Rice Husk Reuse in the Preparation of SnO₂/SiO₂Nanocomposite

Abstract

In this study, biogenic SiO₂ of high purity and high surface area obtained from rice husk was used for prepare a nanostructured SnO₂/SiO₂composite. The predominantly amorphous silica was extracted in an acidic route and then the nanocomposite was done via sol-gel route using ethylene glycol and citric acid followed by heat treatment. SiO₂ content of the rice husk was determined by X-ray fluorescence (XRF) and its specific surface area determined by nitrogen adsorption. The composite nanostructured SnO₂/SiO₂ was structurally characterized by the techniques of X-ray diffraction (XRD), Raman and Fourier transform infrared (FT-IR) spectroscopy. The morphological characteristics were revealed by scanning electron microscope (SEM).

biogenic SiO₂; rice husk; SnO₂/SiO₂ composite; Pechini method

1 Introduction

Agroindustry has generated a large content of residues and a necessity of utilization of these residues might reduce pollution and increase energy savings. Among these residues, rice husks have received attention due to its large volume produced and high content of amorphous silica, approximately 21%^1,2. SiO₂ can be obtained by direct calcination followed by calcination chemical treatment, and by sol-gel route. Silica from rice husk is considered as an alternative to commercial silica³ due to low cost and wide application, such as to obtain silicon carbide⁴, catalysts⁵, adsorbents⁶, zeolites⁷, silicates⁸ and cement⁹.

Compared to conventional materials, nanomaterials have interesting physical and chemical properties, besides several areas of application, and relative easy production carried out by different methods such as microemulsion¹⁰, hydrothermal¹¹and sol-gel¹².

Tin oxide (SnO₂) is an important semiconductor oxide in the industry¹³, but despite its wide usage^14-17 has low thermal stability and tendency to aggregation¹⁸, that can be overcome by incorporating the amorphous SiO₂ to SnO₂^[19], generating the SnO₂/SiO₂nanocomposite. There are several studies using SnO₂/SiO₂, like composite²⁰, xerogel²¹, nanotubes²² and films²³, which application can be in photocatalysis²⁴ and sensors²⁵.

This work aims prepare and characterize SnO₂/SiO₂ nanocomposites from amorphous biogenic silica of high purity extracted from rice husk by Pechini method²⁶, as an alternative to TEOS (tetraethoxysilane). Silica from rice husk was characterized by X-ray fluorescence (XRF) and by physical adsorption of nitrogen. The SnO₂/SiO₂ nanocomposite was characterized by scanning electron microscopy (SEM), by X-ray diffractometry (XRD) and Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR).

2 Experimental

2.1 Extraction of SiO2 and Preparations of nanocomposite

Extraction of silica was made by grinding of the rice husk (RH) followed by chemical treatment with HCl (10% v/v), in the ratio RH:HCl 1:3, for one hour with constant stirring to solubilize organic matter. The solubilized RH was washed with ultrapure water and it was filtered under vacuum, obtaining a pulp (PRH) which was treated with a solution of H₂SO₄ and H₂O₂ in the ratio of 1:2:1 (w/v/v) - PRH:H₂SO₄:H₂O₂ - under constant stirring for one hour to promote the oxidation of organic matter. Finally, the oxidized PRH was washed with ultrapure water and the biogenic SiO₂ was obtained by vacuum filtering and calcinations in an oven for 4h at 600 °C.

SnO₂/SiO₂ nanocomposite was prepared by adding biogenic silica and SnCl₂.2H₂O to a solution of nitric acid (NA)/ethylene glycol (EG), obeying the following proportions 1:3:12 (SnCl₂.2H₂O: CA: EG) and 1:4 (w / w) - (SiO₂: SnCl₂.2H₂O). This mixture was stirred for 1h at 60 °C with subsequent heat treatment at 250 °C for 2 hours, followed by calcination at 400 °C during 1 hour. All steps of calcination and heat treatment were performed without heating rate.

