One-Step Ball-Milling Preparation of Highly Photocatalytic Active

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One-Step Ball-Milling Preparation of Highly Photocatalytic Active CoFe2O4Reduced Graphene Oxide Heterojunctions For Organic Dye Removal Guangyu He, Jiajia Ding, Jianguo Zhang, Qingli Hao, and Haiqun Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504706w • Publication Date (Web): 04 Mar 2015 Downloaded from http://pubs.acs.org on March 10, 2015

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Industrial & Engineering Chemistry Research

One-Step Ball-milling Preparation of Highly Photocatalytic Active CoFe2O4-Reduced Graphene Oxide Heterojunctions For Organic Dye Removal Guangyu Hea, Jiajia Dinga, Jianguo Zhanga, Qingli Haob, and Haiqun Chena,*

a

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University,

Jiangsu Province, Changzhou 213164, China b

Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and

Technology, Ministry of Education, Nanjing 210094, China

AUTHOR INFORMATION

Corresponding Author: *E-mail: [email protected] (Haiqun Chen), Phone: +86 519 86330088, Fax: +86 519 86330086

1

ACS Paragon Plus Environment


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ABSTRACT: We prepared CoFe2O4-RGO composites by ball-milling without using toxic chemical reagents and high-temperature heat treatment. The exfoliation and reduction of graphite oxide and anchoring of CoFe2O4 nanoparticles on graphene sheets were accomplished in one step. The microstructure of the hetero-photocatalyst was characterized by XRD, FTIR, Raman and TEM methods. The photocatalyst exhibited desirable photocatalytic performance with excellent recycling stability for the degradation of methylene blue, rhodamine B and methyl orange under visible-light irradiation. In addition, the CoFe2O4-RGO photocatalyst can be easily separated by external magnetic field. The simple and efficient one-step ball milling strategy for preparing the photocatalysts is more applicable to industrial production.

1. INTRODUCTION Graphene, a two dimensional form of carbon, has attracted numerous investigations into its unique physical, chemical, and mechanical properties, opening up a new research area for materials science and condensed-matter physics, and aiming for a wide-ranging and diversified technological applications1. Because of those exceptional properties, graphene has been used as a support for catalysts. Many metal oxide compounds, such as ZnO, WO3, Fe2O3 and Fe3O4 have been hybridized with graphene and its derivatives2-5. When the spinel-type nanoparticles loaded on graphene sheets, the composites showed significantly enhanced photocatalytic performance under UV irradiation because the introduction of graphene effectively prevented the direct electron-hole recombination of those metal oxides. Recently, interest has been dedicated to the photocatalytic performance of graphene based-metallates, one type of visible-light-driven photocatalysts. Gao and his co-workers6 prepared a Bi2WO6-Graphene photocatalyst by in-situ hydrothermal reaction. It showed enhanced 2


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photocatalytic activity for the degradation of rhodamine B (RhB) under visible light due to the electronic interaction and charge equilibration between graphene and Bi2WO6, which led to the negative shift in the Fermi level and decreased the conduction band potential. In our previous studies, a series of graphene-based nanocomposite photocatalysts was prepared via hydrothermal method,

such

as

CoFe2O4-Graphene,

MnFe2O4-Graphene,

NiFe2O4-Graphene

and

ZnFe2O4-Graphene7-10. These photocatalysts can not only highly activate the degradation of cationic and anionic dyes under visible-light irradiation, but also be separated conveniently by an external magnetic field. However, the widely used preparation methods which involve exfoliation of graphite oxide through sonication or other micro-mechanical ways, such as hydrothermal and solvothermal methods, limited the large scale application of graphene due to the low yields. As is known, mechanical milling method has a wide range of applications in the field of materials. Good results have been reported in the exfoliation of graphene11 and the preparation of metallate12. Therefore, to scale up the yields for practical application of graphene, mechanical ball-milling was applied

in our current study. We prepared magnetically separable

CoFe2O4-reduced graphene oxide (CoFe2O4-RGO) photocatalysts by ball-milling in one step without using toxic chemical reagents and high-temperature heat treatment. The CoFe2O4-RGO composite exhibits good visible-light-induced photocatalytic activity, providing new insights in large scale preparation of graphene-based semiconductor.

2. EXPERIMENTAL SECTION 2.1. Materials. Natural graphite powder (99.9%, 500 mesh), Co(NO3)2·6H2O, Fe(NO3)3·9H2O, NH3·H2O, and other materials were of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were used as received. 2.2. Synthesis of Magnetic CoFe2O4-RGO Composite Photocatalyst. Graphite oxide was 3



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synthesized from purified natural graphite according to the method reported by Hummers and Offeman13. CoFe2O4-RGO nanocomposite photocatalysts with graphene content of 25, 30, 35, 40, 45, 50 and 55 wt%, respectively, were prepared by ball-milling and designated as CoFe2O4-RGO0.25, CoFe2O4-RGO0.30, CoFe2O4-RGO0.35, CoFe2O4-RGO0.40, CoFe2O4-RGO0.45, CoFe2O4-RGO0.50, CoFe2O4-RGO0.55, respectively. Typical synthetic procedure of CoFe2O4-RGO with 45% graphene content is as follows: 0.0743 g Co(NO3)2·6H2O and 0.2063 g Fe(NO3)3·9H2O were added to 2.0 g of graphite oxide colloid (2.5 wt%). The reaction mixture was adjusted to pH 10 by NH3·H2O then milled using Oscillating Mill MM400 (Germany, Retsch) at 25 s-1 for 6 h. The resulting mixture was washed with distilled water five times, and freeze-dried. 2.3. Characterization. X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance Diffractometer with Cu-Kα radiation (λ = 1.54 Ǻ). Raman spectra of different samples were acquired using a Renishaw inVia Reflex Raman microprobe. Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet 370FT-IR spectrometer using pressed KBr pellets. Transmission electron microscopy (TEM) images were taken with a JEOL JEM-2100 microscope. Scanning electron microscope (SEM) images were taken using a field-emission SEM operated at an accelerating voltage of 5 kV. The BET surface area of as-synthesized samples was measured using an ASAP2010C Surface aperture adsorption instrument (Micromeritics Instrument Corporation, USA) by N2 physisorption at 77 K. The UV-vis diffuse reflectance spectra of the samples were recorded by a Shimadzu UV-2700 UV-vis spectrophotometer. BaSO4 was used as a reflectance standard. Photo Luminescence (PL) measurements were performed using a Jobin Yvon SPEX Fluorolog-3-P spectroscope and a 450W Xe lamp was used as the excitation source. 2.4. Photocatalytic Activity Measurement. The photocatalytic performance of the samples was evaluated by the degradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) 4


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at 25 °C. 0.01 g of photocatalyst was suspended in 40 mL of MB, RhB or MO solution (20 mg·L-1). The solution was magnetically stirred for 2 h in the dark to ensure the establishment of an adsorption-desorption equilibrium. The light source was an 800 W Xe lamp equipped with a UV cutoff filter (λ > 420 nm). At given time intervals of irradiation, 3 mL aliquots were withdrawn, and then magnetically separated to remove essentially all the catalyst. The large absorbance at 664 nm for MB, 554 nm for RhB and 463 nm for MO was used respectively to evaluate the variation of organic pollution concentration by a Shimadzu UV-2700 UV-vis spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Characterization of CoFe2O4-RGO Photocatalyst. The XRD diffraction patterns of the CoFe2O4-RGO photocatalyst, CoFe2O4 and graphite oxide were shown in Figure 1 (A). Almost all the diffraction peaks of CoFe2O4-RGO can be assigned to spinel-type CoFe2O4 (JCPDS 22-1086)14. The average diameter of the CoFe2O4 nanoparticles was evaluated by Scherrer equation, which is about 12.08 nm. No obvious diffraction peaks of graphite oxide were observed, suggesting that the regularly stacked graphite oxide were exfoliated after the ball-milling. In the meantime, neither did the diffraction peak of graphene appear in the XRD pattern of CoFe2O4-RGO, which is because the anchoring of CoFe2O4 nanoparticles on the graphene sheets prevented the graphene sheets from restacking orderly15. The Raman spectra of the CoFe2O4-RGO and the pure CoFe2O4 in Figure 1 (B) share similar features in the frequency range between 100 and 1000 cm-1, showing the existence of CoFe2O4 in CoFe2O4-RGO photocatalyst. Compared with those of graphite oxide, the D- and G-bands in the Raman spectra of the CoFe2O4-RGO shifted to lower frequencies. The D-band shifted from 1353 to 1347 cm-1 while the G-band shifted from 1594 to 1587 cm-1, indicating that graphite oxide has been reduced to RGO16-18. In addition, a 2D-band at 2696 cm-1 appeared in the Raman spectrum of the 5



