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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Manipulation of Heteroatom Substitution on Nitrogen and Phosphorus CoDoped Graphene as High Active Catalyst for Hydrogen Evolution Reaction Yu-Han Hung, Dipak Dutta, Chung-Jen Tseng, Jeng-Kuei Chang, Aninda Jiban Bhattacharyya, and Ching-Yuan Su J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04607 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 2, 2019

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The Journal of Physical Chemistry

Manipulation of Heteroatom Substitution on Nitrogen and Phosphorus Co-Doped Graphene as High Active Catalyst for Hydrogen Evolution Reaction Yu-Han Hung,1 Dipak Dutta1, Chung-Jen Tseng1,2, Jeng-Kuei Chang3, Aninda J. Bhattacharyya4, and Ching-Yuan Su1,2* 1 Graduate

Institute of Energy Engineering, National Central University, Tao-Yuan 32001, Taiwan

2 Dept.

of Mechanical Engineering, National Central University, Tao-Yuan 32001, Taiwan

3 Department

of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

4 Solid

State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India

To whom correspondence should be addressed: (C. Y. Su): [email protected]

ACS Paragon Plus Environment

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ABSTRACT Graphene doped with heteroatoms is known to create a unique electronic structure with comparatively much higher active sites by the synergistic coupling effect. However, the earlier attempts wherein the atomic structure of such co-doped graphene could not be altered; thus there lack of reports discussing the influence of the atomic arrangement in the catalytic performance of the co-doped graphene. Here, by co-doping P and N atoms in graphene as a model system,

we

present a facile and two-step process that the sequence of doping helps in manipulating the heteroatom substitution is of great importance in defining better crystallinity and conductivity, favorable elemental functionalities and hence improve catalytic performance. The present method provides a clean, flexible, binder-free, and readily available electro-catalyst that avoids tedious conventional synthesis and device fabrication steps. The highest P-doping percentage (6 at. %) in the present work is superior to previous reports (3 at. %). By altering the sequence of N and P– doping, the co-doped graphene electrode displayed excellent performance, with an increment of 148% in the sp2 domain size and enormous lowering in overpotential and Tafel slope (78%). Further, the efficiency of HER catalyst sustains > 98% for 20 h, which is significantly higher than the well-known MoSx (63%).

Although here the P-N co-doped system was utilized as a proof of

concept, this method could be adapted for other versatile co-doped graphene. This work may pave the way for the development of co-doped graphene-based devices where manipulation of atomic arrangement can result in a structure with properties desirable for catalytic or electronics applications.


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Introduction Hydrogen is a clean source of energy with high energy density. Currently, the electrocatalytic reduction of water to molecular hydrogen through the hydrogen evolution reaction (HER) is a promising technology to meet the market demand of hydrogen production to a solution of present and near future energy crisis.

The most fruitful state-of-the-art technology until date

uses a precious metal catalyst such as Pt to promote low overpotential and sluggish kinetics of HER, as demanded in practical applications.1-2 However, the high cost and low abundance of Pt in addition to its poisoning with CO and sulfur like species limits the practical realization otherwise demanded widespread utility of such technology.3-5 Hence, the replacement of noble metal with a cost-effective material in such devices is highly desirable. Attempts in the past involve the use of transition metals (Co, Ni, Mo, W, etc.) and their derivatives as an alternative.6-9 However, the prone to corrosion and passivation susceptibility of these catalysts substantially inhibit their usage in acidic environment for the production of hydrogen.10 Graphene, an sp2-hybridized 2D honeycomb lattice of carbon, in addition to other advantages of carbonaceous materials, possesses high conductivity, high surface area and excellent mechanical properties, which make it a promising candidate for several technological perspectives including the harsh electro-catalysis.11-13 However, the zero band-gap nature of graphene

limits

many of its applications including the catalytic performance. Current works to address this issue is substitutional doping of graphene (also known as “graphene-alloy”) to realize a “metal free” catalysts.14-16 Co-doping of graphene with two or more heteroatoms of different electronegativity than that of carbon is known to create a unique electronic structure with much more active sites than the pristine or single heteroatom-doped graphene by an effect known as the synergistic coupling effect between two heteroatoms, which enormously enhances the catalytic performance


