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Heterostructured MoS2@Bi2Se3 nanoflowers: A highly efficient electrocatalyst for hydrogen evolution
Journal of Catalysis 381 (2020) 590–598
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Heterostructured MoS2@Bi2Se3 nanoflowers: A highly efficient electrocatalyst for hydrogen evolution Dong Li a,1, Jie Lao a,1, Chunli Jiang b,c,⇑, Yang Shen a, Chunhua Luo a,⇑, Ruijuan Qi a, Hechun Lin a, Rong Huang a,c, Geoffrey I.N. Waterhouse d, Hui Peng a,b a
Key Laboratory of Polar Materials and Devices (MOE), Department of Optoelectronics, East China Normal University, Shanghai 200241, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, Shanghai 200083, China d School of Chemical Sciences, The University of Auckland, Auckland 1142, New Zealand b c
a r t i c l e
i n f o
Article history: Received 1 June 2019 Revised 27 November 2019 Accepted 28 November 2019 Available online 19 December 2019 Keywords: MoS2@Bi2Se3 nanoflowers Electrocatalyst Hybrid Hydrogen evolution reaction
a b s t r a c t The transition metal dichalcogenide MoS2 shows good catalytic properties for the hydrogen evolution reaction (HER). However, the HER activity of 2D MoS2 is limited by its poor electrical conductivity. Bi2Se3 nanosheets are topological insulators possessing metallic surface states, thereby displaying unconventional electron dynamics and excellent conductivity. Therefore, combining Bi2Se3 nanosheets and with MoS2 nanosheets represents a rational approach for improving the HER activity of MoS2. In this work, Bi2Se3 nanoflowers were first synthesized via a hot injection method, followed by the slow growth of MoS2 nanosheets on their surface to form heterostructured MoS2@Bi2Se3 nanoflowers. Compared to pristine Bi2Se3 and MoS2, the MoS2@Bi2Se3 nanoflowers exhibited outstanding HER activity in acidic media with an onset overpotential of 134 mV, an overpotential of 208 mV at 10 mA/cm2, a Tafel slope of 57 mV/dec and remarkable stability. The enhanced HER catalytic activity offered by the MoS2@Bi2Se3 nanoflowers is attributed excellent electron transfer from Bi2Se3 to MoS2, as well as the abundance of edge-rich MoS2 nanosheets vertically aligned on the Bi2Se3 support that act as H2 evolution sites. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Due to its high gravimetric energy density and potential to be produced from a variety of sources (water and biomass feedstocks), hydrogen (H2) is the logical candidate to replace fossil fuels for electricity generation and transportation [1,2]. Currently, electrochemical water splitting is regarded as the most practical and cost effective method for sustainable hydrogen production [3,4]. Various precious metal catalysts, especially platinum (Pt) and Ptgroup metals, show excellent catalytic activity for the hydrogen evolution reaction (HER) in acidic media [5,6]. However, the scarcity and high cost of Pt is a significant obstacle to the application of Pt-based catalysts for industrial scale hydrogen production [7,8]. Accordingly, enormous research effort is currently being directed towards the development of low cost and highlyefficient HER electrocatalysts containing only earth-abundant elements [1,9–12]. ⇑ Corresponding authors. E-mail addresses: [email protected] (C. Jiang), [email protected] (C. Luo). 1 Contribute equally to this work. https://doi.org/10.1016/j.jcat.2019.11.039 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Two dimensional transition metal dichalcogenides (TMDs) are attracting a lot of attention as HER electrocatalysts due to their ease of synthesis, excellent electrochemical activity and high stability [13,14]. Amongst TMDs, molybdenum disulfide (MoS2) is regarded as the best HER catalyst [15–17]. The catalytic properties of hexagonal phase MoS2 (2H-MoS2) for hydrogen production have been widely investigated, owing to its superior stability relative to other phases of MoS2, such as the trigonal 1T-MoS2 phase or rhombohedral 3R-MoS2 phase (the numbers 1, 2 and 3 refer to how many layers are in the unit cell) [18]. Theoretical and experimental studies have shown that the catalytically active sites for HER are located along the edges of the 2H-MoS2 layers, with such edge sites offering a binding energy for hydrogen atoms close to that of platinum. Conversely, and basal planes of 2H-MoS2 are catalytically inert [19,20]. Accordingly, many synthetic strategies have been introduced with the aim of increasing the amount of active edge sites in MoS2 catalysts, including morphological engineering of MoS2 [21–23], enhancing interlayer spacings [24], combining different phases of MoS2 [25], or heteroatom doping [26]. However, 2H-MoS2 is not very conductive, thereby possessing relatively poor electron transport ability which hinders attempts to improve its
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HER performance. To solve this problem, MoS2 is often composited or hybridized with conductive materials to enhance the electron transfer between the MoS2 active sites and the electrode [27–32]. Hybrids of MoS2 with graphene, carbon nanotubes (CNTs), conducting polymers, MoO2 and VS2 have been reported to show very good electrocatalytic performance for HER [27–32]. These studies motivate the search for other new conductive materials that can significantly enhance the HER efficiency of MoS2. Topological insulators are a special class of materials, possessing metallic surface states and a narrow bulk band gap, endowing them with unconventional electron dynamics and also good stability [33–35]. The unconventional electron dynamics of topological insulators can be exploited in a variety of applications, including photodetectors, magnetic devices, field-effect transistors and lasers [33]. Some topological insulators also exhibit good photochemical hydrogen evolution activities, such as Bi2Te3 and Bi2Te2Se [36,37], though the activities of topological insulators for electrocatalytic hydrogen evolution are generally poor. Yang et al. used Bi2Se3 nanosheets, one type of topological insulator, as the substrate to grow perpendicularly-aligned MoSe2 nanosheets [38]. Due to electronic transport channels provided by Bi2Se3, the heterostructured hybrid exhibited excellent HER activity (Tafel slope of 44 mV dec 1 and current density of 85 mA cm 2 at an overpotential (g) of 300 mV [38], far superior to that of pristine MoSe2 nanosheets. Whilst the HER performance of MoSe2 @ Bi2Se3 hybrid is not as good as some other reported heterostructured hybrids [39–42], the work conclusively demonstrated that topological insulators are promising conductive substrates for preparing HER hybrid catalysts with enhanced activity. Bismuth selenide (Bi2Se3), one of most well-known topological insulators, possesses a laminate structure in which the layers are weakly held together through van der Waals interactions [43]. Each layer is formed by covalently bonded Bi and Se atoms in a sequence of Se1-Bi-Se2-Bi-Se1 along the c axis. Bi2Se3 has a narrow bandgap of 0.3 eV and a single Dirac cone on the surface [44]. Previously, we prepared Bi2Se3 nanoflowers composed of numerous thin nanosheets at a relatively low temperature [45]. In this work, we employed these nanoflowers as a substrate to grow MoS2 nanosheets to improve the HER activity of MoS2. The inherent structure of the Bi2Se3 nanoflowers was effective in preventing MoS2 nanosheet agglomeration, whilst also ensuring that the MoS2 nanosheets were well-exposed to the electrolyte and available for electrocatalysis. This, combined with the metallic surface properties of Bi2Se3 nanosheets, afforded the heterostructured MoS2@ Bi2Se3 nanoflowers with greatly improved HER activity compared with pristine MoS2 nanosheets. By varying the MoS2 loading, the activity of the nanoflowers for HER could be optimized. At the optimum MoS2 loading, the onset overpotential for HER and the overpotential at 10 mA/cm2 were 134 mV and 208 mV, respectively. Further, the Tafel slope for the HER reaction was only 57 mV/dec. The overall catalytic performance of the MoS2@ Bi2Se3 nanoflowers was superior to that reported in the literature for MoS2@ Bi2Se3 [38]. First-principle calculations confirmed that electrons can readily be transferred from Bi2Se3 to MoS2 in heterostructured MoS2@ Bi2Se3 catalysts, resulting in a smaller Gibbs free energy of hydrogen adsorption (DGH*). The abundance of exposed MoS2 edge sites in the nanoflowers was also a key feature that enhanced the HER activity.
