Scalable Green Method to Fabricate Magnetically Separable NiFe2O4

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Materials and Interfaces

Scalable green method to fabricate magnetically separable NiFe2O4-Reduced Graphene Oxide nanocomposites with enhanced photocatalytic performance driven by visible light Jianxing Liang, Ying Wei, Jianguo Zhang, Yan Yao, Guangyu He, Bo Tang, and Haiqun Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00218 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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Industrial & Engineering Chemistry Research

Scalable green method to fabricate magnetically separable NiFe2O4-Reduced Graphene Oxide nanocomposites with enhanced photocatalytic performance driven by visible light Jianxing Liang, Ying Wei, Jianguo Zhang, Yan Yao, Guangyu He,* Bo Tang and Haiqun Chen* Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Jiangsu, 213164, China.

ABSTRACT: A reduced graphene oxide (RGO) supported nickel ferrite (NiFe2O4) photocatalyst was prepared by a simple mechanical ball-milling method. No additional solvents, toxic chemical reductants, ultrasonic or high-temperature heat treatment were needed. The exfoliation and reduction of graphite oxide (GO) and the in situ anchoring of NiFe2O4 nanoparticles on graphene sheets were fulfilled simultaneously under the strong shear force. The structure characterization shows that the NiFe2O4 nanoparticles were uniformly dispersed on RGO sheets. Amazingly, after coupling with an appropriate amount of RGO, the photocatalytically inert NiFe2O4 exhibited superior photodegradation performance and recycling stability for the degradation of organic pollutant under visible-light irradiation at room temperature. It suggested that the synergistic effect between RGO and NiFe2O4 improved the photocatalytic performance of the composite. Moreover, the NiFe2O4-RGO is magnetically separable for recycling. Hopefully, this work could shed light on the 1

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environment-friendly large-scale production of graphene-based composites through the efficient ball-milling method.

Keywords: Ball-milling method, NiFe2O4-reduced graphene oxide, Photocatalytic performance, Visible light

1. Introduction Water pollution, a major environmental pollution, has become a significant issue affecting our everyday life.1-3 Over the last few decades, people used to use traditional biological and physical methods to remove chemical pollutants in water, but those methods

are

prone

to

cause

the

secondary

pollution

and

are

highly

energy-consuming.4-6 Lately, photocatalytic degradation technology has attracted widespread attention of researchers, which has great potential in solving environmental problems since it is low energy-consuming, environmental friendly and has excellent catalytic efficiency.7-10 As was known, whether a photocatalytic technology can be industrialized depends to a great extent on the degradation performance as well as recycling stability of the photocatalyst. Recently, spinel-type ferrite has caught many researchers’ eyes since it has good chemical stability.11, 12 More importantly, it is magnetically separable and can be readily recycled. However, it usually possesses a narrow band gap, which lead to the high recombination rate of the photogenerated electron-hole pairs, suppressing its photocatalytic activity.13 Although ferrite alone is a photocatalytically inert material, it is possible to promote photogenerated charge separation by coupling 2


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with a suitable carrier, resulting in excellent photocatalytic performance. Graphene, a two-dimensional one-atomic-thick layered carbon nanomaterial, has been used as an ideal support for nanoparticles due to its large specific surface area (2630 m2·g-1) and high electron mobility (2×105 cm2 V-1·s-1).14-17 Zhang et al. fabricated

reduced

graphene

oxide

(RGO)

based

NiFe2O4

photocatalyst

(NiFe2O4-RGO) using one-step combustion method, which presented excellent photocatalytic activity.18 In our previous work, we studied in detail the preparation of RGO.19, 20 Based on its excellent chemical, thermal stability and good conductivity, we successfully synthesized a variety of graphene-based functional composites.21-27 Among them, a series of graphene-based magnetic spinel photocatalyst prepared by solvothermal method showed promoted photocatalytic performance, which proved that the combination of spinel with graphene effectively suppressed the recombination of electron-hole pairs.28-31 Nevertheless, NiFe2O4-RGO photocatalysts synthesized by combustion and solvothermal methods are difficult to be industrialized, due to complex preparation process, harsh production conditions, such as ultrasonic or extreme heat treatment, and more importantly, the low yields. Hence, an easy and green method to prepare in large scale the photocatalyst is needed for its industrialization. Ball-milling, because of its high efficiency, simple process and low cost, is widely used in industrialized production of materials. It has also been applied to the exfoliation of graphite and the preparation of metal oxide composite.32-34 Inspired by 3



