paper published on journal of nanoscience and nanotechnology … · 2015-02-19 · paper published...

Paper published on Journal of Nanoscience and Nanotechnology Vol. 15, pages 3568–3573 (2015); DOI:10.1166/jnn.2015.9838

http://www.ingentaconnect.com/content/asp/jnn/2015/00000015/00000005/art00032

Optical properties of mixed nanofluids containing carbon nanohorns and silver

nanoparticles for solar energy applications

E. Sani1*, P. Di Ninni2, L. Colla3, S. Barison4, F. Agresti4

1CNR-INO National Institute of Optics, Largo E. Fermi, 6, 50125 Firenze, Italy

2Physics and Astronomy Department, University of Firenze, via P. Sansone 6, 50125 Sesto Fiorentino, Italy

3CNR-ITC Construction Technologies Institute, Corso Stati Uniti, 4, 35127 Padova, Italy

4CNR-IENI Institute for Energetics and Interphases, Corso Stati Uniti, 4, 35127

Padova, Italy

*corresponding author. Tel: +39 055 23081 Fax: +39 055 2337755. E-mail:

[email protected]

Abstract: Different kinds of nanofluids show peculiar characteristics. In this work, a mixed nanofluid consisting of single-wall carbon nanohorns and silver nanoparticles aqueous suspensions is prepared and optically characterized, in the perspective to merge the favorable optical characteristics of carbon nanohorn-based nanofluids to the good thermal properties of silver-nanofluids. For the samples, both the spectral extinction and the scattering albedo at discrete wavelengths have been investigated. The silver nanoparticle plasmonic peak in the visible range further improves the overall nanofluid sunlight absorption properties, opening interesting perspectives for using such mixed nanofluids as solar absorber and heat transfer media in solar thermal collectors. Date of submission: 28 October 2013 Date of acceptance: 17 March 2014

Keywords: Solar absorbers; Carbon nanohorns; Silver nanoparticles; Carbon nanostructures; Concentrating solar power

Paper published on: Journal of Nanoscience and Nanotechnology Vol. 15, pages 3568–3573 (2015), DOI:10.1166/jnn.2015.9838

1. Introduction

The “energy problem”, i.e. supplying with energy the growing world population,

while fossil fuel reserves are declining and global climate changes force mankind to imagine a low-carbon sustainable future, is one the main challenges of the present century. In this framework, renewable energies and, in particular, solar energy exploitation are receiving a considerable and growing interest. As for thermal and thermodynamic sunlight exploitation approach, different concepts for solar receivers have been developed, with peculiar architectures and temperature working ranges [1], from low-medium temperature systems such as linear parabolic collectors to high temperature solar tower plants. In all cases, the key for a wide diffusion of solar energy technologies is the efficiency rise, which entails a corresponding decrease of the cost-per-watt of produced power.

The interest in so-called “nanofluids” (a conventional name indicating fluids with suspended nanometer-sized particles) has dramatically increased in the last years, due to the fact that many of their properties result advantageous with respect to those of the pure base fluid. In particular, among the physical properties, those which usually are evaluated in view of practical applications are thermal conductivity [2-8] and pool boiling heat transfer [9-11]. Different kinds of nanoparticles have been investigated for this purpose, including metallic [4], semiconducting [12] and insulating ones [3, 6, 7]. However, as nanofluids appear promising for thermal solar energy applications, the investigation of their optical properties is needed, in particular when a direct interaction with sunlight is foreseen like the use as direct absorbers. Fluids containing black particles have already been studied in the past for applications in solar thermal collectors, because the use of a black fluid working both as volumetric light absorber and heat exchanger [13- 16] is advantageous over the classical solution of a transparent fluid exchanging heat with a solid absorber (typically, a black-painted or oxidized surface in tight thermal contact with the tubes) [17,18].

