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In Situ Synthesis of MoP Nanoflakes Intercalated N-Doped Graphene Nanobelts from MoO3–Amine Hybrid for High-Efficient Hydrogen Evolution Reaction

Chao Huang, Chaoran Pi, Xuming Zhang,* Kang Ding, Ping Qin, Jijiang Fu, Xiang Peng, Biao Gao, Paul K. Chu, and Kaifu Huo*

C. Huang, C. R. Pi, Prof. X. M. Zhang, K. Ding, P. Qin, Prof. J. J. Fu, Dr. B. GaoThe State Key Laboratory of Refractories and MetallurgyInstitute of Advanced Materials and NanotechnologyWuhan University of Science and TechnologyWuhan 430081, ChinaE-mail: xumzhang@wust.edu.cnC. Huang, Dr. X. Peng, Prof. P. K. ChuDepartment of Physics and Department of Materials Science and EngineeringCity University of Hong KongTat Chee Avenue, Kowloon, Hong Kong 999077, ChinaProf. K. F. HuoWuhan National Laboratory for Optoelectronics (WNLO) School of Optical and Electronic InformationHuazhong University of Science and TechnologyWuhan 430074, ChinaE-mail: kfhuo@hust.edu.cn

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201800667.

DOI: 10.1002/smll.201800667

electrochemical water splitting in large-scale hydrogen production and although noble Pt and its alloy are the best hydrogen evolution reaction (HER) electrocata-lysts,[3] the scarcity and high cost impede wide application.[4] Hence, tremendous efforts have been devoted to developing alternative non-noble-metal electrocata-lysts for efficient hydrogen production.[5,6]

Transition metal phosphides (TMPs) such as molybdenum phosphide (MoP) are promising HER electrocatalysts due to the low cost, abundance, and high cata-lytic activities under both acidic and alka-line conditions.[7,8] The negatively charged P atoms in TMPs could trap positively charged protons during HER leading to high HER activity.[1,9] Previous studies have disclosed that larger P content improves the HER activity of TMPs.[1,9] Nevertheless,

if the P content is too high, electron delocalization of metals is restricted resulting in large electrochemical polarization and sluggish HER kinetics as a result of the lower conductivity.[10] To overcome this hurdle, conductive carbon materials such as porous carbon, graphite, and carbon nanotubes have been intro-duced to construct MoP/carbon nanocomposites to improve the electron conductivity.[11] For examples, Ojha et al.[12] reported that MoP nanoparticles assembled on reduced graphene oxide exhibited a low overpotential of 162 mV at a current density of 10 mA cm−2. Besides the conductivity, the HER activity is asso-ciated with the amount of active sites of the electrocatalysts since hydrogen production via water splitting occurs on the sur-face of the catalysts.[13,14] To this point, 2D MoP nanosheets or nanoflakes are desirable to improve to HER activity due to the increased numbers of surface sites compared to nanoparticles. If the unilamellar MoP nanosheets or nanoflakes and graphene are alternately stacked to form MoP/graphene heterostructure at the molecular scale, the enhanced electrocatalytic properties of MoP could be achieved because of the synergistic utiliza-tion of these 2D materials via the high-quality heterointerfaces. However, to the best of our knowledge, such highly ordered or superlattice-like structures with MoP/graphene hybrid for HER have not been reported.

Herein, we propose a facile strategy to produce alternately stacked MoP nanoflakes and nitrogen-doped graphene layers

Molybdenum phosphide (MoP) is a promising non-noble-metal electrocata-lyst in the hydrogen evolution reaction (HER), but practical implementation is impeded by the sluggish HER kinetics and poor chemical stability. Herein, a novel high-efficiency HER electrocatalyst comprising MoP nanoflakes interca-lated nitrogen-doped graphene nanobelts (MoP/NG), which are synthesized by one-step thermal phosphiding organic–inorganic hybrid dodecylamine (DDA) inserted MoO3 nanobelts, is reported. The intercalated DDA molecules are in situ carbonized into the NG layer and the sandwiched MoO3 layer is converted into MoP nanoflakes which are intercalated between the NG layers forming the alternatingly stacked MoP/NG hybrid nanobelts. The MoP nanoflakes provide abundant edge sites and the sandwiched MoP/NG hybrid enables rapid ion/electron transport thus yielding excellent electrochemical activity and stability for HER. The MoP/NG shows a low overpotential of 94 mV at 10 mA cm−2, small Tafel slope of 50.1 mV dec−1, and excellent elec-trochemical stability with 99.5% retention for over 22 h.

