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Article pubs.acs.org/JPCC

Hydrophobic Carbon-Doped TiO2/MCF‑F Composite as a High Performance Photocatalyst Dianyu Qi,† Mingyang Xing,† and Jinlong Zhang*,†,‡ †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: A novel hydrophobic photocatalyst carbon-doped TiO2/MCF-F was prepared by using silica mesoporous cellular foam (MCF) as host material, glucose as carbon source, and NH4F as hydrophobic modifying agent. It was confirmed that titania nanoparticles were loaded in pore of MCF by XRD, N2 sorption isotherms, and TEM. The loaded titania nanoparticles exhibited higher photocatalytic performance. UV−vis absorption spectra and XPS suggested carbon atoms were doped in the lattice of titania by replacing titanium atoms and narrowed the band gap so that visible light absorption and photocatalytic activity of the photocatalyst were highly promoted. On the other hand, water contact angle measurement and XPS proved that the photocatalyst was endowed with hydrophobic property, which was caused by Si−F bonds. Carbon-doped TiO2/MCFF photocatalyst showed good adsorptive ability and photocatalytic activity in the photodegradation test of methyl orange under visible light.

1. INTRODUCTION In recent years, environment concerns and energy crisis became the most urgent issues. Photocatalysis is a promising technology to solve the above problems because it could degrade pollutants or generate chemical energy at room temperature using only sunlight. TiO2 as one kind of nontoxic, stable, low cost, and highly efficient photocatalyst was applied in water splitting,1,2 solar cells,3,4 destruction of bacteria,5 and air and water purification.6−10 Although notable advances of TiO2 have been found over the past several decades, two challenges still remain: (i) to shift the absorption edge from UV to visible light and (ii) to suppress the recombination of photoelectrons and photoholes. In order to address the first challenge, many studies on doped TiO2 have been carried out. Transition metal doping is effective to improve the photocatalytic activity by decreasing the charge carrier recombination rates. Choi et al.11,12 studied systematically on the photocatalytic activity of TiO2 doped with 21 kinds of transition metals and found that doping with Fe3+, V4+, Re5+, Rh3+, and Os3+ significantly increased the photocatalytic activity. On the other hand, various non-metal-doped TiO2, such as N,13−16 S,17 C,18−21 F,22,23 and B,24,25 have been widely studied for the visible light photocatalytic activities. Among those ions, carbon attracted more attention because it could improve vastly the visible light response and photocatalytic activity. Sakthivel and Kisch21 prepared carbon-doped TiO2 using tetrabutylammonium hydroxide as the carbon source, and the photocatalyst showed efficient photocatalytic activity in the degradation of 4-chlorophenol no matter whether under artificial light or daylight irradiation. Zhang and coworkers19,25 synthesized La, C and B, C codoped TiO2 by the © 2014 American Chemical Society

hydrothermal method and found carbon dopants could not only develop the visible light response but also induce smaller TiO2 nanoparticles, which could lead to higher photocatalytic activity. In addition, dispersing TiO2 photocatalyst on a host material with large surface area is an effective way to solve the second challenge. By this means, the agglomeration of the photocatalyst can be avoided easily so that the particle size became smaller and the surface area became larger.26 This kind of composite was commonly reported for titania. Photogenerated charge migrated to the surface cost less time for small particles so that the recombination rates in bulk could be highly suppressed. Moreover, the large surface area could supply more active sites for further improving the photocatalytic activity. Therefore, mesoporous silica molecular sieve was an ideal choice for loading TiO2 because of its high specific surface area, modifiable framework, and stability. For example, Sun et al.27,28 reported loading TiO2 on Cr-modified molecular sieves (MCM-41, MCM-48, and SBA-15) could improve the visible light photocatalytic activity. Zhang and co-workers29 loaded TiO2 on Cr-KIT-6 and Cr-MCM-48, and the obtained photocatalyst showed high photocatalytic activity. Like other catalytic reactions, photocatalysis started with the adsorption of reactants on catalyst, and improvement in adsorption should enhance the photocatalytic activity.30 Because of the lipophilicity of pollutants, hydrophobic modification on photocatalyst could lead to more adsorption Received: December 18, 2013 Revised: March 16, 2014 Published: March 17, 2014 7329

