Graphene Hybridized Photoactive Iron Terephthalate with Enhanced

Article pubs.acs.org/IECR

Graphene Hybridized Photoactive Iron Terephthalate with Enhanced Photocatalytic Activity for the Degradation of Rhodamine B under Visible Light Caihong Zhang, Lunhong Ai, and Jing Jiang* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Shida Road 1#, Nanchong 637002, P.R. China S Supporting Information *

ABSTRACT: In this study, we report the design and fabrication of a series of visible-light-responsive photocatalysts based on one-dimensional iron terephthalate (MIL-53(Fe)) microrods hybridized with graphene (GR) and experimentally demonstrate their remarkably improved visible-light-induced photocatalytic activity. During the solvothermal process, the reduction of graphene oxide (GO) is accompanied by the MIL-53(Fe) crystallization, which endows them with effective interfacial contact, thus facilitating the transfer of photogenerated charge to lower the recombination rate of excited carriers. The GR/MIL-53(Fe)H2O2 systems exhibit significantly higher photocatalytic activity toward degrading Rhodamine B (RhB) than that of bare MIL53(Fe)-H2O2 under visible light irradiation. The introduced H2O2 induces photosynergistic generation of more amounts of hydroxyl radicals to contribute to the improved photocatalytic activity. This work could open a new way for the exploration and utilization of metal−organic framework (MOF)-based crystalline materials for light harvesting.

1. INTRODUCTION Metal−organic frameworks (MOFs) made of metal ions/ clusters and organic linkers have been known as a fascinating class of porous crystalline materials, currently evoking tremendous research interest.1,2 Their outstanding structure features, such as controlled porosities, large internal surface areas, and tunable cavities, endow them with distinguished properties and potential applications in selective adsorption and separation, gas storage, drug delivery, chemical sensing, and catalysis.3−6 Very recently, MOFs have been explored as a new platform for light harvesting, owing to their structure and composition-induced intrinsic photochemical behaviors.7−10 Interacting with energy-matched incident light, MOFs can be photoexcited reasonably in terms of ligand-to-metal charge transfer (LMCT) 8,9,11 and/or direct metal-oxo cluster excitation.12−14 Some recent studies have highlighted that the interesting photocatalysis phenomenon in a number of MOFs constructed from transition metal ions and carboxylate linkers. For example, a typical MOF-5 reported by Garcia and coworkers has been proven to be an active photocatalyst for phenol degradation under UV light irradiation.15 An aminefunctionalized NH2-MIL-125(Ti) MOF has been employed for CO2 reduction under visible light.16 Some Zr-based UiO66(NH2) MOFs have also displayed visible-light-responsive photocatalysis toward degrading organic pollutants and reduction of Cr(VI).17,18 Of note, although versatile manipulations with photoabsorption based on structure and/or composition design lead to improvements in photocatalytic activity of the MOFs, the activity of reported MOF photocatalysts seems to be very modest. MOFs acting as a photocatalyst still confront with certain limitations similar to the conventional semiconductor. Some MOFs are able to capture visible light in solar spectrum, but their photogenerated electron−hole pairs suffer from fast © XXXX American Chemical Society

recombination. Therefore, efficiently balancing the competition between the transfer and recombination processes of photogenerated charge carriers and rationally engineering the photoactive sites are the critical points to improve performance of MOF photocatalysts. To enhance charge transport in MOFs, many attempts have been paid to the utilization of noble metal (such as Pt, Au, Pd)18−20 as a cocatalyst to suppress electron− hole recombination. This strategy can enhance the photocatalytic efficiency significantly, but the practical applications are restricted by the inevitable disadvantages of high price and limited abundance of noble metal. Alternatively, the combination semiconductor with graphene (GR) to form a heterostructure is a feasible route to promote the separation of photogenerated charge carriers and thus increase their lifetime, owing to the high specific surface area, superior charge carrier mobility, and high electrical conductivity of GR.21,22 Therefore, the design and fabrication of high-performance photocatalytic systems based on GR-related composites are highly desired. It has been reported that the photocatalytic applications of several GR-semiconductor (e.g., TiO2,23 ZnO,24 CdS,25 WO3,26 BiOBr,27 and Bi2WO628) composites. In this background, it is possible that the rational coupling of GR to form composites could favorably enhance the photoreactivity of MOFs, but it has seldom been explored.20,29 MIL-53(Fe) is a three-dimensional porous solids composed of infinite FeO4(OH)2 octrahedra connected by bis-bidentate terephthalate (1,4-benzenedicarboxylate) ligands.30 Due to its flexible structure, nontoxicity, and intrinsically optical absorbance, MIL-53(Fe) has been developed for use in photoReceived: October 18, 2014 Revised: December 1, 2014 Accepted: December 13, 2014

