Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method
© Le et al.; licensee Springer. 2013
Received: 8 November 2012
Accepted: 22 December 2012
Published: 6 February 2013
Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H2SO4 at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m2/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.
Globally, approximately 600 million tons of rice paddies is produced each year. On an average, 20% of the rice paddy is husk, giving an annual total production of 120 million tons . In Vietnam, the average output of the country is 42 billion tons per year, and this country is the second largest manufacturer of rice in the world. Rice husk (RH) is an agricultural waste material that should be eliminated. The chemical composition of RH is similar to that of many common organic fibers, containing cellulose, lignin, hemicelluloses, and silica, which is the primary component of ash. After burning, the organic composition is decomposed and rice husk ash (RHA) is obtained [1–3]. RHA is one of the most silica-rich raw materials containing about 90% to 98% silica and some amount of metallic impurities (after complete combustion) among the family of other agro-wastes [4–8]. It is important that the silica in RHA exists in the amorphous state and has high surface area [9–13]. Because of these features, silica has many applications, such as sources for synthetic adsorption materials [14–16], carriers, medical additives, fillers in composite materials, etc. [17, 18], and demonstrates advantages when achieved at nanometer size.
Silica is a polymer of silicic acid consisting of inter-linked SiO4 units in a tetrahedral fashion with the general formula SiO2. In nature, it exists as sand, glass, quartz, etc. Naturally occurring silica is crystalline, whereas synthetically obtained silica is amorphous in nature. Silica used in chemical applications is synthesized from either silicate solution or silane reagents .
There are various methods to prepare silica nanoparticles. Adam et al.  synthesized spherical nanosilica from agricultural biomass as RH via the sol–gel method. The resulting silica particles were shown to be agglomerates with an average dimension of 15 to 91 nm. Jal et al.  synthesized nanosilica via the precipitation method, and the resulting nanosilica were found to have a particle size of 50 nm in dimension. However, the sol–gel technique [19, 21–23] is the most common method for silica synthesis. It involves simultaneous hydrolysis and condensation reaction. In this process, a sol of sodium silicate or silicon alkoxide or halide gets converted into a polymeric network of gel. During silica synthesis by sol–gel process under certain conditions like restriction of gel growth, silica gets precipitated. In such preparation, the steps involved are coagulation and precipitation from silica solution. In the present investigation, we have focused our effort on preparing stable nanosilica from sodium silicate which was synthesized from Vietnamese rice husk using the sol–gel technique.
Rice husk from the natural rice source of Mekong Delta, Vietnam, was used. Sodium hydroxide, cetyltrimethylammonium bromide (CTAB), cetyl amine (CA), polyethylene glycol (PEG, 10,000), Arkopal, cethyl ammonium chloride (CAC), Aliquat 336, alkyl dimethyl benzyl ammonium chloride (ADBAC), cetylpyridiniumbromide (CPB), and cetyltrimethylammonium chloride (CTAC) were purchased from Merck (Darmstadt, Germany) and used as surfactant agents. Chlorhydric acid, sulfuric acid, and n-butanol were all purchased from Xilong (Guangzhou, China).
Pretreatment of the RHA
The pretreatment of the RHA consisted of acid and thermal treatments. After treating the RH with 10% HCl and 30 wt.% sulfuric acid solution, the material was burned in a muffle furnace at 600°C for 4 h to remove all incorporated hydrocarbons.
An acid washing step was used to remove the small quantities of minerals prior to silica extraction from RHA in the following manner. The calcinated RHA (10 g) was acid-leached with 10% HCl and afterwards 30 wt.% sulfuric acid solution at 100°C for 2 h in a Pyrex three-neck round-bottom flask equipped with a reflux condenser in a hemispherical heating mantle. Then, the slurry was filtered and washed with distilled water for several times until the pH value equaled 7.
Preparation of sodium silicate solution
Sodium hydroxide solution (3.5 mol/L) was added to the pretreated RHA and boiled for 5 h in a Pyrex three-neck round-bottom flask equipped with a reflux condenser in a hemispherical heating mantle to dissolve the silica and to produce a sodium silicate solution. The solution was filtered and washed with boiling distilled water. The final solid sample was cooled to room temperature.
Synthesis of silica nanoparticles
Surfactant (2.0 wt.%) was dissolved in the water/butanol (1:1) solvent. Subsequently, RHA-derived sodium silicate was slowly added into the CTAB/water/butanol solution, and the mixture was stirred at 60°C. Then, 0.5 mol/L sulfuric acid solution was added gradually into the suspension in order to initiate the hydrolysis-condensation reaction at pH ~ 4. The resulting gel mixture was aged at 60°C for 8 h.
Then, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 wt.% of CTAB were dissolved in the water/butanol solvent with 1:1 ratio. Subsequently, RHA-derived sodium silicate was slowly added to the CTAB water/butanol solution that was being stirred at 60°C. Then, 0.5 mol/L sulfuric acid solution was added gradually into the solution in order to initiate the hydrolysis-condensation reaction. The suspension was adjusted until the pH is 4. The resulting gel mixture was aged at different temperatures in the function of time. The aged silica gel was dispersed in butanol and washed with distilled water for several times. Nanosilica was calcinated at 550°C for 4 h in atmospheric condition to remove the surfactant. The final product was obtained and stored in desiccators before further characterizations.
