Solid State Ionics 136–137 (2000) 819–823www.elsevier.com/ locate / ssi

Effects of acoustic waves generated on a positively polarized leadstrontium zirconium titanate substrate upon catalytic activity of a

deposited Ag thin film

*N. Saito, Y. Ohkawara, K. Sato, Y. InoueDepartment of Chemistry, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan

Abstract

A positively polarized lead strontium zirconium titanate (PSZT) substrate was employed for the generation ofthickness-extensional mode resonance oscillation (TERO), and the effects of TERO on the catalytic activity and the surfaceproperties of a 100-nm Ag film catalyst deposited on the substrate were investigated. The catalytic activity for ethanoloxidation increased 18-fold with TERO at 3 W. In low energy photoelectron spectroscopy, a threshold energy forphotoelectric emission from the Ag surface shifted linearly to the lower energy side with increasing power of TERO, thusindicating a decrease in the work function of the Ag surface. On the basis of the behavior of lattice displacement measuredby a laser Doppler method, a model for changes in catalytic activity and work function is proposed. 2000 ElsevierScience B.V. All rights reserved.

Keywords: Thickness-extensional mode resonance oscillation; Ag thin film; Catalytic activity

1. Introduction activity of Pd thin films for ethanol oxidationincreased 1900-fold by thickness-extensional mode

Surface acoustic waves and resonance oscillations resonance oscillation (TERO) of z-cut LiNbO [9].3

generated by a piezoelectric effect have been applied Furthermore, TERO caused an increase in the activi-to thin film catalysts deposited on ferroelectric ty for ethylene production without enhancement ofsubstrates in an attempt to design heterogeneous the activity for acetaldehyde production in ethanolcatalysts which have artificially controllable func- decomposition on a Ag thin film, thus indicating thattions for chemical reactions [1–8]. A single crystal TERO has the capability to change reaction selectivi-of z-cut lithium niobate and a polycrystalline crystal ty [10].of lead strontium zirconium titanates (PSZT) have A ferroelectric PSZT crystal has the advantage ofbeen employed as ferroelectric substrate for the offering two different vibrations of TERO and radial-generation of resonance oscillation. The catalytic extensional mode resonance oscillation (RERO). An

interesting feature is that there are clear differencesin catalyst activation between the two modes of*Corresponding author. Tel.: 1 81-258-479-832; fax: 1 81-resonance oscillation [11]. We have investigated258-479-830.

E-mail address: inoue@analysis.nagaokaut.ac.jp (Y. Inoue). changes in the surface properties of Ag with RERO

0167-2738/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0167-2738( 00 )00506-3

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and TERO [11]. In low energy photoelectron spec- spectrometer (Riken Keiki, AC-1) was composed oftroscopy (LEPS), RERO caused no changes in the a deuterium lamp, a controller, and a gridded airphotoelectron emission pattern, whereas TERO in- counter which can quench the counter discharge andduced a negative shift of photoelectron emission suppress positive ion bombardment. The Ag catalystpattern for Ag deposited on a positively polarized sample was irradiated with monochromatized light ofPSZT. This phenomenon is very important to under- the deuterium lamp scanned in the range 200–360stand the acoustic wave excitation mechanism, since nm, and emitted electrons were counted. Measure-the shift is related to a change in the work function ments were done in air, dried air or oxygen atmos-of the Ag surface. Thus, in the present study, in phere. The excitation energy is nearly proportional to

aorder to reveal the TERO effect upon a Ag surface (CPS) where CPS represents the yield of photoelec-deposited on a positively polarized PSZT substrate, tron emission (count per second). Since the sample isdetailed LEPS measurements have been performed, metal, a was taken as 0.5.together with laser Doppler measurements to evalu-ate lattice displacement caused by TERO.

