Î±-Calcium Sulfate Hemihydrate Nanorods Synthesis: A Method for

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#-Calcium Sulfate Hemihydrate Nanorods Synthesis: a Method for Nanoparticle Preparation by Mesocrystallization Qiaoshan Chen, Caiyun Jia, Yu Li, Jie Xu, Baohong Guan, and Matthew Z. Yates Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00013 • Publication Date (Web): 04 Feb 2017 Downloaded from http://pubs.acs.org on February 7, 2017

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α-Calcium Sulfate Hemihydrate Nanorods Synthesis: a Method for Nanoparticle Preparation by Mesocrystallization Qiaoshan Chen,a Caiyun Jia, a Yu Li, a Jie Xu, a Baohong Guan*a and Matthew Z. Yates*b a

Department of Environmental Engineering, Zhejiang University, Hangzhou 310058, China.

E-mail: [email protected]; Tel: +86-0571-88982026; Fax: +86-0571-88982026. b

Department of Chemical Engineering, University of Rochester, Rochester, New York 14627,

United States. E-mail: [email protected].

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ABSTRACT The past decades have witnessed great advance in nanotechnology since tremendous efforts have been devoted for the design, synthesis and application of nanoparticles. However, for most mineral materials such as calcium sulfate, it is still a challenge to prepare their nanoparticles, especially with uniform size and high monodispersity. In this work, we report a route to regulate the morphology and structure of alpha-calcium sulfate hemihydrate (α-HH) and successfully synthesize and stabilize its mesocrystals for the first time. The ellipsoidal mesocrystals in length of 300 - 500 nm are composed by α-HH nanoparticles arranged in the same crystallographic fashion and interspaced with EDTA. The time-dependent experiments indicate the α-HH aggregates evolve from irregular structure to mesocrystal structure with the subsequent growth of subunits, and then partially fuse into single crystals. Disorganizing the mesocrystal structure before the emergence of fusion reaps α-HH nanorods in a length of 30 - 80 nm and a width of 10 - 20 nm with high monodispersion. This ingenious concept paves an alternative way for nanoparticle preparation and is readily extended to other inorganic systems.

KEYWORDS:

Nanoparticle,

Mesocrystal,

Self-assembly,

Alpha-calcium sulfate hemihydrate, Structure control.


Disorganizing,

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INTRODUCTION The regulation of material structure and size is one of the key topics in modern colloid and material chemistry.1,

2

Herein alignment of nanoparticles into ordered

three-dimensional (3D) superstructure such as mesocrystal structure are of special interests.3,

4

Mesocrystal proposed by Cölfen refers to mesoscopically structured

crystals consisting of crystallographic oriented nanoparticles.5 Different from classical ion-by-ion crystallization, mesocrystals are fabricated through a particle-mediated crystal growth route and usually observed as kinetically metastable species in crystallization reactions leading to single crystals.6,

7

With the help of organic

additives, the development of mesocrystals can be stopped at an intermediary step to form an organic-inorganic hybrid materials, in which the constructing units are well identified.8,

9

Compared with polycrystals, the mesocrystals have much higher

crystallinity and much higher porosity than conventional single crystals.10 The distinct 3D hierarchic structure makes it a promising material for many uses such as catalysts, energy storage, optoelectronics and biomedical applications, drawing great attentions in recent years.11, 12 Calcium sulfate is a naturally abundant and industrially important mineral and alpha-calcium sulfate hemihydrate (α-HH), as an important phase of calcium sulfate, has a broad scope of applications.13, 14 The performance of α-HH is closely associated with the crystal size and structure. α-HH particles in micro size are widely applied in molding and construction industry due to the fast setting time, good workability and high strength of its hydration products,15 while α-HH crystals at nano scale are 3 ACS Paragon Plus Environment

