The situ preparation of silica nanoparticles on the surface of functionalized graphene nanoplatelets
© Li et al.; licensee Springer. 2014
Received: 27 November 2013
Accepted: 27 March 2014
Published: 9 April 2014
A method for situ preparing a hybrid material consisting of silica nanoparticles (SiO2) attached onto the surface of functionalized graphene nanoplatelets (f-GNPs) is proposed. Firstly, polyacrylic acid (PAA) was grafted to the surface of f-GNPs to increase reacting sites, and then 3-aminopropyltriethoxysilane (APTES) KH550 reacted with abovementioned product PAA-GNPs to obtain siloxane-GNPs, thus providing reaction sites for the growth of SiO2 on the surface of GNPs. Finally, the SiO2/graphene nanoplatelets (SiO2/GNPs) hybrid material is obtained through introducing siloxane-GNPs into a solution of tetraethyl orthosilicate, ammonia and ethanol for hours' reaction. The results from Fourier transform infrared spectroscopy (FTIR) showed that SiO2 particles have situ grown on the surface of GNPs through chemical bonds as Si-O-Si. And the nanostructure of hybrid materials was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All the images indicated that SiO2 particles with similar sizes were grafted on the surface of graphene nanoplatelets successfully. And TEM images also showed the whole growth process of SiO2 particles on the surface of graphene as time grows. Moreover, TGA traces suggested the SiO2/GNPs hybrid material had stable thermal stability. And at 900°C, the residual weight fraction of polymer on siloxane-GNPs was about 94.2% and that of SiO2 particles on hybrid materials was about 75.0%. However, the result of Raman spectroscopy showed that carbon atoms of graphene nanoplatelets became much more disordered, due to the destroyed carbon domains during the process of chemical drafting. Through orthogonal experiments, hybrid materials with various sizes of SiO2 particles were prepared, thus achieving the particle sizes controllable. And the factors’ level of significance is as follows: the quantity of ammonia > the quantity of tetraethyl orthosilicate (TEOS) > the reaction time.
KeywordsGraphene SiO2 particles Hybrid material Situ preparation Controllability
Graphene, a single layer carbon material in a close arrangement of honeycomb two-dimensional lattice, has remarkable properties, such as Young's modulus, fracture strength, specific surface area and so on[2–4]. Significantly, graphene is a promising building block material for composites because of its large surface area. Furthermore, decoration of the graphene nanosheets with organic/inorganic materials can bring about an important kind of graphene-based composites[5–10]. However, the two-dimensional structure and huge specific surface area of graphene nanoplatelets made it easy to aggregate, which limited its application. Thus it is necessary to overcome graphene's extreme hydrophobicity which leads to aggregation in polar liquids[12, 13]. Researches indicated that the modification of graphene nanoplatelets is arguably the most versatile and easily scalable method. Meaningfully, the decoration of nanomaterials onto graphene nanosheets is helpful to overcome the aggregation of individual graphene nanosheets and nanomaterials themselves. In recent years, researchers have shown an increasing interest in graphene-based composites[16, 17] in which graphene sheets are used as a wild phase to enhance mechanical properties. Among all these materials, hybrid materials based on GNPs and silica nanoparticles have attracted significant scientific interest because of their remarkable properties that do not exist in the individual components[19–22]. Due to the synergistic effect, graphene nanoplatelets/SiO2 hybrid materials have superior properties compared with bare graphene nanoplatelets and SiO2 particles. Considering the outstanding properties of graphene nanoplatelets and SiO2, graphene/silica composite would be one of the greatly popular and interest topics in the field of nanomaterial and nanotechnology. And this kind of composite materials have been explored as adsorbents[25, 26], catalysts, and fillers into resin for composites along with an excellent application potential[28, 29].
Hao et al. prepared SiO2/graphene composite for highly selective adsorption of Pb (II) ion through a simple two-step reaction, including the preparation of SiO2/graphene oxide and the reduction of graphene oxide (GO). Zhou et al. used a one-pot hydrothermal synthesis to obtain a mesoporous SiO2-graphene hybrid from tetraethyl orthosilicate and graphene oxide without any surfactant. Lu et al. reported on the preparation of well-defined SiO2-coated graphene oxide (GO) nanosheets (SiO2/GO) without prior GO functionalization by combining sonication with solgel technique. And then, the product is decorated with Ag nanoparticles for H2O2 and glucose detection. However, all these abovementioned method did not have the advantage of controlling the size of SiO2. Accordingly, the development of new preparation strategy overcoming the shortcoming is highly desired.
