Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity
Introduction
The research field dealing with the synthesis of metallic nanoparticles offers broad scope, especially in the area of catalysis, and includes fuel cells, solar cells, sensors and photocatalysis applications [1]. It is well known that nanoparticles can exhibit unusual chemical, physical and catalytic properties that are distinctly different from their bulk counterparts. In particular, the preparation of bimetallic nanoparticles has gained considerable interest due to their higher catalytic activity compared with that of monometallic nanoparticles [2], [3]. Amongst different metallic particles, noble metals (Ag, Au, Pt and Pd) have been extensively used to decorate solid supports like CNTs, carbon spheres, graphene oxide, to form very stable and highly dispersed particles on the various substrates [4], [5]. Many researchers have used the Ag–Au bimetallic catalyst in a number of applications [6], [7].
A variety of methods for the preparation of Au–Ag bimetallic nano-structured materials have been reported. Park and Vaia [2] employed a chemical method for synthesizing Ag–Au rods of varying lengths. Radziuk et al. [8] reported the preparation of an alloy form of Au–Ag nano-particles using a sonochemical based technique in the presence of a cationic surfactant. Huang et al. synthesized Au–Ag bimetallic nanostructures through a galvanic replacement reaction [9]. Tang et al. [7] have reported on controllable incorporation of Au–Ag nanoparticles onto carbon spheres and their catalytic properties by monitoring the reduction of 4-nitrophenol. Harish et al. synthesized Au–Ag alloy in a co-reduction chemical method using sodium borohydride as the reducing agent in the presence of a stabilizer [10]. Amongst various methods, sonochemical preparation method is a simple approach for preparing mono- and bimetallic nanoparticles with different architectures without using any added chemicals. Moreover, sonochemically prepared metallic nanoparticles are very stable for a long period of time in the presence of a surfactant [11], [12], which helps to load the catalysts onto a solid support and to design highly active catalysts.
In recent years, graphene oxide (GO) sheets have been used as a potential solid support because of their high carrier mobility, large specific surface area, high thermal and electrical conductivity and potentially low manufacturing cost [13]. GO is a two-dimensional mono-layer of sp2-bonded carbon atoms in the form of sheets of few a layers in thicknesses (3 to 5 layers) and used in applications such as H2 production and storage, drug delivery, sensing, catalysis, nanoelectronics, polymer composites, and photovoltaics [11], [12], [13], [14], [15], [16], [17], [18]. The incorporation of catalyst particles with graphene or reduced graphene oxide (RGO) sheets with good distribution can provide greater versatility in carrying out catalytic processes.
Generally, in a typical sonochemical preparation procedure, a horn type low frequency ultrasonicator produces strong shear forces apart from generating considerable amounts of reactive radicals [19], [20], [21], [22], whereas sonication at high frequency produces a relatively higher amount of reactive radicals [23], [24]. Therefore, applying both frequencies alternatively (dual frequency) at regular time intervals for the synthesis of Au–Ag along with GO may have some advantages. It can be expected that the rate of reduction of GO (to generate reduced GO) as well as the metal ions would be enhanced using high frequency ultrasound and, an improved uniformity of incorporation of metal catalysts on GO sheets on exposure to low frequency ultrasound. The main aim of this study was to prepare Au–Ag-incorporated GO catalysts using dual frequency sonochemistry and to evaluate their catalytic efficiency for the reduction 4-nitrophenol to 4-aminophenol.
Section snippets
Materials and methods
Silver nitrate (AgNO3), chloroauric acid (HAuCl4), polyethylene glycol (PEG), propan-2-ol (C3H8O), 4-nitrophenol (4-NP), sodium hydroxide (NaOH) and sodium borohydride (NaBH4) were purchased from Sigma Aldrich Inc. Graphite powder (99.99%) was purchased from Alfa Aesar, USA and used without further purification. De-ionized water with a resistivity of 18.0 MΩ cm from a 0.22 μm Milli-Q water purifier was used throughout the experiment.
Preparation of GO and Au–Ag–GO
GO was prepared by the modified Hummer’s method [25]. A required
Ag–GO, Au–GO and Au–Ag–GO nano-composites
Ag–GO, Au–GO and Au–Ag–GO nano-composite catalysts with different weight ratios of Au and Ag (1:1, 2:1 and 1:2) were synthesized sonochemically under dual frequency conditions. Both sequential and simultaneous ultrasound exposure procedures were employed to find out the most suitable condition for the preparation of highly active catalysts. The complete details of the preparation conditions of mono-metal and bimetallic Au, Ag with GO are presented in Table 1.
The initial rate of sonochemical
Conclusion
Au–GO, Ag–GO mono-metallic and bimetallic catalysts with different ratios of Au and Ag were prepared by a dual frequency sonochemical method. The catalytic activity of the catalysts was investigated using 4-NP reduction process. The results revealed that the bimetallic catalysts showed higher activity for the reduction of 4-NP. Further, sequential process involving the reduction of Au–GO followed by Ag with 1:2 ratio (Au:Ag) exhibited the highest catalytic activity. The excess electron present
Acknowledgments
We acknowledge financial support from the Australian Research Council (ARC) and the ARC Particulate Fluids Processing Special Research Centre for infrastructure support and also for the partial financial support from DST, India. We thank Professor Franz Grieser for helpful discussion.
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