Green fabrication of magnetic recoverable graphene/MnFe₂O₄ hybrids for efficient decomposition of methylene blue and the Mn/Fe redox synergetic mechanism

Abstract

Herein, we report an environmentally benign synthesis of a high-performance reduced graphene oxide/MnFe₂O₄ (RGO/MnFe₂O₄) catalyst for methylene blue (MB) decomposition in neutral solution using a GO/MnSO₄ suspension from a modified Hummers method and FeSO₄ as the precursors. The as-prepared RGO/MnFe₂O₄ catalyst shows exceptional performance towards the MB decomposition in the presence of H₂O₂. In particular, 10 mL of MB (50 mg L⁻¹) can be thoroughly decolorized in 130 min and 78% mineralized with 5 mg of RGO/MnFe₂O₄ hybrid at room temperature. More interestingly, the catalysts can be magnetically recycled. The good catalytic performance of the RGO/MnFe₂O₄ hybrid is not only attributed to the synergetic effects of RGO, MnFe₂O₄, H₂O₂ and MB molecules, but also related to the redox couples of Fe/Mn ions during the reaction. We have firstly experimentally demonstrated that the catalytic performance of MnFe₂O₄ is dominated by Fe³⁺/Fe²⁺ in the initial stage (<70 min) then by Mn³⁺/Mn²⁺ in the later stage (>70 min), while Fe²⁺/Mn³⁺ redox in turn benefits the redox cycles of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺. Our results not only provide an alternative strategy for green synthesis of high-performance functional nanomaterials, but also promote a deep understanding of the mechanism of MnFe₂O₄ catalyst for MB decomposition.

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Introduction

An ever increasing amount of toxic and hazardous waste water is being generated from industries and consequently results in serious environmental issues.^1–3 Organic dyes are a major type of pollutant widely existing in nature water, and thus the development of powerful and practical methods for the degradation of organic dyes has attracted world-wide attention.⁴ As an effective advanced oxidation technology, catalysts employing the traditional homogeneous Fenton reaction with free Fe²⁺ have been widely used to decompose organic dyes. However, the narrow pH range (pH 2–3) and the polluted iron sludge generated at the end of process prohibit the practical application of this technology.^5,6 To overcome these drawbacks, potential applications of heterogeneous Fenton-like catalysts have been investigated widely. Specifically, iron based Fenton-like catalysts such as Fe₃O₄, Fe₂O₃, FeOOH^7–9 and manganese based ones such as MnO₂, Mn₃O₄ (ref. 10 and 11) are two common kinds of heterogeneous catalysts for the degradation of organic dyes. Nevertheless, the catalytic performance of these mono-metal oxide catalysts is far from satisfaction. Furthermore, the recycling of these heterogeneous catalysts as another crucial issue needs to be addressed (except for Fe₃O₄ and γ-Fe₂O₃, δ-FeOOH).⁴ The introduction of Mn into the structure of Fe₃O₄ to produce magnetic Mn–Fe bimetal oxide catalyst provides new route for the enhancement of Fenton activity.^12–14 In this field, MnFe₂O₄ particles have been intensively investigated mainly because of their high activity, good magnetization, nice biocompatibility and low cost.^15,16 For example, Fang et al. synthesized coated MnFe₂O₄ nanocomposites for the decomposition of Red X-3B with the assistance of microwave.¹⁷ Yang et al. prepared the magnetic MnFe₂O₄ as the adsorbent for the removals of methylene blue (MB) and Congo red in single and binary dye systems.¹⁸ Despite these advances, the unique magnetic property of MnFe₂O₄ particles usually leads to their aggregation, thus decreasing their catalytic efficiency.¹⁹ As a result, extensive efforts have to be made in controlling the size and dispersion of MnFe₂O₄ particles for enhanced catalytic performance.

