Morphological and structural evolution of mesoporous calcium aluminate nanocomposites by microwave-assisted synthesis

Morphological and structural evolution of mesoporous calcium

aluminate nanocomposites by microwave-assisted synthesis

Yen-Po Chang, Po-Hsueh Chang, Yuan-Tse Lee, Tai-Jung Lee, Yen-Ho Lai, San-Yuan Chen

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

a r t i c l e

i n f o

Article history:

Received 3 February 2013

Received in revised form 16 June 2013 Accepted 6 September 2013 Available online 17 September 2013 Keywords: Metal oxides Nanocomposites Calcium aluminates Microwave

a b s t r a c t

Novel special dual-structure calcium aluminate nanocomposites consisting of mesoporous Ca-containing Al2O3nanonetworks with Ca12Al14O33polycrystalline nanorods have been successfully synthesized using

the microwave-hydrothermal (M-H) process in an alcohol solution consisting of calcium nitrate and mes-oporous alumina, followed by calcination at a lower temperature (e.g., less than 600 °C). The mesmes-oporous Ca-containing aluminate nanocomposites possessed a specific surface area of 51 m2_g 1_{and a broad pore}

size distribution of 4–12 nm in diameter, as observed from N2adsorption/desorption measurements. The

results showed that highly dispersed Ca12Al14O33nanorods grown on the mesoporous nanostructure

were obtained by varying the molar ratio of Ca/Al or by controlling microwave heating time, as charac-terized by scanning electron microscopy (SEM), transmission electron microscopy/electronic energy loss spectroscopy (TEM/EELS) and powder X-ray diffraction (PXRD). The Ca12Al14O33nanorods with diameters

of 50–100 nm and lengths of 200–400 nm were grown in and on the one-dimensional mesochannels. A formation mechanism of the mesoporous Ca-containing aluminate nanocomposites involving amorphous mesoporous hydrated-Al2O3was also proposed.

Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction

Calcium aluminates (Ca12Al14O33) have long been known as

refractory mixed oxides in the steel industry and as hydraulic materials in the cement community. In recent years, new applica-tions of calcium aluminates have emerged, such as optical devices [1], oxygen ionic conductors[2], and heterogeneous catalysts[3,4]. Lemonidou et al. reported that the deposition of 5 wt.% Ni on cal-cium aluminate (molar ratio CaO/Al2O3= 1/2) resulted increased

catalytic activity, with lower coke deposition for the reaction of CO2 reforming of CH4 [4]. Traditionally, bulk calcium aluminate

cements have been obtained by fusing or sintering a mixture of CaO or CaCO3 and alumina (Al2O3) at temperatures above

1400 °C, but these materials have very low specific surface areas (<1 m2_{/g) [5]. Risbud et al. reported that amorphous calcium}

aluminate (Ca12Al14O33) powders with high surface areas can be

synthesized by calcining a mixture of aluminum and calcium pre-cursors (such as organic metal or metal salts) below 900 °C[6,7]. On the other hand, crystalline calcium aluminate (Ca12Al14O33)

can also be obtained by evaporative decomposition of a solution made from calcium and aluminum nitrate precursors after

heat-treatment at 900 °C[8]. Recently, Zawrah et al. further re-ported that nanosized calcium aluminate with particle size smaller than 50 nm can be successfully synthesized by thermal decompo-sition treatment above 1000 °C[9]. However, to date, calcium alu-minate nanocrystallites with mesoporous structures, which are used as supports for catalysts and oxygen ion conductors, have not been investigated.

In recent years, microwave-enhanced chemistry has become increasingly important because it can utilize the inherent proper-ties of liquids, solids, and their mixtures to convert microwave en-ergy in situ into heat for promoting reactions [10,11]. Hence, microwave-assisted synthesis of mesoporous materials (such as MCM-41 and SBA-15) has attracted wide attention[12,13]because it offers many advantages such as homogeneous and simultaneous heating through the reaction cell, rapid nucleation and growth, and suppression of undesired phases. To the best of our knowledge, no systematical study has yet been conducted on the synthesis of mesoporous calcium aluminate nanocomposites by microwave-as-sisted processes. In this study, we used microwave-asmicrowave-as-sisted syn-thesis combined with calcination treatment to produce a special dual-structure of polycrystalline nanorods and/or nanonetworks. It was found that the Ca/Al molar ratio plays important roles in the migration of ionic calcium species on the mesostructured sur-face as well as in the morphology evolution of mesostructured cal-cium aluminate grown with Ca12Al14O33 nanorods–nanotube

nanocomposites. Characterization techniques, including nitrogen

1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/j.micromeso.2013.09.013

⇑Corresponding author. Address: Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC. Tel.: +886 3 5731818; fax: +886 3 574727.

