Preparation and formation mechanism of three-dimensionally ordered macroporous (3DOM) MgO, MgSO4, CaCO3, and SrCO 3, and photonic stop band properties of 3DOM CaCO3

Article abstract of DOI:10.1016/j.jssc.2011.06.028

Three-dimensionally ordered macroporous (3DOM) magnesium (Mg) oxide (MgO), MgSO₄, calcium (Ca) carbonate (CaCO₃), and strontium (Sr) carbonate (SrCO₃) were prepared using a colloidal crystal of polymer spheres as a template. Ethanol or ethanol-water solution of metal salts (acetate or nitrate) and citric acid was infiltrated into the void of the colloidal crystal template of a monodispersed poly(methyl methacrylate) (PMMA) sphere. Heating of this PMMA-metal salt-citric acid composite produced the desired well-ordered 3DOM materials with a high pore fraction, which was confirmed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ultraviolet-visible (UV-vis) diffuse reflectance spectra. The presence of citric acid is crucial for production of the 3DOM structures. Reaction of citric acid with metal salt produces metal citrate solid in the void of PMMA spheres, which is necessary to maintain the 3DOM structure during the calcination process. 3DOM CaCO₃ shows opalescent colors because of it's photonic stop band properties.

Full text of DOI:10.1016/j.jssc.2011.06.028

Journal of Solid State Chemistry 184 (2011) 2299–2305
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Preparation and formation mechanism of three-dimensionally ordered
macroporous (3DOM) MgO, MgSO4, CaCO3, and SrCO3, and photonic stop
band properties of 3DOM CaCO3
nn
Masahiro Sadakane a,n, Rika Kato b, Toru Murayama b, Wataru Ueda b,
a Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan
b Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Three-dimensionally ordered macroporous (3DOM) magnesium (Mg) oxide (MgO), MgSO4, calcium (Ca)
carbonate (CaCO3), and strontium (Sr) carbonate (SrCO3) were prepared using a colloidal crystal of
polymer spheres as a template. Ethanol or ethanol–water solution of metal salts (acetate or nitrate) and
citric acid was infiltrated into the void of the colloidal crystal template of a monodispersed poly(methyl
methacrylate) (PMMA) sphere. Heating of this PMMA–metal salt–citric acid composite produced the
desired well-ordered 3DOM materials with a high pore fraction, which was confirmed by powder X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and
ultraviolet-visible (UV–vis) diffuse reflectance spectra. The presence of citric acid is crucial for
production of the 3DOM structures. Reaction of citric acid with metal salt produces metal citrate solid
in the void of PMMA spheres, which is necessary to maintain the 3DOM structure during the calcination
process. 3DOM CaCO3 shows opalescent colors because of it’s photonic stop band properties.
& 2011 Elsevier Inc. All rights reserved.
Received 12 March 2011
Received in revised form
21 June 2011
Accepted 23 June 2011
Available online 1 July 2011
Keywords:
Macroporous material
Template method
Colloidal crystal
Photonic crystal
1. Introduction
porosity, theoretically ca. 74%, which is attractive for catalyst and
photonic materials.
