1. Introduction

Tin oxide (SnO₂) is a semiconductor with optical properties and the semiconductor oxides are important for the development of smart and functional materials, devices and systems [1]. Crystalline tin oxide (cassiterite) structure is a wide band gap (3.6 eV). It is typically transparent n-type semiconductor in its grown state [2]. Semiconductor nanoparticles exhibit change in electronic properties relative to the bulk counterpart and the band gap increases as the solid becomes smaller in particle size. Semiconductor oxides have two unique structural features which are mixed cation valences and an adjustable oxygen deficiency are the basis for creating and changing several novel material properties from chemical, electrical, and optical to magnetic properties. Semiconductor oxide nanoparticles exhibit a change in the electronic properties relative to the bulk counterpart and the band gap increases as the solid becomes smaller in particle size [3]. The research on tin oxide semiconductor is experiencing rapid growth due to wide range of applications. SnO₂ particles are widely used in gas sensing due to their high mobility of conduction of electrons, good chemical and thermal stability under the operating condition of such as gas sensors [2] [4] [5] [7], catalyst [8] [9], photosensors [7], resistors [1], electrodes in glass melting furnaces [11], antistatic coating [12], transparent heating elements [13], batteries [14] [16] [17] [18], photocatalytic degradation of organic dyes [19] [20] [21] [22] [23], photovoltaic devices [8], high electrical conductivity with optical transparent heating elements [9], etc. The aim of this study is to use acid mine drainage (AMD) waste solution to synthesize SnO₂ particles using tin II chloride as the starting reagent. SnO₂ can be synthesized using physical method such as mechanical milling from large particle size to smaller particle sizes [24] [25] [26], and chemical methods such as chemical precipitation [27] [28] [29], hydrothermal [5] [30] [31] [32], microwave [12] [33], microwave [34], sonochemical [35], sol-gel [36] [37], solvothermal [38] [37] [39]. The size, morphology, stability and properties of synthesized SnO₂ nanoparticles are of great importance and should be considered seriously [40]. Oxides of semiconductors like tin oxide have different morphologies depending on the conditions and methods of their synthesis. Their morphologies appears in different nanostructured shapes some of which are nanoflower [41] [42], nanobelt [43], nanotree [44], rods [40] [45], wires [46] [47] tubes [3], belts [43] [48], Chemical vapour deposition (CVD) process [49], needles [50] [51], nanodiskettes [52] [53], etc. Toxicity caused by SnO₂ nanoparticles is very low and it has no adverse health effect which means no carcinogenic effect. The cheap starting materials and other suitable parameters are very important to reduce cost of synthesizing SnO₂ nanoparticles. Therefore, it is very important to design a method of synthesis using cheap and non-toxic reagents.

2. Materials and Methods

2.1. Chemical and Reagents

Raw mine water was collected from Navigation core mining site in Mpumalanga, South Africa was collected in 5L plastic containers and stored in the refrigerator regulated at a temperature of 4°C. The AMD sample was filtered with 0.45 µm membrane filter in order to remove particulate materials present in the mine solution. All chemicals used include absolute alcohol (99.5%) and tin II chloride were reagent grade chemicals purchased from Merck chemicals and used without further treatment or purification.

2.2. Chemical Precipitation

Different concentrations (0.1 M, 0.2 M, 0.3 M, 0.4 M, and 0.5 M) of tin II chloride solution were prepared in five different 250 mL beakers each. A volume of 50 mL of tin II chloride solution was measured into 100 mL acid mine (AMD) solution as received from the mine. The solution mixture was subjected to constant continuous vigorous agitation with a magnetic stirrer at 250 rpm for 60 minutes contact time. Yellow precipitate was formed as the tin II chloride solution was gradually added to the acidic mine solution and the precipitate was filtered and washed with 100 mL of distilled water thrice and dried in a regulated oven at a temperature of 104°C for a period of 3 hours. The weight of the residue was recorded and the drying process continued until a constant weight was obtained. The effect of contact time was investigated by measuring 50 mL of 0.04 M of tin II chloride and added drop wise into 100 mL of raw AMD solution at different contact time of 30, 60 90, 120 and 150 minutes. The same process of rinsing, drying and weighing was carried out for the precipitate formed.

