Effect of different clay minerals and calcination temperature on the morphology and color of clay/CoAl₂O₄ hybrid pigments

Abstract

Clay/CoAl₂O₄ hybrid pigments with different morphologies and colors have been successfully fabricated by combining the coprecipitation technique and a successive calcination step. The morphologies of the obtained hybrid pigments are dependent on the morphologies of the inorganic clays, whereas the colors of the hybrid pigments are largely determined by the successive calcination temperature. The results reveal that the incorporation of the two-dimensional illite–smectite mixed-layer clay is favorable for the protection of the morphologies of the hybrid pigments from the calcination temperature compared with that of one-dimensional attapulgite. The color of the hybrid pigments changes from atrovirens to blue to dark blue with increasing calcination temperature from 800 °C to 1000 °C to 1200 °C, which is similar to the famous “Maya blue” pigment. It can be noted that the introduction of the inorganic clays not only decreases the production cost, but also prevents aggregation and an increase in the size of CoAl₂O₄ nanoparticles. There is no doubt that this strategy can realize the widespread application of CoAl₂O₄ pigments in the relevant fields.

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Introduction

Transition metal oxide spinels are important in numerous application fields due to their high thermal resistance and catalytic, electronic and optical properties.^1–3 Among them, cobalt aluminate (CoAl₂O₄), also referred to as Thenard's blue, is well-known as a thermally and chemically stable pigment with a bright blue color.^4,5 It exhibits excellent resistance to acid and basic attack, solar exposure and atmospheric agents; therefore, it has been widely used for the coloration of plastics, paints, fibres, rubber, glass, cement, ceramic bodies and contrast-enhancing luminescent pigments.^6–9 However, there are two major problems that restrain the wide-spread application of CoAl₂O₄ pigments in the relevant fields. One of them is preparation methods. The earliest known method for the preparation of CoAl₂O₄ is realized through a solid-state reaction between two metal oxides at high temperature (1200–1300 °C), followed by mechanical grinding.¹⁰ Although this process is relatively inexpensive, the obtained sample is always associated with undesired by-products owing to heterogeneity and uncontrollable stoichiometry.¹¹ In recent years, various types of wet techniques have been developed for the preparation of CoAl₂O₄ such as coprecipitation,¹² polymeric gels,¹³ hydrothermal method,¹⁴ microemulsion,¹⁵ combustion¹⁶ and sol–gels.¹⁷ These different methods have different advantages and disadvantages, and the common routes for the preparation of CoAl₂O₄ nanoparticles, in practice, are based on the precipitation of aqueous solutions and sol–gel processes, followed by thermal treatment (up to 1000 °C) to complete the transformation of hydroxides to oxides. However, the particle growth and agglomeration of CoAl₂O₄ nanoparticles are inevitable during thermal treatment. In addition, the production cost is another fatal problem for CoAl₂O₄ pigments due to the scarcity of cobalt ores in nature and the high cost of cobalt compounds. As a result, CoAl₂O₄ pigments are limited to use in a few key fields, such as paints, coatings with super-durability, engineering plastics and phosphor powder coating products for cathode ray tubes.

Recently, hybrid pigments, including organic/inorganic and inorganic/inorganic pigments, have attracted wide interest, especially organic/inorganic hybrid pigments, typical representatives of the “Maya blue” pigment composed of attapulgite (APT) and indigo.^18,19 These pigments exhibit vivid colors from turquoise to blue and have excellent resistance to degradation in adverse environmental conditions such as acids, alkalis, oxidants, reducing agents and organic solvents. APT is a member of the clay mineral family, which is a hydrous layer-ribbon magnesium aluminum silicate with a rod-like structure and a theoretical formula of (Al₂Mg₂)Si₈O₂₀(OH)₂(OH₂)₄·4H₂O.²⁰ Clay minerals, which are familiar to mankind from the earliest days of civilization, are acknowledged to be exceptionally promising candidates in numerous fields due to their low cost, abundance in most continents of the world, non-toxicity and easy modification. The “Maya blue” pigment possibly gives us inspiration to introduce inorganic clays for the fabrication of clay/CoAl₂O₄ inorganic/inorganic hybrid pigments. Therefore, one-dimensional APT, two-dimensional illite–smectite mixed-layer clay (I–S) and montmorillonite (MMT), associated with a small amount of APT, were employed to prepare the clay/CoAl₂O₄ hybrid pigments in this study.

The evolution of the morphology and color of the as-prepared hybrid pigments with the different types of clay and calcination temperatures was investigated in detail.

Experimental

Materials

ATP and I–S were obtained from Jiuchuan Clay Technology Co. of Jiangsu and Shangsi of Guangxi, China, respectively. MMT was purchased from Southern Clay Products Inc. In addition, the composition of the three clays was determined by X-ray fluorescence, as shown in Table S1 (ESI†). Co(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were of analytical reagent grade from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China and were used without further purification. Ultrapure water (18.25 MΩ cm) was used throughout.

Preparation of clay/CoAl₂O₄ hybrid pigments

In a typical procedure, 2.910 g of Co(NO₃)₂·6H₂O and 7.501 g of Al(NO₃)₃·9H₂O were dissolved in 50 mL of water, and then 0.8 g of APT was added. After stirring for 30 min, 3.0 M NaOH was slowly added until the value of the pH was about 9, and the mixture was stirred for 2 h at room temperature. The obtained pink precursor was washed with water several times until neutral and dried in a vacuum at 60 °C for 24 h. Then, it was calcined at 800 °C for 2 h at a heating rate of 10 °C min⁻¹ in a muffle furnace, and the ultimate sample is denoted as APT/CoAl₂O₄-1-800.

To investigate the effect of calcination temperature, content and type of inorganic clays on the color and morphology of the hybrid pigments, clay/CoAl₂O₄ hybrid pigments with different contents and type of clays were prepared, and the conditions for sample preparation are presented in Table 1. The numbers, 1, 2 and 3, in the names of the APT/CoAl₂O₄ hybrid pigments represent APT contents of 45%, 55% and 65%, respectively. As a control experiment, CoAl₂O₄ pigments without clay were also prepared using the same procedure, and the conditions of the sample preparation are also summarized in Table 1.

Table 1 Conditions for sample preparation

Samples	Co(NO3)2·6H2O/g	Al(NO3)3·9H2O/g	Clay	Content of clay^a	Calcination temperature/°C
a Calculated according to the weight of the generated CoAl2O4.
CoAl2O4-800	2.910	7.501	—	—	800
CoAl2O4-900	2.910	7.501	—	—	900
CoAl2O4-1000	2.910	7.501	—	—	1000
CoAl2O4-1100	2.910	7.501	—	—	1100
CoAl2O4-1200	2.910	7.501	—	—	1200
APT/CoAl2O4-1-1000	2.910	7.501	APT	45%	1000
APT/CoAl2O4-2-1000	2.910	7.501	APT	55%	1000
APT/CoAl2O4-3-1000	2.910	7.501	APT	65%	1000
APT/CoAl2O4-2-800	2.910	7.501	APT	55%	800
APT/CoAl2O4-2-900	2.910	7.501	APT	55%	900
APT/CoAl2O4-2-1100	2.910	7.501	APT	55%	1100
APT/CoAl2O4-2-1200	2.910	7.501	APT	55%	1200
I–S/CoAl2O4-900	2.910	7.501	I–S	55%	900
I–S/CoAl2O4-1000	2.910	7.501	I–S	55%	1000
I–S/CoAl2O4-1200	2.910	7.501	I–S	55%	1200
MMT/CoAl2O4-1000	2.910	7.501	MMT	55%	1000

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Characterization

A Bruker IFS 66 v/s IR spectrometer (Bruker, Karlsruhe, Germany) was used for the Fourier transform infrared spectroscopy (FTIR) analysis in the 400–4000 cm⁻¹ range with a resolution of 4 cm⁻¹. The morphology of the as-prepared samples was characterized with a JEM-1200 EX/S transmission electron microscope (TEM) (JEOL, Tokyo, Japan). X-ray diffraction (XRD) analysis was conducted with an X-ray powder diffractometer with Cu anode (PAN analytical Co. X'pert PRO), running at 40 kV and 30 mA. TGA was performed on a Perkin Elmer STA6000 thermogravimetric analyzer at a heating rate of 10 °C min⁻¹ under an oxygen atmosphere. The CIE-L*a*b* colorimetric method was used to described the color of the as-prepared pigments. The suspension stability of the as-prepared pigments was evaluated using the conventional sedimentation technique in a graduated cylinder. Typically, 1.0 g of the as-prepared sample was dispersed in 60 mL of deionized water with a high-speed mixer (GJB-B12K, Qingdao Haitongda Factory) and then transferred to a 50 mL graduated cylinder.

