Article pubs.acs.org/IECR
Green Route for Microwave-Assisted Preparation of AuAg-AlloyDecorated Graphene Hybrids with Superior 4‑NP Reduction Catalytic Activity Hongyu Chen,† Xiaobin Fan, Jingwen Ma, Guoliang Zhang, Fengbao Zhang, and Yang Li* School of Chemical Engineering & Technology, Tianjin University, 300072 Tianjin, P. R. China S Supporting Information *
ABSTRACT: A microwave-assisted green and facile route has been developed to disperse sub-6-nm Au−Ag alloy bimetallic nanoparticles on the surface of partially reduced graphene oxide (PRGO). The properties of the resulting materials were characterized by high-resolution transmission electron microscopy, scanning electron microscopy, EDX line scanning, UV−vis spectroscopy, ICP spectrometry, and X-ray photoelectron spectroscopy. The reduction reaction of 4-nitrophenol (4-NP) was investigated to evaluate the catalytic performance of AuAg−graphene hybrids, and it was found that this unique catalytic system exhibits excellent activity for this reaction. It is hoped that this fast route can be extended to prepare other preferred bimetallic nanoparticle structures with PRGO supports.
■
■
INTRODUCTION
EXPERIMENTAL SECTION Preparation of Graphene Oxide and Graphene Metal Hybrids. Graphite oxide was prepared by the Hummers method17 and exfoliated into graphene oxide (GO) by sonication in water. Typically, trisodium citrate (0.06 g) was added to 30 mL of the graphene oxide suspension (1 mg· mL−1). The mixture was heated to 100 °C in a microwave reactor, and then 1 mL of 10 mM AgNO3, 1 mL of 10 mM HAuCl4, and 1.0 mL of 0.1 M ascorbic acid (AA) were added. The resulting mixture was refluxed at 100 °C with mechanical stirring for 2 min. After being allowed to cool to room temperature, the mixture was washed extensively with deionized water and then centrifuged several times. Hybrids prepared by this method are denoted as AuAg−G (MWI) (where MWI stands for microwave irradiation). For comparison, the same preparation conditions except with heating in an oil bath for 3.5 h (instead of in the microwave oven) were applied to prepare samples denoted as AuAg−G (OB). Graphene-supported monometallic Au NP composites were also prepared by the same MWI route, using 1.5 mL of HAuCl4 to ensure that the same weight percentage of metal was added to the solution. Catalytic Reduction of 4-NP by AuAg−G Hybrids. To investigate the catalytic activity of the as-prepared hybrids, the reduction reaction of 4-nitrophenol (4-NP) was tested. In a standard procedure, chemicals were added into a quartz cuvette at room temperature in the following order: 2.8 mL of 4-NP (1.0 × 10−4 M), 30 × 10−3 mL of AuAg−G hybrids (1 mg· mL−1), and 0.2 mL of NaBH4 (1.5 × 10−1 mol·L−1). The reaction was monitored by observing the UV−visible absorption spectra from 250 to 550 nm. To guarantee the
2
With a unique structure of a one-atom-thick planar sheet of sp bonded carbon atoms, graphene1 combines many advantages such as large surface area,2 impressive mechanical strength,3and high conductivity.4 Thus, graphene is considered a promising candidate for the dispersion of catalytically active metal nanoparticles (NPs)5 and a good support for heterogeneous catalytic processes.6,7 Graphene-supported monometallic NP composites have been widely studied, and a strong metal− graphene interaction was also revealed and might contribute to the enhanced catalytic performances8 of supported monometallic nanoparticles. Compared with individual NPs, bimetallic NPs show enhanced catalytic properties.9,10 Therefore, graphene-supported bimetallic NP composites have attracted increasing attention.11−13 However, difficulties such as a lack of control of bimetallic nanostructures need to be addressed to achieve good dispersion of NPs on the surface of graphene. Most recently, a Pt−Co bimetallic alloy nanodendrite hybrid was successfully dispersed on the surface of graphene by a room-temperature route for more than 24 h.14 Ag@Au core− shell nanostructures with a particle size of around 20 nm were also decorated on the surface of graphene by a two-step sputtering method.15 Now, the ability to plant highly dispersed and well-controlled nanostructures on graphene by simple, green, and fast methods is becoming increasingly important. Motivated by the above situation, we have prepared unique bimetallic AuAg alloy−graphene hybrids (AuAg−G) by a superfast microwave-assisted one-step method. With Au enrichment and well-dispersed sub-6-nm particles, the composites show superior catalytic activity toward the reduction reaction of 4-nitrophenol (4-NP). Moreover, Ag has a lower work function than Au,16 so electrons might shift from Ag to Au, contributing to a prominent synergistic effect for catalytic application and reducing the use of precious Au. © XXXX American Chemical Society
Received: August 18, 2014 Revised: October 19, 2014 Accepted: October 23, 2014
A
dx.doi.org/10.1021/ie503251h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
same reaction time for all reactions, timing was started once NaBH4 was added, and the absorbance evolution was recorded after the same interval (25 or 40 s) within a certain time duration (the zero point was recorded by replacing NaBH4 by the same amount of NaOH). Characterization. The as-prepared AuAg−G hybrids were characterized by high-resolution transmission electron microscopy (HRTEM) (Philips Tecnai G2 F20), scanning electron microscopy (SEM) (Hitachi S-4800), X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer, PHI 1600 spectrometer), X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and EDX line-profile analysis in scanning transmission electron microscopy (STEM) mode (Philips Tecnai G2 F20 and Hitachi S-4800). The alloy compositions of the hybrids were also characterized by inductively coupled plasma (ICP) spectrometry. The catalytic activity of AuAg−G hybrids was measured by UV absorption spectroscopy on a UV2802H system with a temperature controller.
