Structural Engineering of 3D Carbon Materials from Transition Metal

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2018, 30, 3779−3788

Structural Engineering of 3D Carbon Materials from Transition Metal Ion-Exchanged Y Zeolite Templates Gun-hee Moon, Alexander Baḧ r, and Harun Tüysüz* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr D-45470, Germany

Downloaded via LA TROBE UNIV on July 4, 2018 at 18:12:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: A series of three-dimensional ordered microporous carbon materials (3D CMs) were prepared through a nanocasting route by using transition metal ion-exchanged Y zeolite (M-Y) as template and ethylene gas a carbon source. The different d-π coordination and the formation of metal nanoparticles during thermal treatment altered textural parameters of the final carbon products. After a detailed structural analysis and characterization, the most promising cobalt−carbon sample was further treated with NH3 for nitrogen doping and evaluated for oxygen reduction reaction (ORR). This new class of material indicated good electrochemical stability and similar activity in comparison with those of commercial Pt/C (20 wt %) electrocatalyst. The protocol developed here allows in situ incorporation of diverse transition metals as well as the doping of various heteroelements into a three-dimensional carbon framework and has great potential for different catalytic applications.

■

INTRODUCTION Nanostructured carbon materials including 0-dimensional (0D) fullerenes, 1D carbon nanotubes (CNT), 2D graphene, and 3D nanoarchitectures have received great attention. The unique physicochemical properties of these materials arise from the delocalization of π electrons, their electronic structure, surface energy, functionality, and distortion of carbon lattices.1 Graphitic carbon materials have been widely utilized for energy and environmental applications because of their high conductivities that can facilitate the transfer of electrons supplied from the conduction band of semiconductors or the cathodic bias to the desired electron acceptor.2−6 Although a great deal of effort has been made to precisely tailor the structure, the fabrication of 3D carbon materials with wellordered channels preserving a high conductivity still remains a big challenge.7,8 Very recently, Ryoo reported the preparation of microporous 3D carbon materials (3D CMs) with high conductivity from lanthanum-exchanged zeolite templates, demonstrating the role of lanthanum ions as a catalyst for the carbonization of ethylene to graphitic carbons inside the zeolite frameworks.9 Carbon-based composites, such as those containing (i) transition metal species,10−14 (ii) heteroelement-doping (doping or codoping for N, B, S, P, O, etc. elements),15−17 and (iii) both transition metal species and heteroelement-doped structured carbon materials18,19 show unusual catalytic nature from synergistic coupling effects and the formation of more active sites. The high specific surface area of carbon materials makes them good supports for the dispersion of nanoparticles, and the high conductivity enabled by the delocalized π electrons can facilitate charge transfer to enhance various © 2018 American Chemical Society

redox reactions. Furthermore, the doping of heteroatoms changes the energy bandgap, the spin density, and the charge density of surrounding carbon atoms,20 providing many advantages for energy conversion and storage applications such as fuel cells, batteries, photocatalysis, (photo)electrochemical cells, etc. For example, the overpotential for the oxygen reduction reaction (ORR) can be effectively reduced via not only the parallel adsorption of diatomic O2, induced by a charge redistribution arising from N-doping, but also the formation of Co−O−C and Co−N−C bonds between Co3O4 and N-doped graphene in the Co3O4/N-doped graphene hybrid systems.21 In this work, we fabricated microporous 3D CMs using Y zeolites exchanged with transition metal ions (Co2+, Mn2+, Cu2+, and Ni2+) as a hard template. The synthetic procedure can be summarized as follows: (i) the replacement of Na+ in Y zeolite templates with transition metal ions (M-Y, M: Co2+, Mn2+, Cu2+, and Ni2+), (ii) the carbonization and graphitization (M-C) of the ion-exchanged templates, (iii) the removal of the zeolite templates using NaOH, and (iv) the removal of transition metal species and trace amorphous silicates using HCl (M-C-A; noted as 3D CMs) (Scheme 1). We found that the catalytic conversion of ethylene to polymeric carbon bonds was enabled by the transition metal ions (not the zerovalent transition metals), while ethylene polymerization and graphitization proceeded inside zeolite frameworks through a d-π coordination, stabilizing the ethylene and pyrocondensation Received: February 28, 2018 Revised: May 24, 2018 Published: May 25, 2018 3779

DOI: 10.1021/acs.chemmater.8b00861 Chem. Mater. 2018, 30, 3779−3788

Article

Chemistry of Materials

Graphite oxide (GO) was synthesized using the modified Hummer method,24 and the oxygen-containing functional groups were eliminated by heat treatment at 1,000 °C under Ar atmosphere. CNTs were purchased from Hanwha chemical (Multiwalled carbon nanotube, CH-150), and iron impurities were removed using concentrated nitric acid. After base and acid treatment, nitrogen-doping was accomplished for Co-C-A using NH3 treatment at 850 °C for 1 h. To confirm the concentration effect of HCl on the ORR activities, the acid treatment was modified using HCl with different concentrations (0, 0.5, 1, and 2 M) for 4 h before NH3 treatment. The samples of Co-C-A after NH3 treatment were labeled as N-Co-C-A (0 to 2 M) depending on the concentration of HCl. The NH3 treatment was carried out at 1,000 °C for rGO and CNT to effectively introduce nitrogen into the carbon lattices (so-called, N-rGO and N-CNT). All samples were washed by distilled water several times to remove the physisorbed NH3. Electrochemical Activity Measurement. Electrochemical ORR performance was measured (SP-150 potentiostat, Biologic Science Instrument) using a three-electrode system: working electrode, a rotating disk electrode (AFMSRCE, PINE Research Instrumentation); reference electrode, a hydrogen electrode (HydroFlex, Gaskatel); and counter electrode, Pt wire, in 0.1 M KOH as an electrolyte. Oxygen gas was purged for 30 min to saturate the electrolyte before the measurement, and the temperature of the electrolyte cell was fixed at 25 °C. The working electrode was prepared by drop-casting the solution (9.81 μL of suspension containing 0.98 mL of ethanol, 20 μL of Nafion (Nafion 117 solution, 5% in a mixture of lower aliphatic alcohols and water, Aldrich), and 2 mg of the samples) on glassy carbon electrodes (5 mm in diameter, 0.196 cm2 surface area). Each time, the electrode was polished using an Al2O3 suspension (1 and 0.05 μm, Allied High Tech Products, Inc.) and was dried using Ar gas after sonication in distilled water for 1 min. Cyclic voltammetry (CV) was performed for 50 cycles from 0 to 1.0 VRHE with a scan rate of 100 mV/s to stabilize the electrode, and then linear sweep voltammetry (LSV) was continuously conducted from 0 to 1.1 V with a different rotating disk from 2,400 to 1,000 rpm. The number of electrons transferred to dissolved oxygen was calculated by the eqs 1 and 2) as shown below.

