Effective Electrocatalytic Hydrogen Evolution in Neutral Medium

Jul 2, 2019 - The suitable catalyst in the neutral medium for HER requires good conductivity for decreasing ohm resistance, porous structures for weak...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Effective Electrocatalytic Hydrogen Evolution in Neutral Medium Based on 2D MoP/MoS2 Heterostructure Nanosheets Aiping Wu, Ying Gu, Ying Xie, Chungui Tian,* Haijing Yan, Dongxu Wang, Xiaomeng Zhang, Zhicheng Cai, and Honggang Fu* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of China, Heilongjiang University, Harbin 150080, China

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ABSTRACT: The hydrogen evolution reaction (HER) in the neutral medium can avoid the problems caused by strong acid (bases) media and thus is promising for practical application. The suitable catalyst in the neutral medium for HER requires good conductivity for decreasing ohm resistance, porous structures for weakening diffusion resistance, and plentiful active sites, but its synthesis remains a challenge. Here, the 2D MoP/MoS2 heterostructure nanosheets rather than common anion doping supported on carbon cloth (CC) was designed to meet the above criteria. The catalyst only needs a low overpotential of 96 mV to achieve a current density of 10 mA cm−2 (η10) for HER in the neutral medium (without iR correction), which is much lower than 199 mV of the bare MoS2. The good performance is ascribed to plentiful active sites on the heterointerface of MoP/MoS2 for activating H2O, good conductivity of MoP and CC for electron transfer, and pores surrounded by MoP/MoS2 facilitating mass transfer as shown by XPS and density functional theory calculations. The catalyst also exhibits outstanding activity in alkaline (η10 of 54 mV) and acid (η10 of 69 mV) media. The cells by coupling the MoP/MoS2 cathode with a NiFe-LDH anode can deliver a current density of 10 mA cm−2 at 1.51 V in 1 M KOH and 1.98 V in 1 M PBS. The effective overall water splitting can be driven by a solar panel (1.51 V), implying its ability to store solar energy as H2 energy. KEYWORDS: electrocatalytic water splitting, heterostructure nanosheets, molybdenum disulfide, molybdenum phosphides, neutral medium large ohmic loss in the medium.19 Therefore, a catalyst that works well in a neutral-pH electrolyte is wishful thinking. The suitable catalyst should possess good conductivity for decreasing ohm resistance, porous structures for decreasing diffusion resistance, and plentiful active sites for activating H2O. The two-dimensional (2D) nanosheets exhibit some unique chemical, physical, and electronic properties, thus receiving intensive attention in materials science.20−22 Their unique 2D structure and ultrathin characteristic can provide larger accessible active sites for effective contact with reactants, which is beneficial to improving the catalytic performance.23,24 MoS2, a typical 2D metal dichalcogenide, has drawn significant attention as a potential HER catalyst due to its low cost, special structures, and being abundantly available.25−28 Due to the nature of MoS2, the HER performance of MoS2 is general, especially in the neutral medium. Many studies have focused on doping with heteroatoms and tuning the size, crystal phase, and surface defects of MoS2 to regulate its catalytic activity for HER catalysis.29−33 Nevertheless, MoS2 has some disparity in

1. INTRODUCTION Hydrogen is considered as one of the most promising alternative energy sources to nonrenewable fossil fuels.1−3 Electrocatalytic water splitting provides an environmentalfriendly pathway for the production of hydrogen from abundant water.4−6 Actually, the process is of great significance to convert electrical energy from solar energy (wind energy), which cannot be easily connected to a grid, into hydrogen energy for easy storage and utilization. High-effective, low-cost catalysts are persistently pursued to promote the practical application. Currently, Pt-based materials are state-of-the-art catalysts for the hydrogen evolution reaction (HER) in acid medium, but their use has been limited by their high cost and low elemental abundance of Pt.7,8 Intensive efforts have been devoted into the construction of low-cost electrocatalysts, including transition-metal chalcogenides,9−11 nitrides,12−14 and phosphides,15−18 which have good performances in acid or alkaline media. In fact, water splitting in the neutral medium is more promising for practical application because it can eliminate environmental pollution and handle the problems caused by using formidable strong acids or bases. However, the materials that present remarkable HER activity in the neutralpH medium are rare due to the low ionic concentration and © XXXX American Chemical Society

