Effect of CuO and Graphene on PTFE Microfibers: Experimental and Modeling Approaches

The surface of pure polytetrafluoroethylene (PTFE) microfibers was modified with ZnO and graphene (G), and the composite was studied using ATR-FTIR, XRD, and FESEM. FTIR results showed that two significant bands appeared at 1556 cm−1 and 515 cm−1 as indications for CuO and G interaction. The SEM results indicated that CuO and G were distributed uniformly on the surface of the PTFE microfibers, confirming the production of the PTFE/CuO/G composite. Density functional theory (DFT) calculations were performed on PTFE polymer nanocomposites containing various metal oxides (MOs) such as MgO, Al2O3, SiO2, TiO2, Fe3O4, NiO, CuO, ZnO, and ZrO2 at the B3LYP level using the LAN2DZ basis set. Total dipole moment (TDM) and HOMO/LUMO bandgap energy ΔE both show that the physical and electrical characteristics of PTFE with OCu change to 76.136 Debye and 0.400 eV, respectively. PTFE/OCu was investigated to observe its interaction with graphene quantum dots (GQDs). The results show that PTFE/OCu/GQD ZTRI surface conductivity improved significantly. As a result, the TDM of PTFE/OCu/GQD ZTRI and the HOMO/LUMO bandgap energy ΔE were 39.124 Debye and ΔE 0.206 eV, respectively. The new electrical characteristics of PTFE/OCu/GQD ZTRI indicate that this surface is appropriate for electronic applications.


Introduction
PTFE, one of several synthetic polymeric matrices, has excellent corrosion resistance and electrical properties, as well as high temperature resistance and cost-effectiveness [1,2]. Polymers of various forms, particularly fibers influenced by MOs, are frequently used to improve polymer characteristics, resulting in low-cost, high-functioning nano-composites [3][4][5][6]. Because of the mechanical, physical, and chemical stability of PTFE, it may be utilized as a substrate for the development of ZnO nanotubes, allowing for the effective production of various sensors [7] and nanoscale photodetectors used for nano optics applications [8]. Furthermore, because of the unique properties of SiO 2 nanoparticles, such as high hardness, corrosion resistance, and superior electrical insulation, PTFE/SiO 2 might be used in technological applications [9] and for vapor oil purification [10]. Moreover, by increasing the SiO 2 amount, the PTFE/SiO 2 composite possesses good mechanical characteristics [11]. PTFE/SiO 2 /Epoxy composites provide a new hybrid composite with unique properties [12]. The PTFE/Al 2 O 3 nanocomposite shows a high mechanical characteristic with increased thermal conductivity and thermal stability [13,14]. Furthermore, the electrical characteristics of the PANI/PTFE/GO composite have been observed to have increased in the fabrication of electrochemical instruments [15]. Further, the corrosion resistance and insulating characteristics of PTFE/ZnO/SiO 2 on glass have already been considerably enhanced, and this technique provides a novel idea for building an insulator surface on glass that acts as an anti-icing surface [16]. Additionally, there is a Teflon FEP derivative of PTFE that is employed as a thermal control barrier for the Hubble Space Telescope (HST) [17][18][19]. In a space environment in Low Earth orbit (LEO), Teflon FEP suffers from corrosion, exposing the component in space to damage [20][21][22]. As a result, the improvement of Teflon and its derivatives has become an important research topic for space applications [23][24][25]. Physical parameters determined using simulations, such as TDM, HOMO/LUMO band gap energy (∆E), and MESP, are thought to be effective indicators of electronic properties and the responsiveness of such explored interactions [26][27][28]. In addition, it was shown that reactive systems have a high TDM when the energy band gap is minimal, and the charge distribution as contour of the MESP can be compared with the corresponding active sites along the base material [29][30][31][32].
In this work, we investigate the effect of ZnO and G on PTFE microfibers, which are characterized to investigate changes in molecular structure, crystal structure, and morphology. Moreover, in order to study the influence of MOs such as MgO, Al 2 O 3 , SiO 2 , TiO 2 , Fe 3 O 4 , NiO, CuO, ZnO, and ZrO 2 on PTFE electronic characteristics, it is required to analyze the change in TDM, band gap, and MESP. The goal of this theoretical work is to elucidate the influence of GQDs on the electronic characteristics of PTFE/MOs when electronic parameters change.

CuO Nanoparticles Preparation Method
CuO nanoparticles were synthesized according to the usual precipitation procedure-1 M sulphate pentahydrate (CuSO 4 ·5H 2 O) in 100 mL glacial acetic acid with stirring for 2 h at 70 • C. After the solution was completely dissolved, 2 M of Sodium hydroxide was added dropwise in 100 mL of DI water while stirring. The black precipitate was filtrated as well as washed using DI water, then dried at 80 • C for 24 h and later calcined for 2 h at 500 • C.

