Growth of Silver Film on Graphene Oxide Pattern - Krastsvetmet

Aug 2, 2012 - large quantities by the modified Hummers, method, could be reduced to graphene by hydrazine.5−7 This method provides a convenient way ...
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Growth of Silver Film on Graphene Oxide Pattern Yi Cui,†,‡ Tao Wang,†,§ Ding Zhou,† Qian-Yi Cheng,† Chang-Shan Zhang,§ Shu-Qing Sun,*,† Wei Liu,*,‡ and Bao-Hang Han*,† †

National Center for Nanoscience and Technology, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China § School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ‡

S Supporting Information *

ABSTRACT: Graphene oxide (GO) patterns on glass slides are prepared by microcontact printing, on which silver films are produced in situ through a straightforward one-step chemical method in aqueous silver nitrate solution at 60 °C, using glucose as a reductant. The obtained silver films are uniform and well-shaped, which are investigated by optical microscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The surfaceenhanced Raman scattering phenomena of the GO pattern resulting from the silver film give more hyperfine structural characteristics. Using the silver film as a reducing pattern, gold nanoparticles on GO patterns are obtained by simply immersing the silver pattern in a HAuCl4 solution. Our method is a convenient and ecofriendly approach to produce well-shaped silver or other metal nanoparticle patterns on different substrates.

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INTRODUCTION

Raman spectroscopy is a powerful tool to probe the structural characteristics and properties of graphene.8−12 The number of graphene layers can be easily identified by the profile and position of the Raman second-order band and the shift of G-band frequency.8−10 However, graphene is a one-atom-thick flat allotrope of carbon, through which much of the incident light is transmitted.4 Therefore, only a small portion of light is used to generate scattered radiation during Raman measurement. Consequently, some fine structural characteristics, such as a low concentration of defects, vacancies, doping, functional groups, crumpling, and edge structures, cannot be sensitively probed and well-distinguished from the weak Raman spectra of graphene. Silver nanostructures have attracted considerable interest because of their spectacular property known as surface plasmon resonance, which has enabled their widespread use as optical probes, contrast agents, sensors, and substrates for surface-enhanced Raman scattering (SERS) spectroscopy.13−15

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Graphene, a two-dimensional sp carbon network, possesses fascinating electronic, mechanical, and thermal properties.1,2 Because of its extremely high electron carrier mobility, mechanical flexibility, optical transparency, and chemical stability, graphene provides a great opportunity for the development of high-performance electronic devices.3,4 However, the mechanical exfoliation method for graphene production is unrealistic for practical application. It is wellknown that graphene oxide (GO), which could be prepared in large quantities by the modified Hummers’ method, could be reduced to graphene by hydrazine.5−7 This method provides a convenient way to produce a large quantity of graphene or reduced GO (r-GO). The exfoliated GO is a layer of graphene consisting of hydrophilic oxygenic functional groups on their basal planes and edges. The presence of these groups reduces the interlayer forces; therefore, GO could be readily exfoliated and dispersed in aqueous medium or various polar solvents, such as N,N-dimethylfomamide (DMF), N-methyl pyrrolidone (NMP), ethanol, and so on, which provides a reasonable avenue to many applications expediently. © 2012 American Chemical Society

