Effect of the substitution of Co2+ by Mg2+ on the color of the CoCr2O4 ceramic pigment synthesized by solution combustion

Abstract

This research presents the effect on the color of the green ceramic pigment CoCr₂O₄ when Co²⁺ was replaced by Mg²⁺. The objective is to reduce the concentration of cobalt in the pigments, which is considered an expensive and toxic raw material. The pigments were synthesized by solution combustion in one-step. The X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) of the as-prepared powders showed that the spinel structure was obtained during the combustion reaction. The microstructure of the pigments was observed by Scanning Electron Microscopy (SEM) and the powders are porous due to the gases formed during the reaction. Finally, the color change of the powders was evidenced by UV–vis–NIR Diffuse Reflectance Spectroscopy (DRS) and calculation of CIEL*a*b* chromatic coordinates. The largest color change ΔE*_ab of 9.8 was between CoCr₂O₄ and MgCr₂O₄ as a result of the absence of electronic transitions in Mg²⁺. The results of thermal stability of the green pigments using a commercial frit showed that they could be used in ceramic decoration at 1050 °C.

Resumen

Esta investigación presenta el efecto en el color del pigmento cerámico verde CoCr₂O₄ cuando el Co²⁺ se sustituyó por Mg²⁺. El objetivo es disminuir la concentración del cobalto en los pigmentos, el cual se considera una materia prima costosa y tóxica. Los pigmentos se sintetizaron en una etapa por combustión en solución. Los patrones de difracción de rayos X (DRX) y la espectroscopía de infrarrojo (IR) de los polvos mostraron la obtención de la estructura espinela durante la reacción de combustión. La microestructura de los pigmentos se observó por microscopía electrónica de barrido (MEB) y los polvos son porosos debido a los gases formados durante la reacción. Finalmente, el cambio de color se evidenció por espectroscopía de reflectancia difusa UV-Vis y cálculo de las coordenadas cromáticas CIEL*a*b*. El mayor cambio de color ΔE*_ab de 9.8 fue entre CoCr₂O₄ y MgCr₂O₄ como consecuencia de la ausencia de transiciones electrónicas en el Mg²⁺. Los resultados de estabilidad térmica de los pigmentos verdes usando una frita comercial mostraron que podrían ser usados en decoración cerámica a 1050 °C.

Introduction

Ceramic pigments are inorganic compounds that have high thermal and chemical stability to attack of molten glass and are used in ceramic decoration [1,2], classified in category A by Color Pigments Manufactures Association (CPMA) [3]. Spinels with general formula AB₂O₄ are a large family of inorganic compounds that have been used as magnetic materials [4], ceramic pigments [5], anode materials for lithium-ion batteries [6], catalytic materials [7]. Chromites are members of this family which have been studied in detail where CoCr₂O₄ and MgCr₂O₄ are two compositions of great interest due to their thermal stability. Cobalt chromite, CoCr₂O₄, has the normal spinel structure where Co²⁺(3d⁷) divalent cations are in tetrahedral oxygen coordination, while Cr³⁺(3d⁵) trivalent cations occupy the octahedral sites [4] with interest as ceramic pigment [8] and catalyst [9]. In recent years, the ceramic industry has been interested in using pigments with low concentration of cobalt [10], but the new compositions have good colorimetric properties among these as is the color change when a structure is diluted with a cation considered non-toxic and cheap as is the magnesium. Moreover, Magnesium chromite, MgCr₂O₄, also has the normal spinel structure with the Mg²⁺(2p⁶) divalent cations occupy the tetrahedral sites and Cr³⁺(3d⁵) trivalent cations occupy the octahedral sites [11], which is very used as a refractory material in the cement and steel industries. Therefore, it is interesting to explore the formation of a substitutional solid solution between these spinel structures to study the effect on the color when there is a substitution between Co²⁺ and Mg²⁺.

CoCr₂O₄ has been synthesized by various methods such as hydrothermal synthesis [12], coprecipitation technique [13], flame spray pyrolysis [8] and solution combustion [14], among others. On the other hand, MgCr₂O₄ has been synthesized by polyvinyl alcohol solution using microwave [15], sol–gel route [16] and solution combustion [11], among others. These different routes of synthesis show the importance of producing these spinels.

