Why isn't the surface of porous TiO2 I synthesized porous?
I used TALH(Titanium(IV) bis(ammonium lactato) dihydroxide solution) as the precursor, and polystyrene spheres as the templates. Mixing them together and freeze drying the mixture. After that, I did calcination to remove the templates. But from SEM images, the materials are porous inside, but the surface is not porous. Do you have any idea about why the surface is not porous and how to make the surface also porous.
Besides, can a porous surface significantly affect the BET surface area of the products?
Dear Mengqi Duan , yes, a porous surface will affect BET surface area, as it can provide more gas adsorption sites.
However, I don't think your surface is as flat as it looks. Have you tried lowering the voltage while scanning this "flat" surface? It can be that the brightness is not allowing you to see the pores.
When preparing this sample, do you make it as monolith, or powder? To some of my silica monoliths, the region where the monolith is in contact with the tube i use for synthesis offers this smooth sensation, however i can see its roughness when I scan at lower voltage.
Hi Luiz Fernando de Sousa Lima, thanks for your help. I haven't tried lowering the voltage. My sample is powder. I can try to take SEM images at a lower voltage again.
Btw, the BET surface area of the sample is around 35 m2/g, the pore volume is around 0.25 cm3/g. It should have been much higher. So that is why I doubt it is caused by the nonporous surface.
Thank you again!
For how long do you calcinate your sample and at which temperature? Does the powder looks brown after degassing? I also expected a higher surface area by the synthesis/drying you described (from 70 to 150 more or less)
I calcinated the sample in N2 for 2 h at 500 degree and then in air for 10 h at 500 degree. The sample looks white. I also did TGA on the sample after calcination and carbon content is smaller than 2%.
Hi
It could possibly because of the higher concentration of TiO2 precursors, which initially are attached to polystyrene but after that, the content of polystyrene is not available to serve as a template. You can try with a higher concentration of polystyrene and a lower concentration of TiO2... Also, CTAB and other polymeric surfactants are better than polystyrene to make porous structure. Or you can consider adding some content of CTAB... you can see our recent work on silica, where we used CTAB to make coral reef structured silica.
Article Fabrication of coral-reef structured nano silica for self-cl...
Best Regards
Hi Mumtaz Ali , thanks for your opinion! I think that makes sense!
Currently, the mass ratio between TiO2 and Polystyrene nanoparticles is 2:1. The concentration of Ti(IV) in the solution is 0.5 mol/L. Is it a high concentration of Ti(IV)?
Can you explain further about adding CTAB? Do you mean that using CTAB as templates or adding CTAB and Polystyrene at the same time?
Looking forward to your reply!
Thanks!
Hi
From the SEM image, the surface is not porous only, so you can decrease the ratio of TiO2 and Polystyrene nanoparticles to 1.5:1. Also, Please decrease the reaction time by 30%. The surface blocking occurs during last Ostwald's ripening stage, so you can avoid it by decreasing the reaction time.
In addition to above, you can add CTAB and Polystyrene at the same time ... CTAb will make a smaller size of pores. You can keep the concentration of TiO2, Polystyrene nanoparticles, and CTAB to 1.5:1:0.3. Please check that you must heat up to 500 or 600 in order to degrade CTAB. so please check the temperature compatibility of your substrate...
Best regards
Hi Mumtaz Ali , thanks for your suggestions! I will try that and update my progress.
Hi Henrik Romar, thanks for your opinion. But from SEM images, we do see some parts inside the sample are porous and the pore size is around 50 nm.
I still claim that pores cannot be seen with SEM. The porous structure is inside the materia and cannot be seen by SEM.
I'd also suggest avoiding calcinating time as long as you did. You might be sintering your sample, are you reproducing a methodology or adapting a new?
If your synthesis leads you to amorphous TiO2, viscous synthering takes place at low temperatures ~ 400 ºC, and mantaining the sample for longer time can block a few pores.
About the chemicals you use, CTAB and polystyrene will give you different types of pores, polystyrene acts as hard template for bigger pores and CTAB will act as soft templates for mesopores, demanding a basic synthesis method.
About pores being seen by SEM: it's almost impossible to see micropores (< 2nm), however, bigger mesopores (30~50 nm) or macropores (
Hi all, thanks for all of your help again.
I tried shortening the total reaction time from 12 h to 8 h at 500 ℃, but the surface of the materials is still non-porous, which is similar to the previous sample. I am trying to figure out the reason.
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