MOF-Derived Formation of Ni2PâCoP Bimetallic Phosphides with

Article

MOF–Derived Formation of Ni2P–CoP Bimetallic Phosphides with Strong Interfacial Effect towards Electrocatalytic Water Splitting Xin Liang, Bingxia Zheng, Ligang Chen, Juntao Zhang, Zhongbin Zhuang, and Biao-Hua Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06152 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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ACS Applied Materials & Interfaces

MOF–Derived Formation of Ni2P–CoP Bimetallic Phosphides with Strong Interfacial Effect towards Electrocatalytic Water Splitting

Xin Liang,a, b Bingxia Zheng, a, b Ligang Chen,a, b Juntao Zhang,a, b Zhongbin Zhuang, a, b and Biaohua Chen*a a. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029 China. b. Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Beijing, 100029 China. [email protected] Abstract Bimetallic phosphides have attracted research interests for the synergistic effect and superior electrocatalytic activities for electrocatalytic water splitting. Herein, a MOF–derived phosphorization approach was developed to produce Ni2P–CoP bimetallic phosphides as bifunctional electrocatalysts for both hydrogen and oxygen evolution reactions (HER and OER). Ni2P–CoP shows superior electrocatalytic activities to both pure Ni2P and CoP towards HER and OER, revealing a strong synergistic effect. High resolution transmission electron microscopy (HRTEM) and energy dispersive X–ray spectroscopy (EDX) elemental mapping analysis show that in the sample Ni2P–CoP, the Ni2P and CoP nanoparticles with average particle size 10–20 nm were mixed closely in nanoscales, creating numerous Ni2P/CoP interfaces. By comparing with the sample Ni2P+CoP, in which seldom Ni2P/CoP interfaces exist, we documented that the Ni2P/CoP 1

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interface is an essential prerequisite to realize the synergistic effect and to achieve the enhanced electrocatalytic activities in Ni2P–CoP bimetallic phosphides. This founding is meaningful for designing and developing bicomponent and even multicomponent electrocatalysts.

Keywords: Interfaces; Synergistic effect; bi–metallic phosphides; water splitting; electrocatalysis

1. Introduction With the increase of global energy demand and the aggravation of environmental problems, hydrogen is considered as an ideal candidate for the replacement of fossil fuels. The electrocatalytic water splitting reaction is the most potential technology for large scale hydrogen production.1,

2

Hydrogen evolution reaction (HER: 2H+ + 2e− → H2) and oxygen evolution

reaction (OER: 4OH− → O2 + 4e− + 2H2O) are corresponding cathodic and anodic half reactions for the electrocatalytic water splitting, respectively.3-5 The HER and OER are also of great potentials in the fields of proton exchange membrane (PEM) fuel cell and metal–air batteries. Generally, Pt/C was considered as the most efficient HER catalysts,6 while the most efficient OER catalysts are precious metal oxides, such as IrO2 and RuO2.7 However, the high cost and scarcity limits the widespread use of these materials. Thus, developing earth–abundant HER and OER catalysts with low cost and high activity are of significance to overcome the drawbacks and to realize the widely practical applications of water splitting.8 Recently, transition metal phosphides including Ni2P and CoP, have attracted considerable research interests as a new family of promising water splitting electrocatalysts for their high activities and low costs.9, 10 For example, CoP–CNTs were reported that require an overpotential of 122 mV to obtain cathodic current density of 10 mA cm−2 towards HER in acidic medium.11 Ni2P polyhedrons were reported to show a small overpotential of 158 mV at current density of 10 mA cm−2 towards HER.12 CoP hollow polyhedron can serve as a bifunctional electrocatalysts to show an overpotential of 400 mV towards OER and 159 mV towards HER at a current density of 10 mA cm−2.13 To further achieve enhanced electrochemical activity, bimetallic phosphides were 2


