1T Phase MoS2 Research Articles

Miniaturized high-performance metallic 1T-Phase MoS2 micro-supercapacitors fabricated by temporally shaped femtosecond pulses

The recent development of wearable and portable microelectronic devices requires energy storage devices to be miniaturized; micro-supercapacitors (MSCs), as one of the most outstanding candidates, have great potential in future electronic devices. However, in the miniaturization of MSCs, the maintenance of electrochemical performance remains a key challenge. Herein, this study proposes a simple, one-step, mask-free and high-resolution fabrication method for high-performance 1T MoS2 MSCs in atmosphere. The method involves the direct writing of restacked 1T MoS2 films by a temporally shaped femtosecond laser. Specifically, femtosecond laser pulses are temporally shaped to control the transient electron temperature and material absorption for achieving high-resolution fabrication. Excellent electrode material properties and ultrashort ion transfer distance enable the MSCs to exhibit optimal performances with an ultrahigh power density (14 kW cm−3), ultrahigh energy density (15.6 mWh cm−3) and large areal capacitance (36 mF cm−2). Notably, such miniaturized MSCs in a 100 × 100 μm2 area own superior frequency responses (1221 Hz) and time constant (0.82 ms), which are suitable for AC line filters and other high-power demanded electronic devices. This method successfully solves the problem of maintaining performance in the miniaturization of MSCs, allowing next-generation microelectronic devices to be developed.

High-performance asymmetric supercapacitor based on 1T-MoS2 and MgAl-Layered double hydroxides

In this work, carbon fiber clothes (CFC) supported 1T phase MoS2 anode material is synthesized through a one-pot hydrothermal method. The CFC-MoS2 composites has high specific capacitance of 1485.08 F g−1 at the current density of 2 A g−1, and the 54% of initial capacitance can be remained at the current density reached to 15 A g−1. The 87.5% of initial capacitance can be harvested even at the ceaseless cycled for 10,000 cycles at 4 A g−1. In addition, the MgAl layered double hydroxides and graphene oxide (MgAl-LDH-GO) cathode composite material is synthesized. The MgAl-LDH-GO cathode exhibits high specific capacitance (1208.50 F g−1 at 1 A g−1) and long cycle life (76.9% retention during 10,000 cycles). Finally, we successfully assemble the CFC-MoS2//MgAl-LDH-GO asymmetric supercapacitor. It has high energy density of 326.54 Wh kg−1 at power density of 1500 W kg−1, and the energy density can be remained at 279.56 Wh kg−1 even the power density reach to 12 kW kg−1. This excellent performance satisfied the current demand of the high power and energy density. Thus, the lightweight (<0.25 mg/cm2), ultrathin (<0.8 mm), and the favourable electrochemical performance make it suitable for the next generation energy storage device.

Direct growth of high-content 1T phase MoS2 film by pulsed laser deposition for hydrogen evolution reaction

The modified MoS2-based material, with a higher conductivity and rich active sites, is promising one of a variety of nonprecious-metal electrocatalysts for hydrogen evolution reaction (HER). Here, the bulk MoS2 is exfoliated to form small-size MoS2 clusters by pulse laser, which are diluted in solid sulfur and further to form porous film. The 1T phase MoS2 ratio in the prepared film was modulated by adding the different content of sulfur into MoS2 target. Besides, this addition also has an effect on the pore structure of films. Finally, the obtained high-content 1T phase MoS2 film provides the highly metallic conductivity and more active sites, which results in the more enhanced HER catalytic activity with a lower Tafel slope of 38 mV dec−1, a smaller overpotential of 151 mV at 10 mA cm−2, compared with the pure MoS2 film.