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of graphene.17-20 The Tafel slopes for some of these co-doped graphene (B-N-P-S21: 93 mV dec−1; N-P22: 91 mV dec−1; N-F-P18: 136 mV dec-1) which are much lower than their single heteroatomdoped graphene counterpart,18, 21-22 in fact found to be very near to the commercial (20%) Pt/C (87 mV dec−1)21 and RuO2 (141 mV dec-1)18 catalysts. By using the density functional theory (DFT) and the

natural bond orbital (NBO) studies, it was found that the origin of this synergistic effect

is the redistribution of the positive and negative charges in the 2D network of graphene which in turn results in the increase in active sites, carrier concentration and improve the conductivity of graphene.28, 33 Further, the metal-free co-doped graphene catalyst is shown to maintain long-time electrocatalytic performance with high stability in both acid and alkaline electrolytic solution23 which was usually failed by the metallic counterparts.10, 24-25 Despite the vast literature of metal-free co-doped graphene as mentioned above, rarely many report till today describes the manipulation of heteroatom substitution or the influence of the atomic arrangement in the performance of the co-doped graphene systems. In this work, by adopting P and N co-doped graphene as a model system, we tried to highlight some of the salient features about this. We report that the doping sequence of P and N in the graphene which in turn helps in the manipulation of P and N arrangement in the co-doped system. It is of great importance in deciding its catalytic performance by tweaking the crystallinity (sp2 domain size), the quantity of P and N functionalities and distribution of catalytic active sites over the graphene structure. It is noteworthy to mention that the earlier effort of synthesizing NPG (first N- doping follow by P) by incorporating P in NG was not possible in the past22, and it is the novelty of the present methodology to obtain. The results indicate that the alteration of P and N doping sequence lead to vast changes in the crystallinity (148% in the sp2 domain) and catalytic performance (lowering of Tafel slope by 78%). Further, since the white phosphorous which was formed by heating red


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phosphorous at 700oC, at the end of the P-doping process revert to the original red phosphorous state that can be reused for the next P- doping step, ensuring no loss of the starting materials and a cleaner process that avoids tedious after-doping cleaning of the conventional methods.

Experimental section Synthesis of Graphene Oxide (GO)/Carbon Cloth (CC): GO was synthesis from natural graphite powder by following the improved Hummers’ method reported elsewhere.26-27 In a typical procedure, first 3 g of graphite powder was mixed with 270 mL of concentrated sulfuric acid (H2SO4, 96 %) and phosphoric acid (H3PO4, 85%) in a 500 mL flask for 2 minutes. Then 18 g of potassium permanganate (KMnO4) was slowly added to the reaction mixture within an ice-bath for 10 minutes. The reaction temperature slowly increases lower than 50 °C and vigorous stirring continued for 12 h. Subsequently, 400 mL of deionized (DI) water and 30 mL of H2O2 (30 wt. %) were added sequentially to the mixture under an icebath. Finally, the obtained solution was centrifuged at 9000 rpm for 40 minutes and washed in an HCl solution (1:10) to remove the residual metal ion, and followed by rinsing with a large amount of DI water repeatedly until the pH reached neutral. In order to extract fewer layers graphene oxide (GO) from the mixture, the additional centrifugation at 6000~3000 rpm (30 min) was employed for obtaining an aqueous solution (10 mg/mL). To prepare the pre-doped GO/CC assembly sample, the carbon cloth (4 cm  1 cm) was dipped-coated into the as-prepared GO aqueous solution (4 mg/mL), where a half-areal of the carbon cloth (2 cm  1 cm) was covered with GO.