Received: June 7, 2012 Revised: August 1, 2012 Published: August 2, 2012 17698

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GO dispersion in ethanol (0.05 mg mL−1) for 5 min and then blown dry with nitrogen. The stamp was printed onto the substrate surface immediately and left in place for an average contact time of 5 min. The r-GO pattern was obtained from the GO pattern by annealing at 600 °C in the atmosphere of nitrogen. The GO pattern on polycarbonate slides and silicon wafers were prepared by the same procedure above. Growth of Silver Films on GO Pattern. The silver film was prepared by the reduction of silver ions in situ in aqueous silver nitrate solution on GO pattern, where glucose was employed as the reductant. The glass slide with GO patterns was immersed in a beaker containing an aqueous solution of AgNO3 (0.05 M, 10 mL) and glucose (0.05 M, 10 mL), which was purged with nitrogen for 15 min. The system was heated to 60 °C under nitrogen atmosphere and maintained for 30 min. The obtained silver film on the GO pattern was rinsed with ethanol and dried with nitrogen. In addition, the obtained silver film was protected from oxidation by injecting an aqueous MUA solution (2 mM, 50 μL) into the reaction solution during the reaction as the control experiment. The growth of the silver nanoparticle layer on the PAA pattern and r-GO pattern was used the same procedure above as the control experiments. Instrumental Characterization. Optical microscopy observation was carried out by using a Leica DM4000M microscope (Leica Microsystems Ltd., Germany). Scanning electron microscopy (SEM) observation was carried out by using a Hitachi S-4800 microscope (Hitachi Ltd., Japan), which was equipped with a Horiba energy dispersive X-ray spectrometer (EDXS), at an accelerating voltage of 6.0 kV. The GO pattern on the glass slides was subject to observation without any sputter-coating because it shows a good electronic conductivity. Atomic force microscopy (AFM) images were taken in tapping mode with a Dimension 3100 atomic force microscope and a Nanoscope IVa NS4a controller (Veeco Instruments Inc., USA). Raman spectra were recorded with a Renishaw inVia Raman spectrometer (Renishaw plc, UK). All GO patterns on the glass slides were tested without using any solvent. The laser excitation was provided by a regular model laser operating at 514 nm. The GO pattern on glass slide and the silver film on GO pattern were observed by optical microscopy, SEM, and AFM. Meanwhile, the silver pattern was also photographed by a digital camera. The GO pattern on the polycarbonate slide and silver film on GO pattern were also observed by optical microscopy. The SERS spectra of the GO pattern under the silver film were obtained by a Raman spectrometer using the samples on the glass slide. X-ray photoelectron spectroscopy (XPS) surface experiments were achieved with a Thermo Scientific ESCALAB 250Xi with a monochromatic Al source (hν = 1486.6 eV, 15 kV primary energy, 30 mA emission intensity), with 500 × 500 μm2 analysis area. The pass energy was fixed at 30 eV for the element analysis.

Hence, graphene covered with silver nanostructures will show more hyperfine structural characteristics in a Raman spectrum. Microcontact printing is a method for developing a wellshaped pattern of many kinds of materials on the surface of a substrate by using a relief pattern on a polydimethylsiloxane (PDMS) stamp through conformal contact.16,17 Compared with other lithography methods, microcontact printing is a fast, remarkably simple, low energy consuming, and parallel processing method, which could prepare patterns on different substrates and even curved ones. We employ microcontact printing to prepare GO patterns on different kinds of substrates. As we know, there are few reports for fabricating graphene patterns by microcontact printing.18−21 To fabricate metal nanoparticles−GO composite materials as a part of our overall interest,22,23 we have launched an approach for the preparation of GO patterns, on which the silver nanoparticles grow by the reduction of silver(I) ions in situ in aqueous silver nitrate solution, which is similar to the silver mirror reaction to prepare obvious shape evolution of silver nanoparticles24,25 or silver-coated fiber optical nanoprobes.26 In addition, to prepare silver/graphene nanocomposites, the reduction reaction of silver ions is also widely employed with different reductants, such as trisodium citrate,27 glucose,28 ascorbic acid,29 gelatin,30 formaldehyde,31 sodium borohydride,32 hydroquinone,33 or without any reductant.34,35 In summary, we choose the green reductant and gentle condition in this silver mirror reaction, which is a convenient and ecofriendly approach. We select GO patterns to grow silver film, since it is known to contain negative charge groups, such as carboxyl groups, which could adsorb silver ions.36 The resulting flat silver layers on the GO pattern can serve as a good SERS substrate and can readily be used in molecule sensing with high sensitivity and specificity.

