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Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation
Shengying Li,a) Mingkai Chen, Lijun He, Fei Xu, and Guohu Zhao
College of Chemistry and Environmental Science, Lanzhou City University, Lanzhou 730070, China (Received 21 November 2008; accepted 28 April 2009)

A simple and convenient method for preparing visible light response photocatalyst polypyrrole/TiO2 (PPy/TiO2) nanocomposite was developed. The products were characterized by x-ray diffraction, transmission electron microscopy, atomic force microscopy, ultraviolet-visible, and Fourier transform infrared techniques. The results indicated that the nanohybrid was composed of anatase TiO2 and PPy and exhibited an enhanced visible light-capturing ability. Average diameters of TiO2 and PPy/TiO2 were 18 and 35 nm, respectively. The photocatalytic activity of the nanocomposite was evaluated by the degradation of methyl orange under visible light irradiation. In the presence of PPy/TiO2 nanocomposite, the degradation efficiency of methyl orange of 95.54% could be obtained under visible light irradiation within 120 min. The apparent rate constant was 2.19 ? 10?2, which was better than that Degussa P25 nano-TiO2. The sensitization mechanism of PPy/TiO2 photocatalyst was discussed briefly.

It has been demonstrated that semiconducting materials capable of mediating photocatalytic oxidation of organic compounds can be an alternative to conventional methods for the removal of organic pollutants from water.1 Advantages of the photocatalytic process include its mild operating conditions and the fact that it can be powered by visible light, thus significantly reducing the operating costs. Due to the strongly oxidizability, nontoxicity, and long-term photostability, nano-TiO2 exhibits many advantages over the other photocatalysts, even bulk of TiO2.2 However, there are still some shortcomings, such as the lack of a visible light response, a low quantum yield, and lower photocatalytic activity. To overcome these problems, some strategies have been investigated, including noble metal deposition, doping of metal or nonmetal ions, blending with another metal oxide, surface photosensitizing with dye, and compositing with polymer.3 Polypyrrole (PPy) is an especially promising conductive polymer for commercial applications, due to its high conductivity, good environmental stability, and ease in synthesis. Its use as new material has opened up an entirely new field for polymeric material.4 Furthermore, PPy is one of the most familiar conducting polymers that show many advantages in recombining nanomaterials compared with others. The large internal interface area in PPy/inorganic nanocomposite enables an efficient separation of charge, which is

important for photovoltaic application. Therefore, intense research interests have been focused on the PPy/ inorganic nanocomposite.5 PPy/TiO2 nanocomposite combines the merits of PPy and nano-TiO2 to develop the potential applications in many fields. Some studies on the optical and electronic properties of PPy/TiO2 nanocomposite have been reported in recent years.6 To the best of our knowledge, however, the information on the use of PPy/TiO2 nanocomposite in the photocatalytic system is rather scarce. In the present article, a highly visible light response photocatalyst PPy/TiO2 nanocomposite was prepared via sol-gel and emulsion polymerization methods and characterized using the degradation of methyl orange as a probe reaction. The aim of this article is to modify the property of TiO2 by introducing PPy-conducting polymer and to improve the photocatalytic activity of TiO2 under visible light irradiation.

Address all correspondence to this author. e-mail: lisy1966@163.com DOI: 10.1557/JMR.2009.0316
J. Mater. Res., Vol. 24, No. 8, Aug 2009

Pyrrole monomer (Py; Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was doubly distilled under reduced pressure and then stored at 5  C in the absence of light prior to use. Methyl orange was of laboratory reagent grade. Tetrabutyl titanate [Ti(OBu)4], diethanolamine [DEA; NH(CH2CH2OH)2], sodium dodecylbenzene sulfonate [SDBS; CH3(CH2)11C6H4SO3Na], ammonium persulfate [(NH4)2S2O8], and other chemicals were of analytical grade and were used without further purification. Degussa P-25 Nano-TiO2 [(Degussa AG, Dusseldorf, Germany) a mixture of approximately 30% rutile and 70%
? 2009 Materials Research Society


