Efficient Electrocatalytic Hydrogen Evolution from MoS2

Feb 28, 2017 - The electrocatalytic performance of Mo2N–MoS2 composites in the hydrogen evolution reaction (HER) was revealed from the high catalyti...
7 downloads 21 Views 5MB Size
Download PDF
Forum Article www.acsami.org

Efficient Electrocatalytic Hydrogen Evolution from MoS2‑Functionalized Mo2N Nanostructures Kasinath Ojha,† Soumen Saha,† Shivali Banerjee,† and Ashok K. Ganguli*,†,‡ †

Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India Institute of Nano Science and Technology, Mohali, Mohali, Phase-10, Sector-64, Punjab-160062, India

‡

S Supporting Information *

ABSTRACT: Molybdenum-based compounds and their composites were investigated as an alternative to Pt for hydrogen evolution reactions. The presence of interfaces and junctions between Mo2N and MoS2 grains in the composites were investigated to understand their role in electrochemical processes. Here we found that the electrocatalytic activity of Mo2N nanostructures was enhanced remarkably by conjugation with few-layer MoS2 sheets. The electrocatalytic performance of Mo2N−MoS2 composites in the hydrogen evolution reaction (HER) was revealed from the high catalytic current density of ∼175 mA cm−2 (at 400 mV) and good electrochemical stability (more than 18 h) in acidic media. Increasing the amount of MoS2 in the composite, decreases the HER activity. The mechanism and kinetics of the HER process on the Mo2N−MoS2 surface were analyzed using Tafel slopes and charge transfer resistance. KEYWORDS: Mo2N, MoS2, nanocomposites, hydrogen evolution reactions, electrocatalysis

■

INTRODUCTION Earth-abundant transition metal-based compounds, e.g., nitrides, 1−3 carbides, 4−8 phosphides,9−13 and chalcogenides,14−19 have been demonstrated as electrocatalysts for electrochemical water splitting. Mo-based compounds like Mo2C, Mo2N, MoS2, and MoP have also been explored in detail for the hydrogen evolution reaction (HER). However, it is more interesting to look at the electrocatalytic activity of composites having junctions and interfaces. Youn et al.20 have compared the electrocatalytic activities of Mo2C, Mo2N, and MoS2 in composites with carbon nanotube (CNT)−graphene hybrids. They have shown that MoS2 has lower HER activity compared with Mo2C and Mo2N in the composites, as MoS2 has a higher sheet resistance and higher charge transfer resistance (Rct). However, among Mo2C and Mo2N, Mo2C is highly efficient for the HER process. It has been reported that a biomass-derived Mo2C−Mo2N composite synthesized by reacting Mo ions with soybean, which acts as the source of C and N, exhibits much superior HER activity compared with its counterparts.21 Recently, there have been several reports on the role of the heteroanions on the metal center and their activity in hydrogen evolution reactions. It has been shown that doping with heteroanions leads to higher HER activity compared with parent compounds, as in the case of S- and N-codoped Mo2C,22Fe−WCN,23 tungsten carbonitride,24 MoP|S,25 CoP| S,26 S-decorated Mo2C,27 soybean-derived Mo2C/Mo2N composite,21 Mo2C@N-doped carbon,28,29 etc. In the case of “Mo”-based inorganic compounds, the presence of positively © XXXX American Chemical Society

charged Mo induces a downshift in the d-band center that favors the electron donation ability of molybdenum and decreases the hydrogen binding energy.7 This is favorable for hydrogen desorption from the catalyst’s surface and makes it a promising material for electrochemical hydrogen evolution. Therefore, the heterojunctions and interfaces in composites play a very crucial role in controlling the hydrogen binding energy at the interfaces and hence controlling the HER activity. MoS2 sheets have been grown on Mo2C, and the heterojunctions have been demonstrated for hydrogen evolution reactions.29 It has been shown that when MoS2 is embedded on Mo2C surfaces, the HER activity increases drastically compared with bare MoS2 and Mo2C.29 Wang and co-workers have decorated Mo2C with sulfur, which results in a composite of Mo2C coated with MoSx that exhibits a synergistic effect toward the hydrogen evolution reaction.27 However, It has been shown that higher sulfur content reduces the electrochemically active surface area (ECSA) and increases the charge transfer impedance.27 Therefore, heteroanions in transition-metalbased compounds modify the electronic structure of the metal center, affecting its hydrogen binding energy. Again, the electronegativity of the heteroanions also plays a very crucial Special Issue: Focus on India Received: August 25, 2016 Accepted: February 20, 2017

A

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces

Figure 1. Powder XRD patterns of (a) MoS2 synthesized hydrothermally and (b) Mo2N−MoS2 (MN−MS) composites.

