CQDs) nanocomposite films: Structural, optical and catalysis properties

Synthetic Metals 259 (2020) 116218

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

New polyvinyl alcohol/carbon quantum dots (PVA/CQDs) nanocomposite films: Structural, optical and catalysis properties

T

Ahmed G. El-Shamya,*, H.S.S. Zayiedb a b

Physics Department, Faculty of Science, Suez Canal University, Ismailia, Egypt Physics Department, Faculty of Science, Arish University, AL-Arish, Egypt

A R T I C LE I N FO

A B S T R A C T

Keywords: PVA CQD nanoparticles Optical properties FTIR spectra PL properties Methylene blue Cutting-edge isothermal and kinetics study

In this literature, new PVA/CQDs nano-composites films were made-up via solution casting approach for the methylene blue dye removal from wastewater. PVA/CQDs nano-composite films were achieved by the marriage of PVA and the CQDs nano-particles, after the preparation of zero-dimension CQDs nano-particles by microwave heating process. The characterization and the optical properties of PVA/CQDs nano-composite films were reported by using X-ray diffraction (XRD), the Fourier Transformation Infrared Spectroscopy (FTIR) and the Uv–vis spectrophotometer. The application of (PVA/CQDs) nano-composite to remove the wastewater dye was performed. The XRD patterns and FTIR spectra confirmed that the strong interaction between PVA polymer and CQDs nano-particles via hydrogen bonding. The tunable structure and optical properties of PVA/CQDs nanocomposite are attributed to the concentrations of CQDs nano-particles embedded in PVA matrix. The experiments of the adsorption process, at room temperature, showed that (PVA/CQDs 2 wt.%) nano-composites have the ability to eliminate a large concentration of the methylene blue dye (30 mg/L) from aqueous solution with removal efficiency about 97 ± 1% after 40 min. In addition, the adsorption behavior submitted to the linear model of Freundlich isotherm and non-linear pseudo-first-order model. Also, PVA/CQDs can be reused five times after regeneration without any loss of their efficiency (removal ≈97 ± 1%). Finally, PVA/CQDs nano-composite films are expected to find applications in anti-ultraviolet, catalytic water decomposition and desalination.

1. Introduction Recently, with the accelerating growth of industry and economy, the environmental problems increased and this in turn threatens human health. The different synthetic dyes from paper, textiles and plastics industries are considered the most dangerous pollutants released into water, air, soil or environmental media. So, it is urgent to find high efficiency techniques with high quality in eliminating the dyes from wastewater. Multiple techniques are presented to eliminate dyes from wastewater, such as membrane separation, coagulation/flocculation, photocatalysis, biological treatment, electrochemical and adsorption technique [1–3]. But the shortcomings of these techniques lie on how to obtain and design adsorbents with high effective performance and functional applications of the adsorption process [3]. However, the adsorption method has been demonstrated to be an effective procedure with enhanced efficiency and capability to remove dyes on a large scale besides having other benefits such as recovery, and recycling of adsorbents. Numerous types of adsorbents, such as activated carbon, natural and synthetic polymers, MWCNT etc. have been employed to

⁎

remove dyes from wastewater. Recently, the combination between the polymer and inorganic materials as polymeric nano-composite has been displaying promising catalytic performance. Especially, the polymer nano-composites based on the carbon materials have been extremely applied to water treatment [2], due to their excellent adsorption performance, pretty large surface area and high pore volume, thanks to their micrometer-sized overall structure. However, these polymeric based carbon nano-composites have been extensively developed for water treatment, due to their modest shortcomings such as poor stability, poor reusability, their costly regeneration, production of concentrated sludge, costly initial investment, and weak mechanical and electrical properties [1–3]. In this paper, we introduce an economically attractive, easily prepared, rapidly, simply operable and highly efficient polyvinyl alcohol/ carbon quantum dots (PVA/CQDs) nano-composites for removal of dyes from wastewater. In fact, we can define the polymer nano-composite materials as a state of marriage between two lovers (polymer and nanoparticles), to produce a new generation carrying the properties of the parents and also having good healthy and fingerprint properties that

Corresponding author. E-mail address: [email protected] (A.G. El-Shamy).

https://doi.org/10.1016/j.synthmet.2019.116218 Received 25 June 2019; Received in revised form 4 October 2019; Accepted 19 October 2019 0379-6779/ © 2019 Published by Elsevier B.V.

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Sigma–Aldrich Chemicals, D-glucose (99.8% purity) and acetone (99.9%) were obtained from the Al-Nasr Chemicals company, Egypt.

distinguish them. These new generation materials have superior physical properties such as optical, electrical, mechanical properties and others [1–8]. However, these nano-composites have great problems preventing them from being used in industrial applications, such as the ineffective methods for adjusting the dispersion process of the nanoparticles in the matrix of polymer, agglomeration and aggregation processes, which in turn limits the advantage of the size-dependent property of nano-particles, thus hindering the large-scale production and commercialization of nano-composites. Also, another problem faces the nano-composites, which is the poor structure-related properties that limit the application [3]. Today, many polymeric nano-composites based on carbon materials were used for the removal of different organic dyes from the contaminated water. Wasim et al. [9] fabricated the hybrid nano-composites PVDF/ Chitosan/ MWCNTs for the anionic dyeReactive Orange 16 removal. It was found that the removal efficiency of the dye-Reactive Orange-16-dye using the PVDF/ Chitosan/ MWCNTs improved to 91%. Wu et al. [10] used the in-situ polymerization approach to prepare PANI/oxide MWCNTs nano-composites. It was also found that these nano-composites have the ability to eliminate the alizarin yellow R dye from wastewater, and so on. Indeed, among all polymers, polyvinyl alcohol (PVA) is the most specialized polymer that has unique properties and promising functional applications due to its easy preparation, atmospheric stability, low price, optical transparency and others [11–13]. In fact, the PVA polymer has the ability to alter its structure and optical properties once it dopes with different nano-materials, according to the most current researches [14–18]. Also, among all carbon materials, carbon quantum dots (CQDs) nano-particles have appeared as special and important zero-dimensional materials thanks to their unique properties (optical, electronic, spin and so on.) that enhanced by the edge as well as the quantum confinement effects [19–29]. Up to now, there is a large number of modern methods to synthesize CQDs, such as chemical methods, laser ablation [30], hydrothermal treatment and electrochemical methods [31] etc. In fact, CQDs have been considered as an important potential material for promising functional applications in light-emitting, bio-imaging, photo-voltaic devices [32–35], and others. This system PVA/CQDs nano-composite has been successfully prepared for high thermoelectric performance [36]. Also, the same system was used for tunable water-induced shape memory behavior [37] and the PVA/CQDs/C60 nano-composite films also were prepared for high thermoelectric efficiency [38]. Moreover, PVA/CQDs system has been used for the electromagnetic shielding applications [12]. But from our best knowledge there are no literatures on the removal of dye from PVA/CQDs nano-composite are presented, until now. Inspired by these studies, we offer new nano-composite based on PVA doped with CQDs nano-particles (see Scheme 1), which have a high catalytic performance. To achieve our purpose, we first synthesized CQDs nano-particles by using microwave heating method, which were then loaded onto PVA polymer to obtain (PVA/CQDs) nanocomposite by using a casting technique. Using glucose as a carbon source allows the CQDs to have (−OH and −COOH groups) on their surface. In addition, the interaction between the (OH group) of PVA and functional groups on the surface of CQDs allow the increasing of hydrogen bonding in the nano-composite, which can enhance both the crystallinity and optical properties of the nano-composite, also grow up the pore volume and diameter, thus the adsorption process will be enhanced. The PVA/CQDs showed a high performance for the adsorption and catalytic degradation of large concentrations of methylene blue dyes (30 mg/L) in a short time and at room temperature, showing high potential applications in catalytic water decomposition and desalination.

