High-Concentration Niobium-Substituted WS2 Basal Domains with

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High-Concentration Niobium-Substituted WS2 Basal Domains with Reconfigured Electronics Band Structure for Hydrogen Evolution Reaction Mei Er Pam, Junping Hu, Yee Sin Ang, Shaozhuan Huang, Dezhi Kong, Yumeng Shi, Xiaoxu Zhao, Dechao Geng, Stephen J. Pennycook, Lay Kee Ang, and Hui Ying Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08232 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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ACS Applied Materials & Interfaces

High-Concentration Niobium-Substituted WS2 Basal Domains with Reconfigured Electronics Band Structure for Hydrogen Evolution Reaction Mei Er Pam1,2, Junping Hu1, Yee Sin Ang1 ,2, Shaozhuan Huang1, Dezhi Kong1, Yumeng Shi 1,3, Xiaoxu Zhao4, Dechao Geng1, Stephen J. Pennycook5,6, Lay Kee Ang1,2, Hui Ying Yang1* 1Pillar

of Engineering Product Development, Singapore University of Technology and Design, 8

Somapah Road, Singapore 487372, Singapore. 2Science and Math Cluster, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore. 3International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technology of Ministry of Education, Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. 4Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore. 5Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore. 6NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 13 Centre for Life Science, #05-01, 28 Medical Drive, Singapore 117456, Singapore. KEYWORDS: Nb doping, Tungsten disulfides, Hydrogen evolution, Chemical vapor deposition, Band structure

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AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] (H. Y. Yang).

ABSTRACT: Extrinsically controlling the intrinsic activity and stability of two-dimensional (2D) semiconducting materials by substitutional doping is crucial for energy-related applications. However, an in-situ transition metal doping strategy for uniform and large-area chemical vapor deposited 2D semiconductors remains a formidable challenge. Here we successfully synthesize highly uniform niobium substituted tungsten disulfides (Nb-WS2) monolayer, with a doping concentration of nearly 7 % and sizes reaching 100 μm, through a metal dopant precursor route, using salt-catalyzed chemical vapor deposition (CVD). Our results reveal unusual effects in the structural, optical, electronic and electrocatalysis characteristics of Nb-WS2 monolayer. The Nb dopants readily induce a band restructuring effect, providing the most active site with a hydrogen adsorption energy of 0.175 eV, and hence greatly improving its hydrogen evolution activity. The combined advantages of the unusual physics and chemistry by in-situ CVD doping technique open the possibility in designing 2D-material-based electronics and catalysts of novel functionalities.


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INTRODUCTION: Transition metal dichalcogenides (TMDCs) monolayers have attracted intense interests owing to their unique layer-dependent electrical and optical properties, electrocatalytic active properties, and crystal structure tunability, which show great potential for electronics, optoelectronics, valleytronics, spintronics and catalysis applications.1-5 However, the field effect transistors based on layered TMDCs is often plagued by issues such as low field effect mobility and low on/off current ratio due to the high contact resistance at the 2Dsemiconductor/bulk-metal interface.4,5 Other complications such as the limited understanding in charge transport mechanism and the difficulty of achieving orientation-controlled growth in atomically thin 2D TMDCs have posed additional challenges to the development of industrialgrade applications. Recently, tremendous efforts have been devoted to solving the aforementioned problems. The most common strategy to reduce the contact resistance at the 2Dsemiconductor/bulk-metal interface is by fabricating low resistance Ohmic contact 2D van der Waals heterostructure. The van der Waals integration eliminates the lattice mismatch effect and enables great design flexibility in creating devices with custom-made functions. However, the synthesis of large-area 2D van der Waals TMD-based metal-semiconductor heterostructure remains highly challenging due to the chemical instability of atomically thin metallic TMDCs and the limited understanding of the CVD growth mechanism.6-8 Recent efforts based on phase transition surface molecular doping, and in-situ atomistic substitutional doping in 2D semiconductors have been shown to allow enhanced control of the types of majority charge carriers and also to achieve modulation of their band structure, which are the key steps towards high-performance 2D-semiconductor-based electronics, optoelectronics and electrocatalysis devices.9-12 The different localization of the atomic d-orbitals in transition metal dopants plays a critical role in spatial delocalization of their discrete energy levels,


