Network structure-based decorated CPA@CuO hybrid nanocomposite for methyl orange environmental remediation

A unique network core–shell hybrid design-based cross-linked polyaniline (CPA), which was coated with CuO nanoparticles (NPs) and decorated with nitrogen-doped SWCNT/GO/cellulose N-SWCNTS-GO-CE, has been fabricated using the oxidative polymerization technique. This hybrid nanocomposite shows excellent photocatalytic degradation and an acceptable adsorption capability for Methyl Orange (MO) dye in aqueous solutions with a very slight effect for the N-SWCNTS-GO-CE CuO component. The prepared nanocomposites were used for the removal of a carcinogenic and noxious dye, Methyl Orange, from aqueous samples under various adsorption conditions. Approximately 100% degradation of 10 mg/L of Methylene orange dye was observed within 100 min at pH 6.0 using 50 mg/L CPA/N-SWCNTS-GO-CE/CuO nanocomposite under UV radiation. Additionally, significant factors were investigated on the degradation process including the contact time, MO initial concentration (Ci), solution pH, and dosage of the CuO nanocomposite. All investigated experiments were performed under UV radiation, which provided significant data for the MO degradation process. Furthermore, the recovery of the nanocomposite was studied based on the photocatalytic process efficiency. The obtained data provide the high opportunity of reusing CPA/N-SWCNTS-GO-CE/CuO nanocomposite for numerous photocatalytic processes. The CPA/N-SWCNTS-GO-CE/CuO nanocomposite was prepared via chemical oxidative copolymerization of polyaniline (PANI) with p-phenylenediamine (PPDA) and triphenylamine (TPA) in the presence of N-SWCNTS-GO-CE and CuO NPs. The morphology, structure and thermal properties of the CPA/N-SWCNTS-GO-CE/CuO nanocomposite were investigated using various techniques, including FTIR, XRD, RAMAN, SEM, MAP, EDX, TEM, TGA and DTG. Therefore, CPA/N-SWCNTS-GO-CE/CuO nanocomposite can be effectively used as a convenient and reusable adsorbent to remove hazardous dye from wastewater.

www.nature.com/scientificreports/ JASCO spectrometer over the range of 4000-300 cm −1 and used to obtain information on the functional groups of the studied samples. X-ray diffraction (XRD) of the NC crystallinity was examined by using a Bruker model D8 to study the fundamental structures of these nanocomposites. The instrument include reflectometry, highresolution diffraction, in-plane grazing incidence diffraction (IP-GID), small-angle X-ray scattering (SAXS), and residual stress and texture investigations. Raman spectroscopy measurements were performed by using a Raman spectrometer (Lab. RAM-HR Evolution Horiba Co.) with a single visible spectrometer, which was equipped with an air-cooled open electrode 1024 × 256 pixel CCD detector, a 532 nm He-Cd laser with 1800 grating (450-850 nm), and a 10% ND filter, using an acquisition time of 5 s, 5 accumulations without spike filter and delay time, and a 100 × objective. To detect the absorbance of light wavelengths, the UV-visible recording spectrophotometer Shimadzu-Japan was used. Thermal analyses were detected in the form of TGA and DTG measurements using Shimadzu DTA-50 and TGA-50 systems at a heating rate of 10 °C/min in air. The thermal performance was examined to define the degradation temperature of the NC and their thermal stabilities.
Fabrication process. Synthesis of GO. GO was prepared according to a modified Hummers' method 47 .

Synthesis of OXSWCNTs.
According to the literature, the OXSWCNTs were set up 46 . First, 100 mg of SWCNTs were scattered in a mixture (1:3 v/v) of HNO 3 (70%) and H 2 SO 4 (96%).Then, they were ultrasonicated for 4 h. Next, for 2 h at 80 °C, the suspension was refluxed in an oil bath with attractive blending. At that point, deionized water (DIW) was used to dilute the received mixture and dialysed in DIW until the washing arrangement demonstrated pH > 5. Finally, OXSWCNTs were filtered and dried in a stove at 60 °C.
