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Synthesis, characterization and magnetic studies of uniform sized manganosite nanocrystals
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 4403–4411
www.materialstoday.com/proceedings
ISPAN-2015
Synthesis, characterization and magnetic studies of uniform sized manganosite nanocrystals J.S. Sherin a, M. Harris a, D. Shiney Manoj b and J.K. Thomas c* a Department of Physics, Karunya University, Coimbatore, India Department of Physics, Christian College Kattakada, University of Kerala, Thiruvananthapuram, Kerala, India c Department of Physics, Electronic Materials Research Laboratory, Mar Ivanios college, University of Kerala, Thiruvananthapuram, Kerala, India *e-mail: [email protected]. b
Abstract Synthesis of nano-MnO by a modified combustion technique and its suitability for various applications were reported. The structure and phase purity of the sample analyzed by X-ray diffraction, Fourier transform Raman, and infrared spectroscopy show that the sample is phase pure with cubic structure. The particle size from the transmission electron microscopy and scanning electron microscopy is 43nm and specifies spherical shape for MnO nanoparticles. The basic optical properties of the nano MnO were studied using UV-Visible absorption spectroscopy which showed that the material is a wide band gap semiconductor with band gap of 3.15 eV. The sample points out maximum absorbance in visible-near infrared region. Magnetic measurements indicate that the blocking temperature of the MnO structures is 22.4K for Hc = 100 Oe. The magnetization and coercivity of the nanocrystal under FC condition is found to be 0.5709 emu/g and 50.21 G. It is shown that MnO nanoparticles exhibits weak ferromagnetic behaviour due to uncompensated surface spins at low temperature. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Symposium on Photonics Applications and Nanomaterials (ISPAN-2015).
Keywords: Nanostructures; Oxides; Combustion synthesis; Electron microscopy; Magnetic studies
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Symposium on Photonics Applications and Nanomaterials (ISPAN-2015).
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1. Introduction The emerging nanoscience and nanotechnology stimulates extensive research interestin nanostructured metal oxides for potential applications. Particularly, the field of transition metal oxides represent an exciting and rapidly expanding research area that spans the border between the physical and engineering sciences. Among them, Manganese oxides are one of the most versatile cost effective magnetic materials for general use in both low and high frequency devices because of their high resistivity, low dielectric losses and good chemical stability [1-2]. The magnetic properties of manganese oxides strongly dependent on their chemical composition, density, grain size etc. [3]. In particular, MnO (Manganosite) is known to be an efficient electrode material for the preparation of Li-Mn-O electrodes for rechargeable lithium batteries and for soft magnetic materials, such as magnetic ferrite, which is applicable as magnetic cores in transformers for power supplies [4-6]. Manganese oxide is a potential metal oxide which crystallizes with rock salt structure in which the cation Mn2+ and O2- anions are both octahedrally co-ordinated [7-8]. Nano crystalline MnO is ferromagnetic although its bulk counterpart is anti-ferromagnetic with a Neel temperature of TN = 122 – 118 K [9]. The antiferromagnetic transition at Neel temperature is accompanied by a cubic to rhombohedral lattice distortion [10]. Magnetic structure and ordering, antiferromagnetic phase transition of MnO have drawn particular attention in stating precisely its magnetic properties [11-12]. Besides from the magnetic properties, it has been reported that MnO nanoparticles have higher oxidation – reduction activity compared to bulk MnO in an aqueous- alkaline medium[13]. Colloidal MnO nanoparticles prepared by the decomposition of manganese acetyl acetonate in olyelamine showed divergence in the magnetization measured under zero field cooled and field cooled conditions was reported by Seo et al. [14]. Lee et.al. reported that MnO nanoparticles are prepared by the decomposition of Mn2(CO)8 and found that ferromagnetic behaviour in the particles with an average diameter of around 5nm[15]. Park et al. synthesized MnO nanospheres with diameters in the 5 - 40 nm range and nanorods of 7-10 nm diameter by the trioctylphosphine and observed a divergence in the magnetization recorded under ZFC and FC conditions[16]. Yin et al. reported the synthesis of monodisperse nanocrystals of MnO by thermal decomposition of manganese acetate in the presence of Oleic acid at high temperature [17]. The literature contains various proposals for the synthesis of MnO nanomaterials using vapour deposition sol-gel, template direction, hydrothermal electro deposition or thermal decomposition method [18-23]. In this paper we report the synthesis of manganese oxide nanoparticles by facile combustion method as it is simple and cost effective technique. Here the study is aimed to examine the possibility of the synthesized nanopowder as a magnetic material. 2. Experimental Manganese chloride tetrahydrate (MnCl2.4H2O) dissolved in double distilled water were used as starting reagent for the preparation of nano MnO. Citric acid was then added to the based on total valence of the oxidising and reducing agents for maximum release of energyduring combustion. Oxidant to fuel ratio was adjusted to unity (~1) by adding concentrated nitric acid which serves as an oxidiser and ammonium hydroxide solution as a fuel. The precursorsolution of pH ~ 7.0 was stirred well for uniform mixing without any precipitation or sedimentation. The solution was then heated using a hot plate kept at ~250oC in a ventilatedfumehood. The solution boils on continuous heating and undergoes dehydration accompaniedby foam. On persistent heating, the foam gets auto-ignited by giving a voluminous fluffy powder of blackish brown nano MnO. In this typical synthesis, procedure two different concentration of nitric acid carrying from 0.4 and 0.8 M was added to two replicas of manganese chloride tetrahydrate solution and pH of all two final mixtures was measured to 6 and 6.5. Similarly, a different concentration of ammonium hydroxide 0.7 M was added to a replica of manganese chloride tetrahydrate and pH of the final mixture was measured to be 7.5. The solution was then heated using a hot plate kept at ~250oC in a ventilated fumehood. The solution boils on persistent heating and undergoes dehydration accompanied by foam. On continuous heating the foam gets auto ignited by itself giving a voluminous fluffy powder of nano MnO with pH values 6, 6.5 and 7.5.
