Photothermal Effect of Nanomaterials for Efficient Energy Applications

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PHOTOTHERMAL EFFECT OF NANOMATERIALS FOR EFFICIENT ENERGY APPLICATIONS

Yuan Zhao, Andrew Dunn, Jou Lin, Donglu Shi Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States

INTRODUCTION Energy has been a key issue throughout human’s history, especially in the 21st century. The utilization of nonrenewable resources, including petroleum oil, coal, and natural gas, produces large amounts of greenhouse gases. According to the Paris Agreement in 2015, all countries have recognized the global climate change threat and agreed to limit the global temperature rising. To meet the aims in the Paris Agreement, we must develop renewable, low-carbon production resources, including wind power, hydropower, solar energy, geothermal energy, biofuels, and nuclear power. Broadly speaking, all renewable energy, except nuclear power, comes from sun. Better and more efficient use of solar energy will be the most effective way to improve energy issues in the foreseeable future. The photothermal effect has been investigated and developed in the recent decades, especially with the current of nanotechnology. Although the photothermal effect is mainly utilized for medical therapeutics, it can be further developed for efficient energy applications. In this chapter, we will introduce the fundamentals of the photothermal effect, unique materials that exhibit strong photothermal effect, related nanostructures, and their potential applications in energy efficiency.

THE FUNDAMENTALS OF PHOTOTHERMAL EFFECT The photothermal effects of a variety of materials result from photon-irradiated thermal energy. Some materials can absorb substantial radiation in a wide frequency range and convert it to thermal energy. This energy conversion has been characterized by the so-called surface plasmonic resonance for metallic materials, especially those at nanoscales. However, the operating mechanisms for oxides such as Fe3O4 and other semiconducting materials are not yet well identified. In this section, we present the current physical background on the photothermal effect with a short introduction to its applications in medicine and industry.

Novel Nanomaterials for Biomedical, Environmental and Energy Applications. https://doi.org/10.1016/B978-0-12-814497-8.00013-8 Copyright # 2019 Elsevier Inc. All rights reserved.

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PHOTOTHERMAL HEATING EFFICIENCY For nanoparticles displaying photothermal heating properties, the photothermal heating efficiency (η) in solution can be calculated from experimental observation by [1] η¼

hSðTMax  TSurr Þ  QDis
 I 1  10Aλ

(13.1)

where (h) is the heat transfer coefficient from the solution to environment, (S) is the surface area of the container holding the solution, (TMax) is the plateau temperature, (TSurr) is the ambient temperature of the surrounding environment, (QDis) is the heat dissipated from photonic absorption by the solvent and container that can be quantified by taking a system baseline measurement in the absence of nanoparticles, (I) is the intensity of incident light in (W cm2), and (Aλ) is the absorbance at some specific wavelength of incident light. The value of (hS) can be calculated from the relation hS ¼

mD C D τs

(13.2)

where (mD) is the combined mass of nanoparticles and solvent, which can be considered as the mass of solvent for sufficiently dilute systems; (CD) is the combined heat capacity that is regarded as the heat capacity of the solvent for sufficiently dilute systems; and (τs) is a system time constant that can be calculated from τs ¼  θ¼

t lnðθÞ

T  TSurr TMax  TSurr

(13.3) (13.4)

where (θ) is a dimensionless parameter calculated from the cooling curve (after irradiation) with given temperature (T) and time (t) where t ¼ 0 at the onset of cooling.

PHOTOEXCITATION While the previous quantification for (η) is based on empirically observed behavior and may be generally applied, without modification, to many colloidal systems, what is the underlying mechanism for how light, an electromagnetic wave, interacts with the electronic matrix of a nanoparticle to produce heat? General characterization of photon energy comes in the form of a small, well-known, equation directly relating the energy (E) of an incident photon to its frequency (ν) (Eq. 13.5), governed by its wavelength (λ) (Eq. 13.6), where (c) is the speed of light in a vacuum and (h) is Planck’s constant: E ¼ hν v¼

c λ

(13.5) (13.6)

When the energy (E) of an incident photon is equal to the change in energy between a relative ground and some excited state for an electron, absorption of the photon can occur, leading to photoexcitation of the electron; this absorption is wavelength-dependent, that is, energy-dependent, as materials possess specific quantized energy and electronic vibrational levels. An absorption continuum can be observed for materials rather than absorption occurring only at specific wavelengths due to

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417

vibrational sublevels populating electronic states in the material [2]. From a quantum mechanical approach, an excitation is dependent upon the vibrational wavefunction; a transition from a ground state to an excited state is most probable when the wavefunction of the excited state matches that of the ground state [2]. When light interacts with a nanoparticle, it may be scattered or absorbed. Photons absorbed by metal nanoparticles are ultimately converted into heat [3]. The total photonic extinction coefficient (σ ext) for a nanoparticle is a combination of the absorption (σ abs) and scattering (σ sca) crossing sections: σ ext ðλÞ ¼ σ abs ðλÞ + σ sca ðλÞ

(13.7)