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EXPERIMENTAL SECTION Materials. Natural flake graphite with an average particle diameter of 20 μm (99 wt % purity) was obtained from Yingshida graphite Co. Ltd., Qingdao, China. Sulfuric acid (98 wt %), hydrogen peroxide (30 wt %), sodium nitrate (NaNO3), potassium permanganate (KMnO4), silver nitrate (AgNO3), glucose, chlorauric acid (HAuCl4), and ethanol were purchased from Beijing chemical works, China. 3-Aminopropyltriethoxysilane (APTES, 98%) was purchased from Acros. 11Mercaptoundecanoic acid (MUA, 95%) was purchased from Sigma−Aldrich. Polyacrylic acid (PAA, M = 140 000) was purchased from Alfa Aesar. All these reagents were used without further purification. Ultrapure water (18.2 MΩ cm) was obtained by the Millipore−ELIX water purification system. Preparation of GO Pattern. GO was prepared by chemical exfoliation of the natural flake graphite by a modified Hummers’ method.37,38 GO dispersion in ethanol was obtained by a solvent exchange method from the as-exfoliated aqueous GO dispersion.39,40 The substrates, such as glass slides, polycarbonate slides, and silicon wafers were pretreated as follows. Glass slides and silicon wafers were cleaned in a piranha solution (H2SO4 (98%)/H2O2 (30%) = 3:1, v/v) for 1 h at 110 °C, rinsed thoroughly with ultrapure water, and dried under a stream of nitrogen. Caution! This solution is extremely corrosive and reactive with organic materials. Meanwhile, the polycarbonate slides were treated in oxygen plasma for 5 min. The precleaned glass slides were then treated with APTES (1.0 vol % in ultrapure water) overnight.41 The PDMS stamp was cleaned by ultrasonication in ethanol for 5 min and dried with nitrogen. The cleaned PDMS stamp was incubated with

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RESULTS AND DISCUSSION Preparation and Morphology of GO Pattern on Glass Slides. The chemical exfoliated GO contains hydrophilic oxygenic functional groups on their basal planes and edges, which make it possess negative charge. These sites are selective to absorb the metal ions.42 The pretreated substrates possess hydroxyl groups on the surface, on which APTES could be grafted by silicon−oxygen bonding, and the amino groups thus expose on the surface. Therefore, GO on the PDMS stamp could attach on the substrate by electrostatic interaction. This 17699

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Figure 1. Optical microscopy (A) and SEM (B) images of the GO pattern on glass slide.

method has been employed to prepare APTES pattern for adsorption of single-layer GO by Zhang’s group.20 As can be seen in Figure 1, we employ optical microscopy and SEM to observe the obtained GO pattern. The resulting GO pattern can be seen as squares in the optical microscopy image as shown in Figure 1A. The size of the GO pattern is exactly consistent with the PDMS stamp containing squares of 40 μm length. The sharp edges can be clearly seen from the SEM image in Figure 1B. Growth and Morphology of Silver Film on GO Pattern. The silver film is prepared by the reduction of silver ions in situ in aqueous silver nitrate solution on the GO pattern, using glucose as the reductant, which refers to the silver mirror reaction. The influence of the different concentrations of silver nitrate is first optimized. The aqueous silver nitrate solutions of various concentration (0.025−0.10 M) are used to optimize the growth of the silver film on the GO pattern. The result turns out that the silver nitrate solution with the concentration of 0.05 M yields the most uniform morphology of the silver film. The silver pattern produced with the concentration of silver nitrate solution of 0.10 M is outside of the GO square-pattern to form the circled one as shown in Figure S1A. The gap between silver film circle is much smaller than 40 μm (the gap between the GO square pattern). In addition, the silver pattern is not uniform with the concentration of silver nitrate solution of 0.025 M as shown in Figure S1B. We took a picture of the silver film by a digital camera as shown in Figure 2. The pattern is shining brightly against the sunshine, and a very clear pattern can be seen by zooming in. The resulting silver film on the GO pattern reflects light as a metal surface in the optical microscopy images by incident light mode as shown in Figure 3A. The inset in Figure 3A is the zooming-in image, which shows the hyperfine morphology of the silver pattern. As shown in the SEM image in Figure 3B, the silver film on the GO pattern is flat and uniform in each square (40 μm × 40 μm), which is consistent with those prepared by the traditional silver mirror reaction.43 The silver film could grow only on the GO pattern areas, since the areas without GO possess positive charges (from surface-anchored APTES) to prevent the adsorption of the silver ions. The lighter gray silver pattern than the GO pattern in the SEM image (Figure 1B) suggests that the silver growth on the GO improves the electric conductivity. Moreover, the composition of the silver is confirmed by EDX (Figure S2A), which also illustrates the success of the silver growing on the GO pattern. In addition, in

Figure 2. Digital camera image of silver pattern on the glass slide.

