Synthesis of graphene-like carbon from agricultural side stream with magnesiothermic reduction coupled with atmospheric pressure induction annealing

Generally, large-scale production of graphene is currently not commercially viable due to expensive raw materials, complexity and the high-energy consumption of the processes currently used in the production. The use of biomass precursors and energy efficient procedures for carbonization have been proposed to reduce the cost of the graphene materials. However, low-cost graphene production has not been accomplished yet. Herein, we present a sustainable procedure and renewable starting materials to synthesize carbon nanostructures with graphene-like features. First, a SiC/C composite was synthesized from phytoliths and sucrose through magnesiothermic reduction. The phytoliths were obtained from barley husk that is an abundant side stream of agricultural industry. Second, graphene-like structures were achieved by the graphitization of SiC/C composite with high temperature induction annealing at 2400 °C under atmospheric pressure. The formation of graphene-like carbon was initiated by vaporization of silicon from the pre-ceramic SiC/C. Complete transformation of SiC/C to hollow, spherical graphene-like carbon structures and sheets were verified with thermogravimetry, x-ray diffraction, energy dispersive spectroscopy, electron microscopy and Raman spectroscopy. Also, the theoretical thermodynamic consideration of the phase separation of silicon carbide and the role of free carbon in the process has been discussed.


Introduction
Graphene is a single atom layer thick sheet of carbon that was first found in 2004 [1,2]. The unique electrical, thermal and optical properties of graphene combined with its mechanical integrity have attracted attention since its discovery [3]. Variable techniques have been reported to produce graphene but the most common techniques to produce graphene sheets, platelets and flakes still rely on methods such as exfoliation of graphite, epitaxial growth on SiC substrate and chemical vapor deposition (CVD). High-quality graphene is produced mainly with the energy intensive CVD method [4].
From the industrial production point of view these techniques suffer from high cost, low purity, heterogeneous features and limited upscaling [5,6]. For this reason more sustainable and cost-efficient methods and starting materials are needed for the production of high purity graphene [7]. Utilization of biomass would reduce starting material cost but production of graphene from it is challenging particularly, when thermal treatment techniques are used. Thermally treated biomass typically results in partially ordered amorphous carbon/graphene stack composites, and carbon structures composing of randomly organized small crystalline domains. In addition, biomasses contain inherently heteroatoms (H/N/O/S) bound to carbon and other elements (K, Na, Mg, Fe etc), which can be problematic in the synthesis [8][9][10]. Therefore, approaches such as the use of catalysts or the pre-treatment of the biomass are required to obtain ordered graphitic or graphene-like Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. structures [11,12]. Elements such as Mg, Fe, Ni or Si have been reported to direct carbonization towards graphitic structures [11,13]. Especially silicon in the form of silicon carbide has been reported to favor the formation of graphene type carbon structures [2]. This reaction is usually conducted at high temperatures in vacuum. It is based on the sublimation of Si, leaving behind ordered carbon layers [4,14,15]. Pre-ceramic, nanocrystalline SiC is a promising starting material in the production of graphitic carbon structures as it favors the formation of high carbon content materials [16].
Nanostructured silicon carbide (nSiC) can be produced through several synthetic routes such as CVD, solgel process, thermal or laser ablation from organic compounds but also derived from biomass [17,18]. However, current techniques often require expensive precursors, and toxic reactants that result in highly agglomerated nSiC particles [17,19]. Magnesiothermic reduction (MTR) is an alternative method to produce nSiC according to the following pathway: [17]. It enables the production of nanostructured SiC with high purity, uniform size and morphology, while, strong agglomeration of the SiC particles can be prevented [17,[19][20][21]. Furthermore, MTR can be considered a sustainable approach to synthesize nSiC due to its low energy consumption, short reaction time and low temperature compared to the carbothermal reduction process known as the Acheson process [18,22].
Herein, we report an alternative method for production of graphene-like carbon nanostructures from barley husk using magnesiothermic reduction coupled with atmospheric pressure induction annealing. First, the biomass based nanostructured silica (nSiO 2 , phytoliths) was extracted from barley husk. Phytoliths are amorphous silica materials with porous structure that are produced by specific plants like barley, rice and sugarcane. Second, nSiO 2 and a small amount of carbohydrate in the form of sucrose was converted into silicon carbide/carbon composite (nSiC/C) using magnesiothermic reduction. Third, graphene-like structures were formed via nSiC/C dissociation at 2400°C under atmospheric pressure using induction annealing. As the result, the formation of carbon materials through the complete removal of Si was accomplished and the product was a mixture of partially folded graphene-like and multi-layered carbon structures. For comparison, only partial removal of Si was observed with a crystalline, commercial SiC treated in similar conditions.

