Structural Characterization of Loblolly Pine derived Biochar by X-ray

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Structural Characterization of Loblolly Pine derived Biochar by X-ray Diffraction and Electron Energy Loss Spectroscopy Seunghyun Yoo, Stephen Kelley, David Tilotta, and Sunkyu Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04119 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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ACS Sustainable Chemistry & Engineering

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Structural Characterization of Loblolly Pine derived Biochar by X-ray Diffraction and Electron

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Energy Loss Spectroscopy

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Seunghyun Yoo, Stephen Kelley, David Tilotta, and Sunkyu Park*

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a

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27607, USA

Department of Forest Biomaterials, North Carolina State University, 2820 Faucette Blvd, Raleigh, NC

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

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

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Tel: 919-515-0473, E-mail: [email protected]

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Keywords: nanomaterial characterization, biomass-to-crystalline carbon development, electron energy loss spectroscopy, biomass carbonization

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Abstract

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Biochar from lignocellulosic biomass is emerging as a sustainable material with versatile applications,

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but its detailed property is poorly understood due to the structural complexity. We propose a biochar

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structural development model based on the experimental results including composition analysis, surface

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area/pore analysis, x-ray diffraction analysis, electron microscope imaging, and electron energy loss

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spectroscopy. Loblolly pine derived biochars were produced at different carbonization temperatures

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from 300 to 1,000℃. Fixed carbon, sp2 content, and number of graphene layer increased with the

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increased carbonization temperature. Alternating average C-C bond length, interlayer spacing distance,

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and layer coherence length were observed. Bulk plasmon excitation energy was correlated to the average

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C-C bond length that it serves as a good indicator of the carbon structure development when compared

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to the perfect graphitic carbon structure. Based on the experimental results, four different structural

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development phases are identified, which provide a comprehensive understanding of biochar nano-

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carbon crystallite development. Unlike previous biochar structure models, which proposed radially

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growing polyaromatic carbon crystallite, we propose a lengthwise growing polyaromatic carbon

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crystallite model. This experimental-based biochar model should be helpful when determining structure

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of unknown biomass derived carbon materials and disordered pyrolytic carbon materials.

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Biochar is a sustainable carbon material derived from biomass. This study will elucidate the structure of

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sustainable biochar.

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Introduction

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Biochar is a biomass derived sustainable carbon source, which can be utilized in both energy and

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material applications. The traditional role of biochar has been confined to the low value product such as

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soil amendment, adsorption medium, or solid fuel. There are also potential high-end applications for

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biochar such as electrodes for electrochemical energy storage systems1-3 and conductive material for

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sensors and electronic devices.4 For the high-end applications, it is crucial to understand how chemical

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composition and morphology impact the nano-structure of biochar and its resulting properties. Due to

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the inherent structural complexity of biochar and limitations of many common analytical techniques, the

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detailed structure of biochar is not well understood. 2 ACS Paragon Plus Environment

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Advances in nanoscale analytical techniques have enabled characterizations of materials with

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complex structures. Specifically, solid state nuclear magnetic resonance (NMR) can evaluate carbon

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aromaticity and amount of protonated (C-H) / nonprotonated (C-C, C-O) carbon in biochar.5-7 Electron

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energy loss spectroscopy (EELS) can evaluate carbon sp2 content and C-C bond length of biochar.8-12

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Energy-dispersive X-ray spectroscopy (EDS) can quantify molar fraction of atomic elements in

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biochar,10, 13 and X-ray photoelectron spectroscopy (XPS) can quantify amount of carbon functional

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groups (C-O, C=O, COO) in biochar.6, 9, 14-15 X-ray absorption spectroscopy (EXAFS, XANES),14, 16-17

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and Raman spectroscopy10,

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Transmission electron microscopy (TEM),8-9, 19 scanning transmission electron microscopy (STEM),10

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and scanning electron microscopy (SEM)10,

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structures. With these techniques, more details on biochar structure can be detected.

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can now be used to understand the polyaromatic structure of biochar.

