The Influence of Humidity on the Structure of MoS 2 (PEO) 1.3 Nanocomposites
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The Influence of Humidity on the Structure of MoS2 (PEO)1.3 Nanocomposites Philippe Westreich, Datong Yang and Robert F. Frindt Department of Physics, Simon Fraser University Burnaby, B.C. V5A 1S6 Canada ABSTRACT Layered nanocomposites of molybdenum disulfide (MoS2 ) and poly (ethylene oxide) (PEO) were studied using x-ray diffraction of powder and oriented thin film samples. Thermal gravimetric analysis provided information on the composition of the material. The effects of the molecular weight of PEO and the atmospheric relative humidity on the structure were also investigated. A basal plane spacing of 14.3 ˚ A, corresponding to an expansion of 8.1 ˚ A between MoS2 layers, was found for dry samples, in agreement with the literature. At about 30% humidity, the expansion increases to 10.6˚ A. This increase corresponds approximately to the thickness of one monolayer of water. The spacing is constant until around 80% humidity, when the expansion begins to increase again, reaching up to 25 ˚ A near 100% humidity. X-ray coherence lengths in the direction perpendicular to the planes averaged 13, 7 and 2 MoS2 layers for the dry, 58% humid and nearly 100% humid samples, respectively. INTRODUCTION The intercalation of organic compounds into layered transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2 ) has been studied since the 1970s [1]. Potential applications of mechanical, structural and electrical properties of organic/inorganic nanocomposites have sparked recent efforts to incorporate polymers within layered inorganic hosts. As well, such materials are interesting due to their importance in understanding the behaviour of polymers within confined spaces. The polymer poly(ethylene oxide) (PEO) was one of the first and remains one of the most popular poymers studied because PEO is soluble in water and can be readily incorporated between MoS2 layers during the MoS2 exfoliation process. Nanocomposites of MoS2 and PEO are also of interest in battery applications [2]. For the MoS2 (PEO) nanocomposites mentioned above [3, 4, 5], there remain many unanswered questions. In particular, the influence of humidity on the structure of the nanocomposites, which has been mentioned for WS2 (PEO) [6], has yet to be explored in detail. An attempt to understand this influence is made in this work.
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MoS2 (PEO) SAMPLE PREPARATION To make the single layer suspension, MoS2 powder (98%, Johnson Matthey) is added to a 2.5 M solution of n-butyl lithium in hexane in an argon-atmosphere dry box. In 24 hours, lithium intercalates between the MoS2 layers, until a molar ratio of Li:MoS2 ≈ 1 is achieved [7, 8]. For exfoliation, distilled water is added to the LiMoS2 . The lithium reacts strongly with the water, forming LiOH and releasing H2 gas, thereby forcing the MoS2 layers apart. The exfoliated MoS2 is then washed four times with distilled water to remove LiOH. After centrifuging, the resulting material is a black, paste-like suspension of MoS2 platelets in water, with a Li:MoS2 ratio ≈ 0.1. To the suspension, a PEO solution
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