Received: May 1, 2019 Accepted: July 2, 2019 Published: July 2, 2019 A

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces comparison with other non-noble-metal catalysts, such as phosphide, carbides, and borides. Typically, molybdenum phosphide (MoP) is a suitable HER catalyst across the whole pH range.34−38 The performance of MoP can be tuned by regulating its microstructure as small particles, rods, and hollow structures.39,40 Under neutral-phosphate electrolytes, the phosphate can be formed on the surface phosphide, which is active for neutral HER.41 Thus, a 2D MoP will be desirable based on the advantage of MoP and 2D nanosheets. However, the synthesis of 2D MoP remains highly challenging due to volume shrinkage during the preparation process. Phosphorus (P) and sulfur (S) have similar atomic radii. So, it is possible by rational replacement of S in MoS2 with P to give MoP/MoS2 heterojunction materials. In our previous work, the MoS2@MoP heterojunction powder electrocatalysts were synthesized by a similar method, which exhibited good performance for HER in a wide pH range.34 However, for practical use, the powder catalysts should be coated onto a certain substrate, which is complicated and can bring the contact problem between substrate and catalyst thus increasing ohmic resistance. To solve these problems, many researchers directly grow catalysts on the substrates, such as carbon cloth (CC), carbon fiber paper, and Ni foam.42−45 In this work, we have constructed the MoP-based nanosheets by partly replacing S in MoS2 nanosheets grown on carbon cloth with P. To well maintain the sheet-like structure and construct heterostructure, a part of MoS2 should be remained in final materials. The 2D nanosheet structure can offer more active sites for the activation of H2O, and the heterojunctions between MoP and MoS2 are conducive to the transfer of electrons. The good conductivity of the MoP outer layer and their good contact with CC also facilitates the charge transfer. The MoP/MoS2 heterostructure nanosheets as 3D integrated self-supported electrodes show high electrocatalytic HER activity with good stability. It requires an overpotential of 96 mV to obtain a current density of 10 mA cm−2 for HER in the neutral medium. It is exciting that the HER performance of MoP/MoS2 heterostructure nanosheets is higher than that of the commercial Pt/C catalyst in neutral and alkaline media at relatively high current density. The X-ray photoelectron spectroscopy results indicate stronger electronic interaction between MoS2 and MoP, which is beneficial to optimizing the adsorption energy of reaction intermediates. Density functional theory (DFT) calculations show that MoP/MoS2 heterostructures have near-zero Gibbs free energy of H adsorption, which is conducive to achieving high HER activity. By coupling the MoP/MoS2 nanosheets as the cathode and a NiFe-LDH electrode as the anode, a cell voltage of 1.98 V was needed to get a current density of 10 mA cm−2 in the neutral medium. This work offers a valuable strategy for the design and synthesis of low-cost and efficient pH-universal HER electrocatalysts.

Figure 1. Schematic for the formation of MoP/MoS2 heterostructure nanosheets (a); XRD patterns (b), SEM images of MoS2 nanosheets (c,d) and MoP/MoS2-8 (e,f); elemental mapping images of Mo, S, and P for MoP/MoS2-8 (g); TEM and HRTEM images of MoP/ MoS2-8 (h,i) and TEM elemental mapping images of Mo, S, and P for MoP/MoS2-8 (j).

structure nanosheets were obtained by the controllable replacement of S with P. In order to enhance the performance of the catalyst, phosphorization treatment was carried out. As shown in Figure 1b, for the phosphatized sample MoP/MoS2-8, there appears some strong diffraction peaks at around 28.1, 32.1, 43.1, and 57.2° corresponding well to the (001), (100), (101), and (110) planes of MoP (JCPDS no. 89-5110), respectively. Furthermore, it should be noted that the peaks belonging to MoS2 can still be observed, indicating the coexistence of the two phases, which preliminarily proves that the MoP/MoS2 heterogeneous structure on carbon cloth was successfully synthesized. Figure 1e,f and Figure S1b show the SEM images of MoP/MoS2-8. It can be seen that the morphology of 2D sheet-like structures was still intact without obviously collapsing, except for the surface of nanosheets becoming rough compared with MoS2 nanosheets (Figure 1c,d and Figure S1a). The energy-dispersive X-ray (EDX) elemental mapping images of MoP/MoS2-8 and MoS2 nanosheets (Figure 1g, Figure S3) suggest that the Mo, P, and S elements are distributed uniformly in the MoP/MoS2-8 further indicating the formation of MoP/MoS2 heterostructures. Figure S2 and Figure 1h,i show the typical TEM images of MoS2 nanosheets and MoP/MoS2-8 with different magnifications. The low-magnification images (Figure S2a,b) of MoS2 nanosheets further confirm the nanosheet morphology, which is consistent with the SEM observation. For the MoP/MoS2-8 sample (Figure 1h), the surface of the nanosheets begins to become rough, but the morphology and thickness of the nanosheets have barely changed. In the HRTEM image (Figure 1i), a distinct interplanar spacing of 0.64 nm can be

