The Journal of
Physical Chemistry ~
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Q Copyright 1993 by the American Chemical Society
VOLUME 97, NUMBER 49, DECEMBER 9,1993
LETTERS Ti02 Aerogels for Photocatalytic Decontamination of Aquatic Environments Geula Dagan' and Micha Tomkiewicz Physics Department, Brooklyn College of CUNY, Brooklyn, New York 1 1 21 0 Received: March 4, 1993; In Final Form: October 12, 1993"
We describe synthesis and applications of nonsupported semiconducting porous structures that can be used as photocatalysts for photodegradation of organic pollutants in aqueous environments. The photocatalysts, Ti02 aerogels, are prepared through the sol-gel method and supercritical point drying. These materials have a porosity of 85% a BET surface area of 600 m2/g, and a bulk density of 0.5 g/cm3. X-ray diffraction show the anatase crystalline phase with a crystallite size of 50 A. The surface area, pore volume, and pore size distribution of these materials depend on the preparation conditions. The photoassisted oxidation of salicylic acid on these materials was compared to that obtained with commercial (P-25) Ti02 powder. The aerogel shows a much higher photocatalytic activity.
Introduction Illumination of wide-bandgap semiconductingmaterials (such as Ti02 and ZnO) produces highly,potent oxidants (holes) at the semiconductor surface that can be used for photocatalytic detoxificationof aquaticenvironments.' Photocatalyticproperties of semiconductors were investigated using supported and nonsupportedphotocatalysts. It was observed that the photochemical and photophysical properties can be affected by both the particle size and the support. Solid electrodes, powders, colloids, and suspensions were used as nonsupported catalysts.2-' Photocatalysis using Ti02 on a solid support such as glass,8-12 nafion membrane?*J3 clays,13 and metalized s ~ r f a c e s ~was ~ . ~reported. s Aerogels are extremely porous materials that are produced via sol-gel processing and supercritical drying." They are characterized by a low density, very high surface area, translucency or transparency tovisible light, low thermal conductivity,low sound velocity, arid complex microstructure.18-21Aerogels are being studied for a variety of applications, mostly as novel insulators but also as Cherenkov detectors, catalysts and catalyst supports, filters, membranes, and acoustic delay lines. Most of the work in this area is centered around Si02 aerogels. Aerogels of A1203, Ti02,Zr02, MgO, and mixed oxidesl6and borate-based aerogels2' have been prepared. *Abstract published in Aduonce ACS Abstracts, November 15, 1993.
Nonporous Ti02 electrodes have low surface area and high density. These electrodes can be used in closed systems equipped with external illumination in which the contaminated water is circulated.12 If solar illumination is to be utilized, solid electrodes can operate only in shallow water (for example, at the bottom of a container). It is highly desirable, particularly for applications such as oil-spill decontamination, to devise a floating semiconductor that can efficiently absorb the contaminant and photodecompose it to benign products. Recently, attempts along these lines were reported using supported electrodes-Ti02 coated on hollow glass beadslo The very high surface area and pore volume of aerogels make them attractive candidates in catalytic applications. As compared to other catalysts, aerogels were found to be much more active and selective for certain reactions. This was demonstrated by the photocatalytic oxidation of isobutane that was found to be higher on the aerogel than on other forms of anatase (xerogel and aerosol).16 Ti02 aerogels, which were prepared before, exhibit a relatively low porosity and low surface area, and very little is known about the catalytic activity of these materials. In addition, and perhaps equally important to these potential practical advantages, photocatalytically active semiconducting aerogels offer the opportunity to investigate the correlation between the morphology of the photocatalyst and the kinetics of
