Hierarchical Heterostructures of Ultrasmall Fe2O3 Encapsulated

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Energy, Environmental, and Catalysis Applications

Hierarchical Heterostructures of Ultrasmall Fe2O3 Encapsulated MoS2/ N-graphene as an Effective Catalyst for Oxygen Reduction Reaction Nguyen Dinh Chuong, Tran Duy Thanh, Nam Hoon Kim, and Joong-Hee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06485 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

Hierarchical Heterostructures of Ultrasmall Fe2O3 Encapsulated MoS2/Ngraphene as an Effective Catalyst for Oxygen Reduction Reaction Nguyen Dinh Chuonga, Tran Duy Thanha, Nam Hoon Kima*, Joong Hee Lee a,b* a

Advanced Materials Institute of BIN Convergence Technology (BK21 plus Global), Dept. of

BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea. b

Carbon Composite Research Centre, Department of Polymer & Nanoscience and Technology,

Chonbuk National University Jeonju, Jeonbuk 54896, Republic of Korea. *Corresponding author: Tel.: 82-63-270-2301; Fax: 82-63-270-2341. E-mail address: [email protected] (Joong Hee Lee) and [email protected] (Nam Hoon Kim).

ABSTRACT: In this study, a facile approach has been successfully applied to synthesize a hierarchical three-dimensional architecture of ultrasmall hematite nanoparticles homogeneously encapsulated in MoS2/nitrogen doped graphene nanosheets, as a novel non-Pt cathodic catalyst for oxygen reduction reaction in fuel cell applications. The intrinsic topological characteristics along with unique physicochemical properties allowed such catalyst to facilitate the oxygen adsorption, and sped up the reduction kinetics through fast heterogeneous decomposition of oxygen to final product. As a result, outstanding catalytic performance was exhibited with the high electron transferred number of 3.91-3.96, which was comparable to Pt/C product.

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Furthermore, the working stability with retention of 96.1 % after 30,000 s and excellent alcohol tolerance were founed to be significant better than for the Pt/C product. This hybrid can be condidered as a highly potential non-Pt catalyst for practical ORR application in requirement of low cost, facile production, high catalytic behavior, and excellent stability. KEYWORDS: Hematite, molybdenum disulfide, nitrogen doped graphene, electrocatalyst, oxygen reduction reaction. 1. INTRODUCTION Faced with the necessity to reduce global warming effect because of excess greenhouse gas emission by using fossil fuel sources, the growth of cheap, clean, and sustainable energy conversion technologies as a viable global alternative is essential. The fuel cell with facile production, low-cost, and high energy density is a potential energy conversion system that can simply transform chemical energy to electric power.1 In this application, oxygen is used as gas source for reduction reaction at cathode. The slow reaction kinetic of ORR can be well treated using Pt-based catalysts;2–4 but scarcity, high-cost, poor durability, and easy CO poisoning critically hinder the efficient use of these materials for long-term operation, thus significant limiting the performance of fuel cell devices.5–7 To address this issue, the need to discover effective substitutes for Pt-based catalysts remains urgent.8,9 In recent times, two dimensional (2D) metal-based nanocatalysts have been emerging as a favorable topic in electrocatalytic applications, due to their unique topological characteristics along with their unusual properties.10 In this regard, layered-like structure metal dichalcogenides, such as molybdenum disulfide (MoS2), have attracted significant interest in material science research for various applications, especially for ORR.11,12 Many efforts have indicated that the exfoliated MoS2 with single or few-


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layer structure can exhibit exceptional activity for ORR and water splitting as compared to an inert bulk counterpart, due to the increased amounts of exposed active sites, the enhanced intrinsic activity of individual site, and the improved the conductivity of MoS2 catalyst.13,14 Interestingly, the abundant Mo sites at the edges effectively contribute to the 4e reaction pathway for the ORR process.15 Furthermore, very large surface area along with abundant negative charges sites of S atoms on the surface allows exfoliated MoS2 layers serve as a promising anchoring substrate for strong adsorption and uniform dispersion of metal nanostructures, thus providing a novel design of economical and well-performing electrocatalysts.16 However, the critical issue for the utilization of MoS2 is the ease of restacking because of their high surface energy and strong interaction between layers, thereby reducing its electroactive sites and surface area.17 Therefore, the hybridization of MoS2 layers with another prestigious 2D nanomaterial, like graphene nanosheets (GNS), to produce hierarchical Van der Waals heterostructures can remarkably increase their electrocatalytic performance.18 This resulted from the similar structure between MoS2 and GNS, which can produce the uniform assembly of MoS2 layers on surface of GNS with strong interfacial contact; thereby avoiding of MoS2 restacking and significantly modifying the electronic and geometric features, including superior mechanical properties, larger specific surface area, enhanced active site numbers, and superior electrical conductivity.19 In particular, a considerable nitrogen doping level can further change the electronic state of GNS, impressively enhancing the catalytic behavior of MoS2/GNS towards chemical and electrochemical reactions.20,21 Regarding reaction kinetic of ORR, it has been found to occur according to the four-electron pathway in ideal condition, in which the fuel cell generates the highest power output, and then only exhausts water. Although the combination of MoS2 layers with GNS was demonstrated to produce some satisfactory results; the catalytic activity of such


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hybrid need to be further improved because the two-electron pathway still take place to form intermediates as a byproducts, which seriously compromise the efficiency of catalyst.22 In addition, the slow electrochemical decomposition reaction of intermediates at high negative potential is a hostile process, which may harm cell membranes. Considering these issues, the additional hybridization of another metal-based catalyst with MoS2/GNS can be an efficient way to greatly boost the kinetic mechanism of ORR, maximizing the energy yield of the fuel cell devices, as well as reducing the risk of cell degradation. Hematite (α-Fe2O3) is considered to be a potential catalyst, because of its attractive chemical catalysis, good thermodynamic strength, low price, easy synthesis, and high abundance.23–25 Besides, previous reports demonstrated the use of α-Fe2O3 nanoparticles (NPs) to help accelerate the kinetics of the ORR through the rapid chemical decomposition of O2, due to the vigorous redox activities of Fe3+/Fe2+ in its trigonal nanostructure.26–29 Xue et al. reported that α-Fe2O3 nanostructures highly preferred for a 4e pathway towards ORR.29 However, the their problems in terms of low electron mobility and small hole diffusion distance present a barriers for their catalytic use.29 To address the indicated drawbacks, modification of surface characteristics, shape and size control, doping process, and hybridization have been recognized to upgrade electronic and catalytic properties of α-Fe2O3.30– 34

Owning unique structure and properties, the hierarchical MoS2/NGNS heterostructure is

expected for efficiently coupling with ultrasmall α-Fe2O3 NPs to overcome the challenges mentioned above and persistent issues of these materials, thus taking full advantage of catalysts for ORR application. In this study, a facile approach has been developed for preparing a novel hierarchical

architecture

of

ultrasmall

α-Fe2O3

NPs

encapsulated

in

MoS2/NGNS

heterostructures (α-Fe2O3@MoS2/NGNS). The formation of core-shell structures could meritoriously anchor and well disperse α-Fe2O3 NPs, as well as lead to an intrinsic interaction


