High-purity nano silica powder from rice husk using a simple chemical method

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1 Journal of Experimental Nanoscience ISSN: (Print) (Online) Journal homepage: High-purity nano silica powder from rice husk using a simple chemical method R. Yuvakkumar, V. Elango, V. Rajendran & N. Kannan To cite this article: R. Yuvakkumar, V. Elango, V. Rajendran & N. Kannan (2014) High-purity nano silica powder from rice husk using a simple chemical method, Journal of Experimental Nanoscience, 9:3, , DOI: / To link to this article: Published online: 04 Jul Submit your article to this journal Article views: 5258 View Crossmark data Citing articles: 37 View citing articles Full Terms & Conditions of access and use can be found at

2 Journal of Experimental Nanoscience, 2014 Vol. 9, No. 3, , High-purity nano silica powder from rice husk using a simple chemical method R. Yuvakkumar a, V. Elango a, V. Rajendran a * and N. Kannan b a Centre for Nanoscience and Technology, K.S. Rangasamy College of Technology, Tiruchengode , Tamil Nadu, India; b Department of Biotechnology, K.S. Rangasamy College of Technology, Tiruchengode , Tamil Nadu, India (Received 28 July 2010; final version received 8 January 2012) A highly pure, small particle-sized and high surface area nano silica powder was prepared from rice husk using alkali extraction, followed by an acid precipitation method. The composition, phase, morphology, size and surface area of the as-synthesised nano silica powder was investigated by energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, particle size analyser and BET surface area analyser. High-purity nano silica powder was obtained by sodium hydroxide (NaOH) purification treatment (0.5, 1, 1.5, 2 and 2.5 N). The high purity of silica (99.9%) was obtained at 2.5 N NaOH treatment. The pure nano silica powder that is obtained shows an average particle size of 25 nm with a high-specific surface area (SSA) of 274 m 2 g 1, with an average pore diameter of 1.46 nm. Keywords: high purity; nano silica; rice husk; surface area 1. Introduction Nano silica powder is widely used in areas such as ceramics, chemicals, catalysis, chromatography, energy, electronics, coatings, stabilisers, emulsifiers and biological sciences [1 5]. In view of large-scale industrial applications, an enormous quantity of nano silica powder with controlled shape, size and porosity is required [6,7]. The nano silica powder is generally prepared by using vapour-phase reaction, sol gel and thermo-decomposition [8 12] methods. In most of the above-mentioned methods, nano silica powder is synthesised using chemicals as a raw material. In chemical methods, it is easy to control size, shape and purity of the material but the starting reagents are costly. In industrial applications, low costs and large quantities of initial precursor are needed. Rice husk is one of the most abundant by-products produced in the paddy field. The agricultural by-product includes rice husk, rice straw, plant materials and so on. Of all these by-products, rice husk contains more than 95% silica. Hence, several attempts have been made to produce bulk silica from the most eco-friendly and economical source rice husk [13 18]. The review confirms that rice husk is an excellent source for the production of high-grade amorphous bulk silica powder. In recent years attempts have been made to prepare nano silica powder from rice husk [19 24]. To the best of our knowledge, very few attempts have been made to prepare high-purity nano silica powder from rice husk ash (RHA). The amorphous nanostructured silica powder with an average particle size of 60 nm and a SSA of 235 m 2 g 1 has been obtained by *Corresponding author. veerajendran@gmail.com ß 2012 Taylor & Francis

