| In recent years, the design and synthesis of functional materials with controlled size and desired morphology have stimulated great research interest. The simpler method and more effective control was always the immutable target of controllable synthesis domain. Therefore, as an important target materials, ultra-fine boehmite powders with hierarchical structure and nanometer to micrometer dimensions generated great interest due to their special physical and chemical properties which were different from solid particles, owing to their low density, large surface area, special structure and morphology. In this paper, boehmite with hierarchical structure were fabricated via hydrothermal or microwave hydrothermal route. In addition, the morphologies, structures, properties and morphology evolution mechanism of the as-obtained products were investigated systematically. The main results are summarized as follows:(1) Boehmite(γ-Al OOH) hollow microspheres were firstly synthesized by a convenient hydrothermal route. Samples subjected to different reaction durations were characterized by techniques of X-ray diffraction(XRD), transmission electron microscopy(TEM) and scanning electron microscope(SEM) to investigate its crystal form and morphology evolution process. A spontaneous morphology evolution mechanism driven by Ostwald ripening and dissolution-renucleation was proposed based on the experimental facts. The crystal form and morphology evolution process is a very complex process including nucleation, growth, coagulation and self-assembly, all these steps might be influenced significantly by the experimental conditions. We think that systematically understanding and hereby manipulating the morphology evolution process will contribute to the fabrication of materials with novel morphologies.(2) The boehmite(γ-Al OOH) hollow microspheres were synthesized after 120 min reaction time at 150?C for the first time via a microwave hydrothermal route, using Al2(SO4)3 aqueous solution and urea as raw materials and amphiphilic copolymer of P(St)-b-P(HEA) as structure-directing agent. The final product were characterized by techniques of X-ray diffraction(XRD), transmission electron microscopy(TEM) and scanning electron microscope(SEM). The microscope analysis revealed that the boehmite(γ-Al OOH) hollow microspheres were around 1-2 μm in diameter and a shell thickness of approximate 200 nm. To investigate the influencing factors and formation mechanism of the as-obtained boehmite hollow microspheres ultra-fine powders, samples subjected to different reaction durations were also studied by SEM. A self-assembly morphology evolution mechanism was proposed based on the experimental facts.(3) In this paper, we successfully established an additive-free microwave hydrothermal(M-H) route by only using Al2(SO4)3 aqueous solution and urea as raw materials. Core-shell structured boehmite was synthesized at 180 °C for the first time via a M-H route. The final product was characterized by techniques of X-ray diffraction(XRD), transmission electron microscopy(TEM) and scanning electron microscope(SEM). On account of the fact that less reaction time usually means less energy consumption or more eco-friendly design, the M-H reaction time was successfully reduced to only 40 min by utilizing full microwave heating power and appropriate dosage of urea. To investigate the possible mechanism and influencing factors associated with the morphology and crystal form evolution process, samples subjected to different reaction durations were prepared and characterized.(4) By merely using Al(NO3)3 and urea as raw materials, homogeneous lamellar morphology boehmite powders with fine crystalline form were firstly synthesized on a large scale by a convenient additive-free hydrothermal process conducted at 150?C for 24 h in an autoclave of 10 L capacity. Samples subjected to different reaction durations were characterized to investigate the boehmite crystal form and lamellar morphology evolution process. A spontaneous morphology evolution mechanism driven by Ostwald ripening was proposed based on the experimental facts. A conclusion was drawn that systematical understanding of the opportune influence factors and morphology evolution process would contribute to the large-scale fabrication of uniform lamellar morphology boehmite.VII This work provided a sure-enough breakthrough for the successful large-scale and additive-free fabrication of homogeneous lamellar morphology boehmite powders. It is hoped that this easily industrializing hydrothermal route and the detailed property investigations of this special morphology product may offer some opportunities for technical applications in areas of catalysts, catalysts supports, adsorbent, flame retardant, transport and release.(5) As the integration of the three main influence factors, by using full microwave power range of 0-1000 W and 200% theoretical dosage of urea, homogeneous hollow microspheres structured boehmite with fine crystalline form was synthesized at 180?C for only 30 min via an additive-free microwave hydrothermal route. On the basis of the time-dependent experimental results, we propose that the reaction temperature, dosage of urea and microwave power range might be the three pivotal influence factors for the M-H synthesis of hollow microspheres boehmite with hierarchical structure.(6) In this paper, aluminum sulfate was consciously and proportionally introduced as another concomitant raw material into aluminum chloride/aluminum nitrate-urea hydrothermal process to form SO42--Cl- and SO42--NO3- competition system. Boehmite from lamellar assemblies to hollow microspheres morphology was synthesized by succinctly altering the relative proportion of SO42- : Cl- and SO42- : NO3-. In conclusion, we named this morphology control concept as anions competition method. We think our simple but effective morphology control concept to determine the final boehmite morphology is not only an attempt towards a mutually beneficial solution but also a route to win on the starting line.(7) As another concomitant raw material, aluminium acetate(Al(CH3COO)3) was firstly and consciously introduced into aluminum chloride/aluminum nitrate-urea hydrothermal reaction system to not only share the total demand of cations but also provide competitive anions. The key point of this additive-free and morphology self-controlled hydrothermal process was to form CH3COO-:NO3- and CH3COO-:Clcompetition system to realize the aim of morphology control and environmentally friendly. In this study, boehmite from lamellar assemblies to flowerlike assemblies and finally to hollow microspheres morphology was facilely synthesized by succinctly altering the relative ratio of CH3COO-:NO3- and CH3COO-:Cl-. Samples subjected to different reaction durations were prepared and characterized by techniques of SEM and XRD to investigate its crystal form and morphology evolution process.(8) In this paper, aluminum sulfate was consciously and proportionally introduced as another concomitant raw material into aluminum chloride/aluminum nitrate-urea microwave hydrothermal process to form SO42-:Cl- and SO42-:NO3- competition system. Boehmite from lamellar assemblies to hollow microspheres morphology was synthesized by succinctly altering the relative proportion of SO42- : Cl- and SO42- : NO3-. Samples subjected to different reaction durations were prepared to investigate its crystal form and morphology evolution process of hollow microsphere boehmite. The anion competition method was firstly introduced into microwave hydrothermal process, so as to prove its generality and expand its application areas, and also enrich morphology control ideas of boehmite under microwave hydrothermal conditions.(9) In this paper, when aluminium acetate was consciously and proportionally introduced into aluminum chloride/aluminum nitrate-urea microwave hydrothermal process to form CH3COO-:NO3- and CH3COO-:Cl- competition system, the boehmite morphology evolution process proceeded like a game of tug-of-war to some extent, and the increase in the molar ratio of CH3COO-:Cl- resulted in the morphological transformation of boehmite from lamellar assemblies to flowerlike assemblies and finally to hollow microspheres architecture facility. It is hoped that the further detailed morphology control method investigations of this easily handleability microwave hydrothermal route may offer some opportunities for technical applications. |
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