E-mail address:[email protected](S.-Y. Chen).

Contents lists available atScienceDirect

Microporous and Mesoporous Materials

adsorption, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS), were used to exam-ine the crystal structure, morphology and size of nanostructured calcium aluminates. The formation mechanism of the nanostruc-ture grown on the surface of mesoporous materials was proposed to explain the microwave reaction between calcium ion and mes-oporous alumina.

2. Experimental section

2.1. Preparation of mesoporous calcium aluminate nanocomposites First, according to a previous study [14], Yuan et al. reported highly ordered mesoporous alumina (MA) with high thermal sta-bility. In a typical preparation, triblock copolymer HO(CH2CH2O)20

(CH2CH(CH3)O)70–(CH2CH2O)2OH (2 g; Sigma–Aldrich, Mn = 5800,

Pluronic P123) was dissolved in absolute ethanol (20 mL; Sigma– Aldrich, 99.5%) and stirred for 4 h at room temperature. In another solution, aluminum isopropoxide (AIP, 20 mmol; Sigma–Aldrich, 98+ wt.%,) was dissolved in nitric acid (3.2 mL; J.T. Baker, 70 wt.%) and absolute ethanol (10 mL). Then, the AIP solution was slowly added to the surfactant solution, and the mixed solution was vigorously stirred for 5 h and then transferred to an oven to evaporate the solvent at 60 °C for 3 days. The resulting powder was calcined at 700 °C for 4 h with a heating rate of

5 °C min 1_{in an air flow and then cooled in a furnace to ambient}

temperature to give mesoporous Al2O3(M-Al).

Second, the synthesized M-Al and calcium nitrate were used as starting materials. These materials, with a Ca:Al molar ratio of 1:1 and 2:1, were suspended in 100 mL of absolute alcohol solution at room temperature. Then, the solution was transferred to the TFM (tetrafluorometyl) reactor. The reactors with the suspensions were sealed and then placed on a turntable tube for uniform heating by a microwave-accelerated reaction system (model MARSTM, CEM Corporation, Matthews, NC, USA). The suspensions in the TFM vessel were heated at 80 °C for 1 h in a microwave oven with 1600 W of power. The precipitation suspended in the solution was rapidly dried at 50 °C in an oven overnight to re-move the solvent. Finally, the resulting powders were calcined at 600 °C for 3 h with a heating rate of 2 °C min 1_{in an air flow}

to give M-CaAl and M-2CaAl powders with the Ca:Al molar ratio of 1:1 and 2:1, respectively.

2.2. Characterization of materials

The resulting powders were characterized by performing XRD measurements (MAC Science MXP18AHF XRD, with CuKaradiation

source, k = 1.5418 Å). SEM images of the sample were collected with a JEOL-6700 field-emission electron microscope at an acceler-ating voltage of 15 kV. TEM micrographs and electron diffraction patterns were recorded with a JEOL JEM-2100F electron micro-scope equipped with an Oxford energy-dispersive spectrometer

Fig. 1. (a) Small-angle XRD patterns, (b) TEM image with a high resolution image inset, (c) N2adsorption isotherm and (d) pore size distribution of calcined mesoporous Al2O3 (M-Al).

(EDS) analysis system. Elemental Ca or Al distribution mapping was conducted with a TECNAI 30 electron microscope fitted with an electron energy loss spectroscopy (EELS) detector. Samples for TEM measurements were embedded in resin and ultramicrotomed into slices with thicknesses of approximately 50 nm. The surface area (BET) and pore size distribution were calculated using a NOVA 1000e instrument at 77 K nitrogen adsorption isotherms. All sam-ples were degassed under vacuum at 200 °C for 2 h prior to the measurements. The samples were digested with mixed acids and their Ca/Al ratios were determined by ICP-AES (Jarrell-Ash, ICAP-9000). The as-synthesized and calcined M-CaAl samples were also characterized by Fourier transform infrared spectroscopy (FTIR, Bomem DA8.3) and X-ray photoemission spectroscopy (XPS, Ther-mo VG Microlab 350).

