Influence of the pH on Al2O3:CuO pigments prepared by a polymeric precursor method

Influence of the pH on Al₂O₃:CuO pigments prepared by a polymeric precursor method

Carolina Hernandes Marques; Alexandre Mesquita; Vinicius Dantas Araújo; Maria Inês Basso Bernardi*

Instituto de Física de São Carlos – IFSC, Universidade de São Paulo – USP, CP 369, CEP 13560-970, São Carlos, SP, Brasil. *E-mail: [e-mail]

Copper (II) aluminate, CuAl2O4, which is known to be an inverse spinel contains a d⁹ metal ion and possesses interesting electronic, magnetic and catalytic properties and is used in industry as ceramic pigments, coatings and catalysts. Al₂O₃ with 10 mol% Cu (AlCu10) was synthesized by the polymeric precursor method varying the pH of the precursor solution with addition of ammonium hydroxide and ethylenediamine. It was shown that the pyrolysis of the polymeric chain occurs in higher temperatures for samples prepared in higher values of pH. The colorimetric coordinates exhibit dependence with pH and with the increase of annealing temperature. AlCu10 samples prepared with higher values of pH present lower values of colorimetric coordinates, indicating alterations in its color. The luminosity parameters L* increase also as a function of pH.

Keywords: Al₂O₃:CuO, pigments, polymeric precursor method

Introduction

In the last years, the development of pigments for the production of tiles, ceramic coatings or cosmetics has become a need, because the aesthetic aspect and the color frequently represent the parameters of interest¹. These parameters are modified through the changes in the coloring performance of pigments, that depends very much on their thermal stability, their chemical reactivity towards the glaze components, the coordination of host ions, and it also depends on the preparation methodology².

Many studies about nanoscale pigments has been achieved in the last years^3,4. In the area of coatings and pigments, new approaches through nanoscale effects can be used to create coatings with significantly optimized properties. The conventional areas of its application are paints, building materials, plastics, enamel and ceramics³. For this purpose, many methods have been employed, among them the polymeric precursor method³. This method, also referred to as the Pechini method⁵, allows the production of nanocrystalline powder samples at a relatively low temperature. This method consists of formation of a polymeric net starting from a polyhydroxy alcohol and an alpha-hydroxycarboxylic acid, with metallic cations homogeneously distributed throughout the matrix⁶.

However, there are few works in the literature describing the influence of pH on synthesis of powders prepared by the polymeric precursor method^7-9. To better understand this influence, the objective of the present work is to study the influence of pH on the synthesis of pigments.

It is known that copper(II) aluminate, CuAl₂O₄, which is known to be an inverse spinel contains a d⁹ metal ion and possesses interesting electronic, magnetic and catalytic properties and is used in industry as ceramic pigments, coatings and catalysts¹⁰. Various methods have been employed to prepare CuAl2O4 material, such as fusion of the two component oxides at high temperatures, co‑precipitation reactions in solution and sol–gel technique¹⁰. However, there are no studies about CuAl₂O₄ prepared via the polymeric precursor method as far as we know. Thus, Al₂O₃ with 10 mol% Cu was synthesized by the polymeric precursor method varying the pH of the precursor solution with addition of ammonium hydroxide and ethylene diamine. In this study, the samples were characterized by X-ray diffraction (XRD) technique, thermogravimetry (TG) and colorimetric measurement.

Experimental procedure

The polymeric precursor method is based on the polymerization of metallic citrate by ethylene glycol. A hydrocarboxilic acid such as citric acid is normally used to chelate cations in an aqueous solution. The addition of a polyalcohol such as ethylene glycol leads to an organic ester. Polymerization promoted by heating to around 100 °C results in a homogenous resin in which the metal ions are distributed uniformly throughout the organic matrix.

