Adsorption structures of catechol on the ZnO(10-10) surface

The details of catechol interacting with the ZnO surfaces are critical for understanding how the organic chromophores anchor on and hence functionalize the ZnO materials. In this work, we apply low-temperature scanning tunneling microscopy (LT-STM) in combination with density functional theory (DFT) calculations to investigate the adsorption state and interface charge transfer of catechol on a ZnO(10-10) surface. For the molecules deposited at liquid nitrogen temperature, the fully-protonated (FP) status prevails in the isolated molecules despite the synergistic formation of half-protonated (HP) and fully-deprotonated (FD) ones. However, annealing to elevated temperature drives all the molecules to assemble into either linear or a c(2 × 2) superstructure composed of only FD catechols. Such spontaneous deprotonation of catechol is different from the predicted water-assisted process and plays critical roles in switching the catechol from electron acceptors to electron donors relative to the ZnO surface. These results may bring new insights into the design of optoelectronic and biomedical applications of the ZnO-organic compositional system.