Co, N, S Tri-doped Graphene Templated from In-situ

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Co nanoparticles/Co, N, S tri-doped graphene templated from in-situ formed Co, S co-doped g-C3N4 as an active bifunctional electrocatalyst for overall water splitting Geng Zhang, Ping Wang, Wang-Ting Lu, Cao-Yu Wang, YongKe Li, Cong Ding, Jiangjiang Gu, Xin-Sheng Zheng, and Fei-Fei Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08138 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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ACS Applied Materials & Interfaces

Co Nanoparticles/Co, N, S Tri-doped Graphene Templated from In-situ Formed Co, S Co-doped g-C3N4 as an Active Bifunctional Electrocatalyst for Overall Water Splitting

Geng Zhang,a Ping Wang,a Wang-Ting Lu,b Cao-Yu Wang,a Yong-Ke Li,a Cong Ding,a Jiangjiang Gu,a Xin-Sheng Zheng a and Fei-Fei Cao a,*

a

Department of Chemistry, College of Science, Huazhong Agricultural University, 430070, Wuhan, P. R. China.

b

Institute for Interdisciplinary Research, Jianghan University, 430056, Wuhan, P. R. China.

*To whom correspondence should be addressed. E-mail: [email protected] (Fei-Fei Cao)

Keywords: graphitic carbon nitride; cobalt, nitrogen, sulfur tri-doped graphene; template; hydrogen evolution reaction; oxygen evolution reaction

1

ACS Paragon Plus Environment


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Abstract The development of high-performance electrocatalyst with earth-abundant elements for water-splitting is a key factor to improve its cost efficiency. Herein, a noble metal-free bifunctional electrocatalyst was synthesized by a facile pyrolysis method using sucrose, urea, Co(NO3)2 and sulfur powder as raw materials. During the fabrication process, Co, S co-doped graphitic carbon nitride (g-C3N4) was first produced, and then this in-situ formed template further induced the generation of a Co, N, S tri-doped graphene coupled with Co nanoparticles (NPs) in the following pyrolysis process. The effect of pyrolysis temperature (700, 800 and 900oC) on the physical properties and electrochemical performances of the final product was studied. Thanks to the increased number of graphene layer encapsulated Co NPs, higher graphitization degree of carbon matrix and the existence of hierarchical macro/meso pores, the composite electrocatalyst prepared under 900oC presented the best activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with outstanding long-term durability. This work presented a facile method for the fabrication of non-noble metal based carbon composite from in-situ formed template, and also demonstrated a potential bifunctional electrocatalyst for the future investigation and application.

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1. Introduction Splitting water through electrolysis by using renewable energy (e.g., solar and wind energy) is not only an ideal pathway to achieve high-purity hydrogen but also an effective energy storage method to utilize intermittent solar and wind energy.1-3 However, the high cost and scarcity of noble metals inhibit the large-scale deployment of water electrolysis technologies, because noble metals (e.g., Pt, IrO2/RuO2) are currently needed to catalyze HER on the cathode and OER on the anode. Therefore, it is highly attractive to develop efficient water-splitting electrocatalysts with earth-abundant elements.1-2, 4 Due to the features of diverse morphology, high surface area, and tailorable molecular and pore structures, non-noble metal-based carbon composites have attracted great interest in the development of electrocatalysts for HER and OER.1-2 In order to fabricate a composite structure with well-defined morphology, the template-induced strategy has been proved to be an effective method, in which the final product will copy the shape of template. Generally, the template can be divided into two types. One type of template provides a framework to build material and usually should be removed at last, such as silica (e.g., mesoporous silica,5 SiO2 nanoparticles

6-7

) and Te

nanowires.8 The other one is that the template is finally transformed into carbon materials without the need to remove it properly, such as metal-organic frameworks (MOFs),9-10 natural architecture from biomass,11-12 CdS13 and graphitic carbon nitride (g-C3N4).14 One interesting finding is that the great majority of these templates were pre-synthesized and subsequently added into the reaction system, which involves the accurate synthesis of template and the compatibility between template and reactant, thus the difficulty and complexity in the fabrication of final product (i.e., non-noble metal-based carbon composite) was increased inevitably. As a result, if the template can 3



