# Highly Dispersed Re-Doped CoAl2O4 Nanopigments: Synthesis and Chromatic Properties

Authors: Yuping Tong; Hailong Zhang; Shun Wang; Zheng Chen; Baixue Bian
Journal: Journal of Nanomaterials
 (2016)

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## Abstract

Nanosized spinelCoAl2-xRexO4 complex oxides were prepared by self-propagation combustion method. The products have been characterized by XRD, SEM, and EDS. The results indicated that Al3+ can be partly replaced by Re3+ when the doped amount is less than 10%, which forms single solid solution. The NIR reflectance and chromatic properties of samples have also been investigated. The substitution of Re3+ for Al3+ in CoAl2O4 can increase the blueness of pigments. SEM results revealed that the obtained CoAl2-xRexO4 pigments consisted of highly dispersed spherical-like nanoparticles with uniform size distribution. EDS results indicated that the distribution of element was considerably uniform with no chemical segregation phenomenon.


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## Body

## 1. Introduction

Spinel-type structure pigments with a general formula A2B2O4 have attracted extensive attention due to their chemical and thermal stability, which have been applied in decorating porcelains, ceramics, catalysts, paints, and so forth [1–3]. Among them, CoAl2O4 is one of the most important blue pigments, which has classic spinel-type structure and superior properties, such as high resistance to acids, and chemical, color, optical, and thermal stabilities [4–6]. Particularly, for application as optical devices like color filters or pigments, the presence of highly dispersed submicrometer or nano-CoAl2O4 particles is important and indispensable [7].Recent efforts have focused on tailoring a controllable and simple synthetic method for high-quality CoAl2O4 nanopigment. Many new synthetic technologies have been developed to synthesize CoAl2O4 nanopigment, such as organic ligand-assisted supercritical water hydrothermal method [8], polyacrylamide gel method [9], coprecipitation process [6], sol-gel method [5, 10], autoignition technique [11], molten salts method [12], and combustion method [13].The self-propagation combustion method has been developed by our team for preparation of pyrochlore-type and spinel-type nanoparticles [14, 15]. In this paper, we study synthesis and chromatics properties of rare earth ion doped CoAl2O4 nanopigment via self-propagation combustion method, based on the fact that rare earth element as doping ion can change the crystal structure and play an important role in stabilizing the color and changing the color of pigments.


## 2. Experimental

### 2.1. Preparation of Materials

All reagents were of analytical grade and used without further purification. In this work, all pigment samples ofCoAl2-xRexO4 (Re = Y, La, Nd, Sm, and Eu) were synthesized by self-propagation combustion method. CoNO32·6H2O and AlNO33·9H2O were used as the precursors of Co and Al, respectively. ReNO33·nH2O was obtained by dissolving Re2O3 in concentrated HNO3. Urea was used as fuel. According to the formula CoAl2-xRexO4 (where x=0.05, 0.1, 0.15, 0.2, and 0.3), stoichiometric amounts of CoNO32·6H2O, AlNO33·9H2O, and ReNO33·nH2O were added to urea aqueous solution in turn. After a series of steps of magnetic force stirring, evaporating, and self-propagating combustion, the loose precursor was obtained. The precursor was ground into powder and then submitted to calcination at 750°C for 4 h. The synthesis procedure and product of CoAl2O4 nanoparticles are shown in Figure 1.Figure 1
The synthesis procedure of CoAl2O4 nanoparticles.


### 2.2. Instrumentation

The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα radiation. Scanning electron microscopy (SEM) image was recorded on a JSM-7500F scanning electron microscope, and EDS was taken on INCAPentaFET-x3 energy dispersive X-ray detector. The CIE 1976 L∗α∗b∗ colorimetric method was used, as recommended by the Commission Internationale de l’Eclairage (CIE). In this method, L∗ is the lightness axis [black (0) to white (100)], α∗ is the green (−value) to red (+value) axis, and b∗ is the blue (−value) to yellow (+value) axis. The parameter C∗ (chroma) represents saturation of the color. For each colorimetric parameter of a sample, measurements were made in triplicate and an average value was chosen as the result. Typically, for a given sample, the standard deviation of the measured CIE-L∗a∗b∗ values is less than 0.10, and the relative standard deviation is not higher than 1%, indicating that the measurement error can be ignored. UV-vis-NIR reflectance of the obtained pigments was carried out by UV-vis-NIR spectrophotometer (Perkin Elmer Lambda 950), using polytetrafluoroethylene as a white standard.


