Mesoscale Transformation of Amorphous Calcium Carbonate to

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Mesoscale Transformation of Amorphous Calcium Carbonate to Porous Vaterite Microparticles with Morphology Control Rui Sun, Tom Willhammar, Erik Svensson Grape, Maria Strømme, and Ocean Cheung Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00438 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

Mesoscale Transformation of Amorphous Calcium Carbonate to Porous Vaterite Microparticles with Morphology Control Rui Sun,† Tom Willhammar,‡ Erik Svensson Grape, ‡ Maria Strømme,*,† and Ocean Cheung*,†

† Division of Nanotechnology and Functional Materials, Department of Engineering Sciences, Uppsala University, SE-751 21, Uppsala, Sweden

‡ Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91, Stockholm, Sweden

KEYWORDS:

amorphous

calcium

carbonate

microparticles,

mesoscale-transformation,

nanoparticles,

self-assembly,

porous

morphology

vaterite

controllable

synthesis

ACS Paragon Plus Environment

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: The morphology controllable synthesis of porous vaterite microparticles from amorphous calcium carbonate (ACC) nanoparticles via mesoscale transformation and self-assembly is presented. The morphology of vaterite microparticles ranging from ellipsoidal to spherical can be controlled by adjusting the amount of adipic acid (AA) additive during synthesis. Electron microscopy and electron diffraction reveal that the vaterite microparticles are formed by the oriented self-assembly of vaterite nanocrystals. The Brunauer–Emmett–Teller (BET) surface area of the vaterite microparticle varies between ~30 and ~80 m2/g. The coverage of AA on the surface of the ACC nanoparticle plays the pivotal role in the morphology controllable synthesis of vaterite microparticles. 6-aminocaproic acid (6A), benzoic acid (BA), citric acid (CA) and poly(acrylic acid) (PAA) are also tested as additives and their effect on the morphology of vaterite microparticles are presented. Morphology control of functional materials can be beneficial for application where the morphology and porosity are critical, such as drug delivery. This work demonstrates a possible method to finely adjust the morphology of vaterite micropariticles with assistance of additives through mesoscale transformation and self-assembly using amorphous nanoparticles as precursor.


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INTRODUCTION

Crystallization and morphological control of calcium carbonate (CaCO3) are of great interests in biomineralization and for a number of industrial applications. Many organisms have the ability to tailor the shape of various polymorphs of CaCO3 for their natural needs.1-3 CaCO3 is widely used in the manufacture of paper, paint, pharmaceuticals as well as many other products.4 From the synthesis aspect, a number of different approaches have been adopted to control the morphologies of crystalline CaCO3. These include the formation of Langmuir monolayers,5 self-assembled films6-8 and micro-emulsion,9-10 additives assisted crystallization (macromolecules,11 polymer,1216

or low-weight molecules17-18), etc. In particular, additive assisted crystallization of

CaCO3 has been one of the most studied synthesis route in recent years. By influencing the crystallization process using different additives, various morphologies of CaCO3 have been reported. CaCO3 with shapes that can be described as “large micropatterned single

crystals”,19-20

“fibers”,21

“flower-like”,16,

22

“microrings”,23

and

“spherical


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particles”24-26 have been synthesized by researchers. The mechanisms behind the crystallization and the formation of different superstructures of CaCO3 have also been widely discussed. A number of atomistic simulation works have been carried out in order to gain insight into the crystallization process of CaCO3.27-30 With the increasingly assertive demands on the properties of functional materials for various applications, morphology control has become an important aspect of functional materials. One example of the importance of material/particle morphology lies in the development of novel drug carriers. The shape of the carrier on a micro or nanometer scale is one of the main factors affecting the drug uptake efficiency by cells.31-32 Therefore, morphology control of drug carriers, such as CaCO3, has been investigated by a number of researchers.33-34

Mesoscale transformation and spontaneous self-assembly have become a promising strategy to produce nanostructures in recent years.35-36 In this process, metastable amorphous building blocks coated with surfactants/additives go through mesoscale transformation and self-assembly to form crystalline nanostructures.37-38 BaSO4 fibers,39


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SrCO3 nanostructures40 and CaCO3 superstructure10,

41

have been synthesized using

the concept of mesoscale transformation in an emulsion system. Mesostructured calcium phosphate42 and superstructure CaCO343-44 were also prepared in the presence of additives in aqueous solution. The strong interaction between the inorganic amorphous building blocks and additives played the significant role in the mesoscale transformation and self-assembly of the crystalline structure. The intrinsic interface between the inorganic nanocrystals provides the material with high porosity. These porous nanostructured materials have become promising candidate materials for various applications.34, 45-47

CaCO3 is known to exist as different polymorphs. The most common ones are the three anhydrous crystalline polymorphs—calcite, aragonite, vaterite, and two hydrated crystalline polymorphs: monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). A new hydrated crystalline CaCO3 - calcium carbonate hemihydrate, was recently discovered by Zou et al.48 Amorphous calcium carbonate (ACC) is another polymorph of hydrated calcium carbonate and is the precursor of the crystalline polymorphs. It is


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unstable in aqueous solution with a life time of seconds to minutes. It has been reported that ACC synthesized in organic solvent exhibits extended life time49-51 and the crystallization of ACC could be controlled by using mixed solvents (i.e. organic-water mixture solvent).52-55 These studies offer a possible method for morphology control by controlling the crystallization of ACC. It has been documented that water plays the pivotal role in the controllable crystallization and structure formation process.9,