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explored to construct highly active HER and OER activities for their composition diversity and possible synergistic effect. Brock et al. reported that the synthesis of monodisperse CoMnP nanoparticles, which show an overpotential of 330 mV at current density 10 mA cm−2 towards OER.14 Lin et al. reported that Co−Ni−P bimetallic phosphides shows better OER activity than pure CoP and Ni5P4/Ni2P.15 Li et al. synthesized Ni2P–CoP hybrid nanosheet arrays with enhanced HER activities.16 These reported works indicate that there are strong synergistic effect between Ni2P and CoP. Thus, investigating the influencing factors and maximizing the synergistic effect existing in Ni–Co bimetallic phosphides might be of significance for developing bifunctional water splitting electrocatalysts with enhanced performance. Metal–organic frameworks (MOFs), in which the metal atoms and organic ligands were connected by coordination bonds to form periodic structural units, are excellent precursors to produce electrocatalysts due to their high surface areas, mesoporous structure, and abundant organic ligands.17, 18 For example, Zheng et al. produced CoP and CoN porous catalysts by using ZIF–67.19 Sun et al. reported Co–P/N–doped carbon matrices as bifunctional water splitting catalysts by using ZIF–67 as precursors.20 However, the MOF–derived synthetic methods are mainly used to synthesize monometallic phosphide, and the investigations on MOF–derived bimetallic phosphides are limit. MOFs, which can mix different metal atoms in periodic structural units at atomic level by coordination bonds, should be ideal precursors to produce bimetallic phosphides.21 In this work, Ni2P–CoP bi–metallic phosphides have been synthesized by the low temperature phosphorization of the Ni–Co bimetallic–organic frameworks (NiCo–MOFs), in which benzenedicarboxylic acid acted as organic ligands to connect Ni and Co atoms together. The Ni2P–CoP catalysts, which shows an overpotential of 102 mV at mA cm−2 of for HER, and an overpotential of 320 mV at mA cm−2 for OER, is better than most of particulate CoP and Ni2P based catalysts. Compared with the monometallic phosphides, the enhanced electrocatalytic activities for both HER and OER demonstrated the strong synergistic effects in the bimetallic Ni– Co–P system. More importantly, this work demonstrates the advancement of Ni2P/CoP interfaces by comparing with the mechanical mixing sample Ni2P+CoP. 3



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2. Experimental Section Materials. Nickel acetoacetate, Cobaltous acetate tetrahydrate, poly vinylpyrrolidone, (PVP, MW=30000), benzenedicarboxylic

acid,

N,N–Dimethylformamide

(DMF),

deionized

water,

sodium

hypophosphite (NaH2PO2) were purchased from Beijing Chemical Reagent Factory. All chemical reagents were used without any further purification. Synthesis. For the synthesis of the NiCo–MOFs, 100 mg of nickel acetoacetate (Ni(acac)2), 74 mg of Cobaltous acetate tetrahydrate (the Ni:Co molar ratio was 4:3), 50 mg of benzenedicarboxylic acid, 400 mg of polyvinylpyrrolidone (PVP，Mw=30000) were dispersed in 80mL the mixture of dimethylformamide (DMF) and deionized water (VDMF:Vwater=5:3) with the assistant of magnetic stirring. Then, the mixture was transferred to an 80 mL Teflon–lined stainless–steel autoclave. The sealed vessel was sealed and heated at 150 oC for 6 h, and then it was cooled to room temperature naturally. The products were separated via centrifugation and further purified with DMF and deionized water for several times. The products were evaporated at 60 oC to get dry powder. For the preparation of Ni2P–CoP bimetallic phosphide, NiCo–MOFs and NaH2PO2 were placed at two separate positions at a quartz tube in the atmosphere of Ar, and NaH2PO2 was place at the upstream side of the quartz tube. The mass ratio for NiCo–MOFs to NaH2PO2 is 1:5. Then the quartz tube was heated to 350 oC with a ramp rate of 5 oC cm–1, and then maintained at this temperature for 3 h. After naturally cooling to room temperature, the sample was collected and washed with deionized water and ethanol for several times and dried in the oven at 60 oC for 12 h. The synthesis of monometallic CoP and Ni2P is identical with the synthesis of Ni2P–CoP, excepting that Ni–MOF and Co–MOF were used to replace NiCo–MOF as precursors. The synthesis of Ni–MOF and Co–MOF is similar with the synthesis of NiCo–MOF, excepting that 174 mg of nickel acetoacetate or 174 mg of cobaltous acetate tetrahydrate were used to replace the mixture of nickel acetoacetate and cobaltous acetate tetrahydrate in the beginning step for the synthesis of Ni-MOF and Co-MOF, respectively. For the synthesis of the sample of Ni2P+CoP, the synthesized Ni2P and CoP were mechanically 4


