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Controlled crystallization of hierarchical and porous calcium carbonate

crystals using polypeptide type block copolymer as crystal growth modifier in
a mixed solution

Article  in  CrystEngComm · March 2011

DOI: 10.1039/C0CE00202J

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Controlled crystallization of hierarchical and porous calcium carbonate
crystals using polypeptide type block copolymer as crystal growth modifier in
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a mixed solution†
Xiaohui Guo,*ab Lei Liu,a Wanv Wang,b Ji Zhang,b Yaoyu Wangb and Shu-Hong Yu*a
Received 12th May 2010, Accepted 22nd November 2010
DOI: 10.1039/c0ce00202j

Various kinds of nearly spherical calcium carbonate (CaCO3) crystals with hierarchical and porous
structures can be prepared using poly(ethylene glycol)-b-poly(aspartic acid) (PEG-b-pAsp) as a crystal
growth modifier in a mixed solvent composed of N,N-dimethylformamide (DMF) and cyclohexanol.
The results reveal that the porosity or specific surface area of these CaCO3 crystals can be tuned by
altering the volume ratio (R) of DMF/cyclohexanol in solution, and the pore size of the obtained
spherical particles can be ranged from several tens to hundreds of nanometres. Additionally, most of
the obtained calcium carbonate samples can be assigned to vaterite or a mixture of calcite and vaterite,
which are well crystalline and are influenced by the R value. Interestingly, unique hierarchical and
porous microspheres can be prepared at polymer concentrations of  0.5 g L1 and an R value of  1.0,
respectively. It has been proposed that the formation of the specific CaCO3 crystals with hierarchical
and porous structures could be ascribed to the collodial aggregation transition and self-assembly of
calcium carbonate precursor in a desirable mixed solvent. This specific synthesis strategy in a mixed
solvent again emphasizes that it is possible to synthesize other inorganic/organic hybrid materials with
exquisite morphology and specific textures.

1. Introduction or additives has been intensively explored to a remarkable degree

in recent years, since CaCO3 is not only one of the most abun-
In the past few decades, bio-inspired morphogenesis strategies, dant mineral materials in nature, it also has many potential
using self-organized organic molecule superstructures, organic industrial applications as a filler in paints, plastics, rubber and
additives, and/or other templates with specific functionalization paper.6 In addition, Langmuir monolayers,7 self-assembled
patterns or scaffolds, to prepare inorganic or inorganic/organic monomolecular (SAM) films,8 reverse microemulsion or complex
hybrid materials with well-defined and hierarchical morphology micelle,9 and double hydrophilic block copolymers (DHBCs)10,11
and special architecture forms, have attracted considerable have also acted as effective tools or templates to mediate the
attention.1,2 controlled morphology and polymorphs of CaCO3 crystals.
Recently, morphology-controlled synthesis of CaCO3 crystals Previously, it has been demonstrated that DHBCs can exert
has been achieved in the presence of different soluble additives significant influence over the morphogenesis and polymorphs of
and a variety of organized assemblies or soft templates.3,4 Much calcium carbonate during the biomineralization process. In
effort has been devoted to the preparation of carbonate minerals recent years, our group has extended the mineralization reaction
with specific morphology and modification.5 Importantly, in a mixed solvent system, for example, mineralization of highly
inspired by nature mimetic concepts, a novel biomimetic monodisperse vaterite microspheres in a mixed solvent of N, N-
synthesis of CaCO3 crystals in the presence of organic templates dimethylformamide (DMF) and water under control of an
artificial double hydrophilic block copolymer called poly-
a
Division of Nanomaterials & Chemistry, Hefei National Laboratory for (ethylene glycol)-b-poly(L-glutamic acid).12
Physical Sciences at Microscale, School of Chemistry and Materials, Porous calcium carbonate structures have attracted increased
University of Science and Technology of China, Hefei, 230026, P. R.
interest due to their extensive applications as catalyst supports,
China. E-mail: shyu@ustc.edu.cn; guoxh2009@nwu.edu.cn; Fax: + 86
551-3603040 fillers, as well as novel biomimetic scaffolds for target drug
b
Key Lab of Synthetic and Natural Functional Molecule Chemistry of delivery and tissue engineering,13 in which, there have been few
Ministry of Education, The School of Chemistry & Materials Science, reports on the fabrication of porous CaCO3 superstructures until
Northwest University, Xi’an, 710069, P. R. China
now. For example, micro-patterned calcite single crystals can be
† Electronic supplementary information (ESI) available: SEM images,
XRD patterns, FT-IR spectra, TEM images and SAED pattern, and produced through amorphous-to-crystalline transitions on
optical images. See DOI: 10.1039/c0ce00202j micro-patterned templates;14 porous single crystals of hexagonal