3 Journal of Experimental Nanoscience 273 non-isothermal decomposition of rice husk in an oxidising atmosphere [19]. Zaky et al. [20] prepared nano silica particles from semi-burned rice straw ash by a two-step procedure of electro-deposition and reduction, and investigated their electronic and mechanical properties. Real et al. [21] synthesised homogeneous nano-sized silica particles by burning rice husk at 1073 K in a pure oxygen atmosphere. Conradt et al. [22] have investigated the potential and limits of using rice husk as a competitive source of nano-structured silica. The size distribution of secondary particles (agglomerates of two or three primary nano particles) was about 26 nm and the SSA of the primary particles was 250 m 2 g 1. Therefore, it is of great interest and significance to investigate the development of highly pure nano silica powder from naturally available raw material, i.e. rice husk, at low cost with high efficiency. In the present investigation, an attempt has been made to produce highly pure, small particle-size nano silica powder with high-surface area from agricultural by-product, such as rice husk, by using a simple user-friendly, alkali extraction followed by an acid precipitation method. The method is simple, cost-effective, reliable and reproducible. The main difference between work carried out by the conventional method [19 22] and the work reported in this study is the effect of different concentrations of sodium hydroxide (NaOH) on high-purity nano silica powder. The optimum conditions to achieve high-purity nano silica have been studied. In addition, the synthesis of nano silica powder is carried out by muffle furnace without oxygen environment. In conventional methods, nano silica particles are prepared under oxygen atmosphere. The purity obtained by this method is enhanced to currently available conventional methods. Further, the nano silica powder that is prepared, has been comprehensively tested using energy-dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), particle size analyser (PSA) and BET surface area analyser to explore the structure, morphology, size and other properties. 2. Experimental 2.1. Synthesis of high-purity nano silica powder The pure nano silica powder was extracted from RHA by controlling different process parameters. The raw material, rice husk, was obtained from a rice mill and washed thoroughly with distilled water to remove any adhering impurities. The washed rice husk was air-dried at room temperature and then burnt at 973 K for 3 h in a muffle furnace under an inert atmosphere. The obtained RHA was washed with distilled water to neutralise the ph in order to remove the sand, dust, light empty grains and fine dirt. Hence, the repeated washing of RHA neutralises the ph and removes the adhered impurities on the surface of silica. The neutralised RHA was refluxed with 6 N HCl (Merck GR) for 1.5 h and then filtered in order to remove metallic impurities and to extract pure nano silica. The filtered RHA was thoroughly rinsed with hot water repeatedly and then boiled with different concentration of NaOH (Merck GR) solutions at 353 K for 1.5 h, while being stirred magnetically, and was then filtered to obtain sodium silicates, after the reaction, SiO 2 ðashþþnaoh! Na 2 SiO 3 þ H 2 O ð1þ The obtained residue was thoroughly washed with hot water for complete extraction of sodium silicate. The obtained ph of the filtrate (sodium silicate) was reduced to 2.0 using controlled addition of concentrated H 2 SO 4, while being stirred magnetically, to extract the nano silica precipitates. The chemical reaction that takes place during the above process is Na 2 SiO 3 þ H 2 SO 4! SiO 2 þ Na 2 SO 4 þ H 2 O ð2þ

4 274 R. Yuvakkumar et al. The obtained precipitate was washed thrice in warm distilled water and then filtered. The obtained residues were sintered at 973 and 1373 K for 3 h in a muffle furnace. To obtain the desired grain size, the obtained silica powder was pulverised using a milling process Characterisation of nano silica powder EDS (JED 2300, JEOL) analysis was carried out on the prepared samples for qualitative elemental analysis. FTIR (Spectrum 100, Perkin Elmer, USA) was used to confirm the silica functional group. The solid precipitate obtained from rice husk was dried under ambient conditions and then crushed into powder. The obtained powder was mixed with potassium bromide in 1:100 weight ratio and the pellets made from this mixture were used to record the IR spectra in the range of cm 1. XRD patterns of the obtained silica powder were recorded using a powder X-ray diffractometer (X Pert Pro, PANalytical, The Netherlands) with Cu-K (wavelength A ) as a radiation source. The average crystallite size of the nano silica powder was calculated from XRD pattern by using the Scherrer s equation [25]: D p ¼ 0:94 ð3þ 1=2 cos where D p is the size of the particle, the wavelength of X-ray, 1/2 the wavelength of full width half maximum and the peak position. A TEM study was made to examine the morphology and particle size of the nano silica powder (Philips, CM 200, USA). TEM images were formed using transmitted electrons having magnification up to 1,000,000 X with a resolution better than 10 A. The observed images were resolved over a fluorescent screen or a photographic film. The PSA (Nanophox, Sympatec, Germany) was used to measure the size distribution of the particles using dynamic light scattering principle operating with a three-dimensional photon cross-correlation technique in the range of 1 10,000 nm. The SSA of the prepared nano silica powder was measured using BET surface area analyser (Autosorb AS-1 MP, USA). The sample was degassed at 568 K for 3 h and the physi-sorption analysis was carried out with nitrogen gas as an adsorbate and liquid nitrogen as a coolant. The multi-point BET correlation technique was used to measure the SSA of the nano silica powder. 3. Results and discussion A simple, economically viable and more convenient method has been developed to synthesise highly pure nano silica powder using alkaline extraction followed by acid precipitation method. Conventionally, nano silica powders are prepared using the decomposition of RHA in an oxidising atmosphere or using high-energy inputs [19 22]. Hence, in this investigation a new approach is adopted to produce high-purity nano silica powders using the decomposition of RHA without an oxidising atmosphere followed by alkaline extraction acid precipitation method. The production of highly pure nano silica powder is possible because of the strict adoption of the following purification parameters while the synthesis is being done: first, rinsing the rice husk thoroughly with distilled water could remove dust and other impurities; second, moderate thermal treatment (973 K) in an inert atmosphere could remove even more inorganic contaminants through decomposition; third, washing RHA before alkali extraction could avoid impurities; fourth, leaching RHA with a 6 N HCl solution removes the metallic impurities; fifth, repeated washing of RHA with warm distilled water until the filtrate is free from acidic impurities; finally, purifying RHA by an alkaline extraction with different NaOH treatment

