Sulfur Dual-Doped Graphene

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Highly Crumpled Hybrids of Nitrogen/Sulfur Dual-Doped Graphene and Co9S8 Nanoplates as Efficient Bifunctional Oxygen Electrocatalysts Yanping Tang,† Fan Jing,† Zhixiao Xu, Fan Zhang,* Yiyong Mai, and Dongqing Wu* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P.R. China S Supporting Information *

ABSTRACT: A bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly attractive for the manufacture of clean energy conversion devices. In this work, highly crumpled hybrid of nitrogen and sulfur dual-doped graphene and quasi-hexagonal Co9S8 nanoplates (Co9S8/NSGg‑C3N4) is fabricated via a facile ionic assembly approach. The unique structure of Co9S8/NSGg‑C3N4 renders it high specific surface area (288.3 m2 g−1) and large pore volume (1.32 cm3 g−1). As the electrocatalyst for ORR, Co9S8/NSGg‑C3N4 demonstrates excellent performance with the onset potential of −0.02 V vs Ag/AgCl and the limited current density of 6.05 mA cm−2 at −0.9 V vs Ag/AgCl. Co9S8/NSGg‑C3N4 also presents outstanding catalytic activity toward OER by delivering a limited current density of 48 mA cm−2 at 1 V vs Ag/AgCl. The bifunctional catalytic behaviors of Co9S8/NSGg‑C3N4 enable the assembly of a rechargeable Zn−air battery with it as the cathode catalyst, which exhibits stable discharge/charge voltage plateaus upon long time cycling over 50 h. KEYWORDS: nitrogen and sulfur dual-doped graphene, cobalt pentlandite, crumpled architecture, oxygen reduction reaction, oxygen evolution reaction

1. INTRODUCTION The fast-growing negative environmental impact from the combustion of fossil fuel in past decades has boosted the demand on highly efficient renewable energy sources such as fuel cells, metal−air batteries, and water-splitting cells.1−3 As the important half-reactions in these energy conversion technologies, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are both kinetically sluggish and require catalysts to drive their reaction processes.4−8 Generally, platinum (Pt)-based materials are the state-of-art catalysts for ORR. However, their performance toward OER are usually poor. On the other hand, ruthenium or iridium oxides (RuO2 or IrO2) manifest excellent OER catalytic behavior but much diminished ORR activity.9,10 Besides, the limited versatility, natural scarcity, and high cost of these catalysts are also the major bottlenecks that frustrate the development efforts in the above-mentioned clean energy devices.4−10 Very recently, both theoretical calculations and experimental results indicate that cobalt pentlandite (Co9S8) can serve as the electrocatalyst for both ORR and OER.11−15 Nevertheless, the severe aggregations and instinctual low conductivity of Co9S8 prohibited the full utilization of its electrocatalytic activity. To solve these problems, one frequently used strategy is dispersing Co9S8 on graphene substrate, which is an intriguing twodimensional (2D) platform having good electronic conductivity and high mechanical stability.16,17 Especially, the introduction © XXXX American Chemical Society

of heteroatoms such as N and S in graphene can effectively modify the electrochemical behaviors and wettability to electroyte.18−32 Moreover, the synergistic effects between Co9S8 and N/S codoped graphene matrix can further improve the electrochemical activity and durability of the resulting catalysts.23,24,33,35 However, previous research studies mainly focus on the compositions of the hybrids with Co9S8 on graphene and little effort has been devoted to the adjustment of their architectures. In this work, we developed an unprecedented strategy to integrate quasi-hexangular Co9S8 nanoplates on nitrogen and sulfur atoms dual-doped graphene matrix with highly crumpled architectures (Co9S8/NSGg‑C3N4) via the thermal treatment of the ionic-assembled aggregate containing graphene oxide (GO), protonated graphitic C3N4 (g-C3N4−H+), and CoS (CoS/g-C3N4−H+/GO), which is a precursor with highly wrinkled feature. Thanks to the highly corrugated architecture inherited from its precursor, Co9S8/NSGg‑C3N4 manifests high specific surface area (288.3 m2 g−1) and large pore volume (1.32 cm3 g−1), rendering sufficient exposure of the active sites in the hybrid. As the electrocatalyst for ORR, Co9S8/NSGg‑C3N4 demonstrates excellent activity with the onset potential of Received: December 6, 2016 Accepted: March 23, 2017

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DOI: 10.1021/acsami.6b15461 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic demonstration of the preparation process for Co9S8/NSGg‑C3N4..