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 View Article Online

vaterite prisms were easily obtained via a gelatin-mediated into 10 ml NIW or DMF/cyclohexanol mixture solution con-
nanocrystal aggregation approach.15 Moreover, large calcite tained in the glass bottle, with consequent continuous stirring the
single crystals with specific complex pore structures were mixture dissolved completely so as to contain 1g L1 PEG-b-
prepared by templating sea urchin spines.16 Calcite single crystals pAsp in DMF/cyclohexanol mixture solution. After that,
with porous structures can be obtained by using colloidal spheres a desirable amount of calcium chloride (0.5 ml, 0.1M) in aqueous
as soft-templates.17 solution was quickly added into glass bottles containing 5 ml of
Recently, three-dimensionally ordered macroporous calcite PEG-b-pAsp solution under vigorous stirring by using
materials have been readily prepared by means of infiltration of a magnetic stirrer. The bottle was then covered with Parafilm,
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precursors into the interstices of preformed colloidal crystal which was punched with 3 needle holes, and placed in a larger
templates followed by removal of the templates with solvent desiccator. Carbon dioxide was introduced by using three small
extraction or calcination.18 Similarly, porous silica particles have glass bottles (10 ml) of crushed ammonium carbonate, which
been fabricated by means of cetyltrimethylammonium bromide were covered with Parafilm punched with four needle holes and
(CTAB) as a directed-template in a sol–gel process.19 placed at the bottom of the desiccator. The precipitates were
As far as we know, crystallization of calcium carbonate under collected at different time intervals and washed with NIW and
control of DHBCs is mainly carried out in nonionic water dried in air for further characterization. Herein, the concentra-
(NIW).10 In recent years, several research groups have occa- tion of PEG-b-pAsp was varied from 2.0 to 0.25 g L1, the
sionally focused on the use of different solvent media to control concentration of calcium ion was varied from 10 to 50 mM, and
the crystal growth of calcium carbonate and other the crystallization reaction was carried out at ambient tempera-
compounds.12,20 Although CaCO3 crystals with different ture. For the as-synthesized calcium carbonate samples with
morphologies such as solid spheres, flower-like, flake-like, and different R values, such as R ¼ 0, 0.2, 0.5, 1, 5, 10. The corre-
shuttle-like, and even inhomogeneous complex aggregated sponding amounts of DMF and cyclohexanol added in the
structures can be obtained in the presence or absence of reaction solution can also be seen in Table 1.
DHBCs.11,12 So far, controlled synthesis of unique calcium
carbonate crystals with variable surface porous textures in mixed
2.3 Characterization
solvent remains a challenge.
In this study, we report that a series of porous CaCO3 spherical The small pieces of cover slips were examined by sputtering with
aggregates with hierarchical porous surface textures can be gold for scanning electron microscopy (SEM) on a BYBY-1010A
synthesized by using poly(ethylene glycol)-b-poly(aspartic acid)21 microscope and field emission scanning electron microscopy
