Investigation on Synthesis, Stability, and Thermal Conductivity

materials Article

Investigation on Synthesis, Stability, and Thermal Conductivity Properties of Water-Based SnO2/Reduced Graphene Oxide Nanofluids Xiaofen Yu 1 , Qibai Wu 1, *, Haiyan Zhang 1,2, *, Guoxun Zeng 1,2 , Wenwu Li 1 , Yannan Qian 1 , Yang Li 1 , Guoqiang Yang 1 and Muyu Chen 1 1

2

*

School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China; [email protected] (X.Y.); [email protected] (G.Z.); [email protected] (W.L.); [email protected] (Y.Q.); [email protected] (Y.L.); [email protected] (G.Y.); [email protected] (M.C.) Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangzhou 510006, China Correspondence: [email protected] (Q.W.); [email protected] (H.Z.); Tel.: +86-136-5080-6963 (Q.W.); +86-139-2248-1057 (H.Z.)

Received: 17 November 2017; Accepted: 21 December 2017; Published: 27 December 2017

Abstract: With the rapid development of industry, heat removal and management is a major concern for any technology. Heat transfer plays a critically important role in many sectors of engineering; nowadays utilizing nanofluids is one of the relatively optimized techniques to enhance heat transfer. In the present work, a facile low-temperature solvothermal method was employed to fabricate the SnO2 /reduced graphene oxide (rGO) nanocomposite. X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscope (XPS), Raman spectroscopy, and transmission electron microscopy (TEM) have been performed to characterize the SnO2 /rGO nanocomposite. Numerous ultrasmall SnO2 nanoparticles with average diameters of 3–5 nm were anchored on the surface of rGO, which contain partial hydrophilic functional groups. Water-based SnO2 /rGO nanofluids were prepared with various weight concentrations by using an ultrasonic probe without adding any surfactants. The zeta potential was measured to investigate the stability of the as-prepared nanofluid which exhibited great dispersion stability after quiescence for 60 days. A thermal properties analyzer was employed to measure thermal conductivity of water-based SnO2 /rGO nanofluids, and the results showed that the enhancement of thermal conductivity could reach up to 31% at 60 ◦ C under the mass fraction of 0.1 wt %, compared to deionized water. Keywords: SnO2 /rGO nanocomposite; solvothermal; nanofluids; thermal conductivity; dispersion stability

1. Introduction With the rapid development of industry, heat removal and management is a major concern for any technology. Heat transfer plays a critically important role in many sectors of engineering, which especially has been widely used in refrigerators, heat exchangers, automobiles, and electronic devices, etc. Nowadays, utilizing nanofluids is one of the relatively optimized techniques to enhance heat transfer [1,2]. The nanofluid concept was pioneered by Choi in 1995 [2], which are stable colloidal dispersions of solid nanoparticles (typically with sizes in the range of 1–100 nm) in base fluids, compared to traditional base fluids such as water, ethylene glycol, oil, and so on. Nanofluids have many excellent properties due to the large specific surface of nanoparticles. Nanoparticles are expected to greatly enhance the thermal conductivity and improve the stability of nanofluids due to their unique properties [3,4]. Consequently, many scientists have conducted substantial and

Materials 2018, 11, 38; doi:10.3390/ma11010038

www.mdpi.com/journal/materials

Materials 2018, 11, 38

2 of 13

theoretical studies on the different respects of thermal conductivity of nanofluids with various nanoparticles. The hypothetical nanomaterial candidates for nanofluid formulations are metals, metal oxides, and carbon materials. In the past decade, many kinds of the above materials have been studied to produce nanofluids, such as Ag, Au, CuO, Al2 O3 , ZnO, and carbon nanotubes [5]. Patel el al. [6] reported an enhancement in thermal conductivity of about 5–21% for an Au nanofluid at a loading volume fraction of 0.00026% in the temperature range of 30–60 ◦ C, compared to deionized (DI)-water. Zhao et al. [4] found thermal conductivity could be enhanced with an increase of the Al2 O3 nanoparticle volume fraction and temperature, with a maximum enhancement of 28% obtained at a nanoparticle volume fraction of 5.92% and a temperature of 313 K. Chandrasekar et al. [7] experimentally investigated the effective thermal conductivities and viscosities of water-based nanofluids containing Al2 O3 nanoparticles. Karthikeyan et al. [8] found an ethylene glycol-based CuO nanofluid with a 1% volume fraction gave a 54% enhancement in the thermal conductivity. It was reported that the convective heat transfer coefficient of functionalized MWNT nanofluids was enhanced by up to 33–40% at a concentration of 0.25 wt %, compared with that of pure water in laminar and turbulent flows, respectively, at 20 ◦ C [9]. Recently, graphene nanofluids have attracted many researchers’ attentions owing to their variety of remarkable properties, including high thermal conductivities, extraordinary electronic transport property, large specific surface areas, and so on [10–12]. Outstanding thermo-physical characteristic of graphene has made it a potential candidate in the heat-transfer fluid field. The investigations have indicated that graphene-based nanofluids have higher heat transfer and thermal conductivity properties than other carbon materials [10,13–16]. Tessy et al. [10] reported an enhancement in thermal conductivity of water-based graphene nanofluids with a very low volume fraction of 0.056% by about 14% at 25 ◦ C, which increases to about 64% at 50 ◦ C. A substantial thermal conductivity enhancement of graphene nanofluids was obtained even at lower concentrations, the enhancement increased from 10% to 27% with the temperature increasing from 20 to 50 ◦ C at 0.2 vol % concentration [13]. The enhancement also shows strong temperature dependence. The alkaline-functionalized graphene nanofluid also showed good thermal conductivity with the enhancement around 14.1% at 25 ◦ C and 17% at 50 ◦ C compared to water [15]. Nevertheless, due to the high naturally-hydrophobic character of graphene, it cannot be dispersed in any polar solvent, such as ethylene glycol and water. In addition, the van der Waals force and the strong π-π interactions between the planar basal planes give rise to the graphene nanosheets to restack themselves and easily agglomerate in aqueous solution [17]. Therefore, it is much more difficult for graphene to be dispersed in water and suspended in a stable manner. Adding surfactants is considered to be the simplest way to avoid sediment and improve the stability of graphene nanofluids [18,19]. However, low thermal conductivity of the surfactants may decrease the heat-transfer characteristics of the nanofluids [20]. Meanwhile, strong oxidants have been applied successfully to introduce hydrophilic hydroxyl and carboxyl functional groups into graphene to make graphene more hydrophilic [21–24]. However, the structure of graphene will be destroyed during the process of acid treatment, and lead to a reduction of thermal conductivity [15,25]. Recent studies demonstrate that graphene with metal oxide nanocomposites may be one of the effective approaches to solve such problems. Due to the synergistic effect, the metal oxides could not only prevent the graphene nanosheets from restacking, but also improve thermal conductivity of the nanofluids. Mohammad et al. [26] prepared a rGO-Fe3 O4 nanofluid with high stability. The thermal conductivity was enhanced up to 11% at the mass fraction of 0.5%. Hooman et al. [27] studied the functionalized graphene nanoplatelet nanofluid, it was stable and no sedimentation was observed for a long time. The enhancement of thermal conductivity was 16.94% at 20 ◦ C and nearly 22.22% at 40 ◦ C for 0.1% weight concentration. Baby and Sundara reported the enhancement value of thermal conductivities for CuO decorated graphene dispersed nanofluid containing a volume fraction of 0.05% at 25 ◦ C could reach about 28% [28]. Li et al. [29] found better thermal conductivity and stability for SiO2 -coated graphene nanofluids, compared to graphene fluids. Wang et al. [30] reported the