MoS2 sheets coupled with Mo2N nanostructures. Mo2N nanostructures with different morphologies (hexagonal, triangular, and nanowires) as discussed in our earlier report38 were coupled with MoS2 sheets, and the composites were investigated for HER activity. The HER activity of the Mo2N−MoS2 systems was found to be remarkable compared with those of the bare parent compounds.

role in determining the hydrogen binding energy. However, metal doping on MoS2 as well as compounds like M−MoS2 and M−MoS3 (M = Co, Ni) have been investigated for superior HER activity.30,31 In this context, the structure of MoS2 is very important for HER activity. The stable structure of bulk MoS2 has a trigonal-prismatic structure, and this phase is known as 2H-MoS2, where two stoichiometric S−Mo−S layers with hexagonal atomic arrangement are present in the structure. This 2H-MoS2 phase can be further exfoliated into the 1H-MoS2 phase, which contains only one S−Mo−S layer.32 A single layer of MoS2 with an octahedral 1T atomic arrangement is known as the 1T-MoS2 phase. The distorted version of 1T-MoS2 is known as the 1T′-MoS2 phase.32 It has been found that the 1H-MoS2 and 1T-MoS2 phases are not as active as the metastable 1T′-MoS2 phase.32 The edges of MoS2 sheets have been identified as the active sites for its remarkable HER activity.33,34 The HER activity of MoS2 has been correlated linearly with the number of MoS2 edge sites.34 Zheng et al.35 have shown that S vacancies in the MoS2 basal plane are very important for modifying the electronic structure of molybdenum, which creates active sites for hydrogen binding. A lower concentration of S vacancies results in more positive ΔG values, indicating weaker hydrogen binding, whereas a higher concentration of S vacancies in the basal plane favors hydrogen binding on MoS2. The optimum hydrogen binding (ΔG = 0), where hydrogen binds neither too strongly nor too weakly,33 is achieved by the ∼12.5% S vacancies.35 However, in MoS2 the van der Waals interactions between the basal planes play an important role in their charge transfer process during the HER. The conductivity of MoS2 in the direction perpendicular to the basal plane needs to be considered for the insignificant HER activity of bulk MoS2. The resistivity of MoS2 perpendicular to the basal plane has been determined to be 2200 times higher than that parallel to the basal plane.36 The hydrogen binding energy of 0.18 eV for the S edges in MoS2 decreases to 0.1 eV (close to 0.08 eV for the Mo edge) upon doping of Co into the S edges.37 This enables the edges of MoS2 to act as active sites for the HER process. When these MoS2 edges and surfaces are coupled with other Mobased compounds (e.g., Mo2N), it creates a junction between them that might result in a significant contribution to the electronic structure of Mo centers at the interface. Therefore, it is very important to look at the coupling of MoS2 edges with the Mo2N surface to create interfaces and junctions in order to evaluate their activity in hydrogen evolution reactions. In view of the above, it is worthwhile to investigate the role of interfaces and heterojunctions in the composites for hydrogen evolution reactions. Here we synthesized few-layer

■

MATERIALS AND METHODS

Reagents. Aniline (Qualigens) and ammonium molybdate, thiourea, HCl, and melamine (CDH Chemicals) were used without any further purification. Nafion (Sigma-Aldrich) was used for electrode preparation. Water (double-distilled) was used in all of the experiments. MoS2 Synthesis. MoS2 sheets were synthesized using a hydrothermal method. Ammonium molybdate (0.1 mmol) and thiourea (5 mmol) were added to 70 mL of water, and the mixture was kept in a Teflon bomb at 180 °C for 12 h. The product was annealed at 500 °C for 5 h under an atmosphere of N2. Synthesis of Mo2N−MoS2 Composites. Different morphologies of Mo2N (hexagonal, triangular, and nanowires) were synthesized as reported in our earlier paper38 and conjugated with few-layer MoS2 sheets. Briefly, 100 mg of a particular Mo2N nanostructure was added to a mixture of 0.1 mmol of ammonium molybdate, 5 mmol of thiourea, and 70 mL of water. The mixture was kept in a hydrothermal bomb after sonication for 30 min and heated at 180 °C for 12 h. After the hydrothermal reaction, the product was centrifuged and dried in air. Later the product was annealed at 500 °C under N2 for 5 h. We also varied the amount of MoS2 in the composite with triangular Mo2N, where 0.01, 0.1, and 0.25 mmol of ammonium molybdate were used. Characterization Techniques. MoS2 nanostructures and the composites were analyzed by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å). Samples for transmission electron microscopy (TEM) were dispersed in ethanol by sonication for 5 min. A small drop of the dispersion was poured onto a carbon-coated copper grid (200 mesh) and dried in air. The TEM measurements were carried out on a JEOL model JEM2100 transmission electron microscope operated at 200 kV. The X-ray photoelectron spectroscopy (XPS) data were collected from the sample pellet using a Thermo Scientific ESCALAB-MkII photoelectron spectrometer at a pressure of about 1 × 10−9 mbar at room temperature. Shirley background was corrected before the peak deconvolution. Electrochemical Measurements. Electrochemical analyses were carried out on an Autolab (PGSTAT302N) instrument using a catalyst-coated glassy carbon electrode (GCE), a Ag/AgCl (sat. KCl) electrode, and a graphite rod as the working electrode, reference electrode, and counter electrode, respectively. For the preparation of catalyst ink, 2.5 mg of the catalyst was dispersed in a mixture of isopropanol (150 μL), water (90 μL), and Nafion (10 μL). A 5 μL drop of the ink was placed on the GCE (area = 0.03 cm2) and air-dried for 5 min. The mass loading of the catalyst on the electrode was 1.6 B

