Sulfate-Mediated Transformation from Nanoflakes to Hollow

Aug 2, 2010 - P. R. China, and ‡Department of Chemistry, Kent State University, Kent ... single nanoflakes to nanoflake assemblies, flower-like stru...
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DOI: 10.1021/cg100544w

Facile Hydrothermal Synthesis of Hierarchical Boehmite: SulfateMediated Transformation from Nanoflakes to Hollow Microspheres

2010, Vol. 10 3977–3982

Weiquan Cai,† Jiaguo Yu,*,† Shihai Gu,‡ and Mietek Jaroniec*,‡ †

State Key Laboratory of Advanced Technology for Material Synthesis and Processing and School of Chemical Engineering, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, P. R. China, and ‡Department of Chemistry, Kent State University, Kent, Ohio 44242

Received April 24, 2010; Revised Manuscript Received July 20, 2010

ABSTRACT: An interesting morphological evolution from single nanoflakes to nanoflake assemblies, flower-like structures, and hollow microspheres is reported for the first time for boehmite prepared from Al(NO3)3 3 9H2O with addition of urea and different amounts of KAl(SO4)2 3 12H2O under hydrothermal conditions. The microstructure, morphology, and textural properties of the boehmite materials were characterized by X-ray powder diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption. It was found that the resulting hierarchical boehmite materials composed of nanoflakes were transformed from featureless material to irregular particles and solid microspheres. To better understand this transformation, different sulfate additives;AlNH4(SO4)2 3 12H2O, Al2(SO4)3 3 18H2O, Na2SO4, (NH4)2SO4, and MgSO4 3 7H2O;were used to synthesize boehmite and magnesia-incorporated boehmite hollow microspheres. This study shows that the sulfate-mediated transformation strategy can be extended for the preparation of other hierarchical metal oxides with controlled morphological and textural properties for catalysis, adsorption, and separation applications.

*To whom correspondence should be addressed. E-mail: J.Y., jiaguoyu@ yahoo.com; M.J., [email protected].

Boehmite (γ-AlOOH) is an aluminum oxyhydroxide, which is used as a precursor for many aluminum oxide materials, especially for the preparation of catalysts, membranes, coatings, adsorbents, or materials with photoluminescent properties.6 Boehmite is also used as catalyst support,7 catalyst promoter,8 adsorbent,5d crystallization seed,9 or orthopedic or dental material.10 During heating at temperatures between 400 and 700 °C, boehmite undergoes an isomorphous transformation to nanocrystalline γ-Al2O3 without altering the morphology of the parent material.11 Therefore, many efforts have been made to control boehmite morphology. So far, various morphologies of hierarchical boehmite, such as nanofibers and nanotubes,12 nanorods,13 nanoplates and nanowires,14 nanobelts,15 irregular nanoflake assemblies,16 bundles of aligned nanowires,11 flower-like architectures consisting of radial nanostrips,17 cantaloupe-like architectures composed of closely packed nanorods,5c hollow nanospheres,18 microspheres consisting of nanosheets,19 and hollow and self-encapsulated microspheres composed of nanoflakes,20 have been reported. Also, iron-doped boehmite nanofibers and gallium-doped boehmite nanotubes and nanoribbons were studied.21 Despite the aforementioned extensive studies, the current knowledge about formation of hierarchical boehmite structures of desired morphology is still insufficient. In previous works, boehmite hollow microspheres and spindle-like assemblies composed of nanoflakes with enhanced adsorption properties have been synthesized under hydrothermal conditions using aluminum sulfate and aluminum nitrate/ aluminum chloride as precursors, respectively.20,22 Herein, a facile sulfate-mediated hydrothermal synthesis of boehmite materials is proposed to obtain a variety of hierarchical structures with morphologies ranging from single nanoflakes to nanoflakes assemblies, flower-like structures, and hollow microspheres. This unprecedented evolution of boehmite structures was achieved by simply varying the concentration of sulfate ions in the aluminum nitrate/aluminum chloride-urea hydrothermal system. To the best of our knowledge, this work