Materials 2018, 11, 38

3 of 13

water-based TiO2 anchored graphene nanofluids have good dispersion stability and the maximum value of thermal conductivity enhancement was up to 33% at a mass fraction of 0.1%. On the other hand, SnO2 water-based nanofluids show a good enhancement of thermal conductivity as well [31,32]. Habibzadeh et al. [31] found that the thermal conductivity of water-based SnO2 nanofluids at a weight fraction of 0.024% was enhanced up to 8.7%. The electrochemical performance, specific capacitance, and cycling stability of SnO2 /rGO nanocomposite have been investigated as well [33–35]. However, to the best of our knowledge, there are few reports concentrating on the thermal conductivity of SnO2 /rGO nanocomposite dispersed nanofluids. In this work, a novel tin oxide/graphene oxide nanocomposite was synthesized by the solvothermal process. The thermal conductivity properties and stability of water-based nanofluids dispersed by the hybrid nanocomposite with various concentrations was investigated in detail. 2. Experimental 2.1. Materials All of the reagents used in the experiments were of analytical grade and used without further purification. Graphene oxide (GO) with purity ~99% maximum particle diameter of 3 µm and maximum thickness of 1.2 nm was purchased from Chengdu Organic Chemicals Co., Ltd. (Chengdu, China), Chinese Academy of Sciences. Tin chloride pentahydrate (SnCl4 ·5H2 O, 99.0%) and absolute ethanol (99.8%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). 2.2. Synthesis of the SnO2 /rGO Nanocomposites The SnO2 /rGO hybrid composites were prepared by a simple hydrothermal method. Tin chloride pentahydrate (SnCl4 ·5H2 O) (0.1 M) was dissolved in 56 mL absolute ethanol under continuous magnetic stirring and then treated by ultrasonication for 15 min. Then 4 mL GO aqueous solution (2.5 g/mL) was dropped into the above mixture slowly under magnetic stirring. After several minutes, the mixture was transferred into a stainless steel autoclave and kept at 120 ◦ C for 6 h to synthesize tin oxide/reduced graphene oxide composites. The black product was centrifuged and washed several times with deionized water and absolute ethanol, and then dried at 60 ◦ C for 12 h. For comparison, the reduced graphene oxide (rGO) and pure SnO2 were also prepared via the same process without adding SnCl4 and GO, respectively. 2.3. Characterization of the SnO2 /rGO Nanocomposites X-ray diffraction (XRD) was employed to identify the crystalline structure of the gained samples with an X-ray powder diffractometer (Cukα , DMAX-UltimaIV, Rigaku Co., Tokyo, Japan). A transmission electron microscope (TEM, JEM-2100F, JEOL, Tokyo, Japan) was used to study the detailed microstructures and morphology of the samples. An X-ray photoelectron spectroscope (XPS, Kratos AXIS ULTRA DLD, Shimadzu, Kyoto, Japan) was operated to investigate the oxidation states of Sn and graphene, as well as the elemental composition of the SnO2 /rGO nanocomposite. Thermogravimetric analysis (TGA, SDT-2960, TA Instruments, New Castle, DE, USA) measurements were performed to analyze the quality percentage of SnO2 in the SnO2 /rGO compound, the samples were heated from room temperature to 900 ◦ C at the rate of 10 ◦ C/min in the air. The molecular vibration mode and defects of the samples were analyzed by Raman spectroscopy (RM2000, Renishaw, London, UK). Fourier transform infrared spectroscopy (FTIR, Nicolet iz10, Thermol scientific, Waltham, MA, USA) was used to analyze and identify the functional groups on the surface of the samples. 2.4. Preparation and Property Measurement of Water-Based Nanofluids A high-power ultrasonication probe (HNF2000, Huanan Ultrasonic Equipment Co., Ltd., Guangzhou, China) which supplies 2000 W output power and a 20-khz frequency was used to prepare the SnO2 /rGO hybrid water-based nanofluids. SnO2 /rGO nanocomposite was dispersed in DI water

Materials 2018, 11, 38

4 of 13

Materials 2018, 11, 38

4 of 13

to prepare nanofluids with different weight concentrations, including 0.02, 0.04, 0.06, 0.08, and 0.1 wt %, prepare the SnO2/rGO hybrid water-based nanofluids. SnO2/rGO nanocomposite was dispersed in without adding any surfactants. As a comparison, pure SnO2 and rGO water-based nanofluids were DI water to prepare nanofluids with different weight concentrations, including 0.02, 0.04, 0.06, 0.08, also prepared using the same process, respectively. Zeta potential testing was employed to assess the and 0.1 wt %, without adding any surfactants. As a comparison, pure SnO2 and rGO water-based dispersion stability of above nanofluids. A transient heated needle (KD2, Decagon, Device, TPS 500s, nanofluids were also prepared using the same process, respectively. Zeta potential testing was Hot Disk, Uppsala, Sweden) with 5% accuracy was nanofluids. used to measure the thermal conductivity of employed to assess the dispersion stability of above A transient heated needle (KD2, nanofluids based on the method (THW). To avoid enhancement Decagon, Device, TPStransient 500s, Hothot-wire Disk, Uppsala, Sweden) with 5% accuracy was usedoftoeffective measure viscosity the of nanofluids, the low weight concentrations in the range of To 0.02–0.1 thermal conductivity of nanofluids based of on as-prepared the transientnanofluids hot-wire method (THW). avoid wt % viscosity of nanofluids, thetolow of as-prepared wereenhancement chosen, and of theeffective temperature is between from 20 60 ◦weight C. Eachconcentrations sample was repeated five times nanofluids in the range of 0.02–0.1 wt % were chosen, and the temperature is between from to with each measurement interval of 30 min, and the average value of thermal conductivity was20obtained. 60 °C. Each sample was repeated five times with each measurement interval of 30 min, and the average value of thermal conductivity was obtained. 3. Results and Discussion 3. Results and Discussion 3.1. Characterization of Prepared SnO2 /rGO Nanocomposites

The structure information and crystal phase of the samples were investigated by X-ray diffraction 3.1. Characterization of Prepared SnO 2/rGO Nanocomposites (XRD) analysis. Figure 1 shows the XRD patterns of GO, rGO, Pure SnO2 , and SnO2 /rGO The structure information and crystal phase of the samples were◦ investigated by X-ray nanocomposite samples. The diffraction peak observed at 2θ = 10.5 is corresponding to (001) diffraction (XRD) analysis. Figure 1 shows the XRD patterns of GO, rGO, Pure SnO2, and SnO2/rGO ◦ characteristic peak of GO. AThe broad diffraction centered at =25.1 which can nanocomposite samples. diffraction peakpeak observed at 2θ 10.5°appears, is corresponding to contribute (001) to thecharacteristic (002) planepeak diffraction of rGO sheets, indicating a poor degree of graphite-like material of GO. A broad diffraction peak centered at 25.1° appears, which can contribute [36]. This result reveals that the GO has beensheets, reduced into rGO through of oxygenated to the (002) plane diffraction of rGO indicating a poor degreethe of decomposition graphite-like material [36]. This resultduring revealsthe that the GO hasprocess been reduced rGO through decomposition of functionalities solvothermal and the into few-layer structurethe of rGO has been formed. oxygenated functionalities during the solvothermal process and the few-layer structure of rGO has It is clear that the XRD patterns of pure SnO2 and SnO /rGO nanocomposite are basically the same. 2 ◦ , 33.8 been formed. It is at clear XRD◦ , patterns of 64.7 pure◦ SnO 2 and SnO2/rGO nanocomposite are The diffraction peaks 2θ =that 26.5the 51.7◦ , and could correspond to the (110), (101), (211), basically the same. The diffraction peaks at 2θ = 26.5, 33.8, 51.7, and 64.7° could correspond to the and (112) crystalline planes of SnO2 (JCPDS no. 41-1445), respectively [37]. The average size of particles (110), (101), (211), and (112) crystalline planes of SnO2 (JCPDS no. 41-1445), respectively [37]. The calculated by the Debye-Scherrer equation is 4.5 nm, indicating the successful formation of ultra-small average size of particles calculated by the Debye-Scherrer equation is 4.5 nm, indicating the SnO2successful nanocrystals. formation of ultra-small SnO2 nanocrystals.

Figure 1. X-ray diffraction of GO, rGO, SnO2, and SnO2/rGO nanocomposites.

Figure 1. X-ray diffraction of GO, rGO, SnO2 , and SnO2 /rGO nanocomposites.

