Efficient Visible Light-Induced Photocatalytic Degradation of

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Efficient Visible Light-Induced Photocatalytic Degradation of Contaminant by Spindle-like PANI/BiVO4 Meng Shang, Wenzhong Wang,* Songmei Sun, Jia Ren, Lin Zhou, and Ling Zhang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China ReceiVed: July 17, 2009; ReVised Manuscript ReceiVed: October 8, 2009

Photocatalytic active spindle-like BiVO4 modified by polyaniline (PANI) was synthesized via a sonochemical approach for the first time. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images revealed that the photocatalysts are composed of well-crystallized small nanoparticles. The efficient photocatalytic activity in the degradation of a widely used dye, tetraethylated rhodamine (RhB), and phenol under visible-light irradiation (λ > 420 nm) was realized by the spindle-like PANI/BiVO4. Besides decoloring, reduction of the chemical oxygen demand (COD) concentration was also observed in the degradation of RhB, further demonstrating the photocatalytic performance of PANI/BiVO4. Additionally, the notable enhanced photocatalytic performance for phenol oxidation was also realized with the assistance of a little amount of H2O2. The enhanced photocatalytic activity could be attributed to the synergic effect between PANI and BiVO4, which promoted the migration efficiency of photogenerated electron-hole. According to experimental results, the possible photocatalytic mechanisms of the PANI/BiVO4 photocatalyst were proposed in order to guide the further improvement of its photocatalytic performance. 1. Introduction From the viewpoint of the environment issues, the direct use of solar energy such as clean and renewable energy through visible-light-driven photocatalysis for environmental pollution control has attracted great interest in recent years.1-8 It has been well-reported that the photocatalytic activity of the photocatalyst can be promoted by increasing the separation efficiency of photoinduced electron-hole pairs.9 On the other hand, materials with delocalized conjugated structures have been widely studied due to their rapid photoinduced charge separation and relatively slow charge recombination.10 In particular, polyaniline (PANI) as a conducting polymer with an extended π-conjugated electron system has recently showed great promises due to its high absorption coefficients in the visible-light range and high mobility of charge carriers.11 Furthermore, PANI in its undoped or partially doped states are electron donors upon photoexcitation, and it is known as a good hole conductor, which can carry current with several milliamperes.12 Moreover, compared with doped materials such as noble metal, PANI is more valuable in practice for ease of commercial-scale production. Considering the efficient carrier-transfer property of PANI, it is expected that the combination of photocatalyst and PANI seems to be ideal for enhancing the photoactivity under visible light. However, only a few studies have been published on the combination of PANI and simple metal oxide catalysts to improve their photocatalytic performance.13-17 Such study needs further research to extend its applications. Bismuth vanadate (BiVO4) has been recognized as a photocatalyst for water splitting18-21 and the destruction of dye pollutants with a high degree of mineralization22-28 under visible-light irradiation. Besides the decoloring of dye, the photocatalytic degradation of phenol on BiVO4 was also reported.29,30 As is well-known, phenol is a widely used organic * To whom correspondence should be addressed. Tel.: +86 21 5241 5295. Fax: +86 21 5241 3122. E-mail: [email protected].

chemical present in a variety of wastewaters from different industries, which is quite toxic and slowly degradable in the environment.31 It is also of high environmental concern to find a good photocatalyst for the phenol removal.32 Yet the activity of pure BiVO4 is low due to its poor adsorptive performance and difficult migration of electron-hole pairs.29 To enhance the photocatalytic activity and to investigate the mechanism of photodegradation, H2O2 as an efficient electron scavenger was added in the present paper. Additionally, it is worthwhile to note that there was no report on the synthesis and photoactivity in the PANI/BiVO4 system. Even more importantly, it is essential to start the systemic studies on the pathway and mechanism of photoinduced electron-hole pairs under visiblelight irradiation in order to design more efficient visible-lightdriven photocatalytic composite materials, which will meet the requirement of practical environmental application. Herein, for the first time spindle-like PANI/BiVO4 photocatalyst was prepared by a sonochemical method, and the work is mainly focused on the visible-light-induced photodegradation of widely used organic pollutants, tetraethylated rhodamine (RhB) and phenol. Reduction of the chemical oxygen demand (COD) concentration was observed in the degradation of RhB, further demonstrating the photocatalytic performance of PANI/ BiVO4. In addition, the photocatalytic performance for phenol degradation was also much enhanced with the assistance of a little amount of H2O2. The photodegraded mechanism with PANI/BiVO4 photocatalyst was discussed, and the enhanced photoactivity came from the promotion of charge separation efficiency caused by the synergy between PANI and BiVO4. 2. Experimental Section Polyaniline (molecular weight ∼ 105) was purchased from Jilin Zhengji Corp. All the other reagents used in our experiments were of analytical purity and were used as received from Shanghai Chemical Co.

