Amorphous Molybdenum Sulfide on Three-Dimensional Hierarchical

Subscriber access provided by UNIV OF DURHAM

Article

Amorphous Molybdenum Sulfide on Three-dimensional Hierarchical Hollow Microspheres Comprising Bamboo-like N-doped Carbon Nanotubes as a Highly Active Hydrogen Evolution Reaction Catalyst Seung-Keun Park, Jin Koo Kim, and Yun Chan Kang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01843 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Sustainable Chemistry & Engineering

Amorphous

Molybdenum

dimensional

Hierarchical

Sulfide Hollow

on

Three-

Microspheres

Comprising Bamboo-like N-doped Carbon Nanotubes as a Highly Active Hydrogen Evolution Reaction Catalyst

Seung-Keun Park†, Jin Koo Kim‡, and Yun Chan Kang*,‡

† Department of Chemical Engineering, Kongju National University, 1223-24 Cheonan-daero, Seobuk-Gu, Cheonan, 31080, Republic of Korea ‡ Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea

*Corresponding author E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268.

1 ACS Paragon Plus Environment

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

Abstract Novel amorphous-MoSx-coated three-dimensional (3D) hierarchical hollow microspheres comprising one-dimensional bamboo-like N-doped carbon nanotubes (BNCNT/MoSx-HM) are developed as a highly active electrocatalyst for the hydrogen evolution reaction (HER). The 3D hierarchical microspheres are easily prepared via the growth of bamboo-like N-doped CNTs (BNCNTs) on both the inner and outer surfaces of Co3O4-MgO/carbon hollow microspheres, which are prepared by spray pyrolysis, followed by uniform MoSx coating of the surface. Metallic-Co nanocrystals play a key role in the growth of BNCNTs, acting as catalysts for their nucleation. Owing to the electrostatic attraction between the N-doped sites in BNCNTs and the thiomolybdate precursor anions, few-layered amorphous MoSx catalysts are well deposited on BNCNT surfaces, even at low temperature. The 3D hierarchical hollow structure facilitates electrolyte access, and the synergistic effect between the MoSx catalyst material with ample active sites and the conductive Ndoped CNTs increases electrochemical activity for the HER. Accordingly, electrochemical assessment of BNCNT/MoSx-HM reveals a overpotential of 159 mV at a current density of 10 mA cm-2, a low Tafel slope of 41.1 mV dec-1, and excellent stability in acidic conditions.

KEYWORDS: hydrogen evolution reaction, spray pyrolysis, N-doped carbon nanotubes, molybdenum sulfide, hierarchical structure


Page 2 of 28

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


Introduction The development of cost-effective non-noble electrocatalysts for the hydrogen evolution reaction (HER) is a crucial strategy to address environmental concerns regarding the overuse of fossil fuels and the energy crisis.1-7 Accordingly, molybdenum sulfide has received increasing interest as an active non-precious HER catalyst owing to its excellent electrocatalytic properties and chemical stability in acidic or basic conditions.8-11 Both experimental and theoretical studies have indicated that crystalline MoS2 features HER active sites along the coordinated sulfur edge while its basal planes remain catalytically inert.12-14 Conversely, amorphous MoSx possesses ample catalytic active sites on both its edges and basal planes owing to the presence of unsaturated S atoms, which facilitate H+ adsorption and conversion into H2, over its entire surface.13,

15-19

However, the

intrinsically low conductivity of amorphous MoSx hampers electron transport during the HER reaction, resulting in poor overall electrocatalytic performance. Combining amorphous MoSx with carbon materials shows promise as an effective strategy to address the low-conductivity issue.15, 2022

In this strategy, carbon materials with high electrical conductivities, large surface areas, and

strong tolerances to acidic conditions are used to enhance the activity of incorporated electrocatalysts. Recently, hierarchical carbon materials have emerged as promising loading substrates for the preparation of highly active HER catalysts because they offer large surface areas, which facilitate electrolyte access and prevent the aggregation of catalysts, and they also impart improved electronic conductivities to their composites.23-27 Furthermore, there is currently significant research interest in heteroatom-doped hierarchical carbon-based electrocatalysts owing to their high catalytic activities, which originate from the heteroatom doping.28-29 For example, Qiao et al. prepared threedimensional (3D) hierarchically structured WS2 nanolayers/heteroatom-doped graphene films as an HER catalyst using a vacuum-filtration process. The unique structure and heteroatom doping of the graphene sheets was found to imbue them with extraordinary HER performance.30 Furthermore, Kim et al. synthesized highly HER-active MoSx/N-doped CNTs hybrid catalysts via low3 ACS Paragon Plus Environment


temperature precursor decomposition. The abundant catalytic sites on the amorphous MoSx surface and the high conductivity of the N-doped CNTs provided excellent catalytic activity for the HER.20 Moreover, Liu et al. reported hierarchically organized N-doped carbon nanofiber/MoS2 composites as cost-effective electrocatalysts for the HER. The obtained composites exhibited superior electrocatalytic activities owing to the exposed active edge sites on the MoS2, the conductive Ndoped carbon nanofibers, and their unique 3D networks.31 Currently, solution processes are most commonly applied for the preparation of hierarchical structured carbon substrates. However, these processes require harsh synthetic conditions and timeconsuming procedures, leading to low production yields. Thus, a scalable method for the mass production of hierarchical structured carbon substrates is required. Ultrasonic spray pyrolysis, a gasphase reaction processes, has recently emerged as a promising technique for the preparation of various structured nanomaterials owing to its simple apparatus, cost efficiency, and scalability.32-35 Ultrasonic spray pyrolysis has been primarily applied to synthesize hollow, yolk-shell, and porous nanocomposites.36-40 It is suitable for large-scale production of submicron-micron sized spheres having various morphologies with homogeneous size distribution. However, nanomaterials prepared via ultrasonic spray pyrolysis have not yet been utilized as base materials for heteroatomdoped hierarchical carbon substrates. In the present study, we developed a facile method to prepare highly HER-active amorphousMoSx-coated 3D hierarchical hollow microsphere comprising 1D bamboo-like N-doped carbon nanotubes (BNCNT/MoSx-HM). Hollow Co3O4-MgO/carbon (Co3O4-MgO/C) microspheres prepared by one-pot spray pyrolysis were utilized as the base material for hollow 3D hierarchical microspheres. During subsequent thermal treatment with dicyandiamide (DCDA), bamboo-like Ndoped carbon nanotubes (BNCNTs) were grown over the surfaces of the hollow microspheres. After chemical etching of the MgO matrix, MoSx was uniformly coated on the surface of the 3D hierarchical hollow microspheres via low-temperature precursor decomposition. These rationally designed composites offer several advantages as electrocatalysts for the HER. Firstly, the large


Page 4 of 28

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


surface area of the 3D hierarchical hollow microspheres facilitates electrolyte access and prevents the aggregation of the active MoSx material. Secondly, the presence of nitrogen dopants in BNCNTs improves electrocatalytic activity owing to its lone-pair electrons. Lastly, the intimate contact between the active MoSx material and BNCNTs provides direct conductive pathways for fast electron transport from BNCNTs to the catalytic sites. This study suggests the efficient route to construct morphology optimized for HER. Moreover, we witnessed that our result shows remarkable results in terms of electrocatalytic activities compared with the previous reports.