Figure 2. (a) HRTEM image of a single AuAg alloy nanoparticle on graphene. (b) STEM-HADDF image and (inset) HADDF line scan of the nanoparticle.
value.18,19 The formation of a face-centered cubic (fcc) nanostructure of the nanoparticles is further confirmed by the XRD pattern (Figure S2, Supporting Information). To verify the exact structure and composition of the NPs, a high-angle annular dark-field (HAADF) line-scan analysis of the nanoparticle in Figure 2b was performed, and the results are shown in the inset of Figure 2b. The red and black lines represent Au and Ag, respectively. Both Au and Ag have similar composition profiles, and the signal of Au is higher than that of Ag across the whole particle. The HAADF scan results indicate the formation of Au−Ag alloy particles with a higher content of Au. The effective electronegativities of both Au and Au are larger than 1.93, which might help to explain the formation of the Au−Ag alloy structure by the co-reduction synthetic method.20 UV−vis spectroscopic analysis was conducted to further confirm the nanostructure of the graphene-supported bimetallic NPs. Au and Ag NPs have special absorbance bands called surface plasmon resonance (SPR) bands.21 Alloy NPs have only one absorbance peak whose position is between the peaks of the pure NPs, whereas for physically mixed nanoparticles, two absorbance peaks will be observed.22 The characteristic UV−vis peak of Au nanoparticles should be around 520 nm.23 However, in the metal−graphene [graphene oxide (GO), partially reduced graphene oxide (PRGO), or graphene] system, a broadening and red shift of the plasmon band of the Au NPs will occur depending on the size and dispersion of the Au NPs.24,25 In this work, the UV−vis spectra of AuAg−G and Au−G were obtained (Figure S3, Supporting Information). AuAg−G exhibited only one characteristic metal peak that was blue-shifted from ∼530 to 492 nm when compared with that of Au−G. These results indicate the formation of Au−Ag alloy nanoparticles in our system. The coexistence of Ag and Au on the surface of graphene was further confirmed by XPS analysis (Figure 3c), from which the surface atomic ratio of Au to Ag was found to be 0.16:0.2, consistent with the ICP results. Figure 3a,b shows the XPS spectra of the Ag 3d doublet (3d5/2 and3d3/2) and the Au 4f doublet (4f7/2 and 4f5/2) for Ag and Au in AuAg−G, indicating the complete reduction of the Ag26 and Au27 precursors. Notably, the peaks of Au 4f7/2, Au 4f5/2, Ag 3d5/2, and Ag 3d3/2 all shifted slightly to lower binding energies compared with the standard characteristic metallic Ag0 and Au0 peaks.25,28 The negative shift arises from electron transfer from the graphene nanosheets to the Au−Ag alloy, which further confirms that the Au−Ag alloy was anchored onto the surface of the graphene. Chemical and structural changes of GO after the thermal reduction process were also investigated. XPS analysis showed evident and partial removal of the negatively charged oxide
■
RESULTS AND DISCUSSION Synthesis of AuAg−Graphene Hybrids. The SEM image of the purified hybrids [AuAg−G (MWI)] shows that spherical nanoparticles with diameters of less than 6 nm were densely dispersed on the surfaces of graphene sheets (Figure S1, Supporting Information). This result is supported by the HRTEM images shown in Figure 1a,b. The corresponding
Figure 1. (a) Standard and (b) enlarged TEM images of AuAg nanoparticles homogeneously decorated on the surface of graphene. (c) EDX spectrum of a single metallic nanoparticle dispersed on graphene sheets. (Note that the Cu peaks come from the copper grid.)
