Synthesis, characterization and photocatalytic properties of MIL-53(Fe)–graphene hybrid materials

Abstract

MIL-53(Fe)–reduced graphene oxide (rGO) hybrid materials with different rGO contents were prepared through a simple one-step solvothermal method. The products were characterized by X-ray diffraction patterns, thermal analysis, FT-IR spectra, Raman spectroscopy, scanning electron microscopy, UV-vis absorption spectra and photoluminescence spectra. The hybrid with 2.5% rGO content showed optimal photocatalytic performance in degradation of methylene blue under both UV and visible light irradiation and the degradation rate constants under UV and visible light were 1.34 and 1.57 times of those using MIL-53(Fe) respectively. The considerable promotion in photoactivities can be attributed to rGO being able to separate the photogenerated electrons and suppress electron–hole recombination. In addition, the hybrids show increased light absorption intensity and range. This work provides a new concept that MOF–rGO hybrids can be used as high performance photocatalysts and applied in the field of environmental protection.

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1. Introduction

Metal–organic frameworks (MOFs) have been proven to be one of the most exciting new classes of porous materials over the past few decades.¹ They possess large surface area and pore volume. Meanwhile, various combinations of metal ions with organic linkers allows them to have diverse compositions and structure. Therefore, MOFs have potential applications in gas storage,^2–4 separation,^5–9 catalysis,^10–12 drug delivery,^13–15 and so on.

Photocatalysis has been used as a green ecological way to eliminate organic compounds or harmful pollutants. Exploring highly efficient photocatalysts for the utilization of solar energy sources has become an active research area. More recently, several groups tried to exploit new applications of MOFs and demonstrated that MOFs could act as photocatalysts.^16–21 Recently, Du et al. have reported MIL-53(Fe) to be an efficient photocatalyst for the degradation of methylene blue (MB) dye under UV-vis light and visible light irradiation.²⁰ MIL-53(Fe) has a relatively narrow bandgap of 2.72 eV and good photochemical stability. The reactive species generated on the MIL-53(Fe) surface, such as hydroxyl radical (˙OH), superoxide radical (˙O₂⁻) and holes (h⁺), are the main causes of the photodegradation of pollutants. However, the fast electron–hole recombination leads to low efficiency of MIL-53(Fe), which limits its application in photocatalysis.

Due to specific electrical and surface properties, excellent conductivity, and high surface area of graphene, many graphene-based hybrids have been formed.^22–25 Graphene can improve photocatalytic activities of semiconductors since it can promote the electron transfer and charge separation processes. Therefore, during the past few years, various graphene and its derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) based photocatalysts have been reported.^26–31

Recently, there are just several examples of MOF–GO hybrids.^32–38 Petit et al. prepared MOF-5–GO,³² HKUST-1–GO,³³ MIL-100(Fe)–GO³⁴ and these hybrids showed a synergistic effect in adsorption of certain gases, such as hydrogen sulphide, nitrogen dioxide and ammonia. Loh et al. demonstrated that benzoic acid-functionalized graphene could work as a structure-directing template in the crystal growth of metal–organic framework, and the intercalation of graphene in MOF offered new electrical properties.³⁵ However, there are few reports of other applications of MOF–GO hybrids, consequently, exploring more applications of MOF–GO hybrids is an interesting task.

In this paper, MIL-53(Fe)–rGO hybrids were prepared via a facile one-step solvothermal reaction. As-prepared hybrids showed superior photocatalytic performance in degradation of MB under both UV and visible light irradiation relative to bare MIL-53(Fe). The enhancement in photoactivities can be ascribed that the hybrids have increased light absorption intensity and range and that rGO separates the photogenerated electrons and suppresses electron–hole recombination. To our best knowledge, this is the first report of the synthesis of the MIL-53(Fe)–rGO hybrids and the application of MOF–rGO hybrids in the field of photocatalysis.

2. Experimental

2.1 Chemicals

All the reagents and solvents were purchased from Sinopharm Chemical Reagent Co., Ltd and used as supplied without further purification.

2.2 Preparation of graphene oxide (GO)

GO was made from natural graphite powder using the method in the ref. 39. The resulting GO suspension was freeze-dried to get GO powder.

