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Effect of Zn and Ni substitution on structural, morphological and magnetic properties of tin oxide nanoparticles
Author’s Accepted Manuscript Effect of zn and ni substitution on structural, morphological and magnetic properties of tin oxide nanoparticles S. Bhuvana, H.B. Ramalingam, K. Vadivel, E. Ranjith Kumar, Ahmad I. Ayesh www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)31321-X http://dx.doi.org/10.1016/j.jmmm.2016.07.004 MAGMA61616
To appear in: Journal of Magnetism and Magnetic Materials Received date: 4 April 2016 Revised date: 10 June 2016 Accepted date: 1 July 2016 Cite this article as: S. Bhuvana, H.B. Ramalingam, K. Vadivel, E. Ranjith Kumar and Ahmad I. Ayesh, Effect of zn and ni substitution on structural, morphological and magnetic properties of tin oxide nanoparticles, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Zn and Ni substitution on structural, morphological and magnetic properties of tin oxide nanoparticles S. Bhuvana1, 2 H.B. Ramalingam 3, K. Vadivel 3, E. Ranjith Kumar2 Ahmad I. Ayesh4 1
Research and Development Centre, Bharathiar University, Coimbatore-641046, Tamilnadu, India
2
Department of Physics, Dr. NGP Institute of Technology, Coimbatore- 641048, Tamilnadu, India
3
Department of Physics, Government Arts College, Udumalpet - 642126. Tamilnadu, India
4
Department of Math., Stat. and Physics, Qatar University, Doha, Qatar
Abstract Structural, morphological, optical and magnetic properties of Zn and Zn-Ni co-doped tin oxide (SnO2) nanoparticles synthesized by sol-gel method. The influence of doping concentration on phase and particle size of the nanoparticles was determined by X-ray diffraction. The XRD study reveals that the lattice constant and crystallite size of the samples decrease with the increase of doping concentration. The change in the band gap energy of SnO2 nanoparticles influenced more by doping with Zn and Ni. The external morphology and particle size were recorded by SEM and TEM. The results indicated that Ni2+ions would uniformly substituted into the Zn2+ sites of SnO2 lattice. The substitution of Ni creates a vital change in magnetic properties that has been measured by vibrating sample magnetometer (VSM). Keywords: Magnetic materials; Sol-gel; X-ray diffraction; DRS; VSM; TEM 1. Introduction Metal oxide nanoparticles are found to exhibit interesting structural and magnetic properties. The development of semiconductor and magnetic nanoparticles has been intensively pursued due to their technological and fundamental scientific importance [1]. Several interesting phenomena have been discovered in recent years based on the studies of magnetic and non-magnetic nanoparticles [2]. Various physical properties of these nanoparticles are greatly influenced by the distribution of cations, nature of grain, grain boundaries, inhomogeneities, chemical defects and oxygen deficiency, and etc. These nanoparticles have received great attention as a result of their magnetic and electronic properties. Such nanoparticles are currently used in microwave
