Article pubs.acs.org/crystal
High-Speed Morphology Control of Boehmite Nanoparticles by Supercritical Hydrothermal Treatment with Carboxylic Acids Tatsuya Fujii,*,† Shin-ichiro Kawasaki,† Akira Suzuki,‡ and Tadafumi Adschiri§ †
Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551, Japan ‡ New Industry Creation Hatchery Center (NICHe), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-0812, Japan § Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan S Supporting Information *
ABSTRACT: This study demonstrates that the morphology of boehmite (AlOOH) nanoparticles can be controlled over a short timespan by supercritical hydrothermal treatment in the presence of alkyl carboxylic acids including hexanoic, octanoic, decanoic, tetradecanoic, and octadecanoic acids. Boehmite nanoparticles were treated with carboxylic acid in supercritical water at 400 °C and at a water density of 0.35 g/cm3 in a batch-type reactor. When the carboxylic acid was not added, the particles were shaped as rhombic plates. However, the addition of carboxylic acid changed the crystal morphology to hexagonal plates. The aspect ratio (i.e., [length along the aaxis]/[length along the c-axis]) of the rhombic plates increased with a treatment time of 2−30 min, which is a much shorter timespan than that used for conventional hydrothermal crystallization. The aspect ratio of the hexagonal plates increased with increasing concentration of alkyl carboxylic acids. These results clearly indicate that carboxylic acids enhance the dissolution and recrystallization of boehmite. The aspect ratio increased with decreasing length of the alkyl chain of alkyl-carboxylic acid added to the system. Thermogravimetric analysis (TGA) showed that carboxylic acids modified the surface of the boehmite particles. The coverage of the alkyl carboxylic acid on the surface of the nanoparticles was evaluated from the weight loss curve obtained from TGA, and the surface area was evaluated from transmission electron microscopy, which showed that the aspect ratio of the particles increased with increasing the coverage. The results suggest that the carboxylic acid suppresses crystal growth along the shorter axis through surface-capping, thus enhancing dissolution and crystal growth along the a-axis.
1. INTRODUCTION Because of increasing development in industry and technology, the demand for high-performance materials is increasing. It has been becoming difficult to satisfy required performance by single materials such as polymers, ceramics, and metals. Thus, there is a growing need for the development of organic− inorganic hybrid materials. Morphology control of the inorganic particles is important for obtaining high-performance hybrid polymers with inorganic nanoparticles. For example, the mechanical strength of polyurethane-based hybrid polymers containing boehmite (AlOOH) particles is greater if boehmite nanorods are used as the filler rather than boehmite particles with lower aspect ratios (where the aspect ratio is defined as [length along the a-axis]/[length along the c-axis]).1 Also, the thermal conductivity of thermal grease containing CuO microdisks is higher than that of thermal grease containing CuO of other shapes with lower aspect ratio such as microspheres and nanoblocks.2 Supercritical hydrothermal synthesis with in situ organic surface modification is a promising method for controlling the morphology of nanoparticles.3 In this technique, an organic © XXXX American Chemical Society
modifier is added during supercritical hydrothermal synthesis, in which an aqueous metal salt solution is used as the starting material, and various shapes of surface-modified metal oxide nanoparticles can be obtained. For example, Mousavand et al. have conducted supercritical hydrothermal synthesis of boehmite nanoparticles in the presence of organic surface modifiers using aluminum nitrate as the boehmite source.4 They reported that the use of organic modifiers with different functional groups, i.e., amines and aldehydes, resulted in boehmite nanoparticles with different morphologies. Taguchi et al., reported in situ surface modification of CeO2 nanoparticles during supercritical hydrothermal synthesis using Ce(OH)4 as the precursor and a monocarboxylic acid as the modifier.5 They reported that particles obtained without modifier were truncated octahedral structures and that cubic structures were formed in the presence of the modifier.6 Received: November 8, 2015 Revised: January 27, 2016
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2.2. Analytical Procedure. X-ray diffraction (XRD) analysis was performed for the products using an Intelligent XRD system (SmartLab, Rigaku) equipped with a Cu Kα radiation source. The 2θ scan speed was set to 20°/min. Morphology observation was conducted by transmission electron microscopy (TEM, TecnaiG2 30, FEI Co.). The size of the particles was evaluated from the TEM image with no less than 50 particles being inspected to determine the mean length along each axis of the particles (i.e., 100, 010, and 001 axes). The amount of modifier attached to the surface of the particles was evaluated from the weight loss observed in thermogravimetric analysis (TGA, TG-DTA2000, Bruker Corp.). For TGA, the carrier gas used was Ar and the flow rate was set to 100 mL/min. The temperature was first increased to 100 °C and maintained for 1 h to evaporate water and other organic solvents, and then raised to 800 °C at a heating rate of 10 °C/min.