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tion was first prepared by dissolving selenium powder (0.15 mmol) in 2.5 mL of 1-octadecene containing octadecylamine (ODA, 0.05 mmol) and tributylphosphine (TBP, 0.3 mmol). For the synthesis of Bi2Se3 nanoflowers, 1-octadecene (5 mL), bismuth triacetate (0.05 mmol) and oleic acid (1 mmol) were added into a three-neck flask. After removing water and other low-boiling point impurities in the reaction mixture by heating at 110 °C under a nitrogen atmosphere, the temperature was increased 160 °C to completely dissolve the bismuth triacetate. Then, the Se–precursor solution was quickly injected and the reaction maintained at 160 °C for 2.5 h. After quenching the reaction by rapid cooling of the flask to room temperature, the product was collected by centrifugation and washed with chloroform. 2.2. Synthesis of MoS2@Bi2Se3 nanoflowers A S-precursor solution was prepared by mixing 118 lL of tertdodecylmercaptan (t-DDT) and 2.382 mL of oleylamine (OM) in a 10 mL glass vial, which was then purged with nitrogen for 30 min and sealed. 5 mg of the as-prepared Bi2Se3 powder, 0.05 mmol of MoO2(acac)2 and 5 mL of OM were added into three-neck flask. After degassing, the reaction mixture was heated at 120 °C for 30 min under vigorous magnetic stirring. Then the temperature was increased to 200 °C, after which 2 mL of the Sprecursor solution was slowly injected into the reaction mixture (using a syringe pump set at a speed of 0.25 mL/h). After reaction, the product was collected by centrifugation at 6000 rpm for 3 min and then washed three times with a toluene/ethanol mixture. MoS2@Bi2Se3 samples with different MoS2 loadings were prepared by controlling the injection time (1.5 h, 2.5 h, 3.5 h or 4.5 h). These samples are denoted as [email protected] h, [email protected] h, [email protected] h and [email protected] h, respectively. The as-prepared MoS2@Bi2Se3 nanoflowers were hydrophobic, thus a (NH4)2S-assisted phase transfer method was adopted to make them hydrophilic [46]. Briefly, 5 mg of MoS2@Bi2Se3 nanoflowers were dispersed in 5 mL of toluene, then 0.5 mL of a (NH4)2S aqueous solution (10 wt%) and 0.5 mL of methanol were added under vigorous stirring. After 5 min, the MoS2@Bi2Se3 nanoflowers transferred into the aqueous phase. The hydrophilic MoS2@Bi2Se3 nanoflowers were collected by centrifugation, then washed repeatedly with methanol. 2.3. Characterization The morphologies of the products were characterized using scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEM-2100F). The TEM instrument was equipped with an SDD EDS detector (X-Max 80T, Oxford Instruments) for chemical composition analyses. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 X-ray diffractometer equipped with a Cu Ka radiation source (k = 1.541 8 Å). Raman spectra were collected on a micro-Raman spectrometer (Jobin-Yvon LabRAM HR 800UV) and excited using a 514 nm laser. X-ray photoelectron spectroscopy (XPS) data were obtained on a KRATOS AXIS Ultra DLD X-ray photoelectron spectrometer equipped with an Al Ka X-ray source. All binding energies were calibrated against the C 1s peak at 284.6 eV of adventitious hydrocarbons. 2.4. Electrochemical measurements
2. Experimental section 2.1. Synthesis of Bi2Se3 nanoflowers The synthesis of the Bi2Se3 nanoflowers followed a procedure described in our previous work [45]. In brief, a Se–precursor solu-
All electrochemical measurements were conducted on an Autolab NOVA III electrochemical workstation equipped with a threeelectrode cell at room temperature. A graphite rod electrode and a saturated calomel electrode were used as the counter and reference electrodes, respectively. The working electrode was prepared
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by using a polished glass carbon (GC) electrode with a diameter of 3 mm as the substrate. Catalyst (1 mg) was mixed with 180 lL of ethanol, 60 lL of water and 10 lL of 5 wt% Nafion solution and the resulting mixture ultrasonicated for 30 min to form a homogeneous dispersion. Then, 5 lL of the dispersion was dropped onto the surface of the GC electrode (loading amount 0.285 mg/cm2). After drying of the electrode, the electrocatalytic HER performance of the electrode was evaluated using linear sweep voltammetry (LSV) at a scan rate of 50 mV/s in 0.5 M H2SO4 solution. Electrochemical impedance spectroscopy (EIS) measurements were conducted at frequencies ranging from 0.1 to 1 MHz with an amplitude of 10 mV under an overpotential of 240 mV. All polarization curves were iR-corrected against the ohmic resistance of the electrolyte. Polarization curves from all catalysts were iR corrected according to the formula: E = E(measure) - IR, where the R is the ohmic resistance arising from the external resistance of the electrolyte solution measured by electrochemical impedance spectroscopy (EIS). The electrochemical double layer capacitance (EDLC) of the electrode was determined by cyclic voltammetry (CV) at different scan rates (from 20 to 200 mV s 1) in the potential range of 0.15–0.35 V vs. RHE. The turnover frequency (TOF) was calculated according to a literature procedure [47,48]. In all experiments, the electrolyte was purged with N2 for 30 min prior to the experiments in order to remove dissolved oxygen. The potentials with respect to the reversible hydrogen electrode (RHE) were calculated using the formula: E (vs. RHE) = E (vs. SCE) + E0SCE + 0.059 pH. 2.5. Theoretical calculations First-principle calculations were performed using the Vienna Ab initio Simulation Package (VASP), employing the projectoraugmented wave (PAW) method. The generalized gradient approx-
imation in the scheme of Perdew-Burke-Ernzerhof was used for the exchange-correlation functional. The energy cutoff was set to 500 eV, and a Monkhorst-Pack k-point mesh of 5 5 1 was used during all supercell calculations. The residual forces for each ion converged to less than 0.01 eV/A after structure optimization. 3. Result and discussion 3.1. Synthesis and characterization of the heterostructured MoS2@ Bi2Se3 nanoflowers The heterostructured MoS2@Bi2Se3 nanoflowers were synthesized via a two-step hot-injection method, shown schematically in Fig. 1. Bi2Se3 nanoflowers were first synthesized according to the method described in our previous work [45]. The resulting Bi2Se3 nanoflowers were composed of thin Bi2Se3 nanosheets, with a degree of cross-linking existing between the nanosheets. SEM and TEM images for the pristine Bi2Se3 nanoflowers are shown in Fig. 1a and f, respectively. Fig. S1 shows a high-resolution TEM (HRTEM) image and the corresponding selected area electron diffraction (SAED) of a nanosheet (petal) of a Bi2Se3 nanoflower. The thickness of the Bi2Se3 nanosheets was estimated to be around 8 nm. Only one set of diffraction spots was seen in the SAED image, indicating that the individual Bi2Se3 nanosheets were single crystals. The periodic arrangement of Bi and Se atoms seen in the HRTEM image (Fig. S1c) of the Bi2Se3 nanosheets further confirmed that they possessed high crystallinity. Powder X-ray diffraction (XRD) indicated that Bi2Se3 nanosheets in the nanoflowers crystallized in the rhombohedral Bi2Se3 phase (Fig. S2). In order to deposit MoS2 nanosheets of a few layers thickness on the Bi2Se3 nanoflowers, a S-precursor solution (containing tert-dodecylmercaptan, tDDT) was slowly injected to the reaction mixture containing the nanoflowers and MoO2(acac)2 using a syringe pump to control
Fig. 1. Top: Scheme for synthesis of MoS2@ Bi2Se3 nanoflowers. Bottom: SEM images and corresponding TEM images of (a, f) pure Bi2Se3 nanoflowers (b, g) [email protected] h, (c, h) [email protected] h, (d, i) [email protected] h, and (e, j) [email protected] h. The injection time of the sulfur precursor during the synthesis of the MoS2@Bi2Se3 nanoflowers is indicated in the sample name. The MoS2 loading increased from left to right.
