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

Abstract

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.

Porphyrins as Active Components for Electrochemical and Photoelectrochemical Devices

“…270 By and large, chemisorption is the most used method because of the basic character of the ZnO surface. Thus, porphyrins containing functional groups with acidic OH (carboxylic acid, 219,271,272,274,[276][277][278]281,[285][286][287]291,[293][294][295]297,298 sulfonic acid 279,283,284 and cathecol 275,282 ) were easily grafted onto ZnO 2 .…”

“…ZnO nanorods were immersed into an ethanol solution which was prepared with protoporphyrin IX, P-IX, (or its Zn(II), Co(III)Cl complexes) and chenodeoxycholic acid (Figure 80). 287 A different procedure used mother solutions of zinc 5-(4-carboxyphenyl)-10,15,20-tris(3,5-dimethoxylphenyl)porphyrin, stirred with ZnO nanorods in acetonitrile for 16 hours at room temperature. Then, a 2,3-fullerocyclopropane-1,1-dicarboxylic acid (C 60 -acid) solution in the same solvent was added and stirred for an additional 16 hours ( Figure 81).…”

“…This is a problem with ZnO nanorods, which are desirable for devices (because of their crystallinity), that are oriented in the [0001] direction and thus exhibit mainly the nonpolar ZnO(10-10) surfaces where the uptake is small. 287 A problem was observed involving dye uptake in ZnO pores beyond adsorbed sensitizers that led to a small surface area and hence low photoelectrochemical efficiency. 284 The problems of poor pore accessibility were overcome using the so-called re-adsorption method: the electrochemical deposition of ZnO was performed in the presence of Eosin Y, that was subsequently removed, thus providing highly porous ZnO.…”

Chemical Protection of Material Morphology: Robust and Gentle Gas-Phase Surface Functionalization of ZnO with Propiolic Acid

“…19 Thus, an alternative method for surface modification must be developed that produces stable and robust surface linkages and at the same time does not dissolve (or influences the structure of) ZnO features. Hence, the necessity to develop a well-controlled interfacial strategy on ZnO surface is highly desired.…”

Change in spectral properties of dyes upon immobilization on silicon surfaces: a combined theoretical and experimental study

“…Covalent bond formation between the dye molecule and the surface appears to be a better option than other modes of attachments mentioned above, as dyes attached via weaker modes of interaction tends to get removed while washing [5]. Dye molecules are immobilized on solid platforms through some specific functional group on the dye [5,[7][8][9][10][11] and typically carboxylic or phosphoric acid moieties on the dye have served this purpose well [12][13][14][15]. Linkers play an important role in tethering the dye molecules to the surface [16,17].…”