Self-Size-Limiting Nanoscale Perforation of Graphene for Dense

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Self-size-limiting nanoscale perforation of graphene for dense heteroatom doping Uday Narayan Maiti, Ranjit Thapa, Joonwon Lim, Dong Jun Li, Kwang Ho Kim, and Sang Ouk Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08391 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 3, 2015

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ACS Applied Materials & Interfaces

Self-size-limiting nanoscale perforation of graphene for dense heteroatom doping

Uday Narayan Maiti, §,†* Ranjit Thapa,‡ Joonwon Lim,§ Dong Jun Li,§ Kwang Ho Kim¥ and Sang Ouk Kim§*

§

Department of Materials Science & Engineering, KAIST, Daejeon 305-701, Republic of Korea

†

Department of Physics, Indian Institute of Technology Guwahati (IITG), Guwahati-781039,

Assam, India. ‡

SRM Research Institute, SRM University, Kattankulathur 603 203, Tamil Nadu, India

¥

Department of Materials Science & Engineering, Pusan National University, Pusan 609-735,

Republic of Korea. KEYWORDS: Graphene, Perforation, Doping, Oxygen Reduction Reaction, Density Functional Theory.

ACS Paragon Plus Environment

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ABSTRACT: Scalable and controllable nanoscale perforation method for graphene is developed based on the two-step thermal activation of graphene aerogel. Different resistance to the thermal oxidation between graphitic and defective domains in the weakly reduced graphene oxide is exploited for the self-limiting nanoscale perforation in graphene basal plane via selective thermal degradation of the defective domains. The resultant nanoporous graphene with narrow pore size distribution addresses the longstanding challenge for the high level doping of graphene with lattice mismatched large-size heteroatoms (S & P). Noticeably, this novel heteroatom doping strategy is demonstrated to be highly effective for oxygen reduction reaction (ORR) catalysis. Not only the higher level of heteroatom doping but also favorable spin and charge redistribution around the pore edges leads to a strong ORR activity as supported by density functional theory calculation.


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INTRODUCTION: Nanoscale perforation is an emerging strategy to widen the application spectrum of ultrathin layered materials beyond their pristine forms.1-6 Not only the geometrical advantage of perforated structures but also the unusual properties of edge atoms around the pores may offer novel physical, chemical or electrochemical properties in the generic two-dimensional materials.7-10 As a typical example, customized perforation in the basal plane of graphene holds great promise for filtration membrane,11,12,13 single molecule DNA sequencing,14 gas storage,6 electrochemical energy storage and conversion,2,15-17 and so on, taking advantage of the selective molecular/ionic transportation through the porous structures and the effective modification of local electronic structures at pore edges. Nonetheless, how to establish robust principle for the scalable and controllable nanoscale perforation still remains formidable challenge.1-3,18-22

We report a reliable strategy to produce large-scale nanoporous graphene in a highly controllable manner. Chemically modified graphene, producible by the reduction of graphene oxide, is composed of nanoscale amorphous or disordered defective domains surrounded by highly crystalline graphitic domains.23 Different thermal resistance of the two different domains is exploited for the self-size-limiting, highly-specific thermal degradation of the defective domains by two-step open air heat treatment. The resultant nanoporous graphene with uniform pore size and distribution can serve for a unique platform for mass-scale applications. Significantly, the abundant pore edge sites in nanoporous graphene could be utilized for effective high level doping of lattice-mismatched oversize atoms, such as sulfur (S) and phosphorous (P). We note that in contrast to the many successful reports on the substitutional doping of graphene with lattice matching nitrogen (N)24-27 and boron (B),28,29 doping of oversize heteroatoms such as S and P have remained challenging due to the significant distortion in graphitic basal plane and,


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thus, commonly resulted in a limited level of doping principally at contour edges and defects.3040

.

