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of time. Unwanted silicate species are removed via the HF etch, as crystalline quartz etches much slower than other silicate species such as feldspars29. After purification, the sand assumes a bright white appearance in stark contrast to the brown hue of the unpurified sand, as in Fig. 1c. The peaks associated with unpurified sand in the XRD analysis in Fig. 2a confirm that the sample comprises mostly quartz with very minor peaks corresponding to impurities. After purifying the sand, the peaks associated with quartz greatly increase in intens- ity relative to the impurity peaks, confirming that most of the impur- ities have been successfully etched away. After purification, quartz powder and NaCl is ground together in a 1510 SiO2:NaCl weight ratio and ultrasonicated and vigorously stirred for 2 hours. After drying, the SiO2:NaCl Powder is ground together with Mg powder in a 150.9 SiO2:Mg weight ratio. The resultant powder is loaded into Swagelok-type reactors and sealed in an argon-filled (0.09 ppm O2) glovebox. The reactors are imme- diately loaded into a 1" diameter quartz tube furnace purged with argon. The furnace is slowly heated at 5uCmin21 to 700uC and held for 6 hours to ensure complete reduction of all SiO2. After reduction the resulting brown powder is washed with DI water to remove NaCl and then etched with 1 M HCl for 6 hours to remove Mg, Mg2Si, and MgO. The MgCl2 that is produced via HCl etching of MgO can be easily recycled back to Mg via electrolysis, which is the predominant industrial synthesis route for Mg production30. The powder is washed several times with DI H2O and EtOH to remove the etchant and dried overnight under vacuum. A visual comparison, without mag- nification, of unpurified beach sand, purified quartz, and nano-Si stored in glass vials can be seen in Fig. 1d, and the entire synthesis process can be visualized in Fig. 1e. SEM imaging in Fig. 2 reveals the broad size distribution and highly irregular morphology of the milled quartz powder before and after reduction. For the milled quartz powder, the particle size ranges from several microns to 50 nm, as in Fig. 2c and 2d. The quartz powder and nano-Si reduction product are both highly irregu- lar in shape as expected. After reduction, the nano-Si is absent of particles with dimensions in excess several microns and has a much smaller size distribution than the quartz powder, as in Fig. 2e and 2f. We can attribute this to the breakdown of relatively larger particles www.nature.com/scientificreports Figure 2 | (a) XRD plot displaying characteristic peaks of quartz in both pre-reduction samples and Si peaks in the post-reduction nano-Si. (b) EDS analysis with inset displaying weight percent of elements in nano-Si after HCl and HF etching. Low magnification (c) and higher magnification (d) SEM images of quartz powder after purification and milling. Low magnification (e) and higher magnification (f) SEM images of nano-Si after reduction and etching. Scale bars for (c),(d), (e), and (f) are 5mm, 2mm, 2mm, and 500 nm, respectively. SCIENTIFIC REPORTS | 4 : 5623 | DOI: 10.1038/srep05623 3PDF Image | Scalable Synthesis of Nano-Silicon from Beach Sand
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