WS2 with Efficient Charge Separation

Subscriber access provided by University of Sydney Library

Energy, Environmental, and Catalysis Applications 2

2D-2D Heterostructured CdS/WS with Efficient Charge Separation Improving H Evolution under Visible Light Irradiation 2

Ke Zhang, Mamoru Fujitsuka, Yukou Du, and Tetsuro Majima ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

TOC 39x19mm (300 x 300 DPI)

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2D-2D Heterostructured CdS/WS2 with Efficient Charge Separation Improving H2 Evolution under Visible Light Irradiation

Ke Zhang,a,b Mamoru Fujitsuka,a Yukou Du,b,* and Tetsuro Majima a,*

a

The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan.

b

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China.

* Corresponding authors: [email protected] and [email protected]

1


Page 2 of 19

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract: Efficient water splitting for H2 evolution under visible light irradiation has attracted more attention for solving the global environmental and energy issues, but it is still a major challenge to develop an earth-abundant and efficient photocatalyst. Herein, we report a two-dimensional (2D)-2D heterostructured CdS/WS2 (CdS/WS2), composed with nanosheet CdS (CdS) and nanosheet WS2 (WS2), as an efficient photocatalyst for H2 evolution. As a noble-metal-free visible-light driven catalyst for H2 evolution, CdS/WS2 with 10 wt% WS2 exhibited the largest H2 evolution rate of 14.1 mmol g-1 h-1 under visible light irradiation to be 8 times larger than that of pure CdS. The lifetime and dynamics of photogenerated electrons were evaluated by femtosecond time-resolved diffuse reflectance (TDR) spectroscopy, indicating that WS2 works as an electron trapping site and cocatalyst to cause H2 evolution under visible light irradiation. This work suggests that CdS/WS2 has great potential as a low cost and highly efficient photocatalyst for water splitting. Keywords: CdS, WS2, two dimensional heterostructure, photocatalytic H2 evolution, charge separation Introduction As a sustainable and clean energy, H2 has attracted tremendous attention in past decades1-3. Effective conversion from solar energy to H2 energy via water splitting by using semiconductor photocatalysts is a challenging but promising subject 4-6. Among a large number of materials that have the visible-light responses, CdS has fostered considerable interest in H2 evolution owing to its suitable redox potentials and bandgap7-8. However, due to the fast recombination of electrons and holes, bare CdS with poor photocatalytic efficiency is unfavorable for H2 evolution9-10. In order to overcome this difficulty, a facile and cost-effective method has been proposed. Modified CdS with cocatalyst not only reduces the recombination of the photogenerated electron–hole pairs effectively, but also inhibits the photocorrosion11-13. In general, noble metals such as Pt are mostly used as cocatalysts for photocatalytic H2 evolution14-16, but their high price and low abundance limit their wide use17-18. Since the discovery of graphene, 2D materials, such as transition metal oxides19-22, transition metal dichalcogenides23-25, and nitrides26-28, have been widely investigated for various applications in electronics, catalysis29, optics, energy storage30, and so on. Compared with the bulk materials, 2D materials exhibit fascinating physical, electronic, and chemical properties. Especially, several 2D materials have been regarded as cocatalysts to couple with CdS to form heterostructured catalyst31. Among these cocatalysts, a few-layered WS2 has been authenticated to be an efficient cocatalyst for H2 evolution32-34. Up to now, great progress has been achieved in the field of synthesis CdS/WS2, but there still exist some unsolved issues in photocatalytic efficiency improvement. It is generally known that the morphology of catalysts can affect the photocatalytic activity. CdS with large or aggregated structure significantly decreases the photocatalytic efficiency35-36. Unlike regular morphology of CdS, 2D CdS nanosheets possess large surface area to couple with 2D WS2 nanosheets, which can form a large contact interface between CdS and WS2, thus resulting in an increase of photocatalytic efficiency. However, there are rarely reported on lifetimes of electrons and electron transfer kinetics between CdS and WS2. In the present work, we develop a noble metal-free photocatalyst, CdS/WS2 with

