Investigation of Photoelectrochemical Performance under the Piezoelectric Effect Based on Different Zinc Oxide Morphologies

Recently, the piezoelectric effect has been widely used in photoelectrochemical (PEC) water splitting, and the morphology of the piezoelectric material is a critical factor affecting the piezo-photoelectrochemical water splitting performance. Herein, we explored the mechanism of the piezo-photoelectrochemical performance of zinc oxide (ZnO) that is affected by the morphology. Firstly, three different ZnO nanostructures (nanosheets, nanorods, and nanospheres) were synthesized by the electrodeposition, hydrothermal, and sol-gel methods, respectively. Then, the measurements of PEC water splitting performance under the piezoelectric effect revealed a 3-fold increase for the ZnO nanosheets, a 1.4-fold increase for the nanorods, and a 1.2-fold increase for the nanospheres compared to no piezoelectric effect. Finally, finite element simulation showed that nanosheets generated the highest piezoelectric potential (0.6 V), followed by nanorods (0.2 V), and nanospheres the lowest (0.04 V). Thus, among the three morphologies, the ZnO nanosheets exhibited a great improvement in PEC performance under the piezoelectric effect. The great improvement is due to the non-axial vertical homogeneous growth of the ZnO nanosheets, subjecting them to the highest effective deformation stress, which enables the ZnO nanosheets to produce the highest piezoelectric potential to accelerate the carrier separation and limit the recombination of photoelectrons and holes. This work serves as a guide for developing various photoelectrodes that are used in piezo-photoelectrochemical water splitting.

Piezoelectric effect enhanced photocatalysis in environmental remediation: State-of-the-art techniques and future scenarios

Photocatalysis has been widely used as an advanced oxidation process to control pollutants effectively. However, environmental photocatalysis' decontamination efficiency is restricted to the photogenerated electron-hole pairs' rapid recombination. Recently, emerging investigations have been directed to generate internal electric field by piezoelectric effect to enhance the separation efficiency of photogenerated charge carriers for better photocatalytic performance; however, there are still huge knowledge gaps on the rational application of piezo-photocatalysis in environmental remediation and disinfection. Thus, we have conducted a comprehensive review to better understand the physicochemical properties of piezoelectric materials (non-centrosymmetric crystal structures, piezoelectric coefficient, Young's modulus, and etc.) and current study states. We also elucidated the strategy of piezo-photo catalysis system constructions (mono-component, core-shell structure, and etc.) and underlying mechanisms of enhanced remediation performance. Addressing the current challenges and future scenarios (degradation of organic pollutants, disinfection, and etc.), the present review would shed light on the advanced wastewater treatment development towards sustainable control of emerging containments.

Non-linear nanoscale piezoresponse of single ZnO nanowires affected by piezotronic effect

Zinc oxide (ZnO) nanowires (NWs) as semiconductor piezoelectric nanostructures have emerged as material of interest for applications in energy harvesting, photonics, sensing, biomedical science, actuators or spintronics. The expression for the piezoelectric properties in semiconductor materials is concealed by the screening effect of the available carriers and the piezotronic effect, leading to complex nanoscale piezoresponse signals. Here, we have developed a metal-semiconductor-metal model to simulate the piezoresponse of single ZnO NWs, demonstrating that the apparent non-linearity in the piezoelectric coefficient arises from the asymmetry created by the forward and reversed biased Schottky barriers at the semiconductor-metal junctions. By directly measuring the experimental I-V characteristics of ZnO NWs with conductive atomic force microscope together with the piezoelectric vertical coefficient by piezoresponse force microscopy, and comparing them with the numerical calculations for our model, effective piezoelectric coefficients in the range d _33eff ∼ 8.6 pm V^-1-12.3 pm V^-1 have been extracted for ZnO NWs. We have further demonstrated via simulations the dependence between the effective piezoelectric coefficient d _33eff and the geometry and physical dimensions of the NW (radius to length ratio), revealing that the higher d _33eff is obtained for thin and long NWs due to the tensor nature proportionality between electric fields and deformation in NW geometries. Moreover, the non-linearity of the piezoresponse also leads to multiharmonic electromechanical response observed at the second and higher harmonics that indeed is not restricted to piezoelectric semiconductor materials but can be generalized to any type of asymmetric voltage drops on a piezoelectric structure as well as leaky wide band-gap semiconductor ferroelectrics.

A piezoelectric poly(vinylidene fluoride) tube featuring highly-sensitive and isotropic piezoelectric output for compression

Piezoelectric polymers have aroused tremendous attention in self-powered flexible wearable electronics. However, conventional plate-like piezoelectric devices demonstrated insufficient output power and monotonous piezoelectric energy harvesting performance only in one direction, failing to accommodate the complex multi-dimensional stress field. In this study, a poly(vinylidene fluoride) tube featuring all-directional piezoelectric energy harvesting performance with excellent rebound resilience was prepared via a fast industrial extrusion. Benefitting from the isotropic hollow tubular structure, the piezoelectric tube experienced large deformation at a small external load and thus exhibited a strong load-amplifying function to generate the optimized output power at multi-stresses from any direction, with sensitive piezoelectric performance, quick response and mechanical robustness. Finally, the practical potential as a robust energy harvester was evaluated by harvesting small energy from irregular human motions. This study provides a facile structure designing strategy for the preparation of functional piezoelectric devices for multi-directional mechanical energy harvesting applications.