Graphitization Behavior of Loblolly Pine Wood investigated by in situ

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Graphitization Behavior of Loblolly Pine Wood investigated by in situ High Temperature X-ray Diffraction Seunghyun Yoo, Ching-Chang Chung, Stephen Kelley, and Sunkyu Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01446 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Sustainable Chemistry & Engineering

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Graphitization Behavior of Loblolly Pine Wood investigated by in situ High Temperature X-ray

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Diffraction

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Seunghyun Yooa, Ching-Chang Chungb, Stephen S. Kelleya, Sunkyu Parka,*

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a

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b

Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina, 27695, United States Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, 27695, United States

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*

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University, 431 Dan Allen Drive, Campus Box 8005, Raleigh, NC 27607-8005, USA

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E-mail: [email protected]

Corresponding author: Sunkyu Park, Department of Forest Biomaterials, North Carolina State

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ABSTRACT Graphitization is a complex process involving chemical and morphological

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changes, although the detailed mechanism for different starting materials is not well understood. In this

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work, in situ high temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC)

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were used to examine the phase transition occurring between 1,000 and 1,500oC in loblolly pine wood

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derived carbon materials. Electron energy loss spectroscopy (EELS) was also used to study these wood

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derived carbon materials. XRD data showed the disappearance of disordered carbon phase between

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1,300 and 1,400oC, followed by the formation of a crystalline graphitic phase between 1,400 and

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1,500℃. Lattice parameters and crystal structure of the loblolly pine wood derived graphite were

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systematically calculated from the empirical data. The presence of a large endothermic peak at 1,500oC

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in the DSC thermogram supported this observation. Selected area electron diffraction patterns showed

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the growth of graphitic crystallites after heat treatment. EELS spectra also supported the presence of a

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well-developed graphite structure.

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Keywords: Biomass graphite, Graphitization behavior, In situ X-ray diffraction, Electron energy loss

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spectroscopy, High temperature differential scanning calorimetry

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Graphitization behavior of pine biomass. This in situ

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observation will bring a deeper understanding of sustainable

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material conversion to versatile carbon materials.

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Introduction Biomass is a naturally available carbon-rich material that exists in many forms e.g., wood, plant,

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crop, fruit, and animal waste. When biomass is thermally treated under an inert environment, the labile

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oxygen-rich structure decomposes to produce a carbon-rich residue. Biomass has been used as a source

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of carbon materials since ancient time due to the abundance and the easiness in conversion.1 While

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biomass derived carbon has been widely available for centuries, its detailed structure has not been

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clearly elucidated due to its structural complexity. It is generally accepted that the structure of biomass

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derived carbon material is determined by the structure of precursor biomass and the highest treatment

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temperature (HTT). Yet, the exact pathway for the conversion of biomass to carbon is not well

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understood. Recent work on biomass-derived carbon has shown the potential for producing high-end 2 ACS Paragon Plus Environment

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electronic materials such as graphene quantum dot and electrodes for energy storage devices.2 To

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develop advanced biomass-derived carbon for value-added electronic materials, it is imperative to

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understand both the chemical and morphological changes that occur during the thermal treatment.

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Biomass is a complex mixture of cellulose, hemicellulose, lignin, and inorganic elements. The

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ratio among semi-crystalline cellulose, amorphous hemicellulose, polyaromatic lignin, minor

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extractives, and mineral components depends on the specific biomass source. As a result, it is difficult to

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elucidate the specific chemical and morphological changes which take place during the thermal

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conversion of biomass to an ordered carbon material.3-6 During the thermal conversion, long cellulose

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and hemicellulose chains are broken into smaller molecules that are released as bio-oil and pyrolysis

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vapors.7 The thermal conversion process of biomass is thought to be a 3-step process; 1) at a temperature

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range between 300 to 900℃, biomass is converted into a porous-disordered biochar,8-9 2) above 900℃,

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the disordered biochar is turned into turbostratic carbon, which has randomly distributed 2 to 3 layers of

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small graphitic stacking, 10 and 3) finally at temperatures above 1,500oC more ordered graphitic carbon

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is formed.