EDX analysis (Figure 1c) shows the coexistence of Ag and Au elements on graphene with an atomic Ag-to-Au ratio of 35.8:64.2, a value lower than the stoichiometric ratio (1:1) of the metal precursors, which might due to the fact that EDX spectroscopy can only obtain molar ratios of various NPs, rather than the global composition of the hybrids. The composition of the alloys was further investigated by ICP spectrometry. The corresponding results show that the global molar ratio of Ag to Au was about 0.82, which is close to the precursor ratio as initially added. A magnified image of one of the particles (Figure 2a) shows a clear crystal lattice with an interplanar spacing of 0.235 nm. The (111) lattices of both Ag and Au are very close to this B
dx.doi.org/10.1021/ie503251h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 3. (a) Au 4f doublet and (b) Ag 3d doublet XPS spectra in AuAg−G hybrids and (c) XPS spectra of AuAg−G hybrids.
Figure 4. TEM images and corresponding size distributions of (a) AuAg−G (MWI), (b) Au−G (MWI), and (c) AuAg−G (OB).
Catalytic Reaction. The reduction reaction of 4-nitrophenol (4-NP) has attracted great attention because the product 4-aminophenol (4-AP) is an important intermediate for drug manufacture.29 For reasons of energy savings, safe operation, and avoidance of the use of organic solvents, it might be interesting and meaningful to develop a process for the conversion of 4-NP to 4-AP in aqueous solution under mild conditions.30 The catalytic properties of our samples toward this reaction were investigated. The conversion of 4-NP to 4AP can be easily monitored by tracking the changes in the absorbance peak of 4-nitrophenolate ions at 400 nm (Figure S5, Supporting Information). When only 30 × 10−3 mg of AuAg−G catalyst was added, the reduction commenced quickly, and the yellow color of 4-NP under alkali conditions was completely bleached within 1 min (Figure 5). The concentration of BH4− was much higher than that of 4-NP and remained basically constant during the reaction. Therefore, pseudo-first-order kinetics with respect to
functional groups (Figure S4, Supporting Information), which might contribute to the stability of the hybrids in water. To compare the catalytic properties, AuAg−G (OB) prepared by oil-bath heating and Au−G prepared by MWI were also investigated. The amounts of metal precursor were determined to ensure that the same weight percentage of metal element was introduced into each system. The average NP size of AuAg−G prepared by MWI was around 5.3 nm (see Figure 4). However, obvious aggregation was observed for the AuAg− G (OB) samples. The existence of large aggregates makes it hard to obtain a representative average size of the NPs, and the size of the dispersed particles was measured and found to be about 9.0 nm. For the Au−G (MWI) (same amount of Au precursor added as in the AuAg−G hybrids), the average NP diameter was around 6.3 nm, which was close to that of AuAg− G (MWI) and smaller than that obtained by the oil-bath method. C
dx.doi.org/10.1021/ie503251h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
facilitates the uptake of electrons by 4-NP molecules, thus improving the catalytic performance. This effect plus with the superfast MWI reaction leading to smaller particle size compared with single Au can explain why AuAg−G (MWI) has a catalytic activity superior to that of Au−G (MWI).
■
CONCLUSIONS In summary, a simple, green, and fast microwave-assisted method was employed to prepare graphene-supported AuAg bimetallic alloy composites. In this route, the precursor solution is heated evenly and quickly, leading to an outburst of nucleation sites and also higher nucleation rates. Moreover, the galvanic replacement reaction between the zero-valent Ag and Au precursors can further reduce the particle size. As a result, sub-6-nm particles are well-dispersed on the surface of graphene. Moreover, the potential energy of Ag is smaller than that of Au, so electrons can transfer from Ag to Au, contributing to a synergistic effect for the catalytic activity of the hybrids. This superfast microwave-assisted route can be readily changed to prepare other graphene-supported bimetallic nanoparticles with preferred structures and activities toward specific catalytic reactions.
Figure 5. Time-dependent absorbance changes at 400 nm in the presence of MWI AgAu−G, MWI Au−G, and oil bath AgAu−G.
