Metal ion-directed solution-phase tailoring: from large-area graphene oxide into nanoscale pieces
© Wang et al.; licensee Springer. 2013
Received: 22 February 2013
Accepted: 19 March 2013
Published: 14 May 2013
Due to fascinating electronic properties and great potential in various applications, graphene has attracted great interest. Recently, much work have focused on the synthesis of different sizes and properties of graphene or graphene oxides (GOs), for example, graphene nanoribbons, nanosized graphene pieces, and nanosized triangular and hexagonal graphene sheets terminated by zigzag edges. Herein, we have demonstrated a widely available approach to fabricate the nanoscale GO pieces by directly solution-phase cutting a large-area GO sheet into nanoscale pieces via spontaneous redox reactions at room temperature. In this process, GO acts with dual functions as a model and a reducing reagent. With a typical example of silver ions, we have investigated in detail the influence of the reaction time and concentration of metal ions on yield and size of nanoscale GO pieces. Moreover, we also obtain Ag nanoparticle coating on the GO surface. Finally, a possible mechanism is suggested to explain the formation of nanoscale GO pieces.
Since discovered by Andre Geim and Konstantin Novoselov in 2004 , graphene has drawn significant attention to different scientific and technical communities due to its unique electrical, chemical, mechanical, optical, and structural properties . However, large-area graphene remains to be a metallic conductor even at the neutrality point which limits its application in nanoelectronic devices and biological science [3–6]. In addition, for the purpose of drug delivery and biological nanoprobe applications, small-sized graphene or graphene oxides (GOs) can easily be swallowed into organs, tissues, and cells . Recently, quite a lot of researchers have reported about the preparation of graphene ribbons with quantum confinement and edge effect properties by directly tailoring large-area graphene via e-beam lithography , hydrogen plasma etching , scanning tunneling microscope lithography , atomic force microscopy , chemical stripping, or catalytic tailoring (Fe, Ni, and Co nanoparticles as catalysts) [12–16]. Usually, the technologies used for synthesis of graphene ribbons mostly must be operated under ultrahigh-vacuum and high-energy conditions. So it is very difficult to produce large quantities of water-soluble graphene pieces. Moreover, these extreme synthetic conditions will be ultimately bound to affect the properties of graphene ribbon. Till now, direct soluble-phase formation of nanoscale graphene or graphene oxide pieces has been rarely involved . Generally, through selecting small-sized graphite as raw materials to control the size of GO during the synthesis of GO through the Hummers procedure, subsequently complicated treatment with strong sonication treatment and stepwise centrifugation at 4,000 to 10,000 rpm, a small-sized GO can be obtained . However, the procedures are quite complex and the yield of nanoscale fragments is also very low.
Herein, we report a widely available approach to prepare the nanoscale GO pieces directly utilizing some oxidizing metallic ions (Ag+, Ni2+, Co2+, etc.) via spontaneous redox reactions to cut a large-area GO sheet into nanoscale pieces at room temperature. With an example of silver ions, we have investigated the influence of the reaction time and concentration of metal ions on size and properties of nanoscale GO pieces. Meanwhile, the corresponding silver nanoparticles can also be obtained. Finally, a possible mechanism is put forward for explaining the formation of nanoscale GO pieces.
All reagents were of analytical grade and purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Natural graphite powder (800 mesh) was provided by Beijing Chemical Reagents (Beijing, China). All aqueous solutions were prepared with ultrapure water (18 MΩ cm).
Preparation of large-area GO
Water-soluble GO was prepared by oxidizing graphite according to a modified Hummers method just as our previous reports [19, 20]. Briefly, the graphite powder was first oxidized into graphite oxide using KMnO4/H2SO4, and then the graphite oxide was exfoliated into GO sheets in water under ultrasonication for 1 h, followed by centrifugation at 4,000 rpm for 30 min and dispersion in water. The obtained yellow-brown aqueous suspension of GO was stored at room temperature for further characterization and subsequent reaction.