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be in-situ formed during fabrication, the synthesis procedures will certainly be simplified. Graphene-based

composite

has

been

acknowledged

as

a

kind

of

promising

electrochemical-active material due to its featured two-dimensional (2D) morphology, high conductivity and easy functionalization.15-16 As mentioned above, g-C3N4 is an ideal template to fabricate heteroatom-doped graphene owing to its 2D lamella structure and high nitrogen (N) content. It is

reported that g-C3N4

can be generated during the calcination of

melamine/dicyandiamide/urea through thermal polymerization,17 while these reagents happen to be frequently-used nitrogen sources in the synthesis of N-doped carbon material, thus it is reasonable to consider that g-C3N4 can serve as an in-situ produced template to induce the generation of graphene-based material. Pan et al. fabricated N-doped graphene from urea and sucrose, during which g-C3N4 was thought to be in-situ formed at first and then played the role as a 2D template in inducing the generation of graphene in the subsequent decomposition process.18 In contrast, Zou et al. obtained a tubular composite in the form of Co-embedded N-doped carbon nanotubes by calcinating dicyandiamide and cobalt salt, during which Co2+-functionalized g-C3N4 was produced as an intermediate material.19 Our group fabricated Co/N-doped carbon composites by employing cobalt salt, urea and sucrose, and it was found that the morphology of the final product was determined by the content of Co in the composite: the low Co content will give rise to a graphene-like material coupled with Co nanoparticles (NPs), whereas the high Co content will produce tubular composite.20 Therefore, it is feasible to make use of g-C3N4 as an in-situ formed template to induce the production of non-noble metal-based graphene composite under the right conditions. Moreover, it should be pointed out that the aforementioned carbon materials transformed from in-situ formed g-C3N4 are all mono-doped by nitrogen, but it has been 4


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recognized that the dual or tri-doped carbon material is more active in HER or OER than mono-doped material.7, 21-26 Unfortunately, scarce published reports focused on the fabrication of multi-doped carbon material with the assistance of in-situ formed g-C3N4 template. Inspired by the high-performance of N, S dual-doped carbon7, 21-24, 26-28 and Co-based noble metal-free material2,

19, 29-34

for HER and OER, we successfully prepared a composite

electrocatalyst in the form of Co, N and S tri-doped graphene coupled with Co NPs. During the fabrication process, Co, S co-doped g-C3N4 was produced at first, and then this in-situ formed template was further transformed into Co, N and S tri-doped graphene coupled with tiny amount of metallic Co in the subsequent pyrolysis process at 700 oC. If the pyrolysis temperature was increased to 800 or even 900 oC, the content of metallic Co together with the catalytic activity was increased correspondingly. As a result, this Co-N-S-C composite obtained at 900oC exhibited the highest activity for both HER and OER in alkaline media with outstanding long-term durability, indicating a potential non-noble metal-based carbon composite as a bifunctional electrocatalyst for the future application. 2. Experimental section 2.1 Reagents The water used in the experiment was ultrapure water (18.25 MΩ cm). Sucrose, urea, cobalt nitrate hexahydrate, sublimed sulfur, ethanol and potassium hydroxide were provided by Sinopharm Chemical Reagent Co. Ltd. with analytical grade. All chemicals were used as-received without further purification. 2.2 Electrocatalyst preparation In a typical process, urea (3.0 g), sucrose (0.05 g) and cobalt nitrate hexahydrate (0.03 g) was 5