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between α-Fe2O3 and MoS2/NGNS shells along with the enhanced charge/mass transportation. Due to such unique morphology, the highly synergistic effects produced from α-Fe2O3 NPs, MoS2 layers, NGNS resulted in the high performance of α-Fe2O3@MoS2/NGNS towards ORR. The obtained results suggest a new option for developing an effective non-Pt catalyst that meets the requirement of cost-effectiveness, simple synthesis, high catalytic activity, and stability. 2. EXPERIMENTAL SECTION 2.1 Materials. Graphite flake, potassium permanganate (≥ 99%), iron (III) nitrate nonahydrate (99.9%), thiourea (≥ 99%), hydrogen peroxide (35 wt%), ammonium tetrathiomolybdate (≥ 99%), nafion polymer solution (5 wt%), and commercial Pt/C product (20 wt%) were provided by Sigma Co. (USA). Potassium hydroxides (≥ 99.5%), hydrochloric acid (35-37%), sulfuric acid (99%), and methanol (99.9%) were purchased from Samchun Co. (Korea). Carbon black material (HIBLACK 420B) were bought from Korea Carbon Black Co. (Korea). 2.2 Preparation of α-Fe2O3@MoS2/NGNS hybrid. Firstly, 0.06 g graphene oxide (GO), which was prepared from graphite flakes according to a modified Hummers method as reported elsewhere,35 was dispersed in 60 mL DI water containing 0.3 g thiourea by sonication for 1 h. The 0.02 g (NH4)2MoS4 and 0.016 g Fe(NO3)3.9H2O were then slowly added into the solution along with magnetic stirring for 3 h. Subsequently, the obtained solid by a freeze-drying process was calcinated at 500 °C for 1 h under an Ar/H2 (7/3) flow rate of 200 sccm, and then at 900 °C for 3 h under an Ar flow rate of 160 sccm (Scheme 1). In this procedure, (NH4)2MoS4 was employed for synthesis of MoS2 layers through a reaction mechanism reported by Liu et al.36 Thiourea mainly served as the N source for doping into graphene structure. For comparison, pure


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MoS2 layers, NGNS, MoS2/GNS, and α-Fe2O3@MoS2/GNS materials were also prepared, and their synthesis strategies are shown in the supporting information.

Scheme 1. A schematic illustration for α-Fe2O3@MoS2/NGNS fabrication. 2.3 Material Characterization. The morphological characteristics of NGNS and hybrids were investigated by field-emission scanning electron microscopy (FE-SEM) on a Supra 40 VP instrument (Zeiss Co., Germany). Insight into the microstructures was achieved by transmission electron microscopy (TEM) through a JEM-2200FS instrument (JEOL Co., USA), at the Center of the Korea Basic Science Institute in Jeonju (Korea). The crystallinity and phase structure of products were investigated by X-ray diffraction (XRD) on a D/Max 2500 V/PC (Rigaku Co., Japan) installed in the Center for University-Wide Research Facilities (CURF) at Chonbuk National University with Cu target (λ = 0.154 nm), the 2θ range from 5 to 80°, and a scan rate of 2° min-1. X-ray photoelectron spectroscopy (XPS) analysis was conducted by a Theta Probe


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instrument (Thermo Fisher Scientific Inc., USA) to identify elemental composition, as well as the interactions of materials. Raman analysis was performed by Nanofinder 30 instrument (Tokyo Instruments Co., Japan) to provide further structural characteristics of materials. Specific surface area calculation based on the Brunauer-Emmett-Teller (BET) theory was conducted on an ASAP 2020 Plus system (Micromeritics Instrument Corp., USA). Atomic-force microscopy (AFM) analysis was carried out on a Park NX10 (Park System Co., Korea). 2.4 Electrochemical characterization. The electrochemical activities of materials towards ORR were studied by an electrochemical potentiostat (CHI 660D-CH Instruments Inc., USA) integrated with rotating disk electrode rotator (ALS Co., Japan). To make the working electrode, 5 µL of a ink solution, which was prepared by dispersing 2.5 mg of catalyst into 0.5 mL of ethanol in the presence of 5 µL of nafion solution (5 wt%) through ultrasound condition for 1 h, was dropped onto a polished rotating disk electrode (RDE) surface (diameter = 3 mm), and dried at 25 oC for 3 h. Cyclic voltammetric (CV) measurements were carried out in N2/O2-saturated 0.1 M KOH solution at a scan rate of 50 mV.s-1. Linear sweep voltammetry (LSV) was performed in O2-saturated 0.1 M KOH solution at a potential scan rate of 10 mV⋅s-1 and different electrode rotating rates ((400–2,000) rpm). In addition, electrochemical impedance spectroscopy (EIS) measurements were investigated in the frequency range from 0.01 Hz to 105 Hz. In order to evaluate the ORR reaction mechanism for materials, the Koutecky-Levich equations were used:37

= + = + ⁄ ⁄

= 0.62 ⁄


(1)

(2)

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where, ω reflects the angular velocity of the RDE (rpm), JK reflects the kinetic current density (mA⋅cm-2), JL reflects the diffusion-limiting current density, J reflects the measured current density, F (96,485 C⋅mol-1) is the Faraday constant, n is considered as the number of electrons transferred for each oxygen molecule, Co (1.2×10-6 mol⋅cm-3) reflects the oxygen content dissolved in 0.1 M KOH, (0.01 cm2⋅s-1) reflects the viscosity of 0.1 M KOH electrolyte, and Do (1.9×10-5 cm2⋅s-1) reflects the diffusion coefficient of oxygen. 3. RESULTS AND DISCUSSION The microstructures of the as-synthesized materials were firstly investigated by FE-SEM technique. This revealed that the NGNS material was comprised of seriously wrinkled nanosheets, along with folding phenomenon, due to the doping effect of nitrogen heteroatoms into the honeycomb backbone of graphene (Fig. 1a). Meanwhile, the MoS2/NGNS hybrid exhibited a hierarchical architecture, in which MoS2 sheets were vertically assembled on the graphene surface (Fig. 1b). As demonstrated by previous reports, similar sheet-like microstructure between MoS2 nanoplatelets and GNS can lead to worthy structural compatibility along with satisfiable interactions between two these nanomaterials.38 The formation of such hierarchical

architecture

efficiently

suppresses

the

restacking

and

accumulation

of

MoS2 nanoplatelets and GNS, producing a highly electroactive surface area for quick redox reactions, excellent electron transfer rate, and large electrode/electrolyte contact area.39 In the case of the α-Fe2O3@MoS2/NGNS, ultrasmall α-Fe2O3 NPs homogeneously distributed on the surface of 2D supporting materials (Fig. 1c and d). The pure MoS2 sample was also prepared for comparison. SEM (Fig. S1a) and TEM (Fig. S1b) imageries showed a sheet-like structure of the


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as-synthesized MoS2 materials. In addtion, high resolution TEM (HR-TEM) images indicated that MoS2 was formed by the stacking of several layers (Figs. S1c and d).