Fabrication of ordered porous materials in nano-scale order has
been one of the most important topics in recent years, and much
attention has been focused on three-dimensionally ordered macro-
porous (3DOM) materials with pores sized in the sub-micrometer
range because of their applications in photonic crystals, catalysis, and
separation [1–3]. 3DOM materials are produced by the following
procedure: (i) a colloidal crystal template is prepared by ordering
monodispersed spheres, e.g., polystyrene (PS), poly(methyl metha-
crylate) (PMMA) or silica, into a face-centered close-packed array
(opal structure), (ii) interstices in the colloidal crystal are then filled
with liquid metal precursors, either neat or in solution, which solidify
in voids of the sphere templates, resulting in an intermediate
composite structure, and (iii) an ordered form is produced after
removing the template by calcination or extraction. The ordered
(‘‘inverse opals’’) structure synthesized using this method consists of a
skeleton surrounding uniform close-packed macropores. The macro-
pores are interconnected through windows that form as a result of
contact between the template spheres prior to infiltration of the
precursor solution. Furthermore, the 3DOM materials have high
Many 3DOM materials of metals, metal oxides, metal carbides,
salts, and carbon have been prepared. However, 3DOM materials
with group 2 elements of the periodic table [4], such as magnesium
(Mg), calcium (Ca), and strontium (Sr), are rare. Alkaline properties
of the group 2 elements are important factors influencing catalytic
and adsorption properties. Therefore, 3DOM materials containing
such elements are of great interest. Common metal precursors,
such as nitrate or acetate salts, are not suitable for preparation of
the 3DOM structure because of their melting temperature. These
salts should be solidified at a temperature lower than the glass
transition temperature of the polymer template, ca. 380 K for
PMMA and PS, and not melt at a temperature at which the
template polymer decomposes [5]. The first group 2 element
containing 3DOM material, MgO and CaCO3, was reported by
Stein’s group [6]. They infiltrated Mg acetate or Ca acetate into
the void of a PS colloidal crystal template. Then the incorporated
acetate was solidified as metal oxalate by reacting with oxalic acid
in the void (2 step solidification method). Calcination of the PS-
metal oxalate composite produced 3DOM MgO or CaCO3. 3DOM
single crystal of CaCO3 was reported by Qi’s [7]. They infiltrated
amorphous calcium carbonate precursor into the void of colloidal
crystal of poly (styrene–methyl methacrylate–acrylic acid) sphere.
After calcination of the produced polymer–amorphous calcium
carbonate precursor produced single crystal CaCO3 with 3DOM
structure. 3DOM MgO with additional mesopores was reported by
n Corresponding author. Fax: þ81 82 424 5494.
nn Corresponding author. Fax: 81 11 706 9163.
E-mail addresses: sadakane09@hiroshima-u.ac.jp (M. Sadakane),
ueda@cat.hokudai.ac.jp (W. Ueda).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.06.028

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M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
Dai’s group [8]. They infiltrated an ethanol–water mixed solution
of mixture of Mg nitrate, citric acid, and Pluronic F127
2.2. Characterizations
a
(EO106PO70EO106) into the void of a PMMA colloidal crystal.
Calcination of the PMMA–Mg nitrate–citric acid composite pro-
duced 3DOM MgO. 3DOM MgF2 and CaF2 with large surface area
and a strong Lewis acidity were reported by Kemnitz’s group [9].
They infiltrated metal fluoride solution into the voids of PMMA
spheres. After drying of the 3DOM metal fluoride–PMMA compo-
site, the composite was washed with acetone or calcinated to
produce 3DOM CaF2 or MgF2.
In this paper, we describe an easy preparation method,
structural characterization, formation mechanism, and photonic
properties of 3DOM materials with group 2 elements (Mg, Ca, and
Sr). The main purpose is to understand the role of citric acid in the
production of 3DOM MgO and to extend the preparation method
to other group 2 elements such as Ca and Sr.
Powder X-ray diffraction (XRD) patterns were recorded on a
diffractometer (Rigaku, RINT UltimaIII) equipped with a graphite
monochromator using CuKa radiation (tube voltage: 40 kV, tube
current: 20 mA). Diffraction line widths were obtained after subtrac-
tion of the instrumental width determined by the line width of silicon
powder, and crystallite sizes were calculated from the width of the
most intense lines using the Scherrer equation. Scanning electron
microscopy (SEM) images were obtained with a JSM-7400 F (JEOL) or
HD-2000 (Hitachi). Scanning transmission electron microscopy
(STEM) images were obtained with an HD-2000 (Hitachi) using
accelerating voltage of 200 kV. Samples for TEM were prepared by
sonicating small amounts of the powder in 5 ml of ethanol for 1 min
and then depositing a few drops of the suspension on a holey carbon
grid. Nitrogen adsorption measurements were performed on an
Autosorb 6 (Quantachrome, USA) sorption analyzer. Prior to the
sorption measurements, the samples were degassed under vacuum
at 473 K for 1 h. Surface areas were calculated by the
BrunauerꢀEmmetꢀTeller (BET) method. Diffuse-Reflectance (DR)
UV–vis spectra were obtained on a JASCO V-570 spectrophotometer
equipped with an ISN-470 reflectance spectroscopy accessory. Sam-
ples were randomly oriented bulk powders. Thermamogravimetric-
differential thermal analysis (TG-DTA) measurements were per-
formed with a TG-8120 (Rigaku) thermogravimetric analyzer. Each
sample was heated in air at a heating rate of 10 K minꢀ1. Tempera-
ture programmed desorption (TPD) measurement was performed
with a TPD apparatus (BEL Japan Inc.) equipped with a Quadrupole
mass spectrometer (M-100QA; Anelva). Elemental analyses were
carried out by Instrumental Analysis Division, Equipment Manage-
ment Center, Creative Research Institute, Hokkaido University.