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Two-step process for heterogeneous atoms doped graphene P-doped graphene: In a typical procedure, first a 0.5 mg/mL red phosphorous (SHOWA, 98%) dispersion was prepared in water by stirring for few minutes followed by drop-drying 0.7 mL of it onto GO/CC assembly so that the dried weight of red-phosphorous in the assembly is 0.35 g. The redphosphorous-GO/CC assembly was then loaded into a home-build quartz tube with one side sealed while another side having a gas-valve. It is worth noting, the 3 g of iron powder (Alfa, 99.9%, 100 mesh) together with the GO/CC samples were sealed in the quartz tube for absorbing the evolved oxygen species during the annealing process. Before ramping temperature, the tube was evacuated below 0.01 torr, and then it was sealed by closing the gas valve (Fig. 1a). During the annealing process, the furnace was ramped to 700oC for 1 hr thermal treatment. It is to be noted that the red phosphorus sublimes at 416~590oC leading to an increase in chamber pressure to 600 mmHg. However, this is much below the atmospheric pressure ensuring the safety. The tube was subsequently cooled down to room temperature, leading to the excess unreacted phosphorus vapors to condense down at the cooling side of the tube. Based on this separation phenomenon, no phosphide residues have been observed on the sample surface when it was cooled down to room temperature and atmospheric pressure; suggesting this to be a cleaning technique for ready to be used P-doped graphene sample.

N-doped graphene The N-doping was carried out under atmospheric pressure through the pyrolysis method. In a typical procedure, the tubular furnace with graphene sample inside was evacuated to NPG (ID/IG = 2.48) > PG (ID/IG　= 1.8) > PNG (ID/IG　= 1.0), suggesting the P-doping increases crystallinity ( lowers ID/IG ratio) and P-doping prior to N-doping yield the highest crystallinity.

By electrical and electrochemical surface induced doping, recently it has been shown that the Raman G- and 2D-band behaves quite differently concerning hole and electron doping.32,33 The G-band shows a blue-shift for both the hole and electron doping, while the 2D band shows blue- and red-shift for hole and electron doping respectively. On the other hand, the effect of chemical doping (which is usually of two types viz. surface transfer34-35 and substitutional doping 20, 22-23, 28, 36)

is quite different. In the surface transfer chemical doping,34-35 holes results in upshifts


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of both G and 2D and n-type-doping upshift the 2D but downshift the G-band frequencies where electronic effect brings the stiffening or softening of the G-band frequencies, rather than the dynamic effect as observed in electrical surface induced doping.32,33 However, later it was realized that it is not only the doping but there are several other factors including lattice strain,37-38 temperature,39 etc., that affects the shift. While isotopic compression and tension have been shown to bring about phonon hardening and softening respectively, on the other hand, for an applied uniaxial tensile stress, only 1.3% strain leads to lifting the degeneracy and split the G-band in graphene.37-38, 40 Notably, even the pristine graphene shows substrate and external force dependent variation of G and 2D band frequencies which is attributed to a combined effect of induced doping and strain on the graphene, although a vector model for the correlation of the two modes (G and 2D) has been suggested to deconvolute the coupling of strain and doping effects.41-43 However, in the present single and co-doped systems, since the hetero atoms are doped in graphene framework this may lead to even stronger correlation effect of doping and strain on single and co-doped graphene systems. As far our knowledge, there is no detailed analysis of the G and 2D band frequency shift of graphene as a functional of substitutional chemical doping which brings several structural distortions leading to strong overall effect of doping and strain on the honeycomb graphene network.28, 31 The red-shift of 2D band (= 27 cm-1) in NG as compared to GO (= 4 cm-1 compared to rGO) and the blue-shift of

