Atomic layer deposition of amino-functionalized silica surfaces using N-(2-aminoethyl)-3-aminopropyltrimethoxysilane as a silylating agent

Atomic layer deposition (ALD) technique can be used for the preparation of amino-functionalized silica surfaces and for the study of the gas-solid reactions. In the present study N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPS) was used as a silylating agent. The characterization of aminosilylated silica samples was performed by elemental analyses, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and solid-state C-13 NMR. Under saturation conditions, viz. at 180 degreesC and at a pressure of 20-50 mbar, vaporized AAPS molecules were observed to react with the silanols of silica at the silane end of the molecule forming siloxane bridges. Thus, one surface-saturated molecular layer was deposited on the surface. Under these conditions indication of the gas-phase reaction of terminal amino groups with methoxy groups of other AAPS molecules or silanol groups of silica was observed. The surface density of amino groups on the silica surface could be controlled within 2.0-3.4 amino groups/nm(2) silica through the pretreatment temperature of silica, i.e., 200-800 degreesC. The amino group density on silica could also be controlled through a procedure based on sequential gas-phase reactions of AAPS and water. Thus, a high-density aminopropylsiloxane network was grown on the silica surface. With this procedure a surface density of 3.0-5.4 amino groups/nm 2 of silica (pretreated at 450 degreesC) could be obtained depending on the number of AAPS/water cycles. The surface was observed to be saturated with the precursor molecules after four AAPS/water cycles. The gas-solid reactions of AAPS on silica were also compared with those of single-amino-group precursors, viz. gamma-aminopropyltrialkoxysilanes.