Two-Dimensional Corrugated Porous Carbon-, Nitrogen-Framework

Two-Dimensional Corrugated Porous Carbon‑, Nitrogen-Framework/Metal Heterojunction for Efficient Multielectron Transfer Processes with Controlled Kinetics Ken Sakaushi,*,†,‡ Andrey Lyalin,‡ Satoshi Tominaka,§ Tetsuya Taketsugu,‡,¶ and Kohei Uosaki†,‡,§ †

Center for Green Research on Energy and Environmental Materials, ‡Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), and §WPI International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan ¶ Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan S Supporting Information *

ABSTRACT: The material choice for efficient electrocatalysts is limited because it is necessary to be highly active as well as highly stable. One direction to solve this issue is to understand elementary steps of electrode processes and build an unconventional strategy for a conversion of inert and, therefore, stable materials into efficient catalysts. Herein, we propose a simple concept for obtaining catalysts from inert and hence stable materials by forming their heterojunctions, namely, covering inert Au with corrugated carbon−nitrogen-based two-dimensional porous frameworks. It shows more than 10 times better activity for the hydrogen evolution reaction than for the pure Au surface, and it also demonstrates the high catalytic activity for the oxygen reduction reaction (ORR) via an effective four-electron reduction mechanism, which is different from the usual twoelectron reduction typical for ORR on Au surfaces. This activity induced by formation of a heterojunction was analyzed by a conjugation of computational and experimental methods and found to originate from alternative efficient reaction pathways that emerged by the corrugated porous framework and the Au surface. This work provides not only the method for creating active surface but also the knowledge on elementary steps of such complicated multielectron transfer reactions, thereby leading to intriguing strategies for developing energy conversion reactions based on materials which had never been considered as catalysts before. KEYWORDS: electrode process, carbon-, nitrogen-framework, oxygen reduction reaction, hydrogen evolution reaction, gold

U

reactions, we focus on the transformation of inert compounds into active catalytic surfaces because the dramatic transformation of properties is often a key to understand complex chemistry. Recent reports show that even typical inert materials can be highly active catalysts.11,12 This gave us hope to find undiscovered principles to emerge catalytic activities in inert compounds. Herein, we show a corrugated two-dimensional (2D) porous carbon-, nitrogen-framework/metal heterojunction system as a highly efficient electrocatalytic field (Figure 1). Our study indicates that a concert of corrugated porous structure and a metal can trigger efficient electrode processes. The main aim we pursue in the present study is to show that two inert compounds can form an unconventional heterostructure having

nderstanding the basic science of electrode processes related to energy storage/conversion reactions is still one of the most important scientific issues.1,2 In order to solve this, elementary steps in electrode processes should be clarified. A variety of experimental and theoretical studies have been carried out for two well-known reactions: hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), which are basics of other, even more complicated multistep, multielectron transfer reactions, such as CO 2 reduction and methane oxidation. However, even for such basic reactions as HER and ORR, the detailed understanding of their mechanism is still challenging due to the complex nature of the multistep reactions.3−7 Hence, finding a strategy to unveil the HER/ORR electrode process is key for the development of many electrochemical energy conversion systems.5−10 Typically, the improvement of ORR/HER is based on material systems which are already known to work. However, in order to acquire basic understanding of these © 2017 American Chemical Society

Received: November 15, 2016 Accepted: January 30, 2017 Published: January 30, 2017 1770

DOI: 10.1021/acsnano.6b07711 ACS Nano 2017, 11, 1770−1779

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RESULTS AND DISCUSSION Structure of Graphitic Carbon Nitride. Herein, we discuss the structure of the carbon nitride material through the analyses of electron diffraction, X-ray pair distribution function, and X-ray diffraction (XRD). Structures of carbon nitride materials are still open to debate29,30 and may depend on synthetic conditions such as temperature and environment (e.g., open/closed systems): our low-temperature reaction in an open system provides a light yellowish powder,27,28 which is different from the brown powder prepared at a higher temperature in a closed system.31 Our analyses described hereafter reasonably conclude that the yellow material is graphitic carbon nitride (gC3N4) having two-dimensional networks. First, as others reported, the Fourier transform infrared spectroscopy (FT-IR) confirms that the material consists of cyameluric units (C6N7) (peaks at 1640, 1560, 1450, 1410, 1320, and 810 cm−1; details are in Supporting Information, Figure S3). The composition is C 40 H 13 N 45 O 2 , which corresponds to C3N3.4H0.68·0.3H2O and suggests the formation of g-C3N4 with H-terminated defects and/or edges. The selected area electron diffraction (SAED) pattern (Figure 2A) indicates the material has six-fold symmetry. These data strongly suggest that graphitic networks were formed. The simulated pattern (Figure 2B) based on the g-C3N4 model (Figure 2C) can account for the experimental data well, though

