N-doping in monolayer graphene. (a) Raman spectra of graphene upon 0 to 60 s of nitrogenation using ammonia plasma. The spectra are recorded using 2.33 eV (532 nm) laser excitation. (b) Intensity ratio I(2D)/I(G) as a function of nitrogenation times (t N ). (c) Evolution of intensity ratio I(D)/I(G) (black) and I(D)/I(D′) (blue) with respect to t N . (d) Raman mapping of the D band for graphene upon 10 and 30 s of nitrogenation. (e) Scheme of a liquid-gated graphene field effect transistor (GFET). S: source electrode; D: drain electrode. The electrolyte solution is 0.1 M KCl with 10 mM Tris (pH 8). (f) Conductance (G) vs the gate voltage (V g ) curves of graphene upon t N from 0 to 60 s. (g) The carrier mobility of graphene (μ, black square) and charge neutrality point (CNP, blue dot) evolve with t N . The error bars in panels b, c, and g are the standard deviations of the experimental values.

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... spectroscopy, electron microscopy, and transport characterization were performed to reveal the impact of nitrogen dopants on the atomic and electronic structure of monolayer graphene. Raman spectroscopy (Figure 1a) was conducted to evaluate the N-doping process on chemical vapor deposition (CVD) graphene supported by a SiO 2 /Si substrate. For pristine graphene, two main characteristic peaks for monolayer graphene can be found. ...

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... the overtone of the breathing modes of six-atom rings, the sharp 2D peak (∼2670 cm −1 ) is sensitive to the number of graphene layers and doping effects. 24 The monolayer crystallinity was also reflected in the high resolution transmission electron microscopy (HRTEM) images and the fast Fourier transform (FFT) pattern in Figure S1. After more than 2 s of nitrogenation, a D peak appears at ∼1340 cm −1 (see Figure 1a) that corresponds to single phonon intervalley scattering events and is associated with the defects induced by the incorporation of nitrogen atoms into the lattice of graphene (i.e., nitrogen dopants and the edge defects). ...

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... The monolayer crystallinity was also reflected in the high resolution transmission electron microscopy (HRTEM) images and the fast Fourier transform (FFT) pattern in Figure S1. After more than 2 s of nitrogenation, a D peak appears at ∼1340 cm −1 (see Figure 1a) that corresponds to single phonon intervalley scattering events and is associated with the defects induced by the incorporation of nitrogen atoms into the lattice of graphene (i.e., nitrogen dopants and the edge defects). 25 Upon longer nitrogenation times (t N > 6 s), a D′ peak at 1620 cm −1 emerges as a shoulder of the G peak due to the intervalley scattering induced by defects. ...

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... Upon longer nitrogenation times (t N > 6 s), a D′ peak at 1620 cm −1 emerges as a shoulder of the G peak due to the intervalley scattering induced by defects. 26 When t N increases from 0 to 60 s, the intensity ratio I(2D)/I(G) decreases from 2.0 to 0.7 (Figure 1b) and the 2D peak shifts from 2674 to 2665 cm −1 ( Figure S2a); both are in line with an electron (n)-doping effect in nitrogenated graphene. 27−29 As a quantitative reflection of the defect density (n D ) and interdefect distance (L D ), 30 the ratio of I(D)/I(G) in Figure 1c (black line) exhibits a similar growth trend with the peak widths (see n D and L D in Table S1). ...

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... When t N increases from 0 to 60 s, the intensity ratio I(2D)/I(G) decreases from 2.0 to 0.7 (Figure 1b) and the 2D peak shifts from 2674 to 2665 cm −1 ( Figure S2a); both are in line with an electron (n)-doping effect in nitrogenated graphene. 27−29 As a quantitative reflection of the defect density (n D ) and interdefect distance (L D ), 30 the ratio of I(D)/I(G) in Figure 1c (black line) exhibits a similar growth trend with the peak widths (see n D and L D in Table S1). Such consistent saturation trends may correspond to the clustering of nitrogen dopants at a high doping level. ...

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... consistent saturation trends may correspond to the clustering of nitrogen dopants at a high doping level. 11,31 This is reflected by a domain-like defect distribution in graphene after 30 s of nitrogenation ( Figure 1d). The full widths at half-maximum values (fwhm's) for the D, G, and 2D peaks ( Figure S2b) slightly increase upon increasing t N from 0 to 30 s and are saturated at 60 s. ...

