Understanding the Magnesiothermic Reduction to Form Porous Silicon Nanoparticles for Stoichiometric Reactions

Abstract

Porous Si NPs have been explored in applications such as optics, sensing, gas storage, drug delivery, high-capacity anode material, photocatalysts for H2 evolution and for the reduction of CO2. The pore network of porous Si NPs allows the material to possess high surface area which is useful in a wide variety of applications. The magnesiothermic reduction is a facile method to produce porous Si from synthetic, natural, and waste SiO2 sources. Typically, the reaction takes place at 650 ◦C and is held for 6 h. This method is energy intensive, causes sintering of the Si particles and destruction of pore networks which reduces the specific surface area.

To resolve these issues, a two-step magnesiothermic reduction method can be used to prepare porous Si NPs with morphology retention and high specific surface areas at a lower energy requirement. Although the two-step magnesiothermic reduction has resolved many issues with the traditional reaction, it is still extremely sensitive to reaction conditions and can be irreproducible. Attempting to understand and resolve these issues, we discovered that a major factor that influences the reaction is the precursor particle size. To better understand the effect that the precursor size has on the reaction, in-situ X-ray diffraction studies were performed. The effect of the Mg size on the physical properties of the material was then determined through material characterization. The materials performance for stoichiometric H2 evolution applications in various water types was investigated with respect to Mg particle size used for the reduction. Finally, the Si NPs were functionalized with weakly hydridic H’s and investigated for the stoichiometric hydrodefluorination of fluorinated organics.

From these studies, the sensitivity of the magnesiothermic reduction is a benefit as it allows for a high degree of tunability of the final properties of the porous Si material. By changing specific reaction parameters, you can obtain porous Si NPs with desired properties for a given application, such as, for stoichiometric reactions as is highlighted in this study. This work attempts to improve the understanding of the magnesiothermic reduction so we can better design high performing porous Si material for various applications.