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Morphological Controlled Growth of Nanosized Boehmite with Enhanced Aspect Ratios in an Organic Additive-Free Cationic− Anionic Double Hydrolysis Method Wenqian Jiao,†,‡ Xuezhong Wu,† Teng Xue,‡ Gang Li,† Weiwen Wang,† Yangxia Wang,† YiMeng Wang,‡ Yi Tang,*,† and Ming-Yuan He*,‡ †

Department of Chemistry, Laboratory of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China ‡ Shanghai Key Lab of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062 S Supporting Information *

ABSTRACT: Well-crystallized boehmite nanoparticles showing different sizes and morphologies were fabricated in an organic additive-free cationic−anionic double hydrolysis method using inorganic aluminum chloride salt and sodium aluminate as dual aluminum sources. By adjusting the molar ratios of Al3+/AlO2− in the synthesis recipe, the boehmite particles’ shapes could be controllably tuned from twodimensional flakes to one-dimensional (1D) rods, needles, and even fibers with enhanced particles’ aspect ratios. Through X-ray diffraction and high resolution transmission electron microscopy measurements, details of the microstructural features for boehmite particles were gained, and thus the growth habits are discussed, where in strong alkaline synthesis medium with a low Al3+/AlO2− molar ratio, dispersed nanoflakes grew with (010) and (101) faces as basal and lateral surfaces, while 1D nanoparticles, i.e. nanorods, nanoneedles, and nanofibers preferentially grew along the [100] direction with (100) and (101) faces unexposed when Al3+/AlO2− molar ratios were gradually raised. Additionally, the impacts of pH values and Cl− ions in the suspensions to the particles shapes are also discussed. The evolution of boehmite morphologies and the resultant enlargement of aspect ratios led to increased total surface charges (ζ-potential) and higher isoelectric points of boehmite samples which would benefit the stabilization of particles’ suspensions and improvement of surface functionalization abilities.

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structures.13 When considering boehmite nanoparticles’ sizes and shapes, the aspect ratios are frequently mentioned, which are defined as the ratio of length along the a axis to that along the c axis, as the selective exposure of boehmite faces would largely affect their surface properties and functionalization abilities.14−16 Up to now, many approaches have been explored to tailor the sizes and morphologies of boehmite particles. Among those, the soft templating method is the most versatile way in which boehmite nanoparticles with controlled size and morphology can be designed and fabricated. Conventionally, organic surfactants, polymers, or small organic complexing species would be involved as a morphological controller or growth directing agents. For example, polymers sodium polyacrylate (NaPa) 210017−19 and poly(ethylene oxide) (PEO)20,21 were once employed to direct the assembly of aluminum hydrate particles leading to the growth of long boehmite fibers. On the

INTRODUCTION Nowadays, tremendous efforts have been devoted to the preparations of inorganic nanomaterials with desired sizes and morphologies, as the material properties, such as catalytic, optical, and magnetic properties and their potential applications are largely dependent on the particle sizes and/or morphologies.1 Boehmite (γ-AlOOH), as one of the polymorphs of aluminum oxyhydroxide and the primary precursors for the preparation of various aluminas, such as γ-Al2O3 and corundum (α-Al2O3), has been widely applied in areas of adsorption, ceramics, and catalytic processes (catalysts and catalyst supports).2−4 More recently, boehmite nanoparticles have also found some technological utilities as dielectric microelectronics, biomedical materials, and optical devices.5 Because of the close relationship between the morphology and/or size and properties and further applications, considerable attention has been paid to synthesize boehmite samples with different dimensions and shapes, such as dispersed nanosheets,6 nanoflakes,7 nanoribbons,8 nanotubes,9 nanorods,9 nanofibers,10 and even their assembled superstructures, such as flowers,11 hollow microspheres,12 and cantaloupe-like super© XXXX American Chemical Society