Electrocatalysts

1. Introduction

Hydrogen is a clean, abundant, and renewable energy source and may substitute nonrenewable fossil fuels such as coal and petroleum in the future.[1,2] Electrocatalysts play a vital role in

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(MoP/NG) via a one-step process involving phosphiding dodecylamine (DDA, CH3(CH2)11NH2) inserted MoO3 nano-belts, which demonstrate high-efficiency electrocatalytic activity for HER. The intercalated DDA molecules are in situ carbonized into the NG layer and the sandwiched MoO3 layer is converted into island-like MoP nanoflakes which are inter-calated into the carbonized NG interlayers forming the alter-nately stacked MoP/NG hybrid.[15] The layer-by-layer MoP/NG hybrid possesses several advantages boosting the superior electrocatalytic performance. First, the thin MoP nanoflakes are separated and stabilized between the NG layers and the alter-nately stacked structure inhibits restacking and aggregation of MoP nanoflakes even at a high MoP content of 92 wt%, thus producing abundant electroactive sites for HER. Second, the superlattice-like structure of MoP/NG leads to more accessible active sites and promote ion migration. Third, the NG layers improve the overall conductivity enabling fast electron transfer and the smaller charge transfer resistance. Fourth, the alter-nately stacked structure of MoP/NG hybrid prevents oxidation of MoP nanoflakes thereby providing good structural and elec-trochemical durability. Fifth, the high-quality heterointerfaces of MoP and NG layer result in strong chemical interactions via PC bonding and so enhanced HER activity is achieved on account of the partial negative charge on P atoms.[12] Owing to these unique advantages, the MoP/NG delivers excellent HER performance with a small overpotential of 94 mV at 10 mA cm−2, small Tafel slope of 50.1 mV dec−1, and excellent durability with 99.5% current retention for over 22 h. Even at 70 °C, the MoP/NG hybrids retain the high electrochemical sta-bility. Our promising results offer prospective insights into the design and synthesis of high-efficient non-noble-metal electro-catalysts via constructing the alternately stacked structure using layered materials as precursors, such as MoO3, HNb3O8, V2O5 and MoS2.

2. Results and Discussion

Figure 1 shows the synthetic process of the lamellar MoP/NG hybrid nanobelts. The scanning electron microscopy (SEM) images reveal that the MoO3 nanobelts with a diameter of 300 nm and length of ten micrometers are hydrothermally produced as described previously[16] (Figure S1a,b, Supporting Information). MoO3, a layered structure consisting of van der Waals bonded double-[MoO6] octahedron sheets, exhibits the typical (0k0) diffraction peaks with an interlayer spacing d010 of 0.69 nm.[17,18] The protonated DDA (CH3 (CH2)11NH3

+) via the alcoholysis reaction can be easily intercalated into the MoO3 layer (Figure 1a) forming DDA intercalated MoO3 nanobelts

