4-nitrothiophenol SAM on Au(111) investigated by in situ STM, electrochemistry, and XPS

SAMs formed from 4-nitrothiophenol (4-NTP) on Au(lll) were studied in diffe rent electrolytes with in situ STM, cyclic voltammetry, and X-ray photoelec tron spectroscopy. The 4-NTP SAM exhibits structural changes with the elect rode potential, which coincide with the reduction steps of the nitro group. In general, at potentials where no faradaic processes occur, small domains (10 nm x 2 nm) with ordered structures are observed together with unordere d phases, the latter ones occupying ca. 50% of the substrate surface. When subjecting the surface to negative potentials, the ordered structure disapp ears and monatomic-high islands covering 40% of the surface emerge, the nat ure of which could not be elucidated unequivocally. This starts at 20 mV ve rsus saturated calomel electrode (SCE) in 50 mM H2SO4 and at -240 mV versus SCE in 50 mM Na2SO4. Upon further reduction, SAM reductive desorption is o bserved directly in scanning tunneling microscopy. This leads to a bare gol d surface with characteristic triangular gold islands.