Hydrothermal synthesis method

Various methods are applied to the synthesis of titania particles including sol-gel method, hydrothermal method [2], citrate gel method, flame processing and spray pyrolysis [1]. To utilize titania as a photocatalyst, the formation of ultrafme anatase titania particles with large crystallite size and large surface area by various ways has been studied [4]. 

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

Moreira, M.L., Andres, J., Varela, J.A. and Longo, E. (2009) Synthesis of fine microsized BaZr03 powders based on a decaoctahedron shape by the microwave-assisted hydrothermal method. Crystal Growth and Design, 9, 833-839. 

Tomita K, Petrykin V, Kobayashi M, Shiro M, Yoshimura M, Kakihana MA (2006) Water-soluble titanium complex for the selective synthesis of nanocrystaUine brookite, rutile, and anatase by a hydrothermal method. Angew Chem Int Edit 118 2438-2441... 

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

Hydrothermal Method. Iron [Fe(III)], barium, and the dopants are precipitated as their hydroxides and reacted with an excess of sodium hydroxide solution (up to 6 mol/L) at 250-350 °C in an autoclave. This is generally followed by an annealing treatment at 750-800°C to obtain products with the desired magnetic properties. Many variations of the process have been described [5.36]-[5.40], the earliest report being from 1969 [5.41], In later processes, hydrothermal synthesis is followed by coating with cubic ferrites, a process resembling the cobalt modification of iron oxides (see Section 5.1.2). The object is to increase the saturation magnetization of the material [5.42]-[5.44],... 

Solvothermal. The next low-temperature method to be discussed, solvothermal, requires some heating. Like sol-gel, solvothermal is a derivahve method. It comes from the hydrothermal method. Hydrothermal synthesis uses water as a solvent... 

The formation of water (as a reaction product) and local heating under mechanical activation as a result of collisions, friction and plastic friction of solid particles are the conditions for the reactions to occur via a dissolved state (hydrothermal-like synthesis). Therefore, it is reasonable to consider some features of these methods taking into account a large number of works in this direction. 

A large number of investigations on the synthesis of inorganic compounds by hydrothermal method was carried out. At present, the list of compounds that are synthesized under hydrothermal conditions includes aluminates, ferrites, chromates, titanates, zirconates, stannates, antimonates, etc. The works dealing with the synthesis of silicates are most numerous [19-26]. 

A great number of works on the aqueous synthesis of ceria NPs have been reported from very early years, since the ultrafine ceria powders are re-examined from fhe view point of nanotechnology for example, the simple precipitation methods (Zhang et al., 2002a), the microemulsion methods (Masui et al., 1997 Zarur and Ying, 2000), urea assisted hydro-thermal methods (Hirano and Inagaki, 2000 Hirano and Kato, 1999), hydrothermal methods with supercritical conditions (Adschiri et al., 2001). 

The controlled hydrothermal synthesis of rare earth borates results in various nanostructures. The hydrothermal method at 300 °C yields submicron sized GdB03 Eu " powders (Wang et al., 2001a). Well-crystallized YBOsTu NCs were prepared by a mild hydrothermal method in the presence of urea, and a pure hexagonal phase could be obtained at a lower temperature of 200 °C (Jiang et al., 2003). 

Hydrothermal/solvethermal s)mthesis of RMO3 is also extensively adopted, by virtue of the low reaction temperature and well-crystallized products. Vazquez-Vazquez and Lopez-Quintela (2006) reported the solvothermal synthesis of Lai j-A MnOs (A = Ca, Sr, Ba) NPs in benzyl alcohol and acetophenone. The obtained precipitate was annealed to form crystalline products and acetophenone was found to be more suited to obtain clean perovskite phase. Zhu et al. (2008a) prepared single-crystalline YbMnOs and LuMnOa nanoplates via hydrothermal method. The products were found to be hexagonal phases. A possible formation mechanism was proposed, which involves the formation of ROOH phase as intermediate. 

Hydrothermal Synthesis. Hydrothermal synthesis and reactive fluxes have been increasingly used as a method for synthetic actinide chemistry. Unique phases of actinide selenites, tellurites, iodates and periodates have all been synthesized under hydrothermal reaction conditions. New compounds and phases of oxyfluorides, phosphates, and sulfates using organic templates also have been synthesized with hydrothermal methods. " ... 

Hydrothermal methods provide a speciahzed synthesis technique where aqueous solutions are placed in a sealed system and then heated above the normal boiling point of water, often near or shghtly above the critical temperature of the solvent (374 °C). Usually, the charge has a low solubility... 

High-pressure methods are discussed in detail under High Pressure Synthesis of Solids. Hydrothermal methods are discussed in the previous Section. A few examples relative to oxide synthesis are given here. Descriptions of the equipment used are given by Rao and GopaUcrishnan and by Rooymans. ... 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

Blanco-Lopez, M.C., Rand, B., and Riley, RL., J. Ear. Ceram. Soc., 17, 281,1997. Lu, S., Lee, B.I., and Wang, Z., Synthesis of redispersible nano-sized barium titanate powders by hydrothermal method, in Ceramic Transactions, vol. 106, Dielectric Materials Devices, edited by K.M. Narr and A.R. Bhalla, American Ceramic Society, Westerville, OH, 2000. 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

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