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particularly at high doping concentration, resulting in bandgap modification and redistribution of the density of states (DOS).13, 14 Such heavy doping induced band restructuring becomes more significant in reduced dimensionality and directly leads to many exotic properties. Recent theoretical studies also predicted modifications of the electronics, optical, magnetic and catalytic properties with transition metal substitution in 2D TMDCs.15, 16 For examples, Suh et al. have reported Nb doping-induced structural conversion from 2H to 3R stacking in the MoS2 layered structure, accompanied by a renormalization of the valence band structure.14 Recently, Feng et al. have also demonstrated the reduction of Schottky barrier by novel Nb doping in WS2.17 Besides, Jin et al. have further revealed that the degenerate p-type semiconductor behavior of Nb doped WS2 field effect transistors.18 However, high p-doping concentration in WS2 with bandgap modification and redistribution of the density of states (DOS) have rarely been reported. In addition, the lack of controllable synthesis of large-scale high doping concentration in TMDCs monolayers seriously impedes their large-scale applications.19-21 To date, various methods have been employed to produce large-scale TMDCs monolayers, including liquid exfoliation, mechanical exfoliation, chemical vapor deposition (CVD), etc.22, 23Among

these, CVD is the most promising technique for large-scale synthesis of uniform mono-

or multilayer TMDC materials.24-26 For example, various optimized CVD techniques, such as salt-assisted CVD, oxygen-assisted CVD, a separated-induction-growth stage CVD, CVD-grown on a molten glass substrate, sodium-catalyst-assisted CVD, have been successfully applied to synthesize millimeter-scale TMDCs monolayers.24-28 These methods commonly employed the following three strategies, namely: 1) the use of highly reactive metal precursor; 2) the introduction of salt melt synthesis; and 3) the point-to-face metal-precursor supply method.


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In this work, we propose a direct synthesis of Nb-substituted WS2 (Nb-WS2) monolayer through a Nb face-to-face metal dopant precursor route, using salt-catalyzed CVD method. Interestingly, highly uniform p-doped Nb-WS2 monolayer, with an Nb doping concentration of nearly 7 % and a domain size larger than 100 μm was obtained. We observe uncommon effects of degenerate Nb doping in the optical and electronics characteristics of WS2. Our results further reveal that a high concentration direct substitution of Nb atoms for W atoms results in delocalization of the dopant wavefunction and band restructuring, thereby converting the naturally semiconducting property to a semi-metallic property as confirmed by first-principle density functional theory (DFT) simulation and electrical measurement based on a field-effect transistor configuration. Such enhanced metallic characteristics results in an enhanced catalytic activity in hydrogen evolution reaction (HER) compared to the pristine WS2. Our work provides new insights on the electronic and catalysis properties of Nb-doped WS2 monolayer due to the in-situ substitution of transition metal atoms in TMDCs monolayers. The findings presented here shall pave the way for the development of CVD-based TMDCs devices, such as high-efficiency catalyst and van der Waals heterostructures of novel functionalities for various applications. In order to achieve large-area and highly uniform monolayer Nb-WS2 crystals, we explored the combination of a unique face-to-face metal doping method and molten-salt-assisted CVD process. As illustrated in Figure 1a, a Nb oxide foil with dispersed WO3 and NaCl powders was located below the sapphire substrate in a parallel geometry with a 5 mm gap. This configuration is to ensure uniform Nb substitution in the WS2 monolayer. The Nb foil was first oxidized in air at 500 ℃ for 10 mins to form a Nb2O5 oxide layer (Figure S1) as the Nb2O5 layer has a much lower sublimation temperature of about 1512 ℃ compare to pristine Nb foil. In addition, a trace amount of NaCl (3 mg) was used as a low-cost and effective solvent to produce scalable large-


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area TMDCs monolayers. It is worth noting that the molten NaCl has been utilized as an ideal solution medium for the large-scale production of TMDC materials under atmospheric pressure in previous reported works.28, 29, 30 As shown in Figure 1b, reaction at 830 ℃ for 6 mins results in a yield of Nb-WS2 monolayer with crystal size more than 100 µm, demonstrating a fast and efficient CVD. The atomic force microcopy (AFM) line scan at the edge illustrates that the step height is ∼1 nm (Figure S2), indicating the monolayer nature.

