Monodispersity: size means nothing without dispersion
When dealing with nanoparticles, size usually refers to a number average. This is obtained either directly, by measuring the diameter of a sufficiently large number of individual particles using electron or probe microscopy techniques, or indirectly, by analysing data from X-ray diffraction or light scattering.
In both cases, the result is a histogram representing the distribution of nanoparticle sizes, also referred to as dispersity, across a range of intervals. A perfectly monodispersed distribution means that all nanoparticles share the same size: for most applications, this is the ideal condition, as a collection of identical particles is expected to be fully homogeneous in terms of size-dependent physicochemical properties.
Given the complexity of nanoparticle synthesis, achieving narrow size distributions is extremely challenging and should never be taken for granted. For this reason, any indication of size should always be accompanied by a measure of monodispersity, typically expressed as a ± variation.
A broad size distribution implies that, for a given average size, there exist significant populations of both larger and smaller nanoparticles. Each of these tails affects the final properties in distinct ways: larger nanoparticles settle faster, block wider pores and channels, scatter more light, and present a lower specific surface area; smaller nanoparticles are harder to precipitate, exhibit enhanced quantum behaviour, and have a higher specific surface area, leading, for instance, to more pronounced corrosion effects. This inhomogeneity translates directly into processing difficulties and inconsistent performance in the final product.