order to protect the formed silver pattern from oxidation, MUA was added before the reduction reaction as the control experiment. The self-assembled layers of MUA can form on silver nanoparticles surface based on the silver-thiol bonding.44 There is no obvious difference in the morphology of the silver pattern whether or not the MUA is added. The silver pattern, which grew with the MUA, is also uniform and flat as that above as shown in Figure 3C and the inset is the zooming-in image. Since AFM characterization has been one of the most direct methods of quantifying the thickness of exfoliation of graphene, we employ AFM to determine the shape and thickness of the GO pattern and the formed silver film, although the surface of glass slides is not a desirable AFM substrate. As shown in Figure S3A, the cross-sectional view of a typical AFM image of the GO pattern on the glass slide shows that its thickness is about 20 nm at most regions. The thickness of the GO pattern could be controlled by using GO dispersion of different concentrations. We can also see that GO pattern is uniformly covered by the silver film, and the average height is about 10 nm in Figure S3B. After the growth of silver film on the GO pattern, the pattern surface becomes very smooth, that is, because the silver particles occupy the oxygenic functional groups on the GO surface and grow adhering to each other, which is strongly attached to the GO surface. Growth of Silver Pattern on PAA, r-GO, and Polycarbonate Slides. PAA is a polymer with plenty of carboxyl groups, which could absorb silver ion the same as the 17700

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Figure 3. Optical microscopy (A) and SEM (B) images of silver pattern on glass slide. (C) Optical microscopy image of silver pattern grown with the MUA. (D) Optical microscopy image of the silver pattern on the polycarbonate slide. The insets in (A) and (C) show the zooming-in images.

We tentatively speculate that the silver film growth procedure may contain two steps. (1) The oxygenic functional groups at the GO surface are responsible for the initial attachment of the silver ions in solution by electrostatic interactions.45 Moreover, the flat surface of GO is π electron-rich, which may assist the absorption interactions between the silver ions and GO. In addition, the exfoliated GO pattern provides a relatively large flat area on which to disperse the silver nanoparticles. (2) When silver ion is reduced by glucose, silver nanoparticles gradually grow and form films attaching to the GO pattern. SERS Phenomena of GO Pattern Resulted from Silver Film. Raman spectroscopy is a powerful tool to probe the structural characteristics and properties of graphene. Figure 4 shows the Raman spectra of the GO pattern before and after the growth of the silver. As can be seen in Figure 4, there are

carboxyl groups on GO sheets. PAA pattern on glass slide is prepared by the procedure identical to GO, except using the PAA (1.0 vol % in ultrapure water) as the printing material. The growth of silver film on the PAA pattern also employed the procedure identical to the GO pattern. Meanwhile, the r-GO pattern is obtained from the GO pattern by annealing at 600 °C in nitrogen atmosphere, on which the growth of silver film is also prepared as the control experiment. However, the silver film could not be formed on both of the two substrates. The reason is that there is no flat surface of PAA pattern. PAA is a kind of linear polymer, and therefore, the surface of the PAA pattern is discontinuous (cause roughness) and might not support the silver nanoparticles well to form the silver film. The traditional silver mirror reaction always carries out on the continuous flat surface, such as the interior surface of the glass tube. The GO pattern provides a relatively large flat and continuous area to support the silver nanoparticles to form the silver film. Meanwhile, r-GO is obtained from the reduction of GO, so there are much fewer carboxyl groups to adsorb silver ions. Few silver particles grew on the pattern surface, as shown in Figure S4. Therefore, we consider the flat and continuous surface and enough oxygenic functional groups as the two indispensable factors for silver film growth. Polycarbonate is one of the most useful soft substrates in the chip industry. We also used it as the substrate to try to prepare the silver film on the GO pattern, as shown in Figure 3D. The obtained silver patterns are not as good as the patterns obtained on the glass slide. We consider the difference in the silver pattern on these two substrates is due to the difference in binding energy between the GO and the surface of substrates. In addition, the silver pattern on polycarbonate is also wellshaped. Therefore, this method could be used to fabricate patterns on many other substrates, even soft substrates.