S. Li et al.: Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation

anatase, average diameter was 30 nm, BET surface area was 50 m2/g]. Double-distilled water was used throughout the study.
B. Preparation of PPy/TiO2 nanocomposite

TiO2 nanopowder was prepared by the sol-gel method. A total of 8.5 mL of Ti(OBu)4 was dissolved in 34 mL of absolute ethanol with stirring for 5 min, then 2 mL of DEA was added to the above solution, stirring for one additional hour to get the mixture solution A. Mixture solution B containing 5 mL absolute ethanol and 0.5 mL double-distilled water was added dropwise into solution A within 30 min. The resulting mixture was hydrolyzed at room temperature for 1 h under stirring, and then a transparent light yellow sol was obtained, followed by aging in air at room temperature for 24 h. Then, the sample was dried at 80  C for 2 h. After cooling and grinding, the powder xerogel was obtained. The xerogel samples were calcined at 500  C in a muffle furnace for 1 h. Using an emulsion polymerization method, PPy/TiO2 nanocomposite was prepared. A total of 0.5 g TiO2 nanoparticles prepared from the previous step and 0.5 g SDBS were dispersed in 90 mL double-distilled water in a 250-mL, three-neck, round-bottomed flask fitted with ultrasonic vibration for 30 min, 0.5 mL pyrrole monomer was added to the above mixture, and ultrasonic vibration was continued for another 30 min. Under the protection with nitrogen gas, 20 mL 0.4 mol/L (NH4)2S2O8 was added dropwise into the above mixture as an oxidant in 10 min with a separatory funnel. The reaction system was cooled in an ice bath (0$5  C). The mixture was allowed to react for 12 h at 5  C under magnetic stirring, and it was then filtered. The precipitate was washed with water, acetone, and ethanol several times until the washings were found colorless. The solid was dipped in 1 mol/L HCl for 30 min for doping and finally vacuum oven dried 2 h under 60  C. A black PPy/TiO2 nanocomposite was obtained. Pure PPy was also prepared under the same reaction conditions, except for the presence of TiO2 for comparison.
C. Characterization of products

and cast on a carbon-coated copper grid. The AFM images of the products were obtained with a Seiko SPI 3800 scanning probe microscope (Chiba, Japan) in contact mode, using silicon nitride (Si3N4) cantilever and integral tips. The sample used for AFM observation was prepared by dispersing some products in absolute ethanol followed by the ultrasonic vibration for 30 min, placing a drop of dispersion onto a mica sheet, and then calcining the mica substrate at 650  C for 1.5 min in a muffle furnace. A UV-2550 ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu, Kyoto, Japan) with an integrating sphere (F = 6 cm) was used to directly record diffuse reflectance spectra of the products. Baseline correction was done using a calibrated sample of barium sulphate. The absorbance of methyl orange was detected using a 721B spectrophotometer (Shanghai Precision & Scientific Instrument, Shanghai, China). Infrared (IR) spectra were recorded on a Thermo Nicolet 5700 FT-IR Spectrometer (Madison, WI) from 4000 to 400 cm?1. Sample was mixed with KBr powders and pressed into a pellet. Spectra were corrected for the moisture and carbon dioxide in the optical path.
D. Photocatalytic experimental

The phase composition of the products was analyzed by x-ray powder diffraction (XRD), using a Rigaku (Tokyo, Japan) D/max-B x-ray diffraction (XRD) meter ? with Cu Ka radiation (l = 1.5418 A), tube current 40 mA, and voltage 100 kV. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) technologies were used to examine the morphology and microstructure of the products. The TEM images were taken on a JEM-100SX (JEOL, Tokyo, Japan) apparatus with a 100 kV accelerating voltage. The sample powder was dispersed in absolute ethanol

