Xuan Thai Tran,
Sujittra Poorahong and
Mohamed Siaj*
Department of Chemistry and Biochemistry, Université du Quebec à Montréal, Montréal, QC H3C 3P8, Canada. E-mail: siaj.mohamed@uqam.ca
First published on 10th November 2017
The development of a platinum-free electrocatalyst for the hydrogen evolution reaction (HER) is highly essential to the large-scale production and application of water splitting devices. Herein we report a facile one-pot hydrothermal synthesis of composite MoSe2@Cu2Se. The morphology of the obtained material was characterized by scanning electron microscope (SEM) and it was found that the composite material formed a sand rose-like structure. The crystal structure and phase purity of the composite MoSe2@Cu2Se were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Then a selective electrochemical etching of copper from the composite was carried out and the porous MoSe2 rose-like nanosphere was obtained. The robust 3D MoSe2 rose-like structure exhibit remarkable activity and durability for electrocatalytic HER in acid maintaining a small onset overpotential of ∼150 mV and keeping a small overpotential of 300 mV for 6 mA cm−2 current density after 1000 cycles. Based on our data, the obtained porous sand rose-like structure material could improve the active surface area which yields higher HER catalytic activity. The present study provides a simple and effective way for the exploration of efficient Mo-based HER catalysts.
In recent years, many alternatives for non-precious metal-based electrocatalysts have been developed, including transition metal sulfides, selenides, borides, carbides, nitrides, phosphides, and a molecular catalyst family. Among all these alternatives a type of efficient electrocatalysts based on transition metal dichalcogenide (TMD) has recently received significant attention, because of its exotic electronic structure and these physical properties.10 TMD is a family of materials consisting of more than 40 compounds having the generalized formula MX2, wherein M is a transition metal, typically 4–7 groups and X is a chalcogen such as sulfur (S), selenium (Se) and tellurium (Te). These transition metals have an important catalytic behavior in the HER. The latter trend has formed with superimposed layers of weak interactions of van der Waals between two adjacent layers,10 this form has two large areas on the sides (active sites) for the adsorption of ions. It has been proven by further research that TMD slips active sites play an important role in HER. Among them, MoSe2 is a newly emerging catalyst owing to its low cost, high chemical stability, and excellent electrocatalytic activity.11–15 The electrocatalytic HER activity of MoSe2 depends strongly on its active selenium edge sites, while its basal planes were catalytically inert.16 In order to obtain, a high performance MoSe2 electrocatalyst towards the HER is to rationally construct the nanostructure for mass transfer and maximizing the number of active sites. A boost of the catalytic activities could be achieved by reducing the MoSe2 crystal size to nanoscale level and by increasing the exposures of its active edges to the electrolytes. In one aspect, a high surface area with an open structure avail the diffusion of electrolytes, will lead smoothly to the replenishment of the consumed protons and lower the ohmic drop at high reaction rate.17
Herein, we demonstrate the preparation of a 3D hierarchical porous MoSe2 by a combination of hydrothermal and chemical etching methods. To the best of our knowledge, here is the first report of the MoSe2@Cu2Se synthesis by hydrothermal method. After MoSe2@Cu2Se synthesis, the copper is selectively etched from the alloy leading to a 3D porous MoSe2 structure formation (sand rose-like structure). The 3D porous MoSe2 material is tested as a catalyst for HER. A comparison between pure MoSe2 and porous MoSe2 sand rose-like structure shows that the porous one exhibits higher catalytic activity for electrocatalytic HER.
Fig. 1 (A) The procedure of the MoSe2@Cu2Se powder synthesis. (B) The expected morphology after etching process. |
Linear sweep voltammetry (LSV) was performed in 0.5 M H2SO4 saturated using an Ag/AgCl as the reference electrode, and a carbon as the counter electrode. All the potentials were calibrated to a reversible hydrogen electrode (RHE). LSV was recorded by sweeping the potential from 0.2 to −0.8 V vs. RHE with a scan rate of 1 mV s−1 at room temperature. Cyclic voltammetry (CV) was conducted for 1000 cycles between 0.1 V to −0.8 V (vs. RHE) at 100 mV s−1 for the stability test. The Nyquist plots were measured with frequencies ranging from 200 kHz to 100 mHz at an overpotential of −250 mV. The impedance data were then plotted to a simplified Randles circuit to extract the series and charge-transfer resistances.
Fig. 2 (A) The XRD pattern and (B) the SEM morphology of the as-prepared materials grown by solvothermal method (i) MoSe2, (ii) Cu2Se, (iii) MoSe2@Cu2Se and (iv) porous MoSe2 after etching the Cu2Se. |
Fig. 2B-(i) presents the morphology of pure MoSe2 produced by the solvothermal process involving (NH4)6Mo7O24·4H2O as the Mo source and SeO2 as the Se source. The morphology of pristine MoSe2 can be described as the rose-like microsphere that consists of a large number of petals. While Cu2Se has an irregular morphology in the form of dense nanoplates and/or nanocrystals, Fig. 2B-(ii). Even though pure MoSe2 and composite MoSe2@Cu2Se have rose-like microsphere morphology, the petals of pure MoSe2 are thinner than that from MoSe2@Cu2Se composite. Also, compared spaces between the petals of the material composite are denser than MoSe2. Fig. 2B-(iv) shows SEM images indicate that after etching the composite of the rose-like structures remains intact with more porous and opened structure. Such enormous nanosheets could provide a large number of active sites accompanied with a large specific surface area. The dispersion homogeneity of the different components is supported by the selected element mapping of Mo, Cu and Se (ESI Fig. S1†). The mapping of MoSe2@Cu2Se composite sample demonstrates clearly the existence of each element, in addition to being well distributed over the composite material. After the etching process, no copper element was detected. This mapping confirms that the etching process has been done successfully.
The low-resolution TEM images in Fig. 3A and B indicate the rose-like structure formation of porous MoSe2. High-resolution transmission electron microscopy (HRTEM) images of the MoSe2 reveal the microscopic phase information as well as the thickness of the MoSe2. It can be seen that each section of the nanoflowers presenting a shape of a petal is actually an individual stack of 2D MoSe2 thin layers. A large amount of active sites can be attributed to widely distributed petals, which would offer much more active sites for HER. The spacing between two adjacent monolayers is 0.277 nm, which is consistent with the value of MoSe2 interlayer spacing of the (100) plane (Fig. 3C). The selected area electron diffraction (SAED) results also reflect the (002) planes of 2H-MoSe2 clearly in the inset Fig. 3C. Fig. 3D shows the other plane of MoSe2 obtained from the average values for five layers is 0.72 nm, in good accordance with the thickness of the atomic layer of Se–Mo–Se unit where the c-axis orients normal to the (002) lattice plane. Therefore, we conclude that the MoSe2 porous microspheres are composed of MoSe2 monolayer flakes in an incompact way. Furthermore, the comparison between TEM images of pure MoSe2 (ESI Fig. S2A†) and MoSe2 after etching process (ESI Fig. S2B†), the pure MoSe2 looks denser than those MoSe2 after etching. On the other word, after took Cu2Se out from composite material, MoSe2 becomes hollow. To prove that the specific area increased indeed, Brunauer–Emmett–Teller (BET) method was used to measure the surface area of the pure and etched MoSe2. ESI Fig. S3A† shows nitrogen adsorption and desorption isotherms for the porous MoSe2 sample. It showed a hysteresis loop curve, which is the characteristic of a mesoporous material. BET specific surface area for the porous MoSe2 sample was 33.59 m2 g−1 while that the porous MoSe2 was only 12.31 m2 g−1. So, the surface area of the composite increased by about 2.72 times compared with pure MoSe2. Then the density functional theory (DFT) was applied to calculate the pore size distribution from the adsorption isotherm. As you can see in ESI Fig. S3B,† the material possesses the micropores characteristic from the range of 23–54 Å. It has been expected that the relatively large surface area of the as-prepared porous MoSe2 rose-like microspheres not only can provide more active sites but also enhance the conductivity, which may improve the performance for a further application.