Results and discussion

APT/CoAl₂O₄ hybrid pigments

CoAl₂O₄ has a normal spinel structure with Co²⁺ and Al³⁺ ions in tetrahedral and octahedral positions, respectively. It has been reported that the most stable cobalt oxide at room temperature is the mixed valence form of Co₃O₄, which has the same spinel structure as CoAl₂O₄,²¹ and the oxide can be reduced into CoO above 950 °C. During the preparation of CoAl₂O₄, the Co²⁺ precursor is partially oxidized above 400 °C to generate the stable Co₃O₄ phase, and it is then reduced between 700 and 1000 °C, which mainly depends on the preparation technique. The concomitance of Co toward a trivalent oxidative state with the characteristic green can diminish the blue color of CoAl₂O₄. In other words, the coordination of Co²⁺ is one of the most important factors affecting the colorant performance of cobalt pigments.²² XRD patterns of the APT/CoAl₂O₄ hybrid pigments, calcined at different temperatures, reveal the relevant information on the intermediate stages of the formation of CoAl₂O₄. Fig. 1 exhibits the XRD patterns of APT/CoAl₂O₄ hybrid pigments with the APT content of 55% calcined at different temperatures. The Co₃O₄ phase can be observed in the samples calcined at 800 °C and 900 °C. The CoAl₂O₄ phase appears at 1000–1200 °C, which can be distinguished from the presence of the weak peak at 2θ = 49°, belonging to the CoAl₂O₄ phase, as shown in the partially enlarged view of APT/CoAl₂O₄-2-1000, CoAl₂O₄-1000 and APT/CoAl₂O₄-2-1100.^23,24 The other characteristic diffraction peaks at 2θ = 31°, 37°, 44°, 56°, 59°, and 65° are also observed, which can be assigned to the (220), (311), (400), (422), (511) and (440) planes of CoAl₂O₄, respectively. It suggests the crystallization of CoAl₂O₄ with spinel-type cubic structure of Fd3m (JCPDS 44-0160) geometry. Furthermore, it can be clearly found that the relative intensity of these characteristic diffraction peaks gradually increases with the increase in the calcination temperature, suggesting the reduction of the trivalence oxidative state of Co above 900 °C. These results are also in agreement with the visual observation of the color of the APT/CoAl₂O₄ hybrid pigments calcined at different temperatures (Fig. 2). Furthermore, it is obvious that the characteristic diffraction peaks of APT disappear after being calcined at 1000 °C (Fig. 1), implying a destruction of its crystal structure. The new peaks located at 2θ = 20°, 21°, 26°, 28°, 30°, 36° and 57° are related to the formation of new crystalline phases such as augite, Ca(Fe, Mg)Si₂O₆, and the forsterite, MgSiO₄.^25,26

Fig. 1 XRD patterns of (a) APT, (b) APT calcined at 1000 °C, (c) APT/CoAl₂O₄-2-800, (d) APT/CoAl₂O₄-2-900, (e) APT/CoAl₂O₄-2-1000, (f) CoAl₂O₄-1000, (g) APT/CoAl₂O₄-2-1100 and (h) APT/CoAl₂O₄-2-1200 pigments.

Fig. 2 Digital images of (a) the precursor, (b) APT/CoAl₂O₄-2-800, (c) APT/CoAl₂O₄-2-900, (d) APT/CoAl₂O₄-2-1000, (e) APT/CoAl₂O₄-2-1100 and (f) APT/CoAl₂O₄-2-1200.

