J. Phys. Chem. C 2007, 111, 10267-10272
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Characterization and Photoluminescence Properties of MgO Microtubes Synthesized from Hydromagnesite Flowers C. M. Janet, B. Viswanathan,* R. P. Viswanath, and T. K. Varadarajan National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai- 600 036, India ReceiVed: April 1, 2007; In Final Form: May 11, 2007
Monoclinic hydromagnesite (Mg5(CO3)4·(OH)2·4H2O) flower-like structures of 2-3 µm diameter have been synthesized at room temperature by a simple precipitation method without using any template, catalyst, or surfactant. The as-synthesized flowers were further calcined at various temperatures to prepare monodispersed, nanofibrous MgO microtubes of aspect ratio ∼15. Both the precursor flowers and the MgO microtubes formed were characterized by powder XRD, TG-DTA, FT-IR, SEM, EDAX, HR-TEM, SAED, BET analysis and photoluminescence (PL) studies. A plausible mechanism for the formation of cubic MgO microtubes from monoclinic hydromagnesite flowers is proposed. The as-synthesized nanofibrous MgO microtubes have shown intense green and red emission. The more defective MgO microtubes formed in N2 atmosphere at 400 °C showed the highest PL intensity and surface area. An attempt has been made to correlate the lattice parameter and the PL intensity.
1. Introduction Recent interest in the nano and microstructures has given thrust to engineer the materials with morphologies of any design of interest for novel device applications. Various methodologies have been adopted for the precise control over the structure and properties of materials.1-6 Template-assisted methods, vapor deposition techniques, and solvothermal and hydrothermal synthesis are a few among them.7-10 Materials which will selfassemble under the ambient preparation conditions can be finely tuned to get the morphologies of interest by the variation in temperature, pH, rate, and the nature of precipitation.11-16 Synthesis of hydrated magnesium carbonate hydroxide (hydromagnesite) architectures has been given much attention due to the applications in fire-retardants, rubber and plastic industries.17 It is a versatile precursor for the synthesis of MgO nanostructures of different architectures.18-25 MgO is widely used as a refractory material, sorbent, catalyst, and catalytic support in catalysis. Since commercially available MgO is of low surface area, synthesis of MgO with high surface area is of special interest in catalysis.26-28 Defective lattice of MgO are wellknown for its luminescent properties which are being utilized in sensors. MgO substrates are currently being used in IR sensors. MgO with tubular morphologies are recently used as high temperature thermometers. Ga-filled MgO nanotubes have already proved this phenomenon.29 Microtubes of MgO are used as a carrier for the sustained release, encapsulating agents and microreactors.30,31 There are many reports available in the literature where drastic conditions such as high temperatures and pressures are employed in the synthesis of hydromagnesite architectures by hydrothermal route. In this paper, the synthesis of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) flowers at room temperature by the simple precipitation method without using any template, catalyst or surfactant is described. The assynthesized flowers are calcined further at various temperatures * To whom the correspondence should be addressed. E-mail: bvnathan@ iitm.ac.in. Fax: 091-44-22574202.