4-NP can be applied to evaluate the catalytic activity. The rate constant k was determined from a plot of ln(A) (where A is the absorbance at 400 nm) versus reduction time. The corresponding pseudo-first-order rate constants for AuAg−G (MWI), Au− G (MWI), and AuAg−G (OB) were found to be 0.03, 0.011, and 0.0089 s−1, respectively (Figure 5). However, the content of material was based on weight rather than molar or atomic amount, and the particle sizes varied between samples. To obtain a correction factor based on surface area, a single metal nanoparticle was approximated as a sphere with radius r. The volume of that metal nanoparticle should be v = 4/3πr3. The superficial area of a metal nanoparticle equals 4πr2. Therefore, the total specific surface area of the nanoparticle is Stotal = 4Nπr2, where N is the number of metal nanoparticles, which can be expressed as N = vtotal/v. Substituting N and v into the expression for Stotal, gives Stotal = 3vtotal/r. Because the precursor mass added is already known, vtotal for each system can be roughly calculated. Using the average NP diameter, Stotal was obtained for AuAg−G (MWI), AuAg−G (OB), and Au−G (MWI), and the following relationship was found: Stotal[AuAg− G (MWI)] = 1.6Stotal[Au−G (MWI)] = 1.7Stotal[AuAg−G (OB)]. Upon application of correction factors based on Stotal, the corresponding pseudo-first-order rate constants of AuAg− G (MWI), Au−G (MWI), and AuAg−G (OB) changed from 0.03, 0.011, and 0.0089 s−1 to 0.03, 0.0176, and 0.0153 s−1, respectively. After the correction, the catalytic performance of the AuAg−G (MWI) system was still the best. That is, in addition to the size effect, the superior catalytic activity of AuAg−G (MWI) can be attributed to the synergistic catalytic effect arising from the alloy structure. AuAg−G (MWI) showed superior catalytic performance toward the 4-NP reduction reaction, especially considering the small amount catalyst added. The partially reduced graphene support guaranteed the stability of the composite in aqueous solution, and no catalyst aggregation was observed before or after the reaction (Figure S6, Supporting Information). Moreover, during the MWI process, the mixture is heated uniformly and rapidly (preparation finished within 5 min), leading to higher nucleation rates and more uniform and smaller nanoparticles compared with traditional oil-bath method. In addition, Ag has a lower work function than Au.31 Therefore, electrons tend to leave the Ag region into the Au region, which ends up being electron-enriched. The presence of these surplus electrons inside the Au region
■
ASSOCIATED CONTENT
S Supporting Information *
SEM image of the hybrid Nano composites, XRD pattern of AuAg−G hybrid, UV−vis spectra of AuAg−G and Au−G, XPS spectra of GO and the as-prepared AuAg−G hybrids, successive UV−vis absorption spectra showing the reduction of 4-NP by NaBH4 in the presence of different catalysts, optical images before and after the 4-NP reduction reaction. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
* Tel.: +86-22-27890090. E-mail:
[email protected]. Present Address
† Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This study was supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (Nos. 20120032110025, 20130032120017), the National Natural Science Funds for Excellent Young Scholars (No. 21222608), the Research Fund of the National Natural Science Foundation of China (No. 21106099), the Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 201251), the Natural Science Foundation of Tianjin (No. 14JCQNJC05800), and the Program of Introducing Talents of Discipline to Universities (No. B06006).
■
REFERENCES
(1) Novoselov, K. S.; Geim, A. K. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669.
D
dx.doi.org/10.1021/ie503251h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