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2.3. Characterization

The precipitate was characterized by using analytical techniques such as X-ray diffraction (XRD). The precipitate was identified with an X-ray powder diffraction patterns using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.542 Å) with operating current and voltage of 40 mA and 40 kV respectively. The precipitate was scanned at the rate of 0.02° (10 s per step) at diffraction angle 2θ range from 20-80°. The morphology and particle size of precipitate was examined with both high resolution and high resolution scanning electron microscopy (HRSEM). The scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) technique was used to provide available information on morphology and surface texture of individual particle as well as the chemical composition of the elements present in the precipitate. For the SEM analysis, a small quantity of sample was put on carbon coated sample holder stub with a gentle blow applied to remove loosely bound particles and the remaining dust particles were stuck to the carbon coat on the sample holder to dry at room temperature before it was examined under the SEM measurement using a HITACHI S-4700 electron microscope. The precipitate was dried and used for morphological analysis using Phillips Tecnai F20 super-twain HRTEM. Little amount of the precipitate was placed in sample bottle containing 5 mL absolute alcohol and sonicated for 10 minutes to obtain a good particle dispersal on the copper grid and allowed to dry at room temperature. The analysis was carried out to examine the size and morphology of the precipitate. Fourier transform infrared (FTIR) spectroscopy was conducted to identify the functional groups present in the precipitate. The FTIR analysis was conducted with Perkin Elmer RX1 series FTIR spectrometer to record the IR spectra of the sample. The analysis is for phase purity, and to identify related functional group in the precipitate. Small amount of the precipitate was required for the functional group analysis. The sample was mixed with KBr at different ratios to make it in pellets form and dried in a regulated oven at 105°C for 12 hours to remove any trace of moisture present in the precipitate. Brunauer-Emmette-Teller (BET) surface area of the precipitate was measured at a temperature of 77.35K using nitrogen adsorption method with a quantachrome NOVA 2000 surface analyzer. The surface area of the sample was prepared by washing the precipitate with acetone and dried at 100°C for 5-8 hours under constant flow of nitrogen gas. The sample powder was dried in an oven regulated at 105°C for 12 hours to remove any trace of moisture present in the sample.

3. Results

The optimization of tin II chloride concentration and contact time of synthesizing SnO₂ is presented in Figures 1 and 2. The optimized concentration of tin II chloride and contact time in Figures 1 and 2 are 0.04 M and 60 minutes respectively. The optimum concentration yielded a mass of 0.79 g while the contact time was 0.64 g.

Figure 1. Concentration (M) of tin II chloride. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3

Figure 2. Contact time of precipitating tin oxide. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, vol. of SnCl2 = 50 mL, optimum concentration = 0.04 M, n = 3

The precipitate was taken for analyzes so as to assess its quality. Figure 3 presents the result of the XRD diffraction pattern of the precipitates formed from the treatment of AMD with tin II chloride reductant. The mineral phase identified was tin oxide or cassiterite (SnO₂). The diffraction pattern of the crystallite appears at reflection angle 2θ indexed at 27°, 3[°] 51° and 65° which correspond to (110), (101), (211) and (301) lattice plane base on tetragonal phase of cassiterite (SnO₂) (JCPDS File No. 41-1445, a = b = 4.738 Å, and c = 3.178 Å) were indexed to cassiterite (SnO₂) structure. No other characteristic diffraction peaks due to metallic tin (Sn) or tin oxides was observed therefore, the crystal was purely cassiterite (SnO₂). Park et al. [2] reported that similar result was obtained in the synthesis of SnO2. The polycrystalline diffraction of cassiterite (SnO₂) was revealed by the first three predominant peaks exhibited at (110), (101) and (211) planes. The same peaks of the crystallite was also observed to be broad which can be attributed to the small size of the particles [54]. The broadening of the XRD peaks revealed the characteristic of cassiterite (SnO₂). The result obtained from the XRD analysis of cassiterite crystallite agrees with that obtained in the literature [54] [55] [56] [57] [58] [59].

Figure 3. Powder X-Ray diffraction pattern of Cassiterite (SnO2) synthesized from the AMD: Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, conc. of SnCl2 = 0.04 M, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3, CS = cassiterite (SnO2)

The average crystalline size of the tin oxide nanoparticle were calculated using Debye Scherrer formula, D = Kƛ / βCosθ

Where D is the mean crystalline size, K is a grain shape dependent constant (0.9), λ is the wavelength of the incident beam, θ is a Bragg reflection angle and β is the full width half maximum.

Figure 4 presents the SEM morphology (A) and the EDS spectrum (B) of the elements found in the crystals synthesized from the reaction mixture of AMD collected from Navigation coal mine with SnCl₂ as reductant. The surface morphology of the residues is spherical in shape with different sizes and there are some irregular crystals around the spheres. The EDS spectrum revealed that the chemical composition of the cassiterite are Sn, O and Cl. Table 1 presents the EDS of cassiterite (SnO₂) synthesized from the treatment of AMD with tin II chloride solution. The EDS revealed that the chemical composition of the particles was made up of 65.5% of Sn, 31.96% of O, and 2.54% of Cl. The product is composed mainly of tin and the presence of carbon is from the sample stub. The trace of chlorine detected (2.54%) comes from the tin II chloride used in the precipitation process as presented in Table 1. The composition of the elements present in the synthesized sample confirmed that the mineral phase of the sample was cassiterite (SnO₂).

