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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Vertically Aligned N‑Doped Diamond/Graphite Hybrid Nanosheets Epitaxially Grown on B‑Doped Diamond Films as Electrocatalysts for Oxygen Reduction Reaction in an Alkaline Medium Shuguang Wang,† Xixi Ji,† Yu Ao, and Jie Yu*

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Shenzhen Engineering Lab for Supercapacitor Materials, Shenzhen Key Laboratory for Advanced Materials, and Department of Material Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen 518055, China S Supporting Information *

ABSTRACT: Diamond/graphite hybrid nanosheets (DGNSs) have been epitaxially grown on boron-doped diamond (BDD) films from CH4/H2 mixture gas by microwave plasma chemical vapor deposition. The DGNSs are vertically aligned on the crystal facets of the BDD films uniformly, densely, and orderly. The DGNSs are composed of the core diamond sheets and the surface graphitic overlayers, which possess an open edge structure. By posttreatment in NH3 atmosphere in a microwave plasma or a tube furnace, the N-doped DGNSs (NDGNSs) were obtained. The electrocatalytic performance toward oxygen reduction reaction (ORR) for the DGNSs was greatly enhanced after doping with N, and the doped pyridinic N contributes more to the ORR. The electrocatalytic activity for ORR of the NDGNSs doped at 650 °C in NH3 in a tube furnace is the highest in all of the samples, which is comparable to the commercial Pt/C. The present work provides a novel electrocatalyst for the ORR with high performance. KEYWORDS: diamond/graphite hybrid nanosheets, electrocatalysts, NH3, nitrogen-doping, oxygen reduction reaction

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INTRODUCTION A fuel cell is highly attractive because of the high energy density, high energy efficiency, and environmental friendliness.1−3 Oxygen reduction reaction (ORR) in a cathode is crucial to the operation of the fuel cells, which relies on the performance of the electrocatalysts used.4−8 Conventionally, Pt is usually used as the electrocatalyst for ORR as it possess high catalytic activity.8−10 However, Pt is expensive and rare, which hinders its wide application.8,11 Simultaneously, the electrocatalysts based on Pt will be deactivated when CO and methanol are present.8,12 Consequently, developing new ORR electrocatalysts, which are cheap, highly active, and highly tolerable to CO and methanol, is of great importance. Up to now, Pt-free electrocatalysts such as metal chalcogenides,13 metal oxides,4,14,15 nonprecious metals (Fe, Co),16,17 and carbon materials doped with different atoms (e.g., B, N, F, S, and P)7,18−25 have been demonstrated to have electrocatalytic activity for ORR. Among these Pt-free catalysts, N-doped carbon materials, for example, carbon nanotubes (CNT),2 graphene,5,6,22 carbon nanofibers,19,26 and porous carbon,23−25,27 possess high electrocatalytic activity because of their superior electronic properties. Importantly, N-doped carbon materials are highly tolerant to CO poisoning and crossover effects, and exhibit high operation stability.6,12 Resultantly, the N-doped carbon materials are of high promise as low-cost cathodic catalysts for fuel cells. © XXXX American Chemical Society

Nowadays, vertical carbon nanosheets such as graphene nanosheets and diamond nanosheets have been attracting great interest for their high surface area, conspicuous edges, interlaced porous structure, and high mechanical and chemical stability, which make them promising for application in energy storage,28,29 chemical sensing,30,31 catalysis,32,33 and field emission.34,35 Compared with the vertical graphene nanosheets, the vertical diamond nanosheets have been much less investigated. Chen et al. have used Ni, Fe, or Au/Ge films as catalysts to prepare the diamond nanosheets by microwave plasma chemical vapor deposition (MPCVD).36−38 Later, the same group also prepared the diamond nanosheets on nanocrystalline diamond substrates without using catalysts.39 In 2008, Raina et al. prepared the vertical diamond nanosheets (ridge profile) by introducing N2 during the growth process.31,35 Recently, we revealed that the diamond nanosheets arise from epitaxial growth of {111} planes of diamond substrates, resulting in the formation of highly ordered nanosheets arrays.40 Generally, the diamond nanosheets are grown accompanied by the graphitic overlayers, forming a diamond/graphite core/shell hybrid structure. Different applications such as heat dissipation,41 chemical sensing,31 Received: April 15, 2018 Accepted: August 7, 2018 Published: August 7, 2018 A

