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Reduced Graphene Oxide/O–MWCNT Hybrids Functionalized with p–Phenylenediamine as High–Performance MoS2 Electrocatalyst Support for Hydrogen Evolution Reaction Zhanzhao Li, Xiaoping Dai, Kangli Du, Yangde Ma, Mengzhao Liu, Hui Sun, Xingyu Ma, and Xin Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09523 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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The Journal of Physical Chemistry

Reduced Graphene Oxide/O–MWCNT Hybrids Functionalized with p–Phenylenediamine as High– Performance MoS2 Electrocatalyst Support for Hydrogen Evolution Reaction Zhanzhao Li#a, Xiaoping Dai#a*, Kangli Dua,b, Yangde Maa, Mengzhao Liua, Hui Suna, Xingyu Maa and Xin Zhanga* a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

b

Sichuan Tianyi Science & Technology Co. Ltd., Chengdu 610225, China

#

These authors contributed equally to this work.

*

Corresponding author. Prof X. P. Dai: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. E–mail address: [email protected] Prof X. Zhang: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979 E–mail address: [email protected]

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Abstract: Efficient hydrogen evolution through water splitting at low overpotentials is crucial to develop renewable energy technology, which depends on the design of efficient and durable electrocatalyst composed of earth–abundant elements. Herein, a highly and stable electrocatalyst for hydrogen evolution reaction (HER) has been developed on the basis of MoS2 on p–phenylenediamine (PPD)–functionalized reduced graphene oxide/O–containing carbon nanotubes (rGO/O–MWCNT) hybrids via facile and green hydrothermal process. Among the prepared catalysts, the optimized MoS2/rGO/PPD/O–MWCNT with nanosized and highly dispersed MoS2 sheets provides a large amount of available edge sites and the improved electron transfer in 3D conductive networks. It exhibits excellent HER activity with a low overpotential of 90 mV and large current density of 47.6 mA·cm–2 at 200 mV, as well as an excellent stability in an acidic medium. The Tafel slope of 48 mV·dec–1 reveals the Volmer–Heyrovsky mechanism for HER. Thus, this work paves a potential pathway for designing efficient MoS2–based electrocatalysts for HER by functionalized conductive substrates. Keywords: P–phenylenediamine; Functionalized rGO/O–MWCNT; Molybdenum sulfide; Hydrogen evolution reaction


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1. Introduction Hydrogen, as a promising energy carrier in the hydrogen–economy paradigm, is mainly produced from fossil fuels by steam reformation, but increasing concerns over the effect of carbon dioxide on the earth’s climate make sustainable energy technologies attractive. Recently, water splitting for renewable energy has been proposed as an alternative way for hydrogen production in large scale.1 The hydrogen evolution reaction (HER) is one key step in the process of water splitting.2,3 To achieve a high efficiency, high active electrocatalysts for HER are crucial. Currently, the noble metals, such as platinum, exhibit the highest electroactivity toward HER, but the high cost and scarcity of noble metals may limit their large scale application.4,5 The development of highly active, stable electrocatalysts composed of earth–abundant materials still remains a major challenge. Transition–metal chalcogenides, carbides, nitrides, carbonitride and metal alloys have been widely investigated as HER catalysts.6–14 Among all these alternatives, molybdenum disulfide (MoS2) with the earth–abundant composition and high activity has received tremendous attention, but the catalytic performance depends on the exposed edge sites.15-17 Up to now, there are two strategies to optimize the electrochemical performance of MoS2, including (1) tuning the layer number for exposing more edge sites, and (2) increasing the electrical conduction for improving the electron transfer. The coupling of MoS2 nanosheets with carbon–based materials has proven to be an effective way to reduce the restacking and increase the conductivity. Reduced graphene oxide (rGO) and carbon nanotubes (CNTs) are usually used as substrates which exhibit excellent electrical conductivities and large specific surface areas. Dai’s et al. has synthesized MoS2/rGO hybrid by solvothermal method, which showed the superior HER electrocatalytic activity due to its highly exposed edges and excellent electrical property.18 Wang’s et al. has fabricated a networked MoS2/CNT nanocomposite with high number of exposed edge sites and low crystalline, which exhibited high activity towards the HER.19 To maximize the density of edge sites, three dimensional (3D) MoS2 structures have also been designed, but the poor electron transport between layers limits their catalytic efficiency.20 To improve the cathodic current, MoS2 growth on 3D substrates will benefit from the 3D conductive networks of substrates, ultrahigh 3 Environment ACS Paragon Plus


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surface area and porous structure. Furthermore, MoS2 growth on 3D substrates can significantly reduce the self–aggregation of the layered materials, and ensure high utilization efficiency. Tang et al. investigated the electronic structure of 3D graphene–MoS2 (3DGM) hybrid by first–principle calculations, and concluded that the 3DGM could be a promising candidate for HER due to its high stability, huge surface areas and excellent intrinsic properties.21 Hou et al. fabricated a 3D hybrid of layered MoS2/nitrogen–doped graphene nanosheet aerogel, which exhibited superior HER performance with higher current density and excellent stability.22

