Wafer Scale Phase‐Engineered 1T‐ and 2H‐MoSe 2 /Mo Core ... · and reduced free energy so that the Tafel slope of 36 mV dec−1 can also be achieved.[20] Both cases indicate

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Wafer Scale Phase-Engineered 1T- and 2H-MoSe2/Mo Core–Shell 3D-Hierarchical Nanostructures toward Efficient Electrocatalytic Hydrogen Evolution Reaction

Yindong Qu, Henry Medina, Sheng-Wen Wang, Yi-Chung Wang, Chia-Wei Chen, Teng-Yu Su, Arumugam Manikandan, Kuangye Wang, Yu-Chuan Shih, Je-Wei Chang, Hao-Chung Kuo, Chi-Yung Lee, Shih-Yuan Lu, Guozhen Shen, Zhiming M. Wang,* and Yu-Lun Chueh*

Y. D. Qu, Prof. Z. M. WangInstitute of Fundamental and Frontier SciencesUniversity of Electronic Science and Technology of ChinaChengdu 610054, P. R. ChinaE-mail: [email protected]. D. Qu, Prof. Z. M. WangSchool of Microelectronics and Solid-State ElectronicsUniversity of Electronic Science and Technology of ChinaChengdu 610054, P. R. ChinaDr. H. Medina, Y.-C. Wang, C.-W. Chen, T.-Y. Su, A. Manikandan, K. Wang, Y.-C. Shih, Prof. C.-Y. Lee, Prof. Y.-L. ChuehDepartment of Materials Science and EngineeringNational Tsing Hua UniversityHsinchu 30013, TaiwanE-mail: [email protected]. Wang, Prof. H.-C. KuoDepartment of Photonics and Institute of Electro-Optical Engineering National Chiao Tung UniversityHsinchu 30010, TaiwanJ.-W. Chang, Prof. S.-Y. LuDepartment of Chemical EngineeringNational Tsing-Hua UniversityHsinchu 30013, TaiwanProf. G. ShenState Key Laboratory for Superlattices and MicrostructuresInstitute of SemiconductorsChinese Academy of ScienceBeijing 100083, China

DOI: 10.1002/adma.201602697

efficient H2 generation.[2,3] Platinum (Pt)-based electrodes, as the ideal electrodes for the HER process, can provide a Tafel slope of ≈30 mV dec−1, which is close to the theoretical limit of 29 mV dec−1 because of a hydrogen absorption Gibbs free energy (ΔGH*) of zero.[2,4] The high cost and steady increase in the global demand for Pt raises concerns for its use in energy production.[5] Therefore, the search for new materials as the ideal electrode for the HER process is imperative. As an alter-native, transition metal dichalcogenides (TMDs) have recently received great attention as potential catalysts for HER.[6–10] Computational studies have identified that TMDs have as an excellent electrocatalytic activity at edge sites with the ΔGH* located at +0.08 eV for the HER process.[11] However, most of the experimental results from different TMDs acting as a catalyst exhibit Tafel slopes larger than 38 mV dev−1, implying that the HER process is controlled by the Volmer–Heyrovsky mechanism. To improve the HER performance, many efforts have been dedicated to expose more edge sites by engi-neering various morphologies of TMDs, such as core–shell MoO3-MoS2 nanowires,[6] double-gyroid MoS2,[7] and vertically aligned MoSe2.[9] However, these may lead to inefficient elec-tron transfer because of the semiconducting property of the 2H (trigonal prismatic) phase.

To solve this problem, the TMDs are often coupled with a conductive substrate, such as carbon nanotube,[12] gra-phene,[13–16] or 3D metals,[17,18] which not only separate the TMDs to expose more active sites but also accelerate the elec-tron transfer between catalysts and electrode. It has been demonstrated that MoS3/Vulcan C composites have reached a Tafel slope of 36 mV dec−1 because of an increase in electron transfer.[19] In a different approach, MoS2/CoSe2 structures were used as a catalyst by taking advantage of the synergistic effects between MoS2 and CoSe2 with increased catalytic sites and reduced free energy so that the Tafel slope of 36 mV dec−1 can also be achieved.[20] Both cases indicate that a Tafel slope < 38 mV dec−1 can indicate the potential application of TMDs in efficient HER, although it is not yet close to the theoretical limit, indicating that the HER process is still controlled by elec-trochemical desorption (Volmer–Heyrovsky mechanism) rather than Volmer–Tafel recombination. In this regard, we demon-strate the MoSe2/Mo core–shell 3D-hierarchical nanostructures by a low-temperature plasma-assisted selenization process on the Mo 3D-hierarchical nanostructures with controlled shapes