−0.02 V vs Ag/AgCl and the limited current density of 6.05 mA cm−2 at −0.9 V vs Ag/AgCl, outperforming those of commercial Pt/C catalysts and recently reported CoXn (X = O, S, etc.)-based catalysts.6,9,12−14,22−24,34−37,45 On the other hand, Co9S8/NSGg‑C3N4 also presents outstanding catalytic behavior for OER by delivering a limited current density of 48 mA cm−2 at 1 V vs Ag/AgCl. Moreover, the rechargeable Zn−air battery38 with Co9S8/NSGg‑C3N4 as the oxygen cathode exhibits stable discharge/charge voltage plateaus upon long time cycling lasting for 50 h.

ionic interactions between positively charged g-C3N4−H+ and negatively charged GO render the formation of flocculated composite of g-C3N4−H+/GO.40 The dispersion of g-C3N4− H+/GO was then combined with Co(NO3)2·6H2O and Na2S· 9H2O to produce CoS/g-C3N4−H+/GO as dark black precipitate, which was then thermally treated at 550 °C for 3 h under N2 to produce Co9S8/NSGg‑C3N4. For comparison, Co9S8/NSGPANI was prepared via the thermal treatment of the precursor of CoS on PANI-grafted GO. And bare Co9S8 was also obtained by the direct thermal treatment of amorphous CoS. The phase variations of the samples during the fabrication process were first identified by X-ray diffraction (XRD) characterization. In the XRD pattern of CoS/g-C3N4−H+/ GO (Figure S1, Supporting Information), the peak at 27.8° can be assigned to the (002) face of g-C3N4−H+,39 and all the other diffraction peaks are from the Jaipurite phase of CoS (PDF: 658977). In contrast, the diffraction peaks of Co9S8/NSGg‑C3N4 can be indexed perfectly with the fcc phase of Co9S8 (PDF: 656801) with no diffraction peaks of g-C3N4−H+ able to be observed, suggesting the complete decomposition of g-C3N4− H+ during the thermal treatment. Similarly, both bare Co9S8 and Co9S8/NSGPANI also manifest their diffraction peaks corresponding to the fcc phase of Co9S8 (Figures S2 and S3). The morphologies of CoS/g-C 3 N 4 −H + /GO, Co 9 S 8 / NSGg‑C3N4, and the other samples were then investigated. As shown in their scanning electron microscopy (SEM) images (Figure 2a and Figure S4a−d), CoS/g-C3N4−H+/GO has highly wrinkled morphology that is distinctive from GO with few surface fluctuations.17 The large amounts of corrugations of CoS/g-C3N4−H+/GO should be generated by the ionic interactions among the negatively charged GO, positively charged g-C3N4−H+, and CoS.40 The transmission electron microscopy (TEM) images of CoS/g-C3N4−H+/GO (Figure S4e,f) disclose that g-C3N4−H+ nanosheets and CoS nanoparticles are decorated on the crumpled GO sheets. Derived from the thermal treatment of CoS/g-C3N4−H+/GO, Co9S8/ NSGg‑C3N4 has inherited the morphology of its precursor that reserves the highly crumpled architecture with Co 9 S 8 embedded in the wrinkles of graphene (Figure 2b,c and Figure S5). The energy-dispersive X-ray analysis (EDX) and elemental mapping images of Co9S8/NSGg‑C3N4 (Figure S6) indicate that Co, S, N, and C atoms are distributed throughout on its surface, and the sufficient exposure of Co9S8 and heteroatom-doped sites will greatly facilitate the electrochemical performances of

