Formation Of MoS2 Research Articles

Facile synthesis and defect optimization of 2D-layered\xa0MoS2 on TiO2 heterostructure for industrial effluent, wastewater treatments

Current research is paying much attention to heterojunction nanostructures. Owing to its versatile characteristics such as stimulating morphology, affluent surface-oxygen-vacancies and chemical compositions for enhanced generation of reactive oxygen species. Herein, we report the hydrothermally synthesized TiO2@MoS2 heterojunction nanostructure for the effective production of photoinduced charge carriers to enhance the photocatalytic capability. XRD analysis illustrated the crystalline size of CTAB capped TiO2, MoS2@TiO2 and L-Cysteine capped MoS2@TiO2 as 12.6, 11.7 and 10.2 nm, respectively. The bandgap of the samples analyzed by UV–Visible spectroscopy are 3.57, 3.66 and 3.94 eV. PL spectra of anatase phase titania shows the peaks present at and above 400 nm are ascribed to the defects in the crystalline structure in the form of oxygen vacancies. HRTEM reveals the existence of hexagonal layered MoS2 formation on the spherical shaped TiO2 nanoparticles at the interface. X-ray photoelectron spectroscopy recommends the chemical interactions between MoS2 and TiO2, specifically, oxygen vacancies. In addition, the electrochemical impedance spectroscopy studies observed that L-MT sample performed low charge transfer resistance (336.7 Ω cm2) that promotes the migration of electrons and interfacial charge separation. The photocatalytic performance is evaluated by quantifying the rate of Congo red dye degradation under visible light irradiation, and the decomposition efficiency was found to be 97%. The electron trapping recombination and plausible photocatalytic mechanism are also explored, and the reported work could be an excellent complement for industrial wastewater treatment.

Thin Films of Layered-MoS2: ALD of Molybdenum and Sulfurization a Novel Approach

Thin layers of two dimensional (2D) materials mainly transition metal dichalcogenides (TMDs) and more specifically ultra-thin layered-MoS2 semiconductor possess exceptional properties such as electrical, optical, magnetic, mechanical and chemical properties. This allows the exploration of internal quantum degrees of freedom of electrons and their potential for use in semiconductor microelectronics, optoelectronic, energy, and sensor and detector applications. These exciting results are being achieved mostly by using exfoliation of flecks from bulk MoS2 crystal. However, the biggest challenge in realizing TMDs full potential has been the lack of scalable material synthesis methods for such films with high uniformity, conformality and interfacing with other materials such as oxides, metals and its process compatibility. Among the various thin film deposition methods, atomic layer deposition (ALD) offers the best combination of precisely controlled layer-by-layer thin film growth at low temperature with very high conformality on complex substrates. Here we will present the growth of layered-MoS2 thin films. To grow high quality layered–MoS2 thin films, we have developed an ALD-based two step processing approach: firstly the growth of well controlled ultra-thin layer of Mo metal using ALD followed by the sulfurization of the ALD Mo layer at various temperatures. This two-steps processing results in high quality layered-MoS2 thin films on large substrates. For the Mo ALD process we used molybdenum hexafluoride (MoF6) and Si2H6 precursors. We used in-situ QCM measurements to study interfacial and nucleation effects in the formation of continuous ultra-thin metal layer of Mo. The composition of both the ALD Mo and the layered MoS2 layers was determined by X-ray photoelectron spectroscopy (XPS). Further, cross-sectional transmission electron microscopy (TEM) was performed to confirm the formation of layered MoS2 on high aspect ratio trenches and Raman analysis to verify the signature of E12g blue shift and A1g red shift in the MoS2 structure. Here, we will discuss the details of the two-step thin film growth process for creating layered MoS2 layers via ALD Mo and subsequent sulfurization as well as the properties of the MoS2 films.