Received: May 12, 2016 Revised: July 27, 2016

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DOI: 10.1021/acs.cgd.6b00723 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Aluminum sulfate and aluminum nitrate salts were also applied as the cationic aluminum source to synthesize boehmite instead of aluminum chloride, and the synthesis process was all identical. Characterizations. Powder X-ray diffraction (XRD) patterns are collected by a Rigaku-Ultima diffractometer using a Cu Kα radiation source (λ = 0.15432 nm) in the 2θ range from 10 to 80° at a scanning speed of 60°/min. Scanning electron microscopy (SEM) observations were performed on a Hitachi 4800 (Hitachi Ltd. Tokyo, Japan). Transmission electron microscopy (TEM) experiments were conducted on TECNAI G2 F30 operating at 300 kV. pH values of the suspensions were detected using a pH meter (PHSB-260, BOQU Instrument Co., Ltd. Shanghai, China). The supernatant fluids were obtained through centrifugation of the as-prepared mixtures with a rotation rate of 10000 rpm for 3 min. Na, Cl, and Al ions contents in the suspensions (supernatant) were detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES), performed on a PerkinElmer Optima 8000. Surface characterizations of boehmite nanoparticles were carried out through the determination of the isoelectric point (IEP) of the materials, analyzing the variation of zeta potential with the pH in 0.1 g/L suspensions of the samples. A Malvern Zetasizer Nano SZ was used for the zeta potential measurements in a pH range between 2 and 13, keeping the conductivity of the suspensions in the recommended range of 0.01−2 mS/cm.

other hand, the ionic liquid 1-butyl-2,3-dimethyl imidazollium chloride ([bdmim][Cl]) also was applied as a template to tune the morphologies of boehmite dispersed particles and assembled clusters.22 Some small and simple molecules, such as alditols14 and carboxylic acids,23 were used as well in the synthesis media to confine the growth of boehmite nanoparticles. Polyols with a higher number of OH groups (i.e., xylitol) and carboxylic acids can selectively adsorb on the boehmite faces and thus modify the growth behavior of boehmite crystals, leading to boehmite nanoparticles with various morphologies and aspect ratios. In order to avoid the involvement of organic species in the synthesis processes, hydrothermal treatments of the aluminum hydroxide or oxyhydroxide were also explored in the absence of organic additives, and notably the pH values and anions (SO42− and Cl−) in the synthesis media always governed the sizes and morphologies of boehmite samples.24−27 Also, boehmite nanoparticles and their assembled superstructures showing different sizes and shapes can be synthesized through a steamassisted gel conversion method in which the morphologies of the final boehmite samples are largely affected by the pH values and temperatures for gel formation,15,28 the water to gel weight ratios,5 and steam compositions.29 As stated above, numerous procedures for the synthesis of boehmite nanoparticles have been developed and documented to control their sizes and morphologies, yet it is still a main concern to search for a cost-effective and easy to scale up process for production of boehmite nanostructures. Herein, boehmite nanoparticles with different sizes and shapes were synthesized in an organic additive-free cationic−anionic double hydrolysis (CADH) method by taking inorganic aluminum salts and sodium aluminate as dual precursors, and the growth behavior of boehmite nanoparticles were also thoroughly investigated and discussed. The CADH method30−35 was the most adequate for the scale-up production of aluminum hydroxide and oxyhydroxide nanomaterials; nevertheless, not much attention has been gained so far in the synthesis of boehmite nanostructures by this method, and the growth habits, sizes, and shapes of resultant boehmite particles were not examined either before to our best knowledge.

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RESULTS AND DISCUSSION Crystalline Structure of Aluminum Hydroxide. AlCl3· 6H2O solutions of different concentrations were neutralized by the alkaline aluminate solutions with the immediate formation of aluminum hydroxide gels under the conditions of different initial precipitate pH values and varied ionic strength (concentration of Cl− ions). The preparation details are listed in Table 1. XRD experiments were conducted to evaluate the Table 1. Preparation Parameters and Structural Factors of the Obtained Boehmite Samples

