Study of the Formation Mechanism of Boehmite with Different

Jun 29, 2013 - ... Ekaterina A. Brushkova , Maxim I. Morozov , Vladimir V. Vinogradov ... Jaekyoung Lee , Himchan Jeon , Dong Gun Oh , Janos Szanyi , ...
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Study of the Formation Mechanism of Boehmite with Different Morphology upon Surface Hydroxyls and Adsorption of Chloride Ions Yuguo Xia, Xiuling Jiao,* Yongjun Liu, Dairong Chen, Li Zhang, and Zhenhua Qin School of Chemistry & Chemical Engineering, National Engineering Research Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: Identification of the key factors of morphology control in nanomaterial synthesis is important for mechanism understanding of the reactivity−structure relationship and rational design of crystals with desired morphology. Among others, the inorganic ions in solution were found to be a key factor in controlling the final morphology. To reveal the effect of inorganic ions, the boehmite with different morphologies influenced by chloride ions is studied by a combined experimental and computational approach. The quantum mechanical calculations reveal that the chloride ions interact with crystal planes through surface hydroxyls and act as morphology-directing agent. The growth directions and exposed planes of the boehmite determined from HR-TEM images indicate an oriented-aggregation process which is consistent with the DFT calculations. It is envisaged that the defects caused by excessive hydrochloric acid are due to its dissolving property combined with the reduction of adsorption energies. Overall, all the morphologies of boehmite suggested by the calculations are confirmed by experimental results. morphology of γ-AlOOH through the hydrothermal method, whereas most of the conclusions are qualitative explanations for the formation mechanism based on the experimental facts. In the past few years, quantum mechanical studies based on density functional theory (DFT) and molecular mechanics simulation have been conducted with the aim to determine the structure,13 relative stability,14 vibration spectrum,15 and the dehydration process.16 Moreover, the energies of main exposed crystal planes17 and interfaces of boehmite/water18 are also investigated. However, the function form of inorganic ions with the surfaces of γ-AlOOH and its effect on the final morphology are seldom mentioned. Prior to investigating the main factors affecting the final morphology of γ-AlOOH, it is important to be able to confirm the parameters in the synthesis process. Heat treatment is considered to be a useful technique to accelerate the crystallization and aggregation process. Thus, all the treatments in our experiment are conducted at the same temperature. Moreover, the rationality of DFT calculation for hightemperature reaction of γ-AlOOH has been verified.19 Herein, we adopted a combined hydrothermal synthesis and computational approach to investigate the effects of surface energy and inorganic ions. We have performed quantum mechanical calculation using density functional theory (DFT), which is

1. INTRODUCTION As one of the important hydrated alumina, the boehmite (γAlOOH) nanocrystals with morphologies such as fibers,1 rods,2 plates,3 and cubes4 have attracted much attention in recent years. First, the physical, chemical, and catalytic properties of γAlOOH nanomaterials are closely relevant to their morphology. In addition, they can topologically transform to γ-Al2O3 nanocrystals,5 which are widely used as catalyst,6 adsorbent,7 and composite.8 It is well-known that the morphology of γAlOOH depends on not only the intrinsic structure of γAlOOH but also the synthetic parameters, such as temperature, solvent, inorganic ions, and surfactant.9 Studying how these variables affect the final morphology is essential for synthesizing the desired materials and understanding the formation mechanism. Many techniques have been developed to fabricate the inorganic materials with controlled morphologies, including the template method,10 the soft lithography,11 sol−gel technique,12 etc. As a moderate and easy controlled method, the hydrothermal conditions can be adjusted to control the properties of γ-AlOOH, including the morphology, size, structure, etc. In a few cases, nanoscale aluminum hydroxides with different sizes and morphologies have been synthesized by simply regulating the sorts and amount of acid through the hydrothermal method.2 Those variables which have a stabilizing effect on the surface are considered to be the main factors for morphology control. Many experimental researches have been devoted to study the relationship between the parameters and © 2013 American Chemical Society

