Identify the Removable Substructure in Carbon Activation

Article pubs.acs.org/cm

Identify the Removable Substructure in Carbon Activation Zhenyu Xing,† Yitong Qi,† Ziqi Tian,‡ Jing Xu,§ Yifei Yuan,⊥ Clement Bommier,† Jun Lu,*,⊥ Wei Tong,*,§ De-en Jiang,*,‡ and Xiulei Ji*,† †

Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States Department of Chemistry, University of California, Riverside, California 92521, United States § Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Activated carbon plays a pivotal role in achieving critical functions, such as separation, catalysis, and energy storage. A remaining question of carbon activation is which substructures in amorphous carbon are preferentially removed during activation. Herein, we report the first structure−activation correlation elucidated on the basis of unprecedented comprehensive characterization on carbon activation. We discover that activation under CO2 preferentially removes graphenic layers that are more defective. Therefore, the resulting activated carbon contains thinned turbostratic nanodomains that are of a higher local graphenic order. The mechanistic insights explain why more defective soft carbon is “burned” under CO2 at a much faster rate than hard carbon. The mechanism leads to an activation-based design principle of mesoporous carbon. Guided by this principle, a bimodal micromesoporous carbon is prepared simply by CO2 activation. Our findings may cause a paradigm shift for the rational design of nanoporous carbon.

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INTRODUCTION Annually, over one million tons of activated carbon is manufactured world widely. Activated carbon is an indispensable material to facilitate a plethora of advanced functions, such as sorption, separation, catalysis, and energy storage.1−13 Despite large-scale production, its preparation still follows a trial-and-error approach.14−20 One strategy to form activated carbon of high purity is to react pyrolytic carbon with gaseous etchants, such as CO2 and H2O.21−24 The nature of such activation is the preferential gasification of some carbon substructures, leaving other substructures intact, which carves nanoporosity in the carbon framework. Unfortunately, the removable substructures remain unidentified, which has prevented carbon scientists from employing activationthe most scalable and cost-effective methodto design nanoporous carbons.25−31 In this work, we uncover the structure−activation correlation of carbon by monitoring the structural variation upon activation. We focus on activation by CO2, where only one byproduct, gaseous CO, forms in the Boudouard reaction

HC and SC samples in this study are very representative, prepared by pyrolyzing sucrose and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively.32,33

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Δ

C(s) + CO2 (g) → 2CO(g)

In this study, we attempted to activate three major types of low-surface-area bulk carbon: graphite, soft carbon (SC, graphitizable carbon), and hard carbon (HC, nongraphitizable carbon), where the latter two are known as amorphous carbon. © 2017 American Chemical Society

EXPERIMENTAL SECTION

Preparation of Carbon Materials. Graphite was purchased from Sigma-Aldrich (282863−25G). HC was obtained by dehydrating sucrose at 180 °C in air for 24 h, then pyrolyzing at 1100 °C under Ar flow for 6 h with a heating rate of 5 °C/min. SC was prepared by pyrolyzing PTCDA at 900 °C under Ar flow for 5 h, with a heating rate of 5 °C/min. We selected the preparation temperatures of 900 and 1100 °C for soft carbon and hard carbon, respectively, because under such conditions, the resulting structures are very typical for soft carbon and hard carbon, respectively; their corresponding physical properties and electrochemical properties were wellinvestigated by our group.32,33 Indeed, this work is a part of our systematic study to understand the overall reactivity of amorphous carbon materials. For the synthesis of HC-SC composite carbon, sucrose and PTCDA, with a mass ratio of 5/2, were first dissolved in ammonium hydroxide aqueous solution (28% − 30% NH3). The sucrose-PTCDA solution was dried at 90 °C, and dehydrated at 180 °C in air for 24 h before pyrolysis at 1100 °C under Ar flow for 6 h with a heating rate of 5 °C/min. Received: May 11, 2017 Revised: July 24, 2017 Published: July 28, 2017 7288

