At temperatures above 31 ºC and pressures greater than 7.4 MPa, CO₂ becomes a supercritical fluid. Supercritical CO₂ has densities intermediate between those of liquids and gases, but transport properties (diffusivity and viscosity) similar to gases.

Figure 1 shows the PT diagram of CO₂ , showing the regions of coexistance of solid, liquid, vapour, and supercritical fluid (SCF) phase. 

All these properties and the low surface tension of CO₂ with solid surfaces allow to introduce metal precursors dissolved in CO₂ inside porous materials in a very efficient way. Furthermore, the properties of CO₂ can be tuned through small changes of pressure and temperature, changing the final properties of the material.

CO₂ is considered a green solvent because it has moderate critical parameters, it is cheap, non-toxic, inert, nonflammable and it can be recycled. Furthermore, CO₂ is a gas at atmospheric pressure that can be easily released from the material by depressurization and it does not leave any residue.