Hydrothermal Synthesis of Transition Metal Oxides

1.3 Properties of Hydrothermal Solvents

Most of the inorganic solids are prepared by the reaction of a solid with another solid, a liquid (melt) or a gas, usually at high temperatures. Many “solid/solid” reactions are actually “solid/liquid” reactions, because at the high reaction temperature one of the solid can melt to form liquid phase. Therefore it is sometimes difficult to determine what physical phases are involved in a given reaction.

Solids do not react with each other at room temperature, high temperatures are required to reach the suitable reaction rates. The reason [22] for using high temperature mainly is that if there is a large difference between the structure of the starting material and the product, all bonds in the starting material must be broken, and atoms must migrate before new bonds can be formed. This diffusion makes the reactions impossibly slow unless very high temperatures are used. As a rule of thumb two-thirds of the melting temperature of one component is enough to activate diffusion sufficiently and hence to enable the solid state reaction [23]. Most compounds made at high temperature are thermodynamically very stable. Because thermodynamically stable known phases cannot be avoided, the synthesis of new materials becomes difficult at high temperatures [24]. However synthesis of new kinetically stable or metastable compounds can be possible if the proper reaction conditions can be found. Preparation of kinetically stabilized compounds requires relatively lower temperatures because the desired compounds are not thermodynamically stable.

Hydrothermal solvents have different properties at above 100^oC and above 1 atm, especially at critical point. In order to understand hydrothermal reactions the properties of solvent under hydrothermal conditions must be known very well.

In the Figure 1.3, the critical point marks the end of liquid-vapor coexistence curve at the critical temperature, T_c, and pressure, P_c, in a phase diagram for a pure homogenous substance. A fluid is defined as being supercritical if it is maintained at conditions above its critical temperature and pressure. The properties of supercritical fluids (SCFs) vary depending on the pressure and temperature and frequently described as being intermediate between those of a gas and a liquid. As the temperature increases, the liquid becomes less dense due to thermal expansion and at the same time the gas becomes denser. At the critical point the densities of both phases become the same. The compound is neither liquid nor gas any longer above the critical point, and it becomes supercritical fluid. After that, the phases of liquid and gas are not distinguishable and properties of SCF will be between gas and liquid.

The dielectric constant that is defined as the ability of a solvent to charge separate increases sharply with the pressure in the compressible region that refers to the area around the critical point in which compressibility is considerably greater than would be forecasted from the ideal gas law. This behavior is also parallel to a change in density, as shown in Figure 1.4. Density changes sharply but continuously with pressure in the compressible region. One of the most important advantages of hydrothermal solvents is that a change in density affects the solvating power. A decrease in the density results in a significant change in solvating ability.

Diffusivity and viscosity symbolizes transport properties that influence rates of mass transfer. These features are at least an order of magnitude higher and viscosity is lower compared with a liquid solvent. This means that diffusivity of species in SCF will occur faster than that obtained in a liquid solvent, which means that solids can dissolve and migrate more rapidly in SCFs. High diffusivity, low viscosity and intermediate density increases the rate of the reaction [25, 18].

The low viscosity and high mobility of supercritical water allows it to be excellent reaction media for the synthesis of unique metastable phases and the growth of good quality single crystals for analysis. Superheated water has the ability to solvate reagents and is a good reaction media for better transport and for intermixing of reagents.

Conditions between 100-150^oC and 150-375^oC are called superheated and hydrothermal, respectively. The properties of hydrothermal solutions carry its characteristics as the same as its supercritical state.

All properties mentioned above allow us to synthesize new good crystals. During this work, all experiments were carried out in the temperature range between 170-200^oC.

Figure 1.3 Phase diagram of water.

Figure 1.4 Variation of dielectric constant of water with temperature and pressure.

1.4 Water as a Reaction Medium in Hydrothermal Synthesis

Water is one of the most important solvents in nature, and has remarkable properties as a reaction medium under hydrothermal conditions where it acts very differently from water at standard conditions.

One of the biggest advantages of using water is the environmental benefit and cheaper than other solvents, and it can act as a catalyst for the formation of desired materials by tuning temperature and pressure. It is nontoxic, nonflammable, noncarcinogenic, nonmutagenic, and thermodynamically stable. Another advantage is that water is very volatile, so it can be removed from the product very easily [25].

The physical and chemical properties of water and aqueous solutions in the temperature and pressure ranges required for hydrothermal synthesis have been discussed in numerous review articles and are well known. The PVT data for water up to 1000^oC 10 kbar are known accurately enough (within 1% error) [26]. Ion product increases sharply with pressure and temperature. At very high PT conditions (150-200 kbar and 1000^oC), water is completely dissociated into H₃O⁺ and OH^- , behaving like a molten salt, and has a higher density of the order of 1.7-1.9 g/cm³. If the density of water is high enough, nonpolar compounds may be completely miscible with it because water behaves as a nonaqueous fluid. Water is a polar solvent and its polarity can be controlled by temperature and pressure and this can be an advantage over other solvents.

For experimental hydrothermal synthesis, to understand the PT behavior of water, it is first necessary to know how it behaves under various conditions of pressure, volume, and temperature. A detailed study of pressure-temperature behavior of water was reported by Laudise [27]. If the autoclave is filled initially to 32%, the liquid level remains until the critical temperature (as shown in Figure 1.5). At the critical point of water, the density of both the gas and liquid is 0.32 g/cm³. When filled more than 32% with water, the autoclave is filled at temperatures before critical temperature.



Figure 1.5 Presentation of the P-T behavior of water at various degrees of fill.
When filled less than 32%, the liquid level drops as temperature rises and gas fills the autoclave at temperatures below critical temperature and liquid is lost. The higher percentage of fill, the lower the temperature at which the autoclave becomes filled with liquid [27].

Usually, in most routine hydrothermal experiments, the pressure prevailing under the working conditions is determined by the degree of filling and the temperature.