1 Introduction

The recent surge in interest in scientific and industrial societies for the fabrication of nanocomposites is due to their desirable characteristics, such as multi-functional capabilities, multifaceted, low cost, low weight, ease-of-synthesis, and improved thermal, optical, and optoelectronic properties [1, 2]. These nanocomposites are used in optical and optoelectronic devices as well as dielectric materials to create capacitors with improved dielectric permittivity and refractive index [3, 4]. To take advantage of these nanocomposite features it is necessary to adjust the optoelectronic characteristics through a suitable selection of ionic fillers and optimal filler loadings with charge-relocation exchanges between the organic material and the incorporated ions, often resulting in increased concentrations and consequently enhanced refractive index. The need for polymers with a high index of refraction to have excellent photosensitive clarity has attracted much attention [5] due to its potential applications in optoelectronic devices such as OLED devices[6], substrates for advanced display devices [7], photoresistors [8], antireflection optical coatings [9], and microlenses for charge-coupled devices or metal-oxide-semiconductors [10]. However, the implementation of nanofillers often results in a decrease in transparency due to the disruption of ionic agglomeration [11].

Since their discovery in 1991, carbon nanotubes (CNTs) have been seen by researchers as a key material for the nanotechnology revolution of the twenty first century. The enthusiasm for CNTs is due to their remarkable properties; they possess special electronic, chemical, thermal and mechanical characteristics, have low mass density, are highly flexible and boast an extremely high surface area. Furthermore, they have a large aspect ratio and can create a huge interface in polymer nanocomposites [12,13,14,15]. These qualities have led to interest in using CNTs as fillers in polymer composite systems to create ultra-light structural materials with improved electrical, thermal and optical features [16]. In the last decade, the development of new materials, particularly polymer/CNT composites, has become a topic of great interest. However, further research is needed to improve their physical properties and to create potential applications [17]. The optical properties of CNT arrays are based on the atomic structure of each individual CNT and how they are arranged collectively. Single walled carbon nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs) have different optical properties due to their size. Since MWCNTs are larger, they have more uniform optical properties [18] which can be treated as a homogeneous medium for various applications [19].

The purpose of incorporating CNTs into polymers is due to their unique optoelectronic, electronic, and electrical properties, as well as the potential for a variety of interfacial configurations in the nanocomposite [12, 15]. Kumar et al. [14] studied MWCNTs in polyaniline to boost its optoelectrical conductivity and bolster its electromagnetic interference protection. Additionally, Kim et al. [20] looked into the polymethylmethacrylate (PMMA) doped with MWCNTs to prevent microbial growth issues. Hazem et al. [21] determined that the addition of MWCNTs to spinel ferrites made it a possible choice for optoelectronic devices and capacitive storage. Morsi et al. [22] found that the combination of MWCNTs/Au filler with polyethylene oxide matrix improved the electrical, thermal and optical properties of organoelectronic devices.

One of the most investigated conjugated polymers in most optoelectronic devices is poly[bis(4-butypheny)-bis(phenyl)benzidine] (abbreviated as poly-TPD). In the current work, the optoelectronic properties of poly-TPD/MWCNTs composite in various weight ratios prepared by solution blending method, have been studied. Despite many researches on the optical and optoelectronic properties improvement of polymers upon incorporating MWCNTs [12, 13, 23,24,25], there have been no studies presented the MWCNTs effects on the poly-TPD properties. This paper investigates the effects of MWCNTs on the structural and optoelectronic properties of poly-TPD. To do this, XRD patterns, FTIR spectra, absorption and emission spectra were used to analyze the poly-TPD and poly-TPD/MWCNTs composites. Additionally, parameters such as the Stern–Volmer quenching constant (kSV), fluorescence lifetime (τ) of excited poly-TPD in the presence of MWCNTs, photo-induced electron-transfer rate (kET), and bimolecular quenching rate (kq) were determined to evaluate the charge transfer between the poly-TPD and MWCNTs.