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... indicates vacancy defects on the basis of a model of uniform defect distribution without clustering. 32 In our case, the I(D)/ I(D′) ratios vary from 6.5 (10−20 s of nitrogenation) to 5 (more than 30 s of nitrogenation) (Figure 1c, blue dots), indicating that nitrogen dopants behave more like vacancy defects. To conclude, Raman spectroscopy shows that Ndoped graphene has a high, uniform graphitization level and vacancy-like N dopants. ...

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... we studied the electron transport characteristics of graphene in the configuration of an electrochemically gated graphene field effect transistor (GFET) that was fabricated following a previously reported strategy (see the Supporting Information). 33 We used an epoxy substrate to support a clean, pristine graphene surface that was protected by a clean and annealed copper substrate (Figure 1e). Moreover, this graphene surface was never in contact with (and thus not contaminated) any polymer that is generally used for graphene transfer 34 and was only exposed to ambient oxygen for a short period (within 24 to 48 h) before the measurements. ...

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... previous work has confirmed that this graphene surface contains a lower density of charged impurities (i.e., originating from ambient oxidation or trapped impurities) than the polymer transferred one. 33 The conductance (G) of this clean graphene in Figure 1f (black line) demonstrates an ambipolar behavior with respect to the gate voltage (V g ). The G (V g ) curves start to shift negatively after 10 s of nitrogenation, and the charge neutrality point (CNP) shifts by −30 to −60 mV between 20 and 60 s of nitrogenation. ...

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... G (V g ) curves start to shift negatively after 10 s of nitrogenation, and the charge neutrality point (CNP) shifts by −30 to −60 mV between 20 and 60 s of nitrogenation. Such shifts suggest an ndoping effect in graphene (Figure 1g). Using the capacitor model in the electrochemical-gating configuration, 35 we extract the carrier mobility (μ) of graphene, which decreases from ∼3800 to ∼550 cm 2 V −1 s −1 after 30 s of nitrogenation and subsequently levels off at 60 s of nitrogenation (Figure 1g, black). ...

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... shifts suggest an ndoping effect in graphene (Figure 1g). Using the capacitor model in the electrochemical-gating configuration, 35 we extract the carrier mobility (μ) of graphene, which decreases from ∼3800 to ∼550 cm 2 V −1 s −1 after 30 s of nitrogenation and subsequently levels off at 60 s of nitrogenation (Figure 1g, black). Notably, the high carrier mobility value for pristine graphene confirms its intrinsic high quality and low charge impurities. ...

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... the high carrier mobility value for pristine graphene confirms its intrinsic high quality and low charge impurities. Consistent with the saturation trend of I(D)/I(G) ratios in Figure 1c, the evolution of graphene carrier mobility is predicted to be closely related to the distribution of nitrogen dopants. At low doping levels (t N < 30 s), nitrogen dopants independently implant into the carbon lattice, resulting in a rapid and dramatic conductivity degradation of graphene. ...

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... graphene supported on the as-grown copper foil intuitively has two faces: one facing the copper foil (copper face) and the other facing the air (air face) ( Figure 2a). As mentioned above, the copper face of graphene that has been previously confirmed to contain minimized impurities (i.e., oxidation, contaminations) 33 was adopted for the transport measurement ( Figure 1e). To prepare monolayer graphene electrodes for ORR, both faces were employed according to the measurement configurations. ...

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... N/C and O/C ratio increases respectively from 2.0% and 9.0% for 30 s to 3.1% and 21% for 60 s of nitrogenation. Moreover, the dominant forms of pyridand pyrro-N agree well with the observed n-type doping effect in Figure 1. 37,38 The ORR activity was first studied with pristine G supported on the epoxy substrate for both acid and alkaline media. in alkaline media. ...

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... ORR activities are evaluated on the basis of the kinetic catalytic currents obtained at potentials where the diffusion limited condition is not yet achieved. Before that, the CV curves for G@GC samples in a 0.1 M NaOH solution purged with argon and oxygen shown in Figure S10 confirmed the ORR current. The stable ORR current was obtained after ten CV scans in an oxygen saturated solution ( Figure S11). ...

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... that, the CV curves for G@GC samples in a 0.1 M NaOH solution purged with argon and oxygen shown in Figure S10 confirmed the ORR current. The stable ORR current was obtained after ten CV scans in an oxygen saturated solution ( Figure S11). As shown in the LSV curves at a rotation speed ranging from 400 to 1000 rpm in Figure S12, nondoped, single-doped, and dualdoped graphene samples are less active at potentials between 0.1 and 0.4 V vs RHE, while the triple-doped sample (Ar−O−N) is more active with more positive potentials between 0.4 and 0.6 V vs RHE. ...