To evaluate the photocatalytic activity of the products, the degradation of the well known organic azo-dye methyl orange (MO), a typical pollutant in the textile industry, was investigated as a probe reaction under visible light irradiation. Another reason for the selection of methyl orange as model compound was due to the relatively high toxicity and complex structure, which both make it difficult to be treated by physical methods or biological methods. The visible light was obtained by a 100 W tungsten halogen lamp (Institute of Electric Light Source, Beijing, China) with 420 and 800 nm cutoff filters to ensure that irradiation of the methyl orange system occurred only by visible-light wavelengths. Aqueous suspensions of methyl orange (100 mL, 20 mg?L?1) were placed in a Pyrex glass beaker covered with a glass plate, and 0.1 g of photocatalysts was added. The irradiation distance between the lamp and the sample was 20 cm. The radial flux was measured by a power meter from the Institute of Electric Light Source, Beijing. The average light intensity was 44 mW?cm?2. Prior to irradiation, the suspensions were magnetically stirred in darkness for 30 min to establish adsorption-desorption equilibrium. During illumination, the reaction mixture was stirred with a magnetic stirrer to prevent settling of the photocatalyst. The system was cooled by wind and water to maintain the room temperature. At 10-min time intervals during the irradiation, a 2 mL solution was picked out and centrifugalized immediately at 3500 rpm for 10 min and then filtered to

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S. Li et al.: Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation

completely remove catalyst particles. The absorbance A at 470 nm for methyl orange was measured in a 721B spectrophotometer, and the A value was used to estimate the degradation rate D of methyl orange according to Eq. (1): D ? ?A0 ? At ?=A0 ? 100% ; ?1?

where A0 is the initial absorbance of methyl orange, t is the reaction time, and At is the absorbance at time t.

Photocatalytic activity of TiO2 depends on its crystalline structure.7 It is well known that TiO2 has three crystalline forms: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic). Among these crystalline forms, anatase is the mainly used phase for photocatalytic applications.8 Therefore, it is important to study the crystalline structure of the products. Figure 1 shows the XRD patterns of TiO2 nanopowder (a), pure PPy (b), and PPy/ TiO2 nanocomposite (c), respectively. Curve (a) in Fig. 1 indicates that the prepared TiO2 is entirely composed of anatase phase. The rutile and brookite phases of TiO2 are not present. Moreover, the intensity of the diffraction peaks is strong, which means that TiO2 has high crystallization degree. From the wide diffraction peaks, it can be deduced that the TiO2 particles are nanosized. The crystallite size of nano-TiO2 particle was calculated from the Debye–Scherrer equation9 [Eq. (2)]: b ? kl=Dcosy ; ?2? where l is the x-ray wave length, k is the shape factor, D is the average diameter of the crystals (in angstroms), y is the Bragg angle (in degrees), and b is the line-width measured at half-height and expressed in unit of 2y. The value of k depends on the Miller index of the reflecting planes and the shape of the crystals.10 If the shape is

unknown, k is often assigned as a value of 0.89. To analyze the x-ray diffraction pattern of the TiO2 nanopowders, we chose the reflection peaks at 2y = 25.3 to calculate the diameter of products. Thereby, the average diameters of the TiO2 nanopowders obtained is 18 nm. The XRD pattern of pure PPy [Fig. 1(b)] does not show sharp peak, suggesting not-ordered structure in the PPy-conducting polymer. In fact, it displays a diffuse broad peak ranging from 15 to 30 similar to one observed for electrochemically synthesized PPy by Ouyang and Li et al.,11 which can be interpreted due to the ordering in PPy chains at the interplanar spacing.12 PPy has been reported to be a 95% amorphous material, where the controlling of synthesis and types of dopant can make slight ordering. In Fig. 1(c), the positions of all peaks are similar to pure anatase TiO2, which indicates that the crystalline structure of TiO2 can be maintained by coating PPy-conducting polymer because the PPy is polymerized after forming the nano-TiO2 particles. Meanwhile, this gives evidence that PPy in the nanocomposite is also amorphous, and the crystallization of its molecular chains is impeded because of the confinement effect. However, the narrowing of peaks reflects a growth of particle size that indicates TiO2 has been coated with PPy. The average diameter of PPy/ TiO2 is 35 nm. Because of the existing hydrogen bonding action between the hydroxyl groups on the surface of TiO2 and the imine groups (–NH–) in the PPy molecular chains, we can deduce that the PPy in the composite deposits on the surface of TiO2 nanoparticles. An IR study is necessary to confirm further whether the PPy is bonded with the TiO2 nanoparticles or is just deposited on the surface of TiO2. Corresponding results will be seen in Sec. III. D.
B. Morphology analysis

FIG. 1. XRD patterns of the products: (a) TiO2, (b) PPy, (c) PPy/TiO2.