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mixed according to the Ni:Co molar ratio 4:3. Characterization. The powder X–ray diffraction (XRD) measurements were recorded on a Bruker D8 diffractometer with Cu Kα as the radiation source (λ=0.15406 nm). The morphologies of the samples were observed with the by Hitachi S4700 scanning electron microscope (SEM) with an accelerating voltage of 20 kV and a FEI Tecnai G2 20 transmission electron microscope (TEM) with an accelerating voltage of 200 kV. High resolution TEM (HRTEM) were carried out on a by a JEOL JEM-2010 TEM at 200 kV. The Energy dispersion spectra (EDS) were obtained from an attached Oxford Link ISIS energy-dispersive spectrometer fixed on the JEM-2010 electron microscope. Fourier transform infrared (FT–IR) spectroscopy was recorded on Bruker TENSOR27 FTIR Spectrometric Analyzer using KBr pellets. X–ray photoelectron spectroscopy (XPS) measurements were recorded on a Thermo Fisher ESCALAB 250 XPS system with a monochromatic Al Kα X–ray source. Electrochemical measurements. For the preparation of working electrodes, 5mg of Ni2P–CoP bimetallic phosphide powders and 2mg of conductive graphite were dispersed in 1 mL ethanol with 25µL 0.05 wt% Nafion. The mixture was then ultrasonicated for at least 30 min to generate a homogeneous ink. 10µL of the ink was drop–casted onto the glassy carbon electrode with the diameter of 5 mm. The as–prepared catalyst film was dried at room temperature. Electrochemical measurements were performed at room temperature by using CHI 660E electrochemical analyzer (CH Instruments, Inc., shanghai) and three-electrode setup with a saturated calomel electrode (SCE) as the reference electrode. The electrocatalytic activity of samples for OER was studied in 0.1 M KOH solution, and a platinum wire as the counter electrode. The electrocatalytic activity of samples for HER was studied in 0.5 M H2SO4 solution, and a carbon rod as the counter electrode. During the measures, electrolyte was bubbled with a steady oxygen or hydrogen flow for OER or HER measurements respectively. When the measurements were steady, the steady-state linear sweep voltammetry (LSV) Polarization curves were collected at a sweep rate of 5mV s−1. In our work, the potentials are reported vs.the reversible hydrogen electrode (RHE). Polarization curves were expressed as the overpotential (ƞ) 5



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vs. log current (log │j│) to obtain the Tafel plots. By fitting the linear portion of the Tafel plots to the Tafel equation (ƞ=b log (j) + a), the Tafel slope (b) was obtained. 3. Results and Discussions NiCo–MOFs were synthesized by a facile hydrothermal method, in which nickel acetylacetonate, cobaltous acetate, benzenedicarboxylic acid and PVP were mixed in the solvent of DMF and water. The morphological and structural properties of NiCo–MOFs were characterized in Figure 1. TEM and SEM images in Figure 1a–b shows that the NiCo–MOFs are comprised of large and thin nanosheets. These nanosheets show a quite large area with the average width 500 nm and the length over 5 µm, while the thickness of the nanosheets is less than 50 nm.

Figure 1. a–b) TEM and SEM images of NiCo–MOFs, c) XRD patterns of NiCo–MOF, the insert is the SAXS pattern of NiCo–MOFs d) FT–IR spectra of NiCo–MOF and pure benzenedicarboxylic acid

The crystal structure and phase purity of NiCo–MOFs were confirmed by the power XRD patterns in Figure 1c. The XRD patterns of NiCo–MOFs are in good agreements with the previous 6


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data of typical NiCo–MOFS reported by Tang et al.22 Similar to the Ni–based MOFs (no. 985792, Cambridge crystallographic Data Centre),23 NiCo–MOFs adopts a monoclinic layer–like crystal structure, in which [Ni/CoO4(OH)2] and [Ni/CoO2(OH)4] pseudo octahedrons edge/corn shared connected with each other to form 2D layers, and these 2D layers were connected by benzenedicarboxylic ligands to form the final 3D structure. Small angle X–ray scattering (SAXS) pattern insert in Figure 1c shows a strong peak at 2theta value at 8.81° corresponding to the (200) __