DOI: 10.1021/acsami.8b06101 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) SEM image of the BDD films. (b, c) SEM images of the DGNSs grown on BDD films. (d) Cross-section structure of the BDD/ DGNSs films. (e, f) SEM images of the NDGNSs doped in plasma (e) and NDGNSs doped in a tube furnace at 650 °C (f).

field emission,35,39 and cell culturing42 have been reported for the diamond/graphite hybrid nanosheets (DGNSs). A potential application for the DGNSs is electrocatalysis toward the ORR after doping with nitrogen. Nevertheless, as far as we know, this has not been investigated previously. Diamond has been intensively investigated for the application of electrocatalysis because of its chemical inertness, high corrosion resistance, high stability in oxygen atmosphere, high thermal stability, and low background current. An early research indicated that the diamond electrode possesses much higher chemical inertness and corrosion resistance than graphite and glassy carbon (GC) during potential cycling in an acidic solution.43 The diamond materials reported for the electrocatalysis application include CVD-deposited B-doped diamond (BDD) films,44 BDD particles,45,46 BDD rod array,47 diamond-encapsulated glassy carbon particles,48 and grapheneencapsulated diamond particles,49−52 and so forth, which show higher stability than their sp2 bonded graphitic counterparts for both the electrocatalyst and the electrocatalyst support applications. Among the above diamond-based materials, the graphene-encapsulated diamond particles are prepared by annealing the diamond particles in vacuum at high temperature, which have a certain similarity in structure to the DGNSs. Compared with the graphene-encapsulated diamond particles and other diamond-based materials mentioned above, the DGNSs possess prominent advantages such as free of agglomeration, abundant and fully exposed graphitic edges, and strong adhesion with the substrates. For the DGNSs, the surface graphitic layers with open edges favor formation of pyridinic N or pyrrolic N,6,29,40,53 which are favorable for the ORR electrocatalytic activity.26,53 Therefore, we expect that the DGNSs may be a good electrocatalyst toward the ORR. Here, we report preparation, N doping, and ORR electrocatalysis of the DGNSs. The DGNSs were grown by MPCVD on BDD films. The N doping was carried out by posttreatment in NH3. The N-doped DGNSs (NDGNSs) exhibit high electrocatalytic performance for ORR in an alkaline medium,

which is comparable to the commercial Pt/C. The present work provides a novel candidate for the ORR electrocatalysts.

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EXPERIMENTAL SECTION

Synthesis of BDD Films and NDGNSs. BDD films were deposited using a 2.45 GHz MPCVD system on Mo plates. The Mo plates were first abraded for 5 min with diamond paste, followed by cleaning in deionized water and acetone under ultrasonic vibration successively. Then, the substrates were put into the MPCVD system for deposition. During deposition of the BDD films, the parameters for nucleation and growth were different. In the nucleation step, the microwave power of 550 W, H2 flow rate of 200 sccm, CH4 concentration of 3.8%, substrate temperature of 750 °C, chamber pressure of 8 kPa, and duration time of 1 h were adopted. In the growth step, the above parameters were 800 W, 180 sccm, 1%, 900 °C, 20 kPa, and 8 h, respectively. Boron oxide (B2O3) dissolved in ethanol with a molar ratio of 1% was used as the boron source for growing the BDD films. During the growth step, ethanol vapor containing boron oxide was brought into the chamber by 20 sccm H2 flow. Subsequently, the DGNSs growth was carried out with the above parameters changed to 1000 W, 200 sccm, 17.4%, 1100 °C, 24 kPa, and 0.5 h and no introduction of boron oxide. When cooled to room temperature, the BDD films peeled off from the Mo plates automatically, obtaining freestanding BDD/DGNSs films. The N doping was carried out in the following two ways. The first one is treating the BDD/DGNSs films in a tube furnace at 600, 650, 700, and 800 °C in NH3 atmosphere for 60 min. The second one is in situ treating the BDD/DGNSs films in NH3/H2 (20/100 sccm/sccm) plasma at 600, 650, and 700 °C by MPCVD for 60 min. Characterization. The structure and composition of the samples were characterized by the methods described in our previous reports.26,40 A resistance measuring system (2182A, Keithley) was used to determine the electrical conductivity of the samples. Electrochemical Tests. The electrochemical tests were carried out on an electrochemical workstation (CHI760C). During the tests, the BDD/NDGNSs films acted as a working electrode, platinum wire acted as a counter electrode, and Ag/AgCl electrode (saturated KCl, 0.965 V vs reversible hydrogen electrode (RHE) in 0.1 M KOH) acted as a reference electrode. The preparation process of the working electrode is illustrated in Figure S1. The BDD/NDGNSs films B


