When an aqueous metal salt solution is heated, hydroxides and oxides become more thermodynamically stable than in the ionic state. Hydrothermal synthesis reactions can be thought of as hydrolysis and dehydration reactions. Particle synthesis of oxides and hydroxides by hydrothermal treatment takes advantage of this equilibrium shift.
ＭＡx ＋ xＨ₂Ｏ ＝ Ｍ(ＯＨ)x ＋ xＨＡ、Ｍ(ＯＨ)x ＝ ＭＯx/2 ＋ x/2Ｈ₂Ｏ
Where A is an anion like Cl⁻, NO₃⁻, or SO₄⁻.
Followings are the typical examples.
| Raw material | Particle | Diameter [nm] | Temperature [℃] | Pressure [MPa] | Forms |
| Al(NO₃)₃ | AlOOH | 50-600 | 250-400 | 25~40 | Hexagonal plate |
| Ce(NO₃)₃ | CeO₂ | 20-300 | 250-400 | 30 | Octahedron |
| Co(NO₃)₃ | Co₃O₄ | ~100 | 400 | 35 | Octahedron |
| Fe(NO₃)₃ | α-Fe₂O₃ | ~50 | 400 | 35 | 球状 |
| Gd(NO₃)₂ | Gd(OH)₃ | ~20 | – | – | |
| Ni(NO₃)₂ | NiO | ~200 | 400 | 35 | Octahedron |
| Ni(OAc)₂+H₂ | Ni | ~500 | – | – | |
| Ti(SO₄)₂ | TiO₂ | ~20 | 400 | 35 | Spherical |
| Zn(NO₃)₂ | ZnO | ~20 | – | – | |
| ZrOCl₂ | ZrO₂ | ~10 | 400-490 | 30 | Spherical |
| Raw material | Particle | Diameter [nm] | Temperature [℃] | Pressure [MPa] | Form |
| Fe(NH₄)₂H(C₆H₅O₇)₂ | Fe₃O₄ | ~50 | 400 | 35 | Spherical |
| Al(NO₃)₃, Co(NO₃)₂ | CoAl₂O₄ | ~10 | – | – | |
| Ba(NO₃)₂, Fe(NO₃)₃ | BaO₆Fe₂O₃ | 50-300 | 400 | 30 | |
| Ba(NO₃)₂, Ti(SO₄)₂ | BaTiO₃ | 50-1,000 | – | – | |
| Co(OAc)₂, Fe(OAc)₂ | CoFe₂O₄ | 40-70 | 200-400 | 25 | |
| LiOH,Co(NO₃)₂ | LiCoO₂ | 50-1000 | 300-400 | 100 | |
| Li salt, phosphate, etc | LiFePO₄ | 350 | 450 | – | |
| LiOH,Mn(NO₃)₂,H₂O₃ | LiMnO₄ | 20-40 | 400 | 30 | |
| Ni(OAc)₂, Fe(OAc)₂ | NiFe₂O₄ | 30-40 | 190-400 | 25 | |
| Fe(NO₃)₃, Sr(NO₃)₂ | SrFe₁₂O₁₉ | 200 | 400 | 30 |
References: Journal of the Soceity of Inorganic Matericals, Japan 12,429 (2005) Table 1
高圧力の科学と技術 vol.20,3 (2010), 超臨界流体とナノテクノロジー, CMC publishing, p103 table.1 etc. (in Japanese)