Molecular Shape Recognition by a Tin Oxide Chemical Sensor

Mar 18, 2000 - A tin oxide gas sensor was modified by chemical vapor deposition (CVD) of silicon alkoxide using various preadsorbed aldehydes as the t...
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Langmuir 2000, 16, 3858-3865

Molecular Shape Recognition by a Tin Oxide Chemical Sensor Coated with a Silica Overlayer Precisely Designed Using an Organic Molecule as the Template Toyoshi Tanimura, Naonobu Katada,* and Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan Received October 21, 1999. In Final Form: January 19, 2000 A tin oxide gas sensor was modified by chemical vapor deposition (CVD) of silicon alkoxide using various preadsorbed aldehydes as the template. The deposition of silica without the template suppressed the sensitivities for all of the used alkanes, but the deposition under the presence of the preadsorbed template did not affect the sensitivities to n-hexane and octane, linear C6 and C8 alkanes. The use of benzaldehyde as the template suppressed the sensitivities to 2,2-dimethylbutane and -hexane, namely, the isomers with two methyl branches, and cyclohexane and cyclooctane. A n-butyraldehyde template suppressed the sensitivities to these isomers and, in addition, the sensitivities to 2-methylpentane and -heptane, which are the isomers with one methyl branch. The microstructure of the silica surface layer is considered to be precisely controlled by the template molecule to possess the pores whose size and shape were the same as those of the template molecule. Catalytic activity for complete oxidation of the alkanes was also shown to be controlled by the shape and size of pores in the silica layer.

Introduction Much interest has been paid to control the structure of the reaction field in order to create a molecular recognition ability. Discovery and synthesis of supramolecules such as calixaranes,1,2 cyclodextrins,3 C60 fullerene,4 and carbon nanotube5,6 have been carried out on the basis of the carbon chemistry. Molecular imprinting has been attempted to obtain catalytic antibodies7 and imprinted solid polymers8 consisting of organic compounds.9 In the field of inorganic materials, many researchers have tried to modify the surface structures of solid catalysts with organic promoters, including chiral molecules.10 Self-assembled organic monolayers have been applied in order to create a chemical sensor with a molecular recognition property.11-14 Loading of such an inorganic material as zeolite film on functionalized materials has been proposed.15,16 On the other hand, studies in the last decade have given a number of novel micro-17,18 and mesoporous19-21 silicates using organic material as the template17-20 or structural composite18,21 * Corresponding author. Phone: +81-857-31-5684. Fax: +81857-31-5684. E-mail: [email protected]. (1) Lehn, J. M. Supramolecular Chemistry; VCH: Weinheim, Germany, 1995. (2) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713-745. (3) Szejtli, J. Chem. Rev. 1998, 98, 1743-1753. (4) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1975, 318, 162-163. (5) Iijima, S. Nature 1991, 354, 56-58. (6) Thess, A.; Lee, R.; Nikoleav, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rintzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Toma´nek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483487. (7) Schultz, P. G.; Lemer, R. A. Science 1995, 269, 1835-1842. (8) Beach, J. V.; Shea, K. J. J. Am. Chem. Soc. 1994, 116, 379-380. (9) Davis, M. E. CaTTech 1997, 1, 19-26. (10) Hutchings, G. J. Chem. Commun. 1999, 301-306. (11) Reinhoudt, D. N. Sens. Actuators, B 1995, 24-25, 33-35. (12) Rickert, J.; Weiss, T.; Go¨pel, W. Sens. Actuators, B 1996, 31, 45-50. (13) Go¨pel, W. Microelectron. Eng. 1996, 32, 75-110. (14) Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Microelectron. Eng. 1996, 32, 255-268. (15) Feng, S.; Bein, T. Nature 1994, 368, 834-836. (16) Peachey, N. M.; Dye, R. C.; Ries, P. D.; Warren, M.; Olken, M. J. Porous Mater. 1996, 2, 331-339.