These and other previous studies suggest that crystal morphology can be controlled by the in situ organic modification in supercritical hydrothermal synthesis. Since modification occurs immediately after nucleation, nanoparticles with single-nanometer diameters can be produced by this technique.7 Hydrothermal or solvothermal crystallization is a well-known method used to grow inorganic crystals. Unstable particles are dissolved and recrystallized to form more stable particles, leading to crystal growth. Although the processing time depends on the final size of the crystals required, it typically takes hours or days. The shape of the crystals is determined so that their total surface energy can be minimized. An advanced method for particle shape control employing additives in hydrothermal crystallization have been reported. For example, boehmite rods can be obtained by hydrothermal treatment with HCl as an additive, as explained in detail later.8A process for the efficient control of particle morphology at reduced processing times is required. In this paper, we propose a new processing method to quickly control the morphology of nanoparticles under supercritical hydrothermal conditions and by adding organic capping agents as shape controllers. The high temperature used in the supercritical hydrothermal treatment enhances the crystal growth rate. In addition, since supercritical water can dissolve organic capping agents at high concentrations, more effective capping and shape control may be possible. In this study, we focused on the morpholgy control of boehmite. Boehmite is used in various applications such as adsorbents,9 catalysts,10 catalyst supports, 11 and dental materials,12 and as a filler in organic thin films for improving flame retardancy,13 thermal conductivity,14 and toughness.1,14 As discussed above, for hybrid polymers with boehmite particles, shape control is the key to obtaining high performance. The first objective of this study was to demonstrate that boehmite nanorods can be successfully fabricated by supercritical hydrothermal treatment with alkyl carboxylic acid as a capping agent in ca. 10 min using boehmite nanoparticles as starting material. The second objective was to elucidate the mechanism of rapid morphology control, through a detailed study of the crystal growth by changing the treatment time, length of alkyl chain, and additive concentration.
3. RESULTS AND DISCUSSION 3.1. Difference between Products Obtained with and without Modifier. Figure 1 shows TEM images of the products. Rhombic plate boehmite is obtained by supercritical hydrothermal treatment without any modifier (Figure 1a).
2. EXPERIMENTAL SECTION 2.1. Modification Process. Boehmite powder (Pural SCF, Sasol Ltd.) was used as the raw material. Monocarboxylic acids (hexanoic (C6), octanoic (C8), decanoic (C10), tetradecanoic (C14), and octadecanoic (C18) acids) were purchased from Wako Chemicals for use as modifiers. Supercritical hydrothermal treatment was performed using a batch-type reactor made of SUS 316 stainless steel (inner volume: 8.5 cm3). In a typical procedure, 0.15 g of boehmite powder, 3 g of distilled water, and monocarboxylic acid were loaded into the reactor. The concentration of carboxylic acid was 3.3 mol/L in typical modification experiments. The reactor was heated by immersing it into a sand bath (SB160TE, ACRAFT Co. Ltd.) maintained at 400 ± 5 °C. It had been previously confirmed that 2 min heating was necessary for the water temperature to reach 400 °C. The reactor was shaken continuously during the supercritical hydrothermal treatment. At the end of reaction, the reactor was quenched in a water bath, and the products were recovered from the reactor by rinsing it with 5 mL of water and 8 mL of hexane. The solid products were washed repeatedly by centrifugation and decantation using ethanol and/or diethyl ether.
Figure 1. Typical TEM images of the products obtained from 10 min treatment. The images represent products treated without modifier (a), and with hexanoic acid (b), octanoic acid (c), decanoic acid (d), tetradecanoic acid (e), and octadecanoic acid (f).
When a modifier is used in the supercritical hydrothermal treatment, the shape of the particles changes from rhombic plate to rodlike. According to previous research on the morphology of boehmite synthesized under hydrothermal conditions, the long axis of the hexagonal plate is parallel to [100], the short axis is parallel to [001], and the other axis perpendicular to long and short axes is parallel to [010].15 We observed several tilted TEM images and confirmed that images showing apparently rectangular particles are actually side views of rhombic plate or hexagonal plate particles, as previously suggested by Mousavand et al.4 The thickness of the particles B
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suggesting that the alkyl carboxylic acid is attached to the surface of the products. For the sample prepared with the capping agents, TGA was conducted not only in Ar but also in air. When air is used as the carrier gas, the weight loss starts at a lower temperature, which is probably due to the oxidation of the modifier. However, the total weight loss is similar both in Ar gas and in air, suggesting that almost modifiers were gasified in the TGA treatment both in Ar and in air (the TG−DTA result of boehmite modified with octanoic acid in air before and after TGA treatment in Ar in the same method in the Supporting Information). The weight loss observed in Ar gas around 450−500 °C must be due to the thermal decomposition of organic modifiers, as the boiling point of the octanoic acid modifier is much lower at 238 °C. The results suggest that the organic modifier is strongly bound to the surface of boehmite particles with covalent-like bonding. Regarding morphology control under hydrothermal conditions, Xia et al. have studied the morphology change in boehmite with HCl under hydrothermal conditions at 260 °C. Density functional theory calculations suggested that Cl− adsorbed on the (010) and (001) faces to lower their surface energy.8 Some reports have presented discussion of not only the thermodynamic effect but also the kinetic effect of the organic modification on morphology change.5,6 In general, when a surface is covered by modifiers or adsorbates, the rate of the crystal growth on that surface is suppressed. According to previous research, boehmite has hydroxyl groups on the (010) and (001) faces.8 In this study, the carboxylic acid can react/ adsorb on the (010) and (001) surface of boehmite, which should result in suppression of crystal growth on those faces. Thus, faster crystal growth on the (100) surface changes the morphology of the products from rhombic plate to hexagonal plate. As previously mentioned, the data shown in Figure 2b suggest that octanoic acid promotes crystal growth since the volume of particles is greater after treatment in the presence of octanoic acid. Since the initial loading amount of boehmite is the same, this means that the number of particles decreases through this treatment. The results suggest that dissolution and recrystallization occur during the treatment. This process is also known as Ostwald ripening. In order to confirm the effect of Ostwald ripening on the octanoic acid treatment, the concentration of octanoic acid was varied in the range from 0.1 to 4.8 mol/L. The results are shown in Figure 4. As expected, with increasing concentration of octanoic acid, enhancement of the crystal growth becomes more apparent. One possible explanation for the acceleration of crystal growth is the increasing solubility of boehmite by reduction in pH, which is caused by the addition of octanoic acid. Previous
was determined by measuring the length of the short axis of these ostensibly rectangular particles. Figure 2 shows the variation of the aspect ratio and mean volume of particles with time, which are estimated from the
Figure 2. Time profiles of (a) aspect ratio and (b) volume of products. Aspect ratio is defined as [length along a-axis]/[length along c-axis]. (Diamond: without modifier; circle: with octanoic acid.)
mean length along the a-, b-, and, c-axes of the particles obtained in the experiments with and without octanoic acid as a modifier. Aspect ratio is defined as the mean length along the aaxis divided by that along the c-axis. Volume of particles was calculated using lengths of axes assuming that all particles with modification are hexagonal plates (inner angle assumed to be 120° for simplicity) and all particles without modification are rhombic plates (obtuse angle and acute angle assumed to be 120° and 60°, respectively, for simplicity). Figure 2a shows that the aspect ratio increases with time in the presence of octanoic acid, while it remains almost constant when the modifier is not added. Also, as can be seen from the volume increment following 3 h of hydrothermal treatment (Figure 2b), octanoic acid addition enhances the crystal growth. Figure 3 shows the TGA curves of samples obtained by supercritical hydrothermal treatment with and without octanoic acid.
Figure 3. Comparison of TGA curves of products. (Green line: not modified (TGA in Ar gas); red line: modified with octanoic acid (TGA in Ar gas); blue line: modified with octanoic acid (TGA in air).)
Since the weight loss observed below 100 °C is due to the evaporation of water and organic solvents adsorbed on the samples, the weight loss of the samples was calculated based on the weight after 1 h of heat treatment at 100 °C. The weight loss observed for the sample obtained without capping agent around 450−500 °C is because of phase change due to dehydration from boehmite to γ-alumina, as confirmed by the XRD pattern of the product obtained after 2 h calcination of boehmite at 500 °C. The weight loss of the sample is approximately 15%, which is consistent with the change in the molar weight from boehmite to γ-alumina (2AlOOH → Al2O3 + H2O) and the results reported in the literature.16 The weight loss of the sample prepared with a modifier is greater than 15%,
Figure 4. Effect of the concentration of octanoic acid on (a) volume and (b) aspect ratio of products (treatment time: 10 min). C
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It is confirmed that the length along the a-axis decreases with the alkyl chain length of the carboxylic acids. The b- and c-axis lengths also decrease, but to a lesser extent. Thus, the aspect ratio decreased with increasing chain length, as shown in Figure 6b. Figure 7 shows the effect of the alkyl chain length of the modifier on the volume of the particles produced. Larger particles are obtained with shorter modifier.
research suggested that the solubility of boehmite increases with decreasing pH in supercritical water at 400 °C in acidic condition.17 Since dissolution−recrystallization is promoted under high-solubility conditions, faster crystal growth can be observed at lower pH, i.e., under higher octanoic acid concentrations. Another possible reason may be formation of complexes, i.e., Al(OH)n(OCOR)3−n, which could also increase the solubility of boehmite and enhanced crystal growth. The mechanism of the change in aspect ratio with octanoic acid has already been discussed above, i.e., the thermodynamic effect (surface energy change) and the kinetic effect (suppressing crystal growth with capping). Increasing the concentration of octanoic acid is beneficial to both of these mechanisms, and thus the aspect ratio increases with increasing concentration, as shown in Figure 4b. 3.2. Effect of Alkyl Chain Length on the Length of Particles. In this section, the effect of the alkyl chain length on the morphology of the boehmite product is discussed. All data in this section are for 10 min hydrothermal treatment. First, it is confirmed that the product recovered after the hydrothermal treatment is orthorhombic boehmite from the XRD peaks (JCPDS 74-1895), as shown in Figure 5. The peak of boehmite
Figure 7. Relationship between the carbon number of the modifier and the volume of the products.
One possible factor is the difference in the pH of the reaction medium, as discussed in section 3.1. Since the pKa of the modifier increases with increasing alkyl chain length,18 the pH of the reaction medium is probably higher when longer alkyl chain modifiers are used, suggesting that a shorter modifier may facilitate the crystal growth of boehmite more effectively. Another factor is the formation of complex ions, i.e., Al(OH)n(OCOR)3−n, which is also discussed in section 3.1. Complex formation could facilitate the dissolution of boehmite particles and enhance the dissolution−recrystallization process. The ability to form complexes is different among modifiers with different alkyl chain lengths. On this issue, a related mechanism has been reported for the case of germanium, which showed that germanium hydroxide complex formation depended on the organic ligand used.19 A third factor is that the stability effect of the modifier− boehmite interaction may differ for modifier with different alkyl chain lengths; i.e., if the adsorption energy is greater, the growth rate of the surface should be slower. The final factor to consider is that the steric hindrance effect is more pronounced for modifiers with longer alkyl chain lengths. The larger volume of alkyl chain more effectively blocks access of the Al source to the surface of the boehmite particle, more significantly restricting the crystal growth. However, in this specific case, it is unlikely to be the main factor, as the aspect ratio of the hexagonal plate is smaller for the longer alkyl chain, which is inconsistent with the steric effect explained above. The pH, complexation, and modifiersurface stability factors adequately account for the experimentally observed morphology change. Details of such effects will be investigated in future work. To investigate the mechanism in more detail, the coverage by the modifier (molecules/nm2) on the surface of products was estimated using the same method as reported in a previous study,5 i.e., from the weight loss due to the decomposition of organic molecules and the surface area of the particles. In this analysis, the weight loss due to the phase change of boehmite to γ-alumina was assumed to be 0.15 for all cases. The weight losses due to modifier decomposition thus calculated are shown in Table 1.
Figure 5. XRD patterns of raw material and products obtained following 10 min of hydrothermal treatment. Each plot indicates raw material (a), product treated without modifier (b), and that treated with hexanoic acid (c), octanoic acid (d), decanoic acid (e), tetradecanoic acid (f), and octadecanoic acid (g).
with and without modifier after supercritical hydrothermal treatment was sharper than boehmite before the treatment, which suggests that the crystallinity of boehmite was increased through the dissolution−recrystallization process in the supercritical hydrothermal treatment. As shown in Figure 1b−f, the length of particles decreases with increasing alkyl chain length. To investigate the effect of alkyl chain length on the crystal growth in more detail, the mean length along each axis of the recovered particles was evaluated, and the results are shown in Figure 6a.
Figure 6. Comparison of (a) length along each axis (red: a-axis; green: b-axis; blue: c-axis) and (b) aspect ratio. Error bars represent standard deviation of each length. D
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Figure 9 shows the relationship between the coverage ratio and the aspect ratio of the products.