2.3 Synthesis of MIL-53(Fe)[H₂O]

MIL-53(Fe)[H₂O] was prepared following the procedure as previously reported.⁴⁰ Briefly, FeCl₃·6H₂O (10 mmol, 2.7 g), 1,4-benzenedicarboxylic acid (H₂BDC) (10 mmol, 1.66 g), aqueous HF (10 mmol, 0.2 g), N,N-dimethylformamide (DMF) (50 mL) were mixed and stirred for 30 min, then the mixture was placed in a Teflon-lined steel autoclave. The mixture was heated at 150 °C for 3 days in a preheated fan oven. After the autoclave cooled to room temperature, MIL-53(Fe)[DMF] powder was recovered by filtration and washed with MeOH to obtain MIL-53(Fe)[MeOH]. Finally, MIL-53(Fe)[H₂O] was gained after MIL-53(Fe)[MeOH] was washed with water and dried overnight in air. Quantitative elemental analyses of MIL-53(Fe)[H₂O] gave the following results: Fe, 20.7%; C, 37.8%; H, 2.69% which compare well with those calculated from the formula Fe^III(OH)_0.8F_0.2(O₂C–C₆H₄–CO₂)·(H₂O): Fe, 21.9%; C, 37.6%; H, 2.68%. So, MIL-53(Fe)[H₂O] was made successfully.

2.4 Preparation of MIL-53(Fe)–rGO hybrids

A certain amount of GO powder was dispersed in DMF and sonicated for 20 min, and then the GO suspension was added into the mixture of MOF precursors. The rest of the procedure of resulting suspension was the same as that of MIL-53(Fe)[H₂O]. In the reaction process, graphene oxide was reduced to graphene and MIL-53(Fe)–rGO hybrids were got simultaneously. As-obtained hybrids were denoted as FeMG-1, FeMG-2 and FeMG-3, in which the rGO was ca. 1.3, 2.5 and 3.2 wt% respectively. The wt% is the weight ratio of rGO in the hybrids to MIL-53(Fe)[H₂O]–rGO hybrids.

2.5 Characterization

Powder X-ray diffraction (XRD) patterns were recorded using Rigaku D/Max 2400 diffractometer employing Cu Kα radiation. FT-IR spectra were recorded using Bruker EQUINOX55 infrared spectrometer. Thermogravimetric analysis (TGA) was completed by the TG 209C thermal analyzer at a scanning rate of 10 °C min⁻¹ under air atmosphere from 35 to 550 °C. From the TGA curves, differential DTG curves were derived. Diffuse reflectance UV-vis spectroscopy (DR/UV-vis) experiments were performed using a Jasco UV-550 spectrophotometer. Scanning electron microscopy (SEM) images were obtained on the FEI QUANTA 450 scanning electron microscopy. Raman spectroscopy was carried out on the Thermo SCIENTIFIC DXR Raman Microscope, under a 532 nm wavelength. Photoluminescence spectra (PL) were recorded using HITACHI F-7000 fluorescence spectrometer. The Fe contents of the samples were determined quantitatively by inductively coupled plasma (ICP) on Leeman Plasma-Spec-II instrument. C, H contents of the samples were determined by Elemental Analyzer Vario EL III.

2.6 Photocatalytic activity

The photocatalytic activities of MIL-53(Fe)[H₂O] and FeMG hybrids were evaluated by the photodegradation of MB dye under UV and visible light. The reactor that was used in photocatalytic decomposition of MB under UV light was made up of a jacketed quartz tube of 3 cm inner diameter, 4.9 cm outer diameter and 14.3 cm length. A high-pressure Hg lamp of 125 W (λ_main = 365 nm) was placed inside the quartz reactor and average light intensity was 350 mW cm⁻². Incident UV light intensity was measured by radiometer of model UV365 (Photoelectric Instrument Factory of Beijing Normal University). The lamp and the quartz reactor were placed concentrically inside a Pyrex glass outer reactor with 7.2 cm inner diameter and 17.6 cm height. Water was circulated in the annular space of jacketed quartz reactor. 100 mg photocatalyst and 200 mL 30 mg L⁻¹ MB aqueous solution were added to the Pyrex glass outer reactor and the reaction mixture was stirred for 1 h in the dark to reach the adsorption–desorption equilibrium. 2 mL aliquots were withdrawn at given time intervals and then centrifuged to remove all the catalyst. The MB concentration was monitored by measuring its maximum absorbance wavelength at 664 nm with a UV-visible spectrophotometer.

