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C: Plasmonics; Optical, Magnetic, and Hybrid Materials
Thermoplasmonic Heat Generation Efficiency by NonMonodisperse Core-Shell Ag@SiO Nanoparticle Ensemble 0
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Constantinos Moularas, Yiannis Georgiou, Katarzyna Adamska, and Yiannis Deligiannakis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06532 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019
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The Journal of Physical Chemistry
Thermoplasmonic Heat Generation Efficiency by Non-Monodisperse Core-Shell Ag0@SiO2 Nanoparticle Ensemble Constantinos Moularasa, Yiannis Georgioua, Katarzyna Adamskab, Yiannis Deligiannakisa*
aPhysics bInstitute
Department, University of Ioannina, Ioannina 45110, Greece
of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 *
+
, Poland
* Corresponding author. E-mail address:
[email protected] , tel. +3026 51008662
E-mail addresses:
[email protected] (C. Moularas)
[email protected] (Y. Georgiou),
[email protected] (K. Adamska).
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Abstract The plasmon-induced heat generation by core-shell Ag0@SiO2 nanoparticle ensemble, i.e. Ag0 nanoparticles coated with a nanometric, amorphous SiO2 layer, has been studied for nanoparticles dispersed in liquid suspensions or deposited in film. Nonmonodispersed, fractal-like Ag0@SiO2 ensembles were synthesized by Flame Spray Pyrolysis (FSP), varying Ag0 particle size distribution, and SiO2-shell thickness, ranging from 1nm up to 5nm. The particles were characterized by TEM, XRD, XPS and UV/Vis, while the thermoplasmonic heat-generation efficiency was monitored in-situ by measuring the temperature rise over the nanoparticle ensembles by an infrared thermal imager, under UV-Vis irradiation or ambient solar light. We have carried out a systematic investigation of parameters regarding [i] the particle characteristics, [ii] the surrounding medium and [iii] the irradiation characteristics. The data reveal the determinant role played by the SiO2-shell in the plasmonic heating by Ag0@SiO2. Thus, for the thinner SiO2 coating tailored herein ( 1nm), under focused solar light, Ag0@SiO2 films were able to produce a significant temperature rise up to Tmax~400oC. The data are analyzed quantitatively within the theoretical frame of Mie theory as extended by Baffou for multiple plasmonic nano-heaters, where we take into account the fractal dimension of the flamemade Ag0@SiO2 ensemble and the occurring collective thermal effects in this geometry. In this context, we interpreted the observed phenomena in terms of the neighboring Ag0Ag0 coupling within each fractal, and the dual role of SiO2 as the dielectric shell-medium around the metallic core, as well as the plasmonic separator. Accordingly, we provide a consistent
theoretical
frame
which
provides
a
quantitative
hierarchy
of
the
physicochemical parameters, which determine the photoinduced heat generation for realistic non-ideal/non-monodisperse Ag0 ensembles.
