Induced Phosphorization-Derived Well-Dispersed Molybdenum

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Induced Phosphorization-Derived Well-Dispersed Molybdenum Phosphide Nanoparticles Encapsulated in Hollow N‑Doped Carbon Nanospheres for Efficient Hydrogen Evolution Jing-Qi Chi,† Wen-Kun Gao,†,‡ Li-Ming Zhang,†,‡ Bin Dong,*,†,‡ Kai-Li Yan,† Jia-Hui Lin,†,‡ Bin Liu,† Yong-Ming Chai,† and Chen-Guang Liu*,† †

State Key Laboratory of Heavy Oil Processing and ‡College of Science, China University of Petroleum (East China), 66 Changjiang West Road, Huangdao District, Qingdao 266580, P.R. China S Supporting Information *

ABSTRACT: Molybdenum phosphide (MoP) nanoparticles encapsulated in N,P-codoped hollow carbon nanospheres (MoP@ NPC-H) have been synthesized as electrocatalysts for hydrogen evolution reaction (HER) based on an induced phosphorization of an inorganic-organic Mo-P/polyaniline-pyrrole precursor. Notably, the P sources in the polyaniline-pyrrole precursor are responsible for the formation of well-dispersed MoP nanoparticles and enlarged hollow carbon nanospheres through a P-induced process, which can be described as induced phosphorization for realizing the shift and good dispersion of MoP encapsulated in carbon nanospheres. In addition, the polyaniline-pyrrole precursor could avoid the aggregation of MoP nanoparticles and protect MoP from corrosion during the HER process. N,P-codoped carbon layers provide remarkable conductivity for the higher utilization of active sites. Moreover, compared to solid nanostructures (MoP@NPC-S), the MoP@NPC-H possesses a larger BET surface area and implies superior HER performance, which requires an overpotential of only 141 mV in acidic, 176 mV in alkaline, and 198 mV in neutral solution to achieve a current density of 10 mA cm−2. The enhancement mechanisms of HER performances have been discussed. The facile approach may provide a new way for preparing highly dispersed MoP encapsulated in carbon materials based on the induced phosphorization. KEYWORDS: MoP, Induced phosphorization, N,P-codoped hollow carbon spheres, Hydrogen evolution reaction

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INTRODUCTION

amorphous form could show a remarkable catalytic performance for HER.15,16 Currently, researchers are mainly focused on designing unique nanostructures or tuning element components of MoP-based electrocatalysts to enhance the HER activity, such as amorphous MoP NPs,17 MoP nanosheets,18 MoS2(1−x)Px,19 and MoP|S.20 Although some progress has been made for preparing highly active MoP, the exposed active sites are affected by the morphologies and nanostructures.21 Moreover, well-crystallized nanostructured MoP is usually formed at temperatures exceeding 800 °C;22,23 thus, it is still a great challenge to design unique nanostructures to prevent its coalescence and aggregation at high temperature. The use of highly conductive carbon materials as a support can not only improve the conductivity of the active sites but

As a clean, secure, and sustainable energy carrier of hydrogen can satisfy the increased global energy consumption.1−3 Electrochemical water splitting to produce hydrogen, named as the hydrogen evolution reaction (HER), is a sustainable and promising strategy due to its higher efficiency.4−6 However, effective electrocatalysts are demanded to minimize the overpotential derived from HER in such techniques.7,8 To date, state-of-the-art platinum (Pt) or Pt-based alloys give the best HER performance, but the high cost and low abundance have greatly limited their applications.9,10 Consequently, the development of highly efficient electrocatalysts using costeffective non-noble-metal containing materials is an urgent task. Owing to the similar d-band states as Pt-group metals, molybdenum-based compounds such as sulfide,11 carbide,12 nitride,13 and phosphide14 have recently been widely reported as effective catalysts for HER. Among them, molybdenum phosphide (MoP) nanostructures even in the bulk or © XXXX American Chemical Society