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DOI: 10.1021/ie504111y Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. (a) SEM image of MIL-53(Fe), (b) TEM image of GR, (c) SEM, and (d) TEM images of GR/MIL-53(Fe) composite.

catalysis.14,31,32 However, to the best of our knowledge, as of yet there has been no report on using one-dimensional MIL53(Fe) as a high-performance photocatalyst. Herein, we report a novel MOF-based composite photocatalyst derived from onedimensional MIL-53(Fe) microrods hybridized by graphene for the first time. During the solvothermal process, the reduction of graphene oxide (GO) is accompanied by the MIL-53(Fe) crystallization, which endows them with effective interfacial contact, thus facilitating the transfer of photogenerated charge to lower the recombination rate of excited carriers. The GR/ MIL-53(Fe)-H2O2 systems exhibit significantly higher photocatalytic activity toward degrading Rhodamine B (RhB) than that of bare MIL-53(Fe)-H2O2 under visible light irradiation. The introduced H2O2 could induce photosynergistic generation of more amounts of hydroxyl radicals to contribute to the improved photocatalytic activity. Meanwhile, the role of GR in the photocatalytic reaction is demonstrated experimentally as well. This work could open a new way for the exploration and utilization of MOF-based composites for environmental photocatalysis applications.

min, the mixture was transferred into a Teflon-lined stainless steel autoclave with a volume capacity of 20 mL and heated at 150 °C for 12 h. After the heat treatment, the autoclave was allowed to cool naturally to room temperature, and the products were collected by centrifugation at 6000 rpm for 2 min. To remove the solvent, the obtained precipitate was suspended into a 200 mL of distilled water overnight and then centrifuged in water and dried in vacuum at 60 °C for 12 h. The corresponding GR/MIL-53(Fe) composites were designated as GR/MIL-53(Fe)-x, where x (wt %) refers to GR amount in the composites. Among these samples, the GR/MIL-53(Fe)-5 shows the best photocatalytic performance; therefore, if no further notification is provided, the GR/MIL-53(Fe) notation in this study refers to GR/MIL-53(Fe)-5. Characterization. The powder X-ray diffraction (PXRD) measurements were recorded on a Rigaku Dmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). The morphology was observed with a JEOL JSM6510LV scanning electron microscope (SEM) and transmission electron microscopy (TEM, FEI Tecnai G20). Surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C, Al KR). All binding energies were calibrated by using the contaminant carbon (C1S = 284.6 eV) as a reference. The Fourier transform infrared (FTIR) spectroscopy was recorded on Nicolet 6700 FTIR Spectrometric Analyzer using KBr pellets. UV−vis diffused reflectance spectra of the samples were obtained for the dry-pressed film samples using a UV−vis spectrophotometer (UV-3600, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV−vis diffuse reflectance experiment. Raman measurements were carried out by a confocal laser

2. EXPERIMENTAL SECTION Chemicals. All chemicals used in this study were of commercially available analytical grade and were supplied by Kelong Chemical Reagents Company (Chengdu, China) without further purification. Graphene oxide (GO) was prepared from natural graphite powder by a modified Hummers’ method, which is described elsewhere.33 Synthesis of GR/MIL-53(Fe) Composites. In a typical experiment, 1 mmol of FeCl3·6H2O and 1 mmol of 1,4-BDC were added to 5 mL of DMF solution containing an appropriate amount of GO. After ultrasonicating for about 30 B


