Silica-Graphene Oxide Hybrid Composite Particles and Their

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Silica-Graphene Oxide Hybrid Composite Particles and Their Electroresponsive Characteristics Wen Ling Zhang and Hyoung Jin Choi* Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea S Supporting Information *

ABSTRACT: Silica-graphene oxide (Si-GO) hybrid composite particles were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of hydrophilic GO obtained from a modified Hummers method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images provided visible evidence of the silica nanoparticles grafted on the surface of GO, resulting in Si-GO hybrid composite particles. Energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) spectra indicated the coexistence of silica and GO in the composite particles. The Si-GO hybrid composite particles showed better thermal stability than that of GO according to thermogravimetric analysis (TGA). The electrorheological (ER) characteristics of the Si-GO hybrid composite based ER fluid were examined further by optical microscopy and a rotational rheometer in controlled shear rate mode under various electric field strengths. Shear stress curves were fitted using both conventional Bingham model and a constitutive Cho-Choi-Jhon model. The polarizability and relaxation time of the ER fluid from dielectric spectra measured using an LCR meter showed a good correlation with its ER characteristics.

1. INTRODUCTION Graphene oxide (GO), a single-layered material from the chemical exfoliation of graphite, has attracted significant interest as one of the most interesting and potential materials in recent years along with graphene owing to its tremendously large surface area and exceptional thermal and mechanical properties.1 In particular, its good hydrophilic properties due to the presence of oxygen-functional hydrophilic groups, such as hydroxyl, carbonyl, and carboxyl groups, not only makes GO readily dispersible in water to form stable colloidal suspension,2,3 but also facilitates the preparation of GO-based composites in solution.4,5 Furthermore, GO sheets show remarkable improvements in chemical and physical properties when incorporated in composite materials with potential applications in many areas, such as solar cell, resonators, electronic devices, and supercapacitors. 6−8 In addition, graphene-based materials have applications, such as catalyst supports, electronic components, capacitors, and biotechnology.9−12 As a new potential application of GO, both GO/conducting polyaniline nanocomposite13 and GO coated core−shell structured polystyrene microspheres14 were recently reported to show electroresponsive electrorheological (ER) characteristics under an applied electric field. Note that the ER fluids are in general heterogeneous dispersions composed of micrometersized polarizable particles dispersed in low-dielectricity oils, such as silicone oil or mineral oil.15,16 They are considered smart/intelligent materials since their structural and rheological characteristics can change reversibly under external electric fields.17,18 The particles become polarized and behave as electric dipoles to form fibrillar structures owing to their © 2012 American Chemical Society

electrostatic attractive force within the order of milliseconds under an applied electric field, and the flow behaviors change from low-viscosity Newtonian fluids with a no external electric field to high-viscosity non-Newtonian fluids described as Bingham fluids. Since their discovery, these smart materials have a range of applications in the scientific and engineering communities, particularly in the areas of electromechanical devices including shock absorbers, engine mounts, and clutches because of their rapid response to an electrical field and their controllable mechanical properties.19 Extrinsically polarizable particles, such as semiconducting polymers, as well as their derivatives,20−22 high dielectric inorganics including silica and porous silica/polymer nanocomposites, clay/polymer nanocomposites, and TiO2 nanoparticles have been reported extensively as important ER materials.23−26 Many reports are available for the silica particle systems which are decorated on the carbon nanotube (CNT).27 However, compared to the CNT, GO-based composites have attracted increasing attention due to their low cost and fine dispersion, in addition to their rather simply tunable electrical conductivity especially for ER application. In this study, GO was synthesized via the modified Hummers method,28 and silica-GO (Si-GO) composite was fabricated as an ER material that could not only enlarge the application of GO, but also improve its ER properties via the help of silica nanoparticles, because silicate materials themselves exhibit ER characteristics. As a challenging application in electrorheology, the electrical Received: March 3, 2012 Revised: April 7, 2012 Published: April 9, 2012 7055

