Physisorption and Chemisorption of Alkanethiols and Alkyl Sulfides on Au(111)

The energetics and kinetics of the adsorption of a variety of sulfur-containing hydrocarbons onto Au(111) have been explored using helium beam reflectivity and temperature-programmed desorption (TPD) techniques. Simple alkanethiols as well as dialkyl sulfides, dialkyl disulfides, and other sulfur-containing organics were found to adsorb with a low coverage physisorption enthalpy about 20% greater than the heat of vaporization in the bulk. In contrast to the dialkyl sulfides that only physisorb, alkanethiols and dialkyl disulfides also interact chemically with the gold with a chemisorption enthalpy of 126 kJ/mol that is independent of alkyl chain length. The presence of sterically hindering substituent groups on the carbon atom adjacent to the sulfur atom produces, however, a reduction in the chemisorption enthalpy of up to 15%. Temperature-programmed desorption of nonequilibrated, high exposure layers of alkanethiols with eight carbon atoms or longer displayed a second, higher energy, chemisorption peak at 148 kJ/mol which could be eliminated by annealing, leaving only the lower energy chemisorption peak. The chemisorption rate of molecules in the physisorbed precursor state has also been measured. Arrhenius plots for the chemisorption of a series of alkanethiols from their physisorbed precursors show that the chemisorption activation energy is 28.8 kJ/mol and is independent of chain length.