Effect of Calcination Temperature on La0.7Sr0.3MnO3 Nanoparticles Synthesized with Modified Sol-gel Route

Available online at www.sciencedirect.com

ScienceDirect Physics Procedia 54 (2014) 45 – 54

International Conference on Magnetic Materials and Applications, MagMA 2013

Effect of calcination temperature on La0.7Sr0.3MnO3 nanoparticles synthesized with modified sol-gel route S. Ravia*, A. Karthikeyana a

Department of Physics, Mepco Schlenk Engineering College, Sivakasi 626005, India.

Abstract We report a modified route to synthesize La0.7Sr0.3MnO3 nanoparticle with oxalic acid as chelating agent and oleic acid as surfactant at different calcination temperatures. The synthesized nanoparticles were characterized using XRD, SEM, FTIR and VSM. XRD confirms the formation of phase pure perovskite structure with particle size of about 20 nm. SEM and HRSEM reveal a well refined structure with respect to calcination temperature. FTIR confirms the formation of perovskite with broad peak around 520 cm–1. Magnetic study reveals that these nanoparticle with irregular structure exhibit ferromagnetic nature with different value of magnetization except for 500 °C, which shows paramagnetic nature. © by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license © 2014 2014The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati. Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati Keywords: LaSrMnO3 nanoparticle; ferromagnetic nature; oxalic acid; sol-gel route.

1. Introduction Magnetic nanoparticles system receives much attention are those based on iron oxide, alloys of bi-metallic ferromagnets, ferrites, etc., owing to their wide range of applications from biology to magnetic data storage devices (Wu et al. (2008), Kostopoulou et al. (2013), Conaru et al. (2013), Mazur et al. (2013), Ravi and Karthikeyan (2013), Clemente-León et al. (2013), Lei et al. (2013), McKiernan et al. (2010), Namai et al. (2012), Tailhades et al. (1999)). Their bio-compatibility leads to application in drug delivery and treatment of many serious aliments like cancer (Dempsey et al. (2004), Kumar and Mohammed (2011), Thorat et al. (2012), Bhayani et al. (2013), Ramirez (1997)).

* Corresponding author. Tel.: +914562235691; fax: +914562235111. E-mail address: [email protected]

1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of Department of Physics, Indian Institute of Technology Guwahati doi:10.1016/j.phpro.2014.10.035