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preferable for application in the field of biomedicine to act as drug carrier.16 Especially, α-HH nanoparticles have hopes to seal dentinal tubule for treatment of dentin hypersensitivity due to their excellent cementitious property.17 Structure and size control of α-HH is therefore of great importance and many efforts have been devoted for the preparation of α-HH nanoparticles in recent years. The nanoparticles with high dispersion in shape of wire, rod and ellipsoid can be synthesized in aqueous solution but with a large length of ~ 300 nm.16, 18 Kong19 and Song20 both obtained nanoparticles in the size of ~ 50 nm by limiting the crystal growth of α-HH in different organic media, however the particles are irregular-shaped and severely agglomerated. The agglomeration is generally inevitable when nanoparticles are prepared using traditional one-step precipitation method.21 Recently, Cölfen22 obtained α-HH nanorods in size of 50 - 200 nm by quenching aqueous calcium sulfate solutions in ethanol, providing an alternative pathway to get α-HH nanoparticles. To improve the particle uniformity and monodispersity, we design a strategy combined bottom-up route with top-down way for nanoparticle preparation. Since the nanoparticles in mesocrystal commonly have an uniform size distribution, we choose it as the source to get nanoparticles. With the help of EDTA, the self-assembly and development of α-HH aggregates are fine-tuned to reach a mesocrystal structure. Before the emergence of the subunits fusion, disorganizing the mesocrystal structure reaps its subunits, i.e., monodisperse α-HH nanorods with uniform size. We synthesize α-HH mesocrystals for the first time and clearly demonstrate its morphology, structure and orientation evolution. This work deepens the understanding 4 ACS Paragon Plus Environment

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of mesocrystal and endows it with an interesting application, which sheds light on the importance of mesoscopic development in typical crystallization process.

EXPERIMENTAL SECTION Reagents and materials Analytic reagent grade CaCl2 was purchased from Sigma-Aldrich Co., Llc. USA. Analytic reagent grade (NH4)2SO4, Na2EDTA and ethylene glycol (solvent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China. All reagents were used as received. Preparation of α-HH particles In a typical procedure, two homogeneous precursor solutions of Ca2+ and SO42- were first formed separately. The solution of Ca2+ in concentration of 0.4 M was prepared by dissolving CaCl2 and Na2EDTA in 5.0 ml ethylene glycol, and SO42- precursor solution was composed of (NH4)2SO4, Na2EDTA, 8 ml water and 37 ml ethylene glycol. The molar ratio of Ca2+ to SO42- (Ca/S) was kept at 1.0. The two solutions were heated to be transparent and then mixed immediately. The reaction was performed in a 100 ml Teflon reactor with constant magnetic stirring at the temperature of 95oC (with a deviation of ± 0.5oC). After different reaction times of 30 s, 2 min, 5 min, 30 min, 3 h and 12 h, the hot suspension was washed immediately with anhydrous ethanol for three times to terminate the reactions and quickly vacuum filtered to gain samples. To obtain α-HH nanorods, the hot suspension after reaction of certain time was immediately vacuum filtered and washed with boiling water and then


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rinsed with anhydrous ethanol to remove the water. The whole washing and filtering process finish in only 10 - 15 seconds to avoid the phase transformation. Solid products were collected after dried at 60oC in a vacuum oven for 2 h. Characterization The morphology of α-HH particles was examined by the field emission scanning electron microscopy (FESEM, SU8010, Hitachi, Tokyo, Japan). Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained on TEM (Titan Chemi STEM, USA) at an acceleration voltage of 200 kV. The X-ray diffraction (XRD) was performed for the phase identification by a powder XRD analyzer (D/Max-2550 pc, Rigaku Inc., Tokyo, Japan) with Cu Kα radiation at a scanning rate of 8°/min in the 2θ range from 5° to 80°. The thermogravimetry and differential scanning calorimetry (TG-DSC, STA-409PC, NET-ZSCH, Germany) were conducted to distinguish α-HH from the hemihydrate phase. Fourier transform infrared (FTIR) spectra were recorded on a spectrometer (IRAffinity-1, Shimadzu, Japan) with a resolution of 4 cm-1 over the frequency range of 400 - 4000 cm-1. The cross section of α-HH ellipsoids was obtained by ultramicrotomy. The dry powder was embedded in an epoxy resin matrix of Agar100, dodecenyl succinic anhydride (DDSA) and methyl nadic anhydride (MNA) and dried at 60°C in a vacuum oven for 12 h. Then the block was cut in a dry environment at room temperature with a 35° Diatom diamond knife (radius of 5 nm) mounted on Reichert-Jung Ultracut E ultramicrotom.