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Scheme 1. Schematic Diagram of Synthesis Process of Si-GO Hybrid Composite

2.5. Characterization. The morphology of pure graphite, GO, and Si-GO hybrid composite particles was observed by scanning electron microscopy (SEM, S-4300, Hitachi, Japan) with a voltage of 15 kV at a working distance of 15 mm as well as by transmission electron microscopy (TEM) (Philips CM200). The crystal structures of the samples were identified by X-ray diffraction (XRD) (DMAX-2500, Rigaku). The X-ray energy dispersive spectra (EDS) were also obtained using an attached EDAX (couple with Hitachi S-4200) spectrometer. The thermal properties were also examined using a thermal gravimetric analyzer (TGA) (TA Instruments, Q50, USA), in which the samples were heated from room temperature to 800 at 5 °C/min under a nitrogen flow. Fibrillation phenomenon of the Si-GO hybrid composite particle based ER fluid was observed by optical microscopy (OM) (Olympus BX51) with a DC high-voltage source that was used to apply a voltage, in which the gap distance between the two parallel electrodes was approximately 240 μm. The ER performance of the ER fluid was tested using a shear rate controlled rotational rheometer (MCR 300, Physica, Germany), which was equipped with a high-voltage power supply (Fug, HCN 7 × 10−12 500, Germany) and a Couette-type sample loading geometry with a bob and cup (CC 17, gap distance is 0.71 mm). By changing the electric field strength, the flow curves were measured over a wide shear rate range 1−1000 s−1. A dielectric study was carried out using an LCR meter (Agilent HP 4284A) with a frequency range of 20−106 Hz. The experiments were repeated 3 times at 25 °C to obtain valid data.

properties of the polarized particles, such as the conductivity and dielectric constant, have governing influences on their ER performance. Nevertheless, the electrical conductivity of the GO (∼10−1 to 10−5 S/cm depending on the degree of oxidation) is still too high, which can cause an electric short in the ER measurement equipment. The addition of inorganic silica nanoparticles can help solve this problem. The ER properties can also be maximized by adjusting the optimum proportion of GO and tetraethyl orthosilicate (TEOS). Recently, silica nanoparticle-covered GO nanohybrids synthesized in a water−alcohol mixture were also reported regarding their superhydrophilic properties.29

2. EXPERIMENTAL SECTION 2.1. Materials. All chemical regents including strong oxidant KMnO4 (Sigma-Aldrich), NaNO3 (Junsei CO., Japan), 35% HCl (DC Chemical Korea), 98% H2SO4 (DC Chemical Korea), and tetraethyl orthosilicate (TEOS) (reagent grade, Sigma-Aldrich) were used as received from commercial sources without further purification. Distilled water was used in all experimental processes and washing. 2.2. Preparation of GO. GO was prepared via a modified Hummers method28 from pristine graphite powder (flake, ∼20 μm, 100 mesh (≥75% min), Sigma-Aldrich). In a typical procedure, graphite powder, KMnO4, and NaNO3 were added gradually to H2SO4 (98%), and the mixture was stirred vigorously for 3 h. The graphite oxide yielded was treated further with a 30% H2O2 solution until the color turned brilliant brown indicating fully oxidized graphite. The asobtained graphite oxide slurry was exfoliated to generate GO nanosheets by sonication at 60 °C using an ultrasonic generator (28 kHz, 600 W, Kyungil Ultrasonic Co., Korea) for 1 h. Finally, the mixture was separated by centrifugation, washed copiously with 5% HCl and Di water until it reached pH = 7, and dried in a vacuum oven at 60 °C for 24 h. 2.3. Preparation of Si-GO Hybrid Composite. The well-known hydrolysis of TEOS was then employed to fabricate the hybrid composite. Briefly, 0.3 g of the GO prepared via the Hummers method and 5 g of TEOS were dispersed separately in 30 g of ethanol to produce stable suspensions. Subsequently, the two suspensions were mixed together and placed into a water bath at 40 °C for 10 min to ensure a constant temperature. Hydrous ammonia (NH3·H2O, 0.76 g, catalyst) was added quickly into the mixture, and the products were displaced after 15 h, washed 3 times with di water and ethanol each, and finally dried in a vacuum oven at 60 °C 48 h before further use. Scheme 1 shows the synthesis process of the Si-GO hybrid composite via the hydrolysis of TEOS. The hydrophilic GO was prepared using the modified Hummers method in advance. The suspension containing TEOS dispersed in ethanol was placed into a water bath at 40 °C, and hydrous ammonia (NH3·H2O, catalyst) was added for 15 h. 2.4. Preparation of Si-GO Hybrid Composite Based ER Fluid. The synthesized Si-GO hybrid composite particles were shaken through a sieve (100 μm) and stored in an oven at 60 °C for 24 h prior to use. The ER fluid was prepared by dispersing Si-GO composite particles in silicone oil (dynamic viscosity = 30 cS) with the help of an ultrasonicator for better dispersion.

