Chapter 4. Synthesis of LSMO

Transcription

1 Chapter 4 Important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. - Sir William Bragg

2 Nano Big Events happen in small worlds Richard Feynman 4.1. Introduction The main aspect for the successful use of nanoparticles (NPs) in biomedical field is to make them monodispersive and biocompatible. This problem is solved at earlier stage of the time of particles preparation. The suitable method which produces narrow size and biocompatible nanoparticles is highly desirable. As described in Chapter 1, the solution combustion synthesis is employed for the synthesis of LSMO nanoparticles. The combustion methodology of LSMO nanoparticles still requires refinement, including the ability to control many aspects, including suitable fuel choice, colloidal monodispersity, nanoparticle size tuning, cohesive particle sizing and anisotropy. In this chapter, the development of an optimized combustion method to LSMO nanoparticles is presented. Notably the particles made from this study were later shown to be the best ever particles to date for magnetic hyperthermia Combustion technique Combustion synthesis (CS) is becoming one of the most popular methods for the preparation of a variety of materials, ranging from non-oxides, such as borides, nitrides, carbides, etc., to simple and complex oxides [1]. Combustion synthesis processes are characterized by high- temperatures, short reaction times and fast heating rates. These characteristic features make CS an attractive method for the manufacture of technologically useful materials at lower costs compared to conventional ceramic processes [2]. Some other advantages of CS are (i) Use of relatively simple equipment (ii) Formation of high-purity products (iii) Stabilization of metastable phases (iv) Formation of virtually any size ( micro to nano) and shape (spherical to Hexagonal) products (v) Uniform distribution of dopants takes place throught the host material due to the atomic mixing of the reactants in the initial solution. (vi) This process not only give ups nanosize oxide materials but also allows uniform (homogeneous) doping of trace amounts of rare-earth impurity ions in a single step. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 64

3 Depending upon the nature of reactants: elements or compounds (solid, liquid or gas); type and the exothermicity (adiabatic temperature, T), CS is described as: self-propagating high temperature synthesis (SHS); low-temperature combustion synthesis (LCS), solution combustion synthesis (SCS), gel-combustion, sol gel combustion, emulsion combustion, volume combustion (thermal explosion), etc. The solution combustion synthesis (SC) method of preparing oxide materials is a fairly recent development compared to SSC or SHS techniques described above. Today, SCS is being used all over the world to prepare oxide materials for a variety of applications. During the short span (15 years) of SCS synthesis history, hundreds of papers on SCS of oxides have been published. An aqueous solution of a redox system constituted by the nitrate ions of the metal precursor, acting as oxidizer, and a fuel like glycine, urea, citric acid or many others, is heated up to reasonable temperatures and, leading dehydration, the strongly exothermic redox reaction occurs, which is generally self-sustaining and provides the energy for the formation of the oxide. All these fuels serve two purposes: (a) they are the source of C and H, which on combustion form CO 2 and H 2 O and liberate heat; (b) these fuels form complexes with the metal ions facilitating homogeneous mixing of the cations in solution. Oxide materials produced with this method include several ferrites and spinels, tin oxide and antimony tin oxide (ATO), ceria, ferroelectric materials, iron oxide, zinc oxide, protonic conductors and various solid solutions. To understand the highly exothermic nature of this reaction, concepts used in propellant chemistry were employed [3]. A solid propellant contains an oxidizer like ammonium per chlorate and a fuel like carboxy terminated polybutadiene together with aluminum powder and some additives. The specific impulse (Isp) of a propellant, which is a measure of energy released during combustion, is specified by the ratio of thrust produced per pound of the propellant. It is expressed as Tc Isp k (4.1) MolecularWt. of gaseous products The highest heat Tc (chamber temperature in the rocket motor) is produced when the equivalence ratio (φ e = oxidizer/fuel ratio) is unity. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 65

