Structural and optical characteristics of pre- and post ...
34
RESEARCH REPOSITORY This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination. The definitive version is available at: http://dx.doi.org/10.1016/j.jallcom.2017.01.079 Miran, H.A., Rahman, M.M., Jiang, Z-T, Altarawneh, M., Chuah, L.S., Lee, H-L, Mohammadpour, E., Amri, A., Mondinos, N. and Dlugogorski, B.Z. (2017) Structural and optical characteristics of pre- and post-annealed sol-gel derived CoCu-oxide coatings. Journal of Alloys and Compounds, 701 . pp. 222-235. http://researchrepository.murdoch.edu.au/id/eprint/35158/ Copyright: © 2017 Elsevier B.V. It is posted here for your personal use. No further distribution is permitted.
Transcript of Structural and optical characteristics of pre- and post ...
RESEARCH REPOSITORY
This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination.
The definitive version is available at:
http://dx.doi.org/10.1016/j.jallcom.2017.01.079
Miran, H.A., Rahman, M.M., Jiang, Z-T, Altarawneh, M., Chuah, L.S., Lee, H-L,
Mohammadpour, E., Amri, A., Mondinos, N. and Dlugogorski, B.Z. (2017) Structural and optical characteristics of pre- and post-annealed sol-gel derived
CoCu-oxide coatings. Journal of Alloys and Compounds, 701 . pp. 222-235.
http://researchrepository.murdoch.edu.au/id/eprint/35158/
Copyright: © 2017 Elsevier B.V.
It is posted here for your personal use. No further distribution is permitted.
MURDOCH RESEARCH REPOSITORY
This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination.
The definitive version is available at :
http://dx.doi.org/10.1016/j.jallcom.2017.01.079
Miran, H.A., Rahman, M.M., Jiang, Z-T, Altarawneh, M., Chuah, L.S., Lee, H-L, Mohammedpur, E., Amri, A., Mondinos, N. and Dlugogorski, B.Z. (2017)
Structural and optical characteristics of pre- and post-annealed sol-gel derived CoCu-oxide coatings. Journal of Alloys and Compounds, 701 . pp. 222-235.
http://researchrepository.murdoch.edu.au/id/eprint/35158/
Copyright: © 2017 Elsevier B.V. It is posted here for your personal use. No further distribution is permitted.
Accepted Manuscript
Structural and optical characteristics of pre- and post-annealed sol-gel derived CoCu-oxide coatings
Hussein A. Miran, M. Mahbubur Rahman, Zhong-Tao Jiang, MohmmednoorAltarawneh, Lee Siang Chuah, Hooi-Ling Lee, Ehsan Mohammedpur, Amun Amri,Nicholas Mondinos, Bogdan Z. Dlugogorski
PII: S0925-8388(17)30099-3
DOI: 10.1016/j.jallcom.2017.01.079
Reference: JALCOM 40446
To appear in: Journal of Alloys and Compounds
Received Date: 26 October 2016
Revised Date: 4 January 2017
Accepted Date: 8 January 2017
Please cite this article as: H.A. Miran, M.M. Rahman, Z.-T. Jiang, M. Altarawneh, L.S. Chuah, H.-L. Lee,E. Mohammedpur, A. Amri, N. Mondinos, B.Z. Dlugogorski, Structural and optical characteristics of pre-and post-annealed sol-gel derived CoCu-oxide coatings, Journal of Alloys and Compounds (2017), doi:10.1016/j.jallcom.2017.01.079.
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Structural and Optical Characteristics of Pre- and Post-annealed Sol-gel Derived CoCu-1
oxide Coatings 2
3
Hussein A. Miran1,2, M. Mahbubur Rahman1,3, Zhong-Tao Jiang1*, Mohmmednoor 4
Altarawneh4, Lee Siang Chuah5, Hooi-Ling Lee6, Ehsan Mohammedpur1, Amun Amri7, 5
Nicholas Mondinos1, Bogdan Z. Dlugogorski4 6
7
1Surface Analysis & Materials Engineering Research Group, School of Engineering & 8
Information Technology, Murdoch University, Murdoch, Western Australia 6150, Australia 9
2Department of Physics, College of Education for Pure Science - Ibn Al – Haitham, 10
University of Bagdad 10071, Baghdad, Iraq 11
3Department of Physics, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh 12
4School of Engineering & Information Technology, Murdoch University, Murdoch, Western 13
Australia 6150, Australia 14
5Department of Physics, School of Distance Education, Universiti Sains Malaysia, 11800 15
Minden, Penang, Malaysia 16
6School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia 17
7Department of Chemical Engineering, Universitas Riau, Pekanbaru, Indonesia 18
19
*Corresponding authors. Tel.: +618 9360 2867; Email address: [email protected] (Z.-20
T. Jiang), [email protected] (M.M. Rahman) 21
22
23
24
25
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Abstract 26
Sol-gel derived CuCo-oxide coatings as solar selective surfaces, synthesized onto aluminium 27
substrates at various annealing temperatures, are analysed by correlating their structural, 28
chemical bonding states, and surface morphological topographies. As the annealing 29
progressed, all the coatings displayed a Cu0.56Co2.44O4 (ICSD 78-2175) phase with 30
preferential orientation along (400) reflection plane. Rietveld refinement of X-ray diffraction 31
(XRD) data indicate that residual stress and microstrains developed around the coating 32
surfaces are reduced resulting in mechanically stable thin films. Enhancement of the 33
crystallite size and preferred orientation of the surface were confirmed via XRD, field 34
emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM) 35
analysis. X-ray photoelectron spectroscopy (XPS) analysis shows tetrahedral, octahedral and 36
mixed states of Cu and Co ions with a stable atomic ratio of Co/Cu, and an increase of O and 37
C contents but no metal-carbon bonding on the surface of materials. Optical reflectance 38
investigations indicated that solar selectivity of the coatings increased from 3.81 to 24 as the 39
annealing temperature reached up to 500 °C. 40
41
Keywords: Solar selectivity; copper cobalt oxides; density functional theory, solar selective 42
surface, annealing temperature, Bader charges analysis 43
44
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1 Introduction 45
Mixed 3-d metal oxide based thin film composites are promising functional materials due to 46
some superior properties such as a large surface to volume ratio, chemical reactivity, special 47
electronic configuration, catalytic capabilities, mechanical durability and thermal stability [1, 48
2] These materials have a wide range of applications, such as lithium-ion batteries, ion 49
exchange, catalysts, solar energy conversion, recording/memory devices and sensors [3-9]. 50