3. Results and discussion 3.1. Mesoporous Al2O3

Fig. 1(a) illustrates the small-angle XRD patterns of mesoporous Al2O3(M-Al), in which the characteristic reflections of the P6mm

hexagonal structure at 2h = 0.79° (MA) are displayed and the corre-sponding d-spacing is indexed with d100= 11.18°. The hexagonal

unit cell parameter (ao) of 12.9 nm (MA) was calculated by

assum-ing a (1 0 0) reflection for the hexagonal array of pores of the mes-oporous metal oxides, which is indicative of mesostructural formation. This finding also indicates that a hexagonal arrange-ment of the mesoporous structure was evolved upon mesoporous metal oxide synthesis. The large-angle XRD (LA-XRD) pattern of the M-Al sample is shown inFig. 2(a). After calcination at 700 °C, mesoporous Al2O3 (M-Al) displays three broad peaks from the

(3 1 1), (4 0 0), and (4 4 0) reflections, which correspond to those of crystalline

c

c

Al2O3inFig. 1(b) exhibits channels with 2D-hexagonal symmetry

(P6mm) along (1 1 0) with an interplanar space calculated as 7.5 nm, in good agreement with that determined by SA-XRD. Nitrogen adsorption isotherms and the corresponding pore size distribution of mesoporous Al2O3 (M-Al) are shown inFig. 1(c)

and (d). The isotherms can be classified as type IV, indicating typ-ical mesoporous metal oxides[15]. The hysteresis loop of M-Al dis-plays type H1 characteristic and the steepness of the capillary condensation step indicate the uniformity of the mesopores. The Brunauer–Emmett–Teller (BET) surface areas, pore volumes and average pore size of the M-Al sample are measured to be 202 m2g 1; 0.47 cm3g 1; and 5.8 nm, respectively.

3.2. Mesoporous calcium aluminate nanocomposites

Mesoporous calcium aluminate nanocomposites with different molar ratios of M-CaAl and M-2CaAl were synthesized by reacting mesoporous Al2O3with a controlled concentration of calcium

un-der the microwave-assisted hydrothermal process in which the ac-tual Ca/Al molar ratio of 0.96 (M-CaAl) and 1.94 (M-2CaAl) was measured by ICP-AES. The small- and large-angle XRD patterns of the calcined mesoporous nanocomposites (M-CaAl and M-2CaAl) are shown in Figs. 1(a) and 2(a), respectively. The small-angle XRD pattern of the M-CaAl and M-2CaAl samples displayed a single broad diffraction peak at 2h = 1.06° and 2h = 1.33°, respectively. The pore size decreases with increasing molar ratio of Ca/Al, dem-onstrated by the reduction in d-spacing from 8 to 6.7 nm. However, the low intensity and broadness of the diffraction peak in the small-angle region indicate a wormhole framework, which reveals that Ca loading led to the amorphous structure. The existence of the small-angle diffraction peak also indicates the retention of

the mesostructure throughout the calcination process[16]. This finding also demonstrates that the hexagonal arrangement of the mesoporous structure was evolved during the synthesis of the mesoporous calcium aluminates. Additionally, the large-angle XRD patterns of the M-CaAl (Ca/Al = 1:1) and M-2CaAl (Ca/ Al = 2:1) samples that were calcined at 600 °C were characterized, and the main diffraction peaks correspond to crystalline structures of Ca12Al14O33(JCPDS No. 70-2144) and CaO (JCPDS No. 04-0777),

marked as A and B, respectively. The visible diffraction peaks in the XRD pattern of the M-CaAl sample corresponded to the Ca12Al14O33

crystalline phase and the broad peaks may also indicate the forma-tion of mesoporous calcium-containing aluminates nanostructures in this compound. According to the previously reported literature, the Ca12Al14O33 crystalline phase usually formed above 800 °C

