High‐Pressure, High‐Temperature Formation of Phase‐Pure Monoclinic Zirconia Nanocrystals Studied by Time‐Resolved in situ Synchrotron X‐Ray Diffraction

Abstract

Zirconia is one of the most studied metal oxides due to the important mechanical, thermal, electrical and catalytic properties and the many applications they offer. [1] In virtually all applications of zirconia, the key challenge is to control the crystal phase, morphology, and stoichiometry in order to tailor the specific targeted properties. The thermodynamically stable crystal phase of bulk zirconia is the monoclinic structure, which transforms into the tetragonal phase at 1175 8C. The tetragonal phase can be stabilized by cation substitution, which introduces oxygen vacancies in the crystal structure. As early as 1965, Garvie suggested that the tetragonal phase of zirconia is also stabilized for nanoparticles below a critical size of 30 nm due to a lower surface energy when compared to the monoclinic phase.[2] Since then, a lot of research has been conducted to understand this phenomenon, and the many results were recently reviewed by Shukla and Seal.[3] Clearly, it is highly desirable to obtain a fundamental understanding of the formation of zirconia particles on the nanoscale, since this will allow us to design materials with improved functional properties relevant for various industrial and scientific applications. Here, we study the formation of nanocrystals by high-pressure, high-temperature time-resolved in situ X-ray diffraction under hydrothermal conditions. Our data provide striking new insight about the nucleation and formation of zirconia nanocrystals.In situ synchrotron powder X-ray diffraction has had a strong impact on materials science since the method became widely used in the 90's.[4] Many different experimental designs have been developed, for example for controlling atmospheres in studies of catalytic processes, but common for all studies is that they are not carried out under the high pressures and high temperatures needed to bring solvents into the supercritical regime. Studies of reactions in supercritical fluids necessitate highly specialized equipment to withstand the extreme conditions, while still allowing X-ray penetration. Our group recently reported the first in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) study of supercritical fluid reactions using a batch reactor, [5] and novel designs for in situ continuous-flow supercritical reactors have also been described.[6] Studying the formation and growth of zirconia nanoparticles by time-resolved in situ high-pressure, hightemperature X-ray diffraction allows a detailed and simultaneous determination of phase and size development. Zirconia nanoparticles are of particular interest during nucleation and growth due to the size-dependent crystal-phase stability. Lupo et al.

Oxide Nanomaterials: Synthetic Developments, Mechanistic Studies, and Technological Innovations

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In situ total X-ray scattering study of the formation mechanism and structural defects in anatase TiO2 nanoparticles under hydrothermal conditions

“…When different precursors are used for hydrothermal synthesis, it is often observed that the product has specific characteristics, e.g. [9][10][11][12][13] In particular, a few studies have been devoted to the study of TiO 2 crystallization and growth by in situ synchrotron PXRD under solvothermal, hydrothermal or sol-gel conditions. However, the processes occurring during the formation of nanoparticles are not well understood, and this, to a large extent, prevents rational synthesis of new materials.…”

Shaped gold and silver nanoparticles

“…In contrast, high energy synchrotron X-rays represent such an ideal class of probes because they can efficiently penetrate liquid electrolytes. The corresponding timeresolved techniques (such as small-angle X-ray scattering [138][139], X-ray absorption spectroscopy [140][141], wide-angle X-ray scattering [142][143][144][145]) show the potential for real-time monitoring nanoparticle growth in liquid media. Continuous efforts in establishing robust, sensitive, high time-resolution, high-energy X-ray techniques may represent a research direction for understanding the complicated nanoparticles growth process in solution phase and providing valuable knowledge to improve the quality of nanoparticles.…”