devices, sensors and bioprocessing [3]. There are several methods for synthesizing nanoparticles, such as co-precipitation [4-6], solid state reaction method [7], sol-gel method [8, 9], and hydrothermal method [10, 11]. The present work focuses on the preparation of tin oxide based diluted semiconductor material with tunable ferromagnetic property using the sol-gel method. This semiconductor has a wide band gap (3.5 eV - 3.8 eV) with high optical transparency in the visible region and high electrical conductivity which are the most essential features of tin oxide. Room temperature ferromagnetism has been reported in 3d transition metal doped In2O3, Cu2O3, TiO2 and SnO2 [12-15]. Magnetic properties have been reported in Fe, Mn, and Co doped tin oxide by many research groups [16-21]. Here, we make an attempt to synthesize Zn and Zn-Ni co-doped tin oxide nanopowders. The influence of (Zn – Ni) doping level on the structural, optical and magnetic properties of SnO2 nanoparticles is analyzed. 2. Experimental method 2.1 Method of Preparation Zn doped tin oxide and (Zn – Ni) – co-doped tin oxide samples were prepared by simple and low cost sol-gel method. SnCl4.5H2O, NiCl2.6H2O, ZnCl2 analytical grade chemicals were purchased from the Sigma Aldrich. Ethanol HPCL grade was purchased from Merck Scientific and used as received. 1 gm of SnCl4.5H2O and 50 ml of ethanol (HPLC grade) taken in 100 ml beaker and stirred at 300 rpm at 50 ºC for 5 hours, in that solution different concentration of Zn salt was added. The resultant solution beakers covered with aluminum foil sheet with a pin holed, kept in room temperature in un-disturbed condition for 30 days. After the gel formation the gel was annealed in a muffle furnace at 450 ºC for 5 hours, then the material was removed from the furnace after reaching room temperature. Ni co-doping was performed similarly. Separate solutions of Sn1xZnxO2
(x = 0.07 to 0.10) and Sn1-xNixO2 (x = 0.07 to 0.10) were mixed together and
stirred at 50 oC for five hours, aged for one month. The solution was then kept under heat treatment for one hour at 450 oC to obtain nanopowders of Sn (2-(x + y)) NixZnyO2, (x = y = 0.07 to 0.10).
2.2. Equipment used for characterization
The structure and phase identification of the prepared nanomaterial was done through powder X-ray diffraction (PXRD) analysis carried by Rigaku-Miniflex-II using Cu Kα (1.54 Å radiation). The UV-Visible diffuse reflectance spectra (DRS) of the prepared
material
were
measured
by
UV-VIS
Schimadzu
(UV-2550
spectrophotometer) using BaSO4 from 200 to 800nm to estimate their band gap energy. Morphological studies of prepared sample have been performed with a JSM – 6360 scanning electron microscope. High resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) measurments were recorded on a Technai G20-stwin using an accelerating voltage of 200 kV. Magnetic measurements were carried out at room temperature using a (Lake shore- 7410) vibrating sample magnetometer (VSM) equipped with a 1 T magnet. 3. Results and discussion 3.1 X-ray diffraction (XRD) analysis The XRD pattern was used to determine the phase compositions of Zn doped and Zn-Ni co-doped tin oxide nanoparticles are shown in Figs. 1 and 2. The observed peak at (110), (101), (211), and (301) of Zn and Zn-Ni co-doped samples attributed to rutile structure verified from standard JCPDS data.The peak broadening is due to small crystallite size. The intensity of the X-ray peaks for SnO2 samples decreases with increasing Zn-Ni co doping concentration. The crystallite size is estimated using the Scherrer formula D
k cos
(1)
Where D is the grain size, K is a constant taken to be 0.94, is the wavelength of the Xray radiation, is the full width at half maximum, is the angle of diffraction and (hkl) is the planes values. The lattice constant is calculated using Eq. (2).
1 h2 k2 d 2 a 2
l2 2 c
(2)
The crystallite size and lattice constant of the samples are listed in Table 1. The table clearly shows that the crystallite size of the samples decreases with increasing doping concentration as shown in Fig. 3.