Received: March 13, 2013 Revised: June 27, 2013 Published: June 29, 2013 15279

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implemented in the DMol3 module.20 This type of computational method has previously been used to study the physisorption site of H2 on carbon,21 CO adsorption on Pt (211) and (311) planes,22 and the catalysis of Ag12O6/Ag (111).23 However, there are few reports on the calculation of adsorption process of inorganic ions on γ-AlOOH, and it is still unclear how the inorganic ions affect the final morphology. Here, we focus on the relationship of chloride ions and the final morphology of γ-AlOOH in neutral and acidic environments. We present an experimental analysis of the γ-AlOOH using a transmission electron microscope, a high-resolution transmission electron microscope, and X-ray diffraction studies. Additionally, the calculation is analyzed in terms of Mulliken charge population analysis, total charge analysis, and the electron difference density analysis. This draws a detailed picture of reaction type of chloride ions with crystal planes of γAlOOH. Finally, we corroborate the two methods to allow an understanding of the effect of chloride ions on the final morphology of γ-AlOOH.

Å3, the convergence criterion for the force between atoms was 5 × 10−2 eV/Å, and the maximum displacement was 5 × 10−3 Å. Development of the Relaxed and Adsorption γAlOOH Surface Model. Table 1 provides a comparison of Table 1. Optimized Bulk Lattice Parameters of the γ-AlOOH Crystal: Our Atomic Orbital DFT Calculations versus Other DFT and Experimental Results lattice parameter (Å)

this study, GGA+PBE

GGA+PAW

exptl result

a b c

2.90 12.34 3.74

2.90 11.97 3.71

2.87 12.23 3.69

the lattice parameters for the bulk γ-AlOOH calculated in this work versus other DFT29 and experimental results.26 The crystal was then cleaved along the (100), (010), and (001) Miller planes to create γ-AlOOH slabs. To gauge the effective number of atomic layers needed for the surface model, successive supercell slabs comprising the odd number from 3 to 9 layers with a 15 Å vacuum space were tested. Each system was optimized with all the atoms relaxed except the middle layer, and the surface energy was calculated using the equation30 1 slab σ= [Eopt − (Nslab/Nbulk )Ebulk ] (1) 2A where A is the surface area, Eslab opt is the total energy of the relaxed slab, Ebulk is the total energy for single γ-AlOOH bulk, and Nslab and Nbulk correspond to the total atoms number of the relaxed slabs and single γ-AlOOH bulk, respectively. It was found that a slab of 7 atomic layers in thickness, in which 3 surface layers in one side were relaxed, was sufficient to obtain a converged system within a practical simulation time while providing enough layers to effectively study surface reactions. The k-mesh and atoms layers chosen for relaxation were tested, which is provided in the Supporting Information. In order to obtain the surface energies of (100), (010), and (001) planes, supercells with at least three relaxed layers were constructed, corresponding to periodically bounded surfaces, and the length of boxes was increased by 10 Å for both sides as extra vacuum space between the periodic images of the supercells as shown in Figure S2. The surface energies were calculated according to eq 1, and the detailed parameters for calculation are listed in Table 2.

2. EXPERIMENTAL SECTION Synthesis. In a typical synthesis, into 100 mL of water 20.5 g (0.1 mol) of raw γ-AlOOH nanoparticles (Shandong Aluminum Corp.; its XRD pattern and TEM image are shown in Figure S1) was added. After being stirred vigorously for 1 h, a well-dispersed suspension was formed, which was treated by following different steps: (1) adding nothing marked as sample 1, (2) adding 0.08 mol of HCl marked as sample 2, (3) adding 0.08 mol of NaCl marked as sample 3, or (4) adding 0.04 mol of NaCl and 0.04 mol of HCl marked as sample 4. Then the suspension was poured into a Teflon-lined stainless steel autoclave and heated at 240 °C for 24 h. After the autoclave was cooled to room temperature naturally, the white product was separated by centrifugation and washed with deionized water and ethanol. Finally, the products were dried at 100 °C for 24 h. Characterization. X-ray diffraction data were obtained by using a Rigaku D/Max 2200PC powder diffractometer with the tube electric voltage and current of 40 kV and 35 mA for Cu Kα (λ = 1.5418 Å) radiation. The scanning speed and step size were set as 1.0°/min and 0.03° in 2θ. TEM (JEM-100CXII) with an accelerating voltage of 80 kV and HR-TEM (JEOL2010) with an accelerating voltage of 200 kV were used to observe the shape, size, microstructure, and orientation of the nanocrystals. Electronic Structure Calculations. All calculations in the present study were performed using the density functional theory (DFT) method implemented in Dmol3 code20 (DMol3 is available as part of Materials Studio) with the generalized gradient approximation (GGA)24 functional of Perdew, Burke, and Ernzerhof (PBE).25 For closed- and open-shell systems, the spin-restricted and spin-unrestricted algorithms were used, respectively. All-electron double-numerical basis set with polarization functions (DNP) was applied for all atoms. The starting periodic structural model of γ-AlOOH was derived from the experimental structure determined by Corbato et al.26 with positions of the hydrogen atoms with Cmc21 symmetry according to the report of Hill et al.27 Before all calculations were implemented, a full cell relaxation procedure was performed. To achieve the accurate density of the electronic states, the k-space integrations were done with Monkhorst−Pack grids28 setting of 7 × 2 × 6 in the Brillouin zone. The convergence in energy was better than 2 × 10−6 eV/