As a control, the digital images of the CoAl₂O₄ pigments calcined at different temperatures are provided in Fig. S1 (ESI†). They indicate that the color of the CoAl₂O₄ pigments calcined at 900 °C can change from atrovirens to blue, revealing that the introduction of APT can postpone the reduction of the trivalent oxidative state of Co. This might be attributed to the heat-absorbing action of APT during the crystal phase transition. In addition, the colors of the CoAl₂O₄ pigments calcined above 1000 °C are almost the same without any obvious dimming phenomenon, which differs from the changes in the color of the APT/CoAl₂O₄ hybrid pigments (Fig. 2). Therefore, it can be concluded that the changes in the color of the APT/CoAl₂O₄ hybrid pigments with the calcination temperature are ascribed to the changes in the color of APT at different temperatures, as illustrated in Fig. S2 (ESI†). This also can be confirmed by the color coordinates of the as-prepared pigments, which are summarized in Table 2. The L*, a* and b* value indicates the level of lightness or darkness, redness or greenness, and yellowness or blueness, respectively. The degree of blue color is mainly governed by the parameter of b*, the more negative the b* value, the bluer the color hue. Both CoAl₂O₄-800 and APT/CoAl₂O₄-800 exhibit lower negative values of the b* coordinate and the high negative values of the a* coordinate, indicating the color hue of them is green. In the case of CoAl₂O₄ pigments, the L* and b* coordinates gradually increase with the increase in the calcination temperature, whereas the value of a* decreases. However, it can be noted that the b* coordinate reaches a maximum of −30.5 for APT/CoAl₂O₄-1000 and then rapidly decreases to −11.3 at 1200 °C. In addition, the lightness of the APT/CoAl₂O₄ hybrid pigment also exhibits the same variation trend. This phenomenon can be ascribed to the changes in the color of APT at different temperatures, which is also consistent with the digital images of APT/CoAl₂O₄ hybrid pigments calcined at different temperatures.

Table 2 Colorimetric coordinates of the as-prepared CoAl₂O₄ pigments and clay/CoAl₂O₄ hybrid pigments

Samples	L*	a*	b*
CoAl2O4-800	29.1	−16.9	−5.3
CoAl2O4-900	36.6	−9.6	−21.3
CoAl2O4-1000	43.7	−9.1	−32.9
CoAl2O4-1100	46.9	−8.8	34.0
CoAl2O4-1200	47.1	−3.0	−42.0
APT/CoAl2O4-1-1000	44.3	−8.0	−31.1
APT/CoAl2O4-2-1000	43.8	−8.5	−30.5
APT/CoAl2O4-3-1000	43.7	−8.5	−28.8
APT/CoAl2O4-2-800	30.8	−16.8	−2.1
APT/CoAl2O4-2-900	34.8	−8.4	−9.9
APT/CoAl2O4-2-1100	41.7	−8.3	−28.3
APT/CoAl2O4-2-1200	23.5	−3.5	−11.3
I–S/CoAl2O4-900	37.9	−9.2	−18.1
I–S/CoAl2O4-1000	45.7	−8.3	−31.6
I–S/CoAl2O4-1200	39.1	−4.7	23.2
MMT/CoAl2O4-1000	47.7	−7.6	−32.5

