Preparation, infrared emissivity and thermochromic properties of Co doped ZnO by solid state reaction

Article abstract of DOI:10.1016/j.jallcom.2017.05.252

In this paper, green Zn_1-xCo_xO nanopowders were synthesized via solid-state reaction at different calcination temperatures. The effects of sinterring temperature on structure, morphologies, color and infrared emissivity was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV–visible absorption spectroscopy, Raman spectroscopy and IR-2 dual-band infrared emissometer, respectively. The results show that the as-synthesized samples have hexagonal wurtzite structure and meantime the peaks of the secondary phase, Co₃O₄, were observed when the sinterring temperature is below 900 °C. The sintering temperature plays a vital important role in crystallite sizes, lattice parameters, volume of the unit cell as well as microstructure. According to the EDS results, Zn ions were substituted by the Co ions in the ZnO lattice where Co ions present in the +2 state indicated via XPS analysis. Additionally, UV–visible absorption spectra shows that absorption edge have a red shift while the corresponding energy band gap of the semiconductor nanoparticles decrease with the increasing sintering temperature, which affects the thermochromic characteristics of materials significantly. The infrared emissivity is related to the microstructure, grain size, conductivity and lattice vibration.

Full text of DOI:10.1016/j.jallcom.2017.05.252

Journal of Alloys and Compounds 720 (2017) 105e115
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Preparation, infrared emissivity and thermochromic properties of Co
doped ZnO by solid state reaction
*
Yongmei Zhu , Guoyue Xu, Tengchao Guo, Haili Hou, Shujuan Tan
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiang Jun Street 29, Nanjing 211106, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 March 2017
Received in revised form
In this paper, green Zn1-xCo O nanopowders were synthesized via solid-state reaction at different
calcination temperatures. The effects of sinterring temperature on structure, morphologies, color and
infrared emissivity was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM),
energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UVevisible absorption
spectroscopy, Raman spectroscopy and IR-2 dual-band infrared emissometer, respectively. The results
x
19 May 2017
Accepted 24 May 2017
Available online 25 May 2017
show that the as-synthesized samples have hexagonal wurtzite structure and meantime the peaks of the
ꢀ
secondary phase, Co
3
O
4
, were observed when the sinterring temperature is below 900 C. The sintering
Keywords:
temperature plays a vital important role in crystallite sizes, lattice parameters, volume of the unit cell as
well as microstructure. According to the EDS results, Zn ions were substituted by the Co ions in the ZnO
lattice where Co ions present in the þ2 state indicated via XPS analysis. Additionally, UVevisible ab-
sorption spectra shows that absorption edge have a red shift while the corresponding energy band gap of
the semiconductor nanoparticles decrease with the increasing sintering temperature, which affects the
thermochromic characteristics of materials significantly. The infrared emissivity is related to the
microstructure, grain size, conductivity and lattice vibration.
Infrared emissivity
Thermochromic
Co-doped ZnO
Solid-state reaction
©
2017 Elsevier B.V. All rights reserved.
1. Introduction
x
expensive but also dangerous. As for La1-xSr MnO3, it usually has
high solar absorptance (over 0.8) and needs sophisticated in-
struments to control assistantly. Consequently, both of them are not
suitable to fabricate thermochromic materials.
Recently, thermochromic materials have received significant
attention due to the potential utilization in various sectors such as
space, industry and military fields [1,2]. When they are applied in
industry fields, it can not only as temperature indicating coatings to
monitor instruments temperature, but also can as building coatings
to provide cool tones in summer and warm tones in winter and
therefore to decrease the energy consumption of a building envi-
ronment in any seasons [3]. The variable emissivity coatings, which
is based on thermochromism, have great application prospects to
spacecraft thermal control, owing to the lightness, low energy
consumption and facility [4,5]. The smart thermoresponsive coat-
ings can be used as chameleon coatings in military technology to
change the colors of military equipment to match its surroundings,
and then decrease the probability being detected [6,7].
As a non-toxic, cheap, readily available and environmentally
white pigment, zinc oxide with low light absorbance and high ra-
diation dispersion in the visible regio, which can be used in sun-
screen compositions to block UV radiation. Besides, the high
reflectance appears in near - infrared region makes it suitable
candidate for space applications [10]. It has wide band gap energy
(3.3 eV) at room temperature [11] and high excitonic binding en-
ergy (60 meV) [12]. As an important n-type semiconductor mate-
rial, ZnO has received extensive attention because of its remarkable
optical, magnetic, and electrical properties [13e17]. The transition
metal elements doped ZnO matrix is one of the most important
methods to modify the electronic structure and characteristics of
the host material [18e20]. Among all of the transition metals, Co
has been widely employed because of its variable oxidation state,
large magnetic and high solubility limit in ZnO due to the similar
According to the literatures, the materials used to synthesize
thermochromic materials mainly focus on VO
2
[8] and La1-
x
x 3 2
Sr MnO [9]. Nevertheless, it is evident that VO is not only
2
þ
ionic radius (0.072 nm) with Zn atom (0.074 nm) [21e24]. In the
past several years, many studies referred to Co-doped ZnO have
focused on the magnetic properties [25e27]. The properties of
variable emissivity and thermochromic, however, are neglected.
*
Corresponding author.
E-mail address: 1105249397@qq.com (Y. Zhu).
http://dx.doi.org/10.1016/j.jallcom.2017.05.252
925-8388/© 2017 Elsevier B.V. All rights reserved.
0

106
Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
There is no doubt that emissivity is one of the important physical
properties of materials, which have wide applications in various
fields. The materials with high emissivity at high temperature can
be used in furnaces lining, supersonic aircraft and other fields to
save energy or decrease the temperature by radiation [28,29]. Low
infrared emissivity (LIE) materials can prevent targets from being
detected by infrared detectors when coated on the surface of mil-
itary equipments [30e32]. However, thermochromic characteris-
tics and variable emissivity properties of ZnO-based materials were
scarcely reported so far.
In this paper, we conducted a simple solid-state reaction to
synthesize green Zn1-xCo O nanopowders successfully and studied
x
the effects of calcination temperatures on its crystal structures,
morphologiesas well as infraredemissivity. Besides, the colorof Zn1-
x x
Co O nanopowders can graduallychange from green toyellow with
the increase of testing temperature and the changes are reversible.
2
. Experimental details
2 3
Fig. 1. TG-DSC results of Co O .