Considering a spherical particle with diameter smaller than the wavelength of incident light, the absorption and scattering crossing sections for nanoparticles can be equated as follows, where α(λ) is the polarizability [4]: σ abs ðλÞ ¼ 8π 2 ReðiαðλÞÞ=λ

(13.8)

σ sca ðλÞ ¼ 128π 5 jαðλÞj2 =3λ4

(13.9)

Modeled as a sphere, the polarizability can be written in terms of the wavelength-dependent, complex permittivity of the nanoparticle and its diameter: αðλÞ ¼ ½ðεðλÞ  1Þ=ðεðλÞ + 2Þd 3 =8

(13.10)

Substitution of Eq. (13.10) into Eqs. (13.8), (13.9) then gives the following: σ abs ðλÞ ¼ d 3 π 2 Re½iðεðλÞ  1Þ=ðεðλÞ + 2Þ=λ

(13.11)

σ sca ðλÞ ¼ 2d 6 π 5 jðεðλÞ  1Þ=ðεðλÞ + 2Þj2 =3λ4

(13.12)

Eqs. (13.11), (13.12) demonstrate a strong dependence of absorption and scattering on nanoparticle size with sufficiently small nanoparticles interacting with light mainly by absorption [5]. In the case of metals, the probability of photonic absorption is proportional to the square of the local electric field [6]. Upon absorption, an excited electron and respective electron hole are generated. This electron may stimulate an array of effects from photo-induced desorption of surface molecules, electron transfer to an acceptor electrode for current generation, induction of a chemical reaction, electronic doping of a semiconductor, or the generation of thermal energy by relaxation of the excited electron. It is only when the energy of an excited electron surpasses the work function for the material that it may escape confinement and pass to another material. A key difference when juxtaposing nanoparticles versus bulk material is the ability for quantum confinement to play a significant role in the electric properties of nanoparticles. In a general manner, the energy separation between adjacent energy levels increases with decreasing size. This is highly pronounced in semiconductor nanoparticles where bandgap transition shifts to higher energy as the bandgap between the valence and conduction band increases with decreasing particle size [7]. While less pronounced in metals in which the conduction band is heavily populated in the ground state, as opposed to semiconductors, quantum confinement may be observed at very small sizes. In metals, an approximation for the level spacing of one electron states (Δ Es) can be made by dividing the Fermi energy (EF) by the number of atoms (Na) comprising the nanoparticle: ΔEs ¼ EF =Na

(13.13)

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This size-dependent property affects the frequency at which maximal plasmonic resonance is induced by incident light and is demonstrated for colloidal gold nanoparticles of sizes 22, 48, and 99 nm; these sizes possess absorption maxima of 521, 533, and 575 nm, respectively. As nanoparticle size increases, the maximum surface plasmon absorption becomes redshifted [8]. Mie theory allows for the prediction and modeling of photonic interactions in small particles. In the case of spherical nanoparticles in dilute solution where the diameter (d) is rough (d < λ/10), Mie theory for the extinction of light by colloidal nanoparticles reduces to [7] ω ε00 ðωÞ σ ext ðωÞ ¼ 9 ε3=2 V c m ½ε00 ðωÞ + 2εm 2 + ε00 ðωÞ2

(13.14)

where (V) is the particle volume, (ω) is the angular frequency of incident light, (c) is the speed of light, (εm) is the complex permittivity of the surrounding medium, and (ε0 (ω)) and (ε00 (ω)) are the real and imaginary parts of the nanoparticle complex permittivity, respectively. It is therefore the angulardependent, complex permittivity of the nanoparticle that drives its extinction characteristic when considering canonical matrix mediums such as air, aqueous solutions, polymer, or ceramic matrices. For gold nanoparticles, absorption is the dominant attenuation for colloids <10 nm in diameter with scattering becoming significantly larger than  50 nm [3]. For metal nanoparticles illuminated by a plane wave, for a fixed volume sphere, the absorption cross section may be written as proportionally equal to the product of the imaginary part of the nanoparticle permittivity for*some angular frequency of light (εω) and the square of the distance-dependent electric field amplitude (E ðr Þ) integrated over the nanoparticle radius [3]: σ abs ðλÞ ¼

ð

k εo jEo j2

* 2   Imðεω ÞE ðr Þ dr

(13.15)

Np

where (k) is the wave vector (k ¼ 2πn/λ), (n) is the optical index of the surrounding medium, (λ) is the wavelength of incident light, (Eo) is the electric field amplitude of incident light, and E(r) is the total electric field amplitude at some radius (r) within the metal nanoparticle. The heat generation power (Q) can then be calculated as the multiplication of the absorption cross section and the incident irradiance (I): Q ¼ σ abs I

(13.16)

To complete this calculation, the electric field amplitude within the nanoparticle must be known. Calculation of position-dependent electric field amplitude can be calculated from Green’s dyadic method [3,8,9]. From Eqs. (13.3), (13.4), the volumetric power density (q(r)) can be calculated as qðr Þ ¼

 * 2 n2 ω   Imðεω ÞE ðr Þ 2

(13.17)

The temperature distribution (T(r)) along the nanoparticle can be determined by applying Poisson’s equation to the volumetric power density by κr2 T ðr Þ ¼ qðr Þ

