- Email: [email protected]
Investigation of optical properties of SnO2 nanoparticles
Physica E 47 (2013) 257–263
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Investigation of optical properties of SnO2 nanoparticles Pawan Chetri, Amarjyoti Choudhury n Department of Physics, Tezpur University, Napaam 784028, Tezpur, India
H I G H L I G H T S c
c
c
c
We have synthesized SnO2 nanoparticles of below 5 nm size, via a cost effective method. Unit cell volume is found to be decreasing with the increase in annealing temperature. UV–Visible absorption spectra show an irregular type of absorption for SnO2 annealed at 1000 1C. Intensity of photoluminescence emission spectra shows a decrease in intensity with the increase in annealing temperature.
G R A P H I C A L
A B S T R A C T
Summary: the UV–Visible spectroscopy shows interesting band type absorption for high temperature annealed sample and this is due to the oxygen vacancies.
a r t i c l e i n f o
abstract
Article history: Received 7 March 2012 Received in revised form 1 November 2012 Accepted 13 November 2012 Available online 23 November 2012
The preparation of SnO2 nanoparticles with size below 5 nm is achieved using an inexpensive method. The study of structural properties is done by XRD, TEM and FTIR, while the optical properties are observed using UV–Visible and photoluminescence spectroscopy. The prepared SnO2 nanoparticles are annealed at low (200 1C), medium (600 1C) and high (1000 1C) temperature. The UV–Visible spectroscopy shows an interesting band type absorption for high temperature annealed sample. The oxygen vacancies play vital role in the optical properties. These vacancies are the cause of abnormal absorption in high temperature annealed SnO2 nanoparticles. The Urbach energy of all the SnO2 nanoparticles annealed at 200, 600 and 1000 1C is also calculated. A high value of Urbach energy for SnO2 nanoparticles annealed at 1000 1C is found. The band gap of SnO2 annealed at 1000 1C is found to be higher than SnO2 annealed at 200 and 600 1C, which has been explained on the basis of Burstein–Moss shift. The concentration of charge carrier is calculated using Hall effect and found to be increasing as the annealing temperature increases. & 2012 Elsevier B.V. All rights reserved.
1. Introduction Tin oxide is an important metal oxide semiconductor. It has a bulk band gap of 3.6 eV (at room temperature). Both from
n
Corresponding author. Tel.: þ913712267120; fax: þ 913712267005. E-mail address: [email protected] (A. Choudhury).
1386-9477/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2012.11.011
properties and applications it is a very useful and exciting material. It exhibits abnormal variation of its properties with the size and shape variation and carries a wide application in electrical, optical, electrochemical and magnetic properties. Also its potential application in spintronic as a DMS (Dilute magnetic semiconductor) material is enormous. Tin oxide doped with indium [1] in the form of thin films has brought a lot of attention due to its electrical properties as a transparent material.
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P. Chetri, A. Choudhury / Physica E 47 (2013) 257–263
The enhancement of conductivity due to doping of Sb [2] and fluorine [3] in SnO2 nanoparticles is discussed by many authors. Shi and Lin has discussed about SnO2 as ethanol sensor [4]. In addition it has applications in solar cells [5,6], catalytic support materials [7], transparent electrodes [8,9], and solid-state chemical sensors [10–14]. Shi et al. has given many controlled techniques to produce different patterned of SnO2 [15–16] and Sn contained alloys [17]. Researchers have explained many chemical routes to synthesize SnO2 nanoparticles. But to best of our knowledge, our method providing an easy and cost effective route. The synthesized nanoparticles have a size within 5 nm. UV–Vis absorption and photoluminescence spectrum are very useful technique to understand the role played by defects in the optical property of SnO2 nanoparticles. The main defects observed in SnO2 are oxygen vacancies. The structural characterization of the prepared nanomaterials is done using X-ray diffractometer and transmission electron microscope and both the results complement each other. The metal oxygen vibration in FTIR suggests the formation of bond between Sn and oxygen, while the Energy dispersive spectrum (EDS) pattern shows an increase in Sn content as annealing temperature increases. The one of the interesting feature is the appearance of a very abnormal band type absorption in SnO2, annealed at higher temperature. Also its band
gap which should decreases with the increase in size is not observed; rather the band gap increases, which is explained on the basis of Burstein Moss shift. In generally, the Burstein Moss shift is not observed in undoped material but a detailed discussion using Hall measurement is done to show its existence. The work dealt with the investigation of variation of oxygen vacancies in low (200 1C), medium (600 1C) and high temperature (1000 1C) annealing. In this paper SnO2 nanoparticles annealed at 200 1C, 600 1C and 1000 1C are referred to Sn200, Sn600 and Sn1000 respectively. In undoped SnO2 nanoparticles, the presence of defect levels and its effect on band gap is discussed. The discussion of each characterization is linked to other one, so that a clear result can come out.
2. Synthesis of SnO2 nanoparticles The synthesis of SnO2 nanoparticles is started with the addition of 1 gm of SnCl2 2H2O in a mixture of 11 ml deionized water and 5 ml of ethanol and is allowed to stir for 5 min. Then 1 ml HCl is added drop wise to the above solution. After stirring it for 15 min the solution is fixed at pH¼8.5 by drop wise addition of aqueous ammonia. This solution is stirred for 2–3 h at 80 1C. A yellowish solution is formed which is washed with deionized
Fig. 1. (a): XRD of SnO2 annealed at 200 1C, 600 1C and 1000 1C, (b): plot of volume vs annealing temperature, (c): plot of bCos y/l vs Sin y/l, and (d): plot of strain vs annealing temperature.
P. Chetri, A. Choudhury / Physica E 47 (2013) 257–263
water and ethanol to get an impurity (chlorine) free SnO2 nanoparticles. The prepared SnO2 is annealed at low (200 1C), medium (600 1C) and high (1000 1C) temperature.
