Synthesis and Characterization of a Novel Graphene Oxide/titanium Dioxide Solvent-free Nanofluid①

YANG Rui-Lu ZHENG Ya-Ping WANG Tian-Yu WANG Yu-Deng YAO Dong-Dong CHEN Li-Xin

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Synthesis and Characterization of a Novel Graphene Oxide/titanium Dioxide Solvent-free Nanofluid①

YANG Rui-Lu ZHENG Ya-Ping②WANG Tian-Yu WANG Yu-Deng YAO Dong-Dong CHEN Li-Xin

a(School of Natural and Applied Science, Northwestern Polytechnical University, Xi’an 710072, China)

A novel graphene oxide/titanium dioxide (GO/TiO2) solvent-free nanofluid was firstly synthesized by employing GO, which was in-situ deposited by TiO2as the core and (3-Glycidyloxypropyl) trime thoxysilane (KH560) and polyetheramine-M2070 as the shell. The morphology and structure of GO/TiO2nanofluid were verified by Transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectra. These studies confirmed that TiO2has been deposited onto GO with good dispersion, and the organic shell has been grafted onto the core successfully. Thermo gravimetric analysis (TGA) and viscosity analysis indicated that this nanoparticle hybrid material presented a liquid state without solvent at room temperature, and has great fluidity and thermal stability. The solubility investigation of GO/TiO2nanofluid revealed its excellent amphiphilicity and the potential as the functional nanocomposites.

graphene oxide, titanium dioxide, nanofluid;

1 INTRODUCTION

Graphene is a single-layer planar membrane made of carbon atoms with2hybridized orbitals, with only one carbon atom thick. It has many excellent physical and chemical properties, like high specific surface area, superior heat conduc- tivity and remarkable mechanical strength, which attracted the attention of many researchers in physics, chemistry and materials in the world[1-4]. Furthermore, graphene has a two-dimensional planar structure, which is very suitable as the carrier of other nanoparticles, and then the composite material of graphene/nanoparticles is obtained. Many researchers have reported that some nanoparticles, such as Ag[5], Fe3O4[6], SiO2[7,8]and TiO2[9], could be deposited on the surface of graphene. Wu et al.[10]deposited SiO2onto the surface of graphene oxide, and obtained a three-dimensional structure of porous graphene laminator. Adding the laminate to the cathode material of the lithium-sulphur battery improves the battery's cycling stability.Li et al.[11]made Fe3O4coated by chitosan, and then the core-shell structure was loaded to the surface of GO. At last, a magnetic ionic liquid/chitosan/GO hybrid was obtained and applied in water treatment. Among these nanopar- ticles, TiO2is widely concerned because of its excellent photo-catalysis and absorption of ultra- violet. Liu et al.[12]synthesized the hybrid particles composed of reduced graphene oxide sheets (GNSs) coated with Fe3O4nanorods (GNS–Fe3O4), and added it in the bismaleimide. They found that the mechanical and tribological properties of bisma- leimide composites were enhanced due to the good synergy between the graphene and Fe3O4during the wear process. Furthermore, Liu et al.[13]synthesized a ternary nano-particle (GNS-Fe3O4@PZM) consis- ting of graphene, Fe3O4nanoparticles and highly cross-linked polyphosphazene. The bismaleimide (BMI) matrix composites with aligned GNS-Fe3O4@PZM also show superior mechanical properties and thermal stability, owing to the uniform distribution and parallel alignment of graphene in the BMI matrix, as well as good interface interaction between GNS-Fe3O4@PZM and BMI matrix.

However, either graphene or nanoparticle is chemically inertias. Their insolubility in solvents and matrix brings great difficulty to the further study and application of graphene. Therefore, we are facing the opportunities and challenges to solve the dispersion problem of grapheme, and find controllable functionalization method of graphene to meet the actual demands.

Solvent-free nanofluid is a liquid nanoparticle, which can show a liquid state in the conditions of free-solvent, near room temperature and zero vapor pressure[14-17]. It is a kind of organic-inorganic nano-hybrid materials[18, 19]. Solvent-free nanofluids, which were prepared by grafting organic long-chain surfactants onto the nanoparticles, have excellent dispersibility in resin matrix and solvent[20, 21]. Combining in-situ deposition method and the organic long-chain surface modification method, graphene-based solvent-free nanofluids could be obtained. The functionalized graphene solvent-free nanofluids would show great solubility. In addition, the composite function of the graphene/nanopar- ticles would be brought to bear. The GO with a single-layer planar membrane structure was regar- ded as a conductive carbon mat, with TiO2nano- patricles loaded. And the synergy effect between GO and TiO2during the wear process can be produced. The interface interaction between GO and TiO2will enhance the properties of this nanofluid.

