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Synthesis and characterization of Co1-xCaxAl2O4 composite blue nano-pigments by the polyacrylamide gel method
Accepted Manuscript Synthesis and characterization of Co1-xCaxAl2O4 composite blue nano-pigments by the polyacrylamide gel method Nianying Zhou, Yun Li, Yin Zhang, Yan Shu, Shangjiu Nian, Weijing Cao, Zhenning Wu PII:
S0143-7208(17)31405-5
DOI:
10.1016/j.dyepig.2017.08.057
Reference:
DYPI 6219
To appear in:
Dyes and Pigments
Received Date: 23 June 2017 Revised Date:
15 August 2017
Accepted Date: 30 August 2017
Please cite this article as: Zhou N, Li Y, Zhang Y, Shu Y, Nian S, Cao W, Wu Z, Synthesis and characterization of Co1-xCaxAl2O4 composite blue nano-pigments by the polyacrylamide gel method, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.08.057. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Synthesis and characterization of Co1-xCaxAl2O4 composite blue nano-pigments by the polyacrylamide gel method Nianying Zhou1, Yun Li1, Yin Zhang*1,2, Yan Shu1, Shangjiu Nian1, Weijing Cao1 and Zhenning Wu1 1
China 2
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College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009,
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Nanjing Haoqi Advanced Materials Co., Ltd., Nanjing, 211300, China
Abstract
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Blue nano-pigments of Co1-xCaxAl2O4 (x = 0.1, 0.2, 0.4, 0.6, and 0.8) were synthesized by the polyacrylamide gel method. The obtained powders were characterized by XRD, SEM, colorimetry and UV-vis. The XRD patterns indicated the characteristic peaks were attributed to the CoAl2O4 and CaAl2O4 phases with a good crystallinity. The crystallite sizes decreased with the increase of calcium content. The colorimetric data revealed that the increases of blue intensity
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and bright component originated from the enrichment of calcium. When the content of Co was 0.2 (sample P5), the pigment showed the bluest and brightest color. The UV-vis absorption spectra in the range of 430-670 nm decreased with the increase of calcium content thus causing the pigments became brighter and bluer. Incorporation of calcium elements into the CoAl2O4 reduced
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the production costs and also minimized the damage to the environment . These facts show the potential of the obtained powders for cool coatings.
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Keywords: Nano-pigment; Cobalt calcium aluminate; Polyacrylamide; Optical characteristics
1. Introduction
In recent years, the cobalt aluminate oxide is one of the most popular blue inorganic pigments, cobalt ions are located on tetrahedral positions within a conventional, cubic, spinel type structure [1, 2, 3]. Cobalt aluminate has attracted significant attention as a ceramic pigment from scientific research and industry owing to exhibiting a series of superior properties, such as high resistance to acids, chemical and thermal stability, color stability and high refractive index [4-9]. At present, CoAl2O4 pigments have been widely used in coloration of paints, enamels, glass, paper, plastics, rubber, fibers, cement, and ceramics bodies [10]. 1
ACCEPTED MANUSCRIPT It is well known that the method of synthesis plays an important role in influencing the final desired properties of nano-pigments [11]. A variety of techniques, such as solid state reaction [12], combustion [13], sol-gel [14], micro-emulsion [15], co-precipitation [16], polyacrylamide gel method [17] and liquid-feed flame spray pyrolysis method [18], have been developed successfully
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obtaining cobalt aluminate oxides. Among them, the polyacrylamide gel method provides a the mixing at a molecular level during the synthesis process, because the metal ions are dissolved in the polymeric network [19], thus making the calcination temperature lower (1000 ℃) [20] . Taken into consideration that cobalt is scare and expensive, adding suitable metal ions to
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partially replace the Co2+ ions in the spinel structure has been explored [19, 21]. This method can reduce the cost of production, it can also alter and improve the materials properties. Recently, this
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method has been further investigated. Leila Torkian et al. [19] and I.S. Ahmed et al. [21] reported the blue intensity decreased with the increase of the Mg content in Mg2+-doped cobalt. In the Zn2+-doped system, the highest blue intensity was observed in medium Zn content samples [22]. They thought that the blue hue was associated with the absorption bands in the range of 530-670 nm. Ba2+ was added in the blue pigment to improve the optical properties by Mahsa Jafari et al.
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[20]. They put forward a different explanation that the blue hue was ascribed to three spin-allowed and three spin-forbidden electronic transitions of cobalt ions, and the addition of Ba2+ decreased the intensity of the absorption band at 450 nm and yielded a blue pigment with an
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improved color point.