Figure 1. Schematic image of a corrugated 2D porous framework/ metal heterojunction. Because of the corrugated and porous structure, spaces like small containers are formed between the framework and metal. The figure was drawn by VESTA.55

catalytically active sites and to unveil basic electrode processes of this type of heterostructures in order to understand what transforms inert material to active electrocatalysts. In general, a contact of two different flat and hard materials,13 which are typically based on metallic or inorganic materials, is known to exhibit properties different from each component.3,6 This idea was applied to electrocatalysts14 and showed that heterojunctions based on precious metals or atomically thin inorganic materials can improve catalytic activities.11,15−17 However, such hard and/or flat compounds applied to form the heterojunction in previous studies are less flexible at the mesoscopic scale, which is often a key for many efficient reactions.18,19 Materials without long-range ordering may have unique/better functions,20,21 as it is found in nature photosystem II, for example. For the formation of such mesoscopically ordered 2D heterojunctions, we focus on inert materials for ORR/HER: 2D carbon-, nitrogen-based porous frameworks,22 which are flexible because they are polymeric and atomically thin porous materials. Therefore, they are expected to form such unconventional heterojunctions that are accessible for reactants of the reactions, associated with providing an alternative reaction pathway. This strategy is different from the well-known synergic effects in oxide/metal catalysts, where metals are expected to modify the surface electronic structure of oxides in order to activate target processes.23 On the metal surfaces, the corrugated 2D porous materials can form large junction areas between the edges of the pores and the metal support, which can play a role in the catalytically active sites. In addition to this, in the case of the heterojunction based on corrugated 2D porous systems, there are spaces between corrugated frameworks and metals, which are similar to tiny containers where catalytic reactions can take place (Figure 1). This may be partially analogous to the interface effect known in nanocatalysis where reactions often occur at the perimeter interface area between the support and nanoparticles,24 however, in the case of the corrugated porous 2D materials, catalytic activity can be controlled by a combination of sites originating from a corrugated framework/metal interface, the size and composition of pores, rather than the size and composition of nanoparticles. Therefore, use of corrugated porous systems enables us to study the effect of surface modification toward electrocatalysis with a principally nontraditional approach. As a model compound, we selected Au and two-dimensional graphitic carbon nitride (2DCN) whic are less active for both the ORR and HER but stable in acidic media.25,26 We show that graphitic carbon nitride has a corrugated two-dimensional structure, and the heterojunction of 2DCN and Au (2DCN/Au) becomes active for multielectron transfer reactions.

Figure 2. Structure analyses of the carbon nitride material. (A) Experimental SAED pattern along the normal direction of a platelike particle. (B) Simulated ED pattern along the [001] direction of the structure model of graphitic carbon nitride shown in the panel (C). This structure is defined as a 1 × 1 × 1 cell. 1771


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Figure 3. (A) X-ray diffraction patterns of (i) experimental data (blue) and simulated patterns based on structure models: (ii) flat g-C3N4 model of 1 × 1 × 1 cell as shown, (iii) corrugated g-C3N4 model of 3 × 3 × 4 cell as shown, (iv) corrugated g-C3N4 model with a poor ab plane crystallinity. (B) X-ray pair distribution function. The data were analyzed by the reverse Monte Carlo method using the structure model consisting of four layers of A−A stacked graphitic carbon nitride layers having nine C6N7 units in each layer. This structure is a 3 × 3 × 4 cell as shown in (C).