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The graphitic carbon has a long-range crystalline structure with multiple layers of graphitic

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stacking. Emmerich et al. showed a systematic ex situ X-ray diffraction (XRD) pattern observation

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during the graphitization of babassu nut.11 This work tracked the biomass graphitization process at

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temperatures up to 2,200℃, followed by cooling to room temperature. The ex situ XRD data was to

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develop a graphitization model, butdue to recrystallization of graphitized material during the slow

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cooling process, the ex situ observation cannot represent the real phase change during the graphitization

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of biomass at high temperature.12

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To address thelimitations of ex situ observations, the current work used in situ high temperature XRD and DSC to simultaneously observe the structural changes during the thermal treatment of 3 ACS Paragon Plus Environment


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biomass. Both XRD and DSC results revealed simultaneous chemical and morphological changes at

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temperature between 1,200 and 1,600oC. Lattice parameters and crystal structure of the loblolly pine

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wood derived graphite were numerically calculated from the XRD data. The electron diffraction pattern

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allowed for calculation of the reciprocal spacing distance in loblolly pine derived carbon. Finally, EELS

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showed the evolution of electronic structure during the graphitization.

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Experimental

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High temperature in situ X-ray diffraction analysis

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20-mesh size Loblolly Pine (Pinus taeda) particles were pre-carbonized at 800℃ for 15 minutes

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under nitrogen gas flow (1L/min) by using OTF-1200X quartz tube furnace (MTI Corporation,

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Richmond, CA, USA). This pre-carbonization prevents extensive mass loss during the in situ XRD

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measurements graphitization that reduce the intensity of XRD pattern. The pre-carbonized sample was

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labeled N800 biochar. The graphitization of the N800 biochar was analyzed using an Empyrean X-ray

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diffractometer (PANalytical, Westborough, MA, USA) with a HTK2000N heating stage (Anton Paar,

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Graz, Austria) and a Cu-Kα X-ray source (0.15415 nm) in a temperature range of 25 to 1,600℃. To

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avoid chemical reactions between biochar and tungsten heating stage, a thin platinum foil was placed in

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between biochar and tungsten heating stage, and the entire system remained in vacuum. XRD patterns

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were acquired in a 2θ range of 10o to 35o every 100℃ on the heating cycle. The step size and count time

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for the in situ XRD measurements were 0.026 o and 93.8 sec/step, respectively. The heating rate was

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20K/min and the temperature was held constant for 5 minutes prior to each XRD data collection. After

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cooling the sample to room temperature, sample was retained for the further analysis and labeled as


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biomass graphite. A polycrystalline natural graphite was used as a standard to determine the degree of graphitization (Thermo Fischer Scientific, Waltham, MA, USA).

Room temperature x-ray diffraction analysis Room temperature XRD patterns of N800 biochar, biomass graphite, and natural graphite were collected by using a SmartLab X-ray diffractometer (Rigaku, Woodlands, TX, USA) equipped with a Cu X-ray source. in the range of 10o – 50o 2θ at ambient conditions. The step size and count time for the in situ XRD measurements were 0.05o and 3 sec/step, respectively. Background subtraction of the XRD patterns was processed with HighScore Plus 3.0 software (PANalytical, Westboroough, MA, USA). Additional peak fitting and analyses were conducted using MAUD software (Department of Industrial Engineering, University of Trento, Trento, Italy). Thermal analysis with simultaneous thermogravimetry/differential scanning calorimetry High temperature DSC was conducted with a NETZSCH model STA 449 F1 Jupiter® simultaneous thermal analyzer. The sample was heated from 25 to 1,550℃ at heating rate of 20K/min and was remained at 1,550℃ for 10 min, under a constant stream of dry argon. During the heating, sample mass change (thermogravimetry, TG) and energy transformation (differential scanning calorimetry, DSC) were simultaneously recorded.

