Self-Assembly-Assisted Facile Synthesis of MoS2-Based Hybrid

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Energy, Environmental, and Catalysis Applications 2

Self-Assembly-Assisted Facile Synthesis of MoS-Based Hybrid Tubular Nanostructures for Efficient Bifunctional Electrocatalysis Chenyang Zhao, Youfang Zhang, Lunfeng Chen, Chaoyi Yan, Peixin Zhang, Jia Ming Ang, and Xuehong Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04140 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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ACS Applied Materials & Interfaces

Self-Assembly-Assisted Facile Synthesis of MoS2-Based Hybrid Tubular Nanostructures for Efficient Bifunctional Electrocatalysis Chenyang Zhao,a,b Youfang Zhang,b Lunfeng Chen,a Chaoyi Yan,a Peixin Zhang,a Jia Ming Ang b and Xuehong Lu*b a

College of Chemistry and Environmental Engineering, Shenzhen University, 1066

Xueyuan Avenue, Nanshan District, Shenzhen 518071, PR China b

School of Materials Science and Engineering, Nanyang Technological University, 50

Nanyang Avenue, 639798, Singapore

KEYWORDS: dopamine, nanotubes, self-assembly, electrocatalysis, molybdenum disulfide (MoS2)

ABSTRACT: In this work, MoS2-based hybrid tubular nanostructures are facilely synthesized via a self-assembly-assisted process and evaluated as a bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR).

By

simply

mixing

of

the

reactants

under

ambient

conditions,

(NH4)2MoS4/polydopamine (PDA) hybrid nanospheres are formed. The protonated dopamine is linked to tetrahedral [MoS4]2- via weak N-H…S and O-H…S interactions, causing the PDA nanospheres merging together and forming nanorods under stirringinduced shear force. Moreover, the oxidative polymerization of dopamine proceeds on the surface of the nanorods, whereas it is prohibited inside the nanorods owing to lack of oxygen, leading to outward diffusion of dopamine and hence cavitation. After annealing, the tubular morphology is perfectly retained, while ultrafine MoS2 monolayers are formed due to the confinement of the framework. Benefiting from these 1

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unique structural features, the MoS2/C hybrid nanotubes possess abundant active sites and high surface area, as well as boost electronic and ionic transport, remarkably enhancing their electrocatalytic activities. The onset and half-wave potential are 0.91 and 0.82 V, respectively, for ORR, close to those of Pt/C. Moreover, low onset potential and small Tafel slope are also observed for HER, demonstrating the potential of the hybrid nanotubes as a promising non-noble metal bifunctional electrocatalyst.

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INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991, tubular nanomaterials have attracted extensive interest owing to their unique structural merits and fascinating properties. The large specific surface area, well-defined interior voids and functional walls, as well as intrinsic quantum effect of tubular nanomaterials have endowed them with the potential to be used in a variety of fields, such as transistors, catalysis, energy storage and conversion.1-3 As a result, the exploration of nanotube synthesis methods that are simple, scalable, yet capable of controlling of nanotube structure and morphology is of great scientific and practical significance. Generally, the synthesis of tubular nanostructures can be classified into three categories, based on the nanotube formation mechanisms: 1) template-assisted growth,4-5 2) self-assembly via Ostwald ripening, Kirkendall effect, etc.,6-8 and 3) synthesis of nanosheets followed by rolling up via various stimuli.9-11 Although these methods are based on different principles, they typically involve complicated procedures, harsh synthetic conditions or have scaling-up issues, which impede their widespread applicability. As an analogue of graphite, molybdenum disulfide (MoS2) has been a research focus across disciplines because of its remarkably diverse range of unique properties, especially as electrocatalyst and electrode materials.12 In MoS2, the covalently bonded thin nanosheets, each consists of three atomic layers, are held together by weak van der Waals forces. Such structural features allow the intercalation and exfoliation of MoS2, making it a promising host material for lithium ion batteries (LIBs).13-14 Moreover, as a layered semiconductor, the electronic properties of MoS2 are thickness-dependent. The tunable band gap and exposed edge sites endow it with high catalytic activities towards hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR).15-18 To further boost its catalytic and energy storage capabilities, structural engineering of 3