Improvement of Cu2ZnSnS4 thin film performance by using oxygen-containing Cu-Zn-Sn precursor

The CZTS thin film fabricated based on Cu-Zn-Sn (CZT) or Cu-Zn-Sn-S precursors have difficulties in making a pinhole-free and flat CZTS thin film. In this paper, Cu2ZnSnS4(CZTS) thin films were fabricated based on oxygen-containing Cu-Zn-Sn precursors. By intentionally introducing O into Cu-Zn-Sn precursors, sudden volume expansion from precursor to CZTS thin films can be avoided and smooth CZTS film surfaces without pinholes were produced. Oxygen-containing Cu-Zn-Sn precursors were prepared by sputtering ZnO, Cu, and Sn targets successfully. MoS2 formation could be suppressed by introducing ZnO as a barrier layer between CZTS absorber and Mo back contact interface during the sulfurizing process under high sulphur pressure. The short circuit current density (Jsc) and open circuit voltage (Voc) of CZTS solar cells could be increased by optimizing the sulfurization temperature. And the best conversion efficiency of CZTS solar cells from oxygen-containing precursors was 4.97 %, with an open circuit voltage of 566 mV, a short circuit current density of 21.44 mA/cm-2 and a fill factor of 41 %.

Selective demethoxylation of guaiacol to alkylphenols in supercritical methanol over a HT-MoS2 catalyst

The high value-added products such as alkyl substituted phenols (A-Ps) are very important from lignin-derived feedstocks. Herein, the selective conversion of guaiacol, an important chemical of lignin-derived bio-oil, to A-Ps over hydrothermally synthesized bulk MoS2 (HT-MoS2) was examined under supercritical methanol conditions without any gaseous hydrogen input. HRTEM confirms the formation of MoS2 with the nanosheet-like structure and XPS analysis exhibited the presence of Mo5+ and S22− species which are likely the active species for demethoxylation (DMO) and alkylation of guaiacol. The possible reaction pathways along with their plausible intermediates of guaiacol conversion are proposed. The HT-MoS2 catalyst can be reused up to 4 catalytic runs without loss of its catalytic activity.

Mo(VI) dithiocarbamate with no pre-existing Mo–S–Mo core as an active lubricant additive

MoDTC molecular additives reduce friction in boundary lubricated steel-steel contacts through formation of MoS2 sheets. The chemical pathway from MoDTC to MoS2 is investigated for optimizing MoS2 formation. Our experiments show that a MoDTC molecule containing sulfur only in its thiocarbamate ligands forms MoS2 sheets during friction, demonstrating that the presence of peripheral thiocarbamate ligands can be sufficient to provide the required sulfur source. This molecule is not only competitive with MoDTC containing sulfur in the core of the molecule but also more efficient at high contact pressures. Mechanistic investigations on the chemical transformation of MoDTC to MoS2 reveal that thermal activation alone is not sufficient thus suggesting that pressure and/or shear are necessary for MoS2 generation in this system.

DFT calculation of hydrothermal mechanism on preparation of MoS2.

Basing on the simplest hydrothermal system containing deionized water, hexa-ammonium molybdate, and thiourea, hydrothermal mechanism on preparation of MoS2 was studied by DFT calculation. Hydrothermal process was divided into four steps which covered ionization equilibrium, the hydrolysis of CS(NH2)2, the formation of intermediates, and the formation of MoS2. Ionization equilibrium occurs at normal condition and determines the existence of Mo in the form of molybdic acid. Thiourea hydrolysis is rate-determining step in the process of hydrothermal which contains 10 elementary reactions. The formation of intermediates includes hydrogen transfer, dehydration, and vulcanization three steps which contain 18 elementary reactions, and the energy barrier of vulcanization is the highest. The formation of MoS2 is divided into two steps, the first step is that MoO(OH)(SH)3.H2O reacts with MoO (SH)4.H2O to form layer MoS2, and the second step is a very fast process that can affect the morphology of the products.

