Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity Bernaurdshaw Neppolian a,b,⇑, Chang Wang b, Muthupandian Ashokkumar b a b
SRM-Research Institute, SRM University, Kattankulathur, Chennai 603 203, India The School of Chemistry, University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia
a r t i c l e
i n f o
Article history: Received 30 November 2013 Received in revised form 28 January 2014 Accepted 6 February 2014 Available online xxxx Keywords: Au–graphene oxide Ag–graphene oxide Au–Ag–graphene oxide Sonochemical reduction 4-Nitrophenol
a b s t r a c t 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. Ó 2014 Elsevier B.V. All rights reserved.
1. 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
⇑ Corresponding author at: SRM-Research Institute, SRM University, Kattankulathur, Chennai 603 203, India. Tel.: +91 44 2471 7916; fax: +91 44 2745 6702. E-mail address: [email protected] (B. Neppolian).
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
http://dx.doi.org/10.1016/j.ultsonch.2014.02.006 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
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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 [11–18]. 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 [19–22], 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. 2. Experimental details 2.1. 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 MO cm from a 0.22 lm Milli-Q water purifier was used throughout the experiment. 2.2. Preparation of GO and Au–Ag–GO GO was prepared by the modified Hummer’s method [25]. A required amount of graphite powder and NaNO3 were mixed with concentrated H2SO4. KMnO4 was added under ice cold condition and then the mixture was stirred and held at 35 °C for 30 min. Then, deionized water was slowly added into the system. To reduce excess KMnO4, a proper amount of 3% H2O2 aqueous solution was slowly dropped into the mixture. The mixture was then centrifuged to remove the residual impurities. Finally, the product was dried at vacuum oven and used. The precursor solutions for the sonochemical reduction procedure were prepared by mixing 0.2% GO, 2 mM HAuCl4 and 2 mM AgNO3 in a 2% aqueous solution of polyethylene glycol (PEG, MW: 10,000). The solutions were sonicated initially using an ultrasonic reactor (L-3 Communications, ELAC Nautik GMBH) operating at a frequency of 213 kHz in a continues mode for 9 min followed by 20 kHz ultrasonicator (Branson Digital Sonifier-450, tip diameter was 10 mm, USA) that was operated for 1 min using a pulse mode (0.5 s on and 0.5 s off). This sequential sonication was carried out for a total of 3 h. A 35 mL vial containing 30 mL of the precursor solution was immersed in water contained in a borosilicate sonication reactor cell with a volume
capacity of 100 mL. Argon gas was sparged through the reaction sample for 20 min before sonication and an Ar atmosphere was maintained above the solution throughout the experiment. A constant temperature of 28 °C was maintained during the sonication by circulating thermostated water through a jacket surrounding the sonication cell. All experiments were carried out at acoustic (calorimetric) power in the range of 110–125 mW/mL at high frequency and 390 mW/mL at low frequency. At regular time intervals, about 3 mL aliquots were withdrawn from the reactor and UV–vis absorption spectra were recorded on a Varian spectrophotometer (Cary Bio 50). For the synthesis of Ag–Au–GO nano-composites, two separate procedures were used consisting of simultaneous and sequential reduction of GO, Au(III) and Ag(I). In the case of the simultaneous reduction, both Ag(I) and Au(III) solutions were mixed with the GO suspension and sonicated for at least 3 h to ensure that both the GO and the metal ions were fully reduced. In the sequential reduction procedure, the GO and Au(III) were reduced first for one hour and then the Ag(I) solution was added and the sonication continued for another 2 h. 2.3. Characterization studies The morphology, size and size distribution of Au–Ag–GO catalysts were analyzed by transmission electron microscopy (JEOL, JEM 2100). The catalysts were placed on the surface of copper grids and dried under ambient conditions for TEM analysis. The surface topology and thickness of the prepared GO were measured by AFM (Asylum MEP-3D AFM) equipped with a linear variable differential transformer (LVDT)) sensor in the z-direction. XPS analysis was carried out using a M-probe apparatus (Surface Science Instruments). The source was monochromatic Al-Ka radiation (1486.6 eV). UV–vis absorption spectroscopy was carried out on a Varian Cary bio 50 spectrophotometer. 2.4. Catalytic activity-4-nitorphenol reduction The reduction of 4-nitrophenol (4-NP) was used to study the catalytic activities of monometallic and bimetallic catalysts [26]. 4-NP reduction reaction mechanism has been explained by Chen et al. [27]. 1.0–1.2 mg of the catalysts was added to a solution consisted of 4-NP (3.75 mL, 0.12 mM), sodium hydroxide (1.0 mL, 0.4 M), and freshly prepared sodium borohydride (1.0 mL, 0.06 M) under constant stirring at room temperature. The changes in the reaction mixture were recorded by UV–vis absorption spectroscopy at fixed time intervals. 3. Results and discussion 3.1. 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 reduction of both Au(III) and Ag(I) in the presence of GO was determined by UV–vis spectrophotometer. The UV–vis absorption of Ag and Au with GO were monitored around 320 nm and 520 nm, respectively. During the reduction process, 3 mL aliquots of samples were taken from the reactor at regular time intervals to monitor the progress of
Please cite this article in press as: B. Neppolian et al., Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.02.006
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B. Neppolian et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 1 Preparation conditions of Au–GO, Ag–GO monometallic and different ratios of Au and Ag bimetallic catalysts with GO prepared by sonochemical method.