Figure 1 SEM images of (a) NGNS; (b) MoS2/NGNS; and (c and d) α-Fe2O3@MoS2/NGNS nanohybrid. TEM analysis was conducted to further elucidate the micromorphology of the as-synthesized materials. The typical 2D structure of NGNS with crumpled surface showed a high specific surface area (Fig. 2a). In addition, the high transparency along with further HR-TEM revealed that the sheets consisted of NGNS with only single layer. The morphological features of the NGNS were further characterized by 2D and 3D AFM analyses, which indicated the monolayer structure of the NGNS (thickness of ∼1.1 nm) along with the generation of corrugations on its surface, such as crumples, ripples, and wrinkles (Fig. S2). TEM characterization of the MoS2/NGNS hybrid indicated the planar morphology of MoS2/GNS with uniform distribution of


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MoS2 layers on the GNS backbone (Fig. 2b). The corresponding crystal lattices of the inlaid MoS2 layers were confirmed by HR-TEM analysis. The lattice spacing of 0.16, 0.27, and 0.61 nm (Fig. 2c) were in accordance with the (110), (100), and (002) planes of MoS2,36,40 respectively, proving the successful synthesis of inlaid MoS2 layer uniformly dispersed on GNS structure. This architecture undoubtedly favored enhanced electron transfer ability between GNS and MoS2 nanoplatelets,41 thus significantly promoting the electrocatalytic performance. Figure 2d showed a TEM image of the α-Fe2O3@MoS2/NGNS nanohybrid, in which Fe2O3 NPs uniformly attached on the layered heterostructure of MoS2/NGNS hybrid. The high resolution TEM (HR-TEM) analysis, which provided insight into the inner structure of hybrid material, presented the formation of α-Fe2O3–core and outer 2D layer-shell with a thickness of ∼3 nm, clearly observed in Figures 2e-g and Figure S3a-d. It was expected that such a structure could effectively stabilize the hybrid from volume expansion and catalyst dissolution during the electrochemical reaction process.42,43 In addition, the HR-TEM image exhibited a lattice fringe spacing of 0.16 nm, which is consistent with the distance of (116) plane for α-Fe2O3 (Fig. 2h).44 EDAX analysis provided qualitative and half-quantitative information to the existing element in the hybrid. This showed the clear presence of corresponding peaks for C, N, Mo, S, Fe, and O elements in Figure 2i, further confirming the formation of α-Fe2O3 NPs on MoS2/NGNS substrate. The plot of size distribution indicated that the α-Fe2O3 NPs are in the range of 4–12 nm (Fig. 2j). Moreover, the element mapping imagery of the α-Fe2O3@MoS2/NGNS showed a uniform NP distribution on the MoS2/NGNS architecture (Fig. 2k–p).


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Figure 2 TEM images of (a) NGNS and (b) MoS2/NGNS; (c) HR-TEM image of MoS2/NGNS; (d) TEM image, (e-h) HR-TEM image, and (i) EDAX result of α-Fe2O3@MoS2/NGNS nanohybrid; (j) distribution of particle size for α-Fe2O3 NPs in the nanohybrid; (k) STEM and HAAD image of the nanohybrid and color mapping of different element in the nanohybrid: (l) Fe, (m) Mo, (n) S, (o) C, and (p) N. The crystalline characteristics of GO, NGNS, and α-Fe2O3@MoS2/NGNS were examined by XRD technique (Fig. 3a). The specific peak of graphene corresponding to its (002) plane was available for all samples.45 The positive shift of this peak for the NGNS and hybrid to higher 2θ value compared to GO indicated the recovery layer crystallinity of graphene structure due to simultaneous reduction and the N-doping effect;46 therefore, it was expected to increase the charge transfer ability of the material. The peaks located at 2θ of 32.8, 39.4, 50.0, and 58.6°


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corresponded to the (100), (103), (105), and (110) crystal planes of MoS2 phase (PDF 37-1492), respectively.47 Meanwhile, the presence of uniform ultrasmall α-Fe2O3 was identified by appearance of peaks at 34.7, 42.6, 48.9, and 64.1°, which are assigned to the (110), (113), (024), and (214) crystal planes of α-Fe2O3, respectively.48 The structural characteristics of materials were further proven using Raman spectroscopy (Fig. 3b). The ratios for intensity of the D band to G bands (ID/IG) in NGNS and α-Fe2O3@MoS2/NGNS hybrid were measured to be 1.03 and 1.38, which were much higher than that of GO (ID/IG = 0.8), implying more highly disordered structures for the NGNS and hybrid, due to the nitrogen doping effects and the anchoring of MoS2 layer, as well as α-Fe2O3 NPs, on the graphene nanosheets.49 Two specific peaks at 379.8 cm-1 and 404.7 cm-1 corresponded the E12g in-plane and A1g out-of-plane vibrational modes of the MoS2 material, respectively. According to the energy difference of two these peaks, the layer number of MoS2 can be evaluated.50 In this regard, the energy difference was found to be around 24.9 cm-1, consistent with some layer feature of MoS2 in the hybrid. The surface area and the pore size distribution of the as-synthesized α-Fe2O3@MoS2/NGNS hybrid were measured using the nitrogen adsorption-desorption isotherm experiments (Fig. 3c). The obtained curve agreed with a typical type-IV isotherm with a hysteresis loop from the relative pressure of 0.43 to 0.98, suggesting the mesoporous structure of such hybrid.51 The BET surface area was found to be about 216 m2⋅g–1, along with the corresponding average pore width distributed from 30 to 40 nm, indicating such a porous structure can well donate to the electrocatalytic activity, since it can easily hold charges and accelerate the ion diffusion rate.52 The surface characteristics of materials were investigated using XPS technique. Figure 3d showed that the NGNS and α-Fe2O3@MoS2/NGNS hybrid displayed a very much lower intensity ratio of O1s/C1s and the presence of an additional peak for N1s, demonstrating an


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effective reduction process, along with significant nitrogen doping into the carbon backbone. In particular, the XPS survey of hybrid shows the coexistence of Fe2p, Mo3p, and S2p.

