Synthesis and applications of porous silicon-based functional materials

Abstract

Porous silicon (P-Si) is a material with sponge-like structure formed by electrochemically etching the crystalline silicon wafer in hydrofluoric acid solutions. Due to its porous structure, high surface area, controllable porosity and optical response, it has been extensively applied in the field of photonics, micro-system engineering, solid state electronics, biomedical devices, etc. In this dissertation, we study P-Si and P-Si-based nanocomposites and apply them for sensors, photonics, and anodes materials for rechargeable lithium-ion batteries (LIBs). The first chapter gives a briefly review of P-Si with an emphasis on its fabrication, functionalization and applications. The second chapter demonstrates a simple and effective method to fabricate P-Si photonic crystal-based hybrid particles by soft lithographic and microfluidic techniques. The fabricated hybrid particles are composed of pristine P-Si structures strengthened by a polymer infused nanocomposite framework. The pristine P-Si moiety serves as a sensor because of its environmentally sensitive optical features, while the polymer-infused moiety provides a mechanically resilient scaffold and its invariant optical features can be used as both an optical barcode and a built-in self-reference for optical sensing. The third chapter explores the fabrication method of metal-based porous photonic films using P-Si as the sacrificial template. Metals, such as Ni, Co, Co-Ni, Ag, and Sn, have been successfully deposited into the pores of the P-Si template. The Ni-based films thus obtained feature nanoporous structures and distinct optical responses in the visible spectra. Moreover, their optical responses can be conveniently controlled by adjusting the nanostructure of the P-Si template (e.g., its periodcity). The application of the fabricated porous Ni photonic films as optical sensors has been demonstrated. The fourth chapter investigates the application of both pristine P-Si and P-Si-based nanocomposites as the anode materials for LIBs. First, the electrochemical performance of free-standing P-Si films as anode materials for LIBs has been studied: the P-Si electrodes display superior electrochemical properties with a high reversible specific capacity of > 2500 mAh/g and capacity retention of > 83 % after 60 cycles. For further improvement, we have fabricated functionalized freestanding P-Si films via carbonization of the P-Si/polymer composites in an Ar/H2 atmosphere and studied their electrochemical performance as the anode material for LIB. The carbonization treatment coats the walls of the porous structure of P-Si with highly conductive carbon, and considerably increases the electrical conductivity of P-Si. The functionalized P-Si anode materials show significantly enhanced electrochemical performance. Moreover, we have attempted to apply metal (e.g., Ag and Sn) deposited P-Si films as the LIB anode materials. To effectively fabricate Ag deposited P-Si, Ag-mirror reaction was tested but did not generate satisfactory results. However, this failure inspired a related project and the Ag-mirror approach proves to be a facile method to decorate other nanomateirals (e.g., Co3O4 nanowires) with Ag nanoparticles and improve its capability as LIB electrodes, which will be presented in Chapter 5. The fifth Chapter demonstrates that the Ag-mirror method is an effective approach to decorate nanomateirals (e.g., Co3O4 nanowires) with Ag nanoparticles and improve its capability as LIB electrode materials. Highly electrically conductive silver nanoparticles facilitate electron transportation between the current collectors and the active materials. High capacity as well as remarkable rate capability has been achieved through this simple approach. The sixth chapter summarizes the work of the dissertation and suggests the future work.