In this work, nano-sized metal aluminates (CoAl2O4 and CaAl2O4) were synthesized by polyacrylamide gel method. It is well known that Ca2+ can improve the degree of color and
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brightness of glaze and glass. Meanwhile, Ca2+ can weaken the absorption of UV-vis in a certain wavelength range and enhance the reflection properties of the crystal [23]. In addition, the dopant of calcium ions would affect mineralizing agent, which is able to effectively reduce the sintering temperature [24]. Since cobalt is widely considered to be scarce and hazardous, incorporation of non-toxic and inexpensive elements to the CoAl2O4 would reduce the production costs and also minimize the environmental damage [25, 26]. So far little report has been focused on dealing with the use of calcium ion in pigments. Therefore, we comprehensively discussed the effect of Ca2+ on the structure, morphology, chromatic coordinates and optical properties of obtained powders. What’s more, the relationship between the chromatic coordinates and the UV-vis absorbance 2
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2. Experimental procedure 2.1 Materials and Preparation In this work, samples of nano-pigment Ca2+-added alumina/cobalt with the different molar ratio
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of aluminum, cobalt and calcium in composition were prepared by the polyacrylamide gel method. Firstly, cobalt nitrate (Co(NO3)2·6H2O), aluminum nitrate (Al(NO3)3·9H2O) and calcium nitrate (Ca(NO3)2·4H2O) were dissolved in distilled water according to the stoichiometric ratio. Then, acrylamide (C2H3CONH2) and methylene bisacrylamide ((C2H3CONH2)2CH2) were added to the
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above solution at a molar ratio of 22: 1 [17]. After stirring for 45 min, 10% (w/v) ammonium persulfate ((NH4)2S2O8) and 1% (w/v) N, N, N′, N′- tetramethyl ethylene diamide (C6H16N2) were
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added to the mixture, which initiated the polymerization reaction, followed by rapidly forming a transparent gel. The resulted gel was dried at 110 ℃Error! Reference source not found. for 3 h. Finally the formed xerogel was calcined at 1000 ℃ for 2 h and then cool down to room temperature. The sample codes with corresponding molar ratio composition are shown in Table 1. 2.2 Characterization
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XRD data were collected from the synthesized powders for phase identification by an X-ray powder diffractometer (Geigerflex, Rigaku, Japan) using CuKα radiation from 10° to 70Error! Reference source not found. 2θ. In XRD analysis standard silicon was used as a reference to
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define the FWHM (full width at half maximum) values. Measured peak widths were used in Scherrer’s formula to evaluate crystallite sizes. Morphology and particle size were observed by
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SEM (S-4800, JEOL, Tokyo, Japan) and TEM ( Philips CM-10, 60 keV, Holland). The CIE-L*a*b* chromatic coordinates and UV-vis absorption spectra were taken by means of a SC-20 colorimeter and a Jasco-670 spectrophotometer UV-vis, respectively, using standard D65 illumination and barium sulfate as a reference.
3. Result and Discussion 3.1 X-ray powder diffraction Fig. 1 shows the XRD patterns of synthesized samples calcined at 1000 ℃ for 2 h. Compared with other synthetic method [11,31], the polyacrylamide gel method can make the calcination 3
ACCEPTED MANUSCRIPT temperature lower (1000 ℃). At this temperature, the characteristic peaks are correspondent with the JCPDS files 10-458 [27,28] and 23-1036, for CoAl2O4 and CaAl2O4, respectively. This peak is characteristic for the CoAl2O4 phase and the related pattern was investigated in more details at 2θ = 65° [27]. P1 sample represents the pure cobalt aluminate, the characteristic peaks for
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CoAl2O4 spinel structure are broad with maximum at 2θ=31.11°, 36.58°, 59.12° and 64.97°, no secondary phases were observed. With the increase of calcium content, the CaAl2O4 peaks were also observed in the XRD patterns of the samples P2-P5. The intensity of the CoAl2O4 spinel peaks decreased with increasing the concentration of calcium, which was along with gradual
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increasing of peaks of CaAl2O4. The XRD patterns of synthesized samples suggested that they have been prepared for a composite of CoAl2O4 and CaAl2O4 systems.
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The mean crystallite size (D) of the synthesized samples is estimated from the full width at half maximum (FWHM) of the strongest diffraction Bragg peak 220 of the powders using the Scherrer’s formula. The mean crystallite sizes for intermediate and extreme compositions were calculated and listed in Table 1. It is shown that the calcium content influences the mean crystallite size values of the formed oxides and crystallite sizes continuously decrease from 51.0
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nm (P1) to 27.6 nm (P5) with the increase of Ca content. Due to the different effective ionic radius between doping Ca2+ and Co2+ ions, the crystal lattice was distorted after doping. So d(220), d(311), d(511) and d(440) of spinel phases decreased after Ca2+ doping (in Fig.1), which indicated
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that calcium ions entered into the structure of crystal successfully and partially replaced Co2+ to form CaAl2O4, which caused the changes of the crystallinity and the particle size. It is almost consistent with those observed in Fig. 1. In the previous reports, Leila Torkian et al. [19] and
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Chunlan Liu et al. [29] reported that employing Mg2+ to substitute the positions of Co2+ and prepare MgAl2O4, resulted in smaller crystallite sizes in high magnesium contents. It seems that substitution ions entered into the crystallite lattice of CoAl2O4 and formed the secondary phase, which led to the change of crystallization. These results suggested the formation of calcium aluminate in this study. 3.2 Particle size and morphological analysis The SEM images of five samples are shown in Fig. 2 (a) ~ Fig. 2 (e), respectively. As it can be seen in Fig. 2 (a), the morphology of particles was found to be nearly spherical in shape with an 4
ACCEPTED MANUSCRIPT average size of 45 nm. Many researchers reported quasi spherical shapes for the pure CoAl2O4 powders prepared by various chemical methods [17, 30]. It was confirmed the formation of cobalt aluminate nano-particles exhibited with a good crystallinity. When the calcium content reaches 0.6 (P3), the shape of particles (Fig. 2) become a similar dumbbell structure from a nearly
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spherical. In the micrograph of a very high concentration of calcium ions sample P5, the particles of powders were irregular shape and connected together to form a similar dumbbell structure with an average size of 23 nm. Compared five samples images, with the increase of calcium content, the pigments particles are decreased and the shape changed ( nearly spherical in shape to similar
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dumbbell structure ). It may be due to the fact that doping calcium ions had the effect of mineralizing agent. Diffusions were formed between the grain boundary and grain when they
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were calcined, followed by forming the sintering neck. Besides, the calcium ions replaced the Co2+ ions in the tetrahedral positions, which resulted in the decrease of the size of nano-particles. This change in size was in accordance with the results calculated from the X-ray diffraction data. It represented that calcium ions replaced the Co2+ ions in the tetrahedral positions and formed a secondary phase (CaAl2O4).