DOI: 10.1021/acs.chemmater.7b01937 Chem. Mater. 2017, 29, 7288−7295

Article

Chemistry of Materials For CO2 activation, the pyrolysis products and graphite were all subjected to heat treatment at 900 °C under CO2 flow at 60 CCM for different durations. Theoretical Calculation. ReaxFF molecular dynamics simulations were carried out with REAX module in Lammps software package. Parameters for carbon and oxygen were taken from previous work. Four layered graphene models were placed parallel to the XY plane in a box with dimensions of 22.14 Å × 25.56 Å × 75.00 Å. Fifty CO2 molecules were filled in the box. NVT simulations were performed at 2000 K for 1 ns with a time step of 0.25 fs and a temperature damping constant of 100 fs. Characterization Methods. X-ray diffraction (XRD) patterns were recorded by a Rigaku Ultima IV Diffractometer with Cu Kα irradiation (λ= 1.5406 Å). Raman spectra were obtained from WITec confocal Raman spectrometer with a 514 nm laser source. Transmission electron microscopy (TEM) images were recorded by FEI Titan 80−200 TEM. Nitrogen sorption measurements were performed on a Micromeritics TriStar II 3020 analyzer. Neutron total scattering data were collected at the Nanoscale Ordered Materials Diffractometer (NOMAD), Spallation Neutron Source, at Oak Ridge National Laboratory. Samples were loaded into quartz capillaries for analysis. C K-edge X-ray absorption near edge spectra (XANES) were collected on powder samples using beamline 8-2 at the Stanford Synchrotron Radiation Laboratory. Data were acquired under ultrahigh vacuum (10−9 Torr) in a single load at room temperature using total electron yield (TEY) via the drain current.

vastly different behavior under activation has to be attributed to their internal structures. Figure 1 and Figure S2 reveal the carbon surface area and the corresponding burnoff percentage as a function of activation duration for different precursor carbons. The results suggest that only HC can be activated under CO2 to create a large surface area, whereas SC and graphite fail to generate much new surface areas. At burnoff of 35%, the Brunauer− Emmett−Teller (BET) surface area of graphite rises from 1 to only 15 m2/g after 15 h’ activation. Interestingly, SC “burns” very fast under CO2; however, it does not lead to much surface area increase. At burnoff of 87%, the BET surface area of SC is raised from 12 to 166 m2/g. In sharp contrast, burnoff of 72% for hard carbon leads to a considerable surface area of 2530 m2/g. We also measured the evolving conductivity upon activation. We notice that the electrical conductivity of all carbons decreases with CO2 activation (Figure S3). HC exhibited more extent of activation than graphite and SC, which is associated with its contained structural heterogeneity. Pure graphite would contain only sp2-bonded carbon atoms in graphene layers that stack in an ABAB sequence. Both SC and HC are nongraphitic and do not possess a long-range order along the c axis; nevertheless, both carbons exhibit distinctly different structures at the nanometric scale. As shown in the transmission electron microscopy (TEM) images (Figure 2A, B), SC exhibits a typical turbostratic structure, in which graphenic layers are much better aligned than those in HC. In contrast, HC structure can be best described by the combination of the house-of-cards model and the fullerenefragment model, where the graphenic layers are shorter and much more curved, thus enclosing a high level of nanoporosity (Figure 2B).34,35 The structural heterogeneity of HC is embedded in this complex structure. Because activation is preferential gasification, which requires the presence of structural heterogeneity, the contrast between soft carbon and hard carbon is

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RESULTS AND DISCUSSION The surface area of precursor carbon materials is summarized in Table S1, which are 1.2, 12.3, and 18.4 m2/g for graphite, soft carbon, and hard carbon, respectively. The particle size distributions of these materials are shown in Figure S1. Their granularity was measured by taking the average size of 100 particles observed under SEM, which is listed in Table S2 as 22.6, 5.2, and 19.5 μm for graphite, soft carbon, and hard carbon, respectively. It is evident that these precursor carbons are quite similar in their external conditions, and their potentially

Figure 1. Specific surface area and burnoff percentage as a function of activation duration. (A) Graphite, (B) SC, and (C) HC. Surface area and burnoff increase upon longer activation; however, only HC is “activatable” by generating a large surface area. 7289

