Oxidation and metal-insertion in molybdenite surfaces: evaluation of charge-transfer mechanisms and dynamics
© Ramana et al; licensee BioMed Central Ltd. 2008
Received: 21 November 2007
Accepted: 05 June 2008
Published: 05 June 2008
Molybdenum disulfide (MoS2), a layered transition-metal dichalcogenide, has been of special importance to the research community of geochemistry, materials and environmental chemistry, and geotechnical engineering. Understanding the oxidation behavior and charge-transfer mechanisms in MoS2 is important to gain better insight into the degradation of this mineral in the environment. In addition, understanding the insertion of metals into molybdenite and evaluation of charge-transfer mechanism and dynamics is important to utilize these minerals in technological applications. Furthermore, a detailed investigation of thermal oxidation behavior and metal-insertion will provide a basis to further explore and model the mechanism of adsorption of metal ions onto geomedia.
The present work was performed to understand thermal oxidation and metal-insertion processes of molybdenite surfaces. The analysis was performed using atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), and nuclear reaction analysis (NRA).
Structural studies using SEM and TEM indicate the local-disordering of the structure as a result of charge-transfer process between the inserted lithium and the molybdenite layer. Selected area electron diffraction measurements indicate the large variations in the diffusivity of lithium confirming that the charge-transfer is different along and perpendicular to the layers in molybdenite. Thermal heating of molybenite surface in air at 400°C induces surface oxidation, which is slow during the first hour of heating and then increases significantly. The SEM results indicate that the crystals formed on the molybdenite surface as a result of thermal oxidation exhibit regular thin-elongated shape. The average size and density of the crystals on the surface is dependent on the time of annealing; smaller size and high density during the first one-hour and significant increase in size associated with a decrease in density with further annealing.
Sulfide minerals and the associated geological/physical/chemical processes are an active research topic for mineralogists, geochemists, and geotechnical/environmental engineers. Sulfide ores constitute a major source of metals, especially noble metals. In addition, the rich diversity in crystal chemistry, surface reactivity, phase transformations, stability, thermodynamics, and electronic properties makes the sulfide minerals attractive for a wide variety of industrial applications, such as lubricants and catalysts [1, 2]. Molybdenite (MoS2) belongs to the family of the transition-metal dichalcogenide (TMD) minerals with the formula MX2 (where M = Cd, Ti, Mo, Sn and X = I, S, Se). Due to their layered structure, TMDs are often referred to as two-dimensional (2D) solids. The 2D structure of these minerals is due to the strong covalent or ionic bonding within a layer while individual layers are held together by weaker van-der-Waals forces. Even though the latter are often referred to as "van der Waals" type of interactions, some contributions from covalent and ionic interactions are also possible, particularly in the metal inserted complexes [3, 4]. These compounds exhibit anisotropic physical properties, such as different conductivity parallel and perpendicular to the layers, ranking MoS2 the most anisotropic 2D material after graphite .
Molybdenite is the essential ore mineral of the molybdenum industry for production of Mo metal and Mo-based compounds, such as sodium and calcium molybdates, ammonium paramolybdate, and molybdenum trioxide . Molybdenite-based formulations are extensively used in industrial machinery and weapons for lubrication . Molybdenite is a widely used catalyst in CO hydrogenation and hydrodesulfurization processes for the production of cleaner fuels [7, 8]. Mo is a redox-sensitive trace metal and becomes enriched in sulfidic, reducing, and organic rich sediments. Molybdenite is found in deep-sealed veins associated with scheelite, wolframite, topaz, and fluorite .
The objective of the present work is to understand the effects of thermally-induced oxidation and alkali metal (Li) ion insertion in molybdenite surfaces. Understanding the oxidation behavior of MoS2 is crucial because several applications of MoS2 are influenced by its degree of oxidation. Similarly, information on the mechanism of foreign metal-atom insertion into the molybdenite structure is important to understand the associated electronics structure changes. Therefore, the present investigation was made using a wide variety of analytical techniques such as atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (RS), Rutherford backscattering spectrometry (RBS), and nuclear reaction analysis (NRA) to elucidate how the oxidation proceeds in these minerals at higher temperatures. In addition, the structural effects associated with metal insertion at room temperature were evaluated. Our choice of lithium (Li) was due to the following reasons: (1) The chemistry of Li, being a simple metal, is well known, (2) the metal insertion in molybdenite is only possible with small, strongly reducing guests, such as the alkali metals [3, 10] and (3) comparison of the results will be made easy in view of several existing reports on Li in synthetic molybdenite [3, 10]. Using the familiar surface analytical techniques, namely AFM, SEM and TEM, is to probe the changes in surface structure and morphological features due to thermal oxidation and metal-ion insertion in molybdenite. The combined use of RS and TEM measurements allow probing the information on the local chemical structure and bonding and are quite useful to evaluate the impact of metal-ions on the local structural environment in molybdenite. Using the less familiar ion-beam analytical techniques, such and NRA, is mainly to understand the thermal oxidation behavior of molybdenite. High-energy ion-beam analysis using nuclear reactions, particularly NRA measurements, provide an important tool to measure the absolute concentration of lighter elements, such as hydrogen (H), carbon (C), nitrogen (N), and oxygen (O), and makes it quite useful to understand the science and processes at geochemical media [11–13]. Therefore, we have used the NRA to probe the surface oxide growth on molybdenite surface.