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Fig. 3 shows TEM images of three samples (P1, P3, P5). Obviously, the formation of nanocomposites were confirmed further by TEM images. It is clear that morphology of product is nanoparticle. In the polyacrylamide gel method, gelation of the solution is achieved by the
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formation of an organic polymernet work. This organic network-cage can cause an increase in the distance between metal cations [17, 31, 32]. Therefore, weaker interactions among primary particles occur during crystallization, leads to a powder with smaller particle sizes and fewer
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agglomerates. The average particle size of the powders prepared by the polyacrylamide gel method was about 30–80 nm which was relatively in agreement with the crystallite size analysis (Table1). Average particle size observed from TEM images are consistent with SEM images and the value in Table 1.
3.3 Chromatic properties analysis The chromatic coordinates (L*, a* and b*) and optical images of the synthesized pigments are displayed in Table 1 and Fig. 4, respectively. According to the values of L* a* b* in Table 1, L* value increases with the improvement of concentration of calcium, indicating a lighter condition. 5
ACCEPTED MANUSCRIPT The blue hue is mainly governed by the parameter b*: the more b* value increases in negative the more intense blue hue generates [19]. With the increase of calcium content, the value of b* increases in negative, representing a higher blue intensity. In our experiment, more intense blue color was achieved in P5 sample as indicated by the extremely high blue component (b* = -34.51)
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and highest light component (L* = 64.43), in agreement with the visual observations, in Fig. 4. It can be seen that the addition of calcium ions have a great influence on the color change of synthesized pigments. It is well known that Ca2+ can improve the degree of color and brightness of glaze and glass. Meanwhile, Ca2+ can weaken the absorption of UV-vis in a certain wavelength
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range and enhance the reflection properties of the crystal [23]. In addition, in the system of Mg2+-doped [19] and Zn2+-doped [11], the changes of L* value showed the same results that with
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the increasing of contents of metal ions, L* value increased and reached the maximum at the low level of Co content. The blue intensity of Mg2+-doped cobalt blue pigment (-b*) continuously decreased with the increase of the Mg2+ content, but the most negative values of b* were observed in medium Zn content samples in the Zn2+-doped cobalt blue pigment. These results verified that doping of various metal ions had different effects on the chromatic coordinates. Compared with
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the doping of magnesium and zinc ions, calcium ions can improve the brightness of pigments, more importantly, increasing the intensity of the blue hue. Therefore, there is a great advantage for Ca enrichment in structure of cobalt aluminate, and low Co2+ content is preferred.
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3.4 Diffuse reflectance analyse
The UV–vis absorbance spectra of the synthesized pigments were recorded as shown in Fig. 5.
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One side, with the increased of calcium content, red and blue light reflection are enhanced. Fig. 5 showed that the blue reflectance ( R450 ) is highest and the red reflectance ( R600 ) is highest of sample P5 is the highest, but the R450 / R600 values of P5 is higher, i.e., the color purity is better. Sample P1 has the minimum reflectance of red light, but its blue light reflectivity is lower too, so the R450 / R600 value is too small, blue color purity is poor. Compared to the R450 / R600 values of the five samples, it was found that the purity of the P5 sample was best. Fig. 5 showed that the molar ratio of cobalt to calcium should to be selected appropriately for maintain a good blue light color purity. On the other side, the synthesized pigments spectra show similar features: in the wavelength in 6
ACCEPTED MANUSCRIPT the range of 530-670nm, a continuously strong absorption is depicted, corresponding to the absorption of the colors yellow, orange and red. Thus, the reflectance occurs in the complementary colors, namely violet, blue and cyan, centered in the blue [22]. The observed broad band is owing to the spin allowed 4A2(F)Error! Reference source not found.4T1(P)
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transition of the Co2+ ions in tetrahedral sites of spinel structure [19,33]. While the band around 450 nm, which is corresponding to the absorption of blue, is ascribed to the spin forbidden 4
A2(F)Error! Reference source not found.2T(G) transition[34]. In the spectra (Fig. 5), the
intensity of the absorption bands in the range of 430-670 nm weaken gradually with the increase
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of calcium content. With the constant enrichment of calcium ions , the synthesized samples become brighter and bluer, in agreement with the results of the colorimetric coordinates (Table 1)
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and the optical images (Fig. 4). It can be seen that the change of color is decided by the absorption bands in the range of 430-670 nm, which are owing to the substitution of Ca2+ ions for Co2+ ions in the spinel structure. The smaller calcium ions replaced the Co2+ ions in the unit cell, and formed secondary phase CaAl2O4 with smaller crystallite size, suggesting the absorption bands changed. In addition, the formation of CaAl2O4 phase, a cool white pigment with a high NIR solar
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reflectance, would greatly decrease the intensity of the absorption bands [23]. In Fig. 5, it represents that the doping of calcium ions have a great influence on the bands around 430-670 nm and yield a blue pigment with an improved brightness and blue hue.
