Preparation and Characterization

Chapter 3: Co doped SnO 2 based DMS

3.1.1 Preparation and Characterization

Sn_1-xCo_xO₂ samples for x = 0.0, 0.02, 0.05, 0.07 and 0.10 were prepared by following the standard solid state route. Stoichiometric ratio of SnO₂ and Co₃O₄ with 99.9% purity were weighed, mixed under acetone and were presintered at 400^oC. The final sintering in pellet form was carried out in air at 900^oC for 24 hr (air annealed). For a comparison, one of the pellets of x

= 0.02 was annealed at 500^oC for 12 hr under flowing N₂ gas. X-ray diffraction (XRD) patterns were recorded at room temperature using Seifert 3003-TT XRD machine by employing Cu-Kα radiation. Fig.3.1 shows the XRD patterns recorded for Sn_1-xCo_xO₂ samples for x = 0 to 0.10 and they are found to be in single phase form. The patterns could be refined by using P4₂/mnm space group with the aid of Rietveld refinement technique and Fullprof programme. Fig.3.2 shows the refined XRD patterns for all the Co doped SnO2 samples. The obtained lattice parameters from the refinement are tabulated in table-3.1. The lattice parameters for pure SnO₂ are found to be a = b = 4.732 Å and c = 3.184 Å and they reduce to a = b = 4.712 Å and c = 3.142 Å for x = 0.10.

The lattice parameters are found to decrease marginally with increase in doping concentration and it can be understood in terms of Co²⁺ or Co³⁺ replacing Sn⁴⁺ions. The lattice parameters are comparable to those reported by Duan et al. [198] for Mn doped SnO2. The expanded view of (110) peaks are shown in Fig.3.3, where one can see the shift in peak position towards higher 2θ value with increase in doping concentration. The above observation substantiates the argument of lattice parameter variation due to Co-ions replacing Sn ions.

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The average crystallite size was calculated using the Scherrer’s formula

S=λk/β cosθ ... (3.1) where constant k depends upon the shape of the grain size and is taken as 0.89 by assuming the circular grains, λ=1.5406 Å for CuKα radiation, β is the full width at half maximum (FWHM) of diffraction peaks and θ is the glancing angle. The experimental β value was corrected for instrumental broadening using the relation β² = βm

2 - βins 2

. Here βm is the measured FWHM of the XRD peak and βinsis the instrumental broadening. The crystallite sizes for various samples are given in table-3.1. The average crystallite size was found of the order of 60 nm.

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Fig.3.1 XRD patterns recorded at room temperature for Sn1-xCo_xO₂ (x = 0.0, 0.02, 0.05, 0.07 and 0.10).

20 30 40 50 60 70 80

(3 0 1 ) (3 2 1 )

(2 0 2 )

(1 1 2 )

(3 1 0 )

(0 0 2 )

(2 2 0 )

(2 1 1 )

(2 1 0 )

(1 1 1 )

(2 0 0 )

(1 0 1 )

(1 1 0 ) x=0.0

In te n s it y ( a .u )

x=0.02

2θ θ θ (degree) θ

x=0.05 x=0.07 x=0.10

(a)

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Fig.3.2(a) XRD patterns along with Rietveld refinement for x = 0.02 and 0.05.

20 30 40 50 60 70 80

observed points calculated points obs-diff points diffraction peaks

2θ 2θ 2θ 2θ

In te n s it y (a .u )

(degree)

Sn

_0.98

Co

_0.02

O

₂

20 30 40 50 60 70 80

Sn

_0.95

Co

_0.05

O

₂

observed points calculated points obs-diff points diffraction peaks

In te n s it y (a b r. U n it )

2θ 2θ 2θ

2θ ^(degree)

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Fig.3.2(b) XRD patterns along with Rietveld refinement for x = 0.07 and 0.10 samples. The circles represent experimental points and solid line represents Rietveld refined data. The dotted lines at the bottom show the difference between experimental and refined data.

