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A simple route to synthesis and characterization of CoAl2O4 nanocrystalline via combustion method using egg white (ovalbumine) as a new fuel
Materials Research Bulletin 46 (2011) 2548–2553
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
A simple route to synthesis and characterization of CoAl2O4 nanocrystalline via combustion method using egg white (ovalbumine) as a new fuel Ibrahim S. Ahmed * Chemistry Department, Faculty of Science, Benha University, Benha City, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 March 2011 Received in revised form 21 June 2011 Accepted 4 August 2011 Available online 11 August 2011
Single phase of CoAl2O4 nanocrystalline spinel has been synthesized first time successively by combustion method using extracted egg white (ovalbumine) as new fuel. The prepared samples were characterized using XRD, TGA, DSC, TEM, BET, IR, UV–Vis and CIE L*a*b* colorimetric method. The average particle sizes were in the range 10.45–26.58 nm and fine agglomerates in a way that specific surface area up to 188.55 m2 g1 could be obtained. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: Nanocrystalline Thermogravimetric (TG) Combustion Egg white XRD Surface area
1. Introduction Spinel-type oxides AB2O4, where A and B stand for two different cations of comparable ionic sizes, are a class of chemically and thermally stable materials, which are suitable for a wide range of applications, such as magnetic materials, ceramics, and catalysis [1–4]. In the spinel structure, the oxygen ions form cubic close packed structure, and the A and B cations occupy two different crystallographic sites, tetrahedral and octahedral. The distribution of A and B cations in these two sites is affected by the combination and nature of the two cations and depends strongly on the preparation and processing conditions. Studies of cation distribution in spinel have attracted much attention because they may allow better understanding of the correlations between structure and properties such as color, diffusivity, magnetic behavior, catalytic activity and optical properties, which are strongly dependent on the occupation of these two sites by metals [5,6]. Among the class of materials, cobalt aluminates (CoAl2O4) spinel, known as Thenard’s blue, is widely used as catalyst, color filter for automotive lamps or pigment layer on luminescent materials because of its thermal, chemical, photochemical stabilities and peculiar optical properties [7–12]. In recent years, much work has been done on the preparation and the optical properties of CoAl2O4
* Tel.: +20 122408034; fax: +20 1324085. E-mail address: [email protected]. 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.08.005
spinel materials [13–19]. A variety of techniques such as combustion [13], Penchini [14], sol–gel [15,17], and reverse microemulsion [16] have been used for the preparation of cobalt aluminate oxide. Previous studies have shown that the color of CoAl2O4 is strongly affected by the synthesis temperature [17–19]. In more detail, the sample is green when synthesized at relatively low temperature, while it changes to blue when synthesized at relatively high temperature. At present, the origin of the color of CoAl2O4 obtained at low temperature is still under debate [19–21]. The synthesis of nano-crystalline spinel has been investigated intensively due to the unique potential applications in high density magnetic recording, magnetic fluids, and an absorbent material to remove sulfide gases from hot-coal gas [22] and microwave devices. It is well known as thermally and chemically stable pigment of intense blue color for its impressive optical property and widely used applications in the ceramics, glass and paint industries. Also, the wide optical applications are used in color TV tubes as contrast-enhancing luminescent pigment [23–31]. Among these established synthesis methods, it is still critical to find simple and cost-effective routes to synthesize nanocrystalline NiFe2O4 by the utilization of cheap, nontoxic and environmentally benign precursors. Egg white (ovalbumine) proteins are well known for their gelling, foaming and emulsifying characteristics, in addition to their high nutrition quality [32–34]. Due to its solubility in water and its ability to associate with metal ions in solution, egg white has been used as a binder cum gel for shaping material, particularly
I.S. Ahmed / Materials Research Bulletin 46 (2011) 2548–2553
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Fig. 1. Schematic flow chart for the synthesis of CoAl2O4 nanocrystalline powder.
bulk and porous ceramics [35–37]. The low temperature combustion synthesis (LCS) technique has been proved to be a novel, extremely facile, time-saving and energy-efficient route for the synthesis of ultrafine powders [38–40]. Most recently, it was used as a matrix for entrapment of aluminum ions to generate a gelled precursor which resulted in a-alumina particles with crystalline sizes of 15–80 nm after heat treatment above 330 8C [41]. The use of egg white simplifies the process and would provide another alternative process for the simple and economical synthesis of nanocrystalline ceramic particles. The matrix-based precursor method using extracted egg white (ovalbumin) is recognized as a convenient and cost-effective method for the synthesis of various ceramic powders in which egg white is used as the fuels of the combustion synthesis [42–45]. Various oxides and complex oxides with narrow particle size distribution, single phase, and controlled particle morphology have been synthesized based on auto-ignited gel combustion method. However, little attention has been paid to preparing spinel oxides. No literature precedence, to our knowledge, is available on the synthesis of CoAl2O4 by this method. In this paper, the auto-ignited gel low temperature combustion (AIGLTCS) method is introduced to synthesize CoAl2O4 using egg white as fuels. 2. Experimental
Fig. 2. TG/DSC curve of CoAl2O4 spinel (a) (A, B, C and D) precursor gel at different heating rates 2, 5, 7.5 and 10 8C min1 and (b) TG curves of CoAl2O4 (1, 2 and 3 after calcinations at 600, 800 and 900 8C min1 respectively).
