk fibroin biomolecules combined with MoS₂ nanosheets are expected to increase the practical use of such materials in fields related to tissue engineering, biosensors and electrochemical electrodes. Atomic force microscopy and Raman spectroscopy provide the height of the MoS₂ nanosheets spin-cast from MoS₂ /Silk dispersions, showing thicknesses of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction indicate that the outermost surface layer of the hydrophobic MoS₂ crystals interact with oxygen-containing functional groups that exist in the hydrophobic part of silk fibroins. The amphiphilic properties of silk fibroin combined with the MoS₂ nanosheets stabilize dispersions by enhancing solvent-material interactions. The large quantities of exfoliated MoS₂ nanosheets suspended in the as-synthesized dispersions can be utilized for the fabrication of vapor and electrochemical devices requiring high MoS₂ nanosheets contents.

Open image in new window

     

Probing the intrinsic optical quality of CVD grown MoS<Subscript>2</Subscript>

Optical emission efficiency of two-dimensional layered transition metal dichalcogenides (TMDs) is one of the most important parameters affecting their optoelectronic performance.The optimization of the growth parameters by chemical vapor deposition (CVD) to achieve optoelectronic-grade quality TMDs is,therefore,highly desirable.Here,we present a systematic photoluminescence (PL) spectroscopic approach to assess the intrinsic optical and crystalline quality of CVD grown MoS2 (CVD MoS2).We propose the use of the intensity ratio between the PL measured in air and vacuum as an effective way to monitor the intrinsic optical quality of CVD MoS2.Low-temperature PL measurements are also used to evaluate the structural defects in MoS2,via defect-associated bound exciton emission,which well correlates with the field-effect carrier mobility of MoS2 grown at different temperatures.This work therefore provides a sensitive,noninvasive method to characterize the optical properties of TMDs,allowing the tuning of the growth parameters for the development of optoelectronic devices.     

Thermal conduction across the one-dimensional interface between a MoS<Subscript>2</Subscript> monolayer and metal electrode

The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations.Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene,the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene-metal interface.Each covalent bond at the interface serves as an independent channel for thermal conduction,allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacandes).In addition,different Au surfaces form different bonding configurations,causing large ITC variations.Interestingly,the S vacancies in the central region of MoS2 only slightly affect the ITC,which can be explained by a mismatch of the phonon vibration spectra.Further,at room temperature,ITC is primarily dominated by phonon transport,and electron-phonon coupling plays a negligible role.These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces,but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronicdevices.     

Real-time electrical detection of epidermal skin MoS<Subscript>2</Subscript> biosensor for point-of-care diagnostics

Various approaches have been proposed for point-of-care diagnostics,and in particular,optical detection is preferred because it is relatively simple and fast.At the same time,field-effect transistor (FET)-based biosensors have attracted great attention because they can provide highly sensitive and label-free detection.In this work we present highly sensitive,epidermal skin-type point-of-care devices with system-level integration of flexible MoS2 FET biosensors,read-out circuits,and light-emitting diode (LEDs) that enable real-time detection of prostate cancer antigens (PSA).Regardless of the physical forms or mechanical stress conditions,our proposed high-performance MoS2 biosensors can detect a PSA concentration of 1 pg.mL-1 without specific surface treatment for anti-PSA immobilization on the MoS2 surface on which we characterize and confirm physisorption of anti-PSA using Kelvin probe force microscopy (KPFM)and tapping-mode atomic force microscopy (tm-AFM).Furthermore,current modulation induced by the binding process was stably maintained for longer than 2-3 min.The results indicate that flexible MoS2-based FET biosensors have great potential for point-of-care diagnostics for prostate cancer as well as other biomarkers.     

Ultra-small MoS<Subscript>2</Subscript> nanodots with rapid body clearance for photothermal cancer therapy

The clinical translation of many inorganic nanomaterials is severely hampered by toxicity issues because of the long-term retention of these nanomaterials in the body. In this study, we developed a bio-clearable theranostic agent based on ultra-small MoS₂ nanodots, which were synthesized by a facile bottom-up approach through one-step solvothermal decomposition of ammonium tetrathiomolybdate. After modification by glutathione (GSH), the obtained MoS₂-GSH nanodots exhibited sub-10-nm hydrodynamic diameters without aggregation in various physiological buffers. Without showing appreciable in vitro toxicity, such MoS₂-GSH nanodots with strong near-infrared (NIR) absorbance could induce remarkable photothermal ablation of cancer cells. Upon intravenous (i.v.) injection, efficient tumor accumulation of MoS₂-GSH nanodots was observed by photoacoustic imaging, and further confirmed by analysis of the biodistribution of Mo. Notably, the MoS₂-GSH nanodots, in contrast to conventional MoS₂ nanoflakes with larger sizes, showed rather efficient body clearance via urine, where the majority of the injected dose was cleared within just seven days. Photothermal ablation of tumors on mice was then realized with the MoS₂-GSH nanodots, achieving excellent therapeutic efficacy. This study presents a new type of ultra-small nanoparticle with efficient tumor homing/treatment abilities, as well as rapid body clearance behavior, making it promising for cancer theranostics without long-term toxicity concerns.

