Synthesis and characterization of Fe3 + doped Co0.5Mg0.5Al2O4 inorganic pigments with high near-infrared reflectance

    Synthesis and characterization of Fe 3 + doped Co0.5 Mg0.5 Al2 O4 inorganic pigments with high near-infrared reflectance Weiwei Bao, Fei Ma, Yingtang Zhang, Xiaoli Hao, Zhifeng Deng, Xiangyu Zou, Wei Gao PII: DOI: Reference:

S0032-5910(16)30012-2 doi: 10.1016/j.powtec.2016.01.013 PTEC 11450

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

17 November 2015 8 January 2016 14 January 2016

Please cite this article as: Weiwei Bao, Fei Ma, Yingtang Zhang, Xiaoli Hao, Zhifeng Deng, Xiangyu Zou, Wei Gao, Synthesis and characterization of Fe3 + doped Co0.5 Mg0.5 Al2 O4 inorganic pigments with high near-infrared reflectance, Powder Technology (2016), doi: 10.1016/j.powtec.2016.01.013

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ACCEPTED MANUSCRIPT Synthesis and characterization of Fe3+ doped Co0.5Mg0.5Al2O4 inorganic pigments with high near-infrared reflectance

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Weiwei Baoa*, Fei Maa, Yingtang Zhanga, Xiaoli Haoa, Zhifeng Denga, Xiangyu Zoua, Wei Gaob a

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(School of Materials Science and Engineering, Shaanxi University of Technology, Hanzhong

723000, PR China) b (College of Materials Science and Engineering, Heilongjiang University of Science and Technology, Harbin, 150022, PR China)

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Abstract

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In this work, a series of near-infrared reflective inorganic pigments with a general formula Co0.5Mg0.5Al2-xFexO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were successfully prepared by Pechini-type

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sol-gel method. Comprehensive analyses were carried out to characterize the developed pigment

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powders including thermogravimetry and differential scanning calorimetry, X-ray diffraction,

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field emission scanning electron microscopy, ultraviolet-visible near infrared diffuse reflectance spectroscopy, and CIE-L*a*b* 1976 color scales. The results demonstrated that the single-phase

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Co0.5Mg0.5Al2-xFexO4 was synthesized at an optimum temperature of 900 oC. The resulting calcined powders have a well-developed cubic spinel structure. The substitution of Fe3+ for Al3+ in Co0.5Mg0.5Al2-xFexO4 changes the color from blue to black and the band gap shifts from 4.40 eV to 3.50 eV. And Fe doped pigments possess high near-infrared solar reflectance (>43%) in the range of 780-2500 nm. Therefore, these Co0.5Mg0.5Al2-xFexO4 powders have great potential in serving as cool pigments for building coatings. Keywords: Spinel, Pechini-type sol-gel, Doping, Near-infrared reflectance, Cool pigments Corresponding author *

Tel.: 086-0916-2641711; *E-mail address: [email protected]

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ACCEPTED MANUSCRIPT 1.

Introduction Solar radiation consists of ~5% UV radiation, 43% visible radiation and 52% near-infrared

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radiation (NIR; ~780-2500 nm). Solar energy is absorbed by buildings and paved surfaces, which

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leads to temperature rise of urban structures. These surfaces thereby warm up the surrounding, causing measurable ambient temperature rise [1-2]. This phenomenon, termed “urban heat island” (UHI), has been urging a drastic increase in the demand for energy, especially-energy consumed

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by central air conditioning systems in large buildings for cooling. One of the most effective

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methods to alleviate Heat Island Effect involves developing near-infrared reflective (NIR) pigments [3-4]. Induced by energy conservation, these pigments and the materials related have

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been gaining increased attention from researchers over the years. Particularly, one of these

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materials, the spinel-type oxide have been widely studied as attributed to the potential in

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electrochemical, catalytic and pigment applications [5-7]. The spinel-type oxides are generally formulated as A 2  B32 O4 , in which the anions arranged in a cubic close-packed lattice that is