3.2. Diffuse reflectance spectroscopy (DRS) The optical properties of Sn1-xZnxO2 [x = 0.07 to 0.10] and Sn2-(x + y) NixZnyO2, [x = y = 0.07 to 0.10] nanopowders were investigated by UV-Visible diffuse reflectance spectra. For this purpose, we used a two-flux (Kubelka-munk) radioactive transfer model to investigate the absorption and scattering of the radiation in the diluted magnetic semiconductors. It has been shown for a crystalline direct semiconductor that the optical absorption near the band edges follows the equation
( )
(
)1/2, where α, , Eg and A are the absorption
coefficient, light frequency, band gap and a constant, respectively. It is generally admitted that the absorption coefficient (α) can be replaced by the remission function, F(R). The latter can be written in terms of diffused reflectance (R), according to the Kubelka –munk theory, α
( )
(
)
, where ‘s’ is the scattering coefficient. The band gap energies of the
synthesized samples can then be estimated from a plot of [F(R)h ]2 Vs F(h ). From the intercepts of the tangents the band gap of the synthesized samples can be determined from Figure 4and 5. The Kubelka-munk model was used to determine the band gap of powder samples where the band gap energy of the synthesized samples can be estimated from a plot of [F(R) h ]2 Vs F (h ). The band gap of the synthesized samples can be determined from the intercepts of the tangents in Figs. 4 and 5. The estimated band gap energies of the Zn doped tin oxide samples were between 3.6 to 4.0 eV, while forZn-Ni co-doped tin oxide the band gap decreases from 4.6 to 3.2 eV, when Ni concentration increases (see Table.2). The band gap value of the Zn-Ni co-doped samples is obviously smaller than the reported value of undoped tin oxide powders samples. It can be deduced from the decrease in optical band gap that Ni ions have been incorporated into Sn sites of SnO2 lattice homogeneously. 3.3. Morphological analysis
Figures 6(a) - 6(d) show the external morphology of Zn-Ni co-doped nanopowders was recorded by SEM. The morphological change with respect to doping concentration is clearly observed. Figure7(a) shows a TEM image of Zn-Ni co-doped (x=0.1) tin oxide nanopowders. The particle size could be estimated from the TEM images to ~ 6, which is close to that calculated from XRD. The agglomeration is due to the presence of magnetic interactions between the particles [22]. Figure 7(b) reports the corresponding SAED pattern, which indicates the well crystallized nanoparticles.
3.4. Magnetic properties The magnetization measurements (M-H) of samples at room temperature have been studied by VSM. Figure 8 shows the M-H curves of Zn, doped and Fig. 9 shows the Zn-Ni co-doped SnO2 nanoparticles. The variation of magnetic parameters such as saturation magnetization and coercivity with respect to Zn and Zn-Ni co-doping concentration were analyzed. The Zn doped SnO2 exhibits diamagnetic behavior irrespective of different concentration of Zn doping with SnO2. The expected room temperature ferromagnetism is not found in Zn doping in the SnO2 diluted semiconductor material. The hysteresis loop of the Zn-Ni codoped SnO2 is due to changes of the magnetic property from dia to weak ferromagnetism [5]. The magnetization and coercivity values of Ni-Zn co-doped samples are calculated and shown in the Table.2. The table shows that the saturation magnetization decreases with increasing the Zn-Ni concentration. The presence of Zn doping in SnO2 effectively hampers the cluster formation in the SnO2 host. Further it is suggested that the presence of Zn mediates the Ni2+ distribution in the SnO2. Thus, Ni-Zn co-doped tin oxide offer a great deal of interest as a potential candidate for spintronic devices because of its room temperature ferromagnetism behavior. Hence, co-doping plays an important role in inducing ferromagnetism in the sample. 4. Conclusions Zn doped tin oxide as well as Ni-Zn co-doped tin oxide nanopowders were synthesized by sol-gel method and annealed at 450 oC. XRD pattern reveals that all investigated samples are nanocrystallite powder of rutile type tetragonal structure and crystallite size is in the range of 6.0 to 4.77 nm. XRD results also indicate that the crystallite size decreases with increasing the doping concentration. DRS studies expose that the band gap energy decreases with increase of doping concentration.
