Continuous synthesis of surface-modified nanoparticles in supercritical methanol: A facile approach to control dispersibility

Surface-modified cerium oxide (CeO2) nanoparticles, dispersible in either a hydrophilic or a hydrophobic medium, are synthesized continuously in supercritical methanol using methoxy polyethylene glycol (PEG350, H(OCH2CH2)nOCH3, MW = 350 g/mol) or α,ω-Bis(2-carboxymethyl)polyethylene glycol (PEG600, HOOCCH2(OCH2CH2)nOCH2COOH, MW = 600 g/mol) as a surface modifier. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the surface modifiers inhibit the growth of the particles, resulting in smaller size particles (20–25 nm) as compared to unmodified particles (35 nm) synthesized in supercritical methanol or unmodified particles (91 nm) synthesized in supercritical water. At a high concentration of PEG600 (0.3 M), surface-modified particles with a size of 3–4 nm and a low degree of aggregation are produced. An X-ray diffraction analysis reveals that the PEG-modified nanoparticles retain the CeO2 phase. Fourier transform infrared spectroscopy and a thermal gravimetric analysis indicate that the amount of modifier attached to the surface of the nanoparticles is 6.88%, when 0.3 M of PEG350 is used, and 4.49%, when 0.3 M of PEG600 is used. A long-term stability test (40 days) revealed that the PEG350-modified CeO2 nanoparticles have good dispersibility in a hydrophobic medium (oil), while PEG600-modified CeO2 nanoparticles have good dispersibility in a hydrophilic medium (water). This indicates that the nanoparticle dispersibility in either a hydrophilic or hydrophobic medium can be controlled by adjusting the end-group functionality and chain length of the modifiers.

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► One-pot, continuous, surface modified CeO2 nanoparticles in supercritical methanol.
► Polyethylene glycol-based molecules used as the surface modifiers.
► Surface modifier inhibited particle growth.
► Dispersibility in a hydrophobic medium or in a hydrophilic medium can be controlled.