In Situ Synthesis of Metal Nanoparticles on Single-Layer Graphene

May 28, 2009 - layer graphene oxide (GO) and reduced graphene oxide (r-GO) surfaces ... graphene.8,10,11 This method provides an easy way to produce a...
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2009, 113, 10842–10846 Published on Web 05/28/2009

In Situ Synthesis of Metal Nanoparticles on Single-Layer Graphene Oxide and Reduced Graphene Oxide Surfaces Xiaozhu Zhou,† Xiao Huang,† Xiaoying Qi,† Shixin Wu,† Can Xue,† Freddy Y. C. Boey,† Qingyu Yan,† Peng Chen,‡ and Hua Zhang*,† School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore, and School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457, Singapore ReceiVed: April 26, 2009; ReVised Manuscript ReceiVed: May 13, 2009

A straightforward one-step chemical method to in situ synthesis of Ag nanoparticles (Ag NPs) on singlelayer graphene oxide (GO) and reduced graphene oxide (r-GO) surfaces is proposed. After simply heating the single-layer GO or r-GO adsorbed on 3-aminopropyltriethoxysilane (APTES)-modified Si/SiOx substrates in a silver nitrate aqueous solution at 75 °C, Ag NPs are synthesized and grow on the GO or r-GO surface. The obtained Ag NPs are investigated by atomic force microscopy, scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Our method is unique and important since no reducing agent is required in the reaction. Au NPs on a GO surface are obtained by simply immersing the obtained Ag NPs on the GO surface in HAuCl4 solution. Graphene, a single layer of bonded-sp2 carbons compacted into a two-dimensional honeycomb lattice, has attracted intense interest recently.1-3 This material is fascinating in its exceptional electronic, thermal, and mechanical properties with the promise of a range of applications.4-7 However, the mechanical cleavage method used for fabrication of such materials is unrealistic for practical applications since it is impossible to generate a large quantity of graphene. In order to scale up the output of graphene, many methods have been developed.5,8,9 In one of the most promising methods, the graphene oxide (GO) in aqueous solution, generated by the oxidization and subsequent exfoliation of graphite, is reduced to graphene by hydrazine, which transforms electrically nonconductive GO to highly conductive graphene.8,10,11 This method provides an easy way to produce a large quality of graphene. GO has been receiving increased attention due to its nature to be easily reduced to graphene, referred to as reduced graphene oxide (r-GO) in this paper, which has wide applications.1,5-7 For example, the r-GO film has shown applications in solar cells.12-14 Besides the application in chemistry and materials science, r-GO has also played a role in biology. Recently, Chen et al. reported that r-GO films can be used to grow cells and show excellent biocompatibility.15 Nanocomposites or hybrid materials have stimulated intense research over past decades due to their new optical, electronic, thermal, mechanical, and catalytic properties.6,16-19 Besides the applications of GO and r-GO, it is a great desire to fabricate composites or hybrid materials which integrate GO or r-GO with polymers, nanoparticles (NPs) or even nanotubes and * Author to whom correspondence should be addressed. Tel: +6567905175. Fax: +65-67909081. E-mail: [email protected]. Website: http:// www.ntu.edu.sg/home/hzhang/. † School of Materials Science and Engineering. ‡ School of Chemical and Biomedical Engineering.