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1. Introduction Metallic nanoparticles (mNPs) are well-known for their optical properties. The interaction between an electromagnetic (EM) stimulation and the metal's electrons, confined in the nanoscale, drives the phenomenon of Local Surface Plasmon Resonance (LSPR)1,2, i.e. the photoinduced collective oscillation of the metal’s conduction electrons. LSPR can be analyzed in a radiative and a non-radiative process. [i] the radiative plasmon decay describes the scattering of light by the plasmonic particle surface, i.e. resulting to the bright colors of noble metals, and the generation of local amplified electric near-fields due to confinement of surface plasmons and incoming photons in the particle vicinity3. Thus, as described by Mie theory4, the control of particle characteristics, such as size/morphology/dielectric medium, enables tuning of the plasmonic response throughout the EM spectrum5,6, allowing a plethora of applications7–11. [ii] the non-radiative plasmon decay -and the subsequent heat generation- can be interpreted as a dissipation of electromagnetic energy into heat. This phenomenon, gave birth to a new sub-field i.e. Thermoplasmonics12, which exploits the photothermal properties of such mNPs. It is now anticipated that the absorbed photon energy and the subsequent non-radiative dissipation as heat, can cause a significant temperature rise on the near vicinity of the nanoparticle, i.e. local heating in nanoscale13–15, or as homogenized temperature rise in macroscopic particle dispersions in solvents16–18, expanding the field to novel applications19–27. Recently, Baffou et al.28,29 have further elaborated the theoretical frame describing the phenomenon of heat generation by particle arrays with a homogeneous interparticle distance. This theory is outlined in more detail in paragraph 2 hereafter. In more realistic configurations, however, typically we encounter two main types of deviation from the ideal theoretical approach: [a] a distribution of particle size typically occur, rather than a single-size assembly and the particle-particle distance may be determined by a shell, such as SiO2, which surrounds the plasmonic particle, thus the coating defines the dielectric medium, and [b] a fractal-like ensemble of interacting mNPs typically occur in many chemically-prepared particles. Noble metals, such asgold30,31 and silver32 are among the most efficient plasmonic candidates. De Luca et al.18,33,34 have studied in-depth the photothermal efficiency for macroscopic ensembles of Au 3 ACS Paragon Plus Environment
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NPs either dispersed in solution or deposited on organic substrates, controlling the inter-particle distance by mechanical means. In the present work, Ag0 is the chosen material, having the most efficient optical and photothermal performance under visible excitation, in terms of near-field enhancement and the ability to generate heat, according to Baffou et al.32 Encapsulation of Ag0 with a nanometric SiO2 layer is a common strategy adopted by many researchers35–37, since it serves a 3-fold purpose: [i] it protects the Ag0 particles from oxidation. [ii] it prevents the release of toxic Ag+ ions38, enabling the usage of Ag0 in bio-applications. [iii] the SiO2 surface is suitable to functionalization with a well-established chemistry35,39–41. However, to our knowledge, a systematic study on the effect of SiO2 coating to the Thermoplasmonic Heat Generation Efficiency [THGE] for non-monodisperse ensembles of SiO2-coated Ag0 is lacking. Our hypothesis is that a hermetic SiO2 coating will prevent direct Ag0-Ag0 contact and, based on the shell thickness, will control the photothermal conversion mechanism. In the case of a liquid suspension of Ag0@SiO2 NPs, the THGE should take into account the role of the solvent’s thermal properties. In this context, we have studied the THGE for non-monodisperse fractal-like ensembles of Ag0@SiO2 nanoparticles with an amorphous SiO2 coating of controlled thickness varying in the range 1 nm up to 5 nm. Sub-nanometric SiO2 shell was studied in particular detail, as a limiting case for non-hermetically coated Ag0 NPs where Ag0-Ag0 proximity is maximized. As we show, these “patchy” Ag0@SiO2 are the ideal thermoplasmonic nano-ensembles, since they trigger collective thermal effects, which contribute significantly to thermoplasmonic temperature rise. The Ag0@SiO2 synthesis was performed in one-step process by flame spray pyrolysis (FSP). FSP process42,43 allows production of mNPs of high purity and crystallinity with controlled characteristics44,45. Typically, FSP-made nanostructures, including Ag0@SiO2, are non-monodisperse due to high-temperature particle sintering and aggregation35,46 during particle formation. These ensembles can be described as fractal-like structures with a fractal dimension of 1.847. Herein, we have used a single-nozzle FSP reactor to produce Ag0 nanoparticles of size distribution in the range 15-25 nm, hermetically coated with a SiO2 layer up to 5 nm thickness. In previous study, we have shown that in such non-monodisperse SiO2coated Ag0 ensembles, engineering the SiO2-thickness well below 10nm, allows lightinduced plasmonic excitation of the Ag0 core to be sensed by surface-grafted antioxidant organics35. In this way, we have shown that SiO2 of few nanometers is 4 ACS Paragon Plus Environment
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appropriate to hermetically encapsulate the Ag0 core, thus allowing collective plasmon phenomena to be detectable in antioxidant Hydrogen-Atom-Transfer in suspensions of the particles. Herein, we use this knowledge as a guidance for the desired SiO2thickness range, to study the photothermal efficiency of FSP-made Ag0@SiO2 ensembles. Thus, the specific aims of the present research were: [i] to produce and fully characterize hermetically-coated and patchy-coated non-monodisperse Ag0@SiO2 nano-assemblies using FSP, [ii] to study the Thermoplasmonic Heat Generation Efficiency for the Ag0@SiO2 NPs in suspension and on films, and [iii] to discuss the theoretical frame for the description of the THGE for fractal-like ensembles, taking into account the dual role of SiO2 as a dielectric shellmedium and as Ag0-Ag0 spacer.
