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Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10551-10562

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 Parida,*,†,∥ S. K. Singh,† and P. M. Mishra‡

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†

Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110 001, India ‡ Environment & Sustainability Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751 013, Odisha, India § Department of Biotechnology, North Orissa University, Baripada, Mayurbhanj 757003, Odisha, India ∥ Centre for Nano Science and Nano Technology, S‘O’A University, Bhubaneswar 751030, Odisha, India S Supporting Information *

ABSTRACT: Herein, we report a novel single-step hydrothermal synthesis of a photocatalytically stable and magnetically separable g-Fe3O4/RGO nanocomposite in the presence of Averrhoa carambola leaf extract as a natural surfactant for multipurpose water purification application. The Averrhoa carambola leaf extract played a major role in the modification of structural, optical, and electronic properties of the Fe3O4 nanoparticle. At room temperature, the g-Fe3O4/2RGO nanocomposite showed 97% and 76% of Cr(VI) reduction and phenol degradation, respectively. The higher activity of gFe3O4/2RGO was attributed to the in situ loading of RGO, and the synergism developed between RGO and the super magnetic Fe3O4 nanoparticle results in better separation of photoexcited charge carriers (e−/h+) which was concluded from photoluminescence and photocurrent measurements. Further, the g-Fe3O4/2RGO nanocomposite showed better antimicrobial activity against three bacterial pathogens such as Staphylococcus aureous (MTCC-737), Bacillus subtilis (MTCC736), and Escherichia coli (MTCC-443) compared to GO with respect to a standard antibiotic (30 μg). KEYWORDS: Green synthesis, Natural surfactant, Graphene, Electron transfer, Photochemistry

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RGO,13 and 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 utilizing 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 three issues: (1) separation, (2) recovery, and (3) reuse of photocatalyst from the suspended carcinogenic solution after photochemical reaction.16−19

INTRODUCTION

In the present scenario, the 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.2−4 Among them, developments of graphene-based semiconductor photocatalysts have been considered as an emerging candidate to address the budding environmental issues.5−7 In fact, 2D graphene consists of sp2 hybridized 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/RGO,12 α-FeOOH nanorod/ © 2017 American Chemical Society

Received: July 26, 2017 Revised: September 25, 2017 Published: October 10, 2017 10551

DOI: 10.1021/acssuschemeng.7b02548 ACS Sustainable Chem. Eng. 2017, 5, 10551−10562

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Representation of the Hydrothermal Synthesis of g-Fe3O4/2RGO Nanocomposites

Fe3O4/RGO nanocomposite photocatalyst in the 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 the super magnetic Fe3O4 nanoparticle decorated on RGO sheets exhibited superior photocatalytic activity compared to our previous work on Fe3O4,30 α-FeOOH nanorod/RGO,13 αMnO2@RGO,14 and α-Fe2O3 nanorod/RGO composite15 under visible light illumination.

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.20−23 In particular, magnetite (Fe3O4) has been considered as one of the challenging material in this category with regard to its costeffective 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 toward 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 of the above groups claimed that the presence of RGO and surfacing of the synergistic effect between the RGO and Fe3O4 are key factors for the enhanced activity of Fe3O4 toward environmental remediation. In the present context, it is intended to study the role of biosurfactant for the modification of structural, optical, and electronic properties of the Fe3O4/RGO nanocomposite toward environmental remediation. To date, this work will be the first report on the synthesis of the Fe3 O4 /RGO nanocomposite in the 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 (synthesized in the presence of biomolecules) showed better photocatalytic activity compared to C−Fe3O4 (prepared by chemical method).31 To further improve the photocatalytic activity of Fe3O4, we have adopted a single-step hydrothermal synthesis of