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CoFe2O4-RGO, which indicated further the reduction of graphite oxide8. Besides, an increase in D/G intensity ratio (ID/IG = 1.21) of CoFe2O4-RGO indicated increased defects in graphene sheets after ball-milling, which is probably because smaller but more numerous graphitic sp2 domains were created compared with the ones previously present in graphite oxide19-21. The FTIR spectra of graphite oxide and CoFe2O4-RGO composite are shown in Figure 1 (C). Most characteristic peaks of the oxygen-containing functional groups in graphite oxide disappeared in the spectrum of CoFe2O4-RGO. The band at around 1622 cm-1 assigned to C=C skeletal vibration of unoxidized graphitic domains within graphite oxide red shifted to 1567 cm-1, indicating the restoration of π–π conjugation of graphene sheets. The results suggested that graphite oxide has been reduced to RGO in the composite after ball-milling, which is consistent with the results of Raman spectra. Moreover, a new prominent absorption band appeared at about 588 cm-1 in the FTIR spectrum of the CoFe2O4-RGO composite compared with that of graphite oxide, which corresponds to the stretching mode of Fe(Co)-O22. UV-vis absorbance spectra shown in Figure 1 (D) also proved the reduction of graphene oxide (GO) in CoFe2O4-RGO. The characteristic absorption peak of GO at about 228 nm red shifted to 267 nm in CoFe2O4-RGO, which is very close to the characteristic peak of RGO at 272 nm.

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Figure 1. (A) XRD patterns CoFe2O4-RGO, CoFe2O4 and graphite oxide; (B) Raman spectra of CoFe2O4-RGO, CoFe2O4 and graphite oxide; (C) FTIR spectra of graphene oxide and CoFe2O4-RGO; (D) UV-vis absorbance spectra of GO, RGO and CoFe2O4-RGO

The TEM image of the CoFe2O4-RGO composite in Figure 2 (A) showed that CoFe2O4 nanoparticles were decorated on the RGO sheets. The crystal lattice fringes with d-spacing of 0.25 nm shown in the inset of Figure 2 (A) can be assigned to the (311) plane of CoFe2O4. The SEM image in Figure 2 (B) also showed that CoFe2O4 nanoparticles with sizes ranging from 10 nm to 15 nm were evenly fixed on the stacked and wrinkled graphene sheets, which is consistent with the results of TEM and XRD. N2 adsorption-desorption isotherms were applied to investigate the porous structure and surface area of the CoFe2O4-RGO composite. As shown in Figure 2 (C), the N2 isotherm of the CoFe2O4-RGO composite is close to Type IV, revealing the existence of mesopores. The measurements indicated that the sample has a Brunauer-Emmett-Teller (BET, nitrogen, 77 K) surface area of 153.38 m2·g-1 with a pore volume of 0.096 m2·g-1 and a BJH desorption average pore diameter of 4.75 nm. 7



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UV-vis diffusive reflectance spectra are given in Figure 2 (D), which show that pure CoFe2O4 catalyst had very weak absorption in the visible region and only had a strong absorption at the wavelength around 300 nm in ultraviolet light region. On the other hand, the composite material CoFe2O4-RGO photocatalyst showed a strong absorption in the 200-800 nm range, implying the introduction of graphene can broaden the visible light absorption by the composite catalyst.

Figure 2. (A) TEM image of CoFe2O4-RGO; (B) SEM image of CoFe2O4-RGO; (C) Nitrogen adsorption/desorption isotherm of CoFe2O4-RGO; (D) UV-vis diffusive reflectance spectra of CoFe2O4 and CoFe2O4-RGO

3.2. Optimization of Catalyst Preparation Conditions. The photodegradation of MB over pure CoFe2O4 and the as-obtained CoFe2O4-RGO nanocomposite photocatalysts with different graphene content under visible-light irradiation at 25 °C are shown in Figure 3 (A). Greatly enhanced adsorption and photocatalytic activity were both observed when CoFe2O4 was loaded on graphene sheets. This could be mainly attributed to two aspects. One is the large π−conjugational plane of RGO that facilitates a π-π stacking with a face-to-face orientation between RGO and MB molecules. The other is the synergetic interactions between CoFe2O4 and RGO. The existence of 8


