Rapid sonochemical synthesis of spherical silica nanoparticles derived from brown rice husk

Author’s Accepted Manuscript Rapid Sonochemical Synthesis of Spherical Silica Nanoparticles Derived from Brown Rice Husk S. Sankar, Narinder Kaur, Sejoon Lee, Deuk Young Kim www.elsevier.com/locate/ceri

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S0272-8842(18)30388-2 https://doi.org/10.1016/j.ceramint.2018.02.090 CERI17492

To appear in: Ceramics International Received date: 3 January 2018 Revised date: 7 February 2018 Accepted date: 9 February 2018 Cite this article as: S. Sankar, Narinder Kaur, Sejoon Lee and Deuk Young Kim, Rapid Sonochemical Synthesis of Spherical Silica Nanoparticles Derived from Brown Rice Husk, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.02.090 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.

Rapid Sonochemical Synthesis of Spherical Silica Nanoparticles Derived from Brown Rice Husk S. Sankar, Narinder Kaur, Sejoon Lee, and Deuk Young Kim*

Department of Semiconductor Science, Dongguk University-Seoul, Seoul 04620, Republic of Korea

Abstract The nanoparticles of silica (SiO2) were synthesized from brown rice husk via the sonochemical process, during which the sonication time was varied from 0 to 50 min so as to modulate the material properties of the nanoparticles. The synthesized SiO2 nanoparticles were confirmed to have amorphous natures in their structural characteristics. The mean particle sizes of the nanoparticles were increased from 5 to 40 nm with increasing sonication time from 0 to 50 min, respectively. The SiO2 nanoparticles synthesized with the sonication time of 50 min showed to possess the highest porosity among our samples. Accordingly, the sample exhibited a higher effective surface area of 271.22 m2g-1. In addition, the bandgap of the SiO2 nanoparticles was decreased from 5.77 to 5.68 eV with increasing sonication time (i.e., 0 → 50 min). The results suggest that the SiO2 nanoparticles synthesized with an appropriate control of the sonication time (e.g., 50 min) hold promise for future nano-ecosystem applications such as nano-biosensors and energy storage nano-devices.

Keywords: Rice husk; Sonochemical synthesis; Silica; Specific surface area; Porosity.

*

Corresponding Author. E-mail: [email protected] (D. Y. Kim) -1-

1. Introduction The production of silica (SiO2) nanoparticles has attracted enormous attention in the scientific and technological world because of their extensive use in various fields such as electronic devices, catalysts, pigments, pharmacy, thermal insulators, and humidity sensors. According to previous studies, many natural resources (e.g., marine sponges and diatom [1,2], rice husk [3], sand [4], sugarcane bagasse [5], coal fly ash [6] etc.) have been used for the synthesis of SiO2 nanoparticles. Among them, rice husk (RH), i.e., a by-product form of the rice can be easily available in a huge quantity [7,8]. Furthermore, since the reuse of biomass resources could be of great benefit for eco-friend nanoscience and nanotechnology, the synthesis of SiO2 nanoparticles from RH has been widely studied by using various experimental methods. For example, chemical reaction via acid-alkali leaching [9], combustion synthesis [10], microwave [11] and hydrothermal synthesis [12], pressurized hot-water treatment [13], precipitation [14], non-isothermal [15], and sonochemical methods [16] are typical ways that can produce SiO2 nanoparticles from RH (See also Table S1 for the comparison of silica properties prepared by various experimental techniques, Supporting Information). When using the biomass resource of RH as a siliceous material, the silica can be synthesized at an incredible rate of giga tons/year because RH contains more than 90% amorphous silica [17,18]. Thus, it is highly essential to produce a large quantity of high quality silica by using environmental benign-materials of RH. Based on all the above, we have investigated the effective and facile method for obtaining the biogenerated silica using various RH sources [9]. We, here, report on the characteristics of the SiO2 nanoparticles obtained from brown rice husk (BRH) via the rapid sonochemical process, which could be one of the simple techniques for synthesizing high quality silica from biomass siliceous resources. The microstructural size and the

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surface-to-volume ratio of the SiO2 nanoparticles are controlled by changing the sonication time during the sonochemical process. The morphological, structural, textural, and optical properties are thoroughly examined, and the effects of the sonochemical process time on the material properties of the BRH-derived SiO2 nanoparticles are discussed in detail.

2. Experimental Details 2.1. Synthesis of SiO2 Nanoparticles The SiO2 nanoparticles were synthesized via the sonochemical synthesis method by using BRH sources (Fig. 1) produced in Tamil Nadu, India. First, the raw source of BRH was burned in an open environment; then, the ash of BRH was collected. Next, 3.0 g of BRH ash was stirred in 10 % HCl (45 mL) for 2 h so as to exclude metal ions in the prepared material. In the third step, the HCl-leached BRH ash was sonicated in an ultrasonic water bath for different time durations (i.e., 0, 10, 30, and 50 min) under the high-intensity ultrasonic power (240 W at 35 kHz). After the sonication for different time durations, each of HCl-leached BRH ash was filtered, rinsed, and dried at 150 °C for 24 h in an electric oven. To collect semitransparent SiO2 materials, thereafter, the dry powders were baked in the oven and were annealed in a muffle furnace at 700 °C in air ambience for 2 h. For convenience, we denote the SiO2 samples prepared with the different sonication time of 0, 10, 30, and 50 min as S-0, S-10, S-30, and S-50, respectively. 2.2. Characterization of SiO2 Nanoparticles The morphological and the compositional properties of the BRH-derived SiO2 samples were monitored by field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDX) spectroscopy, respectively, using an Inspect-F50 system (FEI Company InspectTM,

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USA). Additionally, the microstructural and the crystallographic properties were confirmed through transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements by using a Titan 80-300 system (FEI Company InspectTM, USA). The structural properties were characterized through X-ray diffraction (XRD) measurements using a D8Advance system (Bruker, USA). The electrochemical properties were examined by Fouriertransform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) using a Scimitar 1000 FT-IR system (Varian, USA) and a ESCALab250Xi system (Thermo Fisher Scientific, USA), respectively. The optical absorption properties were analyzed by using an ultraviolet-visible (UV-VIS) spectrophotometer (Specord 200-plus, Analytik Jena AG, Germany), and the volumetric adsorption properties were measured by using a Brunauer-Emmett-Teller instrument (BELSORP-mini II, MicrotracBEL, Japan).

3. Results and Discussion Figs. 2(a) – 2(d) show the FE-SEM images of the silica samples derived from BRH via the sonochemical synthesis process. All of the samples (S-0 – S-50) consist of spherical grains, which comprise ultra-small SiO2 nanoparticles. By using an ImageJ software (National Institutes of Health, USA) and a dynamic light-scattering technique, the mean particle sizes of the SiO2 nanoparticles were confirmed to increase from 5 to 40 nm (see Fig. S1 and Table S2, Supporting Information) with increasing sonication time from 0 to 50 min (i.e., S-0→ S-50). In addition, the distribution of the silica nanoparticles becomes homogeneous when the sonication time is increased. Namely, compared to other samples, the S-50 sample shows a dense distribution of spherical nanoparticles with the uniform particle size (see the inset of Fig. 2(d)). When the sonication time is increased beyond 50 min, however, the sample exhibited the aggregated

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structure of spherical nanoparticles (see Fig. S2, Supporting Information). To confirm the elemental compositions of the prepared silica samples, we performed EDX measurements. First, we state that the BRH ash contains metal elements (e.g., K, Mg, W etc.) as well as main species of the silica (e.g., Si, O, C) (see Fig. S3 and Table S3, Supporting Information). After the HCl treatment, however, such metal impurities were effectively removed from the HCl-leached BRH ash (see Fig. S4 and Table S4, Supporting Information). From the BRH-derived silica samples (S-0 – S-50), we only observed two strong peaks of O Kα and Si Kα with an additional small peak of Pt Kα, arising from conductive-coating for SEM measurements (Figs. 3(a) – 3(d)). As can be confirmed from the inset of each figure, the weight percentages (wt.%) of Si and O are slightly varied upon the sonication time. Despite the ignorable effect of the sonication time on the elementary composition, the BRH-derived SiO2 nanoparticles involve no precipitates from other species. This indicates that the sonochemically-synthesized silica has high purity even though the material had been made from the biomass siliceous precursor of BRH ashes. After confirming the effective formation of SiO2 nanoparticles through the sonochemical synthesis using BRH, we characterized the microstructural properties of the SiO2 nanoparticles, particularly for S-50, because the sample exhibited the most uniform distribution of spherical nanoparticles among of our samples. As shown in Fig. 4(a), the TEM image of the S-50 sample clearly displays the aggregation of densely-distributed uniform and spherical nanoparticles. However, the SAED pattern indicates the SiO2 nanoparticles to have amorphous natures (Fig. 4(b)), as is typical for normal silica samples. Additionally, the XRD patterns also reveal the amorphous phase of the BRH-derived SiO2 nanoparticles (see Fig. S5, Supporting Information). To verify the configuration of chemical bonds in amorphous SiO2 nanoparticles, we examined the electrochemical properties of the samples through XPS measurements. From the S-

