J Mater Sci: Mater Electron (2014) 25:1564–1570 DOI 10.1007/s10854-014-1769-6
Preparation and ac electrical characterizations of Cd doped SnO2 nanoparticles Feroz A. Mir • Khalid M. Batoo • Indrajeet Chatterjee G. M. Bhat
•
Received: 12 November 2013 / Accepted: 24 January 2014 / Published online: 4 February 2014 Ó Springer Science+Business Media New York 2014
Abstract Nanoparticles (NPs) of the Sn1-xCdxO2 (0.0 B x B 0.04) were synthesized through soft chemistry method. These NPs were characterized for structural, morphological and electrical properties by X-ray diffraction, High resolution transmission electron microscopy and dielectric spectroscopy techniques respectively. Structural analysis confirms that all the NPs are having single phase rutile tetragonal structure. The NPs are of spherical shape and average size of these is found to decrease with Cd doping. Dielectric permittivity and AC conductivity of all the NPs were evaluated as a function of frequency and composition at room temperature. The frequency response of er, ei, tan d and rac show that the dispersion is due to the interfacial polarization and these parameters decrease with doping of Cd in the SnO2 matrix. The possible correlation between observed dielectric properties and size of NPs, and hence disorder in the system are explored.
1 Introduction Nanomaterials have attracted great interest of researchers not only by their unique chemical and physical
F. A. Mir (&) G. M. Bhat University Science Instrumentation Centre, University of Kashmir, Srinagar 190006, J&K, India e-mail:
[email protected] K. M. Batoo King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 2460, Saudi Arabia I. Chatterjee Indian Institute of Science Education and Research, Kolkatta 741252, India
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properties but also due to their potential application in many fields, which has stimulated the search for new synthetic methods for these materials. For last few decades, oxide based semiconductors are at centre stage due to their unique potential properties (like optical, magnetic and electrical) and among them tin dioxide is the predominant one [1]. Bulk SnO2 crystallizes in the tetragonal rutile type of structure, P42/mnm space group with two Sn and four oxygen atoms per unit cell. The lattice parameters are a = b = 0.4737 nm, c = 0.3185 nm. SnO2 is an important n-type wide-energy-gap semiconductor (Eg = 3.64 eV, 300 K) with high carrier concentration (6 9 1020 cm-3) which shows its scope for various potential applications such as in gas sensors [2], transparent conducting electrodes [3], Li batteries [4], and optoelectronic devices [5]. During the past decade, SnO2 based nanostructures have been one of the most important oxide nanostructures due to their properties and potential applications [6, 7]. The structural and optical properties of various geometrical morphologies such as NPs, nanobelts, nanowires, nanorods, nanodiskettes and nanoflowers of SnO2 have been investigated [8–14]. The electrical properties of SnO2 critically depend on the amount of defects, oxygen vacancies and its basic nanostructure [15–17]. There are only few reports present in the literature that deals with electric (dielectric) properties of SnO2. Most of these dielectric studies of SnO2 deal with different formats like doping, film, nanostructure, bulk form as well as their preparation method [18–21]. As per our literature survey, no detailed study on the structural and dielectric properties of Cd doped SnO2 NPs has seen reported in the existing literature. In this paper, structural and dielectric properties of Cd doped SnO2 NPs prepared by citrate gel method have been presented.
