Luminescence Mechanism of Carbon-Incorporated Silica

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Luminescence Mechanism of Carbon Incorporated Silica Nanoparticles Derived from Rice Husk Biomass Zhaofeng Wang, Songshan Zeng, Yezhou Li, Weilin Wang, Zhengguo Zhang, Huidan Zeng, Weixing Wang, and Luyi Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Industrial & Engineering Chemistry Research

Luminescence Mechanism of Carbon Incorporated Silica Nanoparticles Derived from Rice Husk Biomass

Zhaofeng Wang,a,b Songshan Zeng,a Yezhou Li,a Weilin Wang,a,c Zhengguo Zhang,c Huidan Zeng,d Weixing Wang,c,* and Luyi Suna,* a

Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States

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State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China c

Ministry of Education Key Laboratory of Enhanced Heat Transfer & Energy Conservation, School of

Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China d

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

*Authors to whom correspondence should be addressed: Dr. Luyi Sun, Tel: (860) 486-6895; Fax: (860) 486-4745; Email: [email protected] Dr. Weixing Wang, Tel: +86 20 8711 3171; Email: [email protected];

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Abstract In this work, silica-based luminescent materials containing different contents of carbon were synthesized from rice husk biomass. The intrinsic structure, chemical composition, as well as photoluminescent features were investigated. The results suggest that two forms of carbon, i.e., carbon that chemically bonded and non-bonded with silica, exist in the structure of the as-prepared amorphous silica nanoparticles, which are believed to be responsible for the origin and quenching of photoluminescence, respectively. The generation of successive localized energy levels within the band gap of silica by the chemically bonded carbon is believed to be the luminescent mechanism. The insight into the photoluminescence of rice husk derived carbon incorporated silica nanoparticles in this work would be valuable for researchers to further modify the luminescent features for practical applications.

Key words: Rice husk biomass; Luminescent materials; Silica nanoparticles; Localized energy levels

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1. Introduction Rice Husks (RHs) are generally considered a biowaste of rice production.1-2 Millions of tonnes of RHs are produced every year worldwide.3-5 Because of the tough nature and low nutritional value, RHs are usually disposed of by open field burning or land filling, resulting in air pollution, waste of energy, greenhouse gas emission, and occupancy of a large amount of space.6-7 Therefore, it is of great economic and environmental significance to find efficient utilizations for RHs. Since RHs contain a high silica content (15−28 wt. %), the conversion of RHs to valuable silica or other silicon containing materials have attracted much attention.8-12 The resultant products have found widespread application in adsorption,13-14 catalysis,15-18 energy storage,19-21 etc. Because silica has a good biocompatibility, high physicochemical and biochemical stability, the RH derived silica and related materials have also shown promising applications in biomedical fields.22-23 Recently, it was reported that the silica derived from RHs can exhibit photoluminescence (PL) by properly adjusting the experimental conditions during synthesis.24-26 This finding is of particular significance for its potential biomedical applications, as it avoids introducing extra fluorescent dyes or quantum dots into silica, which is complicated, expensive, and not environmentally benign. Although the PL of RH derived materials was observed, the underlying mechanisms, which are critical to optimize and tune the PL and guide the following work, were not clearly revealed in the previous reports.24-26 There is no PL in ideal silica structure because of its wide band gap energy (ca. 9 eV).27 Therefore, the luminescence of the silica-based materials from RHs suggests that there should be some structural change in the prepared silica. To 3



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clearly reveal the underlying mechanisms, in this work, we synthesized a series of silica-based luminescent materials from RHs. By systematically investigating the relationship between the PL features and the intrinsic structures, the origin of the luminescence as well as the corresponding mechanisms of the RH derived silica-based materials are proposed.

2. Experimental section 2.1. Materials The RHs used in this study were obtained from Rice Hull Specialty Products, Inc., Stuttgart, Arkansas, USA. Hydrochloric acid (37 wt. %) was purchased from VWR and used as received. 2.2. Synthesis of carbon incorporated silica nanoparticles The treatment of RHs involved two steps: (1) pretreatment by hydrochloric acid and (2) calcination in a tube furnace (MTI OTF-1200X-III). An alumina crucible was used as the container during the thermal treatment. Typically the water-rinsed RHs (to remove adhering soil and dust) were mixed with a hydrochloric acid solution (5 wt. %) and boiled for 2 h to remove mineral impurities. Then, the acid leached RHs were rinsed with deionized water 3 times and dried at 90 °C for 24 h. Subsequently, the pretreated and dried RHs were calcined at 550 °C for 2 h (heating rate 10 °C/min). The acid treatment was applied to eliminate possible influence by metal cations, particularly K+, which were reported to promote the melting of silica, converting amorphous silica to crystalline phase.12 It was observed in our initial experiments that the carbon residue in the silica samples could be greatly affected by the air flow rate as well as air pressure. As such, the carbon content in the silica samples can 4


