Abstract
A facile method was developed to synthesize Al(OH)3 nanostructures from Al(OH)3 microagglomerates by dissolution in 9.0 mol·L−1 NaOH at 115°C followed by dilution and aging of the solution at room temperature. The influence of Al(OH)3 nanoseed and surfactants as sodium dodecyl sulfate (SDS), polyethylene glycol 6000 (PEG6000), and cetyltrimethylammonium bromide (CTAB) on the formation of the Al(OH)3 nano-structures was investigated. The experimental results indicated that the Al(OH)3 microspheres composed of nanoparticles were prepared in the blank experiment, while dispersive Al(OH)3 nano-particles with a diameter of 80–100 nm were produced in the presence of Al(OH)3 nano-seed and CTAB.
1. Introduction
The synthesis of Al(OH)3 nanostructures has been paid much attention in recent years owing to their unique properties and wide applications in plastics, rubbers, papers, glasses and medicines, and so forth [1–4]. Al(OH)3 microparticles were usually produced in the aluminum metallurgical factories via the Bayer route, which included the leaching of the bauxite ores at alkaline hydrothermal condition (150–250°C), the removing of impurities as Si and Fe at elevated temperatures, and the precipitation of the Al(OH)3 microparticles in the presence of Al(OH)3 seed by gradually cooling of the sodium aluminate solution from 90–100°C to 50–60°C which usually lasted 48–72 h [5–8]. Many wet chemical methods have been developed to synthesize the Al(OH)3 nanostructures. For example, Li et al. [9] produced the Al(OH)3 nanowhiskers with a length of 8–12 m and a diameter of 100 nm by dropping 0.05 moL·L−1 HCl to 1.6 moL·L−1 sodium aluminate at 60°C. Chen et al. [10] synthesized the Al(OH)3 hexagonal plates with a diameter of 100–200 nm and a thickness of 20–30 nm by carbonation of the sodium aluminate solution at room temperature. Li et al. [11] produced the Al(OH)3 nanoparticles with an average size of 30–40 nm from the dilute sodium aluminate solution containing 0.4 moL·L−1 NaOH in the presence of α-Al2O3 nanoseed and polyethylene glycol 20000 (PEG20000). However, some problems still existed in the former methods, such as low efficiency in the dilute system, the use of the expensive organic solvents, and the discharge of waste water or byproducts. Wang et al. synthesized Al(OH)3 particles with an average diameters of 0.5–1.2 m by dissolution of the agglomerated Al(OH)3 microagglomerates with an average diameter of 1.5 m in 2.0 moL·L−1 NaOH solution at 90–100°C and then cooled down to 50–70°C. Up to now, it is still a challenge to develop a moderate, efficient, and environmentally friendly method for the synthesis of Al(OH)3 nanostructures.
Herein, a facile dissolution precipitation method was developed in this paper to synthesize Al(OH)3 nanostructures from Al(OH)3 microagglomerates. The influences of the Al(OH)3 nanoseed and the surfactants including SDS, PEG6000, and CTAB on the formation of Al(OH)3 nanostructures were discussed.
2. Experimental
2.1. Experimental Procedure
Figure 1 shows the morphology and the thermogravimetric analysis of the raw material and the Al(OH)3 seed. Al(OH)3 raw material was Provided by an aluminum metallurgical factory in Shandong province, China. Al(OH)3 seed was synthesized by coprecipitation of 0.5 moL·L−1 Al(NO3)3 and 1.0 moL·L−1 NaOH at room temperature, keeping the initial molar ratio of Al(NO3)3 to NaOH at 1 : 3. As shown in Figure 1(a), Al(OH)3 raw material was composed mainly of the irregular agglomerates with a diameter of 5–20 μm, and the weight loss (36.11%) occurred between 30 and 800°C should be attributed to the removal of the crystal water; thus, the formula for the raw material was deduced as Al2O3·3.20H2O. The data in Figure 1(b) showed that the particle size of the Al(OH)3 seed was 20–40 nm and the weight loss between 30 and 800°C was 35.04%, and then the formula for the seed was deduced as Al2O3·3.15H2O.
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Figure 2 shows the flow chart for the formation of the Al(OH)3 nanostructures from the Al(OH)3 microagglomerates.
Dissolution of Al(OH)3 in NaOH solution. 66.4 g of Al(OH)3 raw material was dissolved in 110 mL of 3.0–11.0 moL·L−1 NaOH at 115°C for 0.5 h to prepare the sodium aluminum solution with or without the existence of the un-dissolved Al(OH)3. The solutions or the supernatants were used to analyze the dissolution of Al(OH)3.
Formation of Al(OH)3 nanostructures. 66.4 g of Al(OH)3 raw material was dissolved in 110 mL of 9.0 moL·L−1 NaOH at 115°C for 0.5 h to get the sodium aluminum solution. The solution was then cooled down to room temperature and diluted with deionized water to get a solution containing 3.0 moL·L−1 NaOH. Then, a certain amount of the Al(OH)3 nanoseed and surfactant (SDS, EDTA, or PEG 6000) were added to the diluted sodium aluminum solution to produce a solution containing 0.125 moL·L−1 Al(OH)3 nanoseed and 13.0 g·L−1 surfactant. The sodium aluminum solution was aged for 0–10.0 h to fabricate the Al(OH)3 precipitate. Then, the Al(OH)3 precipitate was washed by deionized water and ethanol and dried at 105°C for 4.0 h. All of the chemical reagents used in the experiments were analytical grade without further purification.
2.2. Analysis
The morphology of the samples was examined by the field emission scanning electron microscopy (FESEM, JSM 7401F, JEOL, Japan). The structures of the samples were identified by the X-ray powder diffraction (XRD, D8 Advance, Bruker, Germany) patterns using Cu Kα radiation ( nm). The composition of the samples was identified by the thermogravimetric analyzer (TG/DSC, Mettler Toledo, Switzerland). The concentration of the soluble Al ion was analyzed by the EDTA complex-Zn2+ titration method [12].
3. Results and Discussions
3.1. Dissolution of Al(OH)3 Microagglomerates in NaOH Solution
Figure 3 shows the dissolution of the Al(OH)3 microagglomerates in 3.0–11.0 moL·L−1 NaOH solutions. In the experimental conditions, the increase of NaOH concentration from 3.0 moL·L−1 to 11.0 moL·L−1 favored the dissolution of Al(OH)3. It was found that the raw material was dissolved completely if [NaOH] 9.0 moL·L−1. Therefore, 9.0 moL·L−1 NaOH was adapted to dissolve the Al(OH)3 raw material in the later experiments.
3.2. Precipitation of Al(OH)3 at Room Temperature
3.2.1. Influence of Al(OH)3 Nanoseed
Figure 4 shows the morphology of the Al(OH)3 nanostructures formed in the absence and presence of Al(OH)3 nanoseed. As shown in Figures 4(a)–4(c), agglomerated microspheres were prepared in the blank experiment. The microspheres were composed of the plates with a diameter of 0.6–1.0 m and a thickness of 0.2-0.3 μm, and the plates were made up of the nanoparticles with a diameter of 80–100 nm. The diameters of the microspheres formed at 2.0 h, 6.0 h, and 10.0 h were 0.5–1.5 μm, 1.0–3.5 μm, and 1.0–4.0 μm, respectively, revealing that the sizes of the microspheres increased with the increase of the aging time. Dispersive Al(OH)3 nanoplates with a diameter of 250–350 nm and a thickness of 100 nm were produced in the presence of 0.125 moL·L−1 Al(OH)3 nanoseed, and the morphology of the nanoplates formed at 2.0 h, 6.0 h, and 10.0 h were quite similar to each other.
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Figure 5 shows the TG curves and the XRD patterns of the Al(OH)3 nanoplates formed in the presence of Al(OH)3 nanoseed. As shown in Figure 5(a), a weight loss of 35.91% occurred in the temperature range of 30–800°C; thus, the formula of the Al(OH)3 nanoplates was deduced as Al2O3·3.17H2O. The phenomenon that the number of water molecular was slightly higher than 3.0 revealed that some water was absorbed physically by the Al(OH)3 nanoplates. Most of the diffraction peaks in Figure 6(b) can be indexed as gibbsite (PDF no. 74-1775, Å, Å, and Å), and the peak at 2θ = 40.506° revealed that a minor amount of bayerite (PDF no. 20-0011, Å, Å, and Å) also existed in the Al(OH)3 nanoplates.
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Figure 6 shows the influence of Al(OH)3 nanoseed on the precipitation of Al(OH)3 nanostructures. The precipitation of Al(OH)3 was accelerated by the addition of Al(OH)3 nanoseed obviously. 63.0% of the soluble Al ion in the sodium aluminate solution was converted to the Al(OH)3 precipitate after aging for 2.0 h, while only about 1.0% of the soluble Al ion was precipitated after aging for 2.0 h in the blank experiment.
The influence of Al(OH)3 nanoseed on the morphology and the precipitation of the Al(OH)3 nanostructures may be explained as follows. In the absence of Al(OH)3 nanoseed, Al(OH)3 was first precipitated homogeneously and slowly from the sodium aluminate solution, and the Al(OH)3 fine particles formed at the early aging stage acted as the nucleus for the subsequent precipitation of Al(OH)3, leading to the gradual growth of the agglomerated microspheres. Meanwhile, in the presence of Al(OH)3 nanoseed, many Al(OH)3 nanoparticles were added to the initial sodium aluminate solution, and they acted as the nucleus for the later heterogeneous precipitation of Al(OH)3 from the sodium aluminate solution, leading to the fast conversion of most of the soluble Al ion to Al(OH)3 within 2.0 h of aging and producing Al(OH)3 nanoplates with quite similar shapes.
3.3. Influence of Surfactants
The influence of the surfactants including SDS, PEG6000, and CTAB on the morphology of the Al(OH)3 precipitates was shown in Figure 7. Al(OH)3 precipitates were prepared after aging treatment of the sodium aluminate solutions containing 0.125 moL·L−1 Al(OH)3 nanoseed and 13.0 g·L−1 surfactant (SDS, PEG6000, or CTAB) for 10.0 h. Compared with those formed in the sole existence of Al(OH)3 nanoseed (Figure 6(f)), Al(OH)3 nanoparticles with smaller sizes were prepared in the presence of Al(OH)3 nanoseed and surfactants. Al(OH)3 nanoparticles with a diameter of 200–300 nm, 150–250 nm, and 80–100 nm were produced in the presence of SDS, PEG6000, and CTAB, respectively. The order of the influence of the surfactants on the formation of the Al(OH)3 nanoparticles was CTAB>PEG6000>SDBS, and this phenomenon may be related to the interactions between the Al(OH)3 surfaces and the surfactants. The surfaces of Al(OH)3 formed in the concentrated NaOH solutions were negative charged owing to the adsorption of [13] which were easier to absorb the cationic surfactants as CTAB and difficult to absorb the nonionic surfactants as PEG6000 or the anionic surfactants as SDS.
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Figure 8 shows the schematic diagram for the influence of Al(OH)3 nanoseed and the surfactants on the precipitation of Al(OH)3 from the sodium aluminate solution. Generally, the existence of Al(OH)3 nanoseed promoted the heterogeneous precipitation of Al(OH)3, and the adsorption of the surfactants as SDS, PEG6000 and CTAB, especially the cationic surfactant CTAB, on the surfaces of Al(OH)3 inhibited the growth of the Al(OH)3. The coinfluence of the Al(OH)3 nanoseed and CTAB on the precipitation of Al(OH)3 from the sodium aluminate solution led to the formation of the dispersive Al(OH)3 nanoparticles with a diameter of 80–100 nm.
4. Conclusion
Al(OH)3 nanostructures were synthetized from Al(OH)3 microagglomerates by dissolution of the Al(OH)3 microagglomerates in 9.0 moL·L−1 NaOH at 115°C for 0.5 h followed by dilution and aging of the solution at room temperature. The experimental results indicated that the Al(OH)3 microspheres composed of nanoparticles were prepared in the blank experiment, while dispersive Al(OH)3 nanoparticles with a diameter of 80–100 nm were formed in the presence of 0.125 moL·L−1 Al(OH)3 nanoseed and 13.0 g·L−1 CTAB.
Acknowledgment
This work was supported by the National Natural Science Foundation (nos. 51174125 and 51234003).