EXPERIMENTAL SECTION

sample

Al3+/AlO2−

initial pH value

initial Cl− concentration (g/L)

morphologies

AlOOH-1 AlOOH-2 AlOOH-3 AlOOH-4

1:3 1.25:3 1.5:3 1.75:3

12.40 7.80 5.66 4.43

7.38 9.23 11.09 13.03

flakes rods needles fibers

crystalline structures of the final products harvested after crystallization (Figure 1). It showed that pure, highly crystallized boehmite can be formed with the XRD profiles in accordance with the JCPDS powder diffraction pattern 211307. No diffraction peaks ascribed to other aluminum hydroxides or oxyhydroxides (i.e., bayerite, gibbsite, bauxite, etc.36) phases can be observed. The extra tiny peak (denoted in purple circle) appearing in the diffraction profile of the sample of AlOOH-4 in Figure 1d was assigned to halite crystal (NaCl, JCPDS card No. 05-0628), which was probably formed by the residuals of Na+ and Cl− ions on the solid product after centrifugation and washing. With increasing the Al3+/AlO2− molar ratio from 1.0 to 1.75, a clear evolution can be observed from the intense and narrow diffraction pattern of sample AlOOH-1 to the broader, shorter profiles, indicating the reduction of the particle sizes. Besides, the poor split of the two peaks around 2θ of 50° corresponding to facets (051) and (200) of boehmite, respectively, further demonstrated the small sizes of the boehmite particles.29 The crystal dimensions of the boehmite samples were roughly estimated from the XRD (020) and (200)/(002) peaks by using Scherrer’s equation (eq 1).

Reagents. All chemical reagents were of analytical grade and used as received without further purification. NaAlO2 (Al2O3, 52.82 wt %, Na2O, 40.24 wt %) was purchased from Alfa-Aesar. Aluminum chloride hexahydrate (AlCl3·6H2O), aluminum sulfate octadecahydrate (Al2(SO4)3·18H2O), and aluminum nitrate nonahydrate (Al(NO3)3· 9H2O) were provided by Sinopharm Chemical Reagent Co., Ltd. Deionized water was used directly without further purification. Synthesis Procedures. A typical synthesis procedure was as follows: A calculated amount of AlCl3·6H2O was dissolved into 36 g of deionized water at room temperature with stirring, and the resultant solution was denoted as solution A. A total of 1.43 g of NaAlO2 powder (7.5 mmol Al2O3, 18.7 mmol NaOH) was added into 36 g of hot water with stirring until a transparent solution was obtained, and it was denoted as solution B. Then the alkaline solution B was slowly poured into the acidic solution A with a white gel forming immediately, and the mixture was further stirred at room temperature for another 1 h. Finally, the resultant mixture was transferred into a Teflon-lined vessel and hydrothermally treated at 150 °C for 24 h. The composition of the synthetic mixture was 3AlO2−/xAl3+/800H2O (x = 1.0−1.75). After being cooled to room temperature, the product was collected by centrifugation with a rotation rate of 10 000 rpm for 3 min and washed with hot water and then dried at 80 °C overnight. B



