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Improvement of Thermal Stability of Porous Titania Films Prepared by Electrostatic Sol-Spray Deposition (ESSD) Mikihiro Nomura,*,† Ben Meester,‡ Joop Schoonman,‡ Freek Kapteijn,† and Jacob A. Moulijn† Section of Reactor and Catalysis Engineering, and Laboratory for Inorganic Chemistry, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands Received August 5, 2002. Revised Manuscript Received October 23, 2002
A porous titania film with uniform pores of micrometer size was obtained on a dense stainless steel substrate by the electrostatic sol-spray deposition (ESSD) method. A porous film without cracks was obtained from the parent titania sol after more than 5 h of aging. This porous structure comprised an aggregation of fibrous titania. The pore size and the surface morphology were controlled by changing the aging period of the parent sol. The asdeposited films were shown to be amorphous titania by XRD analysis. They were calcined in order to obtain anatase or rutile titania. The structures of the porous films were damaged during the calcination at 873 K in air. Improved thermal stability of the porous titania films was obtained from a parent sol made by mixing fresh reactive titania sol and aged sol. Additionally, small particle structures were observed intergrown with the fibrous structures. The surface morphology of these films was not damaged by calcination at 873 and 1273 K in air. After calcination at 873 K, only the anatase phase was observed by the XRD measurement, whereas only the rutile phase was found after calcination at 1273 K. Porous anatase or rutile films can be produced using ESSD and proper calcination.
Introduction A composite material combining a porous inorganic film on a metal substrate can be used as a support for catalysts in heat exchange reactions or as an inorganic membrane support for a selective separation layer. Even a compact membrane reactor system for highly endothermal or exothermal reactions can be obtained. The morphology of such an inorganic film should be well controlled for different applications. The thermal conductivity of this type of composite is improved compared to that of an isolated inorganic oxide powder, because the thermal conductivity of a metal film in contact with a thin inorganic oxide film improves that of the composite. The thermal conductivity of a nonconductive inorganic oxide is usually low. This film support is suitable for the highly endothermal or exothermal reactions such as steam reforming or oxidation of hydrocarbons. Ioannides and Verykios1 reported the temperature profile control of the partial oxidation of methane by a combination of a hollow ceramic tube and a metal film. Groppi and Tronconi2 claimed the effect of metal support on the dissipation of the reaction heat. * To whom correspondence should be addressed at new location: Department of Advanced Nuclear Heat Technology, Japan Atomic Energy Research Institute, Oarai Research Establishment, Niibori 3607, Narita-cho, Oarai-machi, Higashi-Ibaraki-gun, Ibaraki 311-1394, Japan. Tel. +81-29-264-8324. Fax +81-29-264-8741. E-mail lscathy@ popsvr.tokai.jaeri.go.jp. † Section of Reactor and Catalysis Engineering. ‡ Laboratory for Inorganic Chemistry. (1) Ioannides, T.; Verykios, X. Catal. Today 1998, 46 (2-3), 71. (2) Groppi, G.; Tronconi, E. Chem. Eng. Sci. 2000, 55, 2161.
