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MoSe2 heterojunction for efficient photoelectrocatalytic hydrogen evolution
Materials Science in Semiconductor Processing 107 (2020) 104822
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Construction of WS2/MoSe2 heterojunction for efficient photoelectrocatalytic hydrogen evolution Zhuangzhi Wu a, b, Min Ouyang a, Dezhi Wang a, b, * a b
School of Materials Science and Engineering, Central South University, Changsha, 410083, China Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Changsha, 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photoelectrocatalytic hydrogen evolution WS2 MoSe2 p-n heterojunction
As a typical n-type semiconductor, MoSe2 is limited in the application of photoelectrocatalytic hydrogen evo lution due to the rapid recombination of photo-generated electron-hole pairs. To solve this problem, in this work, a p-type WS2 is selected to design a WS2/MoSe2 hybrid semiconductor catalyst with a p-n heterojunction. The obtained samples were characterized by XRD, Raman, SEM, TEM, UV and XPS. Besides, the photoelectrocatalytic performances for hydrogen production were also evaluated. It was found that the WS2/MoSe2 hybrid catalyst with a loading of 20% WS2 (20W-M) showed the best photocatalytic performance with a photocurrent density of 35 μA cm-2 at -0.6 V (vs SCE), much larger than those of single MoSe2 (20 μA cm-2) and WS2 (3.5 μA cm-2). And the improved performances should be ascribed to the construction of p-n heterojunction, which leads to the change of photoelectron-hole transport mode, accelerates the separation speed of carriers and prolongs the carrier lifetime. This work demonstrates the promoting effect of WS2 as a cocatalyst and this strategy can be extended to other semiconductors.
1. Introduction As one of the most effective routes to solve energy shortage, semi conductor photocatalysis has attracted wide attentions, because it can be used for decomposing water into oxygen and hydrogen under the illumination of sunlight [1]. When an illuminated light source irradiates to the surface of the semiconductor electrode, the electrons (e-cb) in the valence band (VB) of photocatalysts are moved to the conduction band (CB), but the holes (hþ vb) are left in the VB [2]. The excited electrons and holes play an important role in redox reactions to generate H2 and O2. As one of the most representative substances in the transition metal disulfide (TMD), molybdenum diselenide (MoSe2) has excellent perfor mances in the electrocatalytic water splitting application [3]. There are weak van der Waals in its layered structures which resembles graphite. The calculated results of theoretical band structure combined with photoelectron spectroscopy analysis show that the energy gap of MoSe2 (1.4 eV–1.7 eV) can match well with the solar spectrum [4]. Therefore, this new material has a very good prospect of application in photo electrocatalytic hydrogen evolution. However, as a typical n-type
semiconductor, MoSe2 has a fast recombination of the photogenerated electron-hole pairs, resulting in a poor performance of photocatalytic reaction [5,6]. Single phase photocatalysts typically show less flexibility in tuning the electronic structure due to the low efficiency of separating photo-excited charge carriers [7]. But mixed or integrated multi-semiconductors could accelerate the separation of electron-hole pairs, which can make reduction and oxidation reactions happen inde pendently at two different reaction sites. Recently, a lot of researches related to heterojunction structures have been done. Nanoislands of p-type CaFe2O4 integrate with n-type PbBi2Nb1.9W0.1O9 to form pho tocatalytic nanodiodes, demonstrating good stability and high activity for decomposition of water [8]. The photocatalytic activity of n-p (CNO/NiO) junction surface was higher than those of p-n (NiO/CNO) junction, p and n surfaces [9]. Qian Xuefeng et al combined the n-type Ag2O with the p-type Bi2O2CO3, and it was found that the combining rate of the electron-hole pairs inside the complex was much less than that of the monomers [10]. WS2 is extremely similar to MoS2 over crystal structures and chem ical properties, and also has photocatalytic activity for water splitting.
* Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, 410083, China. E-mail address: [email protected] (D. Wang). https://doi.org/10.1016/j.mssp.2019.104822 Received 4 July 2019; Received in revised form 29 October 2019; Accepted 1 November 2019 Available online 9 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. SEM images of (a and b) WS2, (c and d) MoSe2; (e and f) 20W-M.
The WS2 was proved to have high electrocatalytic activity 20 years ago [11], while the photoelectrocatalytic activity of WS2 was rarely researched. Xu Zong et al have discovered that the producing speed of hydrogen can be added by over 28 times when CdS was doped with only 1.0 wt % WS2 [12]. This is mainly because that WS2 and CdS have formed the junction and WS2 is an excellent cocatalyst in catalyzing H2 evolution, which helps CdS to improve the performance. According to the similar strategy, more other WS2-based hybrid catalysts have also been developed, including 2D WS2/carbon dot hybrids [13], WS2/mpg-CN [14], g-C3N4/WS2 [15], WS2 sensitized mesoporous TiO2 [16], ZnS-WS2/CdS [17], Zn0.5Cd0.5S/WS2 [18], etc. Although great efforts have been made, the development of more new and efficient catalysts is still strongly demanded. Considering the fast interlayer energy transfer in MoSe2/WS2 het erostructures [19], herein, we combined p-type WS2 with the n-type MoSe2 matrix to design a hybrid catalyst with a semiconductor p-n junction, so that the combination of electron-hole pairs can be deterred, and then the electrocatalytic performances for hydrogen evolution under visible light can be significantly improved.
2. Experimental section 2.1. Materials Selenium powder (Se), sodium molybdate (Na2MoO4⋅2H2O), oleyl amine (C18H37N), sodium borohydride (NaBH4) and sodium hypo phosphite (NaH2PO2) were obtained from Aladdin. Thioacetamide (CH3CSNH2) was bought from Sinopharm Chemical Reagent Co., Ltd. Absolute ethanol and sodium tungstate (Na2WO4�2H2O) were pur chased from Xilong Co., Ltd. Fluorine tinoxide (FTO) glass was available from Dongxi Co., Ltd. Toluene (C6H3CH3) was purchased from Hen gyang Xinkai Chemical Reagent Co., Ltd. 2.2. Synthesis of WS2 0.99 g of Na2WO4�2H2O, 1.13 g of CH3CSNH2 and 4.77 g of NaH2PO2 with the ratio of 1:5:15 were mixed in 50 mL distilled water and stirred evenly. Afterwards, the mixture was poured to an autoclave (100 mL) and then kept at 220 � C for 24 h. After being cooled naturally, 2
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Fig. 2. TEM and HRTEM images of (a and b) WS2, (c and d) MoSe2, (e and f) 20W-M.
the sample can be collected by washing with water and ethanol thrice, and centrifugation followed by drying at 60 � C for 12 h.
discarding the transparent liquid above the solution, we repeated the steps above to produce the evenly dispersed powders. Next, the gained crude sample was dispersed in 1 mL toluene, and the uniformly stirred solution was dropped onto a 1.5 cm * 1.5 cm FTO glass piece with a pipette. After being dried at room temperature, it can serve as a working electrode.