Received: February 1, 2018 Revised: April 24, 2018 Published: April 30, 2018 A

DOI: 10.1021/acssuschemeng.8b00529 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic View of the Synthesis Process for MoP@NPC-H Hollow Nanospheres

temperature, a precooled 20 mL aqueous solution of APS (8.4 mmol) was slowly added, and then the reaction was left for 12 h at 0 °C. The obtained polyaniline/pyrrole hybrids were washed with water several times. Afterward, the wet polyaniline/pyrrole hybrids were dispersed in a 50 mL aqueous solution containing (NH4)6Mo7O24·4H2O (0.4 mmol) and NaH2PO2·H2O (4 mmol) via ultrasonication for 30 min. The solution was dried at 80 °C, and then the obtained Mo-P polyaniline/pyrrole nanohybrids were annealed at 950 °C in a tube furnace for 5 h under an Ar atmosphere with a heating rate of 5 °C min−1. After cooling to room temperature naturally, the obtained products were washed with distilled water several times to remove impurities followed by drying in a vacuum at 60 °C, resulting in the final MoP@NPC-H. Synthesis of MoP Coated by Solid N,P-Codoped Carbon Spheres (MoP@NPC-S). For comparison and to obtain deep understanding of the relations between nanostructures and electrocatalytic activity of the MoP@NPC, MoP@NPC-S nanospheres were synthesized in the same way as for MoP@NPC-H except for adding (NH4)6Mo7O24·4H2O together with the aniline, pyrrole, and Triton X100 into the polyaniline/pyrrole hybrids. Synthesis of Reference Samples. For comparative purpose, Ndoped carbon spheres (NC) were prepared by pyrolysis of pure polyaniline/pyrrole hybrids with the same temperature program as MoP@NPC-H. N,P-codoped carbon spheres (NPC) and Mo2C@NC were also prepared via the same temperature program as MoP@NPCH without introducing (NH4)6Mo7O24·4H2O or NaH2PO2·H2O, respectively. Characterizations. X-ray diffraction (XRD) data were recorded on an X’Pert PRO MPD diffractometer with Cu Kα radation (λ = 1.54 Å). Raman spectra were recorded using a Renishaw inVia 2000 Raman spectrometer with a 514 nm excitation length. X-ray photoelectron spectroscopy (XPS) measurements were performed on a ThermoFisher Scientific II spectrometer using Al as the exciting X-ray source. The scanning electron microscope (SEM) measurements were carried out on a Hitachi S-4800 equipped with an energy-dispersive X-ray analyzer (EDX, Octane Ultra). The transmission electron microscopy (TEM) measurements were performed on an FEI Tecnai G2 with the accelerating voltage of 200 kV. The BET surface area and pore size were measured on a Tristar II Plus system at liquid N2 temperature. Electrochemical Measurements. The electrochemical measurements were conducted using a Gamry Reference 600 instrument in a three-electrode setup with a carbon rod as counter electrode and a saturated calomel electrode (SCE) as the reference electrode. All of the potentials in the measurements, measured against SCE reference electrode, were referred to the reversible hydrogen electrode (RHE) according to the Nernst eq (eq 1). The working electrode modified by catalyst was prepared as following: 5 mg of catalysts was ultrasonicated with 1 mL of water−ethanol (vwater:vethanol = 1:1) containing 20 μL of 5 wt % Nafion for 1 h. Subsequently, 5 μL of this solution was carefully loaded on a glassy carbon electrode with a diameter of 4 mm and dried in air. The linear sweep voltammetry (LSV) was performed in 0.5 M H2SO4, 1.0 M KOH, and 0.2 M phosphate buffer saline (PBS) at a scan rate of 5 mV s−1, respectively. Electric impedance spectroscopy (EIS) measurements were performed with frequency from 105 to 0.1 Hz at a potential of −0.15 V (vs RHE) with an AC amplitude of 5 mV. The double-layer capacitances (Cdl) were evaluated by CV curves at

