A 29Si and 13C CP/MAS NMR study on the surface species of gas-phase-deposited γ-aminopropylalkoxysilanes on heat-treated silica

The aim of the present solid-state NMR study was to characterize the surface species of gamma-aminopropyltriethoxysilane (APTS), y-aminopropyltrimethoxysilane (APTMS), and gamma-aminopropyldiethoxymethylsilane (APDMS) on porous silica when the deposition was performed via the gas phase. The reaction temperature used, that is, 150-300 degreesC, in an atomic layer deposition reactor at a pressure of 20-50 mbar, was observed to distinctly affect the surface species of aminopropylalkoxysilanes on silica. The gas-solid reactions of the precursors with the silica surface were observed to be surface-limiting at the deposition temperatures of : 150 degreesC. On the basis of Si-29 CP/MAS NMR, the amino ends of APTS and APTMS molecules were observed to react both with alkoxy groups of other precursor molecules and silanols of silica at deposition temperatures of greater than or equal to150 degreesC forming Si-N linkages. The amino groups of APDMS molecules were observed to react at 150 degreesC only on silica heat-treated at 200 degreesC in a similar way. The reaction of amino groups affected also the chemical shifts of the carbon atoms in the propyl chain causing splitting of the peaks in the C-13 CP/MAS NMR spectra. At still higher reaction temperatures, especially at 300 degreesC, decomposition of the surface structures was observed to occur. The bonding modes of trifunctional APTS and bifunctional APDMS on silica heat-treated at 200-800 degreesC were systematically studied by 29Si and 13C NMR when the deposition was performed at 150 degreesC. Bi- and tridentate species of APTS were observed on silica pretreated at 200 degreesC, and mono- and bidentately bound surface structures were observed when silica was heat-treated at 450-800 degreesC. APTMS was also observed to attach onto silica pretreated at 600 degreesC in a similar way. APDMS was bound both mono- and bidentately on silica pretreated at 200 degreesC and at 600-800 degreesC but solely bidentately on silica pretreated at 450 degreesC.