Single-wall carbon nanohorns (NHs) have been discovered in 1999 [19] and have raised expectations in many fields. In addition, their proved non-cytotoxicity [20] makes them appealing for applications requiring nanoparticle handling and in all cases where even accidental leakages into the environment are possible. Recently we characterized the optical properties of NH suspensions for solar collector applications [21, 22, 23]. We found the light extinction properties of such carbon nanostructure favorably compare to those of amorphous carbon [24]. Silver-nanoparticle based nanofluids have been studied mainly as thermal exchange media [25, 26], showing in all cases an improvement of the thermal properties with respect to the base fluid. Authors in [27] obtained an increase of the thermal conductivity from 27% to 80% for volume concentrations in the range 0.3-0.9%. An about 80% decrease of the thermal resistance has been observed in [28] for a heat pipe filled with aqueous nanofluids with Ag concentrations of 0.1 g/l. Similarly low Ag concentrations have been studied in [29], reporting an about 5% increase of the thermal conductivity at 60°C. Composites have been created as well, with Ag nanoparticles (AgNPs) decorating the surfaces of multi-wall carbon nanotubes, and the addition of silver has been proved to be advantageous for the thermal conductivity enhancement of the resulting nanofluid


over simple carbon-nanotube nanofluids [30]. As for optical properties, silver nanoparticles are known to show a light absorption peak at blue wavelengths due to the surface plasmon resonance, with spectral characteristics depending on nanoparticle shape, size and clustering [31]. In last years, due to the growing interest in direct absorption solar collectors, different metal nanoparticles have been investigated in nanofluids for solar applications both as heat transfer medium [32] and direct solar absorber [33, 39, 40]. However, to the best of Authors’ knowledge, the literature does not show works on the optical characterization of mixed nanofluids containing both metal particles and carbon nanohorns as individual particles (not composites). Thus in this paper, we report on the preparation and characterization of mixed NH- AgNP aqueous suspensions, in the perspective to use them as direct sunlight absorbing nanofluids for solar energy applications, taking advantage of their well established heat transfer characteristics. We measured the NH and AgNP mean diameter in water and the suspension stability, correlating them to the spectrally resolved extinction measurements performed on aqueous suspensions. We measured the scattering albedo at two discrete wavelengths and we found an agreement with the Rayleigh scattering regime. This allowed us to estimate the spectral absorption coefficient and to calculate the solar absorbance for the samples. We found that the addition of silver nanoparticles gives an additional degree of freedom for the optimization of the sunlight absorption properties of NH-based nanofluids, with promising results for solar thermal collector applications.

2. NH, silver nanoparticles and nanofluid preparation

Deionized water (Millipore, Billerica MA, USA, 18.2MΩ) was used as solvent and base fluid. AgNO3 (>99 % pure, provided by Sigma-Aldrich) as a metal precursor; PVP-10 (average molecular weight 10000) as surfactant polymer; D-Fructose (99 % pure, provided by Alfa-Aesar) as a reducing agent and NaOH (anhydrous pellets, provided by Carlo Erba) as a catalyst of the metal salt reducing reaction were used in the synthesis of water soluble silver nanoparticles. Sodium Dodecyl Sulfate (SDS) (99 % pure, provided by Alfa-Aesar) was used for the stabilization of NHs suspensions. NHs were supplied by Carbonium Srl with an estimated diameter of 100 nm. A Sonics & Materials VCX130 ultra-sonicator operating at 20 kHz and 130 W, equipped with a 6 mm diameter Ti6Al4V alloy tip was used for dispersing NHs in water. A Malvern Zetasizer Nano ZS, exploiting the Dynamic Light Scattering (DLS) technique, was used for the evaluation of the size distribution of colloids in the dispersions and of the ζ-potential. AgNO3 was reduced to AgNPs exploiting the reducing power of fructose in an alkaline environment. PVP was used as a capping agent. In a typical synthesis, 3 g of PVP + 1.66 g of fructose were dissolved in 40 ml of deionized water; 5 ml of a 2 M solution of NaOH in water was added to this solution. The whole solution was heated under reflux up to 75° C. At this temperature 5 ml of a 0.93 M solution of AgNO3 in water (corresponding to 0.5 g Ag) was rapidly injected. The solution immediately assumed a dark-brown color,