2.3. Synthesis of MoSe2 0.7 g of NaBH4 and 0.32 g of Se powder were placed into 50 mL of distilled water and then stirred for at least 30 min until the solution became transparent and clear. Next, we added 0.49 g of Na2MoO4⋅2H2O to the solution and stir it for 10 min. After that, we placed it into the autoclave and heated it at 180 � C for 24 h. The next step is washing the product with water and ethanol for thrice after being cooled down naturally. Finally, the sample was dried at 60 � C for 12 h in drying oven.
2.6. Characterizations A FEI Sirion 200 scanning electron microscope was used to acquire the scanning electron microscopy (SEM) images. A Tecnai G2 20 trans mission electron microscope (TEM) was used to further analyze the microstructures. The X-ray diffraction (XRD) patterns were collected via a D/max-2500 system by using a Cu-Kα radiation (λ¼1.54 Å). The samples were scanned from 5� to 80� with a speed of 8� min-1. The Raman analysis of the sample was performed using a LabRAM HR-800 Raman spectrometer manufactured by HORIBA. The X-ray photoelec tron spectroscopy (XPS) spectra measurements were implemented using the ESCALAB 250 Xi with an Al Kα source.
2.4. Synthesis of WS2/MoSe2 0.7 g of NaBH4 and 0.32 g of Se powder were mixed in 50 mL distilled water then stirred for at least 30 min until the solution became clear. Various amounts of WS2 powder (0.027 g, 0.057 g, 0.128 g and 0.22 g) were added into solution. Then, 0.49 g of Na2MoO4⋅2H2O was put into the solution with stirring for 10 min. Similarly, it was also placed in an autoclave and kept for 24 h at 180 � C in a drying furnace. Finally, after being washed with water and ethanol thrice and dried at 60 � C for 12 h, the final products can be obtained and donated as 5W-M, 10W-M, 20WM and 30W-M, which correspond to various mass fractions of WS2 in the hybrid, including 5%, 10%, 20% and 30%, respectively.
2.7. Photoelectrochemical measurements UV–Visible–Infrared UV-2600 spectrophotometer manufactured by Shimadzu Corporation was used in the Ultraviolet–Visible-Absorption test. The PEC tests were finished by an electrochemical workstation (CHI 660E) with a typical three-electrode cell. The platinum was used as the auxiliary electrode, saturated calomel electrode (vs SCE) as the refer ence electrode, FTO loaded with the sample film as the working elec trode and 0.1 M Na2SO4 solution as the electrolyte. We used A Xe arc lamp (CEL-HXF-300, 300 W) equipped with a filter (λ > 420 nm) as the
2.5. Preparation of working electrodes 0.05 g of catalyst was mixed with 2 mL toluene, a few drops of oil amine and 6 mL alcohol and then extracted by the centrifuge. After 3
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Fig. 3. XRD patterns of (a) WS2, (b) MoSe2, (c) 20W-M; (d) XRD patterns of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M.
light source for the reactions and its light intensity was maintained at 100 mW cm-2.We set 300 s as the interval of on-off switch of irradiation when drawing the I-T curve of photo-current response.
the WS2 matrix, leading to the formation of a cluster with a middle size between those of pure WS2 and MoSe2, as revealed in Fig. 1e and 1f. In order to observe the microstructures more intuitively, TEM and HRTEM analyses were performed on WS2, MoSe2 and 20W-M, as shown in Fig. 2. Due to the large size of WS2 microspheres, only the edges of microspheres can be observed in the TEM images. It can be found that the edges of microspheres have many fine lines, as displayed in Fig. 2a. From the HRTEM image (Fig. 2b), it can be calculated that the (002) interplanar spacing of WS2 is about 0.98, indicating a remarkable interlayer expansion [20]. Fig. 2c and 2d are the TEM and HRTEM im ages of MoSe2. Clearly, the MoSe2 exhibits a typical nanoflower structure. The obvious lattice fringes can be seen in the HRTEM image of MoSe2 and the interplanar spacing is about 0.28 nm, corresponding to (100) crystal face in hexagonal lattice of MoSe2 [21]. In addition, the expanded structures of MoSe2 is also observed, the other typical lattice fringes of MoSe2 are also found with an interlayer spacing of 0.85 nm, coincident with the (002) crystal plane. The TEM and HRTEM images of 20W-M are dis played in Fig. 2e and 2f, and the interplanar spacing of 0.98 nm and 0.28 nm correspond to the (002) plane of WS2 and (100) plane of MoSe2, respectively. The layered MoSe2 is attached on the surface of WS2, resulting in intimate interfacial contact. According to the related articles [22-25], this kind of structure can confirm the formation of intimate junction between two materials.
3. Results and discussion 3.1. Characterizations The morphologies of the obtained products are depicted in Fig. 1, in which the SEM images (Fig. 1a and 1b) indicate that the obtained WS2 is spherical with a diameter of 2 μm. The SEM images of MoSe2 are dis played in Fig. 1c and 1d. We can see that the obtained MoSe2 is flowerlike granular with a size about 50 nm. In comparison, the morphology of WS2/MoSe2 composite with a WS2 mass fraction of 20% (abbreviated as 20W-M) is displayed in Fig. 1e and 1f. It can be seen that the surface of composite is blocky with an average size from 200 to 300 nm. Obviously, the final heterostructures possess a middle size between the huge WS2 microparticles and the MoSe2 nanoparticles, which means that the MoSe2 is not simply generated on the surface of the WS2 additive, because the finally obtained sample exhibits a much smaller size than that of WS2. Comparing the morphology of pure WS2, MoSe2 and their hybrid, it can be speculated that the original aggregated WS2 is firstly scattered into smaller pellets under the hydrothermal conditions, and then the MoSe2 nanocrystals can be nucleated and grow on the surface of
Fig. 4. Raman spectra of (a) WS2; (b) MoSe2; (c) 20W-M. 4
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Fig. 5. XPS spectra of 20W-M: (a) W 4f; (b) S 2p; (c) Mo 3d; (d) Se 3d.