was evaporated on a hot plate above 130 8C for dehydration and continued until a viscous gel was obtained. The obtained gel was heated at 250 8C in an oven for complete dehydration, resulted in a black foamed mass. The obtained carbon-rich mass, hereinafter referred to as precursor, was ground into powders by a pestle and mortar and subsequently heat-treated in an electric furnace at various temperatures; 400–900 8C for 2 h to obtain carbon-free spinel powder. Fig. 1 shows schematic flow chart for the synthesis of CoAl2O4 nanocrystalline spinel powder. 2.3. Characterization Products were characterized by XRD on SIEMENS D5000 (TGA; DTA Instruments, SDT2960) of the precursor samples were studied using Shimadzu DT-50 thermal analyzer. The morphology and particle shape for the calcinated powder was performed using TEM, model EM 10 Zeiss. The surface area was measured by N2
2.1. Materials and reagents All reagents were of analytical grade and were purchased and used as received without further purification. Cobalt nitrate (Co(NO3)26H2O; Sigma–Aldrich), Aluminum nitrate (Al(NO3)3 6H2O; Sigma–Aldrich), and extracted egg white (ovalbumine). 2.2. Preparation of CoAl2O4 nanocrystalline An appropriate amount of analytical grade Co(NO3)26H2O, Al(NO3)39H2O, egg white were extracted and used as starting materials. A mixed solution of metal nitrates was prepared by dissolving Co(NO3)26H2O, and Al(NO3)39H2O in de-ionized water with the molar ratio of Co2+/Al3+ = 1:2. To prepare a transparent sol, 0.041 mole of cobalt nitrate (12.22 g) and 0.084 mole of aluminum nitrate (31.67 g) were dissolved in 100 ml of de-ionized water. Separately 20, 40 and 60% (w/w) extracted egg white were added. The previous reaction mixture subsequent heating at 85 8C with continuous stirring for 1 h on a hot plate let the ions react with egg white completely resulting in a pale pink color gel. The gel
Fig. 3. Plots log b/T2m vs. 1000/Tm.
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Fig. 5. IR spectra of CoAl2O4 nanocrystals at different calcination temperatures using egg white as fuel.
Fig. 4. XRD patterns of CoAl2O4 nanocrystals annealed at different temperatures using egg white as fuel.
adsorption (ASAP2000, Miromeritics) Infrared spectroscopy (Jasco FT/IR-460 plus). Spectrophotometer measurements were carried out using JASCO V530 UV/Vis Spectrophotometer in the range of 400–800 nm for both calclinated samples using 10 mm matched quartz cells. The diffuse reflectance of fired pigments was measured using JASCO spectrophotometer UV– Visible in 200–800 nm range using standard D65 illumination and barium sulfate as reference and the CIE L*a*b* colorimetric method,
Fig. 6. TEM image of CoAl2O4 nanocrystalline powder heated at different temperatures: (a) 400, (b) 600 and (c) 900 8C.
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Fig. 7. N2 adsorption–desorption isotherms of pure single phase of CoAl2O4 nanocrystalline powder in different media: (i) egg white (ovalbumine), (ii) egg white (ovalbumine) + NH4OH, and (iii) egg white (ovalbumine) + NaOH as fuels.
recommended by the Commission International de l’Eclairage (CIE) [46]. L* is the lightness axis [black (0) ! white (1 0 0)], b* is the blue () ! yellow (+) axis, a* is the green () ! red (+) axis, and DE is the hue variation.