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can also be used for image analysis of nanoscale

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One of the first systematic carbon material structural analyses was performed by Franklin using

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X-ray diffraction (XRD) to analyze various carbon crystallites, and classifying them into two groups.20

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One is graphitizing carbon which is characterized by a combination of narrow and sharp (002)

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reflections XRD patterns after thermal treatment. The other is non-graphitizing carbon which reveals a

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broad (002) reflection XRD pattern after thermal treatment. Several other studies probed additional

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details of carbon structural development during the carbonization and graphitization process.21-22

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Because XRD is a bulk technique, which observes the whole structure of carbon material, Franklin’s

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classification seemed plausible from the macroscopic point of view and many researchers accept

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Franklin’s classification. However, with the development of local technique such as TEM, different

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nanoscale graphitization behaviors in non-graphitizing carbon were examined. Subsequent work on

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nanoscale graphitization behaviors support the idea that there is no absolute graphitizing carbon or non-

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graphitizing carbon.23 Graphitic carbon crystallites could also be synthesized from a non-graphitizing

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carbon precursor (biomass) through thermo-catalytic reactions.24-25 Thus, rather than using Franklin’s

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classification as rigid categories, it is more appropriate to view the carbonization and graphitization

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process as pathways to energetically stable carbon structures.

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The earlier model suggested by Keiluweit et al.16 is intuitive, but it lacked quantitative data

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interpretations to distinguish different phases of biochar structure development. Furthermore, their

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classification of phase components as pyrogenic amorphous carbon, pore space, and turbostratic

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crystallites is inaccurate. The term “amorphous” cannot be used for pyrolytic carbon26 and pore space is

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dependent on precursor and pyrolysis condition, and thus it is irrelevant to the disordered and

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turbostratic structure of carbon material.6 Other prior studies also reported the formation of ordered

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carbon structures during the thermal treatment of biomass, but they lacked a precise quantification of

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both the physical structure and chemical composition profiles.5, 27-28

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This work focused on the systematic characterizations of nano-carbon crystallite development in

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loblolly pine wood derived biochar using tools to characterize both physical structure and chemical

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composition. EELS is a powerful analytical technique, which can simultaneously collect information on

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physical structure (plasmon excitation energy loss and multiple scattering resonance), chemical

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composition data (sp2 content), and visual images (STEM high angle annular dark-field (HAADF)).

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XRD is not solely a nanoscale oriented technique, but extracted XRD patterns give useful information

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about the size and morphology of nano-carbon crystallite. Combined with material composition data,

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this study quantitatively characterized the carbonization behavior of biochar at a series of thermal

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treatment steps. Then, based on the collective interpretation of physical and chemical structure

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developments of nano-carbon crystallite, we propose a schematic model of carbon structural

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development.

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Materials and Methods Biochar sample preparation Air dried Loblolly pine (Pinus taeda) wood chips were milled into 20-mesh size particle using laboratory Wiley Mill and air dried for 24 hours. An OTF-1200X quartz tube furnace (MTI Corporation, Richmond, CA) was used for the carbonization. Before loading a sample, the furnace was preheated to target temperatures (300, 350, 400, 500, 600, 700, 800, 900, and 1000℃). After reaching the target temperature, an alumina boat containing 7-8 g of loblolly pine wood particles was inserted into the quartz tube for 15 minutes under a constant stream of nitrogen gas (1 L/min). Calculated heating rates for each sample were 183, 216, 249, 316, 383, 449, 516, 583, and 649 ℃/s. The resulting biochar was cooled down under nitrogen flow. The nomenclature of biochar sample is Nxxx where xxx indicates the carbonization temperature.

Material Composition Analysis Carbon, hydrogen, and nitrogen content of the biochar and activated carbon samples were analyzed by using PerkinElmer 2400 Series II elemental analyzer (PerkinElmer, Waltham, MA). Oxygen content was calculated by difference. Proximate analysis was done to determine volatile matter, fixed carbon, and ash contents using TA Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE). The specific measurement condition followed modified ASTM D7582-15.29

BET surface area and micropore volume analysis BET surface area and micro pore volume of biochar samples were analyzed with Gemini VII 2390 surface area analyzer (Micromeritics, Norcross, GA, USA). 0.15 ~ 0.20 g of sample was loaded in the quartz tube and the sample was degassed at 220°C with nitrogen for 2 hours. Multipoint BET surface 5 ACS Paragon Plus Environment

