Hydrothermal syntheses

Hydrothermal synthesis is a powerful method used for the fabrication of nanophase materials due to the relatively low temperature during synthesis, facile separation of nanopartides in the product, and ready availability of apparatus for such syntheses. Versatile physical and chemical properties of nanomaterials can be obtained with the use of this method that involves various techniques (e.g., control of reaction time, temperature and choice of oxidant and its concentration). Several extensive reviews are available that discuss the fundamental properties and applications of this method [2, 3]. These reviews cover the synthesis of nanomaterials with different pore textures, different types of composition [2, 4—6], and different dimensionalities in terms of morphology [6-8]. 

Oxidant species Half redox equation (acidic condition) red (V) 

As the name implies, hydrothermal treatments involve the heating of precipitates, gels, or flocculates in the presence of water. These treatments are typically carried out in an autoclave at 100-300 °C, and result in textural and/or structural changes, including crystal and/or particle growth, changes in crystal structure, and transformation of amorphous solids into crystalline ones. 

Ion exchange of the Na+ cations with NH4+, and subsequent drying and calcination, gives the solid acid H-ZSM-5, used in synthetic gasoline manufacture and 

Preparation under hydrothermal conditions (temperatures above 100 °C and the presence of water) can be used to enhance crystallization and it is therefore the method of choice for zeolite synthesis (Chapter 12) but is used for making mixed-metal oxides, too. 

Hydrothermal synthesis has also been used to prepare mixed-metal oxide catalysts. The group of Maier presented already in 1998 the first hydrothermal high-throughput preparation method for such catalytic materials [93]. Corma et al. used a hydrothermal treatment of sol-gel synthesized Ti-silicalite catalyst precursors to accelerate the crystallization [94]. 

The reaction is carried out in water, but the reagents are solids. A solid is precipitated and by reaction with other components of the solid mixture or with dissolved species it is transformed into a new solid. The reaction takes place in a closed vessel (a hydrothermal bomb) usually built in stainless steel lined with Teflon. The reaction takes place at 150-500 °C (depending on the liner used) and high autogenous pressures. Water acts as a pressure transmitter and as a solvent. Seed crystals and a temperature gradient are sometimes used for crystal growth. Under these conditions, solubilization of very insoluble species (e.g., silica) talces place, and the reaction proceeds for instance  

The crystal size generally increases as a result of Ostwald ripening small particles dissolve faster than the large ones and dissolved small particles dynamically and reversibly contribute to growing of the large ones. The process proceeds until the solubilities of the large and the small particles are very close to each other. 

This procedure is generally used to prepare zeolites. A gel is formed by hydro-thermal treatment ( 200 °C) of an aqueous solution of NaOH, NaAl(OH)4, and Na2Si03 (a structure-director agent (SDA) can be added). Depending on the reactants nature and ratio, the SDA used, and the specific experimental conditions (temperature, pH, time, etc.), different structures can be formed. 

The garnet Y3AI5O12 (TAG) can be prepared from Y2O3 and sapphire (a form of AI2O3) in two zones of an autoclave at different temperatures YAG is formed where the two zones meet [9]  

The formation of 2 1 phyllosilicates is performed by using mixtures of either metal oxides or salts, water or organic solvents that provide suitable composition (mainly in stoechiometric one) for crystallization. The process is conducted at a mild temperature (a hydrothermal or solvothermal process). 

Commercially available fluorohectorites are also prepared starting from Na, Mg and Li silicate salts under hydrothermal conditions. In this case, products contain about 5 wt % of fluorine (commercially available as Laponite B ). 

Baerlocher, L.B. McCusker, D.H. Olson, Atlas of Zeolite Framework Types, sixth revised edition, Elsevier B. V., Amsterdam, 2007, also available on the web at www.iza-structure.org/ databases/. 

