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Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by a chemical method
Author’s Accepted Manuscript Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by chemical method S. Sankar, Sanjeev K. Sharma, Narinder Kaur, Byoungho Lee, Deuk Young Kim, Sejoon Lee, Hyun Jung www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(15)02279-8 http://dx.doi.org/10.1016/j.ceramint.2015.11.172 CERI11791
To appear in: Ceramics International Received date: 18 September 2015 Revised date: 13 November 2015 Accepted date: 30 November 2015 Cite this article as: S. Sankar, Sanjeev K. Sharma, Narinder Kaur, Byoungho Lee, Deuk Young Kim, Sejoon Lee and Hyun Jung, Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by chemical method, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.11.172 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by chemical method S. Sankar1, Sanjeev K. Sharma1*, Narinder Kaur1, Byoungho Lee1, Deuk Young Kim1*, Sejoon Lee1, Hyun Jung2 1
Semiconductor Materials and Device Laboratory, Department of Semiconductor Science, Dongguk University-Seoul, Seoul 100715, Republic of Korea 2
Advanced Functional Nanohybrid Materials Laboratory, Department of Chemistry, Dongguk University-Seoul, Seoul 100715, Republic of Korea *Corresponding Author: [email protected] , [email protected] Tel.: +82-2-2260-3183
An inexpensive chemical method was used to synthesize biogenic mesoporous silica (m-SiO2) from rice husk ash (RHA). A comparative study was carried out to produce silica nanoparticles (S-SiO2, R-SiO2, and B-SiO2) from three type of rice husk ashes (sticky, red, and brown). The microstructure of m-SiO2 was dependent on the geographical provenance and the types of RHA. An analysis of the SEM and TEM micrographs reveals that the S-SiO2 nanoparticles had a clustered spherical shape, while R-SiO2 and BSiO2 nanoparticles were found to be purely spherical. The average crystallite size of S-SiO2, R-SiO2 and B-SiO2 nanoparticles evaluated from the TEM measurements were observed to be 50, 20 and 10 nm, respectively. The XRD pattern of silica nanopowders had an absence of sharp peaks that confirmed the amorphous nature of the material. The Fourier transform infrared (FTIR) spectra of silica nanoparticles showed the symmetric Si-O and O–Si–O stretching bond vibrations at 462, 1088, and 1098 cm-1. The surface area of SSiO2, R-SiO2 and B-SiO2 nanopowders were measured to be 7.5513, 201.45, and 247.18 m2g-1, respectively. The surface area of uniformly-distributed spherical nanoparticles of B-SiO2 were observed the highest, which can be applied for the application of energy storage and drug delivery systems.
Key words: Rice husk ash, silica nanoparticles, microstructural analysis, surface and pore area measurements, Thermo gravimetric analysis
1. Introduction Rice husk (RH) is a major by-product of rice milling [1], and is also available an abundant amount from the agricultural waste. RH can be recycled to produce high value eco-materials, such as silicon (Si) [2, 3], silica (SiO2) [4-7], silicon carbide (SiC) [8, 9], silicon nitride (Si3N4) [10] and graphene (G) [11]. The chemical composition (in percentage) of raw rice husk has been reported to contain both organic (74%) and inorganic constituents (26%) [12]. The organic constituents include cellulose, hemi cellulose, lignin, L-arabinose, Methylglucuronic acid, D-galactose and some proteins and vitamins that can be removed from rich husk during the burning process [13]. The major remaining inorganic component is SiO2 (80%), along with some minor inorganic constituents including alumina (3.93%), sulfur trioxide (0.78%), iron oxide (0.41%), calcium oxide (3.84%), magnesium oxide (0.25%), sodium oxide (0.67%), potassium oxide (1.45%), and a loss of ignition (8.56%). Inorganic constituents that are present in SiO2 nanopowder can be removed by acid leaching and high temperature annealing process [14]. Even though the chemical composition of rice husk ash (RHA) can vary according to the geographical location and climatic conditions, type of soils, fertilizers and the paddy [13, 15]. Nanostructured SiO2 can also be synthesized from RHA through chemical means (acid/alkali leaching and post heat treatment) [13, 16, 17] as well as non-isothermal [18], fluidized bed [19], carbonization and combustion [20], pressurized hot-water [21], microwave hydrothermal [22], and precipitation [23]. Of the different synthesis methods, the chemical method consisting of simple acid leaching and post annealing is of the most simple and successful techniques to synthesize the ultrafine SiO2 nanopowder from RHA [24-26]. Silicon dioxide or silica or SiO2 is one of the most common materials used in optical, electrical and medical applications. It has a wide bandgap (~ 9 eV) that results in a high native transparency that extends from UV to infrared [27, 28]. SiO2-polymer composites can also be used in other industrial applications, such as food, rubber, pharmaceutical, and cosmetics productions [29, 30]. Regarding medical applications, mesoporous SiO2 has been used as a carrier for drug delivery and gene transfection [31]. Therefore, SiO2-based composites are important materials for medical additives in bone repair and tissue engineering applications. These also have other clinical uses, such as in bio-glass and glass-ceramics. Fe2O3-doped silica nano-spheres have been used in magnetic targeting [32], and
SiO2 nanoparticles are functionalized with different receptors including naphthalimide, BODIPY, and azobenzene for chemosensor applications to detect lead (Pb2+) and mercury (Hg2+) heavy metals in drinking water [33-35]. For energy storages, the small particle size and high surface area of SiO2-carbon (SiO2-C) nanocomposites allowed to be used as the anode of lithium ion batteries for providing a long cycling life and a high reversible capacity (485 mAhg-1) [36, 37]. To control the microstructure of SiO2 nanopowder synthesized from RHA, the microstructure of SiO2 can be controlled by acid and/or alkali leaching (e.g., HCl, HNO3, H2SO4, NaOH and NH4OH) and subsequently annealing in the temperature range of 5001100 °C for different intervals [4, 6, 38]. Several authors have reported that the SiO2 nanoparticles can be synthesized from rice husk and/or RHA. Wang et al. [24] synthesized biogenic SiO2 nanoparticles and observed a semi-crystalline structure with a diameter of 25 - 30 nm and a surface area of 164 m2/g. Adam et al. [16] synthesized mesoporous SiO2 with a spherical morphology by using a template free sol-gel technique, and they observed the highest specific BET surface area (245 m2 g-1) with a narrow pore size distribution of 5.6 - 9.6 nm. Athinarayanan et al. [39] synthesized nano SiO2 via acid pretreatment, and they observed irregular particle sizes with dimensions of 10 to 30 nm. Van et al. [17] synthesized spherical SiO2 nanoparticles from Vietnamese RH by using a sol-gel method and observed an amorphous structure in the material with an average particle size of 3 nm. Despite the synthesis of small crystallite size and high porosity of SiO2 nanopowders, a comparative study of biogenic silica nanopowders, synthesized from sticky rice husk ash, red rice husk ash, and brown rice husk ash by the least expensive and simple chemical method (using a simple stirring process) have not been studied yet. While, the microstructure of mesoporous silica was also dependent on the geographical provenance and the type of RHAs. Therefore, the uniformly distributed spherical biogenic SiO2 nanopowder have not well matched for the requirement of future prospective applications. The objective of this research work is to synthesize biogenic silica nanopowders from different type of rice husk ashes (sticky rice husk ash, brown rice husk ash, and red rice husk ash). The microstructure, elemental, chemical bonding, functionality, porosity, and mass-loss of mesoporous SiO2 nanopowder were evaluated by using X-ray diffraction spectroscopy (XRD), field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDAX), high resolution-transmission electron microscopy (HR-
TEM), surface area electron diffraction (SAED) patterns, electron diffraction spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) analysis, and thermo gravimetric analysis (TGA). The synthesis mechanism of silica nanopowders from RHAs was also discussed along with its schematic representation.
2. Experimental 2.1 Materials Chemicals and solvents of AR grade were purchased from Sigma Aldrich and Merck and were used without further purification. Sticky rice husk ash was collected in South Korea while brown rice husk ash and red rice husk ashes were collected from Tamil Nadu, India.
