Research Article www.acsami.org
Synergistic WO3·2H2O Nanoplates/WS2 Hybrid Catalysts for HighEfficiency Hydrogen Evolution Lun Yang,† Xiaobin Zhu,† Shijie Xiong,† Xinglong Wu,*,† Yun Shan,†,‡ and Paul K. Chu§ †
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Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics, Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China ‡ Key Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang University, Nanjing 211171, P. R. China § Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China S Supporting Information *
ABSTRACT: Tungsten trioxide dihydrate (WO3·2H2O) nanoplates are prepared by in situ anodic oxidation of tungsten disulfide (WS2) film on carbon fiber paper (CFP). The WO3· 2H2O/WS2 hybrid catalyst exhibits excellent synergistic effects which facilitate the kinetics of the hydrogen evolution reaction (HER). The electrochromatic effect takes place via hydrogen intercalation into WO3·2H2O. This process is accelerated by the desirable proton diffusion coefficient in the layered WO3· 2H2O. Hydrogen spillover from WO3·2H2O to WS2 occurs via atomic polarization caused by the electric field of the charges on the planar defect or edge active sites of WS2. The optimized hybrid catalyst presents a geometrical current density of 100 mA cm−2 at 152 mV overpotential with a Tafel slope of ∼54 mV per decade, making the materials one of the most active nonprecious metal HER catalysts. KEYWORDS: WO3·2H2O, WS2, spillover effect, synergism, hydrogen evolution
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INTRODUCTION Hydrogen is an important carbon-free energy source that can potentially satisfy future energy demands,1 and a myriad of materials such as the boride,2 carbide,2,3 nitride,3 phosphide,4 and sulfide5 of transition metals (Fe, Co, Ni, Mo, and W) have been identified as potential catalysts in the hydrogen evolution reaction (HER).6,7 As an alternative to expensive Pt, MX2 (M = Mo, W; X = S, Se) with various forms like amorphous MoSx and 1T-WS2 has emerged to be an efficient and nonprecious metal catalyst.8,9 Since the edges of 2H-MX2 are catalytically active, much effort has been made to improve the properties of MX2-based catalysts,10,11 for instance, by tailoring the nanostructure to expose more edges,12 doping with cobalt to promote the S-edge (1̅110),13 molecular simulation,14 and single-atom metal doping15 or introducing strained sulfur vacancies16 to activate the basal planes. Besides, heterogeneous composite catalysts like WS2 nanolayers on heteroatom-doped graphene films have also been utilized in hydrogen production.17 However, synergistic MX2-based HER catalysts have mainly been used to increase the quantity of active sites (edges and defects), improve the electron transport, or optimize the Gibbs free energy of adsorbed hydrogen (ΔGH).18,19 Tungsten trioxide and its hydrate are 2D layered oxide with negligible HER activity unless the electronic structure is modulated by changing the chemical constituents or introducing oxygen vacancies.20,21 Nonetheless, they can be applied to © 2016 American Chemical Society
electrochromic devices by taking advantage of the reversible electrochemical injection of both electrons and positive ions into the multivalent host lattice to form tungsten bronze (AxWO3, A = H or Li, Na).22 WO3·2H2O may have faster proton insertion kinetics than WO3 and other hydrates as a result of the larger proton diffusion coefficient.23,24 The hydrogen spillover effect refers to the migration of activated H atoms from the hydrogenrich metal particles to hydrogen-poor surface of the support.25 Hydrogen spillover and the reverse process in heterogeneous composite catalysts such as Pt/WO3 and Co9S8/MoS2 have been investigated in the gaseous and electrochemical environments.26−29 In this work, we design and prepare a new type of hybrid catalyst composed of WO3·2H2O nanoplates on WS2 film. Protons and electrons can be inserted into the WO3·2H2O by electrochromism and hydrogen atoms in the lattice of WO3· 2H2O can be transported to the planar defect or edge sites of WS2 via the hydrogen spillover effect. The hybrid synergistic catalyst delivers outstanding HER performance.