Wang et al. investigated MoS2 nanoparticles

growth on the 3D carbon fiber, and subsequently conducted Li electrochemical intercalation and exfoliation processes to expose more edge sites of MoS2, which exhibited an ultrahigh HER performance.23 Based on the self–assembly of MoS2 nanosheets on rGO, Zhou et al. built a 3D hierarchical framework of MoS2 nanosheets on graphene oxide to achieve remarkable HER performance, which might be ascribed to the good mechanical strength and high electrical conductivity of the 3D frameworks.24 The formation of defect–rich MoS2 nanosheets can be formed due to the abundant nucleation sites of graphene oxide for MoS2 deposition and oxygen incorporation. Qiao’s group developed a 3D hybrid electrocatalyst by decorating N–doped graphene hydrogel film with MoSx clusters, which has successfully combined the desired merits of highly active MoSx sites, excellent mechanical properties, highly hydrated framework for sufficient active site exposure as well as 3D conductive networks.25 The resulted catalyst showed excellent HER performance with high current density, small Tafel slope and strong durability. As mentioned above, rGO and CNTs are widely used as substrates because of their ultrahigh electrical conductivity. However, rGO sheets and CNTs tend to restack together in the reduction process due to strong van der Waals interaction.26 The incorporation of molecular or nanostructured spacer is the most common way to prevent the stacking of graphene sheets and CNTs and produce highly accessible surface area.27–29 The presence of functional groups offer the rGO or CNTs with versatile functionalization, which has been successfully grafted by isocyanates, octadecylamine, polyethylene glycol, p–phenylenediamine and m–phenylenediamine.30-33


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The effective combination of CNTs and rGO can not only inherit full advantages and fabricate new properties of both materials, but also provide 3D pathways for electrons travel and mass transport. Peng et al. have developed a facile method to produce 3D CoS2/rGO–CNT nanoarchitecture by combining hydrothermal treatment and vacuum filtration.34 The unique 3D nanoporous structure and synergistic effects from CoS2, rGO and CNTs led to the high activity and stability in HER. Youn et al. have fabricated a highly active and stable electrocatalysts for HER on the basis of molybdenum compounds on CNTs/rGO hybrid support, indicating that the CNTs–rGO hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.35 Such findings suggest the significance of carbon–based materials in HER electrocatalysis. Although these methods have been established to fabricate 3D hybrids with outstanding properties, most of these hybrids generally were prepared tediously and costly because of multiple synthesis steps. It seems likely that these materials should be of special interest for their easy synthetic accessibility, typically by one–pot syntheses, to fabricate 3D nanostructured architectures as a highly active HER catalyst. Herein, we report a facile wet–chemical strategy to fabricate 3D PPD–functionalized rGO/O– MWCNT supported MoS2 nanosheets (MoS2/rGO/PPD/O–MWCNT) for HER. The synthesis is based on a one–pot hydrothermal process of MoS2 nanosheets growth on PPD–functionalized rGO/O–MWCNT hybrids through electrostatic and π–π stacking interactions by the amine groups of PPD and oxygen–containing groups of rGO/O–MWCNT, which lead to unique 3D nanoarchitectures. As a consequence, the superior HER activity with low onset potential, high current densities and excellent stability are achieved. Such high HER activity is superior to most of MoS2–based catalysts reported so far, indicating a promising cathode catalyst candidate. 2. EXPERIMENTAL SECTION 2.1 Materials Synthesis.



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Chemicals. Ammonium tetrathiomolybdate ((NH4)2MoS4, 99.95%), Graphite flake (99.8%) and commercial Pt/C (20 wt. % Pt on Vulcan carbon black) were purchased from Alfa Aesar. N, N– dimethylformamide (DMF), p–phenylenediamine (PPD) was purchased from Sinopharm Chemical Reagent Co. Ltd. Sulphuric acid, nitric acid and ethanol were purchased from Beijing Chemical Reagent Company. Hydrazine hydrate (N2H4·H2O) and ammonium hydroxide (NH3·H2O) were purchased form Tianjin Fine Chemical Research Institute. Multiwall carbon nanotubes (MWCNT, >95% purity) were purchased from Chengdu Organic Chemicals. All reagents are analysis reagent (A.R.) and used as received without further purification. Deionized water (Milli–Q) was used for the synthesis of catalysts. Preparation of graphene oxide (GO): GO was made from flake graphite powder by a modified Hummers method.36 Typically, 1.0 g of graphite flake and 60 mL of concentrated H2SO4 (98%) was added into the 250 mL round–bottom ﬂask under stirring in an ice water bath. Then, 6.0 g KMnO4 and 1.0 g NaNO3 was immediately added to the ﬂask, and the mixture was stirred for 2 h. After that, the solution was heated to 25 °C, and kept for 4 h. Next, the solution was heated to 40 °C in water bath under stirring for another 72 h. The resultant solution was diluted with deionized (DI) water, and transferred to 500 mL beaker in an ice water bath. 30 % H2O2 were added the solution until the color of the mixture changed to brilliant yellow. The as–obtained mixture was filtered, and redispersed in DI water. The suspension was dialyzed for 48 h, centrifuged at 7000 r/min for 10 min. The upper clear liquid was dried for 48 h with freeze–drying equipment to obtain GO. Preparation of O–functionalized multiwall carbon nanotubes (O–MWCNT): Typically, 2 g of MWCNTs were mixed in a 100 mL round bottle flask with a concentrated sulfuric acid (98%, 37.5 mL) and nitric acid (68%, 12.5 mL) under vigorous stirring for 3 h at 80 oC. After that, the resulting black powder was collected by centrifugation and washed thrice with deionized water, and then the resultant was finally dried at 70 °C for 24 h. The functionalized CNT by sulfuric acid and nitric acid treated was denoted as O–MWCNT. Preparation of PPD–functionalized rGO/O–MWCNT supported MoS2: Typically, the mixture of GO (6 mg) and O–MWCNT (6 mg) in 10 mL of DMF was sonicated for approximately 30 min to form 6 Environment ACS Paragon Plus