Experimental section Synthesis of Hollow Co3O4-MgO and Hollow Co3O4 Microspheres. The sample was prepared using spray pyrolysis as described in our previous reports.38, 40 The spray solution was prepared by dissolving cobalt nitrate, magnesium nitrate, citric acid, and ethylene glycol in 300 mL distilled water. The temperature of the reactor was fixed at 600 °C, and the droplets generated by the ultrasonic nebulizer were carried by N2 gas at a flow rate of 5 L min-1. The powder accumulated on the bag filter was then collected in a fume hood. Synthesis of 3D Hierarchical BNCNT Hollow Microspheres (BNCNT-HMs) and 1D N-Doped CNTs (1D NCNTs). A small alumina boat containing the obtained hollow Co3O4-MgO/C microspheres was placed in a large alumina boat containing DCDA. The boats were then transferred into a tube furnace and heat-treated under Ar atmosphere at 400 °C for the reduction of Co3O4, followed by an additional 1 h treatment at 800 °C for the growth of BNCNTs. After cooling, the powder was etched with diluted HCl solution to remove MgO and unreacted Co. Finally, the sample was collected by centrifugation and washed three times with ethanol. To coat amorphous MoSx onto

Synthesis of BNCNT/MoSx-HMs and 1D NCNT/MoSx. BNCNT-HM,

ammonium

tetrathiomolybdate

dissolved

in

a

small

amount

of

N,N-

dimethylformamide (DMF) was impregnated into the BNCNT-HM using a pipette. The mixture was carefully mixed in an agate mortar to ensure uniform loading and then dried in a vacuum oven. 5 ACS Paragon Plus Environment


The amorphous MoSx was successfully deposited upon subsequent heat treatment at various temperatures under H2/Ar (1:9, v/v) for 30 min. After cooling, the sample was collected in a vial and sealed to prevent surface oxidation. The detailed information of the chemical reagents used and synthetic procedures are precisely described in the supporting information. Characterization and Electrochemical Measurements. X-ray diffractometry was used to confirm the crystal structures. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the morphological characteristics of the samples. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical environment of the composite powders. Thermogravimetric analysis (TGA) was performed to measure the carbon content in the composite powders. The specific surface areas of the composite powders were determined by Brunauer-Emmett-Teller (BET) analysis. Raman spectroscopy was used for analysis of the carbon and MoSx. For electrochemical measurements, homogeneous inks of the samples were prepared and loaded onto a glassy carbon electrode. For comparison, a Pt/C-modified electrode was prepared by the same method using commercial Pt/C. Electrochemical measurements were carried out in a standard three-electrode system using a rotating disk electrode connected to a potentiostat/galvanostat. Electrochemical impedance measurements were conducted in a static potential at −0.2 V (vs. RHE) with an AC amplitude of 10 mV and a scanning frequency between 100 kHz and 10 mHz. The detailed information about the characterization technique and electrochemical measurements are described in the Supporting information.


Page 6 of 28

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


Results and discussion

Figure 1. Schematic illustration of formation mechanism of 3D BNCNT/MoSx-HM.

The procedure for the synthesis of 3D BNCNT/MoSx-HM is shown in Figure 1. First, hollow Co3O4-MgO/C microspheres are synthesized from droplets containing the Co and Mg precursors, citric acid (CA), and ethylene glycol (EG). During spray pyrolysis, an esterification reaction occurs between the citric acid and ethylene glycol inside the droplet, which results in the formation of a well-defined Co3O4-MgO/C composite comprising spherical particles with uniform shell thickness.41 Post-thermal treatment with DCDA in Ar atmosphere at 400 °C for 3 h forms CoMgO/C hollow microspheres. Subsequently, upon increasing the temperature to 800 °C, BNCNTs 7 ACS Paragon Plus Environment


are uniformly grown over the inner and outer surfaces of the hollow microspheres by precursor gases generated by the thermal decomposition of DCDA, in which metallic Co nanoparticles catalyze BNCNT growth with DCDA. The homogeneous distribution of Co nanocrystals in the heat-resistant MgO matrix results in the formation of thin BNCNTs. Subsequent etching of the MgO crystals using aqueous HCl results in the formation of 3D BNCNT-HM. For MoSx deposition, as-obtained BNCNT-HM are physically mixed with an ammonium tetrathiomolybdate precursor solution. Thiomolybdate anions adsorb easily onto the surfaces of BNCNTs owing to electrostatic interactions and then decompose into amorphous MoSx upon annealing.

Figure 2. Morphologies and crystal structure of hollow Co3O4-MgO/C microspheres: a) SEM image, b,c) TEM images, d) HR-TEM image, e) SAED pattern, and f) elemental mapping images.


Page 8 of 28

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


The morphologies and crystal structures of the hollow Co3O4-MgO/C microspheres are displayed in Figure 2 and S1, respectively. The well-defined microspheres, which are formed by the droplets drying and subsequent decomposition during spray pyrolysis, are clearly observed in the SEM and TEM images shown in Figure 2a-c. The high-resolution (HR) TEM image shown in Figure 2d indicates the formation of nanosized Co3O4 and MgO crystals. The XRD and selected area electron diffraction (SAED) patterns shown in Figure S1 and 2e, respectively indicate the formation of hollow microspheres with a mixed composition comprising Co3O4 and MgO nanocrystals. Correspondingly, the elemental mapping images also show homogeneous distributions of carbon (derived from CA/EG), Co3O4, and MgO (Figure 2f).

Figure 3. Morphologies and crystal structure of 3D BNCNT-HM: a,b) TEM images, c) HR-TEM image, d) SAED pattern, and e) elemental mapping images.



