UV-Vis Analysis:
Figure 1(a) and (b) displays the UV-Vis spectra of AuNPs, SWNTs, MWNTs, f-SWNTs, f-MWNTs Au@f-SWNTs, Au@f-MWNTs, neat PANI, Au@PANI, f-SWNTs@PANI, f-MWNTs@PANI, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs nanocomposites. The UV-Vis spectrum of SWNTs shows a van Hove singularity structure but in case of f-SWNTs, the van Hove transitions were completely vanished in the course of functionalization of the surface of SWNTs [25, 26]. In case of MWNTs, the absorption peaks corresponding to 227 and 270 nm is not observed because of low sensitivity of the instrument [27, 28]. The absorption spectrum of f-MWNTs shows a peak at around 346 nm, which is due to the π–π* transition of the carbon skeletal network [29, 30]. The pristine AuNPs shows a characteristic peak centered at 522 nm due to the surface plasmon resonance [31, 32], whereas the absorption peaks corresponding to the AuNPs are completely disappeared due to the low concentration of AuNPs of amount 0.002 wt.% as well as homogeneous dispersion of the AuNPs onto the surface of the CNTs resulted from the strong affinity of the AuNPs for the polar functional groups present on the surface of CNTs. It is noteworthy to mention that the absence of absorption band due to the longitudinal plasmon resonance of AuNPs in the wavelength range of 600–800 nm established the absence of AuNPs agglomerations along with small changes of size from the original spherical AuNPs [33]. In case of pristine PANI, the peak appeared at 332 and 631 nm are owing to the π–π* and n–π* transitions of the benzenoid and quinoid structures, respectively that confirms the emeraldine salt form of the PANI [34]. In case of the Au@PANI nanocomposite, the incorporation of AuNPs into the PANI matrix exhibited the shifting of the peak position to the higher wavelength region that confirms the successful encapsulation of the AuNPs by means of the PANI molecular chains [35]. The f-SWNTs and f-MWNTs decorated PANI matrix show the shifting of the absorption peak towards the higher wavelength region that indicates the formation of f-SWNTs@PANI and f-MWNTs@PANI nanocomposites [36]. Addition of hybrid nanofiller Au@f-SWNTs and Au@f-MWNTs into the PANI matrix resulted in the shifting of the absorption peak positions corresponding to the PANI matrix to the lower wavelength region, which is due to the homogeneous dispersion of the hybrid nanofillers within the PANI matrix. The intensity of the absorption peaks corresponding to the PANI matrix increases in case of the hybrid nanofiller reinforced PANI ternary nanocomposites.
UV-Vis spectroscopy explores electronic transitions between the valence band and the conduction band, which makes it a convenient method to determine the optical band gap for semiconductors [21]. The Tauc plots for direct and indirect optical band gaps of PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs ternary nanocomposites are shown in the Fig. 2(a–d). The Tauc plots exhibit a rapid increase in absorption coefficient with increase in energy. The optical band gap energy is estimated by the extrapolation of the linear region of the Tauc plot towards x-axis. The point of intersection of linear fitted lines with the x-axis gives both direct and indirect band gaps. Table 1 represents the direct and indirect band gaps of neat PANI, Au@f-SWNTs, Au@f-MWNTs, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs ternary nanocomposites. The optical band gaps of PANI matrix, Au@f-SWNTs, and Au@f-MWNTs hybrid nanofillers are shown in the Figure S1. It is reported from the Table 1 that the calculated band gaps for Au@f-SWNTs and Au@f-MWNTs reinforced PANI ternary nanocomposites are below than the optical band gap energy of standard TiO2 (\({E}_{g}\) = 3.22 eV), as a consequence the synthesized ternary nanocomposites can be suitable for application in solar cells [20, 21].
Table 1
Direct and Indirect Optical Band Gaps of the Samples.