Transmission electron microscopy imaging and selected area electron diffraction analysis Samples for the transmission electron microscopy (TEM) were prepared by using UC7 Ultramicrotome (Leica Microsystems Inc. Buffalo Grove, IL, USA). N800 biochar, biomass graphite, 5 ACS Paragon Plus Environment

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and natural graphite were sliced into samples of approximately 100 nm thick. Sliced specimens were transferred to the copper TEM grid (Protochips, Morrisville, NC, USA). TEM images were taken by Titan 80-300 Probe Aberration Corrected Scanning Transmission Electron Microscope (FEI, Hillsboro, OR). TEM images were taken at high magnification using diffracted beam at a voltage of 200kV to provide point-to-point resolution of 0.20nm. Selected area diffraction patterns (SAED) were taken at TEM mode. The reciprocal space distance was measured by ImageJ to determine diffraction planes.

Electron energy loss spectroscopy analysis (EELS) The EELS were collected with a Gatan (Pleasanton, CA, USA) using STEM mode with a high voltage of 200 kV and energy resolution of 0.15 eV. Sample zero energy loss, low energy loss, and carbon Kedge core energy loss spectra were collected, and zero loss peak subtraction, background subtraction, and electron multiple scattering effect removal were conducted using the Gatan Digital Micrograph software.13 Surface plasmon and bulk plasmon energy were obtained from extracted low energy loss spectrum. Corrected carbon K-edge energy loss spectrum was further deconvoluted into three Gaussian spectra to calculate the π to π* transition ratio.14-15

Results and Discussion Graphitization behavior of loblolly pine investigated by in situ X-ray diffraction Figure 1 shows the XRD pattern for the development of graphitic structure from biochar to turbostratic carbon to primitive graphitic carbon. As the temperature increases from 800 to 1,300℃, the intensity of the disordered peak (located at around 24° 2θ) decreases with temperature. The strong and


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broad peak at low 2θ angle is the combination of (002) reflection of few-layer graphitic stacking and disordered carbon structure in biochar. Importantly and unexpectedly, the diffraction peak of disordered carbon diminishes between 1,300 to 1,500℃. Previous ex situ study of babassu nut graphitization at room temperature observed continuous graphitic structure development during the thermal treatment.11 This suggests a model for structural development where the thermally treated biomass is continuously converted from disordered carbon to organized graphitic carbon layers.10 However, the ex situ model ignores the potential for recrystallization of carbon phases during the cooling process that it does not directly represent the biomass graphitization phenomenon.12 As a result, in this work in situ observations suggest a different pathway for the biomass graphitization process. These results suggest that the graphitic structure does not develop continuously from the disordered carbon structure, but the process includes an intermediate process where no solid crystalline graphitic structure exists. XRD pattern suggests that the disordered carbon was nearly disappeared at 1,400℃. After reaching 1,500℃, a minor tiny peak indicative of graphite (002), appears at 26.67°. These observations suggest that the intermediate turbostratic carbon structure re-organizes to create the final graphite structure. These changes are consistent with the DSC results shown below.


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Figure 1. In situ X-ray diffraction patterns at the temperature range of 800 to 1,600℃.

Room temperature XRD patterns of N800 biochar, biomass graphite, and natural graphite are presented in Figure 2. N800 biochar shows a broad pattern at low 2θ angle around 24°. Biomass graphite produced at the highest treatment temperature (HTT) of 1,600℃ shows a sharp (002) reflection with a weak and broad background pattern. Natural graphite shows a strong (002) reflection. These XRD patterns were background subtracted and refined to calculate lattice parameters, layer coherence length, and degree of graphitization (Table 1).


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Lattice parameters were calculated by Bragg’s law and reciprocal lattice vector notation of hexagonal crystal system (equations (1) and (2)) 16-17 where dhkl is the interlayer spacing of (hkl), λ is the wavelength of X-ray radiation, and θ is the Bragg angle.