The morphologies and crystal structure of BNCNT-HM are shown in Figure 3 and S1, respectively. As shown in Figure 3a and b, the TEM images reveal that the hollow microspheres comprise wellgrown 1D BNCNTs. Even after BNCNT growth and etching processes, the unique spherical morphology of BNCNT-HM is still maintained, demonstrating the robustness of the hollow Co3O4MgO/C microspheres. As shown in Figure S2a–S2d, BNCNT-HM containing MgO have similar morphologies to those of MgO-free BNCNT-HM. In addition, encapsulated Co nanocrystals can be clearly observed at the tips of BNCNTs (Figure 3b), suggesting that the nucleation of BNCNTs occurs at the surface of the metallic Co nanocatalysts.42 As revealed in the HR-TEM image in Figure 3c, NCNTs have joints similar to those in bamboo, which is a result of N-doping in the graphite network.43-44 The XRD and SAED patterns (Figure S1 and 3d) indicate the co-existence of highly graphitized BNCNTs and metallic Co nanocatalyst particles. The elemental mapping images and EDX spectrum in Figure 3e and S3, respectively, confirm that BNCNTs are well grown over the hollow microspheres, and they also confirm the complete etching of the MgO nanocrystals and the successful N-doping.


Page 10 of 28

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


Figure 4. Morphologies and crystal structure of 3D BNCNT/MoSx-HM: a,b) TEM images, c) HRTEM image, d) SAED pattern, and e) elemental mapping images.

After MoSx coating, the rough surfaces of BNCNT-HM are slightly smoothed, as shown in Figure S2e and S2f. This result can be attributed to the physical mixing process for the deposition of ammonium tetrathiomolybdate, which partially flattens BNCNTs comprising the hollow microspheres. The TEM images in Figure 4 reveal that the hollow microspheres retain their 3D hierarchical structure during MoSx deposition and that they have thicker shells compared to those of BNCNT-HM (Figure 4a and b). As shown in the HR-TEM image, few-layered amorphous MoSx with a thickness of ~3 nm is uniformly coated onto BNCNTs (Figure 4c, indicated by arrows). The interlayer spacing for MoSx is 0.62 nm, which corresponds to the lattice fringes of the (002) plane in crystalline MoS2. Importantly, the intimate contact between the MoSx and the BNCNTs provides 11 ACS Paragon Plus Environment


direct conductive pathways for fast electron transport from BNCNTs to the catalytic MoSx sites. Only the presence of BNCNTs and the metallic Co nanocatalysts is confirmed by the XRD and SAED patterns (Figure S1 and 4d) owing to the poor crystallinity of amorphous MoSx. Amorphous MoSx has abundant catalytically active sites over its entire surface that originate from the presence of unsaturated S atoms.13, 17 The elemental mapping images confirm the uniform coating of MoSx on the surface of the BNCNTs-HMs (Figure 4e).

Figure 5. High resolution XPS spectra of a) C 1s, b) N 1s, c) Mo 3d, and d) S 2p of 3D BNCNT/MoSx-HM.

XPS analysis was conducted to derive details regarding the chemical composition and bonding states in BNCNT/MoSx-HM (Figure 5 and S4). As shown in Figure S4a, the XPS survey spectrum confirms the presence of C, Mo, S, N, O, and Co. The high-resolution C 1s spectrum (Figure 5a) 12 ACS Paragon Plus Environment

Page 12 of 28

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


can be deconvoluted into three different peaks located at 284.5, 285.3, and 287.6 eV, which correspond to sp2-bonded C-C, C-N and sp 3 -bonded C-C, and -COO groups, respectively.45 The sharp peak for sp2-bonded C-C indicates the graphitic nature of the carbonaceous materials in BNCNT/MoSx-HM.

24

As shown in Figure 5b, a peak corresponding to Mo 3p3/2 is observed at

395.5 eV and there is a shoulder on the left side of the Mo 3p3/2 peak at 396.6 eV, which corresponds to N-Mo bonds. These results indicate the chemical interaction between the N dopant present in the carbon network and Mo,46 which improves electrocatalytic activity for the HER. The peaks centered at 398.6, 400.1, and 401.6 eV are attributed to pyridinic N, pyrrolic N, and graphitic N, respectively.47 According to a previous report, the lone-pair electrons on pyridinic N atoms can attract the MoS42- ions in ammonium tetrathiomolybdate by electrostatic force, which leads to a decrease in the nucleation barrier energy for molybdenum sulfide formation.20 Furthermore, N atoms are inherently favorable for proton interaction owing to their lone-pair electrons, resulting in enhanced intrinsic HER activity. As the annealing temperature increased, the N content of sample gradually decreased from 7.36 to 5.00 at. % (Figure S5). This result indicated that N atoms in the carbon network easily were partially removed at high temperature due to their unstable state. The Mo 3d spectrum shown in Figure 5c shows four deconvoluted peaks located at 227.6, 229.9, 233.1, and 236.1 eV, which are assigned to S 2s, Mo 3d5/2, Mo 3d3/2, and Mo6+, respectively. The peak positions for Mo3d5/2 and Mo3d3/2 indicate an oxidation state of Mo4+. The presence of Mo6+ species indicates partial surface oxidation of MoSx in air. Unlike that of bulk MoS2, the S 2p spectrum (Figure 5d) for our MoSx shows a broad peak with a small shoulder at 163.6 eV, indicating the existence of apical S2-, terminal S2-, bridging S22-, or S22- ligands.48-49 Thus, the XPS results confirm the formation of amorphous MoSx. The Co 2p spectrum shown in Figure S4b shows prominent peaks at 780.1 and 794.7 eV, which correspond to Co 2p3/2 and Co 2p1/2, respectively. The deconvoluted Co 2p spectrum reveals the existence of Co0, Co2+, and satellites. Partial Co2+ species originate from the partially oxidized surface of the metallic Co nanocrystals in the composite.



The morphologies and structures of the materials prepared as comparison samples are shown in Figure S6. The hollow Co3O4 microspheres observed in the SEM and TEM images (Figure S6a and S6c) were synthesized by spray pyrolysis. The SAED pattern reveals that the crystal structure of the Co3O4 microspheres is of pure cubic phase (Figure S6e). Interestingly, the hollow Co3O4 microspheres, unlike the hollow Co3O4-MgO/C microspheres, transform into 1D Co@N-doped CNTs (NCNTs) upon post-treatment under the same conditions (Figure S6b and S6d). The TEM image shown in Figure S6d clearly reveals the formation of 1D NCNTs with corrugated surfaces in which the Co nanocrystals embedded at the tips of the NCNTs are larger than those in BNCNT/MoSx-HM. These results clearly indicate that the MgO matrix minimizes the size of the metallic Co nanocrystals and imparts structural robustness during post-treatment. The SAED pattern of 1D NCNTs indicates the presence graphitic NCNTs and metallic Co crystals (Figure S6f).

.


Page 14 of 28

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


Figure 6. Morphologies and crystal structure of 1D NCNT/MoSx: a) SEM image, b) TEM image, c) HR-TEM image, d) SAED pattern, and e) elemental mapping images.

The morphology and structure of MoSx-coated 1D NCNTs (1D NCNT/MoSx) are shown in Figure 6. Unlike BNCNT/MoSx-HM, 1D NCNTs agglomerate with the MoSx due to their low surface area (Figure 6a and b). These results suggest that the hierarchical structure and large surface area of BNCNT-HM are critical factors for uniform MoSx coating. From the HR-TEM image (Figure 6c), it can be clearly observed that the MoSx in 1D NCNT/MoSx has a well-layered structure caused by weak van der Waals interactions between the layers. Nevertheless, owing to the low MoSx content, the intensities of the peaks and patterns attributed to crystalline MoS2 are very weak (Figure S1 and 6d). The presence of MoSx on the outer surfaces of 1D NCNTs is confirmed by the elemental mapping images (Figure 6e). Raman spectroscopy was used to investigate the electronic structures of the MoSx and carbon species in the composites. None of the composites exhibit peaks characteristic of MoS2 (E12g and A1g) in the range 350–450 cm-1, suggesting the coated MoSx is amorphous (Figure S7a).50 The D and G bands of the composites (Figure S7b), which are attributed to defected graphitic carbon materials, are clearly observed at ~1365 and ~1600 cm-1, respectively. The ID/IG values, used as a measure of the degree of graphitization, for BNCNT/MoSx-HM and NCNT/MoSx are approximately 1.05 and 0.92, respectively. These results suggest that the graphitic carbon materials in BNCNT/MoSx-HM contain more defects than those in 1D NCNT/MoSx. In order to confirm the carbon and MoSx contents of the samples, the TG curves for BNCNT-HM, BNCNT/MoSx-HM, 1D NCNTs, and NCNT/MoSx samples shown in Figure S7c and S7d were obtained under air. As shown in Figure S7c, a prominent weight loss for BNCNT-HM occurs at a higher temperature (470 °C) than that for NCNTs (350 °C), which may be due to the thin, highly crystallized CNTs in BNCNT-HM. The calculated carbon contents of BNCNT-HM and NCNTs are 86.8 and 81.6 wt%, respectively. Based on the results in Figure S7c, we were able to estimate that



the carbon contents of BNCNT/MoSx-HM and NCNT/MoSx are approximately 74.6 and 66 wt%, respectively (Figure S7d). The BET specific surface areas and detailed pore structures of the samples were investigated by N2 adsorption/desorption isotherms and Barrett-Joyner-Halenda (BJH) pore-size distributions (Figure S8). Owing to the interspace between the well-grown N-doped CNTs and their hollow structures, BNCNT-HM shows a higher surface area (71.7 m2 g-1) than that of NCNTs (9.4 m2 g-1), as well as well-developed mesopores. After MoSx coating, the BET surface areas of both samples slightly decrease owing to the low surface area of MoSx. Importantly, mesopores ranging from 10 to 50 nm in size for BNCNT/MoSx-HM still remain, indicating that the unique structure is undamaged upon MoSx coating.

Figure 7. a) LSV curves, b) Tafel plots, and c) Nyquist plots for the labelled electrodes. d) Cycle stability, and e) electrolysis curve for 3D BNCNT/MoSx-HM. 16 ACS Paragon Plus Environment

Page 16 of 28

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


In order to verify the superior HER performance of 3D hierarchical BNCNT hollow microspheres coated with amorphous MoSx, we prepared and compared the samples with different morphology and annealing temperature. The electrocatalytic HER activities of the samples and a commercial Pt/C composite were investigated in 0.5 M H2SO4 aqueous solution using a standard three-electrode system, and the results are presented in Figure 7. As shown in Figure 7a, the commercial Pt/C exhibits a near-zero onset potential and very high current, as previously reported.51 The other samples also show HER activity, which implies that the N-doped CNTs as well as the MoSx have catalytically active sites for the HER. Importantly, BNCNT-HM exhibits superior electrocatalytic activity than that of NCNTs owing to their unique hierarchical structure, which provides a large surface area for easy electrolyte access and more active sites for the HER. After amorphous MoSx coating, the samples show improved HER activities owing to the abundant catalytically active sites in amorphous MoSx. The overpotential of BNCNT/MoSx-HM at a current density of 10 mA cm-2 is only 0.159 V (vs. RHE), which is lower than those of NCNT/MoSx (0.179 V), BNCNT-HM (0.219 V), and NCNTs (0.226 V). The Tafel slope, which is related to the rate-limiting step in the HER, is an important criterion used to assess electrocatalytic activity. In theory, the Tafel slope is determined by the linear segments of the Tafel plot fitted with the Tafel equation.52 As shown in Figure 7b, the commercial Pt/C catalyst exhibits a Tafel slope value of ~30 mV dec-1, which is in agreement with those of previous reports.24, 27 Tafel slopes for BNCNT/MoSx-HM, NCNT/MoSx, BNCNT-HM, and NCNTs are approximately 41.1, 68.7, 100.2, and 133.2 mV dec-1, respectively. The low Tafel slope of BNCNT/MoSx-HM suggests a significant increase in HER rate with increasing overpotential, which is favorable for practical applications. The excellent electrocatalytic performances of BNCNT/MoSx-HM are comparable or superior to those of previously reported molybdenum-sulfide-based catalysts for the HER (Table S1), which could be attributed to the synergistic effect between the hierarchical structured conductive carbon substrate and the highly active MoSx catalyst material. The LSV curve normalized by active mass is shown in Figure S9.