MoS2 confined on graphene by triethanolamine for enhancing electrocatalytic hydrogen evolution performance

The reduction of active sites due to reunion and slow electron transfer rates and low electronegativity greatly reduced the catalytic performance of many two-dimensional materials. In this paper, we synthesized composites for partially reducing graphene oxide and molybdenum disulfide (MoS2@prGO) by one-step hydrothermal method. With the addition of triethanolamine, MoS2 is highly dispersed on the prGO carrier and converted into the 1T phase MoS2 (50.4%). Meanwhile, it helps to increase the electron transfer rate of the MoS2@prGO composites. MoS2@prGO composites presents a high electron cloud density due to the existence of N atoms and prGO, which promotes the occurrence of hydrogen ion conversion hydrogen reaction and decreases the electrocatalytic hydrogen evolution overpotential. MoS2@prGO composites exhibits an overpotential of 263 mV at 10 mA/cm2 and a small Tafel slope of 60 mV/dec. This work is devoted to offer a new prospect and direction for the improvement of electrochemical HER performance.

Insights into the Surface Science, Storage Mechanism and Electrochemical Phase Evolution of MoS2/CNT Hybrid Electrodes for Supercapacitive Application

Lately, layered MoS2 is found to be one of the most researched pseudocapacitive electrode material for Li+ and Na+ ion batteries because of its unique layered structure for initial ion intercalation and de‐intercalation and high conversion chemistry. As far as supercapacitors are considered, the charge storage in nanostructured MoS2 takes place via electric double layer formation, intercalation and de-intercalation pseudocapacitance and redox pseudocapacitance. Recently, a vast majority of studies on layered MoS2, have been focused on designing hierarchical deformed structures instead of regularly ordered crystallites which may accommodate intercalation stress upto some extent while retaining the original structure. In this study, we have synthesized large aspect ratio carbon nanotubes with a high specific surface area and an appropriate superstructure forming a thin layer of molybdenum disulfide nanoflakes hierarchically dispersed on the surface of the CNTs. This novel technique of preparing MoS2@CNT superstructure using a combination of chemical and physical deposition techniques proved to be very efficient in structurally engineering the microstructure of electrode material. Moreover, sputtering as a fabrication technique, provided an opportunity to precisely grow hierarchical structured MoS2 nanoflakes uniformly throughout the matrix, easily scalable and controllable at lengths down to nanometer ranges irrespective of superhydrophobic nature of CVD grown CNTs. The designed hybrid nanostructure form an open porous network with shorter ionic as well as electron diffusion lengths and provides larger number of intercalation sites. The super capacitive measurements of the MoS2@CNT electrode revealed capacity of 337 mF/cm2 at the sweep rate of 5 mV/s. The storage mechanism involves both electrostatic ionic adsorption as well as Na+ intercalation into the MoS2@CNT matrix. The electrochemical performance of the hybrid is far more superior due to the contribution from both faradaic and non-faradaic processes. Also, the cyclic retention have been immensely improved as compared to bare CNT and MoS2 and has shown a retention of 97.6% after 3500 cycles. Further, to elucidate the charge storage mechanism Trasatti’s and Dunn’s analysis are performed which suggest that pseudocapacitive phenomenon is dominantly responsible for superior charge storage in the hybrid. The electrodes subjected to 10, 50, 1000, 2000 and 3500 charging discharging cycles at 100mV/s were then analyzed using X-ray photoelectron spectrophotometer to understand the change in surface science of the material and reason behind cyclic fading. The phase transformation of electrode suggests that the binding energies of MoS2 shits towards shifts towards lower energies after cycling. This shift indicates partial transition of 2H→1T MoS2 phase due to the transverse gliding of S plane upon the intercalation of electrolytic ions. Additionally, we observed the formation of thin MoO3 oxide layer on the surface of electrode which may be due to the presence of dangling bonds generated by ionic intercalation. Thus, this thin insulating layer may be the reason of capacitive fading after 3500 cycles of constant charging and discharging. The all solid state supercapacitor built using symmetric MoS2@CNT electrodes was found to have high areal and volumetric capacitance of 131 mF/cm2 and 2.9 F/cm3 respectively, with high cyclic stability of 97.6% after 2500 cycles. Figure 1

The Origin of High Activity of Amorphous MoS2 in the Hydrogen Evolution Reaction.