DOI: 10.1021/acs.chemmater.7b01937 Chem. Mater. 2017, 29, 7288−7295

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Chemistry of Materials

Figure 2. Representative HRTEM images and XRD patterns of SC, HC, and activated HC samples. (A) SC, (B) HC, (C) HC-10h, (D) HC-20h, (E) HC of high resolution, and (F) XRD patterns of HC and its derived activated carbons after different durations. The results show that activation thins turbostratic nanodomains.

the transition 1s→π* and the oxygen species in carbon, respectively.30 The broad band above 289.6 eV is assigned to sp3-bonded carbon with the transition 1s→σ*. The percentage of sp2-bonded carbon is calculated by the following equation:30

that such heterogeneity seem not exist to a large extent in soft carbon. For the PTCDA-derived soft carbon, its lack of structural heterogeneity may be due to the ordered structure of the precursor molecular solids and the high level of aromaticity of the PTCDA molecule. We set out to reveal the activatability of hard carbon from two aspects: the compositional change and the structural change during the course of activation. One could argue that there is a third aspect of activation, stemming from heteroatoms of hydrogen and oxygen as the residue from precursor pyrolysis. However, the vast disparity of the activation effect between SC and HC and their nearly identical compositions (C/O/H: 97/2.5/0.1 by CHO tests) suggest that the carbon bonding configurations and the resulting atomic/nanometric arrangements are the real activation differentia. In HC, the primary bonding configurations are sp2 and sp3. HC for its name’s sake is mechanically stronger than graphite and soft carbon because it contains a large percentage of sp3 carbon atoms. Carbon atoms with sp3 configuration are likely embedded in the structure as knots that cross-link the sharply curved graphenic layers, where such folding points of graphenic layers are marked by the red circles in a representative highresolution TEM (HRTEM) image (Figure 2E). The question is what the sp2/sp3 ratio is in HC and how this ratio evolves during activation. To quantify the change of sp2/sp3 ratio, we collected XANES of pristine HC and activated HC samples, as shown in Figure 3A. XANES results reflect the global change of the samples without being limited to just the particle surface. The two peaks at 283.5 and 286.4 eV are attributed to sp2-bonded carbon with

%sp2 HC ≈

(π */σ *)HC (π */σ *)HOPG

(1)

Here, π* and σ* are calculated by peak area integration in the binding energy ranges of 282−287 eV and 293−302 eV, respectively, and HOPG refers to highly oriented pyrolytic graphite. To reveal the trend without being affected by the choice of HOPG, we name (π*/σ*)HOPG as a constant s, where the sp2 content is 0.655/s for the pristine HC; it decreased to 0.371/s after the first 10 h’ activation (the sample referred to as HC-10h); it is stabilized at 0.408/s after another 10 h of activation (the sample referred to as HC-20h). It is evident that during the first 10 h of activation, sp2 carbon is preferentially removed. Note that sp2 carbon atoms are constituents of graphenic layers. Does this mean that graphenic layers are removed by initial activation? We also notice that the ratio of the peak area between carbon π* peak and oxygen peak increases upon activation. It may indicate that carbon sections containing oxygen heteroatoms may get oxidized preferentially, or the resulting activated carbon may possess lower capability of absorbing oxygen. To draw a conclusion related to oxygen heteroatoms will need further comprehensive investigation, and is thus beyond the scope of this study. To answer the above question, we employed X-ray diffraction (XRD) and TEM to study the stacking thickness of graphenic layers upon activation. As shown in Figure 2F, the XRD (002) 7290

DOI: 10.1021/acs.chemmater.7b01937 Chem. Mater. 2017, 29, 7288−7295

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Chemistry of Materials

both ratios decrease upon activation, as shown in Figure 4D, indicating that activation removes defects. Particularly, ID/IG ratio is quantitatively correlated to the coherence length of graphenic layers, La, according to the following equation:37 ⎛ ⎞ 4 IG La (nm) = (2.4 × 10−10)λnm ⎜ ⎟ ⎝ ID ⎠