2 Materials and Methods

Poly-TPD, with molecular weight of 15,000 g/mole, and MWCNTs with outer diameter ~ 5–20 nm and length of 1.0 μm, were purchased from American Dye Source, Inc. (Baie d’Urfé, Quebec, Canada) and Chargen Life Sciences LLP (NS Phadke Marg, Mumbai, India), respectively. Poly-TPD was dissolved in toluene by sonicated for 1 h to obtain homogenous liquid and the MWCNTs powder was dispersed into toluene by sonicated for 5 h. MWCNTs with various weight ratios (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 5.0 and 10 wt%) were added to fixed toluene solution of poly-TPD (1.0 mg/ml) by solution blending method under sonication for 2 h to get homogenous specimens. Special quartz cuvette with optical length of 1.0 mm was used for characterization all the samples.

The structural properties of the all specimens were investigated by X-ray diffraction (XRD; Miniflex 600, Rigaku, Japan), Fourier transform infrared spectroscopy (FT-IR; PerkinElmer Spectrum 100, Connecticut, USA), and Transmission electron microscopy (TEM; JEOL JEM-1400, Tokyo, Japan) techniques. The optical properties of all the samples were investigated from the obtained absorption and emission spectra, which collected by UV–Vis spectrophotometer (JASCO V-670, Cremella, Italia) and spectrofluorometer (JASCO FP-8200, Cremella, Italia), respectively.

3 Results and Discussion

The FTIR spectroscopy was used to analyze the interaction between poly-TPD and MWCNT. The FTIR spectra of poly-TPD, MWCNT and poly-TPD/MWCNTs in Fig. 1 demonstrate the formation of the nanocomposite. The broad band at 3440 cm−1 in all samples is attributed to the stretching mode of O–H absorbed by the samples [25, 26]. The FTIR results also show that the aromatic C–H stretching (weak) at 3059 cm−1 for poly-TPD is present in the composites as well. Moreover, a shift by 30 cm−1 to 3029 cm−1 was observed in both composites with 2.5 and 5.0 wt% MWCNT for the aromatic C–H out-of-plane stretch. Additionally, bands at 2860–2960 cm−1 are assigned to the stretching vibrations of –CH2- and –CH3-groups and also to the microstructure formation of the specimen [27]. The peak at 1630 cm−1 in the transmittance spectrum of MWCNT is due to the C=C stretching vibration of the graphite structure in the MWCNT framework [28]. The 1507 cm−1 and 1630 cm−1 peaks are attributed to the C=C stretching of the aromatic structure from poly-TPD, and 695 cm−1 is a characteristic of the benzene ring. The 1265 cm−1 peak in poly-TPD and poly-TPD/MWCNT composites is assigned to C–N stretch band of aromatic amine. Additionally, peak from C–O–C stretching is observed at 1112 cm−1 in MWCNTs and both composites with 2.5 wt% and 5.0 wt% MWCNT content. Lastly, the bands at 957 cm−1 and 855 cm−1 are associated with trans double bond vinylene C–H wagging and phenyl C–H wagging, respectively, which indicates a dipole normal to the phenyl vinyl plane [29, 30]. The peak positions, linewidths, and intensities of the vibrations have been altered by the introduction of MWCNTs into poly-TPD, which implies that MWCNTs have been incorporated into the poly-TPD matrix. The significant changes in linewidth and intensity of the vibrational bands are likely due to the high electronegativity of MWCNTs, which has a major effect on the frequency spectrum of nearby groups [31].

Fig. 1
figure 1

FTIR spectra of poly-TPD, MWCNT, and thier composites

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Figure 2 shows the absorption spectra of the solution of poly-TPD with various incorporation of MWCNTs. A broad absorption band at 366 nm was seen in the pristine poly-TPD due to the π–π* transition of the conjugated polymer. With an increase in MWCNTs, the absorbance of the poly-TPD was increased with a longer absorption tail which would be beneficial for light harvesting in the visible region. No significant wavelength shift or extra peaks were observed, suggesting that there is no chemical interaction between poly-TPD and MWCNTs at ground states [26, 32]. A slight change in intensity of absorption band between pristine poly-TPD and poly-TPD/MWCNT composites can be attributed to good quality dispersion of MWCNTs in the polymer matrix. The MWCNTs act as a photon scattering centers in the nanocomposite. By increasing the amount of MWCNT, more photons are captured by nanocomposite structures, thus resulting in higher absorbance values.