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area was calculated from the linear relative pressure regime of 0.05 < P/P 0 < 0.30. Total pore volume was calculated at P/P0 = 0.98. Micropore volume was calculated by summation of pore volumes with diameter of less than 2 nm.30

STEM sample preparation and EELS analysis Samples for the electron energy loss analysis were prepared by using UC7 Ultramicrotome (Leica Microsystems Inc. Buffalo Grove, IL). Biochar sliced into 100 nm thick samples that were transferred to the copper TEM grid (Protochips, Raleigh, NC). Electron energy loss spectrum was collected with the electron energy loss spectrometer (Gatan, Pleasanton, CA) attached to the Titan 80300 Probe Aberration Corrected Scanning Transmission Electron Microscope (FEI, Hillsboro, OR). High voltage is 200 kV and energy resolution is 0.15 eV. Biochar zero energy loss, bulk plasmon excitation energy loss, and carbon K-edge core energy loss spectra were collected. Electron multiple scattering effect was corrected by using Gatan Digital Micrograph software.31 Corrected carbon K-edge energy loss spectrum was deconvolved into three Gaussian spectra for further calculation.8-9 STEM HAADAF images were taken at 500 nm magnification. Image resolution was 0.07 nm in STEM mode.

X-ray diffraction pattern analysis Biochar X-ray diffraction patterns were collected by using a SmartLab X-ray diffractometer (Rigaku, Woodlands, TX). Operating voltage was 40 kV and operating current was 44 mA. A Cu Kα Xray tube was used to generate X-rays at a wavelength of wavelength was 0.1541 nm. A Graphite monochromator and Kβ filter were used for the collection of diffracted beam. Biochar particles were placed on a quartz supported sample holder. Then, the sample holder was placed inside the x-ray chamber. Measurement of 2θ angle range was set from 10 degrees to 90 degrees. Each step was 0.05


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degree and the X-ray detector remained constant for 4 seconds at each step to collect the diffracted Xrays. Background subtraction and signal smoothing were done with HighScore Plus 3.0 software (PANalytical, Westboroough, MA). Processed XRD patterns were normalized at (002) reflection intensity. Peak position 2θ angle and full width at half maximum (FWHM) of (002) reflection and (100) reflection were extracted for further analysis.

Results and Discussion Biochar composition analysis Figure 1 summarizes the composition analysis result of raw loblolly pine wood and biochar samples. Bar graph indicates the atomic compositions (molar %) of the biochar samples. As the carbonization temperature increases from 300 to 1,000℃, the atomic carbon content of biochar increases from 36.2 to 92.9 mol.%. Conversely, the atomic oxygen content of biochar decreases from 18.1 to 4.2 mol.% and the atomic hydrogen content decreases from 51.8 to 2.2 mol.%. The amount of nitrogen remains nearly constant between 0.1 to 0.8 mol.%. The dotted lines in Figure 1 shows the proximate analysis (weight %) of the biochar samples. As the carbonization temperature increases from 300 to 1,000℃, the amount of fixed carbon increases from 22.2 to 94.9 wt.% while the amount of volatile decreases from 77.2 to 3.4 wt.%. The amount of ash remains nearly constant in between 0.7 to 1.7 wt.%. A major transition in composition occurs at the carbonization temperature between 300 to 350℃. When the carbonization temperature increases from 300 to 350℃, the atomic carbon content increases by 15.0 mol.% while the atomic oxygen and hydrogen content decrease by 7.2 mol.% and 7.8 mol.% each. The amount of fixed carbon also increases by 36.3 wt.% while the amount of volatile decreases 36.5 wt.%. This result matches well-known biomass degradation behavior.32-34 The cellulose and hemicellulose in biomass are rapidly degraded at temperatures between 300 and 400℃.32 Biomass 7 ACS Paragon Plus Environment

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degradation products are emitted as volatiles and the amount of stable fixed carbon increases rapidly. As a result, it can be assumed that the structure of biochar produced at 300℃ is similar to the lignocellulosic structure because only limited degradation of the structural polymers has occurred.32-34 Although the biomass compositional analysis does not imply a specific biochar structure, it clearly differentiates the initial phase of biochar structural development from other subsequent structural development processes.

