*Solar Thermal Conversion of Plasmonic Nanofluids: Fundamentals and Applications DOI: http://dx.doi.org/10.5772/intechopen.96991*

The wet-chemical method is one of the most common bottom-up methods to tune the size or shape of metal NPs. For Au NPs, chloroauric acid (HAuCl4) in aqueous solution is usually used as the precursor and the reducing agent can be polyvinyl pyrrolidone (PVP), Sodium borohydride (NaBH4), ascorbic acid (AA), sodium citrate (SC), and so on. The reduction reaction is also determined by the temperature. And various Au NP shapes can be achieved by introducing the additive, e.g., etyltrimethylammonium bromide (CTAB) for nanorods [20], sodium citrate or trisodium citrate for spheres [21], silver iron (Ag<sup>+</sup> ) for thorns [22]. Au NPs with the size ranged from 5 nm to 150 nm can be obtained by changing the concentration of the precursor and the reducing agent [23], PH value [24], temperature [25] and so on [26]. Seed-medicated method is another effective method to control the size and shape of Au NPs, which divides the reaction into nucleation and growth stages separately, so that the size and morphology of the particles can be controlled in a larger range [27]. By controlling the growth rate of different crystal planes, the final synthesized NPs can deviate from the initial seed crystal structure by the seed growth method. For example, cube Au NPs were prepared by the amount of ascorbic acid (AA) in the growth solution and the size can be controlled by the amount of seed solution or the growth number [28, 29]. Ag NPs also can be prepared by the similar methods using AgNO3 or other silver salts. To scalable and green prepare metal NPs, a rotating electrodeposition and separation (REDS) technique developed, which entails electrochemically depositing NPs onto a continuously rotating metal foil and subsequently harvesting them through mechanical delamination. A wide array of elemental nanoparticles (e.g., Ag, Au, Ni, Cu), alloys nanoparticles (e.g., FeCoNi and FeCoNiW), and metal oxide nanomaterials (e.g., CO3O4) were synthesized by REDS [30].

Besides the bottom-up method, top-down method is another to prepare NPs, which mainly including some physical method, such as: electron beam lithography (EBL), milling, annealing, laser-melting, and so on. The fabrication process of EBL is similar to classical photolithography, an electron beam is used to mark the pattern in the resist instead of light. [31]. Despite the low fabrication throughput and the fact that very small structures may be at the physical limit in terms of electronic function, the technique can find applications in preparation of reproducible largescale arrays of plasmonic NP with arbitrary two-dimensional shapes. Among the disadvantages are the technological requirements such as high vacuum and a scanning electron microscope system, much longer time to write a pattern than photolithography. Using conventional lithography techniques many shapes of nanoparticles on surfaces can be achieved. However, large scale fabrication using reproducible patterns, inverse replication or transfer of NPs between substrates, and three-dimensional nanostructures including deep etching has been increasingly demanded in plasmonic nanostructures [32, 33].

After obtaining different plasmonic NPs, it should be dispersed into the working fluid stability to form various nanofluids. The fabrication of NPs based on the topdown method is expensive and consumed on the materials, which could not meet the scale requirement although the high-quality NPs can be achieved. Wet-chemical method can be an efficient method to achieve plasmonic NPs in the solar thermal conversion applications.

#### **2.3 Nanofluid stability**

Nanofluid is defined as dispersing NPs stability into the base fluid, which is not simply mixing solid NP phase and liquid phase. It's a complex colloid by dispersing specific functional NPs in the base fluid (e.g., water, oil and so on). The main challenge for nanofluid applications is how to produce well-dispersion nanofluids.

