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Quantum-chemical investigations of phenol and larger aromatic molecules at the TiO2 anatase (101) surface

Adsorption of aromatic molecules at the (101) surface of titanium dioxide anatase is studied by quantum-chemical B3LYP computations, where both cluster and periodic calculations were performed and compared. For phenol different adsorption modes at a TiO2 cluster were mapped out and the energetically most favourable conformation was used for investigation of the electronic structure, for periodic calculations, and as a mould for the adsorption modes of phenylmethanol, phenylethanol, naphthalen-2-ol, phenanthren-2-ol, pyren-2-ol and perylen-2-ol. The alcohols form a H-bond to a surface O and a O(molecule)-Ti bond. For the larger aromatic molecules their increasingly higher HOMO levels decrease the effective bad gap of the system. Inclusion of spacer groups as in phenylmethanol and phenylethanol results in higher adsorption energies and larger band gaps. The LUMOs for the adsorbates help visualize the electronic coupling to the surface. Comparison of the cluster with the periodic model indicates that the former describes the electronic coupling in a similar manner as the latter, although the former lacks in the description of the anatase substrate.


Introduction
Dye-sensitized solar cells (DSSC) owe their enhanced photovoltaic capability to organic dye molecules that are chemisorbed in a monolayer on a spongy nanocrystalline metal oxide electrode, which allows for absorption of a wider range in the solar light spectrum [1]. The metal oxide is most commonly titanium dioxide with the anatase or rutile crystal structure. The electron excited by the photon in the dye is subsequently injected into the conduction band of the semiconductor. Part of the efficiency of the cell depends on a sufficiently high rate of this injection, which can be related to the degree of overlap between the dye LUMO level with the metal oxide's conduction band. This makes it of interest to study systematically the electronic coupling between aromatic molecules and the TiO 2 surface. During the last few years several experimental [2][3][4][5][6] and theoretical [7][8][9][10][11][12] studies have been performed for different aromatic organic molecules at the titanium oxide surface, with e.g. various conjugated-system sizes or anchor groups. In this paper we study the adsorption at the anatase surface for aromatic alcohols from phenol to perylenol, and for phenol with CH 2 and C 2 H 4 spacer groups.

Method
The adsorption of molecules on the anatase (101) surface of titanium dioxide was studied both using a cluster and a periodic model. The cluster consists of 8 Ti, 14 O and 4 OH-groups (figure 1a) chosen to represent the outermost atoms of a (101) surface. The OH-groups were added at the corners 1 To whom correspondence should be addressed.  [8,9]. In the periodic calculations all atoms were allowed to relax. The computations were performed using the hybrid density functional B3LYP and the double-zeta basis set used in [7][8][9][10][13][14]. The Gaussian03 [15] program was used for the cluster and the CRYSTAL [16] program for the periodic computations, with visualization of crystal orbitals as described in [10]. Phenol was attached to the cluster at different initial positions and the structure was energyminimized to elucidate the optimal way of adsorption. Both the undissociated and the dissociated (deprotonized) forms of phenol were tested. The energetically most favorable mode was then selected as adsorption mode also for the larger molecules (figure 2) and for phenol in the periodic computations. CH 2 spacer groups were added in phenol (phenylmethanol and phenylethanol, figure 2) to vary the distance between the conjugated system and the surface. Alternatively, the conjugated system was extended by merging additional benzene rings to phenol (naphthalen-2-ol, phenanthren-2-ol, pyren-2ol and perylen-2-ol, figure 2). The molecule plus cluster was geometry-optimized while keeping the surface fixed at the optimized structure from the phenol plus surface calculation. Adsorption geometries, energies, and the electronic structures were analyzed, and the HOMO and LUMO orbitals and the lowest unoccupied orbital mainly located on the adsorbate, LUMO(ads), were visualized and investigated.

