**2.4 Coprecipitation method**

Another classic soft chemistry synthesis method commonly developed in the chemical industry is coprecipitation. This method involves bringing into play several metal cations and a precipitating agent, which is most commonly ammonium bicarbonate (NH4HCO3) or ammonium hydroxide (NH4OH). The use of ammonium bicarbonate results in powders, which are less agglomerated than with ammonium hydroxide or urea [30].

The coprecipitation method could be performed by:


Synthesis by coprecipitation allows obtaining homogeneous ceramics and a well-controlled particle size. Precipitation is characterized by two main processes: nucleation, the birth of crystals, and crystal growth. These two phenomena determine the size distribution of the crystals. Indeed, nucleation corresponds to the formation of the smallest crystalline entity called "germ" or "nucleus". Once supersaturation is established (supersaturation is the driving force behind crystallization processes), it takes a certain time, called the induction time, for germs to appear. Crystal growth corresponds to the spontaneous evolution of germs of critical size towards a state of greater stability [31].

The coprecipitation method offers some advantages:


However, it has some disadvantages:


For example, the synthesis of the Ce0.5Fe0.5O2, cerium and iron nitrates taken in stoichiometric proportions are mixed with stirring. The basic precipitation medium consists of an aqueous solution of ammonia. The precursors are then precipitated dropwise from the reaction medium with vigorous stirring. The solution is left to mature for 2 hours at room temperature before being washed with an aqueous ammonia solution. The powder, strongly hydrated, is then placed in an oven overnight. After a grinding step, a final calcination step at 600 °C for 5 hours, to remove traces of nitrates, would be necessary [32].

#### **2.5 Hydrothermal method**

In a sealed vessel (bomb, autoclave), solvents can be brought to temperatures well above their boiling points by the increase in autogenous pressures resulting from heating. Performing a chemical reaction under such conditions is referred to as solvothermal processing or, in the case of water as solvent, hydrothermal processing.

Hydrothermal reactors are mostly metal autoclaves with Teflon or alloy linings or containing an extra beaker or tube made of Teflon, platinum, or silver to protect the autoclave body from the highly corrosive solvent, which is held at high temperature and pressure.

The influence of the conditions of hydrothermal synthesis (pH, temperature, the nature of the precursors or the presence of surfactant) on the morphology and size of the particles has been investigated by several researchers. Indeed, a high pH favors the formation of nanoparticles. On the other hand, a lower pH favors the formation of nanoparticles, which is explained by the evolution of the nature and the proportion of soluble species in solution. The increase in temperature causes an increase in particle size with a change in morphology [33, 34]. The synergistic effect of high temperature and pressure provides a one-step process to produce high crystalline materials without the need of post-annealing treatments. The hydrothermal method becomes useful when it is difficult to dissolve the precursors at low temperatures or room temperature.it prove to be useful to grow nanoparticles if the material has a high vapor pressure near its melting point or crystalline phases are not stable at melting point [35].

In addition, the size of the particles of the precursor is an important parameter in controlling the morphologies obtained. Nanotubes resulting from the hydrothermal synthesis of particles of small grain size (8–10 nm) have been proven to have a larger outer diameter (10–30 nm), thicker layers and an ill-defined tubular shape or incomplete. In contrast, large particles (~ 200 nm) lead to the formation, not of nanotubes, but rather of layers [36].

The main disadvantage of hydrothermal method is the high cost of equipment. However, it has many advantages; it could generate nanomaterials, which are stable at elevated temperatures. In addition, it creates larger sized and high-quality crystals and nanoparticles. In addition, this method could be combined with other process like microwave, electrochemistry, ultrasound, optical radiation and hot

**Figure 2.** *TEM images of hydrogenotitanate prepared by hydrothermal method (a) nanotubes and (b) nanowires.*

pressing. By varying the synthesis conditions (t, T p, etc.) it is possible to vary phase composition and particle size and to change the morphology. Hydrogenotitanate nanotubes and nanowires (**Figure 2**), used as supports for ruthenium catalysts, were obtained by hydrothermal method using highly concentrated NaOH and KOH, respectively [37].