3. Characterization details Phase structures of the nanoparticles are studied with Rigaku Miniflex X-ray diffractometer equipped with intense CuKa radia˚ at a scanning rate of 11/min and in the scanning tion (l ¼ 1.54 A), range from 101–701. High resolution transmission electron microscope (HRTEM) images of the prepared nanoparticles are obtained with JEOL JEM 2010 transmission electron microscope operating at a voltage of 200 kV. UV–Vis absorption spectra of all SnO2 nanoparticles are taken in diffuse reflectance mode (DRS) in Shimadzu 2450 UV–Vis spectrophotometer. Photoluminescence (PL) spectra are monitored in a Perkin Elmer LS spectrometer. The EDS pattern is obtained from JEOL JSM Model 6390 LV. The electrical measurements are done using Hall effect set up (Scientific Instruments, DHE-21, India).
4. Results and discussion 4.1. Structural characterization Fig. 1(a) represents the XRD pattern of all the prepared SnO2 nanoparticles. The diffraction peak of the samples corresponds to rutile phase of SnO2 (JCPDS 41-1445). The crystallite size is calculated using Scherer’s formula. d ¼ 0:91l=bCosy where d is the crystallite size, l is the wavelength of X-ray used; y is the Bragg angle of diffraction peaks. The crystallite sizes are
Table 1 ˚ for SnO2 annealed at 200 1C, Crystallite size (nm), lattice strain, lattice constant (A) 600 1C and 1000 1C. SnO2 annealed at (1C) Crystallite size (nm)
Lattice strain
Lattice constant ˚ (A)
200 600 1000
41.68 9.05 22.7
a¼ 4.754, c ¼3.19 a¼ 7.732, c ¼3.189 a¼ 4.730, c¼ 3.19
2.53 6.45 12.68
259
calculated for Sn200, Sn600 and Sn1000 are 2.53, 6.45 and 12.68 nm respectively. As the annealing temperature increases the peak gets sharper. This is due to the diffusion of atoms from grain boundary to the grain leading to an increase of crystallite size. Lattice constants of entire sample are determined considering (101) and (110) [18]. Fig. 1(b) gives the volume variation of the unit cell with temperature. We have found that the volume decreases as the temperature increases. Due to stoichiometric variation of oxygen content, SnO2 can have the form SnO2 x. The ˚ is larger than that of Sn4 þ (0.71 A) ˚ so the radius of O2 (1.32 A) variation in oxygen’s concentration is going to bring changes in the lattice structure. Infact an increase in the content of oxygen can increase the lattice volume, while the subsequent decrease can contract the lattice. Xu et al. [19] explained the above fact in the case of ZnO and Capozzi et.al [20] describe the same in the case of Indium doped SnO2. Fig. 1(c) illustrates the variation of bCos y/l with Sin y/l. This shows an expanding strain in Sn200 and Sn600, while a contracting effect in Sn1000 [21]. The gradient of linear fit (which gives the strain) decreases with increase in temperature. Fig. 1(d) provides the information of variation of strain with temperature and it clearly decreases as temperature increases. This is obvious as crystallite size increases. The values of crystallite size, strain and lattice constant is given in Table 1. The TEM image of Sn200 in both low resolution and high resolution is shown in Fig. 2(a) and (b) respectively. These TEM images confirm that the prepared SnO2 is in nano regime. The low resolution image shows the agglomerated nanoparticles, which is very much expected as we have not used any kind of surfactant. The high resolution image shows that the particles are in the range 2.7 nm to 5 nm. The interplanar spacing is found to be 3.57 A˚ from TEM while it was found to be 3.33 A˚ from the XRD pattern. So the XRD and TEM results compliment each other. To know the chemical composition we have performed EDS of SnO2 at different temperatures. Fig. 3(a–c) represents Sn200, Sn600 and Sn1000 respectively. The images show that the prepared SnO2 nanoparticles has very little impurities. This is achieved due to several washing with deionized water and ethanol. The peak intensity of oxygen gets lower while the same of Sn gets sharper, as annealing temperature increases. This might be due to crystallization of SnO2 such that oxygen species were desorbed and trapped electrons were released. This corresponds to the increase in oxygen vacancies [22]. FTIR analysis is done to examine the bonding vibration in all the samples as shown in Fig. 4. In our case our interest is in variation in metal oxygen vibration. Das et.al [23] and Choudhury and Choudhury [24]
Fig. 2. (a): Low resolution TEM image of SnO2 annealed at 200 1C, (b): high resolution TEM image of SnO2 annealed at 200 1C.
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Fig. 4. FTIR plot for SnO2 annealed at 200 1C, 600 1C and 1000 1C.
Fig. 5. UV–Visible absorption spectra for SnO2 annealed at 200 1C, 600 1C and 1000 1C.
the bonding pattern leading to some new vibrations which appear as broad peak as shown in Fig. 4 (by black line arrow). This broad peak can be considered as consisting of different type of vibrations which are not strong enough to show individual intense peaks. 4.2. Optical characterization Fig. 3. (a): EDS pattern of SnO2 annealed at 200 1C, (b): EDS pattern of SnO2 annealed at 600 1C, (c): EDS pattern of SnO2 annealed at 1000 1C.
found that in Co doped TiO2 that increase in oxygen vacancies lead the metal–oxygen vibration to lower values. This is because the bond gets weakened due to unavailability of oxygen. It is reported [25] that the vibrations centered at 620 cm 1 is due to Sn–O–Sn or O–Sn–O or Sn–O vibration. But in Sn1000, we can see a decrease in intensity of 620 cm 1 vibration. This decrease in intensity can be attributed to decrease in Sn–O bonds. This may be due to deficiency of either Sn or oxygen. The deficiency of Sn can occur when Sn moves to interstial position but the evidences that we have obtained from XRD and EDS, indicates that it is due to oxygen deficiency. These oxygen vacancies misbalanced
4.2.1. UV–Vis spectroscopy To have a better understanding of the effect of oxygen vacancies on optical properties of SnO2 at different temperatures, UV–vis spectroscopy is performed. The absorbance is given by FðRÞ ¼ ð1RÞ2 =2 R: The variation of absorbance with wavelength of all the samples is shown in Fig. 5. Fig. 6(a–c) gives the band gap of Sn200, Sn600 and Sn1000 respectively, while the inset figure represents the Urbach energy curves for each samples. The band gap is calculated using Kubel ka Munk equation, where [F(R)hu]2 is plotted against incident photon energy hu and the extrapolated line at [F(R)hu]2 ¼0 gives the value of band gap in electronvolt
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Fig. 6. (a): Band gap determination; inset is the Urbach calculation for SnO2 annealed at 200 1C, (b): band gap determination; inset is the Urbach calculation for SnO2 annealed at 600 1C, (c): band gap determination; inset is the Urbach calculation for SnO2 annealed at 1000 1C.
