Effects of Rayleigh Surface Acoustic Wave upon Adsorptive and

The effects of Rayleigh surface acoustic wave (R-SAW) upon the adsorptive and surface properties of a thin NiO film were studied. A 20 MHz R-SAW devic...
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10602

J. Phys. Chem. B 2000, 104, 10602-10607

Effects of Rayleigh Surface Acoustic Wave upon Adsorptive and Surface Properties of a Thin NiO Film H. Nishiyama, N. Rattana, N. Saito, K. Sato, and Y. Inoue* Department of Chemistry, Nagaoka UniVersity of Technology, Nagaoka 940-2188, Japan ReceiVed: June 2, 2000; In Final Form: July 22, 2000

The effects of Rayleigh surface acoustic wave (R-SAW) upon the adsorptive and surface properties of a thin NiO film were studied. A 20 MHz R-SAW device using a LiNbO3 single crystal was designed to permit measurements of conductivity changes due to O2 adsorption and desorption during SAW propagation. The SAW caused considerable enhancement of conductivity increases due to O2 adsorption in an O2/He flow system and also promoted the desorption of the adsorbed oxygen upon evacuation. Laser Doppler measurements showed that R-SAW had regular standing wave patterns with lattice displacement vertical to the surface which decreased considerably in the presence of 40 nm NiO, compared to a NiO-free surface. The unique feature of promotion of both adsorption and desorption by the SAW is associated with periodic lattice displacement, and a mechanism that the SAW interacts with the carriers in NiO is proposed.

Introduction Surface acoustic waves (SAWs) generated on a poled ferroelectric crystal by a piezoelectric effect are characterized by crystal distortion.1-3 We have demonstrated that the SAWs are useful for the activation of thin film metal catalysts deposited on their propagation path. In ethanol oxidation on thin polycrystalline Ni, Pd, and Ag film catalysts, Rayleigh SAW (RSAW) and a shear horizontal leaky SAW (SH-LSAW) caused considerable enhancements of the catalyst activity.4-10 In the Cu catalyzed ethanol decomposition leading to dehydration and dehydrogenation, SH-LSAW increased the activity for dehydration (ethylene production) without significant changes in that of dehydrogenation (acetaldehyde production).11 These results indicate that the SAWs have the capability not only of increasing the catalytic activity but also of changing the selectivity of catalytic reactions. King and co-workers have shown that R-SAW had an effect on CO oxidation on a Pt{110} single crystal: the activity enhancement by the SAW was associated with the promotion of CO desorption.12-14 The interesting findings in ethanol oxidation on Pd and Ni catalysts were that activation efficiency, defined as the ratio of catalytic activity with SAW-on to that with SAW-off, was remarkably larger when the catalyst surfaces were oxidized, compared to those of the metallic surfaces. The plausible explanation is that the SAW has influences on electron transfer between the adsorbed oxygen and the oxidized catalyst surfaces. To verify this view, and, furthermore, for better understanding of a mechanism with the SAW-induced activation of metal oxide catalysts, it is of importance to reveal the SAW effects on charge transfer between the adsorbed species and the oxide surfaces. For this purpose, the comparison of conductivity changes due to gas adsorption in the absence and presence of SAW would be useful. In the present study, the adsorption of O2 on NiO and the desorption of adsorbed oxygen were investigated in detail. For surface conductivity measurements of a thin metal oxide film during SAW propagation, the electrodes have to be electrically separated from the interdigital transducer (IDT)

