Synergistic ZnFe2O4-carbon allotropes nanocomposite photocatalyst for norfloxacin degradation and Cr (VI) reduction

Journal of Colloid and Interface Science 544 (2019) 96–111

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Synergistic ZnFe2O4-carbon allotropes nanocomposite photocatalyst for norfloxacin degradation and Cr (VI) reduction Arjun Behera, Sriram Mansingh, Kundan Kumar Das, Kulamani Parida ⇑ Centre for Nano Science and Nano Technology, Institute of Technical Education and Research, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar 751030, India

g r a p h i c a l a b s t r a c t Synergistic interaction of one dimensional CNTs with cubic spinel ZFO towards degradation of Norfloxacin and reduction of Chromium under open sun light.

a r t i c l e

i n f o

Article history: Received 28 November 2018 Revised 11 February 2019 Accepted 17 February 2019 Available online 23 February 2019 Keywords: ZFO@Carbon allotropes (CNT, Fullerene, GO) nanocomposites Photocatalytic activity Reusability and degradation of norfloxacin

a b s t r a c t Development of highly efficient robust catalyst for pollutant abetment still remains an ongoing scientific challenge in the field of visible light driven photocatalysis. In this work a series of ZnFe2O4 (ZFO)/carbon derivatives (ZFO@CNT, ZFO@GO, ZFO@Fullerene) nanocomposite were fabricated by one-pot hydrothermal method followed by calcination. The detail anatomical featured such as crystal geometry, morphology, elemental composition, light absorption performance, electron-hole recombination properties and photocurrent density were characterized by XRD, SEM, HRTEM, XPS, UV–Vis DRS, PL and electrochemical analysis respectively. The photocatalytic performances of ZFO@carbon nanocomposites were studied for the degradation of antibiotics (Norfloxacin) and hexavalent Chromium under open sun light illumination and the obtained results suggested that loading of carbon derivatives of ZFO nanoparticles enhance the visible light absorption capacity and excitation separation efficiency. Among the fabricated composites, ZFO@CNT exhibits the highest activity in comparison to other nanocomposites. The highest activity of ZFO@CNT is due to low photoexcited electron-hole recombination and high charge transfer properties of ZFO@CNT as confirmed via PL and impedance measurement. Further, the fabricated ZFO@CNT nanocomposite exhibited highest photocurrent density i.e. 2.25 mA/cm2 which was 225 times higher than that of neat ZFO. The optimal photocatalytic efficiency was shown by ZFO@CNT i.e. 91.36% degradation of 50 ppm norfloxacin and 82% reduction of 10 ppm Cr (VI) in 90 min and 60 min respectively under solar light irradiation. Ó 2019 Published by Elsevier Inc.

⇑ Corresponding author. E-mail address: [email protected] (K. Parida). https://doi.org/10.1016/j.jcis.2019.02.056 0021-9797/Ó 2019 Published by Elsevier Inc.

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1. Introduction The smooth sustainability of our future generation is under a big question mark due to the current growing environmental pollution level of the blue planet ‘Earth’. All three subsystems of the habitable planet i.e. atmosphere, lithosphere and hydrosphere are heavily contaminated with both organic and inorganic pollutants ranging from life-saving antibiotic drugs to the building block element constituting living organism. To neutralize the outcome of this catastrophic phenomenon, trained brains of the whole world trying their best knowledge in this direction. In this respect number of techniques were implemented, out of which photocatalysis driven by semiconducting materials proves to the most demanding one due to their eco-friendly nature [1–4]. In the year 1972 Fujishima and Honda published the photoelectrocatalytic behaviour of TiO2 towards water splitting reaction producing green fuel H2 [5]. However, TiO2 has the ability to harvest UV-light only because of its wide band gap energy i.e. 3.2 eV. In order to absorb a broad range of solar spectrum including UV–Vis light, several semiconductor based transition metal oxides having the electronic configuration d0 to d10, metal nitrides, sulfides, and oxy-nitrides etc. Among all, transition metal ferrite are the show stoppers in the field of photocatalysis, due to its unique absorption property, narrow band gap energy, specific magnetic properties, high thermal conductivity, low fabrication cost and good chemical stability [6]. Above all, its photocatalytic potential is still not upto the mark due to some restricted access together with its unstable nature under powerful reducing condition, reduce conductivity, more photogenerated electron-hole recombination ability etc. These drawbacks have been shutout to a marginal extent by some additional modification skills i.e. forming composites with metal oxides (TiO2 [7], NiO [8], MnO [9]) polymer [10], noble metals (Ag) [11], carbon materials (GO [12], CNT [13], Fullerene, g-C3N4 [14]) and layered materials etc. Out of which ZnFe2O4 (ZFO) combined with carbonious materials shows remarkable photocatalytic activity due to easy of electron migration, enhance light absorption capacity and better exciton separation. Therefore, the theme of my work is to extend its visible light photocatalytic performance to realize the indoor application of the photocatalyst. Now carbon allotropy materials (GR, CNT and Fullerene) are attracted much more interest in the field of photocatalysis because of some exceptional properties such as excellent p skeleton for better electron mobility/separation, formation with good complexation with other materials, large surface area, high thermal conductivity and more optical absorption capacity in the solar spectrum [15]. Taking the above highlighted advantageous features into account many metal oxide-carbon based nanocomposites were fabricated with increase visible light absorptivity and longer lifespan of exciton pairs [16]. In particular, many scientific research state that the carbon derivatives can act as an electron reservoir which helps to trap photoexcited electrons from semiconductors, there by enhancement of lifespan of the excitons, which is a most important factor to improve the photocatalytic activity of semiconductor-carbon nanocomposites [17]. In this regard, Zhang et al. synthesized Pd-Decorated ZnO–Graphene Oxide Nanocomposite, that shows 93% 10 ppm MB degradation within 1 h [18]. Sohail and co-workers have preparedTiO2/ CNTs@CoFe2O4 nanocomposites by high-cost CVD technique and it displays 96% degradation of methylene blue after 90 min [19]. Likewise, Wu and co-authors fabricated ternary Ag/ZnO/ZnFe2O4 nanocomposites by facile calcinations process, which exhibit excellent photodegradation of 10 ppm MB around 93% within 1 h 40 min [20]. Further, Liang et al. synthesized Reduced Graphene Oxide Anchored Magnetic ZnFe2O4 nanoparticles and observed a 99.5% photo degradation of 12 ppm MB after 300 min [21]. Moreover, Movahedi et al. prepared ZnFe2O4 nanoparticles by calcinations