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the nucleation-growth rate. By this approach, MoS2@Bi2Se3 nanoflowers with different MoS2 loadings could easily be prepared simply by varying the injected volume of the S precursor solution (and hence the total amount of S as t-DDT introduced into the reactor). The resulting nanoflowers are labelled MoS2@Bi2Se3-x, where x indicates the length of time for S-precursor injection (the pump rate was fixed, thus a longer injection time means more t-DDT was introduced). SEM and corresponding TEM images for various MoS2@Bi2Se3-x samples are shown in Fig. 1b-e and Fig. 1f-j, respectively. The images confirm that the growth of MoS2 did not alter the morphology of Bi2Se3 nanoflowers appreciably. XRD patterns for all the MoS2@Bi2Se3 nanoflower samples were similar (Fig. S2) and identical to that of the Bi2Se3 nanoflowers, with no obvious diffraction features from MoS2 observed. The data indicates that the MoS2 nanosheets in the MoS2@Bi2Se3 nanoflowers were of very small size and possibly modest crystallinity (which was confirmed by HRTEM below). Small MoS2 nanosheets are highly preferable for electrocatalysis since they will offer an abundance of active edge sites [21,49]. To confirm the successful growth of MoS2 on the Bi2Se3 nanoflowers, the [email protected] h sample prepared with an Sprecursor injection time of 3.5 h was selected for detailed characterization by HRTEM and energy-dispersive X-ray (EDX) elemental mapping. Fig. 2a shows that the [email protected] h nanoflower exposed a large number of nanosheet edges. HRTEM (Fig. 2b) confirmed the presence of MoS2 nanosheets stacked on the surface of Bi2Se3. EDX images (Fig. 2c-g) established that the elements Mo, S, Bi and Se were distributed homogeneously within the [email protected] h nanoflowers. From the EDX spectrum (Fig. S3), the atomic ratio of Mo:Bi was estimated to be ~1:10, suggesting a MoS2 loading of approximately 2.4 wt%. The data above confirms the successful growth of thin MoS2 nanosheets (a few layers thick) on the surface of the Bi2Se3 nanoflowers. Additionally, we prepared a MoS2 reference sample under the same experimental conditions, but in the absence of Bi2Se3 nanoflowers. The TEM image of the
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product is shown in Fig. S4. In the absence of Bi2Se3 nanoflowers as a support, severe aggregation of MoS2 nanosheets occurred through surface energy minimization processes. Therefore, it can be concluded that the internally cross-linked and mechanically robust Bi2Se3 nanoflowers not only provide a stable scaffold for MoS2 nanosheet growth, but also serve prevent MoS2 nanosheet aggregation. Raman spectra for MoS2@Bi2Se3 nanoflowers with different MoS2 loadings are shown in Fig. 3a. The spectra were collected as further evidence of MoS2 growth. Raman spectra of the Bi2Se3 and MoS2 reference samples are also provided for comparison. 2H-MoS2, show Raman peaks at 377 and 402 cm 1, assigned to E12g and A1g vibrational modes, respectively [50,51]. It is evident from Fig. 3a that these peaks were present in the spectra of all the MoS2@Bi2Se3 nanoflower samples and intensified as the S-precursor injection period increased (and hence MoS2 loading increased). The Bi2Se3 nanoflowers showed peaks at 129 and 174 cm 1 (E2g and A1g modes, respectively). The position and relative intensities of these peaks were not affected by the deposition of MoS2. However, the intensities of both the Bi2Se3 peaks decreased slightly as the MoS2 loading increased, indicating that the surface of Bi2Se3 nanoflowers were homogeneously covered by MoS2 (thus diluting the amount of Bi2Se3 in the analysis volume). Additionally, the Raman spectra were collected from the different areas of the sample and the same results were obtained (Fig. S5), which further illustrated the homogeneous cover of Bi2Se3 nanoflowers by MoS2. A previous literature study revealed that the frequency difference (Dk) of E12g and A1g peaks of MoS2 can be used to determine the thickness of MoS2 materials [52,53]. For [email protected] h, the Dk value was 24.0 cm 1, indicating that the average number of layers in the MoS2 nanosheets was less than 6 [52,53]. Meanwhile, the fullwidths at half maximum (FWHMs) of the E12g and A1g peaks were calculated to be 10.5 cm 1 and 9.4 cm 1, respectively, much larger than those of bulk MoS2 with a well-defined crystal structure [52,53]. This suggests that crystal structure of the MoS2 in the MoS2@Bi2Se3 was
Fig. 2. (a) TEM image of the edge of an individual [email protected] h nanoflower, (b) HRTEM and (c, d, e, f, g) EDS elemental maps for the [email protected] h nanoflower.
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Fig. 3. (a) Raman spectra for Bi2Se3, pure MoS2 and MoS2@Bi2Se3 nanoflowers with different MoS2 loadings. High resolution XPS spectra for [email protected] h: (b) Bi 4f, (c) Se 3d and (d) Mo 3d.
not perfect, containing a degree of disorder or defects in the layers [47], consistent with the results of the XRD experiments. The near-surface region elemental composition of the [email protected] h nanoflowers and the chemical states of component elements were probed using X-ray photoelectron spectroscopy (XPS). The survey spectrum of [email protected] h confirmed the presence of Bi, Se, Mo, and S elements in the sample (Fig. S6), in agreement with the EDX findings (in the experiment, some C in the form of adventitious hydrocarbons was also detected). The high-resolution Bi 4f XPS spectrum (Fig. 3b) showed two characteristic peaks at 157.9 and 163.2 eV in a 4:3 area ratio, which were readily assigned to Bi 4f7/2 and Bi 4f5/2 signals of Bi3+ in Bi2Se3, respectively. The Bi 4f signals overlapped with the S 2p signal of MoS2. However, due to the high intensity of the Bi 4f peaks, the S 2p signal could not be resolved. The Se 3d spectrum (Fig. 3c) was deconvoluted into a doublet in a 3:2 area ratio, with the binding energies of the Se 3d5/2 signals and Se 3d3/2 signals being typical for Se2 in Bi2Se3. The high-resolution Mo 3d spectrum (Fig. 3d) was dominated by peaks at 228.7 and 231.8 eV in a 3:2 area ratio, assigned to the Mo 3d5/2 and Mo 3d3/2 signals of Mo4+ in MoS2. Some minor Mo5+ and Mo6+ species were also identified, likely arising from partial the surface oxidation of MoS2 (i.e. Mo5+ in Mo2S5, Mo6+ in MoO3). The feature at 226.2 eV is a S 2 s peak (i.e. for S2 in MoS2). A weak Se 3 s peak is also expected ~230 eV, but was swamped here by contributions from Mo. XPS spectra for the MoS2 sample (no Bi2Se3 support) are provided in Fig. S7. The sample showed multiple Mo 3d signals, with the most intense peaks being located at 228.8 eV and 231.9 eV, typical for Mo4+ in MoS2.
Some Mo5+ and Mo6+ species at higher binding energies were also present, as was also found for the [email protected] h nanoflowers. Interestingly, the Mo 3d peaks for MoS2 in the [email protected] h nanoflower sample were located at slightly lower binding energies (by around 0.1 eV) compared to the corresponding peaks for unsupported MoS2, reasonable experimental evidence for electron density transfer from the Bi2Se3 nanoflowers to the MoS2 nanosheets in the nanoflowers [54]. 3.2. HER electrocatalytic activity of the MoS2@Bi2Se3 nanoflowers Through combining the unique electronic and catalytic properties of Bi2Se3 and MoS2, respectively, the MoS2@Bi2Se3 nanoflowers were expected to show superior HER performance relative to pristine MoS2. The electrocatalytic HER activities of the MoS2@Bi2Se3 nanoflowers were investigated in a 0.5 M H2SO4 solution with a three-electrode electrochemical system. Polarization curves for MoS2@Bi2Se3 nanoflowers with different MoS2 loadings are shown in Fig. S8. The data shows that the [email protected] h nanoflower sample possessed the best HER activity, with a HER onset overpotential (g) of only 134 mV (Fig. S9). Accordingly, the HER performance of the [email protected] h nanoflower sample was investigated in detail and compared with Bi2Se3 nanoflowers, MoS2, and a commercially available Pt catalyst. Fig. 4a shows that the overpotential required to achieve a current density of 10 mA cm 2 was 208 mV (vs. RHE) for the [email protected] h nanoflowers, much lower than the overpotentials required to achieve the same current density using the MoS2 ( 330 mV) and
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Fig. 4. Electrochemical measurements for pristine MoS2, Bi2Se3, and the [email protected] h nanoflowers (a) Polarization curves. (b) Corresponding Tafel plots. (c) Nyquist plots obtained at an overpotential of 240 mV. (Inset) R-C equivalent circuit. (d) Linear fitting of the capacitive currents measured at 0.25 V vs RHE and different scan rates. (e) Calculated TOF values for various catalysts. (f) Stability test for pure MoS2 and the [email protected] h nanoflowers through potential cycling: polarization curves before and after 1000 potential cycles.
