Investigation of the Zinc Oxide Corrosion Impact on the Self-propulsion of Rod-shaped Microswimmers under the Influence of H2O2 Fuel and UV Light Radiation

In recent years, the development and exploration of microrobots with natural motion in liquid environments has made great strides towards making nanoscale materials more intelligent for various applications such as medical, environmental, and diagnostic purposes. To enable movement of these robots at low Reynolds number regimes, which is similar to biological fluids, time symmetry must be disrupted with temperature, concentration, or surface tension gradients. Micro-robots that can move independently under light stimulation and in low concentrations of H2O2 are highly desirable. However, there is still a need for a biocompatible and cost-effective system that can move effectively in biological environments. Due to the complexity of such system, it is conceivable that various factors affect the movement of the microrobots. One such factor is the erosion reaction of metal oxide swimmers when exposed to a common fuel like H2O2, which is often overlooked in photocatalytic reactions. The objective of this investigation is to examine the influence of the corrosion reaction in an oxidizing environment, due to the free radical formation of UV light irradiation and H2O2 fuel decomposing, on the propulsion of the zinc oxide microswimmers. Our approach involved implementing a one-step hydrothermal process to generate the ZnO, followed by assessing the impact of 0.5, 1, 2, 5, 10, and 20 % wt. of H2O2 on morphological, physical, and chemical properties by FTIR, UV-DRS, XRD, SEM, and zeta-potential analyses. Furthermore, the possibility of a ZnO-ZnO2 heterojunction forming within this system was investigated. Clusters of flower-like ZnO were produced using a one-step hydrothermal process. After being exposed to a 35 Hz ultrasonic bath for 30 minutes, the clusters were separated into individual rods and then subjected to a heat treatment at 500 ℃ for 2 hours to stabilize their crystal structures. Next, the rods were introduced to various H2O2 concentrations followed by repeated washing with deionized water until neutral pH. Their functionality as micromotors were then tested in presence and absence of UV light irradiation (365 nm). This was performed in pure water or varying H2O2 concentrations. It was observed that clusters of cubic ZnO2 crystals of 400 nm size were formed on 2 μm ZnO rods with wurtzite structure after exposure to concentrations of more than 1.0 %wt. This heterostructure had a smaller band gap than the pure ZnO structure, which improved its photocatalytic properties. The results showed an increase in the percentage of ZnO2 structures, leading to a positive zeta potential on the surface. When observing the movement of microswimmers, it is possible to distinguish the impact of erosion reaction and heterojunction structure by comparing their movement before and after being modified with H2O2. In the vicinity of 1.0 %wt. of H2O2, a layer of zinc peroxide is formed which enhances the photocatalytic properties, resulting in an increase in the speed and displacement of the micromotor. The surface of the metal oxide micromotor plays a significant role in controlling the speed. Therefore, in studies involving metal oxide micromotors in H2O2 fuel environments, it is crucial to consider the impact of erosion reaction, which is often overlooked. This property can be utilized to create systems that respond to the environment, especially in areas close to cancer cells where the concentration of H2O2 and oxidizing radicals is higher.