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Insight into the Segregation Phenomenon in Metal-Cation-Doped Aluminum Sol during the Drying Process with NO3− as Counterions Jian Zhang,† Yuguo Xia,‡ Li Zhang,‡ Xiuling Jiao,*,†,‡ and Dairong Chen*,†,‡ †

School of Chemistry and Chemical Engineering and ‡National Engineering Research Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: The segregation mechanism of metal ions (Na+, K+, Mg2+, Ca2+, Co2+, Ni2+, La3+, Y3+) in γ-AlOOH was investigated during the sol−gel process by a combination of experiments and density functional theory (DFT) calculations. The DFT calculations showed that all of the adsorption energies of metal ions were smaller than the hydrogen ion in neutral conditions, and the metal ions even could not be adsorbed on the protonated γ-AlOOH surface due to electrostatic repulsion. The segregation of metal ions in γAlOOH sol during the drying process was caused by weak adsorption of metal ions on the γ-AlOOH surface in acidic conditions and mainly occurred at the late period of drying with the removal of adsorbed water. The computational segregation sequence of metal ions was consistent with experimental results, which was La3+ ≈ Y3+ > Ni2+ ≈ Co2+ > Mg2+ ≈ Ca2+ > K+ ≈ Na+. According to the segregation mechanism during sol−xerogel transition, it is speculated that an additive which could strengthen the combination between metal ions and the protonated γ-AlOOH surface could diminish the segregation. On the basis of this speculation, citric acid was added into the γ-AlOOH sol, and the segregation percentage of La3+ decreased to 3.54% from 16.4%.

1. INTRODUCTION

During the sol−gel process, the segregation usually occurs in the drying and calcination steps. So far, the research on segregation has mainly focused on the calcination process,17−19 and that caused by drying has not been investigated to the best of our knowledge. Insight into the segregation mechanism during the sol−gel process is essential for controlling the segregation process and preparing uniform material. However, it is difficult to give an accurate mechanism interpretation only by experimental means. As an important supplementary means of experiment, quantum mechanics and molecular dynamics calculations based on density functional theory (DFT) have been successfully used to study the phase behavior of metal oxides and solution interface in recent years. For example, DFT calculations have been applied to study the structure,20 surface energy,21 dehydration process,22 and effect of inorganic anions (Cl−, SO42−) on γ-AlOOH morphology.23,24 The interactions of Na+ on Si the surface, Cu+ on the Cu surface, and Ag+ on the Ag surface in solution were also successfully analyzed by DFT.25−27 However, the interaction between metal cations and colloidal particles as well as the effect of interaction on the segregation process has not been reported. In this paper, using calculation combined experimental results, the interaction between metal cations (Na+, K+, Mg2+, Ca2+,

As an important structural and functional material, alumina has been widely used in ceramics,1 gate dielectrics,2 catalyst carriers,3 etc., because of its high-temperature resistance, corrosion resistance, large specific surface area, high reaction activity, and good adsorption performance.4 In practical applications, various metal ions are often doped into alumina to optimize the performance.5,6 For example, metal ions such as La3+, Y3+, Mg2+, and Ca2+ are usually used as dopants to enhance the mechanical properties of α-Al2O3, while β-Al2O3 needs to be doped by Na+, K+ for the purpose of increasing the ionic conductivity.7−9 By doping Ni2+ or Co2+ in γ-Al2O3, the catalytic activity of alumina can be significantly enhanced.10 To attain the homogeneous doping of the metal ions in the product, the sol−gel method is usually selected as the preparation technique due to the good dispersion of metal ions in the sol precursor. To date, the sol−gel method has been widely applied in preparing alumina with various forms such as ceramics, coatings, nanopowders, films, fibers, and etc.11−15 In theory, the multicomponent chemical homogeneity at the molecular level can be obtained by the sol− gel method, but the performance of metal oxides with dopant is not as good as expected in reality, mainly due to the dopant segregation in practical situations.16 Therefore, it is of great importance for the study of the segregation mechanism and then the control of the segregation process. © 2015 American Chemical Society

Received: March 11, 2015 Revised: May 24, 2015 Published: May 27, 2015 13915

DOI: 10.1021/acs.jpcc.5b02293 J. Phys. Chem. C 2015, 119, 13915−13921

Article

The Journal of Physical Chemistry C

Figure 1. (a) XRD pattern, (b) FT-IR spectrum, and (c) TEM image of as-prepared γ-AlOOH.