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EXPERIMENTAL SECTION

Materials. Natural graphite powders and potassium permanganate (KMnO4) were purchased from Sigma-Aldrich Chemicals. Ammonium hydroxide (NH4OH), sulfuric acid (H2SO4), hydrochloric acid (HCl), sodium nitrate (NaNO3), and sodium hydroxide (NaOH) were obtained from Finar Chemicals Limited. Ferric chloride anhydrous (FeCl3) and ferrous sulfate heptahydrate (FeSO4·7H2O) were obtained from Spectrochem Chemicals. All of these above chemicals were of analytical grade and were used without further purification. Synthesis of Magnetite in the 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 (Fe3+:Fe2+ = 2:1) was dissolved in 30 mL of distilled water and stirred for 30 min. About 20 mL of leaf extract was added to the 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 a Teflon-lined autoclave for hydrothermal treatment at 180 °C for 6 h. The obtained precipitate was centrifuged, washed with distilled water followed by absolute ethanol for several times to remove heavy biomass of A. carambola leaf extract, and then dried in an oven at 100 °C overnight. The obtained final product was designated as g-Fe3O4. For comparison study, magnetite was also synthesized by following the 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 the g-Fe3O4/RGO nanocomposite, a calculated amount of as prepared GO was dispersed in 40 mL of 10552


Research Article

ACS Sustainable Chemistry & Engineering deionized water for 1 h to obtained exfoliated graphene oxide (EGO). Now, EGO was mixed with the previously prepared suspension of Fe3+/Fe2+ solution and A. carambola leaf extract and stirred for 30 min. Then, the rest of the synthesis process was adopted the same as that of the synthesis of g-Fe3O4. The above synthesis procedure is illustrated in Scheme 1. The as synthesized g-Fe3O4/RGO nanocomposites were named according to the amount of GO loading i.e., 1 wt % (g-Fe3O4/ 1RGO), 1.5 wt % (g-Fe3O4/1.5RGO), 2 wt % (g-Fe3O4/2RGO), and 2.5 wt % (g-Fe3O4/2.5RGO). Photocatalytic Activity. The photocatalytic activity of the as synthesized 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, and 10 in similar conditions. The prepared photocatalysts (1 g/L) were suspended in 20 mL of 50 ppm of Cr(VI) and phenol solution and stirred for 30 min in the dark to obtain an adsorption−desorption equilibrium. Then, the beakers containing Cr(VI)-catalyst and phenolcatalyst suspensions were exposed to visible irradiation. For the analysis purposes, 2 mL of suspension was sampled at a certain time interval and centrifuged to remove the nanocatalyst, and the reduction of Cr(VI) and degradation of phenol were analyzed quantitatively with the filtrates using a 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) and Bacillus subtilis (MTCC-736) and one Gramnegative 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 °C. The slanted pathogenic bacteria were subcultured before use. Disc diffusion analysis was carried out to establish the antibacterial activities of GO and the gFe3O4/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 °C for 24 h 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 the g-Fe3O4/RGO nanocomposite. The maximum zone of inhibition was measured manually. Gentamycin was taken as the standard antibiotic.

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Figure 1. XRD pattern of Fe3O4, green synthesized Fe3O4, and all RGO loaded green synthesized Fe3O4.

sheets occurs, and this is due to the interaction between Fe3+ and Fe2+ ions with the oxygen-related functionalities of GO which leads to the breaking of the regular lamellar structure of GO sheets.34,35 Raman Spectra. In order to confirm the composite formation between g-Fe3O4 and GO and the reduction of GO to RGO, Raman analysis has been carried out. Figure 2a displays the Raman spectra of the as synthesized g-Fe3O4 and gFe3O4/2RGO nanocomposite. The Raman spectra of GO showed two peaks positioned at 1355 and 1596 cm−1 which can be assigned as D and G bands respectively, and two broad peaks positioned at 2695 and 2933 cm−1 which correspond to the 2D band of GO.36,37 Out of which, the appearance of D and G bands is due to disordered atomic arrangement caused by sp3 hybridized carbon atom and plane vibration of sp2 hybridized carbon atoms in 2D GO sheets, respectively, whereas secondorder Raman scattering process results in appearance of the 2D band of GO.38 The Raman spectrum of the g-Fe3O4/2RGO nanocomposite showed all of the above bands, i.e., D, G, and 2D bands, along with all fundamental Raman vibrations of Fe3O4 confirming the composite formation during adopted hydrothermal route. The shifting of D and G bands of GO in the Raman spectrum of the g-Fe3O4/2RGO nanocomposite (Figure 2b) strongly suggests the good incorporation of Fe3O4 nanoparticles in between the 2D GO layers.15 Further, the increase in the intensity ratio of D and G bands (ID/IG) of the 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 the as synthesized neat Fe3O4, g-Fe3O4, and g-Fe3O4/2RGO nanocomposite. From FESEM images, it is observed that the particle size of neat Fe3O4 (Figure 3a) is comparatively larger as compared to that for g-Fe3O4 (Figure 3b) along with the well distribution of gFe3O4 over GO sheets by the adopted hydrothermal process (Figure 3). EDX analysis of the g-Fe3O4/2RGO nanocomposite (Figure 3e) confirms the presence of constituent elements, i.e.,