## 2.1. Preparation of Materials

All reagents were of analytical grade and used without further purification. In this work, all pigment samples ofCoAl2-xRexO4 (Re = Y, La, Nd, Sm, and Eu) were synthesized by self-propagation combustion method. CoNO32·6H2O and AlNO33·9H2O were used as the precursors of Co and Al, respectively. ReNO33·nH2O was obtained by dissolving Re2O3 in concentrated HNO3. Urea was used as fuel. According to the formula CoAl2-xRexO4 (where x=0.05, 0.1, 0.15, 0.2, and 0.3), stoichiometric amounts of CoNO32·6H2O, AlNO33·9H2O, and ReNO33·nH2O were added to urea aqueous solution in turn. After a series of steps of magnetic force stirring, evaporating, and self-propagating combustion, the loose precursor was obtained. The precursor was ground into powder and then submitted to calcination at 750°C for 4 h. The synthesis procedure and product of CoAl2O4 nanoparticles are shown in Figure 1.Figure 1
The synthesis procedure of CoAl2O4 nanoparticles.


## 2.2. Instrumentation

The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα radiation. Scanning electron microscopy (SEM) image was recorded on a JSM-7500F scanning electron microscope, and EDS was taken on INCAPentaFET-x3 energy dispersive X-ray detector. The CIE 1976 L∗α∗b∗ colorimetric method was used, as recommended by the Commission Internationale de l’Eclairage (CIE). In this method, L∗ is the lightness axis [black (0) to white (100)], α∗ is the green (−value) to red (+value) axis, and b∗ is the blue (−value) to yellow (+value) axis. The parameter C∗ (chroma) represents saturation of the color. For each colorimetric parameter of a sample, measurements were made in triplicate and an average value was chosen as the result. Typically, for a given sample, the standard deviation of the measured CIE-L∗a∗b∗ values is less than 0.10, and the relative standard deviation is not higher than 1%, indicating that the measurement error can be ignored. UV-vis-NIR reflectance of the obtained pigments was carried out by UV-vis-NIR spectrophotometer (Perkin Elmer Lambda 950), using polytetrafluoroethylene as a white standard.


## 3. Results and Discussions

### 3.1. XRD Analysis

The XRD patterns ofCoAl2-xLaxO4 (x=0, 0.05, 0.1, 0.2, and 0.3) nanocrystals are shown in Figure 2. From Figure 2, it is clear that all the main peaks when x<0.2 are similar except for a trivial difference of 2θ value. All diffraction peaks of CoAl2-xLaxO4 (x<0.2) are in good agreement with the reflection of spinel CoAl2O4 phase (JCPDS number 44-016) which indicates that Al ion can be replaced by La3+ and the crystal type remains unchanged with the structure of CoAl2O4 only with small crystal distortion. In our present investigation, we found that another phase evolution starts from that composition (x=0.2) onwards. The diffraction peaks at 2θ = 25.51° and 34.03° when x≥0.2 are indexed as LaAlO3, which indicates that more La cannot be accommodated in CoAl2O4. Moreover, compared with pure CoAl2O4, the diffraction peaks of doped products become low. The obtained CoAl2O4 nanocrystals at 750°C have higher crystallinity than that of products via polyacrylamide gel method at the same temperature [9].Figure 2
XRD patterns ofCoAl2-xLaxO4 (x=0, 0.05, 0.1, 0.2, and 0.3) precursor calcined at 750°C for 4 h.For CoAl1.95Re0.05O4 nanocrystallines, we study the effect of the different doped ion on the structure of products. The XRD patterns of CoAl1.95Re0.05O4 (Re = Y, La, Nd, Sm, and Eu) precursor calcined at 750°C for 4 h are shown in Figure 3. It can be found from Figure 3 that all the main diffraction peaks are similar and belong to the standard spinel phase of CoAl2O4. The lattice constants of samples are obtained by Jade 6 program, the average crystal sizes are determined from the XRD patterns according to the Scherrer equation, and corresponding data are listed in Table 1. The average crystal size is about 8~20 nm. From the XRD patterns, it could be noted that doping of CoAl2O4 with Re3+ leads to a marginal shift of diffraction peaks towards lower 2θ angle side only except for the doping of La3+. Due to larger radius of Y3+, Eu3+, Sm3+, Nd3+, and La3+, the lattice constant value has been decreased from 8.09550 to 8.08547.Table 1
Lattice constant and crystal size of CoAl2O4 and CoAl1.95Re0.05O4.
Sample
Lattice constanta
Crystal size/nm
Strain/%
Radius/Re3+
CoAl2O4
8.09881
18.4
−0.081
R(Al3+) = 0.54
CoAl1.95Y0.05O4
8.09550
12.8
−0.094
1.02
CoAl1.95La0.05O4
8.09297
10.3
−0.426
1.18
CoAl1.95Nd0.05O4
8.09050
8.9
−0.591
N/A
CoAl1.95Sm0.05O4
8.08787
11.6
−0.419
1.08
CoAl1.95Eu0.05O4
8.08547
11
−0.125
1.07Figure 3
XRD patterns of CoAl2O4 and CoAl1.95Re0.05O4 (Re = Y, La, Nd, Sm, and Eu).


