High-performance solution-processed plasmonic Ni nanochain-Al2O3selective solar thermal absorbersXiaoxin Wang, Haofeng Li, Xiaobai Yu, Xiaoling Shi, and Jifeng Liu Citation: Appl. Phys. Lett. 101, 203109 (2012); doi: 10.1063/1.4766730 View online: http://dx.doi.org/10.1063/1.4766730 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i20 Published by the American Institute of Physics. Related ArticlesEffect of load variations on exergy performance of solar box type cooker J. Renewable Sustainable Energy 4, 053125 (2012) Intergrain variations of the chemical and electronic surface structure of polycrystalline Cu(In,Ga)Se2 thin-filmsolar cell absorbers Appl. Phys. Lett. 101, 103908 (2012) Experimental investigation of various designs of solar flat plate collectors: Application for the drying of green chili J. Renewable Sustainable Energy 4, 043116 (2012) Effect of solar intensity on efficiency of the convection solar air heater J. Renewable Sustainable Energy 4, 042901 (2012) Nusselt number and friction factor correlations for solar air heater duct with broken V-down ribs combined withstaggered rib roughness J. Renewable Sustainable Energy 4, 033122 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors
High-performance solution-processed plasmonic Ni nanochain-Al2O3
selective solar thermal absorbers
Xiaoxin Wang,a) Haofeng Li, Xiaobai Yu, Xiaoling Shi, and Jifeng Liub)
Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover,New Hampshire 03755-8000, USA
(Received 25 July 2012; accepted 24 October 2012; published online 13 November 2012)
Selective solar thermal absorber coating is an important component of concentrated solar power
systems. It maximizes the absorption of solar spectrum and minimizes thermal radiation losses in
the mid-infrared regime. In this letter, we demonstrate a solution-processed plasmonic Ni
nanochain-Al2O3 selective solar thermal absorber with a high solar absorptance >90% and a low
thermal emittance loss <10%. Unlike conventional graded-index cermet coatings, the spectral
selectivity is tailored by the lengths of Ni nanochains, elimating the requirement of costly vacuum
deposition for stringent thickness control. These results open a path to utilize plasmonics for
low-cost, high-performance solar thermal systems. VC 2012 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4766730]
Solar thermal and solar photovoltaics are two major
approaches to harvest solar energy. Compared to photovol-
taics, solar thermal technology based on concentrated solar
power (CSP) is advantageous in terms of lower cost and
much easier energy storage. Since the heated working fluid,
such as molten salt, can be stored and kept at a high tempera-
ture for an extended period of time, CSP allows stored solar
thermal energy to be dispatched as needed, an attractive so-
lution to the intermittency issue of solar energy.1 A critical
component of CSP systems is selective solar thermal absorb-
ers.2 Ideally, the solar absorber for a CSP system should
absorb all the incident solar energy and convert it to heat
without thermal emittance losses in the mid infrared (MIR)
regime due to black-body radiation. Instead, the MIR ther-
mal radiation is reflected back to the CSP system.
Cermet selective solar absorbers consisting of metal
nanoparticles dispersed in a ceramic matrix have been devel-
oped for applications in CSP systems.3,4 It is well known that
metals have a high absorption coefficient in the solar spec-
trum, yet a continuous layer of metal reflects most of the light
instead of absorbing it due to the refractive index mismatch
with air. Conventionally, cermet structures address this issue
by incorporating very small metal nanoparticles with diame-
ters of 5–10 nm into ceramic layers at different volume frac-
tions to tune the refractive index profile of the coating for
optical impedance matching in the solar spectrum regime. As
illustrated in Fig. 1(a), a graded-index cermet comprises mul-
tiple ceramic layers with increasing volume fractions of small
metal nanoparticles from top to bottom. Sometimes an infra-
red (IR) reflector coating is coupled to enhance spectral selec-
tivity.5 Since the size of metallic nanostructures is much
smaller than the wavelength of incident light, the effective
dielectric function of cermets can be approximated by effec-
tive medium theories (such as Maxwell-Garnet Rule) based
on the dielectric function of metal, ceramic matrix, and metal
nanoparticle volume fraction.6 The design optimization is
based on engineering the effective refractive index and film
thickness of each layer.7 In these structures the cermet film
thicknesses are critical for the optical performance, leading to
stringent requirements on thickness control. Therefore, most
of the existing cermet fabrication techniques rely on vacuum
deposition, such as sputtering, evaporation, or chemical vapor
deposition.3 It was not until recent years that non-vacuum
coating methods, such as electrodeposition8 and solution-
chemical coating,9 have been investigated as less expensive
and more efficient approaches for cermet-based thermal solar
absorbers.