TEM and AFM images of the products are shown in Figs. 2 and 3, respectively. Figures 2(a) and 3(a) indicate that TiO2 nanoparticles are ball-like in shape with diameters of 15 and 20 nm, respectively, which are consistent with the results of XRD. However, TEM image indicates that the TiO2 nanoparticles are aggregated slightly. Figures 2(b) and 3(b) show that a well dispersed, narrow-size spherical nanoparticle was obtained, and the grain size is approximately 40 nm. The formation mechanism of PPy/TiO2 nanocomposite can be described as shown in Fig. 4. First, under the ultrasonic vibration, TiO2 particles will disperse equably into the aqueous solution containing emulsifier SDBS and to distribute in nanosized particles. TiO2 particles were then surrounded by emulsifier molecules to prevent their aggregation. When pyrrole monomer was added to the above mixture, polymerization reaction occurred on the surface of TiO2 nanoparticle, which can be strengthened because the

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S. Li et al.: Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation

FIG. 2. TEM photographs of the products: (a) TiO2, (b) PPy/TiO2.

existence of the hydrogen bonding action between the hydroxyl groups on the surface of TiO2 and imine groups (–NH–) in the PPy molecular chains. Then, when (NH4)2S2O8 was added to the above mixture, PPy resulting from polymerization was coated on the surface of TiO2 and formed PPy/TiO2 nanocomposite.
C. UV-vis spectral analysis

UV-vis absorption spectra of TiO2, PPy, and PPy/ TiO2 are shown in Fig. 5. It indicates that TiO2 powder mainly absorbs ultraviolet rays and the absorption edge is at approximately 370 nm [Fig. 5(a)], which shows a “blue shift” to shorter wave length compared with the bulk anatase TiO2 (385 nm).13 The blue shift is because of the size quantization, and it is observed when there is an increase in the band gap energy between the lowest unoccupied molecular orbital and the highest occupied molecular orbital in the semiconductors. Pure PPy [Fig. 5(b)] has weaker absorption for ultraviolet rays (l < 380 nm) but stronger absorption for visible light and near infrared ray (l = 380$900 nm) than that of

FIG. 3. AFM images of the products: (a) TiO2, (b) PPy/TiO2.

TiO2. The coating of PPy obviously affects the light absorption characteristics of TiO2 as shown in Fig. 5(c), a significant increase in absorption between 200 and 900 nm can be observed. The results indicate that PPy/TiO2 should be capable of responding to visible light. It is a vital prerequisite for visible light response photocatalyst. PPy/TiO2 has two absorption peaks at 335 and 600 nm, respectively. The first peak corresponds to the n-p* transition of PPy,14 and the second is assigned to the polaron absorption of the PPy.15

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S. Li et al.: Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation

FIG. 4. Formation process of PPy/TiO2 nanocomposites.

FIG. 6. FTIR spectra of the products: (a) TiO2, (b) PPy, (c) PPy/TiO2.

FIG. 5. UV-vis absorption spectra of (a) TiO2, (b) pure PPy, and (c) PPy/TiO2 nanocomposites.