2.2 Characterization

Biogenic silica was characterized by x-ray fluorescence (XRF) using an Epsilon 3XL spectrometer. Specific surface area was determined by nitrogen gas adsorption using a Quantachrome Autosorb-iQ equipment by multipoint BET method (Brunauer Emmett Tell) and the average diameter of the silica particles was estimated using the following equation for spherical particles

$d_{BET}=\frac{6}{\left(A_{BET}.{\rho }_{TEO}\right)}$ (1)

where ${\rho }_{TEO}$is the theoretical density²⁷ for the amorphous silica, having a value of 1,92 g cm^–3 and $A_{BET}$ is the specific surface area.

Morphology of SnO₂/SiO₂ nanocomposite was determined by scanning electron microscopy (SEM) using a JEOL model JMS6360-Lv microscope. The crystallinity of the material were characterized by X-ray diffraction (Shimadzu/XRD-7000) with CuKα radiation (λ = 1.542Å), 40 kV and 30 mA. Vibrational spectra were recorded on a Bomem MB-series spectrophotometer (Model B100) and Raman spectra were obtained using Confocal Raman equipment model T64000 Jobin-Yvon with laser excitation at 532 nm. Composition of the biogenic silica and the nanocomposite were identified by XRF, and the crystallite size was determined by Debye-Scherrer equation:

$d=\frac{K\lambda }{\beta cos\theta }$ (2)

where d is crystallite size, K is shape factor (typical value of about 0.89 for spherical crystalline solids with cubic unit cells), λ is CuX-ray wavelength (1.542 Å), θ is Bragg diffraction angle and β is the peak width of the diffraction peak profile at half maximum height.

3 Results and Discussion

3.1 Amorphous SiO2

Composition of the biogenic silica is shown in Table 1. According to the percentage values of the components, the methodology used to extraction provided SiO₂ with purity of approximately 98.6%, higher than commercial SiO₂^[28]. The analysis of the surface area by BET method showed an approximated^29,30 value of 450 m²g^–1 and a mean particle size of 7 nm, given by Equation 1.

Table 1
Composition of SiO₂ determined by XRF.

Component	%
SiO2	98.589
P2O5	0.834
Al2O3	0.390
CaO	0.130
Fe2O3	0.020
Ag2O	0.011
MnO	0.007
TiO2	0.003
Cl	0.016

3.2 Structural study of SiO2 and nanocomposite

XRD diffractogram of the biogenic SiO₂ is presented in Figure 1a and shows a broad peak located approximately at 2θ = 22.5°, that suggests an amorphous characteristic of the sample and agrees with the reported JCPDS data (card No. 01-086-1561).

Figure 1
XRD obtained for (a) biogenic SiO2 from rice husk and (b) SnO₂/SiO₂ nanocomposite.

XRD pattern for the nanocomposite is shown in Figure 1b and reveals a small shoulder at 2θ = 21.7° that can be attributed to amorphous SiO₂ and other peaks assigned to crystalline SnO₂. All values of diffraction peaks are in accordance with JCPDS pattern (card No. 00-041-1485). The main diffraction peaks observed for SnO₂ are centered approximately at 2θ values of 27.1°, 34.2° and 52.1°. In addition, the crystallite size based on the major diffraction peak is 27.8 nm for the nanocomposite.

3.3 Morphology of nanocomposite

Surface morphology of the nanocomposite is presented in Figure 2. SEM micrographs reveal formation of a heterogeneous mixture of agglomerates with irregular shapes and sizes.

Figure 2
SEM images magnified (a) 3500 and (b) 10000 times.

3.4 FTIR and Raman analyses

FT-IR and Raman spectra for the SnO₂/SiO₂ nanocomposite are shown in Figure 3a and 3b, respectively. Comparisons between wavenumber obtained in this work with literature values are exhibited in Tables 2 and 3.

Figure 3
Spectra for the SnO2/SiO₂ nanocomposite (a) FT-IR (b) Raman.

Table 2
Assignments of infrared bands of the SnO₂/SiO₂nanocomposite.

This work	LiteratureRef	Assignment
(cm–1)	(cm–1)
467	46030	δ (Si-O)
548	55832	νsim (Sn-O)
664	67432	νass (Sn-O-Sn)
807	80030	νsim (Si-O-Si)
1097	107430	νass (Si-O-Si)
1628	1620-163029	δ (H-O-H)
3455	3350-345029	ν (O-H)

Table 3
Assignments of the Raman bands for the SnO₂/SiO₂nanocomposite.