*
S. No.
AgNO3 (mM)
HAuCl4 (mM)
Synthesis time (mins)
Sequential vs. simultaneous
1 2 3 4 5 6 7 8
2 – 2 2 1 1 2 2
– 2 2 2* 2 2* 1 I*
90 90 240 240 240 240 240 240
Both Both Sequential Sequential Sequential Sequential Sequential Sequential
Denotes Au–GO reduced first followed by Ag.
the reduction process. The absorption was gradually increased with increasing sonication time and attained a maximum value indicating the completion of the reduction process. On further increasing the sonication time, aggregation of metallic particles occurred leading to settling of the particles. For example, as can be seen from Fig. 1, the UV–vis absorption of Au–GO gradually increases with an increase in the sonication time, attaining a maximum at 150 min, and decreases at longer times. A similar trend was observed for Ag–GO (data are not shown here). The sonochemical reduction of both Ag–GO and Au–GO was investigated initially with two different approaches: (i) simultaneous reduction, i.e., reducing both GO and metal together, and (ii) sequential reduction, i.e., reducing GO first followed by the metal ion, as explained in the experimental section. It was observed from UV–vis spectra that there was very little difference between the above two approaches. This suggests that there was no competition between GO and the metal ions for the secondary reducing free radicals formed during acoustic cavitation. 3.2. Characterization studies AFM analysis can provide information on the surface morphology and thickness of GO sheets prepared by the modified Hummer’s method. It was clearly observed that the thickness of the GO was about 4.0 nm (figure not shown). This implies that GO formed during the sonochemical synthesis has a thickness corresponding to 2–3 monolayers. The size and morphology of Au–Ag–GO particles were analyzed by TEM. The TEM images are shown in Fig. 2. It can be seen that the average particle size of the Au particles is about 50 nm, whereas the Ag particles are about 5 nm. Both Au and Ag nanoparticles were formed separately in the form of islands of particles whether the
preparation was by the simultaneous or sequential method of ultrasound application. Formation of separate particles of both Au and Ag with GO was further confirmed by the TEM–EDX spectra as shown in Fig. 3. Further, the results reveal that there was neither a core–shell nor alloy material formed during the sonochemical method of preparation, whereas Anandan et al. reported that Ag–Au was formed as core–shell structure during sonochemical preparation method without GO [28]. It can be seen that both Au and Ag particles were formed separately, with most of the Ag particles surrounded by Au particles. This is likely due to the larger size of Au particles, causing the smaller Ag particles to aggregate around the larger sized Au particles (probably, pre-formed Au particles catalyzed the reduction of silver ions). TEM images (e.g., Fig. 2) clearly show a high amount of Au and Ag loading onto the GO support using dual frequency ultrasonication. Moreover, for the conditions applied, the size, morphology as well as the shape of the particles remained similar both in the simultaneous and sequential methods as observed from TEM images. X-ray photoelectron spectroscopy was used to determine the oxidation state of silver and gold in GO as well as the presence of epoxy group on GO sheets. XPS results confirmed the existence of the pure metallic state (zero valent) of Au and Ag along with GO based on the binding energies of both Au and Ag (data not shown) after 3 h of sonochemical