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Piezoelectric semiconductors have emerged as redox catalysts, and challenges include effective conversion of mechanical energy to piezoelectric polarization and achieving high catalytic activity. The catalytic activity can be enhanced by simultaneous irradiation of ultrasound and light, but the existing piezoelectric semiconductors have trouble absorbing visible light. A piezoelectric catalyst is designed and tested for the generation of hydrogen peroxide (H2O2). It is based on Nb-doped tetragonal BaTiO3 (BaTiO3:Nb) and is sensitized by carbon quantum dots (CDs). The photosensitizer injects electrons into the conduction band of the semiconductor, while the piezoelectric polarization directed electrons to the semiconductor surface, allowing for a high-rate generation of H2O2. The piezoelectric polarization field restricts the recombination of photoinduced electron–hole pairs. A production rate of 1360 µmol gcatalyst−1 h−1 of H2O2 is achieved under visible light and ultrasound co-irradiation. Individual piezo- and photocatalysis yielded lower production rates. Furthermore, the CDs enhance the piezocatalytic activity of the BaTiO3:Nb. It is noted that moderating the piezoelectricity of BaTiO3:Nb via microstructure modulation influences the piezophotocatalytic activity. This work shows a new methodology for synthesizing H2O2 by using visible light and mechanical energy.

Piezoelectric semiconductors can be polarized and used in mechanoredox systems and photoredox catalysis. Conventional non-piezoelectric semiconductors have limitations when it comes to charge carrier recombination and slow transport rates in catalytic reactions, which can be overcome by piezoelectric polarization processes in piezoelectric semiconductors. Heterostructures based on semiconducting piezoelectrics often offer enhanced catalytic reactivities that are related to their mechanical, piezoelectric, optical, and electronic characteristics. We review how to use such heterostructures to convert mechanical energy into chemical energy, and how the related piezoelectric polarization tunes the band structures and provides advantages in piezophotocatalysis over regular photocatalysis. We discuss fundamental concepts of piezoelectricity, piezoelectric potential, and examine different piezoelectric heterostructures for piezo- and piezophotocatalysis. A review of dynamic investigations of piezo- and piezophotocatalytic processes is presented. The complementary developments in the understanding of the piezotronic and piezophototronic effects are described, which include the induced charge-transfer mechanisms for piezo- and piezophotocatalytic reactions that can occur with piezoelectric heterostructures. Finally, we derive design principles and suggest future research directions in the emerging field of piezo- and piezophotocatalysis employing semiconductive heterostructures.

We report gate bias dependence of the charge due to piezoelectric polarization obtained by using a fully coupled formulation based upon the piezoelectric constitutive equations for stress and electric displacement. This formulation is significant because it fully accounts for electromechanical coupling under the constraint of global charge control. The coupled formulation results in lower charge due to piezoelectric polarization as compared to the uncoupled formulation for a given Al mole fraction. With increasing two dimensional electron gas concentration, that is, for gate biases greater than threshold, the compressive strain along the c axis in the barrier AlGaN layer increases with a concomitant increase of in-plane stress. Current collapse is correlated to the increase in source and drain resistances through their dependence upon surface charge. An alternate explanation of current collapse using local charge neutrality is also presented., The original document contains color images. Sponsored in part by the Air Force Office of Scientific Research (AFOSR) under LRIR 92SN04COR. Published in Applied Physics Letters, v88 n20 p203510, 2006.