in order to control the crystal or bulk structure in nanoand subnanometric scale. However, only a few studies have dealt with precise control of the surface structure which purely consisted of inorganic material with substantial stability. Morihara et al.22,23 and Heilman et al.24 carried out the pioneering works to control the microstructure of the silica surface using organic acids as the templates. We have developed a chemical vapor deposition (CVD) method of silicon alkoxide on zeolite25 and such metal oxides as alumina,26,27 titania, zirconia,28 and tin oxide in order to prepare an ultra thin layer of silica.29 A silica monolayer consisting of a two-dimensional siloxane network was formed on these oxides30,31 and showed novel functions, solid acidity,32,33 and thermal stability.34 More(17) Freyhardt, C. C.; Tsapatsis, M.; Lobo, R. F.; Bulkus, K. J., Jr.; Davis, M. E. Nature 1996, 381, 295-298. (18) Jones, C. W.; Tsuji, K.; Davis, M. E. Nature 1998, 393, 52-54. (19) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (20) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680-682. (21) Stucky, G. D.; Huo, Q.; Firouzi, A.; Chmelka, B. F.; Schacht, S.; Voigt-Martin, I. G.; Schu¨th, F. Stud. Surf. Sci. Catal. 1997, 105, 3-28. (22) Morihara, K.; Kurihara, S.; Suzuki, J. Bull. Chem. Soc. Jpn. 1988, 61, 3991-3998. (23) Morihara, K.; Nishihata, E.; Kojima, M.; Miyake, S. Bull. Chem. Soc. Jpn. 1988, 61, 3999-4003. (24) Heilman, J.; Maier, W. F. Angew. Chem., Int. Ed. Engl. 1994, 33, 471-473. (25) Niwa, M.; Murakami, Y. J. Phys. Chem. Solids 1989, 50, 487496. (26) Niwa, M.; Hibino, T.; Katada, N.; Murakami, Y. J. Chem. Soc., Chem. Commun. 1989, 289-290. (27) Niwa, M.; Katada, N.; Murakami, Y. J. Phys. Chem. 1990, 94, 6441-6445. (28) Niwa, M.; Katada, N.; Murakami, Y. J. Catal. 1992, 134, 340348. (29) Katada, N.; Niwa, M. Adv. Mater., Chem. Vap. Deposition 1996, 2, 125-134. (30) Katada, N.; Toyama, T.; Niwa, M. J. Phys. Chem. 1994, 98, 76477652. (31) Katada, N.; Niwa, M. Res. Chem. Intermed. 1998, 24, 481-494. (32) Katada, N.; Niwa, M.; Murakami, Y. Stud. Surf. Sci. Catal. 1994, 90, 333-338. (33) Katada, N.; Toyama, T.; Niwa, M.; Tsubouchi, T.; Murakami, Y. Res. Chem. Intermed. 1995, 9, 137-149.

10.1021/la991397r CCC: $19.00 © 2000 American Chemical Society Published on Web 03/18/2000

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Scheme 1. Concept of Modification of the Surface by CVD Using (a) n-Butyraldehyde, (b) Benzaldehyde, and (c) r-Naphthaldehyde as the Template

over, we proposed the CVD method using organic material as a template, as shown in Scheme 1. Benzaldehyde forms a stable benzoate anion on the basic metal oxide, i.e., alumina, titania, zirconia, and tin oxide.35,36 After it was preadsorbed on tin oxide37 and alumina,38 the CVD of silicon alkoxide was carried out to form the silica overlayer with pores whose size and shape were controlled by the organic template, followed by calcination in order to convert the alkoxide into an oxide layer and to remove the preadsorbed organics.39 The thus-obtained silica overlayer showed the molecular sieving function as an adsorbent; a molecule larger than benzaldehyde was hardly adsorbed, while a smaller molecule was easily adsorbed.40 Because tin oxide is a semiconductor and therefore has a function as a chemical sensor,41-43 the prepared SiO2/SnO2 was applied as a gas sensor.44 We here report that the clear molecular recognition function for alkanes was found on the SiO2/SnO2 gas sensor which was prepared by changing the size of template molecule as n-butyr-, benz-, and R-naphthaldehydes. Experimental Section Preparation of SiO2/SnO2. Tin hydroxide gel was precipitated from an aqueous solution of SnCl2 (Nacalai Tesque, 97%) by adding an ammonia solution (Wako Pure Chemicals, special grade), followed by washing with water until no chlorine was observed, and the obtained solid was converted to tin oxide by (34) Katada, N.; Ishiguro, H.; Muto, K.; Niwa, M. Adv. Mater., Chem. Vap. Deposition 1995, 1, 54-60. (35) Niwa, M.; Inagaki, S.; Murakami, Y. J. Phys. Chem. 1985, 89, 3869-3872. (36) Niwa, M.; Suzuki, K.; Kishida, M.; Murakami, Y. Appl. Catal. 1991, 67, 297-305. (37) Kodakari, N.; Katada, N.; Niwa, M. J. Chem. Soc., Chem. Commun. 1995, 623-624. (38) Kodakari, N.; Tomita, K.; Iwata, K.; Katada, N.; Niwa, M. Langmuir 1998, 14, 4623-4629. (39) Kodakari, N.; Katada, N.; Niwa, M. Adv. Mater., Chem. Vap. Deposition 1997, 3, 59-66. (40) Kodakari, N.; Katada, N.; Niwa, M. Appl. Surf. Sci. 1997, 121/ 122, 292-295. (41) Seiyama, T.; Kato, A.; Fujii, K.; Nagatani, M. Anal. Chem. 1962, 34, 1502-1503. (42) Yamazoe, N.; Miura, N. Chem. Sens. Tech. 1992, 4, 19. (43) Tamaki, J. Hyomen 1996, 34, 737-736. (44) Kodakari, N.; Sakamoto, T.; Shinkawa, K.; Funabiki, H.; Katada, N.; Niwa, M. Bull. Chem. Soc. Jpn. 1998, 71, 513-519.

