High-quality reduced graphene oxide-nanocrystalline platinum hybrid materials prepared by simultaneous co-reduction of graphene oxide and chloroplatinic acid
© Wang et al; licensee Springer. 2011
Received: 17 September 2010
Accepted: 21 March 2011
Published: 21 March 2011
Reduced graphene oxide-nanocrystalline platinum (RGO-Pt) hybrid materials were synthesized by simultaneous co-reduction of graphene oxide (GO) and chloroplatinic acid with sodium citrate in water at 80°C, of pH 7 and 10. The resultant RGO-Pt hybrid materials were characterized using transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy, and thermogravimetric analysis. Platinum (Pt) nanoparticles were anchored randomly onto the reduced GO (RGO) sheets with average mean diameters of 1.76 (pH 7) and 1.93 nm (pH 10). The significant Pt diffraction peaks and the decreased intensity of (002) peak in the XRD patterns of RGO-Pt hybrid materials confirmed that the Pt nanoparticles were anchored onto the RGO sheets and intercalated into the stacked RGO layers at these two pH values. The Pt loadings for the hybrid materials were determined as 36.83 (pH 7) and 49.18% (pH 10) by mass using XPS analysis. With the assistance of oleylamine, the resultant RGO-Pt hybrid materials were soluble in the nonpolar organic solvents, and the dispersion could remain stable for several months.
Graphene, a flat monolayer of two-dimensional honeycomb-structured carbon atoms, has attracted tremendous attention from both theoretical and experimental studies in recent years [1–6]. Its unique structural, mechanical, and electronic properties make it a promising candidate in many potential applications, such as sensors [7–9], electrodes [10–13], lithium storage [14, 15], and hydrogen storage .
It is well known that carbon nanotubes (CNTs) deposited with platinum (Pt) nanoparticles exhibit outstanding catalytic activity [17–22]. Possessing similar properties, graphene has a larger surface area (theoretical value of ~2,600 m2/g  as unwrapped single-wall CNTs) than that of CNTs. Furthermore, graphene/reduced graphene oxide (RGO) can be produced at a lower cost through large-scale chemical synthesis [24, 25]. All these advantages make the reduced graphene oxide-nanocrystalline platinum (RGO-Pt) hybrid materials even more attractive in engineering applications.
RGO-Pt hybrid materials have been successfully synthesized by several research groups [26–34]. One way to synthesize the RGO-Pt hybrid materials is to reduce the graphene oxide (GO) sheets deposited with Pt precursor by H2[28, 31]. Ethylene glycol [27, 33, 34] and sodium borohydride [29, 30] have also been used as reducing agents to synthesize the RGO-Pt hybrid materials from the mixture of GO sheets and Pt precursors in the one-pot synthesis.
The monodispersity, the faceted uniformity, and the dispersion level of Pt nanocrystals significantly affect the high specificity in a catalytic process . When used in the synthesis of the noble-metal nanocrystals [36–38], sodium citrate, a mild reducing agent and stabilizer, can control well the size and morphology of the nanocrystals.
Materials and methods
Sodium nitrate (NaNO3, 99%), potassium permanganate (KMnO4, 99%), hydrogen peroxide (H2O2, 35%), concentrated sulfuric acid (H2SO4, 98%), oleylamine (C18H36NH2, 70%), sodium citrate dehydrate (Na3C6H5O7·2H2O, 99%), and concentrated hydrochloric acid (HCl, 36.5%) were purchased from Aldrich. Chloroplatinic acid (H2PtCl6·x H2O, with Pt content approximately 40%) was used as received from BDH Chemical. Natural graphite (SP-1) was purchased from Bay Carbon. All solvents were purchased from Merck. The deionized (DI) water was produced using the Millipore Milli-Q water purification system.
Synthesis of RGO-Pt hybrid materials
The GO was synthesized according to the modified Hummers method , a process that had been described earlier . The synthesis of the RGO-Pt hybrid material is a modification of the synthesis method of the Pt nanoparticles . Fifty milligrams of as-prepared dried GO sample and 50 mg of H2PtCl6·x H2O were mixed in 50 mL of DI water under continuous stirring, purged with N2 at room temperature for 2 h, and the pH value of the mixture was approximately 2. The syntheses of two other batches, adjusted to the values of pH 7 and 10 by adding diluted NaOH solution, were carried out. Ten milliliters of 20 g/L sodium citrate solution was injected into the mixture after it was heated to 80°C. After being kept at 80°C for 24 h, the mixture was cooled to room temperature and filtered. The residue was washed with DI water and acetone before freeze-drying, and kept aside until further use. The resultant products were denoted as RGO-Pt-2, RGO-Pt-7, and RGO-Pt-10 for the samples prepared at pH 2, 7, and 10, respectively. Two control syntheses were carried out under the same experimental conditions as RGO-Pt-10 without adding either the Pt precursor or the GO; the dried samples were denoted as RGO-NoPt-10 and Pt-10, respectively.