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methods that depict a photodegradation percentage of 98 for 20 ppm congo-red in about 3 h of irradiation [22]. In another report, Pardeep Singh and co-workers modified graphene oxide incorporated ZnFe2O4 as supermagnetic photocatalyst for mineralization of 90% antibiotics(Oxytetracycline) after 10 h [23]. In this study, we have prepared a series of carbon allotrope (CNT, GO, Fullerene) modified ZnFe2O4 nanoparticles by a facile one-step hydrothermal technique followed by calcination method. The phase purity and morphological modifications of the synthesized photocatalysts were analyzed by XRD, TEM and FESEM study. The existence of the carbon derivatives i.e. graphite-like carbon and the electronic environment was proved by FTIR spectra, Raman and XPS analysis. Additionally, the effective charge separation and migration phenomenon are well demonstrated through PL, photocurrent density and impedance study. Further, the fabricated photocatalysts (ZnFe2O4-carbon nanocomposite) were exposed towards photodegradation of antibiotic norfloxacin under visible light illuminations (k > 420 nm) to finding out the superior ZFOcarbon allotrope composite among the prepared ones. The present investigation may open up a new avenue towards carbon allotrope-metal oxide oriented photocatalytic systems for environmental sustainability. 2. Materials and methods Fe(NO3)3
9H2O (purity-98%, Merck), Zn(NO3)2
6H2O (purity98%, LobaChemie), Fullerene powder (99.5%, Alfa Aesar) and Carbon Nano Tube (95%, Alfa Aesar) were used without purification for the synthesis of the zinc ferrite@carban derivatives. 2.1. Synthesis of ZnFe2O4 and ZnFe2O4@GO, CNT, fullerene by hydrothermal followed by calcination methods In this experimental procedure, 10 mmol of Fe(NO3)3
9H2O and 5 mmol of Zn(NO3)2
6H2O were jointly added to 30 ml of deionized water and subjected to stirring for 20 min. The pH of the mixture was adjusted to 13 by adding 6 M NaOH solutions. Finally, the required amount of carbon derivatives (GO, CNT, Fullerene) were added to the obtained mixture and stirred for another 30 min. The above-mixed suspension was transferred to a Teflon coated autoclave and treated at 180 C for 13 h. The obtained product was collected followed by centrifugation and washed various times with DI water and ethanol, then dried at 70 C for 24 h. The crystalline solids was collected and grounded well with the help of a mortar and pestle. Finally, the sample was annealed at 400 °C for 2 h in a muffle furnace. The obtained Calcined product was designated as ZFO, ZFO@GO, ZFO@CNT and ZFO@Fullerene respectively. 2.2. Photocatalytic degradation process The photocatalytic performance of synthesized materials was evaluated towards photo-degradation of antibiotics (norfloxacin) under sunlight illumination for 90 min. The photodegradation activity of antibiotics was investigated with time under solar irradiation to evaluate the photoactivity of different nanocomposites. The average solar light intensity during the photo chemical reaction is 100,000 lx and the reaction is carried out under the broad range of solar light. Under the experimental condition, 0.02 g of ZFO and its composite with carbon derivative materials were mixed to 20 ml of 50 ppm norfloxacin. The obtained suspension was kept under a dark condition with stirring for 30 min to develop adsorption-desorption equilibrium. After that, the mixed solution was exposed to sunlight illumination. The solution was then analyzed using a UV–VIS spectrometer (JASCO 750).

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2.3. Analytical characterization The XRD pattern of the photocatalyst was analyzed by Rigaku Miniflex instrument using Cu Ka radiation (k = 1.54178 Å, k = 1.54056) to know the phase purity and crystallinity of the materials. The optical properties and light harvesting nature of the prepared ZFO materials were analyzed by a UV–Vis JASCO 750 instrument. JASCO-FP-8300 fluorescence spectrometer was used to investigate the photo-luminescence characteristics with an excitation wavelength of 330 nm. FT-IR spectra analyses were performed to determine the chemical composition of ZFO samples in-between the wave number range of 4000–400 cm1 using JASCO FTIR-4600. Raman polarization effect of the prepared photocatalyst was measured through RENISWAW InVia Raman spectrometer fitted with a 532 nm laser. Surface morphology and topology of the synthesized materials were carried out by scanning electron microscopy (SEM) with a voltage of 5 kV and transmission electron microscopy (TEM) with a high voltage of 200 kV. XPS measurements were executed by using VG Microtech Multilab ESCA 3000 spectrometer to determine the elemental environment and oxidation states of the synthesized photocatalyst. The electrochemical studies of the samples were done by using Iviumn potentiostat. Xe lamp (300w) was used as a light source. 2.4. Electrochemical study Photoelectrochemical performance of synthesized semiconductor is carried out by using three electrode system i.e. counter electrode, a reference electrode, and the working electrode and 300 Xe lamp act as the light source. The photocatalyst acts as the working electrode and it is formed by using the FTO substrate. In the experimental analysis, 0.1 M Na2SO4 used as the electrolyte for the measurements of the photocurrent. 3. Results and discussion 3.1. X-ray diffraction (XRD) study Crystallinity and phase transparency of synthesized material was investigated by XRD analysis. Fig. 1 shows the XRD patterns of pure ZFO and ZFO-Carbon derivative(CNT, Fullerene, GO) nanocomposites. The pure ZFO shows cubic crystal structure hav-

Fig. 1. X-ray diffraction patterns of ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT.

ing diffraction peaks located at 2h = (31.8, 35.5, 42.7, 47.5, 56.6, 62.8, 68.1) representing lattice planes such as (2 2 0), (3 1 1), (4 0 0), (3 3 1), (5 1 1), (4 4 0) and (4 4 2) respectively and the obtained data matches well with JCPDS file no. (01-077-0011). Characteristic XRD peaks of carbon allotropic materials (GO, CNT and Fullerene) are absent in all studied ZFO-Carbon composites, which is attributed to low content and subsequently low peak intensity of carbon derivative samples in the composites. From Fig. 1 it can be clearly seen that the diffraction peaks intensity of ZFO is more in the composite as compared to neat because carbon materials favour the growth of ZFO crystals. Further intense XRD peaks imply a high degree of crystallinity in the composites in reference to neat ZFO. The Scherer formula i.e. D = 0.89k/(b.cosh) was used to evaluate the crystallite size of ZFO where D stands for crystallite size, b as FWHM, k is the wavelength of X-ray and h is the Bragg diffraction angle respectively [24]. The calculated average crystallite size of ZFO particles in the prepared ZFO@Carbon allotrope hybrids was in the range of 30–50 nm that shows good correlation with TEM result. 3.2. SEM SEM measurements were carried out in-order to explore the surface morphology of synthesized samples. Fig. 2 depicts the SEM picture of pure ZFO and ZFO@CNT. It is observed that ZFO nanoparticles were found to be in the agglomerated form both in ZFO and ZFO@CNT. The agglomeration of ZFO nanoparticles may be due to the formation of small crystallites and magnetic property [6]. On magnification, it is found that ZFO exhibits certain spongy type properties as shown in Fig. 2b. Further Fig. 2c and d show the SEM images of ZFO@CNT under different scanning magnification. It can be seen from Fig. 2d that CNTs were well dispersed within ZFO but in an irregular manner leading to a strong interaction between the CNTs and agglomerated ZFO. Significantly the CNTs are twisted around the ZFO nanoparticles which are beneficial for charge separation and transmission that leads to increase in photocatalytic activity as described later part. 3.3. TEM & HRTEM TEM and HRTEM analysis were executed in order to investigate the internal crystal arrangement and topology of synthesized pure ZFO and its respective composites as shown in Fig. 3. The figure reveals that the particle sizes of pristine zinc ferrite are big, nonuniformly shaped and most of the particle remains or found to be in an agglomerated form. These images clearly indicate that ZFO particles are approximate of 30–40 nm in size [6]. High crystallinity and lattice fringes of ZFO nanoparticle can be seen from the HRTEM images with an interlayer d-spacing value of 0.29 nm and 0.25 nm matching to (2 2 0), (3 1 1) crystal planes of the spinel ferrite [6]. The crystalline nature and respective crystal planes of ZFO can be further seen in selected-area electronic diffraction (SAED) pattern. The concentric rings highlighting the polycrystalline feature related to (2 2 0) and (3 1 1) planes of pure zinc ferrite as shown in Fig. S1. Moreover, TEM and HRTEM images of ZFO@GO (Fig. S2) depict that ZFO nanoparticles were uniformly distributed over the GO sheet. Maximum portion of graphene sheet was covered by the ferrite nanoparticles. The HRTEM and SAED pattern exhibits the same type of features as observed for ZFO i.e. lattices fringes of 0.29 nm and 0.25 nm corresponding to (2 2 0) and (3 1 1) crystal planes respectively that explains about the retention of crystalline nature of ZFO nanoparticles that are well dispersed over GO sheet. Further Fig. S3 is the TEM images of ZFO@Fullerene. ZFO nanoparticles with uniform particle size were distributed over Fullerenes. The particle

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Fig. 2. (a, c) SEM images of ZFO and ZFO@CNT (b, d) Enlarge view of ZFO and ZFO@CNT nanocomposites.

Fig. 3. TEM and HRTEM images of (a, b) ZFO@CNT nanocomposites (c) Lattice fringes of ZFO and CNT in the nanocomposite and (d) SAED pattern of (3 1 1) and (2 2 0) plane of ZFO coincide with XRD results.

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size of Fullerene has calculated approximately around 70–85 nm [25]. Large and irregular aggregated fullerene particles were observed from the Fig. S3 TEM image confirms the presence of ferrite nanoparticles on the fullerenes particle. HRTEM and SAED analysis again confirms the high crystallinity of ZFO in the composite. And the characteristic d-spacing with corresponding planes of ZFO are shown in Fig. S3. Additionally, TEM images of ZFO@CNT as depicted in Fig. 3(a) and (b) highlights good interaction between CNT and ZFO particles. The ZFO particles were found to be of uniform size and in an aggregated form which has been well distributed over the nanotubes. The particles may have been aggregated to give small rectangular like structure. It has been observed that some ZFO nanoparticles were firmly anchored to the side wall of CNTs. The reason behind such strong interaction between ZFO and CNTs may be due to strong electrostatic attraction and chemical bonding of ZFO particles with the functional groups that are present on the surface of CNTs [26]. The HRTEM image of ZFO@CNT composite shows clear and distinct crystal fringes with an inter-planar distance of 0.29 nm and 0.25 nm which agrees well with (2 2 0), (3 1 1) facets of cubic ZFO. In addition, the SAED pattern confirms that ZFO@CNT exhibits a polycrystalline structure. The most occurring planes (2 2 0) and (3 1 1) of ZFO were observed from HRTEM image and from the SAED pattern for every synthesized composite as well as for pure ZFO which confirms about the crystallinity nature of the samples. From the SEM and TEM characterization, it was concluded that there is a good interfacial interaction between CNT and ZFO which helps in charge separation and transfer of charge carriers between ZFO@CNT composite. The process of charge separation and charge transfer across the interface plays an important role in improving the photocatalytic performance [27].

3.4. XPS and EDX The electronic arrangement, oxidation states and elemental interaction of the prepared ZFO@CNT were further investigated by X-ray photoelectron spectroscopic characterization. All of the binding energies in XPS study are accurate for specimen charging by reflecting them to the C 1s peak (set at 284.6 eV). XPS spectra of O 1s, C 1s, Fe 2p, and Zn 2p were shown in Fig. 4. Fig. 4a exhibits a survey scan of prepared photocatalyst which confirms the existence of C, O, Zn and Fe elements in the nanocomposites. Fig. 4b depicts the carbon (C1s) peak of ZFO@CNT, showing three carbon derivatives with binding energies 284.58 eV (C@C/CAC of aromatic rings), 285.01 eV (CAO of alkoxy) and 288.51 eV (OAC@O groups) respectively. All peaks of C1s are prominent and clearly visible [28]. Binding energies along with respective fitted peaks of O1s spectrum is shown in Fig. 4c. The peak centered at 529.32 eV can be ascribed towards oxygen present within the lattice of zinc ferrite. Likewise, peak at 530.19 eV reflects FeAOAC bond, providing information about the good interaction between Fe and carbon groups present in ZFO@CNT [28–30]. Whereas peak centered at 531.12 eV can be assigned to oxygen of C@O and that positioned at 532.75 eV stands for singly bonded oxygen of CAO group. The given peaks of oxygen spectrum primarily arise from ZFO and leftover oxygen groups from CNT [28]. Fig. 4d highlights two main peaks for Fe 2p at binding energies 711.06 eV (Fe 2p3/2) and 725.55 eV (Fe 2p1/2) respectively and one satellite peak at 718.79 eV [31]. Further, Fe 2p3/2 spin state is deconvoluted into two peaks in order to explain the detailed characteristic feature of Fe (III) oxidation state in the composite. The obtained deconvoluted peaks at band energies 712.79 eV and 711.06 eV are due to octahedral and tetrahedral site respectively [28]. From the XPS spectra of

