RGO Nanocomposite with Enhanced

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Green synthesis of Fe3O4/RGO nanocomposite with enhanced photocatalytic performance for Cr(VI) reduction, Phenol degradation and antibacterial activity Deepak Kumar Padhi, Tapan Kumar Panigrahi, Kulamani M. Parida, Saroj Kumar Singh, and Pravat Manjari Mishra ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02548 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Sustainable Chemistry & Engineering

Green synthesis of Fe3O4/RGO nanocomposite with enhanced photocatalytic performance for Cr(VI) reduction, Phenol degradation and antibacterial activity Deepak kumar Padhiab, Tapan Kumar Panigrahic, K. M. Paridaad*, S. K. Singha and P. M. Mishrab a

Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and

Industrial Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi-110 001, India b

Environment & Sustainability Department, CSIR-Institute of Minerals and

Materials

Technology, Bhubaneswar – 751 013, Odisha, India c

Department of Biotechnology, North Orissa University, Baripada, Mayurbhanj-757003,

Odisha, India d

Centre for Nano Science and Nano Technology, S‘O’A University, Bhubaneswar—

751030, Odisha, India

*

Corresponding author

E-mail: [email protected] & [email protected] Tel. No. +91-674-2379425, Fax. +91-6 74-2581637.

ACS Paragon Plus Environment


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Abstract Here in, we report a novel single-step hydrothermal synthesis of photocatalytically stable and magnetically separable g-Fe3O4/RGO nanocomposite in presence of Averrhoa carambola leaf extract as a natural surfactant for multipurpose water purification application. The Averrhoa carambola leaf extract played a major role for the modification of structural, optical and electronic properties of Fe3O4 nanoparticle. At room temperature, the g-Fe3O4/2RGO nanocomposite showed 97% & 76% of Cr(VI) reduction and phenol degradation, respectively. The higher activity of g-Fe3O4/2RGO was attributed to the in situ loading of RGO and the synergism developed between RGO and super magnetic Fe3O4 nanoparticle results in better separation of photoexcited charge carriers (e-/h+) which was concluded from photoluminescence & photocurrent measurement. Further, g-Fe3O4/2RGO nanocomposite showed better antimicrobial activity against 3 bacterial pathogens such as Staphylococcus aureous (MTCC -737), Bacillus subtilis (MTCC -736) and Escherichia Coli (MTCC- 443) in compared to GO with respect to a standard antibiotic (30 mcg).

Keywords: Green synthesis, Natural surfactant, Graphene, Electron transfer, Photochemistry.


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Introduction In the present scenario, growing population invariably exerts tremendous pressure on the scientific community to solve several environmental issues. In this context, development of multifunctional materials for environmental remediation is a challenge to the scientific community.1 In particular, implementation of carbon allotropes in environmental remediation has been a hot research field.2-4Among them, developments of graphene-based semiconductor photocatalysts have been considered as emerging candidate to address the budding environmental issues.5-7 In fact, 2D graphene consists of sp2 hybridised carbon atom possesses unique optical, electrical, chemical and mechanical properties.8 Considering the several exceptional properties such as high surface area, high adsorption capacity and high conductivity of graphene, a lot of graphene-based photocatalysts have been already reported for environmental remediation by many researchers including the author’s group.9-15 Among which,

the

author’s

group

have

reported

Gd(OH)3

nanorod/RGO12,

-FeOOH

nanorod/RGO13 & α-MnO2@RGO nanocomposites14 for Cr(VI) reduction and α-Fe2O3 nanorod/RGO15 composite for phenol degradation under visible light irradiation. The enhanced photocatalytic activity of graphene-based inorganic material is attributed to the unique surface property of graphene which suppresses the photoinduced electron-hole recombination by sinking the photoexcited electron from the conduction of corresponding inorganic material and utilising them in the photochemical reaction. Although several graphene-based inorganic materials such as RGO/TiO2, RGO/ZnO, etc. displayed excellent photocatalytic activity but their practical implantation for environmental remediation is limited due to the following 3 issues; (1) separation, (2) recovery & (3) reuse of photocatalyst from the suspended carcinogenic solution after photochemical reaction.16-19 To address the above issues, design and development of magnetically separable photocatalysts has become one of the most exciting and rapidly growing areas of research.20-



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23

In particular, Magnetite (Fe3O4) has been considered as one of the challenging material in

