## 3 Micro- and Nanoparticles Generating Procedure in Sub- and Supercritical Water
### Types of Particles Generated and Their Applications  
Some of the most relevant groups of particles generated by means of hydrothermal synthesis can be seen below [17]:  
* Simple oxides such as, for instance, CoO\({}_{2}\), ZnO, CeO\({}_{2}\), TiO\({}_{2}\), ZnO, ZrO\({}_{2}\), CuO, Al\({}_{2}\)O\({}_{3}\), In\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), etc. Simple oxide particles comprise one of the largest inorganic compound groups. * Multiple oxide particles (La\({}_{2}\)CuO\({}_{4}\), etc.) comprise one of the most important groups with regards to the number of studies and synthesized particles. * Particles with elements from II to VI: CdS, PbS, ZnS, CuS, NiS\({}_{2}\), Bi\({}_{2}\)S\({}_{3}\), CdSe, Bi\({}_{2}\)Te\({}_{3}\), CoSb\({}_{3}\), PbTe, etc. * Phosphates, silicates, borates, and carbonate particles (Zr(PO\({}_{4}\))\({}_{3}\), LiFePO\({}_{4}\), Li\({}_{3}\)Nd\({}_{2}\)(BO\({}_{3}\))\({}_{3}\), Li\({}_{2}\)SiO\({}_{3}\), etc.) were and still are of great interest because of their ceramic, optical, piezoelectric, ionic, luminescent, and magnetic properties. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Principal Investigators** & **Research Centers** & **Articles** \\ \hline \multicolumn{3}{c}{**Europe**} \\ \hline [PERSON], [PERSON] & Christian-Albrechts-Universität zu Kiel, Kiel, Germany & [76] \\ [PERSON], [PERSON] & Universität Heidelberg, Heidelberg, Germany & [77, 78] \\ [PERSON], [PERSON] & University College of London, London, United Kingdom & [79, 80] \\ [PERSON], [PERSON]. & The University of Warwick, Coventry, United Kingdom & [81, 82] \\ [PERSON], [PERSON] & Nottingham University, United Kingdom & [83, 84] \\ [PERSON] & Aarhus Universität, Aarhus, Denmark & [85, 86] \\ [PERSON] & The Institute of Condensed Matter Chemistry of Bordeaux, CNRS and Bordeaux University, France & [87, 88] \\ \hline \multicolumn{3}{c}{**Russia**} \\ \hline [PERSON], [PERSON]. & Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow & [89, 90] \\ \hline \multicolumn{3}{c}{**Taiwan**} \\ \hline [PERSON], [PERSON] & National Taipei University of Technology, Taipei, Taiwan & [91] \\ \hline \multicolumn{3}{c}{**Australia**} \\ \hline [PERSON], [PERSON] & University of Technology Sydney, Sydney & [92, 93] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cont. * Fluoride particles with transition metals are particles with great physical, ionic, and magnetic properties ((TiOF4)2, AlF3, etc.). * Fluorine particles, phosphates and fluorine, and carbonates (M+M(PO4)F, M3 Ln2(CO3)4F) are often materials with high chemical and thermal stability. They also show great magnetic and electrical properties. * Tungsten particles either simple or multiple (M(WO4)2) exhibit great luminescent and electrical properties. * Molybdenum particles (MxMoO4) are closely related to Tungsten and present important optical, magnetic, and electrical properties. * Metal particles such as Ag or Co.
* Metal oxide particles hybridized with organic compounds. Organic and inorganic hybrid nanoparticles are thought to be the most promising type of materials because exhibit a combination of properties between organic and inorganic polymers (high mechanical resistance, high conductivity, and high electricity resistance) [94]. Figure 8 shows the formation process. At first, and after increasing the temperature from the environmental conditions to the near the critical point, the hydrolysis and dehydration of the metal salt occurs (Mn+), as shown in Equations (1) and (2) of the Section 3. Once the supercritical conditions are reached, organic surface modifiers may form a homogeneous phase with supercritical water. After nucleation has taken place and supercritical conditions have been reached, the dissolved tensioactive compounds are adsorbed into the nucleus (hydrophilic metal oxides). Under these conditions an extremely strong bond is formed on the surface, and the stabilization of the nanoparticles leads to size reduction.