Enhanced Lateral to Basal Surface Ratio in Boehmite Nanoparticles Achieved by Hydrothermal Aging

Abstract

Boehmite nanocrystals with tridimensional shape were synthesized by lengthening the aging time of the hydrothermal processing. Boehmite was first precipitated at pH 10 and at 100°C from an aqueous Al 3+ solution, and subsequently hydrothermally annealed at 200°C over the range time between 24 and 168 h. The crystalline structure of all the boehmite nanocrystals prepared showed no significant modifications. On the contrary, its microstructure strongly depended on the aging time, as the growth of the nanoparticles showed two clearly different steps: In the first one, crystallites grew mainly along the ⟨101⟩ directions, usually referred as basal, whereas in the second one the growth was along the baxis, i.e., in the [010] direction. This led to an increase of the proportion of lateral faces parallel to the b-axis in the total surface area of the nanoparticles with an increase of the annealing time. The higher proportion of lateral faces increased the total surface charge of the nanoparticles. Thus, important properties as the stabilization of the boehmite nanocrystals in suspension or their potential functionalization by grafting of organic molecules on these surfaces could be controlled applying the described methodology. ■ INTRODUCTIONNanomaterials technology has attracted great interest in the last two decades. One of the subjects more dealt with in this field is the preparation of inorganic nanoparticles with desired size and morphology, 1 as these materials present interesting properties allowing a number of new applications in fields such as catalysis, 2 biomedicine, 3 and ceramics. 4 Boehmite is one of the polymorphs of the aluminum oxyhydroxide, namely, γ-AlOOH, which is widely used as a precursor for the so-called transitional aluminas, the main one being γ-Al 2 O 3 , and for corundum (α-Al 2 O 3 ), both materials of industrial interest in the catalyst and ceramic fields, respectively. Moreover, and taking advantage of the biocompatibility of aluminum oxide, the only metal oxide approved for intramuscular injections, 5 new applications far beyond its role as vaccine adjuvant 6 are being developed in the medical field. Some examples found in the literature deal with the entrapment of proteins in boehmite, 5 which enhanced their thermal stability, or its use in bone-implant applications, taking advantage of its enhanced osteoblast functions with respect to classic metallic or ceramic materials. 7 Previous works carried out in this group reported the functionalization of boehmite nanoparticles with different molecules to develop potential sensors for cations and anions in aqueous solution by fluorescence spectroscopy 8 as well as potential contrast agents for NMR imaging. 9,10All the above-mentioned applications are based on strict control of the size, shape, and crystallinity of the nanoparticles of boehmite or alumina. It is therefore of capital importance to understand the process giving rise to these materials by the different available methodologies. One of the most used preparation procedures for boehmit...

Synthesis and Characterization of Boehmites Obtained from Gibbsite in Presence of Different Environments

“…Hydrothermal processes from suspensions of initial alkaline pH (processes 1 and 3) resulted in boehmite particles with morphologies "3D" (Figure 4A, cubes, process 1; samples b and c) or "2D" (Figure 4C, thick plates, process 3, samples f and g), while processes where the initial suspension had an acidic pH (processes 2 and 4) resulted in elongated shape (near "1D") boehmite particles ( Figure 4B, process 2, samples d and e; Figure 4D, process 4, samples h and i). This observation is consistent with that reported by Pardo et al 10,20,47 . In these works, in which boehmite is produced from xerogels treated by hydrothermal processes, elongated particles (elongated shape attributed to the increase in the proportion of (100) and/or (001) lateral faces 55 ) are obtained in processes conducted in pH 5.0 aqueous medium, while processes carried out in aqueous medium at alkaline pH (pH = 10.0) lead to "platelet-like" particles.…”

“…Boehmite can also be synthesized from liquid phase by hydrothermal/solvothermal routes 9,11,27,[40][41][42][43][44][45][46][47][48][49][50][51][52] or by sol-gel and precipitation techniques 8,[53][54][55][56][57][58][59] . Experimental conditions such as aluminum source, nature of the precipitating agent, pH, presence and nature of organic additives, temperature and time of thermal treatment (hydrothermal/solvothermal or thermal aging) may have a main influence in the resultant boehmite crystallite size and morphology…”

Crystalline microstructure of boehmites studied by multi-peak analysis of powder X-ray diffraction patterns

“…The image in Figure 2(a), corresponding to sample B024, the diamond-shaped platelets can be easily recognized. With increasing the aging time the particles grew preferentially along [010] crystallographic direction coincident with the thickness of the platelets, as was recently reported (Pardo et al ., 2015). This kind of growth led to the formation of more equiaxial shapes and prismatic forms observed in samples B072, B0120, and B0168, at the expense of the platelets initially formed.…”

“…Aluminium oxyhydroxide occurs in two polymorphs: boehmite (γ-AlOOH) and diaspore, ( α -AlOOH) (Digne et al , 2002). Boehmite [orthorhombic, with approximately: a = 3.694 , b = 12.214 , c = 2.868 Å; with space group: Amam ; (Brühne et al , 2008; Pardo et al , 2015)] has been extensively studied, mainly because its structure enables a topotactic transformation into γ-alumina (γ-Al 2 O 3 ; Krokidis et al , 2001) being an oxide of great industrial interest, widely used as sorbent or catalyst or catalyst support, among others, in cracking (Corma and Fornes, 1990), isomerisation (Sanchez-Valente et al , 2004), and dehydrogenation reactions (Zhao et al , 2014).…”

“…In the topotactic transformation, γ-Al 2 O 3 keeps the microstructure of boehmite that affects critical catalytic features of the former, following from the sizes and distributions of pores, crystallites, and grains (Guzman-Castillo et al, 2001). The microstructure of boehmite can be tuned in the nanoscale through the control of preparation parameters in bottom-up procedures, like the presence of additives (Cai et al, 2010;Chiche et al, 2011;Xia et al, 2013) pH (Mathieu et al, 2007;Chen et al, 2008), temperature (Kaya et al, 2002) or aging time (Pardo et al, 2015). It allows the preparation of nanostructured alumina (Digne et al, 2010;Shen et al, 2012), recently applied in optics, biosensors and biomedicine (Chen et al, 2015a(Chen et al, , 2015b(Chen et al, , 2015c.…”