Nanoparticles of magnetite anchored onto few-layer graphene: A highly efficient Fenton-like nanocomposite catalyst

Journal of Colloid and Interface Science 532 (2018) 161–170

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Nanoparticles of magnetite anchored onto few-layer graphene: A highly efficient Fenton-like nanocomposite catalyst Huan-Yan Xu a,b,⇑, Bo Li a, Tian-Nuo Shi a, Yuan Wang a, Sridhar Komarneni b,⇑ a

School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, PR China Materials Research Institute and Department of Ecosystem Science and Management, 204 Energy and the Environment Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 25 June 2018 Revised 25 July 2018 Accepted 29 July 2018 Available online 30 July 2018 Keywords: Fe3O4 Reduced graphene oxide Fenton-like reaction Kinetics Fe3O4/RGO-H2O2 system

a b s t r a c t Developing a catalyst with high efficiency and recyclability is an important issue for the heterogeneous Fenton-like systems. In this study, magnetic Fe3O4 and reduced graphene oxide (RGO) nanocomposites were prepared by a facile alkaline-thermal precipitation method and employed as a highly effective heterogeneous Fenton-like catalyst for methyl orange (MO) degradation. Characterization of these nanocomposites by XRD, FTIR, Raman, FESEM and TEM revealed that nanoparticles (NPs) of Fe3O4 were tightly anchored on the few-layer RGO sheets. The anchoring of Fe3O4 NPs and the reduction of GO were achieved in one pot without adding any other reducing agents. Based on the measurements of GO surface Zeta potentials, a possible anchoring mechanism of Fe3O4 NPs onto RGO sheets was given. The Fe3O4/RGO nanocomposites exhibited much higher Fenton-like catalytic efficiency for MO degradation than pure Fe3O4 NPs. This degradation process followed the first-order kinetics model, where k1 and T complied with the Arrhenius equation with Ea of 12.79 kJ/mol and A of 8.20 s
1. Magnetic measurements revealed that Fe3O4/RGO nanocomposites were ferromagnetic as indicated by the presence of magnetic hysteresis loops. The Fe3O4/RGO nanocomposites showed good stability and recyclability. Hydroxyl radicals, OH were determined as the dominant oxidative species in Fe3O4/RGO-H2O2 system and the Fenton-like mechanism for MO degradation in water was proposed and discussed. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding author at: School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, PR China.(H.Y. Xu) E-mail addresses: [email protected] (H.-Y. Xu), [email protected] (S. Komarneni). https://doi.org/10.1016/j.jcis.2018.07.128 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

Advanced oxidation processes (AOPs) have been widely accepted as promising approaches to treat refractory organic pollutants in wastewater due to their high degradation efficiencies.

162

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

AOPs usually included ozonation, photocatalysis, sulfate radicalbased oxidation, Fenton and Fenton-like processes, electrochemical and sonochemical oxidation [1–5]. Among these, Fenton reaction involves the in situ generation of highly oxidizing hydroxyl radicals (OH) via the decomposition of H2O2 when catalyzed by iron ions. However, the homogeneous Fenton reagent had some limitations such as the acid working environment, the production of iron-containing sludge and the continuous loss of catalyst [6]. These limitations could be overcome by the application of heterogeneous solid catalysts where the iron ions are embedded in the structure. Therefore, some inorganic minerals and materials could work well as the heterogeneous Fenton-like catalysts because of their suitable crystal structures and chemical compositions with divalent and trivalent iron species. In recent years, magnetite (Fe3O4) has attracted increasing attention as a heterogeneous Fenton-like catalyst because of its low cost, non-toxicity, magnetic property and high stability. More interestingly, it is well-known from Haber-Weiss cycle of Fenton reaction (see Supplementary Information) that Fe3O4 could facilitate the generation of OH radicals in Fenton-like reaction, which is mainly ascribed to the coexistence of Fe (II) and Fe (III) in the octahedral sites of its structure. Furthermore, the smaller nanoparticles (NPs) of Fe3O4 have larger surface-to-volume ratio, which interact with substrate molecules and lead to higher catalytic activity [7]. Nevertheless, Fe3O4 NPs tend to aggregate into large granules or clusters in water, leading to the decrease of surface-to-volume ratio and consequently reduced catalytic activity. To resolve this problem, it is essential to highly disperse Fe3O4 NPs by anchoring onto a solid materials with large surface area, such as pillared bentonite [8], natural maifanite [9], porous silica nanofibers [10], porous carbon microspheres [11], and multiwalled carbon nanotubes [12]. Since Fe3O4 NPs exhibited poor adsorptive characteristics for organic substances in water [10], these supporting materials could also enhance the catalytic activity of Fe3O4 NPs with their synergistic adsorption capability. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.07.128. Graphene is an important allotrope of carbon in the form of two-dimensional structure composed of hexagonally close packed network of carbons [13]. Graphene exhibits unique and superior properties including extraordinarily high carrier mobility, superlative mechanical strength, excellent thermal and electrical conductivities, large specific surface area etc. [14]. These properties impart graphene and its derivatives with the potential to serve as platform of nanocatalysts with enhanced catalytic behavior [15]. Not surprisingly, graphene and its derivatives have been developed as the carriers for magnetic Fe3O4 NPs to catalyze H2O2 and propagate the Fenton circular reaction. For instance, graphene oxide (GO), a graphene derivative was used as the carrier of Fe3O4 NPs and a chemical precipitation route was used for anchoring of Fe3O4 NPs onto GO sheets. The obtained Fe3O4/GO nanocomposites exhibited high degradation efficiency of isatin, which was mainly attributed to the synergistic functionalities of Fe3O4 NPs and GO sheets [16]. Zubir et al. showed that, in heterogeneous Fenton-like reaction, Fe3O4/GO nanocomposites could show 20% higher degradation efficiency of Acid Orange 7 than pristine Fe3O4 NPs [17]. Later, they found that GO was a sacrificial agent in this process because of the oxidation of C@C domains accompanied by electron transfer to Fe3O4 [18]. Moreover, after the attack of highly reactive radicals (e. g. OH and SO
4 ), the overall oxygen-containing groups on GO sheets dramatically declined and sheet-like GO was decomposed into many smaller-size flakes and low-molecular-weight molecules suggesting that the chemical stability of GO in AOPs should be carefully assessed [19]. Therefore, reduced graphene oxide (RGO) was proposed as a support for Fe3O4 NPs. The Fe3O4/RGO nanocomposites were synthesized and employed as heterogeneous

