Revelation of the Excellent Intrinsic Activity of MoS2|NiS|MoO3

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Revelation of its Excellent Intrinsic Activity of MoS|NiS|MoO Nanowire for Hydrogen Evolution Reaction in Alkaline Medium Chuanqin Wang, Bin Tian, Mei Wu, and Jiahai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14827 • Publication Date (Web): 31 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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ACS Applied Materials & Interfaces

Revelation of its Excellent Intrinsic Activity of MoS2|NiS|MoO3 Nanowire for Hydrogen Evolution Reaction in Alkaline Medium Chuanqin Wanga,b, Bin Tian b, Mei Wu b, Jiahai Wang*b a

School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, China.

b

National Engineering Center for Colloid Materials, Shandong University, Jinan, 250100, China. E-mail: [email protected] Tel: +86-15665709672

ABSTRACT Loading electrocatalyst on poor conductive substrate can easily lead to undervaluation of its intrinsic property. In this study, excellent activity of MoS2|NiS|MoO3 nanowires for hydrogen evolution is faithfully revealed. The precursor NiMoO4 synthesized on chemically polished Ti foil can be successfully converted to MoS2|NiS|MoO3 catalyst via gas-phase sulfurization. Without deep polish in sulphuric acid for 2 hours, the as-synthesized materials do not show competitive results. After sulfurization, the surface morphology of the precursor is transformed into rough features and the peripheries of these electrocatalysts are coated by multilayered and misaligned

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MoS2 with a high density of active sites and conductive component NiS. Further analysis shows that defect MoO3 are embedded inside each nanowire, which may facilitate fast electron transfer. Such nanostructured architecture shows promising results for hydrogen evolution reaction in alkaline medium with only 91 mV overpotential for the current density of 10 mA cm−2 and robust long-term stability during more than 20 hours test. KEYWORDS: Hydrogen Evolution Reaction; Hydrogen Energy; Electrocatalysts; Nanowire; Water Splitting; Surface Polishing. INTRODUCTION Electrochemical water splitting holds great promise in providing green and sustainable energy carrier in the form of hydrogen, which is regarded as a real carbon-free content alternative to fossil fuels.1-5 In order to meet the requirements of hydrogen economy based on electrochemical water splitting, the vigorous pursuit of earth-abundant materials with low overpotentials and long-term stability is necessary.5-18 Due to its abundant exposed edge sites with superior electrochemical activities, nanosheet-like MoS2 as electrocatalyst has recently attracted significant attention in the past several years. Several studies has demonstrated that nanostructured MoS2 on various forms presents excellent Hydrogen Evolution Reaction activities.17, 19-22 To lower down the overpotential of HER, superb electrocatalysts also have to possess wonderful conductivity to facilitate the electron transport. In this respect, conformal growth of MoS2 along MoO3 nanotube19 and nanowire,22 etc., were designed to ameliorate the poor conductivity of MoS2, leading to enhanced and competitive HER activities. Nonetheless，there still exists large space for MoS2 to keep up with the electrochemical activity of noble-metal Pt, whose widespread usage is mainly limited by its scarcity and high cost.


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For the increment of the active edge sites and the conductivity of MoS2 all together by using single component, it is very challenging. To solve this problem, more conductive component such as carbon nanotube,23-27 graphene9, 28-30 or metal1, 17, 21, 31, 32 is used as substrate for growth of nanostructured materials. Inefficient electron transfer still cannot be ameliorated because of the semiconducting property of MoS2 in 2H phase. Especially for nanowire/nanotube arrays with highaspect ratio morphology, long distance electron transfer is not favorable for reducing the overpotential. Another delicate strategy several previous studies19, 22, 33 have used is to decouple these requirements by using multicomponent material in the nanowire/nanotube arrays. The first step is to synthesize the MoO3 precursor via different approaches: self-ordering electrochemical anodization (SOA), hot-wire chemical vapor deposition (HWCVD) and e-beam evaporation. More recently, Zheng et al. have synthesized NiMoO4 precursor via hydrothermal approach on the Ti foil.21 After sulfurization or selenization process, molybedenum chacogenides provide abundant active edge sites and defect MoO3 core with conductive property fastens the electron transfer. NiS2 interlaced with MoS2 also contributes significantly to the efficiency of HER.21 In the core/shell design, both active edge sites and basal plane are exposed to electrolyte solution since the MoS 2 shell conformally align along the vertical MoO3 core. Both core/shell and interlaced nanostructures provide interesting approaches to individually deal with the density of electroactive sites and conductive pathway. Nonetheless, the conductivity of the interface between nanowire arrays and Ti foil still not reach the maximum level, leading to undervaluation of the intrinsic activity of the catalysts. In the present study, we synthesized MoS2 nanosheets-stacked nanowires decorated with NiS and MoO3, defined as MoS2|NiS|MoO3, which is converted from the crystallized precursor NiMoO4 nanowire array on chemically polished Ti foil via gas-phase sulfurization. This kind of


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architecture provides a strategy to simultaneously improve the density of active edge sites and the conductivity of electron transfer pathway. These as-prepared catalyst is different from that in the previous study which contain MoS2 and NiS2.

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The experimental results demonstrate the

composite electrocatalyst (MoS2|NiS|MoO3) possesses much higher excellent HER activities in alkaline solution with onset overpotential of 36 mV and a corresponding Tafel slope of 54.5 mV/dec. To reach current densities of 10 and 50 mA/cm2, overpotentials of 91 mV and 135 mV are needed, respectively. Furthermore, long-term duration examination shows this electrocatalyst design is very stable during our test period up to 20 hours. More importantly，this study illustrate a simple approach to synthesize low cost and highly efficient HER electrocatalysts with hierarchical nanostructures. EXPERIMENTAL SECTION Materials: Ni(NO3)2·6H2O, Na2MoO4·2H2O, acetone, sodium hydroxide, ethanol, sulfuric acid were obtained from Sinopharm Chemical Reagent Co., Ltd. Sublimed sulfur was purchased from Aladdin Ltd. (Shanghai, China). Titanium sheet (thickness of 0.1 mm) was purchased from Saiwei metal materials Co., Ltd. The water used throughout all experiments was purified through a Millipore system. Pt/C (20 wt% Pt) were obtained from Shanghai Macklin Biochemical Co., Ltd. Synthesis of NiMoO4@Ti precursor: Precursor composite NiMoO4 was grown on Ti substrate by hydrothermal method. Before the growth of composite, the Ti sheet was sequentially cleaned by sonication in mixed solution (acetone and 10 wt% sodium hydroxide v:v=1:1) for 30 min. After cleaning, Ti sheet was immersed in sulfuric acid at 95 ℃ for 2 h. In a typical synthesis, 0.523 g nickel nitrate (Ni(NO3)2·6H2O), 0.435 g sodium molybdate (Na2MoO4·2H2O), were added to 35 mL of distilled water to form precursor solution. And then, the precursor solution containing the


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above-treated Ti sheet was transferred into a Teflon-lined stainless autoclave (with a volume of 50 mL). The autoclave was sealed and was then heated at 120 ℃ for 10 h. After being cooled to room temperature, the Ti substrate with the oxide precursor was washed 3~5 times with ethanol and deionized water, and then dried in a vacuum oven at 60 ℃ for 6 h. Synthesis of MoS2|NiS|MoO3 composite: The prepared precursors grown on Ti substrate and sublimed sulfur (1g) were put at two separate positions in a porcelain boat with S at the upstream side of the furnace, and then, the samples were sulfurized at 500 °C for 1 h under nitrogen atmosphere with a heating speed of 5 °C/min. After sulfurization, the samples were gradually cooled to ambient temperature. Characterizations: Powder XRD data were acquired on a Bruker D8 ADVANCE diffractometer with Cu Kα radiation (λ = 1.5418 Å). TEM measurements were performed on a JEM-2100 electron microscopy with an accelerating voltage of 200 kV. SEM measurements were carried out on a SU8010 scanning electron microscope at an accelerating voltage of 50 kV. XPS measurements were performed on a Thermo Scientific Escalab 250Xi X-ray photoelectron spectrometer using Al as the exciting source. All electrochemical data were obtained by CHI 660e. Electrochemical measurements: All the electrochemical measurements were carried out in a typical three-electrode system with an electrolyte solution of 1 M KOH, containing a working electrode, a graphite counter electrode and a saturated calomel electrode (SCE) reference electrode. LSV measurements were conducted in 1 M KOH with scan rate of 2 mV/s. All the potentials with respect to SCE reported in our work were calibrated with reference to the reversible hydrogen electrode (RHE) by the following equation: E(RHE) = E(SCE)+0.0591pH+0.2415-0.000761(T298.15). T represents the temperature.