dx.doi.org/10.1021/jp4123979 | J. Phys. Chem. C 2014, 118, 7329−7336

The Journal of Physical Chemistry C

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Scheme 1. Preparation Process of the Photocatalyst Carbon Doped TiO2/MCF-F (Insert Image Is the Photograph of the Photocatalyst)

of pollutants and higher activity. Kuwahara et al.31 used hydrophobically modified hexagonal mesoporous silica (HMS) as host material to load TiO2, and the obtained photocatalyst showed higher adsorptive ability and photocatalytic activity to phenol and 2-propanol. For the above reasons, a novel photocatalyst with visible light response and hydrophobic property was prepared in this study. The detailed synthesis process is shown in Scheme 1. Because of its large mesoporous, thick pore wall, and outstanding stability, siliceous mesoporous cellular foam (MCF) was chosen to be the host material for loading of TiO2. Prepared MCF was immersed in the solution containing titania precursors and glucose which was used as the carbon source. In the following hydrothermal process, carbon species were doped into the lattice of titania as the titania crystals grew. As a result, small size carbon doped titania was synthesized in the pore of MCF. Further hydrophobic modification was applied to develop the adsorptive ability of the photocatalyst by using ammonium fluoride as reagent, which has been mentioned in our previous work.32 Because hydroxyl groups on the surface were replaced by Si−F bonds, the photocatalyst was endowed with superhydrophobic property so that it could adsorb more pollutants. This hydrophobic carbon-doped TiO2/MCF-F showed efficient visible light photodegradation of methyl orange solution because of the synergistic effect of small crystal size, carbon dopants, and hydrophobic modification.

for 20 h under moderate stirring (600 rpm), and then the mixture was transferred into autoclave and kept at 373 K for 24 h. After that, the autoclave was cooled naturally in air, and the precipitate was filtered, washed repeatedly with deionized water, and dried in a vacuum at 333 K overnight. The obtained samples were calcined for 6 h at 823 K, after increasing the temperature to 823 K at 1 °C/min heating rate. The synthesized white powder was MCF. 2.3. Loading C-TiO2 in MCF. Various amounts of glucose were dissolved in 63 mL of double distilled water under magnetic stirring, and then 0.75 g of Ti(SO4)2 was dissolved in the solution; after that, 0.5 g MCF was added to the solution and stirred at room temperature for 1 h. The mixture was transferred into autoclave and kept at 393 K for 7 h. After being filtered and washed, the precipitate was dried in a vacuum at 333 K overnight. The products were marked Cx-TM, where x describes the amount of glucose (0, 0.2, 0.4, 1.5, and 6.0 g). As a comparison, the blank TiO2 without adding MCF and glucose was prepared by the same method. 2.4. Surface Hydrophobic Modification. 0.5 g of prepared Cx-TM samples was dispersed in 70 mL of isopropanol under magnetic stirring, and then 0.13 g of NH4F was added into the mixture. After stirring for 1 h, the suspension was transferred into an autoclave and kept at 393 K for 20 h. The precipitate was filtered, washed, and dried in a vacuum at 333 K overnight. The obtained hydrophobic photocatalysts were marked Cx-TM-F. As a contrast, the sample MCF was marked MCF-F after the same treatment. 2.5. Characterizations. X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/max 2550 VB/PC apparatus using Cu Kα radiation (λ = 0.154 06 nm). The transmission electron microscopy (HRTEM, JEM-2100) was used to observe the surface morphologies of MCF and dispersion of the titania particles. To analyze the light absorption of the photocatalysts, UV−vis diffuse reflectance spectra (DRS) were analyzed using a scan UV−vis spectrophotometer (Shimadzu UV-2450), while BaSO4 was used as a reference. Ti content in composite material was measured by ICP-AES (Varian 710ES). X-ray photoelectron spectroscopy (XPS) was analyzed on a PerkinElmer PHI 5000C ESCA system. The fine spectra of the samples were corrected