G-band (= 6 cm-1 w.r.t. GO) which are in accordance

with the electron doping in NG (Fig. 2b and Table-1).32-33 P-doping on NG ( i.e. NPG) is expected to bring further n-type doping

effect44 over persisting electron-doping effect from N. The blue-

shift of 2D band (21 cm-1 w.r.t NG; 17 cm-1 w.r.t rGO) and red-shift of G-band (8 cm-1 w.r.t. NG) points towards the same (more electron doping) (Fig. 2b and Table-1) if it follows the chemical doping34-35 as explained above. The red-shift of G-bands by very large amount in PG as compared


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to the GO and rGO (= 24 w.r.t GO; 22 w.r.t rGO) as it follows chemical doping further supports the stronger electron donating nature of P atom than even N which is also reported in some of the literatures.35, 45 This may also be an add-on effect of structural strain (that brings about phonon softening38) brought about by longer P-C bond than the C-C bond (1.42 Å), forcing P-atom to adopt an sp3 geometry.31 The red-shift of the 2D band in PG (which do not follow chemical surface doping) by very large degree as compared to GO and rGO (= 54 w.r.t GO; 31 w.r.t rGO) (Fig. 2b and Table-1) may also be explained on the basis of doping-strain couple effects.41-43 On the other hand, by doping N in PG (i.e. in PNG), no shift in G band and only 8 cm-1 blue-shift in 2D band as compared to PG is observed. This minute change in 2D band in PNG may be explained on the basis of relaxation of little structural strain in PNG due to the presence of shorter N-C bonds in the vicinity of P-C bonds as will be explained in the X-ray photoelectron spectroscopy (XPS) section. The weaker n-doping character of N compared to P may also have a contribution in this shift. This is further proved by the fact that in P-N doped graphene, C-N bond internuclear distance is 0.134 nm and the next neighboring C-C bond internuclear distance is 0.138 nm, both of which

are

shorter than the C-C distance (0.14 nm) in pristine graphene.46 Thus, the substitutional chemical doping is quite different from the surface induced doping as explained above32-35 and here the doping-structural strain correlation will be more stronger that allows the G and 2D bands behave quite differently.

A glimpse at the La values (the mean average crystallite size of the sp2 domains in the nanographite system) reveals interesting insights into the crystallinity of the doped-graphene. La, which is reported to vary inversely with respect to the ID/IG can be calculated from the relation 𝐼𝐷⁄𝐼𝐺 = 𝐶()/𝐿𝑎, where C (λ) is a variable scaling coefficient that depends on the laser


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excitation.47-48 The calculated La values for different doped graphene samples are given in Table1. The trend of variation of sp2 domain (La) from GO to PNG can well be correlated to their defect structure. A decrease of sp2 domain size by 15% in NG from GO, is due to the formation of defects and deoxygenation of GO during NG formation at 900oC. On the other hand, an increase of sp2 domain by 37% in PG compared to GO is a consequence of present experimental condition of phosphorylation of the latter in presence of white phosphorous vapor. An increase of sp2 domain (La) of 146% and 80% in PNG and PG as compared to GO, respectively, may be explained based on together effect of phosphorylation,

healing of defects and building of six-fold graphitic rings.

Moreover, the increase of graphitic-N in the vicinity of P atom and the increase of P-C bonds will be discussed in the subsequent section. Another interesting fact is the relative intensity of 2D band (I2D/IG,) which was intensified significantly in PG (I2D/IG = 0.18) and PNG (I2D/IG = 0.23) and the corresponding FWHMs decrease from 270 cm-1 (GO) to 105 (PG) and 98 (PNG) respectively. This crystallization behavior of PG and PNG can well be correlated with the tremendous increase of sp2-domain size in these samples (Table-1). Noteworthy to mention here that the 2D band in NG and NPG is very weak/negligible (Table-1). This again suggests that the healing of defects and building of six-fold graphitic rings which may not be effective in NG and once NG is formed subsequent doping with P to form NPG does not provide any additional help.