crystal planes, and three weak peaks corresponds to the (001), (201) and (201) crystal planes, in well accordance with the reported results and further confirming the crystal structure of NiCo– MOFs.22 Fourier Transform Infrared (FT–IR) spectrum of pure benzenedicarboxylic acid shows a strong peak at 1690 cm−1, which is attributed to the asymmetric stretching vibration of C=O bonds from the carboxylic acid group. Similar with some reported results,24 this asymmetric stretching vibration of C=O bonds in NiCo–MOFs red shifts to 1583 cm−1, and the strong peak at 1357 cm−1 corresponds to the symmetric stretching vibration of C=O bonds,23 demonstrating the coordination of Ni/Co ions with the benzenedicarboxylic acid linkers. In addition, FT–IR spectrum of NiCo– MOFs shows a sharp peak at 3605 cm−1, which can be assigned to the hydroxyl groups coordinated with metal ions, further confirming the coordination structure of NiCo–MOFs. NiCo–MOF, in which Ni and Co atoms were well mixed at atomic level, can serve as an ideal precursor for the fabrication of bimetallic phosphides. As shown in Figure 2a, the products after phosphorization are nanoparticles with particle size about 10–20 nm. The nanoparticles assembled to form a sheet–like array due to the confinement effect of NiCo–MOF nanosheets. XRD pattern in Figure 2b shows that the products were comprised of crystalline Ni2P and CoP, confirming the NiCo–MOFs were successfully converted to Ni2P–CoP bimetallic phosphides by the designed phosphorization process.

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Figure 2. a) TEM image and b) XRD pattern of Ni2P–CoP derived from the phosphorization of NiCo–MOFs TEM images in with high magnification in Figure 3a-b shows that the Ni2P and CoP nanoparticles were embedded at a thin film. Owing to the chemical composition of NiCo–MOF and the oxygen poor condition in the phosphorization process, the thin film can be well attributed to the non–crystalline carbon matrix formed by the organic ligands (benzenedicarboxylic acid) under the phosphorization process.25 To further illustrate the microstructure of the synthesized Ni2P–CoP, HRTEM analysis were performed. HRTEM image in Figure 3c shows the crystalline fringes with d spacing 0.507 nm and 0.283 nm corresponding to Ni2P (100) and CoP (011) planes, respectively, confirming the coexistence of Ni2P and CoP nanoparticles. HRTEM analysis also confirmed the Ni2P and CoP particles swaddled by the carbon matrix closely contacted with each other to form unique Ni2P/CoP interfaces. Selected area electron diffraction (SAED) pattern in Figure 3d shows typical polycrystalline diffraction rings, which are well indexed to Ni2P (111), (300), (400), CoP (211), (111) and (011) planes. The high–angle annular dark field scanning TEM (HAADF–STEM) in Figure 3e was used to investigate the elemental distributions of the Ni2P– CoP matrix. EDX elemental mapping profile in Fig. 3g–h proves that Ni, Co and P atoms evenly distribute in Ni2P–CoP matrix, further confirming the closely assembly of Ni2P and CoP nanoparticles.

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Figure 3. a–b) TEM image and c) HRTEM image of Ni2P–CoP, d) SAED pattern of Ni2P–CoP matrix, e–h) EDX elemental mapping analysis of Ni2P–CoP

Figure 4. a) Schematic of the fabrication of Ni2P–CoP matrix from NiCo–MOF, b) Schematic of the formation of the mechanical mixing Ni2P+CoP from Ni–MOF and Co–MOF, respectively. As the schematic in Figure 4a, the unique structures of NiCo–MOF play a key role on the formation of the Ni2P–CoP matrix. (i) In NiCo–MOF, the Ni and Co atoms connect with each other by organic ligands, and are mixed at an atomic level. Though two different crystal structures: hexagonal Ni2P and orthorhombic CoP, formed at the phosphorization process driven by the different formation energies, the synthesized Ni2P and CoP nanoparticles mixed with each other evenly, benefited from highly uniform distribution of Ni and Co atoms in the template. (ii) The benzenedicarboxylic ligands in NiCo–MOF contain benzene rings, which can readily convert to non–crystalline carbon matrix in the phosphorization process. The carbon matrix swaddles the 9