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Among the magnetic materials, the half-metallic compounds with perovskite structure are probably interesting as they exhibit colossal magnetic resistance (Quijada et al. (1998)). Amazing magnetic and electrical properties of these materials become current research interest for both experimental and theoretical investigations. Lanthanum strontium manganite with formula LaxSr1–xMnO3 (hereafter referred as LSMO) is a typical example. In the crystal, the 'A' sites are occupied by lanthanum and strontium atoms, and the 'B' sites are occupied by manganese atoms. The electrical and magnetic behaviour is largely determined by the electrons in the 3d shell of the manganese atoms. The overall magnetic and electrical behaviour depends on the interaction of the different manganese atoms. LSMO behaves like a half-metal, suggesting its possible use in spintronics. Above its Curie temperature (about 350 K), Jahn-Teller polarons are formed; the material's ability to conduct electricity is dependent on the presence of the polarons (Li et al. (2010)). Its magnetic properties are both temperature and composition (x) dependent leading to wide range of applications. This fascinating property is governed by electrons which ‘hop’ among or are localized on Mn3+/Mn4+ ions (Daengsakul et al. (2009), Yang et al. (2005)). Several reports are available on the synthesis and characterization of LSMO nanoparticles (Sina et al. (2007), Pana et al. (2009), Daengsakul et al. (2009), Grossin and Noudem (2004), Uskokovic and Drofenik (2005), Pang et al. (2003), McGuire et al. (1996), Tian et al. (2006), Nagabhushana et al. (2006), Zhang et al. (2010)). We report a modified sol-gel route to synthesis La0.7Sr0.3MnO3 (LSMO) nanoparticles with oxalic acid as chelating agent, oleic acid as surfactant, in poly acrylic acid matrix. These nanoparticles show a wide range of particle size close to 20 nm and grown in inhomogeneous structures with different sizes. In our work, the sizes are generally greater than 9 nm, so it does not show superparamagnetic behavior, instead it possess ferromagnetic property with sharp switching characteristic suitable for good magnetoresistance for spintronics devices. 2. Experimental methods LSMO nanoparticles were synthesized by a modified sol-gel route from their nitrate precursors. Lanthanum Nitrate, Strontium Nitrate and Manganese Nitrate were purchased from Sigma Aldrich with high purity (99.99%). Appropriate weights of these materials are dissolved in de-ionized water with constant stirring to obtain a homogeneous solution. This solution was known as stock solution. In an another process, Oxalic acid (Sigma Aldrich) equivalent to four times of the metal nitrates was added to de-ionized water to make up 100 ml solution. To this solution, 0.02% of Poly Acrylic Acid (Aldrich) was added and stirred constantly to obtain clear solution. Oxalic acid (H2C2O4) acts as a strong reducing/chelating agent which reduces metal nitrates to metal oxalates. PAA acts as binder to this chelating agent, which act as surface assistant as well. The stock solution was now added to this mixture and heated at the rate of 45 °C with constant stirring (700 rpm) for 3 hours. To this mixture 0.02% of Oleic acid (Sigma Aldrich) was added, which acts as surfactant. As soon as stock solution was added, it turns into white precipitate. After 3 hours, this white precipitate turns into pale brown colour. At this time, the heating rate was increased to 100 °C with reduced stirring (400 rpm). A black colour resin like form is obtained, which is dried in atmosphere to obtain a powder. This powder was grinded finely and kept in an incubator for overnight, which is maintained at 90 °C. This powder was calcinated to 500 °C, 600 °C, 700 °C and 800 °C for 4h to obtain LSMO nanoparticles of different particle size. LSMO nanoparticles were also synthesized without surfactant/PAA with the procedure similar to the above for reference. 3. Result and Discussion The schematic representation of the growth mechanism was presented in the Figure 1. The mixture of metal nitrates was binded with the polymer matrix. Here PAA is used as binder as well as surface assisting agent. In order to form a non-agglomeric particle, Oleic acid was added, which acts as a surfactant. Thus with respect to various calcination temperature, these particles form irregular structure on polymer matrix. However oleic acid has no impact in the formation of non-agglomeric particles for calcination temperature of 500 °C, 600 °C and 700 °C. With calcination temperature of 800 °C, the particles form a refined tablet like structure. For other calcination temperature there is not much change in the structure, with non-uniform distribution. Hence this would be more precise to be called as irregular nanostructure rather than nanoparticles.

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La(NO3)3.6H2O

MnN2O6.XH2O

Sr(NO3)2

Dissolved in Deionized H2O @ 400 rpm stirring Homogeneous white soln. 0.02 % of Poly Acrylic acid

H2C2O4

ȴ = 45°C @700 rpm for 3 hours White precipitate turn into Pale brown colour ȴ = 100°C @400 rpm for 1 hours

0.02 % of Oleic acid

Black colour resin Dried at atm. LSMO Calcination 4 hours 500

600

700

800

Fig. 1. Schematic representation of growth process of LSMO nanoparticles

3.1. TG /DTA of LSMO nanoparticle Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) of as synthesized LSMO nanoparticle is shown in Figure 2. Exothermic one-step decomposition of the nitrate/oxalate complexes takes place in the temperature range 300 °C < T < 400 °C. With the increase in temperature there are three weight loss regions observed in the TGA curve. According to the quantity calculation of the weight loss in each region, the whole thermal decomposition process can be distinguished below. The weight loss region in the temperature range ~35-105 °C is due to loss of coordinate and solvent water molecules. The weight loss between 150–400 °C is due to the decomposition of nitrates/oxalate in around 300 °C and decomposition of C–H organic components. The last stage above 600 °C, the weight loss is very minimum suggesting the formation of crystalline/phase pure perovskite of LSMO. However, this has to be confirmed by XRD/SEM. The weight loss up to temperature 1000 °C is only ~6% suggesting that the nanoparticles are highly stable.

