Microstructural Analysis of Boehmite Nanoparticles Prepared by Rapid Hydrolysis of Aluminum Sec-butoxide

Microstructural properties of six samples, prepared by rapid hydrolysis of aluminum secbutoxide, were investigated at RT using field emission scanning electron microscopy (FE SEM), X-ray powder diffraction (XRD) and Fourier transform infrared (FT-IR) spectrometry. The results of structural analysis show that, regardless of a significant difference in the processing parameters (pH, temperature, time of synthesis), all products contain boehmite as the only crystal phase present. The results of FE-SEM analysis indicate a significant difference in the morphology of obtained boehmites (plates, needles, granules). The results of line-broadening analysis of powder diffraction patterns (Le Bail method – program GSAS) indicate the presence of very small anisotropic crystal domains (around 1.5 to 7 nm in the direction 010; around 3 to 16 nm in the direction perpendicular to 010). In all cases the parameters that contribute to the strain broadening of diffraction lines decrease to nearly zero, which suggests that the obtained boehmites are almost strain-free. (doi: 10.5562/cca1884)


INTRODUCTION
Boehmite (γ-AlOOH) is a major mineral in many types of bauxite which are used as ore in the production of aluminium. 1 On the other hand, synthetic boehmites are utilized as precursors in the production of various Al-oxide ceramics (membranes, catalysts, ceramic fibers, thin film coatings, etc.). Three synthesis routes were utilized in the synthesis of boehmite: precipitation from inorganic Al-salts in aqueous media, 2-5 hydrolysis of Al-alkoxides at different pH, [6][7][8] and thermal decomposition of gibbsite (γ-Al(OH) 3 ). [9][10][11] Generally, synthetic boehmites are not easy to characterize due to variations in their crystallinity, particle size and the presence of an amorphous fraction or other Al-oxide phases. The name pseudoboehmite was introduced in reference literature 1 to describe poor crystalline γ-AlOOH, although the criterion for this name is not defined. Crystallographic studies [12][13][14] indicate that there is no structural difference between pseudoboehmite and boehmite.
Many researchers investigated the conditions of boehmite formation by slow-hydrolysis of Al-oxides. In the present work we have focused on the synthesis of nanosize boehmite particles using rapid hydrolysis of Al-sec-butoxide and the influence of experimental conditions on the properties of the particles synthesized.

EXPERIMENTAL
Rapid hydrolysis of aluminum sec-butoxide (Fluka) was used to prepare six samples of Al-oxides (B1 to B6). Preparation conditions are given in Table 1. All the chemicals used were of analytical grade. Structural and microstructural changes in the obtained products were investigated at RT using Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FE SEM) and X-ray powder diffraction (XRD).
FT-IR spectra were recorded at RT using a Perkin-Elmer spectrometer, model 2000. The FT-IR spectrometer was coupled with a personal computer and operated with the IRDM (IR Data Manager) program. The specimens were pressed into pellets using spectroscopically pure KBr as a matrix.
FE SEM analysis of uncoated samples was made using the field emission scanning electron microscope JSM-7000F (JEOL).
XRD measurements were performed using a Philips counter diffractometer MPD1880 with monochromatized CuKα radiation (graphite monochromator).
The volume-averaged domain size (D V ) and the upper limits of micro-strain (e) of the nanosized boehmites were estimated from the results of Le Bail refine-ments 15 (program GSAS 16 with graphical user interface EXPGUI 17 ) following the procedure proposed in the Size/Strain Round Robin. 18 In the refinement we used a modified pseudo-Voigt function defined by Thompson, Cox and Hastings. 19 The size and strain contribution to the line broadening can be given by the following equation: where λ is the wavelength, D V is the volume-averaged domain size, i represents the upper limits of strain, while β S and β D represent the integral breadths of the Voigt function resulting from size and strain contribution, respectively. In order to obtain pure physically broadened profile parameters, used in the calculation of β S and β D values, the obtained values of refined parameters for samples were corrected by the corresponding values obtained for silicon (Koch-Light Lab. Ltd), used as a standard for the approximation of instrumental profile. The results of the line broadening analysis are summarized in Table 2.

