Development of a novel theory of pressure-induced nucleation in supercritical carbon dioxide

Abstract

Nucleation is the basis of the fabrication of two-dimensional materials in the bottom-up methods such as chemical vapor deposition and atomic layer deposition. Supercritical fluid deposition (SCFD) might provide an alternative to these techniques owing to the excellent physicochemical properties and adjustable solvent power of supercritical fluids. However, classical nucleation theory (CNT) was not suitable for the nucleation in supercritical fluid (SCF) because of the non-ideality of SCF. Herein, a dilute solution system composed of a nonvolatile solute and supercritical CO₂ (scCO₂) was established and the theory of pressure-induced supercritical phase nucleation (PI-SCPN) was proposed. A newly defined solute–solvent correlation function was found to influence the nucleation driving force a lot in this particular process, especially when the supersaturation was small. It was also found that at high temperature and high pressure, the barrier of nucleation was low, which was conducive to the formation of a nucleus. Furthermore, the corresponding experiments were conducted using MoO₂(acac)₂ as the solute and scCO₂ as the solvent to verify the proposed theory. The solubility of MoO₂(acac)₂ was accurately measured by the static equilibrium method. The influence of supersaturation, temperature and pressure on nucleation was found to be consistent with that predicted using the proposed theory. The nucleation rates calculated by PI-SCPN were compared to the experimental values, showing good agreement, and the errors were within one order of magnitude.