Nanostructured Silicon Derived from an Agricultural Residue Bagasse Ash via Magnesiothermic Reduction Method

Abstract

This study presents the magnesiothermic reduction of silica into silicon. This reduction process occurs at a lower reaction temperature than its carbothermal counterpart. Furthermore, silica was extracted from sugarcane bagasse ash via a thermo-chemical treatment method using, for the first time, L-cysteine chloride monohydrate and used as a precursor in the production of silicon using magnesiothermic reduction. The as-synthesized nanocrystalline silicon’s physicochemical properties were investigated using XRD, Raman, FTIR, BET, and SEM. A peak at 2 of 28.2 with a crystallite size of 32 nm was discovered using X-ray diffraction spectroscopy. The pronounced peak around 518 cm⁻¹ was observed from the Raman spectrum, characteristic of crystalline silicon. The FTIR analysis showed two sharp peaks at 446 cm⁻¹ and 1056 cm⁻¹, indicative of the Si-O rocking mode and Si-O-Si stretching mode functional groups present. N₂ physisorption at 77 K reveals that the surface area, pore volume, and pore diameter of the as-synthesized silicon were 73 m²/g, 0.23 cm³/g, and 12 nm, respectively. In this study, we were able to produce silicon from silica extracted from SCBA using the magnesiothermic reduction method in a tube furnace, which has potential for thin-film solar cells.

1. Introduction

Sugarcane bagasse ash, a waste product from the sugarcane industry, was processed into silica (SiO₂) powder in this study. An ideal process for sustainability is the production of manufactured silica particles from low-cost renewable or waste resources. Sugarcane bagasse has emerged as a sustainable resource for both tailored silica particles and bioenergy production. Furthermore, the manufacture of engineered silica particles from sugarcane bagasse ash to produce nanosilicon. Silica is a significant raw material for industrial applications, and much research has been conducted to extract it from agricultural wastes such as sugarcane bagasse, rice husk, etc. [1,2].

Silicon is an inorganic material of great technological importance across many fields owing to its inherent set of physical, chemical, electronic, and optical properties [1,2,3]. Due to its application-specific and superior properties, the performance profile with its synergistic high surface area and small particle size makes it a good candidate for applications in a range of new technological advances, including functional nanomaterials, biotechnology, nanoelectronics, energy production, and storage devices. Another interesting feature is the application of silicon in thermoelectric materials via magnesium alloy technology [4,5].

Presently, the crystalline silicon used in solar cell production is produced from the traditional quartz source of silica. The synthetic route is high energy intensive (around 1900 $℃$), which minimizes all process steps at a high operating cost, resulting in the production of highly expensive solar cells. Several studies report alternative methods with low operating costs, including the metallothermic method, which has gained great attention as a straightforward synthesis of porous Si [6,7].

A metal, such as magnesium, zinc, or aluminum, to name a few, is used in the reduction procedure. Furthermore, the ideal metal for a reduction reaction should be cheap, easy to work with, produce eco-friendly products with a low melting point, and, most importantly, have significant redox properties. Metal oxides may be produced as by-products of these processes, which is undesirable. Nonetheless, metal oxide removal can be hazardous due to a violent reaction with water [8]. Indeed, Zn readily reacts with Si halides; however, the reaction with silica is thermodynamically unfavourable. As a result, Mg and Al are the most commonly-used metallothermic reduction agents for silica. The magnesiothermic reduction has demonstrated the ability to produce silicon from silica in the laboratory. In the temperature range between 500 and 950 °C, the magnesiothermic reduction has demonstrated the ability to synthesize silicon from silica, enabling the template-assisted creation of silicon structures [8,9].

Among the many different natural resources at our disposal, sugarcane is one of the main potential sources for silica extraction and subsequent transformation to nanostructured silicon, as tabulated in

Table 1. Recent studies reporting the magnesiothermic reduction with their results.

Precursor	Reaction Conditions	Properties of As-Produced Silicon
Algae	T = 650 °C	Nanocrystal silicon SSA-500 m2/g, Pore Size Diameter-2 nm. [11]
${\mathrm{SiO}}_2$ (Stober Method)	T = 800 °C t = 12 h Argon atm	Porous nanocrystalline Si [12]
Rice Husk (RH), Bamboo Culm (BC), and Sugarcane Bagasse (SCB)	T = 650 °C t = 30 min (MW-MR)	Porous Silicon, Pore diameter (50–80 nm) SSA [13]
RH	T = 650 °C t = 7 h	Porous silicon SSA 150 m2/g [14]

. This feedstock is traditionally used for sugar and ethanol, and the residue from the process (bagasse) is utilized as the main source of energy. In this study, experiments were carried out to convert silica nanoparticles to nanosilicon using magnesiothermic reduction. Consequently, the as-produced nanopowders were analyzed and compared in terms of physical, chemical composition, morphology, and structural properties [10].

2. Experimental

2.1. Materials Used

All chemicals were purchased from Sigma-Aldrich and used without further purification. L-cysteine hydrochloride monohydrate (98%) and tetrapropylammonium hydroxide (1.0 M) in H₂O were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sugarcane was procured from the Sugar Illovo South Africa Company (KwaZulu-Natal, South Africa). In a typical preparation procedure, sugarcane bagasse was soaked for a period of 24 h in double-deionized water to remove any dust and soil particles. The soaked sugarcane was then oven-dried for a period of 6 h. After being soaked and dried, the sugarcane was burned in the open air to produce black sugarcane bagasse ash (SCBA). The synthesis was done using deionized water from the Milli-Q water purification system (Millipore, Bedford, MA, USA).

The chemical composition test was done using X-ray fluorescence spectroscopy (XRF, (Malvern Panalytical, Malvern, UK) for major oxides present in the SCBA in

Table 2. Chemical composition of acid-leached SCBA from XRF.

Component	Wt%
${\mathrm{SiO}}_2$	79.40
${\mathrm{TiO}}_2$	1.03
${\mathrm{Al}}_2{\mathrm{O}}_3$	8.45
${\mathrm{Fe}}_2{\mathrm{O}}_3$	2.39
$\mathrm{MnO}$	0.04
$\mathrm{MgO}$	0.61
$\mathrm{CaO}$	1.06
${\mathrm{Na}}_2\mathrm{O}$	1.06
${\mathrm{K}}_2\mathrm{O}$	3.86
${\mathrm{P}}_2{\mathrm{O}}_5$	0.22
${\mathrm{SO}}_3$	0.03
${\mathrm{Cr}}_2{\mathrm{O}}_3$	0.04
$\mathrm{NiO}$	0.02
${\mathrm{H}}_2\mathrm{O}$	0.04
$\mathrm{LOI}$	1.18

. The XRF study reveal that the leaching process was efficient as the amount of silica was significantly reported to be 79.40 wt%. The pretreatment step was done using L-cysteine hydrochloride monohydrate as the leaching acid. The loss on ignition is also reported in Table 2.

The significant improvement in the morphology of silica in the powders of the ash is due to the selective removal of the synthesis residues using L-cysteine hydrochloride monohydrate, essentially reducing metallic impurities in the sample. After acid-treatment, the sample was calcined in a muffle furnace and subsequent extraction of silica via hydro-thermal method using tetrapropylammonium hydroxide, thereby minimizing the other metal oxides impurities.

2.2. Magnesiothermic Reduction of Silica to Silicon

The as-synthesized biosilica from sugarcane bagasse ash and Mg chips used for the magnesiothermic reduction were 2.5 g and 3.0 g, respectively, as shown in reaction 1. The mixture was subjected to pyrolysis at 700 $℃$ for 4 h in a tube furnace under nitrogen (at 10$℃$/min heating rate). A portion of dark brownish and greyish powder (as-reduced silica) was immersed in HCl solution, shown in reaction 2, and reflux at 100 $℃$ for 2 h to remove excess Mg and unreacted silica, using a modified method reported in previous studies by Bao Z, et al., 2007 [13].

$${\mathrm{SiO}}_2+2\mathrm{Mg}\to \mathrm{Si}+2\mathrm{MgO}$$

(1)

$$\mathrm{Si}+\mathrm{MgO}+\mathrm{HCl}\to \mathrm{Si}\left(\mathrm{s}\right)\downarrow +{\mathrm{MgCl}}_2(\mathrm{aq})$$

(2)