(13.18)

where (κ) is the thermal conductivity of the nanoparticle and (r2) is the Laplace operator. The increase in temperature at steady state then depends upon the thermal conductivity of the medium (κ 0), the effective radius of the nanoparticle (re), and the heat generation power. Solving for Poisson’s equation

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419

with spherical symmetry and assuming that the temperature of the nanoparticle is homogeneous, the temperature increase of the nanoparticle can be taken to be (T0) T0 ¼

1 Q 4π κ 0 re

(13.19)

The change in temperature (Δ T) of the medium then depends upon the thermal conduction from the medium to the external environment and the surface area through which this heat is diffusing. From Eqs. (13.16)–(13.19), it can be observed that heating of metal nanoparticles depends upon the absorbance of electromagnetic radiation by photons interacting with the electronic matrix of a nanoparticle. This is characterized by the absorption cross-sectional dependence on angular frequency for permittivity of the nanoparticle to be wavelength (energy)-dependent. Photonic excitation in this manner then relaxes largely through electron-electron interactions, releasing heat (thermal energy) that diffuses by conduction to the surrounding medium. It is the local surface plasmon resonance (LSPR) that allows for highly efficient heating juxtaposed to photothermal heating under nonplasmonic resonance.

SURFACE PLASMON RESONANCE (SPR) Local surface plasmon resonance (LSPR) is driven by photonic or electronic excitation when the incident electromagnetic (EM) frequency matches the natural frequency of the oscillating surface electrons. This excitation will cause an inhomogeneity in the electron density of a conductor, generating a local electric field that will tend to drive charge equilibration. Electrons accelerating through this field can pick up enough energy to overshoot equilibrium configuration and effectively switch the local electric field, causing an oscillation. This oscillation is not perpetual, thus requiring EM excitation for maintenance. The oscillation, modeled as a simple electron gas (plasmonic oscillation), possesses a frequency (ωp) governed by the local electron density (N), electronic charge (e), electron mass (me), and permittivity of free space (εo) [6,10]: ω2p ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ne2 =mεo

(13.20)

Light absorption can be enhanced in certain materials by LSPR. When governed by LSPR, heat generation becomes very efficient; LSPR on particles can act as an antenna, effectively capturing more photons than would otherwise be incident to the particle. Normalizing the absorption and scattering cross sections of a particle to the cross-sectional area, the obtained quantities are the dimensionless absorption (Qabs) and scattering (Qsca) efficiencies, respectively. From the Drude formula for the dielectric function of a simple free-electron metal, if the damping coefficient (γ) is much less than ωp, then a size parameter (x) for the nanoparticle can be calculated [10]: pffiffiffi ωF ¼ ωp = 3 00

Qabs ðωF Þ ¼ 12x=ε ðωF Þ

(13.21) (13.22)

For an aluminum nanoparticle in air under plasmon resonance, for example, the aluminum nanoparticle presents an absorption cross section 18 times greater to incident light than its physical area would otherwise suggest [10]. Two methods for electromagnetic decay of excited (hot) electrons in plasmonic nanostructures are by either radiative or nonradiative decay. Nonradiative decay can occur through intra- or interband

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excitations where the photoexcited electron relaxes through electron-electron or electron-phonon interactions [11]. If the work function of standard plasmonic metals is greater than the energy of photoexcited electrons undergoing LSPR, they will not escape confinement, and energy will be distributed among lower energy electrons during relaxation with the final step in relaxation, transferring heat to the surrounding environment [6]. From Eq. (13.14), the condition under which plasmon resonance is fulfilled is when ε0 (ω) ¼ 2εm if " ε (ω) is small or near constant [7]. Eq. (13.13) is valid only under the conditions of homogeneous polarization and assumes that the nanoparticles in solution or some matrix are dilute enough to be noninteracting. Nanoparticles will eventually reach a size at which light can no longer homogeneously polarize the electric field. Plasmon resonance will then explicitly depend on particle size. With this dependence, Mie theory predicts, and experimental results show the photon energy required for plasmonic oscillation redshifts and an increase in the absorption bandwidth. The bandwidth (Γ) may be calculated from a two-level electron model by [12] 1 1 1 ¼ πcΓ ¼ + T2 2T1 T2∗

(13.23)

where (T1) is the relaxation time for the excited electron, (T2) is the total dephasing time, and (T2*) is the dephasing time resulting from collisions that change the plasmon wave vector but not energy. Electron dephasing times computed from the bandwidth suggest that dephasing largely occurs due to electron-electron interactions. Efficient local heating of nanoparticles has seen many applications from localized photothermal cell killing as a potential solid-state tumor therapy [13,14], modification of polymer surfaces, synthetic catalysts, drug delivery, and more [6]. The photothermal effect, commonly found in metal and semiconductor nanoparticles, provides for a multitude of research opportunities in tailored synthesis for medical and industrial applications.

