Dimeric Core Shell Ag-2@Tio2 Nanoparticles For Off-Resonance Raman Study Of The Tio2-N719 Interface

In this paper, we present a nanopartide-enhanced Raman spectroscopic study of TiO2-N719 (bis(tetrabutylammonium) [cis-di(thiocyanato)-bis (2,2'-bipyridy1-4'-carboxylate-4-carboxylic acid)-ruthenium(II)])) interface by employing Ag-2@TiO2 dimeric core shell nanopartides, where the TiO2 shell serves as an active semiconductor surface. The Ag-2@TiO2 nanopartides are synthesized by connecting two Ag nanopartides with thiol-modified complementary DNAs, followed by coating a 2 nm thin layer of TiO2 on the Ag-2 dimeric nanopartides. The Ag-2@TiO2 dimeric core shell nanopartides show a redshift of plasmon resonance frequency compared with Ag@TiO2 core shell nanopartides, and provide significant SERS enhancement over the Ag@TiO, nanopartides, which allows for the investigation of SERS of TiO2-N719 interface under off-resonance conditions using low energy 785 nm laser excitation. Fine potential dependent Raman measurements show that thiocyano and carboxyl groups of the N719 molecule can adsorb on the TiO2 shell competitively: Along with negative potential excursion, the thiocyano is replaced by carboxyl, which is accompanied by a conformationsl change of the molecule involving an orientation change of the bipyridine ring. Furthermore, the potential of maximum intensity of the inter-ring vibration band shifts toward a more positive value with laser excitation wavelengths from 638 to 785 nm, which verifies the photon-driven charge transfer mechanism from the HOMO of N719 to the conduction bands of TiO2. Three-dimensional finite-difference time-domain (3D-FDTD) simulations are performed to evaluate the electromagnetic (EM) enhancement from the Ag-2@TiO2 dimeric core shell nanoparticles. The present work demonstrates that the Ag-2@TiO2 dirneric core shell structures provide significant plasmon resonance, which can be used for Raman investigations of interfaces formed with the semiconductor shell layers of the nanopartides.