The doped oxides Zn1-xCo
action. Raw ZnO powders (9.503 g) and Co O
3 4
x
O were prepared by solid-state re-
powders (0.497 g)
were mixed before calcination. This mixture was gently ground in
an agate mortar for homogenisation. The obtained powders were
ꢀ
The pronounced loss of weight can be observed at 900 C, which is
due to the fact that Co powders decompose into CoO at high
temperature condition. Correspondingly, remarkable endo-
2 3
O
ꢀ
calcined for 20 h in furnace at designed temperatures from 700 C
a
ꢀ
ꢀ
ꢀ
to 1100 C with a step of 100 C with a constant heating rate of 4 C/
min until reach the annealing temperature. After annealed, the
powder was cooled to room temperature gradually in furnace.
thermic peak in DSC curve is observed at the temperature around
00 C. In addition, a small endothermic peak can be observed
ꢀ
9
ꢀ
around 700 C because of the decomposition of Co
2
O
3
into Co
3
O
4
. It
Finally, the pre-treated Zn1-xCo
ing temperature were obtained.
The decomposition temperature of the Co
x
O powders with different anneal-
indicates that there are two step degradation processes in the
sample. The DSC result provides useful information to determine
the appropriate starting temperature for the calcination process.
Therefore, from the DSC, it can be seen that the properly annealing
2 3
O was investigated
by simultaneous thermogravimetry and differential scanning
calorimetry (TG-DSC), which was performed with a temperature-
ꢀ
temperature is 900 C, which ensures the completely thermal
ꢀ
increasing rate of 10 C/min using a simultaneous TG-DSC
decompose of Co
Nan et al. [33].
2 3
O . The result is similar to that reported by Z.D.
ꢀ
STA449C model analyzer in N
2
atmosphere at 1100 C. The crys-
talline phases of the sintered powders were examined by x-ray
ꢀ
diffraction using CuK
a
radiation (XRD Rigaku, D/max-RA) from 20
3.2. X-ray diffraction study (XRD)
ꢀ
to 80 (2
q
). The presence of Zn, Co, and O in the samples was
confirmed by the energy dispersive spectroscopy (EDS). The
chemical state of Co in Zn1-xCo O samples was studied by Thermo
Scientific X-ray Photoelectron Spectrometer (XPS, PHI-5000 Ver-
saprobe) with a monochromatic Al K source with 1486.6 eV of
energy and 150 W of power. The microstructure of samples was
characterized by scanning electron microscopy (SEM, FEI SIRION-
2þ
ions doped in ZnO
Fig. 2 gives the XRD spectrums of Co
x
nanoparticles. Clearly, all the peaks well match with the JCPDS date
of pure ZnO (card no.36-1451) with space group of p63mc, indi-
cating that all samples possess a wurtzite (hexagonal) zincite
crystal structure with a more preferential orientation along the c-
axis perpendicular. However, a small amount of impurity peaks are
a
1
00). To determine the optical band gap of different samples, the
optical absorption measurements were performed with a Shimadzu
450 UVeVIS spectrophotometer and the investigated wavelength
captured arisen from the secondary phase (Co
3
O
4
) when sintered at
ꢀ
8
00 C or below. The secondary phase may be due to the incom-
2
ꢀ
pletely decomposition of Co
2
O
3
below 900 C. This result is
ꢁ1
ranged from 200 to 800 nm. Raman spectra from 100 cm to
consistent with TG-DSC study. The RIR method was employed to
calculate the phase composition of the hybrids quantitatively, ac-
cording to the following equations [34,35]:
ꢁ
1
7
00 cm were acquired at room temperature using a Jobin-Yvon
T64000 Triple-mate instrument. The electrical resistivity of the
Zn1-xCo O nanoparticles in room temperature was measured with
the two-electrode method in AC circuit (FT-303 serial resistivity
tester). The infrared emissivity in the range of 3e5 m in different
x
Ia
W
a
b
¼ I
m
a
þ ðI =ðRIR =RIR
b
b
a
ÞÞ
temperature was detected by an infrared emissometer (IR-2 dual-
band emissometer, Shanghai Institute of Technical Physics, CAS,
China). The temperature dependence of infrared emissivity was
performed using BC-1 temperature control instrument (Shanghai
Institute of Technological Physics, China). The testing temperature
Ib
W
¼
¼ 1 ꢁ W
a
Ib þ ðI
a
=ðRIR
a
=RIR ÞÞ
b
ꢀ
The weight ratio of Co
3
O
4
at calcination temperatures of 700 C
ꢀ
ꢀ
ꢁ1
ꢀ
can range from 20 C to 700 C with a heating rate of 20 K$min
.
and 800 C is 3.7% and 2.8%, respectively. This also implies that
2
þ
higher temperature is helpful for Co doping into ZnO lattice.
Moreover, different starting materials, for example CoO or Co, may
need different calcination temperature to achieve pure single ZnO
phase [36,37].
3
. Results and discussion
3.1. Thermogravimetry and differential scanning calorimetry study
(
TG-DSC)
Fig. 2(b) shows that the higher the temperature is, the diffrac-
tion peaks have a little shift to higher angles, and their values are
tabulated in Table 1. As we all know the radius of Co (0.72 Å) is
2
þ
TG-DSC analysis result of the Co
2
O
3
powders are shown in Fig. 1.

Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
107
lattice parameters ‘a’ and ‘c’ by the following equation [39].
(
ꢀ
ꢁ
!)
2
2
l2
c2
1
d2
4
3
h þ k þ hk
¼
þ
2
a
The calculated lattice parameters ‘a’ and ‘c’ and c/a ratio of pure
2
þ
and Co doped ZnO nanoparticles are shown in Table 1. With the
increasing of temperature, the lattice parameter ‘a’ is found a little
increasing, whereas ‘c’ parameter is found decreasing. The higher
2þ
temperature makes the bigger doping concentration of Co into
ZnO, which results in the shift of 2 into large angles. Actually, the
q
replacement of Zn ions by Co ions decreases the lattice constant c
and this shrink in the z-axis of the unit cell causes an elongation in
the x axis, so it leads to a little increase of the lattice constant a.
These results are in good agreement with those reported earlier
[
40,41].
The average crystallite size (D) of nanoparticles was calculated
from the Debye Scherrer's equation [42].
D ¼ b0
:9l
COS
q
where, D is the crystallite size.
l
is the wavelength of X-ray
is the full width at half
(l
¼ 1.54056 Å),
q
is the Bragg's angle and b
maxima of the most intense diffraction peaks. As shown in Table 1,
the decrease of the full width at half maximum of the diffraction
peak reveals that the attained particle size has increased from
5
6.969 nm to 61.166 nm with the increasing of the calcination
ꢀ
ꢀ
temperature from 700 C to 1000 C, which could be attributed to
the fact that as the temperature went high, many neighbouring
particles tended to fuse together to form larger particle size by
melting their surfaces [43]. When the sintering temperature is at
ꢀ
1
100 C, the average crystallite size is found to decrease to
55.460 nm. This phenomenon could be explained as follows: the
distortion in the host ZnO lattice [44]. The introduction of foreign
impurity contributes to the decrease of nucleation rate and the
growth rate is decreased subsequently. A. Goktasa et al. [45] and
S.D. Birajdar et al. [67] have reported that the crystalline size de-
2
þ
creases with increase of Co content in ZnO.
