3.1 XRD
The crystalline structure of pure PCz and composites PCz/CuO and PCz/Fe2O3 were investigated from XRD patterns. The XRD patterns of the present samples are shown in the below Fig. 1. Some diffraction peaks corresponding to different crystalline planes can be seen. The sharp peaks at 35.77̊, 38.91̊, 48.82̊, 53.54̊, 58.54̊, 61.62̊, 66.50̊, 68.18̊, 72.59̊, 75.35̊ correspond to (0 0 2), (2 0 0), (2 0 2), (0 2 0), (2 0 2), (1 1 3), (0 0 2), (2 2 0), (3 1 1), (0 0 4) planes of monoclinic structure of CuO. Peaks at 24.19̊, 33.15̊, 35.77̊, 40.96̊, 49.68̊, 54.32̊, 62.47̊, 63.96̊ correspond to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4), (3 0 0) planes of rhombohedral structure of Fe2O3. The CuO and Fe2O3 patterns are found to be consistent with JCPDS file Nos. 48-1548 and 89-8104 respectively [19, 23]. The peaks at 19.03̊, 20.01̊, 21.16̊, 23.16̊, 28.12̊ correspond the planes (2 0 -1), (1 2 -1), (2 2 0), (0 1 2), (2 1 0) of polycarbazole [24]. The increase in peak intensity with increasing concentration of CuO and Fe2O3 confirms interaction of dopants with PCz. The crystallinity of the composites enhanced due to increased concentration of nano sized dopants.
The crystallite size, D was determined from XRD patterns using Debye Scherer’s formula showed in Eqn (1) and micro strain, ε using Eqn (2) [25],
D = \(\frac{K\lambda }{ßCos\theta }\) (1)
ε = \(\frac{ß}{4tan\theta }\) (2)
Where, K is a constant called shape factor equal to 0.9 for spherical shaped particles [24], λ is wavelength of X-ray ( ̴ 1.5406Å), ß is full width half maximum and θ position of the peak. The prominent peak for each sample was considered for determining crystallite size and micro strain. The obtained values of crystallite size, average crystallite size and micro strain of the samples are tabulated in Table 1. It can observed that the crystallite size of PCz is less compared to composites, crystallite size is increasing and micro strain is decreasing with wt% of dopants which confirms encapsulation of polycarbazole on the dopant particles. These are in qualitative agreement with the literature on PCz/SnO2 [24].
Table 1
Crystallite size, D average crystallite size and Micro strain, ε of PCz, PCuO and PFO composites.
Sl no
|
Sample
|
2θ
(Degrees)
|
FWHM (ß)
|
Crystallite size
(nm)
|
Average crystallite size
(nm)
|
Micro strain
(ε)
|
1
|
PCz
|
20.04
|
0.33
|
24.07
|
24.07
|
0.47
|
2
|
CuO
|
35.60
|
0.27
|
29.42
|
29.42
|
|
3
|
Fe2O3
|
33.35
|
0.31
|
25.62
|
25.62
|
4
|
PCu10
|
19.43
|
0.30
|
26.51
|
34.14
|
0.44
|
5
|
PCu20
|
19.53
|
0.25
|
31.77
|
0.36
|
6
|
PCu30
|
19.52
|
0.18
|
44.15
|
0.26
|
7
|
PFO10
|
19.49
|
0.26
|
30.06
|
42.89
|
0.38
|
8
|
PFO20
|
19.51
|
0.19
|
41.81
|
0.27
|
9
|
PFO30
|
19.47
|
0.14
|
56.82
|
0.20
|
3.2 FTIR ANALYSIS
The FTIR has been analyzed to know different functional groups developed in the composites due to interaction between constituents such as PCz and CuO and, PCz and Fe2O3. The spectra of the present nanocomposites are shown in the Fig. 2. The IR bands at 726cm−1 and 812 cm−1 are due to C-H deformation of di-substituted and tri-substituted benzene ring of PCz respectively [26, 27]. A sharp band around 3415cm−1 refers to stretching of the N-H bond in PCz. The change in intensity and shifting of 3415cm−1 band evidenced the formation of bond between NH group of PCz and CuO and, Fe2O3 [25]. The presence of the stretching band at 1227 cm−1 is attributed to C=N and the peak at 1316 cm−1 is attributed to C-H out of plane bending vibration of aromatic ring. The sharp band at 1444 cm−1 may be due to ring stretching vibration of carbazole [27]. The bands at 918 cm−1 and 1598 cm−1 are assigned to =CH out of plane and stretching mode of aromatic alkene respectively [21, 25]. A strong absorption band at 562 cm−1 and at 602 cm−1 in the composites confirms incorporation of Cu-O and Fe-O vibrational modes respectively [21, 23]. The assignment of bands of different fuctional groups are tabulated in Table 2.
