Further Understanding of the Electronic Interactions between N719 Sensitizer and Anatase TiO2 Films: A Combined X-ray Absorption and X-ray Photoelectron Spectroscopic Study

Abstract

In this study, the electronic properties of N719 adsorbed onto anatase were comparably investigated by using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) techniques. Sensitized TiO2 films made from two different nanocrystalline anatase powders were investigated: a commercial one (Solaronix) and our synthetic variety produced through aqueous synthesis. This was done to investigate how our aqueous-produced nanocrystalline anatase substrates compared with commercial products and to observe whether both nanocrystalline anatase anodes behaved in a similar manner in terms of their bonding and electronic interactions. Surface coordination changes to Ti−O groups previously reported via Ti K-edge extended X-ray absorption fine structure (EXAFS) data [using transmission or fluorescence yield (FY)] between the pure TiO2 and the adsorbed state were not observed in our measurements via the Ti L or K X-ray absorption near-edge structure (XANES) (nor EXAFS) data for both substrates via a surface-sensitive detection technique (total electron yield, TEY). This is likely due to the probing depth of TEY mode (5−10 nm), in which the coordination changes that occur to the surface groups, which should in turn affect the XANES spectrum, are not observed at Ti K- or L-edge XANES spectrum. The C and N K-edge XANES spectra of the N719 adsorbed onto two TiO2 films were for the first time evaluated in this work. From the C K-edge XANES data, the spectral changes revealed that additional electronic states occur between dye molecules and TiO2 surface. The C K-edge XANES spectra allowed us to propose that electronic interactions do not only occur through the covalent bonding of the anchoring groups but also through the aromatic electron density of the bipyridine groups and the d states found in TiO2. This was further confirmed via XPS analysis by monitoring the N bipyridine groups before and after sensitization. XPS used in combination with XAS (in TEY mode) provided complementary information owing to its higher surface sensitivity. The Ti 2p and O 1s XPS spectra showed that adsorption of the dye on TiO2 leads to a change of the surface dipole and/or a change in the Fermi level position in the band gap, which shifts all the core levels of TiO2. These are not equal for both TiO2 substrates in spite of them being nanocrystallnine anatase. This effect was found to be greater for the N719−aqueous TiO2 system than the respective Solaronix one. For the N 1s and S 2p XPS, the shift toward higher energy indicated that there exists an additional H-bonding interaction of the NCS ligand of the dye molecule with the TiO2 surface groups (OH/H2O).