Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity

Highlights

• Sonochemically prepared Au–Ag–graphene oxide nano-composite shows very high activity for the reduction of 4-nitrophenol.

• A total reduction of 4-nitrophneol takes place with less amount of catalyst.

• With lesser amount of Au along with Ag (1:2) and GO can be able to reduce 4-nitorphenol more effectively.

• Applying dual frequency ultrasonicator considerably enhances the amount of catalysts loading on GO support.

Abstract

Graphene oxide (GO) supported Ag and Au mono-metallic and Au–Ag bimetallic catalysts were synthesized using a sonochemical method. Bimetallic catalysts containing different weight ratios of Au and Ag were loaded onto GO utilizing a low frequency horn-type ultrasonicator. High frequency ultrasonication was used to efficiently reduce Ag(I) and Au(III) ions in the presence of polyethylene glycol and 2-propanol. Transmission electron microscopy (TEM–EDX) and X-ray photoelectron spectroscopy were used to analyze the morphology, size, shape and chemical oxidation states of the prepared metallic catalysts on GO. The catalytic efficiency of the prepared catalysts were compared using 4-nitrophenol (4-NP) reduction reaction and the subsequent formation of 4-aminophenol (4-AP) that was also monitored using UV–vis spectrophotometry. The results revealed that Au–Ag–GO bimetallic catalysts showed high activity for the conversion of 4-NP to 4-AP than their monometallic counterparts. Amongst different weight ratios (1:1, 1:2 and 2:1) between Au and Ag, the 1:2 (Au:Ag) catalyst exhibited very good catalytic performance for the conversion of 4-NP to 4-AP. A total reduction of 4-NP took place within a short period of time if Au–GO was reduced first followed by Ag reduction, whereas a lower reduction rate was observed if Ag–GO was reduced first. The same trend was observed for all the ratios of bimetallic catalysts prepared by this method. The initial unfavorable reduction potential of Ag(I) is likely to be responsible for the above order. It was found that applying dual frequency ultrasonication was a highly effective way of preparing bimetallic catalysts requiring relatively low levels of added chemicals and producing bimetallic catalysts with GO with improved catalytic efficiency.

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.