CRYSTAL GROWTH & DESIGN
Controllable Fabrication of Mesoporous MgO with Various Morphologies and Their Absorption Performance for Toxic Pollutants in Water
2008 VOL. 8, NO. 10 3785–3790
Cuiling Gao,† Wenli Zhang,‡ Hongbian Li,† Leiming Lang,† and Zheng Xu*,† State Key Laboratory of Coordination Chemistry, Laboratory of Solid State Microstructures, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P.R. China, and School of Chemistry and Chemical Engineering, Jiangsu UniVersity, Zhenjiang 212003, P.R. China ReceiVed April 23, 2008; ReVised Manuscript ReceiVed June 24, 2008
ABSTRACT: We have developed a method of controllable synthesis of MgO with various morphologies by adjusting the composition and phase structure of magnesium carbonate hydrate (MCH). The phase structure of the MCH varied from monoclinic Mg5(CO3)4(OH)2(H2O)4 to hexagonal MgCO3 by changing the concentration of Mg2+ and HCO3-; the corresponding morphology of the MCH ranges from nanoflakes and flowerlike microspheres composed of nanoflakes to layer-like rhombohedra and microspheres composed of rhombohedra. After annealing, four kinds of mesoporous cubic MgO micronano structures were obtained with their original morphologies. Because of the high specific surface area, MgO mesoporous nanoflakes and flowerlike microsphers exhibited excellent absorption performance for common toxic heavy metal ions and organic pollutants and are expected to be a potential absorbent in wastewater treatment. Introduction Nanomaterials, with novel size- and shape-dependent properties, as well as their unique applications that complement those of their bulk counterparts, have been extensively investigated for over a decade.1,2 The controllable synthesis of the nanoparticles with desirable morphology has been pursued to realize the design of novel functional materials and devices. Soft 3–5 and hard 6–8 templates are extensively adopted for this end, but preparation and removal of templates are tedious and templateremoving may result in the disorder or destruction of the structures. As we well know, composition and phase structure of the crystals have significant effects on their morphology. If the precursors with different morphologies may be prepared via adjusting the composition and phase structure, the desirable nanoparticles with different morphologies are obtained after simple post-treatment. It might be a way to fabricate the nanoparticles with various morphologies. Unfortunately, there are few reports on the controllable synthesis of the morphology by this approach. Magnesium carbonate hydrate (MCH) is a good candidate for this goal because MCHs have a diversity of crystal morphologies, which vary with their composition and phase structure. For example, Mg5(CO3)4(OH)2 · 4H2O has two morphologies: nanosheets and the self-assembly of nanosheets.10–12 MgCO3 · 3H2O crystallizes into a needle-like structure,9,13 and MgCO3 into a rhombohedral structure.14 So, it is possible to realize the controllable synthesis of different morphologies by adjusting composition and phase structure of MCH. Furthermore, MCH is easy to transform into MgO by simple annealing; therefore, the MgO with different morphology can be obtained after annealing these MCH microstructures. MgO, as a typical wide-band gap insulator (7.2 eV), is an important functional material with various applications in catalysis, refractory material industries, paints, and superconductors.15 Especially, MgO is a weak base with the maximum pH * To whom correspondence should be addressed. E-mail: zhengxu@ netra.nju.edu.cn. Phone: (+86)258-359-3133. Fax: (+86) 258-331-4502. † Nanjing University. ‡ Jiangsu University.
of 10 which meets the Clean Water Act basic limits; the sludge formed in the water treatment process is easier to precipitate and filter than that formed by other alkalis.16 In addition, MgO possesses minimal environmental impact and low solubility and is essential for plant, animal, and human life. Many researchers have adopted MgO and Mg(OH)2 as an absorbent to remove toxic ions and organic pollutants from water.17,18 The Klabunde group has investigated the adsorption of organic species on the surface of nanoscale MgO.19,20 Recently, numerous works have focused on the synthesis of various morphologies of magnesium salts and MgO, such as nanorods,21 fishbone fractal nanostructures induced by Co,22 nanowires,23 nanobelts,24 nanotubes25 and three-dimensional entities,26 and nanocubes.27 However, there are few reports on the self-assembly of MgO into complex structures,12,28,29 let alone the controllable synthesis of mesoporous MgO with various morphologies. Therefore, it is urgent to develop a facile method to fabricate MgO with various morphologies and adopt them as an absorbent to investigate the influence of morphologies on the adsorption capacity for pollutants. In this paper, by changing the concentration of Mg2+ and HCO3-, monoclinic Mg5(CO3)4(OH)2(H2O)4 with nanoflakes and flowerlike microspheres composed of flakes and hexagonal MgCO3 with layer-like rhombohedra and microspheres composed of rhombohedra were synthesized. After annealing, four kinds of mesoporous micronano structures of MgO were obtained with their original morphologies. This is a good example of the controllable synthesis of different morphologies of nanoparticles by adjusting the components and the crystal phases of the precursors. Because of the high specific surface area, mesoporous MgO nanoflakes and flowerlike microspheres exhibit excellent absorption performance for common toxic heavy metal ions and organic pollutants and are expected to be a potential absorbent in wastewater treatment. Experimental Section Preparation. In a typical synthesis of nanoflakes, pluronic P123 (EO20PO70EO20, Aldrich) (1 g), MgCl2 · 4H2O (0.46 g, 0.2 M), and NaHCO3 (0.168 g, 0.2 M) were dissolved into 20 mL of distilled water for 3 h; then the above solution was transferred into a 30 mL Telfon-
10.1021/cg8004147 CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
3786 Crystal Growth & Design, Vol. 8, No. 10, 2008
Gao et al.