2. Experimental
2.1. Materials
All chemicals used were of reagent grade and they were used as
supplied. Suspensions of monodispersed poly(methyl methacrylate)
(PMMA) spheres (diameters: 240, 370, and 410 nm) were synthesized
by literature techniques [10,11]. These spheres were packed into
colloidal crystals by centrifugation. The obtained template was
crushed with an agate mortar and the obtained particles were
adjusted to 0.425–2.000 mm in size using testing sieves (Tokyo
Screen, Co., LTD.) [10,11]. Quartz sand (10–15 mesh) was purchased
from Kokusan Chemical Works (Tokyo).
Table 1
Summary of the preparation conditions and physicochemical properties of the obtained materials.a
Entry
Metal sourceb (amount) Additive (amount)
Solvent (amount)
Calc. temp.
(K)
3DOM
structurec size (nme)
Materiald, crystallite
BET, surf.
area
(m2 gꢀ1
)
1
2
3
Mg(OAc)2 (2.0 g)
Mg(NO3)2 (1.13 g)
EtOH (5 ml)–H2O (5 ml)
EtOH (5 ml)
673
673
673
C
C
C
MgO
MgO
MgO
Mg(NO3)2 (1.28 g)
Ethylene glycol
(ca. 0.5 ml)
MeOH (1 ml)
4
Mg(OAc)2 (2.15 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg(NO3)2 (1.13 g)
Mg3(citrate)2 (0.90 g)
Mg3(citrate)2 (0.90 g)
Mg3(citrate)2 (0.90 g)
Ca(NO3)2 (1.18 g)
Ca(NO3)2 (1.18 g)
Ca(NO3)2 (1.18 g)
Citric acid (2.1 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
HCl
EtOH (5 ml)–H2O (5 ml)
EtOH (5 ml)
773
673
673
773
873
973
1073
1273
1473
773
773
773
673
773
873
A
A
A
A
A
A
A
B
B
A
A
A
A
D
D
MgO
5
MgO
6
EtOH (1.5 ml)
MgO (6)
120
100
92
7
EtOH (1.5 ml)
MgO (7)
8
EtOH (1.5 ml)
MgO (9)
9
EtOH (1.5 ml)
MgO (17)
MgO (23)
MgO
61
10
11
12
13
14
15
16
17
18
EtOH (1.5 ml)
53
EtOH (1.5 ml)
EtOH (1.5 ml)
MgO
EtOH (1.5 ml)
MgO (9)
HNO3
EtOH (1.5 ml)
MgO (9)
H2SO4
EtOH (1.5 ml)
MgSO4 (8)
CaCO3 (Calcite) (18)
CaCO3 (Calcite) (31)
CaCO3 (Calcite
(39)þVaterite)
SrCO3 (12)
SrCO3 (20)
SrCO3 (34)
36
59
23
21
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
MeOH (1.5 ml)–H2O (3.5 ml)
MeOH (1.5 ml)–H2O (3.5 ml)
MeOH (1.5 ml)–H2O (3.5 ml)
19
20
21
Sr(NO3)2 (1.06 g)
Sr(NO3)2 (1.06 g)
Sr(NO3)2 (1.06 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
Citric acid (1.05 g)
MeOH (1.5 ml)–H2O (3.5 ml)
MeOH (1.5 ml)–H2O (3.5 ml)
MeOH (1.5 ml)–H2O (3.5 ml)
673
773
873
A
C
D
54
33
28
a
PMMA with a diameter of 240 nm was used.