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Fig.5 suggested with the increase of the calcium content, the absorption at 450 nm decreases and yields a blue pigment with an improved color point. The change in absorption of cobalt blue pigment can be explained by the influence of Ca2+ on the crystal structure. The radii of Ca2+ and
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Co2+ are different, Ca2+ and Co2+ are 1.12 and 0.74 Å, respectively. Ca2+ can partially replace Co2+ and form solid solution. With adding more Ca, secondary phase (CaAl2O4) is formed. The difference between the radius of Ca2+ and Co2+ leads to a crystal lattice distortion and therefore, the absorption of cobalt blue pigment will be changed. In addition, it seems that the formation of CaAl2O4 phase is also important to decrease the absorption at 450 nm. The effect of CaAl2O4 phase on absorption is more prominent up to 0.8 mol Ca (P5). 3.5 Chemical stability studies of the pigment Chemical stability is very important to pigment properties[34]. And blue CoAl2O4 is a high 7
ACCEPTED MANUSCRIPT chemical stability [3]. In order to evaluate the chemical stability of synthetic pigments (P5), the acid,
alkali
and
water-resistance
of
the
pigment
samples
were
tested
by
6%
H2O/HCl/HNO3/H2SO4/ NaOH [35] and Anhydrous ethanol. The pigment of P5 was first treated with acid/alkali and stirred 1 hour by a magnetic stirrer. Then the samples were filtered, washed
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with distilled water, dried and weighed. Results show that the weight loss of the pigment samples can be ignored by tested in acid, alkali, water and organic solvents. In Table 2, it showed the color coordinates of the pigments after water, acid/alkali and organic solvents treatment. Small values of ∆E (stands for the total color difference, ∆E*= [(∆L)2 + (∆a)2 + (∆b)2]1/2.) indicate that the
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chemical stability of the pigment is better [36, 37]. It is also found that the water resistance, acid resistance and organic solvents resistance of the samples are very good.
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4. Conclusions
Blue inorganic nano-pigments were prepared by the polyacrylamide gel method with calcination at 1000 ℃ for 2 h. XRD and SEM analyse revealed that the synthesized pigments were a composite of CoAl2O4 and CaAl2O4 systems and well crystallized. The crystallite size continuously decreases from 45 nm to 23 nm with the increase of Ca content. Meanwhile,
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according to the comparison and analysis of the chromatic coordinates (L*, a* and b*), optical images and UV-vis absorbance spectra of the synthesized pigments, higher calcium enrichment lead to higher blue intensity and brighter component, and have a great influence on the peaks
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around 430-670 nm. At the same time, the pigment showed good chemical stability. These points can meet the demand of pigment properties, and also reduce the cost. Thus, Co1-xCaxAl2O4
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composite blue nano-pigment have a promising application prospect.
Acknowledgments
This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Jiangsu Collaborative Innovation Center For Advanced Inorganic Function Composites.
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Figures caption list Fig. 1. X-ray diffraction patterns of synthesized samples calcined at 1000 ℃ for 2 h. Fig. 2. SEM images of synthesized pigments: (a) P1, (b) P2, (c) P3, (d) P4 and (e) P5 . Fig. 3. TEM images of synthesized pigments: (a) P1, (b) P3 and (c) P5 .
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Fig. 5. UV-Vis absorbance spectra of synthesized pigments.
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Fig. 4. Optical images of synthesized pigments.
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Table
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Table 1
Corresponding molar ratio composition, crystallite size and color coordinates of synthesized samples Co
Ca
Al
crystallite size (nm)
L*
a*
b*
P1 P2 P3 P4
1 0.8 0.6 0.4
0 0.2 0.4 0.6
2 2 2 2
51.0 47.8 31.2 28.8
46.24 46.49 49.57 57.17
-3.81 -3.45 -2.08 -0.08
-12.87 -13.61 -22.06 -31.95
P5
0.2
0.8
2
27.6
64.43
0.01
-34.51
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Sample code
1/2
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L*
H2O HCl HNO3 H2SO4 NaOH C2H5OH
64.01 63.73 63.82 63.83 63.58 63.81
a*
b*
∆E
-0.10 -0.19 -0.14 -0.15 -0.24 -0.17
-34.01 -33.81 -33.90 -34.01 -33.71 -33.86
0.65 1.01 0.88 0.80 1.19 0.89
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NO.
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Table 2 The chroma of the P5 pigments treated in different solutions for 1 h.
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Figures
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Intensity(a.u.)
P5 P4 P3
♦ ο♦ ♦ ♦ ο ο ♦ ο♦ο
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P2
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Fig. 3. TEM images of synthesized pigments: (a) P1, (b) P3 and (c) P5.
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Fig. 4. Optical images of synthesized pigments.
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1. Blue nano-pigments of Co1-xCaxAl2O4 were synthesized by the polyacrylamide gel method.
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2. It was found the Ca2+ can improve the degree of color and brightness of pigments. 3. Co1-xCaxAl2O4 pigments synthesized would be a new candidates for cool
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pigments.