3 Results and discussion 3.1 Structural analysis Figure 1 shows the XRD patterns of Sn1-xCdxO2 (0.0 B x B 0.04) NPs. The XRD patterns of pure and Cd2? doped SnO2 NPs shows peaks of (110), (101), (200) and (211) orientation at 2h as 26°, 34°, 38° and 52° respectively. In all cases, the rutile phase SnO2 nanocrystallites with tetragonal structure was observed. The observed peaks of the samples were found to be in good agreement with JCPDS file no. 88-0287 and corresponds to the rutile structure of SnO2 NPs [2–5]. With increasing Cd content, none of the samples showed any extra peak corresponding to any metallic cluster or oxide phase of Cd. Therefore, excludes the presence of any secondary phase up to 4 mol% Cd (means within present doping range). However, slight broadening and decreasing in the maximum intensity of the peaks has been noticed. The observed peak
(202)
(221) (301) (311)
(220)
(211) (200)
x=0.0
Intensity(Arbt. Unit)
Sn1-xCdxO2 (0.0 B x B 0.04) NPs were synthesized using a citrate gel method with anlytical grade stannous chloride (SnCl22H2O) and cadmium chloride (CdCl2) as starting material. Stoichiometric amounts of above material were dissolved in deionized water. The mixed metal solution was then added to the citric acid solution in 1:1 molar ratio. The solution was heated at 60 °C to allow gel formation and the gel was dried in air oven at 80 °C for 12 h followed by heating at 400 °C for 6 h to remove any organic material present. The dried material was grinded for half an hour and part of the powder material was then mixed with poly vinyl alcohol (PVA) and left overnight for drying. The PVA mixed material was then pressed into disk-shaped pellets with the help of hydraulic press by applying a pressure of about 4 tones. The pressed samples were sintered in air at 500 °C for 5 h. In order to do dielectric measurement, the opposite faces of the pellets were coated with silver paste by making parallel plate capacitor geometry and were fired at 150 °C for 1 h. Powder X-ray diffraction (XRD) was performed at room temperature by using RIGAKU X-ray powder diffractometer with CuKa ˚ ) at a scanning rate of 2°/min, radiation (k = 1.54056 A ranging from 20° to 75° to study the phase formation and structure of the pure and doped SnO2 NPs at room. The microstructural analysis of the samples was carried out using high resolution transmission electron microscopy (HRTEM) HRTEM-Jeol 2010. Then room temperature dielectric properties of these samples were measured (in frequency range 42–5 MHz) with an HOIKI LCR Hi Tester 3532-50.
(110)
2 Experimental
(002)
1565 (101)
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x=0.01
x=0.02
x=0.04 20
30
40
50
60
70
80
2θ (Degree)
Fig.1 XRD Pattern of Sn1-xCdxO2 (x = 0, 0.01, 0.02 and 0.04) NPs Table1 Different structural parameters of Sn1-xCdxO2 (x = 0, 0.01, 0.02, 0.03 and 0.04) NPs obtained from XRD data X
˚) a (A
˚) c (A
c/a
Grain size (nm)
Strain e x 10-2 (line-2 m-4)
0
4.7368
3.1852
0.67244
12.71
0.1513
0.01
4.7297
3.1845
0.6733
12.06
0.1627
6.87
0.02 0.03
4.7227 4.7181
3.1825 3.1805
0.67387 0.67411
9.81 7.25
0.2001 0.2705
103.91 190.24
0.04
4.7029
3.0215
0.64248
5.81
0.3382
296.24
Dislocation density d x 1014 (line/nm2) 6.19
broadening can be related to particle size decreasing. Since Sn4? and Cd2? have different covalent and ionic radii, and assuming that Sn substitution by Cd would generate oxygen vacancies to keep charge neutrality, a well noticed lattice distortion would be expected if solid solution between the compositions were formed [22]. The lattice parameters a and c (by analyzing XRD data) of pristine and Cd doped SnO2 NPs have been calculated by using the following relation and are given in Table 1. 1=d ¼ ðh2 þ k2 Þ=a þ l2 =c
ð1Þ
where d is lattice spacing and h, k and l are Miller indices. The d values has been estimated by using (101) peak for different values of Cd substitution in SnO2 matrix. It has been observed that the d(101) value decreases with the Cd concentration. Therefore, the decreasing trend of d(101) values indicates that lattice parameters decrease with Cd doping. This observation is in good agreement with the ˚ ) to that earlier reports [22]. The ionic radius of Cd2?(0.97 A 4? ˚ of Sn (0.71 A) could also be one of possible reason [6]. The variations observed in lattice parameters are considerably small but significant. However, dealing with
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NPs, these variations can be considered due to microstrains inducing lattice compression as is previously reported [17–21]. This is because the surface accounts for a large part of the particle and the micro-strains can be generated by surface solid solution of Cd (surface excess). Using the diffraction lines of the (1 1 0), (1 0 1), (200) and (2 1 1) planes, the crystallite or particle size are estimated by Scherer equation [23]: D = 0:89k=b cos h
