Preparation of nanoscaled yttrium oxide by citrate precipitation method

JOURNAL OF RARE EARTHS, Vol. 35, No. 1, Jan. 2017, P. 79

Preparation of nanoscaled yttrium oxide by citrate precipitation method CHEN Jinqing (陈金清)1, HUANG Bin (黄 彬)1,2, HUANG Chao (黄 超)1,2, SUN Xiaoqi (孙晓琦)2,* (1. School of Metallurgy and Chemical Engineering, Jiangxi University of Science & Technology, Ganzhou 341000, China; 2. Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen 361021, China) Received 28 April 2016; revised 31 May 2016

Abstract: The nano-Y2O3 was prepared from YCl3 by the citrate precipitation method. The precursor powders were prepared by 0.1 mol/L YCl3 solution and 0.1 mol/L hydrochloric acid in the presence of 1% surfactant PEG2000, which was dried via an ethanol azeotropic distillation method. The effects of reaction temperature, precursor concentration, hydrochloric acid concentration, surfactant, and calcination temperature on the mean sizes of nano-Y2O3 were studied. It was found that the highest yield of precursor was about 70% at the pH value of 5.0, and the yield decreased rapidly at the pH value below 4 or over 6. The reaction temperature revealed no effect on the size of precursor. The optimized precursor concentration and hydrochloric acid concentration were both 0.1 mol/L. Several typical analytic techniques such as particle size analyzer, X-ray diffraction (XRD), thermogravimetric and differential thermal analyses (TG-DTA) and scanning electron microscopy (SEM) were used to determine the characteristics of the prepared nano powders. Homogeneous torispherical nano-Y2O3 with the smallest size (20 nm) could be obtained by calcining the precursor powders at 800 ºC for an hour. Keywords: nano; Y2O3; citrate precipitation; rare earths

With the booming development of nanotechnology, the nano powders with regular morphology and narrow size distribution have been widely studied[1]. The nanosized oxide particles possessed unique physicochemical properties over common materials, such as optical, catalytic and structural properties[2]. Considerable investigations on the preparation methods of nano-sized oxide particles have been conducted[3]. It is worthwhile to mention that rare earth oxides have been used in many advanced technologies, such as magnets, lightings, sensors, lasers, electronics, batteries, catalysts, alloys and communications[4]. As a result, the development of rare earth (RE) oxides with nano-sizes, larger specific surface areas and higher chemical activities comes to be very important[5,6]. Among the rare earth (RE) oxides, yttrium oxide (Y2O3) possesses excellent heat resistance, corrosion resistance, high temperature and photochemical stability[7]. Its melting point is higher than 2400 ºC and dielectric constant is from 12 to 20[8,9]. As a result, Y2O3 is widely used in ceramics, optical and laser materials. For example, it was used as additive in many high performance ceramics to improve their hardness[10], wear[11,12], corrosion resistance[13], and light transmission[14,15]. As laser host material, the neodymium-doped yttrium aluminum garnet laser was widely used in medical equipment[16]. Besides high purity, uniformity and dispersion of Y2O3

are essential for the high-tech materials. As mentioned previously, the preparation of nano Y2O3 can be summarized to be solid phase method[17,18], liquid phase method (including precipitation method[19–25], sol-gel method[26–29], microemulsion method[30–32], hydrothermal method[33–35]) and gas phase method[36]. Because of low reaction temperature, simple equipment and low energy consumption, the liquid phase precipitation method is the most commonly used technology for the preparation of nano-scale particles on the industrial scale. The common precipitants in liquid phase precipitation method are oxalic acid, carbonic acid, sodium hydroxide, and citrate. The oxalic acid is one of the most commonly used precipitants in industrial production, but it is still expensive for industrial production. In addition, rareearth carbonate and rare-earth hydroxide precipitates are amorphous, the resulting Y2O3 particles are prepared with lower yields and broader size distributions. To develop sustainable and efficient strategy for industrial application, ammonium citrate was used as precipitant for the preparation of nano-scale Y2O3 in this article. In a very recent report, the precipitates revealed crucial influence on the size and morphology of the prepared nano powders[37]. To obtain nano-Y2O3 powders with favorable size and morphology, the effects of reaction temperature, precursor concentration, hydrochloric acid concentration,