DE2 ¼ ðL Þ2 þ ða Þ2 þ ðb Þ2
3. Results and discussion The TG/DSC curves of the dried gel are shown in Fig. 2. The endothermic peak at 201 8C is due to the decomposition of nitrates, accompanied by a significant weight loss. The exothermic peak at 434 8C corresponds to the crystallization of the CoAl2O4 spinel. There is no distinct weight loss at temperature higher than 450 8C. Thermal properties of CoAl2O4 have been studied by TGA and DSC at different heating rates (Fig. 2). One can observe three thermal events: a glass transition (seen as an endothermic baseline shift) followed by two peaks, the position of maxima of the
corresponding peaks depends directly on heating rates and obeys the Kissinger formula [47]: b Ea 1 ln 2 ¼ C R Tm Tm where b, heating rate; Tm, the temperature of the peak’s maximum; R, gas constant; Ea, the activation energy of the crystallization and C, constant. From linear fits of the experimental data plotted in the coordinates ln(b/Tm2) vs. 1/Tm (Fig. 3), the activation energy of the crystallization was determined: 74.55 0.2 kJ mol1. Fig. 4 shows XRD patterns of the CoAl2O4 gels annealed at 400– 900 8C for 5 h. Several diffraction peaks are observed in the patterns and the position of these peaks closely resembles that of cubic CoAl2O4 spinel indicating the formation of cobalt aluminate phase. The presence of spinel phase is also confirmed by the appearance of the weak peak at about 2u = 498, which is characteristic for the CoAl2O4 phase. The intensity of the diffraction
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Table 1 XRD and TEM average particle sizes (nm) data for different CoAl2O4 systems at 900 8C. System
Egg white (A) Egg white + NH4OH (B) Egg white + NaOH (C)
Table 2 Colorimetric data for CoAl2O4 systems at 900 8C. System
Particle size (nm) X-ray
TEM
30.25 20.10 17.33
26.58 19.20 14.42
Egg white (A) Egg + NH4OH (B) Egg + NaOH (C)
CIE method L*
a*
b*
DE
40.20 70.71 74.32
36.57 17.18 7.27
18.90 34.74 40.36
34.80 79.11 69.84
peaks increases with increasing annealing temperature, which is associated with an increase in crystallinity. The average size of the nanocrystallites was also estimated by using the Debye–Scherrer formula: D¼
0:9l b cos uB
(1)
where l, b, uB are the X-ray wavelength, the full width at half maximum (FWHM) of the diffraction peak and the Bragg diffraction angle, respectively. The estimated crystallite size, from XRD data, was calculated to be 10.45–26.58 nm in the temperature range 400–900 8C and shown in Table 1. The IR spectra of the solid phase powders synthesized in range 400, 600 and 900 8C are shown in Fig. 5. The band at 3431 cm1, which was a strong absorption band, is attributed to the OH stretching mode. The band around 1620–1640 cm1 was assigned to the free water molecules. The vibration mode of the CO32 anion appeared around 1384 cm1 [43]. The band around 526–542 cm1 is corresponding to CoO4 and 679–693 cm1 is corresponding to AlO6 vibrations [32]. However, 1080 cm1 band corresponded to vibration mode of the CO32 [44,45]. Transmission electron micrograph of the powders indicated uniform and nonaggregated particles, as seen in Fig. 6. The particles had octahedral morphologies. Particle size distribution in TEM photograph was fairly narrow. Particle size from TEM observation was 10–26.58 nm. The crystallite size of the powders, calculated from Scherrer’s equation, using the broadening of the X-ray diffraction pattern was 10–26.58 nm. The result was in agreement with the TEM observation. Fig. 7 shows that the specific surface area was measured by the BET method which found in range 34.22– 188.55 m2 g1. The nanocrystalline synthesized pigment had normal spinel structure for both precursor obtained and fired samples due to transitions which means presence of few Co2+ cations occupied octahedral sites in the CoAl2O4 spinel lattice [29], while other Co2+ cations occupied in tetrahedral site. Diffuse reflectance spectra for egg white, egg white + NH4OH and egg white + NaOH systems which are represented as symbols A, B and C, respectively as presented in Table 2. Those systems show the appearance of good blue color pigments. Also, the colorimetric data presented in Table 2, reveal that the values of b* increase in the negative direction from A to C systems as well as L* values increase as result of increasing calcination temperatures and time. The decrease in values of a* means the higher intensity of blue color. However, increasing values of b* in negative direction is an indication for the appearance of blue color. Additionally, the decreasing in L* parameter from C to A systems are corresponding to reduce the lightness of sample. The values of a* increase in positive direction and b* increase in negative direction with the depth of blue color as result of calcination temperatures and the increasing of the base media. Colorimetric data show the high value of a* and b* and higher value of hue variation DE up to 900 8C for all samples under investigation. This means that the appearance of good color of blue nano ceramic pigment powders and a good color matching occur up to 900 8C. The UV–Vis spectrum was recorded as shown in Fig. 8 with triple three absorption peaks in visible region, exhibiting bluer
Fig. 8. UV–Vis absorbance spectra of pure single phase of CoAl2O4 nanocrystalline powder in different media: (i) egg white (ovalbumine), (ii) egg white (ovalbumine) + NH4OH, and (iii) egg white (ovalbumine) + NaOH as fuels.
