Synthesis of Noble Metals and Their Alloy Nanoparticles by Laser-induced Nucleation in a Highly Intense Laser Field

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The synthesis of noble metals and their alloy nanoparticles by laser-induced nucleation is described. Femtosecond laser pulses with an energy on the order of mJ were tightly focused to create an intensity of 10¹⁴ W/cm² or more in an aqueous solution of noble metal ions. The intense laser field generated solvated electrons and hydrogen radicals that have a highly reducing ability, resulting in nucleation through the reduction of the noble metal ions and particle growth through ripening. This laser-induced nucleation method can be performed without any reducing agents. Excess irradiation of chloroauric acid solution led to the formation of a stable colloidal solution of gold nanoparticles without any surfactants. Additionally, the irradiation of a mixed solution of different noble metal ions formed solid–solution alloy nanoparticles, even though these metals were immiscible in the bulk. Moreover, the laser-induced nucleation made it possible to form quinary solid-solution alloy nanoparticles of noble metals. The mechanism of superior catalytic activity found for alloy nanoparticles by using Rh–Pd–Pt solid–solution nanoparticles is discussed in terms of elemental distributions inside the particles.