54

Furthermore, porous vaterite can be synthesized by introducing water to ACC nanoparticles covered with additives (polymer or small molecule) in organic solvent.52, 56 From the various additives used to control the synthesis of CaCO3 with different morphologies from ACC,57-61 carboxylic species have been extensively explored as they can form strong interactions with the calcium atom in CaCO3. Note that most of the morphological studies of CaCO3 were carried out in aqueous systems where aqueous solutions containing calcium and carbonate ions were used as the initial precursors. In these systems, ACC particles/clusters would form in the aqueous solution before crystallization would takes place to form vaterite. Zou et al.61-62 investigated the interaction between ACC nanoparticle and the additives and showed that additives


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could be incorporated into an ACC nanoparticle during its formation. The self-assembly and mesoscale transformation ACC nanoparticles to from nanostructured materials has been reported .9, 56, 63-65

Our group recently synthesized highly porous amorphous calcium carbonate (HPACC) with the highest BET surface area report for CaCO3 to date.50 HPACC was obtained by evaporating methanol (i.e. solvent) from an ACC suspension which was synthesized first by dispersing calcium oxide (CaO) in methanol and then pressurizing with CO2 gas for 4 h at 50 °C. The ACC suspension contained ACC nanoparticles with uniform size (< 10 nm) as determined in our previous study. Good storage stability of the ACC suspension was observed. When stored at 0 °C, no aggregation or crystallization of the ACC nanoparticles was detected even after several months. This ACC suspension could be an ideal starting point for studying the crystallization and morphology control of crystalline CaCO3 from ACC nanoparticles.

In this study, we investigated the possibility to use the ACC suspension (in methanol) to produce vaterite microparticles with controlled morphology. We prepared a series of


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these vaterite microparticles using the ACC suspension with adipic acid (AA) as an additive and used a small amount water to induce crystallization. The effect of varying amounts of AA on the morphology of the obtained vaterite microparticles was comprehensively explored. The effect of stirring speed and the concentration of ACC in the suspension were also analyzed. Apart from AA, other additives with similar or different structure of AA, including 6-aminocaproic acid (6A), benzoic acid (BA), citric acid (CA) and poly(acrylic acid) (PAA) were also investigated. The molecular structures of additives investigated in this study are listed in Figure 1.

Figure 1. Molecular structure of additives studied in this work.


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EXPERIMENTAL SECTION

Materials. CaO (Reagent Grade) was purchased from Alfa-Aesar. Methanol (≥99.8%) was purchased from VWR Sweden. AA (≥99.5%), 6A (98%), BA (≥99.5%), CA (≥99.5%) and PAA (average Mw=1800) were purchase from Sigma-Aldrich. CO2 (>99.998%) was purchased from Air Liquide AB. All the chemicals were used without further purification.

Synthesis of ACC suspension. The ACC suspension (in methanol) was synthesized following the procedures detailed in our previous work.50 Typically, 2.5 g of CaO was added to 150 mL methanol at 50 °C under constant stirring. 4 bar CO2 was applied to the reaction system when the mixture appeared homogeneous. After 4 hours at 50 °C, the reaction mixture was centrifuged at 3800 rpm for 15 minutes to remove the unreacted CaO. A suspension containing ACC nanoparticles with particle size of < 10 nm dispersed in methanol (as discussed in our previous work)50 was obtained after centrifugation. This ACC suspension was used for the synthesis of vaterite microparticles.


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Synthesis of vaterite microparticles from ACC suspension. Vaterite microparticles were prepared by adding different amounts of an AA solution in methanol (11.25 mg/mL) to 3 mL ACC suspension (concentration of ACC suspension was determined to be ~25 mg/mL) under stirring at 500 rpm. After stirring the mixture for 1 hour, water with a volume of 50% of the total volume of methanol in the suspension was added to the mixture to induce the crystallization of ACC nanoparticles.56 The volume fraction of water in the final suspension was, hence, 1/3 of the total solvent (methanol + water) in order to ensure that vaterite would be the final phase of CaCO3.55 Vaterite particles formed when the mixture turned white. Four different volumes of AA/methanol (0.354, 1.068, 1.780, 2.667 mL) were added to 3 mL of the ACC suspension in the synthesis of vaterite microparticles. The final AA content in the obtained vaterite samples were 0.053, 0.160, 0.267 and 0.400 g/g CaCO3. These samples were denoted (VateriteAA000 for pure vaterite) Vaterite-AA053, Vaterite-AA160, Vaterite-AA267 and VateriteAA400, respectively. The synthesis of Vaterite-AA053 at different stirring speeds (250 rpm, 500 rpm, 1000 rpm and 1400 rpm) was carried out using the same method.


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To investigate the effect of concentration of ACC suspension on the morphology of vaterite, the ACC suspension was diluted to 50% and 37.5% of the original ACC concertation. These low-concentration ACC suspensions were used to prepare VateriteAA000, Vaterite-AA053 and Vaterite-AA400 using the same method as described above. These samples were labelled in the same way as the above described samples but with a suffix that indicated the dilution factor (i.e. Vaterite-AAXXX-100%, -50% or 37.5%)

The synthesis of vaterite particles with other additives, including 6A, BA, CA and PAA (named Vaterite-6A, Vaterite-BA, Vaterite-CA and Vaterite-PAA, respectively) was carried out using the same method as above.

