International Journal of Photoenergy

International Journal of Photoenergy / 2009 / Article

Research Article | Open Access

Volume 2009 |Article ID 962783 | 8 pages |

Nanosized TiO2 Photocatalyst Powder via Sol-Gel Method: Effect of Hydrolysis Degree on Powder Properties

Academic Editor: Terry Egerton
Received26 Sep 2008
Revised20 Jan 2009
Accepted08 Apr 2009
Published25 Jun 2009


Nanosized powder was synthesized via sol-gel method using titanium tetraisopoxide (TPT) as the precursor. Mol ratios of water to TPT were varied from 1 (Powder A), 2 (Powder B), 3 (Powder C), and 4 (Powder D) to evaluate effect of hydrolysis degree. TG/DTA curves showed that amorphous phase turned to anatase crystal structure at ca. 415, 337, 310, and C for Powders A, B, C, and D, respectively. XRD analysis showed that all the synthesized powders were 100% in anatase form with Powders B and C showing considerably higher crystallinities. The powders obtained at lower water to TPT mol ratios were spherical in shape and they became bar-like shapes higher mol ratios. The lower hydrolysis degree led to higher surface area of the Powder A (24.8  /g) compared to Powder B (14.6  /g). From phenol photocatalytic measurement, Powder B was the most efficient attributed to its higher crystallinity.

1. Introduction

The rapid growth of industries especially in developed countries has lead to the expansion of environmental problems. An awful amount of hazardous waste are continuously released from factories into the uncontaminated river or ground water. An appropriate and a cheap technique to purify the contaminated waste water needs to be developed. Some conventional methods used to purify polluted waste water from the industries are by [1] (1) adsorption on activated carbon technique and (2) oxidative treatment by chlorine. In oxidative treatment, using chlorine is inefficient due to the formation of other toxic products due to the oxidation process such as desethylatrazine which is a degradation product from pesticide type of pollutant known as atrazine [1]. Other methods to remove organic pollutants in the water reported by Kansal et al. [2] were by employing chemical and physical processes such as precipitation, air stripping, flocculation, reverse osmosis, and ultrafiltration [2, 3]. Still, these techniques are nondestructive because they only transfer the nonbiodegradable matter into sludge that will only lead to formation of a new type of pollutant [24]. Other than these two conventional methods; a chemical oxidation method such as addition of potassium permanganate has also considerable potential to act as an oxidant to control biological growth in treatment plants, odor and taste as well as to remove colour. However, potassium permanganate is a poor disinfectant. It is also not advisable to leave a residual of potassium permanganate in purified water after the treatment as it can cause the colour of the water to turn to pink [5]. Potassium permanganate should also be handled with full body protection as it can cause serious injury to the eyes and can generate skin and inhalation irritant if direct contact is occurred. Thus, utilization of semiconductor photocatalyst is a promising approach and has been widely studied for treating the waste water containing harmful chemicals over the past three decades [6, 7].

Photocatalyst has been extensively used for environmental applications because of its high oxidative power, nontoxicity, photostability, and water insolubility properties under most conditions [6, 810]. Photocatalysis process uses ambient oxygen from air and irradiation, basically UV light, to generate oxidation and reduction which can degrade almost all harmful organic and inorganic compounds to nontoxic substances. It is initiated when the amount of UV irradiation absorbs by transcends its band gap energy to induce the separation of electron from the valence band to the conduction band [11]. The electrons will react with the oxygen to produce superoxide anions and parallelly holes will react with water to produce the hydroxyl radicals to degrade and mineralize the organic compound [11]. with anatase crystal structure is more favorable than rutile as it is more active with band gap energy of 3.2 eV than rutile (3.0 eV) [1214]. Higher surface area of anatase than rutile is another factor affecting its higher photocatalytic activity efficiency.

