Reduced Charge-Transfer Threshold in Dye-Sensitized Solar Cells with an Au@Ag/N3/n-TiO₂ Structure As Revealed by Surface-Enhanced Raman Scattering

Abstract

Interfacial information, such as the binding type and charge transfer (CT) processes, is critical for the investigation of dye-sensitized solar cells (DSSCs). Herein, Au@Ag core–shell nanoparticles (NPs) were employed for the fabrication of Au@Ag/N3/n-TiO2 systems. With the help of the CT degree (ρCT), surface-enhanced Raman scattering (SERS) spectra are utilized to evaluate the CT processes upon layer-by-layer addition of TiO2 to metal/N3/n-TiO2 systems based on Au@Ag, Ag, and Au. The layer-dependent SERS intensity and ρCT for Au@Ag/N3/n-TiO2 revealed a CT enhancement involving TiO2 at excitation wavelengths of 488, 514.5, 647, and 785 nm, whereas Ag/N3/n-TiO2 presented such enhancement only at excitation wavelengths of 488 and 514.5 nm. The mechanisms of CT process are proposed to explain such reduced CT energy threshold: In Au@Ag/N3/n-TiO2, an equivalent CT process involving TiO2 is first proposed, in which the electrons are directly transferred from the HOMO level of N3 to the much lower CB3 level of the Au@Ag/TiO2 complex due to energy level equilibration, which reduces the CT threshold involving TiO2 below1.58 eV, extends the CT response region involving TiO2 from 488 to 514.5 nm (Ag/N3/n-TiO2) to 488–785 nm and enhances the CT efficiency in the high-energy region. Finally, Au@Ag1–5 with different Ag/Au ratios were prepared for the fabrication of bimetal/N3/n-TiO2 systems. In the Au@Ag1/N3/n-TiO2 and Au@Ag2/N3/n-TiO2 systems, the decreased CT threshold is induced by energy level equilibration; in the Au@Ag4/N3/n-TiO2 system, it is due to the dual effects of energy level equilibration and activation of Ag at the core–shell surface induced by the inner Au.

Effect of TiO2 arrays on surface enhanced Raman scattering (SERS) performance for Ag/TiO2 substrates

“…As demonstrated in the previous reports [35], the EM enhancement effect and CT effect are largely responsible for SERS activity in Ag/TiO 2 (S) substrate. Figure 10 shows the schematic of Ag/TiO 2 (S) as SERS substrate for the detection of R6G molecules.…”

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.…”