color hues. Regarding the 900 8C calcined powders, they display more like a sky-blue hue. The as-synthesized homogeneous and size controllable nanocrystalline CoAl2O4 spinel pigment is expected to exhibit good transparency and tincting strength as a brilliant, lightfast and inert inorganic coloring agent in different matrices, its coloring performances will be reported later. The minimum absorption was around 500 nm, and the maximum absorption was around 650 nm. The particles size in range 10.45– 26.58 nm after the precursor was sintered at different calcinations temperature. In conclusion, nano-sized CoAl2O4 pigment was successfully synthesized through auto-ignited-gel combustion method. It was thermally stable, and had maximum absorption band around 600 nm. It can be a candidate as a high-grade pigment in color TV tubes and ceramic pigment industries. 4. Conclusions Pure and homogeneous nanocrystalline CoAl2O4 spinel powders have been directly prepared by the auto-ignited gel combustion method. The powder characteristics are strongly dependent on the amount of heat, or the flame temperature generated by fuel ratio used in the starting composition. The CoAl2O4 spinel particle size increases with the enhancement of calcination temperature which affects the color of the pigments on glassy ceramic coatings. The best results were obtained by calcining the spinel CoAl2O4 phase at 400– 900 8C using egg white (ovalbumine) as a fuel. References [1] I.H. Gul, A. Maqsood, M. Naeem, M. Naeem Ashiq, J. Alloys Compd. 507 (2010) 201. [2] F. Tielens, M. Calatayud, R. Franco, J.M. Recio, M. Perez-Ramirez, C. Minot, J. Phys. Chem. B 110 (2006) 988. [3] N. Ballarini, F. Cavani, S. Passeri, L. Pesaresi, A.F. Lee, K. Wilson, Appl. Catal. A: Gen. 366 (2009) 184.
I.S. Ahmed / Materials Research Bulletin 46 (2011) 2548–2553 [4] M. Sakavatu-Niasari, F. Davar, Mater. Lett. 63 (2009) 441. [5] I.B. Bersuker, Electronic Structure and Properties of Transition Metal Compounds, John Wiley & Sons, New York, 1996. [6] R. Burns, Mineralogical Applications of Crystal Field Theory, vol. 5, Cambridge University Press, Cambridge, 1993. [7] C. Maurizio, N. El Habra, G. Rossetto, M. Merlini, E. Cattaruzza, L. Pandolfo, M. Casarin, Chem. Mater. 22 (2010) 1933. [8] A. Walsh, Y. Yan, M.M. Al-Jassim, S.-H. Wei, J. Phys. Chem. C 112 (2008) 12044. [9] X.L. Duan, D.R. Yuan, Z.H. Sun, C.N. Luan, D.Y. Pan, D. Xu, M.K. Lv, J. Alloys Compd. 386 (2005) 311. [10] P.M.T. Cavalcante, M. Dondi, G. Guarini, M. Raimondo, G. Baldil, Dyes Pigments 80 (2009) 226. [11] W.S. Cho, M. Kakihana, J. Alloys Compd. 287 (1999) 87. [12] D. Rangappa, T. Naka, A. Kondo, M. Ishii, T. Kobayashi, T. Adschiri, J. Am. Chem. Soc. 129 (2007) 11061. [13] T. Mimani, J. Alloys Compd. 315 (2001) 123. [14] L. Gama, M.A. Ribeiro, B.S. Barros, R.H.A. Kiminami, I.T. Weber, A.C.F.M. Costa, J. Alloys Compd. 483 (2009) 453. [15] F.L. Yu, J.F. Yang, J.Y. Ma, J. Du, Y.Q. Zhou, J. Alloys Compd. 68 (2009) 443. [16] J. Chandradass, M. Balasubramanian, K.H. Kim, J. Alloys Compd. 06 (2010) 395. [17] M. Zayat, D. Levy, Chem. Mater. 12 (2000) 2763. [18] I.S. Ahmed, S.A. Shama, M.M. Moustafa, H.A. Dessouki, A.A. Ali, Spectrochim. Acta A 74 (2009) 665. [19] U.L. Stangar, B. Orel, M.J. Krajnc, J. Sol–Gel Sci. Technol. 26 (2003) 771. [20] H.S.C. O’Neill, Eur. J. Mineral. 6 (1994) 603. [21] A. Natasuka, Y. Ikda, Y. Yamasaki, N. Nakayama, T. Mizota, Solid State Commun. 128 (2003) 85. [22] A.C. Otero, M.M. Penarroya, P.E. Escalona, X.F.X. Llabresi, J.B. Parra, Mater. Lett. 39 (1999) 22.