Figure 1. Composition analysis result of loblolly pine (raw) and nine biochar samples (top). Magnified atomic nitrogen content and ash content (bottom). Bar graphs represent the atomic compositions (mol.%) of carbon, hydrogen, oxygen, and nitrogen. Line graphs represent the proximate analysis results of fixed carbon, volatile, and ash (wt.%).

BET surface area and micropore volume analysis 8 ACS Paragon Plus Environment

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Figure 2 shows BET surface area and micropore (pore diameters less than 2nm) volume of biochar as a function of carbonization temperature. During the carbonization, BET surface area and micropore volume start to increase from 400℃ (1.98 m2/g and 0.0004 cm3/g each) and maximum values are attained at 600℃ (329.63 m2/g and 0.0101 cm3/g each). Then, additional heat input breaks down the porous network of biochar and the BET surface area and micropore volume start to decrease. N1000 has a BET surface area of 12.34 m2/g and a micropore volume of 0.0007 cm3/g. A similar trend for woody biomass has also been reported.16 Overall graphs of BET surface area/micropore volume as a function of carbonization temperature resemble symmetric bell-shaped curve. Below 400℃, the BET surface area

and micropore volume are not well-developed. Between 400℃ and 800℃, biochar structures develops and the porous structure collapses. Above 900℃, the microporous network of biochar almost disappears.

Figure 2. BET surface area and micropore volume of biochar samples.

Carbon K-edge core energy loss analysis and carbon sp2 content calculation


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Deconvoluted carbon K-edge energy loss spectra of N1000 are presented in Figure 3(a). The sp2 content is a relative value between the sample 1s to π* transition peak area ratio and that of graphite. The equation is given as,35

When the electron beam passes through a thin carbon specimen, three different electronic transitions occur, and these three transitions can be fitted by Gaussian curves.31 The transition at 285.0eV (G1) represents the electronic transition from carbon 1s orbital to C=C π* bonding orbital.8-9, 11, 35-37

The second transition at 292.0eV (G2) represents the electronic transition from carbon 1s orbital to

C-C σ* bonding orbital and the third transition at 298.0eV (G3) represents the electronic transition from carbon 1s orbital to C=C σ* bonding orbital.8-9, 11, 35-37 Figure 3(b) shows changes in the calculated sp2 content of biochar produced at different carbonization temperatures. The calculated sp2 content increases from 58.0 to 77.6 mol.% when the carbonization temperature increases from 300 to 1,000℃. From the sp2 content plot, two noticeable transitions are observed. The first transition was found at 350℃, with a 4.7 mol.% increase in sp2 content, which was larger than any other temperature step. This result also correlates with the composition analysis, where degradation of the carbohydrate content contributes to an increase in atomic carbon and fixed carbon content.16 Another transition is found between 400 and 500℃, where sp2 content increases by 4.2 mol.%. At this interval, residual biomass components are further degraded and the fraction of carbon in biochar increases.38 This interval shows both a change in sp2 chemical structures and organization as measured by XRD, vide infra. The structure development of ideal biochar is related to the thermal treatment temperature.39 However, the precise criterion for the biochar structure development was vague due to lack of numerical indicator. Figure 3(b) suggests a numerical indicator 10 ACS Paragon Plus Environment

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for the biochar structure development, which is sp2 content as a function of carbonization temperature. This criterion will be helpful for further structure analysis of unknown biochar samples.

Figure 3. (a) N800 carbon K-edge energy loss spectrum and deconvoluted Gaussian curves (G1 (red), G2 (green), and G3 (yellow)). (b) Calculated biochar sp2 content. Four to six data points were taken from each sample and the error bars are one standard deviation.