calcination at 773 K in atmosphere for 2 h. The BrunauerEmmett-Teller (BET) surface area45 was determined with nitrogen adsorption experiments at 77 K to be 22.9 m2 g-1. Prior to the preparation of a sensor disk, the adsorption behavior of the template molecule was observed on tin oxide powder by a conventional pulse method. After pretreatment of 50 mg of tin oxide powder set in a Pyrex tube (4 mm i.d.) at 673 K for 1 h in flowing oxygen (50 cm3 min-1), 0.1 mm3 of n-butyraldehyde (butanal, Nacalai Tesque, special grade) was repeatedly injected from the inlet before the reactor at various adsorption temperatures in flowing helium (>99.9%, 50 cm3 min-1) which was purified by passing a liquid-nitrogen trap. The eluted aldehyde was analyzed by a gas chromatography (GC) with a flame ionization detector (FID). After no further adsorption was observed, gaseous ammonia (10 cm3) was repeatedly injected to convert the adsorbed butyric anion into butyronitrile at 673 K. The stoichiometric formation of the nitrile from the adsorbed species was assumed on the basis of our previous study,35 and the surface concentration of adsorbed butyric anion was estimated from the amount of nitrile, which was detected by the FID-GC. By the same method, adsorption behaviors of benz- and R-naphthaldehydes (Wako Pure Chemicals, special grade, and Tokyo Chemical Industries, 1st grade, respectively) were observed, as previously shown.40 In order to prepare the SiO2/SnO2 sensor disk, the powder of tin oxide (1 g) was compressed under a pressure of ca. 30 MPa into a disk (10 mm in diameter and 5 mm in thickness), in which two platinum wires had been buried. The distance between the two wires was ca. 5 mm, and they were connected to an electric circuit in order to measure the electric resistance under 1-5 V of electric voltage. The CVD of silica was carried out in a Pyrex reactor with 18 mm diameter under atmospheric pressure.44 After the pretreatment of the molded disk in oxygen flow (50 cm3 min-1) at 673 K for 1 h, the template aldehyde (5 mm3 as liquid) was injected into a helium flow (50 cm3 min-1) from the inlet installed before the reactor at the adsorption temperature, which was decided based on the adsorption behavior as shown in the following section. The electric resistance of the oxide disk gradually decreased with repetition of the injection of the template compound, and the injection was repeated until the resistance reached a stable value within several injections. Then, tetramethoxysilane vapor [Si(OCH3)4] (Tokyo Chemical Industries, 1st grade, ca. 360 Pa) was fed from a reservoir cooled with an ice bath. A mixture of the vapor and helium (50 cm3 min-1) was introduced into the reactor (45) Bond, G. C. Heterogeneous Catalysis: Principles and Applications, 2nd ed.; Oxford University Press: New York, 1987; p 12.