Effects of plasma conditions on sulfurization of MoO3 thin films and surface evolution for formation of MoS2 at low temperatures

Recently, two-dimensional (2D) MoS2 with the superior properties has been extensively investigated as a next-generation semiconductor channel material to electronic and optoelectronic devices. In order to apply MoS2 thin films to the industrial application, it is important not only to secure excellent properties but also to develop a suitable process for their large area synthesis. Among the various kinds of synthesis methods, the plasma sulfurization of transition metals or oxides has been considered a promising way to form 2D transition metal sulfides at low temperatures. However, there have been an insufficient number of systematic studies of surface phenomena during the plasma sulfurization. Therefore, this study examined the effects of the plasma conditions on the sulfurization process to form uniform MoS2 thin films. It was discovered that the introduction of hydrogen to the H2S gas during the plasma period and the thickness of the initially formed MoO3 thin films had important effects on the obtaining of continuous MoS2 thin films. Furthermore, a model of the surface evolution occurring during the plasma sulfurization process was proposed based on the observations. The results of this work could inspire other researchers and assist in the study of the sulfurization of various other oxides films.

Effect of Adventitious Carbon on Pit Formation of Monolayer MoS2.

Forming pits on molybdenum disulfide (MoS2 ) monolayers is desirable for (opto)electrical, catalytic, and biological applications. Thermal oxidation is a potentially scalable method to generate pits on monolayer MoS2 , and pits are assumed to preferentially form around undercoordinated sites, such as sulfur vacancies. However, studies on thermal oxidation of MoS2 monolayers have not considered the effect of adventitious carbon (C) that is ubiquitous and interacts with oxygen at elevated temperatures. Herein, the effect of adventitious C on the pit formation on MoS2 monolayers during thermal oxidation is studied. The in situ environmental transmission electron microscopy measurements herein show that pit formation is preferentially initiated at the interface between adventitious C nanoparticles and MoS2 , rather than only sulfur vacancies. Density functional theory (DFT) calculations reveal that the C/MoS2 interface favors the sequential adsorption of oxygen atoms with facile kinetics. These results illustrate the important role of adventitious C on pit formation on monolayer MoS2 .

Mo Doping Assisting the CVD Synthesis of Size-Controlled, Uniformly Distributed Single-Layer MoS2 on Rutile TiO2(110)

Molybdenum disulfide (MoS2) has attracted considerable interests due to superior electronic and optical properties which have seen promising applications in optoelectronics and catalysis. Chemical vapor deposition (CVD) method has been successfully applied in synthesizing MoS2 on various substrates. However, it remains a great challenge to fabricate high-quality MoS2 sheets with well-controlled micro/nano size and homogeneous distribution over the functional substrates such as active metal oxides. Herein, we have developed a two-step synthetic strategy via depositing MoO3 first followed by subsequent vulcanization, thus to grow single-layer MoS2 on atomically flat rutile TiO2(110) (r-TiO2(110)) substrate. This method not only very well controls the size as well as the special distribution of MoS2 nanosheets over the TiO2 surface, but also averts the formation of contaminative species at the heterojunction while maintaining the atomic structure of the substrate surface. The extensive characterizations reveal that the formation of MoS2 derives from the sulphurization of the singly dispersed Mo6+ and Mo5+ species in the surface/subsurface region instead of the aggregated MoO3 patches on top of the TiO2 surface. Such mechanism may dictate a general way for synthesizing high-quality transition-metal dichalcogenides (TMDs) over a variety of functional substrates.

Effects of (Cr,Al)N and (Cr,Al,Mo)N coatings on friction under minimum quantity lubrication

In today's mobility applications, efficiency is a key requirement. While in conventional mobility, increased efficiency contributes to reduce greenhouse gas emissions, in e-mobility every increase in efficiency translates also directly into increased battery range, which in turn contributes to reduced CO2 emission. Therefore, different approaches like the reduction of lubricant viscosities and the reduction of lubricant quantities towards minimum quantity lubrication are pursued. A promising approach to meet the increasing demands are (Cr,Al)N and triboactive (Cr,Al,Mo)N coatings.Within this work (Cr,Al)N and triboactive (Cr,Al,Mo)N coatings were investigated at a twin-disk test rig in highly-loaded rolling-sliding contacts. The coatings were deposited by means of cathodic arc evaporation (CAE) on the case hardened steel (AISI 5115) and tribologically investigated under minimum quantity (MQL) lubrication with a low viscosity, sulphur and phosphorous doped, polyalphaolefine oil. Compared to the uncoated steel, the coatings lead to a higher system stability, which can be characterized by a stable friction behavior, and wear reduction. Tribochemical interactions between coatings and lubricant were analyzed by means of Raman spectroscopy and reveal the in situ formation of MoS2. Furthermore, the coatings exhibit lower thermal conductivities compared to the uncoated steel, which might also affect friction reduction. The results show the high potential of (Cr,Al)N and triboactive (Cr,Al,Mo)N coatings under high tribological load.

