Fabrication of palladium/graphene oxide composite by plasma reduction at room temperature
© Yu et al; licensee Springer. 2012
Received: 24 February 2012
Accepted: 26 April 2012
Published: 26 April 2012
Pd nanoparticles were fabricated on graphene oxide (GO) using a deposition-precipitation method with a glow discharge plasma reduction at room temperature. Argon was employed as the plasma-generating gas. The novel plasma method selectively reduces the metal ions. The graphene oxide has no change with this plasma reduction according to the Fourier transform infrared analysis. The Pd nanoparticles on the GO were uniformly distributed with an average diameter of 1.6 nm. The functional groups on the GO not only prevent Pd nanoparticles from further aggregation but also provide a strong hydrophilic property to the Pd/GO composite, which can form stable colloidal dispersions in water.
KeywordsNanoparticles Composite materials Palladium Graphene oxide Glow discharge Plasma
Graphene oxide (GO) is a highly oxidized layered graphene-based material with a large surface area and various functional groups such as -OH, -COOH, etc. [1–4]. These functional groups are chemically active so that GO can be decorated by various substances including biomolecules, metals and metal oxides [3, 5, 6]. GO can also be well dispersed in aqueous solutions due to the hydrophilic functional groups. The high solubility in water makes GO an ideal substrate for catalysts in water phase reactions. However, it is a challenge to reduce metal precursors on GO because GO is easily reduced with conventional reduction using NaBH4, N2H4 and ethylene glycol or irradiation method (microwave and laser pulse).
Here, we present a new fabrication method of Pd/GO composites using the room temperature glow discharge plasma reduction, which has been demonstrated to be a good strategy to reduce noble metal ions into nanoparticles [7–11]. The glow discharge plasma was shown not to affect the GO substrate. In order to maximize the loading efficiency and achieve high dispersion of Pd metal, deposition-precipitation was also carried out to anchor Pd precursor onto GO before the plasma reduction.
GO was prepared by a modified Hummers method. Pd/GO composites were prepared by deposition of palladium hydroxide from hydrolysis of palladium chloride at pH 4 to approximately 10. In a typical synthesis, 4 mL aqueous solution of H2PdCl4 (1 × 10-3 mol/L) was mixed with 4 mL aqueous solution of GO (1 mg/mL). NaOH (1 mol/L aqueous solution) was used to adjust the pH value of the H2PdCl4 and GO mixture. Then, the mixture was aged for 24 h before glow discharge plasma reduction. The details of glow discharge plasmas have been described previously [7, 8].
The morphology of the sample was observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM images were recorded with a Philips TECNAI G2F20 (Philips, Amsterdam, The Netherlands) system. AFM tests were performed by a Veeco Multimode Microscope V (Veeco Instruments Inc., Plainview, NY, USA) with spin coating of the sample on freshly cleaved mica substrates. The Fourier transform infrared (FT-IR) spectra were recorded using a Bruker Tensor 27 (Bruker Optics, Ettlingen, Germany) spectrometer with a resolution of 4 cm-1.
Results and discussion
To see the particles more clearly, a zoom-in image (Figure 1f) and a cross-section analysis were made. The cross-section line went through one of the highest (brightest in color) particles in the whole image and a step between the GO and the mica substrate. It showed that the GO had a thickness of 1.3 nm. The highest particle was around 2 nm, and the majorities were around 1 nm in diameter. When the pH value was further increased to 7 and 10 (Figure 1d,e), large aggregations formed in the solution with little Pd loaded on GO. Thus, the best metal distribution with the highest metal loading efficiency occurred when pH was equal to 6. This sample (denoted as Pd/GO) was further studied by the following characterizations.
The pH effect of the Pd deposition process can be explained by the deposition-precipitation mechanism. GO sheets are highly negatively charged between pH 4 and 10, which can be attributed to the de-protonation of surface hydroxy and carboxy groups at the surface of GO [3–5]. Higher pH resulted in lower zeta potential and more de-protonation of surface groups. For palladium precursors in the solvent, with the addition of NaOH, palladium chloride would hydrolyze into polynuclear palladium (II) hydroxocomplexes (PHCs) with a diameter of 2 nm [14, 15]. The PHC has a positively charged core by Na+ cations and, thus, can be absorbed onto GO by electrostatic forces. In lower pH (4 and 5) (Figure 1a,b), PHCs were not formed, and electrostatic repulsion between GO substrate and PdCl42- complexes kept the palladium off the substrate.
Pd nanoparticles with an average size of 1.6 nm are deposited on graphene oxide following a deposition-precipitation method and glow discharge plasma reduction at room temperature. The pH value has a significant effect on the fabrication of Pd/GO composites. The glow discharge plasma can efficiently reduce Pd precursors while maintaining GO unreduced. Pd/GO composite is successfully fabricated with high dispersion of metal nanoparticles.
Atomic force microscopy
Fourier transform infrared
Transmission electron microscopy
Palladium (II) hydroxocomplex.
The financial support from the National Natural Science Foundation of China (#20990223) is greatly appreciated. We appreciate Dr. Mingdong Dong at Aarhus University, Denmark for his help in the operation of the AFM.
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