This technological challenge could also be understood from the much higher formation

energies of S (3.806 eV) and P (2.813 eV) doping in graphene basal plane as compared to N (0.973 eV) or B (1.265 eV) as predicted by Gholizadeh et al.41 considering 4 × 4 × 1 supercell as model system. In this work, the synergistic interplay between heavily doped heteroatoms and carbon atoms at the pore edges gives rise to a strong oxygen reduction reaction (ORR) catalytic activity, the mechanism of which is supported by DFT calculation in detail.

RESULTS AND DISCUSSIONS: Our procedure for graphene perforation is illustrated in Figures 1a-c. [see experimental details]. The starting material, graphene oxide (GO) has a heterogeneous chemical structure in the basal plane that composed of graphitic crystalline patches surrounded by the percolating disordered areas with oxygen functional groups [Figure 1a].23 A low temperature (typically 250 °C) thermal reduction of a freeze dried GO aerogel can strip off a significant portion of the surface functional groups (G)42 and increases the fraction of graphitic crystalline domains [Figure 1b]. As graphitic domains are much more resistant against oxidation than disordered regions, controlled heating to a high temperature (440-470 °C) under open-air condition can create uniform nanopores in the graphene basal plane (pG) by selective oxidative degradation of the disordered area [Figure 1c]. The macroporous aerogel structure provides the pathway for fluent gas transport during the oxidation and ensures the uniformity of selective etching throughout the bulk volume.


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Comparison of C1s XPS spectra of ‘GO’ and ‘G’ produced at 250 °C (annealing time: 20 min.) [Figure S1a] confirms the significant reduction of oxygen functional groups by thermal treatment. Thermal restoration of graphitic structure is obvious from blue shift of ‘Raman G’ peak (from 1596.3 to 1578 cm-1) [Figure S1b]. Considerable increase of intensity ratio of ‘Raman D’ (1352 cm-1) and ‘Raman G’ peaks (Id/Ig) from 0.93 to 1.38 results from the increase of boundary between graphitic domains and disordered domains by thermal reduction.43 Low magnification SEM image [Figure 1d] of typical pG aerogels (produced at 440 °C) displays macroporous structure, well-maintained after the perforation process.

Figures ‘1e, 1g-h and 1i’ present the high resolution SEM, TEM images and angular dark field scanning transmission electron microscopy (ADF-STEM) of perforated graphene aerogel, respectively. While dense, uniform nanopores are observed throughout the sample, ~86% of the pores lies within the narrow size window of 6 nm between 9 to 15 nm [Figure 1f]. ADF-STEM images in different magnifications further illustrate the size distribution of pores [Figure S2]. Time dependent structural and morphological change may offer an insight into the perforation mechanism. High resolution SEM, HRTEM and ADF-STEM images of ‘G’ do not show any porous structure [Figures S3a-3c]. After 25 min. annealing at 440 °C, pores with 2-5 nm size are formed [Figures S3d-3f] and gradually grow to 12 nm size after 45 min. [Figures S3g-S3i]. Pristine graphene is known to possess a high oxidation resistance (>500 °C)44 in air. Thus, annealing at 440°C maintains the pristine graphitic domain of ‘G’ intact, whereas disordered/functioned domains are degraded by thermal oxidation. Owing to the fairly uniform size of disordered domains in nanometer to sub-nanometer scale,23,45 this perforation mechanism


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intrinsically limits the pore dimension within nanoscale. Intensity of Raman ‘D’ peak (1352 cm1

) is lower in perforated graphene (pG) as compared to before perforation ( G) [Figure S4] which

signifies defect decreases due to gasification of disordered area.