### 3.2. NIR Reflectance of Samples

Figure4 shows the NIR reflectance spectra of the pigments. The sample of CoAl2O4, CoAl1.95Eu0.05O4, CoAl1.9Eu0.1O4, CoAl1.85Eu0.15O4, and CoAl1.8Eu0.2O4 processes the NIR reflectance of about 79.7%, 86.5%, 85.8%, 79.4%, and 82.8%, respectively. It can be seen that the presence of Eu in CoAl2O4 improves the NIR reflectance to some extent except for CoAl1.85Eu0.15O4. The sample CoAl1.95Eu0.05O4 processes the highest NIR reflectance and enhances the NIR reflectance to 86.5%. With the increasing of Eu-doped amount, the NIR reflectance decreases, which may be due to similar results to “fluorescence quenching.”Figure 4
NIR reflectance ofCoAl2-xEuxO4 samples.


### 3.3. Chromatic Properties of Samples

Based on the above discussion, for CoAl1.95Re0.05O4, we study the chromatic properties of the obtained CoAl1.95Re0.05O4 pigment samples, which can be assessed from their CIE 1976 L∗α∗b∗ color coordinate values; the corresponding values are shown in Table 2. With the doping of Re3+, the increasing of b∗ value from −23.9 to −76.5 also presents the enhancement of the blueness of pigments, comparing with undoped samples. At the same time, L∗ value decreases from 34.8 to 20.3 in the presence of Re3+, which indicates that the darkness increases. This result is in agreement with the change of color of the pigments from bright blue to dark blue and then to light blue (Figure 5). It can be concluded that the doping of Re3+ can improve the blueness of pigments. To the best of our knowledge, for cobalt-based pigments, the Co2+ ions can be incorporated as coloring in all kinds of ceramics and enamels where they adopt the tetrahedral coordination. When Al3+ is replaced by Re3+ with larger radius, crystal lattice distortion appears, which may result in the shift of Co2+ from tetrahedral coordination to octahedral one and then cause the change of color. Combining NIR reflectance results with chromatic data, CoAl1.95Eu0.05O4 should be a good candidate as a “colored cool pigment” for use in the surface coating application.Table 2
Color coordinates of the CoAl2O4 and CoAl1.95Re0.05O4 powder pigments.
Pigment composition
Color coordinates
L
∗
a
∗
b
∗
CoAl2O4
34.8
14.7
−23.9
CoAl1.95Y0.05O4
20.8
41.3
−76.5
CoAl1.95La0.05O4
21.9
−4.42
−7.64
CoAl1.95Nd0.05O4
26.1
27.3
−74.2
CoAl1.95Sm0.05O4
20.3
21.6
−75.3
CoAl1.95Eu0.05O4
24.2
25.8
−58.6Figure 5
Photographs of CoAl2O4 and CoAl1.95Re0.05O4 (Re = Y, La, Nd, Sm, and Eu) pigments.
(a)
(b)
(c)
(d)
(e)
(f)


### 3.4. SEM and EDS Analysis

The representative SEM images of the obtained pigments are shown in Figure6. As can be seen, the CoAl2O4 powders (Figure 6(a)) have sphere-like structure with the size of 20 nm, but to some extent, the particles are a bit aggregated. However, the dispersibility of samples is still better than that of samples obtained by sol-gel precursor. Many researchers reported quasi-spheric or platy or irregular shapes for CoAl2O4 powders prepared by soft-chemical methods, and so forth [5, 9]. By La doped into CoAl2O4 (Figure 6(b)), it can be seen that the products are composed of highly dispersed nanoparticles with the size of 30 nm. Figure 6(c) shows that the CoAl1.85Eu0.15O4 samples also consist of well-dispersed uniform nanoparticles. The results reveal that Re-doped CoAl2O4 samples have good dispersibility and uniform size distribution.Figure 6
SEM images of samples: (a) CoAl2O4, (b) CoAl1.85La0.15O4, and (c) CoAl1.9Eu0.1O4.
(a)
(b)
(c)Figure7 gives the EDS results of CoAl1.9Eu0.1O4 samples. It is clear that CoAl1.9Eu0.1O4 nanocrystals are made up of O, Al, Co, Eu, and Si. The ratio Co : (Al + Eu) is approximately equal to 1 : 2, and Al : Eu ≈ 19 : 1, which gives stoichiometric formula of the as-obtained product CoAl1.9Eu0.1O4 with no chemical segregation phenomenon. The Si peak in the spectrum is from the silicon chip for making the sample. From the surface scanning results (Figure 8), it can be seen that the distribution of O, Al, Co, and Eu element is considerably uniform.Figure 7
EDS results of CoAl1.9Eu0.1O4 nanocrystals.Figure 8
Surface scanning images of CoAl1.9Eu0.1O4 nanocrystals.


