Why does silica nanorod growth vary significantly on PDMS substrates prepared and treated under the same conditions?
I am currently studying the growth of SiO₂ nanorod arrays on patterned/wrinkled PDMS substrates using a solution–liquid–solid (SLS)-type method based on PVP/1-pentanol, an aqueous sodium citrate solution, NH₄OH, ethanol, and TEOS.
In some experiments, I obtained a relatively dense and continuous layer of SiO₂ nanorods on the PDMS surface. However, in recent experiments, the SiO₂ structures became much shorter and sparser and appeared mainly as isolated patches or islands, leaving large areas of the PDMS surface nearly uncovered.
What confuses me is that the PDMS substrates were prepared in almost the same way. Two samples were cut from the same cured PDMS film, stretched to 135% simultaneously, and then treated together by UV–ozone under the same conditions. The subsequent growth procedure was also essentially unchanged. The main possible variations were only small differences in the amounts of NH₄OH and TEOS.
Since the reported SLS mechanism for silica nanorod growth strongly depends on the adsorption and stabilization of aqueous catalyst droplets on a sufficiently hydrophilic substrate, I am wondering whether small differences in the actual surface wettability of PDMS after UV–ozone treatment could lead to such a large change in nanorod coverage.
I would particularly appreciate suggestions regarding the following points:
- Can two PDMS samples from the same cured film and treated simultaneously by UV–ozone still exhibit significantly different surface hydrophilicity or surface chemistry?
- How sensitive is SLS-type SiO₂ nanorod growth to small changes in NH₄OH or TEOS concentration/volume?
- Could excessive NH₄OH or TEOS promote bulk-phase silica nucleation and therefore suppress nanorod growth on the substrate?
- What would be the most reliable way to ensure reproducible surface wettability of PDMS before each growth experiment?
- Besides water contact-angle measurements, are there better ways to evaluate the ability of aqueous catalyst droplets to adsorb and remain pinned on PDMS in a 1-pentanol-based emulsion?
Any suggestions on distinguishing between effects from substrate wettability, emulsion stability, and reaction kinetics would be greatly appreciated.