Morphological change of the nanoparticles with respect to doping concentration was clearly visualized by SEM and TEM images. The room temperature ferromagnetism was obtained for SnO2 nanoparticles with increasing Ni concentration which was identified by VSM studies. Thus Ni concentration plays an important role in inducing ferromagnetism in the SnO2 nanopowders
References [1] A. Paul, Pramana – J. Phys 78 (2012) 1-58. [2] Hideo Ohno- Nat. Mater, 9 (2010) 952-954. [3] S.J Liu, C.Y Liu, J.Y Juang, H.W. Fang, J.app.phys. 105 (2009) 013928. [4] Aditya Sharma, Mayora Varshney, Shalendra kumar, K.D. Verma, Ravikumar, Nanomater - nanotechnol, 1 (2011) 29-33. [5] M. Saravanakumar, S. Agilan, N. Muthukumarasamy, V.Rukkumani, Acta. Phys. Pol. A , 127 (2015) 1656-1660. [6] V.Bilovol, C.Herme, S.Jacobo, A.F. Cabrera, Mater. Chem. Phys, 135 (2012) 334339. [7] S.P. Raja, C. Venkateswaran, J. Phys.D: Appl. Phys. 42 (2009) 145001. [8] J. Khemprasitn, S. Kaen-ngam, B.Khumpaitool, P. Kamkhou, J. Magn. Magn. Mater, 323 (2011) 2408. [9] P. Thongbai, T.Yamwong, S. Maensiri, Mater.Chem. Phys. 123 (2010) 56. [10] S. Manna, A. K. Deb, J. Jagannath, S. K. De, J. Phys. Chem. C 112 (2008) 10659. [11] Samar Layek, H. C. Verma, J. Magn. Magn. Mater, 397 (2016) 73-78. [12] K. Vadivel , K. Arivazhagan, S. Rajesh, IJAP 1 (2011) 129-135. [13] M. Venkatesan, C.B Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett 93 (2004) 177206 - 177209. [14] Nguyen Hoa Hong, Antoine Ruyter, W. Prellier, Joe Sakai, Ngo Thu Buon, J. Phys Condens Matter. 17 (2005) 6533-6538. [15] J. Dubowik, I. Goscianska, Y.V. Kudryavtsev, A. Szlaferek, J. Magn. Magn. Mater, 310 (2007) 2773- 2775. [16] C.M. Liu, L. M.Fang, X.T. Zu, W.L. Zhou , Phys. Scripta, 8 (2009) 6. [17] Yuhua Xia, Shi Hui Ge, Lixi, Xueyunzhou, Appl. Surf. Sci. 254 (2008) 74597463. [18] Subhash C. Kashyap, K. Gopinadhan, D.K. Pandya Suject Chaudhary, J. Magn. Magn. Mater, 321 (2008) 957- 962.
[19] Junying Zhang, Qu Yun, Qianghong Wang, MAS, 4(11) (2010) 124 - 130. [20] K.R. Kittilstved, W.L. Liu, D.R. Gamelin, Nat. Mater 2 (2006) 296. [21] R.K. Singhal, Arvind Samariya, Y.T. Xing, sudhish kumar, S.N. Dolia, U.P. Deshpande, T. Shripathi, Elisa B.Saitovitch, J. Alloys. Compd. 496 (1) (2010) 324 -330. [22] T. Prakash, G. Neri, A. Bonavita, E. Ranjith Kumar, K. Gnanamoorthi, J.Mater..Sci, 26 (2015) 4913-4921.
Figure caption Fig.1 XRD patterns of Zn doped doped tin oxide nanopowders Fig. 2 XRD patterns of Zn-Ni co-doped tin oxide nanopowders. Fig. 3 Average crystallite size versus co-doping concentration of (Zn-Ni) co-doped tin oxide nanopowders
Fig. 4 Band gap energy graph of Zn doped tin oxide Fig. 5 Band gap energy of (Zn-Ni) co doped tin oxide Fig. 6 SEM image: (Zn- Ni ) co doped tin oxide nano powders a) concentration of dopant (x=0.07). b) (x=0.08). c) ( x=0.09) d) (x=0.1) Fig.7 (a) TEM micrograph of (Zn–Ni = 0.1) co doping SnO2. (b) SAED Pattern Fig.8 M-H curves of Zn doped with SnO2 Fig.9 M-H curves of (Zn-Ni) co doped with SnO2
Table 1 Effect of doping concentration on crystallite size and lattice constant of SnO2 nanoparticles
Parameters
crystallite Size
Zn=0.