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The formation of the resulting APT/CoAl₂O₄ hybrid pigment and the structural change of the incorporated APT can further be confirmed using FTIR measurements. The FTIR spectra of the CoAl₂O₄ pigments calcined at different temperatures indicate three typical absorption bands in the range of 500–700 cm⁻¹ (Fig. S3, ESI†). In fact, it is usually difficult to identify the presence of the Co-oxide phase as Co₃O₄ from FTIR spectra due to the similar Co–O stretching. Interestingly, in the case of CoAl₂O₄-800, three absorption peaks at around 668 cm⁻¹, 560 cm⁻¹ and 502 cm⁻¹ can be found. With the increase in the calcination temperature, these absorption bands gradually narrow accompanied with the obvious shift, and three typical absorption bands shift to 665 cm⁻¹, 556 cm⁻¹ and 509 cm⁻¹, whereas the calcination temperature increases to 1000 °C, which can be assigned to the vibrational bands of CoAl₂O₄ associated with the vibrations of Co–O, Al–O, and Co–O–Al bonds,²⁷ respectively. Furthermore, the relative intensity of the absorption band at 556–560 cm⁻¹ decreases, whereas the relative intensity of the absorption band at 500–510 cm⁻¹ increases with increasing calcination temperature from 800 to 1200 °C. These results indicate that the trivalent oxidative state of Co has been reduced, which is also consistent with the XRD results. As shown in Fig. 3, the FTIR spectrum of the APT/CoAl₂O₄-800 exhibits two broad bands around 561 and 668 cm⁻¹, which can be assigned to cobalt oxide as observed by Busca et al.,²⁸ and it is almost similar to the FTIR spectrum of CoAl₂O₄-800. With increasing temperature, three typical absorption bands at about 502 cm⁻¹, 556 cm⁻¹ and 674 cm⁻¹ can be observed, indicating the formation of CoAl₂O₄. It is worth noting the differences observed at 3600–2700 cm⁻¹, 1630 cm⁻¹ and 1200–750 cm⁻¹ compared with the FTIR spectra of CoAl₂O₄ pigments (Fig. S3, ESI†). This can be attributed to the structural changes of APT, including the loss of the zeolitic and structural waters, the collapse of the crystal structure and the crystal phase transition, which also can be confirmed by the FTIR spectra of the natural APT and the thermal-treated APT (Fig. S4, ESI†), the hydroxyl vibrational bands at 3612 and 3544 cm⁻¹ decrease in intensity until they disappear completely at 800 °C, forming a broad band centered at 3450 cm⁻¹. In addition, the bands at 1630 cm⁻¹ of the bending vibrations of H₂O, 1423 cm⁻¹ of carbonates and the Si–O vibration bands between 1200 cm⁻¹ and 900 cm⁻¹ also decrease in intensity.²⁵ For the APT/CoAl₂O₄ hybrid pigments calcinated 1000 °C with different APT contents, the FTIR spectra are similar (Fig. S5, ESI†). However, the b* coordinate of APT/CoAl₂O₄ hybrid pigments with different APT contents decreases from −31.1 to −28.8, while the addition of APT increases from 45% to 65%, indicating that the high content of APT affects the color hue of the hybrid pigments (Table 2).

Fig. 3 FTIR spectra of the (a) APT/CoAl₂O₄-2-800, (b) APT/CoAl₂O₄-2-900, (c) APT/CoAl₂O₄-2-1000 and (d) APT/CoAl₂O₄-2-1200.

Fig. 4 shows the TEM images of CoAl₂O₄-1000, APT/CoAl₂O₄-2-800 and APT/CoAl₂O₄-2-1000. The TEM image of CoAl₂O₄-1000 shows that the CoAl₂O₄ nanoparticles are spherical with a diameter of about 10–40 nm accompanied with the submicron aggregates and even large sheet-shape products (Fig. 4a). It is usually difficult to prevent particle growth and agglomeration of CoAl₂O₄ nanoparticles during thermal treatment. Interestingly, it has been reported that the introduction of one-dimensional APT can effectively induce the heterogeneous deposition of carbonaceous species or magnetic nanoparticles on the surface of APT.^29,30 This can also be confirmed from the TEM images of APT/CoAl₂O₄ hybrid pigments calcined at 800 °C. As shown in Fig. 4b, the surface of the rod-like APT becomes rough due to the loading of CoAl₂O₄ or Co₃O₄ nanoparticles compared with that of the natural APT (Fig. S6a, ESI†). In addition, it can also be found that the free nanoparticles or agglomerations decrease with the increase in the addition of APT from 45% to 65%, and the diameter of the loaded nanoparticles is about 10–15 nm (Fig. S7†). However, the rod-like morphology of the hybrid pigment disappears while the precursor is calcined at 1000 °C (Fig. 4c), which might be ascribed to the damage of the destruction of the crystal structure of APT.²⁵

Fig. 4 TEM images of (a) CoAl₂O₄-1000, (b) APT/CoAl₂O₄-2-800 and (c) APT/CoAl₂O₄-2-1000.

Fig. 5 depicts the TGA curves of the precursor, natural APT and APT/CoAl₂O₄-2-1000 hybrid pigment. The weight loss of the natural APT can be assigned to the removal of the physically adsorbed and zeolitic waters located in the channels (100–300 °C)³¹ and the coordinated and structural waters released from the clay framework in the latter temperature range.³² In the case of the as-prepared precursor, the weight loss below 300 °C is mainly attributed to the dehydroxylation of the metal hydroxides. After being calcined at 1000 °C, the as-prepared hybrid pigment has no weight loss in the testing temperature range, which illustrates the excellent thermal stability.