yellow when diluted. The obtained colloid was removed from the heating bath after 1 min. A silver nanopowder easily re-dispersible in water was obtained by diluting the colloid by acetone with a volume ratio 2:1 with respect to the Ag colloid in order to precipitate nanoparticles. The mixture was centrifuged at 4000 rpm for 25 minutes. The supernatant liquid was eliminated and the precipitate was dried in an oven at 70 °C for 35 minutes to eliminate the acetone and partially water. The nanopowder was subsequently ground in an agate mortar. By this method the silver nanoparticles precipitate partially wrapped by PVP10 (a 10 wt% of residual polymer was detected by thermo-gravimetric analyses), thus allowing an easier re-dispersion in water. Suspensions containing AgNPs only and NHs only in deionized water were produced. The suspension containing 0.5 g/L AgNPs in deionized water (sample 8) was obtained by dispersing suitable amounts of Ag nanopowder by heating and periodically shaking the samples at a constant temperature of 55 °C for 4 hours. The suspension containing 0.5 g/L NHs in deionized water (sample 9) was obtained by dispersing the suitable amount of NHs in a 1.5 g/L solution of Sodium Dodecyl Sulfate (SDS) in deionized water; the dispersion was obtained by ultra-sonication at 50% power for 15 min. All other samples discussed in this paper were obtained by suitably diluting and mixing the previously mentioned suspensions and are listed in Table 1.

Label Nanoparticles and concentrations

S1 AgNPs 0.01 g/L S2 NH 0.006 g/L S3 NH 0.01 g/L S4 NH 0.006 g/L, AgNPs 0.002 g/L S5 NH 0.006 g/L, AgNPs 0.01 g/L S6 NH 0.006 g/L, AgNPs 0.1 g/L S7 NH 0.01 g/L, AgNPs 0.01 g/L S8 AgNPs 0.5 g/L S9 NH 0.5 g/L S10 AgNPs 0.1 g/L NH 0.1 g/L

Table 1. Samples under investigation Table 2 shows the ζ-potential values and the hydrodynamic main diameter of samples 8, 9 and 10 (S8, S9 and S10). ζ-potential values show that the three samples have good stability; in fact, suspensions with ζ-potential values higher than 30 mV (absolute value) are generally considered stable [41]. The suspensions with NHs show a higher absolute


value with respect to AgNP suspension. S10, which is the mixture of S8 and S9, shows an intermediate value. The hydrodynamic size of nanoparticle aggregates in S8 is significantly higher with respect to the size evaluated from SEM micrograph reported in Figure 1, which is around 20 nm, because hydrodynamic size also includes the polymer shell wrapped around nanoparticles. Moreover, hydrodynamic size is affected by the resistance to motion caused by the presence of the shell that in this case is made up of long PVP chains. S10 shows hydrodynamic sizes only slightly higher respect to S8 and S9, meaning that aggregation between AgNPs and NHs can be excluded. All the samples had a good time stability over several weeks without additional mixing. Previous experiments on the stability of NH aqueous nanofluids under thermal cycling and without optimization of the surfactant concentration have demonstrated that the samples are stable up to 120°C [23]. We expect the presence of AgNPs do not affect the high temperature stability, while an optimization of the surfactant concentration would considerably improve it, and it will be the subject of further investigations.

Sample ID ζ-potential (mV)

Main peak DLS size (nm)

S8 -35.2 124 S9 -55.4 135 S10 -49.7 142

Table 2 ζ-potential values and main hydrodynamic aggregates size of samples 8, 9, 10. The present results are consistent with our previous size measurements on NH-based nanofluids [24]

Figure 1: SEM micrograph of Ag nanopowder.