Foundation item: Project supported by “Hundreds Talents Program” from Chinese Academy of Sciences, National Natural Science Foundation of China (21571179), Science and Technology Major Project of the Fujian Province, China (2015HZ0101), Xiamen Universities, Research Institutions Jointing Enterprise Projects (3502Z20152009), Research Institutions Jointing Enterprise Projects (3502Z20152009) * Corresponding author: SUN Xiaoqi (E-mail: [email protected]; Tel.: +86-592-6376370) DOI: 10.1016/S1002-0721(16)60164-3

80

JOURNAL OF RARE EARTHS, Vol. 35, No. 1, Jan. 2017

surfactant, and calcination temperature on the mean size of nano-Y2O3 were discussed in this article. It is the first attempt to dry precipitate via an ethanol azeotropic distillation method under reduced pressure of 0.01 MPa, which contributes to the sample dried quickly without agglomeration.

the dried precursor. Crystalline phase present was identified by Rigaku Miniflex 600 XRD system, which generated monochromated Cu Kα radiation with continuous scanning mode at a rate of 8 (°)/min ranging from 5° to 85°, and operating conditions of 40 kV and 15 mA were used to obtain XRD patterns.

1 Experimental

2 Results and discussion

1.1 Reagents and materials

2.1 Effects of pH and molar ratio on precipitation rate

The Y2O3 (99.999%) was provided by Fujian Changting Golden Dragon Rare Earth Co., Ltd., China. Ammonium citrate (purity>98.5%) and PEG2000 were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals used in this study were of analytical grade without further purification.

Citric acid (H3Cit) is hydroxyl tribasic acid, which has multi-level complexing abilities with RE at different pH values and molar ratios. In the acidic solution, the anion complex was formed by RE ions (H2Cit)– and (HCit)3–. When the pH values arrived at 6–8, the precipitation (RECit) of neutral complex salt was produced. In the precipitation (RECit), the molar amount of RE was equal to that of citric acid. When the molar amount of citric acid was higher than RE, [RE2(Cit)3]3– and [RE(Cit)2]3– anions came to be formed. Moreover, hydroxyl of RE citrate was neutralized to form [Y(Cit)’]– in alkaline solution[38,39]. Fig. 1 shows the precipitation rates of precursor [(RECit)·nH2O] at different pH values when the molar ratio (RE3+/Cit3–) is 0.9 or 1. The highest yield is about 70% at the pH value of 5.0, the yield decreases rapidly at the pH value below 4 or over 6. The different molar ratios reveal little effect on the precipitation rates.

1.2 Preparation of nano-Y2O3 (1) The YCl3 solution was prepared by dissolving the Y2O3 in muriatic acid (1:1). The pH value of solution was kept to be 4–5. Y3+ concentration in the solution was determined by EDTA compleximetry. (2) The YCl3 solution was dropped to a stirred ammonium citrate solution, and then ammonia was used to increase the pH until the precipitate disappeared. Subsequently, the hydrochloric acid was dropwisely added with stirring till the generation of precipitation. By adjusting the pH value, the highest rate of precipitation was obtained. Then the precipitate was filtered and washed with deionized water for removing chloride. (3) The precipitate was dispersed in the ethanol by ultrasonic and azeotropic distillation under reduced pressure at 40 ºC to obtain precursor without adsorbed water, which was subsequently heated at various temperatures to get the Y2O3 powder. 1.3 Characterizations The particle size of Y2O3 dispersed in ethanol was measured by dynamic light scattering (Nanobrook Omni, Brookhaven). The morphologies of nano-Y2O3 were observed by scanning electron microscopy (SEM, Hitachi su8010). Simultaneous thermogravimetric and differential thermal analyses (Mettler Toledo, TGA/DSC1) with a heating rate of 10 ºC/min in a static air atmosphere were used to study thermal decomposition behavior of

Scheme 1 Preparation process of nano-Y2O3 powder

2.2 Effect of temperature As can be seen in Fig. 2, the effect of reaction temperature on the mean size (D50) of precursor was studied. It is found that the size of precursor changes slightly with the temperature increased from 25 to 90 ºC. The comparison indicates that the mean size of precursor is not related to the preparation temperature. Accordingly, the precursor powders were prepared at room temperature in this study. 2.3 Effect of precursor and hydrochloric acid According to LaMer model for the crystal nucleation

Fig. 1 Effect of pH values and molar ratios of citric acid with RE on the precipitation rate

