Inorganic nanochemistry: Probing size effects in metal oxide nanoparticles

Theory and indirect evidence have long predicted that reducing metal oxides to ultrasmall sizes (<5 nm) could lead to unexpected properties driven by structural and electronic changes.

However, unlike in metals and semiconductors, the effects of metal oxide size confinement have yet to be directly investigated, largely due to challenges in synthesizing ultrasmall metal oxide nanoparticles with narrow size distributions and high crystallinity.

“Conventional batch syntheses (where all reagents are mixed, heated, and reacted together in closed vessels under static conditions) often produce particles with broad size distributions and unpredictable properties because of uncontrolled side reactions and unsteady heating,” explains Akira Yoko, a member of an AIMR research team. “Therefore, a different synthetic approach is needed to probe the size effects of metal oxide nanoparticles.”

In a 2024 article, Yoko et al. addressed this challenge by developing a continuous-flow supercritical hydrothermal synthesis method that enabled precise control over nanoparticle size down to 1.3 nm. The method was designed both to stabilize the nanoparticles using organic modifiers and to control the growth rate through reaction temperature and modifier concentration.

To achieve this, the team combined a flow-reactor process designed for rapid heating, with organometallic chemistry tailored to control how Ce is introduced to the reaction—enabling high conversion rates, fine size control, and reproducible synthesis of ultrasmall CeO₂ nanoparticles.

The study revealed that CeO₂ nanoparticles smaller than 4 nm exhibited dramatic lattice expansion and stacking faults resulting from particle coalescence during early-stage growth. Moreover, the team observed that even without oxygen vacancies, Ce³⁺ electronic states emerged due to oxygen disordering—a previously unreported phenomenon.

“Our findings are significant because they demonstrate that the structural distortion and electron localization observed in CeO₂ nanoparticles arise from nanoscale effects rather than conventional defect chemistry,” says Yoko. “These results open new possibilities for tuning the electronic and catalytic properties of metal oxides by controlling size and growth dynamics at the atomic scale.”

A personal insight from Dr. Akira Yoko

What result or discovery surprised you the most, and why?

We started out trying to understand the organic modification mechanism in hydrothermal reactions. That led us to study organic–inorganic complex chemistry and eventually improve the precursor for continuous-flow synthesis. What surprised us was that, by precisely controlling the reaction time—down to 40 milliseconds—we could continuously synthesize ultrasmall metal oxides, even smaller than 2 nanometers, which we didn’t expect. The biggest surprise was that these tiny particles had high crystallinity but still showed large structural distortion that caused electron localization without needing lots of oxygen vacancies. We also explored fusion growth, which was fascinating. This method should work for many other metal oxides.

This article was written by Patrick Han, Ph.D. (patrick@sayedit.com).

About the World Premier International Research Center Initiative (WPI)

The WPI program was launched in 2007 by Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) to foster globally visible research centers boasting the highest standards and outstanding research environments. Numbering more than a dozen and operating at institutions throughout the country, these centers are given a high degree of autonomy, allowing them to engage in innovative modes of management and research. The program is administered by the Japan Society for the Promotion of Science (JSPS).

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Advanced Institute for Materials Research (AIMR)
Tohoku University
Establishing a World-Leading Research Center for Materials Science
AIMR aims to contribute to society through its actions as a world-leading research center for materials science and push the boundaries of research frontiers. To this end, the institute gathers excellent researchers in the fields of physics, chemistry, materials science, engineering, and mathematics and provides a world-class research environment.