Modeling of atomic layer deposition on nanoparticle agglomerates

Nanoparticles are increasingly applied in a range of fields, such as electronics, catalysis, energy and medicine, due to their small sizes and consequent high surface-volume ratio. In many applications, it is attractive to coat the nanoparticles with a layer of different materials in order to gain new functionalities. For instance, a coated layer can modify the chemical properties of the nanoparticles, protect the core material resulting in increased stability, facilitate the biofunctionalization, etc. Atomic layer deposition (ALD) is a gas-phase technique that can form an ultrathin solid film on a range of substrates. It utilizes two self-limiting surface reactions applied in an alternating sequence. By controlling the number of applied cycles, the thickness of the coated layer can be controlled with nanometer precision. Several experimental reports in literature have shown that applying ALD to nanoparticles using a fluidized bed is a promising way of producing large quantities of coated nanoparticles. Fluidization is a gas-phase technique that can process large quantities of particles by suspending them in an upward gas stream. It provides good gas-solid mixing, scale-up potential, and allows continuous processing. However, due to the strong cohesive forces between particles, nanoparticles cluster into large agglomerates when fluidized. These agglomerates have a complex, hierarchical structure, which has been commonly described as fractal for their self-similarity under different length scales. During the ALD process, the precursors have to diffuse into such structures to reach the surface of inner particles