silver bimetallic nanoparticles prepared by conventional and ultrasonic-assisted reduction methods

Advanced Powder Technology 25 (2014) 801–810

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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt

Original Research Paper

Structural, electrical, and rheological properties of palladium/silver bimetallic nanoparticles prepared by conventional and ultrasonic-assisted reduction methods Hossein Azizi-Toupkanloo a, Elaheh K. Goharshadi a,b,⇑, Paul Nancarrow c a b c

Dept. of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran Center of Nano Research, Ferdowsi University of Mashhad, Iran Department of Chemical Engineering, American University of Sharjah, Sharjah, United Arab Emirates

a r t i c l e

i n f o

Article history: Received 12 July 2013 Received in revised form 20 November 2013 Accepted 28 November 2013 Available online 12 December 2013 Keywords: Alloy structure Bimetallic nanoparticles Ultrasonic Rheological behavior Electrical properties

a b s t r a c t Polyvinylpyrrolidone stabilized Pd/Ag bimetallic nanoparticles (NPs) with average particle sizes of 9 and 6 nm were synthesized by simultaneous reduction in the presence and absence of ultrasound waves, respectively. The prepared NPs were characterized by six methods including X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution-TEM (HRTEM), UV–vis spectroscopy, scanning tunneling microscopy (STM), and energy dispersive X-ray (EDX) analysis. The rheological properties of Pd/Ag NPs in ethylene glycol as a base fluid with various mass fractions of NPs from 2% to 5% at different temperatures were studied experimentally and theoretically. The experimental results showed that viscosity of Pd/Ag NPs in ethylene glycol increases with increasing particle mass fraction and decreases with increasing temperature. A maximum of 31.58% increase in viscosity of ethylene glycol at 20 °C was observed when 5% Pd/Ag NPs was added. Measurement of the electrical conductivity of nanofluids of Pd/Ag bimetallic NPs in distilled water at different mass fractions and temperatures was performed. A 3841% increase in electrical conductivity of distilled water at 25 °C was observed when 1% Pd/Ag NPs was added. Both the rheological and electrical properties of Pd/Ag bimetallic NPs were measured in ethylene glycol and distilled water, respectively for the first time. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Application of nanotechnology in several fields is driven by the use of a variety of nanostructures. The field of nanomaterials is a fast-growing area and has gained great attention by scientists and industry manufacturers because of its multi-functionality along with processing properties that can be tailored. In recent years, many studies have been carried out on noble metal nanoparticles (NPs) because they display fascinating properties which are different from the bulk materials [1–5]. Among the metallic NPs, bimetallic nanoclusters, in particular, have been demonstrated to be the most attractive ones for a variety of applications [6–9] with several important advantages over the monometallic ones. From not only the scientific but the technological point of view, bimetallic NPs have been investigated in many fields of science and industry. By combining two kinds of metals, some changes such

⇑ Corresponding author at: Dept. of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran. E-mail address: [email protected] (E.K. Goharshadi).

as improving the catalytic quality [10], changing the surface plasmon band [11], and regulating the magnetic properties [12] of the parent metals may occur. Several possible types of structures for bimetallic nanoclusters including core–shell [13–15], random alloy [16,17], crown-jewel [18,19], hollow structure [20,21], and dendritic structures [22] have been reported. In any of these cases, there is direct interaction between the metals. Bimetallic NPs may have unique features including (1) physical and chemical interactions among different atoms, (2) altered miscibility and interactions unique to nanometer dimensions, and (3) morphological variations [23]. Bimetallic palladium-based NPs show unique catalytic activity for hydrogenation of organic compounds. Singh et al. [24] used the Pd/Ni bimetallic NPs as a catalyst for hydrogen generation from decomposition of hydrous hydrazine. They found that the Pd/Ni bimetallic NPs compared with Pd or Ni monometallic NPs have higher H2 selectivity in the reaction. Xiuli and coworkers [25] used Pd/Pt NPs as a catalyst in the hydrogenation of phenyl aldehydes. They also realized that Pd atoms in Pd/Pt NPs promote the activity of the catalytic hydrogenation mainly through geometric and

