Nano Res.
Electronic Supplementary Material
Anisotropic nanowire growth via a self-confined
amorphous template process: A reconsideration on
the role of amorphous calcium carbonate
Li-Bo Mao1, Lei Xue1, Denis Gebauer2, Lei Liu1, Xiao-Fang Yu1, Yang-Yi Liu1, Helmut Cölfen2, and
Shu-Hong Yu1 ()
1
Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale Collaborative Innovation
Center of Suzhou Nano Science and Technology, CAS Center for Excellence in Nanoscience, Department of Chemistry, University of
Science and Technology of China, Hefei 230026, China
2
Department of Chemistry, University of Konstanz, Universitätsstrasse 10, Box 714, D-78457, Konstanz, Germany
Supporting information to DOI 10.1007/s12274-016-1029-6
Figure S1 SEM images representing the effects of different factors while other factors are the same as the typical configuration: (a)
with no OAH; (b) with twelve OAHs; (c) in the absence of Mg2+; (d) in the presence of 6 mM of Mg2+; (e) in the absence of PAA; (f) in
the presence of 1 mM of PAA.
Note: When all air holes were closed, only a few PILP droplets deposited onto the substrate and most parts of
the substrate remained uncovered (Fig. S1(a)). As the number of open air holes (OAHs) increased to twelve, the
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Nano Res.
mineralization process became much faster (Fig. S1(b)). Most of the nanowires overgrew substantially, resulting
in a similar structure as that of the specimen with reaction time of 72 h and four OAHs (Fig. 1(f)). Hence, the
dissipation rate of CO2 acts as the speed limiter of the process to decelerate the overgrowth onto the side
surface of nanowires.
Both Mg2+ ions and PAA can greatly affect the precipitation behavior of CaCO3 (Figs. S1(c)–S1(f)). As the
concentration of Mg2+ increases, the size of the precipitated CaCO3 significantly decreases (from micrometersized blocks to nanometer-sized small particles). Besides, Mg2+ ions are known to be able to stabilize ACC and
restrain its mineralization process [S1]. Therefore the presence of Mg2+ ions provides small particles for the
initial seeds and stabilizes the ACC droplets. Moreover, Mg2+ ions are able to tune the wettability of ACC
droplets, which may be one reason of the anisotropic growth of the nanowires.
There is a strong interaction between Ca2+ ions and PAA, and this polymer is often used to stabilize ACC. In
addition, PAA chains could selectively adsorbed onto specific crystal faces of CaCO3 and facilitate the orientated
attachments of the CaCO3 small particles. In the absence of PAA, only sparse CaCO3 rhombohedrons are
obtained (Fig. S1(e)). As the concentration of PAA increases, angular CaCO3 micrometer-sized crystals are replaced
by a flat layer of CaCO3, because of the existence and precipitation of PAA-stabilized liquid-like ACC droplets.
When the concentration of PAA increases to a higher value, even the small particles would disappear and a
dense layer of CaCO3 then forms on the substrate (Fig. S1(f)).
Figure S2 The nanowires could also grow on (a) polylactic acid- and (b) amylose-coated substrates (other conditions were not changed).
Figure S3 Data plot of the pH value and logCCO2 versus reaction time, showing the number of OAHs could significantly change the
dissipation rate of CO2 and the rate of increase in pH.
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Figure S4 SEM images showing (a) cross section of a small particle layer prior to the growth of nanowires, (b) the seed-like particles,
(c) the abrupt emergence of plenty of nanowires and (d) overgrowth of the nanowires. (e) XRD patterns of the specimens shown in (a)
(pink) and (d) (blue). The specimens were obtained after (a) 24, (b) 30, (c) 36 and (d) 144 h, respectively.
Figure S5 XRD patterns of precipitates at different stages of the mineralization, showing the evolution of crystallinity of the precipitates.
Before 24 h, few precipitates were found on the substrate. The specimens were collected with chitosan substrates.
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Figure S6 HRTEM images of nanowires grown on (a) chitin and (b) collagen substrate (48 h). The measurement was carried out very
carefully to ensure that the direction of the incident electron beam (zone axis) was perpendicular to the nanowire’s growth direction
(Z axis). Three plane sets were demarcated in (a) to calculate its orientation.
Note: Here we take the nanowire in Fig. S6(a) as an example. The direction of the incident electron beam (written
in the form of a vector) could be calculated by the Weiss’ Zone Law
(u, v , w )  ( h1 , k1 , l1 )  ( h2 , k2 , l2 )  (0,0,6)  (1,0, 4)  (0,6,0)
_
The normal direction of (102) plane (written in the form of a vector) was

3a 2  
3a2 
(u, v , w )   2h  k , h  2 k , 2 l    2,1, 2  .
2c  
c 

The reference plane α (written in the form of a vector) which belonged to the [060] (the direction of the incident
_
 3 a 2 
electron beam) and  21 2  (the normal direction of (102) plane) zone axes could also be determined by the
c 

Zone Law


3a 2   18a 2
( h , k , l)  (u1 , v1 , w1 )  (u2 , v2 , w2 )  (0,6,0)   2,1, 2    2 ,0, 12 
c   c


