In situ identification of crystal facet-mediated chemical reactions on tetrahexahedral gold nanocrystals using surface-enhanced Raman spectroscopy.

Direct monitoring of a metal-catalyzed reaction by surface-enhanced Raman scattering (SERS) is always a challenging issue as it needs bifunctional metal structures that have plasmonic properties and also act as catalysts. Here we demonstrate that the tetrahexahedral (THH) gold nanocrystals (Au NCs) with exposed {520} facets give highly enhanced Raman signals from molecules at the interface, permitting in situ observation of chemical transformation from para-aminothiophenol (PATP) to 4,4'-dimercaptoazobenzene (DMAB). The origin of the intense SERS signals of DMAB is carefully investigated based on the comparison of the SERS spectra of PATP obtained with both the THH Au NCs and the Au nanospheres with the exposed {111} facets. It is elucidated that the high-index {520} facet rather than the localized surface plasmons of the THH Au NCs plays a key role in producing a high yield of the product DMAB which is accompanied by the selective enhancement of the characteristic Raman signals.