2



Page 4 of 19

sheet-on-sheet structure composed with CdS and a few-layered WS2, for H2 evolution under visible-light irradiation. Compared with pure CdS, CdS/WS2 showed better photocatalytic efficiency in H2 evolution, and the best photocatalytic efficiency was obtained with CdS/WS2 10 wt% sample. A few-layered WS2 plays a key role to improve the photocatalytic efficiency by trapping the photogenerated electrons from excited CdS, leading to water reduction to generate H2. In order to investigate the electron injection dynamics and charge separation from photoexcited CdS to WS2, femtosecond time-resolved diffuse reflectance (TDR) spectroscopy and electrochemical measurements were carried out. The test results indicate that the charge separation efficiency in 2D/2D CdS/WS2 is favorable for the improvement of photocatalytic H2 evolution performance. Experimental Section Preparation of CdS, WS2, and CdS/WS2. WS2 was purchased from Sigma. N-methyl-2-pyrrolidone (NMP), CdCl2, lactic acid (LA), L-cysteine, sulphur, and diethylenetriamine (DETA) were obtained from Wako. CdS nanosheet was synthesized according to a method similar to that described by Xu et al37. Briefly, 36 L of water, 147 mg of CdCl2, 160mg of sulphur and 30 mL of DETA were added in a 50 mL Teflon-lined autoclave, then kept the reactor at 353K for 48 hours. After cooling to room temperature, the yellow precursors were collected and followed by lyophilization. 20 mg of as-prepared CdS, 10 mg of L-cysteine and 0.1 mL of DETA were added into 40 mL of H2O, then sonicated the mixture for 2 h. The obtained suspensions were centrifuged at 800 rpm for 10 minutes to remove aggregations. A few-layered WS2 was exfoliated by a physical method using NMP as solvent. Firstly, 60 mg of commercial WS2 was added to 30 mL of NMP, then sonicated the dispersion for 3 h by using a tip sonicator (Misonix XL-2000) in an ice bath. Finally, the dispersions were centrifuged for 40 min to remove non-exfoliated WS2. CdS and WS2 were added to 60 mL NMP. After sonication for 2 hours, the mixture was stirred overnight. Finally, the as-prepared sample was collected via high-speed centrifugation and washed several times by deionized water and freeze dried overnight. CdS rods were synthesized by a modified solvothermal method38-41. Dispersing 0.5997 g of Cd(Ac)2 · 2H2O and 0.5638 g of thiourea in 80 mL ethylenediamine, then transfer the homogeneous solution into a Teflon autoclave, and heat to 453.15 K. After 72 h, we collect and dry the powders. Photocatalytic H2 evolution. 3 mg of CdS/WS2 was dispersed in 1 mL of LA solution (used as sacrificial agent) and 4 mL of water, then adding the mixture into a 35 mL glass tube, the system was sealed with a rubber septum. After sonicating the reactor for 10 min, the system was degassed by Ar bubbling for 15 min. Xenon lamp (Asahi Spectra, LAX-C10; output wavelength: 350–1800 nm) with a UV cut-off filter was used as light source. The generated H2 was analyzed by a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector and a MS-5A column. Apparent quantum efficiencies (AQE) were measured at monochromatic wavelength light by using band-pass filters. The value of AQE was calculated by the following formula (1): AQE =

𝑁𝑒 𝑁𝑝

× 100% =

2×𝑛𝐻2 ×𝑁𝐴 ×ℎ×𝑐 𝑆×𝑃×𝑡×𝜆

× 100%

3


(1)

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Np and Ne are the total incident photons and reactive electrons, respectively. NA, h, and c are a constant of Avogadro constant, Planck constant, and light speed, respectively. 𝑛𝐻2 , S, P, t, and λ means the amount of H2, irradiation area, light intensity, photoreaction time, and the wavelength of monochromatic light, respectively. Photoelectrochemical measurements. 0.1 mg of sample was mixed with Nafion in ethanol under ultrasonic to form a uniform suspension. Adding 3 μL of above mixture onto the glassy carbon (GC) electrode surface and dried in an oven. Electrochemical experiments were performed on a 660B electrochemical workstation (ALS). A three-electrode system consisted of a Ag/AgCl electrode, a GC electrode and Pt wire, which was used as counter, working and reference electrodes, respectively. The electrochemical impedance spectroscopy (EIS) and photocurrent response spectra were carried out in Na2SO4 solution under visible light irradiation by using a xenon lamp with a UV cut-off filter. All the photoelectron measurements were performed at room temperature. Time-resolved diffuse reflectance measurements. The TDR spectra were measured by the pump and probe method using a regeneratively amplified titanium sapphire laser (Spectra-Physics, Spitfire Pro F, 1 kHz) pumped by a Nd:YLF laser (Spectra-Physics, Empower 15). The output of an optical parametric amplifier (420 nm, 4 J pulse-1) was used as the excitation pulse. A white light continuum pulse, which was generated by focusing the residual of the fundamental light on a sapphire crystal, was directed to the sample powder coated on the glass substrate, and the reflected lights were detected by a linear InGaAs array detector equipped with the polychromator (Solar, MS3504). All measurements were carried out at room temperature. Characterization of materials. The morphology of samples was characterized by using transmission electron microscopy (TEM) HAADF-STEM-EDX (JEM-3000F, operated at 300 kV) equipped with energy-dispersive X-ray spectroscopy (EDX), TEM (JEOL, 2100, operated at 100 kV). AFM was measured on a Keyence VN-8010 microscope, we prepared samples by spraying suspension on the freshly surface of cleaved mica, and finally dried in air. X-ray photoelectron spectroscopy (XPS) was performed by a JEOL JPS-9010 MC spectrometer. The crystal structure of samples was examined by powder X-ray diffraction (XRD, SmartLab, Rigaku), the equipment was operated at 40 kV/200 mA with Cu Kα radiation. The UV−vis diffuse reflectance spectra (DRS) were investigated by using a UV−vis−near infrared spectrophotometers (JASCO, V-770). Photoluminescence measurements were recorded by a Fluoromx-4 spectrofluorometer (HORIBA, scientific). Results and discussion The morphologies and thickness of the as-synthesized CdS and WS2 nanosheets were investigated by means of SEM (Figure S1), AFM and TEM. As shown in Figures 1A and 1C, WS2 and CdS possess a clear and well-defined ultrathin nanosheet structure. The thicknesses of CdS and WS2 were approximately 4.6 nm and 6.8 - 15.3 nm, respectively (Figure S2). The crystal lattices of CdS and WS2 were explored by HRTEM in Figures 1B and 1D, the interplanar spacings of WS2 were determined to be 0.27 and 0.62 nm, assigned to the (100) and (002) lattices of WS2, respectively, and the crystal lattice of CdS was measured to be 0.34 nm, assigned to the (002) lattice of CdS.