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces mg cm−2. All three electrodes were placed in the cell containing 0.5 M H2SO4 solution (electrolyte). Linear sweep voltammetry (LSV) data were recorded at a sweep rate of 5 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were carried out using the same electrochemical setup at different applied overpotentials at an amplitude of 10 mV. All of the potentials are reported with respect to the reversible hydrogen electrode (RHE).

■

RESULTS AND DISCUSSION Different morphologies of Mo2N (hexagonal, triangular, and nanowires) were synthesized as in our earlier report.38 The powder XRD pattern of bare Mo2N (Figure S1) suggests the formation of Mo2N in a tetragonal structure. In the first step of the reaction, the anilinium ammonium molybdate complex was synthesized in the presence of g-C3N4, and the product mixture was heated at 750 °C for 5 h under an atmosphere of N2. This resulted in the formation of Mo2N nanostructures with various morphologies. Later, the Mo2N nanostructures were added to a mixture of ammonium molybdate and thiourea, and the mixture was treated hydrothermally at 180 °C followed by annealing at 500 °C under N2 to synthesize Mo2N−MoS2 (MN−MS) composites. Pure MoS2 was synthesized hydrothermally at 180 °C followed by annealing at 500 °C under N2. The hexagonal structure of bare MoS2 was confirmed from the powder XRD pattern (Figure 1a), and the crystallinity of MoS2 was enhanced after annealing at 500 °C. The powder XRD pattern of the composite (Figure 1b) confirmed the presence of both Mo2N and MoS2. The TEM image of MoS2 sheets confirmed the presence of five to eight layers (Figure S2a) with an interlayer distance of ∼0.64 nm, which is close to that in bulk MoS2.39 The Raman spectrum of pure MoS2 (Figure S2b) shows the presence of two Raman bands at 378 and 405 cm−1 due to the E2g and A1g Raman-active modes, respectively, and the difference between the two Raman frequencies (A1g − E2g) is equal to 27 cm−1, which corresponds to multilayer/bulk molybdenum sulfide.40 Mo2N nanostructures having hexagonal, triangular, and nanowire morphologies were conjugated with few-layer MoS2 sheets. From the TEM micrographs, it was observed that fourto six-layer MoS2 sheets were embedded on the Mo2N nanostructures (Figure 2). In Figure 2b, the presence of MoS2 sheets has been marked, and these MoS2 sheets were grown on Mo2N surfaces, resulting Mo2N−MoS2 interfaces and junctions. The (200) lattice fringes corresponding to Mo2N were indexed in the high-resolution TEM micrograph (Figure 2c). Figure S3 displays more TEM images of hexagonal MN− MS showing the lattice fringes corresponding to both Mo2N and MoS2. However, the hexagonal shape is not clearly visible because the edges are broken. The high-resolution TEM image shows the presence of (200) lattice fringes of Mo2N along with MoS2 layers. The interlayer distance between MoS2(002) layers is 0.67 nm, which is longer than that in bulk MoS2 (0.62 nm).39 Mo2N having a triangular morphology forms interfaces and junctions with MoS2, as observed in Figure 2d−f. HRTEM images of MN−MS having a triangular morphology of Mo2N show the presence of (200) lattice fringes of Mo2N along with MoS2 layers (Figures S4 and S5). We confirmed the presence of MoS2 in one of the samples by confocal Raman spectroscopy (Figure S6). In the Raman spectrum of triangular MN−MS, the signature A1g and E2g Raman bands of MoS2 were observed at 384 and 410 cm−1, respectively (Figure S6). Similarly, Mo2N nanowires were coupled with MoS2 sheets (Figure 2g,h), and

Figure 2. TEM micrographs of (a−c) hexagonal, (d−f) triangular, and (g−i) nanowire MN−MS composites.