The key factors for the potential applications of such composites also include stoichiometry 51
and homogeneity of composition, crystal morphology and particle size and shape [10, 11]. In 52
the previous reports [6, 12, 13], copper–cobalt oxide thin films synthesised on aluminium 53
substrate via sol–gel dip-coating route displayed unique optical properties with a spectrally 54
selective feature in the visible wavelength of the solar spectrum. It is understood that the 55
thermal treatment during the fabrication procedure of CuCo-oxide thin films has considerable 56
impact on their physicochemical, structural, optical, magnetic and electronic behaviours [14]. 57
The CuCo-oxide compounds demonstrate outstanding thermal stability up to 680 °C, 58
whereupon a structural phase transformation occurs. The spinel CuCo-oxides lean towards a 59
low crystallized single phase together with a moderately inverted spinel configuration and a 60
minor segregation of copper and cobalt oxide matrices dependent on the Cu/Co ratio and the 61
annealing temperature [15, 16]. The gradual increase in annealing temperature contributes to 62
an improving in crystalline phases of CuCo-oxide systems [17]. 63
Solar selective surface is an important component of a solar thermal collector aimed to 64
accumulate solar radiation and transform it into beneficial heat energy for numerous domestic 65
and industrial applications. An ideal solar selective surface (SSS) should, generally, absorb 66
the highest incoming solar radiation in the visible and lowest thermal emittance in the infra-67
red range of the solar spectrum. Industrial SSSs use metal particles based ceramic cermet 68
structures synthesized via vacuum, electroplating, sputtering, and electrochemical procedures 69
[18, 19]. Recently we reported our work on copper-cobalt oxide thin film coatings on the top 70
of highly reflective aluminium substrates using a customized, and yet, cost-effective sol−gel 71
dip-coating method [6, 20]. The new coatings demonstrated distinct optical properties with 72
promising prospects for solar selective absorber application. Details regarding the metal 73
oxidation states as well as the mechanical properties of these coatings are, nonetheless, rather 74
sketchy and currently lacking even though they are vital for design optimization purposes. 75
An area of application of which these cobalt-based oxides are comparatively less studied is 76
optical or solar-based coating whereby optical performance of a surface can be manipulated 77
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by depositing thin films with varying thicknesses and reflective indices. Incidentally, there 78
are certainly many knowledge gaps that need to be filled in terms of fundamental surface 79
characteristics of these thin films especially in regard to their morphologies, binding states of 80
metal oxides and mechanical strengths. A technical understanding of these characteristics is 81
an essential component in the smart design and engineering context of thin film coatings for 82
optical applications. From an atomistic point of view and apart from the experiment, density 83
functional theory (DFT), a quantum mechanics based computational modelling; provide a 84
powerful tool to analysis various material properties, such as electronic structure, thermal and 85
mechanical properties [37]. Numbers of theoretical studies have been conducted to 86
investigate bulk and surface properties, e.g., bonding, band structures, and thermomechanical 87
properties etc., of Cu-oxide [21, 22] and Co-oxide [23, 24] based materials. An earlier study 88
given by Soon et al. reported that the band structures and cohesive properties of copper 89
oxide structure [25]. 90
The physicochemical, optical, magnetic, electrochemical, and thermal properties of these 91
coatings have been extensively studied by several groups which are essential to afford 92
functionalities and enrich application performances of these materials [17, 26-28]. Compared 93
to a large volume of available literature, the post annealing structural features of CuCo-oxide 94
coatings are rarely studied and, till date there are not any integrated experimental reports on 95
temperature dependent structural and optical properties these coatings. Higher degree of 96
thermal stability of a material is an essential condition for its real-world applications. This is 97
also true for metal oxide structures [29]. The particle morphology, crystallite size, grain 98
structure, surfaces roughness and local electronic bonding states of the metal oxides formed 99
via powder synthesis and coatings deposition could differ significantly during thermal 100
treatment and could be essentially different from the regularities established in the 101
unannealed coatings. There are, however, many aspects entailing more in depth analysis to 102
comprehend the impact of annealing temperatures on the structural, morphological and 103
optical characteristics of these coatings. 104
Assessing the above facts, this work is aimed at investigating the structural, morphological 105
and local electronic bonding states, and solar selective profile of CuCo-oxide coatings, 106
synthesized before annealing and annealed at 200, 300, 400, 500°C in air for 1 hour, using 107
XRD, FESEM, AFM, X-ray photoelectron spectroscopy (XPS), ultra-violet visible near 108
infrared (UV–Vis–NIR), and Fourier transform infrared (FTIR) spectroscopic methods. 109
Possible charge distribution given by Bader’s charge calculation for cluster of Cu0.75C2.25O4 110
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matrix will be used to interpret the electronic structure results from experimental 111
characterization. 112
113
2. Experimental 114
2.1 Deposition of coatings 115
Cobalt (II) chloride (CoCl2.6H2O, APS Chemical, >99%), copper (II) acetate monohydrate 116
(Cu(OOCCH3))2.H2O, Alfa Aesar, >98%), propionic acid (C2H5COOH, Chem Supply, 117
>99%), and absolute ethanol (E. Mark of Germany, >99%.) were used to synthesize CuCo-118
oxide coatings onto highly-reflective commercial aluminium substrates (Anofol, size 2 cm × 119
4 cm). Aluminium substrates were cleaned with a hot mixture of Cr(VI)O and phosphoric 120
acid followed by a wash using milli Q water. The cleaned substrates were dried with a flash 121
of N2 gas. 1.502 gm of cobalt (II) chloride (CoCl2·6H2O, 0.25 mol/L), and 1.273 gm of 122
copper (II) acetate monohydrate (Cu(OOCCH3))2.H2O were mixed with absolute ethanol. 123
Propionic acid (C2H5COOH) was used to make a complex solution with the metal ions and to 124
stabilize the solution from unwanted precipitation. After stirring the mixed solution for 2 125
hours, the sol was coated onto aluminium substrates using a dip-coating technique. A dipping 126
and withdrawal rate of 180 mm/min and 60 mm/min, respectively was maintained throughout 127
the synthesis process which was repeated four times to increase the thickness of the film with 128
better uniformity. Finally, the coatings were annealed at 200, 300, 400 and 500 °C in air for 1 129
hour. A constant heating rate of 10 °C/min was maintained throughout. More details about 130
the deposition of coatings is reported elsewhere [12]. 131
132
2.2 Crystal phase structure and Rietveld refinement 133
Rietveld refinement was employed to refine the XRD data as implemented in the TOPAS 134
program. We used a pseudo-Voigt peak shape model comprised of Lorentzian and Gaussian 135
components. The difference between the two patterns was minimized through a process of 136
least-squares. A Bruker Advance D8 X-ray Diffractometer equipped with a LynxEye detector 137
was used to carry out the XRD measurements. The XRD machine was operated at 40 kV and 138
40 mA at room temperature over a 2θ range of 30° to 80° with a step size of 0.01°. The X-139
rays used to characterize the coatings was a combination of CuKα1 (λ = 0.15406 nm) and 140
CuKα2 (λ = 0.15444 nm) radiations. The initial crystal structure including atomic positions of 141
CuCo-oxide phases were obtained from the ICSD database to model the cubic structure in 142
Fm-3m symmetry group. The background was modelled using a Chebychev polynomial 143
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background function with order of 6. The lattice parameter, scale, zero error, sample 144
displacement and background were refined to estimate the lattice parameter, crystallite size 145
(domain size), residual stress, and microstrain of CuCo-oxide coatings before and after 146
annealing. 147
148
2.3 XPS analysis 149
Chemical analysis of the coatings was performed via Kratos Axis-Ultra photoelectron 150
spectrometer. The Kratos XPS machine uses Al-Kα monochromatic X-ray source with beam 151
energy of 1486.6 eV at a power of ~10 mA and ~15 kV. Square size samples (2 mm × 2 mm) 152
were mounted on steel sample holder. A uniform pressure of 2.9 × 10‒9 Torr was maintained 153
in the XPS analyser chamber. The Cu2p, Co2p, O1s, C1s photoelectron lines were recorded 154
with a 2D delay line detector. The photoelectron energy scale was calibrated using C1s 155
(hydrocarbon; C‒H) line at 284.6 eV. CASA-XPS v.2.3.15 software was used for XPS data 156
analysis and deconvolution of the curves. 157
158
2.4 Film surface feature via FESEM analysis 159
FESEM is one of the most popular techniques to investigate the morphological features of 160
coating materials which gives us essential information concerning the growth mechanism, 161
shape and size of the coating particles. The surface morphology of samples was studied 162
utilizing a field-emission scanning electron microscope (FESEM, Nova NanoSem 450) 163
operating at low voltage (10kV) to minimize charging effects. FESEM digitized 164
micrographs were obtained with a magnification 50,000 to 100,000×. A secondary 165
electron imaging (SEI) detector was used for this purpose. 166
167
2.5 Film surface feature via AFM analysis 168
A high resolution atomic force microscopy (AFM, Nanomagnetics Instruments Co.) was used 169
to acquire the two and three-dimensional topographical images of the CuCo-oxide coatings. 170
The AFM was operated in tapping mode at room temperature. A typical rectangular 171
cantilever was employed for the imaging process. This was accomplished by raster scanning 172
the position of the sample with respect to the tip and recording the height of the probe that 173
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corresponds to a constant probe-sample interaction. A LabVIEW software was used to 174
analyse the micrographs. 175
The AFM images are usually quantified by three parameters at the microscopic scale namely, 176
the mean roughness (Ra), rms value (Rq), and z scale. The mean roughness, Ra of an AFM 177
image is estimated from the following relation [30], 178
�� =∑ |���|��� � (1) 179
where hi indicates the surface roughness value of the coating at ‘i’, ℎ� is the mean surface 180
roughness, and N is the number of data points considered for that particular AFM image. But 181
the most common parameter used for estimating the changes in surface topography of a 182
coating is known as the rms value of the surface roughness, Rq. The Rq is a measure of the 183
height deviations taken from the mean data plane and is defined as [30], 184
�� = �∑ |���|���� � (2) 185
The z scale gives the vertical distance between the highest and the lowest point of the image 186
[30]. 187
188
2.6 UV-Vis reflectance studies 189
Solar reflectance of the coatings was determined using a double-beam UV–Vis 190
spectrophotometer (Model: UV-670 UV-Vis spectrophotometer, JASCO, USA) with a 191
unique, single monochromator design covering a wavelength range from 190 to 2500 nm. 192
The monochromator consists of a 1200 grooves/mm grating and a photo multiplier tube 193
(PMT) detector for the UV-Vis measurements. 194
195
2.7 FTIR reflectance studies 196
The solar reflectance of the thin film coatings were measured by using a FTIR spectrometer 197
(Perkin Elmer Spectrum 100 FTIR Spectrometer, USA) in the wavelength of 2.5 to 15.5 µm. 198
The solar absorptance and the thermal emittance of a material can be calculated from 199
measurements of reflectance data from the visible and infrared ranges of the solar spectrum 200
[31]. The total solar absorptance (α) and thermal emittance (ε) of the coatings were estimated 201
via Beckmann-Duffie method as described in Ref. [31]. The solar selectivity (s), the ratio of 202