Open image in new window

     

TiO<Subscript>2</Subscript> nanosheets with exposed {001} facets for photocatalytic applications

TiO₂ nanosheets with highly reactive {001} facets ({001}-TiO₂) have attracted great attention in the fields of science and technology because of their unique properties. In recent years, many efforts have been made to synthesize {001}-TiO₂ and to explore their applications in photocatalysis. In this review, we summarize the recent progress in preparing {001}-TiO₂ using different techniques such as hydrothermal, solvothermal, alcohothermal, chemical vapor deposition (CVD), and sol gel-based techniques. Furthermore, the enhanced efficiency of {001}-TiO₂ by modification of carbon materials, surface deposition of transition metals, and non-metal doping is reviewed. Then, the applications of {001}-TiO₂-based photocatalysts in the degradation of organic dyes, hydrogen evolution, carbon dioxide (CO₂) reduction, bacterial disinfection, and dye-sensitized solar cells are summarized. We believe this entire review on TiO₂ nanosheets with {001} facets can further inspire researchers in associated fields.

Open image in new window

     

Thickness-dependent morphologies of Ag on <Emphasis Type="Italic">n</Emphasis>-layer MoS<Subscript>2</Subscript> and its surface-enhanced Raman scattering

The size and density of Ag nanoparticles on n-layer MoS₂ exhibit thicknessdependent behavior. The size and density of these particles increased and decreased, respectively, with increasing layer number (n) of n-layer MoS₂. Furthermore, the surface-enhanced Raman scattering (SERS) of Ag on this substrate was observed. The enhancement factor of this scattering varied with the thickness of MoS₂. The mechanisms governing the aforementioned thickness dependences are proposed and discussed.

Open image in new window

     

Multidimensional CdS nanowire/CdIn<Subscript>2</Subscript>S<Subscript>4</Subscript> nanosheet heterostructure for photocatalytic and photoelectrochemical applications

Nanomaterial shapes can have profound effects on material properties,and therefore offer an efficient way to improve the performances of designed materials and devices.The rational fabrication of multidimensional architectures such as one dimensional (1D)-two dimensional (2D) hybrid nanomaterials can integrate the merits of individual components and provide enhanced functionality.However,it is still very challenging to fabricate 1D/2D architectures because of the different growth mechanisms of the nanostructures.Here,we present a new solventmediated,surface reaction-driven growth route for synthesis of CdS nanowire (NW)/CdIn2S4 nanosheet (NS) 1D/2D architectures.The as-obtained CdS NW/CdIn2S4 NS structures exhibit much higher visible-light-responsive photocatalytic activities for water splitting than the individual components.The CdS NW/CdIn2S4 NS heterostructure was further fabricated into photoelectrodes,which achieved a considerable photocurrent density of 2.85 mA.cm-2 at 0 V vs.the reversible hydrogen electrode (RHE) without use of any co-catalysts.This represents one of the best results from a CdS-based photoelectrochemical (PEC) cell.Both the multidimensional nature and type Ⅱ band alignment of the 1D/2D CdS/CdIn2S4 heterostructure contribute to the enhanced photocatalytic and photoelectrochemical activity.The present work not only provides a new strategy for designing multidimensional 1D/2D heterostructures,but also documents the development of highly efficient energy conversion catalysts.     

Photocatalytic reduction of CO<Subscript>2</Subscript> with H<Subscript>2</Subscript>O over modified TiO<Subscript>2</Subscript> nanofibers: Understanding the reduction pathway

Nanosized metal (Pt or Pd)-decorated TiO₂ nanofibers (NFs) were synthesized by a wet impregnation method. CdSe quantum dots (QDs) were then anchored onto the metal-decorated TiO₂ NFs. The photocatalytic performance of these catalysts was tested for activation and reduction of CO₂ under UV-B light. Gas chromatographic analysis indicated the formation of methanol, formic acid, and methyl formate as the primary products. In the absence of CdSe QDs, Pd-decorated TiO₂ NFs were found to exhibit enhanced performance compared to Pt-decorated TiO₂ NFs for methanol production. However, in the presence of CdSe, Pt-decorated TiO₂ NFs exhibited higher selectivity for methanol, typically producing ～90 ppmg^?1·h^?1 methanol. The CO₂ photoreduction mechanism is proposed to take place via a hydrogenation pathway from first principles calculations, which complement the experimental observations.