Sl. No. | Sample Name | Optical Band Gap |
| | Direct | Indirect |
1 | PANI | 3.03 | 3.15 |
2 | Au@f-SWNTs | 2.57 | 3.01 |
3 | Au@f-MWNTs | 3.70 | 3.81 |
4 | PANI/Au@f-SWNTs | 3.00 | 3.17 |
5 | PANI/Au@f-MWNTs | 3.03 | 3.16 |
Ftir Analysis:
The chemical interactions of AuNPs, SWNTs, MWNTs, f-SWNTs, f-MWNTs, Au@f-SWNTs, Au@f-MWNTs, neat PANI, Au@PANI, f-SWNTs@PANI, f-MWNTs@PANI, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs nanocomposites are characterized through FTIR spectroscopy as shown in the Fig. 3(a–c). The FTIR spectrum of AuNPs shows absorption peaks at around 1642 and 3316 cm–1 that corresponds to the stretching vibrations of carboxylic carbonyl (> C = O) and hydroxyl (–OH) functional groups present in trisodium citrate reducing-cum-stabilizing agent, which act as an effective surfactant for stabilizing AuNPs in aqueous medium [37]. In case of SWNTs, a small band appears at about 1379 cm–1 is due to the stretching vibration of C–C single bond, whereas the f-SWNTs develops some new bands at around 1382, 1644, 1761, 2380, and 3453 cm–1, which are assigned to the stretching vibration of C–C single bond, carboxyl (COO–), carbonyl (> C = O), and hydroxyl (–OH) groups that established successful incorporation of the polar functional groups on the surface of the f-SWNTs through modification process [38–41]. The FTIR spectrum of f-MWNTs displays vibration peaks at around 1353 cm–1 ascribed to the C–C stretching, 1571 cm–1 due to the infrared active phonon mode of nanotubes, 1688 cm–1 associated to the carboxyl (COO–) groups, and 3401 cm–1 corresponding to the hydroxyl (–OH) groups, which confirmed the successful functionalization of the surface of the MWNTs [28, 42–46]. However, Au@f-SWNTs and Au@f-MWNTs hybrid nanofillers exhibit absorption bands appeared at 3226, 2351, 1724, 1565, 1330, and 1032 cm–1 are ascribed to –OH, COO–, >C = O, and C–C bonds, which are slightly shifted to the lower wavelength region due to the homogeneous dispersion of the AuNPs on the surface of the CNTs [47, 48].
The pristine PANI displays characteristic peaks centered at 1285 and 1126 cm–1 is assigned to the stretching vibration of the C–N bond and the in plane bending vibration of C–H bond of the benzenoid (B) and quinoid (Q) structures of the PANI molecular chains, respectively. The peaks observed at around 1565 and 1461cm–1 are attributed to the stretching vibration of the C = C bond, associated with the quinoid and benzenoid ring of the PANI that established the existence of the emeraldine salt form of the PANI matrix. Furthermore, a small peak observed at 815 cm–1 is ascribed to the out of plane bending vibration of C–H bond [19, 34, 35, 49–52]. The Au@PANI nanocomposite exhibits all the absorption peaks of the neat PANI matrix with a slightly shifting of the band positions, which confirms the development of the interphase interactions between the AuNPs and PANI matrix due to the affinity of the AuNPs for the nitrogen atoms present in the backbone chain of the PANI matrix [35, 53]. Both binary and ternary PANI nanocomposites exhibit all the absorption peaks of the PANI matrix with the shifting of the peak positions of the –OH, C = C, and C–H bonds that pronounces higher absorption intensity as compare to the virgin PANI. The Au@f-SWNTs and Au@f-MWNTs hybrid nanofiller reinforced PANI matrix show absorption peaks corresponds to the individual CNTs and neat PANI molecular chains, which informs the successfully formation of ternary nanocomposites with homogeneous dispersion of the hybrid nanofillers within the PANI matrix [54].
Raman Analysis:
Figure 4(a–b) represents the Raman spectra of SWNTs, MWNTs, f-SWNTs, f-MWNTs, Au@f-SWNTs, Au@f-MWNTs, neat PANI, Au@PANI, f-SWNTs@PANI, f-MWNTs@PANI, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs ternary nanocomposites to evaluate the structural aspects. In Raman spectra of SWNTs and f-SWNTs, the peaks are observed at around 1323 and 1328 cm–1 for disordered mode (D-band) whose intensity is utilized to determine the density of the defects and strong peak appeared at 1581 and 1596 cm–1 corresponding to characteristic of the graphitic nature of the nanotube i.e. graphitic mode (G-band), respectively. Also the radial breathing mode (RBM) is clearly obtained between 100–300 cm–1 for both pristine and f-SWNTs, confirming the presence of high quality SWNTs [40, 41, 55]. The shifting of both D and G bands to the higher wavelength region i.e. red shift during the course of functionalization, which develops large number of polar functional groups and effective surface crystal defects resulted from the strong acid treatment required for surface functionalization of CNTs. The D and G bands of MWNTs are appeared at 1334 cm–1 and 1578 cm–1, which is not significantly changed during functionalization process because of the low surface area of MWNTs as compared to the SWNTs. The Gꞌ band corresponds to the 2D overtone peak appeared at around 2665 cm–1, which is ascribed to the two phonon resonance effects [28, 44–46, 56, 57]. The Raman spectra of Au@f-SWNTs shows two typical D and G bands at 1345 and 1584 cm–1 along with the Gꞌ band at 2681 cm–1, which is significantly reallocated to the higher wavenumber region that confirms the effective incorporation and homogeneous dispersion of AuNPs on to the surface of CNTs. Furthermore, the D and G peaks at 1345 and 1580 cm–1 along with the Gꞌ band at 2681 cm–1 are also substantially changed in case of the Au@f-MWNTs, informing that the AuNPs are successfully decorated on the surface of f-MWNTs due to the strong affinity of the AuNPs with f-MWNTs, resulted from the functional moieties present as well as evenly distribution of AuNPs on the surface of CNTs [58–60].
