Acoustic enhancement of surface reactions
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Introduction Metal and metal oxide surfaces provide unique sites for adsorption, surface reactions, and diffusion processes, and a variety of methods have been developed to enhance the physical and chemical properties of these surfaces. These techniques have typically been based on modifications to geometric structures on the nano- (meaning dispersed atoms or clusters) and macroscopic (various single crystals) scales. Electronic surface structures have been modified via doping with electronegative or electropositive elements, or by the addition of different components. Both the geometric and electronic surface characteristics are determined by the lattice atom arrangement at the surface. Thus, a technique that permits significant lattice distortion such that the atomic arrangements are modified would allow tuning of the surface properties. As an example, this technique could produce metal surfaces that are remarkably active during certain chemical reactions. The application of an electric field to a nonsymmetric crystal such as a ferroelectric material is known to deform the crystal.1,2 This is referred to as the piezoelectric effect. When an alternating voltage is imposed, the piezoelectric effect results in periodic crystal distortions and so generates acoustic waves. If thin films of metal or metal oxide catalysts are attached to the ferroelectric surface undergoing lattice distortion, the distortion can be transferred to the film. This process can produce unique surface sites with different atomic arrangements, since the thin-film surface is more vulnerable to lattice distortion
compared to the bulk material. The manner in which the physical and chemical properties of metal and metal oxide surfaces are affected by dynamic lattice distortions induced by acoustic waves has been previously reviewed.3 However, it is extremely important to have a detailed understanding of the manner in which acoustic waves are able to enhance various surface phenomena, and this article focuses on the acoustic enhancement of adsorption and surface reactions. Two types of acoustic waves are typically used in such processes. One is a surface acoustic wave ([SAW]; Rayleigh or shear horizontal leaky types, whose lattice displacement is perpendicular or parallel to the surface, respectively) and the other is a bulk acoustic wave, otherwise known as resonance oscillation (RO). The basic concepts associated with the acoustic activation of metal surfaces by SAWs are depicted in Figure 1. For the application of ferroelectric crystals as substrates, the most promising candidates are lithium niobate (LiNbO3; LN) and lithium tantalate (LiTaO3; LT) single crystals. In addition to single crystals, highly sintered polycrystalline materials are also available. One such material is lead zirconate tantalate, Pb(Zr,Ti)O3.
SAWs SAW generation For the generation of SAWs, interdigital transducer (IDT) metal electrode “fingers” are used—these are fabricated using photolithography on the surface of a disc composed of a
Yasunobu Inoue, The University of T
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