2.1. Synthesis of PVA films The green PVA film (PCQ0) was made-up via the solution casting technique as follow [2,12,13,36,38–41]: 2 g of PVA were dissolved in 50 mL of bi-distilled water for 2 h under the experimental order (65 ± 3 °C with mechanical stirring), until the solution was fully dissolved. The dry film, with high homogeneous quality, was obtained after the solution poured into a petri-dish and left in air for an average 3 days. 2.2. Synthesis of CQDs CQDs nano-particles were created by means of microwave heating method [2,12,13,36,38–41]. 11.1 g of D-glucose were used in this work, as a carbon basis, and dissolved in 100 mL of a mixture of [bi-distilled water and acetone with a molar ratio (1:1)] for 30 min. Subsequently, the obtained solution was moved to a glass bottle and heated in the home-microwave reaction using a rotor. 700 W and 13 min are the microwave power and the heating time that was used in the reaction. After that, the solution color turns from the colorless solution to the pale yellow color. CQDs solution was centrifuged to obtain the solidstate CQDs. After the preparation of CQDs nano-particles, FTIR analysis was performed to identify the component of CQDs and it was found that the surface of CQDs nano-particles are functionalized by the OH group and the COOH group, see our previous work [36]. 2.3. Synthesis of (PVA/CQDs) nanocomposite films PVA/CQDs nano-composite films were created through the solution casting method [2,12,13,36,38–41]. The sample with 0.25 wt.% of CQDs, labeled PCQ1, was created by dissolving CQDs in bidistilled water. In addition, 2 g of PVA also were dissolved at 65 ± 3 °C in 50 mL of the bi-distilled water for 2 h. Once PVA becomes fully dissolved, CQDs solution was added one drop/second with the mechanical stirring to ensure the well-dispersion of CQDs in the PVA. The solution is poured once the part of the water was evaporated and then the solution was left for an average 3 days to evaporate at room temperature. Then, the obtained films were labeled PCQ0, PCQ1, PCQ2, PCQ3 and PCQ4 corresponding to the concentrations of CQDs nano-particles 0, 0.25, 0.5, 1 and 2 wt.%, respectively. After the preparation of the films, the mechanical and thermal stability properties of the nano-composite (PVA/CQDs) were briefly studied in our previous work [12]. It was found that PVA/CQDs nano-composite exhibited high mechanical properties, whereas Young modulus and tensile strength of PVA/CQDs were found (580 MPa and 560 MPa) at CQDs = 2 wt.% relative to the pure PVA (140 MPa and 470 MPa), respectively. Also, the thermal stability of PVA/CQDs was higher than that of pure PVA. 2.4. Instruments and method X-ray diffraction (XRD) patterns for the green PVA, the prepared carbon quantum dot (CQDs) nano-particles and PVA/CQDs nano-composite films have been recorded using Brucker Axs-D8 Advance powder X-ray Diffractometer with a mono-chromatic beam of wavelength 0.154 nm Cu Kα radiation. The samples were measured in the 2θ range between 5° and 40° at the room temperature. The TEM microscope was used to determine the size of the synthesized CQDs nano-particles. The transmission electron microscope (TEM) Model, (JEOL-JEM-1230) operated at 80−100 kV was used to observe CQDs particles by taking a drop from the synthesized CQDs solution in a carbon-coated copper grid. The size of the CQDs nano-particles was determined by the Image J software. The mean diameter and the associated standard deviation were calculated from the measurements. A JASCO (V-570) UV/VIS/NIR

2. Experimental details PVA (Mw = 17 × 103 g/mole, 99.8% purity) and Methylene blue (C16H18ClN3S, Mw = 319.85 g/mole) were obtained from the 2

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Scheme 1. illustrates the fabrication of PVA/CQDs nanocomposite for removing methylene blue dyes.

methylene blue was performed through the stirring of 20 mg of the (PVA/CQDs 2 wt.%) with 10 mL of the methylene blue dye solution with concentration (30 mg/L). Then, the pH value of methylene blue solutions was tuned from 2 to 12 using the solution (HCl + NaOH) via a pH meter. The mixture was agitated for one day at room temperature by using the mechanical stirrer to increase the adsorption of the methylene blue dye. PVA/CQDs nano-composite was moved from the mixture, after equilibrium achievement.

double beam spectrophotometer was used to record the optical spectra, absorption (A) and transmission (T), of (PVA/CQDs) nano-composite films as a function of wavelength λ (nm) of PVA/CQDs nano-composites in the range from 190 to 800 nm. A Nicolet DTGS TEC detector spectrophotometer was used to measure the Fourier Transform Infrared (FTIR) spectra of PVA/CQDs nano-composite as the films form in the wave number range from 4000 to 400 cm-1. A JASCO FP 6300 spectrofluorometer was used to study the Photoluminescence (PL) spectra of PVA/CQDs nano-composite films at excitation wavelength 500 nm. A (JEOL JSM- 6500 F) FESEM microscope was applied to study the morphology of the said films. For SEM measurements, a vacuum evaporator was applied to cover the films with a thin layer of gold to be conductive. The BET apparatus, model: Micromeritics ASAP 2020, USA, was used to study the porosity of the nano-composite under a liquid nitrogen atmosphere.

2.5.2. Impact of (PVA/CQDs) dosage The effect of the quantity of PVA/CQDs nano-composite was studied via agitating different doses (20, 40, 60, 80, and 100 mg) in 10 mL of methylene blue solution (30 mg/L) for 60 min at room temperature. 2.5.3. Impact of methylene blue concentration To study the effect of initial concentration of methylene blue dye on the adsorption efficiency of PVA/CQDs nano-composites for methylene blue removal; the study was performed using 10 mL of various methylene blue concentrations of the solution (10–100, 150, 200, 250, and 300 mg/L) and 80 mg the PVA/CQDs at pH value 12. Then the solution

2.5. Adsorption studies 2.5.1. Impact of solution pH The effect of pH nature of the solution on the elimination of 3

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

was agitated at room temperature for 60 min by using the mechanical stirrer. 2.5.4. Impact of agitation time In order to study the effect of the interaction time on methylene blue removal, group of experiments with different interaction time (5−50 min) were performed, at room temperature, with (10 mL) of the initial concentration of methylene blue dye (30 mg/L), 80 mg of the nano-composite (PVA/CQDs) and pH 12. 2.5.5. Water resistance properties The percentage degree of swelling (DS%) of the samples was computed as follow [12]: the dry PVA/CQDs composite films were weighed (Wd) and immersed for 24 h in deionized water at room temperature. After that, the swollen samples were taken out and wiped with tissue paper to remove excess water from the film surface, then the films were weighed immediately (Ws). Then, finally samples were dried at 60 °C for 10 h after performing the water dipping test. DS (%) and the weight loss of the samples were computed via the following Equations:

(ws − wd ) ws (wd − wa ) weight loss(%) = wd

Swelling(g/g) =

2.5.6. Reusability of adsorbent After the adsorption experiment which was carried out by stirring the mixture of 80 mg of PVA/CQDs and 10 mL of MB solution (30 mg/L) for 40 min at pH 12, PVA/CQDs was separated from the dye solution and washed with water and utilized to the next adsorption-desorption process. An ethanol solution (30 mL) was employed as the desorption agent. The adsorption-desorption cycle was successively repeated for five times.