0022-3654/93/209712651$04.00/0 0 1993 American Chemical Society
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12652 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE k Reactant Compositions (Molar Ratio) Used, Gelation Time, and Features of the Gels Prepared in This study ne1 H2O/Ti A 2.0 B 3.0 C 4.0 D 3.0 E 3.8 ~
ROH/Ti 20(ethanol 20 (ethanol) 20 (ethanol) 20 (2-propanol) 20 (2-propanol)
gelling time 4 days 1 '/2 days 1 min 4 days 15 min
gel features clear, soft clear, strong opaque, strong cloudy, strong cloudy, strong
the photocatalytic reaction. It was already reported in the literature'aJ2 that with unsupported photocatalyst the kinetics is limited by the adsorption of the organic substrate while for supported photocatalyst the kinetics is diffusion-limited. It can be advantageous if such a comparison can be made on the same class of materials in which the morphology can be modified in a systematic way. Semiconducting aerogels provide perhaps the best opportunity for such a comparison. In this study we report the preparation of highly porous TiOz aerogels that can be used as efficient nonsupported photocatalysts. We have prepared a number of Ti02 materials by the sol-gel method, using different experimental procedures. Studies of the photoactivity of these materials show that the photocatalytic activity of these aerogels is much better than that of a commercial (P-25) Ti02 which is used as benchmark for degradation efficiency.lC.3.5~0 Experimental Section Preparation and Characterizationof Porous TiO2. Ti02 gels were prepared by acid-catalyzed sol-gel methods. The sol was prepared by mixing Ti(1V) isopropoxide (Alfa) with anhydrous ethanol (analytical grade, Aldrich), H20, and HNOs, at room temperature with stirring. A series of gels, with a constant molar ratio (of 1:0.08) between titanium(1V) isopropoxideand the acid, and different alcohol and water contents, were prepared. The compositions of the gels prepared for this study are shown in Table I. The sols were poured into plastic Petri dishes and maintained at room temperature until gelled. The gels were allowed to age in the mother liquor for at least twice the gelation time. Before drying, the gels were successively washed, at least four times for 24 h, with excess of fresh alcohol. The gels were kept in alcohol for periods of a month to several months. Monolithic Ti02 glass that was obtained by air-drying of the gels was heavy and sinks in water. To obtain highly porous materials, we explored other methods for drying the Ti02 gels. Among these were freeze-drying (to form cryogels) and critical point drying (to form aerogels). The physical properties and the photocatalytic activity of these porous materials were compared, under the same experimental conditions, with commercial powder from Degussa. For the freeze-drying, the samples were immersed in liquid nitrogen and dried using a Vitris automatic freeze dryer (Model 10410) for 5-20 h, dependingon thesample'ssize. Supercritical point drying was performed in a SAMDRI-790A (Tousimis) critical point dryer. In this drying method, the alcohol, which is contained in the gel, is replaced with liquid COz. The system is brought to conditions above the C02 critical point (>35 OC and >1200 psi) for a slow removal of the liquid-gas C02. Annealing, at temperatures of 400-500 OC for 1-3 h, was performed to remove residues of organic substances. This could be observed from the color changes during annealing. The materials turn black after few minutes of annealing at 200-400 OC and regain the whitecolor of titania after additional annealing for 1 h at 400-500 OC, indicating complete burning of the organic residues. The annealing procedure cause changes in the morphology. We characterized the annealed and nonannealed materials and compared their adsorption and photocatalytic activity profiles.