0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.11.015

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electronic effects. Pd/Au bimetallic NPs were used as a catalyst in the hydrogenation of imidazolium based ionic liquids [26]. Recently, Smuleac et al. [27] synthesized Fe/Pd bimetallic NPs by a non-toxic green reducing agent, green tea extract, instead of the well-known sodium borohydride. They used the synthesized NPs for reductive degradation of chlorinated organic compounds. Among the various kinds of Pd-based bimetallic NPs, Pd/Ag NPs show special properties. These NPs show catalytic activity for hydrogenation of cis,cis-1,3-cyclo octadiene and methyl acrylate [10,28], electrocatalytic reduction of benzyl chloride [29], electroless copper deposition [30], and catalytic reduction of N2O [31]. A number of methods have been used to prepare the Pd/Ag bimetallic NPs including solvothermal method [32,33], UV irradiation [34,35], heterogeneous reaction [36], reduction of silver ions on the surface of palladium particles [30], reverses micelles [37], laser irradiation [38], c-irradiation [28], microwave-polyol processes [39], and galvanic displacement reaction [40,41]. In addition to the above methods, colloidal bimetallic NPs have been prepared by sonochemical synthetic routes. Suslick and coworkers [42,43] were the first to demonstrate the exploitation of ultrasound to produce bimetallic NPs. They prepared Fe–Co alloys via the sonochemical decomposition of iron pentacarbonyl and cobalt tricarbonylnitrosyl. Mizukoshi and coworkers [44] reported a sonochemical synthesis of core–shell structured Pd/Au NPs in aqueous solutions. Bimetallic Pd/Cu NPs have been prepared by ultrasonic irradiation of Pd and Cu nitrate precursors using ethylene glycol (EG) and polyvinypyrrolidone (PVP) as reducing and stabilization agents, respectively by Nemamcha et al. [45]. Recently, Godínez-García and coworkers [46] synthesized Pd/Ag bimetallic NPs by high intensity ultrasound waves method. They used a mixture of 50% EG and 50% distilled water (DW) as a solvent. The first aim of the present work is to prepare Pd/Ag bimetallic NPs from aqueous solutions of silver nitrate and palladium nitrate dihydrate by simultaneous reduction method in the presence and absence of ultrasound (US) waves. The sodium borohydride (NaBH4) and PVP were used as reducing and stabilizer agents, respectively. The second goal is to prepare the nanofluids of Pd/Ag NPs in DW and EG. Many investigators have studied the various characteristics of fluid flow and heat transfer behavior of nanofluids [47–51]. The viscosity of nanofluids is critical to product performance in many industrial applications and process efficiency. For example, the viscosity of nanofluids is essential for establishing an adequate pumping power as well as the convective heat transfer coefficient. In the actual thermal measurement, the nanofluids are expected to be used under the flow conditions; hence, the rheological properties of nanofluids would affect their heat transfer performance [52,53]. The third goal is the measurement of the rheological properties of Pd/Ag NPs-EG nanofluids at different temperatures and mass fractions. Although the thermal conductivity and viscosity of the nanofluids have been measured widely [41–45], very few studies concerning the electrical conductivity of nanofluids have been done. The electrical conductivity of the nanofluids may give information on the state of dispersion and stability. In this context, the present work was undertaken to explore the electrical transport property of Pd/Ag bimetallic NPs in DW. Towards this purpose, the forth goal of this work, the effect of particle mass fraction and temperature on the electrical conductivity of the Pd/Ag bimetallic NPs-DW nanofluids was analyzed and presented. To the best of our knowledge, this research is the first work which reports the electrical conductivity of suspensions of Pd/Ag bimetallic NPs in DW and the rheological behavior of Pd/Ag-EG nanofluid as functions of both temperature and mass fraction of NPs.