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RESULTS AND DISCUSSION Synthesis of α-HH mesocrystals and nanorods The calcium sulfate ellipsoids were fabricated by CaCl2 and (NH4)2SO4 in ethylene glycol-water solution with Na2EDTA as modifier at 95oC. The phase of calcium sulfate ellipsoids is demonstrated to be hemihydrate (HH) by the diagnostic peaks at 14.68°, 25.62°, 29.70°and 31.89°in x-ray diffraction (XRD) pattern (Figure S1). The endothermic peak at 156oC and the exothermic peak at 184oC on DSC curve identify that the HH phase is α-HH (Figure 1). The first weight loss of - 5.71 wt% on TG curve indicates the elimination of 0.5 crystal water in α-HH, which is lower than the theoretical one (- 6.21 wt%) due to the existence of EDTA ions. The second weight loss (- 3.14 wt%) is judged to be from the pyrolysis of EDTA based on the exothermic peak at 391oC. The subsequent elimination of remained organic materials leads to 5.59 wt % weight loss at third stage, bringing a large exothermic peak at 691oC. The TG/DSC curves of pure Na2EDTA·2H2O are provided in Figure S2 as a comparison.23 The purity of α-HH calculated by subtracting the content of EDTA is 91.27 wt% [(100 - 3.14 - 5.59) = 91.27 wt%], which is consistent with the value calculated from the measured crystal water [(5.71 / 6.21) wt% = 91.95 wt%]. Thus, the as-synthesized ellipsoids are composed of 91.27 wt% α-HH and 8.73 wt% EDTA.


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Figure 1. TG/DSC analysis of the ellipsoids synthesized in ethylene glycol-water solution at 95oC for 3 h to indicate the ellipsoids consist of 91.27 wt% α-HH and 8.73 wt% EDTA.

The α-HH ellipsoids in length of 300 - 500 nm and width of 200 - 300 nm with uniform size and rough surface are formed by self-organization of nanoparticles (Figure 2a). The nanoparticles are well aligned along the vertical axis of the ellipsoids with a rather uniform size of ~ 50 nm (Figure 2b). The ellipsoid is tightly organized (Figure 2c). The lattice fringes which have spacings of 0.23 and 0.60 nm in [021 ] zone-axis HRTEM correspond to (112) and (200) planes of α-HH, showing the nanoparticles share the same orientation (Figure 2d). The single crystal-like selected-area electron diffraction (SAED) pattern further verifies that the ellipsoid is a uniform-structured mesocrystal, although some spots are elongated or slightly extended with arcs due to small misorientation between the subunits (Figure 2d inset). The competition between crystallographic orientation and external force results in a narrow range of slight lattice angle mismatch, which is a typical situation in mesocrystal formation.24, 25 α-HH nanorods and their oriented aggregates were once 8 ACS Paragon Plus Environment

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observed as intermediates in the formation process of gypsum through in situ TEM.26 However, as a metastable phase, the subsequent phase conversion of α-HH to gypsum makes it a big challenge to maintain and separate the ordered structure. It is the first case to synthesize and stabilize the α-HH mesocrystals. The elemental mapping of the cross-sections of α-HH ellipsoids obtained by ultramicrotomy also illustrates the incorporation of EDTA into α-HH mesocrystals, where the elements of carbon and nitrogen assigned to EDTA are in discrete location in comparison with the homogeneous distribution of calcium, sulfur and oxygen element belonging to α-HH (Figure S3). Thus the α-HH ellipsoid is a hierarchically superstructure of inorganic-organic composite with periodic orientation. The complex superstructure suggests a probable transform of crystallization mechanism from classical

ion-by-ion

growth

to

non-classical

oriented

attachment

or

mesocrystallization.27

Figure 2. FESEM images at low magnification (a) and at high magnification (b), TEM (c), HRTEM (d) and SAED (insert) of α-HH ellipsoids synthesized in ethylene glycol-water solution 9 ACS Paragon Plus Environment

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at 95oC for 3 h to exhibit the ellipsoids have a mesocrystal structure and uniform subunits.

