One-Pot Facile Synthesis of Graphene Quantum Dots from Rice Husks

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Materials and Interfaces

One-pot Facile Synthesis of Graphene Quantum Dots from Rice Husks for Fe3+ Sensing Weilin Wang, Zhaofeng Wang, Jingjing Liu, Yikang Peng, Xiaoyuan Yu, Weixing Wang, Zhengguo Zhang, and Luyi Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00913 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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Industrial & Engineering Chemistry Research

One-pot Facile Synthesis of Graphene Quantum Dots from Rice Husks for Fe3+ Sensing

Weilin Wang,1,2 Zhaofeng Wang,2,3 Jingjing Liu,2,4 Yikang Peng,2,4 Xiaoyuan Yu,2,5 Weixing Wang,1 Zhengguo Zhang,1,* Luyi Sun2,4,6,* 1

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Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, United States 3

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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

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

Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, United States

Institute of Biomaterials, College of Materials and Energy, South China Agricultural University, Guangzhou, Guangdong 510642, China 6

Department of Biomedical Engineering, University of Connecticut, Storrs, CT 06269, United States

*Authors to whom correspondence should be addressed: Dr. Luyi Sun, Tel: (860) 486-6895; Fax: (860) 486-4745; Email: [email protected] Dr. Zhengguo Zhang, Tel: 086-87112845; Email: [email protected]

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Abstract In this work, graphene quantum dots (GQDs) with an average size of 3.9 nm were synthesized using rice husk biomass as the raw material via a facile one-step one-pot hydrothermal method. The size and morphology of the rice husk-derived GQDs were characterized by transmission electron microscopy and atomic force microscopy. The GQDs exhibit bright blue photoluminescence under 365 nm ultraviolet irradiation and can be well dispersed in water. The GQDs reach the strongest photoluminescence excitation intensity at ca. 360 nm under an emission wavelength of 466 nm, suggesting that the GQDs were oxidized with oxygenous groups attached. The quenching tests showed that the synthesized GQDs were highly and selectively sensitive towards Fe3+ ions and thus can potentially be used for Fe3+ sensing.

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Introduction Rice is one of the most widely cultivated crops in the world.1 In recent years, more than 715 million tons of paddy rice was produced annually and over 480 million tons rice was generated after milling.2 Millions of tons of rice husks (RHs) are produced after milling due to the high mass percentage of RHs (ca. 20-25%) in paddy rice.3 The traditional disposal methods of RHs are open field burning and land filling, which lead to air pollution and waste of landfill space, respectively.4 Due to the high ash content after burning (resulting from inefficient carbon conversion), RHs are not suitable to be used as a fuel.5 Therefore, finding novel approaches to convert RHs into valuable products is very critical in terms of both environment and economy. RHs contain a high mass concentration of silica (ca. 20-25%) in addition to various organic matters (such as lignin, cellulose, lignocellulose, pentosans, etc.).6-8 Its high silica concentration makes RH an ideal silica source for preparing silicon-containing materials including silica, silicon, silicon carbide, silicon nitride, silicon tetrachloride, and zeolite.9-15 These silicon-containing materials have found applications in catalysis, energy storage, optical devices, etc.16-19 Besides, RHs can be applied as an absorbent for heavy metals and organic pollutants.20, 21 The organic components in RHs are typically treated as a carbon source to prepare activated carbon, etc.22-26 Compared to the silica in RHs, the utilizations of the organic matters are limited. Since RHs mostly consist of organic components, it is important to explore new methods to take advantage of these organic matters. Graphene, a two-dimensional lattice of sp2-hybridized carbon, possesses many unique properties and has found applications in many fields.27-33 However, the zero band gap of graphene limits its applications in optical and photonics field. Reducing the lateral dimensions of graphene into quantum dots could increase its band gap by quantum confinement effect.34 Graphene quantum dots (GQDs) have attracted much attention because of their unique photoluminescence and physicochemical properties.35-38 There are two main approaches to prepare GQDs: top-down and bottom-up methods. The top-down method refers to cutting large sp2 carbon into small pieces, typically involving exfoliation and break-down two steps. The starting materials of this method are usually limited to the ones containing sp2 carbon domain like graphite, graphene, carbon nanotubes, carbon black, carbon fibers, etc.39-43 The top-down method usually lacks a precise control of the size and morphology of the generated GQDs.35 The bottom-up method means converting small organic molecules to GQDs via chemical reactions. This method generally can have a better control of the size and shape of the synthesized GQDs, although the reaction procedures are usually complicated.35 Achieving the synthesis of GQDs via a simple route using the bottom-up method is thus highly desirable. We have managed to synthesize GQDs from RHs via a multi-step process.44 RHs were first pyrolyzed at 700 °C under a nitrogen atmosphere to form RH ash, containing both carbon and silica. The obtained RH ash was then treated with excessive NaOH under a protective atmosphere to remove silica. The resulting RH carbon was then used to synthesize GQDs through the top-down method.44 Furthermore, GQDs grafted silica nanoparticles were synthesized. Instead of removing silica from RH ash as described above, the ash containing both carbon and silica were treated by a mixture of H2SO4 and HNO3, followed by a hydrothermal 3



