2 Nanoporous Films for High Efficient Hydrogen Evolution

ACS Energy Lett. , Just Accepted Manuscript. DOI: 10.1021/acsenergylett.7b00326. Publication Date (Web): May 8, 2017. Copyright © 2017 American Chemi...
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Strained W(SeS ) Nanoporous Films for High Efficient Hydrogen Evolution Kun Liang, Yong Yan, Limin Guo, Kyle Marcus, Zhao Li, Le Zhou, Yilun Li, Ruquan Ye, Nina Orlovskaya, Yong-Ho Sohn, and Yang Yang ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Strained W(SexS1-x)2 Nanoporous Films for High Efficient Hydrogen Evolution Kun Liang†,#, Yong Yan†,#,∆, Limin Guo†,#, Kyle Marcus†, ‡, Zhao Li‡, Le Zhou‡, Yilun Liǁ, Ruquan Yeǁ, Nina Orlovskaya¶, Yong-Ho Sohn‡ & Yang Yang†, ‡,* †

NanoScience Technology Center, University of Central Florida, Orlando, FL 32826,

United States ‡

Department of Materials Science and Engineering, University of Central Florida,

Orlando, FL 32826, United States ǁ

Department of Chemistry, Rice University, 6100 Main Street, Houston, Texas 77005,

United States ¶

Department of Mechanical and Aerospace Engineering, University of Central

Florida, Orlando, FL 32826, United States *E-mail: [email protected]

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ABSTRACT: A W(SexS1-x)2 nanoporous architecture (NPA) was developed by a facile

anodic

and

chemical

vapor

deposition

treatments.

The

ternary

W(SexS1-x)2 NPA offers significant advantages towards high efficiency hydrogen generation: i) nanoporous morphology provides more electrochemically active sites to split water; ii) lattice mismatch and disordering in the mix-phased W(SexS1-x)2 film introduce more defects to enhance the catalytic activity; iii) conducting 1T phase formed in the strained W(SexS1-x)2 facilitates the electron transfer during catalytic reactions. Therefore, an onset overpotential of 45 mV and a Tafel slope of 59 mV dec-1 were achieved using the as-prepared W(SexS1-x)2 NPA.

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TOC

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As a clean and renewable chemical fuel, hydrogen has been widely investigated as a promising alternative to fossil fuels.1-3 In a fuel cell, hydrogen can convert chemical energy into electrical energy with high efficiency while producing no pollutant byproducts.4 Currently, hydrogen is produced mainly through oxidation of methane or pyrolysis of hydrocarbons, which need further purification before delivering to end users.5-6 In recent decades, great effort has been dedicated to researching low cost and high efficiency approaches to generate high purity hydrogen. An commonly effective way of producing hydrogen is water electrolysis (water splitting) using an electrolyzer.7-8 A great deal of research has been devoted to developing efficient electrocatalysts for hydrogen evolution reaction (HER). Platinum is by far the most efficient catalysis for HER, but its high-cost limits large-scale applications.9 Therefore, there is strong motivation to replace Pt-based catalysts with low-cost materials that are rich in abundance and can be viable for HER catalysis. Transition metal dichalcogenides (TMDs) have attracted enormous attention due to their distinct layered structure and interesting physical properties.10-13 Among typical TMDs, tungsten disulfide (WS2) and tungsten diselenide (WSe2) have similar structures with MoS2, where tungsten atoms are inserted between hexagonally packed S/Se atomic layers held together by strong covalent bonds and subsequently, stacking of neighboring layers is accomplished by weak van der Waals interactions.14-15 Recently, WS2 and WSe2 were demonstrated as promising HER catalysts. Zou et al fabricated triangular WSe2 nanoflakes uniformly dispersed on electrospun carbon fibers, showing robust HER activity with a small ηonset of 110 mV.16 4 ACS Paragon Plus Environment

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WS2 and WSe2 show promise as potential HER catalysts, though required improvements in catalytic activity are required if these materials are to be considered for hydrogen generation. Experimental testing and theoretical simulation results have indicated that the HER performance strongly relies upon the amount of available electrochemical active sites. Additionally, the (10-10) edge planes are catalytically active for HER due to a lower Gibbs free energy toward hydrogen absorption.17-20 Additionally, different polymorph structures will influence catalytic activity due to the variations in electrical conductivity. For example, in 2H structures, metal atom bonding is trigonal prismatic and the material displays semiconducting character. In 1T structures, metal atom bonding is trigonal antiprismatic or octahedral, exhibiting a metallic behavior.21 In addition, for 1T structures, the thermodynamic equilibrium is unstable, meaning electrons are easily transferred to the 1T structure, because of the lower energy unoccupied states at the Fermi level.22 Therefore, the distorted 1T structure shows metallic features with enhanced electrical conductivity. Several effective methods can be used to enhance the catalytic performance of 2D TMDs: 1) increase in exposed edge surfaces with respect to the basal portion; 2) improvement of electrical conductivity to facilitate proper diffusion between electron and active sites; 3) introduce defects to serve as active sites, improving HER catalytic performance. In general, the defective sites serve as reaction centers and effectively enhance HER kinetics.23-24 Defects are readily generated in strained TMDs due to the lattice mismatch and disordering generated by either anion doping (WSe2xS2-2x) or cation doping (WxMo1-xS2 and WxMo1-xSe2) and interfacial stress created in 5 ACS Paragon Plus Environment

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mix-phased materials.25-26 These doped and mix-phased TMDs have a greater concentration of defects, which not only can contribute to lower Gibbs free energy for hydrogen adsorption, but also can strongly influence the electronic structure and therefore affect catalytic performance.25-26 Compared with nanotubes (NTs) and nanowires (NWs), materials with a self-assembled nanoporous architecture (NPA), having high surface area and a 3D open framework, can exhibit more exposed edge sites. This is important to consider because an increase in the number of edge sites can better enhance catalytic activity.2, 27

Additionally, partial substitution of chalcogens by other chalcogens leads to

continuous modulation of the d-band electronic structure, resulting in a reduction in hydrogen adsorption free energy and enhancing electrocatalytic activity. Motivated by these advantages, we report here the development of W(SexS1-x)2 NPA by electrochemical anodization and chemical vapor deposition (CVD) treatments. For HER catalysis, the as-prepared W(SexS1-x)2 NPA achieved an outstanding onset overpotential of 45 mV with a Tafel slope of 59 mV dec-1. Figure 1a schematically exhibits a typical experimental process (for details see experimental section) of the strained W(SexS1-x)2 NPA. A WO3 NPA film with thickness of 2.25 µm (Figure 1b) was firstly grown on a tungsten foil by anodization treatment at a constant voltage of 60 V for 1 h. The NPA film presents an average pore size around 50 nm (Figure S1). The as-prepared WO3 NPA was then partially selenized at 350 oC in Ar atmosphere to convert WO3 to WOySex NPA. Afterwards sulfuration was also performed at 350 oC to further substitute the residual oxygen in 6 ACS Paragon Plus Environment

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WOySex NPA to form W(SexS1-x)2 NPA without damaging the highly porous structure (Figure 1c). It should be noted that selenization was performed prior to sulfuration. Because of the lower chemical activity of Se compared to S, sulfuration should proceed more easily than selenization. Transmission electron microscopy (TEM, Figure 2a) was used to further characterize the flake morphology of the W(SexS1-x)2-15 NPA. High resolution TEM (HRTEM) images were used to identify crystalline profiles of the materials. Fine crystal lattice fringes of the layered W(SexS1-x)2 was clearly illustrated in Figure 2b. A layer-to-layer spacing estimated around 0.65 nm denotes the (002) plane of the layered W(SexS1-x)2.28-29 A Fast Fourier Transformation (FFT, Figure 2d) was performed on the selected region (orange dashed box) in Figure 2c. The strong lattice spots in the FFT image corresponds to the parallel planes. The lattice fringe with a distance of 0.27 nm was also observed in Figure 2c, corresponding to the (100) plane of the layered W(SexS1-x)2. Moreover, the FFT image (Figure 2e) selected from selected region (green dashed box) in Figure 2c exhibits the clear honeycomb lattice structure in the (100) planes, confirming the hexagonal lattice structure. It is worth pointing out that large amounts of mismatched crystals and disordered sites are formed (solid blue box in Figure 2c), which might be generated by anion substitution and residual stress in the strained W(SexS1-x)2. These defects can serve as active sites to effectively improve HER performance. Energy-dispersive X-ray spectrum elemental (EDX) mappings of the W(SexS1-x)2 NPA (Figure S2) suggest uniform and homogeneous distribution of W and S across the NPA. However, Se is not 7 ACS Paragon Plus Environment

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homogeneously distributed throughout the entire NPA, owing to the lower chemical activity of Se compared to S and the two-step CVD treatment in this work. That indicates plenty of interfaces may be generated in the mix-phased W(SexS1-x)2 NPA. X-ray diffraction (XRD) analysis was performed to characterize the crystalline structure of the layered 2D materials. Figure 3a shows the XRD profiles of WSe2, W(SexS1-x)2, and WS2, revealing that there is a slight shift of the (002) diffraction peak at about 14o for ternary W(SexS1-x)2 NPA compared to WSe2 (PDF # 06-0080) and WS2 (PDF # 08-0237).17 This could be due to the size difference of Se and S atoms being incorporated during fabrication, which affects interlayer distance. Moreover, the broadened diffraction peak for (002) plane in ternary W(SexS1-x)2 NPA indicates an increased defective structure due to the residual stress in the strained films.30-31 The residual stress can be measured by XRD sin2Ψ method (see Supporting Information and Figure S3). As shown in Figure 3b, W(SexS1-x)2-15 NPA (15 donates 15 min sulfuration time) displays the strongest residual stress of 457 MPa, which might be generated by interfacial stress in the mix-phased W(SexS1-x)2 strained films. Meanwhile, these interfacial stress has been confirmed to have a positive effect on HER.32 Raman spectroscopy (Figure 3c) was employed to analyze in-plane and out-of-plane vibration modes of the layered W(SexS1-x)2. For WSe2 NPA, there is only one broad peak that can be observed due to the small energy difference between the A1g (253 cm-1) and E12g (250 cm-1) for out-of-plane and in-plane modes, respectively.33 Meanwhile, the bands located 350 and 415 cm-1 are associated with 8 ACS Paragon Plus Environment

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E12g and A1g modes for WS2 NPA.17 For the ternary W(SexS1-x)2 NPA with different sulfuration times, it is clear that there is enhancement of E12g mode and A1g mode with gradual increases in S content. Additionally, all peaks are broadened in the ternary W(SexS1-x)2 NPA samples, revealing more elemental disorder. Moreover, the E12g mode and A1g mode intensity ratios were 1.52 and 1.42 for W(SexS1-x)2-15 NPA and W(SexS1-x)2-20 NPA, respectively. This suggests the defective structure of W(SexS1-x)2 NPA was reduced by extending sulfuration time. The larger ratio of E12g/A1g mode also indicated the emergence of 1T phase.32 Raman mapping of the W(SexS1-x)2-15 NPA provides the spatial intensity maps of A1g mode in Figure 3d, demonstrating NPA uniformity of mix-phased TMDs. Electrochemical active sites play a crucial role in catalytic performance. Electrochemically active surface area (EASA) is used to determine the amount of catalytically active sites in a material. Electrochemical double-layer capacitance (Cdl) can be employed to estimate EASA from cyclic voltammetry (CV) curves in the non-Faradaic region. CV curves were tested at various scan rates (10-180 mV s-1, Figure S4) to estimated Cdl of various electrodes. Cdl are calculated to be 1.601, 1.072, 3.000, and 3.112 mF cm-2 for WSe2, WS2 W(SexS1-x)2-15 and W(SexS1-x)2-30 NPA, respectively, suggesting that the active area of W(SexS1-x)2 NPA is about 2-3 times as large as that of WSe2 and WS2. The large EASA of W(SexS1-x)2 NPA indicates more accessible active sites and exposed edges, determining the excellent HER performance. The electrocatalytic activity of W(SexS1-x)2 NPA was evaluated using a standard 9 ACS Paragon Plus Environment

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three-electrode setup in 0.5 M H2SO4 electrolyte bubbled with H2, as described in experimental section. The ηonset and overpotential at the cathodic current density of 10 mA cm-2 (donated as η10) were used to evaluate the HER performance. As determined from linear sweep voltammetry (LSV, Figure 4a), the ηonset and η10 of the W(SexS1-x)2-15 NPA are estimated to be around 45 mV and 110 mV, respectively, significantly lower than those of WSe2, WS2 and W(SexS1-x)2-30 NPA. To further investigate the catalytic activity of W(SexS1-x)2 NPA, the Tafel slope derived from the polarization curve was employed, as shown in Figure 4b. The corresponding Tafel slope of 59 mV dec-1 is achieved for W(SexS1-x)2-15 NPA, while for W(SexS1-x)2-30 NPA, the Tafel slope increases to 62 mV dec-1, which is better than that of most state-of-the-art HER catalysts (for details see Table S1). This result reveals that the Volmer reaction, a process to convert protons into sorbed hydrogen atoms on NPA surface, becomes the rate-determining step in the HER process.2 The exchange current density (J0) is an important parameter to describe the HER activity, as illustrated in Figure 4b. The W(SexS1-x)2-15 NPA exhibits the largest exchange current density of 1259 µA cm-2 for all electrodes and about 4.5 times larger than that obtained from W(SexS1-x)2-30 NPA, indicating prominent hydrogen evolution behavior and fast charge transfer. W(SexS1-x)2-15 NPA and W(SexS1-x)2-30 present a similar EASA, but W(SexS1-x)2-15 NPA shows lower overpotentials/Tafel slope and larger exchange current density, which contributes to better catalytic activity. This is due to the strained W(SexS1-x)2-15 NPA having more defective sites and the addition of the 1T phase (details see XRD and Raman section) promotes a synergistic effect, markedly 10 ACS Paragon Plus Environment

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improving HER performance. The lowest overpotential and Tafel slope and largest exchange current density of W(SexS1-x)2-15 NPA demonstrate the superior HER activity. Retention testing was then performed on the W(SexS1-x)2-15 NPA electrode with a constant current density of 10 mA cm-2 for 30,000 s (Figure 4c). The HER performance remains stable during long term testing and only a slight decay can be observed over time, owing to a reduction in EASA caused by the physical absorption of generated H2 bubbles on the NPA surface. To investigate electrode kinetics of the HER process, electrochemical impedance spectroscopy (EIS) was utilized to examine the electron transfer process. As shown in Figure 4d, the Nyquist plots reveal that the charge-transfer resistances (Rct) of WSe2, WS2, W(SexS1-x)2-15 and W(SexS1-x)2-30 NPA are 84.7, 87.9, 60.3 and 69.7 Ω, respectively. The ternary W(SexS1-x)2-15 NPA possesses the lowest Rct, suggesting a faster electron transfer rate in the active sites among the W(SexS1-x)2-15 NPA electrode. The similar series resistance around 15 Ω denotes a small ohmic loss for all electrodes, suggesting an advantage of the NPA grown on a conductive substrate in that it is unnecessary to use additives required by many other types of materials. Above all, the W(SexS1-x)2-15 NPA electrode demonstrates excellent HER activity, due to several reasons:1) NPA provides a large EASA to contribute more accessible active sites; 2) introducing 1T phase improves electrical conductivity which facilitates fast charge transfer and good diffusion between electron and active sites; 3) interfacial stress in the strained films promote the defect site concentration, which strongly affect 11 ACS Paragon Plus Environment

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the catalytic performance. In summary, W(SexS1-x)2 NPA was grown through substitution reactions of Se and S with anodically formed WO3 NPA on tungsten substrate. These W(SexS1-x)2 films effectively served as a high efficiency HER electrode, exhibiting outstanding electrocatalytic performance. The strained W(SexS1-x)2 NPA provided increased active sites and defect structures, which are beneficial for HER activity. Our experimental results indicate that the ternary W(SexS1-x)2 NPA is a promising alternative for Pt-based catalysts toward HER.

Fabrication: Tungsten foils (0.025 mm, 99.9%, MTI Inc.) were sonicated in acetone, ethanol, and deionized water for 30 min, respectively, before being used as substrate. In a two-electrode setup with Pt as counter electrode, WO3 NPA were anodically grown in an electrolyte of 0.15 M oxalic acid with 0.1 M Na2SO4 and 0.01 M NaF at 60 V of constant voltage for 1 h. After anodization, the samples were rinsed with water three times and then dried in air. To convert WO3 NPA to W(SexS1-x)2 NPA, reactions with Se and S were performed in a CVD system. Briefly, the WO3 NPA was placed at the center of a 1-inch quartz tube furnace. Se powder (Fisher Scientific) was placed at the upstream side of the tube. The tube was evacuated to a pressure of 100 mTorr for 10 min and purged with Ar to remove any residual air. Then, the furnace center was set to 350 oC and Se powder was set to 300 oC to be reached in 10 min. The reaction was performed for 30 min with Ar (100 sccm) as carrier gas, followed by natural cooling. After selenization, the S powder (Fisher Scientific) was used to replace Se powder to repeat the CVD treatment. The temperature for S was designed 12 ACS Paragon Plus Environment

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to 150 oC and the reaction was carried out for differing time durations (15 min to 30 min).

Characterization: A ZEISS ULTRA 55 scanning electron microscope (SEM) was employed to analyze the morphology. An FEI TecnaiF30 high resolution transmission electron microscope (HRTEM) was investigated to observe the morphologies and elemental mapping of the samples. X-ray diffraction (XRD) analysis was performed by a PANalytical Empyrean diffractometer (PANalytical B.V.) configured with a Cu Kα radiation. Raman spectra were recorded with a Renishaw Raman RE01 scope (Renishaw, Inc.) using a 532 nm excitation argon laser.

Electrochemical measurements: Electrochemical performances were performed by a CHI 760E electrochemical workstation (CH Instruments) with a three-electrode cell configuration. The nanoporous thin-film with geometric area of 0.785 cm2 was used as the working electrode, a 3 M Ag/AgCl electrode was used as the reference electrode and Pt wire as the counter electrode. The reference electrode wa calibrated with respect to the reversible hydrogen electrode (RHE) using Pt foil as working and counter electrodes. Linear sweep voltammerty (LSV) with a scan rate of 5 mV s-1 was performed in a H2 saturated 0.5 M H2SO4 electrolyte. The potentials reported herein were converted to RHE, and ERHE = EAg/AgCl + 0.0591pH + 0.21. The electrochemical impendance spectroscopy (EIS) at open circuit potential was collected with frequency range from 10 mHz to 100 kHz and alertnating voltage of 5 mV. Long-term stability tests were performed using galvanostatic method (10 mA cm-2) for 30,000 sec. Acknowledgements 13 ACS Paragon Plus Environment

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This work was financially supported by the University of Central Florida through a start-up grant (No. 20080741). Supporting Information Supporting Information Available: Present Address ∆

Y.Y.

(Yong

Yan)

Department

of Applied Physics, Nanjing University

of Science and Technology, Nanjing, 210094, P.R. China. Author contributions #

K.L., Y.Y. (Yong Yan) and L.G. contributed equally to this work. Y.Y. and Y.Y. (Yong

Yan) designed the experiments. Y.Y. (Yong Yan), L.G. and K.M. performed the materials synthesis and characterization. Y.Y. (Yong Yan) performed the electrochemical test and collected the data. L.Z. and Y.H.S. assisted in TEM. Y.L., R.Y. and N.O. assisted in Raman. K.L. and K.M. wrote the manuscript. Y.Y. corrected the manuscript. All authors approve the manuscript. Additional information Supplementary information is available in the online version of the paper. Correspondence and requests for materials should be addressed to Y.Y. ([email protected]). Competing financial interests The authors declare no competing financial interests.

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Figure 1. (a) Schematic illustration outlining W(SexS1-x)2 nanoporous architecture (NPA) fabrication. (b-c) Cross-sectional and top-view SEM images of NPA.

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Figure 2. (a-b) TEM images of W(SexS1-x)2-15 NPA. (c) HRTEM image from the yellow dashed box in (b). The light blue boxes represent lattice mismatch and defects. (d-e) Fast Fourier Transform (FFT) were performed on the regions marked by orange and light green dashed box, respectively.

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Figure 3. (a) XRD profiles of the as-prepared NPA. (b) Residual stress calculated by XRD sin2Ψ method. (c) Raman spectra of the as-prepared NPA. (d) Raman mapping of W(SexS1-x)2 NPA.

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Figure 4. Electrochemical activity of NPA. (a-b) iR-corrected LSV curves for HER measured at 5 mV s-1 in 0.5 M H2SO4 aqueous solution and corresponding Tafel slopes, respectively. (c) Chronopotentiometric curve of HER with constant current density of 10 mA cm-2. (d) Electrochemical impedance spectroscopy (EIS) of NPA.

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