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Figure 2. (a) TEM image of a DGNS standing on the TEM sample holder. (b) HRTEM image taken from the square area in panel a. (c) TEM image of a DGNS lying flat on the TEM sample holder. (d) HRTEM image taken from the square area in panel c. electrode was suspended in an electrochemical cell by a metal wire for cyclic voltammogram (CV) tests. The BDD/NDGNSs films were adhered onto glassy carbon (GC) disk electrodes (4 mm in diameter) using nafion solution (5%) for rotating-disk electrode (RDE) and rotating-ring-disk electrode (RRDE) tests. For comparison, the electrochemical performance of commercial 20% Pt/C powder at a mass loading of 150 μg cm−2 on a GC electrode was also tested. The electron transfer numbers (n) during the ORR and the kinetic limited current density (Jk) were calculated from Koutecky−Levich (K−L) equation.4,6 The details for the calculation have been described in our previous report.26 During calculation, the current density J was first determined by subtracting the current density measured in N2-saturated electrolyte solution from that measured in O2-saturated electrolyte solution under identical scan rate and rotation speed for the different samples.

structure of the NDGNSs has no pronounced change (Figure 1e,f). In our previous report, the DGNSs were grown on undoped diamond films.40 The present results indicate that the DGNSs can also be successfully grown on the BDD films. The purpose of doping is to make the diamond films electrically conductive for the electrocatalysis application. For verifying boron doping effect, we measured the electrical conductivity of both the undoped diamond and BDD films. During measurements, no current response was observed for the undoped diamond films, indicating that the undoped diamond films are highly insulative. After B doping, the electrical conductivity increased drastically to 13.5 S cm−1, demonstrating the good effect of B doping in increasing the electrical conductivity of the diamond films. Transmission electron microscopy (TEM) analysis was conducted to characterize the structure of the DGNSs. The DGNSs scraped off from the BDD films were used as the TEM samples. Figure 2a,b show the TEM images of a DGNS standing on the TEM sample holder. It can be seen that the DGNSs show a conical edge structure with a thickness of 23 nm at the middle and 9.8 nm at the edge. The high-resolution TEM (HRTEM) image reveals that the structure of the surface part and the inner part is different for the DGNSs and the interplane spacing of the surface part is 0.35 nm, which corresponds to the graphite (002) planes. The surface graphitic layers are open at the edge position, which could serve as active sites absorbing various atoms or species and emitting electrons.29,30 According to our previous results, the inner part is diamond.40 The thicknesses of the inner part and the surface graphitic layer are about 3.5 nm and several to more than 10 nm, respectively. Figure 2c,d show the TEM images of a DGNS lying flat on the TEM sample holder. The fringe spacing of the DGNS is ∼0.21 nm, which is similar to the interplanar spacing of diamond {111} planes, confirming that

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RESULTS AND DISCUSSION The typical scanning electron microscopy (SEM) image of the BDD films grown on Mo substrates is shown in Figure 1a. The BDD films consists of randomly oriented crystal grains with triangle {111} and square {001} facets, and the size range of the grains is 10−20 μm. Figure 1b,c show the SEM images of the vertical DGNSs after growth of 30 min on the BDD films. It is observed that all of the BDD films are uniformly covered with the DGNSs. The DGNSs are interlaced with each other, forming a porous structure. The spacing between the DGNSs is from 30 to 500 nm. Interestingly, the DGNSs show ordered arrays on the facets of the diamond grains, which is caused by the epitaxial growth of the substrate.40 The SEM image of the cross-section of the BDD/DGNSs films (Figure 1d) indicates that the DGNSs form 70.5° angles with the crystal facet of the BDD film substrates. According to our previous results, the underneath crystal facet is diamond (111) plane.40 For convenience, we still call these inclined nanosheets vertical DGNSs. These vertical DGNSs are parallel to each other with an average height of 2.4 μm. After doping with nitrogen, the C


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DGNSs/BDD films shows three prominent peaks at ∼1353, ∼1582, and ∼2708 cm−1, being the D, G, and 2D band of graphite, respectively.6,29 For the NDGNSs/BDD films, the three peaks slightly shift to ∼1351, ∼1581, and ∼2714 cm−1, respectively. The full width at half-maximum (FWHM) and the intensity ratios of G to D peak (IG/ID) for both samples were calculated and shown in Table S1. After doping with N, the values of the FWHM of all of the peaks became larger, and the IG/ID ratio decreases from 2.07 to 0.85, which may be caused by the defect increase due to the incorporation of N atoms into the graphitic lattice.29 It is noted that for the DGNSs/BDD and NDGNSs/BDD films the Raman peaks of diamond disappeared because of the presence of the surface graphitic layers. To analyze the N doping concentration and states, X-ray photoelectron spectroscopy (XPS) measurements were carried out on the N-doped samples. Figure 4a shows the XPS survey spectra, where a C1s peak (285 eV), a weak N1s peak (400 eV), and an O1s peak (538 eV) appear for all of the samples, indicating that N has been doped into the DGNSs.6,22 As the NDGNSs/BDD films doped in a tube furnace at 650 °C show the best electrocatalytic performance in all of the samples, the XPS results of the NDGNSs/BDD films doped in plasma at

the inner part is diamond. Similarly, it is observed that the edge is bared without graphitic layers (Figure 2d). Figure 3a displays the X-ray diffraction (XRD) patterns of the BDD, DGNSs/BDD, and NDGNSs/BDD films doped in a

Figure 3. XRD patterns (a) and Raman spectra (b) of BDD, DGNSs/ BDD, and NDGNSs/BDD films doped in a tube furnace at 650 °C.

tube furnace. As the NDGNSs/BDD films doped in a tube furnace possess much better electrocatalytic performance than those doped in plasma, the following characterization will focus on the samples doped in a tube furnace. The BDD films exhibit the typical pattern of diamond with three peaks corresponding to (111), (220), and (311) planes, respectively. After growing the DGNSs, a new peak appears at 26.3°, corresponding to the (002) plane of graphite. The appearance of this weak peak is due to the presence of the surface graphitic layers of the DGNSs. After doping with nitrogen, the pattern does not change much. Raman measurements were made on the BDD, DGNSs/BDD, and NDGNSs/BDD films, as shown in Figure 3b. The Raman spectrum of the BDD films shows the intense and narrow diamond peak at 1328 cm−1. The weak and wide peaks around 500 cm−1 (peak A) and 1220 cm−1 (peak B) are related to the phonon scattering by structural modifications and the locally disordered crystal structure induced by the boron doping, respectively.54 The Raman spectrum of the

Figure 4. XPS survey spectra (a) and N1s spectra (b) of the NDGNSs doped in plasma at 650 °C for 60 min (I) and in a tube furnace at 600 °C (II), 650 °C (III), 700 °C (IV), and 800 °C (V) for 60 min. D


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Table 1. Fitting Results of the XPS N1s Peaks and the ORR Performance of the Undoped DGNSs, NDGNSs Doped in Plasma at 650 °C, and NDGNSs Doped in a Tube Furnace at 600, 650, 700, and 800 °Ca samples undoped doped in plasma 600 °C 650 °C 700 °C 800 °C

CT (atom %) 0.40 1.57 1.92 1.63 1.20

CPD (atom %) 0.21 1.16 1.41 1.26 0.65

CPR (atom %) 0.16 0.41 0.51 0.32 0.40

CG (atom %)

EP (V)

IP (mA cm−2)

Eonset (V)

0.03 0 0 0.05 0.15

−0.317 −0.241 −0.194 −0.164 −0.185 −0.240

−0.10 −0.32 −0.78 −0.98 −0.84 −0.62

−0.185 −0.147 −0.078 −0.037 −0.075 −0.119

a

The peak potential and peak current density were obtained from the CV curves, and the Eonset was obtained from the RDE curves. (CT, total N concentration; CPD, pyridinic N concentration; CPR, pyrrolic N concentration; CG, graphitic N concentration; EP, peak potential; IP, peak current density.)

