- Email: [email protected]
Nanostructured cobalt oxides (Co3O4 and CoO) and metallic Co powders synthesized by the solution combustion method
Materials Research Bulletin 45 (2010) 672–676
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Nanostructured cobalt oxides (Co3O4 and CoO) and metallic Co powders synthesized by the solution combustion method J.C. Toniolo *, A.S. Takimi, C.P. Bergmann Department of Material Engineering, Federal University of Rio Grande do Sul, Av. Osvaldo Aranha 99, 705, Downtown, 90035190 Porto Alegre, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 February 2009 Received in revised form 9 February 2010 Accepted 3 March 2010 Available online 9 March 2010
The combustion synthesis technique using glycine and urea as fuels and cobalt nitrate as an oxidizer is capable of producing well-crystallized Co3O4, CoO, as well as metallic Co powders. An interpretation based on the thermodynamic viewpoint and the measurement of the combustion temperatures during the reactions occurring for various fuel-to-oxidant ratios was proposed for a study of the nature of combustion and its correlation with the characteristics of as-synthesized powders. The largest measured specific surface area of the powders was 36 m2/g at a 0.14 glycine-to-nitrate ratio. The crystallites were nano-sized ranging from approximately 23 to 90 nm. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics B. Chemical synthesis C. X-ray diffraction D. Thermodynamic properties
1. Introduction In recent years, attention has increasingly been focused on the development of nano-sized cobalt oxide powders (Co3O4 and CoO). These powders have high potential for use as catalysts [1], gas sensors [2], magnetic materials [3], rechargeable batteries [4], pigments for ceramics [5], solar energy absorbers [6], and electrochromic devices [7–9]. Cobalt (II, III) oxide (Co3O4) and cobalt (II) oxide (CoO) particles are largely prepared by a wide assortment of wet chemical techniques like freeze-drying, spray-pyrolysis [10], co-precipitation [11], solubility-controlled synthesis [12], sol–gel [13], and combustion synthesis. In contrast, there are not many studies of cobalt oxides particles obtained by the solution combustion synthesis (SCS) method. To the best of our knowledge, there are only a few reports on the formation of cobalt (II, III) oxide by the SCS method. Co3O4/g-Al2O3 catalysts were prepared by Zavyalova et al. [14] to investigate their catalytic performance during the total oxidation of methane. In the work cited, a series of chemical precursors such as cobalt acetate, cobalt acetate/cobalt nitrate mixtures, cobalt nitrate with citric acid, glycine, and urea fuel mixtures was employed. Additionally, Co3O4 incorporating Li ions was obtained [15]. At present, there are no studies on the fuel-to-oxidant ratio in the formation of cobalt oxides, and information concerning the
* Corresponding author. Tel.: +55 51 33083405; fax: +55 51 33083406. E-mail addresses: [email protected], [email protected] (J.C. Toniolo). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.03.001
process varies. Besides, the knowledge on the influence of these variables on the SCS method of preparation of these powders is scarce. For this reason, in the current work, we report the synthesis of cobalt oxides (Co3O4 and CoO) and metallic Co powders, using glycine and urea as fuels, and cobalt nitrate as oxidizer. Besides, different fuel-to-oxidant molar ratios were employed. The effect of the fuel in controlling particle size and microstructure of the product for combustion with different fuel-to-oxidant ratios was investigated here. 2. Experimental procedure Cobalt nitrate Co(NO3)36H2O (Aldrich, USA), glycine, and urea (Synth Quı´mica, Brazil) of (vendor specification) 98, 98.5, and 99.5% purities, respectively, were employed as starting materials. The components were carefully dissolved in deionized water, and the solution was transferred to a stainless steel container, which was uninterruptedly heated with a Bunsen-type burner without any prior thermal dehydration. After some time, the fuelnitrate solution turned into a transparent viscous gel, which is auto-ignited, giving voluminous foam as product of combustion. The flowchart of this process is shown in Fig. 1. Under continuous intense heating, the precursor mixture autoignited roughly in the 130–180 8C range and resulted in spontaneous ignition forming powder that contained crystalline phases. In fact, all the fuel-to-oxidant ratios evaluated, upon combustion, resulted in a grayish voluminous product that was identified by X-ray diffraction (XRD) to be crystalline phases of Co3O4, CoO or Co, which suggests that a complete combustion
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
673
The stoichiometric composition of the glycine–nitrate redox mixture demanded the presence of 1(10) + n(+9) = 0, or n = 1.11 mol in the reaction. Thus, the chemical precursors were combined in the molar proportion of 1:1.11 (or glycine-to-nitrate molar ratio of 0.56). For urea–nitrate it entailed 1(10) + n(+6) = 0, or n = 1.67 mol. Likewise, reactants were added in the molar proportion of 1:1.67 (or urea-to-nitrate molar ratio of 0.84). SCS reactions with glycine involving stoichiometric, fuel-lean, and fuel-rich combinations can be represented correspondingly, as follows: 1:0CoðNO3 Þ2 6H2 O þ 1:11NH2 CH2 COOH ! 0:33Co3 O4 þ 1:55N2 þ 2:22CO2 þ 8:77H2 O
ðEq: stoichiometric; 0:56Þ
1:0CoðNO3 Þ2 6H2 O þ 3:33NH2 CH2 COOH þ 5:07O2 ! 0:25Co3 O4 þ 0:25CoO þ 2:66N2 þ 6:66CO2 þ 14:32H2 O
ðEq: fuel-rich; 1:67Þ
1:0CoðNO3 Þ2 6H2 O þ 0:28NH2 CH2 COOH ! 0:33Co3 O4 þ 1:14N2 þ 0:56CO2 þ 6:70H2 O þ 1:70O2
Fig. 1. Flowchart for the preparation of Co and cobalt oxide powders.