Bi2Se3 ( 507 mV) catalysts. This data confirms that the growth of MoS2 nanosheets on Bi2Se3 nanoflowers created a synergistic effect that enhanced the electrocatalytic HER activity of MoS2. The most obvious reason for the improved HER activity is efficient electron transfer from Bi2Se3 to MoS2, thus providing an abundance of electrons for H+ reduction to H2 on the edges of the MoS2 nanosheets. A further reason is the open structure of the Bi2Se3 nanoflowers, which allows facile diffusion of H+ to the exposed MoS2 active sites. To get further insights about the HER performance of the [email protected] h nanoflowers, Tafel plots were created (Fig. 4b). The HER Tafel slope of the [email protected] h nanoflowers was 57 mV dec 1, much lower than those of Bi2Se3 nanoflowers (109 mV dec 1) and MoS2 (94 mV dec 1) and reasonably close to the value of 33 mV dec 1 determined for the commercial Pt catalyst. The lower Tafel slope of the [email protected] h nanoflowers (relative to Bi2Se3 and MoS2) confirms a significantly enhanced electrocatalytic activity. The exchange current density (J0) of the
[email protected] h nanoflowers was calculated to be 6.50 10 3 mA cm 2, which is much larger than those of the Bi2Se3 (0.34 10 3 mA cm 2) and MoS2 (1.57 10 3 mA cm 3) (Table S1). In order to better understand the synergetic effect between MoS2 and Bi2Se3 in the heterostructured nanoflowers, the effective active surface area of the Bi2Se3, MoS2 and MoS2@Bi2Se3 nanoflower samples were estimated from the double layer capacitance (Cdl) determined from cyclic voltammetry (CV) curves, which is roughly proportional to the effective active surface area. As shown in Figs. 4d and S10, no Faradaic processes were observed in the potential region of 0.15 V to 0.35 V. Accordingly, the Cdl value for each sample was obtained from the slopes of the curves obtained by plotting the differences between anodic and cathodic current density (Dj = (ja jc)/2 ) at 0.25 V against the scan rate. The calculated Cdl values for MoS2, Bi2Se3 and [email protected] h were 0.14, 0.21, and 1.73 mF cm 2, respectively (Fig. 4d). Since the magnitude
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of the Cdl is proportional to the electroactive surface area, it can be concluded that [email protected] h possesses a much large electroactive surface area compared to the other two samples, which can be traced to the abundance exposed MoS2 active sites in the nanoflowers available for HER. The turnover frequency (TOF) is another important factor for evaluating the intrinsic activity of the HER catalysts, which is defined as the number of hydrogen molecules produced per active site per second. The [email protected] h nanoflower sample had a sharper cyclic voltammogram cycle compared with pristine MoS2 (Fig. S10), consistent with more active edge sites. Fig. 4e shows the calculated TOFs for various samples (see Supporting Information for TOF calculations). The TOF of the [email protected] nanoflowers was 0.0143 s 1 at an overpotential of 210 mV, around 3.44 times that of pristine MoS2 (0.00415 s 1). These results provide addition evidence that synergies between the MoS2 and Bi2Se3 components enhanced the HER activity of MoS2@Bi2Se3 nanoflowers. The electrode kinetics of the [email protected] h nanoflowers and MoS2 during HER were investigated using electrochemical impedance spectroscopy (EIS). Fig. 4c shows electrochemical impedance spectra in the form of Nyquist plots. The spectra were fitted using a common R-C equivalent circuit depicted in the inset of Fig. 4c. The calculated charge-transfer resistances (Rct) were
Fig. 5. Potential vs time plot at a constant current density of 10 mA cm 2 for the [email protected] h nanoflowers. The inset shows H2(g) bubble accumulation and release processes.
53.4 X and 241.1 X for [email protected] h nanoflowers and MoS2, respectively. The much lower Rct value of the [email protected] h nanoflowers confirms that the Bi2Se3 facilitated charge transfer during the HER process. The stability of the [email protected] h nanoflowers as a HER electrocatalyst was evaluated at a scan rate of 100 mV s 1 over 1000 cycles. No significant change in the overpotential or current density was observed over the 1000 cycles for the MoS2@Bi2Se3 nanoflowers (Fig. 4f), suggesting excellent stability. SEM showed that the used MoS2@Bi2Se3 sample still retained a flower-like morphology with vertically aligned nanosheets (Fig. S12). The Raman spectra confirmed that the composition of [email protected] h was not altered (Fig. S13). In addition, the performance of the MoS2@Bi2Se3 nanoflowers was also investigated using chronoamperometry at a current density of 10 mA cm 2 over 10 h. As shown in Fig. 5, the relatively stable current density indicates stable molecular hydrogen evolution. The inset in Fig. 5 (enlarged view) clearly shows the accumulation and release processes of H2(g) bubbles on the surface of the electrode (H2 was observed to evolve vigorously at the electrode surface). These results confirm the excellent long-term stability of MoS2@Bi2Se3 nanoflowers as an electrocatalyst for HER. To obtain even deeper insights about the origins of the excellent HER activity of the MoS2@Bi2Se3 nanoflowers, first-principle calculations were carried out to investigate the electron distribution within MoS2@Bi2Se3 heterostructures. Fig. 6a shows the calculated 3D charge density distribution for a MoS2@Bi2Se3 heterostructure. The pale green and yellow colors represent regions of charge depletion and charge accumulation, respectively. It is clear from Fig. 6a that electrons transfer across the Bi2Se3/MoS2 interface to MoS2, thereby making MoS2 electron-rich. It is generally considered that the Gibbs free energy of adsorbed H should be close to zero for a catalyst with excellent HER performance [20,55]. Accordingly, we calculated the Gibbs free energy of hydrogen adsorption (DGH*) on pure MoS2 and the MoS2@Bi2Se3 heterostructure. For MoS2, the DGH* values for the Mo and S edges are 0.74 eV and 0.60 eV, respectively, suggesting desorption of H* to form H2 would be difficult (Fig. 6b). However, in the MoS2@Bi2Se3 heterostructure, the DGH* values for the Mo and S edges were 0.08 eV and 0.55 eV, suggesting favorable HER activity on the Mo edges. The reduction of DGH* value on the Mo edges through forming the MoS2@Bi2Se3 heterostructure is attributed to the injection of electrons from Bi2Se3 to MoS2. The calculations thus support the earlier findings that Bi2Se3 and MoS2 combine synergistically in the nanoflowers to enhance the HER activity.
Fig. 6. (a) 3D charge density distribution of the MoS2@Bi2Se3 hybrid. The pale green and yellow regions represent areas of charge depletion and charge accumulation, respectively. (b) Free energy diagram versus the reaction coordinate of HER for the pristine MoS2 and the MoS2@Bi2Se3 heterostructure.
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4. Conclusion In summary, heterostructured MoS2@Bi2Se3 nanoflowers were successfully synthesized through a two-step hot-injection method. Growth of MoS2 nanosheets on electrically conductive Bi2Se3 nanoflowers prevented MoS2 nanosheet aggregation, thus offering an abundance of MoS2 edge sites for electrocatalytic reactions. Further, the Bi2Se3 nanoflowers enhanced electron transfer to MoS2, thus greatly improving the HER performance. The rationally designed MoS2@Bi2Se3 nanoflower catalyst displayed outstanding catalytic activity for HER, evidenced by a small HER onset overpotential (134 mV), low Tafel slope (57 mV dec 1), low overpotential at a current density of 10 mA cm 2 ( 208 mV versus RHE) and excellent cycling stability, performance metrics far superior to those determined for pristine MoS2. This work also reveals that topological insulating materials are promising materials for the construction of heterostructured electrocatalysts with excellent HER performance. Declaration of Competing Interest
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There are no conflicts to declare. [20]
Acknowledgements This work was supported by Natural Science Foundation of Shanghai (No. 18ZR1410900), Shanghai Science and Technology Innovation Action Plan (No. 19JC1416700), NSFC (No. 61671206) and China Postdoctoral Science Foundation (No. 2017M621540). The authors thank ECNU Multifunctional Platform for Innovation (004 and 006) for technology support. GINW acknowledges funding support from the MacDiarmid Institute for Advanced Materials and Nanotechnology and the Energy Education Trust of New Zealand.