Synthesis of the N-SWCNTs/GO/cellulose hybrid nanofiller. OXSWCNTs/GO 0.3 g was scattered in DIW100 mL and stirred at 90 °C for 2 h. At that point, 25% aqueous ammonia solution and 0.9 g urea were included and consistently stirred for 12 h at 90 °C. First, because of the formation of the OXSWCNTs/GO hybrid, which dissolved when surplus ammonia was added, the above solution became turbid. Then, in vacuum for 12 h at 90 °C, the transparent solution was evaporated. Cellulose was dissolving in 100 mL H 2 O and 10 g of NaOH. Next, under continuous stirring for another 12 h, the two mixtures were combined at 90 °C. Finally, using ethanol, the final product was washed and dried at 200 °C for 7 h.
Synthesis of cross-linked PANI (CPA). The required pure cross-linked PANI was synthesized as reported in the literature 48,49 .

Synthesis of CPA and CPA/N-SWCNTs-GO-CE/CuO nanocomposites.
The chemical copolymerization of crosslinked PANI with N-SWCNTs-GO-CE and CuO was achieved as follows: In a 250-mL three-neck flask consisting of 120 ml of 0.5 M H 2 SO 4 , 5% N-SWCNTs-GO-CE was added. Next, for 2 h at room temperature, the mixture was ultrasonicated. Then, 5% CuO, 1.8626 mL of (doubly distilled) ANI, 0.043256 gm of PPDA and 0.049064 gm of TPA were added. The mixture was ultrasonicated for 30 min. Afterwards, the solution was cooled in an ice bath with continuous stirring at 0-4 °C. Then, 5 g of a pre-cooled solution of ammonium persulfate (APS) dissolved in 0.5 M H 2 SO 4 40 mL and added dropwise into the previous solution in approximately 30 min with consistent stirring at 0-4 °C under nitrogen N 2 atmosphere for 24 h to maintain the polymerization. Next, using ultracentrifugation, the resulting precipitate was collected. Then, it was washed with DIW numerous times until the filtrate became colourless. Finally, the black fine powder was dried at 60 °C for 24 h. This experiment was repeated for comparison; the pure cross-linked PANI was prepared using an identical method without N-OXSWCNTs-GO-CE and CuO NPs.
Batch method for dye removal. Batch experimentation was afforded to obtain the optimum parameters for the degradation of methyl orange in the presence of the nanocomposite of CPA/N-SWCNTS-GO-CE/CuO as the photocatalyst. The effects of significant variables such as the contact time, pH, nanocomposite dose and initial concentration of methyl orange MO in the medium were exhaustively investigated. To clarify the parameters of the effective reaction, we adjusted one variable at a time. Here, in batch condition, the experiments were conducted with 100 mL 10-mg/mL MO solution in a beaker (250 mL). In the dark, the dye solution with a colloidal suspension of the specified nanocomposite was mixed for 10 min with a magnetic stirrer, and the net solution was centrifuged (4400 rpm/15 min). By using a UV-Vis spectrophotometer, the resulting supernatant was examined. The dye degradation rate was studied using the following equation 50 : where R is the efficiency rate of dye removal, C 0 is the MO dye initial concentration (mg/L), and C t is the concentration of MO dye after the adsorption process per time t (min).