Fenton-like catalysts for the oxidation of methylene blue in a broad operating pH range of 5 to 9. The H2O2-activating ability of Fe3O4/ RGO with 10.0 wt% RGO was found to be six times higher than that of pure Fe3O4 NPs [20]. Furthermore, the Fe3O4/RGO nanocomposites could work as novel catalytic materials with excellent photoFenton catalytic properties [21–23]. However, with regard to these previously reported Fe3O4/RGO nanocomposites, an additional preparation procedure was required for the reduction of GO with toxic and costly reducing agents such as hydrazine hydrate [20] and sargassum thunbergii [21]. So, it is still a challenge to obtain Fe3O4/RGO nanocomposites using a facile one-pot route. In addition, more studies are needed to further understand the kinetics and mechanism of Fenton-like reaction catalyzed by Fe3O4/RGO. Here, we developed a facile alkaline-thermal precipitation method for in situ anchoring of Fe3O4 NPs onto RGO sheets. In this strategy, the anchoring of Fe3O4 NPs and the reduction of GO were simultaneously realized in one pot without adding any other reducing agents. Then, the resulting Fe3O4/RGO nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Raman), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and BET surface area analysis. In addition, the surface Zeta potential (n) of GO was measured. Based on these tests and analyses, the anchoring mechanism of Fe3O4 NPs onto RGO sheets was proposed. The Fenton-like efficiency of Fe3O4/RGO nanocomposites was evaluated using an azo dye, Methyl Orange (MO) as the target pollutant. The various factors affecting dye degradation, kinetics and mechanism of Fe3O4/RGOH2O2 Fenton-like system were discussed. The magnetic property of Fe3O4/RGO catalyst was detected by vibrating sample magnetometer (VSM) and its stability and recyclability were investigated as well.

2. Materials and methods 2.1. Preparation of Fe3O4/RGO A modified Hummers method previously reported in the literature [24] was employed for the preparation of GO using natural flakes of graphite powder (carbon content greater than 99.5%) as the raw material. Fe3O4/RGO nanocomposites were prepared by a facile alkaline-thermal precipitation method. In a typical procedure, 0.2 g GO was treated by ultrasonication for its complete exfoliation in 200 mL deionized water. Then, based on RGO content in the composites (5, 10, 15, 20, and 25 mass ratio (wt.%) in this study), a certain amount of ferrous sulfate heptahydrate (FeSO47H2O) was added into the GO solution while ultrasonicating the mixture. After FeSO47H2O was completely dissolved, the conical flask with the mixed solution was transferred into a water-bath and heated to a set temperature (55, 65, 75, 85, or 95 °C) with mechanical stirring. Afterwards, 100 mL solution of NaOH and NaNO3 with a mass ratio of 2:1 was added dropwise and then the resulting black suspension solution was kept at the set temperature for some time (1, 2, 3, 4, or 5 h) with vigorous stirring during the whole period. The specific amounts of FeSO47H2O, NaOH and NaNO3 are listed in Table S1 (See Supplementary Information). After cooling to room temperature, the black precipitate was separated by a magnet and alternately washed with deionized water and anhydrous alcohol until the filtrate reached neutral pH. After drying at 60 °C in a vacuum oven overnight, Fe3O4/RGO nanocomposites were obtained. The Fe3O4/RGO nanocomposites with 5, 10, 15, 20 and 25 wt% RGO content were labeled as FG-5G, FG-10G, FG15G, FG-20G and FG-25G, respectively. The composites with 10 wt % RGO prepared at 55, 65, 75, 85 or 95 °C in a temperaturecontrolled water-bath and were labeled as FG-55 T, FG-65 T,

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

FG-75 T, FG-85 T and FG-95 T, respectively. The obtained samples with 10 wt% RGO using the water-bath time of 1, 2, 3, 4 or 5 h were labeled as FG-1H, FG-2H, FG-3H, FG-4H and FG-5H, respectively. For comparison, pure Fe3O4 NPs were also prepared using the same procedure without the addition of GO.

163

at reaction time t. The MO degradation percentage was calculated by the following equation: D(%) = (1
C/C0)  100%. Moreover, random experiments were made under different conditions to check the reproducibility of the experimental results. 3. Results and discussion

2.2. Characterization techniques Powder XRD was used to detect the crystalline phases of GO, Fe3O4 NPs and Fe3O4/RGO nanocomposites using a Rigaku D/max-3B X-ray diffractometer (Cu-Ka, k = 0.15418 nm) in the 2 h range of 5-80°. FTIR was employed to confirm the functional groups of the as-prepared samples. The FTIR spectra were recorded on a Nicolet Nexus infrared spectrometer using spectroscopic grade potassium bromide (KBr) as the reference material. Raman spectroscopy was also used to assign the chemical groups of the samples utilizing a Renishaw inVia micro-Raman spectrometer with the laser wavelength of 532 nm. Morphology observation was done on a FESEM using FEISirion200 scanning electron microscope at the working voltage of 20 kV. The nano- and micromorphology, structure of the samples and selected area electron diffraction (SAED) pattern were obtained by a JEOL JEM-2010 transmission electron microscope. The specific surface area of the samples was determined from nitrogen adsorption-desorption data using a Sibata SA-1100 surface area analyzer at liquid nitrogen temperature. The surface Zeta potentials of GO at different solution pHs were tested by a Nano-ZS90 Zeta potential analyzer. The magnetic properties of various samples were analyzed by a lakeshore model 735 VSM with the accuracy of 5  10
6 emu/g. The UV-vis absorption spectra of MO solution were obtained by an USB4000 UV-vis spectrometer. 2.3. Fenton-like experiments The Fenton-like reaction of the as-prepared catalysts was carried out in a 200 mL beaker with mechanical stirring. In a typical run, a known amount of the catalyst was added into 100 mL aqueous solution of MO of known concentration. Then, the Fenton-like reaction was initiated by adding hydrogen peroxide (H2O2, 30% w/w). At regular time intervals (10 min), 2 mL of the reacted solution was collected for the measurement of MO concentration. After the residual catalyst was eliminated by an external magnet, MO concentration was measured using a 752-type spectrophotometer at its maximum absorption wavelength of 503 nm. C/C0 was used to express the degradation degree of MO, where C0 is the initial MO concentration and C is the residual MO concentration

