Metallothermic Reduction of Silica Nanoparticles to Porous Silicon for Drug Delivery Using New and Existing Reductants

Carbon-Free Conversion of SiO2 to Si via Ultra-Rapid Alloy Formation: Toward the Sustainable Fabrication of Nanoporous Si for Lithium-Ion Batteries

Silicon has the potential to improve lithium-ion battery (LIB) performance substantially by replacing graphite as an anode. The sustainability of such a transformation, however, depends on the source of silicon and the nature of the manufacturing process. Today's silicon industry still overwhelmingly depends on the energy-intensive, high-temperature carbothermal reduction of silica─a process that adversely impacts the environment. Rather than use conventional thermoreduction alone to break Si-O bonds, we report the efficient conversion of SiO₂ directly to Mg₂Si by a microwave-induced Mg plasma within 2.5 min at merely 200 W under vacuum. The underlying mechanism is proposed, wherein electrons with enhanced kinetics function readily as the reductant while the "bombardment" from Mg cations and electrons promotes the fast nucleation of Mg₂Si. The 3D nanoporous (NP) Si is then fabricated by a facile thermal dealloying step. The resulting hierarchical NP Si anodes deliver stable, extended cycling with excellent rate capability in Li-ion half-cells, with capacities several times greater than graphite. The microwave-induced metal plasma (MIMP) concept can be applied just as efficiently to the synthesis of Mg₂Si from Si, and the chemistry should be extendable to the reduction of multiple metal(loid) oxides via their respective Mg alloys.

Nanostructured silicon photocatalysts for solar-driven fuel production

Solar-driven production of fuels such as hydrogen, hydrocarbons, and ammonia using semiconducting photocatalysts has the potential to be a sustainable alternative to current chemical processes. In recent years, silicon (Si) nanostructures have been recognized as a promising photocatalyst for hydrogen generation and organic oxidation reactions owing to its abundance, biocompatibility, and cost. While bulk Si has been studied extensively, on the nanoscale, plenty of opportunities exist to understand and engineer optimally performing Si photocatalysts. This perspective will highlight key results on the use of Si nanostructures for photocatalytic H₂ production, CO₂ reduction via light and heat-driven chemical looping, and current challenges in utilizing it for fuel-forming reactions. A brief guide on how these challenges can be addressed in the future and other unexplored questions that remain in the field are also discussed.

Dual Stimuli-Responsive Multifunctional Silicon Nanocarriers for Specifically Targeting Mitochondria in Human Cancer Cells

Specific targeting, selective stimuli-responsiveness, and controlled release of anticancer agents are requested for high therapeutic efficiency with a minimal adverse effect. Herein, we report the sophisticated synthesis and functionalization of fluorescent mesoporous silicon (FMPSi) nanoparticles decorated with graphene oxide (GO) nanosheets. GO-wrapped FMPSi (FMPSi@GO) was loaded with a cisplatin (Cis) anticancer agent, and Cis-loaded FMPSi@GO (FMPSi-Cis@GO) exhibited the dual stimuli (pH and NIR)-responsiveness of controlled drug release, i.e., the drug release rate was distinctly enhanced at acidic pH 5.5 than at neutral pH 7.0 and further enhanced under NIR irradiation at acidic pH condition. Notably, dequalinium-conjugated FMPSi-Cis@GO (FMPSi-Cis@GO@DQA) demonstrated an excellent specificity for mitochondrial targeting in cancer cells without noticeable toxicity to normal human cells. Our novel silicon nanocarriers demonstrated not only stimuli (pH and NIR)-responsive controlled drug release, but also selective accumulation in the mitochondria of cancer cells and destroying them.

From SiO2 to Alkoxysilanes for the Synthesis of Useful Chemicals

The transformation of silica (SiO₂) to useful chemicals is difficult to explore because of the strength of the Si-O bond and thermodynamic stability of the SiO₂ structure. The direct formation of alkoxysilanes from SiO₂ has been explored as an alternative to the carbothermal reduction (1900 °C) of SiO₂ to metallic silicon (Si_met) followed by treatment with alcohols. The base-catalyzed depolymerization of SiO₂ with diols and monoalcohols afforded cyclic silicon alkoxides and tetraalkoxysilanes, respectively. SiO₂ can also be converted to alkoxysilanes in the presence of organic carbonates, such as dimethyl carbonate. Alkoxysilanes can be further converted to useful chemicals, such as carbamates, organic carbonates, and chlorosilanes. An interesting and highly efficient pathway to the direct conversion of SiO₂ to alkoxysilanes has been discussed in detail along with the corresponding economic and environmental implications. The thermodynamic and kinetic aspects of SiO₂ transformations in the presence of alcohols are also discussed.