Graphene nanoplatelets (GNPs) (diameter, 1 to 20 μm; thickness, 5 to 15 nm) were purchased from Xiamen Kona Graphene Technology Co., Ltd. (Xiamen, China). PAA (PH: 1–2) was purchased from Tianjin Damao chemical reagent Co. Ltd. N,N-Dicyclohexyl carbodiimide (DCC) was purchased from Aladdin industrial corporation, Seattle, Washington D.C., USA. 3-Aminopropyltriethoxysilane (APTES) KH550 was purchased from Shanghai Yaohua Chemical Co. Ltd., Shanghai, China. H2SO4 (98%), HNO3 (65%), tetrahydrofuran (analytically pure), TEOS (AR), ammonia solution (AR), and ethanol (AR) were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
Oxidation of graphene nanoplatelets
GNPs (900 mg) were suspended and refluxed in a mixture of concentrated acid H2SO4/HNO3 (30 ml/30 ml) at 140°C for 1 h, followed by diluting with deionized water (3,000 ml). The acid-treated GNPs were retrieved and washed repeatedly with THF until pH = 7 and dried under vacuum. The product was denoted as f-GNPs.
Grafting PAA onto f-GNPs
f-GNPs 50 mg, PAA 100 mg, DCC 100 mg, and THF 50 ml were mixed under dry nitrogen atmosphere and then stirred in a flask for 48 h at 60°C. The solid product was collected and washed repeatedly with THF until pH = 7 and dried under vacuum. The product was denoted as PAAGNPs.
Reaction of PAA-GNPs and KH550
PAA-GNPs 100 mg, DCC 100 mg and THF 100 mg were mixed by sonication for 1 h. Then, the solution of KH550 was added dropwise into suspension at 60°C under nitrogen atmosphere. When completed, the reaction was kept at 60°C and vigorously stirred for 24 h. At last, the solid product was collected and washed repeatedly with THF until pH = 7 and dried under vacuum. The KH550 functionalized GNPs were denoted as siloxane-GNPs.
Preparation of SiO2/GNPs hybrid material
Siloxane-GNPs (50 mg) were added into 10 ml deionized water and stirred for 24 h at room temperature to hydrolyze the alkoxysilane into Si-OH. Then, 0.6 g TEOS, 1.2 g ammonia solution, and 100 ml ethanol were added to the suspension and stirred for 8 h. Finally, the solid product was collected and washed repeatedly with THF until pH = 7 and dried under vacuum. In this process, the quantity of TEOS, the quantity of ammonia, and the time of reaction can be different. Thus, we can control the size of SiO2 particles.
Orthogonal array experimental design
Levels of factor of orthogonal design
NH3 · H2O (g)
Orthogonal arrays for statistical experiment and results
NH3 · H2O (g)
Average particle size (nm)
Fourier transform infrared spectrometer (FTIR, Nexus 670, Valencia, CA, USA) was used to detect the functional groups on the surface of f-GNPs and f-GNPs/SiO2 hybrid materials, which was measured as pellets with KBr. Raman spectroscopy (In Via laser confocal microscope, Renishaw, Wotton-under-Edge, UK) was employed to investigate the ordered or disordered crystal structures and assessing defects of samples, which was recorded using a spectrometer with 532 nm wavelength incident laser light. Thermal gravimetric analysis (TGA, SDTA851e) was used to evaluate the weight loss ratio of the products. The tests were conducted at a heating rate of 10°C/min from room temperature to 900°C under nitrogen. Scanning electron microscopy (SEM, HITACHI SU1510, Chiyoda-ku, Japan) was employed to observe the surface morphology of various products, whose accelerating voltage was 1.0 kV. Transmission electron microscopy (TEM, H-800-1) was employed to observe the microstructure of various products, whose accelerating voltage was 20 kV.
Results and discussion
Fourier transform infrared spectroscopy
Raman spectroscopy is a powerful and useful technique to investigate the ordered or disordered crystal structures and assessing defects of graphene-based materials. It is well known that the typical features of carbon materials in Raman spectra are the G band at 1,580 cm−1 deriving from the E2g phonon of C sp2 atoms and D band at 1,350 cm−1 considered as a breathing mode of k-point photos of A1g symmetry which is assigned to local defects and disorder mostly at the edges of f-GNP platelet[33, 34].
