Hydrothermal Methods

Hydrothermal methods include all chemical reactions in water in high temperatures and high pressures. The hydrothermal methods consist of hydrothermal oxidation, hydrothermal precipitation, precipitation synthesis, hydrothermal deoxidization, hydrothermal decomposition, hydrothermal crystallization, etc. The hydrothermal reactions can be used to prepare nanoparticles as well as nanocapsules. The typical chemical reactions are listed as follows:

(1) the hydrothermal oxidation: mM + «H2O ^ MmO„ + H2, where M = Cr, Fe, and their alloys,

(2) the hydrothermal precipitation: KF + MnCl2 ^ KMnF2,

(3) the precipitation synthesis: FeTiO3 + KOH ^ K2O • nTiO2,

(4) the hydrothermal deoxidization: Mex Oy + yH2 ^ xMe + yH2O, where Me = Cu, Ag, etc.,

(5) the hydrothermal decomposition: ZrSiO4 + NaOH ^ ZrO2 + Na2SiO3,

(6) the hydrothermal crystallite: Al(OH)3 ^ Al2O3 • H2O.

The hydrothermal methods are convenient, lower temperature, and environmentally friendly solution processing routes for synthesis of advanced ceramic materials with desired shapes, sizes, and form, compared with other ceramic materials processing methods such as solid-state reactions, CVD, MOCVD, and physical vapor deposition (PVD) involved using higher temperatures or toxic organometallic precursors, or complicated reaction and post-treatment systems.

Hydrothermal/solvothermal processing of advanced ceramic materials was reviewed [274]. The review focused on the development of the hydrothermal process, its integration with other activation methods such as electrochemistry, microwaves, and its counterpart the solvothermal process for fabrication of ceramic thin films, nanoparticles, and single crystals. The solvothermal process proved to play a significant role in synthesis of advanced nonoxide ceramic materials by using reactions in nonaqueous solutions, which will be an exciting and promising field for preparation of ceramic materials. Shapes, sizes, and phases of the synthesized ceramics can be well controlled by employing different solution processes, which will play a key role in tailoring the properties of ceramic materials.

A series of layered magnesium organosilicate nano-composites containing covalently linked organic functionalities was prepared by a one-step, direct synthesis procedure [275]. The in-situ reactivity of organic moieties (e.g., phenyl, thiol) within the layered inorganic-organic nanocomposites, as well as the potential use of thiol-containing materials as host matrixes for the organized deposition of gold nano-particles, was described.

Semiconductor nanoparticles assembled within a zeolite are an example to create a quantum confined system. Molecular sieves were prepared under hydrothermal conditions from Catapal alumna, aluminosilicate, silicate, and ethylsili-cate gels, respectively, under carefully chosen conditions of pH, temperature, and crystallization time [276]. Coadded alkali metal cations, quaternary alkylammonium cation, and surfactant species functioned as charge balancing spacing-filling, and structure-directing agents to obtain an ultralarge pore molecular sieve.

Nanocrystalline titanium dioxide colloids were synthesized using a sol-gel technique followed by growth under hydrothermal conditions at temperatures between 190 and 270 °C [277]. Thin films were made from aqueous suspensions of these colloids. The rodlike particles self-organized into regular cubic arrays with the long axis of the rods aligned perpendicular to the film surface. This self-organization was dependent upon the base used in colloidal synthesis and also upon the dielectric constant of the medium used during film formation.

Colloidal solutions and redispersible powders of nanocrys-talline, lanthanide-doped YVO4 were prepared via a hydrothermal method at 200 °C [278]. The crystalline particles ranged in size from about 10 to 30nm. The particles exhibit the tetragonal zircon structure known for bulk material.

A varity of synthetic methods of metal sulfide particles were reviewed [279]. Some research work via hydrothermal and solventothermal routes was summarized, concerning the synthesis of metal sulfide nanoparticles and the control of particle size, size distribution, as well as morphology. Nanocrystalline Co9S8 was successfully prepared by a hydrothermal-reduction method at 120 °C [280]. The reducing agents were produced from disproportionation of white phosphorus in alkaline medium.

