DFT Study of the Adsorption and SERS of Pyridine on M₁₀N₁₀(M, N = Cu, Ag) Tetrahedral Clusters

Abstract

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine− M 10 N 10 (M, N = Ag, Cu) tetrahedral (Td) clusters considering two binding positions: vertex (V) and surface (S). In addition to the wellknown monometallic Td structure, we added two different bimetallic Ag−Cu compositions, named Td1 and Td2 geometries. Density functional methodology with the use of BP86 and CAM-B3LYP exchange-correlation functionals (XCs) and LANL2DZ pseudopotential has been employed for analyzing the electronic structure and geometries, the chemical static (CHEM), and resonant Raman mechanisms (RR): charge transfer RR−CT and intracluster excitation RR−CR. The static CHEM mechanism shows an increase in the enhancement factors (EFs) of Py−V concerning Py−S positions, which can also be distinguished by the averaged adsorption energies and bond polarizabilities. The static SERS response for Cu−Py−V junction is from 5 to 10 times greater than Ag−Py−V EFs and up to 28 times greater than Py−S complexes. For the static Raman, we found that the analyses of ν 8a and ν 1 normal modes are related to the EF changes and allow us to distinguish V from S complexes. The TDDFT calculations show striking differences between BP86 and CAM-B3LYP XCs analyzed spectra, and CAM-B3LYP granted a clear distinction between V and S for the location of CT-type transitions. In addition, important differences were obtained from the analysis of the charge transfer excitations between both XCs. Resonant Raman calculations evidenced significant enhancements for RR−CT and RR−CR as compared to the static enhancements, and RR−CT can be distinguished from the RR−CR mechanism, while specific normal modes help to differentiate the vertex from the surface Py-junction. Bimetallic Ag−Cu nanostructures represent promising choices for SERS substrates, showing EFs higher than those of monometallic Ag.

SERS-Based Sensor Using Subnanometric Copper–Silver Mixed Clusters Ag(8–n)Cun (n = 0–8) for Pyridine: A DFT Study

“…In all other cases, the electron and hole are both located on the metallic clusters, making the transition local excitation (LE). This intracluster excitation of the lowest-lying excitation stated by CAM-B3LYP has been previously reported by Seuret-Hernandez et al The atomic contribution to holes and electrons for the lowest excited state (S 1 ) is provided in Table S7 (SI file), and the contribution from fragments (Ag/Cu/Py) to hole and electron is visualized as color heat maps in Figure . As one can see, the metals actively participate in the holes in all cases, and starting from Ag 3 Cu 5 Py, we can clearly observe the active contribution of Cu to th e holes.…”

“…In all other cases, the electron and hole are both located on the metallic clusters, making the transition local excitation (LE). This intracluster excitation of the lowest-lying excitation stated by CAM-B3LYP has been previously reported by Seuret-Hernandez et al 20 The atomic contribution to holes and electrons for the lowest excited state (S 1 ) is provided in Table S7 S8, SI) that helps to define the excitation nature as charge transfer is also computed. A charge-transfer excitation is estimated with a Δr value higher than 2 Å, and a local excitation is considered with a Δr value smaller than 2 Å.…”

“…Similar observations on the performance of the pyramidal Ag 20 cluster were also recognized in the study of chlorpyrifos pesticide. 6 An excellent agreement between experimental and modeling results was also reported on the SERS spectra study of thiram fungicide adsorbed on silver nanoparticles when using the Ag 20 PBE/cc-pVDZ-PP//PBE-cc-pVDZ level of theory. 7 Takenaka et al evaluated the DFT calculations with the dispersion interaction for SERS spectra modeling 2,2′-bipyridyl adsorbed on pyramidal Au 20 , Ag 20 , and Au 10 Ag 10 mixed clusters.…”

“…Recently, Seuret-Hernańdez and Morera-Boado theoretically analyzed in detail the SERS spectra of Py adsorbed on the Ag 10 Cu 10 tetrahedral (T d ) clusters via two binding positions: vertex (V) and surface (S). 20 As a result, the static SERS response for Cu−Py−V complexes is 5 to 10 times greater than that for Ag−Py−V complexes and up to 28 times greater than that for Py−S complexes. The Raman spectra of the Ag−Py−vertex or surface bimetallic clusters are enhanced compared with monometallic clusters, indicating that adding Cu atoms can improve the SERS enhancement of the Ag 20 T d cluster.…”

“…The SERS properties of pyridine (Py) adsorbed on different metals, including silver, , gold, , copper, , nickel, platinum, and cadmium, were intensively investigated in the literature. Recently, Seuret-Hernández and Morera-Boado theoretically analyzed in detail the SERS spectra of Py adsorbed on the Ag 10 Cu 10 tetrahedral ( T d ) clusters via two binding positions: vertex ( V ) and surface ( S ) . As a result, the static SERS response for Cu–Py–V complexes is 5 to 10 times greater than that for Ag–Py–V complexes and up to 28 times greater than that for Py–S complexes.…”