Electrochemical Deposition of Zinc Oxide on the Surface of Composite Membrane Polysulfone-Graphene-Polystyrene in the Presence of Water Soluble Polymers

1Department of Chemical and Biochemical Engineering, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania 2Department of Bioresources and Polymer Science, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania 3National Institute for Research and Development in Microtechnologies (IMT-Bucharest), 126 A, Erou Iancu Nicolae Street, P.O. Box 38-160, 023573 Bucharest, Romania 4Center of Materials Technology and Photonics, School of Engineering, Technological Educational Institute of Crete, 71004 Heraklion, Greece 5Chemistry Faculty, “Alexandru Ioan Cuza” University of Ias,i, 11 Carol I Blvd, 700506 Ias,i, Romania


Introduction
Water purification continues to be considered a major problem of the current century although tremendous efforts were made at industrial level to improve membrane technology.
Although at industrial level polymer membranes such as poly(vinylidene fluoride) (PVDF), polysulfone (PSF), and poly(ethersulfone) (PES) are the most widely used also for their thermal and chemical resistance [2,3], the poor resistance to fouling limits their performance.
For this reason, research studies were driven to increasing the hydrophilicity of the polymer membranes by physical blending with more hydrophilic polymers or inorganic compounds, by chemical and/or surface modification [3][4][5][6].
The filtration membranes were obtained using the classical immersion phase inversion method starting from aliquots of polymer solutions with previously synthesized inorganic particles like zinc oxide (ZnO), titanium dioxide (TiO 2 ), 2 Journal of Nanomaterials alumina (Al 2 O 3 ), silica (SiO 2 ), silver, or carbon [2,4,[7][8][9].This strategy not only determined the modification of pores morphology, but also had an important effect on higher water fluxes and better antifouling properties due to the hydrophilic increased nature of the resulting membranes.
Polymer-inorganic nanocomposites have attracted significant scientific and technological interest.The incorporation of inorganic nanoparticles into the polymer matrix can provide high-performance novel materials that find applications in many industrial fields.As a result of the development in nanotechnology, inorganic nanostructured materials have been designed and fabricated with important cooperative physical phenomena such as superparamagnetism, sizedependent band-gap, ferromagnetism, and electron and phonon transport [10,11].
Carbon nanomaterials possess some unusual size-dependent properties (e.g., morphological, electrical, optical, and mechanical) useful in enhancing energy-conversion, storage performance, and mechanical properties [12,13].Specifically, considerable efforts have been made to utilize the unique properties of fullerenes, carbon nanotubes, and graphene as energy materials, and tremendous progress has been achieved in developing carbon nanomaterials for highperformance energy-conversion and storage devices.
Furthermore, incorporation of pristine carbon nanotubes (CNTs) or graphene oxide (GO) at certain concentrations into the polymer matrix proved to be an advantageous alternative to realize filtration membranes with improved hydrophilicity, thermal stability, and water flux permeability without damaging the mechanical properties of the hybrid membranes [5,[14][15][16].
For complementary purposes composite filtration membranes were obtained using both inorganic nanoparticles and CNTs or GO in order to significantly increase the fouling resistance and in some cases to obtain additional photocatalytic or antimicrobial properties [17,18].
In our study graphene nanosheets were modified with polystyrene in order to favour a better dispersion of the sheets into the polysulfone matrix.The hybrid polysulfonepoly(styrene)-graphene (PSF-G-PST) membranes were synthesized by casting the solution of PSF and G-PST dissolved in N-methyl-pyrrolidone (NMP) on glass plates followed by coagulation in water bath.The electroconductive properties of the synthesized membranes were exploited for the electrochemical deposition of ZnO.In order to control the ZnO morphology, the electrochemical procedure was applied in the presence of water soluble compounds in order to form ZnO nanoparticles on one side of the composite membrane.The PSF membrane is hydrophobic.ZnO was generated in the presence of hydrophile polymers.Thus, ZnO was deposited on the surface of the PSF membrane in order to increase the membrane antifouling properties.
The chemical modification and thermal behaviour of modified graphenes were investigated by Raman spectroscopy, X-ray photon spectroscopy (XPS), and thermogravimetric analysis (TGA).Scanning electron microscopy (SEM) was employed to investigate the pores structures and ZnO nanoparticles modifications depending on the water soluble compounds.Energy dispersive spectroscopy (EDS) evidenced the formation of ZnO on the surface of the hybrid membranes.