Table 2 Band gap (eV), Urbach energy (meV) and electron concentration (cm 3) for SnO2 annealed at 200 1C, 600 1C and 1000 1C. SnO2 annealed at (1C)
Band gap (eV)
Urbach energy (meV)
Electron concentration (cm 3)
200 600 1000
3.85 3.78 3.92
424.38 432.02 627.54
3 1019 3.2 1019 1 1021
(eV). The Urbach energy of each sample is calculated using the equation.
a ¼ a0 exp½E=Eu where a is the absorption coefficient, E is the photon energy and Eu is the Urbach energy [26–30]. Since the absorption coefficient is proportional to absorbance and absorption is proportional to F(R), so we can write F(R) in place of a [29]. For the calculation of Urbach energy; ln F(R) is plotted against photon energy, and linearly fitted. The slope of linearly fitted line gives the reciprocal of Urbach energy. The band gap and Urbach energy values is given in Table 2. An unusual continuum band type absorption is observed for Sn1000 from 538 nm (2.3 eV) to 330 nm (3.75 eV). This is due to the absorption by electrons residing within the band gap in different defect levels. But the concentration of electrons, to cause absorption in the UV range is lesser as compare
to other samples. The absorption for other annealed sample is normal and show single absorption. A slight red shift is observed for Sn600 compared to Sn200. Iijima et al. [31] found that when a TiO2 film is annealed at high temperature in flowing oxygen then the atmospheric oxygen fills the surface oxygen vacancies, but breaks the binding between oxygen and Ti. In this way it pulls out oxygen and hence creates in plane oxygen vacancies. Cox et al. [32] has shown that the in plane oxygen vacancies creates occupied states high in the band gap which extend to Fermi level. These levels can act as donor levels. As temperature increased from 200 1C to 600 1C the band gap decreased. This is due to increase in crystallite size [33], but as temperature increased from 600 1C to 1000 1C the band gap again increased. This increase in band gap is attributed to Burstein Moss shift [34]. Generally it happens in doped material where the dopant creates levels below the conduction band edge. In Sn1000, the increase of in plane oxygen vacancies produce the donor levels (or even very thin band), which can be clearly seen from absorption spectrum. The increase in concentration of oxygen vacancies: increase the electrons near the conduction band edge [35,36], following the Kroner Vink notation as follows: Oo ¼ 1=2 O2 m þ V0 :: þ 2e For this purpose Hall effect is performed and we have found a 21 of increase in concentration of electrons in Sn1000 sample as given in Table 2. The Urbach energy depends on thermal
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Fig. 7. (a): Photoluminescence emission spectra for SnO2 annealed at 200 1C, 600 1C and 1000 1C, and (b): photoluminescence excitation spectra for SnO2 annealed at 1000 1C.
vibrations in the lattice which induce disorder in the lattice [37]. This distortion can bring an Urbach type absorption which occurs due to electron transition between extended band and localized band tail. The structural disorder originates from impurities, defects (oxygen vacancies) [38]. As from band gap calculation we have found that the band gap increases from Sn200 to Sn600 sample, which can be also correlated to the Urbach energy value. The Urbach energy value in Sn600 is very much closer to the Sn200; so the oxygen vacancies could not bring much lattice strain. This is because the band gap decreased with the increase in crystallite size, which is a usual fact. But the Sn1000 has a very large Urbach energy value which signifies the kind of distortion present in it. This distortion is because of large concentration of oxygen vacancies [37]. Srinivas et al. [39] attributed the increase in band gap in Co doped SnO2 with increase in temperature, to either Burstein Moss shift or the distortion in lattice due to annealing temperature. 4.2.2. Photoluminescence spectroscopy Photoluminescence spectroscopy is a technique to investigate the different types of defects present in the material. We have done both excitation and emission spectroscopy to reveal the defect states (Fig. 7). Here we have performed emission spectroscopy for the entire annealed sample while excitation spectroscopy is done only for Sn1000. The emission spectrum is more or less same for the entire annealed sample. The only exception is the decrease in intensity which is due to increase in defects (oxygen vacancies) [40]. The decrease in intensity can also be due to the increase in non radiative recombination [41]. The non radiative emission occurs in a system where lattice distortion is high. The excitation spectrum shows peaks at around 393, 440, 465 and 485 nm. The peak at 393 nm is the most sharper and it may reflects the below conduction band absorption but not the band edge absorption. While other peaks shows potential absorption at different wavelength respectively. The emission studies of all annealed sample show peak at around 430, 450 and 490 nm under 290 nm excitation. The band to band transition is not observed may be due to limitation of instrument [42]. As PLE shows a prominent absorption at 393 nm so it causes the emission at 396 nm. Whereas 440, 465 and 485 nm of absorption corresponds to 450 and 490 nm emissions. Slight red shifts of the peaks are observed as annealing temperature increases. 5. Conclusion The preparation of SnO2 nanoparticles with size below 5 nm is achieved using an inexpensive method. The study of structural
properties verifies the formation of rutile SnO2 nanoparticles. The annealing in low (200 1C) medium (600 1C) and high (1000 1C) temperature created variation in properties of the system. The comparative study shows a well known increase in crystallite sizes with temperature. Also a sudden increase in band gap with increase in annealing temperature from mid to high temperature is observed. The increase in band gap is explained on the basis of Burstein–Moss shift. An increase in charge carrier concentration by 100 times is observed in SnO2 annealed at 1000 1C. The photoluminescence studies show decrease in intensity with increase in annealing temperature, which is due to increase in oxygen vacancies.