Extraction of silicon from barley husk
Nanostructured silica (nSiO 2 ) was produced from barley husk (BH). The BH was leached with 10% HCl at 100°C for 2 h to remove inorganic impurities and hemicellulose. The HCl was removed by vacuum filtration and the solid material, nSiO 2 , was washed with deionized water to remove chlorides after leaching. The washed nSiO 2 was dried at 100°C and calcined in air at 550°C.

Synthesis of nanostructured silicon carbide with high carbon content
Nanostructured silicon carbide/carbon composite (nSiC/C) was synthesized from the purified nSiO 2 and sucrose. First 28.6 g of sucrose was dissolved in 20.2 ml of 2.7 v-% H 2 SO 4 . Then nSiO 2 was added to this reaction solution in a mass ratio 1.2 sucrose/nSiO 2 . This mixture was heated up to 160°C for 5 h to enhance dehydration of sucrose to elemental carbon To remove residual H 2 O and decompose H 2 SO 4 , further treatment under N 2 atmosphere at 700°C for 2 h was performed. Finally, magnesiothermic reduction was conducted in a custom-made reactor under N 2 atmosphere to obtain nSiC/C composite. Magnesium powder was mixed with the nSiO 2 /C composite in mass ratio 2:1 and placed into the reactor. The temperature was increased to 100°C and kept at this temperature for 30 min prior to the reduction. Reduction was initiated with a resistively-heated tungsten wire. The reaction propagated as high temperature reaction through the sample, in which maximum temperature within the powder reached 1000°C. Synthesized nSiC/C was washed with 37% HCl at 70°C for 1 h and 1 M NaOH at room temperature for 16 h to remove byproducts, and dried at 65°C for 2h.

Graphitization of nanostructured silicon carbide/carbon composite by atmospheric pressure induction annealing
Conversion to graphene-like nanostructured carbon was achieved through induction annealing of the nSiC/C composite under atmospheric pressure in argon atmosphere. The induction furnace had 50 kW heating power. The induction annealing is an efficient high temperature treatment due to the high heat transfer capacity of induction [23]. The temperature of the sample was monitored with a Kleiber 730-LO pyrometer at temperatures between 350°C and 2600°C. The previously prepared nSiC/C powder (m=0.5 g) was placed inside a graphite crucible, which was enclosed except for a small pyrometer view hole. The crucible was placed inside the induction oven between heating coils that generate an alternating magnetic field and heats up the crucible. The sample was heated up to 2400°C under atmospheric pressure in Ar atmosphere and kept at this temperature for 20 min. Schematic illustration of the induction furnace is presented in the figure 1. More detailed explanation of induction annealing set up has been described elsewhere [16].

Characterization
Thermogravimetry analysis was performed with TA instruments TGA Q50. The measurements were performed under air, first at 80°C for 30 min and followed by a heating ramp at 20°C min −1 from 80°C to 900°C. The N 2 adsorption/desorption measurements were performed with Micromeritics Tristar II 3020 at 77 K. Before the N 2 sorption measurements the samples were degassed at 120°C under vacuum for 1 h. The specific surface area was determined using the multipoint BET (Brunauer-Emmett-Teller) method.
X-ray powder diffraction examination was carried out with the Bragg-Brentano geometry utilizing the Bruker D8 Discover diffractometer equipped with Cu-tube, λ=1.54 Å. Generator was set to 40 kV and 40 mA and the K β -radiation was removed with a 0.02 mm Ni-filter. The measurement was performed in the 2θ range 20°-90°with the step size of 0.038°and the step time of 1.2 s.
The structure of the particles was analyzed with scanning electron microscopy (SEM, Zeiss Sigma HDVP) and transmission electron microscopy (TEM, JEM-2100F JEOL Ltd). SEM imaging was conducted using 5 kV EHT, Inlens detector and WD 5 mm. Powder samples for SEM were placed on the aluminium stub and analyzed without coating. The size of the particles was estimated by measuring single particle sizes with ImageJ software. HSBD detector was used in the EDS studies. The TEM samples were prepared by pipetting a droplet of ethanol suspension to a holey carbon TEM grid. 200 kV acceleration voltage was used for the imaging.
Raman spectroscopy (Thermo DXR2xi Raman) was conducted at wavelength λ=532 nm, 100× objective and 50 μm confocal pinhole aperture. Laser power of 2.5 mV and exposure time of 0.02 s were used in the analysis.
Thermodynamic equilibrium calculations were carried out with the Equilib-module of the FactSage 6.2 software in order to estimate the decomposition temperature of SiC, and its further reactions during the induction heating experiments. Thermodynamic data of the condensed and gas-phase species were taken from the Fact53 and Ftmisc-LMLQ (liquid light metal alloy) databases. The calculation was carried out at pressure of 1 bar and temperature range of 300°C-3000°C. During induction annealing the substance (SiC) is surrounded by argon gas. Therefore, the global equilibrium was calculated at SiC mixing ratio of 0.1 in argon gas. In addition, a local equilibrium without the presence of Argon gas was calculated.

