A solvothermal reaction can be described as a chemical reaction involving a solvent either in subcritical or supercritical conditions between different precursors. Such a solvent can act as a chemical component or a fluid phase able, through its physico-chemical properties, to induce the synthesis reactions. During the last fifteen years, solvothermal reactions have been used in different scientific areas involving basic or applied research. Several domains have been developed in Materials Chemistry: (i) the synthesis of novel materials, (ii) the development of new low temperature processes able to prepare functional materials, and in Materials Science: (i) new crystal growth processes, (ii) thin film deposition and (iii) sintering processes at low temperature. In Materials Chemistry three illustrations are described: (i) the synthesis through “geomimetism” of a new family of layered oxides: the phyllosiloxides, (ii) the synthesis of inorganic-organic materials, (iii) the potential of solvothermal processes for producing c-BN.

Aluminosilicate nanotubes (imogolite) have been synthesized from highly concentrated inorganic solutions by hydrothermal treatment. These can be converted to microporous nanofibers with a pore radius in the range of 0.3-0.6 nm referring to the results from the nitrogen adsorption isotherm. The water vapor adsorption isotherms indicated that the natural imogolite plotted a proportional isothermal curve where the amount of adsorbed water increased in proportion to P/P0: the maximum amount of adsorbed water was ca. 60 wt%. The synthetic imogolite showed a rapid increase at 0.9-0.95 range of P/P0 and achieved a maximum of ca. 80 wt%, with a better methane storage property than that of the usual compressed natural gas storage. In order to obtain a high ratio of water adsorption and a large methane storage capacity, it is necessary to control the micro/meso porous structure and the hydrophilic/hydrophobic surface affinity. It is expected that the synthetic imogolite might become a multipurpose adsorbent.

Genesis of the structure of zirconia fine particles prepared by precipitation of amorphous hydrated zirconia by ammonia from the ZrO(NO3)2 solution followed by a mild hydrothermal treatment (HTT) of precipitate, washing and calcination under air up to 1000 °C has been studied by HRTEM, X-ray diffraction, Raman and FTIRS. HTT rearranges the structure of amorphous zirconia, which helps to obtain nearly single-phase monoclinic nanozirconia (particle size 5-15 nm) after a mild calcination at 500 °C. Dehydroxilation and sintering of these nanoparticles at higher (600-650 °C) temperatures generate polysynthetic (001) twins. Modeling revealed that reappearance of the (111) “cubic” reflex in XRD patterns of samples calcined at 600-650 °C can be due to these extended defects. In their vicinity, the seven-fold Zr-O coordination sphere is retained, while packing of ZrO7 polyhedra is varied towards more symmetric structures, thus causing disappearance of the Raman spectra.

Rutile and anatase TiO2 films were grown on glass substrates from acidic titanium precursor solutions from 60°C upwards. Anatase synthesized at 60°C had a crystallinity of 15% that increased to 54% for a growth temperature of 200°C. A similar crystallinity by conventional heat treatment of the 60°C material was attained only at 400°C. It is believed that more complete dehydration of titanium complexes at higher growth temperatures led to less disruption of the long range attractive forces required for the formation of the periodic crystalline lattice. Rutile films grown at 60°C were determined to have a refractive index of 2.4. This is lower than the bulk value of 2.65 because the as-synthesized rutile material was only 29% crystalline and also contained nano-sized pores.

A straightforward solvothermal pathway towards anisotropic nanoscale molybdenum, vanadium and tungsten oxides has been established. They are formed quantitatively from one-step procedures within a few days or hours of autoclave treatment in the temperature range between 100 and 220 °C. The addition of straightforward ionic additives (e.g. alkali halides) leads to a versatile interplay between the formation of novel polymolybdates(VI) and the production of oxidic nanoparticles. Key solvothermal features (role of the precursor, solvothermal parameter window, influence of ionic additives) of the individual transition metal oxides are investigated with respect to the development of general synthetic guidelines and predictive concepts.

The use of high temperature furnace assembly used in high–temperature high-pressure machine is proposed and modeled for the infiltration process of metals in opals. The goal here is to provide useful information about the temperature distribution inside the capsule cell in order to save some time and reduce synthesis cost in the real infiltration process. The heat conduction equation is solved for a cylindrical geometry and the variation of the temperature as a function of time and radius distance are reported. The 2-D finite element method for a symmetrical assembly is used and the temperature dependence of heat conductivity of multi elements is considered in the calculations. Temperature dependence of heat conductivity of different elements involved in infiltration process are modeled and also reported here. Such information concerning the temperature distribution inside the infiltration assembly is helpful for both the equilibrium studies and transient conditions. Results of such study help the users to properly select the appropriate temperature gradient for the steady state and transient situations. Such temperature and pressure adjustments are helpful for a successful melt infiltration process.

