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Why does the hot-injection method (rapidly injecting precursors into a hot solvent) readily give nanoparticles of uniform size (monodisperse)?

  1. ANucleation is concentrated in a short burst right after injection; the supersaturation then falls, so no new nuclei form and only growth proceeds
  2. BAdding the precursor little by little to continue nucleation over a long time averages out the sizes of the nuclei born at different times, so they become uniform
  3. CLong holding at high temperature to promote Ostwald ripening lets small particles catch up with large ones, narrowing the distribution
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Answer: A. Nucleation is concentrated in a short burst right after injection; the supersaturation then falls, so no new nuclei form and only growth proceeds

The idea behind making monodisperse particles is explained by the LaMer model. Rapidly injecting precursors into a hot solvent pushes the monomer concentration above the critical supersaturation for nucleation, and many nuclei form all at once in a short time (burst nucleation). Once nuclei form, monomer is consumed and the concentration drops below the critical level, so no new nuclei form afterward, and the existing nuclei grow alike, giving uniform sizes. If nucleation continues for a long time, nuclei born early and late differ in size, so the distribution actually broadens. Ostwald ripening is the dissolution of small particles while large ones grow, not small particles catching up; when it proceeds in the monomer-depleted stage, the distribution generally broadens.

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