THE NANOMATERIALS FOR PHOTOTHERMAL STUDY A variety of materials exhibiting the photothermal effect, such as metallic materials, carbon-based materials, and organic materials, have been reported in literature. Gold nanoparticles have a wide absorption range and are efficient heaters but are cost prohibitive for large-scale productions. Iron oxides are inexpensive by comparison but have a lower photothermal efficiency. In the interest of environmentally friendly alternatives, some organic materials have shown photothermal applicability. However, most organic materials suffer from photobleaching, reducing their long-term applicability. Herein, some typical materials with their photothermal conversion efficiencies are presented.

METAL AND METAL OXIDE NANOMATERIALS Many metal exhibit high photothermal efficiency and can be made with precise sizes and shapes by changing reaction conditions during nucleation and growth. For instance, depending on the size and shape, Au (gold) nanoparticles or Au nanorods have an absorption peak between 500 and 600 nm. This tunable feature appears not only in Au nanoparticles, [15–18] but also in other metal nanoparticles including Ag, Cu, and Pd. This dependence of photothermal conversion efficiency of metal nanoparticles

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on size can be explained by Mie theory. Nanoparticles on the order of and larger than the wavelength of incident light scatter more strongly than their smaller counterparts. The photothermal conversion efficiency is therefore altered by nanoparticle dimensions. Chen et al. reported that when Au nanoparticles are about 8.6 nm, under 809 nm laser irradiation, photothermal conversion efficiency of Au nanoparticles can reach 95% [15]. Jiang et al. reported that decreasing Au nanoparticle sizes can increase photothermal conversion efficiency [18]. Also, Chen et al. synthesized copper nanoparticles via the reactions of copper sulfate pentahydrate and ascorbic acid to form a Cu nanoparticle hydrogel with environmentally friendly polysaccharide. The photothermal conversion efficiency of Cu nanoparticle hydrogel is measured to be 22.3% under 660 nm laser irradiation [19]. Xiao et al. developed porous palladium nanoparticles with promising photothermal conversion efficiency of 93.4% [20]. In metal nanoparticles, high photothermal efficiency can be attributed to surface plasmon resonance. An alternative to metals, metal oxides are also a widely used group of photothermal nanomaterials. Fe3O4 (magnetite) is a common photothermal material studied for cancer therapeutics. Furthermore, Fe3O4, like most metallic/ceramic nanoparticles, can be functionalized by metallic, ceramic, or polymeric coatings. This allows, especially in the case of polymer coatings, derivatization of small molecules or quantum dots to enhance surface and optical properties. Hu et al. reported the photothermal conversion efficiency of Fe3O4, coated with Au, that can reach 88.9% under 808 nm laser irradiation [21]. Tian et al. reported that Fe3O4 coated with Cu2-xS has a photothermal conversion efficiency up to 16% under 980 nm laser irradiation [22]. Zhang et al. observed that the photothermal conversion efficiency of Fe3O4@CuS achieves 20.7% under 808 nm laser irradiation [23]. Some metal oxide nanoparticles such as WxOy have plasmonic effects due to oxygen vacancies contributing free electrons. Therefore, WxOy presents different optical properties based on composition. Fang et al. reported that W18O49 shows a high photothermal conversion efficiency (59.6%) under 808 nm laser irradiation [24]. Chen et al. developed cesium tungsten oxide (Cs0.33WO3); the photothermal conversion efficiency of Cs0.33WO3 was reported about 73% [25].

METAL CHALCOGENIDES QUANTUM DOTS Another group of photothermal materials is semiconductor metal chalcogenide nanoparticles including copper chalcogenide (Cu2-XM, where M can be S, Se, Te, or Po), CdSe/ZnS, CdTe, and Bi2Se3. These semiconductor nanoparticles are also called “quantum dots” [26]. Quantum dots act under the quantum confinement effect. Their size affects optical and photothermal properties. Liu et al. synthesized cysteine-coated CuS nanoparticles. This nanoparticle presented a 38% photothermal conversion efficiency under 980 nm laser irradiation [16]. Cui et al. reported that Cu7S4 has 52.92% photothermal conversion efficiency under 808 nm laser irradiation [27]. Pilla et al. reported that the absorption spectra of CdSe/ZnS core-shell nanocrystal solutions exhibit redshift from 525 to 619 nm due to different core sizes, solvent, and concentration effects [28]. Interestingly, Chu et al. reported that quantum dot color affects photothermal conversion efficiency [29]. They synthesized CdTe solutions and found that darker CdTe solutions have a higher change of temperature (Δ T). Furthermore, concentration was also shown to affect the heating profile upon irradiation. Jia et al. developed Bi2Se3 nanocrystals with small particle size and LSPR-mediated heating that have a high photothermal conversion efficiency (30.06%) under 808 nm laser irradiation [30]. Xie et al. reported a high photothermal conversion efficiency (34.6%), better than Au nanorods (21%), under 808 nm laser irradiation from ultrathin Bi2Se3 nanosheets [31].