as crystal growth modifier in a mixed solvent composed of N,N- (FE-SEM) on a JSM-6700F microscope. The structures of the
dimethylformamide (DMF) and cyclohexanol by the aid of samples were characterized by X-ray diffraction (XRD) pattern,
a slow gas–liquid diffusion reaction at room temperature. Several recorded on a (Philips X’Pert Pro Super) X-ray Powder diffrac-
kinds of CaCO3 samples with variable porosity or pore sizes can tometer with Cu-Ka radiation (l ¼ 1.541874 A). FT-IR spectra
be obtained by tuning the volume ratio (R) of DMF to cyclo- were recorded on a Bruker EQUINOX-55 infrared spectropho-
hexanol and the concentration of reagents. The results demon- tometer on KBr pellets. Thermogravimetry analysis (TGA,
strated that varying micelle based aggregated structures of Entzsch-Sta 449) was used to measure the weight percentage of
polypeptide-type polymer formed at the interface of the micro- the polymer components included in samples. Transmission
emulsion drop can prominently regulate the aggregation and electron microscope (TEM) and selective area electronic
self-assembly process of calcium carbonate precursor formed in diffraction (SAED) were performed on a Hitachi (Tokyo, Japan)
mixed solvent. H-800 transmission electron microscope at an accelerating
voltage of 200 kV. For TEM observation of the samples formed
2. Experimental section at an early stage, copper grids were directly placed in the
mineralization solution in the desiccator, then the copper grid
2.1 Materials taken out from the reaction solution at different time intervals,
Ammonium carbonate and CaCl2 were used as received. A block and washed with NIW and ethanol, respectively, the sample
copolymer containing a poly(ethylene glycol)-b-poly(aspartic deposited on the copper grid for TEM observation. High-reso-
acid) (PEG (110)-b-pAsp(10)) (PEG ¼ 5000 g mol1, pAsp ¼ lution transmission electron microscopy (HRTEM) performed
1000 g mol1), namely PEG-b-pAsp was synthesized as described on a JEOL-2011 HRTEM at an accelerating voltage of 200 kV.
elsewhere.21b the polymer was purified by exhaustive before use in Nitrogen sorption data were obtained with a Micromeritics Tri-
the crystallization of calcium carbonate. N,N-dimethylforma- star 3000 automated gas adsorption analyzer. Isotherms were
mide (DMF) and cyclohexanol were obtained from Shanghai evaluated with the Barrett–Joyner–Halenda (BJH) theory to give
Chemical Reagent Company; NIW was obtained with 18.2 U the pore parameters, including specific surface areas, pore
cm1 from Millipore simplicity 185 type. All chemicals were used volume, and pore size distribution. The structure of the polymer
without further purification. solution was detected by laser light scattering (LLS) on ALV-
5000E with He–Ne laser (l0 ¼ 632 nm) as the source at 298 K.
The polymer solution was filtered through a 0.5 mm Milipore
2.2 Mineralization of calcium carbonate
Millex-LCR filter to remove dust before the LLS experiments.
The mineralization experiments were carried out as described by The average hydrodynamic radius of the polymer micelles is
Addadi et al.22 In a typical procedure, a stock aqueous solution <Rh> ¼ 10 nm and a hydrodynamic radius distribution f (Rh)
of CaCl2 was freshly prepared in NIW. 5 mg polymer was added which can be calculated by using the Stokes–Einstein equation:

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Table 1 Summary of morphology, polymorphs, and structures of the CaCO3 crystals prepared under different experimental conditions.a

No. Polymer concentration (g L1) R* Polymorph Morphology Porosity

1 1.0 0.2 V coral reef- like higher

2 1.0 0.5 V walnut-shaped mezzo
3 1.0 1.0 C+V irregular spherical aggregates mezzo
4 1.0 5.0 A*+C spherical aggregates high
5 1.0 10 Amorphous twinned bread-shaped high
6 0.25 1.0 V flower-like aggregates lower
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7 0.5 1.0 V+C hierarchical porous sphere higher

8 2.0 1.0 V spherical aggregates high
a
Herein, R represents the volume ratio of DMF to cyclohexanol; A, C, and V denoted aragonite, calcite, and vaterite polymorphs, respectively. A*
denoted the presence of a trace amount of aragonite in the case of R  5.

Rh ¼ (kBT/6ph) D1, where T and h are the Boltzmann constant, calcium carbonate crystals, which are analogical to walnut-sha-
the absolute temperature and the solvent viscosity, respectively. ped porous structures, can be obtained (ESI, Fig. S1†). Addi-
tionally, the whole walnut-shaped structure can be formed by
means of closely stacking of flakes building one another. The as-
3. Results and discussion obtained calcium carbonate samples are about 5 mm in diameter.
When the R value increased to 1, then, a kind of layered
3.1 Morphogenesis of CaCO3 crystals in the presence of PEG-
irregular spherical calcium carbonate particles with a high yield
b-pAsp in mixed solution
can be obtained (Fig. 3a). The mean diameter of the particles was
Calcium carbonate crystals with a cubic box-shaped structure around several mm (Fig. 3b). It seems that the large spherical
can be obtained in aqueous solution and in the presence of PEG- aggregates were formed through a closely inter-crossing align-
b-pAsp (Fig. 1). Some obvious defects, edges and/or corners ment of many thick flakes on one another (Fig. 3c, d), The mean
located at crystal surfaces, together with slightly shallow grooves thickness of the individual flake is about 60 nm, a specially, so
distributed on the interface between two crystal faces were called flake-like structure can be formed through oriented
observed (Fig. 1b, c), the mean size magnitude of calcium attachment of numerous smaller nanoparticles (Fig. 3e). While
carbonate crystals was more than 100 mm in length (Fig. 1b). the volume content of DMF in a mixed solvent further increased,
While other experimental conditions were kept constant, only CaCO3 spherical particles and a small amount of semi-spherical
changing the volume ratios (R) of DMF/cyclohexanol in solu- particles with an obvious cavity at the centre were found in the
tion, several different calcium carbonate crystals can be case of R  5 (Fig. 4). The obtained spherical particles are
obtained. For convenience, the samples synthesized at R values approximately several micrometres in diameter (Fig. 4b). Inter-
of 0, 0.2, 0.5, 1, 5, and 10 were referred to as S0, S1, S2, S3, S4, estingly, it was observed that each spherical particle was
and S5 respectively. composed of numerous flake-layered structures (Fig. 4c), and
In the case of R  0.2, the as-made calcium carbonate crystals also the obvious porosity was observed among these flakes. The
almost exhibit multiple layered porous hierarchical structures flake-like structure is also composed of numerous nanoparticles
(Fig. 2). Clearly, the as-made multiple porous sample possess with smaller sizes (Fig. 4d).
higher specific surface area or porosity from Fig. 2c. In addition, However, a kind of calcium carbonate crystals with twinned
it was found that the hierarchical porous structures formed by bread-shaped structure can be obtained when the R value is up to
the ordered stacking of many similar multiple-porous flakes
along parallel orientations (Fig. 2b, c), which is also similar to
a coral reef-like structure. However, the porous layered structure
was actually composed of numerous particles ranging from
several tens to hundreds nm (Fig. 2d). In addition, it is clearly
observed that these building blocks are randomly distributed in
size and shape. Further increasing the R value to 0.5, a kind of

Fig. 1 SEM images of calcium carbonate crystals obtained in aqueous Fig. 2 SEM images of calcium carbonate with layered multiple-porous
solution. The initial concentration of calcium chloride and polymer were structures prepared when the R value is 0.2. The initial concentration of
10 mM and 1 g L1, respectively. calcium chloride and polymer were 10 mM and 1 g L1, respectively.

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 View Article Online

calcium carbonate is very rough (Fig. 5c,d). Based on the above

results, we infer that different nucleation modes and consequent
overgrowth processes mediated by synergetic effect between the
mixed solvent and PEG-b-pAsp contribute to the formation of
distinct morphologies and surface textures of CaCO3. Addi-
tionally, if only using cyclohexanol as a mineralization media,
a class of multilayered cakes with rather rough surfaces can be
formed (ESI, Fig. S2†). These cake-like crystals can even
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undergo further self-aggregating one another to form their cross

