Towards tunable size of silica particles from rice husk

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Towards tunable size of silica particles from rice husk

Usama Zul ﬁ qar ⁎ , Tayyab Subhani, S. Wilayat Husain

Department of Materials Science and Engineering, Institute of Space Technology, Islamabad, Pakistan

a b s t r a c t a r t i c l e i n f o

Article history:

Received 28 June 2015

Received in revised form 26 August 2015 Accepted 28 August 2015

Available online 10 September 2015

Keywords:

Rice husk;

White rice husk;

Sodium silicate solution;

Silica particles;

Tri-modal particles

Amorphous spherical silica particles were produced in a variety of sizes ranging from nanometer to micrometer from rice husk by varying the concentration of white rice husk from 0.7 wt.% to 5.6 wt.% in sodium hydroxide to form sodium silicate solutions. The effect of increased reaction temperature (65 °C) and the addition of water on the size of silica particles was also studied. At low concentrations of white rice husk (0.7 wt.% and 1.4 wt.%), uni- modal silica particles in nanometer size range were produced while at higher concentrations (2.8 wt.% and 5.6 wt.%), bi- and tri-modal particles were produced, which were converted to uni- and bi-modal by changing the reaction conditions, i.e. water and temperature, respectively. X-rayﬂuorescence spectroscopy, inductively coupled plasma optical emission spectroscopy and atomic absorption spectroscopy were employed for the compositional analysis of white rice husk and sodium silicate solution. Scanning electron microscopy was used to study the morphology and particle size of silica while X-ray diffraction conﬁrmed the amorphous state of silica particles. Surface area of silica particles was analyzed by using Brunauer–Emmett–Teller analysis.

1. Introduction

Silica particles have found many applications in industries asfiller material in composites, raw material for the synthesis of shear thicken- ing fluid, electronic components, catalysts, drug delivery systems, thermal insulators, chromatography and the engineeringfields of rub- ber and ceramics[1,2]. The biocompatibility of silica particles exhibiting exceptional biocompatibility with human mesenchymal stem cells has also been reported[3]. With increasing importance of silica in nanometer size range, a variety of approaches have been adopted for their synthesis. A few of these techniques includeflame spray pyrolysis, sol gel process and micro-emulsion technique. However, sol gel process is the common technique for the synthesis of silica nanoparticles because of the ease in controlling the particle size and shape by systematically tailoring the synthesis conditions[4]. Sol gel process allows synthesis of silica nanoparticles by hydrolysis and condensation reaction of alkoxides in the presence of alcohols providing exceptional regulation of reaction kinetics[5]. The formation mechanism in sol gel process involves two steps: (a) a colloidal suspension“sol”isfirst prepared wherein particles are suspended in the liquid (b) whereas the second stage involves the development of a 3D polymeric network, which results in the formation of“gel”[6–8]. Extensive research in thisfield has made it possible to produce silica nanoparticles with controlled size and morphology at low temperature by using silica precursors such as tetraethyl orthosilicate (TEOS)[9]. Another precursor of silica

is the commercially available sodium silicate solution (SSS). The mechanism of the formation of silica particles from SSS after treating with phosphoric acid (H3PO4) is presented in the following expression:

3Na2SiO3þ2H3PO4¼3SiO2þ 2Na3PO4þ 3H2O:

The addition of H3PO4into SSS causes the formation and condensation of`Si–OH, which leads to the formation of`Si–O–Si`; dimers and trimers are formed followed by their growth into particles[10]. In- stead of using H₃PO₄, the reaction of SSS with HCl has also been reported [11]. Moreover, using carbon dioxide as precipitating agent, silica particles have been produced by pressured carbonation[12].

Rice husk (RH) is one of the waste materials, which have been proved beneﬁcial for the production of silica powder and activated carbon [13–16]. In a recent work, the effect of alkali pretreatment on RH was studied and it was observed that the moisture content of RH increased with alkali pretreatment[17]. Silica particles are also synthesized from rice husk ash (RHA) obtained after burning the RH[18,15]. RH contains 15–20% silica along with organic compounds including cellulose, hemicel- lulose and lignin[19–21]. Metallic oxides such as Al2O3,K2O, MgO and CaO are also present in RH, which catalyze the melting of silica during the removal of organic compounds employing thermal treatments[22, 23]. Leaching of RH with acids including HCl is performed for the removal of metallic impurities prior to thermal treatments[14]. The organic compounds degrade during the thermal treatment and form carbonaceous residue, which burn subsequently at high temperatures[21]. The process of thermal decay of RH involves three stages: (a) drying and moisture removal up to 150 °C, (b) elimination of organic compounds in the

⁎ Corresponding author at: Department of Materials Science and Engineering, Institute of Space Technology, Islamabad, Pakistan.

