Atomic Layer Deposition of a High-Density Aminopropylsiloxane Network on Silica through Sequential Reactions of γ-Aminopropyltrialkoxysilanes and Water

A novel gas-phase procedure for the control of amino group density on porous silica through consecutive reactions of aminopropylalkoxysilanes and water vapor was developed. First heat-treated silica was saturated with trifunctional γ-aminopropyltrimethoxysilane (APTMS) or γ-aminopropyltriethoxysilane (APTS) in an atomic layer deposition reactor. During this step, precursor molecules were bound onto the surface both mono- and bidentately forming siloxane bridges with the silanol groups of silica. Then surface densities of 1.8 APTMS or 2.0 APTS molecules/nm2 were achieved. Next the aminosilylated surface was treated with water vapor in order to hydroxylate the free alkoxy groups of chemisorbed aminosilane molecules. At the same time, the silanol groups on the silica surface, which had remained unreacted during the first step, were revealed below the hydrolyzed alkoxy groups. These silanol groups of silica and hydrolyzed alkoxy groups were able to react further with the next feed of aminosilane molecules. The above-mentioned aminosilane/water vapor cycles, that is, two consecutive steps, could be repeated several times, and the amino group content on silica could be controlled through the number of aminosilane/water cycles. After four cycles, the surface was observed to be saturated and maximum amino group density was achieved. Then, by performing four or five cycles, surface densities of up to 3.0 APTS or APTMS molecules/nm2 were obtained. With this procedure, a high-density aminopropylsiloxane network is grown through horizontal polymerization of aminosilane molecules on the surface. With bifunctional γ-aminopropyldiethoxymethylsilane (APDMS), the repetition of aminosilane/water cycles did not increase the amino group content because of a lack of free and reactive ethoxy groups on the aminosilylated silica surface due to the bidentate bonding of APDMS molecules on silica.