3.1. Characterization of Fe3O4/RGO nanocomposites XRD patterns of GO and Fe3O4/RGO nanocomposites with different RGO contents are given in Fig. 1. From Fig. 1(a), it can be seen that GO sample has a strong diffraction peak at 2h = 11.8° corresponding to (0 0 2) crystal plane and a weak diffraction peak at 2h = 42.5° corresponding to the two-dimensional (10) reflection [25]. In comparison with graphite and graphitic oxide, the (0 0 2) diffraction peak of GO shifted to smaller angle region because of the introduction of oxygen containing groups between graphitic sheets resulting in an increase of interlayer spacing of (0 0 2) crystal plane [26,27]. Based on the Scherrer’s equation with the constant of 0.9, the average thickness (height) of stacked GO sheets could be calculated from the full width at half maximum of (0 0 2) diffraction peak [28]. This value was determined as 4.0 nm. Moreover, using the Bragg’s equation, the (0 0 2) interlayer spacing (d(0 0 2)) was calculated to be 0.75 nm, which can be taken as the layer-to-layer distance in GO structure. So, the GO prepared in this work was a few-layer (5–6 layers) graphene based on its stacking structure. As shown in Fig. 1(b), eight X-ray diffraction peaks of magnetite can be found at 2h = 18.3°, 30.0°, 35.6°, 37.1°, 43.1°, 53.4°, 57.3° and 62.8° (JCPDS 65-3107) in Fe3O4 NPs and Fe3O4/RGO nanocomposites, corresponding to (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystal planes, respectively. For Fe3O4/RGO nanocomposites, there exists a broad peak around 10.2° corresponding to (0 0 2) diffraction peak of RGO. The greater full width at half maximum and smaller diffraction angle of (0 0 2) peak suggest a fewer-layer stack of RGO sheets. This could be as a result of the formation of Fe3O4 NPs between RGO sheets, consequently limiting the number of RGO sheets in a stack and resulting in a broad peak of interlayer spacing. XRD patterns of Fe3O4/RGO nanocomposites prepared at different water-bath temperatures and times are displayed in Fig. S1 (See Supplementary Information). Since maghemite (c-Fe2O3) with inverse spinel structure has similar lattice parameters with magnetite, it is hard to differentiate maghemite and magnetite by XRD results only. The direct evidence to clarify the existence of magnetite can be provided by FTIR. As shown in Fig. 2(a), the FeAO stretching mode of tetrahedral and

Fig. 1. XRD patterns of (a) GO and (b) Fe3O4/RGO nanocomposites with different RGO contents.

164

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

Fig. 2. (a) FTIR and (b) Raman spectra of Fe3O4 NPs, GO and FG-10G.

octahedral sites can be found at 580 cm
1 for Fe3O4 NPs, which is different from that of maghemite (630 cm
1) [29]. The absorption band at 580 cm
1 of Fe3O4 still exists in Fe3O4/RGO nanocomposites with a slight blue shift to 576 cm
1. Moreover, there are many oxygen functional groups in GO, i. e., C@O (carbonyl) at 1730 cm
1; C@C (aromatics) at 1621 cm
1; CAO (carboxy) at 1403 cm
1; CAO (epoxy) at 1227 cm
1; and CAO (alkoxy) at 1061 cm
1 [26]. It should be noted that these oxygen functional groups are almost entirely removed in Fe3O4/RGO nanocomposites, implying that GO has been reduced to RGO during the formation of Fe3O4 NPs. In the alkaline-thermal precipitation route of this study, GO could act as an oxidizing agent to partly oxidize Fe2+ to Fe3+ and GO was simultaneously reduced to RGO [22]. On the other hand, a quick deoxygenation was also observed for exfoliated GO in strong alkaline solution at 50–90 °C, which was regarded as a green route to GO reduction [30]. In Raman spectra (Fig. 2(b)), the characteristic bands of Fe3O4 NPs at 664 and 319 cm
1 are assigned to symmetric stretch of oxygen atoms along FeAO bonds (A1g mode) and symmetric bend of oxygen with respect to Fe (Eg mode), respectively [31]. The Raman bands at 292, 691 and 810 cm
1 in Fe3O4/RGO nanocomposites might be related to the formation of FeAO and FeAC bonds [32,33], strongly suggesting a chemical combination between Fe3O4 NPs and RGO sheets. For GO, there exist two obvious bands around 1350 and 1600 cm
1, corresponding to D and G characteristic bands of graphite-like materials, respectively. Besides these two bands, there are 2D (2702 cm
1) and D + D’ (2930 cm
1) bands appearing in Fe3O4/RGO [34]. The ID/IG ratio was commonly employed for the determination of disorder and defects in graphene structure. The higher ID/IG ratio was indicative of more disorder and defects in graphene structure. In this study, the ID/IG ratios were determined to be 0.88 and 0.74 for GO and RGO, respectively. This suggests that RGO in the nanocomposites consisted relatively large carbon ordered domains because of the alkaline-thermal deprivation of oxygen functional groups from GO [35]. The strong and sharp 2D band in Fe3O4/RGO also implied

