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Highly Efficient Photoelectrochemical Hydrogen Generation Reaction Using Tungsten Phosphosulfide Nanosheets Vediyappan Veeramani, Hsin-Chin Yu, Shu-Fen Hu, and Ru-Shi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03692 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Highly Efficient Photoelectrochemical Hydrogen Generation

Reaction

Using

Tungsten

Phosphosulfide Nanosheets Vediyappan Veeramani,†,⊥ Hsin-Chin Yu, ‡,⊥ Shu-Fen Hu,* ‡ and Ru-Shi Liu*,†,§ †

Department of Chemistry, National Taiwan University, Taipei, 106 (Taiwan)

‡

Department of Physics, National Taiwan Normal University, Taipei 116 (Taiwan)

§

Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology,

National Taipei University of Technology, Taipei 106, Taiwan KEYWORDS: Silicon, Tungsten-disulfide, Tungsten-phosphosulfide, Photoelectrochemical performance, Hydrogen

ABSTRACT

The initiation of hydrogen energy production from sunlight through photoelectrochemical (PEC) system is an important strategy for resolving contemporary issues in energy requirement. Although precious Pt and other noble metals offer a desirable catalytic activity for this method, earth-abundant nonprecious metal catalysts must be developed for wide-scale application. In this regard, P-type silicon (P-Si) micro-pyramids (Si MPs) are a favorable photocathode because of

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their effective light-conversion properties and appropriate band gap position. In this study, we developed amorphous tungsten phosphosulfide nanosheets (WS2-xPx NSs) on Si MPs through a simple thermal annealing process for solar-driven hydrogen evolution reaction. The P substitution in the nanostructure effectively produced many defective sites at the edges. The product exhibited an efficient photocurrent density of 19.11 mA cm-2 at 0 V and a low onset potential of 0.21 VRHE compared with tungsten disulfide (WS2; 13.43 mA cm-2). The fabricated catalyst also showed desirable stability for up to 8 h for the WS0.60P1.40@Si MPs photocathode. The extraordinary activity could be due to numerous active sites provided by heteroatoms (sulfur and phosphorus) in the edges, resulting in dwindling reaction kinetics barrier and enhanced PEC activity.

INTRODUCTION Increasing global warming and diminishing nonrenewable fossil fuel resources, such as gas, coal, and oil, have led to negative consequences worldwide.1 In this regard, scholars aim to identify renewable, environment-friendly, and highly efficient energy sources.2,3 Hydrogen (H2) energy, which is produced from solar and wind energy (renewable resources) is a potential alternative to fossil resources. H2 is a promising energy carrier and a clean and sustainable energy source that can be generated by electrochemical/photoelectrochemical (EC/PEC) reactions, such as photo/electrolysis of water.4 Solar-driven PEC hydrogen evolution reaction (HER) is a half-cell reaction in the overall water splitting process (splitting hydrogen and oxygen at thermodynamic potential of 1.23 V vs. RHE).4-6 An efficient photoelectrolysis process requires catalytic materials with suitable band gap, that is, the conduction band (VB) position should be close to null potential (vs. SHE).


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Semiconductor-based photocatalytic materials are effective for solar H2 production due to their efficient light absorption capability in wide ranges.4 Studies have used various semiconductor materials; of which, P-type silicon is widely used as photocathode because it fulfills the band gap requirement (P-Si = 1.12 eV).7,8 However, this material can be easily oxidized (SiO2) in the PEC reaction and adversely influenced the nature and stability of the reaction in the long term.9,11 To overcome this limitation, scholars have introduced noble metal nanoparticles (NPs; such as Pt, Ru, and Ir) to be used as cocatalysts for PEC; these materials are costly and have low abundance, which hinder their application for large-scale H2 production.12-14 Researchers aim to explore abundant and highly active cocatalyst materials for PEC reactions. Nonprecious transition metal–oxides,15-17 metal–carbides,18,19 metal–phosphides,20 and metal–chalcogenides21,22 have been used for fast charge transportation and reaction kinetics. These materials are well-known and promising candidates for EC/PEC water electrolysis because of their stability and high electrical conductivity.15-22 Layered transition metal dichalcogenides (LTMDs) are the most studied cocatalyst materials for PEC water splitting because of their excellent activity toward HER under highacetic conditions.23 In this regard, MX2-based nanomaterials (where M = Mo, W and X2=S, Se) provide the best performance in HER because of their physical structures.23-25 These twodimensional (2D) materials have covalently bonded layers and weak arrangement of S-metal-S, similar to graphite materials.26 Researchers have reported that the edges of molybdenum disulfide (MoS2) possess more catalytic active sites than the basal surfaces because of the electrochemically inert nature of these planes, as confirmed by computational and experimental studies.26,27 Similar to MoS2, tungsten disulfide (WS2) exhibits properties including low reactivity and less electrical contact between the catalyst and the support.27,28 Studies improved