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successively dissolved in ultra-pure water (3.0 mL). Subsequently, this purple red solution was dried at 80 oC and the product was ground thoroughly in a glass mortar. Afterwards, the fine powder was put into a porcelain boat with a cover at the downstream position, and sublimed sulfur powder (1.0 g) was placed at the upstream position in the same porcelain boat. The porcelain boat was treated at 600 oC for 1 h with a heating rate of 3.5 oC min-1 under flowing Ar (~130 mL min-1) in a tubular furnace, and then a brown powder was obtained, which is denoted as SSUCo-p, in which S, S, U, Co and p stands for sulfur, sucrose, urea, cobalt and precursor, respectively. Subsequently, SSUCo-p was further calcined at 900oC for 1 h at a heating rate of 10oC min-1 under flowing Ar (~40 mL min-1), and a black powder was finally produced, which was denoted as SSUCo-900. The preparation process was illustrated in Figure S1. For the sake of investigating the effect of pyrolysis temperature on the properties of catalyst, SSUCo-p was calcined at 700 and 800oC, respectively, and the catalyst obtained was denoted as SSUCo-700 and SSUCo-800 correspondingly. Moreover, a metal-free sample named as SSU-900 was prepared by similar procedures except for that cobalt nitrate hexahydrate was removed. 2.3 Physical characterizations X-ray diffraction (XRD) analysis was conducted on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation. The scanning electron microscopy (SEM) images were recorded by a Hitachi SU8010 field-emission SEM. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2000EX microscope. The high-resolution TEM (HRTEM) and scanning TEM (STEM) analysis was performed on an FEI Tecnai G2 F30 microscope. Raman spectra were collected on a Renishaw inVia spectrometer with 633 nm laser excitation. The thermogravimetric (TG) analyses were tested on a NETZSCH TG 209F3 TG analyzer from room 6


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temperature to 800 oC at a heating rate of 10 oC min-1 under air flow. X-ray photoelectron spectra (XPS) of samples were obtained from a Thermo Scientific ESCALAB 250Xi spectrometer using Al Kα radiation. Specific surface areas and pore size distributions were measured on a Micromeritics ASAP 2460 analyzer by using N2 sorption analysis. 2.3 Electrochemical measurements The electrochemical measurement was carried out in a three-electrode system connected by a CHI-760D electrochemical station (CH Instruments, Inc.). The home-made catalyst or commercial catalyst (20% Pt/C and IrO2, Johnson Matthey) was dispersed in a mixture of ethanol and Nafion (Sigma-Aldrich) by sonication for 1 h. The working electrode was prepared by dipping the catalyst slurry on the carbon cloth (W0S1002, CeTech, Taiwan) followed by drying in air under ambient temperature. The catalyst-loaded carbon cloth was then fixed on a commercial electrode holder (J110, Aida, Tianjin, China). Graphite rod instead of Pt acted as the counter electrode to avoid the possible interference from Pt atoms. Saturated calomel electrode (SCE) was adopted as the reference electrode, but all electrode potentials, unless otherwise noted, were given versus reversible hydrogen electrode (RHE) according to the equation: ERHE = ESCE + [0.241 6.61×10-4×(T-298)] + 1.98×10-4×T×pH.35 The catalytic activity of electrocatalysts towards HER and OER was evaluated by linear sweep voltammetry (LSV) method at a scan rate of 5 mV s-1. Electrochemical impedance spectra (EIS) of catalysts were recorded at a given potential with a frequency range from 105 Hz to 1 Hz. The cyclic voltammetry (CV) curves of electrocatalysts were obtained in a non-Faradaic region with various scan rate (10-160 mV s-1). All the electrochemical measurements were performed in N2-purged 1 M KOH. 7



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3. Results and discussion 3.1 Material structure analysis

Figure 1 Schematic illustration of the preparation process of SSUCo-900. The electrocatalyst was prepared via a two-step pyrolysis strategy (Figure 1 and Figure S1). Firstly, the mixture of sucrose, urea and cobalt nitrate was calcinated at 600 oC in the presence of sulfur vapor to obtain Co, S co-doped g-C3N4 (see below) denoted as SSUCo-p in the form of a brown powder. Next, this SSUCo-p was adopted as an in-situ formed template to induce the formation of final product (SSUCo-900) through the following pyrolysis process at 900 oC in the form of a black powder. The TEM image in Figure 2a shows that SSUCo-900 presents a lamellar structure

in

the

form

of porous

graphene-like

material

coupled

with

NPs.

