Growth Mechanism of 2D Mo 2 C on Cu via CVD (original) (raw)
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Controlled CVD growth of ultrathin Mo2C (MXene) flakes
Journal of Applied Physics, 2022
MXenes combine distinctive properties, including high electrical conductivity, high thermal conductivity, and efficient absorption of electromagnetic waves, which allow them to be utilized in various applications such as electrical energy storage, sensors, and functional composites. This study aims to grow thin and large area Mo 2 C flakes in a controlled manner by using chemical vapor deposition, avoiding surface functionalization, and limited lateral dimensions. Herein, we investigate the effects of CH 4 flow, the precursor/catalyst (Mo/Cu) ratio, and flow rates of carrier gas on the growth of two-dimensional Mo 2 C structures. This study examines the effects of the precursor/catalyst (Mo/Cu) ratio and flow rates of carrier gas on the growth of Mo 2 C structures. Our results show that when the flow rates of CH 4 , catalyst/precursor (Cu/Mo) ratio, and carrier gas (N 2 /H 2) ratio are varied, we can control both thickness (from 7 to 145 nm) and coverage of the substrate surface (from 11% to 68%) of the Mo 2 C flakes. Therefore, this study reveals that it is possible to realize centimeter-scale surface coverage and controllable thicknesses by adjusting the process parameters. The deposited films and flakes are analyzed by optical microscopy, atomic force microscopy, and Raman scattering spectroscopy techniques. The Raman spectra are also compared with the theoretical calculations using density functional theory. Overall, the present work is expected to provide a significant impact for utilization of MXenes in various applications.
Journal of Crystal Growth, 2018
Transition metal carbides is a family of materials with many fascinating properties, especially when studied close to the 2D limit. Molybdenum carbide (Mo 2 C) is a member of this group which has attracted interest especially thanks to its potential application as electrode in energy storage as well as in catalysis. The latest methods development has demonstrated the potential to grow ultrathin Mo 2 C domains of high quality through chemical vapor deposition (CVD) over melted substrates, together with graphene. This progress provides new routes towards the preparation of vertical Van der Waals (VdW) heterostructures. In the present work we provide a detailed study of the dominant growth mechanisms of pure Mo 2 C crystals as well as graphene/Mo 2 C heterostructures. The findings reveal that the methane flow controls the growth of graphene/Mo 2 C heterostructures or pure Mo 2 C domains, while the high growth temperature and presence of hydrogen significantly increase the growth rate. Additionally, the presence of graphene serves as a blocking diffusion layer, permitting the growth of ultrathin crystals and increasing their nucleation density. Atomic Force Microscopy (AFM) characterization demonstrates the growth of stacks of crystals which consist of individual crystals with thickness as low as ∼2 nm.
Journal of Solid State Chemistry, 1998
A molybdenum carbide supported on active carbon for catalytic hydrotreating was prepared by temperature-programmed reaction (TPR) in flowing H 2 of an active carbon impregnated by an heptamolybdate. TPR led at 973 K to the formation of supported Mo 2 C. This new method of preparation avoids the use of methane as carburizing reactant and allows in situ preparation of supported molybdenum carbide without any contact of this pyrrophoric material with air between preparation and catalytic run. The various steps of the carburization process were studied by trapping the solid intermediates at different temperatures during TPR. Two successive reactions were evidenced: the partial reduction by H 2 of the initial molybdenum precursor to MoO 2 , and its subsequent carburization to Mo 2 C. This last step is mainly due to the reduction of MoO 2 and carburization with native methane evolved from the reaction of the carbon support with dihydrogen. Solid materials were characterized by elemental analysis, X-Ray diffraction, transmission electron microscopy and specific surface area measurements.