2. Theoretical Background 2.1.
Mie Theory
According to Drude model48, the conduction electrons in metals, when treated as an ideal free electron with oscillation frequency Np, have a complex dielectric response described by Eq. 149: 2
( )=
+
=1
2
2
+
0
(1)
3
where the plasma frequency Np is given by Np=(ne2/Pome)1/2, n is the free electron density, PQ is the vacuum permittivity and me is the effective electron mass. R0 is the so called Drude relaxation rate R0=1/S49, where S is the mean relaxation time. R0 is related to losses due to electron-electron, electron-phonon and electron-defect/surface interactions1. In Equation 1, the real part Pre corresponds to the dielectric response and the imaginary part Pim describes the optical losses, due to electronic transitions. When the particle size is smaller than the metal’s mean electron free path, then the surface boundaries enhance losses by the confined electron motion1,50. Hence, the relaxation rate obtains a radial dependence ( )=
0
(2)
+
where r is the particle radius and uF is metal’s Fermi velocity49. 5 ACS Paragon Plus Environment
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Assuming a single spherical particle, LSPR can be interpreted by Mie theory and the crosssections for extinction ('ext), scattering ('sca), and absorption ('abs) are calculated as4:
= !"#
2 2
(
=1
=
(! + " )
+ 1)
(3a)
#$!
=
2 2
(
=1
+ 1)(|! |2 + |" |2) (3b)
#$!
(3c)
where an and bn are the factors, describing the behavior of electromagnetic (EM) waves in a spherical surface via the spherical Bessel and spherical Hankel functions and contain the dielectric properties of the medium and the metal. According to eq. 3a, b & c, LSPR can be modulated either by altering particle size or/and the surrounding medium1,51.
2.2 Plasmonic Heating The pioneering work of Baffou et al.28,29 in the field of thermoplasmonics interprets the temperature rise (UV) of an ensemble of W nanoparticles, arranged in a regular particle array with interparticle distance X, via two contributions: [i] a self-contribution due to heat generated by each nanoparticle itself and [ii] a collective contribution from the neighboring NPs. Assuming, a uniform irradiation beam, the total UV for such an ensemble of NPs deposited in a planar substrate is given by a closed expression:
&'(() =
) 4 +
+
!"#,
+
1
(1
-(2
2 (2 -
)
(4a)
Where - is the thermal conductivity (W/ m-1K-1) of the medium surrounding the particle, . the interparticle distance (m), I(Z) the irradiance of the illumination (W/m2) at a particular wave length (Z), D beam’s diameter (m) and r the distance from the heat source. Q is the heat power generated (Watts) by a single NP and is given by:
)=
!"#,
(4b)
where 'abs is the absorption cross-section, described by eq. 3c. In order to determine the dominant term in eq 4a, a dimensionless parameter can be defined as29: 6 ACS Paragon Plus Environment
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. =
( /
(5)
0
where N is the number of particles in the illuminated area, R the particle radius and m the dimensionality of the sample geometry (eg. m=2 for deposited particles in planar substrate, m=3 for dispersed particles in solution). If [m >> 1, the temperature rise is locally confined around the particle, which is acting as an isolated heat source, thus the first term dominates. If [m