Scheme 1. Schematic Diagram for the Fabrication of ThreeDimensional Carbon Materials (3D CMs) by Using Y Zeolite as a Hard Template and Ethylene as Carbon Source

intermediates. The morphology of the NaY zeolite was wellreproduced for 3D CMs obtained from M-Y templates (M: La3+, Co2+, Mn2+, and Cu2+), while only CNTs were formed in the Ni-Y template on account of the preference for 1D growth of carbon moieties when metallic Ni nanoparticles are formed. Although further studies on the fabrication of 3D CMs by La3+ and Ca2+ exchanged zeolites have been reported by Ryoo’s group since 2016,22,23 the transition metal-exchanged zeolites can be widely utilized for various catalytic reactions and are cost-effective. Furthermore, the synthetic concept can be readily expanded for the structural engineering of 3D CMs as starting materials with high flexibility for the target applications. Herein, we focused on in situ incorporation of transition metal species in 3D CMs, using cobalt-exchanged materials as an example of using the synthetic concept to obtain materials as catalysts for electrochemical ORR. Mild acid and NH3 treatments adopted for the selective removal of cobalt species and to enable N-doping, respectively, are shown to have a large effect on the ORR activities of the materials, resulting in a comparable performance with the commercial Pt/C (20 wt %) electrocatalyst.

■

1 1 1 = + jD jk B(w)1/2

MATERIALS AND METHODS

(1)

B = 0.62nFAv(−1/6)Co2Do2(2/3)

Materials Synthesis. Lanthanum and transition metal ions (La3+, LaCl3·xH2O (99.999%, STREM); Co2+, Co(NO3)2·6H2O (ACS reagent, Aldrich); Mn2+, MnCl2·4H2O (>99.0%, Aldrich); Cu2+, Cu(NO3)2·3H2O (99−104%, Aldrich); and Ni2+, Ni(NO3)2·6H2O (>97.0%, Aldrich)) were added to aqueous suspensions containing 1 g of NaY zeolites (Molecular sieve, NaY, Aldrich). The initial concentration of cations was fixed at 0.2 M. The pH was adjusted to around 3.5 using HCl for La3+ and Mn2+, and HNO3 for the other cations. The solution was vigorously stirred at room temperature overnight. After M-Y (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+) samples were collected by centrifugation, they were washed with distilled water several times and then dried at 50 °C overnight. Carbonization and graphitization were then accomplished in three steps: (1) heating up to 600 °C under N2 flow, (2) carbonization at 600 °C for 1 h by flowing ethylene, water vapor, and N2 gases (water vapor was fed through by bubbling N2 carrier gas through a water bath), and (3) graphitization at 850 °C for 2 h under N2 flow. The samples prepared after carbonization and graphitization of M-Y were labeled as M-C (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+). The zeolite templates were removed using NaOH (2 M) at 80 °C for 8 h, and the process was repeated after centrifugation. Then, the samples were washed using distilled water several times. After base treatment, the transition metal species and trace amorphous silicate were removed using HCl (0.1 M) overnight, followed by washing with distilled water several times and drying at 60 °C in the oven. The samples of M-C after base and acid treatment were noted as M-C-A (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+).

(2) 2

where jD = disk current density (mA/cm ), jk = kinetic current density (mA/cm2), ω = electrode rotating speed (rad/s), n = number of electrons, F = Faraday constant (96,485 C/mol), A = surface area of electrode (0.196 cm2), CO2 = concentration of dissolved O2 (1.2 × 10−3 mol/L), ν = kinetic viscosity (0.01 cm2/s), and DO2 = diffusion coefficient of dissolved O2 in 0.1 M KOH (1.9 × 10−5 cm−5/s). Before the durability test, the CV curves from 0 to 1 V were performed for 50 cycles with a scan rate of 100 mV/s, and then chronoamperometry was measured at constant bias of 0.5 V at 800 rpm under O2-saturated conditions.

■

RESULTS AND DISCUSSION 3D CMs were successfully fabricated by a step-by-step procedure starting with the ion-exchange (M-Y; M, Co2+, Mn2+, Cu2+, and Ni2+) of the Na zeolite, following by carbonization and graphitization (M-C), and finally removal of zeolite templates and transition metal species by NaOH and HCl (M-C-A), respectively. As a control experiment, the La-Y template was also used for the synthesis of 3D CMs. In principle, transition metal ions can be localized at different crystallographic sites including the (i) hexagonal prism (Site I), (ii) sodalite cage (Site I′), (iii) supercage (Site II), and (iv) β cage (Site II′) during the ion exchange process. The localization of cations maximizes their interaction with oxygen atoms 3780


Article


S5). The color of Co-Y was changed from pink to purple (not shown), while the original peaks of the Y zeolite were preserved, and metallic Co peaks were not observed in the XRD pattern. Therefore, it makes sense to suppose that the zeolite template was broken by the shrinkage of the carbon structures during heat treatment because of the volumetric decrease of sp3 carbon networks. Hence, cobalt ions not stabilized by the zeolite structure could diffuse out, be reduced, and then agglomerate as Co nanoparticles. Figure S6 shows the XRD pattern and transmission electron microscope (TEM) image with energy-disperse X-ray (EDX) spectroscopies for Co-C after the removal of the zeolite template by hot NaOH (2 M). In the XRD patterns, while the typical peaks of the Y zeolite completely disappeared, the diffraction patterns derived from zeolite-like ordered carbon structure (2θ = 6.6°), cobalt hydroxide (red line), and metallic cobalt (blue line) were clearly measured (Figure S6a). The small particles with a different contrast and the sheet-like morphologies were discovered in the TEM images (Figure S6b). On the basis of the spot EDX analysis, it was verified that the metallic cobalt contained oxygen due to oxide layers on the surface (Figure S6c) and that the atomic ratio of cobalt to oxygen for the sheetlike structure was consistent with that of Co(OH)2 (Figure S6d). With regard to the different interactions between cobalt ions and oxygen atoms, the metallic cobalt species were possibly generated by the reduction of cobalt ions localized in the supercage in that the interaction of Co2+ with the anionic zeolite templates is relatively weak. On the other hand, the cobalt ions captured at hexagonal or sodalite cages with a small window could be stabilized by the strong interaction even at a high temperature. Thus, cobalt hydroxide can be formed by the reaction of Co2+ with hydroxide during NaOH treatment.30 Trace amounts of silicon were also measured in the carbon materials (Figure S6e); however, aluminum elements were not detected in the spot EDX analysis. After the removal of zeolite templates in M-C (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+), the 3D CMs were prepared by overnight acid treatment using 0.1 M HCl to remove transition metal species and remaining trace amorphous silicate. Scanning electron microscope (SEM) images (Figure 1) demonstrated that the duplication of the NaY template was successfully achieved for M-C-A (M: La3+, Co2+, Mn2+, and Cu2+), except for Ni-C-A, which consisted of 3D CMs and thread-like fibers. The morphology of the templated carbon is clearer in TEM images with low (Figure S7) and high (Figure S8) magnifications. The particle sizes of the templated carbon were confirmed in the range of a few hundred nanometers to a few micrometers, and individual metallic nanoparticles were not observed in any of the samples. It should be noted that the result was not limited to a local region but was observed throughout in SEM images with low magnification (Figure S9). Moreover, the surface of the 3D CMs was smooth, and their morphologies and sizes were consistent with those of NaY templates. Meanwhile, the production of CNTs in Ni-C-A was due to the superior catalytic behavior of Ni nanoparticles to promote the 1D growth of carbon species under CVD processes.31 In the XRD data, no impurity peaks, originating from the zeolite templates or transition metal species, were detected in any of the samples (Figure 2a). The reflection at the low angle region (2θ = 6.6°), derived from the ordered zeolite-like structure, was visible for all samples except Ni-C-A (inset in Figure 2a). The two broad peaks centered at 22° (002) and 43°

