Controllable synthesis of graphene oxide–silver (gold) nanocomposites and their size-dependencies

Abstract

Recently, graphene/graphene oxide-based metal composites have opened up an exciting new field in science and technology. From a synthetic point of view, it is highly attractive with controllable synthesis of graphene/graphene oxide-based metal composites. We present here a facile synthesis of graphene oxide (GO) with noble metal (Ag, Au) nanoparticles by mixing GO and metallic nanoparticles in a water–n-butylamine system for controlled local size and properties. The structural details and textural properties of these resultant GO–Ag and GO–Au composites were characterized by X-ray diffraction, transmission electron microscopy, UV-Vis absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The Raman scattering and catalytic experimental results showed that these GO–metal composites exhibit some size-dependencies. The Raman enhancement from GO–Ag samples increased as the Ag nanoparticles size was increased, whereas the catalytic activity of the GO–Au samples decreased in the oxidation of ethylene glycol as the size of Au nanoparticles was increased. This synthetic strategy is generalized and is expected to be applied to decorate GO with a multitude inorganic nanoparticles with desired particle sizes, shapes and properties.

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Introduction

Graphene is a crystalline allotrope of carbon with a two-dimensional and hexagonal lattice form that has triggered considerable interest in recent years due to its amazing electrical, mechanical, thermal and optical properties as well as its unique two-dimensional structure and large surface area.^1–5 However, without any unstable bonds on its surface the structural integrity graphene is highly chemically stable, resulting in an inert surface, weak interaction with other mediums and insolubility in water and common organic solvents, which becomes an obstacle in its further research and application.^6,7 Graphene oxide (GO) synthesized by the oxidation of graphite exhibits good solubility and intercalation properties due to the presence of the hydrophilic polar groups (hydroxyl, epoxy, carbonyl, and carboxyl groups) on the basal plane and sheet edges. These hydrophilic groups support GO as an ideal building block in graphene-based nanocomposites.^8–11

GO–metal nanocomposites, especially GO–noble metals, have been intensively developed and were found to exhibit a range of unique and useful properties in recent years. However, controllable synthesis of GO–metal nanocomposites with desired morphologies and properties is still a challenging task. Nowadays, there are mainly two strategies for fabricating GO–metal nanocomposites.¹² One is the in situ method, which involves forming nanocrystallites in the presence of GO, and then the nanocrystallites directly grow into nanomaterials (such as nanoparticles, nanowires, nanorods, and nanofilms) on the surface of GO sheets. The second is an ex situ method, which involves the pre-synthesis of nanomaterials in the desired dimensions and morphologies that are then attached to the GO sheet surface.

The noble metallic nanoparticles can be anchored to the GO surface by mixing GO with the corresponding metallic salts in a water–ethylene glycol system, which had been found in other researchers works and our previous study.^13,14 The resulting metallic nanoparticles played a pivotal role in catalytic reduction of GO with ethylene glycol, and the extent of deoxygenation depended on the attached metals. In this study, we report a facile and controllable synthesis of GO–silver (gold) nanocomposites, which is similar to the ex situ method. A group of Ag (Au) nanoparticles with different sizes were prepared in advance, after which they were attached onto the GO surface in a water–n-butylamine system. Their size-dependencies were examined, and enhanced Raman signals and catalytic activities were used for determining the size-dependencies.

Experimental section

Materials

Silver nitrate (AgNO₃) and gold chloride trihydrate (HAuCl₄·3H₂O) were purchased from Sigma-Aldrich Co. Sodium borohydride and n-butylamine were all analytical grade and were obtained from Sinopharm Chemical Reagent Co., Ltd. Natural graphite powder (44 μm) was provided by Qingdao Zhongtian Company. The other chemicals were all analytical grade and used without further purification.

Synthesis of Ag, Au nanoparticles in different sizes

Silver and gold colloids were prepared by sodium citrate reduction according to the modified Frens method.^15,16 500 mL of aqueous HAuCl₄ (0.01% by weight) was brought to a boil while being stirred, 1% sodium citrate aqueous solution was quickly added to this solution. The reaction was allowed to run a few minutes until the solution colour changed from glassy yellow to red. The reactions in the series of variable sodium citrate/HAuCl₄ ratios were performed by varying the amount of sodium citrate with a fixed amount of HAuCl₄, and the products were labelled as Au-1 (sodium citrate/HAuCl₄ mole ratio: 2.68 : 1, orange), Au-2 (1.34 : 1, red) and Au-3 (0.56 : 1, dark red). Silver colloids with different particle sizes, Ag-1 (sodium citrate/AgNO₃ mole ratio: 0.64 : 1, colour: yellow), Ag-2 (0.32 : 1, fawn) and Ag-3 (0.16 : 1, brown green), were prepared by the same procedure.