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that, we had fabricated a CoFe2O4-RGO photocatalyst by the ball-milling, which exhibited good photocatalytic performance.35 In order to investigate the applicability and universality of ball-milling for the preparation of graphene-based ferrite photocatalysts, nickel ferrite (NiFe2O4), another one of the most significant magnetic materials, became a candidate. It also has high Curie temperature, high electrical resistivity

and

environmental

stability.36-38

Herein,

we

applied

the

environment-friendly and high-yield ball-milling method to prepare NiFe2O4-RGO and expected an enhanced photocatalytic degradation performance. Neither additional solvents nor toxic chemical reductants were added. Structural characterization suggested that the exfoliation and reduction of graphite oxide (GO), as well as the anchoring of NiFe2O4 nanoparticles on the graphene sheets, were realized simultaneously.

The

obtained

NiFe2O4-RGO

photocatalyst

exhibited

better

performance in the photodegradation of organic pollutants than CoFe2O4-RGO we prepared by ball-milling previously. Good recycling stability was also observed for the magnetically separable NiFe2O4-RGO photocatalyst. This work presents further the potential of ball milling in large-scale production of the graphene-based composites.

2. Experimental section 2.1 Experimental materials Natural graphite powder (99.85%, 500 mesh), nickel nitrate (Ni(NO3)2·6H2O), ferric nitrate (Fe(NO3)3·9H2O), ammonia (NH3·H2O) and other chemical reagents 4


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were purchased from Sinopharm Chemical Reagent Co., Ltd. GO was prepared by Hummers method.39 All reagents were of analytical grade and used as received without further treatment.

2.2 Preparation of NiFe2O4-RGO photocatalyst The ball-milling preparation process of NiFe2O4-RGO photocatalyst was illustrated in Scheme 1. The typical preparation process was as follows: 0.07 g of Ni(NO3)2·6H2O, 0.21 g of Fe(NO3)3·9H2O and 1.26 g of GO colloid (2.5 wt %) were added to the milling tank. The pH of the mixture was adjusted to 10 with aqueous ammonia then milled using Oscillating Mill MM400（Germany, Retsch）at 25 s-1 for 6 h. After that, the mixture was washed with deionized water then dried in a vacuum oven at 60 °C for 12 h (denoted as NiFe2O4-RGO0.35-25-6). Composites with different RGO content (denoted as NiFe2O4-RGOX-Y-Z) were also prepared by the same procedure for comparison, where X was the RGO content (X=0, 0.15, 0.20, 0.35, 0.30, 0.35, 0.40, 0.45, respectively), Y was the ball milling frequency (Y=15, 20, 25 s-1, respectively), and Z was the ball milling time (Z=3, 6, 9 h, respectively).

Scheme 1 Illustration of the preparation process of NiFe2O4-RGO composite photocatalyst. 5



2.3 Characterization of NiFe2O4-RGO photocatalyst The composition of the photocatalysts were characterized by Bruker D8 Advance X-ray powder diffractometer (Germany Bruker), with Cu Kα radiation (λ= 0.154178 nm) and scanning angle ranged from 5 to 80° of 2θ at a scan rate of 0.02 °/s. The functional groups of the as-prepared samples were analyzed with a Nicolet 370 Fourier-transform infrared spectroscopy (FTIR, Thermo Electron Corporation) using pressed KBr pellets. Raman spectra of different samples were recorded on a Renishaw in Via Reflex Raman microprobe. The Brunauer-Emmet-Teller (BET) specific surface area of the samples were acquired using ASAP2010C surface aperture adsorption instrument (Micromeritics Instrument Company, USA) by nitrogen physisorption at 77 K. Transmission electron microscopy (TEM) and Field emission scanning electron microscopy (FE-SEM) images, obtained by JEOLJEM-2100 microscope (JEOL, Japan) and S-900 microscope (Japan), were used to investigate particle size and morphology of the samples. The UV-vis diffuse reflectance spectra (UV-vis DRS) of the samples were detected by Shimadzu UV-2700 UV-vis spectrophotometer and BaSO4 powder was acted as a reflectance standard. The X-ray photoelectron spectra (XPS) were carried out on an RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) endowed with Mg Kα source (hv = 1253.8 eV). 2.4 Photocatalytic activity measurement The photocatalytic property of the obtained photocatalysts were investigated by the degradation of methylene blue (MB), methyl orange (MO) and rhodamine B (RhB) 6


