Chapter 4 Synthesis of LSMO -...

Chapter 4

Synthesis of LSMO

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

Chapter- IV Synthesis of LSMO

Centre For Interdisciplinary Research, D.Y. Patil University, Kolhapur 64

‘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.

4.2. 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.



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 CO2 and H2O 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

MolecularWt. of gaseous products

TcIsp k (4.1)

The highest heat Tc (chamber temperature in the rocket motor) is produced when the

equivalence ratio (φe = oxidizer/fuel ratio) is unity.



φe =

(Coefficient of oxidizing elements in

specific formula) × (Valency)

(Coefficient of reducing elements in( 1)

specific formula) × (Valency)

(4.2)

A mixture is said to be stoichiometric when φe = 1, fuel lean when φe >1, and fuel

rich when φe < 1. Stoichiometric mixtures produce maximum energy. The oxidizer/fuel

molar ratio (O/F) required for a stoichiometric mixture (φe = 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.

4.3. 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 (CO2, H2O) 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 La0.7Sr0.3MnO3 (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 (400-500 ºC) and also

act as oxidants in reaction. The stoichiometric amounts of the nitrate precursors La

(NO3)3.6H2O, Sr (NO3)2 and Mn (NO3)2.4H2O of analytical grade were dissolved in double

distilled water to form the solution of 0.1 M. The equimolar solution of glycine and PVA



were prepared in double distilled water. The mixture of oxidants and fuels kept onto a

magnetic stirrer for 0.5 h at 100 ºC to get uniform mixing and evaporation of water to form

gel of precursors and then the gel was kept onto a heating coil (≈ 300 ºC) 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 La0.7Sr0.3MnO3. Assigning the +4, +1, +3, +2 and +2 valencies to the C, H, La3+, Sr2+, and

Mn2+, reducing elements, respectively, the -2 valency to O2- oxidizer and considering

nitrogen with the valence 0, 4 then the φe is calculated according to the equation 4.2.

Figure 4.1. Flow chart of the preparation of La0.7Sr0.3MnO3 (LSMO) nanoparticles by

solution combustion technique using glycine and PVA fuel.

The combustion reaction mostly influenced by oxidant to fuel equivalent ratio

denoted by Φe (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 Φe is



calculated by taking ratio of the total oxidizing and reducing valencies. When Φe 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

Table 4.2. Thermodynamic data of reactants and products involved in combustion reaction.

Enthalpy ΔfJ.mol-1)

Gibbs free energy

Δ GfJ.mol-1)

Entropy SJ.deg-1.mol-1)

Specific heat CpJ.deg-1.mol-1)

Mn(No3)2 -576.26 -451.0 218.0 -121.0

La(No3)3 -1254.4 ----- ----- ------

Sr(No3)2 -978.22 -780.0 194.56 149.87

N2 0 0 191.609 29.124

H2O -241.826 -228.61 188.835 33.60

Co2 -393.51 394.41 213.785 37.13

Glycine -528.5 -368.6 103.5 99.2

PVA -119.4 ----- ------ 74.82

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 La0.7Ca0.3MnO3 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

Properties Organic fuel components Glycine PVA

Structural formula H2N -CH2- COOH (CH2CHOH)n

Molecular weight (g/mol) 75.0 44.05 (monomer)

Heat of combustion (KJ/g) 13.0 25.056

Decomposition temperature(ºC) 262 228



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 ºC 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.

0.7La3+ (NO3)33-

+ 0.3Sr2+ (NO3)23-

+ Mn2+ (NO3)22-

+ 2.54 (NH2-CH2-COOH) + 1.33O2

La0.7Sr0.3MnO3 + 6.35H2O + 3.62N2 + 5.08CO2 (4.3)

0.7La3+ (NO3)33-

+ 0.3Sr2+ (NO3)23-

+ Mn2+ (NO3)22-

+ 2.29 (-C2H4O) + 1.5O2

La0.7Sr0.3MnO3+4.58H2O + 2.35N2 + 4.58CO2 (4.4)

4.4.Results and discussions

4.4.1. 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 ºC 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 ºC and ends at 550 ºC for both samples. Above the 600 ºC there is

no weight loss in the material for both samples, which implies that the material goes into the

desired stable phase.