Several parameters need to be controlled in order to obtain a highly efficient degradation of organic molecules utilizing photocatalyst. Habibi et al. [15] have developed an efficient way to achieve the optimized photocatalyst process. The most optimal photocatalysis operational parameters such as oxygen, temperature, catalyst loading, UV-light irradiation time, solution pH, and inorganic ions were investigated for the complete decolorization of dyes. Vosooghian and Habibi [16] explained that the photooxidation process of organic sulfides using photocatalyst was better in the presence of pure oxygen rather than in atmospheric oxygen or in the absence of oxygen. The study proved that a specific and controlled oxidative chemistry in organic substrates adsorbed on surface was needed to achieve higher photodegradation rate of the pollutants. In another work, Habibi et al. [17] have also managed to develop thin film for dyes degradation. Investigation on the effect of annealing temperature on the dyes degradation rate found that the annealing temperature of thin film and the substrate nature on which films were deposited played important roles in enhancing the photocatalytic degradation rate of dyes. However, is not the only potential photocatalyst to carry out photodegradation process. Indium tin oxide thin film was one of the promising photocatalysts to degrade textile dyes to replace if the degradation process was done at the optimum condition [18].

In this work we have developed powder photocatalyst via sol-gel method using titanium tetraisopropoxide (TPT) as the precursor. photocatalyst powders with water-to-TPT mol ratios of 1, 2, 3, and 4 were synthesized. The effects of hydrolysis degree on the formation of powders particles were physicochemically investigated. It is expected that the amount of added water will affect the monomer hydrolysis rate resulting in formation of powder particles with different size and shape. The sol-gel derived powders were then tested to evaluate their photocatalytic efficiency in degradation of aqueous phenol solution as a representative model for polluted water.

2. Experimental

2.1. Synthesis of Powder

powders with water-to-TPT mol ratios of 1 (Powder A), 2 (Powder B), 3 (Powder C), and 4 (Powder D) were synthesized via sol-gel method. The synthesis was done by hydrolysis of titanium (IV) isopropoxide (TPT) (Sigma-Aldrich) as a precursor in methanol (R&M chemicals). Both chemicals were used as received. TPT and methanol were mixed and heated until boiling point of methanol was reached. Then, distilled water was added dropwise and it was left for several minutes to let the hydrolysis-polymerization process take place. The obtained white solution was filtered to get the precipitate of gel powder and dried at 200 for 1 hour. The dried powder was calcined at 400 for 1 hour to obtain crystalline sol-gel powder.

2.2. Characterization of Powder

The gel powders were characterized by thermogravimetric and differential thermal analysis (TG/DTA) (Pyris Diamond, Perkin Elmer) to determine the weight loss and phase changing of the powder. X-ray Diffractometer machine (XRD 6000, Shimadzu) was utilized for phase analysis. Fourier Transform Infrared (FTIR) (Spectrum 100, Perkin Elmer) was used to identify the existing bonds before and after calcination of powder. Powder particle size was measured by Zetasizer (Nano S, Malvern Instruments), and surface morphology was examined by a field emission scanning electron microscopy (FESEM) (JSM 6700F, JEOL). Surface area analysis of the powders was measured by a surface area analyzer (BET-Autosorb-1, Quantachrome Instrument). The change in phenol solution concentration was measured using UV-Vis spectrometer (Lambda 35, Perkin Elmer).

2.3. Phenol Degradation Experiment

The sol-gel powder in suspension form was used to degrade aqueous phenol solution. powder (5 g) was added into 250 mL phenol solution of 50 mg/L initial concentration. Phenol photodegradation process was carried out in a reactor consisting of black light bulbs with the wavelength of 369 nm as the UV source with intensity of 2.6 mW/ . The suspension was magnetically stirred during photocatalytic test conducted for 5 hours. The sampling was done every one hour and the decrease in phenol solution concentration was measured by the UV-Vis spectrometer.