S0143-7208(17)31405-5
DOI:
10.1016/j.dyepig.2017.08.057
Reference:
DYPI 6219
To appear in:
Dyes and Pigments
Received Date: 23 June 2017 Revised Date:
15 August 2017
Accepted Date: 30 August 2017
Please cite this article as: Zhou N, Li Y, Zhang Y, Shu Y, Nian S, Cao W, Wu Z, Synthesis and characterization of Co1-xCaxAl2O4 composite blue nano-pigments by the polyacrylamide gel method, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.08.057. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis and characterization of Co1-xCaxAl2O4 composite blue nano-pigments by the polyacrylamide gel method Nianying Zhou1, Yun Li1, Yin Zhang*1,2, Yan Shu1, Shangjiu Nian1, Weijing Cao1 and Zhenning Wu1 1
China 2
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College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009,
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Nanjing Haoqi Advanced Materials Co., Ltd., Nanjing, 211300, China
Abstract
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Blue nano-pigments of Co1-xCaxAl2O4 (x = 0.1, 0.2, 0.4, 0.6, and 0.8) were synthesized by the polyacrylamide gel method. The obtained powders were characterized by XRD, SEM, colorimetry and UV-vis. The XRD patterns indicated the characteristic peaks were attributed to the CoAl2O4 and CaAl2O4 phases with a good crystallinity. The crystallite sizes decreased with the increase of calcium content. The colorimetric data revealed that the increases of blue intensity
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and bright component originated from the enrichment of calcium. When the content of Co was 0.2 (sample P5), the pigment showed the bluest and brightest color. The UV-vis absorption spectra in the range of 430-670 nm decreased with the increase of calcium content thus causing the pigments became brighter and bluer. Incorporation of calcium elements into the CoAl2O4 reduced
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the production costs and also minimized the damage to the environment . These facts show the potential of the obtained powders for cool coatings.
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Keywords: Nano-pigment; Cobalt calcium aluminate; Polyacrylamide; Optical characteristics
1. Introduction
In recent years, the cobalt aluminate oxide is one of the most popular blue inorganic pigments, cobalt ions are located on tetrahedral positions within a conventional, cubic, spinel type structure [1, 2, 3]. Cobalt aluminate has attracted significant attention as a ceramic pigment from scientific research and industry owing to exhibiting a series of superior properties, such as high resistance to acids, chemical and thermal stability, color stability and high refractive index [4-9]. At present, CoAl2O4 pigments have been widely used in coloration of paints, enamels, glass, paper, plastics, rubber, fibers, cement, and ceramics bodies [10]. 1
ACCEPTED MANUSCRIPT It is well known that the method of synthesis plays an important role in influencing the final desired properties of nano-pigments [11]. A variety of techniques, such as solid state reaction [12], combustion [13], sol-gel [14], micro-emulsion [15], co-precipitation [16], polyacrylamide gel method [17] and liquid-feed flame spray pyrolysis method [18], have been developed successfully
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obtaining cobalt aluminate oxides. Among them, the polyacrylamide gel method provides a the mixing at a molecular level during the synthesis process, because the metal ions are dissolved in the polymeric network [19], thus making the calcination temperature lower (1000 ℃) [20] . Taken into consideration that cobalt is scare and expensive, adding suitable metal ions to
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partially replace the Co2+ ions in the spinel structure has been explored [19, 21]. This method can reduce the cost of production, it can also alter and improve the materials properties. Recently, this
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method has been further investigated. Leila Torkian et al. [19] and I.S. Ahmed et al. [21] reported the blue intensity decreased with the increase of the Mg content in Mg2+-doped cobalt. In the Zn2+-doped system, the highest blue intensity was observed in medium Zn content samples [22]. They thought that the blue hue was associated with the absorption bands in the range of 530-670 nm. Ba2+ was added in the blue pigment to improve the optical properties by Mahsa Jafari et al.
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[20]. They put forward a different explanation that the blue hue was ascribed to three spin-allowed and three spin-forbidden electronic transitions of cobalt ions, and the addition of Ba2+ decreased the intensity of the absorption band at 450 nm and yielded a blue pigment with an
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improved color point.