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be controlled by adjusting the above two parameters. Moreover, it was observed that when multiple samples were simultaneously treated in a same quartz tube, the sample at the downstream always possessed a higher concentration of carbon because of the gradient oxygen concentration due to the gradual depletion of oxygen along the air flow direction. In this work, we employed two air flow rates of 50 and 70 mL/min in a quartz tube (44 mm ID × 1219 mm Length), and at each air flow rate, two crucibles containing treated RHs were placed side-by-side along with the air flow direction. Four samples of RH silica containing various carbon contents were synthesized, named as RH-Silica-S1, RH-Silica-S2, RH-Silica-S3 and RH-Silica-S4. We also synthesized silica powders with a high purity by calcining the HCl treated RHs at 700 °C for 2 h in a cubic furnace where sufficient air was present, which was denoted as RH-Silica-700-2 and used as a control sample.12 2.3. Characterization PL measurements were performed at room temperature using a fluorescent spectrometer (Model Fluorolog-3-P) with a Xenon lamp as the excitation source. The microstructure and composition of the samples were characterized by scanning electron microscopy (SEM, FEI Strata 400S, operated at 10 kV), equipped with energy-dispersive spectroscopy (EDS). X-ray diffraction (XRD) patterns were recorded on a Bruker D5 diffractometer with Bragg-Brentano θ-2θ geometry using a graphite monochromator with Cu Kα (λ = 0.1540 nm) radiation. X-ray photoelectron spectra (XPS) of the samples were acquired on an ESCALAB 250XT spectrometer (Thermo Fisher). The X-ray source was Al Kα (1486.6 eV). Elemental analysis was conducted in an Elementar vario MICRO cube analyzer. Fourier transform infrared (FT-IR) spectroscopy (Nicolet Magna 560) was employed to further confirm the 5



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chemical composition and structure of the samples.

3. Results and discussion Figure 1a shows a digital picture of raw RHs. By tailoring the reaction conditions, RHs were converted to carbon incorporated silica powders, exhibiting a slight color change gradually from white to gray as shown in Figure 1b, which indicates that different concentrations of carbon in the synthesized silica samples were achieved. Figure 1c shows the XRD patterns of the as-prepared materials. All samples exhibit a broad diffraction band from ca. 15 to 40º, suggesting the presence of highly amorphous phase of silica. While the four silica samples contain various concentrations of carbon, their overall morphology and particle size are similar (Figure S1). Figure 1d presents a representative SEM image of RH-Silica-S1, showing fine nanoparticles with a diameter of ca. 60 nm. The amorphous structure and nanoparticle morphology can be ascribed to the pretreatment of RHs as well as the subsequent calcination conditions. The removal of metal ions by acid treatment eliminated their possible negative influence to promote the melting of silica at relatively low temperatures.12 The relatively low synthesis temperature (550 ºC) could further prevent the formation of irregular aggregates by fusing, as reported in the previous researches.11,28 Preliminary elemental analysis by EDS showed that all the four samples were composed of C, Si, and O (Figure S2). The above structural, elemental, and morphological analyses suggested that carbon-incorporated amorphous silica nanoparticles were successfully prepared from RHs.

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Figure 1. Digital images of (a) raw RHs and (b) the as-prepared silica-based powders; (c) XRD patterns of the silica-based materials; (d) SEM micrograph of RH-Silica-S1.

The as-prepared silica-based samples can exhibit bright blue-white luminescence when excited by a 365 nm UV lamp, as shown in Figure 2a. Their emission spectra (λex = 365 nm) are presented in Figure 2b, which exhibit broad bands covering the entire visible light region. The color coordinates (x, y) of samples S1, S2, S3 and S4 were calculated to be (0.248, 0.262), (0.253, 0.262), (0.246, 0.263), and (0.270, 0.288), respectively (Figure 2c). Figure 2d depicts the photoluminescence excitation (PLE) spectra of the silica-based materials derived from RHs, which show a broad excitation band from ca. 250~380 nm, with the strongest absorption in the near UV light region. Such broad band features of the PL and PLE spectra suggest that such luminescent silica-based materials are promising for applications in optical devices and intelligent sensors.29-31 7



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Figure 2. (a) Luminescent photograph (λex = 365 nm), (b) emission spectra (λex = 365 nm), (c) CIE color coordinates of the RH derived samples, (d) excitation spectra (λem = 428 nm).

Generally, fine structural silica without modification does not exhibit PL because of its wide band gap energy (ca. 9 eV).27 Luminescence of the silica-based materials in the previous reports was attributed to either the intrinsic defects32-34 or the external doping and surface functionalization.35-36 To further understand the underlying luminescent mechanisms of these samples, high-purity silica powders (RH-Silica-700-2) with virtually no incorporated carbon (which will be discussed below and is shown in Table 1) were prepared for comparison.12 8


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Under the same measuring conditions, RH-Silica-700-2 powders exhibited little luminescence, confirming that the incorporated carbon in the structure of silica plays a key role in the PL features. However, it is highly imperative to understand how the incorporation of carbon endows silica bright luminescence. Table 1 presents the carbon content of all of the samples determined by a detailed elemental analysis on an Elementar vario MICRO cube analyzer. From RH-Silica-S1 to RH-Silica-S4, the mass fraction of carbon gradually increases from 0.14 to 0.33, which is consistent with the color variation of the powders. Along with the increase of the carbon concentration, the intensity of both PL and PLE spectra increases first, and then decreases (Figures 2b and 2d), with RH-Silica-S2 exhibiting the highest intensity in both PL and PLE. This result indicates that the incorporated carbon in silica plays a critical role in both the generation and quenching of luminescence. Table 1. Carbon content in the as-prepared silica-based materials.

Sample Carbon (wt. %)

RH-Silica-700-2

Luminescence Mechanism of Carbon-Incorporated Silica

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