2. Experimental section 2.1. Materials The starting materials used in this work were silver nitrate (AgNO3), palladium nitrate dihydrate (Pd(NO3)22H2O), NaBH4, and PVP (Mw = 2000 g/mol). All chemicals were used as received without further purification. All the solutions were prepared by DW. 2.2. Synthesis procedure A typical synthesis of the Pd/Ag bimetallic NPs was carried out as follows. The total concentration of metal ions was kept constant 5  105 M. In brief, 5  104 mol AgNO3, 5  104 mol Pd(NO3)22H2O, and 0.02 g PVP were dissolved in 100 ml DW under vigorous stirring. PVP protects the NPs from agglomerating. Then, 1.6  103 mol NaBH4 was added. The reaction mixture was sonicated for 60 min with a sonicator (Sonicator-4000) operating at 20 kHz. The power transferred to the solution was 33 W cm2 measured by means of calorimetric method. The reaction temperature was controlled to 30 ± 1 °C with the help of condensation water surrounding the reactor cell. During the reduction reaction, the solution color changed from colorless to black with dark colloidal particles. The fresh samples were centrifuged, washed with DW three times, and dried in the vacuum oven overnight. A similar procedure was done for the Pd and Ag NPs. Control experiments were also carried out in the absence of US waves. The NPs were dispersed in DW and EG to prepare the nanofluids using intensive ultrasonic vibration for 15 min. The stability of a suspension is defined in terms of the change in one or more physical or physicochemical properties over given time period. The nanofluids in DW were fairly stable for a couple of days without visually observable sedimentation. The nanofluids in EG were found to be very stable for several months. 2.3. Characterization techniques The powder phases were determined by means of a Bruker/D8 Advanced diffractometer in the 2h ranging from 20° to 80° by step of 0.04° with graphite monochromatic Cu Ka radiation (k = 1.541 Å). The TEM analyses of the samples were obtained using a LEO 912 AB instrument. The electron beam accelerating voltage was 120 kV. The UV–vis absorbance spectra were obtained for the samples using an Agilent photodiode-array Model 8453 was equipped with glass of 1 cm path length. The spectra were recorded at room temperature in air within the range of 200– 800 nm. The HRTEM analyses were carried out using FEIT ecnai F20 Field emission by the use of an accelerating voltage of 200 kV. The STM images (200  200 nm and 30  30 nm) were provided by STM SS1 with Pt/Ir STM tip, 1.7 nA constant current, and 0.23 V voltage. The EDX analysis was carried out using the type Inca 400 (Oxford Instruments). The electrical conductivity of suspensions of Ag NPs, Pd NPs, and Pd/Ag bimetallic NPs in DW was measured using an EDT instrument BA 380. The viscosity of the nanofluids of Pd/Ag NPs in EG was measured using a Brookfield Viscometer (LV DV-II + Pro EXTRA) with a small sample adaptor. The water jacket was connected to a circulating cooling water bath (BL 7100, Major Science) to control the water temperature. The repeatability of viscometer is ±0.2%. The zeta potential of the Pd/Ag bimetallic NPs NPs in DW was measured using the Zetasizer from Malvern instrument.

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3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD patterns of Pd/Ag NPs in the presence and absence of US waves. In this figure, four diffraction peaks correspond to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of the facecentered cubic (fcc) phase of Pd/Ag NPs. The sharpness of Pd/Ag NPs peaks in the presence of US waves is greater than those in the absence of US waves. Hence, the crystallinity and size of the NPs increased when they were prepared in the presence of US waves. The shock waves and turbulent flows, resulting from numerous microbubble collapses, drive NPs together at velocities of hundreds of meters per second for inelastic impact at the point of meeting, resulting in formation of NPs with changed morphology and crystallinity. Interfacial instabilities of cavitation bubbles, such as capillary surface waves and microjet formation, are thought to nebulize liquid droplets into the hot core of a collapsing bubble for subsequent thermolysis and fusion of metal NPs [54]. That is why the size and crystallinity of the NPs prepared in the presence of US waves are greater. Suslick has shown ultrasonic irradiation of aqueous liquids generates highly reactive free radicals including H and OH and the formation of free radicals by sonolysis of water [55]. These radicals can act as strong oxidants and reductants and utilized for various sonochemical reactions in aqueous solutions. The radical, H , reduces metal ions in bulk solution to form the zero valent metal. Fig. 2 shows the XRD patterns of Ag NPs, Pd NPs, the physical mixture of the corresponding NPs, and Pd/Ag bimetallic NPs in the presence of US waves. The XRD pattern shows Pd and Ag NPs crystallizes in a fcc structure which are similar to those of the bulk metallic Pd and Ag [56]. A comparison between the lattice constants of Pd NPs (a = 3.92 Å), Ag NPs (a = 4.07 Å), and Pd/Ag NPs (a = 4.01 Å) reflects the Pd/Ag bimetallic NPs have alloy structure (Table 1). The physical mixture of Ag and Pd NPs exhibit the characteristic peaks of Ag and Pd without any shift. In contrast, the diffraction peaks of Pd/Ag bimetallic NPs appear to be between those of pure monometallic Ag and Pd NPs. The diffraction angles of (1 1 1) Pd/Ag bimetallic NPs (38.84°) is located between the diffraction angles of (1 1 1) Ag NPs (38.22°) and the diffraction angles of (1 1 1) Pd (39.97°). In addition, the characteristic peaks of metallic Pd and Ag NPs did not appear in the XRD pattern of Pd/Ag bimetallic NPs, suggesting that the prepared bimetallic NPs are composed of a Pd/Ag bimetallic phase rather than a mixture of monometallic Pd and Ag NPs.