(2) McAllister, M. J.; Li, J.-L. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396−4404. (3) Dikin, D. A.; Stankovich, S. Preparation and characterization of graphene oxide paper. Nature 2007, 448, 457−460. (4) Kopelevich, Y.; Esquinazi, P. Graphene Physics in Graphite. Adv. Mater. 2007, 19, 4559−4563. (5) Li, Y.; Fan, X. Gold nanoparticles−graphene hybrids as active catalysts for Suzuki reaction. Mater. Res. Bull. 2010, 45, 1413−1418. (6) Li, Y.; Gao, W. Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010, 48, 1124−1130. (7) Kou, R.; Shao, Y. Stabilization of Electrocatalytic Metal Nanoparticles at Metal−Metal Oxide−Graphene Triple Junction Points. J. Am. Chem. Soc. 2011, 133, 2541−2547. (8) Jasuja, K.; Berry, V. Implantation and Growth of Dendritic Gold Nanostructures on Graphene Derivatives: Electrical Property Tailoring and Raman Enhancement. ACS Nano 2009, 3, 2358−2366. (9) Schneider, C.; Masi, D. Palladium- and Copper-Catalyzed Stereocontrolled Direct CH Fluoroalkenylation of Heteroarenes Using gem-Bromofluoroalkenes. Angew. Chem. 2013, 125, 3328−3331. (10) Rodriguez, B. A.; Delferro, M. Retraction of “Bimetallic Effects for Enhanced Polar Comonomer Enchainment Selectivity in Catalytic Ethylene Polymerization”. J. Am. Chem. Soc. 2013, 135, 17651−17651. (11) Jin, X.; Dang, L. Lattice-Matched Bimetallic CuPd−Graphene Nanocatalysts for Facile Conversion of Biomass-Derived Polyols to Chemicals. ACS Nano 2013, 7, 1309−1316. (12) Yin, H.; Liu, S. Well-Coupled Graphene and Pd-Based Bimetallic Nanocrystals Nanocomposites for Electrocatalytic Oxygen Reduction Reaction. ACS Appl. Mat Interfaces 2014, 6, 2086−2094. (13) Chen, H.; Li, Y. Graphene supported Au−Pd bimetallic nanoparticles with core-shell structures and superior peroxidase-like activities. J. Mater. Chem. 2011, 21, 17658−17661. (14) Yang, P.; Jin, S.-Y. Decorating PtCo Bimetallic Alloy Nanoparticles on Graphene as Sensors for Glucose Detection by Catalyzing Luminol Chemiluminescence. Small 2013, 9, 199−204. (15) Samal, A. K.; Polavarapu, L. Size Tunable Au@Ag Core−Shell Nanoparticles: Synthesis and Surface-Enhanced Raman Scattering Properties. Langmuir 2013, 29, 15076−15082. (16) Zhang, Q.; Xie, J. Synthesis of Ag@AgAu Metal Core/Alloy Shell Bimetallic Nanoparticles with Tunable Shell Compositions by a Galvanic Replacement Reaction. Small 2008, 4, 1067−1071. (17) Hummers, W. S., Jr; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (18) Majimel, J.; Lamirand-Majimel, M. Size-Dependent Stability of Supported Gold Nanostructures onto Ceria: An HRTEM Study. J. Phys. Chem. C 2009, 113, 9275−9283. (19) van der Vegt, H. A.; van Pinxteren, H. M. Surfactant-induced layer-by-layer growth of Ag on Ag(111). Phys. Rev. Lett. 1992, 68, 3335−3338. (20) Wang, D.; Peng, Q. Nanocrystalline intermetallics and alloys. Nano Res. 2010, 3, 574−580. (21) Mulvaney, P. Surface Plasmon Spectroscopy of Nanosized Metal Particles. Langmuir 1996, 12, 788−800. (22) Han, S. W.; Kim, Y. Dodecanethiol-Derivatized Au/Ag Bimetallic Nanoparticles: TEM, UV/VIS, XPS, and FTIR Analysis. J. Colloid Interface Sci. 1998, 208, 272−278. (23) Daniel, M.-C.; Astruc, D. Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 2004, 104, 293−346. (24) Huang, J.; Zhang, L. Nanocomposites of size-controlled gold nanoparticles and graphene oxide: Formation and applications in SERS and catalysis. Nanoscale 2010, 2, 2733−2738. (25) Li, J.; Liu, C.-y. Au/graphene hydrogel: Synthesis, characterization and its use for catalytic reduction of 4-nitrophenol. J. Mater. Chem. 2012, 22, 8426−8430.
(26) He, F.-A.; Fan, J.-T. Fabrication of hybrids based on graphene and metal nanoparticles by in situ and self-assembled methods. Nanoscale 2011, 3, 1182−1188. (27) Yu, K.; Wu, Z. High-Temperature-Stable Au@SnO2 Core/Shell Supported Catalyst for CO Oxidation. J. Phys. Chem. C 2008, 112, 2244−2247. (28) Yeo, Y.-C.; King, T.-J. Metal-dielectric band alignment and its implications for metal gate complementary metal-oxide-semiconductor technology. J. Appl. Phys. 2002, 92, 7266−7271. (29) Hayakawa, K.; Yoshimura, T. Preparation of Gold−Dendrimer Nanocomposites by Laser Irradiation and Their Catalytic Reduction of 4-Nitrophenol. Langmuir 2003, 19, 5517−5521. (30) Kuroda, K.; Ishida, T. Reduction of 4-nitrophenol to 4aminophenol over Au nanoparticles deposited on PMMA. J. Mol. Catal. A: Chem. 2009, 298, 7−11. (31) Wilson, O. M.; Scott, R. W. J. Synthesis, Characterization, and Structure-Selective Extraction of 1−3-nm Diameter AuAg DendrimerEncapsulated Bimetallic Nanoparticles. J. Am. Chem. Soc. 2004, 127, 1015−1024.
E
dx.doi.org/10.1021/ie503251h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