The resulting gray solution was then filtered using a Buchner funnel. The trapped precipitate was then washed with double-deionized water and decanted until the pH of the supernatant reached 6.5. The resulting gray precipitate was then dried in an oven at 40 °C overnight and ground into a fine powder. Subsequently, nanosilicon was produced. The actual dark grey powder silicon was further studied for physico-chemical properties using Raman spectroscopy (Hamamatsu photonics, Japan) for purity, Fourier Transform Infrared spectroscopy (Perkin Elmer, Waltham, MA, USA) for evolution of surface chemistry, Brunauer-Emmett-Teller (BET, Thermofischer Scientific, Waltham, MA, USA) for textural properties (specific surface area, pore volume, and pore diameter), and morphological studies were done using a scanning electron microscope (SEM-EDX), (TESCAN, VEGA, JEOL, Peabody, MA, USA) coupled with EDX (energy X-ray dispersive spectroscopy).

3. Results and Discussion

3.1. X-ray Diffraction (XRD) and Raman Spectroscopy

The XRD and Raman analyses were performed on the material prepared by magnesiothermic reduction to study the crystallinity and vibration as well as structural properties, respectively. The phase identification from XRD analysis was cross-examined on SCBA nano-Si, whose 2$\theta$ angle of 28.2 (Figure 1a) showed peaks of highly crystalline silicon, which can be compared with the JCPDS (No. 27–1402 Si XRD) reference pattern number. The crystallite size estimated by the Scherrer equation based on the peak (inset) at 2$\theta$ of 28.2 was 32 nm.

Scherrer equation: $\mathrm{Dp}=\frac{K\lambda }{\beta \cos \theta }$, whereby Dp-Average crystallite size (nm), K-Scherrer constant. It varies from 0.68 to 2.08 and $K$= 0.94 for spherical crystallites with cubic symmetry, λ-X-ray wavelength. For mini XRD, $CuK\alpha$ average = 1.54178 A, B-FWHM (full Width at Half Maximum) of XRD peak, $\theta$-XRD peak position, one half of $2\theta$.

Despite the fact that the as-synthesized silicon has a well-defined crystalline structure, the presence of a small halo centered at $2\theta =22.5$, characteristic of an amorphous property in the material, is quite interesting; similar results have been reported by [15,16].

This confirms the successful reduction of silica into nano-silicon from sugarcane bagasse ash. The structural properties were carried out using Raman spectroscopy for the as-synthesized SCBA nano-silicon, as presented in Figure 1b. The Raman spectra with a peak at 518 cm⁻¹, characteristic of Si-Si stretching mode, shown in the inset, and the half-full width at half maximum (FWHM) of 6 cm⁻¹ were observed; similar results were reported by Li et al. This was as a result of the scattering of the first-order optical phonon of nanosilicon. In their study of the Raman spectrum for crystalline silicon, they observed a peak at 520 cm⁻¹ and a FWHM of 2.8 cm⁻¹ [17,18].

The other two peaks observed at 302 cm⁻¹ and 935 cm⁻¹, respectively, were emanating from the scattering of two transverse acoustic (2TA) phonons and two optical (2TO) phonons, respectively. The peak at 935 cm⁻¹ is reported in the literature due to the stretching mode of amorphous Si-Si. Interestingly, Li et al. [9] reported 300 ${\mathrm{cm}}^{-1}$ and 970 ${\mathrm{cm}}^{-1}$ as the observations for 2TA and 2TO, respectively [17,18].

As a result, Holzapfel et al. [19] reported a significant feature of silicon signals with the most desirable Raman characteristic at 518 ${\mathrm{cm}}^{-1}$ and a full width at half maximum (FWHM) of 6 ${\mathrm{cm}}^{-1}$, similar to the current study’s results. As a result, the scattering of first-order optical phonon mode (TO) was assigned. The two peaks at 300 ${\mathrm{cm}}^{-1}$, and 900 ${\mathrm{cm}}^{-1}$ were notably assigned to the scattering of the second-order transverse acoustic phonon mode (2TA) and the second-order optical phonon mode (2TO), respectively [17,18,19].

3.2. FTIR Spectroscopy

The evolution of the FT-IR spectra of nanosilicon prepared from the magnesiothermic reduction of silica extracted from sugarcane bagasse ash is presented in Figure 2, and the characteristic bond peaks that appeared in the spectra are tabulated in

Table 3. Typical bond peaks appearing in FTIR spectra for SCBA nano-Si.

Wavelength (cm−1)	Bond	Type
1056	Si-O	Stretching mode
446	Si-O	Rocking vibration

.