Characterization. Infrared (IR) spectra were recorded using a Bruker Tensor 27 spectrometer (Bruker, Bremen, Germany) coupled with a Platinum attenuated total reflection (ATR) diamond sample stage. Powder X-ray diffraction (XRD) data were recorded using a Bruker D8 advance XRD Twin-Twin instrument (Bruker, Bremen, Germany) with Cu-Kα radiation (λ=0.15418 nm) with a step size of 0.04° and 2 s per


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step in the 2Ɵ range of 10 to 70 °. Scanning electron microscope (SEM) secondary electron images were recorded using a Zeiss LEO 1530 scanning electron microscope (Oberkochen, Germany). The samples were coated with a layer of gold-palladium to avoid charging effects. Transmission electron microscopy (TEM) experiments were carried out at room temperature using a FEI Tecnai F30 ST microscope (Hillsboro, Oregon, USA) operated at 300 kV equipped with a Schottky field-emission gun. The fresh sample was diluted with methanol and dispersed on TEM grids with carbon supporting film. Electron diffraction patterns for the continuous rotation electron diffraction (cRED) data were recorded using a JEOL JEM-2100 TEM equipped with a thermal LaB6 gun operated at an acceleration voltage of 200 kV. The diffraction data were recorded using a Timepix detector (ASI) using the instamatic software package.66 The reconstruction of the cRED data was performed using the cREDp software67 and visualized by Tomviz (tomviz.org). N2 adsorption measurements were carried out in a Micromeritics ASAP 2020 (Norcross, GA) volumetric gas adsorption analyzer. Prior to the adsorption measurements, the sample was degassed at 100 °C for 6 hours under dynamic vacuum (1 x 10-4 Pa) using a Micromeritic Smart VacPrep060 (Norcross, GA,


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USA) sample preparation unit. The aspect ratios of the obtained vaterite microparticles were calculated by measuring the lengths and widths of over 20 vaterite microparticles using the SEM images. The average value of the aspect ratios and the standard deviations were calculated.

RESULTS AND DISCUSSION

Figure 2 shows the powder XRD patterns of Vaterite-AA053, Vaterite-AA160, VateriteAA267 and Vaterite-AA400. The diffraction patterns showed that the vaterite phase was clearly observed for all samples (Figure 2b). The presence of other CaCO3 polymorphs (i.e. calcite, aragonite) was not detected in Vaterite-AA267 and Vaterite-AA400. A small amount of calcite was detected together with vaterite in Vaterite-AA053 and VateriteAA160. Calcite was also detected in Vaterite-AA000 (Figure S1a). The calcite content of Vaterite-AA000, Vaterite-AA053 and Vaterite-AA160 was calculated by Rietveld refinement to be 33.3 wt.%, 8.3 wt.% and 1.0 wt.%, respectively (Figure S2, Table S13). The additional diffraction peaks at low angle (< 20 °) in the XRD pattern of Vaterite-


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AA400 could be attributed to calcium adipate as shown in more detail in Figure S3. The appearance of calcium adipate on Vaterite-AA400 is discussed later.

The IR spectra of Vaterite-AAs are shown in Figure 3. The typical IR bands related to CaCO3 were present in all samples. The most intense ν3 band was observed at around 1400 cm-1 and the ν2 band was observed at around 876 cm-1. The location of the ν4 band at around 744 cm-1 matched that observed by Gebauer et al. for vaterite.68 An additional ν4 band observed for Vaterite-AA000 at around 714 cm-1 (Figure S1b) was related to the presence of a small amount of calcite. This second ν4 band for calcite (714 cm-1) was not clearly seen in the IR spectrum of Vaterite-AA053, probably due to the low amount (8.3 wt.%) of calcite that was present in Vaterite-AA053. IR bands related to AA could be observed on Vaterite-AA samples with the most noticeable band being the carbonyl band centered at around 1570 cm-1. The intensity of this IR band (1570 cm-1) was low on Vaterite-AA053, but it increased with increasing AA amount (Figure 3a) and the band is strongly visible in the IR spectra of Vaterite-AA267 and Vaterite-AA400. This carbonyl band of AA was located at 1689 cm-1 (Figure 3b). The


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fact that the band shifted to 1570 cm-1 (Figure 3b) when AA was used as an additive in Vaterite-AA indicate the strong interaction between AA and calcium carbonate69 or the

Vaterite-AA267

(104)

(100)

(110)

(202)

Vaterite-AA160

Vaterite-AA267 PDF 00-001-1033 CaCO3 Vaterite

(002)

Vaterite-AA053

Intensity (a.u.)



(101)

(b)

(a)

(102)

formation of calcium adipate at high AA amounts in Vaterte-AA400 (discussed later).

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vaterite-AA400

20

40 2 ()

60

20

40 2 ()

60

Figure 2. Powder XRD patterns of Vaterite-AAs (a) and XRD pattern of Vaterite-AA267 fitted with PDF 00-001-1033 CaCO3 Vaterite (b). The peak marked with a star in panel (a) corresponds to calcite.


15

876 cm

1088 cm-1

Vaterite-AA400

-1

744 cm-1

Vaterite-AA267 Vaterite-AA160 Vaterite-AA053

3

2000

1

=O

(b)

1406 cm-1

Absorbance (a.u.)

(a) Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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=O


-

-C-C-O

AA Vaterite-AA400

-C-C-OH

2 4

1500 1000 -1 Wavenubmer (cm )

500

2000

1500 1000 -1 Wavenubmer (cm )

500

Figure 3. IR spectra of Vaterite-AAs (a) and comparison of IR spectrum of AA and Vaterite-AA400 (b).

