Nanoscale functional materials represent a unique hybrid of bulk crystalline structure with some degree of disorder due to their large surface area. It is this duality that has made them attractive for a broad range of applications. Among a wide variety of materials, ZnO has proven to be a highly versatile nanomaterial with applications ranging from light-emitting diodes to photocatalysis. For solar energy conversion and storage, photosensitization and photocatalysis the fundamental property is the lifetime and mobility of charge carriers (electrons and holes). Understanding the mechanism by which these charges traverse the nanoparticle, become trapped and/or recombine is therefore of fundamental as well as practical importance. In order to better understand the structure-function relationship and the dynamics of the charge carriers in such devices it is important to be able to measure both electronic and structural properties simultaneously. Here we study the electron-hole dynamics within a colloidal solution of 35 nm ZnO nanoparticles photo-excited with above band-gap UV light (3.2 eV). Using pump-probe X-ray absorption spectroscopy (XAS) combined with a dispersive X-ray emission spectrometer we perform time-resolved resonant X-ray emission spectroscopy (RXES), which yields detailed information on both the occupied and unoccupied electronic states and geometric structure after photo-excitation. Our analysis indicates that the hole is trapped in less than 50 ps at a native oxygen vacancy (V<sub>O</sub><sup>+</sup>) within the nanoparticle, which leads to a structural displacement of the four Zn nearest neighbours away from the trapping site. By extending these techniques into the femtosecond regime at the SACLA XFEL, we measure a hole trapping time of 1 ps.

Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.