Visualizing Interfacial Charge Transfer in Ru-Dye-Sensitized TiO<sub>2</sub> Nanoparticles Using X-ray Transient Absorption Spectroscopy

A molecular level understanding of the structural reorganization accompanying interfacial electron transfer is important for rational design of solar cells. Here we have applied XTA (X-ray transient absorption) spectroscopy to study transient structures in a heterogeneous interfacial system mimicking the charge separation process in dye-sensitized solar cell (DSSC) with Ru(dcbpy)<sub>2</sub>(NCS)<sub>2</sub> (RuN3) dye adsorbed to TiO<sub>2</sub> nanoparticle surfaces. The results show that the average Ru-NCS bond length reduces by 0.06 Å, whereas the average Ru−N(dcbpy) bond length remains nearly unchanged after the electron injection. The differences in bond-order change and steric hindrance between two types of ligands are attributed to their structural response in the charge separation. This study extends the application of XTA into optically opaque hybrid interfacial systems relevant to the solar energy conversion.