Fig. 3 illustrates the XRD patterns of the WS2, MoSe2 and WS2/MoSe2 samples. The (002) peaks of MoSe2 and WS2 shift to 10.4� and 9� due to the probable intercalation of guest ions or molecules [26-29]. The diffraction peaks of MoSe2 are shown in Fig. 3b, and the positions of the diffraction peaks correspond to the standard PDF card (JCPDS card: 20–0757), demonstrating that the obtained sample is MoSe2 without
other impurity. After combination, it seems that there are no visible and significant changes in the XRD pattern compared with that of the MoSe2, as revealed in Fig. 3c, and detailed compositions should be further analyzed by other techniques later. To evaluate the influence of WS2 additive on the MoSe2 matrix, the XRD patterns of WS2/MoSe2 composites with various addition contents
Fig. 6. The optical absorption spectra of (a) WS2, (b) MoSe2, (c) 20W-M; (d) The comparison of optical absorption spectra over various catalysts. 5
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shows the XPS spectrum of Mo 3d, where two characteristic peaks at 231.9 eV and 228.7 eV are assigned to Mo4þ 3d3/2 and Mo4þ 3d5/2, respectively, which coincides with the valence of Mo in MoSe2. The valence states of Mo element corresponding to the double peaks of 235.5 eV and 231.5 eV are Mo6þ and can be assigned to Mo6þ 3d3/2 and Mo6þ 3d5/2, respectively, which should be ascribed to the Mo in MoO3, indicating that Mo4þ has been slowly oxidized due to contact with air [37]. Fig. 5d shows the XPS spectrum of Se 3d, and Se 3d3/2 and Se 3d5/2 are located at 55.3 eV and 54.6 eV, respectively, confirming the exis tence of Se element. 3.2. Photoelectrocatalytic performances The optical properties of the WS2 were studied by the UV-VIS ab sorption spectra, as shown in Fig. 6a. When the wavelength is decreased from 800 nm to 300 nm, the optical absorption shows a monotonic in crease and the light absorption of WS2 starts to appear around 600 nm according to its band absorption edge. Fig. 6b shows the ultra violet–visible absorption spectrum of MoSe2, and the absorption in tensity of MoSe2 is slightly reduced as the wavelength decreases. From the UV–visible absorption spectrum of 20W-M (Fig. 6c), we can see that the absorption edge is about 700 nm. Fig. 6d shows the UV–vis spectra of WS2, MoSe2 and WS2/MoSe2. It can be seen that the absorption ability of WS2/MoSe2 is gradually enhanced with the increase of added WS2 and peaks at 20%, then drops down at a higher additive content (30%). In comparison with other counterparts, the 20W-M possesses the highest absorption intensity, indicating that the 20W-M will exhibit the best photocatalytic performance. Tauc plots were obtained by the Kubelka-Munk function [38] to calculate the bandgap energy, as depicted in Fig. 7. The optical bandg aps can be obtained by Tauc equation ðαhvÞn ¼ Aðhv Eg Þ, where α is the measured absorption coefficient, hv is light energy, A is a constant, Eg is the bandgap energy of the semiconductor, and the value of n is 0.5 for indirect band gap of the semiconductor and 2 for direct band gap of the semiconductor [39,40]. The optic band gaps of WS2 and MoSe2 are 2.0 eV and 1.55 eV, respectively, in agreement with the literatures [41-45]. The energy gaps of 5W-M, 10W-M, 20W-M and 30W-M are 1.6 eV, 1.69 eV, 1.75 eV and 1.84 eV, respectively. The photocatalytic properties of all samples in hydrogen evolution application were measured by conventional PEC experiments. Linear sweep voltammogram (LSV) was obtained in visible light illumination to detect the photoelectrochemical activity. As depicted in Fig. 8a, the cathodic photocurrent density of 20W-M is 6100 μA cm-2 at -1.5 V (vs SCE), remarkably larger than those of common WS2 (900 μA cm-2) and MoSe2 (3500 μA cm-2) under the equal conditions. The LSV curves measured in the dark state are shown in Fig. 8b, and the cathode photocurrent densities of each sample are reduced to various degrees. However, the 20W-M composite still shows the best catalytic perfor mance. The catalytic activity is firstly enhanced with the increased content of WS2, achieving the maximum at 20%, and then drops down at
Fig. 7. Tauc plots of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M.