Fig. 2
figure 2

Absorption spectra of pristine poly-TPD and poly-TPD/MWCNT composites

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The optical energy band gap (Eg) between the valance band and the conduction is a key factor in determining the potential use and performance of organic and inorganic materials in optoelectronic applications. The UV–VIS absorption spectra of composites can be used to detect changes in the band structure and to calculate Eg. The Tauc method is commonly used for this purpose, which involves the equation [33]:

$$\alpha h\nu ={B\left(h\nu -{E}_{g}\right)}^{m}$$
(1)

  

where α is the absorption coefficient, (hν) is the photon energy, B is a constant, and m is a parameter that gives the type of electron transition. This equation defines direct allowed transitions for m = 1/2 and indirect allowed transitions for m = 2; these are illustrated in Fig. 3a, b. The linear regions on the right-hand side of both graphs indicate that the direct (Egd) and indirect (Egi) allowed transition band gap energies of poly-TPD/MWCNT composite can be calculated. The Egd and Egi optical band gap energies were determined from the cut-off point at the (hν)-axis of the linear straight lines in Fig. 3a, b, as shown in Table 1. The existence both types of band gaps in poly-TPD/MWCNT composite implies that the absorption energy range can be extended to enhance photoelectric energy conversion [34]. It is evident that an increase in MWCNT content leads to a slight decrease in both Egd and Egi of poly-TPD. This is due to a slight increase in the localized energy levels acting as electron traps in the band gap, thus reducing the gap between conduction and valance bands [35].

Fig. 3
figure 3

a Direct band gap and b indirect band gap for nanocomposites with various poly-TPD/MWCNTs weight ratio

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The Urbach energy (Band tail) can be used to determine if a sample is crystalline or amorphous after doping [36, 37]. The optical absorption coefficient (α) near the edge of the band typically follows an exponential relationship given by Urbach [38], which is expressed as:

$$\upalpha \,{\mkern 1mu} = \,\upalpha _{0} {\mkern 1mu} {\text{exp}}({\text{h}}\upnu /{\text{E}}_{{\text{u}}} )$$
(2)

Here, α0 is a constant, (hν) is the photon energy and Eu is the band tail energy of localized states in the forbidden band gap. This energy is associated with the amorphous nature of the material and reflects the width of the Urbach tail due to defects in the forbidden band gap between valance and conduction bands. The inverse slope of lnα-(hν) graph according to Eq. (2) gives an indication of Urbach energy. Figure 4 shows how lnα values change according to (hν) for poly-TPD/MWCNT composites. The values of Eu, which are lower than the energy band gap, were determined by the slope of the straight lines in Fig. 4 and listed in Table 1. As the MWCNT content increases, the Eu values also increase exponentially. The decrease in Urbach energy indicates an increase in the crystalline region while an increase in Urbach energy implies a rise in the amorphous part [36, 37]. This means that as MWCNT doping increases, the nanocomposite system becomes more amorphous. Examining Table 1 again reveals that both Egd and Egi values of nanocomposites are reduced with increasing Eu values. The addition of MWCNTs to poly-TPD alters the width of localized states within the optical band gap. The rise in Eu suggests that defects within nanocomposites are increased due to MWCNT doping, thus leading to a decrease in optical energy band gap width [39].

Fig. 4
figure 4

Urbach energy for the poly-TPD with various MWCNTs weight ratio

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The steepness parameter (σ) that described the broadening/shrinkage of optical absorption edge can be altered due to interactions of the electron-phonon or exciton-phonon. As seen in Table 1, a decrease in σ or an increase in Eu is observed when MWCNTs are added, which is attributed to the increased localized density of electronic states within the forbidden band gap of the composites and thus higher disorder degree in the electronic structure of the nanocomposite [40, 41].

Equation (3) can be used to calculate the refractive index (n) as a function of Eg for poly-TPD/MWCNT composites [42, 43]:

$$\frac{{n}^{2}-1}{{n}^{2}+2}=1-\sqrt{\frac{{E}_{g}}{20}}$$
(3)

where n is the linear refractive index for λg corresponding to the Eg. As the MWCNT content increases, the number of scattering centers in the composite rises and light circulates more in the material. This leads to a decrease in the light speed that passing through the composite (n = c/v) and an increase in n values, as seen in Table 1. This increase in n values makes these poly-TPD/MWCNT composites a promising candidate for coatings and optoelectronic applications.

