Controlled preparation of aminofunctionalized surfaces on porous silica by atomic layer deposition

Author
Satu Ek
Year
2004
Abstract & Cover

Because ALD growth is based on the surface reactions of precursor molecules with a substrate, characterization of the surface species on porous substrates is very important. TG, DRIFTS/PLS, and 1H MAS NMR were found to be accurate techniques for the characterization of the surface species. With all these methods, not just the surface silanol groups but also the bulk internal silanols on silica are determined. While TG offers a relatively fast and simple method for the quantification of the total number of silanol groups, the numbers of different types of silanols cannot be determined. Combined DRIFTS/PLS method also allows the determination of the total number of silanol groups. Once calibration has been performed DRIFTS/PLS offers a fast analytical method, that can easily be applied to numerous kinds of samples in process analysis, for example. The advantage of 1H MAS NMR is the quantification of both isolated and hydrogen-bonded silanol groups on silica. It is, however, time-consuming and expensive. The present study showed that amino-functionalized silica surfaces can be prepared by ALD without solvent in a simple, conformal, and reproducible manner. Volatile aminopropylalkoxysilanes studied can be used as precursors because they do not decompose during vaporization or subsequent deposition. The reaction temperature affected the surface species on silica so that the use of relatively high reaction temperatures, i.e. 150-300 °C, led to sidereactions between the amino groups of bi- and trifunctional precursors and silanols groups or alkoxy groups of other precursor molecules on silica. Thus, deposition temperatures less than 150 °C (under pressure of 2-5 kPa) should be used to avoid side-reactions. The deposition and characterization of a single surface-saturated molecular layer on the surface allows study of various surface structures on porous substrates. The number of adsorbed precursor molecules (and terminal amino groups) on silica could be controlled between 1.0 and 3.0 molecules (or NH2 groups/nm2) through heat-treatment of silica (200-800 °C), choice of the precursor (APTMS, APTS, AAPS, APDMS, APDMES), and the number of reaction cycles (from 1 to 4) of gas-phase reactions of aminopropylalkoxysilane precursor and water. Such control is not possible with the liquid-phase methods currently applied to the preparation of aminofunctionalized silica surfaces. The highest amino group density was achieved with the bifunctional precursor, APDMS, on silica pretreated at 200 °C when a single surface-saturated molecular layer was deposited. Still higher amino group densities were obtained when sequential reactions of trifunctional APTMS, APTS or AAPS, and water were applied. A high-density aminopropylsiloxane network, which can be considered as a monolayer, could be deposited on silica because the surface was observed to be saturated with the precursor molecules. The results obtained from these experiments on porous substrates can further be applied to planar substrates, and valuable information on the surface chemistry and deposition process on surfaces can be obtained. As shown in this work, ALD enables a controlled deposition of functionalized surfaces, in addition to the oxides, sulfides, and nitrides earlier deposited on porous supports for catalyst applications. It was also shown that even more complex organic layers, such as polyimides, could be deposited on functionalized surfaces. In the present study polyimide structures were deposited on aminosilylated silica using PMDA and DAH as precursors. Low temperatures are especially desired to prevent decomposition of organic precursors. Pressure within the ALD reactor should be as low as possible, so as to provide the lowest vaporization and deposition temperatures. In addition to the present ALD applications, ALD has great potential for industrial applications in the future in completely novel areas where organic layers may be applied to planar or porous substrates. 

Source of Information
FinALD40 exhibition material, http://www.aldcoe.fi/events/finald40.pdf
University
Helsinki University of Technology, Department of Chemical Technology, Laboratory of Inorganic and Analytical Chemistry
(Espoo, Finland)
External Link
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