Effect of citric acid concentration as emulsifier on perovskite phase formation of nano-sized SrMnO 3 and SrCoO 3 samples

Transcription

1 Cryst. Res. Technol. 45, No. 10, (2010) / DOI /crat Effect of citric acid concentration as emulsifier on perovskite phase formation of nano-sized SrMnO 3 and SrCoO 3 samples M. Khazaei 1, A. Malekzadeh* 1, F. Amini 1, Y. Mortazavi 2, and A. Khodadadi 2 1 School of Chemistry, Damghan University, Damghan, Iran 2 Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, University of Tehran, Tehran, Iran Received 2 May 2010, revised 17 July 2010, accepted 20 July 2010 Published online 20 August 2010 Key words mixed oxides, SrMnO 3, SrCoO 3, nano perovskite, citric acid, emulsifier. In this study the effects of citric acid concentration, used as organic emulsifier, on the perovskite phase formation of the nano strontium manganite or cobaltite samples were studied. Stoichiometric perovskites in the absence and presence of citric acid were prepared by drying a solution containing molar ratio of Sr(NO 3 ) 2 /Mn(NO 3 ) 2 4H 2 O and Sr(NO 3 ) 2 /Co(NO 3 ) 2 6H 2 O = 1 followed by calcination at 900 C for 5 h. Citric acid concentration, selected to be 0.0, 0.3, 0.6, 1.0, 1.3, 2.5 and 5 times of the total number of moles of the nitrate ions. The results revealed that increase in the citric acid concentration, larger than number of moles of the nitrate ions equivalent, deteriorates the perovskite phase formation. Instead, a new phase of carbonates and metal oxides are appeared. 1 Introduction Theoretical and experiment studies have been devoted to studying strontium manganites and cobaltite with the formulas SrMnO 3 and SrCoO 3 during the last several years [1,2]. The SrMnO 3 is known as a G type antiferromagnetic insulator. The properties of the SrMnO 3 sample such as oxygen nonstoichiometry, electronic properties, thermochromism and the others are attributed to the configuration of the skeletal structure that is consisted of a three dimensional network of MnO 6 octahedra [3-5]. The conventional solid state reaction method which has been mainly used for preparation of SrMnO 3 manganite needs a temperature about 1400 C. However, the reaction leading to the formation of perovskite phase is not complete [6,7]. Disadvantages of the soft chemical methods such as sol gel [8,9] or co precipitation [10] for the synthesis of inorganic solids are cited to be; high price of raw material, complex operating procedure, etc. The hydrothermal conditions, as a soft chemical method for preparing high quality ceramic powders, where an aqueous reaction mixture is heated above 100 C in a sealed reaction container, permits a wider range of reaction conditions to be applied for the synthesis of oxides [8-10]. Highly crystallized powders with high purity, controlled morphology and size distribution without high temperature treatment are obtained via hydrothermal synthesis route by adjusting concentration of precursors, reaction time and temperature [11-15]. Organic additives, such as citric acid, allow the formation of amorphous organates of metals with a wide flexibility of composition [16,17]. Thermal decomposition of these organates leads to mixed oxides or solid solutions with high homogeneity. Moreover, organic additives are used as emulsifiers. When citric acid is used, the preparation method is known as citrate method. In this method formation of citrate complexes effectively keep the constituent metal cations dispersed homogeneously and thus make the formation of perovskite type oxides easier. The main advantage of the citrate method is lowering the calcination temperature. For instance, the lowest temperature for perovskite phase formation was reported to be 850 C in the acetate process. However, crystallized perovskites with large specific surface areas are synthesized at temperatures as low as C by calcining amorphous citrate complex precursors, i.e. a calcination temperature C lower than of that acetate process. Attempts have not been made to investigate the effects of emulsifier concentration on structure and extent of crystallinity of perovskites. In this investigation the effects of citric acid concentration on formation of SrMnO 3 and SrCoO 3 perovskite phase via citrate method are explored. * Corresponding author: malekzadeh@dubs.ac.ir