treatment at 200 °C for 10 h.19 While GQDs were successfully synthesized from RHs, the synthetic processes involve multiple steps and are thus time-consuming and hard to scale up. Thus, it is essential to explore a more facile route to synthesize GQDs directly from RHs. In this research, we report a simple one-pot hydrothermal synthesis of GQDs through the bottom-up method using RHs as the raw material. The obtained GQDs are highly and selectively sensitive towards Fe3+ ions and thus can potentially be applied for the detection of iron ions (Fe3+). Experimental Materials The RHs used in this project were obtained from Rice Hull Specialty Products, Inc. (Stuttgart, Arkansas, US). MgCl2, KCl, CrCl3, MnCl2, FeCl2, FeCl3, CoCl2, Ni(NO3)2, CuCl2, Zn(NO3)2, Pb(NO3)2 were purchased from Alfa Aesar. All of these reagents are of analytical grade and were used as received without further purification. Preparation of RH-derived GQDs (RH-GQDs) A sample of 1.0 g of RHs was thoroughly washed by deionized (DI) water, dried, and then ground to powders (100 mesh). The powders were washed by 0.10 M HCl and rinsed by DI water for three times, then dried in vacuum at room temperature for 24 hours. A sample of 100 mg dried RH powders together with 20 mL DI water was transferred into a Teflon® lined autoclave. The hydrothermal reaction was carried out at 150 °C for 5 hours. After reaction, the mixture was filtered. The filtrate was centrifuged at 28,000 g force for 15 min to obtain the supernatant, which contains RH-GQDs. The solid residue from filtration containing a high concentration of silica was dried, and then calcined at 700 °C for 2 h in air to prepare silica nanoparticles. Characterization The size and morphology of the RH-GQDs were characterized by transmission electron microscopy (TEM; FEI Tecnai F30, operated at 300 kV). The elemental analysis was conducted using energy-dispersive spectroscopy (EDS) in the TEM. The size distribution of the RH-GQDs was determined using Nano Measurer (version 1.2) based on their TEM images. The thickness of the RH-GQDs was measured by atomic force microscopy (AFM; Asylum Research MFP-3d). The AFM images were obtained under the tapping mode using a silicon tip coated with chromium/gold with a force constant of 40 N/m. Photoluminescence (PL) mapping images of the as-prepared RH-GQD aqueous dispersion were recorded on a spectrofluorometer (Jobin Yvon SpexFluorolog 3-211), equipped with bath circulators and chillers (Isotemp 4100) to control the temperature. X-ray photoelectron spectroscopy (XPS; Thermo Scientific) was performed using a monochromated Al Kα X-ray source (1486.6 eV) for characterizing surface chemical state of GQDs. The synthesized silica nanoparticles were characterized by X-ray diffraction (XRD; Bruker D2, 40 kV and 30 mA) using a graphite monochromator with Cu Kα radiation (λ = 0.1540 nm) 4


