Article pubs.acs.org/JPCC
Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite Yangming Lin, Danzhen Li,* Junhua Hu, Guangcan Xiao, Jinxiu Wang, Wenjuan Li, and Xianzhi Fu Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou, 350002, P. R. China S Supporting Information *
ABSTRACT: The polyaniline (PANI)/TiO2 nanocomposites have been successfully synthesized via a hydrothermal method and followed by a low-temperature calcination treatment process. We find that such a PANI/TiO2 nanocomposite exhibits higher photocatalytic activity and stability than bare TiO2 and TiO2‑xNx toward the liquid-phase degradation of methyl orange (MO) under both UV and visible light (420 nm < λ < 800 nm) irradiation. More noteworthy, the PANI/TiO2 photocatalyst still perform good activity toward MO and 4-chlorophenol (4-CP) under the longer wavelength of light (550 nm < λ < 800 nm). The total organic carbon (TOC) tests show that the mineralization rate of MO and 4-CP over PANI/ TiO2 are apparently higher than bare TiO2 under the irradiation of both UV and visible light. The presence of synergic effect between PANI and TiO2 is believed to play an essential role in affecting the photoreactivity. At last, the roles of active species in the photocatalytic process are compared by using different types of active species scavengers. Meanwhile, the degradation mechanism of the photocatalysts is proposed. It is hoped that our work could provide valuable information on the design of polymer modified semiconductor with more excellent properties and set the foundation for the further industrial application.
1. INTRODUCTION Semiconductor as a high-profile photocatalyst has been widely applied in various areas ranging from renewable energy to cleanup environment fields.1 In the past years, enormous efforts have been devoted to developing a series of new organic and inorganic photocatalysts, such as C3N4,2 PNDT-DTPyT,3 CaSn(OH)6,4 Sm2Ti2S2O5,5 PbBi2Nb2O9,6 ZnIn2S4,7 Ag3PO4,8 CaBi2O4,9 LaVO4,10 and so on. However, the low quantum efficiency, self-decomposition of photocatalytic reactions, the ineffective utilization of visible light, critical drawback of photocorrosion, secondary pollution on the environment, high costs of rare elements, etc. impair their applications to a great extent. Although the major shortcomings of TiO2 nanoparticles include the low quantum efficiency and the confined utilization of sunlight, it has been still employed widely in solar energy conversion and depollution of the environment due to its environmental friendly, excellent physicochemical properties.1,11−14 To exploit effective visible light active photocatalysts with a slower recombination rate of charge carriers and higher stability, many research activities mainly focus on the modification of the surface or bulk properties of TiO2 materials by doping with nonmetal atoms,15 combining with noble metals,16 narrow band gap semiconductors,17,18 and dye sensitization.19 Therefore, photocatalysis field has attracted intense interest becasue of their low cost, high efficiency, and lack of secondary pollution in the environmental. During the past decade, © 2012 American Chemical Society
polyaniline (PANI) has been the most extensively investigated conducting polymer with good stability, corrosion protection, nontoxicity, facile and low cost synthesis, and high instinct redox properties, and research on it eventually lead to the Nobel Prize in 2000.20−26 Particularly, PANI has shown great potential due to its high absorption coefficients in the visiblelight range and high mobility of charge carriers.27 Furthermore, after the irradiation of light, PANI not only is an electron donor but also itself is an excellent hole acceptor.28 These special characteristics of PANI make it an ideal material to achieve enhanced charge separation efficiency in the photocatalysis field. Recently, more and more attention has been focused on the combination of PANI and semiconductor photocatalysts.29 Zhang et al. and Wang et al. prepared PANI/semiconductor composites via chemisorptions and in situ oxidative polymerization and then found the as-prepared samples have enhanced photocatalytic activity under natural light.30,31 However, in most cases, the photocatalytic degradation of organic pollutants is mainly for dyes including rhodamine B (RhB) and methlene blue (MB), and little research has been performed on phenols, highly toxic and carcinogenic compounds, especially without the assistance of H2O2 under visible light (λ>420 nm), even in the longer wavelength of light. Moreover, the mechanism of Received: November 21, 2011 Revised: January 30, 2012 Published: February 1, 2012 5764