Figure 1. SEM images of (a) Mg5(CO3)4(OH)2(H2O)4 nanoflakes (reaction condition 0.2 M MgCl2 · 4H2O and 0.2 M NaHCO3); (b) Mg5(CO3)4(OH)2(H2O)4 microspheres composed of nanoflakes (1 M MgCl2 · 4H2O and 1 M NaHCO3); (c) MgCO3 layer-like rhombohedra (1 M MgCl2 · 4H2O and 2 M NaHCO3); and (d) MgCO3 microspheres composed of rhombohedra (1 M MgCl2 · 4H2O and 3 M NaHCO3). lined stainless steel autoclave (a Teflon cup in a stainless steel-lined autoclave). The autoclave was maintained at 100 °C for 20 h and allowed to cool to room temperature. The Mg5(CO3)4(OH)2(H2O)4 nanoflakes were obtained. When the concentration of MgCl2 · 4H2O and NaHCO3 increased to 1 M and the reaction temperature raised to 140 °C, the microspheres composed of nanoflakes were obtained. When the concentration of MgCl2 · 4H2O maintained at 1M but the concentration of NaHCO3 increased to 2M, the product became MgCO3 rhombohedra. Under the same experimental condition as for rhombohedra and further increasing the concentration of NaHCO3, MgCO3 microspheres ramified by the rhombohedra were obtained. The products were collected by centrifugation and rinsed more than five times with ethanol and dried at 60 °C for 5 h. Characterization. The products were characterized by scanning electron microscopy (SEM, JEOL JSM-5610 LV), transmission electron microscopy (TEM, JEM-200CX), high-resolution transmission electron microscopy (HRTEM, JEM-400CX), X-ray diffraction (XRD, Shimadzu X-6000 X-ray diffractometer with graphite monochromatized Cu KR radiation), and Fourier transform IR (FTIR, VECTOR 22 from BRUKER). Water Treatment Experiment. In the experiments, K2Cr2O7 was used as the source of Cr(VI). The pH values of the solutions were adjusted using 0.1 M HCl. The as-prepared MgO microspheres composed of nanoflakes were used as adsorbents. The Cr(VI) solution containing 10 mg · L-1 Cr(VI) was prepared and adjusted to pH 4. Then, 15 mg of the adsorbent sample was added to 25 mL of the above solution under stirring. After a given time, the solid and liquid were separated, and Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP, JA1100) was used to measure the chromium concentration remaining in the solutions. Taking Cd(II) and Pb(II) as example of the low valence heavy metal ion, 10 mg of absorbent was added into 10 mL of solution with 100 mg · L-1 Cd(II) and Pb(II) with stirring for 3 h, the solid and liquid were separated, and ICP was used to measure heavy metal ion concentration remaining in the solutions. For the adsorption of organic pollutants, 20 mL of Orange II solution (100 mg · L-1) was mixed with 10 mg of absorbent, and UV-vis adsorption spectra (Shimazu, UV3100) were recorded at different time intervals to monitor the process.
Results and Discussion The SEM images (Figure 1) clearly showed the evolution process of the morphologies with the concentration of reactants increasing. At the lower concentration (0.2 M MgCl2 · 4H2O and 0.2 M NaHCO3), the product was nanoflakes (Figure 1a).
Figure 2. XRD of four structures: (a) Mg5(CO3)4(OH)2(H2O)4 nanoflakes, (b) Mg5(CO3)4(OH)2(H2O)4 microspheres composed of nanoflakes, (c) MgCO3 layer-like rhombohedra, and (d) MgCO3 microspheres composed of rhombohedra; (e) the XRD of these four structures after annealing at 650 °C.
Controllable Fabrication of Mesoporous MgO
Crystal Growth & Design, Vol. 8, No. 10, 2008 3787
Figure 3. IR spectra of (a) monoclinic Mg5(CO3)4(OH)2(H2O)4 microspheres composed of nanoflakes and (b) hexagonal MgCO3 microspheres composed of rhombohedra.