b
c
All compounds were hydrate compounds: Mg(NO3) ꢁ 6H2O, Mg(OAc)2 ꢁ 4H2O, Mg3(citrate)2 ꢁ 9H2O, Ca(NO3)2 ꢁ 4H2O, Sr(NO3)2 ꢁ 4H2O.
A. Well-ordered 3DOM structure was observed by SEM, and the fraction of 3DOM structure was more than 95% (More than 20 particles were randomly chosen by
SEM, and images with magnifications of 5000–10000 were taken. The fraction of 3DOM structure was calculated as follows: number of particles that contain the 3DOM
structure/total number of particles). B. The fraction of 3DOM structure was more than 90%, but the 3DOM structure was not well-ordered. C. 3DOM structure was only
partially found, the fraction of 3DOM being less than 10%. D. No 3DOM structure was observed by SEM.
d
Crystal structure was determined by powder-XRD. MgO (JCPDS: 045–0946), CaCO3 (calcite, JCPDS: 047–1743), CaCO3 (vaterite, JCPDS: 033-0268), SrCO3 (JCPDS: 005–
0418), MgSO4 (JCPDS: 021–0546).
e
Crystallite sizes were calculated from the width of the most intense line using the Scherrer equation.

M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
2301
2.3. Synthesis of 3DOM MgO
overnight. A 0.5-g amount of the sample was mixed with 2.5 g of
quartz sand (10ꢀ15 mesh) and calcined in a tubular furnace (inner
diameter of ca. 21 mm) in an air flow (50 ml minꢀ1). The tempera-
ture was raised at a rate of 1 K minꢀ1 to 673 K and held for 5 h.
Magnesium nitrate hydrate (Mg(NO3)2 ꢁ 6H2O, 1.13 g) and citric
acid (1.05 g) were dissolved with ca. 1.5 ml of ethanol. Then PMMA
colloidal crystals were soaked in the solution for 3 h. Excess solution
was removed from the impregnated PMMA colloidal crystals by
vacuum filtration. The obtained sample was allowed to dry in air at
room temperature overnight. A 0.5-g amount of the sample was
mixed with 2.5 g of quartz sand (10ꢀ15 mesh) and calcined in a
tubular furnace (inner diameter of ca. 21 mm) in an air flow
(50 ml minꢀ1). The temperature was raised at a rate of 1 K minꢀ1
to 873–1073 K and held for 5 h.
2.5. Synthesis of CaCO3 without PMMA template
Calcium nitrate hydrate (Ca(NO3)2 ꢁ 6H2O, 1.18 g) and citric acid
(1.05 g) were dissolved in ca. 1.5 ml of methanol and 3.5 ml of
water. This solution was heated in a muffle oven. The temperature
was raised at a rate of 1 K minꢀ1 to 673 K and held for 5 h.
3. Results and discussion
2.4. Synthesis of 3DOM CaCO3 and SrCO3
3.1. Preparation and formation mechanism of 3DOM MgO
Metal nitrate hydrate (Ca(NO3)2 ꢁ 6H2O, 1.18 g or Sr(NO3)2 ꢁ
6H2O, 1.06 g) and citric acid (1.05 g) were dissolved in ca. 1.5 ml
of methanol and 3.5 ml of water. Then PMMA colloidal crystals were
soaked in the solution for 3 h. Excess solution was removed from the
impregnated PMMA colloidal crystals by vacuum filtration. The
obtained sample was allowed to dry in air at room temperature
Mg(NO3)2 and Mg(OAc)2 were not suitable as starting materi-
als for preparation of a 3-dimensionally ordered macroporous
(3DOM) structure [6] (Table 1, Entries 1 and 2). We previously
reported that our method using ethylene glycol could not produce
3DOM MgO material (Table 1, Entry 3) [5]. However, by adding
Fig. 1. SEM and TEM images of 3DOM MgO prepared using colloidal crystal templates of PMMA (diameter of 240 nm). SEM image of sample prepared using Mg(NO3)2-
citric acid ethanol solution. Calcination temperature was 773 K (a). Sample prepared using Mg(OAc)2-citric acid ethanol–water solution. Calcination temperature was
773 K (b). SEM and TEM images of sample prepared using Mg(NO3)2-citric acid ethanol solution. Calcination temperatures were 973 K (c), (d), (e). SEM image of sample
prepared using Mg(NO3)2-citric acid ethanol solution. Calcination temperature was 1473 K (f).