Synthesis of Graphene-Polystyrene (G-PST) Composite.
A black dispersion of 150 mg of G in 8 mL of ST was added to 10 mL of DMF followed by nitrogen purging for 20 min.The reaction was initiated with 100 mg of PL (lauroyl peroxide) and kept under continuous stirring overnight at 80 ∘ C. The resulting product (G-PST) was purified by precipitation in M. Finally, the gray-black powder was vacuum dried until constant mass.

Synthesis of Polysulfone-Graphene-Poly(styrene) (PSF-G-PST) Composite
Membranes.The composite membranes were obtained by the wet-phase inversion method [19] in which a 15% PSF and 0,2% G-PST (wt.% by PSF content) were dissolved in NMP and kept at room temperature under continuous stirring for 12 h.The casting of the membranes was performed on glass plates immediately followed by coagulation in deionized water bath.The membranes were kept in the water bath until complete detachment of the hybrid membranes and moved afterwards for another 12 h in fresh deionized water in order to remove the entire solvent and to favour the complete formation of the pores.The PSF-G-PST membranes were dried for the next 12 h at room temperature.

Electrochemical Deposition of ZnO on the Surface of the PSF-G-PST Membranes.
The ZnO nanostructures were generated on one side of the hybrid membranes using a modified electrochemical method described elsewhere [20].Hybrid membranes were cut in small pieces (1 × 1 cm) and used as cathodes, while a Pt wire was fixed as the anode.Solutions containing 0.5 g Zn(NO 3 ) 2 ⋅6H 2 O, 0.5 g KCl and 20 mL distilled water for blank experiment were continuously stirred at room temperature for 1 h, at a potential value of −1.5 V.In the case of ZnO generation in presence of water soluble polymers different concentration and polymers were employed according to Table 1.At the end of all experiments, the composite membranes were removed from the electrochemical bath and washed several times with distilled water, followed by drying at 60 ∘ C until constant mass.

Characterization
Graphene chemical functionalization was studied by Raman and XPS spectroscopy.The Raman spectra have been registered on a DXR Raman Microscope from Thermo Scientific with a 633 nm laser.The laser beam has been focused with a 10x objective.The XPS (X-ray photoelectron microscopy) analysis has been performed on a K-Alpha instrument from Thermo Scientific, using a monochromated Al K source (1486.6 eV), at a low pressure of 2 × 10 −9 mbar.Charging effects have been compensated by a flood gun and binding energy has been calibrated by placing the C1s peak at 285 eV as internal standard.
The thermal properties of the material were ascertained using thermogravimetric analysis (TGA).The TGA analysis has been performed on Q500 TA Instruments equipment, under oxygen atmosphere, using a heating rate of 20 ∘ C/min from room temperature to 900 ∘ C.
The morphologies of the membranes have been investigated by SEM (scanning electron microscopy) using FEG-SEM-Nova NanoSEM 630 (FEI).The presence of ZnO nanostructures on the surface of the hybrid materials was confirmed by energy dispersive spectroscopy (EDS).

