p-Si Heterostructure Photocathode Using Direct

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Fabrication of a WS2/p-Si heterostructure photocathode using direct hybrid thermolysis Amirhossein Hasani, Quyet Van Le, Mahider Tekalgne, Min-Ju Choi, Tae Hyung Lee, Sang Hyun Ahn, Ho Won Jang, and Soo Young Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08654 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Applied Materials & Interfaces

Fabrication of a WS2/p-Si Heterostructure Photocathode using Direct Hybrid Thermolysis Amirhossein Hasani,a† Quyet Van Le,b† Mahider Tekalgne,a Min-Ju Choi,c Seokhoon Choi,c Tae Hyung Lee,c Sang Hyun Ahn,a Ho Won Jang c* and Soo Young Kim a* a School

of Chemical Engineering and Materials Science, Integrative research center for twodimensional functional materials, Institute of Interdisciplinary Convergence Research, ChungAng University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea. b Institute

of Research and Development, Duy Tan University, Da Nang 550000, Vietnam.

c Department

of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul, Republic of Korea. † A. Hasani and Q.V. Le contributed equally to this work. *Corresponding authors: E-mail: [email protected] (S. Y. Kim), [email protected] (H. W. Jang).

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ABSTRACT

P-N heterostructures based on TMDs and a conventional semiconductor, such as p-Si, have been considered a promising structure for next-generation electronic devices and applications. However, synthesis of high-quality, wafer-scale TMDs, particularly WS2 on p-Si, is challenging. Herein, we propose an efficient method to directly grow WS2 crystals on p-Si via a hybrid thermolysis process. The WO3 is deposited to prepare the p-Si surface for coating of the (NH4)2WS4 precursor and converted to WS2/p-Si during thermolysis. Moreover, the WS2/p-Si heterojunction photocathode is fabricated and used in solar hydrogen production. The fabricated n-WS2/p-Si heterojunction provided onset potential of +0.022 V at 10 mA/cm2 and a benchmark current density of –9.8 ± 1.2 mA/cm2 at 0 V. This method reliably and efficiently produced high-quality, wafer-scale WS2 crystals and overcame the challenges associated with previous approaches. The approach developed in this research demonstrates a magnificent progress in the fabrication of 2D materials-based electronic devices.

KEYWORDS. WS2, heterostructures, thermolysis, hydrogen, heterojunction, 2D materials.


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INTRODUCTION For a decade, 2D transition metal dichelcongenides (TMDs) have been evaluated as promising semiconducting materials for future electronic devices.1-2 Atomically thin-film TMDs can be used in different applications, such as LEDs, gas sensors, transistors, solar cells, and energy-harvesting devices, because of their tunable and stable electro-optoelectronic and mechanical properties.3-6 Among the TMDs, MoS2 and WS2, which are well-known unique ntype semiconductors, have been considered for electronic devices, particularly for energy conversion. They can provide a high surface area, active edge sites, and tunable band-gap, thereby suggesting that they could be used in photoelectrochemical hydrogen production to overcome the energy crisis and fossil fuel shortage.7-10 With a conventional semiconductor combination, such as p-Si and an n-type semiconductor, TMDs offer an excellent p-n heterojunction for solar hydrogen production.11-12 The narrowband gap (1.14 eV) allows p-Si to cover and absorb visible to infrared wavelengths and efficiently generate hole-electron pairs. In addition, TMD thin films, such as those composed of WS2 and MoS2, significantly increase the hydrogen evolution reaction because of their unique photocatalytic properties.2 Therefore, the combination of WS2 and p-Si to create a heterojunction could provide highly efficient photocatalytic activity for solar hydrogen production. In general, the synthesis of MoS2 and WS2 is based on three processes: (1) chemical vapor deposition (CVD), (2) exfoliation, and the (3) solution process.13-16 Here, p-Si can be easily oxidized in air; therefore, uniform and fully covered TMDs on the p-Si surface are required. Among these introduced processes, CVD is a reliable and large-scale method frequently used to synthesize high-quality MoS2 and WS2 thin films.17 However, the preparation of a uniform, high quality crystal, wafer-scale film of MoS2 and WS2 is challenging, in which the quality of the film likely influences the device performance. For instance, Yue et al. synthesize the high quality of WS2 and applied in the field-effect transistor, in which they proved that the quality 3 ACS Paragon Plus Environment


