High surface area mesoporous silicon nanoparticles prepared via two

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High surface area mesoporous silicon nanoparticles prepared via twostep magnesiothermic reduction for stoichiometric CO to CHOH conversion 2

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Sarah Martell, Yiqi Lai, Emily Traver, Judy MacInnis, D. Douglas Richards, Stephanie L. MacQuarrie, and Mita Dasog ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01207 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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ACS Applied Nano Materials

High surface area mesoporous silicon nanoparticles prepared via two-step magnesiothermic reduction for stoichiometric CO2 to CH3OH conversion. Sarah A. Martell, ‡,1 Yiqi Lai, ‡,1 Emily Traver,1 Judy MacInnis,2 D. Douglas Richards,2 Stephanie MacQuarrie,2 Mita Dasog1* 1Department

of Chemistry, Dalhousie University, 6274 Coburg Road, Halifax, NS, Canada

2Department

of Chemistry, Cape Breton University, 1250 Grand Lake Rd., Sydney, NS, Canada

KEYWORDS: Nanomaterials, porous, solid-state, silicon, sequestration, metallothermic reduction

ACS Paragon Plus Environment

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ABSTRACT

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, CO2 reduction, as well as drug delivery vehicle, sensor, and 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 ºC and then cooled to a lower temperature to minimize heat induced morphological damage. The nanoparticles were characterized using powder X-ray diffraction, scanning and transmission electron microscopies, and N2 adsorption isotherm measurements. Particles prepared using two-step annealing with the initial processing condition of 650 ºC for 30 min followed by 300 ºC for 4 h resulted in crystalline and completely reduced mp-Si with a high specific surface area of 542 ± 18 m2/g. mp-Si nanoparticles generated using these specific parameters were further used for stoichiometric CO2 conversion to CH3OH and the reaction yields were 2.5x higher than prior reports, demonstrating usefulness in effecting an important chemical transformation.


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Introduction Renewable sources of energy such as solar and wind present an inherent intermittency issue that requires the development of efficient energy storage solutions to enable use of such resources. Grid-scale energy storage using technologies such as batteries and electrochemical fuel generating devices requires the development of high-performance materials that are simultaneously non-toxic, earth-abundant and offer scalable production avenues.1,2 Mesoporous silicon (mp-Si) is one such material that is being pursued as an anode material in Li-ion batteries due to its high theoretical capacity (~4200 mAhg-1 based on formation of Li22Si5),3-5 as a hydrogen evolving photocatalyst for solar fuels production,6-8 and for conversion of CO2 to useful, higher-value products.9 The performance of mp-Si for these applications is highly dependent on the surface area, pore size, and crystallinity, which in turn, varies significantly with the preparation method. Battery anodes are typically constructed with relatively low surface area amorphous Si4,5,10-12 whereas the solar-driven water-splitting reactions typically utilize crystalline, porous, and high-surface area Si.6 Stoichiometric conversion of CO2 to CO, CH2O, and CH3OH with hydrogen-terminated Si nanostructures as the reducing agent have been reported.9,13,14 The identity of CO2 reduction products and conversion efficiencies are directly proportional to the amount of hydrogen atoms on the Si surface which in turn is dictated by the surface area.9 Access to high-surface area mp-Si nanoparticles can improve the yields of CO2 reduction products. Common routes to generate mp-Si include electrochemical etching,15-17 metal-assisted chemical etching,18 chemical vapor deposition,19,20 and metallothermic reduction.3,21-27 Of these, metallothermic reduction using Mg metal offers a scalable and straightforward route to prepare mp-Si using inexpensive raw materials such as rich husks, bamboo, sand, glass, and diatoms.3,21,28,29 Magnesiothermic reduction is an exothermic redox reaction between Mg metal


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and SiOx.30 The reaction is typically performed at the melting point of Mg metal (650 ºC) with temperature held constantly at this point to assist in the diffusion of the atoms in the solid-state. However, it has recently been demonstrated that due to the exothermic nature of the reaction, carrying out the entirety of the reaction at 650 ºC results in significant morphological damage and lowering of surface area of mp-Si formed.31 A demonstrated method to circumvent this issue involves the addition of various inorganic salts to the reaction as a heat sink to minimize the sintering of Si nanoparticles,32,33 but, this has not been shown to produce porosity at the same level that can be achieved using methods which involve reduction of SiCl4 precursor in presence of inorganic salt templates.6 Furthermore, addition of an extra reagent to the process can increase the overall cost of the reaction as well as the amount of solvent required to remove the heat sink, rendering such methodologies undesirable for large-scale production. To address this issue, we present in this study a modified magnesiothermic route to prepare high surface area mp-Si nanoparticles (Scheme 1). The magnesiothermic reduction was first initiated at 650 ºC but then the bulk of the reaction was effected at a lower temperature to prevent sintering of Si nanoparticles. Combination of various reaction temperature profiles were investigated to prepare mp-Si nanoparticles in the absence of a heat sink. The most favorable two-step annealing sequence was further optimized for the ideal reaction time. This modified method yields the highest surface area mp-Si nanoparticles reported to date made using magnesiothermic reduction method. The high surface area nanoparticles were etched with HF to generate hydrogen-terminated mp-Si and this material was then applied to effect the stoichiometric reduction of CO2 to CH3OH and the efficacy of this process was explored.


