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Effects of resonance oscillation with a thickness-extensional mode on activation of thin film metal and metal oxide catalysts deposited on poled ferroelectric substrates
Applied Surface Science 144–145 Ž1999. 385–389
Effects of resonance oscillation with a thickness-extensional mode on activation of thin film metal and metal oxide catalysts deposited on poled ferroelectric substrates N. Saito, H. Nishiyama, K. Sato, Y. Inoue
)
Department of Chemistry, Nagaoka UniÕersity of Technology, Nagaoka, Niigata 940-2188, Japan
Abstract The effects of resonance oscillation ŽRO. with a thickness-extensional mode generated in a poled ferroelectric z-cut LiNbO 3 single crystal by rf power were studied on reaction selectivity for the ethanol decomposition of thin film Au, Ag and WO 3 catalysts deposited. For a 100 nm Ag catalyst, a 3.5 MHz RO at 3 W caused an increase in ethylene production at 573 K by a factor of 16 without any change in acetaldehyde production. The selective promotion of ethylene production by RO was also observed at 573 K for a Au catalyst in which the activity for ethylene production increased by a factor of 5. For a WO 3 catalyst the RO increased both the ethylene and acetaldehyde productions, but the former was more largely promoted than the latter. The RO caused decreases in respective activation energy. The selective production of ethylene is associated with the RO-induced strong oxygen–metal Žand metal oxide. surface interactions which are produced by dynamic lattice displacement. q 1999 Elsevier Science B.V. All rights reserved. PACS: 82.65.J Keywords: Resonance oscillation; LiNbO 3 ; Ag catalyst
1. Introduction In heterogeneous catalysis by metals and metal oxides, the design of catalyst surfaces with artificially controllable functions is very important and challenging. Recently, we have demonstrated that resonance oscillation ŽRO. generated on a poled ferroelectric crystal by rf electric power has the feature of dynamic lattice displacement which is effective for the activation of metal catalysts w1–3x. )
Corresponding author. Tel.: q81-258-47-9832; Fax: q81258-47-9830; E-mail: [email protected]
A thickness-extensional mode RO generated at 3 W on a z-cut LiNbO 3 single crystal was found to increase the catalytic activity for ethanol oxidation of a deposited Pd catalyst by a factor of 1880 w4x. More recently, for the same reaction on a Ag catalyst deposited on polycrystalline Pb 0.95 Sr 0.05 Zr0.57 Ti 0.43 O 3 ŽPSZT. crystal, it has been shown that a thickness-extensional mode RO caused unique polarized surface-dependent changes in the reaction behavior, which was completely different from a radial-extensional mode RO w5x. From a viewpoint of high catalyst performance, it is strongly desirable to artificially control the selec-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 8 2 8 - 9
386
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389
tivity of catalytic reactions. Thus, the present study was undertaken to reveal the effects of a thicknessextensional mode RO on catalyst selectivity. The thin film catalysts of Au and Ag metals and of a WO 3 metal oxide were employed for ethanol decomposition to produce ethylene and acetaldehyde. For Au and Ag metal catalysts, RO was found to increase ethylene production without any change in acetaldehyde production which indicated the selective promotion of ethylene production. For a WO 3 catalyst, the activity for both the products were accelerated, but the ethylene production was more significantly promoted than the acetaldehyde production. The RO effects are concluded to be useful for the design of metal and metal oxide surfaces with artificially controllable functions for reaction selectivity.
Fig. 2. RO effects on ethanol decomposition on a Aurz-LN catalyst. T s 573 K, J s 3 W, Pes 4.0 kPa. B; ethylene, v; acetaldehyde.
A ferroelectric single crystal of z-cut LiNbO 3 Žreferred to as z-LN. whose polarization axis is perpendicular to the surface was cut into a rectangle shape and used as substrate for RO generation w4x. Both of the front and back crystal planes were covered with a 100 nm thick Ag or Au film by resistance-heating of a respective pure metal in high vacuum. Au wires were attached to the thin films for rf power introduction. These catalysts are referred to as Agrz-LN and Aurz-LN, respectively. For a
WO 3rz-LN catalyst, z-LN crystal planes were first covered with Al electrodes and then with a 100 nm WO 3 film by reactive sputtering using a WO 3 target in an ArrO 2 atmosphere w6x. For the RO generation, rf power from a network analyzer ŽAnritsu MS3606B. was amplified by an amplifier ŽKalmus, 250FC. and then applied to the sample after impedance adjustment. The catalysts were placed in a glass cell equipped with BNC-junctions for rf-power introduction. The catalytic ethanol decomposition was carried out in a conventional gas circulating vacuum apparatus, and the reactant and products were analyzed by a gas chromatograph directly connected to the reaction system. Measurements of catalyst temperatures were performed by a
Fig. 1. RO effects on ethanol decomposition on a Agrz-LN catalyst. Reaction temperature T s 573 K, RO power J s 3 W. Ethanol pressure Pes 4.0 kPa. B; ethylene, v; acetaldehyde.