The process of solution combustion synthesis (SCS) was discovered in India when aluminum nitrate and urea were heated up to their ignition temperature obtaining alumina in a single step [17], from then until now, the method has been used to obtain simple and complex oxides [18–21] with different properties, such as: superparamagnetism on ZnFe₂O₄[22], electrical properties of calcium phosphates [23], optical properties of green spinel of ZnCr₂O₄[24], and TiO₂:Pt for photocatalytic application [25]. SCS employs a self-sustaining exothermic reaction of oxidation-reduction between a fuel and an oxidant that allows obtaining high temperatures in short reaction times with a simple experimental assembly [26]. The fuel is one the most important parameters of the combustion reaction because plays an important role in controlling the temperature of reaction and therefore to obtain the phase in one-step. Urea, citric acid, and glycine are the fuels most used in the synthesis of chromites, therefore, it is interesting to use other alternative fuels with the high aim to obtain chromites in one step.

In the present paper, we report the preparation of ceramic pigments of Co_1−xMg_xCr₂O₄ (x = 0.0, 0.2, 0.5, 0.7 and 1.0) for the first time a single-step using 6-aminohexanoic acid as an alternative fuel to traditional and study the effect of the substitution of Co²⁺ by Mg²⁺ on the color of CoCr₂O₄.

Experimental procedure

Synthesis of ceramic pigments

The ceramic pigments Co_1−xMg_xCr₂O₄ (x = 0.0, 0.2, 0.5, 0.8 and 1.0) with spinel structures were synthesized by gel-combustion in a single-step using 6-aminohexanoic acid (H₂N(CH₂)₅CO₂H, Sigma–Aldrich, 98.5% of purity) as fuel while Chromium (III) nitrate nonahydrate (Cr(NO₃)₃·9H₂O, Panreac, 98% of purity), Cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, Panreac, 98% of purity), and Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, Aldrich, 98% of purity) are used as oxidants. The stoichiometric equation (1) of the combustion reaction in the synthesis of CoCr₂O₄ and MgCr₂O₄ is as follows:

where M can be Co²⁺ or Mg²⁺.

For obtaining 2 g of CoCr₂O₄. First, stoichiometric amounts of Co(NO₃)₂·6H₂O and Cr(NO₃)₃·9H₂O were taken in a glass beaker of 1000 mL and dissolved in deionized water. Then, 1.21 mole of the fuel 6-aminohexanoic acid was added to the above solution and dissolved with magnetic stirring. Afterward, the solution was slowly evaporated at 90 °C until a gel was formed. Finally, the gel was slowly heated until a reaction with flame and with gases was completed in less of 60 s. Similarly, the previous procedure was also used for the spinel structures of Co_0.8Mg_0.2Cr₂O₄, Co_0.5Mg_0.5Cr₂O₄, Co_0.2Mg_0.8Cr₂O₄, and MgCr₂O₄.

Characterization

Crystalline structure was determined by X-ray diffraction (XRD) with a diffractometer (Model D8Controller, Bruker) equipped with a graphite monochromator using Cu K_α radiation (λ = 1.54184 Å), between 10° and 70° (2θ). The experiments were run with 1 and 6 mm divergence and antiscattering slits, respectively, with a step size of 0.02° (2θ) and an accumulated counting time of 0.2 s.

Fourier transform infrared spectrometer (FTIR, Agilent Cary 630) with attenuated total reflection (ATR) was used between 400 cm⁻¹ and 4000 cm⁻¹. On the other hand, the microstructure of the as-prepared powders was analyzed by Field Emission Scanning Electron Microscopy FESEM at 20 kV (Model S-4100, Hitachi Ltd., Tokyo, Japan). The optical properties of the inorganic pigments were studied using a Jasco V-670 UV-Vis-NIR spectrophotometer with diffuse reflectance and measurement range between 200 and 1800 nm with UV–vis bandwidth of 5 nm and NIR bandwidth of 20 nm, scan speed of 400 nm/min with light source of deuterium arc lamp and Tungsten Halogen Lamp with change of source at 340 nm and change of grating at 340 nm. The color coordinates of the pigments were obtained in CIE 1976 L*,a*,b* color space using the CIE standard illuminant D65 and the CIE Standard observer 10°, where L* coordinate represents darkness (0) and brightness (100), while the a* (red (+)–green (−) axis) and b* (yellow(+)–blue (−)) using the ASTM E308 [27]. The color difference of the powders was calculated using Eq. (2) for chroma C*_ab, Eq. (3) for hue h_ab, and Eq. (4) for the color difference which is defined in the standard practice for calculating of color differences D2244 [28].