(MoO3/DDA) (Figure 1b).[17,18] The MoO3/DDA hybrid nano-belts (Figure S1c,d, Supporting Information) retain the pris-tine morphology of MoO3 nanobelts. The X-ray diffraction (XRD) patterns (Figure S2, Supporting Information) reveal that the MoO3/DDA nanobelts have three sharp (0k0) peaks at small angles (2°–10°) dominating other diffraction peaks.[18] The (010) interlayer distance of MoO3/DDA is measured to be 2.65 nm and larger than that of pristine MoO3 (0.69 nm), cor-roborating that the bi-layers DDA molecules are intercalated in the interlayered MoO3 nanobelts. Fourier transform infrared spectroscopy (FTIR) discloses CH bending (1390 cm−1), NH stretching (1500 cm−1), and CN stretching (1260 cm−1) char-acteristic of DDA in MoO3/DDA (Figure S3, Supporting Infor-mation).[17,19,20] These results verify that the organic–inorganic hybrids of DDA intercalated MoO3 nanobelts are produced by the hydrothermal treatment. The MoP/NG hybrid nanobelts are produced by one-step thermal processing of MoO3/DDA nanobelts using NaH2PO2 as the phosphorus source. The inter-layered DDA molecules are in situ carbonized into the NG layer and the sandwiched MoO3 layer is converted into MoP nano-flakes forming the MoP/NG hybrid nanobelts comprising alter-nating stacked MoP nanoflakes and NG layers as schematically illustrated in Figure 1c.

Figure S4 (Supporting Information) depicts the XRD pat-tern of MoO3/DDA after annealing at different temperature. At 700 °C, all the XRD peaks (Figure 2a) at 27.95°, 32.17°, 43.15°, 57.48°, 64.93°, 67.03°, 67.86°, 74.33°, and 85.67° could be attrib-uted to (001), (100), (101), (110), (111), (200), (102), (201), and (112) planes of hexagonal MoP phase (JCPDS No. 24-0771) and no other peaks could be identified,[5] suggesting complete con-version from MoO3 to MoP. No carbon peaks can be observed due to the poor crystallinity and unstacked interlayers of NG.[21] The Raman scattering spectrum (Figure S5, Supporting Information) shows typical peaks at 1360 and 1590 cm−1 cor-responding to the D and G bands of graphene, respectively, indicating that the interlayer DDA is carbonized to NG nano-belts at 700 °C.[22] The SEM image of the MoP/NG hybrid in Figure 2b reveals that the MoP/NG nanobelt has a width of 300 nm and length up to 10 µm similar to the MoO3/DDA nanobelts (Figure S1d, Supporting Information). The high-resolution SEM image (Figure 2c) indicates that the surface of MoP/NG is coarse with many small nanoflakes embedded into the nanobelts. The high-resolution transmission electron microscopy (HRTEM) image (Figure 2d) discloses that the MoP nanoflakes are anchored on NG and the lattice fringes of 0.336 and 0.213 nm correspond to the (001) and (101) planes of MoP.[23] The elemental maps of MoP/NG (Figure 2e) show that Mo, P, C, and N are homogeneously distributed throughout the nanobelts further corroborating the formation of MoP/

NG hybrid nanobelts. For comparison, MoP nanobelts are also prepared by thermal pro-cessing MoO3 nanobelts but without DDA intercalation. Different from the hybrid MoP/NG nanobelts, the SEM images in Figure S6 (Supporting Information) reveal the MoP nanobelts have obvious mesopores (5–10 nm) due to the volume shrinkage.[24–26] To confirm the alternately stacked MoP/NG hybrid structure, we remove the thin MoP

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Figure 1. Schematic illustration of the synthetic procedures of MoP/NG nanobelts from MoO3. a) MoO3; b) MoO3/DDA; c) MoP/NG.

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nanoflakes layer between the NG layers by immersing the MoP/NG hybrid nanobelts in the 0.5 m KOH solution at 50 °C. The TEM images (Figure S7a,b, Supporting Information) show thin NG nanobelts are produced and NG is amorphous. Energy-dispersive X-ray spectroscopy (EDS) confirms that the nanobelts have no Mo and P (Figure S7c, Supporting Informa-tion). The thickness of the NG is about 3.26 nm by atomic force microscopy (AFM), which is larger than a single-layer graphene due to the restacking of in situ isolated NG after removing MoP in 0.5 m KOH. Since the NG nanobelts have a similar width as the MoP/NG nanobelts, it can be concluded that the NG nanobelts originate from the intercalated NG layers. The NG content in the MoP/NG nanobelts is determined by thermo-gravimetry (TG) as shown in Figure S8 (Supporting Informa-tion). The amount of NG in MoP/NG is about 8 wt% and the Brunauer–Emmett–Teller (BET) areas (Figure S9, Supporting Information) of the MoP and MoP/NG nanobelts are measured to be 31.4 and 48.5 m2 g−1, respectively. The larger BET surface area of MoP/NG is attributed to the thin MoP nanoflakes and lamellared structure of the MoP/NG nanobelts.