r 2010 American Chemical Society

Published on Web 08/02/2010

1. Introduction Among the most challenging topics in the development of materials is the synthesis of hierarchical structures with tunable properties. The assembly of hierarchical structures involves building blocks at different length scales.1 To date, much effort has been devoted to the synthesis of hierarchical materials with various morphologies and sizes to achieve desired electrical, optical, magnetic, catalytic, and adsorption properties.2 Self-assembly has been recognized as a powerful strategy to obtain hierarchical structures. During this process, different building blocks, such as nanoparticles (0D), nanofibers (1D), and nanosheets (2D), self-assemble into regular higher level sophisticated architectures due to hydrogen bonding, electrostatic, and van der Waals forces.3 Some template and surfactant-assisted approaches have been demonstrated to be very effective for the preparation of a variety of hierarchical materials through controlling the reaction rates and selectively altering the growth kinetics of different crystal facets of the final products.3a,4 However, these approaches tend to be rather complicated, with the obvious drawback that the template removal process may compromise the structural integrity of the final products and thus limit their practical applications. Recently, some attractive attempts have been made in the controllable synthesis of hierarchical nanomaterials through simple anion-mediated processes. These simple ions do not leave any impurity and have a pronounced influence on the nucleation process of nanocrystallites because of their smaller size and other unique properties, which can increase the ability to form complexes with reactive species and decrease the reaction rate of reactive ions in the solution.4b,5 However, there is still a big challenge to develop facile and effective simple anion-mediated synthesis routes for the preparation of hierarchical materials with desired chemical composition and controlled morphology.

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is the first report on the evolution of boehmite morphology from nanoflakes to hollow microspheres under hydrothermal conditions. This study provides new insights into the design and synthesis of hierarchical materials, which are of significant interest for catalysis, adsorption, and separation-related applications. 2. Experimental Section Synthesis. All reagents were analytical grade supplied by Shanghai Chemical Reagent Co. Ltd. (P. R. China) and used as received. In a typical procedure, 0.00315 mol of Al(NO3)3 3 9H2O, 0.007 mol of CO(NH2)2, and 0.00035 mol of KAl(SO4)2 3 12H2O were dissolved in 70 mL of distilled water. After being stirred at room temperature for 15 min, the mixed solution was transferred into a 100 mL Teflon-lined stainless autoclave and heated at 180 °C for 3 h under autogenous pressure. After reaching room temperature, the precipitate was filtered, washed three times with distilled water and one time with anhydrous alcohol, and finally dried in a vacuum oven at 80 °C for 12 h. Additional experiments were also conducted using 0-0.00175 mol of KAl(SO4)2 3 12H2O, AlCl3 3 6H2O, and different sulfates such as AlNH4(SO4)2 3 12H2O, Al2(SO4)3 3 18H2O, Na2SO4, (NH4)2SO4, and MgSO4 3 7H2O, while keeping the total Al3þ concentration of about 0.05 M similar to that used in the above presented recipe. Characterization. X-ray powder diffraction (XRD) analysis was conducted on a Rigaku D/Max-RB X-ray diffractometer with Cu KR radiation. The accelerating voltage and applied current were 40 kV and 80 mA, respectively. The crystallite size of boehmite particles was calculated using the Scherrer formula (d = (0.9λ)/ (B cos θ), where d, λ, B, and θ are the crystallite size, Cu KR wavelength (0.15418 nm), full width at half-maximum intensity (fwhm) of the (020) peak in radians, and Bragg’s diffraction angle, respectively).23 Fourier-transform infrared (FT-IR) spectroscopy measurements were performed on a Thermo Nicolet Nexus Fouriertransform spectrometer using KBr pellets. Field-emission scanning electron microscopy (SEM) images were performed on a Hitachi S-4800 microscope operating at 5 kV. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F microscope with an accelerating voltage of 200 kV. Nitrogen adsorption/ desorption isotherms were measured on a Micromeritics ASAP 2020 adsorption analyzer. Since the original Barrett, Joyner, and Halenda (BJH) method underestimates pore sizes in the range of small mesopores, its modified version reported by Kruk, Jaroniec, and Sayari (KJS) was applied for the calculation of pore size distributions using the adsorption branch of the isotherms.24