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100 Decomposition of water

Decomposition of chelating agent

Wt %

98

96 Formation of pervoskite 94 0

250

500

750

1000

Temperature (in °C) Fig. 2 DGA curve of LSMO nanoparticles synthesized with modified sol-gel route

3.2. X-Ray Diffraction analysis of LSMO nanoparticle Figure 2a shows XRD pattern for LSMO nanoparticles calcinated at 500 °C, 600 °C, 700 °C and 800 °C. It is evident that all the samples are formed with single phase with much impurity. Intensity for 800 °C is much higher than others owing to high electron density which may due to high crystalline size (Patterson (1935)). The pattern is indexed with JCPDS # 89-4461 with rhombohedral R3c space group. The broad peak formed at 2ࣄ~32.9, which is prominent for LSMO perovskite structure is taken to calculate the particle size. This was presented as inset and from that it is clear that there is not much change in particle size with calcination temperature, which is rather interesting. The particle size is determined by using Scherer’s formula found to be in the range of 20 nm. Fig 2b is LSMO nanoparticle with 800 °C calcination. This shows high crystalline nature along with all peaks for LSMO perovskite structure without any phase or impurity in accordance with DTA/TGA results. The inset picture of Fig 2b is the EDAX spectrum which shows the purity and elemental composition of LSMO at 800 °C.However, LSMO nanoparticle without PAA/surfactant shows high degree of mixed phase and impurities as seen in Figure 3a. This clearly shows that oleic acid and PAA binds the nanoparticles and prevents from any impurity/phase change. SEM image for this nanoparticle is presented in the right side of Fig. 3b, also shows irregular structure with no definite shape. (a)

Fig 2 (a) XRD pattern for LSMO nanoparticle showing formation LSMO in pure phase for all calcination temperature with intensity variation. (b) XRD pattern for 800 °C showing all prominent peaks in-accordance with the standard for LSMO without any impurity or phase change. Inset picture shows the EDAX spectrum of LSMO confirming the purity and elemental composition

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(b)

(a)

Fig. 3 (a) XRD pattern of LSMO synthesized without PAA/oleic acid showing many impurity/mixed phase (b) SEM image of LSMO showing irregular shape and agglomers without definite structures or shape.

3.3. SEM and HR-SEM analysis of LSMO nanoparticle SEM images of LSMO nanoparticles calcinated at 500 °C, 600 °C, 700 °C and 800 °C was presented in the Figure 4. Close inspection of these images reveal that the nanoparticles has not a definite structure, shape and binded in the polymer matrix with respect to the calcination temperature. Although the nanoparticles are found in agglomers with different irregular structure, the sizes are not seen precisely. Figure 5 was HRSEM of LSMO nanoparticles calcinated at 500 °C. This shows clear morphology of the nanoparticle in two different magnifications. The sizes are distributed un-evenly in the range of 20 nm in accordance with the XRD results. The HRSEM images for LSMO nanoparticles for 800 °C was presented in Figure 6. Four different magnifications were shown and it clearly shows the morphology of the nanoparticles with particle size around 30 nm with irregular tablet shape. Thus from the morphology studies, the particle sizes are not varied drastically but the structures are found to be different for different calcination temperature. 500

700

600 °C

800 °C

Fig 4. SEM micrograph images for LSMO nanoparticles at different calcination temperatures showing nanoparticles aggregates in the polymer matrix with different structures.

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Fig 5 HRSEM of LSMO nanoparticle calcinated at 500 °C showing particle size of 20 nm with agglomers

Fig 6 HRSEM of LSMO nanoparticle calcinated at 800 C at different magnification showing refined tablet structures with particle size of 30 nm

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3.4. FTIR analysis of LSMO nanoparticle FTIR spectra of LSMO calcined from 500 °C to 800 °C are shown in Figure 7. The main absorption band around 524 cm–1 corresponds to stretching of the metal–oxygen bond in the perovskite, which involves the internal motion of a change in Mn–O–Mn bond length in MnO6 octahedral (Gao et al. (2002)). The absorption peaks around 920 cm– 1 and 1050 cm–1 stands for the existence of carbonate. The strong absorption peak around 1381 cm–1 in LSMO 500 °C reveals that the stretching vibration of carbonyl group (COO–) in carbonate, which diminishes with increasing calcination temperature. The absorption peak around 2360 cm –1 is due to O–C–O (Li et al. (1998)). The broad absorption peak around 3350 is the characteristic of absorbed water or hydroxyl group in the alcohol. LSMO 800 °C has a doublet in the main absorption band around 520cm–1 should belong to stretching, 3Ȟ and bending, 4Ȟ of the internal phonon modes of MnO6 octahedral. The stretching mode is related to the change of Mn–O–Mn bond length and the bending mode involves the change of Mn–O–Mn bond angle. The appearance of the stretching and bending modes at transmission spectra indicates that the perovskite structure of LSMO has been formed, which is in agreement with the result of XRD.