However, it is noteworthy to mention that

earlier attempts to synthesize NPG failed22 and it was now possible to obtain only under present set of experimental conditions. Fig. 2c shows the bright field transmission electron microscopy (TEM) images and corresponding selected area electron diffraction (SAED) patterns of PG, NG, NPG and PNG. The images clearly show flat sheets like morphology for all of the doped graphene spreading over


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several microns in size. An interesting point to be noted is that the PG and the PNG show much clear and bright diffraction spot, indicating better crystallinity (polycrystalline) or presence of hexagonal lattices49-50 compared to the mostly amorphous nature of NG or the NPG. Note that the lattice constant is 0.246 nm for an ideal graphene lattice. Based on this calculation, the correlation of lattice constant and lattice spacing with various doping conditions was shown in Fig. S5. The result indicate that the heteroatom substitution of P atoms in graphene lead to lattice expansion.51 This result is consistent with Raman spectra, where PG and PNG show comparatively higher La values and much enhanced I2D/IG value. This also proves that the crystallinity of the co-doped graphene is highly dependent on the sequence of doping the same using the present set of experimental conditions. From XPS analysis of the doped-graphene samples (Fig. 3a and Table S1) it is seen that the maximum attainable P-doping was up to 6 at. % (in PG) and a maximum nitrogen doping attainable was 6.3 at. % (in PNG). Thus while maximum P loading (at. %) is observed in singly doped PG, maximum N-doping is attainable in co-doped PNG (6.3 at. %) and not in singly doped NG (5 at. %). Further, the least N-doping is obtained in NPG (3.5 at. %). This suggests that the order of doping is very crucial and pre-doping graphene with P helps in increasing the doping level of nitrogen in N-P-graphene. It is noteworthy to mention here that the maximum P doping percentage (6 at. %) is obtained in this work, which is the highest value as compared to the literature reports of maximum around 3 at.% in “metal free” doping.21, 31, 44-45 Following the XPS bond junction energy analysis, the N-functionalities in the present singly doped NG can be deconvoluted into three peaks, such as the pyridinic-N (398.3 eV), pyrrolic-N (400 eV) and graphitic-N (401.8 eV), while the P-functionalities in PG can be deconvoluted into two peaks as P-C (133.1 eV) and P-O (134.1) (Fig. 3b and 4).52-55 In the N-P-


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graphene the positions of these functionalities did not change markedly except the graphitic-N which shows at least 0.5 eV lower energy shift in both NPG and PNG compared to NG. This suggests a change of charge states of the elements around the graphitic-N other than carbon and/or N as in NG and it is proximity to a less electronegative element like P (2.19) as compared to C (2.55) and N (3.04).31 Thus in the N-P-graphene, the P atom might be near the graphitic N atom. Since graphitic-N atom leads to lower defect or discontinuity of the graphene six-member ring as compared to pyridinic or pyrrolic-N functionalities, a reasonably high fraction of this graphitic-N together with a higher fraction of P-C functionality in its proximity may bring more defect-free sp2 bonded areas in the N-P-graphene leading to higher conductivity.22,

31

The XPS mapping for

pyridinic-N and the P-C modes on an area of 2 cm2 of the PNG sample (Fig. 3c) clearly shows a homogeneous distribution of these functionalities over the entire mapped region, which is required for the utilization of entire graphene mass and avoid crowding together of the catalytic sites leading to facile throughput for gas-(H2) breathing out. From Fig. 4 it is clear that the fraction of graphitic-N which was 15% in NG remained almost the same in PNG (14%) while it increased to 36% in NPG. The pyridinic-N was 61% in NG, while it becomes 66% and 48% in PNG and NPG, respectively. When phosphorous functionalities are concerned the P-C bond shows a continuous increase (consecutively decrease in P-O bond) in the series PG (52.9%) < NPG (57.8%) < PNG (64.4%). The higher fraction of PC and graphitic-N and if possible in close-proximity are important because this combination will bring less defect state and higher electronic conductivity in the N-P-graphene (only 0.6 at% graphitic-N leads to a large carrier concentration of 2.6  1013 cm-2 ,  4 times higher than that of pristine graphene).56 While considering the La values (obtained from Raman data) in the series NG (1.72) < NPG (2.0) < PG (2.76) < PNG (4.96), the highest La values for PNG can be explained