X-ray photoelectron spectroscopy (XPS) is conducted to determine the composition and chemical states of the MoP/NG nanobelts. The high-resolution spectra of Mo 3d, P 2p, and C1s are exhibited as shown in Figure 3a–c. Two peaks at the low binding energy (228.2/231.2 eV) are assigned to Mo3+ in MoP. The two doublets at 235.5 eV/233.4 eV (Mo6+ 3d3/2/3d5/2) and 231.9 eV/228.8 eV (Mo4+ 3d3/2/3d5/2) in Figure 3a can be assigned to high-oxidation-state Mo (MoO3 and MoO2).[11,15] The peaks of P 2p at 129.4 and 130.4 eV are attributed to low-valence P in MoP[27] and those at higher binding energy (green line in Figure 3b) are attributed to PO4

3− or P2O5 due to oxida-tion.[8,28] Compared to MoP nanobelts, the peaks of MoO and PO bonds acquired from MoP/NG hybrid are weaker, sug-gesting the NG inhibits oxidation of MoP, which is favorable to maintaining the high activity of MoP during the HER process. In the MoP/NG, the PC bond can be identified suggesting

strong interfacial interaction between MoP and NG and it is also reflected by the C1s profile (Figure 3c).[12,29,30] The C signal in the MoP stems from adsorbed gaseous hydrocarbon molecules during XPS analysis, while the CN and CP sig-nals arise from the NG layer and interfacial CP in MoP/NG. The interfacial interaction of CP enhances proton adsorp-tion, electron transfer, and structural stability of the MoP/NG electrode bonding well for the HER activity.[31] The N 1s XPS of MoP/NG is shown in Figure S10 (Supporting Information), indicating the formation of NG.

The HER activities of MoP/NG nanobelts and MoP nanobelts with an active mass of 0.28 mg cm−2 are evaluated by linear sweep voltammetry (LSV) at a scanning rate of 5 mV s−1 in 0.5 m H2SO4 using a three-electrode configuration. All the potentials in polarization curves are iR corrected (Figure S11, Supporting Information) and the polarization curves of the hybrid MoP/NG nanobelts and MoP nanobelts are shown in Figure 4a. For comparison, the polarization curves of commercial 20% Pt/C, NG, and bare glassy carbon electrode (GCE) acquired under the same measurement conductions are also provided. The overpo-tential of Pt/C (46 mV) at a current density of 10 mA cm−2 is measured as reference, which is similar to the previous report (45 mV).[32–36] The MoP nanobelts show a high overpotential of 170 mV at a current density of 10 mA cm−2, whereas the MoP/NG nanobelts exhibit a small overpotential of 94 mV at 10 mA cm−2 in acid media (Figure 4a) as well as an overpoten-tial of 153 mV at 10 mA cm−2 in basic solution (Figure S12, Supporting Information). Moreover, the of MoP/NG nano-belts at 10 mA cm−2 are lower than that of previously reported MoP based electrocatalysts (Table S1, Supporting Information) such as MoP NA/CC (124 mV at 10 mA cm−2),[15] MoP/N, P dual-doped carbon (116 mV at 10 mA cm−2),[33] reduced gra-phene oxide and MoP composites (105 mV at 10 mA cm−2),[12] cluster-like MoP (119 mV at 10 mA cm−2),[11] and hierarchical core–shell MoS2@MoP (108 mV at 10 mA cm−2).[34] The cor-responding Tafel slopes of MoP, MoP/NG, and commercial

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Figure 2. a) XRD pattern, b–d) SEM, TEM, and HRTEM images, and e) elemental maps of the MoP/NG nanobelts.