Graphite and graphene are two common carbon materials for fabricating nanocomposites.^20,21 The former exhibits significant potential in the improvement of thermal conductivities of the composites.^22,23 However, the later with unique electronic property, large surface area, high chemical stability and good mechanical flexibility can not only serve as an ideal substrate for growing functional nanoparticles, but also as a charge transfer medium in the catalytic reactions.^24–26 Nowadays, it is demonstrated that magnetic heterogeneous catalysts can be uniformly dispersed on graphene to fabricate the hybrids towards the degradation of water pollution.²⁷ For example, Bai et al. fabricated graphene/MnFe₂O₄ hybrids from GO, Fe²⁺ and Mn²⁺, which could remove over 75% rhodamine B and 95% MB within 180 min.²⁸ Using the similar method, Yao's group demonstrated that magnetic graphene/MnFe₂O₄ exhibited 97% degradation of MB in 120 min with the assistance of peroxymonosulfate, which was much higher than that of bare MnFe₂O₄.²⁹ These studies indicate that the combination of graphene with magnetic MnFe₂O₄ provides the potential catalytic route which meets the requirements of high catalytic activity, easy recovery and reuse. However, the common methods for the fabrication of graphene/MnFe₂O₄ catalysts often begin with GO as the precursor and additional Mn²⁺ ions as Mn source. Generally, GO produced by the modified Hummers method requires intercalation and oxidation of graphite with a stoichiometric amount of KMnO₄ and H₂SO₄ and results in a large amount of waste containing H⁺, Mn²⁺ and SO₄²⁻ ions.^30,31 For the viewpoint of the whole synthesis, the production of RGO/MnFe₂O₄ will be more valued if the H⁺, Mn²⁺ and SO₄²⁻ ions can be fully utilized, which will not only avoid the tedious purification process of GO as well as the production of large amount of waste, but also contribute to highly atom-economic synthesis.³² We previously fabricated reduced graphene oxide/MnO₂ (RGO/MnO₂) and GO/Mn₃O₄ composites with GO/MnSO₄ suspension as Mn source and the resultant samples exhibited good catalytic performances for 100% degradation of MB in 5 min at 50 °C and 200 min at room temperature, respectively.^31,32 In spite of our efforts, the enhanced catalytic activity, facile recycling treatment and further understanding of the catalytic mechanism are desired.

It is widely reported that hydroxyl radical (˙OH) radical is crucial parameter for understanding the effective degradation of dyes with H₂O₂ assisted heterogeneous Fenton-like reaction.¹² In principle, H₂O₂ activation mechanism is critically depends on the nature of the catalyst. The mono-metal catalyst such as iron oxide, manganese oxide with multiple redox states to activate H₂O₂ into ˙OH has been widely studied.^15,29 However, the exact role of oxidation state of different components for bimetal oxide catalyst during Fenton reaction needs to be further explored based on the experimental data.

In this work, we further develop environmentally benign synthesis of magnetic RGO/MnFe₂O₄ hybrids from the pristine suspension (GO/MnSO₄) with the addition of FeSO₄ as the iron source. The work provides the effective utilization of Mn²⁺ ions in the pristine suspension of GO/MnSO₄, and the introduction of RGO can facilitate the dispersion of the in situ formed magnetic particles, via which their aggregation is significantly alleviated. Due to these structural merits, the as-prepared hybrids show improved catalytic performance towards the MB decomposition in the presence of H₂O₂. In addition, these catalysts can be facilely recycled due to the magnetic property of the MnFe₂O₄ nanoparticles. More importantly, the roles of the oxidation state of Fe(III)/Mn(II) in MnFe₂O₄ with varied redox property (Fe³⁺/Fe²⁺, Mn³⁺/Mn²⁺ and Fe²⁺/Mn³⁺) on the catalytic performance and the involved mechanism are investigated in detail.