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Figure 1. Surface concentration of adsorbed species formed from n-butyr- (3), benz- (0), and R-naphthaldehydes (4) on tin oxide determined by the pulse method. at 423 K for 1.5 h. Water vapor (ca. 2.6 kPa)/helium (50 cm3 min-1) was introduced at the same temperature for 1.5 h. The introduction of Si(OCH3)4 and water vapor was repeated three times. Finally, oxygen flow (50 cm3 min-1) was fed at 673 K for 4 h to decompose the organic residue. Here we mention that the reaction mechanism of Si(OCH3)4 has been studied. It was observed on alumina by means of an infrared technique that one methoxy group reacts with a hydroxyl group on the surface to form the anchored silicon methoxide, and the following reaction of the siloxane network among the multiple silicon methoxides forms a silica layer.30 In the presence of the template aldehyde, no interaction between the template and silicon alkoxide was observed.38 Therefore, it is predicted that the deposition of silica proceeds on the uncovered surface independently of the template. As a comparison, the CVD of silica was carried out on tin oxide disk without the preadsorption of the template aldehyde. In this case the introduction of Si(OCH3)4 and water vapor was carried out only once but not repeated. The obtained sample is termed the nontemplate sample. Measurements of Sensing, Catalytic, and Structural Properties. After the pretreatment of the sample disk in oxygen flow (50 cm3 min-1) at 673 K, dried air was fed into the reactor with 40 cm3 min-1 of the flow rate. After the resistance became stable, 0.5 mm3 of the liquid alkane was injected, and the change of electric resistance was recorded.44 Simultaneously, the products were analyzed by an on-line FID-GC connected to the outlet of the reactor. An X-ray photoelectron spectrum (XPS) was recorded under a reduced pressure of less than 10-5 Pa by a Shimadzu ESCA750 spectrometer with a Mg KR (1254 eV) X-ray excitation source. The measurements were carried out on the external surface of the molded disk and on pieces crashed from the disk. The surface atomic ratio was calculated as follows:

ISi/ASi Si ) Si + Sn ISi/ASi + ISn/ASn where ISi and ASi are the peak intensity and ionization crosssectional area (0.946 nm) of Si 2p electron (103 eV), respectively, and ISn and ASn (14.6 nm) are those of Sn 3d5/2 (487 eV).

Results Adsorption Behavior of Template Molecules. Prior to the preparation of the SiO2/SnO2 sample, the adsorption temperature was decided on the basis of the adsorption behavior of the template molecule. The surface concentration of adsorbed species increased with an increase in the adsorption temperature in all cases of used aldehydes, as shown in Figure 1. This suggests that the adsorption of these molecules under these conditions is classified as chemisorption, which can be controlled by kinetics; therefore, the adsorption amount is enhanced by increas-

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Figure 2. Change in the electric resistance of SnO2 (solid line) and SiO2/SnO2 prepared using benzaldehyde as the template (broken line) by contact with vapor of cyclohexane at 593 K.

ing the temperature. The mechanism of chemisorption of aldehyde has been studied:35 it is considered that the corresponding carboxylate anion was formed from the aldehyde on such a basic metal oxide as alumina and tin oxide. The maximum concentration of adsorbed species was observed above 400, 600, and 450 K to be ca. 2.8, 2.0, and 1.3 molecules nm-2 for n-butyr-, benz-, and R-naphthaldehydes, respectively. These maximum concentrations are in agreement with the simulated value for the surface saturated with the carboxylate anions.39 Therefore, it is estimated that the surface was completely covered by the carboxylate anions at the high temperatures above 400, 600, and 450 K for n-butyr-, benz-, and R-naphthaldehydes, respectively. We assumed the same adsorption behavior also on the inner surface of the molded disk of tin oxide. In order to deposit silica on the uncovered surface as shown in Scheme 1, such a high coverage will be unsuitable. Hence, we selected 353, 423, and 353 K for n-butyr-, benz-, and R-naphthaldehydes, respectively, as the adsorption temperatures in order to make the coverage by the template be 1/2-1/3 on the basis of the temperature dependence of the concentration of adsorbed species. The CVD of silicon alkoxide was carried out after the preadsorption of n-butyr-, benz-, and R-naphthaldehydes at 353, 423, and 353 K, respectively, in the following experiments. Sensing Property. Figure 2 shows an example of the response of a tin oxide gas sensor to the injection of alkane vapor. A high electric resistance was observed in air, and the resistance was quickly decreased by the contact with the organic vapor, followed by slow recovery. In Figure 2, also the response of SiO2/SnO2 prepared by using benzaldehyde as the template was included. The minimum value of resistance was observed to be higher than that of SnO2. Hereafter the sensitivity will be shown as the parameter Ra/Rg, where the constant resistance in air is Ra and the minimum value observed by the contact with organic material is Rg. Figure 3 shows the sensitivity for n-hexane (IUPAC name, hexane) at various temperatures. In this temperature region, the sensitivity monotonously increased with an increase in the temperature. The relationship between the temperature and sensitivity is similar in all of the shown samples prepared by using different template aldehydes. On the contrary, Figure 4 shows the significant effect of the template aldehyde on the sensitivity. The sensitivities to 2,2-dimethylbutane on SnO2 and SiO2/SnO2 prepared using R-naphthaldehyde as the template were relatively high, whereas the sensitivities on SiO2/SnO2