Preparation of nanoscale GO pieces
The experiments of cutting large-area GO were carried out as follows: Firstly, 100-mL GO water solution (0.50 mg/mL) was prepared. Homogeneous suspension (20 mL) of GO was mixed with the desired amount of aqueous metallic ion (Ag+, Ni2+, Co2+, etc.) solution (5 mg/mL). Without heating or ultrasonication, the reaction mixtures were kept at room temperature for 48 h. Then the mixtures were centrifuged to remove the nanoparticles and large-scale GO and particle composites at the rate of 8,000 rpm. The upper solution without further purification was detected by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, UV-vision (UV-vis) spectroscopy, and X-ray photoelectron spectroscopy (XPS). In order to investigate the tailoring mechanism, we selected silver ions as a typical example and elaborately investigate the influence of reaction time and concentration of silver ions on the size and properties of nanoscale GO. All experiments were carried out at 25°C ± 2°C.
Characterization of nanoscale GO
AFM images were obtained on a Nanoscope MultiMode V scanning probe microscopy system (Veeco, Plainview, NY, USA) by tapping-mode imaging. Commercially available AFM cantilever probes with a force constant of approximately 48 N/m and resonance vibration frequency of approximately 330 kHz were used. The scanning rate was usually set at 1 to 1.2 Hz. Freshly cleaved mica with atom-level smoothness was used as the substrates. The samples were coated on the mica surface by spin-coating technology. UV-vis spectra were measured at 20°C with a Shimadzu UV-2450 spectrophotometer equipped with a 10-mm quartz cell (Kyoto, Japan). Zeta potentials were measured with NICOMP 380 ZLS Zeta Potential/Particle Size Analyzer. The XPS measurements were performed on an Axis Ultra DLD XPS (Kratos Analytical, Manchester, UK) using a monochromated Al Kα (1,486.6 eV) source at 15 kV. Scanning electron microscopy (SEM) images were taken on a ZEISS-ULTRA 55 SEM (Oberkochen, Germany) equipped with an X-ray energy-dispersive spectroscope (EDS) at an accelerating voltage of 20 kV (provided in Additional file 1). In addition, the conductive properties of the nanoscale GO film coated on the mica surface were tested using a conductive AFM. The detailed process and results have been given in Additional file 1.
Results and discussion
Tailoring large-area GO by different metal ions
Tailoring large-area GO by silver ions
FTIR spectroscopy has been considered as another powerful tool to analyze surface chemical group changes of GO. As shown in Figure 3c, the characteristic peaks of GO (green line) displayed the C=O stretching vibration peak at 1,730 cm-1, the vibration and deformation peaks of O-H groups at 3,428 and 1,415 cm-1, respectively, the C-O (epoxy groups) stretching vibration peak at 1,220 cm-1, and the C-O (alkoxy groups) stretching peak at 1,052 cm-1. After the reaction is conducted for 48 h (red line), the intensities of the FTIR peaks corresponding to the C-O (epoxide groups) stretching vibration peak at 1,220 cm-1 disappeared nearly, the C=O stretching vibration peak at 1,730 cm-1 decreased dramatically, and the vibration and deformation peaks of O-H groups at 3,428 and 1,415 cm-1, respectively, and the C-O (alkoxy groups) stretching peak at 1,052 cm-1 increased slightly. These results further confirmed that some active functionalities (epoxide groups) in GO have been removed.
The mechanisms of tailoring GO
In summary, we have demonstrated a very simple strategy to obtain nanoscale GO pieces using metal ions as oxidation reagent at mild condition. Without being heated or treated ultrasonically, two kinds of nanoscale GO pieces: GO pieces and nanoparticle-coated GO piece composites, are obtained. Based on systematic investigations of nanoscale GO piece formation by the addition of Ag+ ions as a tailoring reagent, a probable mechanism is suggested to explain the formation of nanoscale GO pieces, which can be mainly attributed to interaction of metal ions (Ag+, Co2+, Ni2+, etc.) with the reducing groups (e.g., epoxy groups) on the basal plane of other GO sheets. Obviously, in this progress a large-scale GO acts with dual functions, as a reducing reagent and a nucleation site of metal or metal oxide nanoparticles. This work provides a good way or chance to fabricate nanoscale GO pieces and GO composites in water solution and more widely apply in nanoelectronic devices, biosensors, and biomedicine.
This work is supported by the National Key Basic Research Program (973 Project; nos. 2010CB933901 and 2011CB933100) and National Natural Scientific Fund (nos. 31170961, 81101169, 20803040, 81028009, and 51102258).
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