4 φ e = (Coefficient of oxidizing elements in specific formula) (Valency) ( 1) (Coefficient of reducing elements in specific formula) (Valency) (4.2) A mixture is said to be stoichiometric when φ e = 1, fuel lean when φ e >1, and fuel rich when φ e < 1. Stoichiometric mixtures produce maximum energy. The oxidizer/fuel molar ratio (O/F) required for a stoichiometric mixture (φ e = 1) is determined by summing the total oxidizing and reducing valencies in the oxidizer compounds and dividing it by the sum of the total oxidizing and reducing valencies in the fuel compounds. In this sort of calculation oxygen is the only oxidizing element; metal cations, hydrogen and carbon are reducing elements and nitrogen is neutral. Reducing elements have negative valencies and oxidizing elements have positive valencies. In solution combustion calculations, the valency of the oxidizing and reducing elements are considered similar to the oxidation number concept familiar to chemists Experimental In the present study the solution combustion technique by using glycine and polyvinyl alcohol (PVA) has been employed for the synthesis of LSMO NPs. Short process duration and the formation of various gases during combustion inhibit particle size growth and favor synthesis of nano-size powders with high specific surface area. To achieve this, choice of organic fuel having lower decomposition temperature with evolution of gases (CO 2, H 2 O) is important. This helps for generating sufficient local heating to system fuel for the completion of combustion synthesis and at the same time creates porosity within the mixture and also prevents particle agglomeration. The perovskite type La 0.7 Sr 0.3 MnO 3 (LSMO) NPs have been prepared with the solution combustion method by using two different fuels such as glycine and polyvinyl alcohol. The representation of the process is graphically shown in the figure 4.1. The nitrates are used as starting materials which decompose at lower temperature ( º C) and also act as oxidants in reaction. The stoichiometric amounts of the nitrate precursors La (NO 3 ) 3.6H 2 O, Sr (NO 3 ) 2 and Mn (NO 3 ) 2.4H 2 O of analytical grade were dissolved in double distilled water to form the solution of 0.1 M. The equimolar solution of glycine and PVA Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 66

5 were prepared in double distilled water. The mixture of oxidants and fuels kept onto a magnetic stirrer for 0.5 h at 100 º C to get uniform mixing and evaporation of water to form gel of precursors and then the gel was kept onto a heating coil ( 300 º C) for the burning process. During the heating process the ignition takes place, followed by the combustion of the reactants mixture with the appearance of a high speed propagating flame. The high temperature reached within the raw material mixture led to the formation of dried fluffy foam of La 0.7 Sr 0.3 MnO 3. Assigning the +4, +1, +3, +2 and +2 valencies to the C, H, La 3+, Sr 2+, and Mn 2+, reducing elements, respectively, the -2 valency to O 2- oxidizer and considering nitrogen with the valence 0, 4 then the φ e is calculated according to the equation 4.2. Figure 4.1. Flow chart of the preparation of La 0.7 Sr 0.3 MnO 3 (LSMO) nanoparticles by solution combustion technique using glycine and PVA fuel. The combustion reaction mostly influenced by oxidant to fuel equivalent ratio denoted by Φ e (O/F). The oxygen content of oxidizer can be completely reacted to oxidize or consume fuel and no more heat exchange is required for the reaction time. The value of Φ e is Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 67

6 calculated by taking ratio of the total oxidizing and reducing valencies. When Φ e becomes unity, it serves maximum heat release at the time of combustion. This kind of combustion is called stoichiometric combustion and is expected to yield desired phase. Table 4.1. Different properties of fuels used for solution combustion synthesis Properties Glycine Organic fuel components PVA Structural formula H 2 N -CH 2 - COOH (CH 2 CHOH)n Molecular weight (g/mol) (monomer) Heat of combustion (KJ/g) Decomposition temperature( º C) Table 4.2. Thermodynamic data of reactants and products involved in combustion reaction. Enthalpy Δ f J.mol -1 ) Gibbs free energy Δ G f J.mol -1 ) Entropy S J.deg -1.mol -1 ) Specific heat Cp J.deg -1.mol -1 ) Mn(No 3 ) La(No 3 ) Sr(No 3 ) N H 2 O Co Glycine PVA According to propellant chemistry for getting the same oxidation and reduction valencies in the solution, 2.29 mole % solution of PVA and 2.54 mole % solution of glycine is required. Glycine is inexpensive and produced combustion heat ( 3.24 Kcal/g) which is more negative as compared to urea ( 2.98 Kcal/g) and citric acid ( 2.76 Kcal/g). Hence, glycine is used as a fuel for synthesis of La 0.7 Ca 0.3 MnO 3 powder. On the other side, polymer such as PVA has not been exploited as a fuel in combustion technique. However, very few authors have reported combustion technique with PVA for preparation of ZnO nanoparticles [4]. Heat source induces polymer bond fission which results into increment in the temperature of polymeric material. The volatile fraction of the resulting polymer Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 68