In this work, nano-sized metal aluminates (CoAl2O4 and CaAl2O4) were synthesized by polyacrylamide gel method. It is well known that Ca2+ can improve the degree of color and
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brightness of glaze and glass. Meanwhile, Ca2+ can weaken the absorption of UV-vis in a certain wavelength range and enhance the reflection properties of the crystal [23]. In addition, the dopant of calcium ions would affect mineralizing agent, which is able to effectively reduce the sintering temperature [24]. Since cobalt is widely considered to be scarce and hazardous, incorporation of non-toxic and inexpensive elements to the CoAl2O4 would reduce the production costs and also minimize the environmental damage [25, 26]. So far little report has been focused on dealing with the use of calcium ion in pigments. Therefore, we comprehensively discussed the effect of Ca2+ on the structure, morphology, chromatic coordinates and optical properties of obtained powders. What’s more, the relationship between the chromatic coordinates and the UV-vis absorbance 2
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2. Experimental procedure 2.1 Materials and Preparation In this work, samples of nano-pigment Ca2+-added alumina/cobalt with the different molar ratio
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of aluminum, cobalt and calcium in composition were prepared by the polyacrylamide gel method. Firstly, cobalt nitrate (Co(NO3)2·6H2O), aluminum nitrate (Al(NO3)3·9H2O) and calcium nitrate (Ca(NO3)2·4H2O) were dissolved in distilled water according to the stoichiometric ratio. Then, acrylamide (C2H3CONH2) and methylene bisacrylamide ((C2H3CONH2)2CH2) were added to the
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above solution at a molar ratio of 22: 1 [17]. After stirring for 45 min, 10% (w/v) ammonium persulfate ((NH4)2S2O8) and 1% (w/v) N, N, N′, N′- tetramethyl ethylene diamide (C6H16N2) were
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added to the mixture, which initiated the polymerization reaction, followed by rapidly forming a transparent gel. The resulted gel was dried at 110 ℃Error! Reference source not found. for 3 h. Finally the formed xerogel was calcined at 1000 ℃ for 2 h and then cool down to room temperature. The sample codes with corresponding molar ratio composition are shown in Table 1. 2.2 Characterization
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XRD data were collected from the synthesized powders for phase identification by an X-ray powder diffractometer (Geigerflex, Rigaku, Japan) using CuKα radiation from 10° to 70Error! Reference source not found. 2θ. In XRD analysis standard silicon was used as a reference to
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define the FWHM (full width at half maximum) values. Measured peak widths were used in Scherrer’s formula to evaluate crystallite sizes. Morphology and particle size were observed by
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SEM (S-4800, JEOL, Tokyo, Japan) and TEM ( Philips CM-10, 60 keV, Holland). The CIE-L*a*b* chromatic coordinates and UV-vis absorption spectra were taken by means of a SC-20 colorimeter and a Jasco-670 spectrophotometer UV-vis, respectively, using standard D65 illumination and barium sulfate as a reference.
3. Result and Discussion 3.1 X-ray powder diffraction Fig. 1 shows the XRD patterns of synthesized samples calcined at 1000 ℃ for 2 h. Compared with other synthetic method [11,31], the polyacrylamide gel method can make the calcination 3
ACCEPTED MANUSCRIPT temperature lower (1000 ℃). At this temperature, the characteristic peaks are correspondent with the JCPDS files 10-458 [27,28] and 23-1036, for CoAl2O4 and CaAl2O4, respectively. This peak is characteristic for the CoAl2O4 phase and the related pattern was investigated in more details at 2θ = 65° [27]. P1 sample represents the pure cobalt aluminate, the characteristic peaks for
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CoAl2O4 spinel structure are broad with maximum at 2θ=31.11°, 36.58°, 59.12° and 64.97°, no secondary phases were observed. With the increase of calcium content, the CaAl2O4 peaks were also observed in the XRD patterns of the samples P2-P5. The intensity of the CoAl2O4 spinel peaks decreased with increasing the concentration of calcium, which was along with gradual
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increasing of peaks of CaAl2O4. The XRD patterns of synthesized samples suggested that they have been prepared for a composite of CoAl2O4 and CaAl2O4 systems.
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The mean crystallite size (D) of the synthesized samples is estimated from the full width at half maximum (FWHM) of the strongest diffraction Bragg peak 220 of the powders using the Scherrer’s formula. The mean crystallite sizes for intermediate and extreme compositions were calculated and listed in Table 1. It is shown that the calcium content influences the mean crystallite size values of the formed oxides and crystallite sizes continuously decrease from 51.0
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nm (P1) to 27.6 nm (P5) with the increase of Ca content. Due to the different effective ionic radius between doping Ca2+ and Co2+ ions, the crystal lattice was distorted after doping. So d(220), d(311), d(511) and d(440) of spinel phases decreased after Ca2+ doping (in Fig.1), which indicated
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that calcium ions entered into the structure of crystal successfully and partially replaced Co2+ to form CaAl2O4, which caused the changes of the crystallinity and the particle size. It is almost consistent with those observed in Fig. 1. In the previous reports, Leila Torkian et al. [19] and
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Chunlan Liu et al. [29] reported that employing Mg2+ to substitute the positions of Co2+ and prepare MgAl2O4, resulted in smaller crystallite sizes in high magnesium contents. It seems that substitution ions entered into the crystallite lattice of CoAl2O4 and formed the secondary phase, which led to the change of crystallization. These results suggested the formation of calcium aluminate in this study. 3.2 Particle size and morphological analysis The SEM images of five samples are shown in Fig. 2 (a) ~ Fig. 2 (e), respectively. As it can be seen in Fig. 2 (a), the morphology of particles was found to be nearly spherical in shape with an 4
ACCEPTED MANUSCRIPT average size of 45 nm. Many researchers reported quasi spherical shapes for the pure CoAl2O4 powders prepared by various chemical methods [17, 30]. It was confirmed the formation of cobalt aluminate nano-particles exhibited with a good crystallinity. When the calcium content reaches 0.6 (P3), the shape of particles (Fig. 2) become a similar dumbbell structure from a nearly
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spherical. In the micrograph of a very high concentration of calcium ions sample P5, the particles of powders were irregular shape and connected together to form a similar dumbbell structure with an average size of 23 nm. Compared five samples images, with the increase of calcium content, the pigments particles are decreased and the shape changed ( nearly spherical in shape to similar
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dumbbell structure ). It may be due to the fact that doping calcium ions had the effect of mineralizing agent. Diffusions were formed between the grain boundary and grain when they
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were calcined, followed by forming the sintering neck. Besides, the calcium ions replaced the Co2+ ions in the tetrahedral positions, which resulted in the decrease of the size of nano-particles. This change in size was in accordance with the results calculated from the X-ray diffraction data. It represented that calcium ions replaced the Co2+ ions in the tetrahedral positions and formed a secondary phase (CaAl2O4).
