III. Results and discussion 40
9. Dyes 105
9.2. Influence of the anchoring group
9.2.1. Comparison of the dyes ID224 and ID94
Valence band
The valence band spectra of the nc-TiO2 substrate and the drop-casted dyes ID224 and ID94 were recorded at a photon energy of 90 eV and are shown normalized to the maximum in Figure 9.12. The shapes of the dye spectra are in very good agreement. This is in accordance to the expectations, as the HOMO of the dyes should not be influenced by a different anchoring group which is located on the opposite end of the molecule.
Intensity [a.u.]
7 6 5 4 3 2 1 0
Kinetic energy [eV]
SE hνννν = 50eV norm. to max.
nc-TiO2 ID224 ID94
Intensity [a.u.]
50 40 30 20 10 0
Binding energy [eV]
VB region hνννν = 90eV norm. to max.
nc-TiO2 Ti3p ID224 ID94
Intensity [a.u.]
3 2 1 0
Binding energy [eV]
HOMO region hνννν = 90eV
nc-TiO2 ID224 I
II
III ID94
Figure 9.12.:The normalized valence band and the normalized detailed HOMO region of the nanocrys-talline TiO2substrate and the drop-casted ID224 and ID94 were recorded at a photon energy of 90 eV.
The secondary edges of nc-TiO2(4.3 eV), ID224 (4.3 eV) and ID94 (4.3 eV) recorded at a photon energy of 50 eV at the TGM7 beamline are taken from other experiments. After the drop-casting of ID224 and ID94, the TiO2features are similarly damped. The HOMO emissions of ID224 and ID94 is decomposed into the HOMO (I) at 2.1 eV, the HOMO-1 (II) at 2.7 eV and the V0gap state (III) of TiO2at ca. 1.3 eV.
Similarly to the additive-dye sequence ID662+ID504 (see Subsection 9.1.1), the Ti3p emission of the nc-TiO2 substrate (at ca. 37.9 eV) is also observable in the valence band measurements of the dyes.
Therefore, thin or not closed layers of ID224 and ID94 are concluded. Furthermore, the emission is damped equally for both dyes, which is why an effect of the anchoring group on the coverage and the layer thickness can be excluded.
The HOMO emissions of ID224 and ID94 can be decomposed equally into a HOMO (2.1 eV) and a HOMO-1 (2.7 eV) molecular orbital. Because of that, a difference in the energetic lineup can be excluded.
Due to the thin or not closed dye layers, a TiO2 gap state contribution is found at ca. 1.3 eV. These gap-states are attributed to oxygen vacancies (VO) as was shown in Subsection 7.1.4. The fits were performed with an HOMO-1/HOMO intensity ratio of 0.46 and an energetic difference between the HOMO-1 and the HOMO and between the HOMO and the VO of 0.65 and 0.93 eV, respectively. The decomposition of the HOMO of organic dyes into two HOMO orbitals was performed in accordance to the literature [215].
The secondary edges are taken from other experiments at the beamline TGM7. The work functions of the nc-TiO2 and the dyes ID224 and ID94 are 4.3 eV. Therefore, no interface dipole can be deduced from these values.
Core levels
The detailed C1s, N1s, O1s and S2p core level spectra, which were measured at a photon energy of 360, 450, 600 and 210 eV, respectively, are shown in Figure 9.13. The C1s, N1s and S2p emissions found at 285.7, 400.7 and 164.1 eV, respectively, are caused mainly by the organic dyes and are composed of the emissions of the different bonds of the respective kind of atom in the molecule. The shapes and the positions of these emissions are the same for both dyes. This reconfirms the conclusion drawn from the HOMO measurements that the lineup is the same in both cases.
Intensity [a.u.]
292 288 284
Binding energy [eV]
C1s hνννν = 360eV
nc-TiO2 ID224 ID94
Intensity [a.u.]
404 402 400 398 Binding energy [eV]
N1s hνννν = 450eV
nc-TiO2 ID224 ID94
Intensity [a.u.]
538 536 534 532 530 528 Binding energy [eV]
O1s hνννν = 600eV
nc-TiO2 ID224 ID94
Intensity [a.u.]
538 536 534 532 530 528 Binding energy [eV]
O1s
difference spectra hνννν = 600eV
ID224 ID94
Intensity [a.u.]
167 166 165 164 163 Binding energy [eV]
S2p hνννν = 210eV
nc-TiO2 ID224 ID94
Figure 9.13.:The C1s (top left), N1s (top right), O1s (middle left), O1s difference (middle right) and S2p (bottom) core level spectra of nc-TiO2, drop-casted ID224 and ID94: the C1s (285.7 eV), N1s (400.7 eV), S2p (S2p3/2 at 164.1 eV) and the O1s main emission (534.1 eV), which can be attributed to TiO2, show no shifts (positions are indicated with a line), which shows that no charge transfer occurs for both dyes.