Fig. 4. X-ray photoelectron spectroscopy of (a) survey scan of ZFO@CNT nanocomposites (b) C, (c) O, (d) Fe and (e) Zn.

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Fe 2p, it was concluded that the oxidation state of Fe did not change from III to II state even after formation of composite ZFO@CNT. This can be further explained, as the binding energy peak for Fe (II) oxidation state is about 709.5 eV which is well below the binding energies of our multiple Fe (III) oxidation state [32]. Moreover Fig. 4e displays the deconvoluted XPS spectrum of Zn 2p. The band energies of Zn 2p3/2 and Zn 2p1/2 are found to be at 1021.18 eV and 1044.98 eV respectively which informs about the existence of zinc in Zn(II) oxidation state within the composite. Narrow and sharp peaks indicate the tetrahedral type of coordination of zinc in the ZFO [32]. Additionally, the presence of elements such as Zn, Fe, O and C were further confirmed from the EDX mapping analysis of ZFO@CNT nanocomposite (Fig. S4) which matches with the XPS survey scan. 3.5. UV–Vis DRS analysis UV  visible diffuse reflectance spectra (DRS) were obtained to explain the optical behaviour and light-absorbing capacity of the

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prepared photocatalysts, as shown in Fig. 5(a) and (b). The optical property of ZFO exhibits a broad absorption range extending from UV to visible region. The band gap energy of neat ZFO is calculated by the following equations.

ahm ¼ Aðhm  Eg Þ1=2 where Eg, m, a and A are the band gap energy, light frequency, absorption co-efficient and A is the constant [33]. From Fig. 5c the direct band gap energy of ZFO is around 1.8 eV. After the addition of carbon derivatives (GO, CNT and Fullerene), the composite reflects stronger absorption peak in both UV and visible light regions. The cause of such behaviour is attributed to black body property of carbon derivatives. However the formed composite of ZFO with GO and fullerene shows a decrease in light absorbance in comparison to neat ZFO, this is because of GO and Fullerene agglomeration that covers the active sites of the neat materials. Interestingly ZFO@CNT displays a red shift with broad light absorbance range and hence behaves as good photocatalyst for the performed photocatalytic reaction.

Fig. 5. (a) Diffuse reflectance spectra of ZFO, ZFO@GO, ZFO@Fullereneand ZFO@CNT nanocomposites, (b) Absorbance plot of neat ZFO photocatalyst (c) Tauc plot of pure ZFO.

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3.6. PL analysis Photoluminescence spectroscopy (PL) is a sophisticated and nondestructive characterization method, frequently availed in photocatalysis chemistry to explore optical, electronic and photochemical features of semiconducting materials. A PL spectrum gives a broad idea about photoinduced electron-hole migration, transfer and charge carrier trapping efficiency of semiconducting materials. Fig. 6 shows the PL of ZFO, ZFO@GO, ZFO@CNT, ZFO@Fullerene. PL emission peaks arise due to recombination of excitons. In the current study, the materials were excited at 330 nm and the corresponding emission peak are observed at 410 nm. According to the addition of different carbonaceous materials i.e. GO, CNT, Fullerene on ZFO samples, a clear emission quenching process was observed i.e. decrease in PL peak intensity which indicates a direct and strong interaction between ZFO and carbon materials. The formed composite exhibits better charge separation compared to neat ZFO because the photoexcited electrons are trapped and channelized in the p skeleton of the added carbon precursor causing better separation. It is clearly observed that CNT modified ZFO shows very low PL signal intensity suggesting delay electron-hole recombination and more charge transfer properties. Further, the above claim manifest good correlation with EIS and photocatalytic result discussed in respective sections. In summary, PL quenching takes place due to segregation of energetic electrons onto GO, Fullerene and CNT framework, indicating the inhibition of photoinduced electron-hole pairing after the combination of ZFO semiconductors [34]. But among all the above-mentioned samples the recombination process is exceptionally low in ZFO@CNTs and hence exhibits good photocatalytic activity in comparison to others. 3.7. FTIR analysis FTIR analysis was used to confirm the functionalization of the synthesized photocatalyst. Fig. 7 shows the FTIR spectra of ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT nanocomposites. The commonly observed characteristic peaks around 1650–1550 cm1 and 3600–3300 cm1 for ZFO represents the bending and stretching band of surface absorbed hydroxyl groups [35]. Further, it was found that vibrational bands located at 1718, 1030, and

Fig. 7. FTIR images of ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT nanocomposites.

1202 cm1 stand for C@O, CAOH and CAOAC, oxygenated functional groups of GO. The IR peak observed at 2350 cm1 depicts the anti-symmetric mode of suspending carbon dioxide, which is common in all the samples [36]. The vibration trough in the region of 590–540 cm1 is due to the symmetric stretching motion of MAO bonds (ZnAO and FeAO) which confirm the existence of ZFO in all prepared nanomaterials. Apart from common stretching and vibration modes, the detail of all obtained FTIR bands for the nanocomposites was discussed as follow. IR spectra of ZFO@GO exhibits two characteristics peaks around 1247 cm1 and 909 cm1 which corresponds to CAO stretching of epoxy and peroxide group respectively [37]. The availability of these stretching confirms the existence of GO within the composite. Similarly, in the case of ZFO@fullerene, weak peaks can be seen for fullerene at 1410 cm1 and 1194 cm1. These peaks can be assigned to the tangential movement of carbon atoms [38]. Moreover, the IR bond vibration of CNT displays a weak peak at 1100 cm1 which can be related to CAO stretching bonds [39]. The intensity of these observed characteristic peaks for GO, Fullerene and CNT are not clearly visible which can be due to following reasons. First of all, low amount of these carbon allotropes and second reason can be attributed to well dispersion of allotropes during the composite formation. The common peaks have been shifted in composites as compared to pristine ZFO which confirms the interaction between ZFO and respective carbon allotropes. 3.8. Raman study

Fig. 6. Photoluminescence Spectra ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT nanocomposite excited with a wavelength of 330 nm.