Results and discussion
In this study, we aimed to fabricate a nanocomposite of cross-linked CPA with N-SWCNTS-GO-CE and CuO NPs. CPA/N-SWCNTS-GO-CE/CuO nanocomposites were prepared using the previous method, which is the chemical oxidative copolymerization of PANI with p-phenylenediamine PPDA and triphenylamine TPA, but in the presence of N-SWCNTS-GO-CE and CuO. Their morphology, structure, and thermal properties of the nanocomposites were examined via several techniques: SEM, TEM, FTIR, XRD, TGA, DTG, RAMAN and dye removal measurements. The result was as follows.    Fig. 3b show that N-SWCNT-GO-CE nanofillers are instilled in the CPA polymer matrix and almost compose a pulp-shell structure. The black core is the GO nanoparticle, the rods are the SWCNTs; the smooth cortex indicate the polymer matrix. Cellulose nanofibres were decorated on the surface of the mixed NPs and matrix. In Fig. 3c, CuO shows a fibrous form. After N-SWCNTS-GO-CE was mixed with CuO, and CPA, the nanofillers are homogenously distributed and inserted in the CPA polymer, and the mixed N-SWCNTS-GO-CE/CuO are obviously immerged in the CPA polymer matrix (Fig. 3d).    Table 1 displays the disintegration temperatures for various percentages. The temperatures for10, 25 and 50% weight losses are T 10 , T 25 and T 50 , respectively. The TGA curve display slight weight losses in the range of 95-100 °C, which are more than 5% of the entire weight loss. This weight loss is attributed to the loss of solvents and absorption of moisture and/or water molecules 51,52 . The TG curves analysis also shows that the studied samples fundamentally decomposed in three decomposition steps. The decomposition begins early in all samples and remain at higher temperatures with decomposition weight loss percentages below 50%. This notice is completely referred to the presence of air atmosphere. The first decomposition step is fundamentally referred to the complete elimination of solvents 14 . The first step was finish at approximately 150 °C and is attributed to the chloride ions linked to the positive positions on the CPA chain, which referred chain degradation. The second decomposition step, which fundamentally occurred at a higher temperature, started at approximately 350 °C and completed at approximately 600 °C and is referred to additional decomposition of CPA chains to smaller chains 52 . Comparable observations are displayed for the CPA/N-SWCNT-GO-CE, and CPA/N-SWCNTS-GO-CE/CuO nanocomposites with small increases in the high temperatures, i.e., increased thermal stability.
Meanwhile, the addition of mixed N-SWCNTS-GO-CE/CuO increases the thermal stability of CPA, which indicates the presence of intermolecular interactions between nanofillers and polymer matrix. T 10 , T 25 and T 50 show important and gradual increases from CPA (the lowest) to CPA/N-SWCNTS-GO-CE/CuO (the highest). From the DTG analysis, the CDT max amount is defined as the maximum temperature at which decomposition occurs 46 , i.e., the composite degradation temperature. These amounts are fully identical in all inspected nanocomposites and are approximately 492 °C ± 30.  56 . The bands due to the C=O stretch are very prominently observed at 1745 cm -1 for the carboxylated SWNT. Other bands are a tiny one at 3452 cm -1 and another at 2952 cm -1 , which indicate O-H and C-H stretches, respectively. The O-H vibration is related to amorphous carbon because amorphous carbon readily shapes a bond with atmospheric air, and there are C-C vibrations due to the internal disorder. An anti-symmetric stretch C-O is also obvious at 1660 cm -1 . The bands at 1236 cm -1 and 1405 cm -1 indicate C=C 57 . In cellulose, the broad band in the 3602-3110 cm -1 region is due to the OH-stretching vibration. The presence of amorphous cellulosic can be ascertained by the shift of the band from 2910 cm -1 , which indicates the C-H stretching vibration. Furthermore, the FTIR absorption band at 1420 cm -1 is attributed to a symmetric CH 2 bending vibration. The FTIR band at 896 cm -1 is attributed to C-O-C stretching 57 . The FT-IR spectra of nanocomposites also display distinguishing bands of GO. Broad bands at1732, 1624, and 3300-3615 cm −1 are referred to the C = O stretch of the carboxylic acid group, C=C, and O-H, respectively. The bands at 1226 cm −1 refer to the epoxy (C-O) ring stretching, and the bands at 642 cm −1 refer to the symmetric ring disfigurement of the epoxy group on the GO nanofillers 15 . In the nanocomposites, there is no remarkable signal for CuO, since the assigned CuO is too small to be detected by FT-IR. XRD analysis. The expected shape of the prepared nanocomposites has been studied using X-ray diffraction techniques. The XRD characterization styles give a clear evidence for the composite fabrication. XRD diffractograms for CPA, N-SWCNT-GO-CE, CuO, CPA/N-SWCNT-GO-CE and CPA/N-SWCNT-GO-CE/CuO nanocomposites are shown in Fig. 6 over the measuring range of 2θ = 10-80°.CPA and N-SWCNT-GO-CE/ CuO nanocomposites physically react with each other via the nanocomposite production. The 2θ scan displays the perfect X-ray diffraction patterns for CPA and mixed N-SWCNT-GO-CE/CuO. Figure 6 displays the XRD pattern for mixed N-SWCNT-GO-CE. Three intense peaks at 49°, 33°, and 31°can be ascribed to the crystalline region of N-SWCNTs 58 . Other distinguishing peaks near 30-40° refer to amorphous cellulose 59 . The diffraction peak with a maximum at approximately25° displays the distinguishing composition of GO 60 . For the CuO sample, sharp intense peaks at 2θ of 35.8° and 39.1° conformable to the (111) plane were noticed with other less intense peaks characteristic of CuO.