Synthesis of GO–Ag and GO–Au nanocomposites

GO was prepared from purified natural graphite by using a modified Hummers' method.¹⁷ 10 mg of the obtained GO powder and 0.5 mL n-butylamine were dispersed in 10 mL of water by sonication and stable GO colloid was formed. After 1 h, 10 mL of the obtained metal colloid (Ag or Au) was added to the GO solution. Subsequently, the mixture was heated to 80 °C with magnetic stirring and aged at that temperature for 12 h. The bulk samples can be obtained by centrifuging the mixture, and then washing with deionized water and ethanol several times. The GO–Ag and GO–Au nanocomposites were obtained after the precipitates were dried at 60 °C under vacuum for 12 h.

Characterization of materials

X-ray diffraction (XRD) patterns of the nanocomposites were recorded on a Bruker D8 Advanced X-ray diffractometer using Cu-Kα radiation (κ = 0.1542 nm). The diffraction data was recorded for 2θ angles between 5° and 80°. X-ray photoelectron spectra (XPS) were obtained on a Thermo ESCALAB250 X-ray photoelectron spectrometer, using Al Kα (hν = 1486.6 eV) radiation. Morphology analyses of the samples were carried out on a JEOL JEM-2100 transmission electron microscope (TEM). The electronic absorption spectra were obtained on a Shimadzu UV-1201 UV-Vis spectrophotometer with standard 1 cm optical cells. Raman spectra were obtained on a Thermo Fisher spectrometer excited by a diode laser beam (532 nm, 1 mW) with a 2 s acquisition time. HPLC was applied to determine the yields of glyoxal using 2,4-dinitrophenylhydrazine as a derivatizing agent. The HPLC separation was performed on a C18 column with a methanol : water (75 : 25) mixture as the mobile phase at a 0.2 mL min⁻¹ flowrate and detected at 440 nm. Inductively coupled plasma-atomic emission spectrometer (ICP, Vista-Mpx) was applied to determine the amount of metal loading on the GO sheets by dissolving GO–Ag and GO–Au samples into nitric acid and aqua regia, respectively.

Results and discussion

The synthetic procedure of GO-based inorganic nanocomposites is similar to the ex situ method, the inorganic nanoparticles were synthesized in advance and then anchored onto GO sheets. In the traditional ex situ approach,^12,18 either the inorganic components or GO (or both) require modification with functional groups before complexation. In our approach, neither metallic nanoparticles nor GO were modified with functional groups in advance, this was only done by mixing Ag (Au) nanoparticles or colloids with GO in a water–n-butylamine system. The synthetic process is very generalized and simple. Our study has encompassed a multitude of variations on this theme.

Characterization of GO–Ag (Au) composites

Fig. 1 shows the XRD patterns of as-synthesized GO–Ag and GO–Au composites. The diffraction peaks in Fig. 1 are all ascribed to the characteristic reflections of silver (JCPDS 4-0783) and gold (JCPDS 76-1802), respectively. The diffraction peak at around 2θ = 10°, corresponding to the (001) reflection of GO, almost disappeared for the GO–Ag and GO–Au samples. It is similar to that in some researchers works and our previous studies,^13,14,19 when GO was hybridized with inorganic particles, the regular layer structures of GO were usually disrupted, and the diffraction peak of GO could hardly be observed. To further investigate the structures of GO–Ag (Au) nanocomposites, morphologic analyses are carried out.

Fig. 1 XRD patterns of as-synthesized GO–Ag and GO–Au composites.

The TEM images of as-synthesized Ag samples and their GO–Ag composites are shown in Fig. 2. Ag particles showed a relatively monodispersed spherical distribution, and the size was found to increase from ca. 21 to 95 nm as the sodium citrate/AgNO₃ ratio decreased. The characteristic absorption peak of the surface plasmon resonances of Ag nanoparticles red shifted from 418 to 452 nm as the size of Ag nanoparticles was increased (ESI, Fig. S1 and S2†). After it was added to the GO colloid, the nano sized Ag particles anchored randomly onto GO sheets and few particles scattered out of the sheets (Fig. 2d–f). The ICP analyses (ESI, Table S1†) showed that about 0.10 g of Ag were loaded per gram of GO in all of GO–Ag samples, and the Ag released from the GO sheets was insignificant (<1%) after sonication in water for 10 min. This indicated that the Ag nanoparticles were firmly attached onto the GO sheets.