Compared with Figure 7a which is normalized by area, the catalytic activities of the samples follow the same trend. In addition, we evaluated the electrochemical behavior of BNCNT/MoSx-HM and NCNT/MoSx during hydrogen evolution using electrochemical impedance spectroscopy (EIS, Figure 7c). The semicircles in the high-frequency region indicate the charge-transfer resistance (Rct) at the electrolyte/electrode interface, which correlates with the electrochemical reaction kinetics.53-56 The smaller Rct value for BNCNT/MoSx-HM indicates a faster reaction rate, which is attributed to decrease in ion-transfer resistance at the electrolyte/electrode interface. Stability is another important criterion for the practical application of a catalyst. To confirm the durability of BNCNT/MoSx-HM under acidic conditions, a long-term cycling test and chronoamperometric electrolysis at static overpotential were performed. As shown in Figure 7d, I-V curves were measured before and after 1,000 cycles between 0.0 and -0.3 V at a scan rate of 0.1 V S-1. The negligible negative shift of the original curve after cycling indicates the superior stability of BNCNT/MoSx-HM for long-term electrochemical processes. Furthermore, chronoamperometric electrolysis at η = 0.18 V reveals stable hydrogen evolution for 15 h, demonstrating the durability of the sample under acidic conditions (Figure 7e). To study the effect of annealing temperature on HER activity, the 3D BNCNT/MoSx-HMs prepared at different annealing temperatures were investigated under same conditions. Their LSV curves and corresponding Tafel plots are displayed in Figure S10. Notably, the overpotential and Tafel slope values of the samples gradually increased as the annealing temperature increased. The deterioration of catalytic activities as the elevation of temperature could be ascribed to the reduction of highly active unsaturated S atoms after the annealing at higher temperatures. As confirmed in Figure S11, the peaks corresponding to bridging S2-2 or apical S2- atoms completely disappeared after the annealing at higher temperatures over 400 oC. Instead, the peaks corresponding to the S 2p3/2 and 2p1/2 of S2- are observed at 162.1 and 163.1 eV which are typical characteristic of S atoms in crystalline MoS2. According to the previous report, the higher HER efficiency of MoSx catalysts


Page 18 of 28

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


is closely related to the presence of unsaturated bridging S2-2 or apical S2- which are mainly confirmed in amorphous MoSx annealed at low temperatures.22

Figure 8. Electrochemical double-layer capacitance results of a) 3D BNCNT/MoSx-HM, b) 1D NCNT/MoSx, c) 3D BNCNT-HM, and d) 1D NCNT and e) summary.

To determine their electrochemically active surface areas, the electrochemical double-layer capacitance (EDLC) values of the samples were determined from the cyclic voltammogram (CV) results (Figure 8). All the CV curves have a nearly rectangular shape, indicating that the samples exhibit ideal capacitive behavior in their electrochemical reactions, and the EDLC values increase with increasing scan rate.57 The positive and negative current density differences of all the samples 19 ACS Paragon Plus Environment


at 0.25 V vs. RHE are plotted against scan rate in Figure 8e. A linear trend is obtained for all samples. BNCNT/MoSx-HM has the largest EDLC value of 12.7 mF cm-2, which is larger than that of BNCNT-HM (5.47 mF cm-2). This result can be attributed to their abundant exposed active sites and large effective electrochemically active area. In contrast, the EDLC value of NCNTs (3.94 mF cm-2) decreases to 3.23 mF cm-2 upon MoSx coating owing to the agglomeration of the MoSx catalyst material, which decreases its electrochemically active area.

Conclusions In summary, a facile strategy has been developed to prepare few-layered-MoSx-coated 3D hierarchical hollow microspheres consisting of 1D bamboo-like N-doped CNTs as highly active electrocatalysts for the HER. The synthetic process involves the growth of BNCNTs on both the inner and outer surfaces of hollow Co3O4-MgO/C microspheres prepared by spray pyrolysis and uniform MoSx coating of their surfaces at a low temperature. The unique hierarchical structure and the intimate contact between the MoSx catalytic material and the conductive N-doped CNTs in the composite improve its electrocatalytic activity for the HER. Thus, BNCNT/MoSx-HM exhibits a low Tafel slope, high current density at low overpotential, and excellent stability in acidic conditions. This novel approach provides a general synthetic route to hierarchical structured Ndoped carbon-based catalysts that can be efficiently used in a wide variety of electrochemical applications.

ASSOCIATED CONTENT Supporting Information XRD patterns of Co3O4-MgO/C-HM, 3D BNCNT-HM, 3D BNCNT/MoSx-HM, and 1D NCNT/MoSx; SEM images of 3D BNCNT-HM before and after acid etching, and 3D BNCNT/MoSx-HM; EDX spectrum of 3D BNCNT-HM; XPS survey scan and high resolution XPS spectra of Co 2p of 3D BNCNT/MoSx-HM; SEM images, TEM images, and SAED patterns of 20 ACS Paragon Plus Environment

Page 20 of 28

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


hollow Co3O4 microsphere and 1D NCNT/MoSx; Raman spectra of 3D BNCNT/MoSx-HM and 1D NCNT/MoSx; TGA curves of 3D BNCNT-HM, 1D NCNT, 3D BNCNT/MoSx-HM, and 1D NCNT/MoSx under air atmosphere; N2 adsorption and desorption isotherms and BJH pore size distributions of 3D BNCNT-HM, 1D NCNT, 3D BNCNT/MoSx-HM, and 1D NCNT/MoSx; LSV curves for the labelled electrodes (normalized by the mass); LSV curves and Tafel plots for 3D BNCNT/MoSx-HMs prepared at different annealing temperatures; Table of electrocatalytic activity for HER of various nanostructured molybdenum sulfide-based catalysts, The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Y.C.K supervised the project. S.-K.P conceived the idea, experimentally realized the idea and wrote the manuscript. J.K.K synthesized the samples. All of the authors discussed the results, commented, and revised the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2017R1A2B2008592). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A4A1014806).