preparation time. Moreover, the epoxy groups present in graphene oxide were substantially reduced in the presence of Au and Ag. These reactions typically involve the reduction of the functional groups such as epoxides (CAO, C@O, ACOOH and etc.) on the surface of GO, and the resulting suspensions of the reduced GO are more graphitic and conductive. The reduction of oxygenated functional groups present on GO surface was remarkably increased from up to 80% from 35%. This suggests the removal of epoxides groups substantially restoring the sp2 graphite structure. Complete reduction of GO to restore the sp2 graphite structure leads to unstable suspensions and GO aggregate themselves. This is very similar to our previous findings in the preparation of Au–GO catalyst [11] where it was observed that the reduction of GO increased in correspondence with an increase in the number of Au particles, as prepared by the sonochemical method. These results further confirmed that the sonochemical method is able to reduce the metallic ions to metal and also reduce GO to reduced graphene oxide (RGO) using dual frequency ultrasonicator. 3.3. Catalytic conversion of 4-NP to 4-aminophenol (4-AP) using Au– GO, Ag–GO and Au–Ag–GO catalysts
Fig. 1. UV–vis absorption spectra of Au–GO prepared by sonochemical method.
The catalytic performance of the prepared metallic catalysts (Au–GO and Ag–GO) with different metal loading ratios was tested by monitoring the reduction rate of 4-nitrophenol in the presence of sodium borohydride with specified amounts of catalyst (1.0 mg). The reduction process of 4-NP and the subsequent formation of 4-AP were monitored by UV–vis spectrophotometry [7]. Fig. 4 shows the rate of reduction of 4-NP with Ag–GO and Au–GO
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Fig. 2. TEM images of Ag–Au–GO (2:1) bimetallic photocatalysts prepared by sonochemical method.
1
Au-GO
Ag-GO 0.8
C/Co
0.6
0.4
0.2
0 0
100
200
300
400
500
4-NP conversion me (sec) Fig. 3. TEM–EDX images of Ag–Au–GO (2:1) bimetallic photocatalysts prepared by sonochemical method.
catalysts in the presence of sodium borohydride. The catalytic performance of both catalysts was very low for the reduction of 4-NP. It is widely reported that the catalytic performance of monometallic particles is not good enough for the total reduction or oxidation process, whereas bimetallic nanoparticles exhibit enhanced catalytic performance for the reduction or oxidation reaction due to various reasons including easy transfer of electrons from one metal to another [29]. Liu et al. have reported that MCM-41 supported monometallic Au and Ag catalyst was less efficient for oxidation of CO than the bimetallic Au–Ag–MCM-41 [29]. Tang et al. also demonstrated that Ag-carbon supported monometallic catalysts showed less catalytic efficiency on the reduction of 4-NP to 4-AP than Au–Ag–carbon supported catalysts [7]. The rate of reduction depends mainly on the morphology of the particles whether those are in alloy form or core–shell or separate particles. Hence, bimetallic particles with specific morphology play an important role in determining the rate of the catalytic reduction reaction. The bimetallic catalysts (Au–Ag with GO) were also synthesized by both simultaneous and sequential methods and the catalytic activity was checked through 4-NP reduction process. It is interesting to note that the rate of reaction of 4-NP was found to be considerably less with the catalysts prepared by simultaneous
Fig. 4. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of both Ag–GO and Au–GO catalysts.