Figure 1. Synthesis of niobium doped tungsten disulfide (Nb-WS2) monolayer. (a) Experimental chemical vapor deposition (CVD) set up for the synthesizing of large-size and highly uniform Nb substituted WS2. (b) Optical image of CVD-grown Nb-WS2 monolayer. (c) Schematic illustrations of the in-situ substitutional doping of Nb atom at the tungsten site by face-to-face Nb foil dopant salt-assisted CVD process. Figure 1c illustrates the growth mechanism of Nb-WS2 monolayer. Firstly, the furnace temperature slowly increased to 830 ℃ under a mixed gas of 200 sccm Ar and 24 sccm H2. At 800 ℃, NaCl powder melts, and reacts with the protective oxide layer to form volatile chloride or oxychloride, resulting in an accelerated corrosion attack known as hot corrosion.31 Generally, these intermediate metal chlorides are regarded as active transition metal species for the CVD


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synthesis of TMDCs. In addition, according to the previous fundamental study,31 WO3-NaCl at 900 K shows a faster weight loss rate as compared to that of Nb2O5-NaCl at 1000K, indicating the less expulsion of volatile niobium chloride with the same mole fraction of NaCl in the reaction mixture. The different expulsion rate of volatile metal chloride with the same mole fraction of NaCl in the reaction mixture will decide the metal dopant species. Since tungsten oxide is more chemically reactive to NaCl than niobium oxide, it will promote a rapid formation of WS2 monolayer compared to NbS2 monolayer and result in the formation of Nb-WS2 monolayer rather than tungsten substituted niobium disulfide. Further, the DFT calculations reveal that the Nb substitution configurations such as single Nb atom substitution (single), two Nb atoms substitution (dimer) and three Nb atoms substitution (trimer-I, trimer-II, and trimer-III) demonstrate a strong tendency to substitute W with a negative exothermic energy in the range from -0.505 eV to -0.665 eV (See Figure S3). This results also reflect the possibility of other dopant configurations besides single point dopants. To confirm the successful Nb-substitution of WS2 monolayer, aberration-corrected scanning transmission electron microscope (STEM) was employed to compare the crystal structures of the transferred WS2 and Nb-WS2 monolayers on the TEM grid (See Figure 2 and S4). In Figure 2a, a STEM image of pristine WS2 is shown where bright spots originate from W with the two stacked S atoms are barely visible in between, revealing the nearly defect-free trigonal prismatic geometry WS2 monolayer. Compared with pristine WS2 monolayer, the Nb-WS2 sample exhibits obviously dimmer spots at the tungsten sites, corresponding to the Nb dopants (See Figure 2b). The larger scale Z-contrast image (Figure 2c) provided shows the doping to be highly uniform. A structural model (Figure 2d) based on the HAADF-STEM image shows the Nb and W sites in green and yellow atoms, respectively. Besides, it should be noted that the Nb atoms substitute the


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W atom sites directly from the observed line intensity scan profile (Figure S4), where the

Figure 2. Comparison of atomic structure of WS2 and Nb-WS2 monolayers. (a-b) Annular Darkfield scanning transmission electron microscopy images (ADF/STEM) of pristine WS2 and NbWS2 monolayer, respectively. (c) Larger scale Z-contrast image of STEM image for Nb-WS2 monolayer. (d) Structure model obtained from Figure 2c demonstrating the Nb dopant’s distribution (White, W sites; Green, Nb site; black hole, metal vacancy). The average Nb concentration is estimated as ~7.3 % and the doping concentration in some local regions is nearly 10 %. (e) Atomic model corresponding to the enlarged view of the white square box in (d). (f) Atomic resolution ADF/STEM images of various Nb substitution configurations present in Nb-WS2 monolayer, including single Nb atom substitution (single), two consecutive Nb atoms substitution (dimer) and three consecutive Nb atoms substitution (trimer-I, trimer-II, and trimerIII). Scale bar: 0.5 nm. (g) Processed atomic resolution ADF/STEM image of various Nb substitution configurations present in Nb-WS2 monolayer. (h) Relaxed structural models of the experimentally observed Nb-WS2 configuration.