Structural stability of α/β-Mo2C during thermochemical processing
Journal of Alloys and Compounds, 2010
The structural stability and its influence on the microstructure of thermochemically synthesized ultrafine Mo 2 C was investigated. The precursors for the thermochemical process were chemically synthesized and consisted of crystalline orthorhombic MoO 3 and different concentrations of amorphous C. Crystalline -Mo 2 C (hexagonal space group: P6 3 /mmc) was obtained on heat treatment of a C-rich precursor at 1000 • C after 1 h of isothermal holding under hydrogen atmosphere. The as-synthesized -Mo 2 C showed spherical grain morphology with less than about 100 nm size and highly agglomerated in plate form. The carbon analysis revealed about 6.24 wt.% of carbon in the -Mo 2 C, which is slightly higher than the equilibrium C-level in Mo 2 C (5.89 wt.%), while the precise lattice parameters, as determined by Rietveld structural refinement, was close to the theoretical values of stoichiometric -Mo 2 C. The precursor containing lower concentration of C, on the other hand, led to crystalline orthorhombic ␣-Mo 2 C with space group: Pnab (non-standard setting of Pbcn) at 1000 • C under identical heat treatment conditions. The as-synthesized ␣-Mo 2 C powders were highly agglomerated with a tendency to acquire spherical morphology with increase in C-level. The preferential formation of hexagonal -Mo 2 C over orthorhombic ␣-Mo 2 C at higher C in the precursor was attributed to the faster kinetics of carbon diffusion due to its higher activity, which possibly led to the disordered distribution of carbon in the octahedral sites of hexagonal Mo sublattice.
Preliminary study of the surface reactivity of 2D α-Mo2 C crystallites
The Canadian Journal of Chemical Engineering, 2018
A preliminary study of the surface reactivity of 2D-α-Mo2C crystallites grown on a copper foil was performed using x-ray photoelectron spectroscopy. Different sample preparation protocols for the as-received materials were explored in order to remove hydrocarbon surface contamination. Annealing in vacuum and in argon led to the formation of graphitic layers, while annealing in O2 lead to almost the complete disappearance of the Mo signal. Gentle argon ion sputtering proved effective at removing the hydrocarbon contamination to reveal pristine molybdenum carbide. XPS spectra were recorded following the exposure of the prepared sample at 297 K to furfural. The results are commented on in relation to deoxygenation and olefin metathesis surface chemistry.
Structures and properties of the Mo-Mo 2 C system
Scripta Metallurgica et Materialia, 1990
The structure-property relationships in a carbon-molybdenum system with carbon contents in the range which produce a mixture of Mo2C in a eutectic mixture of metallic Mo and Mo2C are examined. Physical properties such as microhardness, lattice parameter, strength, ...
In this work, the chemical vapor deposition synthesis of the Mo 2 C/graphene heterostructure above the melting temperature of Cu bias (1356 K) is studied. Two sets of Mo 2 C growth experiments at high CH 4 flow rates (5 SCCM ≥ 3 SCCM) are performed, either using priorgraphene synthesis or having in situ graphitization, for three different Cu bias thicknesses. Raman mappings taken from all six-test samples show graphene covers not only over the Mo 2 C pillars but also over their untransformed Cu bias substrate regions. The only difference is that the Mo 2 C pillar grows over the prior graphene bias; on the other hand, the in situ graphene grown Mo 2 C pillar nucleates and grows over the fresh Cu bias surfaces. A steady-state laminate model for flows of Mo and C species with phase transformations is developed for the radial and vertical growth kinetics of synthesized Mo 2 C/graphene heterostructure. The computer simulation reproduces those experimental observations performed recently in our laboratories on the prior or no-prior graphitized (G) test modules with Cu/G bias, having three different thicknesses at 1363 K. AFM-topography and SEM photos for a prior graphitized test module of 25 μm thick Cu and 4.72 Å graphene bias show a three layered Mo 2 C/graphene heterostructure; the first layer is almost perfect hexagonal flat, and the other two circular shaped layers constitute the whole pillar of 140 nm height. This may be compared to a 250 μm thick Cu/4.7 Å graphene bias sample, which furnishes an ultra-thin single flat layer of 10-13 nm thick Mo 2 C crystallites having a perfect planar hexagonal structure.
Spectroscopic study on the formation of CO−2 on K-promoted Mo2C/Mo(100) surface
Surface Science, 2000
The adsorption of CO 2 on clean and potassium-covered Mo 2 C/Mo(100) surface was studied by high resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TPD), work function measurements and X-ray photoelectron spectroscopy (XPS). The adsorption of CO 2 on Mo 2 C/Mo(100) caused no measurable changes in the work function of Mo 2 C. CO 2 adsorbed weakly on K-free Mo 2 C at 90 K, producing vibration features at 660, 1260-1340 and 2349 cm−1. It desorbed in one peak with T p =190 K. The deposition of potassium on Mo 2 C resulted in a maximum work function decrease of 3.3 eV. The bonding strength of potassium sensitively depended on its surface concentration. At low coverage it desorbed with T p =850 K, this peak temperature was 525 K at monolayer, and 355 K at multilayer. The presence of potassium adatoms greatly increased the binding energy of CO 2 and led to the formation of CO− 2 anion radical characterized by losses at 750-785, 1220-1250 and 1580-1670 cm−1. The activated CO 2 dissociated to adsorbed CO and O at low potassium coverage, even at 90-150 K, and disproportionated into adsorbed CO and CO 3 at and above potassium monolayer. Both compounds were strongly stabilized on the surface by potassium adatoms and were released from the surface coincidentally with a peak temperature of 665-686 K.
