Investigation of Charge Transfer in Ag/N719/TiO₂ Interface by Surface-Enhanced Raman Spectroscopy

Abstract

The interfaces of metal−dye molecule−semiconductor sandwich structure are very important in the investigation of dyesensitized solar cells (DSSCs) where metals are used to enhance absorption. In this work, we first designed and synthesized Ag/N719 and Ag/N719/TiO 2 sandwich systems to investigate the chemical binding type at the interfaces of Ag/N719/TiO 2 . The results of the Raman spectra under the laser excitations of 532, 633, and 785 nm clarified that the SCN groups adsorbed on the Ag surface via the S terminal and the TiO 2 layer possibly bound to Ag/N719 via the ester linkage (OCO) of the COOH group in N719. Then, we optimized the Ag substrate as an SERS detection platform and selected the Ag sol film as the substrate. Last, the relationship between the "degree of CT (ρ CT )" in the SERS spectra and the charge transfer (CT) process was investigated by tuning the contribution from the chemical effect. We found that, owing to the introduction of TiO 2 , the intensity and ρ CT first increased (n = 0−2) and then decreased (n = 2−3) with the increase of the number of TiO 2 layers under 532 and 633 nm laser excitations. However, the intensity decreased (n = 0−3) with the increase of the number of TiO 2 layers under the laser excitation of 785 nm, and there was no obvious change about ρ CT . Meanwhile ρ CT became higher with the increase of the laser excitation energy at the interfaces with the same TiO 2 layer number. In order to explain these variations about ρ CT , we utilize ultraviolet photoelectron spectroscopy (UPS) and UV− vis spectra to calculate energy levels for better understanding the charge transfer (CT) process, and the calculation result is in accordance with the variation tendency of ρ CT .

Defect engineering in semiconductor-based SERS

“…In extension to the monitoring of isolated reactions, the demand for SERS characterization of more complex model systems and devices has also been on the rise. [128][129][130][131][132] It is well established that the CT effect can selectively inuence the enhancement of specic molecules or specic vibrational modes, therefore the relative signal intensity of selected bands in the SERS spectrum can be used to reect the degree of CT (r CT ) and the site of the reaction. This method has been extensively studied in solar-cell-like model systems.…”

Surface Enhanced Raman Scattering Revealed by Interfacial Charge-Transfer Transitions

“… 104 Similarly, the chemical binding types and CT processes, which are critical in dye-sensitized solar cells, were determined by the construction of metal-dye-semiconductor sandwich structures, such as Ag/N719/TiO 2 and Au@Ag/N3/n-TiO 2 systems. 128 , 129 These strategies broaden the scope of SERS applications and make the in situ tracking of the interfacial CT transitions realizable.…”

Controllable Preparation of SERS-Active Ag-FeS Substrates by a Cosputtering Technique

“…EM is an inherent property of metal NPs and is independent of the adsorbed molecules; CM is more complicated than EM. CM occurs via short-range chemical adsorption and is derived from the interactions between the metal substrate and the adsorbed molecules, which change the molecular polarizability [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. The SERS effect is achieved by changing the polarizability of the new CT excited state formed by the interactions between the molecule and the substrate [19,28].…”