The prepared samples are characterized to elucidate the morphological, structural, optical and electrical properties with various characterization techniques.
The effect of the metal oxide, doping acid and oxidant on the morphology of CuO, PANI:CuO:APS hybrid is brought out through the SEM images and shown in Fig. 1(a-e). Fig. 1a shows the surface morphology of the CuO nanopowders which are aggregate easily in aqueous solution due to their high surface energy and irregular shape. A few nanofibres (104 nm) and some clusters (Fig. 1d) are observed in PANI:CuO:APS0.3M hybrid nanocomposite [17-19].
The PANI:CuO:APS0.3M surface with fussy interconnected nanofibers is clearly seen from SEM image of hybrid nanocomposite in Fig. 1(d&e). These SEM images show that the loading of APS oxidant agents has a strong effect on the PANi morphology. The type and concentration of oxidant agents play a major role in determining the surface morphology, which in turn changes with wt. ratio of oxidant.
Fig. 2a shows the TEM micrograph of PANI:CuO hybrid nanocomposite synthesized with 0.3 M of APS oxidant. The presence of CuO phase with varied size in the polymer has also been confirmed. The size of the particle observed in TEM image is in the range 32 nm which agrees the calculated value using Scherrer formula.
Fig. 2b shows the selected area diffraction pattern (SAED) of the as prepare PANI:CuO hybrid
nanocomposite. It shows that all the particles are well crystallized. The diffraction rings on SAED image match with the peaks in the XRD pattern which also proves the monoclinic structure of as prepare CuO nanopowders [20].
Fig. 3a shows the presence on early stoichiometric CuO nanopowders. The calculated weight % of copper and oxide from EDAX is 64.56 wt. % and 35.44 wt. %, which confirms the pure CuO nanopowders are formed. It has no traces of other impurities. Fig. 3b shows the EDAX performed to determine the atomic percentage of PANI:CuO hybrids. The quantitative analysis result indicates the presence of carbon, sulphur, copper and oxide in the formed hybrids.
Fig. 4 shows the FT-IR spectra of PANI, CuO and PANI:CuO composite samples obtained from different synthesis. Their vibrational assignments are presented in Table 1 and Table 2. The formation of polyaniline through chemical synthesis, its HCl doped state and the presence of vibration bands at 3425, 2924, 2330, 1556, 1452, 1296, 1110, 810 and 617 cm-1 are shown in Fig. 4a. The 3425 cm-1 vibration band is attributed to the stretching vibration of secondary amines. The vibration band seen around 2924 cm-1 is ascribed to the aromatic C-H vibration. The 1556 cm-1 vibration band is due to the C=C double bond of quinoid rings, whereas 1452.32 cm-1 vibration band arises due to the vibration of C=C double bond associated with the benzenoid ring. The peak at 1500 cm-1 may be due to strong symmetrical bending band, and it may occur due to secondary aromatic amines. The vibration band at 1110 cm-1 is due to the presence of the C-N double bond and indicative of protonation. The vibration band at 810 cm-1 is attributed to a C-H vibration band of para-linked phenyl ring which confirms the predominance of para-coupling in polymerization of aniline.
Fig. 4b shows the presence of peaks at 453, 494 and 609, 919, 1498, 2353 and 3466 cm−1 correspond to characteristic stretching vibrations of Cu-O bond in the CuO nanopowder [21,22]. The broad absorption peak at 3466 cm-1 is caused by the adsorbed water molecules since the nanocrystalline materials exhibit high surface to volume ratio and absorb moisture. The peaks observed confirm the formation of CuO nanopowders [23]. The FT-IR spectra for PANI: CuO: APSx (x=0.1 to 0.5 M of APS) hybrid nanocomposite is shown in Fig. 4(c-f). It shows some shift in the wave number and change in the peak intensity on comparing PANI with CuO.
The most prominent changes are: (i) shift of =C–H in plane vibration peak to high wave number, i.e., from 1110 cm-1 in PANI to 1142 cm-1 in composite with increased intensity, (ii) shift of C–N structure, bending to higher values i.e., from 1296 to 1305 cm-1 and iii) the presence of 1450 and 1494 cm-1 peaks correspond to benzenoid ring vibration and C=C bond in composite.