The

higher-magnification TEM image (Figure 2b) clearly demonstrates a layered morphology of the carbon matrix with a large amount of wrinkles. It should be noted that SSU-900 prepared in the absence of Co(NO3)2 also presented a porous graphene-like morphology (Figure S2), suggesting that the introduction of Co had unremarkable effect on the apparent morphology of carbon component. The selected area electron diffraction (SAED) pattern (Figure 2c) of SSUCo-900 exhibits a series of diffraction rings that can be ascribed to the face-centered-cubic (fcc) Co 8


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crystal,32 and the lattice fringes displayed (0.177 nm) in the HRTEM image is believed to come from Co(200) crystallographic planes (Figure 2d), illustrating the presence of metallic Co in the composite. Further investigation revealed that many Co NPs were encased in a carbon shell as shown in Figure 2e where a 0.34 nm interlayer distance indicates high graphitization degree of the carbon shell.20 The STEM technique was applied to characterize the element distribution of SSUCo-900. The bright-field image (Figure 2f) shows that dark dots locate not only in Co NPs but also in the carbon matrix, while bright dots locate at the same place in the corresponding high-angle annular dark ﬁeld (HAADF) image (Figure 2g), indicating that Co atoms also distribute in the graphene matrix not just in Co NPs. The EDS analysis demonstrated that SSUCo-900 was composed of C, N, O, S and Co element (Figure S3). The elemental mapping (Figure 2h) derived from EDS shows a homogeneous distribution of C, N and S element over the entire carbon architecture which suggests that the graphite lattice is doped by N and S heteroatoms. Furthermore, it is found that Co signal also releases from graphene matrix, just weaker than that of Co NPs, which is believed to confirm the presence of Co element in the graphene. Considering the composition of SSUCo-900, the Co element may probably exist in the form of single atom coordinating with N and S heteroatoms in the graphene.36

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Figure 2 (a, b) TEM images of SSUCo-900; the arrows in (a) show the existence of pores in the carbon matrix. (c) SAED pattern of SSUCo-900. (d, e) HRTEM images of SSUCo-900. (f) The bright field STEM image of SSUCo-900. (g) HAADF-STEM image of SSUCo-900 taken at the same place with (f). (h) Elemental mapping of C, N, S, Co and O in SSUCo-900.

The crystalline structure of SSUCo-900 was characterized by XRD. The peaks emerging at two theta values of 44.5°, 51.8°, and 76.1° (Figure 3a) can be indexed to the (111), (200) and (220) crystalline plane of FCC Co (PDF #15-0806), respectively, which is consistent with the result of SAED. As for the metal-free material (SSU-900), two broad peaks appear at ~25o and ~43o, which can be assigned to the (002) and (101) plane of graphitic carbon, respectively.14, 18 The peaks from graphitic carbon are not obvious in SSUCo-900, which may be caused by the decreased content of carbon and the shield effect of Co diffraction signals. Raman spectroscopy shows G band (1586 10


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cm-1) and D band (1341 cm-1) in SSUCo-900 (Figure 3b), confirming the presence of sp2 hybridized graphitic structure and structural defects in the graphene matrix, respectively.37-38 The ID/IG ratio is usually applied to evaluate the defect level of doped carbon materials.37-38 Herein, the ID/IG ratio for SSUCo-900 is 1.20, indicating a large number of defects and/or doping atoms in the carbon structure. In addition to D and G bands, two peaks locate at 468 and 672 cm-1 in the Raman pattern of SSUCo-900, which can be attributed to the characteristic modes of CoO.39-40 The presence of CoO in Raman but absence in XRD indicated that Co NPs were oxidized slightly in contact with air and CoO only distributed on the surface.20

Figure 3 (a) XRD patterns and (b) Raman spectra of SSU-900, SSUCo-p and SSUCo-900. High-resolution (c) C 1s and (d) S 2p XPS spectra of SSUCo-900. (e) High-resolution N 1s XPS spectra of SSUCo-900 and SSU-900. (f) High-resolution Co 2p3/2 XPS spectrum of SSUCo-900.

The XPS was applied to analyze the near surface composition and bonding structure of selected element. As shown in Figure S4, five elements, C, N, S, O and Co, can be detected, 11



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consistent with the results from EDS. Figure 3c shows the high-resolution C 1s XPS spectrum of SSUCo-900. The peak centered at 284.7 eV is assignable to the sp2 hybridized C=C bond, while the peak at 285.9, 287.4 and 290.1 eV can be indexed to C=N/C-O/C-S, C-N/C-O-C and O-C=O species, respectively,27, 41-42 confirming the doping nature of the graphene matrix in SSUCo-900. The high-resolution XPS spectrum of S 2p of SSUCo-900 can be deconvoluted into four peaks centered at 163.8 eV (C-S-C), 165.1 eV (C=S) and 168.1 eV (SOn) (Figure 3d), further confirmed the successful sulfur-doping into the carbon lattice,21,