3. RESULTS AND DISCUSSION The distinctive layered structure of GO obtained by sonication and successful decoration of silica nanospheres on layered GO surface by hydrolysis of TEOS in Si-GO composite were confirmed by SEM and TEM images shown in Figure 1 and Figure 2, respectively. The stacking graphite flake in Figure 1a was exfoliated to GO layer (Figure 1b) and spherical nanosized silica particles were distributed on the GO layers in Figure 1c. As shown in the TEM images, the graphite was dark and opaque, whereas the GO showed transparent and lamellar structure, the circular profile in Figure 2c confirmed the existence of silica nanoparticles. It is very interesting to observe that almost all the as-prepared silica nanoparticles are attached on the GO surface. We conjecture that the layered 2-D structure of GO can provide a facile template for the hydrolysis of TEOS. Moreover, the carbonyl group (CO) on GO can be converted to Si−O−C band after the reaction with TEOS which strengthens the interconnection between GO and silica nanoparticles. Figure 3 shows the energy-dispersive X-ray spectra (EDX) spectra of graphite, GO, and Si-GO hybrid composite. As shown in Figure 3a, the element of C (83.60%) is in the majority in pure graphite. The composition of oxygen increased sharply from 4.45 wt % to 31.72 wt % in the GO (Figure 3b), indicating the success of adding oxidative functional groups onto graphene surface. Moreover, there were a small amount of S (3.14 wt %), Si (0.51 wt %), and Pt (10.52 wt %), originating 7056

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Figure 1. SEM images of pure graphite (a), GO (b), and Si-GO hybrid composite (c).

from the oxidants and impurity in the raw material of graphite. Compared with the EDX spectrum of GO, the Si-GO hybrid composite showed a sharp Si peak (45.60 wt %), increase in O (48.61 wt %), and decrease in C (5.79 wt %) elements confirming the existence of silica nanoparticles in the composite. The X-ray photoelectron spectroscopy (XPS) of graphite, GO, and Si-GO hybrid composite were analyzed (Supporting Information Figure S1) to study the difference of the samples further. Compared with pure graphite as shown in Figure S1(a), the intensity of oxygen in GO (Figure S1(b)) was enlarged obviously after the oxidation which confirms the conclusion deduced from the EDX spectra. As shown in the SiGO hybrid composite (Figure S1(c)), the conjecture that the carbonyl group (CO) in the GO was converted to Si−O−C band was supported by not only the appearance of Si2p peak at 104.2 eV27 which is assigned to the Si−O−C, but also the sharp decrease of C1s. Figure 4 shows the X-ray diffraction (XRD) of pure graphite, GO ,and Si-GO hybrid composite. The pure graphite showed a sharp peak at 26.34°, while after sonication, the GO presents a greatly reduced peak at approximately 10.88° which agrees with the reference.13 The Si-GO hybrid composite particles showed a broad peak at 22.96° due to the dominant effect of silica30 and a significant peak at 12.07° which originates from GO. According to Bragg’s law nλ = 2d sin θ (λ = 0.154 nm), the d-spacing of the composite was calculated to 7.32 Å, which decreased more than that of GO of 8.12 Å due to the decoration of silica particles on its surface. In order to prove the decoration of silica on GO surface in Si-GO hybrid composite, Fourier transform infrared (FT-IR)