Figure 5. (a) CV curves of the undoped DGNSs (I), NDGNSs doped in plasma at 650 °C (II), NDGNSs doped in a tube furnace at 650 °C (III) in O2-saturated electrolyte and the NDGNSs doped in a tube furnace at 650 °C in N2-saturated electrolyte (IV). (b) RDE curves of the undoped DGNSs (I), NDGNSs doped in plasma at 650 °C (II), NDGNSs doped in a tube furnace at 650 °C (III), Pt/C (IV), and GC (V) measured at a rotation speed of 1600 rpm in O2-saturated electrolyte. (c, d) RDE curves tested at different rotation speeds (rpm) (c) and the corresponding K−L plots (d) for NDGNSs doped in a tube furnace at 650 °C.

650 °C were used to compare with the samples doped in a tube furnace. The N concentrations for the NDGNSs doped in plasma at 650 °C and those doped in a tube furnace at 600, 650, 700, and 800 °C are 0.40, 1.57, 1.92, 1.63, and 1.20 atom %, respectively (Table 1, see Table S2 for the C and O concentrations). The XPS C1s peak of the DGNSs could be divided into two main peaks by deconvolution with binding energy at 284.6 and 285.4 eV, which are ascribed to the sp2 CC and sp3 C−C bonding, respectively (Figure S2).29 After doping with N, the XPS C1s peak of the NDGNSs doped in a tube furnace at 650 °C could be divided into three peaks at 284.7, 285.5, and 286.4 eV, which are ascribed to sp2 CC, sp3 C−C, and sp2 CN bonding (Figure S2), respectively.12,29 This manifests that most C and N atoms in the NDGNSs are present in the form of a conjugated honeycomb

lattice.12 N1s spectra of the NDGNSs are shown in Figure 4b. The XPS N1s peaks of the NDGNSs doped in plasma at 650 °C and those doped in a tube furnace at 700 and 800 °C were deconvoluted into three peaks at 398.8 ± 0.1, 400.2 ± 0.1, and 401.6 ± 0.2 eV, which arise from pyridinic N, pyrrolic N, and graphitic N, respectively.5,26 The XPS N1s peaks of the NDGNSs doped at 600 and 650 °C in a tube furnace were divided into two peaks at 398.8 ± 0.1 and 400.2 ± 0.1 eV, which result from pyridinic N and pyrrolic N, respectively.5,26 The concentrations of the doped N with different doping forms were computed, as shown in Table 1. In all samples, the pyridinic N and pyrrolic N are the dominant doping forms and the graphitic N is very low or even absent. The reason is that the formation energies of pyridinic N and pyrrolic N are lower.29,55 E


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quantum dots60 without doping, these nitrogen-free carbon materials possess obvious electrocatalytic activity for ORR because of the delocalized π-electrons activated by defects at the graphene edge, which would participate in ORR at low overpotentials.61,62 The dependence of the electrocatalytic activity on the N doping temperature was also investigated. Figure 6 shows the