Table 1 Optical properties of pristine poly-TPD and poly-TPD/MWCNT composite
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Figure 5 illustrates the photoluminescence spectra of poly-TPD with various amounts of MWCNTs when exposed at a 350 nm excitation wavelength. The absorbed radiation energizes the electrons from their ground state, and when they return to the ground state, they emit energy radiation equivalent to the optical bandgap of the material. The primary peak in the emission spectrum of pristine poly-TPD at 419 nm (Near Band edge emission–NBE) is a result of the relaxation of excited π-electrons to their ground state (0–0 vibronic band transition), while the shoulder peaks at 440 and 468 nm are related to interchange states (0–1 and 0–2 vibronic band transitions, respectively). The peaks at 419 and 440 nm indicate violet emission, while the shoulder at 468 nm corresponds to blue emission. It is evident that the emission intensities of all peaks were reduced with the addition of MWCNTs and then shifted to a higher wavelength at high content (≥ 2.5 wt%). The decrease in poly-TPD emission in the composite system was caused by charge transfer from poly-TPD to MWCNTs. This inhibited recombination of excitons in poly-TPD, causing electrons to move across the interface into MWCNTs while holes remain in the poly-TPD. The red shift in the PL spectra is due to an increase in poly-TPD conjugation length [44]. The presence of MWCNTs causes separation between polymer chains, resulting in a change in conjugation length and thus a red shift in the PL spectra. The proposed photocatalysis mechanism in the poly-TPD/MWCNT nanocomposite can involve several steps. Upon exposure to light, poly-TPD can absorb photons and generate electron-hole pairs (excitons) in the polymer. The excitons can then be separated into free electrons and holes, which can migrate to the surface of the MWCNTs due to the strong electron affinity of the nanotubes. The electrons on the MWCNTs can then react with electron-accepting species (e.g. oxygen, water, or organic pollutants) in the solution, leading to the degradation of pollutants or the generation of electrical current. Meanwhile, the holes created in the poly-TPD can react with electron-donating species (e.g. hydroxyl radicals), leading to the degradation of pollutants or the regeneration of the poly-TPD. To prevent the recombination of the electrons and holes, MWCNTs can act as electron sinks, effectively quenching the excitons and enhancing the photocatalytic efficiency. The nanocomposite system of poly-TPD/MWCNT can exhibit enhanced photocatalytic activity compared to the individual components due to the synergistic effects of the two materials. The photocatalysis efficiency and thus the charge transfer mechanism depends on several factors, including the energy offset between the MWCNTs and poly-TPD, the strength of the exciton-MWCNT coupling, and the presence of defects or impurities that can trap the exciton. The more addition of MWCNTs to poly-TPD resulted in a decrease in the energy band gap, which is supported by the data presented in Table 1. This decrease in the band gap led to a closer alignment of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels, facilitating the charges transfer within the system. MWCNTs also have a high surface area and can act as a scaffold for the poly-TPD, providing more reaction sites for the photocatalytic process and increase the efficiency of charge transfer [45]. The photocatalysis mechanism in nanocomposite systems like poly-TPD/MWCNT involves the efficient separation of excitons, charge transfer, and exciton quenching, which lead to enhanced photocatalytic activity. Such this mechanism has promising applications in various environmental and energy-related fields [46,47,48].

Fig. 5
figure 5

Emission spectra of the pristine poly-TPD deposited with various MWCNTs weight ratio at excitation wavelength of 350 nm. The inset is the deconvoluted emission curve of poly-TPD by Gaussian fitting

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The Gaussian function can be used to deconvolute the PL curves into three vibronic bands. An example of a deconvoluted emission curve is given in Fig. 5 inset for poly-TPD, and the results from the deconvolution of all PL curves are summarized in Table 2. The weight fraction of MWCNTs in the starting charge was seen to slightly increase the vibronic spacing energy from 0.154 to 0.160 eV, which is usually associated with a lower degree of chain disorder and a slight rise in π-electron delocalization [26]. Furthermore, as the MWCNT content increases, the stoke shift decreases until it reaches 2 wt%, and then there is a slight increase for ≥ 2.5 wt%, as indicated in Table 1. The change in the stoke shift value can be attributed to the charge transfer states, and as the MWCNT content increases, the stoke shift value also increases due to an increase in the number of charge transfer states [49]. The spectral intensity can be approximated by transitions between vibrational frequencies of the excited and ground electronic states, and this probability can be expressed as [50]:

$${I}_{0\to n}=\frac{{e}^{-S}}{n!}$$
(4)

where S is the Huang–Rhys factor (S-factor), which is related with conformational disorder [51]. As revealed in Table 2, the S-factor was slightly changed with increasing the MWCNTs content, confirming the good distribution of MWCNTs within the poly-TPD.

