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HOMO and LUMO energies of N3 dye on TiO2(110)

Program: Energy and Environment
Department: Physics and Astronomy
Functional DSSCs typically employ anatase TiO2 nanoparticles. To obtain an atomistic view of the dye molecule-TiO2 surface interaction, we are studying the adsorption of N3 dye on the single crystal TiO2(110) surface. A scanning tunneling microscope (STM) images of the atomically clean and well-ordered TiO2(110) surface, shown at the upper left, demonstrates that large, well-ordered, atomically flat terraces can be obtained. The lower image illustrates how the surface can be passivated by exposure to pivalic acid in UHV forming a pivalate layer that retains the surface morphology. Upon removal from the UHV chamber, the surface can be sensitized to N3 dye in solution, where in an exchange reaction adsorbed pivalate ions are replaced by dye molecules. After re-insertion to the UHV chamber, we perform UPS and InvPE measurements to determine the relative alignment of the HOMO and LUMO levels of the molecule, and the conduction and valence band edges of the substrate. The figure below shows photoemission and inverse photoemission of the occupied and unoccupied states, respectively of the clean and N3-dye covered TiO2(110) surface. The HOMO and LUMO energies are 0.9 eV above the TiO2 valence band edge and 0.5 eV below the conduction band edge, respectively. A systematic study of various dye/substrate combinations would enable tailoring DSSC properties.

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