On the diatomite-based nanostructure-preserving material synthesis for energy applications

The present article overviews the current state-of-the-art and future prospects for the use of diatomaceous earth (DE) in the continuously expanding sector of energy science and technology. An eco-friendly direct source of silica and the production of silicon, diatomaceous earth possesses a desirable nano- to micro-structure that offers inherent advantages for optimum performance in existing and new applications in electrochemistry, catalysis, optoelectronics, and biomedical engineering. Silica, silicon and silicon-based materials have proven useful for energy harvesting and storage applications. However, they often encounter setbacks to their commercialization due to the limited capability for the production of materials possessing fascinating microstructures to deliver optimum performance. Despite many current research trends focusing on the means to create the required nano- to micro-structures, the high cost and complex, potentially environmentally harmful chemical synthesis techniques remain a considerable challenge. The present review examines the advances made using diatomaceous earth as a source of silica, silicon-based materials and templates for energy related applications. The main synthesis routes aimed at preserving the highly desirable naturally formed neat nanostructure of diatomaceous earth are assessed in this review that culminates with the discussion of recently developed pathways to achieving the best properties. The trend analysis establishes a clear roadmap for diatomaceous earth as a source material of choice for current and future energy applications.

Photocatalytic hydrogen generation using mesoporous silicon nanoparticles: influence of magnesiothermic reduction conditions and nanoparticle aging on the catalytic activity

In recent years, mesoporous silicon (mp-Si) nanoparticles (NPs) have been recognized as promising materials for sustainable photocatalytic hydrogen (H2) generation, which is both an important chemical feedstock and potential clean energy vector. These materials are commonly prepared via magnesiothermic reduction of silica precursors due to the ease, scalability, and tunability of this reaction. In this work, we investigate how the conditions of magnesiothermic reduction (i.e. reaction temperature and time) influence the performance of mp-Si for photocatalytic H2 generation. The mp-Si NPs were prepared using either the conventional single temperature heating method (650 °C for 3 or 6 h) or a two-temperature method in which the reaction is initially heated to 650 °C for 0.5 h, followed by a second step heating at 100 (mp-Si100), 200 (mp-Si200), or 300 °C (mp-Si300) for 6 h. Of these, mp-Si300 was the best performing photocatalyst and showed the highest H2 evolution rate (4437 μmol h-1 g-1 Si). Our results suggest that crystallinity has a profound effect on the performance of mp-Si photocatalysts. Additionally, high amounts of oxygen and particle sintering lower H2 evolution rates by introducing defect states or grain boundaries. It was also discovered that aging mp-Si NPs under ambient conditions result in continued surface oxidation which deleteriously affects its photocatalytic performance.

Exploiting nanoscale effects enables ultra-low temperature to produce porous silicon

The magnesiothermic reduction (MgTR) of silica has been recently shown to produce porous silicon which can be used in applications such as photocatalysis and energy storage. MgTR typically requires ≥650 °C to achieve meaningful conversions. However, high temperatures are detrimental to the highly desired porosity of silicon, while also raising doubts over the sustainability of the process. In this work we show for the first time that the onset temperature of the MgTR is dependent on the particle size of the feedstock silica. Using both in-house synthesised and commercial silica, we have shown that only particles ≤20 nm are able to trigger the reaction at temperatures as low as 380 °C, well below a previously reported cut-off temperature of 500 °C, producing porous, crystalline silicon. The decrease in temperature requirement from ≥650 °C to 380 °C achieved with little modification to the overall process, without any additional downstream processing, presents significant implications for sustainable and economical manufacturing of porous silicon.

We show the first evidence of reduction of silica occurring at temperatures as low as 380 °C to produce porous silicon without sacrificing the porosity and yield, thus paving the way for sustainable manufacturing.