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Ni2P and CoP nanoparticles closely, thus plays a confined role in the synthesis process, and creating numerous Ni2P/CoP interfaces. (iii) The sheet–like morphology of NiCo–MOF also plays a role on the formation of Ni2P–CoP nanoparticles. The thin thickness of NiCo–MOF sheets (less than 50 nm) lead to the formation of Ni2P–CoP nanoparticles with small particle sizes (10–20 nm). And the formed Ni2P–CoP nanoparticles aggregates to form a sheet–like matrix for the geographical configuration of the NiCo–MOF templates. Compared to the bimetallic template (NiCo–MOF), Ni–MOF and Co–MOF with similar morphology and structure, are promising templates to fabricate monometallic phosphides (Ni2P and CoP). Figure 5 a–b shows TEM images of the Co–MOF and Ni–MOF, respectively. Both Co– MOF and Ni–MOF take on a sheet–like morphology, and convert to nanoparticles with size about 10–20 nm after phosphorization (Figure 5c–d). Careful observations reveal the nanoparticles deriving from Ni–MOF and Co–MOF were also swaddled by a thin film, like the Ni2P–CoP nanoparticles deriving from NiCo–MOF. XRD patterns in Figure 5e confirmed that the Ni–MOF and Co–MOF convert to crystalline Ni2P and CoP after phosphorization. Obviously, the MOF templates play a strong confinement effect on the synthesis of corresponding metal phosphides. The confinement effect makes the Ni2P–CoP nanoparticles deriving from NiCo–MOF contact and assemble closely in the carbon matrix. To further investigate the confinement effect, Ni2P nanoparticles and CoP nanoparticles were mechanically mixed the together according to the Ni:Co molar ratio 4:3. The obtain products were nominated as Ni2P+CoP. TEM image of Ni2P+CoP in Figure 6a. shows two aggregates of nanoparticles. EDX spectrum on aggregates A shows main peaks of Co and P, revealing that aggregates A were mainly comprised of CoP nanoparticles. Meanwhile, EDX spectrum on aggregates B confirmed there are mainly Ni2P nanoparticles. The TEM and EDX analysis confirmed that Ni2P nanoparticles and CoP nanoparticles were mixed at a micrometer level by mechanical mixing. CoP and Ni2P nanoparticles in Ni2P+CoP were separated by carbon matrix, thus there are mainly CoP/CoP and Ni2P/Ni2P interfaces, and seldom CoP/Ni2P interfaces in the sample of Ni2P+CoP. Compared with Ni2P+CoP, the sample of Ni2P–CoP have numerous Ni2P/CoP interfaces, which might be of advantages for the possible Ni2P and CoP synergistic effect.

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Figure 5. TEM images of a) Co–MOF, b) Ni–MOF and corresponding c) CoP nanoparticles and d) Ni2P nanoparticles, respectively, e) XRD patterns of Ni2P and CoP nanoparticles deriving from Ni–MOF and Co–MOF, respectively.

Figure 6. a) TEM image of Ni2P+CoP, b) Enlarged TEM image and c) EDS spectrum of part A, d) Enlarged TEM image and e) EDX spectrum of part B.

Figure 7. XPS spectra of a) Ni 2p, b) Co 2p and c) P 2p performed on NiCo–MOF, Ni2P, CoP, 11



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Ni2P+CoP and Ni2P–CoP.

To investigate the surface states and electron interactions of the samples, the X–ray photoelectron spectroscopy (XPS) of NiCo–MOF, Ni2P, CoP, Ni2P+CoP and Ni2P–CoP were carried out. Figure 7a shows the Ni 2p core level spectra of NiCo–MOF, Ni2P, Ni2P+CoP and Ni2P–CoP. The Ni 2p3/2 spectrum of NiCo–MOF shows a peak at binding energy (B. E.) 857.2 eV along with a shakeup satellite signal, which are ascribed to carboxyl bonded Ni species. No other peaks were observed, implying all Ni atoms are coordinated with benzenedicarboxylic acid in NiCo–MOF. For Ni2P and Ni2P+CoP, a sharp peak at B. E. 853.4 eV were observed, which are ascribed to the Niδ+ (0Ni2P+CoP>Ni2P. Ni2P–CoP shows a strong synergistic effect for both OER and HER, while the performance of Ni2P+CoP were between CoP and Ni2P. The onset potential of Ni2P–CoP is as small as ~1.50 V vs RHE, and require a small η of 320 mV to afford a current density of 10 mA cm−2, which is smaller than 367 mV of Ni2P, 348 mV of CoP, and 360 mV of Ni2P+CoP, and also smaller than many other reported electrocatalysts, such as cobalt phosphorous derived film (354 mV),31 CoP hollow polyhedron (400 mV),13 CoMnP (330 mV),14 CoNiP nanocrystals (340 mV)32 and Ni2P (330 mV).33 Table 1. Summary of the HER and OER catalytic activities of the catalyst. Ni2P–CoP