Dissolving the interspaced EDTA in boiling water gives rise to the disorganization of the mesocrystal structure and the obtaining of monodisperse subunits, the nanorods. The nanorods are detected as pure α-HH by the XRD (Figure S1) and TG/DSC analysis (Figure 3). The weight loss of - 6.23 wt% identical to theoretical one on the TG curve and the endothermic peak at 162oC on DSC curve are attributed to the crystal water elimination. Compared to Figure 1, the flat TG curve without decline after 200oC indicates the complete removal of EDTA through boiling water washing. The characteristic peaks emerging at 601 and 660 cm-1 (γ4 SO42- stretching), 1008 cm-1 (γ1 SO42- stretching), 1096, 1115 and 1156 cm-1 (γ3 SO42- stretching) and 3550 and 3610 cm-1 (vibration of O-H) in Fourier transform infrared (FTIR) spectra further verify the nanorods are made up by α-HH (Figure S4). The disappearance of peaks at 1400 cm-1 and 1621 cm-1 assigned to COO– stretching vibrations of EDTA proves that EDTA, the interspaced organics of mesocrystals, is removed by boiling water washing.


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Figure 3. TG/DSC analysis of the nanorods obtained by synthesizing in ethylene glycol-water solution at 95oC for 3 h and vacuum filtering with boiling water to testify the nanorods contain only phase of α-HH.

The α-HH nanoparticles with high dispersion present in the shape of rod (Figure 4a). The nanorods with intact facets have a length of 30 - 80 nm and a width of 10 20 nm (Figure 4b-c). The diffraction pattern observed from [ 2 01] zone-axis (Figure 4d insert) consists of individual and bright spots, indicating the single crystalline structure of the nanorods. The lattice fringes along two mutually orthogonal directions in the HRTEM correspond to (204) and (020) planes of α-HH, with d-spacing of 0.28 and 0.35 nm, respectively (Figure 4d). Thus the mesocrystal synthesis followed by structure disorganization creates α-HH nanorods, which are subunits of the mesocrystals.


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Figure 4. FESEM image at low magnification (a) and at high magnification (b), TEM (c), HRTEM (d) and SAED (insert) of α-HH nanorods obtained by removing interspaced EDTA in ellipsoidal mesocrystals to show the uniform size and single crystalline structure of the nanorods.

Structure and orientation evolution of α-HH mesocrystals The time-dependent evolution in morphology and microstructure was examined for further understanding of the formation of α-HH ellipsoidal mesocrystals. At the initial stage, only some small particles precipitate in irregular shape without external structure (Figure 5a). The coexistence of α-HH nanoparticles and the amorphous phase is confirmed by the discrete lattice fringes in HRTEM image (Figure 5b) and the splashes in SAED pattern (Figure 5b insert). The nanoparticles start to organize into ellipsoidal aggregates after preliminary growth of 2 min and the surrounding α-HH domains are believed to be the primary building blocks (Figure 5c). The assembly process finishes within a short time of 5 min and a clear boundary of the ellipsoids appears (Figure 5d).


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Figure 5. TEM image (a), HRTEM (b) and SEAD (insert) of α-HH nanoparticles at 30 s and TEM images of α-HH ellipsoids at 2 min (c) and 5 min (d) to illustrate the self-organization process of α-HH subunits.

The ellipsoidal outline remains nearly unchanged after assembly process, while the subsequent growth, orientation and fusion of subunits are undergoing. The ellipsoids are composed of irregularly-shaped nanoparticles in a random arrangement at the end of the self-assembly stage (Figure 6a). After 30 min, the irregular-shaped nanoparticles are partly substituted by rod-like nanoparticles due to crystal growth towards an identical direction (Figure 6b). The morphology of ellipsoids and its subunits keeps almost the same during a long time (Figure 2b). However, at 12 h, the mesocrystals present a column-like outline with a more compact surface, implying the formation of a tighter structure (Figure 6c). Apart from the nanorods, washing the mesocrystals with boiling water at this time leads to some larger particles in length of 200 - 300 nm, including bulk materials and rod aggregates (Figure S5). The mesocrystal structure can not be completely disorganized by removal of EDTA, implying the crystalline fusion highly possible happens after reaction of 12 h. 13 ACS Paragon Plus Environment

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Figure 6. FESEM, TEM and SAED (insert) images of α-HH ellipsoids after reaction for 5 min (a and d), 30 min (b and e), 12 h (c and f) and TEM (g), SAED (insert) and HRTEM (h) of α-HH particles obtained by washing the ellipsoids at 12 h with boiling water to reveal the growth, orientation and fusion of subunits in ellipsoids during the reactions.