this category with regard to its cost-effective preparation, desirable electrical conductivity, paramagnetic property, eco-friendly nature and high absorptive property.24-25 However, high rate of charge carrier recombination and agglomeration features of bare Fe3O4 lowers its photocatalytic efficiency for practical application.26 In this regard, the introduction of graphene to Fe3O4 prevents the agglomeration of Fe3O4 nanoparticles and also decreases the rate of charge carrier recombination by trapping the photoexcited electron from the conduction band of Fe3O4 which results in a better photocatalytic activity of Fe3O4. 27-30 Ming et al. reported that the incorporation of Fe3O4 nanoparticle into RGO sheets results in better photocatalytic activity towards degradation of Methylene blue (MB).27 Das et al. studied photoreduction of Cr(VI) over Fe3O4/RGO nanocomposite.28 Grace et al. reported multifunctional activity such as adsorption, photodegradation and antibacterial of the graphene-Fe3O4 nanocomposite.29 Kar et al. reported enhanced photocatalytic degradation of MB dye over RGO and metal oxide based binary system (RGO-TiO2/RGO-Fe3O4).30 All the above groups claimed that presence of RGO and surfacing of the synergistic effect between the RGO and Fe3O4 are key factors for the enhanced activity of Fe3O4 towards environmental remediation. In the present context, it is intended to study the role of bio-surfactant for the modification of structural, optical & electronic properties of Fe3O4/RGO nanocomposite towards environmental remediation. Till date, this work will be the first report on the synthesis of Fe3O4/RGO nanocomposite in presence of natural biomolecules. The main advantage of using natural biomolecules is that they act as capping agents which prevent the agglomeration of nanoparticles and stabilize the nanoparticles. In our previous work on Fe3O4, the G-Fe3O4 (synthesised in presence of biomolecules) showed better photocatalytic activity in compared to C-Fe3O4 (prepared by chemical method).31 To further improve the photocatalytic activity of Fe3O4, we have adopted a single-


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step hydrothermal synthesis of Fe3O4/RGO nanocomposite photocatalyst in presence of Averrhoa carambola leaf extract as a natural surfactant and implanted them for multipurpose water purification application i.e. photoreduction of Cr(VI), photodegradation of phenol and antibacterial activities against human pathogenic bacteria such as Staphylococcus aureous (MTCC -737), Bacillus subtilis (MTCC -736) and Escherichia Coli (MTCC- 443). The asprepared Fe3O4/RGO nanocomposite composed of super magnetic Fe3O4 nanoparticle decorated on RGO sheets exhibited superior photocatalytic activity in compared to our previous work on Fe3O4,30 -FeOOH nanorod/RGO,13 α-MnO2@RGO14 and α-Fe2O3 nanorod/RGO composite15 under visible light illumination. Experimental Materials Natural graphite powders, Potassium permanganate (KMnO4) were purchased from Sigma Aldrich Chemicals. Ammonium hydroxide (NH4OH), Sulphuric acid (H2SO4), Hydrochloric acid (HCl), Sodium nitrate (NaNO3) and Sodium hydroxide (NaOH) were obtained from Finar chemicals Limited. Ferric chloride anhydrous (FeCl3) & Ferrous sulphate heptahydrate (FeSO4.7H2O) were obtained from Spectrochem chemicals. All these above chemicals were of analytical grade and were used without further purification. Synthesis of Magnetite in presence of Averrhoa carambola leaf extract Averrhoa Carambola (A. Carambola ) leaf extract was prepared by following the same procedure reported by the author’s group.31 In a typical experiment, a calculated amount of FeCl3 and FeSO4.7H2O ( Fe+3: Fe+2 = 2:1) were dissolved in 30 ml of distilled water and stirred for 30 min. About 20 ml of leaf extract was added to solution mixture and again stirred for 30 min. The pH of the resultant solution was maintained to 10 by adding NH4OH solution and then transferred to an in a Teflon-line autoclave for hydrothermal treatment at 180 oC for 6 h. The obtained precipitate was centrifuged, washed with distilled water followed by



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absolute ethanol for several times to remove heavy biomass of A. Carambola leaf extract, and then dried in an oven at 100 ⁰C overnight. The obtained final product was designated as gFe3O4. For comparison study, Magnetite was also synthesised by following above procedure in the absence of A. Carambola leaf extract and the obtained final product was designated as Fe3O4. Green Synthesis of g-Fe3O4/RGO nanocomposite At first graphene oxide (GO) was prepared by modified Hummers method.32 In a typical synthesis of g-Fe3O4/RGO nanocomposite, a calculated amount of as prepared GO was dispersed in 40 ml of deionised water for 1 h to obtained exfoliated graphene oxide (EGO). Now, EGO was mixed with the previously prepared suspension of Fe+3/Fe+2 solution and A. Carambola leaf extract and stirred for 30 min. Then, the rest of the synthesis process was adopted same as that of synthesis of g-Fe3O4. The above synthesis procedure is illustrated in scheme 1. The as synthesised g-Fe3O4/RGO nanocomposites were named according to the amount of GO loading i.e 1wt% (g-Fe3O4/1RGO), 1.5wt% (g-Fe3O4/1.5RGO), 2wt% (gFe3O4/2RGO) and 2.5wt% (g-Fe3O4/2.5RGO). Photocatalytic activity The photocatalytic activity of as synthesised Fe3O4, g-Fe3O4 and all g-Fe3O4/RGO nanocomposites were tested for the reduction of Cr(VI) and degradation of phenol under visible light irradiation in an irradiation chamber (BS-02, Germany) for 2 h. The Cr(VI) reduction ability of as-prepared photocatalysts were studied at different pH i.e 2, 4, 6, 8 & 10 in the similar condition. The prepared photocatalysts (1g/L) were suspended in 20 mL of 50 ppm of Cr(VI) and phenol solution and stirred for 30 min in dark condition to obtain adsorption–desorption equilibrium. Then, the beakers containing Cr(VI)-catalyst & phenolcatalyst suspension were exposed to visible irradiation. For the analysis purposes, 2ml of suspension was sampled at certain time interval, centrifuged to remove the nanocatalyst and