Acknowledgment Author P. Chetri likes to acknowledge DST, Govt. of India for providing Inspire Fellowship. Author A. Choudhury acknowledges the financial support provided by Department of Science and Technology (DST), India, to the project SR/NM/NS-98/2010 (G) and also they thank SAIF, NEHU for helping them in carrying out the TEM measurement. References [1] M. Mizuhashi, Thin Solid Films 70 (1980) 91. [2] C. Terriera, J.P. Chatelona, R. Berjoanb, J.A. Rogera, Thin Solid Films 263 (1995) 37. [3] J.W. Bae, S.W. Lee, G.Y. Yeom, Journal of the Electrochemical Society 154 (2007) D34. [4] L. Shi, H. Lin, Langmuir 27 (2011) 3977. [5] P.G. Harrison, M. Willet, Nature 332 (1988) 337. [6] S. Ferrere, A. Zaban, B.A. Gregg, Journal of Physical Chemistry B 101 (1997) 4490. [7] W.W. Wang, Y.J. Zhu, L.X. Yang, Advanced Functional Materials 17 (2007) 59. [8] J. Zhu, Z. Lu, S.T. Aruna, D. Aurbach, A. Gedanken, Chemisry of Materials 12 (2000) 2557. [9] Y.S. He, J.C. Campbell, R.C. Murphy, M.F. Arendt, J.S. Swinnea, Journal of Materials Research 8 (1993) 3131. [10] Y. Wang, X. Jiang, Y. Xia, Journal of the American Chemical Society 125 (2003) 16176. [11] G. Xi, J. Ye, Inorganic Chemistry 49 (2010) 2302. [12] Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Applied Physics Letters 87 (2005) 1. [13] Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, C. Zhou, Advanced Materials 15 (2003) 1754. [14] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskouits, Nano Letters 5 (2005) 667. [15] L. Shi, Y. Xu, Q. Li, Nanoscale 2 (2010) 2104. [16] L. Shi, H. Lin, Langmuir 27 (2011) 3977. [17] L. Shi, C. Pei, Y. Xu, Q. Li, Journal of the American Chemical Society 133 (2011) 10328. [18] R. Adhikari, A.K. Das, D. Karmakar, T.V. Chandrasekhar Rao, J. Ghatak, Physical Review B 78 (2008) 1. [19] H. Xu, H. Wang, Y.C. Zhang, S. Wang, M. Zhu, H. Yan, Crystal Research and Technology 38 (2003) 429.
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[20] C.J. Capozzi, I.N. Ivanov, S. Joshi, R.A. Gerhardt, Nanotechnology 20 (2009) 1. [21] F. Gu, S.F. Wang, M.K. Lu1, G.J. Zhou, D. Xu, D.R. Yuan, Journal of Physical Chemistry B 108 (2004) 8119. [22] C. Ke, W. Zhu, J.S. Pan, Z. Yang, Current Applied Physics 11 (2011) S306. [23] K. Das, S.N. Sharma, M. Kumar, S.K. De, Journal of Physical Chemistry C 113 (2009) 14783. [24] B. Choudhury, A. Choudhury, Journal of Luminescence 132 (2012) 178. [25] U. Pal1, A. Pe´rez-Centeno, M. Herrera-Zaldı´var, Journal of Applied Physics 103 (2008) 1. [26] K. Boubaker, European Physics Journal Plus 126 (2011) 1. [27] A. Paleari, F. Meinardi, A. Lauria, R. Lorenzi, N. Chiodini, S. Brovelli, Applied Physics Letters 91 (2007) 1. [28] J. Yang, Y.Q. Gao, J. Wu, Z.M. Huang, X.J. Meng, M.R. Shen, J.L. Sun, J.H. Chu, Journal of Applied Physics 108 (2010) 1. [29] H. Deng, J.M. Hossenlopp, Journal of Physical Chemistry B 109 (2005) 66. [30] M.A. Baki, F. El-Diasty, F.A.Abdel Wahab, Optics Communications 261 (2006) 65. [31] K. Iijima, M. Goto, S. Enomoto, H. Kunugita, K. Ema, M. Tsukamoto, N. Ichikawa, H. Sakama, Journal of Luminescence 128 (2008) 911. [32] D.F. Cox, T.B. Fryberger, S. Semancik, Physical Review B 38 (1988) 2072.
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[33] H.X. Deng, S.S. Li, J. Li, Journal of Physical Chemistry C 114 (2010) 4841. [34] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Bulletin of Materials Science 34 (2011) 673. [35] Y.S. Avadhut, J. Weber, E. Hammarberg, C. Feldmann, I. Schellenberg, ¨ R. Pottgen, J. Schmedt auf der Gu¨nne, Chemistry of Materials 23 (2011) 1526. ¨ [36] P. Grosse, F.J. Schmitte, G. Frank, H. Kostlin, Thin Solid Films 90 (1982) 309–315. [37] M. Abdel-Bakia, F.A. Abdel Wahabb, F. El-Diasty, Materials Chemistry and Physics 96 (2006) 201. [38] J. Yang, Y.Q. Gao, J. Wu, Z.M. Huang, X.J. Meng, M.R. Shen, J.L. Sun, J.H. Chu, Journal of Applied Physics 108 (2010) 1. [39] K. Srinivas, M. Vithal, B. Sreedhar, M. Manivel Raja, P. Venugopal Reddy, Journal of Physical Chemistry C 113 (2009) 3543. [40] A.R. Mohmad, F. Bastiman, J.S. Ng, S.J. Sweeney, J.P.R. David, Physica Status Solidi C 9 (2012) 259. [41] H.S. Kwack, Y. Sun, Y.H. Cho, N.M. Park, S.J. Park, Applied Physics Letters 83 (2003) 2901. [42] Qin Kuang, Changshi Lao, Zhong Lin Wang, Zhaoxiong Xie, Lansun Zheng, Journal of the American Chemical Society 129 (2007) 6070.