MoS2 to provide higher conductivity and more active sites is a possible route.19 Synthesis of MoS2-based tubular nanostructures is, therefore, of great interest to researchers. MoS2 nanotubes were first synthesized by Tenne et al. via gas-phase sulfuration of MoO3.20 Later, an alternative route was developed by Walton et al. where MoS2 powder was used.21 Rao et al. demonstrated that the decomposition of an intermediate, MoS3, played a key role in the formation of MoS2 nanotubes.22 Meanwhile, selfassembly of subnanometer-diameter single-wall MoS2 nanotubes was reported by Remskar et al. Their studies showed that the difference in density between Mo6S2I8 and MoS2 led to the cavitation of Mo6S2I8 nanowires.23 Similarly, an anion-exchange reaction between Mo3O10(C2H10N2) nanowires and L-cysteine was reported by Zhang et al. MoS2 nanotubes composed of hierarchical nanosheets were successfully prepared through hydrothermal sulfuration.24 Compared with neat MoS2 nanotubes, however, much less progress has been reported for the controlled synthesis of MoS2-based hybrid tubular nanostructures. Zhi et al. reported multi-step synthesis of MoS2@graphene tubular nanostructures where graphene rolled up into a hollow nanotube and MoS2 nanosheets were standing on the inner surface.25 Tay et al. synthesized coaxial MoS2/CNT nanotubes by depositing MoS2 on pretreated CNTs.26 The incorporation of carbon can greatly improve the electrochemical performances of hybrid nanotube systems because of the synergistic effect of the components. However, the current synthetic methods always involve the synthesis of 2D or 1D building blocks first. Obviously, the direct synthesis of MoS2 hybrid nanotubes from small molecules/salts at mild conditions would be a much simpler and more environmentally friendly approach, which would also facilitate large-scale production.

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Herein, for the first time we report a facile strategy for one-pot synthesis of MoS2/nitrogen-doped carbon hybrid nanotubes (MoS2/C HNT) for bifunctional electrocatalysis of ORR and OER. In this work, organic-inorganic hybrid (NH4)2MoS4/polydopamine (PDA) nanotubes were first prepared in a mixture of water/ethanol at room temperature simply under stirring. The weak interactions between [MoS4]2- and dopamine were found playing a key role in the formation of the hybrid nanotubes, which slowed down the polymerization rate of dopamine and led to a polymerization-driven cavitation. In the subsequent thermal treatment, the tubular structure was well retained. The obtained MoS2/C HNT are composed of oxygenincorporated MoS2 monolayers embedded in highly conductive nitrogen-doped carbon walls with high specific surface area of 153 m2 g-1. This unique architecture of MoS2/C HNT and synergistic effects between MoS2 and carbon promote its conductivities and catalytic activities. As a result, superior ORR and HER performances are achieved, with a small half-wave potential of 0.82 V for ORR and low onset potential of 118 mV for HER, respectively.

EXPERIMENTAL SECTION Synthesis of the MoS2/C nanotubes: The MoS2/C nanotubes were synthesized via a facile and green approach under ambient conditions. Typically, dopamine hydrochloride was firstly dissolved in a mixture of water and ethanol with a volume ratio of 5 to 1. The concentration of dopamine was fixed at 1.0 mg ml-1 based on the volume of water. A certain amount of ammonium tetrathiomolybdate ((NH4)2MoS4, ATTM) was then added, followed by a short period of ultrasonic treatment to assist its dissolution. The pH of the solution was adjusted to ~8.5 by adding 10 mM tris(hydroxymethyl)aminomethane (Tris) buffer. The reaction mixture was stirred for 3 5