Molybdenum disulfide (MoS2) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well‐controlled 2H and Li‐promoted 1T phases of MoS2, as well as an amorphous MoS2 phase, have been investigated and a detailed comparison is made on Mo−S and Mo−Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS2 phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS2 is caused by Li+ intercalation and disappears under harsh HER conditions, amorphous MoS2 maintains its intrinsic short Mo−Mo bond feature and, with that, its high HER activity. Quantum‐chemical calculations indicate similar electronic structures of small MoS2 clusters serving as models for amorphous MoS2 and the 1T phase MoS2, showing similar Gibbs free energies for hydrogen adsorption (ΔG H*) and metallic character.

Plasmon-Free Surface-Enhanced Raman Spectroscopy Using Metallic 2D Materials

Two dimensional (2D) materials-based plasmon-free surface-enhanced Raman scattering (SERS) is an emerging field in nondestructive analysis. However, impeded by the low density of state (DOS), an inferior detection sensitivity is frequently encountered due to the low enhancement factor of most 2D materials. Metallic transition-metal dichalcogenides (TMDs) could be ideal plasmon-free SERS substrates because of their abundant DOS near the Fermi level. However, the absence of controllable synthesis of metallic 2D TMDs has hindered their study as SERS substrates. Here, we realize controllable synthesis of ultrathin metallic 2D niobium disulfide (NbS2) (<2.5 nm) with large domain size (>160 μm). We have explored the SERS performance of as-obtained NbS2, which shows a detection limit down to 10-14 mol·L-1. The enhancement mechanism was studied in depth by density functional theory, which suggested a strong correlation between the SERS performance and DOS near the Fermi level. NbS2 features the most abundant DOS and strongest binding energy with probe molecules as compared with other 2D materials such as graphene, 1T-phase MoS2, and 2H-phase MoS2. The large DOS increases the intermolecular charge transfer probability and thus induces prominent Raman enhancement. To extend the results to practical applications, the resulting NbS2-based plasmon-free SERS substrates were applied for distinguishing different types of red wines.

Novel preparation of high activity 1T-phase MoS2 ultra-thin flakes by layered double hydroxide for enhanced hydrogen evolution performance

In this study, 1T phase molybdenum disulfide Ultra-thin Flakes (1T-MoS2 UFs) with high HER activity was successfully prepared by spatial confinement of layered double hydroxides (LDHs). The growth of MoS2 tends to be thin layer and high 1T phase due to the binding force of LDHs, this novel strategy is different from other phase transition methods, like alkali metal ion intercalation, electron irradiation, and stress induction, etc. The 1T-MoS2 UFs were used as hydrogen evolution reaction (HER) catalysts, when current density is 10 mA/cm2, the potential (vs. RHE) is 254 mV and the Tafel slope is only 64 mV/dec under the optimal condition. The superior HER activity is due to highly active 1T phase and rich defects.

Highly defective 1T-MoS2 nanosheets on 3D reduced graphene oxide networks for supercapacitors

Highly defective molybdenum disulfide (MoS2) nanosheets of 1T phase are synthesized on a three-dimensional reduced graphene oxide (3D RGO) network through a one-pot hydrothermal method. By switching the reaction time, the resulting MoS2's phases (1T or 2H) and defect-density are successfully controlled. Reducing the reaction time from 20 to 6 h increases the defect-density and induces a phase transition from hybrid 2H/1T to 1T phase. High defect-density coupled with high 1T-phase portion in MoS2 nanosheets/RGO heterostructures enhances the electrochemical performance for supercapacitor application by inducing additional active sites, which provide fast charge transfer rate and a large number of ion diffusion channels. Among all samples, the defect rich 1T-phase MoS2 nanosheets/RGO heterostructure synthesized in 6 h (M/RGO-6) gives an outstanding specific capacitance of 442.0 F g−1 at a current density of 1 A g−1. The M/RGO-6 also exhibits excellent cycling stability with capacitance retention of 90.3% over 1000 cycles at 5 A g−1. These make M/RGO-6 attractive to the electrode of high-performance supercapacitors.

