دانلود رایگان مقاله اثرات انرژی سطحی بر ثبات آناتاز و روتایل نانوبلورها

Abstract

Titanium dioxide nanoparticles are widely used for photocatalysis, and the relative fraction of titanium dioxide polymorph, i.e. anatase, rutile, or brookite, significantly affects the final performance. Even though conventional phase diagrams indicate a higher stability for the rutile polymorph, it is well established that nanosizes benefit the anatase phase due to its smaller surface energy. However, doping elements are expected to change this behavior, once changes in both surface and bulk energies may occur. Nb2O5 is commonly added to TiO2 to allow property control. However, the effect of niobium on the relative stability of anatase and rutile phases is not well understood from the thermodynamic point of view. The objective of this work was to build a new predictive nanoscale phase diagram for Nb2O5-doped TiO2. Water adsorption microcalorimetry and high temperature oxide melt solution were used to obtain the surface and bulk enthalpies. The phase diagram obtained shows the stable titania polymorph as a function of the composition and size.

4. Conclusions

A predictive anatase-rutile phase transition diagram was built at nanoscale for Nb2O5-doped-TiO2. Nb2O5-doping postpones the ART, allowing the anatase grains to grow more than undoped TiO2 before phase transition is observed. The stability crossover for undoped TiO2 was found to be 17.3 nm, while 2 mol% Nb2O5- doped-TiO2 crossover is ∼30 nm. The nanophase diagram agrees with the experimental data used in this work. The surface energy for Nb2O5-doped-TiO2 decreases systematically as the Nb concentration increases, confirming the higher stability for doped samples. This result is a powerful predictive tool that can be applied for nanotechnological efforts on Nb-Ti oxide systems.