Raman polarization is employed to confirm the formation of composite between ZFO and CNT entity, highlighting characteristic interaction and defect within the formed composite. Figure displays the Raman images of pure ZFO and ZFO@CNT photocatalyst. ZFO belongs to cubic ferrite family with Fd3m space group having eight formula units per cell. According to group theory, five Raman bands are generally observed for pristine ZFO [40]. From the obtained Raman spectra, it is observed that ZFO exhibits four Raman modes at 340, 488, 636 and 1057 cm1 which can be attributed to symmetric vibrational states of cubic spinel [41]. Additionally, the above highlighted Raman band of ZFO above and below 600 cm1 signifies the presence of tetrahedral AO4 and octahedral BO6 groups respectively [42]. In case of composite ZFO@CNT, three additional peaks are located at 1353, 1580 and 2686 cm1 which is due to the

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Fig. 8. Raman spectra of neat ZFO and ZFO@CNT nanocomposites.

presence of CNTs along with the presence of 317, 476 and 631 cm1 for ZFO. The signature peak located at 1350 cm1 can be identified as ‘‘D band” which may be due to the presence of any local defects or disorder in CNT p-skeleton. Similarly, the Raman peak at 1580 cm1 is identified as ‘‘G band” and this arises because of sp2 moiety present within CNT [43]. The observed peak at 2686 cm1 is attributed to the 2D band which is an overtone of D band [44]. The Raman bands for ZFO have been shifted to lower wave number and the intensities of ZFO bands has been reduced in case of composite ZFO@CNT which indicates about strong interaction between ZFO and CNT. The polarization study validates the existence of ZFO and CNT constituents in the ZFO@CNT nanocomposite Fig. 8. 4. Electrochemical analysis 4.1. Linear sweep voltammetry (LSV) Photocurrent result of prepared photocatalysts (ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT)were measured by observing the LSV curve under visible light irradiation as shown in Fig. 9. As per the obtained LSV graph nature, all the photocatalysts generate

Fig. 9. LSV voltammogram of ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT photocatalyst using 0.1 M Na2SO4 solution.

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photocurrent in an anodic direction under the applied potential. The anodic current with positive bias potential indicates n-type semiconducting behaviour of neat ZFO that shows good consistency with Mott-Schottky data. From the photocurrent plot, it is quite clear that all the photocatalysts could generate a constant photocurrent within the scanned region i.e. 0.8–1.2 V. Further it is very interesting to note that pure ZFO, ZFO@Fullerene and ZFO@GO were able to generate a photocurrent density of 0.01 mA cm2, 0.96 mA cm2 and 0.15 mA cm2 respectively whereas the best photocatalyst i.e. ZFO@CNT generates a photocurrent density of 2.25 mA cm2. The photocurrent density generally depends upon the number of the photogenerated electrons. So larger the concentration of photoelectrons, greater is the current density, which is the characteristic property of n-type semiconductors. Under Xe light illumination, the increase in photocurrent density of carbon allotrope modified ZFO is attributed to (i) strong inhabitation of charge carrier recombination, (ii) better stability of the photocatalysts in the electrolyte medium, (iii) effective charge separation through conjugated p-skeleton of carbon derivates and (iv) high light absorption property of carbonaceous material etc. In summary, compared to ZFO all modified photocatalyst depicts higher photostability and significant improvement in photocurrent generation under visible light irradiation in the performed electrolyte medium. 4.2. Mott–schottky analysis To determine the flat band potential and toproject the mechanism towards performed photocatalytic reaction, Mott-Schottky (MS) measurements were carried out. Fig. 10 represents the MS plot (C2/F2 Vs Applied potential) of pure ZFO studied at a constant applied frequency. The +ve slope obtained by drawing a tangent to the MS plot indicate the n-type behaviour of ZFO and intercept on X-axis gives its flat band position (ECB) i.e. 0.73 eV (vs Ag/AgCl) or 0.13 eV (vs RHE) which matches with the reported literature. According to UV–Vis DRS analysis, the band gap energy of ZFO photocatalyst was found to be 1.8 eV. So, the position of the valence band was calculated as 1.68 eV (vs. RHE). The proposed mechanism of the photocatalytic reaction and the proper band edge alignment has been discussed in latter section. 4.3. Electrochemical impedance study (EIS) analysis Now a day’s numerous techniques are used to characterize photo excited electron-hole separation and transportation

Fig. 10. Mott-Schottky Plots of neat ZFO.

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mechanism which play a vital role in process of multiplying the catalytic efficiency of the developed materials. Out of all those used methods EIS proves to be quite handy in elaborating the smooth flow of charge carrier at electrode/electrolyte interface and their effective separation. In this investigation, the charge transfer resistance and separation of photo excited charge carriers of the prepared ZFO and ZFO@CNT nanocomposites were investigated by EIS analysis and the obtained results were presented through Nyquist impedance plots (imaginary part = Z00 versus real part = Z0 ). As a fundamental, Nyquist graph in the high-frequency region with smaller semicircular geometry i.e. small diameter implies slower charge recombination and high electrical conductivity whereas the larger arc diameter indicates faster electron-hole pairing. From Fig. 11a, it is completely clear that the ZFO@CNT nanocomposite shows a small semicircle suggesting lower interfacial charge transfer and high electrical conductivity of the photocatalyst. On other hand, the large semicircle observed for pure ZFO indicates more charge transfer resistance and hence low electrical conductivity [45]. In detail, the ZFO@CNT nanocomposite displays high electrical conductivity and facilitates the easy flow of photogenerated electrons at the electrode-electrolyte interface i.e. efficient charge separation. The EIS analysis at high frequency indicates the equivalent resistance in the electrode/electrolyte interface, which is provided from both the electrolyte and electronic resistance of the photoelectrodes. The minimum charge-transfer resistance value of ZFO@CNT nanocomposites shows the higher photocatalytic ability of the photocatalysts. Furthermore, the straight line of the Nyquist plot indicates the Warburg resistance which results in the dependence of frequency in ion transport of the electrolyte [46,47]. ZFO@CNT demonstrates a short diffusion path length for ions in the electrolyte, which could be seen from the low resistance of the capacitive part of the Nyquist plot. All the above observation and interpretation are well coordinates with PL study. Additionally, the cause of such separation mechanism in the composite is attributed to the conjugated p-framework of CNT that traps the photoexcited electrons, channelized them in its skeleton and hence leading to more availability of high reducible electrons.