Surface area wasdeterminedby multipoint Brunauer-EmmettTeller (BET) measurement^.^^ Adsorption isotherms were obtained using a Gemini 2360 (Micrometrics) instrument. The samples were preheated for 2 hat 60 OC before the measurement. Total pore volume (VW)was obtained from the N2 adsorption isotherm at a partial pressure of 0.98. Bulk densities (db) for powders were obtained from the weight/ unit volume of the powder. For the aerogels, bulk densities were obtained from the weight and volume of monolithic pieces and by mercury displacement at ambient pressure. Bulk densities, obtained from the geometric volume, represent an average over several measurements, which agree within an error of 10% or less. Skeletal density (dJ were measured by helium pycnometry. Adsorptionand Photodegradationof Salicylic Acid on Ti02. In this study, salicylic acid was used as a prototype substance to study photodegradation on the Ti02 aerogels. Its use as a model molecule for chemisorptionand photodecompositionon other Ti02 surfaces6can serveas a reference for our materials. Stock solutions of 10 mM salicylic acid and a salt solution of 1.1 mM KCl (ionic conductivityof670pS) at pH 3.7 (adjustedbyHC1) wereprepared and used in all experiments. In each experiment, 100 mL of the salt solution was added to Ti02 (0.3 g) and left for 1 h to equilibrate. Salicylic acid was added, and the cell was kept overnight in the dark before the degradation experiment. The initial ratio between salicylic acid and Ti02 was 600 Mmol of salicylic acid for 1 g of Ti02. Adsorption of salicylic acid on Ti02 was studied under the same experimentalconditions (concentrations,pH, ionic strength, temperature) as the photodegradationstudies. These experiments were performed in the dark to avoid photooxidation. Photodegradation and adsorption profiles were obtained from the concentration of salicylic acid, measured on a Perkin-Elmer Lambda 3B spectrophotometer (from the decrease in the peak at 295 nm). The detection limit for salicylic acid was 3 pM. Photodecomposition experiments were performed in a Pyrex cell with a quartz window for illumination, cooled by water circulation (temperature 20 "C) and under stirring. The cell was irradiated by a 85-W Hg lamp. Irradiation intensity between 300 and 400 nm, as measured with an Eppley PSP radiometer and cutoff filters, was 8 W/m2. Oxygen was not deliberately added to the reaction vessel. Thus, oxygen used in the reaction is merely atmospheric oxygen dissolved in the solution. Photodegradation was quenched by filtration of the samples. The initial quantum efficiency (Q) for photodegradation was calculated from the ratio between the number of molecules of salicylic acid that disappear from the solution, during the first 30 min of illumination, to the number of photons that were absorbed by the Ti02 during the same time interval. The number of absorbed photons was calculated, for each sample, from measurements of the light intensity at the front and back sides of a cell that contains the specific sample. Results and Discussion Characterization of the Porous Ti02 Materials. Stable gels were obtained by using molar ratio of 2-4 between water and the Ti precursor. Themost stablegels, which yielded the most porous structure, were obtained by using a ratio of 1:20:3 for Ti:alcohol: H20 (sample B). It is also observed that, in general, longer gelation time are needed when a smaller water:Ti ratio is used. Shorter gelation time with increased water concentrationis usually related toa faster hydrolysisprocess. It isknown25that the balance between the rates of hydrolysis, condensation, and surface ionization determines the structure and homogeneity of the gel. BET surface area (&ET), total pore volume (VW),and skeletal (d,) and bulk densities (db) of these materials are shown in Table 11. The porosity of the aerogel was calculated using two methods:
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The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12653