Open image in new window

     

Controllable synthesis of triangular Ni(HCO<Subscript>3</Subscript>)<Subscript>2</Subscript> nanosheets for supercapacitor

Triangular Ni(HCO₃)₂ nanosheets were synthesized via a template-free solvothermal method. The phase transition and formation mechanism were explored systematically. Further investigation indicated that the reaction time and pH have significant effects on the morphology and size distribution of the triangular Ni(HCO₃)₂ nanosheets. More interestingly, the resulting product had an ultra-thin structure and high specific surface area, which can effectively accelerate the charge transport during charge–discharge processes. As a result, the triangular Ni(HCO₃)₂ nanosheets not only exhibited high specific capacitance (1,797 F·g^-1 at 5 A·g^-1 and 1,060 F·g^-1 at 50 A·g^-1), but also showed excellent cycling stability with a high current density (~80% capacitance retention after 5,000 cycles at the current density of 20 A·g^-1).

Open image in new window

     

Transformation of monolayer MoS<Subscript>2</Subscript> into multiphasic MoTe<Subscript>2</Subscript>: Chalcogen atom-exchange synthesis route

Molybdenum ditelluride (MoTe2),which is an important transition-metal dichalcogenide,has attracted considerable interest owing to its unique properties,such as its small bandgap and large Seebeck coefficient.However,the batch production of monolayer MoTe2 has been rarely reported.In this study,we demonstrate the synthesis of large-domain (edge length exceeding 30 μm),monolayer MoTe2 from chemical vapor deposition-grown monolayer MoS2 using a chalcogen atom-exchange synthesis route.An in-depth investigation of the tellurization process reveals that the substitution of S atoms by Te is prevalently initiated at the edges and grain boundaries of the monolayer MoS2,which differs from the homogeneous selenization of MoS2 flakes with the formation of alloyed Mo-S-Se hybrids.Moreover,we detect a large compressive strain (approximately-10％) in the transformed MoTe2 lattice,which possibly drives the phase transition from 2H to 1T'at the reaction temperature of 500 ℃.This phase change is substantiated by experimental facts and first-principles calculations.This work introduces a novel route for the templated synthesis of two-dimensional layered materials through atom substitutional chemistry and provides a new pathway for engineering the strain and thus the intriguing physics and chemistry.     

Synthesis and structure of multi-layered WS<Subscript>2</Subscript>(CoS), MoS<Subscript>2</Subscript>(Mo) nanocapsules and single-layered WS<Subscript>2</Subscript>(W) nanoparticles

Multi-layered MoS₂ (or WS₂) nanocages stuffed with Mo (or CoS/CoO) nanocrystals have been synthesized by using the reaction between metal nanoparticles and sulfur powders. This simple synthesis method, different from the conventional methods for synthesizing pure inorganic fullerenes, is also potentially important for large-scale synthesis of nanoparticles of other metal dichalcogenide. Besides the multi-layered WS₂ nanocapsules, we have successfully fabricated nanocapsules with a single-layered WS₂ sheet encapsulating W by using the arc-discharge method. We discuss possible mechanisms for the formation of the unique core-shell structured nanocapsules.     

Self-assembly synthesis of LaPO<Subscript>4</Subscript> hierarchical hollow spheres with enhanced photocatalytic CO<Subscript>2</Subscript>-reduction performance

Urchin-like LaPO4 hollow spheres were successfully synthesized by a facile solution route using citric acid (CA) as a structure-directing agent.The size of the three-dimensional (3D) hollow spheres was tuned by changing the concentration of CA.The formation mechanism of the 3D LaPO4 hollow spheres was revealed by studying the time-dependent morphology evolution process.Importantly,compared with monodispersed one-dimensional (1D) LaPO4 nanorods,the 3D LaPO4 hollow spheres self-assembled from nanorods showed a 6.8-fold enhancement in photocatalytic activity for CO2 reduction,which is attributed to the synergistic effect of their hierarchical hollow structure,higher light-harvesting capacity,and faster electron transfer.Our findings provide not only a simple,facile method for the synthesis of hierarchical hollow micro/nanoarchitectures but also an efficient route for enhancing the photocatalytic performance.