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2.3. Characterizations. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The diffuse reflectance spectra (DRS) were performed on Varian Cary 500 UV−vis spectrophotometer with BaSO4 as the background ranging from 200 to 800 nm. Thermogravimetric (TG) analysis was performed on a STA409 thermal analyzer. The atmosphere was N2, and the heating rate was 10 °C/min from room temperature to 800 °C. Fourier transform infrared (FT-IR) spectra of the polymers in KBr pellets were recorded on a Nicolet Avatar 670 FT-IR spectrometer (Nicolet Corp., USA). The spectra were collected from 4000 to 400 cm−1 with a 4 cm−1 resolution over 128 scans. The generation of hydroxyl radicals was investigated by the method of photoluminescence technique (Edinburgh FL/ FS900 spectrophotometer) with 5 mM terephthalic acid (PLTA) and 0.01 M NaOH, and the maximum absorption peak is 426 nm. The TOC values were detected by a Shimadzu TOCVCPH total organic carbon analyzer. The photoelectrochemical experiment was measured on an electrochemical system (CHI660D, China). The P/T and TiO2 catalysts were deposited as a film on a 0.5 cm × 0.5 cm indium−tin−oxide conducting glass to obtain the working electrode. Ag/AgCl and Pt served as the reference electrode and the counter electrode, respectively. The electrolyte was 0.1 M Na2SO4 solution. Electron spin resonance (ESR) spectra were obtained using a Bruker model A300 spectrometer with a 500 W Xe-arc lamp equipped with an IRcutoff filter (λ < 800 nm) and an UV-cutoff filter (λ > 420 nm) as visible light source. 2.4. Tests of Photocatalytic Activity. The visible light source was a 500 W Xe-arc lamp (Institute of Electric Light Source, Beijing). The system was cooled by fan to maintain room temperature. The 420, 550, and 800 nm cutoff filters were placed before the vessel to ensure that irradiation of the MO or 4-CP catalyst system was irradiated only by visible-light wavelengths. A total of 0.08 g of photocatalyst was added to 80 mL of MO solution (10 ppm) and 4-CP solution (1.2 × 10−4 mol/L, 15 ppm) contained in a 100-mL Pyrex glass vessel, respectively. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h in order to reach adsorption− desorption equilibrium between the catalyst and MO or 4-CP. At given time intervals, 3-mL aliquots were sampled and centrifuged to remove the catalyst. The degraded solutions were analyzed using Varian Cary 50 UV−vis spectrophotometer, and the absorption peak at 464 and 225 nm was monitored. UV-light photocatalytic experiments were conducted in a quartz reactor. The catalyst (0.08 g) was suspended in 150 mL of MO solution (20 ppm) in the reactor surrounded by four 4-W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5). The next procedure was similar to that of the above-mentioned visible photocatalytic experiments. To evaluate the stability of the photocatalyst, the photocatalytic process was reused five times to degrade MO under visible light and UV light. 2.5. Measurements of Adsorption. The adsorption process was operated via the method reported by Xu et al.,33 with a slight modification. The concrete procedures were as follows: 0.08 g of catalyst was added to 80 mL of MO solution (10 ppm) and then the suspension was magnetically stirred in the dark. Samples of the solutions were analyzed at selected intervals, 3-mL aliquots were sampled at selected intervals and centrifuged to remove the catalyst as quickly as possible. The change of absorption peak intensity over MO was observed by monitoring with a UV−vis spectrophotometer.
PANI/semiconductor in the visible-light photocatalytic process has not been convincingly explaned, including the excitation mechanism of PANI and questions about semiconductor photocatalytic or dye sensitization under visible light. This paper describes a new method of synthesizing PANI/ TiO2 nanocomposites by combining hydrothermal and low temperature calcination treatment without damaging the main chains of PANI. The as-prepared PANI/TiO2 nanocomposites make full use of the various properties for bare PANI, such as double absorption band region at UV and the visible light region, leading to notable photocatalytic activity. Methyl orange (MO) and 4-chlorophenol (4-CP) are used as target organic pollutants for photocatalytic degradation reactions to evaluate the photocatalytic capability of the PANI modified TiO2 photocatalyst under UV light (λ = 254 nm) and visible light irradiation (420 nm < λ < 800 nm). The results reveal that the PANI/TiO2 nanocomposites possess excellent photocatalytic activities. What is more notable is that the nanocomposite photocatalysts still possess good activities under the longer wavelength of light irradiation (550 nm < λ < 800 nm). Recycled stability tests indicate that the presence of PANI in the photocatalyst contributes to photocorroison inhibition and the higher photocatalytic stability of PANI/TiO2 nanocomposites than the bare TiO2. The possible radical species involved in the degradation of MO and 4-CP are analyzed by means of adding radical scavengers, photoluminescence spectra (PL), electron spin resonance spectra (ESR) techniques, and photoelectrochemical experiments.