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the solar absorptance (�) to the emittance (ε ) [32, 33] (i.e., εα=s ) of the coatings was also 203
estimated from the UV-Vis and FTIR data of the coatings. 204
205
2.8 Theoretical charge distribution analysis via density functional theory (DFT) 206
The charge calculations for Cu0.7C2.28O4 material were initially performed using density 207
functional theory (DFT) as implemented in Vienna ab-initio simulation package (VASP) 208
code [34]. The exchange correlated energies were treated by generalised gradient 209
approximation (GGA) [35]. In order to improve the simulation results, the on-site coulomb 210
interaction correction together with the DFT+U approach [36] was employed for this 211
calculation. 9.5 eV and 0.95 eV were selected for U and J in this simulation approaching 212
because this is the best set of Hubbard parameters (U and J; Ueff =U-J) for reproducing 213
experimental band gaps of monoclinic CuO and cubic CoO clusters. The cut-off energy of 214
500 eV was used to expand the wave function in a form of plane wave. A 4 × 4 × 4 K-points 215
generated by Monkhorst Pack scheme [37] was used to integrate the Brillouin Zone of 216
selected cluster. All atoms were allowed to relax until their energies reached 10-4 eV. Bader’s 217
theory [38] was employed to investigate the charge distribution with the structure. Bader’s 218
method uses electronic charge density to partition continuous molecular charges into 219
individual atomic charges through dividing the space in molecules into volumes. 220
∇����. �� = 0 (3) 221
where ∇���� is three-dimensional gradient operator for the electron density at a position r 222
and �� corresponds to the unit vector perpendicular to the dividing surface. 223
224
3 Results and Discussion 225
3.1 XRD Analysis 226
XRD patterns of the CuCo-oxide coatings before and after being annealed at 200, 300, 400 227
and 500°C are shown in Figure 1. According to the ICSD database, the presents of diffraction 228
peaks at 38.3° (222), 44.8° (400), 65.2° (440), and 78.3° (622) provide the evidences that the 229
major crystalline structure of thin film coatings in the temperature range of study could be 230
classified as Cu0.95Co2.05O4 (ICSD 78-2177), Cu0.75Co2.25O4 (ICSD 78-2176), Cu0.56Co2.44O4 231
(ICSD 78-2175), and/or Cu0.37Co2.63O4 (ICSD 78-2174) phase(s) all with space group of Fm-232
3m (#225). It is seen that the degree of crystallinity of (400) CuCo-oxide phases increases 233
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with the rise in annealing temperatures. It was found that all the annealed coatings had a 234
polycrystalline structure with multiple crystal planes orientations of (222), (400), (440), and 235
(622) respectively. As the annealing progresses, all the coatings demonstrated a preferential 236
orientation along (400) reflection plane together with a significant peak broadening 237
behaviour. It is also clearly shown that with the subsequent annealing, intensities of (222) and 238
(400) peaks are gradually enhanced while a reverse phenomenon was identified with (440) 239
reflection plane. It is assumed that the peak broadening behaviour was instigated from the 240
diminution of grain size and the presence of residual stress induced around the crystal matrix. 241
Rietveld refinement, of the diffraction patterns of crystallized CuCoO coatings analyzed 242
within the Fm-3m space group, produced successful fits with approximated Rwp ≈ 23% and 243
Rexp ≈ 20% (See Fig. 2). The lattice constant was measured as 0.809 nm at room 244
temperature and slightly increased during the annealing. 245
Debye–Scherrer formula (Eq. (4)) was employed to estimate the grain size of the coatings. The results 246
presented in Table 1 show that as the annealing temperature raises, the crystallite size and lattice 247
parameters of the coatings increases considerably. 248
�� = !"#$%& (4) 249
where K is a dimensionless quantity known as the crystallite-shape factor (K = 0.90) [39], ' 250
is the line broadening at half the maximum intensity (FWHM) measured in radians, and θ is 251
the Bragg angle. 252
The strain developed within a material can be estimated by evaluating the d-spacing of the 253
crystal planes using X-ray diffraction [40]: 254
() = *+*,*, (5) 255
where εz is the component of strain normal to the surface, and dn and d0 are the measured and 256
strain free d-spacing values, respectively. For a coating thickness of ~1 µm, the residual stress 257
σz is, generally, taken to be zero [41]. Thus, the strain component normal to the surface is 258
written as [42], 259
() = −.�(/ + (/� = − 12 �3/ + 3/� (6) 260
where ν is Poisson's ratio, E is Young's modulus, and σx and σy are the stresses along x and y 261
directions, respectively. 262
Assuming the coating to be isotropic i.e., σx = σy, and linking Eqs. (5) and (6), we attain 263
23/ =− 21 5*+*,*, 6 (7) 264
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Equation (7) is used to estimate the residual stress developed within the CoCu2O3 matrix at 265
various annealing temperatures. The average E value (100 GPa) [7] has been used to realize 266
the tendency of stress change as the annealing progresses. It is interesting to note that the 267
experimental value of E is very promising with those estimated via first principle 268
calculations, demonstrating that the actual stress developed in the coatings is also in good 269
agreement. From our analysis, it is clearly shown that the residual stress of the CuCo-oxide 270
coatings decreases with the rise in annealing temperature (See Table 1). Diffraction patterns 271
of cubic CuCo-oxide coatings before annealing and after being annealed at various 272
temperatures were analysed within the Fm-3m space group. Rietveld refinement of XRD data 273
produced successful fits with Rwp ≈ 23% and Rexp ≈ 20% (See Fig. 2). 274
The lattice constant was measured as 0.809 nm at room temperature and slightly increased as 275
the annealing progressed. The preferential growth in the as deposited coating was observed at 276
(440) orientation. The reorientation of the coating film from (440) to (400) took place at 200 277