2. EXPERIMENTAL SECTION Preparation of Co9S8/NSGg‑C3N4. The mixed aqueous suspension (∼200 mL) of GO (50 mg) and g-C3N4−H+ (100 mg, see the details for the fabrication process in the Supporting Information) was ultra sonicated for 60 min and then fed with Co(NO3)2·6H2O (180 mg) and stirred at 90 °C for 1 h. The subsequent addition of Na2S·9H2O (300 mg) with stirring at 90 °C for 5 h led to the formation of CoS/gC3N4−H+/GO as black precipitate, which was collected by filtration, freeze-dried, and finally treated at 550 °C for 3 h under N2 with a heating rate of 5 °C min−1 to produce Co9S8/NSGg‑C3N4. Preparation of Co9S8 Powder. The solution of Co(NO3)2·6H2O (0.9 mg mL−1, 200 mL) was stirred at 90 °C and Na2S·9H2O (300 mg) was then added. The resulting amorphous CoS was heated at 550 °C for 3 h under N2 with a heating rate of 5 °C min−1 to obtain powder-like Co9S8. Preparation of Co9S8/NSGPANI. Aniline (500 μL) (PANI, polyaniline) was added in H2O (50 mL) with sodium dodecyl sulfate (15 mg) and then mixed with the GO suspension (20 mL, 2.5 mg mL−1). Subsequently, ammonium persulfate (1 g) in HCl (80 mL, 1 M) was added slowly, and the resulting mixture was vigorously stirred overnight at 0−5 °C. The green suspension was then centrifuged and washed with water three times to obtain PANI/GO, which was redispersed in water (200 mL). The resulting dispersion of PANI/GO was then fed with Co(NO3)2·6H2O (180 mg) and stirred at 90 °C for 1 h. Subsequently, Na2S·9H2O (300 mg) was added and the mixture was stirred at 90 °C for 5 h to generate CoS/PANI/GO as dark green precipitate. The nitrogen content in the PANI coating of CoS/PANI/ GO was controlled as the same level as that in g-C3N4−H+ of the CoS/g-C3N4−H+/GO, PANI, and g-C3N4−H+ are both nitrogen sources. CoS/PANI/GO was collected by filtration, freeze-dried, and finally treated at 550 °C for 3 h under N2 with a heating rate of 5 °C min−1 to produce Co9S8/NSGPANI.

3. RESULTS AND DISCUSSION The fabrication strategy of Co9S8/NSGg‑C3N4 is shown in Figure 1. Typically, g-C3N4−H+39 was first mixed with the aqueous suspension of GO via sonication.40 During this process, the B


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Figure 2. SEM images of (a) CoS/g-C3N4−H+/GO and (b) Co9S8/NSGg‑C3N4; (c, d) TEM images of Co9S8/NSGg‑C3N4; (e) SEM image of CoS/ PANI/GO; (f) TEM image of Co9S8/NSGPANI.

PANI is directly grown on the surface of GO and no strong ionic interactions are involved in the fabrication process. Correspondingly, CoS/PANI/GO does not contain the highly crumpled architecture as CoS/g-C3N4−H+/GO. Besides, small nanoparticles of Co9S8 with the sizes around 2 nm can be observed in the TEM images of Co9S8/NSGPANI (Figure S7b). The residual carbons from the pyrolyzed PANI can inhibit the formation of large Co9S8 nanoplates during the thermal treatment and thus only small Co9S8 nanoparticles can be found on Co9S8/NSGPANI.42 Different from the abovementioned samples, bare Co9S8 without graphene exhibits only arbitrary shapes with irregular sizes (Figure S8). The existence of a large amount of wrinkles on Co9S8/ NSGg‑C3N4 can effectively prevent the restacking of graphene

the hybrid. The TEM images of Co9S8/NSGg‑C3N4 reveal the morphology of the Co9S8 species are quasi-hexagonal nanoplates with the diameters ranging from tens to hundreds of nanometers (Figure 2c and Figure S5d). Their high-resolution TEM (HRTEM) images further disclose that these nanoplates contain uniform lattice planes with interfringe distance of 0.22 nm (Figure 2d), corresponding to the 420 diffraction of Co9S8.41 The selected-area electron diffraction (SAED) pattern of the Co9S8 nanoplate (inset of Figure 2c) shows a set of diffraction spots, indicative of its single-crystalline nature. Different from Co9S8/NSGg‑C3N4, Co9S8/NSGPANI manifests a sheetlike structure with only a few folds (Figure 2f) and such morphology should also be inherited from its precursor (CoS/ PANI/GO, Figure 2e and Figure S7a). It should be noted that C


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Figure 3. (a) Nitrogen adsorption−desorption isotherm and (b) the corresponding pore size distribution of Co9S8/NSGg‑C3N4.

