Gold-ionic liquid nanofluids with preferably tribological properties and thermal conductivity
© Wang et al; licensee Springer. 2011
Received: 8 November 2010
Accepted: 28 March 2011
Published: 28 March 2011
Gold/1-butyl-3-methylimidazolium hexafluorophosphate (Au/[Bmim][PF6]) nanofluids containing different stabilizing agents were fabricated by a facile one-step chemical reduction method, of which the nanofluids stabilized by cetyltrimethylammonium bromide (CTABr) exhibited ultrahighly thermodynamic stability. The transmission electron microscopy, UV-visible absorption, Fourier transform infrared, and X-ray photoelectron characterizations were conducted to reveal the stable mechanism. Then, the tribological properties of these ionic liquid (IL)-based gold nanofluids were first investigated in more detail. In comparison with pure [Bmim][PF6] and the nanofluids possessing poor stability, the nanofluids with high stability exhibited much better friction-reduction and anti-wear properties. For instance, the friction coefficient and wear volume lubricated by the nanofluid with rather low volumetric concentration (1.02 × 10-3%) stabilized by CTABr under 800 N are 13.8 and 45.4% lower than that of pure [Bmim][PF6], confirming that soft Au nanoparticles (Au NPs) also can be excellent additives for high performance lubricants especially under high loads. Moreover, the thermal conductivity (TC) of the stable nanofluids with three volumetric fraction (2.55 × 10-4, 5.1 × 10-4, and 1.02 × 10-3%) was also measured by a transient hot wire method as a function of temperature (33 to 81°C). The results indicate that the TC of the nanofluid (1.02 × 10-3%) is 13.1% higher than that of [Bmim][PF6] at 81°C but no obvious variation at 33°C. The conspicuously temperature-dependent and greatly enhanced TC of Au/[Bmim][PF6] nanofluids stabilized by CTABr could be attributed to micro-convection caused by the Brownian motion of Au NPs. Our results should open new avenues to utilize Au NPs and ILs in tribology and the high-temperature heat transfer field.
Gold nanoparticles (Au NPs) are always the hotspot of scientific research owing to their unique chemical and physical properties [1, 2], high chemical stability and potential applications in optics, catalysts, sensors, and biology . During the past several decades, a number of research groups have focused on the synthesis, characterization, properties, and applications of gold nanomaterials, and great progress in this field has been made [1, 2, 4–8]. To date, Au NP chemistry and physics has emerged as a broad new subdiscipline in the domain of colloids and surfaces . On the other hand, ionic liquids (ILs) have also been widely studied due to their unique physicochemical properties such as negligible vapor pressure, nonflammability, high ionic conductivity, low toxicity, as good solvents for organic and inorganic molecules, high thermal stability, and wide electrochemical window . Thus, ILs have attracted interests as benign solvent systems or green stabilizers for synthesizing gold nanomaterials in the past two decades [5–8]. The Brust-Schiffrin [5, 7], microwave heating , gamma-radiation , sonochemical , seed-mediated , photochemical reduction , and electron beam irradiation  methods have been used to prepare gold nanomaterials in the existence of ILs, of which the Brust-Schiffrin method is most facile and popular.
The stable Au NPs in water or organic solvents have been successfully fabricated using functionalized ILs or surfactants as capping agents and their optical, electrical, catalytic, biological, and thermal properties have been widely studied [4, 5, 16–18]. While Au NPs synthesized in ILs are usually prone to aggregate in the absence of additional stabilizers [11, 14, 15], which greatly restrains their physicochemical properties and applications. Moreover, researchers have paid more attention to synthesize gold nanocrystals, while the Au/IL nanofluids may have more potential applications in various fields. Recently, Dash and Scott  reported that the stable Au NPs and bimetallic PdAu NPs were successfully synthesized in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) by using NaBH4 as reductant and trace 1-methylimidazole as stabilizer. They found that the catalytic activity of the stable PdAu/[Bmim][PF6] nanofluid was remarkably higher than that of the unstable one in which the aggregation of PdAu NPs easily occurred. The pioneer work of Dash et al. indicated that the Au/IL nanofluids were expected to combine the excellent properties and open new avenues to the utilization of Au NPs and ILs. Based on this idea, we would like to make more effort on exploring the fabrication of stable Au/IL nanofluids as well as their properties.