Zn=0.
Zn=0.
Zn=0.
Zn=0.
Zn=0
Zn=0. 07
08
09
Zn=0. 1
07
08
09
.1
Ni=0. 07
Ni=0.
Ni=0.
Ni=0. 1
08
09
3.6
3.9
4.5
5.0
6.1
5.2
5.17
4.8
4.32
4.35
4.34
4.32
4.01
4.01
4
4.01
( t) nm Lattice Constant (a) Å
Table 2 Calculated magnetization, coercivity and band gap for Zn and (Zn-Ni) co-doped tin oxide nanopowders. Zn=0.
Paramete rs
Zn=0.
Zn=0.
Zn=0.
07
08
09
Zn=0. 1
Zn=0. 07
08
Ni=0. 07
Ni=0.
09
08
Band
Zn=0.
Ni=0. 09
Zn=0. 1 Ni=0. 1
3.7
3.9
4.0
4.2
4.7
4.4
3.6
3.3
0.0052
0.0061
0.011
0.0067
0.08
0.075
0.026
0.015
587.79
658.28
1429.9
846.31
480.93
497.62
513.19
1049.4
Gap(eV) Magnetiz ation Coercivit y
Highlights Sn2-(x+y) NixZnyO2, (x = y = 0.07 to 0.10) nano particles are prepared by simple sol gel
method. X-ray diffraction data confirms the single phase rutile tetragonal structure.
The VSM was used to confirm, the codoping of (Ni, Zn) increases the magnetic moment of the sample prepared.
Inducing ferromagnetism in sample makes it suitable for future spintronics applications.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
50 nm
Fig. 7
Fig. 8
Fig. 9
PII: DOI: Reference:
S0304-8853(16)31321-X http://dx.doi.org/10.1016/j.jmmm.2016.07.004 MAGMA61616
To appear in: Journal of Magnetism and Magnetic Materials Received date: 4 April 2016 Revised date: 10 June 2016 Accepted date: 1 July 2016 Cite this article as: S. Bhuvana, H.B. Ramalingam, K. Vadivel, E. Ranjith Kumar and Ahmad I. Ayesh, Effect of zn and ni substitution on structural, morphological and magnetic properties of tin oxide nanoparticles, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Zn and Ni substitution on structural, morphological and magnetic properties of tin oxide nanoparticles S. Bhuvana1, 2 H.B. Ramalingam 3, K. Vadivel 3, E. Ranjith Kumar2 Ahmad I. Ayesh4 1
Research and Development Centre, Bharathiar University, Coimbatore-641046, Tamilnadu, India
2
Department of Physics, Dr. NGP Institute of Technology, Coimbatore- 641048, Tamilnadu, India
3
Department of Physics, Government Arts College, Udumalpet - 642126. Tamilnadu, India
4
Department of Math., Stat. and Physics, Qatar University, Doha, Qatar
Abstract Structural, morphological, optical and magnetic properties of Zn and Zn-Ni co-doped tin oxide (SnO2) nanoparticles synthesized by sol-gel method. The influence of doping concentration on phase and particle size of the nanoparticles was determined by X-ray diffraction. The XRD study reveals that the lattice constant and crystallite size of the samples decrease with the increase of doping concentration. The change in the band gap energy of SnO2 nanoparticles influenced more by doping with Zn and Ni. The external morphology and particle size were recorded by SEM and TEM. The results indicated that Ni2+ions would uniformly substituted into the Zn2+ sites of SnO2 lattice. The substitution of Ni creates a vital change in magnetic properties that has been measured by vibrating sample magnetometer (VSM). Keywords: Magnetic materials; Sol-gel; X-ray diffraction; DRS; VSM; TEM 1. Introduction Metal oxide nanoparticles are found to exhibit interesting structural and magnetic properties. The development of semiconductor and magnetic nanoparticles has been intensively pursued due to their technological and fundamental scientific importance [1]. Several interesting phenomena have been discovered in recent years based on the studies of magnetic and non-magnetic nanoparticles [2]. Various physical properties of these nanoparticles are greatly influenced by the distribution of cations, nature of grain, grain boundaries, inhomogeneities, chemical defects and oxygen deficiency, and etc. These nanoparticles have received great attention as a result of their magnetic and electronic properties. Such nanoparticles are currently used in microwave