Figure 2. TEM images of pure graphite (a), GO (b), and Si-GO hybrid composite (c).

spectra were examined as shown in Figure 5. The typical FT-IR spectrum of GO in Figure 5 is in agreement with previous work. The bands centered at 3426 and 1397 cm−1 were attributed to deformation of the −OH bond of the GO and CO−H groups, respectively. The band centered at 1054 cm−1 was associated with the stretching of the C−O bond. The stretching vibration of the carbonyl or carboxyl groups was observed as a band at 1724 cm−1.31,32After the reaction with TEOS, the characteristic peaks of silica were observed, which indicates that the silica nanospheres were fabricated on the surface of GO. The peak at 469 cm−1 is attributed to Si−O−Si bending vibration.33 The band at 1105 cm−1 attributed to the (Si−O−C/Si−O−Si) asymmetric stretching appeared, while the typical carbonyl group band at 1724 cm−1 disappeared. This evidence proved that the carbonyl groups were converted to Si−O−C bands, which have been reported in [email protected] Additionally, the peak at 810 cm−1 was assigned to the stretching vibration of Si−OH. Figure 6 shows the TGA curves of GO (black line) and SiGO hybrid composite (red line) heated in a TGA instrument to 800 °C at a heating rate 5 °C/min under N2 environment. The initial weight loss up to 150 °C for both samples was attributed to the removal of adsorbed water molecules. GO is thermally unstable; TGA revealed significant weight loss at approximately 290 °C, which was attributed to the decomposition of the labile 7057

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Figure 5. FT-IR spectra of GO and Si-GO hybrid composite.

Figure 3. EDX analysis of pure graphite (a), GO (b), and Si-GO hybrid composite (c).

Figure 6. TGA curves of GO and Si-GO hybrid composite.

Figure 4. XRD patterns of pure graphite, GO, and Si-GO hybrid composite. Figure 7. OM images of the Si-GO hybrid composite based on ER fluid without an electric field (left) and with an electric field (right).

oxygen-containing functional groups.34 Compared to the curve of GO, the weight loss of Si-GO hybrid composite was much lower and tended to be constant up to 200 °C, indicating much greater stability than GO possibly due to the deposited silica particles on GO surface. The Si-GO hybrid composite based ER fluid was prepared by dispersing composite in 30 cS silicone oil. The microstructural changes of the ER fluid were observed by OM using a highvoltage DC source in Figure 7. The gap distance between the two parallel electrodes was fixed at 240 μm. The randomly

dispersed particles moved freely in silicon oil like a Newtonian fluid in no electric field; when an electric field was applied, the particles started to move and transformed to fibrillated chain structures parallel to the electric field between the two sides of electrodes immediately.35 Figure 8a shows the typical flow curves of a controlled shear rate (CSR) mode of the ER fluid based on 9 wt % Si-GO 7058

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model for an ER suspension, the Bingham fluid model39 is expressed as follows: τ = τ0 + η0 γ̇ , τ ≥ τ0 γ̇ = 0

τ < τ0

(1)

where τ is the shear stress, τ0 is the yield stress, which is related to an electric field, γ̇ is the shear rate, and η0 is the shear viscosity. On the other hand, not all the ER fluid curves could be fitted with the simple Bingham model over the whole shear rate region. To fit a range of ER fluids, particularly in the low shear rate region, a familiar model called the Cho-Choi-Jhon (CCJ) model40,41 in terms of six parameters, is described as follows: τ=