Figure 4. Raman spectra of GO pattern before and after the growth of the silver film. 17701

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Figure 5. Optical microscopy (A) and SEM (B) images of gold nanoparticles on silver pattern. The insets show the respective zooming-in images.

two characteristic peaks of GO, namely, the D-band at 1330 cm−1 and the G-band at 1580 cm−1.46 The D-band is ascribed to the edges, defects, and disordered carbon, whereas the Gband arises from the zone center E2g mode, corresponding to ordered sp2-bonded carbon atoms. After silver film grown on the GO pattern surface, the Raman spectra intensity of both the D-band and the G-band peaks are significantly enhanced. Moreover, there are more hyperfine peaks among the D-band and the G-band, which probe more structural characteristics and information of GO, such as the clear characteristic peaks of CC at 1600 cm−1.47 The intensity of the Raman signal is enhanced to 10 times against the normal GO pattern. This Raman spectrum enhancement is attributed to the SERS of silver film on the GO pattern. We attribute this enhancement to chemical enhancement in SERS coming from the modification of the Raman polarizability of the molecule by direct electronic interaction with the metal particles surface.48,49 Our finding provides a feasible mean to study some hyperfine characteristics of graphene-based materials by using SERS. Fabrication of Gold Nanoparticles on Silver Pattern. To further verify the success of silver film grown on the GO pattern, we prepared the gold nanoparticles by the redox reaction between Ag and Au3+. The prepared silver pattern on the glass slide was immersed in aqueous HAuCl4 solution (2.0 mM) for 12 h. As shown by the optical microscopy and SEM images in Figure 5, the gold nanoparticles were formed on the silver film. The gold nanoparticles are not as dense as the silver pattern as shown in the inset images in Figure 5B. This is because three Ag atoms are needed to provide electrons to reduce one Au3+. In addition, the distribution of gold nanoparticles is much less uniform than silver film. It can be explained by the fact that gold nanoparticles formed during the redox process might not strongly attach to the GO surface and are easy to wash away.50 The quantities of the gold nanoparticles on the silver pattern could be regulated by the immersing time and the density of silver layers. It is convenient to find out that the higher density of silver on the surface, the more opportunities of the redox reaction between Ag and Au3+. The composition of the gold nanoparticles is also confirmed by EDX (Figure S2B). The surface composition of the above pattern was also characterized by using XPS. The result of XPS analysis also confirms the growth of the silver film on GO pattern as shown in Figure 6. There are two obvious peaks of Ag 3d fundamental bands at the binding energy of 368.3 (3d5/2) and 374.3 eV (3d3/2), which mainly attributes to Ag0 (368.3 eV), with little amount of Ag+ (367.6 eV).51 After immersion in HAuCl4, Ag

Figure 6. XPS of Ag 3d and Au 4f fundamental bands of the silver pattern before (black) and after (blue) the immersion in HAuCl4.

3d bands decrease in intensity while metallic Au 4f bands at 84.8 (4f7/2) and 88.5 eV (4f5/2) appear as shown in Figure 6. A shift to lower binding energies was also observed in Ag 3d bands as the result of the formation of gold nanoparticles on the silver pattern.52,53 Our above method is a convenient and environmentally friendly route to achieve the GO pattern; moreover, the GO pattern is decorated with silver film, which could be used as antibacterial material chips,54 superhydrophobic material chips,43,55 or photocatalytic activity composites.56 The metal nanoparticles are also used as optical probes to investigate flow dynamics or as reflective components in high resolution displays.57 This simple and convenient method could be also used for decorating other kinds of metal particles on GO pattern.

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CONCLUSIONS Microcontact printing was used to prepare GO patterns on the substrates of glass slides, and then we demonstrated a straightforward one-step chemical method to grow in situ silver film on GO pattern surface in aqueous silver nitrate solution, using glucose as a reductant. The obtained silver pattern is well-shaped and uniform. The SERS phenomena of the GO pattern resulting from the silver film suggest more hyperfine structural characteristics of GO. Gold nanoparticles on a GO pattern are obtained by simply immersing the obtained silver pattern in HAuCl4 solution. Our method is a convenient and environmentally friendly route to produce wellshaped silver or other metal particles patterns on different 17702

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substrates, which could be used as an antibacterial material chip or other applications in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Details of optical microscopy images, AFM images, and EDX data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 8254 5576. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Ministry of Science and Technology of China (National Major Scientific Research Program, Grant No. 2011CB932500), the National Science Foundation of China (Grant Nos. 91023001 and 20973198), and the Chinese Academy of Sciences (Knowledge Innovation Program, Grant No. KJCX2-YW-H21) is acknowledged.

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3055944 | J. Phys. Chem. C 2012, 116, 17698−17704