Fig. 2 TEM images of (a) Ag-1 (ca. 21.0 nm), (b) Ag-2 (ca. 45.5 nm), (c) Ag-3 (ca. 94.5 nm), and (d) GO–Ag-1, (e) GO–Ag-2, (f) GO–Ag-3. The size histograms of the Ag samples are shown in the ESI, Fig. S3.†

The Au samples morphologies show a similar trend to that of the Ag samples (Fig. 3). The average sizes of Au-1, Au-2 and Au-3 were ca. 20 nm, 25 nm and 59 nm, while the sodium citrate/AgNO₃ ratio decreased from 2.68 : 1 to 0.56 : 1. Frens¹⁶ suggested that these metal nanocrystals were grown through a fast nucleation process followed by a diffusion-controlled growth, which is similar to the well-known LaMer model.²⁰ The faster the nucleation rate or the more the nucleation number, the smaller the particles size. Moreover, Yang¹⁵ proposed that the metal size variation was mostly determined by the solution pH that was in turn controlled by the sodium citrate concentration.

Fig. 3 TEM images of (a) Au-1 (ca. 20.2 nm), (b) Au-2 (ca. 24.7 nm), (c) Au-3 (ca. 59.4 nm), and (d) GO–Au-1, (e) GO–Au-2, (f) GO–Au-3. The size histograms of Au samples are shown in the ESI, Fig. S4.†

The almost transparent and crumpled silk-like GO sheets were nicely decorated with these nano sized Au particles. It should be noted that the GO colloid in water–n-butylamine mixture without Ag (Au) nanoparticles was stable, and no obvious aggregations or sediments were found. However, after adding Ag or Au nanoparticles to the above mixture, the GO easily aggregated, and sediments were found at the flask bottom. A reasonable explanation is that the adsorption of the metallic nanoparticles on the GO surface resulted in GO reduction and decreased water solubility, leading to the sedimentation. Based on the abovementioned analyses, these Ag (Au) nanoparticles might act as a catalyst for GO reduction by n-butylamine, and make themselves adhere to the GO surface. To verify this solution, XPS analyses of the surface component variations of GO–Ag (Au) samples were investigated.

X-ray photoelectron spectroscopy (XPS) of GO–Ag-1, GO–Au-1 and GO was examined and spectra are shown in Fig. 4. As shown in Fig. 4a, the C 1s XPS spectrum of GO showed four different peaks of carbon components, centred at 284.6, 286.3, 286.9 and 288.5 eV, corresponding to sp²-hybridized C–C, hydroxyl C, epoxy C and carboxylate C. However, the intensities of some oxygenated functional groups (especially the epoxy groups) on GO sheets in as-synthesized GO–Ag and GO–Au composites were obviously reduced, which confirms a considerable degree of deoxygenation of GO, while the deoxygenation of GO was most likely caused by a reduction process with n-butylamine and metallic nanoparticles. As we know, the reducibility of n-butylamine is between that of ethylene glycol and ethylene diamine. GO sheets in water–n-butylamine mixture was stable and hardly reduced. However, because Ag (Au) nanoparticles were added to the above system, the GO easily aggregated and then deposited. The Ag (Au) nanoparticles acted as catalyst in the reduction of GO by n-butylamine, and were further attached onto the surface of GO sheets.

Fig. 4 C 1s XPS spectra of GO (a), GO–Au-1 (b) and GO–Ag-1 (c) nanocomposites. The intensities of oxygenated functional groups in GO–Ag-1 and GO–Au-1 were reduced as a result of the deoxygenation, indicating the GO reduction.

For comparison, we tried to anchor Au nanoparticles onto the GO sheets by using ethylene glycol (EG) and ethylene diamine (EA), the products labelled as EG–GO–Au and EA–GO–Au, respectively. The TEM images (Fig. 5a and b) show that very few Au particles attached onto GO sheets in EG–GO–Au composites while a serious aggregation appeared in EA–GO–Au composites. The low loading capacity of Au nanoparticles on GO sheets might due to the week reducibility of ethylene glycol, while ethylene diamine could directly reduce GO to reduced graphene oxide (RGO) resulting in intense aggregation. The n-butylamine could enter the GO interlayer but without strong graphene reduction, the amine groups of n-butylamine tended to the surface of the metal nanoparticles and connected with the hydrophilic groups (such as epoxy and carboxyl groups) by a reduction process in the presence of metallic nanoparticles and, subsequently, the metal particles were attached onto the GO sheets.

Fig. 5 TEM images of ethylene glycol–GO–Au (a) and ethylene diamine–GO–Au (b) composites.