cake-like complex. The diameter and thickness of crystals are
about 6–8 mm and 4–6 mm, respectively.
When pure NIW is used as a reaction solvent, the obtained
calcium carbonate sample can be readily indexed as single-crys-
talline calcite polymorph (Fig. 6a) (JCPDS Card No. 86-0174).
Fig. 3 SEM images with different magnification of spherical-shaped With R value increasing, namely, R values equal to 0.2 and 0.5,
CaCO3 complex aggregates formed at R  1 and in the presence of respectively, both samples can be easily indexed to pure vaterite
polymer (1.0 g L1). The initial concentration of calcium chloride was (Fig. 6b,c) (JCPDS Card No. 33-0268). Whereas, when R
10 mM, and the samples were formed by crystallization for 7 days.
changed to 1.0, it was found that both the as-made calcium
carbonate samples can be indexed to a mixture of vaterite and
calcite (ESI, Fig. S3a†). The as-obtained sample can be indexed
as a mixture of calcite and a trace amount of aragonite in the case
of R  5. However, the sample prepared in the case of R 10 is
amorphous calcium carbonate (Fig. 6e), indicating that the
addition of DMF and cyclohexanol into the reaction system can
obviously mediate the polymorphs of calcium carbonate.
Meanwhile, the above phase discrimination for samples was
further determined by FT-IR spectroscopy. For example, the
sample prepared in the case of R  5 is a mixture of calcite and
aragonite phase, which is confirmed by the presence of 862 cm1
(n2 mode), and 1078 cm1 (n1 mode) CO32 absorption bands for
typical aragonite phase, the band at 1425 cm1 was characteristic
of the calcite phase (ESI, Fig. S4†). Moreover, it was easily found
that the polymer is indeed occluded into the synthesized sample
Fig. 4 SEM images of calcium carbonate sphere-shaped complex
aggregates formed in the presence of polymer (1.0 g L1) at R value according to the FT-IR result. This is well consistent with the
of  5. The initial concentration of calcium chloride was 10 mM, the result from the XRD pattern in Fig. 6d. The TGA result of the
samples were formed by crystallization for 7 days. as-prepared sample was shown in Fig. 7, from which it can be
concluded that the sample was a polymer-rich CaCO3–organic
composite. The weight loss 14.3 wt% below 328  C corresponded
10 (Fig. 5). Fig. 5b shows that the two twinned breads closely to the loss of surface and bound water, while that between 328  C
contact with each other by means of the face-to-face mode, and and 490  C can be attributed to the loss of incorporated PEG-b-
also, there are some defects or grooves on the surface of the pAsp. It can be evaluated from the TGA curve (Fig. 7) that the
bread-shaped structure. The whole surface of the bread-shaped

Fig. 5 SEM images of calcium carbonate particles with twinned bread- Fig. 6 XRD patterns of calcium carbonate samples prepared at different
shaped structure formed in the presence of polymer (1.0 g L1) at R value R values and in the presence of polymer (1 g L1), a) pure NIW; b) 0.2; c)
of  10; the initial concentration of calcium chloride was 10 mM, the 0.5; d) 5; e) 10, herein, A denoted aragonite (JCPDS: 41-1475), C denoted
samples were formed by crystallization for 7 days. calcite (JCPDS: 86-0174), V denoted vaterite (JCPDS: 33-0268).

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Fig. 7 TGA curve of the CaCO3 sample prepared in the presence of

PEG-b-pAsp and the R of  5. Fig. 9 SEM images of the flower-shaped calcium carbonate particles
formed in the presence of polymer (0.25 g L1) at R values of  1 after
mineralization for 7 days at room temperature. The initial concentration
of calcium chloride was 10 mM.

Fig. 8 N2 adsorption–desorption isotherms of the obtained CaCO3

sample in the case of R 5 (left up-corner inset is the corresponding pore
size distribution curves).

Fig. 10 SEM images of the porous calcium carbonate spheres formed in

weight percentage of polymer was around 8.8 wt%. The CaCO3
the presence of polymer (0.5 g L1) at R values of  1 after mineralization
sample formed in the case of R 5 behaves with a well-defined for 7 days at room temperature. The initial concentration of calcium
porous nature based on the result depicted above. The BET chloride was 10 mM.
measurement (Fig. 8) shows that the surface area and the pore
diameter are 45.38 m2 g1 and 47 nm, respectively, implying that
the as-prepared sample is of mesoporous feature. show polycrystalline features (ESI, Fig. S6†). Clearly, the
obtained porous calcium carbonate sphere is very analogous to
the porous calcite sphere formed by a polymer colloidal template
3.2 The effect of polymer concentration reported previously.23
When polymer concentration was increased to 2.0 g L1, a kind
It is well known that variation of polymer concentrations can
of complex spherical calcium carbonate aggregate can be easily
also have a significant influence on the growth process and shape
formed (Fig. 11a, b), each sphere is composed of flakes with
evolution in the crystallization process of calcium carbonate
a cross-linked mode (Fig. 11c). Every individual flake is around
crystals.10,11 Similar flower-like aggregates of calcium carbonate
200 nm in thickness. Interestingly, if only increasing the calcium
crystals can be formed at R  1 (Fig. 9a–c), each leaf of larger
aggregates actually consisted of many irregular thick slabs
(Fig. 9c,d). Well-defined hierarchical porous spherical particles
can be observed in the case of polymer concentrations of 0.5 g
L1 (Fig. 10). Wherein, the larger porous CaCO3 spheres were
composed of many nanoparticles, the size of the preliminary
building particles is  20 nm, and the nanoparticles are poly-
crystalline as confirmed by the electron diffraction pattern (ESI,
Fig. S5†). From Fig. 10d, the pore size of the larger pore on the Fig. 11 SEM images of spherical CaCO3 complex aggregates formed in
surface of the sample was around 100–200 nm and the pore size the presence of polymer (2.0 g L1) at R value of  1 after mineralization
of the smaller pore is about several tens of nm. Additionally, the for 7 days at room temperature. The initial concentration of calcium
obtained calcium carbonate particles at different R values also chloride was 10 mM.