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2 𝑑𝑑ℎ𝑘𝑘𝑘𝑘

dℎ𝑘𝑘𝑘𝑘 = =

λ 2 sin(𝜃𝜃)

(1)

4 ℎ2 + ℎ𝑘𝑘 + 𝑘𝑘 2 𝑙𝑙 2 � � + 3 𝑎𝑎2 𝑐𝑐 2

(2)

Two layer coherence lengths, La (orthogonal to c-direction) and Lc (parallel to c-direction), were calculated by Scherrer equation (equation (3)) to show the graphitic ordering 9, 17-18 where β is the FWHM (in radians of theta) of each d spacing, λ is the wavelength of X-ray, and θ is the Bragg angle. 𝐿𝐿𝑎𝑎 =

1.84𝜆𝜆 , 𝛽𝛽cos(𝜃𝜃)

𝐿𝐿𝑐𝑐 =

0.91𝜆𝜆 𝛽𝛽cos(𝜃𝜃)

(3)

The degree of graphitization is calculated by the d002 fractional ratio (equation (4)) of ideal graphite (3.354Å) and non-ideal graphite (3.440Å).17 g� =

3.440 − 𝑑𝑑002 3.440 − 3.354

(4)

Lattice parameters of biomass graphite were calculated as a = 2.619Å and c = 6.735Å. Compared to lattice parameters of natural graphite, lattice parameter a is 6.25% longer and lattice parameter c is 0.02% longer. Both La and Lc dramatically increased by 1,470% and 2,020% from N800 biochar to biomass graphite. The (002) interlayer spacing distance is the largest at N800 biochar. The degree of graphitization value of biomass graphite is similar to that of natural graphite. Furthermore, the thermal expansion in crystal structure is observed from biomass graphite XRD pattern obtained at 1,600℃. Lattice parameter a expanded 2.0% and lattice parameter c expanded 4.7% when compared to lattice parameters obtained at room temperature. Elemental composition and weight yield of N800 biochar, biomass graphite, and natural graphite are given at Table S1. 9 ACS Paragon Plus Environment

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Figure 2. Room temperature XRD patterns of (a) N800 biochar, (b) biomass graphite (patterns collected after cool down to room temperature), and (c) natural graphite obtained at room temperature. Table 1. Quantified lattice parameters of N800 biochar, biomass graphite, and natural graphite.

N800

Biomass Graphite

Measurement Temperature

25℃

a (Å) c (Å) Lc (Å)

N/A N/A 8.3

2.658 7.020 158.3

La (Å) d002 (Å) g̅ (%)

23.5 4.046 N/A

331.5 3.510 N/A

Natural Graphite

Cooled to 25℃

25℃

2.671 7.049 175.9

2.619 6.735 176.3

2.465 6.734 235.8

368.0 3.524 N/A

369.0 3.368 84.12

316.6 3.367 84.83

1500℃ 1600℃

Thermal analysis with simultaneous thermogravimetry/differential scanning calorimetry analysis Figure 3 shows heat flows and mass changes during the high temperature thermal analysis of biochar. From the DSC curve (black line), four phases are distinguished depending on heat input/output status. Below 350℃ (17 min), there is an endothermic peak in the DSC and extensive mass loss in TGA due to the initial degradation of biomass components.19 From 350 to 800℃ (36 min), the DSC shows a broad exothermic region, which is a typical feature of biomass carbonization

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and limited mass loss

supports the formation of the disordered biochar structure above 500℃.7 The XRD in this region is dominated by the broad (002) XRD pattern, which represents a disordered carbon structure. The third 10 ACS Paragon Plus Environment

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phase is initiated at about 800℃, where turbostratic carbon structure starts to develop 8. As the turbostratic carbon structure develops, the DSC heat flow changes from exothermic reaction to endothermic. The slope of DSC curve is modest until the temperature reaches 1,300℃ (62min), where (002) XRD pattern disappears. At about 1,300℃, the slope of DSC curve becomes steeper and a major endothermic peak occurs at 1,530℃ (75min) consistent with the conversion of disordered biochar to ordered crystalline graphite, which requires significant energy input. As mentioned above the XRD pattern also shows a significant increase in order between 1,500 and 1,600oC. Both the DSC and XRD results are consistent with the formation of ordered graphite at temperatures between 1,500 and 1,600oC.