References 21 ACS Paragon Plus Environment


(1) Qu, K. G.; Zheng, Y.; Jiao, Y.; Zhang, X. X.; Dai, S.; Qiao, S. Z., Polydopamine-Inspired, Dual Heteroatom-Doped Carbon Nanotubes for Highly Efficient Overall Water Splitting. Adv. Energy Mater. 2017, 7, 1602068, DOI: 10.1002/aenm.201602068. (2) Zeng, M.; Li, Y. G., Recent Advances in Heterogeneous Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3, 14942-14962, DOI: 10.1039/C5TA02974K. (3) Sun, H. M.; Xu, X. B.; Yan, Z. H.; Chen, X.; Cheng, F. Y.; Weiss, P. S.; Chen, J., Porous Multishelled Ni2P Hollow Microspheres as an Active Electrocatalyst for Hydrogen and Oxygen Evolution. Chem. Mater. 2017, 29, 8539-8547, DOI: 10.1021/acs.chemmater.7b03627. (4) Li, Y. X.; Wang, Y.; Pattengale, B.; Yin, J.; An, L.; Cheng, F. Y.; Li, Y. F.; Huang, J. E.; Xi, P. X., High-index Faceted CuFeS2 Nanosheets with Enhanced Behavior for Boosting Hydrogen Evolution Reaction. Nanoscale 2017, 9, 9230-9237, DOI: 10.1039/C7NR03182C. (5) Huang, G. W.; Liu, H.; Wang, S. P.; Yang, X.; Liu, B. H.; Chen, H. Z.; Xu, M. S., Hierarchical Architecture of WS2 Nanosheets on Graphene Frameworks with Enhanced Electrochemical Properties for Lithium Storage and Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 24128-24138, DOI: 10.1039/C5TA06840A. (6) Li, M. Y.; Zhang, Z. Y.; Lyu, F. Y.; He, X. J.; Liang, Z. H.; Balogun, M. S.; Lu, X. H.; Fang, P. P.; Tong, Y. X., Facile Hydrothermal Synthesis of Three Dimensional Hematite Nanostructures with Enhanced Water Splitting Performance. Electrochim. Acta 2015, 186, 95-100, DOI: 10.1016/j.electacta.2015.10.048. (7) Xie, S. L.; Li, M. Y.; Wei, W. J.; Zhai, T.; Fang, P. P.; Qiu, R. L.; Lu, X. H.; Tong, Y. X., Gold Nanoparticles Inducing Surface Disorders of Titanium Dioxide Photoanode for Efficient Water Splitting. Nano Energy 2014, 10, 313-321, DOI: 10.1016/j.nanoen.2014.09.029. (8) Niu, F. J.; Dong, C. L.; Zhu, C. B.; Huang, Y. C.; Wang, M.; Maier, J.; Yu, Y.; Shen, S. H., A Novel Hybrid Artificial Photosynthesis System using MoS2 Embedded in Carbon Nanofibers as Electron Relay and Hydrogen Evolution Catalyst. J. Catal. 2017, 352, 35-41, DOI: 10.1016/j.jcat.2017.04.027. (9) Yu, X. Y.; Feng, Y.; Jeon, Y.; Guan, B.; Lou, X. W.; Paik, U., Formation of Ni-Co-MoS2 Nanoboxes with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28, 9006-9011, DOI: 10.1002/adma.201601188. (10) 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, 92-97, DOI: 10.1002/adma.201504024. (11) Zhao, Y. F.; Xie, X. Q.; Zhang, J. Q.; Liu, H.; Ahn, H. J.; Sun, K. N.; Wang, G. X., MoS2 Nanosheets Supported on 3D Graphene Aerogel as a Highly Efficient Catalyst for Hydrogen Evolution. Chem.-Eur. J. 2015, 21, 15908-15913, DOI: 10.1002/chem.201501964. (12) Chung, D. Y.; Park, S. K.; Chung, Y. H.; Yu, S. H.; Lim, D. H.; Jung, N.; Ham, H. C.; Park, H. Y.; Piao, Y.; Yoo, S. J.; Sung, Y. E., Edge-exposed MoS2 Nano-assembled Structures as Efficient Electrocatalysts for Hydrogen Evolution Reaction. Nanoscale 2014, 6, 2131-2136, DOI: 10.1039/C3NR05228A. (13) Morales-Guio, C. G.; Hu, X. L., Amorphous Molybdenum Sulfides as Hydrogen Evolution Catalysts. Acc. Chem. Res. 2014, 47, 2671-2681, DOI: 10.1021/ar5002022.


Page 22 of 28

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


(14) Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J., MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299, DOI: 10.1021/ja201269b. (15) Pham, K. C.; Chang, Y. H.; McPhail, D. S.; Mattevi, C.; Wee, A. T. S.; Chua, D. H. C., Amorphous Molybdenum Sulfide on Graphene-Carbon Nanotube Hybrids as Highly Active Hydrogen Evolution Reaction Catalysts. ACS Appl. Mater. Interfaces 2016, 8, 5961-5971, DOI: 10.1021/acsami.5b09690. (16) Lu, A. Y.; Yang, X. L.; Tseng, C. C.; Min, S. X.; Lin, S. H.; Hsu, C. L.; Li, H. N.; Idriss, H. C.; Kuo, J. L.; Huang, K. W.; Li, L. J., High-Sulfur-Vacancy Amorphous Molybdenum Sulfide as a High Current Electrocatalyst in Hydrogen Evolution. Small 2016, 12, 5530-5537, DOI: 10.1002/smll.201602107. (17) Ting, L. R. L.; Deng, Y. L.; Ma, L.; Zhang, Y. J.; Peterson, A. A.; Yeo, B. S., Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction. ACS Catal. 2016, 6, 861-867, DOI: 10.1021/acscatal.5b02369. (18) Vrubel, H.; Moehl, T.; Gratzel, M.; Hu, X. L., Revealing and Accelerating Slow Electron Transport in Amorphous Molybdenum Sulphide Particles for Hydrogen Evolution Reaction. Chem. Commun. 2013, 49, 8985-8987, DOI: 10.1039/C3CC45416A. (19) Tran, P. D.; Tran, T. V.; Orio, M.; Torelli, S.; Truong, Q. D.; Nayuki, K.; Sasaki, Y.; Chiam, S. Y.; Yi, R.; Honma, I.; Barber, J.; Artero, V., Coordination Polymer Structure and Revisited Hydrogen Evolution Catalytic Mechanism for Amorphous Molybdenum Sulfide. Nat. Mater. 2016, 15, 640-646, DOI: 10.1038/nmat4588. (20) Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O., Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction. Nano Lett. 2014, 14, 1228-1233, DOI: 10.1021/nl404108a. (21) Laursen, A. B.; Vesborg, P. C. K.; Chorkendorff, I., A high-Porosity Carbon Molybdenum Sulphide Composite with Enhanced Electrochemical Hydrogen Evolution and Stability. Chem. Commun. 2013, 49, 4965-4967, DOI: 10.1039/C3CC41945B. (22) Chang, Y. H.; Lin, C. T.; Chen, T. Y.; Hsu, C. L.; Lee, Y. H.; Zhang, W. J.; Wei, K. H.; Li, L. J., Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on GrapheneProtected 3D Ni Foams. Adv. Mater. 2013, 25, 756-760, DOI: 10.1002/adma.201202920. (23) Ye, T. N.; Lv, L. B.; Xu, M.; Zhang, B.; Wang, K. X.; Su, J.; Li, X. H.; Chen, J. S., Hierarchical Carbon Nanopapers Coupled with Ultrathin MoS2 Nanosheets: Highly Efficient Largearea Electrodes for Hydrogen Evolution. Nano Energy 2015, 15, 335-342, DOI: 10.1016/j.nanoen.2015.04.033. (24) Park, S. K.; Chung, D. Y.; Ko, D.; Sung, Y. E.; Piao, Y., Three-dimensional Carbon Foam/N-doped Graphene@MoS2 Hybrid Nanostructures as Effective Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2016, 4, 12720-12725, DOI: 10.1039/C6TA03458F. (25) Hou, Y.; Wen, Z. H.; Cui, S. M.; Ci, S. Q.; Mao, S.; Chen, J. H., An Advanced NitrogenDoped Graphene/Cobalt-Embedded Porous Carbon Polyhedron Hybrid for Efficient Catalysis of Oxygen Reduction and Water Splitting. Adv. Funct. Mater. 2015, 25, 872-882, DOI: 10.1002/adfm.201403657. (26) Jiang, Y. M.; Li, X.; Yu, S. J.; Jia, L. P.; Zhao, X. J.; Wang, C. M., Reduced Graphene Oxide-Modified Carbon Nanotube/Polyimide Film Supported MoS2 Nanoparticles for 23 ACS Paragon Plus Environment


