Surface Structure And Reactivity Of Ultrasmall TiO₂Nanocrystals

Ultrasmall nanostructures with the size down to1nm scale in at-least onedimension are the real systems that display nano-effects, and size and surface structureare the key factors determining most enhanced or newly emerging properties. On thebasis of successful synthesis of ultrasmall TiO2nanostructures via wet-chemistrymethods, I studied the stability of ultrasmall nanostructures, solvent-surface interactiondirected evolution habits of nuclei clusters, size-enhanced photocatalytic performanceand size-activated surface modification by H2O2, as well as surface tension capturedinterface enrichment and assembly of TiO2nanocrystals. The roles of surface structureand size effect are emphasized.The ultrathin2D TiO2nanosheets are about1nm thick, and the phases includeanatase, brookite and TiO2(B); without using any surfactants, sub-2-nm anatasenano-clusters were got using a modified sol-gel approach. When studying the stabilityof the ultrasmall nanostructures, I have found the specific interactions at interfaces dueto configuration match can well protect their phase and morphology from evolution.The examples we used were TiO2(B) nanosheets and heterogeneous RuO2/TiO2epitaxial layer. The mechanisms lie in the fact that both phase transition and shapechange are initiated by surface disorder, so maintaining a stable surface is of criticalimportance for ultrasmall nanostructures. Using the sub-2-nm anatase nano-cluster as anuclei model, I studied the roles of solvent-surface interactions in the evolution kineticsof nuclei. From the results of experiments and DFT calculation, we have found thesolvation states of different facets and the surface reaction of solvent with crystal decidethe formation of crystal planes and oriented attachment growth. And the wholeevolution process is size-dependent upon the initial particles.Due to the increased surface areas, the photocatalytic performances of theultrasmall TiO2nanostructures are highly enhanced. And the surface alsobecome more active, because their surfaces can be easily modified by H2O2,resulting in a changed color from white into yellow. After surface modification,the TiO2materials can effectively adsorb various dyes from their solutions. Onthe other hand, TiO2particles with larger sizes like P25cannot be modified byH2O2, meaning that the surfaces of ultrasmall TiO2nanostuctures become activedue to their ultrasmall sizes. On the other hand, because the lattice periodicities of ultrasmall TiO2nanostuctures are quite limited, their lattice are unstableagainst heating. The atoms on the surfaces can move easily and lattice also canexpand. We have found TiO2nanocrystals can be thermochromic at lowtemperatures, and TiO2can easily become black in the presence of H2at lowtemperatures. Using DFT calculation, we find both lattice expansion and surfacedisorder can shrink the band gap of TiO2.When preparing TiO2in water from TiCl3, we found an unusual enrichment andassembly result of TiO2nanocrystals at water/hydrophobic interfaces in the absence ofany surfactants. TiO2nanocrystals formed from TiCl3by oxidative hydrolysis couldspontaneously accumulate at water/hydrophobic interfaces to form macroscopic sheetsand tubes. During the processes, surface tension of the solvent played a critical role inthe floating and assembly of TiO2nanocrystals.