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classified as cubic. In addition, the A and B cations can occupy up to all the octahedral and tetrahedral sites in this lattice. Notably, there are two ideal structures of significance: normal spinel structure and inverse spinel structure [8]. For example, CoAl2O4 is one of the best known members of cobalt spinel-type oxides possessing various uses in ceramic, glass, paint industries; e.g., contrast-enhancing luminescent CoAl2O4 pigments for television tubes to produce Thenard’s blue color [9]. One of the common methods for manufacturing pigment powder involves a solid-state reaction in which the oxides are mechanically ground at high calcination temperatures above 1200 oC for a considerable amount of time. Though the mentioned process is relatively uncostly, undesired

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ACCEPTED MANUSCRIPT byproducts can be produced due to lack of homogeneity indicated by larger and uneven grains in the final product as a result of the weak control [10-12]. Recently, scientists have proposed various

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novel wet-chemical synthetic routes that would alleviate this problem, which include sol-gel

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method (polymer precursor method) [13-14], low temperature combustion technique [15-16], polymer pyrolysis method [17-18], co-precipitation reaction [19-20], emulsion precipitation [21], and hydrothermal crystallization [22].

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In this work, Co0.5Mg0.5Al2-xFexO4 pigment was synthesized using Pechini-type sol-gel method.

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Over the years, other kinds of compound powders, such as LaFe1-xAlxO3 yellow pigment powder [2], Zn1-xMgxFe2O4 [23], CoxZn1-xAl2O4 [24] etc., have also been synthesized using sol-gel method.

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This method has a great many advantages as compared to others; e.g., lower energy-consumption,

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relatively low temperatures of synthesis, good stoichiometric and morphological control. Most

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importantly, these pigment powders exhibit higher homogeneity and stronger chemical activity. CoAl2O4 is one of the commercially important materials which has super properties and

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extensively used as a pigment. But cobalt is scarce and expensive, thus increasing the production costs of cobalt-based pigments. Therefore, in the present work, Mg2+ for Co2+ ions substitution in the CoAl2O4 was done. On the other hand, spinel-type materials have attracted more attention from researchers. Due to their capabilities of accommodating different cations, they display various colors and tonalities [25]. Doping of Fe3+ for Al3+ in Co0.5Mg0.5Al2O4 changes the color properties of pigments. Therefore, in this paper, a series of NIR reflectance inorganic pigments with the formula Co0.5Mg0.5Al2-xFexO4 was used as a cool pigment. Co0.5Mg0.5Al2-xFexO4 samples were synthesized through a sol-gel route within a defined calcination temperature range. The crystal structure, NIR reflective chromatic properties, thermal and chemical stability were

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ACCEPTED MANUSCRIPT investigated. 2. Experimental

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2.1. Materials and methodology

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Pure and substituted Co0.5Mg0.5Al2-xFexO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) powders were synthesized using Pechini-type sol-gel method. The reagents used were cobalt nitrate (Co(NO3)2·6H2O), magnesium nitrate (Mg(NO3)2·6H2O), aluminum nitrate (Al(NO3)3·9H2O),

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ferric nitrate (Fe(NO3)3·9H2O), citric acid (CA) and ethylene glycol (EG). The CA/metal molar

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ratio was 3:1, while the CA/EG mass ratio was 3:2. Initially, citric acid was dissolved in distilled water under constant stirring at a temperature of 80 oC. Later, cobalt nitrate, magnesium nitrate,

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aluminum nitrate or ferric nitrate was added in stoichiometric proportion to the acid solution. After

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being kept at around 80 oC for 1 h, ethylene glycol was then added. The beaker was kept in an

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oven at 120 oC until a viscous gel was obtained. Then the gel undertook a preliminary heat treatment for 2 h at a temperature of 350 oC in an air atmosphere. Following the heat treatment,

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gas liberation during the combustion led to a partial degradation of the organic structure and its expansion. Finally, a fragile and black material, the powder precursor, thus formed. Next the powder precursor was deagglomerated and underwent the final heat treatment at the desired temperature (600-1000 oC) for 6 h to eliminate the remaining organic material. All reagents used in this study were of analytical grade. 2.2. Characterization techniques Thermal analysis of the powder precursor and synthesized pigments were characterized by thermogravimetry and differential scanning calorimetry (TG/DSC1, Mettler Toledo, Switzerland) under air in a temperature range of 50-900 oC at a heating rate of 20 oC/min. Tests were performed

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ACCEPTED MANUSCRIPT with approximately 10 mg of the sample contained in aluminum crucibles. Tabular α-Al2O3 was used as reference weight losses.