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Fig. 3 shows TEM images of three samples (P1, P3, P5). Obviously, the formation of nanocomposites were confirmed further by TEM images. It is clear that morphology of product is nanoparticle. In the polyacrylamide gel method, gelation of the solution is achieved by the
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formation of an organic polymernet work. This organic network-cage can cause an increase in the distance between metal cations [17, 31, 32]. Therefore, weaker interactions among primary particles occur during crystallization, leads to a powder with smaller particle sizes and fewer
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agglomerates. The average particle size of the powders prepared by the polyacrylamide gel method was about 30–80 nm which was relatively in agreement with the crystallite size analysis (Table1). Average particle size observed from TEM images are consistent with SEM images and the value in Table 1.
3.3 Chromatic properties analysis The chromatic coordinates (L*, a* and b*) and optical images of the synthesized pigments are displayed in Table 1 and Fig. 4, respectively. According to the values of L* a* b* in Table 1, L* value increases with the improvement of concentration of calcium, indicating a lighter condition. 5
ACCEPTED MANUSCRIPT The blue hue is mainly governed by the parameter b*: the more b* value increases in negative the more intense blue hue generates [19]. With the increase of calcium content, the value of b* increases in negative, representing a higher blue intensity. In our experiment, more intense blue color was achieved in P5 sample as indicated by the extremely high blue component (b* = -34.51)
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and highest light component (L* = 64.43), in agreement with the visual observations, in Fig. 4. It can be seen that the addition of calcium ions have a great influence on the color change of synthesized pigments. It is well known that Ca2+ can improve the degree of color and brightness of glaze and glass. Meanwhile, Ca2+ can weaken the absorption of UV-vis in a certain wavelength
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range and enhance the reflection properties of the crystal [23]. In addition, in the system of Mg2+-doped [19] and Zn2+-doped [11], the changes of L* value showed the same results that with
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the increasing of contents of metal ions, L* value increased and reached the maximum at the low level of Co content. The blue intensity of Mg2+-doped cobalt blue pigment (-b*) continuously decreased with the increase of the Mg2+ content, but the most negative values of b* were observed in medium Zn content samples in the Zn2+-doped cobalt blue pigment. These results verified that doping of various metal ions had different effects on the chromatic coordinates. Compared with
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the doping of magnesium and zinc ions, calcium ions can improve the brightness of pigments, more importantly, increasing the intensity of the blue hue. Therefore, there is a great advantage for Ca enrichment in structure of cobalt aluminate, and low Co2+ content is preferred.
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3.4 Diffuse reflectance analyse
The UV–vis absorbance spectra of the synthesized pigments were recorded as shown in Fig. 5.
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One side, with the increased of calcium content, red and blue light reflection are enhanced. Fig. 5 showed that the blue reflectance ( R450 ) is highest and the red reflectance ( R600 ) is highest of sample P5 is the highest, but the R450 / R600 values of P5 is higher, i.e., the color purity is better. Sample P1 has the minimum reflectance of red light, but its blue light reflectivity is lower too, so the R450 / R600 value is too small, blue color purity is poor. Compared to the R450 / R600 values of the five samples, it was found that the purity of the P5 sample was best. Fig. 5 showed that the molar ratio of cobalt to calcium should to be selected appropriately for maintain a good blue light color purity. On the other side, the synthesized pigments spectra show similar features: in the wavelength in 6
ACCEPTED MANUSCRIPT the range of 530-670nm, a continuously strong absorption is depicted, corresponding to the absorption of the colors yellow, orange and red. Thus, the reflectance occurs in the complementary colors, namely violet, blue and cyan, centered in the blue [22]. The observed broad band is owing to the spin allowed 4A2(F)Error! Reference source not found.4T1(P)
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transition of the Co2+ ions in tetrahedral sites of spinel structure [19,33]. While the band around 450 nm, which is corresponding to the absorption of blue, is ascribed to the spin forbidden 4
A2(F)Error! Reference source not found.2T(G) transition[34]. In the spectra (Fig. 5), the
intensity of the absorption bands in the range of 430-670 nm weaken gradually with the increase
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of calcium content. With the constant enrichment of calcium ions , the synthesized samples become brighter and bluer, in agreement with the results of the colorimetric coordinates (Table 1)
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and the optical images (Fig. 4). It can be seen that the change of color is decided by the absorption bands in the range of 430-670 nm, which are owing to the substitution of Ca2+ ions for Co2+ ions in the spinel structure. The smaller calcium ions replaced the Co2+ ions in the unit cell, and formed secondary phase CaAl2O4 with smaller crystallite size, suggesting the absorption bands changed. In addition, the formation of CaAl2O4 phase, a cool white pigment with a high NIR solar
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reflectance, would greatly decrease the intensity of the absorption bands [23]. In Fig. 5, it represents that the doping of calcium ions have a great influence on the bands around 430-670 nm and yield a blue pigment with an improved brightness and blue hue.