2. EXPERIMENTAL METHODS 2.1. Materials. Titanium isopropoxide was kindly supplied by Aladdin-reagent. Horeradish peroxidase (POD) and N,Ndiethyl-p-phenylenediamine (DPD) were purchased from J&K Chemical Ltd. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was obtained from Sigma Co. All chemicals are analytical grade without further purification. Deionized water was used for the preparation of all solutions. 2.2. Sample Preparation. The typical preparation of PANI/TiO2 photocatalyst was as follows: 10 mL of titanium isopropoxide (TTIP) and 15 mL of isopropyl alcohol were dissolved in 30 mL of CH3COOH to form a mixture, solution A. Solution A was stirred for 20 h in the water bath at 323 K. A total of 10 mL of 0.09 M aniline (ANI) was dissolved in 8 mL of 1 M HCl to produce a mixture, solution B. After that, 0.205 g of ammonium persulfate (APS) was added to solution B. The molar composition of ANI/APS was 1:1. Then solution B was added dropwise into solution A with continuous stirring under ambient air. After 4 h of stirring, the suspension was transferred into a Teflon-lined stainless steel autoclave with a capacity of 100 mL and maintained at 100 °C for 4 h. The final products were filtered and washed with deionized water and ethanol and dried at 60 °C for several hours in a vacuum oven. The asprepared PANI/TiO2 nanocomposites (marked as P/T) were further calcined at 100, 150, 200, and 300 °C for 3 h to eliminate impurities and some oligomers. The final samples were labeled P/T-100 °C, P/T-150 °C, P/T-200 °C, and P/T300 °C. The same volume of water instead of aniline and HCl was used in the above procedures to synthetize pure TiO2. Solution B was added to 0.205 g of APS to produce polyaniline (PANI) under stirring for 4 h. Nitrogen-doped TiO 2 (TiO2‑xNx) photocatalyst was synthesized as a reference by a typical method.32 5765
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3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of P/T Nanocomposites. Figure 1 shows the XRD spectra of TiO2 and P/T
The first at 284.5 eV is attributed to C atoms bound only to C or H atoms. The two later peaks correspond to C−N and C N of bare PANI, respectively. Compared with PANI, the BE of CN of P/T shifts from 287.75 to 288.54 eV, which indicates that the C element links with O to form the O−CN structure. Another peak at 286.13 originates from CC bonds in the residual organic groups of pare TiO2 or C−N of P/T. As mentioned in the analysis of IR of CC bonds, there is no difference in TiO2 and P/T and then the absence of CC bonds interaction with other elements. However, the BE of C−N of P/T moves from 285.64 to 286.13 which reveals that other structure O−C−N may exist in the P/T system. Meanwhile, as shown in Figure S1b and c, N element binding energy of pure PANI at 399.8 and 401.9 eV can be ascribed to C−N and protonated CN.37 After the PANI is used to modify TiO2, only the CN binding energy shifts to low BE of 401.5 eV. Comparison with Figure S1d shows that the BE of Ti (2p) shifts from 458.5 and 464.19 to 458.72 and 464.4 eV, respectively. There results illustrate that the structure of O− CN−Ti and O−C−N generate in the P/T nanocomposite. Namely, it might be related to the combination of TiO2 nanocrystalline and PANI, which produced stronger binding force due to the interaction between TiO2 nanoparticles and the lone pair electrons of the N atom in the polymer backbone. Moreover, three peaks at 530.0, 531.5, and 532.6 eV for P/T are classified as lattic oxygen, hydroxyl oxygen, and adsorption oxygen. Compared with XRD and the differences of O (1s) spectra of P/T and TiO2, implying the O element of O−C N−Ti and O−C−N is very likely assigned to adsorption oxygen and not lattic oxygen. In conclusion, according to the results of FTIR and XPS, there are likely to contain intensive interaction structures such as O−CN−Ti and O−C−N in the P/T system. A further study of the detailed interaction between PANI and TiO2 is now in progress. However, these interaction structures may not only be beneficial to transfer of carriers but also induce synergetic effect to enhance the photocatalytic activity. To obtain information about thermal stability of the samples, photocatalysts are characterized by TG. As shown in Figure S2, it can be observed that the losses of weight of PANI occurr around three temperature periods, in the range from 100 to 190 to 440 °C. The first two weight losses are mainly attributed to residual water, the elimination of impurities, and some oligomers. Another decrease of mass occurr around 440 °C, which is due to the degradation of the polymer main chain.38 Similarly, there are two regions of loss of weight in the TG curve of P/T. The former from 25 to 460 °C results from desorption of the crystal water, the elimination of impurities, and the degradation of some oligomers.39 The latter ranging from 460 to 600 °C is ascribed to degradation of polymer main chain and the phase transformation of TiO2 from anatase to rutile. Figure 3 shows the optical band gap energy (Eg) of the several photocatalysts by using the method of Li et al,40 which depends on the equation of (F(R)E)1/2 = A(E − Eg). The estimated band gap values of P/T nanocomposites are approximately 3.0, 2.95, and 2.80 eV corresponding to P/T100 °C, P/T-150 °C, and P/T-200 °C, respectively, clearly showing a band gap narrowing as compared to the estimated 3.18 eV of the bare TiO2. Even so, there still exists weak visiblelight absorption phenomenon in the as-prepared TiO2. In addition, as shown in Figure S3, It is also interesting to observe that the P/T nanocomposites possess much better visible light
Figure 1. XRD patterns of TiO2 and P/T nanocomposites.