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reduction of Cr(VI) & degradation of phenol were analyzed quantitatively with the filtrates using Cary-100 (Varian, Australia) spectrophotometer. Antibacterial Analysis The comparative antibacterial activities of GO and g-Fe3O4/RGO nanocomposites were effectively screened against two gram-positive bacteria namely Staphylococcus aureous (MTCC -737), Bacillus subtilis (MTCC -736) and one gram-negative bacteria Escherichia Coli (MTCC- 443). The bacteria used in this analysis were purchased from Indian institute of Microbial Technology, Chandigarh, India with lyophilized form. The pathogenic strains were maintained in nutrient agar media (Himedia, India) and slanted bacterial strains were stored at 4 0C.The slanted pathogenic bacteria were subculture before use. Disc diffusion analysis was carried out to establish the antibacterial activities of GO and g-Fe3O4/RGO nanocomposite against the test pathogens.33 About 50 mg/ml of each sample were dissolved in sterile water subjected to ultrasonication to prevent accumulation. The 4.0 mm size Whatman paper-1 discs were soaked in the solution and the dipped discs were air dried in the aseptic condition. The plates containing nutrient agar media were made by swabbing them with previously cultured human pathogenic bacteria. After that, the previously air-dried discs were put carefully in the cultured petriplate with equal distance. Then, the plates were incubated at 37 0

C for 24 hrs in a bacteriological incubator. After the incubation, a clear zone of inhibition

was observed around the each and confirmed the antibacterial activities of GO and gFe3O4/RGO nanocomposite. The maximum zone of inhibition was measured manually. Gentamycin was taken as the standard antibiotic. Results and discussion XRD The powder X-ray diffraction (PXRD) patterns of all synthesised photocatalysts were recorded to confirm their crystallinity and phase purity and are shown in Figure 1. All the



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observed diffraction patterns for the as-synthesised neat Fe3O4 match well with the characteristics reflections of the inverse-spinel structure corresponding to JCPDS file No 893859.31 The phase purity of Fe3O4 was also maintained in the presence of A. Carambola leaf extract as well as loading of RGO during the hydrothermal synthesis. However, g-Fe3O4 and all g-Fe3O4/RGO nanocomposites showed a gradual decrease in peak intensity and slight broadening of all diffraction peaks of neat Fe3O4. That means hydrothermal synthesis of Fe3O4 in the presence of A. Carambola leaf extract results in decrease of crystallite size and loading of GO also results in further decrease of crystallite size of g-Fe3O4.31&33 The XRD analysis further shows that no diffraction peak was observed for GO in all g-Fe3O4/RGO nanocomposites suggesting exfoliation of GO sheets occur and this is due to interaction between Fe+3& Fe+2 ions with the oxygen-related functionalities of GO which leads to breaking of regular lamellar structure of GO sheets.34-35

Raman Spectra In order to confirm the composite formation between g-Fe3O4 & GO and the reduction of GO to RGO, Raman analysis has been carried out. Figure 2(a) displays the Raman spectra of as synthesised g-Fe3O4 and g-Fe3O4/2RGO nanocomposite. The Raman spectra of GO showed two peaks positioned at 1355 & 1596 cm-1 which can be assigned as ‘D’ & ‘G’ band respectively, and two broad peaks positioned at 2695 & 2933 cm-1 which correspond to ‘2D' band of GO.36-37 Out of which, the appearance of ‘D’ & ‘G’ band is due to disordered atomic arrangement caused by sp3 hybridised carbon atom and plane vibration of sp2 hybridised carbon atoms in 2D GO sheets, respectively, whereas second-order Raman scattering process results in appearance of ‘2D' band of GO.38 The Raman spectrum of g-Fe3O4/2RGO nanocomposite showed all above bands i.e ‘D’, ‘G’ & ‘2D' band along with all fundamental Raman vibration of Fe3O4 confirming the composite formation during adopted hydrothermal route. The shifting of ‘D’ & ‘G’ of GO in the Raman spectrum of g-Fe3O4/2RGO


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nanocomposite (Figure 2(b)) strongly suggests the good incorporation of Fe3O4 nanoparticle in between 2D GO layers.15 Further, the increase in the intensity ratio of ‘D’ & ‘G’ (ID/IG) of g-Fe3O4/2RGO nanocomposite as compared to GO (Table 1) confirms the successful reduction of GO to RGO during hydrothermal synthesis.14-15 FESEM and TEM analysis also support our above observations. Electron microscopy FESEM and TEM analysis has been carried to get a concrete idea regarding the morphological characteristics and particle size of as-prepared photocatalysts. Figure 3 illustrates FESEM analysis of as synthesised neat Fe3O4, g-Fe3O4 and g-Fe3O4/2RGO nanocomposite. From FESEM images, it is observed that the particle size of neat Fe3O4 (Figure 3(a)) is comparatively larger as compared to g-Fe3O4 (figure 3(b)) along with the well distribution of g-Fe3O4 over GO sheets by the adopted hydrothermal process (Figure 3(c) & (d)). EDX analysis of g-Fe3O4/2RGO nanocomposite (Figure 3(e) confirms the presence of constituent elements i.e. C, O & Fe. Further, X-ray elemental mapping of gFe3O4/2RGO nanocomposite (figure 4(a)) was carried to get the information regarding the distribution of C, O & Fe in the matrix as shown in Figure 4(b), (c) & (d), respectively. TEM image of as-prepared GO is shown in figure S1†. Figure 5 represents the TEM images of neat Fe3O4, g-Fe3O4 and g-Fe3O4/2RGO nanocomposite. The average particle size of neat Fe3O4 was observed to be of 46±2 nm (Figure 5(a), whereas this value for g-Fe3O4 was found to be 32±2 nm (Figure 5(b). The particle size distribution of g-Fe3O4 over RGO sheets is shown in figure S2†.That means the particle size of Fe3O4 decreased (∼14 nm) in presence of A. Carambola leaf extract which may due to the interaction of bio molecules present in the leaf extract with Fe+3& Fe+2 ions hinders the particle growth during Fe3O4 formation (scheme 2). The formation mechanism of Fe3O4 nanoparticle in presence of A. Carambola leaf extract has been already reported by the author’s group.31 A control Fe3O4/2RGO nanocomposite was