10.1021/jp9067729 CCC: $40.75  2009 American Chemical Society Published on Web 10/28/2009

PANI/BiVO4 Photocatalyst Mechanisms The typical preparation of spindle-like BiVO4 was as follows: Bi(NO3)3 · 5H2O and NH4VO3 in molar ratio of 1:1 were mixed together in 100 mL solvent containing ethylene glycol (EG) and deionized water. Then the mixture was then stirred for 1 h at room temperature to get a solution. Afterward, the mixture was exposed to high-intensity ultrasonic irradiation (600 W, 20 kHz) at room temperature in ambient air for 2 h. The yellow precipitates were centrifuged, washed with deionized water and absolute ethanol, and then dried at 343 K for 10 h in air. The typical preparation of PANI/BiVO4 photocatalyst was as follows: PANI was dissolved in tetrahydrofuran (THF) to obtain a concentration of 0.081 g · L-1 solution; then a certain amount of BiVO4 powder was added into 100 mL of the above solution. The suspension was ultrasonicated for 30 min, stirred for 6 h, and then filtered. The as-produced precipitate was washed by water for three times and then dried at 343 K for 10 h. The PANI/BiVO4 photocatalysts with different mass ratios ranged from 0.05 to 3% were synthesized by this method. The X-ray diffraction (XRD) patterns of the samples were measured on a D/MAX 2250 V diffractometer (Rigaku) using monochromatized Cu KR (λ ) 0.15418 nm) radiation under 40 kV and 100 mA and scanning over the range of 10° e 2θ e 70°. The morphologies and microstructures of as-prepared samples were analyzed by scanning electron microscope (SEM; JEOL JSM-6700F) and transmission electron microscopy (TEM; JEOL JEM-2100F; accelerating voltage, 200 kV). UV-vis diffuse reflectance spectra of the samples were obtained on an UV-vis spectrophotometer (Hitachi U-3010) using BaSO4 as the reference. The photocatalytic activities of the samples were evaluated by the photocatalytic degradation of rhodamine-B (RhB) and phenol under visible light. A 500 W Xe lamp with a 420 nm cutoff filter was used as the light source to provide visiblelight irradiation. For the degradation of RhB, 0.1 g of photocatalyst was added into 100 mL of RhB solution (1 × 10-5 to 1 × 10-4 M). Before illumination, the solution was stirred for 60 min in the dark in order to reach adsorption-desorption equilibrium between the photocatalyst and RhB. At 30 min intervals, a 4 mL solution was sampled. Then the UV-visible absorption spectrum of the centrifugated solution was recorded using a Hitachi U-3010 UV-visible spectrophotometer. COD was estimated before and after the treatment using the K2Cr2O7 oxidation method. For the degradation of phenol, 0.1 g of the photocatalyst was added into 100 mL of phenol solution (20 mg · L-1). Before illumination, the solution was stirred for 120 min in the dark in order to reach the adsorption-desorption equilibrium between the photocatalyst and phenol. At every 30 min, a 4 mL suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the absorption spectrum of the centrifugated solution was recorded using a Hitachi U-3010 UV-vis spectrophotometer. 3. Results and Discussion 3.1. Characterizations of the PANI/BiVO4 Sample. Figure 1 shows the XRD pattern of the 0.5% PANI/BiVO4 photocatalyst. It was found that the photocatalyst was well-crystallized. All of the diffraction peaks can be well-indexed as pure monoclinic BiVO4 (JCPDS 14-0688), indicating the introduction of PANI did not change the lattice structure of BiVO4. Furthermore, no diffraction peaks assigned to PANI were observed, which also suggested that the PANI layer was very thin.14 Parts A and B of Figure 2 show typical SEM images of the as-prepared 0.5% PANI/BiVO4 photocatalyst. As shown in

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Figure 1. The XRD pattern of the 0.5% PANI/BiVO4 sample.