4



Figure 1. TEM (A, C) and HRTEM (B, D) images of CdS (A, B) and WS2 (C, D) nanosheets. The TEM and HRTEM images of as-synthesized CdS/WS2 are shown in Figure 2. It was shown in Figure 2A and 2B that 2D WS2 with irregular shape are assembled on the surface of CdS. To distinguish the components in CdS/WS2, HRTEM image was obtained as in Figure 2C. In agreement with the d-spacing values in Figure 1, the measured d-spacing values of 0.27 and 0.62 nm are assigned to the (100) and (002) lattices of WS2, respectively. In addition, clear crystal lattice of CdS with d-spacing of 0.34 nm was also observed around the WS2. These results demonstrate that CdS/WS2 was successfully synthesized.

5


Page 6 of 19

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. TEM (A, B) and HRTEM (C) images of CdS/WS2. EDX spectra and element mapping of CdS/WS2 were further measured to analyze the compositions and spatial distribution of CdS and WS2. The signals lower than 1 eV in Figure 3B are C, O, and Cu, respectively, these elements are come from the carbon-coated Cu-grid. EDS spectra indicated that CdS/WS2 has the elements of Cd, S, and W, confirming successful formation of CdS/WS2. The elemental mapping of W, Cd, and S elements indicates obvious images for CdS/WS2 as shown in Figures 3C–3E, demonstrating the successful hybridization between CdS and WS2.

6



Figure 3. HAADF–STEM image (A) and corresponding EDX maps of CdS/WS2 for W (C), Cd (D), and S (E) element mapping. The scale bar in all panels is 250 nm. (B) EDX spectrum of CdS/WS2. The as-prepared CdS/WS2 with different amount of WS2 were explored by XRD. As shown in Figure 4A, the obvious diffraction peaks located at 25.04°, 26.62°, 28.40°, 36.74°, 43.92°, and 48.30°, are assigned to the (100), (002), (101), (102), (110), and (103) planes of CdS, respectively42. The diffraction peaks at 14.34°, 32.74°, 33.54°, 39.52°, 58.38°, and 60.48°, are attributed to the (002), (100), (101), (013), (110), and (112) planes of WS2, respectively43. For all the hybrid catalysts, the peaks belonging to CdS and the peaks assigned to WS2 are unchanged, demonstrating that the structure of CdS and WS2 still maintain the same after mixing42.

7


Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. XRD patterns of CdS and CdS/WS2 (A), XPS spectra of W 4f (B), Cd 3d (C), and S 2p (D). XPS was carried out to investigate the surfaces configurations and the interaction between CdS and WS2 in CdS/WS2. Figures 4B–4D shows the XPS spectra of W 4f, Cd 2p, and S 2p. For WS2, two typical peaks corresponding to the binding energies of W 4f5/2 and W 4f7/2 were detected at about 35.0 and 32.9 eV, respectively 44. In the case of pure CdS, two characteristic peaks of Cd were detected at 404.9 and 411.6 eV to be assigned to Cd 3d5/2 and Cd 3d3/2, respectively45; and the peaks centered at 161.1 and 162.4 eV are assigned to S 2p3/2 and S 2p1/2. On the other hand, for pure WS2, the peaks of S 2p3/2 and S 2p1/2 are located at 162.5 and 163.7 eV. When CdS coupled with WS2, the binding energy of W 4f shifts negatively and the binding energy of Cd 2p shifts positively, these shifts mean a strong interaction between CdS and WS2. In addition, the shifted binding energies means the changes in electron density, indicating that WS2 and CdS act as an electron acceptor and donor in CdS/WS2, respectively. UV–vis absorption spectra were measured to analyze the optical properties of the as-synthesized photocatalysts. As shown in Figure 5A, the as-synthesized pure 2D CdS exhibits a remarkable absorption with the edge at about 520 nm to be relevant to its intrinsic band gap. Figure S3 shows UV–vis absorption spectra of WS2. The two peaks at around 527 and 630 nm can be assigned to direct gap transitions at the K point of the Brillouin zone 43. Compared with pure 2D CdS, all CdS/WS2 exhibited higher absorption ability in wavelengths longer than 520 nm. This increase is mainly attributed to absorption of WS2 sheets.