ð2Þ
˚ for Cu Ka), where, k is the X-ray wavelength (1.543 A b ðfullwidthathalfmaximumÞ ¼ ðb2M b2i Þ1=2 bM and bi are the measured and instrumental broadening in radians respectively and h is the Bragg’s angle in degrees. The calculated average grain size for pure SnO2 NP is about *11.76 nm. As mentioned above, the basic rutile phase remains practically same with doping but the average particle size decreases after doping. This development in the system will also lead to disorder in the system and will affect its various physical properties. Broadening in peaks also reflect the increase in the concentration of lattice imperfection due to the decrease in the internal micro-strain within the matrix [24]. Similar results were also reported by other workers [25]. Smaller grain size maximizes the imperfect regions of the matrix, and is further supported by the smaller strain and dislocation densities. Further the strain (e), was calculated by using the following formula [24]: e ¼ b cos h=4
ð3Þ
The dislocation density, d defined as the length of dislocation lines per unit volume of the nano-crystal, was calculated using d ¼ 1= D2
ð4Þ
As observed here, the calculated strain increases after doping (see Table 1). Further, there is increase in dislocation density after doping. Since dislocation density and strain are the manifestation of dislocation network in the matrix, the increase in dislocation density indicates the formation of high-quality compound. However, strain developed after doping is confirming the reduction in crystallinity and enhancement of disorder in the current system. [Observed increasing value of b for these samples indicate reduction in crystalline quality after doping (see Table 1)]. In addition, the observed inhomogeneous strain component could be localized at the sub grain and sub domain level near grain boundaries, which reveals that the crystalline sizes of the samples show a small significant change, regardless of the Cd2? concentration in the SnO2 host matrix. Figure 2a shows the TEM images of the Cd doping SnO2 NPs (x = 0.02). These NPs are having spherical
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Fig. 2 a HRTEM Image of Sn0.98Cd0.02O2 NPs and b SAED pattern
morphology and the average particle size is around 9.5 nm. It also reflects that they posses low agglomeration and high dispersivity with uniform crystal size. The average particle size calculated by two techniques almost correlate with each other [23]. The selected area electron diffraction (SAED) technique was also used to further study the single phase nature of Cd doped SnO2 NPs (see Fig. 2b). Rings obtained in SAED patterns indicate clearly the randomly oriented single crystals, which rules out the presence of any impurity or secondary phase and indicate that each NP is itself in single phase. Also the observed rings correspond to (1 1 0), (1 0 1), (200) and (2 1 1) planes of the tetragonal rutile phase. 3.2 Dielectric study From the measured capacitance, the dielectric constant (er) can be calculated by the following equation: er ¼ Cd=e0 A
ð5Þ
where C is the capacitance (F), e0 is the free space dielectric constant value (8.854 9 10-12 F/m), A is the capacitor area (m2) and d is the thickness (m) of the material.
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1567 X
12000
0.0 0.01 0.02 0.03 0.04
0.7
0.6
8000
0.5
S
10000
εr
0.4
6000 0.3
4000 0.2 0.0
0.1
0.2
0.3
0.4
x
2000 0 4
6
8
10
12
14
16
18
ln(ω) (Hz)
Fig. 3 Variation of er with frequency of Sn1-xCdxO2 (x = 0, 0.01, 0.02, 0.03 and 0.04) NPs. Inset shows plot between s (frequency exponent) and x (Cd compositions)
The imaginary part of the dielectric constant was calculated using the following relation [26]: ei ¼ er tan d
ð6Þ
where tan d is the dielectric loss. From the dielectric constant and dielectric loss, the ac conductivity (rac) of the present samples was calculated using the following relation [26]: rac ¼ xer e0 tan d
ð7Þ
where x is the angular frequency and rac is a temperature and frequency dependent function.The total conductivity (rtot)of a material is given by [27]: rtot ðxTÞ ¼ r0 ðTÞ þ rac ðxT Þ
ð8Þ
The first term in above equation; r0 (T) is the dc conductivity, it is frequency independent and is due to the band conduction. The second term in above equation; rac (xT) is due to hopping and as well as due dielectric relaxation caused by the localized electric charge carriers which obeys the power law [27]: rac ðxT Þ ¼ Bxs
ð9Þ