probably occurred due to the characteristics of chemical reagents such as the fuels consumed and the oxidant nature of nitrate. The as-synthesized powders were characterized by XRD and scanning electron microscopy (SEM). The combustion reaction was investigated with thermocouple measurements. The temperatures of combustion were measured using an adjustable type K thermocouple. X-ray diffraction was carried out on the combustion-synthesized powders for phase characterization at a rate of 18/min and for crystallite size determination at a rate of 0.48/min, using Cu Ka radiation on a Philips X-ray diffractometer (model X’Pert MPD). The single-line integral-breadth method of crystallite size evaluation (designated as the single-line method [16]) was applied with the Pearson VII profile, assuming characteristic peaks intended for each studied phase, such as Co3O4 (2 2 0), CoO (2 0 0), and Co (1 1 1). Data were analyzed using the WinFit1 software (version 1.2.1). SEM micrographs were recorded on a Jeol (model JSM-5800) instrument after coating the samples with gold. 3. Results and discussion 3.1. Thermodynamic modeling Spontaneous redox reactions are generally exothermic in nature and often lead to explosions when materials are not carefully chosen. Both cobalt nitrate–glycine and cobalt nitrate– urea mixtures underwent a self-propagating reaction showing that they were capable of being heated without causing any explosive exothermic reaction. The initial composition of the solution containing cobalt nitrate and glycine/or urea was derived from the total oxidizing and reducing valences of the oxidizer and fuel using the concepts of propellant chemistry [17]. Carbon, hydrogen, and cobalt were considered as reducing species with corresponding valences of +4, +1, and +2, respectively. Oxygen was considered an oxidizing element with valence of 2, and nitrogen was considered to be 0. The total calculated valences of metal nitrates by arithmetic summation of the oxidizing and reducing valences was 10. The calculated valence of glycine was +9 and of urea +6.
ðEq: fuel-lean; 0:14Þ
The reactions with urea were calculated in the same manner as for glycine. To understand the variations of the adiabatic flame temperature with respect to the fuel-to-oxidant ratio, the following equation was employed [18]. T ¼ T0 þ
DHr0 DH0p Cp
where T0 is 25 8C, DHr0 , and DH0p are the enthalpies of formation of the reactants and products, respectively, and Cp is the heat capacity of the products at constant pressure. Available thermodynamic data in literature [19,20] for various reactants and products are presented in Table 1. Substituting the thermodynamic data from Table 1 in the equation, the enthalpy of combustion and the theoretical adiabatic flame temperatures can be calculated. Thus, all the equations can be simply expressed by some effects shown in Table 2. The thermodynamic calculations of the combustion reaction show that as the fuel-to-oxidant ratio increases the amount of gas produced, the enthalpy, and the adiabatic flame temperature also increase. 3.2. Phase formation and morphology In order to verify the effect of fuel type on combustion behavior, different fuels were employed. Proportions with urea or glycine led to Co3O4 formation in stoichiometric reactions. The glycine-rich reactions corroborate the simultaneous formation of two oxide phases Co3O4 and CoO that combine with Co in the 0.77 fuel-toTable 1 Relevant thermodynamics data. Compounda
DHf (kcal mol1)
Cp (cal mol1 K1)
Co(NO3)26H2O (c) NH2CH2COOH (c) CO(NH2)2 (c) Co3O4 (c) CoO (c) Co (c) CO2 (g) N2 (g) O2 (g) H2O (g) NO2 (g)
111.07 94.03 56.34 0.232 0.059 0.001 94.051 0 0 57.796 33.2
– – – 131.65 + 0.0660Tb 45.26 + 0.0107Tb 4.469 + 0.0299Tb 10.34 + 0.00274T 6.50 + 0.0010T 5.92 + 0.00367T 7.20 + 0.0036T –
a b
(c) = crystalline, (g) = gas. T = absolute temperature.
