Self-Cleaning ZnO Nanosheets for Piezoelectric Sensors

The design of wearable sensors coupling versatile analytical detection to self-cleaning is a highly desired combination, tackling the need of smart devices in response to the recent virus pandemics. To this aim, this work shows ZnO nanostructures obtained by a mild wet-chemistry approach onto ITO/PET flexible supports, resulting in wearable piezoelectric sensors exhibiting photocatalytic activity. ITO surfaces are treated with 0.5 mM KMnO4 aqueous solution (20 minutes, 90°C); ZnO growth is subsequently carried out by a previously shown wet-chemistry method [1]. SEM analysis shows the presence of a good surface coverage of ZnO nanosheets (NSs) (about 1.5 NSs/μm2) in the case of treated ITO, whereas low-density larger flower-like ZnO NSs (about 0.25 NSs/μm2) are observed on the control ITO surface – i.e. not treated by the KMnO4 solution. XPS analysis highlights the presence of zinc and oxygen, whereas manganese traces are detected by depth profile analysis. Chronoamperometric measurements show current reduction upon 1V bias towards bending, in full accordance with previous reports [2]. Electrical impedance spectroscopy (PBS buffer, pH = 7.4) shows that ZnO reduces impedance with respect to the bare ITO electrode at low sampling frequencies (0.1 -1 Hz), being ZnO NSs on treated ITO the system which provides the lowest values. In full accordance to the electrochemical data, piezophotocatalytic tests show that high-density ZnO NSs provide the best results towards photodegradation of methylene blue (25 μM, solar light simulator).