Figure 3 (a) XRD patterns and (b) Raman spectra of GO, NGNS, and α-Fe2O3@MoS2/NGNS; (c) Nitrogen adsorption-desorption isotherm measurement of α-Fe2O3@MoS2/NGNS; (d) XPS spectra of GO, NGNS, and α-Fe2O3@MoS2/NGNS hybrid. Figures 4a, S4a, and S4b showed that the high resolution C1s of the NGNS and αFe2O3@MoS2/NGNS displayed smaller integrated areas consistent with the binding energies of C–O and C=O bonding, indicating effective reduction and N doping on the carbon backbone of graphene. For the α-Fe2O3@MoS2/NGNS material, there were various peaks located at 284.2, 284.6, 285.1, 285.7, 286.9, 288.6, and 291.2 eV, associated to the binding energies of Mo–C–


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Mo, C=C, C–N, C–O, C=O, O–C=O, and π-π* bonding, respectively. In this regard, the presence of binding energy for C–N and Mo–C–Mo bonding exhibited efficient doping of nitrogen on graphene structure, and good interactions between the GNS and MoS2 layers, respectively.52,53 The high-resolution XPS spectra for N1s in the NGNS and hybrid show contribution of pyridinic-N, pyrrolic-N, graphitic-N, and oxide-N (Fig 4b and S4c). The amounts of the pyridinic-N and graphitic-N were around 54 and 64 % (based on total doped N content) for NGNS and α-Fe2O3@MoS2/NGNS hybrid, respectively, which can offer highly electrocatalytic active sites highly valuable for ORR process.51 For the high-resolution Mo3d spectrum, one singlet and two doublets were found at around 235.8; (234.2 and 231.2) and (232.6 and 229.5) eV, which are ascribed to +6, +5, and +4 oxidation states of the Mo3d, respectively (Fig. 4c). Moreover, a chemical state of Mo-S bonding appeared at 228.6 eV nearby the S2s peak, further suggesting the formation of MoS2 layers. In the S2p spectra (Fig. 4d), the binding energies centered at 162.2 and 163.4 eV are ascribed to the S2p1/2 and S2p3/2 orbitals of sulfide ions (S2−), respectively, in agreement with the formation of MoS2 nanostructure.54 Meanwhile two minor peaks at binding energy value higher than 164 eV were consistent with oxidized sulfur,55 resulting from the formation of Mo sulphates and Mo oxysulphides.56 Figure 4f showed the high resolution O1s spectrum of the hybrid. The spectrum was deconvoluted into three components, correlated to the binding energy of Fe-O in α-Fe2O3, O-H in chemisorbed water, and C-O in graphene at 530.9, 532.0, and 533.5 eV, respectively.57


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Figure 4 High-resolution XPS spectra of (a) C1s, (b) N1s, (c) Mo3d, (d) S2p, (e) Fe2p, and (f) O1s in the α-Fe2O3@MoS2/NGNS hybrid. ORR is an important half reaction of the fuel cell system to achieve clean energy. Much attention has been paid to find an efficient strategy to enhance the electrocatalytic behaviour towards ORR. Our research expected the combination of components based on chemical and electrochemical catalysts to produce a unique α-Fe2O3@MoS2/NGNS hybrid that would result in


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promising enhanced ORR activity for fuel cell applications. The electrocatalytic behaviors of layered MoS2, MoS2/GNS, NGNS, α-Fe2O3@MoS2/GNS, and α-Fe2O3@MoS2/NGNS towards ORR in 0.1 M KOH solution saturated with O2 gas were investigated. Figure 5a showed the CV results of α-Fe2O3@MoS2/NGNS hybrid obtained in 0.1 M KOH solution saturated with N2 and O2 gas. Meanwhile, the CV result in N2-saturated KOH solution lacked any peak characteristic of a catalytic reaction, the anodic current density impressively increased in O2-saturated solution when the potential moves to lower negative value, implying a highly catalytic O2 reduction. The LSV results showed that the individual layered MoS2 exhibited inferior ORR behavior (Fig. 5b). In contrast, the formation of hierarchical MoS2/GNS heterostructure, in which MoS2 sheets were homogeneously distributed on the surface of graphene with enhanced active sites and high interactions for rapid charge transfer between components, resulted in much better catalytic performance, as compared to pure MoS2 material. In addition, the incorporation of α-Fe2O3 NPs into MoS2/GNS hybrid led to a significantly positive shift for the onset and half-wave potential, therefore implying that the ultrasmall α-Fe2O3 NPs can effectively boost the catalytic process taking place at MoS2/GNS.


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Figure 5 (a) The CV measurements of the α-Fe2O3@MoS2/NGNS in 0.1 M KOH solution at a scan rate of 0.05 V·s-1; (b) The LSV results and (c) Tafel plots of MoS2, NGNS, MoS2/NGNS, α-Fe2O3@MoS2/GNS, and α-Fe2O3@MoS2/NGNS in O2-saturated 0.1 M KOH solution at rotation speed of 1,600 rpm and potential scan rate of 0.01 V·s-1; (d) EIS of the MoS2, NGNS, MoS2/GNS, α-Fe2O3@MoS2/GNS, and α-Fe2O3@MoS2/NGNS hybrid from 100 kHz to 0.1 Hz at an applied potential of 0.7646 V. In particular, impressively enhanced catalytic behavior was found with a very small onset potential of 0.9476 V and half-wave potential of 0.8546 V, as well as much higher catalytic current density output since the hierarchical nanostructure of MoS2/NGNS hybrid was used to integrate with α-Fe2O3 NPs, as compared to the other samples (Fig. S5). The ORR kinetics of the catalysts mentioned above were identified by the corresponding Tafel plots (Fig. 5c). The Tafel


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slope of the α-Fe2O3@MoS2/NGNS was found to be 73 mV⋅dec-1, whereas the MoS2, MoS2/GNS, NGNS, and α-Fe2O3@MoS2/GNS corresponded to 119, 100, 110, and 103 mV⋅dec1

, respectively. This further indicated the significantly improved ORR performance of α-

Fe2O3@MoS2/NGNS compared to MoS2, MoS2/GNS, NGNS, and α-Fe2O3@MoS2/GNS materials. The formation of a unique hierarchical structure for α-Fe2O3@MoS2/NGNS also leads to the alternation of interfacial charge transfer kinetic. The impedance experiments found the charge transfer resistance (Rct) of the α-Fe2O3@MoS2/NGNS materials to be ∼66 Ω, which is of lower magnitude as compared to that of other materials, such as MoS2 (∼82 Ω), MoS2/GNS (∼75 Ω), and α-Fe2O3@MoS2/NGNS (∼72 Ω) (Fig. 5d). This reduction in the Rct value can result from ultrasmall α-Fe2O3 uniformly dispersed on the unique hybrid architecture, in which MoS2 layers were homogeneously inlaid into highly conductive nitrogen doped graphene surface ((Rct of ∼ 61 Ω)), thus providing extra electroactive sites for reactant molecules. This result also led to the enhanced electrocatalytic behavior of the α-Fe2O3@MoS2/NGNS towards ORR.