The thermal conductivity rate of rutile is 6.0-8.5 W m-1 K-1 and that of SUS 304 is 16.3 W m-1 K-1 at 400 K. On the other hand, the thermal expansion of an inorganic oxide film is usually different from that of a metal film. For example, the thermal expansion rates of tutile and SUS 304 are 7.0-8.1 × 10-6 K-1 and 17.3 × 10-6 K-1, respectively. Flexibility and thermal stability are required properties of an inorganic oxide film to keep the structures on metal substrate, and pore size of micrometer order is suitable as a catalyst support. In this study, we paid attention to titania as an inorganic material. Titania is chemically stable and used as a commercial catalyst support. A porous titania layer can be a good substrate for a molecular sieving membrane such as zeolite membrane. Gora et al. reported that highly selective, high flux zeolite (silicalite-1) membranes have been synthesized on a porous titania layer.3 Porous titania membranes have been prepared by sol-gel4-10 or chemical vapor deposition.11 These meth(3) Gora, L.; Nishiyama, N.; Jansen, J. C.; Kapteijn, F.; Teplyakov, V.; Maschmeyer, T. Sep. Purif. Technol. 2001, 22-23, 223. (4) Burggraaf, A. J.; Bouwmeester, H. J. M.; Boukamp, B. A.; Uhlhorn, R. J. R.; Zaspalis, V. T. In Science of Ceramic Interfaces; Nowony, J., Ed.; North-Holland: Amsterdam, The Netherlands, 1991. (5) Larbot, A.; Fabre, J. P.; Guizard, C.; Cot, L. J. Am. Ceram. Soc. 1989, 72, 257. (6) Kumar, K. P.; Zaspalis, V. T.; Keizer, K.; Burggraaf, A. J. J. Noncrystal. Solids 1992, 147-148, 375. (7) Xu, Q.; Anderson, M. A. J. Am. Ceram. Soc. 1993, 76 (8), 2093. (8) Negishi, N.; Takeuchi, K.; Ibusuki, T. Appl. Surf. Sci. 1997, 121/ 122, 417. (9) Kajiwara, K.; Yao, T. J. Sol-Gel Sci. Technol. 2000, 17 (3), 239.
10.1021/cm0208152 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/20/2003
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Figure 1. Schematic diagram of the electrostatic sol-spray deposition (ESSD) method.
ods have been employed mainly for preparing dense layers or porous layers with nanometer size pores. Recently, pore sizes of the porous titania film were controlled by changing the molecular weight of poly(ethylene) glycol in the parent solution.8,9 Caruso et al.12 reported that titania structure had been made around polymer gel network and the morphology can be controlled by the polymer gel structure. The spray deposition method13-25 is a simple and powerful technique to control the surface morphology of thin films. A parent solution is sprayed onto a substrate by means of air pressure, electrostatic, and ultrasonic force. The surface of the conducing substrate is controlled at a certain temperature where evaporation and chemical reactions occur simultaneously. In this technique, the reaction conditions on the substrate can be controlled independently (substrate temperature, supply rate, and compositions of parent solution). The electrostatic spray deposition (ESD) method15-21 and the electrostatic sol-spray deposition (ESSD) method22-25 are based on electrohydrodynamic atomization for spray-
ing. A precursor solution is used for the ESD method, and a precursor sol is used for the ESSD method. The size distribution of the droplets is narrow when a socalled Taylor cone is created at the top of a nozzle.19 This is one of the advantages of ESD or ESSD in order to make a uniform structure, one of the most important factors for the membrane properties. Many types of unique porous inorganic films (LiCoO2, LiMn2O4, hydroxy-apatite, and ceramic fuel cell electrodes) have been produced by the ESD15-19 or ESSD22,23 method. Chen et al.15-18 showed that the morphology of LiCoO2 could be varying the process conditions in unique reticular structures were ever obtained. Chen et al.18 also claimed that porous titania films having pores of micrometer size could be obtained by using the ESD method. Moreover, the morphology control of the titania layer was not sufficient. In this study, the thermal stability of this type of porous amorphous titania film was investigated in order to obtain a porous anatase or rutile structured titania film on a stainless steel substrate.