The evolution in orientation and fusion of the subunits are clearly exhibited through the electron diffraction analysis. The preliminary α-HH ellipsoids formed at 5 min are loosely organized and diffracted like a polycrystal with slightly preferential orientation towards a certain direction (Figure 6d and insert). After 30 min evolution, 14 ACS Paragon Plus Environment

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the rod-like nanoparticles gradually align themselves towards a same direction illustrated by the well-organized spot pattern, while the intensively elongated spots with arcs indicate that segmental angular deviations still exist (Figure 6e and insert). After 3 h, the nanoparticles complete their orientation to organize into mesocrystals, which is verified by the clear and bright spots in electron diffraction pattern of Figure 2d. The crystallographic orientation of mesocrystals is gradually accomplished along with the growth of subcrystallites rather than one-step oriented attachment or layer-by-layer growth.28,

29

The perfect single crystal-like diffraction pattern of

ellipsoids at 12 h demonstrates a well-aligned interior crystalline structure, implying the probable appearance of crystalline fusion (Figure 6f).30 The particles obtained by disorganizing the ellipsoids contain some bulk materials, which are single crystals confirmed by the spot pattern (Figure 6g and insert) and the complete lattice structure without crystal-ctystal boundary over a range of ~ 60 nm (Figure 6h). The lattice fringes with spacings of 0.44 and 0.60 nm in [111] zone-axis HRTEM are indexed to the (20 2 ) and (1 1 0) planes of α-HH, respectively. The complete disorganization of the ellipsoids into monodisperse nanorods can not be achieved after 12 h due to the crystalline fusion of subunits, shedding light on the fact that disorganizing the mesocrystal structure at suitable time is essentially crucial for nanoparticle obtaining.

Mechanism of α-HH mesocrystal and nanorod formation In general, the high mass fluxes coupled with high reaction rates in the crystallization system of mesocrystals make it difficult to stabilize, isolate and observe the mesocrystallization process.5, 31 Especially for inorganic mesocrystals, the high lattice 15 ACS Paragon Plus Environment

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energy accelerates the crystallographic fusion of three-dimensional oriented nanocrystals to a single crystal.32,

33

The procedures of nucleation, nanoparticle

self-organization and fusion into single crystal usually finish instantly and as an intermediate phase, the mesocrystal can be very short-lived, even with lifetime less than one second.34 However with the help of EDTA, the evolution of mesocrysals is well postponed and the stages are distinguished clearly. The formation mechanism of α-HH mesocrystal and its disorganization into nanorods are schemed in Figure 7. The α-HH nuclei as well as amorphous phase rapidly precipitate in a short time of 30 s and the underdeveloped particles already have the tendency to aggregate. The primary particles start the self-assembly at 2 min for the reduction of facet free energy. The presence of EDTA switches the crystallization mode from ion-mediated growth to the particle-mediated pathway via inducing and stabilizing the nanoparticles as the building blocks,35 which is verified by the fact that only needle-like particles precipitate even after 3 h without EDTA (Figure S6). After 5 min, the self-assembly stage completely finishes and no building blocks can be observed (Figure 5d). The aggregated particles present in shape of ellipsoid and consist of irregular-shaped nanoparticles in size of ~ 10.3 nm calculated by Scherrer equation according to XRD pattern (Figure S1). Although showing an oriented preference towards certain directions, the just assembled ellipsoids still diffract like a polycrystal (Figure 6d). As the prolonging of reaction time, the ellipsoidal profile keeps almost unchanged, while the subunits grow from irregular nanoparticles to rod-like shaped ones. The average crystalline size of the subunits 16 ACS Paragon Plus Environment