4.4. Photocatalytic activity towards degradation of norfloxacin under solar light irradiation As an antibacterial agent i.e. Norfloxacin is widely used antibiotics to treat urinary tract infection, inflammation of the prostate

gland and bladder infections etc. However, it has been detected in water bodies and soil because of human activity, this accumulated antibiotic leads to the evolution of antibiotic resistive pathogens which are more harmful to living beings. This triggers the need to develop material effective to decompose these antibiotics. Therefore, the degradation efficiency of norfloxacin was measured by taking neat ZFO and carbon allotrope (GO, CNT, Fullerene) framed ZFO composites and then illuminating them under solar light. The photodegradation ability of synthesized photocatalysts was estimated by using 50 ppm Norfloxacin solution under solar irradiation. Following steps were followed towards photocatalytic degradation of norfloxacin. The first step involves the study of adsorption mechanism of the catalyst under dark condition. It was found that the effect of surface adsorption is about 5% of total degradation of norfloxacin solution. In the second step, 0.02 g of all the materials were taken to degrade 20 ml of 50 ppm of norfloxacin solution and exposed to open sunlight for 1 h. The above-treated suspension was centrifuged and filtered to separate the catalyst and the so formed clean solution’s optical absorbance was measured with UV–vis spectrometer. Fig. 12a displays the obtained optical spectra of treated norfloxacin. It showed that two peaks are observed at 272 nm and 330 nm. The lower wavelength spectra are generally due to aromatic ring absorption, while the larger peak stands for the electronic transition of n ? p* (HOMOLUMO) of quinolines nitrogen atoms present in the norfloxacin group. After degradation, the light absorption power at 330 nm decreases, which demonstrating the breakdown of germicidal quinoline, while the absorbance reduces at 272 nm that recommend the opening of the aromatic ring [48]. It was observed that composites display better activity compared to neat ZFO which may be attributed to effective charge separation, large light absorbance property and more photostability of the hybrid material. However, among different ZFO@carbon allotrope composites, ZFO@CNT shows less absorbance that means high degradation rate i.e. 91% in 1 h. Further, the catalytic degradation rate of Norfloxacin with respect to time is described in Fig. 12b. Fig. 12c depicts the degradation order of the composites as fallow ZFO@CNT (91.36%) > ZFO@Fullerene (85.1%) > ZFO@GO (78.3%) > ZFO (60%) respectively. The loading of ZFO to CNT increases the light absorption capacity of the composite and the one-dimensional p-skeleton of CNT accepts the photogenerated electrons of ZFO that results in effective charge separation and faster conductivity. Whereas in case of GO and Fullerene due agglomeration their respective composite features a low active compared to ZFO@CNT. The durability of the photocatalyst towards degradation of norfloxacin was analyzed for four consecutive cycles. After each catalytic run, the catalyst was filtered and washed with DI water several times then subjected to further analysis in UV–Vis spectrophotometer. From Fig. 12d, it has been observed that there was no major decrease in photocatalytic activity even subsequent to four cycles, which proves good photostability of the semiconductors in the photocatalytic run. Additionally, to support the photostability of the active composite, the treated catalysts was further characterized through XRD and FTIR to ascertain that even after four cycles the deterioration of the catalyst is marginal as shown in Fig. 12d. 4.5. Kinetics of photodegradation of norfloxacin Fig. 10b represents the kinetic study, which is obtained by plotting a graph between C/C0 Vs. time using the equation given bellow.

Photodegradation rate ¼ ðC  C0 =C0 Þ  100

Fig. 11. Nyquist plot of pure ZFO and ZFO@CNT nanocomposites.

where C0 is the original concentration at time t = 0 min, C is the final concentration after a time laps of ‘t’ min. Photodegradation of

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Fig. 12. (a) Absorbance plot of ZFO, ZFO@GO, ZFO@Fullerene and ZFO@CNT nanocomposites (b) Photocatalytic degradation of Norfloxacin over different photocatalysts (c) rate of degradation of Norfloxacin over various samples and (d) reusability test of the ZFO@CNT photocatalyst (e) kinetics mechanism of neat and all the nanocomposites accompanying by linear fit analysis.

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Table 1 Obtained kinetic results of prepared samples towards Norfloxacin degradation. Catalyst ZFO ZFO@CNT ZFO@Fullerene ZFO@GO

R2 0.85 0.94 0.91 0.90

Kobs (min1) 3

8 * 10 23 * 103 18 * 103 13 * 103

antibiotics follows pseudo 1st order kinetics as per LangmuirHinshelwood i.e. ln (C/C0) = kt. ‘k’ stands for pseudo 1st order rate constant. The kinetic mechanism is supported by plotting a graph between ln (C/C0) vs irradiation time (Fig. 12e). The kobs, t1/2 and % of degradation of all the photocatalyst are given in Table 1. The ZFO@CNT photocatalyst shows highest kobs in comparison to other composite materials and hence shows the best photocatalytic activity.

t1/2 (min)

% of degradation

86.62 30.13 38.5 53.30

60 91 85 78

4.6. Scavenger test involving the active species responsible for the degradation The intermediates that are responsible for the degradation of antibiotics were evaluated by trapping agent experiment. In this work below mentioned scavenging reagents were used to trace the following reactive species such as ethylenediaminetetraacetic acid, p-benzoquinone, isopropyl alcohol, dimethyl sulfoxide for

Fig. 13. (a) Bar diagram demonstrating the function of main active species in the degradation of norfloxacin (b) absorbance spectrum of NBT solution with ZFO@CNT before and after experiment (c) PL spectra of ZFO@CNT solution with TA.

