Energy level alignment of catechol molecular orbitals on ZnO(1 1 image 0) and TiO2(1 1 0) surfaces

The occupied and unoccupied electronic structure of catechol adsorbed onto two single crystal surfaces, rutile TiO2(1 1 0) and wurtzite ZnO(1 1 image 0), have been investigated using UV-photoemission and inverse photoemission spectroscopies (UPS and IPS) in an ultra-high vacuum environment. To aid in assignment of the spectral features, model metal-bound catechol structures were calculated using a DFT approach. From these measurements, the energy alignment of the catechol-related states with respect to the substrates band edges is directly determined and is in good agreement with a direct injection process of the photoexcited electron into the substrate conduction band, resulting in the characteristic absorption properties of adsorbed catechol.