An unexpected interfacial Mo-rich phase in 2D molybdenum disulfide and 3D gold heterojunctions

The interface engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) and metals has been regarded as a promising strategy to modulate their outstanding electrical and optoelectronic properties. Chemical Vapour Deposition (CVD) is an effective strategy to regulate the contact interface between TMDs and metals via directly growing 2D TMDs on a 3D metal substrate. Nevertheless, the underlying mechanisms of interfacial phase formation and evolution during TMD growth on a metallic substrate are less known. In this work, we found a 2D non-van der Waals (vdW) Mo-rich phase (MoNSN+1) during thermal sulfidation of a Mo-Au surface alloy to molybdenum disulfide (MoS2) in a S-poor environment. Systematic atomic-scale observations reveal that the periodic Mo and S atomic layers are arranged separating from each other in the non-vdW Mo-rich phase, and the Mo-rich phase preferentially nucleates between outmost 2D MoS2 and a 3D nanostructured Au substrate which possesses copious surface steps and kinks. Theoretical calculations demonstrate that the appearance of the Mo-rich phase with a unique metallic nature causes an n-type contact interface with an ultralow transition energy barrier height. This study may help understand the formation mechanism of the interfacial second phase during the epitaxial growth of 2D-TMDs on 3D nanostructured metals, and provide a new approach to tune the Schottky barrier height by the design of the interfacial phase structure at the heterojunction.

Sulfidation of Supported Ni, Mo and NiMo Catalysts Studied by In Situ XAFS

Active sites in Mo-based hydrotreating catalysts are produced by sulfidation. To achieve insights that may enable optimization of the catalysts, this process should be studied in situ. Herein we present a comparative XAFS study where the in situ sulfidation of Mo/δ-Al2O3 and Ni/δ-Al2O3 is compared to that of δ-Al2O3 supported NiMo catalysts with different NiMo ratios. The study also covers the comparison of sulfidation of Ni and Mo using different oxide supports as well as the sulfidation conditions applied in the reactor. The XAFS spectra confirms the oxide phase for all catalysts at the beginning of the sulfidation reaction and their conversion to a sulfidized phase is followed with in situ measurements. Furthermore, it is found that the monometallic catalysts are less readily sulfidized than bimetallic ones, indicating the importance of Ni-Mo interactions for catalyst activation. Mo K-edge XAFS spectra did not show any difference related to the support of the catalyst or the pressure applied during the reaction. Ni K-edge XAFS spectra, however, show a more complete sulfidation of the Ni species in the catalyst when SiO2 is used as a support as compared to the Al2O3. Nevertheless, it is believed that stronger interactions with Al2O3 support prevent sintering of the catalyst which leads to its stabilization. The results contribute to a better understanding of how different parameters affect the formation of the active phase of the NiMo catalysts used in the production of biofuel.

Patterned Growth of Transition Metal Dichalcogenide Monolayers and Multilayers for Electronic and Optoelectronic Device Applications

A simple, large area, and cost-effective soft lithographic method is presented for the patterned growth of high-quality 2D transition metal dichalcogenides (TMDs). Initially, a liquid precursor (Na₂ MoO₄ in an aqueous solution) is patterned on the growth substrate using the micromolding in capillaries technique. Subsequently, a chemical vapor deposition step is employed to convert the precursor patterns to monolayer, few layers, or bulk TMDs, depending on the precursor concentration. The grown patterns are characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and photoluminescence spectroscopy to reveal their morphological, chemical, and optical characteristics. Additionally, electronic and optoelectronic devices are realized using the patterned TMDs and tested for their applicability in field effect transistors and photodetectors. The photodetectors made of MoS₂ line patterns show a very high responsivity of 7674 A W^-1 and external quantum efficiency of 1.49 × 10⁶ %. Furthermore, the multiple grain boundaries present in patterned TMDs enable the fabrication of memtransistor devices. The patterning technique presented here may be applied to many other TMDs and related heterostructures, potentially advancing the fabrication of TMDs-based device arrays.