present in aluminosilicate, together with minimizing the electrostatic repulsion among neighboring cations.25 As shown in Figure S1a, the color of NaY was changed from white to pink, blue, and green after exchange with Co2+, Cu2+, and Ni2+, respectively. The exchange with La3+ and Mn2+ did not result in any color change (the color of La3+ and Mn2+ precursor was white and light pink, respectively). In ionexchanged Y zeolites, the cations are mainly placed in the supercage at ambient condition, but they can migrate to the hexagonal or sodalite cage during the dehydration process.26−28 Because of heating up to 600 °C under inert atmosphere, some of the transition metal ions could migrate inside the zeolite, thereby providing uniformly dispersed catalytic active sites for the carbonization of ethylene. Carbonization was carried out under continuous flowing ethylene and water vapor at 600 °C for 1 h, with a mechanistic pathway proposed as follows: (i) La3+ ions can stabilize ethylene as a result of d-π interaction, (ii) carbide was formed by the reaction between La3+ and ethylene, (iii) active carbonaceous moieties were generated by the reaction of carbide with water vapor, and (iv) carbon frameworks were formed inside zeolite by migration and further polymerization of active carbonaceous species.9 Although the color of all M-Y samples was changed to black after the CVD process, that of NaY was gray, indicating that the carbonization of ethylene was not enabled in NaY due to an absence of d-orbitals in Na+ (Figure S1b). After carbonization, all samples were calcined at 850 °C under flowing nitrogen to increase graphitic domains. X-ray diffraction (XRD) patterns of M-C (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+) demonstrate that the peak intensity of the zeolite templates was reduced relative to that of Y-C and that zerovalent metallic species were formed in the M-C samples (M: Co2+, Cu2+, and Ni2+) but not for Mn-C (Figure S2). From a thermodynamic point of view, the reduction potential of Co2+ (E° = −0.28 V), Cu2+ (E° = +0.34 V), and Ni2+ (E° = −0.25 V) to the zerovalent state is more positive than that of Mn2+ (E° = −1.18 V);29 hence, the formation of reduced cobalt, copper, and nickel was observed in the XRD. To verify the carbonization pathway and the formation of transition metal species, Co-C was selected as a standard material for detailed characterization. While the carbonization conditions were kept the same, the graphitization temperature was modified from 600 to 900 °C. As shown in Figure S3, the peak intensity of the Y zeolite was decreased, whereas that of metallic Co became visible with increasing temperature. The trend is more obvious in Figure S4, where the relative peak intensity change at 23.7° (assigned to Y zeolite) and at 44.2° (originated from metallic Co) was plotted with respect to the graphitization temperature. At 600 °C, the change of peak intensity for both Y zeolite and metallic Co was negligible, which implies that the carbonization did not destroy the template and that cobalt ions were still well-localized inside zeolites. This also indicates that the d-π coordination between ethylene and cobalt ions is similar to that which occurs with lanthanum ions. With increasing temperature from 700 to 900 °C, the peak intensity at 23.7° and 44.2° was linearly decreased and increased, respectively. The result suggests that the formation of metallic Co nanoparticles could be attributed to the reduction of the unstabilized metal ions, which stem from the breakage of the zeolite templates. To better understand the influence of the carbonization process on the formation of metallic Co, the same experiment was carried out without the CVD process of ethylene (Figure 3781


Article


scale carbides was too weak to be detected, and the other is that a radical polymerization, initiated by the transition metal ions localized in the zeolite templates, might be involved in the formation of the sp2 carbon networks. In the SAXS profile (Figure 2b), the shift of the Bragg peaks to lower or higher q was not observed among the 3D CMs. Meanwhile, the peak intensity was negligible in Ni-C-A. Although the diffraction intensities were not strong (typical nature of microporous carbon materials), the still visible peaks possessing a higher q (4.80) of 3D CMs than those of NaY (not shown, q = 4.46) demonstrated that the zeolite-like ordered structure was wellmaintained but that the pore size of 3D CMs was a little increased compared with that of NaY.33 As shown in Figure 2c, the Raman spectra of all samples revealed quite similar trends with two broad peaks at 1,588 cm−1 (G-band) and 1,332 cm−1 (D-band). These can be defined as sp2-hybridized carbon atoms with the E2g vibrational mode in-plane and bond disorders and defects such as dangling bonds, functional groups, five- and seven- membered carbon rings, etc. with the A1g breathing mode.34 The small peak around 1,270 cm−1 in Ni-C-A originated from the CNTs.35 Not only the broad peak at 1,332 cm−1 and the narrow peak at 1,588 cm−1 but also the lower peak intensity of IG to ID in 3D CMs predicts the presence of curved structures, functional groups introduced during base and acid treatment, and defect sites on nanographene layers.36−38 The Raman result was not unique in our system, but the broad and high intensity D-band was also measured in a previous report.9 The functional groups of 3D CMs were monitored using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Figure 2d). The peaks at 1,587 and 906 cm−1 were assigned as CC and CH2 vibrations, respectively.39,40 The peaks at 1,054 and 1,222 cm−1 originated from C−O stretching and phenolic C−O vibration, respectively.41 The oxygen-containing functional groups could be introduced during base and acid treatment of the M-C samples. In Ni-C-A, the peak intensities, marked

Figure 1. SEM images of (a) NaY, (b) La-C-A, (c) Co-C-A, (d) MnC-A, (e) Cu-C-A, and (f) Ni-C-A. The samples were obtained after base and acid treatment of M-C.

(100) were caused by the amorphous graphitic carbon.32 These reflections were prominent in Ni-C-A only because of the formation of CNTs, which will be discussed later. Although the formation of the carbides as an intermediate was proposed during carbonization, no transition metal carbide peaks were detected in the XRD data (Figure 2a and Figure S2−S6a). Two scenarios could be possible: one is that the intensity of atomic

Figure 2. (a) XRD, (b) SAXS, (c) Raman, and (d) ATR-FTIR spectra of M-C-A (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+). The peaks at 44.1° (110) and 64.1° (200) in (a) were ascribed to the iron sample holder. Inset in a: magnification of XRD patterns in the low angle region. 3782


Article


Figure 3. (a) STEM image, (b) HRTEM image, and (c) N2 adsorption/desorption isotherm; the black (square) and red (circle) lines are adsorption and desorption isotherms, respectively. The inset shows DFT pore size distribution. (d) High resolution C 1s XPS spectrum for Co-C-A.