3. Optical characterization

Optical transmittance spectra at room temperature have been measured using a double-beam UV-VIS spectrophotometer (PerkinElmer Lambda900). Nanofluids are held in quartz cuvettes, with 10 mm beam path length. For a better evaluation of optical properties, we investigated the spectral scattering albedo using the method described in [22] at 633 nm and 751 nm wavelengths. The investigated samples are listed in Table 2. At first, NH and AgNP optical properties are decoupled investigating nanofluids containing one kind of nanoparticle only, namely, with reference to Table 1, S1-S3 for the extinction spectra and S8-S9 for the scattering measurements. Then, we investigated mixed nanofluids (S4-S7 for extinction spectra and S10 for scattering measurements). Figure 2 shows the spectral extinction coefficient of nanoparticles, obtained from transmittance measurements for a 10 mm path length and decoupled from the base fluid contribution. NHs are characterized by a peak centered at 262 nm, while AgNPs show an extinction minimum located around 322 nm and a maximum peaked at 396 nm. The peak heights linearly scale with the concentration of nanoparticles, both for the AgNPs and NH cases, giving evidence that 1) AgNPs and NH do not interact or aggregate for our sample preparation protocol, thus confirming the results of the previously described hydrodynamic size measurements and 2) the prepared fluids are stable, as they do not show settling or clustering phenomena. The noise in Fig. 2 shown by S6 is due to the divergence of the logarithm function in the regions of near-zero sample transmission. The extinction coefficient of S8, S9 and S10 is not given because of the null transmittance of these samples for 10 mm path length.

Figure 2: Spectral extinction coefficient for S1,…. S7 samples. Nanoparticle type and

concentration for each sample are listed in Table I.


Sample Scattering albedo @ 633 nm

Scattering albedo @ 751 nm

S8: 0.5 g/L AgNPs 0.19 ± 0.03 0.15 ± 0.03

S9: 0.5 g/L NHs 0.06 ± 0.03 below instrumental sensitivity

S10: 0.1 g/L AgNPs

0.1 g/L NHs 0.10 ± 0.03 below

instrumental sensitivity

Table 3. Measured scattering albedo

The experimental scattering results are listed in Table 3. We can appreciate the large difference in the scattering albedo between AgNPs and NHs. The NH albedo agrees with our previous measurements [22]. Following the notation in [34], the extinction and scattering efficiencies are given by:

22 2 2 4 2 2

42 2 2 2

1 1 27 38 8 14 Im 1 Re2 15 2 2 3 3 2ext

m x m m m mQ x xm m m m

⎧ ⎫⎧ ⎫⎡ ⎤⎛ ⎞ ⎛ ⎞− − + + −⎪ ⎪= + +⎨ ⎬⎢ ⎥⎜ ⎟ ⎜ ⎟+ + + +⎝ ⎠ ⎝ ⎠⎪ ⎪⎣ ⎦ ⎪ ⎪⎩ ⎭ ⎩ ⎭

⎪ ⎪⎨ ⎬

(1)

22

42

8 13 2sca

mQ xm

−=

+ (2)

where m is the complex relative refractive index

p p

f f

n ikm

n ik+

=+

(3)

defined in terms of the real (np, nf) and imaginary parts (kp, kf) of the complex refractive indexes of particles and fluid, respectively; and x is the particle size parameter:

2 fn a

xπλ

= (4)

being a the particle radius and λ the light wavelength in vacuum. For polydispersed particles, the scattering albedo is given by [35]:


2,

2,

i sca i ii

i ext i ii

N Q a

N Q a

πω

π

⋅=

⋅

∑∑

(5)

where Ni, Qsca,i and Qext,i are the volume concentration, the scattering efficiency and the extinction efficiency, respectively, of the spheres of radius ai. In the Rayleigh regime |m|x<<1, the expression in brackets in Eq. 1 is approximately unity. The extinction efficiency thus becomes:

22

42 2

1 8 14 Im Re2 3 2ext

m mQ x xm m

2⎧ ⎫⎧ ⎫ ⎛ ⎞− −⎪ ⎪= +⎨ ⎬ ⎨⎜+ +⎩ ⎭ ⎝ ⎠⎬⎟

⎪ ⎪⎩ ⎭

(6)

Figure 3 shows the single-scattering albedo in the Rayleigh hypothesis calculated from Eq. 5 for the three investigated samples, superimposed to the experimental values. The complex refractive indexes we considered were taken from [36, 37, 38]. Calculated data agree with experimental ones for particle radii aAg=14 nm and aNH=31 nm for AgNPs and NHs, respectively, in agreement with the nanoparticle sizes obtained from SEM observations. The agreement with the model appears slightly worse for the mixed nanofluid, probably due to the simplified approach of considering only a bimodal size distribution in Eq. (5). A better agreement would be likely obtained using more general models, e.g. the Mie theory. Anyway, for our purposes, the Rayleigh approximation appears satisfying and it was used for further calculations, as detailed in the following.