CeO2 nanoparticles coated by sodium bis(2-ethylhexyl) sulfosuccinite were prepared by using the microemulsion method [281]. ZnGa2O4 fine particles with a single phase of spinel were synthesized from a mixed solution of gallium sulfate and zinc sulfate in the presence of aqueous ammonia under hydrothermal conditions above 180 °C [282]. The effects of treatment temperature and ZnO/Ga2O3 molar ratio in the starting solution on the crystallite size, morphology, lattice parameter, and chemical composition of the ZnGa2O4 spinel particles were examined. Spinels with different morphologies, cubic nanoparticles, and elongated rodlike particles were thought to be formed based on the structure of amorphous gallium hydroxide and needlelike GaO(OH) particles, respectively.

A hydrothermal CVD process was reported which produced self-assembled patterns of iron oxide nanoparticles [283]. By exposing a planar silica substrate to a prevapor-ized mixture of water, ferrocene [Fe(C5H5)2], and xylene (C8H10), at temperatures around 1000 °C, Fe2O3 nano-particles were deposited on the substrate surface, in regular circular patterns.

Nanosized TiO2 particles coated with dodecylbenzene-sulfonic acid (DBS) and stearic acid (St) were prepared using the hydrothermal procedure [284, 285]. The average size of the DBS/TiO2 and St/TiO2 particles was about 8nm. Raman spectra from DBS/TiO2 and St/TiO2 were measured, and scattering signals from both TiO2 and the coating were observed. In contrast to the redshift of Raman peaks in the nanoparticles with decreasing particle size, a blueshift, namely the Raman peak shift to the higher wavenumber side, in the coated particles was recorded. It was also found that different coatings show different extents of Raman shifts.

Organic monolayer-stabilized copper nanocrystals were synthesized in supercritical water [286]. When water was heated and pressurized above the critical point, it became a suitable solvent to employ organic capping ligands to control and stabilize the synthesis of nanocrystals. Without alka-nethiol ligands, Cu(NO3)2 hydrolyzed to form polydisperse Cu2+ oxide particles with diameters from 10 to 35nm. The use of a different precursor, Cu(CH3COO)2, led to particles with significantly different morphologies. A mechanism was proposed for sterically stabilized nanocrystal growth in supercritical water that described competing pathways of hydrolysis to large oxidized copper particles versus ligand exchange and arrested growth by thiols to produce small monodisperse Cu nanoparticles.

CdS and ZnO-capped CdS semiconductor nanoparticles were synthesized via a hydrothermal method by thermal decomposition of the cysteine-cadmium complex precursor [287]. Compared to CdS nanoparticles, the bandgap emission of CdS/ZnO was greatly improved, meaning that the shell of ZnO modified the surface of the CdS core and reduced the surface defect.

The increasing awareness of the need to create green and sustainable production processes in all fields of chemistry has stimulated materials scientists to search for innovative catalyst supports. These new catalytic supports should allow the heterogenization of most catalytic processes, increasing the efficiency and selectivity of the synthesis and reducing waste. A hexagonal material with large pore diameters and thick walls (4 nm), containing internal microporous silica nanocapsules, was developed [288]. These plugged hexagonal templated silicas had two types of micropores (originating from the walls and the nanocapsules respectively)

and a tunable amount of both open and encapsulated meso-pores. The micropore volumes had a high value (up to 0.3 cm3/g) and the total pore volume exceeded 1 cm3/g. The obtained materials were much more stable than the conventional micellar templated structures and can easily withstand severe hydrothermal treatments and mechanical pressures.

The methods were investigated for preparation of Au/TiO2 catalysts to obtain small gold metal particles (23 nm) and a higher Au loading [289]. The possible mechanisms of deposition of gold on the TiO2 support by the different preparation methods were discussed.

In Mn1-xZnxFe2O4 (x = 0 to 1) nanosize particles prepared through hydrothermal precipitation, a decrease in particle size was observed from 13 to 4 nm with increasing Zn concentration from 0 to 1 [290].

Hematite nanocrystals modified with surface layers of amorphous hydrous iron oxides (Fe2O3-1.64H2O) were prepared by hydrothermal conditions at 130 °C in the absence of alkali [291]. When the temperature was lower than 130 °C, no product was formed, while above this temperature, the amount of amorphous hydrous iron oxides at the surface of hematite nanocrystals was drastically decreased.

Rutile or anatase nanocrystals were prepared by a low temperature dissolution-reprecipitation process in HCl or H2SO4 solutions around room temperature [292]. The repre-cipitation temperature and organic solvent addition in the solution system greatly affected the phase composition of the final powders.