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CARBON-BASED NANOMATERIALS Carbon-based nanomaterials, including carbon dots, carbon nanotubes, and graphene/graphene oxide, show good thermal conductivity but poor transmittance compared with the other categories. Ge et al. reported that carbon dots possess a wide absorption band from visible to the near-infrared light region and their photothermal conversion efficiency to be 38.5% under 671 nm laser irradiation [32]. Li et al. synthesized supracarbon nanodots and indicated that supracarbon nanodots achieve a higher photothermal conversion efficiency up to 53.2% under 808 nm laser irradiation and exhibit a lower transmittance than carbon dots [33]. Lan et al. developed S-doped and Se-doped carbon dots. They found that S/Sedoped carbon dots exhibit a higher photothermal conversion efficiency (58.2%) under 880 nm laser irradiation [34]. Yu et al. synthesized polydopamine-modified reduced graphene oxide (pDA/rGO) nanocomposite with antiarrhythmic peptide 10 (AAP10), and the photothermal conversion efficiency was calculated to be 49.1% under 808 nm laser irradiation [35]. Song et al. proposed carbon nanotube (CNT) rings coated with Au nanoparticles. They reported this nanostructure to have a high photothermal conversion efficiency (76%) under 808 nm laser irradiation [36].

OTHER ORGANIC NANOMATERIALS Some organic compounds including small organic dyes (e.g., indocyanine green and phthalocyanine compounds) and conjugated polymers (e.g., polypyrrole nanomaterials and polyaniline nanoparticles) can also be used as photothermal materials. Lim et al. synthesized nonaggregated indocyanine green solution in water under 671 nm laser irradiation [37]. Phthalocyanine compounds may chelate different metal atoms. The optical properties of phthalocyanine compounds may therefore be modified by chelating different metal elements such as Cu, Zn, Fe, Co, Ni, Ru, Pb, Pt, or Sn [38]. Varying the metal center of phthalocyanine has a promising development for photothermal applications. Natural chlorophyll is a porphyrin derivative that has similar structure to that of phthalocyanine. Chu et al. investigated a Pluronic-encapsulated chlorophyll nanocomposite for cancer photothermal therapeutics [39]. For conjugated polymer systems, Xing et al. developed upconversion nanoparticles coated with a layer of polyaniline nanoparticles (UCNP-PANPs), and they found a conversion efficiency (47.8%) under 808 nm laser irradiation from the UCNP-PANP system [40]. Chen et al. developed polypyrrole nanomaterials with a strong near-infrared absorption and high photothermal conversion efficiency (44.7%) under 808 nm laser irradiation [41]. Table 13.1 shows some recent photothermal materials with their photothermal conversion efficiency, the reported laser wavelength, and power density (or power) from literature.

NANO-TECHNOLOGY BASED ON PHOTOTHERMAL EFFECT The previous section has highlighted different nanoparticles used in photothermal heating alongside some material drawbacks. Some nanoparticles can be improved by better synthesis techniques; others require specific treatments before application. Specific processing methods may also be needed for permanent use of these materials; coating or deposition is a crucial step for most photothermal applications. This section summarizes synthesis, treatment, deposition, and preservation methods for photothermal materials.

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Table 13.1 Some Recent Photothermal Materials and Their Photothermal Conversion Efficiency With Reported Laser Wavelength and Power Density From Literature Photothermal Materials Au nanorods Au nanoparticles Cu Porous Pd Fe3O4/Au Fe3O4/Cu2-xS Fe3O4/CuS W18O49 Cs0.33WO3 Cu7S4 CdSe/ZnS CdSe/CdS Bi2Se3 Carbon dots S, Se-doped carbon dots CNT@Au pDA/rGO Indocyanine green UCNP-PANPs Polypyrrole nanoparticles

Laser Wavelength (nm)

Laser Power or Power Density

Photothermal Conversion Efficiency, η (%)

809 980 800 532 660 808 808 980 808 808 808 808

1.72 W 0.72 W/cm2 20 W/cm2 0.228 W 1.5 W/cm2 8 W/cm2 1.5 W/cm2 0.6 W/cm2 0.3 3.5 W/cm2 2.47 W/cm2 1 W/cm2

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [27] [28]

808 808 671 808 880

1.6 W 1 W/m2

51–95 21.3 93–103 65.0–80.3 22.3 93.4 88.9 16 20.7 59.6 73 52.92 74–78 (in toluene) 57–65 (in water) 36–50 30.06 34.6 38.5 53.2 58.2

808 808 671

0.5 W/cm2 1.5 W/cm2 1.25 W/cm2

76 49.1

[35] [36] [37]

808 808

0.8 W/cm2 1 W/cm2

47.8 44.7

[40] [41]

Notes

[30] [31] [32] [33] [34]

NANOTECHNOLOGY INTRODUCTION Materials with nanoscale structures (approximately 1–100 nm) have unique optical, electronic, or mechanical properties. These unique properties have made nanotechnology widely applied in various fields, such as energy, materials, food, medicine, manufacturing, environmental monitoring, and pollution control. In the above sections, we have introduced different nanomaterials for photothermal studies. Most of them are designed and developed via a series of synthesis and processing steps for specific engineering applications.