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Structure of the as-prepared powders were examined by powder X-ray diffraction (XRD) technique using a Bruker D-8 X-ray diffractometer with nickel filtered Cu radiation. Particulate properties of the combustion product were examined using transmission electron microscopy (TEM, Model-Hitachi H-600 Japan) operating at 200 KV and the morphological study were done by JEOL/EOJSM- 6390 scanning electron microscope (SEM) operated at 5 KV. The Infrared (IR) spectra of the samples were recorded in the range 400–4000 cm−1 on a Thermo-Nicolet Avatar 370 Fourier transform infrared (FTIR) spectrometer using KBr pellet method. The optical measurements of the nanopowder were carried out at room temperature using a Cary 100 BIO UV-VIS spectrophotometer in the wavelength range from 200–700nm. The sample for the analysis was a 2m molar solution of MnO prepared by dispersing the nanopowder in ethanol taken in 1:20 volume ratio. The magnetic studies were done using a Quantum Design Vibrating Sample Magnetometer between 1 KOe and 15 KOe at15 K. ZFC and FC measurements were carried out at 100 Oe and 150 KOe and the blocking temperature was determined. 3. Results and Discussion The XRD pattern of the as-prepared MnO nanopowder at different pH is shown in Figure 1. All the peaks including the minor ones are indexed for a perfect cubic MnO nanoparticles with space group Fm3m. This clearly shows that the MnO phase formation was complete during the combustion process itself without the need for any postcalcinations step. Thus, modified combustion method offers an economic and time saving technique since the as-prepared powder itself is phase pure without any calcination at high temperature. The strong and sharp peaks indicate that the as prepared nanopowders are highly crystalline.
Fig 1. XRD pattern of MnO nanoparticle with different pH and MnO nanoparticle (pH = 7) before and after annealing No secondary phase or impurity peak was seen in the XRD pattern which confirms that as prepared powder itself is phase pure without any further processing. The lattice constants calculated from the XRD are shown in table 1. The calculated values of lattice constants are consistent with that reported in JCPDS NO 07-230. The crystallite size calculated from full width at half maximum (FWHM) using Scherrer formula. The small variations in the values could be due to the quantum size effect of nanoparticles and the rapid formation kinetic energy of MnO during the combustion process, resulting in a small distortion of the lattice. The crystallite size calculated for MnO nanoparticle using the Scherer formula is 31nm, 39nm, 43 nm, and 44nm as pH varies from 6, 6.5, 7 and 7.5 respectively. It has been observed that as the pH value increases the
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particle size also increases and strain decreases. Obtained results clearly indicate that the pH variation affects the crystal lattice, decreasing strain in the MnO nanostructures with the increase of pH. The strain was assumed to be uniform in all crystallographic directions, thus considering the isotropic nature of the crystal, where the material properties are independent of the direction along which they are measured. Based on the above results, it is inferred that the neutral medium (pH-7) of MnO nanopowder is suitable for preparing materials for optical and magnetic studies due to the superior nature of the particulate properties of the products.