8



Figure 5. UV/Vis diffuse reflectance spectra (A) and photoluminescence (B) of 2D CdS and CdS/WS2. To evaluate the separation ability of photogenerated electron−hole pairs, photoluminescence spectra were measured. In general, it was widely believed that a higher photoluminescence intensity means a fast electron–hole pairs recombination rate, leading to inefficient H2 evolution. Figure 5B shows the photoluminescence spectra of the as-prepared CdS/WS2. According to the previous study, a weak peak at about 470 nm corresponding to the excitonic recombination, another broad peak at about 520 nm is assigned to the trap-state recombination46. Obviously, pure CdS presents the highest photoluminescence intensity, demonstrating the fastest recombination of photogenerated charges. When mixed CdS with WS2, the emission intensity was decreased, indicating the photogenerated charges separated efficiently. Moreover, it was shown that the peaks at around 550 nm shift to shorter wavelengths, demonstrating the strong interaction between CdS and WS247. Photocatalytic H2 evolution was examined with CdS/WS2 in the presence of LA under visible light irradiation. Figure 6A shows H2 evolution by using a Xenon lamp with a 420 nm long-pass filter. It was shown that only 15.5 μmol H2 was detected at 3 h for pure CdS due to the fast recombination of photogenerated charge carriers. On the other hand, CdS/WS2 exhibited higher photocatalytic efficiency, indicating WS2 can be used as an efficient cocatalyst for H2 evolution. The effect of WS2 weight on H2 evolution was also investigated. The H2 evolution are 94.3, 126.9, and 88.7 μmol for CdS/WS2 with 5 wt % WS2 (CdS/WS2 5 wt%), CdS/WS2 10 wt%, and CdS/WS2 15 wt%, respectively. The H2 evolution efficiency increased initially with increasing the ratio of WS2, and the 10 wt% sample showed the best activity. However, with further increasing the amount of WS2, the activity decreased. This is because excessive WS2 may decrease the photon absorption by CdS. We also compared the photocatalytic efficiency for H2 evolution with rod-CdS and rod-CdS/WS2 (Figure S5). It is obvious that CdS/WS2 10 wt% photocatalyst possesses better efficiency than others. The photocatalytic stability of CdS/WS2 was investigated by repeating H2 evolution experiments. As shown in Figure 6B, three continues photocatalytic hydrogen evolution were performed. It can be seen that there is a minor decrease in the third cycle. This could be ascribed to the photocorrosion or the decreased concentration of LA during the catalytic reaction. The morphology of CdS/WS2 10 wt% after photocatalytic reaction was characterized by TEM (Figure S11). The AQE was calculated at different wavelength of

9


Page 10 of 19

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


monochromatic light, as shown in Figure S6. It is remarkable that the value of AQE was in agreement with the UV/Vis diffuse reflectance spectrum of CdS. At 460 ± 5 nm, the AQE is as high as approximately 70%. Furthermore, the influence of photocatalyst dose, reaction pH and sacrificial donor concentration were studied, the results indicated that 3mg of CdS/WS2 10 wt% and 20 vol % of LA solution were suitable conditions for hydrogen evolution (Figure S7 - S9). To assess the CdS/WS2 10 wt% photocatalyst for practical applications, we verify the H2 evolution under aerobic conditions. Compared with anaerobic condition, the hydrogen evolution is decreased under aerobic condition, this phenomenon can be ascribed to the inhibited photocatalytic process and back reaction by oxygen molecules.

Figure 6. (A) H2 evolution on 2D CdS and CdS/WS2. (B) Recyclability of CdS/WS2 with 10 wt% WS2 as a photocatalyst for H2 evolution under visible light irradiation. EIS Nyquist plots under visible light irradiation (C) and photocurrent density curves (D) for CdS, WS2 and CdS/WS2 in Na2SO4 solution. According to the above results, the enhanced photocatalytic efficiency can be originated from the effective charge separation between 2D CdS and 2D WS2. As shown in Figure 6C, EIS was investigated to demonstrate the internal resistance and electron mobility of CdS/WS2 under visible light irradiation. Electrodes modified by WS2, CdS, and CdS/WS2 were used as working electrodes. Interfacial electron transport can be evaluated by the diameter of semicircle arc. In the EIS spectra, lower interfacial electron transfer generally exhibits larger semicircle arc. When these electrodes tested under visible light irradiation, CdS/WS2 electrode showed the smallest diameter, indicating a highly efficient interfacial charge transfer under visible light irradiation. Photocurrent

10



responses of above electrodes were measured to clarify charge transport across the heterointerface in CdS/WS2 under visible light irradiation. As shown in Figure 6D, the experimental results show that CdS/WS2 with 10 wt% WS2 has a higher photocurrent than the pure CdS and other CdS/WS2, which is consistent with the above photoluminescence and photocatalytic efficiency. When turned off the light source, the photocurrent densities decreased quickly, demonstrating that photogenerated electrons transferred to the GC electrodes to generate photocurrents under visible light irradiation. The enhanced photoresponse showed that 2D WS2 can improve the separation of electron-hole pairs, and increase the interfacial transfer efficiency of photogenerated charge-carriers. Although photoelectrochemical experimental results show the information on efficient charge transfer in CdS/WS2, charge carrier dynamics of photocatalysts is a critical factor to understand the photocatalytic reactions. In order to analyze charge carrier dynamics, we use the TDR spectroscopy48. A 420 nm pump pulse was used to excite all the photocatalysts, and the spectra of the samples were recorded at different time delays. In the case of CdS, diffuse reflectance signal in NIR was observed upon excitation to be assigned to free and trapped electrons in photocatalysts, while negligible signal was observed from pure WS2, thus the observed in TDR of CdS/WS2 can be assigned to electrons in CdS17. In Figure 7, it can be seen that the signal from CdS/WS2 increased rapidly upon the fs-laser irradiation, and then from 2 to 6 ps TDR signal of CdS/WS2 decreased more rapidly than that of CdS, and CdS/WS2 10% showed the fastest decrease. As shown in Figure S12, the TDR signal of WS2 nanosheet decreased to 0 within 16 ps, this phenomenon can be ascribed to the fast charge recombination.