samples are summarized in Table S1. The surface area and pore volume varied depending on the particular metal ion used, where the highest surface area was observed for the cobalt and manganese samples. It could be possible that the transition metal ions localized in Y zeolites stimulated the activation of 3D CMs, similar to the chemical activation of carbon materials by alkaline earth metals such as AlCl3, ZnCl2, and MgCl2.42 The activation mechanism is still debatable,43 and the question regarding how the wellordered microporous structure could be maintained during the activation process cannot be easily answered at this stage. The small surface area and pore volume of Ni-C-A can be attributed to the formation of CNTs. Figure 3d shows the C 1s core level peaks as measured by X-ray photoelectron spectroscopy (XPS). Four different species were fitted at 284.5, 286.6, 288.4, and 290.6 eV corresponding to C−C, C−O−C, CO, and O− CO, respectively.44 As mentioned in the description of the ATR-FTIR spectra, the oxygen-containing functional groups could be introduced by base and acid treatment. Despite the successful fabrication of the 3D CMs, their applications for energy conversion devices are quite limited since most pores comprised small sized pores. The micropores significantly hinder the access of electrolyte ions or reactant molecules even if the pore structure is well-oriented. This can give rise to a high resistance via diffusion rate limitation and steric interaction.45 Moreover, gas bubbles evolving inside 3D CMs reduce the contact area between the electrolyte and catalytic active sites, and eventually collapse the structure of the electrode materials.46,47 Therefore, the structural modification of 3D CMs is required to overcome these mentioned drawbacks for electrochemical energy conversion devices. Because of the unique structural properties of the 3D CMs prepared here, they should be more suited for the electrochemical ORR application instead of water-splitting since the generation of gas bubbles is not involved during ORR. Consequently, to enhance the electrocatalytic activity of 3D CMs, we introduced active sites including transition metal species and nitrogen elements, together with increasing the pore size. Since the superior electrocatalytic activity of cobalt

with dots, were relatively weak owing to the interference by the background signal of CNTs. The ATR-FTIR analysis suggests that the carbonization of ethylene, catalyzed by different transition metal ions, yielded similar functional groups, containing a small content of C-O groups. Figure 3a shows the scanning transmission electron microscopy (STEM) image and more specific STEM images for different magnifications, dark field (DF) and bright field (BF) modes for Co−C-A; the top side view for a slice of Co-CA with a secondary electron (SE) and transmission electron (TE) modes is presented in Supporting Information (Figure S10−S12). In Figure S10, the STEM images with SE mode clearly show that the smooth surface possessed a high porosity and that no individual impurity particles were identified on the external surface. The porous nature of 3D CMs was clearer in the STEM images with BF mode (Figure S11), revealing that micropores were uniformly formed in the entire 3D CMs without any bulk coke moieties blocking the pores. To investigate the porosity inside of the 3D CMs, the STEM measurement was focused on the slice of Co-C-A (Figure S12). Although the carbon structure was broken during cutting of the samples, thereby providing nonuniform and large pores, it was observed that the inside of the 3D CMs was largely porous. A high resolution TEM (HRTEM) image resolved the relatively well-ordered microporous structure of the 3D CMs (Figure 3b). However, the measurement was limited since the carbon structure rapidly decomposed under high electron beam irradiation. Figure 3c presents the N2 sorption isotherm of Co-C-A at 77 K. The isotherm curves showed not only the sharp increase at low relative pressure (P/P0) but also the near plateau to P/P0 of 1.0 for N2 sorption, which indicates that a majority of pores are micropores. The isotherm curve shape of NaY resembled that of all the M-C-A samples except for Ni-C-A (Figure S13). The nonlocal density functional theory (NLDFT) pore size distribution showed that the maximum peak is around 1.1 nm for the 3D CMs, which is higher than NaY (0.85 nm). The result is consistent with the SAXS data, showing the positive shift of q in 3D CMs. The surface area and pore volume for all 3783


Article


Figure 4. (a) STEM image, EDX mapping for (b) carbon (orange), (c) nitrogen (green), and (d) cobalt (purple) elements corresponding to panel a marked by red box, high resolution XPS spectra deconvoluted for (e) Co 2p and (f) N 1s, and (g) N2 adsorption/desorption isotherm. Black (square) and red (circle) lines are adsorption and desorption isotherms, respectively, and the inset shows DFT pore size distribution for N-Co-C-A (1 M). In e, the S indicates the typical satellite peaks of cobalt.

species for ORR has already been reported,48 and a high surface area and porosity were demonstrated from our synthetic method in Co-C-A, this material (obtained after base treatment of Co-C) was chosen to be systematically modified and evaluated for ORR. To vary the amount of cobalt species within the carbon structure, we simply modified the synthetic procedure by selectively leaching some cobalt by HCl with different concentrations (0 to 2 M) for 4 h. Following the acid treatment, N-doping was introduced by a post-NH3 treatment at 850 °C for 1 h. These samples were labeled as N-Co-C-A (0 to 2 M HCl), where N and A indicate the nitrogen doping and acid treatment, respectively. The acid treatment using different concentrations resulted in morphology change of the N-doped 3D CMs (Figure S14). Without acid treatment, individual particles with a broad size distribution and sheet-like agglomeration appeared inside the 3D CMs. On the other hand, HCl with the highest concentration (2 M) was the most effective for removing the cobalt species; however, this resulted in collapsing of the 3D CMs structure. The morphologies of the 3D CMs were relatively well-maintained when the concentration of HCl was 0.5 and 1 M. The magnetic properties of each sample were confirmed using a niobium magnet (Figure S15). Without acid treatment, the magnetic behavior was obvious regardless of the NH3 treatment due to the existence of metallic cobalt species. However, after the acid treatment, all samples except N-Co-C-A (0.5 M) did not show any magnetic properties, which indicates that the cobalt species were not present in the metallic form anymore. The correlation between the HCl concentration and the cobalt content will be discussed later. To elucidate the effect of HCl on the morphology and the element composition of 3D CMs, the N-Co-C-A (1 M) sample was selected for detailed characterization. The selected area EDX mapping marked in the STEM image (Figure 4a) confirmed the presence of carbon (Figure 4b), nitrogen (Figure 4c), and cobalt (Figure 4d) atoms, which were homogeneously dispersed in the entire N-doped 3D CMs. Similar results were also collected in HRTEM-EDX spot analysis, indicating that cobalt and nitrogen were not occupied in localized domains but rather were uniformly spread out over the sample (Figure S16).

