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Observation of Active Sites for Oxygen Reduction Reaction on Nitrogen-doped Multilayer Graphene

Active sites and catalytic mechanism of nitrogen-doped graphene in oxygen reduction reaction (ORR) have been extensively studied but are still inconclusive, partly due to the lack of an experimental method that can detect the active sites. It is proposed in this report that the active sites on nitrogen-doped graphene can be determined via the examination of its chemical composition change before and after ORR. Synchrotron-based X-ray photoelectron spectroscopy analyses of three nitrogen-doped multilayer graphene samples reveal that oxygen reduction intermediate OH(ads) which should chemically attach to the active sites remains on the carbon atoms neighboring pyridinic nitrogen after ORR. In addition, a high amount of the OH(ads) attachment after ORR corresponds to a high catalytic efficiency and vice versa. These pinpoint that the carbon atoms close to pyridinic nitrogen are the main active sites among the different nitrogen doping configurations.

ACS Nano 8 (7), 6856-6862, 2014 (DOI: 10.1021/nn501506p) that there is a need to develop a method that is able to observe the active sites and elucidate the catalytic mechanism. It was demonstrated before that intermediates of a catalytic reaction could be ex situ analysed on the surface of the catalyst. 20,21 Here, we report that chemically adsorbed oxygen reduction intermediates can be detected on the nitrogen-doped multilayer graphene after ORR using X-ray photoelectron spectroscopy (XPS). This not only implies that nitrogen close to carbon in nitrogen-doped graphene is involved in the catalysis of ORR process and, more importantly, can be used to observe the active nitrogen sites and pinpoint the ORR pathway.

RESULTS AND DISCUSSION
Three multilayer graphene samples were prepared from graphene oxide using different nitrogen  Synchrotron-based XPS of high resolution and high sensitivity was used to investigate the chemical composition change on the three nitrogen-doped multilayer graphene samples before and after ORR. Before ORR, the nitrogen contents in G-NH3·H2O, G-C3N4 and G-NH3 are 6.0%, 5.7% and 6.8%, respectively. The least-square fittings of the XPS spectra in the nitrogen 1s region before ORR are displayed in Figure 2 (upper row). It can be seen that G-NH3·H2O and G-NH3 samples have relatively higher contents of pyrrolic nitrogen at 399.8 eV (red), slightly less pyridinic nitrogen at 398.5 eV (blue), and much less graphitic nitrogen at 401.2 eV (purple). G-C3N4, on the other hand, contains more pyridinic nitrogen and less but comparable amounts of pyrrolic and graphitic nitrogen. After ORR, the three samples show different changes of pyridinic, pyrrolic and graphitic nitrogen contents (lower row in Figure 2).
For G-NH3·H2O, the pyridinic nitrogen peak decreases while the "pyrrolic" peak increases; for G-C3N4, the graphitic content increases and the pyridinic and "pyrrolic" contents are steady; G-NH3 shows a depressed "pyrrolic" peak. As shown below, these changes correspond to very different catalytic behaviors and functions of the different nitrogen configurations during the ORR process. 4 ACS Nano 8(7), 6856-6862, 2014 (DOI: 10.1021/nn501506p) Figure 2. Nitrogen 1s XPS spectra of the three multilayer graphene samples before and after ORR. The least-square fitted peaks are pyridinic nitrogen at 398.5 eV (blue), "pyrrolic" nitrogen at 399.8 eV (red), graphitic nitrogen at 401.2 eV (purple) and nitrogen oxide at 403 eV (green).
The observed changes in nitrogen contents before and after ORR cannot be attributed solely to nitrogen loss. Nitrogen may be lost via a nitrogen oxidation process during ORR, which could change the contents of the different nitrogen configurations. Generally, pyrrolic nitrogen is less stable than pyridinic and graphitic nitrogen and more prone to the oxidation and loss during ORR process. 22,23 In other words, the pyrrolic nitrogen peak should relatively decrease after ORR. However, G-NH3·H2O shows the opposite trend with an increased "pyrrolic" peak after ORR.
