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Open Access 01-04-2023

Investigation of morphological, structural and electronic transformation of PVDF and ZnO/rGO/PVDF hybrid membranes

Authors: Amina Omar, Islam Gomaa, Omar A. Mohamed, Hager Magdy, Hassan Saeed Kalloub, Mohamed H. Hamza, Tarek M. Mohamed, Maisara M. Rabee, Nada Tareq, Haity Hesham, Tamer Abdallah, Hanan Elhaes, Medhat A. Ibrahim

Published in: Optical and Quantum Electronics | Issue 4/2023

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Abstract

Synergistic doping of 2-D Material ZnO nanosheets and reduced graphene oxide (rGO) of Polyvinylidene fluoride (PVDF), PVDF/ZnO, and PVDF/ZnO/rGO Hybrid membranes simply by solution casting technique for raising electronically favored β-phase ratio. Rietveld refinement X-ray diffraction technique, FTIR, Microscopic investigation, SEM, and density-functional theory (DFT) calculations were employed to unravel the atomistic origin of negative piezoelectricity, and increasing reasons for total dipole moment, electrostatic potential and bandgap energy of PVDF hybrid membranes, which arises from the sizeable displacive instability of two-dimensional material coupled with its reduced lattice dimensionality.

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1 Introduction

Polyvinylidene fluoride (PVDF) and related fluoropolymers are the most common commercialized piezoelectric (PZ) polymers that have promising applications in smart biomedical and industrial membranes (Ünsal et al. 2020; Campos et al. 2007; Xin et al. 2018). These polymers are stable at room temperature, simple to process using conventional solvent casting and melt extrusion techniques, chemically inert, biocompatible and exhibit conversion efficiencies higher than those of other piezoelectric polymers (Leszczynski et al. 2010; Sukumaran et al. 2021; Najjar et al. 2017; Nan et al. 2020). These properties make fluoropolymers ideal for use as sustainable electrical generators for powering portable, wearable, and implantable sensors and electrical devices, with or without an integrated energy storage solution(Ünsal et al. 2020; Laroche et al. 1995; Sukumaran et al. 2021; Leszczynski et al. 2010). The behavior for PVDF can be attributed to its crystalline structure, for which five different polymorphs have been observed and are referred to as α, β, δ, γ, and ε (Ünsal et al. 2020; Vatansever Bayramol et al. 2019). The first two are the most common crystalline structures observed in PVDF (Rafeie et al. 2019). Although each PVDF polymer chain has an effective molecular dipole moment, on a molecular scale, only the β and γ phases have dipole moment in the crystalline state (Pariy et al. 2019; Zhang et al. 2018). When a force is applied, an instantaneous electric field is generated parallel to the direction of the polarization vector. This electric field is proportional to the time-differential strain and leads to the separation of positive and negative surface charges on the opposite surfaces of the material, with a fast response time (Shepelin et al. 2019). In contrast, the PVDF chains in α-phase stack with their respective polarizations in alternating directions (Khdary et al. 2020; Rathore et al. 2019), resulting in a paraelectric behavior; therefore, it is nonpolar (Zhou et al. 2021). Consequently, enhancement of the content of the polar β- and γ-phases, and simultaneous suppression of the nonpolar α-phase in PVDF material is of great importance for its applications (Abdullah et al. 2015). It is stated that, the PVDF has a trans-gauche conformation (TGTG´), and hence is nonpolar. The β-phase consists of all-trans conformation (TTTT), meaning most fluorine atoms are separated from hydrogen atoms and, hence, it possesses a dipole moment perpendicular to the polymer chain (Huang et al. 2020; Gopika et al. 2020). The polar and nonpolar phases of this polymer mostly influence its physical properties. The electrical properties of PVDF can be modified for technological needs by making composites with different suitable materials. Otherwise, the solvation factor like dimethylformamide (DMF) as a solvent in our study affects the molecular interaction by including the dipolar interactions between C = O and CH₂–CF₂ in DMF and acetone (C = O double strength), respectively with PVDF, and the presence of weak hydrogen bonding C = O^….H–C, both of which disrupt the inter-chain forces of solid PVDF, and finally dissolve the PVDF (Lakshmi et al. 2018; Kalimuldina et al. 2020; Lin et al. 2006). However, using the same polar solvent can create various phases depending on the temperature of the preparation and evaporation of the solvent (Prasad et al. 2021; Ali et al. 2018). The presence of α-, β- and γ-phases in the PVDF for a specific polar solvent also depends on conformers’ mobility, which is mostly affected by thermal energy (Lim et al. 2020; Ting et al. 2020). When PVDF crystallizes from the solvent, the molecular chains have no fixed structure and can move around freely. The dissolution of PVDF crystalline regions requires interaction energy of the polymer-solvent to go beyond that between the polymer chains. At low temperatures, the reaction energy between polymer molecular chains is greater than the reaction energy of polymer-solvent interaction, so the crystalline region of the PVDF remains practically interactive, and swelling results as the solvent succeeds in entering the amorphous region. At relatively high temperatures, the reaction energy of polymer chain is reduced and the solvent penetrates the crystalline region, causing partial or complete dissolution. When the solution is deposited on a substrate, it solidifies, and the crystalline phase of the PVDF depends on the rate of crystallization and, accordingly, on the rate of solvent evaporation. When an electric field parallel to the polarization is applied in PZ polymer, the positive ions will move along the field direction, and the negative ions will move oppositely. However, the expansion is easier than the compression because of the bond anharmonicity arising from the spontaneous displacement (You et al. 2019). Accordingly, any strain in the lattice impacts the electronic band structure by interfering with the bonding strength in the crystallite structure, and the effect of lattice strain on the relevant optoelectronic properties of material is shifted too (Abdelhamid et al. 2016; Jaleh et al. 2014; Pradhan et al. 2018). Based on the mentioned properties of PVDF, as well as its nanoblends, the present work aims to prepare PVDF films using solution casting technique, then prepare graphene oxide sheets (GO), reduced graphene oxide (rGO) and ZnO nanosheets (ZnO-NS) to be blended with PVDF forming PVDF/ZnO and PVDF/ZnO/rGO hybrid membranes, then characterize all the samples with attenuated total reflection infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD) and scanning electron microscope (SEM). The antimicrobial activity of the prepared samples is then tested using disc diffusion assay antibiotic sensitivity test towards gram-negative bacteria (Escherichia coli) and gram-positive (Bacillus species). Molecular modeling at density functional theory (DFT):B3LYP/LANL2DZ level was utilized to calculate total dipole moment (TDM), band gap energy and mapping the molecular electrostatic potential (MESP).