⎛ τ0 1 ⎞ ⎟⎟γ̇ ⎜ 1 + η + ⎜ ∞ 1 + (t1γ̇)α (t 2γ̇)β ⎠ ⎝

(2)

where τ0 is the yield stress defined as the extrapolated stress at the low shear rate region, and η∞ is the viscosity at a high shear rate and is interpreted as the viscosity in the absence of an electric field. The exponents α and β (0 < β ≤ 1, since dτ/dγ̇ ≥ 0) are related to the decrease and increase in shear stress, respectively. The parameters t1 and t2 are time constants. Note that physical meanings of the flow curve given in Figure 8a along with eq 2 can be explained as follows: the fibrillation of ER particles in shear flow is a breaking and reforming process caused by the cooperation of electrostatic and hydrodynamic interactions, which are reduced by an external electric and flow field. In the low shear rate region, where the electrostatic interaction is dominant, the aligned particles begin to break with shear deformation and the broken structures tend to form chains again. However, the rate of destruction may be faster than the rate of reformation. Therefore, the shear stress generated slightly decreases with increasing shear rate. Hydrodynamic interactions become dominant in the high shear rate region, where fibril particle structures are fully destroyed without reformation, and the suspension behaves like a pseudo-Newtonian fluid. Table 1 lists the optimal fitting parameters used for these two models. A comparison of the two models shows that the six parameter equation of the CCJ model can cover the curves better, particularly for the flow curve at higher electric fields. Figure 8b presents the shear rate versus apparent shear viscosity curve. The ER fluid showed obvious shear-thinning behavior under different electric strengths. The largest increases in shear viscosity occur at small shear rates. Furthermore, the dynamic yield stress obtained from the controlled shear rate

Figure 8. Flow curves for 9 wt % Si-GO hybrid composite based ER fluid under various electric field strengths: (a) shear stress versus shear rate, (b) shear viscosity versus shear rate. Solid and dotted lines in (a) are from CCJ and Bingham equations, respectively.

hybrid composite dispersed in 30 cS silicone oil under a range of electric field strengths using a rotational rheometer. Without the electric field, the suspension exhibited typical Newtonian behavior, in which the shear stress increased linearly with increasing shear rate in the log−log plot. When the electric field was present, the shear stress increased with increasing electric field showing Bingham-like behavior as the particles became polarized and formed chain-like structures.36 The plateau region of the shear stress curves for a wide range of shear rates under an applied electric field could be explained by reformation of the broken chain-like structure. By increasing the electric field strength, the shear stress increased abruptly over the entire shear rate range. This behavior has also been observed in many polymer-based ER fluids.37,38 As a general

Table 1. Optimal Parameters Appearing in Each Model Equation Obtained from the Flow Curves of 9 wt % Si-GO Hybrid Composite Based ER Fluid electric field strength (kV/mm) model

parameters

0.5

1.0

1.5

2.0

2.5

3.0

Bingham

τ0 η0 τ0 t1 α η∞ t2 β

4 0.07 7.3 1.2 0.02 0.05 0.013 0.72

12 0.07 22 0.06 0.62 0.07 0.0068 0.28

26 0.08 35 0.01 0.6 0.09 0.013 0.92

50 0.08 78 0.0028 0.94 0.13 0.22 0.88

130 0.09 175 0.01 0.76 0.18 0.007 0.98

250 0.1 264 0.01 0.66 0.24 0.0065 0.99

CCJ

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mode test (CSR) as a function of various electric fields was plotted in log−log scale, as shown in Figure 9. In general, the

Figure 11. Relaxation modulus G(t) of Si-GO hybrid composite based ER fluid as calculated from G′(ω) and G″(ω).

log−log scale with increasing electric field, which suggests that there is a strong interaction between silica and the GO layers. Figure 9. Dynamic yield stress vs electric field strength for 9 wt % SiGO hybrid composite based ER fluid.