(2)

where λ is the laser wavelength of 514 nm. Upon activation, ID/ IG decreased from 3.63 for the pristine HC to 2.74 and 2.59 for HC-10h and HC-20h, respectively; the corresponding average La values increase from 4.61 to 6.11 nm and 6.46 nm for HC-10h and HC-20h, respectively. Because activation most likely cannot enlarge graphenic layers, the increase of coherence length should arise from the removal of graphenic layers of smaller coherence lengths. Thus, activation selectively removes more defective graphenic domains. This is reflected by the evolution of the ID/IG ratio and the derived average coherence length of the graphenic layers during initial activation. However, after 15 h of activation, highly defective graphenic layers may be completely removed; the continuing activation would have to oxidize graphenic layers with fewer defects. Such further activation would inevitably decrease the average coherence length of graphenic layers, and the ID/IG starts to increase after 15 h of activation, thus indicating that La decreases, as shown in Figure 4D. To confirm activation gasifies more defective graphenic layers, we conducted neutron total scattering and the associated pair distribution function (PDF) studies. PDF studies have been employed to reveal the local structures of nanomaterials and amorphous carbon with the following equation:38−42

Figure 3. XANES spectra and neutron total scattering PDF results of (activated) HC and SC. (A) C K-edge XANES spectra of HC and its activated carbon after activation for 10 and 20 h. (B. Schematic of graphene local structure. (C) PDF profiles of HC and HC-20h. Inset: a magnified region within 5 Å. (D) Comparative PDF profiles of HC and SC. Inset: a magnified region within 5 Å.

G (r ) =

2 π

∫0

∞

Q [S(Q ) − 1]sin(Q r)dQ

(3)

where G(r) is calculated by the Fourier transform of the structure function S(Q), Q is the wavevector, and S(Q) contains both Bragg and diffuse scattering.43 In G(r) plots, the peak positions (r) are the real distances between an arbitrarily chosen central atom and its neighbors averaged over all atoms of the sample. The integrated peak area is proportional to the number of neighboring atoms found at the distance of r. As schematically shown in Figure 3B, the intra-C6-ring distances correspond to the first three peaks, where larger peak areas would indicate a higher degree of graphenic order. In Figure 3C, the PDF peaks’ amplitude rises remarkably from HC to HC-20h, which suggests that activation effectively removes defective sections in the local structure. The slope of the G(r) oscillation at low-r range near zero indicates the atomic density, where a higher slope means a greater atomic density. Thus, the PDF results reveal the expected trend that pristine HC exhibits a higher atomic density than HC-20h, which agrees well with the XRD and TEM results. To further reveal the defect-activation correlation, we conducted carbon activation simulation on two model structures: graphene basal plane (GBP) and graphene nanosheets (GN), where the latter has its edge sites exposed. The calculation is carried out by a reactive force-field molecular dynamics simulation based on the ReaxFF method. After heating up the GBP model under CO2 to 2000 K for 1 ns (ns), GBP remains intact (Figure 5A). In contrast, GN was thoroughly etched off by CO2 in 1 ns (Figure 5B) under the same conditions. Moreover, the burnoff residue atoms could further attack the underlying basal layer, thus leading to carbon defects that are able to react with

peak broadens after activation, which suggests fewer graphenic layers being stacked. TEM results confirm this trend, where 20 h of activation thinned turbostratic nanodomains in HC to single or bilayer graphenic sheets (Figure 2B−D). The XRD and TEM findings confirm that activation removes some graphenic layers. Furthermore, thinning of graphitic layers creates micropores, especially for hard carbon. As shown in Figure S2E, F, the amount of micropores of hard carbon corresponding to the adsorbed volume of N2 gas at very low relative pressure increases upon activation. The remaining question is what causes some graphenic layers to be preferentially removed. The answer to the above question may have to do with the local atomic structure inside the graphenic layers, which we used Raman spectroscopy, neutron total scattering, and the associated pair distribution function (PDF) to characterize. We deconvoluted Raman spectra into four Lorentzian bands: TPA, D, A, and G bands.36 The well-known D band around 1350 cm−1 and the G band around 1580 cm−1 are attributed to the A1g breathing-mode vibration of C6 rings activated by the existence of defects, and the E2g vibration of sp2-bonded carbon pairs, respectively (Figure 4A−C). The TPA band located to the left of the D band is attributed to the transpolyacetylenelike structure, while the A band between the D and G bands is assigned to point vacancy defects. Among these bands, the A and D bands represent defects along ab planes, whereas the G band indicates the graphenic order. Thus, the IA/IG and ID/IG intensity ratios indicate the extent of local disorder. Note that 7291

DOI: 10.1021/acs.chemmater.7b01937 Chem. Mater. 2017, 29, 7288−7295

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Chemistry of Materials

Figure 4. Raman spectra and their deconvolution into TPA (green), D (red), A (orange), and G (blue) bands, showing an increased range of order along ab planes. (A−C) Raman spectra of HC and its activated carbon after durations of 10 and 20 h, respectively. D. Variation in ID/IG and IA/IG values of HC and its activated carbon at different activation time.

