Safe and cost-effective synthesis of porous silicon using Urea: Structural, morphological, and porosity analysis

Abstract

Porous silicon (Si) has gained significant interest in various applications due to its high surface area, tunable pore structure, excellent chemical reactivity, biocompatibility, and surface functionalization potential. Traditional methods for synthesizing porous Si often rely on hydrofluoric acid, a hazardous chemical that poses significant environmental and safety risks, limiting its scalability and sustainability. In this study, a green and scalable approach for synthesizing porous Si microparticles through urea-assisted etching is presented and evaluated as a function of temperature and container conditions (crucible vs. autoclave). The urea etching transformed pristine silicon microparticles, with a non-porous structure and a BET surface area of 2.3 m²/g, into porous silicon with surface areas as high as 26.7 m²/g. The highest porosity was achieved at 400 °C, while higher temperatures (600 °C and 800 °C) led to diminished porosity and surface restructuring. Quantitative analysis revealed a maximum etching yield of 17.5 %, etching rate of 14.6 mg/h, and a pore formation efficiency of ∼43 %. The crystalline structure of silicon remained intact across all treatments, with minor surface disorder observed at higher temperatures. The urea-assisted etching produced a temperature-and environment-dependent surface oxidation and nitrogen incorporation. At 220 °C and 400 °C, a thick oxide layer formed, particularly under high-‍pressure conditions, while oxidation was less pronounced at 600 °C and 800 °C, likely due to rapid thermal decomposition limiting sustained gas-solid interactions. Nitrogen incorporation was most significant in Si-220-HP, where multiple nitrogen environments were detected, including Si–N, NH₂/NH₃⁺, and NO_x species. At higher temperatures, only stable Si–N bonds persisted, while other nitrogen species diminished.