Phosphonium-Based Ionic Liquid: A New ... - ACS Publications

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

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Phosphonium-Based Ionic Liquid: A New Phosphorus Source toward Microwave-Driven Synthesis of Nickel Phosphide for Efficient Hydrogen Evolution Reaction Chenyun Zhang,† Bingwei Xin,‡ Zhucong Xi,§ Baohua Zhang,† Zhuoyu Li,† Hong Zhang,† Zhonghao Li,*,† and Jingcheng Hao† †

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, No. 27, Shanda Nanlu, Jinan 250100, China ‡ College of Chemistry and Chemical Engineering, Dezhou University, No. 566 West University Road, Decheng District, Dezhou 253023, China § School of Materials Science and Engineering, Shandong University, No. 17923, Jingshi Road, Jinan 250061, China S Supporting Information *

ABSTRACT: Employing phosphonium-based ionic liquid, tetrabutylphosphonium chloride [P4444]Cl as novel phosphorus source and reaction medium, a facile approach for fabricating nanostructured Ni2P and Ni12P5 was developed upon microwave heating in 1−2 min or conventional heating at 350 °C for 3 h. In a microwave-driven approach, controlling counteranions of various nickel salts could conveniently tune the phase of as-synthesized nickel phosphides. Ni(acac)2 and Ni(OAc)2·4H2O as Ni source could yield Ni2P nanoparticles, while NiCl2·6H2O and NiSO4·7H2O offered Ni12P5 nanocrystals. The synthesized products were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Their electrocatalytic behavior toward hydrogen evolution reaction in acidic medium was investigated. The assynthesized Ni2P nanoparticles presented more excellent catalytic efficiency than Ni12P5. Ni2P nanoparticles from Ni(acac)2 require overpotentials of only 102 mV to reach 10 mA cm−2 with a small Tafel slope of 46 mV dec−1, showing its best activity among those tested catalysts. The present novel ionic liquid-mediated strategy for the synthesis of nickel phosphide provides the remarkable advantage of operating in very short time by microwave heating, which is of particular interest from the viewpoint of energysaving, fast synthesis, and easy operation. KEYWORDS: Ionic liquid, Tetrabutylphosphonium chloride, Nickel phosphide, Microwave heating, Hydrogen evolution reaction

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INTRODUCTION

red phosphorus, PH3, NaH2PO2, Ni(HPO3H)2, and trioctylphosphine (TOP), etc., were used as P sources. However, most of these syntheses are undergone at high temperature and/or high pressure involving the toxic starting materials or complicated steps.11−15 Moreover these synthesis processes are usually energy-consuming and time-consuming. Therefore, there is a high demand to realize energy-savings and fast synthesis of nickel phosphides by the use of novelly handy P sources. Nowadays, ionic liquids (ILs) have become a hot research topic among multidisciplinary areas, including chemistry, physics, biology, engineering, and so on, and have radically changed the concept of the nature of liquids.16,17 Owing to special physicochemical properties, the ionic liquids provide a

The hydrogen evolution reaction (HER) has become a center of attraction in the search for alternative, clean, and sustainable energy sources, and exploring efficient catalysts for this reaction has been a research focus. Transition metal phosphide nanostructured materials are promising alternative of Pt electrocatalysts for HER because they have been demonstrated to be one kind of the most efficient and stable HER electrocatalysts.1−3 Specifically, nickel phosphide nanocrystals are typical representatives among non-noble metal electrocatalysts because of their low cost, high abundance, and high catalytic activity.4 Generally, nickel phosphides exist in various phases, such as Ni3P, Ni2P, Ni5P2, Ni12P5, and Ni5P4, and their formation is affected by many factors, including P/Ni ratios, reaction temperatures, times, and so on.5−8 Different phases and morphologies exhibit different electrocatalytic performance.9,10 During the past 20 years, many synthetic methods of nickel phosphides were developed, in which flammable white or © XXXX American Chemical Society

Received: October 30, 2017 Revised: November 27, 2017

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DOI: 10.1021/acssuschemeng.7b03954 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. XRD pattern (A), typical low- and high-magnification TEM images (B, C) and particle size distribution (D) of products from the reaction between [P4444]Cl and Ni(acac)2 upon microwave heating.

excellent electrocatalytic activity than Ni12P5, in which Ni2P_Ni(acac)2 nanoparticles exhibit superior electrocatalytic performance while affording long-term cycle stability.