BET analysis reveals that ‘G’ possesses a high surface area of 690 m2/g, and average pore size of 15 nm [Figure S5]. This nanoscale porosity principally originates from the intersheet expansion during the low temperature heat treatment. ‘pG’ obtained by 25 min. annealing exhibits a slight reduction of surface area down to 570 m2/g. Although the thermal annealing does not significantly collapse the aerogel structure of ‘G’ [Figure S6], decrease in surface area related with considerable restacking of graphene sheets, as evident from the decrease in pore volume and average pore diameter [Table S1]. Despite enhanced sheet restacking with higher annealing time, 45 min. annealed sample shows the enlargement of surface area up to 605 m2/g, which is directly associated to the perforation of graphene basal plane. Nanoscale perforation in graphene plane accompanies a considerable amount of nonplanar 7-8 membered rings of sp2 hybrid carbons with a low atomic density such that specific surface area can be effectively enhanced.2 The important of aerogel structure for uniform pore formation is obvious from the

SEM

observation of sample obtained by starting with dried GO bulk sample[Figure S7a-b]. Uniformly perforated graphene is observed in the outermost surface of the sample [Figure S7a] whereas careful observation of samples from inner part shows very less perforation and in addition pores are sparely separated [Figure S7b]. Furthermore, perforated sample from bulk GO possesses much lower BET surface area [298 m2/g] [Figure S8] than perforated from aerogel sample [605 m2/g].


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In our approach, two step heating process starting from ‘GO’ is critical for the pore size uniformity, as the first heating step determines the size distribution of disordered domains. Direct one step heating of ‘GO’ to 440 °C leads to inhomogeneous large pores [Figure S9a] due to the uncontrolled degradation of disordered domains. Oxidation temperature also has a substantial effect on the pore formation. Temperature below 400 °C is unable to drill the basal plane [Figure S9b], whereas 470 °C leads to a rapid etching [Figure S9d] without structural uniformity. Furthermore, 470 °C results in significant sample loss (~60% by weight) by rapid burning.

Our scalable graphene perforation method is exploited to address the fundamental challenges associated to the universal heteroatom doping of graphene. N, S and P are selected as heteroatom dopants in view of their wide variety in atomic radii and electronegativities. These particular choices are also helpful to accommodate different chemical activities. Doping was performed by the high temperature annealing of pG and G samples with N, S and P precursors [see experimental details in Supporting Information]. The resultant doped samples are assigned as NG, S-G, P-G (non-porous) and N-pG, S-pG, P-pG (porous, made with 45 min. annealing at 440 °C). XPS analysis was carried out to characterize the doping level and the bonding configuration. XPS survey scans [Figure 2a] demonstrate much higher doping levels in pG, irrespective of dopant type (Table 1). Notably, the relatively low doping level of P is presumably due to the use of solid dopant source (triphenylphonsphene) in contrast to the gaseous precursors for S and Ndoping (hydrogen sulfide and ammonia, respectively).


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Not only doping level but dopant configuration also significantly differs depending on porosity. Figure 2b shows the deconvoluted N1s spectra of N-G and N-pG and component peaks are associated with three possible configurations of N-dopants; pyridinic (397.9 eV), pyrolic (399.2 eV), graphitic/quaternary (400.6 eV).30 Quarternary N occupies a regular lattice site of graphene coordinated with three nearby carbon atoms [Figure S10]. By contrast, N at the edge and defect sites coordinated with two carbon atoms is termed as pyridinic or pyrolic depending on the involved hexagon or pentagon ring structure [Figure S10]. More than threefold increase of the pyridinic N-doping level in ‘pG’ as compared to ‘G’ is associated with the availability of numerous edge sites around the pores. Deconvoluted high resolution XPS spectra for ‘S2p’[Figure 2c] of S-doped samples signifies two distinct dopant forms of thiophene like sulfide group (-C-S-C-, major peak at 163 eV) and oxidized sulfur groups (-C-SOx-C-, x=2-4, broad peak from 165.5-170 eV) such as sulfate or sulfonate.33,34 Furthermore, asymmetry of the major peak (-C-S-C-) is associated with spin orbit splitting of ‘S2p’ having components S2p3/2 (163 eV) and S2p1/2(164.3 eV).33 P-doping also takes the configurations analogous to S-doping with C-P-C-(130.1eV) and -C-POx-C-(132.2 eV) coordinations, as reveled by deconvoluted P2p XPS spectra [Figure 2d].35,36 Considering the large size of S and P atoms to fit in the regular sp2 carbon basal plane, the aforementioned S/P-doping should principally occur at the (pore) edges (Schematic in Figure S10). XPS analysis verifies that pG is a suitable platform for universal heteroatom doping beyond the intrinsic limit of lattice mismatch.