Lattice Parameter (Å) pH Standard
Calculated
Particle size, D (nm)
Strain,ε
Dislocation density, δ (/m2)
6
4.445
5.05
31
0.117
9.98 X 10 14
6.5
4.445
4.93
39
0.114
6.57 X 10 14
7
4.445
4.44
43
0.113
5.41 X 10 14
7.5
4.445
4.21
44
0.113
5.16 X 10 14
Table1. The variation in the crystallite size, average lattice strain, dislocation density corresponding to different pH values Vibrational spectroscopy is a fine method for investigating the structural details of a compound. In order to understand the degree of structural disorder of the cubic MnO nanopowder FTIR spectra of the sample are recorded at different pH and at pH 7 are given in figure 2.
Fig. 2. FT-IR spectra of MnO at different pH and MnO at pH = 7 FTIR spectra of MnO are taken in the transmittance mode. Since MnO has octahedral symmetry (Oh), only 6 modes are infrared active and according to group theory Γ=2Au+3Eu
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Three main regions appear for the MnO vibration at 3000-2000 cm-1, 1700-900 cm-1 and 700-300 cm-1 which corresponds to the stretching, deformation and lattice modes respectively. The FT-IR peaks at 3437 and 1642 cm-1 corresponds to the δ (OH) bending mode and ν (OH) stretching mode respectively. The presence of adsorbed water molecules is helpful for enhanced supercapacitance. The peak at 942 cm-1 is the Eu mode which corresponds the Mn-OH deformation bending mode. The peaks at 472 and 415 cm-1 are the Au mode. This is assigned to the characteristic symmetric Mn-O stretching mode of the Mn2+cations in the cubic cage. SEM image of the manganese oxide nanocrystals are displayed in figure 3. The morphology of particles is found to be spherical and homogenously agglomerated.
Fig. 3. SEM micrograph of MnO nanoparticle Figure 4 shows the TEM image of as-prepared nano MnO nanoparticles. The average particle size calculated from the TEM micrograph is 45 nm. The particles in the TEM image are agglomerated which points to the crystalline nature of the nanopowder.
Fig. 4. TEM image of MnO nanoparticle
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The UV-Visible absorption spectra of nano-MnO are shown in the Figures 5 and 6. The spectrum reveals that both the samples absorb heavily in the visible region but moderately in the UV region. The high absorbance of light in the visible region indicates the applicability as an absorbing material which makes the material a good candidate in screening off UV portion of electromagnetic spectrum which is dangerous to human health. A film of this material can be used for coating eye glasses for protecting from sunburn caused by UV radiators.
Fig. 5. UV-Vis spectra of nanoMnO
Fig. 6. UV-Vis spectra of nano MnO at pH = 7 A semiconductor is characterised by its electronic band structure. The Wood and Tauc equation was used to estimate the optical band gap of nano-MnO nanopowder. According to this equation, the optical band gap energy is related with absorbance and photon energy by the following equation: αhυ = β(hυ- Eg)n. where α = 2.303A/ t is called the absorption coeffiecient, A is the aborbance, t is the path length of wave which is equal to the thickness of the curvette, β is energy independent constant h is the Planck’s constant , υ is the frequency of the incident photon, Eg is the optical band gap and n is a constant which characterizes the nature of band transition which characterizes the nature of band transition n = 1 2 and 3 2 corresponds to direct allowed and direct forbidden transitions, while m= 1 2 and 3 corrrespond to indirect allowed and indirect forbidden transitions
Sherin et al./ Materials Today: Proceedings 4 (2017) 4403–4411
respectively. The optical bandgap can be obtained by extrapolating the straight line portion of (αhυ) plot to αhυ = 0
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versus hυ
Fig.7. Tauc’s plot of MnO The variation of α with photon energy and the optical band gap is also illustrated in figure 7. The existence of sharp absorption band is an indication of excellent crystalline nature of the nanopowder.The band gap of the sample is found to be 3.1 eV, which is exactly matching with the value reported. As MnO prepared by the present method possess wide band gap along with good transmittance in the visible region is suitable for transparent conducting oxide films for window layers on solar cells. Magnetic properties of the MnO nanocrystals were studied by measuring the magnetization in both zerofield cooled (ZFC) and field cooled (FC) modes under 100 Oe and 150 Oe in the temperature range of 0K200K. Figure 8 shows the magnetization of MnO nanocrystals as a function of temperature. It has been reported that MnO nanoparticles show slight ferromagnetic ordering despite bulk MnO possess antiferromagnetic ordering with a TN of 122 K. It is expected that both a bulk and a nanoparticle system will develop scales on approaching higher temperature. Lee et al has been reported that very small MnO nanoparticles show weak ferromagnetic behaviour at low temperatures [15]. The weak ferromagnetism has been ascribed to the presence of non-compensated surface spins on the antiferromagnetic core of the MnO nanoparticle.