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[23] G. Buxbaum, Industrial Inorganic Pigments, 1st edn, VCH, Weinheim, Germany, 1993, p. 85. [24] P.A. Lewis, Pigment Handbook, Vol. 1, John Wiley and Sons, New York, 1998. [25] J.M. Fernandez, C.O. Arean, J. Solid State Chem. 109 (1994) 43. [26] P.H. Bolt, F.H. Habraken, J.W. Geust, J. Solid State Chem. 135 (1998) 59. [27] T.Y. Tseng, J.C. Lin, J. Mater. Sci. Lett. 8 (1989) 261. [28] T. Tsukada, H. Segawa, A. Yasumori, K. Okada, J. Mater. Chem. 9 (1999) 549. [29] J. Merikhi, H.O. Jungk, C. Feldmann, J. Mater. Chem. 10 (2000) 1311. [30] L.K. Kurihara, S.L. Suib, Chem. Mater. 5 (1993) 609. [31] F. Meyer, R. Hempelmann, S. Mathur, M. Veith, J. Mater. Chem. 9 (1999) 1755. [32] A. Rabenau, Angew. Chem. Int. Ed. Engl. 24 (1985) 1026. [33] W.J. Dawson, Ceram. Bull. 10 (1988) 1673. [34] S. Somiya, K. Hishinuma, T. Akiba, Bull. Mater. Sci. 6 (1995) 811. [35] T.R.N. Kutty, R. Balachandran, Mater. Res. Bull. 19 (1984) 1479. [36] M.M. Lencka, R.E. Riman, Chem. Mater. 5 (1993) 61. [37] S. Chemlal, A. Larbot, M. Persin, Mater. Res. Bull. 35 (2000) 2515. [38] J.J. Kingsley, K.C. Patil, Mater. Lett. 6 (1988) 427. [39] J.J. Kingsley, K. Suresh, K.C. Patil, J. Mater. Sci. 25 (1990) 1305. [40] D.A. Fumo, M.R. Morelli, A.M. Segadaes, Mater. Res. Bull. 31 (1996) 1243. [41] J.T. Kloprogge, R.L. Frost, Appl. Catal. A: Gen. 184 (1999) 61. [42] R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, New York, 1971. [43] F. Kooli, K. Kosuge, A. Tsunashima, J. Mater. Sci. 30 (1995) 4591. [44] P. Jeevanandam, Yu. Koltypin, A. Gedanken, Y. Mastai, J. Mater. Chem. 10 (2000) 511. [45] K. Yamakata, K. Hirota, O. Yamaguchi, J. Am. Ceram. Soc. 8 (1994) 2207. [46] CIE, Recommendations of Uniform Color Spaces, Color Difference Equations, Psychometrics Color Terms, Supplement no. 2 of CIE Publ. No. 15 (E1-1.31), 1971, Bureau Central de la CIE, Paris, 1978. [47] H.E. Kissinger, Anal. Chem. 29 (1957) 1702.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
A simple route to synthesis and characterization of CoAl2O4 nanocrystalline via combustion method using egg white (ovalbumine) as a new fuel Ibrahim S. Ahmed * Chemistry Department, Faculty of Science, Benha University, Benha City, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 March 2011 Received in revised form 21 June 2011 Accepted 4 August 2011 Available online 11 August 2011
Single phase of CoAl2O4 nanocrystalline spinel has been synthesized first time successively by combustion method using extracted egg white (ovalbumine) as new fuel. The prepared samples were characterized using XRD, TGA, DSC, TEM, BET, IR, UV–Vis and CIE L*a*b* colorimetric method. The average particle sizes were in the range 10.45–26.58 nm and fine agglomerates in a way that specific surface area up to 188.55 m2 g1 could be obtained. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: Nanocrystalline Thermogravimetric (TG) Combustion Egg white XRD Surface area
1. Introduction Spinel-type oxides AB2O4, where A and B stand for two different cations of comparable ionic sizes, are a class of chemically and thermally stable materials, which are suitable for a wide range of applications, such as magnetic materials, ceramics, and catalysis [1–4]. In the spinel structure, the oxygen ions form cubic close packed structure, and the A and B cations occupy two different crystallographic sites, tetrahedral and octahedral. The distribution of A and B cations in these two sites is affected by the combination and nature of the two cations and depends strongly on the preparation and processing conditions. Studies of cation distribution in spinel have attracted much attention because they may allow better understanding of the correlations between structure and properties such as color, diffusivity, magnetic behavior, catalytic activity and optical properties, which are strongly dependent on the occupation of these two sites by metals [5,6]. Among the class of materials, cobalt aluminates (CoAl2O4) spinel, known as Thenard’s blue, is widely used as catalyst, color filter for automotive lamps or pigment layer on luminescent materials because of its thermal, chemical, photochemical stabilities and peculiar optical properties [7–12]. In recent years, much work has been done on the preparation and the optical properties of CoAl2O4
* Tel.: +20 122408034; fax: +20 1324085. E-mail address: [email protected]. 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.08.005