However, the copolymer CPA displays the XRD patterns with two broad diffraction peaks at 2θ = 18.4° and 26.4°, which are typical of an amorphous substance 58,60 and as illustrated in Fig. 6. These patterns demonstrate a

Mechanism of polymerization.
According to the investigated results, a suggested mechanismto construct such modern hybrid nanocomposite is proposed to illustrate the fabrication method of CPA/N-SWCNTs-GO-CE/CuO nanocomposite (Fig. 8). The CPA/N-SWCNTs-GO-CE/CuO nanocomposite was synthesized by the softcopolymerization procedures using the chemical oxidative technique. The polymerization occurs in the presence of ANI with TPA and PPDA as cross-linkers with N-SWCNTs-GO-CE in an acidic medium. In the acidic environment, the -COOH groups on the surface of N-SWCNTs-GO-CE becomebe protonated, i.e., they gain H + from the medium 65  The linear poly(ANI-PPDA) oligomer cationic radicals continue to react with TPA cationic radicals after oxidation tocompose a CPA network by polymerization at the three N para-positions due to similar reaction activities. Electrostatic interactions occur between cationic radicals withanions absorbed on both N-SWCNTs-GO-CE and CuO surface. Furthermore, it is conceivable that three types of hydrogen bonding occur: between chains of CPA and oxygen atoms on the N-SWCNTs-GO-CE surface,among the chains of CPA in the nanocomposite of CPA/N-SWCNTs-GO-CE/CuO, and between chains of CPA and oxygen atoms on the CuO surface. Finally, the π-π stacking between the π bonds of N-SWCNTs-GO-CE and the aromatic rings of CPA stabilizes the bound complex structure of the CPA/N-SWCNTs-GO-CE/CuO nanocomposite. The interactions can confirm that N-SWCNTs-GO-CE and CuO are inserted to produce core-shell structures and CPA chains [66][67][68][69][70] . Such above mentioned mechanism enhance the proposed charge transfer due to the use of such fabricated nanocomposites. The reported nanocomposited based on cross-linked PANI and coated with cellulose in the presence of mixed N-SWCNTs-GO nano-filler. In addition to the existence of CuO as well.
UV-Vis spectroscopic measurement for MO dye removal and photocatalytic degradation. MO has dual absorbance peaks at 277 and 466 nm. This dye shows high stability with distinguished absorption peaks in the absence of CPA/N-SWCNTs-GO-CE/CuO nanocatalyst. The rate of decolorization is approximately zero, and the photo-degradation doesnot entirely occur (see Fig. 9a). By using the CPA/N-SWC-NTs-GO-CE/CuO nanocatalyst, we observe a substantial decrease in the main absorption peaks at 277 and 466. These changes are due tothe extensive mineralization of MO,which comprises the phenyl ring degradation. Furthermore, a considerable quenching at 466 nm occurs due to bleaching, which involves the azo bond cleavage (Fig. 9b). For additional investigation, the optical behaviour of the nanocatalyst-based CuO was examined using three conditions: under UV radiation, in the presence of (CPA/N-SWCNTs-GO-CE) and the combination of CPA/N-SWCNTs-GO-CE/CuO nanocatalyst under UV. The data analysis provides the photobleaching and degradation of the MO in the presence of the CPA/N-SWCNTs-GO-CE/CuO nanocatalyst under UV in the maximum time of 100 min. Moreover, the bleaching and photodegradation processes of MO completely disappear in the absence of the CPA/N-SWCNTs-GO-CE/CuO nanocatalyst. The results show that the adsorbability of MO on the nanocomposite surface does not change within 6 min after stirring the solution in the dark, and no further degradation occurs. Thus, with all conditions to investigate the adsorbability balance, the MO molecules and nanocomposite were stirred for approximately 10 min without light sources. Furthermore, Fig. 10 shows a smart summarized overview for the photocatalytic degradation process.