3. Results and Discussion

3.1. Photocatalyst Characterization

Figures 1, 2, 3, and 4 show the TG/DTA curves of Powders A, B, C, and D, respectively. From the TG curves, all powders show first weight loss at temperatures below 100 , indicating that methanol and water started to evaporate from the powders [1921]. At temperature of 100 to 300 , a weight loss of 14.4% for Powder A, 13.9% for Powder B, 22% for Powder C, and 19.7% for Powder D was recorded, which could be attributed to the removal of unhydrolyzed isopropoxide ligands bonded in the titanium. Meanwhile from the DTA curves, a sharp exothermic reaction peak appeared at ca. 414 for Powder A, ca. 337 for Powder B, ca. 310 for Powder C, and ca. 399 for Powder D indicating a phase change of the powder from amorphous to anatase phase. The powder crystallization temperature is summarized in Table 1. With its lower hydrolysis degree, Powder A apparently has smaller individual particle size, thus needing more heat to crystallize. Meanwhile, transformation from amorphous to anatase crystal structure for Powders B, C, and D occurred at lower temperatures (below 400 ) due to their large particles that needed less heat to crystallize. This finding can also be explained that when the mol ratio of water-to-TPT increased, crystallization temperature tends to be lower due to the formation of bigger particles. Figure 5 shows the surface morphology analysis by FESEM for Powders A, B, C, and D, respectively. The pictures showed that Powders A and B consist of spherical shaped particles, with particles B is larger than A. Less amount of water added slow down the polymerization process as the hydrolysis rate became less progressive. When the mol ratio of water-to-TPT increased to 3 and 4, hydrolysis and polymerization became more progressive which accelerated the phase formation rate. Rapid hydrolysis likely formed inhomogeneous nonspherical shape powder particles as shown by Powders C and D. Thus, the amount of water added during the synthesis process plays an important role in controlling the formation of particles.

Sol-gel Mol ratio of water-to-TPTCrystallization temperature ( )

Powder A1414
Powder B2337
Powder C3310
Powder D4399

It is important to note, a slower polymerization process and hydrolysis rate are required to produce smaller particle with more uniform size. It is preferable to have a uniform size of powder as it can control the degradation of organic molecule to happen at faster rate. With uniform particles size, it can increase the amount of pollutants molecules adsorbed on the catalyst’s surface and increase its photocatalytic activity. Meanwhile, smaller powder particles will provide shorter distance to be traveled by the electrons from the valence band to the catalytic site. Hence, more holes will be left at the valence band by the electrons that move to the conduction band. The conduction band electrons are the reduction centers that will react with oxygen while the valence band holes become the oxidation centers that will react with water to produce more hydroxyl radicals.

Figure 6 shows the FTIR spectra for methanol (a), titanium tetraisopropoxide (b), gel powder (c), powder after drying at 200 (d), and powder after calcination (e). In Figure 6(a), absorption peaks of methanol appear at 3314  which is attributed to O-H stretching band and at 1113  attributed to C-O stretching band. Figure 6(b) shows the FTIR spectra of titanium (IV) isopropoxide with absorbance peaks at 2942  and 987  belonged to aliphatic C-H stretching band and C-H alkene bending band, respectively. From Figure 6(c), it is clearly shown that the gel powder after filtration process resembled mixture of spectra of both chemicals. After drying at 200 , all the bands disappeared and only Ti-O bond’s weak stretching band at 394  remained. This peak was stronger after the powder was calcined at 400 showing a densified inorganic phase of . It is important to note that the inorganic bond of Ti-O does not always exist at low wavenumber as reported here. Wang et al. [22] reported that their submicrospheres showed Ti-O stretching mode at higher wavenumber of 568  .

From XRD analysis presented in Figure 7, all powders have the highest peak at in indicating that the powders are purely anatase structure. No other peak other than anatase was identified from the analysis. It is noticed that as the mol ratio of water-to-TPT increased, the powder particles increased both in size and crystallinity. The XRD patterns prove that Powder A has the smallest particle size with low crystallinity while Powders B and C have the highest crystallinity but still maintaining its purity. As for Powder D, it has an intermediate crystallinity between Powders A and B. Theoretically, an increase in powder’s crystallinity will lead to an increase in powder’s particle size. From the XRD pattern, it is assumed that the particle size of Powder A is the smallest as it has the lowest crystallinity compared to Powder B, C, and D. The powder particle size is confirmed by Scherrer’s equation calculation presented in Table 2.

Sol-gel Mol ratio of water-to-TPTDegree in Particle size (nm)

Powder A125.310.8
Powder B225.330.2
Powder C325.437.1
Powder D425.424.1

Comparing Powders A, B, and D for further evaluation on the effect of hydrolysis rate on the powders’ properties, it can be clearly seen that a small change in mol ratio of water-to-TPT affected the particles shape formed. Powder A that experienced slow hydrolysis rate has the smallest particles compared to Powders B and D due to less progressive hydrolysis. Meanwhile, Powder B has a higher crystallinity than Powders A and D while maintaining its purity as shown from XRD analysis. It should be noted that high crystallinity is also an important property for an efficient photodegradation. High crystallinity can produce more charge carriers to the catalytic site to degrade the organic molecules.