11


Page 12 of 19

Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. The TDR spectral changes observed during fs-laser flash photolysis of CdS (A), CdS/WS2 5 wt% (B), CdS/WS2 10 wt% (C), and CdS/WS2 15 wt% (D). To reveal decay kinetics, biexponential decay function was used for fitting the signal from CdS and CdS/WS2, and the calculated date are summarized in Table 1. For pure CdS, τ1 (7.93 ps, 63.6%) corresponds to the charge recombination of electron-hole pairs with shorter distance, while τ2 (215 ps, 36.4%) does the charge recombination with longer distance 49-50. For CdS/WS2, as discussed above, the TDR signals CdS/WS2 are originated from the exited CdS, and WS2 acts as an electron acceptor. Based on the previous research, τ1 of several picoseconds is assigned to the electron transfer from CdS to WS2, while τ2 of hundreds of picoseconds is to the charge carriers’ recombination51-53. It was found that the lifetime of τ1 in CdS/WS2 10% (4.48 ps, 65.2%) is the shortest, and the ratio is the highest among CdS/WS2. These facts suggest that a better contact between CdS and WS2 is realized in CdS/WS2 10%, resulting in a faster and more efficient electron transfer54. In CdS/WS2 15 wt%, the electron transfer was decreased due to the amount of covered WS2. Similar results have been observed in nitrogen-doped hollow-TiO2/g-C3N4 composite photocatalysts54. Compared with pure CdS, the average lifetime was decreased in all CdS/WS2. The shorter average lifetime implies that the photogenerated electrons are transferred more efficiently from CdS to WS2, and the transferred electrons trapped on the surface of WS 2 could participate the subsequent H+ reduction before charge recombination.55-56.

12



Page 14 of 19

Figure 8. Time profiles of diffuse reflectance of electrons monitored at 950 nm for CdS and CdS/WS2 under 420 nm excitation (A). Schematic diagram for photocatalytic H2 evolution by CdS/WS2 under visible light irradiation (B). Table 1. Lifetimes of electrons during TDR of CdS and CdS/WS2. Sample

τ1 (ps)

τ2 (ps)

τav (ps)

CdS CdS/WS2 5 wt% CdS/WS2 10 wt% CdS/WS2 15 wt%

7.93 (63.6%) 6.98 (61.8%) 4.48 (65.2%) 5.01 (63.2%)

215 (36.4%) 101 (38.2%) 104 (34.8%) 112 (36.8%)

83.3 42.6 39.1 44.4

We measured the position of conduction band and valence band of CdS, WS2 and CdS/WS2 10 wt% photocatalysts by Mott-Schottky plots and VB-XPS, respectively. As shown in Figure S13, the conduction band of CdS, WS2 and CdS/WS2 10 wt% photocatalysts were -0.67, -0.57 and -1.06 V, respectively. The valence band of the photocatalysts was obtained by VB-XPS, which are 1.67, 1.11 and 0.66 eV for CdS, WS2 and CdS/WS2 10 wt%, respectively. Based on above experimental results and analysis, the photocatalytic mechanism for H2 evolution by CdS/WS2 under visible irradiation is proposed in Figure 8B. Because of the rapid recombination of photogenerated electrons and holes, pure 2D CdS exhibits a poor photocatalytic efficiency. In CdS/WS2, the 2D nanojunction of photocatalyst is favorable for the improvement of photocatalytic H2 evolution performance. Because the conduction band of CdS nanosheets is more negative than that of WS2, the photoinduced electrons transferred rapidly from CdS nanosheets to the contacted interface, and then migrated to the WS2 efficiently. Finally, the photoinduced electrons reduced water at the edge of WS2. Holes in the valence band of CdS nanosheets are rapidly consumed by LA (hole scavenger) 46. During the photocatalysis processes, the 2D/2D heterostructure exhibited several advantages, such as large contact region, and suppressed charge recombination, which leading to improved photocatalytic activity. Conclusions 2D hybrid photocatalyst was constructed by coupling WS2 with CdS, and used as a noble metal-free catalyst for H2 evolution under visible light irradiation. Due to the unique structure and the addition of WS2, CdS/WS2 10 wt % exhibited the highest activity with a H2 evolution of 14.1 mmol g-1 h-1. The charge separation processes were analyzed by using photoelectrochemical 13


Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


experiment and TDR to show effective charge transport in CdS/WS2 and fast electron transfer from CdS to WS2, respectively. Therefore, efficient separation of the photogenerated charge carriers is accomplished in CdS and WS2 to greatly improve photocatalytic efficiency for H2 evolution. Acknowledgments We thank the members of Comprehensive Analytical Center at SANKEN for their generous support. This work was partly supported by the Cooperative Research Program of "Network Joint Research Center for Materials and Devices", Osaka University, and Innovative Project for Advanced Instruments, Renovation Center of Instruments for Science Education and Technology, Osaka University, and Grants-in-Aid for Scientific Research (Project 25220806 and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, the National Natural Science Foundation of China (No. 51373111), the Suzhou Industry (SYG201636), the project of scientific and technologic infrastructure of Suzhou (SZS201708), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References 1.

Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N-Graphene Films as

Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931-940. 2.

Wang, X. D.; Xu, Y. F.; Rao, H. S.; Xu, W. J.; Chen, H. Y.; Zhang, W. X.; Kuang, D. B.; Su, C. Y.

Novel porous molybdenum tungsten phosphide hybrid nanosheets on carbon cloth for efficient hydrogen evolution. Energy Environ. Sci. 2016, 9, 1468-1475. 3.

Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A., Z-Schematic Water Splitting into H2 and

O2 Using Metal Sulfide as a Hydrogen-Evolving Photocatalyst and Reduced Graphene Oxide as a Solid-State Electron Mediator. J. Am. Chem. Soc. 2015, 137, 604-607. 4.