Fig. 1. shows the X- ray pattern of the green PVA, as synthesized carbon quantum dot (CQDs) nanoparticles and the PVA/CQDs nanocomposite films.

part of the semi-crystalline PVA polymer molecules, which arises from the strong inter/intra-molecular hydrogen bonding between the molecular chains of the PVA polymer [2,12,13,36,38–41]. As seen with increasing CQDs nano-particles concentration, the intensity of this peak (crystallinity of the PVA) increases accordingly. This may be due to increasing the number of hydrogen bonding between CQDs and the molecular chains of the PVA polymer (see Scheme 2). In fact, the increase in the crystallinity of PVA/CQDs nano-composites arises from the strong interaction of CQDs nano-particles with the chains of PVA producing a molecular rearrangement and reordering within the amorphous chains of PVA and thus increasing the crystallinity [12,38–41]. We believed that this was due to the rise in the number of CQDs nano-particles in the PVA matrix, where, CQDs nano-particles interact with the side chain group of PVA mainly with the hydroxyl group, and increase the crystallinity of the nano-composite. This interaction between the matrix of the PVA polymer and CQDs nano-particles is confirmed by the FTIR analysis. Fig. 2 shows the plots of infrared transmission spectra of the nano-composite films at room temperature. For green PVA there is a broad and strong band at 3565.8 cm-1, which could be assigned to the stretching vibration of the hydroxyl groups (ν OHe) with strong hydrogen bonding (intra and/or inter type) between the chains of the PVA due to high hydrophilic forces. The stretching vibrational band of C]O appeared at 1610 cm-1 also provides the evidence that PVA used in this study still has some acetyl groups, that is partially hydrolyzed, and the C]C group appeared at 1570 cm-1. For PVA/CQDs nano-composites with a different concentration of CQDs nano-particles 0.25, 0.5, 1 and 2 wt.%. The FTIR spectrum shows stretching vibration of the hydroxyl groups (ν OHe) at 3566 cm-1, 3566.2 cm-1, 3585 cm-1 and 3596.6 cm-1 for 0.25, 0.5, 1 and 2 wt.%, respectively. The stretching vibration band of C]O appeared at 1654.5 cm-1, 1654.7 cm-1, 1654.9 cm-1 and 1655.1 cm-1. Also, the C]C band appeared at 1566.5 cm-1, 1566.9 cm-1, 1567.1 cm-1 and 1567.4 cm-1 for the concentrations 0.25, 0.5, 1 and 2 wt.%, respectively. This shift proves that CQDs nano-particles are attached to a C]C group as well as to the OH group in the side chain of PVA molecule [42]. Also, these

2.5.7. Data calculation The concentration of methylene blue was calculated by means of the UV–Vis spectroscopy at λmax = 665 nm. The adsorption capacity (qe (mg/g)) was determined from the absorbance results by the Equation.

qe = V

(C0 − Ce ) m

(1)

Where V (L) = the solution volume, Ce (mg/L) = the final concentrations of methylene blue in the solution C0 (mg/L) = the initial concentrations of methylene blue in the solution, and m (g) is the adsorbent (PVA/CQDs) weight. Removal efficiency is stated through the Equation:

Removal efficiency% =

(Co − Ce ) Co

(2)

3. Results and discussion 3.1. X-Ray diffraction and FTIR study The XRD charts of the green PVA, the prepared CQDs and PVA/ CQDs nano-composite films are depicted in Fig. 1. It can be observed that a single broad peak was observed at 2θ = 23.7° for CQDs, which is ascribed to a highly disordered of carbon atoms with a C(002) plane, which characterizes the lattice of hexagonal shape of the structure of graphite (CQDs). The d-spacing for the plane C(002) of the CQDs nanoparticles was calculated and it was around 0.35 ± 0.01 nm. Furthermore, the very small size of the prepared CQDs nano-particles is the reason behind the broadening of the XRD peak [12,36,38]. On the other hand, for the green PVA film (sample PCQ0), there is a broad peak at 2θ = 19.5°. This peak has resulted from the crystalline 4

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Scheme 2. illustrates the interaction mechanism between the CQDs nanoparticles and PVA matrix.

325 nm in the UV region. The band at 200 nm relates to the n–π* transition, which is coming from the unsaturated C]O and/or C]C bonds that exist in the tail–head of PVA and bands at 75 nm, and 325 nm are related to the π–π* transition, which is coming from the charge transfer group [2,12,13,36,38–41]. Also, the green PVA hasn't any absorption peak in the visible regions, due to its high transparency. Fig. 3A shows that the intensities of these three absorption bands increase with the rise in CQDs nano-particles in PVA. This can be illustrated in the term of Beer's law [41], where the increase in the number of the absorbing molecules leads to an increase in the absorption. It is also observed that, as shown in the Fig. 3B, the intensity of the absorption in the visible region at λ = 550 nm increases with increasing the concentration of CQDs nano-particles in the matrix of PVA, due to the presence of CQDs nano-particles in the PVA matrix. As seen, the plots of (αhν)0.5 versus the photon energy (hν), as depicted in Fig. 4, have a linear behavior for the absorption near the fundamental absorption edge. The allowed indirect transition energies were determined by extrapolating the linear portion of the curves to zero absorption as shown in Fig. 4 and calculating the corresponding

shifts prove the existence of hydrogen bonding with the C]O bond via the oxygen atom or refer to the resonating structure from alkene↔alkane and C = O↔C–O- [43,44]. It is also remarkable that the present double bond segments are considered suitable sites for polarons and/or bipolarons. These conjugated polyene sequences are presumably responsible for the color of the fillers treated PVA, (PVA/CQDs nanocomposite). This shifting of the frequency for stretching vibrations is related to the force constant according to Hooke’s Law, and therefore the shifting to the higher frequency indicates an increase in the force constant. The increase in the force constant gives an insight into specific interactions between the dopant CQDs and the polar (OH) groups of the green PVA polymer. 3.2. Optical properties 3.2.1. Absorption spectra The optical absorption spectra for the green PVA and PVA/CQDs nano-composite films were measured and depicted in Fig. 3. For the green PVA, there are three absorption bands at 200 nm, 275 nm, and

Fig. 2. (A) shows the FTIR spectra of the PVA/CQDs nanocomposite films as a function of the wavenumber, (B) Enlarge of the wavenumber regions between (3560−3660 cm-1) to show the change in the spectra. 5

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Fig. 3. (A) shows the optical absorption spectra of the PVA/CQDs nanocomposite films and (B) shows the relation between the absorption intensity at 500 nm and the concentration CQDs nanoparticles.

value of photon energy. For green PVA film, the indirect band lies at 4.55 eV, while for 0.25, 0.5, 1 and 2 wt.% of CQDs nano-particles-doped PVA films, the Eg = 4.12, 3.80, 3.16 and 2.80 eV, respectively. It is clear that, as the concentration of CQDs nano-particles increases the band gap decreases as shown in the figure. This decrease in the optical band gap may be explained on the basis of the fact that incorporation of CQDs dopant forms charge transfer complexes (CTCs) in the trap levels of the PVA polymer between HOMO and LUMO bands of PVA [2,12,13,36,38–41].