In addition, photodegradation of MB under visible light was carried out in a 250 mL beaker. A sodium lamp of 250 W (λ_main = 555 nm) with a 420 nm cutoff filter was used as visible light resource (λ > 420 nm). The sodium lamp was placed inside a jacketed Pyrex tube of 5 cm inner diameter, 7 cm outer diameter and 23 cm length. The average visible light intensity was 52 mW cm⁻², which was measured by radiometer of model FZ-A. Water was circulated in the annular space of Pyrex reactor and the temperature was held at 25 °C. The distance between light source and the solution was 15 cm. 50 mg photocatalyst and 100 mL 30 mg L⁻¹ MB aqueous solution was added into the beaker. Subsequent experiment was similar with that above.

3. Results and discussion

The X-ray diffraction patterns of the parent materials and the hybrids are presented in Fig. 1. The pattern of MIL-53(Fe)[H₂O] is consistent with those reported in the literatures,^20,41 which demonstrates that the material is prepared successfully. As for GO, there is a single peak at 10.8°, suggesting an interlayer distance of 8.1 Å according to Bragg's law. A broad peak at 24.7° is observed for rGO. In the case of the hybrids, almost all of the diffraction peaks of FeMG-1 are the same as those of MIL-53(Fe)[H₂O]. However, for FeMG-2 and FeMG-3, the main diffraction peak of MIL-53(Fe)[H₂O] (2θ of 12.4°) splits into two peaks. One peak is retained at 12.4° and a new peak appears at 11.2°. In addition, the peaks between 23 and 30° as well as the peak at 17° become less sharp. The peak at 9.2° shows higher intensity and another new peak appears at 22.3°. Similar changes of X-ray diffraction patterns were also found in other MOF–GO hybrids, such as MOF-5–GO,³² MIL-100(Fe)–GO,³⁴ MOF–benzoic acid-functionalized graphene.³⁵ Some peaks disappeared and some new peaks emerged in these hybrids, when more GO was added. These changes were attributed that GO caused change of lattice structure of MOFs. So in this work, rGO may result in change of lattice structure of MIL-53(Fe)[H₂O].

Fig. 1 XRD patterns of GO, rGO, MIL-53(Fe)[H₂O], FeMG-1, FeMG-2, FeMG-3 and FeMG-2 after 7 h of visible light irradiation.

Fig. 2 shows that FeMG-1, FeMG-2 and FeMG-3 have similar DTG curves to MIL-53(Fe)[H₂O], so they have similar thermal stabilities. The hybrids and MIL-53(Fe)[H₂O] also have similar FT-IR spectra (Fig. 3). These indicate that graphene layers did not prevent the formation of the MIL-53(Fe)[H₂O] unit and they just affected the lattice structure of MOF (as suggested by XRD).³⁴

Fig. 2 DTG curves of MIL-53(Fe)[H₂O], FeMG-1, FeMG-2 and FeMG-3.

Fig. 3 FT-IR spectra of MIL-53(Fe)[H₂O], FeMG-1, FeMG-2 and FeMG-3.

Fig. 4 presents the Raman spectra of rGO, MIL-53(Fe)[H₂O] and the hybrids. There are two broad peaks at 1595 and 1347 cm⁻¹ in the spectrum of rGO, which are G and D bands of rGO respectively. In the 1800–860 cm⁻¹ range, the spectrum of MIL-53(Fe)[H₂O] is dominated by the vibration modes of the organic part H₂BDC of MOF material (the Raman spectrum of H₂BDC not shown here).⁴² The three hybrids present the similar Raman spectra and the main bands of MIL-53(Fe)[H₂O] are maintained. However, there are some differences between the hybrids and MIL-53(Fe)[H₂O]. A new band appears at 1333 cm⁻¹, which is the D band of rGO. In addition, absorption band at 1610 cm⁻¹ becomes asymmetric and shifts to lower value, because the G band of rGO overlaps with the band of MIL-53(Fe)[H₂O]. These features display the coexistence of rGO and MIL-53(Fe)[H₂O] unit in the hybrids.