Results and discussion
3.1. Production and properties of nanostructured silicon carbide/carbon composite Synthesis of nSiC/C was carried out in two phases: (1) Extraction of nanostructured SiO 2 (nSiO 2 ) from agricultural side stream barley husk and (2) Production of nSiC/C composite through magnesiothermic reduction from nSiO 2 and sucrose. Barley husk is naturally rich in SiO 2 accumulated in phytoliths, which makes it an attractive source of SiO 2 [24]. However, prior to calcination barley husk must be acid-treated to remove metallic impurities, mainly Na and K to ensure high purity of the final product. These metallic impurities can form ternary oxides with SiO 2 , e.g. Na 6 Si 8 O 19 or Na 2 Si 2 O 5 and consequently decrease the melting point of the ash during the heat treatment of the biomass. Presence of ternary oxides enable carbon in the biomass to dissolve and remain in such ternary oxides decreasing the purity of the final nSiO 2 product [25]. The extracted nSiO 2 was mixed with sucrose and treated under magnesiothermic reduction resulting in the formation of a nSiC/C composite. The mixture contained sucrose above the stochiometric amount needed for the formation of SiC in order to increase the amount of free carbon in the composite. This additional carbon improved the formation of graphitic structures in the induction annealing during the graphitization. Figure 2 shows the thermogravimetric analysis, nitrogen adsorption-desorption isotherm and XRD diffractogram of the nSiC/C composite. Based on the analysis the total amount of free carbon in nSiC/C composite before induction annealing was 21 wt% (figure 2(A)). The nitrogen adsorption-desorption isotherm of nSiC/C resembled the type II isotherm with the H3 type hysteresis loop, which indicates mesoporous structures with BET surface area of 234±2 m 2 g −1 ( figure 2(B)).
XRD analysis showed that nSiC/C composed of two different polytypes, namely SiC-3C and SiC-2H (figure 2(C)) [26][27][28]. The average crystallite size calculated from 2θ peaks 60.0°and 71.8°was 16.7 nm. In addition, a single diffraction peak of graphitic carbon was observed at 25.9°. The absence of the other diffraction peaks of graphitic carbon was due to the low crystallinity of the carbon in nSiC/C as can be expected based on the reaction temperature used in the magnesiothermic reduction (1000°C). Figure 3A and 3 show the SEM images of the nSiC/C composite. The nSiC/C appeared to be composed of sharp-edged crystals with the size varying between 100 nm and 600 nm, and spherically shaped primary particles forming larger porous structures. The elemental mapping conducted with energy dispersive x-ray spectroscopy (EDS) coupled with SEM indicated that the sharp-edged crystals composed of SiC and the porous structures were amorphous free carbon ( figure 4(A)). The Mg peak in the EDS spectrum is due to a Mg residue from the magnesiothermic reduction and the Al peak is due to the SEM specimen stub. The oxygen peak may indicate the presence of unreacted SiO 2 , MgO or silicon oxycarbide [29]. Also, some oxygen may be contributed by oxygen surface functional groups on free carbon.