The volume of the unit cell was calculated by using the following
equation:
x
Fig. 2. (a) XRD patterns of Zn1-xCo O nanoparticles at different calcination tempera-
tures; (b) magnified XRD patterns of samples.
pffiffiffi
3
2
2
V ¼
a c
smaller than that of Zn2 (0.74 Å). According to the Bragg's law [38].
þ
where V is the volume of the hexagonal unit cell, ‘a’ and ‘c’ is the
lattice parameter. It was found that the volume of the unit cell
decreases with increasing of sintering temperature, which is dis-
played in Table 1.
2
d sin
q
¼ nl
where
q
is the diffraction angle,
l
is the incident wavelength
From the XRD patterns in Fig. 2(b), it is found that the intensity
of the (101) plane for all the samples gradually decreases with the
increasing of sintering temperature, which indicates that the
dopant of Co ions substitute in the inner lattice of Zn ions [46,47].
(l
¼ 1.5406 Å) and n is the diffraction series (n ¼ 1 in normal
conditions). For hexagonal wurtzite structure of ZnO, the inter-
planar spacing d is related to the Miller indices h, k and l as well as
Table 1
Structure parameters of the sample.
ꢀ
T( C) Position of (002) peak (degree) Position of (101) peak (degree) Intensity of (101) peak (a.u.) FWHM degree D002 (nm) Volume (Å) Lattice
c/a ratio
parameters
a(Å) c(Å)
7
8
9
1
1
00 34.401
00 34.440
00 34.441
000 34.460
100 34.481
36.239
36.260
36.258
36.261
36.271
3328
3209
3046
2944
1849
0.146
0.142
0.138
0.136
0.150
56.969
58.578
60.276
61.166
55.460
47.64
47.59
47.60
47.58
47.56
3.25024 5.20719 1.60209
3.25033 5.20438 1.60119
3.25057 5.20312 1.60068
3.25065 5.20152 1.60015
3.24993 5.19951 1.59988

108
Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
ꢀ
When the sintering temperature is 1100 C, the diffraction peak
intensities is weakened, which may be due to the weakened crys-
talline quality.
And the liquid phase helps facilitate ion diffusion and enhances the
grain growth mechanism during sintering process [48]. The grain
growth could bring about the loss of specific surface area. However,
ꢀ
when the sintering temperature continues rising to 1100 C, the
particle diameter has no apparent growth.
3.3. Scanning electron microscopy (SEM)
Fig. 3AeE shows the morphology of Zn1-xCo
x
O at various tem-
3.4. Energy dispersive spectrometer
peratures with the same magnification. The SEM images imply that
the temperature plays an important role in the technical process of
morphology-controlled synthesis. It can be observed that the
samples consist of spherical particles and construct a loosely
In order to analyze the chemical compositions and amount of Co
contents in doped ZnO, the powders were examined by EDS and
their spectrums are shown in Fig. 4. All samples have one zinc peak
at 1.01 KeV and O signals at 0.52 KeV. The cobalt peaks at 0.78 KeV
and 6.94 KeV are observed in all samples. These verify the presence
of Co ions in ZnO and suggest that the Co ions substituted the Zn
ions in the ZnO lattice. The percentages of Co ions in all samples are
less than the target composition 2.5%. This is probably due to the
ꢀ
porous structure when the sintering temperature is at 700 C. The
grain size of powders appears to increase with higher sintering
temperature, and the block grains are formed when the sintering
ꢀ
temperature reaches 1000 C. It is believed that the increase of
grain size is due to the effects of the liquid phase sintering process.
ꢀ ꢀ ꢀ ꢀ ꢀ
x
Fig. 3. SEM images of the Zn1-xCo O nanoparticles with different sintering temperatures: A 700 C; B 800 C; C 900 C; D 1000 C; E 1100 C.

Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
109
decomposition and volatilization of Co
3
O
4
at high temperatures
In the XPS spectra, all the binding energy of relevant elements in
the sample system is referenced to C 1s hydrocarbon signal at
284.8 eV. As can be seen from the high-resolution Co 2p spectrum
of the sample, the obtained binding energies at 781.0 eV and
796.4 eV are attributed to Co 2p3/2 and Co 2p1/2, respectively. It
indicates that the oxidation number of Co in the ZnO lattice is 2þ, It
should be noted that each of the two peaks in the spectrum is
followed by satellite peak, which is due to the exchange between
valence electrons and core hole, resulting in the change of voltage.
It can be seen that the valence electrons are created by the core hole
and the low detection limitation in SEM-EDS measurement system.
2
þ
ꢀ
The concentration of Co at calcination temperatures of 700 C,
ꢀ ꢀ ꢀ ꢀ
8
00 C, 900 C, 1000 C and 1100 C is about 0.57%, 0.99%, 1.46%,
1.58% and 2.30%, respectively. It can be seen that with the increase
2
þ
become
of calcination temperature, the concentration of Co
larger, which is consistent with XRD study as well.
3.5. XPS analysis
2
þ
The oxidation states of Co, Zn, and O in Co-doped ZnO samples
have been detected via X-ray photoelectron spectra (XPS). The re-
sults of the sample which calcined at 1000 C are shown in Fig. 5(a).
[49], and this is a feature of Co ions. The binding energy of Co 2p3/
3
þ
2
and Co 2p1/2 signals are different from that of Co ions 780.0 eV
and 795.2 eV as well as Co metal 777.8 eV and 792.0 eV. This shows
ꢀ
ꢀ ꢀ ꢀ ꢀ ꢀ
x
Fig. 4. EDS spectra of the Zn1-xCo O powders with different calcination temperatures: A 700 C; B 800 C; C 900 C; D 1000 C; E 1100 C.

110
Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
ꢀ
Fig. 5. (a) XPS spectra of the sample calcinated at 1000 C; (b) the O1s spectra of different samples.
that Co dopants entered into the ZnO lattice only as Co2þ ions [50].
As can be seen from the O 1s signal, more than one state of O in the
ZnO lattice is the main reason causing asymmetry of the O 1s peak.