Fig. 3. Arrhenius plots of ethylene production rate, Ve , in ethanol decomposition on a WO 3 rz-LN catalyst. J s 3 W, Pes1.3 kPa. B: RO-off, I: RO-on.
2. Experimental
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389
Fig. 4. Arrhenius plots of acetaldehyde production rate, Va , in ethanol decomposition on a WO 3 rz-LN catalyst. J s 3 W, Pes 1.3 kPa. v: RO-off, `: RO-on.
387
Fig. 1 shows ethanol decomposition at 573 K on a Agrz-LN catalyst. Without RO, the reaction produced both ethylene and acetaldehyde with constant rates from an initial stage in which the former was slightly largely produced. When RO was generated at 3 W, an immediate increase in the ethylene production occurred, and the activity increased by a factor of 16. The increased activity was maintained as far as RO was turned on and decreased to the original level with RO-off. On the other hand, for acetaldehyde production, no significant change was observed with RO-on and RO-off. The selectivity for ethylene production is defined as the ratio of activity for ethylene production to that for total production of ethylene and acetaldehyde, i.e., S s 100 = Ver Ž Ve q Va .
non-contacting method using a radiation thermometer and by a shift in resonance frequency. These temperatures were calibrated with those measured by a thermocouple directly attached to a sample surface. The temperature of catalyst surface was controlled using an outer electric furnace.
3. Results As demonstrated previously, a z-LN crystal has a thickness mode vibration w4,7x. A sample depositing 100 nm Au thin film showed the same resonance lines at a frequency of 3.5 ŽFirst., 10.5 Žsecond. and 17.5 Žthird. MHz at room temperature as observed previously w4x. These frequencies were in accordance with a series of 2 k y 1 where k s 1, 2, 3. In the present work, the primary resonance frequency of 3.5 MHz Ž k s 1. was used for catalyst activation, unless otherwise specified.
where Ve and Va are ethylene and acetaldehyde production activity, respectively. The value of S was 58% with RO-off and increased to 90% with RO-on at 3 W. Fig. 2 shows a result of ethanol decomposition on a Au catalyst in which acetaldehyde is a major product. Turning the RO on resulted in an immediate increase in ethylene production activity by a factor of 5, whereas the acetaldehyde production remained without changes. The value of S increased from 6% to 10% by RO. It is to be noted that the selective RO effect on the ethylene production occurs for both Ag and Au in which the major product is ethylene and acetaldehyde, respectively. From the Arrhenius plots of ethylene and acetaldehyde production rate over the temperature range 573–653 K, activation energy was compared in the presence and absence of RO. The activation energy for ethylene production decreased from 155 to 145 kJ moly1 and that for
Table 1 Changes in activation energy and ethylene selectivity with RO Catalyst
Activation energy ŽkJ moly1 . Ethylene
Agrz-LN Aurz-LN WO 3rz-LN
S Ž%. at 573 K Acetaldehyde
RO-off
RO-on
RO-off
RO-on
RO-off
RO-on
156 155 115
115 145 64
95 91 46
95 91 21
58 6 13
90 10 30
388
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389
acetaldehyde production, 91 kJ moly1 , remained unchanged. Fig. 3 shows the Arrhenius plots of the ethylene production on a WO 3rz-LN catalyst. Without RO, the slope of the plot yielded an activation energy for ethylene production of 115 kJ moly1 . With RO-on, it decreased to 64 kJ moly1 . Fig. 4 shows the Arrhenius plots for acetaldehyde production. The activation energy decreased from 46 kJ moly1 with RO-off to 21 kJ moly1 with RO-on. The RO effects on the ethanol decomposition over the three catalysts are summarized in Table 1.