nanocomposites prepared in different conditions. The peaks at 2θ values of 25.3, 37.9, 48.1, 54.0, 55.2, and 62.8 can be indexed to (101), (004), (200), (105), (211), and (204) faces of anatase TiO2, respectively. It is obvious that the P/T nanocomposite prepared at different temperatures has not change in peak positions and shapes compared with the pure TiO2, indicating that the presence of PANI do not impact on the lattice structure of TiO2. From the results shown in Figure 2, the peak of 1630 cm−1 is designated as CC bonds of the residual organic groups,
Figure 2. FTIR patterns of PANI, TiO2, and P/T nanocomposites under the condition of different temperature calcinations.
whereas there is not any difference in the TiO2-200 °C and P/T. The band at 1430 cm−1 is corresponding to bending modes of C−H bonds which also come from the residual organic groups in the as-prepared TiO2.34 Moreover, it is clearly seen that the main characteristic peaks of PANI-200 °C are 1596, 1506 (CN for quinone ring and CC for benzene ring, respectively), 1292 (C−N stretching mode for benzene ring), and 1124 cm−1 (quinonoid unit doped PANI).35 However, these peaks in P/T shift to lower wavenumber compared with that in pure PANI-200 °C. And it is obvious that the above four peaks shift to 1540, 1450, 1135, and 1050 cm−1, respectively. Not only that, in XPS spectra, as shown in Figure S1 (Supporting Information), three peaks can be observed at 284.5, 285.64, and 287.75 eV for C (1s) binding energy (BE).36 5766
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displayed in Figure 4b and Table 1, the photodegradation process is fit for pseudofirst-order kinetics by linear transforms Table 1. Variation of Total Organic Carbon (TOC) Removal and Photocatalytic Reaction Rate Constants k (h−1) by Degradation of MO and 4-CP under Irradiation of Light with Different Wavelengths TOC removal (%) k (h−1)
sample
pollutants
light source
time
TiO2-200 °C P/T-200 °C TiO2-200 °C
MO MO MO
125 min 125 min 6h
72.4 81.3 1.1
1.518 2.484 0.166
P/T-200 °C
MO
6h
21.5
0.5316
Figure 3. Eg of P/T and TiO2 nanocomposites.
TiO2-200 °C
MO
6h
0
0.0256
absorption intensity (400−800 nm) than bare TiO2. Moreover, it should be particularly noted that the DRS spectrum of PANI reveals two strong absorption bands centered at around 300 and 600 nm. The former has been assigned to the π → π* transition. The absorption spectrum of PANI has an intense peak at about 600 nm, which is attributed to a charge-transfer excitation-like transition from the highest occupied energy level (HOMO) to the lowest unoccupied energy level (LUMO).41 Thus, the presence of PANI affects significantly the visible light absorption (a red shift to higher wavelength) for the P/T nanocomposites, which is in agreement with the optical color of the samples as exhibited in Figure S4. The unique light absorption property of PANI indicates that PANI modify TiO2 nanocomposites may have dramatic photocatalytic activity for some target reactions under UV or visible light irradiation. 3.2. Photocatalytic Activity and Stability of P/T Nanocomposites. The photocatalytic performance over the P/T nanocomposites for liquid-phase degradation of MO has been measured at room temperature. As can be seen from the Figure 4, blank experiments of organic pollutant without any photocatalyst at the same conditions, labeled by blank, show that no activity is observed under the light irradiation. The photocatalytic activity is enhanced gradually with the increase of the calcining temperature. To further investigate its outstanding activity, nitrogen-doped TiO2 (TiO2‑xNx) which is an efficient photocatalyst under visible light is also used as a reference catalyst.32 The photocatalytic degradation efficiency of MO under visible light follows the order P/T-200 °C > P/T150 °C > TiO2-200 °C > TiO2‑xNx > P/T-100 °C > blank. As