Results and Discussions
In this research study, the porous membranes were designed using an alternative synthesis procedure in order to meet various requirements necessary for membranes used in wastewater treatment applications.For this reason, the materials were modified to ensure good mechanical and thermal resistance and hydrophilic and antifouling properties simultaneously.
Although high amounts of graphene or graphene oxide can decrease mechanical properties of hybrid membranes, these fillers have been used in polymer membranes synthesis in order to increase thermal, hydrophilic, antifouling, and antibacterial properties of different polymer membranes.Due to its poor solubility in common organic solvents, graphene has been chemically modified in order to enhance the compatibility with other materials [21].
In our study, to favour a better dispersion of graphene sheets in polysulfone matrix, the polymer composite G-PST was obtained based on a solution polymerization reaction of styrene.
Raman and XPS analyses were performed in order to put into evidence the chemical modification of graphene with polystyrene, while TGA analysis was employed to establish the thermal properties of the G-PST composite.
The Raman spectrum (Figure 1, black line) confirmed the characteristic signals of graphene: (1) the primary in-plane vibrational mode, the G band (1580 cm −1 ), (2) the secondary in-plane vibrational mode, the D band due to defects presence, and (3) the second-order overtone of different in-plane vibration, namely, the 2D band (2714 cm −1 ).In our case, the peak for G band is slightly shifted and the shape of 2D band is not sharp comparing to single-layered graphenes, which proves a multilayered structure of the nanoplatelets according to Wang et al. [22].For G-PST composite the Raman signals of graphene have lower intensities confirming the chemical modification (Figure 1, red line).The successful chemical functionalization of G is confirmed by the modification of the ratio  G (1580 cm −1 )/ D (1350 cm −1 ) [23] from 4.35 to 2.85 after PST grafting, thus confirming the increased number of defects in the graphene structure.
The XPS analysis from Figure 2(a) shows the high resolution scan for C1s species of the graphene that were deconvoluted into five secondary peaks: the first one at 284.8 eV is assigned to sp 2 carbon atoms involved in C=C type bonds, the second peak at 285.0 eV is attributed to the presence of sp 3 type carbon species which are defects in the structure of graphene lattice, the third peak from 286.3 eV corresponds to the C-O species, while the 288.4 eV peak is known as the signal of O-C=O bonds.There is the fifth peak from 291.5 eV which arises from - * electronic interactions of the graphene aromatic structure [24,25].Meanwhile, oxygen reduction was also clearly observed during the functionalization process; thus, for G-PST sample (Figure 2(b)), the deconvoluted area of C-O groups decreased clearly while the signal of O-C=O is completely lost; also it can be noticed that the amount of C-C sp 3 species at 285 eV increased, indicating that the functionalization of G is mainly achieved via reaction with oxygenous groups, to form PST chains bonded covalently to G surface [26,27].The attack of oxygen functional groups might stem from the fact that the C-O binding energy (351 kJ/mol) is lower than the C-C binding energy (368 kJ/mol) [28].
As exhibited in Figure 3, there is a slight weight loss (approximately 10%) in the case of G nanoplatelets at 500 ∘ C that can be attributed to the labile oxygen groups proving a high carbon content.In the case of G-PST nanocomposite there is also a slight weight loss (around 5%) at temperature below 100 ∘ C due to the removal of adsorbed water on the surface of the composite.The thermal decomposition temperature of G-PST is registered at almost 380 ∘ C, while the complete degradation appears at 450 ∘ C due to the pyrolysis of PST chains.The presence of G nanoplatelets improved the thermal properties of the composite considering that the degradation process of pristine PST starts at around 305 ∘ C and ends at 410 ∘ C. The blank membrane was obtained according to Section 2.2.2 described previously.The phase inversion method used for the synthesis of polysulfone membranes drives the pores structure to asymmetric configuration, with small pores on the top and large pores at the bottom of the ).This could be explained by the presence of polystyrene, which is a more hydrophobic polymer that hinders the exchange between the solvent and the nonsolvent during coagulation time although the concentration of G-PST was kept to 0.2% wt. Figure 4(a) confirms the formation of small pores (around 20-30 nm), while the bottom of the membrane is characterized by large voids (50-100 m) with smaller pores aggregated inside their structure or independent small pores (10-20 m) formed in different areas of the membrane (Figure 4(c)).This configuration confirms that the presence of the G-PST does not alter significantly the asymmetric structure of the final membrane at this loading concentration.
Aiming also at the enhancement of antimicrobial properties of our composite membranes, the next step of this study consisted in the electrochemical generation of ZnO.In order to insert a high content of ZnO in the structure of the composite membrane, flat sheets were cut (1 × 1 cm) and positioned with the bottom pores facing the opposite direction of the electrical field.This strategy allows the growth and aggregation of ZnO structures inside the larger pores.Filling the pores with ZnO will eventually create a compact structure that will favour a better contact between contaminated water and antimicrobial compound.
The electrochemical deposition of ZnO on the composite membrane intended also the attachment of the inorganic particles inside the macrovoids, deeper in the structure of the pores.However, the cross-section image (Figure 5(a)) presents the finger-like pores (close to the skin of the composite membrane) without ZnO particles inside the pores.The bottom configuration shows ZnO nanoparticles attached both on the surface and at the edges of the macrovoids (Figure 5(b)).The detailed images (Figures 5(c) and 5(d)) show on one hand the aggregates of ZnO concentrated on the edges of the larger pores and the attachment of inorganic particles (size less than 1 m) on the walls of the interior pores on the other.