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holes (h+), superoxide (O2) radicals, hydroxyl radicals (OH) and electrons respectively [49]. Fig. 8a shows the active species which are involved in degradation of norfloxacin solution. From the figure, it is observed that superoxide (O2) radical and hydroxyl radicals (OH) play the role of protagonist towards the degradation of antibiotics. The confirmatory test of these active species was examined and discussed in the next section. 4.7. Terephthalic acid test (TA) Terephthalic acid (TA) experiment was carried out to identifying the generation of OH radicals over the catalyst horizon. In this test, TA generally reacts with formed OH to the formation of 2hydroxyterephthalic acid (HTA) [6]. The formation of HTA was occurred by using following procedures, 5 mM of TA was taken and added to weighted amount of catalyst in NaOH solution, as the TA solution was dissolved in basic media. The required suspension was exposed to solar light irradiation for 30 min and then PL measurement was conducted. For PL analysis the excitation was fixed at k = 315 nm, where HTA shows emission at k = 426 nm, while TA does not show any emission. From the Fig. 13c it has been confirmed that the peak intensity at k = 425 nm relates to the formation of HTA complex. The above results prove that OH radicals

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generation is feasible for the prepared catalyst and so supports the degradation of norfloxacin over the photocatalyst. 4.8. Nitroblue tetrazolium test (NBT) In the carried experiment, 5 * 105 M mixture of NBT was taken as identifying reagent to prove the formation of O 2 radicals. To 10 ml of above-prepared molar NBT solution, 0.01 g of as prepared catalyst was added and then illuminated under sunlight for 1 h. After the experiments, the photocatalyst was removed by centrifuge and the solution optical absorbance was measured in a UV spectrophotometer. After the performed UV analysis the obtained data were plotted as shown in Fig. 13b. It was observed that the concentration of NBT decreases which confirms that the formation of O 2 over the photocatalyst surface and hence help in the degradation process [50]. 4.9. Experimental procedure for Cr(VI) reduction The photoreduction efficiency of synthesized materials was further challenged by taking chromium (VI) as probe pollutants and then exposing the catalyst suspended Cr(VI) solution to open sun light, detail as narrated below. The experimental setup includes a

Fig. 14. (a) Cr (VI) reduction rate over different photocatalysts (b) kinetics First order mechanism of neat and all the nanocomposites. (c) Rate of reduction of Cr(VI) over various semiconductors and (d) reusability test of the ZFO@CNT photocatalyst.

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cycle) of the experiment are shown in Fig. 14(d). So, it is concluded that the stability of the prepared photocatalyst has increased than the corresponding neat materials (ZFO).

conical flask containing 20 ml of 10 mg/L Cr (VI) solution along with 0.02 g of the photocatalyst (maintained at pH = 3) placed over a magnetic stirrer. The so formed solution was placed in dark for 30 min to ensure the adsorption and desorption equilibrium. Then the sample solution was kept under solar light irradiation for 1 h to study the photoreduction activity. After completion of the above process, the final solution was centrifuged instantly to remove the samples. Then the decant solution was analyzed by using UV–Vis spectrophotometer to determine the percentage of degradation. All the above condition was maintained following our previously reported literature [16].

4.12. Effect of pH on Cr (VI) reduction pH is a crucial parameter in the reduction process of Cr (VI). At pH value 3, the photocatalyst shows the best catalytic activity i.e. photoreduction of Cr (VI) to Cr (III). Because at lower pH, Cr exist in anionic form i.e. Cr2O27 in the prepared solution. The anionic form of the Cr2O2 7 actively interacts with the positively charged surface of the photocatalyst i.e. ZFO@CNT as both ZFO and CNT have pHPZC value 9.5 and 5.4 respectively, [52,53] which is quite favourable for the reduction of hexavalent Cr to trivalent Cr. The reduction pathway of the Cr (VI) to Cr (III) is equated below:

4.10. Determine the strength of Cr(VI) solution For analysis of remaining Cr(VI) solution, 2 ml of the obtained decant Chromium solution was mixed with 3 M H2SO4 solution and taken in a 10 ml volumetric flask. Further, for colorimetry measurement, the so formed acidic solution was mixed with freshly prepared diphenylcarbazide (DPC) solution referring published article [51]. After addition of DPC, the solution was allowed to stand for 15 min for colour development. Additionally, for reference a blank solution was prepared with untreated 10 ppm Cr(VI) solution but containing all the above reagents (3 M H2SO4 and DPC) under same reaction procedure. During photoreduction process the aliquot was withdrawn from the reaction mixture at a regular time interval such as 10, 20, 30, 40, 50 and 60 min respectively for kinetic mechanism study. The percentage of photoreduction of Cr (VI) was measured in UV–Vis spectra of Cr (VI)–diphenylcarbazide complex in the solution.

Cr2 O7 2 + 14Hþ + 6e ! 2Cr3þ + 7H2 O In order to prove the photostability of ZFO@CNT against the reduction of Cr (VI), reusability experiment was performed. It has been clear that after four cycles of reduction, a small decrease in XRD peak intensity was observed (Fig. 15), which results in a slight reduction in photocatalytic activity. 4.13. Conformation of Cr(VI) to Cr(III) After the photoreduction of Cr (VI) to Cr (III) and the catalyst free light treated solution was analysed by KMnO4 to identify the amount of non-toxic Cr (III). KMnO4 solution was utilized to oxidize Cr(III) to Cr(VI), which is proved by DPC method. Under the experimental condition, 0.02 M KMnO4 solution was prepared, 0.1 ml of which was added to 1.9 ml of Cr(III) solution and placed under solar light for 1 h and similar colorimetric DPC methods were followed toward the detection of Cr(VI). From the above