Gao, M. R.; Liang, J. X.; Zheng, Y. R.; Xu, Y. F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S. H. An efficient

molybdenum disulfide/cobalt diselenide hybrid catalyst for electrochemical hydrogen generation. Nat. Commun. 2015, 6, 5982. 5.

Lu,

Q.;

Yu,

Y.;

Ma,

Q.;

Chen,

B.;

Zhang,

H.

2D

Transition-Metal-Dichalcogenide-Nanosheet-Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions. Adv. Mater. 2016, 28, 1917-1933. 6.

Chen, Z.; Sun, P.; Fan, B.; Zhang, Z.; Fang, X. In Situ Template-Free Ion-Exchange Process to

Prepare Visible-Light-Active g-C3N4/NiS Hybrid Photocatalysts with Enhanced Hydrogen Evolution Activity. J. Phys. Chem. C 2014, 118, 7801-7807. 7.

Jia, T.; Kolpin, A.; Ma, C.; Chan, R. C.; Kwok, W. M.; Tsang, S. C., A graphene dispersed

CdS-MoS2 nanocrystal ensemble for cooperative photocatalytic hydrogen production from water. Chem. Commun. 2014, 50, 1185-1188. 8.

Chen, J.; Wu, X. J.; Yin, L.; Li, B.; Hong, X.; Fan, Z.; Chen, B.; Xue, C.; Zhang, H. One-pot

synthesis of CdS nanocrystals hybridized with single-layer transition-metal dichalcogenide nanosheets for efficient photocatalytic hydrogen evolution. Angew. Chem. Int. Ed. 2015, 54, 1210-1214. 9.

Xiang, Q.; Cheng, F.; Lang, D. Hierarchical Layered WS 2 /Graphene-Modified CdS Nanorods for

Efficient Photocatalytic Hydrogen Evolution. ChemSusChem 2016, 9, 996-1002. 10. Cao, S.; Chen, Y.; Wang, C. J.; Lv, X. J.; Fu, W. F. Spectacular photocatalytic hydrogen evolution

14



using metal-phosphide/CdS hybrid catalysts under sunlight irradiation. Chem. Commun. 2015, 51, 8708-8711. 11. Zhang, S.; Yang, H.; Gao, H.; Cao, R.; Huang, J.; Xu, X. One-pot Synthesis of CdS Irregular Nanospheres Hybridized with Oxygen-Incorporated Defect-Rich MoS2 Ultrathin Nanosheets for Efficient Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Inter. 2017, 9, 23635-23646. 12. Yang, M. Q.; Han, C.; Xu, Y. J. Insight into the Effect of Highly Dispersed MoS 2 versus Layer-Structured MoS2 on the Photocorrosion and Photoactivity of CdS in Graphene–CdS–MoS2 Composites. J. Phys. Chem. C 2015, 119, 27234-27246. 13. Han, B.; Liu, S.; Zhang, N.; Xu, Y. J.; Tang, Z. R. One-dimensional CdS@MoS2 core-shell nanowires for boosted photocatalytic hydrogen evolution under visible light. Appl. Catal. B 2017, 202, 298-304. 14. Zhai, C.; Zhu, M.; Bin, D.; Wang, H.; Du, Y.; Wang, C.; Yang, P. Visible-light-assisted electrocatalytic oxidation of methanol using reduced graphene oxide modified Pt nanoflowers-TiO2 nanotube arrays. ACS Appl. Mater. Inter. 2014, 6, 17753-17761. 15. Wei, L.; Chen, Y.; Lin, Y.; Wu, H.; Yuan, R.; Li, Z., MoS2 as non-noble-metal co-catalyst for photocatalytic hydrogen evolution over hexagonal ZnIn2S4 under visible light irradiations. Appl. Catal. B 2014, 144, 521-527. 16. Yuan, Y. J.; Tu, J.-R.; Ye, Z. J.; Chen, D. Q.; Hu, B.; Huang, Y. W.; Chen, T. T.; Cao, D. P.; Yu, Z. T.; Zou, Z. G. MoS2-graphene/ZnIn2S4 hierarchical microarchitectures with an electron transport bridge between light-harvesting semiconductor and cocatalyst: A highly efficient photocatalyst for solar hydrogen generation. Appl. Catal. B 2016, 188, 13-22. 17. Zhu, M.; Zhai, C.; Fujitsuka, M.; Majima, T. Noble metal-free near-infrared-driven photocatalyst for hydrogen production based on 2D hybrid of black Phosphorus/WS 2. Appl. Catal. B 2018, 221, 645-651. 18. Sun, Z.; Zheng, H.; Li, J.; Du, P. Extraordinarily efficient photocatalytic hydrogen evolution in water using semiconductor nanorods integrated with crystalline Ni2P cocatalysts. Energy Environ. Sci. 2015, 8, 2668-2676. 19. Ko, W.; Lee, S.; Myoung, N.; Hong, J. Solution processed vertically stacked ZnO sheet-like nanorod p–n homojunctions and their application as UV photodetectors. J. Mater. Chem. C 2016, 4, 142-149. 20. Yang, C.; Xiao, F.; Wang, J.; Su, X. 3D flower and 2D sheet-like CuO nanostructures: Microwave-assisted synthesis and application in gas sensors. Senson. Actuat. B 2015, 207, 177-185. 21. Cho, K. M.; Kim, K. H.; Choi, H. O.; Jung, H. T. A highly photoactive, visible-light-driven graphene/2D mesoporous TiO2 photocatalyst. Green Chem. 2015, 17, 3972-3978. 22. Li, F.; Chen, Z. Tuning electronic and magnetic properties of MoO 3 sheets by cutting, hydrogenation, and external strain: a computational investigation. Nanoscale 2013, 5, 5321-5333. 23. Xu, S.; Li, D.; Wu, P. One-Pot, Facile, and Versatile Synthesis of Monolayer MoS2/WS2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2015, 25, 1127-1136. 24. Cheng, L.; Huang, W.; Gong, Q.; Liu, C.; Liu, Z.; Li, Y.; Dai, H. Ultrathin WS 2 nanoflakes as a high-performance electrocatalyst for the hydrogen evolution reaction. Angew. Chem. Int. Ed. 2014, 53, 7860-7863. 25. Zeng, Z.; Tan, C.; Huang, X.; Bao, S.; Zhang, H. Growth of noble metal nanoparticles on single-layer TiS2and TaS2nanosheets for hydrogen evolution reaction. Energy Environ. Sci. 2014, 7,