3. Results and Discussion 3.1. Phase Structures. The composition and phase purity of the boehmite samples obtained by adding different amounts of KAl(SO4)2 3 12H2O were first examined by XRD, as shown in Figure 1. All detectable diffraction peaks can be easily assigned to the orthorhombic phase of boehmite (JCPDS 21-1307) with no impurity.25 A slight gradual increase in the intensities of the XRD peaks was observed with increasing concentration of KAl(SO4)2 3 12H2O from 0 to 0.001 and 0.005 M; however, for concentrations progressively exceeding 0.005 M, the intensities decreased to some extent (curves d and e in Figure 1). This change in the XRD intensity of (020) peak reflects some variation in the crystal size, which increased from 12.7 to 16.1 and 18.5 nm with increasing concentration of KAl(SO4)2 3 12H2O from 0 to 0.001 and 0.005 M, respectively; and at concentrations of 0.013 and 0.025 M, this size slightly decreased to 12.2 and 12.4 nm, respectively. The typical FT-IR spectra of the corresponding samples are shown in Figure S1 (Supporting Information). This figure shows that all absorption bands at 3312, 3098, 2099, 1638, 1518, 1394, 1156, 1067, 753, and 620 cm-1 are consistent with the reported values for

Figure 1. XRD patterns of the samples prepared using Al(NO3)3 3 9H2O with addition of CO(NH2)2 and KAl(SO4)2 3 12H2O: the amounts of KAl(SO4)2 3 12H2O were as follows: (a) 0; (b) 0.001; (c) 0.005; (d) 0.013; (e) 0.025 M.

Figure 2. SEM and TEM images of the samples prepared using Al(NO3)3 3 9H2O with addition of CO(NH2)2 and KAl(SO4)2 3 12H2O. The amounts of KAl(SO4)2 3 12H2O were as follows: (a) 0; (b) 0.001; (c) 0.005; (d) 0.013; (e) 0.025 M.

boehmite,26 strongly supporting the formation of this material. All the FT-IR spectra of the samples studied are practically the same, indicating no significant effect of sulfate concentration on the formation of boehmite crystal and their crystallinity. 3.2. Morphology. Figure 2 shows both SEM and TEM images of the samples obtained by varying KAl(SO4)2 3 12H2O concentration from 0 to 0.025 M. As can be clearly seen, the boehmite sample obtained in the absence of KAl(SO4)2 3 12H2O exhibited mainly nanoflake-like structures having length of ca. 600 nm, width of ∼250 nm, and thickness of ca. 30 nm, accompanied with occasional spindle-like particles composed of nanoflakes (Figure 2a). However, when the concentration of KAl(SO4)2 3 12H2O was increased to 0.001 M, the morphology was changed to well-defined twisted nanoflake assemblies showing larger nanoflakes with length of 800-1000 nm, width of ∼400 nm, and thickness of ca. 40 nm (Figure 2b). A 5-fold increase of KAl(SO4)2 3 12H2O to 0.005 M resulted in the well-dispersed flower-like particles having length of ca. 2 μm and radial width of 1-1.2 μm, which consist of tens of nanoflakes (Figure 2c). A further increase in the concentration of KAl(SO4)2 3 12H2O to 0.013 M led to the formation of ellipsoidallike boehmite hollow microspheres with diameter of ca. 1.5-2 μm and shell thickness of ca. 400 nm (Figure 2d). The external surface of these microspheres consists of closely packed and interconnecting nanoflakes in an ordered fashion such as the sample prepared from aluminum sulfate.20

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Table 1. Effect of KAl(SO4)2 3 12H2O Concentration on the Adsorption Parameters of the Hierarchical Boehmites Prepared by Using Al(NO3)3 3 9H2O-CO(NH2)2 Solutiona sample

conc of KAl(SO4)2 3 12H2O (M)

SBET (m2/g)

Vt (cm3/g)

(a) (b) (c) (d) (e)

0 0.001 0.005 0.013 0.025

70.5 47.2 50.3 105.9 111.0

0.30 0.19 0.32 1.02 0.80

a Notation: SBET, BET specific surface area obtained from adsorption data in the P/P0 range from 0.05 to 0.2; Vt, single-point pore volume calculated from the adsorption isotherm at P/P0 = 0.98.

Figure 3. Nitrogen adsorption-desorption isotherms for the samples prepared using Al(NO3)3 3 9H2O with addition of CO(NH2)2 and KAl(SO4)2 3 12H2O. The amounts of KAl(SO4)2 3 12H2O were as follows: (a) 0; (b) 0.001; (c) 0.005; (d) 0.013; (e) 0.025 M.