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based on the right combination of the highest fraction of P-C bonds in proximity of graphitic-N. The relatively high value of La for PG (2.76 nm) may be due to the healing of defects/vacancies while synthesizing it in the vaporized phosphorous atmosphere at low pressure as will be explained in the later section. Among the co-doped graphene samples, the fraction of pyridinic-N in NPG and PNG are 48% and 66% respectively (Fig. 4). The higher pyridinic-N in co-doped PNG have been reported to be beneficial for creating electro-catalytic active sites (carbon atom with Lewis basicity next to pyridinic-N).57

Thus while

higher amount of graphitic-N in the proximity to P-

C bonds is required for maintaining the electronic conductivity and distribution of catalytic sites, the pyridinic-N might help in generating enormously large number of electroactive sites by creating carbon atoms with Lewis basicity just next to it, both of which are important for better performance of the electrocatalyst.31, 57

Electrocatalytic HER Performance of the doped-graphene samples. The electrocatalytic performance of the N-P-graphene samples in acidic aqueous condition (0.5 M H2SO4) towards HER were tested by linear sweep voltammetry in a three-electrode system, where Pt serves as the counter electrode and Ag/AgCl as the reference electrode and the results are depicted in Fig. 5. From the HER polarization curves of the single and co-doped graphene samples (Fig. 5a), it is clear that the PNG sample outperforms NPG, PG, and NG. While for

PNG

a HER current density of 10 mA/cm2 is achieved at an overpotential of 379.7 mV, the current density of NPG, NG, and PG do not even reach to 10 mA/cm2 for overpotential larger than 500 mV.

The performance of doped-graphene samples follows exactly the same order of their La

values, NG (La = 1.72) < NPG (2.0) < PG (2.76) < PNG (4.96) (Table 1 and 2). The Tafel slope of the doped graphene also follows the same order (except NPG) (Fig. 5b, Table 2). While PNG


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shows the lowest Tafel slope (126 mV/dec), the Tafel slope of NPG (565 mV/dec) is higher than the singly doped NG (405 mV/dec) and PG (348 mV/dec) (Table 2). Thus the order of doping N and P plays an important role in determining the HER performance of the co-doped graphene; The higher performance of PNG may be explained based on the combination of higher crystallinity, PC bonds in proximity to graphitic-N and higher fraction of pyridinic-N, all of which could create a synergistic effect. The HER performance of CC is negligible (Fig. 5a) and its contribution in the hybrid doped-graphene assembly can simply be ignored. Noteworthy to mention here that earlier attempts of synthesizing PNG were unsuccessful22 and hence the flavor of this ordering was not realized.

Even though the performance of PNG is not so good as the nanostructured state-of-the-

art MoS2/WS2,58 but it is comparable with a conventional metallic catalysts such as Au,59 or Mo/No alloy60 systems. The best performance of PNG among others is also depicted in the comparatively lowest charge transfer resistance of only 31.3 Ω for it (Fig. 5c, Table 2 and Fig. S6), which is consistent with the higher crystallinity as mentioned above. Moreover, the long-term stability of HER catalyst is crucial for practical application. Although the MoSx (MoSx/CC) shows outstanding catalytic activity, it was found that a sharp decrease of 37% in its efficiency after 20 h operation, while the PNG sample sustains with an efficiency decrease of only 2% (Fig. 5d) in the same testing condition. This result provide the evidence of highly stability of metal-free of PN-co-doped graphene as HER catalyst. The Tafel slope and overpotential of present work are comparable with the literature reports on similar materials (Table S2).