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prepared without biomolecules to understand the promoting effect of biomolecules on the growth of g-Fe3O4 nanoparticle on RGO sheets, shown in Figure S3†. It can be clearly seen that the distribution of Fe3O4 on RGO sheets is not well. That means the Fe3O4 nanoparticles are getting more agglomerated on the RGO surface in the absence of biomolecules. But good distribution of g-Fe3O4 nanoparticle over RGO sheets was successfully obtained in presence of biomolecules by the adopted hydrothermal route (Figure 5(d)). The strong interaction of Fe+3& Fe+2 with the negatively charged surface of GO not only increased the stability of RGO sheets but also reduced the agglomeration of g-Fe3O4 nanoparticles which played an important role in the enhanced photocatalytic activity. Moreover, the average particle size of g-Fe3O4 in g-Fe3O4/2RGO nanocomposite was observed to be 22±2 nm. That means the introduction of RGO to g-Fe3O4 nanoparticle further reduces its particle size. Several bright continuous concentric rings are observed from selected area electron diffraction (SAED) pattern of g-Fe3O4/2RGO nanocomposite (Figure 5(e)) due to the diffraction from the (110), (130), (111), and (210) planes of g-Fe3O4 nanoparticle indicating its polycrystalline nature which is consistent well with the XRD data. In addition to this, HRTEM analysis also confirmed the crystallinity character of g-Fe3O4 nanoparticle and is presented in Figure 5(f). From Figure 5(f), the obtained lattice fringes spacing 0.485 nm ensures the 111 plane of gFe3O4 and is in well agreement with the XRD analysis.31 Hence, the particle size of g-Fe3O4 obtained by the introduction of RGO and stability of both g-Fe3O4 & RGO sheets played a vital role in the photocatalytic as well as the antibacterial activity of g-Fe3O4/2RGO nanocomposite. Well anchoring of g-Fe3O4 nanoparticles over RGO facilitates better transportation of photoexcited electrons from the CB of g-Fe3O4 nanoparticles to its 2D surface and enhances the utilization of these electrons for better photochemical reaction and simultaneously enhances the photocatalytic activity. On the other hand, the increased stability


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of RGO sheets that is obtained due to the decoration of g-Fe3O4 nanoparticles effectively kills the bacteria as required all over the body with a suitable external magnetic field. Photocatalytic activity The photocatalytic performance of Fe3O4, g-Fe3O4 and RGO loaded g-Fe3O4 nanocomposite was investigated by the reduction of Cr(VI) and phenol under visible light irradiation. Photoreduction of Cr(VI) In a typical photoreduction experiment, optimised amount of photocatalysts (1g/l) and aqueous Cr(VI) solution (50 mg/L) were exposed to visible light irradiation subjected to continuous stirring. Figure 6 (a) displays the reduction ratio(C/C0) as a function of time. In dark condition, no temporal concentrations of changes were observed but upon illumination of visible light. Among all photocatalysts, g-Fe3O4/2RGO nanocomposite showed highest photoreduction efficiency under visible light irradiation i.e. 97% reduction of Cr(VI) (50 mg/L) at pH = 2, whereas Fe3O4 & g-Fe3O4 showed only 22 & 38%, respectively. Synthetic approach greatly influences the photocatalytic activity of Fe3O4. Fe3O4 nanoparticle which has been prepared in presence of A. Carambola leaf extract (g- Fe3O4) showed better activity than chemically synthesised Fe3O4 which is attributed to the surface modification and reduction in particle size of Fe3O4 in presence of biomolecules during hydrothermal synthesis. The Cr (VI) reduction ability of as synthesised photocatalysts exhibit the following order: Fe3O4 < g-Fe3O4 < g-Fe3O4/1RGO < g-Fe3O4/1.5RGO< g-Fe3O4/2.5RGO < g-Fe3O4/2RGO. The above observation suggests the optimum photoreduction efficiency of g-Fe3O4 is achieved at 2% loading of RGO i.e. 97% Cr (VI) reduction occurs in 1h. The higher amount of RGO content i.e. 2.5% in g-Fe3O4/2RGO nanocomposite leads to decrease in the photoreduction ability of g-Fe3O4 which may be attributed to “shielding effect” of RGO in gFe3O4/2.5RGO nanocomposite.39