According to previous studies, the morphology of inorganic particles generated in electrical field can be severely influenced by the presence of water soluble polymers [20,30].In order to favour the formation of ZnO nanoparticles with controlled structure and narrow size distribution of the particles, the electrochemical deposition of ZnO on the PSF-G-PST membrane was performed in the presence of water soluble polymers following the conditions from Table 1.
In Figure 6, the presence of PVA in the electrochemical bath allowed the generation of ZnO structures with cubic shapes (associated with zinc blende crystalline structure of ZnO) of different sizes ranging from 100 to 400 nm.The bottom surface of the composite membrane reveals large inorganic structures of ZnO that were generated on the surface in different areas of the membrane (Figures 6(b) and 6(c)).The presence of PVA at this concentration was unable to inhibit the growth of ZnO particles making it difficult to drive   the growth of the inorganic particles inside the macrovoids or the pore walls.Although spherical ZnO particles are present on the membrane (Figure 6(b)), the cross-section (Figure 6(a)) did not confirm the presence of such particles inside the structure that could have served as nucleation centers for the growth of the inorganic compound.The detailed image of the bottom surface presents the formation of small pores coexisting with larger pores (Figure 6(c)) that do not allow the generation of ZnO inside the structure.
For the next experiment, the generation of ZnO was conducted in the presence of higher concentration of PVA, knowing the fact that polymer solutions with higher viscosity will considerably reduce the size of the particles, affecting the crystal nucleation and growth process [31].
In the case of using 10% wt PVA the bottom surface of the composite membrane PSF-G-PST-PVA10 is characterized by large pores (around 20 m) and small pores (0.5-1 m) (Figure 7(b)).The ZnO particles are drastically changed compared to the previous experiment.ZnO is concentrated on the whole surface of the back-side membrane and aggregated at the edge of the macrovoids.This can be attributed to the edges of the macrovoids which can be assimilated as defects into the composite material/deposition substrate for the nucleation and growth process of inorganic crystallites [32,33].The detailed image (Figure 7(c)) proves that the presence of higher concentration of PVA acts not only as an inhibitor, but also as a stabilizer of the inorganic particles; the spherical shape of ZnO particles is well distinguishable ranging from 0.5 to 1 m in size.
Furthermore, in Figure 7(a) (black circles) the inorganic ZnO particles were obtained on the pores walls for the first time in our experiments.
The presence of well-dispersed graphene inside the composite membrane assisted by the high voltage applied to the water soluble polymer solution, the size of the macrovoids, and the higher concentration of PVA allowed the formation of ZnO nucleus deeper into the membrane structure favouring the growth of inorganic spherical particles (with size in the range of 0.5-1 m) on the porous walls.
Synthesis of inorganic particles in the presence of water soluble polymers is strongly influenced by the molecular weight of the polymer and also by the functional groups [34][35][36][37].
Thus, in the next experiment, the ZnO nanostructures were generated in the presence of PAA at low concentration (2.5% wt.) Similar to PSF-G-PST-PVA 2.5 membrane, the crosssection did not show the presence of any zinc oxide particles inside the porous structure of the membrane.Thus, at this concentration, the amount of PAA inhibited the crystal growth process of the ZnO nanoparticles.Figures 8(b etc.) influences the configuration of ZnO nanostructures confirming that the functional groups of the water soluble compounds have a major influence to the formation of diverse shapes with different sizes [30].
Hydroxyethyl cellulose (HEC) is a water soluble polymer derived from cellulose and usually used as thickening agent in cosmetics, cleaning products, and drug or oilfield chemical products.Recent studies confirmed that HEC has an enhancing inhibitory effect in crystal growth processes in aliquots with poly(N-vinyl pyrrolidone) (PNVP) compared to polyethylene glycol (PEG) and polyacrylamide (PAM) [38].The presence of HEC in 26% NH 4 Cl solution, commonly used for zinc-carbon batteries, led to a decrease of the corrosion process which was attributed to a suppression effect of the anode and cathode reactions due to the adsorbed molecules of HEC that act as a protective layer for the zinc elements [39].
Based on these aspects, our experiment was carried out in the presence of HEC dissolved together with the ZnO precursors.After 1 h deposition time the samples were removed from the electrochemical bath and analysed by SEM (Figure 9).
The SEM analysis shows a clear evidence of the ZnO generated inside the structure of the pores (black circles marked in Figure 9(a)) in the case of the cross-section of the composite membrane.HEC was probably adsorbed on the first nucleus of the inorganic particles which were simultaneously attached to the porous walls embedded with graphene.Thus, HEC acted like a strong stabilizer that also inhibited the crystal growth of ZnO nanostructures.
Compared to PSF-G-PST-PVA10 membrane, where ZnO was also generated inside the porous structure (see Figure 7(a)), in this case the interior walls of the pores were covered with higher amounts of inorganic particles.Related to our previous experiments, the presence of HEC enhanced the deposition rate of ZnO inside the porous structure of the membrane, filling the macrovoids with spherical ZnO nanoparticles and covering the bottom surface on very large areas (Figure 9(b)).Figure 9(c) confirms that sphericalshaped ZnO was not only deposited on the edges of the bottom macrovoids, but also aggregated under electrical voltage filling-up the pores.
In conclusion, the use of HEC improved the deposition rate of ZnO nanostructures and favoured the electrochemical generation of the inorganic spheres in the whole structure of the composite membrane compared to the other water soluble polymer presented/used in this study.
In order to confirm the formation of ZnO on the membranes surface EDS analysis was performed on the bottom side (Figure 10).
The EDS spectrum demonstrates the presence of zinc (Zn) and oxygen (O) peaks together with a strong signal from carbon (C) peak attributed to the composite membrane.Thus, this result validates the deposition of ZnO on the bottom surface of the hybrid membrane.