P/T-200 °C
MO
6h
10.6
0.321
TiO2-200 °C
4-CP
6h
0.2
0.00474
P/T-200 °C
4-CP
UV(λ=254 nm) UV(λ=254 nm) 420 nm ∼800 nm 420 nm ∼800 nm 550 nm ∼800 nm 550 nm ∼800 nm 550 nm ∼800 nm 550 nm ∼800 nm
6h
13.2
0.055
ln(C0/Ct) = kt, where C0 is the adsorption equilibrium concentration of MO, Ct is the concentration of MO at time t, and the apparent rate constant k are 0.5316, 0.166, and 0.0835 h−1 corresponding to P/T-200 °C, TiO2-200 °C, and TiO2‑xNx, respectively. Moreover, the supplements of activity degradation of MO over photocatalysts are displayed in Figure S5A. Clearly, among all of the samples, the P/T-200 °C nanocomposite exhibits the best visible light photocatalytic activity toward the degradation of MO. After 6 h of irradiation, MO is degraded almost completely, the photocatalytic conversion ratio is up to 96%, which exceeds that of TiO2-200 °C and TiO2‑xNx dramatically. Besides the above-mentioned visible light photoactivity, the degradation of MO (20 ppm) over P/T-200 °C under UV light is slightly better than that of TiO2200 °C is shown in Figure S6a. With regard to the best photoactivity catalyst of P/T-200 °C, it has a more stable activity toward degradation of MO in the liquid-phase visible photocatalysis than bare TiO2-200 °C as displayed in Figure 5a. After five times run of degradation reaction, the photocatalytic conversion ratio of MO of the P/T remains 90% under visible light. However, over the bare TiO2-200 °C, it can be clearly found that the photocatalytic conversion ratio of MO decreased from 65% at the first cycle to
Figure 4. Liquid-phase photocatalytic degradation (a) and kinetic linear simulation curves (b) of MO over the P/T, TiO2, and TiO2‑xNx catalysts under visible light irradiation (420 nm < λ < 800 nm). 5767
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Figure 5. Photodegradation stability of MO over (a) the P/T-200 °C nanocomposites and (b) the bare TiO2-200 °C nanocomposites under visible light (420 nm < λ < 800 nm).
Figure 6. (a) Process of photocatalytic degradation of MO, (b) the comparison of kinetic constants over the P/T-200 °C, TiO2-200 °C, and TiO2‑xNx under visible light with different wavelengths (420 nm < λ < 800 nm, 550 nm < λ < 800 nm).
85%, which exceeds the performance of TiO2-200 °C and TiO2‑xNx largely under the same condition. It is no doubt that P/T-200 °C possess much stronger visible light absorption intensity even in the region of longer wavelengths compare with TiO2-200 °C. In this case, it is found that the degradation of MO also accords with pseudofirst-order kinetics. The corresponding kinetic constants (k) are calculated and displayed in Figure 6b and Table 1. Under the same experimental conditions, the apparent rate constant k of P/ T-200 °C is 0.321 h−1 under light irradiation (550 nm < λ < 800 nm), which is about 12.5 times and 46 times higher than that of TiO2-200 °C and TiO2‑xNx photocatalysts, respectively. Figure S8 illustrates the maximum absorption peak of MO is at λ = 464 nm and the absorption spectrum of visible light up to 575 nm, slightly wider than filter light wavelength 550 m, these suggestion that weak activity of TiO2‑xNx toward the degradation of MO at 550 nm < λ < 800 nm most likely due to adsorption. The photocatalytic activity of TiO2-200 °C is ascribed to carbon self-doping and visible photosensitive organic groups on the surface of TiO2-200 °C, which have been detected by IR.43 Therefore, these show that the degradation of P/T system toward organic pollutants is semiconductor photocatalytic not dye sensitization. In order to vividly depict the photocatalytic process, the optical photos of degradation of MO over P/T-200 °C, TiO2-200 °C, and TiO2‑xNx under the visible light with different wavelengths are exhibited in the Figure S7(b).
