Continuous-reactor, pH-controlled synthesis of multifunctional mesoporous nanocrystalline anatase aggregates

• Nanoanatase is synthesized in aqueous solution in a CSTR.
• pH control and low supersaturation promote anatase nucleation.
• Nanocrystals assemble into high surface area, mesoporous aggregates.
• At higher pH, surface area increases, with a decrease in crystallinity.
• Synthesized anatase is successfully utilized in energy applications.

Anatase is an important functional nanomaterial in a wide variety of applications. Despite a breadth of research on novel synthesis methods, there is a lack of studies geared towards developing scalable processes for nanoscale anatase crystalline materials. In this study, anatase secondary nanoparticles are produced in a continuous stirred-tank reactor (CSTR) through forced hydrolysis of pre-diluted aqueous TiCl4 solution by controlling pH. The steady-state supersaturation regime of the CSTR process is shown to be inducive to the formation of phase-pure nanocrystalline anatase aggregates in the order of ∼500–1000 nm possessing a very high surface area ∼200–250 m2/g. The aggregates made of ∼6 nm individual primary nanocrystallites are mesoporous – a highly desirable property in building photocatalytic and thin film electrode (Li-ion and solar) devices with enhanced interfacial functionalities. The new process was tested at a range of conditions, 70–90 °C, 0.1–0.5 M TiCl4, pH 2–5, and 30–60 min. pH control was vital in stabilizing the metastable anatase nanocrystallites and promoting their aggregation that facilitates their handling and application. The process achieves >98% steady-state conversion of TiCl4(aq) to nanoanatase without the use of toxic organic chemicals constituting a cost-effective and green alternative to commonly used sol–gel or hydrothermal technologies.

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