Piezopotential-Driven Redox Reactions at the Surface of Piezoelectric Materials

Themanipulation of charge-carrier conduction characteristics is a critical attribute governing the operation and efficiency of photovoltaic, catalytic, and other energy-converting systems that are based on electrochemical principles. This manipulation is often accomplished through the application of electrical-potential gradients by an external power supply and/or the creation of electronic-state discontinuities by heterojunction-interface engineering. For example, in electrochemical systems, the transport of charge across chemical phases is governed by the energy and density of electronic states within the disparate phases as well as any existing bias between said phases. Piezoelectric materials have long been used as a source of bias and mechanical displacement, relying on their mechanical to electrical coupling character for applications in sensors, actuators, and energy harvesters. In contrast to this historical precedent, contemporary integration of piezoelectric materials in semiconductor heterostructures capitalizes on the capability of the piezoelectric potential to manipulate charge carriers (i.e., piezotronics). For instance, the straining of a piezoelectric element in order to change its semiconducting properties has recently been investigated in zinc oxide nanomaterials, which have opened the doors to strain-gated logic operations and new possibilities for microelectronic circuitry elements. Straining effects in piezoelectric photoelectrochemical cells have also been shown to result in performance enhancements through manipulation of interface energetics. In principle, the piezoelectric modulation of charge carrier energetics should extend beyond the bounds of the buried electronic interfaces explored to date, thus allowing the direct enhancement or suppression of electrochemical processes that occur at the interface of a piezoelectric material and a solution (i.e., piezocatalysis). Preliminary experiments have shown an evolution of H2 and O2 from mechanically agitated piezoelectric BaTiO3 and ZnO microstructures in an aqueous sonication bath. In order to elucidate the intriguing piezocatalytic phenomenon, we report a systematic study of the piezoelectric-potentialdriven electrochemical H2 evolution process that takes place at the electrodes located on the surface of the material. The results compliment the general trends expected from the combinatorial assemblage of piezoelectricity and electrochemistry. The H2 evolution rates were dependent upon the oscillation frequency and amplitude of the piezoelectric material, in accordance with the combination of the direct piezoelectric effect and electrochemical reactions. A study of the piezocatalytic effect was conducted on a single-crystalline ferroelectric Pb(Mg1/3Nb2/3)O3-32PbTiO3 (PMN-PT) cantilever in a sealed chamber (see Figure S1 in the Supporting Information). The voltage output of the PMN-PT slab was first characterized in air (Figure 1a). The cantilever was mechanically oscillated with a fixed frequency and amplitude. When the piezoelectric cantilever was transitioned to the deionized water environment, the voltage amplitude decreased while the mechanical oscillation frequency and amplitude remained constant. In order to encompass the entire piezocatalytic system into a cohesive entity, an analogous circuit was constructed (Figure 1b). In this circuit, opposite and iteratively alternating sides of the piezoelectric material serve as both the working and counter electrodes, respectively. A strained piezoelectric material can be considered a charged capacitor, and thus the change in measured voltage is correlated with a change in piezoelectricity-induced surface charge (DQp). When a strained piezoelectric material is placed within an aqueous medium of finite conductivity and polarizability, its piezoelectricity-induced surface charge can be depleted through two primary pathways: Faradic (If) and capacitive (Ic=dCdVd/dt, in which Cd and Vd are the double layer capacitance and voltage drop across the double layer, respectively) currents: