Zhang, Optical properties across the solar spectrum and indoor ther- mal performance of cool white coatings for building energy efficiency, Energ. Akbari, Solar spectral optical propertie[r]
Trang 1Original Article
Nanostructured zinc aluminates: A promising material for cool roof
coating
Dept of Optoelectronics, University of Kerala, Thiruvananthapuram, Kerala 695581, India
a r t i c l e i n f o
Article history:
Received 15 May 2019
Received in revised form
10 October 2019
Accepted 20 October 2019
Available online 28 October 2019
Keywords:
Zinc aluminate
Nanocrystalline materials
Citrate-sol gel method
NIR reflectance
Cool coating
a b s t r a c t
Nanostructured zinc aluminate pigments were synthesized using a citrate solegel method All the samples were annealed at 600C, 700C, 800C, and 1100C for 6 h, respectively The as-synthesized samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), UVeViseNIR spectroscopy and CIE L*a*b* color analysis The phase purity and structural analysis of the zinc aluminates were confirmed using X-ray diffraction and Rietveld refinement The morphological and microchemical analysis was done using FE-SEM, TEM and EDX The particle size and reflectance properties are found to be sensitive to the annealing temperature The samples exhibited a total solar reflectance of about 85% An attempt was also made to compare the total solar reflectance of the synthesized zinc aluminates with that of commercially available TiO2by applying over a concrete cement surface
Crown Copyright© 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/
4.0/)
1 Introduction
Solar reflective pigments have been a keen study of interest in
recent years, for their applications as thermal-cooling materials in
the current scenario of worldwide warming Solar reflective
coat-ings are characterized by their ability to reflect visible and near
infrared parts of the sunlight, thus reducing the heat absorbed by
buildings, automotives and pavements, leading to energy savings
[1] Therefore, a more comfortable interior climate and electricity
savings with air conditioning during the hot season can be
ach-ieved A substantial portion of the energy expenditure adds up from
the construction sector Also, buildings are responsible for nearly
70% of sulfur oxide emissions, 50% of carbon dioxide emissions [2]
and a large proportion of smog [3] Some recent research reports
have estimated a possible saving of peak cooling load of 11e27% for
air-conditioned buildings depending on the climatic conditions [4
Hence, materials with high solar reflective properties offer an
effective passive cooling method for reducing an urban heat island
effect and greenhouse gas emissions [5,6
Among the solar reflective materials, rutile TiO2 - a white pigment is currently regarded as the best pigment for coating materials, but it is also the most expensive Mixed metal oxide-based materials in nanoscale have been a subject of interest due
to their fascinating optical, structural, magnetic, catalytic and thermal properties Zinc aluminate (ZnAl2O4) is a member of the spinel oxides having the general formula AB2O4, where A repre-sents a divalent metal ion and B reprerepre-sents a trivalent metal ion This is a widely sought material possessing extremely diversified properties such as high thermal and mechanical resistance, hy-drophobic nature, highfluorescence efficiency, high chemical sta-bility, high quantum yields and low sintering temperature [7e9] ZnAl2O4is a direct band gap semiconductor with an optical band gap of around 3.8 eV [10] Thus, ZnAl2O4spinel is a nontoxic, low-cost material and has high thermal stability Various chromophores such as cobalt, chromium, manganese have been introduced into the spinel lattice to produce colored pigments [11e14]
Various methods of synthesis have been employed for the synthesis of ZnAl2O4 The shortcomings of a conventional solid-state reaction method, such as inhomogeneity, lack of stoichiom-etry control, high temperature and low surface area, are amelio-rated when ZnAl2O4is synthesized using a solution-based method Nanomaterials based on ZnAl2O4 have been prepared by various soft chemical routes such as coprecipitation [15], hydrothermal [16], glycothermal [17], solvothermal [18], combustion [19],
* Corresponding author.
E-mail address: gopchandran@yahoo.com (K.G Gopchandran).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.10.003
2468-2179/Crown Copyright © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Journal of Science: Advanced Materials and Devices 4 (2019) 524e530
Trang 2polymeric precursors [20], pyrolysis [21], solegel [8], mechanical
alloying [22] methods and explored for their various properties
And the aid of surfactants and other inorganic materials utilized in
these preparation procedures is costly which limits its industrial
applicability The motivation of this work is to synthesize
high-quality ZnAl2O4 nanomaterial with impressive solar reflectance
compared to the widely used titania as a possible alternative
nontoxic white pigment for reducing the heat build-up
Among the various synthesis methods, the solegel method
en-sures an easy mixing of precursor solutions at the molecular level
and provides a high degree of homogeneity at relatively low
pro-cessing temperatures It enables a higher uniform particle size
dis-tribution and non-agglomeration [23] Citric acid is commonly used
in the solegel method, the advantages being readily available and
cheap Also, citric acid is an effective chelating agent The citric acid
solegel or citrate solegel method is normally used for the synthesis
of binary, ternary and quaternary metal oxides in both crystalline
and amorphous forms The conversion of the‘gel’ to a mixed metal
oxide is accomplished by pyrolysis in air, with the maximum
tem-perature depending on the specific system As the metalecitrate
‘gels’ are heated, the organic component undergoes combustion at
around 300e400C, depending on the metal counterion and the
presence of additives Another impact of the homogeneity of citrate
solegel precursors is on the reaction temperature since the final
crystalline metal oxide may be formed at significantly lower
tem-peratures than those using powder solid-state methods [24]
In the present study, we report on the synthesis of
nanocrystal-line phase pure ZnAl2O4by a simple citrate solegel method The
materials annealed at different temperatures have been
character-ized for their structural and optical properties The near infrared
reflective properties of the samples are emphasized for use as solar
reflective materials Also, we have investigated the solar reflective
property of the as-prepared ZnAl2O4over a concrete surface
2 Experimental
2.1 Synthesis of ZnAl2O4nanoparticles
ZnAl2O4 nanoparticles were prepared via a citrate solegel
method, in which zinc nitrate hexahydrate [Zn(NO3)2$6H2O] (98%,
Sigma Aldrich), aluminium nitrate nonahydrate [Al(NO3)3$9H2O]
(Emplura, Merck) and citric acid (99.5%, Sigma Aldrich) were taken
as the starting materials At first, stoichiometric amounts of
Zn(NO3)2$6H2O and Al(NO3)3$9H2O were dissolved in distilled
water by ultrasonication The citrate solution was prepared by
dissolving the appropriate amount of citric acid in dilute nitric acid
and both solutions were mixed The citric acid to metal molar ratio
wasfixed as 2:1 The mixed solution was heated at 90C in a water
bath for 4 h and then magnetically stirred for 3 h The obtained
solution was kept at room temperature until a gel was formed
which was dried at 200C for 2 h After grinding the dried sample,
it was annealed separately at 600C, 700C, 800C and 1100C for
6 h with a heating rate of 10C/min The obtained powders were
ground in an agate mortar and used for further characterization
2.2 Characterization
The crystalline nature and phase purity of the synthesized
samples were characterized by X-ray powder diffraction (XRD)
using a powder X-ray diffractometer (Bruker D8 ADVANCE) with
Ni-filtered Cu Ka(l¼ 1.5405 Å) radiation Data were collected by
step scanning over a 2qrange from 10to 80with a step size of
0.02 Rietveld analysis of the XRD data was carried out using Topas
program (Version 4.1) Fourier transform infrared spectroscopy
(FT-IR) data were measured using PerkinElmer Frontier FIR
Spectrometer in the range 400e4000 cm1 using Quest single
reflection ATR accessory and KBr window The resolution was
4 cm1with 40 accumulation scans at a speed of 0.2 cm/s Thefield emission scanning electron microscope (FE-SEM) images were obtained by means of afield emission scanning electron micro-scope FEI Nova nano SEM 450 operating at an acceleration voltage
of 15 KV The energy dispersive X-ray analysis (EDX) of the samples was carried out using Carl Zeiss EVO 18 secondary electron mi-croscope with an EDAX attachment of AMETEK EDAX octane series attached to the SEM using the software TEAM The high-resolution micrographs were obtained from JEOL JEM 2100 transmission electron microscope (TEM) using a 200 kV electron beam The ultravioletevisibleenear-infrared (UVeViseNIR) spectra were measured by UVeVisibleeNIR spectrophotometer (PerkinElmer, LAMBDA 950 with an integrating sphere attachment) using spec-tralon as a standard The color coordinates were evaluated by coupling color analytical software UV WinLab to the spectropho-tometer The color of the samples was evaluated according to The Commission Internationale del' Eclairage (CIE) through L*a*b* color scales The measurement conditions were as follows: an illuminant D65 and a 10complementary observer In this system, L* is the color lightness (L* is zero for black and 100 for white), a* is the green ()/red (þ) axis, and b* is the blue ()/yellow (þ) axis The total solar reflectance is expressed as the integral of the percent
reflectance times the solar irradiance divided by the integral of the solar irradiance when integrated over the 300e2500 nm range as shown in the formula,
R¼
ð2500
300 rðlÞiðlÞdl
ð2500
300 iðlÞdl
(1)
where r(l) is the spectral reflectance obtained from the experiment and i(l) is the standard solar spectrum (Wm2mm1) The NIR solar reflectance spectra were determined from ASTM Standard G173-03 [25]
2.3 Application studies
To explore the potential of the above pigments for cool roof applications, the NIR reflectance study of selected pigments was conducted on a concrete cement slab The preparation method of the coating on the concrete cement slab as a cool coat is as follows
Atfirst, the pigment (10 wt.%) was ultrasonicated for 10 min to ensure the complete dispersion in polyurethane (PU diol solution, Sigma Aldrich) which acted as the binder The resulting viscous solution was coated on a 5 cm 4 cm 1 cm concrete cement slab and subsequently dried in air to obtain a coating For comparison, the above process was repeated with rutile TiO2 nanopowder (99.5%, Sigma Aldrich)
3 Results and discussion 3.1 Crystal structure and morphology XRD patterns of the annealed samples are shown inFig 1(a) The XRD data confirms that the samples are phase pure in nature which can be indexed to the cubic spinel ZnAl2O4corresponding to the powder diffractionfile number 05-0669 of ICDD database No extra peaks related to any other impurity was detected The crystallite size was calculated from Debye Scherrer formula
S Sameera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 524e530
Trang 3where D is the crystallite size,lis the wavelength of X-ray used,b
andqare the half-width of X-ray diffraction lines and half
diffrac-tion angle of 2q The diffraction peak of maximum intensity (3 1 1)
was used to determine the crystallite size The crystallite sizes
varied from 13 to 23 nm for the samples annealed from 600C to
1100C
Rietveld refinement of the XRD patterns was employed for the
estimation of structural parameters using the TOPAS program The
XRD powder patterns were simulated employing the space group
Fd3m with Zn at 8a, Al at 16d and O at 32e Wyckoff positions
respectively A fundamental parameter approach was used for
fitting the diffraction peaks The refined parameters are listed in
Table 1 The lattice constant for the nanostructured zinc aluminates
obtained here is in good agreement with the reported value
(a¼ 8.0848 Å) of ZnAl2O4.Fig 1(b) shows the expanded view of the
(311) peak at 2qaround 36 It is observed fromFig 1(b) that the
reflection slightly shifts to higher angles indicating a contraction of
lattice for samples annealed up to 800C The graphical output of
the Rietveld refinement of ZnAl2O4samples is shown inFig S1 It is
seen that the best possiblefits are obtained in all the samples
FT-IR spectra of the samples are shown inFig 2 For the sample
ZnAl2O4annealed at 600C, a small band at 3471 cm1 is seen
which corresponds to OH group indicating slight water content in
the sample The vibrations corresponding to the spinel structure
are well observed around 652, 548 and 475 cm1 The spinels
exhibit stretching bands in the 500e900 cm1range [21], which
are associated with the vibrations of metal-oxygen,
aluminium-oxygen and metal-aluminium-oxygen-aluminium [26] These bands
corre-spond to the regular spinel structure with six-fold coordinated
aluminium [8] No other phases are identifiable in the FT-IR spectra which is consistent with the XRD results
FE-SEM micrographs presented inFig 3reveal that the particles are homogeneous and almost spherical in shape The average sizes
of the particles are found to be in the range of 25e35 nm.Fig 4(a) illustrates the transmission electron microscopic analysis (HR-TEM)
of representative ZnAl2O4sample annealed at 1100C Particle size distribution is in the range of 25e30 nm calculated from HR-TEM images through ImageJ software It can be seen that agglomera-tion observed in the FE-SEM is also detected in TEM InFig 4(b), the HRTEM clearly shows that the spacing between periodic fringes is 0.24 nm which is in agreement with the crystal plane space for (1 1 1) plane of ZnAl2O4and is in agreement with the XRD analysis The corresponding selected area electron diffraction (SAED) pattern shown in Fig 4(c) indicates the nanocrystalline nature of the sample Hence, the TEM, XRD and SEM results are in agreement with each other
3.2 Chemical analysis Elemental analysis using (EDX) revealed that the obtained stoichiometric composition is very close to the theoretical composition in the analyzed regions The peaks at 2.1e2.2 keV in Fig 1 (a) Powder XRD patterns of the zinc aluminates annealed at various temperatures and (b) An expanded view of the (311) peak at 2qaround 36for the same samples.
Table 1
Structural parameters of ZnAl 2 O 4 samples annealed at different temperatures.
Lattice parameters
a (Å) 8.1161(8) 8.0951(1) 8.0098(1) 8.0742(2)
R-factors
The fixed atomic fractional coordinate positions are Zn [8a] (0.125, 0.125, 0.125), Al
S Sameera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 524e530
Trang 4the EDX spectra are due to the gold, which is coated on the samples
before analysis Thus, the formation of pure ZnAl2O4is confirmed
by EDX analysis which corroborates to the XRD analysis.Fig S2
shows the EDX spectra of all the samples
3.3 UVevisible studies and heat reflection performance
The UVevisible absorption spectra of the ZnAl2O4samples are
shown inFig 5 Here the Kubelka Munk reemission function which
is used as a measurement of absorption via powder, to convert
reflectance spectrum to the absorption spectrum [27] An
absor-bance peak around 275 nm is observed for all the samples This is
due to the fundamental band-to-band electron excitations
(elec-tron transition betweenfilled O 2p orbitals and empty Al 3s
or-bitals, with the possibility of mixing of 3s and 3p wavefunctions for
Al3þ) and related to the band structures of intrinsic properties of
spinels A shoulder in the range of 330e400 nm is observed on the
main peak due to the electronic excitation betweenfilled O 2p and
empty Zn 4s orbitals which indicates some defects [28] This
Fig 3 FE-SEM micrographs of the zinc aluminate samples annealed at different temperatures.
Fig 4 (a) TEM (b) HR-TEM images and (c) SAED pattern of of ZnAl 2 O 4 annealed at 1100C.
Fig 5 Absorbance spectra of the zinc aluminate samples annealed at different tem-peratures (Inset: Colorimetric parameters).
S Sameera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 524e530
Trang 5shoulder disappears in the case of ZnAl2O4 sample annealed at
1100C indicating the absence of defects A change in the
absorp-tion edge of the ZnAl2O4samples is seen as the annealing
tem-perature increases
ZnAl2O4is considered to be a direct band gap semiconductor
[29] The band gap energy (Eg) of the ZnAl2O4 samples can be
determined from plots of (ahn)2versus hnusing the Tauc relation
[30], where h is the Planck constant,ais the KubelkaeMunk (KeM)
absorption coefficient, andnis the frequency
The plots of (ahn)2versus hnfor all the samples are shown in Fig S3 The extrapolation of linear regions of the plots give the direct band gap values of 4.07, 3.83, 3.73 and 3.78 eV respectively which is close to that of reported literature [10] The inset ofFig 5 shows the CIE L*a*b* color coordinates of the samples with com-parison to rutile TiO2
Near-infrared irradiation (NIR) lying in the 700e2500 nm ac-counts for 52% of the energy in the solar irradiance spectrum [31] The reflectance of the samples increases with annealing tempera-tures in the infrared wavelength due to the increasing roughness of the surface of the grains The NIR reflectance spectra of ZnAl2O4
samples annealed at various temperatures are shown in Fig 6 Some absorption is also seen in the 1350e2500 nm region which may arise from combination and overtones of fundamental pro-cesses that occur in the mid-infrared region Since nanocrystalline oxides have a high surface area, more water molecules, carbonate ions, etc will be adsorbed and hence exhibit strong absorption features So the reflectance in 700e1300 nm region is taken into account here Also, the particular wavelength at 810 nm is selected because many photonic devices operate around this wavelength [32] The highest NIR reflectance of 86% is achieved in ZnAl2O4
annealed at 1100C (Table 2).Fig 7shows the NIR solar reflectance spectra of the ZnAl2O4 samples determined by ASTM standard G173-03 As mentioned earlier, in the entire solar spectrum, 52% is composed of NIR (700e2500 nm), which can be further carved up into the shortwave NIR (700e1100 nm) and longwave NIR Fig 6 NIR reflectance spectra of ZnAl 2 O 4 samples annealed at various temperatures.
Fig 7 NIR solar reflectance spectra of ZnAl
Table 2
Reflectance values of ZnAl 2 O 4 samples annealed at different temperatures.
S Sameera et al / Journal of Science: Advanced Materials and Devices 4 (2019) 524e530
Trang 6(1100e2500 nm) [33] Among them, shortwave NIR is the main
heat-generating area The NIR solar reflectance spectra displayed in
Fig 7shows the sunlight radiation energy distribution of prepared
samples It can be seen that the radiant energy distribution is
mainly located in the shortwave NIR range (700e1100 nm) with a
distinct decrease occurring only at 1500e2500 nm However, the
prepared ZnAl2O4 samples exhibit a total solar reflectance above
75% This is imputable to the fact that total solar reflectance covers
the entire range of radiation between UV and NIR These results
summarize that the ZnAl2O4samples have the potential to reduce
heat build-up
3.4 Application studies over concrete surface
To assess the utility of the nanopigments for energy saving
ap-plications, their applicability was checked on a building roofing
material like concrete cement The particular ZnAl2O4nanopigment
exhibiting the highest NIR reflectance annealed at 1100 C was
selected to prepare cool coating Single-layer cool white coatings
are more suitable for cooling of reinforced concrete roofs, while
multilayer ones may be better for cooling of metal-based roofs [34]
The most commonly used pigment in the manufacturing of cool
coatings is titanium dioxide (TiO2) Of the available white pigments,
rutile TiO2 is known to exhibit the strongest near-infrared light
reflection [35]
The NIR reflectance spectra of the resulting coatings coated over
bare concrete cement surfaces are shown inFig 8 A bare concrete
surface exhibits a low NIR reflectance of 27%, while the bare con-crete coated with ZnAl2O4and TiO2have NIR reflectances of 90% and 87% in the 700e2500 nm region, respectively The total solar
reflectance and CIE color coordinates were measured in concrete cement as described earlier The CIE color coordinates of ZnAl2O4 coated concrete cement substrates are (L* ¼ 97.98, a* ¼ 2.34, b*¼ 7.52).Fig 9 shows the NIR solar reflectance spectra of bare concrete coated with ZnAl2O4and TiO2 The photographs of the resulting coated samples are shown in the inset ofFig 9 The total solar reflectance of ZnAl2O4 coated concrete is 89% while that of TiO2coated concrete is 87% The results point out that a cool roof coating based on ZnAl2O4 nanopigment can enhance the NIR
reflectance, which leads to a reduction in the surface temperature
of the roof
4 Conclusion The nanostructured ZnAl2O4pigments have been prepared by the citrate solegel method The XRD results confirm the formation
of a single-phase cubic ZnAl2O4 structure Rietveld analysis was performed to determine the structural parameters As the anneal-ing temperature increased, the crystallinity of the samples increased, resulting in a slight increase in grain size Remarkable NIR reflectance of 83e87% was observed in the 700e1300 nm range
in the nanostructured ZnAl2O4pigments The developed pigments could confer their NIR reflecting properties to the concrete sub-strate under study Since these nanopigments act as cool coatings with high total solar reflectance, they may serve as potential energy-saving materials
Declaration of Competing Interest The authors declare no conflict of interest
Acknowledgments S.Sameera acknowledges the financial support from the Uni-versity Grants Commission (UGC), Govt of India, New Delhi to-wards Dr D S Kothari Postdoctoral fellowship program (Ref No F.4-2/2006 (BSR)/CH/16-17/0043) Viji Vidyadharan acknowledges
to SERB, DST (India) for National PDF (PDF/2016/002564/PMS) The authors thank Soumya Valsalam from Central Laboratory for Instrumentation and Facilitation (CLIF), University of Kerala, for the EDX analysis
Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.10.003
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