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. 2013;3:2160.
doi: 10.1038/srep02160.

Fundamental Analysis of Piezocatalysis Process on the Surfaces of Strained Piezoelectric Materials

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Free PMC article

Fundamental Analysis of Piezocatalysis Process on the Surfaces of Strained Piezoelectric Materials

Matthew B Starr et al. Sci Rep. .
Free PMC article

Abstract

Recently, the strain state of a piezoelectric electrode has been found to impact the electrochemical activity taking place between the piezoelectric material and its solution environment. This effect, dubbed piezocatalysis, is prominent in piezoelectric materials because the strain state and electronic state of these materials are strongly coupled. Herein we develop a general theoretical analysis of the piezocatalysis process utilizing well-established piezoelectric, semiconductor, molecular orbital and electrochemistry frameworks. The analysis shows good agreement with experimental results, reproducing the time-dependent voltage drop and H₂ production behaviors of an oscillating piezoelectric Pb(Mg₁/₃Nb₂/₃)O₃-32PbTiO₃ (PMN-PT) cantilever in deionized water environment. This study provides general guidance for future experiments utilizing different piezoelectric materials, such as ZnO, BaTiO₃, PbTiO₃, and PMN-PT. Our analysis indicates a high piezoelectric coupling coefficient and a low electrical conductivity are desired for enabling high electrochemical activity; whereas electrical permittivity must be optimized to balance piezoelectric and capacitive effects.

Figures

Figure 1
Figure 1. Energy diagrams describing the electrochemistry and piezocatalysis process.
(a) The effect of applying a sufficient positive or negative bias to an electrode is to increase anodic and cathodic current, respectively. (b) In the case of a bare piezoelectric, conduction and valence bands act as the reservoirs for electrons donated or accepted from molecules in solution. The piezoelectric polarization (PPZ) applies a variable bias across the material, lifting and lowering valence band and conduction band energies. (c) Applying electrodes to the piezoelectric simplifies the electron reservoir to that of the metal's Fermi energy, while maintaining the piezoelectric potential as the source of bias.
Figure 2
Figure 2. The voltage drop across a strained piezoelectric is dependent upon the rate of capacitive and Faradic effects.
(a) The charged piezoelectric's surface induces both capacitive double layer effects and electron transfer events across the interface. (b) At high negative piezoelectric potentials, the electrode is thermodynamically capable of reducing all reported impurity ions in solution. As the potential increases below -2.0 volts, the majority of side reactions are no longer possible. (c) Depending on tp and the values of kinetic parameters fi,1 and fi,2, the rate of voltage drop experienced as a function of time can take on a wide range of values. (d) Applying a recursion procedure to the data obtained by Wang et al, values of tp = 0.042 s, fi,1 = 0.715 and fi,2 = 0.07 are determined.
Figure 3
Figure 3. The H2 production per straining event predicted as a function of strain magnitude.
(a) In the case of piezoelectric materials with metal electrodes on their surfaces. Inset shows the turn-on strain value is dependent on the ratio of formula image. (b) Comparison between experimental data of hydrogen generation over the course of an oscillating PMN-PT cantilever experiment and the predicted results from simulation under the conditions of poor kinetics (fi = 0.07). Inset shows the envelope of hydrogen generation predicted from simulation under the case of good kinetics (fi = 0.715) and poor kinetics. (c) In the case of bare surfaced piezoelectric materials. Inset shows the turn-on strain values depends upon both the ratio of formula image and the value of formula image.
Figure 4
Figure 4. Free charge can dramatically affect the piezocatalytic ability of a strained piezoelectric.
For a strain = .0.002, the effect of free charge on (a) piezoelectric potential and (b) H2 production for various piezocatalysis systems.

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