The state of Co doping in ZnO has been studied by using two
Gaussian functions to fitting O 1s XPS spectra. The binding energy
of 532.8 eV is attributed to the oxygen which is adsorbed on the
surface of the ZnO powder. The highest peak in the spectra is
42.03% with the improvement of sintering temperature from
700 C to 1100 C.
ꢀ
ꢀ
3.6. Raman spectra
The Raman spectra for different samples are shown in Fig. 6. The
wurtzite structure of ZnO, with space group C4 6v, has four Raman-
active optical phonon modes (A þ 2E ) and one inactive mode
þ E
). The infrared active modes of A and E are split into longitu-
contributed by O2 ions in hexagonal wurtzite structure of ZnO. The
peak at 531.7eV is the signal of oxygen vacancies. The binding en-
ergies of Zn 2p3/2 and Zn 2p1/2 are 1021.3 e V and 1044.4e V, which
is in agreement with the standard data of zinc oxide.
ꢁ
1
1
2
(B
1
1
1
dinal modes (LO) and transverse modes (TO). The spectral peak
ꢁ1
locates at about 333 cm , corresponding to the multi phonon
Fig. 5(b) shows the O1s spectra of different samples. Their
relative areas can be correlated with the contents of oxygen va-
cancies. It is found that the vibration peak at about 530.1 eV de-
creases while another peak at about 531.7 eV increase with the
elevation of calcination temperatures. The area ratio of oxygen
vacancies can be calculated from the XPS spectra. As shown in
Table 2, the area ratio of oxygen vacancies increased from 24.86% to
ꢁ1
mode of ZnO. The peak at 378 cm mode is attributed to the
transverse optical mode (TO). A very weak shoulder locates at the
ꢁ1
low energy side of 437 cm
sponding to the mode of E
peak in pure ZnO sample, corre-
ꢁ1
1
(TO) at 410 cm . It disappears at high
sintering temperature which is due to the disorder induced by the
ꢁ1
Co doping. As shown in Fig. 6, the sharp peak located at 437 cm
2
corresponds to the nonpolar optical phonon E mode, which is the
characteristic peak of wurtzite structure of ZnO, and that is sensi-
tive to the crystalline quality crystal and stress of the samples. With
the increase of sintering temperature, the E (high) modes of the
2
Table 2
The area ratio of each peak in the O 1s XPS spectra for different samples.
samples decreased in different degrees. That is to say, the ZnO
ꢀ
T( C)
Area ratio%
30.1(eV)
crystal structure is destroyed by the Co substitution [51]. When the
ꢀ
5
531.7(eV)
532.8(eV)
sintering temperature increases upto 1100 C, the Raman line of E
2
(
High) becomes very broad and weak, indicating decreased crys-
7
8
9
1
1
00
00
00
000
100
72.56
72.33
62.77
59.10
56.22
24.86
25.44
35.58
37.63
42.03
2.58
2.23
1.65
3.27
1.75
2
tallinity. The E (high) mode redshift is adequately attributed to the
phonon softening caused by the in-plane lattice expansion [52].
The peak at 578 cm is due to the defects such as oxygen vacancies
and Zn interstitials. An additional peak was observed at 545 cm
ꢁ1
ꢁ1

Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
111
x
Fig. 6. Raman spectra of Zn1-xCo O samples with different sintering temperatures.
corresponding to a multiphonon peak due to the substitution of Co
with Zn in the lattice.
3.7. UVevis absorption spectra and optical band gap
To explore the optical properties and band gap of different
samples, UVevis absorption spectra was measured at room tem-
perature. From Fig. 7(a), three absorption peaks can be observed at
5
65 nm, 610 nm, and 660 nm, respectively. These absorptions are
attributed to the ded transitions of the Co ions, corresponding the
4
2
4
2
4
transitions from
A
2
(F) to E(G),
A
2
(F) to
T
1
(P), and
A
2
(F) to
2
2þ
1
A (G), respectively [53]. These absorption peaks indicate that Co
2
þ
ions have partially substituted Zn ions in all the samples [54]. The
absorption edge is found to red shift with increase of the sintering
temperature as exhibited in Fig. 7(b).
The optical band gap of all samples can be obtained by Kubelka-
Munk function [55]:
Fig. 7. (a) UVevisible absorption spectra Zn1-xCo
x
O nanoparticles; (b) the plot of
ꢀ
ꢁ
2
2
ð
a
hvÞ ¼ A hv ꢁ E
g
(
a
hv) verses photon energy (hv) of Zn1-xCo
x
O nanoparticles.
where,
a
is the absorption coefficient, h is the Planks constant, A is a
transiting from apple green to moss green, when the sintering
constant, hv is the photon energy and Eg is the optical energy band
gap. The optical band gap energies were found to be 3.16 eV,
.76 eV, 2.64 eV, 2.58 eV, and 2.44 eV of the Zn1-xCo
ꢀ
ꢀ
temperature increases from 700 C to 1100 C. The color of all the
samples can transformate from green to army yellow at high
temperatures and the phenomenon is much more obvious with the
enhancement of testing temperature. Fig. 8(b) shows the color
2
x
O powders
ꢀ
obtained at various calcination temperature, namely, 700 C,
ꢀ ꢀ ꢀ ꢀ
8
00 C, 900 C, 1000 C and 1100 C. Inspecting Fig. 7(b), it is
under different test temperatures of the sample which calcinated at
obvious that the Eg value of all samples decrease with the increase
of the calcination temperature, which mainly originated from the
sped exchange interactions between the band electrons and the
ꢀ
9
00 C. Besides, it is also interesting to note that the color-changing
temperature has decreased with increasing calcination tempera-
ꢀ
ꢀ
ꢀ
ꢀ
ture from about 400 C at 700 C to around 200 C at 1100 C. The
phenomenon of color-changing could be explained as follows: ZnO
has typical crystal lattice defects such as oxygen vacancies. At high
temperature, more and more oxygen vacancies emerge in ZnO due
to the flowing away of oxygen atoms, which would lead to the
absorbtion of the colour centre and then the samples become yel-
low gradually. However, the oxygen comes back upon cooling and
the color recover at the same time. It was found that the color of
pure crystalline ZnO can change from white to yellow when the
2
þ
2þ
localized d electrons of the Co ions substituting Zn ions. The
exchange interactions between sed and ped orbital give rise to a
negative correction to the conduction-band and a positive correc-
tion to valence-band edges, which results in the decrease in band
gap [56]. Additionally, the high concentration of oxygen vacancy
creates an impurity level near the valence band and induces the
band gap narrowing [57]. This red shift of the band gap may be also
due to large lattice strain and heat treatment [58].