to 5 days. Subsequently, the solid products were isolated by centrifuge, washed with deionized water for 3 times, and then lyophilized. The products obtained were annealed at 800 oC for 2 hours in argon atmosphere to yield the MoS2/C nanocomposites. In this work, the concentration of ATTM was varied from 0 to 4 mg ml-1 to study its effect on the morphologies of the nanocomposites. For comparison, bulk MoS2 was also synthesized by direct pyrolysis of ATTM at 800 oC in argon. Characterization: The morphologies of the samples were studied using a fieldemission scanning electron microscope (JEOL JSM 7600) at an accelerating voltage of 5 kV and a transmission electron microscope (JEOL 2100) at 200 kV. The high resolution TEM (HRTEM) image, energy dispersive X-ray (EDX) mapping and selected area electron diffraction (SAED) were recorded on a Philips CM300 TEM. XPS measurements were conducted on a Kratos Analytical AXIS His spectrometer with a monochromatized Al Ka X-ray source (1486.6 eV photons). The binding energies were corrected by referencing C (1s) binging energy of 285 eV. XRD patterns were recorded on a Bruker GADDS X-ray diffractometer. The compositions of the samples were determined by thermogravimetric analysis (TA Q500). All the samples were heated from room temperature to 800 oC in air. The UV absorption spectra were measured using a Shimadzu UV-3600 UV–vis–NIR spectrophotometer. BET specific surface area was determined using a Micromeritics Tristar II-3020 nitrogen adsorption apparatus. The pore size distribution plot was obtained by the BJH method. Fourier transform infrared spectroscopic (FTIR) measurements were performed using a Shimadzu FTIR IR Prestige-21 with KBr pellets. Electrode preparation and electrochemical measurements: Prior to electrochemical tests, glassy carbon electrodes were pretreated according to a previous report.27 Typically, 2 mg samples were dispersed in a mixed solvent containing 300 µl deionized 6


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water, 100 µl ethanol and 10 µl 5 wt% Nafion solution. The mixtures were ultrasonicated for at least 30 min to get homogeneous inks. 12 µl of the ink were then pipetted onto the glassy carbon electrodes (5mm) and dried naturally in air, leading to a mass loading of ~ 0.3 mg cm-2. The

electrochemical

performances

were

measured

using

an

Autolab

potentiostat/galvanostat (PGSTAT302N) station with a rotating disk electrode. A standard three-electrode-cell setup was used. For the ORR test, 0.1 M KOH was used as the electrolyte, a Hg/HgO electrode as the reference and a Pt wire as the counter electrode. While for the HER test, 0.5 M H2SO4 and Hg/Hg2Cl2 (SCE) reference were used. For the long-term stability tests, a glassy carbon rod of 6 mm was utilized as the counter electrode to avoid Pt contamination. The CV was continuously scanned from 0.2 to 0.5 V at a scan rate of 50 mV s-1. All the potentials were calibrated to reversible hydrogen electrode (RHE) scale according to on the following equations: ERHE =ESCE + 0.241 + 0.059 × pH or ERHE = EHg / HgO + 0.098 + 0.059 × pH . The CV and LSV were recorded in the O2 or N2 saturated electrolyte at a scan rate of 10 mV s-1. For comparison, a commercial Pt/C (20 wt %) with a catalyst loading of 0.1 mg cm-1 was also tested under the same conditions. The number of transferred electrons per O2 molecule in ORR was calculated using Koutecky-Levich equation, and verified by the ring and disk currents using a rotating ring-disk electrode (RRDE). For details, please refer to Supporting Information.

RESULTS AND DISCUSSION Structures and morphology of the hybrid nanotubes To prepare MoS2/C HNT, (NH4)2MoS4/PDA hybrid nanotubes were first prepared from (NH4)2MoS4 and dopamine using a simple solution-based method under ambient 7