4. Discussion The interesting features of RO effects on the ethanol decomposition on the Ag and Au metals are that only the ethylene production was enhanced without giving rise to significant changes in the acetaldehyde production, irrespective of which product is a major one. On the other hand, the RO effect on the same reaction over the WO 3 metal oxide was preferable enhancement of ethylene production, although acetaldehyde activity was also increased to a lesser extent. In measurements of temperature dependence of resonance frequency, it was shown that little temperature rise occurred with a z-LN crystal when 3 W RO was applied at higher temperatures than 520 K. Since the present reactions were carried out above this temperature, a temperature rise during RO-on is negligible. The fact that only ethylene production was accelerated without any increase in acetaldehyde production in spite of as high activation energy as 91-95 kJ moly1 of acetaldehyde production on Ag and Au catalysts gives evidence to little rise in reaction temperatures with RO-on. Therefore, we can conclude that no thermal effect contributes to the selectivity changes. In a previous kinetic study of ethanol oxidation on a positively polarized Pdrz-LN catalyst, the reaction orders with respect to oxygen pressure decreased from 0.5 to y0.1 with RO-on w8x. This change has been related to the RO-induced stronger bond formation of adsorbed oxygen, thus suggesting that the RO affects the electronic and geometric factors of the Pd surface so as to produce strong oxygen-Pd surface
atom interactions. In previous measurements of photoemitted electrons from a Ag thin film deposited on a positively polarized PSZT substrate w5x, a thickness-extensional mode RO caused a negative shift of photoemission threshold energy by 0.12 eV, which is associated with a decrease in work functions of the Ag metal. This result is indicative of RO effects on the electronic or geometric properties of the thin film metal surfaces. It is likely that similar changes take place with the Agrz-LN catalyst. Thus, an increase in ethylene selectivity with RO is associated with a result that RO induces strong oxygen–Ag surface interactions which promote the abstraction of H 2 O from adsorbed ethanol and hence accelerate dehydration reaction to ethylene. Thin film metal oxides deposited on the propagation path of surface acoustic waves ŽSAWs. have been shown to cause not only significant SAW propagation loss, but also decreases in the amplitude of SAW-induced lattice displacement w9x. We have recently showed that the thin films of NiO and ZnO changes their adsorptive properties in the presence of SAWs w10x. These changes are associated with interactions between charge carriers in metal oxides and SAWs w11x. Since RO has a common feature with SAWs in dynamic lattice displacement, it is likely that charge carriers in WO 3 are influenced by RO which affects the density of electrons responsible for the adsorption of ethanol. An increase in ethylene selectivity of the reaction on WO 3 indicates that the strong interactions with oxygen occur, as demonstrated in the metal catalysts, although their extent is rather weak, compared to that of the metals. In conclusion, RO is effective for the control of catalyst selectivity and promising to the design of metal and metal oxide catalyst surfaces which permit the artificial control of selectivity.
References w1x Y. Inoue, J. Chem. Soc., Faraday Trans. 90 Ž1994. 815. w2x Y. Inoue, Y. Ohkawara, J. Chem. Soc. Chem. Commun. Ž1995. 2101. w3x Y. Ohkawara, N. Saito, Y. Inoue, Surf. Sci. 357–358 Ž1996. 777. w4x N. Saito, Y. Ohkawara, Y. Watanabe, Y. Inoue, Appl. Surf. Sci. 121–122 Ž1997. 343.
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389 w5x Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 Ž1998. 502. w6x N. Saito, Y. Ohkawara, K. Sato, Y. Inoue, in: N.M. Rodriguez, S.L. Soled, J. Hrbek ŽEds.., Recent Advances in Catalytic Materials, Vol. 497, MRS, 1998, p. 215. w7x T. Ikeda, Fundamentals of Piezoelectricity, Oxford Univ. Press, 1990.
389
w8x N. Saito, K. Sato, Y. Inoue, Surf. Sci. 417 Ž1998. 384. w9x H. Nishiyama, N. Saito, M. Shima, Y. Watanabe, Y. Inoue, Faraday Discuss. 107 Ž1997. 435. w10x H. Nishiyama, N. Saito, H. Chou, K. Sato, Y. Inoue, Surf. Sci., in press. w11x P. Bierbaum, Appl. Phys. Lett. 2 Ž1972. 15.