It is indicated that the NDGNSs doped in a tube furnace under NH3 gas possess higher N content than the NDGNSs doped in plasma and the NDGNSs doped in a tube furnace at 650 °C possess the highest N content. We speculate that the lower concentration of the doped N obtained in plasma than those in a tube furnace mainly arises from the presence of ion bombardment for the former, which tends to break the C−N bond and thus decreases the N concentration of the NDGNSs. With increasing the temperature from 600 to 650 °C, the total N concentration increases from 1.57 to 1.92 atom %, which should be caused by the enhanced dissociation of NH3 at higher temperature. However, with further increasing the temperature, the total N concentration decreases gradually, which is in agreement with the previous reports.56−58 The decrease of the N concentration with increasing the temperature results from the lower stability of the C−N bond in the graphitic structure at higher temperature. The CV, RDE, and RRDE curves were systematically measured for the different samples in an alkaline electrolyte. Figure 5a presents the CV curves of the undoped DGNSs, NDGNSs doped in plasma at 650 °C, and NDGNSs doped in a tube furnace at 650 °C in O2- and N2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1 (see also Figure S3). It is seen that obvious reduction peaks appear in O2-saturated electrolyte for all of the samples, while no obvious redox peaks appear in N2-saturated electrolyte, indicating that the NDGNSs possess pronounced electrocatalytic activity toward ORR. The ORR peak potential and peak current density of the NDGNSs doped in a tube furnace at 650 °C are −0.164 V (corresponding to 0.801 V vs RHE) and 0.98 mA cm−2, respectively, which are more positive and higher than those of the undoped DGNSs (−0.317 V and 0.10 mA cm−2) and the NDGNSs doped in plasma (−0.241 V and 0.32 mA cm−2) (Table 1 and Figure S3). Figure 5b presents the RDE curves of the undoped DGNSs, NDGNSs doped in plasma at 650 °C, NDGNSs doped in a tube furnace at 650 °C, Pt/C catalysts, and GC measured in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm. The onset potentials (Eonset) of the undoped DGNSs, NDGNSs doped in plasma at 650 °C, NDGNSs doped in a tube furnace at 650 °C, and Pt/C are −0.185, −0.147, −0.037, and −0.029 V, respectively. The steady-state catalytic current density of the NDGNSs doped in a tube furnace at 650 °C is much higher than that of the undoped DGNSs and NDGNSs doped in plasma, even a little higher than that of the Pt/C, making it a promising ORR electrocatalyst. The kinetics of the ORR activity of the undoped DGNSs, NDGNSs doped in plasma, and NDGNSs doped in a tube furnace at 650 °C was further investigated at different rotating rates (400−2500 rpm) (Figure 5c,d and S4). The K−L plots exhibit high linearity and parallelism for the NDGNSs doped in a tube furnace at 650 °C, suggesting the first-order kinetics for the ORR process.6,17 The values of n and Jk of the NDGNSs doped in a tube furnace at 650 °C are 3.95 and 16.21 mA cm−2 at −0.50 V, respectively, suggesting a fourelectron ORR pathway and superior to that of the undoped DGNSs (n = 2.07, Jk = 1.52 mA cm−2) and NDGNSs doped in plasma (n = 2.21, Jk = 2.84 mA cm−2) (Figure S4). According to these results, it is reasonably concluded that N doping results in high electrocatalytic activity and promotes the ORR to a four-electron pathway. Additionally, the undoped DGNSs show obvious electrocatalytic activity for ORR (Figure 5a,b and S4). Just like the pure CNT,2 graphene,6,59 and graphene

Figure 6. CV curves (a) and RDE curves (b) of the NDGNSs doped in a tube furnace at 600, 650, 700, and 800 °C in O2-saturated 0.1 M KOH solution.

CV and RDE curves of the NDGNSs doped in a tube furnace at 600, 650, 700, and 800 °C tested in O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1. All of the NDGNSs show an obvious ORR peak. The ORR peak potential and Eonset become more positive with the increase of the doping temperature from 600 to 650 °C and then become more negative with further increasing the doping temperature. The ORR peaks of the NDGNSs doped in a tube furnace at 600, 650, 700, and 800 °C are located at −0.194, −0.164, −0.185, and −0.240 V, respectively, and their corresponding Eonset are −0.078, −0.037, −0.075, and −0.119 V, respectively (Table 1). The RDE curves measured under different rotation speeds and the obtained K−L plots of the NDGNSs doped in a tube furnace at 600, 700, and 800 °C are displayed in Figure S5. The n values for NDGNSs doped in a tube furnace at 600, 650, 700, and 800 °C are 2.50, 3.95, 2.80, and 2.11 at −0.50 V, respectively, indicating that the ORR process of the NDGNSs doped in a tube furnace at 600, 700, and 800 °C proceeds following both two-electron and four-electron pathways with the former being dominant and that of the NDGNSs doped in a tube furnace at 650 °C proceeds mainly through a fourelectron pathway. The RRDE curves were also measured to probe into the kinetic process, which shows results similar to F


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Figure 7. (a, b) CV curves of the NDGNSs doped in a tube furnace at 650 °C (a) and Pt/C (b) in O2-saturated 1 M methanol/0.1 M KOH solution. (c) Chronoamperometric responses of the NDGNSs doped in a tube furnace at 650 °C (I) and Pt/C (II) in 0.1 M KOH solution before and after adding methanol. (d) Chronoamperometric responses of the NDGNSs doped in a tube furnace at 650 °C (I) and Pt/C (II) tested at −0.4 V in O2-saturated 0.1 M KOH solution.