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fullerenes. With large surface area and the aforementioned properties, GO has been an attractive choice as the matrix for nanocomposites. For example, Stankovich et al. have reported the graphene-based polymer composites by using GO as a support, which showed improved electronic and thermal conductivities after GO was reduced (i.e., r-GO).6 This will open a new kind of composite material and possibly bring in new functionality and properties. Metal oxide NPs have also been integrated in GO to form composites. For example, Co3O420 and TiO221 NPs have been deposited on GO sheets. Due to the catalytic activities of the aforementioned NPs, the resulting composite materials showed promises in catalysis applications. However, few reports used GO or r-GO as a template to directly synthesize metal NPs and directly fabricate metal NP-GO composites on substrates. Metal NPs are of great importance due to their optical, catalytic, and electrical properties.22 It is also a long-term desire to integrate metal NPs into composite materials to explore their properties and applications. Therefore, it is of critical interest if metal NPs can be integrated with GO or r-GO or synthesized by using GO or r-GO as a template. Recently, Muszynski et al. have synthesized Au NPs using chemical reduction of HAuCl4 with NaBH4 in the graphene-octadecylamine suspension.23 However, there is no report to directly synthesize metal NPs on a single-layer GO surface without any reducing agent. Herein, we present a simple method to synthesize metal Ag NPs on GO and r-GO surfaces. Importantly, one-step synthesis of Ag NPs on GO and r-GO surfaces is achieved without any surfactant or reducing agent. Briefly, GO substrate was obtained by adsorption of GO on 3-aminopropyltriethoxysilane (APTES)modified SiOx substrate. Such a substrate was immersed in an aqueous solution of silver nitrate under N2 protection and heating at 75 °C in different reaction time. Ag NPs were obtained on GO. The component of the as-synthesized NPs is assigned to  2009 American Chemical Society

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SCHEME 1a

a (1) GO is adsorbed on APTES-modified SiOx substrate. (2) GO is reduced, and r-GO is obtained. (3) Growth of Ag particles by heating the r-GO substrate in 0.1 M AgNO3 at 75 °C for 30 min. (4) Growth of Ag NPs by heating the GO substrate in 0.1 M AgNO3 at 75 °C for 30 min.

be Ag by X-ray diffraction and EDX. The size is measured by both transmission electron microscopy (TEM) and atomic force microscopy (AFM). The as-synthesized Ag NPs, strongly attached to the GO surface, proved to be SERS active. Au NP-GO composites were obtained by immersing the obtained Ag NP-GO in HAuCl4 solution. Using a similar experiment, r-GO is also used as a template to grow Ag particles, whose size is dramatically different from that of Ag NPs grown on GO. Scheme 1 shows the experimental process for synthesis of Ag NPs on GO and r-GO surfaces. After the synthesized GO was adsorbed on the APTES-modified SiOx substrate (Step 1 in Scheme 1), the substrate was characterized by AFM. Figure 1A shows that the GO sheets with micro-scale size adsorbed on the APTES-modified SiOx substrate. The measured thickness of the GO sheet is ∼1.3 nm, which means that the single layer GO sheet was obtained. This is in agreement with the previous report.8 Note that the adsorbed GO density on APTES-modified SiOx substrates can be controlled by adjusting the concentration of the GO aqueous solution and the incubation time. The r-GO substrate is achieved by reducing the GO substrate in a hydrazine vapor environment (step 2 in Scheme 1). After the thus-generated r-GO substrate is heated in a 0.1 M AgNO3 aqueous solution at 75 °C for 30 min (step 3 in Scheme 1), the SEM image shows there are lots of particles adsorbed on the r-GO surface (Figure 1B), which are confirmed to be Ag particles by XRD and EDX (Figure S1 in the Supporting Information). Importantly, note that in our experiment no additional reducing agent was added. During the experiment, the UV-vis spectra did not show any characteristic evidence of Ag NPs forming in the solution. Therefore, we believe that the r-GO substrate serves as not only a surface template to adsorb Ag NPs but also the donor of electrons for the growth of Ag particles. Very recently, similar to our work, Jung et al. used r-GO films to directly reduce HAuCl4 to get Au NPs without any reducing agent.24 They proposed the mechanism for reduction of Au NPs on r-GO sheets which likely involves Galvanic displacement and redox reaction by the relative potential difference. Similar mechanism has been proposed to explain the decoration of carbon nanotubes with metal particles.25 Based on their description, our observation can be explained in the following reasons. First, since the electrons in the negatively charged substrate can participate in the reduction of metal cations,26 Ag+ could obtain the electrons presenting in the negatively charged r-GO surface to form Ag NPs. Second, since the reduction potential of the r-GO is +0.38 V vs SHE (standard hydrogen electrode),24 which is lower than that of Ag+ (+0.73 V vs SHE in our experimental condition),27 it is therefore