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
674
Table 2 Effects of fuel type and composition based on thermodynamics. Fuel-to-oxidant molar ratio (% fuel)
GLY 1.67 1.11 0.84 0.56 0.42 0.28 0.14
U (+200) (+100) (+50) (25) (50) (75)
2.51 1.67 1.26 0.84 0.63 0.42 0.21
(+200) (+100) (+50) (25) (50) (75)
Total number of mol of gases
DH8
GLY
U
GLY
U
GLY
23.67 18.10 15.30 12.55 12.03 11.06 9.42
27.04 20.36 17.04 13.68 12.46 11.44 9.55
1097 830 696 566 499 435 327
1069 812 688 557 495 430 327
2910 2816 2744 2653 2544 2335 1902
(kcal mol1)
Adiabatic flame temperature (8C) U 2850 2762 2716 2618 2524 2314 1902
oxidant molar ratio, as seen in Fig. 2. On the other hand, for urearich reactions the oxide phases were barely obtained (Fig. 3). Inasmuch as the fuel-to-oxidant ratio increases, variation in the relative concentrations of different chemical species of Co2+ also increases. This augment may be attributed mainly to the differences in flame temperature and as well as to the nature of reactions in the process of combustion. Besides, cobalt oxides are known to release oxygen upon heating, converting Co3O4 into CoO [21].
Fig. 4. Variations of specific surface area of powders for different proportions of glycine-nitrate and urea-nitrate. *The error estimate based on instrument accuracy was 0.2%.
Fig. 4 shows the variation of the specific surface area of powders in relation to the glycine-nitrate and urea-nitrate proportions. Lean-fuel L(75%) reactions in the 0.14 glycine- and 0.21 urea-tooxidant molar ratio yielded the highest specific surface area, while the fuel-rich reactions yielded the lowest.
Fig. 2. Overlay of X-ray diffraction patterns of as-synthesized powders with various glycine-to-oxidant molar ratios: (a) 0.14 fuel-lean (75%); (b) 0.56 stoichiometric; (c) 0.70 fuel-rich (+25%); (d) 0.77 fuel-rich (+37.5%); and (e) 1.67 fuel-rich (+200%).
Fig. 5. Effects of crystallite size and phase formations of cobaltic oxide, cobaltous oxide, and metallic cobalt for different proportions of glycine-nitrate and ureanitrate. *The error estimate based on instrument accuracy was 5.0%.
Fig. 3. Overlay of X-ray diffraction patterns of as-synthesized powders with various urea-to-oxidant molar ratios: (a) 0.21 fuel-lean (75%); (b) 0.84 stoichiometric; (c) 1.15 fuel-rich (+37.5%); (d) 1.26 fuel-rich (+50%); and (e) 2.51 fuel-rich (+200%).
Fig. 6. Effects of fuel-to-oxidant ratios (Co3O4 phase) and maximum temperatures of combustion for different proportions of glycine-nitrate and urea-nitrate.
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
675
Fig. 7. SEM micrographs of cobalt oxide ceramic powders produced from glycine-lean reaction (0.56 ratio): (a) at low-magnification (5000) and (b) at high-magnification (30,000).
There is a probable correlation between the increase in crystallite size and the reduction of specific surface area with regard to the fuel content. In fuel-rich reactions R(200%) in the 1.67 glycine-to-oxidant molar ratio, it was possible to observe the crystallite size contributions of CoO and metallic Co formation by means of the single-line technique (Fig. 5). Specifically, the metallic Co formation was not originated with urea reactions. The presence of metallic Co and CoO indicates that organic compounds reduce partly or completely cobalt oxide, which is formed by decomposition of cobalt nitrate. Moreover, some studies developed by Luisetto et al. [22] revealed a correlation of the reducibility behavior of Co with the particle size (Co3O4 ! CoO ! Co). Fig. 6 shows the formation of smaller crystallites for fuel-lean reactions which followed an expected reducing trend as soon as the temperature decreased for these proportions. The reactions showed a similar linear behavior in proportion to the fuel added; the temperature increases and there is a growth of crystallite sizes in accordance with an increase in the fuel-tooxidant ratio. The values obtained with thermocouple showed to be significantly different from the calculated adiabatic flame temperatures for maximum temperatures. These results are an approach due to a probable presence of inaccuracy measurement. Thermocouple has inertia and it is measured on the punctual form. Nevertheless, the measured flame temperatures are typically much