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Effect of pH on Cr(VI) reduction The effect of pH on the reduction of Cr(VI) over Fe3O4, g-Fe3O4 & g-Fe3O4/2RGO photocatalyst (1g/L) was investigated by varying the pH from 2-10 and is shown in Figure 6(b). It was found that Cr(VI) reduction ability of all photocatalysts decrease with increasing the pH of Cr(VI) solution. At pH= 2, Fe3O4, g-Fe3O4 & g-Fe3O4/2RGO photocatalysts showed 22, 38 & 97% photoreduction ability

under visible light irradiation for 1 h,

respectively. At pH=10, Fe3O4, g-Fe3O4 & g-Fe3O4/2RGO photocatalyst showed 5, 13 & 22% of Cr(VI) reduction, respectively under same experimental condition. Inductively coupled plasma optical emission spectrometry (ICP-OES, ULTIMA2 HORIBA JOBIN YVON) was carried out to measure the concentration of Cr in the aqueous solution after photocatalytic reaction of g-Fe3O4/2RGO nanocomposite (Table S1, ESI†). The obtained results in table S1 represent the concentration of Cr in the supernatant liquids with a standard deviation of ±5%. At lower pH, high reduction efficiency of g-Fe3O4/2RGO photocatalyst can be explained on the basis of following two points i.e (1) HCrO4- and Cr2O42- are the predominant anionic chromate species existing at lower pH & (2) these chromate species can preferably accumulate on the positive surface of g-Fe3O4/2RGO photocatalyst to get reduced to Cr(III).12-13 For better convenience, At pH=2 the temporal decrease in Cr(VI) concentration in form of spectral change with respect to different time interval over g-Fe3O4/2RGO photocatalyst is illustrated in Figure 6(c). Reusability of g-Fe3O4/2RGO photocatalyst In order to examine the stability of as prepared g-Fe3O4/2RGO photocatalyst, recycling study was performed under identical conditions and is shown Figure 6(d). As shown in Figure 6(d), the stability of g-Fe3O4/2RGO photocatalyst was observed up to 3rd cycle with a slight decrease in photoreduction efficiency i.e. 97, 96 & 94% in 1st, 2nd and 3rd cycle, respectively.


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Hence, from above results, it is clear that the as prepared g-Fe3O4/2RGO photocatalyst can be used as an efficient photocatalyst for the reduction of Cr(VI) under visible light irradiation. Further, the superior performance of g-Fe3O4/2RGO nanocomposite towards photoreduction of Cr(VI) ability has been compared to other graphene-based photocatalysts including the ones reported in our previous work (Table S2, ESI†). Mechanism insight for superior photoreduction performance over g-Fe3O4/2RGO photocatalyst Generally, pH plays a major role for the existence of Cr(VI) in acidic and neutral/alkali medium.13 That’s why Cr(VI) exists as HCrO4- in acidic medium, whereas CrO42- is the predominate form in neutral or alkali medium.13 In acidic medium, the photoreduction of Cr(VI) over g-Fe3O4 photocatalyst can be illustrated as following equations, g-Fe3O4 + hυ→ e-(CB) + h+(VB)

(1)

h+(VB) + H2O (OH-) → OH•

(2)

HCrO4- + 7H+ + 3e-(CB) → Cr3+ + 4H2O

(3)

2H2O → O2 + 4H+ + 4 e-

(4)

The net reaction is as follows: 4 HCrO4- + 16 H+ → 4 Cr3+ + 10 H2O +3 O2

(5)

From above equation, it can be speculated that photoreduction is taking place at the conduction band (ECB) of g-Fe3O4 under visible light irradiation. That means the photoreduction efficiency of g-Fe3O4 is directly dependent on the lifetime of photogenerated charge carriers (e-/h+) under light illumination. Based on our above results, enhanced photocatalytic performance of g-Fe3O4 upon loading of RGO which can be explained by



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following two aspects: (1) introduction RGO to g-Fe3O4 increases the light harvesting capacity of g-Fe3O4 concluded from optical absorption spectra (Figure S4†) and (2) better charge carriers separation (e-/h+) concluded from photoluminescence (PL) and photocurrent measurement also supports our above interpretation on efficient photoexcited charge separation (e-/h+), shown in Figure 7 (a) & (b), respectively. The recorded PL spectra of all photocatalysts are represented in Figure 7 (a). The PL intensity measures the extent of radiative recombination of excited e- & h+. More is the recombination of e- & h+, then more will be the PL intensity.40-41 From Figure 7(a), it can be seen that 2 wt% loading of RGO to gFe3O4 showed lowest PL intensity compared to all other samples. Thus, it is confirmed that better separation of photo-excited charge carriers (e-/h+) in g-Fe3O4/2RGO nanocomposite may be due to the sinking of photoexcited e- by 2D RGO Sheets from the ECB of g-Fe3O4 nanoparticles. Further, photocurrent measurement also supports the above evidence. The photocurrent density of Fe3O4, g-Fe3O4, g-Fe3O4/1RGO, g-Fe3O4/1.5RGO, g-Fe3O4/2RGO & g-Fe3O4/2.5RGO was found to be 0.38, 0.65, 1.75, 2.92 & 2.16 mA/cm2, respectively under visible light irradiation (λ ≥ 400 nm). The highest photocurrent density of g-Fe3O4/2RGO compared to all other sample confirms that more electrons are flowing through the circuit upon light illumination. Hence, it can be clearly revealed that appropriate amount loading of RGO (2wt%) to g-Fe3O4 greatly inhabit the recombination of photoexcited charge carriers (e/h+) by trapping the photoexcited e- from ECB of g-Fe3O4 and channelizing through its 2D surface.42 Based on above discussion, the mechanism of Cr(VI) reduction is depicted in Figure 7(c). Under visible light illumination, electrons (e-) get excited from the conduction band (ECB) of g-Fe3O4 to its valence band (EVB) and transferred to 2D RGO sheets. RGO channelizes these electrons through its extended Π-conjugated network resulting in the reduction of Cr6+ to Cr3+, while photogenerated holes (h+) at the EVB of the g-Fe3O4 get reduced by water. For the better convenience of the above-proposed reduction of Cr6+ to Cr3+,