Table 2. Parameters Used for the Calculation of the Surface Energy crystal plane number of atoms slab thickness (Å) surface areas (Å2) surface periodicity k-point mesh

bulk

100

010

001

7 × 2× 6

112 10 92.4 1b⃗ × 2c⃗ 2×3×1

64 10 43.6 2a⃗ × 2c⃗ 3×4×1

80 10 71.8 2a⃗ × 1b⃗ 4×2×1

16

To simulate the reactions occurring on (100), (010), and (001) planes in contact with chloride ions, the (100), (010), and (001) planes of γ-AlOOH with three relaxed layers were decorated with chloride ions, which was placed at least 2.0 Å away from the top surfaces. To keep the integral atoms ratio of the slab to the single γ-AlOOH crystal cell, (1 × 2), (2 × 2), and (2 × 1) surfaces were modeled by a periodically replicated 15280

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slabs to construct the adsorption configuration corresponding to (100), (010), and (001), respectively. After constructing the elemental slabs, the interlayer interactions were minimized by allowing a vacuum width of 15 Å normal to the layers. Additionally, the bottoms of the models constructed were passivated by adding hydrogen atoms. The detailed parameters including the surface areas and total atom number are listed in Table 3. Parameters Used for the Calculation of the Adsorption Energy crystal plane number of atoms slab thickness (Å) surface areas (Å2) surface periodicity k-point mesh a

100 a

80 + (12) 15 92.4 2a⃗ × 2c⃗ 2×3×1

010

001

64 15 43.6 1b⃗ × 2c⃗ 4×3×1

80 + (8)a 15 71.8 2a⃗ × 1b⃗ 4×2×1

Corresponding to the H atoms.

Table 3. DFT calculations were performed to observe the stable geometries and adsorption energies according to eq 2 Eads = (Eslab + adatom − Eslab − χEadatom)/χ

Figure 1. XRD patterns of (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4.

(2)

where Eslab+adatom is the total energy of slab and adsorbed atom after optimization, Eslab is the total energy of slab, Eadatom is the total energy of adsorbed atom, and χ is the number of adsorbed atom. For all the adsorption structures calculation, the convergence in energy was better than 2 × 10−6 eV/Å3, the convergence criterion for the force between atoms was 5 × 10−2 eV/Å, and the maximum displacement was 5 × 10−3 Å. The atomic charges reported here were obtained using the Mulliken charge population analysis. To understand the charge redistribution induced by chloride ion adsorption, the electron density and different density maps were also calculated. By integrating the electronic density within a basin, the partial charge for that atom and the bond type can be estimated.

3. RESULTS AND DISCUSSION Morphology and Structure Analyses of Experimental γ-AlOOH. Figure 1 shows the XRD patterns and TEM images of the products treated by four different processing modes. The XRD reflections of all the products are readily indexed to the orthorhombic boehmite phase γ-AlOOH (JCPDS No. 741895), and no characteristic peaks of impurity are observed. The areas of (020) diffraction peak of the samples are used to compare the crystallinity of the samples, and the different area of the samples indicates the crystallinity difference of these samples. TEM images indicate that different morphologies of γAlOOH were obtained by the four treating methods. As shown in Figure 2, nanoplates, nanorods, nanorods with a small amount of nanoplates, and nanorods were formed for samples 1−4, respectively. To further investigate the composition of final products and identify whether chloride ions and sodium ions remains, EDS analyses were conducted. From the EDS patterns as shown in Figure S6, trace chloride ions were detected only in sample 4, which is considered to be those adsorbed on the particles’ surface. HR-TEM was used to observe the microstructure and orientation of the nanocrystals. As shown in Figure 3, the HRTEM images of a nanorod and a nanoplate show continuous lattice fringes, which demonstrate their single-

Figure 2. TEM images of (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4.

crystalline nature. For sample 1, the interplanar distances are ca. 0.234 and 0.245 nm, which can be ascribed to the (130) and (050) planes of γ-AlOOH, respectively. For sample 2, the interplanar distances of 0.234 and 0.366 nm can be assigned to the (130) and (002) planes. For sample 3, a mixture of nanorod and nanoplate was obtained. The nanorod reveals an interplanar distance of 0.227 nm, which can be ascribed to (130) plane. For the nanoplate in sample 3, the lattice spacings are 0.287 and 0.412 nm, which are consistent with the (100) and (030) interplanar spacings of γ-AlOOH. For sample 4, the lattice spacings of ca. 0.236 and 0.243 nm correspond to (130) and (050) planes. All the planes mentioned above were confirmed further through dihedral calculation and consistent with our previous report.2c Although all the HRTEM images give the continuous lattice fringes, the obvious inhomogeneous contrast and defects imply the oriented-attachment growth of the γ-AlOOH nanoparticles. 15281

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Figure 4. (a) Unit cell of the boehmite bulk containing four AlOOH units. (b) and (c) are the surface structure of boehmite in acidic and alkaline environments. Red, pink, and white spheres represent aluminum atoms, oxygen, and hydrogen atoms, respectively.

crystal parameters listed in Table 1, the optimized parameters of the single γ-AlOOH cell are in good agreement with the experimental values26 and calculation results by the VASP method.29 The slight overestimation of the b parameter due to the difficulty in describing hydrogen bond interaction within DFT has a minor effect on the surface energy and surface properties that involve mainly Al−O and O−H iono-covalent bond breaking or formation.29 In general, pH mainly affects the existing form of surface hydroxyl and ions.31 Here we consider that the surface hydroxyl exists in the form of OH2+ in an acid environment and in the form of O− in an alkaline environment as shown in Figure 4, and the number of OH coordinated to aluminum is taken as constant. By comparing the surface structural parameters optimized in neutral and acidic environments, it is found that both the lengths of O−H bonds and Al−O bonds on (010) and (001) planes increase, as listed in Table 4. It may be attributed

Figure 3. HR-TEM images of (a) sample 1, (b) sample 2, (c) nanorods in sample 3, (d) nanoplates in sample 3, and (e) sample 4. The insets in (a−e) are the corresponding HR-TEM images.

By analyzing the HR-TEM results along with the morphology given by TEM, the oriented-attachment growth direction can be confirmed. For the nanoplate in sample 1, the exposed plane is (001) with the [100] and [010] as the preferential growth direction. For the nanorod in sample 2, the long axis of the rod is parallel to [100], while the short axis is [001], revealing the (100) plane is the most active one. For the nanorod in sample 3, [100] is the preferential growth direction. Meanwhile, [100] and [010] are prior to [001] for the nanoplate in sample 3. In sample 4, the nanorod grows along [100]. Combining the HR-TEM images (inset in Figure 3) with XRD patterns of samples 2 and 4, it is observed that the surface defects of nanorod in sample 2 are more than that in sample 4, and the crystallinity of sample 4 which is illustrated by integrated peaks area is better than sample 2. The reason for the difference in defects and crystallinity will be discussed in the following section. Calculation of Surface Energy and Interaction of Cl−@ γ-AlOOH. As shown in Figure 4a, γ-AlOOH exhibits a lamellar structure with an orthorhombic symmetry, in which the octahedral aluminum is surrounded by four tetrahedral oxygen atoms and two hydroxyl groups. The cohesion of the stacked layers in the b-direction is ensured by hydrogen bonds between hydroxyl groups belonging to two consecutive layers and forming a zigzag chain along the c-direction. According to the