ꢀ
temperature increases to >500 C, and then transform from yellow
3
.8. Colour and thermochromic analysis
to white upon cooling to room temperature. It means ZnO is
reversible thermochromism material. Similar result has been re-
ported in the literature [59]. A decrease in the color-changing
temperature with increasing calcination temperatures is
The colour of pure ZnO powders is white, while Co2þ doped ZnO
powder is green. Fig. 8(a) reveals that the green becomes darker,

112
Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Fig. 8. (a) The color of Zn1-xCo
under different test temperatures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
x
O samples with different sintering temperatures: A 700 C; B 800 C; C 900 C; D 1000 C; E 1100 C; (b) the colour of the sample calcinated at 900 C
attributed to the decreased energy band gap, as evidenced by
UVevis absorption spectra and optical band gap analysis. From the
XPS results, it can also be said that the high concentration of oxygen
vacancies can induce the band gap narrowing and lead to the
enhancement of visible light absorption capability. Similar result is
also available [57]. The temperature dependency of the ZnO band
gap energy (Eg) can be shown by the following equation, given by
Ref. [60].
of 0.2 g were measured with a
The resistivity can be calculated from the following equation:
F2 mm mould under 5 MPa pressure.
r
¼ RA=h
a
T2
T þ
E
g
ðTÞ ¼ E
g
ð0Þ ꢁ
b
where, E
g
ðTÞ is band gap energy of ZnO at TK,
a
¼ ꢁ5.5*10ꢁ4ev/K,
b
¼ ꢁ 900 k and T is the temperature (in Kelvin). Based on the
equation, it is seen that E
ature. Only when the photon energy (hv) is larger than the band
gap E , could the optical absorption be emerged [61]. It probably
g
decreases with the increase of temper-
g
was another reason that yellow ZnO becomes darker at higher
testing temperature. These results confirm that the decrease of
optical energy band gap is very beneficial to thermochromism
properties. Thus, the properties have potential values in application
of visible light and stealth materials.
3.9. Electrical resistivity measurements
Because the samples were in powder form, measured resistance
are affected by the powder density and contacts between grains.
Thus, the resistance of Zn1-xCo O nanoparticles with a fixed weight
x
Fig. 9. The electric resistivity of Zn1-xCo O samples with different sintering
temperatures.
x

Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
113
where
sectional area (mm ) and h is the thickness (mm) of the pressed
Zn1-xCo O slice powders.
As shown in Fig. 9, the influences of sintering temperature on
the electrical resistivity of Zn1-xCo O nanoparticles were investi-
gated at room temperature. It can be observed that the resistivity of
Zn1-xCo O nanoparticles shows a decreasing tendency initially with
r
is the resistivity (M
U
$mm) of the powders, A is the cross
3.10. Infrared emissivity analysis
2
x
Fig. 10 shows the emissivity values at the wavelength of 3e5 mm
of all the samples at various test temperatures from room tem-
ꢀ
ꢀ
x
perature 20 C to high temperature 700 C, with an interval of
ꢀ
50 C. It can be seen that the infrared emissivity decrease with the
ꢀ
ꢀ
x
increase of sintering temperature from 700 C to 1000 C when the
ꢀ
ꢀ
ꢀ
the increase of sintering temperature from 700 C to 1000 C.
test temperature is 20 C. This phenomenon may be caused by
ꢀ
When the sintering temperature was increased to 1100 C, the
different morphology. Particles are loosely agglomerated and
nearly spherical at low sintering temperature while nuclei aggre-
gate interact and nearly block at high calcination temperature.
Nanoparticles with large specific surface area such as sphericity
could enhance the infrared radiation absorption [63]. The model is
shown in Fig. 11. Form Fig. 11a, it can be seen the sample is loosely
spherical particles and most of the infrared radiation have been
absorbed and only little radiation can be reflected. According to
Kirchhoff's law, when a system is in the thermal equilibrium status,
the absorbance is strictly equal to the emissivity. Thus, loosely
spherical particles could lead to higher emissivity. When the par-
ticles aggregate interact and merge each other, as can be seen in
Fig. 11b, the infrared radiation absorption decreases and infrared
reflection increases and consequently lead to a lower emissivity.
However, the infrared emissivity increases when calcination tem-
x
electrical resistivity of Zn1-xCo O nanoparticles is increase, which
means that the electrical conductivity decrease. The primary
reason of the decrease in resistivity at low sintering temperature is
attributed to the strengthened carrier mobility due to the improved
crystalline quality. However, further increase of the sintering
temperature leads to a finer grain and poor crystalline quality as
evidenced by XRD and Raman spectra analysis. Obviously, the fine
grain would result in more grain boundaries, along with more
defects. Surface-roughness scattering and grain boundary scat-
x
tering mainly lower the carrier mobility in Zn1-xCo O semi-
conductor nanoparticles and therefore the resistivity increased.
Besides, the oxygen vacancies are deep donors and have high for-
mation energies in n-type samples [62]. These oxygen vacancies
cannot act as charge carriers to decrease resistivity, even though
there is the highest concentration of oxygen vacancies of the
ꢀ
perature reaches 1100 C. The reason accounting for this phe-
ꢀ
sample calcinated at 1100 C.
nomenon is the poor conductivity arisen from small particle size as
well as poor crystallinity conductivity. The effect of conductivity on
the infrared emissivity has been proved in previous works [64,65].
As was evidenced by electrical resistivity analysis, the poor crys-
talline quality leads to a higher emissivity [66].
The infrared emissivity values of the samples in 3 ~ 5 mm were
measured at different test temperature, and the corresponding
testing results are shown in Fig.10. As seen from Fig.10, the infrared
ꢀ
emission values of the samples which were calcinated at 700 C and
ꢀ
8
00 C, rise at first and then reduce with the increased test tem-
perature. This phenomenon could be explained as follows: the n-
type donor carrier concentration of Zn1-xCo O semiconductor
x
nanoparticles would increase with the increase of test temperature,
which would cause the resistivity decrease [67]. Thus the infrared
emissivity decrease at first with the improvement of conductivity
[68]. Nevertheless, rotation and vibration of atoms and molecules
would increase with the elevated test temperature. These changes
could enhance the lattice vibrational absorption, and consequently
lead to a higher absorption of infrared radiation. When the influ-
ence of lattice vibrational absorption on infrared emissivity is
greater than that of conductivity, the emissivity would increase.
The infrared emissivity increases with the increase of test
ꢀ
ꢀ
temperature from 20 C to 700 C when the powders are annealed
ꢀ
ꢀ
ꢀ
at 900 C, 1000 C and 1100 C. It may be due to the fact that the
x
Fig. 10. Infrared emissivity of Zn1-xCo O samples under different test temperatures.
Fig. 11. Schematic diagram of infrared radiation behavior of samples with different morphology: a sphericity; b block.