The peaks observed at 1577 and 1357 cm–1 for pure PANI are corresponding to the C = C stretching vibrations of the benzenoid and quinoid rings, respectively [52, 61]. Though the peak positions of D and G bands are unaltered but their corresponding peak intensities are noticeably improved by the incorporation of Au@f-SWNTs and Au@f-MWNTs hybrid nanofillers in to the PANI matrix, which is originated due the development of large number of crystal defects resulted from the in situ chemical oxidative synthesis of the pristine PANI. Owing to the surface modification of the CNTs by means of acid treatment and followed by the incorporation of AuNPs, the interactions between Au@f-SWNTs and Au@f-MWNTs nanohybrids and PANI matrix can be significantly improved, as a consequence the surface of the Au@f-SWNTs and Au@f-MWNTs is covalently wrapped with PANI molecular chains [40]
Dls Analysis:
The particle size distribution of the samples is investigated by DLS techniques as represented in Fig. 5(a) and (b). The AuNPs shows a broad peak positioned at 71 nm with Z-average particle size distribution of 40.64 nm and polydispersity index (PDI) of 0.884, which confirms the higher stability of the AuNPs in the aqueous medium due to the strong affinity of the AuNPs for oxygen atoms as well as the presence of the stabilizing agents that act as a strong driving force for the uniform distribution of the individual AuNPs by restricting its aggregation tendencies [62, 63]. The mean hydrodynamic diameter of SWNTs is 449 nm confirming homogeneous dispersion of the SWNTs within the NMP solvent. The repulsive force of interaction developed between nonpolar SWNTs and polar NMP solvent resulted low aggregation tendency of the SWNTs bundles due to the presence of Van der Waals force of attraction and/or π–π stacking. The f-SWNTs exhibit polydispersity of 0.590 with average particle size distribution of 367 nm with two peak positions centered at 210 and 832 nm [64]. After functionalization, the aggregation tendency of the SWNTs is hindered due to the presence of polar functional groups on the surface of the SWNTs as well as on the edge of the SWNTs, which overcome the π–π stacking force. However, a broad peak appeared at about 832 nm is due to the aggregation of the f-SWNTs that attribute strong interfacial interactions developed among polar SWNTs and NMP solvent. The average particle size of the MWNTs is 834 nm, whereas the hydrodynamic diameter of the f-MWNTs is 178 nm, which is lesser as compared to the MWNTs due to the functionalization of the surface of the MWNTs [65]. The hydrodynamic diameter of the Au@f-SWNTs and Au@f-MWNTs hybrid nanofillers are 235 and 246 nm, respectively, which is attributed to the homogeneous dispersion of the hybrid nanofiller. The DLS distribution curve for hybrid nanofiller exhibits narrow distribution as well as the hydrodynamic diameter of Au@f-SWNTs is significantly lower as compared to the f-SWNTs is due to the uniform coating of AuNPs on the surface of the f-SWNTs resulted from the strong affinity of the AuNPs for polar functional groups present on the surface of f-SWNTs, which hinder the aggregation tendency of the SWNTs.
