Present work experimentally characterizes the optical property of blended plasmonic nanofuids based on gold nanorod (AuNR) with different aspect ratios. The existence of localized surface plasmon resonance was verified from measured extinction coefficient of three AuNR solutions, and spectral tunability of AuNR nanofluid was successfully demonstrated in the visible and near-infrared spectral region. The representative aspect ratio and volume fraction of each sample were then calculated from the relation between extinction coefficient and extinction efficiency, which leads to the design of a blended plasmonic nanofluid having broad-band absorption characteristic in the visible and near-infrared spectral region. The results obtained from this study will facilitate the development of a novel volumetric solar thermal collectors using plasmonic nanofluids.
© 2014 Optical Society of America
Localized surface plasmon (LSP) is collective oscillation of free electrons occurring at the surface of sub-wavelength sized metallic particles . It is a resonance phenomenon which can be activated by the light of proper wavelength and the resonance condition mainly depends on the permittivity and geometry of the particle . When LSP is excited, the incident photon energy is resonantly transferred to the plasmon, leading to considerable amount of heat generated in the particle [1, 3]. If the particle is embedded in the medium such as water, the generated heat would be absorbed by the medium resulting in the increase of medium temperature . In other words, the light absorption by the medium is enhanced by the particles supporting LSP excitation .
The light absorption enhancement accompanied by LSP has great potential to be employed in solar energy harvesting applications such as solar thermal collector. For such device, main issue of improving the performance is to enhance the light absorption capability of the heat transfer fluid, and several researchers have tested various nano and microfluids to achieve this goal. For instance, Sani et al. [6, 7] showed that single-wall carbon nanohorns can improve the optical properties of the nanofluid. Also, Tyagi et al.  reported 10% increase of overall efficiency in flat plate solar collector when water with aluminium nanoparticles is used instead of pure water. Similarly, Khullar et al.  showed that Al nanofluid is also applicable to concentrating solar collector. However, these studies have not made full use of surface plasmon. Recent research performed by Lee et al.  focused on the fact that the LSP excited on single species of nanoparticle exhibits narrow absorption peak and attempted to achieve plasmonic nanofluid having broad-band absorption characteristic by blending gold nanoshells with different core sizes and shell thicknesses. Broad-band absorption is highly desirable for solar energy harvesting applications since the solar radiation contains the broad spectral range spanning from the ultraviolet to near-infrared . However, in order to excite the LSP on gold nanoshell in the visible and near-infrared region, the gold shell thickness has to be less than 10 nm, which is extremely difficult to be fabricated, as also note by Taylor et al. . Consequently, nanofluid based on gold nanoshells is not suitable for real-world applications due to great challenges in manufacturing process.
One promising alternative to resolve the fabrication difficulty is the use of gold nanorod (AuNR). AuNRs can be easily synthesized compared to gold nanoshell and its optical property can be tuned simply by adjusting its aspect ratio . Therefore, optimizing the optical property of nanofluid becomes much convenient and inexpensive when AuNRs are used. Although several studies have investigated the light scattering characteristics of AuNR for bioengineering applications [14, 15], little has been done on the optical property of plasmonic nanofluid based on AuNRs nor on the application feasibility of such nanofluid for solar thermal absorption.
The present study experimentally demonstrates the spectral tunability of the optical property of plasmonic nanofluids made of AuNRs. Based on the experimental results of three samples, blended plasmonic nanofluid having nearly uniform absorption characteristic in the visible and near-infrared spectral region is synthesized by properly mixing AuNRs with different aspect ratios. The application feasibility of the synthesized blended plasmonic nanofluid for a novel volumetric solar collector will also be examined.
For the experiment, AuNRs of three different types were first synthesized and suspended in 10 mM cetyltrimethylammonium bromide (CTAB) solution. CTAB is an agent that guides the directional growth of AuNRs while gold ions are reduced and is also a surfactant that stabilizes the synthesized AuNRs . Figure 1 shows the transmission electron microscope (TEM) images of prepared samples. Hereafter, samples will be referred for convenience to ‘Short’, ‘Mid’, and ‘Long’ according to their length.