Fig. 1. XRD patterns of Pd/Ag bimetallic NPs (a) with US waves and (b) without US waves.

Fig. 2. XRD patterns in the presence of US waves for (a) Pd NPs, (b) Pd/Ag bimetallic NPs, (c) Pd/Ag physical mixture and (d) Ag NPs.

Table 1 The crystallite size and lattice constant of Ag [61], Pd [72], and Pd/Ag bimetallic NPs. Samples

D111 (nm) US

Bulk Aga Bulk Pdb Ag NPs Pd NPs Pd/Ag NPs a b

18.4 4.8 4.5

a111 (Å) Without US

15.4 3.5 4.2

4.09 3.89 4.07 3.92 4.01

JCPDC database CAS #7440-22-4. JCPDC database CAS #7440-05-3.

The XRD peaks are strong and sharp and no additional peaks were observed, which shows the high purity of the prepared Pd/ Ag bimetallic NPs. The average crystallite size, D, can be calculated using the well-known Scherrer formula:

Dhkl ¼

kk bhkl  cos hhkl

ð1Þ

where Dhkl is the crystallite size of the NPs perpendicular to the normal line of (hkl) plane, k is a constant (0.9), bhkl is the full width at half maximum of the (hkl) diffraction peak, hhkl is the Bragg angle of (hkl) peak, and k is the wavelength of X-ray. The peak position and the FWHM were obtained by fitting the measured peaks with two Gaussian curves in order to find the true peak position and width corresponding to monochromatic Cu Ka radiation. The average crystallite size and the lattice constant of NPs are given in Table 1. The effect of US waves on the size of NPs is negligible since NaBH4 is a strong reducing agent. The average crystallite sizes of Pd/Ag bimetallic NPs in the presence and absence of US waves were 4.5 and 4.2 nm, respectively. The TEM images and the particle size distributions (PSD) of Pd/Ag bimetallic NPs prepared in the presence and absence of US waves are presented in Fig. 3. We used ‘‘Digimizer Software 4.1.1.0’’ for PSD analysis. To produce the PSD graph, we selected the de-agglomerated particles randomly. The images show that the prepared Pd/Ag bimetallic NPs are nearly monodisperse. The average particle size of Pd/Ag NPs prepared in the presence of US waves is greater than that of the absence of US waves. It is thought that the US waves accelerate effectively the mass transfer and the NPs gain an increased opportunity to collide each other and promote their sizes. The results are in a reasonable agreement with the results obtained from the XRD pattern as shown in Table 1. The HRTEM image of Pd/Ag NPs shows obvious fringes (Fig. 4). The lattice fringes of 0.233 nm is close to the (1 1 1) lattice spacing

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The EDX analysis was performed to determine the chemical composition of samples (Fig. 5). The EDX profile provides a direct proof for the formation of Pd/Ag alloy with high purity. The Pd:Ag atomic ratio of the samples were 57:43 and 53:47 for the NPs prepared by US and without US waves, respectively. The observed carbon peak in the EDX spectrum is related to the carbon coated of the samples. Fig. 6 shows the STM images of Pd/Ag NPs. The average particle size and height were around 9.3–14 nm and 5 nm, respectively. As shown in Fig. 7, the Pd NPs show a broad absorption peak in the UV–vis region because of d–d interband transitions. The Ag NPs show strong absorbance around 400 nm which is attributed to the surface plasmon excitation of silver nanospheres indicating the formation of silver NPs [59,60]. It is obvious that the physical mixture of Ag and Pd NPs exhibits simple addition of two individual spectra of Ag and Pd which has a specific absorption peak near 400 nm. On the other hand, the Pd/Ag bimetallic NPs show no prominent absorption peak. Similar results were reported by others [28,32,33,61]. 3.2. Rheological properties of Pd/Ag -EG nanofluids

Fig. 3. TEM images and the PSD of Pd/Ag bimetallic NPs prepared (a) in the presence of US waves and (b) without US.

of the fcc Pd (about 0.224 nm) and Ag (about 0.235 nm) indicating Pd/Ag NPs have an alloy structure. In contrast to XRD, we cannot make a distinction between Pd and Ag by HRTEM measurement since our prepared bimetallic NPs have alloy structure. Notice that only core–shell NPs have usually distinct HRTEM [57,58]. It mean that in HRTEM image of core–shell structures one metal forms shell and the other metal forms core. The polycrystalline nature of the bimetallic NPs can also be seen in the HRTEM image since various domains can be indentified with different crystallographic orientations positioned around a pore.

