<p>Adsorption of organic molecules at a surface of titanium dioxide (101) anatase is studied using quantum-chemical density functional theory. Anatase can be used in solar cells. For the clean anatase surface the band gap is so large that only UV-light can excite electrons. Different groups with conjugated systems are attached to obtain a more suitable band gap. </p><p>Phenol was attached in different positions to a cluster of anatase and geometry optimized using the B3LYP-functional. The geometry that was energetically most favorable was used to put in phenylmethanol, phenylethanol, naphthol, 2-phenanthrol, 1-pyrol and 2-perylol. To give a more realistic model of phenol at anatase, a study of a two- dimensional periodic anatase surface was also made. </p><p>Molecular orbitals were calculated to study the overlap between HOMO and LUMO orbitals. The calculation shows that phenol will remain as a molecule and will not dissociate. The band gap gets smaller when molecules are attached at the cluster and with 2-perylol it reaches the energy of visible light. </p><p>The molecular orbitals for HOMO, LUMO and LUMO of the adsorbed molecule were investigated. HOMO was localized at the molecule, LUMO at the cluster and LUMO of the adsorbed molecule move closer to the energy of LUMO when the number of rings increases.</p>

Adsorption of organic molecules at a surface of titanium dioxide (101) anatase is studied using quantum-chemical density functional theory. Anatase can be used in solar cells. For the clean anatase surface the band gap is so large that only UV-light can excite electrons. Different groups with conjugated systems are attached to obtain a more suitable band gap. Phenol was attached in different positions to a cluster of anatase and geometry optimized using the B3LYP-functional. The geometry that was energetically most favorable was used to put in phenylmethanol, phenylethanol, naphthol, 2-phenanthrol, 1-pyrol and 2-perylol. To give a more realistic model of phenol at anatase, a study of a two- dimensional periodic anatase surface was also made. Molecular orbitals were calculated to study the overlap between HOMO and LUMO orbitals. The calculation shows that phenol will remain as a molecule and will not dissociate. The band gap gets smaller when molecules are attached at the cluster and with 2-perylol it reaches the energy of visible light. The molecular orbitals for HOMO, LUMO and LUMO of the adsorbed molecule were investigated. HOMO was localized at the molecule, LUMO at the cluster and LUMO of the adsorbed molecule move closer to the energy of LUMO when the number of rings increases.