Figure 5. Representative snapshots in ReaxFF molecular dynamics simulations of CO2 activation of carbon: (A) Two layers of graphene with indefinite size; all CO2 molecules are hidden because no reaction between CO2 and graphene took place. (B) One finite nanographene layer on the surface of one graphene layer with indefinite size and the reaction results after 15 ps and 1 ns.

CO2. This generates vacancies in the graphene basal plane; these vacancies could serve as seeds for further activation. At this point, collectively considering the results of XANES, XRD, TEM, Raman, PDF, and ReaxFF simulation, we postulate the following mechanism of HC activation under CO2: activation preferentially removes more defective graphenic layers, where the graphenic layers containing fewer defects remain as the walls of the resulting nanoporous carbon. The above mechanistic insights if correct should be able to explain the activation-behavior disparity between HC and SC. As aforementioned, SC losses 90% of its mass in just three hours of activation, whereas it takes more than 20 h to see just 80% burnoff in HC. According to the above mechanism, SC should contain graphenic layers more defective than those

Figure 6. (A) Representative HRTEM image of HC-SC. (B) Enlarged image marked in the yellow box of A. (C, D) TEM images of HC-SC-3h.

in hard carbon. To confirm this notion, we compare the PDF files of SC and HC, as shown in Figure 3D, in which, indeed, SC exhibits PDF peaks of lower amplitude than those of HC. The faster reaction rate of SC than HC should also be related to its less structural tortuosity than HC. The fact that SC is more oxidizable not only supports the activation mechanism of HC but presents SC as a good sacrificial “template” in carbon activation. Therefore, we propose a design principle of activated carbon: had one facilitate a homogeneous dispersion of SC nanodomains in a matrix of HC, the size of the SC 7292

DOI: 10.1021/acs.chemmater.7b01937 Chem. Mater. 2017, 29, 7288−7295

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Chemistry of Materials

Figure 7. Surface area and porosity properties of the activated HC-SC samples. (A) Specific surface area and burnoff as a function of different activation durations. (B) Nitrogen sorption isotherms of HC-SC and activated HC-SC samples. (C) PSD of HC-SC and activated HC-SC. (D) The surface area from “micropores” (pore size below 5 nm) and mesopores (pore size between 5 and 30 nm) as a function of the activation time for HC-SC.

HRTEM images reveal that the darker stripes of soft carbon are most likely removed after activation, where by comparing Figure 6A and C, it is evident the overall structure turns more porous. By taking a closer look at the structure after activation, there is much less presence of the soft carbon nanostripes.

nanodomains and its total loading will determine the pore size and pore volume of the resulting activated carbon. To test the above design principle, we prepared a fused HC and SC composite (HC-SC) as the precursor carbon for activation. To investigate the extent of mixing for SC and HC, we looked at HC-SC under HRTEM, where it is evident that darker stripes of SC nanodomains are well dispersed in the matrix of HC phase (Figure 6A). By zooming in, the darker stripes are, indeed, more graphitic domains with well aligned graphenic layers stacked, confirming its soft carbon nature (Figure 6B). We activated HC-SC at the same conditions, where the burnoff and the BET surface area increase linearly with the activation time, as shown in Figure 7A, which is very similar to the activation of HC. However, it only took 3 h to increase the surface area from 72 to 1672 m2/g for HC-SC (Figure 7B), whereas it took 15 h to reach the similar surface area for HC. The resulting activated HC-SC exhibits a bimodal porosity with micropores seen in typical activated carbon and new mesopores sized around 12 nm. The surface area and pore volume from both small nanopores (