Table 4. Structure Parameters for O−H and O−Al Bonds Involved in the Surfaces d(O−H) (Å)

d(O−Al) (Å)

crystal plane

neutral

acidic

neutral

acidic

(010) (001)

0.973 0.927

1.016 1.016

1.918 1.960

2.148 2.577

to the charge redistribution on O, H, and Al atoms when H+ was added, which resulted in the decrease of electrostatic interactions between O, H, and Al atoms according to the Coulomb formula. The details for the relative charge density after redistribution will be discussed below. For the sake of comparison, the surface energies of (100), (010), and (001) planes were first calculated in vacuum and by the COSMO solvation model. It is observed that (010) plane is the most stable plane with a surface energy of 339 mJ/m2, while the surface energies for (001) and (100) planes in vacuum are 1117 and 1963 mJ/m2, respectively. Similarly, by using COSMO solvation model, the surface energy of (010) plane is still the lowest one (254 mJ/m2), and those of (001) and (100) planes are 713 and 1179 mJ/m2, respectively. Comparing the results in two methods, all the surface energies obtained using solvation model are lower than those in vacuum, 15282

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comparison, the cleaved γ-AlOOH plane is also plotted. Mulliken charge population analyses are conducted, and the results are shown in Table 6. Confessedly, the absolute values of atomic charges from the population analysis have little physical meaning, but useful information can be drawn from the relative values of Mulliken population. By comparing the electron difference density maps in Figure 7a*−d*, when the H+ adsorbs on (010) and (001) planes, one can see that the interactions between the H atoms and O atoms become weaker after charge redistribution. The calculated charges of O atoms ((010) and (001) planes, −0.91e) are less than those of the original O atoms ((010) plane, 0.65e; (001) plane, −0.57e). Thus, we attribute the weakening of H−O bonds to the charge transfer from O atom to H+. Figure 7e*,f* shows that partial charges transfer from the adsorbed Cl− to H atoms on (010) and (001) planes. Comparing the charges of Cl atoms adsorbed on (010) and (001) planes, the adhesion force between Cl atom and H atom on (010) plane is stronger than that on (001) plane due to more charges transfer, which is the reason for the lower adsorption energy of (010) plane. To further determine the interaction between Cl− and H atom, the Mayor bond orders were calculated to evaluate bond type as shown in Table S1. Summarizing the bond orders of Cl− and surface atom, the bond nature is covalent bond, which is consistent with the electron difference density analysis. Moreover, the bond order value in neutral environment is bigger than that in acidic environment, illustrating more intense interaction which agrees with the adsorption energy data. Formation Mechanism of γ-AlOOH with Different Morphologies. The intrinsic structure of crystal and experimental factors in the synthesis process especially for solution reactions are so complicated that each of them may play a key role for the final morphology. Identification of the most important factors and corresponding proportion are helpful to get knowledge of the formation mechanism and synthesize crystals with the desired morphology. To determine whether the final morphology mainly controlled by thermodynamic or kinetic process, zeta potentials test and time-dependent experiments were conducted. The zeta potentials of samples 1 and 2 are 24.15 (±3.85) and 36.46 (±4.94) mV, respectively. The positive value of sample 1 indicated the surface of the raw particles was positively charged; the obvious change indicates that the adsorption of H+ is a fast process, which has been completed before aggregation. TEM images of the samples obtained from sample 2 system at 1 and 2 h are shown in Figure S5. The length of nanorod is much shorter than that of the final product, which indicates the aggregation process is a relative slow process in kinetics. Therefore, it is considered that in the present system the formation of the γ-AlOOH nanomaterials is mainly controlled by the thermodynamic process. Here in our experiment, surface energy, surface hydroxyls concentration, and the adsorption energy are considered as three key factors to affect the final morphology of boehmite. First, according to the Gibbs−Curie−Wulff law, linear crystal plane growth rate is proportional to the crystal surface energy and crystal trends to form morphology with lower surface energy planes exposed. Second, the density of surface hydroxyls is an important factor especially for planes with different hydroxyl densities. The dehydration process of γ-AlOOH between nanoparticles is a free energy reducing reaction (ca. 33 kJ/mol) as reported,33 and the driving force is the surface energy reduction.34 Third, according to the DFT results, the

