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I am spin-coating a porous silica solution on a glass microscope coverslip, and then heating the resulting thin film to $\pu{350 °C}$, in an attempt to create a thin film of porous silica sol gel. I am trying to replicate only the porous sol gel portion of the work of Yantasee et al.:
Nanoengineered electrochemical sensor based on mesoporous silica thin-film functionalized with thiol-terminated monolayer

The problem I am encountering is that my heated sol gel is cracking and partially delaminating. This is unacceptable. Can anyone offer advice on how to prevent this cracking when heating to $\pu{350 °C}$?

Relevant Information

  1. The recipe for the silica precursor solution is:
    2.578 g quantity of Pluronic F-127 surfactant was dissolved in 3.430 g of deionized water and 8.846 g of ethyl alcohol. Then 0.108 g of concentrated $\ce{HNO3}$ was added into the mixture. Finally, a 5.0 g quantity of TEOS was added to the solution.
  2. The solution was sonicated for 8 hours and then spin-coated at 3000 rpm onto the surface of a 12 mm glass microscope cover slip.
  3. Film thickness: ~10 μm.

Possible Causes

  • Perhaps I am using the wrong type of glass for the substrate, and the respective thermal expansions are causing cracking at the interface. If so, what relatively cheap glass could I use?
  • Maybe I need to rigorously prepare the glass substrates by cleaning them with a solution of acid (or base) and solvent. Could an ill-prepared substrate contain contaminants acting as nucleation sites for crack formation when heated? If so, what best method is used by these groups to prepare their slides? I can't seemed to find a paper on sol gel thin films where they describe their substrate preparation.
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    • $\begingroup$ I suggest trying the thermal treatment without placing the glass slide in direct contact with the hottest walls of the furnace and focus on a T ramp. These kind of problems are normal and rarely mentioned in experimental parts as for their description and work around are generally looking childish ;) as for most of the users are from disparate fields of chem and phys. You can also contact an author. Normally s/he will answer telling you how no to crack the film. I colleague of mine struggled with TiOx, I suspect he was just making so many films that he did find some of them usable. $\endgroup$ – Alchimista Feb 22 at 9:11
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    The reference you sited has used gold surface to make the thin film. If the method you wanted is the one in that reference, folloeing is how they descibed it in experimental section:

    The modification of a gold electrode array on microchip with a SH-FMS thin film consisted of two steps. First a mesoporous silica film was deposited onto the electrode surface by the spin-coating technique. Then the thiol functional groups were immobilized inside the mesopores of the film by self-assembly chemistry. To prepare the silica film solution, a 2.578 g quantity of Pluronic F-127 surfactant was dissolved in 3.430 g of de-ionized water and 8.846 g of ethyl alcohol. Then 0.108 g of concentrated HNO3 was added into the mixture. Finally, a 5.0 g quantity of TEOS was added to the solution. The molar ratio of TEOS : water : ethanol : HNO3 : Pluronic F-127 was 1 : 8 : 8 : 0.05 : 0.008. The solution was shaken for 24 h, then spin-coated at the speed of 3000 rpm onto the bare surface of a gold microelectrode array on microchip (part No. M1450110, Microsensor System, Inc.) for 30 s. Fig. 1a shows the schematic of the microelectrode array. The specific electrode array had 50 finger pairs (15 microns finger width and finger spacing) with gold as the electrode material and quartz as the substrate. The electrical connection for the array was covered with plastic tape before spin coating with the film solution. After the spin coating, the tape was removed and the film-coated electrode was calcined on a hot plate at 350 °C for 8 min in air.

    Hope this is helpful.

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