Properties of graphitized carbon nanostructures formed by induction annealing at atmospheric pressure
Conversion of nSiC/C to graphene-like carbon was obtained with induction annealing at 2400°C under Ar atmosphere and atmospheric pressure. XRD analysis showed formation of the highly graphitized carbon structures presented in figure 2(C). The diffraction peak at 83.5°particularly indicates the presence of the multilayered carbon sheets as also seen in the TEM images (figures 3(C), (D)) [28]. As expected, the specific surface area of nSiC/C was reduced during the induction annealing as S BET surface area of graphene-like carbon was only 19.0±3 m 2 g −1 . According to the nitrogen sorption-desorption isotherm (figure 2(D)) resembles the type III sorption isotherm referring to the non-porous or macroporous material.
The nSiC/C went through a complete morphological change as can be observed from the SEM images (figures 3(A) -(E)). The nSiC/C transformed mainly to hollow spherically shaped carbon structures with particle sizes varying from 50 to 500 nm in size. In addition to the spherical structures, sheet structures were observed as well as presented in figure 3(C). The atomic plane distance, based on the estimation from TEM images, between the multilayered carbon structures was 0.32 nm. This value is slightly lower than the atomic plane distance of crystalline graphite, which is 0.3354 nm [30].
Based on the Raman analysis shown in figure 3(F), there may be graphene domains present in the sample matrix in addition to the graphitic structures. Peak set up in Raman spectra correlated well with the values for graphene, i.e. dominant single Lorentzian 2D peak band at 2700.1 cm −1 with 24.4 cm −1 full width half maximum (FWHM) and G band at 1581.3 cm −1 . Minor D peak at 1349.8 cm −1 indicates minor defects in the material structure [31].
According to the thermogravimetric analysis complete conversion of nSiC/C composite to high carbon content material was achieved ( figure 2(A)), which corresponds well to the results obtained from XRD and SEM-EDS analysis (figures 2(C) and 4(B)). This is significant improvement compared to the commercial SiC (Sigma-Aldrich) which resulted in only 25 wt% carbon conversion under similar thermal conditions ( figure 2(A)). High conversion of nSiC/C to carbon achieved in this study is in a good agreement with our previous studies conducted with pre-ceramic, nanocrystalline SiC/C produced from organometallic compounds using the same induction annealing approach [16,32]. Figure 5 shows the thermodynamic equilibrium of bulk SiC under Ar atmosphere between 1600°C and 3000°C, based on the thermodynamic data of the condensed and gas-phase species provided in the Fact53 and Ftmisc-LMLQ (liquid light metal alloy) databases. According to the thermodynamic data, the decomposition of SiC

Phase separation of silicon carbide and the role of free carbon
However, the rate of sublimation is typically very slow at these temperatures and complete removal of Si from SiC would take very long time unless low pressures are used. Therefore, sublimation of silicon and subsequent graphitization of SiC has been typically carried out in ultrahigh vacuum conditions (UHV 10 -8 to 10 -9 Torr) around 1470 K [33,34]. According to equilibrium, the maximum solid carbon conversion (26%) with the used SiC mixing ratio is achieved at 2660°C, while further increase in temperature declines the carbon conversion and gaseous compounds, such as SiC 2 (g), Si 2 C, Si (g) and C x (g), become predominant ( figure 5). The calculated maximum carbon conversion is in a very good agreement with the experimentally determined carbon conversion of commercial SiC at 2400°C in which 25 wt% of material was graphitized ( figure 2(A)). Furthermore, the local equilibrium without the presence of Ar-atmosphere indicates that the decomposition of SiC may also lead to a formation C-Si liquid alloy [35]. Induction annealing of nSiC/C at same reaction temperature resulted complete conversion to graphene-like structures. This may indicate lower onset temperature compared to temperature predicted with thermodynamic equilibrium calculations and behavior of commercial SiC. Enhanced reaction kinetics is most likely related to the pre-ceramic, nanocrystalline features of the precursor nSiC, which lower the Si and C phase separation temperature [16]. Si begins to evaporate from the powder due its higher vapor pressure compared to the C leading to the formation of graphene and graphitic structures [2]. It has been suggested that the Si evaporates in a layer by layer fashion but the exact chemical composition of the Si atoms leaving the surface is not fully understood. In addition, the amount of free carbon seems to be an important factor affecting the growth of the graphene layers observed in the study. When the amount of free carbon was increased in pre-ceramic nSiC/C mixture produced in this study, the onset temperature for the phase separation decreased. This is most likely due to Si/C interactions, enabling the formation of the graphene-like structures already at temperatures between 1900°C and 2600°C [16]. The effect of free carbon is in agreement with the simulations carried out by Wang et al (2007) [34]. Based on the simulations the SiC surface is rapidly covered with carbon through cap nucleation of carbon once the top Si atom layer has been removed.

Summary
In summary, the study shows that graphene-like carbon nanostructures can be achieved from biomass-based precursors through energy efficient bottom-up synthesis procedures, magnesiothermic reduction and induction annealing. The nSiC/C composite, used as the precursor in the synthesis of graphene-like carbon nanomaterial, was obtained from sucrose and nSiO 2 . The nSiO 2 was extracted from an agricultural side stream, namely barley husk. The precursor nSiC/C composed of nSiC and 21 wt% of free carbon. Crystalline and low defect graphene-like carbon, composing of nano-sized individual spherical particles 50-500 nm in size and sheet structures, was obtained through magnesiothermic reduction coupled with induction annealing. Reaction mechanism leading to the formation of graphene-like carbon was initiated by the evaporation of Si. We observed significant improvement in the reaction kinetics of nSiC/C compared to commercial SiC. Lowering of the phase separation temperature is related to the nanocrystalline structure of biogenic SiC and the large amount of free carbon in the nSiC/C precursor composite.