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SYNTHESIS AND FABRICATION Most of the nanomethods can be divided into two types, “top-down” and “bottom-up.” The top-down approach refers to cutting, slicing, milling, or laser ablating a bulk material into nanoparticles with controlled shape and size, such as photolithography silicon technology. The application of top-down approach is mainly limited to the semiconductor industry. Top-down approach has advantages on bulk production, but the current cutting technology places limit on resolution. For example, by the year 2017, the most advanced semiconductor device fabrication technology produces chips with 10 nm nodes, limited by 13.5 nm extreme ultraviolet (EUV) lithography [42]. 6.5–6.8 nm beyond EUV photolithography is currently being investigated in research [43], and smaller nodes will be produced in the coming decade. The bottom-up approach refers to assembly of atoms and/or molecules into desired structure. Bottom-up applies to a range of techniques spanning various physical phases such as colloidal synthesis from solutions, plasma synthesis, and chemical vapor deposition (CVD) from gas phase. Theoretically, bottom-up approach allows production with atomic precision; however, upscaling may present cost and uniformity problems.

MODIFICATION There is no “perfect” material, but nanotechnology can greatly improve or adjust various properties, such as roughness, hardness, corrosion resistance, surface energy, surface charge, contact angle, reactivity, and biocompatibility. For instance, magnetic nanoparticles tend to agglomerate because of the magnetic attraction and their large surface-to-volume ratio. Therefore, a core-shell structure by polymer coating has been a common method to prevent aggregation. Furthermore, the shell can also provide protection for the core against different environments and provide a way to control the size, shape, and composition of the core-shell nanoparticles. Wang et al. reported that by using nanotechnology, multiple components can be assembled in one photothermal nanomaterial with multifunctionalities that include multimodality imaging, cell targeting, drug storage, and controlled drug release [44].

COATING Coating methods widely used in nanotechnology include chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), electrochemical coating, spin coating, spray coating, and roll-to-roll coating. The coating method is a crucial process in making thin films from atomic to millimeters thickness. CVD and PVD are deposition methods to process materials from solid phase to vapor phase and then condense the vapor phase as a thin film. These two techniques are widely used in the semiconductor industry. Materials like silicon, carbon, nitrides, and metals, which can form stable vapor or gaseous phase under certain conditions, are suitable for vapor deposition. ALD is a kind of CVD for producing extremely thin films with good control over composition and thickness at atomic level. While it has not been widely applied in manufacturing due to high cost and long reaction times, it has great potential in the microelectronic field. Electrochemical coating includes conversion coating and plating to improve a material’s resistance to corrosion and wear. Materials that are suitable for these methods are mostly limited to metal or salt

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substrates. For spin coating, materials in solution are dropped on a continuously rotating flat substrate and spread out to cover the substrates surface by centrifugal force. The solvent is evaporated during the process to form a thin film on the substrate. The thickness of thin films depends on the viscosity of the solution and the angular speed. Spin coating forms thin films with high uniformity and precise thickness, but most of the fluid is solvated material that is wasted. It’s tolerable for small-scaled research purposes but causes cost problem for industrial manufacturing. Spray coating, however, is common in industry due to the possibility of high throughput. Spray coating includes air gun spraying, thermal spraying, plasma spraying, and high velocity oxygen fuel spraying. The disadvantage of spray coating includes the need for equipment with high pressure or high temperature, which may cause some safety problems. Furthermore, most of the spraying techniques lack an effective monitoring system to control the coating. Roll-to-roll (R2R) is a coating process with relatively low cost and can be applied to a large surface area. R2R can coat materials with specific patterns (by photolithography techniques) like a printer. It has good potential for large semiconductors, like solar cells, but some problems remain, such as flexibility of the materials.

THE APPLICATIONS OF NANOMATERIALS’ PHOTOTHERMAL EFFECT While the photothermal effect and its therapeutic application are well known, it also has a wide range of applications in other areas, such as energy. Some technology like concentrated solar power has been applied in some countries. But there is still a large amount of “free” energy that has not yet been fully utilized. In this section, we introduce some of the latest research and reports on some interesting applications of the photothermal effect such as energy-efficient windows.

SOLAR ENERGY The amount of solar energy that the earth receives is much more than the consumption by all humanity. Humanity has utilized solar thermal energy for thousands of years. During the past century, the percentage of total energy generated by solar has varied due to the fluctuating prices of petroleum oil and natural gas and the concerns about future energy security. In the past decade, the human society has realized the impact of the overuse of fossil fuels, which is the massive emission of greenhouse gases. To limit global climate change caused by these emissions, important decisions such as the Paris agreement have been discussed. The consensus is that while maintaining the progress of industrialization, we need to minimize the use of fossil fuels and replace them by renewable energies, such as solar energy. Besides lower carbon dioxide emissions, there are a lot of advantages to use solar power, including longlasting working life, minimum maintenance requirement, low running costs, and wide installation for both small-scale and large-scale applications. The utilization of solar energy mainly includes photovoltaics and solar thermal energy.