method than those prepared by the sequential method. This is probably due to the completion between both metallic ions for the secondary reducing free radicals during the reduction process. Therefore, the bi-metallic catalysts with various metal loading ratios were obtained by adopting only sequential method (Table 1). It is noteworthy that the rate of conversion of 4-NP was remarkably high with Ag–Au–GO (1:1), i.e., Au(III)-GO is reduced first followed by Ag(I) reduction during the catalyst synthesis (Fig. 5b). It is well known that the reduction potential of Au(III) is more positive than that of Ag(I), and particularly when Ag(I) ions are isolated [30]. As a result, the rate of reduction of Au(III) is very fast in comparison with Ag(I). Chen et al. observed similar phenomenon that the formation of Au nanoparticles was faster than the formation of Ag nanoparticles during the synthesis of Au–Ag bimetallic nanoparticles [31]. Once Au(III) is reduced to its metal state, it receives and stores excess electrons in its outer sphere, and acts as a source (pool) of electrons, which enhances the reduction rate of both GO and Ag(I). The results indicate that the graphite nature of GO was improved substantially in the presence of Au, which further facilitates fast transport of photoformed electrons from Au to GO. As a result, the catalytic activity of Ag–Au–GO bimetallic catalyst
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B. Neppolian et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx
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Fig. 5. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of catalyst: (a) Au–Ag (1:1)–GO (Ag reduced first with GO); (b) Ag–Au (1:1)–GO (Au reduced first with GO).
considerably enhanced in the presence Au and GO. This is similar to our recent work on Pt–GO–TiO2 photocatalysts, which exhibited very high activity for the degradation of organic pollutants [19]. Further, the rate of 4-NP reduction was carried out with higher amount of Ag loading, i.e., 2:1 ratio of Ag and Au along with GO and the results are presented in Fig. 6. At all metal ion ratios, the catalyst where Au–GO was synthesized first achieved higher catalytic activity than Ag–GO prepared first followed by Au (Fig. 6a and b). The 2:1 ratio of Ag–Au–GO has shown exceptionally high catalytic activity for 4-NP reduction (Fig. 6b) amongst the different bimetallic- and mono-metallic-GO catalysts. It is found that Ag–Au with the ratio 2:1 is the optimal blend and the best ratio for a total conversion of 4-NP to 4-AP within a short span of time. It was
observed that within 10 min of reaction time, 4-NP was completely converted into 4-AP. Furthermore, the effect of high amount of Au was carried out with Ag–GO. Ag–Au with 1:2 ratio showed very less activity for the 4-NP conversion in comparison with 2:1 ratio of Ag:Au as presented in Fig. 7. The catalytic efficiency was found to be less with high ratio of Au whether Au reduced first or Ag reduced first with GO. The rate of conversion of 4-NP was notably reduced with high amount of Au loading, almost 3 times less than that of Ag–Au–GO (2:1). The size of Au was relatively larger than that of Ag and Ag nanoparticles were enriched on Au surfaces. As the result, the catalytic efficiency substantially reduced with less amount of Ag. This observation emphasizes that less amount of Au is sufficient
Fig. 6. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of catalyst: (a) Au–Ag (1:2)–GO (Ag reduced first with GO); (b) Ag–Au (2:1)–GO (Au reduced first with GO).
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1 Ag-Au-GO 0.8
Au-Ag-GO
C/Co
0.6
0.4
0.2
0 0
100
200
300
400
500
4-NP conversion me (sec) Fig. 7. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of both catalysts: Au–Ag–GO (Ag reduced first with GO) and Ag–Au–GO (Au reduced first with GO).
for the 4-NP reduction that is added advantage on the cost effective approach in the catalytic reduction studies. 4. 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 in Au assisted for the maximum reduction of Ag and GO during sonochemical preparation. 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. References [1] P.V. Kamat, Meeting the clean energy demand: nanostructure architectures for solar energy conversion, J. Phys. Chem. C 111 (2007) 2834–2860. [2] K. Park, R.A. Vaia, Synthesis of complex Au/Ag nanorods by controlled overgrowth, Adv. Mater. 20 (2008) 3882–3886. [3] J. Luoa, M.M. Mayea, N.N. Kariukia, L. Wanga, P. Njokia, Y. Lina, M. Schadta, H.R. Naslundb, C.J. Zhonga, Electrocatalytic oxidation of methanol: carbonsupported gold–platinum nanoparticle catalysts prepared by two-phase protocol, Catal. Today 99 (2005) 291–297. [4] C. Xu, X. Wang, J. Zhu, Graphene–metal particle nanocomposites, J. Phys. Chem. C 112 (2008) 19841–19845. [5] M.M. Maye, Y. Lou, C.J. Zhong, Core–shell gold nanoparticle assembly as novel electrocatalyst of CO oxidation, Langmuir 16 (2000) 7520–7523.