Figure 4. Core-level XPS spectra of (a) C 1s, (b) N 1s, (c) S 2p, and (d) Co 2p for Co9S8/NSGg‑C3N4.

the thermal treatment via our approach. In contrast, g-C3N4− H+ in CoS/g-C3N4−H+/GO as nitrogen source is completely decomposed after the thermal annealing, which can be confirmed via the XRD, Fourier transform infrared, and TGA (under N2) results (Figures S1, S11, and S12). Further information about the chemical states of the elements in the hybrids were analyzed via X-ray photoelectron spectroscopy (XPS). For Co9S8/NSGg‑C3N4, the C 1s peaks at 284.8 and 285.5 eV are assigned to sp2 graphitic carbon and CN/C−S bonds (Figure 4a).21,32 The obviously attenuated peaks at 286.9 and 291.4 eV (C−O−, OC−O−) suggest the efficient thermal elimination of the oxygen-containing groups in GO.43 High-resolution N 1s spectra (Figure 4b) disclose the presence of pyridinic N (398.3 eV), pyrrolic N (399.4 eV), graphitic N (400.7 eV), and oxidized N (404.0 eV), evidencing the efficient doping of N atoms in graphene framework.32,40 As shown in Figure 4d, the peaks at 780.8 and 796.6 eV correspond to the binding energy of Co 2p3/2 and Co 2p1/2 of Co9S8, respectively. Two weak peaks at 786.1 and 802.8 eV can be the shake-up satellite peaks above the main peaks, implying the presence of Co(II).11 On the other hand, the peaks at 161.1 and 162.1 eV should be derived from the sulfur

and render the exposure of active sites, which thus will be beneficial for its electrochemical applications.33,35 As shown by the nitrogen adsorption−desorption results in Figure 3, Co9S8/ NSGg‑C3N4 displays a typical type IV isothermal curve with a H3 hysteresis loop at relative pressures (P/P0) of 0.45−1.00, suggesting the existence of mesopores. The diameters of the mesopores, according to density functional theory method, are ∼2−20 nm, which may correspond to the crumpled parts of graphene in the hybrid. More importantly, based on the Brunauer−Emett−Teller (BET) analysis, the surface area and pore volume of Co9S8/NSGg‑C3N4 are calculated to be 288.3 m2 g−1 and 1.32 cm3 g−1, respectively, which are much higher than those of Co9S8 (32.8 m2 g−1 and 0.04 cm3 g−1), Co9S8/NSGPANI (142.3 m2 g−1 and 0.57 cm3 g−1), and the recently reported Co9S8 electrocatalysts (Figure S9 and Table S1). Thermogravimetric analysis (TGA) of the hybrids was conducted to probe the compositions of the samples. On the basis of their TGA curves under air (Figure S10), the contents of Co9S8 in Co9S8/NSGg‑C3N4 and Co9S8/NSGPANI are around 65 and 20 wt %, respectively. The much lower weight percentage of Co9S8 in Co9S8/NSGPANI might be due to the insufficient decomposition of PANI with residual carbons after D


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Figure 5. (a) LSVs of Co9S8/NSGg‑C3N4, Co9S8/NSGPANI, Co9S8, and commercial Pt/C catalysts; (b) RRDE voltammograms of Co9S8/NSGg‑C3N4 in O2-saturated 0.1 M KOH at 1600 rpm; (c) electron transfer number and H2O2 as a function of potential calculated from RRDE curve of Co9S8/ NSGg‑C3N4; (d) chronoamperometric responses of Co9S8/NSGg‑C3N4 and commercial Pt/C.