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homogeneous solution at room temperature. 100 mg PPD and 0.06 mL NH3·H2O were added to the above solution, and then 26.4 mg of (NH4)2MoS4 was added and sonicated for approximately 20 min. After that, 0.1 mL N2H4·H2O was added to the above solution and was further sonicated for 10 min. The resulted mixture was transferred to Teflon–lined autoclave (25 mL), and was kept for 24 h at 220 oC. The product was collected by centrifugation and washed with deionized water, and dried with freeze– drying equipment. The resultant was denoted as MoS2/rGO/PPD/O–MWCNT. The various mass ratio of GO/O–MWCNT in the MoS2/rGO/ PPD/O–MWCNT can be prepared by similar process except with various mass of GO and O–MWCNT. The resulted samples were denoted as MoS2/rGO/PPD, MoS2/rGO/PPD/O–MWCNT (1:3), MoS2/rGO/PPD/O–MWCNT (3:1) and MoS2/O–MWCNT/PPD. As a

comparison,

the

MoS2/rGO/O–MWCNT

was

also

prepared

by

similar

process

with

MoS2/rGO/PPD/O–MWCNT except without PPD. 2.2 Materials Characterization. Fourier transform infrared spectroscopy (FTIR) was collected on a Per–kin–Elmer FTIR–2000 spectrometer (4 cm–1 resolution, 32 scans). Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images were obtained with FEI Tecnai G2 F20 electron microscope equipped with a field emission source at an accelerating voltage of 200 kV. X–ray diffraction (XRD) was carried out on a Brüker D8 Advance diffractometer at 40 kV and 40 mA for CuKα (λ=0.15406 nm). N2 adsorption and desorption isotherms were determined on a JW−BK222 Micromeritics adsorption analyzer at −196 °C. Prior to adsorption measurements, the samples were degassed at 150 °C in vacuum overnight. Surface areas were obtained by the Brunauer−Emmett−Teller (BET) method using adsorption data at a relative pressure range of P/P0 = 0.05-0.30. Pore size distributions were calculated using the adsorption branch of the isotherms by Barrett−Joyner−Halenda (BJH) method. Raman spectra were recorded on a Renishaw Micro–Raman System 2000 spectrometer at 532 nm of a He/Cd laser. X–ray photoelectron spectrum (XPS) analysis was performed on a PHI 5000 Versaprobe system using monochromatic Al Kα radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 284.6 eV. 7 Environment ACS Paragon Plus


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2.3 Electrochemical Measurement. All of the electrochemical measurements were carried out on a CHI660E electrochemical workstation in a three–electrode system with a reference electrode (saturated calomel electrode, SCE), a counter electrode (Pt slice) and the working electrode (glassy carbon electrode, GCE) at room temperature. Typically, the mixture of 4 mg catalyst, 1 mL water–ethanol (4:1, v/v) and 80 µL 0.5 wt. % Nafion (Alfa Aesar) was ultrasonicated for 30 min to form ink solution. 5 µL ink solution was dropped onto the GCE (0.07065 cm2) to reach the catalyst loading of 0.262 mg·cm–2. The polarization curves were obtained by linear voltammetry sweeping with scan rate of 5 mV·s −1 in 0.5 M H2SO4. AC impedance measurements were carried out at η = 0.15 V from 106–0.01 Hz. The stability test was performed by continuous cyclic voltanmmetry (CV) from –0.2 to 0.3 V (vs. SCE) at a sweep rate of 100 mV·s–1 for a given number of cycles. The estimation of the effective active surface area of the samples was carried out according to literature by cyclic voltammograms with various scan rates (20, 40, 60 mV/s, etc.) in the 0.1–0.2 V vs. RHE region.37 During the experiments, a flow of nitrogen was maintained over the electrolyte to eliminate dissolved oxygen. All results were calibrated with respect to reversible hydrogen electrode (RHE) by E(RHE) = E(SCE) + 0.273 V. The total number of active sites were calculated by electrochemical approach from cyclic voltammetry measurements in pH = 7 phosphate buffer at a scan rate of 100 mV·s−1 according to the previous methods.19,38,39

N=

Q 1 1 i ⋅ t 1 1 i ⋅V / u 1 5 ⋅ S 1 ⋅ ⋅ = ⋅ ⋅ = ⋅ = ⋅ F 2 m F 2 m F 2 F m

Where N is the total number of active sites (mol/g catalyst), Q is the integrated charge from cyclic voltammograms, F is the Faraday constant (96485 C·mol−1), i, V, t, u, m and S are the current (A), potential (V), sweep time (s), sweep rate (V·s–1), catalyst mass (g) and integrated effective area in cyclic voltammograms recorded in pH=7 phosphate buffer after deduction of the blank value for bare GCE under the same condition, respectively. 3. RESULTS AND DISCUSSION 8 Environment ACS Paragon Plus