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RESULTS AND DISCUSSION The synergistic catalytic mechanism of the WO3·2H2O/WS2 hybrid catalyst in HER is illustrated in Figure 1a. In the acidic Received: April 5, 2016 Accepted: May 23, 2016 Published: May 23, 2016 13966
DOI: 10.1021/acsami.6b04045 ACS Appl. Mater. Interfaces 2016, 8, 13966−13972
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ACS Applied Materials & Interfaces
become protonic in nature.25,30 The electrons transferred from the H atoms reduce the adjacent W atom of O−H bond from W6+ to W5+.30 WO3·2H2O contains both coordinated water bonded to W in the WO3[H2O] layer and structural water between the layers.22 The structural water increases the interlayer distance, and the low energy barrier for swaying the adsorbed proton around the bridging oxygen anion in the WO5(H2O) octahedra network facilitates proton diffusion in WO3·2H2O,31 thereby resulting in more efficient hydrogen intercalation than WO3 and other hydrates like WO3·H2O.23,24 Furthermore, hydrogen atoms can also be adsorbed at defect sites like sulfur vacancies or metallic edge sites of MX2.7,16 The structure of the WS2 film can be tailored to expose plentiful defects and edges. In such cases, an energetically favorable migration route for hydrogen atoms from WO3·2H2O to the active sites of WS2 is expected.25 Thus, the H atoms intercalated in WO3·2H2O can spill over to WS2 and participate in the HER on the defects or edges of WS2. The current WS2 film on CFP is synthesized by a thermolysis method.32,33 Here, the WO3·2H2O/WS2 hybrid catalyst is prepared by a simple anodic treatment of the WS2 film in a 0.5 M H2SO4 solution by cyclic voltammetry (Figure S1), and its schematic structure is presented in Figure 1b. WO3·2H2O is synthesized in situ and forms a close contact with WS2, which could expedite electron transportation and hydrogen migration. The HER property of the hybrid catalyst can be improved by adjusting the synthesis temperature of WS2 and degree of anodic oxidation. With regard to the WS2 films prepared at 600 °C, the sample with a cycle number of Z in anodization is designated as WS2−oxZ, and the WS2−ox0 sample processed by a 1 h cathodic treatment at −0.2 V versus RHE is labeled as WS2−tested. Field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and X-ray powder diffraction (XRD) are employed to characterize the catalysts (Figure 2). The WS2−ox0 sample produced at
Figure 1. (a) Schematic illustration of the synergistic mechanism of the WO3·2H2O/WS2 hybrid catalyst in electrochemical hydrogen evolution. H• represents the H atoms inserted in the lattice of WO3·2H2O forming HxWO3·2H2O by electrochromism. The upper insert shows the action of the electric field of a defect site on the hydrogen atoms. (b) Schematic structure of the WO3·2H2O nanoplates on the WS2 film after anodic treatment.
medium, the intercalation process of protons occurring in WO3· 2H2O can be written as WO3 ·2H 2O + x(H+ + e−) → HxWO3 ·2H 2O
(1)
where x is the mole fraction of hydrogen inserted into the host.24 This process is reversible at a potential positive to the reversible hydrogen electrode (RHE).23,24 The inserted hydrogen atoms are coordinated with terminal oxygen atoms, lose electrons, and
Figure 2. FE-SEM images of (a) WS2−ox0 and (b) WS2−ox6 with the insets depicting the magnified local images. (c) TEM image of WS2−ox6. The red- and blue-framed insets correspond to the WO3·2H2O and WS2 regions, respectively. (d) XRD patterns acquired from tungsten sulfide after different degrees of anodization. The standard patterns of the WO3·2H2O (JCPDS 18-1420) and 2H-WS2 (JCPDS 08-0237) are shown as reference. 13967
DOI: 10.1021/acsami.6b04045 ACS Appl. Mater. Interfaces 2016, 8, 13966−13972
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Figure 3. (a) W 4f and 5p and (b) S 2p XPS spectra obtained from the WS2−ox0 and WS2−ox6 samples. The gray circle line represents the experimental data, and the cyan line shows the fitted curve.
