Environ. Sci. Technol. 2008, 42, 3803–3807
Dramatic Visible Photocatalytic Degradation Performances Due to Synergetic Effect of TiO2 with PANI HAO ZHANG,† RUILONG ZONG,† J I N C A I Z H A O , ‡ A N D Y O N G F A Z H U * ,† Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China and Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R. China
Received December 5, 2007. Revised manuscript received February 1, 2008. Accepted March 3, 2008.
The dramatic visible light photocatalytic activity was obtained for the degradation of Methylene Blue (MB) and Rhodamine B (RhB) under visible light irradiation (λ > 450 nm) after TiO2 photocatalystsweremodifiedwithmonolayerdispersedpolyaniline (PANI ) via a facile chemisorption approach. Under visible light irradiation, PANI generated π-π* transition, delivering the excited electrons into the conduction band of TiO2, and then the electrons transferred to an adsorbed electron acceptor to yield oxygenous radicals to degrade pollutants. Also, the ultraviolet photocatalytic performance was enhanced to about two times compared with that of P-25 TiO2 photocatalyst. The high photocatalytic activity came from the synergetic effect between PANI and TiO2, which promoted the migration efficiency of photogenerated carriers on the interface of PANI and TiO2. Under ultraviolet light irradiation, photoinduced holes in TiO2 valence band could transfer into HOMO orbital of PANI and then emigrate to the photocatalyst surface and oxidize the adsorbed contaminants directly. The optimum synergetic effect was found at a weight ratio of 3.0 wt % (PANI/ TiO2).
Introduction During recent decades, photocatalysis has attracted much attention as an emerging successful technology for purifying wastewater from industries and households (1-3). Recent research shows that TiO2-based heterogeneous photocatalytic oxidation technologies are still the most promising methods because of their outstanding oxidative power and stability (4-6) However, slow reaction rate and poor solar efficiency has hindered the commercialization of this technology (7, 8). To eliminate these drawbacks, many attempts have been carried out to modify surface or bulk properties of TiO2, such as doping (9, 10), codeposition of metals (11), and mixing of two semiconductors (12). Though the above modifications could partly improve the photocatalytic activity of TiO2, there still exist some key problems unresolved, for example, doped materials suffer from a thermal instability and an increase of carrier-recombination probability (9). So from the point of view to remedy the environment, it is very urgent to develop novel photocatalysts with high activity, visible-response, and high stability. * Corresponding author tel: +86-10-62783586; fax: +86-1062787601; e-mail:
[email protected]. † Tsinghua University. ‡ Chinese Academy of Sciences. 10.1021/es703037x CCC: $40.75
Published on Web 04/15/2008
2008 American Chemical Society
Recently, the properties of delocalized conjugated structures in electron-transfer processes have been widely studied to show they can efficiently arouse a rapid photoinduced charge separation and a relatively slow charge recombination (13). We have already carried out the research on photocatalysts (Bi2WO6) modified by materials with delocalized conjugated structures (C60). After modification by C60, the photocatalytic activities of Bi2WO6 were improved remarkably (14). Conducting polymers polyaniline (PANI) composed of benzenoid and quinonoid units with the delocalized conjugated structures has several redox states, which have extensive interesting properties (15). Since the photocatalytic activity of the photocatalyst can be promoted by increasing the separation efficiency of photoinduced electron-hole pairs, the combination of photocatalysts and PANI with the delocalized conjugated structures may be an ideal system to achieve an enhanced charge separation by photoinduced carrier transfer. Additionally, compared with fullerene C60, PANI is more valuable in practice for ease of commercial scale production. Recently, some studies have been published on the combination of PANI and TiO2 to improve their performance of solar energy transfer (16-18) and UV light or sunlight activity (19-22). Wang et al. prepared TiO2/PANI composites by in situ oxidative polymerization and found TiO2 sensitized by PANI had enhanced photocatalytic activity under natural light (22). Herein, monolayer dispersed polyaniline on TiO2 photocatalysts was synthesized by a facile chemsorption approach. Their visible photocatalytic activity (λ > 450 nm) was enhanced dramatically as well as the ultraviolet photocatalytic activity, both of which came from the promotion of charge separation efficiency caused by the synergy between PANI and TiO2.