In this study, the GO/TiO2nanofluid was firstly synthesized and the properties of it were inves- tigated.

2 EXPERIMENTAL

2. 1 Materials

The graphite powder (< 20 μm) was obtained from Xiamen Knanotech Port Co., Ltd, and synthe- sized by mechanical exfoliation. Tetrabutyl titanate (TBOT, AR ≥ 99%) was provided by Xi’an Fuchen Chemical Ind. Ltd. (3-Glycidyloxypro- pyl)trime thoxysilane (KH-560, 98wt.%) was purchased from Chengdu Aikeda chemicals Co., Ltd. Polyetheramine M-2070 (Mw～2000), which was used as polymeric canopy in this study, was obtained from Dalian lianhao Co., Ltd. Other analytical reagents of H2SO4, HCl, KMnO4, H2O2, methanol, ethanol, glacial acetic acid, ammonia, acetone, tetrahydrofuran, toluene, dimethyl formamide and trichloromethane were from Xi’an Fuchen Chemical Ind. Ltd.

2. 2 Synthesis of the GO/TiO2 nanofluid

The synthesizing process of GO/TiO2nanofluid is illustrated in Fig. 1. A modified Hummers method was used to prepare GO[22]. After that, the GO aqueous suspension (1 mg/mL) was obtained under sonication. Then, 50 mL GO aqueous suspension was mixed with ethanol through the ultrasound dispersion for 30 min. Simultaneously, some tetrabutyl titanate was mixed with ethanol and glacial acetic acid through the ultrasound dispersion for 30 min. Next, this tetrabutyl titanate solution was added to the GO solution with stirring at room temperature, and ammonia (25% aqueous solution) was added dropwise into the solution at the same time. After 12 h, the black floccule was observed in the solution. Then the floccule was washed with distilled water three times by centrifuging with a rotating rate of 8000 r/min for 15 min (Anke TGL-20B) to remove the TiO2nanoparticles, which failed to deposit onto the surface of GO. At the end, GO/TiO2hybird was obtained after the black floccule was freeze-dried for 24 h.

Fig. 1. Synthesizing process of GO/TiO2nanofluid

For GO/TiO2nanofluid, firstly, 5 mmol KH560 was added to the diluted solution of 5 mmol polyetheramine-M2070 (15wt.%, diluted with methanol and deionized water) dropwise, and the mixed solution was stirred for 12 h at 45 ℃. Then the GO/TiO2hybrid was added to the mixed solution. Next, the mixture reacted at 25 ℃ for 5 h under mechanicalstirring to ensure complete reaction. The excess organic shell was removed in the dialysis tubing (3.5 k molecular weight cut-off) for 48 h. The unreacted core was removed after centrifuge. Finally, the GO/TiO2nanofluid was obtained after being dried at 70 ℃ in the vacuum oven to remove solvent.

2. 3 Characterization

The morphology of the samples was observed by transmission electron microscopy (TEM-H800, Hitachi Limited, Japan). Samples were dissolved in deionized water to form dispersion. A drop of dispersion was placed on a cooper grid and evaporated the solvent. X-ray diffraction (XRD) analysis was taken on a Rigaku (RD/MAX-Rc, Japan) using Curadiation (= 0.15406 nm), and the measured 2values ranged from 10° to 80°. Fourier transform infrared spectra (FTIR) were recorded from 400 to 4000 cm?1by a WQF-310 FTIR spectrometer. The samples were measured with KBr pellet. X-ray photoelectron spectra (XPS, Kratos Axis Ultra DLD) were used and the curves were obtained after a few samples were placed on a conductive blanket. The wide scan ranged from 0 to 1200 eV. The UV-vis absorption spectra were recorded using a Shimadzu UV1800 spectrometer, and the wavelength ranged from 200 to 800 nm. Thermo gravimetric analysis (TGA) measurements were taken under N2flowing at a heating rate of 10 K/min from 25 to 800 ℃ by using TGA Q50 TA instrument. A Brookfield R/S plus Rotational Rheometer was used to measure the viscosity of samples with a shearing rate of 12.5 °·s-1from 10 to 70 ℃, and the heating rate was 5 ℃·min-1. The temperature was stabilized by using a TC-650 constant temperature water bath.