useful platform for fabricating various nanostructured materials and catalysts, which sometimes are difficult to realize by conventional solvents.18−20 Zhao21 et al. used eutectic mixtures with similar properties to ionic liquids for synthesizing Ni2P nanoparticles from nickel hypophosphite. To date, most of the studies have focused on the imidazolium ILs; however, phosphonium-based ILs are found to exhibit more superiority in organic synthesis, CO2/CO capture, and nanomaterial synthesis than imidazolium ILs in some reactions.22−29 Meanwhile, phosphonium-based ILs, similar to the imidazolium ILs, have strong microwave absorbing ability, and microwavedriven ionothermal synthesis is an alternative approach to offer various opportunities for the synthesis of inorganic nanomaterials.25,30 For example, Ding25 et al. successfully synthesized micro-/nanostructured metal chalcogenides upon microwave heating using phosphonium ILs as solvent. But, so far, to the best of our knowledge, little research has been studied on phosphonium-based ILs acting as a combination of solvent and reactant precursor to synthesize transition metal phosphides. ILs are not only well-known benign solvent and structural templates but also potential precursors to micro-/nanostructured materials. In this work, we employ a tetraalkylphosphonium chloride ionic liquid [P4444]Cl as both reaction medium and novel P source to synthesize nickel phosphide nanocrystals upon microwave radiation for 1−2 min or conventional heating at 350 °C for 3 h. The products from nickel acetylacetonate, nickel acetate, nickel sulfate, and nickel chloride are assigned to Ni2P_Ni(acac)2, Ni2P_Ni(OAc)2, Ni12P5_NiSO4, and Ni12P5_NiCl2, respectively. In the microwave-driven approach, the counteranions of Ni(II) salts are ideal factors to adjust the phase and morphology of nickel phosphides, and then will vary their electrocatalytic properties for HER. The as-synthesized Ni2P nanoparticles present more

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EXPERIMENTAL SECTION

Materials. Tetrabutylphosphonium chloride ([P4444]Cl) was bought from Lanzhou Greenchem, ILS, LICP, CAS, China. Nickel acetylacetonate (Ni(acac)2; purity, 95%) was purchased from Aladdin industrial Corp. Nickel sulfate (NiSO4·7H2O), nickel chloride hexahydrate (NiCl2·6H2O), and nickel acetate (Ni(OAc)2·4H2O) (purity ≥ 98%) were purchased from Sinopharm Pharmaceutical Co Ltd. Johnson-Matthey 20 wt % Pt/C was bought from Alfa Aesar. Nafion solution (5 wt %) was bought from Sigma-Aldrich. All aqueous solutions were prepared with deionized water. Synthesis of Nickel Phosphide Nanocrystals in Microwave Oven. A series of nickel phosphides have been synthesized by phosphonium ionic liquid. In a typical synthesis process, 0.195 mmol of Ni(II) salt was added to 0.5 g (1.69 mmol) of [P4444]Cl in a test tube and was sonicated in an ultrasonic bath for 10 min to obtain a uniform mixture. The test tube was put in the microwave oven (Midea EG72-0EA2-PS). Heating for the indicated time at 50 W (different Ni(II) salts need different radiated times, in which Ni(acac)2 was irradiated for 1 min, Ni(OAc)2·4H2O for 1.5 min, NiCl2·6H2O for 2 min, and NiSO4·7H2O for 1 min 50 s, and those materials were turned to black. After cooling to room temperature naturally, the obtained products were washed with ethanol and water for several times to remove the organic impurities and byproducts and were isolated by centrifugation (10000 rpm). Black precipitates were then obtained and put in a vacuum for 1 day at room temperature. The as-synthesized nickel phosphide nanoparticles are defined as Ni2P_Ni(acac)2, Ni2P_Ni(OAc)2, Ni12P5_NiCl2, and Ni12P5_NiSO4, respectively. The yield of nickel phosphide nanoparticles based on the various Ni salts by microwave heating is provided in Table S1 (Supporting Information). Synthesis of Nickel Phosphide Nanocrystals in Tube Furnace. A 0.195 mmol amount of Ni(acac)2 was added to 0.5 g of [P4444]Cl in a test tube and sonicated in an ultrasonic bath for 10 min. B


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Scheme 1. Possible Mechanism of [P4444]Cl as Phosphorus Source in the Microwave-Driven Preparation of Nickel Phosphide Nanoparticlesa

a

NPs is the abbreviation of nanoparticles.