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M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
citric acid to Mg(NO3)2 or Mg(OAc)2 solution, a 3DOM structure
was successfully obtained (Table 1, Entries 4–6, Fig. 1(a) and (b)).
Recently, Dai’s group also reported preparation of 3DOM MgO
using ethanol solution of Mg(NO3)2 with citric acid [8]. We have
clearly indicated that Mg(OAc)2 is also a suitable Mg precursor.
The well-ordered 3DOM structure was stable at temperatures up
to 1073 K (Table 1, Entries 4–10, Fig. 1(c)). After calcination at
these temperatures, crystalline MgO was obtained (Fig. 2). Crys-
tallite sizes estimated by XRD of 3DOM MgO prepared at 973 K
were ca. 17 nm (Table 1, Entry 9), which is similar to wall
thickness (Fig. 1(e)). These small crystallites constructed 3DOM
walls similar to 3DOM iron based oxide [11,12]. By increasing the
calcination temperature, crystallite size was increased and sur-
face area was decreased. When the temperature was higher than
1273 K, non-well-ordered 3DOM MgO was obtained due to
sintering of crystallites (Fig. 1(f) and Entries 11 and 12).
Fig. 3(a) shows TG-DTA curves of Mg(NO3)2 and citric acid
dissolved in ethanol. Gradual weight loss was observed up
to 600 K, and an exothermic weight loss was observed at tempera-
tures between ca. 700 and 760 K. This TG-DTA behavior indicates that
Mg(NO3)2 reacted with citric acid at a temperature below 430 K,
because an endothermic weight loss at 430 K shown in TG-DTA of
citric acid (Fig. 3(c)) and an endothermic weight loss at ca. 700 K
shown in TG-DTA of Mg(NO3)2 (Fig. 3(b)) were not observed. Table 2
summarizes results of elemental analysis of material obtained after
heating Mg(NO3)2 and citric acid dissolved in ethanol at different
temperatures. C, H, and N contents changed at temperatures from
323 to 423 K (Table 2, Entries 1 and 2) but did not change at heating
temperatures higher than 423 K (Table 2, Entries 3–5). N content
decreased from 323 to 373 K, and N was completely removed at a
temperature of 423 K, indicating that nitrate removal started at a
temperature lower than 373 K and was completed at a temperature
of 423 K. The IR spectrum of material obtained after heating of
Table 2
Elemental analysis of solid material obtained after heating
of ethanol solution of Mg(NO3)2 and citric acid at different
temperatures.
Entry
Temp. (K)
Elemental analysis (%)
C
H
N
1
2
3
4
5
323
373
423
473
573
21
23
32
31
30
4.8
4.8
3.7
3.8
3.5
4.1
3.3
0
Fig. 2. XRD of 3DOM MgO prepared using colloidal crystal templates of PMMA
(diameter of 240 nm). Calcination temperatures were 673 K (a), 773 K (b), 873 K
(c), 973 K (d), and 1073 K (e). Closed circles indicate diffractions corresponding to
quartz, which was used for calcination.
0
0
Fig. 3. TG-DTA curves of Mg(NO3)2–citric acid ethanol solution (a), Mg(NO3)2 (b), citric acid (c), and Mg3(citrate)2 (d).