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Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity Bernaurdshaw Neppolian a,b,⇑, Chang Wang b, Muthupandian Ashokkumar b a b
SRM-Research Institute, SRM University, Kattankulathur, Chennai 603 203, India The School of Chemistry, University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia
a r t i c l e
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Article history: Received 30 November 2013 Received in revised form 28 January 2014 Accepted 6 February 2014 Available online xxxx Keywords: Au–graphene oxide Ag–graphene oxide Au–Ag–graphene oxide Sonochemical reduction 4-Nitrophenol
a b s t r a c t 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. Ó 2014 Elsevier B.V. All rights reserved.
1. 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
⇑ Corresponding author at: SRM-Research Institute, SRM University, Kattankulathur, Chennai 603 203, India. Tel.: +91 44 2471 7916; fax: +91 44 2745 6702. E-mail address: [email protected] (B. Neppolian).
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
http://dx.doi.org/10.1016/j.ultsonch.2014.02.006 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: B. Neppolian et al., Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.02.006
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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 [11–18]. 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 [19–22], 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. 2. Experimental details 2.1. 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 MO cm from a 0.22 lm Milli-Q water purifier was used throughout the experiment. 2.2. Preparation of GO and Au–Ag–GO GO was prepared by the modified Hummer’s method [25]. A required amount of graphite powder and NaNO3 were mixed with concentrated H2SO4. KMnO4 was added under ice cold condition and then the mixture was stirred and held at 35 °C for 30 min. Then, deionized water was slowly added into the system. To reduce excess KMnO4, a proper amount of 3% H2O2 aqueous solution was slowly dropped into the mixture. The mixture was then centrifuged to remove the residual impurities. Finally, the product was dried at vacuum oven and used. The precursor solutions for the sonochemical reduction procedure were prepared by mixing 0.2% GO, 2 mM HAuCl4 and 2 mM AgNO3 in a 2% aqueous solution of polyethylene glycol (PEG, MW: 10,000). The solutions were sonicated initially using an ultrasonic reactor (L-3 Communications, ELAC Nautik GMBH) operating at a frequency of 213 kHz in a continues mode for 9 min followed by 20 kHz ultrasonicator (Branson Digital Sonifier-450, tip diameter was 10 mm, USA) that was operated for 1 min using a pulse mode (0.5 s on and 0.5 s off). This sequential sonication was carried out for a total of 3 h. A 35 mL vial containing 30 mL of the precursor solution was immersed in water contained in a borosilicate sonication reactor cell with a volume
capacity of 100 mL. Argon gas was sparged through the reaction sample for 20 min before sonication and an Ar atmosphere was maintained above the solution throughout the experiment. A constant temperature of 28 °C was maintained during the sonication by circulating thermostated water through a jacket surrounding the sonication cell. All experiments were carried out at acoustic (calorimetric) power in the range of 110–125 mW/mL at high frequency and 390 mW/mL at low frequency. At regular time intervals, about 3 mL aliquots were withdrawn from the reactor and UV–vis absorption spectra were recorded on a Varian spectrophotometer (Cary Bio 50). For the synthesis of Ag–Au–GO nano-composites, two separate procedures were used consisting of simultaneous and sequential reduction of GO, Au(III) and Ag(I). In the case of the simultaneous reduction, both Ag(I) and Au(III) solutions were mixed with the GO suspension and sonicated for at least 3 h to ensure that both the GO and the metal ions were fully reduced. In the sequential reduction procedure, the GO and Au(III) were reduced first for one hour and then the Ag(I) solution was added and the sonication continued for another 2 h. 2.3. Characterization studies The morphology, size and size distribution of Au–Ag–GO catalysts were analyzed by transmission electron microscopy (JEOL, JEM 2100). The catalysts were placed on the surface of copper grids and dried under ambient conditions for TEM analysis. The surface topology and thickness of the prepared GO were measured by AFM (Asylum MEP-3D AFM) equipped with a linear variable differential transformer (LVDT)) sensor in the z-direction. XPS analysis was carried out using a M-probe apparatus (Surface Science Instruments). The source was monochromatic Al-Ka radiation (1486.6 eV). UV–vis absorption spectroscopy was carried out on a Varian Cary bio 50 spectrophotometer. 