2. RESULTS AND DISCUSSION 2.1. Morphology and Structure of 2D MoP/MoS2 Heterostructure Nanosheets. The synthetic route of the self-supported MoP/MoS2 heterostructure nanosheets is illustrated schematically in Figure 1a. First, the treated carbon cloth was used as a self-supported substrate to grow evenly distributed MoS2 nanosheets by a facile hydrothermal process using phosphomolybdic acid and thiourea as precursors. Subsequently, the MoS2 nanosheets were used as templates for the formation of MoP. Then the MoP/MoS2 heteroB

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. XPS spectra of MoS2 nanosheets and MoP/MoS2-8: (a) survey spectrum and the high-resolution spectra of (b) Mo 3d, (c) S 2p, and (d) P 2p.

the electrocatalyst. The weak double peaks at 233.2 and 236.1 eV were ascribed to the Mo−O caused by slight surface oxidation. Figure 2c shows the S 2p spectra of MoS2 nanosheets and MoP/MoS2-8 heterostructure nanosheets. For MoS2 nanosheets, the strong peaks at 163.45 and 162.25 eV correspond to S 2p1/2 and 2p3/2 of S2−, respectively. For MoP/MoS2-8, the peaks of S 2p1/2 and 2p3/2 locate at 163.6 and 162.4 eV, respectively. The formation of MoP can also be verified by the high-resolution P 2p spectrum shown in Figure 2d. It is no doubt that there is no signal of the P element in the spectrum of MoS2. For MoP/MoS2-8, the two peaks at 130.75 and 129.9 eV can be assigned to the P 2p1/2 and P 2p3/2 of P− Mo bonds, respectively, and the broad peak at a high binding energy of 134.2 eV is ascribed to the P−O bonds of PO43−. The ratio of MoS2 to MoP plays an important role in the performance of the final heterostructure nanosheet material. In order to obtain the optimal heterostructure catalyst, the materials were treated by a temperature-controlled phosphorization process. Figure S4a shows the XRD patterns of the asprepared catalysts. As shown in Figure S4, the intensity of the diffraction peaks belonging to MoS2 is growing weaker with the increase of the phosphating temperature, indicating that the contents of MoP and MoS2 in the MoP/MoS2 materials can be adjusted by changing the phosphating temperature. To further confirm the compositions and phase of the as-prepared samples, Raman spectroscopy was carried out. As shown in Figure S4b, the phosphatized samples obtained by different phosphating temperatures all exhibit two dominant Raman peaks at about 384.8 and 409.8 cm−1 arising from E2g1 and A1g vibrational modes of hexagonal MoS2, respectively, which is similar to the MoS2 sample. The result shows that the MoS2 phase is present in all the phosphatized samples further suggesting a MoP/MoS2 heterogeneous structure. Figure S5 shows the SEM images of MoP/MoS2-7 and MoP/MoS2-9.

unambiguously assigned to the (002) crystal plane of MoS2; meanwhile, the lattice fringe with an interplanar distance of 0.28 nm corresponds well to the (100) crystal plane of MoP. In addition, the (100) crystal plane of MoP and the neighboring (002) crystal plane of MoS2 construct the important heterointerfaces in MoP/MoS2 catalysts. The intimate contact of MoP and MoS2 phases in catalysts is conducive to the migration of electrons, which plays an important role in promoting HER performance. In addition, since the thickness of the MoP/MoS2 nanosheets is thin and relatively uniform and the EDX has a certain depth of detection, the TEM elemental mapping images also show that Mo, P, and S elements are all uniformly distributed over the entire nanosheets (Figure 1j). X-ray photoelectron spectroscopy (XPS) was performed to analyze the interaction between MoP and MoS2 in the MoP/ MoS2 heterostructure nanosheets. The survey XPS spectrum of MoS2 (Figure 2a) indicates the presence of Mo, S, C, and O elements, while the MoP/MoS2-8 sample comprises Mo, S, P, C, and O elements. The high-resolution spectrum of Mo 3d of MoS2 nanosheets in Figure 2b presents two peaks at binding energies of 229.45 and 232.55 eV, which correspond to characteristic peaks of Mo 3d5/2 and Mo 3d3/2, respectively. For the MoP/MoS2-8 catalyst, the Mo 3d XPS spectrum can be divided into four distinct peaks. The two peaks at 228.75 and 231.85 eV can be assigned to Mo 3d5/2 and Mo 3d3/2 of Mo3+ species in MoP, respectively. In addition, the peaks located at 229.6 (Mo 3d5/2) and 232.7 eV (Mo 3d3/2) are ascribed to the Mo4+ species in MoS2. Importantly, by comparing with pure MoS2, the peak positions of Mo4+ in MoP/MoS2-8 exhibit an obvious positive-shift. The result is attributed to the electron transfer from MoS2 to MoP indicating a stronger interaction between MoS2 and MoP, which is very favorable to improve the HER performance of C