Effects of resonance oscillation with a thickness-extensional mode on activation of thin film metal and metal oxide catalysts deposited on poled ferroelectric substrates N. Saito, H. Nishiyama, K. Sato, Y. Inoue
)
Department of Chemistry, Nagaoka UniÕersity of Technology, Nagaoka, Niigata 940-2188, Japan
Abstract The effects of resonance oscillation ŽRO. with a thickness-extensional mode generated in a poled ferroelectric z-cut LiNbO 3 single crystal by rf power were studied on reaction selectivity for the ethanol decomposition of thin film Au, Ag and WO 3 catalysts deposited. For a 100 nm Ag catalyst, a 3.5 MHz RO at 3 W caused an increase in ethylene production at 573 K by a factor of 16 without any change in acetaldehyde production. The selective promotion of ethylene production by RO was also observed at 573 K for a Au catalyst in which the activity for ethylene production increased by a factor of 5. For a WO 3 catalyst the RO increased both the ethylene and acetaldehyde productions, but the former was more largely promoted than the latter. The RO caused decreases in respective activation energy. The selective production of ethylene is associated with the RO-induced strong oxygen–metal Žand metal oxide. surface interactions which are produced by dynamic lattice displacement. q 1999 Elsevier Science B.V. All rights reserved. PACS: 82.65.J Keywords: Resonance oscillation; LiNbO 3 ; Ag catalyst
1. Introduction In heterogeneous catalysis by metals and metal oxides, the design of catalyst surfaces with artificially controllable functions is very important and challenging. Recently, we have demonstrated that resonance oscillation ŽRO. generated on a poled ferroelectric crystal by rf electric power has the feature of dynamic lattice displacement which is effective for the activation of metal catalysts w1–3x. )
Corresponding author. Tel.: q81-258-47-9832; Fax: q81258-47-9830; E-mail: [email protected]
A thickness-extensional mode RO generated at 3 W on a z-cut LiNbO 3 single crystal was found to increase the catalytic activity for ethanol oxidation of a deposited Pd catalyst by a factor of 1880 w4x. More recently, for the same reaction on a Ag catalyst deposited on polycrystalline Pb 0.95 Sr 0.05 Zr0.57 Ti 0.43 O 3 ŽPSZT. crystal, it has been shown that a thickness-extensional mode RO caused unique polarized surface-dependent changes in the reaction behavior, which was completely different from a radial-extensional mode RO w5x. From a viewpoint of high catalyst performance, it is strongly desirable to artificially control the selec-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 8 2 8 - 9
386
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389
tivity of catalytic reactions. Thus, the present study was undertaken to reveal the effects of a thicknessextensional mode RO on catalyst selectivity. The thin film catalysts of Au and Ag metals and of a WO 3 metal oxide were employed for ethanol decomposition to produce ethylene and acetaldehyde. For Au and Ag metal catalysts, RO was found to increase ethylene production without any change in acetaldehyde production which indicated the selective promotion of ethylene production. For a WO 3 catalyst, the activity for both the products were accelerated, but the ethylene production was more significantly promoted than the acetaldehyde production. The RO effects are concluded to be useful for the design of metal and metal oxide surfaces with artificially controllable functions for reaction selectivity.
Fig. 2. RO effects on ethanol decomposition on a Aurz-LN catalyst. T s 573 K, J s 3 W, Pes 4.0 kPa. B; ethylene, v; acetaldehyde.
A ferroelectric single crystal of z-cut LiNbO 3 Žreferred to as z-LN. whose polarization axis is perpendicular to the surface was cut into a rectangle shape and used as substrate for RO generation w4x. Both of the front and back crystal planes were covered with a 100 nm thick Ag or Au film by resistance-heating of a respective pure metal in high vacuum. Au wires were attached to the thin films for rf power introduction. These catalysts are referred to as Agrz-LN and Aurz-LN, respectively. For a
WO 3rz-LN catalyst, z-LN crystal planes were first covered with Al electrodes and then with a 100 nm WO 3 film by reactive sputtering using a WO 3 target in an ArrO 2 atmosphere w6x. For the RO generation, rf power from a network analyzer ŽAnritsu MS3606B. was amplified by an amplifier ŽKalmus, 250FC. and then applied to the sample after impedance adjustment. The catalysts were placed in a glass cell equipped with BNC-junctions for rf-power introduction. The catalytic ethanol decomposition was carried out in a conventional gas circulating vacuum apparatus, and the reactant and products were analyzed by a gas chromatograph directly connected to the reaction system. Measurements of catalyst temperatures were performed by a
Fig. 1. RO effects on ethanol decomposition on a Agrz-LN catalyst. Reaction temperature T s 573 K, RO power J s 3 W. Ethanol pressure Pes 4.0 kPa. B; ethylene, v; acetaldehyde.