## 3.1. XRD Analysis

The XRD patterns ofCoAl2-xLaxO4 (x=0, 0.05, 0.1, 0.2, and 0.3) nanocrystals are shown in Figure 2. From Figure 2, it is clear that all the main peaks when x<0.2 are similar except for a trivial difference of 2θ value. All diffraction peaks of CoAl2-xLaxO4 (x<0.2) are in good agreement with the reflection of spinel CoAl2O4 phase (JCPDS number 44-016) which indicates that Al ion can be replaced by La3+ and the crystal type remains unchanged with the structure of CoAl2O4 only with small crystal distortion. In our present investigation, we found that another phase evolution starts from that composition (x=0.2) onwards. The diffraction peaks at 2θ = 25.51° and 34.03° when x≥0.2 are indexed as LaAlO3, which indicates that more La cannot be accommodated in CoAl2O4. Moreover, compared with pure CoAl2O4, the diffraction peaks of doped products become low. The obtained CoAl2O4 nanocrystals at 750°C have higher crystallinity than that of products via polyacrylamide gel method at the same temperature [9].Figure 2
XRD patterns ofCoAl2-xLaxO4 (x=0, 0.05, 0.1, 0.2, and 0.3) precursor calcined at 750°C for 4 h.For CoAl1.95Re0.05O4 nanocrystallines, we study the effect of the different doped ion on the structure of products. The XRD patterns of CoAl1.95Re0.05O4 (Re = Y, La, Nd, Sm, and Eu) precursor calcined at 750°C for 4 h are shown in Figure 3. It can be found from Figure 3 that all the main diffraction peaks are similar and belong to the standard spinel phase of CoAl2O4. The lattice constants of samples are obtained by Jade 6 program, the average crystal sizes are determined from the XRD patterns according to the Scherrer equation, and corresponding data are listed in Table 1. The average crystal size is about 8~20 nm. From the XRD patterns, it could be noted that doping of CoAl2O4 with Re3+ leads to a marginal shift of diffraction peaks towards lower 2θ angle side only except for the doping of La3+. Due to larger radius of Y3+, Eu3+, Sm3+, Nd3+, and La3+, the lattice constant value has been decreased from 8.09550 to 8.08547.Table 1
Lattice constant and crystal size of CoAl2O4 and CoAl1.95Re0.05O4.
Sample
Lattice constanta
Crystal size/nm
Strain/%
Radius/Re3+
CoAl2O4
8.09881
18.4
−0.081
R(Al3+) = 0.54
CoAl1.95Y0.05O4
8.09550
12.8
−0.094
1.02
CoAl1.95La0.05O4
8.09297
10.3
−0.426
1.18
CoAl1.95Nd0.05O4
8.09050
8.9
−0.591
N/A
CoAl1.95Sm0.05O4
8.08787
11.6
−0.419
1.08
CoAl1.95Eu0.05O4
8.08547
11
−0.125
1.07Figure 3
XRD patterns of CoAl2O4 and CoAl1.95Re0.05O4 (Re = Y, La, Nd, Sm, and Eu).


## 3.2. NIR Reflectance of Samples

Figure4 shows the NIR reflectance spectra of the pigments. The sample of CoAl2O4, CoAl1.95Eu0.05O4, CoAl1.9Eu0.1O4, CoAl1.85Eu0.15O4, and CoAl1.8Eu0.2O4 processes the NIR reflectance of about 79.7%, 86.5%, 85.8%, 79.4%, and 82.8%, respectively. It can be seen that the presence of Eu in CoAl2O4 improves the NIR reflectance to some extent except for CoAl1.85Eu0.15O4. The sample CoAl1.95Eu0.05O4 processes the highest NIR reflectance and enhances the NIR reflectance to 86.5%. With the increasing of Eu-doped amount, the NIR reflectance decreases, which may be due to similar results to “fluorescence quenching.”Figure 4
NIR reflectance ofCoAl2-xEuxO4 samples.