The pristine PANI exhibits two narrow peaks centered at 122 and 332 nm with PDI of 0.942 due to the uniform dispersion of the PANI molecular chains within the NMP solvent that resulted from the existence of polar–polar interactions between PANI matrix and solvent, which restrict the agglomeration of the PANI originated from the intermolecular hydrogen bonding present between PANI chains [34, 66]. The Au@PANI shows broad distribution peak positioned at 391 nm because of the decrease in polarity of PANI surface due to the presence of AuNPs, which accelerated the aggregation tendency resulted from the weak interactions among PANI and solvent. Similarly, the f-SWNTs@PANI shows the peak positioned at 299 nm like pristine PANI matrix along with homogeneous dispersion of the f-SWNTs within the PANI matrix that restrict its aggregation tendency. Similarly, the average particle size of the f-MWNTs@PANI nanocomposite showed at around 329 nm. However, PANI ternary nanocomposite results a narrow distribution peak i.e. the hydrodynamic diameter of the PANI/Au@f-SWNTs and PANI/Au@f-MWNTs showed at 332 and 300 nm, respectively, which confirms the homogeneous dispersion of the hybrid nanofillers within the PANI matrix that hinders the aggregation tendency of the PANI matrix by suppressing the polar–polar interactions with solvent molecules [34].
Xrd Analysis:
The nanostructural characteristics of the nanocomposites are studied through XRD analysis, which is displayed in Fig. 6(a) and (b). The XRD patterns of SWNTs and MWNTs show two peaks centered at 2θ value of 25.82° and 42.96°, which are assigned to the (0 0 2) and (1 0 1) diffraction planes that indicates the hexagonal graphitic crystal structure of the carbon nanomaterials [67, 68]. The f-SWNTs and f-MWNTs show a prominent broad peak at around 2θ value of 25.83° and a small peak corresponding to 2θ value of about 42.83° are attributed to the (0 0 2) and (1 0 1) diffraction planes, respectively [45, 69]. In addition to the peaks corresponding to the f-MWNTs, an extra small shoulder appeared at about 52.49° is due to the (0 0 4) diffraction plane that ascribed to the concentric multiple cylindrical layers of the CNTs [69]. The peak positions and intensities of the f-SWNTs and f-MWNTs are slightly changed after functionalization process, which is due to the development of polar functional groups on the surface of CNTs owing to the weak Van der Waals force of interaction/π–π stacking. Both Au doped f-SWNTs and f-MWNTs show diffraction peaks of f-SWNTs and f-MWNTs along with peaks at 38.17, 44.50, 64.50, 77.49 and 81.67° corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) face centered cubic (FCC) crystal structure of the AuNPs, which confirms that the surface of f-SWNTs and f-MWNTs are well decorated with AuNPs [19, 59, 70–72].
The neat PANI exhibits two small peaks at 14.65° and 21.53° along with a broad crystalline peak centered at 25.18° ascribed to (0 1 1), (0 2 0), (2 0 0) diffraction planes, respectively that confirms semi-crystalline nature of the PANI matrix, which is due to the long range regular ordered structures of the PANI molecular chains resulted from the intermolecular hydrogen bonding present among different molecular chains [35, 73, 74]. Incorporation of AuNPs into the PANI matrix is confirmed owing to the presence of small diffraction peaks corresponding to the AuNPs [35]. Au@PANI nanocomposite shows small peaks for AuNPs due to presence of negligible amounts of AuNPs within the PANI matrix [75]. Incorporation of f-SWNTs and f-MWNTs into the PANI matrix show broad peaks corresponding to the 2θ value of 25.50 and 25.67°, respectively, are attributed to (0 0 2) diffraction plane of CNTs. However, the (2 0 0) diffraction plane of PANI matrix may be merged with the diffraction plane of the CNTs or it is considerably diminished due to decrease in crystallinity of the PANI matrix [60]. Both PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites exhibit the peak of CNTs as well as AuNPs along with the peaks of PANI at 2θ value of 14.67°, 20.50°, and 25.67° due to the (0 1 1), (0 2 0), (2 0 0) diffraction planes of the PANI [1, 60]. Furthermore, it is concluded that the Au@f-SWNTs and Au@f-MWNTs hybrid nanofiller are effectively incorporated into the PANI matrix.
