Face-selective etching of ZnO during attachment of dyes

Scanning force microscopy and near-edge X-ray absorption spectroscopy are used to study the attachment of Protoporphyrin IX dye molecules to the low-index single-crystal faces of ZnO, namely, the polar (0001̄) face and the nonpolar (101̄0) face. We found that surface etching depends on the crystal face for various dye immobilization procedures: whereas the polar (0001̄) surface remains nearly unaltered, the nonpolar (101̄0) surface is strongly etched by acidic solutions; these results demonstrate the selective attachment of dye molecules to ZnO surfaces, showing that the dye attachment is extremely sensitive to both the exposed facet and the immobilization protocol. This could be a factor in the surprisingly poor efficiency observed for ZnO-based dye-sensitized solar cells, especially those using ZnO nanorod arrays. © 2013 American Chemical Society.We acknowledge funding support from the Spanish Goverment (MAT2010-21156-C03-01, C03-03, MAT2010-21267-C02-01, PIB2010US-00652), the Basque Government (IT-621-13), the Seneca Foundation (15324/PI/10), and the CONSOLIDER Program ‘Force for Future’ (CSD2010-00024). This work was supported in the United States by the NSF under Awards CHE-1026245 and DMR-0537588 (Synchrotron Radiation Center). R.T.-Z. and E.P.-L. acknowledge support from the Program “Ramon y Cajal” of the MINECO.Peer Reviewe