packings. These features are consistent with the neutron PDF results reported previously.30 The broad peak around 2.9 Å corresponds to the diagonal C−N distance in C3N3 sixmembered rings, indicating that the cyameluric units are buckled as reported for poorly crystalline carbon materials, where graphitic networks are formed though their stacking is disordered and each layer is buckled.35 The PDF in the interatomic distance ranging from 0 to 13 Å was analyzed by the reverse Monte Carlo simulation,36 using a structure model consisting of four layers of A−A stacked graphitic carbon nitride layers having nine cyameluric units in each layer, as shown in Figure 3C (which corresponds to a 3 × 3 × 4 superlattice of the hexagonal unit cell (a = b = 20.55 Å and c = 12.96 Å) composed of a C6N7 unit shown in Figure 2C). This model can simulate the experimental PDF well (Figure 3B), and the obtained structure clearly illustrates that the material is composed of corrugated networks. The buckling appears random, and thus we consider that the material does not have exact periodicity (crystallinity) in-plane, though the connectivity (topology) may be repeated. Using this model, we simulated the XRD pattern using the Mercury program.37 The spherical particle model does not simulate the experimental pattern well, thus we used a uniaxial particle size model using the GSASII program (unique axis = 001 and equatorial size/ axial size ratio = 0.053),38 which can simulate the XRD pattern well (Figure 3A, bottom). The remaining mismatch between experimental XRD and the simulated one is probably attributable to rotational disorders as observed in the electron diffraction experiment (Figure 2A,B). This bulk graphitic carbon nitride (b-g-C3N4) was exfoliated to form 2DCN by following the previous studies.27,28 Using infrared spectroscopy, we confirm that the exfoliation process does not degrade the molecular network (Figure S3). The

the experimental one has streaks probably resulting from rotational disorders. The powder XRD pattern (Figure 3A) of our material is typical for the carbon nitride materials: a large peak exists at 27.5° (d = 3.25 Å) with its higher-order peak at 56.9° (d = 1.6 Å). Generally, the former peak is assignable to the interlayer spacing between the g-C3N4 layers, and the absence of a peak around 15° (d = 6.5 Å) also supports this assignment. There are, however, no XRD peaks assignable to in-plane periodicity of the hexagonal g-C3N4 structure; for example, the 11̅0 peak of the structure (a = 6.99 Å) shown in Figure 2C should be around 14.6° (d = 6.06 Å), but only a broad peak at 13.4° with a shoulder at 13.0° exists in the low-angle region. The d spacing (d = 6.60 Å) for this peak is definitely too large for the in-plane periodicity. Moreover, there are broad intensities due to diffuse scattering underneath the peaks from 10 to 35°. Thus, we used X-ray pair distribution function (PDF) analysis, which can determine local- to middle-range structure of such poorly crystalline or amorphous materials.32−37 The PDF provides information on the relationship between interatomic distance and pair density. The sharp peaks within 5 Å (Figure 3B) reflect the local connectivity of the carbon nitride (i.e., six-membered rings) as found by the IR spectra. The broad oscillation extending up to 30 Å (∼3.28 Å period) corresponds mainly to the interlayer spacing as the large Bragg peak in XRD data indicated. Not only the presence of peaks but also the absence of some peaks is informative; for example, the absence of peaks assignable to the atom pairs in different gC3N4 layers (e.g., 3.25 Å for the nearest interatomic distance between different layers in the A−A stacked model) indicates that the packing of the layers is disordered though the spacing is constant on average. Moreover, there are no sharp peaks attributable to in-plane, long-range orders and interlayer 1772


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effect indicated the well-dispersed state of 2DCN in water (Figure S2), and this can be uniformly distributed on the substrate by the drop-casting method (Figure 4B). This thickness is consistent with the interlayer spacing (d = 3.25 Å) in the bulk sample. The density functional theory (DFT) modeling (see Supporting Information for theoretical methods) of the 2DCN structure fully supports our experimental findings. As found experimentally, the 2D network of C6N7 units linked by nitrogen atoms is strongly corrugated due to rotations of C6N7 units with respect to each other. Such corrugation has been explained by nitrogen lone pair repulsion and a partial rupture of the π delocalization out of the C6N7 rings.39 Based on the above experimental facts and allowing structural relaxations in a hexagonal 2D structure consisting of nine C6N7(N) units results in considerable stabilization of the system if compared with the corresponding plane structure. The optimized lattice parameters a = b = 20.446 Å of the corrugated structure was 1 Å smaller than that of the plane (3 × 3) structure of C6N7(N) units. Our calculations demonstrate that distances between two nitrogen atoms placed at the center of C6N7 units are in the range of 6.73−6.93 Å, which agrees with our experimental results and with the in-plane period of 6.81 Å obtained from the XRD measurements by other studies. We found a good match between the optimized lattice parameter of the corrugated 2DCN structure and the (7 × 7) unit of Au(111), which is

2DCN was found to be 100−500 nm in width and 0.35 nm in average thickness by transmission electron microscopy (TEM) and atomic force microscopy (AFM) (Figure 4). The Tyndall

Figure 4. Representative (A) TEM and (B) AFM images of 2DCN. (C,D) AFM image of 2DCN showing an average thickness of 0.35 nm.