2. EXPERIMENTAL SECTION 2.1. Materials. Tetraethyl orthosilicate (TEOS or Si(OEt)4), titanium sulfate (Ti(SO4)2), hydrochloric acid, 1,3,5trimethylbenzene, ammonium fluoride, glucose, and isopropanol were purchased from Sinopharm Chemical Reagent Company, Ltd. P123 was purchased from Sigma-Aldrich (Shanghai) Trading Company, Ltd. 2.2. Preparation of MCF. First, MCF was prepared according to the reported method.33 In a typical synthesis, 8.0 g of P123 were dissolved in 300 mL of 1.6 M hydrochloric acid under vigorous stirring at 310 K for 1 h, and then 0.093 g of ammonium fluoride and 4.6 mL of 1, 3, 5-trimethylbenzene were added to the solution. After that, 18.3 mL of TEOS was added slowly to the mixture. The suspension was aged at 310 K 7330



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according to the C 1s level at 284.6 eV as an internal standard. The real carbon amounts were measured via thermogravimetric analysis. BET surface areas and pore size distribution curves were obtained by surface area and porosity analyzer (Micromeritics ASAP 2020), and all the samples were degassed in a vacuum at 573 K for 8 h before measurements. The water contact angle was measured by an attension Theta optical tensiometer; the sample powders were dispersed on a glass slide using isopropanol as solvent. Water adsorption isotherms were analyzed at 293 K in a conventional vacuum system. Before measurements, the samples were degassed at 353 K in a vacuum for 8 h to remove water. 2.6. Photocatalytic Performance Experiments. Visible light photocatalytic activities of the samples were estimated by the photocatalytic degradation of methyl orange (MO, 10 mg/ L) under visible light irradiation, using a 500 W halogen lamp as the visible light source. In photocatalytic experiment, 0.14 g of photocatalyst powder was added to 70 mL of MO solution in a quartz tube. The distance between light source and the quartz tube was 10 cm. In order to establish an adsorption/desorption equilibrium, the suspensions were stirred in the dark for 1 h prior to the irradiation. At given time intervals (1 h), 6 mL of suspension was taken out and centrifuged to remove the photocatalyst. The concentrations of the remaining MO solution were then analyzed quantitatively by the scan UV− vis spectrophotometer. UV light photocatalytic activity was evaluated by a similar method, but replaced the light resource with an ultraviolet high pressure mercury lamp.

Figure 2. X-ray diffraction patterns of various samples.

important structure parameters such as specific surface area and pore volume obtained from N2 adsorption−desorption isotherms are listed in Table 1. The curves of all samples showed type IV isotherms with significant hysteresis loops in the relative pressure range from 0.8 to 1.0, indicating that the characteristic MCF mesopores were obtained successfully, and the mesoporous structure still retained after loading TiO2 and further modification. After loading titania, the BET surface area decreased in parallel with the pore volume and average pore diameter. The phenomenon could be explained as the existence of titania in the pore of MCF. Moreover, it was obviously that the pore distribution curves and isotherms of C0-MCF and C0MCF-F were almost same, and the pore diameters hardly changed, indicating that the modification process could not affect the mesoporous structure. This could be explained as the ultrashortness of the Si−F bonds,31 which were generated by the fluorination of NH4F. 3.2. XRD Analysis. Figure 2 shows the wide-angle XRD patterns of TiO2 loaded in MCF with different carbon amount. All the samples showed a characteristic signal of anatase phase, and the peaks at 2θ values of 25.3°, 37.8°, 48.1°, 53.9°, 55.1°, 62.7°, 75.0° were attributed to the (101), (004), (200), (105), (211), (204), and (215) crystal planes of anatase, while the broad peaks between 15° and 25° were attributed to amorphous silica in MCF framework. The average crystallite sizes of blank TiO2 and C0-TM-F sample calculated using the Scherer equation were 21.6 and 10.3 nm, respectively, indicating MCF could induce smaller titania crystals. This could be interpreted as titania seed crystals generated in the pore of MCF, and the growth of titania crystals was limited by the pore wall. In our previous work, titania crystallite size could be reduced greatly by carbon dopants from 21.6 to 14.5 nm.19 However, the crystal size could not change with the addition amount of glucose in this experiment, indicating the small titania crystals could not be reduced further, and the crystal size was mainly determined by the pore size of MCF. 3.3. TEM Analysis. In order to further confirm that titania particles were well loaded in the mesoporous of MCF, a transmission electron microscope was applied to give a detailed view. It could be seen that the opening foamlike mesopores with 20 nm diameter were obtained (Figure 3a). Mesopores were retained after hydrothermal treatment in loading titania process, and it was obviously that the titania particles were loaded successfully in the pore of MCF with well dispersion (Figure 3b). Moreover, the energy dispersive spectrum in