(112) lattice fringes corresponding to Mo2N were identified from the high-resolution TEM image (Figure 2i). X-ray Photoelectron Spectroscopy Analysis. We carried out XPS analysis of one of the composites, i.e., the triangular MN−MS composite, to provide better insights to the compositions and surface electronic states. The high-resolution binding energy spectrum of Mo 3d shows three peaks with binding energies centered at 233.0, 229.7, and 228.7 eV (Figure 3a), which correspond to Mo6+, Mo4+, and Moδ+ (0 < δ < 3) species, respectively. The Mo4+ and Moδ+ species correspond to MoS2 and Mo2N,41,42 respectively. Since there is an overlap of the N 1s and Mo 3p3/2 peaks, deconvolution of the peak shows two different Mo 3p3/2 peaks with binding energies centered at

Figure 3. X-ray photoelectron spectroscopy of the triangular MN−MS composite showing the binding energies of (a) Mo 3d, (b) Mo 3p, and (c) S 2p. C

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces

MoS2 results in fewer MoS2 edges, which have been shown to exhibit remarkable HER activity.47 We also investigated the composites with various Mo2N nanostructure morphologies and found that triangular MN−MS composites exhibit superior HER activity compared with the hexagonal MN−MS and nanowire MN−MS composites (Figure 4b). We found that the composites before heat treatment do not exhibit significant HER performance (Figure S7). A maximum current density of 15 mA cm−2 at 400 mV was observed in the case of MN−MS having triangular Mo2N. The other two composites exhibited very poor HER activity. However, Pt wire exhibits a much superior HER activity, with a current density of 10 mA cm−2 at an overpotential of 72 mV (Figure S8a). The HER activity of the composite was found to be much superior to those of bare MoS2 and bare Mo2N (Figure S9). Pure MoS2 showed a much lower current density (5 mA cm−2) during HER studies (Figure S9), which indicates the formation of the HER-inactive 2HMoS2 phase comprising multilayer sheets with fewer exposed surfaces and active edges. Mechanism of Hydrogen Evolution Reactions. Hydrogen evolution reactions follow either the Volmer−Heyrovsky or Volmer−Tafel reaction mechanism. The reaction steps are given below: Volmer reaction:

398.8 and 395.4 eV (Figure 3b). The binding energy centered at 395.4 eV is due to Mo 3p3/2 for both Mo2N and MoS2,43 and the binding energy at 398.8 eV is due to Mo 3p3/2 of MoO3, which may be due to surface oxidation during the measurement. N 1s peaks corresponding to Mo2N might have overlapped with Mo 3p peaks. Deconvolution of the S 2p peak reveals the presence of S 2p1/2 and S 2p3/2 binding energies at 164.0 and 162.6 eV, respectively (Figure 3c). The S 2p doublet with the higher binding energy can be attributed to bridging S22− and/or apical S2− ligands.44−46 This suggests that there are more bridging and apical S ligands than terminal/edge ones. Therefore, XPS confirms that there is strong bridging of MoS2 with Mo2N surfaces leading to Mo2N−MoS2 interfaces and junctions. Electrochemical Hydrogen Evolution Studies. Electrochemical hydrogen evolution reactions were investigated using Mo2N−MoS2 composites in 0.5 M H2SO4 aqueous solution using a typical three-electrode system, where a graphite rod and a Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Linear sweep voltammetry (LSV) studies at a sweep rate of 5 mV/s (Figure 4a) exhibit the

H+ + e− + M(catalyst) ⇄ MHads

Heyrovsky reaction: H+ + e− + MHads ⇄ M + H 2

Tafel reaction: 2MHads ⇄ 2M + H 2

where M denotes the catalyst. The Tafel slope and exchange current density indicate the reaction mechanism and kinetics of the process. The Tafel slope was calculated from the linear fit of the plot of overpotential versus log(|current density|), as shown in Figure 4c. The triangular MN−MS composite exhibits HER activity with a Tafel slope of 59 mV/decade (Figure 4c) and an exchange current density of 6 μA cm−2. This suggests that Mo2N−MoS2 composites follow the Volmer−Heyrovsky reaction mechanism for hydrogen evolution. The Tafel slopes do not match with the standard values of 29, 38, and 116 mV/ decade corresponding to the Tafel, Heyrovsky, and Volmer reactions, respectively, as the rate-determining step.48 This indicates that the Volmer−Heyrovsky mechanism takes place for hydrogen evolution in acidic media, in which the Volmer reaction is the rate-determining step with very low surface coverage followed by the faster Heyrovsky desorption process. This also agrees with earlier reports.21 However, the Volmer− Heyrovsky reaction mechanism in which the Heyrovsky reaction is the rate-determining step is also possible when the surface coverage with protons is quite high. Cyclic Voltammogram of the Catalyst. The cyclic voltammogram (CV) (Figure 5a) shows a large oxidation current beyond 0.9 V (vs RHE), which completely oxidized the catalyst (mainly Mo2N as mentioned in our earlier report38) and resulted complete loss of the HER activity. Other irreversible reduction and oxidation peaks were observed in the CV. The reduction and oxidation potentials shifted with successive cycles of the cyclic voltammetry studies (Figure 5a). Peak I shifted from 0.4 to 0.37 V, whereas peak II shifted from −0.09 to −0.05 V. They correspond to the redox peaks of