Aluminum nitrate (Al(NO₃)₃×9H₂O) and copper (II) nitrate (Cu(NO₃)₂×3H₂O) were used as precursors. The purity of these reagents was 98% (Synth). Aluminum nitrate and copper nitrate dissolved in water were added to an aqueous citric acid solution (C₆H₈O₇.H₂O – Synth, 99.5%) under constant agitation. Ammonium hydroxide or ethylenediamine (Synth – PA) were added to the solution to adjust the pH to be around 1, 4 and 8. After this, ethylene glycol, HOCH₂CH₂OH (Synth, 98%) was mixed in to promote the polymerization of the citrate by a polyesterification reaction. The citric acid:metal molar ratio was 3:1, while the citric acid:ethylene glycol ratio was 60:40 (mass ratio). The heat treatments to obtain the CuAl₂O₄ pigment powders were carried out in two stages: initial heating of the resin at 400 °C/h (10 °C/min) to pyrolise the organic material, followed by heating at 600, 800 and 1000 °C for 1 hour (10 °C/min) to eliminate residual organic material and formation of crystalline phases.

The thermal decomposition and crystallization process were studied by thermogravimetry (TG) (Netzch STA 409C) technique, after the first stage of the heat treatment, under oxygen atmosphere at a heating rate of 5 °C/min. Al₂O₃ compound was used as the reference material during the thermal analysis.

The powders were structurally characterized using an automatic diffractometer (Rigaku, Rotaflex RU200B) with Cu-Ka radiation (50 kV/100 mA, 1.5405 Å) and a graphite monochromator. The scanning range was between 10 and 70° (2q) with a step size of 0.02° and a step time of 1.0 seconds. For this characterization, the powders were calcined at 600, 800 and 1000 °C during 1 hour to increase the degree of crystallinity.

Colorimetric coordinates and diffuse reflectance of the pigments were measured through the Konica Minolta spectrophotometer CM2600d, in the 350-750 nm range, equipped with standard light sources type A (tungsten lamp, 2800 K), C (halogen lamp, 6775 K), and D50 (day light), following the CIE-L*a*b* colorimetric method recommended by the CIE (Commission Internationale de l'Eclairage)¹¹. In this method, L* is the lightness axis [black (0) - white (100)], b* is the blue (-) - yellow (+) axis, a* is the green (–) - red (+) axis, and DE is defined as the total color difference (DE² = L*² + a*² + b*²).

Results and discussion

In this study, samples with composition 10 mol% Cu in the Al₂O₃ die (labeled as AlCu10) were used to study the influence of pH on the properties of the obtained material. Figure 1a presents the TG curves for samples synthesized in pH 1, 4 and 8 (labeled as P1, P4A and P8A respectively) with the addition of NH₄OH. For all samples, two thermal events are observed. The first, which start at around 100 °C, is attributed to elimination of H₂O and NH₃ formed during the complexation of metals and chain polymerization from isopropyl alcohol, acetic acid and NOx^₁₂. The second event starts at different temperatures for each sample: 390 °C for P1 sample, 435 °C for P4A sample and 460 °C for P8A sample. This thermal event is attributed to the burn of organic materials that were not eliminated during the first thermal treatment, as the -CO- and -COO- groups in polymeric degradation¹².

It should be noted that the organic elimination temperature increases with pH. This fact is because in basic medium the approach of citrate groups is bigger, promoting the polymerization when ethylene glycol is added in the material synthesis13.This bigger approach of citrates groups is related to the higher pitch of dissociating of citric acid molecules, whose dissociating reactions are improved for higher values of pH in water's solutions¹⁴. The same behavior was observed for samples synthesized with ethylenediamine, as can be seen in Figure 1b, which shows the TG curves for samples synthesized in pH 4 and 8 (labeled as P4E and P8E respectively). In these TG curves, the second event starts at 421 °C for P4E sample and 489 °C for P8E sample.

The XRD technique was performed to evaluate the structure in different annealing temperatures and values of pH in the synthesis, as can be seen in Figure 2. Figure 2a shows the X-ray diffraction patterns for sample P1 submitted to different annealing temperatures. The XRD data show that peaks related to crystalline phases are observed for samples annealed at 600 and 800 °C, although amorphous phases predominate for both samples. X-ray diffraction patterns for P1 sample annealed at 1000 °C exhibits a crystalline phase without amorphous phases. This indicates that a complete crystallization occurs at between 800 and 1000 °C.