3. RESULTS AND DISCUSSION 3.1. BET Analysis. Figure 1 shows the N2 sorption isotherms and pore size distribution curves of MCF, C0-TM,

Figure 1. N2 adsorption−desorption isotherms and pore size distribution (inset) of MCF (black), C0-TM (red), and C0-TM-F (blue).

Table 1. Characteristics of MCF, C0-TM, and C0-TM-F

a

sample

BET surf. area (m2/g)

pore vola (cm3/g)

av pore diama (nm)

MCF C0-TM C0-TM-F

549.0 378.4 331.9

2.2 1.4 1.4

18.9 17.8 17.5

Determined from BJH adsorption.

and C0-TM-F. The pore size distribution was calculated from the N2 adsorption isotherm by the BJH method. Several 7331



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Figure 3. Transmission electron microscope images of MCF (a), C0-TM (b), and high resolution transmission electron microscope image of C0-TM (c). The inset in (b) is the energy dispersive spectrum explored in the pink circled area.

Therefore, it could be confirmable that small titania particles were loaded and dispersed well in the mesoporous of MCF from above characterizations. Photogenerated charge transfer distances in small titania particles were much shorter so that the recombination rates in bulk could be highly suppressed. As evidence, the UV light photocatalytic activity of C0-TM was 5 times higher than that of the blank TiO2 sample (Figure S1). 3.4. C 1s XPS Analysis. In order to reveal the chemical states of the carbon dopants in prepared samples, the samples were investigated with XPS C 1s spectra (Figure 4). The C 1s signal of C0-TM-F contained a main peak at 284.6 eV, which was ascribed to adventitious simple substance carbon used in XPS. However, a broad peak which could be fitted by three contributions was observed in the signals of carbon-doped samples. The contribution at 286.4 eV was due to the C−O bonds,19,34 and the peak at 288.4 eV was associated with CO bonds.20 In addition, the peak at 281.2 eV corresponding to Ti−C bonds was not observed.35 These evidence showed carbon atoms tend to bond with lattice oxygen atoms instead of titanium atoms, indicating carbon atoms formed Ti−O−C bonds most likely by substituting titanium atoms rather than forming Ti−C bonds by replacing oxygen atoms. It is obviously that the peaks at 286.4 eV became higher with the addition of glucose (especially for C1.5-TM-F and C6.0-TM-F), indicating that more carbon species could be doped into titania with

Figure 4. C 1s fine X-ray photoelectron spectroscopy spectra of the samples.

Figure 3b confirmed that there was Ti element in the mesopores (Cu signals came from copper wire mesh used in TEM). The high resolution transmission electron microscopy image in Figure 3c shows clearly the titania crystals were surrounded by the amorphous silica, and the 0.35 nm lattice fringes correspond to the d spacing of (101) planes of anatase TiO2.