Intensity ratio of the D and G bands ( I D / I G )
Thermal gravimetric analysis
where A%, B%, C%, D%, and E% were the weight loss percentages at a certain temperature of f-GNPs, SiO2/GNPs hybrid material, siloxane-GNPs, PAA-KH550, and SiO2, respectively. X and Y were denoted as the weight fraction of polymeric species on siloxane-GNPs and content of SiO2 on SiO2/GNPs hybrid material, respectively.
According to our calculation, At 900°C, the residual weight fraction of polymer on siloxane-GNPs was about 94.2% and that of SiO2 particles on hybrid materials was about 75.0%.
Scanning electron microscopy
Transmission electron microscopy
Analysis of orthogonal experiment
Analysis of range of each other
j = 1
I1 = 310
I2 = 280
I3 = 380
II1 = 510
II2 = 520
II3 = 500
III1 = 570
III2 = 590
III3 = 510
k1 = 3
k2 = 3
k3 = 3
According to our analysis, the amount of ammonia affects the size of SiO2 particles most. With the increasing of the amount of ammonia from 0.6 to 1.8 g, the size of SiO2 particles increases continuously. The joining of ammonia can significantly contribute to the occurrence of hydrolysis and polycondensation reaction of TEOS. When adding NH3.H2O to the solution, the OH anion made the silicon atoms negatively charged. As a result, Si-O bond weakened and eventually cracked. The products of hydrolysis reaction such as Si-OH and Si-OR dehydration or dealcoholation in the next polycondensation processing form Si-O-Si chain. Si-O-Si chains cross-linked continuously with each other to fabricate SiO2 particles finally. The hydrolysis rate will increase with the growing amount of ammonia, so the size of SiO2 particles also becomes larger.
With the increasing of the amount of TEOS from 0.3 to 0.9 g, the size of SiO2 particles also increases continuously. From the viewpoint of chemical equilibrium, the increasing of the content of TEOS contributes to the hydrolysis reaction to form SiO2 particles. However, the influence of TEOS is not as significant as ammonia.
The reaction time also had impact on the results. The size of SiO2 particles grew with the increasing of the reaction time from 4 to 8 h. With the time increasing, the cross-linking between Si-O-Si chains strengthened, and the size of SiO2 particles became larger and larger.
According to the above analysis, the controllability of the particle sizes was realized and in a certain range, the quantity of ammonia, the quantity of TEOS and the reaction time all had positive effect on the growing of SiO2 particles.
In this work, SiO2/GNPs hybrid material had been successfully achieved by a facile and controllable method as designed. In this process, firstly, PAA was grafted to the surface of f-GNPs for providing reaction pots, and then KH550 reacted with abovementioned product PAA-GNPs to obtain siloxane-GNPs. Finally, the SiO2/GNPs hybrid material is produced through introducing siloxane-GNPs into a solution of tetraethyl orthosilicate, ammonia, and ethanol for hours' reaction. The new characteristic band from FTIR indicated that those chemical reactions had been occurred as designed, and the results from SEM and TEM indicated that SiO2 nanoparticles were grown on the surface of f-GNPs successfully. Raman spectroscopy proved that after chemical drafting disordered, carbon atoms increased and carbon domains were destroyed. TGA traces suggested the residual weight fraction of polymer on siloxane-GNPs was about 94.2% and that of SiO2 particles on hybrid materials was about 75.0% finally and the SiO2/GNPs hybrid material we have prepared had stable thermal stability. Therefore, it was a feasible and reliable route to produce SiO2/GNPs hybrid material. Through orthogonal experiments, we also got the result that the controllability of the particle sizes was realized and the amount of ammonia had the most important impact on the size of SiO2 particles compared with quantity of TEOS and the reaction time. The next target of our study is to do research on the application of the hybrid material, to prepare epoxy resin composites with hybrid material, and study the influence of the SiO2 particles' size to strengthen epoxy resin composites.
functionalized graphene nanoplatelets
Fourier transform infrared spectra
scanning electron microscopy
transmission electron microscopy
thermal gravimetric analysis.
This work was supported by the National Natural Science Foundation of China (No. 51203062, 51302110). K. J. Yu thanks to Postdoctoral Fund Project of China (No. 2012M520995).
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