conditions (See experimental section). Dopamine is widely used to prepare biomimetic adhesive coatings on various substrates via oxidative polymerization in aqueous media. An unique feature of PDA is that it can self-assemble to form nanospheres in the polymerization process.28 Previous studies also revealed that PDA could be calcined to layered carbon with electrical conductivities close to that of nitrogen-doped graphene.29 In this study, as expected, PDA spheres with a diameter of ~200 nm are formed in the absence of (NH4)2MoS4 (Figure S1). However, when (NH4)2MoS4 was added, surprisingly, (NH4)2MoS4/PDA nanotubes are formed under the same dopamine polymerization conditions. Scanning electron microscopic (SEM) studies show that the obtained (NH4)2MoS4/PDA nanotubes are randomly orientated with diameters ranging from 100 nm to 300 nm (Figure 1a). The variation may be caused by the mild synthesis conditions used, i.e., only stirring at room temperature could not make the reaction system completely homogeneous. Both open-ended and capped nanotubes are observed (Figure 1b). The hollow nature of the (NH4)2MoS4/PDA could be clearly seen from the transmission electron microscopic (TEM) images (Figure 1c and 1d). The formation mechanism and factors that influence the morphology of these hollow nanostructures will be discussed in the following section. In the (NH4)2MoS4/PDA hybrid nanotubes, no MoS2 nanocrystals could be seen from the high resolution TEM (Figure 1d), implying that the Mo and S species are still in the form of [MoS4]2- or its derivatives. The amorphous nature of the (NH4)2MoS4/PDA is confirmed by the X-ray diffraction (XRD) (Figure S2). After carbonization, the tubular structure well retains while the nanotube diameter shrinks slightly. The PDA and (NH4)2MoS4 are converted to nitrogen-doped carbon and MoS2, respectively, forming MoS2/C HNT (Figure 1e). Owing to the release of volatiles and graphitization of carbon, MoS2/C HNT has porous wall structure, as indicated by the white spots on a grey background observed in the 8


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nanotube core regions by TEM (Figure 1e and S3). The existence of MoS2 can be clearly seen from the high resolution TEM image in Figure 1f, in which MoS2 appear as ultrafine dark layers with the lateral dimension of only ~10 nm. These ultrafine MoS2 layers are randomly oriented in the carbon matrix and slightly rich in the inner surface region. In addition, EDX analysis shows that S and Mo atoms are uniformly distributed in the nitrogen-doped carbon matrix (Figure S4). No obvious diffraction ring appears in the SAED pattern of MoS2/C HNT (Figure S5), suggesting that the MoS2 has monolayer structure with low crystallinity. The confined growth of MoS2 is further proved by XRD. As shown in Figure 1g, only two diffraction peaks at 2θ = 33.4o and 58.9o are observed, corresponding to the (100) and (110) planes of MoS2 respectively, which match well with the JCPDS 65-1951. The absence of (002) reflection, which is characteristic for the bulk MoS2, indicates the formation of graphene-like thin MoS2 layers without stacking. The composition of the MoS2/C HNT were determined by TGA (Figure S6), showing a MoS2 content of 42wt%. The specific surface area and porosity of MoS2/C HNT were examined by N2 isothermal adsorption/desorption (Figure 1h). The isotherm of MoS2/C HNT exhibits a typical type-IV curve with a pronounced capillary condensation step at high N2 partial pressure and sharp uptake at low N2 pressure, indicating the coexistence of meso- and micro-pores. The calculated Brunauer-Emmett-Teller (BET) specific surface area of MoS2/C HNT is 153.2 m2 g-1, which is much higher than that of the carbonized PDA (C-PDA) spheres (18.1 m2 g-1). The pores are distributed in the range of 1.9 to160 nm with the peak at ~ 15 nm (insets of Figure 1h). The small shoulder at 105 nm might reflect the average size of the hollow cores. The relatively high specific surface area and abundant pores will not only increase the contact surface area with the electrolyte and facilitate the mass transfer, but also expose more active sites for electrocatalysis. 9