Table 1. Comparison of Weight Losses Observed for Each Sample upon Temperature Change from 100 to 800 °C, As Obtained from TGA under Ar Atmosphere modifier
weight loss [%]
hexanoic acid octanoic acid decanoic acid tetradecanoic acid octadecanoic acid
16.4 16.1 15.8 15.5 17.0
For the analysis of surface coverage ratios, areas of the (010) and (001) surfaces were used, as the (101) surface is much smaller. The areas were estimated based on the mean length obtained from TEM images. Modifier coverage was defined as the number of modifier (N) divided by surface area of the particles (S). The number of modifier molecules was calculated from the weight loss obtained by TGA. Therefore, the following relation is obtained N Coverage = = S
Figure 9. Relationship between the modifier coverage and the aspect ratio.
When the coverage is greater, the rate of crystal growth along [010] and [001] is lower, and crystal growth along the [100] surface is preferred in the dissolution−recrystallization process. Other factors may also affect the morphology of boehmite, since the modifier coverage does not change drastically when the alkyl chain of modifier is long (i.e., C10−C18).
maNA Mw Spm(1 − a) ρVp
where m represents sample mass of TGA, a is the weight loss observed in TGA, NA is the Avogadro number, Mw is the molecular weight of the modifier, Sp is the mean surface area of a particle, ρ is the density of boehmite, and Vp is the volume of a boehmite particle (all units are SI). The effect of the alkyl chain length of the modifier on the modifier coverage thus calculated is shown in Figure 8.
4. CONCLUSIONS High-speed crystal growth of boehmite rods in only several tens of minutes was realized by supercritical hydrothermal treatment in the presence of a carboxylic acid. By adding alkyl carboxylic acid to the treatment, the shape of boehmite particles was changed from rhombic plates to rodlike particles. Alkyl carboxylic acid addition enhanced the dissolution−recrystallization process and suppressed crystal growth along the shorter axis of the particles. The aspect ratio of the particles increased with decreasing length of the modifier alkyl chain due to the increasing modifier coverage of the surface.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01584. TG−DTA results of boehmite modified with octanoic acid in air before and after TGA treatment in Ar (PDF)
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Figure 8. Effect of modifier alkyl chain length on coverage of modifier. The (010) and (001) surfaces are considered.
AUTHOR INFORMATION
Corresponding Author
Figure 8 indicates that the surface coverage tends to decrease with increasing alkyl chain length, except for octadecanoic acid. The value of pKa increases with increasing alkyl chain length of carboxylic acid,18 which means that shorter modifiers (RCOOH) more easily dissociate to their anion (RCOO−). According to a previous report, the zeta potential of boehmite is positive under acidic conditions (the isoelectric point is ca. 9),20 which suggests that the anion has more affinity with the surface of boemite. This may be the reason for higher modifier coverage when shorter alkyl length carboxylic acids are used. In the case of octadecanoic acid modified product, the tendency is reversed. Taguchi et al. observed denser coverage of modifier with longer modifier in the supercritical hydrothermal synthesis of CeO2.5 They suggested that surface modifiers having long alkyl chains can attach to the surface of the products through coordination bonds in an ordered manner. A similar effect may cause the reversed tendency observed in the octadecanoic acid modified sample in Figure 8.
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T. Adschiri thanks JSPS KAKENHI Grant Numbers (20226015, 25249108, 26630397); leading research organizations, namely, NSERC, ANR, DFG, RFBR, RCUK and NSF as Partner Organizations under the G8 Research Councils Initiative for Multilateral Research Funding; and SIP (CrossMinisterial Strategic Innovation Promotion Program) conducted by CSTI (Council for Science, Technology and Innovation), Cabinet Office, the Government of Japan. T. Adschiri is also thankful for the support by NEDO (New Energy and Industrial Technology Development Organization), and the support by World Premier International Research Center Initiative (WPI), MEXT, Japan. The authors thank E
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technological processes: Effect of chain length on the pKa of fatty acid salt solutions. Langmuir 2000, 16, 172−177. (19) Pokrovski, G. S.; Martin, F.; Hazemann, J.-L.; Schott, J. An X-ray absorption fine structure spectroscopy study of germanium-organic ligand complexes in aqueous solution. Chem. Geol. 2000, 163, 151− 165. (20) He, T.; Xiang, L.; Zhu, S. Different nanostructures of boehmite fabricated by hydrothermal process: effects of pH and anions. CrystEngComm 2009, 11, 1338−1342.
Sasol Ltd. for donating boehmite powder (Pural SCF). The authors also thank Dr. Hiromichi Hayashi and Dr. Aritomo Yamaguchi for assistance with analysis, and Ms. Shoko Saito for assistance with experiments and analysis.
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REFERENCES
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