TABLE II: BET Surface Area (Sam, mZ/g), Total Pore Volume ( Vp, cm3/g), Skeletal Density (4,g/cm3), and Bulk Density (4,g/cm3) of Commercial Powder (Degussa) and Porous Ti02 Prepared in This Study material SBET (m2/8) VTP(cm3/g) ds (s/cm3) d b (8/cm3) porosity (a) commercial (Degussa) 54 0.063 3.90 0.1c gel A A1 cryogel 237 0.275 2.51 0.7C A l a A1 annealed 65 0.075 3.92 1.oc A3 aerogel 373 1.345 2.67 0.7; 0.86d 78,' 74b A3a A3 annealed 188 0.639 0.8C gel B B1 aerogel 495 1.336 3.25 0.6; 0 . 6 9 81," 826 Bla B1 annealed 128 0.532 0.9' Blw B1 water washed 607 1.771 3.30 OSc 85,' 8Sb gel C C1 aerogel 465 1SO0 3.01 0.7: 0.74d 82,' 17b gel D D1 aerogel 465 1.876 3.89 0.7c 88," 82b Dlw D1 water washed 378 1.817 3.97 0.3c 88,' 91b gel E El aerogel 316 1.088 3.44 0.3c 79,' 91b Ela El annealed 136 1.092 3.65 0.8c 80,' 78b Calculated from V T and ~ skeletal density (eq 1). Calculated from bulk density and skeletal density (eq 2). From the geometric volume. d From mercury displacement. 1000 ~~~
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Pore Radius(A) Figure 1. Pore size distribution, calculated from the adsorption isotherm and plotted as dV,/d log R,vs R, ( V, = pore volume in cm3/g,R, = pore radius in A), for aerogel (sample A3,O) and cryogel (sample A I , 0). Insert: adsorption-desorption isotherms of nitrogen at 77 K on nonannealed aerogel (sample A3). from the skeletal density (ds)and VTPaccording to
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porosity = vTp/(vTp d;') (1) and from the skeletal density ( d J and bulkdensity (db)according to porosity = 1 - d,,/d, (2) Porosityvalues are shown in Table 11. The aerogelswere produced as monolithic pieces while cryogels were produced as large flakes powders. Porosity was not calculated for powders, as for such materials Vrp and db do not necessarily represent internal pores. Pore size distribution was calculated from the adsorptiondesorption isotherm according to the Pierce method.24 Pore size distribution for nonannealed aerogel (A3) and nonannealed cryogel (Al), as calculated from the adsorption part of the isotherm, are shown in Figure 1. Maxima at pore radii of 70 and 20 A were observed for the aerogel (A3) and the cryogel ( A l ) , respectively. The large surface area and relatively low VTPof the cryogel (Al) as compared to the aerogel (A3) stem from the existence of larger concentration of small pores in A l , which contribute much to the surface area but less to the pore volume. The insert in Figure 1 shows the adsorption-desorption isotherms of the aerogel (A3). TO study the dependence of morphology on the preparation conditions, we compared the pore size distribution of different
aerogels. Figure 2 shows pore diameter distribution, as obtained from the desorption part of the isotherms for samples A3, B1, and C1. Larger pores are obtained when the water concentration in the reaction mixture is higher (C > B > A). The strongest and most transparent aerogels were obtained from gel B (a ratio of 3: 1 between water and the Ti precursor). It is shown in Figure 2 that the aerogel, produced from this gel, has a narrow pore distribution and large pore volume. Figure 3 shows pore distribution of an aerogel (sample D1) and of an aerogel produced from a water-washed gel (sample Dlw). It is observed that the washing procedure results in narrower pore distribution with smaller pores. The N2 adsorption-desorptionisothermsare limited to analyses of pores of radius up to 500 A. However, the agreement between the porosity values, calculated by two different methods (cf. Table 11),indicatesthat the concentrationof macropores is low compared to the mesopores. This means that the total pore volume, calculated from the isotherms, represents basically all the pores contained in the material. X-ray powder diffraction shows that all the aerogels and cryogels have anatase crystalline structure, even before air-
Letters
12654 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
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600
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Time (min.)
Pore Radius ( A ) Figure 3. Pore size distribution,calculatedfrom the adsorption isotherms, for aerogel (samplesD1) and aerogel produced from washed gel (sample Dlw).
Figure 5. Photodegradationprofiles of salicylic acid on annealed (Ela) and nonannealed (El) Ti02 aerogelsas compared to a commercial powder (Degussa).
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Time ( h r s ) Figure 4. Adsorption profile of salicylic acid on annealed (sample Ela) and nonannealed (sampleEl) Ti02 aerogelsas compared to a commercial Ti02 powder (Degussa).
Fipre6. Quantumefficiency(%) us the amount ofsalicylicacid (pmol/l g of Ti02) adsorbed on the various Ti02 materials that are shown in Table 111.