of WS2 are also depicted in Fig. 3d. We can see that with the increased contents of WS2 from 5% to 30%, there are still no significant changes over the XRD patterns of MoSe2 matrix, and no obvious WS2 charac teristic peaks can be seen. Moreover, the indicative (103) peak of the MoSe2 matrix is weakened with the increased proportion of WS2. To get more information over the compositions, the Raman spectra of WS2, MoSe2 and WS2/MoSe2 were also collected, as depicted in Fig. 4. In Fig. 4a, we can see that the representative peaks of WS2 are located at 354 cm-1 and 418 cm-1, respectively. The former is formed by the over lapping of two peaks, corresponding to the 2LA(M)and E [1]2g longi tudinal acoustic phonon modes of WS2, and the latter corresponds to the A1g [30,31]. Fig. 4b shows the representative peaks of MoSe2, and the peaks at 242 cm-1, 288 cm-1 correspond to A1g and E2g longitudinal acoustic phonon modes, respectively [32]. With the addition of WS2, it seems that 20W-M presents similar peaks to those of MoSe2 and no typical peak of WS2 is found at about 418 cm-1. Due to the similar location of peaks at 354 cm-1 for MoSe2 and WS2, it is very hard to distinguish and separate the accurate compositions and contents of them. Hence, we still cannot give a direct evidence to confirm the ex istence of WS2 in the composite. To verify the existence of WS2 in 20W-M and discover the valence states of each element, the XPS tests were conducted. It can be seen that peaks located at around 32.5 eV, 34.5 eV, 36.4 eV and 38.4 eV in Fig. 5a associates strongly with the binding energies of W4þ 4f7/2, W4þ 4f5/2, W6þ 4f7/2 and W6þ 4f5/2 [33,34]. The peak appearing at a higher binding energy (36 eV) demonstrates the presence of W6þ, and the peak intensity is very low, indicating that only a small portion of Wþ4 is oxidized due to air contact. As illustrated in Fig. 5b, in the S 2p region, the doublet located at 161 eV and 167 eV can be ascribed to the Se 3p3/2 and Se 3p1/2 [35]. The double peaks with binding energies of 162 eV and 164 eV correspond to S 2p3/2 and S 2p1/2, respectively, indicating the presence of W–S bonds [36], confirming the existence of WS2. Fig. 5c
Fig. 8. LSV curves of blank FTO glass, WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M: (a) under visible-light irradiation; (b) under dark conditions. 6
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resulting in a narrower range of light absorption spectrum, leading to a certain degree of reduction in photoelectrocatalytic performance. To further understand the enhancement mechanism, we used MottSchottky equation to ascertain the property by calculating the carrier density and flat band potential. Fig. 10a shows the Mott-Schottky curve of WS2. It is obvious that the tangential slope of the curve is negative, indicating that the as-prepared WS2 is a p-type semiconductor. In contrast, the slope of MoSe2 and 20W-M are positive, demonstrating that they are n-type semiconductors. The intersection of the tangent line and the X-axis is the value of the horizontal band potential. The flat-band potentials (EFB) of WS2, MoSe2 and 20W-M have been ascertained to be 0.64 V, -0.79 V and -0.66 V (vs SCE), respectively. Generally, con duction band potential (ECB) is normally more negative by approxi mately 0.1 or 0.2 V than the flat-band potential (EFB) for n-type semiconductors [46,47], but for p-type semiconductors, the valence band potential (EVB) is usually 0.1 or 0.2 V lower than the EFB. To give a direct evidence of heterojunction formation, the photo luminescence (PL) spectra were collected, as shown in Fig. 10d. As we know, the degree of recombination of photo-generated electrons and
Fig. 9. I–T curves of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M at a bias of 0.6 V versus SCE for 300 s of on-off cycles using visible-light.
a higher content of 30%, following the same trend with the optical ab sorption ability. The transient photocurrent responses are also collected by measuring the illumination intensity under the on-off cycles of visible-light irra diation with a bias of -0.6 V (vs SCE), as shown in Fig. 9. When visible light illumination is set to turn on and off at regular intervals (300 s) with multiple times, all samples have shown a photocurrent response. The 20W-M composite exhibits the highest photocurrent response of 35 μA cm-2, which is over twice larger than the sum of the photocurrent responses of single WS2 and MoSe2. Obviously, the addition of WS2 can facilitate the separation of photo-generated carriers and enhance the utilization of carriers, thus improving the photocatalytic performance. Compared with the MoSe2 matrix, the photocurrent responses are gradually enhanced with the increased additive contents, achieving the maximum value at 20W-M, but drops down with a higher content of 30%, even slightly smaller than that of MoSe2. It is speculated that the band gap of 30W-M is larger than other WS2/MoSe2 composite catalysts,
Fig. 11. Energy level diagram of photoelectrocatalytic charge transfer for WS2 and MoSe2.
Fig. 10. Mott-Schottky plots for (a) WS2, (b) MoSe2 and (c) 20W-M in 0.1 M Na2SO4 solution under dark conditions; (d) Photoluminescence (PL) spectra of MoSe2, WS2 and 20W-M. 7
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photocatalytic hydrogen evolution of 20W-M will be more efficient. The long term stability is also an important criterion for evaluating catalysts. The long term stability of various photoelectrodes has been tested under the irradiation of visible light at a bias voltage of -0.6 V (vs SCE), as shown in Fig. 13. It can be seen that the current density de creases slightly after the continuous test of 10000 s, demonstrating a good stability. 4. Conclusions In summary, the heterostructure of WS2/MoSe2 has been successfully prepared using a facile hydrothermal method to construct a p-n junction. With the addition of WS2, the photoelectrocatalytic performance of the MoSe2 is gradually enhanced and achieves the maximum over the 20WM catalyst, and then drops down. The optimized hybrid catalyst exhibits the highest photoelectrocatalytic performance and the photo response is about twice larger than the sum of single WS2 and MoSe2. The perfor mance enhancement should be ascribed to the presence of p-n semi conductor heterojunction, leading to a change in the transfer of photogenerated electron-hole pairs, including more efficient charge separa tion, faster charge transfer to the catalyst and longer lifetime of the charge carriers. This work demonstrates the promotion effect of WS2 on the MoSe2 matrix, which can also be extended to other photocatalysts and electrocatalysts.
Fig. 12. Nyquist plots of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial supports from the National Natural Science Foundation of China (Grants 51572301) and the Innovation-Driven Project of Central South University (Grants 502221802) are gratefully acknowledged.
Fig. 13. The stability tests of various photoelectrodes.
Appendix A. Supplementary data
holes can be determined by photoluminescence (PL) spectrum [48-53]. Evidently, the intensity of 20W-M is smaller than those of WS2 and MoSe2, indicating that the charge recombination of 20W-M is more effectively inhibited, which demonstrates the presence of p-n heterojunction. According to the relation between EVB and ECB for MoSe2 and WS2, a potential energy diagram is obtained, as shown in Fig. 11. The con duction bands of MoSe2 and WS2 are both located at more negative potentials than the potential of Hþ/H2 (-0.653 V), indicating that both of them can satisfy the thermodynamic conditions of water splitting. Since MoSe2 is an n-type semiconductor and WS2 is a p-type semiconductor, they can combine to form a p-n junction. The electrons and holes inside the two semiconductors continuously flow to balance the Fermi levels, and the surface forms a built-in electric field. Driven by the electric field, the holes can be moved to the VB of the p-type semiconductors, and at the same time the electrons can be transferred to the CB of the n-type semiconductors [54]. In this p-n junction, a charge separation and transfer with higher efficiency can be obtained as well as a longer life time of the charge carriers, which endows the p-n type heterostructures with an improved photocatalytic performance. Electrochemical impedance spectroscopy (EIS) measurement was employed to further confirm the separation efficiency of electrons and holes. The EIS Nyquist plots were collected under the visible-light irra diation. The test frequency ranges from 1 MHz to 1 Hz, as shown in Fig. 12. It can be seen that the charge transfer resistance of WS2 is the highest while the minimum resistance can be found in 20W-M. The charge can be transferred faster with a smaller resistance, so the
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Construction of WS2/MoSe2 heterojunction for efficient photoelectrocatalytic hydrogen evolution Zhuangzhi Wu a, b, Min Ouyang a, Dezhi Wang a, b, * a b
School of Materials Science and Engineering, Central South University, Changsha, 410083, China Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Changsha, 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photoelectrocatalytic hydrogen evolution WS2 MoSe2 p-n heterojunction
As a typical n-type semiconductor, MoSe2 is limited in the application of photoelectrocatalytic hydrogen evo lution due to the rapid recombination of photo-generated electron-hole pairs. To solve this problem, in this work, a p-type WS2 is selected to design a WS2/MoSe2 hybrid semiconductor catalyst with a p-n heterojunction. The obtained samples were characterized by XRD, Raman, SEM, TEM, UV and XPS. Besides, the photoelectrocatalytic performances for hydrogen production were also evaluated. It was found that the WS2/MoSe2 hybrid catalyst with a loading of 20% WS2 (20W-M) showed the best photocatalytic performance with a photocurrent density of 35 μA cm-2 at -0.6 V (vs SCE), much larger than those of single MoSe2 (20 μA cm-2) and WS2 (3.5 μA cm-2). And the improved performances should be ascribed to the construction of p-n heterojunction, which leads to the change of photoelectron-hole transport mode, accelerates the separation speed of carriers and prolongs the carrier lifetime. This work demonstrates the promoting effect of WS2 as a cocatalyst and this strategy can be extended to other semiconductors.