(10) Chou, K.-S.; Kao, K. B.; Huang, C. D.; Chen, C. Y. J. Porous Mater. 1999, 6 (3), 217. (11) Huang, S.-C.; Lin, T.-F.; Lu, S.-Y.; Chou, K.-S. J. Mater. Sci. 1999, 34, 4293. (12) Caruso, R. A.; Antonietti, M.; Giersig, M.; Hentze, H.-P.; Jia, J. Chem. Mater. 2001, 13 (3), 1114. (13) Patil, P. S. Mater. Chem. Phys. 1999, 59, 185. (14) Golego, N.; Studenikin, S. A.; Cocivera, M. J. Mater. Res. 1999, 14 (3), 698. (15) Chen, C. H.; Buysman, A. A. J.; Kelder, E. M.; Schoonman, J. Solid State Ionics 1995, 80, 1. (16) Chen, C. H.; Kelder, E. M.; Put, P. J. J. M.; Schoonman, J. J. Mater. Chem. 1996, 6 (5), 765. (17) Chen, C. H.; Kelder, E. M.; Schoonman, J. J. Mater. Sci. 1996, 31, 5437. (18) Chen, C. H.; Kelder, E. M.; Schoonman, J. J. Electrochem. Soc. 1997, 144 (11), L289. (19) Schoonman, J. Solid State Ionics 2000, 135, 5. (20) Kim, S. G.; Choi, K. H.; Eun, J. H.; Kim, H. J.; Hwang, C. S. Thin Solid Films 2000, 377-378, 694. (21) Nguyen, T.; Djurado, E. Solid State Ionics 2001, 138, 191. (22) Chen, C. H.; Kelder, E. M.; Schoonman, J. Thin Solid Films 1999, 342, 35. (23) Chen, C. H.; Emond, M. H. J.; Kelder, E. M.; Meester, B.; Schoonman, J. J. Aerosol Sci. 1999, 30 (7), 959. (24) Choy, K.-L.; Su, B. J. Mater. Sci. Lett. 1999, 18 (12), 943. (25) Su, B.; Choy, K.-L. J. Mater. Sci. Lett. 1999, 18 (20), 1705.
Experimental Section Figure 1 shows the schematic diagram of the ESSD method. A parent sol was introduced to flow through a small metal nozzle that was subjected to an electric field. The sol was atomized into charged droplets at the tip of the nozzle. These aerosol droplets were deposited onto a substrate, the temperature of which was set by a heater. The right side of Figure 1 shows the basic composition of a droplet and the surface of the substrate. The morphology of a product is determined by the shape of the primary building units in the droplets and the reaction on the substrate. A dense stainless steel disk (0.4 mm thick, φ 15 mm, SUS 304) was used as a substrate. The distance between the nozzle and the substrate was kept at 18 mm. The temperature of the substrate was controlled between 473 and 593 K by using a thermocouple and heater mounted in the substrate holder. The parent sol was supplied at 1.6 mL/h by a syringe pump (KD Scientific 100). The electric field strength between the substrate and the nozzle was set at 3.5-4.5 kV to keep a stable Taylor cone aerosol mode. The spray was always stable during these experiments, and an overflow of the parent sol around the nozzle was not observed. The deposition was carried out
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for 30-60 min. The substrate was removed from the substrate holder just after the deposition, and the sample was cooled to room temperature within a minute. Titanium tetraisopropoxide (TTIP, Acros) was used as a precursor of titania, and ethanol (J. T. Baker), butyl carbitol (Aldrich), and acetic acid (Aldrich) were used to prepare the parent sol. The boiling points of ethanol, butyl carbitol, and acetic acid are 351, 508, and 391 K, respectively. First, TTIP was mixed in pure ethanol and stirred for more than 6 h to make a clear sol. The concentration of this sol was 0.036 M. This sol is referred to as “reactive titania sol” in this paper. This reactive titania sol was stable for more than two weeks at room temperature. This sol was diluted by adding ethanol, butyl carbitol, and acetic acid to obtain the proper concentrations. The TTIP concentration of the parent sol was 0.006 M and the composition of the sols were based on ethanol/butyl carbitol/acetic acid ) 16.7:33.3:50.0 [vol %]. Aging was carried out for 0 to 265 h at room temperature. The sol was clear just after the parent sol had been prepared. The color of the sol changed to pale white after 2 h of aging. This color of the sol deepened upon aging until 5 h. The sol aged for more than 5 h is defined as “aged titania sol” in this paper. Calcination was achieved using an air oven (Vulcan 3-550). The calcination temperatures were 873 or 1273 K for 2 h in air. The heating rate was 5 K/min. Thermogravimetric analysis and differential thermal analysis were performed using TGA/ SDTA851e (Mettler Toledo) with a heating rate of 10 K/min. Scanning electric microscopy (JEOL JSM-5800LV) was used to observe the surface morphology of the products. XRD measurements using Cu KR X-ray diffraction were performed by using a D8 Advanced Diffractometer (Bruker). UV absorption of a parent sol was measured using a Cary UV-Vis spectrophotometer.