Experimental

Synthesis of RGO/MnFe₂O₄ hybrids

The procedure for the synthesis of RGO/MnFe₂O₄ hybrids was carried out as follows. First, 10 mL of the homogeneous GO/MnSO₄ suspension (13 mg mL⁻¹ GO and 0.22 mol L⁻¹ MnSO₄), which was synthesized by modified Hummers method according to ref. 32 and 33, was sonicated for 20 min. Then 1.22 g FeSO₄·7H₂O was added and the mixed suspension was stirred at room temperature. Subsequently, 2 mol L⁻¹ KOH was added into the above solution for another 0.5 h under bubbling air and stirring until the pH = 12. The system was then transferred in a 20 mL Teflon-lined autoclave at 150 °C for 15 h. Finally, the black precipitate (RGO/MnFe₂O₄) was collected by filtration, washed thoroughly with distilled water, and fully dried at 80 °C over night.

RGO/MnFe₂O₄ hybrids with different mass contents of MnFe₂O₄ were synthesized by tailoring the ratio of GO/MnSO₄ to FeSO₄. First, GO/MnSO₄ suspensions with different mass ratios of GO to MnSO₄ were prepared according to ref. 33. Then, a certain amount of FeSO₄·7H₂O was added into the above suspension based on the theoretic molar ratio of Fe²⁺ to Mn²⁺ in the MnFe₂O₄ molecule, which was accurately equal to be 2 : 1. The following steps by adding KOH and other procedures are similar to the above process. The resultant samples were named as RGO/MnFe₂O₄-X, where X represented the mass percentage of MnFe₂O₄ in RGO/MnFe₂O₄ hybrids. For comparison, RGO or bare MnFe₂O₄ particle were prepared in a similar way in the absence of MnFe₂O₄ or GO.

Test of the catalytic performance

The activities of RGO/MnFe₂O₄ hybrids for the decomposition of MB dye were investigated. In a typical test, 5 mg of the resulting catalyst and a solution which contained 10 mL of MB dye solution (50 mg L⁻¹) as well as 5 mL of H₂O₂ (30 wt%) solution were mixed under continuous stirring. The concentration of MB dye was determined by UV-vis spectroscopy at 664 nm. Besides, the catalyst was magnetically separated and reused in a fresh MB and H₂O₂ solution after completion of the reaction for the stability tests of RGO/MnFe₂O₄-75. Furthermore, the formation of active ˙OH upon irradiation was chosen to evaluate the catalytic properties of the RGO/MnFe₂O₄ hybrids by using terephthalic acid (TA) as the probe molecule. The detailed process of catalytic activity test and the ˙OH analysis were shown in ESI Sp1 and Sp2,† respectively.

The structure analyses and catalytic measurements of the obtained samples were similar to our previous report.³²

Results and discussion

The characterization of RGO/MnFe₂O₄ hybrids

The environmentally benign synthesis of magnetic RGO/MnFe₂O₄ catalyst for the decomposition of MB is illustrated in Fig. 1. Firstly, we took full use of the pristine GO/MnSO₄ suspension (13 mg mL⁻¹ GO and 0.22 mol L⁻¹ MnSO₄) derived from a modified Hummers method,^34,35 in which Mn²⁺ ions preferably combined with the O atoms of the negatively charged surface of GO sheets via electrostatic interaction.³⁶ Additional FeSO₄ and KOH were successively added into the system. After being hydrothermally treated at 150 °C for 15 h, MnFe₂O₄ was formed (2Mn²⁺ + 12OH⁻ + 4Fe²⁺ + O₂ = 2MnFe₂O₄ + 6H₂O) and anchored firmly on the RGO surface to form RGO/MnFe₂O₄ hybrids. Moreover, the only by-product, namely, K₂SO₄, could be recycled for other purposes (ESI Sp3†). The resultant RGO/MnFe₂O₄ hybrids showed effective catalytic decomposition of MB and could be magnetically recovered.

Fig. 1 Schematic synthesis of magnetically recoverable RGO/MnFe₂O₄ hybrids for the decomposition of MB dye.