114
Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
impacts of lattice vibrational absorption were greater than that of
conductivity in the text temperature range if the annealing tem-
[10] Joel A. Johnson, John J. Heidenreich, Robert A. Mantz, Paul M. Baker, Michael
S. Donley, A multiple-scattering model analysis of zinc oxide pigment for
spacecraft thermal control coatings, Prog. Org. Coat. 47 (3e4) (2003)
ꢀ
perature exceeds 900 C. Besides, when the test temperatures vary
432e442.
ꢀ
ꢀ
from room temperature 20 C to high temperature 700 C, the
changes of infrared emissivity at calcination temperatures of
7
[11] Parmanand Sharma, Amita Gupta, K.V. Rao, Frank J. Owens, Renu Sharma,
Rajeev Ahuja, J. M. Osorio Guillen, B o€ rje Johansson, G.A. Gehring, Ferromag-
netism above room temperature in bulk and transparent thin films of Mn-
doped ZnO, Nat. Mater. 2 (10) (2003) 673e677.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
00 C, 800 C, 900 C, 1000 C and 1100 C is about 0.289, 0.362,
.396, 0.412 and 0.422, respectively, which indicates that the
0
[12] Y.M. Hao, S.Y. Lou, S.M. Zhou, Y.Q. Wang, X.L. Chen, G.Y. Zhu, R.J. Yuan, N. Li,
Novel magnetic behavior of Mn-doped ZnO hierarchical hollow spheres,
J. Nanopart. Res. 14 (2012) 1e9.
changing rates of infrared emissivity has increased with the in-
crease of calcination temperature. These results suggested that the
sintering temperature is really a crucial factor for the infrared ra-
diation properties of materials.
[
13] A.B. Djuri ꢀs i ꢁc , A.M.C. Ng, X.Y. Chen, ZnO nanostructures for optoelectronics:
material properties and device applications, Prog. Quantum Electron 34
(2010) 191e259.
[
14] J.J. Chen, F. Zeng, D.M. Li, J.B. Niu, F. Pan, Deposition of high-quality zinc oxide
thin films on diamond substrates for high-frequency surface acoustic wave
filter applications, Thin Solid Films 485 (1e2) (2005) 257e261.
4
. Conclusions
[
[
15] Y. K o€ seo gꢂ lu, Enhanced ferromagnetic properties of Co-doped ZnO DMS
nanoparticles, J. Supercond. Nov. Magn. 26 (2012) 485e489.
16] L.L. Chen, Z.Z. Ye, J.G. Lu, H.P. He, B.H. Zhao, L.P. Zhu, Paul K. Chu, L. Shao, Co-
doping effects and electrical transport in IneN doped zinc oxide, Chem. Phys.
Lett. 432 (1e3) (2006) 352e355.
x
In summary, the green Zn1-xCo O nanopowders were prepared
by solid-state reaction. XRD results confirm that the as-synthesized
samples have hexagonal wurtzite structure, but the peaks of the
[17] H. Ohno, SEMICONDUCTORS: enhanced: toward functional spintronics, Sci-
ence 291 (5505) (2001) 840e841.
secondary phase, Co
3
O
4
, were observed when sintering tempera-
ꢀ
ꢀ
ture is 700 C and 800 C. Besides, the sintering temperature have
an important impact on crystallite sizes, lattice parameters, volume
of the unit cell and microstructure. EDS results provided strong
evidence that Co ions substituted the Zn ions in the ZnO lattice. XPS
analysis indicates that Co-ions are þ2 state in ZnO lattice. It was
found that with the increase of the sintering temperature, the area
ratio of oxygen vacancies is increased. The optical energy band gap
was found to decrease with the increase of sinterring temperature,
indicating a red shift. The results of colour and thermochromic
[18] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description
of ferromagnetism in zinc-blende magnetic semiconductors, Science 287
(
2000) 1019e1022.
2
þ
[
19] S.D. Birajdar, V. Bhagwat, A. Shinde, K. Jadhav, Effect of Co ions on structural,
morphological and optical properties of ZnO nanoparticles synthesized by sol-
gel auto combustion method, Mater. Sci. Semicond. Process 41 (2016)
441e449.
[20] A.M. El Sayed, S. Taha, G. Said, Ahmed A. Al-Ghamdi, F. Yakuphanoglu,
Structural and optical properties of spin coated Zn1ꢁxCr
Superlatt. Microstruct. 60 (2013) 108e119.
x
O nanostructures,
[
21] N. Srinatha, K.G.M. Nair, Basavaraj Angadi, Microstructure, electronic struc-
ture and optical properties of combustion synthesized Co doped ZnO nano-
particles, Phys. B Condens. Matter 474 (2015) 97e104.
analysis show that the color-changing temperature of Zn1-xCo
nanopowders is concerned with the energy band gap. The electrical
resistivity of Zn1-xCo O nanoparticles relies on the crystallite sizes
and crystallinity. The emissivity of Zn1-xCo O nanopowders not
x
O
[22] M. Bouloudenine, N. Viart, S. Colis, J. Kortus, A. Dinia, Antiferromagnetism in
bulk Zn1-xCo O magnetic semiconductors prepared by the coprecipitation
x
x
technique, Appl. Phys. Lett. 87 (2005) 052501.
x
[23] R.D. Shannon, C.T. Prewitt, Effective ionic radii and crystal chemistry, J. Inorg.
only relies on the morphology but also the electrical conductivity.
In addition, the changing rates of infrared emissivity has increased
with the increase of annealing temperature. As a novel kind of
thermochromic materials, the Zn1-xCo O nanopowders have great
x
potential applications in visible light stealth coatings and spacecraft
thermal control coatings.
Nucl. Chem. 32 (5) (1970) 1427e1441.
[24] Megha Vagadia, Ashish Ravalia, Uma Khachar, P.S. Solanki, R.R. Doshi,
S. Rayaprol, D.G. Kuberkar, Size and grain morphology dependent magnetic
behaviour of Co-doped ZnO, Mater. Res. Bull. 46 (11) (2011) 1933e1937.
[
25] A. Goktas, F. Aslan, I.H. Mutlu, Effect of preparation technique on the selected
characteristics of Zn1-xCo O nanocrystalline thin films deposited by solegel
x
and magnetron sputtering, J. Alloys. Compd. 615 (2014) 765e778.
26] A. Franco, H.V.S. Pessoni, P.R.T. Ribeiro, F.L.A. Machado, Magnetic properties of
Co-doped ZnO nanoparticles, J. Magn. Magn. Mater 426 (2017) 347e350.
[
Acknowledgements
[27] I.A. Sarsari, H. Salamati, P. Kameli, E.S. Razavi, Optical, structural, and magnetic
properties of ZnO: Co nanoparticles prepared by a thermal treatment of ball
milled precursors, J. Supercond. Nov. Magn. 24 (2011) 2295.