Figure 5. Diffuse reflection spectra of Cx-TM-F series (a) and Kubelka−Munk function versus the energy transformed from diffuse reflection spectra (b). 7332



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gap.36 Many works confirmed that the carbon could dope in matrix of titania by the hydrothermal method.19,35 Therefore, we assumed that carbon doped in titania would generated a doping level in the band gap. In order to estimate the location of carbon doping levels, band gap energies were calculated from a plot (αhν)1/2 versus photoenergy (hν) using the Kubelka− Munk function which showed the relationship of band gap Eg and absorption coefficient α. The equation was as follows: (αhν)1/2 ∝ hν − Eg

(1)

where α is the absorption coefficient, ν is the frequency of the light, and h is Planck’s constant. The Tauc plot, (αhν)1/2 versus hν, is shown in Figure 5b. The sample without carbon had a band gap energy of 3.05 eV, and the band gap energy decreased gradually to 2.95, 2.90, 2.75, and 2.3 eV with the increase of the carbon amount, indicating that more carbon dopants induced lower doping levels. From the XPS and optical properties results, it could be confirmed that carbon doped in the matrix of titania by replacing Ti atoms and induced a doping level under the conduction band. The narrowed band gap was sensitive to visible light so that electrons could be excited easily under visible light irradiation. 3.6. F 1s XPS Analysis. The fluorine atoms on the photocatalyst surface and their energy state were studied by XPS analysis. Figure 6 shows F 1s XPS spectra of C0-TM, C0TM-F, and MCF-F. Two peaks at 684.0 and 688.6 eV were observed in C0-TM-F but not in C0-TM. In addition, only one peak at 688.6 eV appeared in MCF-F. So it could be confirmed that the peak at 684.0 eV was ascribed to the Ti−F bonds, and the latter peak at 688.6 eV was attributed to the Si−F bonds, indicating that the F atoms were anchored on the surface of photocatalyst. Fluorine-modified titania attracted much attention in recent years. Many published reports considered that F atoms tend to exist on the TiO2 surface rather than doping into lattice by the solvothermal method,23,37 so F atoms tend to form Ti−F bonds on the surface in our preparation. Some researchers believe that the fluorinated surface was conducive to the generation of free OH radicals, which could improve photocatalytic activity.23 On the other hand, fluorine-modified silica has been also widely studied, and it has been accepted that Si−F bonds could generate hydrophobic properties.31 So it was proposed that Si−F bonds take responsibility to the hydrophobic property of this material. 3.7. Hydrophobic Behaviors of Photocatalyst. As shown in Figure 7, the unmodified photocatalyst settled at the bottom of beaker, and the modified sample could float over the water stably. In order to measure the hydrophobic property, water contact angle measurements were carried out. Obviously, the modification increased the water contact angle from 27.6° to 134.7°, indicating that the material was endowed with hydrophobic property by fluorination treatment. Water adsorption isotherms of the samples were also evaluated by water adsorption measurement (Figure S2), indicating hydrophobic modification could obviously decrease water adsorption capacity of the sample. As we mentioned above, this property was assigned to formation of Si−F bonds on the surface. It has been reported that hydrophobic photocatalysts tend to adsorb more pollutants so that photocatalytic degradation was promoted.31 3.8. Visible Light Photocatalytic Activity Test. The absorptive ability and photocatalytic activity of the samples are

Figure 6. F 1s fine X-ray photoelectron spectroscopy spectra of MCFF (a), C0-TM-F (b), and C0-TM (c).

addition of glucose. As a result, these carbon dopants would induce a doping level under the conduction band of titania so that visible light absorption could be improved. 3.5. UV−vis DRS Analysis. Figure 5a shows the UV−vis absorption spectra of samples with different carbon amounts. It was obviously that carbon dopants endowed titania with visible light response, which was enhanced further with the increase of the carbon amount. This phenomenon indicated that carbon species were responsible for the visible light response. It has been reported that anion doped in titania could increase visible light absorption by introducing localized states in the band 7333



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Figure 7. Water contact angle measurements of C0.4-TM (a), C0.4-TM-F (b), and photographs of C0.4-TM (c) and C0.4-TM-F (d).

