Colloidal TiO2 is easily prepared by the hydrolysis of titanium isopropoxide in aqueous or nonaqueous media. The TiO2 colloids are stable in the pH below pH 3 or greater than pH 11. One needs to use a stabilizer (e.g., 1-2% low mol. wt. polyvinyl alcohol) to obtain stable colloidal solution in the pH range of 3-11. It is advisable to prepare 10% titanium isopropoxide solution in isopropanol as a stock solution.
Add dropwise solution of titanium isopropoxide
to the aqueous solution of 0.1 M HCl or HClO4 with constant
stirring. The final concentration of 5-10 M will yield a transparent
suspension. Higher concentration of TiO2colloidal
suspension can be stabilized by decreasing the pH of the medium.
(Bahnemann, D., Henglein, A. and Spanhel, L., Detection of the intermediates of colloidal TiO2-catalysed photoreactions. Faraday Discuss. Chem. Soc, 1984, 78, pp 151; J. Phys. Chem., 1984, 88, pp 709-11)
Stir the dry acetonitrile solution in a N2
atmosphere. Inject titanium isopropoxide/isopropanol solution with a 25
microliter syringe several times to attain a concentration of 1-5 mM.
process is extremely sensitive to moisture content in acetonitrile.
the addition of titanium isopropoxide once you observe turbidity.
(Kamat, P. V. and Fox, M. A., Photosensitization of TiO2 colloids by erythrosin B in acetonitrile. Chem. Phys. Lett., 1983, 102, pp 379-84.)
In ethanol: This
is by far the best and convenient medium to prepare concentrated (0.01-
M) colloidal TiO2suspension. Colloidal
suspension (0.1 M) in ethanol was prepared by the hydrolysis of
isopropoxide. The procedure involved dropwise addition of 2.97 mL of
titanium isopropoxide solution to an ethanol solution (100mL) kept
vigorous stirring. This stock solution needs to be stored in a closed
vessel under constant stirring. (If the hydrolysis is carried out in
presence of acetic acid (1-2%) the size of the colloidal particles is
small and remains stable even without stirring).
(Kamat, P. V., Bedja, I. and Hotchandani, S., Photoinduced charge transfer between carbon and semiconductor clusters. One-electron reduction of C60 in colloidal TiO2 Semiconductor suspensions. J. Phys. Chem., 1994, 98, pp 9137-9142)
(15%) of SnO2 can be purchased from Alfa
suspensions of WO3
can be prepared in both water and ethanol. Desired
amount of sodium tungstate was dissolved in water. Concentrated HCl was
added dropwise until the precipitation of tungstic acid was
completed. The beaker containing the precipitate is allowed to
settle in an icebath. Once the precipitate is settled the
supernatant was slowly removed and the precipitate was washed with
water. (Decanting the clear solution is better than the filteration
method.) Tungstic acid (WO3.2H2O)
precipitate was the dissolved in water (or ethanol) and the solution is
slowly heated on a hotplate. Solid oxalic acid was added at
elevated temperatures. The concentration
of oxalic acid was varied (0.16-0.31 M) to
obtain colloids of different sizes. The
diameter of these particles is in the
regime ( 50 Å)
as reflected from the blue-shift in their
M. T., Rajh, T., Micic, O. I. and Nozik, A. J., Electron transfer
reactions and flat-band potentials of WO3 colloids. J. Phys. Chem.,
1984, 88, pp 5827-30. Bedja, I., Hotchandani, S. and
Kamat, P. V., Photoelectrochemistry of quantized WO3 colloids. Electron
storage, electrochromic, and photoelectrochromic effects. J. Phys.
Chem., 1993, 97, pp 11064-70)