that fewer defects were formed when GO was reduced by the alkaline-thermal route [36]. Moreover, the number of graphene layers could be determined by the I2D/IG ratio. The single-, double-, triple- and multi- (>4) layer graphene with I2D/IG ratios of 1.6, 0.8, 0.30 and 0.07, respectively, have been reported previously [37]. In this study, the I2D/IG ratio was determined to be 0.54 for Fe3O4/RGO, implying that RGO in nanocomposites was estimated to be 2–3 layers. Based on the XRD analysis above, the number of layers of GO was 5–6. After combination with Fe3O4 NPs, the number of layers of RGO decreased to 2–3. This might be mainly ascribed to the formation of Fe3O4 NPs between graphene sheets, consequently limiting the stacking of graphene sheets. FESEM images of GO, Fe3O4 NPs and Fe3O4/RGO nanocomposites are given in Fig. S2 (See Supplementary Information). The microstructure information of Fe3O4 NPs and Fe3O4/RGO nanocomposites (FG-10G) are displayed in TEM images (Fig. 3). It can be seen that Fe3O4 NPs show a cubic shape with a size of less than 100 nm. In FG-10G, Fe3O4 NPs were well-dispersed when they were deposited on the RGO sheet. In addition, a series of diffraction rings in the SAED pattern can be indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2) and (5 1 1) crystal planes of Fe3O4, consistent with the XRD results. These diffraction rings revealed that Fe3O4 NPs were polycrystalline. In the Fe3O4/RGO nanocomposites, Fe3O4 NPs were still anchored on RGO sheets even after strong ultrasonic dispersion for the preparation of TEM sample, indicating that there was probably a strong chemical interaction between Fe3O4 NPs and RGO sheets, rather than a simple physical adsorption. pattern of Fe3O4 NPs anchored onto RGO sheet; and (d) enlarged nanostructure of FG-10G. 3.2. Anchoring mechanism of Fe3O4 NPs onto RGO sheets In order to gain detailed information for the anchoring process of Fe3O4 NPs onto RGO sheets, the surface Zeta potentials of GO were determined by the electrophoresis experiments. The result

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

165

Fig. 3. TEM images of (a) Fe3O4 NPs and (b) Fe3O4/RGO nanocomposites (FG-10G); (c) SAED.

is illustrated in Fig. S3 (See Supplementary Information), where it can be seen that, at most of the solution pHs, the surface Zeta potentials of GO were more negative than
30 mV. Therefore, the highly negatively charged surface of GO can lead to its good dispersion in water due to the strong electrostatic repulsion [38]. During the preparation process of Fe3O4/RGO nanocomposites, positively charged ferrous ions (Fe2+) could be driven by the electrostatic attraction to anchor onto highly negatively charged GO sheets. When the mixed solution of NaOH and NaNO3 was added dropwise, the suspension solution pH increased very slowly and probably resulting in precipitation of Fe(OH)2 on GO sheets [39,40]. A portion of the ferrous ions could have been oxidized to ferric ions by dissolved oxygen in solution or active oxygen in GO. Thus, Fe3O4 NPs were formed on GO sheets at a certain temperature. At the same time, GO was reduced to RGO by Fe2+ during the alkaline-thermal environment. Therefore, the formation of Fe3O4 NPs and the reduction of GO were simultaneously realized during the alkaline-thermal precipitation route. In addition, Fe3O4 might have been preferentially formed on the defects of RGO forming tight bonds between Fe3O4 NPs and RGO sheets. An illustration of the possible anchoring mechanism of Fe3O4 NPs onto RGO sheets is given in Fig. 4.

3.3. Fenton-like efficiency of Fe3O4/RGO-H2O2 system Control experiments were made to assess the heterogeneous Fenton-like efficiency of Fe3O4/RGO nanocomposites (taking FG10G as the example) and the results are displayed in Fig. 5. It can be seen that, when Fe3O4 was only used as the adsorbent in the absence of H2O2, it exhibited the lowest MO removal percentage (less than 5%) among all the samples. Only in the presence of H2O2, about 5.17% MO could be removed at 60 min reaction time because of the weak oxidative ability of H2O2 without any catalyst. However, Fe3O4/RGO showed higher adsorption capacity for MO molecules than pure Fe3O4, which is mainly ascribed to the larger surface area of FG-10G (56 m2/g) than that of Fe3O4 (13.7 m2/g). The adsorption and desorption could reach equilibrium within 30 min of reaction time and about 57% MO could be adsorbed. Although, in Fe3O4-H2O2 system, Fe3O4 NPs worked as the heterogeneous Fenton-like catalyst, only 50% MO could be degraded within 60 min reaction time. This implied that pure Fe3O4 NPs had poor catalytic activity to trigger the Fenton-like reaction. In Fe3O4/RGO-H2O2 system, nearly 94% MO could be degraded under identical conditions, indicating that Fe3O4/RGO had much higher Fenton-like catalytic activity than pure Fe3O4 NPs. This might be

166

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

Fig. 4. Anchoring mechanism of Fe3O4 NPs onto RGO sheets.

3.4. Kinetic studies of Fe3O4/RGO-H2O2 system The first-order, second-order and Behnajady-ModirshahlaGhanbery (BMG) kinetic models were usually employed for the kinetics studies of Fenton and Fenton-like reactions. The linear equations of these three models are given as follows [41]:

ln

C0 ¼ k1  t Ct

ðFirst
order modelÞ

1 1
¼ k2  t Ct C0

ðSecond
order modelÞ

t ¼ mþbt 1
ðC t =C 0 Þ

Fig. 5. Control experiments on the degradation of 50 mg/L MO at pH = 3 and room temperature in different systems (the dosage of Fe3O4 or Fe3O4/RGO is 1.0 g/L and the initial H2O2 concentration is 9.69 mM in the Fenton-like systems).

explained as follows: firstly, well-dispersed and anchored Fe3O4 NPs on RGO sheets could suppress the aggregation of Fe3O4 NPs and increase the exposure of active Fe sites on catalyst surface to enhance their catalytic abilities towards H2O2; secondly, adsorption had an important effect on the heterogeneous Fenton-like reaction, increasing the contact between MO molecules and OH radicals and thus improving Fenton-like efficiency. So, it is the synergistic effect of Fenton-like reaction and adsorption that governed MO degradation in Fe3O4/RGO-H2O2 system. The impacts of preparation conditions (e. g. RGO mass ratio, water-bath temperature and water-bath time) and catalytic conditions (e. g. initial pH, H2O2 concentration, MO concentration, catalyst dosage, reaction temperature) on the Fenton-like efficiency of Fe3O4/RGO nanocomposites were systematically investigated and the results are illustrated in Fig. S4 (see Supplementary Information). Generally, the Fenton-like efficiency of Fe3O4/RGO increased with the increase of RGO mass ratio and water-bath temperature. The sample prepared using the water-bath time of 2 h exhibited the highest Fenton-like efficiency. The optimum solution pH and H2O2 concentration were determined to be 3 and 9.69 mM, respectively. Moreover, the Fenton-like efficiency of Fe3O4/RGO increased with the increase of catalyst dosage and temperature of reaction and with the decrease of initial dye concentration. The reasons for the above results have been discussed in Supplementary Information.