RESULTS AND DISCUSSION

XRD. The powder X-ray diffraction (PXRD) patterns of all synthesized photocatalysts were recorded to confirm their crystallinity and phase purity and are shown in Figure 1. All of the observed diffraction patterns for the as-synthesized 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 gFe3O4/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 the decrease of crystallite size and loading of GO also results in the 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 10553


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ACS Sustainable Chemistry & Engineering

Figure 2. Raman spectra of as synthesized GO and the g-Fe3O4/2RGO nanocomposite.

concentric rings are observed from the selected area electron diffraction (SAED) pattern of the g-Fe3O4/2RGO nanocomposite (Figure 5e) due to the diffraction from the (110), (130), (111), and (210) planes of g-Fe3O4 nanoparticles indicating its polycrystalline nature which is very consistent with the XRD data. In addition to this, HRTEM analysis also confirmed the crystallinity character of the g-Fe3O4 nanoparticle and is presented in Figure 5f. From Figure 5f, the obtained lattice fringe spacing 0.485 nm ensures the 111 plane of g-Fe3O4 and is in good 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 and RGO sheets played a vital role in the photocatalytic as well as the antibacterial activity of the g-Fe3O4/2RGO nanocomposite. Good 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 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, the optimized amount of photocatalysts (1 g/L) and aqueous Cr(VI) solution (50 mg/L) were exposed to visible light irradiation subjected to continuous stirring. Figure 6a displays the reduction ratio (C/C0) as a function of time. Under dark conditions, no temporal concentrations of changes were observed but upon illumination of visible light. Among all photocatalysts, the g-Fe3O4/2RGO nanocomposite showed the highest photoreduction efficiency under visible light irradiation, i.e., 97% reduction of Cr(VI) (50 mg/L) at pH 2, whereas Fe3O4 and g-Fe3O4 showed only 22 and 38%, respectively. A synthetic approach greatly influences the photocatalytic activity of Fe3O4. Fe3O4 nanoparticle which have been prepared in the presence of A. carambola leaf extract (g- Fe3O4) showed better activity than chemically synthesized Fe3O4 which is attributed to the surface modification and reduction in particle size of Fe3O4 in the presence of biomolecules during hydrothermal

Table 1. Intensity Ratios of D and G Bands (ID/IG) and Variation in the D and G Bands Positions of GO and the gFe3O4/2RGO Nanocomposite sample name

position of D band (cm−1)

position of G band (cm−1)

ID/IG

GO g-Fe3O4/2RGO

1355 1342

1596 1572

0.8327 1.1841

C, O, and Fe. Further, X-ray elemental mapping of the gFe3O4/2RGO nanocomposite (Figure 4a) was carried to get the information regarding the distribution of C, O, and Fe in the matrix as shown in Figure 4b−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 the g-Fe3O4/ 2RGO nanocomposite. The average particle size of neat Fe3O4 was observed to be 46 ± 2 nm (Figure 5a), whereas this value for g-Fe3O4 was found to be 32 ± 2 nm (Figure 5b). 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 the presence of A. carambola leaf extract which may be due to the fact that the interaction of bio molecules present in the leaf extract with Fe3+and Fe2+ ions hinders the particle growth during Fe3O4 formation (Scheme 2). The formation mechanism of Fe3O4 nanoparticles in the presence of A. carambola leaf extract has been already reported by the author’s group.31 A control Fe3O4/2RGO nanocomposite was prepared without biomolecules to understand the promoting effect of biomolecules on the growth of g-Fe3O4 nanoparticles on RGO sheets, shown in Figure S3. It can be clearly seen that the distribution of Fe3O4 on RGO sheets is not good. That means the Fe3O4 nanoparticles are getting more agglomerated on the RGO surface in the absence of biomolecules. However, good distribution of g-Fe3O4 nanoparticles over RGO sheets was successfully obtained in the presence of biomolecules by the adopted hydrothermal route (Figure 5d). The strong interaction of Fe3+ and Fe2+ 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 the gFe3O4/2RGO nanocomposite was observed to be 22 ± 2 nm. That means the introduction of RGO to g-Fe3O4 nanoparticles further reduces its particle size. Several bright continuous 10554