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XPS analysis g-Fe3O4/2RGO photocatalysts (before and after photoreduction experiments) was carried out and is shown in Figure 7(d). The XPS spectra (i) in Figure 7(d) illustrates detail analysis of Fe 2p, O 1s & C1s in g-Fe3O4/2RGO photoreduction of Cr(VI). However, XPS spectra (ii) of g-Fe3O4/2RGO in Figure 7(d) showed a new peak around the binding energy of 580 eV after photoreduction experiment. Further, the individual XPS scan for Cr 2p (Figure 7(e)) showed two photoelectron peaks at 588.6 & 577.9 eV that can be assigned to Cr 2p1/2 & Cr 2p3/2, respectively.13 & 43 From the above it can be concluded that the accumulation of Cr(III) on the surface of RGO sheets after photoreduction of Cr(VI) which is very similar to our previous report on α-FeOOH nanorod/RGO composite.13 That means the above evidence supports our proposed mechanism on the photoreduction of Cr(VI) to Cr(III) over g-Fe3O4/2RGO nanocomposite as described earlier. Further, the superior photoreduction ability of g-Fe3O4/2RGO nanocomposite towards Cr(VI) to Cr(III) has been compared to other graphene-based photocatalysts including the ones reported in our previous work (TableS1, ESI†). The present system (Fe3O4/2RGO nanocomposite) could able to reduce 97% of Cr(VI) (50 mg/L) in 1h and the obtained result is quite impressive in comparison to our previous report including other graphene-based materials. Photodegradation of Phenol Photodegradation of phenol has been carried out over all synthesised samples under visible light irradiation. Briefly, 50 mg of prepared photocatalyst was added to 100 mL of closed pyrex flask containing 50 mL of 10 ppm phenol solution under stirring. The above solution was exposed visible light irradiation in an irradiation chamber (BS-02, Germany) for 150 min and the percentage of phenol degradation was analysed in using Cary-100 (Varian, Australia) spectrophotometer. The reaction time for phenol degradation was optimised over gFe3O4/2RGO nanocomposite and is shown in Figure 8(a). The percentage of phenol degradation was measured at different time intervals such as 30, 60, 90, 120 & 150 min. The



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minor increment in the percentage of phenol degradation was observed after 120 min of light irradiation. So, the photodegradation efficiency of phenol over all the prepared photocatalyst was examined by exposing to visible light for 120 min. The percentage of degradation over Fe3O4, g-Fe3O4, g-Fe3O4/1RGO, g-Fe3O4/1.5RGO, g-Fe3O4/2RGO & g-Fe3O4/2.5RGO was found to be 21, 33, 47, 58, 76 & 69, respectively (Figure 8(b)). The activity of g-Fe3O4 was greatly enhanced with an increase in loading amount of RGO. g-Fe3O4/2RGO showed superior activity as compared to all other samples.

Mechanism insight for superior photodegradation performance over g-Fe3O4/2RGO photocatalyst According to reported literature, hydroxyl radicals formed during the photochemical process are responsible for the degradation of phenol. In this case, the numbers of hydroxyl radicals produced during the photochemical reaction by each prepared photocatalysts were investigated to correlate with degradation efficiency as shown in Figure 8(c). The investigation was carried out with the help of terephthalic acid (TA) fluorescence probe technique. During photochemical reaction, the as-formed hydroxyl radicals by the catalyst will react with TA to give 2-hydroxyterephthalic acid (TAOH) which gives a fluorescent emission signal at 426 nm upon excitation at 315 nm (Figure 8(c)).15&44 The observed higher fluorescent intensity for g-Fe3O4/2RGO from Figure 8(c)) confirmed that more amount of hydroxyl radicals are produced on its surface. That means a perfect amount of RGO loading to g-Fe3O4 nanoparticle plays an important role in the generation of hydroxyl radicals on the surface of the catalysts. Based on the above discussion the proposed mechanism of phenol degradation over g-Fe3O4/2RGO nanocomposite is represented in Figure 8(d).When gFe3O4/2RGO photocatalyst is exposed to visible light irradiation, electrons are excited from its ECB leaving holes in EVB. Now, RGO sinks these photogenerated electrons from the ECB of