ðBMG modelÞ

ð1Þ ð2Þ ð3Þ

where, C0 and Ct are the initial MO concentration and residual MO concentration at reaction time t, respectively; k1 is the reaction rate constant of first-order kinetics model; k2 is the reaction rate constant of second-order kinetics model; m and b are the constants related to oxidation capacities and reaction kinetics of BMG model, respectively. These constants for Fe3O4/RGO-H2O2 system could be calculated by linear regression plots of the three equations mentioned above. The adjusted correlation coefficient (adj. R2) of linear regression was used to determine the kinetics model of Fe3O4/RGO-H2O2 Fenton-like system. All the kinetic constants are listed in Table S2 (Supplementary Information), where it can be found that the linear fittings of first-order model at different conditions are for the most part better than those of second-order and BMG models due to their relatively greater adjusted R2 values. This strongly indicates that MO degradation process in Fe3O4/RGO-H2O2 Fenton-like system follows the first-order kinetic model. Furthermore, the first-order reaction rate constant (k1) and reaction temperature (T) in this system follows the Arrhenius equation because of the linear relationship between lnk1 and 1/T, as presented in Fig. 6. The Arrhenius equation is given as follows [42]:

lnk1 ¼


Ea 1  þ lnA R T

ð4Þ

where, Ea is the activation energy of a reaction, R is the gas constant accepted as 8.314 J/Kmol and A is the collision frequency factor. Comparing the obtained linear equation with Arrhenius equation, the reaction activation energy and collision frequency factor of Fe3O4/RGO-H2O2 Fenton-like system were determined to be 12.79 kJ/mol and 8.20 s
1, respectively.

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

167

This decrease in MO degradation might be caused by the cumulative mass loss of Fe3O4/RGO after each cycle, rather than its deactivation. The mass loss of Fe3O4/RGO was about 40% after 6 cycles, which originated from the adhesion of superfine Fe3O4 particles onto the filter paper during the washing process after each use. Even so, Fe3O4/RGO still exhibited good stability and recyclability. 3.6. Possible Fenton-like mechanism

Fig. 6. Relationship between lnk1 and 103/T.

3.5. Magnetic property and recyclability of Fe3O4/RGO The magnetization curves of Fe3O4 NPs and the Fe3O4/RGO nanocomposites are illustrated in Fig. 7(a), from which it can be deduced that both Fe3O4 NPs and Fe3O4/RGO nanocomposites possessed the typical ferromagnetic property because of the appearance of magnetic hysteresis loops. The saturation magnetization (Ms) of Fe3O4 NPs was determined to be 62.6 emu/g, lower than that of bulk magnetite (92 emu/g). This was probably ascribed to the superfine particle dimension and specific surface effects of Fe3O4 NPs [43]. In addition, when RGO mass ratio increased, the Ms values of Fe3O4/RGO nanocomposites proportionally decreased, as expected. More importantly, as shown in the inset in Fig. 7(a), the strong magnetism of Fe3O4/RGO nanocomposites could result in their easy separation from solution through an external magnet, which might be helpful for the engineering application. The stability and recyclability of Fe3O4/RGO nanocomposites working as Fenton-like catalysts are important requirements for their practical applications. Therefore, the successive use of Fe3O4/RGO was carried out under identical conditions. It can be seen from Fig. 7(b) that, even after 6 cycling times, Fe3O4/RGO still exhibited relatively high catalytic ability with MO degradation percentage of 85.5%.

Fig. 8(a) suggests the decomposition of MO dye in water, and formation of water, carbon dioxide and other small molecules because of the gradual disappearance of UV–Vis absorption peak of MO solution near 503 nm after different reaction periods. Moreover, the inset in Fig. 8(a) shows that the MO solution becomes approximately colorless after 60 min of reaction time. According to the Haber-Weiss Fenton cycle (See Supplementary Information), hydroxyl radicals (OH) and superoxide radicals (HO2/O
2 ) are the main reactive oxidative species in Fenton and Fenton-like reactions. Ethanol and chloroform were usually employed as the scavengers of OH and HO2/O
2 , respectively, to identify the dominant species during this process [44,45]. In Fe3O4/RGO-H2O2 system, when 30 mM ethanol was added, MO degradation was inhibited with an obvious decrease from 94% to 57% after 60 min reaction (Fig. 8(b)). By comparing with Fig. 5, it can be deciphered that only the dye adsorption was responsible for MO removal under these conditions, strongly implying that MO degradation was completely inhibited by ethanol and suggested that OH radicals were the dominant oxidative species in Fe3O4/RGO-H2O2 system. Moreover, with the addition of 30 mM chloroform, nearly 90% MO could be degraded, indicating that HO2 or O
2 might not take part in the direct oxidation of MO. However, HO2/O
2 played an important role in Haber-Weiss cycle and thereby slightly suppressed the generation of OH radicals with a slight decrease in MO degradation. Based on the above results and discussion, the possible Fentonlike mechanism for MO degradation in Fe3O4/RGO-H2O2 system is proposed, as shown in Fig. 9. The control experiments revealed that Fe3O4/RGO with the assistance of H2O2 could lead to much higher MO degradation than Fe3O4/RGO alone (Fig. 5), suggesting that a Fenton-like reaction significantly catalyzed MO degradation by Fe3O4/RGO in H2O2. Since the Fenton-like experiments of this study were performed under acidic conditions (pH 1–5), a small amount of Fe2+ and Fe3+ cloud be dissolved into solution from the surfaces of Fe3O4 NPs. Therefore, in this system, the catalytic decomposition of H2O2 to form OH radicals could be accomplished by both the

Fig. 7. (a) Magnetization curves of Fe3O4 NPs and Fe3O4/RGO nanocomposites (inset: digital image of the magnetic separation of Fe3O4/RGO by an external magnet); (b) recycling experiments on MO degradation (bar chart) and mass loss of Fe3O4/RGO (line graph) in Fe3O4/RGO-H2O2 Fenton-like reaction (experimental conditions: [MO]0 = 50 mg/L, pH = 3, [H2O2]0 = 9.69 mM, catalyst dosage = 1 g/L, t = 60 min and T = 293 K).