In Figure 4e, the Co 2p XPS band shows the presence of cobalt elements, mainly the oxidation state Co2+ as seen from the satellite peak around 786.9 eV.49−51 XPS analysis did not show the existence of any metallic cobalt, which is also confirmed by the nonmagnetic behavior of the sample (Figure S15f). The deconvolution of the N 1s spectrum in Figure 4f provided three peaks at 398.1, 399.8, and 401.4 eV, representing the formation of pyridinic, pyrrolic, and graphitic nitrogen, respectively.52 According to the literature, the NH3 treatment temperature significantly affects the N-doping state, where pyridinic N is a primary state at 550 °C, but graphitic N is starting to be generated with increasing temperature.53 Since the NH3 treatment was carried out at 850 °C, the graphitic N was predominant in N-Co-C-A (1 M). The survey scan spectrum shows the elemental change before and after NH3 treatment, exhibiting the prominent peaks of carbon, oxygen, and silicon elements in both samples, but cobalt and nitrogen elements were only identified after NH3 treatment (Figure S17). The result implies that the cobalt species were located inside 3D CMs before NH3 treatment but that some of them could migrate to the external surface during NH3 treatment. Again, a small amount of silicon always remains when NaOH is utilized as an etching reagent to remove silica and aluminosilicate templates. The ATR-FTIR spectroscopy results showed the change in functional groups in N-doped 3D CMs depending on the concentration of HCl (Figure S18). A broad peak at 1,148 and 1,082 cm−1 specified the CN stretching and C−N vibration, respectively.54,55 The prominent peaks at 1,585 and 960 cm−1 were responsible for CN (and/or CC) and Si−O−Co (and/or Si−O−Si) vibration, respectively.54,56 Without acid treatment, the peak at 960 cm−1 was the only distinct feature (instead of the vibration peaks related with C−N bonds), but other N-doped samples had almost identical functional groups. To better understand the nitrogen and cobalt content in Ndoped 3D CMs, ICP-AES and SEM-EDX analyses were conducted. It should be mentioned that the absolute value of the composition in N-doped 3D CMs cannot be determined by SEM-EDX (a high uncertainty of nitrogen and oxygen) and ICP-AES (a high uncertainty for light elements); however, the relative comparison can be used for a rough estimate. As seen in 3784


Article


Figure 5. (a) LSV curves of N-Co-C-A (1 M) with a different rotating speed. (b) LSV curves of different samples at 1,600 rpm with a scan rate of 10 mV/s. Sample labeling: rGO (reduced graphene oxide), Co-C-A (cobalt−carbon sample after template removal and 1 M HCl treatment), N-rGO (nitrogen doped reduced graphene oxide), and N-Co-C-A (nitrogen doped cobalt−carbon sample after template removal and 1 M HCl treatment). (c) Koutecky−Levich plots for different samples collected at 0.3 V. (d) Number of electrons transferred to dissolved oxygen depending on applied bias. (e) Kinetic limiting current (Jk) for different materials; electron transfer numbers at 0.3 V are noted. (f) Durability test of Pt/C and N-Co-C-A (1 M) under applied bias at 0.5 V at 800 rpm for 17 h.

which is a key and challenging reaction in fuel cell applications. All samples were loaded onto glassy carbon electrodes (GCEs), and cyclic voltammetry (CV) was performed for 50 cycles to stabilize the electrode in 0.1 M KOH electrolyte under O2saturated conditions. The ORR catalytic activities of N-Co-C-A (0 to 2 M HCl) were first investigated by LSV with different rotating speeds from 1,000 to 2,400 rpm (Figure 5a and Figure S22). The current density regularly increased for all samples as the rotating rate was accelerated, whereas not only the curve shape but also the onset potential and the limiting current density were significantly changed depending on the concentration of HCl used in the acid treatment. In particular, without acid treatment, the current density was the lowest compared with that of other the samples, and the wide current plateau over a negative potential region was absent. For N-Co-C-A (0.5 M), the noise fluctuation behavior appeared in the current plateau region, which was different from the other samples. This could be from the predominantly micropore surface area

Figure S19 and Figure S20, the cobalt and nitrogen content were decreased and increased with increasing concentrations of HCl, respectively. It is very notable that the cobalt species remaining in the 3D CMs prohibit N-doping into the carbon lattice. This is due to the fact that NH3 can be decomposed into N2 and H2 over cobalt-based catalysts at a high temperature; thus, it is not reactive anymore.57,58 Accordingly, nitrogen was not detected in N-Co-C-A (0 M) owing to a large amount of cobalt remaining in the sample. In the N2 sorption isotherm (Figure 4g), the sharp increase near P/P0 of 0 was effectively reduced, but there were inflection points observed below P/P0 of 0.5, which indicates that the pores consisted of micro- and mesopores in N-Co-C-A (1 M). As summarized in Table S1 and Figure S21, the surface area and pore volume reached a maximum in N-Co-C-A (0.5 M) and decreased with increasing HCl concentration. After detailed structural analyses, the electrocatalytic performance of the materials was investigated for ORR, 3785


Article


peaks originating from the reduction of oxygen were clearly measured at 0.81 and 0.83 V in Pt/C and N-Co-C-A (1 M), respectively, under O2-saturated conditions, where the current density of the latter was higher than that of the former. To obtain the electron-transfer number and Jk for all samples, the LSV curves were collected at different rotating rates (Figure S27). The Koutecky−Levich plots for each sample are presented in Figure 5c, and the results were well-matched with linearly fitted curves. The electron-transfer number was derived from the Koutecky−Levich equation, which was displayed with a potential applied in Figure 5d. As noted in the literature, the electron-transfer number is recorded as intermediate values from 2.3 to 2.8 in most N-doped carbon samples;61 that for NrGO was estimated around 2.6, which was within the reported range. However, the electron-transfer number of N-Co-C-A (1 M) was calculated to be around 3.7, which is comparable to 3.9 obtained in Pt/C (Figure 5d). Meanwhile, the value was measured at about 1.6 and 1.9 for rGO and Co-C-A, respectively. Fundamentally, the products can be changed by the number of electrons transferred to O2 and applied bias. For example, superoxide anion radicals (O2 + e− → O2−; E° = −0.33 V), hydrogen peroxide (O2 + 2 H+ + 2 e− → H2O2; E° = 0.68 V), and water (O2 + 4 H+ + 4 e− → 2 H2O; Eo = 1.23 V) can be generated by one-, two-, and four-electrons transfer, respectively.62 When two-electron transfer is favorable, hydroxyl radials are produced with further one electron transfer to H2O2 (H2O2 + e− → ·OH + OH−). The hydroxyl radical, a strong oxidant, and H2O2 itself could corrode the membrane and electrode materials in fuel cell.61,63 Therefore, the selective production of water as a result of the reduction of oxygen is highly desired for practical applications. Here, the electrontransfer number of N-Co-C-A (1 M) was recorded close to 4. As shown in Figure 5e, the Jk value of N-Co-C-A (1 M) was higher than that of all samples including Pt/C. Without NH3 treatment, the high Jk value was also measured in Co-C-A, which could be ascribed to the cobalt-based catalysts incorporated in the 3D CMs. According to previous studies, excellent ORR performance is characterized by (i) a high disk and kinetic limiting current density, (ii) an onset potential with a more negative value, (iii) and an electron-transfer number close to 4, all of which were achieved in N-Co-C-A (1 M). The ORR performance was also investigated for 1D CNTs before and after N-doping (Figure S28). The onset potential was more negative, and the current-limiting current was lower relative to those of N-Co-C-A (1 M). Although it was reported that vertically well-aligned N-doped CNTs had high performance for ORR,64 the aggregated CNTs with a low surface area and porosity (Figure S25c,d and Table S1) resulted in low ORR activity. Moreover, the electron-transfer number in the range of 2 to 3 and low Jk values were observed regardless of NH3 treatment. The durability of N-Co-C-A (1 M) was evaluated using chronoamperometry under a constant potential at 0.5 V for 17 h, which revealed that the newly developed catalyst has higher stability than Pt/C (Figure 5f).