3. Results

2. Experimental Fig. 1 shows ethanol oxidation at 383 K over Agdeposited on a positively polarized PSZT substrate.

A poled ferroelectric polycrystalline When acetaldehyde production proceeded steadily,Pb Sr Zr Ti O (referred to here as PSZT) an electric power of 3 W was introduced to generate0.95 0.05 0.53 0.47 3

22with a spontaneous polarization of 28 mC cm and TERO. An immediate increase in acetaldehyde pro-a Curie temperature of 593 K was employed as duction occurred, and the enhanced production ofsubstrate. The sample had the form of a disc (25 mm acetaldehyde continued until power was turned off.in diameter and 0.2 mm in thickness) whose polari- The increased activity returned to an original levelzation axis was normal to the surface, thus exposing with power-off. The ratio of activity with TERO-ona positively polarized surface at one plane and anegatively polarized surface at the other. Both planesof the disk were first covered with catalyticallyinactive Ag paste electrodes for input of highfrequency electric power. Then the positively polar-ized plane was covered with an active Ag filmcatalyst at a thickness of 100 nm by an evaporationmethod with resistance heating of a pure Ag metal inhigh vacuum, whereas the negatively polarized planewas left intact.

High frequency electric power was generated froma network analyzer, amplified, and introduced to asample. Catalyst temperature was monitored by aradiation thermometer through a BaF window and2

controlled by an outer electric furnace. Catalyticoxidation of ethanol was carried out in a gas-circulat-ing apparatus, and the products were analyzed by agas chromatograph connected to the reaction system.

Electrons in metals are emitted from surfaceswhen irradiated by light with energy higher than the

Fig. 1. TERO effects on ethanol oxidation over Ag deposited on awork function. The photoelectron emission was positively polarized PSZT. Ethanol pressure P 5 4.0 kPa, oxygenemeasured by a LEPS method whose principle and pressure P 5 4.0 kPa, reaction temperature T 5 383 K, appliedo

apparatus were reported elsewhere [12]. Briefly, a power J 5 3 W.

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to that with TERO-off showed that the activityincreased 18-fold.

Fig. 2 shows the photoelectron emission patternsof LEPS with TERO-off and TERO-on. With TERO-off, photoelectrons emitted from the surface werenegligible below 4.5 eV, but began to appear at 4.57eV, above which emitted electrons increased remark-ably with increasing photon energy. This resultindicates that threshold energy for electron emissionwas 4.57 eV without TERO. With TERO-on at 1 W,electron emission pattern shifted to a lower photonenergy side, and the threshold energy decreased to

Fig. 3. A decrease in threshold energy with TERO power.

4.55 eV. Further negative shifts of the emissionpattern occurred with increases in TERO power to 2and 3 W. Fig. 3 shows the shift of threshold energyas a function of power. The threshold energy re-mained nearly unchanged at a power of lower than0.5 W and decreased nearly in proportion to TEROpower in the range 0.5–3 W. The threshold energyshift amounted to 2 0.15 eV at 3 W.

Fig. 4 shows threshold energy as a function offrequency applied to a sample. No shift of thresholdenergy occurred at around 10 MHz. With increasingfrequency, threshold energy began to gradually de-crease, followed by a dramatic decrease at 10.1MHz, passed through a minimum at 10.2 MHz, andincreased sharply with increasing frequency. ThisPSZT sample had a resonance frequency of 10.2MHz, and it is to be noted that a frequency corre-sponding to the appearance of the minimum thres-hold energy was exactly the same as that of theresonance frequency to generate TERO.

Lattice displacement was measured by a laserDoppler method in a direction vertical to the surface.Fig. 5 shows changes in lattice displacement withfrequency applied to a sample. Small lattice displace-ment occurred at around 10 MHz, gradually in-Fig. 2. Photoelectron emission pattern at different TERO power.creased with increasing frequency, passed through ah, power-on; j, power-off. Measurements were carried out in air

at room temperature. maximum at 10.2 MHz and then decreased steeply.

