Order-Disorder Transition of Aragonite Nanoparticles in Nacre

Zaiwang Huang and Xiaodong Li

Phys. Rev. Lett. 109, 025501 – Published 9 July 2012

Abstract

Understanding nacre’s bottom-up biomineralization mechanism, particularly, how individual aragonite platelets are formed, has long remained elusive due to its crystallographic peculiarity and structural complexity. Here we report that crystallographic order-disorder transition can be triggered within individual aragonite platelets in pristine nacre by means of heat treatment and/or inelastic deformation, offering a unique opportunity to discriminate mysterious aragonite nanoparticles in transmission electron microscopy. Our findings unambiguously uncover why aragonite nanoparticles in pristine nacre have long been inaccessible under TEM observation, which is attributed to the monocrystal-polycrystal duality of the aragonite platelet. The underlying physical mechanism for why an individual aragonite platelet adopts a highly oriented attachment of aragonite nanoparticles as its crystallization pathway is, for the first time, explained in terms of the thermodynamics. The finding of an order-disorder transition in nacre provides a new perspective for understanding the formation for other biominerals.

Received 7 November 2011

DOI:https://doi.org/10.1103/PhysRevLett.109.025501

Authors & Affiliations

Zaiwang Huang and Xiaodong Li^*

Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208, USA

^*Corresponding author. lixiao@cec.sc.edu

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Issue

Vol. 109, Iss. 2 — 13 July 2012

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Images

Figure 1
Ordered microstructure of an aragonite platelet in pristine nacre. (a) Bright-field TEM image showing the brick-mortar microarchitecture. (b) High-resolution TEM lattice fringe image of individual aragonite platelets from (a) exhibiting a single-crystal diffraction pattern (inset).Reuse & Permissions
Figure 2
XRD spectrum and TEM micrographs of the nacre sample after $350 ° C$ heat treatment. (a) Bright-field TEM image of platelet debris. (b) All XRD peaks can be well indexed as aragonite, showing phase stability of aragonite at $350 ° C$ . (c) Lattice fringe of the platelet exhibiting a hybrid crystallographic feature, through applying the FFT pattern on three typical boxed areas, for example, box areas $D$ , $E$ , and $F$ , the corresponding crystallographic orientation [(d)–(f)] presenting a gradual transition from polycrystalline to monocrystalline appearance.Reuse & Permissions
Figure 3
AFM evolution observation of aragonite nanoparticles under different temperatures. (a) A reference mark was made by Vickers indentation on the cross section of nacre, where an interesting area enclosed by a white box was selected for AFM observation. (b)–(f) are a series of AFM amplitude images by means of heat treatments at $T = 25$ , $100$ , $200$ , $300$ , and $350 ° C$ ; a sharp shift of nanoarchitecture involving nanoparticle coarsening, reorientation can be detected at a relatively high temperature, i.e. $300$ and $350 ° C$ , whereas there is no obvious change at $25$ , $100$ , and $200 ° C$ .Reuse & Permissions
Figure 4
Microstructure observation of aragonite nanoparticles. (a),(b) AFM nanoparticle observation within an interesting area (white box enclosed) from pristine and deformed nacre (three-point bending), showing the change in nanoparticle architecture. After that, a TEM sample was prepared from a severely deformed sample (3% flexural strain). (c) Aragonite platelet fragment adhering to a biopolymer interlay (arrowed). (d) Disordered nanoparticle arrangement from inside the platelet scatters a polycrystalline pattern (inset), showing an order-disorder crystallographic transition upon inelastic deformation.Reuse & Permissions
Figure 5
Order-disorder transition of the aragonite nanoparticle arrangement. (a) Aragonite nanoparticles are self-assembled into a pseudo-single-crystal platelet (sharing the same orientation). (b) Upon imposing external effects, i.e., heat treatment and/or inelastic deformation, nanoparticles are rearranged in a random manner. (c) The disordered colloidal aragonite nanoparticles self-assembled into an ordered aragonite platelet via decreasing the Gibbs energy ( $Δ G_{1} < 0$ ). Upon imposing an external effect, i.e., heat treatment and/or inelastic deformation, the Gibbs energy will accordingly increase ( $Δ G_{2} > 0$ ) but cannot rebound to the previous state ( $| Δ G_{2} | < | Δ G_{1} |$ ). The excess energy ( $Δ G_{3}$ ) can be derived from the amount of grain boundary bonding energy, relative to the free colloidal aragonite nanoparticles.Reuse & Permissions

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