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Figure 3. FESEM images of (a) Fe3O4, (b) g-Fe3O4, (c and d) the g-Fe3O4/2RGO nanocomposite, and (e) EDX spectrum of the g-Fe3O4/2RGO nanocomposite.

2% loading of RGO, i.e., 97% Cr (VI) reduction occurs in 1 h. The higher amount of RGO content, i.e., 2.5% in the g-Fe3O4/ 2RGO nanocomposite, leads to a decrease in the photoreduction ability of g-Fe3O4 which may be attributed to a “shielding effect” of RGO in the g-Fe3O4/2.5RGO nanocomposite.39 Effect of pH on Cr(VI) Reduction. The effect of pH on the reduction of Cr(VI) over Fe3O4, g-Fe3O4, and the g-Fe3O4/ 2RGO photocatalyst (1 g/L) was investigated by varying the pH from 2 to 10 and is shown in Figure 6b. It was found that the Cr(VI) reduction ability of all photocatalysts decreases with increasing the pH of the Cr(VI) solution. At pH 2, Fe3O4, gFe3O4, and g-Fe3O4/2RGO photocatalysts showed 22, 38, and 97% photoreduction ability under visible light irradiation for 1 h, respectively. At pH 10, Fe3O4, g-Fe3O4, and g-Fe3O4/2RGO photocatalysts showed 5, 13, and 22% of Cr(VI) reduction, respectively, under the same experimental conditions. Inductively coupled plasma optical emission spectrometry (ICPOES, 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, Supporting Information). 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 the g-Fe3O4/2RGO photocatalyst can be explained on the basis of the following two points, i.e., (1)

Figure 4. X-ray mapping of the g-Fe3O4/2RGO nanocomposite.

synthesis. The Cr (VI) reduction ability of as synthesized photocatalysts exhibit the following order: Fe3O4 < g-Fe3O4 < g-Fe3O4/1RGO < g-Fe3O4/1.5RGO< g-Fe3O4/2.5RGO < gFe3O4/2RGO. The above observation suggests that the optimum photoreduction efficiency of g-Fe3O4 is achieved at 10555


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Generally, pH plays a major role in the existence of Cr(VI) in acidic and neutral/alkali medium.13 That is 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 the g-Fe3O4 photocatalyst can be illustrated by the following equations: g‐Fe3O4 + hυ → e−(CB) + h+(VB)

(1)

h+(VB) + H 2O(OH−) → OH•

(2)

HCrO4 − + 7H++ 3e−(CB) → Cr 3 + +4H 2O

(3)

2H 2O → O2 + 4H+ + 4e−

(4)

The net reaction is as follows: 4HCrO4 − + 16H+ → 4Cr 3 + + 10H 2O + 3O2

(5)