where B and s are the composition and temperature dependent parameters, respectively. Also s predicts the type of conduction mechanism in the system. According to correlated barrier hopping model s decreases and as per small polaron hopping model, it increases with temperature [27]. Figure 3 shows the variation of real part dielectric constant (er) with ln(x) of Sn1-xCdxO2 (0.0 B x B 0.04). The figure shows that er decreases with increase in frequency and doping. The decrease of dielectric constant
with increasing frequency is a normal behavior for oxide materials [28]. This decrease in er at low frequencies can be attributed to both the deformational (electronic and ionic) and relaxation (orientational and space charge) polarization [29]. The deformational polarization depends on the displacement of electrons and ions but the relaxation polarization depends on the orientational or interfacial effects. The increase in applied field leads to a reduction in orientational polarization, as a result, the molecular dipoles require much time to change their orientation with applied field. This reduces the value of er with increasing frequency. The modification in dielectric behavior in nanomaterials is mostly explained by two dielectric polarization: space charge polarization (SCP) and rotation direction polarization (RDP) process [30, 31]. In the present case, it is believed that both RDP and SCP processes contribute to the dielectric response of the pure and Cd doped SnO2 NPs. The RDP process is an important contribution for higher er of the SnO2 NPs. In case of simple n-type oxide semiconductor, there are various defects (positive, negative or neutral), due to oxygen vacancies which act as shallow donors. Positive oxygen vacancies together with negative oxygen ions give a large amount of dipole moments [22]. These dipole moments will rotate in an external electric field, which leads to the rotational direction of polarization occurring in the interfaces of these n-type SnO2 NPs. Additionally, SCP process can also occur in the these system. It has been found that nanomaterials have around 1,019 interfaces/cm3 and this value is much more than that of their bulk counterparts [32]. Also nanomaterials have large surface area and as a result, these defects or vacancies exist on the interfaces [28, 29]. These defects can cause a change of positive and negative space charge distributions in interfaces [33]. According to electrostatics, these negative and positive charges at the interface move towards positive and negative poles of the electric field respectively. At the interface, they are trapped by defects, which lead to the formation of dipole moments form and hence gives rise to SCP mechanism in these type of ensemble. Also as mentioned earlier, the volume fraction of the interfaces of nano-size sample is larger as compare to the bulk system. Hence SCP is stronger in nanostructures than that in the bulk materials. Thus, er of the SnO2 NPs is higher than that of bulk. However, in the high applied field region, dielectric response of RDP and SCP cannot keep up with changing field or frequency, and hence causing rapid decrease of er in these SnO2 NPs. The decrease in er with doping is due to more and more defects (positive, negative or neutral) or oxygen vacancies which are randomly created (at the surface)
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x
250000 200000
0.0 0.01 0.02 0.03 0.04
x
30
0.0 0.01 0.02 0.03 0.04
25 20
εi
tan δ
150000 100000
15 10
50000 5
0 4
6
8
10
12
14
16
0
18
ln(ω) H z
123
7
9
8
10
11
12
13
14
15
16
17
18
Fig. 5 Variation of tan d versus ln (x) of Sn1-xCdxO2 (x = 0, 0.01, 0.02, 0.03 and 0.04) NPs
-6
-7
-8
-1 -1
and finally disorder in system. As a result of their recombination, the number of dipole decreases which leads to a decrease in the orientational polarization [33]. The decrease in orientational polarization results into the decrease in er. Figure 4 shows the variation of the dielectric loss (imaginary part of dielectric constant ei) of Sn1-xCdxO2 (x = 0, 0.01, 0.02, 0.03 and 0.04) NPs with applied frequency. It can be observed from the figure that ei decreases with increasing frequency. Here for all samples, same behavior (dispersion at low frequency and frequency independent nature at high frequency) is observe. The reduction of ei at low frequencies is due to the migration of ions in the material. Similarly the dielectric loss at low and intermediate frequencies is due to the ionic hopping, conduction losses caused by ion migration. In addition ionic polarization losses can also be a vital factor for this behavior. However, at high frequencies, ion vibrations may be the only reason of dielectric loss for the current NPs. Figure 5 shows the plot of tan d versus ln(x) of Sn1-xCdxO2 (x = 0, 0.01, 0.02, 0.03 and 0.04) NPs. Only pristine sample shows the