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RGO prevented the aggregation of CoFe2O4 nanoparticles, which afforded more active sites for the photocatalysis. Meanwhile, the superior electronic transmission performance of RGO facilitated efficient separation of photogenerated electrons and holes on the catalyst, improving the photocatalytic activity of CoFe2O4-RGO. The adsorption ability increased continuously with the increase of the graphene content in the composite catalyst, while the photocatalytic performance reached its peak when the content of graphene was increased to 45%. Overdose of graphene decreased the content of CoFe2O4 as well as the active sites, which reduced the catalytic activity of CoFe2O4-RGO to some extent. The visible-light photocatalytic activity of pure RGO was also evaluated for comparison. As shown in Figure S1, the pure RGO exhibited negligible photocatalytic activity toward the degradation of MB under visible light irradiation. To further understand the photocatalytic activity, a pseudo-first-order kinetic equation In(Ct /C0 ) = -kt was used to fit the MB degradation reactions, where C0 and Ct are the concentrations of MB when the visible light irradiation time is 0 and t, respectively, and k is the visible-light photodegradation rate constant which was determined using the adsorption-desorption equilibrium solution of MB as starting solution (Figure S2). The rate constant (k) of the photodegradation of MB over catalysts with different graphene content was shown in Figure 3 (B). And CoFe2O4-RGO0.45 gave the fastest photodegradation rate of 0.0144 min-1 at 25 °C (Figure S3), which is comparable with that of the catalyst prepared by hydrothermal method8. The photodegradation of MB under visible-light irradiation at 25 °C on the as-obtained CoFe2O4-RGO0.45 photocatalysts prepared for different ball-milling time are shown in Figure 3 (C). Though the milling time has negligible influence on the adsorption of the photocatalyst, it did affect its photocatalytic activity. CoFe2O4-RGO0.45 prepared by ball-milling for 6 h (designated as CoFe2O4-RGO0.45-6h) exhibited the best photocatalytic performance. Usually, with the increase of 9



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ball milling time, the particle size of the photocatalyst decreased while the specific surface area of the photocatalyst increased. Correspondingly, the number of active sites per unit weight of photocatalyst also increased. As calculated from Figure S4, the grain size of CoFe2O4-RGO0.45 prepared by ball-milling for 3, 6, 9 and 12 h was 14.55, 12.08, 14.57 and 20.05 nm, respectively. But when the ball milling time is longer than the optimum time, the fresh surface formed by high-energy ball milling possess high surface energy and preferred to agglomerate23. Figure 3 (D) shows

the

photodegradation

of

MB

under

visible-light

irradiation

at

25

°C

over

CoFe2O4-RGO0.45-6h photocatalysts at different ball-milling frequency. CoFe2O4-RGO0.45-6h photocatalyst prepared at the ball-milling frequency of 25 s-1 (designated as CoFe2O4-RGO0.45-6h-25) exhibited the best photocatalytic performance. The absorption capacity of catalyst decreased when higher frequency of ball milling was applied. It might be because the graphite oxide was exfoliated and reduced more thoroughly, which decreased the number of MB intercalated in the graphene nanosheets and weakened the electrostatic attraction between GO and MB24. On the other hand, the photocatalytic performance increased when higher frequency of ball milling was applied since the particle size of CoFe2O4 became smaller, which increased the number of active sites and provided better performance of photocatalysis25,26.

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Figure 3. (A) Effect of different catalysts on photocatalytic degradation of MB: (a) pure CoFe2O4, (b) CoFe2O4-RGO0.25, (c) CoFe2O4-RGO0.3, (d) CoFe2O4-RGO0.35, (e) CoFe2O4-RGO0.4, (f) CoFe2O4-RGO0.45, (g) CoFe2O4-RGO0.5 and (h) CoFe2O4-RGO0.55; (B) The rate constant for the photodegradation of MB over different catalysts; (C) Effect of catalyst prepared for different ball-milling time on photocatalytic degradation of MB; (D) Effect of catalyst prepared at different ball-miling frequency on photocatalytic degradation of MB

3.3. Photodegradation of Other Dyes Over CoFe2O4-RGO and the Stability of the Photocatalyst. Photodegradation of MO and RhB under the same experimental conditions as the degradation of MB over CoFe2O4-RGO0.45-6h-25 reached 37.5 and 72.2% (Figure 4 (A)), respectively, after irradiation for 180 min. The photocatalyst showed better photodegradation effect on cationic dyes than anionic dyes. It is because the negatively charged oxygen-containing groups have electrostatic repulsion to anionic dyes, leading to a worse adsorption capacity for anionic dyes, whereas the adsorption is the prerequisite in photocatalytic degradation process8,27. To investigate the stability of the photocatalyst during the photocatalytic reaction, the cycling experiments were performed on the photocatalyst prepared at optimum conditions. As was shown in 11



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Figure 4 (B), the photocatalytic degradation of MB over CoFe2O4-RGO0.45-6h-25 remained at 93.1% after the third cycling run, indicating great stability of CoFe2O4-RGO0.45-6h-25 during the photocatalytic reaction. The characterization of CoFe2O4-RGO0.45-6h-25 after recycled for three times also showed that the catalyst is relatively stable during the reaction (Figure S5).