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50 sample, the only original components of Si and O are detected, as shown in the XPS survey spectrum (Fig. 4(c)), where the existence of C might be associated with the surface contamination of hydrocarbons due to the air exposure [19]. The peaks from Si 2p (Fig. 4(d)) and O 1s (Fig. 4(e)) core levels further corroborate the presence of pure SiO2 for our BRH-derived silica sample [9,20]. Other samples (i.e., S-0 – S-30) showed almost similar characteristics to S-50, but only binding energies of Si and O were slightly varied up on the sonication time because of its dependence on the particle size and the surface area (see Fig. S6 and Table S5, Supporting Information). The chemical bonding structures of Si and O can be confirmed from the FTIR spectrum (Fig. 4(f)). The transmission peaks at 3464 and 1638 cm-1 are attributed to the O-H stretching and O-H bending modes, respectively [21,22]. The band at 1094 cm-1 is responsible for the asymmetric stretching vibration of the Si-O-Si bond [9]. Furthermore, two additional bands at 466 and 800 cm1

can be assigned to the rocking bond and the symmetric (silanol) vibration of the Si-O bond

[9,14,23]. Fig. 5 shows UV-VIS absorption spectra of the samples of S-0 – S-50. The absorption edges of S-0, S-10, S-30, and S-50 are observed at around ~214, 215, 216 and 217 nm, respectively, which correspond to the band-gap energy values of 5.77, 5.73, 5.71 and 5.68 eV. As the sonication time increases from 0 (i.e., S-0) to 50 (i.e., S-50) min, the band-gap energy is monotonically decreased. The observed red-shift upon the sonication time can be ascribed to the increase in the particle size of silica (i.e., 5 → 40 nm for S-0 → S-50, see also Fig. 2). According to previous literatures, the stability for inter-particles of E’ centers could be linearly increased with the increase in the size of SiO2 nanoparticles [24-27]. Namely, the red-shift upon the sonication time in our samples could be expected from the stabilization of E’ centers, which exist in SiO2 nanoparticles, particularly for the larger silica nanoparticles, because of larger numbers of

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neighboring atoms and silanol groups. Next, we evaluated the specific surface area and the pore size of the SiO2 nanoparticles by using the Brunauer Emmett Teller (BET) and Barret Joyner Halenda (BJH) methods. Fig. 6(a) plots the adsorption-desorption isotherm responses of S-0 – S-50 samples. The specific surface areas of S-0, S-10, S-30, and S-50 are observed to be 201.45, 233.83, 242.67, and 271.22 m2g-1, respectively. Compared to S-0 – S-30 samples, the largest specific surface area can be found from the S-50 sample, in which spherical SiO2 nanoparticles with a uniform particle size are densely distributed. The nitrogen adsorption-desorption isotherm of the prepared SiO2 nanoparticle exhibits the category IV model (H2-type hysteresis loop) that justifies an IUPAC classification [21]. Fig. 6(b) shows the results of the pore size measurements for the SiO2 nanoparticles. The pore surface areas of S-0, S-10, S-30, and S-50 are observed to be 176.33, 200.25, 202.58, and 226.56 m2g-1, respectively. With increasing sonication time from 0 to 50 min (i.e., S-0 – S-50), the pore volume increases from 0.2349 to 0.2558 cm3g-1, and the pore diameter decreases from 4.97 to 4.11 nm, as summarized in Table 1. These values are higher than those reported in previous literatures [3,9,15,28], and the high porosity in our BRH-derived silica nanoparticles could be a great use in nano-device systems; for example, nano-biosensors [29,30] and energy storage nano-devices [31-33]. From all of the results provided above, it can be conjectured that the size, the porosity, and the band-gap energy of the SiO2 nanoparticles are easily controlled by changing the sonication time during the sonochemical synthesis using a biomass resource of BRH. Finally, we briefly explain the controllability of the porosity in sonochemically-synthesized silica nanoparticles. The porosity of nanopores can be effectively controlled by changing the biomass resource volume, the acid concentration, the sonication time, and/or the annealing

