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Why do nanoparticles tend to form clumps in composite materials?

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Gold and palladium alloy nanoparticles — nanoparticle Image showing nanoparticles of an alloy of gold (yellow) and palladium (blue) on an acid-treated carbon support (gray). These particles were employed as catalysts for the formation of hydrogen peroxide from hydrogen (white) and oxygen (red).

Nanoparticles are notoriously prone to agglomeration, a process where they form large, difficult-to-disperse clumps when incorporated into composite materials.1

This tendency is primarily driven by the particles' physical properties. Nanoparticles possess an enormous specific surface area, meaning a significant percentage of their atoms are located at the surface. Because of this high surface-to-volume ratio, the particles are highly susceptible to van der Waals forces—weak, attractive intermolecular forces that arise from fluctuations in electron distribution. These forces cause the particles to attract and stick to one another, leading to the formation of aggregates or agglomerates.231

Successfully incorporating nanoparticles into composites is often challenging because these clumps can prevent the material from retaining the unique, size-related properties that make nanoparticles valuable in the first place.1