Figure 1. (A) Tapping mode AFM topographic image and height profile of single-layer GO adsorbed on APTES-modified SiOx substrate. (B) SEM image of Ag particles grown on r-GO surface. (C) Tapping mode AFM topographic image and height profile of Ag NPs grown on single-layer GO surface.

possible for Ag+ in an aqueous solution to be reduced spontaneously on the r-GO surface since the electrons can be donated from the negative-charged r-GO surface, which also has a lower reduction potential. The size of Ag particles is not uniform, ranging from a few tens of nanometers to 1 µm (Figure 1B). The large distribution of formed Ag NPs is similar to that of Au NPs formed on r-GO films with sizes ranging from several nanometers to 200 nm.24 The GO sheet, which contains carboxylic acid, hydroxyl, and epoxide groups, can be considered as the oxidation of a graphene sheet. In other words, we can say the GO sheet is composed of many small aromatic conjugated domains modified with the functional groups mentioned above. After the GO substrate was immersed in 0.1 M AgNO3 aqueous solution at 75 °C for 30 min (step 4 in Scheme 1), AFM (Figure 1C) and SEM (Figure 2A) were used to characterize the surface morphology, where lots of small NPs were observed and were exclusively sitting on GO surface. Note that almost no NP adsorbed on the bare APTES-modified SiOx. The measured size of NPs is 6.0 ( 3.6 nm, based on TEM analysis, Figure S2A. This size distribution is consistent with the measurement from the AFM image in Figure 1C. XRD is used to confirm the component of synthesized NPs, Figure 2B, where it clearly shows that Ag NPs are obtained on GO surface. Interestingly, in Figure 1C and Figure 2A, a much higher density of Ag NPs with smaller sizes are formed on the GO surface compared to the lower-density and bigger Ag particles formed on r-GO. This is reasonable because much more functional groups are present on GO to provide nucleation sites; meanwhile, much quantity of small aromatic

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Figure 2. (A) SEM image of Ag NPs grown on GO. The reaction conditions are same as that mentioned in Scheme 1. (B) XRD pattern of Ag NPs on GO. (C) (i) Raman spectrum of solid p-ATP on SiOx and (ii) SERS spectrum of p-ATP on Ag NPs on GO. 488 nm as excitation source was used.

conjugated domains on GO will act as an electron-donating source to reduce Ag ions and form smaller Ag NPs.28,29 This further supports our proposed Ag NP formation mechanism on r-GO as mentioned above. Therefore, it is reasonable to believe that the carboxylic acid, hydroxyl, or epoxide groups on the GO surface might be used as nucleation sites for growth of the metal particles deposition on GO. This also explains why larger Ag NPs are formed on r-GO with less density. After r-GO is obtained from the reduction of GO, fewer functional groups on r-GO will remain as nucleation sites, resulting in the formation of fewer particles. Meanwhile, the large-restored π-conjugated network will provide more electrons to reduce Ag ions and form larger Ag particles. As is known, without any surface passivation, the Ag NPs will be slowly oxidized to Ag2O once moved out of solution. In our experiment, 16-mercaptohexadecanoic acid (MHA) was added after the reduction reaction was complete. The selfassembled monolayers (SAMs) of MHA can form on Ag NP surface based on the Ag-thiol bonding,30 which protects Ag from oxidation. Importantly, the size of Ag NPs is affected by the reaction time. At a reaction time of 10 min, Ag NPs formed on the GO surface show lower density and smaller diameter (2.7 ( 0.8 nm characterized by AFM, Figure S3), as compared to Ag NPs obtained in the 30-min reaction (Figure 1C and Figure 2A). Surface enhanced Raman spectroscopy (SERS) on Ag NPs is well studied.31,32 Our experiment proved that the as-synthesized Ag NPs on GO are a suitable substrate for SERS. p-Aminothiophenol (p-ATP) is used as a probing molecule.32,33 By immersing Ag NPs on GO in a 10 mM ethanolic p-ATP solution for ∼12 h, SAMs of p-ATP were formed on the surface of Ag NPs. As the incubation time of Ag NPs on GO in a very dilute MHA solution is 1-2 min,34 only loose disordered and very low density MHA SAMs formed on the Ag NP surface.30 Therefore, with immediately overnight incubation in a high concentration of p-ATP (10 mM), the p-ATP can form SAMs in the vacant surface area on Ag NPs unoccupied by MHA. Figure 2C shows the Raman spectrum of the solid p-ATP on SiOx and the SERS spectrum of p-ATP SAMs on Ag NPs on the GO surface. The normal Raman spectrum of solid p-ATP, Figure 2C (i), is in accordance with the previous report.32 By comparing Figure 2C (i) and (ii), it is easily found that the b2 modes at 1432, 1389, and 1140 cm-1 and the a1 mode at 1075 cm-1 are greatly enhanced. We attribute this enhancement to the electromagnetic effect, which has been suggested elsewhere.32,33 Thus, the Ag NPs-GO composites have been successfully synthesized. The effort for fabricating Au NPs-GO composites

