Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic

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Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic Hydrogen Evolution Hanfeng Liang, Huanhuan Shi, Dongfang Zhang, Fangwang Ming, Rongrong Wang, Junqiao Zhuo, and Zhoucheng Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01963 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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Chemistry of Materials

Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic Hydrogen Evolution Hanfeng Liang,†,§ Huanhuan Shi,†,§ Dongfang Zhang,† Fangwang Ming,† Rongrong Wang,† Junqiao Zhuo,‡ and Zhoucheng Wang*,† †

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China; ‡College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

ABSTRACT: We report here for the first time the direct solution growth of VS2 nanoplate arrays on carbon paper and their applications in hydrogen evolution electrocatalysis. The VS2 nanoplate electrodes only required an overpotential as low as 42 mV to achieve a geometric current density of -10 mA cm-2, with a small Tafel slope of 36 mV dec-1 and outstanding long-term stability in acidic media, approaching the performance of Pt. The facile solution synthesis developed in this work enables largescale renewable energy applications, and the excellent activity and stability make the VS2 nanoplate catalyst a promising alternative to Pt for hydrogen fuel production.

KEYWORDS: VS2, nanoplate arrays, solution growth, hydrogen evolution reaction (HER), electrocatalysis

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INTRODUCTION Hydrogen, as a clean, renewable, and secure chemical fuel, is one of the most promising next-generation energy carriers that promises an alternative to meet the future global energy demands.1,2 The electrochemical water splitting (ideally driven by solar) is a highly attractive means to produce hydrogen.3-7 The state-of-the-art electrocatalysts for hydrogen evolution reaction (HER) are noble metals such as Pt,8 however, their high cost and scarcity greatly limit their large-scale applications.9 Therefore, searching for costeffective and earth-abundant materials and nanostructuring existing catalysts, aiming to improve the HER activity and stability while lower the overpotential, have been intensely pursed toward enabling a hydrogen economy.10-13 A number of non-noble metal compounds have emerged as promising HER electrocatalysts, including MoS2, WS2, CoS2, NiSe2, CoSe2, amorphous MoSx, CoP, Ni2P, FeP, WP, MoP|S, CoPS, NiP1.93Se0.07, and Ni-Mo alloys.14-28 Among the various HER catalysts developed, transition metal dichalcogenides (TMDs) have witnessed tremendous progress because of their high catalytic activity toward the HER and low cost.14-18 It is noted that most of recent efforts have been devoted to tuning the electric properties and edge active sites of MoS2 and WS2, both of which are believed to have great impact on the catalytic activity.14,15,29-33 While other TMDs (e.g., TiS2, ZrS2, VS2, NbS2, and TaS2) have rarely been explored despite they also have the potential to catalyze HER.34 It is also worth mentioning that the synthetic MoS2 and WS2 are originally in semiconducting 2H phase and need to be converted into metastable metallic 1T polymorph to further boost their HER performance.14,15,31 Given that high conductivity favors fast electron transport, one can expect that the intrinsic 2 ACS Paragon Plus Environment

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metallic TMDs (e.g., VS2, NbS2, and TaS2) with delicate nanostructures would be potentially good HER electrocatalysts. In fact, metallic CoS2 nanowires have shown to act as a highly active material for both HER and polysulfide/triiodide reduction applications.16 Metallic VS2 has also been used for high-performance supercapacitors.35 Furthermore, it has been theoretically demonstrated that metallic VS2 monolayers exhibit superior HER activity among other TMDs.36,37 A first-principles calculation further showed that the catalytic activity of VS2 nanostructures strongly depends on edge structure and configuration.38 To experimentally investigate the HER performance of VS2, it is highly desirable to synthesize VS2 with well controlled nanostructures, especially those with more edges exposed. Vapor phase methods (e.g., chemical vapor deposition, or CVD) have shown the advantages in synthesizing TMDs such as MoS2 and WS2, and could be generally applied to the synthesis of VS2 with high crystalline quality, but they often require high temperature (normally between 300 and 900 °C), thus are usually more energy costly.14,15,39 Moreover, for CVD and other vapor phase methods, the precursors as well as the byproducts of the reaction can be toxic (e.g., S, H2S) or explosive (e.g., B2H6). In contrast, low temperature liquid phase method is a facile and economic approach, and is therefore much more suitable for large-scale renewable energy applications. Unfortunately, there are very limited reports on the solution growth of VS2 nanostructures.40-42 More importantly, to the best of our knowledge, all of them are prepared in the form of powder,40-42 which requires additional binder (e.g., Nafion)assisted film casting or coating procedures when used for HER. The obtained electrodes suffer from limited active surface areas (being blocked by binders), sluggish diffusion, and poor conductivity, which are unfavorable for electrocatalytic HER.



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To further improve the HER performance of VS2, it is essential to directly grow VS2 nanostructures with large surface area on conductive substrates. In this work, we developed a simple hydrothermal method to grow vertical VS2 nanoplates on carbon paper with high density for the first time (Figure 1). The direct contact between VS2 and current collector (i.e., carbon paper) facilitates the charge carries transport, and the array structure also promotes the release of evolved hydrogen gas bubbles from catalyst surface,16,43 thus enable an enhanced HER performance when compared with common nanoparticles. We demonstrate that the VS2 nanoplate array is a highly active HER catalyst with a small Tafel slope of 36 mV dec-1, a high geometric exchange current density of 0.955 mA cm-2, and an extremely low overpotential of 42 mV at a benchmark current density of -10 mA cm-2 in 0.5 M H2SO4. Furthermore, the VS2 nanoplates also exhibit good long-term operational stability toward the HER.

Figure 1. (A) Side view of the atomic structure of a VS2 crystal showing the 2D layered structure with V (gray sphere) layer sandwiched between two S (yellow sphere) layers. The d-spacing is determined to be 5.75 Å based on XRD result. (B) Schematic of the synthetic procedure of VS2 nanoplate arrays on carbon paper substrate for hydrogen evolution reaction.

EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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Material

preparation. The VS2 nanoplate arrays were synthesized by

hydrothermally reacting Na3VO4 with thioacetamide. In a typical synthesis, 1.1 mmol of Na3VO4 ･10H2O and 5.5 mmol thioacetamide (C2H5NS) were dissolved in 40 mL of deionized water. After being magnetically stirring for 1 h, the above solution was transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL. A piece of carbon paper (1 cm × 2 cm, pre-heated at 700 °C for 10 min) was then immersed into the solution as substrate for the nanoplates growth. The autoclave was sealed and heated at 160 °C for 24 h. After the reaction, the substrate was rinsed with water and ethanol and dried at 60 °C for further use. Material Characterization. X-ray diffraction (XRD) data were collected via a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation. The morphology and structure of the product were characterized by a ZEISS Sigma field-emission scanning electron microscope (FESEM) and a JEOL JEM-2100 high-resolution transmission electron microscope (TEM). Raman spectra were recorded using a Horiba XploRA Confocal Raman microscope fitted with 532 nm excitation laser. X-ray photoelectron spectroscopy (XPS) was performed using a PHI Quantum 2000 ESCA system with monochromatic Al Kα radiation. Electrocatalytic Hydrogen Evolution. All electrochemical measurements were performed using an Autolab PGSTAT302N potentiostat at room temperature in a threeelectrode configuration, where the VS2 nanoplate arrays grown on carbon paper (typically 1 cm × 1 cm) were directly used as the working electrode (WE), a graphite rod and a saturated calomel electrode (SCE) were used as the counter electrode (CE) and the reference electrode (RE), respectively. 0.5 M H2SO4 was used as electrolyte. Linear 5 ACS Paragon Plus Environment


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sweep voltammograms (LSV) were measured from 0.10 to -0.20 V vs reversible hydrogen electrode (RHE) at a scan rate of 3 mV s-1. Electrochemical impedance spectroscopy (EIS) was performed in potentiostatic mode over a frequency range from 100 kHz to 1 mHz with a 10 mV AC dither. Cyclic voltammograms taken at various scan rates (20, 40, 60, 80, and 100 mV s-1) were recorded in the potential range of 0.10-0.36 V vs RHE and were used to estimate the double-layer capacitance (Cdl). All the potentials reported in this work were corrected for the ohmic losses according to Ecorrected = Emeasured – j ×Rs × A/1000, where j (mA cm-2) is the geometric current density, Rs (Ω) is the equivalent series resistance that can be determined form the EIS, and A (cm2) is the geometric area of the electrode.