of the WS2 film influcense directly on device performances. A thermolysis process is an efficient CVD method used to synthesize MoS2. Kwon et al. synthesized MoS2 on a SiO2 wafer and then transferred the MoS2 film to p-Si using a wet-transferring process.18 Similarly, wettransferring process was used to fabricate WS2/p-Si heterojunction reported by the same group.17 Despite the efficiency of the photoelectrochemical hydrogen production, the transferring process restricts the synthesis of a large-scale, reliable, and time-saving process, in which the MoS2 and WS2 films can be damaged during the transfer process. Therefore, a reliable method is required to fabricate the TMDs/p-Si heterojunction that includes the following advantages: a wafer-scale, an easy and fast non-transferring process, reproducibility, and a high crystallinity of the TMD film. Herein, we fabricated a WS2/p-Si heterostructure photocathode via a direct thermolysis process, which is a significant development in the fabrication of 2D-materials based devices, dealing all challenges of previously reported techniques.11, 18-19 We demonstrated an approach to directly synthesize the highly crystalline and wafer-scale (5×5 cm) of WS2 onto p-Si using a thermolysis process. In addition, to determine the applicability of the method, the as-fabricated n-WS2/p-Si heterojunction was used in photoelectrochemical hydrogen production. Because of the direct formation of pristine WS2 on p-Si and the high quality of the p-n heterojunction, the as-prepared photocathode exhibited an efficient performance. The direct fabrication of the p-n heterojunction prepared by this proposed method offers a facile, reliable, and time-effective fabrication process, which is a remarkable development in high-quality semiconductor electronic devices. In this study, the thermolysis of ammonium tetrathiotungstate ((NH4)2WS4)) was carried out to form the WS2 thin film. The preparation of large-scale, high-quality WS2 using an efficient and facile thermolysis process has been previously reported.18, 20-21 Because coating the (NH4)2WS4 precursor onto a substrate with a low hydrophilic surface, such as p-Si, is not possible, we used a hybrid method to directly synthesize WS2 on the p-Si substrate. As shown schematically in Figure 1a, WO3 with different thicknesses (5, 10, 15, 20, 25, and 30 4 ACS Paragon Plus Environment

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nm) was deposited on the p-Si substrate via an evaporation method to increase the hydrophilicity of the Si substrate. Next, the (NH4)2WS4 solution was coated on WO3/p-Si using a spin-coating method (see Figure 1b). Finally, (NH4)2WS4/WO3/p-Si was converted to WS2/pSi at 900 °C under H2 and N2 gas flow via a thermolysis process (see Figure 1c-d). EXPERIMENTAL SECTION Fabrication of a WS2/p-Si heterojunction. p-Si wafers (1.5cm×1.5cm) were cleaned ultrasonically with three conventional steps, followed by sonicated in isopropyl, acetone and deionized water. Then, the p-Si wafers were placed in hydrofluoric acid (Sigma Aldrich, Hf10%) to eliminate the native oxide. Afterwards, various thicknesses (5, 10, 15, 20, 25, and 30 nm) of WO3 were deposited on p-Si via a thermal evaporator. Next, 10 mM of ammonium tetrathiotungstate (NH4)2WS4 dispersed in ethylene glycol was coated on the WO3/p-Si substrates via spin-coater at 3500 rpm for 60 s. To remove the solvent, the samples were putted on a hot plate at 50 ºC for 15 min. The WO3/p-Si wafers were putted in a tube quartz chamber to start the thermolysis process. Here, H2 (40 cm3/min) and N2 (200 cm3/min) gases flowed into the chamber during the process. Initially, the temperature was increased to 500 ºC and, this temperature was maintained for 30 min. The chamber pressure was maintained under N2/H2 gas at 1.2 Torr. Then, the temperature was increased to 900 ºC and maintained for 1 h. Finally, sulfurization was started with 0.6 g of sulfur powder (Sigma Aldrich, 99.5%) in another annealing zone at 330 ºC, and this temperature was kept for 1 h.