7 fragments diffuses into the air and creates a combustible gaseous mixture (also called fuel). This gaseous mixture ignites when the auto-ignition temperature is reached and liberates heat which induces formation of fine nanoparticles. The prepared LSMO powder by combustion technique is annealed at 800 º C for 5h and used for further characterization. The plausible chemical reactions for glycine and PVA fuel assuming complete combustion can be represented by Equations (4.3) and (4.4), respectively. 0.7La 3+ (NO 3 ) Sr 2+ (NO 3 ) Mn 2+ (NO 3 ) (NH 2 -CH 2 -COOH) O 2 La 0.7 Sr 0.3 MnO H 2 O N CO 2 (4.3) 0.7La 3+ (NO 3 ) Sr 2+ (NO 3 ) Mn 2+ (NO 3 ) (-C 2 H 4 O) + 1.5O 2 La 0.7 Sr 0.3 MnO H 2 O N CO 2 (4.4) 4.4.Results and discussions Thermogravimetric Analysis The thermo-gravimetric analysis (TGA) of as synthesized LSMO powder was carried out for phase confirmation of LSMO from room temperature to 1000 º C in air atmosphere. The TGA graphs of as synthesized LSMO are shown in the figure 4.2 (a) and (b). The TG curve of both the samples shows the initial weight loss due to removal of atmospheric water content. The second weight loss is attributed to the decomposition of unburnt carbon content which starts from 200 º C and ends at 550 º C for both samples. Above the 600 º C there is no weight loss in the material for both samples, which implies that the material goes into the desired stable phase. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 69

8 Figure 4.2. Thermogravimetric analysis of La 0.7 Sr 0.3 MnO 3 synthesized by combustion technique (a) by using glycine and (b) by using PVA. LSMO prepared with glycine fuel shows % weight loss due to carbon residue, while LSMO prepared with PVA fuel shows 17.04%. The higher decomposition temperature of glycine burns the maximum carbon content during the combustion which implies lower carbon content in the powder observed from TG analysis. From the figure 4.2 we conclude that the stable phase formation for LSMO occurs above the 600 º C. For the further characterization the LSMO prepared by combustion technique with glycine and PVA fuels is heated at 800 º C for 5h X-Ray diffraction study Figure 4.3 shows the X-ray diffraction patterns of the LSMO samples prepared by using glycine and PVA as fuels. All the reflection peaks are indexed with JCPDS card (reference code: ) and showing pseudo-cubic perovskite structure (spacegroup R-3c). The calculated lattice parameters in both samples are a = Å, and c = Å. The study also shows that the glycine and PVA fuels provide sufficient reaction temperature and help into the formation nuclei of LSMO. Further, the post annealing temperature (800 º C for 5 h) removes all the impurities and develop pure phase LSMO. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 70