Finally, increasing the concentration of KAl(SO4)2 3 12H2O to 0.025 M resulted in the enlargement of these microspheres to ca. 2-3 μm in the diameter and ca. 400-500 nm in the shell thickness (Figure 2e). This study shows that the gradual evolution of boehmite morphology from nanoflakes to nanoflake assemblies, flower-like structures, and hollow microspheres associated with increasing length and reducing length/diameter ratio can be achieved by varying the KAl(SO4)2 3 12H2O concentration from 0 to 0.001, 0.005, 0.013, and 0.025 M. 3.3. Surface Area and Porosity. The porous structures of hierarchical boehmite samples observed by SEM and TEM were further studied by N2 adsorption/desorption analysis. Figure 3 and Figure S2 (Supporting Information) show their N2 adsorption/desorption isotherms and the corresponding pore size distributions (PSD), respectively. The shape of all isotherms (Figure 3) is type IV, as indicated by convex curvature of the isotherm at submonolayer range and by occurrence of a narrow hysteresis loop (H3 type) at high relative pressures; the latter indicates the presence of slitlike mesopores formed between platelike particles.27 The adsorption branches of all isotherms resemble type II, indicating the presence of macropores. In addition, small mesopores (ca. 3-5 nm) are present within nanoflakes. This bimodal nature of porosity is reflected in the PSD curves (Figure 2S, Supporting Information), which show a small mesopore peak located at ∼4 nm and a gradual increase reflecting the presence of large pores. Interestingly, an increase of KAl(SO4)2 3 12H2O concentration from 0 to 0.001 and 0.005 M results in samples with enlarged hysteresis loops shifted toward lower P/P0, which is associated with some reduction of pore sizes. However, a further increase in the KAl(SO4)2 3 12H2O concentration from 0.005 to 0.013 and 0.025 M results in the shift of hysteresis loops toward higher P/P0, which corresponds to some enlargement of pores. The SEM and TEM analyses suggest that the smaller and larger mesopores can be related to the voids formed between primary crystallites within nanoflakes and the interspaces between stacked nanoflakes, respectively. The adsorption parameters of the hierarchical boehmite samples are listed in Table 1. This table shows that the specific surface area of the samples studied depends on the concentration of KAl(SO4)2 3 12H2O additive. The specific surface area of the assemblies prepared with addition of 0.001 M KAl(SO4)2 3 12H2O, as compared with that of the

Figure 4. SEM images of the samples prepared using Al(NO3)3 3 9H2O with addition of CO(NH2)2 and 0.005 M KAl(SO4)2 3 12H2O at different times: (a) 10 min; (b) 0.6; (c) 1.0; (d) 3.0 h.

nanoflakes obtained without KAl(SO4)2 3 12H2O, decreased remarkably from 70.5 to 47.2 m2/g due to the formation of assemblies consisting of larger nanoflakes. However, a further increase of the KAl(SO4)2 3 12H2O concentration to 0.005, 0.013, and 0.025 M resulted in a gradual increase of the specific surface area to 50.3, 105.9, and 110.0 m2/g due to the shape evolution from assemblies of nanoflakes to flower-like particles and hollow microspheres. The observed higher surface areas of samples d and e indicate smaller sizes of crystals, which is in agreement with the XRD intensities obtained for these samples (Figure 1). Among all samples studied, the hollow microspheres showed that the highest adsorption at the relative pressure close to unity, which converted to the volume of liquid nitrogen, reflects the pore volume of 1.02 cm3/g. The above results show that a judicious adjustment of KAl(SO4)2 3 12H2O concentration in the aluminum nitrate-urea system can be used for tailoring the morphology and textural properties of hierarchical boehmite materials, which might be useful for catalysis, adsorption, and related applications. 3.4. Morphology Evolution. The flower-like boehmite structure prepared with 0.005 M KAl(SO4)2 3 12H2O was chosen as an example to elucidate the time-dependent evolution of morphology by SEM (Figure 4) and XRD (Figure S3, Supporting Information). As can be seen from Figure 4a and Figure S3a, irregular amorphous aluminum hydroxide particles having length of ca. 1-2 μm were obtained after 10 min. Increasing the reaction time to 0.6 h resulted in the formation of many small nanoflakes on the surface of the particles and in the spontaneous phase transformation of amorphous solid to boehmite (Figure 4b and Figure S3b,

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Figure 5. Schematic illustration of the morphology evolution for hierarchical boehmite structures formed at different concentrations of KAl(SO4)2 3 12H2O: (I) hydrolysis and polycondensation; (II) nucleation, nuclei growth, and self-assembly; (III) further growth and self-assembly.