and volume, which hindered the access of the electrolyte and dissolved oxygen to the active sites. In principle, the introduction of heteroelements into sp2 carbon lattices induces charge redistribution, and the O−O bond can be weakened by parallel diatomic O2 adsorption, which can lower the overpotential for ORR. The pyridinic N present in graphene can provide one p electron to the neighboring carbon atoms, and their electron-donating nature increases the bonding of O2 with N and/or the adjacent C atoms. Although the mechanism is still controversial, it has been accepted that pyridinic N and graphitic N are responsible for the change of onset potential and limiting current density, respectively.20 Cobalt-based catalysts on carbon materials have been also considered as active sites for ORR, where N-doped carbon supports synergistically facilitate the kinetics.59 On the basis of the characterization results, it was confirmed that the nitrogen and cobalt content increased and deceased with the concentration of HCl used in the treatment, respectively. Therefore, when the LSV curves of N-Co-C-A with different cobalt amounts were compared (Figure S23a), the tendency did not match with the physisorption data. The higher efficiency of N-Co-C-A (1 M) than that of N-Co-C-A (0.5 M) even despite its lower surface area and pore volume can be explained by the optimized ratio of cobalt to nitrogen and the larger pore size as well. An absence of nitrogen in N-Co-C-A (0 M) and low cobalt content in N-Co-C-A (2 M) did give rise to the low efficiency, in addition to its low surface area and pore volume as well. The Koutecky−Levich plots for each sample, obtained at 0.3 V from the LSV curves with different rotating speeds, increased in proportion (J−1 vs w−1/2) (Figure S23b). The electron-transfer number (Figure S23c) and the kinetic limiting current (Jk) (Figure S23d), calculated based on the linearly fitted Koutecky−Levich plots (eqs 1 and 2), indicate that the 4 electron transfer was not favorable and that the dynamic overpotential was relatively high for N-Co-C-A (0 M). The ORR performance was confirmed before and after NH3 treatment, which was further compared with a commercial Pt/ C (20 wt %), 2D CMs (reduced graphene oxide, rGO and NrGO), and 1D CMs (carbon nanotubes, CNT and N-CNT). As shown in Figure 5b, the N-doping effect was obvious in LSV curves, where the broad reduction peak around 0.43 V for rGO and the inflection point for Co-C-A suggest the two-electron ORR process from O2 to OOH−. However, the wide current plateau without unique reduction peaks in N-rGO and N-CoC-A (1 M) reveals the diffusion-controlled ORR process with a tendency of a four-electron transfer to oxygen.60 Compared with N-rGO, the better electrocatalytic activity of N-Co-C-A (1 M) can be attributed to its high surface area and pore volume, the existence of cobalt-based catalysts, and well-defined pore distribution stemming from 3D structure, facilitating the electrolyte and O2 diffusion inside the pore, thereby providing the optimized contact between the active sites and electrolyte. As shown in Figure S24, the sheet-like morphologies of both rGO and N-rGO were not well-ordered, and the materials had small surface area and pore volume (Figure S25a,b and Table S1). Notably, the onset potential and the limiting current density for ORR of N-Co-C-A (1 M) were comparable to those of a commercial Pt/C (20 wt %) catalyst (Figure 5b). A quasirectangular CV curve lacking obvious redox peaks in a range of 0.8 to 1.0 V was obtained in both Pt/C and N-Co-C-A (1 M) under Ar atmosphere (Figure S26). The larger area CV curve in N-Co-C-A (1 M) was attributed to the supercapacitance effect, typically observed in porous CMs. However, the maximum

■

CONCLUSIONS A series of 3D microporous carbon materials with high surface area and pore volume were prepared by using transition metal ion-exchanged zeolites as a hard template. The advantage of the synthetic method is the simple in situ incorporation of transition metal species within 3D carbon materials during the synthesis procedure where the amount of the metallic 3786


Article


oxide composite for photocatalytic CO2 reduction. J. Am. Chem. Soc. 2017, 139, 5660−5663. (7) Mao, J.; Iocozzia, J.; Huang, J.; Meng, K.; Lai, Y.; Lin, Z. Graphene aerogels for efficient energy storage and conversion. Energy Environ. Sci. 2018, 11, 772−799. (8) Moon, G. H.; Shin, Y.; Choi, D.; Arey, B. W.; Exarhos, G. J.; Wang, C.; Choi, W.; Liu, J. Catalytic templating approaches for threedimensional hollow carbon/graphene oxide nano-architectures. Nanoscale 2013, 5, 6291−6296. (9) Kim, K.; Lee, T.; Kwon, Y.; Seo, Y.; Song, J.; Park, J. K.; Lee, H.; Park, J. Y.; Ihee, H.; Cho, S. J.; Ryoo, R. Lanthanum-catalysed synthesis of microporous 3D graphene-like carbons in a zeolite template. Nature 2016, 535, 131−135. (10) Moon, G. H.; Kim, W.; Bokare, A. D.; Sung, N. E.; Choi, W. Solar production of H2O2 on reduced graphene oxide-TiO2 hybrid photocatalysts consisting of earth-abundant elements only. Energy Environ. Sci. 2014, 7, 4023−4028. (11) Moon, G. H.; Park, Y.; Kim, W.; Choi, W. Photochemical loading of metal nanoparticles on reduced graphene oxide sheets using phosphotungstate. Carbon 2011, 49, 3454−3462. (12) Chang, H. X.; Wu, H. K. Graphene-based nanocomposites: preparation, functionalization, and energy and environmental applications. Energy Environ. Sci. 2013, 6, 3483−3507. (13) Wang, G.; Deng, X.; Gu, D.; Chen, K.; Tüysüz, H.; Spliethoff, B.; Bongard, H.-J.; Weidenthaler, C.; Schmidt, W.; Schüth, F. Co3O4 nanoparticles supported on mesoporous carbon for selective transfer hydrogenation of α,β-unsaturated aldehydes. Angew. Chem., Int. Ed. 2016, 55, 11101−11105. (14) Wang, G.; Chen, K.; Engelhardt, J.; Tüysüz, H.; Bongard, H.-J.; Schmidt, W.; Schüth, F. Scalable one-pot synthesis of yolk−shell carbon nanospheres with yolk-supported Pd nanoparticles for sizeselective catalysis. Chem. Mater. 2018, 30, 2483−2487. (15) Deng, D. H.; Novoselov, K. S.; Fu, Q.; Zheng, N. F.; Tian, Z. Q.; Bao, X. H. Catalysis with two-dimensional materials and their heterostructures. Nat. Nanotechnol. 2016, 11, 218−230. (16) Xu, Y.; Kraft, M.; Xu, R. Metal-free carbonaceous electrocatalysts and photocatalysts for water splitting. Chem. Soc. Rev. 2016, 45, 3039−3052. (17) Dai, L. M.; Xue, Y. H.; Qu, L. T.; Choi, H. J.; Baek, J. B. Metalfree catalysts for oxygen reduction reaction. Chem. Rev. 2015, 115, 4823−4892. (18) Wang, G.; Cao, Z.; Gu, D.; Pfänder, N.; Swertz, A.-C.; Spliethoff, B.; Bongard, H.-J.; Weidenthaler, C.; Schmidt, W.; Rinaldi, R.; Schüth, F. Nitrogen-doped ordered mesoporous carbon supported bimetallic PtCo nanoparticles for upgrading of biophenolics. Angew. Chem., Int. Ed. 2016, 55, 8850−8855. (19) Wang, J.; Xu, F.; Jin, H. Y.; Chen, Y. Q.; Wang, Y. Non-noble metal-based carbon composites in hydrogen evolution reaction: Fundamentals to applications. Adv. Mater. 2017, 29, 1605838. (20) Liu, X.; Dai, L. M. Carbon-based metal-free catalysts. Nat. Rev. Mater. 2016, 1, 16064. (21) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 2011, 10, 780−786. (22) Kim, K.; Kwon, Y.; Lee, T.; Cho, S. J.; Ryoo, R. Facile large-scale synthesis of three-dimensional graphene-like ordered microporous carbon via ethylene carbonization in CaX zeolite template. Carbon 2017, 118, 517−523. (23) Lee, H.; Kim, K.; Kang, S. H.; Kwon, Y.; Kim, J. H.; Kwon, Y. K.; Ryoo, R.; Park, J. Y. Extremely high electrical conductance of microporous 3D graphene-like zeolite-templated carbon framework. Sci. Rep. 2017, 7, 11460. (24) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations. Chem. Mater. 1999, 11, 771−778. (25) Frising, T.; Leflaive, P. Extraframework cation distributions in X and Y faujasite zeolites: A review. Microporous Mesoporous Mater. 2008, 114, 27−63.