The sample ‘Short’ and ‘Mid’ were synthesized by seedless reduction of gold ions in CTAB and sodium chloride (NaCl) solution [16,17]. For ‘Short’ AuNRs, 25 mL of 0.18 M CTAB and 0.675 mL of 0.1 M NaCl were put into a 50 mL centrifuge tube, and then 0.53 mL of water was added to adjust the final reaction volume to 30 mL. 0.54 mL of 0.05 M gold(III) chloride trihydrate (HAuCl4 · 3H2O) and 0.54 mL of 0.01 M silver nitrate (AgNO3) were added and gently mixed by inverting the tube. Then, 2.7 mL of 0.3 M L-ascorbic acid was added and gently mixed by inversion again. Finally, 15 μL of 4 mM sodium borohydride (NaBH4) was added and mixed by inversion, and the solution was kept undisturbed at room temperature for 12 hrs for stable growth of nanorods. For ‘Mid’ AuNRs, the protocol for ‘Short’ AuNRs was used and the only differences were the concentration and the amount of the L-ascorbic acid, 0.1 M and 0.54 mL, respectively. The amount of additional water was chosen accordingly.
‘Long’ AuNRs were synthesized by binary surfactant seed-meditated growth method [18, 19]. Seed solution was prepared in a flask by adding 2.5 mL of 1 mM HAuCl4 · 3H2O and 0.6 mL of 10 mM NaBH4 to 7.5 mL of 0.2 M CTAB solution being stirred vigorously. After additional stirring for 2 min, the solution was left undisturbed on the bench. Growth solution was prepared in a capped glass bottle by adding 6.25 mL of 10 mM HAuCl4 · 3H2O solution into a mixture of 37.5 mL of 0.2 M CTAB, 13.75 mL water and 67.5 mL of 0.1 M benzyldimethylhexadecylammonium chloride (BDAC). 2.5 mL of 4 mM AgNO3 was added to the solution and gently mixed by inversion. The process was repeated for 0.75 mL of 0.1 M L-ascorbic acid, and then 0.125 mL of the seed solution was slowly added in the middle of the growth solution and left undisturbed for 12 hrs. Finally, 2.32 mL of 0.1 M L-ascorbic acid was added to 125 mL of the growth solution and gently mixed. The solution turned blue after 3 hrs. Synthesized AuNRs were centrifuged at rcf ∼10,000 g for about 10 ∼ 30 min depending on the type of the centrifugal tubes and the size range of AuNRs. Relatively clear supernatants were discarded and the dark solutions at the bottom of the tubes were collected and re-suspended in 10 mM CTAB solution. This centrifugation/re-suspension step was repeated at least 3 times to wash away most of the residues from the chemical reactions. Concentrated final solutions in 10 mM CTAB were stocked and subjected to appropriate dilution with 10 mM CTAB when used for the experiments in this work. The average diameter and length of synthesized AuNRs were obtained by analyzing TEM image with ImageJ software  and listed in Table 1.
In order to experimentally characterize the optical property of nanofluid based on AuNRs, extinction coefficient of samples was investigated in the wavelength range from 400 nm to 1100 nm, where about 80% of the solar irradiation is contained . In principle, extinction coefficient measures the opacity of a medium and its inverse value represents the penetration depth; that is, the distance traveled by incident light in the medium until its intensity has decreased by a factor of 1/e about its original value. The extinction coefficient of the medium can be calculated from the measured transmittance and Beer-Lambert’s law : Isample/I0 = exp(−sκsample), where I, s and κ stand for intensity measured by detector, path length of the light, and extinction coefficient, respectively. Subscripts refer to the medium through which the light has passed before reaching to the detector and zero subscript denotes that there was no absorbing or diffusing medium on the beam path.
The important aspect of the Beer-Lambert’s law is that it is only applicable to ballistic photons and caution should be taken when dealing with diffusing medium so that the scattered light is not detected by the detector or is negligible. In the present study, Shimadzu UV-3101PC double-beam type spectrophotometer was used to measure transmittance of 1/50 diluted AuNR samples in quartz cuvettes with 10 mm beam path length. The geometry of the spectrophotometer allows the use of Beer-Lambert’s law since the field of view (FOV) of the detector is sufficiently small that the amount of photons scattered to the direction towards the detector is negligible.