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perpendicular to the electron beam.23 With the rise of the Al3+/ AlO2− molar ratio to 1.25, the resultant sample AlOOH-2 mostly consisted of nanorods with a length in the range of ∼40−80 nm and the width of ∼10−15 nm. It was noteworthy that the rods exhibited uneven sides with lots of sags and crests along the sidelines. In the case of sample AlOOH-3 prepared with an Al3+/AlO2− molar ratio of 1.5, needle-like nanorods with a length of ∼60−100 nm and width of ∼6−10 nm were dominating. Each nanoneedle had a wide end and became narrower progressively from one end to the other. Further increasing the Al3+/AlO2− molar ratio to 1.75 would result in boehmite displaying a long fibrous morphology with a length of ∼300 nm and width ∼6−8 nm (Figure 3d). Obviously, boehmite nanoparticles with tunable sizes and morphologies could be fabricated by the CADH method by simply varying the Al3+/AlO2− molar ratios in the synthesis recipe, and increasing the Al3+/AlO2− molar ratio would lead to the particle shape first changing from 2D nanoflakes to 1D nanorods, and second further stretching from short, wide nanorods to long and narrow nanoneedles and even nanofibers. Nevertheless, the length of boehmite fibers would not apparently increase if the Al3+/AlO2− molar ratio were further raised from 1.75 to 2.0 or 2.25 (Figure S1 in Supporting Information). TEM images with high magnifications are shown in Figure 4 to thoroughly investigate the detailed crystalline structures of boehmite samples. It is shown in Figure 4a that the welldeveloped flakes in sample AlOOH-1 exhibited straight edges with angles of ∼104o and 76°, corresponding to the angle between the (101) and (1̅01) planes of the boehmite crystalline structures.16 From the side views of the nanoflakes with less transparency to the electron beam as shown in Figure 4d, distinct lattice fringes were seen with d-spacing of ∼0.61 nm ascribed to the (020) planes in the orthorhombic AlOOH crystals. Therefore, it is convincing from the aforementioned results that the nanoflakes in sample AlOOH-1 grew with the basal planes and lateral faces of (010) and (101), respectively. Furthermore, from the side views of those nanoflakes, the actual thickness of the nanoparticles was about 4−8 nm, very close to the calculated value from the width of the comrresponding XRD diffraction peaks (Table 2). Regarding sample AlOOH-2 prepared at a Al3+/AlO2− molar ratio of 1.25, the particle length and width were in the range of 40−80 and 10−15 nm, respectively, gathered from TEM images (Figures 3b and 4f), which, however, were quite different from the results calculated from the fwhm of the (200) and (002) diffraction peaks (11 and 20 nm, respectively, Table 2). From the careful observations in Figure 4e, it was found that the nanorods in sample AlOOH-2 were actually made up of some linearly aggregated unit particles with a much smaller size, and these unit particles fused incompletely to make the nanorods with very uneven zigzag edges. Zhu and co-workers

Figure 1. XRD patterns of the as-prepared samples (a) AlOOH-1, (b) AlOOH-2, (c) AlOOH-3, and (d) AlOOH-4 with Al3+/AlO2− molar ratios of 1/3, 1.25/3, 1.5/3, and 1.75/3, respectively. Insets are the standard XRD patterns of boehmite (JCPDS no. 21-1307, below in red) and halite (JCPDS Nno. 05-0628, upper in blue).

Dhkl = kλ57.3/[FWHM cos(θ )]

(1)

where Dhkl was the crystallite size in the direction perpendicular to the lattice planes (hkl were the Miller indices of the planes being analyzed), k was a numerical factor (k = 0.9), λ was the wavelength of the X-rays, fwhm was the full-width at halfmaximum of the X-ray diffraction peak, and θ was the Bragg angle. The crystallite sizes were calculated through Scherrer’s equation by the X-ray diffractometry line broadening method without refinement which could be used to qualitatively analyze the dimensions of nanocrystallites in samples. The results listed in Table 2 indicated that the thickness of the boehmite crystallites deduced from XRD (020) peaks1 were almost identical, and the reduction of the crystal size took place mostly in [001] and [100] direction. Morphological and Microstructural Analysis. SEM and TEM observations were therefore performed to further ensure the microstructural features of the boehmite samples inferred from the XRD profiles. Figure 2 depicts the SEM images of the as-prepared boehmite samples with the same magnification. The sample prepared in the system with the lowest Al3+/AlO2− molar ratio displayed uniform-sized, highly dispersed nanoflakes (AlOOH-1, Figure 2a). With increasing of the Al3+/ AlO2− molar ratio, the shape of boehmite samples changed from small nanoflakes to nanorods and then to long nanofibers (Figure 2b−d). Such an evolution of boehmite nanostructures can be further confirmed by TEM images (Figure 3). As for the sample of AlOOH-1 synthesized in the highest alkaline solution with an initial precipitate pH value of 12.40, nanoflakes with a width of ∼30−40 nm can be obtained. Some particles of less transparency to the electron beam were those with lateral faces