Figure 4 shows the SEM secondary electron images of Vaterite-AAs obtained with varying amounts of AA. In all cases, uniformly shaped particles could be observed in the SEM images of Vaterite-AAs. In the AA-free Vaterite-AA000 (Figure 4a-c), prolate spheroid-shaped microparticles were observed. At low amounts of AA (Vaterite-AA053 and Vaterite-AA160), the microparticles appeared to have the similar elongated shape as those found in Vaterite-AA000. Vaterite particles with similar morphology synthesized in mixed solvent were reported by Liu et al.70 and Trushina et al.71 When the amount of AA was increased (i.e. for Vateire-AA267 and Vaterite-AA400), the microparticles


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became more spherical, especially for Vaterite-AA400. As it appeared in the SEM images, the longest dimension of the Vaterite-AAs microparticles ranged from ~1 to ~1.7 μm (on average) depending on the amount of AA. The high resolution SEM images displayed in Figure 4c, f, i, l and o clearly showed that the Vaterite-AAs microparticles were composed of individual nanocrystals. The size of these nanocrystals also collated to the amount of AA. The nanocrystals in Vaterite-AA053 appeared to be larger than those in Vaterite-AA400. The coherence lengths of the crystallites that make up the Vaterite-AAs microparticles were calculated from the (002) and the (100) diffraction peaks in the XRD patterns using the Scherrer equation and the results are shown in Figure S4 and Table S4. The coherence lengths of the crystallites decreased when the amount of AA used was increased. The ratio of the coherence length of crystallites along the [002] and the [100] directions also decreased when the dosage of AA was increased, which may indicate that the size anisotropy of the crystallites was reduced

72

(The size of vaterite nanocrystal was further investigated using TEM and is discussed in detail later). The change in morphology of the vaterite microparticle from prolate spheroid to spherical particles with the increase of additive amount was similar to that of


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CaCO3 synthesized with other kinds of additives (hyperbranched polymers,73 carboxymethyl cellulose 60 and silk fibroin74 etc.) in aqueous solution.

The cross-sectional appearance of Vaterite-AAs microparticles (Figure S5) demonstrated that the assembly of nanocrystals initiated from a center point and propagated in all directions. This may be followed by the spherical growth mechanism of vaterite that has been reported by other researchers.75-77 The assembly of nanocrystals showed some form of orientation in one direction (the longest dimension of the microparticle). The level of orientation appeared to have reduced when the amount of AA was increased (i.e. for Vaterite-AA267 and Vaterite-AA400 the orientation was not easily observed due to the spherical shape of the microparticles). Furthermore, when stored in the synthesis liquid, the nanocrystals seemed to grow bigger as compared with the nanocrystal inside the fresh sample (Figure 4). The shape of these vaterite microparticles remained the same (Figure S6) and with no calcite being detected for up to 2 weeks when stored in the synthesis liquid.


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Figure 4. SEM images of Vaterite-AA000 (a-c), Vaterite-AA053 (d-f), Vaterite-AA160 (gi), Vaterite-AA267 (j-l) and Vaterite-AA400 (m-o).

As there was a clear relationship between the shape (aspect ratio) of the VateriteAAs microparticles and the amount of AA used, we attempted to correlate this


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relationship mathematically. Figure 5 shows that the average aspect ratio (y) - with corresponding standard deviation - of a Vaterite-AA particle as a function of the amount of AA (x, in mg) could be fitted using the equation y = a-b*cx, where a = 0.84, b = -1.08, c = 1.00, R2> 0.97. Kellermeier et al.78 showed that the size of ACC nanoparticle as a function of the amount of added silica could be modelled using a first-order exponential decay equation; y = a0 + a1 exp(-a2*x). The first-order exponential decay equation could also be used to model the data presented in this study and the result is shown in Figure S7. The relationships between the aspect ratio of vaterite microparticles and the amount of AA used in the synthesis could be used as a tool for designing vaterite microparticles with a specific shape. It is important to note that, based on the work presented here, the constants in the equation were not related to any known physical factors associated with the synthesis. These parameters were derived simply from the fitting of the collected data.


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2.0 Experiment data Fitting curve

1.8 Aspect ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6 1.4 1.2 1.0 0

100 200 300 Amount of AA (mg)

400

Figure 5. The relationship between the aspect ratio with its deviation of Vaterite-AAs microparticles and the amount of AA could be fitted y = a - b*cx, a = 0.84, b = -1.08, c = 1.00, R2> 0.97.

In order to gain insight into how the vaterite microparticles were formed, we employed XRD, IR and SEM to monitor the formation process of these microparticles after the addition of water. Figure 6 shows the SEM images, XRD patterns and the corresponding coherence length of crystallites of Vaterite-AA400 microparticles at different times after the addition of water. Aggregation of small nanoparticles could be seen in the SEM image (Figure 6a) 5 minutes after the addition of water. At this time, the sample had similar morphology as the pure HPACC from our previous study. Several spherical particles with an overall size of < 200 nm could be seen at this time, although they appeared to be partially formed on a HPACC particle rather than as stand-alone spherical particles. This material was X-ray amorphous as revealed by the XRD pattern (Figure 6b). 15 minutes after the addition of water, an increased number of spherical