Diaz-Cabanas, J. Martmez-Triguero, F. Rey, J. Rius, Nature, 418, 514-517 

This is a relatively low-temperature, water-based route capable of producing submicron, spherical and uniform sized particles of either high purity or chemically modified BT. Essentially barium, titanium and dopant compounds are reacted in a basic aqueous medium to form hydroxides. Under the hydrothermal conditions, typically in the temperature and pressure ranges 100-250 °C and 100kPa-3MPa ( —30 atm) respectively, sub-micron particles of either pure or modified barium titanate are precipitated. There are many variables which need careful control, especially the reactive areas of the precursors and the degrees of supersaturation of the various species. 

Not surprisingly water in the form of hydroxyl groups can become incorporated into the titanate structure and the evolution of this during subsequent processing stages towards the sintered ceramic has to be planned for. In its fundamentals the situation is identical to that described in Section 4.6.1 concerning the development of proton conductors. 

Developments of the process permit coatings of a range of dopants to be applied to the surfaces of the particles which, during sintering, produce the core-shell structure leading to X7R characteristics (see Section 5.7.1). The coatings may also consist of sintering aids. 

It is also possible to retain the hydrothermally produced barium titanate particles in aqueous suspension to form the basis of a tape-casting slurry capable of producing 3 /mi dielectric layers. This route avoids the risk of the formation of hard agglomerates on drying the precipitates. 

Hydrothermal synthesis is being increasingly adopted for the commercial production of compositionally tailored barium titanates for MLCCs. 

Precipitation of simple and complex oxide nanoparticles from water solutions under hydrothermal conditions (high-temperature hydrolysis) is well known technique for several decades [69]. In the past 20 years, interest has increased to hydrothermal synthesis due to the extensive miniaturization electronic devices and the increasing demand for nanosize high quality powders. 

The process involves heating of reagents, which are usually solutions or suspensions of metal salts, oxides, hydroxides, and even metal powders. The temperatures are between boiling point and critical point of water (100-374 °C) at pressures up to 22.1 MPa (water pressure in its critical point). Typically, the heating occurs in a stainless steel autoclave, the inner surface of which is coated with Teflon to prevent corrosion of the reactor. 

Since the precipitation of the dispersed hydrolysis product occurs under hydrothermal conditions of elevated temperatures and pressures, the output powders receive several useful properties the crystalline phase is usually formed directly in the reactor. This means that no additional annealing is required, satisfactory chemical homogeneity and purity of the particles is achieved and the particle size is 

Incomplete removal of hydroxyl groups after hydrothermal synthesis has several consequences like stabilization of the nonequilibrium allotropic modifications of oxide phases with the hydroxyl groups and substantial residual porosity on sintering. 

The hydrothermal synthesis of ferroelectric nanoparticles like BaTiOs is an industrial concern. This method is a powerful tool for fabricating ultrafine, homogeneous powders of high purity for a large variety of multi-cation oxides [73] as compared to methods based on the decomposition of solid precursors. The main advantage of hydrothermal synthesis is the improved control of the chemical and granular 

Coexistence, intergrowth and thus presumable paragenesis of crystals of diamond, quartz and a- and p-SiC have been found in a diamondiferous kimberlite [34] where crystallization of quartz instead of coesite indicates maximum pressures of 2.0-2.8GPa at temperatures of 1000-1200°C. This might be important in the view of diamond hydrosynthesis from silicon carbides in the metastable range. 

Investigations of hydrothermal processes and development of hydrothermal equipment have their origin in the attempts to understand and mimic nature which produces thousands of well-crystallized minerals at very modest temperatures [19]. The hydrothermal processes in this paper deal with water or aqueous fluids in the supercritical range of water, above its critical point = 374°C, Pc = 22.14 MPa). This is opposed to just boiling in water , and supercritical processes dealing merely with hydrocarbons or halogenated hydrocarbons [35]. 

The discussion of hydrothermal diamond synthesis is divided into two sections, dealing with synthesis from C-H-0 liquids and synthesis based on decomposition of silicon carbide, respectively. Both start with thermodynamic calculations in order to demonstrate the theoretical possibility of carbon formation before the experimental findings are summarized. Naturally, equilibrium calculations do not consider kinetic limitations. 