Rigaku D/Max 2200-PC diffractometer (Cu Kα radiation, λ = 0.15418 nm; graphite monochromator) at ambient temperature with the tube electric voltage and current of 40 kV and 35 mA. The shape and size were characterized by TEM (JEM-100CXII) with accelerating voltage of 80 kV. The zeta potential measurements of boehmite particles were performed using a zeta-potential meter (Delsa Nano C) at 25 °C. The Fourier transform infrared (FT-IR) spectra were measured on a Nicolet 5DX-FTIR spectrometer using the KBr pellet method in the range of 400−4000 cm−1. Thermogravimetric and differential scanning calorimetry (TG-DSC) measurements were conducted on a Mettler Toledo SDTA851e thermogravimetric analyzer with a heating rate of 10 °C min−1 and up to 1000 °C in air atmosphere. Qualitative chemical microanalysis on selected areas was determined using energy-dispersive spectrometry (EDS) equipped on SEM (JEOL JSM-6700F) with 15 kV accelerating voltage. The top and bottom of γ-AlOOH xerogel with the thickness of 1.5 mm were both analyzed by EDS. The surface scanning was adopted with a 40 μm side-length square region as shown in Figure S1 (Supporting Information). The average value of three tests was used to calculate the atomic percentage of metal cations in xerogel. The segregation percentage was calculated according to eq 1.

Co2+, Ni2+, La3+, Y3+) and the γ-AlOOH surface as well as the effect of the interaction on segregation during the sol−gel process were studied. Experimentally, the process was studied based on X-ray diffraction (XRD), transmission electron microscopy (TEM), FT-IR, energy-dispersive spectroscopy (EDS), and zeta potential techniques. In theory, the interaction between metal cations and the γ-AlOOH surface in neutral and acidic conditions was calculated, and the adsorption behavior of metal cations on the γ-AlOOH surface was analyzed by the total charge density. Then, the adsorption sequence and segregation mechanism were given by combining calculation and experimental results. On the basis of the analyses, the pathway to decrease the segregation of metal ions during the sol−gel drying process is proposed and confirmed.

2. EXPERIMENTAL SECTION Preparation. All of the chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., which were of analytical grade and used without any further purification. In a typical process, 0.25 mol of aluminum isopropoxide was added into 175.0 mL of deionized water at 25 °C. After vigorous stirring for 10 min, the suspension was loaded into a 250.0 mL Teflon-lined autoclave and heated at 90 °C for 9 h. After that the autoclave was cooled to room temperature, and the products were washed with alcohol and deionized water two times and then dried at 90 °C for 12 h to obtain the γ-AlOOH precursor. In order to eliminate the effect of different anion species, nitrate was used in all experiments. Amounts of 0.50 g of asprepared γ-AlOOH, 9.50 mL of H2O, and 5.04 × 10−4 mol of HNO3 were mixed to form sol. Then, eight copies of the same sol were prepared, and then individual 3.5 × 10−4 mol of NaNO3, KNO3, Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Y(NO3)3·6H2O, or La(NO3)3·6H2O was added to each sol, respectively. The sol was vigorously stirred for 60 min and then evaporated at 40 °C for 20 h to form hydrogel. At last, the hydrogel was dried at 25 or 100 °C by slow evaporation for 48 h to form xerogel, in which the slow solvent evaporating rate can prevent the gel from cracking. To clarify whether the pH value is the only factor that determines the formation of stable sol, we also adjusted the pH value of γ-AlOOH suspension to 4.0 using M(NO3)x which was the same as that of the stable sol formed by adding HNO3. As a result, stable sol could not be obtained. The result revealed that both the pH value and the adsorption of ions affect the formation of the sol and its stability. Characterization. The phase structure of the product was identified using XRD (scan rate, 2°/min; scan step, 0.02°) on a

CS =

C top − C bottom C theory

× 100% (1)

where CS is the segregation percentage of the metal cation in γAlOOH xerogel, and Ctop and Cbottom are, respectively, the atomic percentage of metal cations at the top and bottom of γ-AlOOH xerogel. Ctheory is the atomic percentage of metal cations in γAlOOH xerogel calculated based on the amounts of added metal ions in the γ-AlOOH sol. From the experiments, the error of the segregation percentage is less than ±0.5%. Calculation Methods. First-principles (ab initio) calculation based on plane-wave pseudopotential density functional theory (DFT) implemented by the CASTEP program setup of Materials Studio was used in all calculations.28 The generalized gradient approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation function was used,29 and a plane-wave cutoff energy of 380 eV was applied to all calculations. The crystal structure of γ-AlOOH was obtained from the experimental structure determined by Corbato et al.,30 and the position of the hydrogen atoms was taken from the work of Hill et al.31 For all geometry optimizations and single-energy calculations, self-consistent convergence accuracy was set at 1 × 10−5 eV per atom; the convergence criterion for the force 13916