[17,18]. However, in our studies, this phase clearly appeared after calcinations at 600 °C. This difference may be attributed to the microwave heating because the microwave can directly interact with the dipole of molecules, resulting in accelerated uniform reac-tions. During the microwave heating, the generated thermal en-ergy evenly distributes Ca ions in this reaction throughout the porous structure and induces a stronger affinity of the Ca ions to the highly active mesoporous Al2O3matrix[19]; thus, the reactions

between Ca and Al–O species could occur at lower temperatures to form mesoporous calcium-containing aluminate nanocomposites. Subsequently, the Ca12Al14O33 polycrystalline phases could be

grown from high-Ca concentration areas such as mesostructured surface and mesochannels during the calcination process. In addi-tion, the XRD patterns of the M-2CaAl sample inFig. 2(a) also show the formation of both CaO and Ca12Al14O33 phases. It is implied

Fig. 2. (a) Large-angle XRD patterns and (b) N2adsorption isotherms and pore size distributions of calcined mesoporous materials.

that the extra unreacted Ca species could be transferred to form calcium oxide after calcination at high temperature. Moreover, in Fig. 2(b), the mesoporous calcium-containing aluminate nanocom-posites showed a hysteresis loop of type H2, which is typical for wormhole-like mesostructures and hierarchical scaffold-like mes-oporous structures[20,21]. Some typical textural parameters of the M-CaAl and M-2CaAl samples were measured by Brunauer– Emmett–Teller (BET): specific surface area of 51 and 16 m2_g 1_,

pore volume of 0.09 and 0.04 cm3_g 1 _{and average pore size of}

11.8 and 3.9 nm, respectively. These results indicate that an in-crease in Ca loading would cause the formation of excessive CaO particles, leading to partial blocking of the mesoporous system and thus a smaller BET surface area and pore volume in the M-2CaAl sample. In contrast, for a sample with less Ca (M-CaAl), Ca ions could be widely distributed throughout the mesoporous Al2O3matrix to produce calcium-containing aluminate

nanocom-posites with larger BET surface area and pore volume after calcin-ing the M-CaAl nanocomposites at 600 °C.

The SEM images of calcined mesoporous calcium-containing aluminate nanocomposites are displayed inFig. 3. For the M-CaAl sample (Ca/Al = 1:1), Fig. 3(a) and (b) show that nanorods 200–400 nm in length and 50–120 nm in width were highly grown on the mesochannel and surface of the mesostructured matrix. The Ca12Al14O33nanorods seem to either directly nucleate

and grow from the surface or initiate from the interior mesostruc-ture. The energy-dispersive spectral analysis (EDS) inFig. 3(c) and (d) also showed that no elements other than Ca, Al and O were detected, and the Ca/Al molar ratio of the mesoporous nanocom-posites was estimated to be ca. 0.833 (spot 1) and ca. 0.892 (spot 2) based on the quantitative EDS analyses, possibly indicating that the mesoporous nanocomposites are characteristic of a highly chemically pure Ca12Al14O33 phase. It is inferred that the

Fig. 3. SEM images of calcined M-CaAl samples, (a) at low magnification (2 K) and (b) high magnification (30 K), displaying nanorods grown on the surface of mesoporous oxide, (c) EDS from spot 1 in (b), (d) EDS from spot 2 in (a) and (e) SEM image of the M-2CaAl sample, presenting a mesoporous structure without nanorods. Note: the prepared M-CaAl sample with Ca/Al ratio = 1, and M-2CaAl sample with Ca/Al ratio = 2.

Ca12Al14O33 nanorod-like crystallites on the mesoporous

Ca-con-taining Al2O3 matrix were formed in situ from the calcinations

and interactions of the Ca species and Al2O3 matrix hybrid

precursor prepared by the microwave accelerated reaction. More-over, a few particles (0.5–1.5

l

in Fig. 3(a), which are attributed to the formation of calcium oxides due to unreacted calcium species on the Al2O3 matrix. In

contrast, the M-2CaAl sample (Ca/Al = 2:1) inFig. 3(e) displays a rougher surface morphology; no uniform nanorods can be de-tected, suggesting that the superabundant Ca species was possi-bly transferred into the aggregation of CaO particles on the mesoporous Ca-containing Al2O3 matrix, which is supported by

the large-angle XRD results (as shown in Fig. 2(a)). Generally, pure Ca12Al14O33 crystallites were formed at high temperature

(up to 800 °C) by following the reaction of 12CaO + 7Al2O3?