in different atmospheres to produce MgO microtubes. Photoluminescence studies have been carried out for the assynthesized MgO microtubes. 2. Experimental Section 2.1. Synthesis of Hydromagnesite Flowers and MgO Microtubes. All reagents used in this experiment are of analytical grade. In a typical procedure, 60 mL of 1M Na2CO3 solution in a burette was added to the 100 mL of 1 M Mg(NO3)2·6H2O solution at a rate of 3 mL min-1 under vigorous stirring. A white precipitate was obtained, and it was washed thoroughly with deionized water by filtration followed by drying at 60 °C in an air oven for 4 h. The dried precipitate was confirmed as hydromagnesite by XRD powder pattern. The assynthesized hydromagnesite was calcined at 400, 600, 700, and 800 °C in air as well as in N2 atmosphere in order to produce nanofibrous MgO microtubes. MgO samples prepared are denoted along with the conditions of preparation in brackets. 2.2. Characterization. Characterization has been carried out using XRD, TG-DTA, SEM, HR-TEM, EDAX, SAED, FTIR, BET surface area, and photoluminescence studies. The specific surface area, pore size and pore volume of the samples were measured using Micromeritics ASAP 2020 instrument at 77 K. Prior to the sorptometric experiment, the samples were degassed at 150 °C for 12 h. Powder X-ray diffraction patterns of samples were recorded using a SHIMADZU XD-D1 diffractometer using Ni-filtered Cu KR radiation (λ ) 1.5406 Å). The surface morphology of both the hydromagnesite and MgO tubes were analyzed by a FEI, Model: Quanta 200 scanning electron microscope operating at 30 kV. High-resolution transmission electron microscopy (HR-TEM) analysis was performed using a JEOL-3010 transmission electron microscope operating at 300 kV. Samples for TEM were prepared by dispersing the powdered sample in ethanol by sonication and then drip drying on a copper grid (400 mesh) coated with carbon film. Photoluminescence (PL) measurements were carried out on a HITACHI 850-type visible ultraviolet spectrophotometer
10.1021/jp072539q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
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Figure 1. XRD powder pattern of as-synthesized hydromagnesite flowers.
Figure 2. XRD powder pattern of MgO samples. (a) MgO (400 °C, N2, 4 h); (b) MgO (600 °C, N2, 4 h); (c) MgO (800 °C, N2, 4 h); (d) MgO (400 °C, air, 30 min); (e) MgO (400 °C, air, 4 h); (f) MgO (700 °C, air, 4 h).
with a Xe lamp as the excitation light source. The FT-IR spectra for the samples were recorded in the PERKIN-ELMER instrument in the range 400-4000 cm-1 at room temperature. The thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed with NETZSCH STA 409 Cell instrument at a heating rate of 10 °C min-1 in static air and N2. 3. Results and Discussion The crystal structures of the as-synthesized hydromagnesite and MgO formed at different temperatures were confirmed by powder X-ray diffractometric analysis. As shown in Figure 1, all diffraction peaks in the XRD pattern of hydromagnesite were indexed to be monoclinic Mg5(CO3)4(OH)2·4H2O with unit cell parameters of a ) 10.10, b ) 8.94, and c ) 8.38 Å and β ) 114.58° as confirmed from the reported data (JCPDS file no. 25-0513).32,33 XRD powder pattern of the MgO samples prepared by the decomposition of hydromagnesite is shown in Figure 2. XRD powder pattern of the MgO samples were indexed to be cubic (NaCl type) and was found to be matching with JCPDS file no. 79-0612. XRD pattern of the MgO (400 °C, air, 30 min) shows one extra peak due to the surface
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Figure 3. FT-IR spectrum of as-synthesized hydromagnesite (Mg5(CO3)4(OH)2·4H2O).
Figure 4. FT-IR spectra of MgO samples. (a) MgO (400 °C, air, 30 min); (b) MgO (400 °C, N2, 4 h); (c) MgO (400 °C, air, 4 h); (d) MgO (600 °C, N2, 4 h); (e) MgO (700 °C, air, 4 h).
adsorbed entity such as carbonate. Even after 4 h treatment in the air at 400 °C, the extra peak was retained. However, after calcination at 700 °C in air, peaks corresponding to only MgO are observed. MgO (400 °C, 4 h, N2) did not show any extra peaks in the XRD pattern. This confirms the source of contaminant for the extra peak that is indicated in the XRD pattern (Figure 2d,e) is the atmospheric air. As the calcination temperature is increased to 600 and 800 °C, irrespective of the atmosphere, there was a decrease in the lattice parameter. This trend is expected due to the contraction of the lattice at high temperatures. The lattice parameter of MgO (400 °C, N2, 4 h) is found to be larger than that of the MgO (400 °C, air, 4 h), as shown in Table 1. This difference can be attributed to the defective lattices formed in the N2 atmosphere due to the lack of sufficient oxygen during the transformation of the monoclinic hydromagnesite to the cubic MgO. In air calcined samples even at lower temperatures, almost optimum lattice dimensions are achieved to get MgO with fewer defects. With the increase in temperature, a crystalline lattice is expected and hence, the lattice contraction as observed for the high-temperature MgO is quite reasonable. Crystallite size was calculated using the Scherrer’s equation,
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TABLE 1: Surface Area and Crystalline Properties of the Hydromagnesite and MgO Samples
sample
crystallite size (nm)
lattice parameter (Å)
Mg5(CO3)4(OH)2.4H2O MgO (air, 400 °C, 30 min) MgO (air, 400 °C, 4 h) MgO (air, 700 °C, 4 h) MgO (N2, 400 °C, 4 h) MgO (N2, 600 °C, 4 h) MgO (N2, 800 °C, 4 h)
9.8 13.7 18.4 10.2 11.8 15.4
4.1784 4.1780 4.155 4.244 4.184 4.172
BET surface area (m2/g)
pore volume (cm3/g)
46.06
0.137
35.47 24.43 84.86 29.21 19.45
0.058 0.039 0.264 0.180 0.032
and given in Table 1. Crystallite size of the MgO samples increases with the increase in calcination temperature due to sintering. FT-IR spectrum of the hydromagnesite flowers has been given in Figure 3. Carbonate ion in the hydromagnesite exists as bicarbonate and a strong band that is splitting into two around 1424 and 1482 cm-1 corresponds to the asymmetric stretching vibrations of bicarbonate ions. Three absorption bands at around 800 (the strongest one), 854, and 885 cm-1 are assigned to the CO32- bending vibration. CO32- symmetric stretching vibration shows its corresponding band at around 1119 cm-1. A very broad band at around 985 cm-1 corresponds to the bending vibrations of adsorbed bicarbonate. Bands corresponding to the water of crystallization are observed around 3516 and 3448 cm-1. An overlapped, free O-H vibration band is observed around 3648 cm-1. A broad shoulder band at around 1645 cm-1 has been attributed the O-H bending mode of water. FT-IR spectral study thus confirms the formation of the hydromagnesite.34,35 In Figure 4, FT-IR spectra of the calcined hydromagnesite at different temperatures and atmospheres are given. It can be seen that the heat treatment at 400 °C for 30 min in air forming an intermediate species where the splitting of antisymmetric stretch of bicarbonate ion disappeared and became a single band. This shows the formation of CO2 from the bicarbonate. CO2 formed again reacts with O2- from MgO surface and will be adsorbed as CO32-. Hence, the bands due to carbonate and OH- groups are unavoidable even at high temperatures. Stretching vibrations of H-bonded hydroxyl groups can be seen as a broad band around 3500 cm-1 in all calcined samples irrespective of the calcination temperature and the environment used for the calcination. At low temperature (c) as well as the sample calcined in N2 environment (b and d), there is a band around 3700 cm-1 due to the stretching of isolated O-H groups. However, at high-temperature this peak vanishes (e). As can be seen from the spectra, increase in temperature results in the decrement in the bicarbonate stretching and a corresponding enhancement in the bands around 860 and 1080 cm-1, which are due to the symmetric stretch of carbonate ions are observed. The bands around 540 cm-1 and 700 cm-1 present in spectra b, d, and e are due to the longitudinal optical (LO) phonon modes characteristic of well-defined MgO crystallite.36 It is well-known that H2O and CO2 molecules are easily chemisorbed onto MgO surface when exposed to the atmosphere. In Figure 4a-c, a very weak band corresponding to the adsorption of gas-phase CO2 is visible at around 2420 cm-1. A shift is observed for the CO32- stretching frequency to higher wavenumbers from 1380 to 1454 cm-1 as the MgO crystallites are formed. In hydromagnesite, CO32- was acting as a triply bridging center whereas after calcination, CO32- chemisorbed will be in a monodentate fashion and hence, a shift is observed towards the higher wavenumbers.37
Figure 5. TGA of as-synthesized hydromagnesite (a) in N2; (b) in static air. Inset: DTA of hydromagnesite (A) in N2; (B) in static air (scan rate: 10 °C min-1).
TGA analysis done under static air and N2 show a considerable difference in the nature of decomposition of the hydromagnesite as expected. Decomposition of Mg5(CO3)4. (OH)2·4H2O was well studied in literature.38,39 A three-step decomposition involving the first removal of the water of crystallization around 200 °C followed by the removal of CO2 at 300 °C and further decomposition of the hydroxide at 350400 °C yields a total of 5 MgO as observed from the TGA in N2 atmosphere (Figure 5a) with a weight lose of ≈ 60%. The percentage of remaining mass theoretically should be ∼43, which correspond to 5 MgO. An observed 3% difference may be due to the presence of the surface adsorbed water molecules. In the static air measurement, around 3% increase in the remaining mass is observed compared to the expected theoretical value (Figure 5b). This occurs due to the involvement of atmospheric H2O and CO2 in the static air. Even after the removal of CO2 in the second step, it is again getting adsorbed on the MgO formed. Hence, in static air measurement a third weight loss is not observed even though it takes place. This observation can be further supported by the presence of extra peak in the XRD pattern of the 400 °C calcined sample in the air. Corresponding N2 treated sample did not show any extra peaks in XRD. From TG-DTA analysis, it is evident that the formation of MgO takes place at a lower temperature in the static air than that of N2 atmosphere and the corresponding temperature difference is 10 °C in both TGA and DTA. BET surface area studies show that the hydromagnesite flowers as-synthesized have a surface area around 46 m2/g as given in Table 1. Among the calcined samples, MgO (400 °C, N2, 4 h) shows a maximum surface area of 85 m2/g and this is a remarkable surface area for the catalytic applications of MgO. Irrespective of the environment used for calcination, the samples calcined at high temperatures show lesser surface area due to sintering. In air atmosphere, sintering and closing of the pores are more pronounced. This leads to the bigger crystallite size and hence lesser the surface area observed. From the BJH pore size analysis, it was found that the MgO samples prepared are found to have a distribution of micro and mesopores. As expected the pore volumes obtained also showed a decrease as the temperature is increased. BET adsorption-desorption isotherms for the MgO samples are given in the Supporting Information. SEM images of the as-synthesized hydromagnesite are shown in Figure 6A,B). A uniform flower like morphology with the
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Figure 7. SEM images of (A) MgO (400 °C, N2, 4 h); (B) MgO (600 °C, N2, 4 h); (C and D) MgO (700 °C, air, 4 h).
Figure 6. SEM images of the (A) as-synthesized hydromagnesite flowers; (B) high magnification and (C and D) intermediate stages of the formation of MgO tubes (400 °C, air, 30 min); (E) MgO (400 °C, air, 4 h); (F) high magnification view of the tubular opening.
diameter in the range 2-3 µm is observed. Along with the spherical flowers, formation of incomplete flowers is also observed. This is an indication that the hydromagnesite flowers are formed through the heterogeneous nucleation during the slow addition of sodium carbonate solution to the Mg(NO3)2 solution. After the calcination at 400 °C or above, irrespective of the atmosphere, the formation of nanofibrous MgO microtubes has been observed as shown in Figure 6C-F and Figure 7A-D. It appears that during the decomposition of hydromagnesite, the flower like morphology collapses and the fibrous sheet like petals constituting them are shrinked and separated. In order to understand the mechanism for the formation of the tubular morphology, the heat treatment was carried out at 400 °C in air for only 30 min (MgO (400 °C, air, 30 min)). The corresponding SEM images are shown in the Figure 6C,D. SEM images show an exploded flower like morphology with each petals made up of nanofibers, folding or rolling to form tubular structures. Since the tubular structures will have less surface tension in comparison with the straight fibrous sheet, formation of tubular morphology at such conditions may be favorable. This mechanism is further confirmed from the observation of many partially folded half tubes in the SEM images. Upon prolonged heating for 4 h in the same condition, the formation of MgO microtubes was observed (Figure 6E). From Figure 6 (E) and (F), it is observed that MgO (400 °C, air, 4 h) shows the tubular morphology with opened mouth, whereas MgO (400 °C, N2, 4 h) shows tubes with the closed mouths (Figure 7A). This shows that O2 present in the air assist the formation of tubes with opened mouth. This may be due to the fast, lowtemperature decomposition in the air atmosphere that leads to
Figure 8. HR-TEM images of MgO (600 °C, N2, 4 h) (A) nanofiber; (B) branched nanofibers; (C) lamellar sheet like MgO; (D) highresolution lattice image of MgO nanofiber. Inset shows SAED patterns of the respective nanofibers.
the crystalline nature of the resulting MgO as evident from the XRD pattern. When the temperature of calcination is increased (600 °C), crystalline tubes are formed as can be seen from the Figure 7B. Energy dispersive analysis of X-rays (EDAX) has been performed to confirm the composition of both the hydromagnesite flowers and MgO microtubes and corresponding EDAX spectra are given in the Supporting Information. HR-TEM images show MgO nanofibers which constitute the microtubes of MgO (600 °C, N2, 4 h) (Figure 8A). The corresponding selected area electron diffraction (SAED) pattern shown in the inset reveals the single crystalline nature of the fibers. The fibers of MgO were having a thickness of around 10-40 nm and the length ranging from 800 nm to 1 µm. Figure 8 (B) shows the image of branched nanofibers of MgO. Figure 8C shows a lamellar sheet like growth in MgO lattice. The lattice fine structure of the MgO (600 °C, N2 4 h) is shown in Figure
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Figure 10. Comparison of lattice parameters of various MgO microtube samples. Sample code is given in the x-axis.
SCHEME 1: Plausible Mechanism for the Formation of “Nest-Like” MgO Microtubes from Hydromagnesite Flowers
Figure 9. Photoluminescence spectra of MgO microtubes synthesized at various conditions: (a) green emission; (b) red emission.
8D. The lattice d spacing calculated is around 0.21 nm and the corresponding plane is [200]. Lattice parameter calculated from the XRD measurement is in good agreement with the TEM results. A tentative mechanism for the formation of tubular structures from the flower like morphology of hydromagnesite is given in Scheme 1. When the precipitating reagent, sodium carbonate, is added to the Mg(NO3)2 solution, Mg2+, NO32-, HCO3-, and OH- ions will be present in the solution (eq 1):
H2O + Na2CO3 f HCO3- + OH- + 2 Na+
(1)
Primarily Mg(OH)2 is precipitated, because it possesses appreciably low solubility product. But as the concentration of carbonate in the medium increases, a parallel competing reaction takes over as shown in eq 2:
HCO3- h CO2 v + OH-
(2)
The first precipitated Mg(OH)2 may act as the nuclei for the growth of hydroxy carbonate, because the solubility product of hydroxy carbonate is still lower than that of the corresponding hydroxide.40 In general, Mg(OH)2 lattice is known to form lamellar flat surfaces with fibrous nature. Under the conditions of preparation, these Mg(OH)2 nuclei will act as the centers for the self-assembly of fibrous lamellar sheets ultimately forming spherical flower like morphology.41 The alternate adsorption of
CO2 and OH- will further enhance the crystallization of hydroxy carbonate in all the directions forming sheet like fibrous petals. This growth continued until the crystal reaches its saturation equilibrium so that its thermodynamical stability is maintained. Hence, almost uniform diameter flower like structures have grown. Upon heating, due to the stress release that happened during the evolution of large amount of CO2 and H2O the flower explodes. Each fibrous petal will be separated and shrunken due to the considerable loss of mass and volume that occurred. In order to reduce the surface tension, these sheets like petals will scroll or fold to give the microtubes as shown in Scheme 1. Optical investigations can reveal very useful information for understanding the physical properties of materials. It also allows the possibility to extend the potential application of MgO nanostructures in optoelectronic devices. Room-temperature Photoluminescence spectra (PL) of MgO microtubes obtained by calcination at various temperatures are given in Figure 9a,b. When excited around 385 nm, MgO microtubes showed a green and red emission centered around 530 and 640 nm respectively. From Figure 9a, it is observed that the emission intensity for the MgO microtubes (400 °C, N2, 4 h) shows the maximum intensity irrespective of the emission wavelengths. Photoluminescence of MgO micro tubes arises due to the oxygen ion vacancies (F and F+ centers). From the observation in the case of N2 treated MgO microtubes, there is a possibility of the formation of nonstoichiometric as well as defective MgO lattice due to the lack of oxygen. If the defect concentration is more, the luminescence intensity also will be high. In general, change in the lattice parameter can be correlated with the concentration of the defects in any undoped material. The green emission may be arising from the emission due to the oxygen ion vacancies (excited F centers).42-46 This is supported by the observation that in 530 nm emission, there is a considerable difference in