DOI: 10.1021/acs.jpcc.5b02293 J. Phys. Chem. C 2015, 119, 13915−13921

Article

The Journal of Physical Chemistry C between atoms was 3 × 10−2 eV/Å; and the maximum displacement was 1 × 10−3 Å. The detailed processes for geometry construction were provided in our previous work, and the parameters are listed in Table S1-2 (Supporting Information). The optimized bulk lattice parameters of the γ-AlOOH crystal are consistent with experimental results32 and other DFT which used the generalized gradient approximation (GGA) with the projector-augmented wave (PAW) method.33 DFT calculations were performed to observe the stable geometries according to adsorption energies (eq 2). Eads = Eslab + adatom + water − Eslab − Eadatom + water

(2) Figure 2. (a) Zeta potential of γ-AlOOH sol during the drying process (adding HNO3 and NaNO3) and (b) segregation percentage of metal cations in xerogel (dry at 25 and 100 °C).

where Eslab+adatom+water is the energy of the γ-AlOOH slab with adsorbed hydrated metal cations after optimization; Eslab is the energy of the γ-AlOOH slab; and Eadatom+water is the energy of hydrated metal cations. To understand the charge redistribution induced by metal cation adsorption, the total electron density was also calculated.

In order to study the segregation at different dry temperatures, the TG-DSC curves of γ-AlOOH xerogel were measured as shown in Figure 3. The dehydration of γ-AlOOH includes two processes: (1) the removal of the physically adsorbed water or free water and (2) the dehydration condensation of surface hydroxyls or structural hydroxyls.43 TG-DSC curves (Figure 3) show that the dehydration of γ-AlOOH xerogel at different temperatures can be mainly divided into two steps. The first step from room temperature to 180 °C can be ascribed to the removal of physisorbed water or free water, as well as a small amount of nitric acid (6.3 wt % in Figure 3a and 1.2 wt % in Figure 3b). The second one from 180 to 450 °C is mainly due to the transformation from γ-AlOOH to γ-Al2O3 (22.39 wt % in Figure 3a and 22.68 wt % in Figure 3b). This is much higher than the theoretical value (15 wt %), which might be due to the removal of a large number of hydroxyls and the decomposition of nitrate, further indicating the pseudoboehmite phase of the precursor. On the basis of the comparison of TG analysis and segregation percentage of metal cations at different drying temperatures, it can be concluded that the segregation mainly occurred at the late period of drying with the removal of the adsorbed water. However, the segregation mechanism of metal cations in γAlOOH sol during the drying process is difficult to be explained by using the existing experimental techniques. Then DFT calculations were carried out, which is helpful to understand the segregation mechanism. Adsorption of Mx+(H2O)3 (M = Na, K, Mg, Ca, Co, Ni, Y, La, x = 1−3) on the (010) Crystal Plane of γ-AlOOH. According to the research of Raybaud on the surface structure and stable morphology of γ-AlOOH in aqueous solution,33 the exposed area percent of (010), (100), (001), and (101) is 44%, 22%, 20%, and 14%, and the corresponding surface energies are 465, 650, 750, and 825 mJ/m2, respectively. The (101) crystal face is unstable because of the existence of a large number of unsaturated bonds and needs to be surface reconstructed. After reconstruction, the total surface energy of the (101) crystal face reduced 23% with ladder surface structure (full of hydroxyl) which was the same as the (010) crystal face.44 The (100) and (001) crystal faces are also unstable due to unsaturated Al−O bonds. In order to decrease the surface energy in solution, the (100) and (001) crystal faces need to be surface hydroxylated (Figure 4a,b). The hydroxylated (001) and (100) crystal faces are positively charged, which is consistent with the positive zeta potential in the γ-AlOOH suspension (Figure 2). Considering the low concentration of the metal ions in the sol, the calculation was based on the monatomic cations. Due to electrostatic repulsion, H3O+ and Mx+(H2O)n could not be absorbed on