Fig. 5 TGA curves of the precursor, natural APT and APT/CoAl₂O₄-2-1000.

I–S/CoAl₂O₄ hybrid pigments

Based on the TEM image of APT/CoAl₂O₄-2-1000, it can be suggested that the introduction of APT cannot effectively prevent the agglomeration of CoAl₂O₄ nanoparticles due to the complete fusion of APT at 1000 °C. Therefore, two-dimensional I–S is employed to fabricate the CoAl₂O₄ hybrid pigment to investigate the morphological change of the hybrid pigments. As shown in Fig. 6a, the XRD pattern of natural I–S presents the diffraction peaks of illite, montmorillonite, kaolin and quartz. For the I–S/CoAl₂O₄ hybrid pigments calcined at 900, 1000 and 1200 °C, the characteristic diffraction peaks of CoAl₂O₄ can be clearly observed, while the diffraction peaks of the inorganic clays disappear. This might be attributed to the crystal phase transition of the inorganic clays to the amorphous silicate. This is also confirmed from the FTIR results (Fig. 6b). In contrast, three obvious changes in the infrared region (3700–3400, 1500–750 cm⁻¹ and 750–400 cm⁻¹) can be observed. The precursor indicates the characteristic absorption bands of the hydroxyl stretching vibration, Si–O–Si vibration, and Al–O or Al–Mg–OH key surface external vibrations in the tetrahedron.³¹ In addition, the absorption bands at 770 and 520 cm⁻¹ are located in the Co–O stretching and M–O–H bending regions, which can be assigned to the generation of Co(OH)₂.³² After being calcinated at 1000 and 1200 °C, three typical absorption bands at about 502 cm⁻¹, 556 cm⁻¹ and 674 cm⁻¹ can be observed, indicating the formation of CoAl₂O₄. The relative intensity located at around 550 cm⁻¹ also decreases with the increase in intensity at about 500 cm⁻¹, while the calcination temperature is increased from 900 to 1200 °C. As shown in Table 2, with the increase in calcination temperature, the change of colorimetric coordinates of the I–S/CoAl₂O₄ hybrid pigments is similar with the results of APT/CoAl₂O₄. Furthermore, I–S/CoAl₂O₄-1000 also exhibits excellent thermal stability with no weight loss in the tested temperature range (Fig. S8, ESI†).

Fig. 6 (a) XRD patterns of the natural I–S, I–S/CoAl₂O₄-900, I–S/CoAl₂O₄-1000 and I–S/CoAl₂O₄-1200. (b) FTIR spectra of the precursor, I–S/CoAl₂O₄-900, I–S/CoAl₂O₄-1000 and I–S/CoAl₂O₄-1200 pigments.

Fig. 7 illustrates the TEM images of I–S/CoAl₂O₄-900 and I–S/CoAl₂O₄-1000. It can be found that the surface of I–S is well decorated by the CoAl₂O₄ nanoparticles with a diameter of about 10–40 nm compared with the smooth surface of natural I–S (Fig. S6b, ESI†). As shown in Fig. 7a, the generated CoAl₂O₄ nanoparticles are confined to the surface of I–S without obvious agglomerations. It can be noted that the two-dimensional morphology of the hybrid pigments is maintained well after being calcined at 1000 °C, and the CoAl₂O₄ nanoparticles also do not aggregate and increase in the size (Fig. 7b). The abovementioned results reveal that the introduction of two-dimensional I–S can realize the control synthesis of CoAl₂O₄ nanoparticles without agglomerations beside the decrease in the production cost. Taking into account the morphological change of the obtained hybrid pigments with different clays, we can speculate that the two-dimensional clay might possess good thermal stability compared with that of one-dimensional APT. Therefore, the hybrid pigments based on MMT associated with a small amount of APT are also prepared to confirm the abovementioned assumption.

Fig. 7 TEM images of (a) I–S/CoAl₂O₄-900 and (b) I–S/CoAl₂O₄-1000.