methanol crossover. However, for the CV curves of the Pt/C catalyst, the ORR peak disappears completely and a new peak corresponding to methanol oxidation appears. Additionally, the stability of the NDGNSs and Pt/C was assessed by measuring the current−time (i−t) chronoamperometric responses at −0.4 V and a rotating speed of 1600 rpm. Figure 7c shows i−t curves measured in the electrolyte with and without introducing methanol for the NDGNSs doped in a tube furnace at 650 °C and Pt/C. When methanol was added into the O2-saturated electrolyte solution, no obvious attenuation of the ORR current was observed for NDGNSs doped in a tube furnace at 650 °C while the current drops sharply for the Pt/C. Figure 7d presents the chronoamperometric responses for the NDGNSs doped in a tube furnace at 650 °C and Pt/C. It is observed that the current retention of the NDGNSs doped in a tube furnace at 650 °C keeps at 95.5% after 10 h, which is higher than that of Pt/C (72.0%). It is indicated that the NDGNSs possess better tolerance to methanol cross-over and better operational stability for ORR than the commercial Pt/C catalyst in an alkaline solution.

those obtained from the RDE curves (Figure S6 and its text). It is obvious that 650 °C is the most suitable doping temperature in terms of the electrocatalytic activity. The electrocatalytic performance of the present NDGNSs doped in a tube furnace at 650 °C is better than most of the carbon nanosheets reported previously,22,63−66 as summarized in Table S3. According to the literature, the pyridinic N, pyrrolic N, and graphitic N play significant roles in the electrolysis of ORR,6,19,22,26,53 although the dominative forms are still controversial. Comparing the ORR electrocatalytic activity of the NDGNSs doped at different temperatures (Table 1), we found that the electrocatalytic activity is more dependent on the concentration of pyridinic N. In particular, the NDGNSs doped in a tube furnace at 650 °C with much higher pyridinic N concentration than other samples possess the highest electrocatalytic activity. However, the dependency relation of the electrocatalytic activity on the concentration of the pyrrolic N and graphitic N was not observed. This indicates that the pyridinic N may be the main active site for ORR electrocatalytic activity. However, the contribution from other forms of nitrogen cannot be excluded. Furthermore, practical application of fuel cells requires the electrocatalysts to have high tolerance to methanol cross-over and durability. Figure 7a,b compare the CV curves tested in the electrolyte with and without methanol for the NDGNSs doped in a tube furnace at 650 °C and Pt/C, respectively. The CV curves of the NDGNSs measured with and without methanol are about similar, indicating their good tolerance to

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CONCLUSIONS In summary, we prepared vertically aligned DGNSs on BDD films by MPCVD, which uniformly grew on the BDD films, forming ordered arrays on the crystal facets of diamond grains. The surface graphitic layers of the DGNSs possess an open edge structure. N doping was achieved for the DGNSs by posttreatment in NH3 atmosphere in microwave plasma or a G


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tube furnace. The ORR electrocatalytic activity of the DGNSs has been significantly improved by N doping, and the pyridinic N plays key roles in ORR electrocatalysis. The NDGNSs doped at 650 °C in NH3 in a tube furnace show higher electrocatalytic activity for ORR, which is comparable to the commercial Pt/C. This work indicates that the vertical NDGNSs grown on BDD films are of high promise for application as the electrocatalysts toward ORR, providing a new candidate for the ORR electrocatalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06101. Raman characterization of the DGNSs and NDGNSs, XPS spectra of C1s of the DGNSs and NDGNSs doped in a tube furnace at 650 °C, element concentrations of the NDGNSs doped under different conditions calculated from the XPS spectra, CV curves of the undoped DGNSs and the NDGNSs doped under different conditions in N2-saturated or O2-saturated 0.1 M KOH solution, RDE curves and the corresponding K− L plots of the NDGNSs doped in plasma and in a tube furnace at 600, 700, and 800 °C, RRDE curves of the NDGNSs doped in a tube furnace at 600, 650, 700, 800 °C, electron transfer number (n) and HO2− yield of the NDGNSs doped in a tube furnace at 650 °C derived from the RRDE tests, and comparison of the ORR performance of the vertical carbon nanosheets (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jie Yu: 0000-0003-2051-9689 Author Contributions †

S.W. and X.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors acknowledge the support from Shenzhen Basic Research Program (JCYJ20160318093244885 and JCYJ20170413112249615) and the National Natural Science Foundation of China (No. 51272057).

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