RESULTS AND DISCUSSION The layered VS2 (see crystal structure in Figure 1A) nanoplate arrays were synthesized by hydrothermally reacting Na3VO4•10H2O with thioacetamide (C2H5NS) in aqueous solution at 160 °C using carbon paper as substrate (Figure 1B, see Experimental Section for details). After the reaction, the carbon paper was coated with a black thin film, suggesting the formation of VS2 (Figure S1 in the Supporting Information). The scanning electron microscopy (SEM) image clearly shows that the VS2 nanoplates are grown uniformly and fully cover the skeletons of carbon paper (Figure 2A). The areal density of the nanoplate arrays was measured to be 1.6 mg cm-2. These vertically standing nanoplates about ~30 nm in thickness (Figure 2B) and ~800 nm in lateral dimension (Figure 2C) with rough surface are interpenetrated with each other, forming a highly open hierarchical network. Transmission electron microscopy (TEM) observation further 6 ACS Paragon Plus Environment

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confirms the hierarchical nanoplate structure (Figure 2D). The corresponding selected area electron diffraction (SAED) pattern and the high-resolution TEM (HRTEM) image show single crystalline structures that can be indexed to vanadium disulfide (Figures 2E and F). The measured lattice spacing of 0.58 nm is in agreement with the d(0001) spacing of VS2 (Figure 2F). The elemental maps reveal that the V and S are distributed homogeneously in an individual nanoplate (Figure 2G) and on the surface of carbon paper skeletons (Figure S2). Furthermore, the energy dispersive X-ray (EDX) spectrum indicates that the molar ratio of V:S is 1:1.96, consistent with the stoichiometry of VS2 (Figure S2). These results clearly suggest that the VS2 nanoplates were grown on the carbon paper substrate. Note that the powder collected from the suspensions also shows the same nanoplate morphology but more disordered and aggregated, which should be the result of homogenous nucleation growth mechanism (Figure S3).44 It is worth mentioning that the morphology and the mass loading can be easily tuned by adjusting the precursor concentration (Figure S4) or the reaction temperature (Figure S5). For example, when we decreased the Na3VO4 concentration to 0.5 mmol (the molar ratio of Na3VO4:C2H5NS kept at 1:5), sparse nanoplates with similar morphology were obtained, but the VS2 loading is much lower and cannot cover the whole surface of carbon paper (Figure S4).

Figure 2. Structural characterization of the VS2 nanoplate arrays. (A) Low-magnification 7 ACS Paragon Plus Environment


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SEM image, (B) high-magnification top-view and (C) side-view SEM images, (D) TEM image, (E) SAED pattern, (F) HRTEM image, and (G) elemental maps of the VS2 nanoplate arrays.

We further verified the phase and composition of the as-obtained VS2 nanoplates using X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The diffraction peaks of the product grown on carbon paper as well as those from the powder sample collected from suspensions can be indexed to VS2 (space group: P-3m1, lattice parameter a = 3.221 Å, c = 5.755 Å, JCPDS No. 89-1640) (Figure 3A). Raman spectrum of the nanoplates grown on carbon paper shows two distinct peaks, at 280 cm-1 and 404 cm-1 (Figure 3B), corresponding to the vibration Eg mode and the out-of-plane (Ag) vibration mode of layered VS2, respectively.45 XPS was further performed to probe the elemental composition and chemical state of the VS2 sample. The V peaks at 516.3 and 523.7 eV are attributed to V 2p3/2 and V 2p1/2 levels, indicating the V4+ oxidation state (Figure 3C).46,47 The peak fitting analysis of S 2p confirms the presence of S2- with two peaks located at 162.7 and 163.9 eV that can be assigned to S 2p3/2 and S 2p1/2, respectively (Figure 3D).48 The observation of the peak around 168.4 eV that can be assigned to sulfate indicates the surface of the VS2 is oxidized.48 These results strongly suggest the successful growth of VS2 on carbon paper after the hydrothermal reaction.


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Figure 3. Structural and compositional characterization of VS2 nanoplate arrays. (A) XRD patterns of the VS2 nanoplates grown on carbon paper (red curve), and the VS2 powder collected from the suspensions (black curve). The stars mark the diffraction peaks from carbon paper. (B) Raman spectrum of the VS2 nanoplates. (C, D) High resolution XPS spectra for V 2p (C) and S 2p (D) and peaking fitting analysis of the VS2 nanoplates.

We measured the electrical conductivity of the bulk VS2 powder using a four-point probe meter and the result suggests that the VS2 is high conductivity (102 S cm-1 at room temperature), which should favor the electron transfer thus the HER catalytic activity. In addition, the direct growth of VS2 nanoplate arrays on carbon paper allows us to conveniently evaluate the HER performance without using any extra binders. We thus carried out the HER measurements in 0.5 M H2SO4 using a standard three-electrode configuration, where the carbon paper with VS2 nanoplates grown on it (mass loading:



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1.6 mg cm-2) was used as the working electrode (WE), a graphite rod and a saturated calomel electrode (SCE) served as the counter electrode (CE) and the reference electrode (RE), respectively. Carbon paper and a Pt wire were also examined for comparison. Figure 4A shows the IR-corrected (the series resistances are generally smaller than 2 Ω in all of the measurements) polarization curves recorded at a scan rate of 3 mV s-1 (see original data in Figure S6). The pure carbon paper shows negligible cathodic current densities within the potential window investigated. In contrast, the VS2 nanoplate arrays grown on carbon paper show high activity for HER as indicated by the near zero onset overpotential and the rapidly rising cathodic current at more negative potentials, approaching the performance of Pt. Specifically, the overpotentials required to drive geometric current densities of -10, -50 and -100 mA cm-2 are 42, 83 and 97 mV, respectively. It is worth mentioning that the HER over Pt electrode is purely diffusion limited,49 and its geometric area is quite different from that of VS2, therefore, the comparison between Pt and the VS2 electrodes may not be accurate. Nonetheless, the overpotentials achieved on VS2 are better than or at least favorably compare to those of recently reported high-performance non-noble metal HER electrocatalysts, including metal chalcogenides and phosphides (see comparison in Table S1). Note that the VS2 samples with other morphologies and mass loadings synthesized using different precursor concentrations (Figure S7) and reaction temperatures (Figure S8) show inferior catalytic activity (Figure S9), suggesting the VS2 nanoplate array is the optimal catalyst in our system. Here, we like to emphasize that in comparison with powder sample, which needs extra polymer binders to fabricate electrodes, our directly grown VS2 nanoplate arrays on carbon paper enables good electrical connection between them, thus facilitates the


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electron transport. Moreover, the highly open hierarchical structure allows easy diffusion of the ionic species and enables more efficient utilization of active sites (which are not being blocked by binders). These features make the integrated nanoplate arrays more attractive than powder sample for HER. Evidently, the VS2 nanoplate arrays grown on carbon paper exhibit higher HER activity than its powdery counterpart even with a much lower catalyst loading (Figure S10). Note that their Tafel slopes are reasonably close to each other (36 and 42 mV dec-1 for nanoplate array and powder sample, respectively), indicating that the disparity in morphology doesn’t affect their intrinsic activity, and the different performance should be due to their different electrochemically active surface area (ECSA) and other factors (e.g., conductivity, density of active sites) as discussed (Figure S10 and Figure S11).

Figure 4. Electrochemical properties of VS2 nanoplate arrays for HER in 0.5 M H2SO4. (A) IR-corrected polarization curves of carbon paper, Pt wire, and VS2 nanoplate catalyst at a scan rate of 3 mV s-1. (B) Polarization curves derived Tafel plots of the Pt and VS2



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catalysts. (C) Difference in current density (ja - jc) at 0.24 V vs RHE plotted against scan rate fitted to a linear regression showing the extraction of double-layer capacitance (Cdl) for the estimation of relative electrochemically active surface area. Inset: voltammograms of the VS2 electrode at various scan rates. (D) Long-term stability test carried out under a constant current density of -10 mA cm2. Inset: polarization curves of VS2 nanoplate catalyst recorded before and after 2000 sweeps from 0.36 to -0.20 V vs RHE.

The polarization curves were then fitted to the Tafel equation and the plots are shown in Figure 4B. The apparent Tafel slope of Pt electrode is 31 mV dec-1, consistent with previous reports.14-16 For the VS2 nanoplate array electrode, the Tafel slope is 36 mV dec-1. The pathway of HER over TMDs is not yet determined. A theoretical calculation based on nudged elastic band (NEB) method shows that the activation barrier is as high as ~1.5 eV for H2 desorption for both the basal plane of 1T-MoS2 and the Mo-edge (the active sites for HER) of 2H-MoS2, thus is insurmountable under the HER conditions performed at room temperature.37 Given the very low overpotential required for 1T-MoS2, as well as the VS2 nanoplate arrays in our case, to drive the HER, it is highly likely that the HER over VS2 proceeds via the Volmer-Heyrovsky but not Volmer-Tafel mechanism as for Pt.37 We also determined the geometric exchange current density (j0) of VS2 nanoplate array electrode as 0.955 mA cm-2 (also see ECSA normalized exchange current density for VS2 in Table S1) by extrapolating the Tafel slope to the overpotential of 0 V (Figure S12). The normalized exchange current density is one to two orders of magnitude higher than those of other TMDs such as CoS2,16 outperforming most of the non-noble metal HER catalysts (Table S1).


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It should be mentioned that unlike the MoS2 and WS2, which are only edge active,14,15,30,33 the VS2 is active in terms of both the basal planes and the edges as predicted by Pan et al.,36 therefore more active sites are accessible during the HER. More importantly, the reaction free energy of hydrogen adsorption (∆GH) of VS2 is around the optimum value of 0, as predicted previously by the density function theory (DFT) calculation, which means the optimal catalytic activity toward the HER can be achieved on the VS2.36,37 The metallic nature of VS2 also enables the fast electron transfer, which favors the reaction kinetics. Moreover, compared to the VS2 nanoplates with several hundred nanometers in thickness made by CVD,39 our solution grown VS2 nanoplates are much thinner (~30 nm), which should be beneficial to the HER. We thus estimated the ECSA using a simple cyclic voltammetry method.14-16 The current in the investigated potential window of 0.10-0.36 V vs the reversible hydrogen electrode (RHE) should be only due to the charging of the double-layer (inset of Figure 4C), and the double-layer capacitance (Cdl) should be proportional to the ECSA.14-16 The Cdl is measured to be 93 mF cm-2, suggesting a high ECSA of the VS2 electrode, which certainly contributes to the high catalytic activity. Note that this value is much higher than those of other reported metal chalcogenide HER catalysts.14-16 We further evaluated the stability of the VS2 catalyst by a constant current measurement. As shown in Figure 4D, the overpotential required to drive the current density of 10 mA cm-2 for VS2 nanoplate array is very stable and only shows minimal change after a 12 h-test. The polarization curve recorded after 2000 sweeps merely shows a shift compared to the initial one. The SEM images after HER further verify the stability of the VS2 catalyst. The overall nanoplate morphology is well maintained except for some cracks are observed in the originally compact layers,



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which could be due to severe catalysis process and bubble release during the HER (Figure S13). The XRD and EDX results confirm that the catalyst remains VS2 after the HER, indicating the good stability of the VS2 nanoplate arrays for HER (Figure S13) under room temperature. The VS2 catalyst was also found to be reasonably stable even under more severe conditions. We performed the chronopotentiometry test at under 50 °C and didn’t observe significant change in the potential required to drive the current density of 10 mA cm-2 (Figure S14A). Moreover, the corrosion of VS2 during the HER was monitored using ICP-MS (Figure S14B). The corrosion of the V seems to be fast at the first 8 h (3.1 wt.% of V was etched) but gradually slows down and about 4.7 wt.% of the V was leached out after 24 h catalysis. Note that this corrosion rate is found to be higher than other reported HER catalysts such as Ni2P in alkaline solution,50 thus further surface coating may be needed to protect the VS2 from being etched.

CONCLUSION In summary, we have developed a facile hydrothermal route to directly grow VS2 nanoplate arrays on carbon paper with high density for the first time. When evaluated as an electrocatalyst for HER, the VS2 nanoplate arrays exhibit outstanding catalytic activity and excellent long-term stability in acidic media superior to that recently reported for metal chalcogenides, and are among the most active non-noble metal HER catalysts reported so far. The low cost and scalable solution synthesis and the high catalytic activity of the VS2 catalyst make it a promising alternative to Pt for large-scale hydrogen production. The VS2 nanoplate arrays reported in this work may also find other applications in energy storage and conversion, such as supercapacitors and batteries. 14 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Additional SEM and TEM images, PXRD patterns, optical photographs, electrochemical data, and comparison of catalytic performance, as noted in the main text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] (Z.W.) Notes §

These authors contributed equally to this work.

ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (No. 51372212) for financial support.

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Liang, H.; Li, L.; Meng, F.; Dang, L.; Zhuo, J.; Forticaux, A.; Wang, Z.; Jin, S. Porous

Two-dimensional

Nanosheets

Converted

from

Layered

Double

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Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900.

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Morales-Guio, C. G.; Hu, X. Amorphous Molybdenum Sulfides as Hydrogen Evolution Catalysts. Acc. Chem. Res. 2014, 47, 2671-2681.



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Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem. Int. Ed. 2014, 53, 5427-5430.

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Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270.

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Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Ni2P as a Janus Catalyst for Water Splitting: The Oxygen Evolution Activity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 2347-2351.

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Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. A Costeffective 3D Hydrogen Evolution Cathode With High Catalytic Activity: FeP Nanowire Array as the Active Phase. Angew. Chem. Int. Ed. 2014, 53, 12855-12859.

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McEnaney, J. M.; Crompton, J. C.; Callejas, J. F.; Popczun, E. J.; Read, C. G.; Lewis, N. S.; Schaak, R. E. Electrocatalytic Hydrogen Evolution Using Amorphous Tungsten Phosphide Nanoparticles. Chem. Commun. 2014, 50, 11026-11028.

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Kibsgaard, J.; Jaramillo, T. F. Molybdenum Phosphosulfide: An Active, Acidstable, Earth-abundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 53, 14433-14437.

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Yu, Y.; Huang, S.-Y.; Li, Y.; Steinmann, S. N.; Yang, W.; Cao, L. Layerdependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553-558.

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Cheng, L.; Huang, W.; Gong, Q.; Liu, C.; Liu, Z.; Li, Y.; Dai, H. Ultrathin WS2 Nanoflakes as a High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 53, 7860-7863.

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Tan, C.; Zhang, H. Two-dimensional Transition Metal Dichalcogenide Nanosheet-based Composites. Chem. Soc. Rev. 2015, 44, 2713-2731.

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Feng, J.; Sun, X.; Wu, C.; Peng, L.; Lin, C.; Hu, S.; Yang, J.; Xie, Y. Metallic Few-Layered VS2 Ultrathin Nanosheeets: High Two-Dimensional Conductivity for In-Plane Supercapacitors. J. Am. Chem. Soc. 2011, 133, 17832.

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Feng, L.; Vrubel, H.; Bensimon, M.; Hu, X. Easily-Prepared Dinickel Phosphide (Ni2P) Nanoparticles As an Efficient and Robust Electrocatalyst for Hydrogen Evolution. Phys. Chem. Chem. Phys. 2014, 16, 5917.

Table of Contents Graphic


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Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic

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