TABLE 111: Amount of Salicylic Acid (pmol/g of TiOz) Adsorbed on the Various Ti02 Materials after 1 h and at Equilibrium and the Quantum Efficiency (Q) for Photodegradation of Salicylic Acid on Ti02 adsorption (&mol/g) material l h eauilib 0 (%) Degussa (powder) 42 43 8 A1 (cryogel) 23 114 16 A l a (cryogel) 11 75 8 aerogels B1 141 359 47 Blw 266 388 65
Photodegradation of Salicylic Acid on TiOz. Adsorption of salicylic acid is indicated by a bright yellow color on the titania surface (nocolor in thesolution). Thiscolor stemsfromformation of a complex on the Ti02surface that enhance electron charge transfer to the solution.6*26Under illumination, the yellow color changes gradually to dark brown, indicating oxidation of the organic material on the Ti02 surface. When oxidation of salicylic acid is completed, the white color of Ti02 is restored. Figure 5 shows photodegradation profiles for the aerogel before (sample E l ) and after (sample E l a ) annealing, as compared to the commercial (Degussa) powder. Figure 5 shows a much higher photocatalytic activity for the nonannealed aerogel. The numbers in the ordinate represent the solution concentration of salicylic acid, after illumination for the specific time interval and overnight equilibration. The quantum efficiencies for photodegradation of salicylic acid on these materials are shown in Table 11. High quantum efficiencies are observed for the nonannealed materials. Direct photodegradation of salicylic acid in the absence of Ti02 does not take place under these experimental conditions. This was checked by illumination of a solution of salicylic acid, under the same experimental conditions used for the photocatalytic experiments. No decrease in the concentration was observed even after 5 h. Figure 6 shows the correlation between the quantum efficiency and the amount of salicylic acid adsorbed on the photocatalyst for the samples that are shown in Table 111. The quantum efficiency was calculated taking into account the number of photons that were actually absorbed by the different samples. It is clear that under these experimental conditions, where the ratio of catalyst to pollutant is constant and no oxygen is added, the important factor for activity is the adsorption capacity of the
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annealing. Crystallite size, obtained from peak width, is 50 8, for the nonannealed aerogels and 70 A for the annealed ones. Adsorption of Salicylic Acid on Ti02. To be able to monitor the disappearance of salicylic acid from the solution, either by adsorption on Ti02 surface or by photodecomposition, we studied the adsorption profiles on the various Ti02 samples. The amounts of salicylic acid adsorbed on TiO2, after 1 h and at equilibrium (when no further increase in adsorption is observed), are shown in Table 111. The adsorption profiles of salicylic acid on Ti02 aerogel before (sample E l ) and after (sample E l a ) annealing at 400 OC for 1 h and on commercial (Degussa) powder are shown in Figure 4. It is clear from thesedata that adsorption on the aerogel is about an order of magnitude higher than on the commercial material. This correlates with the larger surface area.
Letters photocatalyst. Preliminary photodecomposition experiments of other organic contaminants that do not form a complex on the surfaceof Ti02 (such as phenol) also show a better photocatalytic activity for the aerogels as compared to commercial powders and other less porous structures (Dagan et al., to be published). Conclusions The synthesis of efficient, nonsupported, highly porous photocatalysts has a potential for solar-assisted decomposition of organic compounds. These materials have a large surface area and very good adsorption properties. The correlation between surface area and activity was shown. This, taken together with the bandgap and band edge position, makes the Ti02 aerogel suitable for many photocatalytic reactions. The sol-gel preparation method is advantageous as the desired morphology (and perhaps also electrical properties) of such devices can be tailored by modification of the preparation conditions. This opens new possibilities for formation of aerogels of various doped and nondoped wide-bandgap semiconducting materials that can be used for heterogeneous catalyses. Acknowledgment. This work was supported by the US. Department of Energy. We thank Ms. Zhu-Zhu for the X-ray measurements and Prof. C. Forest for facilitating the critical point drying. References and Notes (1) For example: (a) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Photocatalysis-Fundamentals andApplications; Wiley: New York, 1989;Chapter 18. (b) Kamat,P. V. In KineticsandCatalysisinMicroheterogeneousSystems; Gratzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991. (c) Bahnemann, D.; Bockelmann, D.; Goslich, R. Sol. Energy Mater. 1991, 24, 564. (2) Frank, S.N.; Bard, A. J. J . Phys. Chem. 1977,81, 1484. (3) Peterson, M. W.; Turner, J. A.; Nozik, A. J. J . Phys. Chem. 1991, 95, 222. (4) Palmans, R.; Frank, A. J. J . Phys. Chem. 1991, 95,9338. ( 5 ) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J . Photochem. Photobiol. 1989, 48, 161: Environ. Sci. Technol. 1988, 22, 798.
