XRD peak is showing rutile phase of TiO2 during nanorod formation. But other than nanorod TiO2 means in the case of bulk or nanoparticles it is showing anatase phase. what is the actual reason behind this? kindly comment based on the physics point of view with reference. Thank you so much.
Your question is not clear especially the second sentence in the main bulk (beginning ‘but other than’). Please rephrase.
I think this depends on the method that is followed in preparation of TiO2 nanorods, if it performs at high temperature this can lead to rutile formation as it is the more stable form of titania at high temperatures.
With my very best regards.
As compared with anatase, bulk rutile is more stable at all temperatures and pressures; this is due to the lower bulk free energy of rutile. However, the planes of anatase have lower surface energy as compared with those of rutile. Thus, anatase becomes more stable than rutile when extremely small size crystallites, which have sizes below a critical value, are formed due to the high surface areas of such system.
Based on that, rutile is stable when the bulk free energy dominates the total free energy of the system, while, anatase is stable when the surface energy is the dominated. According to that, one can say that the preferred morphologies for rutile, of a given size, are those give higher bulk free energy and lower surface energy. This means that spherical shape is more preferred by rutile, as compared with rod at any aspect ratio that have the same size, because it gives the lowest surface area to volume ratio. Thus, I think that the stability of the rutile nanarods, which are commonly synthesized by hydrothermal method, is related to their size rather than their morphology.
For references, please refer to the valuable review by Dorian A H Hanaon and Charles Sorrell:
Article Review of the Anatase to Rutile Phase Transformation
As mentioned by Imad, the growth of rutile instead of anatase depends on many factors. In your case you might first have anatase growth which then turns into rutile. You could notice it if you heat-treat your sample for small time interval.
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