Figure 5. HER activity test with conjugation of electrochemistry and a first-principle calculation. (A) LSV at a scan speed of 1 mV/s (iR corrected) and (B) Tafel plots for Au, 2DCN/Au, and Pt. The measurements were carried out in Ar-saturated 0.05 M H2SO4 solution. (C) Free-energy diagram for HER on 2DCN/Au(111) calculated within DFT at equilibrium potential. The most stable configuration of H* on 2DCN/Au(111) (D) and the configuration of H* closest to the thermo-neutrality condition (E). Only area in the vicinity of one pore is shown for (D,E). (F) Most stable configuration of two H* adsorbed at the nearest sites at the edge of 2DCN pore on Au(111). Gold substrate is not shown for simplicity. Large distance between adsorbed hydrogen atoms (2.23 Å) prevents recombination via a proton discharge−chemical desorption mechanism. 1773


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ACS Nano Table 1. Summary of the HER Electrode Processes on Au, 2DCN/Au, and Pt catalyst

J0 (10−6 A cm−2)

η at 10 mA cm−2 (V)

Tafel slope (mV/dec)

α

mechanisma

Au 2DCN/Au Pt

4 32 404

−0.38 −0.21 −0.05

116 41 32

0.5 1.5 2.0

PD(RDS)−CD PD−ED(RDS) PD−CD(RDS)

a PD−CD, proton discharge−chemical desorption mechanism; PD−ED, proton discharge−electrochemical desorption mechanism; RDS, ratedetermining step.

k3

20.349 Å. This becomes a model surface to evaluate this heterojunction system in the ideal situation (Figure S6); therefore, 2DCN can be deposited on the Au surface with less noticeable lattice deformations, and this is the key for activation of the electrocatalytic reaction, as shown later. Controlled Kinetics of a Hydrogen Multistep Multielectron Process. First, the 2DCN/Au, 2DCN, and Au were examined for the HER (2H+ + 2e− → H2, the equilibrium potential = 0 V versus reversible hydrogen electrode (vs RHE)) in order to understand the change of electrode processes. We selected Ar-saturated 0.05 M H2SO4 solution (pH 1) because we can obtain an intrinsic value for reaction kinetics.40 In our system, the Tafel data were reproducible between independent and sequential measurements in a steady-state condition (Figure S18). The liner sweep voltammetry (LSV) on a rotating disk electrode (RDE) at a rotating speed of 1600 rpm and at a scan rate of 1 mV/s showed an activity of 2DCN/Au much higher than that of Au and 2DCN (Figure 5 and Table 1). In order to test the activity of 2DCN, we used a glassy carbon electrode. The loading of 2DCN for each electrode was fixed to 1 μg. We confirmed that the 2DCN is available even after it underwent electrochemical measurements (Figure S7). We included the results of the Pt electrode since it has been well studied, thus Pt is an ideal electrode to confirm reliability of our experiments and to discuss kinetics and mechanisms of the electrode processes by comparing previous data.9,41 The transfer coefficient (α) was obtained from the Butler−Volmer equation with the quasi-equilibrium assumption for the proton discharge step.8 The overpotential (η) of 2DCN/Au for the HER is −0.21 V at 10 mA cm−2, and η = −0.38 V at 10 mA cm−2 for the Au (Figure 5A). Indeed, the exchange current density (J0) for the reaction was also dramatically improved from 4 × 10−6 A cm−2 for Au to 3 × 10−5 A cm−2 for 2DCN/ Au, showing 1 order of magnitude higher J0 (Table 1). In order to understand the principle behind this change of kinetic values, the electrode processes of both systems were characterized with conjugation of a physical chemistry approach and first-principle calculations. The Tafel slope (TS) for Au in acid solution was measured to be ca. 120 mV/dec, which agrees well with previous reports;41 thus the electrode process is governed by proton discharge as the rate-determining process (α = 0.5), and the overall mechanism can be described as the proton discharge (rate-determining step) followed by chemical desorption. However, it was measured that TS = 40 mV/dec (α = 1.5) for the 2DCN/Au. Note that the α = 1.5 for the HER is possible only if the overall reaction can be described by proton discharge followed by rate-determining electrochemical desorption. The proton discharge−electrochemical desorption (RDS) mechanism in acidic conditions can be described with the following two steps