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The crystalline nature and phase purity of the synthesized samples were determined using a

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Rigaku Ultima ⅣX-RAY diffractometer employing Ni-filtered Cu Kα radiation (λ=0.15406 nm), which was operated with voltage and current settings of 40 kV and 40 mA, respectively. Data were collected by step scanning over a 2θ range from 10 to 80o, with a step size of 0.02o and 5s

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counting time at each step.

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The morphologies of the pigment samples were obtained by field emission scanning electron microscopy (FESEM, S-4800, Japan). The samples used for SEM characterization were dispersed

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in absolute ethanol and ultrasonicated before preparation.

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The diffuse reflectance of the pigment powders were measured using a UV-vis-NIR

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spectrophotometer (Lambda 950, Perkin Elmer, America) with integrating sphere, where poly-tetrafluoroethylene (PTFE) was used as the white standard. The reflection spectra were

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scanned in a range of 300-2500 nm with a 5 nm interval. The spectral reflectance data were used to calculate the solar reflectance of each sample. The optical band gap energy was calculated using Tau relation [26]. The Kubulka-Munk reemission function [27] was generally applied to convert the diffused reflectance into equivalent absorption coefficient and mostly used for analyzing the powder samples [28]. The function K-M was given by 2  1- R   F ( R) 

(1)

2R

where F(R) is Kubulka-Munk function, α is the absorption coefficient, R is the reflectance of samples. Thus the Tauc relation becomes, 5

ACCEPTED MANUSCRIPT F R hv  A( hv  Eg )n

(2)

where n=1/2 and 2 for direct and indirect transitions, respectively, thereby giving direct and

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indirect band gaps [29]. Extrapolation of linear regions of these plots to (F(R)hv)2 = 0 gives the

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direct band gap values. A plot of K-M and wavelength is drawn and the absorption edge is determined from the plot. The value of the wavelength is substituted in formula

1236 Wavelength nm 

(3)

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E ev 

The color parameters of the pigments were calculated from the visible light reflection data

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using the color CIE chromaticity analysis software from Perkin Elmer. The CIE 1976 L*a*b*

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colorimetric method was used, as recommended by the Commission internationale de l’Eclairage.

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In this method, L* is the color lightness (L*=0 for black and L*=100 for white), a* is the green (-)/red (+) axis, and b* is the blue (-)/ yellow (+) axis. The parameter C* (chroma) represents

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saturation of the color and is defined as C*= (a*2+b*2)1/2 [30]. The NIR solar reflectance (R*) in the wavelength ranged from 780 to 2500 nm was calculated

R*

 

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according to measurement standard JG/T 235-2014. The function R* is determined by 2500

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

r ( )i ( )d ( )

2500

780

(4)

i ( ) d ( )

where r(λ) is the spectral reflectance (Wm-2) obtained from the UV-vis-NIR spectrophotometer and i(λ) is the solar special irradiance (Wm-2nm-1) obtained from measurement standard JG/T 235-2014. 3. Results and discussion 3.1. Thermal analysis of Co0.5Mg0.5Al2-xFeO4 pigment precursor The synthesis of the typical pigment samples was followed by the thermal analysis (TG/DSC) 6

ACCEPTED MANUSCRIPT before calcination, and the results were presented in Fig. 1. Three different stages with total weight loss of approximately 70% can be observed from the TG curve. As observed from the TG

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curve, when the temperature reaches 285 oC, the sample shows weight loss of about 21%, which is

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associated with the removal of the residual water, un-polymerized citric acid and ethylene glycol molecular in the precursor. When the temperature increases from 285 to 600 oC, the mass loss accelerates, and more than 44% mass loss was recorded at this stage. Meanwhile, a high and

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intense exothermic peak can be clearly identified at 450 oC in the DSC curve, which likely to be

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associated with the decomposition of polymeric precursor into Co0.5Mg0.5Al2O4 particles [31]. When the decomposition temperature is higher than 600 oC, the mass loss is less than 5%,

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indicating the formation of final Co0.5Mg0.5Al2O4 particles.