electrodes to which radio frequency electric power is applied for SAW generation. In our preliminary study, thin ZnO and NiO films were deposited on the propagation path of SH-LSAW. The conductivity changes due to the adsorption of ethanol on ZnO and of O2 on NiO during SAW propagation were examined,15 and it has been shown that conductivity measurements can be made during SAW propagation without electric interference with each other. On the basis of these results, the experimental approach was improved so as to achieve high sensitivity in conductivity measurements. In the fabrication of a SAW sample, there are two choices in the position of the electrodes for conductivity measurements. One is that the electrodes are placed on the top of a metal oxide surface which was first attached on a ferroelectric substrate, and the other is that the electrodes are sandwiched between the substrate and the metal oxide film. The latter structure has an advantage in providing a large geometric surface area of the metal oxide films exposed, because of no covering of the surface by the electrodes for conductivity measurements, and was employed in the present study. The other variation made in the present study was the employment of a flow system, instead of a previous static vacuum system,15 for O2 adsorption and desorption. Lattice displacement caused by the propagation of R-SAW was determined by a laser Doppler system developed in our laboratory for measurements of the three-dimensional patterns.16 In the present study, the patterns were obtained under in situ conditions over the temperature and pressure ranges similar to those of adsorption experiments. The distributions of lattice displacement and their changes were compared in the absence and presence of a thin NiO. The contact potential difference was measured to examine surface potential changes caused by SAW propagation. Experimental Section Figure 1 shows the structure of a sample for SAW-combined conductivity measurements. A pair of the IDT electrodes was photolithographically fabricated on both the ends of a poled

10.1021/jp002009t CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

R-SAW Effects upon Properties of a Thin NiO Film

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Figure 1. Structure of a SAW sample combined with electrodes for conductivity measurements.

Figure 3. XRD pattern of a 100 nm NiO film deposited on a 128° rotated Y-cut LiNbO3 and heated in a vacuum at 473 K.

Figure 2. Electric circuits for SAW propagation and surface conductivity measurements.

ferroelectric 128° rotated Y-cut LiNbO3 single crystal which generates R-SAW. In the middle of the two IDT electrodes, 100 nm thick Au electrodes for conductivity measurements were first deposited by resistance-heating of a pure Au metal. The direction of the Au electrodes was designed so that conductivity current was at right angles to SAW propagation in order to avoid interfering influences between them. On the Au electrodes, a thin NiO film was deposited at a thickness of 40 nm by radio frequency (rf) sputtering of a Ni target in a mixture of O2 (0.04 kPa) and Ar (0.16 kPa). Prior to adsorption experiments NiO was heated at 473 K in an O2/He atmosphere. Figure 2 shows a combined electric circuit for the SAW generation and conductivity measurements. Radio frequency power was generated from a network analyzer (Anritsu 3606B), amplified by a power amplifier (Kalmus 250FC), and then introduced to the samples after impedance adjustment through a network tuner. The surface conductivity changes with gas adsorption were obtained by measuring currents at constant voltages. All the data were accumulated in a computer.

Adsorption experiments were carried out in a flow system of atmospheric pressure. A SAW sample was first exposed to a flow of pure He gas, and the flow was switched to a mixture of He (60 cm3 min-1) and O2 (10 cm3 m-1) gas for the adsorption of O2. For oxygen desorption, the flow was changed to a pure He gas stream. Care was taken to control the surface temperatures of the sample through adsorption experiments in the absence and presence of R-SAW. For temperature measurements, two methods were employed. One was direct measurement by a noncontacting method using a calibrated radiation thermometer: a sample was placed in a glass cell equipped with a BaF2 window, through which the temperature of a NiO surface was monitored. The other was the use of a sensitive and good linear relationship between the frequency shifts of SAW and temperature changes,13,15 by which the fluctuation of sample temperature was evaluated from a frequency shift. The temperature rises were corrected by an outer electric furnace and maintained within (0.1 K. Description of the use of the laser Doppler system for measurements of lattice displacement has been given elsewhere.16 Briefly, a NiO sample was placed in the reaction cell for conductivity measurements and irradiated with a Ne-He laser beam through the BaF2 window. The wavelength shift of the reflected beam was monitored with a vibrometer (Ono Sokki LV 5300). Measurements were performed in a He flow in the temperature range 403-473 K. The contact potential difference was measured in air at room temperature by a dynamic condenser electrometer (Ando Elec. Co., AA2404). A sample was placed in parallel to the probe of the electrometer at a distance of 3 mm.17 Results Figure 3 shows an X-ray diffraction pattern of a 100 nm NiO film on the substrate. Two main peaks appeared at 2θ ) 37.2 and 43.3°, which were consistent with the major peaks of NiO powder and assigned to the reflection from the (101) and (012) planes, respectively. Figure 4 shows conductivity changes with time when a NiO film was first exposed to an O2/He mixture and then to He at 433 or 453 K. Without SAW, conductivity enhancement at 433 K occurred gradually and reached a nearly constant level. At