Then the Raman spectroscopy of all samples has been carried out. The observation of the Raman spectra of MoSe2 (Fig. 4A-(i)) can be noted that there are two resonance peaks at 238 cm−1 and 283 cm−1 which can be well indexed to the out-of-plane A1g and in-plane E12g modes of 2H-MoSe2, respectively.18 In the Cu2Se Raman spectrum (Fig. 4A-(ii)), an active mode A1g at 257 cm−1 is observed. It is corresponding to Cu–Se vibration and is in good agreement with the literature.19 In the Raman spectrum of MoSe2@Cu2Se composite (Fig. 4A-(iii)), it can be noted that the two-mode vibration at 237 cm−1 MoSe2 and at 256 cm−1 Cu2Se appears in this spectrum. After the etching process (Fig. 4A-(iv)), the sample exhibits two Raman peak at 237.1 cm−1 and 283.1 cm−1. In addition, as can be seen, the A1g peak intensity is much higher than the E12g peak at pure MoSe2 and MoSe2 composite. This Raman peak corresponding to the out-of-plane Mo–Se phonon mode is preferentially excited for the edge-terminated perpendicularly-oriented nanosheets.11 Moreover, after etching process, the E12g peak intensity is much higher than A1g peak where the E12g peak is preferentially excited for terrace-terminated film. It can be supported the porous structure MoSe2 formation. These make a relatively weak layer–layer interactions in the porous MoSe2 nanosheets happen and in-plan Mo–Se phonon occurs. Photoluminescence spectroscopy was used to investigate the optical emission properties of porous MoSe2. As shown in ESI Fig. S4† the porous MoSe2 shows bandgap energy of 1.54 eV.
Chemical compositions on the surface and valence states of the porous MoSe2 rose-like microspheres were further investigated by X-ray photoelectron spectroscopy (XPS) measurements. Fig. 4B shows the XPS survey spectrum of MoSe2 after etched Cu2Se out. In which the peaks derived from Mo, Se, C and O elements were detected; no Cu peak was observed compared to composite materials sample (ESI Fig. S5†). Generally, a small amount of oxygen may be due to surface adsorption of oxygen, and the C 1s peak located at 284.6 eV mainly results from the contamination from the used carbon conductive tabs. The core-level XPS spectra of Mo 3d shows the binding energies at 229 eV and 232.1 eV belong to Mo 3d5/2 and 3d3/2 spin orbit peaks of MoSe2, confirming the elemental chemical state of Mo is mainly the Mo4+ oxidation state in the hexagonal 2H phase of MoSe2. In case of Se, two fitted peaks at 55.4 and 54.5 eV attributable to the core levels of Se 3d3/2 and Se 3d5/2, respectively, are further illustrating Se2− of the MoSe2.
All of the above characterization results prove that the hybrid MoSe2@CuSe2 is the individual compound and formation can be described as followed. Under the solvothermal condition, as the temperature and pressure increase, Mo(VI) reduced to Mo(IV) by C2H8N2 and then reacts with Se from the decomposition of SeO2 to form MoSe2. At the same time, the Cu2Se nanoparticles also form via stacking of a redox reaction occurred between the copper and the Sex2− to form Cu2Se crystal.19 The generated MoSe2 and Cu2Se nucleus accumulates and leads to nanosheets growth. With the increasing time, the nanosheets of both materials tend to aggregate to form 2D layer and several layers stack under the influence of the hydrogen bonding interaction and thermodynamic stability20 and finally formed 3D hierarchical microsphere-like of composite MoSe2@Cu2Se. As resulting in the elements are well distributed over the composite material. Furthermore, the etching process does not inhibit the MoSe2 properties.
Impedance spectroscopy (Fig. 6A) revealed that the pure MoSe2 films themselves contributed significantly to the series resistance in addition to the substrate and solution resistances. Consequently, this may contribute to the observed trends in turnover frequency. The high degree of resistance from the pure MoSe2 is not surprising, given that MoSe2 possesses semiconducting as well as anisotropic charge transport properties. However, the charge transfer resistance of the porous MoSe2 was much smaller. The material showed a charge transfer resistance of about 150 Ω. These results suggest that the conductivity of the porous MoSe2 is much higher than that of pure MoSe2, which indicates a much faster electron transfer process during electrochemical reaction. Other than high electrocatalytic activity, good durability is another important criterion in the selection of electrocatalysts. Here, MoSe2 porous microsphere catalyst was continuously cycled for 1000 times in 0.5 M H2SO4. The polarization curves before and after 1000 cycles were compared as shown in Fig. 6B. Little HER activity loss is discernible, indicating that the 3D porous structure of MoSe2 rose-like microspheres is highly stable and no significant electrocatalytic active sites were lost during the cycles.
Fig. 6 (A) EIS Nyquist plots of pure MoSe2 and porous MoSe2. (B) Polarization curves of MoSe2 porous microsphere catalyst before and after 1000 potential cycles in 0.5 M H2SO4. |
Footnote |
† Electronic supplementary information (ESI) available: One-pot hydrothermal synthesis and selective etching method of porous MoSe2 sand rose-like structure for electrocatalytic hydrogen evolution reaction. See DOI: 10.1039/c7ra10001a |
This journal is © The Royal Society of Chemistry 2017 |