Effect of the solution pH on the catalytic process. The photocatalytic experimentation of the pH effect was investigated at a range of pH 4-10 using 50 mg of nanocomposite in 100 mL MO dye solution of 10 mg/L concentration at adjusted contact time. Moreover, different pH solutions were obtained using standard solutions of hydrochloric acid and sodium hydroxide. Figure 11 shows the pH effect on the proficiency of MO degradation and bleaching. The highest degradation and bleaching rates are detected at pH 6 (Fig. 11). These pH significances show that the degradation percentage increases with increasing pH up to 6; then, itbegins to quench. Generally, the pH effect on the degradation depending on the nanocomposite has been assorted with the establishment of acid-base equilibria to monitor the chemical behaviour of the nanocomposite surface 71,72 . Therefore, all following experiments were performed at pH 6. The pH effect on the adsorption and removal proficiency can be ascribed to the dissimilarity of Coulomb interactions between the CuO-based nanocomposite surfaces and MO.
Additionally, MO has a pKa value of approximately 3.8 75 . In excess of this pKa and with increased pH, MO provides anions, which make it more easily bind to the surface of the CuO-nanocomposite. Thus, when the pH of the medium increases to 6, the adsorbability and removal efficiencies increase. At pH > 6, the bleaching and degradation were enhanced, which can be attributed to the quenching of the oxidation potential of hydroxyl radical due to increased pH 76 . However, the increase in hydroxyl ions at pH > 6 fights with MO anions at the nanocomposite surface 77 .   Fig. 12. The data emphasize that the bleaching and degradation efficiencies increased with increasing nanocomposite content to 50 mg/L, exhibited no notable alteration when the nanocomposite weight increased to 100 mg/L, and extinguished when the nanocomposite quantity was 200 mg/L. This behaviour can be assigned to the presence of active sites, which increased with the increase in nanocomposite quantity. Consequently, the produced hydroxyl radicals enhanced the photocatalytic proficiency of the CuO-nanocomposite. With excess nanocomposite, the presence of a viscous colloid medium prevents UV light from penetrating the nanocomposite surface. Therefore, this effect inhibits the production of hydroxyl radicals and decreases the adeptness of MO deterioration and staining of the medium 78,79 . Thus, the nanocomposite quantity was adjusted to be 50 mg/L for further experimentation.

Effect of C i of MO.
To investigate the effect of MO dye on the degradation process, MO concentrations were utilized in the range of 5-20 mg/L throughout a reaction time of 100 min at pH of 6. In addition, the amount of nanocomposite is 50 mg/L. The results are shown in Fig. 13. The obtained data demonstrate that the bleaching and degradation decreased with increasing C i of MO. This result can be related to the decrease in number of active sites of the CPA/N-SWCNTs-GO-CE/CuO nanocatalyst surface. Thus, the introduction of hydroxyl radicals quenches and reduces the competence of the photo-catalytic reaction. Meanwhile, the increase in C i of MO reduces the photon path length that diffuses through the MO medium. With more MO molecules, the dye can absorb significantlymore light than the CPA/N-SWCNTs-GO-CE/CuO nanocomposite, which reduces the competence of the photo-catalytic degradation process 80,81 . Thus, the suitable C i of MO is 10 mg/L. The studied solution was conducted to air and nitrogen atmosphere to study the oxygen effect on the degradation process. The experimentations were accomplished by monitoring several parameters: time of 100 min, pH 6, C i of MO 10 mg/mL, CPA/N-SWCNTs-GO-CE/CuO nanocomposite concentration of 50 mg/mL, and the solution was exposed to nitrogen gas for 2.5 min. Figure 14 shows the oxygen gas effect on the rates of bleaching and degradation. Furthermore, the analysis of the obtained results indicates that the adequacy of bleaching and degradation enhances with oxygen. This resultcan be attributed to the introduction of reactive species including hydroxyl radicals, oxygen radical and hydrogen peroxide 82 . Thus, the resulting radicals increase the efficiency of bleaching and degradation of MO.