This work is financially supported by the National Natural Sci-
ence Foundation of China (Grant No: 51403102), Fundamental
Research Funds for the Center Universities (Grant No: NS2014058),
Natural Science Foundation of Jiangsu Province, China (Grant No:
BK20140811) and a Project Funded by the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions (PAPD).
[28] Fei Yi, Hai Zhu, Jason C. Reed, Ertugrul Cubukcu, Plasmonically enhanced
thermomechanical detection of infrared radiation, Nano Lett. 13 (4) (2013)
1
638e1643.
29] N. Adak, N. Heybeli, C. Ertekin, Infrared drying of strawberry, Food Chem. 219
2016) 109e116.
[
(
[30] W.G. Zhang, G.Y. Xu, R.Y. Ding, K.G. Duan, J.L. Qiao, Nacre biomimetic
designea possible approach to prepare low infrared emissivity composite
coatings, Mater. Sci. Eng. C 33 (2013) 99e102.
[
31] H.J. Yu, G.Y. Xu, X.M. Shen, X.X. Yan, R. Huang, F.L. Li, Preparation of leafing Cu
References
and its application in low infrared emissivity coatings, J. Alloys Compd. 484
(2009) 395e399.
[
[
1] L.-O. Hennerdal, M. Berggren, Picture-to-picture switching in full-color ther-
mochromic paper displays, Appl. Phys. Lett. 99 (2011), 183303e183303-3.
2] O. Yarimaga, J. Jaworski, B. Yoon, J.-M. Kim, Polydiacetylenes: supramolecular
smart materials with a structural hierarchy for sensing, imaging and display
applications, Chem. Commun. 43 (2012) 2469e2485.
3] S.J. Zheng, Y. Xu, Q.H. Shen, H. Yang, Preparation of thermochromic coatings
and their energy saving analysis, Sol. Energy 112 (2015) 263e271.
4] C.H. Wu, J.W. Qiu, J.B. Wang, M. Xu, L.X. Wang, Thermochromic property of
[32] J. Chen, Y.M. Zhou, Q.L. Nan, X.Y. Ye, Y.Q. Sun, F.Y. Zhang, Z.Q. Wang, Prepa-
ration and properties of optically active polyurethane/TiO
derived from optically pure 1,1 -binaphthyl, Eur. Polym. J. 43 (2007)
4151e4159.
2
nanocomposites
0
[33] Z.D. Nan, Q.Z. Jiao, Z.C. Tan, L.X. Sun, Investigation of thermodynamic prop-
[
2 3
erties of Co O powder, Thermochim. Acta 404 (2003) 245e249.
[34] F.T. Li, Q. Wang, J.Y.J. Hao, X.J. Wang, D. Zhao, S.Z. Qiao, Ionic liquid self-
combustion synthesis of BiOBr/Bi24O31Br10 heterojunctions with excep-
tional visible-light photocatalytic performances, Nanoscale 7 (2015) 1116.
[
La0.8 Sr0.2 MnO
06 (2010) L22eL24.
5] D.Q. Liu, W.W. Zheng, H.F. Cheng, H.T. Liu, Thermochromic VO
prepared by post annealing treatment of V thin film, Adv. Mater. Res.
9e82 (2009) 747e750.
3
thin-film material sputtered on quartz glass, J. Alloy. Compd.
5
[35] J. Cao, B. Luo, H. Lin, B. Xu, S. Chen, Thermodecomposition synthesis of WO
WO heterostructures with enhanced visible light photocatalytic properties,
Appl. Catal. B-Environ. 111 (2012) 288e296.
3
/H
[
2
thin film
2
4
2 5
O
7
[36] M. Shatnawi, A.M. Alsmadi, I. Bsoul, B. Salameh, G. Alnawashi, F. Al-Dweree,
Magnetic and Optical properties of Co-doped ZnO nanocrystalline particles,
J. Alloy. Compd. (2015) 655.
[37] B. Pal, P.K. Giri, Defect mediated magnetic interaction and high Tc ferro-
magnetism in Co doped ZnO nanoparticles, J. Nanosci. Nanotechnol. 11 (2011)
9167e9174.
[38] Richa Bhargava, Prashant K. Sharma, Ranu K. Dutta, Sanjeev Kumar, Avinash
C. Pandey, Naresh Kumar, Influence of Co-doping on the thermal, structural,
and optical properties of solegel derived ZnO nanoparticles, Mater. Chem.
[
[
[
[
6] D.J. Watts, L. Battista, J. Zunino, N. Colon, J. Federici, G. Thomas, Smart Coating
System, 2006. US, US 20060182160 A1.
7] R.F. Reynolds, M.J. Kinsella, Thermal and Visual Camouflage System, 2002. US,
US6338292 [P].
8] C.G. Granqvist, Electrochromics and thermochromics: towards a new para-
digm for energy efficient buildings, Mater Today 3 (2016) S2eS11.
9] D.S. Fan, Q. Li, Y.M. Xuan, Tailoring the solar absorptivity of thermochromic
3
material La0.7Ca0.2Sr0.1MnO , Quant. Spectrosc. Ra 112 (2011) 2794e2800.

Y. Zhu et al. / Journal of Alloys and Compounds 720 (2017) 105e115
115
Phys. 120 (2e3) (2010) 393e398.
prepared by microwave-assisted hydrothermal method, Mater. Sci. Semicond.
[
[
39] A. Khorsand Zak, W.H. Abd Majid, M.E. Abrishami, Ramin Yousefi, X-ray
analysis of ZnO nanoparticles by WilliamsoneHall and sizeestrain plot
methods, Solid State Sci. 13 (1) (2011) 251e256.
40] S.D. Birajdar, V. Bhagwat, A. Shinde, K. Jadhav, Effect of Co ions on structural,
morphological and optical properties of ZnO nanoparticles synthesized by sol-
gel auto combustion method, Mater. Sci. Semicond. Process 41 (2016)
57 (2017) 233e238.
[54] H.A. Weakliem, Optical spectra of Ni2 , Co , and Cu in tetrahedral sites in
þ
2þ
2þ
crystals, J. Chem. Phys. 36 (1962) 2117e2140.
2þ
[55] Faheem Ahmed, Nishat Arshi, M.S. Anwar, Rehan Danish, Bon Heun Koo, Effect
of transition Metal (Co, Ni and Cu) doping on lattice volume, band gap,
morphology and saturation magnetization of ZnO nanostructures, J. Korean
Phys. Soc. 62 (10) (2013) 1479e1484.
441e449.
