Department of Chemistry, University of Maine , Orono, Maine 04469, United States.
Laboratory for Surface Science and Technology, University of Maine , Orono, Maine 04469, United States.
Langmuir. 2016 Jul 19;32(28):7170-9. doi: 10.1021/acs.langmuir.6b01669. Epub 2016 Jul 8.
An in situ FTIR thin film technique was used to study the sequential atomic layer deposition (ALD) reactions of SiCl4, tetraethyl orthosilicate (TEOS) precursors, and water on nonporous silica powder using supercritical CO2 (sc-CO2) as the solvent. The IR work on nonporous powders was used to identify the reaction sequence for using a sc-CO2-based ALD to tune the pore size of a mesoporous silica. The IR studies showed that only trace adsorption of SiCl4 occurred on the silica, and this was due to the desiccating power of sc-CO2 to remove the adsorbed water from the surface. This was overcome by employing a three-step reaction scheme involving a first step of adsorption of triethylamine (TEA), followed by SiCl4 and then H2O. For TEOS, a three-step reaction sequence using TEA, TEOS, and then water offered no advantage, as the TEOS simply displaced the TEA from the silica surface. A two-step reaction involving the addition of TEOS followed by H2O in a second step did lead to silica film growth. However, higher growth rates were obtained when using a mixture of TEOS/TEA in the first step. The hydrolysis of the adsorbed TEOS was also much slower than that of the adsorbed SiCl4, and this was overcome by using a mixture of water/TEA during the second step. While the three-step process with SiCl4 showed a higher linear growth rate than obtained with two-step process using TEOS/TEA, its use was not practical, as the HCl generated led to corrosion of our sc-CO2 delivery system. However, when applying the two-step ALD reaction using TEOS on an MCM-41 powder, a 0.21 nm decrease in pore diameter was obtained after the first ALD cycle whereas further ALD cycles did not lead to further pore size reduction. This was attributed to the difficulty in removal of the H2O in the pores after the first cycle.
采用原位傅里叶变换红外（FTIR）薄膜技术，使用超临界二氧化碳（sc-CO2）作为溶剂，研究了 SiCl4、正硅酸乙酯（TEOS）前体和水在无孔二氧化硅粉末上的顺序原子层沉积（ALD）反应。无孔粉末的 IR 研究用于确定使用基于 sc-CO2 的 ALD 来调整介孔二氧化硅孔径的反应顺序。IR 研究表明，SiCl4 在二氧化硅上仅发生微量吸附，这是由于 sc-CO2 的干燥能力从表面去除吸附水所致。通过采用三步反应方案克服了这一点，该方案包括第一步吸附三乙胺（TEA），然后是 SiCl4 ，然后是 H2O。对于 TEOS，使用 TEA、TEOS 然后是水的三步反应序列没有优势，因为 TEOS 只是将 TEA 从二氧化硅表面置换出来。两步反应涉及首先添加 TEOS，然后在第二步中添加 H2O，确实导致了二氧化硅膜的生长。然而，当在第一步中使用 TEOS/TEA 的混合物时，获得了更高的生长速率。吸附的 TEOS 的水解也比吸附的 SiCl4 慢得多，通过在第二步中使用水/TEA 的混合物克服了这一点。虽然三步法用 SiCl4 显示出比两步法用 TEOS/TEA 更高的线性生长速率，但由于生成的 HCl 导致我们的 sc-CO2 输送系统腐蚀，因此其使用不切实际。然而，当在 MCM-41 粉末上应用两步法的 TEOS 原子层沉积反应时，第一次 ALD 循环后获得了 0.21nm 的孔径减小，而进一步的 ALD 循环不会导致孔径进一步减小。这归因于第一次循环后孔中 H2O 的去除困难。