Fig. 8. Magnetization of MnO nanocrystal as function of temperature at 100 Oe and 150 Oe
The blocking temperature, TB is defined as the temperature corresponding to the peak point of ZFC curve and is a small length scale phenonmenon where the energy required to change the direction of the magnetic moment of particle. The transition temperature is found to have a value in the range of 21.5 - 22.8K. The curve indicates a ferromagnetic to paramagnetic phase transition at 22.5K. The above magnetic measurement shows that the MnO nanoparticle with
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diameters less than 45nm shows ferromagnetic behaviour at low temperatures is consistent with previous reports. The anomalous ferromagnetic behaviour can be related to not only the size effect but also the surface spin effect. It is likely that MnO nanoparticle with higher surface to volume ratio that exhibits much larger proportion of uncompensated surface spins on the antiferromagnetic core and thus reveals higher TB and magnetization values than those of larger nanoparticles. The magnetization curve of MnO nanoparticle was investigated in the range of 5K-20K with an applied field of 15 KOe to +15 KOe is shown in the figure 9.
Fig.9. H vs M of MnO The curve shows conclusively that MnO nanoparticles exhibit ferromagnetic behaviour at low temperatures. The hysteresis loop at different temperatures are found to 10.5, 12.6, 25.6 and 57.1 G respectively and remanence value of 1.05, 1.85, 5.25 and 10.59 emu/g which are the typical phenomena for a ferromagnetic material.These results are consistent with superparamagnetic theory which predicts a non-zero value of Hc at temperature lower than TB (22.5 K). The MnO nanocrystals synthesized in the present study have a greater magnetization than the larger nanoparticles.The sample possesses excellent magnetic behaviour which makes as a promising candidate for electrochromic devices and magnetic materials. 4. Conclusions Nanocrystalline semiconducting MnO was synthesized through a modified combustion process. The X-ray diffraction studies showed that the nanopowder was single phased with cubic structure. The FTIR spectral analysis confirms that the as-prepared powder itself is phase pure without any distortion. The SEM image of the sintered sample indicates that the material achieved high densification and particles are spherically agglomerated. TEM analysis confirms that the nanocrystalline nature of the sample has a mean size of 45nm. The UV-VIS spectra analysis revealed that the material is a wide band semiconductor of band gap 3.1eV along with good transmittance in the visible region which makes it suitable for transparent conducting oxide films for window layers on solar cells, warming coatings, and antireflection coatings. The magnetic properties of MnO nanoparticles were investigated by measuring the magnetization in both zero field cooled and field cooled modes under 100 Oe and150 Oe showed the superparamagnetic behaviour. The transition temperature for MnO is found to be 22.8 K, above which the material is paramagnetic. The hysteresis loop studies on MnO are the typical phenomena for ferromagnetic material which predicts a non-zero value of Hc at temperature lower than Tc. The sample possesses excellent optical and magnetic behaviour which makes this material a promising candidate for electrochromic devices and magnetic materials. Acknowledgements The authors acknowledge The Electronic materials research laboratory (EMRL), Mar Ivanios College, Thiruvananthapuram; Central Instrumentation Facility, Karunya University, Coimbatore and Central Instrumentation Centre, Pondicherry Central University, Pondicherry for the facilities provided by them.
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References