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Investigation of optical properties of SnO2 nanoparticles Pawan Chetri, Amarjyoti Choudhury n Department of Physics, Tezpur University, Napaam 784028, Tezpur, India
H I G H L I G H T S c
c
c
c
We have synthesized SnO2 nanoparticles of below 5 nm size, via a cost effective method. Unit cell volume is found to be decreasing with the increase in annealing temperature. UV–Visible absorption spectra show an irregular type of absorption for SnO2 annealed at 1000 1C. Intensity of photoluminescence emission spectra shows a decrease in intensity with the increase in annealing temperature.
G R A P H I C A L
A B S T R A C T
Summary: the UV–Visible spectroscopy shows interesting band type absorption for high temperature annealed sample and this is due to the oxygen vacancies.
a r t i c l e i n f o
abstract
Article history: Received 7 March 2012 Received in revised form 1 November 2012 Accepted 13 November 2012 Available online 23 November 2012
The preparation of SnO2 nanoparticles with size below 5 nm is achieved using an inexpensive method. The study of structural properties is done by XRD, TEM and FTIR, while the optical properties are observed using UV–Visible and photoluminescence spectroscopy. The prepared SnO2 nanoparticles are annealed at low (200 1C), medium (600 1C) and high (1000 1C) temperature. The UV–Visible spectroscopy shows an interesting band type absorption for high temperature annealed sample. The oxygen vacancies play vital role in the optical properties. These vacancies are the cause of abnormal absorption in high temperature annealed SnO2 nanoparticles. The Urbach energy of all the SnO2 nanoparticles annealed at 200, 600 and 1000 1C is also calculated. A high value of Urbach energy for SnO2 nanoparticles annealed at 1000 1C is found. The band gap of SnO2 annealed at 1000 1C is found to be higher than SnO2 annealed at 200 and 600 1C, which has been explained on the basis of Burstein–Moss shift. The concentration of charge carrier is calculated using Hall effect and found to be increasing as the annealing temperature increases. & 2012 Elsevier B.V. All rights reserved.
1. Introduction Tin oxide is an important metal oxide semiconductor. It has a bulk band gap of 3.6 eV (at room temperature). Both from
n
Corresponding author. Tel.: þ913712267120; fax: þ 913712267005. E-mail address: [email protected] (A. Choudhury).
1386-9477/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2012.11.011
properties and applications it is a very useful and exciting material. It exhibits abnormal variation of its properties with the size and shape variation and carries a wide application in electrical, optical, electrochemical and magnetic properties. Also its potential application in spintronic as a DMS (Dilute magnetic semiconductor) material is enormous. Tin oxide doped with indium [1] in the form of thin films has brought a lot of attention due to its electrical properties as a transparent material.
258
P. Chetri, A. Choudhury / Physica E 47 (2013) 257–263
The enhancement of conductivity due to doping of Sb [2] and fluorine [3] in SnO2 nanoparticles is discussed by many authors. Shi and Lin has discussed about SnO2 as ethanol sensor [4]. In addition it has applications in solar cells [5,6], catalytic support materials [7], transparent electrodes [8,9], and solid-state chemical sensors [10–14]. Shi et al. has given many controlled techniques to produce different patterned of SnO2 [15–16] and Sn contained alloys [17]. Researchers have explained many chemical routes to synthesize SnO2 nanoparticles. But to best of our knowledge, our method providing an easy and cost effective route. The synthesized nanoparticles have a size within 5 nm. UV–Vis absorption and photoluminescence spectrum are very useful technique to understand the role played by defects in the optical property of SnO2 nanoparticles. The main defects observed in SnO2 are oxygen vacancies. The structural characterization of the prepared nanomaterials is done using X-ray diffractometer and transmission electron microscope and both the results complement each other. The metal oxygen vibration in FTIR suggests the formation of bond between Sn and oxygen, while the Energy dispersive spectrum (EDS) pattern shows an increase in Sn content as annealing temperature increases. The one of the interesting feature is the appearance of a very abnormal band type absorption in SnO2, annealed at higher temperature. Also its band
gap which should decreases with the increase in size is not observed; rather the band gap increases, which is explained on the basis of Burstein Moss shift. In generally, the Burstein Moss shift is not observed in undoped material but a detailed discussion using Hall measurement is done to show its existence. The work dealt with the investigation of variation of oxygen vacancies in low (200 1C), medium (600 1C) and high temperature (1000 1C) annealing. In this paper SnO2 nanoparticles annealed at 200 1C, 600 1C and 1000 1C are referred to Sn200, Sn600 and Sn1000 respectively. In undoped SnO2 nanoparticles, the presence of defect levels and its effect on band gap is discussed. The discussion of each characterization is linked to other one, so that a clear result can come out.
2. Synthesis of SnO2 nanoparticles The synthesis of SnO2 nanoparticles is started with the addition of 1 gm of SnCl2 2H2O in a mixture of 11 ml deionized water and 5 ml of ethanol and is allowed to stir for 5 min. Then 1 ml HCl is added drop wise to the above solution. After stirring it for 15 min the solution is fixed at pH¼8.5 by drop wise addition of aqueous ammonia. This solution is stirred for 2–3 h at 80 1C. A yellowish solution is formed which is washed with deionized
Fig. 1. (a): XRD of SnO2 annealed at 200 1C, 600 1C and 1000 1C, (b): plot of volume vs annealing temperature, (c): plot of bCos y/l vs Sin y/l, and (d): plot of strain vs annealing temperature.
P. Chetri, A. Choudhury / Physica E 47 (2013) 257–263
water and ethanol to get an impurity (chlorine) free SnO2 nanoparticles. The prepared SnO2 is annealed at low (200 1C), medium (600 1C) and high (1000 1C) temperature.