indicating the stabilization effect of water as reported.32 The surface energy results are in accordance with those obtained by dynamic simulation.29 To evaluate the most stable geometry of Cl−@γ-AlOOH, two main interaction sites, right above a hydroxyl (site A) and between two hydroxyls (site B) as shown in Figure S3, were considered in both neutral and acidic environments. Without hydroxyls on surface, the (100) plane was also calculated for energy comparison and the chloride ion was set above the aluminum atom for Coulomb force. The surface periodicity and the number of chloride ions set in the calculations were decided by calculating the ratio of chloride ion to total surface hydroxyls based on our experiment, and the computational details for the surface hydroxyls are shown in Figure S4. For adsorption energy calculations, the configurations with Cl− located at the top or between H atoms were calculated. Here, only the energetically most favorable configurations are listed in Figure 5a−c. In neutral environment, the adsorption

Figure 5. Adsorption geometry optimized in neutral environment: (a) (100) plane, (b), (010) plane, (c) (001) plane. The partial structures are also shown, and the unit of length is angstroms.

energy Eads of Cl− on (010) and (001) planes calculated based on eq 2 are negative as −2.58 and −1.37 eV, indicating the exothermic adsorption nature, which result in stable structures with chemical bonding between chloride ion and surface hydroxyl. Meanwhile, the Eads of (100) plane calculated for comparison is 16.2 eV. Thus, the adsorption energy order of Cl− on different planes in neutral system is (100) > (001) > (010), revealing Cl− prefers to adsorb on (010) plane in neutral environment. In acidic environment, the adsorption energy Eads for (010) and (001) planes are −4.86 and −2.40 eV, respectively, indicating that the surface decorated by chloride ion in acidic environment are more stable. To investigate the effect of chloride ion number, models with two chloride ions were constructed as shown in Figure 6c,d. The average adsorption energies for (010) and (001) planes are −4.01 and −1.68 eV, respectively, more positive than that with only one chloride ion. It is considered that the repulsion force between chloride ions weakens the interaction between chloride ion and surface hydroxyls. To understand the charge redistribution induced by H+ and adsorption of Cl−, we calculated the total electron density and electron difference density of γ-AlOOH in acidic environment and the structures with chloride ion adsorption. For a 15283

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Figure 6. Adsorption geometries of chloride ions on γ-AlOOH crystals in acidic environment: (a) one chloride ion on (010) plane, (b) one chloride ion on (001) plane, (c) two chloride ions on (010) plane, and (d) two chloride ions on (001) plane. The partial structures are also shown, and the unit of length is angstroms.

Table 5. Surface Energies and Electronic Properties of Cl−@ AlOOH plane

σvac (mJ/m2)

σCOSMO (mJ/m2)

Eneutral ads (eV)

Eacid ads (eV)

Eacid ads * (eV)

(100) (010) (001)