Table 1. Energy dispersive spectroscopy (EDS) chemical composition of Cassiterite (SnO2) synthesized from AMD. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, conc. of SnCl2 = 0.04 M, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3, SnCl2

Figure 4. SEM morphology (A) and EDS spectrum (B) of cassiterite (SnO2) synthesized from AMD. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, conc. of SnCl2 = 0.04 M, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3

Figure 5 presents the HRTEM micrograph of SnO₂ nanoparticle (A), selected area electron diffraction [SAED] (B) results of SnO₂ crystallite formed from the treatment of AMD with tin II chloride as reductant. The morphology of the SnO₂ showed high resolution fringes which indicate that the crystallite was crystalline with an average crystal size range from 1.5 - 2 nm as measured with imageJ software. The selected area electron diffraction ring showed tiny spots round the ring which indicates that it is polycrystalline. The lattice fringes spacing of SnO₂ nanoparticles shows contribution mainly from (110) and (101) planes. The selected area electron diffraction ring showed tiny spots round the ring which indicates that it is polycrystalline. The lattice fringe spacing of SnO₂ nanoparticles shows contribution mainly from (110) and (101) planes. The selected area electron diffraction ring showed tiny spots round the ring which indicates that it is polycrystalline. Fast Fourier transform (FFT) analysis of selected regions of the coating reveals details of the local SnO₂ structure. The corresponding ring pattern of SAED (B) confirmed the presence of single crystalline cassiterite (SnO₂) phase nanoparticles. It reveals the presence of fluffy particles that looks like agglomerated tin oxide nanoparticle with uniform particle size ranging from 2 to 3 nm which confirms the presence of single-crystal SnO₂ nanostructure.

Figure 5. HRTEM morphology (A), SAED, (B) of cassiterite (SnO2) synthesized from AMD. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, conc. of SnCl2 = 0.04 M, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3

Figure 6 presents the FTIR absorption spectrum of cassiterite obtained from reaction of AMD with tin II chloride solution. Fourier transform infrared (FTIR) analysis of selected regions of the coating reveals the details of the local SnO₂ structure. The cassiterite crystal showed absorption peaks at 483.5 cm^-1, 496.5 cm^-1, 1039.75 cm^-1, 1131.75 cm^-1, 1485.5 cm^-1, 1692.5 cm^-1 and 3000 cm^-1. The Sn-O bond exhibits its stretching vibration mode at absorption band of 464 cm^-1 and 503.3 cm^-1. The absorption band at 483.3 cm^-1 and 503.3 cm^-1 in the FTIR spectrum can be assigned to lattice vibration stretching mode of Sn-O and OSnO respectively. Studies have shown that SnO and OSnO absorption bands at 400-800 cm^-1 are characteristic of nanocrystalline cassiterite structure which contain bound OH group [60]. The broad water absorption band with OH group vibration stretching was observed at 2800-3450 cm^-1 and the bending vibration of water OH group at 1643-1665 cm^-1. The absorption band at 1031 cm^-1 and 1117.3 cm^-1 resemble those assigned to Sn-OH modes [61]. The result obtained in this study is within the range attributed to tin oxide in the literature [40] [61] [62] [63]. The SnO particle is metastable and will decompose to SnO₂ and H₂ when subjected to drying or evaporation at a high temperature above 100-150°C.

Figure 6. FTIR absorption spectrum of cassiterite synthesized from AMD. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, conc. of SnCl2 = 0.04 M, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3

The SnO particle is metastable and will decompose to SnO₂ and H₂ when subjected to drying or evaporation at a high temperature above 100-150°C. The SnO particle is metastable and will decompose to SnO₂ and H₂ when subjected to drying or evaporation at a high temperature above 100-150°C. The SnO particle is metastable and will decompose to SnO₂ and H₂ when subjected to drying or evaporation at a high temperature above 100-150°C. BET surface area of cassiterite from the treatment of AMD with tin II chloride is presented in Figure 7.

Figure 7. Nitrogen adsorption-desorption isotherm of cassiterite (SnO2) synthesized from AMD. Experimental conditions: pH of AMD = 2.14, vol. of AMD = 100 mL, conc. of SnCl2 = 0.04 M, vol. of SnCl2 = 50 mL, contact time = 60 minutes, n = 3

Figure 7 presents the nitrogen adsorption-desorption isotherm of cassiterite (SnO₂) synthesized from the reaction between AMD and tin II chloride salt solution. The hysteresis loop of the SnO₂ sample adsorption classification was Type II and the adsorption-desorption classification was H3 and the BET surface area obtained for the SnO₂ particle was 7.7 m²/g. The BET surface area obtained in this study was very low compared to 35 m²/g reported in the literature for the reagent grade synthesized cassiterite [56] [57] [58] [64].

ACKNOWLEDGMENTS

This research was supported in part by the Water Research Commission, South Africa and technical support was provided by contributions to this research work: Water Research Commission (WRC) South Africa, Dr. Remy Bucker National Research Fund (NRF) Ithemba-labs, Western Cape, South Africa, Mrs. Ilse Wells, University of The Western Cape (UWC), South Africa and Mrs. Miranda Waldron, University of Cape Town (UCT), South Africa, and Dr. Avoseh Opeyemi, Lagos State University.