3. Characterization details Phase structures of the nanoparticles are studied with Rigaku Miniflex X-ray diffractometer equipped with intense CuKa radia˚ at a scanning rate of 11/min and in the scanning tion (l ¼ 1.54 A), range from 101–701. High resolution transmission electron microscope (HRTEM) images of the prepared nanoparticles are obtained with JEOL JEM 2010 transmission electron microscope operating at a voltage of 200 kV. UV–Vis absorption spectra of all SnO2 nanoparticles are taken in diffuse reflectance mode (DRS) in Shimadzu 2450 UV–Vis spectrophotometer. Photoluminescence (PL) spectra are monitored in a Perkin Elmer LS spectrometer. The EDS pattern is obtained from JEOL JSM Model 6390 LV. The electrical measurements are done using Hall effect set up (Scientific Instruments, DHE-21, India).
4. Results and discussion 4.1. Structural characterization Fig. 1(a) represents the XRD pattern of all the prepared SnO2 nanoparticles. The diffraction peak of the samples corresponds to rutile phase of SnO2 (JCPDS 41-1445). The crystallite size is calculated using Scherer’s formula. d ¼ 0:91l=bCosy where d is the crystallite size, l is the wavelength of X-ray used; y is the Bragg angle of diffraction peaks. The crystallite sizes are
Table 1 ˚ for SnO2 annealed at 200 1C, Crystallite size (nm), lattice strain, lattice constant (A) 600 1C and 1000 1C. SnO2 annealed at (1C) Crystallite size (nm)
Lattice strain
Lattice constant ˚ (A)
200 600 1000
41.68 9.05 22.7
a¼ 4.754, c ¼3.19 a¼ 7.732, c ¼3.189 a¼ 4.730, c¼ 3.19
2.53 6.45 12.68
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calculated for Sn200, Sn600 and Sn1000 are 2.53, 6.45 and 12.68 nm respectively. As the annealing temperature increases the peak gets sharper. This is due to the diffusion of atoms from grain boundary to the grain leading to an increase of crystallite size. Lattice constants of entire sample are determined considering (101) and (110) [18]. Fig. 1(b) gives the volume variation of the unit cell with temperature. We have found that the volume decreases as the temperature increases. Due to stoichiometric variation of oxygen content, SnO2 can have the form SnO2 x. The ˚ is larger than that of Sn4 þ (0.71 A) ˚ so the radius of O2 (1.32 A) variation in oxygen’s concentration is going to bring changes in the lattice structure. Infact an increase in the content of oxygen can increase the lattice volume, while the subsequent decrease can contract the lattice. Xu et al. [19] explained the above fact in the case of ZnO and Capozzi et.al [20] describe the same in the case of Indium doped SnO2. Fig. 1(c) illustrates the variation of bCos y/l with Sin y/l. This shows an expanding strain in Sn200 and Sn600, while a contracting effect in Sn1000 [21]. The gradient of linear fit (which gives the strain) decreases with increase in temperature. Fig. 1(d) provides the information of variation of strain with temperature and it clearly decreases as temperature increases. This is obvious as crystallite size increases. The values of crystallite size, strain and lattice constant is given in Table 1. The TEM image of Sn200 in both low resolution and high resolution is shown in Fig. 2(a) and (b) respectively. These TEM images confirm that the prepared SnO2 is in nano regime. The low resolution image shows the agglomerated nanoparticles, which is very much expected as we have not used any kind of surfactant. The high resolution image shows that the particles are in the range 2.7 nm to 5 nm. The interplanar spacing is found to be 3.57 A˚ from TEM while it was found to be 3.33 A˚ from the XRD pattern. So the XRD and TEM results compliment each other. To know the chemical composition we have performed EDS of SnO2 at different temperatures. Fig. 3(a–c) represents Sn200, Sn600 and Sn1000 respectively. The images show that the prepared SnO2 nanoparticles has very little impurities. This is achieved due to several washing with deionized water and ethanol. The peak intensity of oxygen gets lower while the same of Sn gets sharper, as annealing temperature increases. This might be due to crystallization of SnO2 such that oxygen species were desorbed and trapped electrons were released. This corresponds to the increase in oxygen vacancies [22]. FTIR analysis is done to examine the bonding vibration in all the samples as shown in Fig. 4. In our case our interest is in variation in metal oxygen vibration. Das et.al [23] and Choudhury and Choudhury [24]
Fig. 2. (a): Low resolution TEM image of SnO2 annealed at 200 1C, (b): high resolution TEM image of SnO2 annealed at 200 1C.
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Fig. 4. FTIR plot for SnO2 annealed at 200 1C, 600 1C and 1000 1C.
Fig. 5. UV–Visible absorption spectra for SnO2 annealed at 200 1C, 600 1C and 1000 1C.
the bonding pattern leading to some new vibrations which appear as broad peak as shown in Fig. 4 (by black line arrow). This broad peak can be considered as consisting of different type of vibrations which are not strong enough to show individual intense peaks. 4.2. Optical characterization Fig. 3. (a): EDS pattern of SnO2 annealed at 200 1C, (b): EDS pattern of SnO2 annealed at 600 1C, (c): EDS pattern of SnO2 annealed at 1000 1C.
found that in Co doped TiO2 that increase in oxygen vacancies lead the metal–oxygen vibration to lower values. This is because the bond gets weakened due to unavailability of oxygen. It is reported [25] that the vibrations centered at 620 cm 1 is due to Sn–O–Sn or O–Sn–O or Sn–O vibration. But in Sn1000, we can see a decrease in intensity of 620 cm 1 vibration. This decrease in intensity can be attributed to decrease in Sn–O bonds. This may be due to deficiency of either Sn or oxygen. The deficiency of Sn can occur when Sn moves to interstial position but the evidences that we have obtained from XRD and EDS, indicates that it is due to oxygen deficiency. These oxygen vacancies misbalanced
4.2.1. UV–Vis spectroscopy To have a better understanding of the effect of oxygen vacancies on optical properties of SnO2 at different temperatures, UV–vis spectroscopy is performed. The absorbance is given by FðRÞ ¼ ð1RÞ2 =2 R: The variation of absorbance with wavelength of all the samples is shown in Fig. 5. Fig. 6(a–c) gives the band gap of Sn200, Sn600 and Sn1000 respectively, while the inset figure represents the Urbach energy curves for each samples. The band gap is calculated using Kubel ka Munk equation, where [F(R)hu]2 is plotted against incident photon energy hu and the extrapolated line at [F(R)hu]2 ¼0 gives the value of band gap in electronvolt
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Fig. 6. (a): Band gap determination; inset is the Urbach calculation for SnO2 annealed at 200 1C, (b): band gap determination; inset is the Urbach calculation for SnO2 annealed at 600 1C, (c): band gap determination; inset is the Urbach calculation for SnO2 annealed at 1000 1C.