species can be controlled by a post selective leaching. Among the diverse possible electrochemical applications of the carbonbased materials, the main emphasis was given ORR. A high disk and kinetic limiting current density, an onset potential comparable to a commercial Pt/C (20 wt %), and an electron-transfer number close to 4 were achieved with Ndoped cobalt−carbon material. Furthermore, this new class of material indicates very good durability and superior activity in comparison with those of 1D CNTs and 2D graphene. This methodology can be extended to diverse catalytic applications due to the simple and flexible synthetic procedure.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00861. Characterization conditions, analysis results of asprepared samples (XRD, TEM, STEM, SEM, EDX, BET isotherm and DFT pore size distribution, XPS, ATR-FTIR, and ICP-AES), and the different electrochemical ORR results induced by the different concentrations of acid, and the dimension of carbon materials (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gun-hee Moon: 0000-0003-2931-1083 Harun Tüysüz: 0000-0001-8552-7028 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the MAXNET Energy research consortium of the Max Planck Society and the Carbon2Chem project funded by the Bundesministerium für Bildung und Forschung (BMBF) of the German government. We thank Mr. B. Spliethoff, Mr. H.-J. Bongard, and Ms. S. Palm for assistance with electron microscopy measurements and EDX analysis, Dr. C. Weidenthaler for XPS data analysis, and Professor C. K. Chan (Arizona State University) for assistance with revising the manuscript.

■

REFERENCES

(1) Yu, Z. N.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy Environ. Sci. 2015, 8, 702−730. (2) Xiang, Q.; Yu, J.; Jaroniec, M. Graphene-based semiconductor photocatalysts. Chem. Soc. Rev. 2012, 41, 782−796. (3) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen evolution by a metal-free electrocatalyst. Nat. Commun. 2014, 5, 3783. (4) Hu, C.; Dai, L. Carbon-based metal-free catalysts for electrocatalysis beyond the ORR. Angew. Chem., Int. Ed. 2016, 55, 11736− 11758. (5) Duan, X.; Xu, J.; Wei, Z.; Ma, J.; Guo, S.; Wang, S.; Liu, H.; Dou, S. Metal-free carbon materials for CO2 electrochemical reduction. Adv. Mater. 2017, 29, 1701784. (6) Xu, Y.-F.; Yang, M.-Z.; Chen, B.-X.; Wang, X.-D.; Chen, H.-Y.; Kuang, D.-B.; Su, C.-Y. A CsPbBr3 perovskite quantum dot/graphene 3787


Article


(47) Wang, Z. Y.; Li, J. T.; Tian, X. C.; Wang, X. P.; Yu, Y.; Owusu, K. A.; He, L.; Mai, L. Q. Porous nickel-iron selenide nanosheets as highly efficient electrocatalysts for oxygen evolution reaction. ACS Appl. Mater. Interfaces 2016, 8, 19386−19392. (48) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Highperformance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011, 332, 443−447. (49) Dominguez, M.; Taboada, E.; Idriss, H.; Molins, E.; Llorca, J. Fast and efficient hydrogen generation catalyzed by cobalt talc nanolayers dispersed in silica aerogel. J. Mater. Chem. 2010, 20, 4875− 4883. (50) Yao, Z. W.; Zhu, A. I.; Chen, J.; Wang, X. K.; Au, C. T.; Shi, C. Synthesis, characterization and activity of alumina-supported cobalt nitride for NO decomposition. J. Solid State Chem. 2007, 180, 2635− 2640. (51) Zhang, Y. Q.; Ouyang, B.; Xu, J.; Jia, G. C.; Chen, S.; Rawat, R. S.; Fan, H. J. Rapid synthesis of cobalt nitride nanowires: Highly efficient and low-cost catalysts for oxygen evolution. Angew. Chem., Int. Ed. 2016, 55, 8670−8674. (52) Kondo, T.; Casolo, S.; Suzuki, T.; Shikano, T.; Sakurai, M.; Harada, Y.; Saito, M.; Oshima, M.; Trioni, M. I.; Tantardini, G. F.; Nakamura, J. Atomic-scale characterization of nitrogen-doped graphite: Effects of dopant nitrogen on the local electronic structure of the surrounding carbon atoms. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 035436. (53) Lai, L. F.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C. H.; Gong, H.; Shen, Z. X.; Jianyi, L. Y.; Ruoff, R. S. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ. Sci. 2012, 5, 7936−7942. (54) Khai, T. V.; Na, H. G.; Kwak, D. S.; Kwon, Y. J.; Ham, H.; Shim, K. B.; Kim, H. W. Influence of N-doping on the structural and photoluminescence properties of graphene oxide films. Carbon 2012, 50, 3799−3806. (55) Takanabe, K.; Kamata, K.; Wang, X. C.; Antonietti, M.; Kubota, J.; Domen, K. Photocatalytic hydrogen evolution on dye-sensitized mesoporous carbon nitride photocatalyst with magnesium phthalocyanine. Phys. Chem. Chem. Phys. 2010, 12, 13020−13025. (56) Bhoware, S. S.; Singh, A. P. Characterization and catalytic activity of cobalt containing MCM-41 prepared by direct hydrothermal, grafting and immobilization methods. J. Mol. Catal. A: Chem. 2007, 266, 118−130. (57) Schuth, F.; Palkovits, R.; Schlogl, R.; Su, D. S. Ammonia as a possible element in an energy infrastructure: catalysts for ammonia decomposition. Energy Environ. Sci. 2012, 5, 6278−6289. (58) Lendzion-Bielun, Z.; Narkiewicz, U.; Arabczyk, W. Cobalt-based catalysts for ammonia decomposition. Materials 2013, 6, 2400−2409. (59) Bezerra, C. W. B.; Zhang, L.; Lee, K. C.; Liu, H. S.; Marques, A. L. B.; Marques, E. P.; Wang, H. J.; Zhang, J. J. A review of Fe-N/C and Co-N/C catalysts for the oxygen reduction reaction. Electrochim. Acta 2008, 53, 4937−4951. (60) Zheng, Y.; Jiao, Y.; Chen, J.; Liu, J.; Liang, J.; Du, A.; Zhang, W. M.; Zhu, Z. H.; Smith, S. C.; Jaroniec, M.; Lu, G. Q.; Qiao, S. Z. Nanoporous graphitic-C3N4@carbon metal-free electrocatalysts for highly efficient oxygen reduction. J. Am. Chem. Soc. 2011, 133, 20116− 20119. (61) Daems, N.; Sheng, X.; Vankelecom, I. F. J.; Pescarmona, P. P. Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2014, 2, 4085−4110. (62) Moon, G. H.; Fujitsuka, M.; Kim, S.; Majima, T.; Wang, X. C.; Choi, W. Eco-friendly photochemical production of H2O2 through O2 reduction over carbon nitride frameworks incorporated with multiple heteroelements. ACS Catal. 2017, 7, 2886−2895. (63) Moon, G. H.; Kim, S.; Cho, Y. J.; Lim, J.; Kim, D. H.; Choi, W. Synergistic combination of bandgap-modified carbon nitride and WO3 for visible light-induced oxidation of arsenite accelerated by in-situ Fenton reaction. Appl. Catal., B 2017, 218, 819−824. (64) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 2009, 323, 760−764.