Xps Analysis:
The elemental composition and chemical interactions of Au@f-SWNTs, Au@f-MWNTs, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs ternary nanocomposites are examined through XPS analysis to study the C, N, O, and Au elements at different chemical environments and their different possible binding interactions as shown in the Figs. 7 and 8. In case of Au@f-SWNTs and Au@f-MWNTs hybrid nanofillers as shown in Figures S2 and S3, the peaks for Au 4f core level of AuNPs deconvoluted into two peaks appeared at Eb values of 88.12 and 84.48 eV, which are due to the 4f5/2 and 4f7/2 energy levels of Au, respectively that confirms the presence of zero oxidation state of AuNPs [59, 76]. The core level spectrum of C1s deconvoluted into 3 peaks that shown at Eb values of 284.82, 285.5, and 286.7 eV assigning to the C–C, C–O, and O–C = O bonds, respectively. The core level spectra of O1s deconvoluted into 2 peaks displayed at Eb values of 531.49 and 533.4 eV corresponding to the O–C and O = C bonds, respectively [32, 59]. The XPS spectra of PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs ternary nanocomposites are studied to evaluate Au4f, C1s, O1s, and N1s core levels. The spectrum for Au4f core level of AuNPs deconvoluted into two peaks positioned at Eb values of 88.1 and 84.45 eV, which are attributed to the 4f5/2 and 4f7/2 energy levels of Au, respectively that reveals the zero oxidation state of the AuNPs [19, 77]. The core level spectrum of C1s deconvoluted into three peaks, shown at Eb values of 284.7, 285.4 and 286.8 eV are owing to C–C, C–N, and C–O bonds, respectively. The core level spectra of O1s deconvoluted into two peaks displayed at Eb values of 531.6 and 532.9 eV corresponding to the O–C and O = C bonds, respectively. The core level spectrum of N1s deconvoluted into three different peaks showed at Eb values of 399.85 assigned to immine (= N–) as well as 400.5 and 401 eV are ascribed to amine (–NH–, and N+) bonds present in the backbone molecular chain of the PANI matrix [78]. The immine (= N–) group in the ternary nanocomposite shows the presence of emeraldine salt form of PANI that attributed to the higher electrical conductivity of the ternary nanocomposites [79].
Sem Analysis:
The nanostructure morphology of the nanocomposites is studied through SEM analysis as shown in the Figure 9. The micrograph of f-SWNTs and f-MWNTs show three dimensional interconnected network structures of CNTs along with both individual CNTs layers and cluster like structures. In the modification process, the CNTs bundles are separated into individual entities [80]. The nanostructure morphology of the hybrid nanofiller are changed by the incorporation of AuNPs onto the surface of the CNTs [44]. The Au@f-SWNTs reinforced PANI ternary nanocomposites shows granular morphology due to the encapsulation of the hybrid nanofillers by means of the PANI matrix. The globular morphology of the PANI matrix is modified into the granular morphology in the presence of hybrid nanofillers. In the same contest, Au@f-MWNTs shows fibrous entities, which are well separated from each other along with absence of clusters. Au@f-MWNTs reinforced PANI matrix exhibits granular morphology like Au@f-SWNTs reinforced PANI matrix.
The elemental detection of the PANI/Au@f-SWNTs ternary nanocomposite is studied from the energy dispersive X-ray (EDX) spectra and elemental mapping micrographs from SEM, which are shown in Fig. 10(a) and (b(i-v)). The corresponding EDX study and mapping images from SEM show the presence of C, N, O, S and Au elements that confirmed the successful fabrication of the Au@f-SWNTs hybrid nanofiller reinforced PANI ternary nanocomposites [34].
Fesem Analysis:
The Fig. 11 displays the FESEM images of PANI/Au@f-SWNTs ternary nanocomposites at different magnification and resolution at an acceleration voltage of 15 kV, observed that the AuNPs are uniformly distributed on the f-SWNTs surface, which are uniformly embedded on the surface of PANI matrix [6, 60].
Tga Analysis:
The thermal stability of the nanocomposites is studied through TGA as shown in the Fig. 12(a) and (b). The thermal degradation of the SWNTs and MWNTs occurred at around 541 and 471°C, which is due to the breakdown of C–C and C = C bonds that leads to the degradation of the graphene walls with weight loss of about 5 and 6 wt.%, respectively [81]. The thermal stability of the CNTs are substantially decreased owing to the development of the polar functional groups in the course of oxidation–reduction reactions during functionalization process, which is completely decomposed during thermal degradation process as compare to the pristine CNTs. Furthermore, it is inferred that the surface of the CNTs is successfully modified through functionalization process owing to the different way of decomposition for both pristine and functionalized CNTs [82]. The AuNPs reinforced f-SWNTs and f-MWNTs hybrid nanofillers exhibit retarded thermal decomposition process as compared to the functionalized CNTs. The thermal stability of the hybrid nanofillers are noticeably higher in comparison with the functionalized CNTs due to the high thermal stability of AuNPs as well as uniform distribution of AuNPs on the surface of CNTs, which act as a strong barrier effect resulted from the development of the tortuous path for the propagation of the thermal energy into the core of the CNTs. Furthermore, the synergistic effect exhibited by AuNPs as well as f-SWNTs enhances the thermal stability of the hybrid nanofillers [83].