3. RESULTS AND DISCUSSION Structure Characterization of As-Prepared γ-AlOOH Nanoparticles and Xerogel. Figure 1a shows the XRD pattern of the as-prepared γ-AlOOH. All the reflections can be indexed to orthorhombic boehmite (JCPDS, No. 21-1307), indicating the phase-pure nature of the precursor. The wide profile of the diffraction peaks was caused by small crystallite size.34 According to the experimental observations,35−37 γ-AlOOH exhibits three exposed facets, which is (100), (010), and (001). Taking into account the reasonable shape of γ-AlOOH crystallites, based on the widths (200), (020), and (002) diffraction peaks, the average crystalline size along a-, b-, and c-directions can be estimated by the Scherrer formula,38 which are a = 5.40 nm, c = 5.30 nm, and b = 3.00 nm. The shortest dimension of the crystal was along the b axis, as reported in the literature.39,40 The TEM image of asprepared γ-AlOOH shows that the particles are about 5.0 nm (Figure 1c), which is in good agreement with the calculated results. The IR spectrum shown in Figure 1b is consistent with that in the literature.41,42 As shown in the IR spectrum, the broad peaks at 3300−3500 cm−1 can be ascribed to symmetric stretching vibration of surface and structural hydroxyls and hydroxyls from absorbed water in pseudoboehmite, while the bending vibration of absorbed water in pseudoboehmite appears at 1641 cm−1. Moreover, the typical vibration peak at 3082 cm−1 is attributed to the structural water existing in the γ-AlOOH layers. The vibrations of isopropanol do not appear, indicating that there is no isopropanol existing in the as-prepared γ-AlOOH precursor. The effect of isopropanol on segregation can be omitted. The zeta potential of γ-AlOOH sol with different drying time is shown in Figure 2a. Taking γ-AlOOH sol with NaNO3 as an example, the zeta potential of γ-AlOOH suspension (22.69 mV) increased to 33.78 mV after adding HNO3 and then to 35.61 mV after the further addition of NaNO3. The zeta potential of γAlOOH sol at different drying time was also measured, which decreased along with extending the drying time. The segregation percentage of metal cations in γ-AlOOH xerogel (Figure 2b) shows the sequence of Y3+ > La3+ > Ni2+ > Co2+ > Mg2+ > Ca2+ > K+> Na+ after drying at 25 °C while La3+ > Y3+ > Ni2+ > Co2+ > Mg2+ > Ca2+ > K+ > Na+ at 100 °C. The segregation of Y3+ and La3+ is the most serious in xerogel, while that of Na+ is the least obvious. The segregation increased with the increasing dry temperature. In general, the sequence of segregation is trivalent > divalent > monovalent metal cations. 13917

DOI: 10.1021/acs.jpcc.5b02293 J. Phys. Chem. C 2015, 119, 13915−13921

Article

The Journal of Physical Chemistry C

Figure 3. TG-DSC curves of γ-AlOOH xerogel with NaNO3 as additive dried at (a) 25 °C and (b) 100 °C.

Figure 4. Optimized structure of the (a) hydroxylated (001) crystal face and (b) hydroxylated (100) crystal face.

Figure 5. Initial adsorption geometry and adsorption energies of Mx+(H2O)3 on different adsorption positions of the (010) crystal face. (a, d) T-site, (b, e) M-site, and (c, f) B-site.

positively charged (001) and (100) crystal faces, while NO3− can be adsorbed on these two crystal faces to form the electric double-layer structure to stabilize γ-AlOOH colloid particles. Therefore, the DFT calculations in our research were mainly based on the (010) crystal face for the adsorption of H3O+ and Mx+(H2O)n. The adsorption energy of H3O+ on (010) is −16.26 eV, while NO3− is 1.53 eV. Therefore, the adsorption of NO3− on the (010) face is not considered. In order to compare the adsorption ability of H3O+ and Mx+(H2O)n on the γ-AlOOH

surface, the Calculation section was divided into two parts, the adsorption in the neutral and protonated γ-AlOOH (010) surface. Considering the solvent effect of metal cations in aqueous solution, the adsorption of Mx+(H2O)n on the γ-AlOOH surface was calculated with one to six water molecules (Figure S2, Supporting Information). In all cases, the geometry of Mx+(H2O)n was optimized, and the geometrical parameters are in agreement with the experimental and theoretical works.25−27 13918

DOI: 10.1021/acs.jpcc.5b02293 J. Phys. Chem. C 2015, 119, 13915−13921

Article

The Journal of Physical Chemistry C

Figure 6. (a) Initial and (b) optimized structures of Na+(H2O)3 and H3O+ on the T-site adsorption position and (c) total charge density of Na+(H2O)3 and H3O+ on T-site adsorption position after optimization. The blue and yellow regions represent charge accumulation and depletion, respectively.