From the particle size analysis as shown in Figure 8, Powder A’s mean particle size was 118 nm with particle size distribution ranging from 50 to 300 nm. The particle size of Powder B was ranging from 80 to 400 nm with the mean particle size of 167 nm. Powder D particles size range was from 150 nm to 500 nm with mean particle size of 199 nm. It is worthy to note that the particle size measured from this analysis was quite big in size due to the possibility that powder’s agglomerates were measured instead of individual particles. In fact, most particles were dispersed into a primary size in aqueous suspension [23]. In addition, small particles tend to agglomerate due to its high surface energy to attract other particles to combine together and form large primary particles. Hence, it is presumed that particles are aggregated into agglomerates in this suspension.

BET surface area analysis shows that the sol-gel powders have Isotherm type II which was evaluated from the adsorption curves of Powders A and B shown in Figure 9. Type II isotherms are normal forms of isotherms obtained with a nonporous and macroporous adsorbent. The hysteresis loop formed by the adsorption-desorption curve indicates that both powders had type E hysteresis loop that can be attributed to “ink-bottle’’ pores. From BET surface area analysis, Powders A and Powder B have surface area of 24.8  and 14.6  , respectively. This BET result is consistent with the results from XRD, FESEM, and particle size analysis that Powder A has much smaller mean particles size than Powder B.

3.2. Photocatalytic Measurement

Photocatalytic degradation of phenol using Powders A, B, and D has been conducted under UV irradiation of 2.6  intensity. In this case, 5 g of powders were mixed with 250 mL of phenol solution with initial concentration of 50 mg/L. The suspension was stirred to obtain the maximum adsorption of organic pollutant molecules on the photocatalyst surface and then exposed to UV irradiation for 5 hours. The sampling was taken every one hour and filtered to remove the excess of powders to get a clear solution prior to UV-Vis spectrometer measurement.

The plots showing phenol degradation by these three powders are presented in Figure 10. It was found that Powder B showed the fastest degradation compared to Powders A and D. Powder B managed to degrade ca. 50% of the phenol initial concentration within the first 60 minutes. It can be observed that rate of phenol degradation by Powder A was very slow and almost constant for the whole 5 hours. About 77% of the initial concentration has been completely degraded by Powder B after 5 hours compared to only 26% by Powder A and 57% by Powder D. Some researchers claimed that smaller particle size of powder can lead to faster degradation of organic compound than bigger particles. However, Kanipus et al. [24] reported that degradation rate did not depend on specific surface area of . The influence of the crystalline structure is stronger for the transformations of inorganic anions whereby the rate constants can differ in 30 times. In this research Powder B showed the fastest degradation of phenol in solution due to its high crystallinity than Powder A and D. This is due to the Powder B’s optimum crystallinity and particle size that could lead to an efficient removal of phenol. This optimum crystallinity was responsible to form more charge carriers at the photocatalyst’s surface to enhance the photocatalytic efficiency. Thus, higher crystallinity of Powder B could compensate its large particles to perform faster photocatalytic process. The low efficiency of Powder A in photodegradation process of phenol molecules can be explained by its small particle size. The solution was clouded by small particles of Powder A and did not permit the maximum UV to penetrate into the solution. If the powder particles are too small, it can create “screening effect’’ in the solution that can limit the penetration of UV light into the solution and slow down the degradation rate [25]. This argument is also supported by Hosseini et al. [26] that the presence of and the radicals depend both on photocatalyst surface and solution composition. This research proves that powder with high crystallinity also shows high potential to degrade organic compound. Kinetic analysis for photocatalytic degradation of phenol under optimal conditions parameters still needs to be carried out for further investigation.