600 °C without anodization has a nonuniform and cracked feature with a typical dimension of ∼20 μm (Figure 2a and Figure S2a). The magnified WS2 surface in the inset of Figure 2a is relatively smooth and nonporous. All the XRD peaks of WS2− ox0 can be indexed to the 2H-WS2 phase except for those from the CFP substrate (Figure 2d and Figure S3).32 The characteristic XRD peaks of WS2 are a bit broad, attributable to the small crystalline size and poor crystallinity.34 In all the samples, the (002) diffraction peak of the 2H-WS2 shifts to a lower angle of 2θ = 14.0° compared with the standard value (14.3°). This implies about 2% lattice expansion due to the tensile strain caused by the distorted structure.35 The XRD analysis reveals that there are abundant planar defects and edges in WS2.34 After anodic oxidation, new peaks corresponding to the monoclinic WO3· 2H2O phase appear in the XRD patterns (Figure 2d).36 Other phases like WO2 or WO2.9 cannot be found by XRD.20,21 With increasing cycle number, the peak intensity of WO3·2H2O increases and reaches a maximum for WS2−ox6 and then decreases, but the peak intensities of the WS2 phase hardly change. The cathodic treatment is unable to produce the WO3· 2H2O phase because the XRD pattern of WS2−tested is almost the same as that of WS2−ox0 (Figure S3). As shown by the SEM images, nanoplates emerge from WS2 after anodization attributable to WO3·2H2O according to XRD (Figure 2b and Figure S4). The quantity of nanoplates in WS2−ox6 is also the largest, indicating that an excessively large cycle number leads to corrosion such as that observed from WS2−ox12. Besides, slowing down the scan rate and decreasing the acidic concentration are both adverse to the formation of the WO3· 2H2O nanoplates (Figure S5). With regard to WS2−ox6, the nanoplates with a thickness of 20−90 nm and length of about 500 nm are densely and homogeneously distributed on the WS2 film (Figure 2b and Figure S2b). The TEM image of WS2−ox6 shows the morphology and microstructure (Figure 2c). In the redframed inset in Figure 2c, the lattice fringes with measured interplanar spacings of 0.377 and 0.370 nm are assigned to the
(200) and (001) planes of the monoclinic WO3·2H2O, respectively.22 The lattice spacing of 0.630 nm in the blueframed inset corresponds to the (002) plane of 2H-WS2, which is a little larger than the standard value of 0.618 nm.35 This is consistent with the above XRD results. The grain sizes of WS2 are between several to dozens of nanometers. Here, we would like to mention that, in all the XRD spectra (Figure 2d), the diffraction peaks of the WS2 nanoplates are at the same position. This indicates that the introduction of the strain does not occur in the process of anodization. Thus, the WO3·2H2O nanoplates grown during anodization do not show the observable lattice expansion. To investigate the chemical nature and bonding states on the surface of products, X-ray photoelectron spectroscopy (XPS) is performed. The atomic ratio of W to S rises from 1:2.30 in WS2− ox0 to 1:1.06 in WS2−ox6 and then decreases to 1:1.98 in WS2− ox12 (Figure S6). The characteristic W4+ (5p3/2 = 38.4 eV, 4f5/2 = 34.7 eV, 4f7/2 = 32.6 eV) and S2− (2p1/2 = 163.4 eV, 2p3/2 = 162.2 eV) signals are indicative of the 2H-WS2 phase (Figure 3 and Figures S7 and S8).7,32 The two components (2p1/2 = 170.0 eV, 2p3/2 = 168.9 eV) are assigned to the orbitals of S atoms in a +6 form originating from sulfuric acid and oxidation of S2−.13,37 The peak at ∼532.0 eV is associated with chemisorbed oxygen species and water molecules in WO3·2H2O (Figure S9).37,38 The signal of O2− bonded to W in WO3·2H2O is at 530.5 eV and absent from the WS2−ox0.38 The constituents of W6+ (5p3/2 = 41.8 eV, 4f5/2 = 38.1 eV, 4f7/2 = 36.0 eV) and SO42− species are rather small in WS2−ox0 (Figure 3).32 However, after anodization, the intensities of W6+, SO42−, and O2− escalate to be the highest in WS2−ox6, and then decline (Figure 3 and Figures S7−S9). The portion of W6+ is even more than that of W4+ in WS2−ox6 especially. Moreover, W5+ or S−, which exists in the phases like WO2.9 or amorphous WSx, is not observed from the materials in this work.21,33 Previous reports have demonstrated that tungsten trioxide and its hydrate can be produced by oxidation of WS2.37,39 Here, the 2H-WS2 in the sample exposes plenty of defects and edges, which 13968
DOI: 10.1021/acsami.6b04045 ACS Appl. Mater. Interfaces 2016, 8, 13966−13972
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Figure 4. Polarization curves obtained from the catalyst films on CFP in 0.5 M H2SO4 at a scanning rate of 2 mV s−1: (a) WS2 synthesized at 600 °C with different degree of anodization and (b) WS2 fabricated at different temperature with the same anodizing treatment. All polarization curves are corrected for iR losses except for Pt. (c) Tafel plots of the WS2−ox0, WS2−ox6, and Pt wire samples. (d) Nyquist plots acquired by EIS for WS2−ox0 and WS2− ox6. The data are fitted by the simplified equivalent circuit displayed in the inset, and the fitted results are shown by solid lines.