Figure 4. (a) Linear sweep voltammetry (LSV) curves showing the current densities of triangular MN−MS composites with various MoS2 contents. (b) LSV curves of Mo2N−MoS2 composites with different morphologies, where 0.1 mmol of ammonium molybdate was used to get MoS2. (c) Tafel plot calculated from the LSV curve of triangular MN−MS showing a Tafel slope of 59 mV/decade.

current density as a function of applied potential. We investigated the HER activity of triangular MN−MS composites, in which Mo2N nanostructures with triangular morphology were conjugated with MoS2 sheets. We varied the amount of MoS2 in the composite by controlling the amount of ammonium molybdate (e.g., 0.01, 0.1, or 0.25 mmol). Figure 4a suggests that the composite where 0.1 mmol of ammonium molybdate was used shows the highest HER activity, with a maximum current density of ∼175 mA cm−2 at an overpotential of 400 mV. It requires only overpotentials of 128 and 190 mV to drive current densities of 1 and 10 mA cm−2, respectively. When we increased the amount of MoS2, the HER activity decreased, perhaps as a result of the higher amount of free multilayer MoS2 sheets and lower amount of Mo2N−MoS2 interfaces and junctions. A larger amount of free multilayer D

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces

Figure 7. Electrochemical impedance spectroscopy (EIS) measurement: (a) Nyquist plots and (b) Bode plots of triangular MN−MS at different applied bias potentials (−20 to −200 mV vs RHE).

Figure 5. Cyclic voltammograms of Mo2N−MoS2. (a) CV up to a potential of +1.2 V (vs RHE). The inset shows a large oxidation current at 0.9 V. (b) CV up to +0.8 V.

(−20 to −200 mV vs RHE). At higher applied potential, the radius of the semicircle decreases, signifying a lower charge transfer resistance (Rct) and higher rate of hydrogen evolution. Rct was determined from semicircle fit of the EIS data using an equivalent circuit of Rs − Rct|Cdl. Rct decreases with increasing applied potential (Figure S11a), and the Tafel slope calculated from the Rct values is 50 mV/decade (Figure S11b), which matches with the Tafel slope calculated from LSV. The triangular MN−MS composite exhibits an Rct value of 76 Ω at an applied potential of 200 mV (Figure S11a). The EIS studies of MN−MS having different morphologies of Mo2N (Figure S12) suggest that the MN−MS composite with a triangular morphology of Mo2N exhibits a lower Rct compared with the other composites (with hexagonal and nanowire morphologies of Mo2N). The MN−MS composites exhibit the lowest Rct of 76 Ω and the highest double-layer capacitance of 463 μF at an overpotential of 200 mV (Table S1). However, MN−MS having hexagonal Mo2N and MN−MS having nanowire Mo2N were found to exhibit charge transfer resistances of 458 and 623 Ω, respectively, and their double-layer capacitances were found to be 376 and 142 μF, respectively, which are lower than that of MN−MS having triangular Mo2N (Table S1). Again, the solution resistance is lower in the case of MN−MS having triangular Mo2N. Our earlier report suggests that only Mo2N nanostructures exhibit Rct ≈ 4000 Ω at an applied potential of 200 mV, which is much higher than that of the composites.38 This clearly indicates that the lower HER activities of both MoS2 and Mo2N have been enhanced remarkably by coupling them together in a composite (Mo2N−MoS2) and creating active interfaces and junctions. From the Bode diagram (Figure 7b), it is observed that the value of the maximum phase angle decreases and the frequency at the maximum shifts to higher values at higher applied potential. This suggests faster kinetics of the hydrogen evolution reactions.

Mo3+/4+ to Mo0 (peak II) and Mo6+ to Mo4+ (peak I). However, the catalyst retained its HER activity when the cyclic voltammogram was run up to 0.8 V, where complete oxidation does not takes place (Figure 5b). Although there was partial oxidation at 0.8 V, there was no significant loss in the HER activity. This clearly indicates that Mo2N contributes to the HER process. However, cyclic voltammetry measurements over a lower potential range where partial oxidation does not take place are suggested. This simply confirms the contribution of Mo2N to the HER activity of the composites. Electrochemical Stability of the Catalyst. To check the stability of the triangular MN−MS composite, we compared the LSV curves before and after 3000 LSV cycles. It can be observed that after 3000 LSV cycles there is only a change of 57 mV in the overpotential at a current density of 10 mA cm−2 and a change of 82 mV in the overpotential at a current density of 100 mA cm−2 (Figure 6a). We also investigated the