[1] Armstrong, A.R.; Bruce, P. G. Nature 381 (1996) 499-500. [2] Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J.T. Angew. Chem., Int. Ed. 2004, 43, 1115-1117. [3] Ghosh, M.; Biswas, K.; Sundaresan, A.; Rao, C. N. R. J. Mater. Chem. 16 (2006) 106-111. [4] Zhang, Y. C.; Qiao, T.; Hu, X. Y. J. Solid State Chem. 177 (2004) 4093-4097. [5] Yang, L. X.; Zhu, Y. J.; Tong, H.; Wang, W. W.; Cheng, G. F. J. Solid State Chem. 179 (2006) 1225-1229. [6] Na, C. W.; Han, D.S.; Kim, D. S.; Park, J.; Tae Jeon, Y.; Lee, G.; Jeon, Y. T.; Lee, G.; Jung, M. H. Appl. Phys. Lett. 87(2005) 142504-1-142504-3. [7] Palache, C.; Herman, H.; Frondel, C. Dana’s system of mineralogy. 1944, 7th edn, 501-502. [8] Gavarri, J. R.; Arabski, J.; Jasienska, S.; Janowski, J.; Garel, C. J. Solid state Chem., 1985, 58, 56-70. [9] Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.; Kim, S. H.; Hyeong; Ri, C. J. Am. Chem. Soc. 124 (2002) 12094 [10] Cheng, F.; Shen, J.; Zhanliang, T.; Chen, J. App. Mater. and Inter. 460 (2009). [11] Zhang, L. C.; Liu, Z. H.; Lv; Tang, X.; Ooi, K. J. Phys. Chem. C. III(2007) 8418. [12] Winter, M.; Brodd, R. J. Chem. Rev. 108 (2004) 13594. [13] Zhong, X.; Xie, R.; Sun, L.; Lieberwirth.; Knoll, W. J. Phys. Chem. B. 110 (2006) 2. [14] Seo, J. W.; Jeen, Y. W.; Ko, S. J.; Cheon, J. J. Phys. Chem. B. 109 (2005) 5389. [15] Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.; Kim, S. H.; Ri, H. C. J. Am. Chem. Soc. 124 (2002) 12094. [16] Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J.Y.; Park, J.H.; Hwang, N. M.; Hyeon, T. J. Phys. Chem. B. 108 (2004) 13596. [17] Yin, M.; O’ Brien. S. J. Am. Chem. Soc. 125 (2003) 10180. [18] Ahmad, T.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. J. Mater. Chem. 14 (2004) 8406 [19] Zhao, N.; Nie, W.; Liu, X.; Tian, S.; Zhang, Y.; Ji, X. Small 4 (2008) 77. [20] Mutin, P. H.; Vioux, A. Chem. Mater. 21 (2009) 582. [21] Liu, J.; Cai, J.; Son, Y. C.; Suib, S. L.; Aindow, M. J. Phy. Chem. B. 106 (2002) 976. [22] Yuan, J.; Li, W.N.; Gomez, S.; Suib, S.L. J. Am. Chem. Soc. 127(2005) 14184. [23] Wu, M. S.; Lee, J. T.; Wang, Y. Y.; Wan, C.C. J. Phys. Chem. B. 108 (2004) 16331.
ScienceDirect Materials Today: Proceedings 4 (2017) 4403–4411
www.materialstoday.com/proceedings
ISPAN-2015
Synthesis, characterization and magnetic studies of uniform sized manganosite nanocrystals J.S. Sherin a, M. Harris a, D. Shiney Manoj b and J.K. Thomas c* a Department of Physics, Karunya University, Coimbatore, India Department of Physics, Christian College Kattakada, University of Kerala, Thiruvananthapuram, Kerala, India c Department of Physics, Electronic Materials Research Laboratory, Mar Ivanios college, University of Kerala, Thiruvananthapuram, Kerala, India *e-mail: [email protected]. b
Abstract Synthesis of nano-MnO by a modified combustion technique and its suitability for various applications were reported. The structure and phase purity of the sample analyzed by X-ray diffraction, Fourier transform Raman, and infrared spectroscopy show that the sample is phase pure with cubic structure. The particle size from the transmission electron microscopy and scanning electron microscopy is 43nm and specifies spherical shape for MnO nanoparticles. The basic optical properties of the nano MnO were studied using UV-Visible absorption spectroscopy which showed that the material is a wide band gap semiconductor with band gap of 3.15 eV. The sample points out maximum absorbance in visible-near infrared region. Magnetic measurements indicate that the blocking temperature of the MnO structures is 22.4K for Hc = 100 Oe. The magnetization and coercivity of the nanocrystal under FC condition is found to be 0.5709 emu/g and 50.21 G. It is shown that MnO nanoparticles exhibits weak ferromagnetic behaviour due to uncompensated surface spins at low temperature. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Symposium on Photonics Applications and Nanomaterials (ISPAN-2015).
Keywords: Nanostructures; Oxides; Combustion synthesis; Electron microscopy; Magnetic studies
2214-7853© 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Symposium on Photonics Applications and Nanomaterials (ISPAN-2015).