cations and the protonated (010) crystal face will be discussed in the following section (Figure 6). To study the interaction between metal cations and the γAlOOH surface, the metal cations and H3O+ existing in the (010) crystal face were calculated. Figure 5 shows that the adsorption energy of H3O+ was much bigger than those of metal cations. The pH value of γ-AlOOH sol was 3.85 after adding metal cations, indicating that there was still a small amount of dissociative H3O+ in sol, whose molar concentration is about 1/5 of the metal cations. Figure 6a shows the initial structure of Na+(H2O)3 and H3O+ on T-site adsorption position, and the initial distances of H−O and Na−O were all 2.0 Å. The optimized structure in Figure 6b indicated that the distance of H−O was 0.979 Å, while Na−O was 6.01 Å, showing that H+ can hinder the surface adsorption of sodium ions. The distance of Na−O (6.01 Å) also indicates that Na+(H2O)3 could not be absorbed on the γAlOOH surface. In order to better understand the interaction between the Na+ and γ-AlOOH surface, the total charge density figure is shown in Figure 6c. The charge accumulated mostly at the interface between H3O+ and the (010) crystal face. There was little interaction between Na+(H2O)3 and the (010) crystal face, and the fully protonated surface was a limited case. When the (010) crystal face was fully protonated, the positive surface potential would increase, and the repulsive interaction between Na+(H2O)3 and the γ-AlOOH protonation surface would be aggravated. The higher positive valence the metal cations have, the bigger exclusion the metal ions will get. The sequence of adsorption energy difference (AED) of the (010) crystal face before and after protonation in the presence of H+ is Y3+ > La3+ > Ni2+ > Co2+ > Ca2+ > Mg2+ > K+ ≈ Na+ (Figure S5, Supporting Information), indicating that the existence of H+ in the (010) crystal face can reduce the adsorption energy of metal cations drastically. On the basis of the above calculation analysis, the trivalent metal cations with the biggest adsorption energy could suffer the biggest repulsion during protonation. However, the monovalent metal cations are on the contrary. The segregation sequence in experiment is in good agreement with calculation, which is trivalent > divalent > monovalent. Considering the partial protonation and electrostatic attraction between the metal cation and NO3−, the metal cations can be close to the Stern layer. As for a single metal cation, there is a little deviation caused by NO3− in solution. Considering the calculation amount and the effect of long-range force, it is difficult to use quantitative calculation to solve the coexistence of cation and anion on segregation during

Besides, considering the stereogeometries of hydrated metal cations in the γ-AlOOH surface, the geometry of Mx+(H2O)n was also taken into consideration when there was one water molecule between Na+(H2O)n and the γ-AlOOH surface. However, the water molecule was repelled, and Na+ interacted with the γAlOOH surface directly as seen in Figure S3 (Supporting Information). Generally, the convergence criterion for binding energy changes below 0.1 eV/atom was considered to be well achieved.45,46 At the same time, the other convergence criterions described in the Experimental Section should be satisfied. Herein, with n ≥ 3, the binding energy changes were smaller than 0.1 eV/atom as shown in Table S3 (Supporting Information). According to the DFT calculations (Figure S2b, Supporting Information, and Figure 5), the adsorption energy sequences of H+ and metal ions when n = 3 and n = 6 are almost the same. Considering the different adsorption positions and steric hindrance of Mx+(H2O)n on the γ-AlOOH surface, our calculations are based on n = 3. As for the hydrogen ion, H3O+ was used to calculate the interaction. In order to study the adsorption, three adsorption positions on the γ-AlOOH (010) crystal face are shown in Figure 5a−c, which are, respectively, T-site (directly above a surface oxygen atom), M-site (between two oxygen atoms), and B-site (center of three oxygen atoms). The adsorption energy sequence on the three adsorption positions was almost the same, which was H+ > La3+ > Y3+ > Ni2+ > Co2+> Mg2+ > Ca2+ > Na+ > K+ (Figure 5d−f). As for a single cation, the adsorption energy sequence basically performed as B-site > M-site > T-site (Figure 5 and Table S4, Supporting Information). Taking Na+(H2O)3, for example, the total charge density figures of cations were calculated for the sake of studying the adsorption energy difference in the three positions (Figure S4, Supporting Information). The charge accumulated mostly between Na+ and one O on the T-site of the (010) crystal face and between two O on the M-site and among three O on the B-site. The adsorption energy difference of the three adsorption positions was caused by the numbers of oxygens in the γ-AlOOH surface which interacted with cations. However, the difference percentage was small ( divalent > monovalent. Considering the partial protonation and electrostatic attraction between the metal cation and NO3−, the metal cations can be close to the Stern layer. The segregation of metal cations in γAlOOH sol during the drying process is caused by weak adsorption on the γ-AlOOH surface and mainly occurred at the late period of drying with the removal of the adsorbed water. To strengthen the combination between metal ions and the γAlOOH colloid surface, citric acid was added in sol acting as “double faced adhesive tape”. The segregation of La3+ during the drying process was decreased to 3.54% from 16.4%.