Transmittance (%)

100

85 800 °C 500 °C 600 °C 700 °C 70 300

800

1300

1800

2300

Wavenumber

2800

3300

3800

(cm-1)

Figure 7 FTIR of LSMO nanoparticles at all calcination showing the formation of pervoskite structure

3.5. Magnetic studies of LSMO nanoparticle The magnetic property of synthesized LSMO nanoparticles synthesized at different temperatures was presented in the Figure 8. Except for 500 °C, all shows ferromagnetic nature with different emu values. For 500 °C, it is almost like a paramagnetic. At very low field, it shows marginal loop like behavior, may be due the spin exchanges. This shows that at 500 °C, the Mn3+/4+ exchanges with O2– are not prominent and hence the Mn property dominates, leading to paramagnetic feature. For 600 °C and 700 °C, it looks like ferromagnetic with very low Msat value of 0.004 emu/g and 0.02 emu/g with different loop area. Both behave like soft magnetic material with different switching characteristic. LSMO nanoparticles calcinated at 800 °C shows hysteresis like behavior and close look up of this plot clearly shows that it has ferromagnetic nature at low field. The inset picture clearly shows that it exhibit hysteresis. But with increase in the field, no saturation was found up to 16000 G. The Msat value for this sample is found to be maximum and this may be due to the presence of more Mn4+ ion content in the Mn4+–O–Mn3+ exchange mechanism (Urban et al. (2004), Roy et al. (2004)). For 600 °C the Mn3+ dominates resulting in low value Msat. The value of saturation magnetization increases with increase in calcination temperature and was maximum at 800 °C. However, this value is very low to the other reports for LSMO (Rostamnejadi et al. (2011), Yanga et al. (2005), Chen et al. (2005), Rabelo et al. (2011)). The decrease in the magnetization may be due to the PAA–oleic acid capping, which are non-magnetic [Reddy et al. (2008)]. LSMO nanoparticles synthesized with our procedure does

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not exhibit superparamagnetic property, hence it could not be used in biomedical applications. However, with different switching characteristic, different loop area and magnetization this could be a potential candidate for spintronics applications. 0.4 0.3

Moment (emu/g)

0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -16000

-8000

0

8000

16000

Field (G) 4

0.03

0.02

Moment (emu/g)

Moment (emu/g)

2

0.01

0

-0.01

0

-2

-0.02 -4

-0.03 -16000

-8000

0 Field (G)

8000

16000

-16000

-8000

0

8000

16000

Field (G)

Fig. 8. M–H behavior of LSMO nanoparticle at different calcination temperature. All of these (except 500 °C) show ferromagnetic behavior with drastic difference in their loop area and magnetization values enabling them to use it in wide range of spin based applications.

4. Conclusion We have successfully synthesized LSMO nanoparticles with oxalic acid as chelating agent and PAA/oleic acid as polymer/surfactant. XRD reveals the phase pure formation of LSMO perovskite, which is further confirmed by FTIR. HRSEM reveals that these particles exhibit agglomers of well definite structures. Magnetic property shows all exhibit ferromagnetic nature with different magnetization and loop area except for 500 °C which shows paramagnetic nature. References Wu, W., He, Q., Jiang, C., 2008. Magnetic Iron Oxide Nanoparticles: Synthesis and Surface Functionalization Strategies, Nanoscale Research Letters 3, 397–415. Kostopoulou, A., Tsiaoussis, I., Lappas, A., 2013. Magnetic iron oxide nanoclusters with tunable optical response, Photonics and Nanostructures 9, 201–206. Simona Liliana conaru, Alina Mihaela Prodan, Philippe Le Coustumer, Daniela Predoi, 2013. Synthesis and Antibacterial and Antibiofilm Activity of Iron Oxide Glycerol Nanoparticles Obtained by Coprecipitation Method, Journal of Chemistry 2013. Mazur, M., Barras, A., Kuncser, V., Galatanu, A., Zaitzev, V., Turcheniuk, K.V., Woisel, P., Lyskawa, J., Laure, W., Siriwardena, A., Boukherroub, R., Szunerits, S., 2013. Iron oxide magnetic nanoparticles with versatile surface functions based on dopamine anchors, Nanoscale 5, 2692–2702.

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