26

while the peak at 162.0 eV can be

assigned to the Co-S bonding in the composite material.43-44 Moreover, four N species can be identified in the graphene matrix by XPS (Figure 3e): pyridinic N (398.7 eV), graphitic N (401.1 eV), pyrrolic N (399.8 eV) and oxidized N (402.1 eV).18 In comparison with SSU-900, the N content of SSUCo-900 was increased from 9.56 at.% to 10.1 at.%, meanwhile a new peak centered at 399.0 eV emerged (Figure 3e), which can be assigned to the Co-N species formed by the bonding between N and Co atoms.33, 45-46 The presence of Co-N species was further confirmed by the Co 2p3/2 spectrum (Figure 3f), because two peaks centered at 780.0 and 781.6 eV were observed in addition to the metallic Co at 778.4 eV, which can be indexed to Co species with higher-valence (such as CoNx, CoCxNy, Co-S and CoO).33, 45, 47-48 The above results proved that Co element is inserted in the graphene matrix in the form of single atom, which correlates well with the result from STEM analysis.

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Figure 4 (a) High-resolution Co 2p3/2 XPS spectra of SSUCo-900, SSUCo-800, SSUCo-700 and SSUCo-p. (b) The Co weight percent and atomic percent of SSUCo-900, SSUCo-800 and SSUCo-700. (c) The proportion of various N, Co and S species in SSUCo-900, SSUCo-800 and SSUCo-700 derived from XPS; Co(X) stands for CoNx, CoCxNy, CoO and Co-S. (d) The TEM images of SSUCo-p, SSUCo-700, SSUCo-800 and SSUCo-900, which show the evolution of morphology of Co-N-S-C composite from 700oC to 900oC.

In order to investigate the formation mechanism of Co-N-S-C composite, the structure of SSUCo-p was characterized in detail firstly. As reported previously, the pyrolysis of urea at elevated temperature in inert atmosphere will produce g-C3N4. In this work, the absence of G-band and D-band in the Raman spectrum of SSUCo-p indicates that graphitic carbon structure has not formed yet (Figure 3b). The N 1s XPS spectrum of SSUCo-p (Figure S5a) is very close to that of g-C3N4 reported in the literature: the peak at 398.4 eV and 400.0 eV originates from the sp2-bonded N in triazine rings (C-N=C) and the bridging N atoms in N(-C)3 or N bonded with H 13



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atoms in g-C3N4, respectively.49-51 Moreover, the peaks centered at 287.8 eV, 286.1 eV and 284.8 eV in the C 1s spectrum of SSUCo-p (Figure S5b) can be ascribed to the sp2-bonded C in g-C3N4 framework, C-O from insufficiently decomposed sucrose and C-C/C=C from adventitious graphite carbon from the XPS instrument, respectively.14, 49 As for the Co element in SSUCo-p, the Co atom probably coordinates with two adjacent pyridinic-N atoms from two separate triazine units of g-C3N4.52 There are three evidences to support this conclusion: 1) a single peak centered at 781.0 eV appears in the Co 2p3/2 XPS spectrum (Figure 4a), which can be ascribed to the CoNx structure;19, 50 2) the presence of intense shake-up satellite peak at the high binding-energy side of the main peak proves the presence of high-spin Co(II) species (Figure 4a);50 3) XRD fails to detect any crystalline Co species (Figure 2a) and TEM does not find obvious NPs (Figure S6). Additionally, the existence of S element in SSUCo-p was also evidenced by XPS (Figure S5c). The XPS peak centered at 164.4 eV comes from C-S bonds formed by substituting N by S in the framework of g-C3N4,53 and the peak at 168.5 eV probably results from the SO42- group formed by binding S with O atoms on the surface of g-C3N4.53-54 The XRD pattern of SSUCo-p (Figure 2a) shows a broad diffraction peak around 27.0o, which is assignable to the (002) interlayer diffraction of a CN graphitic-like structure.49 This value is slightly smaller than the normal 2theta degree of g-C3N4 (27.4o), which may be caused by the introduction of Co and S atoms into the lattice of g-C3N4. Furthermore, the TEM image of SSUCo-p (Figure S6 and Figure 4d) shows a porous network which is believed to be constructed by g-C3N4 nanolayers. On the basis of XPS, Raman spectroscopy, XRD and TEM, it can be concluded that the dominant phase of SSUCo-p is Co, S co-doped g-C3N4. In order to produce electrochemical-active Co-N-S-C composite, SSUCo-p was then 14