3 RESULTS AND DISCUSSION

The TEM images of GO and GO/TiO2nanofluidsare shown in Figs. 2(a), 2(b) and 2(c). From Fig. 2(a), the sheet-like structure and fold structure of GO are observed clearly.This observation indicates that the graphite is oxidized well. Compared with Fig. 2(a), the GO/TiO2nanofluid shown in Fig. 2(b) is formed by depositing a larger number of TiO2nanoparticles on the surface of GO. In addition, it can be observed that theTiO2nanoparticles are well-dispersed without aggregation. Such a good dispersity is attributed to the protection role of organic layerswrapped on two sides of GO/TiO2core. The sphere structures of TiO2nanoparticles can be observed visibly in Fig. 2(c), which are coated with a layer of organisms. The chance of contacting between TiO2is reduced by perturbation of organic molecular chains, whichprovides lubrication between TiO2simultaneously.

Fig. 2. TEM images of (a) GO and (b), (c) GO/TiO2nanofluid

To further confirm that TiO2nanoparticles are deposited onto the surface of GO successfully, XRD curves of GO and GO/TiO2are shown in Fig.3(a). The diffraction peak at 2= 11.4° corresponds to the (002) reflection of GO. In addition, the characteristic diffraction peaks at 2=25°, 38°, 48°, 53°, 62°, 69°, 79° and 75° are assigned to the (110), (004), (200), (105), (204), (116), (220) and (215) planes of TiO2anatase, respectivelyaccording to JCPDS no. 89-4921. These characteristic diffraction peaksreveal that the GO is decorated with the TiO2nanoparticles successfully. Furthermore, compared with the characteristic peak (002) of GO at 11.4°, the (002) peak for GO/TiO2is reductive, which suggests that some GO has already been reduced to graphene. This indicates that the GO/TiO2hybrid material is not the simple mixture of GO and TiO2. TiO2is deposited onto the surface of GO with chemical bonds.

Included in Fig.3(b) are the FTIR spectra of GO, GO/TiO2hybrid and GO/TiO2nanofluid, respec- tively. Obviously, peaks at 3440 and 1720 cm-1of GO and GO/TiO2hybrid are associated to the vibration of -OH and C=O, which are from the acidic groups of GO. While the peak at 629 cm-1of GO/TiO2hybrid and GO/TiO2nanofluid is originated from the Ti–O–Ti vibrations. Addi- tionally, forthe data of GO/TiO2nanofluid, peaks at 1150and 850 cm-1are assigned to the vibrationof C–O–C, which belongs to the structure of M2070. Besides, peaks appearingat 998and 950 cm-1are designated to Si–Oand Si–O–Ti vibrations, respec- tively, which are from the groups of KH560. Most importantly, the peakat 1680 cm-1is designated to -NH- vibrations, which confirms the reaction between KH560 and M2070. All of these evidences indicate that the organic layer has been grafted onto the surface of inorganic nanoparticles successfully in GO/TiO2nanofluid.

The curves in Fig. 4 are wide scan spectra of GO, GO/TiO2hybrid and GO/TiO2nanofluid. The peaks at 284 and 532 eV correspond to C1and O1, respectively. Moreover, the presence of Ti2at 456 eV of GO/TiO2hybrid and GO/TiO2nanofluid confirm that TiO2nanoparticles are deposited onto the surface of GO, which is consistent with the results of XRD and FTIR. Furthermore, the peaks at 100 and 150 eV, which only exist in GO/TiO2nanofluid curve, are designated to Si2and Si2, respectively. This illustrates that the corona layers KH-560 have been grafted to the hybrid nanopar- ticles successfully. In addition, the peaks at 400 eV in GO/TiO2nanofluid curve are designated to N1. This reveals that the canopy layers M2070 have been grafted to the KH560 successfully.

Fig. 3. (a) XRD curves of GO and GO/TiO2hybird, (b) FTIR curves of GO, GO/TiO2hybird and GO/TiO2nanofluid

Fig. 4. XPS curves of GO, GO/TiO2hybird and GO/TiO2nanofluid

Fig.5 shows the UV-Vis spectra of GO, GO/TiO2hybrids and GO/TiO2nanofluid. The absorption peak at 235 nm is caused by-* transition of C=C. Obviously, this absorption peak exists in all three curvesbecause of the structure of GO. Furthermore, the absorption peak at 300 nm only in GO curve is assigned to→*transition of C=O because the crystalline structure of graphite is destroyed by strong oxidation. It is worth noting that there is almost no peak at 300nm in both GO/TiO2hybrid and GO/TiO2nanofluid. This might be the result of the consumption of C=O during the deposition and grafting reaction process.