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Figure 6 (a) The LSV curves of α-Fe2O3@MoS2/NGNS in O2-saturated 0.1 M KOH at potential scan rate of 0.01 V⋅s-1 and various rotation speeds of RDE; (b) Plot of J-1 vs ω-1/2 consistent with the Koutecky-Levich equation at different values of potential; (c) The electron transfer number of MoS2, NGNS, MoS2/GNS, α-Fe2O3@MoS2/GNS, and α-Fe2O3@MoS2/NGNS at different applied potentials; (d) The working stability of α-Fe2O3@MoS2/NGNS towards ORR after 1,000 cycles. The rotation speed dependency of the LSV response was examined by changing the rotation rate of RDE from 400 to 2,000 rpm to evaluate the ORR mechanism kinetic of the as-synthesized materials (Fig. 6a and Figs. S6a, S6c, and S6e). According to the Koutecky-Levich plots of J–1 against ω–1/2 at different applied potentials, the slopes of linear fitting curves were verified and utilized for determination of the electron transferred number (n) (Fig. 6b, S6b, S6d, and S6f). The n values for the α-Fe2O3@MoS2/NGNS catalyst were found to be 3.91–3.96, approaching 4, in the range of the surveyed potential, impressively higher than those of other materials (Fig. 6c). This indicated that the ORR process for the α-Fe2O3@MoS2/NGNS is fast, and it mainly takes place via four electron pathway processes. The stability of the electroactive sites for catalysts during ORR process is a key concern to evaluate the catalytic performance of materials. In this regard, the long-term stability of the αFe2O3@MoS2/NGNS was recorded by CV measurements at 0.05 V⋅s-1 in O2-saturated 0.1 M KOH solution for 1,000 cycles. Figure 6d showed that there was a negligible drop in the current density along with almost no change of onset and half-wave potentials for the hybrid after 1,000 continuous potential cycles, suggesting that the electroactive sites in our catalyst were highly


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stable. Table S1 showed a comparison of the catalytic performance of our αFe2O3@MoS2/NGNS catalyst with other previously reported research, where it can be seen that our catalyst possesses better behavior in terms of the onset potential, half-wave potential, and number of electron transferred. The good catalytic performance of the proposed catalyst can be assumed to the synergistic effect from the physico-chemical properties of well dispersed Fe2O3 NPs encapsulated in the hierarchical heterostructures based on the uniform assembly of MoS2 layers on NGNS. In this context, because of the structural similarity, hybridizing MoS2 and NGNS to be hierarchical Van der Waals heterostructures with ideal interface contact is an effective approach to modify the electronic and geometric characteristics, thereby improving interfacial charge transfer, and producing a larger number of adsorption sites and catalytic active centers.58,59 In addition, the presence of S, N-containing sites on surface of the heterostructure and the formation of core-shell architecture could efficiently anchor and uniformly disperse Fe2O3 NPs, as well as result in unique electronic interaction between the Fe2O3 NPs and MoS2/NGNS. These phenomena led to the changes in the surface electronic states of the NPs,60 thus well amplifying the electrochemical surface and improving the electroactive site numbers for a better ORR behavior. In an effort to demonstrate the superior performance of the as-synthesized catalyst towards ORR, its catalytic behavior was compared to a commercial Pt/C product (20 wt %). Figure 7a showed the LSV curves of the as-synthesized catalyst and Pt/C, which evident that these two catalysts showed almost the same limit current density. Notwithstanding the minor negative values of onset potential and half-wave potential for the hybrid, it suggests the relative catalytic activity of the the proposed hybrid material, compared with Pt/C. The good catalytic performance of catalyst was further seen when the Tafel slope of the catalyst (73 mV⋅dec-1) was


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similar to that of the Pt/C (75 mV⋅dec-1) (Fig. 7b). To study whether α-Fe2O3@MoS2/NGNS is an effcient catalyts for oxygen reduction in acidic environment or not, the LSV curves of the αFe2O3@MoS2/NGNS and Pt/C were measured in O2-saturated 0.5 M H2SO4 solution. The obtained results indicated that α-Fe2O3@MoS2/NGNS exhibited more negative onset and half wave potential (Fig. S7a), as well as much higher Tafel slope (Fig. S7b) as compared to those of Pt/C product. This implies that the catalytic behavior of proposed catalyst is unfavoured for ORR in acid condition. Methanol crossover is a critical factor that prohibits the catalytic activity and stability of a cathodic electrocatalyst in fuel cell applications, thus the performance of our materials was evaluated the alcohol tolerance ability. In order to study the alcohol tolerance of the αFe2O3@MoS2/NGNS, the chronoamperometric current density was recorded in O2-saturated 0.1 M KOH solution with the addition of 0.5 M methanol. Compared with the Pt/C product, αFe2O3@MoS2/NGNS was found to be inert to methanol, an important advantage for an efficient catalyst towards ORR (Fig. 7c). The durable comparability between the hybrid materials and Pt/C was evaluated using chronoamperometric measurement at 0.7646 V. It was clear that the chronoamperometric current density of the hybrid retained impressive stability with 96.1 %, whereas there was a fast decrease in ORR activity of Pt/C with current density retention of 81.4 % after 30,000 s (Fig. 7d). Furthermore, such hybrid materials also exhibited much better retention of current density as compared to pure α-Fe2O3 NPs after long-term chronoamperometric measurement in alkaline medium (Fig. S8). This implied the superior stability of the as-synthesized hybrid catalyst over that of the Pt/C and α-Fe2O3 NP catalyst under similar experimental conditions in alkaline electrolyte. The obtained results were due to that α-Fe2O3 NP and Pt NPs on carbon matrix undergo the loss of active area because of


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significant dissolution and precipitation;5,6 However, the formation of α-Fe2O3-core and MoS2/NGNS-shell nanostructure in the as-synthesized catalyst, as previously indicated by TEM observation, effectively avoided the detachment, as well as the agglomeration of α-Fe2O3 NPs, thus impressively improving the stability of material. In another regard, concerning the catalytic potential of α-Fe2O3@MoS2/NGNS for hydrogen evolution reaction (HER). The LSV measurements of α-Fe2O3@MoS2/NGNS and Pt/C were surveyed in both alkaline (0.1 M KOH) (Fig. S9a) and acid electrolyte (0.5 M H2SO4) (Fig. S9c). The obtained results of the overpotential at 10 mA⋅cm-1 (insets of Figs. 9a and c ) and tafel slope values (Figs. S9b and d) for α-Fe2O3@MoS2/NGNS and Pt/C imply that the proposed hybrid possessed relatively good performance towards HER, suggesting a new option with high promise for water splitting application.