Figure 6. Electrochemical stability of the triangular MN−MS composite. (a) LSV curves of the composite before and after 3000 LSV cycles at 50 mV/s. (b) Chronopotentiometric study of the composite at an applied current density of 10 mA cm−2.

chronopotentiometric stability at a fixed current density of 10 mA cm−2 for more than 18 h (Figure 6b). The overpotential required to drive a current density of 10 mA cm−2 (η10) increases from 183 to 214 mV after 18 h of chronopotentiometric studies. This indicates that the composite exhibits good stability during hydrogen evolution in acidic media. We checked the TEM analysis after 3000 LSV cycles and found that the morphology of the nanostructure remained the same. However, formation of some pores after catalysis was seen after cycling studies (Figure S10). There is some etching of Mo2N particles, which creates a porous structure and decreases the number of Mo2N−MoS2 interfaces, which in turn decreases the HER activity. Electrochemical Impedance Spectroscopy Analysis. EIS studies indicate very interesting phenomena of hydrogen adsorption on the catalyst’s surface and its evolution from the surface. Figure 7a shows the Nyquist plots of the Mo2N−MoS2 composite (triangular MN−MS) at different applied potentials

■

CONCLUSIONS We have demonstrated the presence of interfaces and junctions between MoS2 and Mo2N in Mo2N−MoS2 composites. These non-precious-metal-based, environmentally friendly composite nanostructures were found to be very efficient catalysts for water electrolysis in acidic electrolyte. Our study of the electrochemical hydrogen production from water reveals remarkable enhancement of the electrocatalytic activity of the MoS2-based composites. Interfaces and junctions between Mo2N and MoS2 contributed significantly in hydrogen evolution reactions, as observed in the charge transfer resistance (only 76 Ω at 200 mV for the composite). Cyclic voltammetry studies confirmed the contribution of Mo2N to the HER activities of the composites. The HER activity of Mo2N−MoS2 composites suggests that the composites of E

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces

(10) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Closely Interconnected Network of Molybdenum Phosphide Nanoparticles: A Highly Efficient Electrocatalyst for Generating Hydrogen from Water. Adv. Mater. 2014, 26, 5702−5707. (11) Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. A CostEffective 3D Hydrogen Evolution Cathode with High Catalytic Activity: FeP Nanowire Array as the Active Phase. Angew. Chem., Int. Ed. 2014, 53 (47), 12855−12859. (12) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am. Chem. Soc. 2014, 136 (50), 7587−7590. (13) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem., Int. Ed. 2014, 53, 5427−5430. (14) Zhang, H.; Li, Y.; Zhang, G.; Xu, T.; Wan, P.; Sun, X. A Metallic CoS2 Nanopyramid Array Grown on 3D Carbon Fiber Paper as an Excellent Electrocatalyst for Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 6306−6310. (15) Morales-Guio, C. G.; Stern, L.-A.; Hu, X. Nanostructured Hydrotreating Catalysts for Electrochemical Hydrogen Evolution. Chem. Soc. Rev. 2014, 43 (18), 6555−6569. (16) Naresh Kumar, T.; Chandrasekaran, N.; Lakshminarasimha Phani, K. Structural and Electronic Modification of MoS2 Nanosheets Using S-Doped Carbon for Efficient Electrocatalysis of Hydrogen Evolution Reaction. Chem. Commun. 2015, 51, 5052−5055. (17) Zhang, H.; Lei, L.; Zhang, X. One-Step Synthesis of Cubic Pyrite-Type CoSe2 at Low Temperature for Efficient Hydrogen Evolution Reaction. RSC Adv. 2014, 4, 54344−54348. (18) Xu, C.; Peng, S.; Tan, C.; Ang, H.; Tan, H.; Zhang, H.; Yan, Q. Ultrathin S-Doped MoSe2 Nanosheets for Efficient Hydrogen Evolution. J. Mater. Chem. A 2014, 2 (16), 5597−5601. (19) Lukowski, M. a; Daniel, A. S.; English, C. R.; Meng, F.; Forticaux, A.; Hamers, R. J.; Jin, S. Highly Active Hydrogen Evolution Catalysis from Metallic WS2 Nanosheets. Energy Environ. Sci. 2014, 7, 2608−2613. (20) Youn, D. H.; Han, S.; Kim, J. Y.; Kim, J. Y.; Park, H.; Choi, S. H.; Lee, J. S. Highly Active and Stable Hydrogen Evolution Electrocatalysts Based on Molybdenum Compounds on Carbon Nanotube À Graphene Hybrid Support. ACS Nano 2014, 8 (5), 5164−5173. (21) Chen, W.-F.; Iyer, S.; Iyer, S.; Sasaki, K.; Wang, C.-H.; Zhu, Y.; Muckerman, J. T.; Fujita, E. Biomass-Derived Electrocatalytic Composites for Hydrogen Evolution. Energy Environ. Sci. 2013, 6 (6), 1818−1826. (22) Ang, H.; Tan, H. T.; Luo, Z. M.; Zhang, Y.; Guo, Y. Y.; Guo, G.; Zhang, H.; Yan, Q. Hydrophilic Nitrogen and Sulfur Co-Doped Molybdenum Carbide Nanosheets for Electrochemical Hydrogen Evolution. Small 2015, 11 (47), 6278−6284. (23) Zhao, Y.; Kamiya, K.; Hashimoto, K.; Nakanishi, S. Hydrogen Evolution by Tungsten Carbonitride Nanoelectrocatalysts Synthesized by the Formation of a Tungsten Acid/polymer Hybrid in Situ. Angew. Chem., Int. Ed. 2013, 52, 13638−13641. (24) Chen, W. F.; Schneider, J. M.; Sasaki, K.; Wang, C. H.; Schneider, J.; Iyer, S.; Iyer, S.; Zhu, Y.; Muckerman, J. T.; Fujita, E. Tungsten Carbide-Nitride on Graphene Nanoplatelets as a Durable Hydrogen Evolution Electrocatalyst. ChemSusChem 2014, 7 (9), 2414−2418. (25) Kibsgaard, J.; Jaramillo, T. F. Molybdenum Phosphosulfide: An Active, Acid-Stable, Earth- Abundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 14433−14437. (26) Cabán-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H.-C.; Tsai, M.-L.; He, J.-H.; Jin, S. Efficient Hydrogen Evolution Catalysis Using Ternary Pyrite-Type Cobalt Phosphosulphide. Nat. Mater. 2015, 14 (12), 1245−1251. (27) Tang, C.; Wang, W.; Sun, A.; Qi, C.; Zhang, D.; Wu, Z.; Wang, D. Sulfur-Decorated Molybdenum Carbide Catalysts for Enhanced Hydrogen Evolution. ACS Catal. 2015, 5, 6956−6963.