NaCl-Assisted Chemical Vapor Deposition of Large-Domain Bilayer MoS2 on Soda-Lime Glass

In recent years, two-dimensional molybdenum disulfide (MoS₂) has attracted extensive attention in the application field of next-generation electronics. Compared with single-layer MoS₂, bilayer MoS₂ has higher carrier mobility and has more promising applications for future novel electronic devices. Nevertheless, the large-scale low-cost synthesis of high-quality bilayer MoS₂ still has much room for exploration, requiring further research. In this study, bilayer MoS₂ crystals grown on soda-lime glass substrate by sodium chloride (NaCl)-assisted chemical vapor deposition (CVD) were reported, the growth mechanism of NaCl in CVD of bilayer MoS₂ was analyzed, and the effects of molybdenum trioxide (Mo) mass and growth pressure on the growth of bilayer MoS₂ under the assistance of NaCl were further explored. Through characterization with an optical microscope, atomic force microscopy and Raman analyzer, the domain size of bilayer MoS₂ prepared by NaCl-assisted CVD was shown to reach 214 μm, which is a 4.2X improvement of the domain size of bilayer MoS₂ prepared without NaCl-assisted CVD. Moreover, the bilayer structure accounted for about 85%, which is a 2.1X improvement of bilayer MoS₂ prepared without NaCl-assisted CVD. This study provides a meaningful method for the growth of high-quality bilayer MoS₂, and promotes the large-scale and low-cost applications of CVD MoS₂.

Understanding the Synergistic Oxidation in Dichalcogenides through Electrochemiluminescence Blinking at Millisecond Resolution

The oxidation of transition metal dichalcogenides (TMDCs) has been extensively studied and applied in electronics, optics, and energy sources because of its tunable structure and performance. However, due to the lack of appropriate technology, dynamically observe the oxidation process remains an arduous task. Herein, the synergistic oxidation between edge and basal plane in molybdenum disulfide (MoS₂ ) is observed through electrogenerated chemiluminescence (ECL) blinking with a millisecond resolution. In addition, the ECL method provides a simple, convenient, and quick way to judge structural changes. The transient elevation of the ECL intensity proved the intermittent doping of oxygen at MoS₂ , which generates O-atom active sites. High ECL intensity enhanced from the produced hydroperoxide intermediates eases the monitoring of MoS₂ particles. Further study shows that the formation of sulfur vacancies at MoS₂ , by the edge activation of hydrogen peroxide and the migration of oxygen to the basal plane, is more conducive to oxygen doping that favors the formation of MoOMo as new active sites to induce bursts. The revealing of sulfur vacancy-governed blinking from MoS₂ indicates a complex interaction between oxygen and MoS₂ . The same phenomenon is observed on tungsten disulfide (WS₂ ), which provides new information about the oxidation feature of 2D dichalcogenides.

Correlation between structure, surface defect chemistry and 18O/16O exchange for La2Mo2O9 and La2(MoO4)3

The La₂Mo₂O₉ and La₂(MoO₄)₃ powders were synthesized using a solid-state reaction method and used to prepare dense ceramics. X-ray photoelectron spectroscopy was used to study the chemical composition and charge numbers of the elements in the subsurface area of dense ceramics of lanthanum molybdates. The spectra were measured under an ultra-high vacuum of 7 × 10^-11 atm at 30 °C and 600 °C, and under an oxygen atmosphere at 2 × 10^-3 atm at 600 °C and 825 °C. High resolution spectra for La 3d, Mo 3d and O 1s states were obtained and analyzed. The kinetics of oxygen exchange were considered in the framework of a two-step model including the consecutive steps of dissociative adsorption and the incorporation of oxygen. The oxygen adsorption (r_a) and incorporation (r_i) rates were calculated. Correlations between the oxide surface defect chemistry and the rates of individual oxygen-exchange steps were discussed.