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Scheme 1. Two-step magnesiothermic reduction process to prepare mp-Si nanoparticles.

Results and Discussion Monodisperse Stöber silica nanoparticles with an average diameter of 292 ± 5 nm as indicated by scanning electron microscopy (SEM) were prepared using a sol-gel method (Figure S1, Supporting Information).34 The specific surface area of these silica nanoparticles was determined to be 6 ± 1 m2/g using the Brunauer-Emmett-Teller (BET) method. The silica nanoparticles were ground together with Mg powder using a mortar and pestle until a uniform grey powder was obtained and was then processed in a tube furnace under Ar atmosphere. Initially, the reaction mixture was heated to 650 ºC and maintained at that temperature for 15, 30, 45, or 60 min and cooled to room temperature. The reaction product was washed with 1.0 M HCl solution for 4 h to remove MgO and any unreacted Mg. The reduced product was collected via suction filtration and dried in an oven overnight at 100 °C.


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Figure 1. (A-D) Scanning electron microscopy images of mp-Si nanoparticles generated via indicated processing conditions. (E) Powder X-ray diffraction patterns of silica and reduction reaction products for indicated conditions.

The SEM image of the product obtained after 15 min of reduction showed unreacted silica (smooth particles) along with porous Si nanoparticles (Figure 1A). After 30 min of reduction the presence of unreacted silica particles was not observed by SEM and after 45 and 60 min of reduction a high degree of morphological damage was observed wherein the spherical particles were broken into smaller and denser components (Figure 1C-D). Powder X-ray diffraction (XRD) analysis (Figure 1E) indicated that the sol-gel prepared silica precursor was amorphous as evidenced by a broad reflection at ca. 22º in the pattern.35 XRD analysis also showed that for both the 15 and 30 min reaction times, amorphous silica remained in addition to the crystalline Si product indicated by reflections at 28º, 47º, and 56º corresponding to 111, 220, and 311 planes.31 The pattern observed for the 45 and 60 min reaction presented only crystalline Si reflections. The specific surface areas were determined using the BET method to be 58 ± 12, 98 ± 11, 171 ± 15,


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and 164 ± 14 m2/g for 15, 30, 45, and 60 min reactions, respectively. The 15 and 30 min reactions yielded incompletely reduced product whereas the 45 and 60 min reaction caused sintering and morphological damage. The incomplete reduction at 15 and 30 min reaction indicates that the heat released due to exothermic nature of magnesiothermic reduction in that duration is insufficient to keep the reaction self-propagating. However, conducting the entirety of the reaction at 650 ºC results in morphological damage.

Figure 2. Powder XRD patterns of the mp-Si nanoparticles obtained under various processing conditions.

In order to effect complete reduction while preserving morphological integrity, a two-step reduction process was explored wherein the reduction was initially performed at 650 ºC for 15 or


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30 min and then in the subsequent step the temperature was lowered to 100, 200, 300, or 400 ºC and maintained for 6 h. After the reduction, the product was washed with 1.0 M HCl solution and dried overnight. Figure 2 shows the powder XRD patterns of the resulting products under various reaction conditions. All the products showed characteristic reflections corresponding to formation of crystalline Si. Only the product processed at 650 ºC for 15 min followed by heating at 100 ºC showed presence of amorphous silica precursor. The SEM images of mp-Si showed presence of spherical nanoparticles for most processing conditions (Figure 3). A higher degree of morphological damage and sintering was observed for mp-Si nanoparticles prepared at 650 ºC for 30 min followed by heating at 400 ºC for 6 h (Figure 3H).

Figure 3. (A-H) SEM images of mp-Si nanoparticles obtained by two-step reduction using the indicated conditions.