3D flower-like defected MoS2 magnetron-sputtered on candle soot for enhanced hydrogen evolution reaction

Exploring highly efficient, cost-effective and stable electrocatalysts for hydrogen evolution reaction (HER) based on naturally abundant elements is of great interest but still a challenge. As is generally known, molybdenum disulfide (MoS2) represents a promising catalyst to replace the state of the-art platinum (Pt) for HER, but an effective way of applying it is yet unknown due to a significant resistance between basal planes. Herein, for the first time, a novel nanostructure with three-dimensional (3D) flower-like defected MoS2 nanosheets grown directly on candle soot by magnetron sputtering with Ni foam as a supporter was developed, and it displayed excellent catalytic activity and stability. On the one hand, the bombardment of Ar ions in the deposition system could effectively etch the inert basal plane, leading to the formation of defects and steps to increase the active edge sites. On the other hand, the resistance of MoS2-based electrocatalysts decreased due to the unique topographic structure and the existence of 1T-MoS2 phase, and also the composite was superhydrophilic which is beneficial to the contact of the solid-liquid interface. The as-prepared 3D MoS2/candle soot/Ni foam (NF + CS + MoS2) composite has an overpotential of 56 mV at the current density of 10 mA cm−2 and a small Tafel slope of 49 mV decade-1. This present work provides new insight into the large-scale production of chemically active MoS2-based catalysts for HER.

Confining Free Radicals in Close Vicinity to Contaminants Enables Ultrafast Fenton-like Processes in the Interspacing of MoS2 Membranes.

Heterogenous Fenton-like reactions are frequently proposed for treating persistent pollutants through the generation of reactive radicals. Despite great efforts to optimize catalyst activity, their broad application in practical settings has been restricted by the low efficiency of hydrogen peroxide or persulfate decomposition as well as ultrafast self-quenching of the activated radicals. Theoretical calculations predicted that two-dimensional (2D) metallic 1T phase MoS2 materials with exposed (001) surfaces and (100) edges should have remarkable affinity towards crucial intermediates in the peroxymonosulfate (PMS) activation process. X-ray photoelectron spectroscopy and in situ Raman spectroscopy were used to show that the exposed metallic Mo sites accelerate the rate-limiting step of electron transfer. A lamellar membrane made from a stack of 2D MoS2 with tunable interspacing was then designed as the catalyst. The non-linear transport between the MoS2 nanolayers leads to high water diffusivity so that the short-lived reactive radicals efficiently oxidize contaminants.

Confining Free Radicals in Close Vicinity to Contaminants Enables Ultrafast Fenton‐like Processes in the Interspacing of MoS2 Membranes

AbstractHeterogenous Fenton‐like reactions are frequently proposed for treating persistent pollutants through the generation of reactive radicals. Despite great efforts to optimize catalyst activity, their broad application in practical settings has been restricted by the low efficiency of hydrogen peroxide or persulfate decomposition as well as ultrafast self‐quenching of the activated radicals. Theoretical calculations predicted that two‐dimensional (2D) metallic 1T phase MoS2 materials with exposed (001) surfaces and (100) edges should have remarkable affinity towards crucial intermediates in the peroxymonosulfate (PMS) activation process. X‐ray photoelectron spectroscopy and in situ Raman spectroscopy were used to show that the exposed metallic Mo sites accelerate the rate‐limiting step of electron transfer. A lamellar membrane made from a stack of 2D MoS2 with tunable interspacing was then designed as the catalyst. The non‐linear transport between the MoS2 nanolayers leads to high water diffusivity so that the short‐lived reactive radicals efficiently oxidize contaminants.