are favorable for oxidation.37 The WO3·2H2O is formed on the WS2 film after anodization, and the quantity of WO3·2H2O nanoplates in WS2−ox6 is the largest. The formation mechanism of WO3·2H2O is described as follows:
is improved significantly. The performance is in direct correlation with the WO3·2H2O content, and WS2−ox6 shows the best result, indicating the importance of WO3·2H2O in the HER. The onset potential of WS2−ox6 is only ∼60 mV, and it shows a geometrical current density of 100 mA cm−2 at η = 152 mV, which is 10 times larger than that of WS2−ox0 at the same voltage (10 mA cm−2). Meanwhile, the influence of the fabrication temperature of WS2 on the HER performance is illustrated in Figure 4b. The products are all treated for six cycles during anodic oxidation. The HER activity increases initially and then decreases as the synthesis temperature increases, having an optimal case at 600 °C. Moreover, the anodic peak can be utilized to estimate the concentration of active sites since the defects and edges in 2H-WS2 are easier to oxidize than those in the (0001) basal plane.13 The anodic current for WS2−ox6 is a little smaller than WS2−ox0, but still much larger than bulk WS2 powder, suggesting that the amount of active sites in the WS2−ox6 is close to that in WS2−ox0 (Figure S10b). The intrinsic activity of each active site is usually evaluated by the turnover frequency (TOF). Utilizing Cdl and anodic current data, the per-site TOF values at 150 mV overpotential for WS2−ox0 and WS2−ox6 are estimated to be 0.077 and 0.85 s−1, respectively (see the details in the caption of Figure S11).33,41 The durability of samples is assessed by constant-voltage electrolysis, and a sustained current is observed for at least 180 min at 170 mV overpotential for WS2− ox6 (Figure S13a). The current decay may result from inactivation of WS2 film and loss of WO3·2H2O, because the WO3·2H2O phase might slowly dehydrate to the WO3 phase and the WO3·2H2O nanoplates are apparently detached from WS2 due to the damage caused by hydrogen bubbles (Figure S13b).24,33 Electrochemical treatment is widely used to activate electrocatalysts by removing surface carbon or forming active
WS2 + 13H 2O → WO3 ·2H 2O + 2SO4 2 − + 18e− + 22H+
(2)
The generated H+ could fast migrate to the electrolyte due to the desirable proton diffusion coefficient in WO3·2H2O. Moreover, WO3·2H2O is easier to form in an acidic solution,40 especially by cyclic voltammetry.23 To examine the HER property of the catalysts, linear sweep voltammetry is carried out in 0.5 M H2SO4 at 2 mV s−1. Figure 4a presents the polarization curves of the samples prepared at 600 °C and different degree of anodization. These curves are corrected for the ohmic potential drop (iR) loss. The bulk WS2 powder sample and commercial Pt wire electrode are measured for comparison. Pt shows the extraordinary HER activity with an onset potential of almost zero, whereas both the bulk WS2 powder and WO3·2H2O particles deliver much poorer catalytic performance compared to WS2−ox0 (Figure S10a). The WS2− ox0 sample exhibits a rather large geometrical current density of 100 mA cm−2 at an overpotential (η) value of 281 mV. Besides, it is known that the electrochemically active surface area (ECSA) is linearly proportional to the double layer capacitance (Cdl).4,6 Thus, cyclic voltammetry is conducted to measure Cdl (Figure S11). Cdl of WS2−ox0 is obtained to be 20 mF cm−2, which is about 59 times larger than that of the bulk WS2 (0.34 mF cm−2). This indicates many more defects and edges in the twisted WS2 nanocrystal.34 After anodization, a small reduction peak is observed prior to hydrogen production in the first polarization scan but disappears in the subsequent scan (Figure S12),41,42 and the HER efficiency 13969