3.2.2. Transmission spectra The transmittance spectra of PVA/CQDs nano-composite films as a function of wavelength are depicted in Fig. 5 that the green PVA (PCQ0) has three absorption peaks at 200 nm, 275 nm, and 325 nm (UV region), as seen in absorption spectra, and the intensity of the transmission spectrum is constant (85%) from 400−700 nm (visible region). It is clear that with increasing CQDs nano-particles in the matrix of PVA, the three absorption bands (200 nm, 275 nm, and 325 nm) are twinned and coincide together and the intensity of the transmission

Fig. 4. shows the energy gap of the PVA/CQDs nanocomposite films as a function of the photon energy. 6

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

composites at 608.5 nm and the intensity of this peak increases with increasing the concentration of CQDs nano-particles in the PVA matrix. Furthermore, the PL peak was shifted to a higher wavelength (red shift behavior), as shown in the (Fig. 6B) We hypothesize that the functional groups (hydroxyl group, −OH) of polyvinyl alcohol polymer can share in the electron donation process from the (−OH) group to the CQDs depend on enhancing electron withdrawal, producing π-conjugated structure with more attracted or acquired electrons (electrophilic nature) [45]. In fact, CQD has a large π-conjugated structure according to the organic chemistry; the group of the electron donator can be excited to the aromatic rings to create p-π conjugated structure, hence expanding the π-conjugated structure [45]. The interaction between polyvinyl alcohol, π-conjugated structure and strong orbital interaction, raises the primary HOMO to the orbital with a higher energy. Where, the (−OH) groups increase the densities of the delocalized electron, producing a narrow and small optical band gap. Also, the PL emission spectra of PVA/CQDs nano-composite can be tuned by varying the concentration of CQDs nano-particles. It is clearly observed that, the peak of the PL spectra is shifted from 608.5 to 611.3 nm with increasing the concentration of CQDs nano-particles, which is attributed to the quantum confinement effect; this behavior was shown in other quantum dots [46]. In addition, the detected emission peak from the CQDs comes from the changing in the sp2 bonding nature and density available at CQDs nano-particles [47].

Fig. 5. shows the optical transmission spectra of the PVA/CQDs nanocomposite films as a function of the wavelength from 190−800 nm.

spectra from 190−450 nm goes to zero value (zero transmission). This is due to the strong interaction between CQDs nano-particles and the molecular chains in the PVA polymer that produce a large number of the hydrogen bonding (inter/intra types), thus elevating the electronic transitions and hence the absorption in the UV region [40,41]. In addition, the intensity of the transmission spectra decreased to 55% with respect to the green PVA from 450−800 nm (visible region), due to the increase in the concentration of CQDs nano-particles.

3.4. BET analysis and TEM study Fig.7(A) shows the adsorption-desorption isotherms of the nanocomposite PVA/CQDs films under the N2 environment. As seen from the figure, the meso-porous structure was observed from the hysteresis loop of the IV type of isotherm. Table 1 shows the BET analysis of the samples. The PVA/CQDs nano-composite has the BET surface area (82 × 10-2 m2/g) larger than that of green PVA (1.6 × 10-3 m2/g). This increase in the surface area can offer more surface active sites, producing an enhancement in the efficiency of adsorption capacity. It can be also seen that PVA has pore volume = 3.7 × 10-3 cm3/g and pore diameter = 3.08 nm. On the other side, the PVA/CQDs (2 wt.%) nanocomposite has pore volume = 191 × 10-3 cm3/g and pore diameter = 9.23 nm. These results show that PVA/CQDs (2 wt.%) nano-

3.3. Photoluminescence (PL) spectra Fig. 6A shows the photoluminescence (PL) spectra of PVA/CQDs nano-composites. It is very clear that, the green PVA polymer has not any PL peak in the visible region, as the PVA polymer itself has not any absorption peak in the visible region (visible clarity) [40,41]. From the figure, it can be seen that the only one PL peak for PVA/CQD nano-

Fig. 6. (A) shows the PL spectra of the PVA/CQDs nanocomposite films as a function of the wavelength. (B) shows the relation between the PL intensity and PL peak position against the concentration of the CQDs nanoparticles. 7

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Fig. 7. (A) BET analysis of the green PVA and the PVA/CQDs 2 wt.% nanocomposite. (B); shows the FESEM micrograph of the (PVA/CQDs) nanocomposite films with the CQDs concentration (A) 0.25 wt.%. (B) 0.5 wt.%. (C) 1 wt.%. (D) 2 wt.%.

show that the increase in the small CQDs nano-particles in the whole matrix of the PVA polymer, indicating the increase in the surface area inside the matrix. Moreover, the sample (PCQ4) shows a high packing density and higher surface area. The size and shape of CQDs nano-particles were studied using the TEM microscope, as shown in Fig S1and S2. From the TEM micrograph, there are many islands of tiny carbogenic nano-particles that are isolated from each other, and have a regular shape. From the histogram, it can be seen that the size distribution ranging from 1 to 5 nm and the average size of CQDs was about 3 nm.

Table 1 shows the BET data of the green PVA and PVA/CQDs (2 wt.%) nanocomposite films. Sample

BET surface areas (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

Green PVA PVA/CQDs (2 wt. %)

1.6 × 10-3 82 × 10-2

3.7 × 10-3 191 × 10-3

3.08 9.23

composite has a meso-porous structure, which is useful for the elimination efficiency of the dye. As can be seen from the results that PVA/ CQDs nano-composite has relatively small surface area, however the capacity of adsorption of this nano-composite is high. This is owing to the presence of a large number of the functional groups (hydroxyl −OH groups of the PVA) on the surface of the nano-composite as well as on the surface of CQDs nano-particles (hydroxyl −OH and −COOH groups). The abundant −OH groups on the surface of the PVA and also −COOH and −OH groups on the surface of CQDs nano-particles are ionized at the pH value 12. This leads to boosting the negative charge density on PVA/CQDs nano-composite surface, producing an increase in the attraction electrostatic force between the adsorption sites of PVA/CQDs nano-composites and the + ve charged molecules of methylene blue dye, which improve the adsorption of methylene blue dye from the solution. The FE-SEM micrograph was applied to explore the dispersion property of CQDs into the matrix of the PVA polymer, Fig. 7(B). It is clear that the sample (PCQ1) contains some gallery space in the matrix of PVA/CQDs nano-composites and also the higher density of the insulator (PVA) is observed due to the small concentration of CQDs nanoparticles in the nano-composites, in addition to, there is a large density of pores in the nano-composite. Alternatively, with increasing the concentration (number) of CQDs nano-particles in the nano-composite (PVA/CQDs) films, the density of CQDs nano-particles in the matrix increases and increasing the pore volume and size in the matrix, samples (PCQ2-PCQ4). Reaching to the sample (PCQ4) the micrographs

3.5. Water resistance properties Water resistance properties of PVA/CQDs nano-composites were assessed via calculation of the degree of swelling (%) and the weight loss through the water-dipping test. The swelling behavior of the fabricated PVA/CQDs composites is depicted in Fig. 8A. As observed; the pure PVA demonstrated higher degree of swelling compared to PVA/ CQDs composites. The (−OH) groups of PVA which are responsible for its hydrophilicity, were consumed because of the reaction with CQDs thus PVA becomes less capable of H-bonding with H2O, leading to the lower degree of swelling [12]. Also, the degree of swelling of PVA/ CQDs composites decreased with the enhancement of CQDs amount which is attributed to the fact that addition of CQDs enhances rigidification which reduces the mobility and free volume of PVA. Also, the formation of efficient H-bonding between CQDs and PVA matrix reduces the hydrophilic nature and ability of PVA to form an H-bonding with water molecules and this in turn decreases the degree of swelling of the obtained composites [12]. PVA can be dissolved in cold water slowly and in hot water rapidly. The (−OH) groups of PVA reacted with (−OH) group of CQDs and the whole PVA forming a strong H-bonding, therefore it becomes water insoluble. The (−OH) groups of PVA can form an H-bonding with water, therefore it can absorb a large amount of water and consequently it can swell. Fig. 8B demonstrates the weight loss percentage of prepared composites. As observed, the weight loss percentage of PVA/CQDs composites is 0% and they are completely 8