Figure 3. Simultaneous TG/DSC curve of loblolly pine wood.

Transmission electron microscopy image and selected area electron diffraction pattern analysis Figure 4(a, b, c) shows SAED patterns of N800 biochar, biomass graphite, and natural graphite, respectvely. Natural graphite shows clear polycrystalline diffraction rings while N800 biochar and natural graphite exhibit poorly defined diffraction rings. From SAED patterns of N800 biochar and biomass graphite, a strong diffraction ring from (002) reflection and weak diffraction rings from (100) and (011) 11 ACS Paragon Plus Environment

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are observed. Existence of three diffraction rings also corresponds to the observation from XRD patterns. A comparison between N800 biochar and biomass graphite shows that as the N800 biochar is heated the (112) reflection develops and the radius of diffraction ring becomes larger. Overall SAED pattern becomes clearer at biomass graphite. TEM images taken at high magnification show the development of graphitic structure within samples. N800 biochar shows a relatively large piece of carbon material without any layered structure (Figure 4(d)). Biomass graphite shows a primitive layered structure related topology, which is consistent with the formation of graphitic stacking (Figure 4(e)). Natural graphite clearly shows single graphene layer and few layers of graphitic stacking (Figure 4(f)). Together the SAED patterns and TEM images suggest that the biomass graphite produced at 1,600℃ has a significant portion of crystalline graphite that coexists with disordered carbon. Combined with the XRD analysis result, the crystalline part of the biomass graphite has a similar structural property as natural graphite. However, the disordered part of biomass graphite confirmed by the XRD pattern at low 2θ angle region and TEM analysis still differentiates the biomass graphite from natural graphite.


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Figure 4. SAED patterns (a-c) and TEM images (d-f) of N800 biochar (a, d), biomass graphite produced at 1,600℃ (b, e), and natural graphite (c, f). TEM images were obtained at 50 nm magnification. Electron energy loss spectroscopy analysis (EELS) EELS is a powerful analytical technique that reveals details of both the electronic and chemical structure of material at the molecular orbital level. Figure 5 shows low energy loss region (a, b, c) and carbon K-edge energy loss (d, e, f) spectra of the three samples. At low energy loss region, plasmonic features, collective oscillations of electrons in material, are observed. At 5 to 6 eV region, a surface plasmon peak is observed. At 22 to 27 eV region, a bulk plasmon peak is observed. As the material has more ordered graphitic structure, the peak position of surface and bulk plasmon shift to higher energy region (Table 2). The bulk plasmon is an indicator of different carbon structure

13, 22

. Diamond bulk

plasmon peak appears at 33 eV, graphite bulk plasmon peak appears at 27 eV, and amorphous and disordered carbon bulk plasmon peaks appear from 22 to 25 eV.9, 13, 23 Quantified bulk plasmon peak values found in this work correspond to values from these references.


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Figure 5. Low energy loss and Carbon K-edge core energy loss spectra of (a, d) N800 biochar, (b, e) biomass graphite, and (c, f) natural graphite.

From the carbon K-edge electron energy loss spectrum, an electronic transition from carbon 1s (core shell orbital) to π* (valence shell orbital) is detected (Figure 5(d, e, f)). This transition is closely related to the aromatic sp2 ratio in the carbon materials (Add a reference). Specifically, when an electron beam passes through a thin carbon specimen, three different electronic transitions occur and these three transitions can be fitted with Gaussian curves (shown in Figure S1, S2, S3) 13. The transition at 285.0eV (G1) represents the electronic transition from carbon 1s core orbital to C=C π* bonding orbital.14-15, 22-25 The second transition at 292.0eV (G2) represents the electronic transition from carbon 1s core orbital to C-C σ* bonding orbital and the third transition at 298.0eV (G3) represents the electronic transition from carbon 1s core orbital to C=C σ* bonding orbital.14-15, 22-25 The ratio of sp2 bonding is given as, C 1s to π∗ transition ratio =

𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎(𝜋𝜋 ∗ ) 𝐺𝐺1 = 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎(𝜋𝜋 ∗ + 𝜎𝜎 ∗ ) 𝐺𝐺1 + 𝐺𝐺2 + 𝐺𝐺3

C 1s to π* transition ratio of N800 biochar is 0.093 and as the sample is heat treated at 1,600℃, the transition ratio increases to 0.100 (Table 2). The transition ratio of biomass graphite is about 80% of that of natural graphite.