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the catalytic activity of MX2 materials by activating the basal surfaces to introduce various metal doping or by doping with heteroatoms to produce numerous defects on the sheet surface and achieve chemical exfoliation.24-32 Phosphatation on the WS2 surface lead to enhanced HER activity. Kibsgaard et al.33 reported that non-noble molybdenum phosphosulfide (MoSP) is one of the most efficient catalysts for HER given that heteroatoms provide numerous active sites (S and P). Caban-Acevedo et al.34 reported the use of cobalt phosphosulphide (CoPS) co-catalyst on the pyramid-Si for EC and PEC HER. This cocatalyst offers superior activity and stability over the reaction. Thus far, the use of WS2 for EC/PEC-HER catalyst has been investigated,24-32,35,36 and there was no report available concerning on the PEC performance of tungsten– phosphosulfide (WPS) based nanomaterials. Herein, we developed highly active tungsten–phosphosulfide nanosheets (WPS NSs) as cocatalyst on pyramid p+-Si as photocathode for PEC HER for the first time. WS2-xPx NSs and pure WS2 NSs samples were prepared using a simple thermal annealing process (Figure 1). To tune the PEC performance of the photocathode, we varied the phosphorous (x = 1.40 and 1.60) doping percentages in WS2 NSs (tungsten disulfide). In this sample, some P particles were occupied with sulfur, which could enable HER, in contrast to that in pure WS2. The WS0.60P1.40 photocathode electrode exhibited low onset potential of 0.21 VRHE and provided 19.11 mA cm-2 photocurrent density at 0. V; the photocurrent density of this electrode is higher than that of pure WS2 (13.43 mA cm-2). The improved performance could be due to sufficient active sites in the photocathode and numerous defects as a result of doping with P.

EXPERIMENTAL SECTION Fabrication of Si-pyramid photocathode (Si MPs)


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Silicon wafer (boron-doped p-type Si wafer; resistivity: 1–25 Ω cm) was used to prepare Si pyramids through a simple wet-chemical etching process. In a typical procedure, 10 mL of isopropyl alcohol (IPA, 5 vol%) solution was mixed with 190 mL of deionized (DI) water containing potassium hydroxide (KOH; 4.02g) and placed on a hot plate at 90 °C for 15 minutes. The wafer was dipped into the heated solutions for 30 minutes, washed with DI water, and dried under nitrogen gas flow. Fabrication of Si-pyramid/ tungsten phosphosulfide nanosheet (WS2-xPx NSs) photocathode WS2-xPx co-catalyst was prepared directly on the surface of the Si pyramid photocathode through simple thermal annealing. Ammonium tetrathiotungstate [(NH4)2WS4; 0.5 mmol] and sodium hypophosphite (NaH2PO2; 220 mg; 2.5 mmol) were mixed in solution containing 10 mL of methanol and hexamethyldisilazane (HMDS, 20 µL). The mixed solution was dispersed in an ultra-sonication bath for 15 minutes. The dispersed solution was drop-casted on the Si-MPs and evaporated methanol solution and annealed at 500 °C under N2/H2 atmosphere for 2 h. Finally, nanosheet-like WS0.60P1.40@Si MPs photocathode was obtained for PEC measurement. The NaH2PO2 (440 mg; 5 mmol) amount in the preparation process was to tune the photocathode performance, and the product was denoted as WS0.40P1.60@Si MPs photocathode. Pure nanosheet-like WS2@Si MPs photocathode was prepared for comparison. Ammonium tetrathiotungstate [(NH4)2WS4; 0.5 mmol] was mixed with 10 mL of methanol and hexamethyldisilazane (HMDS; 20 µL) solution and used to prepare WS2@Si MPs photocathode.