also embed, disperse, and protect active nanoparticles from aggregation and erosion so that carbon-coated electrocatalysts could expose more active sites and maintain stability for a long time.24−26 Hollow carbon nanospheres with special morphology in particular possess advantages of large interior void space fraction, low density, and fast mass transport and are thus considered as an excellent support for the purpose of improving electrocatalyst activity and stability for HER.27−30 Moreover, heteroatoms such as nitrogen and phosphorus with different electronegativities doped into carbon supports can further tune electronic structures and enhance the activity through transfer of electrons from N or P to real active sites of adjacent C, thus weakening H-binding energy.31−33 On the other hand, Yin et al. reported that the rapid oxidation of P at high temperature could provide a strong driving force for the outward diffusion of P so that the hollow parts can be further enlarged to produce a larger surface area.34 Moreover, electrochemical reactions usually occur at the interface between a liquid electrolyte and a solid electrode; thus, induced phosphorization-derived welldispersed MoP nanoparticles encapsulated in highly hollow Ndoped carbon spheres are an ideal choice to enhance the HER performance. In this regard, preparing highly dispersed MoP NPs with induced phosphorization embedded in N,P-codoped highly hollow spheres is an ideal choice to improve the HER activity. On the basis of this consideration, we have prepared the MoP NPs encapsulated in N,P-codoped hollow nanospheres through the phosphorization reaction by employing inorganicorganic Mo-P/polyaniline-pyrrole as a precursor for the first time (Scheme 1). The synthesis route relies on the induced phosphorization reaction between the host polyaniline/pyrrole and guest Mo-P that lies in the pores of the hollow spheres. The as-prepared MoP@NPC-H requires an overpotential of 141 mV in acidic, 176 mV in alkaline, and 198 mV in neutral solution to drive a current density of 10 mA cm−2, respectively, which exhibits superior HER performance to that of the reference MoP@NPC-S electrocatalysts. The remarkable HER performance of MoP@NPC-H can be ascribed to the MoP NPs being well embedded in the hollow N,P-codoped carbon with a high specific surface area (122.4 m2 g−1) and the synergistic effect between the active MoP NPs and NPC. Moreover, the P dopants in NC can not only tune the electron structures but also be responsible for the formation of larger voids through induced phosphorization.

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EXPERIMENTAL SECTION

Synthesis of MoP Coated by Hollow N,P-Codoped Carbon Spheres (MoP@NPC-H). For synthesizing Mo-P polyaniline/pyrrole precursors, 4 mmol aniline, 4 mmol pyrrole, and 0.1 mmol Triton X100 were dissolved in 60 mL of distilled water. After magnetic stirring for 0.5 h and strong ultrasonication for another 0.5 h at room B


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ACS Sustainable Chemistry & Engineering various scan rates ranging from 40 to 120 mV s−1. The long-term stability tests were performed by CV from −0.3 to 0.1 V vs RHE at 100 mV s−1 for 1000 cycles and by chronoamperometry technique at −0.2 V vs RHE for 10 h.

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E(vs RHE) = E(vs SCE) + (0.059 V)pH + 0.243 V

corresponding to the (001), (100), (101), (110), (002), (111), (200), (102), and (201) planes of hexagonal MoP (PDF 03065-6487), respectively. The structural information on MoP@ NPC-H is also reflected from the Raman spectrum. As shown in Figure 1b, the strong peaks located at 1570 and 1363 cm−1 are assigned to the G and D bands of amorphous carbon relevant to the sp2-hybridized graphitic carbon and the defective carbon, respectively. The intensity ratio of ID/IG can reflect the disordered degree of the carbon matrix. The results show that ID/IG of MoP@NPC-H is 0.86, indicating good graphitization of the sample, which may be beneficial to improve the conductivity of the MoP@NPC-H. XPS analysis of the MoP@NPC-H composite has been tested for obtaining electronic states and elemental compositions. As illustrated in Figure 2a, the typical peaks of Mo, P, C, N, and O can be clearly detected. Notably, the existence of O element may be ascribed to the surface oxidation during the phosphorization process. Figure 2b shows that the Mo 3d profiles can be deconvoluted into three species. The fitting peaks centered at 228.4 and 231.9 eV in the Mo 3d region are typical peaks for MoP.35,36 Binding energies at 233.5 eV correspond to Mo 3d3/2 of MoO2.37 The binding energy doublet (Mo 3d3/2 and Mo 3d5/2) at 235.6 and 232.4 eV can be indexed to MoO3.38 This implies that the surface of the MoP@ NPC-H composite is contaminated with Mo oxides, which are inactive species toward HER. In the P 2p region (Figure 2c), the doublet peaks at 129.8 and 130.6 eV powerfully demonstrate the characteristic peaks of Mo bonded to P in the MoP.39 Moreover, the peak at 133.5 eV in the P 2p spectrum for the C−P bond implies the successful incorporation of P into carbon.40 Another peak at 134.2 eV represents the typical oxidation peak of phosphide, such as PO43− and H3PO3.41 In the C 1s region (Figure 2d), the peaks at 284.3, 284.8, 285.1, and 285.9 eV can be assigned to C−C/CC, C− O, C−P, and C−N species, respectively.42,43 The N 1s XPS spectrum (Figure 2e and f) exhibits three types of nitrogen species including pyridinic-N (398.2 eV), pyrrolic-N (399.7 eV), and quaternary-N (401.3 eV), which indicates that N is doped into the coated-carbon matrix.44,45 It is worth noting that P atoms with vacant 3d orbitals and N atoms with lone pair electrons introduced into the carbon matrix can largely tune the electronic structure of the catalyst.46,47 In this case, the incorporation of P and N into carbon layers may be favorable for HER performance of MoP@NPC-H. As a comparison, XPS analyses of NPC and Mo2C@NC are also displayed as shown in Figures S2 and S3. XPS results of NPC in Figure S2 show that N and P elements are successfully doped into the carbon matrix. For Mo2C@NC, Figure S3a shows that the Mo 3d profiles can be deconvoluted into three species including Mo2C, MoO2, and MoO3. As is consistent with MoP@NPC-H, MoO2 and MoO3 oxides are mainly derived from the oxidation during the heating process. C 1s and N 1s XPS spectrum in Figure S3b and c show that N elements are successfully doped into the carbon matrix. Furthermore, SEM and TEM images indicate that the asprepared MoP@NPC-H is assembled from the MoP NPs and N,P-codoped carbon. The SEM image in Figure S4a shows that the as-prepared Mo−P polyaniline/pyrrole precursor nanostructures exhibit a well-defined spherical-like morphology with an average diameter of 120 nm. Inheriting from the 0D Mo-P polyaniline/pyrrole, NC, NPC, Mo2C@NC, and MoP@NPCH remain with intact spherical morphology even after harsh thermal treatment, confirming the stable organic−inorganic