Thermogravimetric analysis (TGA, SDT-2960, TA Instruments, New Castle, DE, USA) measurements were performed analysis the quality of SnO 2 in Castle, the SnODE, 2/rGOUSA) Thermogravimetric analysis to (TGA, SDT-2960, TA percentage Instruments, New compound.were As shown in Figure 2, there two percentage obvious weight loss processes for GO and measurements performed to analysis the are quality of SnO in the SnO /rGO compound. 2 2 SnO 2/rGO. The weight loss in the range of room temperature to 150 °C is owing to the dislodgement As shown in Figure 2, there are two obvious weight loss processes for GO and SnO2 /rGO. The weight of absorbed water and carbon combustion. The mass loss from 150 to 400 °C corresponds to the loss in the range of room temperature to 150 ◦ C is owing to the dislodgement of absorbed water decomposition of oxygen-containing groups. At high temperatures, from 400 to 500 °C, the weight and carbon combustion. The mass loss from 150 to 400 ◦ C corresponds to the decomposition of loss is attributed to the destruction of the carbon skeleton. This ◦decomposing process causes oxygen-containing groups. At high that temperatures, fromor400 500 C, the weight loss[35]. is attributed significant weight loss, indicating there is no GO rGOtoremaining in the samples The to theresidual destruction of the carbon skeleton. This decomposing process causes significant mass fraction for the GO sample still reaches 14.3%, which is probably due weight to the loss, indicating that there is no GO or rGO remaining in the samples [35]. The residual mass fraction for the GO sample still reaches 14.3%, which is probably due to the introduction of impurities during

Materials 2018, 11, 38

5 of 13

introduction of impurities during preparation process. As for the SnO2/rGO compound, the mass loss equals to 45% in the range of room temperature to 500 °C. That is, the remaining weight Materials 2018, of 11, the 38 hybrid sample stays 55%. Therefore, according to the TGA results, the calculated 5 of 13 percentage weight percentage of SnO2 is about 40% in the SnO2/rGO composites. preparation process. As11,for the SnO2 /rGO compound, the mass loss equals to 45% Materials 2018, 38 5 of 13 in the range of room temperature to 500 ◦ C. That is, the remaining weight percentage of the hybrid sample stays 55%. introduction of impurities during preparation process. As for the SnO2/rGO compound, the mass Therefore, according to the TGA results, calculated of SnO2weight is about 40% in the loss equals to 45% in the range ofthe room temperatureweight to 500 °C.percentage That is, the remaining percentage of the hybrid sample stays 55%. Therefore, according to the TGA results, the calculated SnO2 /rGO composites. weight percentage of SnO2 is about 40% in the SnO2/rGO composites.

Figure 2. Thermogravimetric curves of SnO2/rGO compound and GO. Figure 2. Thermogravimetric curves of SnO2/rGO compound and GO. Figure 2. Thermogravimetric curves of SnO2 /rGO compound and GO.

XPS is a forceful tool to investigate the surface chemistry of the samples. As shown in Figure 3a, the as-prepared 2/rGO nanoparticles contain C,chemistry O, and Sn elements. the core-level XPS is a forceful tool toSnO investigate the surface of theInsamples. AsXPS shown in Figure signals oftool Sn3dto (Figure 3b), the Sn3d and Sn3dchemistry 5/2 peaks are observed at 495.8 andAs 487.4 eV, XPSas-prepared is a forceful investigate the3/2surface of the samples. shown in FigureXPS 3a, 3a, the SnO 2/rGO nanoparticles contain C, O, and Sn elements. In the core-level respectively, corresponding to Sn4+ ions in the tetragonal rutile structure of SnO2 [38]. For the the as-prepared SnO contain C, O, and are Sn3/2 elements. the core-level XPS 2 /rGO signals of Sn3d 3b),nanoparticles the the Sn3d 5/2 peaks observed atIn495.8 SnO(Figure 2/rGO nanocomposites, shift3/2of and 0.4 eV Sn3d is observed for the Sn3d and Sn3d5/2 peaks, which and 487.4 eV, may be due to3b), the reaction of4+Sn4+ 3/2 ions with the active5/2 sitespeaks of rGO. Figure 3c displays the O1s XPS and 487.4 eV, signals of Sn3d (Figure the Sn3d and Sn3d are observed at 495.8 respectively, corresponding to Sn ions in the tetragonal rutile structure of SnO 2 [38]. For the spectrum of the SnO2/rGO nanocomposites can be divided into two peaks. One is at the binding 4+ ions in the respectively, corresponding to Sn tetragonal rutile structure of SnO [38]. For the 2 SnO2/rGO nanocomposites, the shift of 0.4 eV is observed for the Sn3d 3/2 and Sn3d 5 / energy of 531 eV, which corresponds to the C=O or Sn=O group of SnO2. Another peak at 532.5 eV 2 peaks, which SnO the shift of 0.4the eV isabove observed the Sn3d andinSn3d attributed to the of C–OH and C–O–C groups. The results that the Sn element exists 5 /O1s 2 peaks, 3/2 may2 /rGO be duenanocomposites, toisthe reaction Sn4+ ions with active sites verify offor rGO. Figure 3c displays the XPS 4+ ions the compound state of SnO2.of C1sSn core-level XPS signals of GO and SnO 2/rGO can be deconvoluted which may be due to the reaction with the active sites of rGO. Figure 3c displays the spectrum of the SnO 2 /rGO nanocomposites can be divided into two peaks. One is at the binding into four components, including C–C/C=C, C–O, C=O carbonyl and carboxylic groups, and O–C=O O1s XPSofspectrum of thecarbon SnO2groups, /rGO respectively, nanocomposites can divided into twopeaks peaks. One is at the carboxylate as or shown in be Figure 3d,e [39]. of energy 531 eV, which corresponds to the C=O Sn=O group of SnO 2.The Another peak at 532.5 eV oxygen-containing groupscorresponds for SnO2/rGO have been greatly suppressed compared with GO, which binding energy of 531 eV, which to the C=O or Sn=O group of SnO . Another peak at 2 is attributed tohas theabundant C–OHoxygenated and C–O–C groups. The above results verify that the Sn element exists in groups. This demonstrates that the majority of oxygenated functional 532.5 eV is attributed to SnO the C–OH and C–O–C groups. The above results verify thatbethe Sn element the compound statehave of . C1s core-level XPS signals GO and SnO 2/rGO can deconvoluted groups been 2removed after the solvothermal process.of However, a small amount of residual oxygenated groups that GO has been partially reduced. These hydrophilic exists in the compound statestillofremain, SnO . C1sC–O, core-level XPS signals of GO and SnO2 /rGO can be 2indicating into four components, including C–C/C=C, C=O carbonyl and carboxylic groups, and O–C=O functional groups are advantageous for SnO2/rGO to be stably dispersed in aqueous solution for a deconvoluted into four components, including C–C/C=C, C–O, C=O carbonyl and carboxylic groups, carboxylate carbon long time. groups, respectively, as shown in Figure 3d,e [39]. The peaks of and O–C=O carboxylate carbon groups, respectively, as shown in Figure 3d,e [39]. The peaks of oxygen-containing groups for SnO 2/rGO have been greatly suppressed compared with GO, which oxygen-containing groups for SnO /rGO have been greatly suppressed compared with GO, which has 2 has abundant oxygenated groups. This demonstrates that the majority of oxygenated functional abundant oxygenated groups. This demonstrates that the majority of oxygenated functional groups groups have been removed after the solvothermal process. However, a small amount of residual have been removed solvothermal process. However, a small amount of residual oxygenated groupsafter still the remain, indicating that GO has been partially reduced. These oxygenated hydrophilic groups still remain, indicating that GO has been partially reduced. These hydrophilic functional functional groups are advantageous for SnO2/rGO to be stably dispersed in aqueous solution for a groups are advantageous for SnO /rGO to be stably dispersed in aqueous solution for a long time. 2 long time.

Figure 3. Cont.