S(e−) + H+ + SH → 2S + H 2↑

where S(e ) denotes the free sites on an electrode in order to discharge protons. Here, we introduce the Butler−Volmer equation for step 2. i = 2Fk 3c H+k′θe−βFη / RT

where k′θ is the concentration of adsorbed hydrogen atoms at a coverage of θ and k′ is the concentration when θ = 1. The F, cH+, R, T, η, and β indicate Faraday constant, concentration of proton, gas constant, temperature, overpotential, and symmetric factor, respectively. With a quasi-equilibrium assumption to step 1, we obtain θ=

k2

k1c H+e−Fη / RT k1c H+e−Fη / RT + k 2k′

≅

k1c H+e−Fη / RT k 2k′

(k1c H+e−Fη / RT ≪ k 2k′at low η)

Therefore, the Butler−Volmer equation for step 2 can be written i = 2F

k1k 3 (c H+)2 e−(1 + β)Fη / RT k2

Hence α = 1 + β = 1.5 (β = 0.5). Furthermore, we found that the HER activity of 2DCN/Au depends on the loading of 2DCN (Figure S8). This result suggests that the formation of the 2DCN/Au heterojunction is key for emergence of high activity. From these results and considerations, it was indicated that the activation of the HER on the 2DCN/Au surface emerges by the change of electrode process associated with modification of the catalytic surface. The quantum chemistry approach gives a clear view for the above description. As mentioned before, the corrugation of 2DCN on the Au surface is a key feature that defines the unique catalytic activity of the 2DCN/Au system. A network of C6N7(N) units forms porous a 2DCN structure with six pyridinic-nitrogen atoms at the edge of each pore. It is wellknown that the pyridinic-N moiety in graphene often exhibits high chemical and catalytic activity.42−46 However, in the case of the 2DCN/Au system, each of the pyridinic-nitrogen atoms at the edge of the pore is located at considerably different distances from the Au surface. Thus, typically two pyridinicnitrogen atoms possess strong chemical bonding with the surface Au atoms, forming three kinds of states; first, it forms N−Au bonds with a length of 2.6−3.0 Å; second, another pair of N atoms is located at the intermediate distances of 3.6−4.0 Å from the surface; and finally, the third pair of N atoms is directed upward at large distances 4.6−5.3 Å from the surface (Figure S9). The chemical activity of the above-mentioned pyridinic-N centers in 2DCN is tuned by the strength of the interaction with the gold surface as presented in the free-energy diagram for HER with DFT at equilibrium potential (Figure 5C). Therefore, each pore in 2DNC on Au can provide a

k1

S(e−) + H+ ⇄ SH

(step 2)

−

(step 1) 1774


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Figure 6. ORR activity test with conjugation of electrochemical experiments and a first-principle calculation. (A) LSV at a scan speed of 5 mV/ s and a rotating speed of 1600 rpm (iR corrected). (B) Tafel plots and (C) electron transfer number (n) for Au and 2DCN/Au. (D) H2O yield (%) obtained by the RRDE technique. The experiments were carried out in O2-saturated 0.05 M H2SO4 solution. (E) Free-energy diagram for ORR on the 2DCN/Au(111) (solid line) and Au(111) (dashed line) surface. Inset shows optimized geometry of O* adsorbed on the 2DCN/ Au(111) heterojunction.

with the Au surface (Figure 5E). In this case, ΔGH* = −0.18 eV. In the configuration closest to the thermo-neutrality (ΔGH* = −0.02 eV), H* adsorbs on the N atom, which directly interacts with the surface Au atom (Figure 5E). Such an adsorption site guarantees the best kinetics for the HER, and it provides an electron transport from the gold electrode to the adsorbed proton due to orbital mixing of interacting Au and N atoms. Our calculations demonstrate that reactions over several sites are nearly close to the thermo-neutrality condition,47 suggesting that the 2DCN/Au(111) heterojunction can be a good electrocatalyst for HER. On the other hand, the pure Au(111) surface is a poor catalyst for HER because hydrogen is unstable on the surface (ΔGH* is positive); therefore, proton transfer is difficult, and bare 2DCN is not active for HER because it cannot provide the electron transport to the catalytically active sites. Thus, the 2DCN system itself provides

variety of non-equivalent catalytic sites with different activity. The considered model 2DCN/Au(111) system contains nine non-equivalent pores per unit cell due to the lack of periodicity within the cell. Hence, 2DCN/Au(111) heterojunctions possess a unique ensemble of non-equivalent sites with different catalytic activity. Contrary to the ideal metal surface, where only one or a few adsorption geometries for H* are possible, 2DCN/Au(111) provides a large variety of nonequivalent adsorption sites, with ΔGH* dispersed in the wide energy range of −0.20 to +0.45 eV. In order to get deep inside of the HER mechanism on the 2DCN/Au(111) heterojunction, we have analyzed preferable adsorption sites of H*. Figure 5D,E demonstrates the most stable configuration of H* on 2DCN/Au(111) and the configuration closest to the thermoneutrality condition. It is seen that H adsorbs most strongly on the N atom located at the edge of pore and does not interact 1775


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ORR process along the possible reaction pathways (Figure 6E). Similar approaches were used by previous studies.49−52 Due to the large complexity of the considered 2DCN/Au(111) system, having a corrugated surface which has more than 100 initial configurations for each intermediate, it has been optimized on the surface to find preferred adsorption sites. Optimized geometries of the most stable ORR intermediates are shown in Supporting Information (Figures S12−S15). Figure 6E demonstrates the change in free energy, ΔG, calculated for ORR on 2DCN/Au(111) (solid line) and Au(111) systems with an assumption of independent adsorption of ORR intermediates.11 Here, ΔG includes zero-point energy corrections, entropy, and solvent effects on the adsorption energies of all ORR intermediates as described in previous works.52 Only the most stable configurations of the adsorbed ORR intermediates are considered for energy analysis. Adsorption and activation of the molecular oxygen is the first and one of the most important steps of the complicated multistep ORR process.6,53 Such adsorption is accompanied by the first electron transfer from the metal to the antibonding 2π* orbital of the adsorbed oxygen. It is well-known that O2 binds weakly to the pure Au(111) surface.25 Our DFT calculations demonstrate that adsorption energy of O2 on the 2DCN/ Au(111) system is very similar to that of the pure Au(111) surface. Analysis of the possible adsorption sites shows that oxygen in the highly activated state adsorbs directly on the Au(111) surface in the pores of 2DCN, while the corrugated 2DCN layer itself remains inert for O2 adsorption. Figure 6E demonstrates that, in the case of the pure Au(111) surface, ORR proceeds only via the two-electron mechanism with reduction of the OOH* intermediate to H2O2, whereas the four-electron process is not energetically favorable, due to the stability of OOH* toward dissociation to OH* and O* on Au(111). The stability of OOH* lowers the O−O bond energy, and this appears as a result of a weak binding of O* on the Au(111) surface. However, the 2DCN/Au(111) heterojunction provides a large variety of active sites at the edge of pores and space originated by the corrugated structure (Figures 1 and 3 and Figures S14 and S15), where O* can be stabilized, promoting OOH* dissociation. An appropriate stability of O* is key for undergoing a four-electron process.54 Such an effect enables the effective 4e− pathway for ORR with formation of H2O. Thus, the 2DCN/Au(111) heterojunction provides complementary adsorption sites at the 2DCN/metal interface, where O2 adsorbs on the metal surface, O* bridges the 2DCN and Au(111) surface, while OH* adsorbs at the edge of 2DCN pores.

energetically favorable adsorption sites for HER, while the 2DCN−Au heterojunction enables the electron transfer to the catalytically active sites due to the Au−N interaction. Therefore, the effective proton discharge−electrochemical desorption mechanism is possible only at the 2DCN/Au heterojunction. Analysis of the most stable adsorption sites for hydrogen on 2DCN/Au(111) shows that H* + H* recombination on the surface is not likely to happen. These adsorption sites are located at the edge of pores in 2DCN, which are strongly corrugated. Such corrugation results in large H*−H* interatomic distances, even when both H* species are located at the nearest adsorption sites (Figure 5F), preventing hydrogen recombination via a chemical desorption mechanism. Thus, we suggest that HER on 2DCN/Au(111) occurs via the proton discharge−electrochemical desorption mechanism (rate-determining step), in full accord with our experimental findings (Table 1). As such, it was clearly shown that the corrugated feature of 2DCN is a key to understand the catalytic activity of 2DCN/Au heterojunctions for electrocatalytic reactions. Controlled Kinetics of the Oxygen Multistep Multielectron Process. The ORR activity of catalysts was obtained in O2-saturated 0.05 M H2SO4 solution using a LSV on a RDE at a rotating speed of 1600 rpm and at a scan rate of 5 mV/s (Figure 6 and Table 2). The equilibrium potential of the ORR Table 2. Summary of the ORR Activities and Kinetics on Au and 2DCN/Au catalyst