Multiple scattering in near-edge structure and average C-C bond length calculation In Figure 3(a), a broad hump, physically related to multiple scattering resonance of the ejected core electron was detected. When the core electron is ejected from the excited atom, it behaves as a standing wave such that multiple scattering occurs.11 Understanding the multiple scattering is crucial for 11 ACS Paragon Plus Environment

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the analysis of electron energy loss near-edge structure (ELNES) because a large volume of backscattering reflects the local structure of the specimen.31 To get better understanding of the multiple scattering phenomenon, an imaginary shell concept is proposed (Figure 4(a)). The excited core atom is surrounded by imaginary circles with radius define as the distance from the core atom to nth nearest neighbor atom. The ejected core electron is backscattered at the boundary between two adjacent shells. The contribution of backscattered ELNES modulations is effective within 1nm from the excited core atom.40 The wave number of the ejected electron k obeys the relationship “kR = constant” where R is bond length.17 Therefore, the multiple scattering resonance energy (EMSR) is proportional to the inverse square of R as shown in equation (2).11-12, 41 Here, RMSR is the radius of the secondary shell (2.467 Å ) and KMSR is a constant value (1980.8904eVÅ 2). The average C-C bond length is the radius of the primary shell, so RMSR value from equation (2) fits into equation (3).

Calculated average C-C bond lengths in biochar is plotted in Figure 4(b) as a function of carbonization temperature. Three significant transition points are found in the data. Between 350℃ and 500℃, the average C-C bond length decreases from 0.143 nm to 0.142 nm. Then, the C-C bond length is constant until 800℃ when the average C-C bond length increases from 0.143 nm in N700 to 0.143 nm in N800. From 900℃ to 1,000℃, the average C-C bond length decreases from 0.143 nm to 0142 nm. Drastic collapse and degradation of surface area and pore volume above 800℃ seem to have a relationship with decreasing C-C bond length. The fluctuation in the average C-C bond length implies that biomass carbonization includes various phase changes. More details will be compared with plasmon excitation results of biochar samples.


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Figure 4. (a) Visualization of multiple elastic scattering structure by excited core electron. (b) Calculated average C-C bond length based on ideal graphene structure. 4 to 6 data points were taken from each sample and the error bar corresponds to one standard deviation.

Low energy loss and quantum mechanical interpretation of bulk plasmon excitation Bulk plasmon excitation energy loss spectrum appears at the low energy loss region (0 ~ 100 eV). The low energy loss region spectrum of N900 is presented in Figure 5(a). The bulk plasmon exciatation energy loss peak (Ep) provides a measure of the order for different carbon materials. The diamond plasmon peak appears at 33 eV, the graphite plasmon peak appears at 27 eV, and the amorphous (or disordered) carbon plasmon peak appears at 25 eV.31, 37 Figure 5(b) shows the bulk plasmon excitation energy peak values of biochar. Biochar bulk plasmon excitation energy values are placed in between 2223 eV, consistent with known disordered carbon.8 The plasmon excitation energy increases until the carbonization temperature reaches 500℃ and then it gradually shifts to a lower energy region. A similar trend from disordered carbon was also reported.8

An electron energy loss peak related to π-π*

bonding molecular orbital transition is also observed (5-6 eV), but due to its weak intensity (dielectric nature of biochar) and overlap with the zero-loss peak, further data interpretation is not practical.


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Physical interpretation of plasmon excitation requires a quantum mechanical approach. Equations for the resonance frequency of plasma oscillation and the plasmon energy can be derived from the Drude model, which explains displacement of a quasi-free electron in a local electric field.

In these equations, n is the electron density (valence electrons per unit volume), e is the elementary charge of an electron, m is the mass of electron, and ε0 is the vacuum permittivity.31 The resonance frequency of plasma directly related to the bulk plasmon energy, which value is proportional to the square root of the electron density. However, in a real solid, the plasma resonance is strongly confined by the damping due to single electron transitions.31 For the analysis of insulating or semiconducting materials like biochar, the band structure (for crystalline material) or HOMO (highest occupied molecular orbital) – LUMO (lowest unoccupied molecular orbital) structure (for non-crystalline materials) should be considered. As a result, the oscillator strength term, f, is introduced to modify the equation (4).42-43

Eg is an energy gap and ani is an atomic dipole matrix element for the excitation. The energy gap in the nano-sized material is determined by the quantum confinement effect.44 The equation of changing energy gap (ΔEg, equation (7)) induces an equation of changing plasmon energy (ΔEp, equation (8)).43


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m* is the effective mass of electron, d is the average distance between carbon atoms, and the subscript (b) refers to the bulk properties. Usually, d relates to nanocluster size or polymer chain length (for conductive polymer). However, the exact nanocluster size of disordered carbon cannot be measured (XRD only quantifies the average size of crystalline region) so d is replaced by the average C-C bond length as an indirect measurement of the disordered carbon nanocluster size. Based on the equation (8), the inverse square of the average C-C bond length is plotted as a function of plasmon excitation energy in Figure 5(c). The bulk plasmon excitation energy is physically related to the average C-C bond length so it indicates the development of a disordered carbon structure during the carbonization and graphitization of biomass. Perfect graphitic structure has a bulk plasmon excitation energy of 27.0 eV with the inverse square of the average C-C bond length (49.5738 nm-2).