15


Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


797-803. 26. Zhu, M.; Kim, S.; Mao, L.; Fujitsuka, M.; Zhang, J.; Wang, X.; Majima, T. Metal-Free Photocatalyst for H2 Evolution in Visible to Near-Infrared Region: Black Phosphorus/Graphitic Carbon Nitride. J. Am. Chem. Soc. 2017, 139, 13234-13242. 27. Ding, Y.; Wang, Y. Surface functionalization of molybdenum dinitride nanosheets by halogen and alkali atoms: a first-principles study. J. Mater. Chem. C 2017, 5, 683-689. 28. Zhang, C.; Liu, J.; Shen, H.; Li, X. Z.; Sun, Q. Identifying the Ground State Geometry of a MoN 2 Sheet through a Global Structure Search and Its Tunable p-Electron Half-Metallicity. Chem. Mater. 2017, 29, 8588-8593. 29. Wu, T.; Fan, J.; Li, Q.; Shi, P.; Xu, Q; Min, Y. Palladium Nanoparticles Anchored on Anatase Titanium Dioxide‐Black Phosphorus Hybrids with Heterointerfaces: Highly Electroactive and Durable Catalysts for Ethanol Electrooxidation. Adv. Energy Mater. 2018, 8, 1701799. 30. Wei, H.; Zhang, Q.; Wang, Y.; Li, Y.; Fan, J.; Xu, Q.; Min, Y. Baby Diaper‐Inspired Construction of 3D Porous Composites for Long‐Term Lithium‐Ion Batteries. Adv. Funct. Mater. 2018, 28, 1704440. 31. Min, Y.; He, G.; Xu, Q.; Chen, Y. Dual-functional MoS2 sheet-modified CdS branch-like heterostructures with enhanced photostability and photocatalytic activity. J. Mater. Chem. A 2014, 2, 2578. 32. Zhong, Y.; Zhao, G.; Ma, F.; Wu, Y.; Hao, X. Utilizing photocorrosion-recrystallization to prepare a highly stable and efficient CdS/WS2 nanocomposite photocatalyst for hydrogen evolution. Appl. Catal. B 2016, 199, 466-472. 33. Zong, X.; Han, J.; Ma, G.; Yan, H.; Wu, G.; Li, C. Photocatalytic H 2 Evolution on CdS Loaded with WS2 as Cocatalyst under Visible Light Irradiation. J. Phys. Chem. C 2011, 115, 12202-12208. 34. Gopannagari, M.; Kumar, D. P.; Reddy, D. A.; Hong, S.; Song, M. I.; Kim, T. K. In situ preparation of few-layered WS2 nanosheets and exfoliation into bilayers on CdS nanorods for ultrafast charge carrier migrations toward enhanced photocatalytic hydrogen production. J. Catal. 2017, 351, 153-160. 35. Zhai, C.; Zhu, M.; Pang, F.; Bin, D.; Lu, C.; Goh, M.; Yang, P.; Du, Y. High efficiency photoelectrocatalytic methanol oxidation on CdS quantum dots sensitized Pt electrode. ACS Appl. Mater. Inter. 2016, 8, 5972-80. 36. Zhai, C.; Hu, J.; Sun, M.; Zhu, M. High‐Performance Visible‐Light‐Driven Pt/CdS/Graphene Photoelectrocatalysts for Methanol Oxidation. Energy Technol. 2017, 5, 1292-1299. 37. Xu, Y.; Zhao, W.; Xu, R.; Shi, Y.; Zhang, B. Synthesis of ultrathin CdS nanosheets as efficient visible-light-driven water splitting photocatalysts for hydrogen evolution. Chem. Commun. 2013, 49, 9803-9805. 38. Zhai, C.; Sun, M.; Zhu, M.; Zhang, K.; Du, Y. Insights into photo-activated electrode for boosting electrocatalytic methanol oxidation based on ultrathin MoS2 nanosheets enwrapped CdS nanowires. Int. J. Hydrogen Energy 2017, 42, 5006–5015. 39. Yue, Z.; Liu, A.; Zhang, C.; Huang, J.; Zhu, M.; Du, Y.; Yang, P. Noble-metal-free hetero-structural CdS/Nb2O5/N-doped-graphene ternary photocatalytic system as visible-light-driven photocatalyst for hydrogen evolution. Appl. Catal. B: Environ. 2017, 201, 202-210. 40. Lou, Z.; Zhu, M.; Yang, X.; Zhang, Y.; Whangbo, M.; Li, B.; Huang, B. Continual injection of photoinduced electrons stabilizing surface plasmon resonance of non-elemental-metal plasmonic photocatalyst CdS/WO3−x for efficient hydrogen generation. Appl. Catal. B: Environ. 2018, 226, 10-15.