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between DMF and cyclohexanol,24 as shown in Fig. 13a, the

micro-emulsion liquid-drop can be firstly observed at the DMF/
cyclohexanol solution surface or corner (ESI, Fig. S7†). Addi-
tionally, the calcium ion can undergo diffusion and aggregation
around the polymer micelles to form a higher calcium ion
concentrated area due to the binding effect between the calcium
ion and the –COOH group from PEG-b-pAsp, resulting in the
formation of complex colloidal aggregates at the interface of O/
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W micro-emulsion (Fig. 13b, c).

Remarkably, the polymer micelle formation was directly evi-
denced by the specific Tyndall phenomena occurring in polymer
solution in the absence of Ca2+ (Fig. 14a), and also, the size
distribution of the polymer micelle formed can be evaluated by
means of dynamic light scattering (DLS) measurement, the
average size of the polymer micelles was  10 nm (Fig. 14b).
Fig. 12 SEM images of solid calcium carbonate complex microspheres Herein, we proposed that polymer aggregates sequester Ca2+
formed in the presence of polymer (0.5 g L1) at R value of  1 after ions and thus serve as localized nucleation centers due to the
mineralization for 7 days at room temperature. The initial concentration higher local concentration of Ca2+ ions after the addition of
of calcium chloride was 50 mM. a source of CO32 ions, which is consistent with previous
observation during hydroxyapatite whiskers growth in the
presence of aggregates of PEO-b-PMAA-C12 (PEO: poly-
ion concentration to 50 mM, with other conditions constant,
(ethylene oxide), PMAA: poly(methacrylic acid)).25 The forma-
a class of unique monodisperse solid CaCO3 microspheres can be
tion of polymer micelles is also confirmed from the case of R  1
obtained in the presence of a polymer concentration of  0.5 g
(Fig. 15a). With reaction proceeding, the colloidal aggregates
L1 (Fig. 12). It was found that the average diameter of the
containing calcium ion undergo heterogeneous nucleation and
spheres is about 5 mm (Fig. 12c). The results demonstrated that
crystallization to form CaCO3 preliminary nanoparticles at an
varying polymer concentrations can have a significant influence
on the early nucleation and self-organization mode of prelimi-
nary particles in a mixed solvent, which can further regulate the
nucleation and crystallization mode of calcium carbonate caused
by the specific inhibition actions between the function groups of
polymer including the amine group and the carboxyl group and
the calcium carbonate crystal faces.12
The XRD patterns of the as-prepared samples in the presence
of polymer with different concentration confirmed that the
calcium carbonate samples can be indexed as pure vaterite or
a mixture of vaterite and calcite (ESI, Fig. S3†) and the samples
are well crystalline if the polymer concentrations are 0.25 and
2.0 g L1, respectively. If the polymer concentrations are 0.5 and
1.0 g L1 respectively, the products are a mixture of vaterite
and a small amount of calcite (JCPDS Card No. 33-0268 and 86-
0174) (ESI, Fig. S5†).
Table 1 shows a list of the summary of the morphology and
structural features of the CaCO3 samples obtained under Fig. 13 Schematic illustration of the possible formation process of
different conditions. From Table 1, it is obviously seen that CaCO3 samples with hierarchical and porous surface textures formed at
calcium carbonate crystals obtained under various experimental different R values.
conditions, including various R values and polymer concentra-
tions can play a key role in mediating the morphologies, poly-
morphs, and structures of the porous CaCO3 crystals.24

3.3 Possible formation mechanism of the porous CaCO3

crystals formed at different R values
A possible formation mechanism for the porous calcium
carbonate particles formed at various R values, i.e., colloidal
aggregates based transition and self-assembly mechanism, has Fig. 14 (a) The optical photograph of the micelle formation of polymer
been proposed. Firstly, polymer and calcium precursor solution in the case of R  1. (b) the hydrodynamic radius distribution of the
can form an analogous O/W micro-emulsion system in DMF/ polymer solution in the case of R  1. No calcium ion was added into the
cyclohexanol solution that results form the solubility difference polymer solution and the polymer concentration is 1.0 g L1.