4. Conclusion

Sol-gel powders with the water-to-TPT mol ratios of 1 (Powder A), 2 (Powder B), 3 (Powder C), and 4 (Powder D) were synthesized and their physicochemical properties were evaluated. All powders are 100% in anatase structure. Powder A with the smallest individual particles needed more heat energy to become crystalline due to its smaller particle size than Powders B, C, and D. The powder started to form homogeneous spherical shape to inhomogeneous shape as the water-to-TPT mol ratio increased. It was proven that the amount of water added during synthesis process could affect the hydrolysis-polymerization rate and consequently the particle size and shape. Powder’s crystallinity became higher when the mol ratio of water-to-TPT was increased due to the acceleration of hydrolysis rate during the synthesis process. From the photocatalytic degradation of phenol, Powder B showed the fastest phenol degradation and higher efficiency than Powders A and D due to its high crystallinity that was capable of producing more charge carriers at the catalyst’s surface to degrade the adsorbed phenol molecules.


The authors acknowledge the Ministry of Higher Education of Malaysia (MOHE) for their financial support (Research no. FRGS 0106-22).


  1. O. Zahraa, L. Sauvanaud, G. Hamard, and M. Bouchy, “Kinetics of atrazine degradation by photocatalytic process in aqueous solution,” International Journal of Photoenergy, vol. 5, no. 2, pp. 87–93, 2003. View at: Publisher Site | Google Scholar
  2. S. K. Kansal, M. Singh, and D. Sud, “Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts,” Journal of Hazardous Materials, vol. 141, no. 3, pp. 581–590, 2007. View at: Publisher Site | Google Scholar
  3. M. Krichevskaya, A. Kachina, T. Malygina, S. Preis, and J. Kallas, “Photocatalytic oxidation of fuel oxygenated additives in aqueous solutions,” International Journal of Photoenergy, vol. 5, no. 2, pp. 81–86, 2003. View at: Publisher Site | Google Scholar
  4. V. Augugliaro, C. Baiocchi, A. B. Prevot et al., “Azo-dyes photocatalytic degradation in aqueous suspension of TiO2 under solar irradiation,” Chemosphere, vol. 49, no. 10, pp. 1223–1230, 2002. View at: Publisher Site | Google Scholar
  6. K. M. Parida, N. Sahu, N. R. Biswal, B. Naik, and A. C. Pradhan, “Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light,” Journal of Colloid and Interface Science, vol. 318, no. 2, pp. 231–237, 2008. View at: Publisher Site | Google Scholar
  7. T. Kudo, Y. Nakamura, A. Ruike, and A. Hasegawa, “Design and development of highly efficient rectangular column structured titanium oxide photocatalysts anchored onto silica sheets,” Topics in Catalysis, vol. 35, no. 3-4, pp. 225–229, 2005. View at: Publisher Site | Google Scholar
  8. M. N. Rashed and A. A. El-Amin, “Photocatalytic degradation of methyl orange in aqueous TiO2 under different solar irradiation sources,” International Journal of Physical Sciences, vol. 2, no. 3, pp. 73–81, 2007. View at: Google Scholar
  9. O. Prieto, J. Fermoso, and R. Irusta, “Photocatalytic degradation of toluene in air using a fluidized bed photoreactor,” International Journal of Photoenergy, vol. 2007, Article ID 32859, 8 pages, 2007. View at: Publisher Site | Google Scholar
  10. W. Su, J. Chen, L. Wu, X. Wang, X. Wang, and X. Fu, “Visible light photocatalysis on praseodymium(III)-nitrate-modified TiO2 prepared by an ultrasound method,” Applied Catalysis B, vol. 77, no. 3-4, pp. 264–271, 2008. View at: Publisher Site | Google Scholar
  11. F. Chen, J. Zhao, and H. Hidaka, “Highly selective deethylation of rhodamine B: adsorption and photooxidation pathways of the dye on the TiO2/SiO2 composite photocatalyst,” International Journal of Photoenergy, vol. 5, no. 4, pp. 209–217, 2003. View at: Publisher Site | Google Scholar