Table 2
Assignment of bands to different functional groups in FTIR spectra of PCz, PCuo and PFO composites.
S.No
|
FTIR bands in PCz (cm−1)
|
FTIR bands in PCz/CuO composites (cm−1)
|
FTIR bands in PCz/Fe2O3 composites (cm−1)
|
Assignment of bands
|
1
|
|
|
562-563
|
Fe-O stretching vibratio mode[23]
|
2
|
|
602 To 610
|
|
Vibration of Cu-O bond[21]
|
3
|
726
|
717 To 723
|
720
|
Ring deformation of substituted aromatic structure[26]
|
4
|
812
|
812
|
814
|
C-H deformation in tri substituted benzene ring[27]
|
5
|
918
|
918
|
918
|
= CH out of plane vibrations[21]
|
6
|
1227
|
1227
|
1232
|
C=N stretching[27]
|
7
|
1316
|
1316
|
1316
|
C-H out of plane bending vibration of aromatic ring[27].
|
8
|
1444
|
1444
|
1444
|
Ring stretching vibration of carbazole moiety[27].
|
9
|
1598
|
1598
|
1598
|
stretching mode of aromatic alkene [25]
|
10
|
3415
|
3415-3419
|
3415-3420
|
Stretching of N-H bond [25]
|
3.3 MORPHOLOGY
Figure 3 show3.3 MORPHOLOGYs typical SEM images of PCz, PCu10 and PFO10 nanocomposites. It is evident from the images that the polycarbazole has homogeneous surface morphology with nodular nature and the particles are agglomerate. It can be observed from the images of the PCz/CuO, PCz/Fe2O3 nanocomposites the morphological changes occurring upon adding the CuO/Fe2O3 nanoparticles. The added nanofillers lead to branching of polymer chain in the polycarbazole and that intern create network like structure in composites, Polycarbazole in PFO composites.
3.4 UV-VIS ABSORPTION ANALYSIS
Figure 4 (a & b) depicts optical absorption spectra of the samples PCz, PCu10, PCu20, PCu30, PFO10, PFO20 and PFO30. A broad band is observed at 279nm in pure polycarbazole is assigned to bonding and antibonding (π-π*) transition of the benzoid ring and small peak around 347nm is corresponding to polaronic energy level (n-π*) transition of the quinoid ring. The polaronic energy level is created by the formation of defects during polymerization process [24, 26]. It is observed that the peaks are slightly shifted to blue end about 4nm for CuO composites and to 8nm for Fe2O3 composites of spectrum and also there is variation in the intensity with different concentration of CuO and Fe2O3. This is because CuO or Fe2O3 nanoparticles absorbs partly incident radiation by their free electrons and due to the strong interaction between polymer and dopant nanopartiles. The blue shift on a small scale with increase in CuO wt% is in agreement with the reports, PCz/SnO2 [25].
The optical absorption gives information about band gap and electronic transitions. Optical energy gaps can be determined using Mott-Davis-Tauc’s equation [28].
(αhν)1/n = \(\frac{2.303}{d}\) = B (hν-Eg) (3)
Where, α is the absorption coefficient, B the absorption constant, hν the energy of the photon, Eg the optical energy gap and d the sample thickness. The exponent (1/n) represents different electronic transitions and it takes values \(\frac{1}{2}\), 2, \(\frac{3}{2}\) and 3 corresponding to allowed direct, indirect, forbidden direct and forbidden indirect transitions respectively. The direct and indirect energy gaps are determined from the transition of electrons from valance band to conduction band when photons interact with them in the valance band.
The Tauc’s plots for direct and indirect transitions were made and tangents to the band edges were extrapolated on to the hν-axis. The intersecting values on hν-axis gave band gap values corresponding to direct or indirect transitions as the case may be. The typical plots of direct band gap for one sample in each series and for pure PCz are shown in Fig. 5 and for indirect band gap in Fig. 6. To save space, Tauc’s plots for all the corresponding are not shown in the Fig. 5 & 6.
The results tabulated in Table 3 revealed that the intended direct and indirect band gap values of pure PCz were 3.32 eV and 3.42 eV respectively. For PCu10, direct and indirect gaps are found to be 3.47eV and 3.54 eV respectively. It implies that band gap values increases on doping PCz with CuO. Similarly, for PFO10, direct and indirect gaps are 3.49 eV and 3.53 eV. These results are also suggest that band gap of PCz increases when doped with Fe2O3. This may be due to strong interaction between the polymer matrix and dopant oxides. Increase of CuO or Fe2O3 from 10 wt% to 30 wt% decreases band gaps slightly. Similar nature of results were reported for PCz/SnO2 and PVA/CuO [24, 28].