E-mail address:usamazulﬁ[email protected](U. Zulﬁqar).

Contents lists available atScienceDirect

Journal of Non-Crystalline Solids

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j n o n c r y s o l

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temperature range of 215–350 °C and (c) burning of carbonaceous materials at 350–690 °C[24–26].

For the recovery of silica from rice husk, acid-leaching methods have been studied[27,28]. In a previous study, the highest content of silica was achieved after treatment with 2 M HCl at 60 °C and it was observed that impurities were completely dissolved after 2 h[28].

In the present study, white rice husk (WRH) was produced from locally available RH. Four different concentrations of WRH were added to NaOH to produce four different SSSs, which were later treated with phosphoric acid in the presence of ethanol to produce silica particles in different sizes. The effect of WRH concentration was studied on particle size and the development of different particle size modals.

Moreover, the effect of the addition of distilled water and increased reaction temperature (65 °C) upon the particle size and their size modals was investigated.

2. Experimental 2.1. Materials

A local rice mill near Islamabad provided RH. Ethanol (AR Grade, C2H5OH) was procured from Lab-Scan while hydrochloric acid (37%, HCl) was purchased from Fisher Scientiﬁc. Phosphoric acid (85%,

H3PO4) was purchased from Sigma Aldrich. Sodium hydroxide (NaOH) was acquired from local market. Distilled water was used during the synthesis of silica particles.

Table 1

Details of experiments including WRH concentrations used for the synthesis of silica particles and their particle sizes.

No. WRH

(wt%) SSS (ml)

Ethanol (ml)

Water (ml)

Temperature (°C)

Average particle size

1 0.7 50 10 – 25 181 ± 17 nm

2 1.4 50 10 – 25 256 ± 33 nm

3 2.8 50 10 – 25 339 ± 58 nm

1.7 ± 0.7μm

4 5.6 50 10 – 25 352 ± 77 nm

1.8 ± 0.3μm 7.1 ± 1.3μm

5 1.4 50 10 – 65 259 ± 41 nm

6 2.8 50 10 – 65 211 ± 52 nm

7 5.6 50 10 – 65 179 ± 19 nm

0.87 ± 0.35μm

8 1.4 50 10 10 25 96 ± 10 nm

9 2.8 50 10 10 25 250 ± 49 nm

10 5.6 50 10 10 25 275 ± 77 nm

Fig. 1.Graphical representation of the scheme of experiments.

Fig. 2.Flow chart representing the process to produce silica particles of different sizes from rice husk.

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2.2. Manufacturing

100 g of RH was treated with 1000 ml of 2 M HCl solution at 90 °C for 2 h. It was then continuously washed andﬁltered with distilled water until it was neutralized to pH 7. Later, RH wasﬁltered from the distilled

water and thermally treated at 600 °C for 4 h in a muffle furnace to produce WRH in an amount of approximately 13.5 g. Four different loadings of WRH (0.7 wt.%, 1.4 wt.%, 2.8 wt.% and 5.6 wt.%) were sepa- rately mixed with 100 ml of 2 M NaOH solution at 90 °C with continuous stirring for 2 h to prepare sodium silicate solutions (SSS). The solutions were filtered to remove unwanted residue and neutralized with concentrated H3PO4 under continuous stirring in the presence of 10 ml ethanol as co-solvent and aged for 30 min either at 25 °C or 65 °C to explore the effect of high temperature on the morphology and size of silica particles. In addition, distilled water (10 ml) was added in selected solutions tofind the effect of water on the morphology and size of silica particles; the details are provided inTable 1. A gel was formed during the reaction and the tendency to form gel increased with increasing amount of RH. However, gel formation was avoided through continuous stirring. The neutralized solutions were later centri- fuged to produce silica particles. Finally, silica particles were washed with hot water to remove the impurities including sodium phosphate (Na3PO4) and dried. For comparison, RHA was produced by burning as-received RH in open air. The complete scheme of experiments is shown graphically inFig. 1. Aflow chart of the process is provided in Fig. 2.