Mo2N and MoS2 exhibit superior HER activity with good electrochemical stability compared with bare MoS2 and Mo2N nanostructures.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10717. Characterization of MoS2 and Mo2N, HRTEM images and Raman spectrum of triangular MN−MS, LSV curve of Pt, LSV curves of MN−MS composites before annealing, Nyquist plots of MN−MS having various morphologies of Mo2N, HRTEM images after catalysis, and Rct values (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 91-11-26591511. Fax: 91-11-26854715. ORCID

Kasinath Ojha: 0000-0002-3811-8579 Ashok K. Ganguli: 0000-0003-4375-6353 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.K.G. thanks DST and DeitY, Government of India, for financial support. K.O. and S.S. thank UGC and CSIR, respectively, for fellowships.

■

REFERENCES

(1) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G. Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-Noble Metal Electrocatalysts for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 19186−19192. (2) Xie, J.; Li, S.; Zhang, X.; Zhang, J.; Wang, R.; Zhang, H.; Pan, B.; Xie, Y. Atomically-Thin Molybdenum Nitride Nanosheets with Exposed Active Surface Sites for Efficient Hydrogen Evolution. Chem. Sci. 2014, 5 (12), 4615−4620. (3) Chen, W.-F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-Evolution Catalysts Based on Non-Noble Metal Nickel-Molybdenum Nitride Nanosheets. Angew. Chem., Int. Ed. 2012, 51 (25), 6131−6135. (4) Xiao, P.; Yan, Y.; Ge, X.; Liu, Z.; Wang, J. Y.; Wang, X. Investigation of Molybdenum Carbide Nano-Rod as an Efficient and Durable Electrocatalyst for Hydrogen Evolution in Acidic and Alkaline Media. Appl. Catal., B 2014, 154−155, 232−237. (5) Wan, C.; Regmi, Y. N.; Leonard, B. M. Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 6407−6410. (6) Michalsky, R.; Zhang, Y.; Peterson, A. A. Trends in the Hydrogen Evolution Activity of Metal Carbide Catalysts. ACS Catal. 2014, 4, 1274−1278. (7) Chen, W.-F.; Muckerman, J. T.; Fujita, E. Recent Developments in Transition Metal Carbides and Nitrides as Hydrogen Evolution Electrocatalysts. Chem. Commun. 2013, 49 (79), 8896−8909. (8) Liao, L.; Bian, X.; Xiao, J.; Liu, B.; Scanlon, M. D.; Girault, H. H. Nanoporous Molybdenum Carbide Wires as an Active Electrocatalyst towards the Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2014, 16 (21), 10088−10094. (9) Popczun, E. J.; Mckone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. F