lower than calculated values as a result of radioactive losses, incomplete combustion, and heating of air. As can be seen, the estimated flame temperature was systematically higher for glycine as a fuel compared to the case when urea was used, and this conclusion agrees well with the experiment. At the same time, the crystallite size was systematically larger for samples prepared from mixtures containing urea even though one should expect better crystallinity for the powders prepared at higher temperature. This result may indicate a strong chelation of Co by glycine, while in the case of urea one deals with the decomposition of plain cobalt nitrate in the presence of fuel without a substantial complexation. Although this transition metal is satisfactory and is mostly effectively complexed by the amino groups and carboxylic acid of the glycine, it is possible that a steric hindrance occurs caused by the zwiterionic character (amino and carboxylic acid presence) of the glycine molecule that could serve as a barrier to the growth of the crystallites. Another factor that can explain the reason for the lower formation of crystallites with glycine than with urea is the vigorous activity generated by the two amino ions from urea,
rendering it more reactive than the amino and carboxylic acid pairs of glycine, in spite of the combustion reactions with urea yield lower maximum temperatures. The SEM morphology of the agglomerates of the cobalt oxide is shown in Fig. 7. It exhibited foamy agglomerated particles with a wide distribution (low-magnification) and presence of large voids in its structure (high-magnification). The formation of these features is attributed to the large volume of gas evolved during combustion. No significant differences came out of the fuel-to-oxidant ratios from all morphologies examined with the SEM. 4. Conclusions Glycine-nitrate and urea-nitrate combustion syntheses present a remarkable potential for producing Co and cobalt oxide powders. The metallic Co powder was formed only in fuel-rich reactions with glycine, while the cobaltous oxide phase was formed in fuelrich reactions of both fuels. This difference may be attributed to the chemical nature of the fuels. The smallest crystallite size obtained was 23 nm and the highest specific surface area was 36 g/m2 under fuel-lean reactions of glycine. Thermodynamic modeling of the combustion reaction shows that when the fuel-to-oxidant ratio increases the amount of gas produced and the temperature of the adiabatic flame also increase. The same type of increment was obtained in the actual flame temperature and crystallite size. Acknowledgments Thanks are due to undergraduate student Frederico Wallauer for combustion synthesis measurements and graduate student Cibele M. Halmenschlager for providing support in scanning electron microscopy. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
J.H. Choy, et al. Chem. Mater. 14 (2002) 3823–3828. M. Ando, T. Kobayashi, S. Iijima, M. Haruta, J. Mater. Chem. 7 (9) (1997) 1779. M. Verelst, et al. Chem. Mater. 11 (1999) 2702–2708. S.A. Needham, Development of Advanced Electrode Materials for Use in Rechargeable Lithium Batteries, Doctoral Thesis, University of Wollongong, 2007. J.D. Albani, A.C. Antunes, S. Cava, A.S. Pianaro, S.R.M. Antunes, 3rd Brazil MRS Meeting, 2004, H-O0222. E. Barrera, I. Gonza´lez, T. Viveros, Sol. Energy Mater. 51 (1998) 69–82. S. Weichel, P.J. Moller, Surf. Sci. 399 (1998) 219. C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. F. Svegl, B. Orel, M.G. Hutchins, K. Kalcher, J. Electrochem. Soc. 143 (5) (1996) 1532–1539.
676
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
[10] Z.W. Zhao, K. Konstantinov, L. Yuan, H.K. Liu, S.X. Dou, J. Nanosci. Nanotechnol. 7 (4) (2004) 861–866. [11] J.B. Wu, J.P. Tu, X.L. Wang, W.K. Zhang, Int. J. Hydro. Energy 32 (5) (2007) 606–610. [12] T. He, D. Chen, X. Jiao, Y. Wang, Y. Duan, Chem. Mater. 17 (2005) 4023–4030. [13] N.R.E. Radwan, M.S. El-Shall, H.M.A. Hassan, Appl. Catal. A 331 (2007) 8–18. [14] U. Zavyalova, P. Scholz, B. Ondruschka, Appl. Catal. A 323 (2007) 226–233. [15] F. Gu, C. Li, Y. Hu, L.J. Zhang, J. Cryst. Growth 304 (2007) 369–373. [16] R. Delhez, T.H. Keijser, E.J. Mittemeijer, Fresen. Z. Anal. Chem. 312 (1982) 1–16.