Table 2 Band gap (eV), Urbach energy (meV) and electron concentration (cm 3) for SnO2 annealed at 200 1C, 600 1C and 1000 1C. SnO2 annealed at (1C)
Band gap (eV)
Urbach energy (meV)
Electron concentration (cm 3)
200 600 1000
3.85 3.78 3.92
424.38 432.02 627.54
3 1019 3.2 1019 1 1021
(eV). The Urbach energy of each sample is calculated using the equation.
a ¼ a0 exp½E=Eu where a is the absorption coefficient, E is the photon energy and Eu is the Urbach energy [26–30]. Since the absorption coefficient is proportional to absorbance and absorption is proportional to F(R), so we can write F(R) in place of a [29]. For the calculation of Urbach energy; ln F(R) is plotted against photon energy, and linearly fitted. The slope of linearly fitted line gives the reciprocal of Urbach energy. The band gap and Urbach energy values is given in Table 2. An unusual continuum band type absorption is observed for Sn1000 from 538 nm (2.3 eV) to 330 nm (3.75 eV). This is due to the absorption by electrons residing within the band gap in different defect levels. But the concentration of electrons, to cause absorption in the UV range is lesser as compare
to other samples. The absorption for other annealed sample is normal and show single absorption. A slight red shift is observed for Sn600 compared to Sn200. Iijima et al. [31] found that when a TiO2 film is annealed at high temperature in flowing oxygen then the atmospheric oxygen fills the surface oxygen vacancies, but breaks the binding between oxygen and Ti. In this way it pulls out oxygen and hence creates in plane oxygen vacancies. Cox et al. [32] has shown that the in plane oxygen vacancies creates occupied states high in the band gap which extend to Fermi level. These levels can act as donor levels. As temperature increased from 200 1C to 600 1C the band gap decreased. This is due to increase in crystallite size [33], but as temperature increased from 600 1C to 1000 1C the band gap again increased. This increase in band gap is attributed to Burstein Moss shift [34]. Generally it happens in doped material where the dopant creates levels below the conduction band edge. In Sn1000, the increase of in plane oxygen vacancies produce the donor levels (or even very thin band), which can be clearly seen from absorption spectrum. The increase in concentration of oxygen vacancies: increase the electrons near the conduction band edge [35,36], following the Kroner Vink notation as follows: Oo ¼ 1=2 O2 m þ V0 :: þ 2e For this purpose Hall effect is performed and we have found a 21 of increase in concentration of electrons in Sn1000 sample as given in Table 2. The Urbach energy depends on thermal
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Fig. 7. (a): Photoluminescence emission spectra for SnO2 annealed at 200 1C, 600 1C and 1000 1C, and (b): photoluminescence excitation spectra for SnO2 annealed at 1000 1C.
vibrations in the lattice which induce disorder in the lattice [37]. This distortion can bring an Urbach type absorption which occurs due to electron transition between extended band and localized band tail. The structural disorder originates from impurities, defects (oxygen vacancies) [38]. As from band gap calculation we have found that the band gap increases from Sn200 to Sn600 sample, which can be also correlated to the Urbach energy value. The Urbach energy value in Sn600 is very much closer to the Sn200; so the oxygen vacancies could not bring much lattice strain. This is because the band gap decreased with the increase in crystallite size, which is a usual fact. But the Sn1000 has a very large Urbach energy value which signifies the kind of distortion present in it. This distortion is because of large concentration of oxygen vacancies [37]. Srinivas et al. [39] attributed the increase in band gap in Co doped SnO2 with increase in temperature, to either Burstein Moss shift or the distortion in lattice due to annealing temperature. 4.2.2. Photoluminescence spectroscopy Photoluminescence spectroscopy is a technique to investigate the different types of defects present in the material. We have done both excitation and emission spectroscopy to reveal the defect states (Fig. 7). Here we have performed emission spectroscopy for the entire annealed sample while excitation spectroscopy is done only for Sn1000. The emission spectrum is more or less same for the entire annealed sample. The only exception is the decrease in intensity which is due to increase in defects (oxygen vacancies) [40]. The decrease in intensity can also be due to the increase in non radiative recombination [41]. The non radiative emission occurs in a system where lattice distortion is high. The excitation spectrum shows peaks at around 393, 440, 465 and 485 nm. The peak at 393 nm is the most sharper and it may reflects the below conduction band absorption but not the band edge absorption. While other peaks shows potential absorption at different wavelength respectively. The emission studies of all annealed sample show peak at around 430, 450 and 490 nm under 290 nm excitation. The band to band transition is not observed may be due to limitation of instrument [42]. As PLE shows a prominent absorption at 393 nm so it causes the emission at 396 nm. Whereas 440, 465 and 485 nm of absorption corresponds to 450 and 490 nm emissions. Slight red shifts of the peaks are observed as annealing temperature increases. 5. Conclusion The preparation of SnO2 nanoparticles with size below 5 nm is achieved using an inexpensive method. The study of structural
properties verifies the formation of rutile SnO2 nanoparticles. The annealing in low (200 1C) medium (600 1C) and high (1000 1C) temperature created variation in properties of the system. The comparative study shows a well known increase in crystallite sizes with temperature. Also a sudden increase in band gap with increase in annealing temperature from mid to high temperature is observed. The increase in band gap is explained on the basis of Burstein–Moss shift. An increase in charge carrier concentration by 100 times is observed in SnO2 annealed at 1000 1C. The photoluminescence studies show decrease in intensity with increase in annealing temperature, which is due to increase in oxygen vacancies.