2 Materials and methods

2.1 Chemicals and reagents

All chemicals were used without any further purification. Zinc Acetate di-hydrate (Oxford), Ascorbic Acid 99% (Fisher Chemical), PVDF (MW ~ 180.000 by GPC), DMF (MW 73.09 g/mol, Fisher Chemical), acetone 99% (Fisher Chemical), ammonia solution 35% (Fisher Chemical), sodium hydroxide ≥ 97% (Fisher Chemical), graphite powder (Fluke, Germany). Aqueous solution of H₂SO₄ 98%, HCl 33% and H₂O₂ 30% were purchased from El-Nasr Pharmaceutical company, Egypt. Besides, KMnO₄ (Alfa Aesar 98%, Germany). Deionized (DI) Milli-Q water was used during this experiment.

2.2 Synthesis of graphene Oxide (GO)

GO was synthesized by using the Hummers method (Pradhan et al. 2018). In this method, 1 g of graphite was first mixed with 35 mL of H₂SO₄ and 3 g of KMnO₄, and afterward stirred for about 1 hr in an ice-water bath at a temperature of less than 20 °C. Subsequently, about 105 mL of H₂O₂ 30% was carefully added to and mixed with the above solution for an hour, and heated to around 100 °C. Finally, this combination was diluted by adding 280 ml of distilled water, then filtered and kept in the dryer at 70 °C overnight.

2.3 Synthesis of reduced graphene Oxide (rGO)

The proper and simple physical method of probe-sonication to reduce GO to rGO (Abdelhamid et al. 2016; Sabbaghan et al. 2019; Mei et al. 2011), was done according to the following procedure. First, suspension aqueous colloids of GO were prepared from the dried GO by mechanical stirring and heat treatment at 60^oC. 1 g of GO was stirred in 100 mL of distilled water. This dispersion was then stirred using a Fisher mechanical stirrer until it became a clear solution with no visible particulate material. After that, the solution was probe-sonicated for 20 minutes in pulsed mode (operating at an amplitude of 40%) (20 KHz), then finally dried in a wide glass dish in a vacuum oven at 80 °C overnight.

2.4 Synthesis of ZnO nanosheets

ZnO-NS were synthesized using the semi-solid method (Hosny and Hosny 2011). 2 g of Zinc Acetate were added to 2.4 g of ascorbic acid as ligand and flip well, then 1 mL of deionized water and properly mixed for 1 h. During mixing, vinegar odor aroused and then declined gradually. Moreover, the color of the reaction turned yellowish white. The solution was then left in the dryer at 100 °C for 2 h. Finally, the dried powder was calcined inside the furnace at 400 °C for 2 h.

2.5 Preparation of PVDF membrane

PVDF has minimal solubility in the common organic solvents. PVDF is typically dissolved in polar solvents such as DMF, dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP) or dimethylacetamide (DMAc). Here, 5 g of PVDF were dissolved in a mixed solvent of DMF and Acetone (1:1), typically at 5% wt/v and dissolution temperature of 70 °C. At low temperatures, tiny crystallites exist due to partial dissolution or refolding of the polymer chains, which serve as nuclei when the solution is recrystallized (Li et al. 2013). The nuclei cannot be destroyed with a longer dissolution time but only by raising the dissolution temperature. We state this temperature as optimum for 2 h to produce gel form. Then cast uniformly in a petri dish and dried in an oven at 80ºC for 6 h.