less than 45% after cycling runs. The unstable performance of TiO2 for degradation of MO can be ascribed to the blockage of photocatalytic active sites by the strong adsorption of dye intermediate.42 To further investigate its photodegradation stability, analogous the process of P/T-200 °C under UV light (254 nm) is shown in Figure S6b. Undoubtedly, in this case, the P/T-200 °C also exhibits excellent photocatalytic stability. Therefore, this illustrates that the presence of PANI for P/T photocatalyst not only enhances photocatalytic performance in degradation of organic dye pollutants for TiO2 but also exhibits excellent photocatalytic stability in the visible light range. The pictures of color change of recycled photodegradation are displayed in Figure S7 (a). As above-mentioned about the special light absorption property of PANI under visible light and mean that P/T nanocomposites may have dramatically photoactivity even in the longer wavelength of light, to clearly show excellent photocatalytic activity, process of degradation of MO over different photocatalysts under visible light with different wavelengths is shown in Figure 6a. It can be easily found that the degradation ratio of MO over the TiO2‑xNx significantly decreases from 40% at 420/800 nm to 3% at 550/800 nm system after 6 h of illumination. Analogously, over the TiO2200 °C, within 6 h after the light irradiation (550 nm < λ < 800 nm), MO is only degraded by 14% compare with 65% in the shorter wavelength (420 nm < λ < 800 nm). However, after the light is filtered by a 550/800 nm combined filters, the degradation ratio of MO over the P/T still reaches to around 5768
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indicating that considerable intermediate products are oxidized rapidly by P/T-200 °C, which possesses a higher mineralizing ability than TiO2-200 °C under UV and visible light. Regarding the photocatalytic performance under visible light with different wavelengths, it is necessary to start to study the influence factors in photoactivity. It is well-known that the adsorption capacities of organic pollutants have important influences on the photocatalytic performance. However, what is the relationship between the adsorption capacity and photocatalytic performance? For that reason, we discuss the interaction between them preliminarily. From the results displayed in Figure 8b, the adsorption capacity of MO on the surface of P/T-200 °C is better than TiO2-200 °C, which is consistent to photocatalytic activity as displayed in Figure 8a. The result represents that adsorption capacity can have an positive effect on photocatalytic performances, which is in accordance with the report of Xu et.al illustrating that the adsorbability influence photocatalytic degradation ability to some degree.44 Therefore, let us briefly summarize the above results about the P/T photocatalyst for liquid phase degradation of organic pollutants. The stable and higher photocatalytic performance over P/T for the degradation of MO and 4-CP under visible light can be primarily attributed to the three characteristic features: the first point is due to have more excellent adsorption capacity than bare TiO2 in the reaction process. Second, P/T nanocomposites possess much stronger visible light absorption intensity (400−800 nm) than bare TiO2 and PANI by itself has special light absorption properties. Another factor may be ascribed to the synergistic effect between PANI and TiO2 under the light irradiation. The detailed mechanism under visible light will be discussed in the following part. 3.3. Discussion of Photocatalytic Mechanism. Owing to PANI has charge-transfer excitation-like transition from the HOMO to the LUMO can lead to that itself excited photogenerated electrons transfer to the conduction band of TiO2 and it accepts the holes from the valence band of TiO2. Consequently, the lifetime of charge carrier over the photocatalyst is prolonged, which in tune with causes the formation of a large number of radical species with strong oxidation capability, such as hydroxyl radical and superoxide radical species in the degradation process. In order to further confirmed that, ESR spin-trapping technique with DMPO is carried out to detect the active species in this system. As shown in Figure 9a, under light irradiation (420 nm < λ < 800 nm), the stronger and more obvious peaks for DMPO-O2•− species
Moreover, 4-CP is chosen as another representative model pollutant to further evaluate photocatalytic performance of P/T-200 °C, as shown in Figure 7. After 6 h of illumination, no
Figure 7. Process of photocatalytic degradation of 4-CP (1.2 × 10−4 mol/L, 15 ppm) over the photocatalysts under visible light (550 nm < λ < 800 nm).
degradation of 4-CP is observed with P/T-200 °C nanocomposite in the dark or direct photolysis of 4-CP under visible light (550 nm < λ < 800 nm). The photocatalytic conversion ratio of 4-CP in the presence of bare TiO2-200 °C under visible light is only about 2%. However, even after 6 h of illumination with 550 nm < λ < 800 nm, approximately 30% of 4-CP is degraded when bare TiO2 is modified by PANI. To investigate the mineralization of the organic pollutants in the photocatalytic oxidation, the TOC removal efficiency is operated and the result is shown in Table 1. Within 125 min after UV (λ = 254 nm) irradiation, the mineralization rate of MO over P/T-200 °C is higher than bare TiO2-200 °C, which derives from the fact that synergic effect between PANI and TiO2 improves the photocatalytic activity. According to comparison, a similar phenomenon of total organic carbon (TOC) removal also expresses in vislble light regions (420 nm < λ < 800 nm, 550 nm < λ < 800 nm) toward MO. These results indicate that the photocatalytic performance of bare TiO2-200 °C fully in terms of decolorization is quite different from visible light degradation of MO over P/T which depends on the mineralization to some degree. As shown in Figure 7, the photocatalytic degradation rate of P/T toward 4-CP is 30% corresponding to the TOC removal reach to 13.2%, which is about 66 times that of TiO2. In a word, the TOC removal of 4-CP and MO over P/T-200 °C far exceed that of TiO2-200 °C,
Figure 8. (a) Photocatalytic performances of P/T-200 °C and TiO2-200 °C under visible light (420 nm < λ < 800 nm) and (b) adsorption capacity of MO on the surface of P/T-200 °C and TiO2-200 °C. 5769
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Figure 9. DMPO spin-trapping ESR spectra of P/T-200 °C and TiO2-200 °C in methanol dispersion for DMPO-O2•−(a) and (b) the comparison plot of temporal changes in •OH-trapping PL spectra of P/T-200 °C and TiO2-200 °C under visible light (420 nm < λ < 800 nm) for 6 h.