Furthermore, the Roman spectrum of MoS2/C HNT in Figure 1i exhibits two bands at 1348 and 1593 cm-1, corresponding to the disordered (D band) and graphite (G band) sp2 carbon, respectively. The high D/G intensity ratio (ID/IG = 1.10) indicates a highly defective carbon structure owing to the incorporation of nitrogen and MoS2, which is beneficial to electrocatalytic activities.30 Two characteristic bands of MoS2 are found at 380 (E12g) and 401 cm-1 (A1g) with a Δ value of only 21 cm-1. The small energy difference compared with that of bulk MoS2 further confirms the ultrathin nature of the MoS2 in the hybrid nanotubes. X-ray photoelectron spectroscopy (XPS) analysis was then performed to obtain insights into the chemical states and compositions of the (NH4)2MoS4/PDA and MoS2/C tubular nanostructures (Figure 2). XPS spectra of both (NH4)2MoS4/PDA and MoS2/C nanotubes display distinctive bands of C, N, O, Mo and S (Figure S7), indicating the successful incorporation of Mo and S into the hybrids. For (NH4)2MoS4/PDA, the high resolution C 1s spectrum in Figure 2a could be deconvoluted into three components centered at 285, 286.3, and 289 eV, corresponding to the sp2 C, and sp3 C in C-O/C-N and C=O, respectively. Only one doublet with a Mo 3d5/2 binding energy of 232.5 eV are detected in the Mo region (Figure 2b), indicating that the Mo ions are still hexavalent, as in the (NH4)2MoS4. The N 1s spectrum shown in Figure 2c consists of three types of N centered at 398.4, 400.1 and 402.0 eV, which can be assigned to the tertiary, secondary and primary amines of PDA, respectively. After thermal annealing, the C 1s spectrum remains almost the same except the absence of C=O, while an overlap between N 1s and Mo 3p (395.9 eV) is observed, indicating the successful reduction of Mo6+ (Figure 2d-2f). In the Mo 3d region, the dominant doublet centered at 229.9 and 233 eV could be assigned to the 3d5/2 and 3d3/2 of Mo4+ in MoS2 (Figure S8).14 The smaller doublet at 233.4 and 236.5 eV are related to the 3d5/2 and 3d3/2 of Mo6+, 10


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indicating the bonding of oxygen to Mo. The formation of Mo-O bond is also confirmed by the XPS spectrum of O 1s (Figure S9). The bands at 533.4 eV and 531.3 eV are assigned to the C-O and Mo-O bonds, respectively. Compared with MoO3 (O 1s at ~530.6 eV), the Mo-O bonds in MoS2/C HNT exhibits a higher binding energy, indicating that they have a different chemical environment. What’s more, the FTIR spectrum of MoS2/C HNT (Figure S10) shows a distinct band at 900 cm-1, which corresponds to the vibration of the Mo-O bond, and it shifts to a higher wavelength compared with the Mo-O bond in MoO3.31 No C=O group is detected. Therefore, a covalent bond is probably formed at the carbon/MoS2 interface.32 Thus, based on previous reports,32-34 we propose that the ultrafine MoS2 monolayers are covalently bonded to the surrounding carbon through the Mo-O-C bond, as illustrated in Figure 2g. This will increase the stability of MoS2. For N 1s, three peaks centered at 398.6, 399.6 and 401.5 eV are observed after deconvolution, which could be assigned to pyridinic, pyrrolic and graphitic N, respectively. The N heteroatoms will not only alter the charge density of adjacent C atoms, but tune the surface chemistry (e.g., basicity and hydrophilicity) of the hybrids, thus enhancing its electrocatalytic activities.27, 35 It is reported that the limiting current density and onset potential of ORR can be greatly improved by the graphitic and pyridinic N.36 Here, the contents of pyridinic and graphitic N of MoS2/C HNT are up to 83 % based on the integral areas, which would be beneficial to the performance of ORR. Usually, dopamine undergoes oxidative polymerization in alkaline solutions and forms nanospheres within 24 h (Figure S1). With the addition of (NH4)2MoS4, however, the polymerization behavior of dopamine changes dramatically and (NH4)2MoS4/PDA hybrid nanotubes are formed after several days under stirring. The much slower polymerization rate of dopamine could be attributed to its secondary bonding with 11



[MoS4]2- anions. The protonated dopamine, which is a stronger organic base than ammonium, displaces the ammonium in (NH4)2MoS4 and is linked to the tetrahedral [MoS4]2- anion by weak N-H…S and O-H…S interactions.37-38 These interactions are confirmed by the UV-Vis spectra. As shown in Figure S11a, dopamine has a characteristic absorption at 280 nm, while the bands at 320 and 470 nm can be assigned to the [MoS4]2- anions. After mixing dopamine with (NH4)2MoS4, the intensities of the bands are weakened, especially the one at 280 nm, indicating the charge transfer interactions between dopamine and the [MoS4]2-. As a result, the transitions of dopamine to dopaminequionone and leukodopaminechrome (Figure S12), an essential pathway to the formation of PDA, are restricted due to the limited mobility of the functional groups, leading to reduced polymerization activity.39