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experimental intensity ratio of W atom to the Nb atom at the W site is nearly 1.5 times, showing good agreement with previous reported result.32 Furthermore, by counting individual sites of the Nb and W atoms, the Nb concentration is estimated as 7.3% and the doping concentration in some local regions is nearly 10 %. The enlarged view based on Figure 2e-h further reveals various Nb substitution atomic arrangements in Nb-WS2 monolayer, such as a single Nb atom substitution (single), two Nb atoms substitution (dimer) and three Nb atoms substitution (trimerI, trimer-II, and trimer-III), showing good agreement with our DFT calculation on the Nb substitution configurations on WS2. Besides, transition metal vacancy defects indicated by the dark hole in the STEM figures are also observed in the as-grown Nb-WS2 crystals. Raman and photoluminescence (PL) characterizations were further employed to examine the Nb doping effects on the chemical composition and local optical properties of the as-grown samples. As shown in Figure 3a, the Raman spectrum of WS2 exhibits two characteristic peaks of WS2 monolayer, corresponding to the E2g (351 cm-1) and A1g (416 cm-1) vibration mode, respectively.33,

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An additional weak Raman mode at 381 cm-1 was observed, which may be

assignable to the A1g mode of NbS2.17, 35, 36 After Nb substitution, the Nb-WS2 shows a detectable upshift of the A1g Raman peak and an increased A1g/E2g peak ratio, further suggesting the p-type doping effect induced by Nb substitution.17, 36 As shown in Figure 3b, the experimental PL of Nb-WS2 monolayer shows a high intensity, broader and lower energy PL peak position in contrast to that of pristine WS2 monolayer. The PL peak shift is generally induced by the doping and dependent on the gradient in doping concentration.18, 39 Therefore, the almost uniform spatial dependence of PL peak position in the Nb-WS2 monolayer, together with its narrow spread (1.795-1.803 eV) confirms the uniform doping concentration (see Figure S5).


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Figure 3. Optical and structural characterization of WS2 and Nb-WS2 monolayers. (a-b) Comparison of Raman and photoluminescence (PL) spectra of WS2 and Nb-WS2 monolayer crystals (c-d) XPS spectra of (c) Nb 3d core level region and (d) W 4f core level region core level region for pristine WS2 and Nb-WS2 monolayers. X-ray photoelectron spectroscopy (XPS) was performed on the pristine WS2 and Nb-WS2 to further confirm the Nb substitution (Figure 3c-d and S6). As illustrated in Figure 3c, the XPS spectra show the Nb 3d core level of the WS2 and Nb-WS2. The presence of peaks at 206.7 eV and 209.4 eV in Nb-WS2 sample indicates the Nb substitution in WS2. In Figure 3d, the pristine WS2 shows dominant peaks at 38.0 eV and 35.8eV, which are attributed to W 4f5/2 6+ and W 4f7/2


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6+,

indicating a large amount of WO3 in these as-grown monolayers. While the W4f dominant

peaks shift from W6+ rich WS2 to W4+ rich Nb-WS2 with two dominants peaks at 34.5 eV and 32.4eV, which can be attributed to the efficient reduction of the WO3 precursor and conversion to WS2 in the presence of hydrogen gas during the synthesis. To confirm the experimental results, the Nb atom substitution effects on the electronic band structure of WS2 monolayer were studied using the crystal structure in Figure 4a. As shown in Figure 4b, it can be observed that the conduction band minimum (CBM) and the valence band maximum (VBM) are at the same high symmetry K point for monolayer WS2, indicating that monolayer WS2 is a direct band gap that agree well with previous reported works. The calculated band gap is 1.811 eV. This value is slightly smaller than our measured PL peak position due to the choices of plane-waves cutoff and k-point grid. When the W atoms of pure WS2 is substituted by one Nb atoms, the calculated band structure (0.4%) shows significant band restructuring in monolayer Nb-WS2, suggesting a strong band-anticrossing interaction. Since Nb has one less valence electron as compare to six for W, an extra electron is removed from the doped system. This results in the creation of new electronic states near the Fermi level and an upshift of the VBM (see Figure 4c). The new electronic states near the VBM introduced by Nb-doping results in an enhanced, broad band and redshifted PL emission at room temperature (RT) from the WS2 monolayer (Figure 3b). As shown in Figure 4 and S7, the DFT calculation reveals the electronic band structures under various types of Nb substitution in the supercell. In the presence of Nb substitution, the band gap remains direct at the K and K’ points. A significant band gap reduction is observed, i.e. from 1.811 eV of pristine WS2 to 1.73 eV, 1.608 eV, 1.576 eV, 1.572 eV and 1.573 eV for single, dimer, trimer-I, trimer-II and trimer III Nb doped configuration, respectively. The obtained results are consistent with the formation energy calculation for various Nb doping concentration on the presence of Nb clusters