4.11. Photoreduction and kinetics of Cr (VI) solution The photoreduction of hexavalent chromium were carried out by using the following photocatalyst viz. neat ZFO, ZFO@CNT, ZFO@Fullerene and ZFO@GO under solar photon irradiation. Before the photoreduction process, the sample solution is kept under dark condition for adsorption study. It was observed that 2–5% of Cr (VI) was adsorbed on the surface of the different photocatalyst which is quite a low value and can be neglected. After completion of all reduction experiments and colorimetry analysis of each photo reduced Cr (VI) solution, it was found that the photocatalytic performance of ZFO modified CNT showed better reduction ratio 82% after 60 min in comparison to neat ZFO and other carbon allotrope modified ZFO. Fig. 14(a) depicts the reduction rate of different photocatalyst which is calculated by plotting the graph between C/C0 Vs. time. The kinetic mechanism study was monitored in every 10 min time interval to study the photoreduction rate of Cr (VI) is represented in Fig. 14(b). The reduction process goes by first order kinetics and ZFO@CNT composite shows highest rate constant i.e. 27 * 103. Moreover, the rate constant value, half-life period and percentage of Cr (VI) reduction of all the prepared photocatalyst are displayed in Table 2. Further, the reduction percentage of the different photocatalyst are shown in Fig. 14(c) and hence the order of reduction is as follows ZFO@CNT > ZFO@Fullerene > ZFO@GO > ZFO. In the repeating experimental study, it was found that the ZFO@CNT shows better photocatalytic activity and retain its catalytic activity even up to more than three cycle (4

Fig. 15. Reusability test of ZFO@CNT and after Cr(VI) reduction.

Table 2 Obtained kinetic data of synthesized photocatalyst towards photoreduction of Cr (VI). Catalyst ZFO ZFO@CNT ZFO@Fullerene ZFO@GO

R2 0.991 0.995 0.995 0.998

Kobs (min1) 3

10 * 10 27 * 103 22 * 103 16 * 103

t1/2 (min)

% of Cr reduction

69.3 25.66 31.5 43.31

48 82 75 64

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Scheme 1. Reaction mechanism of the degradation of norfloxacin and reduction of Chromium over pure ZFO.

experiment, it was confirmed that hexavalent Cr (VI) is reduced to trivalent Cr(III)via the designed photocatalyst (ZFO@CNT) under light illumination [50]. 4.14. Mechanism of photocatalytic degradation of norfloxacin solution and Cr (VI) reduction Over the horizon of ZFO@CNT is depicted in Scheme 1. The cause behind the enhanced photodegradation activity and the  intermediates (O 2 and OH) responsible to carry out the degradation process is narrated below. Under solar light agitation, electrons in the valence band (VB) of ZFO are excited to its conduction band (CB) by absorbing energy Eg and then get channelized to CNT surface. As per Mott-Schottky analysis of ZFO the conduction band edge potential (ECB) = 0.13 eV and valence edge potential ((EVB) = + 1.68 eV) respectively. The photoexcited electrons get transferred from the CB of ZFO to CNT framework and get trapped in its p-network where these photoelectrons interact with adsorbed oxygen to generate superoxide radical as illustrated in Scheme 1. This is possible because the conduction band potential of ZFO is more negative that of the redox potential of O2/O 2 (0.046 eV) [6]. Similarly, the holes residing at the VB of the ZFO oxidize the water molecules to OH radical. The redox potential of OH/OH is 1.99 eV which is higher than that of VB potential of ZFO. So it is clear from the band edge potential value that ZFO can indirectly form OH though H2O2 which helps in the degrada tion process. The main active species i.e. O 2 and OH are solely responsible degrade of norfloxacin under visible light source which is again well justified through scavenger test. CNT proves to be quite fruitful in increasing the catalytic performance of ZFO. The high photocatalytic activity of ZFO@CNT is attributed to high photocurrent density, low rate of photogenerated electron-hole recombination and high exciton separation efficiency that are well supported by LSV, PL and EIS study respectively. Further to justify the superiority and novelty of the prepared ZFO@carbon allotrope nanocomposite system, a comparison table is included in ESI(S5). Additionally, the photoreduction of Cr (VI) to Cr (III) was brought about the photoexcited electrons and the reaction pathway is equated below. 4.15. Norfloxacin degradation mechanism

ZFO@CNT + hm ! ZFO@CNT(e ) + ZFO@CNT(hþ )

ZFO@CNT(e ) + O2 !  O2  ZFO@CNT(e ) +  O2  + 2Hþ ! H2 O2 ZFO@CNT(e ) + H2 O2 !  OH + OH Norfloxacin +  OH ! degradedproduct Norfloxacin +  O2  ! degradedproduct Hence the net reaction is

Norfloxacin +  OH +  O2  ! degradedproduct 4.16. Chromium reduction mechanism

ZFO@CNT + hm ! ZFO@CNT(hþ ðVBÞ + e ðCBÞ ) CrO4 2 + 8Hþ + 3e ðCBÞ ! Cr3þ + 4H2 O 2H2 O ! O2 + 4e + 4Hþ The net reaction is as follows:

CrO4 2 + 20Hþ ! 4Cr3þ + 10H2 O + 3O2 5. Conclusion In brief, we have modified the ZFO with carbon derivatives (CNT, GO and Fullerene) through hydrothermal followed by calcinations method. We have evaluated the photocatalytic efficiency of these prepared materials i.e. ZFO and ZFO@Carbon derivative nanocomposites towards degradation of Norfloxacin and reduction of Cr (VI) as model pollutants. The loading of carbon allotropes to ZFO results in high visible light absorption activity, better charge separation efficiency and enhanced photocatalytic performance which is well supported by UV–Vis DRS, PL, EIS and carried catalytic study. Furthermore, ZFO@CNT displays a photocurrent density of 2.25 mA/cm2, which is significantly higher than neat ZFO (225 times) and some reported literature [54,55,31]. Out of all these synthesized photocatalysts, ZFO@CNT proves to be more effective nanocomposites for the degradation of norfloxacin (91.36% degradation i.e. 1.5 times higher than parent ZFO) and

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photoreduction of Cr(VI) solution (82% in 60 min i.e. 1.7 times higher than parent ZFO) under open sunlight. These results are due to low electron-hole recombination, better electron-hole separation and effective charge transfer efficiency. The main cause behind the superior photocatalytic performance of these nanocomposites are attributed to the electron trapping and channelizing property of conjugated p-skeleton present in the carbon derivatives. It is believed that the present investigation gives new insight into the research community towards the development of ferrite materials with carbon derivatives for solving energy and environmental issues.

Conflicts of interest There is no conflict of interest. Acknowledgments The authors are very much grateful to S ‘O’ A (Deemed to be university) management for their encouraging support toward research work and publication. Mr. Sriram Mansingh is thankful to CSIR New Delhi India for awarding SRF.

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