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Figure 3. Sensitivity to n-hexane on SnO2 (O) and SiO2/SnO2 prepared using n-butyraldehyde (3) and benzaldehyde (0) as the template. Figure 6. Sensitivity to C8 alkanes at 643 K on SnO2 (9) and SiO2/SnO2 prepared using R-naphthaldehyde (dark gray box), benzaldehyde (light gray box), and n-butyraldehyde (0) as the template.

Figure 4. Sensitivity to 2,2-dimethylbutane on SnO2 (O) and SiO2/SnO2 prepared using n-butyraldehyde (3), benzaldehyde (0), and R-naphthaldehyde (4) as the template.

Figure 5. Sensitivity to C6 alkanes at 643 K on SnO2 (9) and SiO2/SnO2 prepared using R-naphthaldehyde (dark gray box), benzaldehyde (light gray box), and n-butyraldehyde (0) as the template and without the template (cross-hatched box).

prepared by using the small aldehydes, namely, nbutyraldehyde and benzaldehyde, were low over the temperature range. Similarly, the difference in sensitivity to other C6 alkanes was observed, as summarized in Figure 5. The sensitivity to the C6 alkanes on SnO2 was approximately constant, 65-95 under these conditions. To n-hexane, all of the SiO2/SnO2 samples showed sensitivities comparable

to that on SnO2, but the nontemplate sample, i.e., SiO2/ SnO2 prepared without template, showed low sensitivity. The nontemplate sample showed low sensitivities for all of the C6 alkanes. These results point out that the silica layer loaded on tin oxide, which probably covers the surface completely on this sample because of the lack of inhibitor for the silica deposition, i.e., the template, during the CVD, is inactive for the reaction with alkane. On the other hand, SiO2/SnO2 prepared using nbutyraldehyde as the template showed low sensitivities to 2-methylpentane, 2,2-dimethylbutane, and cyclohexane; sensitivities were slightly higher than those on the nontemplate sample. The sensor disk prepared using benzaldehyde as the template showed high sensitivities to n-hexane and 2-methylpentane, while sensitivities to 2,2-dimethylbutane and cyclohexane were small. Sensitivities to all of the C6 alkanes on the sample prepared using R-naphthaldehyde as the template were comparable to those on tin oxide. Similar phenomena were also observed for C8 alkanes, as shown in Figure 6. Similarly to the case of C6 alkanes, SnO2 showed relatively high sensitivities to all the used alkanes, while the use of benzaldehyde template suppressed the sensitivity to 2,2-dimethylhexane and cyclooctane. n-Butyraldehyde template suppressed the sensitivities to them and 2-methylheptane also. Catalytic Activity. Simultaneous measurements of the electric resistance and the reaction products were made by the on-line FID-GC to evaluate the catalytic activity of the present samples as an oxidation catalyst. Because no byproducts were detected in all of the experiments, we assumed that only CO2 was a product and calculated the conversion of alkane into CO2 on the basis of the consumption amount of alkane. On tin oxide, we have confirmed the formation of CO2 by analyzing the products using a GC with a thermal conductivity detector (TCD). Figure 7 shows an example of the temperature dependence of conversion, corresponding to the catalytic activity of tin oxide for complete oxidation of 2,2-dimethylbutane. The activity increased with an increase in the temperature, and for this branched alkane, the activities on SiO2/SnO2 prepared using the small aldehydes, n-butyr- and benzaldehyde, as the templates were lower than that on SnO2. Figure 8 summarizes the activities for the C6 alkanes at 643 K. The change in activity was similar to the change in sensitivity; SnO2 showed the high activities for all of

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Figure 7. Conversion of 2,2-dimethylbutane on SnO2 (O) and SiO2/SnO2 prepared using n-butyraldehyde (3), benzaldehyde (0), and R-naphthaldehyde (4) as the template.

Figure 8. Conversion of C6 alkanes at 643 K on SnO2 (9) and SiO2/SnO2 prepared using R-naphthaldehyde (dark gray box), benzaldehyde (light gray box), and n-butyraldehyde (0) as the template. Table 1. Surface Atomic Ratio of Si/(Si + Sn) Determined from the X-ray Photoelectron Spectrum

template n-butyraldehyde benzaldehyde R-naphthaldehyde

inside of the disk, i.e., surface of particle crashed from the disk 0.702 0.733

external surface of the disk 0.741 0.763 0.815

the alkanes, while the use of the benzaldehyde template suppressed the sensitivity to 2,2-dimethylbutane and cyclohexane. The n-butyraldehyde template suppressed the sensitivities to them and 2-methylpentane also. Similar results were obtained for the C8 alkanes. Surface Composition. The XPS showed almost the same peak positions of Si, Sn, and O elements on the present SiO2/SnO2 samples as those observed on pure tin and silicon oxides. Almost 3/4 of the surface atomic ratio of Si/(Si + Sn) was observed on all of the samples of SiO2/ SnO2 and on both external surface and inside of disk, as shown in Table 1. Discussion Molecular Recognition in Sensing Property. It is often observed that the sensitivity of the semiconductor gas sensor shows a volcano-shape relationship against the temperature.42,43 In such a case the sensitivity is

controlled by multiple factors, e.g., the reaction rate of the reactant and the sensor surface, the adsorption density, the desorption rate of product, and the rate of the reoxidation of surface, because the sensing property of the semiconductor is believed to be due to the change of the surface concentration of the oxygen anion. However, as shown in Figures 3 and 4, the sensitivity to the used alkanes on tin oxide simply increased with an increase in the temperature similar to the increase of the oxidation rate shown in Figure 7 in the experimental temperature region. This suggests that the sensitivity is mainly dependent on the reaction rate between the alkane and the oxygen anion on the tin oxide surface, but other complex factors did not have a significant effect. The sensitivity to all of the C6 alkanes on SnO2 was approximately constant (Figure 5), showing that the chemical reactivities of the used C6 alkanes on the uncovered tin oxide surface are intrinsically similar. On the contrary, SiO2/SnO2 prepared without a template showed low sensitivities for all of the C6 alkanes. This points out that the surface silica layer is substantially inactive for the reaction with alkane. This is rational because silica is an insulator but not a semiconductor. Therefore, the difference in the sensitivity discussed below should be related with the molecular shape and the size of pore in a silica overlayer. To n-hexane, all of the SiO2/SnO2 samples prepared using the template showed sensitivities (Figure 5) and oxidation activities (Figure 8) comparable to those on SnO2. The size of molecules used in this study is roughly estimated, as shown in Figures 9 and 10.46-49 The size of n-hexane was estimated to be 0.48 × 0.36 nm, which is similar or smaller than any template molecules presently used. No suppression of the sensitivity to n-hexane by silica is therefore consistent with the pore size in the silica layer which is expected to be similar to the molecular size of the template; it should be possible for the linear alkane to pass the pore in a silica overlayer to react with the tin oxide surface. The SiO2/SnO2 sample prepared using n-butyraldehyde as the template showed low sensitivities to 2-methylpentane, 2,2-dimethylbutane, and cyclohexane. Because this sample is expected to possess pores whose size is close to 0.48 × 0.36 nm on the basis of the molecular size of the template shown in Figure 9a, the observed low sensitivity to 2-methylpentane is consistent with the explanation that at least 0.47 nm of the pore size is necessary to pass this branched alkane as predicted from the projection of 2-methylpentane onto the x-y plane shown in Figure 10b. Also, 2,2-dimethylbutane and cyclohexane should have projections (0.62 × 0.57 and 0.61 × 0.43 nm, respectively, as shown in parts c and d of Figure 10) larger than the expected pore size, in agreement with the fact that this sensor disk showed the low sensitivities also to these molecules. The controlled sensitivities to the large alkane molecules were slightly higher than those on the nontemplate sample, suggesting the control for sensitivity is (46) It was assumed that the bond lengths of C-C and C-H single bonds are 0.154 and 0.107 nm, respectively, and those of the C-C bond in the aromatic ring and C-H attached to the aromatic ring are 0.140 and 0.110 nm, respectively; all of the bond angles of H-C-H, C-C-H, and C-C-C in alkanes are 105.9°; all of the bond angles of H-C-C and C-C-C in aromatic molecules are 120°; the van der Waals radius of aromatic carbon is 0.18 nm, and that of hydrogen atom is 0.12 nm.47,48 (47) Cotton, F. A.; Wilkinson, G. Basic Inorganic Chemistry; John Wiley & Sons, Inc.: New York, 1976. (48) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; John Wiley & Sons, Inc.: New York, 1966. (49) Pine, S. H.; Hendrickson, J. B.; Cram, D. J.; Hammond, G. S. Organic Chemistry, 4th ed.; McGraw-Hill: New York, 1981.

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Figure 9. Estimated shape of template molecules. The covalent radius of carbon atom is shown by the closed symbol, and the van der Waals radius is shown by the open circle.

Figure 10. Estimated shape of C6 alkane molecules.

not complete but the efficiency of the use of the template is considered to be large enough. We emphasize that the

presence of only one methyl branch drastically controlled the sensitivity.

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Table 2. Molecular Sizes of Used Templates and C6 Alkanesa template

size/nm

n-hexane (0.36 × 0.48 nm)

2-methylpentane (0.37 × 0.6 nm)

2,2-dimethylbutane (0.57 × 0.62 nm)

cyclohexane (0.43 × 0.61 nm)

SnO2 R-naphthaldehyde benzaldehyde n-butyraldehyde non

∞×∞ 0.37 × 0.92 0.37 × 0.67 0.36 × 0.48 0×0

O O O O ×

O O O × ×

O O × × ×

O O × × ×

a

The symbol O shows high sensitivity, while × shows low.

The sensor disk prepared using benzaldehyde as the template showed high sensitivities to n-hexane and 2-methylpentane, while sensitivities to 2,2-dimethylbutane and cyclohexane were small. From the size of the benzene ring, this sample is expected to possess 0.67 × 0.37 nm of the pore size (Figure 9b). The suppression of sensitivity to 2,2-dimethylbutane (0.62 × 0.57 nm) and cyclohexane (0.61 × 0.43 nm) is in agreement with the expected pore size. On the other hand, the high sensitivity to 2-methylpentane (0.61 × 0.47 nm) points out the possibilities that the alkane molecule and/or silica wall can move to pass the alkane through the pore whose size is slightly smaller than that of the alkane molecule; it seems possible to push the molecule into a square of 0.6 × 0.37 nm shown in Figure 10b by moving some hydrogen atoms. Another possible explanation is that the copresence of the template benzoate anion controlled the shape of the siloxane network containing methoxy groups during the CVD operation, and after calcination, pores slightly larger than the template were formed because the methoxy groups were removed together with the template or substituted by hydroxyl groups. The use of R-naphthaldehyde as the template allowed high sensitivities to 2,2-dimethylbutane and cyclohexane. These alkane molecules are estimated to have the size 0.62 × 0.57 and 0.61 × 0.43 nm, respectively. It is also explained that this molecule could pass the pore expected to have 0.92 × 0.37 nm (Figure 9c) because of the movement of molecule and silica wall and/or the pore size slightly larger than the used template molecule, as above. Table 2 again summarizes the sensitivities to the C6 alkanes. Generally, the shape selectivity expected from the molecular size of the template was obtained. The effects of template molecules on the sensitivities to the C8 alkanes (Figure 6) were similar to those to the C6 alkanes shown above. Tin oxide revealed relatively high sensitivities to all of the used alkanes, while the use of benzaldehyde suppressed the sensitivity to the isomers with two methyl branches and cyclic molecule. The n-butyraldehyde template suppressed the sensitivities to them and the isomer with one methyl branch also. In summary, the presence of the template molecule during the CVD of silica resulted in the selective suppression of sensitivity to the alkane with large molecular size, while the sensitivities to the molecules smaller and comparable to the used template molecule, including the ones slightly larger than the template, were almost maintained. Thus, clear molecular recognition ability was obtained. It is proposed to combine the sensors which possess characteristic response with specific compounds in order to establish a “chemical imaging system” for “electronic nose”.50 For this purpose, the present series of chemical sensors with adjustable molecular shape recognition properties is presumed to have a large potential. Molecular Recognition in Catalysis. As shown in Figure 8, the change in activity was similar to the change (50) Go¨pel, W. Sens. Actuators, B 1998, 52, 125-142.

in sensitivity. Tin oxide showed high activities for all of the alkanes, while the use of benzaldehyde suppressed the sensitivity to the alkane with two methyl groups and a cyclic one, i.e., 2,2-dimethylbutane and cyclohexane. The small template molecule, i.e., n-butyraldehyde, suppressed the sensitivities to them and the singly branched isomer, namely, 2-methylpentane, also. Such a shape selectivity in catalysis has never been reported on these nonzeolitic solid catalysts and should be promising for the precise design of a highly selective solid catalyst. Surface Structure. We have clarified that the shapeselective adsorption ability appeared after the monolayer of silica completely covered the tin oxide39 and alumina38 surface uncovered by the template. In other words, complete coverage by the monolayer or accumulation of the multiple layer of silica is needed to generate the shape selectivity. This should be related to the expected scheme of the control of the surface structure in which the shape of a silica overlayer is controlled by filling the uncovered surface with silica (Scheme 1). The XPS showed almost 3/4 of the surface atomic ratio of Si/(Si + Sn) on all of the samples of SiO2/SnO2 prepared using the template, on not only the external surface but also the inside of the disk, as shown in Table 1. This ratio 3/ is consistent with an assumption that 3/ of the tin 4 4 oxide surface is covered with the silica layer whose thickness is larger than the escape depth of a photoelectron (1.5 and 2 nm in tin oxide and silica, respectively39). Because the thickness of the silica monolayer was estimated to be ca. 0.3 nm,51 the thickness of the present silica layer (>1.5 or 2 nm) would correspond to the multiply accumulated layer (5-7 layers). It is therefore estimated that the multiple layer of silica covers 3/4 of the surface. We adjusted the conditions for the adsorption of template to cover 1/3-1/2 of the surface with the template, as shown above; hence, the coverage by silica is expected to be 1/2 to 2/3. However, the coverage by silica determined by the XPS was 3/4, exceeding the expected value. It is suggested that a small fraction of the template molecule was desorbed or decomposed during the CVD of silicon alkoxide. Although the unexpected phenomenon was observed, the resulted high coverage is consistent with the concept shown in Scheme 1, in which the structure of silica was controlled by filling the uncovered surface with silica. On the other hand, the uniformity in distribution of Si on both the internal and external surfaces of the sample disk is emphasized. The sample disk was molded from the small particles with the high surface area (22.9 m2 g-1); the deposition of silica should proceed homogeneously on all of the surfaces of the small particles. This is in agreement with the mechanism of the CVD of silica, in which the kinetics of formation of the siloxane network controls the whole process, but the mass transfer does not affect the reaction in these conditions.29,30 Such a homo(51) The length of the Si-O-Si covalent bond is estimated to be ca. 0.3 nm, because the covalent radii of Si and O are 0.12 and 0.07 nm, respectively.47-49

Tin Oxide Chemical Sensor

geneous deposition of the silica layer must be required for the precise control of the surface structure. Conclusion The SiO2/SnO2 chemical sensor prepared by the CVD using an aldehyde molecule as the template showed high sensitivity to the alkane whose molecular size is slightly larger, comparable, or smaller than the used template,

Langmuir, Vol. 16, No. 8, 2000 3865

while sensitivities to the larger alkanes were significantly suppressed. The oxidation activity was also suppressed for alkane molecules larger than the used template. A clear molecular recognition function was thus observed on the present gas sensor.

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