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g-Fe3O4 which results in better separation of photoexcited charge carriers and utilizes them for the formation of hydroxyl radicals. Simultaneously, hydroxyl radicals are also formed at the EVB of g-Fe3O4 due to the reaction of holes with hydroxyl groups attached to its surface. Our proposed mechanism is similar to our previous report on α-Fe2O3 nanorod/RGO composite15 and also with other reported literature.45-46 The mechanism of phenol degradation can be summarised by following equations; g-Fe3O4/2RGO + hυ → e-cb + h+vb → RGO (e-) + h+vb

(1)

RGO (e-) + O2 → O.2-

(2)

RGO (e-) + O.2- + H2O → H2O. + OH

(3)

h+vb + H2O → OH. + H+

(4)

RGO (e-) + H2O. + H+ → H2O2

(5)

RGO (e-) + H2O2

(6)

→ OH

.

+ OH-

Phenol + OH. → →Intermediates→ CO2 +H2O

(7)

Further, the superior performance of g-Fe3O4/2RGO nanocomposite towards phenol degradation has been compared to other graphene-based photocatalysts including the ones reported in our previous work (Table S3, ESI†). Table S3 (ESI†) confirms better activity of g-Fe3O4/2RGO as compared to our previous work on α-Fe2O3 nanorod/RGO15 and the obtained result is quite impressive in comparison to other graphene-based materials. Antibacterial activity The antibacterial activities of GO and g-Fe3O4/2RGO nanocomposite were evaluated against three human pathogenic bacteria (two gram-positive and one gram-negative) which are shown in table-2 & Figure 9. The results of the antibacterial activity against each pathogenic strain (2 gram-negative and one gram-positive) with the zone of inhibition ranging from 7.0 mm to 16.0 mm are shown in table-2. In the present study, the results obtained for antibacterial activity of GO and g-Fe3O4/2RGO nanocomposite were compared with respect



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to a standard antibiotic (30 mcg) in which g-Fe3O4/2RGO showed better antimicrobial activity against 3 bacterial pathogens (two gram-positive & one gram-negative) whereas, GO showed less activity (Table-2 & Figure 9). After complete incubation, zones of inhibition were observed for both the sample (GO and g-Fe3O4/2RGO) at 50 mg/ml (Figure 9). Out of which a strong bactericidal activity of g-Fe3O4/2RGO was observed for Staphylococcus aureus bacteria. The principal mechanism behind the antibacterial activity may be attributed to formation of oxidative stress by reactive oxygen species (ROS) as well as superoxide radicals (O2-), hydroxyl radicals (OH.), hydrogen peroxide(H2O2) and the singlet oxygen (1O2), which are mainly responsible for damaging the proteins and DNA in bacteria.47-48 Kim et al. reported a similar process that Fe2+ reacted with O2 to produce hydrogen peroxide (H2O2), as a result, the same H2O2 further effectively reacted with ferrous irons through the Fenton’s reaction and produced hydroxyl radicals. These radicals damaged the biological micromolecules.49 Moreover, according to literature very small size nanoparticles (10-80nm size) can easily penetrate the bacterial membrane which leads to bactericidal effects in the bacterial species.47 The present work reports that synergic effect of RGO and g-Fe3O4 (particle size of 22±2 nm) result in the generation of more number of ROS in compared to GO which plays a major role in the inhibition of a wide range of human pathogenic bacteria including Staphylococcus aureus. Kim et al have reported that the concentration of silver nanoparticle plays an important role for bacterial activities towards Staphylococcus aureus and Escherichia Coli.50 In our present investigation, we also observed that the concentration of GO and g-Fe3O4/2RGO nanocomposite was an important aspect of bactericidal activity. Conclusions The present work demonstrates a novel, eco-friendly, cost-effective & single-step fabrication of magnetically separable and photocatalytically stable g-Fe3O4/RGO nanocomposite in presence of Averrhoa carambola leaf extract. The adopted hydrothermal process results in


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well incorporation of g-Fe3O4 nanoparticles at an average size of 22±2 nm in to 2D RGO sheets. Among all, g-Fe3O4/2RGO nanocomposite showed superior photocatalytic performance towards the reduction of 50 mg/L of Cr(VI) (97%) in 1 h and 10 mg/L of phenol degradation (76%) in 2 h at room temperature under visible light illumination. The gFe3O4/2RGO nanocomposite showed better bactericidal properties against gram-positive bacteria in contrast to gram-negative bacteria. The disc diffusion analysis result concludes that Fe3O4/RGO nanocomposite showed a higher zone of inhibition as compared to GO and which is also more or less comparable to a silver nanoparticle of topical applications. Our investigation reveals the potential application of g-Fe3O4/RGO nanocomposite as a good antibacterial agent which may be used for topical applications in the field of pharmaceuticals as well as in biomedical sectors. Acknowledgement The authors are very much gratified to Prof. B.K. Mishra, Director, CSIR-IMMT for all support to publish the work. One of the authors, Mr. D. K. Padhi is also thankful to CSIRNew Delhi, India for awarding him SRF. References (1) Cha, C.; Shin, S. R.; Annabi, N.; Dokmeci, M. R.; Khademhosseini, A. Carbon-Based Nanomaterials: Multi-Functional Materials for Biomedical Engineering. ACS Nano, 2013, 7, 2891–2897. DOI: 10.1021/nn401196a



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and

Oxidative

Stress.