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RESULTS AND DISCUSSION For efficient PEC conversion, Si MPs were used as photocathode to absorb solar light and prepared by a simple wet-chemical etching process. On the planar surface, the light source incident on the surface and that directly come out of the surface cause low conversion efficiency and light absorption assets. This limitation was overcame by the Si-MPs due to their multiple incident paths and efficient light absorption. The morphologies of the WS2-xPx NSs@Si MPs and powder samples were analyzed using scanning electron microscope (SEM, Figures 2 and 3a). The bare etching surface showed micro-pyramid arrays with size of 8–15 µm (Figure 2a). The WS0.60P1.40@Si MPs sample showed uniform distribution of the nanosheets on the MP surface (Figure 2c). The thickness of WS2-xPx NSs is slightly higher than that of the pure WS2@Si MPs (Figure 2b) because P was occupied with sulfur. The morphology of the nanosheets changed into microsheets. The high percentage of P further increased the thickness of the sheet (Figure 2d). Besides, the top view SEM images and the corresponding EDX spectra of the samples shown in Figures S1 and S2. Subsequent testing on the prepared powder sample further confirmed the nanosheet morphology (Figure 3a). To verify elements present in the WS0.60P1.40 sample, we conducted energy-dispersive X-ray spectroscopy (EDX) mapping studies. The results show the signals of W, S, and P in the mixture (Figure 3b-d). The EDX mapping of WS2 and WS0.40P1.60 samples are shown in Figures S3 and S4, respectively. In addition, we further calculated the elemental composition of materials (WS2-xPx NSs) form EDX analysis and the analogous results shown in Table 1. The crystal structure and purity of the prepared samples were analyzed using X-ray diffraction patterns (XRD). Figure 4a shows the XRD patterns of the WS0.60P1.40@Si MPs and the pure WS2@Si MPs photocathode. A sharp, high-intensity peak was obtained at 32.6° and


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could be due to the presence of Si MPs. No other peaks were obtained for the WS2 sample because of its weak crystallinity. No peaks were also found in the WS0.60P1.40@Si MPs photocathode because of the amorphous nature of the WS2-xPx NSs on the Si MP surface. Raman spectra were recorded to analyze the bond structures of the samples. The predominant peaks at the Raman intensity of 350 and 417 cm-1 are attributed to the in-plane (E12g) and out-plane (A1g) vibration modes of W-S bonds, respectively (Figure 4b).30 For WS2, the intensity of the A1g plane is two times smaller than that of the E1g plane, thereby confirming the monolayer of ultrathin WS2 nanosheets.29 After doping with P, the intensity of the A1g plane slightly increased with increasing layered thickness of WS2-xPx NSs (Figure 4b).35 To explore vacancies in the unoccupied electronic states of WS2, WS0.60P1.40, and WS0.40P1.60, we examined the samples through X-ray absorption near the edge structure (XANES) analysis. For all synchrotron X-ray measurement, Ti foil were utilized as a substrate to prepare co-catalysts materials because we can’t measure the XAS analysis using the Si photocathode due to the strong diffraction of Si photocathode in the synchrotron hard X-ray. As shown in Figure 5, the W L3-edge, S K-edge, and P K-edge of the absorption spectra shifted from 4f to 5d states, from 1s to 3p states, and from 1s to 3p states, respectively. The W L3-edge XANES spectra of WS2 showed strong intensity absorption peak, indicating the presence of high vacancy states.37 Moreover, the WS0.60P1.40, and WS0.40P1.60, samples exhibited slightly lower intensity peaks than the WS2 sample due to the electron transference from S and P to W (Figure 5a) and the distortion of the W surface caused by P substitution.38 Therefore, the W vacancy was diminished because of electron transference.37-39 By contrast, the S K-edge XANES spectra of WS0.60P1.40 showed slightly higher intensity peak than that of the WS0.40P1.60 sample. This result suggests that the S-3p orbital possesses more vacancies in the WS0.60P1.40 than that in the