The neat PANI exhibits three stage of degradation, the first stage of degradation occurred at around 98°C with weight loss of 10 wt.% corresponding to the removal of the volatile components as well as presence of small molecules during the course of synthesis of PANI. The second stage of degradation reported at 222°C with a weight loss of 19 wt.% corresponding to the breakdown of amine C–N polar bond present in the PANI backbone molecular chains. The third stage of degradation happened at 526°C with a weight loss of 44 wt.% is attributed to the decomposition of PANI molecular backbone into small organic moieties [34]. The thermal stability of AuNPs reinforced PANI matrix exhibits higher thermal stability up to 400°C, after that it decreases as compared to the PANI matrix. The higher affinity of the AuNPs for the nitrogen active sites present on the PANI backbone, which act as an effective driving force for the homogeneous dispersion of AuNPs within the PANI matrix that attributes higher thermal stability of Au@PANI nanocomposites [35, 72]. The thermal stability of f-MWNTs reinforced PANI matrix are significantly higher, whereas the f-SWNTs reinforced PANI matrix are retained as compared to the PANI matrix because of the higher thermal stability of f-MWNTs than that of f-SWNTs with more number of concentric cylindrical nanotubes [84]. The hybrid nanofillers reinforced PANI matrix exhibit more thermal stability as compared to the pure PANI matrix. The synergistic effect of AuNPs and CNTs significantly improved the thermal stability of PANI matrix in the ternary nanocomposites [6, 85]. The stages of thermal degradations of nanofillers are tabulated in Table 2.
Table 2
Thermal Degradation Data of Nanofillers.
Sl. No. | Name of the Sample | First Stage Degradation (°C) | Second Stage Degradation (°C) | Third Stage Degradation (°C) | Residual Weight (wt.%) |
1 | SWNTs | 541 | – | – | 95.69 |
2 | MWNTs | 471 | – | – | 95.39 |
3 | f-SWNTs | 68 | 331 | – | 89.73 |
4 | f-MWNTs | 54 | 351 | – | 92.98 |
5 | Au@ f-SWNTs | 70 | 362 | – | 96.99 |
6 | Au@ f-MWNTs | 59 | 151 | 371 | 96.65 |
A crossover point has arisen below which the nanocomposites system exhibits higher thermal stability than that of PANI matrix, whereas the thermal stability of the nanocomposites decreases above 500°C. PANI and PANI nanocomposites exhibit three stage of degradations as tabulated in Table 3 and shown that the PANI/Au@f-SWNTs ternary nanocomposite exhibits higher thermal stability than others.
Table 3
Thermal Degradation Data of PANI and Its Nanocomposites.
Sl. No. | Name of the Sample | First Stage Degradation (°C) | Second Stage Degradation (°C) | Third Stage Degradation (°C) | Residual Weight (wt.%) |
1 | PANI | 98 | 222 | 526 | 56 |
2 | Au@PANI | 76 | 252 | 520 | 54 |
3 | f-SWNTs@PANI | 84 | 286 | 538 | 53.15 |
4 | f-MWNTs@PANI | 73 | 264 | 540 | 54 |
5 | PANI/Au@f-SWNTs | 94 | 276 | 556 | 62.5 |
6 | PANI/Au@f-MWNTs | 74 | 264 | 527 | 55 |
Figure 13 represents the DTG thermograms of PANI and PANI nanocomposites. The thermal degradation corresponding to the first (TP1) and second peaks (TP2) significantly shifted to the high temperatures by the incorporation of nanofillers. However, the thermal degradation at high temperature corresponding to the PANI nanocomposites decreases because of the higher thermal conductivity of the nanofillers that retained with the nanocomposites, which can accelerate the high temperature decomposition more rapidly as compared to the pristine PANI matrix.