Table 2. Quantified parameters from electron energy loss spectra


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N800 0.093 ± 0.002

C1s to π* Transition Ratio Surface Plasmon 5.10 ± 0.57 Energy (eV) Bulk Plasmon 22.94 ± 0.34 Energy (eV) C-C Bond Length (Å) 1.4242 ± 0.0004

Biomass Graphite 0.100 ± 0.001

Natural Graphite 0.125 ± 0.003

5.66 ± 0.33

6.35 ± 0.06

23.68 ± 0.44

26.78 ± 0.42

1.4232 ± 0.0013

1.4207 ± 0.0003

* 5 samples measurements were done for N800 and biomass graphite and 4 sample measurements were done for natural graphite

At carbon K-edge electron energy loss spectrum, a broad feature known as a multiple scattering resonance (MSR) structure appears at 325 to 327 eV (Figure 5(d, e, f)). The MSR structure is generated by the local electron resonance in carbon-carbon bonding.22 When an atom is excited by electron beam, core electrons are ejected and backscatter from first and second nearest neighbor atoms. The backscattering in electron energy loss near-edge structure (ELNES) is effective within 1nm from the excited core atom and appears as MSR structure.26 The wave number of ejected electron k follows the below relationship, kR = constant

(5)

where R is bond length 27. The MSR energy (EMSR) is proportional to the inverse square of R as shown in equation (6).22, 28-29 The distance from excited core carbon atom to the second nearest neighbor carbon atom, RMSR, is given as 2.467 Å and the constant, KMSR, is given as 1980.8904 eVÅ2.22 The average C-C bond length is the radius of the primary shell, so RMSR value from equation (6) fits into equation (7). 𝐸𝐸𝑀𝑀𝑀𝑀𝑀𝑀 =

𝐾𝐾𝑀𝑀𝑀𝑀𝑀𝑀 2 𝑅𝑅𝑀𝑀𝑀𝑀𝑀𝑀

C − C bond length =

𝑅𝑅𝑀𝑀𝑀𝑀𝑀𝑀 2 sin(60°)

(6)

(7)

Calculated average C-C bond lengths of N800 biochar, biomass graphite, and natural graphite are given at Table 2. Average C-C bond length is the longest at N800 biochar which value is 1.424Å. Biomass 15 ACS Paragon Plus Environment

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graphite has shorter average C-C bond length of 1.423Å. Natural graphite has the shortest average C-C bond length of 1.421Å which is identical to known graphite C-C bond length.30 The plasmonic features can be interpreted by the quantum mechanical approach. By using the Drude model, the displacement of a quasi-free electrons in a local electric field can be modeled. Equations for the resonance frequency of plasma oscillation and the plasmon energy are given as, 𝑛𝑛𝑒𝑒 2 𝜔𝜔𝑝𝑝 = � 𝜀𝜀0 𝑚𝑚

𝐸𝐸𝑝𝑝 = ħ𝜔𝜔𝑝𝑝

(10)

where n is the electron density (valence electrons per unit volume), e is the elementary charge of electron, m is the mass of electron, and ε0 is the vacuum permittivity 13. The resonance frequency of plasma directly notates that the bulk plasmon energy is proportional to the square root of the electron density. However, plasma resonance in a real solid is strongly confined by the damping factor occurred by the single electron transition 13. As a result, oscillator strength term f is added to the equation 31-32. f=