16



Page 18 of 19

41. Hu, J.; Yu, C.; Zhai, C.; Hu, S.; Wang, Y.; Fu, N.; Zeng, L.; Zhu, M. 2D/1D heterostructure of g-C3N4 nanosheets/CdS nanowires as effective photo-activated support for photoelectrocatalytic oxidation of methanol. Catal. Today 2018, DOI: 10.1016/j.cattod.2018.02.043. 42. Yue, Z.; Liu, A.; Zhang, C.; Huang, J.; Zhu, M.; Du, Y.; Yang, P. Noble-metal-free hetero-structural CdS/Nb2O5/N-doped-graphene ternary photocatalytic system as visible-light-driven photocatalyst for hydrogen evolution. Appl. Catal. B 2017, 201, 202-210. 43. Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal synthesis of 1T-WS2 and 2H-WS2 nanosheets: applications for photocatalytic hydrogen evolution. J. Am. Chem. Soc. 2014, 136, 14121-14127. 44. Zhang, Y.; Shi, J.; Han, G.; Li, M.; Ji, Q.; Ma, D.; Zhang, Y.; Li, C.; Lang, X.; Zhang, Y.; Liu, Z. Chemical vapor deposition of monolayer WS2 nanosheets on Au foils toward direct application in hydrogen evolution. Nano Res. 2015, 8, 2881-2890. 45. Yu, J.; Yu, Y.; Zhou, P.; Xiao, W.; Cheng, B. Morphology-dependent photocatalytic H2-production activity of CdS. Appl. Catal. B 2014, 156-157, 184-191. 46. Yin, X. L.; He, G.-Y.; Sun, B.; Jiang, W.-J.; Xue, D. J.; Xia, A. D.; Wan, L. J.; Hu, J. S. Rational design

and

electron

transfer

kinetics

of

MoS 2/CdS

nanodots-on-nanorods

for

efficient

visible-light-driven hydrogen generation. Nano Energy 2016, 28, 319-329. 47. He, J.; Chen, L.; Yi, Z. Q.; Au, C. T.; Yin, S. F. CdS Nanorods Coupled with WS2 Nanosheets for Enhanced Photocatalytic Hydrogen Evolution Activity. Ind. Eng. Chem. Res. 2016, 55, 8327-8333. 48. Elbanna, O.; Fujitsuka, M.; Majima, T. g-C3N4/TiO2 Mesocrystals Composite for H2 Evolution under Visible-Light Irradiation and Its Charge Carrier Dynamics. ACS. Appl. Mater. Interfaces 2017, 9, 34844-34854. 49. Furube, A.; Asahi, T.; Masuhara, H.; Yamashita, H.; Anpo, M. Charge Carrier Dynamics of Standard TiO2 Catalysts Revealed by Femtosecond Diffuse Reflectance Spectroscopy. J. Phys. Chem. B 1999, 103, 3120-3127. 50. Peng, P.; Sadtler, B.; Alivisatos, A. P.; Saykally, R. J. Exciton Dynamics in CdS−Ag2S Nanorods with Tunable Composition Probed by Ultrafast Transient Absorption Spectroscopy. J. Phys. Chem. C 2010, 114, 5879-5885. 51. An, C.; Feng, J.; Liu, J.; Wei, G.; Du, J.; Wang, H.; Jin, S.; Zhang, J. NiS nanoparticle decorated MoS2 nanosheets as efficient promoters for enhanced solar H2 evolution over ZnxCd1−xS nanorods. Inorg. Chem. Front. 2017, 4, 1042-1047. 52. Zhang, Y.; Tang, Y.; Liu, X.; Dong, Z.; Hng, H. H.; Chen, Z.; Sum, T. C.; Chen, X. Three-dimensional CdS-titanate composite nanomaterials for enhanced visible-light-driven hydrogen evolution. Small 2013, 9, 996-1002. 53. Wu, K.; Zhu, H.; Liu, Z.; Rodriguez-Cordoba, W.; Lian, T. Ultrafast charge separation and long-lived charge separated state in photocatalytic CdS-Pt nanorod heterostructures. J. Am. Chem. Soc. 2012, 134, 10337-10340. 54. Shi, X.; Fujitsuka, M.; Lou, Z.; Zhang, P.; Majima, T. In situ nitrogen-doped hollow-TiO2/g-C3N4 composite photocatalysts with efficient charge separation boosting water reduction under visible light. J. Mater. Chem. A 2017, 5, 9671-9681. 55. Zhang, P.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Atomic Layer Deposition-Confined Nonstoichiometric TiO2 Nanocrystals with Tunneling Effects for Solar Driven Hydrogen Evolution. J. Phys. Chem. Lett. 2016, 7, 1173-1179. 56. Li, X.; Bi, W.; Zhang, L.; Tao, S.; Chu, W.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Single-Atom Pt as

17


Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co-Catalyst for Enhanced Photocatalytic H2 Evolution. Adv. Mater. 2016, 28, 2427-2431.

18


WS2 with Efficient Charge Separation

Recommend Documents