Figure 2. XPS spectra of the Ni 2p (A) and P 2p (B) regions for Ni2P_Ni(acac)2 nanocrystals. Then the bottle was transferred to a tube furnace and thoroughly flushed with N2 gas for 30 min. The temperature increased linearly at a ramp of 5 °C/min to 350 °C. The reaction was held at 350 °C for 3 h. The black products were obtained. The product was washed thoroughly with ethanol and water for several times. Finally, black precipitates were then obtained and put in a vacuum for 1 day at room temperature. Synthetic methods of other Ni(II) salts were the same as that for Ni(acac)2. Material Characterization. X-ray diffraction (XRD) characterization was performed on a Rigaku Dmax-rc X-ray diffractometer. Transmission electron microscopy (TEM) characterization was examined on a JEM 1400 TEM. X-ray photoelectron spectroscopy (XPS) measurement was conducted with a photoelectron spectrometer ESCALAB 250. Electrochemical Measurements. The electrochemical measurements were conducted by an electrochemical workstation (CHI model 760E) in a standard three-electrode setup. In our experiment, we used a saturated calomel electrode (SCE) as reference electrode, graphite rod as counter electrode, and a glassy carbon electrode (GCE, 3 mm in diameter) as working electrode. The as-synthesized products were modified on a GCE at a mass loading of 0.35 mg cm−2. The modified electrodes were dried in a vacuum at room temperature. After that, 0.2% Nafion was modified on the GCE, drying in a vacuum at room temperature. All date were gotten in 0.5 M H2SO4 with respect to the reversible hydrogen electrode (RHE). iR compensation was performed. Linear sweep voltammetry (LSV) was obtained by sweeping at a potential sweep rate of 5 mV s−1. The onset potentials were given from the beginning of the linear regime in Tafel plot. A stability test was carried out in 0.5 M H2SO4 by cyclic voltammograms (CV) with 50 mV s−1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) characterizations were tested at overpotential η = 200 mV (vs RHE) with the frequency range of 0.01 Hz to 100 kHz.

The electrochemically active surface areas (ECSAs) of nickel phosphide were obtained by CV which could measure the doublelayer capacitance from 0.312 to 0.412 V (vs RHE) using scanning rate of 10−100 mV s−1. The current density−time (I−t) study was performed at overpotential of 110 mV for 12 h.

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RESULTS AND DISCUSSION Microwave as a type of electromagnetic wave can result in a homogeneous temperature within samples by the way of polarization of the electromagnetic waves on polar molecules. Herein, using Ni(acac)2 as the model Ni source, we employed Ni(acac)2 and [P4444]Cl to synthesize nickel phosphide upon microwave radiation. In a typical process, the mixture of [P4444] Cl and Ni(acac)2 was sonicated to uniformity, and then the reaction was performed for 1 min upon 50 W microwave radiation. The crystalline phase structure of as-synthesized black product nanoparticles was characterized by XRD pattern, as shown in Figure 1A. The main diffraction peaks’ positions clearly appear around 40.8°, 44.6°, 47.3°, 54.2°, 54.9°, and 74.7°, which correspond to the (111), (201), (210), (300), (211), and (400) crystal faces, respectively. All of the diffraction peaks match very well with hexagonal Ni2P (JCPDS No. 030953), suggesting the successful formation of pure hexagonal Ni2P. These results unambiguously demonstrate that [P4444]Cl can act as an efficient P source for the formation of Ni2P. Panels B and C of Figure 1 show the typical low- and highmagnification TEM images of as-prepared Ni2P nanocrystals, respectively. Obviously, small spherical particles with uniform size were formed. The size of the particles is measured to be C


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Figure 3. XRD patterns and the typical low- and high-magnification TEM images for NixPy from different Ni(II) salts reacted with [P4444]Cl upon microwave heating. XRD (A) and TEM (B, C) for Ni2P_Ni(OAc)2; XRD (D) and TEM (E, F) for Ni12P5_NiCl2; XRD (G) and TEM (H, I) for Ni12P5_NiSO4.