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Sherin et al./ Materials Today: Proceedings 4 (2017) 4403–4411
1. Introduction The emerging nanoscience and nanotechnology stimulates extensive research interestin nanostructured metal oxides for potential applications. Particularly, the field of transition metal oxides represent an exciting and rapidly expanding research area that spans the border between the physical and engineering sciences. Among them, Manganese oxides are one of the most versatile cost effective magnetic materials for general use in both low and high frequency devices because of their high resistivity, low dielectric losses and good chemical stability [1-2]. The magnetic properties of manganese oxides strongly dependent on their chemical composition, density, grain size etc. [3]. In particular, MnO (Manganosite) is known to be an efficient electrode material for the preparation of Li-Mn-O electrodes for rechargeable lithium batteries and for soft magnetic materials, such as magnetic ferrite, which is applicable as magnetic cores in transformers for power supplies [4-6]. Manganese oxide is a potential metal oxide which crystallizes with rock salt structure in which the cation Mn2+ and O2- anions are both octahedrally co-ordinated [7-8]. Nano crystalline MnO is ferromagnetic although its bulk counterpart is anti-ferromagnetic with a Neel temperature of TN = 122 – 118 K [9]. The antiferromagnetic transition at Neel temperature is accompanied by a cubic to rhombohedral lattice distortion [10]. Magnetic structure and ordering, antiferromagnetic phase transition of MnO have drawn particular attention in stating precisely its magnetic properties [11-12]. Besides from the magnetic properties, it has been reported that MnO nanoparticles have higher oxidation – reduction activity compared to bulk MnO in an aqueous- alkaline medium[13]. Colloidal MnO nanoparticles prepared by the decomposition of manganese acetyl acetonate in olyelamine showed divergence in the magnetization measured under zero field cooled and field cooled conditions was reported by Seo et al. [14]. Lee et.al. reported that MnO nanoparticles are prepared by the decomposition of Mn2(CO)8 and found that ferromagnetic behaviour in the particles with an average diameter of around 5nm[15]. Park et al. synthesized MnO nanospheres with diameters in the 5 - 40 nm range and nanorods of 7-10 nm diameter by the trioctylphosphine and observed a divergence in the magnetization recorded under ZFC and FC conditions[16]. Yin et al. reported the synthesis of monodisperse nanocrystals of MnO by thermal decomposition of manganese acetate in the presence of Oleic acid at high temperature [17]. The literature contains various proposals for the synthesis of MnO nanomaterials using vapour deposition sol-gel, template direction, hydrothermal electro deposition or thermal decomposition method [18-23]. In this paper we report the synthesis of manganese oxide nanoparticles by facile combustion method as it is simple and cost effective technique. Here the study is aimed to examine the possibility of the synthesized nanopowder as a magnetic material. 2. Experimental Manganese chloride tetrahydrate (MnCl2.4H2O) dissolved in double distilled water were used as starting reagent for the preparation of nano MnO. Citric acid was then added to the based on total valence of the oxidising and reducing agents for maximum release of energyduring combustion. Oxidant to fuel ratio was adjusted to unity (~1) by adding concentrated nitric acid which serves as an oxidiser and ammonium hydroxide solution as a fuel. The precursorsolution of pH ~ 7.0 was stirred well for uniform mixing without any precipitation or sedimentation. The solution was then heated using a hot plate kept at ~250oC in a ventilatedfumehood. The solution boils on continuous heating and undergoes dehydration accompaniedby foam. On persistent heating, the foam gets auto-ignited by giving a voluminous fluffy powder of blackish brown nano MnO. In this typical synthesis, procedure two different concentration of nitric acid carrying from 0.4 and 0.8 M was added to two replicas of manganese chloride tetrahydrate solution and pH of all two final mixtures was measured to 6 and 6.5. Similarly, a different concentration of ammonium hydroxide 0.7 M was added to a replica of manganese chloride tetrahydrate and pH of the final mixture was measured to be 7.5. The solution was then heated using a hot plate kept at ~250oC in a ventilated fumehood. The solution boils on persistent heating and undergoes dehydration accompanied by foam. On continuous heating the foam gets auto ignited by itself giving a voluminous fluffy powder of nano MnO with pH values 6, 6.5 and 7.5.
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Structure of the as-prepared powders were examined by powder X-ray diffraction (XRD) technique using a Bruker D-8 X-ray diffractometer with nickel filtered Cu radiation. Particulate properties of the combustion product were examined using transmission electron microscopy (TEM, Model-Hitachi H-600 Japan) operating at 200 KV and the morphological study were done by JEOL/EOJSM- 6390 scanning electron microscope (SEM) operated at 5 KV. The Infrared (IR) spectra of the samples were recorded in the range 400–4000 cm−1 on a Thermo-Nicolet Avatar 370 Fourier transform infrared (FTIR) spectrometer using KBr pellet method. The optical measurements of the nanopowder were carried out at room temperature using a Cary 100 BIO UV-VIS spectrophotometer in the wavelength range from 200–700nm. The sample for the analysis was a 2m molar solution of MnO prepared by dispersing the nanopowder in ethanol taken in 1:20 volume ratio. The magnetic studies were done using a Quantum Design Vibrating Sample Magnetometer between 1 KOe and 15 KOe at15 K. ZFC and FC measurements were carried out at 100 Oe and 150 KOe and the blocking temperature was determined. 3. Results and Discussion The XRD pattern of the as-prepared MnO nanopowder at different pH is shown in Figure 1. All the peaks including the minor ones are indexed for a perfect cubic MnO nanoparticles with space group Fm3m. This clearly shows that the MnO phase formation was complete during the combustion process itself without the need for any postcalcinations step. Thus, modified combustion method offers an economic and time saving technique since the as-prepared powder itself is phase pure without any calcination at high temperature. The strong and sharp peaks indicate that the as prepared nanopowders are highly crystalline.
Fig 1. XRD pattern of MnO nanoparticle with different pH and MnO nanoparticle (pH = 7) before and after annealing No secondary phase or impurity peak was seen in the XRD pattern which confirms that as prepared powder itself is phase pure without any further processing. The lattice constants calculated from the XRD are shown in table 1. The calculated values of lattice constants are consistent with that reported in JCPDS NO 07-230. The crystallite size calculated from full width at half maximum (FWHM) using Scherrer formula. The small variations in the values could be due to the quantum size effect of nanoparticles and the rapid formation kinetic energy of MnO during the combustion process, resulting in a small distortion of the lattice. The crystallite size calculated for MnO nanoparticle using the Scherer formula is 31nm, 39nm, 43 nm, and 44nm as pH varies from 6, 6.5, 7 and 7.5 respectively. It has been observed that as the pH value increases the
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particle size also increases and strain decreases. Obtained results clearly indicate that the pH variation affects the crystal lattice, decreasing strain in the MnO nanostructures with the increase of pH. The strain was assumed to be uniform in all crystallographic directions, thus considering the isotropic nature of the crystal, where the material properties are independent of the direction along which they are measured. Based on the above results, it is inferred that the neutral medium (pH-7) of MnO nanopowder is suitable for preparing materials for optical and magnetic studies due to the superior nature of the particulate properties of the products.
Lattice Parameter (Å) pH Standard
Calculated
Particle size, D (nm)
Strain,ε
Dislocation density, δ (/m2)
6
4.445
5.05
31
0.117
9.98 X 10 14
6.5
4.445
4.93
39
0.114
6.57 X 10 14
7
4.445
4.44
43
0.113
5.41 X 10 14
7.5
4.445
4.21
44
0.113
5.16 X 10 14
Table1. The variation in the crystallite size, average lattice strain, dislocation density corresponding to different pH values Vibrational spectroscopy is a fine method for investigating the structural details of a compound. In order to understand the degree of structural disorder of the cubic MnO nanopowder FTIR spectra of the sample are recorded at different pH and at pH 7 are given in figure 2.
Fig. 2. FT-IR spectra of MnO at different pH and MnO at pH = 7 FTIR spectra of MnO are taken in the transmittance mode. Since MnO has octahedral symmetry (Oh), only 6 modes are infrared active and according to group theory Γ=2Au+3Eu
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Three main regions appear for the MnO vibration at 3000-2000 cm-1, 1700-900 cm-1 and 700-300 cm-1 which corresponds to the stretching, deformation and lattice modes respectively. The FT-IR peaks at 3437 and 1642 cm-1 corresponds to the δ (OH) bending mode and ν (OH) stretching mode respectively. The presence of adsorbed water molecules is helpful for enhanced supercapacitance. The peak at 942 cm-1 is the Eu mode which corresponds the Mn-OH deformation bending mode. The peaks at 472 and 415 cm-1 are the Au mode. This is assigned to the characteristic symmetric Mn-O stretching mode of the Mn2+cations in the cubic cage. SEM image of the manganese oxide nanocrystals are displayed in figure 3. The morphology of particles is found to be spherical and homogenously agglomerated.
Fig. 3. SEM micrograph of MnO nanoparticle Figure 4 shows the TEM image of as-prepared nano MnO nanoparticles. The average particle size calculated from the TEM micrograph is 45 nm. The particles in the TEM image are agglomerated which points to the crystalline nature of the nanopowder.
Fig. 4. TEM image of MnO nanoparticle
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The UV-Visible absorption spectra of nano-MnO are shown in the Figures 5 and 6. The spectrum reveals that both the samples absorb heavily in the visible region but moderately in the UV region. The high absorbance of light in the visible region indicates the applicability as an absorbing material which makes the material a good candidate in screening off UV portion of electromagnetic spectrum which is dangerous to human health. A film of this material can be used for coating eye glasses for protecting from sunburn caused by UV radiators.