Fig. 8 shows the viscosity, g, of pure EG as a function of shear rate, c°, at different temperatures. The viscosity of EG is independent of shear rate in the range of 20–50 °C. Hence, EG acts as a Newtonian fluid and its viscosity depends strongly on temperature. The results show that with increase of 10 °C in temperature, the viscosity of EG decreases by about 30%. For example, the viscosity of EG at 30 °C and 40 °C is 12.38 cP and 8.85 cP, respectively. As temperature increases, the intermolecular interactions between the molecules weaken and therefore the viscosity decreases. The viscosity of Pd/Ag-EG nanofluids for different mass fractions, fm, (between 0.02 and 0.05) and various temperatures (20– 50 °C) was measured and the results for two mass fractions are shown in Fig. 9. The Newtonian behavior of EG as a base fluid is mostly changed to non-Newtonian for the nanofluids because of the new interactions between the EG and Pd/Ag NPs. The nonNewtonian behavior becomes prominent at lower temperatures, lower shear rates, and higher mass fractions [52]. At low shear rates (0–50 s1), the viscosity of the nanofluids decreases with increasing shear rate (Fig. 9(a) and (c)). i.e. the nanofluids behave as non-Newtonian fluid. The decrease in viscosity is due to alignment of suspended NPs in the direction of flow. Fig. 9(b) and (d) shows that the nanofluids are Newtonian at higher shear rates (60–190 s1). At high shear rates, the viscosity of nanofluid attains a limiting constant value. At high shear rates, the intermolecular interactions weaken or even break and hence the nanofluid acts as a Newtonian fluid.

Fig. 4. HRTEM images of Pd/Ag NPs prepared in the presence of US waves with two magnifications.

H. Azizi-Toupkanloo et al. / Advanced Powder Technology 25 (2014) 801–810

805

Fig. 5. The energy spectrum analysis of the Pd/Ag NPs prepared (a) with and (b) without US.

The viscosity of EG by loading of 2% Pd/Ag NPs at 30 °C and 40 °C under constant shear rate of 45.18 s1 (163.68 s1) is 13.43 cP (12.49 cP) and 10.10 cP (9.04 cP), respectively. Hence, the viscosity of the nanofluid (2%) at 40 °C for shear rate 45.18 s1 (163.68 s1) is 1.3 (1.02) times greater than that of the EG at 40 °C. Similarly, the viscosity of EG by loading of 5% of Pd/ Ag NPs at 30 °C and 40 °C under constant shear rate of 45.18 s1(163.68 s1) is 15.28 cP (14.42 cP) and 11.16 cP (10.70 cP), respectively. The viscosity of the nanofluid (5%) at 40 °C for shear rate 45.18 s1 (163.68 s1) is 1.3 (1.2) times greater than that of the EG at 40 °C. This is an advantage for EG-based materials like aerosol paint concentrates, agricultural chemicals, antifreeze preparations, automobile body polish and cleaners, disinfectants, glass window cleaning preparations, lubricating oils; i.e. the decrease in the viscosity with increasing temperature is compensated by loading the Pd/Ag NPs. Also, this figure shows the viscosity of nanofluids with higher mass fraction of NPs is greater than that of lower mass fraction keeping the temperature and shear rate constant. The maximum enhancement in viscosity (ME) described by the following equation:

ME ¼

gnFðmaxÞ  gF  100 gF

ð2Þ

where (gnF(max) is the maximum viscosity of nanofluid and gF is that of the EG at the same temperature) of the Pd/Ag-EG nanofluids at

different temperatures and mass fractions under constant shear rate were summarized in Table 2. At 20 °C (50 °C) and shear rate of 57.88 s1, by increasing loading of the NPs from 2% to 5%, the viscosity enhances from 6.65% (3.17%) to 31.58% (13.67%). Hence, the magnitude of enhancement of viscosity because of increase of mass fraction of NPs become less important at higher temperatures. Similar trend is observed for all shear rates. The plots of shear stress, s, versus shear rate for both EG and the nanofluids at 40 °C are shown at Fig. 10. The nanofluids present Bingham plastic behavior, i. e. they have a linear shear stress/shear rate relationship which require a finite yield stress before they begin to flow (the plot of shear stress against shear rate does not pass through the origin). The temperature dependence of the values of viscosity of the base fluid as well as the nanofluids was shown to fit well with the Vogel–Fulcher–Tammann (VFT) equation [50]:

gðTÞ ¼ A exp



B T þ T0

 ð3Þ

where g is the shear viscosity, T is the temperature, and A, B, and To are constants given in Table 3. A is the value of g at the infinite temperature. B corresponds to the energy barrier associated with the so-called ‘cage’ confinement due to the close packing of liquid molecules, implying any structural rearrangement of liquid molecules would need to overcome the energy barrier. The experimental data at a constant shear rate were fitted by Eq. (3) and the results are

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Fig. 6. (a) STM images of Pd/Ag NPs prepared in the presence of US waves for the area of 500 nm  500 nm. (b) Height profile along the white line.