spinel materials [13–19]. A variety of techniques such as combustion [13], Penchini [14], sol–gel [15,17], and reverse microemulsion [16] have been used for the preparation of cobalt aluminate oxide. Previous studies have shown that the color of CoAl2O4 is strongly affected by the synthesis temperature [17–19]. In more detail, the sample is green when synthesized at relatively low temperature, while it changes to blue when synthesized at relatively high temperature. At present, the origin of the color of CoAl2O4 obtained at low temperature is still under debate [19–21]. The synthesis of nano-crystalline spinel has been investigated intensively due to the unique potential applications in high density magnetic recording, magnetic fluids, and an absorbent material to remove sulfide gases from hot-coal gas [22] and microwave devices. It is well known as thermally and chemically stable pigment of intense blue color for its impressive optical property and widely used applications in the ceramics, glass and paint industries. Also, the wide optical applications are used in color TV tubes as contrast-enhancing luminescent pigment [23–31]. Among these established synthesis methods, it is still critical to find simple and cost-effective routes to synthesize nanocrystalline NiFe2O4 by the utilization of cheap, nontoxic and environmentally benign precursors. Egg white (ovalbumine) proteins are well known for their gelling, foaming and emulsifying characteristics, in addition to their high nutrition quality [32–34]. Due to its solubility in water and its ability to associate with metal ions in solution, egg white has been used as a binder cum gel for shaping material, particularly
I.S. Ahmed / Materials Research Bulletin 46 (2011) 2548–2553
2549
Fig. 1. Schematic flow chart for the synthesis of CoAl2O4 nanocrystalline powder.
bulk and porous ceramics [35–37]. The low temperature combustion synthesis (LCS) technique has been proved to be a novel, extremely facile, time-saving and energy-efficient route for the synthesis of ultrafine powders [38–40]. Most recently, it was used as a matrix for entrapment of aluminum ions to generate a gelled precursor which resulted in a-alumina particles with crystalline sizes of 15–80 nm after heat treatment above 330 8C [41]. The use of egg white simplifies the process and would provide another alternative process for the simple and economical synthesis of nanocrystalline ceramic particles. The matrix-based precursor method using extracted egg white (ovalbumin) is recognized as a convenient and cost-effective method for the synthesis of various ceramic powders in which egg white is used as the fuels of the combustion synthesis [42–45]. Various oxides and complex oxides with narrow particle size distribution, single phase, and controlled particle morphology have been synthesized based on auto-ignited gel combustion method. However, little attention has been paid to preparing spinel oxides. No literature precedence, to our knowledge, is available on the synthesis of CoAl2O4 by this method. In this paper, the auto-ignited gel low temperature combustion (AIGLTCS) method is introduced to synthesize CoAl2O4 using egg white as fuels. 2. Experimental
Fig. 2. TG/DSC curve of CoAl2O4 spinel (a) (A, B, C and D) precursor gel at different heating rates 2, 5, 7.5 and 10 8C min1 and (b) TG curves of CoAl2O4 (1, 2 and 3 after calcinations at 600, 800 and 900 8C min1 respectively).
was evaporated on a hot plate above 130 8C for dehydration and continued until a viscous gel was obtained. The obtained gel was heated at 250 8C in an oven for complete dehydration, resulted in a black foamed mass. The obtained carbon-rich mass, hereinafter referred to as precursor, was ground into powders by a pestle and mortar and subsequently heat-treated in an electric furnace at various temperatures; 400–900 8C for 2 h to obtain carbon-free spinel powder. Fig. 1 shows schematic flow chart for the synthesis of CoAl2O4 nanocrystalline spinel powder. 2.3. Characterization Products were characterized by XRD on SIEMENS D5000 (TGA; DTA Instruments, SDT2960) of the precursor samples were studied using Shimadzu DT-50 thermal analyzer. The morphology and particle shape for the calcinated powder was performed using TEM, model EM 10 Zeiss. The surface area was measured by N2
2.1. Materials and reagents All reagents were of analytical grade and were purchased and used as received without further purification. Cobalt nitrate (Co(NO3)26H2O; Sigma–Aldrich), Aluminum nitrate (Al(NO3)3 6H2O; Sigma–Aldrich), and extracted egg white (ovalbumine). 2.2. Preparation of CoAl2O4 nanocrystalline An appropriate amount of analytical grade Co(NO3)26H2O, Al(NO3)39H2O, egg white were extracted and used as starting materials. A mixed solution of metal nitrates was prepared by dissolving Co(NO3)26H2O, and Al(NO3)39H2O in de-ionized water with the molar ratio of Co2+/Al3+ = 1:2. To prepare a transparent sol, 0.041 mole of cobalt nitrate (12.22 g) and 0.084 mole of aluminum nitrate (31.67 g) were dissolved in 100 ml of de-ionized water. Separately 20, 40 and 60% (w/w) extracted egg white were added. The previous reaction mixture subsequent heating at 85 8C with continuous stirring for 1 h on a hot plate let the ions react with egg white completely resulting in a pale pink color gel. The gel
Fig. 3. Plots log b/T2m vs. 1000/Tm.