Nitrogen adsorption and desorption isotherms were measured to determine the specific surface area and analyze the pore characteristics of mp-Si nanoparticles in detail. Silica


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nanoparticles first processed at 650 ºC for 15 min and then further processed at range of lower temperatures showed type IV adsorption-desorption isotherms with H3-type hysteresis loop (as classified by IUPAC) which indicate the presence of mesopores (Figure 4A).36,37 The average pore size increased from 5.3 nm to 9.0 nm as the temperature of the second step was increased from room-temperature (no explicit second step) to 200 ºC (Figure S2). When the mp-Si nanoparticles were synthesized using 650 ºC, 15 min and then cooled to 300 and 400 ºC, the pore size distribution was broad, and two average sizes were observed (Figure S2); 6.4 and 18.0 nm for the second annealing temperature of 300 ºC and 5.6 and 25.5 nm for 400 ºC. The specific surface area was determined using the BET method and was found to be 207 ± 13, 376 ± 22, 432 ± 18, and 456 ± 19 m2/g for mp-Si nanoparticles prepared with second annealing temperature of 100, 200, 300, and 400 ºC, respectively (Figure 5). Silica nanoparticles processed at 650 ºC for 30 min and then cooled to various temperatures showed type IV adsorption-desorption isotherms with H3/H4-type hysteresis loop which indicate the presence of mixture of pore shapes (Figure 4B).37 When processed at 650 ºC for 30 min and cooled to the room temperature (single-step process), an average pore size of 6.0 nm was obtained (Figure S3). Two pore sizes with an average diameter of 3.3 and 14.8 nm were observed for mpSi nanoparticles with second annealing temperature of 100 ºC and 3.7 and 9.0 nm average pore diameters were obtained for annealing temperature of 200 ºC. At 300 ºC, three pore sizes with average diameter of 4.6, 9.0, and 17.8 nm were observed (Figure S3) but at 400 ºC a broad pore distribution between 5 – 140 nm was observed (Figure S3). This is likely due to the extensive damage and sintering observed in the mp-Si nanoparticles processed under those conditions (650 ºC, 30 min followed by 400 ºC, 6 h). The specific surface area was determined using the BET method and was found to be 364 ± 18, 412 ± 15, 527 ± 21, and 428 ± 31 m2/g for mp-Si


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nanoparticles prepared with second annealing temperature of 100, 200, 300, and 400 ºC, respectively (Figure 5). A decrease in surface area was observed for the mp-Si nanoparticles processed at 400 ºC consistent with the sintering and morphological damage observed in the SEM image (Figure 3H).

Figure 4. Nitrogen adsorption and desorption isotherms of mp-Si nanoparticles initially processed at (A) 650 ºC for 15 min and (B) 650 ºC for 30 min.


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Figure 5. Specific BET surface area of mp-Si nanoparticles processed at different annealing conditions.

The two-step process involving first reacting at 650 ºC for 30 min followed by continued reaction at 300 ºC for 6 h resulted in material that exhibited a surface area of 527 ± 21 m2 which was the highest of all reaction conditions initially examined herein. This combination of annealing was further optimized for the reaction time at the lower temperature. Transmission electron microscopy (TEM) was performed on the mp-Si nanoparticles at various time intervals to monitor the progress of the reaction (Figure 6). The size of the silica core decreases with the reaction time and complete reduction is observed at 4 h. Beyond 4 h, the primary particles appeared to grow and in certain areas sinter resulting in darker regions scattered throughout the nanoparticle. The magnified image shows these dark regions to contain larger structures that likely result from fusing of the smaller particles. The specific BET surface area of 1, 2, 3, 4, 5, and 6 h reactions were 173 ± 10, 312 ± 15, 503 ± 12, 542 ± 18, 532 ± 23, and 527 ± 21 m2/g, respectively. This demonstrates


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that both the annealing temperatures and reaction time can be manipulated to determine the surface area and pore sizes of mp-Si during the two-step magnesiothermic reduction process. Moreover, the reaction at 650 ºC for 30 min followed by continued reaction at 300 ºC for 4 h resulted in the highest surface area observed to-date for mp-Si obtained via magnesiothermic reduction using a non-porous precursor (Table S1).