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Figure 5. (a) Low energy loss spectrum of N900. (b) Peak positions of biochar bulk plasmon excitation energy loss spectrum. (c) Correlation between plasmon excitation energy loss and inverse square of the average C-C bond length. 4 to 6 data points were taken from each sample and the error bars are one standard deviation.

X-ray diffraction analysis of biochar crystalline structure Background subtracted biochar XRD patterns are plotted together in Figure S1(a) and S1(b) (supporting information). The N300 biochar XRD pattern is dominated by the crystalline cellulose in the original biomass16, 45 and the pattern is not included in the biochar analysis. Increasing intensity at the (110) reflection is also observed, but the broad FWHM and weak (110) reflection intensity limit the detailed interpretation of this peak. The interlayer spacing distance is derived from the Bragg’s law.46 The layer coherence length of d100 spacing and the average number of graphene layer are derived from the Scherrer equation.8, 47-48 (002) interlayer spacing (average distance between two adjacent planes), (100) layer coherence length (La, average size of carbon crystallite in a plane), and average number of ) of biochar are calculated.48

graphene layers in an ordered graphitic stacking, ( From the Bragg’s law,


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where d is the interlayer spacing, λ is the wavelength of X-ray, and θ is the diffraction angle.46 The interlayer spacing d is inversely proportional to the incident angle θ. The (100) layer coherence length and the average number of graphene layers were derived from the Scherrer equation,

where t is the layer coherence length (mean size of ordered domain), K is the dimensionless shape factor, β is the FWHM (in radians of theta) of each d spacing, λ is the wavelength of X-ray, and θ is the diffraction angle.8, 47-48 The K value for d002 spacing equals 0.9 and that of (100) spacing equals 1.84.47 A schematic of the graphitic structure is shown in Figure 6(d). The FWHM and peak position 2θ angle of (002) and (100) reflection (Figure 6(a) and 6(d)) are used to calculate the interlayer spacing distance and the layer coherence length. From the XRD patterns, three phase transitions of biochar are observed. The initial phase transition appears at 350℃ and can be called “transition char”. As the carbonization temperature increases from 300 to 350℃, the crystalline cellulose XRD pattern disappears and single-broad XRD (002) pattern appears (Figure S1(a)). The disappearance of the crystalline cellulose XRD pattern during biomass pyrolysis is well-known.7, 16 This change is also consistent with the results of the composition analysis and EELS analysis. The secondary phase transition appears between 350 and 500℃ as the structure moves from “disordered char” to “composite char”. Indicative of this transition, the coherence length decreases dramatically from 70.2 to 2.0 nm. Above 500℃, the (100) layer coherence length starts to increase again as the carbonization temperature increases (Figure 6(c), length values of N350 and N400 biochar were too big that not plotted together but given as written numbers). N350 and N400 biochar have much longer (100) layer coherence length compared to other biochar samples. It is assumed that long range disordered structure, which is a mixture of carbon cluster and residual biomass components, dominated during this phase. The substantial decrease in the (100) layer coherence length supports the theory that increasing carbonization temperature induces structural 17 ACS Paragon Plus Environment

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changes from disordered carbon clusters to more ordered carbon clusters. The last phase transition is found at 900℃. At the carbonization temperature step from 800 to 900℃, a crossover between the (002) interlayer spacing distance and the (100) layer coherence length is observed (Figure 6(c)). The decrease in the (002) interlayer spacing is consistent with a transition from disordered stacking of graphite layers to more ordered turbostratic stacking. The increase in the (100) layer coherence length supports growth of nano-carbon crystallites.