Fig. 4 Raman spectra of rGO, MIL-53(Fe)[H₂O], FeMG-1, FeMG-2 and FeMG-3.

Fig. 5 displays SEM images of parent materials and the hybrids. In Fig. 5a, it can be clearly seen that GO sheets are curled and corrugated, which seem like chiffon. The crystals of MIL-53(Fe)[H₂O] are blocky granular shape and not homogeneous. For the hybrids, the particles of MIL-53(Fe)[H₂O] are coated by rGO layers, which leads to the surface of the particles more rough.

Fig. 5 SEM images of GO (a), MIL-53(Fe)[H₂O] (b) and the hybrids (c and d).

The light-absorbance characteristics of all the samples are explored with UV-vis diffuse reflectance spectra (DRS), as shown in Fig. 6. Absorption edge of MIL-53(Fe)[H₂O] is about 498 nm and its bandgap energy is 2.49 eV, which is calculated simply by the equation E_g = 1240/λ. With increment of rGO content, an obvious red shift in the absorption edge of FeMG hybrids in contrast with MIL-53(Fe)[H₂O] is observed. Besides, an enhanced absorbance is found in the visible-light region (>400 nm), which is in accordance with the change in color of the samples from light yellow to gray-green. When absorbance in the visible-light region increases, solar energy can be utilized more effectively. Consequently, we can speculate that the introduction of rGO in MIL-53(Fe)[H₂O] particles can enhance its performance in photodegradation of MB.

Fig. 6 UV-vis spectra of MIL-53(Fe)[H₂O] and the hybrids: FeMG-1, FeMG-2 and FeMG-3.

Fig. 7 illustrates PL spectra of MIL-53(Fe)[H₂O], FeMG-1, FeMG-2 and FeMG-3. When MIL-53(Fe)[H₂O] is excited by 300 nm laser, its emission spectrum shows two major strong peaks at 417 and 470 nm. While, the intensity of the hybrids is lower than that of MIL-53(Fe)[H₂O], because rGO is introduced into the hybrids and electron–hole recombination is suppressed. It means that the separation of electrons and holes in the hybrids is more efficient than that in MIL-53(Fe)[H₂O].

Fig. 7 PL spectra of MIL-53(Fe)[H₂O], FeMG-1, FeMG-2 and FeMG-3.

The photocatalytic activities of MIL-53(Fe)[H₂O] and the hybrids in the degradation of MB under UV light were firstly investigated (Fig. 8). Simple photolysis of MB without catalyst was also carried out for a comparison. MIL-53(Fe)[H₂O] degraded 82% of MB after 80 min of irradiation. The photocatalytic efficiencies of FeMG hybrids were gradually improved with the increase of rGO content and the maximum value reached 95% for FeMG-2. Further increasing rGO content resulted in decreased degradation rate, but it still remained higher than that of MIL-53(Fe)[H₂O].

Fig. 8 Photodegradation of MB as a function of UV irradiation time over MIL-53(Fe)[H₂O] and the hybrids.

The degradation reaction was fitted to a pseudo first-order kinetic:

ln(c₀/c) = kt

where c₀ = initial concentration, c = concentration at certain time t, k = degradation rate constant, t = reaction time.

As shown in Table 1, the average rate constants k for degradation of MB using MIL-53(Fe)[H₂O] and FeMG-2 were 2.13 × 10⁻² and 2.85 × 10⁻² min⁻¹ respectively. That was, the latter was 1.34 times of the former.