5 Journal of Experimental Nanoscience 275 (0.5, 1, 1.5, 2 and 2.5 N) and rinsing the silica powder with hot water eliminates carbonates or hydroxides of alkali metal, such as sodium Effect of concentration of NaOH on high-purity nano silica powder To evaluate the effect of purification parameter and to confirm the presence of silica, EDS analysis was carried out on nano silica powder for 0.5, 1, 1.5, 2 and 2.5 N NaOH treatments. NaOH has an effect on the production of high-purity nano silica powder from RHA and the influence of NaOH on purity is summarised in Figure 1(a) (e). Figure 1(d) shows the EDS pattern of nano silica powder at 2 N NaOH purification treatment. Majority of the elements present are silicon and oxygen with some impurities such as sodium and carbon. The EDS pattern of nano silica powder at 2.5 N NaOH purification treatment is shown in Figure 1(e). The 2.5 N NaOH purified nano silica powder predominantly contains elements such as Si and O. The observed Si and O peaks confirm the presence of silica. The purified nano silica powder contains wt% of silicon and wt% of oxygen. It is inferred that the actual composition of silicon in silica is 46%. The observed EDS result shows that the sample contains 66.29wt% of silicon. It can be confirmed from EDS analysis that the sample contains more than 99% purity of silica with free silicon. In addition, the EDS measurement of purified silica powder shows that the content of Si and O is, respectively, 99.9 (% in wt%) and 99.9 (% in at%). The abovementioned results further indicate the absence of other elements such as Na and C, which, in turn, confirms the formation of pure silica structures. In addition, the major chemical groups present in the nano silica powder at 2 and 2.5 N NaOH purification treatments have been identified from the FTIR spectra. Figure 2(a) shows the FTIR spectrum of nano silica powder at 2 N NaOH purification treatment. The broad absorption peaks between 452 and 951 cm 1 are because of the silica structures and the other peaks observed in the range of cm 1 are because of impurities such as carbonate and sodium groups. On the other hand, at 2.5 N NaOH treatment, the sodium and carbonate groups were absent in the FTIR spectrum of pure nano silica powder, as is evident from Figure 2(b). The strong absorption peaks absorbed at 497, 623 and 795 cm 1, respectively, indicate Si O Si bending, Si H and symmetric Si O Si stretching modes of vibrations. Thus, it is confirmed that one can produce a highly pure nano silica powder from RHA using 2.5 N NaOH purification treatment. Figure 3(a) shows the Raman spectra of nano silica powder at 2 N NaOH purification treatment. The presence of broad peaks indicates the formation of silica along with other impurities. In contrast, Figure 3(b) and (c) show the Raman spectra of pure nano silica powder at 2.5 N NaOH purification treatment, sintered at 973 and 1373 K, respectively. The characteristic sharp silica peak observed at 220 and 420 indicates the structure of the cristobalite silica phases [26]. The studies at 2 and 2.5 N NaOH purification treatments of EDS, FTIR and Raman spectra show that the purification treatment at 2.5 N NaOH is valid to achieve high-purity silica powders. The conditions necessary for achieving high-purity nano silica include the optimum time, number of washing cycles, temperature and concentration of NaOH solutions. The purity of silica increases when the number of cycle increases. However, the enhancement in the purity reaches the maximum when the sodium content in the silica is completely removed. The optimisation of washing has been performed with respect to the content of sodium present in the product. All these parameters prevent the resulting nano silica powder from both impurities and larger particle sizes. Hence, the above purification treatment is more essential and viable for obtaining high-purity nano silica powder from RHA. Following this procedure, 99.98% pure silica can be obtained.

6 276 R. Yuvakkumar et al. Figure 1. EDS pattern of nano silica powder (a) 0.5 N, (b) 1 N, (c) 1.5 N, (d) 2 N and (e) 2.5 N NaOH purification treatments.