2.6 Casting of PVDF/ZnO hybrid membrane

The as-prepared ZnO-NS were added (1% wt/wt) to PVDF solution prepared before the step of gel formation (Sect. 2.5). The temperature was raised to 70 °C for 8 h until the proper formation of dissolution and gel phase forms. The solution was then uniformly casted in a petri dish, and dried in an oven at 80 °C for 6 h.

2.7 Casting of PVDF/ZnO/rGO hybrid membrane

PVDF-composite films were prepared by dispersing 2 wt% and 0.4% nanofillers of ZnO-NS and rGO-sheets, respectively, in a mixed solvent through a bath sonication of 0.5 hr in continuous mode. As a second step, the PVDF powder was added to the obtained suspension and stirred for 12 hr. Finally, flexible and self-standing membranes were obtained by casting the nano-filled solution on to a clean glass plate, followed by subsequent solvent evaporation in an oven at 80°C for 12 hr. The preparation procedure is illustrated schematically as in Fig. 1.

2.8 Characterization techniques

The FTIR spectra were collected using Vertex 70 FTIR spectrometer (Bruker, Germany) by attenuated total reflection (ATR) method. The spectra were recorded in the spectral range of 4000–400 cm⁻¹ with a spectral resolution of 4 cm⁻¹. X-ray diffraction (XRD) of the as-prepared samples was obtained using Malvern Panalytical Empyrean 3 diffractometer to determine the phase composition and crystal structure. The surface microstructure was investigated using inverted microscope (Zeiss Axio Observer 5). Scanning Electron Microscopy (SEM-QUANTA FEG-250) was utilized to analyze the morphology of the prepared samples.

2.9 Antimicrobial activity study

A piece with the same dimensions and thickness was obtained from each of PVDF, PVDF/ZnO and PVDF/ZnO/rGO prepared membrane and submitted for studying the antimicrobial activity using Disc Diffusion Assay (Kirby-Bauer Disc method) antibiotic sensitivity test towards gram-negative bacteria (Escherichia coli) and gram-positive bacteria (Bacillus species) species. Each piece was placed on the agar plate of microorganism solutions that had been swabbed uniformly. The diameter of the inhibition zone around each sample was measured at 37 °C.

2.10 Calculation details

Molecular modeling calculations were run on GAUSSIAN 09 software at the Molecular Spectroscopy and Modeling Unit, National Research Centre, Egypt (Frisch 2010). Geometry optimization of PVDF and its interaction with ZnO and GO was done using the DFT method at the B3LYP (Becke 1993; Lee et al. 1988; Vosko et al. 1980) level with the LANL2DZ basis set. TDM, HOMO/LUMO band gap energy, and MESP mapping of the model structures were obtained using the same level of theory.

3 Results and discussions

3.1 Microstructures of PVDF, PVDF/ZnO and PVDF/ZnO/rGO membranes

Bright field optical microscope images of the prepared PVDF, PVDF-ZnO, and PVDF-ZnO-rGO membranes are shown in Fig. 2 Homogenous distribution of ZnO nanoparticles on the surface PVDF polymer membrane is noted. By adding rGO for PVDF-ZnO, crystals were embedded from the surface of PVDF into its layers, increasing the surface area of the prepared composite membranes for further use in different electronic and smart biomedical applications.

3.2 XRD results

In XRD pattern of PVDF pellets Fig. 3a, there were three permanent and characteristic peaks observed at 2θ = 18.4° (020), 19.8° (110), and 26.4°(021) corresponding to the primitive structure of the α-phase of PVDF contained as a part of the polymer (Xiong et al. 2020), and sequenced 2θ values at 2θ = 20.2° (200/110), 20.3° (200/110) and 20.4° (200/110) attributed to the β-phase (Abdullah et al. 2015), enhanced by presence of two semi-haze wide peaks located approximately at 2θ ranges of 17.9°26° and 30°–45°for the first and second peak, respectively (Abdullah et al. 2015; Vosko et al. 1980). Low-intensity overlapped 2θ at 18.50° and 19.2° are noted and ascribed to γ-phase, which is attributed to the gauche conformation in the structure of PVDF. The XRD method can serve as qualitative analysis to confirm the presence of the pure β-phase and distinguish it from other commonly found phases corresponding to JPCDS card number: 00-038-1638 (Xiong et al. 2020; Satapathy et al. 2011), because a lot of haziness in the original diffractogram treatment (smoothing) was done by HighScore Plus software®, and this is also a good reason for the high degree of amorphicity of the diverse polymorphs phases (α, β, and γ) of the commercial pellets.