devices, sensors and bioprocessing [3]. There are several methods for synthesizing nanoparticles, such as co-precipitation [4-6], solid state reaction method [7], sol-gel method [8, 9], and hydrothermal method [10, 11]. The present work focuses on the preparation of tin oxide based diluted semiconductor material with tunable ferromagnetic property using the sol-gel method. This semiconductor has a wide band gap (3.5 eV - 3.8 eV) with high optical transparency in the visible region and high electrical conductivity which are the most essential features of tin oxide. Room temperature ferromagnetism has been reported in 3d transition metal doped In2O3, Cu2O3, TiO2 and SnO2 [12-15]. Magnetic properties have been reported in Fe, Mn, and Co doped tin oxide by many research groups [16-21]. Here, we make an attempt to synthesize Zn and Zn-Ni co-doped tin oxide nanopowders. The influence of (Zn – Ni) doping level on the structural, optical and magnetic properties of SnO2 nanoparticles is analyzed. 2. Experimental method 2.1 Method of Preparation Zn doped tin oxide and (Zn – Ni) – co-doped tin oxide samples were prepared by simple and low cost sol-gel method. SnCl4.5H2O, NiCl2.6H2O, ZnCl2 analytical grade chemicals were purchased from the Sigma Aldrich. Ethanol HPCL grade was purchased from Merck Scientific and used as received. 1 gm of SnCl4.5H2O and 50 ml of ethanol (HPLC grade) taken in 100 ml beaker and stirred at 300 rpm at 50 ºC for 5 hours, in that solution different concentration of Zn salt was added. The resultant solution beakers covered with aluminum foil sheet with a pin holed, kept in room temperature in un-disturbed condition for 30 days. After the gel formation the gel was annealed in a muffle furnace at 450 ºC for 5 hours, then the material was removed from the furnace after reaching room temperature. Ni co-doping was performed similarly. Separate solutions of Sn1xZnxO2
(x = 0.07 to 0.10) and Sn1-xNixO2 (x = 0.07 to 0.10) were mixed together and
stirred at 50 oC for five hours, aged for one month. The solution was then kept under heat treatment for one hour at 450 oC to obtain nanopowders of Sn (2-(x + y)) NixZnyO2, (x = y = 0.07 to 0.10).
2.2. Equipment used for characterization
The structure and phase identification of the prepared nanomaterial was done through powder X-ray diffraction (PXRD) analysis carried by Rigaku-Miniflex-II using Cu Kα (1.54 Å radiation). The UV-Visible diffuse reflectance spectra (DRS) of the prepared
material
were
measured
by
UV-VIS
Schimadzu
(UV-2550
spectrophotometer) using BaSO4 from 200 to 800nm to estimate their band gap energy. Morphological studies of prepared sample have been performed with a JSM – 6360 scanning electron microscope. High resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) measurments were recorded on a Technai G20-stwin using an accelerating voltage of 200 kV. Magnetic measurements were carried out at room temperature using a (Lake shore- 7410) vibrating sample magnetometer (VSM) equipped with a 1 T magnet. 3. Results and discussion 3.1 X-ray diffraction (XRD) analysis The XRD pattern was used to determine the phase compositions of Zn doped and Zn-Ni co-doped tin oxide nanoparticles are shown in Figs. 1 and 2. The observed peak at (110), (101), (211), and (301) of Zn and Zn-Ni co-doped samples attributed to rutile structure verified from standard JCPDS data.The peak broadening is due to small crystallite size. The intensity of the X-ray peaks for SnO2 samples decreases with increasing Zn-Ni co doping concentration. The crystallite size is estimated using the Scherrer formula D
k cos
(1)
Where D is the grain size, K is a constant taken to be 0.94, is the wavelength of the Xray radiation, is the full width at half maximum, is the angle of diffraction and (hkl) is the planes values. The lattice constant is calculated using Eq. (2).