Figure 6. TEM images of silica and mp-Si nanoparticles processed at 650 °C for 30 min followed by 300 °C at times indicated in the images.

mp-Si have recently been investigated for the reductive conversion of CO2 into highervalue products, as summarized in Table S2.9,13 In this application, the specific surface area of the particles can strongly influence the reaction yield. Previously, hydrogen terminated mp-Si nanoparticles with specific surface area of 165 m2/g were shown to convert CO2 to CH3OH at 150


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°C and 10 bar pressure and reaction yield of 0.84 mmol of CH3OH per g of Si was observed.9 The mp-Si nanoparticles with the highest surface area prepared in this study (650 °C, 30 min; 300 °C, 4 h) were explored for CO2 reduction under similar reaction conditions. Hydrogen-terminated mpSi nanoparticles were obtained by treating Si with HF acid. The characteristic Si–Hx stretches were observed in the IR spectrum (Figure S4) centered around 2100 cm-1, confirming the presence of hydrogen terminated Si surface.13 The SEM image of mp-Si after HF etching showed no significant change in the morphology of the nanoparticles (Figure S5A). The surface area and pore size of hydrogen-terminated mp-Si nanoparticles was determined to be 539 ± 17 m2/g and 17.2 nm, respectively (Figure S5B). CO2 reduction was performed at 150 °C and 10 bar pressure for an hour in a steel autoclave. The gas phase products were analyzed using gas chromatography and liquid phase products were analyzed using 1H NMR. No gas phase CO2 reduction products were observed via gas chromatography. CH3OH was the only liquid phase reduction product that was obtained as confirmed using 1H NMR studies (Figure S6). The amount of CH3OH was quantified using an internal standard (acetone) and a reaction yield of 2.1 ± 0.2 mmol of CH3OH per g of Si was observed. This is ~2.5x higher CH3OH yields than previously observed for mp-Si nanoparticles.9 The mp-Si nanoparticles were analyzed after the CO2 reduction reaction. The powder XRD pattern showed presence of crystalline Si reflections without any significant oxidation (Figure S7A). However, the morphology of the nanoparticles was substantially damaged (Figure S7B) and the specific surface area decreased to 480 ± 12 m2/g. N2 adsorption and desorption isotherm showed loss of H3/H4 type hysteresis loop that was originally observed for these nanoparticles (Figure S7C). After CO2 reduction reaction, the original pore size distribution was lost, and a broad peak was observed between 10 – 80 nm.


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Other mp-Si nanoparticles of 650 °C, 30 min series were examined for CO2 reduction reactions. The yield of CH3OH increased from 0.23 ± 0.06 mmol to 2.1 ± 0.2 mmol as the specific surface area of the mp-Si nanoparticles increased from 98 ± 11 to 542 ± 18 m2/g (Figures S8). This indicates that the CH3OH yield is proportional to the surface area of the mp-Si as higher area can accommodate higher number of hydrogen atoms after etching with HF.

Conclusions In conclusion, we report a modified two-step magnesiothermic reduction method that enabled the synthesis of high-surface area mp-Si nanoparticles. Processing the silica nanoparticles initially at 650 °C for 30 min followed by 300 °C for 4 h results in mp-Si nanoparticles with specific surface area of 542 ± 18 m2/g. The surface area is higher than other magnesiothermic reduction of non-porous SiO2 precursors and also higher than the template method (reduction of SiCl4 around salt crystals).6 All the two-step annealing conditions resulted in higher specific areas than carrying out the entirety of the reduction at 650 °C. Significantly less morphological damage was observed for two-step annealing process compared to the single-step reduction. Hydrogen termination of the high surface area mp-Si nanoparticles was effected using HF treatment and the resultant material was investigated in CO2 reduction application wherein stoichiometric conversion to CH3OH was accomplished with product yields 2.5x higher than described in previous reports. Consistent with earlier investigations, CO2 reduction was selective to formation of CH3OH and no other products were observed. Thus, this method allows for straightforward synthesis of high-surface area, crystalline mp-Si nanoparticles that can be used for fuel forming


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reactions. These materials may additionally find utility in other energy conversion and storage applications such as water-splitting and as anodes for Li-ion batteries.