PHOTOVOLTAIC (PV) SOLAR CELLS Photovoltaic (PV) solar cells are the most common way we utilize solar energy to generate electric power. While the percentage of global power generated by PV solar cells was around 1.3% in 2016, this percentage has doubled in only 3 years [45]. PV refers to the conversion of light to electricity

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Photons Antireflective coating Positive P-type semiconductor

Electron holes P-N junction Electrons

Negative N-type semiconductor

Substrate

FIG. 13.1 Schematic of p-n junction solar cell structure.

through the photovoltaic effect of semiconductor materials. Photovoltaic effect and photoelectric effect are quite similar. The difference is that electrons are ejected from the material during the photoelectric effect but contained within the material at a higher energy state relative to the ground state during the photovoltaic effect. A typical PV cell is made through joining two different types of semiconductor, a p-type and an n-type, to form a p-n junction structure. Fig. 13.1 shows a typical p-n junction solar cell structure. The traditional material of PV cells (also known as first generation) is crystalline silicon, including monocrystalline silicon and polycrystalline silicon. Currently, more than half of commercial solar cells are made of polycrystalline silicon. Monocrystalline silicon solar cells take about a third of the market. The second-generation cell is a thin-film solar cell, which has not been used in large-scale applications due to low efficiency compared with traditional solar cells. The third-generation PV cell, also known as emerging PV cell, includes CZTS solar cell, dye-sensitized solar cell, organic solar cell, quantum dot solar cell, and Perovskite solar cell. These third-generation PV cells are made of multiple layers created through nanotechnology and thin-film coating. The thickness of each layer varies from nanometers to micrometers, much thinner than a traditional solar cell wafer with thickness up to 200 μm. This feature allows the thin-film cells to be light in weight, flexible in shape, and semitransparent, greatly widening its application. These PV cells also have the potential to overcome the Shockley-Queisser limit with an efficiency that can be >30%. Fig. 13.2 shows a typical organic solar cell structure. In general, the basic idea is to raise the conversion efficiency of light to electricity to as high as possible. The presence of photothermal heating leads to a corresponding reduction in PV efficiency. Furthermore, high temperatures may damage the cell irreversibly. Under most circumstances, the

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+ −

Cathode Photoactive layer

Anode Flexible transparent substrate Light

FIG. 13.2 Schematic diagram of a typical organic solar cell [46] (open access).

photothermal effect of the cell material is undesirable and should be avoided. Commercialized solar cell panels usually require a cooling system and regular cleaning to maintain relatively high efficiency. Thus, the lack of water is a main problem for developing solar power in arid areas. If the photothermal effect of PV solar cells can be effectively limited, PV cells can be more easily applied and maintained. Therefore, during the experimental phase, not only the efficiency but also the photothermal effect of the materials should also be studied. While in most solar cells the photothermal effect is unwanted, some researchers are utilizing the photothermal effect as an advantage for solar cells. The thermoelectric effect, based on so-called Seebeck effect, has been recently applied on solar cells for heat recycling, and therefore, a new type of solar cell based on the so-called photothermoelectric (PTE) effect was investigated. Fig. 13.3 shows a typical PTE device schematic. The Seebeck effect describes an electric voltage or electromotive force (EMF) induced by a temperature gradient. The relationship between the EMF and the temperature difference can be represented as E ¼ σ  ΔT

(13.24)

where σ is the average Seebeck coefficient for the material. Compared with relatively mature PV technology, a PTE device usually has much lower efficiency (<5%) [48]. But, it broadens the applications for solar energy, such as heat recycling, energy-efficient windows, solar cooling systems, and flexible and transparent solar cells [49].

CONCENTRATED SOLAR POWER AND CONCENTRATED PHOTOVOLTAICS Concentrated solar power (CSP) and concentrated photovoltaics (CPV) are conversions of solar light to heat or electricity in the similar way that conventional solar power or PV cells do but utilize curved optical systems to focus sunlight to small areas for maximum efficiency (Fig. 13.4). CSP and CPV may have a broader future compared with previous semiconductor-based cells as the advantages include higher conversion efficiency and lower cell material cost (the semiconductor materials are much more expensive than curved mirrors). However, CSP and CPV have some drawbacks. They require frequent

FIG. 13.3 Schematic diagram of a PTE device based on solar thermoelectric effect [47]. From Royal Society of Chemistry.

Reflector

Receiver engine

Absorber tube Solar field piping

(A)

(B)

Reflector

Solar tower Curved mirrors

(C)

Absorber tube and reconcentrator

(D)

Heliostats

FIG. 13.4 Schematic diagrams of concentrating solar power with different types of collectors: (A) parabolic trough, (B) disk/engine, (C) linear Fresnel, and (D) heliostats and central receiver [50]. Modified from Elsevier.

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cleaning as dust can seriously impact performance, power output is more sensitive to solar radiation that may change by weather and solar angle, and an accurate tracking system for focusing solar power is required. To address these issues, nanotechnology is key in making precise cells and tracking systems under higher working temperature.