Simultaneous sulfidation of Mo and Co oxides supported on Au(111)

Here we present the results of a study carried out to investigate the simultaneous sulfidation of Co and Mo oxide nanoparticles on Au(111) as a synthesis strategy to prepare a model catalyst for hydrodesulfurization (HDS). We make use of scanning tunneling microscopy and X-ray photoelectron spectroscopy to track the changes in morphology and chemistry during the synthesis of a mixed Mo and Co oxide precursor and the sulfidation thereafter, to the respective sulfides. We investigated the effects of temperature and the duration of sulfidation on the completeness of the sulfidation process. Our study shows that the formation of MoS₂ with the CoMoS edge (the desired model catalyst) is not affected by the time or the temperature of sulfidation. However, the yield of the Co-promoted MoS₂ slabs is limited by the formation of large clusters due to the spreading of Mo and Co oxide phases upon sulfidation. Complete sulfidation of the mixed oxide precursor to Co-promoted MoS₂ can be accelerated by increasing the sulfidation temperature to 730 K due to the thermally activated nature of Mo oxide sulfidation. Thus, we demonstrate that using a mixed Mo and Co oxide precursor as a starting point for the Co-promoted MoS₂ phase for fundamental catalytic studies is a viable strategy.

Sulfurization of MoO3 in the Chemical Vapor Deposition Synthesis of MoS2 Enhanced by an H2S/H2 Mixture

The typical layered transition metal dichalcogenide (TMDC) material, MoS₂, is considered a promising candidate for the next-generation electronic device due to its exceptional physical and chemical properties. In chemical vapor deposition synthesis, the sulfurization of MoO₃ powders is an essential reaction step in which the MoO₃ reactants are converted into MoS₂ products. Recent studies have suggested using an H₂S/H₂ mixture to reduce MoO₃ powders in an effective way. However, reaction mechanisms associated with the sulfurization of MoO₃ by the H₂S/H₂ mixture are yet to be fully understood. Here, we perform quantum molecular dynamics (QMD) simulations to investigate the sulfurization of MoO₃ flakes using two different gaseous environments: pure H₂S precursors and a H₂S/H₂ mixture. Our QMD results reveal that the H₂S/H₂ mixture could effectively reduce and sulfurize the MoO₃ reactants through additional reactions of H₂ and MoO₃, thereby providing valuable input for experimental synthesis of higher-quality TMDC materials.

Sulfidation of MoO3/γ-Al2O3 towards a highly efficient catalyst for CH4 reforming with H2S

The structural evolution of MoO₃/γ-Al₂O₃ during sulfidation and a subsequent CH₄/H₂S reforming reaction is revealed, and the structure–performance relationships are established.

Deep desulfurization of sintering flue gas in iron and steel works based on low-temperature oxidation

The deep desulfurization method of sintering flue gas based on the low-temperature oxidation method is studied. Based on the analysis of the main principle of deep desulfurization of sintering flue gas, a deep desulfurization system of sintering flue gas is constructed, which is composed of an absorption washing unit and a washing solution treatment unit. Sodium hydroxide solution is used as the desulfurizing absorbent to mix with the sintering flue gas entering the reaction tower. Sulfur dioxide in the sintering flue gas reacts with sodium hydroxide to generate sodium sulfite, and sodium sulfite is oxidized to produce sodium sulfate; ozone is produced by ozone generator, nitrogen oxide compounds are oxidized by ozone to generate oxyacid, which is easy to be removed by sodium hydroxide washing solution, and the detergent is the same as that used to remove sulfur dioxide and dust. The experimental results show that the highest desulfurization rate and denitrification rate of the proposed method are 90% and over 22%, and the reaction efficiency and economy are significantly better than that of the comparative method, which shows that the method is reasonable and effective.

Solid-state synthesis of few-layer cobalt-doped MoS2 with CoMoS phase on nitrogen-doped graphene driven by microwave irradiation for hydrogen electrocatalysis

The high catalytic activity of cobalt-doped MoS₂ (Co-MoS₂) observed in several chemical reactions such as hydrogen evolution and hydrodesulfurization, among others, is mainly attributed to the formation of the CoMoS phase, in which Co occupies the edge-sites of MoS₂. Unfortunately, its production represents a challenge due to limited cobalt incorporation and considerable segregation into sulfides and sulfates. We, therefore, developed a fast and efficient solid-state microwave irradiation synthesis process suitable for producing thin Co-MoS₂ flakes (∼3-8 layers) attached on nitrogen-doped reduced graphene oxide. The CoMoS phase is predominant in samples with up to 15 at% of cobalt, and only a slight segregation into cobalt sulfides/sulfates is noticed at larger Co content. The Co-MoS₂ flakes exhibit a large number of defects resulting in wavy sheets with significant variations in interlayer distance. The catalytic performance was investigated by evaluating the activity towards the hydrogen evolution reaction (HER), and a gradual improvement with increased amount of Co was observed, reaching a maximum at 15 at% with an overpotential of 197 mV at -10 mA cm^-2, and a Tafel slope of 61 mV dec^-1. The Co doping had little effect on the HER mechanism, but a reduced onset potential and charge transfer resistance contributed to the improved activity. Our results demonstrate the feasibility of using a rapid microwave irradiation process to produce highly doped Co-MoS₂ with predominant CoMoS phase, excellent HER activity, and operational stability.

Bimetallic Nanoparticles as a Model System for an Industrial NiMo Catalyst

An in-depth understanding of the reaction mechanism is required for the further development of Mo-based catalysts for biobased feedstocks. However, fundamental studies of industrial catalysts are challenging, and simplified systems are often used without direct comparison to their industrial counterparts. Here, we report on size-selected bimetallic NiMo nanoparticles as a candidate for a model catalyst that is directly compared to the industrial system to evaluate their industrial relevance. Both the nanoparticles and industrial supported NiMo catalysts were characterized using surface- and bulk-sensitive techniques. We found that the active Ni and Mo metals in the industrial catalyst are well dispersed and well mixed on the support, and that the interaction between Ni and Mo promotes the reduction of the Mo oxide. We successfully produced 25 nm NiMo alloyed nanoparticles with a narrow size distribution. Characterization of the nanoparticles showed that they have a metallic core with a native oxide shell with a high potential for use as a model system for fundamental studies of hydrotreating catalysts for biobased feedstocks.

Defect Healing in Layered Materials: A Machine Learning-Assisted Characterization of MoS2 Crystal Phases

Monolayer MoS₂ is an outstanding candidate for a next-generation semiconducting material because of its exceptional physical, chemical, and mechanical properties. To make this promising layered material applicable to nanostructured electronic applications, synthesis of a highly crystalline MoS₂ monolayer is vitally important. Among different types of synthesis methods, chemical vapor deposition (CVD) is the most practical way to synthesize few- or mono-layer MoS₂ on the target substrate owing to its simplicity and scalability. However, synthesis of a highly crystalline MoS₂ layer remains elusive. This is because of the number of grains and defects unavoidably generated during CVD synthesis. Here, we perform multimillion-atom reactive molecular dynamics (RMD) simulations to identify an origin of the grain growth, migration, and defect healing process on a CVD-grown MoS₂ monolayer. RMD results reveal that grain boundaries could be successfully repaired by multiple heat treatments. Our work proposes a new way of controlling the grain growth and migration on a CVD-grown MoS₂ monolayer.

Revealing the Role of Gold in the Growth of Two-Dimensional Molybdenum Disulfide by Surface Alloy Formation

The formation of Mo/Au surface alloy during Au-assisted chemical vapor deposition (CVD) of MoS₂ is confirmed by a series of control experiments. A metal-organic chemical vapor deposition (MOCVD) system is adapted to conduct two-dimensional MoS₂ growth in a controlled environment. Sequential injection of Mo and S precursors, which does not yield any MoS₂ on SiO₂ /Si, grows atomically thin MoS₂ on Au, indicating the formation of an alloy phase. Transmission electron microscopy of a cross-section of the specimen confirms the confinement of the alloy phase near the surface only. These results show that the reaction intermediate is the surface alloy, and that the role of Au in the Au-assisted CVD is the formation of an atomically thin reservoir of Mo near the surface. This mechanism is clearly distinguished from that of MOCVD, which does not involve the formation of any alloy phases.

Pulsed laser deposition of single-layer MoS2 on Au(111): from nanosized crystals to large-area films

Molybdenum disulphide (MoS₂) is a promising material for heterogeneous catalysis and novel two-dimensional (2D) optoelectronic devices. In this work, we synthesized single-layer (SL) MoS₂ structures on Au(111) by pulsed laser deposition (PLD) under ultra-high vacuum (UHV) conditions. By controlling the PLD process, we were able to tune the sample morphology from low-coverage SL nanocrystals to large-area SL films uniformly wetting the whole substrate surface. We investigated the obtained MoS₂ structures at the nanometer and atomic scales by means of in situ scanning tunneling microscopy/spectroscopy (STM/STS) measurements, to study the interaction between SL MoS₂ and Au(111)-which for example influences MoS₂ lattice orientation-the structure of point defects and the formation of in-plane MoS₂/Au heterojunctions. Raman spectroscopy, performed ex situ on large-area SL MoS₂, revealed significant modifications of the in-plane E12g and out-of-plane A_1g vibrational modes, possibly related to strain and doping effects. Charge transfer between SL MoS₂ and Au is also likely responsible for the total suppression of excitonic emission, observed by photoluminescence (PL) spectroscopy.

Non-equilibrium fractal growth of MoS2 for electrocatalytic hydrogen evolution

Non-equilibrium fractal growth of MoS₂ was induced by establishing an extremely Mo rich chemical vapor deposition (CVD) environment using a rapid heating rate in a confined reaction space.

Role of H Transfer in the Gas-Phase Sulfidation Process of MoO3: A Quantum Molecular Dynamics Study

Layered transition metal dichalcogenide (TMDC) materials have received great attention because of their remarkable electronic, optical, and chemical properties. Among typical TMDC family members, monolayer MoS₂ has been considered a next-generation semiconducting material, primarily due to a higher carrier mobility and larger band gap. The key enabler to bring such a promising MoS₂ layer into mass production is chemical vapor deposition (CVD). During CVD synthesis, gas-phase sulfidation of MoO₃ is a key elementary reaction, forming MoS₂ layers on a target substrate. Recent studies have proposed the use of gas-phase H₂S precursors instead of condensed-phase sulfur for the synthesis of higher-quality MoS₂ crystals. However, reaction mechanisms, including atomic-level reaction pathways, are unknown for MoO₃ sulfidation by H₂S. Here, we report first-principles quantum molecular dynamics (QMD) simulations to investigate gas-phase sulfidation of MoO₃ flake using a H₂S precursor. Our QMD results reveal that gas-phase H₂S molecules efficiently reduce and sulfidize MoO₃ through the following reaction steps: Initially, H transfer occurs from the H₂S molecule to low molecular weight Mo _xO _y clusters, sublimated from the MoO₃ flake, leading to the formation of molybdenum oxyhydride clusters as reaction intermediates. Next, two neighboring hydroxyl groups on the oxyhydride cluster preferentially react with each other, forming water molecules. The oxygen vacancy formed on the Mo-O-H cluster as a result of this dehydration reaction becomes the reaction site for subsequent sulfidation by H₂S that results in the formation of stable Mo-S bonds. The identification of this reaction pathway and Mo-O and Mo-O-H reaction intermediates from unbiased QMD simulations may be utilized to construct reactive force fields (ReaxFF) for multimillion-atom reactive MD simulations.