Ca12Al14O33with Ca/Al molar ratio = 0.857; however, as excessive

Ca was added, as in M-2CaAl (Ca/Al = 2), large CaO particles may have been easily formed on the mesostructural Al2O3matrix after

calcinations, as clearly evidenced by the sharp diffraction peaks of the CaO crystal phase in the XRD pattern.

The TEM images of the calcined calcium aluminate nanocom-posites (M-CaAl and M-2CaAl samples) are shown in Figs.4 and 5. For the M-CaAl sample, nanorods with diameter of 50–120 nm and length of 200–400 nm were distributed on the surface of the mesoporous Ca-containing Al2O3 matrix, as shown in Fig. 4(a).

On the other hand, nanorods were observed growing outward from the pore structure inside a mesochannel in the magnified TEM image inFig. 4(b), where the pore size distribution of the meso-structured matrix was ca. 10–50 nm. It is suggested that when

the Ca species were loaded into the ordered mesoporous Al2O3

dur-ing microwave-assisted synthesis, a reaction between the meso-porous Al2O3 frameworks and the Ca ions could occur, forming

an intermediate compound that was then transformed into the mesoporous calcium aluminates with Ca12Al14O33nanorods during

the subsequent calcinations. During the synthetic process, the or-dered mesoporous frameworks would be consumed, forming the larger mesopores. The high-resolution TEM (HRTEM) and FT (Fou-rier transform) diagram inFig. 4(d) reveal that the nanorods are corresponding to Ca12Al14O33but seem to be poorly crystalline in

structure because the indices of diffraction rings are consistent with the XRD peaks of the (2 1 1), (3 1 0), (3 2 1), and (4 2 0) reflections.

When the Ca/Al molar ratio was increased to 2, the TEM image of the M-2CaAl sample inFig. 4(c) showed that no nanorods can be disclosed on the mesoporous frameworks, and the pore size distri-bution became non-uniform, with large nanopore sizes greater than 50 nm. Furthermore, most of the pores seem to be covered by excessive and large-scale CaO on the surface of the larger nano-porous calcium aluminates, limiting the growth of nanorods under calcinations at high temperature. Furthermore, TEM, EELS map-ping, and FT (Fourier transform) diagrams were used to character-ize the crystal structure and elemental distribution of the calcined mesoporous calcium aluminate nanocomposites (M-CaAl) inFig. 5. The M-CaAl sample shows a mesoporous structure with nanorods forming the nanocomposites inFig. 5(a). In addition, two images of the EELS mapping for Ca and Al were shown inFig. 5(b) and (c), respectively.Fig. 5(d) is the overlapped image ofFig. 5(b) and (c), which can be clearly divided into two parts: (1) the Ca or Al atoms may be homogeneously dispersed in the mesoporous structure

Fig. 4. TEM images of a calcined mesoporous M-CaAl sample, (a) at low magnification and (b) high magnification, displaying nanorods grown from inside a pore channel of mesoporous oxides, and (c) TEM images of a calcined mesoporous M-2CaAl sample. Image (d) shows the high resolution TEM image and FT pattern of the boxed area of M-CaAl in image (b). Note: the prepared M-CaAl sample with Ca/Al ratio = 1, and M-2CaAl sample with Ca/Al ratio = 2.

with elemental compositions of Ca-containing Al2O3frameworks;

(2) the heterogeneous and aggregative Ca atoms are observed for Ca12Al14O33nanorods that grow either from the inside

mesostruc-tural channel or outside mesostrucmesostruc-tural surface. Hence, the Ca12Al14O33nanorods could be formed by a nucleation process

be-tween calcium species and mesoporous Al2O3, with subsequent

growth in the later annealing process.

The Al 2s core level and valence band of the as-synthesized sample (after microwave hydrothermal treatment) and calcined sample (M-CaAl) were further analyzed using an XPS measure-ment. The XPS spectra inFig. 6(a) indicate that the microwave-heated sample mainly displays the Al 2s of hydrated alumina (Al–OH and Al–OOH) at 116 eV of binding energy. In contrast, the XPS spectrum of the calcined sample (M-CaAl) exhibited the Al 2s sharp peak at 123 eV of binding energy, indicating the for-mation of calcium–aluminum oxides after calcination at high temperature. Further, the FTIR spectra of the as-synthesized and calcined samples (M-CaAl) in Fig. 6(b) evidenced the difference between microwave-assisted and calcined treatments. For the microwave-assisted treatment, the pair of bands at 953 and 1020 cm 1 _{may be associated with the characteristic vibrations}

of the Al–OH bonding. The O–H stretching vibration bands (Ca/Al–OH) can be clearly observed at 3650 and 835 cm 1_{. The}

broad peaks at 625 and 2100 cm 1 _{could be due to stretching}

and bending modes of AlO–(OH)[22]. The broad absorption band in the region 3200–3600 cm 1_{and the intense bands at 1380 and}