The DSC thermograms as reported from TG-DSC instrument are represented in Fig. 14(a) and (b), which show a plateau region of heat flow with increase in temperature up to 300°C due to the absence of any first ordered degradation with modified and unmodified nanofillers but it gradually increases with increase in temperature up to 700°C due to the endothermic first ordered transition that can be attributed to the liberation of functional groups associated with the surface of CNTs. The endothermic peaks associated with the functionalized nanotubes and AuNPs embedded hybrid nanofillers show significantly higher heat capacity as compared to the pristine nanotubes due to the presence of polar functional groups that developed in the modification process. The pristine PANI matrix shows two endothermic peaks within 300°C corresponding to the first ordered thermal transitions due to the liberation of moistures and volatile organic compounds centered at 75°C along with scissoring of PANI molecular backbone chains occurred at 136°C [86]. However, the peak positions corresponding to these two endothermic peaks are not significantly changed for nanocomposites because the heat flow associated with these endothermic peaks are very low and it does not affect structural changes linked with the PANI matrix. Furthermore, the endothermic heat flow above 300°C gradually increases up to 700°C attributed to the permanent changes associate to the PANI matrix. The AuNPs reinforced PANI matrix exhibits an exothermic peak at around 600°C due to the liberation of heat, which is absorbed by the highly thermally conducting AuNPs in the due course of heating [34]. The amount of heat liberated in case of PANI nanocomposites are higher than that of the PANI matrix because of the higher heat absorbing capability of nanofillers due to their large active surface area [34].
Dielectric Property:
The Fig. 15, 16, and 17 represents the variation of AC conductivity, dielectric permittivity, and dielectric loss of the nanocomposites with applied frequency at room temperature ranging from 100–107 Hz. The ability of the dielectric material is that when an external electric field is applied, the dipoles are oriented in a particular direction and show alignment of polar charges. In the dielectric study, AC conductivity increases with increasing frequency, the dielectric permittivity, and dielectric loss decreases with increasing frequency, which is due to the Maxwell-Wagner-Sillar effect i.e. because of the interface charge polarization and intrinsic electric dipole polarization [34, 87, 88].
The ability to carry or conduct electric current in an electric field is AC conductivity or electrical conductivity. Figure 15 shows the dependency of AC conductivity of the (a) SWNTs, MWNTs, f-SWNTs, f-MWNTs, Au@f-SWNTs, and Au@f-MWNTs, and (b) PANI, Au@PANI, f-SWNTs@PANI, f-MWNTs@PANI, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs nanocomposites on an applied frequency. With increase in frequency, the AC conductivity increases. In Fig. 15(a), the AC conductivity of the AuNPs decorated f-SWNTs and f-MWNTs hybrid nanofiller shows higher conductivity as compared to the pristine f-SWNTs and f-MWNTs due to the effective incorporation and uniform dispersion of the AuNPs on the f-SWNTs and f-MWNTs surface. The Au@f-SWNTs has higher conductivity value than the Au@f-MWNTs hybrid nanofiller due to the presence f-SWNTs, which is highly conducting than the f-MWNTs. Similarly, in Fig. 15(b) the Au@f-SWNTs and Au@f-MWNTs hybrid nanofiller reinforced PANI nanocomposite shows the higher conductivity as compared to the pure PANI and CNTs/PANI nanocomposites due to the homogeneously distributed hybrid nanofillers in the PANI matrix that combines the synergistic effect of AuNPs, CNTs, and the conducting polyaniline matrix and formed interconnecting network structures between the hybrid nanofiller and PANI matrix [87–89].
Dielectric constant or dielectric permittivity is the ability of the material to store electrical energy in an electric field, which is directly related to the electronic, atomic, and dipolar polarizations. With the increasing frequency, the dielectric permittivity decreases. Figure 16 displays the variation of dielectric permittivity of (a) SWNTs, MWNTs, f-SWNTs, f-MWNTs, Au@f-SWNTs, and Au@f-MWNTs, and (b) PANI, Au@PANI, f-SWNTs@PANI, f-MWNTs@PANI, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs nanocomposites with applied frequency. Owing to the higher polarization in low frequency region, the dielectric permittivity is high but the dielectric permittivity is lower in high frequency region due to the minimum dipole polarization. In Fig. 16(a), the Au@f-SWNTs hybrid nanofiller show high dielectric permittivity value than the others and in Fig. 16(b), pure PANI and CNTs@PANI nanocomposite shows lower dielectric permittivity value as compared to the Au@f-SWNTs hybrid nanofiller reinforced PANI nanocomposite due to the dipolar interaction of the PANI molecular chain and the hybrid nanofiller surface as well as transfer of charge in between them [87, 88, 90, 91, 92].