shown in Figure S2 (Supporting Information). Based on the results, it can be seen that the other elements are negligible and the mole ratio of Ni to P is 2, which agrees well with Ni2P. As a unique P source in our system, [P4444]Cl should take part in reaction due to decomposition upon microwave radiation during the synthesis of nickel phosphide. Many studies have explored that some ionic liquids are not stable in high temperature. 31 The decomposition of quaternary phosphoniums has been found in many reactions using both traditional heating and microwave radiation. In 2007, Tseng et al.32 found that reactions of trihexyl(tetradecyl)phosphonium chloride and substituted sodium benzoates afforded aryl ketones via microwave heating 30 W at 180 °C, in which phosphonium was reduced to the obtained trialkylphosphine. Likewise, Ramnial et al.33 found that decomposition of trihexyl(tetradecyl)phosphonium chloride occurred and gave trialkylphosphine when they reacted with Grignard reagents. Inspired by these reactions, we speculate the possible mechanism using [P4444]Cl as the precursor to fabricate nickel phosphide (Scheme 1). Upon microwave heating, [P4444]Cl should decompose to tributylphosphine (TBP), an organophosphine reagent for transition metal phosphide.34 Based on

Scheme 2. Typical Schematic Illustration for Synthesizing Various Nickel Phosphides Controlled by the Counter Anions of Nickel Salts

12.0 ± 3.3 nm (Figure 1D). The Ni2P nanocrystals were further studied by energy dispersive X-ray spectroscopy (EDX), as D


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Figure 4. XRD patterns and typical low- and high-magnification TEM images for NixPy from different Ni(II) salts reacted with [P4444]Cl under conventional heating method. XRD pattern (A) and TEM images (B, C) for Ni2P, from Ni(acac)2; XRD pattern (D) and TEM images (E, F) for Ni2P from Ni(OAc)2·4H2O; XRD pattern (G) and TEM images (H, I) for metallic Ni from NiSO4·7H2O.

Figure 5. Polarization curves of the as-synthesized NixPy catalysts (A) and Tafel plots of catalysts (B).

the similar mechanism of TOP reacting with Ni(II) salt,6 TBP possesses a strong coordination effect. Therefore, it could adsorb on nickel nanoparticles to form Ni−TBP complexes. It is well-known that metals can cause cleavage of the P−C bond in trialkylphosphine and then lead to the formation of phosphorus atoms. The diffusion of phosphorus and nickel atoms will reach a balanced state from all directions and leads metal precursors to be phosphorized, offering nickel phosphide

nanocrystals.1 In an IL-mediated system, ionic liquids themselves are excellent stabilizers for nanoparticles35 and can avoid aggregation of nickel phosphide nanocrystals, inducing ideal dispersity. In order to gain further insight into the chemical states of P and Ni, XPS characterization of Ni2P_Ni(acac)2 NCs is performed. Figure 2 shows the XPS spectra of Ni 2p and P 2p regions. In Figure 2A, Ni 2p XPS peaks are observed. The E