Figure 2b

Figure 2c presents the X-ray diffraction pattern for AlCu10-P1 sample annealed at 1000 °C. Al₂O₃^[15] and CuAl₂O₄^[16] phases were indexed and the peaks marked with asterisk are related to crystallographic plans of the CuAl2O4 phase. According to the literature, the Al₂O₃ presents a structure with a rhombohedral symmetry of R-3c space group and the CuAl2O4 presents a structure with a cubic symmetry of Fd3m space group. No other phase was identified and the amount proportion of Al₂O₃ and CuAl₂O₄ phases are 66% and 34% respectively.

Colorimetric analyses according to the CIE-L*a*b* standard colorimetric method were performed for AlCu10 samples synthesized at different pH annealing temperatures, whose results are presented in Table 1. As can be seen, AlCu10 samples annealed at 600 and 800 °C exhibit negative values of colorimetric coordinates a* and b*, which is located in the third quadrant indicating a mixture of green and blue colors. AlCu10 samples annealed at 1000 °C present positives values for the colorimetric coordinates (first quadrant), indicating a brown color. Also, colorimetric coordinates a* and b* are smaller for samples AlCu10-P4A, AlCu10-P8A, AlCu10-P4E and AlCu10-P8E in comparison with sample AlCu10-P1, indicating that the synthesis pH and the addition of NH4OH or ethylenediamine influences the final color of the samples. The luminosity parameter L* shows values near to the black and the highest values were founded to samples with highest values of pH. It was also observed that L* decreases as a function of the annealing temperature. This dependence of L* with annealing temperature can be attributed to the higher crystallinity degree, as can be seen in XRD results.

The total color difference (DE) is a parameter used to compare two samples of the same color17.One important information obtained here is that each pigment powder showed very close values of DE even for the different excitation light sources (tungsten lamp in 2800 K, halogen lamp in 6775 K and day light). This result shows that each pigment presents the same visual color in differents types of illumination, a desirable characteristic in applications as ceramic pigments.

Figure 3 shows the diffuse reflectance spectra for AlCu10 samples which were calcined at 800 °C for 1 hour. It is possible to observe that the samples present a wide reflectance band around 500-520 nm, indicating a predominance of blue and green colors. According to the literature, the Cu²⁺ cation, with a 3d⁹ electronic configuration, exhibits one of the following structures: octahedral (blue color) or tetrahedral coordination (green color)¹⁸. Thus, it can be supposed that these two kinds of Cu²⁺ cation coordination are present at AlCu10 samples.

The reflectance band shows dependence with addition of ammonium hydroxide or ethylenediamine in the synthesis of AlCu10 samples. Samples prepared with ammonium hydroxide present the highest enlargement of reflectance band than samples prepared with ethylenediamine. The AlCu10-P1 exhibits the lowest enlargement.

Conclusion

Ceramics pigments of Al₂O₃ with 10 mol% Cu prepared by a polymeric precursor method (Pechini) present different optical properties when synthesized at different pH values. It was shown that the pyrolysis of the polymeric chain occurs in higher temperatures for samples prepared in higher values of pH. However, alterations in the crystalline structure of the material are not observed as a function of the pH, or as a function of ethylenediamine or ammonium hydroxide addition.

The samples of this study heat treated at 600 and 800 °C present green and blue colors, whereas samples annealed at 1000 °C presents brown color. These differences are attributed to the fact that a complete crystallization occurs only in 1000 °C, whereas amorphous phases are still observed in samples annealed at 800 °C.

The colorimetric coordinates exhibit dependence with the value of pH and annealing temperature. AlCu10 samples prepared with higher values of pH present lower values of colorimetric coordinates, indicating alterations in its color. The luminosity parameters L* increase also as a function of pH.

Acknowledgements

The authors gratefully acknowledge the financial support of the Brazilian research funding institutions CNPq and FAPESP.

Received: December 12, 2011

Revised: August 19, 2012

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Publication Dates

• Publication in this collection
  02 Nov 2012
• Date of issue
  Feb 2013