In some articles I can see calcination of TiO2 at 400 C for 5 hrs. What is the role of 5 hrs? What happens when we reduce or increase the time duration ?
300 - 400 C is a temperature where water and OH groups are generally lost (as well as organics) from solution-prepared materials. Also in the TiO2 system, if the material is made in the amorphous form, it will transition to the anatase phase around 350 - 400 C, while a phase transition to rutile occurs at 500 - 600 C depending on the form of the material (thin film vs nanoparticles vs nanorods etc). So I would guess that 400 C is a good temperature to consolidate the loss of water and organics as well as the crystallinity of the anatase phase. Since there is not very high mobility of the ions at that temperature (Ti and O, I mean) it could take some time to improve crystallinity to a great extent. Anyway that is a long way of saying that shorter time may not give good crystallinity or leave some OH groups or organic carbon (if the calcining is very short), and if it is very long you might get better crystallinity but a point would come where no more changes occur. But probably the exact time is somewhat arbitrary.
300 - 400 C is a temperature where water and OH groups are generally lost (as well as organics) from solution-prepared materials. Also in the TiO2 system, if the material is made in the amorphous form, it will transition to the anatase phase around 350 - 400 C, while a phase transition to rutile occurs at 500 - 600 C depending on the form of the material (thin film vs nanoparticles vs nanorods etc). So I would guess that 400 C is a good temperature to consolidate the loss of water and organics as well as the crystallinity of the anatase phase. Since there is not very high mobility of the ions at that temperature (Ti and O, I mean) it could take some time to improve crystallinity to a great extent. Anyway that is a long way of saying that shorter time may not give good crystallinity or leave some OH groups or organic carbon (if the calcining is very short), and if it is very long you might get better crystallinity but a point would come where no more changes occur. But probably the exact time is somewhat arbitrary.
+the time has direct effect with the crystalline grow, more time more large grains, but is important the method of preparation.
+change phase temperature depend if the sample is bulk or nanostructured.
+if your sample is on sodo calcic glass it can be contaminated by Na.
Thermodynamically, rutile is favourable for larger crystallite size, while anatase and brookite are more stable at smaller crystallite sizes. Thus growth often implies phase change from anatase (or brookite) to rutile. Check the references:
J. Phys. Chem. B 104 (2000) 3481
Chem Mater 14 (2002) 4145
J. Sol. Gel. Sci. Technol. 64 (2012) 27
Thus the structure (crystallographic phase) and the particle size are often connected to each other! Hope this is useful! Dirk
There is generally an equivalence between time and temperature. For a physical (and even for some chemical) transformation, the higher the temperature, the lower the heating time for the same effect. But some crystalline changes may happen above some temperatures, so if they have to be avoided, lower temperatures should be used but require higher treatment times. So the couple (temperature, time) is generally a compromise between what you want to get and what you want to avoid. Synthesis of zeolites is a good example: too long or too hot syntheses lead to more stable phases, thus not zeolites at all or at least not the one you wanted to prepare.
Alain
Can bulk amorphous TiO2 readily available in the market be converted to nanocrystalline form (anatase) by merel calcinating it at about 400 deg C? Or is there soemthing else that needs to be done to first convert the bulk phase into nanoparticles and then calcine it to convert to anatase form??
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