J0 (10−11 A cm−2)

V at 1 mA cm−2 (V vs RHE)

Tafel slope (mV/dec)

n

Au 2DCN/Au

3 20

0.17 0.40

131 118

1.9 3.6

with four-electron transfer (O2 + 4H+ + 4e− → 2H2O) is 1.23 V versus RHE. The 2DCN/Au catalyst could supply 1 mA cm−2 of current density at 0.4 V versus RHE and 0.17 V versus RHE for the Au catalyst (Figure 6A). Indeed, 2DCN/Au showed a sufficient stability on ORR (Figure S19). The 2DCN was confirmed to be inactive for the ORR, which agrees well with our theoretical considerations shown later. The ORR J0 was obtained by Tafel plots (Figure 6B). Surprisingly, by just forming the 2DCN/Au heterojunction, the J0 for the ORR was improved from 3 × 10−11 to 2 × 10−10 A cm−2, which is again the enhancement of 1 order of magnitude (Table 2). A further important indication was obtained by checking the electron transfer number per dioxygen molecule (n), which can be obtained by Koutecky−Levich (K−L) equation and K−L plots (Figure 6C and Figures S10 and S11).48 This result shows the dramatic change of the ORR electrode process in 2DCN/Au. The Au electrode is well-known to show a two-electron transfer reaction (n = 2). On the other hand, the 2DCN/Au electrode can process the four-electron transfer reaction by calculating n and measuring H2O yield (%) by using the rotating ring-disk electrode (RRDE) setup (Figure 6C,D). This is a quite impressive change since the inert catalytic surface of Au was converted to an active surface with the change of the electrode process by just forming a heterojunction with a corrugated porous 2D material. The first-principle study was carried out for further understanding of this phenomenon via investigation of the adsorption preferences of O2, OOH, O, and OH intermediates for a model catalyst and analysis of the overall energetics of the

CONCLUSION It was shown that the formation of a heterojunction with inert materials having corrugated surface can activate the key electrochemical reactions by using Au and 2DCN as model compounds. This finding is important for advancing the electrocatalysts because these results serve as a fundamental platform in order to obtain highly efficient electrode processes. We show that an inactive surface can have functions for specific electrochemical reactions by choice/combination of corrugated porous frameworks and metals in order to form a heterojunction having appropriate pore structures and spaces formed by corrugated structures, which work as reaction sites. Therefore, with this corrugated porous 2D framework/metal heterojunction strategy, it could be possible to find a catalyst system which could allow inert materials to have a high 1776


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where Id is disk current, Ir is ring current, and N is a collection efficiency which was determined to be 0.31. Therefore

electrocatalytic activity. In addition to using 2D materials, the possibility to form a heterojunction can be expanded by using other modern materials having unique geometrical structures, for instance, many other covalent organic frameworks and metal−organic frameworks.

H 2O yield (%) = 100 (%) − H 2O2 yield (%) PDF Analysis and RMC Method. The X-ray total scattering data were collected with Ag Kα (λ = 0.560883 Å) operated at 50 kV, 40 mA using a Rigaku Rapid-S diffractometer at room temperature (27 °C). The detector distance was calibrated using a NIST standard CeO2 (674B, a = 5.411651(6), phase purity 91%). The samples were sealed in 1 mmϕ Lindeman glass capillaries (Hilgenberg) in air. The data were collected on a curved imaging plate for 2 h, and more than 24 data were integrated to obtain sufficient intensities at the magnitude of scattering vector, Q, of 20.8 Å−1. Background subtraction, X-ray polarization correction, absorption correction, and Compton scattering correction were performed using the PDFgetX2 program. The structure functions, S(Q), were further treated to remove the slowly changing oscillation, which is not associated with the structure of the materials. In brief, the signal causing termination ripples within 1.0 Å was filtered out to form smooth PDF data, which was composed of a liner function of −4πρ0r (where ρ0 is the number density of atoms in the system and r is the interatomic distance) and oscillation from the first peak located around 1.3 Å simulated as the combination of a Gaussian function and a sinc function. In order to suppress the truncation ripples, S(Q) was treated using a revised Lorch function as reported by Soper and Barney (Δ = π/Qmax was used). Finally, the structure functions (Qmax = 20.7 Å−1) were converted into PDFs through the discrete sine transform:

METHODS Material Synthesis. The graphitic carbon nitride (g-C3N4) was obtained by thermal condensation of melamine (TCI; >98%), which is a well-known method to synthesize this compound. In brief, 1 g of melamine was put into a ceramic container and was left into an oven in N2 atmosphere. The oven was continuously heated under N2 flow at 5 K/min to 550 °C. The final temperature was kept for 1 h. The 2D gC3N4 was obtained by following protocols of the previous report: the bulk g-C3N4 was added in the Milli-Q water, and the mixture was sonicated for 5 days. Electrochemistry. The electrochemical measurement was conducted using a RDE and RRDE setup with an electrochemical measurement system (HZ-7000, HOKUTO DENKO) in a custommade two-compartment electrochemical glass cell at 298 K. The cells were cleaned by boiling in a mixture of 1:1 concentrated sulfuric and nitric acids and before each experiment by boiling in ultraclean water (Milli-Q water). The three-electrode setup consisted of a high-surface carbon-based counter electrode, a Ag/AgCl (NaCl sat.) reference electrode, and a commercial fixed glassy carbon or Au or Pt working electrode (HOKUTO DENKO) with a diameter of 0.5 cm (0.196 cm2). The 2DCN/Au electrode can be obtained by a simple dropcasting method. We prepared a 2DCN solution in Milli-Q water with a concentration of 1 g/L, and we dropped this solution on electrodes. Then, the electrodes were dried slowly at 20 °C in order to form homogeneously dispersed 2DCN on substrates. All measurements were carried out in 0.05 M H2SO4 solution (pH 1). The solution was bubbled with O2 or Ar for 30 min before the ORR or HER experiments to prepare the oxygen-saturated condition or argonsaturated condition, respectively. A reversible hydrogen electrode (RHE) potential can be calculated by following equation:

G(r ) = 4πρ0 r(g (r ) − 1) =

2 π

∫Q

Q max

Q (S(Q ) − 1)sin(Qr )dQ

min

where G(r) is the reduced pair distribution function and g(r) is the pair distribution function. The structures of the materials were analyzed by the real-space Rietveld analysis using the PDFgui program in the r range of 0−30 Å. For the RMC method, we employed the flat structure with four layers (C96N128) as the initial structure.

V vs RHE = V vs Ag/AgCl(NaCl sat.) + 0.197(V)

ASSOCIATED CONTENT

+ 0.059 × pH(V)

S Supporting Information *

In this report, the ORR limiting current was not observed. Therefore, the Tafel slopes were obtained by calculating the kinetic currents (Jk) from K−L plots (1/J vs 1/ω0.5) or analyzing the kinetic control regions. The log Jk versus overpotential was plotted as Tafel plots. K−L plots can be obtained by the Koutecky−Levich equation:

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07711. Standard analytical techniques and theoretical methods (PDF) X-ray data for C3N4 (CIF)

1/J = 1/Jk + 1/Bω0.5

AUTHOR INFORMATION

Jk = nFkcO2

Corresponding Author

*E-mail: [email protected].

where J is current density, n is electron transfer number, F is Faradaic constant, k is reaction rate constant, cO2 is the concentration of dissolved oxygen, B is the Levich factor, ω is rotation rate. B can be written:

ORCID

Ken Sakaushi: 0000-0003-4797-9087 Andrey Lyalin: 0000-0001-6589-0006 Tetsuya Taketsugu: 0000-0002-1337-6694

B = 0.620nFcO2D2/3ν−1/6

Author Contributions

where D is oxygen diffusivity and ν is the solution’s viscosity. Tafel plots were obtained by plotting the kinetic currents of the ORR (Jk) by using the above equations. The RRDE tests were carried out using a RRDE setup (HOKUTO DENKO). The disk electrode was scanned at 10 mV/s, while the ring electrode was kept at 1.2 V vs RHE. The n and H2O2 yields were determined by the following equations:

K.S. conceived the idea, organized the project, and performed material synthesis, initial characterization of materials, and electrochemical experiments. A.L. performed the first-principle calculations. S.T. performed the PDF analysis and other methods in order to analyze the structure of materials. K.S., A.L., and K.U. discussed the results of the electrochemistry. K.S. and S.T. discussed the crystal structure of the material. K.S., A.L., and S.T. discussed the whole results and wrote the manuscript. All authors discussed and commented on the manuscript.

n = 4{Id /(Id + Ir /N )}

H 2O2 yield (%) = 200 × Ir /N /(Id + Ir /N ) 1777


Article

ACS Nano Notes

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The authors declare no competing financial interest.

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Two-Dimensional Corrugated Porous Carbon-, Nitrogen-Framework

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