Attachment of protoporphyrin dyes to nanostructured Zno surfaces: Characterization by near edge X-ray absorption fine structure spectroscopy

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. © 2011 American Chemical Society.We acknowledge funding through Spanish research projects PET2008-109, BIO 2007-67523, intramural project 200960I159, and HOPE CSD2007-0007 (Consolider-Ingenio 2010). This work was supported by the Spanish MICINN (PIB2010US 00652, MAT2010 21156 C03 01, and MAT2010 21156 C03 03) and the Basque Government (IT 257 07). This work was supported in the U.S. by the NSF under the awards CHE-1026245 and DMR-0537588 (SRC) and by the DOE under the contracts DEFG02-01ER45917 (end station), DEAC03-76SF00098 (ALS), and ZnO Surface Functionalization was supported by DOE Basic Energy Sciences Grant DE-FG02-09ER16122 (RJH and RER).Peer Reviewe