1 h2 k2 d 2 a 2
l2 2 c
(2)
The crystallite size and lattice constant of the samples are listed in Table 1. The table clearly shows that the crystallite size of the samples decreases with increasing doping concentration as shown in Fig. 3.
3.2. Diffuse reflectance spectroscopy (DRS) The optical properties of Sn1-xZnxO2 [x = 0.07 to 0.10] and Sn2-(x + y) NixZnyO2, [x = y = 0.07 to 0.10] nanopowders were investigated by UV-Visible diffuse reflectance spectra. For this purpose, we used a two-flux (Kubelka-munk) radioactive transfer model to investigate the absorption and scattering of the radiation in the diluted magnetic semiconductors. It has been shown for a crystalline direct semiconductor that the optical absorption near the band edges follows the equation
( )
(
)1/2, where α, , Eg and A are the absorption
coefficient, light frequency, band gap and a constant, respectively. It is generally admitted that the absorption coefficient (α) can be replaced by the remission function, F(R). The latter can be written in terms of diffused reflectance (R), according to the Kubelka –munk theory, α
( )
(
)
, where ‘s’ is the scattering coefficient. The band gap energies of the
synthesized samples can then be estimated from a plot of [F(R)h ]2 Vs F(h ). From the intercepts of the tangents the band gap of the synthesized samples can be determined from Figure 4and 5. The Kubelka-munk model was used to determine the band gap of powder samples where the band gap energy of the synthesized samples can be estimated from a plot of [F(R) h ]2 Vs F (h ). The band gap of the synthesized samples can be determined from the intercepts of the tangents in Figs. 4 and 5. The estimated band gap energies of the Zn doped tin oxide samples were between 3.6 to 4.0 eV, while forZn-Ni co-doped tin oxide the band gap decreases from 4.6 to 3.2 eV, when Ni concentration increases (see Table.2). The band gap value of the Zn-Ni co-doped samples is obviously smaller than the reported value of undoped tin oxide powders samples. It can be deduced from the decrease in optical band gap that Ni ions have been incorporated into Sn sites of SnO2 lattice homogeneously. 3.3. Morphological analysis
Figures 6(a) - 6(d) show the external morphology of Zn-Ni co-doped nanopowders was recorded by SEM. The morphological change with respect to doping concentration is clearly observed. Figure7(a) shows a TEM image of Zn-Ni co-doped (x=0.1) tin oxide nanopowders. The particle size could be estimated from the TEM images to ~ 6, which is close to that calculated from XRD. The agglomeration is due to the presence of magnetic interactions between the particles [22]. Figure 7(b) reports the corresponding SAED pattern, which indicates the well crystallized nanoparticles.