From the 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, the enhanced photocatalytic performance of g-Fe3O4 upon loading of RGO can be explained by the following two aspects: (1) introduction of RGO to g-Fe3O4 increases the light harvesting capacity of gFe3O4 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 7a,b, respectively. The recorded PL spectra of all photocatalysts are represented in Figure 7a. The PL intensity measures the extent of radiative recombination of excited e− and h+. The greater the the recombination of e− and h+ is , then the greater will be the PL intensity.40,41 From Figure 7a, it can be seen that 2 wt % loading of RGO to g-Fe3O4 showed the lowest PL intensity compared to all other samples. Thus, it is confirmed that better separation of photoexcited charge carriers (e−/h+) in the gFe3O4/2RGO 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 densities of Fe3O4, g-Fe3O4, g-Fe3O4/1RGO, g-Fe3O4/1.5RGO, g-Fe3O4/2RGO, and g-Fe3O4/2.5RGO were found to be 0.38, 0.65, 1.75, 2.92, and 2.16 mA/cm2, respectively, under visible light irradiation (λ ≥ 400 nm). The highest photocurrent density of g-Fe3O4/ 2RGO compared to all other samples confirms that more electrons are flowing through the circuit upon light illumination. Hence, it can be clearly revealed that an appropriate amount loading of RGO (2 wt %) to g-Fe3O4 greatly inhabits the recombination of photoexcited charge carriers (e−/h+) by trapping the photoexcited e− from ECB of gFe3O4 and channeling through its 2D surface.42 Based on the above discussion, the mechanism of Cr(VI) reduction is depicted in Figure 7c. 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-

Figure 5. TEM images of (a) Fe3O4, (b) g-Fe3O4, and (c and d) the g-Fe3O4/2RGO nanocomposite. (e and f) SAED pattern and HRTEM image of the g-Fe3O4/2RGO nanocomposite, respectively.

HCrO4− and Cr2O42− are the predominant anionic chromate species existing at lower pH and (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 the form of spectral change with respect to different time interval over the g-Fe3O4/2RGO photocatalyst is illustrated in Figure 6c. Reusability of the g-Fe3O4/2RGO Photocatalyst. In order to examine the stability of the as prepared g-Fe3O4/ 2RGO photocatalyst, a recycling study was performed under identical conditions and is shown Figure 6d. As shown in Figure 6d, the stability of the g-Fe3O4/2RGO photocatalyst was observed up to a third cycle with a slight decrease in photoreduction efficiency, i.e., 97, 96, and 94% in the first, second, and third cycle, respectively. Hence, from the 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 the g-Fe3O4/2RGO nanocomposite toward photoreduction of Cr(VI) ability has been compared to other graphene-based photocatalysts including the ones reported in our previous work (Table S2, Supporting Information). Mechanism Insight for Superior Photoreduction Performance over the g-Fe3O4/2RGO Photocatalyst. 10556


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Scheme 2. Schematic Representation of Interaction between Biosurfactant and in Situ Formed Fe3O4 over RGO Sheets

proposed reduction of Cr6+ to Cr3+, XPS analysis g-Fe3O4/ 2RGO photocatalysts (before and after photoreduction experiments) was carried out and is shown in Figure 7d. The XPS spectra (i) in Figure 7d illustrates the detail analysis of Fe 2p, O 1s, and C 1s in g-Fe3O4/2RGO photoreduction of Cr(VI). However, XPS spectra (ii) of g-Fe3O4/2RGO in Figure 7d shows a new peak around the binding energy of 580 eV after the photoreduction experiment. Further, the individual XPS scan for Cr 2p (Figure 7e) showed two photoelectron peaks at 588.6 and 577.9 eV that can be assigned to Cr 2p1/2 and Cr 2p3/2, respectively.13,43 From the above, the accumulation of Cr(III) on the surface of RGO sheets after photoreduction of Cr(VI) can be concluded, which is very similar to our previous report on the 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 the g-Fe3O4/ 2RGO nanocomposite as described earlier. Further, the superior photoreduction ability of the g-Fe3O4/2RGO nanocomposite toward Cr(VI) to Cr(III) has been compared to other graphene-based photocatalysts including the ones

reported in our previous work (Table S1, Supporting Information). The present system (Fe3O4/2RGO nanocomposite) could be able to reduce 97% of Cr(VI) (50 mg/ L) in 1 h, and the obtained result is quite impressive in comparison to our previous report including other graphenebased materials. Photodegradation of Phenol. Photodegradation of phenol has been carried out over all synthesized samples under visible light irradiation. Briefly, 50 mg of prepared photocatalyst was added to a 100 mL closed pyrex flask containing 50 mL of 10 ppm phenol solution under stirring. The above solution was exposed to visible light irradiation in an irradiation chamber (BS-02, Germany) for 150 min, and the percentage of phenol degradation was analyzed in using Cary100 (Varian, Australia) spectrophotometer. The reaction time for phenol degradation was optimized over the g-Fe3O4/2RGO nanocomposite and is shown in Figure 8a. The percentage of phenol degradation was measured at different time intervals such as 30, 60, 90, 120, and 150 min. The minor increment in the percentage of phenol degradation was observed after 120 10557


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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) photoreduction over Fe3O4, g-Fe3O4, and g-Fe3O4/2RGO and (c and d) UV−visible spectral evolution with time during reduction and Reusability mechanism of photocatalytic reduction of Cr(VI) over the g-Fe3O4/2RGO nanocomposite photocatalyst, respectively.