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increases to about 32.5 nm at 1 h and the orientation gets better with the crystalline growth. As a chelator of metal ions, the EDTA ions constrain plenty of calcium ions in the form of EDTA-Ca complex in solution at the self-assembly stage, and the complex gradually release the calcium ions to supply for the growth of subunits at the subsequent evolution.36,

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The crystallite fusion also accounts for the increase in

crystalline size of the subunits, which is common in aggregation-based crystal growth.38 After 3h reaction, the subunits grow to the size of ~ 47.8 nm and complete the orientation, forming the mesocrystal structure (Figure 2). In the ellipsoid, the subunits interspaced by EDTA have substantial spaces and possibilities to wiggle around until eventually, a lattice mismatch angle < 15o is reached.31 The EDTA molecules in aggregates have chances to form an ordered organization through intermolecular hydrogen bonds with adjacent molecules, which is considered to be the force for subunit orientation.39,40 Upon the mesocrystal formation, disorganizing the ellipsoids through EDTA removel gives rise to monodisperse α-HH nanorods with high crystallinity in a size of 30 - 80 nm (Figure 4). This strategy creates a facile method to prepare nanoparticles with uniform particle size and high dispersity, which is possibly extended from α-HH nanorods to other mineal nanoparticles.


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Figure 7. Schematic illustration of the stepwise formation of α-HH mesocrystals to demonstrate the evolution in crystalline structure and orientation of α-HH aggregates from irregular to well-ordered with the subunit growth. Before the fusion starts, disorganazing the mesocrystal structure through EDTA removal gives rise to α-HH nanorods with uniform size and high dispersion.

The evolution of EDTA content in the ellipsoids is tracked by TG analysis in Figure S7. The initially aggregated ellipsoids at 5 min contain ~ 12.36 wt% EDTA. After self-assembly stage, the EDTA content shows a slow decrease to 9.19 wt% at 1 h, 8.73 wt% at 3 h and 6.59 wt% at 6 h. At this stage, the surplus EDTA are gradually squeezed from the ellipsoids, while the remained ones guide the orientation of α-HH subunits until the formation of mesocrystal structure. A remarkable decrease of EDTA content to 2.46 wt% is observed at 12 h. As the rapid excretion of EDTA from mesocrystals, a part of α-HH subunits fuse together by the short-range interactions between adjacent surfaces.41 In aggregation-based crystal growth, the reduction in surface free energy is achieved by the removal of pairs of surfaces.38 Since the subunits partially fuse into single crystalline structure, the disorganization of ellipsoids can not hit the mark to gain the uniform and monodisperse α-HH naorods at this time. The formation, stabilization and disorganization of mesocrystals at suitable time enables the nanoparticle preparation.

CONCLUSIONS This work reports a simple and facile route to fabricate α-HH mesocrystals followed by structure disorganization for the preparation of a-HH nanoparticles. The preliminarily aggregated ellipsoids consist of irregular nanoparticles and diffract like a polycrystal. The subunits of the ellipsoids turn from irregular shape to rod-like 18 ACS Paragon Plus Environment

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shape, and the orientation gets better with the subsequent crystalline growth until the mesocrystal forms. The EDTA in aggregates optimize the spatial distance of the subunits for the orientation adjustment as well as avoiding an ultrafast fusion. The nanorods of α-HH with uniform size and high monodispersity can be obtained through mesocrystal structure disorganization at the right stage. The gradual excretion of EDTA from ellipsoids and partial fusion of the subunits emerge after the stabilization of mesocrystal for several hours, which restricts the complete disorganization of the ellipsoids. This novel strategy provides chemically flexible control over the mesocrystallization process and allows for targeted fabrication of the inorganic nanoparticles.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. XRD patterns, TG/DSC curves of pure Na2EDTA·2H2O, elemental distribution analysis of a-HH ellipsoids, FTIR spectra, FESEM images and TG curves (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID 19 ACS Paragon Plus Environment

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Baohong Guan: 0000-0001-6183-3979 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support is provided by The National Key Research and Development Program of China (2016YFB0301800) and National Science Foundation of China (Project 21176219).

