The present invention is for a porous silicon powder comprising silicon particles wherein the outermost layers of said particles are porous. The present invention is also directed to a method of making this porous silicon powder using a stain etch method. The present invention is also directed to a method of making silicon nanoparticles from the porous silicon powders using a process of ultrasonic agitation. The present invention also includes methods of processing these silicon nanoparticles for use in a variety of applications.