Acknowledgment Author P. Chetri likes to acknowledge DST, Govt. of India for providing Inspire Fellowship. Author A. Choudhury acknowledges the financial support provided by Department of Science and Technology (DST), India, to the project SR/NM/NS-98/2010 (G) and also they thank SAIF, NEHU for helping them in carrying out the TEM measurement. References [1] M. Mizuhashi, Thin Solid Films 70 (1980) 91. [2] C. Terriera, J.P. Chatelona, R. Berjoanb, J.A. Rogera, Thin Solid Films 263 (1995) 37. [3] J.W. Bae, S.W. Lee, G.Y. Yeom, Journal of the Electrochemical Society 154 (2007) D34. [4] L. Shi, H. Lin, Langmuir 27 (2011) 3977. [5] P.G. Harrison, M. Willet, Nature 332 (1988) 337. [6] S. Ferrere, A. Zaban, B.A. Gregg, Journal of Physical Chemistry B 101 (1997) 4490. [7] W.W. Wang, Y.J. Zhu, L.X. Yang, Advanced Functional Materials 17 (2007) 59. [8] J. Zhu, Z. Lu, S.T. Aruna, D. Aurbach, A. Gedanken, Chemisry of Materials 12 (2000) 2557. [9] Y.S. He, J.C. Campbell, R.C. Murphy, M.F. Arendt, J.S. Swinnea, Journal of Materials Research 8 (1993) 3131. [10] Y. Wang, X. Jiang, Y. Xia, Journal of the American Chemical Society 125 (2003) 16176. [11] G. Xi, J. Ye, Inorganic Chemistry 49 (2010) 2302. [12] Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Applied Physics Letters 87 (2005) 1. [13] Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, C. Zhou, Advanced Materials 15 (2003) 1754. [14] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskouits, Nano Letters 5 (2005) 667. [15] L. Shi, Y. Xu, Q. Li, Nanoscale 2 (2010) 2104. [16] L. Shi, H. Lin, Langmuir 27 (2011) 3977. [17] L. Shi, C. Pei, Y. Xu, Q. Li, Journal of the American Chemical Society 133 (2011) 10328. [18] R. Adhikari, A.K. Das, D. Karmakar, T.V. Chandrasekhar Rao, J. Ghatak, Physical Review B 78 (2008) 1. [19] H. Xu, H. Wang, Y.C. Zhang, S. Wang, M. Zhu, H. Yan, Crystal Research and Technology 38 (2003) 429.
P. Chetri, A. Choudhury / Physica E 47 (2013) 257–263
[20] C.J. Capozzi, I.N. Ivanov, S. Joshi, R.A. Gerhardt, Nanotechnology 20 (2009) 1. [21] F. Gu, S.F. Wang, M.K. Lu1, G.J. Zhou, D. Xu, D.R. Yuan, Journal of Physical Chemistry B 108 (2004) 8119. [22] C. Ke, W. Zhu, J.S. Pan, Z. Yang, Current Applied Physics 11 (2011) S306. [23] K. Das, S.N. Sharma, M. Kumar, S.K. De, Journal of Physical Chemistry C 113 (2009) 14783. [24] B. Choudhury, A. Choudhury, Journal of Luminescence 132 (2012) 178. [25] U. Pal1, A. Pe´rez-Centeno, M. Herrera-Zaldı´var, Journal of Applied Physics 103 (2008) 1. [26] K. Boubaker, European Physics Journal Plus 126 (2011) 1. [27] A. Paleari, F. Meinardi, A. Lauria, R. Lorenzi, N. Chiodini, S. Brovelli, Applied Physics Letters 91 (2007) 1. [28] J. Yang, Y.Q. Gao, J. Wu, Z.M. Huang, X.J. Meng, M.R. Shen, J.L. Sun, J.H. Chu, Journal of Applied Physics 108 (2010) 1. [29] H. Deng, J.M. Hossenlopp, Journal of Physical Chemistry B 109 (2005) 66. [30] M.A. Baki, F. El-Diasty, F.A.Abdel Wahab, Optics Communications 261 (2006) 65. [31] K. Iijima, M. Goto, S. Enomoto, H. Kunugita, K. Ema, M. Tsukamoto, N. Ichikawa, H. Sakama, Journal of Luminescence 128 (2008) 911. [32] D.F. Cox, T.B. Fryberger, S. Semancik, Physical Review B 38 (1988) 2072.
263
[33] H.X. Deng, S.S. Li, J. Li, Journal of Physical Chemistry C 114 (2010) 4841. [34] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Bulletin of Materials Science 34 (2011) 673. [35] Y.S. Avadhut, J. Weber, E. Hammarberg, C. Feldmann, I. Schellenberg, ¨ R. Pottgen, J. Schmedt auf der Gu¨nne, Chemistry of Materials 23 (2011) 1526. ¨ [36] P. Grosse, F.J. Schmitte, G. Frank, H. Kostlin, Thin Solid Films 90 (1982) 309–315. [37] M. Abdel-Bakia, F.A. Abdel Wahabb, F. El-Diasty, Materials Chemistry and Physics 96 (2006) 201. [38] J. Yang, Y.Q. Gao, J. Wu, Z.M. Huang, X.J. Meng, M.R. Shen, J.L. Sun, J.H. Chu, Journal of Applied Physics 108 (2010) 1. [39] K. Srinivas, M. Vithal, B. Sreedhar, M. Manivel Raja, P. Venugopal Reddy, Journal of Physical Chemistry C 113 (2009) 3543. [40] A.R. Mohmad, F. Bastiman, J.S. Ng, S.J. Sweeney, J.P.R. David, Physica Status Solidi C 9 (2012) 259. [41] H.S. Kwack, Y. Sun, Y.H. Cho, N.M. Park, S.J. Park, Applied Physics Letters 83 (2003) 2901. [42] Qin Kuang, Changshi Lao, Zhong Lin Wang, Zhaoxiong Xie, Lansun Zheng, Journal of the American Chemical Society 129 (2007) 6070.