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Figure 6. Change in the saturation conductivity ratio, (σe)on/(σe)off, as a function of SAW power at 453 K. (σe)on and (σe)off are saturation conductivity with SAW-on and SAW-off, respectively.

Figure 4. Changes in surface conductivity of NiO with O2 adsorption and desorption at 453 and 473 K. SAW power: J ) 0.8 W.

Figure 7. Increase in the desorption rate of adsorbed oxygen with increasing SAW power at 453 K. (Vd)on/(Vd)off is the ratio of the desorption rate with SAW-on to that with SAW-off.

Figure 5. Larger conductivity increases with O2 adsorption by R-SAW: J ) 0.8 W, Adsorption temperature: Ta ) 453 K.

453 K, a larger conductivity change was obtained. The slow uptake of oxygen and larger conductivity changes at a higher temperature suggest that the O2 adsorption occurs via an activated process. With the O2/He mixture being switched to a pure He flow at 433 K, a slight decrease occurred. On the other hand, in the presence of SAW at 0.8 W, exposure to an O2/He mixture at the same temperature resulted in a fast and large conductivity increase. Interestingly, the removal of O2 from the flow (O2-off) caused a remarkable decrease in the achieved conductivity. A clear difference in conductivity changes between SAW-off and SAW-on was also observed at 453 K. To confirm reproducibility, the run of O2 adsorption was repeated with SAW-off and with SAW-on. As shown in Figure 5, the presence of SAW always led to larger conductivity increases. Figure 6 shows the SAW power dependence of the saturation conductivity ratio, (σe)on/(σe)off, which is defined as the ratio of saturation conductivity at adsorption equilibrium of O2 with SAW-on to that with SAW-off. The values increased gradually with increasing power in a low-power region below 1 W and became considerably large at 1.4 W. Changes in O2 desorption rate, Vd, with SAW were examined, and the extent of desorption

rate enhancement was expressed by the ratio of O2 desorption with SAW-on to that with SAW-off, (Vd)on/(Vd)off. As shown in Figure 7, the ratio increased in a slightly nonlinear manner with SAW power and attained a 5-fold larger value at 1.4 W than that without SAW. The presence of a thin NiO film on the propagation path caused the loss of SAW propagation. Figure 8 shows SAW propagation loss as a function of resistance of a thin NiO film deposited on the path. With decreasing resistance, the propagation loss increased significantly. Figure 9 shows the three-dimensional lattice displacement patterns of a NiO-free and NiO-containing surface at 403 K. The free surface exhibited a regular standing wave pattern with a periodic array: the average peak-to-peak distance of the array toward the x-direction was 102 µm, whereas the highest lattice displacement vertical to the surface along the z-direction was about 10 nm. The displacement pattern of the 40 nm NiOcontaining surface showed a standing wave pattern with the same array as that of the free surface with a considerably smaller vertical lattice displacement. Figure 10 shows the displacement patterns of the NiO-containing surface at 433 and 453 K. No significant changes in the patterns were observed, although there are slight decreases in the vertical displacement with increasing temperatures. At a raised temperature of 473 K, the pattern still provided a clear periodic array structure for both the free and the 40 nm NiO-containing surface, as shown in Figure 11. Figure

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Figure 8. Relation between SAW propagation loss and resistivity of NiO film deposited.