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The TEM images show that the size of Ni 12 P 5_NiCl 2 nanocrystals is about 122.5 nm with aggregation to a certain extent (Figure 3E,F), while Ni12P5_NiSO4 particles exhibit a sponge shape (Figure 3H,I). Obviously, the significant influence of counteranions on the tailoring phase and morphology is presented in microwave-driven IL-mediated synthesis of nickel phosphides. The typical schematic illustration for various nickel phosphides tuned by counteranions of nickel salts is shown in Scheme 2. Many groups have highlighted the important role of counteranions of nickel salts for synthesis of inorganic materials.36−39 Deng and coworkers36 speculated that anions of nickel salts, such as acac−, Cl−, SO42−, and OAc−, had different coordination capabilities with Ni2+ during the crystal growth stage and then influence the adsorbing and desorbing rate during the nucleation process for the formation of nickel phosphides, largely influencing the final crystal structure and morphologies. Therefore, various phase crystals of nickel phosphides with different morphology and size were easily obtained in our case. The control experiments were further performed using the same agents by the conventional heating method. The reactions between [P4444]Cl and Ni(II) salts underwent at 350 °C for 3 h in the tube furnace. The XRD pattern of product Ni2P_Ni(acac)2 prepared via tube furnace heating is shown in Figure 4A. The result confirms that the crystalline phase is pure hexagonal Ni2P. The main diffraction peaks’ positions are identical to the microwave-fabricated Ni2P. However, the tube heating gives larger spherical Ni2P particles with diameter up to 1.2 μm (Figure 4B), as shown in the TEM images. Highmagnification TEM image testifies that they consist of relatively small spheres with diameter about 62 nm, forming hierarchical structured Ni2P crystals (Figure 4C). Obviously, conventional heating method results in significantly larger Ni2P than the microwave method, and severe aggregation is observed. Xue’s40 group has studied the effect of different heating methods on the size of nanostructured materials. Usually, the tube furnace heating easily results in severe sintering of particles, while microwave radiation offers promising dispersion due to homogenious heating. The products from Ni(OAc)2·4H2O exhibit results similar to those of Ni(acac)2, and Ni2P with the average size of 180.6 nm is obtained (Figure 4D−F). However, no product is obtained when using NiCl2·6H2O and metallic nickel (Figure 4G) with the shape of nanoplates is given from NiSO4·7H2O (Figure 4H,I). For the formation of metallic nickel nanoplates, it can be explained as follows. It is reported that the tetraalkylphosphonium ionic liquids can act as solvent and template for the nanoparticle formation.41 Specifically, the tetraalkylphosphonium ionic liquids exhibit a unique structure of parallel layers.27 Thus, such parallel layer structure can work as a template for the growth of nanoplate particles and result in the formation of metallic Ni nanoplates. Therefore, the microwave-driven strategy can break through the limitations of conventional heating methods and efficiently reduce the reaction time. The HER electrocatalytic activities of the as-synthesized NixPy catalysts by microwave heating were investigated in 0.5 M H2SO4 with a three-electrode system. For comparison, 20 wt % Pt/C was also studied, which is known to be a highly active electrocatalyst for HER. Based on Figure 5, the onset potential, Tafel slope, and overpotential η values at 10 mA cm−2 (η10) and 20 mA cm−2 (η20) can be obtained. Figure 5A shows the polarization curves of different catalysts. Ni2P from Ni(acac)2 and Ni(OAc)2·4H2O possesses low onset potentials of 49 mV

Table 1. Comparison of HER Performance for the AsSynthesized Ni2P with Other Reported Electrocatalysts catalyst

current density (mA cm−2)

overpotential (mV)

Tafel slope (mV dec−1)

ref

WS2 MoO2 MoP Fe4.5Ni4.5S8 Ni2P NiS2 P-WN/rGO MoO2/MoSe2 CoSe2 FeP/CC CoP Cu3P/CF NixWP Cu3P-CoP/CC Co2P/Ti N,P-Co2P/GO Ni2P

20 10 10 10 10 10 10 20 50 10 10 10 20 10 10 10 10

275 169 105 280 172 174 85 200 188 34 110 143 110 59 95 103 102

55 58 126 72 62 63 54 49.1 34 29.2 54 67 39 58 45 58 46

46 18 47 48 49 50 51 52 53 54 55 56 57 58 59 60 our work

peaks centered at 853.3 and 870.4 eV are assigned to Niδ+ for Ni 2p3/2 and Ni 2p1/2 in Ni2P, respectively. Peaks at 856.5 and 874.3 eV correspond to oxidized Ni species of Ni 2p3/2 and Ni 2p1/2, formed by exposure to air. The peaks at 860.7 and 879.2 eV belong to the satellite peak for Ni 2p3/2 and Ni 2p1/2 of divalent Ni2+ species. The XPS spectra in Figure 2B represented P 2p, which could be divided into two peaks. The peak at 129.2 eV can be assigned to P in Ni2P nanocrystals, and the binding energy value is lower than elemental P (130.2 eV), indicating that P species has a small negative charge (Pδ−).6 Combining Niδ+ with Pδ− peaks in Figure 2A,B, we can deduce that there is electron transfer between Ni and P in microwave-assisted Ni2P_Ni(acac)2 NC phases. The other peak observed at 133.1 eV in Figure 2B is attributed to the oxidized P species. To further study the reactive property of various Ni(II) salts with [P4444]Cl, other Ni(II) salts with different counteranion were screened, including Ni(OAc)2·4H2O, NiCl2·6H2O, and NiSO4·7H2O. When Ni(OAc)2·4H2O reacted with [P4444]Cl for the same time (1 min) as Ni(acac)2 at 50 W, no product was obtained. The optimal radiation time was found to be 1.5 min for this reaction system. Figure 3A shows the XRD results for the products from Ni(OAc)2·4H2O. All of the characteristic diffraction peaks can be assigned to hexagonal Ni2P nanocrystal. From the TEM images (Figure 3B,C), quasi-spherical particles formed. The size of the particles is measured to be 30.2 ± 8.0 nm (Figure S1, Supporting Information). This suggests that their morphologies and size are different with the products from Ni(acac)2. When Ni source is replaced to NiCl2·6H2O and NiSO4·7H2O, the microwave radiation times are optimized to 2 and 1 min 50 s at 50 W, respectively. The experimental results present that the counteranions induce the remarkable change compared with Ni(acac)2 as well as Ni(OAc)2·4H2O. First, the phase of the products is not the Ni2P phase. According to the analysis of XRD patterns (Figure 3D,G), the products synthesized from NiCl2·6H2O and NiSO4·7H2O can all be indexed as tetragonal Ni12P5 (JCPDS No. 22-1190). The XRD results suggest that controlling the counteranions of nickel salt can simply obtain various nickel phosphide phases (Ni2P vs Ni12P5). Second, noticeable changes in morphologies and the size of nickel phosphide nanoparticles are observed. F


