Spectroelectrochemical behavior of 4-aminobenzenethiol on nanostructured platinum and silver electrodes

Abstract

The adsorption of 4-aminobenzenethiol (4-ABT) on Ag and Pt nanoparticles is studied by spectroelectrochemical means (cyclic voltammetry, Surface-Enhanced Raman Spectroscopy (SERS) and Surface-Enhanced Infrared Reflection Absorption Spectroscopy (SEIRAS). Similar SERS spectra are obtained when 4-ABT is adsorbed on platinum and silver nanostructured substrates. In addition, unless a low power density of the laser is used, these spectra show, both under open circuit conditions and when applying electrode potentials above -0.50 V, bands not observed in the normal Raman spectrum of 4-ABT. These bands disappear when the electrode potential is shifted to more negative values. Conversely, the SEIRA spectra of 4-ABT adsorbed on Ag do not show any significant change with the electrode potential, which indicates that there are not new species electrochemically formed in the range of potentials considered, which include some anodic and cathodic processes as shown in the corresponding cyclic voltammograms. In this regard, SERS measurements put in evidence the time dependence of the spectra obtained at potentials above -0.50 V just after switching on the laser suggesting the formation of new species, probably dimercaptoazobenzene (4-4'-DMAB), formed by a photochemical process of the adsorbed 4-ABT.

Spectroelectrochemical Study of the Photoinduced Catalytic Formation of 4,4′-Dimercaptoazobenzene from 4-Aminobenzenethiol Adsorbed on Nanostructured Copper

“…An increase of the intensity of the bands at 1629 and 1106 cm -1 and the upward shift and broadening of the band initially at 1264 cm -1 are the main changes observed in the spectra collected at potentials above -0.90 V. The potential-dependent behaviour of the spectra in Figure 4 is rather similar to that reported for 4-ABT adsorbed on silver. 23 In this latter case, an additional small band was observed at ca. 1200 cm -1 which is obscured by the silicon feature at 1216 cm -1 .…”

“…23 DFT calculations show that there are four bands that appear at 1135, 1388, 1422 and 1579 cm -1 for adsorbed 4,4'-DMAB on Cu which are absent for adsorbed 4-ABT likewise on Cu, and that a similar lack is also observed not only when the experimental Raman but also the SERS spectra are compared (see Tables 1 and 2). …”

“…The main features of the SEIRA spectra for 4-ABT adsorbed on a copper thin film electrode, shown in Figure 4, are similar to those previously obtained for silver. 23 In this way, the spectrum collected at -0.9 V shows bands at 1012, 1106, 1180, 1216, 1264, 1390, 1491, 1597 and 1629 cm -1 . The bands at 1106, 1216 and 1629 cm -1 do not appear in the SERS spectrum of 4-ABT at this electrode potential (see Table 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1216 cm -1 in Figure 4 does not seem to be related to adsorbed 4-ABT.…”

“…This behaviour also contrasts with that observed with Au and Pt. [23][24] It is known that Cu nanoparticles exhibit a plasmon absorption band with the maximum at wavelengths about of 580 nm (the exact values depending on their size, shape and aggregation). 61 Taken into account that aggregation shifts the plasmon resonance to higher values of wavelengths we have also recorded the SERS spectra of a self-assembled monolayer of 4-ABT on this active copper SERS substrate with a laser of 632.8 nm.…”

“…The spectra obtained and shown in Figure 4 are similar to those previously reported by our group for 4-ABT adsorbed on a silver thin film electrode. 23 …”