Experimental Section Sample Preparation. Polyaniline (PANI, molecular weight ∼105) was purchased from Jilin Zhengji Corp (P.R. China), TiO2 (P-25, particle diameter 30 nm, surface area 50 m2 g-1) was obtained from Degussa Corp (Germany). All other reagents were analytical pure and used without further purification. The typical preparation of PANI/TiO2 photocatalysts was as follows: PANI was dissolved in tetrahydrofuran (THF) to obtain a concentration of 0.45 g L-1 solution, then an amount of TiO2 powder was added to 100 mL of the above solution, sonicated for 30 min, and stirred for 24 h. The suspension was filtered. The precipitate was washed by water three times and transferred to oven to dry at 80 °C for 24 h. According to this method, different mass ratios of PANI/ TiO2 photocatalysts from 0.5% to 7.5% were synthesized. ITO/ TiO2/PANI and ITO/TiO2 electrodes were prepared as follows: 0.20 mg of TiO2 and PANI/TiO2 (3.0%) were added in ethanol (20 mL) and sonicated for 30 min, then indium tin oxide (ITO) glass substrates (3 cm × 2 cm) with a sheet resistance of 15 Ω were dipped into the complex precursor for 3 min and then were pulled out with a velocity of 3 cm · min-1. Then the sample was dried at 100 °C for 12 h. Characterization. Diffuse reflectance spectroscopy was carried out on a Hitachi U-3010 instrument. BaSO4 was the reference sample. TG and DTA analyses were performed on a STA409 thermal analyzer. The atmosphere was N2 and the heating rate was 10 °C/ min. The Brunauer-Emmett-Teller (BET) surface area measurements were performed by a Micromeritics (ASAP2010 V5.02H) surface area analyzer. FTIR spectra were measured by using a Perkin-Elmer System 2000 infrared spectrometer with KBr as the reference sample. Raman spectra were recorded on a RM 2000 microscopic confocal Raman spectrometer (Renishaw Company) with an VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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excitation of 633 nm laser light. HRTEM images were obtained by a JEM 1200EX transmission electron microscope operated at an accelerating voltage of 100 kV and by a TecnaiTF20 transmission electron microscope operated at an accelerating voltage of 200 kV. The photoelectrochemical experiment was measured on an electrochemical system (CHI-660B, China). Photocatalytic Reactivity Test. The photocatalytic activities were evaluated by the decomposition of MB and RhB under visible light (λ >450 nm) and UV light (λ ) 254 nm). The visible light was obtained by a 500 W xenon lamp (Institute of Electric Light Source, Beijing) with a 450 nm cutoff filter to ensure the desired irradiation light. The radial flux was measured by a power meter from the Institute of Electric Light Source, Beijing. The average light intensity was 14 mW cm-2. Aqueous suspensions of MB or RhB (100 mL, 10 mg L-1) were placed in a vessel, and 50 mg of PANI/TiO2 photocatalysts was added. Prior to irradiation, the suspensions were magnetically stirred in dark for about 30 min. The suspensions were kept under constant air-equilibrated conditions before and during illumination. At certain time intervals, 2 mL aliquots were sampled and centrifuged to remove the particles. The filtrates were analyzed by recording variations of the maximum absorption band (633 nm for MB and 554 nm for RhB) using a Hitachi U-3010 UV-vis spectrophotometer. The UV light was obtained by an 11 W germicidal lamp (Institute of Electric Light Source, Beijing) and the average light intensity was 0.8 mW cm-2. The method was similar to the visible light test. The active species generated in the photocatalytic system could be measured through trapping by tert-butyl alcohol (tBuOH) and EDTA and ESR tests. Photoelectrochemical measurements were carried out in a conventional three-electrode, single-compartment glass cell, fitted with a synthesized quartz window, using a potentiostat. The quartz electrolytic cell was filled with 0.1 M Na2SO4 and 10-5 M MB solution (100 mL). The ITO/TiO2/ PANI or ITO/TiO2 electrodes served as the working electrode. The counter and the reference electrodes were platinum black wire and saturated calomel electrode (SCE), respectively. A 500 W xenon lamp and an 11 W germicidal lamp were used as the excitation light source for visible and ultraviolet irradiation, respectively.