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The ideal reaction of citric acid, ethylene glycol and metal nitrates for synthesis

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Co0.5Mg0.5Al2O4 was presented as the following equation:

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1 1 CoNO3 2  6 H 2O  Mg NO3 2  6 H 2O  2 Al NO3 3  9 H 2O  3C6 H 8O7  H 2O  C2 H 6O2 2 2  10O2  Co0.5 Mgo .5 Al2O4  42 H 2O  24CO2  4 N 2 3.2. Powder X-ray diffraction analysis The optimum calcination temperature required for the formation of the typical pigment sample Co0.5Mg0.5Al2O4 was ascertained by calcining the precursor oxides at various temperatures ranging from 600 to 1000 oC for 6 h. Fig. 2 displays the XRD patterns of xerogel precursor calcined in air at different temperature. The result suggested that higher temperatures were beneficial for the system crystallinity. The patterns also capture a spinel phase formation at 600 oC, according to the standard JCPDS card No. 44-0160 for CoAl2O4. The characteristic peaks of all the samples at 2θ values 31.2o, 36.7o, 44.7o, 55.5o, 59.2o, and 65.0o are indexed to (220), (311),

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ACCEPTED MANUSCRIPT (400), (422), (511), and (440) crystallographic planes of cobalt aluminate spinels, respectively. In addition, the intensity of the peak is relatively strong at 900 oC. So in the next step, Fe3+ doped

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Co0.5Mg0.5Al2O4 pigments were calcined at 900 oC for 6 h. The average crystallite size (D) of samples is estimated from X-ray diffraction line broadening using the Scherrer formula [32],

0.89 cos

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D

(5)

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where λ is the wavelength of the X-ray source, θ is the diffraction angle and β is the corrected full width at half maximum of peaks. It was observed that the sizes of powdered samples increased

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from 20.1 nm at 600 oC to 36.8 nm at 1000 oC which was attributed to the enlargement of

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Co0.5Mg0.5Al2O4 grains and reunion of the grains with the raised temperature. The grain of sample

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calcined at 900 oC holds an average size of 35.1 nm. Fig. 3a shows the XRD patterns of xerogel precursor, after grinding and calcination at 900 oC

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for 6 h. It can be seen that the diffraction peaks identified all belong to the standard cubic spinel structure of CoAl2O4, and the doping of Fe3+ did not change the crystal structure of the CoAl2O4

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phase. The addition of Fe3+ ions results in a slight shift of XRD peaks toward lower diffraction angles in contrast with those of the parent oxide. For example, Fig. 3b reports shifts of the Bragg reflections (220), (311) and (400). The observed 2θ shift is due to a change in the cell volume. Table 1 shows that the increase in cell volume with the increase in Fe concentration in Co0.5Mg0.5Al2-xFexO4, which declares that a larger fraction of iron replaces larger Al3+ (0.051 nm) as larger Fe3+ (0.064 nm). The regular increase in the cell volume with the increase in Fe concentration indicates successful replacement of the aluminum by iron. 3.3 Particle size and morphological analysis FE-SEM was used to examine the size and morphology of the prepared particles. As shown in 8

ACCEPTED MANUSCRIPT Fig. 4, the images of all Co0.5Mg0.5Al2O4 samples reveal that the particles have irregular shaped morphology and agglomerations are observed. Besides, the average particle diameter of

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Co0.5Mg0.5Al2O4 samples annealed at 700-900 oC is about 40-100 nm. It can be seen that the

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particle size of Co0.5Mg0.5Al2O4 increases with increasing calcination temperature. 3.4. Diffuse reflectance and chromatic properties analysis

Fig. 5 depicts the diffused reflectance spectra obtained for the Co0.5Mg0.5Al2-xFexO4 pigments.

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The spectra of metal oxide samples show similar features: in the visible region, three-structured

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bands centered at approximately 550, 580 and 620 nm are attributed to the spin allowed 4A2 (F)  4T1 (P) transition of the Co2+ ions in tetrahedral sites of spinel structure [33-34]. Meanwhile,

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the reflectance spectra show a stronger absorption in the visible region with increasing number of

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ferric ions, suggesting band gap shrinkage.