Metallic 1T phase MoS2/MnO composites with improved cyclability for lithium-ion battery anodes

Transition metal oxides, such as manganese monoxide (MnO), have the potential to provide higher reversible capacities via conversion reactions than carbon-based anodes. However, one issue hindering MnO's use in lithium ion batteries is its low electrical conductivity. Here, we fabricate a metallic 1T phase MoS2 by molten lithium salts assisted method and describe a simple and effective ball-milling strategy to form a composite lithium ion battery anode consisting of two-dimensional metallic 1T phase MoS2 single crystal nanosheets and MnO nanoparticles. The nanocomposite electrode initially delivers 729.8 mAh g−1 and 493.7 mAh g−1 at 0.5 C and 10 C, respectively. At 2000th cycled at 0.5 C, charge/discharge capacities of electrode delivered 584.8 mAh g−1 and 589.6 mAh g−1. Even at the 2020th cycle, the charge/discharge capacities of electrode present 583.9 mAh g−1 and 588.2 mAh g−1, respectively, which still are more than 80% retention of the initial charge/discharge capacities. The good rate performance could be attributed to the improved electrical conductivity provided by the 1T MoS2. In the composite, there appears to the formation of Mo–O bonds between the 1T MoS2 and MnO. Such bonding may be the reason for the cyclability.

Intercalation of Solvated Magnesium-Ions into Layered Transition Metal Sulfide for Fast Mg Storage

Rechargeable magnesium (Mg) batteries (RMBs) are potential candidates for new generation battery technologies due to the attractive properties of Mg as a metal anode, especially the intrinsic nature of homogeneous Mg deposition and abundant Mg reserves. Compared to the monovalent counterpart (e.g. Li+), the divalent Mg ions have a greater charge density and thus need to overcome much stronger electrostatic forces for insertion and diffusion inside the host structures. One of the strategies to enhance Mg2+ ion intercalation kinetics is to reduce its charge density. Herein, we present the intercalation chemistry of the solvated ions [Mg(DME)3]2+, in which the improved ion mobility has been proven by various electrochemical and spectroscopic characterizations.1 The novel concept was realized with Mg[B(hfip)4]2/DME electrolyte2, 3 and a layered MoS2 cathode coupled with a Mg anode. The cell configuration exhibited a long-term cycling stability (>200 cycles) and high rate capability (47 mA h g-1 at 0.5 A g-1). In addition, the mechanism of Mg storage has been investigated in detail, revealing the reversible 2H to 1T MoS2 phase transition during Mg ions de-/intercalation. Compared to other reported intercalation with [Mg(H2O)x]2+ or MgCl-,4, 5 the DME molecules will neither passivate the anode nor corrode the cell components, which makes this approach more practical. Moreover, the concept of solvated Mg2+ ions can be potentially extended to other intercalation cathode materials and applied to other multivalent ions intercalation. References Z. Li, X. Mu, Z. Zhao-Karger, T. Diemant, R. J. Behm, C. Kübel and M. Fichtner, Nature Communications, 2018, 9, 5115.Z. Zhao-Karger, M. E. Gil Bardaji, O. Fuhr and M. Fichtner, J. Mater. Chem. A, 2017, 5, 10815-10820.Z. Zhao-Karger, R. Liu, W. Dai, Z. Li, T. Diemant, B. P. Vinayan, C. Bonatto Minella, X. Yu, A. Manthiram, R. J. Behm, M. Ruben and M. Fichtner, ACS Energy Letters, 2018, 3, 2005-2013.K. W. Nam, S. Kim, S. Lee, M. Salama, I. Shterenberg, Y. Gofer, J.-S. Kim, E. Yang, C. S. Park, J.-S. Kim, S.-S. Lee, W.-S. Chang, S.-G. Doo, Y. N. Jo, Y. Jung, D. Aurbach and J. W. Choi, Nano Letters, 2015, 15, 4071-4079.H. D. Yoo, Y. Liang, H. Dong, J. Lin, H. Wang, Y. Liu, L. Ma, T. Wu, Y. Li, Q. Ru, Y. Jing, Q. An, W. Zhou, J. Guo, J. Lu, S. T. Pantelides, X. Qian and Y. Yao, Nature Communications, 2017, 8, 339.

Ultra-high-aspect-ratio vertically aligned 2D MoS2-1D TiO2 nanobelt heterostructured forests for enhanced photoelectrochemical performance

Vertically aligned one-dimensional (1D) materials offer numerous advantages for photoelectrochemical (PEC) energy conversion due to highly accessible surface area, direct charge transport pathway along the aligned direction, and suppressed electron-hole recombination. Hierarchical 1D-2D materials generate heterogeneous interfaces that can modify intrinsic chemical and physical material properties. Here, we report an ultra-high-aspect-ratio perfectly aligned 2D MoS2-1D TiO2 nanobelts (NBs) heterostructured nanoforest electrode and demonstrate enhanced PEC performance. The vertically aligned nanoforest electrode with an ultra-high aspect ratio of approximately 3000 was achieved using high-voltage electrophoretic deposition (EPD). The thickness of the vertical nanoforest correlated well with the length of the NBs and there was no evidence of vertical stacking. In addition, both semi-conductive 2H phase and metallic 1T phase MoS2 were discovered to form on TiO2 (011) and (001) surfaces, respectively. The nanoforest drastically enhanced light penetration into the interstice of the electrode and increased the probability of photon–semiconductor interaction due to reduced band gap as well as local surface plasmonic resonance (LSPR) evidenced by numerical studies. A remarkable 98% enhancement of the PEC photocurrent was achieved for the MoS2–TiO2 NB forest electrode compared with its un-aligned counterpart.

Rare-Earth Elements Modified 1T Phase Mos2 Synergy with Defects for Enhanced Hydrogen Evolution

Earth-abundant MoS2 has drawn continuous attentions in many fields, especially hydrogen evolution reaction (HER), which has been regarded as an effective technology for renewable energy. However, its activity is limited by edge sites, to the disadvantage of atomic economy. Herein, combination with theoretical calculation, we propose that rare-earth element (Ce, La, and Tb) synergy with defect can optimize bulk site of MoS2 for HER. Furthermore, self-supporting electrode was designed for integrating the optimized activity site, leading to an excellent HER performance. Compared to La, Tb, and pristine MoS2, the Ce-based MoS2 synergy with defect has a better activity with an overpotential of 165 mV to achieve 100 mA cm−2, close to the state-of-the-art noble metal Pt.

One-pot synthesis of porous 1T-phase MoS2 integrated with single-atom Cu doping for enhancing electrocatalytic hydrogen evolution reaction

Molybdenum sulfide (MoS2) has attracted great interest as a promising non-precious-metal catalyst candidate to replace the precious-metal Pt catalysts for the hydrogen evolution reaction (HER). Nevertheless, the catalytic efficiency of MoS2 is significantly restricted by its density of catalytic active sites and inert basal plane. In this work, we have designed a facile one-pot solvothermal method to synthesize porous 1T-MoS2 that is integrated with atomic doping of Cu atoms. The as-prepared Cu@MoS2 sample exhibits enhanced HER performance with a low overpotential of 131 mV at the current density of 10 mA/cm2, a small Tafel slope of 51 mV/dec and as well as a good long-term stability. Enhanced HER performance can be ascribed to the synergistic effect of 1T-MoS2 metallic phase, single atom Cu doping and numerous sulfur vacancies. Theoretical calculations indicates that the adsorption energy of Cu atom on 1T-MoS2 surface (−3.68 eV) is much higher than that on 2H-MoS2 surface (−1.94 eV), moreover, the Cu atom adsorbed on the surface of the 1T-MoS2 has larger charge transfer (−0.38e), which can be contributed to further enhance HER performance of 1T-MoS2.