Figure 3: Calculated spectral single-scattering albedo in the Rayleigh regime (red dashed line: AgNPs with 14 nm radius; black dotted line: carbon nanoparticles CNPs with 31 nm radius; continous blue line: polydispersion of AgNP 14nm radius-CNP 31 nm radius) and


experimental values for the different samples (S8: red triangles, S9: black square and S10: blue circle). From the obtained experimental extinction coefficient µext(λ) and from the calculated spectral scattering albedo ω(λ) for the various samples (Eq. 5), we estimated the spectral absorption coefficient of nanofluids µabs(λ) as:

[ ]( ) ( ) 1 ( )abs extμ λ μ λ ω λ= − (7)

The solar absorbance for a given path length l within the fluid, which represents the solar energy being absorbed across the first fluid layer of thickness l [42] is defined as:

max( )

minmax

min

( )(1 )

( )

abs l

s

I e dA

I d

λμ λ

λλ

λ

λ λ

λ λ

−−=∫

∫ (8)

where I(λ) is the incident solar irradiance [43], λMAX=2000 nm and λmin=280 nm are the maximum and minimum considered wavelengths of the solar spectrum. Table 4 lists the solar absorbance for the various samples, calculated for l=1 cm and for air mass m=1.5.

Sample As (l=1 cm) Nanoparticles S1 0.37 AgNPs 0.01 g/L S2 0.56 NH 0.006 g/L S3 0.70 NH 0.01 g/L S4 0.58 NH 0.006 g/L, AgNPs 0.002 g/L S5 0.67 NH 0.006 g/L, AgNPs 0.01 g/L S6 0.92 NH 0.006 g/L, AgNPs 0.1 g/L S7 0.79 NH 0.01 g/L, AgNPs 0.01 g/L

Pure water 0.18 ----

Table 4. Solar absorbance for 1 cm path length.

From Table 4 we can appreciate the different characteristics of nanoparticles. If sunlight absorption is concerned, NHs are more efficient that AgNPs (0.70 vs 0.37 absorbance at the same weight concentration for 1 cm path length). However, in solar receivers, a 100% sunlight absorption on the whole fluid volume is preferred over an absorption peaked near the surface, as it reduces the thermal emission losses. Mixing two different kinds of


nanoparticles provides an additional degree of freedom in tuning the absorption properties (Table 4). Moreover, as underlined above, the addition of Ag is expected to improve the nanofluid thermal characteristics [26-29]. The evidenced possibility to obtain optimal sunlight absorption over practically every desired fluid length opens interesting perspectives for the development of novel customized geometries for the solar receiver. Other parameters, like the heat transfer coefficient can be optimized as well by combining the thermal properties of different kinds of nanoparticles.

4. Conclusions In this work we report on the preparation and optical characterization of nanofluids based on aqueous suspensions of NHs and AgNPs. A synthetic protocol was developed for the synthesis of AgNPs partially wrapped on a polymeric surfactant, leading to a nanopowder easily re-dispersible in water by avoiding aggregation. Measurements of the ζ-potential evidenced the absence of aggregation phenomena between the two kinds of nanoparticles, as further confirmed by the optical spectra. The spectral absorption coefficient of the nanofluids is obtained from the measurement of the spectral extinction coefficient and the calculation of the spectral scattering albedo in the Rayleigh approximation, validated by albedo experimental data at discrete wavelengths. Then, the nanofluid application as direct sunlight absorber in solar receivers is evaluated calculating the solar absorbance. Thanks to the favorable and different spectral characteristics of NHs and AgNPs, the optical properties of the resulting nanofluid can be tuned by changing both absolute and relative concentrations of nanoparticles, according to the desired absorption length. This result is interesting for designing solar receivers with novel geometries and optimized performances.

Acknowledgments

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