(1)

RESULTS AND DISCUSSION The crystalline diffraction patterns of all the samples are examined by XRD (Figure 1a and Figure S1a). NC and NPC

Figure 1. (a) XRD patterns of NPC, Mo2C@NC, and MoP@NPC-H and (b) Raman spectrum of MoP@NPC-H.

samples showing a similar broad peak at ∼24° are ascribed to the amorphous carbon matrix. The main peaks of Mo2C@NC at 34.5°, 38.0°, 39.4°, 52.2°, 61.7°, 69.7°, 74.9°, and 75.9° are in good agreement with (002), (020), (211), (221), (203), (231), (223), and (014) facets of orthorhombic Mo2C (PDF no. 01077-0720), and no other peaks can be detected, implying the high purity of active Mo2C structures. After introducing a phosphorus source heated at 950 °C in Ar flow, the hybrids of more active hexagonal MoP@NPC-H nanostructures are formed, which may be converted from Mo oxides with low crystallinity as intermediates (Figure S1b). As shown in Figure 1a, the typical sharp peaks at 27.9°, 32.0°, 43.0°, 57.1°, 57.7°, 64.8°, 67.0°, 67.6°, and 74.1° are observed for MoP@NPC-H C


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Figure 2. (a) XPS high-resolution scans for MoP@NPC-H in (b) Mo 3d, (c) P 2p, (d) C 1s, and (e) N 1s regions. (f) Schematic illustration of the three kinds of N dopants present in MoP@NPC-H based on the XPS analysis.

framework but with narrow particle size of ∼80 nm and more rough surface compared with that of Mo-P polyaniline/pyrrole precursors (Figure 3a and Figure S4b−d). The TEM image (Figure 3b) shows that MoP@NPC-H possesses a uniform hollow core 35 nm in diameter, implying that the shell thickness is ∼22 nm. Moreover, the HRTEM image (Figure 3c) indicates that the MoP composite is encapsulated within NPC and retains monodispersity without obvious aggregation. Carbon layers coating on the surface of MoP can not only protect the MoP NPs from erosion in solution but also accelerate the charge transfer rate. The d spacing of 0.28 nm can be ascribed to the (100) facet of MoP, which is highly consistent with the XRD results. The good dispersion of the MoP NPs derives from the confining effect of the NPC matrix

on the Mo−P units, which can prohibit the MoP NPs from aggregating during the phosphorization process. Other contrast samples for NC and NPC, obtained under the same thermal conditions, certainly possess the unique hollow spherical morphology (Figure 3d and e). However, the as-prepared NC with a shell thickness of ∼30 nm is displayed, which is more thick than MoP@NPC-H with a shell thickness of ∼22 nm. Furthermore, Mo2C@NC without adding phosphorus source exhibits a nearly solid nanospherical morphology, which may be due to the homogeneous diffusion of MoO42− into hollow interiors (Figure 3f). Notably, it can easily be seen from the TEM images that, when only the phosphorus source is added into the polyaniline/pyrrole solution, regular NPC hollow spheres with increased hollow interiors (∼50 nm in D


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Figure 3. (a) SEM, (b) TEM, and (c) high-magnification TEM images of MoP@NPC-H. TEM images of (d) NC, (e) NPC, and (f) Mo2C@NC. (g) EDX spectra of MoP@NPC-H. (h) TEM image and elemental mapping of Mo, P, C, N, and O for MoP@NPC-H.