CHEN Jinqing et al., Preparation of nanoscaled yttrium oxide by citrate precipitation method

81

hydrochloric acid. 2.4 Effect of calcination temperature

Fig. 2 Particle sizes obtained at different temperatures

process[40,41], nanocrystal formation comprises the following three steps: (1) the atoms start to aggregate into nuclei via self-nucleation with the increase of reactant concentration; (2) the reactant aggregates on the pre-existing nuclei or seed, leading to gradual decrease of reactant concentration; (3) the nuclei continue to grow into nanocrystals. To obtain a high yield of shape-monodisperse nanocrystals, nucleation must occur rapidly and instantaneously, and then the concentration of reactants must be kept below the critical level without nucleation happens. At the same time, the nanocrystals grow slowly to uniform monodisperse nano-particles. As can be seen in Fig. 3, the concentration effects of precursor and hydrochloric acid on the mean size of precursor were studied. It is obvious that the higher concentrations of precursor solution and hydrochloric acid result in pronounced increases of the mean sizes. When the concentration of precursor solution was increased from 0.05 to 0.4 mol/L, the sizes of Y2O3 increased from 322 to 1130 nm. Similar increasing trend of Y2O3 size from 538 to 1624 nm could also be observed by increasing the concentration of hydrochloric acid. It is worthwhile to mention that there existed serious agglomeration phenomena among the prepared Y2O3 powders when the sizes of precursors were below 400 nm. Based on the above mentioned studies, the optimized conditions were 0.1 mol/L precursor solution and 0.1 mol/L

Fig. 3 Concentration effect of precursor and hydrochloric acid

Fig. 4 shows the typical TG and DTA curves of dried precursors. There are endothermic peak at 116 ºC and exothermic peak at 400 ºC within the DTA curve. The strong endothermic peak at 116 ºC can be attributed to the loss of crystal water and residual ethanol. The exothermic peak at 400 ºC is due to the combustion reaction of organic groups and formed oxide. Subsequently, there is no exothermic peak below 100 ºC, indicating that no absorbed water remains in the precursor. The ethanol azeotropic distillation method can remove the absorbed water effectively at low temperature. It can be seen from TG curve that the powders of precursors reveal two times of obvious weightlessness in the whole roasting process. The total weight loss of the precursor is around 60%. The weight loss above 100 ºC can be mainly attributed to the removal of crystal water and residual ethanol, which can be observed by the existence of endothermic peak centered at 116 ºC in the DTA curve. The weight loss in the temperature range of 350–650 ºC is due to the burnout of the organic group of yttrium citrate. Moreover, there is no obvious weight loss calcined upon 700 ºC. To achieve a complete calcination, the results from TG-DTA analysis reveal that the calcination temperature needs to be above 700 ºC. The mean sizes of Y2O3 calcined from 700 to 1000 ºC for an hour are revealed in Fig. 5(a). In this study, both the precursor solution and hydrochloric acid concentrations were 0.1 mol/L. As revealed in Fig. 5(a), the nanoY2O3 with the smallest size can be obtained at 800 ºC. Fig. 5(b) shows the XRD pattern of Y2O3 calcined at different temperatures. With the increase of calcination temperature, the intensities of characteristic peaks were increased and peak widths at half height were narrowed. The temperature effects reveal that higher calcination temperature contributes to the crystals growth. The powder calcined at 700 ºC was incompletely crystallized, and the completely crystallized form can be obtained at 800 ºC. As revealed in Fig. 5(b), the nano-Y2O3 is apt to form hard agglomeration when the temperature arrives

Fig. 4 TG-DTA plots of precursor powder

82

JOURNAL OF RARE EARTHS, Vol. 35, No. 1, Jan. 2017

The effect of surfactant contributes to the reduced agglomeration of precipitate and collapse of the network during calcination procedures, which would help reduce the size of final product[43]. As can be seen in Table 1, the 182 nm (D50) nano-Y2O3 powders can be obtained by adding 1 wt.% PEG2000 into the precursor solution. 2.6 Particle sizer and SEM analysis of nano-Y2O3

Fig. 5 Mean size (a) and XRD pattern (b) of Y2O3 calcined at different temperatures

above 800 ºC. The agglomeration tendency results in the change of nano-particle morphology. Furthermore, the obtained intensities and positions of XRD diffraction peaks were compared with those of JCPDS card. It is found that the Y2O3 product was cubic crystal structure, which was consistent with the Y2O3 standard diffraction data (JCPDS card No.: 65-3178). Calculated from the full width at half maximum (FWHM) using Scherrer’s equation[42], the crystallite sizes of Y2O3 calcined at 800 ºC were found to be around 15 nm.