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calcinated at an elevated temperature (≥700oC). After heat treatment at 700oC, a carbon-based architecture was obtained because D and G bands were observed by Raman spectroscopy (Figure S7). The TEM image of SSUCo-700 (Figure 4d) shows a porous and lamellar carbon structure, but Co NPs are hardly observed, which is consistent with the XRD pattern where the Co diffraction peak is quite broad (Figure S8), suggesting the low content and small particle size of Co NPs. Moreover, XPS further illustrated that nearly all of Co atoms were in high valence (i.e., CoNx and CoCxNy) and the number of zero-valence Co atoms was negligible (Figure 4a). With the increment of pyrolysis temperature from 700 to 900 oC, the metallic Co XRD peaks of Co-N-S-C composite were sharpened (Figure S8), and the density of Co NPs together with the average particle size of Co NPs were increased (Figure 4d and Figure S9). The variation of chemical structure of N, Co and S species in the product at different temperature was analyzed in detail. Firstly, the content ratio of graphitic N/Co-N in the final product was increased gradually with pyrolysis temperature, which indicated the preferential decomposition of Co-N bond in the doped graphene, because graphitic N had high thermal stability (left panel in Figure 4c and Figure S10).18 Secondly, the Co weight percent in Co-N-S-C composite measured by TG was increased from 33.9% of SSUCo-700 to 39.4% of SSUCo-900, whereas the Co atomic percent in the material from XPS presented an opposite trend from 4.43% to 2.16% (Figure 4b, Figure S11 and Table S1). The increase of Co weight percent from TG was due to the reduction of unstable species in the product with the increase of pyrolysis temperature, while the decrease of Co atomic percent from XPS can be explained by the fact that more and more Co atoms released from doped graphene at higher temperature and then assembled to form Co NPs, thus only the near surface Co atoms on the NP can be detected by XPS. Further analysis verified the above assumption: the content of 15



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high-valence Co in the Co-N-S-C composite was decreased, whereas the content of Co(0) was increased (Figure 4a and middle panel of Figure 4c). Finally, the content of C-S-C/C=S species with higher thermal stability55 was increased with the increase of pyrolysis temperature (right panel of Figure 4c and Figure S10). All of the above results demonstrated a clear process of the transformation from SSUCo-p to Co-N-S-C composite: the in-situ formed Co, S co-doped g-C3N4 nanolayers acted as templates to result in graphene-like Co-N-S-C composite after pyrolysis. At relatively low pyrolysis temperature, the state of Co atom in the composite tended to copy that of Co in SSUCo-p, i.e., Co atoms doped in the graphene matrix in the form of single atom coordinated with pyridinic N. In contrast, the Co-N bond will be destroyed at higher pyrolysis temperature 56, and the produced Co atoms assembled and formed Co NPs.

Figure 5 (a-c) SEM images of SSUCo-900. (d) N2-sorption isotherms for SSUCo-900 and the inset shows the corresponding pore size distribution based on the BJH model using the data from adsorption curve.

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The porosity of electrocatalyst and electrode played an important role in the energy storage and conversion devices.5, 57 The pore structure of SSUCo-900 was analyzed by SEM and N2 sorption technique. The low magnified SEM image (Figure 5a) demonstrates that SSUCo-900 presents a lamellar structure, and the higher magnified images (Figure 5b and 5c) clearly shows the existence of macropores in the Co-N-S-C composite, which is in line with the TEM image (Figure 2a). The N2-sorption isotherms of SSUCo-900 (Figure 5d) are of characteristic type-IV with distinct H3-type hysteresis loops, indicating the presence of large amounts of mesopores.58 The absence of steep initial uptake (p/p0

Co, N, S Tri-doped Graphene Templated from In-situ

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