Electrochemical water splitting is an ideal eco-friendly way to produce clean energy though hydrogen (H2) production.[1] As part of the process of hydrogen generation known as hydrogen evolution reaction (HER), an electrode acting as a catalyst is needed for efficiently converting a pair of protons and elec-trons into H2. In general, the Tafel slope is used to evaluate the efficiency of the HER process. The HER process is slow because hydrogen absorption on the active sites (Volmer reac-tion) will normally result in a Tafel slope > 116 mV dec−1, although it can be enhanced with a Tafel slope < 38 mV dec−1 by ion exchange via electrochemical desorption (Heyrovsky reaction). The most efficient and fastest HER process should be determined by hydrogen recombination (Tafel reaction) with a Tafel slope of ≈29 mV dec−1, which is desirable for more

Adv. Mater. 2016, DOI: 10.1002/adma.201602697

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grown by a glancing angle deposition system (GLAD). Phase engineering of MoSe2 can be achieved at different plasma sele-nization temperatures. The metallic 1T-MoSe2/Mo core–shell 3D-hierarchical nanostructures were used for HER measure-ments. In particular, 1T-MoSe2/Mo core–shell nanoscrews exhibit the highest catalytic activities with a Tafel slope as low as 34.7 mV dec−1, which is close to the theoretical value of 29 mV dec−1 dominated by a Volmer–Tafel reaction for the ideal HER process. More importantly, the demonstration provides a pathway toward low-cost and more efficient TMD catalysts as the electrode for HER applications.

The complete fabrication processes are schematically shown in Figure 1a (see experimental section for details). First, the Mo hierarchical nanostructures were grown on a Mo/glass substrate by a GLAD process.[21,22] The substrate can be tilted and rotated with a rotational speed (Δφ) at a vapour flux incidence angle (θ) as schematically shown in Figure 1a1. Because of the self-shadow effect provided by the initial nucleated islands, different porous 3D-nanostructure arrays can be formed when engineering the deposition con-ditions. Sub sequently, the sample surface was selenized by a plasma-assisted selenization method, as shown in Figure 1a2. A vertical furnace equipped with an ICP coil is powered by a 13.56 MHz RF supply to generate the plasma. The gaseous selenium (Se)-bearing molecules flowing into the chamber were cracked by high-energy electrons, from which the cracked Se atoms became Se radicals that are more active than Se molecules.[23] Note that without the plasma treatment, the chemical reaction between Mo and Se will not take place until the selenization temperature becomes higher than 500 °C (Figure S1, Supporting Information). Therefore, the Se radi-cals could accelerate the chemical reaction for the formation of MoSe2 at the lower selenization temperature. Consequently,

the MoSe2/Mo core–shell 3D-hierarchical nanostructures can be formed on a conductive Mo substrate, which is a beneficial configuration as an electrocatalytic electrode for hydrogen evo-lution. Here, a few shell-MoSe2 monolayers act as the active catalysts with enhanced surface area, and the core Mo pro-vides a fast electron transfer channel to the bottom Mo con-ductive layer, as schematically shown in the inset of Figure 1b.