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ACS Applied Materials & Interfaces composite.43,44 However, pure WO3·2H2O is inert in HER. To explore the role of WO3·2H2O nanoplates in this work, the WS2−ox6 and WS2−ox0 samples are annealed at 600 °C in Ar gas again. After the thermal treatment, the HER property, XRD peaks, and SEM features change very little for WS2−ox0 (Figure S14), but the WO3·2H2O nanoplates in the WS2−ox6 sample turn into WO3 nanocrystals. The HER performance of the annealed WS2−ox6 sample is obviously inferior to that of WS2− ox6 but close to that of WS2−ox0, revealing that the WO3·2H2O nanoplates indeed participate in the process of hydrogen evolution. Furthermore, Tafel analysis is conducted to elucidate the HER mechanism. The exchange current density (j0) and Tafel slope (b) are determined by fitting the linear segments of the Tafel plots to the Tafel equation (η = a + b log|η|).3 The smaller Tafel slope is expected because a lower overpotential is required to produce a given current and the Pt wire displays a Tafel slope of 30 mV dec−1 (Figure 4c).15 The Tafel slope of WS2−ox6 is 54 mV dec−1, suggesting the Volmer−Heyrovsky mechanism in HER and the rate limiting reaction is the electrochemical desorption step (H3O+ + e− + cat.−H → H2 + cat. + H2O).45 The slope value is comparable to the current levels of MX2-based catalysts such as 1T-WS2 (55 mV dec−1),7 amorphous MoSx (40 mV dec−1),41 and WS2@P,N,O-graphene (52.7 mV dec−1).17 The WS2−ox0 sample has a Tafel slope of 87 mV dec−1. On account of nearly the same concentrations of the active sites in WS2−ox0 and WS2−ox6, the decreased Tafel slope of WS2−ox6 can be ascribed to the synergistic effects between WO3·2H2O and WS2. The exchange current density of 137 μA cm−2 observed from WS2−ox6 is nearly one-third the value of 450 μA cm−2 determined from Pt (111), and so the WO3·2H2O/ WS2 hybrid is one of the most active nonprecious metal HER catalysts.7 Electrochemical impedance spectroscopy (EIS) is performed to study the electrode kinetics.18 The Bode and corresponding Nyquist plots of WS2−ox6 obtained at various overpotentials are displayed in Figure S15a−c. In the Nyquist plots, WS2−ox6 shows one capacitive semicircle, revealing that the equivalent circuit for the electrocatalysis can be characterized by one time constant and the reaction is dynamically controlled.34 The Bode plots, which depict |Z| (modulus of impedance) or phase angle as a function of frequency, indicate a series of resistors in addition to one unit of parallel resistor and capacitor. The simple equivalent electrical circuit constructed to fit the EIS data is presented in the inset of Figure 4d, which consists of a series resistance (Rs), constant phase element (CPE), and charge transfer resistance (Rct).6 The Rs values of WS2−ox0 and WS2−ox6 are 0.95 ± 0.05 Ω cm2, confirming the close contact between the catalysts and CFP electrode. Rct is a key parameter related to the electrocatalytic kinetics at the catalyst/electrolyte interface, and the Rct values at 130 mV overpotential are 11.8 and 2.0 Ω cm2 for WS2− ox0 and WS2−ox6, respectively (Figure 4d).17 The smaller Rct of WS2−ox6 demonstrates faster HER kinetics, which is also obviously due to the synergistic effects between WO3·2H2O and WS2. In addition, with increasing overpotentials, the radius of a semicircle in the Nyquist plots for WS 2 −ox6 reduces significantly, and the peak frequency in the Bode phase plot goes up, suggesting a higher reaction rate and a smaller reaction time constant.6 The Rct diminishes from 21.9 Ω cm2 at 70 mV to 0.6 Ω cm2 at 210 mV in WS2−ox6 (Figure S15d). The Tafel slope of 48 mV dec−1 at low overpotential for WS2−ox6 can be obtained from the plot of overpotential versus −log Rct,46 which is consistent with the value of 54 mV dec−1 in the Tafel analysis.
