Nanobelt Formation of Magnesium Hydroxide Sulfate Hydrate via a

Jun 20, 2006 - The nanobelt formation of magnesium hydroxide sulfate hydrate (MHSH) via a soft chemistry approach using carbonate salt and magnesium ...
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J. Phys. Chem. B 2006, 110, 13387-13392

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Nanobelt Formation of Magnesium Hydroxide Sulfate Hydrate via a Soft Chemistry Process Zhengzhi Zhou, Qunhui Sun, Zeshan Hu, and Yulin Deng* School of Chemical and Biomolecular Engineering, IPST at GT, Georgia Institute of Technology, 500 10th Street, N.W., Atlanta, Georgia 30332-0620 ReceiVed: February 26, 2006; In Final Form: April 13, 2006

The nanobelt formation of magnesium hydroxide sulfate hydrate (MHSH) via a soft chemistry approach using carbonate salt and magnesium sulfate as reactants was successfully demonstrated. X-ray diffraction (XRD), energy dispersion X-ray spectra (EDS), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analysis revealed that the MHSH nanobelts possessed a thin belt structure (∼50 nm in thickness) and a rectangular cross profile (∼200 nm in width). The MHSH nanobelts suffered decomposition under electron beam irradiation during TEM observation and formed MgO with the pristine nanobelt morphology preserved. The formation process of the MHSH nanobelts was studied by tracking the morphology of the MHSH nanobelts during the reaction. A possible chemical reaction mechanism is proposed.

1. Introduction One-dimensional nanostructured materials such as nanotubes,1 nanowhiskers,2,3 nanowires,4 and nanobelts5 have attracted considerable research interest in the past decades due to the promising potential applications of these materials in biological, electronic, and optoelectronic devices.6 One-dimensional MgO nanomaterials have attracted particular research interest due to their high melting point (∼2852 °C), high specific heat capacity (870-880 J K-1 kg-1), inertness toward many superconductors,7 and their low cost relative to other one-dimensional nanomaterials. Yang and Lieber reported significant enhancements in the critical current density at elevated temperatures when MgO nanorods were incorporated into high-temperature superconductors.8 The general ways to synthesize MgO nanowires are from chemical vapor deposition (CVD)9-13 or through a dehydration process using one-dimensional nanostructured precursor, such as magnesium hydroxide or magnesium hydroxide sulfate hydrate (MHSH) at an elevated temperature.14-18 Unfortunately, the latter approach can produce, to date, only one-dimensional MHSH nanowhiskers, rather than nanobelts, with limited aspect ratios.17,18 In addition to the important application of MHSH nanowires or nanobelts as the precursor for the synthesis of onedimensional MgO nanomaterials, MHSH itself has unique applications as flame retardants, fillers, polymer reinforcement agents, and heat-insulating materials. Thus, the exploration of wet chemical processes to synthesize MHSH with desired structures is of significant importance. There are several reports on the preparation of nanoparticles,6,17-19 nanoplates,20 nanoflowers,21 and nanowhiskers22,23 of MHSH materials via hydrothermal processes. Unfortunately, the reports are focused on only the MHSH nanoparticles and nanowhiskers,6,18,19,22,23 but are not concerned with the synthesis of MHSH nanoribbons or nanobelts, due, most probably, to the vagueness in the mechanism governing nanobelt formation in the field, and the technical difficulties to achieve this. Yang and co-workers, * To whom correspondence should be addressed. Phone: 404-894-5759. Fax: 404-894-4778. E-mail: [email protected].

however,24 reported the formation of MHSH nanoribbons with a typical length of less than 2 µm via a precipitation process. In this article, we report for the first time the synthesis of MHSH nanobelts with a length of a few hundred micrometers via a facile soft chemistry approach in a MgSO4 aqueous solution in the presence of carbonate salts. The reaction was carried out at ambient pressure with mild reaction temperature (∼100 °C), in sharp contrast with that requiring higher temperature (usually g160 °C) and higher pressure (in an autoclave) reported in previous MHSH whisker preparation.17,18 We examined the influence of reaction parameters, such as molar ratio of reagents, reaction temperature, stirring mode, and inert gas bubbling, on the nanobelt formation. Meanwhile, we tracked the nanobelt morphology during various stages of the reaction, via optical or electron microscopy, to support the above studies. Based on the above observation and research results, we proposed a possible chemical process governing the dynamics in the process, to illustrate a possible mechanism of nanobelt formation. 2. Materials and Methods 2.1. Materials. Magnesium sulfate heptahydrate, MgSO4‚ 7H2O (98%), Na2CO3 (>99.5%, powder), and CaCO3 (>99%) were purchased from Aldrich and used without further treatment. All other reagents were from Sigma-Aldrich and used as received. The deionized (DI) water was milli-Q grade with a conductivity of 18 mΩ S-1. 2.2. Synthesis of MHSH Nanobelt. A prototypical procedure is described below. To a 250 mL three-neck round-bottom flask equipped with a magnetic stirrer, an air-inlet bubbling tubing, and a refluxing column, 50.52 g of magnesium sulfate heptahydrate MgSO4‚7H2O (ca. 0.20 mol), 2.1 g of sodium carbonate Na2CO3 (ca. 0.02 mol), and 180 mL of DI water were loaded. The mixture in the flask was heated to its boiling temperature (∼102 °C) while maintaining gentle agitation with a magnetic stirrer. Compressed air at a pressure of 0.5 psi was charged at a speed of 20 L/h and bubbled through the inlet tubing in the mother liquor.

10.1021/jp0612228 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/20/2006

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Figure 1. Morphologies of the MHSH nanobelts: (A) a typical bending MHSH nanobelt and the stacking mode; (B) a zoom-in image of the nanobelts.

The morphology of the nanobelts formed was tracked during various stages of the reaction using an optical microscope equipped with a Leica DC 100 camera shot atop or on a SEM machine, by sampling the mother liquor at intervals (∼4.0 h) by a plastic pipet and casting on a glass cover without washing. When the reaction was over, the suspension was vacuum-filtered and washed extensively with DI water to remove the excess magnesium sulfate and other water-soluble impurities. The final product was dried in a vacuum oven at 60 °C overnight and in white powder form. 2.3. Characterization. SEM observation was conducted on a LEO 1530 thermally assisted field emission (TFE) scanning electron microscope with an acceleration voltage of 3 kV. The samples were lightly sputter-coated with a thin layer of Au/Pd prior to observation. The EDS spectra were recorded on the LEO 1530 TFE SEM machine with the same manipulation parameters. SAED and TEM analysis was carried out on a JEOL 100C type TEM machine with an accelerating voltage of 100 kV. Optical microscopy observations were performed on a Leica DMLM microscope equipped with a Leica DC 100 camera shot atop. XRD patterns were recorded on a PW 1800 X-ray diffractometer (Philips, USA) using Cu KR ray (λ ) 1.54056 Å) as the radiation source. A step size of 0.01° and a scan step time of 0.5 s were used to record the spectra in a range of 2θ from 0.2° to 80°. 3. Results and Discussion 3.1. Morphology Observation. As shown in Figure 1, the nanobelts exhibited a random arrangement and a long curled morphology. However, the accurate average length of the nanobelts could not be defined because they are physically entangled with each other with no measurable start or end points. Branching points, either splitting from the subunit in the center of the nanobelt or separating as two individual branches of belts, were visible, as shown in Figure 1B. It was found that the nanobelts possessed a thin layered structure, with a thickness of 20-50 nm and a width of 200-300 nm, respectively. It is interesting to note that the surface appearance of the nanobelts is not smooth, but consists of a bunch of subunit nanowires

Figure 2. XRD pattern of the MHSH nanobelts prepared. The Arabic numbers from 1 to 10 indicate the main peaks and the asterisks refer to minor peaks. These data matched very well with those listed in the database25 with a formula of Mg6(OH)10SO4‚3H2O.

with a wavelike morphology. One nanobelt is normally composed of 4-7 subwires with a diameter of ca. 50 nm for each wire, approximately equal to the thickness of one belt. A magnified picture for a number of nanobelts, as shown in Figure 1B, further revealed the surface unevenness of the MHSH nanobelts, in comparison with those of ZnO or MgO cousins via a CVD process with a smooth and even surface morphology.5 3.2. Structural Characterization. The XRD analysis of the MHSH prepared exhibited a very good match with that as listed in the database. As shown in Figure 2, a set of diffraction peaks as numbered from 1 to 10 sequentially in the plot, at 2θ: 12.83°, 17.38°, 23.11°, 29.42°, 34.62°, 39.41°, 43.25°, 45.48°, 47.61°, and 57.42° were recorded as strong peaks, corresponding to d spacings of 6.89, 5.10, 3.84, 3.03, 2.59, 2.28, 2.09, 1.99, 1.91, and 1.60 Å, respectively. These data matched very well with that of 2θ (intensity): 12.91° (45), 17.31° (95), 22.78° (60), 29.96° (50), 34.44° (60), 39.47° (6), 43.49° (4), 45.57° (25), 47.70° (4), and 57.21° (12), as listed for MHSH with a formula of Mg6(OH)10SO4‚3H2O in the database.25 In addition to that, other minor peaks with less strengths appeared at 2θ (d): 10.96° (8.06), 26.11° (3.41), 30.13° (2.96), 36.38° (2.47), 48.42° (1.88),

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Figure 3. SAED pattern of the MHSH nanobelts under electron beam irradiation (A), and the bright (B) and dark field (C) TEM images of the nanobelts.

51.63° (1.77), 53.68° (1.71), 54.92° (1.67), and 61.26° (1.51), as marked with asterisks sequentially from small to large in the plot, also matched well with that of 2θ (intensity): 11.092° (6), 25.955° (8), 29.960° (50), 36.434° (8), 47.995° (8), 51.594° (10), 53.955° (6), 54.828° (14), 55.114° (40), and 61.164° (18) in the database.25 The cell parameters for the MHSH nanobelts synthesized were found to be a ) 15.94 Å, b ) 3.11 Å, and c ) 13.38 Å, respectively, in contrast with that reported before.17,24 A selected area electron diffraction pattern (SAED), as shown in the inset of Figure 3A, indicated that the pristine MHSH nanobelts no longer existed after exposure to electron beam irradiation and decomposed to MgO. The SAED pattern includes a strong set of spots corresponding to the [001] pattern of cubic MgO, which suggests a well-textured structure for the irradiated nanobelts.14 This decomposition to MgO was analogous, to some extent, to the dehydration of the MHSH under thermal conditions. The magnesium hydroxide sulfate dehydration at different temperatures has been studied using TGA and DSC.17,18,26 It was found that MgO was the final product at the temperature above 800 °C.

MgSO4‚5Mg(OH)2‚2H2O f MgSO4‚5Mg(OH)2 + 2H2O (∼260 to 360 °C) MgSO4‚5Mg(OH)2 f MgSO4 + MgO + 5H2O (∼360 to 800 °C) MgSO4 f MgO + SO3 (>800 °C) It was interesting to note that although the MHSH nanobelts experienced a decomposition process, the final MgO crystal still preserved the original nanobelt morphology. The bright and dark field TEM images of the MgO, as seen in Figures 3B and 3C, exhibited a textured MgO nanobelt with a porous surface morphology, which may suggest other potential applications as porous nanobelt materials. The bright dots in Figure 3C suggested the existence of oriented MgO nanocrystals within the bulk nanobelt.

Figure 4. Comparison of morphologies of a typical bundle of MHSH nanobelt with less (A) and more electric beam irradiation under SEM observation conditions.

The surface roughness observation, as shown in Figure 4, further revealed that the MHSH nanobelt was partially decomposed by exposure to the electron beam with a porous surface morphology left, as indicated by the arrows in the plot (B). EDS analysis, as can be seen from Figure 5, clearly demonstrated the existence of O, Mg, and S, which is in agreement with the XRD analysis as discussed above. A rough atomic ratio estimation of Mg to S from EDS analysis was 11.13/1.81 = 6.1, very close to the ratio of Mg/S ) 6.0 defined in the chemical formula 5Mg(OH)2‚MgSO4‚3H2O of MHSH, in good agreement with the chemical structure of the MHSH proposed. The extra peaks of C and Cu from the picture resulted from the holey copper grid which was used to hold the samples for the observation. 3.3. Tracking the Formation Process of the MHSH Nanobelts. The process of nanobelt growth was tracked during

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Zhou et al. TABLE 1: Ksp of Magnesium-Based Precipitatesa

compound

Ksp

minimum concentration (M × 105) of counterions at precipitation, presuming [Mg2+] ) 0.2 M

MgCO3 MgCO3‚3H2O MgCO3‚5H2O Mg(OH)2

6.82 × 10-6 M2 2.38 × 10-6 M2 3.79 × 10-6 M2 5.61 × 10-12 M3

3.41 1.19 1.90 0.53 (pH ) 11.3)

a

The data are obtained at 25 °C.

SCHEME 1: Schematic Illustration of the Reaction Processes Involved in the System for the Formation of the MHSH Nanobelt

Figure 5. EDS spectrum of the MHSH nanobelt.

the experiments by taking samples out at intervals for optical microscope and SEM observations. Within the first 6 h of the reaction, no clear nanobelt formation was observed except sphere-like solids, which were apparently precipitated out from the mother liquid comprised of unreacted MgSO4 and MgCO3, as shown in Figure 6A. After an approximately 6-10 h reaction, as shown in Figure 6B, burgeoned MHSH nanobelts comprised mainly of sub-nanowires in a separated and loosely bonded form embedded in the unreacted MgSO4 particles were found. The diameter of a single sub-nanowire was ca. 50 nm, in accordance with the results obtained above. With further progression of the reaction, for example, in the period between 10 and 20 h, more MHSH nanobelts were formed, as shown in Figure 6C. After 20-24 h of reaction, as shown in Figure 6D, large quantities of MHSH nanobelts with little unreacted residue were observed. It was estimated that the yield was more than 90%.27 3.4. Possible Formation Mechanism. It was noted that the nanobelts could be formed neither at reaction time less than 6 h nor at reaction temperature lower than 90 °C, irrespective of other reaction conditions. The long nanobelt morphology could only be obtained under magnetic stirring with a magnetic stir bar (length was ca. 2.54 cm). With use of a downward twoblade mechanical stirrer at ca. 300 rpm, the product form was mainly nanowhiskers with lengths around ca. 20-50 µm. When equimolar amounts of MgSO4 and Na2CO3 were used, the final products were primarily in short whiskers, with trace amount of nanobelts formed. By changing the molar ratios of MgSO4 to Na2CO3 to within the range of 5-20, the products formed were changed to nanobelts. With further increases in the ratio of MgSO4 to Na2CO3 the proportion of nanobelts in the product decreased. Although a clear picture of the growth mechanism cannot be given, a possible mechanism governing the parameters for the formation of the MHSH nanobelts could be proposed. It was observed that supersaturation and alkaline conditions in the reaction were necessary for the formation of the nanobelts. We studied the influence of varieties of carbonate sources besides Na2CO3, for example, CaCO3 and MgCO3, on the

formation of the nanobelts and found that all of them are effective in controlling the nanobelt formation if all the reaction conditions were kept the same as in the systems described above. It was interesting to note that no nanobelts were formed by simply adding NaOH into MgSO4 solution with the same ratios used above, although a possible product in this system was Mg(OH)2, a close cousin of the final MHSH. The solubility products (Ksp) of the reactants involved in the system determines, to some extent, the sequence of the crystals to be precipitated out in the system. As is well-known, MgSO4 and Na2CO3 can be ionized in water to form a variety of hydrolyzed ion species, such as Mg2+, SO42-, CO32-, and HCO3-. The dissociation and association processes of these species then may result in the formation of bicarbonium and OH- ions. Furthermore, the reactions between these ion pairs can be reorganized into a range of precipitates, for example, Mg(OH)2, MgCO3, MgCO3‚3H2O, and MgCO3‚5H2O, depending on the Ksp of the pairs to be formed as shown in Table 1.28 It was easy to calculate the minimum concentration of CO32([CO32-]) required for the formation of magnesium carbonate and its hydrates based on the data given in Table 1. It was found that it is much lower than the amounts that we used in the experiments (ca. 0.02-0.04 M). That means the possible initial precipitates of magnesium salts would be either MgCO3 or MgCO3‚3H2O and MgCO3‚5H2O, but not Mg(OH)2 at the concentrations used in our study. However, it should be noted that CO32- will take the following reactions in an aqueous solution

CO32- + H2O ) HCO3- + OHHCO3- h CO2 v + OHTo facilitate shifting the equilibrium of the above reactions to the direction of forming Mg(OH)2, the reaction systems were heated to 100 °C and the compressed air was continually bubbled through the reaction medium to remove the CO2 gas formed during the reaction, as shown in Scheme 1. It is worthy to note that no MHSH could be produced if NaOH, and not NaCO3, was used and all other conditions were kept the same. Instead, many irregular-shaped particles of Mg(OH)2 were formed immediately when a NaOH solution was mixed with MgSO4 at 100 °C. Compared with NaOH completely dissolved

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Figure 6. Morphology development along with the reaction process at the following reaction times: (A) 12-16 h; (B) and (C) 16-20 h; (D) 24 h.

in an aqueous phase, the release of OH- ion from the MgCO3 in water was much slower in terms of hydrolysis process. It is thus believed that the slow release of OH- ion from the hydrolysis of MgCO3 plays an important role in controlling the growth of MHSH nanobelt. In comparison, however, the role of the MgSO4 in the reaction is not clear yet and is still under investigation.28-30 It should be noted that the mechanism of MHSH formation discussed above was based on the formation of Mg(OH)2 rather than 5Mg(OH)2‚MgSO4‚3H2O. However, we believe that a similar mechanism should be applied to the latter because the content of MgSO4 in 5Mg(OH)2‚MgSO4‚3H2O nanobelts is very low, so the Mg(OH)2 formation should play a key role in the final nanobelts formation. 4. Conclusion In conclusion, the MHSH nanobelt formation via a soft chemistry approach using carbonate salts and magnesium sulfate as reactants was demonstrated for the first time. SEM and TEM

observations revealed that the nanobelts formed possessed a length up to a hundred micrometers, with a thin belt structure and rectangular surface profile. The MHSH was easily subjected to decomposition under electron beam irradiation during the TEM observation to form MgO, a very important candidate for superconductor fabrication and a brand new class of nanofiller materials in polymer composite engineering with very high aspect ratio. A possible mechanism, a dynamic but not a thermodynamic control mechanism, was proposed to elucidate chemically the phenomena observed. Acknowledgment. Assistance from Dr. P. Gao and Dr. Y. Ding for TEM observation and helpful discussions are greatly acknowledged. The authors thank Dr. H. Gu, Mr. S. Y. Soon, and Dr. Z. Yan for their help during the fulfillment of the work. References and Notes (1) Law, M.; Goldberger, J.; Yang, P. ReV. Mater. Res. 2004, 34, 83122.

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