M. Sadakane et al. / Journal of Solid State Chemistry 184 (2011) 2299–2305
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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