EDX (Figure S5) and SEM mapping (Figure S6) images of prepared NPC and Mo2C@NC are also employed to confirm the uniform elements ratio and distribution, respectively. The HER electrocatalytic activity of MoP@NPC-H is first investigated by LSV in 0.5 M H2SO4 (Figure 4a). For comparison, the electrocatalytic activity of bare GCE, NC, NPC, Mo2C@NC, and commercial 20 wt % Pt/C are also measured under the same conditions. As expected, the Pt/C catalyst exhibits enhanced performance toward HER with an onset potential close to zero, but the bare GCE possesses no noticeable HER activity. Impressively, MoP@NPC-H shows dramatically superior HER performance compared with those of Mo2C@NC, NPC, and NC, implying the intrinsic activity of highly active MoP and synergistic effect between MoP and NPC. The overpotential driving a current density of 10 mA cm−2 for MoP@NPC-H is 141 mV, which is much lower compared to Mo2C@NC (233 mV), NPC (567 mV), NC (658 mV), and many of the currently active Mo- or P-based nonnoble metal electrocatalysts for the HER (Table S1). Moreover, NC shows an inferior onset potential compared with that of NPC. This result indicates that the introduction of P into NC can enhance the electrocatalytic activity. The Tafel slope is an important kinetic parameter in electrocatalysis, which is fitted from the polarization curve by

inner diameter and 15 nm in outer diameter) are seen. The formation of nanostructures with different hollow degrees may be due to the induced phosphorization. It is considered that the rapid oxidation of P at high temperature provides a strong driving force for the outward diffusion of P. Larger voids form during the reaction of MoO42− with P source in their surroundings, where the preferred diffusion of P source from core to the reaction interface results in the coalescence of larger vacancies inside the nanospheres. Thus, the introduction of a phosphorus source into polymer solution can not only tune the electron structures but also be responsible for the formation of highly hollow nanospheres, thus leading to a large surface area. Moreover, induced phosphorization for realizing the shift and good dispersion of MoP near to the surface of carbon nanospheres can further enhance the HER performance because the electrochemical reactions usually occur at the interface of a solid electrode and a liquid electrolyte. The EDX spectrum in Figure 3g demonstrates that the MoP@NPC-H composite is composed of Mo, P, C, N, and O elements, and the ratio of Mo/P is estimated to be 1:1, which is consistent with the composite of MoP. The TEM mapping image of MoP@NPC-H shows that Mo and P encapsulated in N-doped carbon nanospheres are well dispersed (Figure 3h), which may expose rich active sites for enhanced HER. For comparison, E


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Figure 4. (a) Polarization curves of GCE electrode, NC, NPC, Mo2C@NC, MoP@NPC-H, and Pt/C electrodes in 0.5 M H2SO4 with a scan rate of 5 mV/s. (b) Tafel plots of NC, NPC, Mo2C@NC, MoP@NPC-H, and Pt/C. (c) Nyquist plots of NC, NPC, Mo2C@NC, and MoP@NPC-H. (d) Polarization curves of MoP@NPC-H before and after 1000 CV sweeps. Inset: chronoamperometry i-t curve of MoP@NPC-H.