Table 2. Crystal Sizes of Boehmite Estimated by Application of the Scherrer’s Equation vs. Actual Particle Sizes Observed from TEM Imagesa crystal sizes (nm) along different directions

a

sample

D020

db

D002

dc

D200

da

aspect ratio (da/dc)

AlOOH-1 AlOOH-2 AlOOH-3 AlOOH-4

10 10 10 9

4−8 4−6 4−6 4−6

20 20 11 11

30−40 10−15 6−10 6−8

14 11 10 9

30−40 40−80 60−100 300

∼1 4−6 6−10 50

Dhkl: crystal size calculated through Scherrer’s equation from XRD (hkl) peaks; dx: sizes of boehmite particles observed from TEM images. C



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Figure 2. SEM images of the as-prepared samples (a) AlOOH-1, (b) AlOOH-2, (c) AlOOH-3, and (d) AlOOH-4 with Al3+/AlO2− molar ratios of 1/3, 1.25/3, 1.5/3, and 1.75/3, respectively.

once reported almost the same growth mode of boehmite fibers, in which an assembling process of stable boehmite crystals, acting as the basic building units, was involved.20 As shown in Figure 4e,f, the sizes of the unit particles were about ∼8−15 nm, in good agreement with the estimated crystal sizes of XRD results (Table 2). Besides those nanorods with uneven edges, a few of the narrow and straight particles can also be viewed with very smooth edges and less transparency (red square in Figure 4e), and the lattice fringes of (020) planes with d-spacing of ∼0.61 nm were clearly identified on them. It was believed that those exposed faces with lattice fringes were the lateral planes of the nanorods, and therefore the thicknesses of the boehmite particles in sample AlOOH-2 were also measured in the range of ∼4−6 nm, very similar to that of sample AlOOH-1. In sample AlOOH-3, the particles appeared as nanoneedles with arrow ends and uneven edges as denoted in the white circle (Figure 4g,h), and each needle was seemed to be composed of several small unit particles, just like those in sample AlOOH-2. The unit particles were better fused to each other and had even smaller sizes ranging from 4 to 6 nm. Also, some straight and smooth nanorods coexisted as denoted in red squares in Figure 4g. After magnification, clear lattice fringes ascribed to d-spaces of (020) planes were observed (Figure 4h). Nearly identical phenomena of the particle morphologies were observed in sample AlOOH-4 (Figure 4i,j), wherein the long fibers with arrow ends but uneven edges were simultaneously found with those with regular and smooth edges (Figures 4i and S2 in Supporting Information), and the lattice fringes ascribed to d spaces of (020) planes were observed on the lateral faces of AlOOH-4. The measured depths of the particles of AlOOH-3 and AlOOH-4 indicated that the thicknesses of the boehmite samples were almost the same though with various sizes and shapes, further confirming the results elucidated from XRD diffraction peaks. Moreover, the decreases in crystallite sizes derived from XRD (200) and (002) peaks (Table 2) were ascribed to the reduction of building unit particles’ dimensions.

The observations of different particle appearances and the presence of lattice fringes on the lateral surfaces of the samples enabled an assumption of growth habit of boehmite with different morphologies and sizes (Figure 5). For all the boehmite samples displaying varied morphologies, the (010) faces were always dominating (basal surfaces), and as their shape evolved from nanoflakes to nanorods, nanoneedles, and nanofibers, the width of basal surface was gradually decreased, while the total length was increased. Discussions on Discrepancies of Boehmite Growth Habit. In boehmite crystalline structures (Amam space group; unit cell parameters a ≈ 0.370, b ≈ 1.223, c ≈ 0.287 nm), each Al atom was coordinated by four O atoms and two OH groups with the formation of AlO4(OH)2 octahedral units (Figure 6a). The octahedrons linked each other through sharing O atoms along the [100] and [001] directions leading to the construction of a double layer structure. The layers piled up through the interaction of hydrogen bonds along the [010] direction as shown in Figure 6b−d. Conventionally, boehmite crystals tended to grow along the [100] and [001] directions under neutral or basic conditions, resulting in the formation of 2D structures such as flakes and platelets etc., as the interactions of covalent bonds between oxygen and aluminum atoms along the a and c axis were much stronger than the hydrogen bonds between octahedron double layers along the b axis.37 Meanwhile, Jolivet et al. pointed out that the discrepancies in sizes and/or shapes of boehmite were tightly related to the variations of the electrostatic surface charge densities of the particles, which would induce variations of the oxide-solution interfacial tensions and thus decrease the surface energy.1 As the interfacial tension of the boehmite (010) facet was the lowest and independent of the pH, boehmite crystals always grew with (010) as the basal faces, which was consistent with the experimental results in this work. In the alkaline synthesis media, the interfacial tensions for (101), (100), and (001) faces kept increasing and reached the maximum at pH 10, and thus the platelet-like particles would form15 to decrease the surface energies by enlarging the exposure of (010) faces D