The powder X-ray diffraction (XRD) patterns of RGO/MnFe₂O₄-75, bare MnFe₂O₄ particles and RGO were shown in Fig. 2a. MnFe₂O₄ particles exhibit the intense peaks corresponding to (220), (311), (400), (511) and (440) planes, which match well with the standard MnFe₂O₄ (JCPDS 38-0430).¹⁷ For RGO/MnFe₂O₄-75, the intense peaks are weaker than those of bare MnFe₂O₄, which may be attributed to the small size and good distribution of MnFe₂O₄ in the hybrid. Furthermore, RGO/MnFe₂O₄-75 and RGO show the broad and weak peak at around 2θ = 25° as the result of the hydrothermal reduction of GO to RGO.²⁹

Fig. 2 (a) XRD patterns and (b) Raman spectra of RGO/MnFe₂O₄-75, MnFe₂O₄, and RGO.

The Raman spectra of the obtained samples were shown in Fig. 2b. It is well known that the G band at 1596 cm⁻¹ is usually assigned to sp² carbon domains, while the D band located at 1345 cm⁻¹ is associated with the defects and disorder carbon in the graphitic layers.^31,36 The intensity ratio of D band to G band (I_D/I_G) is usually used as the measure of the ordering quality of carbon materials. A lower intensity I_D/I_G ratio of RGO/MnFe₂O₄-75 hybrid, i.e., 1.11 is observed compared with 1.34 of RGO, suggesting that MnFe₂O₄ in RGO/MnFe₂O₄-75 can help to increase the size of the sp² domains. In addition, the characteristic peak at 612 cm⁻¹ of RGO/MnFe₂O₄-75 matches completely with that of bare MnFe₂O₄ particles.^29,37

XPS analysis was further employed to elaborate the surface chemical bonding states of the RGO/MnFe₂O₄ hybrid. The full-scale XPS spectra with C 1s, O 1s, Mn 2p, and Fe 2p spectra were shown in Fig. 3a. The spectrum of Fe 2p (Fig. 3b) has two main peaks at binding energies of 711.3 and 725.1 eV, which are related to Fe 2p_3/2 and Fe 2p_1/2, respectively. The satellite peaks at the position of 719.8 and 733.3 eV are also visible, confirming the presence of Fe³⁺ chemical state within MnFe₂O₄.^29,38 As shown in Fig. 3c, it is found that the XPS spectrum of Mn 2p presents two main peaks of Mn 2p_3/2 (641.2 eV) and Mn 2p_1/2 (652.8 eV) together with a satellite peak (644.9 eV), which are in good agreement with the Mn²⁺ chemical state.^39,40 The C 1s peak at 284.6 eV is commonly assigned to the elemental carbon (Fig. 3d).⁴¹ When comparing to the peaks of GO (the inset in Fig. 3d), the C–C/C=C peak becomes predominant (284.6 eV) while the peaks of C–O and C=O decrease drastically. This means that GO has been reduced to RGO except for a small amount oxygen-containing groups after being hydrothermally treated, which agrees with the XRD result.

Fig. 3 XPS spectra of RGO/MnFe₂O₄-75, MnFe₂O₄ and RGO: (a) the survey scan; (b–d) Fe 2p region, Mn 2p region and C 1s region of RGO/MnFe₂O₄-75; the inset in (d) is C 1s region of GO.

SEM images of RGO/MnFe₂O₄-75 and bare MnFe₂O₄ were shown in Fig. 4. Sandwich-like RGO layers are decorated by MnFe₂O₄ spheres with the size of around 200 nm, indicating that the presence of RGO effectively inhibits the aggregation of MnFe₂O₄ particles (Fig. 4a and b). Meanwhile, the existence of MnFe₂O₄ can prevent RGO from stacking.⁴² Such synergistic interaction plays a positive role in the catalytic decomposition of organic pollutants due to the improved transportation of electrons between MnFe₂O₄ and RGO.³⁰ For comparison, the bare MnFe₂O₄ particles (Fig. 4c) with diameters ranging from 100 to 500 nm seriously aggregated in the absence of RGO.

Fig. 4 SEM images of (a and b) RGO/MnFe₂O₄-75 and (c) the bare MnFe₂O₄.

TGA was applied from room temperature to 700 °C in air flow to analyze the thermal stability and quantified composition of RGO/MnFe₂O₄ hybrids. As shown in Fig. 5a, the gradual weight loss in RGO/MnFe₂O₄-75 hybrid occurs at 20–500 °C, which is corresponding to the loss of the physically adsorbed water and the burning of RGO.³¹ At last, MnFe₂O₄ with the mass percent of 75% is left after RGO is fully burned up at 700 °C (evidenced by XRD analysis in Fig. 5b). Therefore, it is calculated that the content of MnFe₂O₄ in RGO/MnFe₂O₄ is 75%. The mass ratios of MnFe₂O₄ in RGO/MnFe₂O₄ with 80% and 55% can be further tuned by adjusting the weight ratio of pristine GO/MnSO₄ suspension to FeSO₄ (shown in Experimental Section).

Fig. 5 (a) TGA curves of RGO/MnFe₂O₄ hybrids with different MnFe₂O₄ contents and (b) XRD pattern of residual MnFe₂O₄ after the burning of RGO of RGO/MnFe₂O₄-75.

Catalytic activities and stability test of RGO/MnFe₂O₄ hybrids

The catalytic activities of the as-prepared samples were evaluated at a given reaction interval in the presence of H₂O₂. The UV-vis absorption spectra of MB solution treated by RGO/MnFe₂O₄-75 hybrid were shown in Fig. 6a. The two characteristic absorption peaks appearing at 614 and 664 nm are contributed to the typical MB peaks in visible region at the starting solution.⁴³ After adding both RGO/MnFe₂O₄-75 and H₂O₂, the absorption peaks of MB solution obviously decrease. It can be seen that the absorption of MB is relatively rapid in the initial 50 min and its gradual blue shift is observed with prolonged reaction time, indicating the breakdown of chromophore structure of MB.³² As a result, the solution turns colorless gradually within 130 min and the corresponding photo images were shown in the inset of Fig. 6a. A series of comparative experiments were designed to verify the catalytic performance of the obtained samples. Fig. 6b shows the result of the decomposition degree of MB dye within 130 min for different samples. For comparison, the degree of MB decolorization is 21% without catalyst (only MB and H₂O₂), which is slightly higher than 17.3% in the absence of H₂O₂ (only MB and RGO/MnFe₂O₄-75) because of the low specific surface area (29 m² g⁻¹) of RGO/MnFe₂O₄-75 hybrid. Therefore, the synergetic effect of catalyst and H₂O₂ plays an important role in efficient decomposition of MB dye from water. To further explore the synergistic effect of MnFe₂O₄ and RGO in RGO/MnFe₂O₄ hybrids, the catalytic activities of RGO/MnFe₂O₄ with different MnFe₂O₄ contents as well as pure RGO in the decomposition of MB were compared. It is found that 27% and 24% of MB are decolorized by bare MnFe₂O₄ particles and RGO, respectively. As the mass ratio of MnFe₂O₄ in RGO/MnFe₂O₄ hybrids varies from 55 to 75 wt%, the decolorization degree of MB increases from 71.2 to 100% within 130 min, which is more effective than that of the reported RGO/MnFe₂O₄ hybrid (95% MB decomposition within 180 min)²⁸ and our previous work on RGO/MnO₂ and GO/Mn₃O₄ compositions (100% MB decomposition within 5 min at 50 °C and 200 min at room temperature, respectively).^31,32 The further increased MnFe₂O₄ content to 80 wt%, however, results in a low MB decolorization rate of 82% under the same condition. Thus, 75 wt% is the best content for MnFe₂O₄ in RGO/MnFe₂O₄ hybrid for MB decolorization. The mechanism of the significant activity enhancement of RGO/MnFe₂O₄ hybrids is illustrated as below. First, RGO can prevent the aggregation of MnFe₂O₄ particles as well as serve as the electron transfer bridges to enhance the MB decomposition based on the XRD, Raman and SEM analyses. Considering 2D planar structure and giant π-conjugation system of RGO, the MB molecules can be adsorbed on the surface of RGO and retained in close proximity to the active sites of MnFe₂O₄, thus providing great opportunities to degrade the contaminants.⁴⁴ Second, the uniformly dispersed MnFe₂O₄ particles can increase the number of active sites for the catalytic decomposition of MB as well. Besides, Mn²⁺ also plays an important role in accelerating the electron transfer process between Fe²⁺/Fe³⁺, and thus improves the catalytic activity to some extent,^45,46 which will be discussed in the following part.

Fig. 6 (a) Absorption spectra of MB solution (50 mg L⁻¹, 10 mL) of RGO/MnFe₂O₄-75 hybrid in different time intervals (the inset is the photo image); (b) time profiles of MB decomposition under different conditions; (c) the decomposition of MB with RGO/MnFe₂O₄-R, the inset is its SEM image; (d) the photo image of magnetically recyclable RGO/MnFe₂O₄.

Undoubtedly, the stability of the catalyst is quite important for practical applications. Thus, the stability test of RGO/MnFe₂O₄-75 is shown in Fig. 6c. Four recycling runs of RGO/MnFe₂O₄-75 were performed without any noticeable loss of catalytic ability. The catalyst named as RGO/MnFe₂O₄-75-R which was recovered after the test was further studied by SEM (the inset of Fig. 6c) and XRD analyses (Fig. 2a), which indicates that the size, morphology, and structure of the catalysts remains almost unchanged, further demonstrating their excellent stability. The concentration of the slightly leached Mn ions (about 0.5 wt%) from RGO/MnFe₂O₄-75 was further detected by inductive coupled plasma emission spectrometer (ICP),³² indicating the excellent stability of RGO/MnFe₂O₄-75 hybrid. It's worth mentioning that the catalyst can be completely recovered with a permanent magnet after each run (as shown in Fig. 6d), which not only simplifies operation steps and improves working efficiency, but also meets the demands of green chemistry.

As is known to all, the SO₄²⁻ mineralization is widely used to evaluate the MB degradation efficiency.³¹ In this work, the SO₄²⁻ concentration in the final solution was measured to be 7.1 mg L⁻¹ (0.4 mg L⁻¹ of the SO₄²⁻ comes from the H₂O₂ reagent). According to the theoretically calculated concentration (about 8.5 mg L⁻¹) after 100% decolorization of MB by RGO/MnFe₂O₄-75, we draw the conclusion that MB dye has been effectively decolorized and about 79% has been mineralized over RGO/MnFe₂O₄-75 hybrid, which is higher than those of previously reported GO/Mn₃O₄ composite (77%),³² RGO/MnO₂ composite (66%)³¹ and Mn₃O₄/graphene hybrid (27%).³⁶

Possible catalytic mechanism

In order to deeply understand the catalytic performance of as-prepared samples, the cumulative amount of ˙OH radicals was measured by photoluminescence (PL) technique with the similar method according to our previous work.³² The TA solution was selected as a probe molecule to react with ˙OH to produce highly fluorescent product 2-hydroxyterephthalic acid.¹⁰ It can be observed from Fig. 7a that a gradual increase of the PL intensity appears at 445 nm under UV light irradiation with the reaction time, suggesting that fluorescence presents because of the reactions of TA with ˙OH, showing a proportional relationship between PL intensity and the amount of generated ˙OH.³² Fig. 7b presents a comparison of the PL intensities of RGO/MnFe₂O₄-75, MnFe₂O₄ and RGO samples in the assistance of H₂O₂. As shown in Fig. 6b, the amount of produced ˙OH radicals by the tested samples decreases in the same order of their MB catalytic performances (RGO/MnFe₂O₄-75 > MnFe₂O₄ > RGO). This agreement between catalytic performance and hydroxyl radical measurement further clarifies that positive effect between MnFe₂O₄ and RGO on the MB decomposition in this study.

Fig. 7 (a) PL spectra of 5 × 10⁻⁴ M basic TA solution treated by RGO/MnFe₂O₄-75 irradiated by UV light under different time (excitation at 325 nm) and (b) PL spectra of 5 × 10⁻⁴ M basic TA solution treated by various samples irradiated by UV light at 50 min.

The mechanism of the enhanced catalytic performance for RGO/MnFe₂O₄ may be explained by the possible electron transfer process of MnFe₂O₄ during the reaction. According to the experimental results of XPS analysis, iron and manganese of MnFe₂O₄ exist mainly in the oxidation state, i.e. Fe(III) and Mn(II), respectively. Thus, Fe₂O₃ and MnO samples were prepared for further analysis (as shown in Sp4†). The catalytic performances of individual Fe₂O₃, MnO and MnFe₂O₄ were evaluated under the same condition (Fig. 8a). It is observed that MB decomposition rate is dominated by Fe₂O₃ in the initial stage (<70 min) then by MnO in the later stage (>70 min). As a result, the performance of MnFe₂O₄ is superior to that of individual MnO and Fe₂O₃ during the whole process. The competitive PL intensities of MnO and Fe₂O₃ are shown in Fig. 8b–d. The results demonstrate that the performance of above catalysts is related to the amount of ˙OH. In the starting period (20 min), the amount of ˙OH produced by Fe₂O₃ is more than that of MnO (as shown in Fig. 8b), which exhibits a contrary result when the reaction time is more than 70 min (Fig. 8c and d). The amount of ˙OH produced by MnFe₂O₄ is larger than those of MnO and Fe₂O₃ in the whole process, which are in good agreement with their catalytic performances as described in Fig. 8a.

Fig. 8 (a) Time profiles of MB decolorization (50 mg L⁻¹, 10 mL) treated by different samples under the same condition; (b–d) PL spectra of various samples in a 5 × 10⁻⁴ M basic TA solution under UV light irradiation.

The performance of Fe₂O₃ may be attributed to the significant effect of Fe³⁺/Fe²⁺ cycling to yield ˙OH radicals for the degradation of organic pollutants. At first, Fe³⁺ is reduced to Fe²⁺ which subsequently reacts with H₂O₂ to produce ˙OH and Fe³⁺. Consequently, such process accelerates the Fe³⁺/Fe²⁺ cycle to form the large amount of strong oxidant ˙OH for the degradation of MB.^27,47 The main process may be described as eqn (1) and (2).

Fe³⁺ + H₂O₂ → Fe²⁺ + ˙OOH + H⁺ (1)

Fe²⁺ + H₂O₂ → Fe³⁺ + ˙OH + OH⁻ (2)

The reactions between Mn²⁺ and H₂O₂ can also contribute to the generation of ˙OH, which would be responsible for the remarkable increase on the catalytic activities,⁴⁶ as shown in eqn (3) and (4).

Mn²⁺ + H₂O₂ → Mn³⁺ + ˙OH + OH⁻ (3)

Mn³⁺ + H₂O₂ → Mn²⁺ + ˙OOH + H⁺ (4)

Beside the Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ cycles, the reaction between Fe²⁺ and Mn³⁺ is thermodynamically favorable, which benefits the redox cycles of Fe³⁺/Fe²⁺ (E^θ(Fe³⁺/Fe²⁺) = 0.77 V) and Mn³⁺/Mn²⁺ (E^θ(Mn³⁺/Mn²⁺) = 1.51 V) (as shown in eqn (5)).

Fe²⁺ + Mn³⁺ → Fe³⁺ + Mn²⁺, E^θ = 0.74 V (5)

Given the better catalytic performance of MnFe₂O₄ than those of individual Fe₂O₃ and MnO, it is anticipated that the synergetic effects of the redox couples of Mn³⁺/Mn²⁺, Fe³⁺/Fe²⁺ and Fe²⁺/Mn³⁺ during the reaction is responsible for the enhanced performance.