was carried out as well. After the freshly prepared Ag NPs on GO substrate (Figure 1C and 2A) was immersed in a 2 mM aqueous HAuCl4 solution overnight, Au NPs were formed on GO due to the redox reaction between Ag and Au3+, Figure 3. Compared to Ag NPs on GO (Figure 1C and Figure 2A), the number of Au NPs is fewer. This is reasonable because 3 Ag atoms are needed to provide electrons to reduce 1 Au3+ ion. In addition, the distribution of Au NPs is much less uniform than Ag NPs. It can be explained by the fact that Au NPs formed during the redox process might not firmly attach to GO and are relatively easy to be removed or washed away. This also explains why there are a few Au NPs adsorbed on the bare APTES-modified SiOx without any GO sheet. The component of the formed Au NPs was confirmed by EDX and XRD (insets in Figure 3). Conclusions In summary, we have demonstrated that the Ag NPs can be directly reduced on GO and r-GO substrates without adding any reducing agent. This approach is of considerable interest and importance as it offers a facile way to synthesize Ag NPs with easy control of size which only involves heating in an aqueous solution of silver nitrate. The as-synthesized Ag NPs may have many applications in biosensing, catalysis, etc. This method is very important, and it could be used for synthesis of other noble metals, such as Pt, Pd, etc. Experimental Section Materials. Nature graphite (SP-1) was purchased from Bay Carbon (Bay City, MI) and used for synthesizing graphene oxide (GO). 3-Aminopropyltriethoxysilane (APTES), 98% H2SO4, 30% H2O2, potassium permanganate (KMnO4), silver nitrate (AgNO3, 99.9%) and 16-Mercaptohexadecanoic acid (MHA) were purchased from Sigma-Aldrich (Milwaukee, WI) and used as received. Silicon oxide wafers were purchased from Bonda Technology, Singapore. Ultrapure Milli-Q water (Milli-Q System, Millipore, Billerica, MA) was used in all experiments. Synthesis of Graphene Oxide (GO). Graphite oxide was synthesized from natural graphite (SP-1) by a modified Hummer’s method.8,10,35 In brief, 0.3 g graphite was added into a mixture of 2.4 mL 98% H2SO4, 0.5 g K2S2O8, and 0.5 g P2O5, the solution was maintained at 80 °C for 4.5 h. The resulting preoxidized product was cleaned by water and dried. After the preoxidized product was added into 12 mL 98% H2SO4, followed by slowly added 1.5 g KMnO4 with the temperature

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Figure 3. SEM image of Au NPs on GO. Insets: (A) XRD pattern of Au NPs and (B) EDX confirmation of Au element.