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under the visible-light irradiation at 25 °C The photocatalytic reaction was carried out using 800 W Xe lamp with a UV cutoff filter (λ > 420 nm) in XPA series photochemical reaction instrument. 10 mg of the photocatalyst was dispersed respectively into 40 mL of MB, MO or RhB solution (20 mg·L-1) and magnetically stirred for 2 h in dark to ensure the adsorption-desorption equilibrium between the organic dye and photocatalyst. Subsequently, 3 mL of the suspension was taken out at regular intervals, and centrifuged. The concentration variation of the supernatant was monitored by an UV-vis spectrophotometer at 664 nm for MB, 554 nm for RhB, and 464 nm for MO, respectively. Besides, the removed photocatalyst and the supernatant were returned to the reaction tank after the test to reduce the experimental error.

3. Results And Discussion 3.1 Characterization of NiFe2O4-RGO photocatalyst

Figure 1 UV-vis diffuse reflectance spectra of NiFe2O4, NiFe2O4-RGO0.2-25-9 and NiFe2O4-RGO0.35-25-9.

UV-vis diffuse reflectance could directly reflect the absorption characteristics of semiconductor photocatalysts in different regions. The UV-vis diffuse reflectance spectra of NiFe2O4, NiFe2O4-RGO0.2-25-9 and NiFe2O4-RGO0.35-25-9 photocatalysts are 7



shown in Figure 1. NiFe2O4 showed a strong absorption edge in the ultraviolet region while NiFe2O4-RGO had a relatively intense absorption in the visible region, which means the introduction of graphene enhanced the light absorption especially in the visible light region 40, 41. In addition, when the RGO content was increased from 0.2 to 0.35, the visible-light absorption of the composite was enhanced. This could be attributed to the presence of RGO, reducing reflection of visible light.42, 43 Therefore, better utilization of visible light by the composite catalyst was expected, which should improve its photocatalytic degradation activity under visible light irradiation.2, 44

Figure 2 TEM images of (A) NiFe2O4-RGO0.35-25-9 and (B) NiFe2O4-RGO0.35-25-3.

The microstructure of NiFe2O4-RGO photocatalyst was investigated by TEM. As were presented in Figure 2A, after being ball milled for 9 h, NiFe2O4 nanoparticles with an average size of about 11 nm were uniformly dispersed on the transparent silk veil-like RGO sheets. It showed that GO was exfoliated efficiently by ball milling, meanwhile, the NiFe2O4 nanoparticles formed in situ. The dispersion of NiFe2O4 nanoparticles on RGO sheets prevented both the restacking of RGO and the aggregation of NiFe2O4 nanoparticles. Whereas, the composite prepared by ball 8


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milling for 3 h (Figure 2B) had a sparse loading of seriously agglomerated NiFe2O4 particles on RGO sheet. It seemed that the composite prepared by a longer time of ball milling would exhibit a better photocatalytic degradation performance due to smaller size and better dispersion of NiFe2O4 nanoparticles. The SEM image of the composites shown in Figure S1 indicated that NiFe2O4 nanoparticles were dispersed on the RGO sheets, which was in accord with the results of TEM.

Figure 3 (A) XRD pattern of GO, NiFe2O4 and NiFe2O4-RGO0.35-25-9; (B) Raman spectra of GO, NiFe2O4-RGO0.35-25-3, NiFe2O4-RGO0.35-25-9 and NiFe2O4; (C) FTIR spectra of NiFe2O4-RGO0.35-25-9 and GO; (D) Nitrogen adsorption-desorption isotherm of NiFe2O4-RGO0.35-25-9 (The inset is the pore size distribution plots of NiFe2O4-RGO0.35-25-9).