Semiconductor selenides of MSe (M = Cd, Hg) nanocrys-talline powders were synthesized through the reactions between metal chlorides and sodium selenosulfate in the ammoniacal aqueous solution at room temperature for 6-10 h [293]. The average diameters of CdSe and HgSe nanocrystallites were 4 and 8 nm, respectively. The storage and an interesting phase transition under hydrothermal conditions were presented. The absorption spectrum of the as-prepared samples exhibited an obvious blueshift due to the size confinement

Cerium oxide nanoparticles coated by cetyltrimethylam-monium bromide were prepared using reverse micelles and the microemulsion method [294]. A chemical transformation from an amorphous Ce3+ component to a crystalline Ce4+ component was found in the temperature range of 200 to 400 °C. For the as-prepared nanoparticles without annealing (25 °C), a single shell (10 x 2.59 A) was sufficient to describe its local structures, while two shells had to be used for the nanoparticles annealed at 200 and 350 °C. One shell corresponded to the Ce3+ component. Another corresponded to the Ce4+ component. A core-shell model was used to explain the structural parameters around cerium for the two samples annealed at 200 and 350 °C.

The covalent binding of yeast alcohol dehydrogenase to magnetic nanoparticles via carbodiimide activation was studied [295]. The magnetic nanoparticles Fe3O4 were prepared by coprecipitating Fe2+ and Fe3+ ions in an ammonia solution and treating under hydrothermal conditions. The resultant magnetic nanoparticles were superparamag-netic characteristics, and their saturation magnetization was reduced only slightly after enzyme binding. The kinetic measurements indicated the bound yeast alcohol dehydrogenase retained 62% of its original activity and exhibited a 10-fold improved stability than did the free enzyme.

Cerium oxide (CeO2) nanoparticles were prepared sono-chemically, by using cerium nitrate and azodicarbonamide as starting materials, and ethylenediamine or tetraalkylam-monium hydroxide as additives [296]. The additives had a strong effect on the particle size and particle size distribution. CeO2 nanoparticles with small particle size and narrow particle size distribution were obtained with the addition of additives, while highly agglomerated CeO2 nanoparticles were obtained in the absence of additives. Blueshifts of the absorption peak and the absorption edges of the products were observed in the UV-vis absorption spectra as a result of the quantum size effect.

Millerite NiS nanocrystals were prepared through the solvent (hydro)thermal method [297]. Using ethylenediamine and hydrazine hydrate as solvent resulted in the formation of rodlike nanocrystalline NiS, whereas spherical nanoparticles were obtained using aqueous ammonia as solvent.

The hydrolyze process consists of hydrolyzing compounds in water to form precipitates, which can be used to prepare nanoparticles/nanocapsules. Usually, the starting materials of the hydrolyze process are water and metal inorganic salts like chlorates, sulphates, nitrates, ammonium salts, etc. The metal organic salts, like metal alcoholic salts, which can be solved in organic solvents, can be used for the starting materials of the hydrolyze process. The products of the hydrolyze reactions are hydroxids and hydrates. The difference between the hydrolyze process and the hydrothermal process is that high temperature and pressure are not needed in the former. The following are several examples for the hydrolyze process. Pseudolatexes of the biodegradable polyesters poly(D,L-lactide) (PLA) and poly(e-caprolactone) (PCL) were developed as potential aqueous coatings for sustained release [298]. The influence of the surfactant system, temperature, pH, and particle size on the chemical stability of the polymers as aqueous colloidal dispersions was investigated. The room-temperature reactions of the chemical warfare agents VX [O-ethyl S-2-(diisopropylamino) ethyl methylphosphonothioate], GD (3,3-dimethyl-2-butyl methylphosphonofluoridate, or Soman), and HD (2,2'-dichloroethyl sulfide, or mustard) with nanosize MgO were studied [299]. All three agents hydrolyzed on the surface of the very reactive MgO nanoparticles. Procedures for the preparation at low temperature (80 °C) of uniform colloids consisting of Mn3O4 nanoparticles (about 20 nm) or elongated a-MnOOH particles with length less than 2 /m and width 0.4 /m or less, based on the forced hydrolysis of aqueous Mn2+ acetate solutions in the absence (Mn3O4) or the presence (a-MnOOH) of HCl, were described [300]. The role that the acetate anions played in the precipitation of these solids was analyzed.

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