Rationale for the crystallization of titania polymorphs in solution

Abstract

We use in situ X-ray absorption and diffraction studies to directly monitor the crystallization of different titania polymorphs in one and the same solution. We find that, despite the commonly accepted polymorphic-crossover from anatase to rutile triggered by the critical size of nanoparticles, in the solution their respective nucleation and growth are independent processes. Moreover, we find that 5.9 nm rutile nanoparticles are formed prior to the formation of 8.4 nm anatase nanoparticles. Our results suggest that the origins of this crystallization mechanism lie in the formation of an intermediate non-crystalline phase and in time-dependent changes in the chemical environment.

Mechanistic Aspects in the Formation, Growth and Surface Functionalization of Metal Oxide Nanoparticles in Organic Solvents

“…The controlled hydrolysis of TiCl 4 by the in situ generatedw ater molecules from the dehydration of acetonel ed to the formation of TiO 2 .K r änzlin et al studied the crystallization mechanism of different polymorphs of titania in the TiCl 4 -acetone system. [233] However,t hese results are discussed in more detailsi nS ection5 .L ie tal. synthesized TiO 2 microspheres via ao ne-pot template-free solvothermalr eaction of tetra-n-butyl titanatew ith acetylacetone.…”

“…Kränzlin et al. studied the crystallization mechanism of different polymorphs of titania in the TiCl 4 ‐acetone system . However, these results are discussed in more details in Section 5.…”

“…If the synthesis was performed in acetone rather than in benzyl alcohol, titania precipitated in the anatase and rutile modification. The crystallization was monitored by using in situ X‐ray absorption and diffraction studies . The origin of this crystallization mechanism lies in the formation of an intermediate, noncrystalline phase and in the time‐dependent changes in the chemical environment.…”

“…The crystallization was monitored by using in situ X-ray absorption and diffraction studies. [233] The origin of this crystallization mechanism lies in the formation of an intermediate, noncrystalline phase andi n the time-dependent changes in the chemical environment. In spite of the commonly accepted polymorphic-crossoverf rom anatase to rutile, triggered by the criticals ize of nanoparticles, the authors found that the respective nucleation and growth of rutile and anatase in solution were independentp rocesses.…”

The chemistry of ZnWO4 nanoparticle formation

“…29 This suggests that it is not the thermodynamic equilibration which determines the TiO 2 polymorph, but instead it is the time-dependent changes in the chemical environment. 31 Collectively this evidence stresses that no universal model can explain the mechanisms for nucleation and growth of metal oxide nanoparticles. The chemistry and reaction mechanisms vary significantly from system to system and they are time dependent.…”