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and scanning electron microscopy (SEM; FEI Strata 400S, operated at 10 kV). Sensing of Fe3+ and other ions Fe3+ aqueous solutions of various concentrations (0.02 to 0.2 mM, interval of 0.02 mM; 0.2 to 2.0 mM, interval of 0.2 mM) together with other metal ion solutions (2.0 mM) were freshly prepared. To evaluate the sensitivity of the RH-GQDs towards Fe3+, Fe3+ solutions of different concentrations were mixed with 0.1 mg/mL RH-GQDs aqueous dispersion at a volume ratio of 1:1. Then, the mixture solutions were characterized by a fluorescence spectrophotometer after 5 min of equilibration. The photoluminescence (PL) spectra were recorded using an excitation wavelength of 360 nm. Other metal ion solutions were similarly mixed with RH-GQDs and characterized. Results and discussion As discussed above, RHs contain both inorganic silica (ca. 20-25%) and organic lignocellulose, pentosans, etc.6, 7 These organic matters could be hydrolyzed into small molecules such as glucose, aromatic aldehydes (vanillin, syringaldehyde, etc.), and organic acids (acetic acid, lactic acid, etc.).45, 46 According to the previous reports, some of these biomass derived small molecules can serve as precursors to prepare GQDs or carbon nanoparticles through a hydrothermal route.47 For example, Yang et al. synthesized carbon nanoparticles (ca. 5 nm in diameter) via hydrothermal carbonization using chitosan as a precursor.48 As such, we expect similar nano-carbon materials can be hydrothermally synthesized from RHs directly, and our TEM and AFM characterizations proved that GQDs were indeed synthesized from RHs. The TEM image (Figure 1a) shows that the synthesized RH-GQDs have a uniform dispersion without apparent aggregation. As shown in Figure 1c, those RH-GQDs have a size distribution of 3.9 ± 0.7 nm. The high resolution TEM image (Figure 1b) reveals that the RH-GQDs have a lattice spacing of 0.24 nm, which corresponds to the (1120) lattice fringes of graphene.43 The AFM image and the corresponding height profiles (Figure 2) show that the RH-GQDs have a thickness of ca. 1.2-1.6 nm, corresponding to 2-3 layers of graphene.44, 49, 50 The energy dispersive spectroscopy of the RH-GQDs is shown in Figure 3. Only carbon and oxygen elements were detected (Cu was from the Cu grid supporting the sample), suggesting that no other elements were introduced in the GQDs. The GQDs were synthesized via a one-step hydrothermal reaction under mild conditions, without requiring high temperature and strong oxidants. This could be the reason that no nitrogen or silicon was introduced into GQDs.

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Figure 1. (a) TEM and (b) high resolution TEM image of RH-GQDs; (c) size distribution of the RH-GQDs.

Figure 2. (a) AFM image of the RH-GQDs and the corresponding height profiles along the (b) blue and (c) red line.

Figure 3. Energy dispersive spectroscopy of the RH-GQDs.

It is proposed that the synthesis of RH-GQDs consists of two steps: (1) degradation of large organic molecules to small molecules through hydrothermal hydrolysis;35, 36 (2) hydrothermal carbonization of these small molecules to form GQDs.47, 48 Overall, it is a facile bottom-up 6