M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
2303
Mg(NO3)2 and citric acid dissolved in ethanol at 423 K shows
carbonyl peaks, which was also observed in the IR spectrum of
Mg3(citrate)2 (Figure S1). TG-DTA of Mg3(citrate)2 shows an exother-
mic weight loss at ca. 700 K (Fig. 3(d)). These results indicate that
heating of ethanol solution of Mg(NO3)2 and citric acid produced Mg
citrate solid in the void of the PMMA template at 423 K. In order to
maintain the 3DOM structure, solidification should occur at a
temperature lower than the glass transition temperature (ca. 378 K)
of PMMA [5]. Reaction of Mg nitrate with citric acid started at a
temperature lower than 373 K. Therefore, we propose solidification of
Mg(NO3)2 started at a temperature lower than 378 K (glass transition
temperature of PMMA) in the presence of citric acid. Li’s group
reported that citric acid is crucial for preparation of 3DOM CeO2 and
La2O3 [13]. They reported that the chelating effect of citric acid
facilitated the formation of a 3DOM structure.
Commercially available Mg3(citrate)2 can also be used as a
starting material. Mg3(citrate)2 has low solubility in ethanol, and
acids were added to increase solubility. 3DOM MgO was obtained
using ethanol solution of Mg3(citrate)2 with HCl or HNO3 (Table 1,
Entries 13 and 14, and Figure S2), and 3DOM MgSO4 was obtained
using ethanol solution of Mg3(citrate)2 with H2SO4 (Table 1, Entry
15, and Figures S2 and S3). This is the first report on production of
a 3DOM MgSO4 material. Crystallite size of 3DOM MgSO4 esti-
mated by XRD was ca. 8 nm, which is similar to crystallite sizes
(7–9 nm) of 3DOM MgO at same calcination temperature (Table 1,
Entries 7, 13–15). However, surface area of 3DOM MgSO4 was
surprisingly smaller than that of 3DOM MgO, and we are now
investigating reasons. IR spectra of 3DOM MgO materials show a
peak corresponding to CO23ꢀ, which form on the surface of MgO
by a reaction of basic MgO with CO2 (Figure S4). On the other
hand, IR spectrum of 3DOM MgSO4 shows no peak corresponds to
CO23ꢀ but peak corresponds to SO24ꢀ (Figure S4(e)) because sur-
face of MgSO4 is not basic and does not react with CO2.
3.2. Preparation of 3DOM CaCO3 and SrCO3
Sr(NO3)2 or Ca(NO3)2 was used as a starting compound to
prepare 3DOM material with Sr or Ca. For both cases, well-
ordered 3DOM CaCO3 or SrCO3 was produced after calcination
at 673 K (Table 1, Entries 16 and 19, Figs. 4(a), (d), (e), (f), (i), (j),
and 5). Heating at higher temperature resulted in collapse of the
3DOM structure due to crystal growth (Table 1, Entries 17, 18, 20,
21, and Fig. 4 (b), (c), (g), (h)).
TG-DTA analysis of 3DOM CaCO3 material prepared after
calcination at 673 K showed an exothermic weight loss starting
at ca. 673 K and an endothermic weight loss starting at ca. 873 K
(Figure S4(a)). The first exothermic weight loss corresponded to
combustion of organic residue from PMMA, because this exother-
mic weight loss is not observed in TG-DTA of CaCO3 prepared at
673 K without a PMMA template (Figure S4(c)). The endothermic
weight loss corresponded to decomposition of CaCO3 to CaO.
Evolution of CO2 and H2O at the first exothermic weight loss and
evolution of CO2 were confirmed by temperature programmed
desorption (TPD) analysis (Figure S5(b) and (d)). Decomposition
of SrCO3 to SrO started at ca. 1000 K (Figure S6).
Fig. 4. SEM images of CaCO3 and SrCO3 prepared using colloidal crystal templates
of PMMA. CaCO3 materials prepared using PMMA with a diameter of 240 nm.