Materials 2018, 11, 38

6 of 13

Materials 2018, 11, 38

6 of 13

Materials 2018, 11, 38

6 of 13

Figure 3. XPS of GO and SnO2/rGO composite. (a) XPS survey spectra of GO and SnO2/rGO; (b) XPS Figure 3. XPS of GO and SnO2 /rGO composite. (a) XPS survey spectra of GO and SnO2 /rGO; (b) XPS spectrum of Sn3d of SnO2/rGO and SnO2; (c) O 1s spectra of SnO2/rGO; and (d,e) C1s spectra of GO spectrum of Sn3d of SnO2 /rGO and SnO2 ; (c) O 1s spectra of SnO2 /rGO; and (d,e) C1s spectra of GO and SnO2/rGO. and SnO2 /rGO. Figure 3. XPS of GO and SnO2/rGO composite. (a) XPS survey spectra of GO and SnO2/rGO; (b) XPS The structure disordered degree of the carbonaceous materials can be gained by Raman spectrumdisordered of Sn3d of SnO2degree /rGO and SnO ; (c) Ocarbonaceous 1s spectra of SnO2/rGO; and (d,e) C1s of GO The structure of 2the materials canspectra be gained by Raman spectroscopy. and It is that there are two prominent peaks at around 1351 and 1591 cm−1, SnOobvious 2/rGO. −1 spectroscopy. It is obvious that there are two prominent peaks at around 1351 and 1591 cm corresponding to the D-band for carbon structure and the G-band for defects, respectively, as , corresponding to the D-band for to carbon structure and reduction the G-band defects, respectively, asisshown in The structure disordered degree of the carbonaceous materials can the be gained by Raman shown in Figure 4. Attributed the solvothermal offorGO, peak position shifted −1, spectroscopy. It is obvious that there are two prominent peaks at around 1351 and 1591 cm Figureshows 4. Attributed to theofsolvothermal of GO, theofpeak position is shifted which shows which the damage rGO networkreduction and the formation defects [29]. Moreover, the intensity corresponding to the D-band for carbon structure and the G-band for defects, respectively, as the damage of rGO network and the formation of defects [29]. Moreover, the intensity ratio the ratio of theshown D toinGFigure bands4. (IAttributed D/IG) normally indicates the defectofdegree theposition materials. The ID/Iof G for to the solvothermal reduction GO, the of peak is shifted D to G bands (I /I ) normally indicates the defect degree of the materials. The I /I for GO, rGO, D 2G/rGO D intensity G rGO in the shows the damage of rGO are network the formation of respectively, defects [29]. Moreover, GO, rGO, which and SnO composite 0.88,and 0.91, and 0.96, whichthe means and SnO /rGO composite are 0.88, and 0.96, respectively, which means rGO in the SnO ratio of the D to G bands (I D/IG) 0.91, normally indicates the defect degree of the materials. The I D/I G for 2 SnO2/rGO nanocomposites has more defects compared with the pure rGO, revealing the2 /rGO SnO2 GO, rGO,has andmore SnO2/rGO composite are 0.88,with 0.91, and 0.96, respectively, which means rGO2innanoparticles the nanocomposites defects compared the pure rGO, revealing the SnO nanoparticles intercalating into the rGO sheets. SnO2/rGO nanocomposites has more defects compared with the pure rGO, revealing the SnO2 intercalating into the rGO sheets.

nanoparticles intercalating into the rGO sheets.

Figure 4. Raman spectra of GO, rGO, and SnO2/rGO.

Figure 4. Raman spectra of GO, rGO, and SnO2 /rGO.

Figure 4. Raman spectra of GO, rGO, and SnO2/rGO.

Materials 2018, 11, 38

7 of 13

Transmission Materials 2018, 11, 38electron microscopy (TEM) has been employed to further characterize 7 of 13 the morphologies and crystal structure of the SnO2 /rGO composite. Figure 5a shows the rGO sheets Transmission microscopy (TEM) has surface been employed to further the TEM. are almost transparentelectron with some wrinkles on the and folding at thecharacterize edges under morphologies and crystal structure of the SnO 2/rGO composite. Figure 5a shows the rGO sheets are For the SnO2 /rGO nanocomposite, as shown in Figure 5b,c, numerous nanoparticles are uniformly almost transparent with some wrinkles on the with surface and folding particle at the edges the It is distributed on the wrinkled rGO nanosheets the average sizeunder fromTEM. 3 to For 5 nm. SnO2/rGO nanocomposite, as shown in Figure 5b,c, numerous nanoparticles are uniformly consistent with the result we calculated from XRD. High-resolution TEM imagery (Figure 5d) reveals distributed on the wrinkled rGO nanosheets with the average particle size from 3 to 5 nm. It is that the nanoparticles are crystalline, having a calculated lattice spacing of 0.33 and 0.27 nm, which consistent with the result we calculated from XRD. High-resolution TEM imagery (Figure 5d) are consistent the (110) and planeshaving of rutile SnO2 [37]. Thespacing corresponding pattern reveals thatwith the nanoparticles are(101) crystalline, a calculated lattice of 0.33 andSAED 0.27 nm, (insetwhich of Figure 5d) further confirms the presence of the (110), (101), (111), and (211) lattice planes are consistent with the (110) and (101) planes of rutile SnO2 [37]. The corresponding SAED of SnO (JCPDS.no. 41-1445) [40,41]. Those results are also in good agreement with the pattern (inset of Figure 5d) further confirms the presence of the (110), (101), (111), and (211) lattice XRD 2 planes of SnO2 (JCPDS.no. 41-1445) [40,41]. are also in theElemental XRD results, indicating SnO2 nanoparticles haveThose beenresults synthesized ongood the agreement surface ofwith rGO. results, indicating SnOthat 2 nanoparticles have SnO been2 synthesized on dispersed the surface homogenously of rGO. Elemental mapping results confirm the ultrasmall nanoparticles as well. mapping in results confirm theand ultrasmall SnO2 nanoparticles dispersed as well. As illustrated Figure 5e, thethat C, O, Sn elements distribute uniformly inhomogenously the selected area with high As illustrated in Figure 5e, the C, process O, and Sn elements distribute uniformly in the selected area with density. Considering the ultrasonic used during the sample preparation for TEM observation, high density. Considering the ultrasonic process used during the sample preparation for TEM these results clearly demonstrate that the SnO2 nanoparticles have been successfully prepared and are observation, these results clearly demonstrate that the SnO2 nanoparticles have been successfully anchored firmly on the rGO surface with high packing density. prepared and are anchored firmly on the rGO surface with high packing density.

Figure 5. (a) TEM images of rGO; (b–c) TEM images of SnO2/rGO under different magnification; (d)

Figure 5. (a) TEM images of rGO; (b,c) TEM images of SnO2 /rGO under different magnification; HRTEM images of SnO2/rGO, and the inset is the corresponding SAED pattern; and (e) elemental (d) HRTEM SnO the inset is the corresponding and (e) elemental 2 /rGO, and /rGO depicting the even distribution of C (e1), O (e2), SAED and Snpattern; (e3). mappingimages of SnO2of mapping of SnO2 /rGO depicting the even distribution of C (e1), O (e2), and Sn (e3).

3.2. Stability of the Water-Based SnO2/rGO Nanofluids

3.2. Stability the Water-Based SnO2 /rGO Nanofluids Zetaofpotential measurements and the sedimentation have been applied to identify the stability of thepotential SnO2/rGOmeasurements nanofluids as they arethe common and important to evaluate the dispersion Zeta and sedimentation havemethods been applied to identify the stability behavior of nanoparticles in a liquid environment. In general, the nanofluid is referred to have good of the SnO2 /rGO nanofluids as they are common and important methods to evaluate the dispersion stability when the zeta potential values are higher than 30 mV [16]. The zeta potential values of behavior of nanoparticles in a liquid environment. In general, the nanofluid is referred to have good rGO, SnO2, and SnO2/rGO nanofluids with mass fractions of 0.04% and 0.06% are shown in Figure stability when the zeta potential values are higher than 30 mV [16]. The zeta potential values of 6. It is clear that the absolute zeta potential values of both rGO and SnO2/rGO nanofluids are higher rGO,than SnO30 nanofluids with mass fractions of 0.04% and 0.06% are shown in Figure 6. 2 , and 2 /rGO mV, SnO in which SnO 2/rGO nanofluids has higher zeta potential value (above 50 mV) than the It is clear that the absolute both rGOthe and SnOis2less /rGO nanofluids are higher rGO nanofluids (about 33zeta mV).potential However,values for SnOof 2 nanofluid, value than 30 mV implying than poor 30 mV, in whichstability. SnO2 /rGO nanofluids hasthe higher zeta potential 50 mV) than dispersion The results indicate suspension stability value of SnO(above 2/rGO nanofluid is the than the rGO 33 andmV). SnO2However, nanofluids. sedimentation photographs of the nanofluids after rGO better nanofluids (about forThe SnO nanofluid, the value is less than 30 mV implying 2 30 days The also results clearly indicate show thatthe only a small amount be poorquiescence dispersionfor stability. suspension stabilityof ofprecipitation SnO2 /rGOcould nanofluid is observed for rGO and SnO 2/rGO nanofluids, and SnO2 nanofluids present a pronounced better than the rGO and SnO2 nanofluids. The sedimentation photographs of the nanofluids after precipitation over time, as exhibited in insets of Figure 6. This phenomenon may be due to the fact quiescence for 30 days also clearly show that only a small amount of precipitation could be observed that on the surface of rGO still remain a small amount of hydrophilic functional groups, such as

for rGO and SnO2 /rGO nanofluids, and SnO2 nanofluids present a pronounced precipitation over time, as exhibited in insets of Figure 6. This phenomenon may be due to the fact that on the surface

Materials 2018, 11, 38

8 of 13

Materials 2018, 11, 38

8 of 13

of rGO still remain a small amount of hydrophilic functional groups, such as hydroxyl, carboxyl, and carbonyl groups,and after the hydrothermal reaction process. Furthermore, as the ultrasmall SnOas 2 hydroxyl, carboxyl, carbonyl groups, after the hydrothermal reaction process. Furthermore, nanoparticles are decorated on the surface of reduced graphene oxide, the graphene sheets could avoid the ultrasmall SnO2 nanoparticles are decorated on the surface of reduced graphene oxide, the overlapping or stacking. Hence, this ensures the largeHence, specificthis surface area rGO,specific good stability, graphene sheets could avoid overlapping or stacking. ensures theoflarge surface and thermal conductivity for the nanofluids. area of rGO, good stability, and thermal conductivity for the nanofluids. Materials 2018, 11, 38

8 of 13

SnO2

rGO

SnO2/rGO

SnO2

rGO

SnO2/rGO

hydroxyl, carboxyl, and carbonyl groups, after the hydrothermal reaction process. Furthermore, as the ultrasmall SnO2 nanoparticles are decorated on the surface of reduced graphene oxide, the graphene sheets could avoid overlapping or stacking. Hence, this ensures the large specific surface area of rGO, good stability, and thermal conductivity for the nanofluids.

SnO2

rGO

SnO2/rGO

SnO2

rGO

SnO2/rGO

Figure 6. 6. Zeta Zeta potential potential value valueofofSnO SnO2/rGO /rGO composite, composite, rGO, rGO, and and SnO SnO2 nanofluids nanofluids with with different different Figure 2 2 2/rGO, and SnO2 concentration; (a) 0.04 wt % and (b) 0.06% insets are photographs of rGO, SnO concentration; (a) 0.04 wt % and (b) 0.06% insets are photographs of rGO, SnO2 /rGO, and SnO2 nanofluids after after quiescence quiescence for for 30 30 days. days. nanofluids

Figure 6. Zeta potential value of SnO2/rGO composite, rGO, and SnO2 nanofluids with different 2/rGO, and SnO2 concentration; 0.04 wt % and (b) 0.06% are photographs of rGO, Temperature and (a)concentration play aninsets important role on the SnO stability of nanofluids. As nanofluids after quiescence for 30play days.an important role on the stability of nanofluids. As plotted Temperature and concentration

plotted in Figure 7a, it is obvious that the absolute zeta potential values of the SnO2/rGO nanofluids in Figure 7a, it is obvious that the absolute zeta potential values of the SnO2 /rGO nanofluids with with mass Temperature fractions of and 0.02%, 0.04%, 0.06%, andimportant 0.08% are than 40 mV at temperatures concentration play an roleall onhigher the stability of nanofluids. As mass fractions of 0.02%, 0.04%, 0.06%, and 0.08% are all higher than 40 mV at temperatures from 30 to plotted in °C. Figure 7a, itindicates is obvious that that the absolute zeta potential values of the SnO2/rGO nanofluids from 30 to 70 This the prepared SnO 2/rGO nanofluids possess much better 70 ◦ C. This that the prepared SnO much dispersion stability. 2 /rGO withindicates mass fractions of 0.02%, 0.04%, 0.06%, and nanofluids 0.08% arenanofluids allpossess higher than 40 better mV at temperatures dispersion stability. Sedimentation observation of the after quiescence for 60 days Sedimentation of the nanofluids quiescence for nanofluids 60 days demonstrate clearly that the from 30 observation to 70 °C. This that theafter prepared SnO2/rGO possess much demonstrate clearly that the indicates SnO2/rGO nanofluids exhibit good long time dispersionbetter stability for dispersion stability. Sedimentation observation of the nanofluids after quiescence for days SnO2 /rGO nanofluids exhibit good long time dispersion stability for each prepared 60 concentration, each prepared concentration, which also coincide with zeta potential values, as shown in Figure 7b. demonstrate SnO2/rGO nanofluids exhibitin good long7b. time dispersion for which also coincideclearly with that zetathe potential values, as shown Figure No obviousstability sedimentation is No obvious sedimentation is observed forcoincide all nanofluids whereas little agglomeration and each prepared concentration, which also with zetasamples, potential values, as shown in Figure 7b. observed for all nanofluids samples, whereas little agglomeration and sedimentation are found in the sedimentation aresedimentation found in theissamples concentration, especially at 0.1 wt %.and No obvious observedwith for allhigher nanofluids samples, whereas little agglomeration samplessedimentation with higherare concentration, especially at 0.1 wt %. found in the samples with higher concentration, especially at 0.1 wt %.

Figure 7. (a) Effect of temperature on the absolute Zeta potential values at different concentrations;

Figure 7. (a) Effect of temperature on the absolute Zeta potential values at different concentrations; after Photographs 0.02, 0.04, 0.06, 0.08,absolute and 0.1 wt % water-based nanofluids of SnO2/rGO Figure (b) 7. (a) Effect of of temperature on the Zeta potential values at different concentrations; (b) Photographs of 60 0.02, 0.04, 0.06, 0.08, and 0.1 wt % water-based nanofluids of SnO2 /rGO after quiescence for days. (b) Photographs of 0.02, 0.04, 0.06, 0.08, and 0.1 wt % water-based nanofluids of SnO2/rGO after quiescence for 60 days. quiescence forConductivity 60 days. of the Water-Based SnO2/rGO Nanofluids 3.3. Thermal

3.3. ThermalThermal Conductivity of the Water-Based Nanofluids conductivity is one of the SnO most /rGO important values for nanofluids. In this work, the 3.3. Thermal Conductivity of the Water-Based SnO22/rGO Nanofluids thermal properties analyzer (KD2 pro, Decagon, Device, TPS 500s, Hot Disk, Uppsala, Sweden) was Thermal conductivity is one of the most important values which for nanofluids. In this work, the thermal employed to measure the of nanofluidsvalues has 5% accuracy. In Thethis theory Thermal conductivity is thermal one ofconductivity the most important for ananofluids. work, the properties analyzer (KD2 Decagon, Device, TPS 500s, Hot(THW). Disk, Uppsala, Sweden) employed measurement was pro, based on the transient hot-wire method measuring the was thermal thermalofproperties analyzer (KD2 pro, Decagon, Device, TPS 500s, Before Hot Disk, Uppsala, Sweden) was to measure the thermal conductivity of nanofluids which a 5% accuracy. theoryatof measurement conductivity of nanofluids, the thermal conductivity of has deionized water was The measured 20–60 °C employed to measure the thermal conductivity of nanofluids which has a 5% accuracy. The theory to calibrate experimental apparatus, using the standard thermal conductivity value of deionized was based on thethe transient hot-wire method (THW). Before measuring the thermal conductivity of of measurement was based the transient hot-wire (THW). Before measuring the thermal water which comes fromonRamires et al. [42]. It is clearmethod the results are gained a maximum of error conductivity of nanofluids, the thermal conductivity of deionized water was measured at 20–60 °C to calibrate the experimental apparatus, using the standard thermal conductivity value of deionized water which comes from Ramires et al. [42]. It is clear the results are gained a maximum of error

Materials 2018, 11, 38

9 of 13

nanofluids, the thermal conductivity of deionized water was measured at 20–60 ◦ C to calibrate the experimental apparatus, using the standard thermal conductivity value of deionized water which comes from Ramires et al. [42]. It is clear the results are gained a maximum of 9error 3.48%, Materials 2018, 11, 38 of 13 as demonstrated in Figure 8. Therefore, it can be concluded that the KD2 Pro worked within its 3.48%, as demonstrated in Figure 8. Therefore, it can beof concluded that the KD2 Pro worked withinthe low designed accuracy. In order to avoid enhancement effective viscosity of nanofluids, its designed accuracy. In order to avoid enhancement of effective viscosity of nanofluids, the low weight concentrations of the as-prepared nanofluids in the range of 0.02–0.1 wt % were chosen, weight concentrations of the as-prepared nanofluids in the range of 0.02–0.1 wt % were chosen, and and the temperature was set between 20 and 60 ◦ C. Each sample was repeated five times with each the temperature was set between 20 and 60 °C. Each sample was repeated five times with each measurement interval of 30ofmin, and thetheaverage ofthermal thermal conductivity was obtained. measurement interval 30 min, and average value value of conductivity was obtained.

Figure 8. Comparison of the measured values of thermal conductivity using KD2 Pro for distilled

Figure 8. Comparison of the measured values of thermal conductivity using KD2 Pro for distilled water with the standard values presented by Ramires [42]. water with the standard values presented by Ramires [42]. The thermal conductivity properties of water-based SnO2/rGO nanofluids at different weight concentrations as a functionproperties of temperature are shown inSnO Figure 9. The thermal conductivity of The thermal conductivity of water-based 2 /rGO nanofluids at different weight deionized water and water-based pure rGO nanofluids with the same mass fraction have been concentrations as a function of temperature are shown in Figure 9. The thermal conductivity of measured as well for comparison. It is obvious that the thermal conductivity of SnO2/rGO deionized water are andhigher water-based rGO nanofluids the same mass fraction have been nanofluids than thatpure of deionized water. Thewith percentage enhancement in thermal measured as well for comparison. It is obvious that the thermal conductivity of SnO /rGO nanofluids conductivity (Keff) is calculated through the formula ((K − K0) × 100%)/K0, where K0 and K2 represents the thermal conductivity of thewater. base fluid water) and nanofluid, As shown (Keff ) are higher than that of deionized The(deionized percentage enhancement inrespectively. thermal conductivity in Figure 9a,b, the fraction have significant influence on the thermal is calculated through the mass formula ((K and − Ktemperature 0 ) × 100%)/K0 , where K0 and K represents the thermal conductivity of SnO2/rGO nanofluids. The thermal conductivities of nanofluids enhance at various conductivity of the base fluid (deionized water) and nanofluid, respectively. As shown in Figure 9a,b, degrees as concentrations of SnO2/rGO nanocompounds and temperatures increase. There are the massseveral fraction and temperature have significant influence on the thermal conductivity of SnO /rGO possible suggested mechanisms to explain the enhancement of thermal conductivity for 2 nanofluids. The thermal conductivities of nanofluids enhance at various degrees as concentrations nanofluids [43,44]. When the temperature of nanofluids increases, the Brownian motion of of SnO2nanoparticles /rGO nanocompounds andleads temperatures increase. There are possible suggested enhances which to higher thermal conductivity of several nanofluids. With the increasing of nanoparticles weight concentration, the distance between particles [43,44]. (free path) mechanisms to explain the enhancement of thermal conductivity for nanofluids When the decreases. More nanoparticles are in contact with each other,of resulting in the frequency of the latticeleads to temperature of nanofluids increases, the Brownian motion nanoparticles enhances which vibration increasing and the heat transfer of electrons and phonons improving [28]. As a result, the higher thermal conductivity of nanofluids. With the increasing of nanoparticles weight concentration, higher the concentration of nanoparticles, the higher the thermal conductivity, which is consistent the distance between theory particles path) decreases. Morethat nanoparticles are in contact with with Maxwell’s [45].(free In addition, it is well known there is a nano-layered structure at each the other, resulting in the frequency of the lattice vibration increasing and the heat transfer of electrons and solid-liquid interface when the liquid molecules are close to the solid surface. This solid-like phonons improving [28].acts Asasa aresult, the the concentration ofthe nanoparticles, themay higher the nano-layer structure medium of higher heat transport from the solid to bulk liquid which a major contributing to the enhancement thermal[45]. conductivity in the it nanofluid thermalbeconductivity, which mechanism is consistent with Maxwell’softheory In addition, is well known [46]. the temperature risesatfrom 20 to 60 °C, the enhancement of thermal conductivity that there is When a nano-layered structure the solid-liquid interface when the liquid molecules are close icnreases from 2% to 7% at a mass fraction of 0.02 wt %, and reaches 7% to 16% at a mass fraction of to the solid surface. This solid-like nano-layer structure acts as a medium of heat transport from the 0.06 wt %. The maximum enhancement of thermal conductivity approaches to 31% at 60 °C for the solid to concentration the bulk liquid which a major contributing enhancement of thermal of 0.1 wt %.may This be phenomenon of non-linearmechanism enhancementto is the consistent with others’ ◦ conductivity in studies the nanofluid When the temperature risesdisplayed from 20 in toFigure 60 C,9c,d thethat enhancement previous [10,30]. It[46]. can be seen clearly from the results the thermal conductivities of SnO2/rGO areathigher than rGO nanofluids under mass 7% to of thermal conductivity icnreases fromnanofluids 2% to 7% a mass fraction of 0.02 wt %, various and reaches It is suggested SnO 2 nanoparticles are decorated on the surface of rGO, which can 16% at afractions. mass fraction of 0.06 that wt %. The maximum enhancement of thermal conductivity approaches block the stacking of graphene sheets. As a result, water-based SnO2/rGO nanofluids have better ◦ to 31% at 60 C for the concentration of 0.1 wt %. This phenomenon of non-linear enhancement is stability and exhibit better thermal conductivity.

consistent with others’ previous studies [10,30]. It can be seen clearly from the results displayed in Figure 9c,d that the thermal conductivities of SnO2 /rGO nanofluids are higher than rGO nanofluids under various mass fractions. It is suggested that SnO2 nanoparticles are decorated on the surface of rGO, which can block the stacking of graphene sheets. As a result, water-based SnO2 /rGO nanofluids have better stability and exhibit better thermal conductivity.

Materials 2018, 11, 38

10 of 13

Materials 2018, 11, 38

10 of 13

Figure 9. Thermal conductivity of water-based rGO and SnO2/rGO nanofluids as a function of

Figure 9. Thermal conductivity of water-based rGO and SnO2 /rGO nanofluids as a function of temperature and the weight fraction of nanoparticles. (a) SnO2/rGO water-based nanofluids at temperature and the weight (b) fraction of temperature nanoparticles. SnO2conductivity /rGO water-based nanofluids at different concentrations; Effect of on the(a) thermal enhancements at different concentrations; (b) Effect of temperature the and thermal conductivity enhancements at 2/rGO rGO water-based nanofluids at different concentrations; (c,d) Comparison of SnOon differentdifferent concentrations; (c,d)ofComparison SnO and rGO water-based nanofluids at different concentrations 0.04 wt % and of 0.06 wt 2%/rGO . concentrations of 0.04 wt % and 0.06 wt %. It is evident that the thermal conductivity of the nanofluids relies on temperature and concentration when the nanoparticles and the base fluid are assigned. Consequently, it is clear that It the is evident that the thermal conductivity of the nanofluids relies on temperature and thermal conductivity and dimension of the solid-liquid interfacial layer have significant impacts concentration when thethermal nanoparticles andofthe base fluid are assigned. Consequently, is clear on the enhanced conductivity nanofluids. The typical theoretical models that ithave been that the thermaldeveloped conductivity dimension of theofsolid-liquid have significant for and thermal conductivity nanoparticlesinterfacial suspendedlayer fluids only consider impacts thermal on the conductivities of the base fluid, particles, andThe volume fraction of particles, whilethat particle shape, enhanced thermal conductivity of nanofluids. typical theoretical models havesize, been developed the distribution and motion of dispersed particles and micro convection are having important for thermal conductivity of nanoparticles suspended fluids only consider thermal conductivities of influences on thermal conductivity enhancement as well [47]. Therefore, the experimental results the base fluid, particles, and volume fraction of particles, while particle size, shape, the distribution sometime could not be compared with the correlated values of the theoretical models [30,48]. and motion of dispersed particlesbetween and micro convection are having important influences oninthermal Furthermore, the comparison graphene-based nanofluids in most recent works is shown conductivity enhancement well [47]. Therefore, the experimental results sometime could not be Table 1, it is apparent as that water-based SnO2/rGO nanofluids with low additive concentration possess pretty good thermal conductivity enhancement, compared to those of other works with compared witha the correlated values of the theoretical models [30,48]. Furthermore, the comparison higher concentration ofnanofluids nanoparticles. results, it canisbeshown concluded that the1,SnO between graphene-based inFrom mostabove recent works in Table it 2/rGO is apparent nanocomposite could possibly be a useful candidate obtain satisfactory thermal conductivity that water-based SnO2 /rGO nanofluids with low additive concentration possess a pretty good enhancement for medium-temperature coolant applications.

thermal conductivity enhancement, compared to those of other works with higher concentration of nanoparticles. From above results, it can be concluded thatas-prepared the SnOnanofluid Table 1. Comparison of thermal conductivity enhancements of our with 2 /rGO nanocomposite reported nanofluids.obtain satisfactory thermal conductivity enhancement for could possibly begraphene-based a useful candidate medium-temperature coolant applications. Materials Base Fluid Loading Enhancement % References graphene graphene

water water

0.2 vol% 0.05 wt%

27 17

[13] [15]

Table 1. Comparison of thermal conductivity enhancements of our as-prepared nanofluid with reported rGO water 0.03 wt% 10 [16] water 0.05 vol% 16 [23] graphene-basedf-HEG nanofluids. rGO-Fe3O4 GNP–Ag Materials graphene-CuO graphene-SiO2 graphene rGO/TiO2 graphene SnO2/rGO

rGO f-HEG rGO-Fe3 O4 GNP–Ag graphene-CuO graphene-SiO2 rGO/TiO2 SnO2 /rGO

water water Base Fluid water waterwater water water water

water water water water water water water water

0.5 vol% 0.1 wt% Loading 0.5 vol% 0.1 wt% 0.2 vol % 0.1 wt % 0.05 wt % 0.1 wt % (0.078 vol %)

0.03 wt % 0.05 vol % 0.5 vol % 0.1 wt % 0.5 vol % 0.1 wt % 0.1 wt % 0.1 wt % (0.078 vol %)

11 22 Enhancement % 28 20 27 33 17 31

10 16 11 22 28 20 33 31

[26] [27] References [28] [29] [13] [30] [15] This work

[16] [23] [26] [27] [28] [29] [30] This work

Materials 2018, 11, 38

11 of 13

4. Conclusions In this paper, SnO2 /rGO nanocomposites with ultrasmall-sized SnO2 nanoparticles anchored on the surface of rGO have been synthesized successfully via a simple solvothermal reaction. The well-dispersed water-based SnO2 /rGO nanofluids with different weight concentrations have been prepared without any surfactant as well. The results reveal that the SnO2 /rGO nanofluids have good dispersion stability. The absolute values of zeta potential for water-based SnO2 /rGO nanofluids are basically high than 30 mV and only a few sedimentation can be observed after quiescence for 60 days. The thermal conductivity of SnO2 /rGO nanofluids increases with increasing temperature, as well as weight concentration. The enhancement of thermal conductivity of water-based SnO2 /rGO nanofluids could reach up to 31% at 60 ◦ C under the mass fraction of 0.1 wt %, comparing to deionized water. The SnO2 /rGO nanofluids also possess higher thermal conductivity than the water-based rGO nanofluid. The above results show the water-based SnO2 /rGO nanofluids have a bright prospect in the industrial application of heat exchangers systems. However, the electrical conductivity, viscosity, and convective heat transfer coefficient of nanofluids have not yet been studied widely compared to their thermal conductivity, which are important for nanofluid research. Additionally, investigations about theoretical models of the mechanism of thermal conductivity and heat exchange enhancement are needed for further study. Acknowledgments: This work is supported by the link project of the National Natural Science Foundation of China and Guangdong Province (grant no. U1401246), by the National Natural Science Foundation of China (grant no. 21701030), by the Science and Technology Program of Guangdong Province of China (grant no. 2015A050502047, 2016A020221031, 2017B050504004), and by the Science and Technology Program of Guangzhou City of China (grant no. 2016201604030040, 201607010089). Author Contributions: Qibai Wu and Haiyan Zhang conceived and designed the experiments; Guoxun Zeng prepared reagents and synthesized the SnO2 /rGO nanocomposite; Yang Li and Guoqiang Yang carried out the characterization of the SnO2 /rGO nanocomposites; Wenwu Li contributed the preparation of nanofluids; Muyu Chen participated in the implementation of the experimental setup to measure thermal conductivity Xiaofen Yu analyzed the data and wrote the paper; Yannan Qian coordinated the redaction of the manuscript; and all authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

2. 3.

4. 5. 6.

7.

8.

Kasaeian, A.; Daneshazarian, R.; Mahian, O.; Kolsi, L.; Chamkha, A.J.; Wongwises, S.; Pop, I. Nanofluid flow and heat transfer in porous media: A review of the latest developments. Int. J. Heat Mass Transf. 2017, 107, 778–791. [CrossRef] Eggers, J.R.; Kabelac, S. Nanofluids revisited. Appl. Therm. Eng. 2016, 106, 1114–1126. [CrossRef] Agromayor, R.; Cabaleiro, D.; Pardinas, A.A.; Vallejo, J.P.; Fernandez-Seara, J.; Lugo, L. Heat transfer performance of functionalized graphene nanoplatelet aqueous nanofluids. Materials 2016, 9, 455. [CrossRef] [PubMed] Zhao, N.; Li, Z. Experiment and artificial neural network prediction of thermal conductivity and viscosity for alumina-water nanofluids. Materials 2017, 10, 552. [CrossRef] [PubMed] Devendiran, D.K.; Amirtham, V.A. A review on preparation, characterization, properties and applications of nanofluids. Renew. Sustain. Energy Rev. 2016, 60, 21–40. [CrossRef] Patel, H.E.; Das, S.K.; Sundararajan, T.; Sreekumaran, N.A.; George, B.; Pradeep, T. Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects. Appl. Phys. Lett. 2003, 83, 2931–2933. [CrossRef] Chandrasekar, M.; Suresh, S.; Chandra, B.A. Experimental investigations and theoretical determination of thermal conductivity and viscosity of Al2 O3 /water nanofluid. Exp. Therm. Fluid Sci. 2010, 34, 210–216. [CrossRef] Karthikeyan, N.R.; Philip, J.; Raj, B. Effect of clustering on the thermal conductivity of nanofluids. Mater. Chem. Phys. 2008, 109, 50–55. [CrossRef]

Materials 2018, 11, 38

9.

10. 11. 12. 13. 14. 15.

16. 17. 18. 19.

20. 21. 22.

23. 24. 25. 26.

27.

28. 29. 30.