Preparation of the PTFE/CuO/G Nanocomposite
To start the preparation process, Graphene was prepared in a laboratory following the Hammer method, as indicated earlier in the literature under code (IFP-KKU-2020/10) [33]. PTFE was cut into microfibers to be used in the nanocomposite fabrication. 0.01 gm of CuO combined with 0.02 g of G were dissolved using (100 mL) DMSO solution, and then PTFE microfibers (70%) were mixed in the solution for 24 h with continuous stirring. The composite fibers were then removed and dried in air.

Characterization Techniques
For the intermolecular investigation of pure and composite materials, an Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectrometer (Vertex 70, Bruker, Billerica, MA, USA) with a spectral range of 4000-400 cm −1 and a spectral resolution of 4 cm −1 was utilized. Furthermore, X-ray Diffraction (XRD) was used to determine the crystal structure and phase composition of samples using a Malvern Panalytical Empyrean 3 diffractometer (Malvern, UK). Furthermore, Field-emission Scanning Electron Microscopy (FESEM, Quattro S, Thermo Scientific, Waltham, MA, USA) was utilized to analyze the morphology of the produced samples.

Calculation Details
The GAUSSIAN 09 software (Gaussian, Inc.: Wallingford, CT, USA) was used to design model designs for four PTFE units and their interactions with MOs such as MgO, Al 2 O 3 , SiO 2 , TiO 2 , Fe 3 O 4 , NiO, CuO, ZnO, and ZrO 2 at the Molecular Spectroscopy and Modeling Unit, National Research Centre, Egypt [34]. The HOMO/LUMO band gap energy, TDM, and MESP as contour were calculated for model structures using DFT theory at the B3LYP level with the LANL2DZ basis set [35][36][37].

FTIR Result
FTIR spectra for pure PTFE, CuO, G and PTFE/CuO/G composite are illustrated in Figure 1. The characteristic bands for pure PTFE microfibers are shown as only transmittance bands of F 2 stretching at 1204 cm −1 , 1152 cm −1 , and 635 cm −1 , respectively [38]. Moreover, CuO nanoparticles spectra displays a prominent band at 598 cm −1 , indicating CuO generation [39]. The PTFE/CuO/G spectrum reveals the recognized bands of PTFE microfibers and even a new band at 1556 cm −1 , reflecting the C−C of G with lower intensity in respect to a lower ratio and good distribution of the composite, and a CuO band which moved to a lower wavenumber at 553 cm −1 , confirming the composite creation of PTFE, CuO, and G.

XRD Result
As shown in Figure 2, the XRD pattern of PTFE microfibers was observed at around 2θ = 18.02 • and 31.60 • , relative to (100) and (110) diffraction plan [40]. In addition, the XRD pattern of monoclinic crystal CuO nanoparticles appearing around 2θ = 32.50 • (110) [41]. Then, a characteristic diffraction peak of G was at about 2θ = 25.12 • , which is related to the (002) reflection plan [42]. Finally, PTFE/CuO/G composite diffraction peaks were seen at 2 = 18.16 • , 24.93 • , 31.79 • , 37.15 • 41.49 • , and 72.89 • in relation to the (100), (222), (110), (107), (108), and (311) reflection planes. The intensity of the G peak was so small according to the interaction of CuO on the two G sheet surfaces, indicating that the PTFE/CuO/G nanocomposite was formed with high purity. Particle size calculation from X-ray diffraction and by considering the peak at degrees, average particle size was estimated by using the Debye-Scherrer formula [xx]: D = 0.9λ/β cos θ (1), where λ is the wavelength of the X-ray (0.15406 nm), β is FWHM (full width at half maximum), θ is the diffraction angle, and 'D' is the particle size diameter; thus, D = 111 nm as particle size. This would be more accurate in individual CuO because the composite would be on a micro scale, invading the limits of equations and accuracy, as estimated from the FESEM [43,44].  Figure 3 shows an electron microscope image of the surface of PTFE microfibers, CuO, G, and PTFE/CuO/G composite. For a pure PTFE image, the diameters of the PTFE fibers were measured by using an image-analysis system consisting of an FESEM, a highresolution monitor, and image-analysis by image j ® Program. The estimated diameter size is 100 nm. The thickness size distribution curve is indicated in Figure 4. The distribution curve indicates a narrow unimodal size distribution in the range from 20 to 220 nm with an average size of 100 nm [45]. For CuO-Nps, similar to the above analysis, the average particle size distribution was found to be in the range of (101-196 nm) with mean of 102 nm, as indicated in Figure 4b, ensuring the estimated value obtained by FESEM. Small spherical CuO nanoparticles with homogenous distribution were illustrated in the morphological SEM image [46]. Moreover, the G SEM image illustrates agglomeration layers. As a result, PTFE/CuO/G illustrates uniform distribution of the quantities of CuO and G on the PTFE microfibers' surface, which confirms the formation of the PTFE/CuO/G composite.

