I am actually working on synthesis of black TiO2 using sol gel method. When I directly calcined the gel in the muffle furnace at 300 deg C, they turned black in colour but when I first dried it in oven at 250 and then calcined it in muffle furnace at 300 deg C, they turned into a mixture of black and white with majority of them being white. What could be the reason for this?
There are many reasons why things are the color they are, but mainly it is due to the absorption and the scattering properties of the material being different from that of the incoming wavelengths of the light that illuminate it. This effect is due to the release of surface oxygen.
TiO2 is semiconductor, it can absorb only light with energies that exceed its absorption edge that is defined by the band gap of the semiconductor. However, impurities in the lattice of TiO2 can change its absorption properties.
Notable that the defectiveness of the TiO2 crystals could be evaluated by their color- the most reduced, at the limit of the TiO2 homogeneity range [corresponding to ~TiO1.992[1]], crystals are dark blue or black in color, while the stoichometric dioxide is paled yellow [2]. In short, defects in TiO2 generate energy states in the band gap of TiO2, so, photons with less energies than the band gap of TiO2 could be absorbed.
In order to obtain the calcined sample, at the first stage, appearance moisture must be removed in oven. At the second stage, sample calcined in furnace. By the way, It should not be oxidized and the furnace must be completely cooled before removing the sample.
I guess it is due to impurity and unregulated ramp rate, temperature of the muffle furnace. Try it drying using vacuum dryer or in hot air oven at 60 degree Celsius before calcination.
Pure white nano anatase TiO2 is prepared simply through slow and controlled hydrolysis of titanium isoproxide , aged for overnight, filtered by nanomembranes , dried at 90 C for 2 hours then finally calcined at 400 for 4 hours.
Considering your sol-gel synthesis method, some factors can explain your problem. The most probable is a consequence of the methodology: to obtain the gel you usually employ an organic precursor. If you did not wash properly the solid obtained (before calcination) and attending to the low temperature used, some carbonaceous deposits can be generated in solid, coloring it in black. The parts in "white" would be carbon-free TiO2, consequence of the low temperature combustion of organic matter from precursor.
Is the calcination temperature is over 400ºC, in oxidant environment, most of carbon would be removed by combustion.
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