Fabrication, characterization, and thermal property evaluation of silver nanofluids
© Noroozi et al.; licensee Springer. 2014
Received: 14 August 2014
Accepted: 15 November 2014
Published: 29 November 2014
Silver nanoparticles were successfully prepared in two different solvents using a microwave heating technique, with various irradiation times. The silver nanoparticles were dispersed in polar liquids (distilled water and ethylene glycol) without any other reducing agent, in the presence of the stabilizer polyvinylpyrrolidone (PVP). The optical properties, thermal properties, and morphology of the synthesized silver particles were characterized using ultraviolet-visible spectroscopy, photopyroelectric technique, and transmission electron microscopy. It was found that for the both solvents, the effect of microwave irradiation was mainly on the particles distribution, rather than the size, which enabled to make stable and homogeneous silver nanofluids. The individual spherical nanostructure of self-assembled nanoparticles has been formed during microwave irradiation. Ethylene glycol solution, due to its special properties, such as high dielectric loss, high molecular weight, and high boiling point, can serve as a good solvent for microwave heating and is found to be a more suitable medium than the distilled water. A photopyroelectric technique was carried out to measure thermal diffusivity of the samples. The precision and accuracy of this technique was established by comparing the measured thermal diffusivity of the distilled water and ethylene glycol with values reported in the literature. The thermal diffusivity ratio of the silver nanofluids increased up to 1.15 and 1.25 for distilled water and ethylene glycol, respectively.
The potential of thermal properties of nanofluids, colloidal dispersion of nanomaterial in a base fluid, is a candidate for various uses, which range from biological and biomedical applications to the new class of heat transfer fluids . The thermal conductivity of the nanofluids is the main thermal property that has been focused in the majority of the experimental studies such as hot wire technique and parallel plate geometry [2–4]. Recently, however, a few techniques have been developed to measure the thermal diffusivity of nanofluids [4–6]. High electrical and thermal conductivity of silver nanofluids, as well as their extraordinary optical properties and their thermo-optical properties, could lead to their use in a variety of applications in the biological and biomedical fields, for example, in photothermal therapies. The surface plasmon resonance of silver nanoparticles (Ag NPs) also makes these materials candidates for applications in electromagnetic hyperthermia therapies [7, 8]. However, the particle shape, size, and size distribution are crucial factors in these applications ; different Ag NPs can display different optical and thermal properties [10, 11], and it is therefore important that during the preparation process, these parameters can be controlled and adjusted.
Different methods have been used for the synthesis of Ag NPs, including hydrothermal , sol-gel , and electrochemical  techniques. The microwave (MW) irradiation method offers many advantages, including simple processes, a short reaction time, a rapid heating rate, and a high and uniform heat distribution that is produced by the dielectric heating . In a MW device, heating is caused by the interaction of the permanent dipole moment in a molecule with high frequency (2.45 GHz) electromagnetic waves. In short, the polarity of a solvent plays a significant role in the reaction process, and the higher polarity value of the solvent, the more efficiently a solvent couples with the MW energy. Many factors affect the polarity of a solvent, such as the dielectric constant (ϵ’), dielectric loss (ϵ”), and tangent delta (tan δ = ϵ”/ϵ’). The dielectric loss is the most indicative factor. The MW radiation can heat a material through its dielectric loss, which converts the MW energy into heat energy. Heat is generated in this way, and this heat may affect the particle morphology and particle size, as well as the particles' physical properties. The stability and thermal properties of a Ag nanofluid can therefore be modified by choosing the correct solvent for the formation of the Ag NPs in the MW heating method . However, no detailed studies that investigate the effects of the solvent properties on the formation and size of NPs and their thermal properties exist in the literature. Motivated by this, distilled water (DW) and ethylene glycol (EG) were selected as solvents to study the thermal diffusivity and dispersion of Ag nanofluids under MW irradiation as a function of time.