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interface to mediate charge transfer during photocatalytic process. The crystal structures of GO, GR, MIL-53(Fe), and GR/ MIL-53(Fe) composite were determined by X-ray diffraction (XRD). As shown in Figure 2, GO shows a sharp and intense

micro-Raman spectrometer (Thermo DXR Microscope, U.S.A.). The laser was 633 nm with a 5 mW. Photocatalytic Experiment. The photocatalytic activities of GR/MIL-53(Fe) composites were tested by photocatalytic degradation of RhB aqueous solution under visible light irradiation in the presence of a small amount of H2O2, which was conducted in a cylindrical Pyrex vessel reactor using a 500 W Xe arc lamp with a 420 nm cutoff filter as the light source. Typically, 0.01 g of photocatalyst sample was added into 25 mL of 20 mg·L−1 RhB aqueous solution. The suspension was magnetically stirred in the dark for 30 min to establish the adsorption−desorption equilibrium, followed by the addition of 20 mmol·L−1 H2O2 to the mixture solution. After visible light illumination, 2 mL of samples were taken out at predetermined time intervals and separated by centrifugation. The concentration of RhB left in the supernatant solution was determined by using a Shimadzu UV2550 UV−vis spectrophotometer at its maximum absorption wavelength of 554 nm. Electrochemical Measurements. An electrochemical workstation (CHI660E Instruments) connected to a computer was used in our electrochemical experiment. The electrodes were prepared according to Zhang’s method.34 For the preparation of composite electrodes, GR/MIL-53(Fe) composites were dispersed in chitosan solution to form a 10 mg·mL−1 solution and ultrasonicated for 10 min; 0.5 mL of colloidal solution was dropped on the conductive side of the ITO glass and allowed to dry for 24 h at room temperature in the air. The photocurrents were measured by an electrochemical analyzer in a standard three-electrode system with the GR/MIL-53(Fe) composite as the working electrodes, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as a reference electrode. A 500 W Xe arc lamp with a 420 nm cutoff filter was utilized as a light source. Amperometric I−t curves were tested at a 0.5 V bias voltage potential (vs SCE). The Mott−Schottky measurements were carried out with impedance-potential model to evaluate the band positions of the MIL-53(Fe). Electrochemical impedance spectroscopy (EIS) tests were performed under dark condition at open circuit potential over the frequency range between 105 and 10−2 Hz, with an AC voltage magnitude of 5 mV. All of the measurements were performed in 0.5 M Na2SO4 solution at room temperature.

Figure 2. XRD patterns of GO, GR, MIL-53(Fe), and GR/MIL53(Fe).

diffraction peak at 2θ = 10.3° associated with the interplanar spacing of GO sheets, corresponding to the characteristic (001) reflection of GO. After solvothermal treatment in the DMF solution, the characteristic peak of GO completely disappears, and a weak, broad peak centered at 2θ = 25.0° appears instead, indicating the efficient conversion of GO to GR during the solvothermal reduction process. The XRD patterns of MIL53(Fe) are in good agreement with the previously reported MIL-5335,36 as well as the simulated patterns (Figure 2). The well-defined peaks indicate its well-crystalline feature. Similarly, the characteristic peaks of MIL-53(Fe) still dominate the XRD pattern of GR/MIL-53(Fe) composite. However, no apparent diffraction peaks belonging to GR are detected, which could be due to the relatively low content and crystallinity of GR in the composite. This phenomenon is consistent with the previous report for GR-semiconductor composites.37,38 To further identify structural properties of the GR/MIL53(Fe) composites, the infrared and Raman spectroscopy measurements were carried out. Figure 3a shows the FTIR spectra of GO, GR, MIL-53(Fe), and GR/MIL-53(Fe) composites. The characteristic absorption peaks of GO can be observed at 3439 cm−1 (the O−H stretching mode), 1730 cm−1 (the CO stretching mode), 1629 cm−1 (the CC stretching mode), 1389 cm−1 (the C−OH stretching mode), and 1065 cm−1 (the C−O−C stretching mode).39−41 The absorption intensities of these peaks decrease dramatically after the solvothermal reduction process, suggesting that most of oxygen-containing functional groups situated at the edges of the GO have been removed. For MIL-53(Fe), the characteristic absorption peaks at 1542, 1387, and 746 cm−1 can be clearly seen, which are similar to those of reported data in the literatures.42,43 The two intense peaks at 1542 and 1387 cm−1 correspond to asymmetric and symmetric vibrations of carboxyl groups, respectively, confirming the presence of the dicarboxylate linker within frameworks. The peak at 746 cm −1 corresponds to C−H bending vibrations of the benzene in the organic linkers. In the case of GR/MIL-53(Fe) composite, the characteristic absorption peaks of MIL-53(Fe) dominate the spectrum. Compared with the pure MIL-53(Fe), the peak

