Ternary Titania–Cobalt Ferrite–Polyaniline Nanocomposite - American

Jul 4, 2013 - Binhai, Xu,. ∥ and Xin Wang*. ,‡. ‡. Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and...
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Ternary Titania−Cobalt Ferrite−Polyaniline Nanocomposite: A Magnetically Recyclable Hybrid for Adsorption and Photodegradation of Dyes under Visible Light† Pan Xiong,‡,⊥ Lianjun Wang,*,§ Xiaoqiang Sun,*,⊥ Binhai, Xu,∥ and Xin Wang*,‡

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‡

Key Laboratory for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Ministry of Education, Nanjing 210094, China § Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, China ⊥ Key Laboratory of Fine Petrochemical Engineering, Changzhou University, Changzhou 213164, China ∥ Jiangsu Hehai Nanometer Science & Technology Co., Ltd, Taixing 225401, China S Supporting Information *

ABSTRACT: A straightforward strategy is designed for the fabrication of magnetically recyclable ternary titania−cobalt ferrite− polyaniline (P25-CoFe2O4-PANI) photocatalysts with differing P25/CoFe2O4 ratio. The pseudo-second-order and Langmuir models are found to be most suitable for describing the adsorption of methyl orange (MO) onto the photocatalysts. The photocatalytic activity of P25-CoFe2O4-PANI is evaluated by the degradation of various dyes under visible light irradiation, and the results show that the ternary P25-CoFe2O4-PANI photocatalyst exhibits high photocatalytic activity due to the good adsorption capacity of the hybrid and the introduction of P25, which can further improve the separation of the light-induced electron−hole pairs. The degradation of anionic dyes is much more effective than that of cationic dyes due to the negatively charged groups of anionic dyes undergo electrostatic attraction with the positively charged backbone of PANI, and such an effective adsorption helps in promoting the degradation.

1. INTRODUCTION Water pollution has become a global issue of concern as human activities expand and climate change threatens to cause large alterations to the hydrological cycle. For this reason, intense research attention has been paid to the removal of pollutants from wastewater.1−3 Among the various techniques, the method related to solar photocatalytic degradation of contaminants in water has been considered as a promising solution to environmental problems.4−7 In the last two decades, there has been growing interest in developing conjugated polymers for applications in many areas.8−11 They have been successfully used in organic electronic devices such as light emitting diodes, field effect transistors, and solar cells.12,13 Among many conjugated polymers, polyaniline (PANI) has been one of the most extensively investigated conducting polymers due to its good stability, corrosion protection, nontoxicity, facile and low cost synthesis, and high instinct redox properties.14−17 Particularly, it has been found that PANI has high absorption coefficients in the visible-light range and high mobility of charge carriers.18 It has also been found that PANI not only is an electron donor but also itself is an excellent hole acceptor after light irradiation.19 These special characteristics of PANI make it a promising material for improved charge separation efficiency in the photocatalytic process. Titanium dioxide (TiO2) is the most used catalyst in photoinduced reactions mainly due to its chemical stability, high photocatalytic activity, nontoxic nature, and inexpensiveness. However, only UV irradiation is active in the photoexcitation processes using pure TiO2.20−22 As UV © 2013 American Chemical Society

light constitutes only a small fraction (about 3−5%) of the solar spectrum, a large amount of the solar photons are useless for TiO2 photocatalysts. It has been reported that the combination of PANI and TiO2 can extend the photoresponse of TiO2 into the visible spectrum of light.23−27 On the other hand, recovery and reuse of suspended photocatalysts after reaction are of great significance for sustainable process management. Introducing the magnetic nanoparticles to the allowable photocatalysts can be used for easy handling of magnetic separation.28 Ferrite (MFe2O4) is a well-known cubic spinel material where oxygen forms an fcc close packing, and M2+ and Fe3+ occupy either tetrahedral or octahedral interstitial sites.29 It has been widely used in electronic devices, information storage, magnetic resonance imaging (MRI), and drug-delivery technology.30 In the past decades, the application of the spinel ferrite material in photocatalysis has been reported.31 Recently, we reported the ferrite-containing magnetic photocatalysts fabricated by combining of ferrite nanoparticles with the delocalized conjugated materials such as multiwalled carbon nanotubes (MWCNTs),32 graphene,33−35 and PANI,36 etc. These magnetic photocatalysts not only show high photoactivity under UV or visible light irradiation but also can be separated easily by an external magnetic field after degradation. Received: Revised: Accepted: Published: 10105