D. Fourier transform infrared (FTIR) spectral analysis

Figure 6 shows the FTIR spectra of (a) TiO2 nanopowder, (b) pure PPy, and (c) PPy/TiO2 nanocomposite. Figure 6(b) indicates that characteristic absorption bands of PPy appear in the pure PPy, which are assigned as follows: the bands at 3400, 1450, and 1045 cm?1 have been attributed to N–H, C–N, and C–O–C stretching vibration, respectively,16 whereas the peak at 1548 cm?1 represents the stretching vibration of C=C in pyrrole ring. The peaks at 1652 and 1324 cm?1 correspond to C–N in-plane bending vibration and C–N bending vibration. The C–H in-plane bending vibration and C–H out-of-plane bending vibration are observed at 1175 and 910 cm?1, respectively. All of these bands are essentially the same as a conventional polypyrrole.17 In addition, the band at 790 cm?1 is assigned as the characteristic absorption band of five-membered heterocyclic compounds a-substitution,18 which indicates that PPy produced in this study has a a-position conjugation chain structure.

In addition, Fig. 6(c) reveals that the characteristic absorption bands of PPy and the maximum peak of TiO2 (571 cm?1) occur in the composite. However, the absorption peak at 3400 cm?1 disappeared, which belongs to stretching vibration of N–H indicating interaction of Ti with PPy backbone. Furthermore, all bands of PPy and TiO2 were found to be shifted to a lower wave number compared with pure PPy and TiO2. These results indicate that a strong interaction exists between the interface of PPy and nano-TiO2. That is, when the (NH4)2S2O8 was added to the reaction system, the polymerization proceeded on the surface of the TiO2 nanoparticle. It led to adhesion of PPy to the TiO2 nanoparticles. Because titanium is a transition metal and titanic has an intense tendency to form coordination compound with nitrogen atom in PPy molecule, such adhesion will not only constrain the motion of PPy chains but also restrict the modes of vibration in PPy molecule. Finally, the strong interaction causes the shifts of bands. Moreover, hydrogen bond between the hydroxyl groups on the surface of TiO2 nanoparticle and the imine group in the PPy molecular chain also contributed to the shift of bands. In summary, FTIR measurements show that PPy and TiO2 nanoparticles are not simply blended or mixed up, but a strong interaction exists at the interface of nano-TiO2 and PPy.
E. Photocatalytic activities

Control experiments were carried out on methyl orange solutions containing no TiO2 and PPy/TiO2 with the visible light irradiation, under otherwise identical experimental conditions, showed that in the absence of photocatalyst there was no change in absorbency with irradiation time. Results of the photocatalytic degradation efficiency of methyl orange are shown in Fig. 7. Because Degussa P-25 Nano-TiO2 (P25) is considered as an excellent photocatalyst, the photocatalytic behavior

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S. Li et al.: Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation

of P25 was also measured as a reference to compare with that of the prepared catalysts. It can be seen that within 60 min, only 40.26 and 31.25% of methyl orange were degraded by visible light in the presence of nano-TiO2 [Fig. 7(c)] prepared and P25 nano-TiO2 [Fig. 7(d)], respectively. The results indicate that there is nearly similar activity between the P25 and the proposed TiO2. If using PPy as photocatalyst, the degradation efficiency reaches up to 50.25% in the same conditions. It is noteworthy that the degradation efficiency reaches 40.35 and 75.42% after 10 min and 30 min by using PPy/TiO2 as photocatalyst under the visible light irradiation. Because methyl orange is an azo dye, and the conjugative system made up of azo group and aryl rings is the main chromophore of methyl orange, the decoloration of methyl orange was mainly caused by damage to the conjugative system,19 and it is difficult to destroy the N–N bond completely. However, after 120 min, degradation efficiency was up to 95.54%. This result suggests that PPy

plays an important role in the photocatalytic reaction. A conclusion may be drawn that coating TiO2 with PPy can remarkably enhance the photocatalytic activity of TiO2. The photocatalytic degradation of methyl orange is a pseudo-first-order reaction,20 and its kinetics may also be expressed as the following Eq. (3)21: ln?A0 =At ? ? kt ; ?3?

where k is the apparent rate constant, and a linear regression program can be used to calculate it. In this study, the value of k is 2.19 ? 10?2, which is even higher than that of P25 (k = 4.19 ? 10?3).22
F. Mechanism of the promoted photocatalysis

FIG. 7. Results of photocatalytic degradation of methyl orange solution (20 mg/L) under different conditions, (a) with PPy/TiO2 and visible light, (b) with PPy and visible light, (c) with TiO2 and visible light, (d) with P-25 TiO2 and visible light, and (e) without any catalyst and with visible light. c(catalyst) = 1 g/L.