Cetyltrimethylammonium bromide (CTABr) is a commercially available surfactant, which has been widely used as capping agent of Au NPs and shape controller of gold nanorods in aqueous systems . To our best knowledge, CTABr has not yet been used as stabilizer for the synthesis of Au NPs in the ILs. In the present article, we synthesized Au NPs in [Bmim][PF6] using CTABr as capping agent and NaBH4 as reductant. The Au NPs modified by CTABr exhibit ultrahigh stability and homogeneity in [Bmim][PF6] for more than 5 months. We investigated the tribological and thermal conductivity (TC) properties of the novel Au/[Bmim][PF6] nanofluids, and two major strategies are pursued in our studies: (1) the effects of the stability of nanofluids on their properties, and (2) the improvements of properties of [Bmim][PF6] induced by the introduction of low amount of Au NPs.
[Bmim][PF6] has been used as high performance lubricant since 2001 . The nanomaterials and ILs have both been widely used as effective additives for base lubricants in the past decade [21, 22], whereas the research on soft metal as additives of base ILs has not been developed yet. Therefore, the tribological properties of the Au/[Bmim][PF6] nanofluids with changeable stabilities were detailedly evaluated in our present work. Due to their potential applications as next generation heat transfer fluids, the TC of Au nanofluids has been studied as a function of temperature and Au NP content [16–18]. Patel et al.  found the temperature-dependent TC of Au/water nanofluids were greatly enhanced especially at high temperature, whereas Putnam et al.  and Shalkevich et al.  did not find this phenomenon and the TC of Au/ethanol, Au/methanol, and Au/water nanofluids were no obvious enhancements in their investigation under low temperature (≤40°C). The experimental differences of Au nanofluids and the controversy on whether the Brownian motion of nanoparticles is an important heat transfer mechanism of the nanofluids or not are always existent. Herein, we first measured the TC of Au/[Bmim][PF6] nanofluids using a transient hot-wire method as a function of temperature (33 to 81°C) and Au NP amount. The work conducted here is hopeful to supply experimental support and theoretical explanation on heat transfer mechanism in nanofluids.
[Bmim][PF6] with high purity was synthesized in our laboratory according to Ref.  with several small modifications. Chloroauric acid tetrahydrate (HAuCl 4 ·4H 2 O, 99.7%), hexadecyl trimethyl ammonium bromide (CTABr, 99%), and 1-Methylimidazole (98%) were purchased from Shanghai Sinopharm Chemical Regent Co., Ltd (China), 1-Methylimidazole was distilled under vacuum before used. Sodium borohydride (NaBH 4 , 98%), dichloromethane (99.5%), and anhydrous ethanol (99.7%) obtained from Tianjin Chemical Regent Co., Ltd (China) were used as received.
The experimental parameters and stabilities of different samples.
More than 5 months
Characterization and property measurements
Surface Plansmon Resonance (SPR) spectra were recorded on a U-3010 UV-visible spectrometer using a quartz cell of 1 cm path length. Fourier transformation infrared (FT-IR) spectra were recorded on a Bruker IFS 66v/S FTIR spectrometer using the KBr disk method. X-ray photoelectron spectroscopy (XPS) analysis was obtained on a PHI-5702 multifunctional XPS. Transmission electron microscopy (TEM) analysis was conducted on a JEM-2010 transmission electron microscope at 200 kV. To prepare sample of TEM, a drop of sample 4 solution was placed on a holey-carbon coated Cu TEM grid (200 mesh). Then, the grid was rinsed with dichloromethane and dried under room temperature. The SEM/EDS analysis was performed on a JSM-5600LV scanning electron microscope.
The tribological measurements were evaluated on an Optimol SRV-IV oscillating friction and wear tester in a ball-on-disc contact configuration. The upper test piece is ϕ 10 mm GCr15 bearing steel (AISI-52100) ball, and the lower test piece is ϕ 24.00 × 7.88 mm GCr15 bearing steel (AISI-52100) flat disc. All the tests were conducted at the frequency of 25 Hz, amplitude of 1 mm, and 30 min of test duration. Prior to the friction and wear test, two drops of the sample were introduced to the ball-disc contact area. The friction coefficient curve was recorded automatically with a chart attached to the SRV-IV test rig. The wear volumes were conducted by a MicroXAM 3 D surface profilometer (ADE Phase-Shift).