The XRD patterns of RGO, GO, NiFe2O4 (prepared by ball milling for 9 h) and NiFe2O4-RGO0.35-25-9 are presented in Figure 3A. The characteristic peaks at 2θ=18.3°, 9



30.1°, 35.3°, 43.0°, 53.5°, 56.3°, 62.4° and 74.6° were associated respectively with (111), (220), (311), (400), (422), (511), (440) and (533) planes of the spinel-type NiFe2O4 (JCPDS No.54-0964), revealing that the NiFe2O4 nanoparticles was prepared by ball milling and the structure of NiFe2O4 remained after the composition with RGO.45 The average particle size of NiFe2O4 nanoparticles loaded on RGO was calculated by the Scherer equation, which was about 11 nm and consistent with the TEM result. Whereas, no characteristic peak of GO at 2θ=11.2° was observed in the XRD pattern of NiFe2O4-RGO0.35-25-9. It proved that the regular layered structure of GO disappeared during the ball-milling exfoliation of GO and the growth of NiFe2O4 on RGO sheet.28, 35, 46 XPS spectra of NiFe2O4-RGO0.35-25-9 (Figure S2) confirmed the presence of elements.31, 55 The peaks at 855.0 and 872.9 eV corresponding to Ni 2p3/2 and Ni 2p

1/2

indicated the existence of Ni2+.29, 57 The peaks of Fe 2p1/2 and Fe 2p3/2

appeared at 724.9 and 711.1 eV corresponds to Fe3+.28 The peak intensities of oxygen-containing functional groups in NiFe2O4-RGO0.35-25-9 was much lower than those in GO, indicating the reduction of GO via ball-milling.29 As shown in Figure 3B, the Raman spectra of NiFe2O4-RGO0.35-25-3, NiFe2O4-RGO0.35-25-9 and NiFe2O4 exhibited similar characteristic peaks of NiFe2O4 in the range of 100~1000 cm-1, which agree with the literatures.47, 48 At the same time, two strong characteristic peaks located at 1353 cm-1 and 1594 cm-1 were observed in the spectrum of GO, corresponding respectively to the D and G bands of carbon-based materials. Compared with those of GO, the D and G bands of NiFe2O4-RGO0.35-25-9 10


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appeared at lower frequencies. The D band shifted from 1353 cm-1 to 1345 cm-1, while the G band shifted from 1594 cm-1 to 1584 cm-1, indicating that GO has been reduced to RGO. Besides, the peak appeared at 2694 cm-1 in the Raman spectrum of the NiFe2O4-RGO corresponds to the 2D band of single-layer graphene, which suggested further the reduction of GO and the exfoliation of the graphite structure during the ball-milling.26, 49-52 In addition, the ID/IG value of NiFe2O4-RGO0.35-25-3 and NiFe2O4-RGO0.35-25-9 were both higher than that of GO (ID/IG=0.985), implying that the degree of reduction of GO increased. Meanwhile, more oxygen-containing groups were removed when the ball milling time was extended, leading to a greater reduction degree of GO. The difference between the functional groups of GO and NiFe2O4-RGO was analyzed by FTIR and displayed in Figure 3C. GO showed a broad and intense absorption peak at about 3402 cm-1, corresponding to the stretching vibration of O-H.53, 54 Two intense absorption bands at 2919 cm-1 and 2850 cm-1 were ascribed to the anti-symmetrical and symmetrical vibration of CH2, respectively. In addition, the bands at about 1716 cm-1 and 1622 cm-1 represented the stretching vibration of C=O and C=C, respectively. The peaks at 1380 cm-1 and 1090 cm-1 could be attributed to the bending vibration of C-O, which corresponded with our previous researches.20, 55 Whereas, as shown in the spectrum of NiFe2O4-RGO, the peak at 1622 cm-1 corresponds to the stretching vibration of C=C shifted to 1628 cm-1, illustrating the recovery of π-π conjugation in graphene sheets.35 Almost all the absorption peaks 11



correspond to the oxygen-containing functional groups in GO disappeared in the spectrum of NiFe2O4-RGO, suggesting that the GO in the composite was reduced to RGO after ball-milling, which was consistent with the Raman results.56 In the meantime, two new prominent absorption bands at about 532 cm-1 and 415 cm-1 were observed in the FT-IR spectrum of NiFe2O4-RGO0.35-25-9 composite, , which belong to the stretching vibrations of the metal-oxygen bond.37, 57, 58 The specific surface area and pore structure of NiFe2O4-RGO photocatalyst was studied using N2 adsorption-desorption isotherm. As was shown in Figure 3D, the nitrogen adsorption desorption isotherm of NiFe2O4-RGO0.35-25-9 photocatalyst was close to a typical IUPAC type IV pattern at a relative pressure of 0.4 to 1.0, proving the existence of mesopores. The average pore size of NiFe2O4-RGO0.35-25-9 composites was calculated to be 4.34 nm via the BJH equation and the BET (nitrogen, 77 K) specific surface area was 178.57 m2·g-1. Its BET specific surface area was significantly higher than that of NiFe2O4 nanoparticle.58 It revealed that the composite obtained more adsorption sites and active sites after NiFe2O4 nanoparticles were supported on RGO due to remarkably increased specific surface area.59 Furthermore, improved photocatalytic degradation performance of the composite could be predicted since the graphene has excellent electron mobility, which is favorable for the electron transfer during the photocatalytic reaction. 3.2 Photocatalytic activity and reaction kinetics of NiFe2O4-RGO