This change is due to the conjugation loss and molecular order in the synthesized PANI with CuO. Since the frequency of vibration is directly proportional to force constant of bonds, a shift in =C–H in plane vibration to lower side indicates the localization of electrons in the benzenoid ring. This also indicates the shortening of N–C bonds and suggests the localization of p-orbital electrons on the N. In addition, the enhancement of the peak intensity (C–H in plane vibration) indicates the dipole moment increase.
With an equal mole ratio of PANI and CuO nanopowders and the addition of APS lead the localization of electrons in the formed PANI composites. Moreover, the growth ratio of the PANI backbone could be influencing the presence of the CuO and APS, which has the ability to influence the dopant anions.
X-ray diffraction patterns of CuO nanopowders, PANI, PANI:CuO:APSx hybrid nanocomposite are shown in Fig. 5 (inset the Fig.). All the peaks in diffraction pattern show the monoclinic structure of CuO and follow the JCPDS No. (89-5895, 048-1548 and 80-1917). The Miller indices are identified and structural parameter values are listed in Table 3. The lattice parameters are calculated from XRD data. The crystallite size of the CuO nanopowders is calculated using Debye-Scherer formula given in eqn. 1 [24].
where, D is the crystalline size, λ is the wavelength (= 0.1540 nm), β is the full width at half maximum of the peak in radian, θ is the Bragg diffraction angle at peak position in degrees. The maximum and minimum crystallite sizes are found to be 46.36 nm and 15.8 nm.
Fig. 5a shows the amorphous nature of PANI. The peak at 25.33° is ascribed to the periodicity perpendicular to the polymer chain, while those other peaks are likely due to the branches of HCl doped PANI [25,26].
The characteristic peaks ascertained from the XRD of PANI:CuO:APSx hybrid nanocomposites formed at different mole ratios (x=0.1, 0.15, 0.3 and 0.5 M) are shown in Fig. 5(b-f). The presence of sharp and distinct peaks indicates the nanocrystalline nature of CuO. The 2θ values 25.2o, 35.43o, 38.6o, 48.66o, 58.6o, 61.48o and 68.8o are consistent with the JCPDS No. (89-5895, 048-1548, 80-1917), which demonstrates the monoclinic structure of CuO. PANI:CuO hybrid nanocomposites structural parameter values are listed in Table 4.
Fig. 5 shows a small peak around 25o indicates the crystallinity of PANI. As the APS content is increased, the amorphous nature of PANI slightly changes and the composite powder becomes strongly oriented along 35o. The strong peaks in the XRD pattern of nanocomposites verify the presence of copper nanopowders in the polyaniline matrix. Obviously, these differences should have some influence on the crystallinity of the prepared hybrid composite. Moreover, it is concluded that the degree of crystallinity is associated with the supermolecular organization of PANI chains. The fraction of crystalline phase increases with molecular weight of the grown PANI.
Fig.6 shows the optical absorbance with respect to wavelength for PANI, CuO and PANI:CuO:APSx hybrid nanocomposites. Two absorption bands at 355 nm and 602 nm are observed for PANI. The first absorption band is usually attributed to the 𝜋-𝜋* electronic transition of the benzene rings and the second absorption band is assigned to the benzenoid to quinoid excitonic transition. The position of the peak is related to the degree of conjugation between adjacent phenylene rings in the polymer chain [27, 28]. The forced planarization of 𝜋-system induced by aggregation leads to the increased conjugation with decreased band gap.
The absorption bands of PANI appear in the range ∼320-370 nm and 542 - 642 nm for CuO and APS loadings indicate the shifting of absorption bands and the presence of strong interaction between PANI and CuO. However, the band at ∼602 nm almost disappears in the composite formed with CuO and increased content of APS. It is interesting to note that the absorption at ∼602 nm is due to the benzenoid to quinoid excitonic transition and is shifted to 642 nm (Fig. 6e) for 0.5M of PANI:CuO hybrid nanocomposite. The optical band gap of all samples is estimated according to the
relation [29]:
αhγ= B (hv-Eg)2 (2)
where, hν is the incident photon energy, α is the absorption coefficient, B is a materials dependent constant and Egis the optical band gap.