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12655 (6) (a) Tunesi, S.; Anderson, M. A. J. Phys. Chem. 1991,95,3399. (b) Tunesi, S.; Anderson, M. A. Chemosphere 1987, 16, 1447. (7) Minoura, H.; Katoh, Y.;Sugiura, T.; Ueno, Y.; Matsui, M.; Shibata, K. Chem. Phys. Lett. 1990, 173, 220. (8) Fox, M. A. Res. Chem. Intermed. 1991, 15, 153. (9) (a) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J . Phys. Chem. 1985.89.732. Ib) Ueno. A.: Kakuta. N.: Park. K. H.: Finlavson. M. F.: Bard. J.; Campion, A..;’Fox, M. A.; Webber, S. E.iWhite, J. M. J . Phys. Chem: 1985, 89, 3828. (10) Jackson, N. B.; Wang, C. M.; Luo, Z.; Schwitzgebel, J.; Ekerdt, J. G.;Brock, J. R.; Heller, A. J . Electrochem. SOC.1991, 138, 3660. (11) (a) Sakka, S.; Kamiya, K.; Yoko, T. In Inorganic and Organic Polymers; Zeldin, M., Wynne, K. J., Allcock, H. R., Eds.; ACS Symposium Series No. 360; American Chemical Society: Washington, DC, 1988. (b) Yoko, T.; Kamiya, K.; Sakka, S. Denki Kagaku 1986,54, 284. (12) Sabate, J.; Anderson, M. A.; Kikkawa, H.; Edwards, M.; Hill, Jr., C. G. J. Catal. 1991, 127, 167. (13) Fan, F. F.; Liu, H. Y.; Bard, A. J. J. Phys. Chem. 1985, 89, 4418. (14) Hetrick, R. E. Appl. Phys. Commun. 1985, 5 , 177; J. Appl. Phys. 1985, 58, 1397. (15) Vlachopoulos, N.; Liska, P.; Gratzel, M. J . Am. Chem. SOC.1988, 110, 1216. (16) (a) Teichner, S. J.; Nicolaon, G. A.; Vicarini, M. A.; Gardes, G. E. E. Ado. Colloid Interface Sci. 1976, 5, 245-273. (b) Teichner, S. J. In ref 16, pp 22-30. (17) Kisler, S. S. J. Phys. Chem. 1932, 36, 52. (18) Aerogels, Proceedings of the 1st International Symposium; Fricke, J., Ed.; Springer-Verlag: Berlin, 1986. (19) Gesser, H. D.; Goswami, P. C. Chem. Rev. 1989, 89, 765. (20) Better Ceramics Through Chemistry III; Brinker, C. J., Clark, D. E., Ulrich, D. R., Mater. Res. SOC.Symp. Proc. Vol. 121, Elsevier Science Publishing: Pittsburgh, PA, 1989; pp 678-716. (21) Ayen, R. J.; Jacobucci, P. A. Rev. Chem. Eng. 1988, 5 , 157. (22) Turchi, C. S.; Ollis, D. F. J . Phys. Chem. 1988,92,6852. Mattews, R. W. J. Phys. Chem. 1988, 92, 6853. (23) Brinker, C. J.; Ward, K. J.; Keefer, K. D.; Holupka, E.; Bray, P. J.; Pearson, R. K. In ref 13, p 57. (24) Gregg, S. J.; Sing, K. S . W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982; Chapter 3. (25) (a) Lofftus, K. D.; Sastry, K. V. S.; Hunt, A. J. In Aduanced Materials Proceedings; SME: Littleton, CO, 1990; Vol. 90, Chapter 26. (b) Hunt, A. J.; Lofftus, K. D. In ref 16. (26) Moser, J.; Punchihewa, S.;Infelta, P. P.; Gritzel, M. Langmuir 1991, 7 , 3012.