## 3.3. Chromatic Properties of Samples

Based on the above discussion, for CoAl1.95Re0.05O4, we study the chromatic properties of the obtained CoAl1.95Re0.05O4 pigment samples, which can be assessed from their CIE 1976 L∗α∗b∗ color coordinate values; the corresponding values are shown in Table 2. With the doping of Re3+, the increasing of b∗ value from −23.9 to −76.5 also presents the enhancement of the blueness of pigments, comparing with undoped samples. At the same time, L∗ value decreases from 34.8 to 20.3 in the presence of Re3+, which indicates that the darkness increases. This result is in agreement with the change of color of the pigments from bright blue to dark blue and then to light blue (Figure 5). It can be concluded that the doping of Re3+ can improve the blueness of pigments. To the best of our knowledge, for cobalt-based pigments, the Co2+ ions can be incorporated as coloring in all kinds of ceramics and enamels where they adopt the tetrahedral coordination. When Al3+ is replaced by Re3+ with larger radius, crystal lattice distortion appears, which may result in the shift of Co2+ from tetrahedral coordination to octahedral one and then cause the change of color. Combining NIR reflectance results with chromatic data, CoAl1.95Eu0.05O4 should be a good candidate as a “colored cool pigment” for use in the surface coating application.Table 2
Color coordinates of the CoAl2O4 and CoAl1.95Re0.05O4 powder pigments.
Pigment composition
Color coordinates
L
∗
a
∗
b
∗
CoAl2O4
34.8
14.7
−23.9
CoAl1.95Y0.05O4
20.8
41.3
−76.5
CoAl1.95La0.05O4
21.9
−4.42
−7.64
CoAl1.95Nd0.05O4
26.1
27.3
−74.2
CoAl1.95Sm0.05O4
20.3
21.6
−75.3
CoAl1.95Eu0.05O4
24.2
25.8
−58.6Figure 5
Photographs of CoAl2O4 and CoAl1.95Re0.05O4 (Re = Y, La, Nd, Sm, and Eu) pigments.
(a)
(b)
(c)
(d)
(e)
(f)


## 3.4. SEM and EDS Analysis

The representative SEM images of the obtained pigments are shown in Figure6. As can be seen, the CoAl2O4 powders (Figure 6(a)) have sphere-like structure with the size of 20 nm, but to some extent, the particles are a bit aggregated. However, the dispersibility of samples is still better than that of samples obtained by sol-gel precursor. Many researchers reported quasi-spheric or platy or irregular shapes for CoAl2O4 powders prepared by soft-chemical methods, and so forth [5, 9]. By La doped into CoAl2O4 (Figure 6(b)), it can be seen that the products are composed of highly dispersed nanoparticles with the size of 30 nm. Figure 6(c) shows that the CoAl1.85Eu0.15O4 samples also consist of well-dispersed uniform nanoparticles. The results reveal that Re-doped CoAl2O4 samples have good dispersibility and uniform size distribution.Figure 6
SEM images of samples: (a) CoAl2O4, (b) CoAl1.85La0.15O4, and (c) CoAl1.9Eu0.1O4.
(a)
(b)
(c)Figure7 gives the EDS results of CoAl1.9Eu0.1O4 samples. It is clear that CoAl1.9Eu0.1O4 nanocrystals are made up of O, Al, Co, Eu, and Si. The ratio Co : (Al + Eu) is approximately equal to 1 : 2, and Al : Eu ≈ 19 : 1, which gives stoichiometric formula of the as-obtained product CoAl1.9Eu0.1O4 with no chemical segregation phenomenon. The Si peak in the spectrum is from the silicon chip for making the sample. From the surface scanning results (Figure 8), it can be seen that the distribution of O, Al, Co, and Eu element is considerably uniform.Figure 7
EDS results of CoAl1.9Eu0.1O4 nanocrystals.Figure 8
Surface scanning images of CoAl1.9Eu0.1O4 nanocrystals.


## 4. Conclusions

A series of Re-doped CoAl2O4 nanosized blue pigments have been synthesized. XRD results indicated that CoAl2O4 had limited accommodation for Re3+ only when x<0.2. When x≥0.2 in CoAl2-xRexO4, the impurity phase will be formed. It can be concluded from the chromatic data that the doping of Re3+ can improve the blueness of pigments. SEM images revealed that the doped samples had good dispersibility and uniform size distribution. Combining NIR reflectance results with chromatic data, CoAl1.95Eu0.05O4 can be considered as a good “colored cool pigment” candidate for use in the surface coating application.



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*Source: 4169673-2016-03-16.xml*