Nano-tribology studies of reduced graphene oxide films in air and in aqueous solutions with different pH values
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hua Chenga) School of Mechanical and Power Energy Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China; and State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People’s Republic of China (Received 2 July 2016; accepted 1 November 2016)
In this study, three types of graphene films—hydrothermal reduced graphene oxide (GO) film, thermal reduced GO film, and GO film—on silicon substrate by using 3-aminopropyl triethoxysilane (APTES) as the interface adhesive layer were prepared for investigation. The chemical compositions of the samples were characterized by x-ray photoelectron spectroscopy (XPS). Surface morphologies, adhesive forces, and nano friction forces in air and aqueous solutions with different pH values were investigated by atomic force microscopy (AFM). Results showed that capillary force dominated the adhesive force in air condition, and adhesive force was much smaller in aqueous solution than in air due to the disappearance of the capillary force. Adhesive force and friction coefficient of the three samples slightly decreased with the increase of pH values. Hydrothermal reduced GO film exhibits the best lubricity both in air and in liquids among those three films.
I. INTRODUCTION
Graphene has been proved to hold novel tribological performances at the nano-scale,1–3 owing to its excellent mechanical properties.4,5 Meanwhile, due to its nanometer thickness, graphene sheets can be applied to prepare ultra-thin lubricant films6–8 for contact surfaces or to act as lubricant additives9 in micro machines, for instance, MEMS. However, because of the chemical inertness of graphene, the bonding strength between graphene and protected surface is not strong enough to prevent wear. Graphene oxide (GO), as the precursor to obtain graphene, can be used to prepare a covalently bonded GO film on pretreated substrates, and obtain graphene films through surface reduction treatment of GO film.10–13 Those films performed well as lubricate films owing to their strong bonding strength. Noticeably, the surface reduction process played an important role in improving tribological performance by reducing the surface energy of the films. Thus, such a high efficiency method for GO reduction may have the potential for improving the nanotribological properties of reduced GO films.
Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.450
A variety of approaches have been developed to reduce GO.14–19 However, few of them can be directly applied to reduce GO films for lubrication. There are mainly three reasons: introducing noncarbon impurities into the film,14–16 damaging the structure of the graphene platelets,17,18 or weakening the bonding strength between GO films and substrates.15,17,19 In contrast, hydrothermal10,20 and thermal11,13,21 methods have been shown to be lowcost and effective methods to reduce GO films, and bring less impurities and harm to the film. Besides, Zhou20 and Pei22 found that the addition of HBr, HC
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