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Figure 4. High magnification TEM and HRTEM images of (a−d) sample AlOOH-1, (e−f) AlOOH-2, (g−h) AlOOH-3, and (i−j) AlOOH-4 prepared with Al3+/AlO2− molar ratios of 1/3, 1.25/3, 1.5/ 3, and 1.75/3, respectively. Figure 3. TEM images of the as-prepared samples (a) AlOOH-1, (b) AlOOH-2, (c) AlOOH-3, and (d) AlOOH-4 with Al3+/AlO2− molar ratios of 1/3, 1.25/3, 1.5/3, and 1.75/3, respectively.

while only dispersed nanoparticles with a size of around 30−40 nm could be obtained when the molar ratio of Al3+/AlO2− was 1.0 with aluminum sulfate as the cationic aluminum source, which was the same as that when aluminum chloride was applied (Figure S3 in Supporting Information). It was stated that inorganic anions, such as sulfate, chloride, and nitrate ions, can selectively adsorb on boehmite (010) and (001) planes through interacting with surface hydroxyls on crystal planes,25,38,27 resulting in the preferential growth of boehmite crystals along the [100] direction. On the basis of the adsorption tendency of the anions (SO42− > Cl− > NO3−), nanofibers and nanorods with different aspect ratios were prepared in an acidic environment, whereas in alkaline solutions, no adsorption of anions occurred on boehmite faces, irrespective of the anion types and only nanoflakes can be obtained. The pH values of the synthesis media played an important role in the surface charges of boehmite, positive in acidic and negative in alkaline solutions, and consequently

(AlOOH-1). In addition, when boehmite was crystallized in acidic media with a pH value 95%) of boehmite can be precipitated, especially for sample AlOOH-2 (∼99%) prepared with an initial pH value near 7. Determinations of ζ-Potential and Surface Chemistry. In order to illustrate the impact of boehmite particle sizes and morphologies to the surface properties, the ζ-potential of aqueous suspensions of beohmite were measured as a function of pH, and the results are shown in Figure 7 and Table S2 in

Figure 5. Illustration of the preferential growth of boehmite samples with different morphologies.

Figure 6. Schematic representations of (a) AlO4(OH)2 octahedral unit and (b−d) the lattice planes (010), (001), and (100) in boehmite crystalline structures.

facilitating or inhibiting the adsorption of anions on the boehmite faces.20,21 Starting from boehmite nanoparticles, Xia et al. also prepared boehmite nanoplates and nanorods through an aggregation process, and the pH values and chloride ions contents of synthesis suspensions were tuned by adding HCl or NaCl or the mixture thereof. Through density functional theory calculations, it was pointed out that chloride ions can absorb on boehmite crystal (010) and (001) planes to lower the surface energy and control the morphologies of the boehmite particles.27 The pH values and the ions contents of the suspensions for different boehmite samples before and after hydrothermal treatment were measured as listed in Table S1 and shown in Figure S4 in Supporting Information. It turned out that for sample AlOOH-1 the final pH value was just slightly lowered compared to the initial value and still remained in a strong alkaline region. Concurrently, the Cl− and Na+ ions almost remained in the suspension after crystallization, showing hardly adsorption of ions on boehmite solid during hydrothermal

Figure 7. Zeta potential as a function of pH for the as-prepared samples (a) AlOOH-1, (b) AlOOH-2, (c) AlOOH-3, and (d) AlOOH-4 with Al3+/AlO2− molar ratios of 1/3, 1.25/3, 1.5/3, and 1.75/3, respectively.