  12. H. Yang, K. Zhang, R. Shi, X. Li, X. Dong, and Y. Yu, “Sol-gel synthesis of TiO2 nanoparticles and photocatalytic degradation of methyl orange in aqueous TiO2 suspensions,” Journal of Alloys and Compounds, vol. 413, no. 1-2, pp. 302–306, 2006. View at: Publisher Site | Google Scholar
  13. H. Yin, Y. Wada, T. Kitamura et al., “Hydrothermal synthesis of nanosized anatase and ruffle TiO2 using amorphous phase TiO2,” Journal of Materials Chemistry, vol. 11, no. 6, pp. 1694–1703, 2001. View at: Publisher Site | Google Scholar
  14. M. Sakanoue, Y. Kinoshita, Y. Otsuka, and H. Imai, “Photocatalytic activities of rutile and anatase nanoparticles selectively prepared from an aqueous solution,” Journal of the Ceramic Society of Japan, vol. 115, no. 1348, pp. 821–825, 2007. View at: Google Scholar
  15. M. H. Habibi, A. Hassanzadeh, and S. Mahdavi, “The effect of operational parameters on the photocatalytic degradation of three textile azo dyes in aqueous TiO2 suspensions,” Journal of Photochemistry and Photobiology A, vol. 172, no. 1, pp. 89–96, 2005. View at: Publisher Site | Google Scholar
  16. H. Vosooghian and M. H. Habibi, “Photooxidation of some organic sulfides under UV light irradiation using titanium dioxide photocatalyst,” International Journal of Photoenergy, vol. 2007, Article ID 89759, 7 pages, 2007. View at: Publisher Site | Google Scholar
  17. M. H. Habibi, N. Talebian, and J.-H. Choi, “The effect of annealing on photocatalytic properties of nanostructured titanium dioxide thin films,” Dyes and Pigments, vol. 73, no. 1, pp. 103–110, 2007. View at: Publisher Site | Google Scholar
  18. M. H. Habibi and N. Talebian, “Photocatalytic degradation of an azo dye X6G in water: a comparative study using nanostructured indium tin oxide and titanium oxide thin films,” Dyes and Pigments, vol. 73, no. 2, pp. 186–194, 2007. View at: Publisher Site | Google Scholar
  19. C.-W. Lee and J.-S. Lee, “Formation of TiO2 hollow nanoparticles via gas phase synthesis using titanium oxide acetylacetonate as a precursor,” Journal of the Ceramic Society of Japan, vol. 114, no. 1335, pp. 923–928, 2006. View at: Google Scholar
  20. M. Inada, K. Kamada, N. Enomoto, and J. Hojo, “Microwave effect for synthesis of TiO2 particles by self-hydrolysis of TiOCl2,” Journal of the Ceramic Society of Japan, vol. 114, no. 1334, pp. 814–818, 2006. View at: Publisher Site | Google Scholar
  21. M. Zhang, J. Wang, and H. Fu, “Preparation and photocatalytic activity of nanocrystalline TiO2 with uniform shape and size,” Journal of Materials Processing Technology, vol. 199, no. 1, pp. 274–278, 2008. View at: Publisher Site | Google Scholar
  22. H. Wang, B. Li, Z. Yan, Z. Lu, R. Cheng , and D. Qian, “Fast synthesis of monodisperse TiO2 submicrospheres via a modified sol-gel approach,” Rare Metals, vol. 27, no. 1, pp. 1–4, 2008. View at: Publisher Site | Google Scholar
  23. J. Imasu and Y. Sakka, “Large-scale patterning of TiO2 nano powders using micro molds,” Journal of the Ceramic Society of Japan, vol. 115, no. 1347, pp. 697–700, 2007. View at: Google Scholar
  24. E. I. Kapinus, T. A. Khalyavka, V. V. Shimanovskaya, T. I. Viktorova, and V. V. Strelko, “Photocatalytic activity of spectro-pure titanium dioxide: effects of crystalline structure, specific surface area and sorption properties,” International Journal of Photoenergy, vol. 5, no. 3, pp. 159–166, 2003. View at: Publisher Site | Google Scholar
  25. S. K. Kansal, M. Singh, and D. Sud, “Comparative evaluation of UV/solar light induced photodegradation of phenol in aqueous solutions,” Indian Chemical Engineer, vol. 49, no. 1, pp. 11–20, 2007. View at: Google Scholar
  26. S. N. Hosseini, S. M. Borghei, M. Vossoughi, and N. Taghavinia, “Immobilization of TiO2 on perlite granules for photocatalytic degradation of phenol,” Applied Catalysis B, vol. 74, no. 1-2, pp. 53–62, 2007. View at: Publisher Site | Google Scholar

Copyright © 2009 Nor Hafizah and Iis Sopyan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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