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RESULTS AND DISCUSSION The straightforward fabrication process is presented in Figure 1 (more details in experimental section). The crystallized precursor (NiMoO4) was successfully synthesized on chemically polished titanium foil via well-developed hydrothermal approach. The SEM image show the arrayed nanowires (NiMoO4) have pretty smooth morphology (Figure 2a). After the subsequent sulfurization with sulfur powder located in the upstream of the furnace tube under the protection of N2 (more details in experimental section), each nanowire in these arrays was transformed into morphology with distinguished rough surface. By a close inspection of the nanowire in the scanning microscopy imaging (Figure 2b), it is disclosed that many flakes protrude from the nanowire.

Figure 1. Schematic of the experimental process. X-ray diffraction (XRD) characterization was carried out to analyze the components of the above-prepared products (Figure 2c), and it was revealed that crystalized NiMoO4 was converted to multiple components including MoS2, NiS and MoO3. The strong diffraction peaks corresponding to (110), (220) and (510) crystal facets of NiMoO4 (JCPDS card no. 33-0948) were all found in the X-ray spectra (Figure 2c). After sulfurization of the NiMoO4 precursor, the crystallized precursor was broken down into different phases. Diffraction peaks pattern changed dramatically and several main peaks indexed to (003), (104) and (119) crystal facets of MoS 2


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(JCPDS card no. 17-0744), appeared in the X-ray spectra. Notably, the diffraction peaks for NiS (JCPDS card no. 02-1280) also exist, corresponding to (100), (101), (102) and (200) planes. In combination with the analysis of SEAD obtained from high resolution transmission electron microscopy (TEM, Figure 2f), the peaks located at 26o, 35o and 53o were ascribed to the (210), (310) and (420) crystal facets of MoO3 (JCPDS card no. 21-0569).

Figure 2. a) Typical SEM image of the NiMoO4 nanowires on Ti sheet; b) Typical SEM image of the MoS2|NiS|MoO3 nanowire on Ti sheet; c) XRD patterns of the precursor and sulfurization product scratched down from Ti sheet; d) TEM image of the nanowire structure of MoS2|NiS|MoO3; e) High-resolution TEM image of MoS2|NiS|MoO3; f) Selected area electron diffraction patterns of MoS2|NiS|MoO3.

Further insights into the morphology and composition of the electroactive materials were obtained from transmission electron microscopy. It can be observed that the surface of the nanowire is composed of many protruded sheets with lattice fringe of 0.61 nm in agreement with MoS2 structures (Figure 2e), which further confirm the rough surface observed in SEM images. All MoS2 nanosheets are well packed together to retain the morphology of nanowire. By a close


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examination of the high resolution TEM, abundant edge sites were directly exposed to the surrounding and accessible to the electrolyte, leading to efficiency enhancement of water splitting. Instead of growth of MoS2 nanosheet conformably along the conductive surface nanowire22 and nanotube,19 the architecture of sulfurized nanowire in a stacking format is totally different, inside which conductive NiS and MoO3 exist. (104), (102) and (310) crystal facets corresponding to MoS2, NiS and MoO3 which has been found in XRD spectra, can be confirmed in selected area electron diffraction (SAED) pattern (Figure 2f). The detailed elemental ratio acquired from energy dispersive spectra (EDS) was shown in supporting information (Figure S1-2, supporting information), which qualitatively claim the existence of Ni, Mo, S and O elements. In this study (Figure S3, supporting information), NiS instead of NiS2 was found in the final electroactive material,21 nevertheless, the total efficiency of the present catalyst in this scenario was much higher. The surface chemical nature and composition of MoS2|NiS|MoO3 were further investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Peak fitting of Mo 3d region (Figure 3A) reveals that two covalent states for Mo element including Mo6+ species (236 eV, 233.2 eV) and Mo4+ species (232.4 eV, 229.8 eV) are indicative of MoO3 (or NiMoO4) and MoS2 existence in line with the XRD and SAED results. As compared to the precursor (NiMoO 4), new peaks corresponding to Mo4+ species assigned to molybdenum in MoS2 appear, which is also consistent with the Raman spectra (374 and 405.6 cm-1) in Figure S4 (Supporting information). In the Ni 2p region (Figure 3B) after sulfurization, two new peaks (855 and 872 eV) show up in addition to the peaks corresponding to nickel in NiMoO4 (Figure S5, supporting information), which can be assigned to the nickel in NiS phase. Based on these results, it is also revealed that the precursor was not completely converted and sulfurized, which is confirmed by both surfacesensitive Raman spectroscopy (Figure S4, supporting information) and XPS techniques. But in


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XRD and SAED patterns, no peaks related to NiMoO4 have been identified possibly because after sulfurization, the precursor amount has decreased significantly.