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3.1 Materials synthesis and characterization The synthetic scheme of the PPD–functionalized rGO/O–MWCNT supported MoS2 nanosheets is presented in Figure 1A. The GO and O–MWCNT are sonicated to form homogenous solution, which are further functionalized by the reaction between amine groups of PPD and oxygen–containing groups of GO and O–MWCNT.40 The O–MWCNT and rGO sheets are pillared with the assistance of PPD driven by electrostatic and π–π stacking interactions between the abundant oxygen–containing functional groups of GO/O–MWCNT and the positively charged PPD with numerous pores.41 PPD could also be grafted onto the GO/O–MWCNT through the nucleophilic substitution reaction between the epoxide groups of GO/O–MWCNT and the amine groups of PPD during the reduction process.42 The interaction between PPD and GO/O–MWCNT is confirmed by FTIR spectroscopy (Figure S1). The stretching vibration of C−N−C mode at ca. 1100 cm–1, the stretching of N−H in −C−NH2 groups at ca. 1573 cm–1 and bending vibration mode at ca. 940 cm–1 indicate the presence of nitrogen–containing groups.40,43 Basically, there are two main interactions: one is the H−bonding interaction between the proton in the carboxylic group of GO/O–MWCNT and N atoms of PPD, another one is the π–π* interaction between benzenoid ring of PPD and the π−system of O–MWCNT/GO.44 The C–N–C bond in the MoS2/rGO/PPD/O–MWCNT originates from the PPD graft and N–doping in the rGO/O– MWCNT, which has been supported by the significantly higher ratio of N/C (0.123) in MoS2/rGO/PPD/O–MWCNT (Figure S2A–C).45 The formation of C−N−C bond by the reaction between the amine groups (−NH2) in the PPD and the containing−oxygen groups in the GO/O– MWCNT implies the successful graft of PPD. The morphologies of the as–prepared composites are identified from TEM and HRTEM. Most of MoS2 nanosheets anchor on graphene from Figure 1B. For O–MWCNT, the O–containing MWCNT act as a bridge and substrate to allow the coat of the MoS2 nanosheets along the O–MWCNT longitudinal axis (Figure 1C). The unique 3D structure are built by the O–MWCNT and/or rGO with PPD functionalization, and together serve as the good substrate for the uniform growth of MoS2 nanosheets with few layers (1 to 3 layers) in the hydrothermal environment (Figure 1D). PPD molecules could insert the space between the graphene sheets/O–MWCNT through 9 Environment ACS Paragon Plus


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the reactions of –NH2 on the para–position of benzene ring and the different graphene sheets or O– MWCNT to build a 3D structure.46 From the HRTEM images in Figure 1E and Figure S3A, the thick MoS2 nanosheets with several layers form on the rGO surface for the MoS2/rGO/PPD.47 Only little amount of MoS2 sheets grow along the longitudinal axis, and the others form thick MoS2 for the MoS2/O–MWCNT/PPD (Figure 1F and Figure S3B). The HRTEM image in Figure 1G shows that the MoS2 nanosheets uniformly grow on rGO and O–MWCNT substrates with few–layer MoS2 (1 to 3 layers). PPD molecules are as spacers to alleviate the aggregation and restacking of graphene or O– MWCNT, and form loose layered structures, which is favorable to the diffusion of electrolyte ion, leading to a great improvement of the electrochemical performance.46 It is worthwhile pointing out that the few–layer MoS2 along crystal fringes are discontinuous, which is attributed to the existence of rich defective sites.16 The 3D structures by PPD molecules and O–MWCNT as spacers are further verified by SEM (Figure S4), which shows that an interconnected 3D networks are clearly discerned. On the other hand, in virtue of the porous architecture and the ultrathin nanosheets, a relatively large specific surface area of 140 m2·g–1 and average pore size of 11.5 nm are achieved for the MoS2/rGO/PPD/O– MWCNT, which are slightly higher than those of the MoS2/rGO/PPD (122 m2·g–1, 10.2 nm) and MoS2/O–MWCNT/PPD (119 m2·g–1, 10.6 nm) in Figure S5. The unique 3D structure has two main advantages: (1) it provides a large surface area for the growth of layered MoS2, resulting in more edge active sites and defective sites,15 which play an important role for HER; (2) the hybrids by the PPD functionalization has high electrical conductivity, and improves the efficiency of electron transfer by the quick electron conducting pathway in 3D structure.34 Figure 2A compares the XRD patterns of the as–prepared MoS2/rGO/PPD, MoS2/O– MWCNT/PPD and MoS2/rGO/PPD/O–MWCNT. The peaks at 2θ =32.5, 42.6, 56.5

o

are

attributed to the (100), (105) and (110) planes of the hexagonal MoS2, respectively (JCPDS card 41–1471). The absence of (002) reflection suggests the unapparent restacking of MoS2 sheets in the composites.48 The peak at 25.5 o is indexed to the (002) plane of O–MWCNT or rGO with a d–spacing of 0.35 nm. It is noteworthy that the peak at 25.5

o


is significantly weakened on the

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MoS2/rGO/PPD/O–MWCNT, indicating that the restacking degree of graphene and O–MWCNT decreases on the MoS2/rGO/PPD/O–MWCNT due to the dual spacer of 1D MWCNT and PPD.42,49 Raman spectroscopy reveals the characteristic peaks of MoS2 at 376 cm–1 and 404 cm– 1