168

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

Fig. 8. (a) UV–Vis absorption spectra and color change (inset) of MO solutions after different periods during the Fenton-like reaction; (b) the effects of radical scavengers on MO degradation (experimental conditions: [MO]0 = 50 mg/L, pH = 3, [H2O2]0 = 9.69 mM, catalyst dosage = 1 g/L, t = 60 min and T = 293 K).

Fig. 9. Possible Fenton-like mechanism for MO degradation in Fe3O4/RGO-H2O2 system.

dissolved iron ions in solution and the exposed iron ions on Fe3O4 surface. The former is known as homogeneous catalysis while the latter is known as heterogeneous catalysis. As a metal oxide, the solubility of Fe3O4 increased with decreasing pH [46]. So, the homogeneous and heterogeneous catalyses might govern the catalytic generation of OH radicals at lower and higher pH ranges, respectively. Attacked by OH radicals, MO molecules could be oxidized and decomposed into H2O, CO2 and other smaller molecules. In addition, graphene played a bifunctional role in the Fenton-like process, namely as a substrate and co-catalyst of Fe3O4 NPs. As the substrate for Fe3O4 NPs, graphene could help to highly disperse and anchor Fe3O4 NPs onto its sheet matrix and effectively restrain their aggregation. In this case, more active Fe sites could be exposed on the surface of Fe3O4 NPs and their catalytic abilities towards H2O2 could be correspondingly enhanced. As the cocatalyst of Fe3O4 NPs, graphene could improve the adsorption abil-

ity of nanocomposite catalyst towards the reactants such as MO molecules, H2O2 and OH radicals. During the heterogeneous Fenton-like processes, the catalytic decomposition of H2O2 and degradation of organic compounds mainly happened on or near the catalyst surface [47,48]. Fe3O4 NPs had poor adsorption ability but graphene had great adsorption ability leading to their synergistic effect. The adsorption could lead to a high concentration of reactants on or near the catalyst surface and thus, increase their contacts and reactions, thereby improving the efficiency of Fenton-like reaction. On the other hand, graphene has extraordinarily high electron mobility [14], which could speed up the redox reactions of Fe2+ and Fe3+ adsorbed on graphene sheets or exposed on surfaces of Fe3O4 NPs and thus facilitate the generation of OH radicals. So, the catalytic enhancement of MO degradation in Fe3O4/RGO-H2O2 Fenton-like system might be partially contributed by the great electron transfer ability of graphene support.

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

4. Conclusions In this work, a facile alkaline-thermal precipitation method was developed to prepare Fe3O4/RGO nanocomposites. The surface Zeta potentials of GO were found to be more negative than
30 mV at most solution pHs, so we utilized this feature to assemble positive Fe2+ ions onto GO sheets and simultaneously achieved the anchoring of Fe3O4 NPs and the reduction of GO in one pot without adding any other reducing agents. GO had been reduced to RGO during the formation of Fe3O4 NPs and RGO in nanocomposites was estimated to be about 2–3 layers. It appeared that a strong chemical interaction between Fe3O4 NPs and RGO sheets existed, rather than a simple physical adsorption. The obtained Fe3O4/RGO nanocomposites exhibited higher Fenton-like efficiency for MO degradation than pure Fe3O4 NPs. MO degradation process in this system followed the first-order kinetics model. Fe3O4/RGO nanocomposites could be easily separated from solution by an external magnet because of the ferromagnetic property of Fe3O4. After 6 cycling times, Fe3O4/RGO still exhibited relatively high catalytic ability with MO degradation percentage of 85.5%. With the present study, OH radicals were determined as the dominant oxidative species in Fe3O4/RGO-H2O2 system. Attacked by OH radicals, MO molecules could be oxidized and decomposed into H2O, CO2 and other smaller molecules. This work would open up a new way to prepare graphene-based nanocomposites and could lead to a comprehensive understanding of heterogeneous Fenton-like reaction with Fe3O4/RGO nanocomposites. Acknowledgements This work was financially supported by Natural Science Foundation of Heilongjiang Province, China (E2015065). Huan-Yan Xu and Sridhar Komarneni would like to give their thanks to China Scholarship Council (201708230069). References [1] Z.T. Hu, W.D. Oh, Y.Q. Liu, E.H. Yang, T.T. Lim, Controllable mullite bismuth ferrite micro/nanostructures with multifarious catalytic activities for switchable/hybrid catalytic degradation processes, J. Coll. Interf. Sci. 509 (2018) 502–514. [2] S.X. Liang, Z. Jia, W.C. Zhang, X.F. Li, W.M. Wang, H.C. Lin, L.C. Zhang, Ultrafast activation efficiency of three peroxides by Fe78Si9B13 metallic glass under photo-enhanced catalytic oxidation: a comparative study, Appl. Catal. BEnviron. 221 (2018) 108–118. [3] C.M. Chen, X. Yan, B.A. Yoza, T.T. Zhou, Y. Li, Y.L. Zhan, Q.H. Wang, Q.X. Li, Efficiencies and mechanisms of ZSM5 zeolites loaded with cerium, iron, or manganese oxides for catalytic ozonation of nitrobenzene in water, Sci. Total Environ. 612 (2018) 1424–1432. [4] Y.P. Hou, Z.B. Peng, L. Wang, Z.B. Yu, L.R. Huang, L.F. Sun, J. Huang, Efficient degradation of tetrabromobisphenol A via electrochemical sequential reduction-oxidation: degradation efficiency, intermediates, and pathway, J. Hazard. Mater. 343 (2018) 376–385. [5] A. Hassani, G. Çelikdag˘, P. Eghbali, M. Sevim, S. Karaca, Ö. Metin, Heterogeneous sono-Fenton-like process using magnetic cobalt ferrite reduced graphene oxide (CoFe2O4-rGO) nanocomposite for the removal of organic dyes from aqueous solution, Ultrason. Sonochem. 40 (2018) 841–852. [6] H.H. Wang, L.L. Zhang, C. Hua, X.K. Wang, L. Lyu, G.D. Sheng, Enhanced degradation of organic pollutants over Cu-doped LaAlO3 perovskite through heterogeneous Fenton-like reactions, Chem. Eng. J. 332 (2018) 572–581. [7] L.Z. Gao, J. Zhuang, L. Nie, J.B. Zhang, Y. Zhang, N. Gu, T.H. Wang, J.D. Feng, L. Yang, S. Perrett, X.Y. Yan, Intrinsic peroxidase-like activity of ferromagnetic nanoparticles, Nat. Nanotechnol. 2 (2007) 577–583. [8] D. Wan, G.H. Wang, W.B. Li, X.B. Wei, Investigation into the morphology and structure of magnetic bentonite nanocomposites with their catalytic activity, Appl. Surf. Sci. 413 (2017) 398–407. [9] H. Zhao, L. Weng, W.W. Cui, X.R. Zhang, H.Y. Xu, L.Z. Liu, In situ anchor of magnetic Fe3O4 nanoparticles onto natural maifanite as efficient heterogeneous Fenton-like catalyst, Front. Mater. Sci. 10 (2016) 300–309. [10] X.K. Tang, Q.M. Feng, K. Liu, Z.S. Li, H. Wang, Fabrication of magnetic Fe3O4/ silica nanofiber composites with enhanced Fenton-like catalytic performance for Rhodamine B degradation, J. Mater. Sci. 53 (2018) 369–384.