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Solvent and at the Air/Solution Interface Using a Double Hydrophilic Block Copolymer as a Crystal Modifier. Cryst. Growth Des. 2007, 7 (2), 354-359. 30. Liang, X.; Gao, L.; Yang, S.; Sun, J. Facile Synthesis and Shape Evolution of Single-Crystal Cuprous Oxide. Adv. Mater. 2009, 21 (20), 2068-2071. 31. Cölfen, H.; Antonietti, M. Mesocrystals and Nonclassical Crystallization; John Wiley & Sons, 2008. 32. Schwahn, D.; Ma, Y.; Cölfen, H. Mesocrystal to Single Crystal Transformation of D, L-Alanine Evidenced by Small Angle Neutron Scattering. J. Phys. Chem. C 2007, 111 (8), 3224-3227. 33. Li, H.; Guan, M.; Zhu, G. X.; Yin, G.; Xu, Z. Experimental Observation of Fullerene Crystalline Growth from Mesocrystal to Single Crystal. Cryst. Growth Des. 2016, 16, 1306-1310. 34. Judat, B.; Kind, M. Morphology and Internal Structure of Barium Sulfate-Derivation of a New Growth Mechanism. J. Colloid Interface Sci. 2004, 269 (2), 341-353. 35. Zhu, G.; Yao, S.; Zhai, H.; Liu, Z.; Li, Y.; Pan, H.; Tang, R. Evolution from Classical to Non-Classical Aggregation-Based Crystal Growth of Calcite by Organic Additive Control. Langmuir 2016, 32 (35), 8999-9004. 36. Liu, D.; Savino, K.; Yates, M. Z. Microstructural Engineering of Hydroxyapatite Membranes to Enhance Proton Conductivity. Adv. Funct. Mater. 2009, 19 (24), 3941-3947. 37. Xie, R.; Feng, Z.; Li, S.; Xu, B. EDTA-Assisted Self-Assembly of Fluoride-Substituted Hydroxyapatite Coating on Enamel Substrate. Cryst. Growth Des. 2011, 11 (12), 5206-5214. 38. Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Aggregation-Based Crystal Growth and Microstructure Development in Natural Iron Oxyhydroxide Biomineralization Products. Science 2000, 289 (5480), 751-754. 39. Jiang, W.; Pan, H.; Cai, Y.; Tao, J.; Liu, P.; Xu, X.; Tang, R. Atomic Force Microscopy Reveals Hydroxyapatite-Citrate Interfacial Structure at the Atomic Level. Langmuir 2008, 24 (21), 12446-12451. 40. Wu, Y. J.; Tseng, Y. H.; Chan, J. C. Morphology Control of Fluorapatite Crystallites by Citrate Ions. Cryst. Growth Des. 2010, 10 (10), 4240-4242. 41. André, A.; Zherebetskyy, D.; Hanifi, D.; He, B.; Samadi Khoshkhoo, M.; Jankowski, M.; Chassé, T.; Wang, L. W.; Schreiber, F.; Salleo, A. Toward Conductive Mesocrystalline Assemblies: PbS Nanocrystals Cross-Linked with Tetrathiafulvalene Dicarboxylate. Chem. Mater. 2015, 27 (23), 8105-8115.


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Table of Contents only α-Calcium Sulfate Hemihydrate Nanorods Synthesis: a Method for Nanoparticle Preparation by Mesocrystallization Qiaoshan Chen,a Caiyun Jia, a Yu Li, a Jie Xu, a Baohong Guan*a and Matthew Z. Yates*b

Synopsis: Fabrication of α-HH nanorods with uniform particle size and high dispersion by synthesizing, stabilizing and disorganizing of its mesocrystals.