The growth direction of the nanowire was the normal direction of reference plane α. It was given by (written in
the form of a vector)
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
3a 2   36 a2 18a 2 18 a2 
(u , v , w )   2 h  k , h  2 k , 2 l    2 , 2 , 2  .
2c   c
c
c 

___
_
_
This growth direction was actually the [211] direction (labeled to be [1011] in the four-index scheme).
The growth direction of the nanowire in (b) as well as that of other nanowires could be calculated alike.
Figure S7 HRTEM images of the initial nanowire grown on chitin. The growth direction of this nanowire can be calculated in the
same way as the nanowires in Fig. S6.
Figure S8 Linear fitting of d-spacing of calcite {104} planes versus the molar ratio of Mg to the sum of Mg and Ca. The data for
plotting were retrieved from JCPDS-ICDD (card No. 83-1762, 89-1304, 89-1305, 71-1663 and 86-2336).
Note: The result shown in Fig. S8 could give the molar ratio of Mg to Ca of a sample with specific d-spacing [S2].
As the d-spacing of {104} planes could be directly measured from the HRTEM image to be 3.010 Å, x was
calculated to be
x
3.03484  3.010
 0.08592
0.28911
and the molar ratio of Mg to Ca was
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0.08592 : (1  0.08592)  1 : 10.64
This result excluded the adsorbed Mg2+ ions, while EDS data calculated total Mg content. Therefore the percentage
of molar ratio of doped Mg to total Mg content was estimated to be
10.17
 100%  95.6% ,
10.64
which meant almost all Mg2+ ions were doped rather than adsorbed.
Figure S9 HRTEM images of the transformation (from (a) to (b)) of the amorphous layer in a nanowire (42 h). Marked areas were the
crystallized regions in the layer which were induced by the intense electron beam (200 kV, 60 s).
Note: After being exposed to the intense electron beam of HRTEM for 60 s, the ACC layer started to crystallize
into calcite (the white circles). This transformation process proved that the amorphous layer was amorphous
calcium carbonate. In addition, we did not observed the destruction of the layer in the HRTEM test, so the layer
should not be an organic layer, i. e. a low-molecular-weight PAA layer (this layer would shrink or be destroyed
by the electron beam otherwise).
Figure S10 Enlarged view of the immature nanowire shown in Fig. 4 (42 h). The black arrow in each image indicates the growth
direction of the nanowire. The tip area was composed of polycrystalline and amorphous domains and an amorphous periphery was
absent on any surfaces of this area. The amorphous and polycrystalline domains in the interior part decreased while the thickness of
amorphous shell increased from (a) to (d). It could be found that some crystalline domains appeared in the amorphous shell in (b) and
(c), but in (d) the shell layer became purely amorphous.
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Figure S11 SEM images of the 48 h specimen at a low CO2 dissipation rate, showing the fusion of liquid droplets.
Note: We did not directly observe that the ACC nanodroplets was liquid (such as the distortion under external
force and the flow inside the particle). However, enough evidences could be found to prove that the nanoparticles
were liquid-like (Fig. 2—the spherical-shaped nanoparticles, Fig. S10(a)—the smooth tip, Fig. S11 and Movie
S1—the fusing droplets).
Figure S12 (a) HILIC spectra of specimens containing 10 mM CaCl2 and 2 mM MgCl2 (black line) and 10 mM CaCl2, 2 mM MgCl2
and 0.2 mM PAA (red line), respectively; the peaks appeared at 1 min was assigned to small inorganic ions (including Ca2+, Mg2+ and
Cl– ions) and the peak appeared at a retention time of 4.875 min was assigned to PAA. (b) HILIC spectra of specimens obtained after
different reaction times, showing the decreasing peaks of PAA. (c) Time-resolved profile of PAA peak areas calculated by integration. It
should be noticed there were two rapid declines, where the first decline might due to the formation of amorphous nanodroplets, and the
second one might due to the emergence and fast growth of nanowires.
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Movie S1 The fusion of liquid-like droplets under optical microscope. The solution was colored with
Rhodamine 6G, a positively charged pigment, to improve the visibility and to neutralize the surface charge of
the nanodroplets to accelerate their fusion.
Table S1 ICP-OES data showing the concentration variation of Ca and Mg before and after the mineralization with a typical
configuration
Time (h)
Ca (mM)
Mg (mM)
0
10.43
2.01
48
7.72
1.98
72
6.35
1.96
Table S2 Zeta-potential data of the solution
Time (h)
0
12
24
36
48
60
72
Zeta-potential (mV)
0.20 ± 0.06
–9.53 ± 0.27
–9.06 ± 0.15
–8.23 ± 0.31
–8.12 ± 0.22
–6.17 ± 0.24
–6.04 ± 0.11
References
[S1] Nishino, Y.; Oaki, Y.; Imai, H. Magnesium-mediated nanocrystalline mosaics of calcite. Cryst. Growth Des. 2009, 9, 223–226.
[S2] Celotti, G.; Nobili, D.; Ostoja, P. Lattice parameter study of silicon uniformly doped with boron and phosphorus. J. Mater. Sci. 1974,
9, 821–828.
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