2.3. Characterization

The compositions of RHA and WRH were determined by X-rayfluo- rescence (XRF) (WD XRD PANalytical). SSS obtained from 1.4 wt.% WRH was subjected to inductively coupled plasma (ICP-OES) (IRIS ICP-OES, Thermo Jarrel Ash) analysis to perform elemental analysis. Sodium content in SSS was determined by using atomic absorption spectroscopy (AAS). The microstructural observation of produced silica particles was performed by afield emission gun scanning electron microscope (FEG-SEM) (Mira 3 TESCAN) while X-ray diffraction (XRD) was performed for phase analysis (Broker D8 Advance). For SEM, silica particles were dispersed in ethanol after sonication for 5 min. After sonication, selected quantity of suspension was dropped on a glass slide followed by heating to evaporate ethanol. The dried glass slide was coated with carbon by using sputter coater. Silica particle size measurement was performed using SEM images along with Image J software. A least 1000 measurements were performed tofind the average size of a single type of silica particles. The samples of silica nanoparticles were washed with 8 M HCl before XRD analysis; washing with HCl lowers the sodium content in the produced silica[29]. For the determination of surface area, nitrogen adsorption desorption isotherm measurements were exploited at 77 K. The samples were degassed a 50 °C prior to analysis.

Brunauer–Emmett–Teller (BET) analysis (Micromerics, Tristar 2, Smartprep) was exploited by using isotherm adsorption data for rela- tive pressure p/p0from 0.05 to 0.3. Surface area was measured in two Fig. 3.TGA curves of rice husk (1) and rice husk ash (2) in nitrogen atmosphere.

Table 2

Compositions of RHA and WRH analyzed by XRF.

Compound RHA (%) WRH (%)

SiO2 90.5 95.9

Al2O3 0.36 0.22

Fe2O3 0.36 0.09

K2O 3.49 0.04

MgO 1.61 0.98

CaO 1.77 0.43

Table 3

Composition of SSS derived from 1.4 wt.% WRH in 2 M NaOH analyzed by ICP-OES.

*Sodium was analyzed by using AAS.

Element Na* Si Al Fe Mg

μg/ml 68,800 2155 40.7 0.71 0.39

Fig. 4.SEM images of uni-modal silica particles produced by sodium silicate solution (SSS) containing 0.7 wt.% white rice husk (WRH).

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cases to generate a comparison, i.e., WRH and the particles obtained from 0.7 wt.% solution (lowest concentration).

3. Results

Fig. 3shows TGA results of RH and RHA. It can be seen that the moisture removed from RH until 126 °C as indicated by ~5% sample mass loss at this temperature. The second stage of thermal decomposition of organic compounds started at 258 °C andﬁnished at 345 °C with ~ 47%

mass loss. The third stage started at 345 °C andﬁnished at 522 °C with

~ 79% mass loss, which corresponds to removal of carbonaceous

materials and no appreciable decrease in the mass of sample was observed after this temperature. In comparison to RH, the TGA curve of RHA shows the removal of moisture until ~ 177 °C with 4% mass loss and the process of the oxidation of carbonaceous materials started at 356 °C andﬁnished at 577 °C with ~53% mass loss. No further decrease in mass was observed after rise in temperature up to 1000 °C. It is to be mentioned here that based upon these TGA results, the temperature for thermal treatment of acid-leached RH was chosen to be 600 °C to convert RH into WRH. Moreover, TGA results showed that the organic compounds and carbonaceous materials were degraded before 600 °C;

therefore, RH was not converted into RHA in the present work.

Fig. 5.SEM images of uni-modal silica particles produced by sodium silicate solution (SSS) containing 1.4 wt.% white rise husk (WRH).

Fig. 6.SEM images of bi-modal silica particles (a and b) produced by sodium silicate solution (SSS) containing 2.8 wt.% white rice husk (WRH); (c and d) small sized silica particles in the bi- modal distribution.