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The band gap was determined by Tanc relation shown above. The plots of (F(R) hv)2 versus hv for all samples are shown in Fig. 6. It is clear that the replacement of Fe3+ for Al3+ in

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Co0.5Mg0.5Al2O4 shifts the absorption edge of samples from 323 to 353 nm. The estimated values of the band gap of Co0.5Mg0.5Al2-xFexO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) particles are 4.40, 3.77, 3.50, 3.50, 3.67 and 3.69 eV respectively (Fig. 6 and Table 2). With the increase of iron concentration, the samples show a red shift in the band gap transition. It can be attributed to the additional sub-band-gap energy levels that are caused by abundant surface and interface defects in the agglomerated particles [35]. As a result, the color of pigment samples changes from blue to black (Fig. 7). The chromatic properties of the synthesized Co0.5Mg0.5Al2-xFexO4(x=0.0-1.0) pigment samples can be assessed from their CIE1976 L*a*b* color coordinate values listed in Table 2. It can be

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ACCEPTED MANUSCRIPT noticed that more Fe3+ added results in the values of L* decreasing from 55.64 to 30.05 suggesting that the doped Co0.5Mg0.5Al2O4 pigment turns dimmer in color. On the other hand, more and more

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substitution of Fe3+ for Al3+ increases the color coordinate a* value from -15.67 to 2.61, which

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presents the loss of green hue of the pigments. Moreover, the b* values increase from -38.84 to 6.35, which denotes the weakness in the intensity of blueness of the pigment samples. In consequence, the C* values, which present the richness of the color hue, drop from 41.88 to 6.87.

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3.5. NIR reflectance analysis

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The NIR reflectance spectra of the ferric doped Co0.5Mg0.5Al2O4 powder pigment samples are given in Fig. 8, Fig. 9 shows the result of NIR reflectance of pigments handled with measurement

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standard JG/T 235-2014, in which it multiplies the spectral reflectance by normalized solar

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spectral hemispherical irradiance i(λ). Remarkably, the sample of Co0.5Mg0.5Al2O4 possesses the

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highest NIR solar reflectance of about 54.2%. At the same time, it is interesting to observe that no absorption band between 780 and 1100 nm is remarkable for all samples which lead to a higher

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reflection in this range. This wavelength region of interest for solar radiation plays the most important role in the heat generation [32]. With Fe3+ ions doped to substitute Al3+ ions, the NIR solar reflectance values of the powdered samples climb up over 43.0% with maximum 52.7% (x=0.2), as a result of the change of Co2+ surrounding electron environment. Table 3 presents the detailed information of NIR solar reflectance of powdered pigments. However, the solar reflectance values of commercially available roofing products are in the range of 5-33% [36]. Thus, all the newly designed blue to black pigments synthesized by the Pechini-type sol-gel method display high NIR solar reflectance, thus they can serve as interesting candidates for cool colorants.

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ACCEPTED MANUSCRIPT 3.6. Thermal and chemical stability studies of the pigments The TG/DSC curves of the typical obtained pigments Co0.5Mg0.5Al2O4 and Co0.5Mg0.5AlFeO4

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are displayed in Fig. 10. It is demonstrated that there is negligible change of weight and phase

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transition of the pigments in the temperature ranging from 50 to 900 oC, indicating that these pigments are thermally stable. To evaluate the chemical stability of the synthesized pigments, the acid/alkali and water resistance of the pigment samples was tested in 5% HCl/HNO3/H2SO4

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NaOH and H2O. A pre-weighed amount of the pigments was treated with acid/alkali and then

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soaked in solution for half an hour with constant stirring by a magnetic stirrer. Afterwards, the pigment samples was filtered, washed with water, dried and weighed. Negligible weight loss of

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the pigment samples was noticed for the acid, alkali and water tested. Table 4 presents the color

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coordinates of the samples after acid/alkali and water treatment. △E* stands for the total color

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difference. Small values of △E* indicated that the pigments are chemically stable towards the acid/alkali and water tested [37].