Dielectric loss or tangent loss determines the amount of energy dissipated in an applied electric field. Figure 17 represents the variation of dielectric loss of (a) SWNTs, MWNTs, f-SWNTs, f-MWNTs Au@f-SWNTs, and Au@f-MWNTs, and (b) PANI, Au@PANI, f-SWNTs@PANI, f-MWNTs@PANI, PANI/Au@f-SWNTs, and PANI/Au@f-MWNTs nanocomposites with applied frequency. With increasing the frequency, the dielectric loss gradually decreases. In Fig. 17(a), the Au@f-SWNTs hybrid nanofiller shows low dielectric loss as compared to the f-CNTs and in Fig. 17(b), the Au@f-SWNTs hybrid nanofiller reinforced PANI nanocomposite shows low dielectric loss as compared to the pure PANI and PANI/Au@f-MWNTs ternary nanocomposites. The PANI/Au@f-SWNTs ternary nanocomposites exhibits high AC conductivity and dielectric permittivity along with low dielectric loss as compared to the other nanocomposites that permits it as a good dielectric material which is favorable for the application in the energy storage devices [34, 87, 88].
Sensing Property:
Figure 18(a) and (b) represents the selectivity in sensing of cations by UV-Vis spectra of PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites with adding 1 µl of 10–4 M concentration of various metal ions such as K+, Na+, Ca+ 2, Ba+ 2, Mg+ 2, Sr+ 2, Cr+ 6, Cr+ 3, Fe+ 3, Fe+ 2, Co+ 2, Mn+ 2, Cu+ 2, Zn+ 2, Pb+ 2, Cd+ 2, Ni+ 2, and Hg+ 2. The graph demonstrates that both PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites exhibit excellent selectivity towards the toxic Cr(VI) metal ion, whereas other metal ions are silent. Figure 18(a) and (b) shows that the addition of Cr(VI) metal ion with PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites results in red shifting of the absorbance peak position to λmax = 637 and 636 nm due to the interaction of the nanohybrid reinforced PANI ternary nanocomposites with Cr(VI) metal ion [87, 93].
The sensing behavior of the PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites with Cr(VI) metal ion are shown in the Fig. 19(a) and (b). The Au@f-SWNTs nanohybrid reinforced PANI matrix exhibits a strong broad peak at around 627 nm due to the n–π* transitions. To study the interaction of the Cr(VI) ion with ternary nanocomposites, a concentration of 10–4 M aqueous solution was added into the PANI/Au@f-SWNTs prepared in NMP solvent in drop wise manner with a variation of volume from 1–32 µl. The peak height decreases with increase in amount of Cr(VI) ion solution. At around Cr(VI) ion concentration of 32 µl, it gives optimum decrease in absorbance intensity. Peak height decreases due to the formation of complex between Cr(VI) ion and ternary PANI/Au@f-SWNTs nanocomposites, which quenches the absorption peak corresponding to the PANI matrix. It is noteworthy to mention that the ternary PANI nanocomposites can sense Cr(VI) ion at very low concentration of 32 µl. The complex is formed due to the strong affinity of the Cr(VI) with electron rich ternary nanocomposites. In the similar manner, PANI matrix reinforced Au@f-MWNTs hybrid nanofiller also exhibits the sensing of the Cr(VI) ion [23, 93, 94].
The binding constant calculated from Benesi–Hildebrand plot for PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites are found to be 1.028×105 and 3.46×104 M− 1, respectively as shown in the Fig. 20(a) and (b), which also gave the information regarding the complex formation and stability in the course of sensing. It is found that the PANI/Au@f-SWNTs shows more stable complex than the PANI/Au@f-MWNTs ternary nanocomposite [95, 96].
Figure 21(a) and (b) represents the time variation UV-Vis spectra of PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites in the presence of Cr(VI) ion. From Fig. 21(a) and (b), it is informing that the saturation point for the complete removal of the Cr(VI) ion from the PANI/Au@f-SWNTs and PANI/Au@f-MWNTs ternary nanocomposites are 312 and 144 hr, respectively. However, the λmax value of the absorption peak significantly shifted to the lower wavelength region as well as drastic fall in intensity of the absorption peak with time, confirming high removal efficiency of the Cr(VI) ion using ternary nanocomposites.