Table 2 Optical energy properties for poly-TPD/MWCNT composite
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The parameters of the Stern–Volmer quenching constant (kSV), fluorescence lifetime (τ) of excited poly-TPD in the presence of MWCNTs, photo-induced electron-transfer rate (kET), and bimolecular quenching rate (kq) can be adjusted to further enhance the efficient charge transfer between the poly-TPD and MWCNTs [52].

The Stern–Volmer equation for collisional quenching can be expressed as [53]:

$$\frac{I_0}I\;=\;1\;+\;k_{SV}\left[Q\right]$$
(5)

  

where I and I0 are the emission intensities in the presence and absence of quenching, respectively, and [Q] is the quencher concentration. There is no change in the absorption spectrum of the poly-TPD when MWCNTs are added, meaning there are no ground-state interactions between the two. This suggests that the emission quenching is due to dynamic quenching [53], which is supported by the linear relationship between \(\frac{{I}_{0}}{I}\) and [Q] in the Stern–Volmer plot (Fig. 6) [54]. The slope of this linear fit gives a Stern–Volmer constant of 177 M−1, which implies that 50% of fluorescence was quenched at a concentration of 5.65 mM. This homogeneous quenching may be due to good distribution of MWCNTs and sufficient coverage of poly-TPD on their surface.

Fig. 6
figure 6

Stern–Volmer plot for emission quenching of poly-TPD by MWCNTs

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The Stern–Volmer equation can be used to calculate the kq and kET values for poly-TPD/MWCNTs hybrid thin films, where \(\frac{{I}_{0}}{I}\) is replaced by \(\frac{{{\uptau }}_{0}}{{\uptau }}\) [53]. This is demonstrated by Table 3, which shows that the fluorescence lifetime of poly-TPD decreases significantly with increasing quencher (MWCNT) concentration. The pristine poly-TPD had a lifetime (τ0) of 1.36 ns [55], and all composites had shorter lifetimes (τ) than this, indicating efficient charge transfer [56] and the absence of static quenching for the poly-TPD/MWCNTs [57].

The values of kq and kET for poly-TPD/MWCNTs composites can be calculated using the following equations [58, 59]:

$${k}_{ET}=\frac{1}{\tau }-\frac{1}{{\tau }_{0}}$$
(6  )
$${k}_{SV}={\tau }_{0}{k}_{q}$$
(7)

The kq value of 1.30 × 1011 M−1 S−1 is much higher than the minimum value for efficient quenching, 1 × 1010 M−1 S−1 [53], which shows that the combination of poly-TPD and MWCNTs is successful. As more MWCNTs were added to the poly-TPD, the kET values increased from 0.44 × 108 to 5.89 × 108 S−1 (as seen in Table 3). This indicates that there is an enhancement in quenching efficiency and charge transfer in the composites with increasing MWCNT content. In other words, by adjusting the amount of MWCNTs, it is possible to control the kET values. A higher content of MWCNTs leads to a larger surface area and more defects, which increases the kET values and allows for more efficient optoelectrical devices to be made with poly-TPD/MWCNT composites.

Table 3 Photo-induced electron transport rate and lifetime values of poly-TPD/MWCNT nanocomposites
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4 Conclusion

The photophysical properties of the poly-TPD thin film were successfully altered by incorporation MWCNTs in varying ratios, without changing its structure. As the amount of MWCNTs increased, the refractive index, energy tail, stoke shift, and vibronic splitting also increased, while the energy band gap and steepness parameter decreased. The incorporation of MWCNTs caused dynamic quenching in poly-TPD, as evidenced by the linear Stern–Volmer plot where 50% of fluorescence was quenched at a concentration of 5.65 mM. Additionally, the poly-TPD/MWCNTs nanocomposite thin films showed enhanced fluorescence lifetime, photo-induced electron-transfer rate, and bimolecular quenching rate, indicating effective charge transfer. The effectiveness charge transfer played unique role for enhancement optoelectrical devices based on poly-TPD/MWCNT nanocomposites.