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Figure 1 | (a) Flow chart showing conventional synthesis routes of nano-Si, including the introduction of our synthesis route from sand. Optical images of (b) unpurified sand, (c) purified sand, and (d) (from left to right) vials of unpurified sand, purified sand, and nano-Si. (e) Schematic of the heat scavenger-assisted Mg reduction process. via magnesiothermic reduction of rice husks (SiO2), an abundant by- product of rice production measured in megatons per year24. Thermic reduction of SiO2 can be accomplished via a few well- known mechanisms including carbothermal, magnesiothermic, alu- minothermic, and calciothermic reduction. Carbothermal reduction utilizes electric arc furnaces operating at .2000uC and is the primary mode for metallurgical silicon production25. However, this process is very energy intensive and liquefies the silicon, thus destroying any original morphology of the SiO2. Recently, magnesiothermic reduc- tion has gained attention due its much lower operating temperatures (,650uC). Typically, Mg powder is placed adjacent to SiO2 powder and the furnace is heated until the Mg vaporizes. However, this reduction scheme produces zonal variations in composition with Mg2Si forming near the Mg powder, Si in the middle, and unreacted SiO2 furthest from the Mg23. Luo et al. have shown that adding a relatively large amount of NaCl to the reduction process aids in scavenging the large amount of heat generated during this highly exothermic reaction. NaCl effectively halts the reaction temperature rise at 801uC during fusion, preventing the reaction from surpassing the melting point of silicon and thus aiding in preserving the original SiO2 morphology26. Herein, we propose a facile and low cost alterna- tive to production of nano-Si with excellent electrochemical per- formance using a highly abundant, non-toxic, and low cost Si precursor: sand. Results The majority constituent of many sands is quartz (SiO2) and sand is easily collected since it is predominantly found on the surface of the earth’s crust. The sand used in this analysis was collected from the loamy surface of the shores of Cedar Creek Reservoir in the Claypan region of Texas. The soil of this region is classified as an Alfisol, specifically a Paleustalf, comprising .90% quartz with minor amounts of feldspars and chert27,28. The sand grains utilized herein have a grain size of ,0.10 mm, as in Fig. 1b. Further mechanical milling in an alumina mortar easily reduces the grain size to the micrometer and nanometer scale within minutes. Organic species are removed via calcining in air at 900uC, and the sand is then sequentially washed with HCl, HF, and NaOH for varying amounts www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 5623 | DOI: 10.1038/srep05623 2PDF Image | Scalable Synthesis of Nano-Silicon from Beach Sand
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