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. HER performance characterization. (a) Polarization curves for MoS2, MoP/MoS2-7, MoP/MoS2-8, MoP/MoS2-9, and Pt/C in 1 M PBS; (b) overpotentials required to reach different current densities for Pt/C, MoS2, MoP/MoS2-7, MoP/MoS2-8, and MoP/MoS2-9; (c) corresponding Tafel plots; (d) polarization curves for MoP/MoS2-8 with a scan rate of 100 mV s−1 before and after 2000 cycles. Inset: current−time (I−t) curves of MoP/MoS2-8 for 24 h. (e) XRD pattern and (f) SEM image of the MoP/MoS2-8 catalyst after the stability test.

phosphating temperature (700 °C). With increasing phosphating temperature, the content of MoP gradually increased (Figure S11). The results confirm that MoP/MoS2 heterogeneous structures on carbon cloth were successfully synthesized and the contents of MoP and MoS2 can be tuned by adjusting the phosphating temperature, which is consistent with the above XRD, EDX, and TEM characterizations. 2.2. Electrocatalytic Performance of 2D MoP/MoS2 Heterostructure Nanosheets. Electrocatalytic hydrogen evolution in neutral conditions is of greater practical value that can eliminate environmental pollution and handle the problems from using formidable strong acids or bases. Therefore, the electrochemical HER performances of the asprepared materials and commercial Pt/C were first evaluated in neutral electrolytes (1 M phosphate buffer solution PBS) at room temperature using a typical three-electrode setup. Figure 3a shows the polarization curves of the as-prepared samples. Impressively, all MoP/MoS2 samples deliver higher current density and lower onset potential than bare MoS2, which indicates that constructing heterostructures can sufficiently enhance the catalytic performance toward HER in neutral conditions. The MoP/MoS2-8 shows remarkable HER activity with a small overpotential of 96 mV to achieve a current density of 10 mA cm−2 (η10), lower than those of MoS2 (199 mV), MoP/MoS2-7 (100 mV), MoP/MoS2-9 (99 mV), and most recently reported non-noble-metal HER catalysts, such as MoP/CNT (102 mV),39 MoP NA/CC (187 mV),46 MoP/Mo2C@C (136 mV),47 MoP NPs@NC (136 mV),48 Ni0.1Co0.9P-CFP (125 mV),19 MoP NWs/CC (232 mV),49 Co-Fe-P nanotubes (130 mV),50 CoP-400 (161 mV),51 and Fe17.5%-Ni3S2/NF (145 mV).52 Moreover, it is obvious that the HER activity of MoP/MoS2-8 is even better than that of commercial Pt/C at the high potential region (Figure 3b). The