For clues to the reason for the relative change of the nitrogen contents, we can turn to the carbon and oxygen XPS results. Both the oxygen and carbon 1s XPS spectra reveal that the content of -OH, one of the intermediate products of ORR, increases on G-NH3·H2O after ORR. Figure 3 compares the oxygen 1s XPS spectra of G-NH3·H2O before and after ORR. It can be seen that after ORR the C(aliphatic)-OH/C(aliphatic)-O-C(aliphatic) peak at 532.0 eV (red) relatively decreases in intensity and the C(aromatic)-OH at 533.3 eV (blue) increases in intensity. 24,25 This suggests that compared to the other oxygen-containing groups, the relative content of -OH group attached to aromatic carbon rises after ORR. The carbon 1s XPS spectra also show that the C-OH peak at 285.4 eV 24 (purple) dramatically strengthens in G-NH3·H2O after ORR (Figure 4). 5 ACS Nano 8(7), 6856-6862, 2014 (DOI: 10.1021/nn501506p)  Based on these evidence, the changes of the nitrogen 1s XPS profiles in Figure 2 could be caused by the chemical environment change of the doped nitrogen during ORR, i.e. the attachment of -OH to the carbon atoms bond to the nitrogen. It has been reported that the attachment of -OH to pyridone can cause an upshift of the nitrogen binding energy from 398.8 to 400.2 eV in XPS. 22,26,27 Similarly, when an -OH attaches to the carbon neighboring pyridinic nitrogen in graphene, the nitrogen binding energy should shift upwards to an energy very close to the 399.8 eV position of the pyrrolic peak. Therefore, the observed intensity decrease of the pyridinic peak and intensity increase of the "pyrrolic" peak in G-NH3·H2O after ORR could 6 ACS Nano 8(7), 6856-6862, 2014 (DOI: 10.1021/nn501506p) be caused by the change of the pristine pyridinic nitrogen to the pyridinic nitrogen with a neighboring carbon attached to -OH, as illustrated in Figure 5. In other words, the increase of the "pyrrolic" nitrogen after ORR in XPS is not caused by the increase of pyrrolic content, but the increased content of the -OH attached pyridinic nitrogen which has a very similar XPS peak position to the pyrrolic nitrogen. The existence of the -OH attached pyridinic nitrogen is also supported by the increased full width at half maximum (FWHM) of the least-square fitted "pyrrolic" peak at 399.8 eV from 1.61 eV before ORR to 1.71 eV after ORR, suggesting the existence of the -OH attached pyridinic nitrogen peak in the region. The unchanged and decreased intensities of the 399.8 eV XPS peak for G-C3N4 and G-NH3 suggest that much less -OH attachments are formed to the carbon atoms neighboring pyridinic nitrogen in the two graphene samples and pyrrolic nitrogen is slightly lost during ORR. The above interpretation on the changed nitrogen contents observed in XPS is in excellent agreement with Fourier transform infrared spectroscopy (FTIR) analyses. The contents of -OH on the three samples before and after ORR have been measured by FTIR. 28,29 In Figure 6, the characteristic -OH peak centered at ~3440 cm -1 becomes much stronger for G-NH3·H2O after ORR; in contrast, G-C3N4 and G-NH3 have eithor unchanged or decreased -OH contents after ORR. It should be mentioned that the -OH signals in FTIR are not due to water, because all 7 ACS Nano 8 (7), 6856-6862, 2014 (DOI: 10.1021/nn501506p) the samples were heated at 65 °C in vacuum for 24 h. Additionally, solid-state nuclear magnetic resonance (NMR) spectroscopy shows that most hydrogen is in the form of C-OH in G-NH3·H2O after ORR and no H2O signal was detected (see Supporting Information, Figure   S2). 30  to graphitic nitrogen should not be the main active site in the three tested samples, because according to XPS, the content change of graphitic nitrogen before and after ORR is much less significant and therefore presents no strong relation with the catalytic performance. 