Figure 7. (a) PL spectra of Fe3O4 (i), g-Fe3O4/1RGO (ii), g-Fe3O4/1.5RGO (iii), g-Fe3O4/2.5RGO (iv), and g-Fe3O4/2RGO (v). (b) Photocurrent spectra of the g-Fe3O4 and RGO loaded g-Fe3O4 under illumination of visible light (λ ≥ 400 nm). (c) Mechanism of photocatalytic reduction of Cr(VI) over and g-Fe3O4/2RGO. (d) XPS of g-Fe3O4/2RGO: before (i) and after (ii) photoreduction experiment. (e) Survey for Cr 2p.

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Figure 8. (a) Time variation plot for g-Fe3O4/2RGO. (b) Photocatalytic degradation of phenol by all synthesized 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). (d) Mechanism of photodegradation of phenol over the gFe3O4/2RGO nanocomposite.

excited from its ECB leaving holes in EVB. Now, RGO sinks these photogenerated electrons from the ECB of 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 the α-Fe2O3 nanorod/RGO composite15 and also with other reported literature.45,46 The mechanism of phenol degradation can be summarized by the following equations:

min of light irradiation. So, the photodegradation efficiency of phenol over all of the prepared photocatalyst was examined by exposing to visible light for 120 min. The percentages of degradation over Fe3O4, g-Fe3O4, g-Fe3O4/1RGO, g-Fe3O4/ 1.5RGO, g-Fe3O4/2RGO, and g-Fe3O4/2.5RGO were found to be 21, 33, 47, 58, 76, and 69, respectively (Figure 8b). The activity of g-Fe3O4 was greatly enhanced with an increase in the loading amount of RGO. g-Fe3O4/2RGO showed superior activity as compared to all other samples. Mechanism Insight for Superior Photodegradation Performance over the g-Fe3O4/2RGO Photocatalyst. According to the 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 the degradation efficiency as shown in Figure 8c. The investigation was carried out with the help of the terephthalic acid (TA) fluorescence probe technique. During the 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 8c).15,44 The observed higher fluorescent intensity for g-Fe3O4/2RGO from Figure 8c confirmed that more hydroxyl radicals are produced on its surface. That means a perfect amount of RGO loading to gFe3O4 nanoparticles 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 the g-Fe3O4/2RGO nanocomposite is represented in Figure 8d.When the g-Fe3O4/2RGO photocatalyst is exposed to visible light irradiation, electrons are

g‐Fe3O4/2RGO + hυ → e−cb + h+ vb → RGO(e−) + h+ vb (6) −

RGO(e ) + O2 → O2

•−

(7)

RGO(e−) + O2•− + H 2O → H 2O• + OH

(8)

h+ vb + H 2O → OH• + H+

(9)

RGO(e−) + H 2O• + H+ → H 2O2 −

•

RGO(e ) + H 2O2 → OH + OH

−

phenol + OH. → intermediates → CO2 + H 2O

(10) (11) (12)

Further, the superior performance of the g-Fe3O4/2RGO nanocomposite toward phenol degradation has been compared to other graphene-based photocatalysts including the ones reported in our previous work (Table S3, Supporting Information). Table S3 (Supporting Information) confirms the better activity of g-Fe3O4/2RGO as compared to our previous work on α-Fe2O3 nanorod/RGO,15 and the obtained 10559