12


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Figure 4 (A) Effect of graphene content in NiFe2O4-RGOX-25-9 on photocatalytic degradation of MB (X = 0, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40 and 0.45, from a to h, respectively). (B) Rate constant for photocatalytic degradation of MB over NiFe2O4-RGOX-25-9.

MB was selected as an organic pollutant to evaluate the photocatalytic activity of NiFe2O4-RGO photocatalyst. As shown in Figure 4A, the adsorption of MB in dark increased greatly after NiFe2O4 was supported on graphene sheets, and the adsorption kept increasing when RGO content was raised. It is because RGO has a large specific surface area and π conjugate plane, offering more adsorption sites and facilitating the π-π stacking between RGO and MB molecules.35,

60

After reaching the

adsorption-desorption equilibrium, the MB solution was irradiated under visible light. Compared with that of NiFe2O4, the photocatalytic degradation performance of NiFe2O4-RGO was improved dramatically. Meanwhile, with the increase of graphene content in NiFe2O4-RGO photocatalyst, faster photodegradation of MB was observed. The highest photodegration rate was observed when the graphene content increased to 0.35, which was approximately 99.1%. The possible reason accounts for this phenomenon is that the existence of graphene, which not only hindered the agglomeration of NiFe2O4 particles, but also effectively suppressed the recombination of the photogenerated electron-hole pairs by promoting the transfer and separation of 13



photogenerated charge carries of the photocatalyst. Moreover, the adsorption of MB molecules on the photocatalyst is a prerequisite of degradation. Therefore, with the increase of RGO content in the photocatalyst, the adsorption sites increased, leading to improved photocatalytic degradation performance.61 However, when the RGO content exceeded 0.35, photocatalytic degradation performance of the composite started to decrease. It might be because the content of NiFe2O4 decreased as the RGO content was raised, and the NiFe2O4 nanoparticles might be wrapped up by the excess RGO. Therefore, the photocatalytic active sites in the photocatalyst decreased, resulting in the decreased photocatalytic degradation performance. In order to understand the photodegradation activity, the pseudo-first-order kinetic equation lnCt ⁄C0 = -kt was proposed to fit the degradation of MB, where C0 and Ct are respectively the concentration of MB at the irradiation time 0 and t. k is the photocatalytic degradation rate constant under visible-light irradiation at 25 °C. As can be seen in Figure 4B, when the RGO content was increased to 0.35, the NiFe2O4-RGO0.35-25-9 sample exhibited the best photocatalytic activity among all the NiFe2O4-RGOX-25-9 samples. And the rate constant reached 0.0199 min-1 after 180 min visible-light irradiation. In addition, the good photocatalytic performance of NiFe2O4-RGO0.35-25-9 was highlighted by comparing it with those of related photocatalysts, as was illustrated in Table 1. 28, 29, 31, 35, 36 Apparently, NiFe2O4-RGO0.35-25-9 we prepared in this work either shows superior photocatalytic performance or obtained through an easier, less 14


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energy-consuming

and

more

environmental-friendly

way

with

comparable

photocatalytic activity, which possesses more potential for large scale pollutant treatment.

Table 1 Comparison of the photocatalytic performance of NiFe2O4-RGO0.35-25-9 with related photocatalysts in literatures.