All the results above indicate that the synergy between WO3· 2H2O and WS2 leads to the enhanced catalytic activity, because pure WO3·2H2O is inert in HER, and the amounts of the active sites hardly change in both the WS2−ox0 and WS2−ox6 samples. The hydrogen spillover effect has been proposed to explain the improved HER performance of Ni−Mo alloy and Pt/Ti|RTO SiO2|p-Si photocathode.28,29 Here, the hydrogen migration from the WO3·2H2O nanoplates to the defects and edges of WS2 increases the surface coverage of hydrogen (ΘH) on WS2. The HER dynamics on the active sites is accelerated by drastically increasing the per-site TOF and obviously diminishing Rct. As a consequence, the Volmer reaction no longer influences the HER rate, and the Tafel slope becomes smaller.6,45 Hence, the WO3· 2H2O/WS2 hybrid catalyst exhibits an excellent HER property, which is comparable to that of other WS2-based catalysts (Table S1). To theoretically elucidate the hydrogen spillover effect, we calculate the dynamics of hydrogen diffusion from WO3·2H2O to the planar defect and edge sites of WS2. The spillover hydrogen atoms on the basal plane of WS2 are attracted toward the defect sites (W or S vacancy with positive or negative charge) by the electric field of charges on these sites. The charge produces a spokewise field on the plane with the site at the center. The hydrogen atoms on the plane are polarized by the field, and the induced dipoles are pulled by the field toward the defect site due to the gradient of the field of the site no matter if the charge is positive or negative. This process is illustrated in the upper inset of Figure 1a. The polarization D of an H atom in a field E can be estimated from the perturbation theory. The first-order correction for the 4ceE wave function is |ψ1⟩ = − 3u ε |pE ⟩, where |pE⟩ is the 1p orbital of 0
hydrogen atom along the direction of electric field E, μ0 = 1 Ry =13.6 eV, ε is the dielectric constant, e is the electron charge, and 2π
π
3 cos2 φ 1 − sin 2 θ
3
c = A ∫ dφ ∫ sin 2 θ dθ = 6 A , with A 4π 0 0 being the integration of the radial function A=
1 6 a3
4 a 6
∞
∫0 r 3 dre−3r /2a( ar ) = ( 43 )
. Here, a is the Bohr
radius. This correction produces a dipole in the hydrogen atom which can be calculated as D =
8c 2e 2E , 3u0ε
and the force acting on a
hydrogen atom at a distance R from a defect site with charge Q is F=
16k 2c 2e 2Q 2 , 3u0ε 2R5
which produces acceleration of the hydrogen
atom toward the defect site ah =
16k 2c 2e 2Q 2 3u0ε 2mR5
=
0.74k 2a 2e 2Q 2 u0ε 2mR5
with k
and m being the Coulomb constant and the mass of hydrogen atom, respectively. According to the calculation, in the case of R = 1 μM and Q = 10 electron charge and with ε as the dielectric constant of water, acceleration of the hydrogen atom toward the defect site is about 6.3 mM/s2. Although there is random scattering during diffusion of the hydrogen atoms, this force is still large enough to drive them toward the defect sites. At these sites, two hydrogen atoms are combined to form a hydrogen molecule to complete the HER process.