1. Introduction As one of the most effective routes to solve energy shortage, semi conductor photocatalysis has attracted wide attentions, because it can be used for decomposing water into oxygen and hydrogen under the illumination of sunlight [1]. When an illuminated light source irradiates to the surface of the semiconductor electrode, the electrons (e-cb) in the valence band (VB) of photocatalysts are moved to the conduction band (CB), but the holes (hþ vb) are left in the VB [2]. The excited electrons and holes play an important role in redox reactions to generate H2 and O2. As one of the most representative substances in the transition metal disulfide (TMD), molybdenum diselenide (MoSe2) has excellent perfor mances in the electrocatalytic water splitting application [3]. There are weak van der Waals in its layered structures which resembles graphite. The calculated results of theoretical band structure combined with photoelectron spectroscopy analysis show that the energy gap of MoSe2 (1.4 eV–1.7 eV) can match well with the solar spectrum [4]. Therefore, this new material has a very good prospect of application in photo electrocatalytic hydrogen evolution. However, as a typical n-type
semiconductor, MoSe2 has a fast recombination of the photogenerated electron-hole pairs, resulting in a poor performance of photocatalytic reaction [5,6]. Single phase photocatalysts typically show less flexibility in tuning the electronic structure due to the low efficiency of separating photo-excited charge carriers [7]. But mixed or integrated multi-semiconductors could accelerate the separation of electron-hole pairs, which can make reduction and oxidation reactions happen inde pendently at two different reaction sites. Recently, a lot of researches related to heterojunction structures have been done. Nanoislands of p-type CaFe2O4 integrate with n-type PbBi2Nb1.9W0.1O9 to form pho tocatalytic nanodiodes, demonstrating good stability and high activity for decomposition of water [8]. The photocatalytic activity of n-p (CNO/NiO) junction surface was higher than those of p-n (NiO/CNO) junction, p and n surfaces [9]. Qian Xuefeng et al combined the n-type Ag2O with the p-type Bi2O2CO3, and it was found that the combining rate of the electron-hole pairs inside the complex was much less than that of the monomers [10]. WS2 is extremely similar to MoS2 over crystal structures and chem ical properties, and also has photocatalytic activity for water splitting.
* Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, 410083, China. E-mail address: [email protected] (D. Wang). https://doi.org/10.1016/j.mssp.2019.104822 Received 4 July 2019; Received in revised form 29 October 2019; Accepted 1 November 2019 Available online 9 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. SEM images of (a and b) WS2, (c and d) MoSe2; (e and f) 20W-M.
The WS2 was proved to have high electrocatalytic activity 20 years ago [11], while the photoelectrocatalytic activity of WS2 was rarely researched. Xu Zong et al have discovered that the producing speed of hydrogen can be added by over 28 times when CdS was doped with only 1.0 wt % WS2 [12]. This is mainly because that WS2 and CdS have formed the junction and WS2 is an excellent cocatalyst in catalyzing H2 evolution, which helps CdS to improve the performance. According to the similar strategy, more other WS2-based hybrid catalysts have also been developed, including 2D WS2/carbon dot hybrids [13], WS2/mpg-CN [14], g-C3N4/WS2 [15], WS2 sensitized mesoporous TiO2 [16], ZnS-WS2/CdS [17], Zn0.5Cd0.5S/WS2 [18], etc. Although great efforts have been made, the development of more new and efficient catalysts is still strongly demanded. Considering the fast interlayer energy transfer in MoSe2/WS2 het erostructures [19], herein, we combined p-type WS2 with the n-type MoSe2 matrix to design a hybrid catalyst with a semiconductor p-n junction, so that the combination of electron-hole pairs can be deterred, and then the electrocatalytic performances for hydrogen evolution under visible light can be significantly improved.
2. Experimental section 2.1. Materials Selenium powder (Se), sodium molybdate (Na2MoO4⋅2H2O), oleyl amine (C18H37N), sodium borohydride (NaBH4) and sodium hypo phosphite (NaH2PO2) were obtained from Aladdin. Thioacetamide (CH3CSNH2) was bought from Sinopharm Chemical Reagent Co., Ltd. Absolute ethanol and sodium tungstate (Na2WO4�2H2O) were pur chased from Xilong Co., Ltd. Fluorine tinoxide (FTO) glass was available from Dongxi Co., Ltd. Toluene (C6H3CH3) was purchased from Hen gyang Xinkai Chemical Reagent Co., Ltd. 2.2. Synthesis of WS2 0.99 g of Na2WO4�2H2O, 1.13 g of CH3CSNH2 and 4.77 g of NaH2PO2 with the ratio of 1:5:15 were mixed in 50 mL distilled water and stirred evenly. Afterwards, the mixture was poured to an autoclave (100 mL) and then kept at 220 � C for 24 h. After being cooled naturally, 2
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Fig. 2. TEM and HRTEM images of (a and b) WS2, (c and d) MoSe2, (e and f) 20W-M.
the sample can be collected by washing with water and ethanol thrice, and centrifugation followed by drying at 60 � C for 12 h.
discarding the transparent liquid above the solution, we repeated the steps above to produce the evenly dispersed powders. Next, the gained crude sample was dispersed in 1 mL toluene, and the uniformly stirred solution was dropped onto a 1.5 cm * 1.5 cm FTO glass piece with a pipette. After being dried at room temperature, it can serve as a working electrode.