The crystallite size was estimated by using the Scherrer Equation (Dhand et al. 2019):

$$\begin{aligned} {\text{D}} = 0.{\text{9}}\lambda /\beta {\text{cos}}\theta \end{aligned}$$

(1)

where λ is the wavelength of the X-ray (0.15406 nm), β is the full width at half maximum, θ is the diffraction angle, and D is the Crystallite size with average 63 Å. Also crystal lattice strain is calculated according to Williamson–Hall approach (Dhand et al. 2019) using the formula:

$$\begin{aligned} \in = \beta /\left( {4 \times \tan \theta } \right) \end{aligned}$$

(2)

This indicates crystal imperfection related to crystallite size and strain contribution, which are responsible for diffraction line broadening tabulated in Table 1.

Table 1

Crystallite size and lattice strain of the studied PVDF polymer

No.	B obs. [°2θ]	B std. [°2θ]	Peak pos. [°2θ]	B struct. [°2θ]	Crystallite size [Å]	Lattice strain [%]
1	1.683	0.008	32.151	1.683	49	2.549
2	0.641	0.008	35.274	0.641	132	0.879
3	13.022	0.008	79.434	13.022	8	6.84

The structural characterization and phase identification of synthesized ZnO-NPs were performed by XRD, as shown in Fig. 3b, with the structural refinement performed by the Rietveld method using HighScore Plus software® measured at 0.008 step size and a time per step of 1:00 s. ZnO-NPs consist of sharp, intense peaks that confirm the crystalline phase formation. From this XRD pattern analysis, the diffraction peaks located at 31.84°, 34.52°, 36.33◦, 47.63°, 56.71°, 62.96°, 68.13°, and 69.18° were indexed as hexagonal Zincite phase of ZnO with lattice constants a = b = 0.324 nm and c = 0.521 nm, corresponding to JPCDS card number: 96-900-4179. It also confirms that the synthesized powders were free of other impurity phases, as it does not contain any characteristics of XRD peaks other than ZnO peaks, according to the Scherrer equation and lattice strain calculation mentioned above. The values are tabulated in Table 2.

Table 2

Crystallite size and lattice strain of ZnO-NPs

No.	B obs. [°2θ]	B std. [°2θ]	Peak pos. [°2θ]	B Struct. [°2θ]	Crystallite size [Å]	Lattice strain [%]
1	4.657	0.008	5.004	4.649	17	46.504
2	0.46	0.008	34.482	0.452	184	0.646
3	0.247	0.008	36.228	0.239	349	0.33
4	0.288	0.008	36.389	0.28	299	0.382
5	0.758	0.008	47.609	0.75	116	0.75
6	0.531	0.008	56.662	0.523	173	0.43
7	0.515	0.008	62.886	0.507	184	0.367
8	0.377	0.008	66.4	0.369	257	0.252
9	0.52	0.008	68.005	0.512	187	0.336
10	0.566	0.008	69.152	0.557	173	0.358
11	0.324	0.008	71.206	0.316	309	0.197
12	0.522	0.008	72.563	0.514	192	0.31
13	0.618	0.008	77.021	0.61	167	0.339
14	0.665	0.008	81.501	0.657	160	0.337

Figure 3c attributed to PVDF-sheet without additives (PVDFO), PVDF/ZnO, and PVDF/ZnO/rGO membranes which showed remarkable changes in terms of both XRD-charts disappearance of the reflection peak located at 26.0° which was observed in the raw sample. This observation signified that a major structural change had taken place. Moreover, a distinguished peak located at 18.4° as a signature of α-phase was transformed into a broad peak, offering additional evidence of structural modification. Detailed analysis of the diffractograms revealed a characteristic peak located at 2θ = 20.4°(110) (Ali et al. 2018)In both diffractograms, this peak appeared as the main peak, suggesting the creation of a β-phase in the material. Since this peak was quite broad, it strongly overlapped with the remaining α-phase. One of the important parameters to enhance the crystallization of β-phase in PVDF is the molecular interaction between PVDF, solvent and specific materials (Lim et al. 2020). Therefore, this good indication of incorporating the ZnO-NPs and 2D-rGO sheet could be effectively utilized for the formation of crystalline β-phase and improved piezoelectric properties in membrane gradually with remarkable lattice strain and crystallite size, as tabulated in Table3.