[17] V.C. Sousa, A.M. Segada˜es, M.R. Morelli, R.H.G.A. Kiminami, Int. J. Inorg. Mater. 1 (1999) 235–241. [18] J.C. Toniolo, M.D. Lima, A.S. Takimi, C.P. Bergmann, Mater. Res. Bull. 3 40 (2005) 561–571. [19] R.H. Perry, C.H. Chilton, Chemical Engineers Handbook, 5th ed., McGraw-Hill, New York, 1973, p. 197. [20] O.V. Dorofeeva, O.N. Ryzhova, J. Chem. Thermodyn. 41 (2009) 433–438. [21] R. Flamand, High Temp.–High Pres. 3 (16) (1984) 323–328. [22] I. Luisetto, F. Pepe, E. Bemporad, J. Nanopart. Res. 10 (2008) 59–67.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Nanostructured cobalt oxides (Co3O4 and CoO) and metallic Co powders synthesized by the solution combustion method J.C. Toniolo *, A.S. Takimi, C.P. Bergmann Department of Material Engineering, Federal University of Rio Grande do Sul, Av. Osvaldo Aranha 99, 705, Downtown, 90035190 Porto Alegre, RS, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 February 2009 Received in revised form 9 February 2010 Accepted 3 March 2010 Available online 9 March 2010
The combustion synthesis technique using glycine and urea as fuels and cobalt nitrate as an oxidizer is capable of producing well-crystallized Co3O4, CoO, as well as metallic Co powders. An interpretation based on the thermodynamic viewpoint and the measurement of the combustion temperatures during the reactions occurring for various fuel-to-oxidant ratios was proposed for a study of the nature of combustion and its correlation with the characteristics of as-synthesized powders. The largest measured specific surface area of the powders was 36 m2/g at a 0.14 glycine-to-nitrate ratio. The crystallites were nano-sized ranging from approximately 23 to 90 nm. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics B. Chemical synthesis C. X-ray diffraction D. Thermodynamic properties
1. Introduction In recent years, attention has increasingly been focused on the development of nano-sized cobalt oxide powders (Co3O4 and CoO). These powders have high potential for use as catalysts [1], gas sensors [2], magnetic materials [3], rechargeable batteries [4], pigments for ceramics [5], solar energy absorbers [6], and electrochromic devices [7–9]. Cobalt (II, III) oxide (Co3O4) and cobalt (II) oxide (CoO) particles are largely prepared by a wide assortment of wet chemical techniques like freeze-drying, spray-pyrolysis [10], co-precipitation [11], solubility-controlled synthesis [12], sol–gel [13], and combustion synthesis. In contrast, there are not many studies of cobalt oxides particles obtained by the solution combustion synthesis (SCS) method. To the best of our knowledge, there are only a few reports on the formation of cobalt (II, III) oxide by the SCS method. Co3O4/g-Al2O3 catalysts were prepared by Zavyalova et al. [14] to investigate their catalytic performance during the total oxidation of methane. In the work cited, a series of chemical precursors such as cobalt acetate, cobalt acetate/cobalt nitrate mixtures, cobalt nitrate with citric acid, glycine, and urea fuel mixtures was employed. Additionally, Co3O4 incorporating Li ions was obtained [15]. At present, there are no studies on the fuel-to-oxidant ratio in the formation of cobalt oxides, and information concerning the
* Corresponding author. Tel.: +55 51 33083405; fax: +55 51 33083406. E-mail addresses: [email protected], [email protected] (J.C. Toniolo). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.03.001
process varies. Besides, the knowledge on the influence of these variables on the SCS method of preparation of these powders is scarce. For this reason, in the current work, we report the synthesis of cobalt oxides (Co3O4 and CoO) and metallic Co powders, using glycine and urea as fuels, and cobalt nitrate as oxidizer. Besides, different fuel-to-oxidant molar ratios were employed. The effect of the fuel in controlling particle size and microstructure of the product for combustion with different fuel-to-oxidant ratios was investigated here. 2. Experimental procedure Cobalt nitrate Co(NO3)36H2O (Aldrich, USA), glycine, and urea (Synth Quı´mica, Brazil) of (vendor specification) 98, 98.5, and 99.5% purities, respectively, were employed as starting materials. The components were carefully dissolved in deionized water, and the solution was transferred to a stainless steel container, which was uninterruptedly heated with a Bunsen-type burner without any prior thermal dehydration. After some time, the fuelnitrate solution turned into a transparent viscous gel, which is auto-ignited, giving voluminous foam as product of combustion. The flowchart of this process is shown in Fig. 1. Under continuous intense heating, the precursor mixture autoignited roughly in the 130–180 8C range and resulted in spontaneous ignition forming powder that contained crystalline phases. In fact, all the fuel-to-oxidant ratios evaluated, upon combustion, resulted in a grayish voluminous product that was identified by X-ray diffraction (XRD) to be crystalline phases of Co3O4, CoO or Co, which suggests that a complete combustion
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
673
The stoichiometric composition of the glycine–nitrate redox mixture demanded the presence of 1(10) + n(+9) = 0, or n = 1.11 mol in the reaction. Thus, the chemical precursors were combined in the molar proportion of 1:1.11 (or glycine-to-nitrate molar ratio of 0.56). For urea–nitrate it entailed 1(10) + n(+6) = 0, or n = 1.67 mol. Likewise, reactants were added in the molar proportion of 1:1.67 (or urea-to-nitrate molar ratio of 0.84). SCS reactions with glycine involving stoichiometric, fuel-lean, and fuel-rich combinations can be represented correspondingly, as follows: 1:0CoðNO3 Þ2 6H2 O þ 1:11NH2 CH2 COOH ! 0:33Co3 O4 þ 1:55N2 þ 2:22CO2 þ 8:77H2 O
ðEq: stoichiometric; 0:56Þ
1:0CoðNO3 Þ2 6H2 O þ 3:33NH2 CH2 COOH þ 5:07O2 ! 0:25Co3 O4 þ 0:25CoO þ 2:66N2 þ 6:66CO2 þ 14:32H2 O
ðEq: fuel-rich; 1:67Þ
1:0CoðNO3 Þ2 6H2 O þ 0:28NH2 CH2 COOH ! 0:33Co3 O4 þ 1:14N2 þ 0:56CO2 þ 6:70H2 O þ 1:70O2
Fig. 1. Flowchart for the preparation of Co and cobalt oxide powders.