ACS

Nano,

DOI: 10.1021/nn202451x


2011,

5,

6971-6980.

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Zhang, B.; Xu, X. X.; Fu, Y. H. Graphene Oxide Coated Coordination

Polymer Nanobelt Composite Material: A New Type of Visible Light Active and Highly Efficient Photocatalyst for Cr(VI) Reduction. Dalton Trans., 2015, 44, 11155-11164. DOI: 10.1039/C5DT01190F List of Schemes, Figures, and Tables with Captions Scheme 1:

Schematic representation of the hydrothermal synthesis of g-Fe3O4/2RGO nanocomposite.

Scheme 2:

Schematic representation of interaction between bio-surfactant and in-situ formed Fe3O4 over RGO sheets.

Table 1.

The intensity ratio of “D” and “G” band (ID/IG) and Variation in the “D” and “G” band position of GO and g-Fe3O4/2RGO nanocomposite.

Table 2.

Zone of inhibition (in nm) of antibacterial activity of GO and gFe3O4/2RGO nanocomposite compared with standard antibiotic.

Figure 1.

XRD pattern of Fe3O4, green synthesised Fe3O4 and all RGO loaded green synthesised Fe3O4.

Figure 2.

Raman spectra of as synthesised GO and g-Fe3O4/2RGO nanocomposite.

Figure 3.

FESEM images of (a) Fe3O4, (b) g-Fe3O4, (c) & (d) g-Fe3O4/2RGO nanocomposite and (e) EDX spectrum of g-Fe3O4/2RGO nanocomposite.

Figure 4.

X-ray mapping of g-Fe3O4/2RGO nanocomposite.



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Figure 5.

TEM images of (a) Fe3O4, (b) g-Fe3O4, and (c) & (d) g-Fe3O4/2RGO nanocomposite. (e) & (F) SAED pattern and HRTEM image of gFe3O4/2RGO nanocomposite, respectively.

Figure 6.

(a) Photocatalytic reduction of Cr(VI) by Fe3O4, g-Fe3O4 and RGO loaded g-Fe3O4 nanocomposite under Visible-light irradiation[catalyst dose = 1g/L; [Cr(VI)] = 50 mg/L; time = 60 min], (b) Effect of pH on the initial rate of Cr(VI) photo-reduction over Fe3O4, g-Fe3O4 & g-Fe3O4/2RGO and (c) & (d) UV−visible spectral evolution with time during reduction and Reusability mechanism of photocatalytic reduction of Cr(VI) over gFe3O4/2RGO nanocomposite photocatalyst, respectively.

Figure 7.

(a) PL spectra of Fe3O4 (i), g-Fe3O4/1RGO (ii), g-Fe3O4/1.5RGO (iii), gFe3O4/2.5RGO (iv) & g-Fe3O4/2RGO (v); (b) Photocurrent spectra of the gFe3O4 and RGO loaded g-Fe3O4 under illumination of visible light (λ ≥ 400 nm) ;(c) Mechanism of photocatalytic reduction of Cr(VI) over & gFe3O4/2RGO; (d) XPS of g-Fe3O4/2RGO:Before (i) & after (ii) photoreduction experiment and (e) Survey for Cr 2p.

Figure 8.

(a) Time variation plot for the g-Fe3O4/2RGO; (b) Photocatalytic degradation of Phenol by all synthesised photocatalysts under visible-light irradiation [catalyst dose = 1g/L; Phenol = 10 mg/L; time = 120 min]; (c) PL spectral changes during visible light illumination for all the photocatalysts in 5×10-5 M basic solution of terephthalic acid (excitation at 315 nm) and (d) Mechanism of photodegradation of phenol over gFe3O4/2RGO nanocomposite.


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Figure 9.

Antimicrobial activity (zone of inhibition) of GO and g-Fe3O4/2RGO nanocomposite against S. Aureous (a&b), Bacilus subtilis (c&d) and Escherichia Coli (e&f).

Scheme 1



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Scheme 2

Sample name

GO g-Fe3O4/2RGO

Position

of

“D” Position of “G” ID/IG

band(cm-1)

band(cm-1)

1355

1596

0.8327

1342

1572

1.1841

Table 1


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Name of the strain

Zone of inhibition in mm of Standard antibiotics nanocomposite (50mg/ml)

(30µl/disc)

GO

(g-Fe3O4/2RGO) Gentamycin

Staphylococuss aureus (+)

10.0

16.0

23.0

Bacilus subtilis (+)

7.1

10.0

18.0

Escherichia coli (-)

7.9

11.2

17.0

Table 2

Figure 1



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Figure 2

Figure 3


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Figure 4



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Figure 5


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Figure 6

Figure 7



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Figure 8

Figure 9


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“For Table of Contents Use Only”

Fe3O4/RGO nanocomposite have been synthesised by a sustainable hydrothermal rout in presence of biomolecules for multipurpose water purification application.


RGO Nanocomposite with Enhanced

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