3. RESULTS AND DISCUSSION The microscopic morphologies of GR, MIL-53(Fe), and GR/ MIL-53(Fe) composites were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 1a shows a typical SEM image of MIL53(Fe), which is composed of large quantities of onedimensional rod-like structure. These microrods with smooth surfaces have widths of 4−10 μm and lengths of several tens of micrometers. The TEM image (Figure 1b) of GR presents typical near-transparent two-dimensional nanosheet with chiffon-like ripples and wrinkles. The formation of GR/MIL53(Fe) composite could be evidenced by the SEM and TEM observations. As shown in Figure 1c, the as-formed rod-shaped structures are enwrapped with flexible nanolayers, and the characteristic crinkles of GR nanosheets can be clearly distinguished. The TEM image shown in Figure 1d further confirms this phenomenon, where the silk-like GR nanolayers closely wrap on the MIL-53(Fe) microrod, leading to intimate contact between MIL-53(Fe) microrod and GR nanosheet. Of note, such long-ranged contact is favorable to form sufficient C


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intensity and location have no obvious change in the spectrum of GR/MIL-53(Fe) composite, indicating the framework structure of MIL-53(Fe) is not affected by GR wrapping. Figure 3b shows the Raman spectra of GO, GR, MIL-53(Fe), and GR/MIL-53(Fe) composites. In the case of GO, two prominent bands at around 1350 and 1590 cm−1 can be observed, corresponding to the local defects or disorder atomic arrangement of sp3-carbon (D-band) and the plane vibration of the sp2-carbon in the two-dimensional lattice (G-band), respectively.44 In comparison with GO (ID/IG = 0.97), it is evident that GR displays the increased intensity ratio of the Dto G-band (ID/IG = 1.09), confirming the efficient reduction of GO during solvothermal process.37 The Raman spectrum of MIL-53(Fe) shows the bands at about 1613, 1450, 1139, 863, and 632 cm−1, corresponding to the characteristic vibrations of metal terephthalate.16,32,45 The GR/MIL-53(Fe) displays the similar Raman spectrum to that of MIL-53(Fe). Additionally, the enhanced absorption at around 1350 cm−1 due to contribution of GR is also reflected in the Raman spectrum of GR/MIL-53(Fe) composites. These results confirm the effective combination between GR and MIL-53(Fe). The elemental composition and electronic structure of GR/ MIL-53(Fe) composite was further analyzed by X-ray photoelectron spectroscopy (XPS). Figure 4 shows the XPS survey spectrum and high-resolution XPS spectra of the GR/MIL53(Fe) composite. XPS survey spectrum (Figure 4a) reveals the GR/MIL-53(Fe) composite mainly consists of Fe, C and O elements. Figure 4b shows the high-resolution XPS spectra of Fe 2p. The peaks at binding energy of 711.8 and 725.8 eV with a satellite signal at 718.4 eV are characteristic of Fe3+ in MIL53(Fe).31 The high-resolution XPS spectrum (Figure 4c) of C

Figure 3. FTIR (a) and Raman (b) spectra of GO, GR, MIL-53(Fe), and GR/MIL-53(Fe).

Figure 4. XPS spectra of GR/MIL-53(Fe) composite: (a) survey, (b) Fe 2p, (c) C 1s, and (d) O 1s. D