Hads + Hads → H 2

the Tafel equation (η = blog j + a, with a and b as the constants). Typically, three main reactions named as the Volmer (eq 2), Heyrovsky (eq 3), and Tafel reactions (eq 4) are supposed to elucidate the HER mechanism in acid listed in eqs 2−4.48 As shown in Figure 4b, the commercial Pt/C with the Tafel slope of 29 mV dec−1 is consistent with previously reported literature.49 The Tafel slope of MoP@NPC-H is 59 mV dec−1, implying a Volmer−Heyrovsky mechanism in which the electrochemical desorption is the rate-limiting step.50 Moreover, higher Tafel slopes of 93, 204, and 126 mV dec−1 are observed for Mo2C@NC, NPC, and NC, respectively, which is in accordance with their lower HER activity. EIS plots of all the samples are performed to provide further insight into the kinetics for the HER (Figure 4c). By fitting the equivalent circuit intercalated in Figure 4c, the charge-transfer resistance values (Rct) of all samples are shown in Table S2. It can be seen from the Nyquist plot and Rct value of samples that MoP@ NPC-H exhibits a much smaller semicircle and Rct value of 24.09 Ω, suggesting a faster charge transfer of MoP@NPC-H for the HER. The small resistance of MoP@NPC-H might be derived from the relatively high graphitization (ID/IG = 0.86). It can be clearly seen from Figure 3e that the P-dopants accelerate the graphitization of carbon, thus leading to higher conductivity. H3O+ + e− → Hads + H 2O

(2)

Hads + H3O + e− → H 2 + H 2O

(3)

(4)

The electrocatalytic durability is a key factor for practical applications. Thus, the stability of the MoP@NPC-H catalyst is investigated by measuring 1000 continuous CV between −0.3 and 0.1 V vs RHE with a scan rate of 100 mV dec−1. As shown in Figure 4d, the MoP@NPC-H shows no measurable loss of current density after 1000 sweeps in 0.5 M H2SO4, manifesting the excellent stability of MoP@NPC-H. Furthermore, the chronoamperometric curve of MoP@NPC-H at an overpotential of 200 mV vs RHE suggests that MoP@NPC-H maintains its current density after a 10 h test. Both results demonstrate that MoP@NPC-H possesses robust long-term electrochemical durability in acid. The excellent stability of MoP@NPC-H can be ascribed to the N,P-codoped hollow carbon encapsulation protecting the highly active MoP NPs from corrosion. Thus, N,P-codoped hollow graphitic carbon encapsulation can not only improve the conductivity but also maintain the excellent durability of MoP NPs. The performance of MoP@NPC-H in 1.0 M KOH and 0.2 M PBS electrolytes is further investigated. Notably, the obtained MoP@NPC-H also exhibits excellent electrocatalytic performance with an overpotential of only 176 mV for a current density of 10 mA cm−2 (Figure 5a), which is superior to many of the currently reported Mo- or P-based electrocatalysts for HER (Table S1). The Tafel slope of MoP@NPC-H is 61 mV dec−1 in 1 M KOH, suggesting the Volmer−Heyrovsky mechanism for HER (Figure 5b). MoP@NPC-H also exhibits F


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Figure 5. Polarization curves of GCE electrode, NC, NPC, Mo2C@NC, MoP@NPC-H, and Pt/C electrodes in (a) 1.0 M KOH and (e) 0.2 M PBS with a scan rate of 5 mV/s. Tafel plots of NC, NPC, Mo2C@NC, MoP@NPC-H, and Pt/C in (b) 1.0 M KOH and (f) 0.2 M PBS. Nyquist plots of NC, NPC, Mo2C@NC, and MoP@NPC-H in (c) 1.0 M KOH and (g) 0.2 M PBS. Polarization curves of MoP@NPC-H before and after 1000 CV sweeps in (d) 1.0 M KOH and (h) 0.2 M PBS and chronoamperometry i-t curve of MoP@NPC-H in (inset of d) 1.0 M KOH and (inset of h) 0.2 M PBS. G


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Figure 6. (a) XRD patterns of MoP@NPC-H and MoP@NPC-S. (b) Raman spectrum of MoP@NPC-S. (c) N2 adsorption−desorption isotherms of MoP@NPC-H and MoP@NPC-S. (d) SEM, (e) TEM, and (f) HRTEM images of MoP@NPC-S.