2.4. Catalytic activity-4-nitorphenol reduction The reduction of 4-nitrophenol (4-NP) was used to study the catalytic activities of monometallic and bimetallic catalysts [26]. 4-NP reduction reaction mechanism has been explained by Chen et al. [27]. 1.0–1.2 mg of the catalysts was added to a solution consisted of 4-NP (3.75 mL, 0.12 mM), sodium hydroxide (1.0 mL, 0.4 M), and freshly prepared sodium borohydride (1.0 mL, 0.06 M) under constant stirring at room temperature. The changes in the reaction mixture were recorded by UV–vis absorption spectroscopy at fixed time intervals. 3. Results and discussion 3.1. 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 reduction of both Au(III) and Ag(I) in the presence of GO was determined by UV–vis spectrophotometer. The UV–vis absorption of Ag and Au with GO were monitored around 320 nm and 520 nm, respectively. During the reduction process, 3 mL aliquots of samples were taken from the reactor at regular time intervals to monitor the progress of
Please cite this article in press as: B. Neppolian et al., Sonochemically synthesized mono and bimetallic Au–Ag reduced graphene oxide based nanocomposites with enhanced catalytic activity, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.02.006
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B. Neppolian et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx Table 1 Preparation conditions of Au–GO, Ag–GO monometallic and different ratios of Au and Ag bimetallic catalysts with GO prepared by sonochemical method.
*
S. No.
AgNO3 (mM)
HAuCl4 (mM)
Synthesis time (mins)
Sequential vs. simultaneous
1 2 3 4 5 6 7 8
2 – 2 2 1 1 2 2
– 2 2 2* 2 2* 1 I*
90 90 240 240 240 240 240 240
Both Both Sequential Sequential Sequential Sequential Sequential Sequential
Denotes Au–GO reduced first followed by Ag.
the reduction process. The absorption was gradually increased with increasing sonication time and attained a maximum value indicating the completion of the reduction process. On further increasing the sonication time, aggregation of metallic particles occurred leading to settling of the particles. For example, as can be seen from Fig. 1, the UV–vis absorption of Au–GO gradually increases with an increase in the sonication time, attaining a maximum at 150 min, and decreases at longer times. A similar trend was observed for Ag–GO (data are not shown here). The sonochemical reduction of both Ag–GO and Au–GO was investigated initially with two different approaches: (i) simultaneous reduction, i.e., reducing both GO and metal together, and (ii) sequential reduction, i.e., reducing GO first followed by the metal ion, as explained in the experimental section. It was observed from UV–vis spectra that there was very little difference between the above two approaches. This suggests that there was no competition between GO and the metal ions for the secondary reducing free radicals formed during acoustic cavitation. 3.2. Characterization studies AFM analysis can provide information on the surface morphology and thickness of GO sheets prepared by the modified Hummer’s method. It was clearly observed that the thickness of the GO was about 4.0 nm (figure not shown). This implies that GO formed during the sonochemical synthesis has a thickness corresponding to 2–3 monolayers. The size and morphology of Au–Ag–GO particles were analyzed by TEM. The TEM images are shown in Fig. 2. It can be seen that the average particle size of the Au particles is about 50 nm, whereas the Ag particles are about 5 nm. Both Au and Ag nanoparticles were formed separately in the form of islands of particles whether the