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particles with an overall size of ~200 – 400 nm were noted from the SEM images (Figure 6a). These particles appeared to have well-defined shapes. Several weak diffraction peaks (ascribed to vaterite) emerged in the XRD pattern (Figure 6b). The appearance of these XRD peaks confirmed that ACC had partially crystallized to vaterite. The crystalline vaterite in this sample existed as the spherical particles that can be seen in the SEM image (Figure 6a). The other part of the sample remained as ACC as shown by the SEM images and was implied by the sample’s low overall crystallinity (demonstrated by the XRD pattern). These results suggested that the formation of the spherical vaterite particles occurred simultaneously with the formation of the vaterite nanocrystals, and not as a separate step after crystallization. If the formation of spherical vaterite particles took places as a separate step, we would first see the sample with vaterite nanocrystal with high crystallinity and no spherical particles, followed by the formation of the spherical particles at a later stage. This was clearly not observed here. As the crystallization proceeded, ACC nanoparticles continued to transform to vaterite nanocrystals and the assembled vaterite particles grew bigger to form (micro)particles of around 1 µm. This was observed between 15 min to 45 min after the addition of water. These spherical vaterite microparticles also appeared to be more uniformly shaped over time. As expected, the intensity of diffraction peaks belonging to vaterite also increased with time. Between 45 and 120 minutes after the addition of water, the overall morphology of the spherical vaterite microparticles did not change, but some additional diffraction peaks appeared (Figure 6b). These additional diffraction peaks (also observed in Figure 2) were attributed to calcium adipate. The formation of calcium adipate was probably related to the release of AA when ACC crystallized to vaterite. Similar release of adsorbed molecules triggered by the crystallization of ACC has been observed when ACC was tested as a carrier for pharmaceutical molecules.34,

50

The presence of calcium adipate was


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observed only on high AA amounts due to the high abundance of AA adsorbed on ACC before crystallization. The presence of calcium adipate was observed with fiber-like morphology from the SEM images shown in Figure S8. Note that the fiber-like calcium adipate was not observed in the SEM images shown in Figure 6a. This was due to the different sample preparation procedure used for SEM recordings (Supporting Information, Figure S8). In order to understand the formation of the individual vaterite nanocrystals that made up the vaterite microparticles, we calculated the coherence length of the crystallites using the Scherrer equation at different times after the addition of water. The results are shown in Figure 6c. The coherence length of the crystallites along the [002] and the [100] directions increased with time after the addition of water for up to 60 min and then remained steady up to 120 min. The IR spectra of vateriteAA400 at different times after the addition of water also confirmed the evolution process of Vaterite-AA400 (Figure S9). The formation of Vaterite-AA053 microparticles was also followed in a similar manner as demonstrated for Vaterite-AA400. The SEM images, IR spectra and XRD patterns of VateriteAA053 at various times after the addition of water are shown in the Supporting Information, Figure S10. The formation of Vaterite-AA053 required a shorter time (< 1 min) than the formation of Vaterite-AA400. Furthermore, the formation of the prolate spheroids vaterite microparticles appeared to be completed after 15 min. The formation of the prolate spheroids vaterite microparticles was completed as indicated by SEM, XRD, IR and the calculated coherence length of crystallites from the (002) and the (100) diffraction peaks. Vaterite-AA053 was consisted of prolate spheroids microparticles while Vaterite-AA400 was consisted of spherical microparticles. This indicated that the amount of AA used in synthesis played a pivotal role in the formation of vaterite microparticles with different shapes.


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Figure 6. SEM images (a), powder XRD patterns (b) and coherence length of crystallites calculated from the (002) and the (100) diffraction peaks using the Scherrer equation (c) of Vaterite-AA400 formed at different times after addition of water.

In order to investigate other factors that may affect the morphology of these VateriteAAs microparticles, we varied the stirring speed and the concentration of the ACC suspension during synthesis. Figure S11 in the Supporting Information shows SEM images of Vaterite-AA053 synthesized at different stirring speeds during synthesis. No


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significant differences in the morphology and aspect ratios (Figure S12) could be observed when the stirring speeds were varied during the synthesis of Vaterite-AA053. Similarly, no remarkable changes to the morphology and aspects ratios were observed when the concentrations of the ACC suspension were varied during the synthesis (Figure S13-16). While in many studies of morphology control of CaCO3 from aqueous solution, the concentration26,

71

and the stirring speed14 usually played a significant

effect on the shape of the obtained CaCO3. The morphology and aspect ratios of Vaterite-AAs synthesized under different conditions are discussed further in the Supporting Information.

The TEM images of Vaterite-AA053 and Vaterite-AA400 are shown in Figure 7. Like the SEM images shown in Figure 4, the TEM images of Vaterite-AA053 and VateriteAA400 (Figure 7 and Figure S17) showed that the microparticles were made of assembled nanocrystals with a preferred orientation along the elongated direction (fast Fourier transform (FFT) images in Figure S17c and f). The alignment was also clearly observed in the TEM images of Vaterite-AA000 (Figure S17g, h). The nanocrystals


25


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within the Vaterite-AAs microparticles also appeared to have different shapes. VateriteAA000 had the largest individual prolate spheroids vaterite nanocrystal (around 90 nm in the longest dimension). The nanocrystals within Vaterite-AA053 had the same prolate spheroid-shape (indicated by white dotter lines in Figure 7c) as Vaterite-AA000 but they appeared to be much smaller in size than those found on Vaterite-AA000. The Fourier filtered79-80 images (Figure S18) for the three nanocrystals marked with dot line in Figure 7c (denoted as i, ii and iii) also illustrated that the individual vaterite nanocrystal had dimensions of around 9 nm in length and 5 nm in width. In contrast, the nanocrystals of Vaterite-AA400 appeared to be more spherical (but still elongated in one dimension) (Figure 7e, f and the Fourier filtered images in Figure S19) when compared with those in Vaterite-AA053. The diameter of the nanocrystals in Vaterite-AA400 was estimated to be ~7-8.5 nm from the Fourier filtered images of Figure 7f (shown in Figure S19b-d, note that we are aware of the potential loss of information at the edges of the crystals due to Fourier filtering). These dimensions were comparable to the coherence length of the crystallites calculated from (002) and (100) diffraction peaks (Table S4), which may


26

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imply that the nanocrystals observed in the TEM images are in fact crystallites of the microparticles.