Figure 2A, the product exhibited a spindle-like morphology. A close-up view of the spindles (Figure 2B) demonstrated that the majority of the crystals possess a uniform spindle-like shape with center diameter of about 400 nm and length of about 800 nm. In addition, such spindle-like structure was also revealed by the TEM investigation, as shown in Figure 2C. It was explored that this spindle-like PANI/BiVO4 actually consisted of many smaller nanoparticles with the size of about 50 nm. The polycrystalline rings resulting from the aggregation of the nanoparticles with preferred orientation were obtained from the selected area electron diffraction (SAED), as shown in Figure 2C. The high-resolution TEM (HRTEM) image (Figure 2D) was recorded on the edge of the nanoparticle. The clear lattice fringe indicates the high-crystallinity and single-crystalline nature of the nanoparticles. The interplanar spacing is 0.307 nm, which corresponds to the (121) plane of monoclinic BiVO4. It has also been reported that small grain size and high crystallinity endow higher photocatalytic activity for the increased reactive sites and the promoted electron-hole separation efficiency.33,34 Thus, the as-prepared nano-PANI/BiVO4 was expected to show enhanced photocatalytic performance. Optical absorption of the as-prepared BiVO4 and 0.5% PANI/ BiVO4 samples were measured using an UV-visible spectrometer. As shown in Figure 3, the BiVO4 sample has photoabsorption from UV light to visible light, and the wavelength of the absorption edge is 525 nm. The absorption of the 0.5% PANI-modified BiVO4 sample increases over the whole range of the spectrum. On the basis of the equation ahν ) A(hν Eg)n/2,35 the band gaps of the samples were estimated to be 2.39 and 2.29 eV from the onset of the absorption edges, corresponding to the pure BiVO4 and 0.5% PANI/BiVO4 samples, respectively. It shows that the band gap energy of the 0.5% PANI/BiVO4 sample is lower than that of the pure BiVO4. Therefore, the PANI-modified BiVO4 sample can be excited to produce more electron-hole pairs under the same visible-light illumination, which could result in higher photocatalytic activity. 3.2. Visible-Light-Induced Photocatalytic Performance and Mechanism on Spindle-like PANI/BiVO4. 3.2.1. Photocatalytic Degradation of Dye. RhB was chosen as a representative hazardous dye to evaluate the photocatalytic performance of as-prepared PANI/BiVO4 first, which showed a major absorption band at 553 nm. The photodegradation efficiencies of RhB mediated by different mass ratios of PANI/BiVO4

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Figure 2. Spindle-like 0.5% PANI/BiVO4 sample: (A) low-magnification SEM image; (B) high-magnification SEM image; (C) TEM image (inset, SAED pattern); (D) HRTEM image.

photocatalyst showed the highest activity, which photodegraded 100% RhB after only 60 min under the same condition. To quantitatively understand the reaction kinetics of the RhB degradation in our experiments, the Langmuir-Hinshelwood model was applied as expressed by eq 1, which is wellestablished for the photocatalytic experiments when the pollutant is in the millimolar concentration range.36,37

( )

- ln

Figure 3. UV-vis diffuse reflectance spectra of the (a) BiVO4 sample and (b) 0.5% PANI/BiVO4 sample.

photocatalysts, as well as without photocatalysts (photolysis of RhB), under visible-light illumination (λ > 420 nm) with otherwise identical conditions were displayed in Figure 4A. First, it was demonstrated that the photolysis of RhB was extremely slow without a photocatalyst under visible-light illumination. The adsorption of RhB on the PANI/BiVO4 sample in the dark was also checked. After 120 min, the concentration of RhB decreased 4% only, suggesting the decolorizing of RhB by spindle-like PANI/BiVO4 is mainly caused by photodegradation but not adsorption. Moreover, only 62% RhB can be photodegraded by BiVO4 under visible light in 120 min. However, all the PANI/BiVO4 samples exhibited higher photocatalytic activities than pure BiVO4. Among them, the 0.5% PANI/BiVO4

C ) kt C0

(1)

where C0 and C are the concentrations of dye in solution at times 0 and t, respectively, and k is the apparent first-order rate constant. It can be seen clearly and directly from Figure 4B that the loading amount of PANI had a great influence on the photodegraded rate (k) of the as-prepared samples. The sample with 0.5% PANI exhibited the highest photodegraded efficiency, which was about 12-fold compared to that of the pure BiVO4 sample. To confirm the decolorization is really originated from the photocatalysis, the percentage change of COD which reflects the extent of degradation or mineralization of organic species was studied as a function of irradiation time in the photodegradation of RhB (10-4 M) under visible light, as shown in Figure 5. If the dye was not degraded completely, the residual colorless organic molecules could be oxidized by K2Cr2O7; thus, the oxygen demand could be more. In other words, the COD value would be higher than that of the completely mineralized dye.38 The initial COD concentration of the RhB solution is 226.7 mg · L-1, and the T% (measured at 500 nm) is 0.9%. After 4 h visible-light irradiation, the COD concentration decreased to 70.1 mg · L-1, and the T% at 500 nm reached 96%. The reduction of COD (70%) and the increase of the T% further confirm that