Fabrication of the WS2/p-Si photocathode. To improve contact, the back of the WS2/p-Si samples was scratched with a blade, and then, InGa alloy (Sigma-Aldrich, 99% purity) was coated on the scratched back of the samples. Subsequently, a copper wire was attached to the back of the WS2/p-Si samples via silver paste and placed on a hot plate at 50 ºC for 1 h. At last, the samples were passivated by epoxy resin for PEC measurements. The effective areas of the sample was about 5mm×4mm (see Figure 1e). 5 ACS Paragon Plus Environment


Characterization. X-ray photoelectron spectroscopy (XPS) (K-Alpha plus, ThermoFisher Scientific, USA) was carried out under a vacuum at QJQ .R5 mbar using a constant pass energy of 5.Q 6 and Mg 2S radiation > 5.Q 6@3 X-ray diffraction (XRD) instrument (D8-Advance, Bruker, Germany) to analyze crystal structures of the samples with a Cu 2S target having a wavelength of .3 5+ Q

3 Raman spectra were examined by Raman spectroscopy analyzer (HR,

Horiba, Japan) at a wavelength of 5 +Q

3 To evaluate the morophology of the sample surfaces

the Field-emission scanning electron microscopy (Zeiss 300, Germany) was used at an acceleration voltage of 5.Q,63 Transmission electron microscopy (TEM) was carried out with an instrument (2100F, Japan) and having 50 kV for acceleration voltage. Atomic force microscopy (AFM, XE-100/PSIA) in non-contact-mode (scan rate of 0.5 Hz) was employed to evaluate the roughness and thickness of the thin films. To examine by UV-vis spectroscopy (V670, Germany), the as-synthesized WS2 thin films were transferred onto glass substrates via polymethyl methacrylate (PMMA) spin-coating. Next, the PMMA/WS2/p-Si substrates was placed in a hydrochloric aide (HF, Sigma 10%) for 2 minutes to detach the WS2 layer and then transferred onto glass and Copper grid. Finally, acetone was used to remove PMMA in 30 minutes and 50 oC.


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Photoelectrochemical measurements. Electrochemical measurements were implemented in 0.5M H2SO4 with a three-electrode quartz electrochemical cell using a carbon-rod as the counter electrode and saturated calomel as the reference electrode and it was connected to a potentiostat (Ivium 5612, Netherlands). A solar simulator (Oriel 150W) was utilized and adjusted to an output of 100 mW/cm2 irradiation with scan rate of 10mV/s. Electrochemical impedance spectroscopy (EIS) was conducted by applying a small constant potential of –0.065 V near to onset potential compared to the open-circuit potential to show the HER performance of the photo catalyst. The sweeping frequency was applied from 0.1 Hz to 250 kHz with a 10mV AC voltage.

RESULTS AND DISCUSSIONS Material characterizations. The 2D-TMD material uniformity is one the most important parameters that has a direct effect on the device performance. Various methods, such as epitaxial growth, Li-intercalation, sputtering, exfoliation, chemical vapor deposition, and thermolysis have been introduced to fabricate large-scale and uniform WS2 and MoS2.13-14, 1718, 22

For instance, Dumcenco et al. demonstrated the formation of mono-layer MoS2 via

epitaxial growth.22 However, the random triangle-shape formation of MoS2 with an uncontrollable orientation can limit the reliability of the process and commercialized application. FE-SEM was carried out to evaluate the morphology of the WS2 thin film on the p-Si substrate, as shown in Figure 2a. From the FE-SEM image, the as-synthesized WS2 film appeared uniform, pinhole-free and continuous on the entire surface of the p-Si substrate. AFM was utilized to inspect the morphological and topological characteristics and thickness of the WS2 thin films. Figure 2b shows the obtained thickness (38 nm) using 20-nm deposited WO3 on p-Si, incorporating (NH4)2WS4 and WO3 layers to form the WS2 thin film. The roughness of WS2 was 0.3 nm for the 20-nm deposited WO3 on p-Si (see Figure 2c), which, to the best our knowledge, is an improvement compared to other studies.18, 23 The results confirmed that 7 ACS Paragon Plus Environment


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the obtained WS2 thin film on p-Si using this method was pristine and ultra-clean, resulting in a high-quality p-n heterojunction and enhancing the device performance. Furthermore, AFM was performed under other conditions to evaluate the roughness and thickness (see Figure S1 and S2), confirming the controllability of the process. Previous report suggested the average height of monolayer WS2 is measured to be approximately

