Abstract
Adsorption and thermal reaction of alkanethiols (RSH; R = H, CH3, C2H5, and C4H9) on the Cu(110) and Si(100) surfaces were studied by means of temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Desorption profiles of evolved gases were detected by quadrupole mass. Intermediate species adsorbed on surface were characterized with XPS using synchrotron radiation. Alkanethiols tended to dissociatively adsorb on Cu or Si surface to form surface hydrogen and alkanethiolate (RS) at 115 K. Upon raising the temperature, all of surface RS species further decomposed via the scission of the S-C bond, resulting in evolution of hydrocarbons and deposition of sulfur. The surface condition and the number of carbon atoms of alkyl groups of RSHs’ significantly affected their decomposition on the surfaces. On the Cu surface, the evolved products from CH3SH adsorption varied with the initial coverage. At low coverages, CH3 decomposed from CH3S tended to combine with surface hydrogen to evolve CH4. At high coverages, the CH3 also recombined to form C2H6. The surface C2H5 decomposed from C2H5S produced C2H6 through hydrogenation and C2H4 through b-hydride elimination; the products ratio between C2H6 and C2H4 increased with the coverage of adsorbed C2H5SH. The surface C4H9 group decomposed from C4H9S underwent an exclusive elimination of b-hydride to form C4H8. The alkyl moiety on Cu surface also underwent extensive dehydrogenation become to surface carbons. The degree of carbon deposition decreases on increasing the number of carbon atoms in the alkyl group. Silicon surface was less active than Cu toward R-S scission. Alkyl groups decomposed on the scission tend to dehydrogenate, instead of hydrogenation into alkane, into olefins and carbon deposit. Deposited sulfur further reacted with Si and desorbed in the form of SiS when the surface was heated above 820 K.