PANI/BiVO4 Photocatalyst Mechanisms

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Figure 4. (A) Photocatalytic degradation of RhB by PANI-modified BiVO4 and BiVO4 under visible-light irradiation (λ > 420 nm); (B) rate constant k as a function of PANI content.

Figure 5. Variation of COD and transmittance of RhB (10-4 M) aqueous solutions with irradiation time.

Figure 6. Cycling runs in the photocatalytic degradation of RhB in the presence of 0.5% PANI/BiVO4 under visible light.

RhB was truly photodegraded by the 0.5% PANI/BiVO4 photocatalyst. Besides the enhanced photocatalytic activity resulting from PANI, the photostability of the photocatalyst was also retained.15 The circulating runs in the photocatalytic degradation of RhB in the presence of 0.5% PANI/BiVO4 under visible light (λ > 420 nm) were checked (Figure 6). After five recycles for the photodegradation of RhB, the catalyst did not exhibit any significant loss of activity. It indicates that the PANI/BiVO4 photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of the model pollutant molecules. PANI is also cheaper than noble metals; thus, the PANI/BiVO4 photocatalyst is promising for practical application in water purification.

3.2.2. Photocatalytic Degradation of Phenol and the Effect of H2O2. In addition to the dye removal, phenol was also chosen as a representative model pollutant to evaluate the photocatalytic performance of the as-prepared PANI/BiVO4. Figure 7A shows the photodegradation efficiencies of phenol as a function of irradiation time with the photocatalyst of 0.5% PANI/BiVO4 and different amounts of H2O2 under visible-light illumination (λ > 420 nm). It has been reported that phenol is hardly adsorbed on the catalysts such as TiO2 or BiVO4 in aqueous solution;29 the decrease of phenol concentration in the presence of pure BiVO4 under visible light is thus very slow, only about 4% in 120 min. However, the performance had been improved when it was modified by PANI. About 24% of phenol was reduced in 120 min in the case of 0.5% PANI/BiVO4. Due to the synergic effect between PANI and BiVO4, the photocatalytic activity was enhanced, but the efficiency was still relatively low. In spite of this, the degradation rate was greatly promoted to 100% in 60 min when 0.25 mL of H2O2 was introduced into the photoreaction suspension. Moreover, it was also found that the system with 0.25 mL of H2O2 exhibited the highest activity, which was increased about 18-fold compared to that of the without-H2O2 system (Figure 7B). However, no degradation of phenol was observed with only H2O2 under visible light or with PANI/BiVO4/H2O2 system in the dark. 3.2.3. Photocatalytic Mechanism. The above experiments have shown the excellent photocatalytic performance of the asprepared spindle-like PANI/BiVO4 on the degradation of the widely used dye and phenol. It follows that the PANI modified BiVO4 photocatalyst may have highly potential applications in the conservation of the environment. Not only limited to the experimental results, the photodegraded mechanism of phenol in the visible light/H2O2/PANI/BiVO4 system was necessary to investigate and guide the further improvement of its photocatalytic performance. The possible photocatalytic mechanism (Scheme 1) was proposed as follows: hν

BiVO4 /PANI f h+ + e-

(2)

On the basis of the relative energy level of PANI (π-orbital and π*-orbital) and BiVO4 (conduction band, CB, and valence band, VB),13,29 which leads to synergic effect, the photogenerated holes in VB can directly transfer to the π-orbital of PANI. Simultaneously, the photogenerated electrons can transfer to the CB of BiVO4, which results in charge separation and stabilization, thus hindering the recombination process. PANI is a good material for transporting holes, and the grain size of the

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Figure 7. (A) Photocatalytic degradation of phenol with 0.5% PANI/BiVO4 under visible-light irradiation by different amounts of H2O2; (B) rate constant k as a function of H2O2 content.

