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).

Fabrication of hierarchical porous anodized titania nano-network with enhanced active surface area: Ruthenium-based dye adsorption studies for dye-sensitized solar cell (DSSC) application

“…Other possible -COOH binding modes are described in detail in [54]. Regarding the aforementioned bonding mechanisms, the majority of studies have reported that N719 dye adsorption occurs via two carboxylic groups with the surface oxygen atoms of anatase titania, thereby producing the strongest binding state [57,58]. In our case of hierarchically porous anatase titania nanotubes with higher active surface areas (i.e., higher than the conventional TNAs), the number of bonding states are also much higher.…”

“…Generally, the carboxylic groups of the dye interact with the TiO 2 surface in different binding modes involving two or three of the -COOH groups anchoring with the metal oxide surface [54]. Additionally, the interaction of the NCS groups with the titania surface, as well as hydrogen bonding with surface hydroxides of titania, can also occur [55][56][57], as described in Fig. 8(c).…”

Investigation of the influence of coadsorbent dye upon the interfacial structure of dye-sensitized solar cells

“…[9][10][11][12][13][14][15][16] However, it is unclear whether the behavior of dye molecules on a single crystal surface is faithfully reproduced by molecules on nanocrystalline TiO 2 , which is the material used for photovoltaic cell electrodes. Direct observations are very difficult because of the complex structure of nanocrystalline TiO 2 , so a high-resolution method with high specificity is required for observing the microscopic structure of the surface morphology of these porous materials.…”

Modeling of adsorption isotherms of dye N719 on titanium oxide using the grand canonical ensemble in statistical physics for dye sensitized solar cells

“…Dye sensitized solar cells (DSSCs) are promising technology for low cost solar energy conversion, with reasonable efficiency values, a good stability and cheap manufacturing methods (Lee et al, 2011a(Lee et al, , 2011b These devices have attracted wide interest in order to find an environmentally friendly alternative for energy production. In this context, it is especially important to choose sensitizers allowing to obtain maximum efficiency.…”

“…Moreover, these anchoring groups act also as electron acceptors, given their electron-with drawing capabilities Several methods, including UV-Vis absorption spectroscopy (León et al, 2006;Durr et al, 2005) Raman scattering spectroscopy (León et al, 2006;Durr et al, 2005;Greijer et al, 2001) Infra-Red absorption spectroscopy (Greijer et al, 2001;Finnie et al, 1998) and photoelectron spectroscopy (Tian et al, 2010;Likodimos et al, 2009) have been used to study the interfacial behaviors between sensitizing dyes and nanoporous TiO 2 surfaces. Structural and electronic properties of these complex materials have been also reported using theoretical approaches that are based on first-principle molecular dynamic methods (Lee et al, 2011a(Lee et al, , 2011b. Raman scattering and Infra-Red absorption spectroscopy are powerful tools because they can directly probe the bond orders and bond strengths of the groups of interest (León et al, 2006;Greijer et al, 2001;Finnie et al, 1998).…”