Glucose-Induced Synthesis of 1T-MoS2 /C Hybrid for High-Rate Lithium-Ion Batteries.

1T phase MoS2 possesses higher conductivity than the 2H phase, which is a key parameter of electrochemical performance for lithium ion batteries (LIBs). Herein, a 1T-MoS2 /C hybrid is successfully synthesized through facile hydrothermal method with a proper glucose additive. The synthesized hybrid material is composed of smaller and fewer-layer 1T-MoS2 nanosheets covered by thin carbon layers with an enlarged interlayer spacing of 0.94 nm. When it is used as an anode material for LIBs, the enlarged interlayer spacing facilitates rapid intercalating and deintercalating of lithium ions and accommodates volume change during cycling. The high intrinsic conductivity of 1T-MoS2 also contributes to a faster transfer of lithium ions and electrons. Moreover, much smaller and fewer-layer nanosheets can shorten the diffusion path of lithium ions and accelerate reaction kinetics, leading to an improved electrochemical performance. It delivers a high initial capacity of 920.6 mAh g-1 at 1 A g-1 and the capacity can maintain 870 mAh g-1 even after 300 cycles, showing a superior cycling stability. The electrode presents a high rate performance as well with a reversible capacity of 600 mAh g-1 at 10 A g-1 . These results show that the 1T-MoS2 /C hybrid shows potential for use in high-performance lithium-ion batteries.

Charge Storage by Electrochemical Reaction of Water Bilayers Absorbed on MoS2 Monolayers

It is well-known that in neutral and acidic aqueous electrolytes, MoS2 monolayers can store charges by adsorption of cations on to the electrode-electrolyte interface as its analog of graphene. Restricted by its low conductivity and the charge storage mechanism, the electrochemical performance of MoS2 monolayer supercapacitor electrode is not satisfactory. It is reported here that water bilayers absorbed on MoS2 monolayers can be involved in charge storage. One proton of each absorbed water molecule can intercalate/de-intercalate the water bilayers during charging/discharging in the alkaline aqueous electrolyte. For two water molecules are present for every Mo atom, the water bilayers can endow MoS2 monolayers an ultrahigh specific capacitance. In this paper, 1T phase MoS2 nanosheets with three monolayers were synthesized by hydrothermal reaction. It presents a specific capacitance of 1120 F g−1 at a current density of 0.5 A g−1 in KOH. As it is assembled with active carbon into a hybrid supercapacitor, the device has an energy density of 31.64 Wh kg−1 at a power density of 425 W kg−1, and gets a specific capacitance retention of 95.4% after 10,000 cycles at 2 A g−1.

Mechanical characterization of phase-changed single-layer MoS2 sheets

Phase transitions in 2D materials, such as MoS2, result in drastic changes in the physical properties of the material. A change in the phase of MoS2 from 2H to 1T transforms it from a semiconductor to conductor, thus broadening its potential applications in electronics. The structural transformation associated with the phase change may significantly affect the mechanical properties. Here we statistically characterized the elastic modulus and fracture strength of phase-changed single-layer MoS2 sheets using the atomic force microscopy (AFM) nanoindentation technique. MoS2 sheets were subjected to lithium intercalation, which transformed them from the 2H phase to the 1T phase. The transformed sheets were again annealed to make a partially phase-changed sample. The elastic modulus and fracture strength of 1T phase MoS2 were about 50% lower than those of 2H phase MoS2. Annealing of the 1T phase MoS2 sheet resulted in a good recovery of the mechanical properties, up to 90% of those of the original 2H phase MoS2. This result was attributed to a mixture of phases in the annealed product, and the ratio of the phases was evaluated spectroscopically. Interestingly, while the fracture strengths differed for the three phases, their average fracture strains were quite similar, suggesting that the overall structure of the material maintained its integrity during the phase transition process.