CIE 1976 chroma:

CIE 1976 hue angles:

where

if astandard*bB*>aB*bstandard* then

s = 1

else

s = −1

Results and discussion

Adiabatic temperatures for the combustion reactions on the synthesis of CoCr₂O₄ and MgCr₂O₄

Fig. 1 shows digital images of the combustion reactions of all the reactions as can be seen there is the presence of a flame permits where the least intense is for the synthesis of MgCr₂O₄. The heats of combustion ΔH⁰ for the synthesis of CoCr₂O₄ and MgCr₂O₄ were calculated using the thermodynamic data in Table 1 which can be expressed using Eq. (5)

where n is the number of moles and ΔH_f⁰ is the standard enthalpy of formation for the reactants and products.

Fig. 1.

Digital images of the combustion reactions: (a) CoCr₂O₄, (b) Co_0.8Mg_0.2Cr₂O₄, (c) Co_0.5Mg_0.5Cr₂O₄, (d) Co_0.2Mg_0.8Cr₂O₄, (e) MgCr₂O₄.

Table 1.

Thermodynamic data.

Compound	ΔHf^0 (kJ mol^−1)	Cp (J mol^−1 K^−1)	Source
Cr(NO3)3*9H2O	−3631.63	–	[29]
Co(NO3)2*6H2O	−2212.68	–	[35]
Mg(NO3)2*6H2O	−2614.57	–	[36]
C6H13NO2	−637.77	–	[37]
CoCr2O4	−1432.81	130.25	[38]
MgCr2O4	−1763.19	227.38	[39]
CO2	−393.77	37.26	[35]
H2O	−241.95	30.14	[35]
N2	0	27.21	[35]

The adiabatic flame temperatures were estimated using Eq. (6) where m is the number of moles of each product, T_ad is the adiabatic flame temperature and C_p represents the specific heat of the products.

In this process, the combustion reaction occurs when the precursor mixture is dehydrated into gel above the room temperature until its ignition temperature.

The heat of the reactions are ΔH = −1761 kJ and ΔH = −1689 kJ for CoCr₂O₄ and MgCr₂O₄, respectively, and the adiabatic flame temperatures are 1482 K and 1347 K for CoCr₂O₄ and MgCr₂O₄, respectively. These results show that the high temperature during the reactions can be sufficient for the formation of the spinel phase in a single state where the temperature of the reaction when MgCr₂O₄ was synthesized is lower than CoCr₂O₄. Recently, Chamyani et al. [29] estimated adiabatic flame temperatures of the combustion reactions in the synthesis of CoCr₂O₄ using different fuels, and they obtained a T_ad = 1266 K when the citric acid was used as fuel, but if the mixture of fuels citric acid and ethylenediamine was used the T_ad = 1412 K. However, in our case when the 6-aminohexanoic acid as single fuel, the adiabatic flame temperature estimated is higher than the mixture of fuels used by Chamyani et al. On the other hand, De Andrade et al. [11] calculated the adiabatic flame temperature in the synthesis of MgCr₂O₄ using glycine and urea as fuels, T_ad = 1300 K and T_ad = 1200 K, respectively. In our case using a 6-aminohexanoic acid as single fuel, the adiabatic flame temperature is also higher than the mixture of fuel and extra-oxidant used by De Andrade et al.

X-ray diffraction patterns and IR spectra of the ceramic pigments

Fig. 2 shows the XRD patterns of the as-prepared powders with compositions CoCr₂O₄, Co_0.8Mg_0.2Cr₂O₄, Co_0.5Mg_0.5Cr₂O₄, Co_0.2Mg_0.8Cr₂O₄, and MgCr₂O₄. The patterns obtained confirm the formation of the cubic spinel structure with Space Group Fd3m (227) and with well-defined peaks around 18.5°, 30.4°, 35.8°, 37.4°, 43.5°, 53.9°, 57.6° and 63.2° (2θ) which are respectively ascribed to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of CoCr₂O₄ (JCPDS 22-1084) and MgCr₂O₄ (JCPDS 10-0351). The intensity of the main peak (311) of the as-prepared powder decreases with the increase of the concentration of Mg this is due at the different temperatures during the combustion reaction as it was shown in the calculation of flame temperature. Therefore, in the pattern of CoCr₂O₄ is observed that there is a higher crystallization than MgCr₂O₄. It is interesting to observe the slight shift to right in the peak associated with the main plane (331) in the XRD patterns with respect to the standard, this is due to differences in ionic size, The cristal radii of Co²⁺ in four coordination and High Spin is 72 pm, while for the Mg²⁺ cation in four coordination is 71 pm [30]. Using the Bragg's law, the lattice parameter was determined for CoCr₂O₄ and its value is 0.8314 nm, whereas for MgCr₂O₄ was 0.8312 nm which shows a slight variation when Co²⁺ is replaced by Mg²⁺, this observation is in agreement with the small difference of their crystal radii. These results agree with those reported by Hu et al. [31]. Moreover, the different ionic sizes and chemical behavior of Mg²⁺ and Co²⁺ could originate different crystallization rates for each chromite. For example, Gilabert et al. [21] found that there is a huge change in the crystallinity of the spinel as one cation is substituted by another.