SCHEME 1: Photocatalytic Mechanism for PANI/BiVO4

photocatalyst is also relatively small;12 therefore, the photogenerated charges can emigrate to the surface of photocatalysts easily and photodegrade the adsorbed contaminations. -

•

-

H2O2(added) + PANI/BiVO4(e ) f HO + OH

(3) phenol + HO• f degradation product

(4)

H2O2, as an efficient electron scavenger and a source of HO• with high oxidizing ability (eq 3), prevented the recombination of electron-hole and led to a faster degradation of phenol (eq 4) (Scheme 1). Therefore, the appropriate amount of additive H2O2 can enhance the photocatalytic degradation efficiency. However, when the amount of H2O2 was deficient, as shown in Figure 7A, the phenol was hardly photodegraded after 30 min due to the lack of electron scavenger, which means a lack of oxidizer. On the other hand, when more than optimum H2O2 is added, the photocatalytic activity also decreases. The excess H2O2 molecules scavenge the valuable HO• and generate a much weaker hyperoxyl radical, HO2•, which can further react with the remaining strong HO• to form ineffective oxygen and water, as shown in eq 5 and eq 6.39

H2O2 + HO• f HO2• + H2O

(5)

HO2• + HO• f H2O + O2

(6)

As a result, the photocatalytic activity was decreased when more H2O2 was added (Figure 7A). 4. Conclusions For the first time the spindle-like PANI/BiVO4 photocatalyst was prepared by a sonochemical method. On the basis of the

small grain size, the intrinsic property of PANI, and the synergic effect between PANI and BiVO4, a rapid electron-hole separation and slow recombination came true. As a result, both the photodegradation of RhB and phenol with PANI-modified BiVO4 photocatalysts under visible light (λ > 420 nm) were enhanced remarkably. In particular, the photocatalytic efficiency was also improved by the appropriate hydroxyl radical concentration generated by H2O2. Besides the enhanced photoactivity, the photostability of the photocatalyst was also retained. This work not only provides the mechanism of photocatalytic degradation but also opens new possibilities to provide some insight into the design of new modified photocatalysts with high activity for environmental purification and other applications. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 50672117 and 50732004), National Basic Research Program of China (973 Program, Grant No. 2007CB613302), and the Nanotechnology Programs of Science and Technology Commission of Shanghai (Grant No. 0852 nm00500). References and Notes (1) Liu, G. M.; Wu, T. X.; Zhao, J. C.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 1999, 3, 2081. (2) Cong, Y.; Zhang, J. L.; Chen, F.; Anpo, M. J. Phys. Chem. C 2007, 111, 6976. (3) Chen, C. C.; Zhao, W.; Li, J. Y.; Zhao, J. C.; Hidaka, H.; Serpone, N. EnViron. Sci. Technol. 2002, 36, 3604. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (6) Zhao, W.; Ma, W. H.; Chen, C. C.; Zhao, J. C.; Shuai, Z. G. J. Am. Chem. Soc. 2004, 126, 4782. (7) Kohtani, S.; Tomohiro, M.; Tokumura, K.; Nakagaki, R. Appl. Catal., B 2005, 58, 265. (8) Kohtani, S.; Koshiko, M.; Kudo, A.; Tokumura, K.; Ishigaki, Y.; Toriba, A.; Hayakawa, K.; Nakagaki, R. Appl. Catal., B 2003, 46, 573. (9) Wang, X.; Yu, J. C.; Ho, C.; Hou, Y.; Fu, X. Langmuir 2005, 21, 2552. (10) Zhu, S. B.; Xu, T. G.; Fu, H. B.; Zhu, Y. F. EnViron. Sci. Technol. 2007, 41, 6234. (11) Shaheen, S. E.; Brabec, C. J.; Padinger, F.; Fromherz, T.; Hummelen, J. C.; Sariciftci, N. S. Appl. Phys. Lett. 2001, 78, 841. (12) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. (13) Li, J.; Zhu, L. H.; Wu, Y. H.; Harima, Y.; Zhang, A. Q.; Tang, H. Q. Polymer 2006, 47, 7361. (14) Zhang, H.; Zong, R. L.; Zhao, J. C.; Zhu, Y. F. EnViron. Sci. Technol. 2008, 42, 3803. (15) Zhang, H.; Zong, R. L.; Zhu, Y. F. J. Phys. Chem. C 2009, 113, 4605. (16) Li, X. Y.; Wang, D. S.; Cheng, G. X.; Luo, Q. Z.; An, J.; Wang, Y. H. Appl. Catal., B 2008, 81, 267. (17) Gemeay, A. H.; El-Sharkawy, R. G.; Mansour, I. A.; Zaki, A. B. Appl. Catal., B 2008, 80, 106.

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