Fig. S9† presents the XRD patterns of the natural MMT and MMT/CoAl₂O₄-1000, it can be also found that the characteristic diffraction peaks of CoAl₂O₄ present accompanied with the disappearance of the diffraction peaks MMT. The natural MMT indicates the typical two-dimensional lamellar morphology associated with the one-dimensional nanorod (Fig. 8a). After being calcined at 1000 °C, the two-dimensional lamellar morphology remains well, whereas the one-dimensional nanorods disappear (Fig. 8b), indicating that the two-dimensional clay exhibits the good thermal stability compared with that of one-dimensional APT. The MMT/CoAl₂O₄-1000 hybrid pigments also present the two-dimensional morphology loaded CoAl₂O₄ nanoparticles. In addition, the colors of the as-prepared hybrid pigments with different types of clays are almost same as that of CoAl₂O₄ pigments (Fig. 9), which is also consistent with the color coordinates of CoAl₂O₄-1000 and clay/CoAl₂O₄-1000. As shown in Table 2, CoAl₂O₄-1000, APT/CoAl₂O₄-2-1000, I–S/CoAl₂O₄-1000, and MMT/CoAl₂O₄-1000 exhibit almost same color coordinates.

Fig. 8 TEM images of (a) the natural MMT, (b) MMT calcined at 1000 °C and (c) MMT/CoAl₂O₄-1000.

Fig. 9 Digital images of CoAl₂O₄-1000, APT/CoAl₂O₄-2-1000, I–S/CoAl₂O₄-1000 and MMT/CoAl₂O₄-1000.

Fig. 10 shows the sedimentation behaviour of the suspension of CoAl₂O₄-1000, APT/CoAl₂O₄-2-1000, I–S/CoAl₂O₄-1000 and MMT/CoAl₂O₄-1000. It can be clearly seen that three clay/CoAl₂O₄ hybrid pigments show excellent suspension stability compared with that of CoAl₂O₄-1000, no sedimentation behaviour can be observed for clay/CoAl₂O₄ hybrid pigments after 24 h, but CoAl₂O₄-1000 pigment almost completely sedimentates after 30 min (Fig. S10†). This difference might be attributed to the introduction of the inorganic clay. As shown in Fig. S11,† the three clays used possess good colloidal properties. Therefore, the introduction of the inorganic clays not only decreases the production cost, but also prevents aggregation and the increase in the size of CoAl₂O₄ nanoparticles. In addition, it is in favor of improving the suspension stability of hybrid pigments, which is important in applications such as water-based paints and inks.

Fig. 10 Digital images of the suspension of (a) CoAl₂O₄-1000, (b) APT/CoAl₂O₄-2-1000, (c) I–S/CoAl₂O₄-1000 and (d) MMT/CoAl₂O₄-1000.

Conclusions

In conclusion, we develop a facile strategy to fabricate clay/CoAl₂O₄ hybrid pigments by incorporating low-cost inorganic clays. The morphology of the hybrid pigments is dependent on the morphologies of the inorganic clays, and the introduction of two-dimensional illite–smectite mixed-layer clay favors the protection of the morphologies of the hybrid pigments from the calcination temperature compared with that of one-dimensional attapulgite. The color of the hybrid pigments changes with the increase in the calcination temperature due to the reduction of trivalent oxidative state of Co and the color change of the inorganic clay, which is similar to the color of the famous “Maya blue” pigment. It can be noted that inorganic clay with a content of up to 55% almost has no effect on the color of the hybrid pigments. The introduction of the inorganic clays not only decreases the production cost, but also prevents aggregation and the increase in the size of CoAl₂O₄ nanoparticles. This study is expected to realize the widespread application of the CoAl₂O₄ pigments in the relevant fields.

Acknowledgements

The authors gratefully acknowledge the financial support of the “863” Project of the Ministry of Science and Technology, P. R. China (No. 2013AA032003).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The composition of three clays, the color of CoAl₂O₄ calcined at different temperatures, the color of APT calcined at different temperatures, FTIR spectra of CoAl₂O₄ pigments, FTIR spectra of APT and the calcined APT, FTIR spectra of the hybrid pigment with different content of APT, TEM images of the natural APT and I–S, TEM images of APT/CoAl₂O₄-1-800 and APT/CoAl₂O₄-3-800, TGA curves of the natural I–S, the precursor, and I–S/CoAl₂O₄-1000, XRD patterns of the natural MMT and MMT/CoAl₂O₄-1000, digital photographs of the suspension of CoAl₂O₄-1000, digital photographs of the suspension of the used three clays. See DOI: 10.1039/c5ra19955g

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