20 T= 20°C 25 30 35 40 45 50

18

Viscosity (cP)

16 14 12 10 8 6 Fig. 7. UV–vis spectra (a) Pd/Ag physical mixture, (b) Pd NPs [72], (c) Ag NPs [61] and (d) Pd/Ag bimetallic NPs.

shown in Fig. 11. The fitting parameters and the correlation coefficient between the fitted data and the measurements are summarized in Table 3. The experimental data display a very good fit to the VFT equation. Yaws [62] proposed the following equation for the viscosity:

logðgnf Þ ¼ A þ BT 1 þ CT þ DT 2

ð4Þ

0

50

100

150

200

250

Shear rate (s-1) Fig. 8. Viscosity of EG as a function of shear rate at different temperatures.

For determining the stability of the prepared nanofluids in EG, the zeta potential was measured and found to be 43.8 mV. Nanofluids with zeta potentials more positive than +30 mV or more negative than 30 mV are normally considered stable [64,65].

where gnf is the viscosity of nanofluid and A, B, C, and D are fitting parameters. Reid and Sherwood [63] suggested the equation:

3.3. Electrical conductivity measurement

gnf ¼ A expðB=TÞ

The electrical conductivity of suspensions of Pd/Ag NPs in DW in different mass fraction, fm was measured for various temperatures (Fig. 12). The electrical conductivity of DW at the temperature range of the experiment (25–50 °C) varies from 1.2 lS cm1 to 3.3 lS cm1. The results show that for every 10 °C increase in temperature, the electrical conductivity of DW increases by a

ð5Þ

where A and B are fitting parameters. The curve fitting parameters of Eqs. (4) and (5) were listed in Table 4. By comparison of the correlation coefficients of Eqs. (3)–(5), we found that the experimental data were fitted very well by Eq. (3).

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22

22

(a)

Viscosity (cP)

18 16 14 12

20

(b)

18 16

Viscosity (cP)

T= 20 °C 25 30 35 40 45 50

20

14 12 10

10 8 8 6 6 4 20

30

40

50

60

60

Shear rate (s-1)

100

120

140

160

180

Shear rate (s-1)

25

(c)

(d)

20

Viscosity (cP)

Viscosity (cP)

25

80

15

20

15

10

10

5

5 20

30

40

50

60

60

Shear rate (s-1)

80

100

120

140

160

180

Shear rate (s-1)

Fig. 9. Viscosity of (a) 2% Pd/Ag NPs – EG nanofluids for shear rates 0–60 s1 at different temperatures (b) same as (a) for 60–190 s1(c) 5% Pd/Ag NPs – EG nanofluids for shear rates 0–60 s1 at different temperatures (d) same as (c) for 60–190 s1.

Table 2 The maximum enhancement in viscosity of the Pd/Ag-EG nanofluidsat different temperatures and mass fractions under constant shear rate.

57.88

112.87

180.57

T (°C)

Enhancement in viscosity (%) fm = 0.02

0.03

0.04

0.05

20 25 30 35 40 45 50

6.65 3.57 5.69 2.41 11.62 7.71 3.17

17.97 5.18 5.91 12.61 12.80 17.39 8.58

25.38 9.18 16.06 18.50 19.05 22.54 10.04

31.58 17.91 19.89 21.33 25.86 25.51 13.67

20 25 30 35 40 45 50

6.10 1.67 5.12 3.72 7.39 4.98 0.68

14.89 2.33 5.56 6.68 8.01 12.00 6.29

24.04 7.38 15.45 16.61 14.31 19.23 7.81

28.02 17.11 21.52 19.24 23.85 24.33 15.63

20 25 30 35 40 45 50

6.43 2.27 1.90 1.32 6.27 4.57 1.26

14.57 2.49 3.14 6.18 8.51 11.49 7.79

23.89 7.61 14.78 17.30 16.28 19.74 9.37

26.97 16.84 19.11 19.18 24.80 26.03 17.34

25

EG fm = 0.02 0.05

20

τ (dynes/cm2)

Shear rate (s1)

factor of approximately 1.5. It is of interest to examine the enhancement in electrical conductivity of the Pd/Ag bimetallic nanofluid with respect to DW. The electrical conductivity of DW

15

10

5

0

20

40

60

80

100

120

140

160

180

γ o (s-1) Fig. 10. Shear stress versus shear rate for EG and Pd/Ag-EG nanofluids for different mass fractions at 40 °C.