In order to further demonstrate the oxidation state of Mn and Fe of MnFe₂O₄ during the reaction, XPS spectra of the cycled RGO/MnFe₂O₄-75-R were further recorded (see Sp5†). Comparing to the Mn 2p and Fe 2p in Fig. 3b and c, the RGO/MnFe₂O₄-75-R hybrid exists the same peaks with the fresh one. However, there are some slight differences of the binding energies and the full widths at half maximum (FWHM) values of Fe 2p as well as Mn 2p before and after Fenton reaction, which have been displayed in Tables S1 and S2,† respectively. Such results may be attributed to the electron transfer of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ in the catalytic process. H₂-TPR measurement was further applied in the MnFe₂O₄ catalyst with Mn₃O₄ and Fe₂O₃ as the references (Sp6†). It is found that the reduction step of Fe³⁺ to Fe²⁺ starts at a higher temperature (320 °C) comparing with that of Mn³⁺ to Mn²⁺ (210 °C).⁴⁸ This observation suggests that Mn³⁺/Mn²⁺ presents higher oxidation capacity than Fe³⁺/Fe²⁺, which is in agreement the reaction described in eqn (5).

Accordingly, the mechanism schematic of MB decomposition on RGO/MnFe₂O₄ catalyst with the assistance of H₂O₂ was shown in Fig. 9. At first, MB molecules are adsorbed on the RGO and its giant π-conjugation system and 2D planar structure help to promote the electron transfer during the catalytic reaction.^32,49 Besides, MnFe₂O₄ nanoparticles anchored on the RGO surface catalyze H₂O₂ to produce large amount of ˙OH radicals and constantly facilitate the decomposition of MB molecules into CO₂, H₂O and SO₄²⁻ etc. During the catalytic process, the coexistence of the internal electron transfer in Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox couples have prominent contributions to the significant catalytic performance, wherein the former help to that of the earlier 70 min and the later plays a positive role in the last period. Furthermore, the reduction of Mn³⁺ by Fe²⁺ during the reaction is thermodynamically favorable, which in turn benefits the redox cycles of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺. Therefore, the coupling contributions of RGO/MnFe₂O₄ catalyst result in an appreciable improvement in the decomposition of MB.

Fig. 9 Schematic of the mechanism of MB decomposition on RGO/MnFe₂O₄ hybrid with the assistance of H₂O₂.

Conclusions

In summary, we have successfully synthesized magnetic RGO/MnFe₂O₄ hybrids with pristine GO/MnSO₄ suspension derived from a modified Hummers method and ferrous sulfate as the main precursors. Such a process not only alleviates the tedious purification process for GO production, but also contributes to a high atom-economic synthesis. The combination of magnetic MnFe₂O₄ particles with RGO plays an important role in controlling the size and distribution of the MnFe₂O₄ nanoparticles, thus allowing excellent catalytic performance for the decomposition of MB. In particular, MB dye molecules in neutral solution can be fully decomposed within 130 min in the presence of H₂O₂ at room temperature. The sequential synergetic actions of Fe³⁺/Fe²⁺, Mn³⁺/Mn²⁺ and Fe²⁺/Mn³⁺ redox couples at different stages contribute to the excellent catalytic activity. Besides, the magnetic property of the MnFe₂O₄ nanoparticles greatly simplifies the recycling performance. This work not only offers a new approach to fabricate graphene-based functional hybrids, but also benefits understanding the mechanism of transition metals on the heterogeneous Fenton system.

Acknowledgements

This work is supported by the NSFC (No. 51372277, 50902066), China Postdoctoral Science Foundation (2013M530922, 2014T70253), Program for Liaoning Excellent Talents in University (LJQ2014118), the Fundamental Research Funds for the Central Universities (No. 15CX08005A), Natural Science Fund of Liaoning Province (201602458).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24320g

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