(inset of Figure 2B), which are corresponding to the in–plane E12g and out–of–plane A1g

vibrational modes, respectively. The relative larger width and the weaker intensity of E12g peak suggest that the crystal structure of MoS2 may contain substantial defect sites.19,50 All the samples exhibit two distinct bands at 1342 and 1575 cm–1 in Figure 3B, matching well with the D and G band of graphene or O–MWCNT in the hybrids.51 The calculated ID/IG value of MoS2/rGO/PPD/O–MWCNT (1.25) is slightly higher than those of the MoS2/O–MWCNT/PPD (1.02) and MoS2/rGO/PPD (1.13), indicating the presence of more defects in the MoS2/rGO/PPD/O–MWCNT.42,53 Figure 3 depicts the X–ray photoelectron spectroscopy (XPS) of the MoS2/rGO/PPD/O–MWCNT. The survey spectra indicates the presence of Mo, S, N, C and O elements. It gives the atomic S/Mo ratio of 2.57:1 (Figure 3A), indicating the presence of some unsaturated sulfur atoms. The high–resolution S 2p XPS spectrum in Figure 3B further confirms the existence of the unsaturated sulfur atoms (S22–) with the deconvolution peaks at 163.2 and 164.1 eV for S 2p3/2 and S 2p1/2, respectively. The peaks at 161.7 and 162.7 eV are corresponded to the S 2p3/2 and 2p1/2 orbitals of divalent sulfide ions (S2–), and the peak at 168.4 eV is attributed to presence of SO42–.39,54 In the Mo 3d region, six peaks are observed, and one of those at 226.3 eV actually corresponded to S 2s of MoS2. The four peaks at low binding energy are assigned to Mo 3d5/2 and Mo 3d3/2 of MoS2 (Figure 3C), respectively, which are relevant with the Mo4+ and Mo5+, indicating that Mo6+ can be partially reduced to Mo5+.55 The peak at 235.6 eV is associated with the Mo6+, as a result of the formation of MoS3.56 The deconvolution of high–resolution N 1s–Mo 2p3/2 spectra in Figure 3D yields four classic peaks near 394.6, 397.6, 399.8 and 401.5 eV, which are associated with Mo 2p3/2 and N–containing functional groups, such as pyridinic–N, pyrrolic– N and graphitic–N on the MoS2/rGO/PPD/O–MWCNT.57.58 We also investigate that the molar ratios of N/C are 0.0286, 0.0473 and 0.099 for MoS2/rGO, MoS2/O–MWCNT and MoS2/rGO/O–MWCNT 11 Environment ACS Paragon Plus


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without PPD, respectively (Figure S2A–C), which are significantly lower than that of the MoS2/rGO/PPD/O–MWCNT (0.123). The nitrogen may originate from the N–doped rGO/O–MWCNT by NH3·H2O and N2H4·H2O as co–reductant, and N–functionalized rGO/O–MWCNT by PPD during the hydrothermal process.46,59,60 The higher ratio of N/C can enhance the electrode wettability and facilitate the access to electrolytes in HER.51 A characteristic peak of C−N−C group at 285.4 eV confirms that PPD is grafted onto rGO nanosheets or O–MWCNT (Figure S6),52 agreeing well with the FTIR spectra (Figure S1). 3.2 PPD–functionalized rGO/O–MWCNT supported MoS2 nanosheets toward HER 3.2.1 HER activity and stability The catalytic activity of the MoS2/rGO/PPD, MoS2/O–MWCNT/PPD and MoS2/rGO/PPD/O– MWCNT toward HER are shown in Figure 4A. For comparison, the curves recorded on bulk MoS2 and commercial 20 wt. % Pt/C are also displayed. As a reference point, the Pt/C exhibits the highest activity for HER with a near−zero overpotential and high current density, but the MoS2/rGO/PPD/O–MWCNT exhibits higher HER catalytic performance and lower onset overpotential in the as−prepared catalysts. The current density on the MoS2/rGO/PPD/O–MWCNT at an overpotential of 200 mV is 47.6 mA·cm−2, which is significantly higher than those of the MoS2/O–MWCNT/PPD (19.8 mA·cm−2) and MoS2/rGO/PPD (20.5 mA·cm−2). Although the HER activity on the MoS2/rGO/PPD/O–MWCNT is lower than that of MoS2/N–MWCNT in our previous work,7 the preparation procedure is facile and green hydrothermal method without NH3 emission. Furthermore, the MoS2/rGO/PPD/O–MWCNT shows much lower onset potential (~90 mV) and higher current density (at 200 mV) than most of representative MoS2−based HER electrocatalysts reported up to date (Table S1), such as exfoliated MoS2 nanosheets,37 amorphous MoSx,59 defect–rich MoS2,15 oxygen–incorporated MoS2 nanosheets,16 MoS2/rGO,18,62 MoS2/CNTs,19 MoS3/CNTs,63 MoS2/Mo2C–NCNTs.64 This performance can be favorably compared with most of recently reported efficient non–precious MoS2−based electrocatalysts. The MoS2/rGO/PPD/O–MWCNT catalyst also exhibits excellent performance than that of the MoS2/rGO/O–MWCNT without PPD (Figure S7), indicating that the PPD plays a positive role in HER. 12 Environment ACS Paragon Plus