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Fig. 8. (A) shows the Degree of Swelling of PVA/CQDs nanocomposites (B) Weight loss percentage of PVA/CQDs nanocomposites.

composites was calculated over a range from 3 to 12, Fig. 9(A). From the figure, the supreme adsorption of methylene blue is observed at pH 12 and the efficiency of the adsorption declines with the decrease in the pH value. It is known that methylene blue is a cationic dye and presents in solutions as positive ions. In the basic nature pH 12, the (OH) groups of PVA and (OH) groups that located on the surface of CQDs are ionized and resulted in increasing the negative charge density on the nanocomposites surface. This leads to an increase in the interactions (electrostatic type) between the adsorption sites of PVA/CQDs nano-composites and the + ve charged methylene blue, thus enhancing methylene blue uptake from solution. On the other hand, at acidic pH value, the lower dye uptake was obtained and this is maybe due to the competition between the cations of methylene blue and the spare of H+ ions for the adsorption sites of PVA/CQDs nano-composites. Moreover, the electrostatic repulsion force has happened in acidic medium between PVA/CQDs (which in the existence of H+, its surface has a positive

water insoluble and they can be used in water-based applications [12]. 3.6. The applicability to remove methylene blue dyes 3.6.1. Methylene blue adsorption from aqueous solutions onto PVA/CQDs nano-composites The adsorption of methyl blue dye by the carbon materials can be explained by two reasons, van der Waals and the attraction force (electrostatic type) between the + ve charged methyl blue dye and -ve charged CQDs. The pH value of the solution, the amount of the adsorbent, the initial dye concentration, and the contact time are affected on the adsorption process. In the next section we study the effect of these parameters on the adsorption process. 3.6.2. Effect of pH The pH effect on the adsorption capacity of PVA/CQDs nano-

Fig. 9. (A) shows the Effect of pH on the removal of methylene blue dye by the PVA/CQDs nanocomposite (B) The effect of adsorbent amount on the removal of methylene blue dye using PVA/CQDs nanocomposite. 9

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

composite was estimated in the range of 1−50 min. It is clear from the results that adsorption of methyl blue dye increases with increasing the time of interaction between the dye and the nano-composite in the range of 1−40 min and then it reaches to the stable state (constant level).

charge) and the cationic methylene blue. This interaction decreases the uptake of methylene blue [48]. It is clear that the adsorption capacity of the composite material is largly limited by pH value. Thus, we postulate a better electrostatic interaction between the dye and the catalyst surface at higher pH value. Moreover, it has also been postulated that H2O2 loses its oxidizing capabilities in basic media since it decomposes to molecular oxygen and H2O. Also, it has been claimed that the dye is more prone to undergo degradation in the presence of *OH and *OOH radicals.

3.6.6. Adsorption isotherm studies In order to explain and understand how the interaction between the solute and the (PVA/CQDs) adsorbent takes place, it is vital to study of the isotherm data. The Langmuir and the Freundlich are the best isotherms to examine the adsorption equation (Fig. 11). The limited adsorption site, monolayer, and homogenous nature are studied from the Langmuir isotherm. Langmuir isotherm in the nonlinear form is defined via the Equation:

3.6.3. Effect of adsorbent dosage on methylene blue dye adsorption Fig. 9(B) shows the effect of the adsorbent (PVA/CQDs) quantity on the removal of methylene blue dye. The typical results show that the improvement in the removal percentage with increasing the quantity of (PVA/CQDs) adsorbent. Hence the adsorption capacity of methylene blue was increased by increasing the number of adsorption sites when raising the quantity of (PVA/CQDs) adsorbent. When the adsorbent (PVA/CQDs) quantity is 80 mg (or larger), the maximum elimination percentage reached (98 ± 1%).

Qe = Q m

KL Ce 1+KL Ce

(6)

Where Ce (mg/g), is the concentration at the equilibrium state, Qe (mg/g), is the adsorbed quantity of dye/unit mass of (PVA/ CQDs), and Qm (mg/g) is a constant linked to the maximum capacity of adsorption and KL (L/mg) is the adsorption energy. Langmuir isotherm in the linear form is defined as:

3.6.4. Impact of initial concentration of methylene blue dye The impact of the initial concentration of the methylene blue dye was studied by changing the concentrations of methyl blue dye from 10 mg/L to 300 mg/L, as shown in Fig. 10(A). It is clear from the results that the capacity of the adsorption of methylene blue on PVA/CQDs nano-composites depends on the initial concentration of methylene blue and increasing the initial concentration of methylene blue dye reduces the adsorption percentage. When the concentrations of methylene blue dye are low (lower concentrations), the ratio of the available sites quantity on the adsorbent surface to the adsorption sites on methylene blue dye was reduced. Indeed, PVA/CQDs nano-composites molecules have more chance, at lower methylene blue dye concentrations, to interact with the active sites on methylene blue dye and, hence, the capacity of the adsorption improved [49].

Ce 1 C = + e Qe Q m KL Qm

(7)

The equilibrium parameter or the separation factor (RL) is a dimensionless constant and it is an important parameter to study the Langmuir isotherm, which is calculated from the Equation:

RL =

1 1+KL Co

(8)

Where,"C0 is the initial concentration of methylene blue. The RL parameter determines the isotherm nature. When RL = 0 the adsorption is irreversible, when RL = 1 the adsorption is linear, 0 < RL < 1 the adsorption is favourable, and the adsorption is unfavourable when RL > 1 [50]."In our case study and at all initial methylene blue concentrations, the calculating RL value shows the adsorption is favorable (0 < RL < 1). The Freundlich isotherm denotes that the surfaces with

3.6.5. Impact of agitation time The impact of interaction time on the removal efficiency of methylene blue dye using PVA/CQDs nano-composite is established in Fig. 10(B). The time that the dye takes place to adsorb on the nano-

Fig. 10. (A) shows the effect of initial concentration of methylene blue on the removal of methylene blue dye via the PVA/CQDs nanocomposite (B) Influence of agitation time on removal of methylene blue dye using the PVA/CQDs nanocomposite. 10

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Fig. 11. Adsorption kinetics: (A) linear Pseudo-first-order model, (B) Nonlinear Pseudo-firstorder model, (C) Linear Pseudo-second-order model, and (D) Nonlinear Pseudo-second-order model for removal of methylene blue via PVA/CQDs nanocomposite. n

heterogeneous nature. Freundlich isotherm in the nonlinear form is defined via the Equation:

Qe =

1 KF Cen

R2 =

∑i= 1 (q e,exp − q e,model )i2 n

∑i= 1 [(q e,exp − q e,model )i2 + (q e,exp − q e,model )i2 ] n

(9)

χ2 =

∑i= 1 (q e,exp − q e,model )i2

The linear form of this isotherm can be expressed via the following Equation:

RMS =

log Qe = log KF +

1 log Ce n

(11)

q e,exp 1 n−2

(12)

n

∑i=1 (q e,exp − q e,model )i2

(13)