To alleviate the stringent film thickness constraint of tra-
ditional cermet solar selective absorbers and facilitate
solution-chemical processing, we explore metal nanochains
that exhibit inherent spectral selectivity for CSP applications
instead of relying on cermet thickness design. In this letter
we demonstrate a solution-processed plasmonic Ni nano-
chain-Al2O3 selective solar thermal absorber with a high so-
lar absorptance >90% and a low thermal emittance <10%
for low-cost, high performance CSP systems. As shown in
Fig. 1(b), the Ni nanochains consist of Ni nanoparticles,
�100 nm in diameter, an order of magnitude larger than
nanoparticles in traditional cermets. This nanoparticle size is
chosen such that absorption and scattering in the solar spec-
trum can be significantly enhanced by optical excitation of
surface plasma polaritons (SPPs) in metal nanostructures.
Strong SPP scattering from Au nanoparticles (50–100 nm in
diameter) towards Si has been utilized to increase the photo-
current in Si photodiodes and improve the energy conversion
efficiency in thin film photovolatic devices.10–12 A challenge
for solar absorber applications, though, is that the plasmonic
resonances in noble metals (like Au and Ag) are too narrow
to cover the entire solar spectrum. For broadband plasmonic
absorbers, Ag crossed gratings have been designed and fabri-
cated by e-beam lithography.13 In contrast to noble metals,
ferromagnetic metals such as Ni and Fe have a higher damp-
ing coefficient so that the SPP resonance appears broader in
spectrum.14 This feature is highly advantageous for solar
thermal absorbers. Furthermore, the optical response can be
a)Electronic mail: [email protected])Electronic mail: [email protected].
0003-6951/2012/101(20)/203109/5/$30.00 VC 2012 American Institute of Physics101, 203109-1
APPLIED PHYSICS LETTERS 101, 203109 (2012)
optimized by tailoring the length of Ni nanochains. The
nanochains can also form a 3D network, which offers even
stronger overall solar absorption due to multiple scatterings.
On the other hand, the �100 nm diameter Ni nanoparticles
are small enough such that long wavelength MIR photons
from thermal radiation cannot resolve them. Consequently,
these MIR photons see the Ni nanochain network as a contin-
uous metal sheet and are reflected back to the CSP system,
minimizing thermal emittance losses. The optical perform-
ance is mainly determined by the structure of plasmonic Ni
nanochains instead of layer thicknesses, which greatly facili-
tates solution-chemical processing.
In order to understand the plasmonic effect of Ni nano-
chains, the absorption, scattering, and extinction efficiency
factors of individual Ni nanochains are calculated by a 3D
finite-element method (FEM) and shown in Fig. 2. The nano-
chains are composed of a series of connected Ni nanospheres
with a diameter of 80 nm, and the chains are 1-, 2-, 6-, and
10-nanosphere long. The efficiency factors presented here are
absorption/scattering/extinction cross-sections normalized by
the geometric area of the nanostructures and thereby unit-
less. The dielectric functions of Ni and Al2O3 are obtained
from Ref. 15. To improve the computation efficiency based
on the symmetry of the structure, only one quarter of the
domain is calculated using adequate boundary conditions. To
check the validity of FEM method, the absorption/scattering/
extinction efficiency factors of one Ni nanosphere are calcu-
lated and found to be highly consistent with analytical solu-
tions based on Mie scattering theory (Fig. 2(a)).16 We have
analyzed incident plane waves propagating in the z direction
with linear x-polarization (along the nanochain) and y-
polarization (perpendicular to the nanochain), respectively.
As summarized in Fig. 2, the optical response of the individ-
ual nanochain is dependent on the polarization direction. For
incident light polarized in x-direction along the nanochain,
the optical response spectrum is extended from k� 1000 nm
to k� 2500 nm with the increase of nanosphere numbers,
covering >99% of optical energy in the solar spectrum. Such
spectrum broadening saturates when the number of nanopar-
ticles in a nanochain is large enough, as evidenced by the
similarities of optical response from 6- and 10-nanosphere
chains in Fig. 2(a). To further elucidate the mechanism of
extended absorption spectrum with the number of spheres in
a nanochain, we investigated the optical field distribution at
k¼ 2500 nm under x-polarization. The optical field is found
to be strongly enhanced at the narrow gaps between nano-
spheres due to near-field plasmonic effect, an advantage over
nanorod structures in terms of SPP enhancement. The 6-
nanosphere chain shows a stronger field enhancement than
the 2-nanosphere one at k¼ 2500 nm, confirming that the
increase in absorption at longer wavelengths (k> 2000 nm) is
indeed related to the SPP effect. It is likely that plasmonic
coupling among a larger number of nanospheres leads to a
stronger field enhancement at the gaps between nanospheres.
When the number of nanospheres becomes large enough the
plasmonic coupling among them reaches a steady state, which
qualitatively explains the saturation in spectral broadening.
On the other hand, for incident light polarized in y-direction
perpendicular to the nanochain, Fig. 2(b) shows that the
absorption/scattering/extinction efficiency factors are not sen-
sitive to the nanochain length, as one would expect. In con-
trast to x-polarization, the optical field distribution between
nanospheres is almost the same for 2-nanosphere and 6-
nanosphere chains. In the real cermet structures with ran-
domly oriented nanochain networks formed in 3D, the optical
response is polarization-independent. It is an average of the
two cases in Fig. 2. The multi-scattering among nanochains
may also enhance the overall absorption. The broad tunable
optical response in the solar spectrum (300–2500 nm) is par-
ticularly beneficial to solar selective absorbers. Unlike con-
ventional graded-index cermets, it is not necessary to use
multilayers with precise thickness control of each layer.
These advantages greatly facilitate large-scale fabrication of
high-performance solar selective absorbers by solution-
chemical techniques.
To demonstrate this concept, Ni nanochain-Al2O3 cermet
coatings are fabricated by a solution-chemical process. First
Ni nanochains with a diameter of �80 nm and a length of
2–3 lm are synthesized by reducing Ni2þ with N2H4,17 as
shown by the scanning electron microscopy (SEM) image in
Fig. 3(a). It is observed that Ni nanochains do not separate
into individual nanoparticles even under strong ultrasonic
vibration. Then the Ni nanochains are dispersed in an Al2O3
sol for spin-coatings on 25-lm-thick, 20� 20 mm2 stainless
steel substrates.18 Finally, the samples are annealed at 400 �Cfor 1 h in N2 to form �1-lm-thick cermet coating. More
FIG. 1. Schematic structures of solar selective absorbers. (a) Conventional
graded-index Ni-Al2O3 cermet composed of 5–10-nm-diameter Ni nanopar-
ticles, with increasing volume fractions of nanoparticles from top to bottom.
(b) Proposed coatings with Ni nanochains embedded in Al2O3 matrix. Nano-
chains comprise a series of connected Ni nanospheres with a diameter of
�100 nm.
203109-2 Wang et al. Appl. Phys. Lett. 101, 203109 (2012)
details about the fabrication process are provided in the sup-
plementary material.19 The surface of the Ni nanochain-
Al2O3 cermet layer visually looks black, as expected for solar
thermal coatings. The SEM image in Fig. 3(b) shows that
Al2O3 nicely covers Ni nanochains.
Fig. 4 shows the x-ray diffraction data of the Ni nano-
chain-Al2O3 layer in comparison with as-synthesized Ni
nanochains alone. The bump at �20� corresponds to amor-
phous Al2O3 matrix, while the three peaks correspond to Ni
(111), Ni (200), and Ni (220), respectively. Compared to as-
sythesized Ni nanochains, it is clear that there is no modifica-
tion in crystal structure or formation of Ni oxide during the
annealing process.
The reflection of the Ni nanochain-Al2O3 cermet coating
in the wavelength range of 0.2–2.5 lm is measured using a
UV-VIS-NIR spectrometer with an integrating sphere to col-
lected both specular and diffuse reflection,20 and the reflec-
tion in the infrared range of 2.5–20 lm is obtained using a
Fourier transform infrared spectroscopy (FTIR) equipment.
Fig. 5 shows the reflectance spectrum of the Ni nanochain-
Al2O3 cermet coating in the wavelength range of 0.3–15 lm.
The AM 1.5 solar spectrum and black-body radiation spec-
trum at 400 �C are also shown as a reference. According to
Kirchhoff’s law, at each wavelength k the absorptance aðkÞis equal to the emittance eðkÞ under thermal equilibrium.4
Since the transmittance through the stainless steel substrate
FIG. 2. 3D finite-element calculation of
absorption (magenta line), scattering (red
line), and extinction (green line) efficiency
factors vs. wavelengths for individual Ni
nanochains with 1, 2, 6, and 10 nanospheres.
