Piezo-electrochemical coupling of AgNbO3 piezoelectric nanomaterials

In this work, the AgNbO3 piezoelectric nanomaterials are hydrothermally synthesized, and they have an average particle size of~1 m, which is obtained from scanning electron microscopy pattern. The AgNbO3 nanomaterial possesses an orthorhombic crystal structure with an mm2 point group symmetry, indicated by the X-ray powder diffraction analysis result. The piezo-electrochemical coupling of AgNbO3 is characterized, and its physical mechanism is discussed. Under an external mechanical vibration, the surfaces of the piezoelectric AgNbO3 nanomaterials will generate a large number of positive and negative electric charges. Due to the existence of spontaneous polarization, these positive and negative electrical carriers are respectively distributed on the top surface and bottom surface of AgNbO3 and can further induce the generation of some strong oxidation middle active species such as hydroxyl radicals in solution on the basis of some special chemical redox reactions, realizing the piezo-electrochemical coupling. Therefore, we can consider the piezo-electrochemical coupling as the product of the piezoelectric effect and the electrochemical redox effect. Utilizing the strong piezo-electrochemical coupling, a practical application in mechano-catalysis is further developed to decompose dye solution under a driven vibration. After experiencing~60 min vibration with AgNbO3 nanomaterial as mechano-catalyst,~70% rhodamine B (~5 mg/L) is decomposed. Prior to the vibration, the rhodamine B solution with the addition of AgNbO3 catalyst is slowly stirred for 30 min to ensure the establishment of the physical adsorptiondesorption equilibrium between catalyst and dye. It is difficult to directly exert a mechanical stress on the micro/nanoparticles. Here, an ultrasonic source with a vibration frequency of~40 kHz is employed to exert a stress to compress and stretch the AgNbO3 particles through utilizing micro-bubble collapse forces during ultrasonic cavitations, which needs the AgNbO3 particle size to be roughly identical with the diameter (~m) of micro-bubble. Our mechanocatalytic dye decomposition experiment is conducted at room-temperature and in a dark environment to avoid the influence of photocatalysis. The slight increase of temperature of the dye solution in the ultrasonic vibration process has no obvious influence on the dye decomposition efficiency, which has been confirmed from our experiment. Through a technology of fluorescence spectrum trapping, the intermediate active product in the piezo-electrochemical coupling process-the strongly oxidized hydroxyl radicals, is successfully observed. With the increase of vibration time, the number of hydroxyl radicals obviously increases, which proves that the piezo-electrochemical coupling plays a key role in our mechano-catalytic process. After using AgNbO3 catalyst in cyclic decomposition of rhodamine B 5 times, no obvious reduction in the piezo-electrochemical coupling performance occurs. The AgNbO3 nanomaterial possesses an efficient piezo-electrochemical coupling for mechano-catalysis, and it has the advantages of high decomposition efficiency and reusability, and potential applications in vibration decomposing dye.