0.9 nm.24 Therefore, the layer

number of WS2 thin films were determined by the AFM results as shown in Figure S1. The results demonstrated that the thickness and roughness of WS2 could be easily controlled by the thickness of the deposited WO3. The thicknesses of WS2 were approximately 9.5, 16, 24, 38, 47.5, and 57 nm for the deposited 5, 10, 15 ,20, 25, and 30-nm WO3 on p-Si, respectively (see Figure S1, S2, and 2d). Moreover, Figure S3a-f indicate the FE-SEM images of the assynthesized WS2 on p-Si with different thicknesses (9.5, 16, 24, 38, 47, and 57 nm). To clarify the high purity of the 2H-WS2 crystal, an X-ray diffraction (XRD) measurement was used (see Figure 3a). The corresponding sharp peaks were located at 14 and 41.2°, which were attributed to the (002) and (103) planes, respectively.25 From the XRD data, three sharp domain peaks were detected, which were ascribed to the high crystallinity of the WS2 film on p-Si. However, some small peaks in 57nm of WS2 were observed which is attributed some formation of WOx as a result of imperfection and lack of sulfur during process, in which the quality of crystal can be improved by giving more time to synthesis process and increasing the amount sulfur powder. Figure 3b shows the TEM image of the microstructure of the WS2 thin film and there are likely some wrinkles on copper grid. HR-TEM images are shown in Figure 3c,d. Obviously, there are predominantly a-domains (layer-by-layer stacked) and c-domains (vertically aligned) in (001) orientation of the 2H-WS2 phase (see Figure 3c,d). In addition, from some a-domain regions in WS2 thin film, Moire´ fringes were observed in a-domains (in-plane),in which it is defined as rotation between two basal planes (see Figure S4). The active sites for hydrogen evolution reaction (HER) are located at the c-domains due to existing dangling bonds at the surface.16 Therefore, the presence of vertically aligned c-domains of the 2H-WS2 thin film is 8 ACS Paragon Plus Environment

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the main merits of the proposed approach, suggesting higher active sites for HER. The hexagonal crystal structure of WS2 was observed with the [002] and [103] orientations (see inset of Figure 3d), which was confirmed by the XRD results. Figure 4a shows the Raman spectra of the WS2 thin films with different thicknesses. There were two vibrational modes of A1g and E12g ascribed to the out-of-plane and in-plane orientations, respectively.26 The distance between A1g and E12g increased as the thickness of the WS2 film increased. This phenomenon was attributed to becoming stiff of the A1g mode and relaxing of the E12g mode.18 Therefore, the layer number of WS2 thin film can be efficiently determined by the Raman frequency difference between A1g and E2g modes. Figure S5 shows the difference in A1g and E2g modes as a function of layer number. XPS was performed to study the elemental component and atomic ratio, as shown in Figure 4b. and it confirmed the existence of W and S and the formation of the WS2 film. In addition, the proportion of W to S was 29.58% to 70.42%, which agreed with the WS2 structure (see inset of Figure 4b). To study the effects of the (NH4)2WS4 precursor on the formation of single crystal WS2 on p-Si, XPS was performed on the WS2 prepared with and without the (NH4)2WS4 precursor. As a result, no peaks corrospondant of W or S were detected due to being very weak bonds between the WO3 layer and the p-Si substrate (see Figure 4c, brown and red curves). On the other hand, the W 4f and S 2p peaks were strongly observed after deposition of the (NH4)2WS4 precursor on WO3/p-Si. The (NH4)2WS4 precursor assists in the formation of WS2 and probably performs as a shielding layer to prevent evaporation of the WO3 layer during thermolysis. Figure S6 indicates the high resolution of W4f peaks of all thicknesses of WS2. As can be seen from Figure S6, W4f7/2 (oxide) is located at about 36 eV. As the oxide peaks are so small compared with two dominant peaks (W 4f5/2, W 4f7/2 ), therefore those oxide peaks can be ignored, suggesting almost fully transformation of WO3 to WS2.

Photoelectrochemical properties. To confirm the applicability of this method, the WS2/p-Si photocathode was fabricated. The as-fabricated photocathode was utilized as the working 9 ACS Paragon Plus Environment


