Magnesiothermic reduction of silicon oxide can result in the formation of nanostructured, mesoporous elemental silicon (mp-Si), which has been explored in a variety of energy applications such as Li-ion battery anodes, photocatalytic water splitting, CO 2 reduction, drug delivery vehicles, and sensors as well as for gas storage. The physical properties of the resultant mp-Si generated via magnesiothermic reduction, and thus the potential utility, are highly dependent on the specific reduction conditions utilized. Herein, we report a modified magnesiothermic reduction method which allows for the synthesis of high surface area mp-Si nanoparticles. The reaction was initiated at 650 °C and then cooled to a lower temperature to minimize heat-induced morphological damage. The nanoparticles were characterized by using powder X-ray diffraction, scanning and transmission electron microscopies, and N 2 adsorption isotherm measurements. Particles prepared by using two-step annealing with the initial processing condition of 650 °C for 30 min followed by 300 °C for 4 h resulted in crystalline and completely reduced mp-Si with a high specific surface area of 542 ± 18 m 2 /g. mp-Si nanoparticles generated by using these specific parameters were further used for stoichiometric CO 2 conversion to CH 3 OH, and the reaction yields were 2.5 times higher than prior reports, demonstrating usefulness in effecting an important chemical transformation.
Exfoliation can be used to weaken and break van der Waals interactions within layered materials and produce small monolayered counterparts with remarkable properties. In the current study, liquid-phase exfoliation (LPE) using ultrasound and organic solvents is explored as a method to break down layered structures in biochars. Unfortunately, preferred solvents that can effectively disperse and stabilize the sheets produced during exfoliation often possess several health risks. In this work, we show that LPE in greener solvents can be used to access nanostructures of biochars to further improve the applications of this biobased material. Herein, pristine and oxidized biochars are exfoliated in a range of solvents to allow the identification of benign alternatives, which have been classified as nonhazardous or less hazardous by various solvent guides. The majority of biochar nanostructures produced consists of stacked nanosheets containing between two and eight layers with 15 nm thickness in average. Correlations between the LPE of biochars and different solvent parameters are established, and surface modification of biochars has potential to increase their exfoliation in more benign solvents. The LPE of oxidized biochars is more efficient in hydrogen-bond-accepting solvents due to the increased concentration of functional groups on their surface. Dispersions containing 0.20−0.75 mg/mL exfoliated oxidized biochars were obtained in solvents such as polyethylene glycols and ε-caprolactone. The LPE of pristine biochars in dimethyl carbonate and ethyl acetate gives similar yields to the most commonly used solvent for this process, N-methyl-2-pyrrolidone.
<div> <div> <div> <div> <p>Liquid-phase exfoliation (LPE) is a process frequently used to overcome the interactions between layers in layered materials to produce small sheets of material, with remarkable properties and high value applications. Materials are prepared via direct or indirect sonication in a solvent that must be able to effectively disperse and stabilize the sheets produced. Unfortunately, the preferred solvents for exfoliation processes are often toxic and possess several health risks. In this work, we show that LPE in greener solvents can be used to access nanostructures of biochar and further improve the applications of this renewable and bio-based material. Herein, pristine and oxidized biochars prepared from hardwood and softwood biomass waste (e.g. sludge, bark, and sawdust) are exfoliated in a range of solvents to allow the identification of benign alternatives that could afford highly concentrated dispersions. The majority of biochar nanostructures produced after exfoliation are stacked nanosheets containing between 2-8 layers (average 15 nm thickness). Correlations between effective LPE of biochar in solvents and different solvent parameters, including Kamlet-Taft, were established and allowed greener solvents to be used. Surface modification of biochars (e.g. via oxidation) has potential to increase their dispersibility in more benign solvents. LPE of oxidized biochars is more efficient in hydrogen-bond accepting solvents due to the increased concentration of carboxylic acid and alcohol functional groups on the surface of particles, when compared to non- functionalized biochars. Dispersions containing 0.20-0.75 mg/mL exfoliated oxidized biochar were obtained in solvents such as polyethylene glycols, glycerol formal and e-caprolactone. Moreover, LPE of pristine biochars in dimethyl carbonate, ethyl acetate, and solketal gave similar yields to more commonly used solvent for this process, N-methyl-2-pyrrolidone (NMP) a known reprotoxic molecule. </p> </div> </div> </div><br></div>
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