March July 2, July 4, July 4,

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Figure 1. TEM images of (a and b) P25-PANI and (c and d) P25-CoFe2O4-PANI(0.10), (e) the EDS pattern of P25-CoFe2O4-PANI(0.10), (f) XRD patterns of CoFe2O4, P25, P25-PANI, and P25-CoFe2O4-PANI.

property, the photocatalyst can be readily separated out from a slurry system under an external magnetic field after the reaction.

It has been found that the combination of two different semiconductors can provide an approach to achieve more efficient charge separation, leading to an enhanced photocatalytic activity.37−39 Recently, the graphene-based ternary composites, reported by Kamat’s group40 and Amal’s group,41 show new insight into the development of novel multicomponent nanoarchitectures with versatile and extraordinary properties. So it is of great interest to fabricate the multicomponent PANI-based nanocomposites for better catalytic performances. Herein, we report a magnetically recyclable ternary P25-CoFe2O4-PANI heteroarchitecture prepared by in situ oxidative polymerization. The adsorption of methyl orange (MO) was analyzed using the adsorption kinetics and isotherm models. It is found that P25-CoFe2O4PANI exhibits enhanced photocatalytic activity for dye degradation under visible light compared with the binary P25-CoFe2O4, CoFe2O4-PANI, or P25-PANI hybrid. As CoFe2O4 nanoparticles themselves have a good magnetic

2. EXPERIMENTAL SECTION 2.1. Synthesis of the P25-CoFe2O4-PANI Catalysts. All the reagents used in our experiments were of analytical purity and were used without further purification. TiO2 (P25, 20% rutile, and 80% anatase) was purchased from Degussa. CoFe2O4 nanoparticles were synthesized by the hydrothermal method.36 The P25-CoFe2O4-PANI hybrids were prepared by in situ oxidative polymerization method with differing P25/CoFe2O4 ratio. A typical experiment procedure for the synthesis of P25CoFe2O4-PANI with P25/CoFe2O4 ratio of 0.10, marked as P25-CoFe2O4-PANI(0.10), is as follows: P25 (50 mg), CoFe2O4 nanoparticles (500 mg), and ammonium peroxydisulfate (APS, 1.25 g) were dispersed in 100 mL of 1 mol/L HCl 10106

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Figure 2. (a) Survey XPS spectra of PANI, P25-PANI, and P25-CoFe2O4-PANI(0.10), (b) N(1s) spectrum of P25-PANI, (c) N(1s) spectrum of P25-CoFe2O4-PANI(0.10), and (d) UV−vis spectra of P25-CoFe2O4-PANI(0.10).

into the flask containing Methyl Orange (MO) solutions with an initial concentration of 40 mg/L under magnetic stirring in the dark. Samples were collected at different time intervals, and the concentration of dye was determined by a Shimadzu UV2550 UV−vis spectrophotometer at 463 nm. Adsorption isotherm was found by agitating MO solution of different concentrations with the known amount of photocatalyst until the equilibrium was achieved. The equilibrium adsorption capacity of MO onto photocatalysts was evaluated by using the mass balance equation:

aqueous solution with ultrasonic vibrations for 30 min to obtain a uniform suspension. Then, aniline (0.5 mL) was added into this mixture dropwise under vigorously stirring in an ice water bath, after which the resulting mixture was allowed to polymerize under stirring for 6 h in the ice−water bath. Finally, the P25-CoFe2O4-PANI hybrid was filtered out and washed with large amount of deionized water, and then dried at 60 °C until the constant mass was reached. The P25-PANI and P25-CoFe2O4 were prepared without the addition of CoFe2O4 or aniline, respectively. The CoFe2O4-PANI system was also prepared for comparison.36 2.2. General Characterization. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advanced diffractometer with Cu Ka radiation and the scanning angle ranged from 15° to 70° of 2θ. Transmission electron microscopy (TEM) images were taken with a JEOL JEM2100 microscope. X-ray photoelectron spectra (XPS) were carried out on a PHI Quantera II system (Ulvac-PHI, INC, Japan). The UV−vis spectra was conducted on a Shimadzu UV-2550 UV−vis spectrophotometer. The photocurrent responses were measured on an electrochemical system (CHI-660B). The visible light (λ > 420 nm) was obtained by a 500 W Xe lamp with a 420 nm cutoff filter (JB420) to completely remove any radiation below 420 nm. The photocatalysts were dispersed in ethanol and dropped onto the ITO substrates (1 cm × 5 cm) with a sheet resistance of 15 Ω. Then, the electrodes were dried in nitrogen at 200 °C for 30 min. 2.3. Adsorption Experiments. Adsorption experiments were conducted by adding different amounts of photocatalysts

qe =

(C0 − Ce)V W

(1)

where qe is the equilibrium adsorption capacity, C0 is the initial concentration of the dye solution, Ce is the equilibrium concentration of the dye solution in milligrams per liter, V is the volume of the solution in milliliters, and W is the mass of catalyst in milligrams taken for the experiments. 2.4. Photocatalytic Activity Measurement. In all the photocatalytic degradation experiments, 25 mg of catalyst was added to 100 mL dye solution (40 mg/L). Before irradiation, the suspensions containing dyes and photocatalysts were magnetically stirred in the dark for 120 min to ensure establishment of adsorption/desorption equilibrium. At a fixed time interval, 5 mL aliquots were sampled and then magnetically separated to remove essentially all the catalyst. The filtrates were analyzed by recording variations in the maximum absorption band using a Shimadzu UV-2550 UV−vis spectrophotometer. The active species generated in the photocatalytic system could be measured through trapping by 10107

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Figure 3. (a) Weber’s intraparticle diffusion plots of adsorption capacity qt versus the square root of time t1/2 for the adsorption of MO onto P25CoFe2O4-PANI(0.10), (b) Langmuir adsorption isotherm of the adsorption of MO onto P25-CoFe2O4-PANI(0.10), (c) adsorption capacity of anionic and cationic dyes onto P25-CoFe2O4-PANI(0.10), (d) reusability of P25-CoFe2O4-PANI(0.10).

tert-butyl alcohol (t-BuOH) and disodium ethylenediaminetetraacetate dehydrate (EDTA-2Na).

peaks of P25 at 2θ values of 25.4 and 27.5 are ascribed to anatase (101) and rutile (110) crystalline phases, respectively.42,43 The peaks at the 2θ values of 18.3, 30.1, 35.3, 43.0, 56.3, and 62.8, can be indexed to (111), (220), (311), (400), (511), and (440) crystal planes of spinel CoFe2O4 (JCPDS 221086), respectively.30,36 In the ternary P25-CoFe2O4-PANI hybrid, the relative intensity of diffraction peaks for P25 and CoFe2O4 varies with the ratio of the two components and only the diffraction peaks of CoFe2O4 can be observed when the ratio of P25/CoFe2O4 is low to some extent. However, no diffraction peak of PANI appeared in the diffraction patterns of ternary P25-CoFe2O4-PANI, implying amorphous PANI in the hybrid. The survey XPS spectra give the chemical composition information of PANI, P25-PANI, and P25-CoFe2O4-PANI (Figure 2a). The N 1s spectrum of PANI can be deconvoluted into three distinct curves: the peak appeared at 398.7 eV is associated with pyridinic-N,44 the peak at 400.4 eV may be assigned to pyrrolic-N,45 and the high binding energy of N+ at 401.8 eV is due to the interaction between N+ and protons introduced by the acid doping.46 While in the decomposed N 1s spectrum of P25-PANI or P25-CoFe2O4-PANI, a new peak at about 399.5 eV can be observed and the peak of pyrrolic-N was decreased remarkably in comparison to pure PANI (Figure 2b and c). The peak at 399.5 eV may result from the interaction between the metal ions and nitrogen.36,47,48 The UV−vis spectrum of ternary P25-CoFe2O4-PANI in N-methyl-2pyrrolidone (NMP) revealed absorption maxima at 630 and 330 nm (Figure 2d), originating from the charge-transferexcitation-like transition from the highest occupied energy level