Table 3

Crystallite size and lattice strain of PVDF/ZnO/rGO

No.	B obs. [°2θ]	B std. [°2θ]	Peak pos. [°2θ]	B struct. [°2θ]	Crystallite size [Å]	Lattice strain [%]
1	0.813	0.008	18.268	0.805	100	2.207
2	2.107	0.008	20.206	2.099	38	5.158
3	1.661	0.008	36.444	1.653	51	2.202
4	4.319	0.008	39.536	4.311	20	5.244
5	0.826	0.008	42.387	0.818	104	0.929
6	0.727	0.008	56.791	0.719	126	0.587

Furthermore, the XRD intensity is dictated by the crystallinity and revealed increasing crystallinity as shown in Fig. 3c and texture structure of the membrane. Subsequently, methods to perform strain engineering are proposed for different materials by addressing the affecting parameters (Kumar et al. 2018). On the other hand, in the PVDF₀chart which showed difference than that of PVDF pellet, the polarity of the solvent has good impact on the crystallinity of the PVDF. Polar solvents, like in our study, causes the strong dipoles of the molecular chain C–F bond in the PVDF to rotate and decrease the energy barrier for creating the more expanded all trans conformation (TTT), whereas solvents with a lower dipole moment always favor alternate trans and gauche conformation (TGTG) (Prasad et al. 2021; Arshad et al. 2019) and it effect to some extent as compared with the other sharp peaks of ZnO/PVDF and ZnO/rGO/PVDF.

3.3 FTIR Spectroscopy

Figure 4 presents the FTIR transmittance spectra for (i) ZnO-NPs, (ii) Graphene oxide before and after reduction (a) GO and (b) rGO (iii) (a) PVDF, (b) PVDF/ZnO, (c) PVDF/ZnO/rGO membranes respectively. In Fig. 4i which depicts the FTIR transmittance spectrum of ZnO nanoparticles, the spectrum mainly gave information characteristic of the chemical bonding between Zn and O. The spectrum showed a broad peak around 453 cm⁻¹ and a shoulder around 567 cm⁻¹, corresponding to ZnO nanoparticle (Xia et al. 2010; Nath et al. 1980). The remainder of the spectrum was relatively smooth, with a few peaks of adsorbed humidity and traces of residual organic impurities. The absorption peak at 1388 cm⁻¹ is assigned to the bending vibration of C-H. The peaks at 731 cm⁻¹, 650 cm⁻¹, and 590 cm⁻¹ are due to vibrations of free water molecules.

Figure 4ii shows the FTIR spectra of GO and rGO. The spectrum of GO shows a peak at 1031 cm⁻¹ that is attributed to C-O stretching. The peak at 1225 cm⁻¹ is corresponding to C–O–C bending, while C-OH bending is observed at 1403 cm⁻¹. The carbonyl groups C=O stretching are also shown at 1726 cm⁻¹, and the broad peak at 3590 cm⁻¹is attributed to O–H stretching vibration of the C-OH groups in GO-phase (Abdelhamid et al. 2016; Hu et al. 2014; Beygmohammdi et al. 2020). In the spectrum of rGO, there is reduction in the intensity or removal of the bands of oxygen-containing groups on GO by probe sonication. The bands in the fingerprint region centered at 482, 594, and 870 cm⁻¹ disappeared, and the intensity of the two bands at 1726 and 3590 cm⁻¹ decreased dramatically. This is an indication of the removal of oxygen-containing functional groups in the resulting rGO, although some residual oxygen-functional groups of GO are still present on the surface of rGO with weaker intensity after the reduction process. The band at 1612 cm⁻¹ shifted to 1580 cm⁻¹ in rGO and showed a strong intensity; suggesting the recovery of sp²lattice (Rathore et al. 2019).

The FTIR spectra in Fig. 4(iii) of (a) pure PVDF sheet, (b) PVDF/ZnO, and (c) PVDF/rGO/ZnO membrane demonstrated vibrational bands at 520 cm⁻¹ corresponding to CF₂ bending, 601 and 767 cm⁻¹ corresponding to CF₂ bending and skeletal bending, respectively, and 802 cm⁻¹ corresponding to CH₂ rocking, all attributed to the α -phase [37,53, 54]. Due to the similarity between the β- and γ-phase-specific conformations, their FTIR bands are so close to each other or coincide. Therefore, it is difficult to distinguish between both phases, and some bands appear at similar wavenumbers (Zhang et al. 2014; Karimi et al. 2019). For instance, in the case of the band at 513 cm⁻¹ for the γ-phase of PVDF polymer, it is also very close to the band at 507 cm⁻¹ attributed to CF₂ bending of the β-phase. Similarly, some articles classify the strong peak at 841 cm⁻¹ (CF₂ rocking) as characteristic of the β-phase, while others attribute it to both phases. It has been recently accepted that the band around 840 cm⁻¹ is common to both polymorphs but it is a strong band just for the β-phase, whereas for the γ-phase it appears as a shoulder of the 838 cm^− 1 band. In addition, there are some distinguishable γ-phase FTIR absorption bands. The bands at 431 and 812 cm⁻¹are individual of the γ-phase (Nuamcharoen et al. 2020). The significant differences between FTIR spectra of pure PVDF and PVDF nanocomposites are the decreasing intensity of the vibrational modes characteristic for α-phase, and the appearance and increase in intensity of the vibrational modes characteristic for β-phase (at 513 cm^− 1 and 830 cm^− 1). The results of FTIR showed a polymorph transformation from α- to β-polymorph as a result of the insertion of rGO and ZnO, with disappearance of the most common bands of ZnO and rGO because of the deep embedding in the PVDF membrane matrix, being beyond the limit of the penetration depth of ATR technique. However, it should be noted that FTIR cannot be used alone to quantify the presence and relative proportions of the phases due to the peak at 833 cm^− 1 from γ-phase PVDF overlapping with the peak at 840 cm⁻¹for β-phase PVDF. The polymorph change induced by the dipole interaction between C-F band of PVDF with rGO and ZnO appeared in previous reports (Sorayani Bafqi et al. 2015; Golzari et al. 2017). Therefore, and as an attempt for better understanding of the molecular-level mechanism, DFT calculations were carried out on the doping characteristics of a PVDF film on a single-layer graphene sheet as a function of the polarization direction of PVDF. Since the β-phase is the only PVDF phase whose permanent polarization can make it useful for ferroelectric applications, it would be very necessary using molecular modeling to investigate the change in dipole moment and electronic properties after nanocomposite addition.