9 Figure 4.3. XRD patterns of LSMO prepared by solution combustion technique with glycine and PVA as fuels and annealed at 800 º C for 5 h. The Gaussian fit of the most intense peak (110) was used to calculate the full width at half maxima for determination of crystallite size (D) by equation D = 0.9λ/βcosθ, where λ = Å wavelength of incident X-ray, θ is the corresponding Bragg s diffraction angle and β is full width at half maxima of the (110) peak. The average crystallite size obtained by above equation is about 25 and 20 nm for LSMO prepared by glycine and PVA, respectively. From the XRD patterns of both samples it is seen that the reflection peaks are quite broad, suggesting their nanocrystallinity FT-IR analysis The FTIR spectra of LSMO powder annealed at 800 o C for 5h and are shown in the figure 4.4. The characteristic band around 600 cm -1 observed in both samples corresponding to Mn-O. This indicates both samples strongly contain the metal-oxygen bonds which involves the internal motion of a change in Mn-O-Mn bond length. The stretching mode is associated with the change of Mn-O-Mn bond length while the bending mode involves the change of Mn-O-Mn bond angle. The peaks at 1630 and 3440 cm -1 are due to surfaceadsorbed water on the particle of LSMO. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 71

10 Figure 4.4. FTIR spectra of LSMO prepared by using glycine and PVA Morphological and elemental analysis Figure 4.5. FE-SEM images of LSMO samples prepared by (a) glycine and (b) PVA. The surface morphologies of the samples were analyzed by FE-SEM and corresponding images for two different samples are shown in the figure 4.5 (a and b). From the images one can observe that most of the grains are spherical in shape with unvarying distribution. It is further observed that the grain size in sample prepared by glycine is distributed from nm, whereas sample prepared by PVA is in the range of nm. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 72

11 Figure 4.6. EDAX spectra of LSMO nanoparticles prepared by (a) glycine and (b) PVA. The EDAX spectra were used for quantitative elemental analysis and composition of the LSMO prepared by using glycine and PVA fuels (Figure 4.6(a) and (b)). Spectra indicate that both samples are consistent with their elemental signals and stoichiometry as expected. The corresponding peaks are due to the La, Sr, Mn and O elements, whereas not any additional impurity peak is observed and it implies that the prepared samples are pure in nature. The detailed analysis of both samples shows the atomic weight ratio of (La, Sr): Mn 1.0 and suggests the obtained LSMO samples are stoichiometric. The observed atomic percentage from EDAX is presented in the table 4.3. Table 4.3. Elemental compositions of La, Sr, Mn and O atoms evaluated by using EDAX Technique. Element LSMO by Glycine Mass% At% LSMO by PVA Mass% At% O Mn Sr La Total Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 73

12 TEM and DLS studies Figure 4.7. TEM images of LSMO nanoparticles prepared by solution combustion synthesis (a) glycine (b) PVA. (Inset: there corresponding SAED pattern). Figure 4.8. DLS histogram of LSMO nanoparticles prepared by solution combustion synthesis (a) glycine and (b) PVA. The TEM micrographs of LSMO nanoparticles prepared by solution combustion synthesis by using glycine and PVA fuel are shown in figure 4.7 (a) and (b), respectively. The average particle size of LSMO by using glycine as a fuel is of the order of 30 nm while Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 74

13 by using PVA fuel is about 20 nm. The smaller particle size of LSMO synthesized by PVA fuel is attributed to lower decomposition temperature of PVA compared to glycine which controls the combustion flame temperature. At the same time, it creates the porosity in the powder to prevent agglomeration. In both samples the particle size distribution is almost homogeneous. The particle diameters are slightly larger than the observed crystal sizes obtained from XRD, due to the presence of noncrystalline surface layers as well as high temperature calcinations (800 º C) which causes the grain growth and it results into increasing the particle size that is not determined by XRD. The corresponding selected area electron diffraction (SAED) patterns (inset in the figure 4.7 (a) and (b)) show bright rings, indicating the polycrystalline nature of the LSMO MNPs. The hydrodynamic diameter of nanoparticles was finally determined by DLS (Figure 4.8 (a, b)). The main peaks were centered on nm for sample prepared with PVA and nm for sample prepared by glycine, which are consistent with TEM results. Moreover, one should observe the absence of large aggregates in the sample prepared by PVA Magnetization study Figure 4.9. (a) M-T measurements of LSMO nanoparticles prepared with glycine and PVA as fuels, respectively at 500 Oe. (b) dm/dt curves obtained from M-T measurement of both samples from which Curie temperature is determined. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 75