3. RESULTS AND DISCUSSION 3.1. Characterization of the Photocatalysts. The typical TEM images of P25-PANI and P25-CoFe2O4-PANI are shown in Figure 1a and b and Figure 1c and d, respectively. As shown in Figure 1a, the P25 nanoparticles with an average diameter of 30 nm are well dispersed in the P25-PANI system. From the HRTEM image (Figure 1b), it can be clearly seen the lattice fringes with a d-space of about 0.36 nm corresponding to the (101) plane of anatase TiO2 and a PANI layer with noncrystal structure adsorbed evenly on the TiO2 surface with the thickness of about 2.5 nm. For the ternary P25-CoFe2O4-PANI, the TEM images (Figure 1c) show that the hybrid consists of well dispersed CoFe2O4 particles with an average diameter of about 20 nm and P25 nanoparticles with an average diameter of 30 nm in the PANI matrix. The as-prepared P25-CoFe2O4PANI shows two different lattice structures with d-space of about 0.36 and 0.25 nm corresponding to the (101) plane of P25 and the (311) plane of CoFe2O4 nanoparticles, respectively (Figure 1d). These observations indicate that the CoFe2O4 and P25 nanoparticles are successfully enveloped by the PANI layers. As shown in Figure 1e, there are N, O, C, Ti, Co, and Fe elements in the ternary P25-CoFe2O4-PANI composite and an approximate Co/Fe atom ratio of 1:2 is in fair agreement with the theoretical value for CoFe2O4. The XRD diffraction patterns of CoFe2O4, P25, P25-PANI, and P25-CoFe2O4-PANI are shown in Figure 1f. The main 10108

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Table 1. Isotherm Parameters for the Adsorption of MO onto P25-CoFe2O4-PANI(0.10) Hybrids Langmuir catalysts P25-CoFe2O4PANI(0.10)

Freundlich

Temkin

qm (mg·g−1)

b (L·mg−1)

R2

Kf (mg·g−1)

n

R2

AT (L·g−1)

BT (J·mol−1)

R2

168.57 ± 0.18

1.8360 ± 0.0019

0.9979

112.81 ± 1.45

5.7244 ± 0.0737

0.9744

1520.9 ± 20.1

22.807 ± 0.301

0.9738

Figure 4. (a) Photodegradation of MO on P25, PANI, CoFe2O4, P25-CoFe2O4, CoFe2O4-PANI, P25-PANI, and P25-CoFe2O4-PANI(0.10) (from 1 to 7). (b) Photodegradation of MO: blank, P25-CoFe2O4-PANI(0.01), P25-CoFe2O4-PANI(0.03), P25-CoFe2O4-PANI(0.05), P25-CoFe2O4PANI(0.07), P25-CoFe2O4-PANI(0.30), and P25-CoFe2O4-PANI(0.10) (from 1 to 7). (c) Photodegradation of various dyes on P25-CoFe2O4PANI(0.10). (d) Photodegradation of MO on P25-CoFe2O4-PANI(0.10) without irradiation and under visible light irradiation. (e) Photodegradation of MO on P25-CoFe2O4-PANI(0.10) in cycles. The inset shows the magnetic separation after photodegradation. (f) XRD patterns of P25-CoFe2O4-PANI(0.10) before and after photocatalysis.

to the lowest unoccupied energy level and the π−π* transition of PANI, respectively.49 3.2. Adsorption Kinetics of P25-CoFe2O4-PANI Photocatalyst. MO molecules with negatively charged sulfonic groups were chosen as a model dye to evaluate the adsorption capacity of the ternary P25-CoFe2O4-PANI photocatalysts. A plot of adsorption capacity qt versus time t for the adsorption of MO onto P25-CoFe2O4-PANI(0.10) is shown in Supporting Information Figure S1a, and it reached the plateau value after approximately 120 min. The kinetic process of MO adsorption on P25-CoFe2O4-PANI was described by pseudo-first-order, pseudo-second-order, and Elovich kinetic equations, resepectively (Supporting Information Figure S1b−d and Table S1). It can be seen that neither the pseudo-first-order model nor