Conclusions
The aim of this work was to develop an alternative synthesis procedure for hybrid polysulfone membranes endowed combined properties (i.e., mechanical and thermal resistance, antifouling and antimicrobial characteristics) in one single membrane designed for water purification.In this work, graphene nanoplatelets were modified with PST in order to obtain polysulfone composite membranes relevant for electrochemical deposition of ZnO.The deposition procedure was performed on one side of the composite membrane, at high voltage, in the presence of water soluble polymers.
At low concentrations (2.5% wt) of water soluble polymers, such as PVA and PAA, the generation of ZnO nanostructures was influenced by the functional groups.In this case, the inorganic structures were generated mainly at the edges of the bottom macropores hardly protruding inside the pore channels.
The experiments confirmed that ZnO nanostructures can be embedded inside the porous structure of the composite membrane at high concentrations of PVA (around 10% wt.).
The use of water soluble polymers with strong inhibitory effect on the crystal growth process, such as HEC, improved the deposition rate, decreased the size of inorganic structures, and increased the amount of ZnO deposited in the whole structure of the composite PSF-G-PST-membrane.These results are promising and further research will be performed to reach the goal of obtaining new composite materials for improved efficiency membranes for water purification systems.

Figure 1 :
Figure 1: Raman analysis of G (black line) and modified G-PST (red line).

Figure 3 :
Figure 3: TGA analysis of the G and G-PST nanocomposite.

Figure 6 :
Figure 6: SEM images of ZnO deposition on PSF-G-PST in the presence of PVA (2.5% wt.):(a) cross-section; (b) bottom; (c) detailed image of the bottom surface.

Figure 7 :
Figure 7: SEM analysis of ZnO deposited on PSF-G-PST membrane in the presence of PVA (10% wt.):(a) cross-section; (b) bottom; (c) detailed image of the bottom surface.

Figure 8 :
Figure 8: SEM analysis of ZnO deposited on PSF-G-PST membrane in the presence of PAA (2.5% wt): (a) cross-section; (b) bottom; (c) detailed image of the bottom surface.

Figure 9 :Figure 10 :
Figure 9: SEM image of ZnO deposited on PSF-G-PST in the presence of HEC: (a) cross-section; (b) bottom; (c) detailed image of the bottom surface.

Table 1 :
Electrochemical deposition conditions for ZnO generation on the composite membrane.