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temperature. In the sonochemical process, the porosity can be easily controlled by changing the sonication time and the ultrasonic condition (e.g., frequency, power etc.). The ultrasonic sounds provide the kinetic energy and enhance the collision, nucleation, porosity, and crystal growth rate during the sonochemical synthesis of silica nanoparticles. Hence, the kinetic energy can be controlled by changing the reaction time. For example, we used the high-power continuous wave source for the ultrasonic irradiation of the solution. Compare to the low-power sonication, the high-power ultrasonic irradiation produces large pores during the synthesis of silica nanoparticles because of large cavitation bubbles during the sonication process [34-39]. When using a higher power of the ultrasonic wave source, therefore, the pore size could be effectively controlled by changing the sonication time because the total kinetic energy during the sonochemical synthesis process can be determined by the time duration of the high power sonication.

4. Conclusions The SiO2 nanoparticles were successfully synthesized from BRH through the sonochemical synthesis method. By changing the sonication time (0 – 50 min), the mean particle sizes and the pore volumes of the SiO2 nanoparticles were facilely controlled from 5 to 40 nm and from 176.33 to 226.56 m2g-1, respectively. As a result, the nanoparticles synthesized for the sonication time of 50 min exhibited a high porosity (i.e., pore volume 0.2558 cm3g-1). In addition, the band-gap energy of the BRH-derived SiO2 nanoparticles was controlled from 5.77 to 5.68 eV by changing the sonication time from 0 to 50 min, respectively. Consequently, the size-controllable SiO2 nanoparticles via changing the sonication time during sonochemical synthesis could be of great benefit for nano-ecodevice systems such as nano-biosensors and energy storage nano-devices.

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Acknowledgements This research was supported by the National Research Foundation of Korea through the Basic Science Research Program (2016R1A6A1A03012877) funded by the Korean government of the Ministry of Education.

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Table 1. Specific surface area and pore characteristics of the BRH-derived silica nanoparticles.

Samples

BET Surface Area (m2g-1)

S-0

Pore size characteristics (BJH) Surface Area of Pores (m2g-1)

Pore Volume (cm3g-1)

Average Pore Diameter (nm)

201.45

176.33

0.2349

4.97

S-10

233.83

200.25

0.2564

4.92

S-30

242.67

202.58

0.2661

4.62

S-50

271.22

226.56

0.306

4.11

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Figure Captions

Fig. 1. Schematic of the sonochemical synthesis process for the formation of silica nanoparticles from BRH.

Fig. 2. FE-SEM images of the silica nanoparticles for (a) S-0, (b) S-10, (c) S-30, and (d) S-50 samples. The inset of (d) displays a high magnification image of the silica nanoparticles for the S-50 sample.

Fig. 3. EDX spectra of the silica nanoparticles for (a) S-0, (b) S-10, (c) S-30, and (d) S-50 samples. The inset of each figure lists the weight percentages (wt.%) of Si, O, Pt involved in the BRH-derived silica nanoparticle samples.

Fig. 4. (a) TEM image, (b) SAED pattern, (c) XPS survey spectrum, (d) Si 2p core level spectrum, (e) O 1s core level spectrum, and (f) FTIR spectrum of the silica nanoparticles for the S-50 sample.

Fig. 5. UV-VIS absorption spectra of the silica nanoparticles for S-0 – S-50 samples.

Fig. 6. (a) Absorption-desorption isotherm and (b) porosity of the silica nanoparticles for S-0 – S50 samples.

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