Elovich model is appropriate for predicting the best-fit kinetics of MO adsorption onto the photocatalyst. While using the pseudo-second-order model, the obtained qe,cal values were very close to the qe,exp values with higher R2, implying that the pseudo-second-order model is suitable for describing the adsorption kinetics of MO onto P25-CoFe2O4-PANI photocatalyst.50 The kinetic results were further analyzed by Weber’s intraparticle diffusion model to elucidate the diffusion mechanism, which is expressed as51

q = kit 1/2 + Ci

(2)

−1

where qt (mg·g ) is the amount of sorption at time t (min) and ki (mg·g−1·min−1/2) is the rate constant of the intraparticle 10109

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diffusion model. As shown in Figure 3a, the plot of qt versus t1/2 for the adsorption of MO onto the P25-CoFe2O4-PANI(0.10) consists of two linear sections attributed to mesopore and micropore diffusion, respectively. The rate constants for the first phase (ki,1) and second phase (ki,2) were obtained from the plot of qt versus t1/2 and the results are given in Figure 3a. The lower value of ki,2 indicates that the intraparticle diffusion step is a more important step for the adsorption of MO onto the photocatalyst. 3.3. Adsorption Isotherms of P25-CoFe2O4-PANI Photocatalyst. The Langmuir, Freundlich, and Temkin isotherm models were applied to understand the mechanism of the adsorption process. The linear equation of the Langmuir, Freundlich, and Temkin isotherm models are expressed as eqs 3−5, respectively:52−54 Ce C 1 = e + qe qm bqm

(3)

1 log Ce n

(4)

qe = BT ln A T + BT ln Ce

(5)

log qe = log K f +

The stability and regeneration ability of the adsorbent is crucial for its practical application. After every 3 h of adsorption, the separated photocatalysts were washed with enough deionized water to desorb the adsorbed dye molecules and then dried. As shown in Figure 3d, the adsorption capacity of MO using the recycled photocatalyst can be retained to a great extent after five cycles, indicating that the ternary P25CoFe2O4-PANI nanocomposite is a stable and effective hybrid for the adsorption of dyes. 3.4. Photocatalytic Activity of P25-CoFe2O4-PANI Photocatalyst. The photocatalytic performance of ternary P25-CoFe2O4-PANI photocatalysts is evaluated by degradation of MO under visible light irradiation. It can be seen from Figure 4a, PANI, P25, CoFe2O4, or P25-CoF2O4 show very poor catalytic activity under visible light irradiation. Meanwhile, P25PANI and CoFe2O4-PANI display higher visible-light photocatalytic activity compared to pure P25 and CoFe2O4. This may be due to the fact that the separation efficiency of photogenerated electron−hole pairs was increased after modified with PANI.23,36 Furthermore, the ternary P25-CoFe2O4-PANI photocatalyst exhibits enhanced photoactivity compared to the binary CoFe2O4-PANI or P25-PANI, indicating that the combination of two different semiconductors can achieve the photosensitized charge separation more efficiently. It is possible that the P25/CoFe2O4 ratio of P25-CoFe2O4PANI may influence the reaction, photocatalytic activities of P25-CoFe2O4-PANI with differing P25/CoFe2O4 ratio were evaluated by the degradation of MO under visible light irradiation and the results are dispalyed in Figure 4b. It can be seen that the P25/CoFe2O4 ratio of 0.10 was found to be an optimum in photoactivity. The photocatalytic activity of P25-CoFe2O4-PANI(0.10) was further evaluated by degradation of various dyes under visible light irradiation. It can be seen from Figure 4c that P25CoFe2O4-PANI(0.10) shows remarkable photocatalytic activity for degradation of MO, TB, and BBR, while only poor activity for MB, MG, and NR. Moreover, P25-CoFe2O4-PANI(0.10) also exhibits high activity for the photodegradation of pchlorophenol under visible light irradiation (Supporting Information Figure S2). In order to confirm that the dyes were photodegraded rather than adsorbed on the photocatalysts, a blank experiment was carried out and the result showed there was no noticeable change in the dye concentration after 7 h stirring without visible light irradiation (Figure 4d), which demostrated that the observed photobleaching in the process should come from the photodegradation of the dye moleculesrather than adsorption. The residual total organic carbon (TOC) is about 55% after photocatalytic reaction (Supporting Information Figure S3), indicating that a significant fraction of dye molecules can be mineralized. The ternary P25-CoFe2O4-PANI photocatalyst can be easily separated from the photocatalytic system by a magnet or an applied magnetic field as a result of the good magnetic property of CoFe2O4 (see the inset of Figure 4e). The stability of photocatalysts is an important issue for their practical applications.58−60 After every 7 h of photodegradation, the separated photocatalysts were washed with deionized water and dried. As shown in Figure 4e, the photodegradation of MO still retained a high rate after three cycles, indicating that P25CoFe2O4-PANI(0.10) is stable and effective for the degradation of dyes in water. The structural stability of ternary P25CoFe2O4-PANI(0.10) was further confirmed by XRD analysis