Figure 7 (a) LSV results for the α-Fe2O3@MoS2/NGNS and Pt/C in 0.1M KOH saturated with oxygen gas at a potential scan speed of 0.01 V·s-1 and RDE rotation of 1,600 rpm; (b) Tafel plots


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for the α-Fe2O3@MoS2/NGNS and Pt/C; (c) Methanol tolerance and (d) electrochemical stability for α-Fe2O3@MoS2/NGNS and Pt/C. 4. CONCLUSIONS A simple and cost-effective approach was developed for doping ultrasmall α-Fe2O3 NPs into hierarchical MoS2/NGNS heterostructures. Due to the synergistic effect of excellent catalyst materials and the unique structural feature, α-Fe2O3@MoS2/NGNS exhibited good electrochemical properties, including comparable catalytic behavior and excellent methanol tolerance as compared to Pt/C. Particularly, it showed the improved working stability, in which the chronoamperometric current density of hybrid remain 96.1 %, even after a long period of time (30,000 s), outperforming Pt/C and other previously reported materials. Therefore, αFe2O3@MoS2/NGNS hybrid with cost-efficience and good performance is highly potential for ORR in fuel cell applications. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. SEM images of MoS2 sheets, AFM analysis of NGNS, TEM and HR-TEM images of MoS2 sheets and αFe2O3@MoS2/NGNS, results of electrochemical measurements, and Table. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Joong Hee Lee) and [email protected] (Nam Hoon Kim)


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Notes The authors declare no competing financial interest ACKNOWLEDGMENTS We thank support from the Basic Research Laboratory Program (2014R1A4A1008140) and Nano-Material Technology Development Program (2016M3A7B4900117) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT of Republic of Korea.


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REFERENCES (1)

Liu, M.; Zhang, R.; Chen, W. Graphene-Supported Nanoelectrocatalysts for Fuel Cells:

Synthesis, Properties, and Applications. Chem. Rev. 2014, 114 (10), 5117–5160. (2)

Liu, J.; Jiao, M.; Lu, L.; Barkholtz, H. M.; Li, Y.; Jiang, L.; Wu, Z.; Liu, D. J.; Zhuang,

L.; Ma, C.; Zeng, J.; Zhang, B.; Su, D.; Song, P.; Xing, W.; Xu, W.; Wang, Y.; Jiang, Z.; Sun. G. High Performance Platinum Single Atom Electrocatalyst for Oxygen Reduction Reaction. Nat. Commun. 2017, 8, 1–9. (3)

Neergat, M.; Rahul, R. Unsupported Cu-Pt Core-Shell Nanoparticles: Oxygen Reduction

Reaction (ORR) Catalyst with Better Activity and Reduced Precious Metal Content. J. Electrochem. Soc. 2012, 159 (7), F234–F241. (4)

Lv, H.; Li, D.; Strmcnik, D.; Paulikas, A. P.; Markovic, N. M.; Stamenkovic, V. R.

Recent Advances in the Design of Tailored Nanomaterials for Efficient Oxygen Reduction Reaction. Nano Energy 2016, 29, 149–165. (5)

Ferreira, P. J.; la O’, G. J.; Shao-Horn, Y.; Morgan, D.; Makharia, R.; Kocha, S.;

Gasteiger, H. A. Instability of Pt/C Electrocatalysts in Proton Exchange Membrane Fuel Cells. J. Electrochem. Soc. 2005, 152 (11), A2256–A2271. (6)

Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Kostka, A.; Schüth, F.;

Mayrhofer, K. J. J. Degradation Mechanisms of Pt/C Fuel Cell Catalysts under Simulated StartStop Conditions. ACS Catal. 2012, 2 (5), 832–843. (7)

Hien, H. Van; Thanh, T. D.; Chuong, N. D.; Hui, D.; Kim, N. H.; Lee, J. H. Hierarchical

Porous Framework of Ultrasmall PtPd Alloy-Integrated Graphene as Active and Stable Catalyst for Ethanol Oxidation. Compos. Part B Eng. 2018, 143, 96–104. (8)

Ben Liew, K.; Daud, W. R. W.; Ghasemi, M.; Leong, J. X.; Su Lim, S.; Ismail, M. Non-


25


Page 26 of 33

Pt Catalyst as Oxygen Reduction Reaction in Microbial Fuel Cells: A Review. Int. J. Hydrogen Energy 2014, 39 (10), 4870–4883. (9)

Gautam, J.; Thanh, T. D.; Maiti, K.; Kim, N. H.; Lee, J. H. Highly Efficient

Electrocatalyst of N-Doped Graphene-Encapsulated Cobalt-Iron Carbides towards Oxygen Reduction Reaction. Carbon 2018, 137. 358–367. (10)

Thanh, T. D.; Chuong, N. D.; Balamurugan, J.; Van Hien, H.; Kim, N. H.; Lee, J. H.

Porous Hollow-Structured LaNiO3 Stabilized N,S-Codoped Graphene as an Active Electrocatalyst for Oxygen Reduction Reaction. Small 2017, 13 (39), 1–11. (11)

Amiinu, I. S.; Pu, Z.; Liu, X.; Owusu, K. A.; Monestel, H. G. R.; Boakye, F. O.; Zhang,

H.; Mu, S. Multifunctional Mo–N/C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn–Air Batteries. Adv. Funct. Mater. 2017, 27 (44), 1702300. (12)

Thanh, T. D.; Chuong, N. D.; Hien, H. Van; Kshetri, T.; Tuan, L. H.; Kim, N. H.; Lee, J.

H. Recent Advances in Two-Dimensional Transition Metal Dichalcogenides-Graphene Heterostructured Materials for Electrochemical Applications. Prog. Mater. Sci. 2018, 96, 51–85. (13)

Zhang, H.; Lv, R. Defect Engineering of Two-Dimensional Materials for Efficient

Electrocatalysis. J. Mater. 2018, 1–13. (14)

Laursen, A. B.; Kegnæs, S.; Dahl, S.; Chorkendorff, I. Molybdenum Sulfides—efficient

and Viable Materials for Electro-and Photoelectrocatalytic Hydrogen Evolution. Energy Environ. Sci. 2012, 5 (2), 5577–5591. (15)

Zhang, X.; Shi, S.; Gu, T.; Li, L.; Yu, S. The catalytic activity and mechanism of oxygen

reduction

reaction

on

P-doped

MoS2.

Phys.

Chem.

Chem.

Phys.

2018,

DOI:

10.1039/C8CP01294F. (16)

Rawal, T. B.; Le, D.; Rahman, T. S. MoS2-Supported Gold Nanoparticle for CO


26

Page 27 of 33 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


Hydrogenation. J. Phys. Condens. Matter 2017, 29 (41), 415201. (17)

Leng, K.; Chen, Z.; Zhao, X.; Tang, W.; Tian, B.; Nai, C. T.; Zhou, W.; Loh, K. P. Phase

Restructuring in Transition Metal Dichalcogenides for Highly Stable Energy Storage. ACS Nano 2016, 10 (10), 9208–9215. (18)

Wang, J.; Liu, J.; Chao, D.; Yan, J.; Lin, J.; Shen, Z. X. Self-Assembly of Honeycomb-

like MoS2 Nanoarchitectures Anchored into Graphene Foam for Enhanced Lithium-Ion Storage. Adv. Mater. 2014, 26 (42), 7162–7169. (19)

Li, M. Y.; Chen, C. H.; Shi, Y.; Li, L. J. Heterostructures Based on Two-Dimensional

