Hierarchical tubular MoP/MoS2 composite with enhanced electrochemical hydrogen evolution activity

Electrocatalytic production of hydrogen from water is a promising and sustainable strategy. Herein, hierarchical tubular MoP/MoS2 composites with the wall composed of loosely stacked nanosheets were fabricated through partially phosphating the tubular MoS2. As an electrocatalyst for hydrogen evolution reaction (HER), the hierarchical tubular MoP/MoS2 composite displayed a superior HER activity with a low overpotential (101 mV) to obtain a current density of 10 mA/cm2, small Tafel slope (56 mV/dec). Moreover, the MoP/MoS2 composite demonstrate superior long-term durability in acid electrolytes. The excellent catalytic activity of MoP/MoS2 composite may be ascribed to its hierarchical structure: hierarchical porous structure can offer mass diffusion pathways, and the nanosheets with MoP/MoS2 heterojunctions can provide rich active sites for HER. Density functional theory calculations reveal that more favorable H* adsorption kinetics on the surface of the MoP/MoS2 composite during the HER process than pristine MoP and MoS2. This work can offer a strategy to design high performance electrocatalysts for HER applications.


Introduction
Due to its renewability, environmental friendliness, and highest mass-specific energy density, hydrogen has been recognized as the promising alternative to fossil fuel [1]. Among the hydrogen production approaches, the electrocatalytic water splitting using electrical energy is believed to be a safe and efficient process [2]. Currently, Pt-group metals have been proved to be the state-of-the-art electrocatalyst towards Hydrogen evolution reaction (HER) [3]. However, their high cost and low earth-abundance hinder the scale-up application. Therefore, it is necessary to develop the cost-effective electrocatalysts that drive HER with high reaction rate at low overpotential.
Due to the low-cost and earth-abundance, metal oxides have been intensively studied as good candidates for HER electrocatalysis. Various strategies, such as morphology engineering, oxygen vacancy control, phase structure engineering and non-metal or metal doping have been adopted to enhance the metal oxides electrocatalytic performance [4]. Besides of the metal oxides, metal chalcogenides and phosphides, such as MoS 2 [5], WSe 2 [6], CoP [7], MoP [8] and Ni 2 P [9], have generated great interest for their catalytic activity toward HER. Among them, two-dimensional (2D) MoS 2 has been investigated as the most potential alternative to Pt-based HER catalyst [10]. Both theoretical and experimental studies have illustrated that the exposed edges of MoS 2 were active sites for HER [11]. However, the low conductivity and limited exposed active sites of 2D MoS 2 hinder its practical application. So far, several strategies were applied to enhance the HER activity of MoS 2 , such as, fabricating nanostructure MoS 2 to increase the active sites [12][13][14], doping transition metals in MoS 2 to increase the intrinsic catalytic activity [15,16], or coupling with conductive substrates to increase the conductivity [17,18].
Besides MoS 2 , MoSe 2 [19], MoP [8], Mo 2 C [20], MoN [21], and MoO 2 [22] also exhibit catalytic performance of HER. In addition, the Mo-based composites, such as MoS 2 /MoSe 2 [23], MoP/Mo 2 C [24], MoN/Mo 2 C [25] and MoP/MoS 2 [26][27][28] displayed excellent HER activity due to the synergistic effect. Especially MoP/MoS 2 composites have been studied a lot due to their high electrocatalytic activity. For example, MoS 2(1-x) P x (x = 0-1) solid solution prepared by annealing bulk particulate MoS 2 with red phosphorus needed an overpotential of & 150 mV to deliver a current density of 10 mA/cm 2 [29]. The hierarchical MoS 2 @MoP core-shell heterojunction electrocatalysts exhibited excellent electrocatalytic activity for HER with a low onset overpotential of 29 mV and g of 108 mV at 10 mA/cm 2 in 0.5 M H 2 SO 4 and retains its good activity for 30 h [30]. In addition, the catalyst showed excellent activity in 1 M KOH with an onset overpotential of 42 mV and g of 119 mV at 10 mA/cm 2 . The MoP/MoS 2 nanosheets on carbon cloth only needed a low overpotential of 96 mV to achieve a current density of 10 mA/cm 2 for HER in the neutral medium, ascribing to plentiful active sites on the heterointerface of MoP/MoS 2 , good conductivity of MoP and CC for electron transfer, and pores surrounded by MoP/MoS 2 nanosheets [31]. Due to the synergistic effect of MoP and MoS 2 , most of the MoP/MoS 2 composites show extraordinary electrochemical properties.
In the past, the MoP/MoS 2 composite as HER catalysts studied are mostly based on bulk MoS 2 or MoS 2 nanosheets materials. The hierarchical structures composite with MoS 2 /MoP heterojunction, especially hollow tubular structural materials, have rarely been reported. As is known that the electrochemical performances of the electrode material are associated with transportation of electrolyte and electrochemical reaction. The hierarchical hollow nanostructure is beneficial to the transport of electrolyte [32,33], and the 2D MoS 2 nanosheets with tubular hierarchical structure can provide rich accessible active sites. Therefore, a hierarchical porous structure catalyst composed of MoP/MoS 2 nanosheets with rich heterojunctions may be a superior electrocatalyst for HER.
Based on the above consideration, a hollow tubular MoS 2 with the wall composed of nanosheets was prepared. Using it as template, a series of MoP/MoS 2 composites were fabricated through partially phosphating the tubular MoS 2 . In the obtained MoP/MoS 2 composites, besides of the inherited hollow tubular, nanosheet structure from MoS 2 , the rich heterojunctions between MoP and MoS 2 can be found. As HER electrocatalyst, MoP/MoS 2 -1 displayed a superior HER activity with an onset potential about 37 mV and an overpotential of 101 mV to obtain a current density of 10 mA/cm 2 , and the Tafel slope was 56 mV/dec. The superior catalytic activity of MoP/ MoS 2 -1 may be ascribed to its hierarchical structure: hierarchical porous structure can offer mass diffusion pathways, the nanosheets with MoP/MoS 2 heterojunctions can provide rich active sites for HER.   [34]. Secondly, MoP/MoS 2 composites were prepared by phosphating the as-prepared tubular MoS 2 . Typically, 100 mg MoS 2 and 100 mg NaH 2 PO 2 were separately placed in two ceramic boats, then the boats were put in an alumina tube. The samples were calcinated in the tubular furnace with NaH 2 PO 2 at the upstream side in a sealed system under Ar atmosphere at 800°C for 2 h. The total reactive mechanism is as follows [35] 2NaH 2 PO 2 ! PH 3 " þNa 2 HPO 4 The product was denoted as MoP/MoS 2 -1. In order to adjust the phosphidation degree of the MoS 2 , a series of weight ratio of NaH 2 PO 2 to MoS 2 were placed in the furnace, and the products were denoted as MoP/MoS 2 -x, where x represents the weight ratio of NaH 2 PO 2 to MoS 2 .

Characterization methods
Powder X-ray diffraction (XRD) patterns of the prepared samples were conducted on a Bruker D8 advance X-ray diffractometer equipped with Cu Ka (k = 1.5418 Å ). The morphologies of the prepared samples were investigated with scanning electron microscopy (SEM, JSM-6490LV), field emission scanning electron microscope (FESEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100). The X-ray photoelectron spectroscopy (XPS) was recorded on a Thermo Scientific ESCAlab 250Xi photoelectron spectrometer equipped with a monochromatic Al Ka source (k = 1486.7 eV).

Electrochemical measurements
Electrochemical measurements were performed in a typical three-electrode setup in 0.5 M H 2 SO 4 on a CHI 660E electrochemical workstation. The prepared MoP/MoS 2 composites coated on the glass carbon electrode, a saturated calomel electrode and a graphite rod were used as the working electrode, the reference electrode and the counter electrode, respectively. The working electrode was prepared as follows: firstly, catalyst ink was prepared by adding 5 mg of catalyst in 1 mL the mixture of water and isopropanol (v:v = 1:1), and 50 lL Nafion solution (5 wt%) was added into the solution, followed by ultrasonication to obtain a homogeneous ink. Then an aliquot of 20 lL dispersion was pipetted onto GCE (0.196 cm 2 ) and dried at room temperature. The catalyst loading was about 0.486 mg/cm 2 . The durability test was performed at a static overpotential for 12 h, during which the current variation with time was recorded.

Results and discussion
The tubular MoP/MoS 2 composite assembled from nanosheets was synthesized by partially phosphating of tubular MoS 2 . Figure 1a shows the XRD patterns of MoP/MoS 2 -1. It is obvious that the diffraction peaks at 14.1°, 33.0°, 39.5°and 58.6°are corresponding to (002), (100), (103) and (110)  plane of MoP, respectively. The hollow tubular structure of MoP/MoS 2 -1 is favorable for the abundant mass diffusion, the nanosheets can provide active sites and short diffusion path, and the rich heterojunctions between MoS 2 and MoP on the nanosheets may be favorable in promoting the electrocatalytic performance for HER because the migration of electrons become easily [30,31].
X-ray photoelectron spectroscopy (XPS) was conducted to analyze the chemical compositions and states of the MoP/MoS 2 -1. Figure 2a shows the survey XPS spectrum, and it is clear that MoP/MoS 2 -1 comprises Mo, S, and P elements. The XPS spectrum of Mo 3d can be divided into four peaks as shown in  Figure 2c shows two distincted peaks at 162.9 and 161.7 eV, which are correspond to S 2p 1/2 and 2p 3/2 of MoP/ MoS 2 -1, respectively. Figure 2d shows the P 2p spectra of MoP/MoS 2 -1 with two characteristic peaks at 130.0 eV for P 2p 1/2 and 129.1 eV for P 2p 3/2 of P-Mo bonds, respectively. These results further confirm that the coexistence of MoP and MoS 2 in hierarchical tubular MoP/MoS 2 -1.
In order to get the optimal electrocatalyst for HER, a series of composites with different ratios of MoP to MoS 2 were prepared by adjusting the weight ratio of NaH 2 PO 2 to MoS 2 . Figure 3 shows the XRD patterns of MoP/MoS 2 -0, MoP/MoS 2 -1/3, MoP/MoS 2 -5 and MoP/MoS 2 -40. It is can be observed that the intensities of the diffraction peaks of MoP become stronger with the increase of the weight ratio of NaH 2 PO 2 to MoS 2 . Especially when the 40 times of NaH 2 PO 2 to MoS 2 was used in the synthesis process, the diffraction peaks of MoS 2 totally disappeared, and only  (Fig. S4). The EDX results showed that the phosphorus content of the synthesized materials increased with the increase of the mass ratio of NaH 2 PO 2 to MoS 2 (Table S1). Figure 3a  The HER activities of MoS 2 , MoP and a series of MoP/MoS 2 composites were evaluated in a threeelectrode electrochemical cell. As the control, the electrocatalytic performance of Pt/C was tested. Figure 5a shows the polarization curves of the samples in N 2 -saturated 0.5 M H 2 SO 4 . It is obvious that Pt/C displays superior HER electrocatalytic performance with a nearly zero onset potential and a high current density. Compared to other samples, MoS 2 displayed the poorest activity with onset potential of 165 mV (obtained at 1 mA/cm 2 ) and an overpotential of 260 mV to obtain a current density of 10 mA/cm 2 . The MoP/MoS 2 composites and MoP displayed better electrochemical performances compared with the tubular MoS 2 as shown in Fig. 5a (Fig. 5a) [27,30,31]. This may be attributed to the proper proportion of MoP and MoS 2 in MoP/MoS 2 -1, which endows the material MoP/ MoS 2 -1 with higher intrinsic properties and higher electrochemical area.
Tafel slopes are commonly used to illustrate the inherent properties of the catalyst and reaction kinetic mechanism for HER, which are fitted to Tafel equation g ¼ b log j þ a (b is the slope and j is the current density). Figure 5b shows the Tafel slopes of Pt/C and the as-prepared samples, The Tafel slopes of Pt/C was 32 mV/dec, in accordance with reported values [30]. The Tafel  In order to study the intrinsic performance of the synthesized samples, the exchange current density (j 0 ) was obtained by extrapolation from the Tafel plots. As shown in Fig. 5c (Fig. 5e). The C dl value of the MoP/MoS 2 -1 electrode is larger than those of the other samples, which indicates MoP/MoS 2 -1 electrode has a large ECSA with rich active sites towards the HER. As a HER catalyst, MoP/MoS 2 -1 outperformed the other as-prepared samples and recently reported molybdenum-based and non-noble-metal catalysts due to its low overpotential and small Tafel slope [18,[37][38][39]. The long-term durability of MoP/ MoS 2 -1 is evaluated by the long-term CV cycling and the time dependence of the current density at an overpotential of 100 mV. As shown in inset of Fig. 5f, the polarization curves of MoP/MoS 2 -1 after 500 CV cycles almost overlaps with that before the cycle. Furthermore, the current density slightly declined during the chronoamperometry operation for 12 h in 0.5 M H 2 SO 4 (Fig. 5f). All the above results indicate that the catalyst has good cycle stability.
Generally, the HER activity of the catalyst is closely related to the relative free energies of H* absorption on the catalyst (DG H* ). So density functional theory (DFT) calculations were conducted to get the adsorption energies of H* on the surfaces of MoS 2 , MoP/MoS 2 composite and MoP to understand the activity origin of the catalyst. According to Sabatiers' principle, the closer to zero the DG H* is, the higher HER activity it will have [40]. Figure 6 shows the geometric models of H* adsorbed on catalysts. The calculated DG H* for H* adsorption on MoS 2 and MoP are 2.20 eV and -1.15 eV, respectively. While the value for absorption on MoP/MoS 2 composite is about -0.35 eV. It can be concluded that the more favorable H* adsorption kinetics on the surface of the MoP/MoS 2 composite during the HER process.
A good catalyst should have excellent HER activity in a wide pH range. To explore the application of the hierarchical tubular MoP/MoS 2 -1 composite under alkaline and neutral conditions, the polarization curves and Tafel plots of the samples were tested in 1 M KOH (Fig. S7a, b) and 1 M PBS (Fig. S7c, d). The Tafel slopes of MoP/MoS 2 -1 under alkaline and neutral conditions were 78 and 93 mV/dec, respectively. The current densities of the catalyst can reach 10 mA/cm 2 at overpotential of 210 mV in 1 M KOH and 257 mV in 1 M PBS, respectively. The results demonstrate that hierarchical tubular MoP/MoS 2 composites are promising catalysts for the HER over a broad pH range.

Conclusion
A series of tubular MoP/MoS 2 composites were synthesized using the tubular MoS 2 as template. As a HER catalyst, MoP/MoS 2 -1 exhibited excellent HER activity with a small overpotential of 101 mV in 0.5 M H 2 SO 4 at current density of 10 mA/cm 2 and a small Tafel slope of 56 mV/dec. The outstanding electrocatalytic activity of MoP/MoS 2 -1 may be benefit from its hierarchical porous structure and abundant MoS 2 / MoP heterojunctions on the nanosheets, which ensure the fast mass transport and electron transfer kinetics for HER. DFT calculation results shows that MoP/MoS 2 composite has more favorable H* adsorption kinetics during the HER process than pristine MoP and MoS 2 . Both experimental results and the theoretical calculations demonstrate that the tubular hierarchical structure with a MoP/MoS 2 heterojunctions can facilitate reaction kinetics to boost the HER performance. We believe that this work can offer a strategy to design electrocatalysts with high performance for HER applications.