A General Method for the Synthesis of Hybrid Nanostructures Using MoSe2 Nanosheet-Assembled Nanospheres as Templates.

The layered transition metal dichalcogenides (TMDs) and transition metal phosphides are low-cost, earth-abundant, and robust electrocatalysts for hydrogen evolution reaction (HER). Integrating them into hybrid nanostructures is potentially promising to further boost the catalytic activity toward HER based on their synergistic effects. Herein, we report a general method for the synthesis of a series of MoSe2-based hybrid nanostructures, including MoSe2-Ni2P, MoSe2-Co2P, MoSe2-Ni, MoSe2-Co, and MoSe2-NiS, by postgrowth of Ni2P, Co2P, Ni, Co, and NiS nanostructures on the presynthesized MoSe2 nanosheet-assembled nanospheres, respectively, via a colloidal synthesis method. As a proof-of-concept application, the as-synthesized hybrid nanostructures are used as electrocatalysts for HER, exhibiting high activity and stability in acidic media. Among them, the MoSe2-Co2P composite shows the highest HER activity with an overpotential of 167 mV at 10 mA cm-2.

1. Introduction

With increasing concerns on the global environmental contamination and energy shortage caused by the excessive consumption of fossil fuels, hydrogen as a promising chemical fuel has been considered as a clean and sustainable alternative [1-4]. However, the massive and sustainable hydrogen production from the electrocatalytic water splitting requires highly efficient and robust catalysts [5-9]. Although platinum (Pt) and other precious metals have shown superior catalytic performance in the hydrogen evolution reaction (HER) at low overpotentials in acidic media, their scarcity and high cost limit their practical applications [10-12]. Therefore, it is still a great challenge to find cost-effective and earth-abundant electrocatalysts with high HER activity, low overpotential, and excellent stability to replace the rare and expensive noble metal electrocatalysts.

Among various non-noble-metal HER catalysts, transition metal semiconductor nanomaterials, e.g., dichalcogenides and phosphides, have been extensively studied [13-21], because of their low cost, high abundancy, and high HER catalytic activity [22-28]. Recently, theoretical calculations and experimental studies on MoS2 nanosheets suggested that the exposed edge is one of the catalytically active sites for hydrogen evolution [29, 30], which inspired researchers to prepare edge-rich MoS2 nanostructures to enhance the HER performance [31-34]. Furthermore, based on the density functional theory, Tsai et al. found that the HER performance of MoSe2 can be comparable to or even better than that of MoS2 [35]. However, there are only a few reports on designing the catalytically active MoSe2 nanostructures for HER [36-41]. Besides transition metal dichalcogenides (TMDs), the emerging transition metal phosphides have also drawn extensive attention as effective HER catalysts due to their good durability, corrosion resistance, and high current density at low overpotential [42-50]. Among them, the Co- and Ni-based phosphide nanostructures have exhibited their great potential as the electrocatalysts for HER [26, 51–54]. Therefore, it is very important to design and synthesize MoSe2-based hybrid HER electrocatalysts by combining the advantages of both MoSe2 and transition metal phosphides. Herein, we report a general colloidal method for the synthesis of a series of hybrid nanostructures using the MoSe2 nanosheet-assembled nanospheres as templates, including MoSe2-Ni2P, MoSe2-Co2P, MoSe2-Ni, MoSe2-Co, and MoSe2-NiS (Scheme 1). First, MoSe2 nanospheres were prepared by a hot-injection method. Then, the obtained MoSe2 nanospheres mixed with transition metal cations, oleylamine, and trioctylphosphine in a three-necked flask. Because the surface potential of the freshly prepared MoSe2 nanospheres is negative, the transition metal cations in the solution can be easily adsorbed on the surface of MoSe2 nanosheets via the electrostatic interaction. As shown in Scheme 1, when the reaction temperature was increased to about 220°C, Ni or Co nanoparticles were formed. If the reaction temperature was further increased to 320°C, the trioctylphosphine would react with transition metals to form the transition metal phosphides on MoSe2 nanospheres. However, if the reaction temperature was 220°C, after addition of the S precursor, the transition metal sulfide, such as NiS, nanoparticles were formed on MoSe2 nanospheres. As a proof-of-concept application, the as-prepared hybrid nanostructures exhibit high electrocatalytic HER activity and stability in acidic media.

Scheme 1

Schematic illustration of synthesis of the MoSe2 nanosphere-based hybrid nanostructures.