SEM Result
According to the experimental results of enhancing PTFE microfibers with CuO and G, a theoretical analysis of PTFE with different Mos, including MgO, Al 2 O 3 , SiO 2 , TiO 2 , Fe 3 O 4 , NiO, CuO, ZnO, and ZrO 2 , might be performed to study the change in electronic characteristics.

Designed Model Structures
Nanocomposites are of great interest currently for a wide range of applications. The addition of nanofillers to polymer matrices improves their main properties, leading to better electrical and optical mechanical functionality [47]. As a result, the model structure assumes four monomers of PTFE interacting with a variety of MOs, including MgO, Al 2 O 3 , SiO 2 , TiO 2 , Fe 3 O 4 , NiO, CuO, ZnO, and ZrO 2 . It is worth mentioning that for each metal oxide MO, there are two modes of interaction between the MO and the PTFE-one from the side of the metal atom and the other from the oxygen atom. Figures 5-7 presents the PTFE; it first interacts with MgO and then Omg; this is repeated for all the studied MOs.      Figure 5. The HOMO/LUMO orbital dispersion is spread throughout the whole chain of the four PTFE monomers. When MOs interacted with the PTFE surface, HOMO/LUMO orbitals delocalized all over MO atoms. According to this, increasing TDM while lowering ∆E improved electrical characteristics, in addition to structure stability and reactivity [48]. Table 1 Figure 6 illustrates the MESP of PTFE and PTFE interactions with numerous MOs, such MgO, Al 2 O 3 , SiO 2 , TiO 2 , Fe 3 O 4 , NiO, CuO, ZnO, and ZrO 2 . MESP is another significant characteristic for understanding the electrical properties of chemical interactions. MESP is significant because it may connect between changes in total charge and the influence on physical and chemical characteristics for studied structures. The distribution of MESP on the molecule's surface was illustrated by a map with the colors Red > Orange > Yellow > Green > Blue, with the red color representing the greatest charge zone, the blue color representing the lowest charge zone, and the green color representing zero charge zone [49]. The F atom was revealed to be the active side of the low reactivity of PTFE. The colour red intensified along the polymer chain's up and down branches due to the F atom, which is an indication for the PTFE active side. Because of the influence of MOs, the red colour increased and relocated then around the oxygen atom of the MO, indicating an improvement in PTFE reactivity. According to the MESP observations, PTFE's electrical properties increased, allowing it to be used in a wide range of applications.

GQDs Interaction with PTFE/OCu
According to earlier findings, the four GQD forms-ATRI, AHEX, ZTRI, and ZHEX-should be examined with PTFE/OCu. Owing to the GQDs' features of large surface area and effective edge atoms, they have great interaction with the surrounding molecules [50]. The most electronically improved active structure is PTFE/OCu, so it was chosen to interact with the four GQDs forms, as illustrated in Figure 7. From Table 2

Conclusions
This work combined both experimental and DFT:B3LYP/LANL2DZ calculations to gain better insight into the molecular structure of the studied polymer PTFE as well as its graphene and Mos-modified structure.
Thus, the PTFE microfibers were reinforced with CuO and graphene G, then studied with some characterization techniques such as FTIR, XRD, and SEM. The FTIR results confirmed that the PTFE/CuO/G is formed as a composite structure, and the SEM image showed the uniform distributed of nanoparticles on the PTFE microfibers' surface.
DFT calculations as consulted to study PTFE interacted with various MOs. Throughout the calculations, the HOMO/LUMO orbitals were mapped. It could be concluded that MOs are responsible for reducing the HOMO/LUMO band gap by changing it from broad to small band gap semiconductor.
The addition of various MOs to PTFE creates and controls a wide range of band gaps, allowing for several applications, including as a solar cell, sensor, and capacitor.
Another important mapping throughout the calculations is the molecular electrostatic potential map, which indicated an active site starting from the PTFE step by step during the interaction with graphene as well as other MOs.
The structure is considered active as its calculated total dipole moment increased with a decrease in its HOMO/LUMO band gap energy.
It could thus be concluded that CuO was the most effective MO to increase the electronic characteristics of PTFE. Thus, it was chosen to interact with the four GQDs forms, namely ATRI, AHEX, ZTRI, and ZHEX.
The results confirmed that, the PTFE/OCu/GQD ZTRI C46 composite increases PTFE's ability to perform in nanoelectronics devices, which are important in space applications.