Designing hierarchical hollow nanostructures of Cu₂MoS₄ for improved hydrogen evolution reaction

Abstract

Layered Cu₂MoS₄, consisting of earth-abundant elements, is regarded as a potential catalyst for the hydrogen evolution reaction (HER). Herein, we demonstrate a Cu₂O-based template strategy to synthesise hierarchical hollow nanostructures of Cu₂MoS₄. The characterizations reveal that the electrochemically active surface of the hollow Cu₂MoS₄ is largely enhanced, in contrast to the nanosheet or nanoparticle structures. As the direct outcome, the designed hierarchical hollow structures display excellent HER activities with a low overvoltage and small Tafel slope. This study may provide new inspiration for the research of other ternary sulphide materials as well as subsequently accelerating their applications in the field of catalysis.

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Introduction

The overuse of fossil fuels and the continuous deterioration of the environment have seriously hindered the sustainable development of the modern society.¹ It is urgent to explore new ways to face these challenges. Hydrogen, as a clean energy source, has been considered as one of the most promising candidates to replace traditional energies and to help human beings to solve the above problems in the future.² In the past few decades, tremendous efforts had been carried out to develop effective approaches for practical hydrogen production.³ Among these, the electrocatalytic hydrogen evolution reaction (HER) is treated as a feasible and important method. Pt and its alloys are the best known HER catalysts with low overpotentials and high current densities.⁴ However, high costs and scarce resources makes it difficult for them to be widely used in practical applications. So it's necessary to explore new noble-metal-free HER electrocatalysts. Up to now, much attention has been focused on exploiting earth-abundant solid-state inorganic materials for HER; for example, transition metal alloys, carbides, nitrides, phosphides, sulphides, and so on.^5–9

Recently, increasing interest has been devoted to two dimensional (2D) transition metal chalcogenide (TMC) nanomaterials which display exceptional mechanical, catalytic, optical and electronic properties, due to both their layered structure and the presence of transition metal d-electrons.¹⁰ Many binary TMCs have been widely investigated, such as MoS₂, TiS₂, WS₂, MoSe₂, etc.¹¹ The above TMCs play important roles for HER. It was demonstrated that MoS₂ nanoparticles exhibit excellent HER activities, mainly owing to the number of exposed edge sites which were suggested to be catalytically active.¹² But as far as we know, there is little research about the synthesis and applications for ternary layered TMCs.¹³ Inspired by the great success of binary TMCs, it is significant to develop new methods to prepare ternary layered TMCs for further applications. Cu₂MoS₄ is a new ternary layered TMC material, which has two allotropes with the space groups: P4̄2m and I4̄2m, respectively.¹⁴ Previous reports have shown that Cu₂MoS₄ would be a promising HER electrocatalyst because of its low overvoltage requirement and good stability in an aqueous solution with a wide pH range.^15,16 However, just like MoS₂ or its analogues, the exposure of the catalytically active sites of Cu₂MoS₄ is the bottleneck. Herein, we develop a new hard-template strategy to controllably fabricate Cu₂MoS₄ hierarchical hollow nanostructures (HHNSs), and it is found that the synthetic Cu₂MoS₄ HHNSs perform well for HER catalysis.

Cu₂MoS₄ HHNSs are prepared by a solvothermal method where Cu₂O nanocrystals are used as sacrificial templates. Cu₂O, a p-type semiconductor (2.2 eV), has been widely used in photocatalysis, solar cells or other areas by the controllable synthesis of Cu₂O.^17,18 What's more, as excellent sacrificial templates, Cu₂O nanocrystals have been widely adopted to prepare many kinds of hollow nanostructures for various applications.¹⁹ We adopt Cu₂O as the copper source, as well as the template, with sodium molybdate (Na₂MoO₄) and thioacetamide (C₂H₅NS) as the molybdenum and sulfur source, respectively. All three chemicals are environmentally-friendly and low-priced. In addition, we use ethylene glycol (EG) as a solvent, which is widely adopted to synthesize a variety of nanocrystals, such as noble metals, transition metal oxides and sulfides.²⁰ On the other hand, the reducibility of EG can prevent the oxidation of Cu⁺ effectively. It has been demonstrated that the proposed approach is practical to obtain high quality Cu₂MoS₄ nanostructures.²¹

Experimental

Preparation of Cu₂MoS₄ hierarchical hollow nanostructures

To prepare Cu₂MoS₄ hierarchical hollow nanostructures, the Cu₂O template was firstly prepared according to previous research.²² 3.3 mg the Cu₂O powder was dispersed in 20 mL ethylene glycol with stirring for 10 min. Then 10 mg thioacetamide and 5 mg sodium molybdate were added to the mixed solution. After 30 min stirring, the solution was transferred to a 45 mL Teflon-lined autoclave and heated at 160 °C for 10 h. The products were centrifuged and washed with deionized water and pure ethanol several times. Finally, the samples were dried at 60 °C under vacuum for further use.