Layered Materials and Their Potential Applications. Mater. Today 2016, 19 (6), 322–335. (20)

Sheng, Z. H.; Shao, L.; Chen, J. J.; Bao, W. J.; Wang, F. Bin; Xia, X. H. Catalyst-Free

Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis. ACS Nano 2011, 5 (6), 4350–4358. (21)

Lin, Z.; Waller, G.; Liu, Y.; Liu, M.; Wong, C. P. Facile Synthesis of Nitrogen-Doped

Graphene via Pyrolysis of Graphene Oxide and Urea, and Its Electrocatalytic Activity toward the Oxygen-Reduction Reaction. Adv. Energy Mater. 2012, 2 (7), 884–888. (22)

Shimizu, K.; Sepunaru, L.; Compton, R. G. Innovative Catalyst Design for the Oxygen

Reduction Reaction for Fuel Cells. Chem. Sci. 2016, 7 (5), 3364–3369. (23)

Dalle Carbonare, N.; Cristino, V.; Berardi, S.; Carli, S.; Argazzi, R.; Caramori, S.; Meda,

L.; Tacca, A.; Bignozzi, C. A. Hematite Photoanodes Modified with an FeIII Water Oxidation Catalyst. ChemPhysChem 2014, 15 (6), 1164–1174. (24)

Wheeler, D. A.; Wang, G.; Ling, Y.; Li, Y.; Zhang, J. Z. Nanostructured Hematite:

Synthesis, Characterization, Charge Carrier Dynamics, and Photoelectrochemical Properties. Energy Environ. Sci. 2012, 5 (5), 6682–6702.


27


(25)

Page 28 of 33

Gurudayal, G.; Chiam, S. Y.; Kumar, M. H.; Bassi, P. S.; Seng, H. L.; Barber, J.; Wong,

L. H. Improving the Efficiency of Hematite Nanorods for Photoelectrochemical Water Splitting by Doping with Manganese. ACS Appl. Mater. Interfaces 2014, 6 (8), 5852–5859. (26)

Sun, M.; Dong, Y.; Zhang, G.; Qu, J.; Li, J. α-Fe2O3 Spherical Nanocrystals Supported

on CNTs as Efficient Non-Noble Electrocatalysts for the Oxygen Reduction Reaction. J. Mater. Chem. A 2014, 2 (33), 13635–13640. (27)

Sun, M.; Zhang, G.; Liu, H.; Liu, Y.; Li, J. α- and γ-Fe2O3 Nanoparticle/Nitrogen Doped

Carbon Nanotube Catalysts for High-Performance Oxygen Reduction Reaction. Sci. China Mater. 2015, 58 (9), 683–692. (28)

Zhao, H.; Wang, J.; Chen, C.; Chen, D.; Gao, Y.; Saccoccio, M.; Ciucci, F. A Bi-

Functional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions from Used Baby Diapers: α-Fe2O3 Wrapped in P and S Dual Doped Graphitic Carbon. RSC Adv. 2016, 6 (69), 64258–64265. (29)

Xue, Y.; Jin, W.; Du, H.; Wang, S.; Zheng, S.; Zhang, Y. Tuning α-Fe2O3 Nanotube

Arrays for the Oxygen Reduction Reaction in Alkaline Media. RSC Adv. 2016, 6 (48), 41878– 41884. (30)

Garino, N.; Sacco, A.; Castellino, M.; Muñoz-Tabares, J. A.; Armandi, M.; Chiodoni, A.;

Pirri, C. F. One-Pot Microwave-Assisted Synthesis of Reduced Graphene Oxide/Iron Oxide Nanocomposite Catalyst for the Oxygen Reduction Reaction. ChemistrySelect 2016, 1 (13), 3640–3646. (31)

Xu, X.; Cao, R.; Jeong, S.; Cho, J. Spindle-like Mesoporous α-Fe2O3 Anode Material

Prepared from MOF Template for High-Rate Lithium Batteries. Nano Lett. 2012, 12 (9), 4988– 4991.


28

Page 29 of 33 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


(32)

Bassi, P. S.; Gurudayal; Wong, L. H.; Barber, J. Iron Based Photoanodes for Solar Fuel

Production. Phys. Chem. Chem. Phys. 2014, 16 (24), 11834. (33)

Yang, Y.; Ma, H.; Zhuang, J.; Wang, X. Morphology-Controlled Synthesis of Hematite

Nanocrystals and Their Facet Effects on Gas-Sensing Properties. Inorg. Chem. 2011, 50 (20), 10143–10151. (34)

Xi, L.; Chiam, S. Y.; Mak, W. F.; Tran, P. D.; Barber, J.; Loo, S. C. J.; Wong, L. H. A

Novel Strategy for Surface Treatment on Hematite Photoanode for Efficient Water Oxidation. Chem. Sci. 2013, 4 (1), 164–169. (35)

Uddin, M. E.; Layek, R. K.; Kim, H. Y.; Kim, N. H.; Hui, D.; Lee, J. H. Preparation and

Enhanced Mechanical Properties of Non-Covalently-Functionalized Graphene Oxide/Cellulose Acetate Nanocomposites. Compos. Part B Eng. 2016, 90, 223–231. (36)

Liu, K. K.; Zhang, W.; Lee, Y. H.; Lin, Y. C.; Chang, M. T.; Su, C. Y.; Chang, C. S.; Li,

H.; Shi, Y.; Zhang, H.; Lai. C. S.; Li, L. J. Growth of Large-Area and Highly Crystalline MoS2 thin Layers on Insulating Substrates. Nano Lett. 2012, 12 (3), 1538–1544. (37)

He, D.; Xiong, Y.; Yang, J.; Chen, X.; Deng, Z.; Pan, M.; Li, Y.; Mu, S. Nanocarbon-

Intercalated and Fe–N-Codoped Graphene as a Highly Active Noble-Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution. J. Mater. Chem. A 2017, 5 (5), 1930–1934. (38)

Jiang, L.; Lin, B.; Li, X.; Song, X.; Xia, H.; Li, L.; Zeng, H. Monolayer MoS2-Graphene

Hybrid Aerogels with Controllable Porosity for Lithium-Ion Batteries with High Reversible Capacity. ACS Appl. Mater. Interfaces 2016, 8 (4), 2680–2687. (39)

Zhang, Z.; Li, W.; Yuen, M. F.; Ng, T. W.; Tang, Y.; Lee, C. S.; Chen, X.; Zhang, W.