Ten milligrams of the prepared RGO-Pt hybrid materials was dispersed in 10 mL of 1,2-dichlorobenzene (DCB) with the addition of OA (0.1-0.2 mL) under sonication for 30 min. The clear solution of RGO-Pt hybrid materials in DCB can remain stable for several months.
Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained using a JEOL JEM-2010 operated at 200 kV. The samples for the analysis were prepared by dropwise addition of dilute DCB solutions of the RGO-Pt hybrid materials onto carbon-coated copper grids. Powder X-ray diffraction (XRD) patterns were obtained using Bruker AXS D8 Advance (Cu Kα λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) analysis was performed in an ultrahigh vacuum chamber, with a base pressure below 2.66 × 10-7 Pa at room temperature. Photoemission spectra were recorded using a Kratos Axis Ultra spectrometer equipped with a standard monochromatic Al Kα excitation source (h ν = 1486.71 eV). Fourier-transform infrared spectroscopy (FTIR) analysis was performed using a Perkin Elmer GX with potassium bromide dye for the preparation of the sample pellet. Thermogravimetric analysis (TGA) was carried out under air using thermogravimetric analyzer Perkin Elmer TGA 7. The samples were heated from room temperature to 900°C at a ramp rate of 10°C/min.
Results and discussion
The RGO-Pt hybrid materials were prepared from the co-reduction of GO and H2PtCl6 at 80°C at different pH values. The as-prepared RGO-Pt-2 appears brownish yellow and slightly darker than the pristine GO, while RGO-Pt-7 and RGO-Pt-10 show black color. This difference in color could be attributed to the reduction degree of the original GO sheets and the formation of the Pt nanoparticles. The RGO-Pt-2 can be easily dispersed in DI water, while RGO-Pt-7 and RGO-Pt-10 can be stably dispersed in nonpolar organic solvents assisted by OA.
The average mean diameters of the Pt nanoparticles are 1.76 and 1.93 nm for RGO-Pt-7 and RGO-Pt-10, respectively. The high-resolution TEM (HRTEM) images are shown in Figure 3c, d. The lattice spacing in both samples (RGO-Pt-7 and RGO-Pt-10) is measured as 0.226 nm, which corresponds to the (111) planes of the face-centered cubic Pt .
The characterizations confirm that the RGO-Pt hybrid materials can be produced by a simultaneous co-reduction process at pH 7 and 10. In the acidic (pH 2) condition, neither GO nor the Pt precursor could be reduced by the reductant sodium citrate; however, in neutral (pH 7) and basic (pH 10) conditions, sodium citrate was able to reduce the GO to graphene and the Pt precursor to Pt nanoparticles. This could be explained by the release of H+ and CO2 during the reduction process involving sodium citrate as the reducing agent . The increase of the pH values could consume the H+ and hence favors the reduction reaction. However, in the lower pH region, the high concentration of H+ would inhibit the reduction reaction from taking place.
The as-prepared RGO-Pt hybrid materials could not be dispersed well in any polar and nonpolar solutions because the oxygen-containing groups were decreased in the reduction process. However, the RGO-Pt hybrid materials assisted by OA could be dispersed well in nonpolar solvents such as DCB, toluene, and chlorobenzene. The dispersion can remain stable for several months. The dispersion of the RGO-Pt hybrid materials in DCB is caused by the coordination between OA and Pt nanoparticles, which is similar to the OA-assisted dispersion of CNT [51–53]. The weak nucleophilicity of OA ensures that the Pt nanoparticles are kept bonded together with the reduced GO. In order to investigate the interaction between OA and reduced GO, the dispersion of the RGO-NoPt-10 was attempted in DCB with the assistance of OA. It is worth mentioning that the RGO-NoPt-10 could not be dispersed satisfactorily in DCB even with the assistance of OA. Therefore, it is believed that the intercalation of Pt nanoparticles into the stacked graphene sheets and the coordination with OA are crucial for the redispersion of RGO-Pt hybrid materials. As a weak ligand, OA can also be very easily washed away by common solvents, like ethanol [54, 55], hexane , and acetic acid .
It has been shown that RGO-Pt hybrid materials can be synthesized by simultaneous co-reduction of GO and H2PtCl6 with sodium citrate at modest temperatures in neutral and basic conditions. The attachment of Pt nanoparticles onto reduced GO sheets can effectively prevent the aggregation of GO during reduction. The reduced GO can work as excellent supporters for dispersing and stabilizing the Pt nanoparticles. The resultant RGO-Pt hybrid materials exhibit high thermal stability, and are soluble in nonpolar organic solvents with the assistance of OA. The dispersion of RGO-Pt hybrid material composite is important for future catalytic applications. This material could be useful for the fabrication of the thin film that can replace the Pt coating in the counter electrode of the dye-sensitized solar cells. It could also be used as the electrode material for proton exchange membrane fuel cells.
Fourier-transform infrared spectroscopy
reduced graphene oxide-nanocrystalline platinum
transmission electron microscopy
- X-ray diffraction (XRD):
X-ray photoelectron spectroscopy (XPS).
The authors would like to acknowledge the Clean Energy Research Program under the National Research Foundation of Singapore for their financial support (Grant No. NRF2007EWT-CERP01-0420).
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