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The compositional analysis results of WRH and RHA, as performed by XRF, are given inTable 2. RHA contains 90.5% silica along with impurities including Al2O3,K2O, MgO and CaO. The composition of RHA depends upon many factors including the agricultural practice and geographical location; as a result, a range of compositions of RHA have been reported[30,31]. The silica content in WRH increased to

95.9% while the fractions of impurities also lowered signiﬁcantly, which may be due to acid-leaching prior to thermal treatment. For example, the amount of K2O in RHA was 3.49%, which reduced to 0.04%

in WRH; the contents of other impurities also decreased. In our study, WRH with a purity level of 95.9% silica was obtained after thermal treatment of acid-leached RH at 600 °C for 4 h (Table 2). However, in a Fig. 7.SEM images of (a) tri-modal silica particles produced by sodium silicate solution (SSS) containing 5.6 wt.% white rice husk (WRH); (b, c and d) silica particles of two different sizes in tri-modal distribution.

Fig. 8.Graph showing different particle sizes along with the presence of uni-, bi- and tri- modals of silica particles formed by increasing white rice husk (WRH) concentration in NaOH to form sodium silicate solutions (SSS).

Fig. 9.XRD patterns of RHA and silica particles produced by using 0.7 wt.%, 1.4 wt.%, 2.8 wt.% and 5.6 wt.% WRH in NaOH to form SSS.

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different study, 72.1% silica was reported in RHA obtained from the RH of Brazil, which increased to 94.9% after heating RHA at 700 °C in a furnace for 6 h[32].

Table 3shows the elemental analysis of SSS derived from 1.4 wt.%

WRH in 2 M NaOH, as obtained by ICP-OES and AAS. Sodium in SSS was derived from NaOH while silicon came from WRH; the impurities including aluminum, iron and magnesium were also carried in SSS from WRH, as found in XRF results (Table 2). The presence of silicon has conﬁrmed the formation of SSS. The amount of silicon is expected to increase in the elemental analysis of SSS with higher loading of WRH. The elemental analysis of SSSs with higher loadings of WRH was not performed in this study.

Fig. 4shows the SEM images of silica particles at two different magniﬁcations synthesized from SSS, which was formed from NaOH containing 0.7 wt.% WRH. The particles are spherical in nature with the average diameter of 181 ± 17 nm. Surface areas of WRH and silica nanoparticles produced after adding 0.7 wt.% WRH in NaOH were calculated by BET analysis; and were found to be 60.3 m²/g and 169.7 m²/g, respectively. In a different investigation, the mesoporous silica nanoparticles were produced from RH with an average size of 50.9 nm and surface area of 245 m²/g[33].

When 1.4 wt.% WRH was added to NaOH to form SSS, the average particle size of silica increased to 256 ± 33 nm while retaining the spherical morphology, as shown inFig. 5. However, when 2.8 wt.%

Fig. 10.SEM images of silica particles produced by sodium silicate solution (SSS) at 65 °C containing (a and b) 1.4 wt.% WRH (c and d) 2.8 wt.% and (e and f) 5.6 wt.% WRH; bi-modal distribution is visible in e and f.

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WRH was added to NaOH to form SSS, bi-modal distribution of silica particles was produced with spherical morphology, as observed in SEM images inFig. 6. The presence of silica particles at two different sizes can clearly be seen, i.e. 339 ± 58 nm and 1.7 ± 0.7μm inFig. 6(a and b). Small particles are shown inFig. 6(c and d) at higher magniﬁca- tions for comparison with silica particles produced after adding 0.7 wt.%

WRH and 1.4 wt.% WRH in NaOH (Figs. 4 and 5). When 5.6 wt.% WRH was added to NaOH to form SSS, a tri-modal distribution of silica particles was formed with spherical morphology, as observed in SEM images inFig. 7a. The average particle sizes of three different distributions were: 352 ± 77 nm, 1.7 ± 0.3μm and 7.1 ± 1.3μm. Silica particles of two different sizes are shown inFig. 7b, while for comparison at the same magniﬁcation with silica particles produced at different loadings of WRH in NaOH (Figs. 4, 5 and 6c and d), SEM images of the silica particles produced after adding 5.6 wt.% WRH in NaOH are shown in Fig. 7c and d.