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20% Pt/C are determined to investigate the HER kinetics (Figure 4b). The Pt/C catalyst shows an excellent polariza-tion curve with a small Tafel slope of 31.4 mV dec−1, which is similar to the previous report.[35,36] The Tafel slope of MoP/NG

is 50.1 mV dec−1 and smaller than that of the bare MoP nano-belts (86.1 mV dec−1) and many other phosphide electrocata-lysts such as MoP nanoparticles (54 mV dec−1),[37] MoP NA/CC (58 mV dec−1),[15] MoP/rGO (111 mV dec−1),[12] and Mo2C@C (60 mV dec−1),[38] suggesting the superior HER kinetics. The improved HER activity of MoP/NG stems from enriched redox sites of 2D MoP nanoflakes, more electronegative P atoms flourishing the electrochemical activity, and the sandwiched structure favoring the transfer of protons and electrons.[12]

Electrochemical impedance spectroscopy (EIS) is employed to determine the charge transfer resistance (Figure 4c) and the equivalent circuit is presented in Figure S13 (Supporting Information). The MoP/NG nanobelts have a resistance of 13.8 Ω cm2, Table S2, Supporting Information) that is smaller than that of the MoP nanobelts (238.5 Ω cm2), implying effi-cient electron transport and favorable HER kinetics at the MoP/NG interfaces.[15] The stability is also critical to durable per-formance. The long-term stability of MoP/NG is evaluated by CV at 200 mV s−1. The polarization curves (Figure 4d) show slight deterioration at the cathodic current after 10 000 cycles and chronoamperometry (Figure S14, Supporting Information) shows that the cathodic current is maintained for over 22 h with 99.5% retention at a current density of 10 mA cm−2 confirming the high stability of MoP/NG in acidic media. On the other hand, the polarization curves of MoP (Figure S15, Supporting Information) indicate poor stability with obvious degradation from −0.26 to −0.34 V at a current density of 40 mA cm−2. To further evaluate the structural and chemical stability, the XPS spectra of MoP/NG and MoP after 22h duration are shown in Figures S16 and S17 (Supporting Information). The amount of MoP bond in MoP/NG has no obvious changes after 22 h duration, however, the MoP content decreases and PO con-tent increases in MoP nanobelt after 22 h due to the surface oxidation of MoP, suggesting the enhanced oxidation resistance due to the protection of the inserted NG layer in the MoP/NG hybrid. The double-layer capacitance (Cdl) at the solid–liquid interface is proportional to the effective electrochemical sur-face.[23] As shown in Figure 4e,f, Cdl of the MoP/NG hybrid nanobelts is 9.10 mF cm−2, which is 35 times larger than that of the MoP nanobelts (0.26 mF cm−2) (Figure S18, Supporting Information), indicating more exposed active sites on MoP/NG consistent with the enhanced catalytic activity.[39] We investi-gate the HER activity of the alternately stacked MoP/NG nano-belts at different temperature from 25 to 70 °C. As shown in Figure S19 (Supporting Information), the polarization curves and corresponding Tafel slopes of the MoP/NG hybrid nanobelts reveal satisfactory HER activity even at 70 °C, further indicating that the superior HER kinetics stems from the increased active sites and enhanced conductivity of the sandwiched structure.

3. Conclusion

In summary, a high-efficient HER electrocatalyst composed of alternately stacked MoP/NG lamellar nanobelts is prepared by one-step thermal phosphide of DDA intercalated MoO3 nano-belts. The in situ formed MoP nanoflakes are confined between carbonized NG layers. The 2D MoP nanoflakes provide abun-dant edge sites to improve the HER activity, while the NG layer

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Figure 3. XPS spectra: a) Mo 3d, b) P 2p, and c) C1s of the MoP nano-belts and hybrid MoP/NG nanobelts.