3.4 SEM and EDX results

SEM has investigated the Morphology of ZnO-NS and different hybrid membrane in Fig. 5. It reveals the clear morphology of ZnO in different nano flakes. In Fig. 5a, it has a nanosheet appearance with a straight edge graphene-like with discrete irregular morphology structure, different sizes and thickness, and act as filler pieces that appear in a 2D plane. Moreover, in Fig. 5b PVDF porous membrane reveals a homogeneous surface with irregular pore distribution and size, held together as a building-blocks array. The porosity of the surface is decreasing gradually and filled by ZnO-NPs in Fig. 5c and enormously filled and complied by graphene and Zincite Nanosheets in Fig. 5d. The flakes of both graphene and zincite nanosheets act as pore fillers and connectors embedded into the surface of PVDF membrane, appearing as cement for cracks and pores.

The elemental composition of the synthesized nanoparticles and hybrid membranes was confirmed through energy-dispersive X-ray spectroscopy (EDX) analysis and is shown in Fig. 6 for ZnO, PVDF/ZnO, and PVDF/ZnO/rGO hybrid membranes, in rigid elucidation of the interaction of ZnO and graphene into the matrix of PVDF, with good ratios comparable to those used in the experimental procedure.

3.5 Antimicrobial activity study

All Samples have been neutral regardless of the inhibition zone that appeared on bacterial strains of gram-positive and gram-negative bacteria as illustrated in Table 4. This gives a good indication of a neutral surface without the ability for reactive oxygen species (ROS) generation and/or the release of metallic cations, which damage the microbial cell membrane (Hosny et al. 2020). With high profile, safety needs higher ZnO content or a biologically active doping material in future.

Table 4

Biological activity of different hybrid membranes against strains gram-positive and gram-negative

Sample	Inhibition Zone (mm)
Sample	E. coli (Gram-negaive bacteria)	Bacillus (Gram positive bacteria)
ZnO (1 mg /ml)	0.1	0.03
PVDF₀	− ve	− ve
PVDF/ZnO	0.002	0.005
PVDF/ZnO/rGO	− ve	− ve

3.6 Molecular modeling study

3.6.1 Building Model Molecules

To describe the modeling results, it is important to describe how the model molecules were built. As indicated in Fig. 7, the polymer modified by nanoparticles played an important role in improving the polymer’s electrical and optical characteristics to be used in different applications. To modify the properties of PVDF, two monomers of PVDF interacted through fluorine atoms with ZnO (Alghunaim et al. 2017) and GO as shown in Fig.7. It must be noted that these models are constructed based on the previous work our research group. For example, GO structure is constructed based upon the previously published work at (Ezzat et al. 2021), while ZnO and its two forms of interaction with polymers were presented at (Hegazy et al. 2022). The effective functional group in PVDF is F, which was used to form a hydrogen bonds during the interaction with ZnO. PVDF interacts by hydrogen bonding with ZnO through Zn to form PVDF-ZnO and through O to form PVDF-OZn. There are several possibilities for GO to interact with PVDF/ZnO, but interacting through the COOH group of GO with PVDF/ZnO via physical interaction (H-bonding) is selected based upon the conclusion of the work presented at (Ezzat et al. 2021). The OH of COOH of GO interacted with O of PVDF/ZnO to form PVDF/ZnO/GO and with Zn to form PVDF/OZn/GO.