°C. The XRD patterns of annealed coatings were dominated by peak broadening and 278
sharpening. The maximum line broadening and peak sharpening occurred to coating annealed 279
at 500 °C. The contribution of crystallite size, lattice constant, residual stress, and microstrain 280
to the peak broadening were identified using the Lorentzian and Gaussian components of the 281
fitted peaks. Table 1 show that the crystallite size and lattice constants of the coatings are 282
systematically increased as the annealing rises. On the other hand, the residual stress and 283
lattice micorstrain are reduced at higher temperatures which indicates the internal stress 284
release in the coating resulted from the reduction of defects, such as dislocations and 285
vacancies. 286
287
3.2 XPS Analysis 288
Figure 3 shows the XPS survey scans of CuCo-oxide coatings before and after annealing in 289
the binding energy range of 0-1200 eV. Elemental compositions of the coatings before 290
annealing and after annealing as estimated via XPS studies are listed in Table 2. From Table 291
2, it is evident that annealing has a prominent effect on the elemental compositions of metal 292
oxide coatings. From quantitative analysis of high resolution XPS spectra, the O and C 293
contents increase as annealing temperature increases. The atomic ratio between Co and Cu is 294
constant within experimental error as annealing temperature increases to 500 ºC. The possible 295
bonding states include C-C, C-H, C-O and C=O due to high temperature reaction in 296
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atmospheric conditions (O2, CO2, H2O, etc.). Definitely, there is no metal-carbide bonding. 297
Since the oxygen content is higher at high annealing temperature, the oxidation layer may be 298
thicker and more predominant at the surface. 299
The deconvolution curve of Cu2p3/2 photoelectron line and its satellite, before and after 300
annealing, of CuCo-oxide coatings is shown in Figure 4. The curve-fitting results of all 301
coatings as deducted from XPS data analysis are enumerated in Table 3. From Figure 4, it is 302
clearly seen that features arising from Cu2p3/2 photoelectron line lies in the binding energy 303
range of 931.7–943.8 eV. It also indicates that the gradual increase of annealing temperature 304
does not have significant effect on the copper bonding structure around the coatings surface. 305
Cu2p3/2 feature seen at 931.7-932.6 eV (denoted as ‘i’) is originated from tetrahedral Cu+ 306
ions. The second component detected at 932.5–933.5 eV (denoted as ‘ii’) can be assigned as 307
the contribution from the octahedral Cu2+ ions. The third segment observed at 933.7-934.8 eV 308
are assumed to be originated from a mixture of Cu+ ions and Cu2+ ions. The absence of a 309
component at the low binding energy side of the Cu2p3/2 peak indicates that natural cooling 310
overnight to room temperature inside the closed oven furnace may prevent the reduction of 311
octahedral Cu2+ in contrast to the relatively faster cooling outside the furnace as reported in 312
our previous work [20]. 313
The deconvolution of high resolution XPS profile and the corresponding satellite of Co2p3/2 314
photoelectron line (five components), of CuCo-oxide coatings, before annealing and after 315
annealing are presented in Figure 5. The manifestation of a typical satellite on the high 316
energy side of Co2p3/2 peak reveals the presence of CoII+ ions. Relatively lower intensity of 317
these satellites indicates a partial spinel-type lattice arrangement of cobalt ions. It is also 318
predicted that lower intensity of these satellites might correspond to a mixer of CoIII+ and 319
CoII+ ions [12, 42]. The first components seen at 778.8-779.7 eV are basically arising from 320
the CoIII+ ions in octahedral coordination. The second fitting curve is believed to be a 321
contribution from the mixed Co+III and Co+II bonding states. The third fitting components in 322
the binding energy range of 782.2-783.1 eV are the characteristic of CoII+ ions in tetrahedral 323
coordination. The corresponding binding energy positions and the percentage of each 324
component together with the satellite positions are set out in Table 4. From the Cu2p3/2 325
features, it is recognised that the CuIII+ ions partially substitute the CoII+ ions thus forming a 326
lower degree of crystallization of CuII+Co2III+O4 spinel systems [12, 13]. The satellite peaks 327
above 785.00 eV are well-known to be the contribution from cobalt oxide bonds. The gradual 328
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increase in the annealing temperatures, generally, does not have significant influence on the 329
bonding state of cobalt atoms. 330
Figure 6 shows the deconvolution of O1s XPS spectra of CuCo-oxide coatings synthesized 331
before annealing and after being annealed at different temperatures. In all five coatings, the 332
O1s spectrum displays a prominent peak with a shoulder at the higher binding energy side 333
above 530.0 eV. The curve-fitting of O1s photoelectron line gives out three fittings 334
components. The first components at 528.6–529.6 eV (symbolized as ‘i’) is ascribed to lattice 335
O2− ions, while the second components at 529.1–530.1 eV (symbolized as ‘ii’) may be due to 336
the surface oxygen originated from OH-like species such as hydroxyl, and carbonate groups 337
[26, 43, 44]. The third and final component at 530.7–531.7 eV (symbolized as ‘iii’) is 338
attributed to the sub-surface O− species. The apparent are assumed to be the distinctive 339
feature of the CuCo-oxide system which differentiates them from the O1s on Co3O4 phase. 340
However, there is no apparent alteration in the surface compositions of the CuCo-oxide 341
coatings as they undergo at different annealing temperatures. 342
3.3 FESEM analysis of the coatings 343
The impact of thermal treatment on the surface morphology of CuCo-oxide coatings was 344
investigated via FESEM imaging. As observed from FESEM images shown in Figure 7(a-e), 345
the annealing plays a significant role modifying the morphological features of the synthesized 346
coatings. Before annealing the CuCo-oxide coatings showed mould-like structures; after 347
thermal treatment they were transformed into compact and smooth morphologies together 348
with the homogeneously distributed particles. This might be due to the consequence of the 349
grain size’s change associated with amalgamation of grains. It is also expected that after 350
consolidation of the grains, agglomerates are formed and the new grains attain a certain 351
specific shape (see Figure 7(e)). The particles showed well-defined grain boundaries with the 352
particle size in the range of 25-70 nm. It was also confirmed that the average particle size of 353
the coatings monotonically increased with the gradual increase of annealing temperature of 354
the coatings. The overall quality and morphology of the coating are substantial and 355
remarkable. Morphological studies on Cu2O–CoO composites annealed at indicate the 356
formation of dense nanostructured particles without any distinguishable landscapes. 357
However, upon the increase of temperature to 500 °C, particles were agglomerated to 358
irregularly shape larger sizes [45]. 359
3.4 AFM analysis of the coatings 360
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Surface topographical features of the CuCo-oxide coatings, imaged using AFM technique are 361
presented in Figure 8. The estimated surface roughness values, together with the image 362
surface area of the CuCo-oxide coatings before and after annealing are presented in Table 6. 363
The Ra and Rq values of the CuCo-oxide coatings before annealing are found to be lower than 364
the annealed ones. The mean surface roughness, Ra and the rms value of the surface 365
roughness, Rq have been improved by 121 and 142%, respectively after the coatings been 366
annealed, within an average image surface area of 117 μm2. This clearly indicates that 367
annealing has remarkable impact on the surface topographical features of CuCo-oxide 368
coatings. Annealing results in the occurrence of major grain growth around the coating 369
surface which in turn is responsible for the enhanced surface roughness of the coatings. This 370
is because as the annealing progresses, the atoms have adequate activation energy to occupy 371
the correct site in the crystal lattice and grains. It is also assumed that, during annealing, due 372
to the higher ionic mobility, and densification of the materials grains were growth on the z 373
direction, perpendicular to the substrate surface and surface roughness of the coatings was 374
increased. This is consistent with the crystalline properties of these coatings as seen in 375
FESEM and XRD analysis of this manuscript. The peaks and valleys indicate the quantitative 376
surface roughness and absorptancy of these coatings. With close examination of images, 377
grain-like particles in range of 20 - 80 nm are observed. These grain-like particles 378
morphologies were also reported by Amun et al. [13]. This is consistent with FESEM 379
analysis, section 3.3 above. 380
381
3.5 Solar selectivity studies 382
The solar selective properties of CuCo-oxide coatings before and after annealing were 383
evaluated on the basis of solar reflectance spectra, acquired using UV-Vis and FTIR 384
techniques in the wavelength range of 190-2500 nm and 2.5-15.4 μm, respectively shown in 385
Figures 9 and 10. Using the UV-Vis and FTIR reflectance spectra, the solar absorptance and 386
thermal emittance values of these coatings were assessed by Duffie and Beckman method, as 387
described in [24]. The corresponding solar selectivity values of the coatings as computed 388
using 7 = 89 are displayed in Table 7. The prepared coatings exhibit low to moderate 389
reflectance together with interference peaks in the lower wavelength regions and sharp 390
absorption edges that basically form solar selective absorber curve profiles surrounded by the 391
UV–Vis–NIR range of the solar spectrum. Gradual increase in annealing temperature, 392
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generally, has a tendency to shrink the interference peaks and the absorption edges. 393
Consequently, the solar absorptance values should be improved. Substantial reductions of the 394
interference peak and the absorption edge were identified by the coatings annealed at 400 °C 395
and 500 °C and the corresponding absorptance values reached to the maximum. It is, 396
generally, assumed that the reflectance behaviours of CuCo-oxide coatings, in the NIR range, 397
is governed by three aspects: (i) thickness of the coatings, (ii) inherent properties of the 398
coating materials, and (iii) the reflective nature of the substrates used to deposit the coatings 399
[13, 46]. In the present case all the coatings having the similar thicknesses, the reflectance 400
profiles are governed by the combined effect of solar absorptions/scattering by the coating 401
materials and the back-reflections of the near-infrared radiations passed through the coatings 402
by the substrate. Furthermore, high temperature annealing also boosts the crystallinity of the 403
coatings material that consequently results in increasing the scattering leading to enhance the 404
absorption. 405
Following Beckman-Duffie method [24], using the FTIR reflectance data presented in Figure 406
10, the estimated thermal emittance values of CuCo-oxide coatings before and after annealing 407
presented in Table 7 shows that the thermal emittance values are significantly reduced from 408
13.6 to 3.8% with the gradual increase in annealing temperatures. It is established that the 409
solar absorptance and thermal emittance of coatings strongly depend on their corresponding 410
band-gap. Annealing effectively modifies the overall band structure of these coatings and 411
thereby the solar selectivity values are also improved. The selection of substrate materials 412
also has a considerable impact on the reflective behaviour of coatings. The longer the near-413
infrared wavelength, the more radiation will be transmitted through the coating due to the less 414
energy preserved by the optical photons. As a result, it is easier for them to pass through to 415
the coating without being absorbed and then reflected back by the substrate. This absorber–416