1640 cm 1 _{are due to the vibration of H}

2O molecules that took

part in hydrogen bonding with the Al2O3 surface[23]. It is

im-plied that a possible hydrated formation of Ca–Al2O3occurred

be-tween mesoporous Al2O3 and calcium species during microwave

hydrothermal treatment. Additionally, according to the previous report [24,25], FTIR spectra of inorganic aluminates varied with aluminum coordination number, the state of the coordination group (‘‘isolated’’ or ‘‘condensed’’) and the vibrational coupling between neighboring groups. The above-mentioned results led to a conclusion that the characteristic absorption regions of Al–O stretching vibrations are 650–800 cm 1 _{for ‘‘isolated’’ and}

700–900 cm 1_{for ‘‘condensed’’ AlO}

4tetrahedra and 400–530 cm 1

and 500–680 cm 1 _{regions for ‘‘isolated’’ and ‘‘condensed’’ AlO} 6

octahedra. In FTIR spectra, the Al–O stretching vibrations for tetrahedral AlO4are observed in the 750–850 cm 1spectral region.

At the same time, for octahedral AlO6, they were observed at

500–750 cm 1_{. In the OH-stretching region of the FTIR spectrum}

of the calcined sample, both a sharp peak at 3650 cm 1(Ca–OH and Al–OH bond) and a broad band at 3450 cm 1_(H

2O bond) are

observed. The FTIR analysis of the calcined sample (M-CaAl) is consistent with the XRD results. Additionally, in order to under-stand the formation of nanorods–nanotubes, XRD and SEM analy-ses were performed. The XRD pattern in Fig. 6(c) showed that the as-synthesized samples after microwave hydrothermal treat-ment exhibited an amorphous crystallite, indicating it could be hydroxylated species (Ca-containing Al2O3matrix). The SEM image

inFig. 6(d) of the as-synthesized sample revealed that no nano-rods–nanotubes appeared. The results can provide the evidence

Fig. 5. (a) TEM image of the M-CaAl sample with Ca/Al ratio = 1, and the corresponding qualitative element mapping for (b) Al and (c) Ca by EELS analysis, (d) the overlap image of the (b) and (c) resulting images. (Note: green dots indicate Ca, and red dots indicate Al). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. (a) X-ray photoelectron and (b) FTIR spectra of the M-CaAl sample (Ca/Al ratio = 1) prepared by microwave hydrothermal treatment for 1 h and calcined at 600 °C, (c) large-angle XRD pattern and (d) SEM image of the M-CaAl sample after microwave hydrothermal treatment.

Fig. 7. (a) Large-angle XRD patterns of calcined samples (M-CaAl) with different reactive times at 80 °C during microwave hydrothermal (MH) reaction, followed by calcination at 600 °C, (b) and (c) SEM images of calcined samples (M-CaAl-MH-2 h) taken from two different areas. (Note: the prepared M-CaAl sample with Ca/Al ratio = 1.)

that the as-synthesized sample appeared as hydroxylated species after hydrothermal treatment step but would be transformed into nanorods–nanotubes after high-temperature calcinations. 3.3. Formation mechanism of mesoporous calcium aluminate nanocomposites