the drying process. Therefore, sol systems with the anions being fixed as NO3− were selected to study the segregation mechanism. Segregation Mechanism of Metal-Cation-Doped Aluminum Sol during the Drying Process. On the basis of the experimental and calculation results, the segregation mechanism of metal cations in γ-AlOOH sol during the drying process is summarized. The zeta potential of the γ-AlOOH suspension was positive (Figure 2a), but it was not positive enough to form the sol. After adding HNO3, the H3O+ could be adsorbed in the (010) crystal face, which resulted in the increase of zeta potential (Figure 2a). As to NO3−, it could exist around γ-AlOOH particles due to electrostatic attraction to form the electric double-layer structure to stabilize sol. Because the protonated γ-AlOOH particle was positively charged, the higher positive valence the metal cations have, the bigger exclusion the metal ions will get. However, metal ions should be in the double electric layer and close to the Stern layer due to the partial protonation of the (010) crystal face and electrostatic attraction between the metal cation and NO3−. During the drying process, the dissociative water in sol was evaporated first and then was the adsorbed water in the double electric layer structure. The segregation of metal ions increased obviously at the late period of drying with the removal of the adsorbed water. According to Figure S2 (Supporting Information), the binding energies of trivalent metal ions with water molecules were the biggest, while that of monovalent metal ions was the smallest. The trivalent metal ions had a strong combination with water molecules and moved with water easily. Hence, the segregation difference of trivalent metal ions dried at different temperatures was much bigger than that of monovalent metal ions (7.38% for La3+ and 2.62% for Na+), which is in good agreement with the calculation results. On the basis of the above analyses, the segregation of metal cations was caused by weak adsorption of metal ions on the γAlOOH surface. The metal ions mainly existed in the colloid diffusion layer. At the late period of drying with the removal of the adsorbed water, the metal ions moved with water, and segregation occurred. Therefore, the segregation will be diminished by strengthening the combination between metal ions and the γ-AlOOH surface. Adding an additive which can combine metal ions and the γ-AlOOH surface simultaneously should be a good way to reduce the segregation during the drying process. The citric acid is an excellent metal ion chelating agent. On one hand, citrate ions can be adsorbed on γ-AlOOH due to the positively charged surface of γ-AlOOH sol particles. On the other hand, the carboxylate radical in citric acid can chelate with many metal cations. So the citric acid can act as “double faced adhesive tape” to fix metal cations on the γ-AlOOH surface. It is inferred that adding citric acid to the sol system can effectively reduce the segregation during drying. Further experiments were conducted to confirm this point. In the experiment, the citric acid (7 × 10−4 mol) was added into γ-AlOOH sol to diminish segregation, which can be adsorbed on the γ-AlOOH surface and even washed two times with water (Figure S6, Supporting Information). Then the movement of metal ions with water is limited during the drying process by the strong coordination between citric acid and metal ions. The segregation of La3+ decreased to 3.54% from 16.4% after adding citric acid.

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ASSOCIATED CONTENT

S Supporting Information *

Optical photograph, SEM, FT-IR spectra of γ-AlOOH xerogel, calculated results of geometric parameters, adsorption energy difference, and total charge density figures. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02293.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Major National Science and Technology Projects of China (2012ZX04007-021-04) and the Taishan Scholars Climbing Program of Shandong Province.

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REFERENCES

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4. CONCLUSIONS The segregation mechanism of metal ions in aluminum sol during the drying process can be well explained by combining density functional theory calculations and experiments. The DFT calculations showed that the adsorption energy of the 13920

DOI: 10.1021/acs.jpcc.5b02293 J. Phys. Chem. C 2015, 119, 13915−13921

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DOI: 10.1021/acs.jpcc.5b02293 J. Phys. Chem. C 2015, 119, 13915−13921