The one-step method is to disperse NPs in the NP synthesis process by using the

The fraction processes of plasmonic NPs can be divided into two main categories in **Figure 3b**: the top-down based on the lithography, etching or milling, and the bottom-up, including the seed-mediated growth, chemical reduction, electrochemical method, and so on. Given that the NP shape can significantly affect the way it interacts with light and its SPRs, researchers have made great efforts to develop preparation methods for plasmonic NPs with the reproducible control of the size and shape. Nowadays, it's possible to fabricate high-quality plasmonic NPs (e.g., Au or Ag) with the target SPR wavelengths or near-field enhancement by enabling a systematic study of SPR dependencies on the size, shape, and structure of

physical method or wet-chemical method. The prepared nanofluid is relatively stable by avoiding the NP separation process (e.g., centrifuge or drying) and redisperse process (e.g., stirring or ultrasonic oscillation). Hence, the one-step method can reduce NP agglomeration or sedimentation in the nanofluids. For example, an ultrasound-assisted one-step method was used to prepare spherical and plate-shaped Au NPs with the NP size of 10 300 nm [19]. But the dispersed base fluid has limitations, which usually consists of the residual chemical reagent in the NP synthesize process and the nanofluid could not be applied in a large-scale range. The two-step method separates the NP synthesis process from the dispersion process, which is widely used in the large-scale applications. In this method, various NPs, such as: nanospheres, nanorods, nanotubes, and so on, are in the state of dry powders and then these NPs can be dispersed into different base fluids by stirring or ultrasonic oscillation for different applications. Due to the redispersion process of NPs, the stability is worse than that of the one-step method, but the two-step method has a simple preparation process, a high NP controllability, and wide application ranges. It can be seen from the two-step method that the NP parameters, such as the size, morphology, and dielectric environment of the base fluid determine their unique SPRs. Therefore, for solar thermal conversion applications of plasmonic nanofluids, preparing plasmonic NPs with the controllable morphology and size is the prerequisite and basis for their applications. And the fabrication

*Preparation methods of (a) plasmonic nanofluids and (b) nanoparticles.*

*Advances in Microfluidics and Nanofluids*

of these plasmonic NPs will be discussed in the next section.

**2.2 Plasmonic nanoparticle fabrication**

plasmonic NPs.

**112**

**Figure 3.**

Owing to the interaction among different NPs at the nanoscale and gravity at Earth, NPs are usually agglomerated due to Van der Waals force and then trend to be sediment at the bottom [34]. As a result, the agglomeration and sedimentation of NPs in the base fluid would affect the optical absorption and heat transfer performance, weakening the system efficiency. In addition, recent studies showed that the agglomeration or sedimentation can be worse under harsh operating conditions, such as: high temperature and pressure [35, 36]. Many methods were used to evaluate the stability of nanofluids, the simplest and direct method is the sedimentation method [18]. Interface electromotive force analysis is another common method to observe the stability of nanofluids, but this method is limited by the viscosity and concentration of the fluid [37]. Wang et al. [38] used an ultraviolet– visible spectrophotometer to study the stability of nanofluids. The NP concentration can be obtained by measuring the change in the light absorption rate of the system with the sedimentation time because the NP concentration is a linear relationship with the absorbance of nanofluid at the low concentration.

firstly. And the dielectric function of materials are required for the optical simulation, which is taken from the experimental data of bulk materials (e.g., Johnson and

method). The Drude model is the simplest of all, but disregards radiation damping. Even today, mainly because of the simplicity, the Drude model is still used to describe the dielectric functions in many calculations. In some problems, the classical models of dielectric functions are unsatisfactory but, at the same time, full quantum theories involve a very complex treatment including non-local effects [45], polarizabilities including non-linear terms [46], electron densities calculation using mean-field theories [47] and temperature dependent effects [48]. The need for quantum treatment of the optical properties of small particles has been

evidenced in recent experimental studies [49]. In large particles the resonances are influenced by retardation effects and are strongly dependent on the size of particles, but the dielectric function can be assumed as that of bulk. Based on the dielectric function, Au, Ag and Al are the three most used materials in plasmonics. Their SPR wavelengths are at visible or UV spectral bands and, therefore, of great potential in

**Mie theory:** Mie theory is a simple and theoretical method to calculate the optical properties of sphere NPs in a homogenous medium, which uses a series of coefficients *a*<sup>n</sup> and *b*<sup>n</sup> for the scattered fields and *c*<sup>n</sup> and *d*<sup>n</sup> for the internal fields to determine the scattering fields. The scattering and extinction cross sections can be