The morphologies of the grown Mo hierarchical structures can be modified by controlling a ratio given by ρ = r/Δφ, namely, a deposition rate (r) divided by a substrate rotational speed (Δφ).[21] As can been seen in Figure 2a–c, nanohelices, nano-screws and nanorods were grown by changing Δφ at 0.25, 0.5, 2 deg s−1 with a constant deposition rate (r = 0.1 nm s−1) and ρ values of 144, 72, and 18 nm rev−1, respectively (Figure S2, Supporting Information). Effectively, electrochemical surfaces are used to represent the surface area of the nanostructures esti-mated by double-layer capacitance (Cdl) by a cyclic voltammetry method.[8] The corresponding cyclic voltammograms were col-lected with regions of 0.25–0.35 V versus reversible hydrogen electrode (RHE), where the current changes are due to the charges of the double layer (Figure S3, Supporting Information). As a result, the capacitance can be calculated from the scan rate as the function of the current density at E = 0.3 V versus RHE, which is twice the Cdl, by extracting slopes from ΔJ versus scan rates as shown in Figure 2d. Table S1 of the Supporting Infor-mation lists the capacitance values of the flat and different Mo 3D-hierarchicalstructures. Because of the relatively rough sur-face of Mo/glass (Figure S2a, Supporting Information), the Cdl of 1.495 mF cm−2 extracted from the flat Mo is higher than the expected value for the flat electrode (typically tens of μF cm−2).[7] However, after the growth of Mo 3D-hierarchical structures on the Mo/glass, a more than fourfold enhancement of the capacitance can be achieved (Figure S2b1,c1,d1, Supporting



Figure 1. a) Schematics of MoSe2/Mo core–shell 3D-hierarchical nanostructures fabricated by glancing angle deposition (GLAD) followed by the plasma-assisted selenization process. The inset (a1) illustrates the GLAD of e-beam evaporation, and (a2) illustrates the plasma-assisted selenization process. b) Schematics of hydrogen evolution reaction and charge transfer.

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Information). In particular, the largest capacitance was obtained from the Mo nanoscrews (Cdl of 6.855 mF cm−2), which is approximately 4.6 times larger than that of the flat Mo. Based on the largest surface areas and curved surface morphology, we selected Mo nanoscrews as the base structure for the HER test.

Using Mo nanoscrews as a precursor for the HER electrode, the plasma-assisted selenization was performed at different temperatures from 300 to 550 °C with a fixed plasma power of 300 W for 8 min to convert metallic Mo into MoSe2, forming the MoSe2/Mo core–shell nanoscrews. Figure 3a,b shows

scanning electron microscopy (SEM) images of MoSe2/Mo core–shell nanoscrews after the plasma-assisted selenization process at the selenization temperature of 400 °C. Interest-ingly, the plasma-assisted selenization pro-vides a low-temperature process for main-taining the morphology of the nanoscrews, of which the average diameter and spacing are 60 and 30 nm, respectively, without any significant change. In addition, an optical image of MoSe2/Mo core–shell nanoscrews on a 4 in. wafer scale with plasma-assisted selenization at 400 °C is shown in the inset of Figure 3a, indicating that our process is a scalable process that depends on GLAD and plasma-assisted selenization systems and MoSe2/Mo core–shell nanoscrews uniformly distribute on the 4 in. wafer confirmed by Raman spectra (Figure S4, Supporting Infor-mation). The core–shell structure can be con-

firmed by transmission electron microscopy (TEM), as shown in Figure 3c, and the corresponding high-resolution TEM images taken from two places from Figure 3c are shown in Figure 3d,e. These MoSe2 monolayers epitaxially grown on the curved Mo surface may suffer out-of-plane strain and lattice mismatch, leading to exposure of more edge sites and defects (marked by red arrows in Figure 3d,e) because of the disordered atomic arrangement. By a careful examination of the edge site of the nanostructure under TEM, three different lattices were found (Figure S5, Supporting Information). The lattice with a distance of 0.22 nm (Figure S5b,c, Supporting Information) is in agree-ment with expected (001) plane for the diffraction of body-cen-tered cubic (bcc) Mo.[24] The lattice with a distance of 0.65 nm is in agreement with the interlayer distance of 2H-MoSe2 in the (002) plane.[25] Furthermore, a larger interlayer distance of 0.69 nm was also found in the TEM images of the sample synthesized at 400 °C mixed with 0.65 nm layer structure. As a result, other more metallic polymorphs of Mo–Se such as Mo3Se4 cannot be found in our case under the TEM observa-tion.[26] The plasma selenization temperature also significantly affects the number of converted MoSe2 layers, resulting in dif-ferent densities of active sites. For example, if we reduce the selenization temperature to as low as 300 °C, the density of exposed edges decreases because only a few MoSe2 monolayers can be formed on a few regions of Mo nanoscrews (Figure S6, Supporting Information). However, a few MoSe2 monolayers become continuous layers on the surface of the Mo nano-screws once the selenization temperature becomes >500 °C, resulting in the reduction of the exposed edge sites (Figure S7, Supporting Information). By further increasing the seleniza-tion temperature to 550 °C, we found that the rate of reaction increases sharply and that all Mo nanoscrews have almost com-pletely selenized (Figure S8, Supporting Information).

The crystallinity of a few MoSe2 monolayers at different plasma-assisted selenization temperatures was confirmed by Raman spectroscopy as shown in Figure 4a. The character-istic Raman peaks associated with A1g (out-of-plane) and E1

2g (in-plane) of 2H-MoSe2 were identified at 238 and 289 cm−1, respectively. In addition, the inset in Figure 4a shows the



Figure 2. Cross-section SEM images of a) nanohelices, b) nanoscrews, and c) nanorods depos-ited at substrate rotation speeds of Δφ = 0.25, 0.5, and 2 deg s−1, respectively, corresponding to ρ = 144, 72, and 18 nm rev−1; the scale bar is 250 nm. d) Scan-rate dependence of the current density at E = 0.3 V versus RHE for the three different Mo hierarchical nanostructures.

Figure 3. The top a) and cross-sectional b) SEM images of Mo nano-screws selenized at 400 °C; c,d,e) TEM images of Mo nanoscrews sele-nized at 400 °C. The inset in (a) shows a photograph of Mo nanoscrews grown on a 4 in. Si wafer.


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full width at half maximum (FWHM) of the A1g peak as the function of temperature. Clearly, the FWHM decreases as the Raman intensity increases, indicating that the enhanced crys-tallinity of the MoSe2 can be achieved as the selenization tem-perature increases, which is consist with the TEM observation. Glancing incident angle X-ray diffraction (GIXRD) was also carried out to further ascertain the structural information, as shown in Figure 4b. The XRD spectra for Mo nanoscrews after the plasma-assisted selenization temperature of 550 °C show strong diffraction peaks corresponding to the (002), (004), (100), (103), (105), and (110) crystal facets of the MoSe2, and the XRD spectra for Mo nanoscrews with the plasma-assisted seleniza-tion temperatures of 300, 400, and 500 °C also present broad (002) and (100) peaks, revealing the nanocrystalline character-istics of MoSe2. Note that the strong diffraction peaks from Mo (110) and (200) were found in all XRD spectra. The peak at the MoSe2 (002) plane slightly shifts by a smaller angle compared with the bulk MoSe2 (JCPDS No. 77–1715), and the (100) peak shifts to a larger angle, indicating that the crystal structure of the shell MoSe2 is different from that of the bulk MoSe2. Using Bragg’s equation, a calculated interlayer spacing of ≈0.7 nm for the (002) plane was obtained from the Mo nanoscrews at the selenization temperature of 400 °C, which is consistent with the average layer spacing of 0.69 nm confirmed by TEM results (Figure 3d,e) and larger than that of bulk 2H-MoSe2 (0.65 nm).

We found that the average of the internal lattice spacing is the largest at the plasma-assisted selenization temperature of 400 °C. It decreases with an increase in the seleniza-tion temperature and reaches a bulk value of MoSe2 (0.65 nm) by selecting 10 layers on average from high-resolution transmission electron microscopy (HRTEM) images at dif-ferent plasma-assisted selenization tempera-tures (Figure S9, Supporting Information). We believe that the difference in internal spacing is expected to be from the result of the phase transition of MoSe2 from 1T to 2H and the internal strain between MoSe2 and Mo caused by the lattice mismatch during the plasma-assisted selenization process. Fur-thermore, X-ray photoelectron spectroscopy (XPS) was used to measure the chemical states of Mo and Se to confirm the existed phases of MoSe2 at different plasma-assisted selenization temperatures. Figure 4c,d shows the high-resolution XPS spectra of Mo 3d and Se 3d at different plasma-assisted sele-nization temperatures. For Mo nanoscrews selenized at 300, 400, and 500 °C, the Mo 3d spectra show large similar double peaks at 228.3 eV (3d5/2) and 231.4 eV (3d3/2) and small double peaks at 229.0 eV (3d5/2) and 232.1 eV (3d3/2) as solid evidence for the for-mation of MoSe2 consisting of the metallic (1T) and semiconducting (2H) phases.[27,28] Note that Raman spectra of purely 1T-MoSe2 previously demonstrated by Rao and co-workers[29] was not observed in our case.

Instead, our Raman results are consistent with those presented by Pumera and co-workers[27] due to the presence of mixed phases of 1T and 2H-MoSe2

[27] and in agreement with our XPS data. We suggest that the weak Raman intensity of the 1T phase is faded by the strong intensity of the 2H phase.

Moreover, very small double peaks at 230.3 and 233.4 eV were also observed, which are attributed to amorphous MoSe3. Other peaks located at 231.8 and 234.8 eV correspond to Mo6+, and the single peak at 230 eV represents Se 3S.[30] When the temperature increases to 550 °C, the 1T phase disappears, and the 2H phase dominates. In addition, the small double peaks at 232.9 and 236.1 eV are attributed to MoO3.[30] Similarly, Se 3d spectra show two large peaks at 53.8 and 54.7 eV, which are attributed to the 1T form of Se2−, and small double peaks at 54.4 and 55.2 eV corresponding to the 2H form of Se2−.[27,28] At the plasma-assisted selenization temperature of 550 °C, only one double peak was found at 54.4 and 55.2 eV, confirming the existence of 2H MoSe2 only. Note that the Se2

2− form in MoSe3 is not considered here.

HER catalytic activities of the MoSe2/Mo nanostructures at different plasma-assisted selenization temperatures from 300 to 500 °C were studied in a 0.5 m H2SO4 solution using a three-electrode electrochemical setup (see methods for details). Figure 5a,b shows typical cathodic polarization curves and the corresponding Tafel plots (Table S2, Supporting Information).



Figure 4. a) Raman spectra were measured using a 514 nm laser; the inset shows the full width at half maximum (FWHM) of the A1g peak as a function of temperature. b) XRD patterns of the Mo nanoscrews selenized at different temperatures. c) Mo 3D and d) Se 3D XPS spectra.


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Note that the data were modified by iR-loss correction, and the current was normalized to the geometric electrode area (Figure S10a, Supporting Information). As a reference, the cata-lytic performance of pristine Mo nanoscrews exhibits a much lower catalytic activity than the selenized Mo nanoscrews, indicating an intrinsically high catalytic activity of MoSe2 (Figure S10b, Supporting Information). Clearly, the Mo nano-screws selenized at 300 °C show a good catalytic result, with an onset potential of −105 mV versus RHE and an operating poten-tial of −177 mV at the cathodic current density of 10 mA cm−2. By increasing the plasma-assisted selenization temperature to 400 °C, the Mo nanoscrews clearly show the best catalytic performance, exhibiting an onset potential of −89 mV versus RHE and an operating potential of −166 mV. This enhanced catalytic reaction can be attributed to the larger density of active sites (edges) without losing electrical conductivity because of the metallic phase. However, the operating potential increases once the plasma-assisted selenization temperature increases to 500 °C because of fewer active sites due to the formation of continuous MoSe2 layers. The HER process can be defined by a series of reactions as follows. First, the adsorbed hydrogens ions are formed on the active sites in the discharge step (Volmer reaction, Equation (1)). Then, the process is followed by an elec-trochemical desorption step (Heyrovsky reaction, Equation (2)) or a recombination step (Tafel reaction, Equation (3))[3]

+ → ++ −H O e H H O3 ads 2 (1)

+ + → ++ −H H O e H H Oads 3 2 2 (2)

→2H Hads 2 (3)