Figure 7. TEM images of Vaterite-AA053 (a, b) and Vaterite-AA400 (d, e), highresolution TEM (HRTEM) images of Vaterite-AA053 (c) and Vaterite-AA400 (f). The particle denoted as i, ii and iii in panel c and f indicated specific nanocrystals in VateriteAA053 and Vaterite-AA400, respectively. The images generated by Fourier filtering of the particles i, ii and iii in Figure 7c and f are shown in Figure S18 and Figure S19, respectively.


27


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We further employed electron diffraction (ED) to investigate the alignment of the vaterite nanocrystals seen in the SEM and TEM images above. cRED data were collected for Vaterite-AA000, Vaterite-AA053 and Vaterite-AA400 confirmed that the phase of the Vaterite-AAs samples was indeed vaterite. The reconstructed reciprocal lattice from the cRED data exhibited a single-crystal-like pattern, indicating that the nanocrystals were aligned with respect to each other throughout an entire microparticle. An individual nanocrystal was always aligned with its crystallographic c-axis along the longest dimension of the microparticle. The arcs in the ED displayed in Figure 8 showed that nanocrystal alignment showed a small amount of misorientation. The degree of misorientation was similar for all three studied samples as reflected in the length of the arcs produced around each reflection. The indices of the two reflections marked with red and green were 100 and 002, respectively. The three dimensional (3D) reconstruction of the reciprocal lattice of a microparticle in Vaterite-AA053 is shown in Figure S20 and the cRED movie can be found in the Supporting Information.


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Figure 8. Sections through the 3D reciprocal lattice reconstructed from cRED data showing the h0l family of reflections for Vaterite-AA000 (a), Vaterite-AA053 (c) and Vaterite-AA400 (e) (c*-axis horizontal) and TEM images of the microparticle in VateriteAA000 (b), Vaterite-AA053 (d) and Vaterite-AA400 (f). The cRED data were taken from the particles shown in (b), (d) and (f).

The N2 adsorption-desorption isotherms (Figure 9) revealed that the obtained Vaterite-AAs microparticles were porous. Vaterite synthesized without AA (i.e. VateriteAA000) had the lowest BET specific surface area of around 16 m2/g. The addition of AA in the synthesis of vaterite had an effect on the BET surface area. The BET surface


29


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area of Vaterite-AA053 increased to 35 m2/g. Vaterite-AA160 had the highest BET surface area of ~85 m2/g. When the AA dosage was further increased as in VateriteAA267 and Vaterite-AA400, the BET surface area of the vaterite microparticles decreased. The decrease in BET surface area may be related to the shape and size of the nanocrystals, or possibly the high amount of AA that was adsorbed on the surface of the vaterite nanocrystals and the existence of small amounts of calcium adipate in Vaterite-AA400 as seen in the powder XRD (Figure 2) and IR spectroscopy (Figure 3) analyzes. The pore size of the Vaterite-AAs was also affected by the amount of AA. As it appeared from the SEM and TEM images, the pores within the Vaterite-AAs microparticles arose from the space between the nanocrystals that have aggregated to form a microparticle. As mentioned earlier, the orientations of nanocrystals within a microparticle showed some form of alignments along the longest dimension of the microparticle and therefore, the packing of these nanocrystals within the microparticles would have some form of regularity. As a result, the shape and size of these nanocrystals were related to the pore dimensions detected by N2 soprtion. Table 1 shows that Vaterite-AA000 had the largest pores of all the Vaterite-AAs with a peak


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pore size of ~34 nm (density functional theory (DFT) pore size distributions shown in Figure 9b), which was due to the presence of large nanocrystals (~90 nm as observed in the TEM image Figure S17g, h). The pore size distributions of Vaterite-AA053 and Vaterite-AA160 were comparable (peak pore size of ~9.4 nm for both samples), suggesting that both samples contained vaterite nanocrystals of similar size and shape. The low BET surface area observed on Vaterite-A053 could be related to the low AA coverage on the nanocrystals, allowing some levels of particle intergrowth. Intergrown particles may have blocked some of the pore volumes on Vaterite-AA053. By considering the pore size distributions as well as the SEM and TEM images (Figure 4 and 7), it was clear that the Vaterite-AA053 and Vaterite-AA160 had similar nanocrystal shape (prolate spheroids) and size. Similar comparison could be made for VateriteAA267 and Vaterite-AA400. Vaterite-AA267 and Vaterite-AA400 showed similar DFT peak pore size of ~13.4 nm, they also had nanocrystals with comparable shape (sphere-like) and size. The pore structure properties of the samples studied in this work are listed in the Table 1. The pore volume of the Vaterite-AAs increased with increased dosage of AA. We believe that this was related to the density of AA on the surface of


31


the nanocrystals – high AA density would hinder the nanocrystals intergrowth which would increase the overall porosity. The high porosity of the obtained vaterite microparticles with specific aspect ratios is interesting for certain application where

(a)

3

5

Vaterite-AA000 Vaterite-AA053 Vaterite-AA160 Vaterite-AA267 Vaterite-AA400

4 3 2 1 0 0.00

dV/dlog(W) Pore Volume (cm /g)

morphology and porosity are important.