Supporting Information). For the reaction time equal to 1.0 h, newly formed nanoflakes tended to grow larger epitaxially, resulting in the formation of flower-like boehmite particles having length of ca. 2.0 μm and width of 1.5 μm, respectively (Figure 4c). A further increase in the reaction time to 3.0 h led to the formation of more uniform flower-like particles (Figure 4d), with quite complex internal structure. The current findings along with the KAl(SO4)2 3 12H2O concentration-dependent experimental result (Figure 2) and our previous findings5d,20,22 show that a facile template-free hydrothermal synthesis is suitable for tailoring the morphology of hierarchical boehmite structures from nanoflakes to hollow microspheres with high yield. It is anticipated that in this weakly basic system the concentration of SO42- is a key factor affecting the morphology of the final material. This SO42--mediated process involves the formation of boehmite crystallites followed by cooperative assembly and localized ripening, which is illustrated in Figure 5. First, sequential hydrolysis and polycondensation of Al3þ and urea kinetically favor deposition of amorphous aluminum hydroxide, which undergoes a series of morphological transformations with increasing concentration of KAl(SO4)2 3 12H2O additive. It is noteworthy that only a featureless material was obtained in the absence of SO42-, while uniform microspheres were formed at sulfate concentrations exceeding the value of 0.013 M. Because SO42- has higher affinity to bridge polymeric hydroxylated aluminum complexes than NO3-,28 the hydrothermal system with higher SO42- concentration favors the formation of spherical amorphous particles; thus, an increase in the KAl(SO4)2 3 12H2O concentration from 0 to 0.005 and 0.025 M resulted in the morphological transformation of boehmite from an amorphous featureless material to irregular particles and finally to solid microspheres. Second, the in situ phase transformation of amorphous aluminum hydroxide to hierarchical boehmite occurred spontaneously under certain alkaline conditions, where the aforementioned hydroxide might act as both the aluminum source and the sacrificial template. This phase transformation occurred first on the surface of amorphous solid, followed by the growth of boehmite crystallites along the main crystallographic [001] axis and by the primary self-assembly of these crystallites into 2D nanoflakes of higher thermodynamic stability.5c,8,13,14,22 It was reported

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elsewhere29 that boehmite crystals prefer to grow into nanoflakes as a result of their distinctly AlO6 octahedral layered structure with plenty of surface-located OH-groups under the weakly basic hydrothermal conditions using either aluminum nitrate or aluminum chloride. The aforementioned transformation seems to proceed gradually from the outside to the inside with decreasing supersaturation due to the exchange of reacting species through intercrystallite interstitials, which finally leads to the formation of hierarchical structures composed of nanoflakes.4a In particular, the primary self-assembly of SO42--stabilized nanoparticles into nanoflakes around irregular amorphous particles proceeds gradually together with crystallization in the range of increasing concentrations of KAl(SO4)2 3 12H2O up to 0.005 M. The hierarchical flower-like structures, composed of nanoflakes, have been reported for the synthesis of bismuth tungstate, iron oxide, and ceria.30 It was shown that the growth of these flower-like structures started with formation of their cores by aggregation of amorphous primary particles, which was induced by either poly(sodium 4-styrenesulfonate) or ethylene glycol. However, in the system reported here, irregular amorphous primary particles of ca. 1-2 μm (Figure 4a) are the cores of the flower-like structures. Since the external surface of these spherical particles is protected by adsorbed SO42- ions, the progressive dissolution and redistribution of matter from the interior to the exterior proceeds until formation of the hollow structure by numerous interconnected surface nanoflakes.4a Similar preferential dissolution of the interior with relatively high surface energy was also achieved by stabilization of the surface with phosphate ions.31 Finally, a complex process involving Ostwald ripening, anisotropic growth, and oriented attachment proceeds simultaneously within individual particles.30 Particularly, the oriented attachment of adjacent flakelike nanocrystals and other accompanying processes that occur in the system results in the formation of the well-defined hierarchical boehmite structures from nanoflakes through nanoflakes assemblies, flower-like structure, and hollow microspheres. In short, this unique morphological evolution with increasing concentration of sulfate can be a result of several synergistic effects, which include interplay between the hydroxylated surface of layered boehmite structures, coordinating ability of SO42- ions, aluminum nitrate-urea hydrothermal system, and other experimental conditions. Among the aforementioned factors, the morphologies of amorphous primary particles and final hierarchical assemblies are highly dependent on the concentration of sulfate, the formation of nanoflakes followed by their assembly into hierarchical structures, the intrinsic crystal structure of boehmite, and the weakly basic environment of the aluminum potassium sulfate-aluminum nitrateurea system. 3.5. Generality of the Sulfate-Mediated Morphological Transformation. To demonstrate the generality of the present strategy, similar experiments were also carried out by using reaction mixtures containing aluminum chloride instead of aluminum nitrate. As can be seen in Figure 6, the addition of KAl(SO4)2 3 12H2O has a pronounced effect on the morphology of the resulting boehmite materials. However, in this case, an analogous morphology evolution to that observed for aluminum nitrate can be obtained by increasing the AlCl3 3 6H2O concentration from 0.005 to 0.025 M. The proposed strategy seems to be quite general because analogous morphological changes are observed by adding