Thermal conductivity of the suspension was measured using a Decagon KD2 pro thermometer. The KD2 is based on transient hot wire method having a probe of length 6 cm and diameter 0.13 cm. This probe integrates in the interior, a heating element and a thermoresistor, which is connected to a microprocessor for controlling as well as conducting measurements. The KD2 was calibrated using distilled water before use. In order to study the temperature effect on TC of nanofluids, a thermostat bath was used, which maintained temperature within the range of ±0.1°C. Five measurements were taken at each temperature to ensure uncertainty in the measurement within ±5%.
Results and discussion
Characterization and stabilization mechanism
The volumetric fraction of Au NPs of sample 4 in Table 1 with ultrahigh stability is about 1.02 × 10-3% as mentioned above. The Au/[Bmim][PF6] nanofluids with concentrations of 2.55 × 10-4 and 5.1 × 10-4% were also fabricated by diluting the sample 4 before the TC measurements. Compared with traditional heat transfer oil, the [Bmim][PF6] possesses slightly higher TC, much higher thermal stability, lower volatility, and nonflammability, which make it be a potential high-temperature heat transfer fluid in the future. However, the poor TC of [Bmim][PF6]  still needs to be enhanced. Moreover, the temperature is a key factor for the investigation of heat transfer mechanism in nanofluids. Thus, the TC of Au/[Bmim][PF6] nanofluids was measured as a function of temperature in our following work.
The TC enhancement of nanofluids showing a strong sensitivity to the temperature was also found by some other researchers [16, 29–33]. Among the various proposed mechanisms of ballistic heat transfer of nanoparticles, nano-layers of liquid molecules around nanoparticles, clustering of nanoparticles, and the Brownian motion of nanoparticles for the anomalously enhanced TC of nanofluids compared to that of base liquids , the micro-convection caused by the Brownian motion of nanoparticles is the most reliable explanation for low concentration nanofluids . In our experiments, these Au nanofluids with low concentrations exhibit little enhancements under low temperature but obvious enhancements under high temperature. The relationship between the concentration and the TC enhancement is negligibly and sharply relative under low and high temperature, respectively. All these phenomena verify that the micro-convection caused by the Brownian motion of nanoparticles plays the most important role in the TC enhancement of Au nanofluids compared with other heat transfer mechanisms of the nanofluids. Then, it is not difficult to understand the results in our work. The viscosity of base liquids and temperature are two factors influencing the Brownian motion of Au NPs. The increase of temperature would cause large viscosity reduction of [Bmim][PF6] with a large viscosity-temperature exponent and aggravate the Brownian motion of Au NPs. These changes of Au/[Bmim][PF6] nanofluids with the increase of temperature can be the reason why the TC of nanofluids is conspicuously temperature-dependent and greatly enhanced especially at high temperatures.
The Au/[Bmim][PF6] nanofluids with changeable stabilities were synthesized by a facile Brust-Schiffrin method at room temperature. The reliable encapsulation mechanism was proposed for the nanofluids with ultrahigh stability by UV-visible, TEM, FT-IR, and XPS characterizations of Au NPs. The electrostatic repulsion and steric hindrance between Au NPs modified by CTABr make the Au NPs keep stable in [Bmim][PF6] for a long time. In comparison with pure [Bmim][PF6], the stable nanofluids exhibited excellent friction-reduction and anti-wear properties even if the addition concentration of Au NPs was very low, which indicated that the stability of the nanofluids is of key importance. Moreover, the TC of stable Au/[Bmim][PF6] nanofluids were also measured as a function of temperature. The TC of nanofluids is sharply temperature-dependent and greatly enhanced compared to that of pure [Bmim][PF6], which can be attributed to the micro-convection caused by the Brownian motion of Au NPs. To sum up, the additions of stable Au NPs with low concentrations can greatly improve the physicochemical properties of [Bmim][PF6]. Therefore, more Au/IL nanofluids with high stability need to be prepared and their other properties also need to be exploited in the future, which might broaden their potential applications in the fields of photonics, optoelectronics, sensor, catalysts, lubricants, heat transfer liquids, information storage, and medicine.
This work was supported by the NFSC (grant nos. 20803087 and 21033005) and the Major State Basic Research Development Program of China (973 Program, 2007CB607606).
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