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Figure 6. Electrochemical impedance spectra at 200 mV vs RHE (A), cyclic voltammograms of Ni2P_Ni(acac)2 (B), Ni12P5_NiSO4 (C), Ni2P_Ni(OAc)2 (D), and Ni12P5_NiCl2 (E) at scan rates of 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV s−1. The corresponding differences in current density variation (ΔJ = ja − jc) at 0.1 V vs RHE as a function of scan rate fitted to evaluate the Cdl for as-produced NixPy catalysts (F).

Figure 7. Polarization curves of the Ni2P_Ni(acac)2 before and after 2000 cycles (A); current density−time (I − t) curve for Ni2P_Ni(acac)2 at overpotential of 110 mV (B).

and 62 mV, while Ni12P5 from NiSO4·7H2O and NiCl2·6H2O has 59 mV and 96 mV, respectively. The Ni2P_Ni(acac)2 electrodes require overpotentials of only 102 mV and 143 mV to obtain 10 mA cm−2 and 20 mA cm−2, respectively, whereas Ni12P5_NiCl2 produced the same current density at larger

overpotential (182 mV and 211 mV). To further clarify the HER properties, the Tafel curves [overpotential vs log(current density)] (Figure 5B) are investigated. The Tafel slope is an intrinsic property of electrocatalysts, and Pt/C electrode shows a Tafel slope of 29 mV dec−1. Ni2P_Ni(acac)2 displays a Tafel G


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ACS Sustainable Chemistry & Engineering slope of 46 mV dec−1, while Ni2P_Ni(OAc)2 shows 51 mV dec−1. These are comparable to the values reported for other state-of-the-art nanostructured HER catalysts (Table 1). For Ni12P5 from NiSO4·7H2O and NiCl2·6H2O, the Tafel slopes are 52 mV dec−1 and 80 mV dec−1. The observed Tafel slopes of all catalysts in our studies indicate that the HER reaction obeys Volmer−Heyrovsky HER mechanism according to classic theory on HER in acidic media.42,43 Parameters based on both the overpotential and Tafel slope reveal the HER rate of the Ni2P nanocrystals is faster than that of the Ni12P5 NCs, in which Ni2P_Ni(acac)2 nanocrystals exhibit the highest catalytic activity, only second to Pt/C. Many studies have verified that the phase of nickel phosphide plays crucial roles for electrocatalytic efficiency,6 and Ni2P displays a significant improvement in electrocatalytic HER activity over Ni12P5 in both acidic media6,44 and alkaline media.45 The phenomenon is attributed to higher levels of phosphorus species in Ni2P than that of Ni12P5. It has been found that phosphorus species can create negative charge to trap protons and influence the desorption of H2. Therefore, the increasing atomic percentage of phosphorus is beneficial to HER activity for Ni2P. Moreover, small size and high dispersity of Ni2P electrocatalysts are also important keys for the resulted higher HER activity. The electrochemical impedance spectra (EIS) were further used to understand the electrode kinetics and interfacial properties of the catalysts. As shown in Figure 6A, the Nyquist plots show the charge transfer resistance (Rct) of as-obtained products. Rct of Ni2P_Ni(acac)2 nanocrystals is 45 Ω, and Rct of Ni2P_Ni(OAc)2 is 186 Ω. These results suggest that Ni2P_Ni(acac)2 nanocrystals possess the lower hydrogen adsorption impedance with faster charge transfer kinetics on the electrode than that of Ni 2 P_Ni(OAc) 2 . Meanwhile, the R ct of Ni12P5_NiSO4 is 82 Ω. Ni12P5_NiCl2 NCs have larger impedance (290 Ω), which is not active for the HER, agreeing well with the results from both the overpotential and Tafel slopes. Furthermore, we compared ECSAs of the studied catalysts by measuring the double-layer capacitance (Cdl) (Figure 6), which is in proportion to the ECSA. The Cdl of Ni2P_Ni(acac)2 is determined to be 8.65 mF cm−2, significantly higher than that of Ni2P_Ni(OAc)2 (2.95 mF cm−2), Ni12P5_NiSO4 (4.6 mF cm−2), and Ni12P5_NiCl2 (1.2 mF cm−2). Unambiguously, this demonstrated that Ni2P_Ni(acac)2 could allow the more effective accessibility of active sites. The stability test for Ni2P_Ni(acac)2 nanocrystals was carried out in 0.5 M H2SO4 by cyclic voltammograms (CVs) for 2000 cycles and the current density−time (I−t) study. Based on the polarization curve with 2000 cycles (Figure 7A) and the I−t curve for 12 h (Figure 7B), only an ignorable decay is observed, indicating that Ni2P_Ni(acac)2 NCs have an excellent durability in 0.5 M H2SO4 environment. Therefore, our Ni2P_Ni(acac)2 nanocrystals could be used as an efficient electrocatalyst with good long-term cycle stability for the hydrogen evolution reaction.