The adsorption geometry for phenol
The different adsorption geometries for phenol at the cluster surface and the corresponding energies are given in figure 3  In the energetically most favorable mode phenol forms a monodentate bond to Ti1 and a H-bond to O6 in figure 1a. The adsorption energies (table 1) from the periodic slab computations (figure 4) for the two most stable modes agree well with those from the cluster. For mode "a" the distance from the O in the hydroxyl group to Ti1 is 2.37 Å/2.32 Å and the H-O6 distance is 1.85 Å/2.16 Å for the cluster and periodic model, respectively. The surface Ti1-O6 distance is 3.89 Å/3.93 Å. The H in the OH group is somewhat more twisted out from the benzene ring plane in the cluster than in the periodic case (7° and 2° respectively). For both models the benzene ring is tilted ~30° from the normal of the surface.

Adsorption of larger molecules
The adsorption energies for the molecules with spacer groups and larger aromatic ring systems are shown in Table 2. Increasing the aromatic systems does not change the adsorption energy compared to phenol significantly. Interestingly, inclusion of CH 2 or C 2 H 4 spacer groups results in notably higher adsorption energies, possibly due to the breakage of conjugation to the alcohol O. This was not seen in previous studies where phosphonic and carboxylic instead of alcoholic anchor groups were used [10].
The molecular orbitals HOMO, LUMO and LUMO(ads) for phenylethanol, naphthalenol and perylenol adsorbed at anatase are shown in figure 7. The situation is similar to that of phenol in that the HOMO is localized on the adsorbate and the LUMO on the surface, and LUMO(ads) is well localized on the molecule but with a surface Ti d coupling. The DOS for the molecules with spacer groups ( figure 8) show that the HOMO level is stabilized compared to that in phenol, resulting in a slightly larger band gap. The LUMO(ads) is approximately unaffected by the spacer group. In the DOS for the larger aromatic molecules (figure 9) more occupied adsorbate levels with successively higher energies are introduced into the anatase band gap with increasing aromatic system size, thereby reducing the LUMO-HOMO difference. For example, for the pure anatase cluster the band gap is 5.59 eV (table 3), and with phenol respective perylenol adsorbed the band gap becomes 4.12 eV and for 2.83 eV, respective. One can also see from figure 9 that the LUMO(ads) moves to lower energies with increasing aromatic size. For phenol LUMO(ads) is located 32 orbitals above the LUMO level but for the perylenol it lies only three orbitals above. From also appears that exciting an electron from HOMO to LUMO(ads) will require approximately the same amount of energy in the adsorbate-surface system as in the free molecule.

Rotation of the benzene ring in phenol
The effect of how the relative ring position in phenol influences the band gap and the molecular orbitals was studied. The OH-group was kept fixed at the cluster surface while the benzene ring was rotated around the CO-axis. The energy variation relative the energy minimum upon rotation is shown in figure 10. The minimum is at 0° , and at approximately 75° and 115° the rotation energy has   The molecular orbitals HOMO, LUMO and LUMO(ads) for phenol upon rotation of the benzene ring are shown in figure 12. The appearance of the HOMO and LUMO are not much affected by the rotation, but for LUMO(ads) the orbital distribution changes from being located at the molecule to be delocalized over both the molecule and the anatase cluster. When the ring is rotated by 90° and 135° a noticeable part of the orbital is delocalized to the anatase cluster.

Conclusions
In the energetically most favored adsorption mode of phenol at the anatase (101) surface according to the present B3LYP calculations, phenol is chemisorbed in an undissociated manner and forms an O(alcohol) -Ti(surface) bond and a H-bond with a surface oxygen. The HOMO is localized at the adsorbed molecule while LUMO is localized at the surface (Ti d orbitals). If spacer groups are introduced between the alcoholic group and the benzene ring, the adsorption energy increases. The effective band gap also increases due to stabilization of the HOMO orbital. When the aromatic ring system of the molecule is augmented the adsorption energy is relatively unaffected. The band gap decreases due to the increased amount of higher energy adsorbate orbital levels introduced into the anatase band gap. When the benzene ring of the phenol adsorbate is rotated, the degree of localization of the LUMO(ads) orbital changes from being mainly localized to phenol to being significantly delocalized to the surface, which points to the possibility of manipulating the electronic coupling by molecular construction [7]. This may be of importance when novel dye-sensitized solar cell devices are to be created. When the rather crude cluster model is compared to the periodic model, it is seen that the cluster model for example exaggerates the band gap, but it still manages to capture some of the essential features of the electronic LUMO(ads)-surface coupling.