The proposed photocatalytic degradation mechanism. Schematic illustration explains the expected photocatalytic degradation mechanism for CPA/N-SWCNTs-GO-CE/CuO nanocomposite against MO dye has been given shortly in Fig. 15. Such photocatalytic decomposition of the azo dye on the CuO nanoparticles doped CPA/N-SWCNTS-GO-CE nanocatalysts under the UV radiation is presented in the following Fig. 15. Typically, the UV-radiation induces the valance band electrons of the CuO nanocatalysts convey to the conduction band. This equivalent energy is higher than the band gap of the doped CuO nanoparticles (2.43 eV) 83   Reversibility of the CPA/N-SWCNTs-GO-CE/CuO nanocatalyst. The reversibility of the CPA/N-SWCNTs-GO-CE/CuO nanocatalyst has a noteworthy effect from the economic viewpoint. Therefore, it is essential to study the competence of the nanocatalyst throughout the recovery process. Thus, the used CPA/N-SWCNTs-GO-CE/ CuO nanocomposite was collected after the bleaching and degradation processes using a centrifuge with an adapted rate of 4400 rpm. Then, the centrifuged nanocatalyst yield was washed three times with ethanol and bi-distilled water. The separated nanocatalyst was dried and reused in more processes as shown in Fig. 16. The effectiveness of the MO bleaching and degradation reduce with the amount of recovered CPA/N-SWCNTs-GO-CE/CuO catalysts. In addition, the quenching of the degradation rate is elevated than the bleaching process.
Furthermore, comparable studies of using different adsorbent against methyl orange removal at the optimum conditions have been given in Table 2. The results confirm the extreme higher efficiency of CPA/N-SWCNTs-GO-CE/CuO nanocomposite compared to the other reported materials.

Conclusion
In this work, a novel CPA/N-SWCNTS-GO-CE/CuO nanocomposite was successfully prepared via an oxidative chemical polymerization method. The core-shell structure was clearly visualized through TEM, SEM, XRD, FT-IR and RAMAN studies. These synthesized nanocomposites were utilized to remove a hazardous dye (MO) and found to be highly efficient in its removal. Various parameters that affect the adsorption process, including C i of MO, pH and dosage of CuO-based nanocomposite, were optimized. In addition, the oxygen content has a significant effect on the degradation process. The obtained data clarified that the dye was successfully degraded in the presence of CPA/N-SWCNTS-GO-CE/CuO compared to CPA/N-SWCNTs-GO-CE under UV radiation, which indicates the enhanced effect of CuO as a reinforced agent. The degradation process was investigated under the optimal conditions of pH of 6, 50 mg/L of CPA/N-SWCNTS-GO-CE/CuO photocatalyst, C i of MO of 10 mg/L and under stirring at room temperature in atmosphere. This high efficiency of the CuO NP-based nanocomposite as a photocatalyst may provide a promising application for the degradation of dyes from aqueous solutions. In addition, the CPA/N-SWCNTS-GO-CE/CuO catalyst exhibits significant reversibility and highly www.nature.com/scientificreports/ recovered by effortless methods. This CuO-based photocatalyst provide an effective procedure with considerable effect in the wastewater treatment from dye pollution. Therefore, the present study has shown a simple and facile way for the synthesis of Cu NPs for the treatment of wastewater problems.