Direct Measurement of the Absolute Orientation of N3 Dye at Gold and Titanium Dioxide Surfaces with Heterodyne-Detected Vibrational SFG Spectroscopy

Christopher C. Rich, Max A. Mattson, Amber T. Krummel

Research output: Contribution to journalArticlepeer-review

13 Scopus citations

Abstract

Light-harvesting dyes in dye-sensitized solar cells (DSSCs) must be designed not only to effectively harvest visible light but also to maintain an adsorption geometry at the solvent/TiO2 interface that encourages electron injection. Electron injection is encouraged when the dye is adsorbed to the TiO2 surface such that the LUMO of the dye is spatially near the surface. Furthermore, deleterious recombination pathways between the surface and dye are suppressed if the HOMO of the dye is spatially well-separated from the surface. Thus, measuring the configuration of dyes at these interfaces is important for understanding why some dyes perform better than others as well as providing insight into designing more ideal dyes. We investigate the adsorption geometry of N3 dye on gold and TiO2 using heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Incorporating heterodyne detection into our VSFG experiment provides both enhanced SFG signal and phase sensitivity, which enables the measurement of the absolute orientation of molecules at interfaces. On gold, we find that N3 adsorbs to the surface by binding through one of its isothiocyanate ligands at a 36° tilt angle from the surface normal. The other isothiocyanate ligand exhibits a tilt angle of 82° and thus does not interact with the interface as strongly. Conversely, on TiO2, we find that N3 adsorbs to the surface through three carboxylic acid groups with both isothiocyanate ligands facing away from the surface at a 180° tilt angle from the surface normal. This adsorption geometry of N3 on the TiO2 is arranged such that its LUMO, which resides primarily on the bipyridine ligands, is positioned near the surface while the HOMO, which resides primarily on the isothiocyanate ligands, is oriented far away from the surface. This study presents the first HD-VSFG spectra of N3 on nanoparticulate TiO2 and on gold. (Graph Presented).

Original languageEnglish (US)
Pages (from-to)6601-6611
Number of pages11
JournalJournal of Physical Chemistry C
Volume120
Issue number12
DOIs
StatePublished - Mar 31 2016

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