The Si-O stretching mode (1056 ${\mathrm{cm}}^{-1}$) and Si-O rocking vibration (446 ${\mathrm{cm}}^{-1}$) are observed in the powder samples. A major contribution to the increase of the Si-O bond comes from the aging of the Si nanoparticles. It is highly probable that these bonds are formed by the combination of O₂ present in the DI water; a similar conclusion was made in the literature [20,21].

3.3. Nitrogen Physisorption Studies

The interfacial properties of the as-synthesized SCBA nanosilicon were studied via the N₂ adsorption–desorption method, and the results are reported in

Table 4. Textural properties of sugarcane bagasse ash nano silicon.

Sample ID	BETSSA (m2/g)	Vp (cm3)	Dp (nm)
SCBA nano-silicon	74	0.23	12

below.

The values for the specific surface area obtained by the BET analysis were 74 m²/g, a pore volume of 0.23 cm³, and a pore diameter of 12 nm for the SCBA nanosilicon. When the hysteresis loop was present at a range of 0.6 < P/P₀ < 1.0 from the graph, the degree of porosity was observed. And the loop is associated with large pores [22].

It is interesting to note that the material prepared via the thermo-chemical treatment method was observed to have a moderate increase in specific surface area, porosity, pore volume, and average pore diameter in relation to the increasing P/P₀ at the lower P/P₀ regions, as shown in Figure 3. Moreover, the adsorption isotherm of the as-prepared SCBA nanosilicon had similar features as those presented in [22,23,24].

3.4. Scanning Electron Microscopy (SEM) Analysis

The SEM images are presented in Figure 4a,b at different magnifications. At higher magnifications, agglomerates of SCBA nanosilicon of irregular shapes can be seen, as well as the presence of some spherical particles distributed in a porous structure. This transformation is as a result of the particles having sizes in the nanometric scale consistent with the XRD results, the grain size was in the range 30 nm to 500 nm [24].

The surface morphologies of nano-silicon produced show nanoparticle agglomeration, which is favoured by the magnesiothermic reduction step. As shown in Figure 4c, the process efficacy was assessed using energy dispersive X-ray spectroscopy (EDX). The strong intensity of oxygen and silicon shown in the EDX spectra confirms the silica element is present in the sample. This is a negligible impurity commonly associated with the green method. This may be due to the resultant surface of highly active silicon upon exposure to the aqueous solution of HCl as well as the small presence of unreacted silica in the sample during the reduction method [24,25]. The morphological changes are in agreement with the surface chemistry discussed earlier in the FTIR analysis.

The SEM-EDX analysis presented in Figure 4c confirms that magnesiothermic reduction of silica into silicon is an effective tool in harnessing sugarcane bagasse ash for valuable materials such as silicon nanoparticles. The predominantly as-synthesized silicon content in the sample is indicative that the agro-waste SCBA is rich, useful, and cost-effective for the synthesis of silicon.

4. Conclusions

Based on the findings, silica extracted from sugarcane bagasse ash by L-cysteine chlorination-hydrothermal treatment was used as a precursor for the preparation of nano silicon by magnesium thermal reduction method. The nanosilicon nanoparticles were successfully synthesized from extracted biosilica from sugarcane bagasse ash (SCBA). The physical characteristics and quantitative elemental composition of nano-silicon were investigated. The surface area, pore volume, and pore diameter of the as-synthesized silicon were 74 m²/g, 0.23 cm⁻¹, and 12 nm, respectively. According to Raman spectroscopy, a frequency downshift is caused by a decrease in particle size as a result of particle surface oxidation. The FTIR spectrum confirms the increase in oxidation of the particles during washing with deionized water, with peaks at 446 ${\mathrm{cm}}^{-1}$ and 1056${\mathrm{cm}}^{-1}$, respectively, corresponding to rocking and stretching modes. The morphological, textural, and structural properties determined in this study indicated that the as-produced material exhibits highly rich silicon with a well-defined crystalline structure with a diameter of 32 nm. Moreover, these results showed that nanostructured silicon can be produced from sustainable sources of silica. Magnesiothermic process serves as a potential method for the production of silicon due to its low operating costs, and due to its high violent reactivity, sodium chloride is often used as a stabilizer for an efficient and stable process, and can rival a conventional carbothermal process.