Electrocatalytic Hydrogen Evolution. Adv. Funct. Mater. 2015, 25, 2693-2700, DOI: 10.1002/adfm.201500194. (27) Peng, S. J.; Li, L. L.; Han, X. P.; Sun, W. P.; Srinivasan, M.; Mhaisalkar, S. G.; Cheng, F. Y.; Yan, Q. Y.; Chen, J.; Ramakrishna, S., Cobalt Sulfide Nanosheet/Graphene/Carbon Nanotube Nanocomposites as Flexible Electrodes for Hydrogen Evolution. Angew. Chem. Int. Ed. 2014, 53, 12594-12599, DOI: 10.1002/ange.201408876. (28) Jin, H. Y.; Wang, J.; Su, D. F.; Wei, Z. Z.; Pang, Z. F.; Wang, Y., In Situ Cobalt-cobalt Oxide/N-doped Carbon Hybrids as Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 2688-2694, DOI: 10.1021/ja5127165. (29) Ying, J.; Jiang, G. P.; Cano, Z. P.; Han, L.; Yang, X. Y.; Chen, Z. W., Nitrogen-doped Hollow Porous Carbon Polyhedrons Embedded with Highly Dispersed Pt Nanoparticles as a Highly Efficient and Stable Hydrogen Evolution Electrocatalyst. Nano Energy 2017, 40, 88-94, DOI: 10.1016/j.nanoen.2017.07.032. (30) Duan, J. J.; Chen, S.; Chambers, B. A.; Andersson, G. G.; Qiao, S. Z., 3D WS2 Nanolayers@Heteroatom-doped Graphene Films as Hydrogen Evolution Catalyst Electrodes. Adv. Mater. 2015, 27, 4234-4241, DOI: 10.1002/adma.201501692. (31) Lai, F. L.; Miao, Y. E.; Huang, Y. P.; Zhang, Y. F.; Liu, T. X., Nitrogen-doped Carbon Nanofiber/Molybdenum Disulfide Nanocomposites Derived from Bacterial Cellulose for HighEfficiency Electrocatalytic Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 3558-3566, DOI: 10.1021/acsami.5b06274. (32) Skrabalak, S. E.; Suslick, K. S., Porous MoS2 Synthesized by Ultrasonic Spray Pyrolysis. J. Am. Chem. Soc. 2005, 127, 9990-9991, DOI: 10.1021/ja051654g. (33) Xu, H. X.; Guo, J. R.; Suslick, K. S., Porous Carbon Spheres from Energetic Carbon Precursors using Ultrasonic Spray Pyrolysis. Adv. Mater. 2012, 24, 6028-6033, DOI: 10.1002/adma.201201915. (34) Park, G. D.; Cho, J. S.; Kang, Y. C., Novel Cobalt Oxide-Nanobubble-Decorated Reduced Graphene Oxide Sphere with Superior Electrochemical Properties Prepared by Nanoscale Kirkendall Diffusion Process. Nano Energy 2015, 17, 17-26, DOI: 10.1016/j.nanoen.2015.07.026. (35) Zhu, Y. J.; Choi, S. H.; Fan, X. L.; Shin, J.; Ma, Z. H.; Zachariah, M. R.; Choi, J. W.; Wang, C. S., Recent Progress on Spray Pyrolysis for High Performance Electrode Materials in Lithium and Sodium Rechargeable Batteries. Adv. Energy Mater. 2017, 7, 1601578, DOI: 10.1002/aenm.201601578. (36) Hong, Y. J.; Son, M. Y.; Kang, Y. C., One-Pot Facile Synthesis of Double-Shelled SnO2 Yolk-Shell-Structured Powders by Continuous Process as Anode Materials for Li-ion Batteries. Adv. Mater. 2013, 25, 2279-2283, DOI: 10.1002/adma.201204506. (37) Ko, Y. N.; Choi, S. H.; Kang, Y. C., Hollow Cobalt Selenide Microspheres: Synthesis and Application as Anode Materials for Na-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 64496456, DOI: 10.1021/acsami.5b11963. (38) Choi, S. H.; Ko, Y. N.; Lee, J. K.; Kang, Y. C., 3D MoS2-Graphene Microspheres Consisting of Multiple Nanospheres with Superior Sodium Ion Storage Properties. Adv. Funct. Mater. 2015, 25, 1780-1788, DOI: 10.1002/adfm.201402428.