From the table, the values (R , χ and RMS) for the linear and nonlinear forms of the Langmuir and Freundlich isotherms were compared with each other. It is known that from the linear isotherms of the Langmuir and Freundlich, the equilibrium data can be calculated to ensure the best outcomes. It can be seen that from the comparison between the linear isotherm of Freundlich and the Langmuir models, also the Freundlich isotherm has a greater value of R2 and very low values of χ2 and RMS. Hence, the Freundlich isotherm with its linear model is more suitable for illustrating the elimination of methylene blue dye using PVA/CQDs nano-composite. Due to the huge surface area and the effects of (PVA and CQDs) nano-composite, the multilayer of methylene blue dye will handle on the active energy surface of PVA/ CQDs without saturation of energetic binding sites, producing different mechanisms of the elimination of methylene blue dye using PVA/CQDs 2

(10)

Where KF Freundlich constant is related to the capacity of adsorption, and n is Freundlich constant is related to the intensity of adsorption. When n < 1 the adsorption is poor, 1 < n < 2 the adsorption is relatively difficult, and the adsorption is excellent when 2 < n < 10 [51].""In our case study, the calculating n values are 2.419 ± 0.235 for the linear Freundlich isotherm and 2.449 ± 0.198 for the nonlinear Freundlich isotherm showing the process of the dye removal is favorable. From the linear and non-linear regression achieved from the fitting of the curve, the parameters of the adsorption isotherm were calculated and depicted in Table 2. In addition to the R2, χ2 and RMS values were determined from the following equations:

11

2

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Table 2 The adsorption isotherm and kinetics parameters for methylene blue removal by PVA/CQDs nanocomposite. Isotherm and kinetic Models

Parameters

Error Functions

Langmuir Linear Nonlinear Freundlich Linear Nonlinear Pseudo-first order Linear Nonlinear Pseudo-second order Linear Nonlinear

Qm(mg/g) 15.932 ± 0.867 17.872 ± 1.11 KF(L/g) 1.61 ± 0.237 2.021 ± 0.234 qe,cal 3.266 ± 1.2 3.12 ± 0.302 qe,cal

KL(L/mg) 0.051 ± 0.015 0.035 ± 0.005 n 2.419 ± 0.235 2.449 ± 0.198 qe,exp 1.3 1.3 qe,exp

3.142 ± 0.497 3.529 ± 0.602

1.3 1.3

R2 0.982 0.961

χ2 3.557 3.913

RMS 0.538 0.656

0.985 0.982

0.0302 6.978

0.048 0.968

k1 0.131 ± 0.032 0.032 ± 0.009 K2

0.551 0.987

0.914 0.011

5.058 0.032

0.012 ± 0.022 0.007 ± 0.001

0.698 0.988

4.988 0.014

5.112 0.035

Fig. 12. the reusability of the PVA/CQDs for the degradation of methylene blue by during five cycles (each cycle stand up for 40 min).

blue dye were listed in Table 2. From the table, the values (R2, χ2 and RMS) for the non-linear forms of the pseudo-first and second-order kinetic models were compared with each other. It is known that from the nonlinear kinetic models, data can be calculated to ensure the best results. The values of (R2, χ2 and RMS), show insignificant differences for non-linear pseudo-first and second order kinetic models. However, the values of the q that calculated from the pseudo-first order model (qe,cal) are close to the values of q that experimentally obtained (qe,exp). Thus, the removal of methylene blue dye by PVA/CQDs nano-composite submitted to the non-linear pseudo-first-order model shows that the removal of the methylene blue dye is only based on the adsorption site population.

nano-composite [52]. 3.6.7. Kinetics of adsorption In order to explore the kinetic mechanism of the removal of methylene blue dye by PVA/CQDs nano-composite, the linear and nonlinear formulas of pseudo-first and second order kinetic models were evaluated (Fig. 11). Eq (14) explains the formula of the linear of pseudo-first-order kinetic model

ln (q e − qt) = ln q e − K1t

(14)

Eq (15) explains the formula of the non-linear of pseudo-first-order kinetic model

qt = q e (1 − exp)(−K1t)

(15)

3.6.8. Reusability of adsorbent An ideal adsorbent should not only have high adsorption capacity, but also demonstrate good reusability, which can considerably decrease the total cost of the adsorbent. After the desorption experiment, it was discovered that all the adsorbed methylene blue was released from the adsorbent and the capacity for the methylene blue adsorption remained unchanged, (see Fig. 12A). Therefore, the adsorbent can be reused for at least five times after regeneration by ethanol without any loss of efficiency (removal ≈ 100%) suggesting that PVA/CQDs nano-composite films are stable enough for long-time recycling. Therefore, PVA/CQDs nano-composite films with cost-efficiency and long-time stability have a promising application in practical wastewater treatment.

-1

Where qe(mg g ) denotes to the concentrations of the adsorbed methylene blue at equilibrium, qt (mg g-1) denotes to the concentrations of the adsorbed methylene blue at time t, and K1 (min-1) is the rate constant of pseudo-first-order, Eq (16) explains the formula of the linear of pseudo-second-order kinetic model

t 1 t = + qt qe K2q 2e

(16)

Eq (17) explains the formula of the non-linear of pseudo-secondorder kinetic model

qt =

q e K2t 1 + q e K2t

3.6.9. Effect of ionic strength on adsorption Generally, heavy metal wastewaters from many industries contain various types of salts. The presence of ions leads to a high ionic strength, which may significantly affect the performance of the

(17)

Where K2 (g mg-1 min-1) is the rate constant at equilibrium [52]. The kinetic parameters obtained for the elimination of methylene 12

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

Table 3 shows a comparative study of previous works on removal the methylene blue dye. Reference

Results

Material

[54] [55] [56] [57]

Adsorption, reaction conditions: 2 g of adsorbent/25 mL of the dye olution at 25 °C Adsorption, best reaction conditions: 24 h, 37 °C, 100 rpm Initial concentration of MB 50 mg/L at 1 h 80% Adsorption using 0.5 g, 2.5 h

Sulfonated graphene/PVA Activated carbon/PVA Chitosan/TiO2 Composite Octa(maleimidophenyl) silsesquioxane–SiO2/TiO2

adsorption process. The effect of Na+ and Zn2+ concentration on adsorption is investigated by adding different amount of sodium chloride or zinc nitrate to a series of methylene blue solutions, and the result is shown in Fig. 12B. It is clear that both metal ions cause inhibition of adsorption. The effect of Zn2+ is particularly strong. On one hand, high concentration of metal ions will significantly reduce the solubility of methylene blue, which brings about a decrease of adsorption capacities [53]. On the other hand, unlike Zn2+, certain amount of Na+ in the solution causes little impact on the swelling properties of the nanocomposite. Thus, it is much more difficult for methylene blue molecules to enter the inner part of the composite in the solution of Zn2+ than Na+. Also, this decrease in adsorption efficiency may be ascribed to the neutralization of the surface charge of adsorbent by electrolyte ions, which competes with methylene blue molecules for the adsorption on the surface of electrolyte. However, this decrease in adsorption capacity is not very high indicating that PVA/CQDs can effectively remove methylene blue from the aqueous solution even in the presence of high salt concentration. Table 3, shows a comparative study of previous works on removal the methylene blue dye. From this comparative study it could be seen that our nano-composite has some advantageous than other reported. Our (PVA/CQDs) nano-composite works as an excellent adsorbent owing to its unique properties includes low cost, simple, easy processing and feasible method of synthesis. In addition, the strong interaction between PVA and CQDs allows the increasing of hydrogen bonding in the nano-composite, which can enhance both the pore structure of the nano-composite, grow up the pore volume and diameter, thus enhanced the adsorption capacity of the dyes on the nano-composite. Also, the PVA/CQDs have a high performance for the adsorption and catalytic degradation of large concentrations of methylene blue dyes (30 mg/L) in a short time and at room temperature when compared with other reported materials.