Figure 4.2. Thermogravimetric analysis of La0.7Sr0.3MnO3 synthesized by combustion

technique (a) by using glycine and (b) by using PVA.

LSMO prepared with glycine fuel shows 13.64 % 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 ºC. For the further

characterization the LSMO prepared by combustion technique with glycine and PVA fuels is

heated at 800 ºC for 5h.

4.4.2. 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: 00-051-0409) and showing pseudo-cubic perovskite structure (spacegroup

R-3c). The calculated lattice parameters in both samples are a = 5.4907 Å, and c = 13.324 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

ºC for 5 h) removes all the impurities and develop pure phase LSMO.



Figure 4.3. XRD patterns of LSMO prepared by solution combustion technique with

glycine and PVA as fuels and annealed at 800 ºC 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 =

1.5405Å 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.

4.4.3. FT-IR analysis

The FTIR spectra of LSMO powder annealed at 800 oC 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 surface-

adsorbed water on the particle of LSMO.



Figure 4.4. FTIR spectra of LSMO prepared by using glycine and PVA.

4.4.4. 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 50-60 nm, whereas sample prepared by PVA is in the range of 40-50 nm.



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 3.13 20.74 4.23 20.27

Mn 30.98 39.33 29.24 40.85

Sr 7.28 6.57 6.53 5.72

La 58.61 33.36 60.00 33.15 Total 100.00 100.00 100.00 100.00



4.4.5. 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



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 ºC) 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 20-25 nm for sample prepared with PVA and

30-40 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.

4.4.6. 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.



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 (TSEP) which is different for two samples. TSEP 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 TSEP 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 rc, the

mathematical expression of the same is presented in chapter 2 (equation 2.3) The typical

values for rc is about 15 nm for Fe, 35 nm for Co, 30 nm for γ-Fe2O3 [5] and about 40 nm for

LSMO, [6] thus La0.7Sr0.3MnO3 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

(TB). The temperature at which the magnetic anisotropy energy of a nanoparticles system is

overcome by thermal energy and the whole system becomes superparamagnetic above TB or

magnetization of the particle is blocked below TB [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 TB, 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 TB = KV/25kB where K is modified magnetic anisotropy



constant and for LSMO nanoparticles and it is 2.25×104 erg/cm3 [10], V is a particle volume

calculated from TEM images for LSMO NPs. Calculated values are given in the table 4.4.

The TC 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 4.10. 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 La0.7Sr0.3MnO3

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 La0.7Sr0.3MnO3 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 La0.7Sr0.3MnO3.

The M-H curve at 300 K for sample prepared using glycine reports the magnetization

value MS ~ 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 MS value of 34.91emu/g for



sample prepared using PVA. This implies that the effect of fuel choice strongly influences

not only on morphology, but also magnetic properties of the La0.7Sr0.3MnO3.

Table 4.4. Magnetization parameters of LSMO observed from SQUID measurement.

4.5. Summary

The structural, magnetic and dispersion properties of the La0.7Sr0.3MnO3 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 TC 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.

Magnetization (emu/g)

5 K 300 K

Coercivity (Oe)

5K 300 K

Remanent magnetization

(emu/g)

5 K 300 K

Blocking temperature

(K)

Curie temperature

(K)

LSMO by

Glycine 84.43 54.91 76 7 16 1 ~75 350

LSMO by PVA

71.22 34.90 70 1 10 0 ~30 350



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) 3126.

[8] A. K. Pramanik and A. Banerjee, J. Phys.: Condens. Matter, 20 (2008) 275207.

[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) 235214.

[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) 3072.

[11] S.Daengsakul, C.Thomas, I. Thomas, C. Mongkolkachit, S. Siri, V. Amornkitbamrung

and S. Maensiri, Nanoscale Res Lett. 4 (2009) 839.

Chapter 4 Synthesis of LSMO -...

Documents

Transcript of Chapter 4 Synthesis of LSMO -...