G(t ) ≅ G′(ω) − 0.560G″(ω/2) + 0.200G″(ω)

Furthermore, the dielectric spectra of the 9 wt % Si-GO hybrid composite based ER fluid were examined with a LCR meter in the frequency range 20−106 Hz and are plotted in Figure 12. The experimental data were fitted using the wellknown Cole−Cole equation shown in eq 5;46 the parameters were obtained to help understand the ER properties of the interfacial polarization of suspensions consisting of polarizable phases dispersed in insulating media.

dependency of the dynamic yield stress on the electric field strength can be represented by the power law relationship as follows:

τy ∝ Em

(4)

(3) 42,43

where m = 2 is suggested by the polarization model. The yield stress is approximately proportional to the square of the electrical field strengths, where the slope of the single line in this paper was 2.0 corresponding to the polarization model of the ER mechanism. Figure 10 describes the change in storage modulus (G′) and loss modulus (G″) as a function of frequency under a range of

ε* = ε′ + iε″ = ε∞ +

Δε (1 + i ωλ)1 − α

(0 ≤ α < 1) (5)

The dielectric constant (ε′) and loss factor (ε″) are typical results for the interfacial polarization of ER fluids, ε0 is the dielectric constant when the frequency (ω) is close to 0, whereas ε∞ is the dielectric constant at a high frequency limit, Δε = ε0 − ε∞ is the achievable polarizability of an ER fluid, which provides a positive effect on ER performance. The exponent (1 − α) characterizes the broadness of the relaxation time distribution. The relaxation time, λ = (1/2)πf max, is the dielectric relaxation time, which is related to the yield stress and stress enhancement under an applied electric field, where f max is the maximum of the dielectric loss of an ER fluid. The higher stress enhancement is achieved when λ becomes smaller within the adequate range.47 In the fitting result, the value of α, λ, ε0, and ε∞ are 0.71, 0.05, 5.10, and 2.90, respectively. Therefore, Δε is 2.30, indicating a faster response to be polarized in the direction of the long axis in a short time, which can be conducive to the ER effect.48

Figure 10. Frequency sweeps for 9 wt % Si-GO hybrid composite based on ER fluid under different electric fields. Storage modulus: closed symbol. Loss modulus: open symbol.

4. CONCLUSIONS This study developed a relatively simple, inexpensive, and fast method for preparing Si-GO hybrid composite particles as an ER smart material in the presence of GO synthesized via modified Hummer method. SEM and TEM confirmed the successful synthesis of silica particles on the GO surface, the XRD and FT-IR data show the structural differences between pure GO and Si-GO hybrid composite, the latter showed superior thermal stability to GO. The Si-GO hybrid composite based ER fluid shows typical ER characteristics and behaved as a Bingham fluid in the presence of electric fields. The ER fluid

electric field strengths. In addition, the storage modulus is always higher than the loss modulus over the frequency range. The stress relaxation behavior was examined to confirm the phase change in ER fluid from a liquid-like phase to a solid-like phase. The stress relaxation modulus (G(t)) was calculated from the values of G′(ω) and G″(ω) from Figure 11 using the common formula known as the Schwarzl equation given in eq 4 for numerical analysis to predict the relaxation behavior of the material.44,45 As shown in Figure 11, the G(t) became linear on 7060

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ACKNOWLEDGMENTS This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea (2011).

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Figure 12. (a) Permittivity and (b) dielectric loss factor as a function of the frequency (c) Cole−Cole arc of 9 wt % Si-GO hybrid composite based ER fluid. The solid lines are obtained from eq 5.

presents a very short relaxation time in dielectric analysis. The CCJ model fitted the flow curves better than the Bingham model, indicating the importance of the CCJ model in analyzing the ER behaviors. Overall, this study can open a new area for graphene applications in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7061

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dx.doi.org/10.1021/la3009283 | Langmuir 2012, 28, 7055−7062