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Table 3 The empirical constants of Eq. (3) and the correlation coefficients between the equation and the measured values of viscosity of nanofluids at different mass fractions, fm, and constant shear rate (108.6 s1). A

B

R2

0 2 3 4 5

4.0609 4.0739 4.0976 4.3809 4.6763

31.0867 32.0398 33.1053 33.1269 32.6993

0.97 0.98 0.99 0.98 0.99

70

) uctivity (µs/cm Electrical cond

fm

60 50 40 30

Viscosity (cP)

20

55

on

cti

0 50

0.02 0.03 0.04 0.05

15

45

40

Temp

35

eratu

10

1.2 1.0 0.8 0.6 0.4 0.2 0.0

10

sf ra

0.02 0.03 0.04 0.05 fm = 0.00 (VFT equation)

as

fm = 0.00

20

M

25

30

re (°C

)

25

20

Fig. 12. Electrical conductivity enhancement of suspension of Pd/Ag NPs in DW at different mass fractions and temperatures.

20

30

40

50

The enhancement in electrical conductivity of DW by dispersing the NPs is a result of net charge effect on the surface of NPs and relevant electrical double layer (EDL) interactions [69,70]. In addition, the presence of uniformly dispersed NPs increases electrophoretic mobility which consequently increases the electrical conductivity of the DW. By increasing mass fraction of the NPs, the availability of conductivity pathways increases in the solution which in turn increases the overall electrical conductivity of solution (Fig. 12). With an increase in temperature, EDL thickness decreases leading to increase electrical conductivity of the solution. Also, increasing the temperature lowers the viscosity of the suspension which increases the electrophoretic mobility which increases the conductivity of the solution [71]. Fig. 13 shows the electrical conductivity of suspensions of Ag NPs [61], Pd NPs [72], and Pd/Ag NPs in DW for fm = 0.001 at different temperatures. The suspensions of the Pd NPs in DW show the maximum increase of electrical conductivity. This maximum increase is due to the small size of Pd NPs. The average particle sizes of Ag NPs, Pd NPs, and Pd/Ag NPs were 18.4, 4.8, and 9.0 nm, respectively. By decreasing the particle size, the surface charge increases and hence the electrical conductivity can effectively increase. To gain additional knowledge on the electrical conductivity of nanofluids, the experimental values of electrical conductivity of Pd/Ag-DW nanofluids as a function of mass fraction were fitted by an exponential equation:

Temperature (°C) Fig. 11. Viscosity of Pd/Ag NPs-EG nanofluids as a function of temperature at a constant shear rate (108.6 s1), the symbols and lines are the measured and calculated values using Eq. (3), respectively.

is enhanced by adding NPs and increasing temperature. The highest value of electrical conductivity, 61.7 lS cm1, was obtained for a mass fraction of 0.01 of NPs at 50 °C. The electrical conductivity of this nanofluid was 47.3 lS cm1 at 25 °C .The similar results were also observed for silver NPs suspended in DW [61]. The temperature dependence of electrical conductivity of the nanofluids is much less than that of the mass fraction of NPs. The percentage enhancement of the electrical conductivity, defined as the difference between the electrical conductivity of the nanofluid and that of DW divided by the electrical conductivity of DW, was calculated. The maximum value of the percentage enhancement was 3841% for the nanofluid containing 1% of Pd/Ag NPs at 25 °C. The minimum percentage enhancement was 775% for mass fraction of 0.1% of Pd/Ag NPs at 25 °C. The zeta potential for the Pd/Ag-DW bimetallic nanofluid at different pH values of 1.8, 5.5 (used nanofluid for electrical properties measurement), and 9.5 was measured. The zeta potential of these colloids was 3.07, 28.8, and 34.4 mV, respectively. Hence, the nanofluid at pH = 9.5 is the most stable and the nanofluid used for electrical conductivity measurement is fairly stable [66–68].

r ¼ c þ að1  expðbfm ÞÞ

ð6Þ

Table 4 The empirical constants of Eqs. (4) and (5) and the correlation coefficients between the equations and measured values of viscosity of nanofluids at different mass fractions and constant shear rate (108.6 s1). fm