Formation mechanism of the hybrid nanotubes To reveal the formation mechanism of the hybrid nanotubes, the effect of (NH4)2MoS4 concentration on the polymerization of dopamine was first studied. It is intriguing that the morphology of the hybrids actually changes with the (NH4)2MoS4 concentration it evolves from nanospheres to nanotubes and finally nanoflakes as the (NH4)2MoS4 concentration increases. The formation of the hybrid structures was monitored via SEM, pH and UV-Vis. As shown in Figure 3a, the (NH4)2MoS4/dopamine solution goes through similar polymerization process as neat dopamine solution when the concentration of (NH4)2MoS4 is low. After 15 h, sphere-like nanoparticles with fuzzy boundaries are recognized and the (NH4)2MoS4/PDA nanospheres with two different sizes are formed after polymerization for 25 h (Figure 3a1 and a2). Clearly, although the polymerization rate is not much affected due to the existence of considerable amount of free dopamine, the addition of (NH4)2MoS4 alters the size distribution of the 12


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(NH4)2MoS4/PDA. After carbonization, MoS2/C nanospheres are formed (Figure 3a3). Small MoS2 domains are uniformly distributed in the C-PDA spheres, as confirmed by the Energy-dispersive X-ray spectroscopy (EDS) and linear elemental analysis (Figure S13). With an increment in (NH4)2MoS4 concentration (dopamine: (NH4)2MoS4 ratio is reduced to 1:1), all the dopamine species are linked to the [MoS4]2- anions and hence the transition of dopamine has to overcome the secondary forces between protonated dopamine and [MoS4]2- anion, leading to a lower degree of polymerization (oligomerization) in limited time. As a result, only bulk aggregations of oligomers are formed by 15 h, and small particles with irregular shapes are found after 25 h (Figure 3b1 and 3b2). The slow and incomplete polymerization of dopamine is confirmed by the time-dependent UV-Vis spectra (Figure S11b-d). The characteristic absorption of dopamine at 280 nm is retained throughout the reaction, while this band almost disappears after 64 h when the (NH4)2MoS4 concentration is low. Due to the presence of secondary forces, the neighboring particles stick to each other through the N-H…S and O-H…S interactions and the aggregates exhibit rod-like morphology because of the shear force created by stirring.40 As the polymerization goes on, the spheres are merged together gradually, forming the PDA-based hybrid nanorods (Figure 3b3). The growth of the outer shell continues thereafter due to the continuous supply of the dopamine species from the solution. The inner core is, however, isolated by the shell and the oxidation polymerization of dopamine is therefore largely inhibited. With increasing the polymerization degree of dopamine, the content of dopamine monomers/oligomers in the outer region reduces. As a result, the dopamine species in the core region diffuse outwards, leading to a polymerization-driven cavitation. It is worth noting that even with the cavities, the inner surface still has relatively poor accessibility to air, and hence outward diffusion of dopamine species dominates. With 13



the continuous enlargement of cavities at the center, the (NH4)2MoS4/PDA HNT is finally formed, as indicated by the arrows in Figure 3b4. The red cycles highlight some partially formed HNT, which are formed by the combination of nearby particles and therefore exhibit interconnected necklace structure. Based on the evidence presented above, the evolution of the hollow structures is illustrated in Figure 3b5. With even higher (NH4)2MoS4 concentration (dopamine: (NH4)2MoS4 ratio is decreased to 1:4), slow decomposition of (NH4)2MoS4 occurs. As shown in Figure 3c1, sheet-like structures are formed after 15 h, which may be attributed to MoS3. The dopamine is then polymerized and coated on the surface of MoS3 in the presence of (NH4)2MoS4 (Figure 3c2). After carbonization, multilyerd-MoS2 are formed owing to the high concentration of (NH4)2MoS4 (Figure 3c3), as confirmed by the XRD pattern (Figure S14). Other than (NH4)2MoS4 concentration, the effect of a series of other factors, such as pH, ionic concentration, surfactant, stirring, oxygen and complexation (strong interactions) were also investigated to further verify the mechanism proposed. The results are shown in Figure S15-18. They confirm that the slow polymerization of dopamine induced by the weak N-H…S and O-H…S interactions, adequate supply of oxygen to the reaction system and shear force created by stirring are the keys for the formation of the hybrid nanotubes.