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boehmite hollow microspheres were also obtained when appropriate amounts of other sulfates such as AlNH4(SO4)2 3 12H2O, Al2(SO4)3 3 18H2O, Na2SO4, (NH4)2SO4, and MgSO4 3 7H2O were used in the aqueous phase of aluminum nitrate and urea. Thus, this new general strategy seems to be applicable for the synthesis of hierarchical boehmite structures and related materials with controlled morphology and enlarged surface area for various applications such as catalysis, adsorption, and separation.

Figure 6. SEM images of the samples prepared using AlCl3 3 6H2O with addition of CO(NH2)2 and different amounts of KAl(SO4)2 3 12H2O: (a) 0.005; (b) 0.009; (c) 0.013; (d) 0.025 M.

Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (50625208, 20773097, and 20877061), the China Postdoctoral Science Foundation Project (20080440142), and the National Basic Research Program of China (2007CB613302). The China Scholarship Council is also acknowledged for support of this research under the State Scholarship Fund program. Supporting Information Available: Five figures showing FTIR spectra, pore size distributions, XRD patterns, and SEM images of the selected boehmite samples. This information is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 7. SEM images of the samples prepared using Al(NO3)3 3 9H2O with addition of CO(NH2)2 and different sulfates ([Al3þ] = [SO42-] = 0.05 M): (a) AlNH4(SO4)2 3 12H2O; (b) Al2(SO4)3 3 18H2O; (c) Na2SO4; (d) (NH4)2SO4; (e) MgSO4 3 7H2O.

other sulfates. The SEM images (Figure 7a-d) of the samples prepared at 0.05 M SO42- concentration by using different aluminum sulfates such as AlNH4(SO4)2 3 12H2O and Al2(SO4)3 3 18H2O, and other sulfates such as Na2SO4 and (NH4)2SO4, indicate that the uniform and well-dispersed boehmite hollow microspheres having diameters of 2-3 μm and shell thicknesses of 500-700 nm were successfully obtained. Interestingly, using MgSO4 3 7H2O instead of the above sulfates resulted in hollow microspheres with a bigger shell thickness of ca. 1 μm and a higher length-to-diameter ratio of the primary nanoflakes (Figure 7e). Correspondingly, the introduction of Mg species resulted in an obvious change of the framework crystal, as reflected by the diffraction lines of MgAl hydrotalcite (Figure S4 of the Supporting Information). However, the formation of such hollow structures may be closely related to the concentrations of Al3þ and SO42- and to the overall composition of the synthesis mixture. For example, only boehmite shell fragments can be obtained by using reaction mixtures containing 0.2 M Al3þ and 0.3 M SO42- arising from Al(NO3)3 3 9H2O and Na2SO4, respectively (Figure S5 of the Supporting Information). 4. Conclusions This work shows that the morphology of hierarchical boehmite structures formed in aqueous solutions of aluminum salts and urea can be controlled by adjusting the sulfate concentration. The morphology of these structures evolves with increasing KAl(SO4)2 3 12H2O concentration from nanoflakes, nanoflakes assemblies, and flower-like structures to hollow microspheres. Although the effect of sulfate concentration was systematically studied for KAl(SO4)2 3 12H2O, the well-dispersed boehmite and magnesia-incorporated

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