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One of the promising applications of heavily doped pG is the electrocatalytic activity towards oxygen reduction reaction (ORR) for fuel cell or metal-air battery applications. Cyclic voltamograms of the doped graphene samples in O2 saturated 0.1 M KOH solution are presented in Figures 3a-c. All samples illustrate pronounced ORR peaks in -0.2 to -0.4 V region. A careful observation discloses larger peak currents in the low peak potential region for porous samples, which verifies higher catalytic activities of the porous structures. For further understanding of ORR activity, linear sweep voltammetry (LSV) was performed at different disc rotation speeds [Figures S11 and S12]. Figure 3d compares LSV curves at a fixed rotation rate (1500 rpm) for porous and non-porous samples. S-pG exhibits the lowest onset potential (0.07 V) [Figure S12b], which is followed by the order N-pG (0.9 V), N-G & P-pG (0.10 V), S-G (0.11 V) and P-G (0.14 V). Notably, the onset potential and limiting current value (4.61 mA/cm2) of S-pG are comparable to those of commercially available ‘Pt/C’ (0.02 V & 4.59 mA/cm2).

Electron transfer number and kinetic current density (Jk) were determined using KouteckyLevich equation (S.I.), which is a direct probe of ORR kinetics as given by 1 1 1 = + / where B = 0.62nFC0(D0)3/2v-1/6, J is the experimentally measured current density, is the electrode rotation speed (rpm), n is the electron transfer number, F is the Faraday constant (96485 C/mol), C0 is the bulk concentration of dissolve oxygen (1.2 × 10 mol/L), D0 is the diffusion coefficient of O2 (D = 1.9 × 10 / ) and v is the kinetic viscosity of the electrolyte (0.1 m2/s). ORR can occur either via 2e- or 4e- transfer process, ‘Jk’ is the kinetic current density


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which is a measure of catalytic efficiency. Number of transferred electron, as obtained from the slope of K-L (Figure 3e) together with kinetic current density, is compared in Figure 3f. Among all samples, S-pG shows the highest electron transfer number (3.78) and kinetic current density (14.06 mA/cm2), which is comparable to those of commercial ‘Pt/C’ (14.43 mA/cm2) in the same experimental condition. Irrespective of dopants, Figure 3 illustrates that the higher the doping level, the higher the catalytic activity. The catalytic activity also depends on the dopant configurations. For N-dopant, pyridinic N at edges is believed to induce 4e- electron pathways due to beneficial oxygen adsorption; a key step in ORR process.46-49

Observed excellent catalytic activity with different dopant species could further be understood by DFT calculations. It is well-recognized that heteroatom dopants at graphene with high charge or spin density may act as active sites for ORR.48,50 N with a stronger electronegativity can induce local positive charge at the neighboring C atoms, which lead to catalytic active sites.24,46,48,51 For S or P-dopant with an electronegativity similar to C, spin density rather than charge density play the principal role in the activation of catalytic sites. In our calculation (see Supporting Information for details), edge doped thiophene like (2S-graphene) or sulfur oxospecies (SO2Graphene) are found to induce a high spin density in the neighboring C atoms [Figures S13a & 13c]. P-doping also redistributes spin [Figures S13b & 13d] and thereby provides the active sites for ORR. We focused on S-doping in more detail, which demonstrates the most promising ORR activity in the experiments.


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Free energy profiles for the ORR at the edges of 2S-graphene and SO2-graphene are shown in Figures 4(a) and 4(e), respectively. The calculations were performed assuming alkaline conditions, i.e., at pH=14. The equilibrium potential, denoted as U0, is hence 0.40 V vs SHE. The free energy change can be defined as ∆G = ∆E − T ∆S − neU , where ∆E represents the change in enthalpy (binding energy) and ∆S is the change in entropy. More details regarding effect of PH in free energy profile is given supporting information. The optimized geometries of intermediate reaction steps are presented in Figures 4(b)-(d) (2S-graphene) and Figures 4(f)-(h) (SO2-graphene); each inset represents the corresponding step number. At zero cell potential (U = 0V) the first electron transfer leads to endothermic energy step, while all other electron transfer steps followed exothermic steps in both doping configurations (Figures 4(a) & 4(b)). O2 is adsorbed with end-on fashion on the C atoms neighboring S-dopants (for 2S-graphene) and two carbon atoms (for SO2-graphene), as shown in the relaxed structures (Figures S14a & S14c).

In the case of 2S-graphene, reaction step 1 results in OOH* structure (Figure 4(b)) by adsorbing one hydrogen from water and thereby produces one OH-. The energy barrier for this first hydrogen atom adsorption is ~0.4 eV. The O-O bond is slightly elongated to 1.489 Å (20% elongation from a free O2 molecule in triplet state). Desorption of the OOH into vacuum is unflavored, since the C-O bond breaking energy is ~1.53 eV. For the following stages of reactions, we have considered both the possibilities of well-known 2e- process (generates H2O2) and 4e- process (benchmark ORR pathway). First, we have investigated the possible formation of H2O2 from the adsorption of a second H onto OOH*. Various pathways were considered but H2O2 formation is found to be generally unflavored. Instead, the reaction goes with transfer of the second electron leading to formation of OH- and O*intermediates (Figure 4(c)). Subsequent


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attachment of the second hydrogen atom (from H2O(l)) forms OH* intermediate state (Figure 4(d)), along with one more OH-. Finally, the fourth electron transfer transforms OH* into OH- ion (totally four OH-). The operating potential for 2S-graphene surface is found to be about -0.4 V (0.43 V) with respect to SHE (RHE).

In the case of SO2-Graphene, the first hydrogen adsorption with a very low barrier energy (0.085 eV) leads to the structure as shown in Figure 4(f) (denoted by OHO*). The H atom is strongly bound to the O atom (bond length: ~1.01 Å), and the O-O bond is broken. Formation of the intermediate OOH is intrinsically limited due to the breakage of the O-O bond upon the introduction of the first hydrogen. Subsequent adsorption of H does not lead to the development of H2O2. Therefore, the graphene edge doped with SO2, leads to proficient direct 4e- process via subsequent steps with intermediate configurations, as presented in Figures 4(g)-4(f).52 The operating potential for 2SO2-graphene surface is found to be about -0.085V (0.745V) with respect to SHE (RHE). This calculation indicates that incorporation of S in the form of SO2 at the edge of graphene suppresses H2O2 formation and also increases the operating potential (decreases the overpotential). The optimized structures of O2 adsorption on 2P-graphene (Figure S14b), PO2-graphene (Figure S14d), and 3N-graphene sheet (Figure S15) reveal similar behaviors that also suggest 4e- processes. Overall, our calculation reveals that not only doping level but also dopant configuration is highly significant for an efficient 4e- process. In this regard, perforated graphene offers abundant pore edge doping sites for high catalytic activity towards ORR.


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CONCLUSIONS: A scalable robust approach is demonstrated for the controlled nanoscale perforation of graphene basal plane. This novel protocol based on the self-limiting thermal oxidation of defective domains in graphene basal plane ensures fairly narrow pore size distribution around 10 nm as well as arbitrary scale-up to a mass-production. The resultant nanopores resolve longstanding challenge for the effective high level doping of graphene with lattice mismatched large heteroatoms. Irrespective of heteroatom size, the doping level of porous graphene increases by more than two fold from its pristine form. A heavy doping is possible at the numerous pore edge sites, where oversize heteroatom can reside without significant lattice strain in the neighboring graphene plane. Our experiments and DFT simulation further present that the edge dominant doping around the pores offers highly active catalytic sites for ORR. The collective figure of merits of our approach, including scalability, simplicity and cost effectiveness for porous graphene are promising for versatile applications beyond ORR catalysis, such as supercapacitor, lithium ion battery, DSSC, composites and so on.

METHODS Synthesis of nanoporous graphene: Graphene oxide (GO) was synthesized following modified Hummer’s method. GO aerogel was prepared by the freeze drying of 1 mg/ml GO aqueous solution. The GO aerogel was subjected to two stages thermal annealing in open air to get porous graphene network in the form of aerogel. In the first stage, GO aerogel (40 mg) within a alumina crucible was inserted within a preheated muffle furnace at 250 °C and kept for 20 min. The second annealing step is similar to the first one with the increased temperature around 440 °C. The annealing time was varied from 0-45 min.


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Universal doping of heteroatoms: Heteroatom doping was done by annealing G/pG with an appropriate dopant precursor in a tube furnace at high temperature optimized for different dopant type (750 °C for N/S and 850 °C for P). Before heating the furnace was evacuated upto pressure level down to 1 × 10-3 mbar. For three different choices of dopant precursors optimized doping condition is given below. N/S doping: N-doping was performed at 750 °C under the flow of NH3 (60 SCCM) and H2 (20 SCCM) mixture gas. Temperature rise to 750 °C took about 1h where the sample was kept at 750 °C for 2 h and air-cooled. For S-doping NH3 is replaced by H2S (60 SCCM) as s sulfur source. P-doping: Triphenyl phosphine (TPP) is selected as the precursor for P-doping. 60 mg G/pG was stirred for 2 h in the 40 mg solution of TPP in ethanol. Ethanol was evaporated at 50 °C to collect the dry mixture of TPP and graphene. Finally the mixture was annealed at 850 0C for 2 h under the inert atmosphere of Argon (50 SCCM) and air-cooled.

Characterizations: Morphology of the samples was characterized by using field emission scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (FEI Tecnai F20). ADF-STEM characterization was done with double Cs corrected TEM (FEI, Titan cubed G2 60-300). X-ray photoemission spectroscopy was conducted using Sigma Probe with monochromatic X-ray source(Al Kα) (Thermo VG Scientific, Inc.). Raman spectroscopic measurement was performed with ARAMIS, Horiba Jobin Yvon, France using 514.5 nm laser source. Surface area was measured by Brunauer-Emmet-Teller (BET) N2 adsorption-desorption


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(TriStar II 3020 V1.03). The specific surface area, pore volume and the pore size of the samples were determined by Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) models applied to the adsorption branch of the isotherm, respectively.

Electrode preparation of electrochemical measurements: Rotary disc electrode (RDE) was polished using aluminium oxide (Al2O3, 0.3 µm) powder followed by though rinsing with D.I water. After successive sonication in D.I water and ethanol, the electrodes were again rinsed with D.I water and dried at room temperature. 1 mg of doped nanoporous graphene were dispersed in 0.5 ml solvent mixture of Nafion (5 wt%) and ethanol (V:V ratio =1:4) for 1h sonication. 10 µl of each suspension was dropped on the disc surface of RDE. For comparison, commercially available catalyst of 20 wt% Pt/C powder (E-TEK) was used as a reference and ink is prepared through the same procedure of similar loading.

Electrochemical measurements: Cyclic voltammetry and linear sweep voltammetry were performed by computer controlled electrochemical analysis instrument (VersaSTAT3 electrochemical workstation, Princeton Applied Research, USA) with a three electrode cell system. A glassy carbon RDE after loading the electocatalyst was used as the working electrode, an Ag/AgCl (KCl 3M) electrode as a reference and a Pt wire as the counter electrode. The electrochemical experiments were conducted in O2 saturated 0.1M KOH electrolyte for the oxygen reduction reaction. Cyclic voltammetry was performed within voltage window of 0.2 to 0.8V (vs Ag/AgCl) at a scan rate 100 mV/s. Linear sweep voltammetry was performed in the voltage range from 0 to -0.8V (vs Ag/AgCl) at various disc rotation rate from 500 to 2500 rpm.


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FIGURES

Figure 1. (a-c) Schematic for two step graphene perforation mechanism. (a) GO composed of graphitic crystalline and disordered areas. (b) rGO having reduced disordered domains. (c) pG. (d-e) SEM images of pG; (f) Size distribution of pores (size1nm). (g-h) TEM images of pG. (i) ADF-STEM image of pG.


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Figure 2. XPS analysis. (a) Survey scan. (b) Deconvoluted N1s spectra for N-G (lower) and NpG (upper). (c) Deconvoluted S2p XPS spectra for S-G (lower) and S-pG (upper). (d) Deconvoluted P2p XPS spectra for P-G (lower) and P-pG (upper).


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Figure 3. CV scans in O2 saturated 0.1M KOH solution for (a) N-doped (b) S-doped (c) P-doped samples, respectively. (d) Comparison of linear sweep voltammetry curves at electrode rotation speed of 1500 rpm. (e) K-L plot for different doped samples. (f) Comparison of kinetic current density and electron transfer number of N, S or P-doped pG (blue) and G (yellow) with commercial Pt/C catalyst.


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Figure 4. DFT calculation for S-doping: (a) & (e) calculated ‘Free energy’ profiles of intermediate reaction steps for the 4e-ORR process at 2S and 2SO2 edge doped graphene, respectively, in the alkaline medium. (b) OOH*, (c) O* and (d) OH* represent the optimized structures of the intermediate steps 1, 2 and 3 of figure 4(a), respectively. (f) OHO* (g) O* and (h) OH* represent the optimized structures of the intermediate steps 1, 2 and 3 of Figure 4(e), respectively. Star sign (*) indicates the adsorbed state of the molecules on the respective surface. The red, yellow, green and grey balls represent O, S, H and C atoms, respectively. The dotted lines are only guide to the eye.


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Table 1: Heteroatom doping levels for porous (pG) and non-porous graphene (G) Sample

N%

S%

P%

G

4.76

3.2

1.4

pG

11.56

7.2

3.2

Supporting Information: Numerical calculation details; Comparative XPS and Raman spectra of G and GO [Figure S1]; Additional ADF-STEM image of pG [Figure S2]; SEM, TEM, ADFSTEM images of time evolution of pore [Figure S3]; Comparative Raman spectrum of G and pG [Figure S4]; BET analysis for time evolution of pore [Figure S5 & Table S1]; SEM of pG aerogel structure [Figure S6]; Pore formation and BET for sample obtained with dried GO bulk instead of aerogel [Figure S7 & Figure S8]; Effect of temperature on pore formation, SEM images [Figure S9]; Schematic of heteroatom configurations [Figure S10]; Linear sweep voltammetry for doped sample and Pt/C [Figure S11 & Figure S12]; Spin density plot of doped sample [Figure S13]; Optimized structure of O2 adsorbed over doped samples [Figure S14 & Figure S15]. “This material is available free of charge via the Internet at http://pubs.acs.org.”


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AUTHOR INFORMATION Corresponding Authors *[email protected] [Sang Ouk Kim] *[email protected] [Uday Narayan Maiti]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS This research was supported by the Multi-Dimentional Directed Nanoscale Assembly Research Center (Creative Research Initiative), and the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014M3A6B1075032), and the Asian Office of Aerospace Research and Development (AOARD FA2384-14-1-4013).


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Table of Content: A straightforward strategy is developed for the dense size-limiting nanoscale perforation of graphene basal plane via two stage thermal activation of graphene oxide aerogel. The resultant uniform size nanopores at graphene basal plane resolve the longstanding challenge for effective high content doping of graphene with lattice mismatched large heteroatoms, such as S and P. Heteroatom dopants around the pore edges highly activate the neighboring carbon atoms for high performance oxygen reduction reaction.

TOC figure


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Self-Size-Limiting Nanoscale Perforation of Graphene for Dense

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