relaxation peak. For all under investigation NPs, the dielectric loss is found to decrease with the increase in applied frequency. The relaxation parameters may be extracted by analyzing tan d as a function of frequency. For highest dielectric loss at a particular temperature, the absorption peak is expressed by the relation xms = 1 ? xm = 1/s = 1/(RC), where s is relaxation time and xm is the angular frequency (at highest loss peak) of the applied field [27]. It indicates that the dielectric constant enhances with the volume fraction of SnO2 NPs. For pure SnO2 NPs, within measured frequency range, this dielectric relaxation peak is observed at room temperature. This indicates that SnO2 contributes largely to the dielectric response. Further, the
6
ln(ω) Hz
ln σ ac ( Ω . c m )
Fig. 4 Variation of e// with frequency of Sn1-xCdxO2 (x = 0, 0.01, 0.02, 0.03 and 0.04) NPs
5
-9
-10
x
-11
0.0 0.01 0.02 0.03 0.04
-12 4
6
8
10
12
14
16
18
ln(ω) Hz
Fig. 6 Variation of ln(rac) with frequency of Sn1-xCdxO2 (x = 0, 0.01, 0.02 and 0.04) NPs
photoemission experiment [34] and microscopic studies [35] conducted on the samples have established that tin ions with Sn2? state mostly exists at the surface. In addition, computational studies like first principles density function calculations have also established that the oxygen deficient surfaces of SnO2 behave like SnO [36]. These oxygen vacancies and the existence of a mixed valence state of Sn (like Sn4?, Sn2?) can induce local lattice distortion in nano sized SnO2 matrix [37]. The existing lattice distortions can create various potential minima. Therefore, the observed dielectric relaxation at high frequency may be due to thermally activated motion of charges within the potential minima. The observed activation energy associated with the dielectric relaxation is consistent with the shallow levels along with intrinsic defects within SnO2.
J Mater Sci: Mater Electron (2014) 25:1564–1570
Figure 6 shows the variation of ln(rac) versus ln(x) of Sn1-xCdxO2 (0.0 B x B 0.04) NPs. At lower frequency, this system frequency independent behavior. It is also observed that the conductivity decreases with increasing Cd doping. As the particle size is decreases, the surface– volume ratio increases resulting large surface scattering. This results, a reduction in the electronic conductivity [38]. This may be due to the disordered structure and hence causing large scattering at grain boundaries for these nanoscaled materials. A similar trend was also seen in various metal doped oxide semiconducting NPs [39, 40].The high conductivity at higher frequencies confirms the short range intrawell hopping of charge carriers between localized states. But on doping with Cd, the effective number of charge carriers involved in doping mechanism was reduced, hence ac conductivity has been found to decrease. The frequency variation of ac conductivity component (rac) is expressed by Eq. 9. The exponent s has been calculated by fitting ln(rac) versus ln(x) (figure not shown here).The fitted values of s are shown in inset of Fig. 3. It is well established that s takes values between 0 and 1 (under limit 0 B s B 1). When s = 0, the electrical conduction is frequency independent (dc conduction) and when s [ 0, the conduction is frequency dependent or becomes the ac conduction [41]. In case of present samples, the value of s ranges from 0.7109 (for x = 0) to 0.2417 (for x = 0.04), which suggests that the conduction phenomenon in under study material is due to the correlated barrier hopping [42]. Similar trend in ac conductivity has been observed in many nanocrystallized semiconductor materials [42, 43].
4 Conclusion The tetragonal rutile phase of Cd doped SnO2 NPs were successfully synthesized by citrate gel method. The average crystallite size of these NPs was found to vary between 7 and 11 nm. The dielectric properties show normal behavior with frequency at room temperature. The dielectric and the conducting behavior have been explained in the light of interfacial space charge polarization and hopping of charges. It has been found that at higher frequencies, these NPs exhibit a constant dielectric loss behavior, which predicts that these materials possess a lossless nature. Keeping in view the above observations, it is further suggests that these NPs could be used in high frequency electronic device applications. Acknowledgments F. A. M would like thank University Grants Commission (UGC) for awarding the UGC-Dr. D. S. Kothari Postdoctoral Fellowship. We are also thankful to the King Abdullah
1569 Institute for Nanotechnology, King Saud University, Riyadh, Saudi Arabia for providing some experimental facilities.
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