Fig. 7(a) shows the PXRD patterns of calcined samples (Ca/Al molar ratio = 1) prepared by the microwave assisted process at 80 °C with hydrothermal times ranging from 15 min to 2 h. Firstly, no diffraction peaks were detected in PXRD patterns for samples MH-15 min and MH-30 min, indicating that these samples may ex-ist in a poorly crystalline or amorphous Ca–Al–O solid solution. Upon increasing the microwave hydrothermal time to 1 h, a Ca12Al14O33phase can be clearly detected from the XRD pattern,

which is supported by the SEM image inFig. 3(b) which showed Ca12Al14O33 nanorods grown on the mesoporous surface. As the

microwave hydrothermal time was extended to 2 h, Fig. 7(b) shows that more nanorods and fewer nanotubes were found to be dispersed on the mesoporous Ca-containing Al2O3matrix, but

some aggregative and large CaO particles were also observed in the same sample, as shown in Fig. 7(c). It can be inferred that longer microwave heating times may decrease the attractive force between Ca2+_{and Al–OH, and some of the calcium ions remaining}

outside the surface of mesoporous Al2O3 rapidly migrated and

aggregated to form the larger CaO particles during subsequent cal-cination, as supported by the PXRD results ofFig. 7(a). On the other hand, if the microwave-assisted reaction was allowed to proceed for more than 3 h, it was found that more crystalline Ca12Al14O33

nanotubes may be formed, indicating that growth of the crystalline Ca12Al14O33 nanorods or nanotubes varies with microwave

reac-tion condireac-tions.

The formation mechanism of mesoporous calcium aluminate nanocomposites is further illustrated in Scheme 1. The process for the formation of mesoporous Al2O3is based on so-called

‘‘evap-oration induced self-assembly (EISA)’’[26,27]. When subjected to calcination at high temperature (700 °C) to remove the surfactant, a polycrystalline

c

mesostructure and high thermal stability (designated as mesopor-ous Al2O3), which corresponds to the PXRD inFig. 2(a). As

meso-porous

c

in alcohol solution during the microwave hydrothermal reaction, the formation pathway of the C12Al14O33nanorods may be related

to Al2O3 hydration and a nucleation process between Ca species

and Al2O3, according to the PXRD, SEM, XPS and FTIR results. In

the solution, the mesoporous Al2O3 crystallites may be hydrated

to form amorphous mesoporous hydrated-Al2O3possessing highly

concentrated oxy-hydroxide and hydroxide groups (such as AlO–OH and AlOH) on the Al2O3 surface. During the microwave

synthesis, Ca ions were attracted and promoted toward the hy-drated-Al2O3 matrix, depositing inside or outside the surface of

the mesoporous hydrated-Al2O3 surface and leading to the

pre-crystal formation (such as AlO–Ca–OH or CaAlO(OH)x;

desig-nated as mesoporous Ca-bonded Al2O3). Next, the solid-state

reac-tions (calcinareac-tions) could promote the phase transformation of the mesoporous calcium aluminate nanocomposites, in which two types of calcium aluminates (amorphous Ca-containing Al2O3 or

crystalline C12Al14O33 nanorods) could be formed depending on

Ca ion content. The Ca-rich pre-crystal complexes inside the mes-ochannel pore or outside the surface of Al2O3mesochannel could

react with more Al2O3nano-frameworks to grow pure Ca12Al14O33

nanorods after calcination at high temperature. In contrast, the Ca-poor complexes deposited on the Al2O3surface may be doped into

the Al2O3frameworks to form an amorphous Ca–O–Al solid

solu-tion. In summary, hydrothermal time and elemental composition (Ca/Al) play important roles in forming mesoporous calcium alu-minate nanocomposites via the microwave hydrothermal process. 4. Conclusion

Novel mesoporous calcium aluminate (Ca12Al14O33)

nanocom-posites with nano-scale rods and mesoporous frameworks have been successfully synthesized using the microwave-hydrothermal process and calcination below 600 °C. The mesoporous calcium aluminate nanocomposites possessed a specific surface area of 51 m2_g 1 _{and a broad pore size distribution of 4–12 nm. The}

highly dispersed Ca12Al14O33nanorods on the mesoporous

frame-works can be obtained by controlling the Ca/Al molar ratio to be 1:1; in such samples, the polycrystalline-like C12Al14O33nanorods

were grown in and on the one-dimensional mesochannels. A for-mation mechanism of the mesoporous calcium aluminate nano-composites involving amorphous mesoporous hydrated-Al2O3

was also proposed. Mesoporous Ca-Al metal oxide materials with highly dispersed C12Al14O33nanorods inside or outside the

meso-porous framework could be used to treat natural gas for hydrogen gas production for applications in fuel cells, power plants or fuel combustion in vehicles.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Science Council in Taiwan through Contract NSC-101-3113-E-009-003.

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