2 Cryst. Res. Technol. 45, No. 10 (2010) Experimental Samples of SrMnO 3 and SrCoO 3 were prepared by the so called citrate method similar to the recipe reported by L. Abadian et al. [18]. A solution containing mol Sr(NO 3 ) 2, mol Mn(NO 3 ) 2 4H 2 O or Co(NO 3 ) 2 6H 2 O and 0.0, , , , , or mol of citric acid in 10 ml distillated water were used for preparation of SrMnO 3 or SrCoO 3 samples, respectively. Solutions were evaporated at 60 C overnight. Thus obtained viscous material was subsequently dried at 80 C overnight. The resulting pink spongy and friable materials were powdered and were kept at 150 C overnight. The dark brown spongy and friable material was powdered and calcined at 900 C for 5 h. X ray diffraction patterns of the freshly calcined samples were recorded in a Bruker AXS diffractometer D8 ADVANCE with Cu Kα radiation filtered by a nickel monochromator and operated at 40 kv and 30 ma. Diffraction patterns were recorded in the range 2θ = The FTIR spectra of samples were recorded by mixing a small amount of each sample with potassium bromide (KBr) in a PERKIN ELMER FTIR spectrometer. Diluted samples were pelletized into 13 mm diameter thin pellets. Each pellet was then located in a plate holder inside the FTIR unit. The spectra were obtained over a wave number range of cm -1. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis were done by a Philips XL30 instrument to investigate the particle size and morphology as well as the elemental analysis of the samples. 3 Results and discussion Fourier transformed infrared (FTIR) spectroscopy results of fresh SrCoO 3 and SrMnO 3, prepared by addition of different amounts of citric acid as emulsifier, are shown in figure 1. A completely different spectrum is observed for samples prepared by citric acid addition equal to 2.5 and 5.0 times of total number of moles of nitrate ions. The results are clearly indicative of formation of a segregated new phase. Fig. 1 FTIR spectrum of SrCoO 3 (A) and SrMnO 3 (B) samples, prepared by addition of different amounts of citric acid as emulsifier. (a) 0.0, (b) 0.3, (c) 0.6, (d) 1.0, (e) 1.3, (f) 2.5 and (g) 5.0 times of the total number of moles of nitrate ions. In figure 1, one notes the presence of a segregated phase in form of carbonate similar to the FTIR spectra of strontium carbonate, reported in literature [19]. The bands at 703 and 857 cm -1, observed in f and g spectrum of figure 1, are assigned to correspond to the in plane bending and out of plane bending modes of carbonate anion. The peak at 1450 cm -1 was assigned to the asymmetrical lengthening of O C O vibration. The most important band of the perovskite structure, which is clearly seen at 578 cm -1 in SrCoO 3 sample, corresponds to the asymmetrical lengthening of the B O bond of the octahedrons BO 6 of the ABO 3 perovskite structure [20]. The appearance of a shoulder or widening of this band indicates a structure with lower symmetry. This peak is appeared at 520 cm -1 in SrMnO 3 sample and is attributed to the Mn O vibrations of MnO 6 octahedra. Similar peak is also reported in MnO 2 crystals [21,22]. Presence of the other bands in SrCoO 3