Fig. 3. Arrhenius plots of ethylene production rate, Ve , in ethanol decomposition on a WO 3 rz-LN catalyst. J s 3 W, Pes1.3 kPa. B: RO-off, I: RO-on.
2. Experimental
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389
Fig. 4. Arrhenius plots of acetaldehyde production rate, Va , in ethanol decomposition on a WO 3 rz-LN catalyst. J s 3 W, Pes 1.3 kPa. v: RO-off, `: RO-on.
387
Fig. 1 shows ethanol decomposition at 573 K on a Agrz-LN catalyst. Without RO, the reaction produced both ethylene and acetaldehyde with constant rates from an initial stage in which the former was slightly largely produced. When RO was generated at 3 W, an immediate increase in the ethylene production occurred, and the activity increased by a factor of 16. The increased activity was maintained as far as RO was turned on and decreased to the original level with RO-off. On the other hand, for acetaldehyde production, no significant change was observed with RO-on and RO-off. The selectivity for ethylene production is defined as the ratio of activity for ethylene production to that for total production of ethylene and acetaldehyde, i.e., S s 100 = Ver Ž Ve q Va .
non-contacting method using a radiation thermometer and by a shift in resonance frequency. These temperatures were calibrated with those measured by a thermocouple directly attached to a sample surface. The temperature of catalyst surface was controlled using an outer electric furnace.
3. Results As demonstrated previously, a z-LN crystal has a thickness mode vibration w4,7x. A sample depositing 100 nm Au thin film showed the same resonance lines at a frequency of 3.5 ŽFirst., 10.5 Žsecond. and 17.5 Žthird. MHz at room temperature as observed previously w4x. These frequencies were in accordance with a series of 2 k y 1 where k s 1, 2, 3. In the present work, the primary resonance frequency of 3.5 MHz Ž k s 1. was used for catalyst activation, unless otherwise specified.
where Ve and Va are ethylene and acetaldehyde production activity, respectively. The value of S was 58% with RO-off and increased to 90% with RO-on at 3 W. Fig. 2 shows a result of ethanol decomposition on a Au catalyst in which acetaldehyde is a major product. Turning the RO on resulted in an immediate increase in ethylene production activity by a factor of 5, whereas the acetaldehyde production remained without changes. The value of S increased from 6% to 10% by RO. It is to be noted that the selective RO effect on the ethylene production occurs for both Ag and Au in which the major product is ethylene and acetaldehyde, respectively. From the Arrhenius plots of ethylene and acetaldehyde production rate over the temperature range 573–653 K, activation energy was compared in the presence and absence of RO. The activation energy for ethylene production decreased from 155 to 145 kJ moly1 and that for
Table 1 Changes in activation energy and ethylene selectivity with RO Catalyst
Activation energy ŽkJ moly1 . Ethylene
Agrz-LN Aurz-LN WO 3rz-LN
S Ž%. at 573 K Acetaldehyde
RO-off
RO-on
RO-off
RO-on
RO-off
RO-on
156 155 115
115 145 64
95 91 46
95 91 21
58 6 13
90 10 30
388
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389
acetaldehyde production, 91 kJ moly1 , remained unchanged. Fig. 3 shows the Arrhenius plots of the ethylene production on a WO 3rz-LN catalyst. Without RO, the slope of the plot yielded an activation energy for ethylene production of 115 kJ moly1 . With RO-on, it decreased to 64 kJ moly1 . Fig. 4 shows the Arrhenius plots for acetaldehyde production. The activation energy decreased from 46 kJ moly1 with RO-off to 21 kJ moly1 with RO-on. The RO effects on the ethanol decomposition over the three catalysts are summarized in Table 1.