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Figure 3. XPS spectrum (A) Mo 3d, (B) Ni 2p, (C) S 2p of MoS2|NiS|MoO3 and (D) Ni 2p of NiMoO4.

The presence of sulfur in the as-synthesized electrocatalysts was verified by XPS. There is a broad peak related to S 2s next to Mo 3d region. Furthermore, peak fitting of the S 2p region shows the existence of three doublets and one separate broad peak. The first doublet (162.5 and 163.8 eV) is assigned to sulfide species and the second doublet (163.9 and 165 eV) is indicative of sulfur in thiolate-type environment.1 The third doublet (165 and 166.2 eV) informs us that the sulfur bridge 2-

2-

(S 2 ) exist in the as-synthesized materials. The broad peak (170 eV) is related to SO 4 species formed from oxidation of the surface sulfur by exposure to the ambient air. These sulfur species


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are very similar to those reported in MoS2 conformally growing on MoO3 nanotubes.19, 22

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MoS2|NiS|MoO3@Ti-2 MoS2|NiS|MoO3@Ti-1

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-100 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 Potential (V vs. RHE) Figure 4. Linear sweep voltammograms of MoS2|NiS|MoO3 on titanium substrate after different treatments. Ti foil treated in 1 M H2SO4 for 2 hours at 95 oC is defined as Ti-1; Ti foil treated in 1 M HCl for short time at room temperature is defined as Ti-2.

HER catalytic activities of MoS2|NiS|MoO3 was evaluated in alkaline solution (1 M KOH) with a three-electrode system, in which MoS2|NiS|MoO3 nanowires on titanium foil was directly used as the working electrode. As compared to the Ti foil treated in normal way,21 the MoS2|NiS|MoO3 on the Ti foil, treated with 1 M H2SO4 for 2 hours at 95 oC, shows excellent efficiency with 200 mA/cm2 at overpotential of 183.6 mV (Figure 4). As shown in the supporting information (Figure S6-7), after polishing the substrate, the electric resistance was greatly decreased as compared to conventional treatment. To reach the same current density, the electrocatalyst need overpotential of more than 299 mV, which is much larger than that corresponding to the long time treatment in 1 M H2SO4. Therefore, seamless integration of electrocatalyst onto the conductive substrate is extremely important for the improvement of the


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whole performance. Loading of electrocatalyst onto a poor conductive substrate leads to undervaluation of its performance.

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Figure 5. A) Linear sweep voltammograms of MoS2|NiS|MoO3 (blue), NiMoO4 (red), Ti (black) and Pt/C (green) collected in 1 M KOH at a scan rate of 2 mV s−1; B) The corresponding Tafel plots for A and MoS2|NiS|MoO3 on normally treated Ti foil; C) Stability test for MoS2|NiS|MoO3 at η=106 mV; D) Polarization curves before and after continuous potential sweeps at 50 mV s−1. Unless specifically noted, MoS2|NiS|MoO3 grow on Ti foil treated in 1 M H2SO4 for 2 hours.

In contrast with titanium substrate and NiMoO4 precursor, as shown in Figure 5A, the IRcorrected polarization curve of MoS2|NiS|MoO3 shows excellent electroactivity with onset potential of 36 mV. Under the overpotential ranges between 0 and 0.2 mV, both Ti substrate and NiMoO4 precursor show only negligible activities. The polarization curves have been normalized with respect to the geometric area of the working electrode. To reach current densities of 10 mA/cm2 and 100 mA/cm2 need 91 mV and 157 mV overpotential in alkaline medium, where the electrocatalyst performs best (Figure S8, supporting information). As compared to previous study


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regarding to MoS2-NiS2,21 the enhanced HER activity of MoS2|NiS|MoO3 might be ascribed to the resistance decrease in the interface between electrocatalyst and substrate as well as the intrinsic activity of MoS2|NiS|MoO3 (Figure S9, supporting information). For comparison, other excellent molybdenum sulfide electrode need overpotential more than 100 mV to reach 10 mA/cm2. 9, 23, 3437