Electrocatalytic performance of the hybrid nanotubes for ORR and HER The unique tubular structure, large specific surface area, relatively high contents of N dopants and oxygen-bonded MoS2 monolayers endow MoS2/C HNT with high ionic and electronic conductivities and abundant active sites, rendering it a promising nonnoble metal electrocatalysts for ORR and HER. As shown in Figure 4a, cyclic voltammetry (CV) of the MoS2/C HNT shows a pronounced cathodic peak at 0.80 V in 14


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the O2-saturated 0.1 M KOH solution, which is much more positive than that of C-PDA (0.67 V) and bulk MoS2 (0.59 V). The linear sweep voltammetry (LSV) polarization curve exhibits an onset potential (defined as the potential at which the current reaches 0.1 mA cm-2) of 0.91 V and half-wave potential of 0.82 V for the MoS2/C HNT (Figure 4b), close to those of Pt/C (0.97 V and 0.84 V). Clearly, the much improved electrocatalytic performance could be attributed to the synergetic effect between the doped N and MoS2.27, 35, 41-42 The presence of pyridinic and graphitic N improves the limiting current density and onset potential of ORR, while the incorporation of MoS2 further alters the charge density of adjacent nitrogen atoms. This high ORR activity achieved in this work is compared with those of reported state-of-arts electrocatalytic materials in Table S1. The ORR perforamnces of the MoS2/C nanospheres and nanosheets are shown in Figure S19. The MoS2/C nanospheres displays similar ORR activity to that of CPDA due to its relatively low MoS2 contents and similar spherical morphology. As for the MoS2/C nanosheets, though higher ORR activity is achieved, it is still inferior to that of MoS2/C HNT, which may be caused by the stacking of MoS2 and its limited specific surface area (Figure S20). To gain further insight into the pathway and reaction kinetic, polarization curves of the MoS2/C HNT at different rotation speeds were recorded using rotating disk electrode (RDE) at a scan rate of 10 mV s-1 (Figure 4c). With the increasing rotating speed, the onset potentials remain almost constant, while the current densities increase sharply due to the enhanced mass transport, indicating a kinetics-controlled ORR process. The corresponding KouteckyLevich (K-L) plots in Figure 4d exhibit excellent linearity and almost consistent slopes in the range of 0.28 to 0.68 V, which validates first-order kinetics of the ORR and similar electron transfer number(n) throughout the voltage window.43 The average n value is calculated to be 4.07 for MoS2/C HNT, very close to that of Pt/C (N = 4.05) 15



and much higher than that of C-PDA (n = 2.8) (Figure 4e and Figure S21-22). The n number calculated using Koutecky-Levich equation is further verified by the RRDE test. The results (Figure S23) indicate that the ORR process follows a desirable fourelectron pathway. To evaluate the durability of the MoS2/C HNT catalyst, a currenttime chronoamperometric test was carried out at 0.73 V and 1600 rpm (Figure 4f). The relative current density retains more than 80% after 36000 s, better than that of Pt/C (Figure S24), showing the MoS2/C HNT is a promising noble-metal free catalyst for ORR. In addition to ORR, the electrocatalytic properties of MoS2/C HNT for HER were also evaluated to demonstrate its bifunctionality in electrocatalysis. As shown in Figure 4g, bulk MoS2 and N-doped carbon show almost no HER activity, while MoS2/C HNT exhibits a low onset potential of 118 mV and high current density of 10 mA cm-2 at 190 mV, which could be attributed to the uniformly distributed MoS2 monolayers and highly conductive carbon framework.44-46 It has been reported that the HER activity of MoS2 mainly comes from its edge and defect sites.47 Herein, the MoS2 in MoS2/C HNT are single layers with lateral dimension of only ~ 10 nm, which possess much more edge/defect sites than its bulk counterpart. Furthermore, though N-doped carbon framework is inert for HER, it promotes electron and proton transport, and hence also contributes to the high HER activity. The HER activities of the MoS2/C nanospheres and nanosheets are compared in Figure S25. Among them, the MoS2/C nanospheres show the worst HER performance because of their lowest MoS2 content. The performance of MoS2/C nanosheets is still inferior to that of MoS2/C HNT, showing the importance of edge and defect sites in promoting the HER activity. The Tafel slope of the MoS2/C lowHNT is calculated to be 71 mV dec-1, higher than that of Pt/C (31 mV dec-1, Figure 4h), indicating a combination of the Volmer-Tafel mechanism in the 16


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HER process.48 The long-term cycling stability of the MoS2/C HNT was investigated by taking continuous scans at 50 mV s-1. As shown in Figure 4i, the overpotential increases only 5 mV after 4000 cycles, showing excellent durability and great potential as an alternative for noble-metal-based HER electrocatalyst. For comparison, the HER performance of the MoS2/C HNT is also summarized in Table S1 and compared with some state-of-art electrocatalysts.

CONCLUSIONS Different from the hollow nanostructures formed by template-assisted growth or via Kirkendall effect under harsh conditions, in this work, unique hybrid tubular nanostructures were facilely prepared for the first time by simply mixing of the reactants under ambient conditions. Systematical studies reveal that the formation of the hybrid nanotubes is probably driven by oxidation polymerization of dopamine, and assisted by the weak N-H…S and O-H…S interactions between protonated dopamine and [MoS4]2-. The subsequent thermal treatment has not only preserved the tubular nanostructures, but also introduced N and MoS2 dopants into the carbon framework. To evaluate the electrocatalytic activities of the MoS2/C HNT for ORR and HER, a series of electrochemical tests have been performed. The results show superior ORR activity of MoS2/C HNT, which is close to that of Pt/C, and low onset potential of 118 mV and small Tafel slope of 71 mV/decade for HER.

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Figure 1. (a, b) SEM and (c, d) TEM images of (NH4)2MoS4/PDA nanotubes; (e) TEM image, (f) HRTEM image, (g) XRD patterns, (h) BET isotherm and BJH pore size distribution and (i) Raman spectrum of MoS2/C HNT.

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Figure 2. The XPS C 1s, Mo 3d and N 1s spectra of (a-c) the (NH4)2MoS4/PDA nanotubes and (d-f) MoS2/C HNT; schematic diagrams showing (g) the interaction between MoS2 and carbon, and (h) configurations of N heteroatoms.

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Figure 3. Structural evolution of the (NH4)2MoS4/PDA hybrids with different (NH4)2MoS4 concentrations: (a) 0.25 (b) 1 and (c) 4 mg ml-1. The numbers in a1-a2, b1b4 and c1-c2 indicate the polymerization time in hour; a3 and c3 show the morphologies after annealing at 800 oC; b5 illustrates the formation process of the tubular structure.

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Figure 4. (a) CV curves of the Pt/C, C-PDA, bulk MoS2 and MoS2/C HNT in O2saturated 0.1 M KOH (dash line: N2-saturated), (b) LSV curves of the Pt/C, C-PDA, bulk MoS2 and MoS2/C HNT at a rotation speed of 1600 rpm, (c) LSV curves of the MoS2/C HNT at different rotation speeds, (d) the K-L plots and (e) calculated electron transfer number derived from the RDE data, (f) ORR stability test of the MoS2/C HNT at 0.73 V and 1600 rpm, (g) LSV curves of the Pt/C, C-PDA, bulk MoS2 and MoS2/C HNT in N2-saturated 0.5 M H2SO4, (h) the corresponding Tafel plots and (i) HER stability test of the MoS2/C HNT.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: X.XXX/acsami.XXXXX. SEM and TEM of the PDA and CPDA, XRD and XPS of the (NH4)2MoS4/PDA, TEM, XPS, TGA, XRD and EDX of the MoS2/C, UV-Vis, pH and SEM of the dopamine/(NH4)2MoS4 solutions, SEM of the dopamine/SDS, dopamine/(NH4)2SO4, dopamine/Na2MoO4 solutions, electrocatalytic performances of CPDA, Pt/C and MoS2/C hybrids. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for the financial support from the Natural Science Foundation of Shenzhen University (2018034).

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Self-Assembly-Assisted Facile Synthesis of MoS2-Based Hybrid

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