preparation was by the simultaneous or sequential method of ultrasound application. Formation of separate particles of both Au and Ag with GO was further confirmed by the TEM–EDX spectra as shown in Fig. 3. Further, the results reveal that there was neither a core–shell nor alloy material formed during the sonochemical method of preparation, whereas Anandan et al. reported that Ag–Au was formed as core–shell structure during sonochemical preparation method without GO [28]. It can be seen that both Au and Ag particles were formed separately, with most of the Ag particles surrounded by Au particles. This is likely due to the larger size of Au particles, causing the smaller Ag particles to aggregate around the larger sized Au particles (probably, pre-formed Au particles catalyzed the reduction of silver ions). TEM images (e.g., Fig. 2) clearly show a high amount of Au and Ag loading onto the GO support using dual frequency ultrasonication. Moreover, for the conditions applied, the size, morphology as well as the shape of the particles remained similar both in the simultaneous and sequential methods as observed from TEM images. X-ray photoelectron spectroscopy was used to determine the oxidation state of silver and gold in GO as well as the presence of epoxy group on GO sheets. XPS results confirmed the existence of the pure metallic state (zero valent) of Au and Ag along with GO based on the binding energies of both Au and Ag (data not shown) after 3 h of sonochemical preparation time. Moreover, the epoxy groups present in graphene oxide were substantially reduced in the presence of Au and Ag. These reactions typically involve the reduction of the functional groups such as epoxides (CAO, C@O, ACOOH and etc.) on the surface of GO, and the resulting suspensions of the reduced GO are more graphitic and conductive. The reduction of oxygenated functional groups present on GO surface was remarkably increased from up to 80% from 35%. This suggests the removal of epoxides groups substantially restoring the sp2 graphite structure. Complete reduction of GO to restore the sp2 graphite structure leads to unstable suspensions and GO aggregate themselves. This is very similar to our previous findings in the preparation of Au–GO catalyst [11] where it was observed that the reduction of GO increased in correspondence with an increase in the number of Au particles, as prepared by the sonochemical method. These results further confirmed that the sonochemical method is able to reduce the metallic ions to metal and also reduce GO to reduced graphene oxide (RGO) using dual frequency ultrasonicator. 3.3. Catalytic conversion of 4-NP to 4-aminophenol (4-AP) using Au– GO, Ag–GO and Au–Ag–GO catalysts
Fig. 1. UV–vis absorption spectra of Au–GO prepared by sonochemical method.
The catalytic performance of the prepared metallic catalysts (Au–GO and Ag–GO) with different metal loading ratios was tested by monitoring the reduction rate of 4-nitrophenol in the presence of sodium borohydride with specified amounts of catalyst (1.0 mg). The reduction process of 4-NP and the subsequent formation of 4-AP were monitored by UV–vis spectrophotometry [7]. Fig. 4 shows the rate of reduction of 4-NP with Ag–GO and Au–GO
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Fig. 2. TEM images of Ag–Au–GO (2:1) bimetallic photocatalysts prepared by sonochemical method.
1
Au-GO
Ag-GO 0.8
C/Co
0.6
0.4
0.2
0 0
100
200
300
400
500
4-NP conversion me (sec) Fig. 3. TEM–EDX images of Ag–Au–GO (2:1) bimetallic photocatalysts prepared by sonochemical method.
catalysts in the presence of sodium borohydride. The catalytic performance of both catalysts was very low for the reduction of 4-NP. It is widely reported that the catalytic performance of monometallic particles is not good enough for the total reduction or oxidation process, whereas bimetallic nanoparticles exhibit enhanced catalytic performance for the reduction or oxidation reaction due to various reasons including easy transfer of electrons from one metal to another [29]. Liu et al. have reported that MCM-41 supported monometallic Au and Ag catalyst was less efficient for oxidation of CO than the bimetallic Au–Ag–MCM-41 [29]. Tang et al. also demonstrated that Ag-carbon supported monometallic catalysts showed less catalytic efficiency on the reduction of 4-NP to 4-AP than Au–Ag–carbon supported catalysts [7]. The rate of reduction depends mainly on the morphology of the particles whether those are in alloy form or core–shell or separate particles. Hence, bimetallic particles with specific morphology play an important role in determining the rate of the catalytic reduction reaction. The bimetallic catalysts (Au–Ag with GO) were also synthesized by both simultaneous and sequential methods and the catalytic activity was checked through 4-NP reduction process. It is interesting to note that the rate of reaction of 4-NP was found to be considerably less with the catalysts prepared by simultaneous
Fig. 4. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of both Ag–GO and Au–GO catalysts.