Figure 4. (A) The photodegradation of MB, RhB and MO over CoFe2O4-RGO0.45-6h-25 under visible-light irradiation; (B) Cycling experiments of visible-light photocatalytic degradation of MB

3.4. Mechanism of Photocatalytic Degradation. Dyes and organic pollutants can be photodegraded via photocatalytic reactions. Active oxygen species have been implicated in the photocatalytic degradation of dyes, such as superoxide radical anion (O2−), photogenerated holes and hydroxide radical (·OH)8,28. To understand the photocatalytic mechanism, the main active oxidant in the photocatalytic reaction process was identified by trapping them with disodium ethylenediamine tetraacetate (EDTA-2Na) and tert-butyl alcohol (t-BuOH) under visible-light irradiation at 25 °C29,30. As shown in Figure 5, the addition of t-BuOH to scavenge ·OH greatly reduced the degradation of MB over CoFe2O4-RGO0.45-6h-25, whereas the addition of EDTA-2Na as hole-scavenger also worked with less effect. Therefore, ·OH played as the main oxidant in the CoFe2O4-RGO catalyzed photodegradation.

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Figure 5. Photocatalytic degradation of MB with different radical scavengers in the presence of CoFe2O4-RGO

The possible mechanism of CoFe2O4-RGO catalyzed phtodegradation of MB was speculated as follows. Under visible light irradiation, photogenerated electrons and holes were yielded from charge separation within CoFe2O4-RGO. The introduction of graphene efficiently suppressed the recombination of electron-hole pairs in the valence band (VB) and conduction band (CB) via a percolation mechanism due to the superior electronic conductivity of graphene. The CB electrons at the catalyst surface were trapped by the ubiquitously present O2 to yield O2− first, which was then protonated to yield the HOO· radical. The VB holes were mostly scavenged as ·OH radicals by oxidizing either OH− anions and/or H2O molecules at the catalyst surface. The ·OH radical can also be formed from the trapped electron after formation of the HOO· radical31. The active species (holes, O2−, HOO· or ·OH radicals) oxidized the MB molecules adsorbed on the active sites of the CoFe2O4-RGO system via π–π stacking and/or electrostatic attraction. The Photo Luminescence (PL) spectra of CoFe2O4 and CoFe2O4-RGO (Figure S6) showed that the emission intensity of the hybrid catalyst CoFe2O4-RGO is much lower than that of CoFe2O4, which suggests that the introduction of graphene in the composite as a photoexcited electron-trapping site prevented the fast recombination of photoinduced charge carriers and prolonged their lifetime, hence improved the photocatalytic activity. The degradation could be ascribed to the excitation of CoFe2O4-RGO under visible light irradiation. However, there is also 13



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possibility of the concomitant presence of photosensitized degradation of dyes over CoFe2O4-RGO31, which is worth to being further explored in our future work.

4 CONCLUSION In summary, we reported a facile fabrication method for the synthesis of magnetically separable

CoFe2O4-RGO

nanocomposite

photocatalysts

by

ball-milling

in

one

step.

Characterization results indicated that graphite oxide was reduced, exfoliated and decorated with CoFe2O4 nanocrystals in one step. The photocatalytic degradation experiments showed that CoFe2O4-RGO was a highly active catalyst for the degradation of MB under visible-light irradiation. The method we applied here, using ball-milling to prepare graphene-based photocatalyst, avoided the exfoliation of graphite oxide by inefficient ultrasonication, the reduction of GO with toxic chemical reducing agent, and the high-temperature heat treatment, which has great potential for the industrial application in the field of catalyst fabrication.

ACKNOWLEDGEMENTS The financial supports from the National Natural Science Foundation of China (No. 51202020, 51472035), the Science and Technology Department of Jiangsu Province (BY2012099, BY2013024-04, BE2014089), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), and the PAPD of Jiangsu Higher Education Institutions are gratefully acknowledged.

ASSOCIATED CONTENT Supporting Information Experimental details and analytic data (XRD patterns of photocatalysts prepared by different ball-milling time; TEM and SEM images, XRD and FTIR spectra of the recycled photocatalyst; PL spectra of CoFe2O4 and CoFe2O4-RGO photocatalyst). This material is available free of charge via 14


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the Internet at http://pubs.acs.org.

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One-Step Ball-Milling Preparation of Highly Photocatalytic Active

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