potential: 0.02 V, half-wave potential of −0.15 V). Moreover, Co9S8/NSGg‑C3N4 also delivers a strikingly higher diffusionlimiting current density (6.05 mA cm−2 at −0.9 V) than that of Pt/C (5.56 mA cm−2 at −0.9 V). The superior ORR activity of Co9S8/NSGg‑C3N4 should be benefited from its structure and composition features achieved via our synthetic strategy. First, the heteroatoms in graphene, especially the enriched pyridinic N, graphitic N, and thiophene S atoms, together with the exposed Co9S8 nanoplates, can provide abundant active sites for ORR catalysis.18−22 Besides, the highly crumpled morphology of Co9S8/NSGg‑C3N4 with high surface area can ensure the exposure of the active sites and prevent the restacking of the graphene sheets.33,35 Moreover, with the complete decomposition of g-C3N4−H+, the efficiently N- and S-doped graphene substrate could possess high electronic conductivity and enhanced interactions with the anchored Co9S8 nanoplates.23 To further assess the ORR performance of Co9S8/NSGg‑C3N4, its rotating ring-disk electrode (RRDE) test was conducted in O2-saturated KOH (0.1 M) at a rotation rate of 1600 rpm (Figure 5b). On the basis of the ring current (Ir) and disk current (Id), the electron-transfer number (n) of Co9S8/ NSGg‑C3N4 is calculated to be 3.67−3.92 over the potential range from −0.10 to −1.0 V with a low H2O2 yield of 15.0%, indicative of a four-electron transfer pathway for ORR (Figure 5c). The cycling stability of Co9S8/NSGg‑C3N4 was then evaluated in O2-saturated KOH (0.1 M) by the chronoamperometric measurement at −0.4 V with a rotation speed of 1600 rpm. As indicated in Figure 5d, the current of Co9S8/NSGg‑C3N4 can reserve ∼98% of the initial value after 20 000 s. In contrast, commercial Pt/C electrode shows a much faster current decrease (∼12%) under the same conditions. The excellent stability should be benefited from the strongly integrated Co9S8 nanoplates on the N and S codoped graphene with good distribution, suppressing detachments and aggregations. The crossover methanol tolerance of Co9S8/NSGg‑C3N4 was also measured by the addition of 3 M methanol to the reaction

atoms in Co9S8. And the existence of different binding states for sulfur at 163.8 eV (C−S−C), 164.8 eV (CS), 168.7 eV (C− SO2−C), and 169.8 eV (C−SO3−C) confirms that S atoms have also been successfully incorporated into the carbon framework (Figure 4c).21,26 The sulfur dopants in graphene should be donated from CoS during its transformation to Co9S8 nanocrystals. Calculated from the integral areas of the corresponding peaks, the amount of N and S atoms in Co9S8/NSGg‑C3N4 is 7 and 5 at. %, respectively. In contrast, the N and S dopants in Co9S8/NSGPANI are 7 and 0.7 at. % (Figure S13). The crumpled architecture together with high surface area, high content of Co9S8, and efficient doped N and S atoms make Co9S8/NSGg‑C3N4 an appealing electrocatalyst for ORR. Correspondingly, the cyclic voltammogram (CV) curves of Co9S8/NSGg‑C3N4 in O2 and N2 saturated 0.1 M KOH solutions were first recorded with Ag/AgCl as the reference electrode. As shown in Figure S14a, a single cathodic reduction peak appears at −0.2 V in an O2 saturated solution and no reduction peaks in N2 saturated solution can be observed, confirming the electrocatalytic activity of Co9S8/NSGg‑C3N4 toward ORR.6 Subsequently, the linear sweep voltammograms (LSVs) of Co9S8/NSGg‑C3N4, Co9S8/NSGPANI, Co9S8, and commercial Pt/ C (20 wt % platinum) in the O2 saturated KOH were recorded using a rotating disk electrode (RDE) at a rotation rate of 1600 rpm to further study their ORR activities. As indicated by Figure 5a, bare Co9S8 exhibits sluggish ORR catalytic activity with onset potential (E0) of −0.25 V and diffusion-limiting current density with 2.86 mA cm−2 at −0.9 V. With the N and S dual-doped graphene, Co9S8/NSGPANI manifests much improved ORR activity with E0 of −0.15 V and diffusion-limiting current density of 5.43 mA cm−2 under the same conductions. Impressively, the ORR polarization curve of Co9S8/NSGg‑C3N4 shows a sharp current increase with E0 of −0.02 V and halfwave potential at −0.1 V, which are superior to those of the other reported CoX n (X = O, S, etc.)-based catalysts,6,9,12−14,22−24,34−37,44,45 and the commercial Pt/C (onset E