Supporting Information. Distinct differences in the profile shapes and the IEPs for the samples were observed, wherein IEP was defined as the pH value at which a particle as a whole carried no electrical charges.16 It revealed in Figure 7 that sample AlOOH-1 with a nanoflake shape possessed the lowest ζ-potential and presented a progressive transition from positive to negative charges with an increase of pH values. Otherwise, the other samples, showing 1D morphologies such as nanorods, F



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nanoneedles, or nanofibers all possessed high and positive charges in wide pH ranges and displayed abrupt slopes in the change from positive to negative potentials in alkaline solution. Besides, it also demonstrated that the IEP gradually changed from pH ≈ 7.6 for sample AlOOH-1 to pH ≈ 11 for the other three samples. As described above, all the aluminum and oxygen atoms in (010) basal face of boehmite were fully saturated, and thus hydroxyl groups in this surface were inert, and no surface charge or acid−base properties were exposed by this surface.15 Contrarily, crystal faces (101) and (001)/(100) (lateral faces) displayed surface charges because of the presence of active hydroxyl groups resultant from the surface reconstruction and protonation of relaxation of Al ions with coordination frustration. Therefore, the higher IEPs for the samples displaying 1D morphologies were attributed to the enhancement of the proportions of areas for lateral to basal faces, which was consistent with the crystalline structural observations. Moreover, the increased proportions of lateral surfaces in boehmite could be a benefit for potential technological applications, as they were strongly related to the suspension stabilities and surface functionalization abilities.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (M.-Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was also supported by National Key Basic Research Program of China (2013CB934101), NSFC (21433002, 21573046), National Plan for Science and Technology of Saudi Arabia (14-PET827-02), and Sinopec (X514005).

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CONCLUSIONS By the CADH method, the well-crystallized boehmite nanoparticles were successfully synthesized using aluminum chloride and sodium aluminate as dual aluminum sources without addition of any organic additives. The sizes and shapes of boehmite particles were easily tuned by adjusting the molar ratios of Al3+/AlO2− in the synthesis recipes. Different Al3+/ AlO2− molar ratios in the synthesis recipes would lead to varied initial precipitation pH values and ionic strength in the suspensions, which contributed to the diverse growth behavior of boehmite crystals and thus the evolutions of morphologies. In strong alkaline synthesis medium (Al3+/AlO2− = 1.0) with initial pH value of 12.40, dispersed boehmite 2D nanoflakes formed with (010) and (101) faces as basal and lateral surfaces, and no Cl− anions adsorption on boehmite solids was observed during the crystallization process because of the negatively charged surface properties. With gradually increasing the Al3+/ AlO2− molar ratios (1.25−1.75) in the synthesis recipes with decreased initial precipitate pH values (7.80−4.43), resultant boehmite nanoparticles appeared as 1D nanorods, nanoneedles, and nanofibers with promoted particles’ aspect ratios. The selective adsorptions of Cl− anions on boehmite (010) and (001) surfaces through interacting with surface hydroxyls on crystal planes facilitated by the spontaneously lowering of pH values were noticed during crystallization process and thus led to preferential growth of boehmite particles along the a axis. The evolution of boehmite sizes and morphologies and the resultant promotion of aspect ratios led to increased total surface charges (ζ-potential) and higher IEPs of boehmite samples, which would be of benefit for the stabilization of particle suspensions and improvement of surface functionalization abilities. Besides, the high solid yields of boehmite after crystallization make the process a potential for large-scale practical productions.

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Table of chemical compositions of the clear suspension, zeta potential as a function of pH for different samples. TEM images for boehmite (PDF)

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