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under visible light, attributing to the photolysis of H2O2 to form reactive •OH (H2O2 + visible light → •OH + OH−). The GR/ MIL-53(Fe) composite presents the weak adsorption ability toward RhB (9.3% in 120 min) in the dark and the adsorption process reaches equilibrium within 60 min (Figure S1, Supporting Information). Considering that iron-based catalyst probably activate H2O2 in this catalytic system, the contrast experiment was further carried out in the presence of H2O2 and GR/MIL-53(Fe) composite in the dark, where the degradation efficiency of RhB is found to be about 33.5% (curve c: designated as GR/MIL-53(Fe)-H2O2-dark). Meanwhile, visible light irradiated GR/MIL-53(Fe) composite shows moderate photocatalytic activity without H2O2, and about 41.6% degradation efficiency of RhB is achieved (curve d: designated as GR/MIL-53(Fe)-Vis), which is mainly attributed to the generated reactive charge carriers from the photoexcited GR/ MIL-53(Fe) composite. Intriguingly, after introducing small amount of H2O2 into the photocatalytic system of GR/MIL53(Fe)-Vis, 20 mg·L−1 of RhB can be completely decomposed within 60 min (curve e: designated as GR/MIL-53(Fe)-H2O2Vis), confirming the superior performance of this catalytic system. The whole RhB degradation in GR/MIL-53(Fe)-H2O2Vis system is witnessed by the time-dependent UV−vis absorption spectra. As shown in Figure 6B, during the reaction process, the absorption intensity of RhB at 554 nm decreases gradually with the irradiation time, which indicates the successful degradation of RhB. We further quantitatively analyze the rate constant of above catalytic systems and interestingly find that the rate constant in the catalytic system of GR/MIL-53(Fe)-H2O2-Vis (kGMHV: 7.772 × 10−2 min−1) is much higher than that of the sum of GR/MIL-53(Fe)-Vis (kGMV: 1.498 × 10−2 min−1) and GR/MIL-53(Fe)-H2O2-dark (kGMHD: 0.852 × 10−2 min−1). This phenomenon indicates a favorable synergistic effect between H2O2 and photoexcited GR/MIL-53(Fe) composite. The synergetic index (SI), SI = kGMHV/(kGMV + kGMHD), is further calculated to be 3.31, implying synergistic effect plays an important role in the H2O2mediated photocatalytic process. Also, as a H2O2-mediated photocatalysis process, the photocatalytic activity of GR/MIL53(Fe)-H2O2-Vis system is found to be closely dependent on the solution pH (Figure 6C) and H2O2 concentration (Figure 6D). As widely accepted, ·OH radicals as an important active species dominate the performance of H2O2-containing catalytic system. Apparently, if synergistic effect does determine the catalytic activity, more amount of ·OH radicals should be produced in the GR/MIL-53(Fe)-H2O2-Vis system. To support this hypothesis, we first explore the amounts of ·OH radicals in various reaction system by photoluminescence (PL) method. In general, PL intensity is proportional to the amounts of produced ·OH radicals. Figure 7 shows the PL intensity as a function of reaction time under different conditions. It is clear from Figure 7a that the negligible PL intensity is observed in the GR/MIL-53(Fe)-Vis system, indicating that ·OH radicals are scarcely generated in the GR/MIL-53(Fe)-Vis system. In contrast, the GR/MIL-53(Fe)-H2O2-dark system enables the production of considerable PL intensity (Figure 7b), owing to the intrinsic characteristics of iron-containing composition. This suggests that the GR/MIL-53(Fe) can activate H2O2 to induce •OH radicals in the dark through following process: FeIIIGR/MIL‑53(Fe) + H2O2 → H+ + •HO2 + FeIIGR/MIL‑53(Fe), FeIIGR/MIL‑53(Fe) + H2O2 → OH− + •OH + FeIIIGR/MIL‑53(Fe), which is similar to that of previous reported iron-based

1s can be deconvoluted into three surface components, corresponding to the sp2 carbon of GR and benzoic rings of BDC linkers at binding energy of 284.8 eV, the carboxylate (OCO) groups of BDC linkers at binding energy of 288.8 eV, and a very small amount of unreduced GO at binding energy of 286.0 eV.46 Figure 4d shows the high-resolution XPS spectra of O 1s, which could be fitted by two peaks at binding energies of around 532.0 and 530.8 eV, which are attributed to the oxygen atoms on the carboxylate groups of the BDC linkers and the FeO bonds of MIL-53(Fe), respectively. These results are consistent with the above FTIR and Raman analyses, which further confirm the effective solvothermal synthesis of GR/MIL-53(Fe) composites. The optical properties of the MIL-53(Fe) and GR/MIL53(Fe) composites were probed by UV−vis diffuse reflectance spectra (Figure 5). The pure MIL-53(Fe) displays a wide range

Figure 5. UV−vis diffuses reflectance spectra of MIL-53(Fe) and GR/ MIL-53(Fe) composites with different content of GR; inset shows the corresponding colors of MIL-53(Fe) and GR/MIL-53(Fe) composites.

