Isolation and Characterisation of High Grade Nanosilicon from Coastal Landform in Ilaje Local Government Area of Ondo State, Nigeria.

The application of silicon and silicon based materials in industries and optoelectronic devices cannot be over emphasized due to their unique properties. The availability of high grade silicon has been subject of interest for manufacturers and industrialist as the material has from discovery been very much insu ﬃ cient to cater for the numerous demands for di ﬀ erent uses and utilization. Numerous e ﬀ orts have been put together to adapt the properties of silicon through the manufacture and fabrication of semiconductor compounds to replace silicon in functionality. Notwithstanding, the quest for more silicon in the environment is yet unabated. This research centers on the isolation and characterisation of high grade nanosilicon from coastal landform in Ilaje Local Government Area of Ondo State, Nigeria due to the attractive nature of the landforms in terms of colour and texture. Sand from the selected study area were categorised into di ﬀ erent forms according to their colour and silicon isolated using the magnesiothermic reduction while the nanosilicon was obtained using an appropriate ball milling process. The morphology of nanosilicon from Zion, Micheal-Ugbonla, and Oluwa Glass coastal landforms shows an agglomeration of particles with irregular shapes having average particle sizes of 58.98 nm, 77.82 nm, and 37.27 nm, respectively. The XRD spectra of the nanosilicon showed sharp, distinct peaks that indicate crystallinity of the samples. The percentages of nanosilicon value obtained ranges from 65.23%-80.30% and considered high enough to ﬁnd speciﬁc useful industrial applications in lithium ion batteries, biomedical devices, optoelectronic device utilization and computer industries.


Introduction
Silicon is a metalloid and one of very few elements that have properties of both metals and non-metals. In nature, silicon can be found as either silica (quartz, sand) or silicates (feldspar, kaolinite). The production of computer chips, transistors, integrated circuits, solar cells, and automotive all often utilize silicon. Due to their unique optoelectronic properties, silicon nanoparticles have recently attracted the attention of various industries. In the bulk state, research indicate that silicon nanoparticles exhibit effective visible photoluminescence attributed to quantum confinement resulting from a reduced particle size smaller than the excitonic Bohr radius and their higher surface area to volume ratio [1,2]. The worldwide demand for potential uses for nanosilicon serves as a significant driving force to reduce the size of silicon materials to the nanometer range [3,4]. Quantum confinement has a substantial impact on silicon's performance and properties when its size is reduced to the 1-100 nm range [5]. Nanostructured materials have electrical, magnetic, and chemical properties that are significantly different from those of the corresponding bulk materials due to the reduced size of their basic building blocks. In comparison to bulk materials, nanostructured materials have been discovered to have increased strength and hardness, higher electrical resistivity, enhanced diffusivity, reduced density, etc. Bulk silicon has limited optoelectronic applications due to the indirect nature of its band gap. However, the development of nanostructured silicon has opened up a wide range of potential applications [6][7][8]. The best material for high density electronics is nanosilicon. The most effective method for converting silicon into a photonic material is by nanostructuring. The small size results in new quantum phenomena that yield extraordinary properties. Materials properties change drastically at the Nano scale because quantum confinement effects arise from the confinement of electrons and holes in the material. Additionally, at the Nano scale, short-range forces like van der Waals forces are dominant [9,10]. As prospective low-cost emitters for optoelectronics, silicon nanoparticles have drawn a lot of interest. Nanosilicon can be utilized as an anode material of Li-ion batteries (LIBs) with the highest gravimetric capacity (4200 mA h g −1 ) among anode candidates created up to this moment, which is a highly probable application. There have been several attempts to use nanostructured silicon for LIB applications in order to overcome silicon's drawbacks, such as volume expansion, pulverization, and evolving electrolyte decomposition, which are in fact observed with macro-sized silicon materials under repeated charge/discharge cycles [11][12][13]. The use of nanosilicon (Si) is the most promising routes for boosting the capacity of modern Li-ion batteries. Rechargeable Li-ion batteries (LIBs) offer a great energy storage solution for clean transportation, local energy storage systems, portable power and electronic devices [14].
The basic method of producing metallurgical silicon is through carbothermal reduction, which uses electric arc furnaces that operate at temperatures above 2000 o C [15]. However, silicon is liquefied during this energy-intensive process, which also destroys the SiO 2 original morphology. Magnesiothermic reduction has received attention recently because of its significantly lower operating temperatures (650 o C). It is possible to sufficiently accommodate the strain and prevent fracture by reducing silicon to the nanoscale [16][17][18][19]. Various researches have been carried out on extraction of elemental silicon using different materials like rice husk, sugarcane bagasse, corn cob as their starting material but there is little or no detailed information on the isolation of nanosilicon from any of these materials. Interestingly, the landform in the study areas have different colours and are used for limited agricultural and industrial purposes. Aside the fact that the sand in the area may contain high grade silicon, it is believed that the colour of the landforms would have effect on the silicon contents. It is hoped that studies on the isolation and characterisation of nanosilicon will increase the industrial value of the landforms. In addition, the research will serve as a data base for the development of new approaches in converting landforms to useful applications as well as provide available source of high grade silicon/nanosilicon for industrial applications.

Collection of samples
The sand samples used in the analysis were collected from different coastal landforms in Ilaje Local Government Area of Ondo State, Southwestern Nigeria. The samples were collected at a depth of about 30 cm from the surface into non-reacting polyethylene bags. The sources and location of the areas where the samples were collected are presented in Table 1.