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4. Conclusions

A series of NIR reflective inorganic pigments with the general formula Co0.5Mg0.5Al2-xFexO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) having a spinel structure and displaying varying colors from blue to black have been synthesized by the Pechini-type sol-gel. The result demonstrated that a wide range of colors can be achieved by the incorporation of suitable chromophore metal ions (Fe3+) in Co0.5Mg0.5Al2-xFexO4 the matrix by engineering the band gap. Most importantly, the designed pigments not only exhibit various color hues in the visible region but also possess high NIR solar reflectance (>43%). The current synthesized pigments also possess good thermal and chemical stability. Consequently, these pigments are considered as new candidates for cool pigments.

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ACCEPTED MANUSCRIPT Acknowledgment This work was supported by the Doctoral Scientific Research Foundation of Shaanxi University

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of Technology, China (Grant No. SLGQD 2013(2)-15) and the Foundation of Shaanxi Educational

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Committee, China (Grant No. 15JK1165). References

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Scheller, R.S. Angelica, C.E.F. Costa, Blue pigments based on CoxZn1-xAl2O4 spinel synthesized by the polymeric precursor method, Dyes and pigments 81 (2009) 187-192. [25] R.A. Candeia, M.A.F. Souza, M.I.B. Bernardi, S.C. Maestrelli, I.M.G. Santos, A.G. Souza, E.

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Longo, Monoferrite BaFe2O4 applied as ceramic pigment, Ceram. Int. 33 (2007) 521-525.

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[26] G. Theophil Anand, L. John Kennedy, J. Judith Vijaya, Microwave combustion synthesis, structural, optical and magnetic properties of Zn1-xCoxAl2O4 (0≤x≤0.5) spinel nanostructures, J.

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Alloys Comp. 581 (2013) 558-566.

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[27] K.J. Sreeram, C.P. Aby, B.U. Nair, T. Ramasami, Colored cool colorants based on rare earth

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metal ions, Sol. Energy Mater. Sol. Cells 92 (2008) 1462-1467. [28] J. Ng, S. Xu, X. Zhang, H.Y. Yang, D.D. Sun, Hybridized nanowires and cubes: a novel

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architecture of a heterojunctioned TiO2/SrTiO3 thin film for efficient water splitting, Adv. Funct. Mater. 20 (2010) 4287-4294. [29] E.M.A. Jamal, D. Sakthi Kumar, M.R. Anantharaman, On structural, optical and dielectric properties of zinc aluminate nanoparticles, Bull. Mater. Sci. 34 (2011) 251-259. [30] M. Zhao, A. Han, M. Ye, T. Wu, Preparation and characterization of Fe3+ doped Y2Ce2O7 pigments with high near-infrared reflectance, Sol. Energy 97 (2013) 350-355. [31] J.L. Wang, Y.Q. Li, Y.J. Byon, S.G. Mei, G.L. Zhang, Synthesis and characterization of NiTiO3 yellow nano pigment with high solar radiation reflection efficiency, Powder Technol. 235 (2013) 303-306.

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color tunable nano-pigment, Dyes and Pigments 113 (2015) 588-595.

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Amiri Zarandi, Synthesis and characterization of Co1-xZnxCr2-yAlyO4 as a near-infrared reflective

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[34] L. Torkian, M. Daghighi, Effects of b-alanine on morphology and optical properties of

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CoAl2O4 nanopowders as a blue pigment, Ad. Powder Technol. 25 (2014) 739-744. [35] G.L. Fan, J. Tong, F. Li, Visible-light-induced photocatalyst based on cobalt-doped zinc

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ferrite nanocrystals. Eng. Chem. Res. 51 (2012) 13639-13647.

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[37] S. Jose, M.L. Reddy, Lanthanum-strontium copper silicates as intense blue inorganic pigments with high near-infrared reflectance, Dyes and Pigments 98 (2013) 540-546.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. TG/DSC analysis of Co0.5Mg0.5Al2O4 pigment precursors.

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Fig. 2. The XRD patterns of Co0.5Mg0.5Al2O4 calcined at different temperatures.

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Fig. 3. (a) XRD patterns of Co0.5Mg0.5Al2-xFexO4 pigments and (b) shift of the Bragg reflections (220), (311) and (400) for the different studied compositions.