Two sets of naturally-occurring molybdenite samples were used in this work. The minerals, found in the mineral storage/collection at the Department of Geological Sciences at the University of Michigan, of unknown origin were employed for thermal oxidation experiments. Molybdenite samples obtained from Japan were employed for lithium-insertion measurements. Atomic force microscopy measurements were performed using a Digital Instruments AFM (NanoScope IV). The measurements were made in tapping mode. Scanning electron microscopy observations were made using two different instruments. A JEOL (model 6150) electron microscope was used to image the oxidation behavior of the molybdenite surfaces as a function of heating time. SEM imaging experiments on lithium-reacted molybdenite samples were made using a high-resolution electron microscope (Hitachi S-4700). Raman scattering (RS) spectra were measured using a Jobin-Yvon U1000 double-pass spectrometer equipped with a cooled, low-noise photomultiplier tube (ITT FW130). The incident light used for the experiments was the 515 nm Ar ion laser. TEM analysis was performed using a JEOL TEM 2010F at a 200 kV acceleration voltage. Phase and structure of the material were monitored using selected area electron diffraction (SAED). Specimens for TEM analysis were prepared by dispersing the MoS2 sample on 3-mm Cu grid with a hole size of 1 × 2 mm. High resolution transmission electron microscopy (HRTEM) image processing including the fast Fourier transformation (FFT) was carried out using a Gatan Digital Micrograph 3.4.
Lithium intercalated LixMoS2 samples ware prepared from natural 2H-MoS2 treated with dilute butyllithium in hexane in a controlled water and oxygen-free environment. Prolonged treatment (~10 days) with very dilute solutions (~0.005 M) was used to produce the lithium-inserted samples for analytical experiments. On cleaving the surface was seen to be broken into mm sized regions of crystal, either shiny, or matt black (poor surface quality) indicating the intercalated areas.
Thermal oxidation experiments were performed in air. Molybdenite surfaces were heated to 400°C in a furnace. Various analytical measurements were performed as a function of heating time to understand the thermally-induced effects. The ion-beam experiments namely Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) measurements were carried out in the accelerator facility at the Environmental Molecular Sciences Laboratory (EMSL) of the Pacific Northwest National Laboratory (PNNL), Richland, Washington, USA. A helium (He+) ion beam of 2 MeV was incident on the sample surface at near normal and the scattered ions were detected at an angle of 170° from the sample normal for RBS measurements. A beam of 0.94 MeV deuterium (d+) ions was incident on the molybdenite sample and the reaction products were detected at a scattering angle of 170° from the sample normal for NRA measurements. A thin aluminized mylar film covered the detector to stop backscattered deuterium ions, allowing only the more energetic reaction products to enter the detector. The detected particles for these measurements were protons from the 16O(d,p)17O reaction.
Results and Discussion
Pristine molybdenite surfaces
The thermally-induced oxidation behavior and lithium metal-insertion reaction effects on molybdenite surfaces are studied using a combination of structural imaging and spectroscopic measurements. Starting from pristine and well-ordered molybdenite layers, local disordering of the structure occurs as a result of charge-transfer process between the inserted metal ions and the molybdenite layer. Heating of the molybdenite surface in air at 400°C induces oxidation. Oxidation in the first hour of heating is initially slow and then increases significantly.
The authors at the University of Michigan acknowledge the support of the National Science Foundation (NSF-NIRT, EAR-0403732). We are thankful to Professor Eric J. Essene and Dr. Martin Reich for stimulating discussions about this project. Ion beam analysis was performed at the EMSL, a national scientific user facility located at PNNL and supported by the U.S. Department of Energy's Office of Biological and Environmental Research. PNNL is a multi-program national laboratory operated for the U.S. DOE by Battelle Memorial Institute under contract no. DE-AC06-76RLO 1830.
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