2.3. Synthesis of MoSe2 0.7 g of NaBH4 and 0.32 g of Se powder were placed into 50 mL of distilled water and then stirred for at least 30 min until the solution became transparent and clear. Next, we added 0.49 g of Na2MoO4⋅2H2O to the solution and stir it for 10 min. After that, we placed it into the autoclave and heated it at 180 � C for 24 h. The next step is washing the product with water and ethanol for thrice after being cooled down naturally. Finally, the sample was dried at 60 � C for 12 h in drying oven.
2.6. Characterizations A FEI Sirion 200 scanning electron microscope was used to acquire the scanning electron microscopy (SEM) images. A Tecnai G2 20 trans mission electron microscope (TEM) was used to further analyze the microstructures. The X-ray diffraction (XRD) patterns were collected via a D/max-2500 system by using a Cu-Kα radiation (λ¼1.54 Å). The samples were scanned from 5� to 80� with a speed of 8� min-1. The Raman analysis of the sample was performed using a LabRAM HR-800 Raman spectrometer manufactured by HORIBA. The X-ray photoelec tron spectroscopy (XPS) spectra measurements were implemented using the ESCALAB 250 Xi with an Al Kα source.
2.4. Synthesis of WS2/MoSe2 0.7 g of NaBH4 and 0.32 g of Se powder were mixed in 50 mL distilled water then stirred for at least 30 min until the solution became clear. Various amounts of WS2 powder (0.027 g, 0.057 g, 0.128 g and 0.22 g) were added into solution. Then, 0.49 g of Na2MoO4⋅2H2O was put into the solution with stirring for 10 min. Similarly, it was also placed in an autoclave and kept for 24 h at 180 � C in a drying furnace. Finally, after being washed with water and ethanol thrice and dried at 60 � C for 12 h, the final products can be obtained and donated as 5W-M, 10W-M, 20WM and 30W-M, which correspond to various mass fractions of WS2 in the hybrid, including 5%, 10%, 20% and 30%, respectively.
2.7. Photoelectrochemical measurements UV–Visible–Infrared UV-2600 spectrophotometer manufactured by Shimadzu Corporation was used in the Ultraviolet–Visible-Absorption test. The PEC tests were finished by an electrochemical workstation (CHI 660E) with a typical three-electrode cell. The platinum was used as the auxiliary electrode, saturated calomel electrode (vs SCE) as the refer ence electrode, FTO loaded with the sample film as the working elec trode and 0.1 M Na2SO4 solution as the electrolyte. We used A Xe arc lamp (CEL-HXF-300, 300 W) equipped with a filter (λ > 420 nm) as the
2.5. Preparation of working electrodes 0.05 g of catalyst was mixed with 2 mL toluene, a few drops of oil amine and 6 mL alcohol and then extracted by the centrifuge. After 3
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Fig. 3. XRD patterns of (a) WS2, (b) MoSe2, (c) 20W-M; (d) XRD patterns of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M.
light source for the reactions and its light intensity was maintained at 100 mW cm-2.We set 300 s as the interval of on-off switch of irradiation when drawing the I-T curve of photo-current response.
the WS2 matrix, leading to the formation of a cluster with a middle size between those of pure WS2 and MoSe2, as revealed in Fig. 1e and 1f. In order to observe the microstructures more intuitively, TEM and HRTEM analyses were performed on WS2, MoSe2 and 20W-M, as shown in Fig. 2. Due to the large size of WS2 microspheres, only the edges of microspheres can be observed in the TEM images. It can be found that the edges of microspheres have many fine lines, as displayed in Fig. 2a. From the HRTEM image (Fig. 2b), it can be calculated that the (002) interplanar spacing of WS2 is about 0.98, indicating a remarkable interlayer expansion [20]. Fig. 2c and 2d are the TEM and HRTEM im ages of MoSe2. Clearly, the MoSe2 exhibits a typical nanoflower structure. The obvious lattice fringes can be seen in the HRTEM image of MoSe2 and the interplanar spacing is about 0.28 nm, corresponding to (100) crystal face in hexagonal lattice of MoSe2 [21]. In addition, the expanded structures of MoSe2 is also observed, the other typical lattice fringes of MoSe2 are also found with an interlayer spacing of 0.85 nm, coincident with the (002) crystal plane. The TEM and HRTEM images of 20W-M are dis played in Fig. 2e and 2f, and the interplanar spacing of 0.98 nm and 0.28 nm correspond to the (002) plane of WS2 and (100) plane of MoSe2, respectively. The layered MoSe2 is attached on the surface of WS2, resulting in intimate interfacial contact. According to the related articles [22-25], this kind of structure can confirm the formation of intimate junction between two materials.
3. Results and discussion 3.1. Characterizations The morphologies of the obtained products are depicted in Fig. 1, in which the SEM images (Fig. 1a and 1b) indicate that the obtained WS2 is spherical with a diameter of 2 μm. The SEM images of MoSe2 are dis played in Fig. 1c and 1d. We can see that the obtained MoSe2 is flowerlike granular with a size about 50 nm. In comparison, the morphology of WS2/MoSe2 composite with a WS2 mass fraction of 20% (abbreviated as 20W-M) is displayed in Fig. 1e and 1f. It can be seen that the surface of composite is blocky with an average size from 200 to 300 nm. Obviously, the final heterostructures possess a middle size between the huge WS2 microparticles and the MoSe2 nanoparticles, which means that the MoSe2 is not simply generated on the surface of the WS2 additive, because the finally obtained sample exhibits a much smaller size than that of WS2. Comparing the morphology of pure WS2, MoSe2 and their hybrid, it can be speculated that the original aggregated WS2 is firstly scattered into smaller pellets under the hydrothermal conditions, and then the MoSe2 nanocrystals can be nucleated and grow on the surface of
Fig. 4. Raman spectra of (a) WS2; (b) MoSe2; (c) 20W-M. 4
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Fig. 5. XPS spectra of 20W-M: (a) W 4f; (b) S 2p; (c) Mo 3d; (d) Se 3d.