To characterize the prepared nano-Y2O3 powders, size distribution of the nano-Y2O3 powders are given in Fig. 6(a) and their SEM pictures are revealed in Fig. 6(b) on the scales of 200 and 60 nm. In this study, the precursor precipitates were prepared by 0.1 mol/L solution and hydrochloric acid in presence of 1 wt.% PEG2000. Then the precursor precipitate was calcined at 800 ºC for an hour. As shown is Fig. 6(a), the size distribution of nano-Y2O3 powders is monodisperse and very narrow. The product powder has a mean size of 180 nm. As for the morphologies of nano-Y2O3 powders, SEM pictures shown in Fig. 6(b) reveal that the product is torispherical particle and has a good size distribution around 20 nm. Nanoparticles are the particles between 1 and 100 nanometers in size, the sizes of ultrafine particles are the same as those of nanoparticles. Fine particles are sized between 100 and 2500 nanometers, and coarse particles cover a range between 2500 and 10000 nanometers[44,45]. In this study, the final products are nano-Y2O3 powders of 20 nm with torispherical morphologies, which can be called nanoparticles or ultrafine particles. Their sizes are smaller than the latest report on nano-Y2O3 powders[37].

2.5 Effect of surfactant on nano-Y2O3 Table 1 lists the effect of surfactant on size distribution of nano-Y2O3. The Y2O3 powers were calcined at 800 ºC, which were prepared by the same precursor solution and hydrochloric acid but different surfactants. The comparison in Table 1 indicates that the surfactants can decrease the surface tension of water contained in the pores. Table 1 Effect of surfactant on size distribution of nano-Y2O3 No.

Surfactant/wt.%

D10/nm

D50/nm

D90/nm

1

–

170.10

293.30

505.83

2

3% Ethanol

173.38

260.94

392.72

3

5% Ethanol

147.28

230.79

361.65

4

10% Ethanol

147.54

227.00

349.27

5

1% PEG2000

118.65

182.87

281.85

6

3% PEG2000

184.51

277.90

418.55

7

5% PEG2000

193.37

321.39

534.17

Fig. 6 Size distribution (a) and SEM photos (b) of Y2O3 product

CHEN Jinqing et al., Preparation of nanoscaled yttrium oxide by citrate precipitation method

3 Conclusions The preparation of nano-sized RE oxide is crucial for the development of RE material, therefore, it is of importance to develop new and simple method to obtain nano powder of RE oxide. Nano-Y2O3 powders were prepared by the citrate precipitation method in this study. The precipitate was dried via an ethanol azeotropic distillation method, which contributed to the sample dried quickly at a low temperature of 40 ºC without agglomeration. As indicated by TG-DTA analysis, the ethanol azeotropic distillation method removed absorbed water effectively to obtain well dispersive precursor. The nanoY2O3 product of 20 nm was obtained using 1 wt.% surfactant PEG2000, ammonium citrate and YCl3 by controlling pH value and calcining at 800 ºC for an hour. XRD and SEM results revealed that the morphology of final Y2O3 product was torispherical and uniform-sized particle with a well-developed crystal structure.