electrode in an electrochemical cell with 0.5 M sulfuric acid as the electrolyte under simulated solar irradiation (100 mW/cm2). Figure 5a shows the photocurrent onset potential reversible hydrogen electrode (RHE) versus the current density of the different sample conditions. The successful fabrication of the WS2/p-Si heterojunction was confirmed, and the heterojunction behaved like a p-n junction. The dark current is the gray dashed line. Because of the cost effectiveness, suitable band gap, and crystallinity of Si, the bare p-Si is frequently utilized for solar hydrogen production.18, 27 However, to absorb the H2 ions on the p-Si surface, a large voltage should be used. Thus, combination of a catalyst, such as WS2 with a conventional semiconductor such as p-Si, is required to achieve an efficient hydrogen production performance. Here, WS2 (38 nm)/p-Si showed the highest performance with the current density benchmark of –9.8 mA/cm2 at 0 V and the onset potential of +0.022 V, which is better than that of bare p-Si with 0.1 mA/cm2 at 0 V and the onset potential of –0.34 V. The thickness of WS2 thin film plays a vital role in optimizing performance. As a result, the current density at 0V was reduced by exceeding the WS2 thickness from 38nm to higher thicknesses which is attributed to its lower film transmittance and the light absorption in the underlying pSi. To evaluate the catalytic activity and mechanism of the WS2 layers, the WS2 thin films were synthesized on glassy carbon (GC) substrate. Figure 5b shows linear sweep voltammetry (LSV) curves of WS2 thin films prepared on glassy carbon substrate and (inset) Tafel slopes for WS2 thin films obtained from LSV curves. From the electrochemical HER measurement results, the WS2 (38 nm)/p-Si sample can show the highest catalytic activity due to having a numerous active sites in the 38nm-thick WS2 compared with the lower thicknesses. WS2 (38 nm)/p-Si showed the lowest Tafel slope of 92 mV/dec, corresponding to the Volmer-Heyrovsky electrochemical mechanism (see Figure S7).28 The results state that optimization of the thickness of WS2 plays a vital role in achieving the highest performance, in which the critical thickness for the best performance is 38 nm-thick WS2.


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Due to its controllability of the synthesis method, the WS2 thin film can be easily optimized. EIS is another method used to evaluate the photocatalytic activity. The small circle in the EIS Nyquist plot is the figure of merit for the HER activity, as shown in Figure 5c. From the EIS results, the smallest circle obtained was the WS2 (38 nm)/p-Si sample. The fastest movement of the electrons between the electrolyte and the working electron was obtained using the optimized WS2 (38 nm)/p-Si photocathode. In the inset of Figure 5c, the equivalent circuit included the charge-transfer resistance (Rct) and capacitors, in which R1, R2, and R3 represent the resistance of contact/p-Si, p-Si/WS2, and WS2/electrolyte, respectively. Here, R3 was 88.5 Ohm cm2 for the optimized WS2 (38 nm)/p-Si photocathode, which was in agreement with the highest current density as shown in Table S1. Because the WS2 thin film is transparent, the incident of solar light passed through and absorbed into p-Si, thereby producing hole-electron pairs and current generating through electrons and moving to the WS2 thin film and electrolyte. Figure S8 illustrates the UV-Vis absorbance of WS2 film with difference thickness. There is an exitonic peaks related to the transformation of direct gap near K point at the Brillouin zone which is in a good agreement with 2H-WS2 semiconductor published previously.18-19,

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Ultraviolet photoelectron spectroscopy (UPS) measurement was used to analyze the band diagram and energy levels. The energy diagram of the optimized WS2/p-Si heterojunction is shown in Figure 5d. From the UPS results, the work functions were 4.45 and 4.73 eV for the optimized WS2/p-Si and p-Si, respectively (see Figure S9). As shown in Figure 5d, for the optimized WS2, the fermi level is located lower than the conduction band (Ec), demonstrating an n-type semiconductor. In fact, equalization of fermi level and band-bending were formed via n-WS2/p-Si heterojunction. Therefore, photogenerated electrons can be easily transferred from p-Si to the n-WS2 layer. In addition, the proximity of Ec in WS2 to the reduction potential (EH+/H2 = 4.5 eV) can facilitate electron transfer from WS2 to the electrolyte without an obstacle.18 The stability of the as-optimized WS2 (38 nm)/p-Si photocathode was examined for 20 cycles to evaluate the durability of the device for photoelectrochemical HER (see Figure 11 ACS Paragon Plus Environment


S10a). Current density shifted after 20 cycles >YA = ±1.2 mA/cm2), which can be neglected and confirmed excellent stability of the as-fabricated WS2 (38 nm)/p-Si photocathode for solar hydrogen production. In addition, to clarify the passivation effect of the WS2 thin film, the stability of the photocathode was evaluated using a fritted Pt counter electrode, which is quite stable in the acidic electrolyte (see Figure S10b). For the optimized WS2/p-Si photocathode, a considerable degradation current density was not observed after 40 hours, representing that the as direct-synthesized WS2 thin film performs as not only a photocatalyst for hydrogen evolution reaction but also a remarkable passivation layer that preserves the WS2/p-Si photocathode from photocorrosion.16 Gas chromatography measurement was used to examine the faradaic efficiency and the generation H2. As a results, the as-optimized WS2/p-Si generated about 6.3 Z

with exhibited high faradaic efficiency of 86 ± 5 % as shown in Figure S10c-d.