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Appendix A. Supplementary material [26]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.11.039. [27]
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Heterostructured MoS2@Bi2Se3 nanoflowers: A highly efficient electrocatalyst for hydrogen evolution Dong Li a,1, Jie Lao a,1, Chunli Jiang b,c,⇑, Yang Shen a, Chunhua Luo a,⇑, Ruijuan Qi a, Hechun Lin a, Rong Huang a,c, Geoffrey I.N. Waterhouse d, Hui Peng a,b a
Key Laboratory of Polar Materials and Devices (MOE), Department of Optoelectronics, East China Normal University, Shanghai 200241, China Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, Shanghai 200083, China d School of Chemical Sciences, The University of Auckland, Auckland 1142, New Zealand b c
a r t i c l e
i n f o
Article history: Received 1 June 2019 Revised 27 November 2019 Accepted 28 November 2019 Available online 19 December 2019 Keywords: MoS2@Bi2Se3 nanoflowers Electrocatalyst Hybrid Hydrogen evolution reaction
a b s t r a c t The transition metal dichalcogenide MoS2 shows good catalytic properties for the hydrogen evolution reaction (HER). However, the HER activity of 2D MoS2 is limited by its poor electrical conductivity. Bi2Se3 nanosheets are topological insulators possessing metallic surface states, thereby displaying unconventional electron dynamics and excellent conductivity. Therefore, combining Bi2Se3 nanosheets and with MoS2 nanosheets represents a rational approach for improving the HER activity of MoS2. In this work, Bi2Se3 nanoflowers were first synthesized via a hot injection method, followed by the slow growth of MoS2 nanosheets on their surface to form heterostructured MoS2@Bi2Se3 nanoflowers. Compared to pristine Bi2Se3 and MoS2, the MoS2@Bi2Se3 nanoflowers exhibited outstanding HER activity in acidic media with an onset overpotential of 134 mV, an overpotential of 208 mV at 10 mA/cm2, a Tafel slope of 57 mV/dec and remarkable stability. The enhanced HER catalytic activity offered by the MoS2@Bi2Se3 nanoflowers is attributed excellent electron transfer from Bi2Se3 to MoS2, as well as the abundance of edge-rich MoS2 nanosheets vertically aligned on the Bi2Se3 support that act as H2 evolution sites. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Due to its high gravimetric energy density and potential to be produced from a variety of sources (water and biomass feedstocks), hydrogen (H2) is the logical candidate to replace fossil fuels for electricity generation and transportation [1,2]. Currently, electrochemical water splitting is regarded as the most practical and cost effective method for sustainable hydrogen production [3,4]. Various precious metal catalysts, especially platinum (Pt) and Ptgroup metals, show excellent catalytic activity for the hydrogen evolution reaction (HER) in acidic media [5,6]. However, the scarcity and high cost of Pt is a significant obstacle to the application of Pt-based catalysts for industrial scale hydrogen production [7,8]. Accordingly, enormous research effort is currently being directed towards the development of low cost and highlyefficient HER electrocatalysts containing only earth-abundant elements [1,9–12]. ⇑ Corresponding authors. E-mail addresses: [email protected] (C. Jiang), [email protected] (C. Luo). 1 Contribute equally to this work. https://doi.org/10.1016/j.jcat.2019.11.039 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Two dimensional transition metal dichalcogenides (TMDs) are attracting a lot of attention as HER electrocatalysts due to their ease of synthesis, excellent electrochemical activity and high stability [13,14]. Amongst TMDs, molybdenum disulfide (MoS2) is regarded as the best HER catalyst [15–17]. The catalytic properties of hexagonal phase MoS2 (2H-MoS2) for hydrogen production have been widely investigated, owing to its superior stability relative to other phases of MoS2, such as the trigonal 1T-MoS2 phase or rhombohedral 3R-MoS2 phase (the numbers 1, 2 and 3 refer to how many layers are in the unit cell) [18]. Theoretical and experimental studies have shown that the catalytically active sites for HER are located along the edges of the 2H-MoS2 layers, with such edge sites offering a binding energy for hydrogen atoms close to that of platinum. Conversely, and basal planes of 2H-MoS2 are catalytically inert [19,20]. Accordingly, many synthetic strategies have been introduced with the aim of increasing the amount of active edge sites in MoS2 catalysts, including morphological engineering of MoS2 [21–23], enhancing interlayer spacings [24], combining different phases of MoS2 [25], or heteroatom doping [26]. However, 2H-MoS2 is not very conductive, thereby possessing relatively poor electron transport ability which hinders attempts to improve its
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HER performance. To solve this problem, MoS2 is often composited or hybridized with conductive materials to enhance the electron transfer between the MoS2 active sites and the electrode [27–32]. Hybrids of MoS2 with graphene, carbon nanotubes (CNTs), conducting polymers, MoO2 and VS2 have been reported to show very good electrocatalytic performance for HER [27–32]. These studies motivate the search for other new conductive materials that can significantly enhance the HER efficiency of MoS2. Topological insulators are a special class of materials, possessing metallic surface states and a narrow bulk band gap, endowing them with unconventional electron dynamics and also good stability [33–35]. The unconventional electron dynamics of topological insulators can be exploited in a variety of applications, including photodetectors, magnetic devices, field-effect transistors and lasers [33]. Some topological insulators also exhibit good photochemical hydrogen evolution activities, such as Bi2Te3 and Bi2Te2Se [36,37], though the activities of topological insulators for electrocatalytic hydrogen evolution are generally poor. Yang et al. used Bi2Se3 nanosheets, one type of topological insulator, as the substrate to grow perpendicularly-aligned MoSe2 nanosheets [38]. Due to electronic transport channels provided by Bi2Se3, the heterostructured hybrid exhibited excellent HER activity (Tafel slope of 44 mV dec 1 and current density of 85 mA cm 2 at an overpotential (g) of 300 mV [38], far superior to that of pristine MoSe2 nanosheets. Whilst the HER performance of MoSe2 @ Bi2Se3 hybrid is not as good as some other reported heterostructured hybrids [39–42], the work conclusively demonstrated that topological insulators are promising conductive substrates for preparing HER hybrid catalysts with enhanced activity. Bismuth selenide (Bi2Se3), one of most well-known topological insulators, possesses a laminate structure in which the layers are weakly held together through van der Waals interactions [43]. Each layer is formed by covalently bonded Bi and Se atoms in a sequence of Se1-Bi-Se2-Bi-Se1 along the c axis. Bi2Se3 has a narrow bandgap of 0.3 eV and a single Dirac cone on the surface [44]. Previously, we prepared Bi2Se3 nanoflowers composed of numerous thin nanosheets at a relatively low temperature [45]. In this work, we employed these nanoflowers as a substrate to grow MoS2 nanosheets to improve the HER activity of MoS2. The inherent structure of the Bi2Se3 nanoflowers was effective in preventing MoS2 nanosheet agglomeration, whilst also ensuring that the MoS2 nanosheets were well-exposed to the electrolyte and available for electrocatalysis. This, combined with the metallic surface properties of Bi2Se3 nanosheets, afforded the heterostructured MoS2@ Bi2Se3 nanoflowers with greatly improved HER activity compared with pristine MoS2 nanosheets. By varying the MoS2 loading, the activity of the nanoflowers for HER could be optimized. At the optimum MoS2 loading, the onset overpotential for HER and the overpotential at 10 mA/cm2 were 134 mV and 208 mV, respectively. Further, the Tafel slope for the HER reaction was only 57 mV/dec. The overall catalytic performance of the MoS2@ Bi2Se3 nanoflowers was superior to that reported in the literature for MoS2@ Bi2Se3 [38]. First-principle calculations confirmed that electrons can readily be transferred from Bi2Se3 to MoS2 in heterostructured MoS2@ Bi2Se3 catalysts, resulting in a smaller Gibbs free energy of hydrogen adsorption (DGH*). The abundance of exposed MoS2 edge sites in the nanoflowers was also a key feature that enhanced the HER activity.