probably occurred due to the characteristics of chemical reagents such as the fuels consumed and the oxidant nature of nitrate. The as-synthesized powders were characterized by XRD and scanning electron microscopy (SEM). The combustion reaction was investigated with thermocouple measurements. The temperatures of combustion were measured using an adjustable type K thermocouple. X-ray diffraction was carried out on the combustion-synthesized powders for phase characterization at a rate of 18/min and for crystallite size determination at a rate of 0.48/min, using Cu Ka radiation on a Philips X-ray diffractometer (model X’Pert MPD). The single-line integral-breadth method of crystallite size evaluation (designated as the single-line method [16]) was applied with the Pearson VII profile, assuming characteristic peaks intended for each studied phase, such as Co3O4 (2 2 0), CoO (2 0 0), and Co (1 1 1). Data were analyzed using the WinFit1 software (version 1.2.1). SEM micrographs were recorded on a Jeol (model JSM-5800) instrument after coating the samples with gold. 3. Results and discussion 3.1. Thermodynamic modeling Spontaneous redox reactions are generally exothermic in nature and often lead to explosions when materials are not carefully chosen. Both cobalt nitrate–glycine and cobalt nitrate– urea mixtures underwent a self-propagating reaction showing that they were capable of being heated without causing any explosive exothermic reaction. The initial composition of the solution containing cobalt nitrate and glycine/or urea was derived from the total oxidizing and reducing valences of the oxidizer and fuel using the concepts of propellant chemistry [17]. Carbon, hydrogen, and cobalt were considered as reducing species with corresponding valences of +4, +1, and +2, respectively. Oxygen was considered an oxidizing element with valence of 2, and nitrogen was considered to be 0. The total calculated valences of metal nitrates by arithmetic summation of the oxidizing and reducing valences was 10. The calculated valence of glycine was +9 and of urea +6.
ðEq: fuel-lean; 0:14Þ
The reactions with urea were calculated in the same manner as for glycine. To understand the variations of the adiabatic flame temperature with respect to the fuel-to-oxidant ratio, the following equation was employed [18]. T ¼ T0 þ
DHr0 DH0p Cp
where T0 is 25 8C, DHr0 , and DH0p are the enthalpies of formation of the reactants and products, respectively, and Cp is the heat capacity of the products at constant pressure. Available thermodynamic data in literature [19,20] for various reactants and products are presented in Table 1. Substituting the thermodynamic data from Table 1 in the equation, the enthalpy of combustion and the theoretical adiabatic flame temperatures can be calculated. Thus, all the equations can be simply expressed by some effects shown in Table 2. The thermodynamic calculations of the combustion reaction show that as the fuel-to-oxidant ratio increases the amount of gas produced, the enthalpy, and the adiabatic flame temperature also increase. 3.2. Phase formation and morphology In order to verify the effect of fuel type on combustion behavior, different fuels were employed. Proportions with urea or glycine led to Co3O4 formation in stoichiometric reactions. The glycine-rich reactions corroborate the simultaneous formation of two oxide phases Co3O4 and CoO that combine with Co in the 0.77 fuel-toTable 1 Relevant thermodynamics data. Compounda
DHf (kcal mol1)
Cp (cal mol1 K1)
Co(NO3)26H2O (c) NH2CH2COOH (c) CO(NH2)2 (c) Co3O4 (c) CoO (c) Co (c) CO2 (g) N2 (g) O2 (g) H2O (g) NO2 (g)
111.07 94.03 56.34 0.232 0.059 0.001 94.051 0 0 57.796 33.2
– – – 131.65 + 0.0660Tb 45.26 + 0.0107Tb 4.469 + 0.0299Tb 10.34 + 0.00274T 6.50 + 0.0010T 5.92 + 0.00367T 7.20 + 0.0036T –
a b
(c) = crystalline, (g) = gas. T = absolute temperature.