10272 J. Phys. Chem. C, Vol. 111, No. 28, 2007 the PL intensities of MgO (400 °C, N2, 4 h) and MgO (700 °C, air, 4 h). Movement of the atoms and ions from the lattice sites results in the creation of defects or excess surface states. Hence, the red emission can be due to the relaxation luminescence of such defect centers created by mechanical stress during fracture and rapid crystallization. The observed trend in the luminescence was in good agreement with the lattice parameter values calculated from the XRD data. A decrease or increase from the optimum lattice parameter is an indication of the defect concentration in the respective samples.47-48 Hence, it very well explains the observed higher intense emissions for the samples of MgO (400 °C, N2, 4 h) and MgO (700 °C, air, 4 h), which have shown a marked deviation from the optimum lattice parameters. A comparison of the lattice parameters of the different MgO samples is shown in the Figure 10. The order of the PL intensities of the MgO samples are MgO (400 °C, N2, 4 h) > MgO (700 °C, air, 4 h) > MgO (600 °C, N2, 4 h) > MgO (800 °C, N2, 4 h) > MgO (400 °C, air, 4 h). This trend confirms a good correlation of PL intensity and the lattice parameter. Hence, it is possible to tune the PL intensity of MgO by varying the calcination conditions. 4. Conclusions A simple, room-temperature chemical precipitation method was utilized for the synthesis of hydromagnesite flower like architectures in micrometer dimensions. No template, catalyst or surfactant was employed in this process. Thermally stable, nano fibrous MgO microtubes have been synthesized in bulk amounts through the calcination of hydromagnesite precursor. In the case of nanomaterials, the architectures adopted by them have not been rationalized in most of the cases. These architectures should have originated from the species present in the precursors and the preferential growth process that will set in while forming nano materials. A plausible mechanism has been proposed for the formation of cubic MgO microtubes on the basis of the SEM analysis of the intermediate species. Hence this communication attempts to rationalize the flower like and tube like morphologies adopted by magnesium based materials. Tuning of the surface area of MgO by varying the calcination conditions has been demonstrated. Photoluminescence measurements showed a variation in PL intensity in the observed red and green emission for the MgO microtubes prepared under different calcination conditions, corresponding to the defect concentration present in them. A clear correlation was made with PL intensity and the lattice parameter of the MgO microtubes, which will open up the possibilities for predicting the optical properties by looking into the X-ray diffractometric data. The photoluminescent MgO microtubes may have potential application on the design of safe, inexpensive and environmentally benign optical materials. High surface area MgO formed can be a promising candidate as support in catalytic applications. Acknowledgment. The authors thank CSIR for a research fellowship to C.M.J. The funding from the Department of Science and Technology (Government of India) for NCCR is gratefully acknowledged. We also acknowledge DST nanotechnology centre, IIT Madras for HR-TEM analysis. Supporting Information Available: SEM images of the flower like hydromagnesite, MgO microtubes obtained from different calcination conditions, HR-TEM images of MgO nanofibers, and BET adsorption-desorption isotherms for all the samples whose surface area is given and the UV absorption spectra for MgO samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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