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Fig. 5. IR spectra of CoAl2O4 nanocrystals at different calcination temperatures using egg white as fuel.
Fig. 4. XRD patterns of CoAl2O4 nanocrystals annealed at different temperatures using egg white as fuel.
adsorption (ASAP2000, Miromeritics) Infrared spectroscopy (Jasco FT/IR-460 plus). Spectrophotometer measurements were carried out using JASCO V530 UV/Vis Spectrophotometer in the range of 400–800 nm for both calclinated samples using 10 mm matched quartz cells. The diffuse reflectance of fired pigments was measured using JASCO spectrophotometer UV– Visible in 200–800 nm range using standard D65 illumination and barium sulfate as reference and the CIE L*a*b* colorimetric method,
Fig. 6. TEM image of CoAl2O4 nanocrystalline powder heated at different temperatures: (a) 400, (b) 600 and (c) 900 8C.
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Fig. 7. N2 adsorption–desorption isotherms of pure single phase of CoAl2O4 nanocrystalline powder in different media: (i) egg white (ovalbumine), (ii) egg white (ovalbumine) + NH4OH, and (iii) egg white (ovalbumine) + NaOH as fuels.
recommended by the Commission International de l’Eclairage (CIE) [46]. L* is the lightness axis [black (0) ! white (1 0 0)], b* is the blue () ! yellow (+) axis, a* is the green () ! red (+) axis, and DE is the hue variation.
DE2 ¼ ðL Þ2 þ ða Þ2 þ ðb Þ2
3. Results and discussion The TG/DSC curves of the dried gel are shown in Fig. 2. The endothermic peak at 201 8C is due to the decomposition of nitrates, accompanied by a significant weight loss. The exothermic peak at 434 8C corresponds to the crystallization of the CoAl2O4 spinel. There is no distinct weight loss at temperature higher than 450 8C. Thermal properties of CoAl2O4 have been studied by TGA and DSC at different heating rates (Fig. 2). One can observe three thermal events: a glass transition (seen as an endothermic baseline shift) followed by two peaks, the position of maxima of the
corresponding peaks depends directly on heating rates and obeys the Kissinger formula [47]: b Ea 1 ln 2 ¼ C R Tm Tm where b, heating rate; Tm, the temperature of the peak’s maximum; R, gas constant; Ea, the activation energy of the crystallization and C, constant. From linear fits of the experimental data plotted in the coordinates ln(b/Tm2) vs. 1/Tm (Fig. 3), the activation energy of the crystallization was determined: 74.55 0.2 kJ mol1. Fig. 4 shows XRD patterns of the CoAl2O4 gels annealed at 400– 900 8C for 5 h. Several diffraction peaks are observed in the patterns and the position of these peaks closely resembles that of cubic CoAl2O4 spinel indicating the formation of cobalt aluminate phase. The presence of spinel phase is also confirmed by the appearance of the weak peak at about 2u = 498, which is characteristic for the CoAl2O4 phase. The intensity of the diffraction
I.S. Ahmed / Materials Research Bulletin 46 (2011) 2548–2553
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Table 1 XRD and TEM average particle sizes (nm) data for different CoAl2O4 systems at 900 8C. System
Egg white (A) Egg white + NH4OH (B) Egg white + NaOH (C)
Table 2 Colorimetric data for CoAl2O4 systems at 900 8C. System
Particle size (nm) X-ray
TEM
30.25 20.10 17.33
26.58 19.20 14.42
Egg white (A) Egg + NH4OH (B) Egg + NaOH (C)
CIE method L*
a*
b*
DE
40.20 70.71 74.32
36.57 17.18 7.27
18.90 34.74 40.36
34.80 79.11 69.84
peaks increases with increasing annealing temperature, which is associated with an increase in crystallinity. The average size of the nanocrystallites was also estimated by using the Debye–Scherrer formula: D¼
0:9l b cos uB
(1)
where l, b, uB are the X-ray wavelength, the full width at half maximum (FWHM) of the diffraction peak and the Bragg diffraction angle, respectively. The estimated crystallite size, from XRD data, was calculated to be 10.45–26.58 nm in the temperature range 400–900 8C and shown in Table 1. The IR spectra of the solid phase powders synthesized in range 400, 600 and 900 8C are shown in Fig. 5. The band at 3431 cm1, which was a strong absorption band, is attributed to the OH stretching mode. The band around 1620–1640 cm1 was assigned to the free water molecules. The vibration mode of the CO32 anion appeared around 1384 cm1 [43]. The band around 526–542 cm1 is corresponding to CoO4 and 679–693 cm1 is