Attachment of Protoporphyrin Dyes to Nanostructured ZnO Surfaces: Characterization by Near Edge X-ray Absorption Fine Structure Spectroscopy

The attachment of Hr and metal (Co- and Zn-) protoporphyrin IX molecules to ZnO nanorods and single-crystal surfaces is investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The carboxyl groups of the protoporphyrin are found to be essential for anchoring the molecules to ZnO surfaces. The crystallographic orientation of the exposed ZnO face has an influence on the dye immobilization, with the highest uptake observed for the oxygen-terminated ZnO (000-1) surface. The preparation conditions are crucial for the dye immobilization. Under certain preparation conditions, there is a Zn atom exchange between the H-2-protoporphyrin and the ZnO surface, i.e., a metalation of H-2 protoporphyrin IX to form Zn-protoporphyrin. Moreover, in the presence of chenodeoxycholic acid as coabsorber, the ZnO single-crystal surfaces are etched, as indicated by the loss of the orientation-dependent spectral features. These results help to pinpoint the chemical reactions that are responsible sensitized solar cells, especially those built from ZnO nanorod arrays. for the poor efficiency of ZnO-based dyesensitized solar cells, especially those built from ZnO nanorod arrays.