Normally, the Tafel slope is used as an indicator of the lim-iting reaction for HER. A lower Tafel slope means a stronger enhancement of the HER rate at a constant increase of overpo-tential or better chemical reactivity. To evaluate the Tafel slope of the 1T-MoSe2/Mo core–shell nanoscrews, linear portions of Tafel plots under a small overpotential were fitted to the Tafel equation, η = b log(j/j0), where b, j0, and j represent the Tafel slope, the exchange current density and the current density, respectively. For Mo nanoscrews selenized at 300 and 400 °C, the Tafel slopes of 36.4 and 34.7 mV dec−1 were calculated, and they increase to 41.5 mV dec−1 at a plasma-assisted selenization temperature of 500 °C (Figure 5b, and Table S2, Supporting Information). The lower Tafel slopes of 36.4 and 34.7 mV dec−1 achieved by MoSe2/Mo core–shell nanoscrews at the plasma-assisted selenization temperatures of 300 and 400 °C suggest that the recombination-limited Volmer–Tafel mechanism domi-nates the HER process because of the denser catalytic points with reduced ΔGH* offered by a high density of MoSe2 edges. In addition, the Mo core in the MoSe2/Mo core–shell nano-screws also provides an Ohmic contact with the MoSe2 shell layer, resulting in low resistance and a rapid carrier transpor-tation path along the axial Mo core, which can be confirmed by electrochemical impedance spectroscopy (EIS) performed at an overpotential of 0.2 V as shown in Figure 5c. The inset in Figure 5c shows the corresponding equivalent circuit of the solid–liquid interface by a one-time constant. Clearly, the fitting results confirm that the lowest charge transfer resistance (Rct) of MoSe2/Mo core–shell nanoscrews formed at the plasma-assisted selenization temperature of 400 °C is only 47 Ω, indi-cating the fastest electrocatalytic reaction rate under all condi-tions (Table S2, Supporting Information). For comparison, the



Figure 5. a) Typical cathodic polarization curves and b) corresponding Tafel plots before and after the iR correction. c) The electrode kinetics of Mo nanoscrews selenized at different temperatures by Nyquist plots. d) Electrochemical stability test of Mo nanoscrews selenized at 400 °C before the iR correction.


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structures with a fixed selenization temperature of 400 °C was also investigated (Figure S11 and Table S3, Supporting Infor-mation). Clearly, the performance of the MoSe2/Mo core–shell nanoscrews is the best.

Normally, three primary approaches have been applied to optimize the catalysts by increasing the number of active sites, improving the carrier transfer and reducing the free energy of hydrogen (ΔGH*), whose corresponding results, including onset potential, Tafel slope, and overpotential at 10 mA cm−2, are listed in Table 1. Although it is difficult to determine which TMD-based configuration has the best HER performance due to dif-ferent TMD material systems and measured conditions, we still found that the Tafel slope was mainly located at 40–70 mV dec−1 without reduction of ΔGH* by enhancing active sites and carrier transfer. However, the MoSe2/Mo core–shell nanoscrews con-tain three enhanced factors, successfully resulting in the lowest Tafel slope (as low as 34.7 mV dec−1), which is close to the theo-retical limit of 29 mV dec−1 for the ideal Volmer–Tafel reaction. Furthermore, the faradaic efficiency reaches 98.4% as a further proof of the outstanding behavior of the MoSe2/Mo core–shell nanoscrews electrodes for HER. Details of the gas chromato-graphy measurement and estimation of the faradaic efficiency calculation are given in Figure S12 of the Supporting Informa-tion. To investigate the durability of catalysts, continuous cyclic voltammetry (CV) was conducted at a scan rate of 100 mV s−1, as shown in Figure 5d. Notoriously, there is only a small degen-eration after 1000 cycles, possibly caused by some strain relaxa-tion and oxidation of MoSe2 during the HER process.[18,31] Pre-vious works reported the low stability of Pt under several cyclic

voltammetry after several cycles in acidic media.[6,32] In order to avoid this problem, the Pt counter electrode was separated by a Teflon frit. After EDS measurements, only 1% Pt was found on the sample after the stability test as shown in Figure S13, Supporting Information, confirming that the stability is indeed attributed to the MoSe2/Mo core–shell nanoscrews electrode. Figures S14a and 4b compare the SEM images of MoSe2/Mo core–shell nanoscrews before and after stability test where no obvious changes in the morphology of the nanoscrews can be found. In addition, the Raman spectra collected after the test does not show any particular shift or intensity decrease, further confirming the stability of our electrode.

Although the overpotential at 10 mA cm−2 for the MoSe2/Mo core–shell nanoscrews still cannot compare favorably with those of some phosphide electrodes such as P8W48/rGO mate-rial,[33] it is at the level of other state-of-the-art noble-metal-free HER catalysts, such as carbides, nitrides, and chalcogenides material systems.[34–38] It is believed that the catalytic perfor-mance can be further enhanced as the length of nanoscrews increases due to the larger surface area, whereas the length of the nanoscrews in the current study was fixed at 400 nm to simplify our concept demonstration. Moreover, by cou-pling with other materials, the other parameters such as onset potential and overpotential can be further improved. We believe that the proposed process can also be applied to other TMD-based materials, opening up a step forward in the devel-opment of a low-cost and high energy efficiency catalyst for H2 production.

In conclusion, we have successfully demonstrated, for the first time, the MoSe2/Mo core–shell hierarchical nanostruc-

tures by a low-temperature plasma-assisted selenization process on the Mo 3D-hierar-chical nanostructures with controlled shapes deposited by a GLAD system. The mixed phases of MoSe2/Mo core–shell hierarchical nanostructures can be achieved at the low selenization temperature of 400 °C, whereas the 2H-MoSe2/Mo core–shell 3D-hierar-chical nanostructures dominate once the plasma-assisted selenization temperature exceeds 500 °C. In particular, the mixed MoSe2/Mo core–shell nanoscrews exhibit the highest catalytic activities, with a Tafel slope as low as 34.7 mV dec−1, which is close to the theoretical value of 29 mV dec−1 and a low potential of −166 mV at J = 10 mA cm−2 dominated by a Volmer–Tafel reaction for the ideal HER process. The high catalytic activities are attributed to the increased den-sity of exposed edges and the metallic phase of the MoSe2 shell, providing a fast charge transfer with optimized ΔGH*. In addition, the proposed structure is composed of low-cost precursors that can be used for large-scale practical applications and provides a pathway toward low-cost and more effi-cient TMD catalysts as electrodes for HER applications.



Table 1. Comparison of Tafel slopes for different samples.

Samples Onset potential [mV]

Tafel slope [mV dec−1]

Overpotential (mV) at 10 mA cm−2

Ref.

MoO3-MoS2 core–shell nanowiresa) −200 50–60 250 [6]

Double-gyroid MoS2a) −200 to −150 50 240 [7]

MoSe2/carbon fiber papera,b) −110 59.8 250 [9]

Amorphous MoS2/Vulcan Cb) N/A 36 N/A [19]

MoS2/CoSe2 hybrida,c) −11 36 68 [20]

MoSx/GN-protected Ni foamsa,b) −130 42.8 N/A [17]

MoS2/N-doped CNT foresta,b) −75 40 110 [12]

MoSe2/RGO hybrida,b) −50 69 115 [14]

MoS2/RGOa,b) N/A 41 140 [16]

MoSe2/GN nanosheetsa,b) −50 61 159 [13]

Strained chemically exfoliated WS2c) N/A 55 N/A [34]

Metallic WS2 nanosheetsc) −75 70 142 [35]

Exfoliated metallic MoS2 nanosheetsc) −135 43 187 [36]

Monolayer MoS2/3D golda–c) −118 46 226 [18]

P8W48/rGOa–c) (phosphides) N/A 38 28 [33]

NiMoNx/C (nitrides) −157 35.9 225 [37]

Nanoporous Mo2C nanowires (carbide) −70 53 125 [38]

MoSe2/Mo core–shell nanoscrewsa–c) −89 34.7 166 This work

a)Increasing the number of active sites; b)Improvement of electron transfer; c)Optimization of ΔGH*.


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Experimental SectionMaterials Preparation: Mo film with a thickness of 400 nm was

fabricated on soda-lime glass by a two-stage sputtering processes to achieve better adhesion, higher thermal stability, and lower contact resistance. The Mo/glass was used as a substrate to grow the Mo-MoSe2 core–shell nanostructures for HER measurements. The Mo nanostructures were first deposited on the Mo/glass by e-beam evaporation using a glancing angle deposition process whose substrate temperature (Tsub), deposition rate (r), deposition thickness (t), and oblique angle (α) were fixed at −40 °C, 0.1 nm s−1, 400 nm, 80°, respectively. Nanohelices, nanoscrews and nanorods were formed when changing the substrate rotation speed (Δφ) to 0.25, 0.5, 2 deg s−1, respectively.

Low-Temperature Plasma-Assisted Selenization Process: Selenium pellets were placed into the selenium heater located at the top of the chamber. The temperature was set at 220 °C, followed by flowing with a N2/H2 mixture (N2:H2 = 100:40, 2 Torr) into the selenization chamber for the selenization process. The heating of the substrate to the reaction temperatures was from 300 to 550 °C at a ramping rate of ≈12.5 °C min−1. Once the temperature reached the targeted temperature, the plasma was turned on for 8 min during the selenization process.

Materials Characterization: Morphologies and microstructures were investigated by field-emission scanning electron microscopy (HITACHI UHR FE-SEM SU8010) and HRTEM (JEOL, JEM-3000F FEGTEM, 300kV). Micro-Raman spectroscopy (HORIBA, LabRAM, HR800) equipped with a 514 nm laser was used to examine the formation of MoSe2. Crystal structures of the MoSe2 were characterized by a Grazing Incident Angle XRD (GIAXRD) with Cu Kα (λ = 0.154 nm) as the radiation source. X-ray photoemission spectroscopy equipped with a monochromatic Al Kα X-ray source (XPS, Ulvac-PHI 1600) was carried out to obtain the bonding information and to distinguish between different phases at different selenization temperatures.

Electrochemical Characterizations: Electrochemical measurements were performed in a three-electrode system using a Bio-Logic VSP potentiostat in a cylindrical cell made of Teflon with an O-ring in the bottom with a working electrode area of 0.264 cm2. The MoSe2/Mo core–shell hierarchical nanostructures on the Mo/glass substrates were used as the working electrode with a platinum wire counter electrode and a Ag/AgCl (3 m NaCl) reference electrode in a 0.5 m H2SO4 electrolyte solution. The working electrode, contacted by a copper tape (3 m), was pasted on a bare sputter Mo after scratching upper nanostructures. All the potentials were converted to RHE using the equation given by ERHE = EAg/AgCl + 0.0591 × PH + 0.194, resulting in a shift of −0.2117 V versus RHE. Cyclic voltammograms were acquired in a non-faradaic region between 0.25 and 0.35 V (vs RHE) at various scan rates (20, 40, 60, 80, and 100 mV s−1 vs RHE) to evaluate the double-layer capacitance (Cdl). The capacitance of Mo nanostructures can be calculated from the scan rate dependence of charging current density at E = 0.3 V versus RHE, where the slope of the ΔJ versus scan rate curve is twice that of the Cdl. Polarization curves were performed under ambient conditions under potentials between +0.2 and −0.3 V versus RHE at a 5 mV s−1 scan rate. The data were collected after applying 3–5 potential sweeps in order to avoid the redeposition of Pt onto the working electrode after several cycles. The electrochemical impedance spectra were taken from 105 to 0.1 Hz with an amplitude of 5 mV under a potential of −0.2 V versus RHE. The impedance data were fitted to a simplified Randles circuit to extract the series resistances (Rs) and charge-transfer resistances (Rct). The stability of the electrocatalyst was examined by continuously cycling the potential between +0.2 and −0.3 V at a scan rate of 100 mV s−1.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThe research was supported by the National Program on Key Basic Research Project (973 Program) of China (Grant Nos. 2013CB933301 and 2015CB358600) and the National Natural Science Foundation of China (Grant No. 51272038). Y.-L.C. would like to thank the Ministry of Science and Technology for support through Grant Nos. 104-2628-M-007-004-MY3, 104-2221-E-007-048-MY3, 104-2633-M-007-001, and 104-2622-M-007-002-CC2 and the National Tsing Hua University for support through Grant No. 104N2022E1. Y.-L.C. greatly appreciates the use of the facility at CNMM, National Tsing Hua University through Grant No. 104N2744E1 and the support of glancing angle deposition system (EBS-500-GLASD) from Jun Sun Tech Co., Ltd.

Received: May 21, 2016Revised: July 26, 2016

Published online:

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