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method to synthesize GQDs in mild conditions (150 °C for 5 hours). It is also an environmentally friendly and economic approach to convert RH biomass into a value-added product. The as-prepared RH-GQD aqueous dispersion is very stable. No sign of precipitation was observed even after the aqueous dispersion was left stand still for 6 months. The dispersion exhibits intense blue luminescence (Figure 4a) under the irradiation of 365 nm ultraviolet (UV) light at room temperature. The quantum yield of the RH-GQDs at 20 °C was calculated to be ca. 8.8% (the detailed calculation method is presented in the Supporting Information) based on the irradiation of 400 nm, which is comparable to the values of many other GQDs in the literature.41, 51 PL 3D mapping of the RH-GQDs was conducted with an increasing excitation wavelength from 320 to 400 nm, and the emission wavelength was recorded from 425 to 550 nm under each excitation wavelength (Figure 4b). It is clear that the position of the emission peak varies with the change of excitation wavelength. For example, as the excitation wavelength was increased from 320 to 340, 360, 380, and further to 400 nm, the emission peak was shifted to longer wavelength with an intensity increased initially then decreased (Figure 4c). This result suggests that the photoluminescence (PL) emission spectrum of the RH-GQDs is related to the photoluminescence excitation (PLE) energy. In general, PL property of GQDs is highly related to the size, shape and surface structure (functional groups and defects) of the GQDs.37, 52 According to the previous research, the number of oxygenous defects on GQDs has a significant influence on the position of the strongest PLE peak.53 Liu et al synthesized GQDs and graphene oxide quantum dots (GOQDs) with a diameter of ca. 4 nm. The GQDs have the strongest PLE peak at ca. 290 nm while the GOQDs have the maximum PLE intensity at a higher wavelength (~350 nm).53 The existence of oxygenous functional groups (like carboxyl group) is able to decrease the band gap by introducing more conjugate structures. The synthesized RH-GQD sample has an average diameter of 3.9 nm and reaches the strongest PLE intensity at ca. 360 nm (under the emission wavelength of 466 nm). This suggests that the RH-GQDs were oxidized with oxygenous groups attached.

Figure 4. (a) Digital picture of a RH-GQD aqueous dispersion under the irradiation of 365 nm UV light, (b) PL 3D mapping of the RH-GQD aqueous dispersion, (c) PL spectra of the RH-GQD aqueous dispersion under the excitation wavelength of 320, 340, 360, 380, and 400 nm. 7



To further determine the chemical structure and understand the formation mechanism of the RH-GQDs, XPS was employed. Figure 5 shows the high resolution C 1s XPS spectrum of the RH-GQDs. Besides sp2 carbon, the GQDs contain various oxygenous groups including hydroxyl groups, epoxide groups, and carbonyl groups. The result is consistent with the PL 3D mapping, indicating that oxygenous groups formed on the GQDs.

Figure 5. High resolution XPS spectrum of C1s of the RH-GQDs.

GQDs have been applied in biological and medical fields owing to their low toxicity, high solubility, excellent biocompatibility, as well as special optical and electronic properties.35, 36 Recently, ion sensing via GQDs has attracted high attention. Ananthanarayanan et al. prepared GQDs by the chemical vapor deposition method and the synthesized GQDs were applied for Fe3+ detection.54 Ju et al. used citric acid as the precursor and synthesized GQDs for Fe3+ sensing.55 Wang et al. derived GQDs from graphite via the top-down method for detecting Cu2+ .56 Although these GQDs were synthesized from different raw materials and methods, the prepared GQDs all performed well as ion sensors. Since transition metals have open d orbitals, chelation is very likely to happen when encountering with an electron donor. As for GQDs, functional groups containing oxygen are able to act as electron donors and combine with ions to form non-fluorescence complexes.56 As a result, PL intensity would be significantly decreased. This makes GQDs effective sensors to some transition metal ions. In order to evaluate the selective quenching property of RH-GQDs, a series of experiments were conducted and the results are shown in Figure 6. Figure 6a-6c demonstrates the luminescence quenching after adding 1.0 mM Fe3+, Co2+, or Mn2+ ions. Compared to other two ions, quenching was much more obvious after adding Fe3+ ions under 365 nm UV irradiation and can be easily recognized through naked eyes. Additional various metal ions were mixed with the RH-GQD dispersion and evaluated. As shown in Figure 6d, the most effective quenching was by Fe3+ ions. No other ions could create equally significant quenching, therefore the quenching of 8


