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.
Highly doped diamond films are new candidates for electrodes
in
reactive environments, such as electrocatalytic interfaces. Here the
electronic structure of such films is investigated by X-ray absorption
spectroscopy at the C 1s and B 1s edges, combined with X-ray and ultraviolet
photoelectron spectroscopy, as well as optical measurements. A diamond
surface functionalized covalently with Ru(tpy)2, a model
complex similar to ruthenium-based molecules used in photocatalysis
and photovoltaics, is compared to a hydrogen-terminated diamond surface
as a reference. Bulk-sensitive absorption spectra with photon detection
reveal diamond gap states, while surface-sensitive spectra with electron
detection reveal the adsorbate states and π-bonding at the diamond
surface. The positions of the frontier orbitals of the dye relative
to the band edges of diamond are inferred from the spectroscopic data.
The implications of using diamond films as inert electron donors in
photocatalysis and dye-sensitized solar cells are discussed.
We present a unique remodeling of flat copper terraces induced by organic molecules. In particular, we show how metal−phthalocyanines (MPc, M = Zn and Cu) tailor the formation of regular arrays of Cu nanostripes on its (110) surface. The MPc mediated reshaping of Cu(110) terraces is found to involve a massive reorganization of Cu adatoms, at variance with the conventional changes of metal reconstructions upon molecular adsorption observed so far. By combining experimental and theoretical surface science techniques, we reveal the key role played by the metal atom at the molecular center in reshaping the terrace structure. Moreover, we demonstrate that extra Cu adatoms surrounding the molecules stabilize the molecular decoration of the Cu nanostripes. The massive surface reshaping is thus due to molecular mediated unidirectional blocking of adatom surface diffusion followed by adatom capture and accumulation.
A combination of variable temperature scanning tunneling microscopy, near edge X-ray adsorption fine structure and density functional theory has been used to investigate the chemisorption and self-assembly of metal-free protoporphyrin IX molecules on Cu(110) surface. The molecules in contact with the substrate suffer irreversible molecular transformations, mainly deprotonation of the carboxylic groups and metalation of the pyrroline subunits. The carboxylate group has been revealed as the anchored group versus the tetrapyrrole ring. We study the role played by the carboxylic acid groups in the surface-molecular bonding and how its presence affects the supramolecular structure.
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