Fig. 8graphically shows different particle sizes of silica produced after increasing the concentration of WRH from 0.7 wt.% to 5.6 wt.% in NaOH to form SSSs. It can be seen that the size of smaller particles in nanometer range increased by rising the loading of WRH; at 0.7 wt.%, 1.4 wt.% and 2.8 wt.%, the particle sizes were 181 ± 17 nm, 256 ± 33 nm, 339 ± 58 nm, respectively, while further increase in WRH to 5.6 wt.% did not signiﬁcantly increase the size of the smaller nanometer silica particles, i.e. 352 ± 77 nm. Moreover, at 2.8 wt.% and 5.6 wt.%

loadings of WRH, micrometer sized silica particles were also produced that had similar size ranges, i.e. 1.7 ± 0.7μm and 1.8 ± 0.3 μm, respectively. However, at 5.6 wt.% WRH a third range of micrometer sized particles was observed, i.e. 7.1 ± 1.3μm.

Fig. 9shows XRD patterns of RHA and silica nanoparticles produced from 0.7 wt.% WRH to 5.6 wt.% WRH in NaOH. It can be seen that the RHA and the produced silica particles were amorphous in nature and no evidence of the presence of crystalline phases was detected from the XRD patterns. Previous studies have also conﬁrmed the amorphous nature of silica particles derived from RH[34,35,14]. Silica in RH remains amorphous up to 800 °C during thermal treatment and devitriﬁcation occurs after this temperature[21,36].

When SSSs were formed by adding 1.4 wt.%, 2.8 wt.% and 5.6 wt.%

WRH in NaOH and neutralized with H3PO4in the presence of ethanol at 65 °C, the particle sizes decreased in comparison to those formed at room temperature, as observed inFig. 10. Moreover, bi-modal system reduced to uni-modal in 2.8 wt.% WRH and tri-modal reduced to bi- modal in 5.6 wt.% WRH. For example, uni-modal silica particles of size 211 ± 52 nm were found (Fig. 10a and b) instead of bi-modal of 339 ± 58 nm and 1.8 ± 0.7μm in the case of 2.8 wt.% WRH (Fig. 6) and bi-modal silica particles (179 ± 19 nm and 0.8 ± 0.3μm) were found (Fig. 10c and d) instead of tri-modal particles (352 ± 77 nm, 1.8 ± 0.3μm and 7.1 ± 1.3μm) in the case of 5.6 wt.% WRH (Fig. 7).

In case of 1.4 wt.% WRH, no signiﬁcant reduction in the particle size was observed, i.e. 259 ± 41 nm (Fig. 10e and f) in comparison to 256 ± 33 nm (Fig. 5). The effect of temperature and water addition was not tested in SSS containing 0.7 wt.% WRH. The morphology of the particles treated at 65 °C was spherical like those prepared at room temperature. A graphical representation of the particle sizes is shown inFig. 11; a declining trend in nanometer sized silica particles has been observed by increasing the loadings of WRH.

The silica particles of uni-modal were produced despite variation in the loadings of WRH when distilled water was added in SSSs in addition to ethanol and H₃PO₄. Moreover, an appreciable reduction in the size of particles was observed (Fig. 12). For example, in the case of 1.4 wt.%

WRH, the particle size reduced to 96 ± 10 nm (Fig. 12a and b) from 256 ± 33 nm (Fig. 5). Similarly the particle sizes reduced to 250 ± 49 nm (Fig. 12c and d) and 275 ± 77 nm (Fig. 12e and f) from bi-modal of 339 ± 58 nm and 1.7 ± 0.7μm (Fig. 6) and tri-modal of 352 ± 77 nm, 1.7 ± 0.3μm and 7.1 ± 1.3μm (Fig. 7), in SSSs of 2.8 wt.% WRH and 5.6 wt.% WRH, respectively. The morphology of silica particles after treating with distilled water was again spherical.Fig. 13

graphically shows the particle sizes produced after treatment with distilled water. It can be seen that the increase in the WRH loading from 1.4 wt.% to 2.8 wt.% signiﬁcantly increased the particle size while further increase in the contents of WRH to 5.6% did not signiﬁcantly increase the particle size.