Understanding the underlying mechanism of better performance of PNG: 1. Exchange current density: The exchange current density as obtained here for the singly doped and co-doped graphene (Table 2) are much higher than most of the reported HER catalysts.23 These


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values are even comparable or higher than Pt catalyst (60 mA ∙ cm-2).61 In this work, the better performance of these doped-graphene samples as indicated by the exchange current density may be a result of the flexible and porous framework provided by the CC support, which in turn facilitates easy H2 gas throughput and the electrolyte inlet to the active sites.

2. Higher crystallinity and performance of PG and PNG as compared to the NG and NPG: The overall HER pathways, in general, are represented as 2H+ + 2e-

→ H2 and described by a

three-state diagram comprising of an initial state H+ + e- , an intermediate adsorbed H* and the final product 1/2H2.

The Gibbs free-energy of the hydrogen adsorption │𝐺𝐻 ∗ │ is considered

as the main signifier of HER activity and and negative values of

𝐺𝐻 ∗

𝐺𝐻 ∗

= 0 is considered to be optimal. The positive

signifies that the formation of intermediate H* and desorption of

H* to form H2 are the rate determining steps respectively.22 By using first principle calculations, it has been shown that the 𝐺𝐻 ∗ values of PG and NG can be arranged as pyridinic-N >> graphitic-N > PG. However, in the co-doped graphene the arrangement according to the 𝐺𝐻 ∗ values are (graphitic-N and P co-doped graphene) > PG > (pyridinic-N and P co-doped graphene).22 Further, it has been reported that the separation distance of P-N, the N-N and the dopants concentration and spatial distribution in the graphene structure plays a crucial role in deciding the catalytic performance of the co-doped graphene.62 This shows that having higher fraction of pyridinic-N in singly doped NG (61%) is not beneficial for HER application and consequently NG shows the worst performance (highest overpotential and Tafel slope = 405 mV/dec) among PG, NPG and PNG (Table 2). The relatively higher performance of the PG (Tafel slope = 348 mV/dec) as compared to NG and NPG may be explained by its higher crystallinity (La = 2.76 nm) and optimum 𝐺𝐻 ∗

value.


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The higher crystallinity of PG might arise from several factors. As mentioned earlier the red phosphorous that we used as the precursor for P-doping of graphene converts to white phosphorous (tetrahedral molecule) at the present experimental condition (700oC). These tetrahedral molecules of white phosphorous under the present set of experimental condition might attack a carbon atom with a dangling bond similar to the situation where reduction and healing of graphene was shown to occur by annealing GO at high temperature under alcohol atmosphere.63 Since doping takes place at the defect sites rather than at the pristine graphene sites64 and healing of defects is known to occur through removal of functional groups,65 vacancy healing in presence of gaseous species,66 migration and self-healing of vacancy,67 one or more of these might contribute to the crystallinity of PG. Since the doping was performed at high temperature, this also might have some effect in enhancing the crystallinity of this sample. The best performance of PNG might be explained on the basis of higher crystallinity (graphitic-N in the vicinity of P-C bond) and the lowest 𝐺𝐻 ∗ value because of the presence of high the est fraction of pyridinic-N (66%) and P-C bonds (64.4%). The poor performance of NPG (Tafel slope = 565.1 mV/dec) may be explained based on lower crystallinity (La = 2.0 nm), higher 𝐺𝐻 ∗ (due to lower fraction of pyridinic-N) and might have segregation of dopants too.

Conclusion In summary, here we proposed a two-step strategy for the fabrication of P and N-co-doped graphene electrode, which is a flexible, binder-free, and readily available for electro-catalyst that avoids conventional tedious multistep processes. For the first time, we demonstrate that the elemental stoichiometry and heteroatomic structure of such co-doped graphene can be manipulated, thus the correlation between the atomic arrangement and their catalytic performance


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can be systematically investigated. We found out that the sequence of N and P–doping, resulted in dramatic influence on the electro-catalytic properties of the co-doped graphene electrode. The HER performance for PNG (10 mA/cm2 at overpotential of 379 mV; Tafel slopes of 125 mV/dec) was surperior than

that of NPG. Moreover, the efficiency of HER catalyst sustains > 98% for 20

h, implying the highly stability of metal-free of PN-co-doped graphene as HER catalyst. The improved performance of PNG was attributed to the high higher crystallinity and well-distribution of large amount of catalytic active sites (the pyridinic-N). Although the P and N-co-doped graphene has been selected as a model in this study, this method could be adapted for other versatile co-doped graphene system for exploring new and similar phenomenon, which was potential for future applications on advanced catalyst and electronic devices.