Characterization of the synthetic samples

X-ray powder diffraction (XRD) patterns were carried out by a RigakuTTR-III X-ray diffractometer with Cu K_α radiation (λ = 1.5418 Å). The elemental valence of the as-prepared products were characterized by X-ray photoelectron spectroscopy (XPS, ThermoESCALAB250). The morphologies of the samples were obtained by using scanning electron microscopy (SEM, JSM-6700F, 5 kV) and transmission electron microscopy (TEM, JEOL JEM-2100F microscope, 200 kV).

Electrochemical measurements

A conventional three-electrode cell with a workstation (CHI 660E) was used to carried out the electrochemical measurements. In a typical process, 4 mg of the sample mixed with 30 μL Nafion solution (Sigma Aldrich, 5 wt%) was dispersed in a mixture of 750 μL Milli-Q water and 250 μL ethanol for at least 1 h. Then, 5 μL of this ink was dropped onto a glassy carbon electrode (GCE) with a diameter of 3 mm to form a catalyst loading of 0.285 mg cm⁻². The reference electrode was an Ag/AgCl (3.0 M KCl) electrode and we used a graphite rod as the counter electrode. Linear sweep voltammetry was recorded with a scan rate of 5 mV s⁻¹ in 0.5 M H₂SO₄ (purged with pure N₂ for 30 min). All the potentials in this work were quoted against a reversible hydrogen electrode (RHE) unless noted otherwise. The electrochemical active surface areas (ECSA) were determined by taking cyclic voltammetry measurements at various scan rates ranging from 20 to 120 mV s⁻¹ in the potential window of −0.10 to 0.10 V versus Ag/AgCl. Then, linear fitting of the charging current density differences (Δj = j_a − j_c) at the potential of 0.00 V versus Ag/AgCl was carried out. The linear slope is twice that of the double-layer capacitance C_dl. All the above measurements were carried out without ohmic-drop correction.

Results and discussion

The synthetic procedure for Cu₂MoS₄ HHNSs is shown in Fig. 1. In our design, the Kirkendall effect plays an important role in the formation process. It means that the MoS₄²⁻ ions in the solution diffuse onto the surface of the Cu₂O nanocrystals, while the Cu⁺ cations migrate from Cu₂O to the forming Cu₂MoS₄ interface, and this exchange reaction is thermodynamically favourable. The lasting supply of MoS₄²⁻ dianions from the solution phase is an important factor for the formation of Cu₂MoS₄ HHNSs. In the solvothermal reaction system, MoS₄²⁻ is derived according to the following reaction:

MoO₄²⁻ + C₂H₅NS → MoS₄²⁻ + NH₄⁺ + OH⁻

Fig. 1 Illustration showing the formation mechanism of Cu₂MoS₄ hierarchical hollow nanostructures.

The concentration of MoS₄²⁻ is controlled by the reaction above. Because the reaction itself takes time, a longer presence of this anion is anticipated for the formation of the shell. Owing to the simultaneous etching effect in the reaction solution, hierarchical structures are obtained.

As expected, the morphology and size of the Cu₂O template have a significant effect on the final product of Cu₂MoS₄ HHNSs. We prepared different Cu₂O nanocrystals by tuning the reaction conditions (ESI,† Table S1). The SEM and TEM images of the synthetic Cu₂O samples are shown in Fig. S1 (ESI†). It is observed that hollow nanoflowers are obtained when Cu₂O nanospheres with a size of about 60–80 nm are used (shown in Fig. 2). These hollow nanoflowers have a diameter of about 100–150 nm and they are composed of many primary nanocrystals with a size of about 20–30 nm, according to the SEM (Fig. 2A) and TEM (Fig. 2B and C) images. The HRTEM image is also given in Fig. 2D and the measured lattice distance is 5.39 Å, corresponding to the (010) face of Cu₂MoS₄ with a P-phase. Cu₂MoS₄ HHNSs with other morphologies and sizes were also constructed, such as octahedral HHNSs and cubic HHNSs, which are shown in Fig. 3. Herein, we select the hollow nanoflower to be the reference, in order to investigate the features and potential applications of the series of Cu₂MoS₄ HHNSs. In addition, Cu₂MoS₄ nanosheets and nanoparticles were also prepared as comparative samples by a method that was proposed in a previous work (ESI,† Fig. S2).^21,23

Fig. 2 The morphologies and structures of Cu₂MoS₄ hollow nanoflowers. (A) SEM image; (B and C) TEM images with different magnifications; (D) high resolution TEM image.

Fig. 3 SEM and TEM images of octahedral (A, B, C, D) and cubic (E, F, G, H) hierarchical hollow nanostructures and of Cu₂MoS₄. A, B, E and F are SEM images with different magnifications; C and H are typical TEM image of single hollow structures while D and G show the detail of the shells.

The composition and elemental valence state of the synthetic flower-like sample were obtained using X-ray photoelectron spectroscopy (XPS). Fig. 4A gives the binding energies of Cu 2p, Mo 3d and S 2p, and the data suggests that the elemental valence states of Cu, Mo and S are +1, +6 and −2 valences, respectively. The energy dispersive X-ray (EDX) elemental mapping results (Fig. 3B–D and ESI,† Fig. S3) for Cu₂MoS₄ hollow nanoflowers suggest that these three elements in the samples are homogeneously distributed.

Fig. 4 High resolution XPS spectra of Cu₂MoS₄ hollow nanoflowers: Cu 2p, Mo 3d and S 2p; (A) Cu 2p, Mo 3d and S 2p photoemission spectra; EDX elemental mapping images of (B) Cu K_α1, (C) Mo K_α1 and (D) S K_α1.

The structures of the Cu₂MoS₄ samples (hollow nanoflower, nanosheet and nanoparticle) were analyzed by XRD and the patterns are shown in Fig. 5. It reveals that the as-prepared Cu₂MoS₄ hollow nanoflower and nanoparticle structures are both P-phase, while the nanosheet is I-phase. The diffraction peaks of the nanosheet pattern are strong and sharp, suggesting that the nanosheets are well-crystalline. The accurate structure of the I-Cu₂MoS₄ nanosheets has been defined in previous work.²¹ Owing to the reduced size all of the diffraction peaks of the Cu₂MoS₄ nanoparticle, and especially those of the hollow nanoflower, are broadened to some degree. Combined with the SEM and TEM data, the XRD results indicate that the primary particles of the hollow nanoflowers are smaller than the simple nanoparticles. In addition, it can be seen that the (001) peak of the nanoparticle sample is stronger than the (010) peak. However, it is quite different for the hollow nanoflower, whose (001) peak almost disappears, while the (010) feature is obvious. A similar result was also found in previous work.²³

Fig. 5 XRD patterns of Cu₂MoS₄ nanostructures: hollow nanoflower (P phase), nanosheet (I phase) and nanoparticle (P phase).

The hydrogen evolution reaction (HER) was carried out to demonstrate the potential application of these Cu₂MoS₄ HHNSs in the H₂ energy field. The HER activities of the Cu₂MoS₄ hollow nanoflower, nanosheet and nanoparticle structures were investigated in 0.5 M H₂SO₄ solution using a typical three-electrode setup. The result is shown in Fig. 6 and Table 1. The polarization curve from the Cu₂MoS₄ hollow nanoflower displays an onset overpotential of −165 mV for HER, and this is lower than that of the nanoparticle and nanosheet. What's more, the Tafel slope of the hollow nanoflower is also the smallest among the three samples, and is only 64 mV dec⁻¹. The electrochemical results indicate that the Cu₂MoS₄ hollow nanoflower exhibits better HER activity than the nanosheet and the nanoparticle. In addition, the durability of the Cu₂MoS₄ samples was assessed by an accelerated cyclic test, from −0.3 V to 0.2 V vs. RHE for 1000 cycles. It was found (shown in the ESI,† Fig. S4) that the polarization curve of the nanosheet catalyst shows no evident change from the initial test after continuous long-term cycling in an acidic condition. However, the current density of both the nanoflower and the nanoparticle degrades to some degree after 1000 CV cycles. Nevertheless, the nanoflower showed better stability than the nanoparticle.

Fig. 6 The polarization (A) and Tafel (B) curves of HER electrodes composed of Cu₂MoS₄ hollow nanoflowers, nanosheets and nanoparticles. Electrolyte: 0.5 M H₂SO₄, scan rate: 5 mV s⁻¹.