Results and Discussion Photocatalytic Performances. The visible light photocatalytic performances of the PANI/TiO2 photocatalysts were evaluated by degradation of MB and RhB (Figure 1). Under visible light irradiation with λ > 450 nm, the absence of a catalyst and the mechanical mixture samples of PANI and TiO2 had no notable effect on degradation of MB and RhB, while the PANI/ TiO2 samples exhibited excellent visible light photocatalytic activity. The photocatalytic activity was enhanced gradually with increasing the proportion of PANI. When the ratio got to 3:100, the as-prepared photocatalyst had an optimal photocatalytic activity that could degrade MB by 88% in 5 h and RhB by 97% in 100 min. The photodegradation process was fit for pseudo-first-order kinetics, and the apparent rate constant k was 0.0071 min-1 and 0.023 min-1, respectively. However, further increasing the ratio of PANI, the degradation rate decreased gradually though it remained larger than that of TiO2 (P-25). The above results showed that the loading amount of PANI had a great influence on the photocatalytic activity of the as-prepared photocatalysts. It can be inferred that at the mass ratio of 3.0%, the as-prepared photocatalyst may have the fastest transferring of the photoinduced carriers and lowest recombination of electron-holes pairs. The UV light photocatalytic performances of the PANI/ TiO2 photocatalysts are described in Figure 2. It revealed that the absence of a catalyst and the mechanical blend samples of PANI and TiO2 had no notable effect on degrada3804
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FIGURE 1. Plots of degradation of (1) MB and (2) RhB over PANI/TiO2 photocatalysts under visible light irradiation with λ > 450 nm: (a) blank; (b) TiO2; (c) mechanical mixture of PANI and TiO2 (3:100); (d) PANI/TiO2 (1.0%); (e) PANI/TiO2 (5.0%); (f) PANI/TiO2 (2.0%); and (g) PANI/TiO2 (3.0%). tion of MB and RhB, while after modification by PANI the photocatalysts presented improved ultraviolet light activity than TiO2 (P-25). Moreover, at the ratio of 3:100, the PANI/ TiO2 samples exhibited the best value of photodegradation of MB by 99.6% in 60 min and RhB 96.5% in 30 min. The degradation process followed pseudo-first-order kinetics, and the apparent rate constant k was 0.091 min-1 and 0.099 min-1, respectively, which was about 2.0 and 1.7 times of that of TiO2 (P-25) photocatalyst. Photoelectrochemical Performances. The photogenerated charge separations process on PANI/TiO2 could be investigated through the electrochemical impedance spectroscopy (EIS) (see Supporting Information Figure S1). Figure S1 shows EIS response of ITO/TiO2 film and ITO/TiO2/PANI film under visible light (λ > 450 nm) irradiation and UV light irradiation (λ ) 254 nm). The radius of the arc on the EIS Nynquist plot reflects the reaction rate occurring at the surface of electrode. The arc radius on the EIS Nynquist plot of ITO/TiO2/PANI film was smaller than that of ITO/TiO2 both under visible and UV irradiation, which suggested that a more effective separation of photogenerated electronhole pairs and faster interfacial charge transfer occurred on the ITO/TiO2/PANI film (14, 23). These photoelectric characteristics proved that the combination of TiO2 and PANI was an effective way to improve photocatalytic efficiency. Stability of Photocatalysts. The PANI/TiO2 photocatalysts could maintain chemical stability after 9 days UV light illumination (light intensity 2 mW · cm-2) (see Supporting Information Figure S2). The visible light photocatalytic activity of the photocatalysts decreased a little and the apparent photodegradation rate constant k reduced from 0.0071 to 0.0069 min-1 with the photocatalytic activity decreased only by 2.5%. The UV light photocatalytic activity of the composite also decreased a little and the apparent photodegradation rate constant k reduced from 0.0 to 0.0
FIGURE 3. HRTEM image of PANI/TiO2 photocatalyst (3.0%) and its partial magnified image.