Associated contents Supporting Information Available: The experimental details for Raman, sheet resistance, SEM images, lattice constant, corresponding equivalent circuit, percentage of various elements from XPS, and literature reports comparison table. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This research was supported by Ministry of Science and Technology Taiwan (MOST 106-2923E-008-003-MY3).


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Figures and Tables

Table 1. Conclusion on peak positions, 2D FWHM, ID/IG ratios, I2D/IG ratios and crystallite size (La) as obtained from Raman spectroscopy. Sample

D band

G band

Position

Position

(cm-1)

(cm-1)

2D band

2D FWHM

ID/IG

I2D/IG

La (nm)

Position (cm-1)

GO

1343

1584

2722

270

2.46

0.08

2.02

rGO

1341

1582

2699

--

2.09

--

2.37

NG

1341

1588

2695

--

2.88

--

1.72

NPG

1343

1580

2716

--

2.48

--

2.00

PG

1333

1560

2668

105

1.80

0.18

2.76

PNG

1335

1560

2676

98

1.00

0.23

4.96

Table 2. The HER properties for variously doped graphene. Sample

Onset potential 2

 2

Tafel slope (mV/dec)

EIS (Ω)

Exchange Current density (mA/cm2)

@0.5 mV/cm (mV)

@10 mA/cm

PG

536.2

--

347.6

346.5

76.3

NG

498.7

--

405.0

364.7

86.0

NPG

398.8

--

565.1

260.0

265.0

PNG

247.4

379.73

125.5

31.3

21.0


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Fig. 1. Schematic Illustrations of the two step-process for (a) P- and (b) N-co-doped graphene. While P-doping was carried out in a semi-closed tube operating at 700 oC under low pressure (a), the N-doping was carried out at 900 oC under normal pressure in continuous flow of NH3 gas (b). SEM image of a fiber of (c) the bare carbon cloth and (d) GO/CC hybrid. The schematic representation depicting the bare and graphene-coated portion of a carbon cloth flexible electrode is also shown.


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Fig. 2. (a) Raman spectra of GO, rGO, NG, NPG, PG and PNG showing the D-band, G-band and 2D-bands (especially in PG and PNG, the 2D band is more distinct). (b) The integral area ratios of the D-band to G-band (ID/IG) which clues about the crystallinity of graphene are also indicated. (c) HRTEM images and the corresponding SAED patterns recorded at the cross mark for PG, NG, NPG and PNG samples.


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Fig. 3. (a) Bar chart based on XPS data, showing the percentages of carbon, oxygen, phosphorous and nitrogen in the heterogeneously doped graphene samples. (b) The schematic of different Nfunctional groups in the graphene plane constructed based on the XPS elemental composition analysis. (c) The XPS mapping on the active area (2 cm  1 cm) for Pyridinic-N and P-C functionalities of PNG sample.


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Fig. 4. Compositional characterization for various N- and P-functional groups on heterogeneously doped graphene (PG, NG, NPG and PNG) samples based on the data from high-resolution XPS N 1s and P 2p spectra.


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Fig. 5. HER electrochemical characterizations for various doped-graphene samples in 0.5 M H2SO4 solution. (a) Polarization curves (at scan rate of 5 mV/s), (b) corresponding Tafel plots and (c) Nyquist plots for various doped-graphene samples. The dotted lines in (a) indicate overpotential of around 0.38 V at current density of 10 mA/cm2 for PNG. (d) HER stability test for MoSx and PNG.


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TOC Graphic


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