Effect of the Molybdenum Substrate Shape on Mo2C Coating Electrodeposition
Coatings, 2018
By electrochemical synthesis in the NaCl-KCl-Li2CO3 (1.5 wt.%)-Na2MoO4 (8.0 wt.%) melt on molybdenum, substrates with different configuration Mo2C coatings with the hexagonal lattice were obtained. The influence of the substrate form on the structure of Mo2C cathodic deposits was studied. The molybdenum carbide coatings on a molybdenum substrate (Mo2C/Mo) show a catalytic activity in the water–gas shift (WGS) reaction by at least three orders of magnitude higher than that of the bulk Mo2C phase. The catalytic activity remained constant during 500 h for the water–gas shift reaction.
Carbon coated nano molybdenum carbide (Mo 2 C) has been synthesized at 800 °C through single step reduction route using molybdenum trioxide (MoO 3) as a precursor, polypropylene (P.P) as a carbon source and magnesium (Mg) as a catalyst in an autoclave. The synthesized samples were characterized by X-ray diffraction (XRD), thermal analysis techniques (TG/DTA/DTG), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Williamson-Hall (W-H) analysis has been done to estimate various parameters like strain, stress and strain energy density. Multi-stage kinetic analysis of the product phase has been studied to establish the nature of the thermal decomposition. Coats-Redfern method applied to determine the mechanism involved in the decomposition of the product phase shows that initial and final stage follow F1 mechanism whereas middle stage follow F3 mechanism. The activation energy (E a) and pre-exponential factor (A) has also been determined. The morphological studies shows that the particles have partially spherical/faceted shape, with carbon coated having wide particle size distribution. The surface chemistry and surface area analysis were studied by X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmet-Teller (BET), respectively. The formation mechanism of carbon coated Mo 2 C nano particles has been predicted based on the XRD, TG/DTA & DTG and microstructural results. The industrial demand of transition metal carbides (TMCs) is increasing due to their remarkable physical and chemical properties. The high melting point, good conductivity, thermal stability, excellent corrosion, wear resistance and catalytic properties similar to noble metals of Mo 2 C find many industrial applications 1–5. The Mo 2 C particles are highly active HER (Hydrogen Evolution Reaction) catalyst and has stability in both acidic and basic mediums 6, 7. Because of these extensive applications of Mo 2 C, synthesis of nanocrystalline Mo 2 C utilizing low cost carbon source by simple route at lower temperatures is eminently desirable. The catalytic activity of Mo 2 C is mostly influenced by surface structure and elemental composition, which are dependent on the synthesis route 8. Traditionally, the micron size Mo 2 C is produced by direct carburization of molybdenum and molybdenum oxide powders at higher temperatures 9. However, for the synthesis of nano Mo 2 C powder different procedures are adopted 10–17. The structure and crystallite size of carbides mainly depends on the synthesis temperature, type and concentration of carbon source 18–20. The morphology of particles depends upon nature of the carbon and reaction time that plays an important role. Chen et al. 21 synthesized Mo 2 C at 600 °C using MoO 3 in an autoclave in presence of Mg and CH 3 COOK as reducing agent and carbon source, respectively. However, the authors did not establish the mechanism of reduction. Moreover, the processing parameters have not been optimized. In this paper, synthesis of Mo 2 C through a simple reduction and carburization of MoO 3 in an autoclave is reported. For the synthesis, polypropylene and Mg are used as carbon source and catalyst, respectively. Polypropylene is a thermoplastic polymer and used for manufacturing variety of plastics. In order to recycle the plastics and to conserve the natural products, the current path is followed 22. Furthermore, the kinetic analysis involved in the thermal decomposition process is crucial to understand the thermal stability of materials for wide range of applications. Numerous researchers have evaluated various kinetic parameters (activation energy, pre-exponential factor and co-relation factor) and proposed the reaction mechanism, by adopting well known thermal kinetic models 23–27. However, so far no kinetic study has been done to determine the kinetic parameters