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WS0.40P1.60 sample. Hence, the S atom donated electrons to the W-5d orbital, creating numerous vacancies in the S-3p orbital (Figure 5b).40 In addition, a small peak was observed at approximately 2482 eV, denoting the negligible amount of S atoms with +6 oxidation state (sulfate ion SO42-). As shown in Figure 5c, both samples exhibited almost negligible peak intensities in the P K-edge XANES spectra, indicating the stronger covalency among P particles in WS2 materials.40 Furthermore, the result reveals that the presence of phosphate ion (PO43-) in the both samples has identical oxidation states even it can’t change with high amount.41 The Fourier transform result of the EXAFS oscillation spectra is shown in Figure S5. The detected peaks at ~1.98 and ~1.91 Å are due to the W-S coordination shell and the W-W coordination shell, respectively.42 After the substitution of P with S, the peak intensity marginally increased and the W-S bond length was shortened to ~0.08 eV compared with those in WS2. P substitution could have enhanced the formation of numerous edge defects in the sample, thereby increasing the number of active sides toward PEC HER. The photogenerated electrons transferred from the heteroatoms (S and P) to the photocathode surface and generated H2 gas. Photoelectrochemical studies. The prepared photocathode was utilized to evaluate the solar-driven PEC HER by using 0.5 M H2SO4 electrolyte solution as support at 110 mW cm-2 simulated solar light source. Figures 6a and S5 represents the linear sweep voltammetry (LSV) profile of different photocathodes, such as bare Si MPs, WS2@Si MPs, WP1.0@Si MPs, WS0.60P1.40@Si MPs, and WS0.40P1.60@Si MPs scanned at 20 mV/s. The onset potential of the bare Si MPs photocathode is -0.38 V at 1 mA cm-2 current density, indicating the low kinetics of the HER over pure Si photocathode. The bare Si MPs provides poor hydrogen evolution performance owing to the fast recombination of the photogenerated electron–hole pairs and the insulting nature of the oxidized substrate over the reaction. To improve the HER kinetics, we


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prepared WS2 and WS2-xPx NSs nanocomposite materials through a simple thermal annealing process. Ding et al.8 reported that cocatalysts directly grown on the photocathode exhibited improved catalytic performance than those fabricated through drop-cast method. The cocatalysts were prepared directly on the Si MPs to solve the onset potential and current density (0 V) issues. The onset potential was dramatically decreased after the WS2 NSs were decorated on Si MPs due to the decreased kinetic barrier and promoted fast charge transportation between the photoelectrode and the electrolyte interface of the photocathode. The onset potential of the WS2 NSs@Si MPs photocathode is 0.12 V at 1 mA cm-2 and 0 V at -5.8 mA cm-2. The saturated current density is -15.0 mA cm-2. The results reveal that WS2 NSs provide numerous active sites toward photocatalytic HER. As shown in Figures 6b and S5, the onset potential of the WS0.60P1.40@Si MPs, WS0.40P1.60@Si MPs, and WP1.0@Si MPs photocathodes are 0.22, 0.17 and 0.11 V at 1 mA cm-2, respectively. During P doping (x=1.40) with WS2 NSs cocatalysts, the onset potential further decreased due to the creation of many defects on the surface, leading to enhanced PEC HER and saturated current density (-21.2 mA cm-2). When the doping percentage was further increased (x=1.60), the photocatalytic activity decreased because the excess P did not contribute to HER and may block the S activity. To understand the catalytic activity, the polarization curves were converted to Tafel plots as shown in Figure S6 and the data was collected and evaluated in the linear portion of the curves. The Tafel slope value of 69.31 mV dec-1 for WS0.60P1.40@Si MPs photocathode that indicate the reactions occurred through Volmer-Heyrovsky mechanism.43 For the comparison studies, the pure WP@Si MPs photocathode performed for PEC activity (Figure S7). The durability of the WS0.60P1.40@Si MPs photocathode was also studied (Figure 7a). The photocathode provides acceptable long-term stability for up to 8 h, and noise was obtained due to


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the accumulation of the bubbles on the surface. Therefore, the photocathode (WS0.60P1.40@Si MPs) showed an excellent stability with small dropping of photocurrent density (1.8 mA cm-2). The photocurrent-time response curve explains the behavior of the photogenerated electron-holes in the WS0.60P1.40@Si MPs photocathode (Figure 7b). The photocathode provides high magnitude of photocurrent density, indicating that it exhibits efficient charge transportation property and negligible recombination. The photocurrent density decreased when the solar irradiation was turned-off, indicating that the photocurrent was produced under light irradiation by moving the electrons toward the Si substrate. Figure S7 represents the EIS spectra of WS2@Si MPs, WS0.60P1.40@Si MPs, and WS0.40P1.60@Si MPs photoelectrodes under light illumination. The result reveals that the arc of WS0.60P1.40@Si MPs is much smaller than the WS2@Si MPs and WS0.40P1.60@Si MPs, which indicates that WS0.60P1.40@Si MPs electrode provide more catalytic active sites. The result indicates that the P substitution can effectively create more defective sites in the fabricated WS0.60P1.40@Si MPs photocathode compared to the WS2@Si MPs and as a result improved the PEC activity.