where qe (mg·g−1) is the amount of the dye adsorbed per unit mass of adsorbent, Ce (mg·L−1) is the equilibrium dye concentration, qm (mg·g−1) is the maximum adsorption capacity, b (L·mg−1) is the Langmuir constant, Kf (mg·g−1) is the Freundlich constant, 1/n is an empirical parameter, BT (J·mol−1) is the Temkin constant, and AT (L·g−1) is the equilibrium binding constant. The adsorption isotherm parameters are summerized in Table 1, and the Langmuir isotherm plot for the adsorption of MO onto P25-CoFe2O4-PANI(0.10) is presented in Figure 3b. The high correlation coefficient (R2 = 0.9979) indicates that the Langmuir model is suitable for describing the adsorption process. The essential characteristics of the Langmuir isotherm can be expressed by means of a dimensionless constant, called the separation factor or equilibrium parameter RL, defined by the following equation:52 RL =

1 1 + bC0

(6)

The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0).52 The calculated RL value for the P25CoFe2O4-PANI(0.10) photocatalyst is 0.013, indicating quite favorable conditions in the adsorption process and the relatively strong interaction between the MO molecules and the photocatalyt.53 The variation of adsorption capacity of six dyes onto P25CoFe2O4-PANI(0.10) with adsorption time is shown in Figure 3c. Among the six types of dyes, Methyl Orange (MO), Trypan Blue (TB), and Brilliant Blue R (BBR) are anionic dyes with negatively charged groups in their molecules, while Methylene Blue (MB), Malachite Green (MG), and Neutral Red (NR) are cationic dyes with positively charged groups in their molecules (Supporting Information Table S2). A noticeable adsorption of anionic dyes is observed (Figure 3c) due to the electrostatic attraction between the negatively charged dye anions and the positively charged PANI backbone.55−57 10110