Results and Discussion Preparation of Porous Titania Films. Figure 2 shows the SEM images of the products obtained at 533 K from the parent sol aged for 0-23 h. The surface morphologies depend on the aging period of the parent sol. The structure from a parent sol without aging represents a smooth surface, whereas the structures from the aged sol show an aggregation of sticks (2 h) or fibers (5 h, 23 h). These SEM images reveal that the period of aging affects considerably the morphology of the final product and indicate that the titania precursor polymerized during aging. The polymerization of titania in the parent sol was finished after 5 h of aging, because the obtained films from 5 and 23 h of aging represent similar microstructures. The UV-Vis absorption spectra of the parent sol were measured during the aging period. The absorption at 265 nm, representative for a titania polymer,27 gradually increased during aging between 0 and 5 h. Especially under acid catalysis coexistence, less branched polymer chains are obtained by hydrolysis and condensation of titania alkoxide.28,29 This might be the reason for the fibrous structures made after 5 and 23 h of aging. From lower magnification pictures, it is apparent that the films having uniform pores of ca. 5 µm without cracks were obtained after 5 and 23 h of aging. Many cracks were observed on the surfaces of the products (26) Balachandran, W.; Miao, P.; Xiao, P. J. Electrostatics 2001, 50, 249. (27) Facca, F.; Puccetti, G.; Leblanc, R. M. Colloids Surf., A 1999, 149, 89. (28) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, MA, 1990; p 48. (29) Yoldas, B. E. J. Mater. Sci. 1986, 21, 1087.
Figure 2. SEM images of the surface of the products prepared at 533 K at high and low magnification: Effect of aging period of the parent sol. The bars in high and low magnification images indicate 5 µm and 50 µm, respectively.
for 0 and 2 h aging. Cracks might be due to the difference of thermal expansion between the titania layer and stainless steel when the samples were rapidly cooled after deposition. The sample temperature was decreased from 533 K to room temperature in less than a minute by removing the sample holder from the heater. The products from 0 or 2 h aged sol did not have enough flexibility to keep their structures. The porous structures obtained from an aged titania sol are stable under the rapid temperature change. These porous titania films were produced repeatedly from parent sols aged for more than 5 h. Despite the difference in surface morphologies, the size of pores is similar for the samples in Figure 2. The effect of the substrate temperature was studied using an aged titania sol (aging period 72-75 h). Figure 3 shows the SEM image of the surfaces of the products. The surface structures obtained at 533 and 593 K are aggregations of fibers. The structure made at 533 K (aging period 72 h) is similar to that in Figure 2 (5 or 23 h of aging) which indicates that the aged parent sol has properties similar to those of a sol after 5 h of aging. Porous structures of a few micrometers in size are observed for 533 K, whereas a nonuniform structure is deposited at 593 K. The boiling point of butyl carbitol
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Figure 3. SEM images of the surface of the products from the aged (72-75 h) titania sol deposited at different temperatures.
Figure 4. XRD patterns of the substrate, the as-deposited sample (5 h aging, 533 K), and the calcined sample (873 K).
is 504 K and the substrate temperature (593 K) is much higher. The evaporation of solvents was very fast. The fast evaporation should break the deposited porous structure. So, the big holes were observed with the SEM image. On the other hand, the structure obtained at 473 K represents a dense film with a rough surface and without pores. The fibrous structure is already mixed in the dense film. For comparison, the parent sol aged for 5 h was only dried at 473 K in air on the substrate. The surface of the dried product is dense without porous structures up to a magnification of 10 000× by SEM. The boiling point of butyl carbitol is higher than the substrate temperature. The deposition of the titania layer by ESSD at 473 K occurred in the liquid phase, and the deposition in the liquid phase leads to a dense structure without pores of micrometer size. A substrate temperature of 533 K is adequate to obtain uniform porous titania films. The porous titania film without any cracks (533 K, 5 h aging) and a substrate were characterized by XRD (Figure 4). The as-made film shows only peaks that are the same for the substrate and gold, used for the sample preparation. Peaks at around 43° and 51° show SUS 304. Only after calcination at 873 K, a peak at 2θ ) 25.3° representing the anatase phase is observed. No indications were found for a rutile phase (2θ ) 27.8°). A titania as-made film is an amorphous phase, and a calcination procedure is necessary to obtain anatase or rutile titania. Calcination Temperature. Thermogravimetric analysis was performed on the sample made by drying the aged parent sol at 473 K. Figure 5 shows the weight loss and the differential thermal analysis (DTA) of this sample. The total weight loss was about 40 wt %. There are five regions of weight loss and/or heat effects around 350, 550, 700, 750, and 950 K. The weight loss around 350 K (3.8 wt %) is endothermic and should be water desorption. The weight losses around 550 and 700 K
Figure 5. TG measurements for the dried aged sol sample.