3.6.2 Energy calculations

Calculations of the total energy were conducted for the constructed model molecules, and their various interaction probabilities through hydrogen bonding formation at the high theoretical level of DFT at B3LYP/ LANL2DZ level. The calculated physical and electronic parameters are listed in Table 5, including total energy (E), TDM, and HOMO/LUMO bandgap energy ($\varDelta$E). E as one of the physical indicators of structures’ stability, measured − 15.1131, − 3.8288 and − 75.3871 KeV for PVDF, ZnO, and GO, respectively. The interaction of PVDF with ZnO through Zn (PVDF/ZnO) and O (PVDF/OZn) resulted in a lower energy of -18.9404 and − 18.9399 KeV, respectively and, hence, much higher stability. Then the interaction of GO with PVDF/ZnO and PVDF/OZn produced two structures with roughly the same energies (~ -94.32 KeV), indicating the same level of stability. This ensures that PVDF interacted with ZnO blends represent stable structures, forming a highly firm interaction with GO, which is energetically preferred. TDM is considered as one of the physical indicators of the structure’s reactivity. The obtained TDM values were 4.869, 5.570 and 14.146 Debye for PVDF, ZnO and GO. The fact that GO has three active functional groups (hydroxyl, carboxyl, and epoxy) explains its high reactivity. This represents a highly reactive chemical structure, which increases its potential for use in various applications. Interaction of PVDF with ZnO through Zn (PVDF/ZnO) increased the reactivity, in terms of higher TDM of 14.440 Debye, compared to PVDF/OZn, which gave TDM value of 3.508 Debye. Therefore, adding GO to PVDF/ZnO (PVDF/ZnO/GO) increases its reactivity by increasing the TDM to 28.812 Debye, on the contrary to the lower reactivity of PVDF/OZn/GO with TDM of 16.443 Debye. ∆E is considered an important physical parameter, reflecting the electrical conductivity of the proposed structures. ∆E measured 11.242 eV for PVDF, while it measured 2.087 and 0.521 eV for ZnO and GO, respectively. The addition of ZnO to PVDF lowered its calculated ∆E, thus increased its conductivity. Also, the interaction of GO with PVDF/ZnO and PVDF/OZn decreased ∆E to 0.559 and 0.522 eV, respectively. Therefore, the calculated physical and electronic parameters ensured the feasibility of interaction of PVDF with ZnO through Zn and their ability to interact with GO, yielding a highly stable and extremely reactive structure.

Table 5

Calculated total energy (E) as KeV, total dipole moment (TDM) as Debye and HOMO/LUMO bandgap energy (ΔE) as eV of PVDF, ZnO, GO, PVDF/ZnO, PVDF/OZn, PVDF/ZnO/GO and PVDF/OZn/GO at DFT: B3LYP/LANL2DZ.

Structure	E (KeV)	TDM (Debye)	∆E (eV)
PVDF	− 15.1131	4.869	11.242
ZnO	− 3.8288	5.570	2.087
GO	− 75.3871	14.146	0.521
PVDF/ZnO	− 18.9404	14.440	3.730
PVDF/OZn	− 18.9399	3.508	3.418
PVDF/ZnO/GO	− 94.3283	28.812	0.559
PVDF/OZn/GO	− 94.3262	16.443	0.522

3.6.3 Distribution of HOMO/LUMO molecular orbitals for PVDF interacted with ZnO and GO

Figure 8 presents the HOMO/LUMO orbital distribution with its ∆E values for PVDF, ZnO, GO, PVDF/ZnO, PVDF/OZn, PVDF/ZnO/GO and PVDF/OZn/GO.

HOMO/LUMO molecular Orbitals (MOs) are extensively used to foresee the reactivity of PVDF and its interaction with ZnO and GO. Both reactivity and selectivity are based on how intense the interactivities are among the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Two units of PVDF are assumed to interact through F atom with ZnO and GO. The distribution of HOMO/LUMO MOs changed because of the interaction of PVDF with ZnO and GO, as seen in Fig. 8. The HOMO/LUMO MOs dispersion is spread throughout PVDF. With the addition of ZnO to PVDF, HOMO/LUMO MOs became delocalized on ZnO.

3.6.4 Molecular electrostatic potential (MESP)

MESP map is an effective tool owing to its ability to represent the electrostatic charge distribution and transportation across a chemical structure, and even its chemical active sites. [64, 65]. The MESP of PVDF and its associated hypothetical structures with ZnO and GO are shown in Fig. 9. MESP map values were represented by colors that covered the molecular surface, ordered from high to low in the sequence of Red, Orange, Yellow, Green, and Blue. The colors are described such that the red color represents the highest electron density region, the blue color represents the lowest electron density region, and the green color represents the neutral surface. Firstly, the GO MESP map is divided into four major zones, each with its electric charge, resulting in a structure with a separate active site. In the right side of the structure, an electronegative area is shown in red, which is accompanied by a lower electric charge region in yellow. These two locations are the most probable to conduct nucleophilic reactions. The lower side, on the other hand, has a dark blue color, indicating a positively charged active site.