a much lower Rct value of 18.92 Ω, suggesting a faster charge transfer rate (Figure 5c and Table S2). Furthermore, MoP@ NPC-H retains a stable electrocatalytic property in 1 M KOH after continuous CV or chronoamperometric test (Figure 5d). In the neutral media, MoP@NPC-H exhibits enhanced HER performance with an overpotential of 198 mV to drive a current density of 10 mA cm−2 (Figure 5e and Table S1), a smaller Tafel slope of 94 mV dec−1 (Figure 5f), and a lower Rct (Figure 5g and Table S2). Moreover, MoP@NPC-H also retains favorable long-term stability as shown in Figure 5h. Overall, these relatively small overpotentials, low Tafel slopes, and Rct demonstrate that MoP@NPC-H is a favorable electrocatalyst in a wide pH range. As we know, hollow carbon structures with unique morphology possess charming properties such as larger surface area, lower density, and more rapid mass-transfer kinetics compared with those of solid nanostructures. Solid structure encapsulation in particular may result in mass loss due to the large amount of unexposed active sites, whereas mass loss can be minimized in a hollow counterpart.51 In this regard, we subsequently compare the MoP@NPC-H with MoP@NPC-S by facilely changing the order of MoO42− into solution in the synthesis process. As can be seen from Figure 6a, the XRD pattern for MoP@NPC-S is in good agreement with the standard crystalline phase of hexagonal MoP (PDF no. 03-0656487) and MoP@NPC-H, implying that the order in which MoO42− is added has no obvious effect on the formation of highly pure MoP. As shown in Figure 6b, the intensity ratio ID/ IG of MoP@NPC-S is 1.03. The larger ID/IG intensity as compared to that of MoP@NPC-H indicates less graphization and inferior conductivity of MoP@NPC-H. Benefitting from the hollow nanostructures, MoP@NPC-H retains a highly specific BET surface area of 122.4 m2 g−1 and 53.9 m2 g−1 for MoP@NPC-H measured by the N2 adsorption isotherms, as shown in Figure 6c. The insertion of MoP NPs into the hollow nanostructures reduces the relative BET surface area. The poresize distribution curve in Figure S7a and b displays a wide poresize distribution ranging from 2 to 60 nm, indicating

mesoporous characteristics for MoP@NPC-H and MoP@ NPC-H, respectively. The SEM image in Figure 6d suggests that MoP@NPC-S is also composed of uniform destructive nanospheres with a size of 80 nm, which is the same with MoP@NPC-H. The TEM image (Figure 6e) shows that MoP NPs with severe aggregation couples with carbon layers and hollow structures of carbon nanospheres have disappeared. The lower BET surface area of MoP@NPC-S could be ascribed to the disappearance of hollow structures compared with MoP@ NPC-H. The clear lattice fringes with d-spacing of 0.28 nm are assigned to the (100) plane of MoP in the HRTEM image in Figure 6f, which excludes the effect of crystalline structure on the HER performance. As shown by EDX spectra and SEM mapping images in Figures S8 and S9, the MoP@NPC-S including elements of Mo, P, C, N, and O are uniformly distributed in the whole carbon matrix. XPS also visualizes the elemental compositions of Mo, P, C, N, and O in line with the EDX mapping (Figure S10a), and the deconvolution of the Mo 3d and P 2p spectrum clearly shows the existence of MoP (Figure S10b and c). The C 1s and N 1s spectrum of MoP@ NPC-S (Figure S10d and e) is consistent with the results observed in Figure 2d and e for MoP@NPC-H, imply that the adding order of MoO42− has no obvious effect on the composition of MoP@NPC. The formation of two kinds of distinct nanostructures but with the same crystalline structure can be explained as follows: when MoO42− is added in the first step of participating polymerization, the strong coalescence between MoO42− and aniline/pyrrole limits the diffusion of MoO42−, and the driving force of P is not strong enough to extract effectively from the MoP to produce hollow structures. For the significance of constructing hollow nanostructures coating highly active MoP NPs to be further confirmed, the HER performances of MoP@NPC-H and MoP@NPC-S samples are measured over a wide pH range. The overpotentials of MoP@NPC-S in 0.5 M H2SO4, 1.0 M KOH, and 0.2 M PBS driving a current density of 10 mA cm−2 are 209, 257, and 286 mV, respectively, which is much higher than MoP@NPC-H driving the same current density (Figure 7a−c). The difference H


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Figure 7. Polarization curves of MoP@NPC-H and MoP@NPC-S electrodes in (a) 0.5 M H2SO4, (b) 1.0 M KOH, and (c) 0.2 M PBS. Tafel plots of MoP@NPC-H and MoP@NPC-S in (d) 0.5 M H2SO4, (e) 1.0 M KOH, and (f) 0.2 M PBS. The double-layer capacitance (Cdl) of the MoP@ NPC-H and MoP@NPC-S in (g) 0.5 M H2SO4, (h) 1.0 M KOH, and (i) 0.2 M PBS.