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acts as the conductive layer for fast electron transport and also against agglomeration and corrosion of MoP during HER. The alternately stacked MoP and NG layers have strong chemical interactions via PC bonding causing a partial negative charge on P and attracting protons to the electrocatalyst surface to enhance the HER activity. The MoP/NG hybrid nanobelts have a small overpotential of 94 mV at a current density of 10 mA cm−2, small Tafel slope of 50.1 mV dec−1, small charge transfer resist-ance (13.8 Ω cm2), and excellent cycling stability. The results not only reveal an attractive electrocatalyst with excellent HER activity, but also offer prospective insights into the design and synthesis of efficient non-noble-metal electrocatalysts.

4. Experimental SectionSynthesis of Hybrid MoP/NG Nanobelts: All the chemicals were

purchased from Sigma and used without purification. The MoO3

nanobelts were synthesized by hydrothermal reaction as previous report.[16,40] The DDA (CH3(CH2)11NH2) was used to thrust into the interlamination of two neighboring layers of MoO3. First, 0.2 g of MoO3 and 0.066 g of CH3(CH2)11NH2 (DDA) were dissolved in 30 mL alcohol under stirring for 5 min and transferred to a 50 mL Teflon-lined stainless steel autoclave heated at 100 °C for 24 h. The white product (MoO3/DDA) was obtained after washing with ethanol several times and dried at 80 °C in vacuum overnight. Then, the MoO3/DDA nanobelts were placed at the downstream side of sodium hypophosphite (NaH2PO2) in the furnace tube and the distance between them was about 20 cm. After being kept at 700 °C for 2 h in Ar, the hybrid MoP/NG nanobelts were collected when the furnace was naturally cooled down to room temperature. The pure MoP nanobelts were obtained via the similar method without adding DDA molecule.

Materials Characterization: The morphology, structure, and composition of the samples were determined by XRD using the Cu Kα radiation (λ = 1.5418 Å) (Philips X′ Pert Pro), field-emission scanning electron microscopy (FESEM, FEI nanoSEM 450), TEM (FEI Titan 60-300 Cs), and XPS (ESCALB MK-II). Raman scattering spectroscopy was conducted using a 514.5 nm argon laser as the excitation source

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Figure 4. a) Polarization curves; b) Tafel plots; c) Nyquist plots (at η = 100 mV); d) cycling stability of the hybrid MoP/NG nanobelts before and after 10 000 cycles in 0.5 m H2SO4; e) CV curves of MoP/NG at different scanning rates from 10 to 100 mV s−1; f) corresponding anodic current densities (measured at 0.2 V vs RHE) versus scanning rates.

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on the HR RamLab Raman Microscope. TG analysis (STA449) and differential thermal analysis (DTA, STA449) were conducted to determine the carbon content as well as the chemical reactions in the heating process. The specific area was determined by the BET method using an automated chemisorption/physisorption surface area analyzer Micromeritics SASP 2020 at 77 K and the pore size distribution was determined by the Barrett–Joiner–Helenda (BJH) method. AFM was performed on SPM-9700 analytical instruments. FTIR spectra were performed on IFS-85 (Bruker) spectrometer.

Electrochemical Measurements: The commercial 20% Pt/C catalyst was purchased from Sigma-Aldrich. The electrochemical measurements were carried out on a three-electrode system (CHI 760e, Shanghai CHI Company, China) in 0.5 m H2SO4 solution with a carbon sheet (1 cm × 1 cm) as the counter electrode, saturated calomel electrode (SCE) as the reference electrode, and modified GCE as the working electrode. All the potentials were referenced to the reversible hydrogen electrode (RHE) according to the equation: ERHE = ESCE + 0.0591 × pH.[1,13] The SCE also calibrated referenced to the RHE is shown in Figure S20 (Supporting Information). It was present in the hydrogen-saturated electrolyte with a Pt wire as the working electrode. Hence, a value of 0.23 V was added in an overpotential of RHE. In order to prepare the working electrode, 12 mg of samples were dissolved in 3 mL of deionized water under sonication for 30 min and 5 µL of solution was loaded onto the GCE with a diameter of 3 mm at a loading density about 0.28 mg cm−2. After drying in air, 5 µL of diluted Nafion (0.5% nafion-water-ethanol solution) covered the GCE to fix the electrocatalyst. LSV was acquired by CHI760e at a scanning rate of 5 mV s−1. The EIS was tested at overpotential of 100 mV in a frequency range between 100 kHz and 0.1 Hz with an AC perturbation of 5 mV. The stability of the catalyst was measured by cyclic voltammetry (CV) at a scanning rate of 200 mV s−1. All the potentials in polarization curves and Tafel plots were checked by iR correction using Rs in EIS.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsC.H. and C.R.P. contributed equally to this work. This work was financially supported by National Natural Science Foundation of China (Nos. 51572100 and 61434001), HUST Key Interdisciplinary Team Project (2016JCTD101), Fundamental Research Funds for the Central Universities (HUST: 2015QN071), Wuhan Yellow Crane Talents Program, and City University of Hong Kong Applied Research Grant (ARG) No. 9667122 and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 11205617. The authors also acknowledge the Nanodevices and Characterization Centre of WNLO-HUST and Analytical and Testing Center of HUST.