14 Figure 4.9 (a) shows the variation of magnetization M as a function of temperature (T) of LSMO prepared with glycine and PVA in the range 5 to 375 K in an external magnetic field of 500 Oe recorded in zero-field-cooled (ZFC) and field-cooled (FC). From both the curves it is clearly observed the superimposition of the ZFC and FC curves take place at a certain temperature (T SEP ) which is different for two samples. T SEP for LSMO prepared with glycine (30 nm) is 230 K while for LSMO prepared with PVA (20 nm) is 175 K, which indicates that the T SEP is a function of particle size. The superimposition of ZFC and FC curves is one of the characteristic features of superparamagnetic system [5] and from the two graphs it has been observed that the LSMO prepared with glycine and PVA is superparamagnetic in nature. The superparamagnetism is induced in the system when the system comes from maultidomain to single and uniformly magnetized domains. Then, the overall system is in a state of uniform magnetization and its phase transition occurs from ferromagnetic to superparamagnetic and system behaves like a small permanent magnet. The size of the single-domain particle depends on the material and its anisotropy energy constant. Transformation from multidomain behavior (ferromagnetic) to single domain (superparamagnetic) occurs at a certain radius of particle called critical radius r c, the mathematical expression of the same is presented in chapter 2 (equation 2.3) The typical values for r c is about 15 nm for Fe, 35 nm for Co, 30 nm for γ-fe 2 O 3 [5] and about 40 nm for LSMO, [6] thus La 0.7 Sr 0.3 MnO 3 nanoparticles prepared by solution combustion method with glycine and PVA fuels are below the critical size (i.e. single domain). The important parameter, involved in our measurements, is the blocking temperature (T B ). The temperature at which the magnetic anisotropy energy of a nanoparticles system is overcome by thermal energy and the whole system becomes superparamagnetic above T B or magnetization of the particle is blocked below T B [7]. The experimental blocking temperature observed for two different samples are given in Table 4.4. Rostamnejadi et al. used the AC magnetic susceptibility measurements for the estimation of blocking temperature on the principle that the below T B, the magnetization direction of nanoparticles can follow the direction of the applied field and the total magnetization increases with decreasing temperature. However, we estimated the blocking temperature based on earlier reports of the magnetic measurements for ferrite nanoparticles [8, 9]. The theoretical blocking temperature is estimated from the equation T B = KV/25k B where K is modified magnetic anisotropy Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 76

15 constant and for LSMO nanoparticles and it is erg/cm 3 [10], V is a particle volume calculated from TEM images for LSMO NPs. Calculated values are given in the table 4.4. The T C is calculated by taking the differentiation of ZFC curve. The observed nature is shown in the figure 4.9 (b). The observed value is about 350 K for both samples, which is less than the reported value for the same composition (~ 370 K). Figure M-H curves of LSMO prepared by glycine and PVA (a) at 300 K (b) at 5 K. To understand the physics behind the superparamagnetic behavior of La 0.7 Sr 0.3 MnO 3 system we carried out the M versus H measurements as a function of applied field and temperature. Figure 4.10 (a) and (b) shows the M-H curves of La 0.7 Sr 0.3 MnO 3 nanoparticles at 300 and 5 K prepared by glycine and PVA, respectively. Magnetization value of sample prepared using glycine at the particular magnetic field is more than that using PVA due to the particle size effect. Former sample has the larger particle size as compared to the latter one. The similar observation is reported by Dyakonov et al. [10]. The magnetization increases with decreasing temperature from 300 to 5 K due to overcome of the magnetic anisotropic energy over the thermal effect. The hysteresis parameter (coercivity) of both samples is almost zero at 300 K, which reveals the superparamagnetic nature of La 0.7 Sr 0.3 MnO 3. The M-H curve at 300 K for sample prepared using glycine reports the magnetization value M S ~ 54.91emu/g, which is slightly higher than the value 46.8 emu/g reported by Daengsakul et al.[11] for the same composition. We observed M S value of 34.91emu/g for Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 77