12 shows the distributions of lattice displacement at 473 K. The free surface provided a broad distribution having a maximum at 5 nm: the average lattice displacement was 4.4 nm. On the other hand, a 40 nm NiO surface exhibited a much sharper distribution with a maximum at 2.7 nm: the average displacement was 2.6 nm. It is to be noted that the presence of 40 nm NiO decreased the lattice displacement significantly. Measurements of the contact potential difference showed that no voltages were generated by R-SAW propagation even under the conditions of SAW power up to 1 W. Discussion NiO is a p-type semiconductor metal oxide, and the transfer of electrons from the oxide surface to an adsorbed species leads to the increase of hole density, thus resulting in conductivity enhancement. In other words, the surface conductivity increases upon exposure to an O2/He atmosphere are indicative of electron transfer from a NiO surface to an adsorbed oxygen. The conductivity increases of the NiO surface upon O2 adsorption were enhanced by the SAW. Interestingly, the desorption of the adsorbed oxygen from the NiO surface upon evacuation was also promoted by the SAW. Thus, these results clearly indicate that the SAW has the capability of enhancing not only conductivity increases due to O2 adsorption but also conductivity decreases with the desorption of the oxygen species. The enhancement effects of the SAW on both adsorption and desorption are characterized by nonlinear increases with increasing power (Figures 6 and 7). The unique feature of the SAW effects on the promotion of both adsorption and desorption is considered to be related to a periodic surface change generated by R-SAW. The SAW effects on conductivity increases correspond to an increase in the amount of adsorbed oxygen and/or to the enhancement of efficiency in electron transfer from a NiO surface to an adsorbed oxygen. The latter assumes that different kinds of the adsorbed oxygen such as neutral and differently polarized oxygen species are formed on the NiO surface, and the charge density is changed by R-SAW. In previous studies, the effects of SAW on the catalytic activity have been demonstrated to be different between the metallic and the oxidized surfaces. For a 200 nm Pd film catalyst deposited on the propagation path of 10 MHz R-SAW, the SAW caused an

Figure 9. Three-dimensional patterns of lattice displacement measured in a He flow at 403 K by a laser Doppler method for a free surface (a) and a NiO-depositing surface (b): J ) 0.8 W. Sharp spiky signals involved in the patterns are due to noise.

increase in activity by a factor of 87 for ethanol oxidation on an oxidized Pd surface, but a slight increase of 1.3-fold for a metallic Pd surface.9 For the same reaction on a 47 nm Ni film catalyst, the effect of 20 MHz SH-LSAW was 3.6-fold larger for an oxidized than for a metallic Ni surface.7 Thus, it is evident that the SAW effects are significantly larger for the oxide surfaces. Furthermore, a kinetic study showed that reaction order with respect to oxygen pressure decreased from +0.5 with SAW-off to -0.5 with SAW-on for the oxidized Ni, whereas no significant changes in the reaction order occurred for the metallic Ni surface.7 These reaction order changes indicate that stronger oxygen adsorption is induced by the SAW on the oxidized Ni surfaces. This result suggests that large conductivity increases with oxygen adsorption by the SAW are associated with increases of strongly adsorbed oxygen. The three-dimensional images obtained by laser Doppler measurements showed the regular standing wave patterns of lattice displacement which was characteristic of R-SAW. The average distance of peak-to-peak along the propagation direction was 102 µm, which is in good agreement with half of the wavelength, 100 µm, of the R-SAW. At a high temperature of 473 K, the characteristic pattern images were still observed with a clear line separation for both NiO-free and 40 nm NiO-

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Figure 10. Three-dimensional patterns of lattice displacement measured in a He flow at 433 (a) and 453 K (b) by a laser Doppler method for a NiO-depositing surface: J ) 0.8 W.