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tion was first prepared by dissolving selenium powder (0.15 mmol) in 2.5 mL of 1-octadecene containing octadecylamine (ODA, 0.05 mmol) and tributylphosphine (TBP, 0.3 mmol). For the synthesis of Bi2Se3 nanoflowers, 1-octadecene (5 mL), bismuth triacetate (0.05 mmol) and oleic acid (1 mmol) were added into a three-neck flask. After removing water and other low-boiling point impurities in the reaction mixture by heating at 110 °C under a nitrogen atmosphere, the temperature was increased 160 °C to completely dissolve the bismuth triacetate. Then, the Se–precursor solution was quickly injected and the reaction maintained at 160 °C for 2.5 h. After quenching the reaction by rapid cooling of the flask to room temperature, the product was collected by centrifugation and washed with chloroform. 2.2. Synthesis of MoS2@Bi2Se3 nanoflowers A S-precursor solution was prepared by mixing 118 lL of tertdodecylmercaptan (t-DDT) and 2.382 mL of oleylamine (OM) in a 10 mL glass vial, which was then purged with nitrogen for 30 min and sealed. 5 mg of the as-prepared Bi2Se3 powder, 0.05 mmol of MoO2(acac)2 and 5 mL of OM were added into three-neck flask. After degassing, the reaction mixture was heated at 120 °C for 30 min under vigorous magnetic stirring. Then the temperature was increased to 200 °C, after which 2 mL of the Sprecursor solution was slowly injected into the reaction mixture (using a syringe pump set at a speed of 0.25 mL/h). After reaction, the product was collected by centrifugation at 6000 rpm for 3 min and then washed three times with a toluene/ethanol mixture. MoS2@Bi2Se3 samples with different MoS2 loadings were prepared by controlling the injection time (1.5 h, 2.5 h, 3.5 h or 4.5 h). These samples are denoted as [email protected] h, [email protected] h, [email protected] h and [email protected] h, respectively. The as-prepared MoS2@Bi2Se3 nanoflowers were hydrophobic, thus a (NH4)2S-assisted phase transfer method was adopted to make them hydrophilic [46]. Briefly, 5 mg of MoS2@Bi2Se3 nanoflowers were dispersed in 5 mL of toluene, then 0.5 mL of a (NH4)2S aqueous solution (10 wt%) and 0.5 mL of methanol were added under vigorous stirring. After 5 min, the MoS2@Bi2Se3 nanoflowers transferred into the aqueous phase. The hydrophilic MoS2@Bi2Se3 nanoflowers were collected by centrifugation, then washed repeatedly with methanol. 2.3. Characterization The morphologies of the products were characterized using scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEM-2100F). The TEM instrument was equipped with an SDD EDS detector (X-Max 80T, Oxford Instruments) for chemical composition analyses. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 X-ray diffractometer equipped with a Cu Ka radiation source (k = 1.541 8 Å). Raman spectra were collected on a micro-Raman spectrometer (Jobin-Yvon LabRAM HR 800UV) and excited using a 514 nm laser. X-ray photoelectron spectroscopy (XPS) data were obtained on a KRATOS AXIS Ultra DLD X-ray photoelectron spectrometer equipped with an Al Ka X-ray source. All binding energies were calibrated against the C 1s peak at 284.6 eV of adventitious hydrocarbons. 2.4. Electrochemical measurements
2. Experimental section 2.1. Synthesis of Bi2Se3 nanoflowers The synthesis of the Bi2Se3 nanoflowers followed a procedure described in our previous work [45]. In brief, a Se–precursor solu-
All electrochemical measurements were conducted on an Autolab NOVA III electrochemical workstation equipped with a threeelectrode cell at room temperature. A graphite rod electrode and a saturated calomel electrode were used as the counter and reference electrodes, respectively. The working electrode was prepared
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by using a polished glass carbon (GC) electrode with a diameter of 3 mm as the substrate. Catalyst (1 mg) was mixed with 180 lL of ethanol, 60 lL of water and 10 lL of 5 wt% Nafion solution and the resulting mixture ultrasonicated for 30 min to form a homogeneous dispersion. Then, 5 lL of the dispersion was dropped onto the surface of the GC electrode (loading amount 0.285 mg/cm2). After drying of the electrode, the electrocatalytic HER performance of the electrode was evaluated using linear sweep voltammetry (LSV) at a scan rate of 50 mV/s in 0.5 M H2SO4 solution. Electrochemical impedance spectroscopy (EIS) measurements were conducted at frequencies ranging from 0.1 to 1 MHz with an amplitude of 10 mV under an overpotential of 240 mV. All polarization curves were iR-corrected against the ohmic resistance of the electrolyte. Polarization curves from all catalysts were iR corrected according to the formula: E = E(measure) - IR, where the R is the ohmic resistance arising from the external resistance of the electrolyte solution measured by electrochemical impedance spectroscopy (EIS). The electrochemical double layer capacitance (EDLC) of the electrode was determined by cyclic voltammetry (CV) at different scan rates (from 20 to 200 mV s 1) in the potential range of 0.15–0.35 V vs. RHE. The turnover frequency (TOF) was calculated according to a literature procedure [47,48]. In all experiments, the electrolyte was purged with N2 for 30 min prior to the experiments in order to remove dissolved oxygen. The potentials with respect to the reversible hydrogen electrode (RHE) were calculated using the formula: E (vs. RHE) = E (vs. SCE) + E0SCE + 0.059 pH. 2.5. Theoretical calculations First-principle calculations were performed using the Vienna Ab initio Simulation Package (VASP), employing the projectoraugmented wave (PAW) method. The generalized gradient approx-
imation in the scheme of Perdew-Burke-Ernzerhof was used for the exchange-correlation functional. The energy cutoff was set to 500 eV, and a Monkhorst-Pack k-point mesh of 5 5 1 was used during all supercell calculations. The residual forces for each ion converged to less than 0.01 eV/A after structure optimization. 3. Result and discussion 3.1. Synthesis and characterization of the heterostructured MoS2@ Bi2Se3 nanoflowers The heterostructured MoS2@Bi2Se3 nanoflowers were synthesized via a two-step hot-injection method, shown schematically in Fig. 1. Bi2Se3 nanoflowers were first synthesized according to the method described in our previous work [45]. The resulting Bi2Se3 nanoflowers were composed of thin Bi2Se3 nanosheets, with a degree of cross-linking existing between the nanosheets. SEM and TEM images for the pristine Bi2Se3 nanoflowers are shown in Fig. 1a and f, respectively. Fig. S1 shows a high-resolution TEM (HRTEM) image and the corresponding selected area electron diffraction (SAED) of a nanosheet (petal) of a Bi2Se3 nanoflower. The thickness of the Bi2Se3 nanosheets was estimated to be around 8 nm. Only one set of diffraction spots was seen in the SAED image, indicating that the individual Bi2Se3 nanosheets were single crystals. The periodic arrangement of Bi and Se atoms seen in the HRTEM image (Fig. S1c) of the Bi2Se3 nanosheets further confirmed that they possessed high crystallinity. Powder X-ray diffraction (XRD) indicated that Bi2Se3 nanosheets in the nanoflowers crystallized in the rhombohedral Bi2Se3 phase (Fig. S2). In order to deposit MoS2 nanosheets of a few layers thickness on the Bi2Se3 nanoflowers, a S-precursor solution (containing tert-dodecylmercaptan, tDDT) was slowly injected to the reaction mixture containing the nanoflowers and MoO2(acac)2 using a syringe pump to control
Fig. 1. Top: Scheme for synthesis of MoS2@ Bi2Se3 nanoflowers. Bottom: SEM images and corresponding TEM images of (a, f) pure Bi2Se3 nanoflowers (b, g) [email protected] h, (c, h) [email protected] h, (d, i) [email protected] h, and (e, j) [email protected] h. The injection time of the sulfur precursor during the synthesis of the MoS2@Bi2Se3 nanoflowers is indicated in the sample name. The MoS2 loading increased from left to right.
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the nucleation-growth rate. By this approach, MoS2@Bi2Se3 nanoflowers with different MoS2 loadings could easily be prepared simply by varying the injected volume of the S precursor solution (and hence the total amount of S as t-DDT introduced into the reactor). The resulting nanoflowers are labelled MoS2@Bi2Se3-x, where x indicates the length of time for S-precursor injection (the pump rate was fixed, thus a longer injection time means more t-DDT was introduced). SEM and corresponding TEM images for various MoS2@Bi2Se3-x samples are shown in Fig. 1b-e and Fig. 1f-j, respectively. The images confirm that the growth of MoS2 did not alter the morphology of Bi2Se3 nanoflowers appreciably. XRD patterns for all the MoS2@Bi2Se3 nanoflower samples were similar (Fig. S2) and identical to that of the Bi2Se3 nanoflowers, with no obvious diffraction features from MoS2 observed. The data indicates that the MoS2 nanosheets in the MoS2@Bi2Se3 nanoflowers were of very small size and possibly modest crystallinity (which was confirmed by HRTEM below). Small MoS2 nanosheets are highly preferable for electrocatalysis since they will offer an abundance of active edge sites [21,49]. To confirm the successful growth of MoS2 on the Bi2Se3 nanoflowers, the [email protected] h sample prepared with an Sprecursor injection time of 3.5 h was selected for detailed characterization by HRTEM and energy-dispersive X-ray (EDX) elemental mapping. Fig. 2a shows that the [email protected] h nanoflower exposed a large number of nanosheet edges. HRTEM (Fig. 2b) confirmed the presence of MoS2 nanosheets stacked on the surface of Bi2Se3. EDX images (Fig. 2c-g) established that the elements Mo, S, Bi and Se were distributed homogeneously within the [email protected] h nanoflowers. From the EDX spectrum (Fig. S3), the atomic ratio of Mo:Bi was estimated to be ~1:10, suggesting a MoS2 loading of approximately 2.4 wt%. The data above confirms the successful growth of thin MoS2 nanosheets (a few layers thick) on the surface of the Bi2Se3 nanoflowers. Additionally, we prepared a MoS2 reference sample under the same experimental conditions, but in the absence of Bi2Se3 nanoflowers. The TEM image of the
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product is shown in Fig. S4. In the absence of Bi2Se3 nanoflowers as a support, severe aggregation of MoS2 nanosheets occurred through surface energy minimization processes. Therefore, it can be concluded that the internally cross-linked and mechanically robust Bi2Se3 nanoflowers not only provide a stable scaffold for MoS2 nanosheet growth, but also serve prevent MoS2 nanosheet aggregation. Raman spectra for MoS2@Bi2Se3 nanoflowers with different MoS2 loadings are shown in Fig. 3a. The spectra were collected as further evidence of MoS2 growth. Raman spectra of the Bi2Se3 and MoS2 reference samples are also provided for comparison. 2H-MoS2, show Raman peaks at 377 and 402 cm 1, assigned to E12g and A1g vibrational modes, respectively [50,51]. It is evident from Fig. 3a that these peaks were present in the spectra of all the MoS2@Bi2Se3 nanoflower samples and intensified as the S-precursor injection period increased (and hence MoS2 loading increased). The Bi2Se3 nanoflowers showed peaks at 129 and 174 cm 1 (E2g and A1g modes, respectively). The position and relative intensities of these peaks were not affected by the deposition of MoS2. However, the intensities of both the Bi2Se3 peaks decreased slightly as the MoS2 loading increased, indicating that the surface of Bi2Se3 nanoflowers were homogeneously covered by MoS2 (thus diluting the amount of Bi2Se3 in the analysis volume). Additionally, the Raman spectra were collected from the different areas of the sample and the same results were obtained (Fig. S5), which further illustrated the homogeneous cover of Bi2Se3 nanoflowers by MoS2. A previous literature study revealed that the frequency difference (Dk) of E12g and A1g peaks of MoS2 can be used to determine the thickness of MoS2 materials [52,53]. For [email protected] h, the Dk value was 24.0 cm 1, indicating that the average number of layers in the MoS2 nanosheets was less than 6 [52,53]. Meanwhile, the fullwidths at half maximum (FWHMs) of the E12g and A1g peaks were calculated to be 10.5 cm 1 and 9.4 cm 1, respectively, much larger than those of bulk MoS2 with a well-defined crystal structure [52,53]. This suggests that crystal structure of the MoS2 in the MoS2@Bi2Se3 was
Fig. 2. (a) TEM image of the edge of an individual [email protected] h nanoflower, (b) HRTEM and (c, d, e, f, g) EDS elemental maps for the [email protected] h nanoflower.
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Fig. 3. (a) Raman spectra for Bi2Se3, pure MoS2 and MoS2@Bi2Se3 nanoflowers with different MoS2 loadings. High resolution XPS spectra for [email protected] h: (b) Bi 4f, (c) Se 3d and (d) Mo 3d.
not perfect, containing a degree of disorder or defects in the layers [47], consistent with the results of the XRD experiments. The near-surface region elemental composition of the [email protected] h nanoflowers and the chemical states of component elements were probed using X-ray photoelectron spectroscopy (XPS). The survey spectrum of [email protected] h confirmed the presence of Bi, Se, Mo, and S elements in the sample (Fig. S6), in agreement with the EDX findings (in the experiment, some C in the form of adventitious hydrocarbons was also detected). The high-resolution Bi 4f XPS spectrum (Fig. 3b) showed two characteristic peaks at 157.9 and 163.2 eV in a 4:3 area ratio, which were readily assigned to Bi 4f7/2 and Bi 4f5/2 signals of Bi3+ in Bi2Se3, respectively. The Bi 4f signals overlapped with the S 2p signal of MoS2. However, due to the high intensity of the Bi 4f peaks, the S 2p signal could not be resolved. The Se 3d spectrum (Fig. 3c) was deconvoluted into a doublet in a 3:2 area ratio, with the binding energies of the Se 3d5/2 signals and Se 3d3/2 signals being typical for Se2 in Bi2Se3. The high-resolution Mo 3d spectrum (Fig. 3d) was dominated by peaks at 228.7 and 231.8 eV in a 3:2 area ratio, assigned to the Mo 3d5/2 and Mo 3d3/2 signals of Mo4+ in MoS2. Some minor Mo5+ and Mo6+ species were also identified, likely arising from partial the surface oxidation of MoS2 (i.e. Mo5+ in Mo2S5, Mo6+ in MoO3). The feature at 226.2 eV is a S 2 s peak (i.e. for S2 in MoS2). A weak Se 3 s peak is also expected ~230 eV, but was swamped here by contributions from Mo. XPS spectra for the MoS2 sample (no Bi2Se3 support) are provided in Fig. S7. The sample showed multiple Mo 3d signals, with the most intense peaks being located at 228.8 eV and 231.9 eV, typical for Mo4+ in MoS2.
Some Mo5+ and Mo6+ species at higher binding energies were also present, as was also found for the [email protected] h nanoflowers. Interestingly, the Mo 3d peaks for MoS2 in the [email protected] h nanoflower sample were located at slightly lower binding energies (by around 0.1 eV) compared to the corresponding peaks for unsupported MoS2, reasonable experimental evidence for electron density transfer from the Bi2Se3 nanoflowers to the MoS2 nanosheets in the nanoflowers [54]. 3.2. HER electrocatalytic activity of the MoS2@Bi2Se3 nanoflowers Through combining the unique electronic and catalytic properties of Bi2Se3 and MoS2, respectively, the MoS2@Bi2Se3 nanoflowers were expected to show superior HER performance relative to pristine MoS2. The electrocatalytic HER activities of the MoS2@Bi2Se3 nanoflowers were investigated in a 0.5 M H2SO4 solution with a three-electrode electrochemical system. Polarization curves for MoS2@Bi2Se3 nanoflowers with different MoS2 loadings are shown in Fig. S8. The data shows that the [email protected] h nanoflower sample possessed the best HER activity, with a HER onset overpotential (g) of only 134 mV (Fig. S9). Accordingly, the HER performance of the [email protected] h nanoflower sample was investigated in detail and compared with Bi2Se3 nanoflowers, MoS2, and a commercially available Pt catalyst. Fig. 4a shows that the overpotential required to achieve a current density of 10 mA cm 2 was 208 mV (vs. RHE) for the [email protected] h nanoflowers, much lower than the overpotentials required to achieve the same current density using the MoS2 ( 330 mV) and
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Fig. 4. Electrochemical measurements for pristine MoS2, Bi2Se3, and the [email protected] h nanoflowers (a) Polarization curves. (b) Corresponding Tafel plots. (c) Nyquist plots obtained at an overpotential of 240 mV. (Inset) R-C equivalent circuit. (d) Linear fitting of the capacitive currents measured at 0.25 V vs RHE and different scan rates. (e) Calculated TOF values for various catalysts. (f) Stability test for pure MoS2 and the [email protected] h nanoflowers through potential cycling: polarization curves before and after 1000 potential cycles.
Bi2Se3 ( 507 mV) catalysts. This data confirms that the growth of MoS2 nanosheets on Bi2Se3 nanoflowers created a synergistic effect that enhanced the electrocatalytic HER activity of MoS2. The most obvious reason for the improved HER activity is efficient electron transfer from Bi2Se3 to MoS2, thus providing an abundance of electrons for H+ reduction to H2 on the edges of the MoS2 nanosheets. A further reason is the open structure of the Bi2Se3 nanoflowers, which allows facile diffusion of H+ to the exposed MoS2 active sites. To get further insights about the HER performance of the [email protected] h nanoflowers, Tafel plots were created (Fig. 4b). The HER Tafel slope of the [email protected] h nanoflowers was 57 mV dec 1, much lower than those of Bi2Se3 nanoflowers (109 mV dec 1) and MoS2 (94 mV dec 1) and reasonably close to the value of 33 mV dec 1 determined for the commercial Pt catalyst. The lower Tafel slope of the [email protected] h nanoflowers (relative to Bi2Se3 and MoS2) confirms a significantly enhanced electrocatalytic activity. The exchange current density (J0) of the
[email protected] h nanoflowers was calculated to be 6.50 10 3 mA cm 2, which is much larger than those of the Bi2Se3 (0.34 10 3 mA cm 2) and MoS2 (1.57 10 3 mA cm 3) (Table S1). In order to better understand the synergetic effect between MoS2 and Bi2Se3 in the heterostructured nanoflowers, the effective active surface area of the Bi2Se3, MoS2 and MoS2@Bi2Se3 nanoflower samples were estimated from the double layer capacitance (Cdl) determined from cyclic voltammetry (CV) curves, which is roughly proportional to the effective active surface area. As shown in Figs. 4d and S10, no Faradaic processes were observed in the potential region of 0.15 V to 0.35 V. Accordingly, the Cdl value for each sample was obtained from the slopes of the curves obtained by plotting the differences between anodic and cathodic current density (Dj = (ja jc)/2 ) at 0.25 V against the scan rate. The calculated Cdl values for MoS2, Bi2Se3 and [email protected] h were 0.14, 0.21, and 1.73 mF cm 2, respectively (Fig. 4d). Since the magnitude
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of the Cdl is proportional to the electroactive surface area, it can be concluded that [email protected] h possesses a much large electroactive surface area compared to the other two samples, which can be traced to the abundance exposed MoS2 active sites in the nanoflowers available for HER. The turnover frequency (TOF) is another important factor for evaluating the intrinsic activity of the HER catalysts, which is defined as the number of hydrogen molecules produced per active site per second. The [email protected] h nanoflower sample had a sharper cyclic voltammogram cycle compared with pristine MoS2 (Fig. S10), consistent with more active edge sites. Fig. 4e shows the calculated TOFs for various samples (see Supporting Information for TOF calculations). The TOF of the [email protected] nanoflowers was 0.0143 s 1 at an overpotential of 210 mV, around 3.44 times that of pristine MoS2 (0.00415 s 1). These results provide addition evidence that synergies between the MoS2 and Bi2Se3 components enhanced the HER activity of MoS2@Bi2Se3 nanoflowers. The electrode kinetics of the [email protected] h nanoflowers and MoS2 during HER were investigated using electrochemical impedance spectroscopy (EIS). Fig. 4c shows electrochemical impedance spectra in the form of Nyquist plots. The spectra were fitted using a common R-C equivalent circuit depicted in the inset of Fig. 4c. The calculated charge-transfer resistances (Rct) were
Fig. 5. Potential vs time plot at a constant current density of 10 mA cm 2 for the [email protected] h nanoflowers. The inset shows H2(g) bubble accumulation and release processes.
53.4 X and 241.1 X for [email protected] h nanoflowers and MoS2, respectively. The much lower Rct value of the [email protected] h nanoflowers confirms that the Bi2Se3 facilitated charge transfer during the HER process. The stability of the [email protected] h nanoflowers as a HER electrocatalyst was evaluated at a scan rate of 100 mV s 1 over 1000 cycles. No significant change in the overpotential or current density was observed over the 1000 cycles for the MoS2@Bi2Se3 nanoflowers (Fig. 4f), suggesting excellent stability. SEM showed that the used MoS2@Bi2Se3 sample still retained a flower-like morphology with vertically aligned nanosheets (Fig. S12). The Raman spectra confirmed that the composition of [email protected] h was not altered (Fig. S13). In addition, the performance of the MoS2@Bi2Se3 nanoflowers was also investigated using chronoamperometry at a current density of 10 mA cm 2 over 10 h. As shown in Fig. 5, the relatively stable current density indicates stable molecular hydrogen evolution. The inset in Fig. 5 (enlarged view) clearly shows the accumulation and release processes of H2(g) bubbles on the surface of the electrode (H2 was observed to evolve vigorously at the electrode surface). These results confirm the excellent long-term stability of MoS2@Bi2Se3 nanoflowers as an electrocatalyst for HER. To obtain even deeper insights about the origins of the excellent HER activity of the MoS2@Bi2Se3 nanoflowers, first-principle calculations were carried out to investigate the electron distribution within MoS2@Bi2Se3 heterostructures. Fig. 6a shows the calculated 3D charge density distribution for a MoS2@Bi2Se3 heterostructure. The pale green and yellow colors represent regions of charge depletion and charge accumulation, respectively. It is clear from Fig. 6a that electrons transfer across the Bi2Se3/MoS2 interface to MoS2, thereby making MoS2 electron-rich. It is generally considered that the Gibbs free energy of adsorbed H should be close to zero for a catalyst with excellent HER performance [20,55]. Accordingly, we calculated the Gibbs free energy of hydrogen adsorption (DGH*) on pure MoS2 and the MoS2@Bi2Se3 heterostructure. For MoS2, the DGH* values for the Mo and S edges are 0.74 eV and 0.60 eV, respectively, suggesting desorption of H* to form H2 would be difficult (Fig. 6b). However, in the MoS2@Bi2Se3 heterostructure, the DGH* values for the Mo and S edges were 0.08 eV and 0.55 eV, suggesting favorable HER activity on the Mo edges. The reduction of DGH* value on the Mo edges through forming the MoS2@Bi2Se3 heterostructure is attributed to the injection of electrons from Bi2Se3 to MoS2. The calculations thus support the earlier findings that Bi2Se3 and MoS2 combine synergistically in the nanoflowers to enhance the HER activity.
Fig. 6. (a) 3D charge density distribution of the MoS2@Bi2Se3 hybrid. The pale green and yellow regions represent areas of charge depletion and charge accumulation, respectively. (b) Free energy diagram versus the reaction coordinate of HER for the pristine MoS2 and the MoS2@Bi2Se3 heterostructure.
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4. Conclusion In summary, heterostructured MoS2@Bi2Se3 nanoflowers were successfully synthesized through a two-step hot-injection method. Growth of MoS2 nanosheets on electrically conductive Bi2Se3 nanoflowers prevented MoS2 nanosheet aggregation, thus offering an abundance of MoS2 edge sites for electrocatalytic reactions. Further, the Bi2Se3 nanoflowers enhanced electron transfer to MoS2, thus greatly improving the HER performance. The rationally designed MoS2@Bi2Se3 nanoflower catalyst displayed outstanding catalytic activity for HER, evidenced by a small HER onset overpotential (134 mV), low Tafel slope (57 mV dec 1), low overpotential at a current density of 10 mA cm 2 ( 208 mV versus RHE) and excellent cycling stability, performance metrics far superior to those determined for pristine MoS2. This work also reveals that topological insulating materials are promising materials for the construction of heterostructured electrocatalysts with excellent HER performance. Declaration of Competing Interest
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There are no conflicts to declare. [20]
Acknowledgements This work was supported by Natural Science Foundation of Shanghai (No. 18ZR1410900), Shanghai Science and Technology Innovation Action Plan (No. 19JC1416700), NSFC (No. 61671206) and China Postdoctoral Science Foundation (No. 2017M621540). The authors thank ECNU Multifunctional Platform for Innovation (004 and 006) for technology support. GINW acknowledges funding support from the MacDiarmid Institute for Advanced Materials and Nanotechnology and the Energy Education Trust of New Zealand.
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Appendix A. Supplementary material [26]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.11.039. [27]
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