Figure 6. XRD pattern and derived physical profiles of biochar. (a) FWHM and peak 2θ angle of the (002) reflection. (b) FWHM and peak 2θ angle of the (100) reflection. (c) (002) interlayer spacing distance and (100) layer coherence length of biochar produced above 500℃. (002) interlayer spacing distances of N350 and N400 are 0.48 and 0.40 nm each. (100) layer coherence lengths of N350 and N400 are 70.2 and 10.0 nm each. (d) Visualization of crystalline graphitic structure. Numerical values


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for d002 and d100 spacing distance are given. Compared to graphite, biochar has wider (002) interlayer spacing and shorter (100) layer coherency length because ordered graphitic structure is not formed yet.

STEM HAADF imaging and modeling of nano-carbon crystallite in biochar STEM HAADF images (all at 500 nm magnification) are shown in Figure 7. Images of N300 and N400 biochar show a dense structure with parallel and repeating lines consistent with a dense morphology from the original wood. These images correspond with the BET surface area data, because the BET surface area and micropore are hardly developed below 400℃. N500 biochar shows splitting of the dense structure and the initial formation of primitive fibril-like structures. With biochar N700, there is clear separation of carbon fibril-like structures represented as raised nodes. The N800 and N900 biochars show more separation, twisting, and breaks as the length of carbon fibril structure is shortened. These STEM HAADF images suggest a path for the structural development of nano-carbon crystallites in biochar. Previous biochar structural development models proposed radially growing polyaromatic carbon crystallite.5-6 However, the STEM HAADF images suggest lengthwise growth of carbon crystallite because carbon fibril-like structure is observed from biochar. Increasing (001) layer coherence length also supports a lengthwise growth model.


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Figure 7. HAADF images of biochar at 500 nm magnification. (a-f) N300, N400, N500, N700, N800 and N900.

Combining all the information from STEM HAADF, EELS, and XRD analysis, a new biochar structural development model is proposed, as shown schematically in Figure 8. The descriptive nomenclature for each phase follows the work of Keiluweit et al.16 Transition char: The original biomass structure dominates the biochar structure. Transition char has a high fraction of hydrogen, oxygen, and volatiles. The XRD pattern is dominated by the crystalline cellulose structure. The initial degradation of biomass structure between 300 to 350℃ is clearly observed and the increases in carbon, fixed carbon and sp2 content are the highest among any other carbonization temperature step. BET surface area and micropore are not well-developed at this transition char stage. The XRD pattern associated with crystalline cellulose disappears during this transition. There is no evidence for multiple-layer carbon stacking. Disordered char: A disordered carbon structure appears at 350℃. Residual biomass components only occupy a minor structural fraction. BET surface area and micropore volume start to increase. The average C-C bond length decreases from 0.14339 nm to 0.14246 nm. The sp2 content of biochar gradually increases and the FWHM of the (002) reflection narrows due to degradation of biomass 20 ACS Paragon Plus Environment

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components. Decrease in (100) layer coherence length supports formation of nano-carbon crystallites. The term “amorphous” in carbon material should be used carefully due to the above mentioned reasons. Composite char: Nano-carbon crystallite begins to grow at 500℃. BET surface area and micropore volume reach their maximum values at 600℃ and then start to decrease. The average C-C bond length remains still at about 0.142 nm range. The sp2 content continues to increase, while the (100) layer coherence length gradually increases from 2.0 to 2.3 nm and the average number of graphene layer increases from 1.4 to 2.0 layers. The initial nano-carbon crystallite molecular structure can be detected within the composite char phase. Turbostratic char: Primitive graphitic structure appears at about 900℃. The microporous structure of biochar mostly disappears. Turbostratic char still does not have completely ordered graphitic structure, but it has two to three layers of ordered graphitic stacking. The average C-C bond length decreases from 0.14301 nm to 0.14235 nm. The cross over between (002) interlayer spacing distance and (100) layer coherence length clearly differentiates turbostratic char from composite char. Biochar sp 2 content reaches the maximum value seen in this work (78.6 mol.%).



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Figure 8. Schematic of the chemical and morphological changes that occur during the carbonization of biomass at different temperatures.