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Fig.5 suggested with the increase of the calcium content, the absorption at 450 nm decreases and yields a blue pigment with an improved color point. The change in absorption of cobalt blue pigment can be explained by the influence of Ca2+ on the crystal structure. The radii of Ca2+ and
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Co2+ are different, Ca2+ and Co2+ are 1.12 and 0.74 Å, respectively. Ca2+ can partially replace Co2+ and form solid solution. With adding more Ca, secondary phase (CaAl2O4) is formed. The difference between the radius of Ca2+ and Co2+ leads to a crystal lattice distortion and therefore, the absorption of cobalt blue pigment will be changed. In addition, it seems that the formation of CaAl2O4 phase is also important to decrease the absorption at 450 nm. The effect of CaAl2O4 phase on absorption is more prominent up to 0.8 mol Ca (P5). 3.5 Chemical stability studies of the pigment Chemical stability is very important to pigment properties[34]. And blue CoAl2O4 is a high 7
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and
water-resistance
of
the
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samples
were
tested
by
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H2O/HCl/HNO3/H2SO4/ NaOH [35] and Anhydrous ethanol. The pigment of P5 was first treated with acid/alkali and stirred 1 hour by a magnetic stirrer. Then the samples were filtered, washed
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with distilled water, dried and weighed. Results show that the weight loss of the pigment samples can be ignored by tested in acid, alkali, water and organic solvents. In Table 2, it showed the color coordinates of the pigments after water, acid/alkali and organic solvents treatment. Small values of ∆E (stands for the total color difference, ∆E*= [(∆L)2 + (∆a)2 + (∆b)2]1/2.) indicate that the
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chemical stability of the pigment is better [36, 37]. It is also found that the water resistance, acid resistance and organic solvents resistance of the samples are very good.
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4. Conclusions
Blue inorganic nano-pigments were prepared by the polyacrylamide gel method with calcination at 1000 ℃ for 2 h. XRD and SEM analyse revealed that the synthesized pigments were a composite of CoAl2O4 and CaAl2O4 systems and well crystallized. The crystallite size continuously decreases from 45 nm to 23 nm with the increase of Ca content. Meanwhile,
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according to the comparison and analysis of the chromatic coordinates (L*, a* and b*), optical images and UV-vis absorbance spectra of the synthesized pigments, higher calcium enrichment lead to higher blue intensity and brighter component, and have a great influence on the peaks
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around 430-670 nm. At the same time, the pigment showed good chemical stability. These points can meet the demand of pigment properties, and also reduce the cost. Thus, Co1-xCaxAl2O4
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composite blue nano-pigment have a promising application prospect.
Acknowledgments
This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Jiangsu Collaborative Innovation Center For Advanced Inorganic Function Composites.
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Figures caption list Fig. 1. X-ray diffraction patterns of synthesized samples calcined at 1000 ℃ for 2 h. Fig. 2. SEM images of synthesized pigments: (a) P1, (b) P2, (c) P3, (d) P4 and (e) P5 . Fig. 3. TEM images of synthesized pigments: (a) P1, (b) P3 and (c) P5 .
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Fig. 5. UV-Vis absorbance spectra of synthesized pigments.
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Fig. 4. Optical images of synthesized pigments.
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Table
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Table 1
Corresponding molar ratio composition, crystallite size and color coordinates of synthesized samples Co
Ca
Al
crystallite size (nm)
L*
a*
b*
P1 P2 P3 P4
1 0.8 0.6 0.4
0 0.2 0.4 0.6
2 2 2 2
51.0 47.8 31.2 28.8
46.24 46.49 49.57 57.17
-3.81 -3.45 -2.08 -0.08
-12.87 -13.61 -22.06 -31.95
P5
0.2
0.8
2
27.6
64.43
0.01
-34.51
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Sample code
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L*
H2O HCl HNO3 H2SO4 NaOH C2H5OH
64.01 63.73 63.82 63.83 63.58 63.81
a*
b*
∆E
-0.10 -0.19 -0.14 -0.15 -0.24 -0.17
-34.01 -33.81 -33.90 -34.01 -33.71 -33.86
0.65 1.01 0.88 0.80 1.19 0.89
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Table 2 The chroma of the P5 pigments treated in different solutions for 1 h.
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Figures
♦
ο
Intensity(a.u.)
P5 P4 P3
♦ ο♦ ♦ ♦ ο ο ♦ ο♦ο
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P2
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220
♦ ο
CaAl2O4 CoAl2O4
ο
P1
20
30
40
50
60
70
2θ(°)
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Fig. 1. X-ray diffraction patterns of synthesized samples calcined at 1000 ℃ for 2 h.
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P2
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P1
P3
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P4
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P5
Fig. 2. SEM images of synthesized pigments: (a) P1, (b) P2, (c) P3, (d) P4 and (e) P5 .
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Fig. 3. TEM images of synthesized pigments: (a) P1, (b) P3 and (c) P5.
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P
P4
P5
P
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Fig. 4. Optical images of synthesized pigments.
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100
P2
P1
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Absorbance(a.u.)
80
60
40
200
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20
400
600
P3 P4
P5
800
Wavelength(nm)
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Fig. 5. UV-Vis absorbance spectra of synthesized pigments.
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ACCEPTED MANUSCRIPT Highlights
1. Blue nano-pigments of Co1-xCaxAl2O4 were synthesized by the polyacrylamide gel method.
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2. It was found the Ca2+ can improve the degree of color and brightness of pigments. 3. Co1-xCaxAl2O4 pigments synthesized would be a new candidates for cool
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pigments.