Similarly, the rest of the GO structure, which appears to have a less positive charge than the dark blue site, represents a region favoring electrophilic interactions. In the case of PVDF polymer chains, there are two separate charges; negative charges emerge clearly around the electronegative F atoms, indicating a tendency for nucleophilic reactions there. In contrast, the less electronegative sites occur in light blue-colored regions surrounding H atoms, as well as in the chain. The interaction of the PVDF active site (F atom) with ZnO is proposed to occur in through one of two ways, one through the Zn atom and the other through the O atom. The electronegativity of the hypothetical interaction through O atom decreased as one moved away from the interaction site, reaching a dark blue zone surrounding the Zn atom. However, the calculated MESP map for the other postulate of interaction through Zn atom appears to be highly electronegative in the vicinity of the proposed interaction site, which contains a red color around O atom of ZnO, referring to a high electron mobility region, implying that the proposed interaction is more probable.

Similarly, as illustrated in Fig. 9, the MESP map of the interaction of GO with PVDF/ZnO via the COOH group was calculated. The map revealed that the yellow and green colors were distributed along the polymer chain and that a portion of the GO surface constituted an area of significantly positive charge with respect to the light blue site on one side of the GO sheet. At the same time, the other side of the GO sheet has an orange color site for higher electronegativity. This refers to GO’s capacity to draw electrons from structures that interact with it, resulting in an area of high electron mobility. When GO is combined with PVDF/ZnO, an electric charge distribution gradient is formed, ranging from a highly positive region surrounding the polymer chain to fairly negative locations on the GO side (looking orange). This refers to GO’s exceptional capacity to extract electrons from PVDF/ZnO, which may be explained by GO’s high electron mobility, which improves the electrical characteristics.

4 Conclusion

The preferential β-phase formation of PVDF is one of the parameters to be maximized for the increased electrical output of fluoropolymers. Hence, enhancements of the electrical output of PVDF-based flexible membranes induced by simple solvation and casting process is introduced. The experimental approach is represented by XRD, FTIR, and SEM with EDX analysis for the prepared PVDF hybrid membranes. Results of the experimental FTIR spectra and XRD ensured the proper synthesis of PVDF/ZnO and PVDF/ZnO/rGO membranes. The theoretical approach is represented by DFT calculations B3LYP/LANL2DZ level. PVDF/ZnO and PVDF/ZnO/rGO hybrid membranes have highly reactive chemical structures with enhance their potential for various biological applications. DFT calculations illustrated that the calculated physical and electronic parameters ensured the feasibility of the interaction of PVDF with ZnO through Zn, and their ability to interact with GO yielding a highly stable and highly reactive structure.

Acknowledgements

This paper is carried out under supervision of Prof. Medhat Ibrahim during the Nano Club activity that conducted at the Nanotechnology Research Centre (NTRC), The British University in Egypt (BUE), Cairo, Egypt.

Declarations

Conflict of interest

We declare that there is no conflict of interest.

Ethical approval

There is no human and/or animal studies in this work.

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Title: Investigation of morphological, structural and electronic transformation of PVDF and ZnO/rGO/PVDF hybrid membranes
Authors: Amina Omar
Islam Gomaa
Omar A. Mohamed
Hager Magdy
Hassan Saeed Kalloub
Mohamed H. Hamza
Tarek M. Mohamed
Maisara M. Rabee
Nada Tareq
Haity Hesham
Tamer Abdallah
Hanan Elhaes
Medhat A. Ibrahim
Publication date: 01-04-2023
Publisher: Springer US
Published in: Optical and Quantum Electronics / Issue 4/2023
Print ISSN: 0306-8919
Electronic ISSN: 1572-817X
DOI: https://doi.org/10.1007/s11082-023-04663-6

Springer Professional

Abstract

Publisher’s Note

1 Introduction

2 Materials and methods

2.1 Chemicals and reagents

2.2 Synthesis of graphene Oxide (GO)

2.3 Synthesis of reduced graphene Oxide (rGO)

2.4 Synthesis of ZnO nanosheets

2.5 Preparation of PVDF membrane

2.6 Casting of PVDF/ZnO hybrid membrane

2.7 Casting of PVDF/ZnO/rGO hybrid membrane

2.8 Characterization techniques

2.9 Antimicrobial activity study

2.10 Calculation details

3 Results and discussions

3.1 Microstructures of PVDF, PVDF/ZnO and PVDF/ZnO/rGO membranes

3.2 XRD results

3.3 FTIR Spectroscopy

3.4 SEM and EDX results

3.5 Antimicrobial activity study

3.6 Molecular modeling study

3.6.1 Building Model Molecules

3.6.2 Energy calculations

3.6.3 Distribution of HOMO/LUMO molecular orbitals for PVDF interacted with ZnO and GO

3.6.4 Molecular electrostatic potential (MESP)

4 Conclusion

Acknowledgements

Declarations

Conflict of interest

Ethical approval

Publisher’s Note

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