1963 339 1117

1179 254 713

16.2 −2.58 −1.37

−4.68 −2.40

−4.01 −1.68

and the surface hydroxyls density are two factors competing with each other. As explained above, plate-like γ-AlOOH nanocrystals with preponderant plane of (001) are formed as indicated in the HR-TEM image. The specific weight of surface energy and surface hydroxyls concentration involving dynamics process will be discussed in our subsequent work. According to the Eads calculated in neutral environment, the nanoparticles tend to aggregate along [100] direction to form a rod-like product without chloride ions, while inclining to form a plate-like product under the dehydration process of surface hydroxyls. As shown in the TEM image of sample 3, nanorods with a few plates are obtained, which indicates the chloride ions are of top priority to the final morphology than surface energy and surface hydroxyls concentration not only for its stabilized effect to crystal planes but also for its hindering effect to dehydration process, even if the adsorption energies are smaller (−2.54 and −1.37 eV for (010) and (001) plane, respectively) compared to those in acidic environment (−4.68 and −2.40 eV for (010) and (001) plane, respectively). The small amount of plates formed in sample 3 is conjectural due to the insufficient concentration of chloride ions, and the formation mechanism for plates is the same as for sample 1. As for samples 2 and 4, the existence of chloride ions made it hard to aggregate along [010] and [001] directions for high adsorption energy in acidic environment, and adsorption of chloride ions is the most important factor for its high adsorption energy. Thus, only rod-like product was obtained growing along [100] direction as shown in Figure 2. Comparing the single and double adsorption energies in Table 5, it is concluded that increasing chloride ions will reduce the adsorption energy for Coulomb repulsion. The corresponding XRD peaks and the HR-TEM images also illustrate the excess of chloride ions do hinder the oriented-aggregation process instead of facilitating crystallization. The reason for this phenomenon is inferred to be an acidification and dissolution mechanism. That is, the aggregated nanoparticles might be decomposed with the action of hydrochloric acid.

Table 6. Mulliken Charge Population on the H, O, Al, and Cl Atoms for Pure Phase and Adsorption Systems charge population species AlOOH AlOOH/H+ AlOOH/H+/ Cl−

Al

Cl

(010) (001) (010) (001) (010)

0.30 0.30 0.29, 0.30 0.36, 0.35 0.31, 0.50

H

−0.91, −0.91, −0.65, −0.57, −0.78,

O −1.29 −1.17 −1.22 −1.12 −1.25

1.89 1.79 1.88 1.44 1.91

−0.28

(001)

0.39, 0.42

−0.63, −1.09

1.56

−0.58

negative adsorption energies on (010) and (001) planes indicate the stabilization effect of Cl−, while the (100) plane without hydroxyl is the disadvantageous plane for its high surface energy. As reported, the surfactant usually can absorb on certain crystal planes to lower the surface energy and act as a morphology-directing agent.35 Similar to the effect of surfactant in crystal growth, the inorganic ions are considered to play a similar role in oriented-attachment growth. As shown in HR-TEM images, sample 1 gives a plate-like morphology of γ-AlOOH with (001) planes exposed. It seems to go against the surface energy results, from which it is inferred that (010) should be the exposed plane because of the lowest surface energy both in vacuum (339 mJ/m3) and in COSMO model (254 mJ/m3). However, the density order of surface hydroxyls of γ-AlOOH is (010) > (001) > (100), which makes it easier to aggregate along the [010] direction, resulting in a faster integration of (010) planes. Obviously, surface energy 15284

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Figure 7. Total electron density maps (left) and the electron difference density maps (right) for (a, a*) (010) plane and (b, b*) (001) plane of γAlOOH, (c, c*) (010) plane, (d, d*) (001) plane in acidic environment, (e, e*) (010) plane, and (f, f*) (001) plane with chloride ions adsorbed.

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4. CONCLUSIONS A combination of density functional theory calculation and experimental data was employed to get insight into the possible formation mechanism of γ-AlOOH with different morphologies. From these results, we are now able for the first time to not only indentify the interaction form of Cl−@γ-AlOOH and the main factors affecting the final morphology but also give out the experimental evidence. Our data suggest that the Eads is disparate in neutral and acidic environment affecting by the form of hydroxyls on the surface. The chloride ions play the similar role as surfactant on the aggregation of γ-AlOOH nanoparticles. The amount of chloride ions exceeding the gelation point may play a contrary effect for oriented aggregation. In the aggregation process controlled by thermodynamic, the final morphologies are affected by surface energies, surface hydroxyls concentration, and the adsorption energy of chloride ions.

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ACKNOWLEDGMENTS This work is supported by the Major National Science and Technology Projects of China (2012ZX04007-021-04), the Key Projects in the Science & Technology Pillar Program of China (2011BAE33B01), and the Natural Science Foundation of Shandong Province (ZR2011BZ002).

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ASSOCIATED CONTENT

S Supporting Information *

XRD pattern and TEM image of raw materials, acid amount used in experiment, adsorption site considered for geometry optimization, and Mayor bond orders. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.J.). Notes

The authors declare no competing financial interest. 15285

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