reflector tandem concept allows them to behave similar to a semiconducting object. The solar 417
selectivity values of these coatings also depend on the materials used, synthesis conditions 418
and techniques, coatings thickness, surface roughness of the coatings, and so on. It has been 419
clearly seen that annealing leads to lattice and grain refinements (XRD results), modifications 420
in residual stress and microstrain values (XRD data), formation of new bonding and changes 421
in chemical bonding states (XPS results), microstructural modification (from FESEM 422
studies), variations in surface roughness of the coatings (AFM studies), and band structure 423
change (band-gap analysis data). All these factors substantially affect the scattering and the 424
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reflection of incident solar radiation and therefore the optical characteristics of the coatings 425
are, in turn, are improved. Consequently, the solar selectivity values are boosted. 426
427
3.6 Bader’s charge analysis 428
Table 8 lists Bader’s charges on Cu, Co and O atoms in bulk CuxCo3-xO4 system. As seen 429
from Table 8, the Cu and Co atoms in all the stoichiometries hold positive charges while the 430
O atoms are associated with negative charges. Bader charge values also reveal a covalent 431
character for Cu-Co bond in all the system that have both of these atoms and ionic 432
characteristics for Cu-O and Co-O bonds. Moreover, our results show that Co atoms loss 433
more electrons when Cu is introduced into the system, while O atoms gains electrons less 434
than what they did when Cu ratios increased. 435
436
4. Conclusions 437
The CuCo-oxide coatings, deposited on aluminium substrates, were investigated for their 438
temperature dependent structural and solar selectivity analysis via XRD, XPS, FESEM, 439
AFM, UV–Vis, and FTIR approaches. FESEM, AFM and XRD show increase in the 440
crystalline domains of the coatings with increasing annealing temperatures while the residual 441
stress systematically decreasing indicating mechanically stable material. XPS analysis 442
determined (i) tetrahedral, octahedral and mixed states of Cu and Co ions, (ii) stable Co/Cu 443
ratio, (ii) increasing surface C and O ratio and (iv) no metal-carbon bonding. Optical studies 444
via UV–Vis and FTIR reflectance spectrum confirmed an excellent solar selectivity of 24 445
attained by the coating annealed at 500 °C. 446
447
5. Supporting Information 448
Figure S1 shows the 2D AFM images of CuCo-oxide coatings before annealing and after 449
being annealed at various temperatures while Figure S2 represents the FESEM images of the 450
same samples at same conditions that demonstrate the average particle size of CuCo-oxide 451
coatings. 452
453
6. Acknowledgements 454
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The authors acknowledge funding support provided by IRU-MRUN Collaborative Research 455
Program, as well as grants of computing time from the National Computational Infrastructure 456
(NCI) in Canberra and the Pawsey Supercomputing Centre (iVEC) in Perth. Hussein Miran 457
wishes to thank for the PhD scholarship provided by the Ministry of Higher Education and 458
Scientific Research of Iraq. 459
460
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541
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Annealing temperature
(°C)
Crystallite size
(±20%), Dg (nm)
Lattice constant, a
(nm)
Along (400) plane Along (440) plane
Microstrain 2θ
Residual stress, σx (GPa)
2θ Residual stress, σx
(GPa) Room
temperature 310 0.809 44.74 -1.15 65.12 0.19 0.14
200.00 330 0.810 44.70 0.05 65.07 0.11 0.12 300.00 350 0.810 44.71 0.05 65.08 0.08 0.11 400.00 420 0.811 44.72 0.03 65.09 0.08 0.10 500.00 530 0.812 44.72 0.02 65.09 0.07 0.08
Table 1. Variation of crystallite size, lattice parameters, residual stress and microstrain of CuCo-oxide coatings with different annealing temperatures.
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Annealing temperatures (°C) Atomic percentages of elements
Cu Co O C Before annealing 13.96 24.57 59.32 2.15
200°C 11.99 25.10 60.25 2.67 300°C 11.35 23.89 61.54 3.22 400°C 10.77 17.86 63.67 7.70 500°C 6.43 12.41 53.04 28.13
Annealing temperature
(°C)
Binding energy positions and percentage of bonding states Cu2p photoelectron line Satellites
i ii iii iv v Before
annealing 932.6 eV (35.5%)
933.5 eV (20.6%)
934.5 eV (27.0%)
941.0 eV (10.9%)
943.8 eV (6.0%)
200 931.7 eV (31.8%)
932.5 eV (29.4%)
933.7 eV (22.6%)
940.1 eV (9.4%)
942.7 eV (6.7%)
300 931.8 eV (35.1%)
932.7 eV (27.8%)
934.1 eV (16.6%)
940.1 eV (10.9%)
942.6 eV (9.6%)
400 932.3 eV (49.3%)
933.3 eV (9.33.3%)
934.7 eV (934.7%)
940.7 eV (940.7%)
943.3 eV (943.3%)
500 932.2 eV (23.2%)
933.1 eV (50.3%)
934.8 eV (11.6%)
940.9 eV (11.3%)
943.4 eV (3.6%)
Table 2. Elemental compositions of CuCo-oxide coatings before and after annealing in air.
Table 3. The deconvolution results of high resolution XPS spectra at Cu2p photoelectron
line and its satellite.
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Annealing temperature
(°C)
Binding energy positions and percentage of bonding states Co2p photoelectron line Satellites
i ii iii iv v Before
annealing 779.7 eV (32.7%)
781.2 eV (22.7%)
783.1 eV (21.6%)
786.3 eV (14.0%)
788.5 eV (8.9%)
200 778.8 eV (32.0%)
780.3 eV (24.4%)
782.3 eV (18.4%)
785.2 eV (14.8%)
787.6 eV (10.4%)
300 778.8 eV (27.0%)
780.3 eV (33.7%)
782.5 eV (15.9%)
785.2 eV (13.8%)
787.4 eV (9.7%)
400 779.0 eV ((23.7%)
780.2 eV (27.9%)
782.2 eV (16.9%)
785.6 eV (25.6%)
788.5 eV (5.9%)
500 779.3 eV (33.5%)
780.7 eV (18.4%)
782.2 eV (19.3%)
785.5 eV (20.6%)
788.3 eV (8.1%)
Annealing temperature (°C) Binding energy positions and percentage of bonding states
of O1s photoelectron line i ii iii
Before annealing 529.6 eV (48.0%)
529.9 eV (42.7%)
531.7 eV (9.3%)
200 528.6 eV (40.7%)
529.1 eV (49.7%)
530.7 eV (9.6%)
300 528.8 eV (47.5%)
529.1 eV (38.6%)
530.8 eV (13.9%)
400 529.1 eV (49.5%)
529.5 eV (34.95)
531.2 eV (15.7%)
500 529.3 eV (61.5%)
530.1 eV (20.1%)
531.5 eV (18.4%)
Table 4. The deconvolution results of high resolution XPS spectra at Co2p photoelectron
line and its satellite.