The chemical compositions of the RHA before and after the treatment by acid were determined by adsorption atomic spectroscopy (AAS), and the results are presented in Table 1. Unlike conventional organic silicon compounds, the RHA is an agriculture waste, which contains several main extraneous components. The thermal and acid treatments are efficient, resulting in a material with high reduction in K2O, Al2O3, Fe2O3, CaO, and MgO contents. The silica (SiO2) in the RHA is not dissolved in the H2SO4 treatment. The silica nanoparticles are obtained via the following reactions:
NaOH + SiO2 → Na2SiO3 + H2O
Na2SiO3 + H2SO4 → SiO2 + Na2SO4 + H2O
Chemical compositions of the RHA analyzed by AAS
Effect of surfactant on the particle size distribution of silica nanoparticles
The results show that the cationic surface-active substances do not coat uniformly the particle surface. In addition, due to the high surface energy and free OH groups on the silica surface which produce the hydrogen bond with water molecules, when the dispersed silica was isolated from the solvent, this hydrogen bond was also removed forming a Si-O-Si liaison and resulting to larger size particles which were agglomerated.
For surface-active substances of group 1, the mixture, after being synthesized, was dispersed completely in butanol phase and became transparent. The results show that the size distribution of silica particles is more uniform. In the case using Aliquat 336, ADBAC, and CPC, silica particles can be obtained in the form of a rope having an average particle size of 20 nm. Especially, when using the CTAB agent, the dispersion of the sample was much better with the smallest size of particles of about 2 to 4 nm. The result indicates that the CTAB surfactant has coated uniformly the surface of the material giving it much better dispersion in suspension.
Effect of surfactant concentration on the particle size distribution of silica nanoparticles
In order to optimize the formation condition of silica nanoparticles, the effect of the CTAB concentration was investigated. The experiments were performed varying its concentration from 0 to 3 wt.% of total mass of silica, and the aging time and aging temperature condition are fixed at 8 h and 60°C, respectively.
Effect of aging temperature and time on the particle size and its distribution of silica nanoparticles
Achieving the particle size and its distribution of silica nanoparticles depends on the stability of silica sol. Derjaguin  had distinguished three types of stability of colloidal systems: (1) phase stability, analogous to the phase stability of ordinary solutions; (2) stability of disperse composition, the stability with respect to the change in dispersity (particle size distribution); and (3) aggregative stability, the most characteristic for colloidal systems. Colloidal stability means that the particles do not aggregate at a significant rate. As explained earlier, an aggregate is used to describe the structure formed by the cohesion of colloidal particles. So, in this investigation, we have proved the effect of aging temperature and time on the stability of silica nanoparticles.
Effect of aging temperature
Survey results on the influence of temperature on the particle size showed that the best condition in the survey area to obtain good dispersion and uniform particle size is at a temperature of 60°C with 2 wt.% CTAB.
Effect of aging time
RHA material was successfully synthesized from the abundant Vietnamese rice husk. A new synthetic method for spherical silica nanoparticles using RHA as the silica source and CTAB as the surfactant via the sol–gel technique in water/butanol was investigated. This method is a simple and effective route for preparing ultrafine powders on a nanometer scale and with a homogeneous particle size distribution. The specific surface area is reached at 340 m2/g, and the silica product obtained is amorphous. This leads to the low-cost production of silica nanoparticles for various practical applications such as pollution treatment, nanocomposite materials, etc. Furthermore, using this source for the production of RHA provides a way to solve the waste problem of rice husk pollution in the Mekong Delta of Vietnam.
VHL graduated and received his Bachelor of Science in Organical Chemistry in 2005, and after that, he received his M.S. in Physical Chemistry in 2011 from the University of Science, HoChiMinh City, Vietnam. His research interests include nanomaterials and polymers.
CNHT is currently the Vice Dean of the Faculty of Materials Science, University of Science-National University of HoChiMinh City, Vietnam. He graduated with the degree B.S. in Physical Chemistry from the University of Science, HoChiMinh City, Vietnam, in 2004. He received his M.S. in Physico-chemistry of Materials from the University of Maine, Le Mans, France, in 2005 and received his Ph.D. in Materials Science and Engineering from the University of Savoie, Chambéry, France, in 2008. His research interests include polymers, nanocomposites based on polymers, and biodegradable polymers.
HHT is an associate professor in the Faculty of Chemistry, University of Science, Vietnam National University in HoChiMinh City, Vietnam. He is also the Chief of the Polymer Laboratory, Faculty of Chemistry, University of Science, Vietnam National University in HoChiMinh City, Vietnam. Moreover, he is visiting Professorships at Université du Maine, Le Mans, France; Université de Savoie, France; Polytechnic University - Vietnam National University in HoChiMinh City, Vietnam; and Can Tho University, Vietnam. He received his Bachelor of Chemistry degree from the Faculty of Sciences, Saigon University, Vietnam, in 1971 and then received his Master of Science in Organic Physical Chemistry from the Faculty of Sciences, Saigon University in 1972. He graduated with a Ph.D. degree from the University of HCM City in 1992. He was the Dean of the Faculty of Chemistry, University of Science-Vietnam National University in HoChiMinh City, Vietnam, from 2002 to 2007. His research interests include modification of natural polymers (rubber, chitosan, etc.), natural or synthetic polymer-controlled degradation, synthesis of systems containing free and/or linked plant growth stimulator molecules in rubber/polymer matrix, living polymers, polymer blends, composite materials, nanocomposites, and graphene.
The authors wish to thank the Laboratory of Polymer of the University of Science, HoChiMinh City, and Bui Van Ngo Company for the rice husk used in this investigation and the Laboratory of Nanotechnology and Institute of Material Science and Technology, HoChiMinh City, for the different analytical techniques of XRD, TGA, DMTA, TEM, and SEM analyses.
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