References: [1] Gai S L, Li C X, Yang P P, Lin J. Recent progress in rare earth micro/nanocrystals: soft chemical synthesis, luminescent properties, and biomedical applications. Chem. Rev., 2013, 114: 2343. [2] Weng G Q, Sun J W, Zhang C X. The rare-earth nanomaterials and its applications. Rare Met. Cem. Carbides (in Chin.), 2006, 34: 42. [3] Hong G Y. Syntheses and assemblies of rare earth nanomaterials. J. Chin. Soc. Rare Earths (in Chin.), 2006, 24: 641. [4] Eliseeva S V, Bünzli J C G. Rare earths: jewels for functional materials of the future. New J. Chem., 2011, 35: 1165. [5] Azimi G, Dhiman R, Kwon H M, Paxson A T, Varanasi K K. Hydrophobicity of rare-earth oxide ceramics. Nat. Mater., 2013, 12: 315. [6] Rao T D, Karthik T, Asthana S. Investigation of structural, magnetic and optical properties of rare earth substituted bismuth ferrite. J. Rare Earths, 2013, 31: 370. [7] Huo D, Sun X D, Xiu Z M, Li X D. Influence of processing conditions on morphologies and sinterability of ultrafine Y2O3 powders. J. Chin. Soc. Rare Earths (in Chin.), 2007, 25: 566. [8] Paton M G, Maslen E N. A refinement of the crystal structure of yttria. Acta. Crystallogr, 1965, 19: 307. [9] Xu Y N, Gu Z Q, Ching W Y. Electronic, structural, and optical properties of crystalline yttria. Phys. Rev. B, 1997, 56: 14993. [10] Chen Y H, Jiang L, Zhang L L, Huang Z K, Wu L E. Effect of rare-earth oxides additive on liquid phase sintered SiC. Key Eng. Mater., 2014, 602-603: 407. [11] Liu J C, Wu B L. Effects of Eu2O3 addition on microstructure, grain-boundary cohesion and wear resistance of high-alumina ceramics. J Alloy Compd., 2014, 50: 49. [12] Mu B C, Li M, You X Q, Liu B Y, Gu Z G, Liu D W. Effect of rare earths on mechanical properties and micro-

83

structure of Si3N4 ceramic. J. Chin. Soc. Rare Earths (in Chin.), 2000, 18: 38. [13] Di S C, Guo Y P, Lü H W, Yu J, Li Z W. Microstructure and properties of rare earth CeO2-doped TiO2 nanostructured composite coatings through micro-arc oxidation. Ceram. Int., 2015, 41: 6178. [14] Kruk A, Wajler A, Mrozek M, Zych L, Gawlik W, Brylewski T. Transparent yttrium oxide ceramics as potential optical isolator materials. Opt. Appl., 2015, 45: 585. [15] Jin L L, Jiang Z J, Zhang J, Wang S W. Research progress of yttria transparent ceramics. J. Chin. Ceram. Soc. (in Chin.), 2010, 38: 521. [16] Li J, Pan Y B, Zhang J J, Huang L P, Guo J K. Synthesizing yttrium aluminum garnet (YGA) nano-powder by co-precipitation method. J. Chin. Ceram. Soc. (in Chin.), 2003, 31: 490. [17] Shuang X, Li L, Zhao R G T, Ga R D. Preparation and characterization of nanocrystalline MgO-La2O3 prepared by solid state reaction. Chin. Rare Earths (in Chin.), 2007, 28: 115. [18] Sun M, Yu L, Yu J, Hao Z F, Yu Q, Peng L Q. Preparation of nanometer MgO by microwave solid-state method. J. Funct. Mater. (in Chin.), 2006, 37: 1978. [19] Xiong X B, Liu L S, Li M D, Jia T, Ma S F, Hu W H. Status in study of nanometer rare earth oxides prepared by precipitation methods. Chin. Rare Earths (in Chin.), 2013, 34: 33. [20] Wang A Q, Cui L K, Xie J P, Li J W. Thermodynamic analysis of ybco powder by oxalate co-precipitation preparation. J. Chin. Soc. Rare Earths (in Chin.), 2014, 32: 322. [21] Mei Y, Wang W, Sun H, Nie Z R. Preparation and morphology of nano-size ceria by a stripping precipitation using oxalic acid as a precipitating agent. J. Rare Earths, 2012, 30: 1265. [22] Jeong J Y, Park S W, Moon D K, Kim W J. Synthesis of Y2O3 nano-powders by precipitation method using various precipitants and preparation of high stability dispersion for backlight unit (BLU). J. Ind. Eng. Chem., 2010, 16: 243. [23] Wang H, Gao L, Niihara K. Synthesis of nanoscaled yttrium aluminum garnet powder by the co-precipitation method. Mater. Sci. Eng. A, 2000, 288: 1. [24] Wang Y, Ma J, Luo M, Fang P, He M. Preparation of highsurface area nano-CeO2 by template-assisted precipitation method. J. Rare Earths, 2007, 25: 58. [25] Zhu W, Qiu D X, Pei H Y, Zhou X Z, Zhu W C, Li J, Liu Y Z, Li D P, Zhou X M, Li Y X. Precipitation crystallization process of yttrium carbonate and size controlling synthesis of ytterium oxide particles. J. Chin. Soc. Rare Earths (in Chin.), 2016, 34: 180 [26] Sreethawong T, Chavadej S, Ngamsinlapasathian S, Yoshikawa S. Sol-gel synthesis of mesoporous assembly of Nd2O3 nanocrystals with the aid of structure-directing surfactant. Solid State Sci., 2008, 10: 20. [27] Chen W F, Li F S, Liu L L, Li Y X. Synthesis of nanosized yttria via a sol-gel process based on hydrated yttrium nitrate and ethylene glycol and its catalytic performance for thermal decomposition of NH4ClO4. J. Rare Earths, 2006, 24: 543.