The HER activity of Pt/C material as the gold standard was also been employed for direct

comparison. The Tafel slope is as an important indicator of chemical activity, which has been normally derived from the linear part of Tafel plot under a small overpotential (Figure 5B). Tafel slope can be influenced by three main factors: the number of active sites, efficiency of charge transfer (conductivity) and free energy of hydrogen on the electrocatayst surface. A value of 54.5 mV/dec for MoS2|NiS|MoO3 has been obtained in alkaline solution, which is much better than those of bare Ti substrate and NiMoO4 precursor and is close to the value of 41.3 mV/dec obtained for platinum particle in the same solution. The Tafel data (70 mV/dec) for MoS2|NiS|MoO3 on Ti foil treated in normal way is much higher. Therefore, the electrochemical parameter of MoS2|NiS|MoO3 on poor substrate cannot reflect the intrinsic property of the electrocatalyst. The partial conversion of the NiMoO4 precursor is good enough to provide high activity. With the synergistic contribution of NiS, the as-synthesized material demonstrates superior stability during the 20 hours test (Figure 5C). Furthermore, after 1000 cyclic voltammograms, the polarization curve only shifts slightly and especially at large current density of 100 mA/cm2, the change is much less (Figure 5D). The morphology of each nanowire is preserved after 1000 cyclic voltammograms (Figure S10, supporting information).


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Figure 6. A) Cyclic voltammetry of MoS2|NiS|MoO3 at different scan rate of 10 mV s-1, 20 mV s-1, 30mV s-1, 40 mV s-1, 50 mV s-1, 60 mV s-1, 70 mV s-1, 80 mV s-1, 90 mV s-1, 100 mV s-1, 110 mV s-1, 120 mV s-1, 130 mV s-1 and 140 mV s-1 in 1 M KOH in the non-faradaic potential region, loading amount: 2 mg cm-2; B) The cathodic and anodic currents measured at 0.3 V vs. RHE and the average of the absolute value as a function of the scan rate. The slope is taken as the double-layer capacitance of the electrode; C) Niquist plot obtained at 0.2 V vs. RHE in 1 M KOH; D) Quantitative H2 measurement via water displacement.

By using the deeply polished substrate, the intrinsic property of MoS2|NiS|MoO3 is faithfully revealed without impact from the voltage drop across the interface and the impact leads to undervaluation of the electrocatalysts. The excellent performance of this electrocatalyst can be contributed from three respects: high double-layer-capacitance, fast electron transfer and hydrogen absorption energy. The double-layer capacitance is calculated to be 45.125 mF cm-2, which guarantees a large electrochemical active surface area (ECSA) equal to 1128 cm2 by using a specific capacitance (Cs) of 0.040 mF cm-2 (Figure 6A-B) and high HER efficiency. Furthermore, from the electrochemical impedance spectra (Figure 6C), the MoS2|NiS|MoO3 shows very low electron transfer resistance in comparison to other materials. The quantitative H2 measurement shows that faradic efficiency is close to 100% (Figure 6D). All the fast electron transfer, and


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synergistic cooperation inside the heterostructure, abundant active sites work together to contribute the boosted electrochemical activity of MoS2|NiS|MoO3 in alkaline medium. For comparison, other electrocatalysts are listed in the Table S1 and it shows that our multicomponent catalyst in the current scenario is very competitive in alkaline medium. CONCLUSIONS In summary, excellent HER activity of MoS2|NiS|MoO3 nanowires has been achieved via high temperature sulfurization of NiMoO4 precursor grown on long-time treated titanium substrate. The excellent conductivity, abundant exposed active sites and synergistic effect of this kind of heterostructure contribute significantly to the amelioration of the HER activity in alkaline solution. Electrochemical studies showed that the obtained 3D electrode exhibited excellent HER activity with an overpotential of 91 mV at 10 mA cm-2, a small Tafel slope of 54.5 mV dec-1 and prominent electrochemical durability. The results presented herein may offer a new methodology for the design and engineering of effective HER catalysts based on earth-abundant and inexpensive components. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: EDS spectra, Raman spectra, LSV and XRD of the electrocatalysts prepared at different temperatures. LSV of titanium substrate, NiMoO4 on titanium substrate and MoS2|NiS|MoO3 under different pH condition. SEM images of MoS2|NiS|MoO3 prepared at different temperatures or after stability test. Table for HER activities of different materials (PDF) ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 21575078).


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Revelation of the Excellent Intrinsic Activity of MoS2|NiS|MoO3

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