Spectroscopic measurements were performed with the reference cuvette holder kept empty. In such case, the effect of reflection at the air-cuvette and cuvette-sample interfaces should be separately accounted and eliminated. This was achieved by using ray tracing method proposed by Large et al.  who calculated total transmittance of multi-layer system composed of different materials. When the cuvette filled with nanofluid is modeled as a three-layer system as shown in Fig. 2, the transmittance of sample cuvette can be expressed in terms of the reflectance at each interface as:Eq. (1), the extinction coefficient of nanofluid can be obtained. In the present study, the refractive index of the quartz cuvette was determined by matching the transmittance of an empty cuvette calculated from ray tracing method to the measured value assuming that quartz does not attenuate light in the considered wavelength region. Also, n2 of the base fluid (i.e., CTAB solution) is assumed to be the same as that of pure water, which will be confirmed by the experiment later.
3. Results and discussion
Figure 3 shows the measured extinction coefficient of 1/50 diluted AuNR nanofluids. We also plot the extinction coefficient of CTAB solution (10 mM) and deionized (DI) water to clarify the effect of AuNR itself. It is shown in the figure that the extinction coefficients from CTAB solution and DI water is almost indistinguishable, and both exhibit small extinction peak at around 1000 nm. It is thus confirmed that the optical constants of CTAB solution can be assumed to be the same as those of the water. If the contribution by CTAB solution is ignored, two peaks are observed in each AuNR nanofluids: one near 520 nm and the other at longer wavelength depending on the sample. Among them, the peaks placed at the longer wavelength can be interpreted as a clear evidence to the excitation of LSP, based on the fact that they show red-shifting tendency as the aspect ratio of nanorod increases, which is the typical behavior of LSP excited on nanorod structure . The resonance wavelength of LSP in each AuNR samples was 645 nm, 756 nm, and 1021 nm for ‘Short’, ‘Mid’, and ‘Long’, respectively, which lie well within the dominant wavelength range of the solar irradiation.
It is widely known that the LSP splits into two bands for non-spherical nanostructure such as nanorod: one associated with the transverse mode and the other associated with the longitudinal mode of LSP . In general, the longitudinal LSP exhibit larger extinction due to the longer distance between polarized atoms which results in the increased dipole moment . Therefore at first glance, the secondary peaks existing in the vicinity of 520 nm in Fig. 3 appear to be due to the transverse LSP mode. For the considered AuNRs, however, it was found that the transverse LSP mode dose not generate noticeable peaks in the extinction coefficient. Rather, the smaller peaks in each AuNR sample are originated from the immature nanorods that have failed to grow to their aimed length . The presence of such particles in the samples can be verified from the histograms shown in Figs. 1(d)–1(f), where number of particles having aspect ratio close to the unity is significant. Assuming that those immature nanorods with aspect ratio of 1 are approximated to spherical nanoparticles, Mie scattering theory  predicts its extinction peak to be occurred at 516 nm, which is nearly consistent with the position of observed secondary peaks. On the other hand, there seems one more peak exist for ‘Short’ AuNR around 580 nm, which may be due to the agglomeration of particles because the spectroscopic measurements conducted right after the sample synthesis did not show such peak.
As noted from Fig. 3, LSP resonance condition is a very sensitive function of the aspect ratio of AuNR. Therefore, by matching the resonance wavelength, one can estimate the aspect ratio of AuNR. For such fitting analysis, Fig. 4 plots the calculated extinction efficiency of randomly oriented AuNR of diameter 16 nm while length L varies from 16 nm to 96 nm (i.e., aspect ratio varies from 1 to 6). Here, we intentionally set the diameter of AuNR to be 16 nm, which is the average of three AuNR samples presented in Table 1. The 1 nm difference in diameter will not significantly alter the resonance condition of LSP associated with AuNR; thus, the result of calculation shown in Fig. 4 can serve as a general guideline for the fitting analysis. The calculation was performed using discrete dipole approximation (DDA) scheme where subject structures are replaced by an array of discrete dipoles and Maxwell’s equation for such dipole array is solved exactly without employing further simplifications. The open source software DDSCAT developed by Draine and Flatau  was used, and inter-dipole distance was set to be 1 nm in the calculation. In order to model a suspension of randomly oriented AuNRs in base fluid, the direction- and polarization-average have been performed. Optical constants of gold and water were obtained from tabulated data in Palik’s handbook . In general, if the size of Au nanostructures is much smaller than the mean-free-path of the conduction electrons, the size-dependent dielectric function of Au can be included by using the Drude model with modified scattering rate . However, it has been shown that Au thin film whose thickness is as small as 20 nm can be well modelled by the bulk property . Therefore, as far as the longitudinal mode of LSP considered, the intrinsic size effect in the dielectric function of gold does not need to be incorporated because of the length greater than 27 nm.
In Fig. 4(a), the calculated extinction efficiencies of randomly oriented AuNRs in water are shown by solid lines, and scattering efficiencies are indicated by dashed lines. It is shown in the figure that both extinction and scattering efficiencies increase as L gets larger. In addition, the DDA calculation also confirms that the transverse mode of LSP does not induce noticeable peak near 520 nm. Calculation also plots the scattering efficiency of AuNRs with various aspect ratios. For short AuNRs with L/D ≤ 2, the scattering is indiscernible (< 5% of the extinction). On the other hand, as the length increases the scattering efficiency can contribute to 10% of the extinction for AuNR with L/D = 3 and 15% of the extinction for AuNR with L/D = 6.
The peak wavelength of the extinction efficiency of AuNRs is plotted with respect to the aspect ratio in Fig. 4(b). There clearly exists linear relation between peak position and nanorod aspect ratio, consistent with other works [23, 29]. Hence, the least square method is applied to the data sets of aspect ratio and corresponding peak position to give linear equation shown in Fig. 4(b). The representative aspect ratio for AuNR samples are then obtained by substituting the peak position of Fig. 3 and indicated on the histograms given in Figs. 1(d)–1(f) after modified slightly to make better agreement with experiment result. The representative aspect ratios are larger than the average value for ‘Mid’ and ‘Long’ AuNRs because longer nanorods in the solution will exhibit higher peak value and thus can cause the shift of peak position to longer wavelength even if their portions in the solution are less. On the other hand, for ‘Short’ AuNR, the representative aspect ratio is slightly smaller than the average. This is probably due to the fact that the aspect ratio distribution of ‘Short’ AuNR does not clearly show strong peak, different from other samples. Hence, neither the average nor the representative aspect ratio can precisely predict the extinction peak. In accordance to the aspect ratio distribution, the extinction coefficient of ‘Short’ AuNR nanofluid exhibits the widest peak among three samples, as can be seen in Fig. 3.
In designing a blended nanofluid with desirable absorption capability, the relation between volume fraction and the extinction coefficient of diluted AuNR samples should be known. However, since AuNRs were chemically synthesized in the solution from the beginning, it is not easy to determine the volume fraction of AuNR samples directly. Therefore, we estimated the volume fraction of 1/50 diluted AuNR samples using relation between extinction coefficient and extinction efficiency :Fig. 3. Using Eq. (2), the volume fraction of 1/50 diluted AuNR samples is estimated as 6.8×10−7 for ‘Short’, 5.3×10−7 for ‘Mid’, and 4.1 × 10−7 for ‘Long’. These values were substituted again into Eq. (1) with extinction efficiency to obtain calculated extinction coefficients of AuNR samples and the result is compared with the measured value in Fig. 5. It is shown in the figure that the magnitude and the position of the peaks are in good agreement between calculated and measured extinction coefficient except for ‘Long’ AuNR where the water absorption shifts the peak to the shorter wavelength region. On the other hand, the measured extinction coefficients exhibit moderate amount of inhomogeneous broadening caused by non-uniform size distribution of synthesis AuNRs.
As the second step in designing the blended nanofluid, the target total volume fraction was first decided and the portions of each AuNR samples in the solution were calculated such that the peak values of the samples become nearly uniform. It is important to keep the total volume fraction as low as possible so that the hydrodynamic properties of nanofluid remain unchanged from those of the water. In this work, we set the total volume fraction to be 0.0001%, which would yield the extinction coefficient value close to 1.8 cm−1 in the visible spectrum. The portions of each AuNR samples required to achieve κ ≈ 1.8 cm−1 are 58.5% for ‘Short’, 25.5% for ‘Mid’, and 16.0% for ‘Long’. The portions tend to decrease for AuNR sample of larger aspect ratio because longer AuNRs exhibit higher peak value at the same volume fraction. After the designing process, the expected extinction coefficient of designed blended plasmonic nanofluid was obtained by combining measured extinction coefficient of AuNR samples after weighting them according to their relative volume fractions in designed nanofluid as shown in Fig. 6. Here, the extinction caused by CTAB solution should be accounted separately before being weighted because its effect depends only on the concentration of CTAB solution itself.
Figure 7 shows the measured extinction coefficient of blended plasmonic nanofluid together with the expected extinction coefficient. In the visible spectrum, the average extinction coefficient of blended nanofluid is 1.77 cm−1, meaning that 63.2% of solar radiation will be extinct (i.e., either absorbed or scattered) after passing through 0.56 cm of the nanofluid. As seen from Fig. 4(a), the scattering is negligible for ‘Short’ AuNR, but occupies 10% for ‘Mid’ and 15% for ‘Long’. Considering relative portion of each AuNR samples, the scattering contribution of blended nanofluid is about 8%. However, ‘Long’ AuNR mainly affects the absorption in the wavelength region greater than 800 nm, as seen from Fig. 6. As far as the visible spectrum is concerned, we can say that the portion of the absorbed light would be approximately 95% of the total attenuated light; thus, the absorption coefficient of blended plasmonic nanofluid can be expected to be approximately 1.68 cm−1. The measurement using an integrating sphere also suggests less than 2% of the scattering contribution of blended nanofluid in the visible spectrum. There exists undesirable region where extinction coefficient becomes very low between 800 nm and 1000 nm which attributes to the absence of AuNR exhibiting extinction peak inside that range. The appropriate length of such AuNR would be around 72 nm, estimated from the linear equation given in Fig. 4(b).
It should be noted that the non-negligible scattering of ‘Long’ AuNR sample does not necessarily lower the solar collector efficiency in real application. Lee et al. , for instance, investigated a plasmonic nanofluid whose scattering efficiency is nearly 30% of its absorption coefficient. In such a case, the incident solar radiation can be back-scattered to the top cover and may leave the solar collector. However, the performance degradation turned out to be insignificant due to the fact that a large portion of the back-scattered radiation will eventually re-entered to the collector because of the reflection from the top cover glass. Furthermore, scattering will also lengthen the optical path of photon bundles in the volumetric solar collector, so that optical absorption may be increased to some extent.
Another important point to consider is that temperature of AuNRs in water can reach as high as the boiling temperature when the solution works as solar-thermal fluid. Thermal expansion of AuNR would be negligible as the linear thermal expansion coefficient of gold is very small (1.4×10−5 K−1 at 20°C). However, a prevalent concern is that the shape of AuNRs turns spherical at high temperature through restructuring of Au atoms so that the aspect ratio and resonance peak wavelength decrease. The time scale for the change depends on the temperature and the amount of supplied energy to the AuNRs. Much gradual change in aspect ratio is expected when CTAB-stabilized AuNR is in water at T < 100°C . Geometrical change of AuNRs can be limited also by choosing appropriate capping ligand . If CTAB needs to be avoided for any reasons, AuNRs can be more stably suspended in water by forming covalent bonding with charged ligand molecules like mercaptocarboxylic acid and charge-neutral thiol-modified polyethylene glycol .
4. Concluding remark
Present study has characterized the optical property of blended plasmonic nanofuids based on AuNR with different aspect ratios. The existence of LSP on AuNRs was verified from measured extinction coefficient of the samples, and the spectral tunability of AuNR nanofluid was successfully demonstrated in the visible and near-infrared spectral region. Based on the experimental results of three AuNR solutions, the blended nanofluid was designed to have nearly uniform absorption in the visible and near-infrared spectral region. The measured extinction coefficient of the blended plasmonic nanofluid was about 1.77 cm−1 in the visible spectrum when its volume fraction was 0.0001%. The results obtained from this study will facilitate the development of a volumetric solar thermal collectors using plasmonic nanofluids. Furthermore, the spectral tunability of AuNR nanofluids will also enable a novel application such as nanofluid-based optical filters .
This research was supported by the Pioneer Research Center Program ( NRF-2013M3C1A3063046) and by the Basic Science Research Program ( NRF-2013-027065) through the National Science Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.
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