method than those prepared by the sequential method. This is probably due to the completion between both metallic ions for the secondary reducing free radicals during the reduction process. Therefore, the bi-metallic catalysts with various metal loading ratios were obtained by adopting only sequential method (Table 1). It is noteworthy that the rate of conversion of 4-NP was remarkably high with Ag–Au–GO (1:1), i.e., Au(III)-GO is reduced first followed by Ag(I) reduction during the catalyst synthesis (Fig. 5b). It is well known that the reduction potential of Au(III) is more positive than that of Ag(I), and particularly when Ag(I) ions are isolated [30]. As a result, the rate of reduction of Au(III) is very fast in comparison with Ag(I). Chen et al. observed similar phenomenon that the formation of Au nanoparticles was faster than the formation of Ag nanoparticles during the synthesis of Au–Ag bimetallic nanoparticles [31]. Once Au(III) is reduced to its metal state, it receives and stores excess electrons in its outer sphere, and acts as a source (pool) of electrons, which enhances the reduction rate of both GO and Ag(I). The results indicate that the graphite nature of GO was improved substantially in the presence of Au, which further facilitates fast transport of photoformed electrons from Au to GO. As a result, the catalytic activity of Ag–Au–GO bimetallic catalyst
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Fig. 5. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of catalyst: (a) Au–Ag (1:1)–GO (Ag reduced first with GO); (b) Ag–Au (1:1)–GO (Au reduced first with GO).
considerably enhanced in the presence Au and GO. This is similar to our recent work on Pt–GO–TiO2 photocatalysts, which exhibited very high activity for the degradation of organic pollutants [19]. Further, the rate of 4-NP reduction was carried out with higher amount of Ag loading, i.e., 2:1 ratio of Ag and Au along with GO and the results are presented in Fig. 6. At all metal ion ratios, the catalyst where Au–GO was synthesized first achieved higher catalytic activity than Ag–GO prepared first followed by Au (Fig. 6a and b). The 2:1 ratio of Ag–Au–GO has shown exceptionally high catalytic activity for 4-NP reduction (Fig. 6b) amongst the different bimetallic- and mono-metallic-GO catalysts. It is found that Ag–Au with the ratio 2:1 is the optimal blend and the best ratio for a total conversion of 4-NP to 4-AP within a short span of time. It was
observed that within 10 min of reaction time, 4-NP was completely converted into 4-AP. Furthermore, the effect of high amount of Au was carried out with Ag–GO. Ag–Au with 1:2 ratio showed very less activity for the 4-NP conversion in comparison with 2:1 ratio of Ag:Au as presented in Fig. 7. The catalytic efficiency was found to be less with high ratio of Au whether Au reduced first or Ag reduced first with GO. The rate of conversion of 4-NP was notably reduced with high amount of Au loading, almost 3 times less than that of Ag–Au–GO (2:1). The size of Au was relatively larger than that of Ag and Ag nanoparticles were enriched on Au surfaces. As the result, the catalytic efficiency substantially reduced with less amount of Ag. This observation emphasizes that less amount of Au is sufficient
Fig. 6. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of catalyst: (a) Au–Ag (1:2)–GO (Ag reduced first with GO); (b) Ag–Au (2:1)–GO (Au reduced first with GO).
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1 Ag-Au-GO 0.8
Au-Ag-GO
C/Co
0.6
0.4
0.2
0 0
100
200
300
400
500
4-NP conversion me (sec) Fig. 7. UV–vis absorption spectra of 4-NP reduction with 1.0 mg of both catalysts: Au–Ag–GO (Ag reduced first with GO) and Ag–Au–GO (Au reduced first with GO).
for the 4-NP reduction that is added advantage on the cost effective approach in the catalytic reduction studies. 4. 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 in Au assisted for the maximum reduction of Ag and GO during sonochemical preparation. 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. References [1] P.V. Kamat, Meeting the clean energy demand: nanostructure architectures for solar energy conversion, J. Phys. Chem. C 111 (2007) 2834–2860. [2] K. Park, R.A. Vaia, Synthesis of complex Au/Ag nanorods by controlled overgrowth, Adv. Mater. 20 (2008) 3882–3886. [3] J. Luoa, M.M. Mayea, N.N. Kariukia, L. Wanga, P. Njokia, Y. Lina, M. Schadta, H.R. Naslundb, C.J. Zhonga, Electrocatalytic oxidation of methanol: carbonsupported gold–platinum nanoparticle catalysts prepared by two-phase protocol, Catal. Today 99 (2005) 291–297. [4] C. Xu, X. Wang, J. Zhu, Graphene–metal particle nanocomposites, J. Phys. Chem. C 112 (2008) 19841–19845. [5] M.M. Maye, Y. Lou, C.J. Zhong, Core–shell gold nanoparticle assembly as novel electrocatalyst of CO oxidation, Langmuir 16 (2000) 7520–7523.
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