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(Figure 4f). Zero-, first-, and second-order kinetic plots were made to determine the rate constant of photodegradation reaction and the results are shown in Supporting Information Figure S4 and Table S3. 3.5. Possible Mechanism of Photocatalysis with the Ternary Photocatalyst. It is well-known that the adsorption capacity is an important factor that may influence the photocatalytic activity.36,61,62 The positively charged backbone of PANI can adsorb anionic dyes more effectively, leading to higher rates of degradation. This is why PANI-based catalyst shows poorer photocatalytic activity for cationic dyes. The significant enhancement in photocatalytic activity can also be attributed to the remarkable synergistic effect between the semiconductor and PANI.23−27,36,58−60 As is well-known, the high separation efficiency of photogenerated electron−hole pairs based on the photosynergistic effect among the individual components may play an important role in the photodegradation of dyes. Transient photocurrent measurements can be useful in helping to understand the synergistic effect of the individual components.62 The transient photocurrent responses of electrodes were recorded via several on−off cycles under visible light irradiation (Figure 5a). The highest photocurrent of the P25-CoFe2O4-PANI(0.10) electrode indicates the highest separation efficiency of photogenerated electron−hole pairs based on the photosynergistic effect among P25, CoFe2O4, and PANI. It is vitally important to identify the main active oxidant in the photocatalytic reaction process for better understanding the mechanism. The active oxidants generated in the photocatalytic process can usually be measured through trapping by disodium ethylenediaminetetraacetate dehydrate (EDTA-2Na) (hole scavenger) and tert-butyl alcohol (t-BuOH) (·OH radical scavenger).63,64 As shown in Figure 5b, in the P25-PANI photocatalyst, the photodegradation of MO under visible light irradiation was obviously restrained after the injection of tBuOH or EDTA-2Na, giving almost the same tendency to decrease with time. This phenomenon suggests that both radicals and holes are the main active species in this system. For P25-CoFe2O4-PANI, the photodegradation of MO was obviously restrained after the injection of EDTA-2Na, while the photodegradation changed indistinctively after the addition of t-BuOH (Figure 5c). Therefore, we can conclude that the dye degradation by P25-CoFe2O4-PANI under visible light irradiation is driven mainly by the participation of hole and to a lesser extent by the contribution of ·OH radical. On the basis of the above discussion, a possible mechanism of photocatalysis with binary P25-PANI and ternary P25CoFe2O4-PANI under visible light irradiation is proposed (Figure 6). For binary P25-PANI, PANI absorbs visible light to induce photoelectrons and holes.23,24,60 The excited state electrons in the lowest unoccupied molecular orbital (LUMO) of PANI can readily migrate into the conduction band (CB) of P25, and subsequently transfer to the surface to react with water to yield hydroxyl radicals, which would oxidize the organic molecules. As PANI is a good material for transporting holes,19,65 the holes can migrate easily to the surface of the photocatalysts and photodegrade the adsorbed MO molecules. While under the visible light irradiation, both CoFe2O4 and PANI can be excited to yield photogenerated carriers (electrons and holes) in the ternary P25-CoFe 2 O 4 -PANI syetem.23,24,34,36,58−60 The excited state electrons in LUMO of PANI can readily migrate into conduction band (CB) of both CoFe2O4 and P25.23,24,36 On the other hand, the photo-

Figure 5. (a) Photocurrent transient responses of P25-CoFe2O4PANI(0.10), P25-PANI, CoFe2O4-PANI, CoFe2O4, P25-CoFe2O4, P25, and PANI (from 1 to 7) under visible light irradiation. Plots of photogenerated carriers trapping in the system of photodegradation of MO under visible light on (b) P25-PANI and (c) P25-CoFe2O4PANI(0.10).

Figure 6. Possible mechanism of ternary P25-CoFe2O4-PANI.

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generated holes in valence band (VB) of CoFe2O4 can directly transfer to HOMO of PANI36 and consequently the photogenerated holes can migrate easily to the surface of the P25CoFe2O4-PANI photocatalysts. Most of holes can directly photodegrade the dye, and only a few of them can react with the adsorbed water (or hydroxide anions) to yield hydroxyl radicals (Figure 6).

4. CONCLUSIONS In summary, P25-CoFe2O4-PANI photocatalysts with differing P25/CoFe2O4 ratio were successfully prepared by in situ oxidative polymerization method. Due to the magnetic properties of CoFe2O4, the P25-CoFe2O4-PANI photocatalysts can be easily separated by an external magnetic field from an aqueous suspension. The P25-CoFe2O4-PANI system shows significant adsorption of anionic dyes, which can help in promoting the photodegradation of the dyes. The high photocatalytic activity of P25-CoFe2O4-PANI can be attributed to the synergic effect among PANI, P25, and CoFe2O4 in the photocatalytic process.

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ASSOCIATED CONTENT

S Supporting Information *

Adsorption kinetic process of MO adsorption on P25CoFe2O4-PANI. The structure of the dyes. The degradation of p-chlorophenol under visible light irradiation. Evolution of TOC removal of MO with irradiation time. The photodegradation kinetics according to zero-, first-, and second-order kinetic. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-84305667. Fax: +86-25-8431-5054. E-mail address: [email protected] (X.W.), [email protected]. edu.cn (L.J.W.) or [email protected] (X.Q.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This investigation was supported by NNSF of China (No. 21171094), RFDP (No. 20123219130003), STPP of Jiangsu (No.BE 2012151), the Fundamental Research Funds for the Central Universities (No.30920130122002), and PAPD of Jiangsu.

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DEDICATION This work is dedicated to Professor Xiao-Zeng You on the occasion of his 80th birthday. †

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