Chapter 3: Co doped SnO 2 based DMS
3.4 Co doped SnO 2 by Ball Milling
3.4.1 Preparation and Characterization
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Sn1-xCoxO2 samples for x = 0.0, 0.02, 0.07 and 0.10 were prepared by mechanical alloying method. The stoichiometric ratio of SnO2, Co3O4 with 99.9% purity were weighed and mixed and, then ball milled using a hardened steel vial containing steel balls of 8 mm diameter and weight of each ball is approximately 2.1g. The weight of the vial is 15.5kg with a volume of 203cm3.The milling speed was 500 rpm and ball to powder weight ratio was 10:1. The milling was carried out for duration of 10 hrs. The milled powders were pressed into cylindrical pellets and were annealed in air at 900oC for 10 hrs.
XRD patterns of ball milled Sn1-xCoxO2, for x = 0.0, 0.02, 0.07 and 0.10 are shown in Fig. 3.30.
In addition to the allowed XRD peaks for tetragonal rutile structure of SnO2, a minor XRD peak at 2θ ≈ 35.5o is observed. The origin of the minor extra peak was found to be from Fe3O4 or SnFe2O4 based spinel structure. Since, no Fe was doped in the above samples; we can infer that the contamination from the steel vial could be the source of the spinel structure. So, the patterns were refined to the two phase model of crystal structure corresponding to Sn1-xCoxO2 and the spinel phase by using Rietveld refinement technique and Fullprof programme. The refinements were carried out by choosing P42/mnm space group for (Sn,Co)O2 and Fd3m space group for the spinel phase. Typical XRD pattern for x = 0.10 sample along with Rietveld refined data are shown in Fig. 3.31. The volume fraction of the spinel phase obtained from the XRD refinement was found to around 6 %, without any appreciable variation with increase in doping concentration. From the obtained value of volume fraction of spinel phase and by taking the theoretical density, the weight % of spinel phase was determined for different samples and the values are found to be close to 4.5 wt %. The lattice parameters and particle size values determined from XRD analysis are given in Table-3.11. We can see that the values of the lattice parameters decrease marginally up to 7 % of Co-doping and can be explained in terms of Co3+
(0.61 Å) replacing some of Sn4+ (0.69 Å) ions. The above variation of lattice parameters is comparable to those of Co doped SnO2 samples prepared by conventional solid state route as discussed in section 3.1.2. The refined occupancy values of Sn and Co are found to be comparable to the nominal starting composition. Typical values of Sn/Co occupancy are
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0.923/0.077 for x=0.07 sample. The lattice parameters of spinel structure phase is found to be around 8.40 Å, and is comparable to the reported value of ferrites [263]. The crystallite size values are found to be close to 40 nm for all the samples.
Table-3.11. List of parameters obtained from the Rietveld refinement and analysis of XRD patterns.
Samples/Parameters x=0.0 x=0.02 x=0.07 x=0.10
a=b(Å) 4.7322 4.7327 4.7310 4.7329
c(Å) 3.1848 3.1836 3.1824 3.1830
χ χχ
χ2(%) 1.31 1.9 2.2 2.21
Rp (%) 14.9 19.9 18.5 17.7
Crystallite Size(nm) 40 39 42 41
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Fig. 3.30 XRD patterns of Sn1-xCoxO2 samples for x = 0.0, 0.02, 0.07 and 0.10 prepared by ball milling method. Star marked peaks correspond to spinel phase.
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(321)
(202)
(301)
(112)
(310)
(002)
(220)
(211)
(210)
(111)
(200)
(101)
(110)
SnO
2(Unmilled)
x=0.0
*
* x=0.02
x=0.07
*
x=0.10
In te n s it y ( a .u )
2θ θ θ θ (degree)
*
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Fig. 3.31 XRD pattern along with Rietveld refinement for x=0.10 sample.
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2θ 2θ
2θ 2θ(((( degree)
In te n s it y (a b r. U n it )
x=0.10
observed points calculated points obs-diff points diffraction peaks
(s p in e l)
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The EDX spectra recorded for the pure and the Co-doped samples show the presence of Fe along with Co, Sn and O peaks as shown in Fig.3.32. This can be understood in terms of Fe contamination due to the ball milling process using the steel vial. It is also in agreement with the observed spinel phase from XRD analysis. Similar Fe contaminations were also reported in Co- doped ZnO based materials prepared by mechanical synthesis route [264]. The EDX spectra analyses for all the samples are tabulated in table-3.12. From the observed Fe content in EDX analysis and by assuming that they are from the spinel phase, the weight percentage of spinel phase was estimated by taking the theoretical densities of (SnCo)O2 and Fe3O4 and by normalizing them, the value was found to be close to 6 wt %. They are comparable to the results of XRD analysis.
Fig. 3.32 SEM images for x=0.02 and 0.10 with EDX spectra.
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Table-3.12 Cationic ratio obtained from EDX measurement for x =0.0, 0.02, 0.07 and 0.10 samples.
Typical TEM image of x = 0.02 sample is shown in Fig. 3.33, where we can see the nanometric size of the particles. The high resolution TEM (HRTEM) images recorded at different locations show the presence of (110) plane as shown in Fig. 3.33(b). The respective Fast Fourier Transform (FFT) image is shown in the inset of Fig. 3.33(b). Typical selected area diffraction pattern (SAD) recorded for x = 0.02 sample is shown in Fig. 3.33(c), where a regularly arranged bright spots are observed and it highlights the single crystalline nature of the particle. The crystal structure observed at the microscopic scale was found to be comparable to the bulk crystal structure obtained from XRD analysis.
Fig. 3.33 TEM, HRTEM and SAD images of x=0.02 sample.
(110)
(b)
(110) (101)
(200)
(a) (c)
Sample Calculated Cationic Ratio from EDS
Sn Co Fe
x=0.0 0.93 --- 0.06 x=0.02 0.91 0.023 0.077 x=0.07 0.87 0.067 0.072 X=0.10 0.74 0.098 0.062
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The Raman spectra recorded for all the Co-doped samples are shown in Fig.3.34. All the samples show the Raman shift at ~ 640 cm-1 which corresponds to A1g (symmetric Sn-O stretching). This indicates the tetragonal rutile structure. The other peak observed at 693 cm-1 is because of the A2u(LO), LO=longitudinal optic mode; which is basically IR-active but because of some disorder this become Raman active [259-261]. The samples exhibit the peak broadening effect and it depicts the presence of some amorphous phase. The observed peak at 475 cm-1corresponds to the Eg mode of vibration. As the Eg mode is the result of two oxygen atoms vibrating parallel to the c axis, but in opposite direction; it is more sensitive to oxygen vacancies than other modes[262]. So the vacancy related defects are dominant in the samples.
Fig.3.34 Raman Spectra recorded for Sn1-xCoxO2 x=0.0, 0.02, 0.07, 0.10 samples at room temperature.
200 300 400 500 600 700 800
In te n s it y (a .u )
Raman Shift(cm -1 ) x=0.0
x=0.02 x=0.07 x=0.10
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