Improved electrochemical performance of interface modified MoS2/CNT nano-hybrid and understanding of its lithiation/delithiation mechanism

Efficient interfacial interaction between MoS2 nanosheets and functionalized carbon nanotube (CNT) has been achieved via adopting an engineered, but facile hydrothermal process using ammonium hepta molybdate and thiourea as precursors. The importance of the route lies in enabling strong and easy coupling between Mo7O246− anion and functionalized CNT at low pH, since the zeta potential of CNT has been found to be positive at low pH. The strong interfacial interaction between MoS2 and CNT enhances the overall electronic conductivity and structural stability which leads to better electrochemical performance. The as-developed composite, with just ∼8% carbon, possesses high Li-storage capacity, excellent rate capability and good cycle-life. Impedance spectra indicate least charge transfer resistance in the case of CNT-containing composites, with post-cycling Scanning Electron Microscopic (SEM) images confirms maintenance of integrity after repeated cycling. The cyclic voltammograms, along with x-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS), confirm that during lithiation step, MoS2 gets converted to Li2S and Mo via formation of LixMoS2 (0 < x ≤ 1). The detection of formation of lithiation MoS2 (LixMoS2) is very important as the reaction processes through this intermediate. The as-formed Mo does not transform back completely to MoS2 during delithiation step, with only Li2S gets delithiated to form elemental sulfur. In subsequent cycles, it is elemental sulfur which primarily gets lithiated and delithiated.

Structural, morphological, optical, and photocatalytic properties of Ag-doped MoS2 nanoparticles

Nanoparticles of Ag doped molybdenum disulfide (Ag–MoS2) were synthesized by hydrothermal process. The structural, morphological, and optical characterisics of these nanoparticles were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and photoluminescence spectroscopy (PL), respectively. Raman spectra confirmed the formation of MoS2 and Ag-doped MoS2 nanoparticles. SEM images indicated that sheet-like structure is prepared in undoped MoS2, while Ag–MoS2 has a particle-like structure. PL peaks are observed at 600 nm (2.06 eV), 610 nm (2.03 eV), 700 nm (1.77 eV), 725 nm (1.71 eV), and 730 nm (1.69 eV) for pure, 1% and 2% Ag– MoS2. When the doping amount of Ag is increased further (i.e., to 3% Ag-doped MoS2), new peaks at 636 nm (1.9 eV) and 663 nm (1.87 eV) are observed due to an increase in structural defects. The photocatalytic activity (PCA) of undoped and Ag–MoS2 is estimated by monitoring the degradation of rhodamine blue (RhB) and methylene blue (MB) dyes. The PCA of Ag–MoS2 samples are greater than that of undoped MoS2 when irradiated with visible light, which is attributed to increased electron–hole pair separation.

Ordered mesoporous carbon provoked dimensionally varied molybdenum dichalcogenide: A striking sensing matrix for electrochemical detection of hydrazine

Rational design and construction of two-dimensional transition metal dichalcogenides (TMDs) with minimal size are vital for improving the sensitivity and selectivity of the electrochemical sensors. Here we demonstrated a vast surface triggered size reduced MoS2 formation on ordered mesoporous carbon (OMC) support. The OMC support material was synthesized through hard templating approach using mesoporous silica SBA-15. Subsequently, the MoS2-OMC nanocomposite material was obtained through hydrothermal route. The structural properties of the prepared materials prove the existence of a well-defined mesoporous nature of carbon and flower-like MoS2 on the surface of OMC. The developed MoS2-OMC composite was deployed as a sensing matrix for the detection of hydrazine in neutral medium. The electrochemical results illustrated that the prepared composite materials exhibited excellent activity towards hydrazine detection with a wide linear detection range (1–9 µM) and very low detection limit of 0.4 µM with a sensitivity of 0.25 µA/µM respectively.

