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The attachment of H2- 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 H2-protoporphyrin and the ZnO surface, i.e., a metalation of H2-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 for the poor efficiency of ZnO-based dye-sensitized solar cells, especially those built from ZnO nanorod arrays.

Using a combination of X-ray photoelectron spectroscopy (XPS) and core level shift first principles calculations, we determine the formation of an unusual intermolecular interaction between porphyrin molecules. We show that protoporphyrin IX molecules (H2PPIX) adsorbed on Cu surfaces at low temperature (LT) form molecular adlayers stabilized by a strong (1.3 eV) tetragonal coordinated H-bond. This unconventional H-bonding involves the four nitrogen atoms in the tetrapyrrole macrocycle of one molecule, four hydrogen atoms, and one of the hydroxyl oxygen atoms from the carboxylic acid groups of an adjacent molecule. The calculations demonstrate that the corresponding fingerprint of this bond is observed in both the nitrogen XPS data, showing a unique peak, and the almost unchanged 1s core level energy of the hydroxyl oxygen atoms. We explain the formation of this bond by a charge rearrangement mechanism that includes proton sharing and migration.