Ni2P+CoP

14


Ni2P

CoP

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Overpotential at 2 mA/cm2 for HER (mV) Overpotential at 10mA/cm2 for HER (mV) Tafel slope for HER (mV/dec) Overpotential at 10mA/cm2 for OER (mV) Tafel slope for OER (mV/dec)

55 105 64 320 69

105 150 65 360 71

89 137 67 367 72

135 184 66 348 63

More importantly, the studies demonstrated that Ni2P–CoP exhibits superior electrocatalytic activities than pure Ni2P and CoP for both HER and OER reactions, while no such synergistic effect were observed in Ni2P+CoP, proving the Ni2P–CoP interfaces were the main factor leading to the high HER and OER activies. This unique interfacial effects can be explained from the following points: (i) Electron transportation is essential for electocatalysis.26 In Ni2P–CoP, Ni2P and CoP nanoparticles compacted with each other closely, thus creating numerous Ni2P–CoP interfaces. The interfaces provide electron transport channel between Ni2P and CoP, ensuring a prerequisite for the possible synergistic effect. (ii) as XPS analysis documented, Ni2P–CoP adopts a higher electron density around Co, Ni and P atoms. (iii) the Ni2P–CoP interfacial interactions might lower the bandgap, and favor the electron transportation, therefore bring a synergistic effect. To further prove the importance of the Ni2P/CoP interfaces for the enhanced electrocatalytic activities, we further synthesized the Ni doped CoP and investigated its electrocatalytic activities (Figure S2 in supporting information). For the sample Ni doped CoP, the Ni atoms were doped into the crystal structure of orthorhombic CoP. The Ni and Co atoms can be viewed as mixing at atomic level in Ni doped CoP. As shown in Figure S4 in supporting information, Ni doped CoP requires an overpotential of 140 mV for HER and an overpotential 358 mV for OER to obtain the current density 10 mA cm‒2. Obviously, Ni doped CoP shows worse electrocatalytic activities than Ni2P-CoP. The comparison further confirmed the synergistic effect between Ni2P and CoP, and the importance of Ni2P/CoP interfaces. 4. Conclusions In summary, Ni2P–CoP bimetallic phosphides were synthesized through a MOF derived phosphorization approach, and this material exhibited high electrocatalytic activity towards water splitting, which shows an overpotential of 105 mV for HER and 320 mV for OER to achieve a current density 10 mA cm−2, respectively. The Ni2P–CoP interface was found to be important for 15



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the electrochemical catalysis, which exhibiting a strong synergistic effect. This work demonstrates that the interfaces in the materials are important for catalysis and enhanced catalytic activity can be achieved by controlled formation of interfaces.

Acknowledgments This work was financially supported by National Natural Science Foundation of China (NSFC) (Major Program 91534201, and General Program No. 21571012, 21476012)

Supporting Information Elemental contents data determined by XPS analysis, EDX spectra and TEM images of Ni2P–CoP, and XRD pattern, TEM image and electrocatalytic activities of Ni doped CoP.

References

1.

Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H., Co3O4 Nanocrystals on

Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 2011 10, 780-786. 2.

Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H., MoS2 Nanoparticles Grown on

Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 2011 133, 7296-7299. 3.

Gong, M.; Wang, D.-Y.; Chen, C.-C.; Hwang, B.-J.; Dai, H., A Mini Review on

Nickel-Based Electrocatalysts for Alkaline Hydrogen Evolution Reaction. Nano Res. 2016, 2016 9, 28-46. 4.

Wang, H.; Tsai, C.; Kong, D.; Chan, K.; Abild-Pedersen, F.; Norskov, J. K.; Cui, Y.,

Transition-Metal Doped Edge Sites in Vertically Aligned MoS2 Catalysts for Enhanced

16


Page 16 of 22

Page 17 of 22

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


Hydrogen Evolution. Nano Res. 2015, 2015 8, 566-575. 5.