The surface morphology of MoP/MoS2-7 has no obvious change, similar to MoS2 nanosheets (Figure S5a). However, as the phosphating temperature rises to 900 °C, the surface of the nanosheets becomes rough and the formation of many nanoparticles on the nanosheets can be obviously seen (Figure S5b). The EDX elemental mapping images of MoP/MoS2-7 (Figure S6) and MoP/MoS2-9 (Figure S7) also demonstrate the uniform distribution of Mo, S, and P elements in the MoP/ MoS2 heterostructure nanosheets. In addition, the mole ratios of MoS2 to MoP calculated from EDX results in MoP/MoS2-7, MoP/MoS2-8, and MoP/MoS2-9 are about 1.52, 0.60, and 0.11, respectively, which further proves that the ratio of MoP to MoS2 in MoP/MoS2 heterostructure nanosheets can be adjusted (Table S1). Figures S8 and S9 show the typical TEM images of MoP/MoS2-7 and MoP/MoS2-9 heterostructure nanosheets. As shown in Figure S8, the sheet-like structure is well preserved, and the microstructure of the MoP/MoS2-7 nanosheet is almost identical to the MoS2 nanosheets indicating that the content of MoP is low when the phosphating temperature is at a lower temperature (700 °C). As the temperature of the phosphorization increases to 900 °C, lots of nanoparticles are loaded on the surface of the nanosheets, which destroy the morphology of the nanosheets (Figure S9). The above results indicate that the phosphorization degree and the microstructure of the MoP/MoS2 materials can be adjusted by a temperature-controlled phosphorization process. The ratio of MoP to MoS2 and the microstructure of MoP/MoS2 are important parameters that affect the performance of the final heterostructured material. The XPS spectra can also provide some information about the change of composition and content. Figures S10 and S11 show the XPS spectra of MoP/MoS2-7 and MoP/MoS2-9. As shown in Figure S10, a small amount of MoP was generated at low D

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Polarization curves for MoS2, MoP/MoS2-7, MoP/MoS2-8, MoP/MoS2-9, and Pt/C in (a) 1 M KOH and (d) 0.5 M H2SO4. Corresponding Tafel plots of samples in (b) 1 M KOH and (e) 0.5 M H2SO4. Polarization curves for MoP/MoS2-8 with a scan rate of 100 mV s−1 before and after 2000 cycles in (c) 1 M KOH and (f) 0.5 M H2SO4. Inset: current−time (I−t) curves of MoP/MoS2-8 for 24 h. (g) Geometric model of MoP/MoS2 heterostructures. (h) HER free-energy diagram. (i) H2O adsorption energy on the surfaces of MoP/MoS2 heterostructures, MoP, and MoS2.

Tafel slopes were obtained by fitting the linear regions of Tafel plots to illustrate the reaction kinetic mechanism. Figure 3c presents the Tafel plots of MoS2, MoP/MoS2-7, MoP/MoS2-8, MoP/MoS2-9, and commercial Pt/C catalyst in 1 M PBS. MoP/MoS2-8 shows a Tafel slope of 48 mV dec−1, suggesting that the HER on the MoP/MoS2-8 material should undergo the Volmer−Heyrovsky mechanism and the electrochemical desorption (H2O discharge and desorption of H from the catalyst surface) is the rate-determining step. The Tafel slope of MoP/MoS2-8 is smaller than those of MoS2 (100 mV dec−1), MoP/MoS2-7 (54 mV dec−1), and MoP/MoS2-9 (51 mV dec−1). A small Tafel slope leads to a strongly enhanced HER rate at a moderate increase of overpotential. Meanwhile, the exchange current densities (j0) are also calculated through an extrapolation method from the Tafel plots (Figure S12 and Table S2). The value of j0 is highly dependent on the nature of the catalyst, which indicates the intrinsic electron transfer rates

between the catalyst and the electrolyte. The exchange current density for MoP/MoS2-8 is 0.26 mA cm−2, which is larger than those of MoS2 (0.11 mA cm−2), MoP/MoS2-7 (0.21 mA cm−2), and MoP/MoS2-9 (0.23 mA cm−2), also suggesting that constructing the effective heterostructures can adjust the intrinsic catalytic activities of MoS2 for neutral HER. The MoP/MoS2-8 catalyst has the lowest overpotential, smallest Tafel slope, and highest exchange current density among all the as-prepared catalysts indicating the best HER performance. Compared with other recently reported molybdenum-based, sulfide-based, phosphide-based, and non-noble-metal catalysts (Table S3), the MoP/MoS2-8 heterostructure nanosheet catalyst exhibits remarkable HER performance, which is superior to most other noble-metal-free catalysts. To obtain deep insight into the intrinsic catalytic activity of MoP/MoS2 heterostructure nanosheets, the turnover frequency (TOF) values were calculated (Figure S13 and Table S2). As shown in E

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

dec−1, lower than those of MoS2 (102 mV dec−1), MoP/MoS27 (63 mV dec−1), and MoP/MoS2-9 (62 mV dec−1). A similar result to neutral and alkaline electrolyte tests further indicates that constructing the MoP/MoS2 heterostructures is an effective way to improve the HER performance. As shown in Figure 4f, negligible degradation was observed for MoP/MoS28, suggesting the excellent stability of the MoP/MoS2-8 catalyst in acid conditions. To further identify the intrinsic activity of the as-prepared samples toward HER, the electrochemical double-layer capacitance (Cdl) was performed to estimate the electrocatalytically active surface area. The Cdl’s for the MoS2 nanosheets, MoP/MoS2-7, MoP/MoS2-8, and MoP/MoS2-9 are 31, 35, 47, and 41 mF cm−2, respectively (Figure S15). Since Cdl is expected to be linearly proportional to the electrocatalytically active surface area, the results demonstrate that constructing effective MoP/MoS2 heterostructures favors increasing the catalytic active sites, thus contributing to the enhanced HER performance. In order to gain further insight into the outstanding HER performance of MoP/MoS2 heterostructures, density functional theory (DFT) calculations were carried out to understand the activity origin of the catalyst. Generally, the activity of the HER electrocatalyst is closely related to its ability to adsorb H* in the acidic electrolyte, while, in alkaline or neutral media, it is determined by the adsorption of H2O.5 The relative free energies of H* and H2O absorption on the surface are commonly used to evaluate the catalytic activity of HER catalysts. Therefore, the adsorption energies of H* and H2O on the surfaces of MoP/ MoS2 heterostructures, MoS2, and MoP were calculated. The geometric models of H* and H2O adsorbed on the MoP/ MoS2 heterostructures, MoS2, and MoP are presented in Figure 4g and Figures S16 and S17, respectively. As shown in Figure 4h, the calculated ΔGH* for H* adsorption on MoP/ MoS2 heterostructures is about −0.12 eV, while the values for absorption on MoS2 and MoP are 1.79 and −0.57 eV, respectively. The result indicates the more favorable H* adsorption kinetics on the surface of the MoP/MoS 2 heterostructures during the HER process. Figure 4i shows the calculated H 2 O adsorption energy on MoP/MoS 2 heterostructures, MoS2, and MoP catalysts. The MoP/MoS2 material has a lower H2O adsorption energy (−1.38 eV) than MoS2 (0.25 eV) and MoP (−0.72 eV), indicating that H2O can be easily adsorbed on the surface of MoP/MoS2 to accelerate the HER process in neutral or alkaline solution. Moreover, as shown in Figure S18, the density of states of MoP/MoS2 heterostructures near the Fermi level is larger than that of MoS2 indicating enhanced electron mobility in the MoP/MoS2 heterostructure for fast electron transport in the HER process. Electrochemical impedance spectroscopy results in Figure S19 show that MoP/MoS2 nanosheets deliver a lower reaction resistance than MoS2, indicating that faster and more efficient charge transfer is achieved in MoP/MoS2 nanosheets. In addition, the strong interaction in the heterostructures and the interwoven nanosheet structure may facilitate the diffusion of the reactants and intermediates during the HER process. The fast charge transfer and mass diffusion give rise to accelerated hydrogen evolution kinetics. Based on the structural and theoretical analysis, the superior HER performance of MoP/MoS2 heterostructures in a wide pH range should be mainly ascribed to the following aspects: (1) the synergistic effect between MoS2 and MoP facilitates H* and H2O adsorption on the surface of MoP/MoS2

Figure S13, with the increase of negative voltage, the TOF value of MoP/MoS2-8 increased faster than that of Pt/C around an overpotential of 85 mV and the TOF value of MoP/ MoS2-8 is larger than that of Pt/C, meaning that MoP/MoS2-8 has higher intrinsic HER activity than Pt/C, which is due to the proper surface electronic structure that facilitates electron and mass transfer. Additionally, the stability is another important parameter for evaluating the quality of HER electrocatalysts. The long-term durability of MoP/MoS2-8 is evaluated by the long-term CV cycling and chronoamperometric response measurements. As shown in Figure 3d, the polarization curves of MoP/MoS2-8 almost overlap each other before and after the 2000 cycle CV test. In addition, the I−t test suggests that the current density shows a slight decline over the duration of 24 h (inset in Figure 3d). In addition, after electrolysis, the structure and chemical state of MoP/MoS2-8 were examined by XRD and SEM, which indicate that the sheet-like structure and chemical state are maintained well (Figure 3e,f). All the above results demonstrate that the MoP/ MoS2-8 heterostructure nanosheet electrocatalyst exhibits remarkable HER performance and good structural and chemical stability for neutral-pH water splitting. Additionally, the MoP/MoS2-8 catalyst also shows high HER activity and excellent durability in 1 M KOH. As shown in Figure 4a, the MoP/MoS2-8 exhibits better HER performance than other as-prepared samples and even better than the commercial Pt/C catalyst at the high potential region. The MoP/MoS2-8 electrocatalyst requires a low overpotential of 58 mV to drive 10 mA cm−2 current density, lower than those of MoS2 (189 mV), MoP/MoS2-7 (59 mV), and MoP/MoS2-9 (60 mV), as shown in Table S4. Figure 4b indicates that MoP/ MoS2-8 possessed the fastest dynamics with a Tafel slope of 58 mV dec−1, smaller than those of MoS2 (102 mV dec−1), MoP/ MoS2-7 (63 mV dec−1), and MoP/MoS2-9 (62 mV dec−1). Compared to other reported noble-metal-free HER catalysts tested in 1 M KOH, the MoP/MoS2 heterostructure catalyst shows an enhanced HER performance, as shown in Table S5. In addition, the j0 for MoP/MoS2-8 is 1.41 mA cm−2, larger than those of MoS2 (0.14 mA cm−2), MoP/MoS2-7 (1.13 mA cm−2), and MoP/MoS2-9 (1.05 mA cm−2), as shown in Figure S14a and Table S4. In addition, continuous CV scanning and chronoamperometric curves (Figure 4c) indicate the considerably good long-term durability of MoP/MoS2-8 toward HER in the alkaline medium. In order to meet the needs of different fields, an excellent HER electrocatalyst should have better HER activity over a broad pH range. Therefore, the HER performances of the asprepared samples were also measured in 0.5 M H2SO4. Figure 4d shows the polarization curves of MoS2, MoP/MoS2-7, MoP/MoS2-8, MoP/MoS2-9, and commercial Pt/C catalyst measured in a 0.5 M H2SO4 electrolyte. The results of electrochemical measurements are similar to those in neutral and alkaline conditions. As expected, the Pt/C catalyst exhibits the best activity in the 0.5 M H2SO4 electrolyte, while MoS2 shows the lowest HER activity. In contrast to MoS2, MoP/ MoS2 heterostructure nanosheets show enhanced HER activity. The η10 for MoP/MoS2-8 is 61 mV, much smaller than those of MoS2 (87 mV), MoP/MoS2-7 (79 mV), and MoP/MoS2-9 (74 mV), as shown in Table S6, and also better than or comparable to most other noble-metal-free HER electrocatalysts measured in the acid medium (Table S7). The corresponding Tafel plots of the samples are shown in Figure 4e. The MoP/MoS2-8 sample exhibits a Tafel slope of 58 mV F

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. (a) Digital photograph of the electrolytic cell. Polarization curves for MoP/MoS2∥NiFe-LDH/NF in (b) 1 M KOH and (c) 1 M PBS. Inset: current−time (I−t) curves of MoP/MoS2∥NiFe-LDH/NF for 24 h; (d) schematic for the solar power assisted water splitting. (e) Solar panel with a voltage of about 1.51 V under the illumination of the sun; (f) digital photograph of solar power assisted water splitting device (electrode area: 2 × 2 cm2); (g) magnified picture of the electrode.

three-electrode system at a scan rate of 2 mV s−1. Figure S22 shows the polarization curves for NiFe-LDH/NF. As expected, the NiFe-LDH/NF exhibits outstanding OER performance with a low potential of 1.43 V to achieve a current density of 20 mA cm−2 in a 1 M KOH electrolyte. In addition, the NiFeLDH/NF also presents better OER activity in 1 M PBS, which requires a small potential of 1.79 V to reach a current density of 10 mA cm−2. Based on the above results, we used the MoP/ MoS2 heterostructure nanosheet electrode as the cathode and the NiFe-LDH/NF as the anode for overall water splitting. The digital photograph of the electrolytic cell is illustrated in Figure 5a. The polarization curve (Figure 5b) indicates that the MoP/MoS2∥NiFe-LDH/NF electrodes exhibit superior overall water splitting activity. Remarkably, it only needs a cell voltage of 1.51 V to achieve a current density of 10 mA cm−2 in a 1 M KOH electrolyte. Such activity is better than most previously reported catalysts (Table S8). Furthermore, the long-term stability of overall water splitting measurement was evaluated. The chronoamperometric curves indicate the excellent stability of MoP/MoS2∥NiFe-LDH/NF for overall water splitting (inset in Figure 5b). Figure 5c presents the polarization curve of MoP/MoS2∥NiFe-LDH/NF tested in 1 M PBS. As shown in Figure 5c, it only requires a cell voltage of 1.98 V to afford a current density of 10 mA cm−2. In addition, the long-

heterostructures shown by DFT calculation improving the HER activity; (2) the 2D heterostructure nanosheets increase the active sites of the catalysts and the porous structure conducive to the effective contact between electrolyte and catalyst and the release of the formed H2 from the catalyst surface; (3) the good electrical conductivity of phosphide facilitates the rapid transfer of electrons; (4) the close contact between MoP/MoS2 heterostructure nanosheets and carbon cloth improves the transfer of electrons in the HER process and enables the good mechanical adhesion enhancing the stability. According to the results discussed above, the MoP/MoS2 heterostructure nanosheet electrode can serve as a high-active electrocatalyst for HER in neutral and alkaline media. Therefore, it is a promising HER electrocatalyst to combine with an oxygen evolution reaction (OER) electrocatalyst for overall water splitting. Here, a NiFe-LDH/NF electrode was prepared by a simple hydrothermal reaction to be used as an OER electrocatalyst. As shown in Figure S20, the vertically aligned NiFe-LDH nanosheets were uniformly grown on the surface of the Ni foam substrate with high coverage. Its phase purity was confirmed by the XRD measurement (Figure S21). The OER performance of NiFe-LDH/NF was evaluated in 1 M KOH and 1 M PBS aqueous electrolytes using a standard G

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

21805073, 21571054), the Basic Research Fund of Heilongjiang University in Heilongjiang Province (RCYJTD201801), the Fundamental Research Project of Provincial Higher Education Institutions in Heilongjiang Province (RCCXYJ201807), the Natural Science Foundation of Heilongjiang Province (YQ2019B005).

term potentiostatic electrolysis test suggests that the MoP/ MoS2∥NiFe-LDH/NF electrolyzer also exhibits excellent stability in 1 M PBS. In order to reflect the practical application of solar water electrolysis, a solar power assisted water splitting device was used (Figure 5d). As shown in Figure 5e, the voltage of the solar panel is about 1.51 V under the illumination of the sun. Figure 5f shows the optical photo of the solar power assisted water splitting device. The evolution of gas bubbles could be clearly observed at the surface of both electrodes by powering with the solar panel under the illumination of the sun (Figure 5g and Movie S1). Such a solar power assisted water splitting device makes it possible to convert low-voltage electricity into chemical energy.

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3. CONCLUSIONS In conclusion, we have demonstrated the successful synthesis of sheet-like MoP/MoS2 heterostructure catalysts by controllable substitution of S in MoS2 with P. The heterostructure catalysts have plentiful active sites for effectively activating H2O, good conductivity for electron transfer, and porous structures facilitating the mass transfer as shown by DFT and other tests. As a result, the MoP/MoS2 catalysts exhibited outstanding activity for HER in neutral solution (η10 of 96 mV), as well as alkaline (η10 of 54 mV) and acid (η10 of 69 mV) media. The MoP/MoS2 cathode can couple with the NiFe-LDH anode to decompose water to reach a current density of 10 mA cm−2 at 1.51 and 1.98 V in alkaline and neutral media and can be easily driven by solar panels for storage of solar energy as H2 energy. This work affords a valuable route for constructing the efficient and robust HER catalyst with large potential to convert plentiful solar energy into H2 energy.

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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/acsami.9b07415. Solar power assisted water splitting (MP4) Experimental section; SEM images, EDX mapping images, XRD patterns, Raman spectra, TEM images, and XPS spectra of MoS2 nanosheets and MoP/MoS2 heterostructure nanosheets; exchange current density, TOF, and Cdl measurements; SEM images, XRD pattern, and polarization curves of NiFe-LDH/NF; comparison tables of recently reported HER electrocatalysts in neutral, alkaline, and acid media (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.T.). *E-mail: [email protected] (H.F.). ORCID

Chungui Tian: 0000-0003-4846-3605 Honggang Fu: 0000-0002-5800-451X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of this research by the National Key R&D Program of China (2018YFB1502401), the National Natural Science Foundation of China (21631004, H

DOI: 10.1021/acsami.9b07415 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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