Chemical vapor deposition of molybdenum disulphide on platinum foil

Transition metal dichalcogenides (TMDCs) of group -VIB (MX2, M = Mo, W; X = S, Se, Te, etc.) with sizeable bandgap exhibit different interesting physical and chemical properties which open up new avenues to their technological applications. However, refinement and expansion of growth of TMDCs are required to realize commercializable products and to bring these technological applications to consumer's market. Here, we present growth of MoS2 catalyst by ambient pressure chemical vapor deposition (CVD). A Pt foil substrate was used to grow MoS2 TMDC from the reaction at high temperature (~800 °C) between molybdenum trioxide (MoO3) and sulfur (S) in the presence of argon flow. The area coverage of 4 cm2 of MoS2 layers was achieved on the Pt substrate, which is promising for application requiring large area coverage. The Pt foil was reusable in subsequent growth experiments although there was signature of surface sulphurization that occurred due to elevated temperature needed in the growth process. To confirm the growth of MoS2, Raman spectroscopy was performed. The frequency difference between the featured modes (E12g at 384.7 cm−1 and A1g at 409.3 cm−1) of MoS2 was 24.6 cm−1, which confirmed the multilayer growth of MoS2 on Pt surface. The chemical composition of the as-grown MoS2 film was obtained by X-ray photoelectron spectroscopy (XPS): Mo 3d and S 2s doublets were observed which supported MoS2 formation.

Room-Temperature Patterning of Nanoscale MoS2 under an Electron Beam.

Molybdenum disulfide (MoS2) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the shape and size of MoS2 crystals that can be patterned precisely at required positions. Here, we describe a method of direct nanoscale patterning of MoS2 at room temperature by exposing a molybdenum thiocubane single-source precursor to a beam of electrons. Molybdenum thiocubanes with various alkylxanthate moieties [Mo4S4(ROCS2)6, where R = alkyl] were prepared using a "self-assembly" approach. Micro-Raman and micro-FTIR spectroscopic studies suggest that exposure to a relatively smaller dose of electrons results in the breakdown of xanthate moieties, leading to the formation of MoS2. High-resolution transmission electron micrographs suggest that the growth of MoS2 most likely happens along (100) planes. An electron-beam-induced chemical transformation of a molybdenum thiocubane resist was exploited to fabricate sub-10 nm MoS2 lines and dense dots as small as 13 nm with a pitch of 33 nm. Since this technique does not require the liftoff and etching steps, patterning of MoS2 with interesting shapes, sizes, and thicknesses potentially leading to tunable band gap is possible.

Mechanistic and Experimental Study of the Formation of MoS2/HKUST-1 Core–Shell Composites on MoS2 Quantum Dots with an Enhanced CO2 Adsorption Capacity

In this study, a new “one-pot” synthesis method was developed for the fabrication of HKUST-1 on MoS2 quantum dots (QDs) to form a core–shell structure. It is proved that the HKUST-1 self-assembles ...

3D hierarchical microspheres constructed by ultrathin MoS2-C nanosheets as high-performance anode material for sodium-ion batteries

MoS2/C composites are considered to have great application potential in sodium-ion batteries (SIBs). It is a challenging and meaningful subject that developing high-performance anode materials via combining MoS2 and carbon effectively to give free rein to their advantages in sodium ion storage. In this work, a novel MoS2-C material was designed by using cellulose nanocrystals (CNCs) as low-cost and green carbon source. 3D hierarchical microspheres (200–250 nm) constructed by ultrathin MoS2-C nanosheets were synthesized by synchronizing the pre-carbonization of CNCs with the formation of MoS2 in hydrothermal reaction and subsequent pyrolysis process. It is found that the ultrathin MoS2-C nanosheets were composed of CNCs-derived short-range ordered carbon and few-layered MoS2. Benefiting from the unique structure and robust combination of MoS2 and CNCs-derived carbon, the ultrathin MoS2-C nanosheets composite was proved to have excellent cycling stability and superior rate performance in sodium-ion half-cell test and have high first reversible specific capacity of 397.9 mAh/g in full-cell test. This work provides a significant and effective pathway to prepare MoS2-C materials with excellent electrochemical performance for the application in large-scale energy storage systems.