2. Results

Briefly, the colloidal MoSe2 nanosheet-assembled nanospheres were prepared by injection of the selenium-octadecene precursor into a mixture of octadecene and stearic acid containing MoCl5 at 300°C, which was kept for 30 min (see Materials and Methods for details). The morphologies of the obtained colloidal MoSe2 nanospheres were characterized by a transmission electron microscope (TEM). As shown in Figures 1(a) and 1(b), the prepared MoSe2 nanospheres with size of 75 ± 7 nm (inset in Figure 1(b)) were formed by the assembly of MoSe2 nanosheets. High-resolution TEM (HRTEM) image of a typical edge of MoSe2 nanosheet confirms its single-crystalline nature (Figure 1(c)). The lattice distances of 2.88 Å and 6.89 Å can be assigned to the (100) and (002) planes of 2H phase MoSe2 [37], respectively (Figures 1(c) and 1(d)). The powder X-ray diffraction (XRD) pattern () further confirms that the nanosphere consists of crystalline 2H phase MoSe2 (JCPDS No. 15-0029, hexagonal, a = 3.288 Å, c = 12.89 Å). As shown in , the peaks located at 31.69°, 37.38°, and 56.10° correspond to the (100), (103), and (110) planes of the hexagonal MoSe2, respectively.

Figure 1

TEM and HRTEM measurements of the as-prepared MoSe2 nanospheres. (a, b) TEM images and (c, d) HRTEM images. Inset in (b): statistical analysis of the size of 50 MoSe2 nanospheres measured from TEM images.

The as-prepared MoSe2 nanosheet-assembled nanospheres were then used as templates for the growth of hybrid nanostructures. For example, Ni2P nanoparticles have been grown on the surface of MoSe2 nanospheres to form the MoSe2-Ni2P hybrid nanostructures (see Materials and Methods for details). From the TEM image (Figure 2(a)), it can be seen that the Ni2P nanoparticles are coated on the surface of MoSe2 nanospheres. Based on the XRD analysis (), the as-obtained hybrid nanostructure is composed of crystalline 2H phase of MoSe2 (JCPDS No. 15-0029) and hexagonal phase of Ni2P (JCPDS No. 65-1989, a = 5.859 Å, c = 3.382 Å). The HRTEM image (Figure 2(b)) further confirms that the Ni2P nanoparticles are highly crystalline with a lattice spacing of 2.31 Å attributing to the (111) planes of the hexagonal phase Ni2P. The measured lattice distance of 6.89 Å corresponds to the (002) plane of 2H phase MoSe2. The size of Ni2P nanoparticles in the hybrid nanostructures is 8.7 ± 1.3 nm (). The energy-dispersive X-ray spectroscopy (EDS) spectrum () shows that the estimated molar ratios of Mo/Se and Ni/P are 1/2.2 and 1.6/1, respectively, close to the calculated stoichiometric ratios. The corresponding EDS elemental mapping () confirms the homogeneous distribution of Mo, Se, Ni, and P, further revealing the successful growth of Ni2P on the surface of MoSe2 nanospheres.

Figure 2

TEM and HRTEM measurements of the MoSe2-Ni2P and MoSe2-Co2P hybrid nanostructures. (a) TEM and (b) HRTEM images of MoSe2-Ni2P hybrid nanostructures. (c) TEM and (d) HRTEM images of MoSe2-Co2P hybrid nanostructures.

Importantly, our method is general, which can be used to grow other nanostructures on MoSe2 nanospheres. For example, Co2P nanoparticles can also be grown on the surface of MoSe2 nanospheres to form MoSe2-Co2P hybrid nanostructures (Figure 2(c)). The measured lattice fringes of 6.89 Å and 2.23 Å match well with the (002) planes of MoSe2 and (121) planes of Co2P, respectively. The size of Co2P nanoparticles in the hybrid nanostructures is 7.5 ± 1.4 nm (). The XRD analysis () demonstrates the coexistence of MoSe2 (JCPDS No. 15-0029) and Co2P (JCPDS No. 32-0306, orthorhombic, a = 5.6465 Å, b = 6.6099 Å, c = 3.513 Å). The molar ratios of Mo/Se and Co/P are 1/1.5 and 2.2/1, respectively, as characterized by EDS (). The presence of Mo, Se, Co, and P and their homogeneous distributions can be clearly observed in the EDS elemental mapping of MoSe2-Co2P hybrid nanostructures ().

Besides Ni2P and Co2P nanoparticles, metallic Ni and Co nanoparticles have also been successfully grown on the surface of MoSe2 nanospheres (see Materials and Methods for details). Figure 3(a) shows that the Ni nanoparticles have been grown on the surface of MoSe2 nanospheres to form the MoSe2-Ni hybrid nanostructures. The size of Ni nanoparticles in the hybrid nanostructures is 7.4 ± 1.5 nm (). In the HRTEM image (Figure 3(b)), the measured interplanar distances between lattice fringes are estimated to be 6.89 Å and 2.18 Å, which match well with the (002) planes of MoSe2 and (111) planes of Ni, respectively. The cubic Ni (JCPDS No. 01-1258, a = 3.54 Å) was confirmed by the XRD pattern (). confirms the magnetic property of Ni nanoparticles in the prepared MoSe2-Ni hybrid nanostructures. The EDS () and corresponding elemental mapping () demonstrate the presence of Mo, Se, and Ni and their homogeneous distribution. Using the similar method, the MoSe2-Co hybrid nanostructure can also be prepared (Figure 3(c)). The corresponding HRTEM image reveals that the observed lattice spacing for MoSe2 (002) planes and Co (114) planes are 6.89 Å and 2.26 Å, respectively. The size of Co nanoparticles in the hybrid nanostructures is 6.4 ± 1.4 nm (). The XRD pattern identifies that the MoSe2 (JCPDS No. 15-0029) and hexagonal phase Co (JCPDS No. 65-9722, a = 8.288 Å, c = 10.542 Å) coexist (). confirms the magnetic property of Co nanoparticles in the prepared MoSe2-Co hybrid nanostructures. Furthermore, the presence of Mo, Se, and Co and their homogeneous distributions have been shown in the EDS spectrum () and corresponding elemental mapping ().

Figure 3

TEM and HRTEM measurements of the MoSe2-Ni, MoSe2-Co, and MoSe2-NiS hybrid nanostructures. (a) TEM and (b) HRTEM images of the MoSe2-Ni hybrid nanostructures. (c) TEM and (d) HRTEM images of the MoSe2-Co hybrid nanostructures. (e) TEM and (f) HRTEM images of the MoSe2-NiS hybrid nanostructures.

Moreover, after the sulfurization of the MoSe2-Ni hybrid nanostructure, the MoSe2-NiS hybrid nanostructure can also be synthesized by adding sulfur precursor to the reaction solution (see Materials and Methods for details). The as-synthesized nanostructure was characterized by XRD (). The XRD analysis of the final product reveals a mixture of hexagonal MoSe2 (JCPDS No. 15-0029) and hexagonal NiS (JCPDS No. 02-1273, a = 3.440 Å, c = 5.350 Å). The TEM image (Figure 3(e)) shows that the NiS nanoparticles are uniformly coated on the surface of the MoSe2 nanospheres. The size of NiS nanoparticles in the hybrid nanostructures is 11.7 ± 1.4 nm (). As shown in the corresponding HRTEM image, the lattice spacing of 6.89 Å corresponds to the (002) planes of MoSe2, while the other one of 2.92 Å is attributed to the (100) planes of NiS (Figure 3(f)). The EDS spectrum in shows that the Mo/Se and Ni/S molar ratios are estimated to 1/2.4 and 1/1, respectively, close to the corresponding stoichiometric ratios. The EDS elemental mapping () further demonstrates the homogeneous distributions of Mo, Se, Ni, and S in the as-prepared nanostructure.

As a proof-of-concept application, the aforementioned synthesized hybrid nanostructures, including MoSe2-Ni2P, MoSe2-Co2P, and MoSe2-NiS, were used as catalysts for the electrochemical HER. The electrochemical HER activities were tested in 0.5 M H2SO4 aqueous solution using a standard three-electrode system. Figure 4(a) shows typical polarization curves of these as-prepared hybrid nanostructures and the 20 wt% Pt/C. The MoSe2 nanosphere shows the lowest HER activity with overpotential of 245 mV at 10 mA cm−2. After growth of NiS, Ni2P, and Co2P on the MoSe2 nanospheres, the catalytic activities of hybrid nanostructures were significantly enhanced. The overpotentials at 10 mA cm−2 are 231, 211, and 167 mV for the MoSe2-NiS, MoSe2-Ni2P, and MoSe2-Co2P hybrid nanostructures, respectively. Furthermore, the Tafel slopes were used to evaluate the HER kinetics. As shown in Figure 4(b), the measured Tafel slope of Pt/C is 30.0 mV dec−1, close to the reported value [10-12]. The Tafel slope of MoSe2 nanospheres is 70.1 mV/dec, which decreases to 68.8, 61.5, and 53.2 mV/dec for MoSe2-NiS, MoSe2-Ni2P, and MoSe2-Co2P hybrid nanostructures, respectively. In addition, as shown in Figure 4(c), the electrochemical impedance spectroscopy (EIS) reveals that these hybrid nanostructures exhibit a faster electron/charge transfer rate than does the MoSe2 nanosphere (), suggesting that the growth of NiS, Ni2P, and Co2P on MoSe2 nanospheres can significantly enhance the electrical conductivity. The good electron-transfer kinetics is important for electrocatalysts to exhibit high activity [55-57]. Note that the performance of MoSe2-Co2P is comparable to or even better than those previously reported similar materials for the acid HER ().

Figure 4

HER electrochemical performances of the hybrid nanostructures. (a) Polarization curves of the 20 wt% Pt/C, MoSe2-Co2P, MoSe2-Ni2P, and MoSe2-NiS hybrid nanostructures, and MoSe2 nanosphere with iR correction. (b) The corresponding Tafel plots. (c) Nyquist plots of MoSe2-Co2P, MoSe2-Ni2P, and MoSe2-NiS hybrid nanostructures and MoSe2 nanosphere collected at a bias voltage of -150 mV. Inset: the equivalent circuit used for fitting the Nyquist plots. (d) Chronopotentiometry response for MoSe2-Co2P hybrid nanostructures at 10 mA cm−2 for 12 h.

It is important to understand the mechanism for the improved HER performance of MoSe2-Co2P hybrid nanostructures. First, the more postgrowth nanostructures mean the more exposure of active sites, enabling a high utilization ratio of catalysts [58-61]. The effect of the surface area on HER was evaluated through the electrochemical double-layer capacitance (Cdl, ). The fitted Cdl of MoSe2-Co2P is close to that of MoSe2, revealing that the promoted HER performance should be derived from the enhanced intrinsic activity rather than the increased electrochemical active specific area or active sites. On the other hand, the Rct value of the MoSe2-Co2P hybrid nanostructures is smaller than the pure MoSe2 nanospheres (), indicating the fastest charge transfer process. These results clearly show the crucial role of the synergetic effect between MoSe2 and Co2P in MoSe2-Co2P hybrid nanostructures, which is responsible for the enhanced electrochemical hydrogen evolution.

The stability of the MoSe2-Co2P hybrid nanostructures for HER catalysis was tested using the chronopotentiometry method. As shown in Figure 4(d), only a slight voltage drop is observed even after 12 h electrolysis of water at current density of 10 mA cm−2, indicating the excellent stability of the MoSe2-Co2P hybrid nanostructures. Moreover, after the stability test, the polarization curve only shows a slight negative shift () as compared with the initial one. These results imply the long-term stability of MoSe2-Co2P catalyst for HER.

3. Discussion

In summary, by using the synthesized MoSe2 nanospheres as templates, nickel and cobalt-based nanomaterials can be synthesized on them to form a series of the MoSe2 nanosphere-based hybrid nanostructures, including MoSe2-Ni2P, MoSe2-Co2P, MoSe2-Ni, MoSe2-Co, and MoSe2-NiS. Importantly, as a proof-of-concept application, when used as electrocatalysts, the MoSe2-Ni2P, MoSe2-Co2P, and MoSe2-NiS hybrid nanostructures exhibited high HER activities in acidic environment. Among them, the MoSe2-Co2P hybrid nanostructures exhibit excellent stability and highest HER activity with an overpotential of 167 mV at 10 mA cm−2. We believe that as a general method for hybridizing layered TMD nanostructures with transition metal chalcogenide/phosphide nanocrystals, this strategy is applicable for growth of many other nanomaterials to form hybrid nanostructures with enhanced electrochemical activity and stability.

4. Materials and Methods

4.1. Chemicals

Molybdenum chloride (MoCl5) was purchased from Alfa Aesar. Sulfur, selenium, nickel acetylacetonate (Ni(acac)2), cobalt acetylacetonate (Co(acac)2), trioctylphosphine (TOP), octadecene (ODE), stearic acid (SA), oleylamine (OLA), and toluene were purchased from Aldrich. All the chemicals were used as received without further purification.

4.2. Synthesis of MoSe2 Nanosheet-Assembled Nanospheres

In a typical procedure, 0.5 mmol of MoCl5, 1 g of SA, and 9 mL of ODE were added into a 100 mL three-necked flask. The aforementioned mixture, denoted as solution A, was degassed under a vacuum at 120°C for 10 min. Then, the temperature was heated up to 300°C under nitrogen. At the same time, 1 mmol of selenium powder was dissolved into 2 mL of ODE at 250°C, which was denoted as solution B. Solution B was cooled to 130°C and subsequently injected into solution A when the temperature of mixed solution finally reached 300°C, which was then kept at 300°C for 30 min. After it was cooled down to room temperature, the product, i.e., MoSe2 nanospheres, was collected by centrifugation (10000 rpm, 5 min) and washed several times with toluene and acetone (technical grade) before further characterization. All products in this work were collected and washed by the same method.

4.3. Synthesis of MoSe2-Ni2P, MoSe2-Co2P, MoSe2-Ni, MoSe2-Co, and MoSe2-NiS Hybrid Nanostructures

In order to synthesize the MoSe2-Ni2P hybrid nanostructures, the freshly prepared MoSe2 nanospheres were redispersed into 10 mL of OLA containing 0.5 mmol Ni(acac)2 and 2 mL TOP. The mixture was degassed under a vacuum at 110°C for 10 min. Then, the temperature was increased to 320°C under nitrogen atmosphere and kept for 1 h before cooling down to room temperature. The MoSe2-Co2P hybrid nanostructures were synthesized by the same method, except that Co(acac)2 was used instead of Ni(acac)2. The preparation procedure of MoSe2-Ni and MoSe2-Co hybrid nanostructures was the same as the aforementioned procedure except that the reaction temperature was changed to 220°C from 320°C. For the synthesis of MoSe2-NiS hybrid nanostructures, the procedure was the same as that used for synthesis of MoSe2-Ni hybrid nanostructures, except that after the reaction was proceeded at 220°C for 1 h, 2 mL of sulfur powder (0.5 mmol) in oleylamine (OLA) was injected. The resulting mixed solution was kept at 220°C for another 1 h.

4.4. Characterization

The XRD were recorded on a Bruker D8 diffractometer (German) with a slit of 1/2° at a scanning rate of 2° min−1, using Cu K radiation (λ = 1.5406 Å). TEM, HRTEM, and EDS mapping characterizations were performed on JEOL 2010F (Japan) and JEOL 2100F (Japan) with an acceleration voltage of 200 kV.

4.5. Electrode Preparation

Experimentally, 4 mg of acetylene black was first mixed with 5 mL of hexane and then sonicated to form a uniform suspension. Then, 8 mg of MoSe2, MoSe2-Ni2P, MoSe2-Co2P, or MoSe2-NiS in hexane was added into acetylene black suspension under sonication for 1 h. The mixed catalysts were separated by centrifugation, then washed with ethanol. The prepared catalysts were annealed at 400°C for 30 min under a flow 5% H2/Ar to remove surfactants. 4 mg of the respective catalyst powders was dispersed in 1 mL of 1 : 1 (v/v) water/ethanol mixed solvents under ultrasonication for 1 h. 5 μL of the resulting solution was dropped onto the surface of a cleaned glassy carbon electrode by a microliter syringe and dried at room temperature. After drying at room temperature, the surface of the catalyst-based electrode was covered by 5 μL of 1% Nafion solution.

4.6. Electrochemical Measurements

All the electrochemical experiments were performed on an electrochemical workstation (CHI 760C, CH Instruments Inc., USA), using a conventional three-electrode system, i.e., a Hg/Hg2Cl2 (saturated KCl, SCE) reference electrode, a carbon rod counter electrode, and the prepared working electrode. 0.5 M H2SO4 aqueous solution was used as the electrolyte throughout the experiments. Before the electrochemical measurement, the electrolyte was degassed by bubbling N2 for at least 30 min. The polarization curves were obtained by sweeping the potential from 0 to -0.6 V (vs. SCE) at a sweep rate of 2 mV s−1. Electrochemical impedance spectroscopy (EIS) was recorded in 0.5 M H2SO4 aqueous solution using an alternating current (AC) voltage of 5 mV and direct current (DC) voltage of -0.15 V (vs. RHE) within the frequency range from 100 kHz to 0.1 Hz. Voltage-time responses were monitored by chronopotentiometry measurement at 10 mA cm−2 for 12 h.