2𝑚𝑚𝐸𝐸𝑔𝑔 𝑒𝑒 2 ħ2

2 𝑎𝑎𝑛𝑛𝑛𝑛

(11)

𝑓𝑓𝑓𝑓𝑒𝑒 2 𝐸𝐸𝑝𝑝 = ħ� 𝜀𝜀0 𝑚𝑚

(12)

Eg is an energy gap and ani is an atomic dipole matrix element for the excitation. Then, equation of changing energy gap (ΔEg, equation (13)) induces equation (14).32 2𝜋𝜋 2 ħ2 ∆E𝑔𝑔 = ∗ 2 𝑚𝑚 𝑑𝑑

∆𝐸𝐸𝑝𝑝

(𝑏𝑏)

𝐸𝐸𝑃𝑃

𝜋𝜋 2 ħ2 1 = (𝑏𝑏) 2 𝑚𝑚∗ 𝐸𝐸 𝑑𝑑 𝑔𝑔

(13)

(14)

m* is the effective mass, d is the average distance between carbon atoms, and the subscript (b) remarks the properties of bulk. Equation (14) implies a physical relationship between molecular bond length and plasmonic features. The inverse square of average C-C bond length is plotted as a function of bulk plasmon 16 ACS Paragon Plus Environment

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energy in Figure 6. The bulk plasmon energy is physically related to the average C-C bond length. The inverse square of average C-C bond length values shows linear correlation with increasing bulk plasmon energy. N800 biochar with its disordered structure and longer average C-C bond length reflects the smaller value of bulk plasmon energy. As the degree of graphitization increases, the average C-C bond length decreases and the bulk plasmon energy increases. It is known that a theoretically ‘perfect’ graphitic structure has 27.0 eV of the bulk plasmon energy with 0.496Å-2 of the inverse square of the average C-C bond length.22 In this work the average C-C bond length and bulk plasmon energy of natural graphite are almost the same as that of theoretically perfect graphite. This quantum mechanical relationship can be used as an indicator of the carbon structure development during the carbonization and graphitization of biomass.

Figure 6. Correlation between bulk plasmon excitation energy and inverse square of the average C-C bond length

Conclusions Phase transitions occurring during the loblolly pine wood graphitization was quantitatively analyzed based on empirical observations from a series of complimentary techniques. From the in situ XRD measurement, disappearance of disordered carbon phase was observed between 1,300 to 1,400℃ where no crystalline structure exists. But with additional heating to 1,500 to 1,600oC this non-ordered


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state is rapidly converted into a highly order graphite carbon. Previous ex situ studies suggested a continuous structure development model for the progressive formation of graphitic structure. However, the observations from this work strongly suggest a new model, where there is a non-continuous process for the formation of graphitic structure. The endothermic nature of graphite formation was confirmed by the DSC measurement. The results from analysis of graphitic lattice parameters, electron diffraction, electron microscopy imaging, sp2 ratio, average C-C bond length, and plasmonic features all provide compelling evidence for the formation of graphitic structure from loblolly pine wood. This combination of tools, and the results of this work help to clarify the detailed mechanisms involved in the formation of biomass derived graphitic materials.

Acknowledgement. This project is supported by the USDA National Institute of Food and Agriculture (Award 2011-68005-30410) and the US-DOE Office of Energy Efficiency and Renewable Energy (Award Number DE-EE0006639). The characterization was performed at the Analytical Instrumentation Facility (AIF), supported by the State of North Carolina and the National Science Foundation (Award ECCS-1542015). A special appreciation to the NETZSCH Instruments Applications Laboratory (Burlington, MA, USA) for running the high temperature DSC measurement.

Supporting Information Supplementary Table S1: Yield and elemental composition of samples Supplementary Figure S1: EELS spectrum and deconvolution of N800 biochar Supplementary Figure S2: EELS spectrum and deconvolution of biomass graphite Supplementary Figure S3: EELS spectrum and deconvolution of natural graphite

References 18 ACS Paragon Plus Environment

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Graphitization Behavior of Loblolly Pine Wood investigated by in situ

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