Compositions are provided that can include nanoscale particles including metal cations such as cerium having an average particle size of less than 10 nm. The nanoscale particles can include cerium and oxygen.Methods for forming nanoparticles are provided. The methods can include exposing a metal cation within a solution to radiation to form metal nanoparticles that can include metal cations. The methods can include exposing a cerium salt solution to radiation to form the nanoparticles. The methods can include exposing solvated metal cations to radiation to precipitate nanoparticles that include metal cations such as Ce. The methods can include exposing the homogeneous solution to radiation to precipitate nanoparticles. The methods can include: providing an aqueous solution comprising metal cations； and increasing the pH of the aqueous solution with radiation to form nanoparticles that include metal cations.Nanoparticle generators are provided. The generators can include: a reactant reservoir comprising a metal cation in solution； a fluid cell in fluid communication with the reactant reservoir； a radiation source operatively aligned with the fluid cell； and a product reservoir in fluid communication with the fluid cell.