Synthetic

Mass (mg)

Degradation

Concentration/

time /

Volume

efficiency

Light

Photocatalyst method

MB

k (min-1)

Reference

source

NiFe2O4-RGO0.35-25-9

Ball milling

10

800 W a

20 mg·L-1/ 40 mL

3 h / 99.1%

0.0199

This work

CoFe2O4-RGO

Ball milling

10

800 W a

20 mg·L-1/ 40 mL

3 h / 96.8%

0.0144

35

NiFe2O4-graphene

Hydrothermal

25

500 W a

20 mg·L-1/ 100 mL

3 h / ~100%

0.0242

29

MnFe2O4-graphene

Hydrothermal

25

500 W a

20 mg·L-1/ 100 mL

6 h / ~99%

0.0097

31

CoFe2O4-graphene

Hydrothermal

25

500 W a

20 mg·L-1/ 100 mL

4 h / ~100%

0.0148

28

NiFe0.5Nd1.5O4

One-pot

100

solar light

10 mg·L-1/ 100 mL

3 h / 93.4%

—

36

a

Light source: Xe lamp (λ > 420 nm)

3.3 Effect of ball-milling time on photocatalytic degradation of MB

15



Figure 5 Effect of ball-milling time used for the preparation of NiFe2O4-RGO0.35-25-Z (Z=3, 6, 9 h, respectively) on its degradation activity towards MB.

As was shown in Figure 5, varying ball-milling time affected the degradation performance of NiFe2O4-RGO0.35-25-Z towards MB under visible-light irradiation at 25 °C. The photocatalytic performance of the photocatalysts was improved with the gradually increased ball-milling time from 3 to 9 h. The result was delightfully in accordance with what was expected. With the extension of ball-milling time, the particle size of NiFe2O4 decreased (Figure S3), thereby enlarged its specific surface area, which enhanced the photocatalytic performance due to an increase in the number of active sites per unit weight of photocatalyst. 3.4 Effect of ball-milling frequency on photocatalytic degradation of MB

Figure 6 Effect of different ball-milling frequencies used for the preparation of 16


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NiFe2O4-RGO0.35-Y-9 (Y=15, 20, 25 -1s, respectively) on the degradation of MB.

MB photodegradation was performed at 25 °C under visible light irradiation in the presence of NiFe2O4-RGO0.35-Y-9 prepared at different ball-milling frequency. The results are shown in Figure 6. Apparently, as the ball-milling frequency increased, the photocatalytic performance was gradually enhanced. NiFe2O4-RGO0.35-25-9 prepared at the ball-milling frequency of 25 s-1 displayed the best photodegradation performance. The reason accounts for this phenomenon is that the particle size of NiFe2O4 became smaller with the increase of the ball-milling frequency (Figure S4), therefore more active sites were created, which improved the degradation performance of the photocatalyst. 3.5 Influence of degradation temperature on photocatalytic degradation of MB

Figure 7 Effect of degradation temperature on the rate constant for photocatalytic degradation of MB over NiFe2O4-RGO0.35-Y-9 (Y=10, 15, 20, 25, 30, 40 °C, respectively).

As shown in Figure 7, the influence of reaction temperature for photocatalytic degradation of MB was studied. The rate constant (k) increased at first with the temperature rise. It may be attributed to the increase of the collision probability 17



between the photocatalyst and the MB molecules at higher temperature, which enhanced the photocatalytic activity. However, after the rate constant reached the maximum point at 25 °C, it started to decrease. The catalytic activity of the photocatalyst was suppressed when the temperature exceeded the optimum temperature, which might be because the adsorption-desorption equilibrium of MB molecules on the photocatalyst surface started to be dominated by desorption.62-65

3.6 Stability and selectivity of NiFe2O4-RGO photocatalyst

Figure 8 (A) Cycling tests of photocatalytic degradation of MB using NiFe2O4-RGO0.35-25-9 as the photocatalyst under visible-light irradiation. The inset illustrates that the NiFe2O4-RGO0.35-25-9 photocatalyst was easily separated by an externally applied magnetic field; (B) Photocatalytic degradation of MO, RhB and MB in the presence of NiFe2O4-RGO0.35-25-9 under visible-light irradiation.

The stability was one of the most important properties to evaluate the practical application potential of a photocatalyst. Thus, the stability of the photocatalyst prepared at optimum conditions was evaluated by three cycling degradation of MB 18


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under visible light illumination. As revealed from Figure 8A, the photocatalytic activity of NiFe2O4-RGO0.35-25-9 did not exhibit a noticeable decline after three runs of the photocatalytic degradation, revealing the good stability of the photocatalyst. In addition, the NiFe2O4-RGO photocatalyst is magnetically separable (inset of Figure 8A), which can be easily recycled. Figure 8B shows the photodegradation performance of NiFe2O4-RGO towards different dyes. The degradation rates of MB (99.1%) and RhB (82.2%) are superior to that of MO (47.1%) under visible-light irradiation for 180 min. It is because the existence of negatively charged oxygen-containing functional groups on RGO is unfavorable for the adsorption of MO, an anionic dye, on the NiFe2O4-RGO0.35-25-9 surface due to electrostatic repulsion. Nevertheless, the existence of negatively charged oxygen-containing functional groups greatly boosted the adsorption capacity of the photocatalyst toward cationic dyes (MB and RhB) due to electrostatic attraction. As we know, the adsorption ability of a photocatalyst is one of the key factors influencing its photocatalytic performance.35, 60 Therefore, the degradation of cationic dyes was more sufficient than that of anionic dyes. 3.7 Mechanism of photocatalytic degradation