Fig. 4. FE-SEM images of Co0.5Mg0.5Al2O4 particles annealed at (a) 700oC, (b) 800oC and (c)

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900oC.

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Fig. 5. UV–vis diffuse reflectance spectra of Fe3+ doped Co0.5Mg0.5Al2O4 compounds. Fig. 6. Kuelka-Munk transformed reflectance spectra of Co0.5Mg0.5Al2-xFexO4 (x=0.0-1.0)

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powdered pigments.

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Fig. 7. Photographs of Co0.5Mg0.5Al2-xFexO4 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) powdered pigments.

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Fig. 8. NIR reflectance of Co0.5Mg0.5Al2-xFexO4 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) powdered samples. Fig. 9. NIR solar reflectance of Co0.5Mg0.5Al2-xFexO4 (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) powdered

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samples.

Fig. 10. TG/DSC analysis of powdered (a) Co0.5Mg0.5Al2O4 and (b) Co0.5Mg0.5AlFeO4 pigments.

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8.1011 8.1316 8.1475 8.1694 8.2417 8.2719

8.0981 8.1231 8.1579 8.3259 8.2586 8.2674

8.0857 8.1236 8.1508 8.3299 8.3507 8.2464

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Average(Å)

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400

8.0861 8.1278 8.1602 8.1636 8.2508 8.2587

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311

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a=b=c(Å) a=b=c(Å) a=b=c(Å) a=b=c(Å) a=b=c(Å) a=b=c(Å)

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0.0 0.2 0.4 0.6 0.8 1.0

hkl

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Fe(x)

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Table 1 Cell parameters of the cubic cell of Co0.5Mg0.5Al2-xFexO4.

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8.0908 8.1233 8.1532 8.2228 8.2515 8.2600

Volume(Å3) 529.63 536.04 541.98 555.98 561.82 563.56

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Color coordinates L

a

b

-15.67 -16.09 -9.84 -9.45 -3.74 2.61

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*

-38.84 -10.48 1.74 0.74 6.92 6.35

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55.64 49.79 43.53 45.27 40.84 30.05

*

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x=0.0 x=0.2 x=0.4 x=0.6 x=0.8 x=1.0

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Pigment composition

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Table 2 Color coordinates of the Co0.5Mg0.5Al2-xFexO4 (x=0.0-1.0) powdered pigments and band gap values. C

Band gap (eV)

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41.88 19.20 9.64 9.48 7.87 6.87

4.40 3.77 3.50 3.50 3.67 3.69

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x=0.0

x=0.2

x=0.4

NIR solar reflectance of powdered pigments

54.2%

52.7%

51.4%

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x=0.8

x=1.0

43.1%

43.0%

45.7%

x=0.6

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Pigment Composition

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Table 3 The detail values of NIR solar reflectance of the Co0.5Mg0.5Al2-xFexO4(x=0.0-1.0) powdered pigments.

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Co0.5Mg0.5Al2O4 L

*

a

*

b

*

Co0.5Mg0.5AlFeO4

△E

55.04

-15.57

-39.44

0.9

HCl HNO3 H2SO4

54.94 56.15 56.08

-15.67 -15.23 -15.27

-39.84 -39.72 -39.69

1.2 1.1 1.0

54.54

-15.57

-39.94

NaOH *

* 2

* 2

* 2 1/2

1.6

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△E =[(△L ) +(△a ) +(△b ) ]

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H2O

*

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a*

b*

△E*

29.55

2.61

5.85

0.7

29.25 29.36 29.43

2.71 3.12 2.98

5.45 5.42 5.57

1.2 1.3 1.1

29.05

2.71

5.25

1.5

L

*

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5% Acid/alkali

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Table 4 The color coordinates of the (a) Co0.5Mg0.5Al2O4 and (b) Co0.5Mg0.5AlFeO4 powder pigments after chemical resistance tests.

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Fig. 9.

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Graphical abstract

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ACCEPTED MANUSCRIPT Research highlights (1) High NIR reflecting pigments Co0.5Mg0.5Al2O4 were synthesized by the Pechini-type sol-gel

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method.

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(2) The doping of Fe3+ changed the pigments color from blue to black.

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(3) Pigment properties make it a potential candidate for use as cool pigments.

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