12 of 13

Amrollahi, A.; Rashidi, A.M.; Lotfi, R.; Emami, M.M.; Kashefi, K. Convection heat transfer of functionalized mwnt in aqueous fluids in laminar and turbulent flow at the entrance region. Int. Commun. Heat Mass Transf. 2010, 37, 717–723. [CrossRef] Baby, T.T.; Ramaprabhu, S. Investigation of thermal and electrical conductivity of graphene based nanofluids. J. Appl. Phys. 2010, 108, 124308. [CrossRef] Marcano, D.C.; Dmitry, V.K.; Berlin, J.M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L.B.; Lu, W.; Tour, J.M. Improved synthesis of graphene oxide. ACS Nano 2010, 4, 4806–4814. [CrossRef] [PubMed] Aravind, S.S.J.; Ramaprabhu, S. Graphene-multiwalled carbon nanotube-based nanofluids for improved heat dissipation. RSC Adv. 2013, 3, 4199–4206. [CrossRef] Sen, G.S.; Manoj, S.V.; Krishnan, S.; Sreeprasad, T.S.; Singh, P.K.; Pradeep, T.; Das, S.K. Thermal conductivity enhancement of nanofluids containing graphene nanosheets. J. Appl. Phys. 2011, 110, 084302. Kole, M.; Dey, T.K. Investigation of thermal conductivity, viscosity, and electrical conductivity of graphene based nanofluids. J. Appl. Phys. 2013, 113, 084307. [CrossRef] Ghozatloo, A.; Shariaty-Niasar, M.; Rashidi, A.M. Preparation of nanofluids from functionalized graphene by new alkaline method and study on the thermal conductivity and stability. Int. Commun. Heat Mass Transf. 2013, 42, 89–94. [CrossRef] Kamatchi, R.; Venkatachalapathy, S.; Abhinaya, S.B. Synthesis, stability, transport properties, and surface wettability of reduced graphene oxide/water nanofluids. Int. J. Therm. Sci. 2015, 97, 17–25. [CrossRef] Liao, Q.; Li, N.; Jin, S.; Yang, G.; Wang, C. All-solid-state symmetric supercapacitor based on Co3 O4 nanoparticles on vertically aligned graphene. ACS Nano 2015, 9, 5310–5317. [CrossRef] [PubMed] Sarsama, W.S.; Amiri, A.; Kazi, S.N.; Badarudin, A. Stability and thermophysical properties of non-covalently functionalized graphene nanoplatelets nanofluid. Energy Convers. Manag. 2016, 116, 101–111. [CrossRef] Uddin, M.E.; Kuila, T.; Nayak, G.C.; Kim, N.H.; Ku, B.-C.; Lee, J.H. Effects of various surfactants on the dispersion stability and electrical conductivity of surface modified graphene. J. Alloys Compd. 2013, 562, 134–142. [CrossRef] Leong, K.; Mohd, N.H.; Mohd, S.; Amer, N. The effect of surfactant on stability and thermal conductivity of carbon nanotube based nanofluids. Therm. Sci. 2016, 20, 429–436. [CrossRef] Lin, Y.; Zhang, H.; He, C.; Li, Y.; Wang, S.; Hong, H. A new kind of water-based nanofluid with a low loading of three-dimensional porous graphene. J. Mater. Sci. 2017, 52, 10485–10496. [CrossRef] Tchoul, M.N.; Ford, W.T.; Lolli, G.; Resasco, D.E.; Arepalli, S. Effect of mild nitric acid oxidation on dispersability, size, and structure of single-walled carbon nanotubes. Chem. Mater. 2007, 19, 5765–5772. [CrossRef] Baby, T.T.; Ramaprabhu, S. Enhanced convective heat transfer using graphene dispersed nanofluids. Nanoscale Res. Lett. 2011, 6, 289. [CrossRef] [PubMed] Liu, M.; Yang, Y.; Zhu, T.; Liu, Z. Chemical modification of single-walled carbon nanotubes with peroxytrifluoroacetic acid. Carbon 2005, 43, 1470–1478. [CrossRef] Zhang, L.; Ni, Q.; Fu, Y.; Natsukia, T. One-step preparation of water-soluble single-walled carbon nanotubes. Appl. Surf. Sci. 2009, 255, 7095–7099. [CrossRef] Mehrali, M.; Sadeghinezhad, E.; Akhiani, A.R.; Tahan, L.S.; Metselaar, H.S.C.; Kherbeet, A.S.; Mehrali, M. Heat transfer and entropy generation analysis of hybrid graphene/Fe3 O4 ferro-nanofluid flow under the influence of a magnetic field. Powder Technol. 2017, 308, 149–157. [CrossRef] Yarmand, H.; Gharehkhani, S.; Ahmadi, G.; Shirazi, S.F.S.; Baradaran, S.; Montazer, E.; Zubir, M.N.M.; Alehashem, M.S.; Kazi, S.N.; Dahari, M. Graphene nanoplatelets-silver hybrid nanofluids for enhanced heat transfer. Energy Convers. Manag. 2015, 100, 419–428. [CrossRef] Baby, T.T.; Sundara, R. Synthesis and transport properties of metal oxide decorated graphene dispersed nanofluids. J. Phys. Chem. C 2011, 115, 8527–8533. [CrossRef] Li, X.; Chen, Y.; Mo, S.; Jia, L.; Shao, X. Effect of surface modification on the stability and thermal conductivity of water-based SiO2 -coated graphene nanofluid. Thermochim. Acta 2014, 595, 6–10. [CrossRef] Wang, S.; Li, Y.; Zhang, H.; Lin, Y.; Li, Z.; Wang, W.; Wu, Q.; Qian, Y.; Hong, H.; Zhi, C. Enhancement of thermal conductivity in water-based nanofluids employing TiO2 /reduced graphene oxide composites. J. Mater. Sci. 2016, 51, 10104–10115. [CrossRef]

Materials 2018, 11, 38

31.

32.

33.

34.

35. 36. 37. 38. 39. 40.

41. 42. 43. 44. 45. 46. 47. 48.

13 of 13

Habibzadeh, S.; Kazemi-Beydokhti, A.; Khodadadi, A.A.; Mortazavi, Y.; Omanovic, S.; Shariat-Niassar, M. Stability and thermal conductivity of nanofluids of tin dioxide synthesized via microwave-induced combustion route. Chem. Eng. J. 2010, 156, 471–478. [CrossRef] Mariano, A.; Pastoriza-Gallego, M.J.; Lugo, L.; Camacho, A.; Canzonieri, S.; Piñeiro, M.M. Thermal conductivity, rheological behaviour and density of non-newtonian ethylene glycol-based SnO2 nanofluids. Fluid Phase Equilib. 2013, 337, 119–124. [CrossRef] Zhang, D.; Liu, J.; Chang, H.; Liu, A.; Xia, B. Characterization of a hybrid composite of SnO2 nanocrystal-decorated reduced graphene oxide for ppm-level ethanol gas sensing application. RSC Adv. 2015, 5, 18666–18672. [CrossRef] Jiang, S.; Yue, W.; Gao, Z.; Ren, Y.; Ma, H.; Zhao, X.; Liu, Y.; Yang, X. Graphene-encapsulated mesoporous SnO2 composites as high performance anodes for lithium-ion batteries. J. Mater. Sci. 2013, 48, 3870–3876. [CrossRef] Chen, L.; Ma, X.; Wang, M.; Chen, C.; Ge, X. Hierarchical porous SnO2 /reduced graphene oxide composites for high-performance lithium-ion battery anodes. Electrochim. Acta 2016, 215, 42–49. [CrossRef] Xu, C.; Wang, X.; Zhu, J. Graphene-metal particle nanocomposites. J. Phys. Chem. 2008, 112, 19841–19845. [CrossRef] Wang, Y.; Ding, J.; Liu, Y.; Liu, Y.; Cai, Q.; Zhang, J. SnO2 @reduced graphene oxide composite for high performance lithium-ion battery. Ceram. Int. 2015, 41, 15145–15152. [CrossRef] Zhang, X.; Huang, X.; Zhang, X.; Xia, L.; Zhong, B.; Zhang, T.; Wen, G. Cotton/rGO/carbon-coated SnO2 nanoparticle-composites as superior anode for lithium ion battery. Mater. Des. 2017, 114, 234–242. [CrossRef] Gu, Y.; Xing, M.; Zhang, J. Synthesis and photocatalytic activity of graphene based doped TiO2 nanocomposites. Appl. Surf. Sci. 2014, 319, 8–15. [CrossRef] Li, F.; Song, J.; Yang, H.; Gan, S.; Zhang, Q.; Han, D.; Ivaska, A.; Niu, L. One-step synthesis of graphene/sno2 nanocomposites and its application in electrochemical supercapacitors. Nanotechnology 2009, 20, 455602. [CrossRef] [PubMed] Wu, Q.-H.; Wang, C.; Ren, J.-G. Sn and SnO2 -graphene composites as anode materials for lithium-ion batteries. Ionics 2013, 19, 1875–1882. [CrossRef] Ramires, M.L.V.; de Castro, C.A.N. Standard reference data for the thermal conductivity of water. J. Phys. Chem. Ref. Data 1995, 24, 1377–1381. [CrossRef] María, J.P.-G.; Luis, L.; Legido, J.L.; Piñeiro, M.M. Thermal conductivity and viscosity measurements of ethylene glycol-based Al2 O3 nanofluids. Nanoscale Res. Lett. 2011, 6, 1–11. Keblinski, P.; Phillpot, S.R.; Choi, S.U.S.; Eastman, J.A. Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids). Int. J. Heat Mass Transf. 2002, 45, 855–863. [CrossRef] Yu, W.; Choi, S.U.S. The Role of Interfacial Layers in the Enhanced Thermal Conductivity of Nanofluids: A Renovated Maxwell Model. J. Nanopart. Res. 2003, 5, 167–171. [CrossRef] Yang, L.; Xu, X. A renovated hamilton–crosser model for the effective thermal conductivity of cnts nanofluids. Int. Commun. Heat Mass Transf. 2017, 81, 42–50. [CrossRef] Patel, H.E.; Sundararajan, T.; Pradeep, T.; Dasgupta, A.; Dasgupta, N.; Das, S.K. A micro convection model for thermal conductivity of nanofluid. Pramana 2005, 65, 863–869. [CrossRef] Mehrali, M.; Sadeghinezhad, E.; Latibari, S.T.; Kazi, S.N.; Mehrali, M.; Zubir, M.N.; Metselaar, H.S. Investigation of thermal conductivity and rheological properties of nanofluids containing graphene nanoplatelets. Nanoscale Res. Lett. 2014, 9, 15. [CrossRef] [PubMed] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).