The optical band gaps 2.64 eV and 2.17 eV to 2.7 eV are obtained for CuO and PANI:CuO nanocomposite from the Fig. 7 (inset Fig.). The increase of CuO band gap is due to the addition of APS and the formation of polaron in the nanocomposite.
The PL emission peaks of PANI and PANI:CuO:APSx hybrids are shown in Fig. 8 (a-e). The presence of an excitation band at 342 nm and emission peak at 410 nm (blue) for PANI as shown as in Fig. 8a. The hybrid PANI:CuO:APSx hybrid nanocomposites are perceived at different excitation wavelengths ~330-335 nm along with an emission peak at 520 nm as shown in Fig. 8 (b). Addition of small quantity of CuO nanopowders leads to the intensity increase at blue and green regions of visible spectrum due to the development of more density of states in the energy band.
The increase of APS from 0.1 M to 0.5 M develops peaks at two wave lengths as shown in Fig. 8. The electrons donated by PANI can move easily through the CuO nanopowders and this combination enhances the electron mobility in the nanocomposites [30].
The I-V characteristics (Fig. 9(a-e)) of PANI:CuO:APSx hybrid nanocomposite (x=0.1, 0.15, 0.3 & 0.5M) show a linear increase in current with applied voltage and temperature (40 to 150 oC) and confirm the ohmic behavior. It is observed from the Fig. 10 that the electrical conductivity of PANI:CuO:APS0.1M hybrid increases with APS content and attains its maximum for PANI:CuO:APS0.3M hybrid nanocomposites.
Further increase of APS (0.5 M) content leads the conductivity decrease as shown in the Fig.10. However, the polaron bands of these polymers remain unchanged even with the increase of oxidant concentration which also decreases the conductivity to a large extent.
Fig. 11 (a & e) show the photoconductivity change of PANI and PANI:CuO:APS0.5M hybrid nanocomposites. The photo conducting is due to the reduction of charge carriers in the presence of radiation. When the sample is exposed to light irradiation, recombination of electrons and holes takes place which reduces the charge carrier concentration with increased positive photoconductivity as shown in Fig. 11. The above study concludes the better response of PANI:CuO nanocomposites compared with HCl doped PANI.
The C-V curves of PANI, CuO and PANI:CuO hybrid composites are recorded at a scan rate 10 mVs‑1 in room temperature and are shown in Fig. 12(a-c).The above
An optimized conditions different weight percentage of oxidant (x=0.1, 0.15, 0.3 & 0.5M) PANI:CuO hybrid composites samples. Electrical studies reveal the electrical conductivity of PANI:CuO:APS0.1M hybrid increases with APS content and attains its maximum for PANI:CuO:APS0.3M hybrid nanocomposites.
Fig. 12a shows the broad redox couple of PANI observed at positive and negative potentials at 0.90 V and −0.27 V, respectively. Fig. 12b & c show the absence of peaks for CuO and the occurrence of redox couples for CuO incorporated PANI composites. Among these redox peaks, an individual sharp oxidative and a broad reductive peak are recorded at 1.25 V and −0.57 V, respectively. The redox couple of CuO is found to be shifted from 0.90 to 1.25 V (oxidative peak) and −0.27 to −0.57 V (reductive peak) due to electrostatic and hydrogen bonding interactions, when PANI:CuO composite is formed.
The mobility of the counter ions through the charge transport supporting electrolyte and the presence of redox process are established. The calculated specific capacitance (C) values are listed in Table. 5
The specific capacitance value of the supercapacitor with PANI:CuO composite electrolytes is maximum compared with acid doped PANI and pure CuO nanopowder due to the high cycling reversibility. The capacitance of the polymeric materials is remarkably enhanced due to the synergistic effect between CuO and PANI. Moreover, the composite demonstrates good capacitive and charge/discharge properties [31].