Quantity Adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 54

0.25 0.50 0.75 Relative Pressure (P/P0)

1.00

(b)

0.3

Vaterite-AA000 Vaterite-AA053 Vaterite-AA160 Vaterite-AA267 Vaterite-AA400

0.2

0.1

0.0 0

20 40 Pore Width (nm)

60

Figure 9. N2 adsorption-desorption isotherms of Vaterite-AAs (a) and the corresponding density functional theory (DFT) pore size distributions (b).

Table 1. The porosity characteristics of Vaterite-AAs

Sample

BET surface area

Peak pore sizea

Pore volumeb

(m2/g)

(nm)

(cm3/g)

Vaterite-AA000

15.9

34.3

0.06

Vaterite-AA053

31.9

9.3

0.08


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Vaterite-AA160

84.7

9.3

0.14

Vaterite-AA267

73.1

14.8

0.16

Vaterite-AA400

70.7

14.8

0.17

a, Pore size distribution was calculated by applying the DFT to the adsorption points using the N2 slit pore model. b, Pore volume was taken at the last adsorption point at around P/P0=0.98.

Based on the data collected in this study, it was possible to propose a formation mechanism for the Vaterite-AAs microparticles (Figure 10). In this proposed formation mechanism, ACC nanoparticles acted as precursors for the formation of individual vaterite nanocrystals. The addition of water to the ACC nanoparticles stabilized with AA in suspension in methanol induced the crystallization of these ACC nanoparticles to form vaterite nanocrystals. The shape of the resulting vaterite microparticles appeared to be affected by the amount of AA used in the synthesis as explained below: When the amount of AA was high with respect to the amount of ACC nanoparticles in suspension, ACC nanoparticles were likely to be highly covered by AA molecules (for the calculation of the coverage on the surface of ACC, see Figure S21) due to the strong interaction between AA and ACC as discussed earlier. The coverage of AA on the surface of ACC could also be adjusted by varying the amount of AA used. When the


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amount of AA on the surface of an ACC nanoparticle is sufficient and the AA molecules are close together, hydrogen bonds can form between the AA molecules adsorbed on the surface of the ACC nanoparticles. The hydrogen bonded AA molecules on the surface of an ACC nanoparticle could effectively shield the ACC nanoparticle from contact with water and retard the crystallization step. As water was added to the ACC suspension dropwise, crystallization of individual ACC nanoparticles occurred when it came into contact with water (i.e. crystallization took place inhomogeneously, only in parts of the ACC suspension as only a small amount of water was available). The crystallization of an ACC nanoparticle covered by AA may go through a partial dissolution-recrystallization process.44,

56, 61, 81

The partial dissolution-recrystallization

process in this case, happened in a confined space inside the “AA shield” created by the hydrogen-bonded AA molecules on the surface of an ACC nanoparticle. The appearance of this AA shell could be seen from the STEM images in the Supporting Information (Figure S22). The dissolution of ACC occurred when water diffused through the AA shield and came into contact with the surface of the ACC particle. This would cause a local dissolution of ACC and result in a saturated CaCO3 in solution within the


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AA shield. The crystallization of the dissolved CaCO3 to form vaterite would then take place spontaneously.82 Some researchers have reported that hollow vaterite structures can form by partial dissolution-recrystallization from ACC nanoparticles that have a size of hundreds of nanometer.56, 61, 83 Li et al.9 have reported that the vaterite nanocrystal formed from ACC nanoparticle (5 nm in size) appeared to be solid. We were not able to deduce the structure of the vaterite nanocrystal in this study, however, given the size of these nanocrystals ( 5 min). The vaterite formed at low amounts of AA grew into bigger and prolate spheroids nanocrystals (similar to Vaterite-AA000) which could be the result of the absence of an AA shield on the surface of the ACC nanoparticles. The formation of prolate spheroids vaterite nanocrystals and these nanocrystal aligned with its crystallographic c-axis along the longest dimension of the microparticle may also be related to the high energy of the (001) face due to the fact that only CO32– or Ca2+ ions exist in this face as earlier reported.57 Additionally, as reported by Wang el al.37 a dipole field could be established between two adjacent (001) faces along the c axis, which would result in an orientated assemble of crystals. In summary, the formation of vaterite microparticles from ACC nanoparticles was considered to take place via three possible steps (Step I, II, and III, described in Figure 10). I) The crystallization of individual ACC nanoparticles was initiated by dropwise addition of water into the ACC methanol suspension. ACC nanoparticles crystallized to form vaterite nanocrystal through a partial dissolution-recrystallization process. The newly formed vaterite nanocrystal aggregated into spherical particles simultaneously and facilitated the spherical growth of vaterite particles. II) With increased time, the remaining ACC nanoparticles crystallized into vaterite nanocrystals with the formation of more vaterite microparticle simultaneously. III) When all ACC crystallized to vaterite, the vaterite nanocrystal may undergo Ostwald ripening to increase its crystallinity.