7 Journal of Experimental Nanoscience 277 (a) (b) Transmittance (%) Transmittance (%) Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure 2. FTIR spectrum of nano silica powder (a) 2 N and (b) 2.5 N NaOH purification treatments. Figure 3. (Colour online) Raman spectra of nano silica powder (a) 2 N NaOH purification treatment, (b) and (c) 2.5 N NaOH purification treatment sintered at 973 and 1373 K, respectively. Various classic methods have been proposed to obtain bulk silica powders from RHA [13 18], though these models unpredicted the step-by-step procedure to obtain pure silica powders. Based on the classic studies, in the present investigation, a new methodology has been proposed to produce highly pure nano silica powder from RHA employing the abovementioned purification parameters. The purity, particle size and surface area values of nano silica powder obtained in this investigation is compared with the conventional methods [19 22] and is given in Table 1 for easy understanding.

8 278 R. Yuvakkumar et al. Table 1. Characteristic comparison of the prepared nano silica with conventional nano silica. Nano silica product Purity (%) Average particle size (nm) Surface area (m 2 g 1 ) [19] [20] [21] [22] Present study (b) Intensity (a.u.) (a) θ (Degree) Figure 4. XRD pattern of pure nano silica powder (a) sintered at 973 K and (b) sintered at 1373 K. The purity (99.9%), particle size (around 25 nm) and BET surface area (274 m 2 g 1 ) values of 2.5 N NaOH-treated nano silica are found to be enhanced in all aspects than the reported values. It is inferred from the above studies that 2.5 N NaOH treatment is best suited for the synthesis of high-purity nano silica powders with a low-synthesis technique. The concentration of NaOH had strong effect on the dissolution, particle size and surface area of silica and also removed some impurities which were not dissolved from the main product. The XRD patterns of the obtained pure nano silica powder were sintered at 973 K, as shown in Figure 4(a). The characteristic broad silica peak observed at 2 ¼ 22 indicates the presence of amorphous silica. The absence of sharp peaks at this temperature indicates the absence of crystallinity. In contrast, to achieve the crystalline phase and to measure the crystallite size of the pure nano silica, the product is sintered at 1373 K. The crystalline phase is evident from the sharp peaks that are observed (Figure 4b). The average crystallite size calculated from the broadening of the corresponding sharp peaks by the Scherrer formula is 25 nm. The sharp peaks observed from the XRD pattern reveal the formation of cristobalite structures [27]. A similar observation was made in the Raman study.

9 Journal of Experimental Nanoscience 279 Figure 5. TEM image of pure nano silica powder. 6 Density distribution (q) Particle size (nm) Figure 6. Particle size distribution of pure nano silica powder. In addition, TEM and PSA studies have been carried out on the sintered samples at 1373 K to correlate the particle size of the obtained pure nano silica powder. The TEM image of the nano silica powder is shown in Figure 5. The obtained nano silica powder is uniform and agglomerated. The shape of the silica grains is spherical with an average homogeneous particle size distribution of about 25 nm, which is in good agreement with the average crystal size estimated from XRD pattern. Further, the particle size distribution of the nano powder was measured using photon cross-correlation spectroscopy. The particle size distribution curve of the nano silica powder is shown in Figure 6. The average particle size distribution of the product obtained by the present process is 29 nm (designated as d 10 % ¼ 24.19, d 50 % ¼ 29.17

10 280 R. Yuvakkumar et al /(W((P0/P)-1)) (P/P 0 ) Figure 7. BET surface area plot of nano silica powder. and d 90 % ¼ nm). The particle size obtained from PSA is reasonable and is consistent with the results of TEM and XRD. The SSA of the material was measured using multiple-point BET surface area analyser. The SSA of the nano silica powder is approximately 274 m 2 g 1 with an average pore diameter of approximately 1.46 nm (Figure 7). BET analysis confirmed that the obtained nano silica powder is a microporous material with high SSA. The results that are obtained by the proposed method confirm the quality of the production of high-purity nano silica powder from rice husk. Most of the other conventional methods are more expensive and require high-energy inputs. To overcome this and to produce high purity, the proposed method can be implemented and is suitable for producing high-purity nano silica powder from rice husk using a simple chemical method. 4. Conclusion Highly pure nano silica powder is synthesised from rice husk using a simple precipitation technique. The nano silica powder that is obtained is characterised in terms of its content, particle size, SSA and pore diameter. The 2.5 N NaOH treatment resulted in high-purity silica content. The purity of silica that is obtained is about 99.9% (wt%) and 99.9% (at%) with an average particle size of 25 nm. The successful synthesis of high-purity nano silica powder is possible by using the method described in this study. Acknowledgements The authors are very thankful to Defence Research and Development Organisation (DRDO), New Delhi, for the financial support to carry out this project (ERIP/ER/ /M/01/1015 dt ).

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