effect of the NPC shell on MoP, the stability tests of continuous CV for MoP@NPC-S in acidic, alkaline, and neutral solution are also conducted to make a comparison with MoP@NPC-H. As shown in Figure S13a−c, compared with MoP@NPC-H, the MoP@NPC-S suffers from severe performance degradation over a wide pH range, especially in neutral solution. The inferior stability of MoP@NPC-S may be ascribed to the MoP NP coupling with destructive nanospheres in which a large amount of MoP NPs would escape from interiors to the surfaces, so many MoP NPs derived from MoP@NPC-S lose the protection from NPC. Therefore, the selection of unique porous structures could ensure a large surface area, facilitate the charge and mass transfer, and improve the stability of the catalyst during the HER. On the basis of the experiments and analysis, the enhanced HER performance of MoP@NPC-H over a wide pH range can be attributed to the following explanations: (1) the polyanilinepyrrole precursor could also avoid the aggregation of MoP NPs and protect MoP from corrosion under harsh conditions, leading to the formation of well-dispersed MoP encapsulated by NPC. (2) The high specific surface area (122.4 m2 g−1) of the MoP@NPC-H is prone to exposing abundant active sites and

in performance between MoP@NPC-H and MoP@NPC-S is mainly attributed to the different morphologies of intact hollow nanostructures and irregular solid shapes, thus leading to the distinct exposure of MoP. The Tafel slopes of MoP@NPC-S are larger by 81, 71, and 120 mV dec−1 in acidic, alkaline, and neutral media, respectively, than those of MoP@NPC-H, suggesting the intrinsically inferior activities compared with those of MoP@NPC-H (Figure 7d−f). Panels a−c in Figure S11 exhibit the EIS data of MoP@NPC-H and MoP@NPC-S. The results are highly consistent with the LSV and Tafel slope results, suggesting a lower charge transfer rate of MoP@NPC-S. For the high surface area of hollow nanostructures to be further verified, the electrochemical surface area (ECSA) is determined from the electrochemistry double-layer capacitance (Cdl) (Figure S12a−c). As shown in Figure 7g, the Cdl values of MoP@NPC-H and MoP@NPC-S are 5.7 and 2.1 mF cm−2 in acid, respectively. The higher Cdl of MoP@NPC-H might be attributed to the P-modification to enlarge the hollow degree of carbon nanospheres. The results in alkaline and neutral media are in agreement with the results in acidic media, verifying the outstanding properties of constructing unique hollow structures (Figure 7h and i). Furthermore, for revealing the protection I


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accelerating mass transfer. (3) Induced phosphorizationderived well-dispersed MoP NPs are encapsulated by highly hollow N-doped carbon spheres benefiting for the electrochemical interfacial reaction. (4) Heteroatom doping (N, P) in the MoP@NPC-H catalyst, which optimizes the electronic configurations, thus enhances the HER performance. (5) The synergistic effects between MoP and N, P dopants boost excellent catalytic activity to the C atoms adjacent to N, P dopants in the carbon matrix and making MoP@NPC-H an efficient HER catalyst.

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CONCLUSIONS In summary, we have developed an induced phosphorization method for preparing well-dispersed MoP NPs encapsulated within N,P-codoped hollow carbon nanospheres as an excellent HER electrocatalyst. The precursors consisting of polyaniline/ pyrrole spheres and Mo-P sources are key for the formation of MoP@NPC-H. During the high-temperature phosphorization process, the induced phosphorization-derived well-dispersed MoP NPs are encapsulated by highly hollow N-doped carbon spheres. In addition, the enlarged hollow carbon spheres can also be obtained. The superior HER activity of as-prepared MoP@NPC-H may be derived from the well-dispersed MoP nanoparticles coated by highly N,P-codoped carbon layers and the synergistic effect between MoP nanoparticles and hollow NPC. Therefore, this strategy of induced phosphorization based on a polyaniline-pyrrole precursor may provide a facile method for well-dispersed MoP-based electrocatalysts for HER.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00529. XRD pattern, XPS spectra, SEM and elemental mapping images, EDX spectra, BJH pore-size distribution plots, Nyquist plots, CV polarization, and LSV curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; Tel: +86-532-86981376; Fax: +86532-86981787. ORCID

Bin Dong: 0000-0002-4817-6289 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Key Research and Development Program of China (No. 2017YFB0306600), Shandong Provincial Natural Science Foundation (ZR2017MB059 and ZR2016BL22), and the Fundamental Research Funds for the Central Universities (No. 17CX02061).

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Induced Phosphorization-Derived Well-Dispersed Molybdenum

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