methylene blue, thus enhancing methylene blue uptake from solutions. In addition the linear Freundlich isotherm and non-linear pseudo-firstorder model agreed well with the experimental data. The adsorbent can be reused for at least five times after regeneration with ethanol without any loss of efficiency. Finally, this work may be a scientific horizon for a wide range of potential functional applications in anti-ultraviolet application, catalytic water decomposition and desalination. Declaration of Competing Interest There is no conflict of interest. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116218. References [1] D. Zhang, H. Zhang, C. Song, W. Yang, J. Deng, Chiral microspheres constructed by helical substituted polyacetylene: A new class of organocatalyst toward asymmetric catalysis, Synth. Met. 162 (2012) 1858–1863. [2] Ahmed Gamal Abed El-Azim Khalil El-Shamy (January 23rd 2019). Polymer/Noble Metal Nanocomposites, Nanocomposites - Recent Evolutions, Subbarayan Sivasankaran, IntechOpen, DOI: https://doi.org/10.5772/intechopen.79016. Available from: https://www.intechopen.com/books/nanocomposites-recent-evolutions/polymer-noble-metal-nanocomposites. [3] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [4] A. Haryono, W.H. Binder, Controlled arrangement of nanoparticle arrays in blockcopolymer domains, Small 2 (2006) 600. [5] W.T. Kim, J.H. Jung, T.D. Kim, D.I. Son, Current bistability and carrier transport mechanisms of organic bistable devices based on hybrid Ag nanoparticle-polymethyl methacrylate polymer nanocomposites, Appl. Phys. Lett. 96 (2010) 253301. [6] M.S. Mehata, M. Majumder, B. Mallik, N. Ohta, External electric field effects on optical property and excitation dynamics of capped CdS quantum dots embedded in a polymer film, J. Phys. Chem. C 114 (2010) 15594. [7] M. Majumder, A.K. Chakraborty, B. Biswas, A. Chowdhury, B. Mallik, Indication of formation of charge density waves in silver nanoparticles dispersed poly(methyl methacrylate) thin films, Synth. Met. 161 (2011) 1390. [8] D.I. Son, D.H. Park, J.B. Kim, J.W. Choi, T.W. Kim, B. Angadi, et al., Bistable organic memory device with gold nanoparticles embedded in a conducting poly(N-vinylcarbazole) colloids hybrid, J. Phys. Chem. C 115 (2011) 2341. [9] M. Wasim, S. Sagar, A. Sabir, M. Shafiq, T. Jamil, Decoration of open pore network in poly(vinylidene fluoride)/MWCNTs with chitosan for the removal of reactive orange 16 dye, Carbohydr. Polym. 174 (Supplement C) (2017) 474–483. [10] K. Wu, J. Yu, X. Jiang, Multi-walled carbon nanotubes modified by polyaniline for the removal of alizarin yellow R from aqueous solutions, Adsorpt. Sci. Technol. 36 (2018) 198–214. [11] A.A. Syed, M.K. Dinesan, Review: polyaniline—a novel polymeric material, Talanta 38 (1991) 815–837. [12] A.G. El-Shamy, Novel conducting PVA/Carbon quantum dots (CQDs) nanocomposite for high anti-electromagnetic wave performance, J. Alloys. Compd. 810 (2019) 151940. [13] A.G. El-Shamy, W. Attia, K.M. Abd El-Kader, The optical and mechanical properties of PVA-Ag nanocomposite films, J. Alloy. Comp 590 (2014) 309–312. [14] A.N. Ananth, S. Umapathy, J. Sophia, T. Mathavan, D. Mangalaraj, On the optical and thermal properties of in situ/ex situ reduced Ag NP’s/PVA composites and its role as a simple SPR-based protein sensor, Appl. Nanosci. 1 (2011) 87–96. [15] C.M. Luk, B.L. Chen, K.S. Teng, L.B. Tang, S.P. Lau, Optically and electrically tunable graphene quantum dot-polyaniline composite films, J. Mater. Chem. C 2 (2014) 4526. [16] B. Biswas, A. Chowdhury, B. Mallik, Tuning of electrical conductivity and hysteresis effect in poly(methyl methacrylate)–carbon nanotube composite films, RSC Adv. 3 (2013) 3325. [17] A. Sleiman, M.F. Mabrook, R.R. Nejm, A. Aayesh, A.A. Ghaferi, M.C. Petty, et al.,

4. Conclusion(s) In conclusion, we have used the solution casting technique to prepare PVA/CQDs nano-composite films for the methylene blue dye removal from wastewater. It was observed that the structure and the optical properties of PVA/CQDs nano-composite films strongly depend on the concentration of the CQDs nano-particles in the films. The optical absorption spectra confirmed that the strong interaction between PVA and CQDs nano-particles. In addition, the transmission spectra results also showed the presence of a cut-off frequency in the entire ultraviolet regions and in a significant part of the visible region. From the obtained results the energy gap was decreased with increasing CQDs nano-particles. The results confirmed the tunable PL behaviors for CQDs inserted into the matrix of PVA. The π-conjugated structure interaction between PVA and CQDs was the reason behind the PL red shift in the nano-composite. The ideal environments for the adsorption of methylene blue dye were found to be the initial quantity of methylene blue dye 30 mg/L, 80 mg PVA/CQDs, and pH = 12. In the basic nature pH 12, the (−OH) groups of PVA and the (−OH) or (−COOH) groups that located on the surface of CQDs are ionized leading to an increase in the negative charge density on the nano-composites surface. This leads to increasing interactions (electrostatic type) between the adsorption sites of PVA/CQDs nano-composites and the + ve charged 13

Synthetic Metals 259 (2020) 116218

A.G. El-Shamy and H.S.S. Zayied

[18]

[19] [20] [21] [22] [23]

[24]

[25]

[26] [27] [28] [29] [30] [31] [32] [33] [34]

[35]

[36]

[37]

[38]

[39]

Organic bistable devices utilizing carbon nanotubes embedded in poly(methyl methacrylate), J. Appl. Phys. 112 (2012) 024509. M. Rinkio, M.Y. Zavodchikova, P. Torma, A. Johansson, Effect of humidity on the hysteresis of single walled carbon nanotube field-effect transistors, Phys. Status Solidi B 245 (2008) 2315. D. Pan, J. Zhang, Z. Li, M. Wu, Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots, Adv. Mater. 22 (2010) 734. H. Feng, Z. Qian, Functional carbon quantum dots: a versatile platform for chemosensing and biosensing, Chem. Rec. 18 (2018) 491–505. X. Sun, Y. Lei, Fluorescent carbon dots and their sensing applications, Trends Analyt. Chem. 89 (2017) 163. S.Y. Lim, W. Shen, Z. Gao, Carbon quantum dots and their applications, Chem. Soc. Rev. 44 (2015) 362. R. Liu, D. Wu, X. Feng, K. Mullen, Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology, J. Am. Chem. Soc. 133 (2011) 15221. Y. Li, Y. Hu, Y. Zhao, G. Shi, L. Deng, Y. Hou, L. Qu, An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics, Adv. Mater. 23 (2011) 776. S. Zhu, J. Zhang, C. Qiao, S. Tang, Y. Li, W. Yuan, et al., Strongly green-photoluminescent graphene quantum dots for bioimaging applications, Chem. Commun. (Camb.) 47 (2011) 6858. J. Shen, Y. Zhu, C. Chen, X. Yang, C. Li, Facile preparation and upconversion luminescence of graphene quantum dots, Chem. Commun. (Camb.) 47 (2011) 2580. K.A. Ritter, J.W. Lyding, The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons, Nat. Mater. 8 (2009) 235. J. G¨uttinger, T. Frey, C. Stampfer, T. Ihn, K. Ensslin, Spin states in graphene quantum dots, Phys. Rev. Lett. 105 (2010) 116801. B. Trauzettel, D.V. Bulaev, D. Loss, G. Burkard, Spin qubits in graphene quantum dots, Nat. Phys. 3 (2007) 192. X. Yan, B. Li, X. Cui, Q. Wei, K. Tajima, L. Li, Independent tuning of the band gap and redox potential of graphene quantum dots, J. Phys. Chem. Lett. 2 (2011) 1119. X. Yan, X. Cui, B. Li, L. Li, Large, solution-processable graphene quantum dots as light absorbers for photovoltaics, Nano Lett. 10 (2010) 1869. X. Yan, X. Cui, L. Li, Synthesis of large, stable colloidal graphene quantum dots with tunable size, J. Am. Chem. Soc. 132 (2010) 5944. L. Li, X. Yan, Colloidal graphene quantum dots, J. Phys. Chem. Lett. 1 (2010) 2572. J. Shen, Y. Zhu, X. Yang, J. Zong, J. Zhang, C. Li, One-pot hydrothermal synthesis of graphene quantum dots surface-passivated by polyethylene glycol and their photoelectric conversion under near-infrared light, New J. Chem. 36 (2012) 97. V. Ramar, S. Moothattu, K. Balasubramanian, Metal free, sunlight and white light based photocatalysis using carbon quantum dots from Citrus grandis: a green way to remove pollution, Sol. Energy 169 (2018) 120–127. A.G. El-Shamy, New free-standing and flexible PVA/Carbon quantum dots (CQDs) nanocomposite films with promising power factor and thermoelectric power applications, Mater. Sci. Semicond. Process 100 (2019) 245–254, https://doi.org/10. 1016/j.mssp.2019.04.004. G. Yang, X. Wan, Y. Liu, R. Li, Y. Su, X. Zeng, J. Tang, Luminescent poly(vinyl alcohol)/Carbon quantum dots composites with tunable water-induced shape memory behavior in different pH and temperature environments, ACS Appl. Mater. Interfaces 8 (2016) 34744–34754. A.G. El-Shamy, Novel hybrid nanocomposite based on poly(vinyl alcohol)/carbon quantum dots/fullerene (PVA/CQDs/C60) for thermoelectric power applications, Composites B 174 (2019) 106993, , https://doi.org/10.1016/j.compositesb.2019. 106993. A.G. El-Shamy, W. Attia, K.M. Abd El-Kader, Enhancement of the conductivity and

[40]

[41] [42]

[43]

[44]

[45]

[46] [47]

[48] [49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

14

dielectric properties of PVA/Ag nanocomposite films using γ irradiation, J. Mater. Chem. Phys. 191 (2017) 225–229. A.G. El-Shamy, A.A. Maati, W. Attia, K.M. Abd El-Kader, Promising method for preparation the PVA/Ag nanocomposite and Ag nano-rods, J. Alloys Compd. 744 (2018) 701. A.G. El-Shamy, Composite (PVA/Cu nano) films: two yield points, embedding mechanism and thermal properties, Prog. Org. Coat. 127 (2019) 252–259. R.F. Bhajantri, V. Ravindrachary, A. Harisha, V. Crasta, S.P. Nayak, B. Poojary, Microstructural studies on BaCl2 doped poly(vinyl alcohol), Polymer 47 (2006) 3591–3598. A.N. Krkljes, M.T.M. Cincovic, Z.M.K. Popovic, J.M. Nedeljkovic, Radiolytic synthesis and characterization of Ag-PVA nanocomposites, Eur. Polym. J. 43 (2007) 2171–2176. H.M. Ragab, Spectroscopic investigations and electrical properties of PVA/PVP blend filled with different concentrations of nickel chloride, Physica B 406 (2011) 3759–3767. R.T. Morrison, R.N. Boyd, Electrophilic Aromatic Substitution in Organic Chemistry, 6th edn, Prentice Hall International, Inc., Upper Saddle River, NJ, 1992, pp. 517–546. D.V. Melnikov, J.R. Chelikowsky, Quantum confinement in phosphorus-doped silicon nanocrystals, Phys. Rev. Lett. 92 (2004) 046802. J. Peng, W. Gao, B.K. Gupta, Z. Liu, R. Romero-Aburto, L. Ge, L. Song, et al., Graphene quantum dots derived from carbon fibers, Nano Lett. 12 (2) (2012) 844–849. N.S. Tabrizi, M. Yavari, Methylene blue removal by carbon nanotube-basedaerogels, Chem. Eng. Res. Des. 94 (Supplement C) (2015) 516–523. Z. Derakhshan, M.A. Baghapour, M. Ranjbar, M. Faramarzian, Adsorption of methylene blue dye from aqueous solutions by modified pumice stone: kinetics and equilibrium studies, J. Health Scope 2 (3) (2013) 136–144. E.A. Mohamed, A.Q. Selim, A.M. Zayed, S. Komarneni, M. Mobarak, M.K. Seliem, Enhancing adsorption capacity of Egyptian diatomaceous earth by thermo-chemical purification: methylene blue uptake, J. Colloid Interface Sci. 534 (2018) 408–419, https://doi.org/10.1016/j.jcis.2018.09.024. A. El Hamidi, S. Arsalane, M. Halim, Kinetics and isotherm studies of copper removal by brushite calcium phosphate: linear and non-linear regression comparison, E J. Chem. 9 (3) (2012) 1532–1542. S. Mallakpour, F. Motirasoul, Use of PVA/a-MnO2-stearic acid nanocomposite films prepared by sonochemical method as a potential sorbent for adsorption of Cd(II) ion from aqueous solution, Ultrason. Sonochem. 37 (Supplement C) (2017) 623–633. Y. Tang, R. Yang, D. Ma, B. Zhou, L. Zhu, J. Yang, Removal of methyl orange from aqueous solution by adsorption onto a hydrogel composite, Polym. Polym. Compos. 26 (2018) 161. H. Safardoust-Hojaghan, M. Salavati-Niasari, Degradation of methylene blue as a pollutant with N-doped graphene quantum dot/titanium dioxide nanocomposite, J. Clean. Prod. 148 (2017) 31–36. H. Li, J. Fan, Z. Shi, M. Lian, M. Tian, J. Yin, Preparation and characterization of sulfonated graphene-enhanced poly (vinyl alcohol) composite hydrogel and its application as dye absorbent, Polymer 60 (2015) 96–106. S.R. Sandeman, V.M. Gun’ko, O.M. Bakalinska, C.A. Howell, Y. Zheng, M.T. Kartel, G.J. Phillips, S.V. Mikhalovsky, Adsorption of anionic and cationic dyes by activated carbons, PVA hydrogels, and PVA/AC composite, J. Colloid Interface Sci. 358 (2011) 528–592. M.H. Farzana, S. Meenakshi, Synergistic effect of chitosan and titanium dioxide on the removal of toxic dyes by the photodegradation technique, Ind. Eng. Chem. Res. 53 (2013) 55–63.