Size--and morphology-dependent structural transformations in anatase Ti[O.sub.2] nanowires under high pressures

Titanium dioxide (Ti[O.sub.2]) nanowires with two different dimensions (i.e., < 100 nm and ~200 nm in diameter) were synthesized and studied under high pressure up to 37 GPa by Raman spectroscopy and synchrotron X-ray diffraction. Direct anatase to baddeleyite phase transitions were observed in both samples upon compression, but with different onset pressures. The observed phase transitions are in contrast to bulk Ti[O.sub.2], where the anatase phase transforms to α-Pb[O.sub.2] phase and then the baddeleyite phase. Compressibility of the anatase and baddeleyite phases was found different than both nanocrystals and the corresponding bulk materials. Our comparative study demonstrated not only that the morphology of Ti[O.sub.2] nanowire substantially influences the high pressure behaviors, but dimensions play a determining role in terms of transformation pressures, phase stability regions, and compressibility. Key words: high pressure, Ti[O.sub.2] nanowires, phase transformation, Raman spectroscopy, synchrotron radiation. Resume: Des nanofils de dioxyde de titane (Ti[O.sub.2]) de deux dimensions differentes (c.-a-d. de diametres < 100 nm et ~200 nm) ontete synthetises et analyses sous de hautes pressions allant jusqu'a 37 GPa par spectroscopie Raman et diffraction des rayons X synchrotron. Des transitions de phase directes anatase-baddeleyite ont ete observees dans les deux echantillons sous l'effet d'une compression, mais avec une pression exercee differente pour chaque echantillon. Les transitions de phase observees contrastent avec celles du Ti[O.sub.2] brut avec lequel la phase anatase se transforme en une phase α-Pb[O.sub.2] puis en une phase baddeleyite. La compressibilite des phases anatase et baddeleyite s'est averee differente de celle des deux nanocristaux et des materiaux bruts correspondants. Notre etude comparative a montre que non seulement la morphologie du nanofil de Ti[O.sub.2] influe grandement sur les comportements a haute pression mais egalement que leurs dimensions joue un role determinant en ce qui concerne les pressions de transformation, les domaines de stabilite des phases et la compressibilite. [Traduit par la Redaction] Mots-cles: haute pression, nanofils de Ti[O.sub.2], transformation de phase, spectroscopie Raman, rayonnement synchrotron.
Introduction Titanium dioxide (Ti[O.sub.2]), or titania, is a well-known semiconductor with a wide band gap. Ti[O.sub.2] exhibits a rich phase diagram including more than seven polymorphs. The most abundant forms, [...]