In Situ SERS Study of Azobenzene Derivative Formation from 4-Aminobenzenethiol on Gold, Silver, and Copper Nanostructured Surfaces: What Is the Role of Applied Potential and Used Metal?

The aromatic mercapto derivative 4-aminobenzenethiol (4-ABT) is a substance that can be easily adsorbed on Au, Ag, and Cu surfaces, but in some studies, formation of 4,4′-dimercaptoazobenzene (4,4′-DMAB) on Ag and Au is described. We have studied 4-ABT on all three SERS-active metals in a spectroelectrochemical cell aiming at the role of the metal and electrode potential on formation of 4,4′-DMAB at 785-nm excitation. In the case of Au, intense bands of 4,4′-DMAB are observed in a potential range from +0.2 to −0.8 V. Only at very negative potentials do these bands almost disappear and only spectral features of 4-ABT are observed. In the case of Ag, a similar spectral behavior is observed, but relative bands intensities are weaker than on Au. In the case of Cu, there is no spectral evidence of 4,4′-DMAB at any potential value. Only characteristic bands of 4-ABT are observed in the whole potential range; the highest signals are obtained at potentials around −0.6 V. Experimental results are supported by DFT calculations. We can conclude that the crucial aspect of surface photocatalytic formation of 4,4′-DMAB from 4-ABT is the metal. The reaction is very effective on Au, and it is inhibited on Cu.