Page 24 of 28

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


(39) Fan, X. L.; Gao, T.; Luo, C.; Wang, F.; Hu, J. K.; Wang, C. S., Superior Reversible Tin Phosphide-Carbon Spheres for Sodium Ion Battery Anode. Nano Energy 2017, 38, 350-357, DOI: 10.1016/j.nanoen.2017.06.014. (40) Kim, J. K.; Park, G. D.; Kim, J. H.; Park, S. K.; Kang, Y. C., Rational Design and Synthesis of Extremely Efficient Macroporous CoSe2-CNT Composite Microspheres for Hydrogen Evolution Reaction. Small 2017, 13, 1700068, DOI: 10.1002/smll.201700068. (41) Park, G. D.; Lee, J. H.; Lee, J. K.; Kang, Y. C., Effect of Esterification Reaction of Citric Acid and Ethylene Glycol on the Formation of Multi-Shelled Cobalt Oxide Powders with Superior Electrochemical Properties. Nano Res. 2014, 7, 1738-1748, DOI: 10.1007/s12274-014-0533-9. (42) Sumpter, B. G.; Huang, J. S.; Meunier, V.; Romo-Herrera, J. M.; Cruz-Silva, E.; Terrones, H.; Terrones, M., A Theoretical and Experimental Study On Manipulating the Structure and Properties of Carbon Nanotubes Using Substitutional Dopants. Int. J. Quantum Chem. 2009, 109, 97-118, DOI: 10.1002/qua.21893. (43) Wang, J.; Wu, H. H.; Gao, D. F.; Miao, S.; Wang, G. X.; Bao, X. H., High-density Iron Nanoparticles Encapsulated within Nitrogen-Doped Carbon Nanoshell as Efficient Oxygen Electrocatalyst for Zinc-Air Battery. Nano Energy 2015, 13, 387-396, DOI: 10.1016/j.nanoen.2015.02.025. (44) Ding, Y. L.; Kopold, P.; Hahn, K.; van Aken, P. A.; Maier, J.; Yu, Y., Facile Solid-State Growth of 3D Well-Interconnected Nitrogen-Rich Carbon Nanotube-Graphene Hybrid Architectures for Lithium-Sulfur Batteries. Adv. Funct. Mater. 2016, 26, 1112-1119, DOI: 10.1002/adfm.201504294. (45) Zhang, Y. C.; You, Y.; Xin, S.; Yin, Y. X.; Zhang, J.; Wang, P.; Zheng, X. S.; Cao, F. F.; Guo, Y. G., Rice Husk-Derived Hierarchical Silicon/Nitrogen-Doped Carbon/Carbon Nanotube Spheres as Low-Cost and High-Capacity Anodes for Lithium-Ion Batteries. Nano Energy 2016, 25, 120-127, DOI: 10.1016/j.nanoen.2016.04.043. (46) Cai, Y. S.; Yang, H. L.; Zhou, J.; Luo, Z. G.; Fang, G. Z.; Liu, S. N.; Pan, A. Q.; Liang, S. Q., Nitrogen-Doped Hollow MoS2/C Nanospheres as Anode for Long-Life Sodium-Ion Batteries. Chem. Eng. J. 2017, 327, 522-529, DOI: 10.1016/j.cej.2017.06.146. (47) Ai, W.; Jiang, J.; Zhu, J. H.; Fan, Z. X.; Wang, Y. L.; Zhang, H.; Huang, W.; Yu, T., Supramolecular Polymerization Promoted In Situ Fabrication of Nitrogen-Doped Porous Graphene Sheets as Anode Materials for Li-Ion Batteries. Adv. Energy Mater. 2015, 5, 1500559, DOI: 10.1002/aenm.201500559. (48) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L. S.; Jin, S., Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274-10277, DOI: 10.1021/ja404523s. (49) Shi, Y. M.; Wang, Y.; Wong, J. I.; Tan, A. Y. S.; Hsu, C. L.; Li, L. J.; Lu, Y. C.; Yang, H. Y., Self-assembly of Hierarchical MoSx/CNT Nanocomposites (2 < x < 3): Towards High Performance Anode Materials for Lithium Ion Batteries. Sci. Rep. 2013, 3, 2169, DOI: 10.1038/srep02169. (50) Hsu, C. L.; Chang, Y. H.; Chen, T. Y.; Tseng, C. C.; Wei, K. H.; Li, L. J., Enhancing the Electrocatalytic Water Splitting Efficiency for Amorphous MoSx. Int. J. Hydrogen Energ. 2014, 39, 4788-4793, DOI: 10.1016/j.ijhydene.2014.01.090.



(51) Kuang, M.; Wang, Q. H.; Han, P.; Zheng, G. F., Cu, Co-embedded N-Enriched Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions. Adv. Energy Mater. 2017, 7, 1700193, DOI: 10.1002/aenm.201700193. (52) Park, S. K.; Kim, J. K.; Kang, Y. C., Metal-Organic Framework-Derived CoSe2/(NiCo)Se2 Box-in-Box Hollow Nanocubes with Enhanced Electrochemical Properties for Sodium-Ion Storage and Hydrogen Evolution. J. Mater. Chem. A 2017, 5, 18823-18830, DOI: 10.1039/C7TA05571D. (53) Yan, M.; Pan, X.; Wang, P.; Chen, F.; He, L.; Jiang, G.; Wang, J.; Liu, J. Z.; Xu, X.; Liao, X.; Yang, J.; Mai, L., Field-Effect Tuned Adsorption Dynamics of VSe2 Nanosheets for Enhanced Hydrogen Evolution Reaction. Nano Lett. 2017, 17, 4109-4115, DOI: 10.1021/acs.nanolett.7b00855. (54) Mo, R. W.; Du, Y.; Rooney, D.; Ding, G. Q.; Sun, K. N., Ultradispersed Nanoarchitecture of LiV3O8 Nanoparticle/Reduced Graphene Oxide with High Capacity and Long-Life Lithium-Ion Battery Cathodes. Sci. Rep. 2016, 6, 19843, DOI: 10.1038/srep19843. (55) Kaneti, Y. V.; Zhang, J.; He, Y. B.; Wang, Z. J.; Tanaka, S.; Hossain, M. S. A.; Pan, Z. Z.; Xiang, B.; Yang, Q. H.; Yamauchi, Y., Fabrication of an MOF-derived Heteroatom-Doped Co/CoO/Carbon Hybrid with Superior Sodium Storage Performance for Sodium-Ion Batteries. J. Mater. Chem. A 2017, 5 (29), 15356-15366, DOI: 10.1039/C7TA03939E. (56) Lee, K.; Shin, S. Y.; Yoon, Y. S., Fe3O4 Nanoparticles on MWCNTs Backbone for Lithium Ion Batteries. J. Korean Ceram. Soc. 2016, 53 (3), 376-380, DOI: 10.4191/kcers.2016.53.3.376. (57) Peng, Y. Y.; Akuzum, B.; Kurra, N.; Zhao, M. Q.; Alhabe, M.; Anasori, B.; Kumbur, E. C.; Alshareef, H. N.; Ger, M. D.; Gogotsi, Y., All-MXene (2D Titanium Carbide) Solid-State Microsupercapacitors for On-Chip Energy Storage. Energy Environ. Sci. 2016, 9 (9), 2847-2854, DOI: 10.1039/C6EE01717G.


Page 26 of 28

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


Table of Contents Graphic Novel amorphous-MoSx-coated 3D hierarchical hollow microspheres comprising 1D bamboo-like N-doped carbon nanotubes are first introduced as a highly active electrocatalyst for HER. The unique structure facilitates electrolyte access, and the synergistic effect between the MoSx catalyst material with ample active sites and the conductive N-doped CNTs were responsible for the superior electrochemical activity of 3D BNCNT/MoSx-HMs for the HER.


ToC ACS Sustainable Chemistry & Engineering

H2O 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

H2O

3D BNCNT/MoSx Hollow Microsphere

Page 28 of 28

H2

Hydrogen Evolution Reaction

ACS Paragon Plus Environment

Amorphous Molybdenum Sulfide on Three-Dimensional Hierarchical

Recommend Documents