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CONCLUSION A WO3·2H2O/WS2 hybrid catalyst is produced by a facile anodizing method. The hybrid displays superior activity due to the synergistic effects. The HER dynamics on the planar defects and edges of WS2 is facilitated by proton intercalation in the WO3·2H2O followed by migration of hydrogen from WO3·2H2O 13970
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to WS2. The HER property can be optimized by the preparation temperature and degree of anodic oxidation. The best sample, namely, WS2−ox6, exhibits an excellent exchange current density of 137 μA cm−2 with a Tafel slope of ∼54 mV dec−1, and the performance renders it to be one of the best non-noble metal catalysts. The results indicate that the WO3·2H2O/WS2 hybrid catalyst has large potential in water splitting devices, and our study provides a new route to enhance the HER efficiency by utilizing the synergistic effects based on structure design.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Basic Research Programs of China under Grants 2014CB339800 and 2013CB932901 and National Natural Science Foundation of China (11374141, 21203098, 61521001, and 21375067). Partial support was provided by the Qing Lan Project of Jiangsu Province and China Postdoctoral Science Foundation (2013M530247) as well as City University of Hong Kong Applied Research Grant (ARG) 9667104.
METHODS
Sample Preparation. In a typical procedure, a 20 wt % solution was formed by dissolving ammonium tetrathiotungstate [(NH4)2WS4, Alfa Aesar] in dimethylformamide (DMF, Sinopharm Chemical Reagent Co., Ltd.), followed by ultrasonic treatment for 20 min. The solution was dropped onto a CFP (HCP030N, Shanghai Hesen Electric Co., Ltd.) and dried in a vacuum desiccator at room temperature. The amount of precursor was 16 mg cm−2. For the bulk WS2 sample, the (NH4)2WS4 was replaced by tungsten sulfide powder (6 μm, Aladdin Industrial Corporation). Afterward, the CFP was put into a quartz tube, heated to the predesigned temperature in 30 min, and kept at this temperature for 60 min under argon at a flow rate of 100 standard-state cubic centimeter per minute (sccm). The anodization treatment was performed between 0.0 and +0.9 V versus Ag/AgCl at a rate of 50 mV s−1 in a 0.5 M H2SO4 solution by cyclic voltammetry. The degree of anodic oxidation was controlled by the cycle number. WO3·2H2O particles were synthesized by the Zocher method.40 A 120 mL portion of 1 N HCl solution and 80 mL of 70 mg mL−1 NaWO4·2H2O solution were cooled to 5 °C and then added together. The mixture was stirred for 90 min in an ice−water bath and for 30 min at room temperature. After that, the supernatant liquid was removed by centrifugation, and the precipitate was washed with deionized water. A 30 mg portion of WO3·2H2O particles is dissolved in 30 mL of deionized water, mixed with 1 mL of 5 wt % Nafion solution. The dispersed suspension was then treated in a sonic bath for about 20 min, dropped onto a CFP, and dried at room temperature. Characterization. The samples were characterized by X-ray powder diffraction (XRD, Philips, Xpert), field-emission scanning electron microscopy (FE-SEM, Hitachi S4800), high-resolution transmission electron microscopy (HR-TEM, JEOL-2100), and X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe). Electrochemical Measurements. The electrochemical measurements were carried out on a three-electrode system on the CHI 660D workstation (CH Instrument) at ambient temperature. The sample deposited on CFP was the working electrode, whereas the Ag/AgCl (in 3.5 M KCl solution) electrode and platinum mesh served as the reference and counter electrodes, respectively. The geometrical current density was normalized by geometrical area of CFP, and the area of Pt wire was calculated using a cylinder model with S = 2πrh. In the HER measurements, all of the potentials reported were calibrated to the RHE using the following equation: E(RHE) = E(Ag/AgCl) + 0.059 V × pH + 0.205 V. The polarization curves were obtained from the catalyst films in 0.5 M H2SO4 at a scanning rate of 2 mV s−1. Electrochemical impedance spectroscopy (EIS) was performed at the set potential from 104 to 0.03 Hz with an ac voltage of 5 mV. The series resistance (Rs) was determined from the impedance experiments, and iR correction was performed with ηcorr = η − jRs. In the constant-voltage electrolysis, the headspace in the cathodic compartment was purged with high-purity nitrogen for 60 min under vigorous stirring before the experiment.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b04045. Figures S1−S15; Table S1; and additional details on sample characterization, measurements, and analyses (PDF) 13971
DOI: 10.1021/acsami.6b04045 ACS Appl. Mater. Interfaces 2016, 8, 13966−13972
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b04045 ACS Appl. Mater. Interfaces 2016, 8, 13966−13972