Conflict of InterestThe authors declare no conflict of interest.

Keywordselectrocatalysts, hydrogen evolution reaction, layer-by-layer structure, molybdenum phosphides, sandwiched nanobelts

Received: February 17, 2018Revised: March 28, 2018

Published online: May 10, 2018

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Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

for Small, DOI: 10.1002/smll.201800667

In Situ Synthesis of MoP Nanoflakes Intercalated N-DopedGraphene Nanobelts from MoO3–Amine Hybrid for High-Efficient Hydrogen Evolution Reaction

Chao Huang, Chaoran Pi, Xuming Zhang,* Kang Ding, PingQin, Jijiang Fu, Xiang Peng, Biao Gao, Paul K. Chu, andKaifu Huo*

Supporting Information

In situ Synthesis of MoP Nanoflakes Intercalated N-doped Graphene Nanobelts

from MoO3-Amine Hybrid for High-Efficient Hydrogen Evolution Reaction

Chao Huang, Chaoran Pi, Xuming Zhang*, Kang Ding, Ping Qin, Jijiang Fu, Xiang

Peng, Biao Gao, Paul K. Chu, and Kaifu Huo*

C. Huang, C. R. Pi, Prof. X. M. Zhang, K. Ding, P. Qin, J. J. Fu, B. Gao

The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced

Materials and Nanotechnology, Wuhan University of Science and Technology,

Wuhan 430081, China.

E-mail: xumzhang@wust.edu.cn (X.M. Zhang).

Prof. K. F. Huo

Wuhan National Laboratory for Optoelectronics (WNLO) School of Optical and

Electronic Information Huazhong University of Science and Technology Wuhan

430074, China.

E-mail: kfhuo@hust.edu.cn (K.F. Huo);

C. Huang, X. Peng, P. K. Chu

Department of Physics and Department of Materials Science and Engineering, City

University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China.

Figure S1. SEM images of pristine MoO3 nanobelts (a-b) and MoO3/DDA hybrid

nanobelts (c-d).

Figure S2. The XRD patterns of MoO3 and MoO3/DDA. The insert pictures are

illustrated the fabrication of MoO3 and sandwich MoO3/DDA, respectively.

Figure S3. The FTIR spectra of MoO3/DDA nanobelts, MoO3 nanobelts and DDA

molecules.

Figure S4. The small-angle XRD (a) and large-angle (b) of MoO3/DDA nanobelts

with annealing temperatures.

Figure S5. Raman spectrum of MoP/NG nanobelts.

Figure S6. (a) XRD pattern, (b) SEM, (c) TEM, (d) HRTEM of mesoporous MoP.

Figure S7. (a) TEM and (b) HRTEM images of graphene nanobelts after dissolving

the MoP from the hybrid MoP/NG nanobelts in 0.5 M KOH at 50 oC; (c) The

corresponding EDS of NG.

Figure S8. TG curves of mesoporous MoP nanobelts and MoP/NG nanobelts.