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4. Discussion

As shown in Fig. 3, the LEPS results demonstratedthat the threshold energy of electron emission pat-terns shifted to the lower energy side with increasingpower for the generation of TERO. Since the thres-hold energy corresponds to a parameter related to thework function of the Ag surface, this shift indicatesthat the work function of Ag decreases linearly withincreasing power in the higher power range. Theinteresting feature is that the work function of thethin film metal surface can be controlled in anartificial manner through TERO.

In a previous study on measurements of contactpotential difference, TERO produced negative volt-ages on the Ag surface deposited on a positivelypolarized PSZT, whereas no changes occurred withRERO [11]. Similar changes in surface potential

Fig. 4. Changes in threshold energy with TERO frequency. J 5 3were observed for Pd deposited on a positivelyW.polarized z-cut LiNbO substrate [13]. These results3

suggest that dynamic lattice movement vertical to thesurface is useful for the generation of negativevoltages. A mechanism that the combined effects ofsonic wave-electron interactions and the direction ofthe spontaneous polarization axis in PSZT are in-volved has been proposed [11]. Namely, the interac-tions of the sonic wave with electrons facilitate themovement, while a strong field due to polarizationaxis controls the direction of the electron movement.Thus, the positively polarized PSZT surface accumu-lates electrons at the surface, and hence the Ag filmin contact with this plane turns out to produce anegatively charged surface. The negative voltagesgenerated might be pointed out to be responsible forchanges in the work function. However, the extent ofits effect on work function was calculated to be0.008 eV or less. This value was too small to explainthe observed shift of 0.15 eV. Thus, the observedwork function shift is not explained in terms ofmerely static electric field effects at surface.Fig. 5. Lattice displacement vertical to the surface with TERO.

Laser Doppler measurements showed that theJ 5 3 W. Measurements were carried out in air at room tempera-ture. largest lattice displacement vertical to the surface

was induced at a frequency at which TERO wasgenerated. Thus, it is suggested that the changes inwork functions with TERO are associated with

Note that the frequency for maximum displacement dynamic behavior of lattice displacement at a nega-was in a good agreement with that observed for the tively charged surface. Work function of metals islargest threshold energy shift. controlled by two factors: cohesive energy of metal

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atoms in bulk and a double electric layer formed at a mum. However, the Ag thin films employed aresurface [14]. In particular, the latter electric layer polycrystalline whose surfaces have many imperfec-plays an important role in transition metals and is tions such as grain boundaries, dislocations, anddetermined by the extent of ‘spill-out’ electrons. A vacancies. Thus, the possibility of the geometricmodel is proposed that the dynamic lattice displace- effects still remains in case that the acoustic wave isment vertical to the surface has an influence on the concentrated on imperfect sites to such an extent thatdensity of the ‘spill-out’ electrons so as to reduce a it influences the arrangement of specific local struc-barrier of the double electric layer. tures.

Another possibility of work function decrease In conclusion, the TERO effects are characterizedwould be the desorption of a negatively-charged by dynamic lattice displacement vertical to theadsorbed species such as oxygen or water with surface and by changes in the work function-relatedTERO, since LEPS measurements were not per- electronic states which are proposed to be respon-formed in vacuum. However, the repetition of mea- sible for the catalytic activity enhancement.surements produced very good reproducibility, andexactly the same results were obtained when theatmosphere in the measurements was changed to air, Acknowledgementsdry air or pure oxygen. These results indicate that thecontribution of the desorption is small. This work was supported by a Grant-in-Aid for

A rate-determining step of ethanol oxidation on Scientific Research from the Ministry of Education,metal catalysts such as Pd and Ag is proposed to be Science, Sports and Culture.the abstraction of a hydrogen atom from an adsorbedethanol [4,13]. The previous study using surfaceacoustic waves has demonstrated that a strongly Referencesadsorbed reactant is predominantly influenced by theacoustic wave excitations [6] and that negatively [1] Y. Inoue, M. Matsukawa, K. Sato, J. Am. Chem. Soc. 111

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