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
674
Table 2 Effects of fuel type and composition based on thermodynamics. Fuel-to-oxidant molar ratio (% fuel)
GLY 1.67 1.11 0.84 0.56 0.42 0.28 0.14
U (+200) (+100) (+50) (25) (50) (75)
2.51 1.67 1.26 0.84 0.63 0.42 0.21
(+200) (+100) (+50) (25) (50) (75)
Total number of mol of gases
DH8
GLY
U
GLY
U
GLY
23.67 18.10 15.30 12.55 12.03 11.06 9.42
27.04 20.36 17.04 13.68 12.46 11.44 9.55
1097 830 696 566 499 435 327
1069 812 688 557 495 430 327
2910 2816 2744 2653 2544 2335 1902
(kcal mol1)
Adiabatic flame temperature (8C) U 2850 2762 2716 2618 2524 2314 1902
oxidant molar ratio, as seen in Fig. 2. On the other hand, for urearich reactions the oxide phases were barely obtained (Fig. 3). Inasmuch as the fuel-to-oxidant ratio increases, variation in the relative concentrations of different chemical species of Co2+ also increases. This augment may be attributed mainly to the differences in flame temperature and as well as to the nature of reactions in the process of combustion. Besides, cobalt oxides are known to release oxygen upon heating, converting Co3O4 into CoO [21].
Fig. 4. Variations of specific surface area of powders for different proportions of glycine-nitrate and urea-nitrate. *The error estimate based on instrument accuracy was 0.2%.
Fig. 4 shows the variation of the specific surface area of powders in relation to the glycine-nitrate and urea-nitrate proportions. Lean-fuel L(75%) reactions in the 0.14 glycine- and 0.21 urea-tooxidant molar ratio yielded the highest specific surface area, while the fuel-rich reactions yielded the lowest.
Fig. 2. Overlay of X-ray diffraction patterns of as-synthesized powders with various glycine-to-oxidant molar ratios: (a) 0.14 fuel-lean (75%); (b) 0.56 stoichiometric; (c) 0.70 fuel-rich (+25%); (d) 0.77 fuel-rich (+37.5%); and (e) 1.67 fuel-rich (+200%).
Fig. 5. Effects of crystallite size and phase formations of cobaltic oxide, cobaltous oxide, and metallic cobalt for different proportions of glycine-nitrate and ureanitrate. *The error estimate based on instrument accuracy was 5.0%.
Fig. 3. Overlay of X-ray diffraction patterns of as-synthesized powders with various urea-to-oxidant molar ratios: (a) 0.21 fuel-lean (75%); (b) 0.84 stoichiometric; (c) 1.15 fuel-rich (+37.5%); (d) 1.26 fuel-rich (+50%); and (e) 2.51 fuel-rich (+200%).
Fig. 6. Effects of fuel-to-oxidant ratios (Co3O4 phase) and maximum temperatures of combustion for different proportions of glycine-nitrate and urea-nitrate.
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
675
Fig. 7. SEM micrographs of cobalt oxide ceramic powders produced from glycine-lean reaction (0.56 ratio): (a) at low-magnification (5000) and (b) at high-magnification (30,000).
There is a probable correlation between the increase in crystallite size and the reduction of specific surface area with regard to the fuel content. In fuel-rich reactions R(200%) in the 1.67 glycine-to-oxidant molar ratio, it was possible to observe the crystallite size contributions of CoO and metallic Co formation by means of the single-line technique (Fig. 5). Specifically, the metallic Co formation was not originated with urea reactions. The presence of metallic Co and CoO indicates that organic compounds reduce partly or completely cobalt oxide, which is formed by decomposition of cobalt nitrate. Moreover, some studies developed by Luisetto et al. [22] revealed a correlation of the reducibility behavior of Co with the particle size (Co3O4 ! CoO ! Co). Fig. 6 shows the formation of smaller crystallites for fuel-lean reactions which followed an expected reducing trend as soon as the temperature decreased for these proportions. The reactions showed a similar linear behavior in proportion to the fuel added; the temperature increases and there is a growth of crystallite sizes in accordance with an increase in the fuel-tooxidant ratio. The values obtained with thermocouple showed to be significantly different from the calculated adiabatic flame temperatures for maximum temperatures. These results are an approach due to a probable presence of inaccuracy measurement. Thermocouple has inertia and it is measured on the punctual form. Nevertheless, the measured flame temperatures are typically much lower than calculated values as a result of radioactive losses, incomplete combustion, and heating of air. As can be seen, the estimated flame temperature was systematically higher for glycine as a fuel compared to the case when urea was used, and this conclusion