Liu, S.; Zhang, X.; Zhang, J.; Lei, Z.; Liang, X.; Chen, B., MoS2 with Tunable Surface

Structure Directed by Thiophene Adsorption toward Hds and Her. Sci. China Mater. 2016, 2016 59, 1051-1061. 6.

Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J.

A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R., Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 2014 343, 1339-1343. 7.

Pei, J.; Mao, J.; Liang, X.; Chen, C.; Peng, Q.; Wang, D.; Li, Y., Ir-Cu Nanoframes:

One-Pot Synthesis and Efficient Electrocatalysts for Oxygen Evolution Reaction. Chem.

Commun. 2016, 2016 52, 3793-3796. 8.

Saadi, F. H.; Carim, A. I.; Verlage, E.; Hemminger, J. C.; Lewis, N. S.; Soriaga, M. P., Cop

as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance Evaluation. J. Phys. Chem. C 2014, 2014

118, 29294-29300. 9.

Zhu, Y.-P.; Ren, T.-Z.; Yuana, Z.-Y., Mesoporous Phosphorus-Doped G-C3N4

Nanostructured Flowers with Superior Photocatalytic Hydrogen Evolution Performance. ACS

Appl. Mater. Interfaces 2015, 2015 7, 16850-16856. 10. Pu, Z.; Liu, Q.; Asiri, A. M.; Sun, X., Tungsten Phosphide Nanorod Arrays Directly Grown on Carbon Cloth: A Highly Efficient and Stable Hydrogen Evolution Cathode at All Ph Values.

ACS Appl. Mater. Interfaces 2014, 2014 6, 21874-21879. 11. Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X., Carbon Nanotubes 17



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Decorated

with

Cop

Nanocrystals:

A

Highly

Active

Non-Noble-Metal

Page 18 of 22

Nanohybrid

Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 2014 126, 6828-6832. 12. Yan, L.; Dai, P.; Wang, Y.; Gu, X.; Li, L.; Cao, L.; Zhao, X., In Situ Synthesis Strategy for Hierarchically Porous Ni2p Polyhedrons from Mofs Templates with Enhanced Electrochemical Properties for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2017, 2017 9, 11642-11650. 13. Liu, M.; Li, J., Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional Electrocatalysts for the Evolution Reaction of Hydrogen and Oxygen. ACS Appl. Mater.

Interfaces 2016, 2016 8, 2158-2165. 14. Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L., Efficient Water Oxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 2016 138, 4006-4009. 15. Fu, S.; Zhu, C.; Song, J.; Engelhard, M. H.; Li, X.; Du, D.; Lin, Y., Highly Ordered Mesoporous Bimetallic Phosphides as Efficient Oxygen Evolution Electrocatalysts. ACS

Energy Letters 2016, 2016 1, 792-796. 16. Wang, A.-L.; Lin, J.; Xu, H.; Tong, Y.-X.; Li, G.-R., Ni2p–Cop Hybrid Nanosheet Arrays Supported on Carbon Cloth as an Efficient Flexible Cathode for Hydrogen Evolution. J. Mater.

Chem. A 2016, 2016 4, 16992-16999. 17. Yang, J.; Zhang, F.; Wang, X.; He, D.; Wu, G.; Yang, Q.; Hong, X.; Wu, Y.; Li, Y., Porous Molybdenum Phosphide Nano-Octahedrons Derived from Confined Phosphorization in Uio-66 for Efficient Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 2016 55, 12854-12858. 18. Tao, Z.; Wang, T.; Wang, X.; Zheng, J.; Li, X., Mof-Derived Noble Metal Free Catalysts for Electrochemical Water Splitting. ACS Appl. Mater. Interfaces 2016, 2016 8, 35390-35397. 19. Weng, B.; Wei, W.; Yiliguma, Y.; Wu, H.; Alenizi, A. M.; Zheng, G., Bifunctional Cop and 18


Page 19 of 22

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


Con Porous Nanocatalysts Derived from Zif-67 in Situ Grown on Nanowire Photoelectrodes for Efficient Photoelectrochemical Water Splitting and Co2reduction. J. Mater. Chem. A 2016, 2016 4, 15353-15360. 20. You, B.; Jiang, N.; Sheng, M.; Gul, S.; Yano, J.; Sun, Y., High-Performance Overall Water Splitting Electrocatalysts Derived from Cobalt-Based Metal–Organic Frameworks. Chem.

