Reversal of triboelectric charges on sol-gel oxide films annealed at different temperatures

Using Kelvin probe force microscopy, we studied the contact-electrification (CE) induced charge density on the surfaces of organic-inorganic composite oxide films, such as Nb₂O₅, ZrO₂, and HfO₂ films, prepared by a sol-gel method and annealed at different temperatures. The results show that positive triboelectric charges on the film surface gradually turn into negative ones with the increase in the annealing temperature. This phenomenon is attributed to the broken valence bonds between organics and organic-inorganic composites in the film at high temperature, which causes oxygen vacancy defects and consequently changes the surface states of the film. A surface states model is established to understand the process of CE, which further confirms that electron transfer is the dominant mechanism of CE. Moreover, it indicates that in addition to oxygen vacancies, there are also many other factors that can influence CE, including but not limited to non-stoichiometric ratios, impurities, and other types of defects. These factors will modify the surface states through changing the potential barrier energy of the bound electrons on the surface and thus impact the amount and even the direction of electron transfer.