Here, we report our study of the formation and thermal properties of Ag nanofluids produced using two different solvents (DW and EG) and different MW irradiation times. DW and EG have different properties, and the influence of these different solvent properties on the particle size, size distribution, and thermal diffusivity of the Ag nanofluids were investigated. The optical properties and morphology of nanoparticles were characterized using ultraviolet-visible (UV-vis) spectroscopy and transmission electron microscopy (TEM). A photopyroelectric (PPE) technique, an accurate and non-destructive method [17–20], was chosen to investigate the effects on the thermal diffusivity of silver nanofluid.
AgNO3 (99.98%, Merck, Darmstadt, Germany) was used as the Ag precursor, polyvinylpyrrolidone (PVP: MW =29,000, Sigma Aldrich, St Louis, MO, USA) was used as a stabilizer for the fabrication of Ag NPs. In the first set of experiments, AgNO3 (0.3 g) and PVP (0.3 g) were each dissolved separately in 25 ml of DW and were stirred for 15 min. The transparent AgNO3 solution was then added to the surfactant solution. The resultant solution (W0) was colorless and was stirred for 10 min; after cooling to a room temperature, the solution was placed in a Panasonic microwave oven (NN-K574MF, 1100 W, 2.45 GHz) for four different times of 20, 40, 60, and 90 s (W1-W4); the heating was paused after each 20 s to prevent intense boiling of the solvents and aggregation of the Ag NPs. The same procedure was followed for the preparation of Ag NPs in the EG suspension at four different times 20, 40, 60, and 90 s (EG1-EG4).
The fabricated colloidal Ag NPs were characterized using UV-vis spectroscopy (UV-1650 PC, Shimadzu, Kyoto, Japan) in the range of 200 to 800 nm. Microscopic TEM observations of the Ag NPs (in a dry condition) were performed using a TEM (Hitachi H-7100 electron microscopy, Chiyoda, Tokyo, Japan), and the individual particle size and particle size distributions were determined using UTHSCSA ImageTool (version 3.0) software. The TEM samples were prepared by dispersing a few drops of Ag colloid on a carbon film supported by a copper grid. The morphology of the NP samples was also studied using a field emission scanning electron microscope (FE-SEM, S-4700, Hitachi, Tokyo, Japan) operating at 5.0 kV.
Photopyroelectric technique setup
Results and discussion
Preparation of stable Ag nanofluids in different solvents
The spectra obtained in Figure 3 with those in Figure 2 clearly show that the adsorption of EG was higher in comparison to the water solution. In both Figures 2 and 3, the peaks were sharp and symmetrical, signifying a narrow distribution in the size and morphology of the Ag NPs . The high concentration and uniform distribution in the EG solution was attributed to the size of the Ag NPs. Some aggregation was observed, but the metallic Ag NPs prepared in EG were well dispersed and high in concentration, and the EG dispersions were much more stable than those prepared in water under identical MW irradiation conditions.
Comparison between Ag NPs in water and ethylene-glycol as a solvent
Boiling point: The boiling point of the solvent has an important effect on the crystallinity of Ag NPs . The high boiling point of EG (197.3°C) was advantageous for the high crystallinity of the Ag NPs and may have been the reason why EG was the favored solvent for the formation of Ag crystals under MW irradiation. The low boiling point of water did not favor the growth and ripening of crystals (Figure 4).
Molecular weight: In general, liquids with large molecules have high viscosity. In the case of EG, which has a large molecular weight, the long EG chains were easily adsorbed into the surface of the particles, owing to the higher viscosity of EG (1.61 × 10−2 Pa s) compared with that of water (8.94 × 10−4 Pa s) ; this assisted in the stabilization of the particles to achieve a narrow size distribution. This could also lead to the growth of crystals, and larger silver particles were also formed at high concentrations (Figures 5 and 6).
Dielectric loss: The higher dielectric loss of EG (49.950) compared with that of water (9.889) may have been the reason that the reaction proceeded at a much faster rate. The particles grew larger in EG as a result of the continuous nucleation of Ag.
As a result, for both solvents (DW and EG), changes in the MW irradiation time mainly affected the particles' stability, rather than their size. This was probably because longer irradiation times produced a large number of nuclei, leading to the formation of a narrow particle size distribution with a small average size.
Enhancement of thermal diffusivity
Summarized results for thermal diffusivity ratio of Ag nanofluids with varying MW irradiation time for two solvents DW and EG
Thermal diffusivity(cm2/s) × 10−3
Thermal diffusivity ratio (αsample/αbase fluid)
Thermal diffusivity (cm2/s) × 10−3
Thermal diffusivity ratio (αsample/αbase fluid)
1.476 ± 0.022
0.969 ± 0.009
1.532 ± 0.011
1.036 ± 0.011
1.624 ± 0.021
1.101 ± 0.006
1.729 ± 0.032
1.158 ± 0.007
Figure 9 shows the dependence of the thermal diffusivity ratio (αsample/αbase fluid) for the Ag/EG and Ag/ DW nanofluids on the MW irradiation time. Here, when the MW irradiation time was increased, the total number of particles in the solution increased, and the thermal diffusivity ratio (αsample/αbase fluid) of the nanofluids also increased, because of increases in the surface-to-volume ratio of the NPs, which led to decreases in the specific heat of the nanofluid . Similar results have been reported in the literature for the thermal conductivity of Ag nanofluids, as measured using the transient hot-wires technique . However, the enhancement of the thermal diffusivity ratio was smaller for the Ag/DW nanofluids than for the Ag/EG nanofluids. This dependence was attributed to the smaller particle size, the higher concentration, the uniform distribution, and the high surface area of the Ag NPs in the EG solution. These results confirmed that the Ag/EG nanofluids had good stability, and that they could be good candidates for bio-applications, including their use as heat-transfer nanofluids.
Previous studies have shown that the viscosity (not investigated here) and thermal properties of nanofluids increase when the concentration of NPs increases [35, 36]. It is important to note that the methods used for the synthesis and preparation of nanofluids play a major role in improving the thermal properties of nanofluids, while the viscosity of nanofluids is an important transport property for applications in engineering systems. The ideal nanofluid should possess not only good thermal properties, but also low viscosity . Maddah et al. showed that the viscosity increased slightly when the NP concentration was increased, and the enhancement in thermal diffusivity was larger than the enhancement in viscosity for Ag nanofluids . Furthermore, Xie et al. studied ethylene glycol-based nanofluids containing five types of nanoparticles and discovered that nanofluids containing particles with an average diameter of less than 30 nm are appropriate for pumping in a heat exchanger setup . The MW-assisted method therefore provides a promising route for the fabrication of NPs with small diameters, which result in monodispersed nanostructures, and which may in turn improve the thermal characteristics and decrease the viscosity of nanofluids.
Stable and homogeneous Ag nanofluids were prepared in the distilled water and ethylene glycol, and the enhanced thermal diffusivity of these nanofluids was demonstrated. The synthesized Ag colloid was stable even after 6 months, and the average particle size in the different preparations was found to be between 7 and 12 nm. The results for the Ag NP dispersions indicated that the Ag ions interacted with the ethylene glycol much more strongly than with the water. This result, and the narrow size distribution, indicated that the higher dielectric loss, higher boiling point, and higher molecular weight of ethylene glycol (compared with water) may have had an important influence on the degree of crystallinity and growth of the Ag NPs. Ethylene glycol was a good medium for the synthesis, because of the high penetration depth of microwaves that it permitted. The thermal diffusivity of these nanofluids was investigated. The results showed that the thermal diffusivity was enhanced by the presence of the Ag NPs, compared with the results for the pure solvents. The thermal diffusivity ratio was found to increase with increases in the MW irradiation time; this led to increases in the nanoparticle concentration in the solvents. The thermal diffusivity ratio increased inversely with the thermal diffusivity of the nanofluid solvent. The enhancement of the thermal diffusivity ratio, when it was determined experimentally as a function of concentration for the Ag nanofluids, showed behavior similar to that observed in literature studies using different techniques. The PPE technique therefore provides a promising alternative method for the measurement of the thermal diffusivity of nanofluids with high accuracy.
The authors are grateful to the Ministry of Science, Technology and Innovation for supporting this work under the Research University Grant Scheme No. 05-02-12-1878RU. The financial support from the Universiti Kebangsaan Malaysia (UKM) with project code DPP-2014-055 is acknowledged. We also acknowledge the financial assistance of the government of Malaysia.
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