This work	LiteratureRef	Assignment
(cm–1)	(cm–1)
1592	158040	G band
1376	136040	D band
752	77434, 77535	B2g
681	68734	A2u LO*
620	63834, 63135	A1g
540	51234	A2u TO†
495	47534, 47435	Eg
347	37734	Eu (2) LO
242	23634	Eu (1) TO
125	10034	B1g

• ^*
  LO – longitudinal optical phonons.
• ^†
  TO – transverse optical phonons.

Infrared spectrum shows a broad band at 3455 cm^–1 attributed to O-H stretching from hydroxyl groups presents on surface of the material and the band at 1628 cm^–1 is associated with bending H-O-H bond groups of adsorbed water molecules³¹. Bands at 467, 807 and 1097 cm^–1 are assigned respectively to vibrational modes of O-Si-O bending, to symmetric stretching of Si-O-Si group and to the asymmetric stretching of Si-O-Si structural bond of siloxane³². Vibrations in the range from 500 to 700 cm^–1 are assigned to Sn-O-Sn group as result of condensation reactions³³. The bands characterized by the peaks 548 and 664 cm^–1 are assigned to the Sn-O stretching vibration and Sn-O-Sn asymmetric vibration, respectively³⁴.

SnO₂ has a tetragonal rutile crystalline structure with point group D_4h^{[35]. A}ccording to Li et al. there are three typical modes to SnO₂ in Raman spectrum (474 cm⁻¹ (Eg), 631 cm⁻¹(A_1g) and 775 cm⁻¹ (B_2g). When the particle size decreases, A_1g and B_2g modes of SnO₂ are shifted to lower wavenumbers and E_g mode is shifted to higher wavenumber³⁶. Obtained results have shown that mode at 475 cm^–1 is assigned to translational mode (E_g) of the oxide. On the other hand, mode at 620 cm^–1 is assigned to symmetric O-Sn-O stretching (A_1g) and the third mode at 752 cm^–1 is assigned to asymmetric O-Sn-O stretching (B_2g)³⁷.

The position of SnO₂ peak in Raman spectrum is dependent of particle size and a fourth vibrational mode (B_1g) peak appears only to nanomaterials. Thus, in the Raman spectrum the presence of an intense peak centered at 125 cm^–1 is associated to non degenerated B_1g mode of SnO₂ and is assigned to rotation of the oxygen atoms, with all oxygen atoms participating in the vibration at tetragonal unit cell of rutile³⁶. All modes and assignments are presented in Table 3.

The effects of particle size and disorder in the material lead to a relaxation of the Raman selection rule, and some modes that are usually inactive in Raman become actives. Furthermore, the peaks at 242 cm^–1, 347 cm^–1, 540 cm^–1and 681 cm^–1were assigned to optical phonon modes of SnO₂, E_u (1) TO, E_u (2) LO, A_2uTO and A_2uLO of SnO₂, where LO and TO are longitudinal and transverse optical phonons, respectively^34,38-40.

Peaks at 1354 and 1587 cm^–1 are termed D and G bands, respectively⁴¹, and confirm the presence of amorphous carbon that is typical of the temperature and of the sol-gel route used for obtain the nanocomposite.

4 Conclusion

Biogenic SiO₂ extracted from rice husk and the SnO₂/SiO₂nanocomposite were characterized by XRF, XRD, SEM, FTIR and Raman. The method used in SiO₂ extraction was efficient to obtain amorphous biogenic SiO₂ of high purity in nanometer scale. SnO₂/SiO₂ nanocomposite behaved as a solid mixture of SiO₂ of low crystallinity and crystalline SnO₂. Therefore, from rice rusk is possible obtain biogenic SiO₂ of high purity that added to SnO₂ provide the SnO₂/SiO₂ nanocomposite.

Acknowledgements

The authors acknowledge financial assistance from FAPEAM (Grant # 2985/2012) and the Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ-UNICAMP/FAPESP Grant # 2009/54066-7) for use of Raman equipment.

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Publication Dates

• Publication in this collection
  May-Jun 2015

History

• Received
  22 Apr 2015
• Reviewed
  09 June 2015