agrees well with the experiment. At the same time, the crystallite size was systematically larger for samples prepared from mixtures containing urea even though one should expect better crystallinity for the powders prepared at higher temperature. This result may indicate a strong chelation of Co by glycine, while in the case of urea one deals with the decomposition of plain cobalt nitrate in the presence of fuel without a substantial complexation. Although this transition metal is satisfactory and is mostly effectively complexed by the amino groups and carboxylic acid of the glycine, it is possible that a steric hindrance occurs caused by the zwiterionic character (amino and carboxylic acid presence) of the glycine molecule that could serve as a barrier to the growth of the crystallites. Another factor that can explain the reason for the lower formation of crystallites with glycine than with urea is the vigorous activity generated by the two amino ions from urea,
rendering it more reactive than the amino and carboxylic acid pairs of glycine, in spite of the combustion reactions with urea yield lower maximum temperatures. The SEM morphology of the agglomerates of the cobalt oxide is shown in Fig. 7. It exhibited foamy agglomerated particles with a wide distribution (low-magnification) and presence of large voids in its structure (high-magnification). The formation of these features is attributed to the large volume of gas evolved during combustion. No significant differences came out of the fuel-to-oxidant ratios from all morphologies examined with the SEM. 4. Conclusions Glycine-nitrate and urea-nitrate combustion syntheses present a remarkable potential for producing Co and cobalt oxide powders. The metallic Co powder was formed only in fuel-rich reactions with glycine, while the cobaltous oxide phase was formed in fuelrich reactions of both fuels. This difference may be attributed to the chemical nature of the fuels. The smallest crystallite size obtained was 23 nm and the highest specific surface area was 36 g/m2 under fuel-lean reactions of glycine. Thermodynamic modeling of the combustion reaction shows that when the fuel-to-oxidant ratio increases the amount of gas produced and the temperature of the adiabatic flame also increase. The same type of increment was obtained in the actual flame temperature and crystallite size. Acknowledgments Thanks are due to undergraduate student Frederico Wallauer for combustion synthesis measurements and graduate student Cibele M. Halmenschlager for providing support in scanning electron microscopy. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
J.H. Choy, et al. Chem. Mater. 14 (2002) 3823–3828. M. Ando, T. Kobayashi, S. Iijima, M. Haruta, J. Mater. Chem. 7 (9) (1997) 1779. M. Verelst, et al. Chem. Mater. 11 (1999) 2702–2708. S.A. Needham, Development of Advanced Electrode Materials for Use in Rechargeable Lithium Batteries, Doctoral Thesis, University of Wollongong, 2007. J.D. Albani, A.C. Antunes, S. Cava, A.S. Pianaro, S.R.M. Antunes, 3rd Brazil MRS Meeting, 2004, H-O0222. E. Barrera, I. Gonza´lez, T. Viveros, Sol. Energy Mater. 51 (1998) 69–82. S. Weichel, P.J. Moller, Surf. Sci. 399 (1998) 219. C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. F. Svegl, B. Orel, M.G. Hutchins, K. Kalcher, J. Electrochem. Soc. 143 (5) (1996) 1532–1539.
676
J.C. Toniolo et al. / Materials Research Bulletin 45 (2010) 672–676
[10] Z.W. Zhao, K. Konstantinov, L. Yuan, H.K. Liu, S.X. Dou, J. Nanosci. Nanotechnol. 7 (4) (2004) 861–866. [11] J.B. Wu, J.P. Tu, X.L. Wang, W.K. Zhang, Int. J. Hydro. Energy 32 (5) (2007) 606–610. [12] T. He, D. Chen, X. Jiao, Y. Wang, Y. Duan, Chem. Mater. 17 (2005) 4023–4030. [13] N.R.E. Radwan, M.S. El-Shall, H.M.A. Hassan, Appl. Catal. A 331 (2007) 8–18. [14] U. Zavyalova, P. Scholz, B. Ondruschka, Appl. Catal. A 323 (2007) 226–233. [15] F. Gu, C. Li, Y. Hu, L.J. Zhang, J. Cryst. Growth 304 (2007) 369–373. [16] R. Delhez, T.H. Keijser, E.J. Mittemeijer, Fresen. Z. Anal. Chem. 312 (1982) 1–16.
[17] V.C. Sousa, A.M. Segada˜es, M.R. Morelli, R.H.G.A. Kiminami, Int. J. Inorg. Mater. 1 (1999) 235–241. [18] J.C. Toniolo, M.D. Lima, A.S. Takimi, C.P. Bergmann, Mater. Res. Bull. 3 40 (2005) 561–571. [19] R.H. Perry, C.H. Chilton, Chemical Engineers Handbook, 5th ed., McGraw-Hill, New York, 1973, p. 197. [20] O.V. Dorofeeva, O.N. Ryzhova, J. Chem. Thermodyn. 41 (2009) 433–438. [21] R. Flamand, High Temp.–High Pres. 3 (16) (1984) 323–328. [22] I. Luisetto, F. Pepe, E. Bemporad, J. Nanopart. Res. 10 (2008) 59–67.