Figure 11. Three-dimensional patterns of lattice displacement measured in a He flow at 473 K by a laser Doppler method for a free surface (a) and NiO-depositing surface (b): J ) 0.8 W.

containing surfaces, thus indicating that lattice displacement was induced to a considerable extent even at the high temperatures. The distributions of lattice displacement showed that a 40 nm NiO-containing surface had smaller lattice displacement than did a NiO-free surface (Figure 12). This demonstrates that a part of the SAW energy for elastic deformation is expended for the interactions with the charge carriers in the NiO. In a previous study, an electroluminescent Cu-doped ZnS film was deposited on the SAW propagation path, and it was shown that a pulse R-SAW through the film produced strong acoustoluminescence.5 This demonstrated that a strong electric field was generated at the surface by R-SAW propagation. A correlation between acoustoluminescent intensity and SAW power was consistent with Lakin’s equation18 derived on the basis of perturbation theory for electromagnetic to elastic surface waves. According to the equation, an electric field generated by R-SAW was calculated to be 106 V m-1 at a power of 1 W. When we measured surface potential by the contact potential difference, however, no significant voltage generation was detected. These two contradictory results are explained in terms of the characteristic lattice displacement of R-SAW. A positive and a negative electric field are generated according to the geometric deformation of lattice displacement. R-SAW is composed of regular lattice displacement in which the densities

of the top and bottom positions in the wave are nearly the same. Thus, it follows that such equivalent distributions of the opposite fields lead to no voltage appearance as total voltage. This corresponds to the results observed in measurements of the contact potential difference. On the other hand, a positive or negative field can be detected in acoustoluminescent measurements. This explanation is considered to be rational when one sees that irregular lattice displacement caused by the SH-LSAW gave rise to the generation of total voltages in measurements of the contact potential difference,11 in addition to acoustoluminescence.19 From these results, it is evident that the R-SAW induces the positive and negative electric fields which have significant influences on the behavior of the charge carriers in semiconductor NiO. Considerable propagation loss of R-SAW occurred by the deposition of 40 nm NiO, compared to a NiO-free surface. Bierbaum showed that the intensity of SAW attenuated by the deposition of thin metal films on the propagation path.20 The propagation loss of the SAW was associated with the resistivity of the metal films: with increasing resistivity, propagation loss increased, passed through a maximum, and decreased.20 The changes are explained in terms of interactions of the electric fields accompanying the surface wave with the charge carriers in the metal films. The results of Figure 8 showed that R-SAW

R-SAW Effects upon Properties of a Thin NiO Film

J. Phys. Chem. B, Vol. 104, No. 45, 2000 10607 NiO surface facilitate electron transfer from the NiO surface to an adsorbed oxygen. Under the conditions where the gaseous oxygen is in equilibrium with adsorbed oxygen, this situation leads to large conductivity increases. On the other hand, the electron deficient sites give rise to the weak adsorption of O2, which results in easier desorption of the adsorbed oxygen when gas-phase oxygen is evacuated. This mechanism explains the SAW-induced activation of both adsorption and desorption. In conclusion, the characteristic feature is that the SAW has remarkable effects on the activation of both O2 adsorption on and desorption from NiO which are associated with the periodic properties of lattice displacement by the SAW. Acknowledgment. This work was supported by a Grantin-Aid of Scientific Research (B) from The Ministry of Education, Science, Sports and Culture. References and Notes

Figure 12. Different distributions of lattice displacement in the absence (a) and presence (b) of NiO at 473 K.

propagation decreased with decreasing resistivity of a NiO film, which demonstrates that the R-SAW has interactions with the charge carriers in the NiO. A decrease in lattice displacement in the presence of a NiO film, compared to that of a NiO-free surface, gives support to this view, since the decrease is understood as a consequence of the SAW-carrier charge interactions. On the basis of these results, a mechanism of SAWactivation effects on both O2 adsorption and its desorption is proposed. In the R-SAW where lattice displacement accompanies electric fields, the SAW-carrier interactions lead to electron accumulation and scattering according to a maximum amplitude (top) and a minimum (bottom) of the wave. The accumulation and scattering occurs alternatively with periodicity, which repeats electron enrichment and deficiency at the same site. It is highly plausible that the electron-enriched sites of a

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