The term hydrothermal usually refers to heterogeneous reactions in the presence of a solvent (aqueous or non-aqueous) under high pressure and tanpera-ture conditions to dissolve and then recrystallize materials in a closed systan. Although there is no exact lower limit for the pressure and temperature conditions, the majority of authors fix hydrothermal synthesis at conditions above 100 C and above 1 atm. 

Hydrothermal synthesis is also known mainly by chemists as solvothermal synthesis, a boarder term meaning any chemical reaction in the presence of a solvent in supercritical or near supercritical conditions. Likewise, there are other terms such as glycothermal, alcothermal, ammonothermal, depending npon the type of solvent nsed in such reactions. However, the purpose behind using these different solvents in the chemical reactions is essentially to bring down the pressure and tanperature conditions. 

Under hydrothermal conditions, reactions that only occur at high temperatures can occur under fairly normal conditions. It is used to increase reaction velocities between solids, dissolve or crystallize substances, and promote phenomena 

Kopp Alves et al., Novel Synthesis and Characterization ofNanostructured Materials, Engineering Materials, DOI 10.1007/978-3-642-41275-2 6, 

In order to facilitate the solubilization of chemically inert materials it is often necessary to use chemical additives called mineralizers generally represented by an electrolyte. They modify the solubility of solids by the formation of intermediate compounds that usually are not present in the water in the absence of this agent [1, 2], representing another system variable to corroborate the capability of the method, controlling not only the chemical composition of the studied material, but also the morphology and microstructure [2, 5,6]. 

Tflbk 5. t Preparative methods and surface aiea of metaJ oxides nanotubes 

Starting Materials Reaction condilion Washing dolulion ProducL p Ditt. (nm) Length (i m) Smt (raVsl Ref 

Nanopartides from using soUgel, Siirj TWt given 5 15 M NaOH 100 1fiOTfor4K hr 0.1 M HCI T1O3 5-30 -1000 2157- 265 46 

Nanopanicles tironi precipitates by dropping (NH4j S04 orHCl into Tin 4 solution and then NH OHneinraliyarinn. S(d-r = liWm /tf 10 Xf NaOH. for 20 lir 0.1 M HCI IitJi -10 5U0 107- 451 47 

Nanopartides of P25of commercial Degussn AO. TiClj liydiulysis in HVO flame . Sui t SO m /ii 10 MNaOH 110 l50 Cfoi24lii 0.1 M HCI TiOi 10.10 200 400 48 

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Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Most hydrothermal synthesis processes are carried out at moderate (in the range of 100 to 300°C) temperatures and at the corresponding solution... 

Fig. 1. General process operations for hydrothermal synthesis. Feedstocks may be oxides, hydroxides or salts, gels, organic materials, or acids or bases. The atmosphere within the reactor may be oxidising or reducing. To convert MPa to psi, multiply by 145.

The most significant commercial product is barium titanate, BaTiO, used to produce the ceramic capacitors found in almost all electronic products. As electronic circuitry has been rniniaturized, demand has increased for capacitors that can store a high amount of charge in a relatively small volume. This demand led to the development of highly efficient multilayer ceramic capacitors. In these devices, several layers of ceramic, from 25—50 ]lni in thickness, are separated by even thinner layers of electrode metal. Each layer must be dense, free of pin-holes and flaws, and ideally consist of several uniform grains of fired ceramic. Manufacturers are trying to reduce the layer thickness to 10—12 ]lni. Conventionally prepared ceramic powders cannot meet the rigorous demands of these appHcations, therefore an emphasis has been placed on production of advanced powders by hydrothermal synthesis and other methods. 

Another important class of titanates that can be produced by hydrothermal synthesis processes are those in the lead zirconate—lead titanate (PZT) family. These piezoelectric materials are widely used in manufacture of ultrasonic transducers, sensors, and minia ture actuators. The electrical properties of these materials are derived from the formation of a homogeneous soHd solution of the oxide end members. The process consists of preparing a coprecipitated titanium—zirconium hydroxide gel. The gel reacts with lead oxide in water to form crystalline PZT particles having an average size of about 1 ]lni (Eig. 3b). A process has been developed at BatteUe (Columbus, Ohio) to the pilot-scale level (5-kg/h). 

Hydrothermal Synthesis Systems. Of the unit operations depicted in Figure 1, the pressurized sections from reactor inlet to pressure letdown ate key to hydrothermal process design. In consideration of scale-up of a hydrothermal process for high performance materials, several criteria must be considered. First, the mode of operation, which can be either continuous, semicontinuous, or batch, must be determined. Factors to consider ate the operating conditions, the manufacturing demand, the composition of the product mix (single or multiple products), the amount of waste that can be tolerated, and the materials of constmction requirements. Criteria for the selection of hydrothermal reactor design maybe summarized as... 

Zeolites. A large and growing industrial use of aluminum hydroxide and sodium alurninate is the manufacture of synthetic zeoHtes (see Molecular sieves). ZeoHtes are aluminosiHcates with Si/Al ratios between 1 and infinity. There are 40 natural, and over 100 synthetic, zeoHtes. AH the synthetic stmctures are made by relatively low (100—150°C) temperature, high pH hydrothermal synthesis. For example the manufacture of the industriaHy important zeoHtes A, X, and Y is generaHy carried out by mixing sodium alurninate and sodium sHicate solutions to form a sodium alurninosiHcate gel. Gel-aging under hydrothermal conditions crystallizes the final product. In special cases, a small amount of seed crystal is used to control the synthesis. 

The two fluids most often studied in supercritical fluid technology, carbon dioxide and water, are the two least expensive of all solvents. Carbon dioxide is nontoxic, nonflammable, and has a near-ambient critical temperature of 31.1°C. CO9 is an environmentally friendly substitute for organic solvents including chlorocarbons and chloroflu-orocarbons. Supercritical water (T = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. 

Thiele and co-workers 389) prepared the only known palladium chalcogenide halides, PdTel and Pd Sels, by hydrothermal synthesis in HI (see Section II,D,2) at 300°C, starting with the elements. Crystalline PdzSelj is better obtained by reaction of Pdl2 with Se and an excess of iodine in a closed ampoule at 250°C (reaction time, 2 days). 

Melt growth Vapor growth Hydrothermal synthesis Solid-state reactions... 

Preparation of Mono-Dispersed MFI-type Zeolite Nanoerystals via Hydrothermal Synthesis in a Water/Surfactant/Oil Solution... 

The zeolite nanocrystals have attracted the considerable attention of many researchers [1-5]. The syntheses of several types of zeolites with different nanometer sizes, such as silicalite-1, ZSM-5, A-type and Y-type, have been reported. Recently, micellar solutions or surfactant-containing solutions have been used for the preparation of zeolite nanoerystals [4,5], We have also successMIy prepared silicalite nanoerystals via hydrothermal synthesis using surfactants. In this study, we demonstrate a method for preparing mono-dispersed silicalite nanoerystals in a solution consisting of surfiictants, organic solvents and water. 

In order to prepare ZSM-5 zeolite nanocrystals, an A1 source of aluminium isopropoxide was added into solution A, and hydrothermal synthesis of the solution A containing Si and A1 sources was carried out in an 0-15/cyclohexane solution at 120 degree C for 50 h. Figures 4 show ac-NHj-TPD spectra and a SEM photograph of the ZSM-5 zeolite nanocrystals. Nanocrystals with a diameter of approximately 150 nm were observed, and the NH3-TPD spectrum showed desorption of NHj above 600 K, indicating that the nanocrystals possessed strong acid sites. 

Hydrothermal synthesis of titanium dioxides using acidic and basic peptizing agents and their photocatalytic activity on the decomposition of orange II... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Recently, Somorjai reported the hydrothermal synthesis of SBA-15 in the presence of PVP-stabilized Pt nanoparticles [22]. This is a one-step synthesis of composites of metal nanoparticles and mesoporous silica. 

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