The adsorption and thermal decomposition of alcohols (CH3OH, C2H5OH, and C4H9OH) on Ge(100) were investigated with temperature-programmed desorption and X-ray photoelectron spectra. At 105 K, CH3OH adsorbs both molecularly and dissociatively on Ge(100). Chemisorbed CH3OH molecules dissociate to form surface CH3O and hydrogen in a temperature range 150-300 K. Surface CH 3O can dehydrogenate to yield CH2O as two desorption features, which depend on coverage. At small coverage, surface CH3O undergoes mainly α-hydrogen elimination to desorb CH2O at 490 K. At large coverage, another desorption of CH2O occurs predominantly at 525 K, which is initiated by a recombinative desorption of CH3OH. A calculation with density functional theory at the B3LYP/6-311+G** level shows that the dissociation of the O-H bond has a much smaller barrier (<40 kJ/mol) than those for C-O bond cleavage (>150 kJ/mol). Desorption of CH2O results from the moderate barriers (∼110 kJ/mol) for cleavage of the C-H bond of surface CH3O and weak adsorption energy of CH2O (-56 kJ/mol). The recombination of surface CH3O with H occurs at large coverage with an energy barrier 127-140 kJ/mol. Similarly to CH3OH, C2H5OH and C4H 9OH undergo the mechanism of thermal reactions through formation of alkoxyl intermediates. The longer-chain alkoxyl decomposes to desorb aldehyde at lower temperature because the interaction of its alkoxyl chain with the surface is stronger. On annealing to ∼570 K, all alkoxyl groups are completely removed from the surface via dehydrogenation and recombination to desorb aldehyde and alcohol, respectively. At a large coverage, the longer-chain alkoxyl undergoes dehydrogenation to a larger extent than recombinative desorption.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films