In-Situ Formation of MoS2 and WS2 Tribofilms by the Synergy Between Transition Metal Oxide Nanoparticles and Sulphur-Containing Oil Additives

This works investigates the in-situ formation of MoS2 and WS2 tribofilms by the synergy between transition metal oxide nanoparticles and conventional sulphur-containing anti-wear and extreme pressure additives. The formation of these low friction tribofilms can be obtained under reciprocating sliding contact and under extreme pressure conditions, as evidenced using X-ray photoelectron spectroscopy. Under reciprocating sliding conditions, the synergy between transition metal oxide nanoparticles and the ZDDP leads to coefficients of friction around 0.06 before they rise as consequence of oxidation. The synergy is more outstanding in extreme pressure conditions, particularly for MoO3 nanotubes combined with extreme pressure additive. This combination outperforms base oil mixtures containing EP additive or MoS2 nanotubes. While MoS2 nanotubes build superb extreme pressure tribofilms containing iron and molybdenum oxides and sulphides, MoO3 nanotubes are able to build similar tribofilms that can continuously re-sulphurize in the presence of the extreme pressure additive. Despite having a similar chemistry, MoO3 nanotubes are observed to sulphurize more easily when compared to WO3 nanoparticles. The work highlights the tribological potential of these nanoparticles otherwise typically used as precursors for the synthesis of transition metal dichalcogenide nanoparticles.

Effect of Adding TiO2 Nanoparticles to a Lubricant Containing MoDTC on the Tribological Behavior of Steel/Steel Contacts Under Boundary Lubrication Conditions

The tribological performance of lubricants containing TiO2 nanoparticles and Molybdenum DiThioCarbamate (MoDTC) was investigated using a reciprocating ball-on-flat tribometer in steel–steel contacts. Lubricants containing only MoDTC were used for comparison. The influences of the additive concentration (0.1 wt% and 0.5 wt%) and of the roughness of the counterparts (Ra from 10 to 200 nm) on the performance of the lubricant were studied. Improved friction modification and anti-wear properties were found when TiO2 nanoparticles were blended with MoDTC compared to MoDTC alone, even at low concentration and with rough surfaces. XPS characterizations and FIB-TEM analyses of tribofilms were performed and suggested that the formation of MoS2 from MoDTC is favored in the presence of TiO2 nanoparticles. The results are discussed, taking into account the tribocatalytic properties of TiO2 nanoparticles.

Effect of Cd source on photocatalytic H2 evolution over CdS/MoS2 composites synthesised via a one-pot hydrothermal strategy

As a prototypical non-noble metal photocatalytic composite, CdS/MoS2 has attracted increasing attention for its efficient visible-light H2 evolution. Owing to the simplicity and high reliability, the one-pot hydrothermal strategy has been widely used for CdS/MoS2 synthesis. However, Cd source effect on the hydrothermal synthesis of CdS/MoS2 composites has rarely been investigated. In this study, CdS/MoS2 is synthesised by the one-pot hydrothermal strategy using different Cd sources (CdCl2, Cd(Ac)2, Cd(NO3)2, and CdSO4), and the corresponding visible-light photocatalytic performances of resulting composites are evaluated by hydrogen evolution experiments. The results reveal that Cd source strongly influences the crystallisation, morphology, MoS2 loading, and visible-light absorption of the resulting CdS/MoS2 composites. The CdS/MoS2 composite synthesised using Cd(Ac)2 as precursor exhibited the highest hydrogen evolution rate of 19.34 mmol‧g−1‧h−1, which could be attributed to its larger specific surface area, stronger visible light absorption, and inhibited recombination of photogenerated carriers. In contrast, the use of cadmium nitrate as the Cd source hinders the formation of MoS2 because of the oxidation ability of nitrate groups in the hydrothermal process, which results in its low photocatalytic efficiency. We believe that these findings will provide guidance for the one-pot hydrothermal synthesis of CdS/MoS2 or related composites.