Experimental details Materials. Tetraethyl orthosilicate (TEOS, 99.9%), ammonium hydroxide (NH4OH, 28%), and magnesium -325 mesh powder (Mg, 99.8%) were purchased from Alfa Aesar. Hydrochloric acid (HCl, 36.5-38.0%) was purchased from Anachemia. Hydrofluoric acid (HF, 48%), nitric acid (HNO3, 70%), and deuterated water (D2O) were purchased from Sigma-Aldrich. Water (≥18.2 MΩ cm resistivity) was obtained from a Barnstead E-Pure system. All the reagents were used as received without further purification. Synthesis of Stöber silica nanoparticles. TEOS (30 mL) was added to 95% EtOH (700 mL) with stirring at 400 rpm followed by 28% NH4OH (60 mL). The reaction vessel was sealed with parafilm and left to stir for 18 h. The solution was initially clear but quickly changed to a white, opaque solution. The ambient temperature and humidity were 21 ± 2 °C and 60 ± 8%. The solution was centrifuged at 3300 rpm for 25 mins and decanted. The nanoparticles were then washed twice with EtOH, centrifuged for 15 mins at 3300 rpm and decanted. The resulting white product was dried overnight in an oven at 90 °C. The average reaction yield was 89%. Magnesiothermic reduction of silica nanoparticles. Stöber silica nanoparticles (0.20 g, 3.33 mmol) and -325 mesh Mg powder (0.18 g, 7.3 mmol) were ground together using a mortar and pestle, resulting in a light grey powder (Figure 7B). The mixture was transferred to a ceramic reaction boat and annealed under an argon atmosphere at pre-determined temperature and time with a ramp rate of 10 °C min-1. For two-step annealing process, the reaction mixture was annealed


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at 650 °C for 15 or 30 min under argon atmosphere and then cooled to pre-determined temperature via air-cooling. The reaction product was treated with HCl (25.0 mL, 1.0 M) for 4 h with stirring at 300 rpm. The resulting product was collected through suction filtration and washed with deionized water (150 mL). The product was transferred to a beaker and dried overnight in an oven at 90 °C.

Figure 7. Stöber silica nanoparticles and Mg powder (a) before and (b) after grinding with a mortar and pestle.

CO2 reduction with porous silicon nanoparticles. Porous silicon nanoparticles (0.20 g) were suspended in 1:1 mixture (5.0 mL) of water and ethanol. 48% HF (5.0 mL) was added to the nanoparticle slurry followed by 100 μL of concentrated HNO3 acid (caution: hydrofluoric acid can be extremely dangerous and must be handled with great care) and was stirred for 10 min. The mixture was carefully transferred to polypropylene tubes and centrifuged at 3200 rpm for 15 min and the supernatant was discarded directly into saturated calcium chloride solution. The hydrogen terminated porous silicon nanoparticles were redispersed in toluene (10.0 mL) were transferred to a 50 mL Bergoh high pressure steel autoclave. The reaction vessel was evacuated and refilled with


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argon thrice and finally filled with 10 bar CO2. The reaction mixture was heated at 150 oC for 3 hours. The reaction flask was cooled to room temperature and the pressure was released. The reaction mixture was stirred with D2O (2.00 mL) and catalytic amounts of DCl for 30 min and D2O layer was extracted. 200 µL of the extracted D2O was transferred to an NMR tube and methanol content was quantified using 1H NMR with acetone as an internal standard. Characterization. Powder X-ray powder diffraction (XRD) patterns were collected using a Rigaku Ultima IV X-Ray diffractometer with CuKα radiation (λ = 1.54 Å). The samples were placed on to a zero-background Si wafer and the spectra were collected at 3 counts/s. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4700 electron microscope. Transmission electron microscopy (TEM) images were collected using a Hitachi 7700 transmission electron microscope. The powdered samples were dispersed ultrasonically in ethanol for 15 minutes and the resulting suspension was drop-casted onto formvar/carbon coated copper grids using a micropipette. The electron microscopy images were analyzed using Image J software (version 1.45). Surface area and pore size were measured using either an ASAP 2020 analyser (Micromeritics, Norcross, GA, USA) or Micromeritics Flowsorb II 2300 BET surface area analyzer. Fourier transform infrared (FTIR) spectra were collected with an Agilent Cary-630 ATR using a Ge crystal. NMR spectra were recorded on a Bruker AV-500 MHz spectrometer.

ASSOCIATED CONTENT Supporting Information. Figures - SEM image of silica nanoparticles, pore size distribution in various mp-Si samples, IR spectrum, SEM image, and pore size distribution of hydrogen terminated mp-Si nanoparticles, 1H NMR of the CO2 reduction products, and powder XRD, SEM


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image, N2 adsorption isotherm and pores size distribution of mp-Si after CO2 reduction. Tables literature summary of mp-Si syntheses conditions and surface areas and CO2 reduction with Si nanostructures.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; Tel: 1-902-494-4245 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT The authors acknowledge funding from the Natural Sciences and Engineering Research Council of Canada (RGPIN 2017-05143), Canada Foundation for Innovation (35692), and Dalhousie University. SM thanks Nova Scotia Graduate Scholarship for funding. Patricia Scallion and Clean Technologies Research Institute are thanked for access and assistance with the SEM analysis.

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High surface area mesoporous silicon nanoparticles prepared via two

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