Calcination temperatures were 673 K (a), 773 K (b) and 873 K (c). CaCO3 materials
prepared using PMMA with a diameter of 370 nm. Calcination temperature was
673 K (d). CaCO3 materials prepared using PMMA with a diameter of 410 nm.
Calcination temperature was 673 K (e). SrCO3 materials prepared using PMMA
with a diameter of 240 nm. Calcination temperatures were 673 K (f), 773 K (g) and
873 K (h). SrCO3 materials prepared using PMMA with a diameter of 370 nm.
Calcination temperature was 673 K (i). SrCO3 materials prepared using PMMA
with a diameter of 410 nm. Calcination temperature was 673 K (j).
3.3. Photonic properties
The colors of 3DOM CaCO3 prepared using PMMA with
diameters of 240, 370 and 410 nm (Fig 4 (a), (d), and (e)) were
opalescent violet, blue and green, respectively, under room light,
which is visible evidence of their long-range 3D order (Fig. 6). The
3DOM CaCO3 materials were characterized using DR UV–vis
spectroscopy (Fig. 7), and photonic stop bands centered at 440,
492 and 526 nm corresponding to violet, blue and green colors,
respectively, were observed. 3DOM CaCO3 was first reported by
Stein’s group [6], but photonic stop bands and opalescent colors
were not reported.
An approximate expression for the position of the stop band is
given by
l
¼ 2dhklnavg
,
ð1Þ

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M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
Fig. 6. Photographs of 3DOM CaCO3 prepared using PMMA templates with
diameters of 240 nm (a), 370 nm (b), and 410 nm (c). Calcination temperature
was 673 K.
Fig. 5. XRD of CaCO3 (A) and SrCO3 (B) materials prepared using a colloidal crystal
template of PMMA with a diameter of 240 nm. Calcination temperatures were
673 K (a), 773 K (b) and 873 K (c). Closed circles and squares indicate diffraction
corresponding to calcite (JCPDS: 047–1743) and vatelite (JCPDS: 033-0268) ,
respectively.
Fig. 7. Absorption spectra of 3DOM CaCO3 prepared using PMMA templates with
diameters of 240 nm (solid line), 370 nm (solid line with open circles), and 410 nm
(solid line with open squares). Calcination temperature was 673 K.
where
l is the wavelength of the stop band minimum, dhkl is the
interplanar spacing, and navg is the average refractive index of the
materials [10,14]. navg is given by
È
É
1=2
navg
¼
ð12aÞn2wall þanv2oid
,
ð2Þ
diameter. 3DOM MgO, MgSO4, and SrCO3 did not show clear opal
opalescent color, because their stop band wavelength was not in
the visible light range.
where a is pore fraction of the material, nwall is refractive index
of CaCO3, and nvoid is refractive index of air (nvoid¼1.00). The
interplanar spacing of the (1 1 1) set planes d111 is given by
d111 ¼ ð2=3Þ1=2D,
ð3Þ
4. Conclusion
where D is pore size.
Pore sizes (D) of 3DOM CaCO3 prepared using PMMA spheres
with diameter of 240, 370 and 411 nm were 190, 270 and 309 nm,
respectively. The pore size is usually smaller than the original
PMMA sphere diameter because of shrinkage caused by melting
of PMMA, crystallization of walls, and removal of chelating
materials. The photonic stop band position is tunable by changing
PMMA diameter.
Well-ordered 3DOM MgO, MgSO4, CaCO3, and SrCO3 materials
were obtained using a colloidal crystal template method in a high
pore fraction. The presence of citric acid in metal salt solution is
crucial for production of a 3DOM structure. The citric acid reacts
with the metal cation to form metal–citrate solid in the void of
the colloidal crystal. High porosity and high order of the 3DOM
materials, which are attractive features for application as future
catalysts, filters and electrode material, were confirmed by using
SEM, TEM, and photonic stop band properties.
As shown in Eqs. (1)–(3), stop band wavelength depends on
the refractive index of the materials, pore fractions, and pore

M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
2305
Acknowledgments
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We thank Mitsubishi Rayon Co. for financial support and
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Appendix A. Supplementary materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.06.028.
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