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that of R  1, then, the flake units contain relatively little poly-

mer colloidal component, and hence, the self-assembly of these
flakes may not fully perform the organization into spherical
aggregates, herein, the spherical aggregates can be confirmed
from the evidence obtained at an early reaction stage in the case
of R  1, (ESI, Fig. S8†), therefore, resulting in the formation of
similar porous spherical aggregates with a cavity at the centre of
the sphere or semi-spherical aggregates (Fig. 13k,l). Additionally,
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the flake-shaped unit is a single-crystalline structure, as evi-

denced by Fig. 15d. Based on the discussion for the porous
CaCO3 sample, we therefore consider that the colloidal aggre-
gation transition behaviour governed by the micro-emulsion
liquid drop can induce the formation of hierarchical and porous
CaCO3 structures. Of course, Ostwald ripening process may
favor the formation of calcium carbonate with distinct porous
surface textures in the intermediate reaction stages of CaCO3
mineralization.

4. Conclusions
Fig. 15 TEM images and HRTEM images of calcium carbonate crystals In summary, we have synthesized hierarchical porous calcium
obtained in the presence of polymer (1 g L1) at an R value of  1. Herein, carbonate crystals using PEG-b-pAsp as a crystal growth
[Ca2+] ¼ 10 mM, the samples were prepared by crystallization for 7 days. modifier in a DMF/cyclohexanol mixed solution. A series of
(a) TEM image of similar spherical aggregates formed at the early reac- calcium carbonate samples with distinct porous surface textures
tion stage. (b) TEM image of individual thick flake-like complex aggre- can be prepared by mediating the experimental reaction regimes.
gate after ultrasonic treatment. (c) TEM image of individual flake-like The results demonstrated that selecting suitable R values and
structure with corners. Inset shows the corresponding ED pattern of the polymer concentrations can result in the formation of unique
sample shown in (c). (d) HRTEM image of a single flake-like structure.
hierarchical and porous spherical aggregates. It is proposed that
the kinetic regimes, the polymer micelle aggregated structures
formed14 and solvent effect can exert significant influence on
early stage (Fig. 13d). The calcium carbonate preliminary controlling the crystallization process of calcium carbonate. A
nanoparticles are composed of many polymer molecules and colloidal aggregation transition and self-assembly mechanism for
behave specifically as colloidal in nature in the present system the formation of such hierarchical and porous structures has
according to a previous report,12 thus, the preliminary colloidal been proposed. The synergistic effects between peptide with
particles can undergo further preferable self-assembly to form variable secondary structures in a desirable mixed solvent can
flake or rod-shape aggregated structures resulting from the facilitate the formation of porous structures. This synthetic route
reduction of the total surface energy (Fig. 13e,f).19 When the may provide a facile and feasible strategy for preparation of
system includes much more cyclohexanol molecules compared to other mineral composites with specific porosity and unique
DMF, then, more micro-emulsion drops can be stably main- structure features.
tained in the reaction system, which leads to the formation of
numerous colloidal calcium carbonate particles and drives them Acknowledgements
into forming layered stacking aggregated structures (Fig. 13g).
Obviously, the self-assembly of rods into flakes can also be This work is supported by the funding support from the National
observed in Fig. 15b,c. With further mineralization, the so-called Basic Research Program of China (2010CB934700), the National
layered stacking structure can undergo further overgrowth and Science Foundation of China (NSFC) (Nos. 91022032,
be followed by destruction of the micro-emulsion drop to form 50732006). X. H. Guo thanks the NSFC (No. 21001087), the
a novel multiple layered porous CaCO3 structure (Fig. 13h), Education committee of Shanxi Province (Grant No. 09JS089)
which is very similar to gingili staff-like rods formed through for financial support, the Research startup Fund of Northwest
a parallel orientation growth at the solution–air interface.11a University of China (No. PR09047), and the National Basic
In the same way, with the R value further increasing, the Science Cultivate Fund of China (No. J0830417).
number or concentration of the colloidal calcium carbonate
particles reduces a lot accordingly, and thus, these preformed Notes and references
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