Factors that affect the stability, type and morphology of Pickering emulsion stabilized by silver nanoparticles/graphene oxide nanocomposites Mingyi Tang a , Tao Wu b , Xiaoyang Xu b, *, Lei Zhang b , Fei Wu b a Department of Applied Chemistry, School of Science, Tianjin University of Commerce, Tianjin 300134, PR China b Department of Chemistry, Tianjin University, Tianjin 300072, PR China A R T I C L E I N F O Article history: Received 25 March 2014 Received in revised form 5 August 2014 Accepted 11 August 2014 Available online 17 August 2014 Keywords: A. Interfaces A. Composites B. Chemical synthesis D. Catalytic properties A B S T R A C T Silver nanoparticles/graphene oxide (AgNPs/GO) nanocomposites were easily fabricated by a green method without using any additional reductant and the prepared nanocomposites were then used to stabilize Pickering emulsions. The properties of the emulsions stabilized by the AgNPs/GO prepared with different AgNO3/GO mass ratios were investigated and the effects of the oil/water ratio, the AgNPs/GO concentration, and the pH on the stability, type and morphology of the emulsions were studied. In addition, the effects of adding different types and concentrations of electrolytes on the emulsion stability were studied. Adding electrolytes to the systems improved the stability of the Pickering emulsions due to the reduction in the particle zeta potentials. Polystyrene–AgNPs/GO composites were also prepared by Pickering emulsion polymerization and their catalytic performance for the reduction of4-nitrophenol was investigated. ã 2014 Elsevier Ltd. All rights reserved. 1. Introduction Graphene is a monolayer of carbon atoms arranged in a hexagonal lattice [1,2]. It has received considerable attention due to its unique two-dimensional (2D) structure and its excellent mechanical, electronic and optical properties [3–5]. Large amounts of graphene are easily produced via the reduction of graphene oxide (GO) which is obtained by the intense oxidation of graphite [6,7]. GO sheets are oxygenated graphene sheets that are covered with epoxy, hydroxyl, and carboxyl groups and they have large surface areas, so they have many potential applications, such as adsorption [8,9], separation of pollutants [10], and catalytic carriers for chemical reactions [11,12]. Their unique structures also offer opportunities to modify the GO and make functiona- lizedgraphene-based materials. Recently,Ag-based catalysts have attracted much attention because of their relatively low cost and their broad applications in various reactions [13]. To prevent the aggregation of the nanoparticles, supports are often used to anchor and stabilize the metal nanoparticles. It has been found that GO is an ideal support material for growing and anchoring metal nanoparticles such as Au, Ag, Pd, Pt, and Ru nanoparticles (NPs) [14–17]. GO supported metal nanoparticles have excellent electrocatalytic and electrochemical properties. Pickering emulsions or solid-stabilized emulsions are colloidal emulsions stabilized by solid particles instead of organic surfac- tants [18,19]. They have broad and important applications in the food [20], cosmetics [21], and pharmaceutical fields [22,23]. Solid particles have been shown to assemble at the fluid interface due to the reduction of the inter-facial energy of the system that occurs when a new interface is created between the particles and the liquid phases instead of at the liquid–liquid interface. The particles should be partly wetted by both phases for an effective stabilization of Pickering emulsions [24]. In general, the predomi- nate hydrophilic or hydrophobic nature of the particles determines whether they tend to stabilize in oil-in-water (o/w) or water-in-oil (w/o) emulsions, respectively [25,26]. Multiple emulsions can be obtained by adding two types of particles with different hydro- phobicities to oil and water mixtures. In some cases, multiple emulsions can form around a phase inversion of emulsions in a system containing only one particle type [27,28]. The differences in * Corresponding authors. Fax: +86 22 274 034 75. E-mail addresses: mingyitang2012@163.com (M. Tang), winy5t@163.com (T. Wu), xiaoyangxu2012@163.com (X. Xu). http://dx.doi.org/10.1016/j.materresbull.2014.08.019 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved. Materials Research Bulletin 60 (2014) 118–129 Contents lists available at ScienceDirect Materials Research Bulletin journa l homepage: www.elsevier.com/locate/matresbu