Theoretical Background
The interplanar spacing (d-spacing) was determined using the Bragg equation [20,21]: where n is the order of reflection, λ is the wavelength of the incident X-ray (m), d is the interplanar spacing of the crystal, and θ is the Bragg's angle ( o ).
Scherrer's equation was used to calculate the crystallite size (L). It is given as: where K is a constant value given as 0.91, the incident X-rays' wavelength is λ, the Bragg's angle ( o ) is θ, the intensity of the Full Width at Half Maximum (FWHM) proportional to a high intensity peak of the diffraction plane is B.

Preparation of samples
Fifty (50) grams of each sand sample was washed successively with 250 ml of 1 M hydrochloric acid (HCl) at a temperature of 75 o C and 1 M sodium hydroxide (NaOH) at room temperature for 3 hours. Distilled water was used after each step to remove any trace of acid and alkali. The washed sample was dried in an oven at 105 o C for 2 hours. A mechanical test sieve shaker was also used to sieve the sand samples into 63 µm for a homogenous size distribution at the Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure. 2.4. Magnesium reduction of the silica and synthesis of nanosilicon 16.0 g of sand (silica) and 15.0 g of magnesium powder (180 µm) were mixed thoroughly for effective reaction to take place. This was subsequently transferred to platinum crucible and heated at a temperature of 800 o C for 30 minutes [22,23]. The samples were then allowed to cool down gradually to room temperature. The equation of reaction is given as: When silica is heated with magnesium powder, MgO and Si are formed. The unreacted Mg and its products is completely removed by washing one gram (1 g) of silicon (Si) sample using 20 ml of 5 M HCl at 100 o C and stirring using a magnetic stirrer for 60 minute for effective leaching to take place. The hydrochloric acid leached product was washed thoroughly with distilled water. The washed Si samples are then dried in an oven at 105 o C for 5 hours and stored in desiccator. Dried silicon particle were ball-milled into nanoparticle by using high energy ball-mill. The isolated nanosilicon was analysed using EDX, SEM (JOEL-JSM 7600F NanoSEM equipment) and XRD (using GBC EMMA XRD equipment, Australia)

X-ray Diffraction (XRD) Analysis
As shown in Table 2, the values of interplanar spacing (d-spacing) and band position of angle 2-Theta (2 θ) in the nanosilicon spectra closely matched those reported in the International Centre for Diffraction Data (ICDD). Welloriented and distinct crystalline peaks were obtained, as shown in Figures 2-4. It was noticed that the silica had a pronounced peak in the sample. This could be due to the powder's contact with air, especially during storage. Also, the presence of oxygen, which could be caused by silica that was not removed via HCl leaching [24]. Table 2 shows the band position (2θ), FWHM, crystallite size, and d-spacing of nanosilicon. Calculations using Scherrer's equation (2) revealed that the sizes of crystallite were 48.34 nm, 51.92 nm, and 23.89 nm for Zion, Micheal-Ugbonla, and Oluwa glass coastal landforms, respectively. XRD patterns of the nanosilicon for the coastal landforms revealed that the diffraction peak planes are in good agreement with the International Centre for Diffraction Data (ICDD), as shown in Table 2.

Scanning Electron Microscopy (SEM) Analysis
As shown in Figures 5-7, the morphology of the nanosilicon for Zion, Michael-Ugbonla, and Oluwa glass coastal landforms revealed the existence of agglomerations of particles with irregular shapes having average particle sizes of 4  58.98 nm, 77.82 nm, and 37.27 nm, respectively. The presence of agglomerates is an inevitable phenomenon that is associated with the interaction between particles of small size due to electrostatic attraction or van der Waals force [25]. The small size of the silicon nanoparticles obtained indicates that they can be usefully exploited in a suitable

Energy Dispersive X-ray (EDX) Analysis
The elemental composition of the raw samples from the coastal landform was determined by energy dispersive Xray (EDX), as shown in Table 3. EDX indicates that silicon has the highest percentage among the elements observed in the spectra. Impurities in quartz sands like Na, K, Al, and Fe are presumed to emanate from minerals like mica and feldspar [26]. This range of silicon content (i.e., 58.60%-63.73%) in the raw samples as obtained from the respective sources (i.e., Zion, Micheal-Ugbonla, and Oluwa glass landforms) is considered relatively low for meaningful industrial utilization. However, the EDX analyses after the process of isolating nanosilicon revealed that the nanosilicon obtained from  the samples ranged from 65.23% to 80.30%, as presented in Figures 8-10. The coastal landform obtained from Zion with a whitish colour had the highest percentage, while that obtained from Oluwa glass with a brownish colour had the lowest percentage value of nanosilicon. This could be due to the chemical composition of the samples, as earlier reported by [27]. These values are high enough and can find useful applications in material fabrication and industrial utilization. Although the percentages of all other elements in the landforms decreased considerably, the magnesium contents were found to increase. This could be due to the magnesium powder used as a reducing agent. The presence of metallic contaminants may have detrimental effects for some applications, such as solar cells, nonetheless, these metallic impurities may increase the conductivity of nanosilicon in battery applications [28,29]. Sulphur and potassium were not noticed in the raw samples, their appearance in the isolated samples could have been injected into the materials via the EDX machine.

Conclusion
It was established in the research that the percentages of nanosilicon value obtained ranged from 65.23%-80.30% and were high enough to find useful industrial applications in areas like lithium ion batteries, biomedical devices, photovoltaic/solar cells, computer industries, etc. Silicon and oxygen were prominent among the samples characterized.
In conclusion, the percentage of isolated nanosilicon from the coastal landform is pure enough to be further purified and used as a starting material for the production of semiconductor-grade silicon.