Figure S9. (a) The pore size distribution of mesoporous MoP nanobelts and hybrid

MoP/NG nanobelts; (b) N2 adsorption/desorption isotherms.

Figure S10. The XPS of N 1s of MoP/NG.

Figure S11. Polarization curves of MoP/NG hybrid nanobelts electrocatalyst with and

without iR correction.

Figure S12. (a) Polarization curves and (b) Tafel plots of MoP/NG acquired in 1 M

KOH.

Figure S13. Electrical equivalent circuit used to simulate the Nyquist plots in Figure

4c, where Rs is the electrolyte resistance, Rct is the charge-transfer resistance, and Cdl

represents the double-layer capacitance.[1-3]

Figure S14. Time-dependent current density of MoP/NG nanobelts at 10 mA cm-2

for

22 h.

Figure S15. The stability of MoP nanobelts with an initial polarization curve and after

10,000 cycles in 0.5 M H2SO4 at room temperature.

Figure S16. The XPS of MoP/NG nanobelts is obtained after 10,000 cycles at a scan

rate of 200 mV s-1

.

Figure S17. The XPS spectra of P 2p in (a) MoP/NG and (b) MoP are obtained after

22 h of durability test.

Figure S18. (a) CV curves of MoP at different scan rates from 10 mV s-1

to 100 mV

s-1

; (b) Corresponding anodic current densities (measured at 0.2 V vs. RHE) plotted

against the scanning rates.

Figure S19. (a) Polarization curves of MoP/NG hybrid nanobelts at different

temperatures; (b) Corresponding Tafel plots of MoP/NG nanobelts.

Figure S20. CV of SCE calibrated with respect to RHE at a scanning rate of 1 mV s-1

.

The inset is the enlarged CV curve. The average of the two potentials at which the

current crosses zero is taken to be the thermodynamic potential of the hydrogen

electrode reaction.[4]

Table S1. Performance compaarison of MoP/NG with other precious-metal-free HER

electrocatalysts.

Samples

Mass

loading

(mg cm-2

)

media 10 mA cm

-2

(mV)

Tafel slope

(mV dec-1

) Ref.

MoP/NG

nanobelts 0.28

0.5 M

H2SO4 94 50.1 This work

MoP/NG

nanobelts 0.28 1 M KOH 153 62.9 This work

MoP

nanobelts 0.28

0.5 M

H2SO4 170 86.1 This work

MoP/N,P

dual-doped

carbon

3.0 0.5 M

H2SO4 116 51

[5]

MoSe2/rGO 0.16 0.5 M

H2SO4 ~120 69

[6]

MoP

NA/CC --

0.5 M

H2SO4 124 58

[7]

MoP NPs 0.36 0.5 M

H2SO4 125 54

[8]

MoC-Mo2C

HNWs 0.14

0.5 M

H2SO4 126 43

[9]

Porous

Mo2C 0.8

0.5 M

H2SO4 142 53

[10]

MoP/rGO 1.6 0.5 M

H2SO4 152 88

[11]

MoP/rGO 1.6 1 M KOH 162 57 [11]

MoS2@

N-doped

carbon

1 0.5 M

H2SO4 165 55

[12]

Mo2C/GCSs 0.36 0.5 M

H2SO4 200 62.6

[13]

MoP 0.071 0.5 M

H2SO4 246 60

[14]

c-Mo2C 0.102 1 M KOH 293 58 [15]

γ-Mo2N 0.102 1 M KOH 353 108 [15]

MA-MoS2 0.28 0.5 M

H2SO4 -- 104

[16]

Mo2C/CNT 2 0.1 M

HClO4 -- 55.2

[17]

Mo2C 2 0.1 M

HClO4 -- 87.6

[17]

Table S2. Fitted parameters of the electrocatalysts at an overpotential of 100 mV

according to the equivalent circuit in Figure S13.

Electrocatalysts Rs (Ω cm2) Rct (Ω cm

2)

MoP/NG 0.54 13.8

MoP 0.56 238.5

Pt/C 0.52 2.8

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