of light absorption with an absorption edge onset around 470 nm, corresponding to the optical bandgap of 2.64 eV. The band at 220 nm is assigned to the ligand-to-metal charge transfer, implying the bonding of carboxylate oxygen to metal. The bands in the range 300−500 nm are ascribed to the spinallowed d−d transition ([6A1g → 4A1g + 4Eg(G)]) of Fe3+ in MOFs.47,48 It can be seen that the absorption edge of GR/MIL53(Fe) composites is similar to that of pure MIL-53(Fe), but the optical absorption intensity in the range 500−800 nm obviously increases after combination with appropriate amount of GR, attributing to the background absorption of GR in the visible light region.38 Correspondingly, the color change of MIL-53(Fe) is observed; the yellow−orange color of MIL53(Fe) turns to dark orange and even becomes dark when the relatively large amount of GR is introduced (inset in Figure 5), which is in agreement with previous observations in other GRbased composites.49,50 The photocatalytic performance of the GR/MIL-53(Fe) composites was evaluated by degradation of RhB under visible light irradiation. Figure 6A shows the variation of RhB concentration (C/C0) with reaction time over different catalytic system. In the absence of catalysts, no significant degradation of RhB is observed under visible light irradiation within 60 min (curve a), revealing that RhB is quite stable toward incident light. After adding a small amount of H2O2 to the RhB solution, the degradation efficiency of RhB increases to 12.3% (curve b) E


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Figure 6. (A) Variation of RhB concentration (C/C0) as a function of reaction time in different catalytic systems. Experimental conditions: RhB, 20 mg·L−1; GR/MIL-53(Fe), 0.4 g·L−1; H2O2, 20 mM; and initial pH 5. (B) UV−vis absorption spectra of RhB aqueous solution taken at regular time intervals over the GR/MIL-53(Fe)-H2O2-Vis system. (C) Effect of initial pH and (D) H2O2 concentration on the degradation of RhB on GR/MIL53(Fe)-H2O2-Vis system.

catalysts.51,52 In the case of GR/MIL-53(Fe)-H2O2-Vis system (Figure 7c), more •OH radicals are indeed produced, which is solidly evidenced by the significantly increased PL intensity. Furthermore, we carefully compare the variation of fluorescent intensities in different reaction times (Figure 7d). The synergetic index calculated from the fluorescent intensity produced in 60 min is about 1.52, which is perfectly matches the magnitude of synergetic index for RhB degradation rate. To clarify the photocatalytic process of GR/MIL-53(Fe)H2O2-Vis system, we have performed active species trapping experiments to obtain the insightful information on the role of active species in such system. The trapping experiment results are shown in Figure 8. The addition of CCl4 (an electron scavenger), tert-butyl alcohol (TBA, a •OH scavenger) and EDTA (a hole scavenger) could induce the depression effect on the photodegradation of RhB, clearly indicating that photogenerated hole, electron and •OH radicals participate in the photocatalytic reaction together. Moreover, the obvious inhibitory effect of TBA and CCl4 reflects that photogenerated electron and •OH radicals may be the main contributors to the photocatalytic process of GR/MIL-53(Fe)-H2O2-Vis system. To this end, we further carried out electrochemical technique to determine the band structure of photoactive MIL-53(Fe). Figure 9a shows a typical Mott−Schottky plot of MIL-53(Fe) measured at a frequency of 1000 Hz in dark, which reveals the typical n-type characteristics for MIL-53(Fe). The flat band potential of MIL-53(Fe) derived from Mott−Schottky plot is about −0.63 V vs SCE (equivalent to −0.39 V vs NHE). Accordingly, the conduction band potential (ECB) of MIL53(Fe) is determined to be about −0.49 V vs NHE.

Combination of the bandgap value of MIL-53(Fe) obtained by DRS analysis (Figure 5), the valence band potentials (EVB) of MIL-53(Fe) is calculated to be 2.15 V vs NHE. The diagram of band structure of MIL-53(Fe) schematically illustrated in Figure 9b. By carefully comparison of EVB potential (2.15 V vs NHE) and redox potential of •OH/OH− (2.38 V vs NHE), we believe that GR/MIL-53(Fe) is incapable for the generation of •OH radicals by hole oxidation of hydroxyl ion or water molecules, consistent with the result of PL measurement (Figure 7a). On the basis of above results, it is reasonably expected that produced more large amount of •OH radicals in GR/MIL-53(Fe)-H2O2-Vis system would be derived from the reduction of H2O2 by photogenerated electron (H2O2 + e−CB → OH− + •OH),53−55 which should be the origin of the synergistic effect existed in the H2O2-containing catalytic system. We further tested the photocatalytic activity of GR/MIL53(Fe)-H2O2-Vis systems with different GR amounts for the photodegradation of RhB. As shown in Figure 10, the photocatalytic performance of MIL-53(Fe) is indeed affected by the GR incorporation. It can be clearly seen that introducing an appropriate amount of GR could enhance the photocatalytic activity of MIL-53(Fe), and the photocatalytic rate constant follows sequence GR/MIL-53(Fe)-5 > GR/MIL-53(Fe)-7 > MIL-53(Fe) > GR/MIL-53(Fe)-10. Obviously, the GR/MIL53(Fe)-5 exhibits the highest photocatalytic activity, which could completely degrade 20 mg·L−1 of RhB within 60 min. The apparent rate constant of GR/MIL-53(Fe)-5 is 0.078 min−1, which is 3.1 times that of bare MIL-53(Fe). Therefore, rationally controlling GR content in the GR/MIL-53(Fe) F


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Figure 7. •OH-trapping PL spectra of different catalytic systems: (a) GR/MIL-53(Fe)-Vis, (b) GR/MIL-53(Fe)-H2O2-dark, (c) GR/MIL-53(Fe)H2O2-Vis. (d) Comparison of the PL intensities recorded every 10 min in these systems.

dependent photocurrent density is observed. The photocurrent density increases continuously with GR amount reaching to 5 wt % and decreases significantly with further increasing GR amounts due to increased light-blocking and scattering effect of photons through excess graphene in the photosystem.56 The remarkably enhanced photocurrent density over GR/MIL53(Fe)-5 reveals the more efficient separation and prolonged lifetime of photoinduced charge carriers than that of other GR/ MIL-53(Fe) and bare MIL-53(Fe), which is critical to the observable excellent photocatalytic activity. To gain deeply insight into how GR affect the charge transport behaviors of GR/MIL-53(Fe) composites, the electrochemical evaluation of GR/MIL-53(Fe) composites was investigated by the electrochemical impedance spectroscopy (EIS). Figure 12a shows EIS Nyquist plots of bare MIL53(Fe) and GR/MIL-53(Fe) composites in 0.5 M Na2SO4 solution under dark condition. The observed semicircle part at high frequencies for all electrodes corresponds to the charge transfer-limiting process due to the charge transfer resistance at the contact interface between the electrode and electrolyte solution. The charge transfer resistance can be directly characterized by the semicircle diameter. Normally, the smaller radius means the better ability to transfer charge. In comparison with bare MIL-53(Fe), the radius of the Nyquist arc gradually decreases with the increase of GR amounts, reflecting that the GR loading is benefit to the improvement of charge transfer efficiency in MIL-53(Fe) due to its excellent electrical conductivity. To better understand the charge transfer in photochemical process, EIS measurements on GR/MIL-53(Fe) composite were conducted under visible light irradiation. As shown in Figure 12b, the visible light irradiated electrode

Figure 8. Photodegradation of RhB on GR/MIL-53(Fe)-H2O2-Vis in the presence of trapping systems (scavenger amount: 12 mM). Experimental conditions: RhB, 20 mg·L−1; GR/MIL-53(Fe), 0.4 g·L−1; H2O2, 20 mM; and initial pH 5.

composites is essential to achieve the optimal photocatalytic performance. In terms of the photocatalytic process, the efficient separation of photogenerated active carriers is necessary to the subsequent redox reaction on the catalyst surface. To confirm the separation efficiency of photoinduced charge carriers, the transient photocurrent response behaviors of GR/MIL-53(Fe) composites with different GR contents were investigated. Figure 11 shows the chopped I−t curves of GR/ MIL-53(Fe) electrodes under visible light irradiation in 0.5 M Na2SO4 electrolyte solution. Similarly, the GR amount G


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Figure 9. (a) Mott−Schottky plot for the MIL-53(Fe) electrodes, (b) estimated energy level diagram of the MIL-53(Fe).

Figure 10. Photodegradation curves of RhB on GR/MIL-53(Fe) of different GR content under visible light irradiation. Experimental conditions: RhB, 20 mg·L−1; GR/MIL-53(Fe), 0.4 g·L−1; H2O2, 20 mM; and initial pH 5.