169

[11] L.C. Zhou, Y.M. Shao, J.R. Liu, Z.F. Ye, H. Zhang, J.J. Ma, Y. Jia, W.J. Gao, Y.F. Li, Preparation and characterization of magnetic porous carbon microspheres for removal of methylene blue by a heterogeneous Fenton reaction, ACS Appl. Mater. Interf. 6 (2014) 7275–7285. [12] H.Y. Xu, T.N. SHi, H. Zhao, L.G. Jin, F.C. Wang, C.Y. Wang, S.Y. Qi, Heterogeneous Fenton-like discoloration of methyl orange using Fe3O4/MWCNTs as catalyst: process optimization by response surface methodology, Front, Mater. Sci. 10 (2016) 45–55. [13] C.L. Tan, X.H. Cao, X.J. Wu, Q.Y. He, J. Yang, X. Zhang, J.Z. Chen, W. Zhao, S.K. Han, G.H. Nam, M. Sindoro, H. Zhang, Recent advances in ultrathin twodimensional nanomaterials, Chem. Rev. 117 (2017) 6225–6331. [14] V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N.D. Kim, K.C. Kemp, P. Hobza, R. Zboril, K.S. Kim, Functionalization of graphene: covalent and noncovalent approaches, derivatives and applications, Chem. Rev. 112 (2012) 6156–6214. [15] D.S. Su, S. Perathoner, G. Centi, Nanocarbons for the development of advanced catalysts, Chem. Rev. 113 (2013) 5782–5816. [16] Z.Y. Zhou, M.H. Su, K.M. Shih, Highly efficient and recyclable graphene oxidemagnetite composites for isatin mineralization, J. Alloy. Compd. 725 (2017) 302–309. [17] N.A. Zubir, C. Yacou, J. Motuzas, X.W. Zhang, J.C.D. Costa, Structural and functional investigation of graphene oxide-Fe3O4 nanocomposites for the heterogeneous Fenton-like reaction, Sci. Rep. 4 (2014) 4594. [18] N.A. Zubir, C. Yacou, J. Motuzas, X.W. Zhang, X.S. Zhao, J.C.D. Costa, The sacrificial role of graphene oxide in stabilizing a Fenton-like catalyst GO-Fe3O4, Chem. Commun. 51 (2015) 9291–9293. [19] Z.H. Wang, L.Y. Sun, X.Y. Lou, F. Yang, M. Feng, J.S. Liu, Chemical instability of graphene oxide following exposure to highly reactive radicals in advanced oxidation processes, J. Coll. Interf. Sci. 507 (2017) 51–58. [20] W. Liu, J. Qian, K. Wang, H. Xu, D. Jiang, Q. Liu, X.W. Yang, H.M. Li, Magnetically separable Fe3O4 nanoparticles-decorated reduced graphene oxide nanocomposite for catalytic wet hydrogen peroxide oxidation, J. Inorg. Organomet. Polym. Mater. 23 (2013) 907–916. [21] X.Y. Jiang, L.L. Li, Y.R. Cui, F.L. Cui, New branch on old tree: Green-synthesized RGO/Fe3O4 composite as a photo-Fenton catalyst for rapid decomposition of methylene blue, Ceram. Int. 43 (2017) 14361–14368. [22] P.K. Boruah, B. Sharma, I. Karbhal, M.V. Shelke, M.R. Das, Ammonia-modified graphene sheets decorated with magnetic Fe3O4 nanoparticles for the photocatalytic and photo-Fenton degradation of phenolic compounds under sunlight irradiation, J. Hazard. Mater. 325 (2017) 90–100. [23] B.C. Qiu, Q.Y. Li, B. Shen, M.Y. Xing, J.L. Zhang, Stöber-like method to synthesize ultradispersed Fe3O4 nanoparticles on graphene with excellent photo-Fenton reaction and high-performance lithium storage, Appl. Catal. B-Environ. 183 (2016) 216–223. [24] Z.G. Xiong, L.L. Zhang, J.Z. Ma, X.S. Zhao, Photocatalytic degradation of dyes over grapheme-gold nanocomposites under visible light irradiation, Chem. Commun. 46 (2010) 6099–6101. [25] L. Stobinski, B. Lesiak, A. Malolepszy, M. Mazurkiewicz, B. Mierzwa, J. Zemek, P. Jiricek, I. Bieloshapka, Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods, J. Electron Spectrosc. Relat. Phenom. 195 (2014) 145–154. [26] K. Krishnamoorthy, M. Veerapandian, K. Yun, S.J. Kim, The chemical and structural analysis of graphene oxide with different degrees of oxidation, Carbon 53 (2013) 38–49. [27] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z.Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (2010) 4806–4814. [28] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Preparation and characterization of graphene oxide paper, Nature 448 (2007) 457–460. [29] X.Z. Wang, Z.B. Zhao, J.Y. Qu, Z.Y. Wang, J.S. Qiu, Fabrication and characterization of magnetic Fe3O4-CNT composites, J. Phys. Chem. Solids 71 (2010) 673–676. [30] X.B. Fan, W.C. Peng, Y. Li, X.Y. Li, S.L. Wang, G.L. Zhang, F.B. Zhang, Deoxygenation of exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation, Adv. Mater. 20 (2008) 4490–4493. [31] O.N. Shebanova, P. Lazor, Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum, J. Solid State Chem. 174 (2003) 424–430. [32] X.B. Hu, B.Z. Liu, Y.H. Deng, H.Z. Chen, S. Luo, C. Sun, P. Yang, S.G. Yang, Adsorption and heterogeneous Fenton degradation of 17a-methyltestosterone on nano Fe3O4/MWCNTs in aqueous solution, Appl. Catal. B-Environ. 107 (2011) 274–283. [33] L. Yu, X. Yang, Y. Ye, D. Wang, Efficient removal of atrazine in water with a Fe3O4/MWCNTs nanocomposite as a heterogeneous Fenton-like catalyst, RSC Adv. 5 (2015) 46059–46066. [34] D. López-Díaz, M.L. Holgado, J.L. García-Fierro, M.M. Velázquez, Evolution of the Raman spectrum with the chemical composition of graphene oxide, J. Phys. Chem. C 121 (2017) 20489–20497. [35] S.D. Perera, R.G. Mariano, N. Nijem, Y. Chabal, J.P. Ferraris, K.J. Balkus, Alkaline deoxygenated graphene oxide for supercapacitor applications: an effective green alternative for chemically reduced grapheme, J. Power Sour. 215 (2012) 1–10. [36] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653–2659.