[
41] Jacek Wojnarowicz, Sylwia Kusnieruk, Tadeusz Chudoba, Stanislaw Gierlotka,
Witold Lojkowski, Wojciech Knoff, Malgorzata I. Lukasiewicz, Bartlomiej
S. Witkowski, Anna Wolska, Marcin T. Klepka, Tomasz Story,
Marek Godlewski, Paramagnetism of cobalt-doped ZnO nanoparticles ob-
tained by microwave solvothermal synthesis, Beilstein J. Nanotechnol. 6
[56] S.K. Saha, M.A. Rahman, M.R. Sarkar, M. Shahjahan, M.K. Khan, Effect of Co
doping on structural, optical, electrical and thermal properties of nano-
structured ZnO thin films, J. Semicond. 36 (2015) 35e40.
[57] J.P. Wang, Z.Y. Wang, B.B. Huang, Y.D. Ma, Y.Y. Liu, X.Y. Qin, X.Y. Zhang, Y. Dai,
Oxygen vacancy induced band-gap narrowing and enhanced visible light
photocatalytic activity of ZnO, Acs. Appl. Mater. Inter 4 (2012) 4024e4030.
[58] Manuel García-M eꢁ ndez, Alvaro Bedoya-Calle, Ricardo Rangel Segura,
(
2015) 1957e1969.
ꢁ
[42] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley Pub-
lishing Company Inc, Reading, MA, 1978, pp. 277e279, 100-105.
Víctor Coello, Investigation of the annealing effects on the structural and
optoelectronic properties of RF-sputtered ZnO films studied by the Dru-
deeLorentz model, Appl. Phys. A 120 (4) (2015) 1375e1382.
[
43] K. Maaz, S. Karim, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, Synthesis and
magnetic characterization of nickel ferrite nanoparticles prepared by co-
precipitation route, J. Magn. Magn. Mater. 321 (12) (2009) 1838e1842.
44] G. Vijayaprasath, R. Murugan, T. Mahalingam, G. Ravi, Comparative study of
structural and magnetic properties of transition metal (Co, Ni) doped ZnO
nanoparticles, J. Mater. Sci. Mater. Electron. 26 (9) (2015) 7205e7213.
45] A. Goktasa, F. Aslan, A. Tumbul, S.H. Gunduz, Tuning of structural, optical and
dielectric constants by various transition metal doping in ZnO: TM (TM¼Mn,
Co, Fe) nanostructured thin films: a comparative study, Ceram. Int. 43 (2017)
[59] G. Auer, W.D. Griebler, B. Jahn, Industrial Inorganic Pigments, third ed., Wiley-
VCH Verlag GmbH & Co. KGaA, Weinheim, 2005.
[
[
€
ꢂ
[60] Ü. Ozgür, YaI. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.-
J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices,
J. Appl. Phys. 98 (2005), 11-1.
[61] J.P. Wang, Z.Y. Wang, B.B. Huang, Y.D. Ma, Y.Y. Liu, X. Qin, X.Y. Zhang, Y. Dai,
Oxygen vacancy induced band-gap narrowing and enhanced visible light
photocatalytic activity of ZnO, ACS Appl. Mater. Inter 4 (2012) 4024e4030.
[62] Anderson Janotti, Chris G. Van de Walle, Native point defects in ZnO, Phys.
Rev. B 76 (2007) 165202.
[63] Y.M. Zhu, Y.X. Han, X.Y. Wang, X. Wang, Preparation of nano- MgO and
investigation of its infrared absorption properties, Adv. Mater 58 (2008)
115e120.
704e713.
[
[
[
[
[
46] S. Benramache, B. Benhaoua, H. Bentrah, Preparation of transparent,
conductive ZnO: Co and ZnO: in thin films by ultrasonic spray method,
J. Nanost. Chem. 3 (2013) 54.
47] C.J. Cong, J.H. Hong, K.L. Zhang, Effect of atmosphere on the magnetic prop-
erties of the Co-doped ZnO magnetic semiconductors, Mater. Chem. Phys. 113
(
1) (2009) 435e440.
[64] Y.H. Yao, Q.X. Cao, Effect of preparation techniques on infrared emissivities of
Zn0.9Co0.1O powders, Mater. Sci. Semicond. Process 16 (2013) 759e764.
[65] K.K. Wang, B. Cheng, B. Wu, C. Defranoux, P. Basa, C. Song, G. Han, Y. Liu, Study
48] N.M. Alhada, E.B. Saion, A.H. Shaari, M.A. Kamarudin, M.H. Flaifel, S.H. Ahmad,
S.A. Gene, A. Facile, Thermal-treatment route to synthesize ZnO nanosheets
and effect of calcination temperature, PLoS One 9 (2014) 103e134.
49] G. van der Laan, C. Westra, C. Haas, G.A. Sawatzky, Satellite structure in
photoelectron and Auger spectra of copper dihalides, Phys. Rev. B 23 (9)
2 x y
of annealing effects upon the optical and electrical properties of SnO :F/SiC O
low emissivity coatings by spectroscopic ellipsometry, Thin Solid Films 571
(2014) 720e726.
(
1981) 4369e4380.
[66] H.J. Yu, G.Y. Xu, X.M. Shen, X.Y. Yan, W.G. Chen, Low infrared emissivity of
polyurethane/Cu composite coatings, Appl. Surf. Sci. 255 (2009) 6077e6081.
[67] S.D. Birajdar, P.P. Khirade, T.S. Saraf, R.C. Alange, K.M. Jadhav, Sol-gel auto
50] H.-J. Lee, C.H. Park, S.-Y. Jeong, K.-J. Yee, C.R. Cho, M.-H. Jung, D.J. Chadi,
Hydrogen-induced ferromagnetism in ZnCoO, Appl. Phys. Lett. 88 (6) (2006)
0
62504.
combustion synthesis, electrical and dielectric properties of Zn1-xCo
x
O (0.0 ꢂ
[
[
[
51] Z.W. Dong, C.F. Zhang, H. Deng, G.J. You, S.X. Qian, Raman spectra of single
micrometer-sized tubular ZnO, Mater. Chem. Phys. 99 (1) (2006) 160e163.
52] Z.K. Heiba, L. Arda, XRD, XPS, optical, and Raman investigations of structural
changes of nano Co-doped ZnO, J. Mol. Struct. 1022 (2012) 167e171.
x ꢂ 0.36) semiconductor nanoparticles, J. Alloys. Compd. 691 (2017) 355e363.
[68] F.L. Du, N. Wang, D.M. Zhang, Y.Z. Shen, Preparation, characterization and
infrared emissivity study of Ce-doped ZnO films, J. Raer Earth 28 (2010)
391e395.
53] M. Fang, Z.W. Liu, Structure and properties variations in Zn1-xCoxO nanorods