In Situ Formation of Nanoporous Silicon on a Silicon Wafer via the Magnesiothermic Reduction Reaction (MRR) of Diatomaceous Earth

Successful direct route production of silicon nanostructures from diatomaceous earth (DE) on a single crystalline silicon wafer via the magnesiothermic reduction reaction is reported. The formed porous coating of 6 µm overall thickness contains silicon as the majority phase along with minor traces of Mg, as evident from SEM-EDS and the Focused Ion Beam (FIB) analysis. Raman peaks of silicon at 519 cm^-1 and 925 cm^-1 were found in both the film and wafer substrate, and significant intensity variation was observed, consistent with the SEM observation of the directly formed silicon nanoflake layer. Microstructural analysis of the flakes reveals the presence of pores and cavities partially retained from the precursor diatomite powder. A considerable reduction in surface reflectivity was observed for the silicon nanoflakes, from 45% for silicon wafer to below 15%. The results open possibilities for producing nanostructured silicon with a vast range of functionalities.

Crystallisation Behaviour of Pharmaceutical Compounds Confined within Mesoporous Silicon

The poor aqueous solubility of new and existing drug compounds represents a significant challenge in pharmaceutical development, with numerous strategies currently being pursued to address this issue. Amorphous solids lack the repeating array of atoms in the structure and present greater free energy than their crystalline counterparts, which in turn enhances the solubility of the compound. The loading of drug compounds into porous materials has been described as a promising approach for the stabilisation of the amorphous state but is dependent on many factors, including pore size and surface chemistry of the substrate material. This review looks at the applications of mesoporous materials in the confinement of pharmaceutical compounds to increase their dissolution rate or modify their release and the influence of varying pore size to crystallise metastable polymorphs. We focus our attention on mesoporous silicon, due to the ability of its surface to be easily modified, enabling it to be stabilised and functionalised for the loading of various drug compounds. The use of neutron and synchrotron X-ray to examine compounds and the mesoporous materials in which they are confined is also discussed, moving away from the conventional analysis methods.

A carbon network strategy to synthesize silicon–carbon anodes toward regulated morphologies during molten salt reduction

In this work, a carbon network strategy was proposed to prepare Si/SiO_x/C anodes with regulated morphologies during molten salt reduction.

The influence of hydrofluoric acid etching processes on the photocatalytic hydrogen evolution reaction using mesoporous silicon nanoparticles

H2 has been identified as one of the potential energy vectors that can provide a sustainable energy supply when produced through solar-driven water-splitting reaction. Si is the second most abundant element in the Earth's crust and can absorb a significant fraction of the solar spectrum while presenting little toxicity risk, making it an attractive material for photocatalytic H2 production. Hydrogen-terminated mesoporous Si (mp-Si) nanoparticles can be utilized to effectively drive the hydrogen evolution reaction using UV-to-visible light. In this work, the response of the photocatalytic activity of mp-Si nanoparticles to a series of HF acid treatments was investigated. A two-step magnesiothermic reduction method was used to prepare crystalline mp-Si nanoparticles with a specific surface area of 573 m2 g-1. The HF etching process was optimized as a function of the amount of acid added and the reaction time. The reaction time did not influence the H2 evolution rate substantially. However, the amount of HF used did have a significant effect on the photocatalytic activity. In the presence of ≥1.0 mL HF acid per 0.010 g of Si, morphological damage was observed using electron microscopy. N2 adsorption measurements indicated that the pore size and surface area were also altered. Solution-phase 19F{1H} NMR studies indicated the formation of SiF5- and SiF62- when larger volumes of HF were used. Both factors, morphological damage and the presence of byproducts in the pores, likely result in a lowering of the photocatalytic H2 evolution rate. Under the optimized HF treatment conditions (0.5 mL of HF per 0.010 g of Si), a H2 evolution rate of 1398 ± 30 μmol g-1 h-1 was observed.

Systematic evaluation of inorganic salts as a heat sink for the magnesiothermic reduction of silica

In this study, the effectivity of a series of inorganic salts, sodium chloride, calcium chloride, magnesium chloride, potassium chloride, and sodium bromide as heat sinks during magnesiothermic reduction of silica to porous silicon was investigated. The salts were chosen based on cost, thermal stability, ability to remain chemically inert during the reduction process, and ease of removal after the reaction. The structural integrity of the spherical porous silicon nanoparticles was observed using scanning electron microscopy, the surface area was determined via nitrogen adsorption experiments, and the crystallite size was determined using powder X-ray diffraction analysis; together, these were used to determine the efficacy of each salt. The ability of a salt to act as an effective heat sink was found to be highly correlated and principally dependent on the heat capacity of the salt. Calcium chloride was found to be the most effective heat sink overall among the five heat sinks investigated here.