The absorption cross section can be obtained as: *Cabs* ¼ *Cext* � *Cscat*. Despite to the less computation load, it is possible to obtain cross-sections for many wavelengths in a few seconds, using a common PC. However, a large number of terms is required for accurate cross-section calculations of spheres with very large size parameter [51]. The Mie theory has been extended to permit calculations for

**DDA:** To calculate the light scattering of an arbitrary shape NP, discrete dipole approximation (DDA) was first presented by Purcell and Pennypacker [53] by using a grid of dipoles. To occupy by the scattering target, DDA method discretizes the volume by an array of *N* dipoles using Clausius–Mossotti polarizability *α <sup>j</sup>* for each dipole, which interacts with the incident field and the neighbors. The polarization of dipole *j* located at *rj* can be determined by *P <sup>j</sup>* ¼ *α jE <sup>j</sup>*, and the field can be

> *<sup>j</sup>* �<sup>X</sup> *k*6¼*j*

To achieve accurate and reproduce the calculation results, two validity conditions should be verified in DDA: (a) the dipole lattice spacing *d* should be small enough, i.e., j j *m kd* ≤1, where *m* is the complex refractive index of the scattering target. (b) *d* must be small enough to refabricate accurately the NP shape. For small plasmonic NPs, or small inter-particle separations, *d* must be smaller than

*<sup>E</sup> <sup>j</sup>* <sup>¼</sup> *Einc*

ð Þ <sup>2</sup>*<sup>n</sup>* <sup>þ</sup> <sup>1</sup> j j *an* <sup>2</sup> <sup>þ</sup> j j *bn* <sup>2</sup> � � (1)

ð Þ 2*n* þ 1 Re ð Þ *an* þ *bn* (2)

*AjkPk* (3)

. *Ajk* is the matric of dipole interaction and retardation effect.

*Cscat* <sup>¼</sup> <sup>2</sup>*<sup>π</sup> k*2 X∞ *n*¼1

*Cext* <sup>¼</sup> <sup>2</sup>*<sup>π</sup> k*2 X∞ *n*¼1

ellipsoidal shape, multilayer or several spheres [52].

solar thermal applications.

calculated as: [50].

calculated as:

where *Einc*

1 nm.

**115**

*<sup>j</sup>* <sup>¼</sup> *<sup>E</sup>*0*eikr*�*iω<sup>t</sup>*

Christy [44],) or a model approximating experimental results (e.g., Drude

*Solar Thermal Conversion of Plasmonic Nanofluids: Fundamentals and Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96991*

The stability of plasmonic nanofluids is also one of the major issues limiting the applications of nanofluids. Many researchers have made much efforts to improve the stability of the plasmonic nanofluids from the aspect of long-time and hightemperature dispersion [39]. For example, Au@SiO2 and Ag@SiO2 core-shell NPs were synthesized using a low-temperature two-step solution process. Results showed that the synthesized metal@SiO2 nanofluids exhibited excellent dispersion stability of 93.7% for Au@SiO2 and 100% for Ag@SiO2 in 6 months without using any surfactants, and they also showed a good thermal stability after thermal exposure at 150° C for an hour [40]. An ultrastable nanofluids with the broadband photothermal absorption was achieved using citrate and polyethylene glycol-coated Au NPs, circumventing the need for free surfactants. Electrostatic stabilization provided superior colloidal stability and more consistent optical properties; chemical and colloidal stability was verified for 16 months, the longest demonstration of stable nanofluids under ambient storage in the solar literature [41]. Besides the base fluid water used above, the base fluid oil was also studied to improve the stability. A facile and effective strategy, including controlled high-temperature synthesis of nanoparticles, surface modification of particles, and post-modification particle size partition, was designed to prepare stably dispersed silicone-oil-based nanofluids that enable high-temperature operation [42]. A low cost, and scalable method was reported to synthesize solar selective nanofluids from 'used engine oil' with the excellent long-term stability and photothermal conversion efficiency. Results showed that their stability and functional characteristics can retain even after extended periods (72hours) of high temperature (300°C) heating, ultra violet light exposure and thermal cyclic loading [43].