Fig. 5. UV-Vis spectra of nanoMnO
Fig. 6. UV-Vis spectra of nano MnO at pH = 7 A semiconductor is characterised by its electronic band structure. The Wood and Tauc equation was used to estimate the optical band gap of nano-MnO nanopowder. According to this equation, the optical band gap energy is related with absorbance and photon energy by the following equation: αhυ = β(hυ- Eg)n. where α = 2.303A/ t is called the absorption coeffiecient, A is the aborbance, t is the path length of wave which is equal to the thickness of the curvette, β is energy independent constant h is the Planck’s constant , υ is the frequency of the incident photon, Eg is the optical band gap and n is a constant which characterizes the nature of band transition which characterizes the nature of band transition n = 1 2 and 3 2 corresponds to direct allowed and direct forbidden transitions, while m= 1 2 and 3 corrrespond to indirect allowed and indirect forbidden transitions
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respectively. The optical bandgap can be obtained by extrapolating the straight line portion of (αhυ) plot to αhυ = 0
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versus hυ
Fig.7. Tauc’s plot of MnO The variation of α with photon energy and the optical band gap is also illustrated in figure 7. The existence of sharp absorption band is an indication of excellent crystalline nature of the nanopowder.The band gap of the sample is found to be 3.1 eV, which is exactly matching with the value reported. As MnO prepared by the present method possess wide band gap along with good transmittance in the visible region is suitable for transparent conducting oxide films for window layers on solar cells. Magnetic properties of the MnO nanocrystals were studied by measuring the magnetization in both zerofield cooled (ZFC) and field cooled (FC) modes under 100 Oe and 150 Oe in the temperature range of 0K200K. Figure 8 shows the magnetization of MnO nanocrystals as a function of temperature. It has been reported that MnO nanoparticles show slight ferromagnetic ordering despite bulk MnO possess antiferromagnetic ordering with a TN of 122 K. It is expected that both a bulk and a nanoparticle system will develop scales on approaching higher temperature. Lee et al has been reported that very small MnO nanoparticles show weak ferromagnetic behaviour at low temperatures [15]. The weak ferromagnetism has been ascribed to the presence of non-compensated surface spins on the antiferromagnetic core of the MnO nanoparticle.
Fig. 8. Magnetization of MnO nanocrystal as function of temperature at 100 Oe and 150 Oe
The blocking temperature, TB is defined as the temperature corresponding to the peak point of ZFC curve and is a small length scale phenonmenon where the energy required to change the direction of the magnetic moment of particle. The transition temperature is found to have a value in the range of 21.5 - 22.8K. The curve indicates a ferromagnetic to paramagnetic phase transition at 22.5K. The above magnetic measurement shows that the MnO nanoparticle with
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diameters less than 45nm shows ferromagnetic behaviour at low temperatures is consistent with previous reports. The anomalous ferromagnetic behaviour can be related to not only the size effect but also the surface spin effect. It is likely that MnO nanoparticle with higher surface to volume ratio that exhibits much larger proportion of uncompensated surface spins on the antiferromagnetic core and thus reveals higher TB and magnetization values than those of larger nanoparticles. The magnetization curve of MnO nanoparticle was investigated in the range of 5K-20K with an applied field of 15 KOe to +15 KOe is shown in the figure 9.
Fig.9. H vs M of MnO The curve shows conclusively that MnO nanoparticles exhibit ferromagnetic behaviour at low temperatures. The hysteresis loop at different temperatures are found to 10.5, 12.6, 25.6 and 57.1 G respectively and remanence value of 1.05, 1.85, 5.25 and 10.59 emu/g which are the typical phenomena for a ferromagnetic material.These results are consistent with superparamagnetic theory which predicts a non-zero value of Hc at temperature lower than TB (22.5 K). The MnO nanocrystals synthesized in the present study have a greater magnetization than the larger nanoparticles.The sample possesses excellent magnetic behaviour which makes as a promising candidate for electrochromic devices and magnetic materials. 4. Conclusions Nanocrystalline semiconducting MnO was synthesized through a modified combustion process. The X-ray diffraction studies showed that the nanopowder was single phased with cubic structure. The FTIR spectral analysis confirms that the as-prepared powder itself is phase pure without any distortion. The SEM image of the sintered sample indicates that the material achieved high densification and particles are spherically agglomerated. TEM analysis confirms that the nanocrystalline nature of the sample has a mean size of 45nm. The UV-VIS spectra analysis revealed that the material is a wide band semiconductor of band gap 3.1eV along with good transmittance in the visible region which makes it suitable for transparent conducting oxide films for window layers on solar cells, warming coatings, and antireflection coatings. The magnetic properties of MnO nanoparticles were investigated by measuring the magnetization in both zero field cooled and field cooled modes under 100 Oe and150 Oe showed the superparamagnetic behaviour. The transition temperature for MnO is found to be 22.8 K, above which the material is paramagnetic. The hysteresis loop studies on MnO are the typical phenomena for ferromagnetic material which predicts a non-zero value of Hc at temperature lower than Tc. The sample possesses excellent optical and magnetic behaviour which makes this material a promising candidate for electrochromic devices and magnetic materials. Acknowledgements The authors acknowledge The Electronic materials research laboratory (EMRL), Mar Ivanios College, Thiruvananthapuram; Central Instrumentation Facility, Karunya University, Coimbatore and Central Instrumentation Centre, Pondicherry Central University, Pondicherry for the facilities provided by them.
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