19



Figure 9 Photocatalytic degradation of MB under visible-light irradiation using NiFe2O4-RGO as photocatalyst in the presence of different radical scavengers.

The main oxidative species in the photocatalytic reaction was investigated in order to explore the photodegradation mechanism. tert-Butyl alcohol (t-BuOH) and disodium ethylenediamine tetraacetate (EDTA-2Na) were added to the photocatalytic reaction system at 25 °C as hydroxyl radical and photogenerated hole scavenger, respectively.66-68 As illustrated in Figure 9, greater reduction of the photodegradation rate was observed when t-BuOH was added, suggesting that the hydroxyl radical acted as the main oxidative species. Only a small part of the photogenerated holes directly oxidized MB molecules, while most of the holes reacted with water or hydroxide ions to generate hydroxyl radicals. Thus, the photodegradation reaction catalyzed by NiFe2O4-RGO was proved to be dominated by radical oxidation. A possible photocatalytic degradation mechanism was proposed and illustrated in Scheme 2.

20


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Scheme 2 Mechanism of photocatalytic degradation of MB in the presence of NiFe2O4-RGO.

Visible-light irradiation induced the charge separation in NiFe2O4 and generated the electron hole pairs.69 The photogenerated electrons rapidly migrate to the surface of RGO via the permeation mechanism because of the excellent electron mobility of RGO, which suppressed the recombination of the photogenerated electrons on the conduction band (CB) with the holes on the valence band (VB). The photogenerated ·-

electrons reacted with O2 in the water to form O2·-, meanwhile, the holes in VB -

reacted with OH- in the solution to formed ·OH.70, 71 The MB molecules adsorbed on the NiFe2O4-RGO surface by π-π stacking and electrostatic attraction were oxidized +

·-

by all of the active oxides (h , ·OH and O2·-).72, 73

4. Conclusion In conclusion, a green and environmentally benign synthetic process for the fabrication of the magnetically separable NiFe2O4-RGO photocatalysts with different graphene content via a one-step ball-milling method was reported. The NiFe2O4 nanoparticles with an average particle size of 11 nm were uniformly distributed on the ultrathin graphene sheet. Amazingly, NiFe2O4 exhibited a dramatical change in the 21



photodegradation

performance

after

coupling

with

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graphene.

The

NiFe2O4-RGO0.35-25-9 photocatalyst showed both excellent photocatalytic degradation performance and recycling stability. 99.1% of the MB molecules was removed after visible light irradiation for 180 min at 25 °C. The degradation rate was remained at 95% after three cycling experiments. The successful ball-milling preparation of NiFe2O4-RGO photocatalyst with desirable photocatalytic property implied that large-scale production of graphene based photocatalysts could be achieved. It may put out an alternative way in industry to produce highly efficient visible-light-driven graphene-based photocatalysts for the degradation of organic pollutants in waste water.

Associated Content Supporting Information Experimental details and analytic data (SEM image and XPS spectra of the NiFe2O4-RGO0.35-25-9; XRD patterns of photocatalysts prepared by different ball-milling time and frequencies).

Author Information Corresponding Author *Haiqun Chen. E-mail: [email protected]. Phone: +86 519 83293890. Notes The authors declare no competing financial interest. 22


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Acknowledgments The financial support from the National Natural Science Foundation of China (No. 51572036, 51472035, 51506012); the Science and Technology Department of Jiangsu Province (BY2016029-12, BY2015027-18); the Natural Science Foundation of Jiangsu Province (BK20150266); the Science & Technology Bureau of Changzhou (CE20160001-2, CM20153006); and the PAPD of Jiangsu Higher Education Institutions is gratefully acknowledged.

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Mechanism of photocatalytic degradation of MB in the presence of NiFe2O4-RGO.

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Scalable Green Method to Fabricate Magnetically Separable NiFe2O4

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