4. Discussion

It has been reported previously that mono-dispersed silica particles were produced by the hydrolysis of low concentration of a different precursor, i.e. TEOS, in ethanol and ammonium hydroxide and it was observed that the size of particles increased with rising concentration of TEOS[37,38]. In our work, the average size of nanometer sized silica particles increased continuously, i.e. 181 ± 17 nm, 256 ± 33 nm, 339 ± 58 nm and 352 ± 77 nm after increasing the concentration of WRH i.e. 0.7 wt.%, 1.4 wt.%, 2.8 wt.% and 5.6 wt.%, respectively. There- fore, the concentration of a precursor has a direct effect on the particle size of silica. Moreover, uni-modal of silica particles was disturbed when the concentration of WRH increased from 1.4 wt.% WRH to 2.8 wt.% WRH and bi-modal distribution of silica particles was found;

particles of nanometer and micrometer size were produced. Further increase in the concentration of WRH from 2.8 wt.% to 5.6 wt.% produced a tri-modal distribution. It has been discussed previously that the concentration of precursor has direct effect on the concentration of nuclei produced in the reaction system; at high concentration of precursor, more primary particles are produced and aggregate to form multi modal distribution of particles[10]. It has been observed in the present study that the particle size and distribution modals change directly with increasing amount of WRH in the precursor. Moreover, aggregation of smaller particles on larger particles can be observed in SEM images (Figs. 6 and 7).

Besides the concentration of WRH, the co-solvents also play a critical role in the morphology and size of nanoparticles. For example, the addition of ethanol as co-solvent generally results in spherical particles with increased dispersion, which is attributed to the miscibility of ethanol with the precursor[39,29]. In addition to ethanol, distilled water also affects the particle size of silica[40]. The decrease in particle size by increasing the water content was observed using TEOS in ethanol and ammonia[40]. In the present work, a decrease in the particle size was also observed after introducing distilled water in the precursor system of SSS. Due to high polarity of water, solubility of intermediate species in the solution increases during the reaction and it may lead to long nucleation period[41]. This effect results in the production of smaller particles as more nuclei are produced. It is observed that the addition of water not only reduces the size but bi- and tri-modal distributions of Fig. 11.Graph showing the particle size of silica formed at 65 °C with increasing concentration of white rice husk (WRH) in NaOH to form sodium silicate solution (SSS).

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silica particle are also reduced to uni-modal silica particles. This may be attributed to increased nucleation period as compared to growth period because of increased solvent polarity.

It wasﬁnally observed in the present study that the effect of a high reaction temperature (65 °C) decreased the size of silica particles. In addition, tri-modal reduced to bi-modal and bi-modal reduced to uni- modal distribution of silica particles in the cases of 2.8 wt.% WRH and 5.6 wt.%WRH, respectively. The nucleation rate of particles increases when the reaction temperature is increased[29]. This effect leads to the formation of smaller particle sizes as observed in the present study. Tri-modal particles are reduced to bi-modal and bi-modal are reduced to uni-modal due to increased rate of nucleation and reduced aggregation. Hence, the increased temperature and water content reduces

the particle size by increasing the rate of nucleation and extending the nucleation period respectively.

5. Conclusions

White rice husk with a surface area of 60.3 m²/g was recovered from rice husk after acid-leaching followed by thermal treatment. Different sizes of amorphous silica particles ranging from nanometer to micrometer were produced by varying the concentration of white rice husk from 0.7 wt.% to 5.6 wt.% in sodium hydroxide to form sodium silicate solutions. It was observed that the silica particle size increased with increasing concentration of white rice husk, and bi- and tri-modals of silica particles were observed at higher concentrations, i.e. 2.8 wt.%

Fig. 12.SEM images of silica particles produced by sodium silicate solution (SSS) treated with water containing (a and b) 1.4 wt.% WRH (c and d) 2.8 wt.% WRH (e and f) 5.6 wt.% WRH; uni- modal particles are visible in all images.

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and 5.6 wt.%, respectively. Increase in the reaction temperature to 65 °C not only reduced the size of silica particles but also converted their tri- and bi-modals to bi- and uni-modals, respectively. The addition of water reduced the tri- and bi-modals of silica particle to uni-modal along with the decrease in particle size.

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Fig. 13.Graph showing the silica particle sizes with increasing WRH concentration in the presence of water.