sizes, and crystal phases of nickel phosphides are facilely adjusted. (3) The use of microwave heating has the remarkable advantage of operating in less than 2 min. The as-synthesized Ni2P_Ni(acac)2 nanocrystals show excellent electrocatalytic activity with good long-term stability for HER in acidic medium. Comparing with other catalysts prepared from this strategy, it further proves that the morphology, size, and phase of nickel phosphide are all important for realizing high catalytic properties. The present microwave-driven, ionic liquidmediated method for the synthesis of HER catalysts shows great potential as an efficient alternative strategy to realize highly active HER catalysts.

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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.7b03954. EDX pattern of Ni2P nanocrystal, reaction products’ particle size distribution, and nickel phoshide yields (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhonghao Li: 0000-0003-0699-300X Jingcheng Hao: 0000-0002-9760-9677 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant No. 21673128). REFERENCES

(1) Shi, Y.; Zhang, B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem. Soc. Rev. 2016, 45, 1529−1541. (2) Callejas, J.; Read, C.; Roske, C.; Lewis, N.; Schaak, R. Synthesis, characterization, and properties of metal phosphide catalysts for the hydrogen-evolution reaction. Chem. Mater. 2016, 28, 6017−6044. (3) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148−5180. (4) Popczun, E.; McKone, J.; Read, C.; Biacchi, A.; Wiltrout, A.; Lewis, N.; Schaak, R. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (5) Muthuswamy, E.; Savithra, G.; Brock, S. L. Synthetic levers enabling independent control of phase, size, and morphology in nickel phosphide nanoparticles. ACS Nano 2011, 5, 2402−2411. (6) Pan, Y.; Liu, Y.; Zhao, J.; Yang, K.; Liang, J.; Liu, D.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. Monodispersed nickel phosphide nanocrystals with different phases: synthesis, characterization and electrocatalytic properties for hydrogen evolution. J. Mater. Chem. A 2015, 3, 1656− 1665. (7) Wang, J.; Johnston-Peck, A. C.; Tracy, J. B. Nickel phosphide nanoparticles with hollow, solid, and amorphous structures. Chem. Mater. 2009, 21, 4462−4467. (8) Layan Savithra, G. H.; Muthuswamy, E.; Bowker, R. H.; Carrillo, B. A.; Bussell, M. E.; Brock, S. L. Rational design of nickel phosphide hydrodesulfurization catalysts: controlling particle size and preventing sintering. Chem. Mater. 2013, 25, 825−833.

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CONCLUSION In summary, we have developed a facile strategy that ionic liquid tetraalkylphosphonium chloride works as both novel P source and reaction medium to synthesize nickel phosphide nanocrystals upon microwave irradiation within 2 min. Comparing with traditional reagents and fabrication method, our approach has some advantages: (1) Using ionic liquid [P4444]Cl as P source provides a handy strategy. (2) By controlling anions of nickel-based reactants, the morphologies, H


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