can be observed over the P/T-200 °C than that of TiO2200 °C. Unfortunately, we have not detect the presence of •OH over the P/T-200 °C and TiO2-200 °C by ESR technique. This may results from that the strong signal intensity of other species such as HO2•, e−aq make the signal of •OH may be covered45or the ESR technique is not very sensitive to detect the formation of the •OH radicals in this system.46 To further investigate the generation of •OH, the PL-TA technique is used in the detection. As shown in Figures S9 and 9b, after the light irradiation for 6 h, clearly, the PL intensity increases steadily with time, and the •OH radicals produced at the photocatalyst surface are proportional to the light irradiation time. Thus, the formation rate of O2•−and •OH radicals on P/T-200 °C is higher than that of bare TiO2-200 °C, which corresponds to the result that P/T possesses higher visible light photocatalytic activity than TiO2 toward the degradation of MO and 4-CP. Meanwhile, the faint peaks of TiO2 appear in ESR and PL spectra also side reflect that the electron/hole exist in the photocatalytic process under visible light. H2O2 as another important intermediate species in the photocatalysis process can occur mainly from reactions of superoxide anion radical. The DPD method employed for peroxide measurements is used for the detection of H2O2 that formed during the photodegradation of the organic pollutant.47 It indicates that no H2O2 is detected in the absence of photocatalysts as shown in Figure S10. Compared to TiO2 with faint peaks, TiO2 is modified by PANI possesses more obvious absorption peaks at 510 and 551 nm when DPD and POD are added in the system. Therefore, it not only verified the existence of O2•− species again, but also provided another inevitable source for •OH radicals. Moreover, photocurrent and electrochemical impedance spectroscopy (EIS) as a common electrochemical method have been widely used in evaluating the interface charge transfer efficiency and separation of photogenerated electron− hole pairs over the photocatalyst. In Figure S11a, when visible light source is turned on or off, the instantaneous photocurrent of pristine PANI and bare TiO2 are in a small degree. The photocurrent of PANI results from its high mobility of charge carriers under the irradiation of visible light. That of TiO2 is derived form carbon self-doping and visible photosensitive groups on the surface of TiO2 can excite to genenrate the electron/hole pairs under visible light, which is also corresponding to the weak photodegradation of organic pollutants, but much smaller than after P/T. Combined by the above discussion about ESR and PL, we can infer that a series of active
species over the bare TiO2-200 °C appear in the photocatalytic process and consequently affect the photocatalytic performance under visible light. Under light irradiation, the photocurrent of P/T-200 °C has dramatically increased, which is about 3.2 times compared with that of the TiO2-200 °C. This further confirmes that the synergistic effect between PANI and TiO2. As shown in Figure S11b, it is so clearly that the radius of the arc on the EIS Nynquist plot of P/T-200 °C is smaller than that of the TiO2-200 °C, which reflects that P/T-200 °C possesses the faster interfacial charge transfer. The results of photoelectrochemical tests are well correspond to that of photocatalytic experiments. According to the photocurrent graphs and EIS, it is indicated that the presence of PANI in the P/T-200 °C nanocomposites is capable of improving separation efficiency and effectively inhibit the electron−hole pair recombination. To further evaluate the role of these active species such as electrons/holes, •OH and O2•−, different types of active species scavengers are added in catalyst system. Figure 10
Figure 10. Photocatalytic degradation of MO over P/T-200 °C under different conditions with exposure to visible light (420 nm < λ < 800 nm).