9 ACS Nano 8(7), 6856-6862, 2014 (DOI: 10.1021/nn501506p) Another very interesting point from this study is that the dominant type of nitrogen doping configuration may not necessarily correlate with the ORR performance of nitrogen-doped graphene. Before ORR, G-NH3·H2O and G-NH3 have similar contents of nitrogen and oxygen (about 13% in G-NH3·H2O and 14% in G-NH3). The contents of pyridinic, pyrrolic and graphitic nitrogen in the two samples are similar too (Figure 2a and e). In addition, the three samples show similar Raman spectra and electrochemical impedance values, indicating their similar crystallinity (defect levels) and electrical conductivity (see Supporting Information, Figure S4 and S5). The electrochemical impedance tests also imply comparable wettability of the three samples, consistent to their capability to be dispersed in water. However, the two samples show dramatically different catalytic properties (Figure 7). This clearly shows that the pyridinic or graphitic nitrogen content before ORR cannot be used to predict or explain ORR performance. This also raises a question: why do the two samples with similar pyridinic contents show dramatically different ORR performance if the pyridinic sites are the main catalytic active sites? One presumption is that pyridinic nitrogen in graphene may have different ORR properties. It has been demonstrated by theoretical calculations that the microstructure of nitrogen doping could lead to different ORR activities. 16,37,38 So, although the total amounts of pyridinic nitrogen are similar in G-NH3·H2O and G-NH3, it is likely that the pyridinic nitrogen in G-NH3·H2O and G-NH3 have different dominant microstructures. For example, our oxygen 1s XPS spectra in Figure 3 show that the -OH is mainly attached to aromatic carbon rather than aliphatic carbon in the ORR, suggesting that the pyridinic nitrogen on the edge should be more likely to involve in the catalytic process. So it is possible that G-NH3·H2O has more pyridinic nitrogen on the edge than G-NH3. According to theoretical calculations, 37 other microstructures, such as the edge type (zig-zag or arm-chair) and neighboring nitrogen doping, may also affect the ORR activity of pyridinic nitrogen.
Unfortunately, the microstructure of pyridinic nitrogen in graphene is very difficult to 10 ACS Nano 8 (7) 150). In the scans, the excitation energy was 700 eV for better signal-to-noise ratios and the E-pass was set to 5 eV for optimum energy resolution. The excitation photon energies were calibrated by the photon energy measured on a reference Au sample and the binding energies were normalized by the C-C peak at 284.6 eV. To evaluate the signal contribution from Nafion and the gold plate, the XPS spectra of pure Nafion and a clean gold plate were also recorded. It is found that for all samples, the N and C contributions from the gold plate are negligible (less than 5% and 1%, respectively).
Transmission infrared spectra were collected using a Bruker Vertex 70 infrared spectrometer.
The multilayer graphene samples before and after ORR were vacuum dried at 65 °C for 24 h and then mixed with potassium bromide (KBr, Sigma-Aldrich) by grind before pressed to pellets. Total 32 scans were collected for both the reference (pure KBr) and the samples. The reference spectrum was then subtracted from the sample spectrum to reduce the interference 12 ACS Nano 8(7), 6856-6862, 2014 (DOI: 10.1021/nn501506p) from the atmosphere and the equipment. The resolution is 4 cm -1 and the scan range is from 600 to 4,000 cm -1 .
All electrochemical measurements were performed under identical conditions (the same catalyst mass loading). Taking G-NH3·H2O electrode as an example, the catalyst was first ultrasonically dispersed in Milli-Q water. Aqueous catalyst solution dispersion of 20 µL (2.0 mg/mL, with 0.5 wt% Nafion ® solution) was then transferred onto a glassy carbon electrode (GC, 0.196 cm 2 , Pine Research Instrumentation, USA) via a controlled drop casting approach then dried in air for 12 h served as a working electrode. The reference electrode was an Ag|AgCl in saturated AgCl-KCl solution and the counter electrode was a platinum wire. A flow of O2 was maintained over the electrolyte (0.1 M KOH) during electrochemical measurements for continued O2 saturation. Cyclic voltammograms (CVs), linear sweep voltammograms (LSVs) and rotating disk electrode (RDE) tests were carried out using a glassy carbon rotating disk electrode. The scan rate of CVs was kept at 100 mV/s while that for LSVs and RDE tests was both 10 mV/s. The data were recorded using a CHI 760D potentiostat (CH Instruments Inc., USA).

Supporting Information
Solid-state NMR, calculation of catalytic performance and subtraction of Nafion's contribution to the XPS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. 13