maintained at below 20 °C in order to avoid overheating and explosion, the solution temperature was increased to 35 °C and maintained for 2 h. Then 25 mL H2O was added. After 2 h, additional 70 mL H2O was added to dilute the solution, and 2 mL 30% H2O2 was injected into the solution to completely react with the excess KMnO4. A bright yellow solution was obtained. Then the resulting mixture was washed by HCl and H2O and the graphite oxide was obtained. The obtained graphite oxide was dispersed in water with a certain concentration and subsequently sonicated to give GO. Adsorption of GO on Substrates. Silicon oxide substrates (1 × 1 cm2) were cleaned in a piranha solution (H2SO4 (98%)/ H2O2 (30%) ) 7: 3) at 100 °C for 30 min (CAUTION: strongly corrosive). The cleaned substrates were thoroughly rinsed with water and dried under nitrogen flow, then treated with APTES (0.5 wt % in ethanol).36 The obtained APTES-modified substrates were used to adsorb single-layer GO by immersing them in a GO aqueous solution (Note: few two layers of GO may appear on APTES-modified SiOx due to the overlap of 2 pieces of GO sheets, see Figure 1B). Reduced-graphene oxide (r-GO) was obtained by vapor reduction of the GO substrates in hydrazine environment. The resulting GO and r-GO substrates were used for templated growth of metal particles. Growth of Ag NPs on GO and r-GO. GO or r-GO substrates were immersed in a bottle containing 5 mL aqueous solution of 0.1 M AgNO3 which is purged with N2 for 15 min and under N2 protection during the heating. The system was heated to 75 °C and maintained for 30 min. The reaction condition is same throughout the whole experiment unless otherwise stated. After the reaction is complete, the synthesized Ag NPs were protected from oxidation by injecting 50 µL of 2 mM 16-Mercaptohexadecanoic acid (MHA) ethanolic solution into the reaction solution and incubation for 1-2 min. The obtained MHA-passivated Ag NPs on GO were rinsed with water and dried with N2. The Ag NPs on GO were immersed in a 10 mM paminothiolphenol (p-ATP) ethanolic solution for ∼12 h prior to the Raman measurement. Au NPs on GO were obtained by immersing the freshly prepared Ag NPs on GO in a 2 mM HAuCl4 aqueous solution overnight.

Characterization. All AFM images were obtained by using Dimension 3100 (Veeco, CA) in tapping mode using a Si tip (Veeco, resonant frequency: 320 kHz; spring constant: 42 N m-1) under ambient conditions with a scanning rate of 1 Hz and scanning line of 512. Scanning electron microscopy (SEM) was performed using a JEOL JSM-6700 field-emission scanning electron microanalyzer at an accelerating voltage of 12 kV. Transmission electron microscopy (TEM) image was obtained on a JEOL JEM-2010 transmission electron microscope with an accelerating voltage of 200 kV. The GO sheets decorated with Ag NPs were scratched off from the SiOx substrate and dispersed in ethanol, which were used for TEM characterization. The MHA-passivated Ag NPs on GO were analyzed by X-ray diffraction (XRD) using an X-ray diffractometer (Rigaku Dmax 2200) with Cu Ka radiation (1.5406 Å). The accelerating voltage and the applied current are 40 mV and 40 mA, respectively. The scanning angle ranges from 35 to 85°. Surface enhanced Raman spectroscopy (SERS) spectrum was recorded with a WITec CRM200 confocal Raman microscopy system with the excitation line of 488 nm and an air cooling charge coupled device (CCD) as the detector (WITec Instruments Corp, Germany). The Raman band of a silicon wafer at 520 cm-1 was used as a reference to calibrate the spectrometer. EDX Measurements were performed on a JEOL 6360A scanning electron microscope. Acknowledgment. We thank the financial support from NTU (the Start-Up Grant), MOE (AcRF Tier 1, RG 20/07) in Singapore. P.C. thanks the financial support from A*STAR SERC (Grant No. 072 101 0020) in Singapore. Supporting Information Available: Additional XRD, EDX, TEM, electron diffraction, and AFM data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669.

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