4. Discussion The interesting features of RO effects on the ethanol decomposition on the Ag and Au metals are that only the ethylene production was enhanced without giving rise to significant changes in the acetaldehyde production, irrespective of which product is a major one. On the other hand, the RO effect on the same reaction over the WO 3 metal oxide was preferable enhancement of ethylene production, although acetaldehyde activity was also increased to a lesser extent. In measurements of temperature dependence of resonance frequency, it was shown that little temperature rise occurred with a z-LN crystal when 3 W RO was applied at higher temperatures than 520 K. Since the present reactions were carried out above this temperature, a temperature rise during RO-on is negligible. The fact that only ethylene production was accelerated without any increase in acetaldehyde production in spite of as high activation energy as 91-95 kJ moly1 of acetaldehyde production on Ag and Au catalysts gives evidence to little rise in reaction temperatures with RO-on. Therefore, we can conclude that no thermal effect contributes to the selectivity changes. In a previous kinetic study of ethanol oxidation on a positively polarized Pdrz-LN catalyst, the reaction orders with respect to oxygen pressure decreased from 0.5 to y0.1 with RO-on w8x. This change has been related to the RO-induced stronger bond formation of adsorbed oxygen, thus suggesting that the RO affects the electronic and geometric factors of the Pd surface so as to produce strong oxygen-Pd surface
atom interactions. In previous measurements of photoemitted electrons from a Ag thin film deposited on a positively polarized PSZT substrate w5x, a thickness-extensional mode RO caused a negative shift of photoemission threshold energy by 0.12 eV, which is associated with a decrease in work functions of the Ag metal. This result is indicative of RO effects on the electronic or geometric properties of the thin film metal surfaces. It is likely that similar changes take place with the Agrz-LN catalyst. Thus, an increase in ethylene selectivity with RO is associated with a result that RO induces strong oxygen–Ag surface interactions which promote the abstraction of H 2 O from adsorbed ethanol and hence accelerate dehydration reaction to ethylene. Thin film metal oxides deposited on the propagation path of surface acoustic waves ŽSAWs. have been shown to cause not only significant SAW propagation loss, but also decreases in the amplitude of SAW-induced lattice displacement w9x. We have recently showed that the thin films of NiO and ZnO changes their adsorptive properties in the presence of SAWs w10x. These changes are associated with interactions between charge carriers in metal oxides and SAWs w11x. Since RO has a common feature with SAWs in dynamic lattice displacement, it is likely that charge carriers in WO 3 are influenced by RO which affects the density of electrons responsible for the adsorption of ethanol. An increase in ethylene selectivity of the reaction on WO 3 indicates that the strong interactions with oxygen occur, as demonstrated in the metal catalysts, although their extent is rather weak, compared to that of the metals. In conclusion, RO is effective for the control of catalyst selectivity and promising to the design of metal and metal oxide catalyst surfaces which permit the artificial control of selectivity.
References w1x Y. Inoue, J. Chem. Soc., Faraday Trans. 90 Ž1994. 815. w2x Y. Inoue, Y. Ohkawara, J. Chem. Soc. Chem. Commun. Ž1995. 2101. w3x Y. Ohkawara, N. Saito, Y. Inoue, Surf. Sci. 357–358 Ž1996. 777. w4x N. Saito, Y. Ohkawara, Y. Watanabe, Y. Inoue, Appl. Surf. Sci. 121–122 Ž1997. 343.
N. Saito et al.r Applied Surface Science 144–145 (1999) 385–389 w5x Y. Ohkawara, N. Saito, K. Sato, Y. Inoue, Chem. Phys. Lett. 286 Ž1998. 502. w6x N. Saito, Y. Ohkawara, K. Sato, Y. Inoue, in: N.M. Rodriguez, S.L. Soled, J. Hrbek ŽEds.., Recent Advances in Catalytic Materials, Vol. 497, MRS, 1998, p. 215. w7x T. Ikeda, Fundamentals of Piezoelectricity, Oxford Univ. Press, 1990.
389
w8x N. Saito, K. Sato, Y. Inoue, Surf. Sci. 417 Ž1998. 384. w9x H. Nishiyama, N. Saito, M. Shima, Y. Watanabe, Y. Inoue, Faraday Discuss. 107 Ž1997. 435. w10x H. Nishiyama, N. Saito, H. Chou, K. Sato, Y. Inoue, Surf. Sci., in press. w11x P. Bierbaum, Appl. Phys. Lett. 2 Ž1972. 15.