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the RH-GQDs is highly selective to Fe3+ ions and thus potentially suitable for sensing Fe3+ ions. It was reported that phenolic hydroxyl groups from the surface of GQDs could form complexes with Fe3+ ions thus facilitating charge transfer. As a result, the exciton recombination could be restrained and significant fluorescence quenching would occur.57 According to Figure 6e, the quenching percentage was basically proportional to the concentration of Fe3+ ions at low concentrations (below 0.3 mM), but the growth of the quenching percentage slowed down when the concentration was further increased. As shown in the inset of Figure 4e, two RH-GQD dispersions (with and without Fe3+ ions) remained distinguishable even when the concentration of Fe3+ was as low as 0.1 mM, which shows the excellent sensitivity of the RH-GQDs towards Fe3+. The detection limit is estimated to be 5.8 nM at a signal-to-noise ratio of 3 (3σ/m, σ was the standard deviation of the blank signal and m was the slope of the linear calibration plot).

Figure 6. PL spectra of the RH-GQD aqueous dispersion containing 1.0 mM (a) Fe3+, (b) Co2+, (c) Mn2+; (d) PL response of the RH-GQD aqueous dispersion towards different metal ions (excitation wavelength: 360 nm). I0 and I are the fluorescence intensities of the RH-GQD aqueous dispersion with and without metal ions, respectively; (e) PL response as a function of the concentration of Fe3+ in the RH-GQD aqueous dispersion. The insets are the digital pictures of the RH-GQD aqueous dispersions under 365 nm UV irradiation (the left one is the control sample without metal ions, the right one is the sample with metal ions).

In addition to the synthesis of GQDs from RHs from the supernatant after centrifuge, the collected residue after centrifuge containing a high percentage of silica can be utilized subsequently to prepare silica nanoparticles by calcining the residue at 700 °C for 2 h in air. These nanoparticles appear to be white powders (Figure 7a inset). The XRD pattern (Figure 7a) 9



clearly shows that the synthesized silica nanoparticles are amorphous. The SEM image (Figure 7b) of the synthesized silica shows that these nanoparticles have a narrow size distribution of ca. 25-35 nm. Such silica nanoparticles can be further used to prepare other silicon-based materials, such as silicon, silicon carbide, zeolite, etc.9, 10, 58 Overall, both GQDs and silica nanoparticles can be synthesized from RHs to achieve a comprehensive utilization of RH biomass.

Figure 7. (a) XRD pattern of the silica nanoparticles prepared from RH residue; the inset is a digital photograph of the as-prepared silica; (b) SEM image of the prepared silica nanoparticles.

Conclusions In summary, we presented a facile one-step one-pot synthesis method of GQDs from RH biomass. The synthesized GQDs have an average size of ca. 3.9 nm with 2-3 graphene layers. The RH-GQDs can be stably dispersed in water, exhibiting bright blue PL under 365 nm UV light. The GQDs reach the strongest PLE intensity at ca. 360 nm under an emission wavelength of 466 nm. The position of the luminescence center suggests that the GQDs were oxidized with oxygenous groups attached. The RH-GQDs showed highly selective quenching to Fe3+ ions, which makes these GQDs a promising sensor for Fe3+ ions. Meanwhile, the residue from the synthesis of the GQDs can be used to prepare amorphous silica nanoparticles. Overall, this facile and new route allows for the synthesis of two value-added products from RH biomass, demonstrating significant economic and environmental benefits. ASSOCIATED CONTENT Supporting Information Available: UV-Vis spectrum of the RH-GQDs; PL intensity of the RH-GQDs as a function of pH value; photoluminescence quantum yield calculation of the RH-GQDs. Acknowledgements This work was sponsored by the Guangzhou Science and Technology Planning Project (No. 201704030022). Z.W. thanks the support from the CAS Pioneer Hundred Talents Program. W.W. thanks the China Scholarship Council for offering her a scholarship to conduct research at University of Connecticut.

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One-Pot Facile Synthesis of Graphene Quantum Dots from Rice Husks

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