Fig. 6 is the TGA curves of GO, GO/TiO2hybrid, GO/TiO2nanofluid and KH560+M2070. As shown in Fig. 6, weight loss is observed for both GO and GO/TiO2hybrid from about 200 ℃ owing to the decomposition of hydroxy, carboxyl and carbonyl of GO. However, it is noticed that the reducing rate of GO/TiO2hybrid is much lower than GO. This may be due to the addition of TiO2nanoparticles, which display great heat resistance and are deposited onto the surface of the GO. In addition, it is clearly seen that there is few weight loss for GO/TiO2nanofluid below 300 ℃. This indicates that the GO/TiO2nanofluid is a solvent-free state and presents a liquid-like state at room temperature due to the long, flexiblechains of the polyethera- mine canopy. As can be seen from the curve of KH560+M2070, a great mass loss was observed above 300 ℃ owing to the decomposition of organic layers. Then the weight of organics is up to 0.5% at 450 ℃. This reveals that the organic shell is completely decomposed in the region of 300～450 ℃. Thereby, the residuary mass of GO/TiO2nanofluid above 450 ℃ is the weight percentage of nanoparticles, which is as high as 8.2wt%.

Fig. 5. UV curves of GO, GO/TiO2hybird and GO/TiO2nanofluid

Fig. 6. TGA curves of GO, GO/TiO2hybrid and GO/TiO2nanofluid

The viscosity of GO/TiO2nanofluid in the range of 10～70 ℃ is demonstrated in Fig. 7. We can see that the viscosity is decreased with the increased temperature. It can be concluded that the higher temperature makes the long organic chains more active. The data in Fig. 7 illustrate that the viscosity of GO/TiO2nanofluid is 25.5 Pa·s at room temperature and 11.5 Pa·s at 50 ℃. This indicates that the GO/TiO2nanofluid has good fluidity whether at room temperature or higher temperature. Pristine TiO2nanoparticles tend to agglomerate in water or other solvents owing to the large specific surface area and high surface energy. Almost all of the nanoparticles are solid in normal circumstance. The GO/TiO2nanofluidshows a liquid-like state at room temperature with the organic covering in the absence of solvents because the long organic chains enhance the steric hindrance between TiO2nanoparticles and reduce the chance of contacting between TiO2.

Fig. 7. Viscosity of GO/TiO2nanofluid from 10 to 70 ℃

The solubility and dispersion of GO/TiO2nano- fluid in some common organic solvents are illustrated in Fig. 8. It can be seen that GO/TiO2nanofluid is homogeneously dispersed in water, ethanol, acetone, tetrahydrofuran, toluene, dimethyl formamide, trichloromethane, methanol, etc. After one month, they still display good dispersions without precipitate. This great dispersity of the GO/TiO2nanofluid is provided by the oxy- gen functional groups from the long organic chains of the GO/TiO2nanofluid.

Fig. 8. Images of GO/TiO2nanofluid in a) water, b) ethanol c) acetone, d) tetrahydrofuran, e) toluene, f) dimethyl formamide, g) trichloromethane and h) methanol

Based on all these investigations, TiO2nanopar- ticles are deposited on the surface of GO and the organic long chains are grafted on the surface of hybrid nanoparticles successfully. This GO/TiO2nanofluid hybrid shows good fluidity and excellent solubility and dispersion in lots of common organic solvents. This study may provide potential appli- cations of GO derivatives in UV absorption com- posite material, photo-catalysis material, conductive polymer material, and maybe some composite materials possesses great properties of both gra- phene and TiO2.

4 CONCLUSION

The GO/TiO2solvent-free nanofluid was synthe- sized by employing GO/TiO2as core, and (3-glyci- dyloxypropyl) trimethoxysilane (KH560) and polyetheramine-M2070 as shell. It can be observed that GO exists a single sheet-like structure and TiO2nanoparticles are dispersed on the surface of GO without aggregation. In addition, organic polymer shell has been grafted onto the inorganic core successfully. TGA and Rheology analyses indicated that the GO/TiO2solvent-free nanofluid has great fluidity and thermal stability, and the weight percentage of nanoparticles is 8.2wt%. The great solubility in some common organic solvents revealed its composite material potential which may possess great properties of both graphene and TiO2.

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10 November 2017;

30 January 2018

① This project was supported by the National Natural Science Foundation of China (51373137), the International cooperation project of Shaanxi Province (2016KW-053), and the Natural Science Basic Research Plan in Shaanxi (2017JQ2002)

. E-mail: zhengyp@nwpu.edu.cn

10.14102/j.cnki.0254-5861.2011-1886