Size dependence of SERS effect from GO–Ag

As is known, noble metal nanoparticles can exhibit the optical phenomenon called surface enhanced Raman scattering (SERS), which provides a spectral intensity often enhanced by many orders of magnitude for molecules adsorbed thereon.^21,22 Previous studies have shown that Ag (Au) nanoparticles can enhance the Raman signal of only 1 nm thickness of GO/graphene sheets,^14,23 which is similar to the SERS effect. In our experiment, a 10 μL of GO–Ag dispersion were dropped onto a silicon wafer and air-dried before the Raman measurements. In Fig. 6, we show the Raman spectra of the GO–Ag samples with three different sizes of Ag nanoparticles together with the normal Raman spectrum of GO sheets. Obviously, the intensities of the D band at ∼1590 cm⁻¹ and G band at ∼1350 cm⁻¹ of GO increased for each spectrum of the GO–Ag sample compared with that of the original GO under the same measurement conditions. It was noted that the Raman enhancement showed an increasing trend as the size of the Ag nanoparticles was increased. The intensities of the Raman bands from GO on GO–Ag-3 were almost five times than that of GO–Ag-1 and ten times than that of GO. The ICP spectrometer demonstrated that the amounts of Ag loading of GO–Ag samples were almost the same (ESI, Table S1†). According to recent research,^24,25 the optimal size of Ag particles for SERS effect was around 70 nm under 514 or 532 nm light excitation, which is in agreement with the results for the GO–Ag samples. In addition, Raman enhancement is proportional to the concentration of metal distribution. Because the amount of Ag loading per gram of GO is the same, the larger size of Ag particles should have the lower concentration on GO sheets, and the smaller Raman enhancement. However, the SERS experiments showed that GO–Ag-1 with the largest size of Ag particles had the highest enhancement. Therefore, the Raman enhancements are most likely to depend on the metal size.

Fig. 6 Raman spectra of GO–Ag nanocomposites: (a) GO–Ag-1; (b) GO–Ag-2; (c) GO–Ag-3, and (d) the Raman spectrum of GO.

Size dependence of catalytic oxidation of ethylene glycol by GO–Au

As we know, bulk Au is chemically inert, however, the nanosized Au particles have chemical and catalytic activities. Recent studies^26,27 have shown that the catalytic activity encounters a large increase when Au nanoparticles are anchored onto GO sheets. In our experiment, 0.5 g of GO–Au sample and 4 g of ethylene glycol was added to a three-necked flask to form a 100 mL mixture with ventilating oxygen at a 0.5 L min⁻¹ speed. The mixture was heated to 50 °C, and then dropwise added a 30% concentration of NaOH solution until the pH value of the mixture was maintained at 7.8. In the catalytic oxidation process from ethylene glycol to produce glyoxal, GO–Au with three sizes of Au particles were used as catalysts (labelled as GO–Au-1, GO–Au-2 and GO–Au-3), and the yields of glyoxal are listed in Table 1. For comparison, the catalytic oxidation reactions with Au nanoparticles (Au-1, Au-2 and Au-3) and GO only were also carried out. As observed from Table 1, the yield of glyoxal increased markedly after Au particles were attached onto GO sheets in each of the GO–Au samples. The maximum yield was ∼47% for GO–Au-1, the minimum yield was 35% for GO–Au-3. The yields show size-dependence, with the smaller size producing a better yield and higher catalytic activity. After the catalytic experiment, the catalysts were recollected and washed with ethanol and deionized water, and then reused for catalytic oxidation of ethylene glycol to produce glyoxal. The yields of glyoxal showed a small, but not significant, decrease. The ICP spectrometer showed that the amount of Au leached from GO was negligible (<4.5%). The small decrease could be attributed to the inevitable small amount of catalyst loss from the recycling process.

Table 1 The yields of glyoxal oxidized from ethylene glycol with the help of GO–Au catalysts

Number	Catalyst	Glyoxal yields
1	GO–Au-1	47%
2	GO–Au-2	42%
3	GO–Au-3	35%
4	Au-1	26%
5	Au-2	25%
6	Au-3	22%
7	GO	—

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Conclusions

In summary, the series of GO–Ag (Au) composites were prepared by mixing GO colloid and Ag (Au) nanoparticles directly in the water–n-butylamine system. The Raman scattering properties and catalytic activities of these GO-based metal composites were correlated with the size of metallic nanoparticles, exhibiting size-dependency. The SERS enhancement from GO–Ag samples increased as the size of the Ag nanoparticles increased, whereas the catalytic activities of GO–Au samples for the oxidation of ethylene glycol to glyoxal decreased as the size of the Au nanoparticles increased. This study may be helpful to control the synthesis of graphene-based composites for desired structures and properties.

Acknowledgements

We gratefully acknowledge the support of the National Natural Science Foundation of China (51302113), the Natural Science Foundation of Jiangsu Province (No. BK20130512), China postdoctoral Science Foundation (2012M510123) and Natural Science of Jiangsu Education (15KJB430008). We are also indebted to the Jiangsu Training Programs for Innovation and Entrepreneurship for Undergraduate (201613986010Y).

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis absorption spectra of Ag and Au nanoparticles in different sizes. The amount of metal loaded per gram of GO for GO–Ag and GO–Au samples. The size distribution of GO–Ag and GO–Au samples was calculated from the TEM images. See DOI: 10.1039/c6ra11291a

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