RESULTS AND DISCUSSION
FT-IR spectra of all samples ( Figure 1) contain the bands at approximately 3300, 3090, 1160, 1070, 775, 735, 615, and 490 cm -1 , typical of boehmite. 20 Beside the bands typical of boehmite all spectra contain very broad bands resulting from the vibration of adsorbed water molecules. Samples B1 and B2 also contain bands at 1560, 1420 and 1349 cm -1 , which indicate the presence of an acetate group 21 that remained from the acetic acid used in the preparation of the sample (Table 1).  β S (φ = 90º) 462(9) 485(7) 443 (7) 117(6)   FE-SEM micrographs of samples B1 and B2, obtained at low pH, contain a conglomerate of particles of very different shapes and sizes (Figure 2). Boehmite crystallites appear to be very small (few nanometers).  High magnification (100,000 times) of sample B1 indicates the presence of scratches or channels on the sur-face of the grains. The presence of some very small (few nanometers) fiber-like particles indicates the anisotropic growth of boehmite crystallites. In case of sample B2, even at very high magnification individual boehmite particles could not be clearly seen.
FE-SEM micrographs of samples B3 to B6 ( Figure  3) indicate a significant difference in the morphology of obtained boehmite crystallites. In sample B3 (obtained at pH ≈ 6.0) the interconnection between needle-shape boehmite crystallites forms a three-dimensional star morphology. Sample B4 contains the agglomerates of small boehmite granules of almost spherical symmetry. Sample B5 contains soft agglomerates (low bulk density) of irregular plates, while sample B6 contains more compact agglomerates of regularly shaped plates (Figure 3).
The results of XRD analysis show that boehmite (ICDD PDF-2 card No. 21-1307) is the only crystal phase present in all prepared samples. Significant broadening of the boehmite diffraction lines was used for size-strain line broadening analysis (estimation of D V and e values). Figure 4 shows the results of Le Bail 15 wholepowder-pattern profile refinements. Small differences between the observed and calculated patterns and very low values of the R wp index indicate a relatively good reliability of the extracted values. The obtained results indicate a very small contribution of the micro-strain parameters (the refined values of the profile parameters U, Y and Ye were nearly 0) to the broadening of the boehmite diffraction lines. The obtained results indicate that boehmite crystallites are generally larger with the increase of pH, temperature or time of hydrothermal treatment. However, regardless of the difference in processing parameters, boehmite crystallites in all products are of nanometric size with a pronounced size anisotropy. 22,23 In all diffraction patterns the size contribution to the line broadening appeared to be anisotropic with a significantly smaller D V value in the direction parallel to the broadening axis 010 than in the directions perpendicular to the broadening axis 010 (Table 2).

CONCLUSION
The results of XRD analysis show that, regardless of significant changes in the processing parameters (pH, temperature, time of synthesis), rapid hydrolysis of aluminum sec-butoxide resulted in the formation of nanosize boehmites. The results of FE-SEM analysis indicate a significant difference in the morphology of obtained boehmites (plates, needles, granules). The sizes of boehmite crystallites increase with the increase in pH, temperature or the duration of hydrothermal treatment. However, the results of line-broadening analysis of powder diffraction patterns (Le Bail methodprogram GSAS) indicate that boehmite crystallites in all products are of nanometric size with a pronounced size anisotropy. D V values of the obtained boehmite products appeared to be about two times smaller in the direction parallel to the axis 010 (around 1.5 to 7 nm) than in the directions perpendicular to the axis 010 (around 3 to 16 nm). In all cases the parameters that contribute to the strain-broadening of the diffraction lines decrease to nearly zero, which suggests that the obtained boehmites are almost strain-free.