Table 1 Degradation rate constants for MIL-53(Fe)[H₂O] and the hybrids under UV light

No.	Catalysts	Rate constant (min^−1)
1	Blank	0.27 × 10^−2
2	MIL-53(Fe)[H2O]	2.13 × 10^−2
3	FeMG-1	2.36 × 10^−2
4	FeMG-2	2.85 × 10^−2
5	FeMG-3	2.54 × 10^−2

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In addition, the photocatalytic activities of MIL-53(Fe)[H₂O], the hybrids and P25 in the degradation of MB under visible light are shown in Fig. 9. P25 showed rather poor photocatalytic activity and only degraded 30% of the initial MB after 420 min of irradiation, because its photoresponding range is limited. MIL-53(Fe)[H₂O] displayed slightly higher activity than P25 and decomposed 32% of the initial MB during the same time.

Fig. 9 Photodegradation of MB as a function of visible light irradiation time over P25, MIL-53(Fe)[H₂O] and the hybrids.

However, when rGO was added, all the hybrids demonstrated better photocatalytic performance than P25 and bare MIL-53(Fe)[H₂O]. Especially for FeMG-2, it showed optimal photocatalytic performance and lessened 49% of the MB. Moreover, the intensities of X-ray diffraction pattern peaks for FeMG-2 were unchanged after 420 min of irradiation, as shown in Fig. 1, which demonstrated that the photocatalyst was stable.

Table 2 shows the kinetic of the degradation reaction. The average rate constants k were 0.86 × 10⁻³ and 0.89 × 10⁻³ min⁻¹ for P25 and MIL-53(Fe)[H₂O] respectively. While, the rate constant k for FeMG-2 was 1.40 × 10⁻³ min⁻¹, which was 1.63 and 1.57 times as high as that of P25 and MIL-53(Fe)[H₂O].

Table 2 Degradation rate constants for P25, MIL-53(Fe)[H₂O] and the hybrids under visible light

No.	Catalysts	Rate constant (min^−1)
1	Blank	0.78 × 10^−3
2	P25	0.86 × 10^−3
3	MIL-53(Fe)[H2O]	0.89 × 10^−3
4	FeMG-1	1.05 × 10^−3
5	FeMG-2	1.40 × 10^−3
6	FeMG-3	1.09 × 10^−3

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The FeMG hybrids display preferable photocatalytic performance than MIL-53(Fe)[H₂O] in the degradation of MB under UV and visible light irradiation. The significant promotion in photoactivities can be attributed that rGO can accept the photogenerated electrons and suppress electron–hole recombination. Beyond that, the light absorption intensity and range are increased in the hybrids. The causes can be certified by PL spectra and UV-vis spectra of MIL-53(Fe)[H₂O] and the hybrids. Fig. 10 shows the possible photocatalytic mechanism of FeMG hybrids. Under light irradiation, MIL-53(Fe)[H₂O] absorbs light and is excited. The photo-induced electrons are transferred from conduction band of MIL-53(Fe)[H₂O] to rGO sheet, which can efficiently separate the electrons and suppress electron–hole recombination. The electrons (e⁻) can be trapped by molecular oxygen to generate superoxide radical (˙O₂⁻). At the same time, the holes (h⁺) react with hydroxyl ion (OH⁻) or water molecules to form hydroxyl radical (˙OH). The ˙OH and ˙O₂⁻ have strong oxidative ability to degrade the MB molecules to CO₂, H₂O or other products.

Fig. 10 Schematic illustration of photodegradation of MB using FeMG hybrids.

However, when rGO content exceeds its optimal value, the photocatalytic efficiency becomes poor. There may be two reasons: on the one hand, more rGO shows stronger absorption to light, so that less light is harvested by MIL-53(Fe)[H₂O], thus less electrons are produced; on the other hand, excessive rGO may promote recombination of electrons and holes.⁴³

4. Conclusions

In this paper, a simple one-step solvothermal method was used to prepare MIL-53(Fe)–rGO hybrids with different graphene contents. The hybrids show enhanced light absorption intensity and range and rGO in the hybrids suppresses the recombination of electrons and holes, so they exhibit superior photocatalytic performance in degradation of MB under both UV and visible light irradiation. The hybrids of other MOF and rGO can be prepared using similar method and applied in photocatalysis field.

Acknowledgements

The authors are grateful for the financial support from program for New Century Excellent Talents in University (NCET-04-0270).

Notes and references

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