16 sample prepared using PVA. This implies that the effect of fuel choice strongly influences not only on morphology, but also magnetic properties of the La 0.7 Sr 0.3 MnO 3. Table 4.4. Magnetization parameters of LSMO observed from SQUID measurement. Magnetization (emu/g) Coercivity (Oe) Remanent magnetization (emu/g) Blocking temperature (K) Curie temperature (K) 5 K 300 K 5K 300 K 5 K 300 K LSMO by Glycine LSMO by PVA ~ ~ Summary The structural, magnetic and dispersion properties of the La 0.7 Sr 0.3 MnO 3 nanoparticles (20~30nm) prepared with novel solution combustion method have been studied in great detail. The properties of the LSMO have been affected by the fuel used for combustion. The glycine and PVA both produce pure-phase LSMO with almost identical particle size distribution. FC and ZFC measurements strongly support the superparamagnetic behaviour of the LSMO and the T C determined is smaller than bulk value of the same composition. The coercivity observed is almost zero for both samples at room temperature, which is a characteristic feature of superparamagnetism. It is observed that the magnetization influenced by the fuel used in the combustion method. LSMO prepared by PVA shows lower aggregation and well dispersion stability. So, we conclude from FE-SEM, TEM and DLS, study that the LSMO prepared by using PVA as a fuel is excellent candidate for the biomedical application especially hyperthermia. However, magnetization is lower as compared to LSMO prepared by glycine, but the observed magnetization is enough for the successful application in hyperthermia treatment. Hence, in further study, surface functionalization is done only on LSMO prepared by PVA and results are presented in the next chapters. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 78

17 References [1] M. Epifani, E. Melissano, G. Pace and M. Schioppa, J. Eur. Ceram. Soc. 27 (2007) 115. [2] K. C. Patil, S.T. Aruna and T. Mimania, Curr. Opin. Solid State Mater. Sci. 6 (2002) 507. [3] Chemistry of Combustion Synthesis, Properties and Applications Nanocrystalline Oxide Materials (chapter 1 and 2) K. C. Patil, M. S. Hegde T. Rattan and S. T. Aruna, World Scientific (2008). [4] S. K. Sharma, S. S. Pitale, M. M. Malik, R. N. Dubey, M. S. Qureshi and S. Ojha, Physica B, 405 (2010) 866. [5] Supermagnetism in magnetic nanoparticle systems Ph. D. thesis submitted by Subhankar Bedanta to the University of Duisberg-Essen (2006). [6] S. Daengsakul, C. Mongkolkachit, C. Thomas, S. Siri, I. Thomas, V. Amornkitbamrung and S. Maensir, Appl Phys A, 96 (2009) 691. [7] A. Rostamnejadi, H. Salamati, P. Kameli and H. Ahmadvand, J. Magn. Magn. Mater. 321 (2009) [8] A. K. Pramanik and A. Banerjee, J. Phys.: Condens. Matter, 20 (2008) [9] S. K. Sharma, R. Kumar, S. Kumar, M. Knobel, C. T. Meneses, V. V. Siva Kumar, V. R. Reddy, M. Singh and C. G. Lee, J. Phys.: Condens. Matter, 20 (2008) [10] V. Dyakonov, A. Slawska-Waniewska, N. Nedelko, E. Zubov, V. Mikhaylov, K. Piotrowski, A. Szytua, S. Baran, W. Bazela, Z. Kravchenko, P. Aleshkevich, A. Pashchenko, K. Dyakonov, V. Varyukhin and H. Szymczak, J. Magn. Magn. Mater. 322 (2010) [11] S.Daengsakul, C.Thomas, I. Thomas, C. Mongkolkachit, S. Siri, V. Amornkitbamrung and S. Maensiri, Nanoscale Res Lett. 4 (2009) 839. Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 79