Table 3
Optical Band gap energy (direct and indirect) and Urbach energy values for PCz, PCuo and PFO composites.
Sl no
|
Sample
|
Direct band gap
Eg (eV)
|
Indirect band gap
Eg (eV)
|
Urbach energy
Eu (eV)
|
1
|
PURE PCz
|
3.32
|
3.42
|
0.32
|
2
|
PCu10
|
3.47
|
3.54
|
0.30
|
3
|
PCu20
|
3.48
|
3.53
|
0.31
|
4
|
PCu30
|
3.46
|
3.52
|
0.32
|
5
|
PFO10
|
3.49
|
3.53
|
0.30
|
6
|
PFO20
|
3.47
|
3.52
|
0.32
|
7
|
PFO30
|
3.46
|
3.51
|
0.33
|
The Urbach energy (Eu) was determined by plotting ln(α) versus hν as depicted in Fig. 7. Urbach energy (Eu) of pure PCz is determined to be 0.325 eV [Table 3]. This value decreased to 0.307 eV when 10 wt% of CuO or Fe2O3 are doped to PCz. Since Urbach energy is a measure of defects in the sample, present results indicate that samples improve their quality in terms of defects when they were doped with 10 wt% of dopant oxides. On increasing dopants beyond 10 wt% Urbach energy increases. This reveals that higher amounts of dopant oxides increases concentration of structural defects in the samples. Similar results were quoted for PVA/CuO composits [28].
3.5 CONDUCTIVITY
Conductivity, σ of pure PCz and the composites is observed to be increasing with increase of temperature and is of the order of 10−5 (Ωm)−1. This reveals semiconducting behavior of the samples. In composites, conductivity increased with increase of CuO or Fe2O3 contents. Conductivity of the composites is found to be less than that of pure PCz at all the temperatures of interest. Increase in conductivity with increase in CuO/Fe2O3 concentration may be due to formation of well organized network for transportation of charge carriers by the added dopants. Raj et al studied temperature dependent conductivity of pure PCz and their conductivity was in the order of 10−5 (Ωm)−1[26].
The temperature variation of electrical conductivity is analyzed using Arrhenius expression,
σ = σ0 exp (Ea/kBT) (4)
Where, σ is conductivity, Ea the activation energy and kB the Boltzman constant.
Figure 8 and Fig. 9 shows the plots of ln(σ) versus (1/T) for pure PCz and PCuO and PFO composites respectively. The linear lines were fit to the data at higher temperatures and the obtained slopes of the fits were used to determine the activation energy Ea. Fig. 10 shows activation energy (Ea) and σ at 400 K versus wt% of CuO/Fe2O3 composites, it can be seen that activation energy Ea decreased and conductivity increased with increase of dopant concentration and, it may be due to the decrease in the scattering rate of polarons with increase of CuO/Fe2O3 concentration. The conductivity and activation energy values of PCz, PCz/CuO and PCz/Fe2O3 nanocomposites at 350 K and 400 K are tabulated in Table 4. To emphasize conductivity behavior with filler content its value at two different temperatures are shown in Table 4. Similar kind of behavior in Ea and σ has been observed by J. Selvi et al [29] for PVA/CuO composites and noticed enhancement in conductivity and reduced activation energy in PVA doped CuO. Mohammad Shakir et al [30] have noticed increase in conductivity with TiO2 content in PCz/TiO2 nanocomposite. Syed Abthagir et al [31] compared conductivity of polyindole, polycarbazole and their derivatives and found that polycarbazole had higher conductivity than polyindole.
Table 4
DC conductivity, σ at 350 K and 400 K and activation energy, Ea for conduction for PCz and PCuO and PFO composites .
sl no
|
Sample
|
Ea
(meV)
|
σ (350 K)
(×10−5) (Ωm)−1
|
σ (400 K)
(×10−5) (Ωm)−1
|
1
|
PCz
|
4.91
|
3.19
|
3.26
|
2
|
PCu10
|
15.11
|
1.95
|
2.07
|
3
|
PCu20
|
8.29
|
2.07
|
2.13
|
4
|
PCu30
|
7.25
|
2.45
|
2.55
|
5
|
PFO10
|
8.68
|
1.20
|
1.25
|
6
|
PFO20
|
8.42
|
1.80
|
1.83
|
7
|
PFO30
|
5.99
|
2.22
|
2.27
|