FIGURE 2. Plots of degradation of (1) MB and (2) RhB over PANI/TiO2 photocatalysts under UV light irradiation: (a) blank; (b) TiO2; (c) mechanical blend of PANI and TiO2 (3:100); (d) PANI/TiO2 (1.0%); (e) PANI/TiO2 (5.0%); (f) PANI/TiO2 (2.0%); and (g) PANI/TiO2 (3.0%). min-1 with the photocatalytic activity decreased only by 3.3%. So herein, the prepared photocatalysts are highly stable under the studied conditions and the PANI component could hardly be decomposed, which ensures secondary pollution is not caused. Structure of PANI/TiO2 Photocatalysts. It was wellknown that the photocatalytic activity was governed by various factors such as surface area, phase structure, interfacial charge transfer, and separation efficiency of photoinduced electrons and holes (24). XRD patterns of TiO2 showed no change before and after modification by PANI, indicating the absorption of PANI did not influence the lattice structure of TiO2. No XRD diffraction peaks assigned to PANI were observed because the PANI layer was too thin. There was no appreciable change in the surface area (BET surface area of TiO2 and PANI-modified TiO2 was 50.1 and 49.7 m2 · g-1, respectively). Since the surface area and phase structure of TiO2 remained almost the same before and after being modified by PANI, it could be inferred that the enhancement of photocatalytic activity may be attributed to the high separation efficiency of electron and hole pairs. The high separation efficiency of charge carriers is mainly related to the interface structure of PANI/TiO2 photocatalysts. DRS of PANI/TiO2 samples (see Supporting Information Figure S3) showed that the absorption intensity increased sharply when the ratio of PANI to TiO2 was no more than 3:100, while it increased much slower when the ratio was in excess of 3:100. This suggested that this ratio may be the threshold quantity to get the monolayer dispersed PANI on TiO2 surface (14). When the amount of PANI surpassed this threshold value, the superfluous PANI molecules tended to aggregate on the surface of TiO2. The typical HRTEM images of PANI/TiO2 photocatalysts (3.0%) are shown in Figure 3, revealing clearly that a PANI layer with noncrystal structure adsorbed evenly and stably on the TiO2 surface with the thickness of about 0.7-0.8 nm. Since the PANI molecule has
a fold-line frame structure and the diameter of benzene structure is about 0.5 nm, the PANI layer of the obtained product (3.0%) was approximately a monomolecular layer. The TG-DTA curves of TiO2 and PANI/TiO2 (3.0%) samples (see Supporting Information Figure S4) revealed that nanoTiO2 particles were very stable and almost no decomposition took place in the range of 100-800 °C. However, two weight loss regions appeared in the TG curve of PANI/TiO2 photocatalysts. The first weight loss of 2.0% occurring from 150 to 320 °C could be attributed to desorption of the solvent, and the second one of about 2.8% from 350 to 600 °C could be attributed to the thermal decomposition of the PANI. This was nearly consistent with the amount of monolayerdispersed PANI on TiO2 (3.0%). In the DTA curve of TiO2, the endothermic peak at 230 °C was attributed to the loss of solvent and another endothermic peak at 530 °C was attributed to the phase transform of TiO2 (25). In the DTA curve of PANI/TiO2 photocatalysts, the endothermic peak at 255 °C was due to the loss of solvent and the very wide endothermic peak from 350 to 800 °C was related to thermal decomposition and chemical desorption of PANI and phase transformation of TiO2. So, based on the results of HRTEM and TG-DTA, it is clear that when the ratio was equal to 3:100, the PANI was chemically adsorbed on surface of nanoTiO2 particles and formed monolayer dispersion. The novel monolayer-dispersed PANI structure of photocatalysts may cause the interface interaction between PANI and TiO2. In FT-IR (see Supporting Information Figure S5), it can be clearly seen that the main characteristic peaks of PANI and TiO2 all appear in the FT-IR spectra of PANI/TiO2 photocatalysts. However, compared with the main bands of pure PANI at 1560 cm-1 (CdN and CdC stretching mode), 1296 and 1240 cm-1 (C-N stretching mode) (25), all bands shifted to lower wavenumbers. And it was obvious the peak at 1240 cm -1 shifts to 1224, 1218, and 1218 cm-1 with the ratio of PANI to TiO2 from 1:100, 3:100, and 5:100, respectively. The red shift of the bands at 1560 cm-1 suggested that the bond strengths of CdN and CdC were weakened. Also in Raman spectra (see Supporting Information Figure S6), the peaks of pure PANI at 1596 cm-1 (benzenoid units vibration mode), 1408 cm-1 (quinonoid units vibration mode), and 1172 cm-1 (C-H bend mode) (25) all moved to lower wavenumbers for about 3-8 cm-1. According to the results of IR and Raman, the conjugated system of PANI was weakened, and there existed an intensive interaction between PANI and TiO2. This chemical bond interaction may be useful to transfer