2.2. Synthesis of silica nanoparticles Silica nanopowders were synthesized from various type of rice husk ashes through a simple acid pretreatment (chemical method). All three type of rice husks were burned in an open environment to collect their ashes. 3.0 g of each type of rice husk ash was first stirred with 45 mL of 10% HCl for 2 h to remove the metal ions inside. The metal ions were then removed from the rice husk ash, and these are denoted as leached rice husk ash (LRHA). The LRHA were filtered and washed with a large amount of deionized (DI) water and dried at 150 °C for 24 h in an electric oven. The obtained dry powders were transferred from a Petri dish to an alumina crucible and annealed at 700 °C with a ramp rate of 5 °C min-1 in a muffle furnace at atmospheric pressure for 2 h. Finally, we obtained a white-colored mesoporous silica (m-SiO2) nanopowder. The various silica (SiO2) nanopowders are denoted as sticky SiO2 (S-SiO2), red SiO2 (R-SiO2) and brown SiO2 (B-SiO2) synthesized from sticky RHA, red RHA, and brown RHA, respectively. 2.3. Microstructural analysis
A microstructural analysis of synthesized silica nanopowders, S-SiO2, R-SiO2 and B-SiO2, was carried out to experimentally assess the properties of the different samples by using X-ray diffraction spectroscopy (XRD), field emission-scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDAX), and transmission electron microscopy (TEM). The XRD (XRD, XPERT-PRO), patterns were obtained by using CuKα1 as a radiation source (λ = 1.5405 Å) operating under a constant current of 30 mA at 40 kV with a diffraction angle (2θ) scan range of 5 to 80°. The surface morphology and the chemical composition of the prepared SSiO2, R-SiO2 and B-SiO2 nanoparticles were examined by using FE-SEM (Hitachi, S-4800), EDAX (S-4800), and HR-TEM. The functional groups of the S-SiO2, R-SiO2 and B-SiO2 nanoparticles were analyzed by mixing spectral-grade KBr powder at a weight ratio of 1:100 mg with different S-SiO2, R-SiO2 and B-SiO2 nanopowders in an agate mortar. The powders were pressed into pellets with a diameter of 13 mm and thickness of 0.5 mm. The infrared (IR) absorption spectra of the S-SiO2, R-SiO2 and B-SiO2 nanopowder composite pellets were obtained by using FTIR spectroscopy (Scimitar 1000 FT-IR, Varian) over the wave number range from 4000 to 400 cm1. The chemical bonding and chemical composition of S-SiO2, R-SiO2 and B-SiO2 nanoparticles were evaluated via electron diffraction spectroscopy (EDS). The amorphous structure of silica nanopowder was further confirmed from the selected area electron diffraction (SAED) pattern. A PerkinElmer thermo gravimetric analyzer (model STA 6000) was used to determine the residue content of the silica samples. About 4.0 mg of each sample were heated from 30 to 900 °C at a heating rate of 10 °C/min in a highly purified air (99.999%, San-Fu Chem. Co.). The variation in the remaining sample mass ratio (W/Wo) was recorded according to the temperature with Wo and W representing the initial and the instantaneous mass of the samples. 2.4. Surface area and pore size analysis Adsorption-desorption isotherm tests were carried out for all nano silica powders, S-SiO2, R-SiO2 and B-SiO2, by using a Brunauer-Emmett-Teller (BET) (BELSORP-mini II, Japan) analyzer with the N2 absorption technique at 77 K. The pore size distributions were derived from the adsorption branch of the isotherms by the Barrett-Joyner-Halenda (BJH) method. The total pore
volume of each sample was estimated from the N2 adsorbed at a relative pressure of P/Po = 0.990. This value was then used to determine the specific surface area, pore specific surface area, pore diameter and pore volume by using the BET equation [40] below.
P Po P V 1 Po
1 C 1 P Vm Vm Po
(1)
Where V is the volume of nitrogen (cm3 g-1) adsorbed at equilibrium pressure P, Vm is the monolayer capacity (cm3 g-1), P is the partial vapor pressure of the adsorbate gas (Pa), Po is the saturation vapor pressure (Pa), and C is the BET constant. 3. Results and Discussion The corporeal difference of the biomass rice husk ashes (RHAs) is presented by taking some snapshots of the original and intermediate substances, including raw rice, rice, rice husk, rice husk ash and mesoporous silica nanopowders, as shown in Fig. 1. These original and intermediate digital snapshots varied according to the geographical location from where the rice husk was collected, that is, sticky rice husk is from Korea and red and brown rice husk are collected from India. After milling the raw rice, the color of the rice and the crushed rice husk could be differentiated in terms of sticky, red and brown rice husk. The impact of the collected RHA can also be understood in terms of the microstructure of SiO2 nanoparticles synthesized via similar processes and chemical reactions. The digital snapshots and SEM images clearly indicate the variation in terms of the microstructure of the silica nanoparticles. Fig. 2 shows FE-SEM images of mesoporous silica nanopowders synthesized from three different biomass RHAs through the corresponding chemical reactions. Fig. 2 (a) shows an image of the mesoporous silica nanopowder synthesized from sticky rice husk ash, which is referred to as S-SiO2. The morphology of S-SiO2 consisted of cluster-type spherical nanoparticles while the other two mesoporous silica nanoparticles synthesized from red rice husk ash (R-SiO2) and brown rice husk ash (B-SiO2) exhibited uniform
spherical nanoparticles with a narrow size distribution, as shown in Figs. 2 (b) and (c), respectively. The SiO2 nanoparticles that were obtained had a uniform surface morphology with respect to the particle size distribution, and the size of the particles of the SiO2 nanopowders decreased from 50 nm to 10 nm as the RHA changed from sticky RHA to brown RHA. The mesoporous SiO2 nanopowder synthesized from brown rich husk ash was observed to have the lowest particle size distribution of 5 - 10 nm, and the silica nanopowders presented in this study have better characteristics than those reported elsewhere [24]. In order to eliminate and understand the impurities from RHA, leached RHA (LRHA), and m-SiO2 nanopowder, we measured the EDAX spectra (Supplementary information, Fig. S1, Fig. S2 and Fig. S3). For raw rice husk ashes, (sticky: R1, red: R2, and brown: R3), the main elemental composition were observed to be C, O, Si, Mg and K along with few other of very low amount of inorganic constituent (wt.%: Table S1). The chemical composition of rice husk ash (RHA) varied as per the geographical location and types of raw rice husks. The elemental composition of LRHA (named L1, L2, and L3) were observed the major inorganic elements such as C, O, Si and K, while
other lower content of inorganic constituents were completely removed during leaching and filtration (Table S2). While the final product, mesoporous silica nanopowders, S-SiO2, R-SiO2, and B-SiO2, were observed only Si and O content (wt.%) and confirmed EDAX-mapping analysis (Table S3). Figs. 3 (a), (b), and (c) show the EDAX-mapping for all three mesoporous silica nanopowders, SSiO2, R-SiO2, and B-SiO2, respectively. The elemental composition of Si and O determined from EDAX-mapping are mentioned in the inset of the microstructure. For the S-SiO2 nanopowder, the elemental composition of Si and O was observed to be Si : O = 51.10 : 48.90 in terms of weight percentage (wt.%). For the R-SiO2 nanopowder, the elemental composition of Si and O was observed to be Si : O = 51.11 : 48.89 wt.%. For the B-SiO2 nanopowder, the elemental composition of Si and O was observed to be Si : O = 52.07 : 47.93 wt.%. The elemental composition of Si and O for mesoporous silica nanopowders varied a little according to the variation of the type of RHAs. For the purity and bonding energy of SiO2 nanopowder, we additionally measured the XPS spectra (X-ray photoelectron spectroscopy). The full scale survey of XPS spectra confirmed the purity and the chemical-bonding of mesoporous SiO2 nanopowders (Fig. S4 & Table S4). The different electron core level spectra of mesoporous silica, Si2p, Si2s, and O1s, indicate the presence of silicon and oxygen components in the powder. The prepared mesoporous silica, S-SiO2, R-SiO2 and B-SiO2 nanopowders observed
the Si2p strong peak at 102.6, 101.8 and 102.2 eV, respectively, which revealed the existence of pure SiO2 powder. The absorbed Si2s signal peak at 153.8 eV corresponds to the presence of Si species in the sample. The O1s peak of m-SiO2 nanopowders were observed at ~531.5 for all 3 samples, which is clearly indicated the higher contribution of oxygen. In addition, the C element peak was also observed at 284.5 eV due to the charging of the samples was setting the binding energy of the adventitious carbon (C 1s) [41, 42] . The binding energy of Si and O for mesoporous silica nanopowders varied a little due to the type of RHAs. The XRD pattern of the mesoporous SiO2 nanopowders synthesized from different RHAs, S-SiO2, R-SiO2 and B-SiO2, is shown in Fig. 4. The absence of sharp peaks in the XRD pattern of all three mesoporous silica nanopowders indicated the amorphous nature of the material. The prepared nanopowders exhibited a broad intense peak at 2θ = 22°, which indicated the presence of silica nanoparticles. No other impurities were detected, even though the materials were characterized in triplicate. The absence of sharp peaks in the XRD pattern of silica nanopowders confirmed an amorphous nature. Therefore, the economically-synthesized silica nanopowder is useful for various applications [4, 5, 24, 43]. The crystallite size and structural properties of the mesoporous silica nanopowders were determined from HR-TEM and SAEDpattern. Figs. 5 (a), (b) and (c) show the bright field TEM images, HE-TEM images and SAED patters of S-SiO2, R-SiO2 and B-SiO2, respectively. A low and high magnification of bright-field TEM images of the S-SiO2 nanoparticles (Figs. 5 a1 & a2) showed the clusters of primary particles with an irregular geometry and a spherical shape along with a wide size distribution. The average diameter of S-SiO2 nanoparticles was observed to be 50 nm. Every primary particles were interconnected and adhered with each other. For the RSiO2 (Figs. 5 b1, b2) and B-SiO2 (Figs. 5 c1, c2) nanoparticles, the bright-field TEM images exhibited uniform and spherical nanoparticles along with the narrow size distributions. The average particle size of R-SiO2 and B-SiO2 nanopowders was observed to be 20 nm and 10 nm, respectively. The shape of the nanoparticles were detected to consist of uniform and agglomerated species. When compared to the particle size of the S-SiO2 and R-SiO2 nanopowder, the particle size of the B-SiO2 nanopowder was the smallest and had a spherical in shape with a uniform narrow size distribution.
The HR-TEM images of the S-SiO2, R-SiO2 and B-SiO2 samples are shown in Figs. 5 (a3), (b3), and (c3), respectively. The absence of uniform and periodic lattice spacing in the HR-TEM images also confirmed the amorphous nature of the mesoporous silica nanopowders. The selected area electron diffraction (SAED) pattern of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2 nanoparticles are shown in Figs. 5 (a4), (b4), and (c4), respectively. The electron diffraction rings of the synthesized mesoporous silica nanopowders indicated the amorphous phase of the main phase of silica nanoparticles. Thus, the XRD, HR-TEM & SAED results clearly indicate that all three types of mesoporous silica nanopowders were obtained in an amorphous state. An elemental composition analysis of the S-SiO2, R-SiO2 and B-SiO2 nanoparticles was carried out by using energy dispersive X-ray spectroscopy (EDS). The EDS spectra of the S-SiO2, R-SiO2 and B-SiO2 nanopowders are shown in Fig. 6. The EDS spectra showed the characteristic peaks for the two elements at 0.5 keV and 1.73 keV, which correspond to the spectral lines of O and Si. In addition, the Cu element peak was also observed at 8.0 and 8.8 keV due to the conductive copper holder mounted for the EDS measurement. The synthesized mesoporous silica nanoparticles were formed with the minimal weight percentage (wt.%), and the EDS analysis confirmed that the synthesized materials formed in a good stoichiometric ratio. An experimental evaluation of the weight percentage of Si and O detected from S-SiO2, R-SiO2 and B-SiO2 nanopowders are provided in the inset of Fig. 6. Thus, the results finally confirmed that the synthesized mesoporous silica nanopowders consisted of pure silica nanoparticles. The prepared mesoporous S-SiO2, R-SiO2 and B-SiO2 nanopowders were shown to have structural variations that depend upon the chemical composition of the rice husk. Every type of rice husk has a different chemical composition that might be varied due to the type of soils, geographical and climatic conditions, fertilizers used and paddy conditions [1, 12, 13, 43]. Fig. 7 shows the schematic illustration for the synthesis mechanism of silica nanoparticles from rice husk ash. We synthesized mesoporous silica nanoparticles from three different RHA (i.e., sticky RHA, red RHA and brown RHA) using a simple chemical method. The raw rice husk is anticipated to be constituted from both organic (cellulose, hemi cellulose, and lignin) and inorganic (mostly silica) ingredients. To remove the organic constituents from the raw rice husk, the rice husk was burned in an open environment, and the ashes were collected. Some of the remaining organic constituents and unwanted inorganic metal ions in the rice husk ash can be
removed by leaching the rice husk ash in 10% HCl for 2 h. After leaching the rice husk ash, the sol was filtered using WhatmanTM filter paper (Grade 1 with a pore size of 11 µm) and washed with a large amount of deionized (DI) water. Therefore, the organic constituents and the unwanted inorganic metallic ions were separated as a liquid from the HCl leached suspensions. The inorganic constituents, particularly the silica nanoparticles, were obtained as solid ingredients. The solid ingredients, which are referred to as leached RHA (LRHA), were dried in an electric oven at 150 C for 24 h. The chemical reaction for the synthesis of silica nanopowders along with the high-temperature annealing process can be described according to the following chemical reactions given below. h) SiO 2 (Ash) HCl RT (2 SiO 2 MCl H 2O
( where M : Na, K, Ca, Fe, and Mg)
C (2 h) SiO 2 (Colloidal ) 700 SiO 2 (nanoparticle)
(1)
(2)
The dry colloidal powder was further annealed at 700 °C with a ramp rate of 5 °C min-1 in a muffle furnace under atmospheric pressure for 2 h. During the high temperature annealing process, most of the unwanted inorganic constituents were removed from the colloidal nanopowder. Finally, white precipitates, that is, silica nanopowder were observed. The purity and phase of the silica nanopowders were confirmed by using various characterization tools (Fig. 2: FE-SEM, Fig. 3: EDAX-mapping, Fig.4: XRD, Fig. 5: TEM, HR-TEM, SAED, Fig. 6: EDS, and Fig. 8: FTIR). To evaluate the chemical bonding of the pure SiO2 nanopowder, Fourier transform infrared (FTIR) transmittance spectra were taken for all mesoporous silica nanopowders. The FTIR spectra of the mesoporous silica nanopowders are shown in Fig. 8. The transmission peaks were detected at 462 and 800 cm-1 corresponding to the rocking bond and symmetric bond (silanol) vibrations of the Si–O elements, respectively. The bands located at 1097 and 1088 cm-1 are related to the vibrational stretching of asymmetric Si–O–Si in S1O4 tetrahedron, showing a stoichiometric silicon dioxide (SiO2) structure. The O–H bending and O–H stretching vibration modes
also appeared in the absorption band region at 3440 and 3463 cm−1 and at 1647 and 1643 cm−1, respectively. The transmittance wave number for the S-SiO2 nanoparticles is slightly different from that of R-SiO2 and B-SiO2 nanoparticles, and it was dependent on the oxygen levels shown in the SiO2 nanoparticles. The transmittance spectra of the three mesoporous SiO2 nanopowders (S-SiO2, R-SiO2 and B-SiO2) are summarized in Table 1. The mesoporous silica nanoparticles could also be discriminated in terms of their surface morphology and the particle size distribution. The mesoporous S-SiO2 nanopowder showed a cluster-type spherical morphology while the R-SiO2 and B-SiO2 nanopowders were observed to have uniform spherical nanoparticles with a narrow size distribution. The surface area and the pore characteristics of the mesoporous silica nanoparticles were determined using the Brunauer Emmett Teller (BET) and Barret Joyner Halenda (BJH) methods. The adsorption-desorption isotherm of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2, nanopowders are shown in Fig. 9. The specific surface area of the mesoporous silica nanopowders were observed to be 7.5513, 201.45 and 247.18 m2 g-1, respectively. Compared to the surface area of S-SiO2 and R-SiO2 nanopowders, the surface area of the B-SiO2 nanopowder was observed to be the highest specific surface area (247.18 m2 g-1) due to the smallest size of SiO2 nanoparticles with a spherical shape and a uniform size distribution. The nitrogen adsorption-desorption isotherm of the mesoporous silica, B-SiO2 and R-SiO2, nanopowders showed a category IV model (H2-type hysteresis loop) that can be justified per the IUPAC classification [4, 18]. The nitrogen adsorption-desorption isotherm indicated that the S-SiO2 nanopowder was the least porous material. The characteristics of the S-SiO2 nanopowder were detected from the non-hysteresis loop of the III-type model. During nitrogen adsorptiondesorption, the hysteresis loop generally appeared between the adsorption and desorption branches, which indicates capillary condensation of the porous structure [4]. The mesoporous R-SiO2 and B-SiO2 nanopowders were observed to have a hysteresis loop (H2) with a partial pressures range of 1 > P/Po > 0.4 [44]. Thus, the BET analysis of the R-SiO2 and B-SiO2 nanopowders proved that the uniformly distributed spherical nanoparticles had a higher porosity than the cluster-type spherical nanoparticles (S-SiO2). The pore size distribution of the mesoporous silica nanopowders were evaluated from Barret Joyner Halenda (BJH) method, and the pore volume versus pore diameter of the mesoporous silica nanopowder are shown in Fig. 9 (b). The pore surface area of the
mesoporous silica nanopowders were determined to be 6.77, 178.54, and 206.75 m2 g-1, respectively. Therefore the pore surface area of the S-SiO2 nanopowder was also observed the lowest surface area because it was the least porous material. The pore volume of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2, nanopowders was increased from 0.0708 to 0.2852 cm3 g-1 and the average pore diameter decreased from 37.53 nm to 4.33 nm as the silica nanoparticles changed from S-SiO2 to B-SiO2. The pore diameter of the B-SiO2 (4.33 nm) nanopowders was observed to be the smallest, with a uniform pore diameter, when compared to the average pore diameters of S-SiO2 (37.53 nm) and R-SiO2 (5.93 nm) nanopowders. In general, the pore size distribution of the mesoporous materials was detected in a narrow range from 2 to 22 nm with an average pore diameter 2 - 9 nm [24, 45]. Thus, the BJH analysis for the pore diameter of the B-SiO2 and R-SiO2 nanopowders was observed to have a higher specific pore surface area, uniform pore diameters and narrow size distribution. The pore diameter of the S-SiO2 nanopowders was observed to have the highest pore diameter and a non-uniform fever porous material. Finally, the mesoporous SiO2 nanopowders, the brown rice husk ash (B-SiO2) was indicated to have the highest specific surface area, pore volume, pore surface area, and uniform pore diameter along with a narrow size distribution. The specific surface area, pore specific surface area, pore diameter and pore volume of the three mesoporous silica nanopowders are summarized in Table 2. The mass loss of the mesoporous silica nanopowders was analyzed using a thermo gravimetric analyzer (TGA). The weight ratio (W/Wo) of the mesoporous silica nanopowders versus the temperature is shown in Fig. 10. The TGA curves of the synthesized mesoporous silica nanopowders represent the variation in the weight remaining (W/Wo) according to the reaction temperature. The thermal decomposition of the mesoporous silica nanopowders was tested in the range 30 - 900 °C. The mass-loss of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2, nanopowders slowly decreased with respect to increasing the reaction temperature. This trend was closely matched to the trend for the pretreatment with HCl [4]. The decrease in the weight of the mesoporous SiO2 nanopowders can be attributed to the removal of combustible volatiles, and the TGA curves for the R-SiO2 and B-SiO2 nanopowders exhibited a variation point during thermal decomposition, and two main stages in the temperature region from 30 to 200 °C and 200 to 900 °C, respectively.
However, the mesoporous S-SiO2 nanoparticles did not show a variation in mass loss during thermal decomposition. The moisture content of the mesoporous R-SiO2 and B-SiO2 nanopowders was observed in the temperature range from 30 to 200 °C. For the S-SiO2 nanoparticles, less moisture was detected in the temperature range from 30 to 200 °C under similar conditions. The mesoporous S-SiO2 nanopowder exhibited the highest mass-loss during high temperature reactions since it was the least porous material with spherical clusters. Nevertheless, the mesoporous R-SiO2 and B-SiO2 nanoparticles displayed a smaller mass-loss during high temperature reactions due to higher porosity and smaller spherical nanoparticles. Therefore, the mesoporous silica nanopowders need a longer heating duration to release the water molecules. When compared to S-SiO2 and R-SiO2 nanopowders, the mesoporous B-SiO2 nanopowders showed the smallest mass-loss silica nanopowder. In conclusion, a thorough synthesis and analysis of mesoporous silica nanopowders confirms that the mesoporous B-SiO2 nanopowders exhibited the highest porosity and the most narrow size distribution. Therefore, the B-SiO2 nanopowder is the most compatible material for use in energy storage devices and drug delivery applications. 4. Conclusions Biogenerated mesoporous SiO2 nanopowders were successfully synthesized from sticky RHA, red RHA and brown RHA through a simple acid pretreatment method. The microstructural and elemental composition and/or chemical bonding analysis confirmed the spherical shape and the purity of SiO2 nanopowders. The SEM and TEM analyses of the silica nanopowders revealed that B-SiO2 had the smallest nanoparticle size (3 - 10 nm) and the highest surface area (247.18 m2g-1). The absence of sharp peaks in the XRD pattern and the electron diffraction rings in the SAED pattern confirmed the amorphous nature of the material. The thermo gravimetric analysis of the spherical B-SiO2 nanoparticles indicates that these had the least mass-loss with the highest porosity and narrow size distribution. The biogenerated B-SiO2 nanoparticles synthesized from brown rich husk ash have the lowest nanoparticle size and the highest surface area, and so are considered to be the most compatible material for energy storage and drug delivery applications.
Acknowledgements This research was supported by the Basic Science Research Program (NRF-2013R1A1A2059900), funded by the Korean government of Ministry of Education (MoE).
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Figure caption Fig. 1 Digital and SEM images of raw rice, rice, rice husk, rice husk ash and synthesized silica nanopowders from sticky, red and brown rice.
Fig. 2 FE-SEM images of synthesized mesoporous silica, S-SiO2, R-SiO2 and B-SiO2 nanoparticles. Fig. 3 EDAX-mapping for elemental compositions of S-SiO2, R-SiO2 and B-SiO2
nanoparticles, the percentage values of Si
and O are in the inset.
Fig. 4 XRD pattern of the synthesized porous S-SiO2, R-SiO2, and S-SiO2 nanopowders. Fig. 5 Microstructural analysis of the S-SiO2, R-SiO2, and B-SiO2 nanoparticles: (a1), (b1), and (c1) low magnification TEM images; (a2), (b2), and (c2) high magnification TEM images; (a3), (b3), and (c3) HR-TEM images; (a4), (b4), and (c4) SAED pattern.
Fig. 6 EDS-spectra of the obtained silica S-SiO2, R-SiO2, and B-SiO2 nanopowders. Fig. 7 Schematic illustration of the synthesis mechanism for mesoporous silica nanopowders from raw rice husk.
Fig. 8 FT-IR spectra of S-SiO2, R-SiO2, and B-SiO2 nanoparticles.
Fig. 9 Surface area analysis of S-SiO2, R-SiO2, and B-SiO2 nanopowders: (a) hysteric loops to evaluate the surface area measurements of silica (BET-measurement), (b) pore size analysis (BJH-measurement) curves and the pore diameter variation is an inset of the figure.
Fig. 10 TGA-analysis of S-SiO2, R-SiO2, and B-SiO2 nanopowders.
Table 1. FT-IR spectral data of S-SiO2, R-SiO2 and B-SiO2 nanopowders.
Frequency (cm−1) S-SiO2 R-SiO2 462 462
B-SiO2 462
800
800
800
1097
1088
1088
1647
1643
1643
3440
3463
3463
Position assignment
Literature value Reference
Si–O bond rocking
465-475
[23]
Symmetric Si-O bending (silanol) Asymmetric Si–O–Si stretching in S1O4 tetrahedron O–H bending
790-805
[4, 17, 39]
1050-1115
[18, 46, 47]
1630
[23, 48]
O–H stretching and adsorbed water
3000-3800
[4, 46]
S: Sticky RHA, R: Red rice RHA, B: Brown rice RHA
Table 2. The surface area and pore characteristics of S-SiO2, R-SiO2 and B-SiO2 nanopowders.
Samples
S- SiO2
BET surface area (m2 g-1)
7.5513
Pore characteristics (BJH) Surface area of pores (m2 g-1)
Pore volume (cm3 g-1)
Average pore diameter (nm)
6.7782
0.0708
37.53
Figure 1:
R- SiO2
201.45
178.54
0.2507
5.93
B- SiO2
247.18
206.75
0.2852
4.33
Figure 2: FE-SEM
Figure 3: EDAX-Mapping
Red Color – Si, Green Color - O Si - 51.10 % O - 48.90 %
Si - 51.11 % O - 48.89 %
Si - 52.07 % O - 47.93 %
Figure 4: XRD
Figure 5: TEM, HR-TEM, SAED
Low Magnification
High Magnification
(a3)
(a2)
(a1)
HRTEM
20 nm
20 nm
20 nm
100 nm
50 nm
(c2)
(c1) 10 nm 10 nm 10 nm
(c3)
SAED
Figure 6: EDS
Figure 7: Schematic illustration - mechanism
Bio-silica
Lignin
SiO2 HCl (10 wt.%) R.T (2 h) Leaching
Cellulose
Hemicellulose
Acid treated - RHA
Rice Husk Ash (RHA) Leached - RHA
Figure 8: FTIR
30
Figure 9 (a): BET surface analysis
31
Figure 9 (b): BJH pore analysis
32
Figure 10: TGA analysis
33
Supplementary Information
34
Fig. S1: EDAX spectra of raw rice husk ashes, named sticky rice husk ash (R1), red rice husk ash (R2), and brown rice husk ash (R3).
Table S1: Elemental compositions of raw rice husk ash from EDAX analysis of R1, R2, and R3. Elements
Weight (%) R1
R2
R3
C
13.74
52.28
50.39
O
39.88
27.56
26.35
Si
24.62
5.95
2.46
Mg
0.60
1.79
4.18
K
6.26
2.64
3.71
S
0.40
0.34
-
Cl
0.48
-
-
W
9.76
-
2.03
35
Ir
2.60
9.21
-
Ca
0.56
-
-
Mn
1.10
-
-
Al
-
0.23
-
P
-
-
10.88
Total
100
100
100
Fig. S2: EDAX spectra of leached rice husk ashes (LRHAs) denoted as leached sticky rice husk ash (L1), leached red rice husk ash (L2), and leached brown sticky rice husk ash (L3).
Table S2: EDAX analysis of L1, L2, and L3.
Elements
Weight (%) L1
L2 36
L3
C
44.84
52.16
55.12
O
33.36
28.32
28.92
Si
19.27
19.52
15.96
K
2.53
-
-
Total
100
100
100
Fig. S3: EDAX spectra of mesoporous silica nanopowders, S-SiO2 (S1), R-SiO2 (S2), and BSiO2 (S3).
Table S3: EDAX analysis of S1, S2, and S3. 37
Elements
Weight (%) S1
S2
S3
Si
51.10
51.11
52.07
O
48.90
48.89
47.93
Total
100
100
100
38
Fig. S4: Full scale survey of XPS spectra of SiO2 nanopowders.
Table S4: XPS analysis of S-SiO2, R-SiO2, and B-SiO2. Elements
Binding Energy (eV) S-SiO2
R-SiO2
B-SiO2
Si
102.6
101.8
102.2
O
531.8
531.1
531.5
39
PII: DOI: Reference:
S0272-8842(15)02279-8 http://dx.doi.org/10.1016/j.ceramint.2015.11.172 CERI11791
To appear in: Ceramics International Received date: 18 September 2015 Revised date: 13 November 2015 Accepted date: 30 November 2015 Cite this article as: S. Sankar, Sanjeev K. Sharma, Narinder Kaur, Byoungho Lee, Deuk Young Kim, Sejoon Lee and Hyun Jung, Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by chemical method, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.11.172 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biogenerated silica nanoparticles synthesized from sticky, red, and brown rice husk ashes by chemical method S. Sankar1, Sanjeev K. Sharma1*, Narinder Kaur1, Byoungho Lee1, Deuk Young Kim1*, Sejoon Lee1, Hyun Jung2 1
Semiconductor Materials and Device Laboratory, Department of Semiconductor Science, Dongguk University-Seoul, Seoul 100715, Republic of Korea 2
Advanced Functional Nanohybrid Materials Laboratory, Department of Chemistry, Dongguk University-Seoul, Seoul 100715, Republic of Korea *Corresponding Author: [email protected] , [email protected] Tel.: +82-2-2260-3183
An inexpensive chemical method was used to synthesize biogenic mesoporous silica (m-SiO2) from rice husk ash (RHA). A comparative study was carried out to produce silica nanoparticles (S-SiO2, R-SiO2, and B-SiO2) from three type of rice husk ashes (sticky, red, and brown). The microstructure of m-SiO2 was dependent on the geographical provenance and the types of RHA. An analysis of the SEM and TEM micrographs reveals that the S-SiO2 nanoparticles had a clustered spherical shape, while R-SiO2 and BSiO2 nanoparticles were found to be purely spherical. The average crystallite size of S-SiO2, R-SiO2 and B-SiO2 nanoparticles evaluated from the TEM measurements were observed to be 50, 20 and 10 nm, respectively. The XRD pattern of silica nanopowders had an absence of sharp peaks that confirmed the amorphous nature of the material. The Fourier transform infrared (FTIR) spectra of silica nanoparticles showed the symmetric Si-O and O–Si–O stretching bond vibrations at 462, 1088, and 1098 cm-1. The surface area of SSiO2, R-SiO2 and B-SiO2 nanopowders were measured to be 7.5513, 201.45, and 247.18 m2g-1, respectively. The surface area of uniformly-distributed spherical nanoparticles of B-SiO2 were observed the highest, which can be applied for the application of energy storage and drug delivery systems.
Key words: Rice husk ash, silica nanoparticles, microstructural analysis, surface and pore area measurements, Thermo gravimetric analysis
1. Introduction Rice husk (RH) is a major by-product of rice milling [1], and is also available an abundant amount from the agricultural waste. RH can be recycled to produce high value eco-materials, such as silicon (Si) [2, 3], silica (SiO2) [4-7], silicon carbide (SiC) [8, 9], silicon nitride (Si3N4) [10] and graphene (G) [11]. The chemical composition (in percentage) of raw rice husk has been reported to contain both organic (74%) and inorganic constituents (26%) [12]. The organic constituents include cellulose, hemi cellulose, lignin, L-arabinose, Methylglucuronic acid, D-galactose and some proteins and vitamins that can be removed from rich husk during the burning process [13]. The major remaining inorganic component is SiO2 (80%), along with some minor inorganic constituents including alumina (3.93%), sulfur trioxide (0.78%), iron oxide (0.41%), calcium oxide (3.84%), magnesium oxide (0.25%), sodium oxide (0.67%), potassium oxide (1.45%), and a loss of ignition (8.56%). Inorganic constituents that are present in SiO2 nanopowder can be removed by acid leaching and high temperature annealing process [14]. Even though the chemical composition of rice husk ash (RHA) can vary according to the geographical location and climatic conditions, type of soils, fertilizers and the paddy [13, 15]. Nanostructured SiO2 can also be synthesized from RHA through chemical means (acid/alkali leaching and post heat treatment) [13, 16, 17] as well as non-isothermal [18], fluidized bed [19], carbonization and combustion [20], pressurized hot-water [21], microwave hydrothermal [22], and precipitation [23]. Of the different synthesis methods, the chemical method consisting of simple acid leaching and post annealing is of the most simple and successful techniques to synthesize the ultrafine SiO2 nanopowder from RHA [24-26]. Silicon dioxide or silica or SiO2 is one of the most common materials used in optical, electrical and medical applications. It has a wide bandgap (~ 9 eV) that results in a high native transparency that extends from UV to infrared [27, 28]. SiO2-polymer composites can also be used in other industrial applications, such as food, rubber, pharmaceutical, and cosmetics productions [29, 30]. Regarding medical applications, mesoporous SiO2 has been used as a carrier for drug delivery and gene transfection [31]. Therefore, SiO2-based composites are important materials for medical additives in bone repair and tissue engineering applications. These also have other clinical uses, such as in bio-glass and glass-ceramics. Fe2O3-doped silica nano-spheres have been used in magnetic targeting [32], and
SiO2 nanoparticles are functionalized with different receptors including naphthalimide, BODIPY, and azobenzene for chemosensor applications to detect lead (Pb2+) and mercury (Hg2+) heavy metals in drinking water [33-35]. For energy storages, the small particle size and high surface area of SiO2-carbon (SiO2-C) nanocomposites allowed to be used as the anode of lithium ion batteries for providing a long cycling life and a high reversible capacity (485 mAhg-1) [36, 37]. To control the microstructure of SiO2 nanopowder synthesized from RHA, the microstructure of SiO2 can be controlled by acid and/or alkali leaching (e.g., HCl, HNO3, H2SO4, NaOH and NH4OH) and subsequently annealing in the temperature range of 5001100 °C for different intervals [4, 6, 38]. Several authors have reported that the SiO2 nanoparticles can be synthesized from rice husk and/or RHA. Wang et al. [24] synthesized biogenic SiO2 nanoparticles and observed a semi-crystalline structure with a diameter of 25 - 30 nm and a surface area of 164 m2/g. Adam et al. [16] synthesized mesoporous SiO2 with a spherical morphology by using a template free sol-gel technique, and they observed the highest specific BET surface area (245 m2 g-1) with a narrow pore size distribution of 5.6 - 9.6 nm. Athinarayanan et al. [39] synthesized nano SiO2 via acid pretreatment, and they observed irregular particle sizes with dimensions of 10 to 30 nm. Van et al. [17] synthesized spherical SiO2 nanoparticles from Vietnamese RH by using a sol-gel method and observed an amorphous structure in the material with an average particle size of 3 nm. Despite the synthesis of small crystallite size and high porosity of SiO2 nanopowders, a comparative study of biogenic silica nanopowders, synthesized from sticky rice husk ash, red rice husk ash, and brown rice husk ash by the least expensive and simple chemical method (using a simple stirring process) have not been studied yet. While, the microstructure of mesoporous silica was also dependent on the geographical provenance and the type of RHAs. Therefore, the uniformly distributed spherical biogenic SiO2 nanopowder have not well matched for the requirement of future prospective applications. The objective of this research work is to synthesize biogenic silica nanopowders from different type of rice husk ashes (sticky rice husk ash, brown rice husk ash, and red rice husk ash). The microstructure, elemental, chemical bonding, functionality, porosity, and mass-loss of mesoporous SiO2 nanopowder were evaluated by using X-ray diffraction spectroscopy (XRD), field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDAX), high resolution-transmission electron microscopy (HR-
TEM), surface area electron diffraction (SAED) patterns, electron diffraction spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) analysis, and thermo gravimetric analysis (TGA). The synthesis mechanism of silica nanopowders from RHAs was also discussed along with its schematic representation.
2. Experimental 2.1 Materials Chemicals and solvents of AR grade were purchased from Sigma Aldrich and Merck and were used without further purification. Sticky rice husk ash was collected in South Korea while brown rice husk ash and red rice husk ashes were collected from Tamil Nadu, India.
2.2. Synthesis of silica nanoparticles Silica nanopowders were synthesized from various type of rice husk ashes through a simple acid pretreatment (chemical method). All three type of rice husks were burned in an open environment to collect their ashes. 3.0 g of each type of rice husk ash was first stirred with 45 mL of 10% HCl for 2 h to remove the metal ions inside. The metal ions were then removed from the rice husk ash, and these are denoted as leached rice husk ash (LRHA). The LRHA were filtered and washed with a large amount of deionized (DI) water and dried at 150 °C for 24 h in an electric oven. The obtained dry powders were transferred from a Petri dish to an alumina crucible and annealed at 700 °C with a ramp rate of 5 °C min-1 in a muffle furnace at atmospheric pressure for 2 h. Finally, we obtained a white-colored mesoporous silica (m-SiO2) nanopowder. The various silica (SiO2) nanopowders are denoted as sticky SiO2 (S-SiO2), red SiO2 (R-SiO2) and brown SiO2 (B-SiO2) synthesized from sticky RHA, red RHA, and brown RHA, respectively. 2.3. Microstructural analysis
A microstructural analysis of synthesized silica nanopowders, S-SiO2, R-SiO2 and B-SiO2, was carried out to experimentally assess the properties of the different samples by using X-ray diffraction spectroscopy (XRD), field emission-scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDAX), and transmission electron microscopy (TEM). The XRD (XRD, XPERT-PRO), patterns were obtained by using CuKα1 as a radiation source (λ = 1.5405 Å) operating under a constant current of 30 mA at 40 kV with a diffraction angle (2θ) scan range of 5 to 80°. The surface morphology and the chemical composition of the prepared SSiO2, R-SiO2 and B-SiO2 nanoparticles were examined by using FE-SEM (Hitachi, S-4800), EDAX (S-4800), and HR-TEM. The functional groups of the S-SiO2, R-SiO2 and B-SiO2 nanoparticles were analyzed by mixing spectral-grade KBr powder at a weight ratio of 1:100 mg with different S-SiO2, R-SiO2 and B-SiO2 nanopowders in an agate mortar. The powders were pressed into pellets with a diameter of 13 mm and thickness of 0.5 mm. The infrared (IR) absorption spectra of the S-SiO2, R-SiO2 and B-SiO2 nanopowder composite pellets were obtained by using FTIR spectroscopy (Scimitar 1000 FT-IR, Varian) over the wave number range from 4000 to 400 cm1. The chemical bonding and chemical composition of S-SiO2, R-SiO2 and B-SiO2 nanoparticles were evaluated via electron diffraction spectroscopy (EDS). The amorphous structure of silica nanopowder was further confirmed from the selected area electron diffraction (SAED) pattern. A PerkinElmer thermo gravimetric analyzer (model STA 6000) was used to determine the residue content of the silica samples. About 4.0 mg of each sample were heated from 30 to 900 °C at a heating rate of 10 °C/min in a highly purified air (99.999%, San-Fu Chem. Co.). The variation in the remaining sample mass ratio (W/Wo) was recorded according to the temperature with Wo and W representing the initial and the instantaneous mass of the samples. 2.4. Surface area and pore size analysis Adsorption-desorption isotherm tests were carried out for all nano silica powders, S-SiO2, R-SiO2 and B-SiO2, by using a Brunauer-Emmett-Teller (BET) (BELSORP-mini II, Japan) analyzer with the N2 absorption technique at 77 K. The pore size distributions were derived from the adsorption branch of the isotherms by the Barrett-Joyner-Halenda (BJH) method. The total pore
volume of each sample was estimated from the N2 adsorbed at a relative pressure of P/Po = 0.990. This value was then used to determine the specific surface area, pore specific surface area, pore diameter and pore volume by using the BET equation [40] below.
P Po P V 1 Po
1 C 1 P Vm Vm Po
(1)
Where V is the volume of nitrogen (cm3 g-1) adsorbed at equilibrium pressure P, Vm is the monolayer capacity (cm3 g-1), P is the partial vapor pressure of the adsorbate gas (Pa), Po is the saturation vapor pressure (Pa), and C is the BET constant. 3. Results and Discussion The corporeal difference of the biomass rice husk ashes (RHAs) is presented by taking some snapshots of the original and intermediate substances, including raw rice, rice, rice husk, rice husk ash and mesoporous silica nanopowders, as shown in Fig. 1. These original and intermediate digital snapshots varied according to the geographical location from where the rice husk was collected, that is, sticky rice husk is from Korea and red and brown rice husk are collected from India. After milling the raw rice, the color of the rice and the crushed rice husk could be differentiated in terms of sticky, red and brown rice husk. The impact of the collected RHA can also be understood in terms of the microstructure of SiO2 nanoparticles synthesized via similar processes and chemical reactions. The digital snapshots and SEM images clearly indicate the variation in terms of the microstructure of the silica nanoparticles. Fig. 2 shows FE-SEM images of mesoporous silica nanopowders synthesized from three different biomass RHAs through the corresponding chemical reactions. Fig. 2 (a) shows an image of the mesoporous silica nanopowder synthesized from sticky rice husk ash, which is referred to as S-SiO2. The morphology of S-SiO2 consisted of cluster-type spherical nanoparticles while the other two mesoporous silica nanoparticles synthesized from red rice husk ash (R-SiO2) and brown rice husk ash (B-SiO2) exhibited uniform
spherical nanoparticles with a narrow size distribution, as shown in Figs. 2 (b) and (c), respectively. The SiO2 nanoparticles that were obtained had a uniform surface morphology with respect to the particle size distribution, and the size of the particles of the SiO2 nanopowders decreased from 50 nm to 10 nm as the RHA changed from sticky RHA to brown RHA. The mesoporous SiO2 nanopowder synthesized from brown rich husk ash was observed to have the lowest particle size distribution of 5 - 10 nm, and the silica nanopowders presented in this study have better characteristics than those reported elsewhere [24]. In order to eliminate and understand the impurities from RHA, leached RHA (LRHA), and m-SiO2 nanopowder, we measured the EDAX spectra (Supplementary information, Fig. S1, Fig. S2 and Fig. S3). For raw rice husk ashes, (sticky: R1, red: R2, and brown: R3), the main elemental composition were observed to be C, O, Si, Mg and K along with few other of very low amount of inorganic constituent (wt.%: Table S1). The chemical composition of rice husk ash (RHA) varied as per the geographical location and types of raw rice husks. The elemental composition of LRHA (named L1, L2, and L3) were observed the major inorganic elements such as C, O, Si and K, while
other lower content of inorganic constituents were completely removed during leaching and filtration (Table S2). While the final product, mesoporous silica nanopowders, S-SiO2, R-SiO2, and B-SiO2, were observed only Si and O content (wt.%) and confirmed EDAX-mapping analysis (Table S3). Figs. 3 (a), (b), and (c) show the EDAX-mapping for all three mesoporous silica nanopowders, SSiO2, R-SiO2, and B-SiO2, respectively. The elemental composition of Si and O determined from EDAX-mapping are mentioned in the inset of the microstructure. For the S-SiO2 nanopowder, the elemental composition of Si and O was observed to be Si : O = 51.10 : 48.90 in terms of weight percentage (wt.%). For the R-SiO2 nanopowder, the elemental composition of Si and O was observed to be Si : O = 51.11 : 48.89 wt.%. For the B-SiO2 nanopowder, the elemental composition of Si and O was observed to be Si : O = 52.07 : 47.93 wt.%. The elemental composition of Si and O for mesoporous silica nanopowders varied a little according to the variation of the type of RHAs. For the purity and bonding energy of SiO2 nanopowder, we additionally measured the XPS spectra (X-ray photoelectron spectroscopy). The full scale survey of XPS spectra confirmed the purity and the chemical-bonding of mesoporous SiO2 nanopowders (Fig. S4 & Table S4). The different electron core level spectra of mesoporous silica, Si2p, Si2s, and O1s, indicate the presence of silicon and oxygen components in the powder. The prepared mesoporous silica, S-SiO2, R-SiO2 and B-SiO2 nanopowders observed
the Si2p strong peak at 102.6, 101.8 and 102.2 eV, respectively, which revealed the existence of pure SiO2 powder. The absorbed Si2s signal peak at 153.8 eV corresponds to the presence of Si species in the sample. The O1s peak of m-SiO2 nanopowders were observed at ~531.5 for all 3 samples, which is clearly indicated the higher contribution of oxygen. In addition, the C element peak was also observed at 284.5 eV due to the charging of the samples was setting the binding energy of the adventitious carbon (C 1s) [41, 42] . The binding energy of Si and O for mesoporous silica nanopowders varied a little due to the type of RHAs. The XRD pattern of the mesoporous SiO2 nanopowders synthesized from different RHAs, S-SiO2, R-SiO2 and B-SiO2, is shown in Fig. 4. The absence of sharp peaks in the XRD pattern of all three mesoporous silica nanopowders indicated the amorphous nature of the material. The prepared nanopowders exhibited a broad intense peak at 2θ = 22°, which indicated the presence of silica nanoparticles. No other impurities were detected, even though the materials were characterized in triplicate. The absence of sharp peaks in the XRD pattern of silica nanopowders confirmed an amorphous nature. Therefore, the economically-synthesized silica nanopowder is useful for various applications [4, 5, 24, 43]. The crystallite size and structural properties of the mesoporous silica nanopowders were determined from HR-TEM and SAEDpattern. Figs. 5 (a), (b) and (c) show the bright field TEM images, HE-TEM images and SAED patters of S-SiO2, R-SiO2 and B-SiO2, respectively. A low and high magnification of bright-field TEM images of the S-SiO2 nanoparticles (Figs. 5 a1 & a2) showed the clusters of primary particles with an irregular geometry and a spherical shape along with a wide size distribution. The average diameter of S-SiO2 nanoparticles was observed to be 50 nm. Every primary particles were interconnected and adhered with each other. For the RSiO2 (Figs. 5 b1, b2) and B-SiO2 (Figs. 5 c1, c2) nanoparticles, the bright-field TEM images exhibited uniform and spherical nanoparticles along with the narrow size distributions. The average particle size of R-SiO2 and B-SiO2 nanopowders was observed to be 20 nm and 10 nm, respectively. The shape of the nanoparticles were detected to consist of uniform and agglomerated species. When compared to the particle size of the S-SiO2 and R-SiO2 nanopowder, the particle size of the B-SiO2 nanopowder was the smallest and had a spherical in shape with a uniform narrow size distribution.
The HR-TEM images of the S-SiO2, R-SiO2 and B-SiO2 samples are shown in Figs. 5 (a3), (b3), and (c3), respectively. The absence of uniform and periodic lattice spacing in the HR-TEM images also confirmed the amorphous nature of the mesoporous silica nanopowders. The selected area electron diffraction (SAED) pattern of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2 nanoparticles are shown in Figs. 5 (a4), (b4), and (c4), respectively. The electron diffraction rings of the synthesized mesoporous silica nanopowders indicated the amorphous phase of the main phase of silica nanoparticles. Thus, the XRD, HR-TEM & SAED results clearly indicate that all three types of mesoporous silica nanopowders were obtained in an amorphous state. An elemental composition analysis of the S-SiO2, R-SiO2 and B-SiO2 nanoparticles was carried out by using energy dispersive X-ray spectroscopy (EDS). The EDS spectra of the S-SiO2, R-SiO2 and B-SiO2 nanopowders are shown in Fig. 6. The EDS spectra showed the characteristic peaks for the two elements at 0.5 keV and 1.73 keV, which correspond to the spectral lines of O and Si. In addition, the Cu element peak was also observed at 8.0 and 8.8 keV due to the conductive copper holder mounted for the EDS measurement. The synthesized mesoporous silica nanoparticles were formed with the minimal weight percentage (wt.%), and the EDS analysis confirmed that the synthesized materials formed in a good stoichiometric ratio. An experimental evaluation of the weight percentage of Si and O detected from S-SiO2, R-SiO2 and B-SiO2 nanopowders are provided in the inset of Fig. 6. Thus, the results finally confirmed that the synthesized mesoporous silica nanopowders consisted of pure silica nanoparticles. The prepared mesoporous S-SiO2, R-SiO2 and B-SiO2 nanopowders were shown to have structural variations that depend upon the chemical composition of the rice husk. Every type of rice husk has a different chemical composition that might be varied due to the type of soils, geographical and climatic conditions, fertilizers used and paddy conditions [1, 12, 13, 43]. Fig. 7 shows the schematic illustration for the synthesis mechanism of silica nanoparticles from rice husk ash. We synthesized mesoporous silica nanoparticles from three different RHA (i.e., sticky RHA, red RHA and brown RHA) using a simple chemical method. The raw rice husk is anticipated to be constituted from both organic (cellulose, hemi cellulose, and lignin) and inorganic (mostly silica) ingredients. To remove the organic constituents from the raw rice husk, the rice husk was burned in an open environment, and the ashes were collected. Some of the remaining organic constituents and unwanted inorganic metal ions in the rice husk ash can be
removed by leaching the rice husk ash in 10% HCl for 2 h. After leaching the rice husk ash, the sol was filtered using WhatmanTM filter paper (Grade 1 with a pore size of 11 µm) and washed with a large amount of deionized (DI) water. Therefore, the organic constituents and the unwanted inorganic metallic ions were separated as a liquid from the HCl leached suspensions. The inorganic constituents, particularly the silica nanoparticles, were obtained as solid ingredients. The solid ingredients, which are referred to as leached RHA (LRHA), were dried in an electric oven at 150 C for 24 h. The chemical reaction for the synthesis of silica nanopowders along with the high-temperature annealing process can be described according to the following chemical reactions given below. h) SiO 2 (Ash) HCl RT (2 SiO 2 MCl H 2O
( where M : Na, K, Ca, Fe, and Mg)
C (2 h) SiO 2 (Colloidal ) 700 SiO 2 (nanoparticle)
(1)
(2)
The dry colloidal powder was further annealed at 700 °C with a ramp rate of 5 °C min-1 in a muffle furnace under atmospheric pressure for 2 h. During the high temperature annealing process, most of the unwanted inorganic constituents were removed from the colloidal nanopowder. Finally, white precipitates, that is, silica nanopowder were observed. The purity and phase of the silica nanopowders were confirmed by using various characterization tools (Fig. 2: FE-SEM, Fig. 3: EDAX-mapping, Fig.4: XRD, Fig. 5: TEM, HR-TEM, SAED, Fig. 6: EDS, and Fig. 8: FTIR). To evaluate the chemical bonding of the pure SiO2 nanopowder, Fourier transform infrared (FTIR) transmittance spectra were taken for all mesoporous silica nanopowders. The FTIR spectra of the mesoporous silica nanopowders are shown in Fig. 8. The transmission peaks were detected at 462 and 800 cm-1 corresponding to the rocking bond and symmetric bond (silanol) vibrations of the Si–O elements, respectively. The bands located at 1097 and 1088 cm-1 are related to the vibrational stretching of asymmetric Si–O–Si in S1O4 tetrahedron, showing a stoichiometric silicon dioxide (SiO2) structure. The O–H bending and O–H stretching vibration modes
also appeared in the absorption band region at 3440 and 3463 cm−1 and at 1647 and 1643 cm−1, respectively. The transmittance wave number for the S-SiO2 nanoparticles is slightly different from that of R-SiO2 and B-SiO2 nanoparticles, and it was dependent on the oxygen levels shown in the SiO2 nanoparticles. The transmittance spectra of the three mesoporous SiO2 nanopowders (S-SiO2, R-SiO2 and B-SiO2) are summarized in Table 1. The mesoporous silica nanoparticles could also be discriminated in terms of their surface morphology and the particle size distribution. The mesoporous S-SiO2 nanopowder showed a cluster-type spherical morphology while the R-SiO2 and B-SiO2 nanopowders were observed to have uniform spherical nanoparticles with a narrow size distribution. The surface area and the pore characteristics of the mesoporous silica nanoparticles were determined using the Brunauer Emmett Teller (BET) and Barret Joyner Halenda (BJH) methods. The adsorption-desorption isotherm of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2, nanopowders are shown in Fig. 9. The specific surface area of the mesoporous silica nanopowders were observed to be 7.5513, 201.45 and 247.18 m2 g-1, respectively. Compared to the surface area of S-SiO2 and R-SiO2 nanopowders, the surface area of the B-SiO2 nanopowder was observed to be the highest specific surface area (247.18 m2 g-1) due to the smallest size of SiO2 nanoparticles with a spherical shape and a uniform size distribution. The nitrogen adsorption-desorption isotherm of the mesoporous silica, B-SiO2 and R-SiO2, nanopowders showed a category IV model (H2-type hysteresis loop) that can be justified per the IUPAC classification [4, 18]. The nitrogen adsorption-desorption isotherm indicated that the S-SiO2 nanopowder was the least porous material. The characteristics of the S-SiO2 nanopowder were detected from the non-hysteresis loop of the III-type model. During nitrogen adsorptiondesorption, the hysteresis loop generally appeared between the adsorption and desorption branches, which indicates capillary condensation of the porous structure [4]. The mesoporous R-SiO2 and B-SiO2 nanopowders were observed to have a hysteresis loop (H2) with a partial pressures range of 1 > P/Po > 0.4 [44]. Thus, the BET analysis of the R-SiO2 and B-SiO2 nanopowders proved that the uniformly distributed spherical nanoparticles had a higher porosity than the cluster-type spherical nanoparticles (S-SiO2). The pore size distribution of the mesoporous silica nanopowders were evaluated from Barret Joyner Halenda (BJH) method, and the pore volume versus pore diameter of the mesoporous silica nanopowder are shown in Fig. 9 (b). The pore surface area of the
mesoporous silica nanopowders were determined to be 6.77, 178.54, and 206.75 m2 g-1, respectively. Therefore the pore surface area of the S-SiO2 nanopowder was also observed the lowest surface area because it was the least porous material. The pore volume of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2, nanopowders was increased from 0.0708 to 0.2852 cm3 g-1 and the average pore diameter decreased from 37.53 nm to 4.33 nm as the silica nanoparticles changed from S-SiO2 to B-SiO2. The pore diameter of the B-SiO2 (4.33 nm) nanopowders was observed to be the smallest, with a uniform pore diameter, when compared to the average pore diameters of S-SiO2 (37.53 nm) and R-SiO2 (5.93 nm) nanopowders. In general, the pore size distribution of the mesoporous materials was detected in a narrow range from 2 to 22 nm with an average pore diameter 2 - 9 nm [24, 45]. Thus, the BJH analysis for the pore diameter of the B-SiO2 and R-SiO2 nanopowders was observed to have a higher specific pore surface area, uniform pore diameters and narrow size distribution. The pore diameter of the S-SiO2 nanopowders was observed to have the highest pore diameter and a non-uniform fever porous material. Finally, the mesoporous SiO2 nanopowders, the brown rice husk ash (B-SiO2) was indicated to have the highest specific surface area, pore volume, pore surface area, and uniform pore diameter along with a narrow size distribution. The specific surface area, pore specific surface area, pore diameter and pore volume of the three mesoporous silica nanopowders are summarized in Table 2. The mass loss of the mesoporous silica nanopowders was analyzed using a thermo gravimetric analyzer (TGA). The weight ratio (W/Wo) of the mesoporous silica nanopowders versus the temperature is shown in Fig. 10. The TGA curves of the synthesized mesoporous silica nanopowders represent the variation in the weight remaining (W/Wo) according to the reaction temperature. The thermal decomposition of the mesoporous silica nanopowders was tested in the range 30 - 900 °C. The mass-loss of the mesoporous silica, S-SiO2, R-SiO2 and B-SiO2, nanopowders slowly decreased with respect to increasing the reaction temperature. This trend was closely matched to the trend for the pretreatment with HCl [4]. The decrease in the weight of the mesoporous SiO2 nanopowders can be attributed to the removal of combustible volatiles, and the TGA curves for the R-SiO2 and B-SiO2 nanopowders exhibited a variation point during thermal decomposition, and two main stages in the temperature region from 30 to 200 °C and 200 to 900 °C, respectively.
However, the mesoporous S-SiO2 nanoparticles did not show a variation in mass loss during thermal decomposition. The moisture content of the mesoporous R-SiO2 and B-SiO2 nanopowders was observed in the temperature range from 30 to 200 °C. For the S-SiO2 nanoparticles, less moisture was detected in the temperature range from 30 to 200 °C under similar conditions. The mesoporous S-SiO2 nanopowder exhibited the highest mass-loss during high temperature reactions since it was the least porous material with spherical clusters. Nevertheless, the mesoporous R-SiO2 and B-SiO2 nanoparticles displayed a smaller mass-loss during high temperature reactions due to higher porosity and smaller spherical nanoparticles. Therefore, the mesoporous silica nanopowders need a longer heating duration to release the water molecules. When compared to S-SiO2 and R-SiO2 nanopowders, the mesoporous B-SiO2 nanopowders showed the smallest mass-loss silica nanopowder. In conclusion, a thorough synthesis and analysis of mesoporous silica nanopowders confirms that the mesoporous B-SiO2 nanopowders exhibited the highest porosity and the most narrow size distribution. Therefore, the B-SiO2 nanopowder is the most compatible material for use in energy storage devices and drug delivery applications. 4. Conclusions Biogenerated mesoporous SiO2 nanopowders were successfully synthesized from sticky RHA, red RHA and brown RHA through a simple acid pretreatment method. The microstructural and elemental composition and/or chemical bonding analysis confirmed the spherical shape and the purity of SiO2 nanopowders. The SEM and TEM analyses of the silica nanopowders revealed that B-SiO2 had the smallest nanoparticle size (3 - 10 nm) and the highest surface area (247.18 m2g-1). The absence of sharp peaks in the XRD pattern and the electron diffraction rings in the SAED pattern confirmed the amorphous nature of the material. The thermo gravimetric analysis of the spherical B-SiO2 nanoparticles indicates that these had the least mass-loss with the highest porosity and narrow size distribution. The biogenerated B-SiO2 nanoparticles synthesized from brown rich husk ash have the lowest nanoparticle size and the highest surface area, and so are considered to be the most compatible material for energy storage and drug delivery applications.
Acknowledgements This research was supported by the Basic Science Research Program (NRF-2013R1A1A2059900), funded by the Korean government of Ministry of Education (MoE).
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Figure caption Fig. 1 Digital and SEM images of raw rice, rice, rice husk, rice husk ash and synthesized silica nanopowders from sticky, red and brown rice.
Fig. 2 FE-SEM images of synthesized mesoporous silica, S-SiO2, R-SiO2 and B-SiO2 nanoparticles. Fig. 3 EDAX-mapping for elemental compositions of S-SiO2, R-SiO2 and B-SiO2
nanoparticles, the percentage values of Si
and O are in the inset.
Fig. 4 XRD pattern of the synthesized porous S-SiO2, R-SiO2, and S-SiO2 nanopowders. Fig. 5 Microstructural analysis of the S-SiO2, R-SiO2, and B-SiO2 nanoparticles: (a1), (b1), and (c1) low magnification TEM images; (a2), (b2), and (c2) high magnification TEM images; (a3), (b3), and (c3) HR-TEM images; (a4), (b4), and (c4) SAED pattern.
Fig. 6 EDS-spectra of the obtained silica S-SiO2, R-SiO2, and B-SiO2 nanopowders. Fig. 7 Schematic illustration of the synthesis mechanism for mesoporous silica nanopowders from raw rice husk.
Fig. 8 FT-IR spectra of S-SiO2, R-SiO2, and B-SiO2 nanoparticles.
Fig. 9 Surface area analysis of S-SiO2, R-SiO2, and B-SiO2 nanopowders: (a) hysteric loops to evaluate the surface area measurements of silica (BET-measurement), (b) pore size analysis (BJH-measurement) curves and the pore diameter variation is an inset of the figure.
Fig. 10 TGA-analysis of S-SiO2, R-SiO2, and B-SiO2 nanopowders.
Table 1. FT-IR spectral data of S-SiO2, R-SiO2 and B-SiO2 nanopowders.
Frequency (cm−1) S-SiO2 R-SiO2 462 462
B-SiO2 462
800
800
800
1097
1088
1088
1647
1643
1643
3440
3463
3463
Position assignment
Literature value Reference
Si–O bond rocking
465-475
[23]
Symmetric Si-O bending (silanol) Asymmetric Si–O–Si stretching in S1O4 tetrahedron O–H bending
790-805
[4, 17, 39]
1050-1115
[18, 46, 47]
1630
[23, 48]
O–H stretching and adsorbed water
3000-3800
[4, 46]
S: Sticky RHA, R: Red rice RHA, B: Brown rice RHA
Table 2. The surface area and pore characteristics of S-SiO2, R-SiO2 and B-SiO2 nanopowders.
Samples
S- SiO2
BET surface area (m2 g-1)
7.5513
Pore characteristics (BJH) Surface area of pores (m2 g-1)
Pore volume (cm3 g-1)
Average pore diameter (nm)
6.7782
0.0708
37.53
Figure 1:
R- SiO2
201.45
178.54
0.2507
5.93
B- SiO2
247.18
206.75
0.2852
4.33
Figure 2: FE-SEM
Figure 3: EDAX-Mapping
Red Color – Si, Green Color - O Si - 51.10 % O - 48.90 %
Si - 51.11 % O - 48.89 %
Si - 52.07 % O - 47.93 %
Figure 4: XRD
Figure 5: TEM, HR-TEM, SAED
Low Magnification
High Magnification
(a3)
(a2)
(a1)
HRTEM
20 nm
20 nm
20 nm
100 nm
50 nm
(c2)
(c1) 10 nm 10 nm 10 nm
(c3)
SAED
Figure 6: EDS
Figure 7: Schematic illustration - mechanism
Bio-silica
Lignin
SiO2 HCl (10 wt.%) R.T (2 h) Leaching
Cellulose
Hemicellulose
Acid treated - RHA
Rice Husk Ash (RHA) Leached - RHA
Figure 8: FTIR
30
Figure 9 (a): BET surface analysis
31
Figure 9 (b): BJH pore analysis
32
Figure 10: TGA analysis
33
Supplementary Information
34
Fig. S1: EDAX spectra of raw rice husk ashes, named sticky rice husk ash (R1), red rice husk ash (R2), and brown rice husk ash (R3).
Table S1: Elemental compositions of raw rice husk ash from EDAX analysis of R1, R2, and R3. Elements
Weight (%) R1
R2
R3
C
13.74
52.28
50.39
O
39.88
27.56
26.35
Si
24.62
5.95
2.46
Mg
0.60
1.79
4.18
K
6.26
2.64
3.71
S
0.40
0.34
-
Cl
0.48
-
-
W
9.76
-
2.03
35
Ir
2.60
9.21
-
Ca
0.56
-
-
Mn
1.10
-
-
Al
-
0.23
-
P
-
-
10.88
Total
100
100
100
Fig. S2: EDAX spectra of leached rice husk ashes (LRHAs) denoted as leached sticky rice husk ash (L1), leached red rice husk ash (L2), and leached brown sticky rice husk ash (L3).
Table S2: EDAX analysis of L1, L2, and L3.
Elements
Weight (%) L1
L2 36
L3
C
44.84
52.16
55.12
O
33.36
28.32
28.92
Si
19.27
19.52
15.96
K
2.53
-
-
Total
100
100
100
Fig. S3: EDAX spectra of mesoporous silica nanopowders, S-SiO2 (S1), R-SiO2 (S2), and BSiO2 (S3).
Table S3: EDAX analysis of S1, S2, and S3. 37
Elements
Weight (%) S1
S2
S3
Si
51.10
51.11
52.07
O
48.90
48.89
47.93
Total
100
100
100
38
Fig. S4: Full scale survey of XPS spectra of SiO2 nanopowders.
Table S4: XPS analysis of S-SiO2, R-SiO2, and B-SiO2. Elements
Binding Energy (eV) S-SiO2
R-SiO2
B-SiO2
Si
102.6
101.8
102.2
O
531.8
531.1
531.5
39