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Synopsis: Fabrication of α-HH nanorods with uniform particle size and high dispersion by synthesizing, stabilizing and disorganizing of its mesocrystals. 128x72mm (300 x 300 DPI)

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Figure 1. TG/DSC analysis of the ellipsoids synthesized in ethylene glycol-water solution at 95oC for 3 h to indicate the ellipsoids consist of 91.27 wt% α-HH and 8.73 wt% EDTA. 107x77mm (300 x 300 DPI)


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Figure 2. FESEM images at low magnification (a) and at high magnification (b), TEM (c), HRTEM (d) and SAED (insert) of α-HH ellipsoids synthesized in ethylene glycol-water solution at 95oC for 3 h to exhibit the ellipsoids have a mesocrystal structure and uniform subunits. 114x80mm (300 x 300 DPI)


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Figure 3. TG/DSC analysis of the nanorods obtained by synthesizing in ethylene glycol-water solution at 95oC for 3 h and vacuum filtering with boiling water to testify the nanorods contain only phase of α-HH. 107x76mm (300 x 300 DPI)


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Figure 4. FESEM image at low magnification (a) and at high magnification (b), TEM (c), HRTEM (d) and SAED (insert) of α-HH nanorods obtained by removing interspaced EDTA in ellipsoidal mesocrystals to show the uniform size and single crystalline structure of the nanorods. 114x81mm (300 x 300 DPI)


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Figure 5. TEM image (a), HRTEM (b) and SEAD (insert) of α-HH nanoparticles at 30 s and TEM images of αHH ellipsoids at 2 min (c) and 5 min (d) to illustrate the self-organization process of α-HH subunits. 109x73mm (300 x 300 DPI)


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Figure 6. FESEM, TEM and SAED (insert) images of α-HH ellipsoids after reaction for 5 min (a and d), 30 min (b and e), 12 h (c and f) and TEM (g), SAED (insert) and HRTEM (h) of α-HH particles obtained by washing the ellipsoids at 12 h with boiling water to reveal the growth, orientation and fusion of subunits in ellipsoids during the reactions. 230x327mm (300 x 300 DPI)


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Figure 7. Schematic illustration of the stepwise formation of α-HH mesocrystals to demonstrate the evolution in crystalline structure and orientation of α-HH aggregates from irregular to well-ordered with the subunit growth. Before the fusion starts, disorganazing the mesocrystal structure through EDTA removal gives rise to α-HH nanorods with uniform size and high dispersion. 89x35mm (300 x 300 DPI)


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Figure S1. XRD patterns of HH nanorods and ellipsoids formed at reaction times of 5 min, 1 h and 3 h to demonstrate the crystalline growth of subunits in the ellipsoids and indicate the obtained nanorods by mesocrystal structure disorganization are subunits of the ellipsoids. 142x113mm (300 x 300 DPI)


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Figure S2. TG/DSC curves of pure Na2EDTA·2H2O provided as a comparison for TG/DSC curves of α-HH mesocrystals to verify the presence of EDTA in the ellipsoids. 101x68mm (300 x 300 DPI)


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Figure S3. TEM image (a), STEM image (b) and elemental distribution analysis of Ca (red), S (green), O (blue), C (yellow) and N (purple) (c) at the cross-section of α-HH ellipsoids to further confirm the existence of EDTA inside the mesocrystals. 117x91mm (300 x 300 DPI)


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Figure S4. FTIR spectra of α-HH ellipsoids, α-HH nanorods and Na2EDTA to show the presence of EDTA in the mesocrystals and its removal by boiling water washing. 138x106mm (300 x 300 DPI)


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Figure S5. FESEM images at low magnification (a) and high magnification (b) of particles obtained by washing the ellipsoids with boiling water to illustrate the emergence of crystalline fusion of partial subunits in the ellipsoids after reaction of 12 h. 114x162mm (300 x 300 DPI)


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Figure S6. FESEM image of particles formed in ethylene glycol-water solution without EDTA at 95oC for 3 h to demonstrate that EDTA participates in the self-assembly of α-HH for obtaining of the ellipsoidal shape. 88x61mm (300 x 300 DPI)


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Figure S7. TG curves of α-HH ellipsoids formed at 5 min, 1 h, 6 h and 12 h to show the gradual excretion of EDTA from the ellipsoids after the self-assembly stage. 135x101mm (300 x 300 DPI)


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Î±-Calcium Sulfate Hemihydrate Nanorods Synthesis: A Method for

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