Fig. 2.

XRD patterns for the as-prepared powders.

Fig. 3 shows the FTIR spectra of the as-prepared powders where the characteristic absorption bands of the structure are shown in Table 2. Kumar et al. [16] report the FTIR spectrum of MgCr₂O₄ and observed absorption bands at υ₁ = 632 and υ₂ = 430 cm⁻¹ arising from the vibration absorption of octahedral for Cr–O and tetrahedral for Co-O, respectively. On the other hand, Maczka et al. [5] obtained the FTIR spectrum of CoCr₂O₄ and found absorption bands at υ₁ = 631 and υ₂ = 493 cm⁻¹ which can be attributed to the vibration absorption of tetrahedral group CoO₄ and octahedral group CrO₆, respectively. All the observed vibrations have an increase in wavenumber when cobalt is replaced by magnesium which is due to differences in ionic sizes while the shift in band positions not is observed for the octahedral group CrO₆, this can be associated with normal spinels synthesized. Therefore, these results confirm the formation of the spinel structure.

Fig. 3.

Infrared spectra for the as-prepared powders.

Table 2.

Absorption vibration modes of as-prepared powders.

Compound	Absorption vibration (cm^−1) octahedral group	Absorption vibration (cm^−1) tetrahedral group
CoCr2O4	464	602
Co0.8Mg0.2Cr2O4	468	614
Co0.5Mg0.5Cr2O4	469	614
Co0.2Mg0.8Cr2O4	470	614
MgCr2O4	472	622

Fig. 4 shows the SEM micrographs of the as-prepared powders and is evident the formation of porous powders, which is due to the gases released during the reaction of combustion as has occurred in other combustion syntheses [26], there is also the formation of agglomerates of primary particles with irregular shape, this is a consequence of the high temperatures during the combustions. Images of SEM show that it is not possible to find an effect on the morphology with the increase of the concentration of Mg²⁺ in the structure.

Fig. 4.

SEM micrographs for the as-prepared powders: (a) CoCr₂O₄, (b) Co_0.8Mg_0.2Cr₂O₄, (c) Co_0.5Mg_0.5Cr₂O₄, (d) Co_0.2Mg_0.8Cr₂O₄, and (e) MgCr₂O₄.

Optical absorption and color coordinates L*a*b*

Fig. 5 shows the digital images of the as-prepared powders where is evident the change in the color in the powders between CoCr₂O₄ and MgCr₂O₄ due to the absorption of the visible light, because when Co²⁺ is present the green color is darker than with Mg²⁺.

Fig. 5.

Digital images for the as-prepared powders: (a) CoCr₂O₄, (b) Co_0.8Mg_0.2Cr₂O₄, (c) Co_0.5Mg_0.5Cr₂O₄, (d) Co_0.2Mg_0.8Cr₂O₄, and (e) MgCr₂O₄.

Fig. 6 shows the spectra of the as-prepared powders with compositions Co_1−xMg_xCr₂O₄ (x = 0.0, 0.2, 0.5, 0.7 and 1.0) which have bands around 272, 345, 423, 609, 657 and 1560 nm. These spectra are very similar to the spectra of MgCr₂O₄ and CoCr₂O₄ reported in the literature [32–34]. Therefore, the band around 272 nm can be attributed to the charge-transfer transition between O²⁻ and Cr³⁺[32–34]. Bands in the range 350–500 nm are associated to ⁴A_2g → ⁴T_1g and ⁴A_2g → ²T_2g transitions of the octahedral coordinated Cr³⁺ ion [32–34]. The bands around 609 and 657 nm can be assigned with ⁴A₂(⁴F) → ⁴T₁(⁴P) transitions of the Co²⁺ ion in tetrahedral coordination as well as ⁴A_2g → ⁴T_2g and ⁴A_2g → ²T_1g transitions of Cr³⁺[32–34]. Finally, bands in the 1300–1620 nm range are due to ⁴A₂(⁴F) → ⁴T₁(⁴F) transitions of Co²⁺[32–34]. Spectra of the powders are very similar, but the spectrum of MgCr₂O₄ is different due to the absence of the electronic transitions of Co²⁺. Therefore, our absorption spectra show very clearly the effect of substituting the chromophore on the optical properties of the powders.