The impact of powdered oxides of Mg, Ca, La, Ti, Al or their nitrate salts on the properties of MeOx/Al2O3/Al composites obtained via the hydrothermal treatment (HTT) of Al-based blends put in a closed space of the die allowing the free water vapor access has been studied. Additives were found to affect the process of aluminum oxidation mainly through variation of the density of formed aluminum hydroxide layer. In some cases, incorporation of dopant cations into the structure of alumina formed after calcination of composite in air was revealed by 27 Al MAS NMR. Variation of the Al oxidation degree and associated change of the solid phase volume in a closed die caused by dopants affects also the pore structure and texture of composites determining the mechanical strength of produced monoliths. The processes of the redistribution of soluble additives between the solid phase and autoclave solutions were revealed and their consequences are considered.

Well-shaped single crystals of RNiO3 perovskites (R= Nd, Ho, Y) have been grown under high oxygen pressure conditions, in a piston-cylinder press at 2 GPa or in a belt-type press at 4 GPa. The reaction took place in sealed platinum capsules, in the presence of KClO3 as oxidizing agent. It seems that the choice of hydroxides of the involved cations as precursors is crucial for the success of the crystal growth, via water vapor transport reactions. The perovskites have been characterized by X-ray diffraction, scanning microscopy and DSC measurements, suitable to identify the metal insulator transition. The transition temperatures of the as-grown crystals show a significantly lower hysteresis in the heating and cooling runs than those reported for powder materials, suggesting a lower density of defects.

Abstract:Mercury Cadmium Telluride (MCT) nanoparticles were synthesized by Solvothermal method for the first time. Mercury, Cadmium and Tellurium in compound/elemental form were added to Ethylenediamine (solvent) in the Teflon-Stainless-Steel autoclave along with a reducing agent. Different compositions of MCT were synthesized by varying the proportions of Hg and Cd to obtain Hg1-xCdxTe with x varying from 1 to 0. X-ray diffraction patterns revealed the formation of MCT nanoparticles with predominant cubic (111) phase. TEM studies revealed that the average particle size ranged from 10 to15 nm for different compositions.

We offer a synthesis of novel nanocrystalline ZnBiVO4 using hydrothermal method. The same novel catalyst was synthesized using solid-state route for the first time. We have exemplified the hydrothermal synthesis of this new compound using zinc nitrate, bismuth nitrate and ammonium metavanadate. The ZnBiVO4 was synthesized using zinc oxide, bismuth oxide and vanadium oxide by solid-state route. X-ray difractometry for its structural study and Scanning Electron Microscopy for the particle morphology characterized the resultant product. The prima facie observations revealed the formation of tetragonal crystallites of hydrothermal ZnBiVO4 ranging from 40-50nm. The BET surface area of hydrothermal ZnBiVO4 was increased 9 times as compared to solid state ZnBiVO4. The steepness of the UV-visible DRS Diffuse Reflectance Spectra) absorption edge, suggests the good crystalline nature of the material. From the photodecomposition of H2S, it is noteworthy that the hydrogen evolution was enhanced by 70% in hydrothermal ZnBiVO4.

Large hydrothermal ZnO crystals were grown using 3N NaOH, 1N KOH and 0.5N Li2CO3 mineralizer. The crystals were studied by cathodoluminescence (CL), showing a good crystalline quality. Different growth regions were identified by CL imaging. These regions were characterized by their corresponding luminescence spectra, showing that the incorporation of impurities and non radiative recombination centers depend on the growth sector. The surface is shown to introduce band tailing modifying the high energy part of the spectrum. The main spectral signatures of each sector are discussed.

Control of the synthesis of nanomaterials to produce morphologies exhibiting quantized properties will enable device integration of several novel applications including biosensors, catalysis, and optical devices. In this work, solvothermal routes to produce zinc oxide nanorods are explored. Much previous work has relied on the addition of growth directing/inhibiting agents to control morphology. It was found in coarsening studies that zinc oxide nanodots will ripen to nanorod morphologies at temperatures of 90 to 120 °C. The resulting nanorods have widths of 9-12 nm average dimension, which is smaller than current methods for nanorod synthesis. Use of nanodots as nuclei may be an approach that will allow for controlled growth of higher aspect ratio nanorods.

III-Nitrides (GaN, AlN and other compounds) have attracted vast interest due to their unique properties and potential applications in optoelectronic devices operating in the blue and UV spectral regions and for the construction of electronic devices capable of operating under high power and high temperature conditions.

Nanocrystalline AlN powder were obtained by AMMONO method, in which nitridization of Al metal occurs in highly chemical active supercritical ammonia using both NH4Cl as mineralizer. The experiments were performed in the temperature range of 350-550 °C and pressure of 80-120 MPa in stainless steel autoclaves for up to 5 days. Nanocrystalline AlN were spontaneously nucleated on the lower walls of the autoclaves. The obtained AlN powder was characterized by X-ray diffraction. Nanocrystalline AlN powders with average crystallite size 20-30 nm were produced in the temperature range of 450-550 °C.

Using a continuous-flow reactor, barium titanate (BaTiO3) nanoparticles have been successfully synthesized at temperatures ranging from 150 to 380°C and 16 MPa. Ba-Ti alkoxide solutions were used as precursors and water as reagent. The influence of synthesis parameters on the powder characteristics was investigated. Results showed that the purity, crystallinity and stoichiometry of the as-synthesized BaTiO3 powder depend mainly on the reactor temperature, quantity of water injected into the reactor and the Ba:Ti molar ratio of alkoxide solutions.