3.4. Magnetic properties The magnetization measurements (M-H) of samples at room temperature have been studied by VSM. Figure 8 shows the M-H curves of Zn, doped and Fig. 9 shows the Zn-Ni co-doped SnO2 nanoparticles. The variation of magnetic parameters such as saturation magnetization and coercivity with respect to Zn and Zn-Ni co-doping concentration were analyzed. The Zn doped SnO2 exhibits diamagnetic behavior irrespective of different concentration of Zn doping with SnO2. The expected room temperature ferromagnetism is not found in Zn doping in the SnO2 diluted semiconductor material. The hysteresis loop of the Zn-Ni codoped SnO2 is due to changes of the magnetic property from dia to weak ferromagnetism [5]. The magnetization and coercivity values of Ni-Zn co-doped samples are calculated and shown in the Table.2. The table shows that the saturation magnetization decreases with increasing the Zn-Ni concentration. The presence of Zn doping in SnO2 effectively hampers the cluster formation in the SnO2 host. Further it is suggested that the presence of Zn mediates the Ni2+ distribution in the SnO2. Thus, Ni-Zn co-doped tin oxide offer a great deal of interest as a potential candidate for spintronic devices because of its room temperature ferromagnetism behavior. Hence, co-doping plays an important role in inducing ferromagnetism in the sample. 4. Conclusions Zn doped tin oxide as well as Ni-Zn co-doped tin oxide nanopowders were synthesized by sol-gel method and annealed at 450 oC. XRD pattern reveals that all investigated samples are nanocrystallite powder of rutile type tetragonal structure and crystallite size is in the range of 6.0 to 4.77 nm. XRD results also indicate that the crystallite size decreases with increasing the doping concentration. DRS studies expose that the band gap energy decreases with increase of doping concentration.
Morphological change of the nanoparticles with respect to doping concentration was clearly visualized by SEM and TEM images. The room temperature ferromagnetism was obtained for SnO2 nanoparticles with increasing Ni concentration which was identified by VSM studies. Thus Ni concentration plays an important role in inducing ferromagnetism in the SnO2 nanopowders
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Figure caption Fig.1 XRD patterns of Zn doped doped tin oxide nanopowders Fig. 2 XRD patterns of Zn-Ni co-doped tin oxide nanopowders. Fig. 3 Average crystallite size versus co-doping concentration of (Zn-Ni) co-doped tin oxide nanopowders
Fig. 4 Band gap energy graph of Zn doped tin oxide Fig. 5 Band gap energy of (Zn-Ni) co doped tin oxide Fig. 6 SEM image: (Zn- Ni ) co doped tin oxide nano powders a) concentration of dopant (x=0.07). b) (x=0.08). c) ( x=0.09) d) (x=0.1) Fig.7 (a) TEM micrograph of (Zn–Ni = 0.1) co doping SnO2. (b) SAED Pattern Fig.8 M-H curves of Zn doped with SnO2 Fig.9 M-H curves of (Zn-Ni) co doped with SnO2
Table 1 Effect of doping concentration on crystallite size and lattice constant of SnO2 nanoparticles
Parameters
crystallite Size
Zn=0.
Zn=0.
Zn=0.
Zn=0.
Zn=0.
Zn=0
Zn=0. 07
08
09
Zn=0. 1
07
08
09
.1
Ni=0. 07
Ni=0.
Ni=0.
Ni=0. 1
08
09
3.6
3.9
4.5
5.0
6.1
5.2
5.17
4.8
4.32
4.35
4.34
4.32
4.01
4.01
4
4.01
( t) nm Lattice Constant (a) Å
Table 2 Calculated magnetization, coercivity and band gap for Zn and (Zn-Ni) co-doped tin oxide nanopowders. Zn=0.
Paramete rs
Zn=0.
Zn=0.
Zn=0.
07
08
09
Zn=0. 1
Zn=0. 07
08
Ni=0. 07
Ni=0.
09
08
Band
Zn=0.
Ni=0. 09
Zn=0. 1 Ni=0. 1
3.7
3.9
4.0
4.2
4.7
4.4
3.6
3.3
0.0052
0.0061
0.011
0.0067
0.08
0.075
0.026
0.015
587.79
658.28
1429.9
846.31
480.93
497.62
513.19
1049.4
Gap(eV) Magnetiz ation Coercivit y
Highlights Sn2-(x+y) NixZnyO2, (x = y = 0.07 to 0.10) nano particles are prepared by simple sol gel
method. X-ray diffraction data confirms the single phase rutile tetragonal structure.
The VSM was used to confirm, the codoping of (Ni, Zn) increases the magnetic moment of the sample prepared.
Inducing ferromagnetism in sample makes it suitable for future spintronics applications.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
50 nm
Fig. 7
Fig. 8
Fig. 9