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Besides the HER activity, high stability is of importance for a good electrocatalyst. The polarization curves in Figure 4B show the negligible loss of the cathodic current after 1000 CV sweeps in acidic media. The superior durability is probably attributed to the tight binding between the MoS2 and rGO/PPD/O–MWCNT by the in−situ fabrication of MoS2 and robust feature of the 3D structure. 3.2.2 The mechanism and kinetics of HER The corresponding Tafel plots obtained from overpotential versus log current density are shown in Figure 4C, which are often used to predict the rate–limiting step of HER.65 Generally, the HER includes two possible steps: Volmer−Tafel and Volmer−Heyrovsky.66 The commercial Pt/C catalyst shows the Tafel slope of 32 mV·dec−1, which is close to the reported values (30 mV·dec−1).67 The observed Tafel slopes are 40, 41 and 48 mV·dec−1 for MoS2/rGO/PPD, MoS2/O–MWCNT/PPD,

MoS2/rGO/PPD/O–MWCNT,

suggesting

that

electrochemical

desorption reaction is the rate–limiting step, and thus that the Volmer–Heyrovsky HER mechanism is responsible for the HER. Electrochemical impedance spectroscopy (EIS) study is usually used to investigate the electrode kinetics in HER. In Figure 4D and Figure S8, only one semicircle at η=150 mV in each EIS nyquist plots suggests that the equivalent circuit for the electrocatalysis is characterized by one time constant. A constant phase element (CPE) in the equivalent circuit from fitting the impedance data represents the deviation from the ideal capacitance behavior corresponding to a frequency–dependent phase–angle, as depicted in the inset of Figure 4D. The MoS2/rGO/PPD/O–MWCNT exhibits the lowest internal resistance (Rs, 0.8 Ω·cm2 vs. 1.2, 1.4 and 2.0 Ω·cm2) and charge transfer resistance (Rct, 18.4 Ω·cm2 vs. 57.7, 46.6 and 32.1 Ω·cm2) than those of MoS2/O–MWCNT/PPD, MoS2/rGO/PPD and MoS2/rGO/O–MWCNT (Table 1), indicating the fast charge transfer during the electrocatalytic reaction on the MoS2/rGO/PPD/O– MWCNT. This means that the growth of MoS2 on the PPD–functionalized rGO/O–MWCNT provides an additional electron−transport path besides the graphene layers or O–MWCNT,



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which improve the electrical connectivity between the graphene layer and O–MWCNT by PPD functionalization, suggesting favorable charge transport in the 3D path.35,68,69 3.3. The origin of superior HER performance We also investigate the effect of the mass ratio of GO to O–MWCNT in the MoS2/rGO/PPD/O–MWCNT on HER activity. The polarization curves and current density at 200 mV with various mass ratio of GO to O–MWCNT indicates the shape of a volcano in Figure 5A–B. The MoS2/rGO/PPD/O–MWCNT (1:1) exhibits the highest HER activity among the MoS2/rGO/PPD/O–MWCNT catalysts with various mass ratio. The superior performances of the MoS2/rGO/ PPD/O–MWCNT (1:1) could be mainly ascribed to two origins: (1) the lower Rct and Rs of the MoS2/rGO/PPD/O–MWCNT is beneficial to achieve efficient charge transport in 3D path (Figure 5C and Table 1); (2) the appropriate proportion between GO and O– MWCNT by PPD functionalization would help to generate more edge–exposed sites for HER. The calculated total number of active sites by cyclic voltammetry measurements in pH=7 phosphate buffer (Figure 5D) confirms that the MoS2/rGO/PPD/O–MWCNT (1:1) shows the highest density of active sites (0.783 mmol·g−1) among as–prepared MoS2/rGO/PPD/O– MWCNT samples (Figure 5B), giving direct evidence of the predominant enrichment effect of active edge sites. The intrinsic activity can also be described by exchange current densities (j0). The MoS2/rGO/PPD/O– MWCNT (1:1) also exhibits much higher exchange current density (8.32 µA·cm−2), which is 5.7−34.7 times larger than those of as−prepared samples (Figure S9 and Table 1), suggesting the excellent intrinsic activity. The electrochemical double–layer capacitances capacitance (Cdl) can be used to further estimate the effective surface area of the solid–liquid interface, assuming that the two quantities are linearly proportional.37 The MoS2/rGO/PPD/O–MWCNT (1:1) shows the highest Cdl (221.2 mF·cm–2) than those of as−prepared samples (Figure 6, Figure S10 and Table 1). The results indicate the high exposure of effective active sites for MoS2/rGO/PPD/O–MWCNT (1:1) by PPD functionalization, which is responsible for the excellent HER activity. Thus, the fast charge transfer during the 14 Environment ACS Paragon Plus

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electrocatalytic reaction coupling with large exposed active surface area could contribute to the superior electrocatalytic activity of in the 3D the MoS2/rGO/PPD/ O–MWCNT architecture composite.62,70 4. Conclusion A facile and environmentally friendly hydrothermal approach is proposed to fabricate PPD– functionalized rGO/O–MWCNT supported MoS2 electrocatalyst for HER. Benefiting from the unique 3D structure for fast charge and mass transport, the MoS2/rGO/PPD/O–MWCNT hybrid exhibits a large output current density (47.6 mA·cm–2 at 200 mV) and low overpotential (~90 mV) in HER. The as– prepared MoS2/rGO/PPD/O–MWCNT electrocatalyst also shows an excellent stability during 1000 cycles. Tafel slope of 48 mV·dec−1 reveals the rate–limiting step of the electrochemical desorption in HER. The excellent HER performances have been attributed to the fast charge transfer during the electrocatalytic reaction coupling with large exposed active surface area. This work highlights the significance of the formation of 3D structure based on the in–situ growth of MoS2 nanosheets on the PPD–functionalized rGO/O–MWCNT hybrid in the enhancement of HER electrocatalytic activity. Acknowledgments The authors acknowledge the financial supports from the NSFC (No. 21576288, 20903119, 21173269 and 91127040), Ministry of Science and Technology of China (No. 2011BAK15B05), and Specialized Research Fund for Doctoral Program of Higher Education (20130007110003). ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E–mail: [email protected]; [email protected] Author Contributions