The heterogeneous photocatalysis degradation of methyl orange by semiconductors has been widely investigated, and the photocatalytic mechanism of TiO2 has been reported.23 As a conducting polymer, previous experimental and theoretical works have shown that PPy has semiconductor capability and large band gap (Eg = 2.2 eV).24 This effect is similar to the TiO2 semiconductor. TiO2 particles can absorb UV light (l < 387 nm) to create mobile electrons (e?) and holes (h+) in the conduction band and valence band, respectively. If the electrons and holes cannot be captured in time, they will recombine with each other within a few nanoseconds, which will reduce the photocatalytic efficiency of TiO2. However, PPy/TiO2 nanocomposite reveals a different case, which is shown in Fig. 8. If PPy/TiO2 nanocomposite is irradiated by visible light, electrons in valence bands of PPy and TiO2 can be excitated to their conduction bands and leave holes at their surface. Because the oxidation potential of PPy is approximately ?1.15 V versus NHE,25 and the conduction band of TiO2 is approximately ?0.5 V versus NHE,26 the conduction band of PPy is higher than that of TiO2, so it is thermodynamically possible for electrons to be moved from the conduction band of PPy into the conduction band of TiO2, whereas holes in the valance band of TiO2 will inject into the valance band of

FIG. 8. The energy-level diagram for TiO2 and PPy and the charge transfer processes.
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S. Li et al.: Preparation and characterization of polypyrrole/TiO2 nanocomposite and its photocatalytic activity under visible light irradiation

PPy, and the efficient charge separation at the interface between TiO2 and PPy takes place [Eqs. (4)–(7)]: PPy ? hv ! PPy: ; ; ?4? ?5? ?6? ?7?

bination rate, increasing the quantum yield of the photocatalytic process and promoting the photocatalytic activity of TiO2.

PPy: ? TiO2 ! PPy? ?TiO2 ?e? ? TiO2 ? hv ! TiO2 ?e ?h ?
? ?


PPy ? TiO2 ?e? ?h? ? ! PPy? ?TiO2 ?e? ? :

More importantly, due to the existence of the interface between PPy and TiO2, separated electrons and holes have little possibility to recombine again. This result ensures higher charge separation efficiency and better photo-oxidation capacity for the nanocomposite. In succession, oxygen molecules adsorbed on the surface of PPy/TiO2 can capture electrons, producing O2?, O, and O? species [Eqs. (8)–(10)]. O2ads ?e? ! O2ads? O2ads ! 2Oads ; : ; ?8? ?9? ?10?

In summary, PPy/TiO2 nanocomposite exhibits higher photocatalytic activity than pure anatase TiO2, as well as P25 photocatalysts. The excellent property has been evaluated by the degradation of methyl orange under visible light irradiation. Meanwhile, TiO2 modified by PPy can widen the visible optical absorption region and increase the photocatalytic activity in the visible region. The different conduction band, valence band, and forbidden zone width of TiO2, PPy overlapped, and the separate rate of charges in the composite was enhanced. The proposed method may be used for the synthesis of the nanocomposite of PPy with various inorganic nanoparticles. Conducting polymer-sensitized TiO2 composite gives a potential and promising way to solve environmental purification.

Oads ?e? ! Oads?

At the same time, photogenerated holes can be trapped by hydroxyl ions or water adsorbed on the photocatalyst surface, producing hydroxyl radicals, ?OH [Eqs. (11) and (12)], which play important roles in photocatalytic reactions. OH? ?h? ! ?OH ; H2 Oads ?h? ! ?OH ? H? : ?11? ?12?

This work was supported by the Natural Science Foundation of Gansu Province (Grant 0710RJZA073), the Educational Department Foundation of Gansu Province (Grant 0611B-01), and the Scientific Research Foundation of Science & Technology Bureau of Lanzhou (Grant 08-1-12), People’s Republic of China.
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