ENERGY EFFICIENT WINDOW As the most important component of resident and commercial buildings, windows are required to take roles for both optical and thermal purposes. However, glass, the main material for windows, is a poor thermal insulator. A 2014 report states that 1.16  1012 kWh, which is about 4% of total annual energy consumption of the United States, was lost through windows [51]. U-factor is introduced by the National Fenestration Rating Council as the parameter to describe the insulation capability of a window assembly. A lower value indicates better thermal resistance. U-factor can be expressed by the following equation [52]: U¼

1 1

1 + + RL  0:25  4 4 8:07  v0:605 Th  Tg Th  Tg 1:46  + σel H Th  Tg

(13.25)

where Th is the indoor air temperature, Tg is the inner surface glass temperature, H is the window height, σ is the Stefan-Boltzmann constant, e is the emissivity, l is the window glass thickness, v is the wind speed, and RL is the resistance of the windowpane assembly. The current methods to lower U-factor are mainly on structural and spectral designs [53]. For structural design, the most popular approach is multipane glazing that focuses on reducing heat conductivity. A multipane glazing usually contains two or more glass panes to make a glass sandwich with spaces between the panes filled with air, argon, water, aerogel, or even vacuum. An important criterion for filling materials is that they have to be mostly visibly transparent and good insulators. Another idea to lower U-factor is by using spectral design to lower radiative heat transfer. Glass is a material with high emissivity up to 0.84. With a low-emissivity (low-e) coating or tint, the emissivity can drop to 0.03 [54]. Other technologies include spectrally selective film, antireflection film, chromic technology, angular selective coating, liquid crystal, and electrophoretic and suspendedparticle technologies [53]. Khandelwal et al. reported an infrared regulating window using cholesteric liquid crystal [55]. This kind of design makes reflectivity of infrared radiation tunable by temperature, electric fields, or incident light intensity. A parameter called specific absorption rate (SAR) is also commonly used and can be expressed by the following Eq. [56]:


SAR ¼

 cg mg + cn mn ΔTcoated  cg mg ΔTuncoated mn Δt

(13.26)

where c is heat capacity and m is sample mass. Subscripts ( g) and (n) correspond to glass and nanoparticle coating. ΔT is the temperature differences between the maximum glass temperature and the ambient temperature; Δt is the time for the glass to reach the maximum temperature. According to U-factor and SAR, temperature is the key parameter for heat resistance and solar absorption of a window. Therefore, the photothermal effect should be considered since it’s an effective

Lo

s la s G

3O 4

w

-e

co

at

in

Conductive heat to increase inner surface temperature

la ye r

g

CHAPTER 13 PHOTOTHERMAL EFFECT OF NANOMATERIALS FOR ENERGY

red

Fe

430

fra

-in lar

So

Reflect long-wave radiation

Reflect long-wave radiation

Well-tuned NIR absorption Surface 2

Surface 1

FIG. 13.5 Schematic diagram of a typical glazing structure with thin films based on spectral selectivity and photothermal effect [53]. From Elsevier.

means for raising window surface temperature. Heating of a window can effectively reduce the U-factor, translating to less heat loss through windows from building interiors and therefore improving energy savings. It should be noted that although the photothermal effects of nanomaterials have been extensively studied for therapeutic applications, their potentials on energy applications, such as in energy-efficient window coatings, have not been fully utilized. Fig. 13.5 shows the schematic diagram of a typical window design based on the photothermal effect and spectral selectivity. Recent research by Zhao et al. investigated the photothermal effects of Fe3O4 nanoparticles in a thin-film coating on glass irradiated by white light, highlighting that inclusion of the photothermal effect to create energy-efficient windows will be a promising research new direction [57]. Specifically, surface-functionalized Fe3O4 nanoparticles are coated as thin films on glass substrates. Glass samples coated with an area density of 0.5 mg/cm2 Fe3O4 nanoparticles are irradiated by simulated sunlight, and as can be seen in the Fig. 13.6, these thin-film samples rapidly rise in temperature within minutes, quickly reaching a temperature plateau due to steady-state conditions between heat generation and thermal dissipation to the environment.

SOLAR COOLING SYSTEM Similar to solar heating during cold weather, cooling systems for energy savings are researched for applications during hot weathers. Solar cooling systems are usually based on a vapor compressed system, electricity through PV panels, or solar-assisted heat pumps. It is believed to be a promising

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34 Uncoated Fe3O4

Temperature / °C

32

PAA-Fe3O4 PS-Fe3O4

30

Polymer film

28

Light ON

26 0

5

Light OFF 10 15 Time/min

20

25

FIG. 13.6 Temperature versus time for the thin-film samples containing various Fe3O4 nanoparticles. Poly(acrylic acid) (PAA) and polystyrene (PS). Heating power, 0.1 W/cm2 [57]. Reproduced with permission from Elsevier.

FIG. 13.7 Schematic of a typical "closed circuit" solar cooling system [60] (open access). From Elsevier.

alternative to reduce the peak energy consumption during summer seasons. Fig. 13.7 shows a schematic diagram of a solar cooling system. An application in Jordan was reported for a nanotechnology-based solar cooling generator to improve the performance of solar cooling systems [58]. Zhai et al. also reported roll-to-roll manufactured glass microspheres and a polymer hybrid material for its radiative cooling through absorption and emission of infrared radiation [59]. Not just limited to heat generation, nanotechnology-based solar cooling systems present exciting new avenues and opportunities in research.

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FURTHER READING Yamada M, Foote M, Prow TW. Therapeutic gold, silver, and platinum nanoparticles. Wiley Interdiscip Rev Nanomed Nanobiotechnol 2015;7:428–45.