(a) Incident light with x-polarization (along
the nanochain). The dotted dark gray, gray,
and light gray lines correpond to the calcu-
lated absorption/scattering/extinction effi-
ciency factors of one Ni nanosphere based on
the analytic Mie scattering theory, respec-
tively. (b) Incident light with y-polarization
(perpendicular to the nanochain). The insets
show the magnitude of the electric field jExjfor x-polarization and jEyj for y-polarization
at k¼ 2500 nm in the cases of 2-nanosphere
and 6-nanosphere chains.
FIG. 3. SEM images of Ni nanochains: (a)
as-synthesized; (b) as-coated and annealed
Ni-Al2O3 composite at an annealing temper-
ature of 400 �C.
203109-3 Wang et al. Appl. Phys. Lett. 101, 203109 (2012)
is 0, the absorptance/emittance at each wavelength is equal
to one minus the reflectance at that wavelength4
aðkÞ ¼ eðkÞ ¼ 1� RðkÞ � TðkÞ ¼ 1� RðkÞ: (1)
Here RðkÞ and TðkÞ are reflectance and transmittance at
wavelength k, respectively. Consequently, a low reflectance
is desirable in the solar spectrum regime of k¼ 0.3–2.5 lm
for high solar absorptance, while a high reflectance is desira-
ble in the mid infrared regime at k> 3 lm for low thermal
emittance. The reflectance of the Ni nanochain-Al2O3 cermet
coating is 0.06–0.09 in the wavelength range of k¼ 0.3–
2.1 lm, leading to high solar absorptance. The reflectance
starts to increase significantly at k> 2.1 lm, indicating a
drastic decrease in absorption. This transition point corre-
sponds well to the roll-off in the calculated absorption/scat-
tering/extinction spectra of Ni nanochains at k> 2 lm, as
shown earlier in Fig. 2(a). Therefore, the optical performance
of this Ni nanochain-Al2O3 cermet structure is consistent
with theoretical predictions. The reflectance is >0.9 at
k> 3.5 lm, leading to low thermal emittance in the MIR re-
gime. The overall solar absorptance, asol, and overall thermal
emittance, etherm, are derived from the reflectance spectrum
using the following equations:9
asol ¼
ð2:5lm
0:3lm
IsolðkÞaðkÞdk
ð2:5lm
0:3lm
IsolðkÞdk
¼
ð2:5lm
0:3lm
IsolðkÞ½1� RðkÞ�dk
ð2:5lm
0:3lm
IsolðkÞdk
;
(2a)
etherm ¼
ð20lm
2lm
IPðkÞeðkÞdk
ð20lm
2:5lm
IPðkÞdk
¼
ð20lm
2lm
IPðkÞ½1� RðkÞ�dk
ð20lm
2:5lm
IPðkÞdk
: (2b)
Here IsolðkÞ is the radiation intensity at wavelength k in
AM 1.5 solar spectrum and IpðkÞ is the radiation intensity at
wavelength k in 400 �C black-body radiation spectrum. The
overall solar absorptance is determined to be 93%, and the
overall thermal emittance is 9% for the current structure. A
small amount of solar energy may be scattered by Ni nano-
chains and absorbed by the stainless steel substrate, which
also contributes to the overall solar-thermal energy conver-
sion. This optical performance is comparable to vacuum de-
posited multilayer cermets,7 while the fabrication process is
much less expensive. The performance can be further opti-
mized by fine tuning the nanoparticle sizes to better match
the solar spectrum and the 400 �C black-body radiation
spectrum.
In conclusion, plasmonic Ni nanochain-Al2O3 cermet
structures are fabricated by cost-effective solution-chemical
approach for solar thermal applications. Unlike conventional
multilayer graded-index cermet coatings, SPP enhanced so-
lar absorption in these nanostructures are tailored by the
lengths of Ni nanochains instead of cermet layer thicknesses,
elimating the requirement of costly vacuum deposition for
stringent thickness control. High solar absorptance >90%
and low thermal emittance losses <10% have been demon-
strated in these Ni nanochain-Al2O3 cermet coatings, compa-
rable to the performance of vacuum deposited selective
cermet absorbers. These results open a path to utilize plas-
monics for low-cost, high-performance performance solar
thermal systems.
This work was supported by New Hampshire Innovation
Research Center (NHIRC) and Axisol, Inc. We would also
like to thank Dr. Xing Sheng at Massachusetts Institute of
Technology for assistance with some optical measurements.
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line) and 400 �C black-body radiation spectrum (magenta line) are also
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203109-5 Wang et al. Appl. Phys. Lett. 101, 203109 (2012)
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