Scheme 2. Photocatalytic Process of Carbon-Doped TiO2/ MCF-F

Figure 8. Adsorption and photodegradation rate of methyl orange solution under visible light irradiation.

shown in Figure 8. C0-TM sample hardly adsorbed dyes and showed the lowest photocatalytic activity. After hydrophobic modification, the photocatalyst could adsorb more dyes and the photocatalytic activity was improved as a result. In addition, the photocatalytic activity was further improved with the doping of carbon and C0.4-TM-F sample showed the highest photocatalytic activity. That was because carbon doped into the lattice of TiO2 and narrowed the band gap so that the photocatalysts could be sensitive to the visible light and generate photoelectrons and photoholes under visible light irradiation. However, excess carbon dopants were harmful to the visible light photocatalytic activity, and similar phenomena were also observed in other elements doping.13,16,19 Excess dopants could induce recombination centers, which could largely decrease the photocatalytic activity. It was notable that the absorption was improved when amount of glucose is 6.0 g. That was because excess amount carbon could not be entirely doped into the lattice of titania, but covered on the surface of photocatalyst as coke or graphite carbon, which might lead to

more dyes absorption.25,34 However, the coating carbon prevented light from irradiating on photocatalyst, resulting in a decrease of photocatalytic activity. 3.9. Mechanism of the Photocatalysis. A detailed photocatalytic mechanism is presented in Scheme 2. At first, photocatalyst was dispersed in methyl orange (MO) solution. Because of hydrophobic property of the photocatalyst, methyl orange molecules tent to concentrate on the surface of the photocatalyst. This property could also improve the adsorptive capacity for other pollutants. As a result, the concentration of 7334



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MO on photocatalyst surface was increased, and that could directly accelerate the reaction rate. As a contrast, pure TiO2 could not degrade pollutants under visible light irradiation because of its large band gap. Carbon dopants induced a doping level and narrowed the band gap of titania so that the photocatalyst could absorb visible light. Under visible light irradiation, electron in valence band was stimulated and transit to conduction band, leaving a hole in the valence band. Ti4+ could trap photoelectrons and formed Ti3+ so that the recombination of photocharges could be suppressed. Photocharges tend to migrate to the surface of photocatalyst. Because of the limit of host material MCF, further growth of titania crystals was suppressed and titania particles could be small. Small titania particles were in favor of the migration of charge carriers so that the recombination rate of them could be decreased largely. After migrating to the surface of the photocatalyst, photoelectrons reacted with dissolved oxygen and generated O2− radicals, while photoholes reacted with water and generated •OH radicals. Moreover, it was mentioned above that the fluorination process generated Ti−F bonds on the surface of titania, which were favorable to trap the photoinduce holes to generate free •OH radicals. Finally, the preconcentrated MO molecules were degraded into CO2 and H2O by these radicals.

ASSOCIATED CONTENT

S Supporting Information *

Ultraviolet light photocatalytic activity, carbon amounts of the samples, and water adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS In conclusion, carbon-doped TiO2/MCF-F photocatalysts were prepared and hydrophobically modified by the hydrothermal method. Titania particles were small and highly dispersed in MCF, and the photocatalyst exhibited absorption in visible light region. In addition, the photocatalyst showed outstanding hydrophobic property due to the fluorination of NH4F. Because of the synergistic effect of these three aspects, the photocatalyst showed efficient photodegradation of methyl orange under visible light irradiation. We hope our research can offer a useful source of reference on fabricating and designing hydrophobic photocatalysts for their applications in environment protection.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 86-021-64252062 (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Nature Science Foundation of China (21173077, 21377038 and 21237003, 21203062), the National Basic Research Program of China (973 Program, 2010CB732306, 2013CB632403), the Project of International Cooperation of the Ministry of Science and Technology of China (No. 2011DFA50530); Science and Technology Commission of Shanghai Municipality (12230705000, 12XD1402200), the Research Fund for the Doctoral Program of Higher Education (20120074130001), and the Fundamental Research Funds for the Central Universities. 7335



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