correspond to those reported for the decomposition during the production of titania powders from TTIP in alcohol solutions with low water concentration.30 The exothermal weight loss around 550 K (-32 wt %) is the oxidation of residual organic components after hydrolysis of TTIP. This indicates that about 40% of -O-C3H7 groups in TTIP had been removed for the as-made titania layer. The weight loss around 700 K (3.2 wt %) is the crystallization of amorphous hydrated titania to anatase.30 The weight loss around 750 K (2.6 wt %) was exothermic and this might be the oxidation of carbonaceous material. A small amount of carbon remained in the structure. The endothermic peak without any weight change observed at 950 K represents the phase transformation from anatase to rutile. The anatase phase without carbon impurities would be obtained from calcination in air between 750 and 950 K. It is necessary to calcine over 950 K to prepare pure rutile titania. Therefore, the calcination temperatures of the coated samples were set at 873 K for the anatase phase and at 1273 K for the rutile phase. The porous titania film (533 K, 5 h aging) was calcined at 873 K for 2 h. The XRD pattern of this calcined sample (Figure 4) shows a sharp reflection at 2θ ) 25.3° indicating anatase diffraction pattern. The peak at 2θ ) 33.5° related to gold was sharpened by the calcination process. The gold particles from the sputtering for SEM sintered during the calcination. Figure 6 shows the SEM image after the calcination. The porous structure had shrunk and large cracks are observed. A fibrous structure was still observed at (30) Song, K.-C.; Pratsinis, S. E. J. Colloid Interface Sci. 2000, 231, 289.
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Figure 6. SEM image of the calcined (873 K) product (5 h aging, 533 K).
Figure 8. SEM images of the surface of the as-made products prepared at 533 K from mixtures of aged sol and fresh sol in different ratios.
Figure 7. Schematic diagrams of the formation of porous titania films by the ESSD method.
higher magnification, while the density of the product after calcination is obviously higher than that of the as-made sample shown in Figure 2 (5 h of aging). This porous structure was not stable during calcination at 873 K. Improvement of the Thermal Stability of Porous Titania Film. The porous amorphous titania films prepared from the aged titania sol shrank during calcination at 873 K. This phenomenon is schematically described in Figure 7 (upper diagram). The titania precursor in the parent sol consists only of titania polymeric species, because the polymerization of titania finished before 5 h of aging. This titania polymer accumulates on the substrate by the ESSD method. The macro pore size can be controlled by changing the operation conditions. This porous structure is flexible enough to stay intact during the rapid temperature change from 533 K to room temperature. The shrinkage by the high-temperature treatment at 873 K may be due to the low interconnectivity of the polymer chains. To obtain a better thermal stability at higher temperatures, the preparation procedure was changed as described in Figure 7 (bottom). A parent sol is made by mixing a reactive titania sol with an aged titania sol. The reactive titania precursor in a fresh sol is envisaged to act as a connection between titania polymers during the deposition. The pore structure is similar to that without fresh titania sol, but, the polymer chains are now connected to each other to preserve the porous structure during calcination. Figure 8 shows SEM images of the surface morphologies of the products prepared from the mixtures of the aged titania sol (260-265 h) and the reactive titania sol in different ratios. The ratios of the mixtures were aged titania sol/reactive titania sol ) 100:0, 87.5:12.5, and 66.7:33.3 [vol %], respectively. The same samples with different calcination temperatures were observed as shown in Figures 8-11. The surface structures at low magnification (500×) are similar to that of Figure 2 (5 or 23 h of aging). No significant difference is observed. These films are resistant against rapid tem-
Figure 9. SEM images of the surface after calcination at 873 K.