shows the photocatalytic activity of P/T-200 °C toward the degradation of MO under the different conditions. Without the addition of the scavengers, the photocatalytic degradation rate of MO is 96% after 6 h of visible irradiation. Since the PANI itself is not only electron donors, but also the hole acceptors, a special kind of similar to the circulatory system between the PANI and TiO2 will form when P/T is exposured to light. After 0.1 g of ammonium oxalate (AO) as a hole-scavenger are added into the reaction system,48 the rate for degradation of MO over P/T is remarkably decreased. There are the following two 5770
dx.doi.org/10.1021/jp211222w | J. Phys. Chem. C 2012, 116, 5764−5772
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Scheme 1. Mechanism Diagram of Photocatalytic Reaction of MO and 4-CP over P/T-200°C under Visible Light
sum up, through the comparison, we can conclude that the MO oxidation is driven mainly by the participation of holes and O2•− radicals, and to a lesser extent by the contribution of •OH radicals. Therefore, the synergistic effect in the P/T system operate by PANI constantly donating electronic and accepting holes from TiO2, a series of active species such as O2•−, •OH, H2O2, successively generate in the photocatalytic process. The detailed degradation process of photocatalyst has been exhibited in the scheme 1.
reasons: one is that PANI as a hole acceptor will produce a series of active species on its surface under visible light irradiation. Another is the presence of high mobility of charge carriers in the circulatory system of P/T by constantly donating electronic and accepting holes. Once the AO is added into the system, the generation of active species and high mobility of charge carriers of the circulatory system are aborted, which seriously inhibits the photocatalytic degradation process. The benzoquinone (BQ) has the ability to trap O2•− by a simple electron transfer mechanism.49 The addition of BQ (1 mg) provokes partial inhibition of the MO degradation as shown in Figure10. A combination of the results of ESR, DPD, and the addition of BQ indicates that O2•− plays an important role in the photocatalytic process. Moreover, dissolved O2 as an efficient electron scavenger to produce a variety of active oxygen species significantly promotes the degradation process. In order to further evaluate the role of dissolved O2 in the reaction, N2 is bubbled through the suspension at the rate of 20 mL/min to ensure that the reaction is operated without O2. As displayed in Figure 10, after the light irradiation, although the final photocatalytic conversion ratio of MO still reach to 95%, whereas the absorption spectra are significantly changed and a new peak appears in 243 nm as shown in Figure S12a, which is attributed to a reduction product, hydrazine.50 This indicats that photoreduction of MO is well catalyzed by P/T possibly through electron and hole transfer, but the hydrazine does not continue to be degradated without O2. Namely, in the absence of O2, MO molecules are not completely degradated and the degradation process is actually a reduced phenomenon in the P/T system. In the air-equilibrated conditions shown in Figure S12b, both of the intensities of the two main absorption peaks are decreased. Therefore, the electrons or the active species generated by electrons play an important role in the photocatalytic reaction. After 2 mL of tert-butyl alcohol (TBA) as a scavenger for •OH is added in the system, it does not obviously affect the decomposition rate at all over P/T system. So, the MO oxidation is driven by the contribution of •OH radicals to a lesser extent. However, as mentioned in Figures S9 and 9b, we can still detect the signal of •OH on the surface of P/T by PL-TA method. To
4. CONCLUSION The P/T nanocomposites have been prepared via a new method of a simple hydrothermal and low-temperature treatment approach. Moreover, the stable and higher photocatalytic performance over P/T for the degradation of MO and 4-CP without the assistance of H2O2 under the irradiation both UV and visible light (420 nm < λ < 800 nm, 550 nm < λ < 800 nm) can be primarily attributed to two characteristic features associated with P/T composite material. The key point is a wide and strong absorption band of PANI in the visible region makes it easy to excite charge transfer from HOMO to LUMO and then offer an electron to the CB of TiO2 and itself accept a hole from VB of TiO2 leading to a restraining of the recombination of the electron−hole pair and finally promoting the migration efficiency of photogenerated electron−hole on the interface. The other factor is the increased adsorptivity of pollutants over photocatalyst and consequently affecting the photocatalytic performance. The experiments of TOC also reflect that PANI has a special role in the photocatalytic process. At last, the roles of active species in the photocatalytic process are compared by using different types of active species scavengers and the degradation mechanism process of photocatalysts toward organic pollutants is proposed. In conclusion, P/T nanocomposites possess the high potential ability for photodecomposition of organic contaminants under UV and visible light irradiation. 5771
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Additional data, experimental method descriptions, and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS This work was financially supported by the NNSF of China (21173047, 21073036, and 20873023), National Basic Research Program of China (973 Program, 2007CB613306), and the Science Foundation of Fujian, China (0330-033070, JA07001).
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