37


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Figure 10. Scheme of the possible formation mechanism of vaterite microparticles with morphology from prolate spheroids to spherical microparticle. I-II) Mesoscale transformation of ACC nanoparticles to vaterite nanocrystal and spontaneous assembly to vaterite microparticles; III) possible Ostwald ripening of vaterite nanocrystals after formation of well-shaped vaterite microparticles

In order to confirm the formation mechanism of the Vaterite-AAs microparticles, we tested the synthesis of vaterite microparticles using other additives. Specifically, we replaced AA with 6A in the synthesis of vaterite microparticles. Since 6A has a similar structure to AA with functional groups that can form hydrogen bond, we found that the


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aspect ratios of the vaterite microparticles could also be regulated by varying the amount of 6A (Figure S23-24). The overall relationship between the amount of 6A and the aspect ratio of the microparticles was comparable to that established for AA in this study. BA was also tested as an additive for the synthesis of vaterite. According to Figure S25 and S26, BA could also be used to control the morphology of vaterite microparticles. The aspect ratios of the vaterite microparticles and the amount of BA showed more of a linear relationship when compared with AA and 6A. This was probably due to the molecular structure of BA, which has only one functional group available for hydrogen bonding (two hydrogen bonding functional groups are available on AA and 6A).

Additionally, we tested two other additives (CA and PAA) with multiple carboxylic groups in the synthesis of vaterite microparticles. Our results showed that the microparticle morphology could not be controlled by varying the amount of CA and PAA (Figure S27 and Figure S28). This was probably due to the ability of the multiple carboxylic groups on CA and PAA to interact with more than one ACC nanoparticle.


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When CA and PAA additives were used, ACC crystallized to form a mixture of different CaCO3 polymorphs.

CONCLUSION

Porous

vaterite

microparticles

with

controlled

morphology

were

successfully

synthesized by introducing water to an ACC suspension in methanol and using AA as an additive. The obtained vaterite microparticles have morphologies that vary between prolate spheroids and spherical microparticles. The aspect ratio of these microparticles could be finely adjusted by varying the amount of AA. The relationship between the aspect ratio of the vaterite microparticles and the amount of additive could be represented using a mathematical equation. Stirring speed and concentration of ACC suspension had no significant effect on the shape of the obtained vaterite microparticles. Electron diffraction revealed that the nanocrystals within each microparticle were aligned in a certain crystallographic orientation. A formation mechanism of the vaterite microparticles in this study was proposed based on our


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results - the ACC nanoparticles went through mesoscale transformation to form vaterite nanocrystals, and these nanocrystals spontaneously assembled together to form the vaterite microparticles. The coverage of additive AA on ACC nanoparticle played a critical role in accurate morphology control of vaterite microparticles. Apart from AA, other additives with similar molecular structure to AA could also be used to control the morphology of vaterite. In summary, we demonstrated that the particle morphology of vaterite could be finely controlled using organic additives. As the particle morphology has a noticeable effect on the performance of many functional materials (i.e. drug uptake by cells), this work opens up the possibility for tuning the functionality of various functional materials by morphology control. The morphology control method used in this study could potentially be used for morphology control of other systems from amorphous nanoparticles, such as amorphous calcium phosphate and amorphous magnesium carbonate. By controlling the coverage of additives on the surface of precursor nanoparticles, the crystallization process could be adjusted and, as a result, porous nanostructures may be acquired.


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ASSOCIATED CONTENT

Supporting Information. IR, powder XRD of Vaterite-AA000; Rietveld refinement of Vaterite-AA000, Vaterite-AA053 and Vaterite-AA160; the evolution of Vaterite-AA053; effect of stirring speed and concentration of ACC suspension on the formation of vaterite microparticles; calculation of the coverage of AA on ACC nanoparticles; the effect of other additives on the formation of vaterite microparticles (PDF) and cRED of Vaterite-AA053 (video).

AUTHOR INFORMATION

Corresponding Author *[email protected], *[email protected]

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The Swedish Research Council (grant # 2014-3929) and the Swedish Research Council for Sustainable Development (FOMAS, grant #2018-00651) are acknowledged for funding this work. Rui Sun thanks the China Scholarship Council (CSC) for financial support. Dr. Lars Riekehr of Uppsala University is acknowledged for his help with TEM measurement and valuable discussion. Jie Zhao of Uppsala University is acknowledged for drawing molecular structure. TW acknowledges a grant from the Swedish research council (VR, 2014-06948) as well as financial support from the Knut and Alice Wallenberg Foundation through the project grant 3DEM-NATUR (no. 2012.0112) for the electron microscopy experiments at Stockholm University.

ABBREVIATIONS ACC, amorphous calcium carbonate; HPACC, highly porous amorphous calcium carbonate; BET, Brunauer–Emmett–Teller; DFT, density functional theory; IR, infrared; XRD, X-ray diffraction; SEM, scanning electron microscope; TEM, transmission electron


43


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microscopy; HRTEM, high resolution TEM; ED, electron diffraction; cRED, continuous rotation electron diffraction; FFT, fast Fourier transform.

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“For Table of Contents Use Only” Mesoscale Transformation of Amorphous Calcium Carbonate to Porous Vaterite Microparticles with Morphology Control Rui Sun, Tom Willhammar, Erik Svensson Grape, Maria Strømme, and Ocean Cheung

Porous vaterite microparticles with controlled morphology have been synthesized from amorphous calcium carbonate nanoparticles through mesoscale transformation and self-assembly.


54

Mesoscale Transformation of Amorphous Calcium Carbonate to

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