The Journal of Physical Chemistry #

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These authors contributed equally to this work. The manuscript was written through contributions of

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Bifunctional Natural Catalysts and Their Applications as Scaffolds for High–Rate Lithium–Sulfur Batteries. Adv. Mater. 2014, 26, 6100–6105 (62) Zheng, X. L.; Xu, J. B.; Yan, K. Y.; Wang, H.; Wang, Z. L.; Yang, S. H. Space–Confined Growth of MoS2 Nanosheets within Graphite: The Layered Hybrid of MoS2 and Graphene as an Active Catalyst for Hydrogen Evolution Reaction. Chem. Mater. 2014, 26, 2344–2353 (63) Lin, T. W.; Liu, C. J.; Lin, J. Y. Facile Synthesis of MoS3/Carbon Nanotube Nanocomposite with High Catalytic Activity toward Hydrogen Evolution Reaction. Appl. Catal. B: Environ. 2013, 134, 75–82 (64) Zhang, K.; Zhao, Y.; Zhang, S.; Yu, H. L.; Chen, Y. J.; Gao, P.; Zhu, C. L. MoS2 Nanosheet/Mo2C–Embedded N–doped Carbon Nanotubes: Synthesis and Electrocatalytic Hydrogen Evolution Performance. J. Mater. Chem. A 2014, 2, 18715–18719 (65) Chen S.; Duan J. J.; Jaroniec M.; Qiao S. Z. Three–Dimensional N–Doped Graphene Hydrogel/NiCo Double Hydroxide Electrocatalysts for Highly Efficient Oxygen Evolution. Angew. Chem. Int. Ed. 2013, 52, 13567–13570 (66) Duan, J. J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N−Graphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931–941 (67) Duan, J. J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Heteroatom−Doped Graphene-Based Materials for Energy−Relevant Electrocatalytic Processes. ACS Catal. 2015, 5, 5207−5234 (68) Huang, Z.; Chen, Z.; Chen, Z.; Lv, C.; Humphrey, M. G.; Zhang, C. Cobalt Phosphide Nanorods as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction. Nano Energy 2014, 9, 373–382 (69) Huang, Z.; Lv, C.; Chen, Z.; Chen, Z.; Tian, F.; Zhang, C. One–pot Synthesis of Diiron Phosphide/Nitrogen–Doped Graphene Nanocomposite for Effective Hydrogen Generation. Nano Energy 2015, 12, 666–674 (70) Merki, D.; Vrubel, H.; Rovelli, L.; Fierro, S.; Hu, X. Fe. Co and Ni Ions Promote the Catalytic Activity of Amorphous Molybdenum Sulfide Films for Hydrogen Evolution. Chem. Sci. 2012, 3, 2515–2525 23 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 24 Environment ACS Paragon Plus

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FIGURE CAPTIONS

Figure 1 (A) Synthetic scheme of the MoS2/rGO/PPD/O–MWCNT; representative TEM and HRTEM images of (B, E) MoS2/rGO/PPD, (C, F) MoS2/–MWCNT/PPD and (D, G) MoS2/rGO/PPD/O– MWCNT.



Intensity (a.u.)

A

(100)

(105)

C(002)

(110) MoS2/rGO/PPD/O-MWCNT MoS2/O-MWCNT/PPD MoS2/rGO/PPD

10

20

30

40

50

60

70

o

2θ ( )

B

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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360

390

420

M oS2/ rGO /P P D/O -MWCNT

MoS 2/rG O /P PD

MoS2 /O-MWCNT/ P PD

1000

1250

1500

175 0

200 0

Raman shift (cm-1)

Figure 2 (A) XRD patterns and (B) Raman spectra of MoS2/rGO/PPD, MoS2/O–MWCNT/PPD and MoS2/rGO/PPD/O–MWCNT.


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Element S C N O Mo Atom, % 12.6 65.7 8.1 8.7 4.9

C 1s N 1s Mo 3p

O 1s

B

Intensity (a.u)

Intensity (a.u)

A

600

500

400

2-

2-

S2 2p1/2

S 2p3/2

2-

S2 2p3/2

2-

S 2p1/2

Mo 3d S

S 2p

300

200

100

175

6+

170

Binding energy (eV)

165

160

155

Binding energy (eV)

C

D IV

Mo 3d3/2 V

Mo 3d3/2

V

Mo 3d5/2

VI

Mo 3d S 2s

240

235

230

225

Binding energy (eV)

Intensity (a.u)

Mo 3d5/2

IV

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mo2p3/2

Pyridinic N Pyrrolic N Graphic N

408 406 404 402 400 398 396 394 392 390 388 386

Binding energy (eV)

Figure 3 (A) XPS survey spectrum, and high–resolution spectra of (B) Mo 3d–S 2s, (C) S 2p, (D) N 1s– Mo 2p3/2 of MoS2/rGO/PPD/O–MWCNT catalyst.