0 2 3 4 5

Eq. (4)

Eq. (5)

A

B

C

D

R2

A

B

R2

1.344 0.471 0.4382 0.8086 0.4523

3.4153 12.8232 23.2016 26.5379 23.3579

0.0126 0.014 0.0395 0.0546 0.0432

0.0000 0.0002 0.0005 0.0006 0.0005

1.00 1.00 1.00 0.99 1.00

4.0609 4.0739 4.0976 4.3809 4.6763

31.0867 32.0398 33.1053 33.1269 32.6993

0.97 0.98 0.99 0.98 0.99

Electrical conductivity (µs/cm)

H. Azizi-Toupkanloo et al. / Advanced Powder Technology 25 (2014) 801–810

22

4. Conclusions

20

Pd/Ag NPs were prepared by both conventional and ultrasonic assisted reduction method. The NPs were characterized by TEM, UV–vis, XRD, HRTEM, STM, and EDX. The rheological properties and electrical conductivity of the Pd/Ag NPs in EG and DW, respectively were measured. The rheological and electrical properties of a nanofluid usually depend on different parameters including shape, size, and morphology of NPs. According to our results, these factors do not differ significantly for the NPs prepared by two methods. Hence, the viscosity and electrical conductivity of nanofluids containing the NPs synthesized in the presence and absence of US waves were not compared. This study contains the following main points:

18 16 14 12 10 8 20

30

40

50

Temperature (°C) Fig. 13. Electrical conductivity of suspensions of Ag NPs, Pd NPs, and Pd/Ag NPs in DW for fm = 0.1% as a function of temperature.

where a, b, and c are the parameters of fitting. Table 5 represents the parameters and the correlation coefficient, R2 at different temperatures. The parameters are functions of temperature. Eqs. (7)– (9) show the temperature dependence of these parameters:

a ¼ a0 þ b expðcTÞ

ð7Þ

c ¼ c0 þ g expðhTÞ

ð8Þ

b ¼ b0  eT

ð9Þ

The values of the constants of Eqs. (7)–(9) are given in Table 6. The correlation coefficients for of Eqs. (7)–(9) are 0.9978, 0.9984, and 0.9926, respectively. The ability of the above equations to predict the electrical conductivity of Pd/Ag-DW nanofluidsat different mass fractions and temperatures was evaluated by the absolute average deviation (AAD) which is defined as follows:

AAD ¼

809

  N rexp  rcal  1X  100 N i¼1 rexp 

 The HRTEM images confirmed the formation of Pd/Ag alloy NPs where the lattice fringe spacing of 0.233 nm locates between the (1 1 1) plane of metallic Pd and Ag.  The highly and fairly stable suspensions of Pd/Ag NPs in EG and DW were prepared, respectively.  Although EG behaves as a Newtonian fluid, the nanofluids of Pd/Ag NPs in EG show non-Newtonian behavior. The non-Newtonian behavior becomes prominent at lower temperatures, lower shear rates, and higher mass fractions. The viscosity of Pd/Ag NPs in EG increases with increasing the particle mass fraction and decreases with increasing temperature. A maximum of 31.58% increase in the viscosity of EG at 20 °C was observed when 5% Pd/ Ag NPs was added.  A considerable enhancement in electrical conductivity with both increase in mass fraction and temperature was observed. A 3841% increase in electrical conductivity of DW at 25 °C was observed when 1% Pd/Ag NPs was added.

Acknowledgments The authors express their gratitude to Ferdowsi University of Mashhad for support of this Project (Grant no. B/437). References

ð10Þ

where rexp and rcal represent the experimental and predicted electrical conductivity, respectively. The AAD was 0.75%. Hence, our experimental and predicted values for the electrical conductivity are in a good agreement with each other.

Table 5 The parameters of Eq. (6). Temperature (°C)

c (lS cm1)

a (lS cm1)

b (lS cm1)

R2

25 30 35 40 45 50

1.1510 1.6493 2.0781 2.5394 2.9888 3.6056

54.8631 56.8214 61.2790 65.1492 69.7407 75.5665

1.8506 1.8394 1.7036 1.6324 1.5725 1.4606

0.9998 0.9999 0.9963 0.9998 0.9998 0.9998

Table 6 The fitting parameters of Eqs. (7)–(9).

a0

b

c

c0

g

h

b0

e

38.6020

6.8949

0.0336

6.7332

6.0752

0.0106

2.2352

0.0151

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