20 30 40 50 60 70 80

Sn

0.93

Co

0.07

O

2

observed points calculated points obs-diff points diffraction peaks

In te n s it y (a .u )

(degree) 2θ 2θ

2θ 2θ

20 30 40 50 60 70 80

Sn

_0.90

Co

_0.10

O

₂

observed points calculated points obs-diff points diffraction peaks

In te n s it y (a .u )

2θ 2θ 2θ

2θ (degree)

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Fig.3.3 Expanded view of (110) peak of XRD pattern.

Table-3.1. List of parameters obtained from the Rietveld refinement and analysis of XRD patterns of Sn_1-xCo_xO₂ samples prepared by solid state route.

Sample/Parameters x = 0.0 x = 0.02 x = 0.05 x = 0.07 x = 0.10 Space group P4₂/mnm P4₂/mnm P4₂/mnm P4₂/mnm P4₂/mnm

a = b (Å) 4.732 4.731 4.723 4.717 4.712

c (Å) 3.184 3.182 3.178 3.164 3.142

Volume (ų) 71.3 71.2 70.9 70.4 69.8

χχχ

χ² (%) 3.17 1.38 2.26 1.87 1.95

Rp (%) 28.1 19.9 20.0 18.5 16.7

Crystallite Size(nm) 66 68 66 65 63

26.4 26.7 27.0

2θ θθ θ (degree)

x=0.0 x=0.02 x=0.05 x=0.07

Intensity(a.u)

(110)

(b)

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The microstructure and compositional analysis were studied by using LEO SEM machine with energy dispersive X-ray spectrum (EDX) facilities. The SEM images show uniform surface morphology. The cation ratio obtained from EDX analysis are given in table-3.2 and are found to be comparable to that of nominal starting composition.

Fig.3.4 SEM images recorded for x= 0.02, 0.05 and 0.10 samples along with EDX spectra for x=0.10 Co-doped sample.

Table-3.2 Cationic ratio obtained from EDX measurement for different Co-doped SnO₂ samples.

(x=0.02) (x=0.05)

(x=0.10)

Sample Calculated Cationic Ratio from EDS

Sn Co

x=0.02 0.97 0.03

x=0.05 0.94 0.05

x=0.07 0.92 0.08

x=0.10 0.87 0.12

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Fig. 3.5 shows the TEM images of x = 0.02 sample taken in a carbon coated copper grid. The average particle size is found to be around 50 nm. Fig. 3.5(b) shows the selected area electron diffraction pattern and it depicts the polycrystalline behavior of the sample. The absence of ring like pattern suggests that the crystalline grains are in some preferred orientation.

Fig. 3.5 TEM, SAD and HRTEM image recorded for Sn_0.98Co_0.02O₂ sample annealed in air at 900⁰C.

The EDS measurement using TEM facility was carried out and it showed the presence of Co within the crystallites. The high resolution transmission electron microscope (HRTEM) image recorded at different locations shows continuous (101) atomic plane as can be seen in Fig.3.5(c).

The Fast Fourier Transform (FFT) image is shown in the inset of Fig. 3.5(c), where we can see the uniform (101) plane.

The samples were also characterized by recording Raman spectra by using LabRam HR 800 spectrometer. The typical Raman spectra recorded for x=0.0, 0.02 and 0.10 samples are shown in Fig.3.6. The main peak at 633 cm^-1 corresponds to A1g mode, i.e. symmetric Sn-O stretching.

The observed minor peak at ~693 cm^-1 can be compared to the reported Raman spectrum in literature for SnO2 [253, 254]. It may be noted that other than reduction in intensity no additional peak has been observed. So, the samples are basically free from Co₃O₄ and other related impurity phases.

(110) (101)

(101)

(a) (b) (c)

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Fig.3.6 Room temperature Raman spectra of Sn1-xCoxO2 for x= 0.0, 0.02, 0.10 sample.