Table 5. The deconvolution results of high resolution XPS spectra at O1s photoelectron line
and its satellite.
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Table 6. Surface roughness parameters of CuCo-oxide coatings before and after annealing.
Annealing temperatures
(°C)
Mean roughness, Ra
(nm)
rms value of the surface roughness,
Rq(nm) z Scale (nm)
Image surface area (μm2)
Before annealing
19.4 24.8 210 105
200 27.8 38.6 530 121 300 32.1 44.7 704 134 400 37.0 52.1 820 157 500 43.0 60.0 910 167
Table 7. Solar absorbance, thermal emittance and solar selectivity values of CuCo-oxide coatings before annealing and after annealing at different temperatures in air for 1 hour.
Annealing condition Solar absorptance (%) Thermal emittance (%) Solar selectivity Before annealing 51.72 13.60 3.81
200 °C 74.45 9.72 7.66 300 °C 80.90 7.45 10.86 400 °C 86.30 6.49 13.30 500 °C 91.22 3.80 24.00
Table 8. Bader’s charges on Cu, Co and O atoms in e on CuxCo3-xO4 (x = 0, 0.75, 1.5, 2.25 and 3) coatings.
Stoichiometry Charge transfer (electrons) Cux Co3-x O4 Cu Co O 0 3 4 0 1.58 -1.18
0.75 2.25 4 1.10 1.61 -1.11 1.5 1.5 4 1.30 1.66 -1.11 2.25 0.75 4 1.28 1.66 -1.03
3 0 4 1.12 0 -0.84
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Fig. 1. XRD spectra of CuCo-oxide coatings before and after annealing.
(222
)
(400
)
(440
)
(622
)
200°C
300°C
400°C
500°C
Before annealing
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400°C
020040060080010001200
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
Fig. 3. XPS survey spectra of CuCo-oxide coatings before and after annealing.
Before annealing
200°C
300°C
500°C
O
C
Cu Co
400°C
Fig. 2. Rietveld refinement of XRD spectra of CuCo-oxide coatings before and after annealing.
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Fig. 4. Decoupling of high resolution XPS spectra of Cu2p3/2 peak of CuCo-oxide coatings (a) before
annealing, and annealed at: (b) 200°C, (c) 300°C, (d) 400°C, and (e) 500°C.
928930932934936938940942944946
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(a)
(i)
(ii)
(iii)
Sat. (i) Sat. (ii)
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Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(b) (i)
(ii) (iii)
Sat. (i) Sat. (ii)
928930932934936938940942944946
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(d)
(i)
(ii)
(iii)
Sat. (i) Sat. (ii)
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Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(c)
(i)
(ii)
(iii)
Sat. (i) Sat. (ii)
928930932934936938940942944946
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(e)
(i) (ii)
(iii)
Sat. (i) Sat. (ii)
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Fig. 5. Decoupling of high resolution XPS spectra of Co2p3/2 peak of CuCo-oxide coatings (a) before
annealing, and annealed at: (b) 200°C, (c) 300°C, (d) 400°C, and (e) 500°C.
774776778780782784786788790792794
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
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Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(a) (b)
(i) (ii)
(iii) Sat. (i)
Sat. (ii)
(i) (ii)
(iii) Sat. (i)
Sat. (ii)
774776778780782784786788790792794
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(c)
(i)
(ii) (iii)
Sat. (i)
Sat. (ii)
774776778780782784786788790792794
Inte
nsi
ty (
A.U
.)
Binding energy (eV)
(d) (i)
(ii)
(iii) Sat. (i)
Sat. (ii)
774776778780782784786788790792794
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nsi
ty (
A.U
.)
Binding energy (eV)
(e) (i)
(ii)
(iii) Sat. (i)
Sat. (ii)
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Fig. 6. Decoupling of high resolution XPS spectra of O1s peak of CuCo-oxide coatings (a) before
annealing, and annealed at: (b) 200°C, (c) 300°C, (d) 400°C, and (e) 500°C.
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nsi
ty (
A.U
.)
Binding energy (eV)
(a)
(i)
(ii)
(iii)
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nsi
ty (
A.U
.)
Binding energy (eV)
(b)
(i)
(ii)
(iii)
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.)
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(c)
(i)
(ii)
(iii)
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.)
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(d) (i)
(ii) (iii)
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.)
Binding energy (eV)
(e)
(i)
(ii)
(iii)
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Fig. 7. FESEM images of CuCo-oxide coatings (a) before annealing, and annealed at: (b) 200°C, (c)
300°C, (d) 400°C, and (e) 500°C.
(a) (b)
(c) (d)
(e)
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Fig. 8. AFM images of CuCo-oxide coatings (a) before annealing, and annealed at: (b) 200°C, (c) 300°C,
(d) 400°C, and (e) 500°C.
(a) (b)
(c)
(d)
(e)
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Fig. 10. FTIR reflectance spectra of CuCo-oxide coatings before and after annealing.
Fig. 9. UV-Vis reflectance spectra of CuCo-oxide coatings before and after annealing.
0
20
40
60
80
100
190 690 1190 1690 2190
Re
fle
cta
nce
, R
(%
)
Wavelength, λ (nm)
Before annealing
200°C
300°C
400°C
500°C
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Highlights
� The effect of thermal treatment of CoCu-oxide thin film coatings on mechanical and optical properties was investigated
� Crystallinity and grain size of thin films increase with the increase of annealing temperature
up to 500 °C
� Residual stress and microstrains of thin films decrease with the increase of annealing temperature up to 500 °C
� A high solar selectivity of 24 was achieved through thermal treatment at 500 °C