Conclusions We propose a biochar structural development model based on the experimental results including their compositions, BET surface area, micropore volume, sp2 content, average C-C bond length, bulk plasmon excitation energy, interlayer spacing, and layer coherence length. As pine biomass is thermally treated, four different structural development phases are identified, which provide a comprehensive understanding of biochar nano-carbon crystallite development. Unlike previous biochar structure models, which proposed radially growing polyaromatic carbon crystallite, we propose lengthwise growing polyaromatic carbon crystallite model. This experimental-based biochar model should be helpful when determining structure of unknown biomass derived carbon materials. Future study will include quantification of surface functional groups in biochar and reconstruction of disordered carbon structure.


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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).

Supporting Information Supplementary Figure S1(a): XRD patterns 1 Supplementary Figure S2(b): XRD patterns 2


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Figure 1. Composition analysis result of loblolly pine (raw) and nine biochar samples. Bar graphs represent the atomic compositions (mol.%) of carbon, hydrogen, oxygen, and nitrogen. Line graphs represent the proximate analysis results of fixed carbon, volatile, and ash. 323x221mm (96 x 96 DPI)



Figure 2. BET surface area and micropore volume of biochar samples. 282x169mm (150 x 150 DPI)


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Figure 3. (a) N800 carbon K-edge energy loss spectrum and deconvoluted Gaussian curves (G1 (red), G2 (green), and G3 (yellow)). 279x152mm (96 x 96 DPI)



Figure 3. (b) Calculated biochar sp2 content. 4 to 6 data points were taken from each sample and the bar follows a standard deviation. 303x174mm (150 x 150 DPI)


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Figure 4. (a) Visualization of multiple elastic scattering structure by excited core electron. 329x190mm (150 x 150 DPI)



Figure 4. (b) Calculated average C-C bond length based on ideal graphene structure. 4 to 6 data points were taken from each sample and the bar follows a standard deviation. 297x166mm (150 x 150 DPI)


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Figure 5. (a) Low energy loss spectrum of N900. 301x177mm (150 x 150 DPI)



Figure 5. (b) Peak positions of biochar bulk plasmon excitation energy loss spectrum. 293x172mm (150 x 150 DPI)


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Figure 5. (c) Correlation between plasmon excitation energy loss and inverse square of the average C-C bond length. 4 to 6 data points were taken from each sample and the bar follows a standard deviation. 288x166mm (150 x 150 DPI)



Figure 6. (a) FWHM and peak 2θ angle of (002) reflection. 179x118mm (96 x 96 DPI)


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Figure 6. (b) FWHM and peak 2θ angle of (100) reflection. 179x118mm (96 x 96 DPI)



Figure 6. (c) (002) interlayer spacing distance and (100) layer coherence length of biochar produced above 500℃. (002) interlayer spacing distances of N350 and N400 are 0.48 and 0.40 nm each. (100) layer coherence lengths of N350 and N400 are 70.2 and 10.0 nm each. 255x151mm (96 x 96 DPI)


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Figure 6. (d) Visualization of crystalline graphitic structure. Numerical values for d002 and d100 spacing distance are given. Compared to graphite, biochar has wider (002) interlayer spacing and shorter (100) layer coherency length because ordered graphitic structure is not formed yet. 389x362mm (96 x 96 DPI)



Figure 7. HAADF images of biochar at 500 nm magnification. (a) N300 1040x1075mm (25 x 25 DPI)


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Figure 7. HAADF images of biochar at 500 nm magnification. (b) N400 270x280mm (96 x 96 DPI)



Figure 7. HAADF images of biochar at 500 nm magnification. (c) N500 1040x1075mm (25 x 25 DPI)


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Figure 7. HAADF images of biochar at 500 nm magnification. (d) N700 270x280mm (96 x 96 DPI)



Figure 7. HAADF images of biochar at 500 nm magnification. (e) N800 1040x1075mm (25 x 25 DPI)


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Figure 7. HAADF images of biochar at 500 nm magnification. (f) N900 1040x1075mm (25 x 25 DPI)



Figure 8. Schematic of the chemical and morphological changes that occur during the carbonization of biomass at different temperatures. 309x190mm (150 x 150 DPI)


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Structural Characterization of Loblolly Pine derived Biochar by X-ray

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