A

0

Bulk MoS2 -20 MoS2/O-MWCNT/PPD

2

Current density (mA/cm )

-40 -60 -80 MoS /rGO/PPD 2 -100 Pt/C

-120 MoS2/rGO/PPD/O-MWCNT

-140 -0.4

-0.3

-0.2

-0.1

0.0

B

-20 -40 -60 -80 -100 MoS2/rGO/PPD/O-MWCNT

-120

MoS2/rGO/PPD/O-MWCNT 1000 cycles

-140 -0.3

0.1

-0.2

Potential (V vs. RHE) 0.40

-0.1

800

C

D

0.35

CPE MoS2/O-MWCNT/PPD

0.30

Rs

600

Rct

0.15

Bulk MoS2 MoS2/rGO/PPD MoS2/rGO/PPD/O-MWCNT

0.10

-Z'' (ohm)

0.25 0.20

0.0

0.1

Potential (V Vs. RHE)

-Z'' (ohm)

2

Current density(mA/cm )

0

Overpotential (V Vs. RHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15 10 5 0

400

0

10 20 30 40

Z' (ohm)

MoS2/O-MWCNT/PPD

200 MoS2/rGO/PPD/O-MWCNT

0.05 0.00 0.1

Pt/C MoS2/rGO/PPD

0

1

10

100

0

200

2

Current density (mA/cm )

400

600

800

Z' (ohm)

Figure 4 (A) Polarization curves, (B) Durability for the MoS2/rGO/PPD/O–MWCNT, (C) Tafel plots, (D) EIS nyquist plots (symbol), and fitted data (solid line) by equivalent electrical circuit diagrams (left inset) of bulk MoS2, MoS2/rGO/PPD, MoS2/O–MWCNT/PPD, MoS2/rGO/PPD/O–MWCNT.


2.0

B

2

Current density(mA/cm )

60

A

0

50 1.5

2

Current (mA/cm )

-20 -40 -60 GO/O-MWCNT=1:3

-80 -100

GO/O-MWCNT=3:1

40 30

1.0

20 0.5 10

-120

GO/O-MWCNT=1:1

-140 -0.3

0

-0.2

-0.1

0.0

0.1

0.0

400

300

15

0.0002 O-MWCNT

5

10

20

30

Z' (ohm)

200

GO/O-MWCNT=1:1 0 100

200

300

400

500

600

GCE

-0.0002 GO/O-MWCNT=1:1 GO/O-MWCNT=3:1

-0.0004

100

GO/O-MWCNT=1:3

0.0000

GO/O-MWCNT=3:1

0

O-MWCNT

1:3

D GO

10

0 0

GO/O-MWCNT=1:3

1:1

The mass ratio of GO to O-MWCNT 0.0004

Current (A)

-Z'' (ohm)

C

3:1

GO

Potential (V vs. RHE) 500

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Density of Active sites (mmol/g)

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-0.0006 -0.2 -0.1

0.0

Z' (ohm)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Potential (V vs. RHE)

Figure 5 (A) Polarization curves, (B) Current density at 200 mV and density of actives sites, (C) EIS nyquist plots recorded in 0.5 M H2SO4, and (D) Cyclic voltammograms (–0.1~0.6 eV) recorded in recorded in pH=7 phosphate buffer (scan rate: 100 mV·s–1) on the MoS2/rGO/PPD/O–MWCNT with various mass ratio of GO to O–MWCNT.



4

6

A

B

20-180 mV/s

3

MoS2/GO/PPD MoS2/GO/PPD/O-MWCNT(3:1)

5

MoS2/GO/PPD/O-MWCNT(1:1) MoS2/GO/PPD/O-MWCNT(1:3)

2 4

∆j / mA cm-2

Currrent/ mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 0 -1

MoS2/O-MWCNT/PPD MoS2/rGO/O-MWCNT

3

2

-2 1

-3 0

-4

0

0.10

0.12

0.14

0.16

0.18

50

0.20

Potential / V vs.RHE

100

150

200

Scan rate / mV s-1

Figure 6 (A) Cyclic voltammograms with various scan rate from 20 to 180 mV·s–1 at E = 0.1– 0.2 V vs. RHE over the MoS2/rGO/PPD/O–MWCNT, (B) The differences in current density variation (∆j) at an overpotential of 0.15 V plotted against scan rate fitted to a linear regression enables the estimation of Cdl.


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Table 1 Electrochemical Parameters of the MoS2−based hybrids. Log|j, mA·cm-2| (η=0 V) −3.41

j0 (µA·cm−2) 0.39

Cdl (mF·cm−2)

Rct (Ω·cm2)

Rs (Ω·cm2)

63.8

46.6

1.4

MoS2/rGO/PPD/O-MWCNT(3:1)

−2.84

1.45

111.1

23.0

0.9

MoS2/rGO/PPD/O-MWCNT(1:1)

−2.08

8.32

221.2

18.4

0.8

MoS2/rGO/PPD/MWCNT(1:3)

−3.30

0.50

101.9

38.4

1.0

MoS2/O-MWCNT/PPD

−3.61

0.24

51.4

57.7

1.2

MoS2/rGO/O-MWCNT

−2.99

1.02

61.6

32.1

2.0

Catalysts MoS2/rGO/PPD



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Table of contents graphic


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O-MWCNT Hybrids ... - ACS Publications

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