84 [28] Zhou L Y, Shi J X, Gong M L. Red phosphor SrY2O4:Eu3+ synthesized by the sol-gel method. J. Lumin., 2005, 113: 285. [29] Laishram K, Mann R, Malhan N. Microwave gel combustion synthesis and sinterability of Nd:GGG nanopowders. J. Rare Earths, 2014, 32: 521. [30] Zhu W Q, Xu L, Ma J, Ren J M, Chen Y S. Size controlled synthesis of CeO2 nanoparticles by a microemulsion method. Acta. Phys-Chim. Sin. (in Chin.), 2010, 26: 1284. [31] Masui T, Fujiwara K, Peng Y, Sakata T, Machida K I, Mori H, Adachi G Y. Characterization and catalytic properties of CeO2-ZrO2 ultrafine particles prepared by the microemulsion method. J. Alloys Compd., 1998, 269: 116. [32] Wu P P, Zhang Z, Song G P. Preparation of Nd2O3 nanorods in SDBS micelle system. J. Rare Earths, 2014, 32: 1027. [33] Lu C H, Wang H C. Formation and microstructural variation of cerium carbonate hydroxide prepared by the hydrothermal process. Mater. Sci. Eng. B, 2002, 90: 138. [34] Shen G, Wang Q, Wang Z, Chen Y. Hydrothermal synthesis of CeO2 nano-octahedrons. Mater. Lett., 2011, 65: 1211. [35] Tok A I Y, Boey F Y C, Dong Z, Sun X L. Hydrothermal synthesis of CeO2 nano-particles. J. Mater. Process. Technol., 2007, 190: 217. [36] Hu Y J, Li C Z. Progress on flame aerosol synthesis of nanomaterials. Mater. Chin. (in Chin.), 2012, 31: 44. [37] Li S S, Liu B L, Li J, Zhu X W, Liu W B, Pan Y B, Guo J K. Synthesis of yttria nano-powders by the precipitation method: The influence of ammonium hydrogen carbonate

JOURNAL OF RARE EARTHS, Vol. 35, No. 1, Jan. 2017 to metal ions molar ratio and ammonium sulfate addition. J. Alloys Compd., 2016, 678: 258. [38] Wu S, Chang Z, Wang K, Xiong W. Preparation and thermal behaviour of rare earth citrate hydrates. J. Therm. Anal. Calorim., 1995, 45: 199. [39] Zhou X M. Preparation of super-fine powders cerium oxide by complexed-precipitation method. J. Chin. Soc. Rare Earths (in Chin.), 2002, 20: 19. [40] Huang Y, Pemberton J E. Synthesis of uniform, spherical sub-100 nm silica particles using a conceptual modification of the classic LaMer model. Colloid Surf. A, 2010, 360: 175. [41] Watzky M A, Finke R G. Transition metal nanocluster formation kinetic and mechanistic studies. A new mechanism when hydrogen is the reductant: slow, continuous nucleation and fast autocatalytic surface growth. J. Am. Chem. Soc., 1997, 119: 10382. [42] Drits V A, Srodon J, Eberl D D. XRD measurement of mean crystallite thickness of illite and illite/smectite: Reappraisal of the Kubler index and the Scherrer equation. Clays Clay Miner., 1997, 45: 461. [43] Yuan W J, Zhou Y M. Reasons for aggregation of nanoparticles and solutions. Mater. Rev., 2008, 22: 59. [44] Buzea C, Pacheco I I, Robbie K. Nanomaterials and nanoparticles: sources and toxicity. Biointerphases, 2007, 2: MR17. [45] Taylor R, Coulombe S, Otanicar T, Phelan P. Small particles, big impacts: a review of the diverse applications of nanofluids. J. App. Phy., 2013, 113: 011301.