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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), and 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 the literature very small size nanoparticles (10−80 nm size) can easily penetrate the bacterial membrane which leads to bactericidal effects in the bacterial species.47 The present work reports that the synergic effect of RGO and g-Fe3O4 (particle size of 22 ± 2 nm) results in the generation of more ROS 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 nanoparticles plays an important role for bacterial activities toward Staphylococcus aureus and Escherichia coli.50 In our present investigation, we also observed that the concentrations of GO and g-Fe3O4/2RGO nanocomposite were an important aspect of bactericidal activity.

result is quite impressive in comparison to other graphenebased 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 and Figure 9. The Table 2. Zone of Inhibition (in nm) of Antibacterial Activity of GO and the g-Fe3O4/2RGO Nanocomposite Compared with Standard Antibiotic zone of inhibition in mm of nanocomposite (50 mg/mL)

standard antibiotics (30 μL/disc)

name of the strain

GO

(g-Fe3O4/2RGO)

gentamycin

Staphylococuss aureus (+) Bacilus subtilis (+) Escherichia coli (−)

10.0 7.1 7.9

16.0 10.0 11.2

23.0 18.0 17.0

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CONCLUSIONS The present work demonstrates a novel, eco-friendly, costeffective, and single-step fabrication of a magnetically separable and photocatalytically stable g-Fe3O4/RGO nanocomposite in the presence of Averrhoa carambola leaf extract. The adopted hydrothermal process results in good incorporation of g-Fe3O4 nanoparticles at an average size of 22 ± 2 nm in 2D RGO sheets. Among all of those studied, the g-Fe3O4/2RGO nanocomposite showed superior photocatalytic performance toward 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 g-Fe3O4/ 2RGO nanocomposite showed better bactericidal properties against Gram-positive bacteria in contrast to Gram-negative bacteria. The disc diffusion analysis result concludes that the 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 the 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.

Figure 9. Antimicrobial activity (zone of inhibition) of GO and the gFe3O4/2RGO nanocomposite against S. aureous (a and b), Bacilus subtilis (c and d), and Escherichia coli (e and f).

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ASSOCIATED CONTENT

S Supporting Information *

results of the antibacterial activity against each pathogenic strain (two 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 the g-Fe3O4/2RGO nanocomposite were compared with respect to a standard antibiotic (30 μg) in which g-Fe3O4/2RGO showed better antimicrobial activity against three bacterial pathogens (two Gram-positive and one Gram-negative), whereas GO showed less activity (Table 2 and Figure 9). After complete incubation, zones of inhibition were observed for both samples (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 Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02548. Additional experimental results as discussed in the text and shown in Figures S1−S4 and Tables S1−S3 as well as two additional references. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; kulamaniparida@ soauniversity.ac.in. Tel.: +91-674-2379425. Fax: +91-6 742581637. 10560