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Figure 6. (a) LSVs of Co9S8/NSGg‑C3N4, Co9S8/NSGPANI, Co9S8, and commercial Pt/C catalysts in O2-saturated 0.1 M KOH in the range of 0.0−1.0 V vs Ag/AgCl. (b) Typical discharge and charge curves and (c) cycling performance over 50 h at 5 mA cm−2 of the Zn−air battery using Co9S8/ NSGg‑C3N4 as catalyst. (d) Digital image of the LEDs lighted by the Zn−air battery.

during the charge process, which should be benefited from the outstanding stability of Co9S8/NSGg‑C3N4.

system. As indicated in Figure S14b, the ORR current of Pt/C decreased sharply after the addition of methanol, while Co9S8/ NSGg‑C3N4 shows no noticeable changes in the current. To investigate the electrocatalytic behavior of the samples for OER, their polarization curves were measured in O2-saturated KOH aqueous solution (0.1 M) at 1600 rpm with Ag/AgCl as the reference electrode. As indicated by Figure 6a, bare Co9S8 shows an onset potential of 0.6 V and a current density of 25 mA cm−2 at 1.0 V. In comparison, Co9S8/NSGPANI exhibits poorer activity with an onset potential of 0.9 V, which might be due to the low loading amount of Co9S8.45 Remarkably, Co9S8/ NSGg‑C3N4 possesses the lowest onset potential of 0.5 V and the highest oxygen-evolution current at all potentials (48 mA cm−2 at 1.0 V) among the samples in this work, which also outperforms the previously reported cobalt-based OER catalysts.13,22,37,45 It is worth noting that the overvoltage between ORR and OER is a very important parameter for a bifunctional electrocatalyst.9,23,45 With both appealing ORR and OER activities, Co9S8/NSGg‑C3N4 possesses low overvoltage (0.76 V) between the ORR (voltage at 1 mA cm−2) and OER (voltage at 10 mA cm−2), which is much less than those for commercial Pt/C (0.94 V), IrO2 (1.32 V), RuO2 (1.10 V), and previously reported CoxOy/NC (0.86 V) and MnxOy/NC (0.87 V).9 The outstanding ORR and OER catalytic activities of Co9S8/ NSGg‑C3N4 render it to be promising as the oxygen electrode catalyst in a rechargeable Zn−air battery. As shown in Figure 6b, the Zn−air battery with Co9S8/NSGg‑C3N4 delivers an opencircuit voltage of around 1.48 V. Moreover, with the current density of 5 mA cm−2, the device shows almost constant discharge voltage of ∼1.24 V and charge voltage of ∼2.04 V after the charge/discharge cycles for 50 times (over 50 h, Figure 6c,d). Notably, stable voltages are revealed during the discharge process and a slight decrease in the voltages can be observed

4. CONCLUSIONS Herein, highly crumpled Co9S8/NSGg‑C3N4 was obtained via a facile ionic assembly of precursors with different charges. Benefiting from the unique wrinkled architecture, exposed Co9S8 nanoplates, together with the enriched N and S dopants in graphene, Co9S8/NSGg‑C3N4 demonstrated excellent catalytic activities and durability toward both ORR and OER. More importantly, the ionic interactions assisted fabrication strategy in this work opens up an unprecedented avenue to construct highly corrugated hybrid precursor and then the formation of highly crumpled products with high surface area and enriched active sites for much improved performance.

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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.6b15461. Additional experimental details, XRD, SEM, EDS, elemental mapping, nitrogen adsorption−desorption isotherm, pore size distribution, TGA, XPS, CV, and current−time chronoamperometric results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fan Zhang: 0000-0003-2319-6133 Yiyong Mai: 0000-0002-6373-2597 Dongqing Wu: 0000-0003-4975-6178 F


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†

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the Natural Science Foundation of China (21572132 and 21372155), Shanghai Committee of Science and Technology (16JC1400703), and MPI-SJTU Partner Group Project (Polymer Chemistry of Graphene Nanoribbons).

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Sulfur Dual-Doped Graphene

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