Therefore, the optimized device prepared by a direct synthesis process can be applied to HER activity in acidic solutions with high stability and low degradation under solar light. Therefore, the proposed method can provide reliable and facile fabrication of a 2D material-based p-n junction and can be efficiently applied to electronic devices. Table S2 shows a comparison between the I-V parameters of the as-direct synthesized WS2/p-Si heterojunction photocathode with that of previously reported other similar design based on 2D material /p-Si CONCLUSIONS In conclusion, we introduced a controllable method to directly synthesize WS2 on p-Si. The thermolysis process was an efficient method for synthesizing WS2. However, to deposit the (NH4)2WS4 precursor on non-oxide substrates, a super hydrophilic surface is required. Therefore, deppostion of WO3 was added as a prestep to transform the p-Si wafer from a lowhydrophilic to super-hydrophilic surface, making it suitable for a precursor coating. Therefore, the as-fabricated (NH4)2WS4/WO3/p-Si was converted to WS2/p-Si via a thermolysis process. In addition, the WS2/p-Si heterojunction was applied to solar hydrogen production to demonstrate the suitability of the method. Because of the significant advantages of wafer-scale 12 ACS Paragon Plus Environment

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thickness, controllability, excellent crystallinity, and high reproducibility, the proposed method could revolutionize 2D electronic industries in the near future.

ACKNOWLEDGMENTS

This research was supported by the Bio & Medical Technology Development Program (2018M3A9H1023141); Creative Materials Discovery Program through the NRF funded by Ministry of Science and ICT (2017M3D1A1039379); and the Basic Research Laboratory of the NRF funded by the Korean government (2018R1A4A1022647).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: AFM images and height profiles of the as-fabricated WS2/p-Si samples, FE-SEM images of the directly synthesized WS2 thin films on p-Si with different thicknesses, TEM images of Moiré fringes from in-plane rotation between two basal planes, Difference in peak frequencies of Raman modes as a function of layer number, high-resolution of W4f XPS peaks, Tafel slopes of WS2 thin films on glassy carbon, the values of charge transport resistances, UV-Vis absorbance spectra, UPS spectra, Stability test for the as-fabricated n-WS2 /p-Si photocathode, comparison of the performances between our work heterojunction photocathode and other similar 2D materials on p-Si photocathodes, Video file is presented in the ESI for showing our PEC measurement system and device operation.

REFERENCES (1) Hasani, A.; Tekalgne, M.; Van Le, Q.; Jang, H. W.; Kim, S. Y. Two-dimensional Materials as Catalysts for Solar Fuels: Hydrogen Evolution Reaction and CO2 Reduction. J. Mater. Chem. A 2019, 7, 430–454. (2) Hasani, A.; Nguyen, T. P.; Tekalgne, M.; Van Le, Q.; Choi, K. S.; Lee, T. H.; Park, T. J.; Jang, H. W.; Kim, S. Y. The role of Metal Dopants in WS2 Nanoflowers in Enhancing the Hydrogen Evolution Reaction. Appl Catal A Genl 2018, 567, 73-79. 13 ACS Paragon Plus Environment


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(b)

(a) Intensity (arb. units)

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57 nm 47 nm 38 nm 24 nm 16 nm 9.5 nm

(002)

(103)

500 nm

5 10 15 20 25 30 35 40 45 50 55 60

2; (degree)

(c)

(d) a-domains

c-domains

002

103

5 nm

2 nm

10 nm

Figure 3. (a) XRD spectra of the as-fabricated WS2 with different thicknesses of WS2, (b) HRTEM measurement of the as-fabricated WS2 (38 nm), (c) c-domains with vertically stacked (001) planes, (d) a-domains with horizontally stacked (001) planes obtained from the WS2 crystalline thin films.


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p-Si Heterostructure Photocathode Using Direct

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