Table 1 The onset potentials and Tafel slopes of different catalysts

Catalyst	Onset potential [mV]	Tafel slope [mV dec^−1]
Hollow nanoflower	−165	64
Nanoparticle	−175	71
Nanosheet	−240	98

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Based on Tran's research,¹⁵ Cu₂MoS₄ is confirmed to act as an active catalyst, not as a precursor of MoS₂ or MoS₃, and Cu₂MoS₄ remains intact under the catalytic hydrogen evolution condition. Regardless of the two potential intermediate states for Cu₂MoS₄: [H-Mo^VI(μ-SH)Cu^I] and [Mo^V{(μ-SH)Cu^I}₂], the key point lies in the exposure of potential active sites, which induces the activity difference of the Cu₂MoS₄ hollow nanoflower, nanosheet and nanoparticle. To better understand the different catalytic performances of these three types of Cu₂MoS₄ nanostructure, we further compared the ECSA (electrochemically active surface areas) of these three samples using their double-layer capacitances (C_dl), as shown in Fig. 7. The ECSA of the hollow nanoflower is 0.160 ± 0.007 mF cm⁻², which is 1.5-fold the ECSA of the nanoparticle and 2.7-fold the ECSA of the nanosheet, which does not account for the dramatic catalytic improvement in HER. Therefore, we later compared the nanostructures of the three samples. Among the three samples, although the thickness of the Cu₂MoS₄ nanosheet is in the nanoscale, both the length and width are on a micrometer scale. So we can treat the nanosheet as the so-called bulk material, compared to the hollow nanoflower and nanoparticle, and it is easy to understand that the large nanosheet is less active through the size effect, which says that a nanomaterial with a smaller size usually has a better activity than the larger one. However, it is hard to compare the HER activity of the Cu₂MoS₄ hollow nanoflower and nanoparticle simply using the size effect, although the primary particle of the hollow nanoflower is actually smaller than the nanoparticle, which is certified by both electron microscopy (SEM, TEM) and XRD data. In contrast, smaller particles aggregate easier and this hinders further enhancement of the catalytic activity. So the excellent performance of the Cu₂MoS₄ hollow nanoflower may derive from its unique hierarchical hollow structure. The three-dimensional (3D) feature of the hollow nanoflower with high surface area not only gives more active sites for catalytic reaction, but also prevents the disadvantageous effect of particle aggregation effectively. What's more, the robust hollow frame is also beneficial to maintain the stability. So, the construction of Cu₂MoS₄ HHNSs provides a new opportunity to enhance the HER performance. However, up to now many questions about the accurate mechanisms of Cu₂MoS₄-based HER catalysts are still unanswered. More theoretical and experimental research should be carried out in the future.

Fig. 7 (A) Typical cyclic voltammetry curves of the Cu₂MoS₄ hollow nanoflower electrode in 0.5 M H₂SO₄ with different scan rates; (B) ΔJ (J_a − J_c) of the Cu₂MoS₄ hollow nanoflower, nanoparticle and nanosheet plotted against scan rates. All the error bars represent the standard deviations of three replicate measurements. The slopes (2C_dl) were used to represent ECSA. The unit of the slopes is mF cm⁻².

Conclusions

In summary, we constructed a novel hierarchical hollow nanostructure of Cu₂MoS₄, which is earth-abundant and inexpensive. A facile and environmentally-friendly solvothermal method with Cu₂O nanocrystals as hard templates was used in the synthetic process. With unique structural features and highly exposed reactive sites, the designed Cu₂MoS₄ hierarchical hollow nanostructures exhibited an excellent activity for HER. In addition, the synthetic strategies may give new inspiration for the research of other ternary sulphide materials, which will largely accelerate their applications in the fields of catalysis and clean energy.

Acknowledgements

We acknowledge the financial support from the ministry of science and technology of China (2014CB848900, 2015CB932301), the National Natural Science Foundation of China (U1232131, U1532112, 11375198, 11574280), the Fundamental Research Funds for the Central Universities (WK2310000053, WK6030000031), User with Potential from CAS Hefei Science Center (2015HSC-UP020). L. S. thanks the recruitment program of global experts and the CAS Hundred Talent Program of China. The authors thank Dr Hongliang Jiang for useful discussions, MCD and Photoemission Endstations in NSRL for helps in X-ray measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp07269k

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