CONCLUSIONS Highly active WS2-xPx NSs catalyst was directly prepared on Si MPs through a simple thermal annealing process for solar-driven HER. The pure WS2 NSs photocathode delivered only 13.43 mA cm-2 photocurrent density. The photoelectrocatalytic activity was improved after the P substitution in the WS2. After varying the doping percentages (x = 1.40 and 1.60), the WS0.60P1.40@Si MPs photoelectrode provides low onset potential of 0.21 VRHE and 0 V and excellent stability. Therefore, the heteroatoms doped with the WS2-xPx NSs exhibit potential for solar-driven water splitting application.


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ASSOCIATED CONTENT

Supporting Information Supporting Information is available free of charge on the ACS Publication website. SEM and elemental mapping images of WS2 and WS0.40P1.60 samples. EXAFS spectra of the WS2, WS0.60P1.40, and WS0.40P1.60 samples. AUTHOR INFORMATION Corresponding Authors *Shu-Fen Hu, Email:[email protected]; Ru-Shi Liu, Email: [email protected] Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. ⊥V.

Veeramani and H. C. Yu contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully appreciate the ﬁnancial support from the Bureau of Energy (BOE), Ministry of Economy Aﬀair (MOEA) of Taiwan (105-D0114) and the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M-002-012-MY3 and MOST 106-2112-M-003-007MY3). Conflict of interest


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Figures and Captions

Figure 1. Simple synthesis procedure for tungsten phosphosulfide nanosheets (WS2-xPx) on p+-Si micro-pyramids (Si MPs).


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Figure 2. Cross-sectional SEM images of (a) bare Si MPs, (b) WS2@Si MPs, (c) WS0.60P1.40@Si MPs, and (d) WS0.40P1.60@Si MPs photoelectrodes.


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Figure 3. (a) SEM image and elemental mapping images of (b) W, (c) S, and (d) P of the WS0.60P1.40 sample. Table 1. Present elemental compositions of the prepared WS2-xPx from EDX measurement.

Notation of the samples

Measured composition Nominal composition by EDX 0.5 mmole (NH4)2WS4

WS2

W1.00 ± 0.05S2.00 ± 0.03 (mole ratio of W: S = 1:4 )

WS(2-x)Px

0.5 mmole (NH4)2WS4 + 2.5 mmole NaH2PO2

(x = 1.40)

(mole ratio of W:S:P = 1:4:5 )

WS(2-x)Px

0.5 mmole (NH4)2WS4 + 5 mmole NaH2PO2

(x =1.60)

(mole ratio of W:S:P = 1:4:10 )

W1.0 ± 0.10S0.60 ± 0.05P1.40 ± 0.05

W1.0 ± 0.2S0.40 ± 0.05P1.60 ± 0.06


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Figure 4 (a) Raman and (b) XRD pattern of the WS2@Si MPs, WS0.60P1.40@Si MPs, and WS0.40P1.60@Si MPs samples.


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Figure 5. (a) W L3-edge, (b) S K-edge, and (c) P K-edge XANES spectra of the WS2, WS0.60P1.40, and WS0.40P1.60 samples.


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Figure 6. PEC hydrogen generation using WSP/Si MPs photocathodes in 0.5 M H2SO4 electrolyte solution. (a) J–V curves of the bare Si MPs, WS2@Si MPs, WS0.60 P1.40@Si MPs, and WS0.40P1.60@Si MPs electrodes; and (b) corresponding electrode onset potential, and current density at 0 V vs. RHE.


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Figure 7. Chronoamperometry studies of the WS0.60 P1.40@Si MPs photocathode at 0 V vs. RHE. (a) PEC stability measurement and (b) photoresponses for On/OFF irradiation cycles.


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Table of Content


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Highly Efficient Photoelectrochemical Hydrogen ... - ACS Publications

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