The O1s diffence spectra show the dye emissions of the dyes ID94 and ID224 which can be attributed mainly to double-bonded oxygen. C1s spectra were recorded at a photon energy of 360 eV, N1s at 450 eV, O1s at 600 eV and S2p at 210 eV.
The anchoring of the dyes to the TiO2 substrate occurs via O-Ti bonds. Therefore, the oxygen emissions can provide information about the anchoring mechanism. The O1s emission is a combination of emis-sions originating from the TiO2 substrate and the dyes. The maximum of the O1s emissions at 530.7 eV which is attributed to the TiO2substrate does not shift after drop-casting of both dyes. The O1s difference spectra, which show the O1s dye emissions, were obtained by subtracting the TiO2 substrate emission from the respective dye emission. The emission maxima of both dyes are located at 532.3 eV. Therefore, no charges are transferred at the interface. As the oxygen emissions of both dyes originate mainly from similarly double-bonded and single-bonded oxygen (see Figure 9.11) the curve shape is similar but not exactly the same.
Ti2p / gap-states
Figure 9.14 shows the Ti2p emissions of the nc-TiO2 substrate, ID224 and ID94. After drop-casting ID224 and ID94, the energetic positions of the Ti2p1/2 (465.1 eV) and Ti2p3/2 (459.3 eV) emissions do not change. Similar to the additive/dye sequence ID662+ID504 (see Subsection 9.1.1), the normalized Ti2p spectra show a small reduced Ti3+ species. It can be seen as a low binding energy shoulder in the spectra andis also verified by the appearance of gap-states which are mentioned later in this paragraph.
After the deposition steps, this species is damped but not quenched completely. Therefore, an interaction of the dye with the gap-states of TiO2 can be excluded.
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Intensity [a.u.]
468 464 460 456
Binding energy [eV]
Ti2p hνννν = 600eV norm. to max.
Intensity [a.u.]
4 3 2 1 0
Binding energy [eV]
GS region hνννν = 465eV Ti2p-Ti3d resonance
nc-TiO2 ID224 ID94
Figure 9.14.:The Ti2p core level spectra, the normalized Ti2p spectra and the gap-states region measured in resonance (Ti2p-Ti3d resonance) of (from the bottom up) nc-TiO2, drop-casted ID224 and ID94: no shift can be observed in the Ti2p spectra and also a reduced Ti3+ species in the Ti2p spectra. The measurements in resonance does not show a complete damping of the TiO2 gap states. The detailed Ti2p and gap-states region spectra were recorded with a photon energy of 600 and 465 eV, respectively.
The gap-states region was recorded at a photon energy of 465 eV. The photon energy was chosen to match the Ti2p-Ti3d resonance and enhance the intensity of the gap-states as explained in Subsection 7.1.4.
The spectra show two kinds of TiO2 gap-states: one just below the Fermi level and one at approximately 1.3 eV, as was shown in Subsection 7.1.4. Upon drop-casting the gap-states are not completely damped.
The region shows besides the gap-states the HOMO emission of ID224 and ID94, respectively. Therefore, it is concluded that neither of the dyes preferentially docks at the oxygen vacancies and thus forming O-Ti bonds at these sites, as one could expect.
Lineup
The energetic lineups of both dyes to nc-TiO2 are shown in Figure 9.15. The results are equal and, therefore, the lineups are the same. The nc-TiO2/dye interfaces do not show a band bending in neither of the layers. Also no interface dipole is concluded, as the work functions (4.3 eV) are the same for TiO2, ID224 and ID94.
The calculated optical band gap of ID224 and ID94 (2.1 eV for both) obtained from the project partner BASF is the difference of the HOMO and LUMO onset. The band gap was calculated according to Equation 8.1, which transforms the optical band gap into the band gap which would be measured by photoelectron spectroscopy and inverse photoelectron spectroscopy. Due to the band gap of 3.4 eV, the theoretical binding energy positions of the LUMO is 1.3 eV above the the Fermi level and thus 1.1 eV above the conduction band minimum of TiO2. Again the value of the conduction band maximum was taken from Subsection 7.1.4 as the determination of the valence band maximum of TiO2 is very difficult due to the underestimation of minor valence band emissions and the interference of TiO2gap-states.
Figure 9.15.:The band diagrams of the TiO2/ID224 and the TiO2/ID94 interfaces as formed after drop-casting of ID224 and ID94. The LUMO maximum positions of ID224 and ID94 are determined by the calculated energy gap of the dyes (3.4 eV) and result 1.3 eV above the Fermi level. The offsets of the conduction band minimum of TiO2 and the LUMO of ID224 and ID94 are 1.1 eV, whereas the work functions of all materials are the same, which indicates no interface dipole.