Evolution of atomic structure during nanoparticle formation

Abstract

Understanding the mechanism of nanoparticle formation during synthesis is a key prerequisite for the rational design and engineering of desirable materials properties, yet remains elusive due to the difficulty of studying structures at the nanoscale under real conditions. Here, the first comprehensive structural description of the formation of a nanoparticle, yttria-stabilized zirconia (YSZ), all the way from its ionic constituents in solution to the final crystal, is presented. The transformation is a complicated multi-step sequence of atomic reorganizations as the material follows the reaction pathway towards the equilibrium product. Prior to nanoparticle nucleation, reagents reorganize into polymeric species whose structure is incompatible with the final product. Instead of direct nucleation of clusters into the final product lattice, a highly disordered intermediate precipitate forms with a local bonding environment similar to the product yet lacking the correct topology. During maturation, bond reforming occurs by nucleation and growth of distinct domains within the amorphous intermediary. The present study moves beyond kinetic modeling by providing detailed real-time structural insight, and it is demonstrated that YSZ nanoparticle formation and growth is a more complex chemical process than accounted for in conventional models. This level of mechanistic understanding of the nanoparticle formation is the first step towards more rational control over nanoparticle synthesis through control of both solution precursors and reaction intermediaries.

Keywords: EXAFS; PDF; in situ; nanoparticle; total scattering.

Figure 1

Local atomic ordering as revealed by total scattering. PDF (black line) and structural modeling (red line) for (a) the precursor solution prior to nucleation, (b) the amorphous precipitates formed after nucleation and (c) nanocrystalline domains present after prolonged reaction. (d) Time-resolved view of the local structural region (0–10 Å) of the PDF.

Figure 2

Structural stages observed during yttria-stabilized zirconia formation. (a) Structural model of the zirconia double-chain existing in the precursor solution part of a (b) polymeric chain. (c) Amorphous structure formed after precipitation containing distinct rigid units (yellow). (d) Mature cubic crystalline structure of YSZ. Oxygen: red; zirconium: green and yellow; nitrogen: blue. Yttrium is here structurally equivalent to zirconium.

Figure 3

Initial nucleation mechanism. Total scattering local environment PDF and visualized transformation route of polymeric precursor species into amorphous matrix. Zirconium polyhedra are colored green and yellow with dotted lines indicating cluster orientation.

Figure 4

Changing local environment of zirconium and yttrium as observed by total scattering PDF and EXAFS. (a) Change in the two shortest interatomic distances as obtained through total scattering PDF analysis. For clarity, only every 15th datapoint is shown. (b) Change in the two shortest interatomic distances as obtained through EXAFS analysis. Dotted lines correspond to the change in the local structure surrounding yttrium, while solid lines correspond to the local structure of zirconium. Explanations of the stages I, II and III may be found in the text.

Figure 5

Post-nucleation structural transformation of amorphous precipitates. (a) Crystallite diameter growth curve obtained from total scattering. (b) Expansion of the nearest Zr—Zr distance as a function of growth in the coherent domain diameter. (c) Depiction of the size regime of individual crystalline domains.