Mater. 2015, 2015 27, 7636-7642. 21. Zhang, T.; Du, J.; Xi, P.; Xu, C., Hybrids of Cobalt/Iron Phosphides Derived from Bimetal-Organic Frameworks as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2017, 2017 9, 362-370. 22. Zhao, S.; Wang, Y.; Dong, J.; He, C.-T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z., Ultrathin Metal–Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution. Nat. Energy 2016, 2016 1, 16184. 23. Mesbah, A.; Rabu, P.; Sibille, R.; Lebegue, S.; Mazet, T.; Malaman, B.; Francois, M., From Hydrated Ni3(OH)2(C8H4O4)2(H2O)4 to Anhydrous Ni2(OH)2(C8H4O4): Impact of Structural Transformations on Magnetic Properties. Inorg. Chem. 2014, 2014 53, 872-881. 24. Zhang, Z.; Chen, Y.; He, S.; Zhang, J.; Xu, X.; Yang, Y.; Nosheen, F.; Saleem, F.; He, W.; Wang,

X.,

Hierarchical

Zn/Ni-MOF-2 Nanosheet-Assembled

Hollow

Nanocubes for

Multicomponent Catalytic Reactions. Angew. Chem., Int. Ed. 2014, 2014 53, 12517-12521. 25. Zhao, D.; Shui, J.-L.; Grabstanowicz, L. R.; Chen, C.; Commet, S. M.; Xu, T.; Lu, J.; Liu, D.-J., Highly Efficient Non-Precious Metal Electrocatalysts Prepared from One-Pot Synthesized Zeolitic Imidazolate Frameworks. Adv. Mater. 2014, 2014 26, 1093-1097. 19



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

26. Liang, X.; Liu, B.; Zhang, J.; Lu, S.; Zhuang, Z., Ternary Pd-Ni-P Hybrid Electrocatalysts Derived from Pd-Ni Core-Shell Nanoparticles with Enhanced Formic Acid Oxidation Activity.

Chem. Commun. 2016, 2016 52, 11143-11146. 27. Yang, X.; Lu, A.-Y.; Zhu, Y.; Hedhili, M. N.; Min, S.; Huang, K.-W.; Han, Y.; Lin, L.-J., Cop Nanosheet Assembly Grown on Carbon Cloth: A Highly Efficient Electrocatalyst for Hydrogen Generation. Nano Energy 2015, 2015 15, 634-641. 28. Wang, J. L.; Yang, Q.; Zhang, Z. D.; Sun, S. H., Phase-Controlled Synthesis of Transition-Metal Phosphide Nanowires by Ullmann-Type Reactions. Chem.-Eur. J. 2010, 2010 16, 7916-7924. 29. Benito, N.; Palacio, C., Growth of Ti–O–Si Mixed Oxides by Reactive Ion-Beam Mixing of Ti/Si Interfaces. J. Phys. D: Appl. Phys. 2014, 2014 47, 015308. 30. Dong, G.; Fang, M.; Wang, H.; Yip, S.; Cheung, H.-Y.; Wang, F.; Wong, C.-Y.; Chu, S. T.; Ho, J. C., Insight into the Electrochemical Activation of Carbon-Based Cathodes for Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 2015 3, 13080-13086. 31. Jiang, N.; You, B.; Sheng, M.; Sun, Y., Electrodeposited Cobalt-Phosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem., Int. Ed. 2015, 2015 54, 6251-6254. 32. Liang, Q.; Huang, K.; Wu, X.; Wang, X.; Ma, W.; Feng, S., Composition-Controlled Synthesis of Ni2-XCoxP Nanocrystals as Bifunctional Catalysts for Water Splitting. RSC Adv. 2017, 2017 7, 7906-7913. 33. Menezes, P. W.; Indra, A.; Das, C.; Walter, C.; Goebel, C.; Gutkin, V.; Schmeisser, D.; Driess, M., Uncovering the Nature of Active Species of Nickel Phosphide Catalysts in 20


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High-Performance Electrochemical Overall Water Splitting. ACS Catal. 2017, 2017 7, 103-109.

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89x60mm (300 x 300 DPI)


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MOF-Derived Formation of Ni2PâCoP Bimetallic Phosphides with

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