(26) Hutta, P. J.; Lunsford, J. H. Spectroscopic evidence for the location of Co2+ ions in zeolite V upon dehydration. J. Chem. Phys. 1977, 66, 4716−4717. (27) Praliaud, H.; Coudurier, G. Optical spectroscopy of hydrated, dehydrated and ammoninated cobalt (II) exchanged zeolites X and Y. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2601−2616. (28) Bae, D.; Seff, K. Structures of cobalt(II)-exchanged zeolite X. Microporous Mesoporous Mater. 1999, 33, 265−280. (29) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solutions. CRC Press, 1985. (30) Wang, Y.; Wu, H. L.; Zhang, Q. H.; Tang, Q. H. Cobalt nanoparticles prepared in faujasite zeolites by borohydride reduction. Microporous Mesoporous Mater. 2005, 86, 38−49. (31) Che, G.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R.; Ruoff, R. S. Chemical vapor deposition based synthesis of carbon nanotubes and nanofibers using a template method. Chem. Mater. 1998, 10, 260−267. (32) Wang, R. T.; Wang, P. Y.; Yan, X. B.; Lang, J. W.; Peng, C.; Xue, Q. J. Promising porous carbon derived from celtuce leaves with outstanding supercapacitance and CO2 capture performance. ACS Appl. Mater. Interfaces 2012, 4, 5800−5806. (33) Kockrick, E.; Schrage, C.; Borchardt, L.; Klein, N.; Rose, M.; Senkovska, I.; Kaskel, S. Ordered mesoporous carbide derived carbons for high pressure gas storage. Carbon 2010, 48, 1707−1717. (34) Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095−14107. (35) Stephan, C.; Nguyen, T. P.; Lahr, B.; Blau, W.; Lefrant, S.; Chauvet, O. Raman spectroscopy and conductivity measurements on polymer-multiwalled carbon nanotubes composites. J. Mater. Res. 2002, 17, 396−400. (36) Nishihara, H.; Yang, Q. H.; Hou, P. X.; Unno, M.; Yamauchi, S.; Saito, R.; Paredes, J. I.; Martinez-Alonso, A.; Tascon, J. M. D.; Sato, Y.; Terauchi, M.; Kyotani, T. A possible buckybowl-like structure of zeolite templated carbon. Carbon 2009, 47, 1220−1230. (37) Su, F. B.; Poh, C. K.; Chen, J. S.; Xu, G. W.; Wang, D.; Li, Q.; Lin, J. Y.; Lou, X. W. Nitrogen-containing microporous carbon nanospheres with improved capacitive properties. Energy Environ. Sci. 2011, 4, 717−724. (38) Zhan, D.; Ni, Z. H.; Chen, W.; Sun, L.; Luo, Z. Q.; Lai, L. F.; Yu, T.; Wee, A. T. S.; Shen, Z. X. Electronic structure of graphite oxide and thermally reduced graphite oxide. Carbon 2011, 49, 1362−1366. (39) Svatos, A.; Attygalle, A. B. Characterization of vinyl-substituted, carbon-carbon double bonds by GC/FT-IR analysis. Anal. Chem. 1997, 69, 1827−1836. (40) Anariba, F.; Viswanathan, U.; Bocian, D. F.; McCreery, R. L. Determination of the structure and orientation of organic molecules tethered to flat graphitic carbon by ATR-FTIR and Raman spectroscopy. Anal. Chem. 2006, 78, 3104−3112. (41) Beidaghi, M.; Wang, C. L. Micro-supercapacitors based on interdigital electrodes of reduced graphene oxide and carbon nanotube composites with ultrahigh power handling performance. Adv. Funct. Mater. 2012, 22, 4501−4510. (42) Dai, X. D.; Liu, X. M.; Qian, L.; Yan, Z. F.; Zhang, J. A novel method to synthesize super-activated carbon for natural gas adsorptive storage. J. Porous Mater. 2006, 13, 399−405. (43) Ahmadpour, A.; Do, D. D. The preparation of active carbons from coal by chemical and physical activation. Carbon 1996, 34, 471− 479. (44) Xu, L. M.; Xiao, G. Y.; Chen, C. B.; Li, R.; Mai, Y. Y.; Sun, G. M.; Yan, D. Y. Superhydrophobic and superoleophilic graphene aerogel prepared by facile chemical reduction. J. Mater. Chem. A 2015, 3, 7498−7504. (45) Chang, B. B.; Wang, Y. L.; Pei, K. M.; Yang, S. M.; Dong, X. P. ZnCl2-activated porous carbon spheres with high surface area and superior mesoporous structure as an efficient supercapacitor electrode. RSC Adv. 2014, 4, 40546−40552. (46) Berglund, S. P.; Flaherty, D. W.; Hahn, N. T.; Bard, A. J.; Mullins, C. B. Photoelectrochemical oxidation of water using nanostructured BiVO4 films. J. Phys. Chem. C 2011, 115, 3794−3802. 3788


Structural Engineering of 3D Carbon Materials from Transition Metal

Recommend Documents