170

H.-Y. Xu et al. / Journal of Colloid and Interface Science 532 (2018) 161–170

[37] T.F. Jiao, Y.Z. Liu, Y.T. Wu, Q.R. Zhang, X.H. Yan, F.M. Gao, A.J.P. Bauer, J.Z. Liu, T. Y. Zeng, B.B. Li, Facile and scalable preparation of graphene oxide-based magnetic hybrids for fast and highly efficient removal of organic dyes, Sci. Rep. 5 (2015) 12451. [38] D. Li, M.B. Müller, S. Gilje, R.B. Kanerand, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nat. Nanotechnol. 3 (2008) 101–105. [39] H. Iida, K. Takayanagi, T. Nakanishi, T. Osaka, Synthesis of Fe3O4 nanoparticles with various sizes and magnetic properties by controlled hydrolysis, J. Coll. Interf. Sci. 314 (2007) 274–280. [40] Z.P. Cheng, X.Z. Chu, J.Z. Yin, H. Zhong, J.M. Xu, Surfactantless synthesis of Fe3O4 magnetic nanobelts by a simple hydrothermal process, Mater. Lett. 75 (2012) 172–174. [41] H.Y. Xu, T.N. Shi, L.C. Wu, S.Y. Qi, Discoloration of methyl orange in the presence of schorl and H2O2: kinetics and mechanism, Water Air Soil Pollut. 224 (2013) 1740. [42] G. McKay, F.L. Rosario-Ortiz, Temperature dependence of the photochemical formation of hydroxyl radical from dissolved organic matter, Environ. Sci. Technol. 49 (2015) 4147–4154.

[43] L.J. Xu, J.L. Wang, Fenton-like degradation of 2,4-dichlorophenol using Fe3O4 magnetic nanoparticles, Appl. Catal. B-Environ. 123–124 (2012) 117–126. [44] W.Y. Huang, M.Q. Luo, C.S. Wei, Y.H. Wang, K. Hanna, G. Mailhot, Enhanced heterogeneous photo-Fenton process modified by magnetite and EDDS: BPA degradation, Environ. Sci. Pollut. Res. 24 (2017) 10421–10429. [45] Z.H. Diao, J.J. Liu, Y.X. Hu, L.J. Kong, D. Jiang, X.R. Xu, Comparative study of Rhodamine B degradation by the systems pyrite/H2O2 and pyrite/persulfate: reactivity, stability, products and mechanism, Sep. Purif. Technol. 184 (2017) 374–383. [46] V.K. Mittal, S. Bera, S.V. Narasimhan, S. Velmurugan, Adsorption behavior of antimony(III) oxyanions on magnetite surface in aqueous organic acid environment, Appl. Surf. Sci. 266 (2013) 272–279. [47] C.M. Zheng, C.W. Yang, X.Z. Cheng, S.C. Xu, Z.P. Fan, G.H. Wang, S.B. Wang, X.F. Guan, X.H. Sun, Specifically enhancement of heterogeneous Fenton-like degradation activities for ofloxacin with synergetic effects of bimetallic FeCu on ordered mesoporous silicon, Sep. Purif. Technol. 189 (2017) 357–365. [48] J. Zhang, G.D. Liu, P.H. Wang, S.J. Liu, Facile synthesis of FeOCl/iron hydroxide hybrid nanosheets: enhanced catalytic activity as a Fenton-like catalyst, New J. Chem. 41 (2017) 10339–10346.