Figure 4. (a) SEM image of MgO nanoflakes; (b) the TEM image of an individual nanoflake; (c) the HRTEM image of nanoflakes; (d) SAED pattern of individual nanoparticle fallen apart from the flake; (e) the SEM image of MgO microspheres composed of nanoflakes; (f) the magnified FESEM image of the MgO microsphere composed of nanoflakes.
Keeping the ratio of MMg2+/MHCO3- (1:1) constant and increasing the concentration of both reactants to 1M, these nanoflakes self-assembled into microspheres (Figure 1b). When the concentration of MgCl2 · 4H2O was maintained at 1M but the concentration of NaHCO3 was increased to 2 M (MMg2+/MHCO3) 1:2), the product became MgCO3 rhombohedra (Figure 1c). Further increasing the concentrations of both NaHCO3 and Mg2+, but keeping their ratio constant (1:2), MgCO3 microspheres assembled by the rhombohedra were obtained (Figure
1d). The crystal phase of the product was measured by XRD. The XRD patterns (Figure 2) showed that the nanoflakes (Figure 2a) and the flowerlike microspheres composed of nanoflakes (Figure 2b) can be indexed as monoclinic symmetry Mg5(CO3)4(OH)2(H2O)4 (JCPDS Card No. 70s0361), while layerlike rhombohedra (Figure 2c) and the microspheres assembled by rhombohedra (Figure 2d) can be assigned to hexagonal symmetry MgCO3 (JCPDS Card No. 80-0042). Figure 3 showed the infrared (IR) spectra of the microspheres composed
3788 Crystal Growth & Design, Vol. 8, No. 10, 2008
Gao et al.
Figure 5. (a) SEM image of rhombohedra of MgO; (b) the TEM image of an individual rhombohedron; (c) the SEM image of MgO microspheres composed of rhombohedra; (d) the FESEM image of a part of a MgO microsphere.
Figure 6. SEM images of the product reacted for (a) 0.5 h, (b) 2 h, (c) 6 h, and (d) 12 h. Table 1. Absorption Capacities for Removing Cr(VI) and Orange G of MgOa commercial MgO BETb surface area (m2 · g-1) Cr(VI)(mg · g-1) Orange G(mg · g-1)
product I
product II
product III
38
135
118
35
10.2 81
15.2 122
19.8 156
12.9 95
product IV 46 13.9 102
a Product I: Nanoflakes; II: mesoporous microspheres composed of nanoflakes; III: rhombohedra; IV: microspheres composed of rhombohedra. b Brunauer-Emmett-Teller.
of nanoflakes and rhombohedra, respectively. In Figure 3a, the sharp bands between 3600 and 3400 cm-1 were attributed to free O-H vibration. The bands at 1484 and 1422 cm-1 were characteristic absorption bands of CO32-.29 All the absorption bands were in agreement with the standard IR spectrum of Mg4(CO3)3(OH)2.3H2O (No. 41581P) which should have the same bands of the CO32-, OH-, and H2O with the Mg5(CO3)4(OH)2(H2O)4. For the rhombohedras, the sharp bands between 3600 and 3400 cm-1 disappeared, and the IR spectrum was in accordance with the standard IR spectrum of MgCO3 (No. 28543K). This further confirms that different building block nanoflakes and rhombohedra possessed different composition. After annealing Mg5(CO3)4(OH)2(H2O)4 and MgCO3 at 650 °C, the cubic phase MgO with the same morphologies as the precursors was obtained (Figures 4a, 4e, 5a, 5c). The XRD patterns of the products shown in Figure 2e can be assigned as cubic MgO phase (JCPDS Card No. 78-0430). The SEM image of MgO nanoflakes (shown in Figure 4a) indicated that they were irregular with the thickness about 30 nm. The TEM image
of the sample (Figure 4b) indicated that the nanoflake was composed of nanoparticles and there were many pores on it with an aperture of about 20-50 nm. The high-resolution TEM (HRTEM) image of the nanoparticles constituting the nanoflakes displayed clear lattice fringes with a spacing of 0.214 nm (Figure 4c), in good agreement with the (200) plane of cubic MgO. Selected area electron diffraction (SAED) pattern of the nanoparticle (Figure 4d) showed the single crystalline nature. The SEM images of the flowerlike MgO microspheres shown in Figure 4e,f indicated that they were composed of nanoflakes with the diameter ranging from 20 to 30 um. SEM (Figure 5a) and TEM (Figure 5b) images of rhombohedra showed the layerlike structure and mesoporous feature. The SEM image of MgO microspheres self-assembled by rhombohedra was shown in Figure 5c. The magnification SEM image of these microspheres (Figure 5d) displayed that the rhombohedra consisted of nanoparticles. The experimental results indicated that the concentrations and mole ratio of the reactants had a crucial influence on the morphologies and phase structures of the precursors. When MHCO3-/MMg2+ < 2:1, Mg5(CO3)4(OH)2(H2O)4 nanoflakes were formed with a monoclinic phase structure. When the concentrations, under a constant mole ratio of MHCO3- to MMg2+ (