3 1066 M. Khazaei et al.: Perovskite phase formation of nano-sized SrMnO 3 and SrCoO 3 and SrMnO 3 samples is related to the segregated phases of Co 3 O 4, Mn 3 O 4 and SrO [23-25]. The results show more affinity of Sr cation to cobalt cation than to the manganese cation. The vibration frequencies in the infrared region are necessary for determination of crystalline structures. FTIR spectra of the as grown samples, presented in figure 1, show that increase in citric acid concentration deteriorate the crystalline perovskite phase formation and segregate the phases in form of carbonate and metal oxide. Figure 2 displays the XRD patterns of SrMnO 3 and SrCoO 3 samples, prepared by various amount of citric acid. Results are consistent with those observed in figure 1 and show the formation of strontium carbonate and metal oxide phases by an increase in citric acid concentration, accompanied by a decrease in the perovskite phase. A single hexagonal structure is determined for the formed perovskite phase in both of the SrMnO 3 and SrCoO 3 samples after 5 h calcination at 900 C [20]. Fig. 2 XRD patterns of SrCoO 3 (A) and SrMnO 3 (B) samples, prepared by addition of different amounts of citric acid as emulsifier. (a) 0.0, (b) 0.3, (c) 0.6, (d) 1.0, (e) 1.3, (f) 2.5 and (g) 5.0 times of total number of moles of nitrate ions. The SEM micrographs of the fresh SrMnO 3 and SrCoO 3 samples are presented in figures 3 and 4, respectively. The energy dispersive spectroscopy (EDS) spectra are also included to show the elemental composition of the prepared samples. The SEM micrographs indicate that the samples prepared by addition of citric acid equal to 1.0 times of total moles of nitrate ions, have large particles. However, the samples having citric acid equal to 5.0 times of total moles of nitrate ions have smaller particles. In fact presence of larger amounts of citric acid prevents agglomeration of particles. It is expected that large amount of gases is evolved during calcination at 900 C when citric acid decomposes. The gas evolvation, however, dose not makes the samples porous. The average crystallite size of SrCoO 3 and SrMnO 3 samples, estimated by XRD data using Scherrer and Williamson Hall equations, is shown in table 1. a simple relationship between crystallite size D and peak broadening is offered by Scherrer equation [26,27]: D = Kλ / Bcosθ, (1) where K is known as Scherrer constant whose value is approximately 0.9, λ is the wavelength of X ray, θ is the Bragg angle in radians and B is the width, in radians, of the peak due to size effects and was defined as the width at half maximum of the peak (FWHM). For an X ray profile, B is calculated as B = (B obs B s ), where B obs is the width at half maximum measured for the specimen and B s is the width at half maximum of an standard sample, defined as width due to instrument. Contribution of strain to the diffraction line broadening, however, is not negligible. The effect of the size and strain on peak broadening has been separated and developed by Williamson and Hall [23]. It is now known as the Williamson Hall plot. The Williamson Hall equation is expressed as Kλ Bcosθ = + 2εsin θ, (2) D where ε is the lattice strain and other symbols have been defined above. In this equation the width B, in radians, of the peak is due to size and strain effects, i.e. B = B size+strain and is calculated as Scherrer equation. A linear

4 Cryst. Res. Technol. 45, No. 10 (2010) 1067 extrapolation to the plot of Bcosθ against 2sinθ gives the particle size in form of (Kλ/D) and the slope gives the strain ε. Although the particle size changes, the crystallite size of nano SrCoO 3 or SrMnO 3 samples is observed to be independent of the emulsifier concentration (see figures 3 and 4 and table 1). Table 1 Crystallite size of SrMnO 3 and SrCoO 3 samples, calculated using Scherrer (S a ) and Williamson Hall (WH b ) equations c. (a: S is abbrivated for Scherrer, b: WH is abbrivated for Williamson Hall, c: Calculated using peaks positioned at 2θ = 18, 27, 33, 35, 43, 44, 55, 59, 60 and 70 o, d: E. C. is abbrivated for Emolsifier concentration, e: calculated using FWHM, obtained directly from the instrument, f: calculated using FWHM, obtained by the lorentzian fitting of the XRD peaks, g: calculated using FWHM, obtained by the gauss fitting of the XRD peaks) SrMnO 3 SrCoO 3 Average crystallite size (nm) Average crystallite size (nm) E. C. d S a, e S a, f S a, g WH b, e WH b, f WH b, g S a, e S a, f S a, g WH b, e WH b, f WH b, g Fig. 3 Energy dispersive spectroscopy (EDS) analysis spectra and SEM micrographs of synthesized SrCoO 3 samples, prepared by addition of citric acid equal to 1 (A) and 5 (B) times of total number of moles of nitrate ions. Fig. 4 Energy dispersive spectroscopy (EDS) analysis spectra and SEM micrographs of synthesized SrMnO 3 samples, prepared by addition of citric acid equal to 1 (A) and 5 (B) times of total number of moles of nitrate ions. Table 2 Energy Dispersive Spectroscopy (EDS) metal elemental analysis of SrCoO 3 samples, prepared by addition of citric acid equal to the 1 (A) and 5 (B) times of total number of moles of nitrate ions. Sample A B Oxide Nominal (wt%) Experimental (wt%) (±1.5) Nominal (wt%) Experimental (wt%) (±2) Sr Co Table 3 Energy Dispersive Spectroscopy (EDS) metal elemental analysis of SrMnO 3 samples, prepared by addition of citric acid equal to the 1 (A) and 5 (B) times of total number of moles of nitrate ions. Sample A B Oxide Nominal (wt%) Experimental (wt%) (±1) Nominal (wt%) Experimental (wt%) (±1.5) Sr Mn

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