These days, I have tried to synthesize TiO2 nanotubes but failed again and again. I attached SEM images of as-synthesized TiO2 samples. (Most parts of TiO2 sample were not made to nanotubes (1). Very small parts were made to nanotubes (2).) My experimental conditions are as follows. 1. 500 grit-polished Ti metal (3 cm * 3cm * 1mm) 2. Cathode : SUS plate (3 cm*3cm*1mm) 3. Electrolyte : HF(0.4wt%)/H3PO4(1M) 4. 10-30 V for 2 h 5. Ethanol washing after anodization (not using a sonicator) Can you please advice me what is wrong with my works?
1. Purity of Ti foil could play a role. Higher the purity (>99.8%) better will be the results.
2. You may not need more than 40 minutes to achieve about 500 nm long tubes in your phosphoric acid + fluoride solution at 20 V
3. In stead of 0.4 wt% HF, use 2-4 wt% NaF
4. Finer the polishing (electropolishing is the best) better will be the ordering of nanotubes. You may polish down to 1200 or finer grit.
5. Pre-anodization surface cleaning is more important than post-anodization cleaning. Clean the surface free of grease, finger print, etc.
6. If you leave the stainless steel (S.S) counter electrode in open circuit condition, fluoride will attack the S.S and contaminate the electrolyte. Immerse the S.S electrode only after connecting to the negative terminal and switching on the DC power source with the desired potential. In brief, the S.S electrode should be always cathodically polarized. Otherwise use Pt or graphite counter electrode. Ti can also be used but always cathodically polarized.
7. If you are reusing the electrolyte, then use PTFE container not glass. Do not store your acidified fluoride aqueous solution in glass containers for extended period of time.
8. Fresh solution will always give better results.
If you have followed all the above and still do not get good results, then you have a problem with your power source! repair it!!
In our work Article TiO2 nanostructures: voltage influence in corrosion resistan...
you can find a similar methodology to the one you propose and that works quite wel
I recommend another type of electrolyte such as Ehtylenglycol+Water (1-15%) and NH4F. We have recently published an article where we address the study of some of the structural issues related with TiO2 nanotubes synthetized by electrochemical anodization
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