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... stable ORR current was obtained after ten CV scans in an oxygen saturated solution ( Figure S11). As shown in the LSV curves at a rotation speed ranging from 400 to 1000 rpm in Figure S12, nondoped, single-doped, and dualdoped graphene samples are less active at potentials between 0.1 and 0.4 V vs RHE, while the triple-doped sample (Ar−O−N) is more active with more positive potentials between 0.4 and 0.6 V vs RHE. Therefore, the currents at 0.4 V vs RHE were selected for the activity comparison between different samples. ...

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... the ORR currents at 0.4 V vs RHE for the less active graphene samples can be influenced by the oxygen-containing groups (the extra reduction peak at 0.45 V vs RHE). Correspondingly, the currents at 0.2 V vs RHE were also compared to gain more reliable insights into the activity trends for less active samples (see Figure S13). Compared with G in Figure 3a, N30 exhibits a similar or even lower current (∼0.6-fold@0.4 ...

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... highest atom% for pyridinic N, pyrrolic N, and graphitic N is found, respectively, in O−N (1.1%), Ar−O−N (3.5%), and Ar−N (0.6%). In addition, XPS characterizations were also conducted for other single-doped (Ar30, O10) and dual-doped (Ar−O) samples ( Figure S14) for comparison. ...

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... highest current at ∼0.45 V of O10 compared to other samples, illustrated in Figure 3a, is further confirmed by the highest oxygen content and superior ORR activity for O10 shown in Figure 3c. It is of note that the Ar−O sample has a much higher C−O% than Ar30 but a similar (@ 0.2 V in Figure S13) or even lower activity (@0.4 V in Figure 3f). Therefore, it is hypothesized that the contributions of oxygen dopants and vacancy defects to ORR do not have a synergetic effect. ...

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... intercepts of the K-L plots summarized in Figure 3i are consistent with the summary of the activities in Figure 3f, which further supports the correlation between ORR activity and atomic ratios. Using the ring current I r and disk current I d collected from the RRDE data (see Figure S10), the electron transfer number N e can be derived using eq 1 ...

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... novel finding in this work is that the intentional or unintentional doped oxygen groups in the lattice of monolayer graphene are the prerequisite for the N-doped carbon system to show enhanced ORR activity. In brief, the predoped oxygenated defects create the activation center integrating nitrogen heteroatoms (illustrated in the scheme in Figure S14a) to lower the kinetic barrier of the active sites in graphene, thus enhancing the ORR activity and selectivity toward water production. Further incorporation of nitrogen dopants contributes to optimize the electronic structure of the predoped graphene system. ...

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... Raman spectra show that the defect density reflected by the I(D)/I(G) ratio drops from 1.7 for Ar−O−N to 0.8 for 500−Ar−O−N (Figure 4b). Moreover, N1s spectra of 500−Ar−O−N ( Figure S14) shows the absence of Ndopants and an obvious rise in the atom% of sp 3 C compared to Ar−O−N (from 9.4 to 14.9, Figure 4c and Table S2). Therefore, it is assumed that the decrease of defect density should be related to the removal of N-dopants. ...

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... defects and oxygen and nitrogen dopants were doped in different orders into graphene to further evaluate their specific roles in improving ORR ( Figure S15). The comparison in Figure S15a shows that O−N exhibited a higher activity than N−O over a wide range of overpotentials (0.4 to −0.2 V). ...

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... defects and oxygen and nitrogen dopants were doped in different orders into graphene to further evaluate their specific roles in improving ORR ( Figure S15). The comparison in Figure S15a shows that O−N exhibited a higher activity than N−O over a wide range of overpotentials (0.4 to −0.2 V). Such a contrast is closely related to the different compositions. ...

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... the comparison further confirms that predoped oxygen dopants in graphene are beneficial for a higher nitrogen doping level and a higher ORR activity. Nitrogenation in graphene samples predoped with oxygenation and vacancy defects (Ar−O−N and O−Ar−N) still contribute to higher activities ( Figure S15b). In comparison, nitrogenation followed by oxygenation and vacancy introduction, namely, N−Ar−O and N−O−Ar, contribute to relatively lower activities. ...