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces (28) Liu, Y.; Yu, G.; Li, G.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites. Angew. Chem., Int. Ed. 2015, 54, 10752−10757. (29) Zhang, K.; Zhao, Y.; Zhang, S.; Yu, H.; Chen, Y.; Gao, P.; Zhu, C. MoS2 nanosheet/Mo2C-Embedded N-Doped Carbon Nanotubes: Synthesis and Electrocatalytic Hydrogen Evolution Performance. J. Mater. Chem. A 2014, 2, 18715−18719. (30) Yu, L.; Xia, B. Y.; Wang, X.; Lou, X. W. General Formation of M-MoS3 (M = Co, Ni) Hollow Structures with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28 (1), 92−97. (31) Yu, X.-Y.; Feng, Y.; Jeon, Y.; Guan, B.; Lou, X. W. D.; Paik, U. Formation of Ni-Co-MoS 2 Nanoboxes with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28 (40), 9006− 9011. (32) Fan, X.-L.; Yang, Y.; Xiao, P.; Lau, W.-M. Site-Specific Catalytic Activity in Exfoliated MoS 2 Single-Layer Polytypes for Hydrogen Evolution: Basal Plane and Edges. J. Mater. Chem. A 2014, 2 (48), 20545−20551. (33) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308−5309. (34) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317 (5834), 100−102. (35) Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J.; Han, H. S.; Manoharan, H. C.; AbildPedersen, F.; et al. Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat. Mater. 2015, 15 (1), 48−53. (36) Laursen, A. B.; Kegnæs, S.; Dahl, S.; Chorkendorff, I. Molybdenum Sulfidesefficient and Viable Materials for Electro and Photoelectrocatalytic Hydrogen Evolution. Energy Environ. Sci. 2012, 5 (2), 5577−5591. (37) Bonde, J.; Moses, P. G.; Jaramillo, T. F.; Nørskov, J. K.; Chorkendorff, I. Hydrogen Evolution on Nano-Particulate Transition Metal Sulfide. Faraday Discuss. 2009, 140, 219−231. (38) Ojha, K.; Saha, S.; Kumar, B.; Hazra, K. S.; Ganguli, A. K. Controlling the Morphology and Efficiency of Nanostructured Molybdenum Nitride Electrocatalysts for the Hydrogen Evolution Reaction. ChemCatChem 2016, 8, 1218−1225. (39) Peng, K.; Fu, L.; Ouyang, J.; Yang, H. Emerging Parallel Dual 2D Composites: Natural Clay Mineral Hybridizing MoS2 and Interfacial Structure. Adv. Funct. Mater. 2016, 26 (16), 2666−2675. (40) Patil, S.; Harle, a; Sathaye, S.; Patil, K. Development of a Novel Method to Grow Mono-/few-Layered MoS2 Films and MoS2Graphene Hybrid Films for Supercapacitor Applications. CrystEngComm 2014, 16 (47), 10845−10855. (41) Wei, Z. B. Z.; Grange, P.; Delmon, B. XPS and XRD Studies of Fresh and Sulfided Mo2N. Appl. Surf. Sci. 1998, 135, 107−114. (42) Liu, J.; Tang, S.; Lu, Y.; Cai, G.; Liang, S.; Wang, W.; Chen, X. Synthesis of Mo2N Nanolayer Coated MoO2 Hollow Nanostructures as High-Performance Anode Materials for Lithium-Ion Batteries. Energy Environ. Sci. 2013, 6 (9), 2691−2697. (43) Sanjines, R.; Wiemer, C.; Almeida, J.; Levy, F. Valence Band Photoemission Study of the Ti-Mo-N System. Thin Solid Films 1996, 290−291, 334−338. (44) Vrubel, H.; Merki, D.; Hu, X. Hydrogen Evolution Catalyzed by MoS3 and MoS2 Particles. Energy Environ. Sci. 2012, 5 (3), 6136− 6144. (45) Wang, T.; Liu, L.; Zhu, Z.; Papakonstantinou, P.; Hu, J.; Liu, H.; Li, M. Enhanced Electrocatalytic Activity for Hydrogen Evolution Reaction from Self-Assembled Monodispersed Molybdenum Sulfide Nanoparticles on an Au Electrode. Energy Environ. Sci. 2013, 6, 625− 633.

(46) Gao, M.-R.; Liang, J.-X.; Zheng, Y.-R.; Xu, Y.-F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S.-H. An Efficient Molybdenum Disulfide/cobalt Diselenide Hybrid Catalyst for Electrochemical Hydrogen Generation. Nat. Commun. 2015, 6, 5982. (47) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W. D.; Xie, Y. Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2013, 25 (40), 5807−5813. (48) Thomas, J. G. N. Kinetics of Electrolytic Hydrogen Evolution and the Adsorption of Hydrogen by Metals. Trans. Faraday Soc. 1961, 57, 1603−1611.

G

DOI: 10.1021/acsami.6b10717 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX