Nanostructured zinc aluminates: A promising material for cool roof coating

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Original Article

Nanostructured zinc aluminates: A promising material for cool roof

coating

S. Sameera, Viji Vidyadharan, Sajesh Sasidharan, K.G. Gopchandran

*

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 600
_{C, 700}
_{C, 800}
_{C, and 1100}
_{C 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 ( />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:(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

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polymeric 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 300e400
C, 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 90
_{C 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 200
C for 2 h. After grinding the dried sample,
it was annealed separately at 600
C, 700
C, 800
C and 1100
C for
6 h with a heating rate of 10
C/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(

l

¼ 1.5405 Å) radiation. Data were collected by
step scanning over a 2

q

range from 10
to 80
with 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 10
complementary observer. In this system, L* is the
color lightness (L* is zero for black and 100 for white), a* is the
green ()/red (ỵ) axis, and b* is the blue ()/yellow (ỵ) 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,

Rẳ
2500

300 r

l

ịi

l

ịd

l

2500
300 i

l

ịd

l

(1)

where r(

l

) is the spectral reectance 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

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where D is the crystallite size,

l

is the wavelength of X-ray used,

b

and

q

are the half-width of X-ray diffraction lines and half
diffrac-tion angle of 2

q

. 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 600
C to
1100
C.

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
(a_{¼ 8.0848 Å) of ZnAl}2O4.Fig. 1(b) shows the expanded view of the

(311) peak at 2

q

around 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 800
C. 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 600
C, 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 1100
C. 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 36
for the same samples.

Table 1

Structural parameters of ZnAl2O4samples annealed at different temperatures.

Sample 600
_C ₇₀₀
_C ₈₀₀
_C ₁₁₀₀
_C

Lattice parameters

a (Å) 8.1161(8) 8.0951(1) 8.0098(1) 8.0742(2)

V(Å)3 _534.61 _530.49 _513.89 _526.39

R-factors

Rexp(%) 9.14 9.70 9.54 9.25

Rp(%) 6.87 7.36 7.10 6.87

Rwp(%) 9.58 10.18 10.15 9.92

RBragg(%) 9.64 16.56 8.88 13.56

GOF 1.05 1.05 1.06 1.07

Thefixed atomic fractional coordinate positions are Zn [8a] (0.125, 0.125, 0.125), Al

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the 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
Al3ỵ) 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 ZnAl2O4annealed at 1100
C.

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shoulder disappears in the case of ZnAl2O4 sample annealed at

1100
C 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 (

a

h

n

)2versus h

n

using the Tauc relation
[30], where h is the Planck constant,

a

is the KubelkaeMunk (KeM)
absorption coefficient, and

n

is the frequency

a

h

n

¼ A(h

n

Eg)n ₍₃₎

The plots of (

a

h

n

)2versus h

n

for 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 1100
C (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 ZnAl2O4samples annealed at various temperatures.

Fig. 7. NIR solar reflectance spectra of ZnAl2O4samples annealed at various temperatures.
Table 2

Reflectance values of ZnAl2O4samples annealed at different temperatures.

ZnAl2O4 600
C 700
C 800
C 1100
C

Average reflectance in Vis region (%) 75 72 78 85

Average reflectance in 700e1300 nm region (%) 83 82 81 87

NIR reflectance at 810 nm (%) 83 81 80 86

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(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 roo_fing
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
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