corresponding to AlO6 vibrations [32]. However, 1080 cm1 band corresponded to vibration mode of the CO32 [44,45]. Transmission electron micrograph of the powders indicated uniform and nonaggregated particles, as seen in Fig. 6. The particles had octahedral morphologies. Particle size distribution in TEM photograph was fairly narrow. Particle size from TEM observation was 10–26.58 nm. The crystallite size of the powders, calculated from Scherrer’s equation, using the broadening of the X-ray diffraction pattern was 10–26.58 nm. The result was in agreement with the TEM observation. Fig. 7 shows that the specific surface area was measured by the BET method which found in range 34.22– 188.55 m2 g1. The nanocrystalline synthesized pigment had normal spinel structure for both precursor obtained and fired samples due to transitions which means presence of few Co2+ cations occupied octahedral sites in the CoAl2O4 spinel lattice [29], while other Co2+ cations occupied in tetrahedral site. Diffuse reflectance spectra for egg white, egg white + NH4OH and egg white + NaOH systems which are represented as symbols A, B and C, respectively as presented in Table 2. Those systems show the appearance of good blue color pigments. Also, the colorimetric data presented in Table 2, reveal that the values of b* increase in the negative direction from A to C systems as well as L* values increase as result of increasing calcination temperatures and time. The decrease in values of a* means the higher intensity of blue color. However, increasing values of b* in negative direction is an indication for the appearance of blue color. Additionally, the decreasing in L* parameter from C to A systems are corresponding to reduce the lightness of sample. The values of a* increase in positive direction and b* increase in negative direction with the depth of blue color as result of calcination temperatures and the increasing of the base media. Colorimetric data show the high value of a* and b* and higher value of hue variation DE up to 900 8C for all samples under investigation. This means that the appearance of good color of blue nano ceramic pigment powders and a good color matching occur up to 900 8C. The UV–Vis spectrum was recorded as shown in Fig. 8 with triple three absorption peaks in visible region, exhibiting bluer
Fig. 8. UV–Vis absorbance spectra of pure single phase of CoAl2O4 nanocrystalline powder in different media: (i) egg white (ovalbumine), (ii) egg white (ovalbumine) + NH4OH, and (iii) egg white (ovalbumine) + NaOH as fuels.
color hues. Regarding the 900 8C calcined powders, they display more like a sky-blue hue. The as-synthesized homogeneous and size controllable nanocrystalline CoAl2O4 spinel pigment is expected to exhibit good transparency and tincting strength as a brilliant, lightfast and inert inorganic coloring agent in different matrices, its coloring performances will be reported later. The minimum absorption was around 500 nm, and the maximum absorption was around 650 nm. The particles size in range 10.45– 26.58 nm after the precursor was sintered at different calcinations temperature. In conclusion, nano-sized CoAl2O4 pigment was successfully synthesized through auto-ignited-gel combustion method. It was thermally stable, and had maximum absorption band around 600 nm. It can be a candidate as a high-grade pigment in color TV tubes and ceramic pigment industries. 4. Conclusions Pure and homogeneous nanocrystalline CoAl2O4 spinel powders have been directly prepared by the auto-ignited gel combustion method. The powder characteristics are strongly dependent on the amount of heat, or the flame temperature generated by fuel ratio used in the starting composition. The CoAl2O4 spinel particle size increases with the enhancement of calcination temperature which affects the color of the pigments on glassy ceramic coatings. The best results were obtained by calcining the spinel CoAl2O4 phase at 400– 900 8C using egg white (ovalbumine) as a fuel. References [1] I.H. Gul, A. Maqsood, M. Naeem, M. Naeem Ashiq, J. Alloys Compd. 507 (2010) 201. [2] F. Tielens, M. Calatayud, R. Franco, J.M. Recio, M. Perez-Ramirez, C. Minot, J. Phys. Chem. B 110 (2006) 988. [3] N. Ballarini, F. Cavani, S. Passeri, L. Pesaresi, A.F. Lee, K. Wilson, Appl. Catal. A: Gen. 366 (2009) 184.
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