Hierarchical Composite Structure of Few-Layers MoS2 nanosheets Supported by Vertical Graphene on Carbon Cloth for High-Performance Hydrogen Evolution Reaction. Nano Energy


29


Page 30 of 33

2015, 18, 196–204. (40)

Yang, L.; Zhou, W.; Hou, D.; Zhou, K.; Li, G.; Tang, Z.; Li, L.; Chen, S. Porous Metallic

MoO2 -Supported MoS2 Nanosheets for Enhanced Electrocatalytic Activity in the Hydrogen Evolution Reaction. Nanoscale 2015, 7 (12), 5203–5208. (41)

Ma, L.; Hu, Y.; Zhu, G.; Chen, R.; Chen, T.; Lu, H.; Wang, Y.; Liang, J.; Liu, H.; Yan,

C.; Tie, Z.; Jin, Z.; Liu, J. In Situ Thermal Synthesis of Inlaid Ultrathin MoS2/Graphene Nanosheets as Electrocatalysts for the Hydrogen Evolution Reaction. Chem. Mater. 2016, 28 (16), 5733–5742. (42)

Cui, X.; Li, Y.; Bachmann, S.; Scalone, M.; Surkus, A. E.; Junge, K.; Topf, C.; Beller, M.

Synthesis and Characterization of Iron-Nitrogen-Doped Graphene/Core-Shell Catalysts: Efficient Oxidative Dehydrogenation of N-Heterocycles. J. Am. Chem. Soc. 2015, 137 (33), 10652–10658. (43)

Thanh, T. D.; Chuong, N. D.; Hien, H. Van; Kim, N. H.; Lee, J. H. CuAg@Ag Core-

Shell Nanostructure Encapsulated by N-Doped Graphene as a High-Performance Catalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2018, 10 (5), 4672–4681. (44)

Townsend, T. K.; Sabio, E. M.; Browning, N. D.; Osterloh, F. E. Photocatalytic Water

Oxidation with Suspended Alpha-Fe2O3 Particles-Effects of Nanoscaling. Energy Environ. Sci. 2011, 4 (10), 4270–4275. (45)

Vinayan, B. P.; Nagar, R.; Ramaprabhu, S. Solar Light Assisted Green Synthesis of

Palladium Nanoparticle Decorated Nitrogen Doped Graphene for Hydrogen Storage Application. J. Mater. Chem. A 2013, 1 (37), 11192–11199. (46)

Haque, E.; Islam, M. M.; Pourazadi, E.; Hassan, M.; Faisal, S. N.; Roy, A. K.;

Konstantinov, K.; Harris, A. T.; Minett, A. I.; Gomes, V. G. Nitrogen Doped Graphene via Thermal Treatment of Composite Solid Precursors as a High Performance Supercapacitor. RSC


30

Page 31 of 33 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


Adv. 2015, 5 (39), 30679–30686. (47)

Qu, B.; Sun, Y.; Liu, L.; Li, C.; Yu, C.; Zhang, X.; Chen, Y. Ultrasmall Fe2O3

Nanoparticles/MoS2 Nanosheets Composite as High-Performance Anode Material for Lithium Ion Batteries. Sci. Rep. 2017, 7, 42772. (48)

Cherian, C. T.; Sundaramurthy, J.; Kalaivani, M.; Ragupathy, P.; Kumar, P. S.; Thavasi,

V.; Reddy, M. V.; Sow, C. H.; Mhaisalkar, S. G.; Ramakrishna, S.; Chowdari, B. V. R. Electrospun α-Fe2O3 Nanorods as a Stable, High Capacity Anode Material for Li-Ion Batteries. J. Mater. Chem. 2012, 22 (24), 12198. (49)

Park, S. H.; Chae, J.; Cho, M.-H.; Kim, J. H.; Yoo, K.-H.; Cho, S. W.; Kim, T. G.; Kim,

J. W. High Concentration of Nitrogen Doped into Graphene Using N2 Plasma with an Aluminum Oxide Buffer Layer. J. Mater. Chem. C 2014, 2 (5), 933–939. (50)

Li,X. L.; Qiao, X. F.; Han, W. P.; Zhang, X.; Tan, Q. H.; Chen, T.; Tan, P. H.

Determining layer number of two-dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrates. Nanotechnology 2016, 27, 145704. (51)

Chen, M.; Wu, P.; Chen, L.; Yang, S.; Yu, L.; Ding, Y.; Zhu, N.; Shi, Z.; Liu, Z. Three-

Dimensional Multi-Doped Porous Carbon/Graphene Derived from Sewage Sludge with Template-Assisted Fe-Pillared Montmorillonite for Enhanced Oxygen Reduction Reaction. Sci. Rep. 2017, 7 (1), 1–10. (52)

Zhang, Y. K.; Sun, Z.; Wang, H.; Wang, Y. D.; Liang, M.; Xue, S. Nitrogen-Doped

Graphene as a Cathode Material for Dye-Sensitized Solar Cells: Effects of Hydrothermal Reaction and Annealing on Electrocatalytic Performance. RSC Adv. 2015, 5 (14), 10430–10439. (53)

Zhu, Y.; Wang, S.; Zhong, Y.; Cai, R.; Li, L.; Shao, Z. Facile Synthesis of a MoO2-

Mo2C-C Composite and Its Application as Favorable Anode Material for Lithium-Ion Batteries.


31


Page 32 of 33

J. Power Sources 2016, 307, 552–560. (54)

Zhu, H.; Zhang, J.; Yanzhang, R.; Du, M.; Wang, Q.; Gao, G.; Wu, J.; Wu, G.; Zhang,

M.; Liu, B.; Yao, J.; Zhang, X. When Cubic Cobalt Sulfide Meets Layered Molybdenum Disulfide: A Core-Shell System Toward Synergetic Electrocatalytic Water Splitting. Adv. Mater. 2015, 27 (32), 4752–4759. (55)

Qie, L.; Chen, W.; Xiong, X.; Hu, C.; Zou, F.; Hu, P.; Huang, Y. Sulfur-Doped Carbon

with Enlarged Interlayer Distance as a High-Performance Anode Material for Sodium-Ion Batteries. Adv. Sci. 2015, 2 (12). (56)

Jagminas, A.; Niaura, G.; Žalneravičius, R.; Trusovas, R.; Račiukaitis, G.; Jasulaitiene,

V. Laser Light Induced Transformation of Molybdenum Disulphide-Based Nanoplatelet Arrays. Sci. Rep. 2016, 6, 2–10. (57)

Bandgar, D. K.; Navale, S. T.; Naushad, M.; Mane, R. S.; Stadler, F. J.; Patil, V. B. Ultra-

Sensitive Polyaniline–iron Oxide Nanocomposite Room Temperature Flexible Ammonia Sensor. RSC Adv. 2015, 5 (84), 68964–68971. (58)

Geim, A. K.; Grigorieva, I. V. Van Der Waals Heterostructures. Nature 2013, 499

(7459), 419–425. (59)

Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts

for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134 (15), 6575–6578. (60)

Hansgen, D. A.; Vlachos, D. G.; Chen, J. G. Using First Principles to Predict Bimetallic

Catalysts for the Ammonia Decomposition Reaction. Nat. Chem. 2010, 2 (6), 484–489.


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Graphic abstract


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Hierarchical Heterostructures of Ultrasmall Fe2O3 Encapsulated

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