Fig. 3 illustrates the XRD patterns of the WS2, MoSe2 and WS2/MoSe2 samples. The (002) peaks of MoSe2 and WS2 shift to 10.4� and 9� due to the probable intercalation of guest ions or molecules [26-29]. The diffraction peaks of MoSe2 are shown in Fig. 3b, and the positions of the diffraction peaks correspond to the standard PDF card (JCPDS card: 20–0757), demonstrating that the obtained sample is MoSe2 without
other impurity. After combination, it seems that there are no visible and significant changes in the XRD pattern compared with that of the MoSe2, as revealed in Fig. 3c, and detailed compositions should be further analyzed by other techniques later. To evaluate the influence of WS2 additive on the MoSe2 matrix, the XRD patterns of WS2/MoSe2 composites with various addition contents
Fig. 6. The optical absorption spectra of (a) WS2, (b) MoSe2, (c) 20W-M; (d) The comparison of optical absorption spectra over various catalysts. 5
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shows the XPS spectrum of Mo 3d, where two characteristic peaks at 231.9 eV and 228.7 eV are assigned to Mo4þ 3d3/2 and Mo4þ 3d5/2, respectively, which coincides with the valence of Mo in MoSe2. The valence states of Mo element corresponding to the double peaks of 235.5 eV and 231.5 eV are Mo6þ and can be assigned to Mo6þ 3d3/2 and Mo6þ 3d5/2, respectively, which should be ascribed to the Mo in MoO3, indicating that Mo4þ has been slowly oxidized due to contact with air [37]. Fig. 5d shows the XPS spectrum of Se 3d, and Se 3d3/2 and Se 3d5/2 are located at 55.3 eV and 54.6 eV, respectively, confirming the exis tence of Se element. 3.2. Photoelectrocatalytic performances The optical properties of the WS2 were studied by the UV-VIS ab sorption spectra, as shown in Fig. 6a. When the wavelength is decreased from 800 nm to 300 nm, the optical absorption shows a monotonic in crease and the light absorption of WS2 starts to appear around 600 nm according to its band absorption edge. Fig. 6b shows the ultra violet–visible absorption spectrum of MoSe2, and the absorption in tensity of MoSe2 is slightly reduced as the wavelength decreases. From the UV–visible absorption spectrum of 20W-M (Fig. 6c), we can see that the absorption edge is about 700 nm. Fig. 6d shows the UV–vis spectra of WS2, MoSe2 and WS2/MoSe2. It can be seen that the absorption ability of WS2/MoSe2 is gradually enhanced with the increase of added WS2 and peaks at 20%, then drops down at a higher additive content (30%). In comparison with other counterparts, the 20W-M possesses the highest absorption intensity, indicating that the 20W-M will exhibit the best photocatalytic performance. Tauc plots were obtained by the Kubelka-Munk function [38] to calculate the bandgap energy, as depicted in Fig. 7. The optical bandg aps can be obtained by Tauc equation ðαhvÞn ¼ Aðhv Eg Þ, where α is the measured absorption coefficient, hv is light energy, A is a constant, Eg is the bandgap energy of the semiconductor, and the value of n is 0.5 for indirect band gap of the semiconductor and 2 for direct band gap of the semiconductor [39,40]. The optic band gaps of WS2 and MoSe2 are 2.0 eV and 1.55 eV, respectively, in agreement with the literatures [41-45]. The energy gaps of 5W-M, 10W-M, 20W-M and 30W-M are 1.6 eV, 1.69 eV, 1.75 eV and 1.84 eV, respectively. The photocatalytic properties of all samples in hydrogen evolution application were measured by conventional PEC experiments. Linear sweep voltammogram (LSV) was obtained in visible light illumination to detect the photoelectrochemical activity. As depicted in Fig. 8a, the cathodic photocurrent density of 20W-M is 6100 μA cm-2 at -1.5 V (vs SCE), remarkably larger than those of common WS2 (900 μA cm-2) and MoSe2 (3500 μA cm-2) under the equal conditions. The LSV curves measured in the dark state are shown in Fig. 8b, and the cathode photocurrent densities of each sample are reduced to various degrees. However, the 20W-M composite still shows the best catalytic perfor mance. The catalytic activity is firstly enhanced with the increased content of WS2, achieving the maximum at 20%, and then drops down at
Fig. 7. Tauc plots of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M.
of WS2 are also depicted in Fig. 3d. We can see that with the increased contents of WS2 from 5% to 30%, there are still no significant changes over the XRD patterns of MoSe2 matrix, and no obvious WS2 charac teristic peaks can be seen. Moreover, the indicative (103) peak of the MoSe2 matrix is weakened with the increased proportion of WS2. To get more information over the compositions, the Raman spectra of WS2, MoSe2 and WS2/MoSe2 were also collected, as depicted in Fig. 4. In Fig. 4a, we can see that the representative peaks of WS2 are located at 354 cm-1 and 418 cm-1, respectively. The former is formed by the over lapping of two peaks, corresponding to the 2LA(M)and E [1]2g longi tudinal acoustic phonon modes of WS2, and the latter corresponds to the A1g [30,31]. Fig. 4b shows the representative peaks of MoSe2, and the peaks at 242 cm-1, 288 cm-1 correspond to A1g and E2g longitudinal acoustic phonon modes, respectively [32]. With the addition of WS2, it seems that 20W-M presents similar peaks to those of MoSe2 and no typical peak of WS2 is found at about 418 cm-1. Due to the similar location of peaks at 354 cm-1 for MoSe2 and WS2, it is very hard to distinguish and separate the accurate compositions and contents of them. Hence, we still cannot give a direct evidence to confirm the ex istence of WS2 in the composite. To verify the existence of WS2 in 20W-M and discover the valence states of each element, the XPS tests were conducted. It can be seen that peaks located at around 32.5 eV, 34.5 eV, 36.4 eV and 38.4 eV in Fig. 5a associates strongly with the binding energies of W4þ 4f7/2, W4þ 4f5/2, W6þ 4f7/2 and W6þ 4f5/2 [33,34]. The peak appearing at a higher binding energy (36 eV) demonstrates the presence of W6þ, and the peak intensity is very low, indicating that only a small portion of Wþ4 is oxidized due to air contact. As illustrated in Fig. 5b, in the S 2p region, the doublet located at 161 eV and 167 eV can be ascribed to the Se 3p3/2 and Se 3p1/2 [35]. The double peaks with binding energies of 162 eV and 164 eV correspond to S 2p3/2 and S 2p1/2, respectively, indicating the presence of W–S bonds [36], confirming the existence of WS2. Fig. 5c
Fig. 8. LSV curves of blank FTO glass, WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M: (a) under visible-light irradiation; (b) under dark conditions. 6
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resulting in a narrower range of light absorption spectrum, leading to a certain degree of reduction in photoelectrocatalytic performance. To further understand the enhancement mechanism, we used MottSchottky equation to ascertain the property by calculating the carrier density and flat band potential. Fig. 10a shows the Mott-Schottky curve of WS2. It is obvious that the tangential slope of the curve is negative, indicating that the as-prepared WS2 is a p-type semiconductor. In contrast, the slope of MoSe2 and 20W-M are positive, demonstrating that they are n-type semiconductors. The intersection of the tangent line and the X-axis is the value of the horizontal band potential. The flat-band potentials (EFB) of WS2, MoSe2 and 20W-M have been ascertained to be 0.64 V, -0.79 V and -0.66 V (vs SCE), respectively. Generally, con duction band potential (ECB) is normally more negative by approxi mately 0.1 or 0.2 V than the flat-band potential (EFB) for n-type semiconductors [46,47], but for p-type semiconductors, the valence band potential (EVB) is usually 0.1 or 0.2 V lower than the EFB. To give a direct evidence of heterojunction formation, the photo luminescence (PL) spectra were collected, as shown in Fig. 10d. As we know, the degree of recombination of photo-generated electrons and
Fig. 9. I–T curves of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M at a bias of 0.6 V versus SCE for 300 s of on-off cycles using visible-light.