perature change after the deposition. At higher magnification (5000×) some differences between the structures are observed. There are no particles observed from the parent sol without reactive titania sol. On the other hand, the image of 66.7:33.3 [vol %] represents aggregation of fibers and small particles. The small particles were made by adding reactive titania sol as in the diagram shown in Figure 7. Those particles were not found in the film prepared from 12.5 vol % of reactive titania sol. This structure was similar to that without reactive titania sol (aged titania sol/reactive titania sol ) 100:0). The amount of reactive titania sol was too small to obtain powders among the fibrous structure. Figure 9 shows the surface structures after calcination at 873 K in air. The titania film prepared without reactive titania sol shrunk upon calcination, resulting in large cracks. The porous structures stayed intact when the products were made with a mixture of the reactive and the aged titania sol. Thermal stability was obviously improved by adding a reactive titania sol for both 12.5 and 33.3 vol %. Figure 10 shows the XRD patterns of the products obtained from a mixture of aged titania sol and reactive titania sol in a ratio of 66.7:33.3. There are no titania reflections observed for the as-made sample (bottom line in Figure 10), but sharp reflections at 2θ ) 25.3° were found from the sample calcined at 873 K in air (third line in the Figure) indicating anatase titania. Samples prepared from mixtures of aged sol and the fresh titania sol in a ratio of 87.5:12.5 and 66.7:33.3 were calcined at 1273 K to obtain porous rutile structures. Figure 11 represents the SEM images of the products. The porous structure from a ratio of 87.5:12.5 was damaged by calcination. There was no porous structure found in the high magnification image. The porous structure remained in the sample prepared from a
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Figure 10. XRD patterns of the ESSD products prepared at 533 K from a mixture of aged and fresh titania sol in a ratio of 66.7:33.3 before and after calcination at 873 K and 1273 K.
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%. The XRD patterns of the sample and the substrate calcined at 1273 K are present in Figure 10. A sharp reflection is observed at 2θ ) 27.8, representative for the rutile phase. There was no indication for an anatase phase with a reflection at 2θ ) 25.3, so this is a pure porous rutile film. This result is different from that of the titania films made by the ESD method.22 Chen et al.22 reported that the phase transformation from anatase to rutile occurred between 1273 and 1373 K. Their conclusion was that the anomalous high-temperature phase transformation was caused by the silicon impurity from the silicon tubes in the apparatus. In our case, polymeric tubing was used to eliminate the possibility of silicon contamination. The temperature of the phase transformation from anatase to rutile was similar to that observed for the powder.30 There are minor reflections found at 2θ ) 24.5, 29.8, 33.6, 35.2, and 36.2 from the substrate, and these are identified as NiMnO2 and Cr2O3. The stainless steel substrate was partially oxidized by the calcination at 1273 K. Conclusions
Figure 11. SEM images of the surface after calcination at 1273 K.
mixture with a ratio of 66.7:33.3. The 3D pores of 5 µm were observed in the high magnification image. The microstructure of the pores was different in the as-made sample (Figure 8, right and bottom). The aggregation of fibers and particles turned into the aggregation of sticks. The macro pore size, however, was not changed significantly. To keep the porous structure requires the addition of 33.3 vol % of reactive sol in need of 12.5 vol
A porous titania film having uniform micrometersized pores without cracks was obtained on dense stainless steel disks by the ESSD method. The film morphology depended strongly on the aging period of the parent sol. Porous amorphous films without cracks were obtained from the parent gel at 533 K after more than 5 h of aging. These films are XRD analyzed and stable under the rapid temperature change upon cooling from 533 K to room temperature. They shrunk by calcination at 873 K for 2 h resulting in crack formation. Improved thermal stability of the porous titania films was obtained by employing mixtures of an aged titania sol and a reactive titania sol. The porous structures obtained from the mixed sol by ESSD remained intact by calcination at 873 and 1273 K, resulting in a macroporous anatase or rutile film, respectively, with uniform pores of a few micrometers in size. CM0208152