carriers and induce synergetic effect to enhance photocatalytic activity. VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. (a) Plots of photogenerated carriers trapping in the system of photodegradation of MB by PANI/TiO2 (3.0%) under visible light irradiation (λ > 450 nm); and (b) mechanism scheme of visible light photocatalysis process. Oxidative Species and Photocatalytic Mechanisms. The main oxidative species in the photocatalytic process could be detected through the trapping experiments of radicals and holes. As shown in Figure 4a, under visible light irradiation the photodegradation of MB was hardly inhibited by the addition of EDTA (holes scavenger) (26) while it was intensively suppressed when tBuOH (radicals scavenger) (26) was put in. This indicates that radicals were the main active species that could oxidize the adsorbed organic pollutants. To further confirm these results, the ESR/DMPO spintrapping experiments were carried out to detect the active species in this system under visible light irradiation (27, 28). Results (see Supporting Information Figure S7) revealed that no DMPOs · OH and DMPOs · O2- spin adducts had been detected before irradiation, while after irradiation the DMPOs · OH characteristic peaks (in aqueous solution) and DMPOs · O2- characteristic peaks (in methanol solution) were observed obviously. Moreover, the intensity of ESR signals increased with prolonged irradiation time. It could be inferred that the pollutants were eliminated mainly by means of oxygenous radical’s oxidation under visible light irradiation. The relative energy level of PANI (π-orbital and π*-orbital) and TiO2 (conduction band, CB, and valence band, VB) (21, 22) iss shown in Figure 4b. Based on the results of photocatalytic and photogenerated carriers trapping tests, the photocatalytic mechanics under visible light irradiation can be proposed as follows: PANI absorbs visible light to induce π-π* transition, transporting the excited-state electrons to the π*-orbital; The d-orbital (CB) of TiO2 and π*orbital of PANI match well in energy level and have chemical bond interaction, which can cause synergic effect; based on the synergic effect, the excited-state electrons could readily inject into the d-orbital (CB) of TiO2 and subsequently transfer to the surface to react with water and oxygen to yield hydroxyl and superoxide radicals, which would oxidize the organic pollutants. So herein, a rapid photogenerated charge separation and relatively slow charge recombination is achieved, which significantly enhances the photocatalytic activity of the as-prepared photocatalysts. 3806
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FIGURE 5. (a) Plots of photogenerated carriers trapping in the system of photodegradation of MB by PANI/TiO2 (3%) under UV light irradiation; and (b) mechanism scheme of UV photocatalysis process. For photocatalytic performance under ultraviolet light irradiation, the radicals and holes trapping experiments (Figure 5a) showed that under UV light irradiation the photodegradation of MB was scarcely restrained after adding tBuOH while it was obviously prevented after the injecting of EDTA. This suggested that photogenerated holes were the main active species in this system. The improvement of UV light photocatalytic activity is also mainly due to high efficiency of charge separation induced by the synergetic effect of PANI and TiO2. The energy level of the HOMO in PANI is between the CB and VB of TiO2 (Figure 5b), and these bands are tending to combine to produce synergetic effect. When TiO2 absorbs UV light to generate electron-hole pairs, the holes in VB can directly transfer to HOMO of PANI because the VB of TiO2 matches well with the HOMO of PANI (21, 22). Since PANI is a good material for transporting holes (17), the photogenerated holes can emigrate to the photocatalysts surface easily and oxidize the adsorbed contaminations directly (29). Consequently electron-hole pairs are separated effectively, and the photocatalytic ability is improved remarkably. In summary, based upon the synergetic effect between PANI and TiO2, a rapid charge separation and slow charge recombination came true and both the visible and ultraviolet photocatalytic activities of PANI-modified TiO2 photocatalysts were remarkably enhanced. The modified photocatalyst is a promising photocatalytic material with higher efficiency which has good potential applications for environmental purification.
Acknowledgments This work was partly supported by Chinese National Science Foundation (20433010, 20673065) and National Basic Research Program of China (2007CB613303).
Supporting Information Available EIS Nynquist plots, the stability experiment of photocatalysts, diffuse reflectance spectra, TG-DTA curves, FT-IR and Raman
spectra, ESR data. This material is available free of charge via the Internet at http://pubs.acs.org.
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