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<p>Titanium dioxide (TiO₂) nanoparticles were synthesized via a supercritical anti-solvent (SAS) process using carbon dioxide (CO<sub>2</sub>) and isopropanol (IPA), and the relationships between processing conditions, particle aggregation, and crystallinity were systematically elucidated. The influence of reaction temperature, pressure, surfactant concentration, holding time, and depressurization rate were examined to establish a process–structure–crystallinity framework for SAS-derived TiO₂ nanoparticles. The results reveal that both aggregation state and crystallinity are predominantly determined during in-reactor particle formation, governed by the coupled effects of thermodynamics, interfacial phenomena, and kinetic processes. Elevated temperatures favor anatase-phase crystallization, whereas increased pressures promote particle aggregation and suppress crystallinity, which is attributed to reduced molecular diffusivity and enhanced collision frequency in the supercritical medium. Surfactant concentration exhibits a non-monotonic effect on dispersion behavior: insufficient surface coverage leads to poor stabilization. At the same time, excessive loading induces polymer-mediated bridging, with an intermediate concentration providing optimal dispersion. Holding time emerges as a critical kinetic parameter: short residence times favor nucleation-dominated particle formation, whereas prolonged residence promotes irreversible aggregation via condensation-driven necking. In contrast, depressurization rate and precursor concentration exert secondary influences within the studied parameter space. Thermal and surface characterization confirm high specific surface areas and residual surfactant incorporation, while post-synthesis calcination enhances crystallinity without fundamentally altering aggregation states established during SAS processing. Based on these findings, a qualitative process map, together with a conceptual framework and processing insights, is proposed for controlling aggregation and phase formation in SAS-derived TiO₂ nanoparticles.</p>

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Controlling particle aggregation behavior and crystallinity of TiO₂ nanoparticles in supercritical anti-solvent processing

  • 期刊论文

  • 发表日期：

    2026年4月15日

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  • Rajiv Kamaraj,
  • Hansung Lee,
  • Yi-Jen Huang,
  • Yu-Wei Chang,
  • Ruey-Chi Hsu,
  • Byungmin Ahn

摘要

Titanium dioxide (TiO₂) nanoparticles were synthesized via a supercritical anti-solvent (SAS) process using carbon dioxide (CO2) and isopropanol (IPA), and the relationships between processing conditions, particle aggregation, and crystallinity were systematically elucidated. The influence of reaction temperature, pressure, surfactant concentration, holding time, and depressurization rate were examined to establish a process–structure–crystallinity framework for SAS-derived TiO₂ nanoparticles. The results reveal that both aggregation state and crystallinity are predominantly determined during in-reactor particle formation, governed by the coupled effects of thermodynamics, interfacial phenomena, and kinetic processes. Elevated temperatures favor anatase-phase crystallization, whereas increased pressures promote particle aggregation and suppress crystallinity, which is attributed to reduced molecular diffusivity and enhanced collision frequency in the supercritical medium. Surfactant concentration exhibits a non-monotonic effect on dispersion behavior: insufficient surface coverage leads to poor stabilization. At the same time, excessive loading induces polymer-mediated bridging, with an intermediate concentration providing optimal dispersion. Holding time emerges as a critical kinetic parameter: short residence times favor nucleation-dominated particle formation, whereas prolonged residence promotes irreversible aggregation via condensation-driven necking. In contrast, depressurization rate and precursor concentration exert secondary influences within the studied parameter space. Thermal and surface characterization confirm high specific surface areas and residual surfactant incorporation, while post-synthesis calcination enhances crystallinity without fundamentally altering aggregation states established during SAS processing. Based on these findings, a qualitative process map, together with a conceptual framework and processing insights, is proposed for controlling aggregation and phase formation in SAS-derived TiO₂ nanoparticles.