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Characterisation and Photocatalytic Reduction of Cr(VI). Chem. Eng. J. 2014, 255, 78−88. (13) Padhi, D. K.; Parida, K. M. Facile Fabrication of α-FeOOH nanorod/RGO Composite: A Robust Photocatalyst for Reduction of Cr(VI) under Visible Light Irradiation. J. Mater. Chem. A 2014, 2, 10300−10312. (14) Padhi, D. K.; Baral, A.; Parida, K. M.; Singh, S. K.; Ghosh, M. Visible Light Active Single-Crystal Nanorod/Needle Like α-MnO2@ RGO Nanocomposites for Efficient Photoreduction of Cr(VI). J. Phys. Chem. C 2017, 121, 6039−6049. (15) Pradhan, G. K.; Padhi, D. K.; Parida, K. M. Fabrication of αFe2O3 Nanorod/RGO Composite: A Novel Hybrid Photocatalyst for Phenol Degradation,. ACS Appl. Mater. Interfaces 2013, 5, 9101−9110. (16) Liu, X.; Pan, L.; Lv, T.; Zhu, G.; Lu, T.; Sun, Z.; Sun, C. Microwave-Assisted Synthesis of TiO2-Reduced Graphene Oxide Composites for the Photocatalytic Reduction of Cr(VI). RSC Adv. 2011, 1, 1245−1249. (17) Liu, X.; Pan, L.; Zhao, Q.; Lv, T.; Zhu, G.; Chen, T.; Lu, T.; Sun, Z.; Sun, C. UV-Assisted Photocatalytic Synthesis of ZnO− Reduced Graphene Oxide Composites with Enhanced Photocatalytic Activity in Reduction of Cr(VI). Chem. Eng. J. 2012, 183, 238−243. (18) Liu, X.; Pan, L.; Lv, T.; Zhu, G.; Sun, Z.; Sun, C. MicrowaveAssisted Synthesis of CdS-Reduced Graphene Oxide Composites for Photocatalytic Reduction of Cr(VI). Chem. Commun. 2011, 47, 11984−11986. (19) Du, Y.; Tao, Z.; Guan, J.; Sun, Z.; Zeng, W.; Wen, P.; Ni, K.; Ye, J.; Yang, S.; Du, P.; Zhu, Y. Microwave-Assisted Synthesis of Hematite/Activated Graphene Composites with Superior Performance for Photocatalytic Reduction of Cr(VI). RSC Adv. 2015, 5, 81438− 81444. (20) Wang, D.; Astruc, D. Fast-Growing Field of Magnetically Recyclable Nanocatalysts. Chem. Rev. 2014, 114, 6949−6985. (21) Polshettiwar, V.; Luque, R.; Fihri, A.; Zhu, H.; Bouhrara, M.; Basset, J. M. Magnetically Recoverable Nanocatalysts. Chem. Rev. 2011, 111, 3036−3075. (22) Laurent, S.; Forge, D.; Port, M.; Roch, A.; Robic, C.; Vander Elst, L.; Muller, R. N. Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications. Chem. Rev. 2008, 108, 2064−2110. (23) Metin, O.; Aydogan, S.; Meral, K. A New Route for the Synthesis of Graphene Oxide−Fe3O4 (GO−Fe3O4) Nanocomposites and Their Schottky Diode Applications. J. Alloys Compd. 2014, 585, 681−688. (24) Akbarzadeh, A.; Samiei, M.; Davaran, S. Magnetic Nanoparticles: Preparation, Physical Properties, and Applications in Biomedicine. Nanoscale Res. Lett. 2012, 7, 144−156. (25) Ghandoor, H. E.; Zidan, H. M.; Khalil, M. M. H.; Ismail, M. I. M. Synthesis and Some Physical Properties of Magnetite (Fe3O4) Nanoparticles. Int. J. Electrochem. Sci. 2012, 7, 5734−5745. (26) Chakraborty, S.; Sharma, K. S.; Rajeswari, A.; Vimalnath, K. V.; Sarma, H. D.; Pandey, U.; Jagannath, J.; Ningthoujam, R. S.; Vatsa, R. K.; Dash, A. Radiolanthanide-Loaded Agglomerated Fe3O4 Nanoparticles for Possible Use in the Treatment of Arthritis: Formulation, Characterization and Evaluation in rats. J. Mater. Chem. B 2015, 3, 5455−5466. (27) Peik-See, T.; Pandikumar, A.; Ngee, L. H.; Ming, H. N.; Hua, C. C. Magnetically Separable Reduced Graphene Oxide/Iron Oxide Nanocomposite Materials for Environmental Remediation. Catal. Sci. Technol. 2014, 4, 4396−4405. (28) Boruah, P. K.; Borthakur, P.; Darabdhara, G.; Kamaja, C. K.; Karbhal, I.; Shelke, M. V.; Phukan, P.; Saikia, D.; Das, M. R. Sunlight Assisted Degradation of Dye Molecules and Reduction of Toxic Cr(VI) in Aqueous Medium Using Magnetically Recoverable Fe3O4/ Reduced Graphene Oxide Nanocomposite. RSC Adv. 2016, 6, 11049− 11063. (29) Santhosh, C.; Kollu, P.; Doshi, S.; Sharma, M.; Bahadur, D.; Vanchinathan, M. T.; Saravanan, P.; Kim, B. S.; Grace, A. N. Adsorption, Photodegradation and Antibacterial Study of Graphene−

Kulamani Parida: 0000-0001-7807-5561 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are very much gratified to Prof. B. K. Mishra, Director, CSIR-IMMT for all support to publish this work. One of the authors, D.K.P., is also thankful to CSIR-New Delhi, India for awarding him SRF.

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