Fig. 6.

UV–vis–NIR absorbance spectra for the chromites.

Table 3 shows the change in the color coordinates CIEL*a*b* of the as-prepared powders are green-blue where the a* and b* coordinates decrease when the Co²⁺ is substituted by Mg²⁺ which is due to the disappearance of electronic d-d transitions in Co²⁺. Therefore, the chroma decreases when the substitution by Mg²⁺ increases. On the other hand, the hue shows a change of color toward a green which is due to a decrease in the blue coordinate. However, all the powders have not changed in the L* values, which can be associated with a few influences of the morphology in the scattering of the light. The highest color change (ΔE*_ab) using the as-prepared powder of CoCr₂O₄ as reference was observed when there is a substitution complete between Co²⁺ and Mg²⁺. Moreover, the change in the color (ΔE*_ab) for the substitutions of Mg between x = 0.2 and x = 0.8 was 3.2 which shows that the dilution of CoCr₂O₄ permits to obtain green pigments without a change in the color when the cobalt is in the structure.

Table 3.

Color coordinates CIEL*a*b* for the ceramic pigments.

Compound	L*	a*	b*	Chroma C*ab	ΔHab	ΔE*ab
CoCr2O4 (standard)	63.7	−13.3	−8.1	15.6
Co0.8Mg0.2Cr2O4	64.3	−11.9	−5.3	13.0	−1.82	3.2
Co0.5Mg0.5Cr2O4	64.2	−11.3	−5.4	12.5	−1.41	3.4
Co0.2Mg0.8Cr2O4	64.2	−11.4	−5.2	12.5	−1.66	3.5
MgCr2O4	64.7	−6.8	−0.9	6.9	−4.26	9.8

The thermal stability of the pigments prepared by solution combustion was evaluated in a commercial frit as is shown in Fig. 7. The pigments were mixed with an industrial CaO–ZnO–SiO₂ frit (5 wt% of the pigment) and cylindrical pellets have been made and after drying the samples were fired at 1050 °C for 5 min. The heating rate up to 1050 °C has been 10 °C min⁻¹. As can be seen good thermal and chemical stability into the frit was obtained in all samples and the pigments had green color after glazing. Therefore, the pigments can be classified in the category A of CPMA which is important in the production of inorganic pigments with a lower concentration of cobalt for applications in ceramic decoration.

Fig. 7.

Digital images (a) frit without pigment, and frit with the pigments, (b) CoCr₂O₄, (c) MgCr₂O₄, (d) Co_0.8Mg_0.2Cr₂O₄, (e) Co_0.5Mg_0.5Cr₂O₄, and (f) Co_0.2Mg_0.8Cr₂O₄.

Conclusions

Ceramic pigments with compositions Co_1−xMg_xCr₂O₄ (x = 0.0, 0.2, 0.5, 0.7 and 1.0) were synthesized by gel combustion in one-step using 6-aminohexanoic acid. The adiabatic flame temperatures are 1482 K and 1347 K for CoCr₂O₄ and MgCr₂O₄, respectively, which is important for the control of crystallization during the reaction. Moreover, the micrographs in SEM showed agglomerated particles and porosity in the powders. On the other hand, the effect of substituting Co²⁺ by Mg²⁺ change of the color on the powders with the highest color change (ΔE*_ab) using the as-prepared powder of CoCr₂O₄ as reference was observed when there is a substitution complete between Co²⁺ and Mg²⁺. Furthermore, these results show the possibility of using gel-combustion in the synthesis of ceramic pigments. The results of thermal stability of the green pigments using a commercial frit showed that they could be used in ceramic decoration at 1050 °C.

Acknowledgments

Authors are very grateful to Colciencias - Colombia. Project “Fellowship for National Doctorates calls 647” for Doctoral Training Award to E. A. Chavarriaga. Fellowship Jóvenes Investigadores 2017 - Universidad de Valencia, Spain.