a higher content of 30%, following the same trend with the optical ab sorption ability. The transient photocurrent responses are also collected by measuring the illumination intensity under the on-off cycles of visible-light irra diation with a bias of -0.6 V (vs SCE), as shown in Fig. 9. When visible light illumination is set to turn on and off at regular intervals (300 s) with multiple times, all samples have shown a photocurrent response. The 20W-M composite exhibits the highest photocurrent response of 35 μA cm-2, which is over twice larger than the sum of the photocurrent responses of single WS2 and MoSe2. Obviously, the addition of WS2 can facilitate the separation of photo-generated carriers and enhance the utilization of carriers, thus improving the photocatalytic performance. Compared with the MoSe2 matrix, the photocurrent responses are gradually enhanced with the increased additive contents, achieving the maximum value at 20W-M, but drops down with a higher content of 30%, even slightly smaller than that of MoSe2. It is speculated that the band gap of 30W-M is larger than other WS2/MoSe2 composite catalysts,
Fig. 11. Energy level diagram of photoelectrocatalytic charge transfer for WS2 and MoSe2.
Fig. 10. Mott-Schottky plots for (a) WS2, (b) MoSe2 and (c) 20W-M in 0.1 M Na2SO4 solution under dark conditions; (d) Photoluminescence (PL) spectra of MoSe2, WS2 and 20W-M. 7
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photocatalytic hydrogen evolution of 20W-M will be more efficient. The long term stability is also an important criterion for evaluating catalysts. The long term stability of various photoelectrodes has been tested under the irradiation of visible light at a bias voltage of -0.6 V (vs SCE), as shown in Fig. 13. It can be seen that the current density de creases slightly after the continuous test of 10000 s, demonstrating a good stability. 4. Conclusions In summary, the heterostructure of WS2/MoSe2 has been successfully prepared using a facile hydrothermal method to construct a p-n junction. With the addition of WS2, the photoelectrocatalytic performance of the MoSe2 is gradually enhanced and achieves the maximum over the 20WM catalyst, and then drops down. The optimized hybrid catalyst exhibits the highest photoelectrocatalytic performance and the photo response is about twice larger than the sum of single WS2 and MoSe2. The perfor mance enhancement should be ascribed to the presence of p-n semi conductor heterojunction, leading to a change in the transfer of photogenerated electron-hole pairs, including more efficient charge separa tion, faster charge transfer to the catalyst and longer lifetime of the charge carriers. This work demonstrates the promotion effect of WS2 on the MoSe2 matrix, which can also be extended to other photocatalysts and electrocatalysts.
Fig. 12. Nyquist plots of WS2, MoSe2, 5W-M, 10W-M, 20W-M and 30W-M.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial supports from the National Natural Science Foundation of China (Grants 51572301) and the Innovation-Driven Project of Central South University (Grants 502221802) are gratefully acknowledged.
Fig. 13. The stability tests of various photoelectrodes.
Appendix A. Supplementary data
holes can be determined by photoluminescence (PL) spectrum [48-53]. Evidently, the intensity of 20W-M is smaller than those of WS2 and MoSe2, indicating that the charge recombination of 20W-M is more effectively inhibited, which demonstrates the presence of p-n heterojunction. According to the relation between EVB and ECB for MoSe2 and WS2, a potential energy diagram is obtained, as shown in Fig. 11. The con duction bands of MoSe2 and WS2 are both located at more negative potentials than the potential of Hþ/H2 (-0.653 V), indicating that both of them can satisfy the thermodynamic conditions of water splitting. Since MoSe2 is an n-type semiconductor and WS2 is a p-type semiconductor, they can combine to form a p-n junction. The electrons and holes inside the two semiconductors continuously flow to balance the Fermi levels, and the surface forms a built-in electric field. Driven by the electric field, the holes can be moved to the VB of the p-type semiconductors, and at the same time the electrons can be transferred to the CB of the n-type semiconductors [54]. In this p-n junction, a charge separation and transfer with higher efficiency can be obtained as well as a longer life time of the charge carriers, which endows the p-n type heterostructures with an improved photocatalytic performance. Electrochemical impedance spectroscopy (EIS) measurement was employed to further confirm the separation efficiency of electrons and holes. The EIS Nyquist plots were collected under the visible-light irra diation. The test frequency ranges from 1 MHz to 1 Hz, as shown in Fig. 12. It can be seen that the charge transfer resistance of WS2 is the highest while the minimum resistance can be found in 20W-M. The charge can be transferred faster with a smaller resistance, so the
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