Figure 12. (a) EIS Nyquist plots of bare MIL-53(Fe) and GR/MIL53(Fe) composites in 0.5 M Na2SO4 solution under dark condition. (b) EIS Nyquist plots of GR/MIL-53(Fe)-5 composite in dark and under light irradiation.

performance of MIL-53(Fe), which could be ascribed to following reason. GR with excellent conductivity can indeed promote interfacial charge transfer and retard charge recombination. However, excess amounts of GR could cause the increased light-blocking and scattering effect of photons, leading to the lower photogenerated rate of charge carriers. Noticeably, the enhanced conductivity cannot compensate for the lower photogeneration rate of charge carriers, which is why the GR/MIL-53(Fe)-7 and GR/MIL-53(Fe)-10 composites with higher conductivity shows relatively lower photocatalytic activity and photocurrent density. Therefore, the photo-

Figure 11. Transient photocurrent responses of MIL-53(Fe) and GR/ MIL-53(Fe) composites in 0.5 M Na2SO4 aqueous solutions under visible light irradiation.

presents smaller arc radius than that obtained in the dark, indicating the charge transfer could be more favorable under visible light irradiation. On the basis of the above discussion, we think that the optimal GR amounts in the photocatalytic composite system should be critical for the improvement of photochemical H


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Figure 13. Schematic illustration of photocatalytic process of GR/MIL-53(Fe) composite in the presence of H2O2.

Figure 14. (a) Repeated photocatalytic degradation of RhB on GR/MIL-53(Fe)-H2O2-Vis system. (b) The stability of photocurrent responses of GR/MIL-53(Fe) composite in 0.5 M Na2SO4 aqueous solutions over 3500 s long times under visible light irradiation. (c) FTIR spectra and (d) PXRD patterns of GR/MIL-53(Fe) composite before and after reaction.

chemical process of GR/MIL-53(Fe) composites is schematically illustrated in Figure 13. The pure organic linker (terephthalic acid) does not absorb any visible light (Figure 5), so an intramolecular electron transfer from the ligand to Fe(III) (a LMCT mechanism) could be ruled out in our catalytic system. Therefore, the direct excitation of iron-oxo cluster under visible light irradiation could contribute to the photoreactivity of MIL-53(Fe), which is also confirmed by above DRS analysis. Upon visible light irradiation, the reactive electrons and holes are generated in the excited MIL-53(Fe). The photoinduced electrons on the CB of MIL-53(Fe) transfer energy-favorably to conductive GR nanolayers, leading to efficient separation of electron−hole pairs and thus prolonging

the lifetime of charge carriers. In this situation, the photoinduced electrons on GR could be captured and reacted with H2O2 to produce large amounts of •OH radicals. On the other hand, the possible iron species exposure on the surface of GR/ MIL-53(Fe) composites could also react with H2O2 to produce •OH radicals, which is another contributor for the enhanced photocatalytic activity. The recycled experiments were also conducted to further evaluate the photostability of GR/MIL-53(Fe)-H2O2-Vis system. As shown in Figure 14a, the GR/MIL-53(Fe) composite displays favorable cycle performance. After three cycling runs of photodegradation of RhB, the photocatalytic ability of the GR/MIL-53(Fe) composite did not show any I


Article


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loss. Similarly, the photocurrent measurement (Figure 14b) of the GR/MIL-53(Fe) composite reveals that a stable photocurrent over the course of thousands of seconds. In addition, after photocatalytic reaction, there is no measurable alteration in crystal and molecular structures of the GR/MIL-53(Fe) composite, as evidenced by FTIR and XRD measurements (Figure 14c and d). These results demonstrate that the GR/ MIL-53(Fe) composite is stable during photocatalytic process.

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CONCLUSIONS In summary, the highly efficient visible light photocatalysts based on the GR hybridized MIL-53(Fe) microrods have been rationally designed and fabricated by a facile one-pot solvothermal process. The GR/MIL-53(Fe)-H2O2 systems exhibit significantly higher photocatalytic activity toward degrading Rhodamine B (RhB) than that of bare MIL53(Fe)-H2O2 under visible light irradiation. The introduced H2O2 could induce photosynergistic generation of more amounts of hydroxyl radicals to contribute to the improved photocatalytic activity. This work could open a new way for the exploration and utilization of MOF-based crystalline materials for environmental applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: Time profiles of adsorption of RhB over the GR/ MIL-53(Fe) composite in the dark. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +86-817-2568 081. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21103141, 21207108), Sichuan Youth Science and Technology Foundation (2013JQ0012) and the Research Foundation of CWNU (11A036).

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Graphene Hybridized Photoactive Iron Terephthalate with Enhanced

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