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observed in experiments by Jin et al.18 This suggests that the increasing Nb doping concentration leads to the upshifting of the VBM. Importantly, the Fermi level dwells into the upshifted VBM as a result of the Nb-doping, thus indicating a highly p-doped nature of the Nb-WS2. In addition, the optical absorption spectra of the WS2 and Nb-WS2 monolayer are presented in Figure S8 There are two characteristic WS2 absorption peaks at 614 nm (2.01 eV) and 514 nm (2.42 eV) arising from the direct transition from valence band to conduction band at the K point of the Brillouin zone were observed for pristine WS2.40 While the two adsorption characteristic peaks of Nb-WS2 have shown a redshift compared to the pristine WS2, further showing good agreement with the obtained PL characterization and DFT calculation. To further understand the electronic properties of Nb-WS2, a back-gated field-effect transistor device was fabricated based on Nb-WS2 monolayer (See Figure 4d for detailed device schematics). The source-drain current voltage characteristics exhibits non-rectifying behavior even at zero gate-voltage Vg, thus indicating the formation of Ohmic contact between Nb-WS2 and the gold electrode due to the highly p-doped nature of the monolayer (see Figure 4e and f). This result shows good agreement with previous report on Nb-WS2 FET devices.18 In addition, this result is in strong contrast to the pristine monolayer WS2 where a rectifying Schottky contact is typically formed at the WS2/gold interface.34,

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To summarize, the Nb-doping of WS2

monolayer leads to: (i) the reduction of the direct bandgap at K and K’ points; (ii) the creation of doping-induced electronic states near the VBM which enhances PL emission; (iii) the upshifting of the VBM that results in highly p-doped characteristic; and (iv) the formation of non-rectifying Ohmic contact at the Nb-WS2/gold interface which can be beneficial for electronic and optoelectronic applications.


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Figure 4. Effect of Nb substitution configurations on the electronic band structure and electrical properties of WS2 monolayer. (a) Crystal structure of 2H-WS2 and Nb-WS2 (b) DFT calculated electronic band structures of pristine 2H-WS2 and single Nb substituted WS2 based on the 5 × 5 supercell. (c) Schematic band structure of monolayer WS2 and Nb-WS22. (d) Schematic diagram of Nb-WS2 device. (e, f) Output characteristics and transfer characteristics of Nb-WS2 monolayer device. The results demonstrate that Nb-WS2 device exhibits linear ID-VD relationship, indicating its ohmic like contact behavior (more metallic properties).


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Lamellar TMDCs have been regarded as a promising catalysts towards HER because of their


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layer-dependent electronic properties that can vary from metallic to semiconducting behavior. In


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order to evaluate the HER performance of pristine WS2 and Nb-WS2 monolayers, Nb-WS2 and


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WS2 samples were first transferred onto the indium tin oxide (ITO) coated glass by a wet


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chemical transfer method. Then, the HER performance of Nb-WS2 and WS2 nanosheets was


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systematically performed using a three-electrode system in a solution of 0.5 M H2SO4. As


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Figure 5. HER performance of WS2 and Nb-WS2 nanosheets supported on ITO substrate. (a) LSV curves of Nb-WS2, WS2 nanosheets, bare ITO and 20% Pt-ITO in 0.5 M HsSO4 for HER. (b) Tafel plots obtained from the LSV curve for Nb-WS2 and WS2 nanosheets. (c) Polarization curves of Nb-WS2 nanosheets obtained before and after 48-hour scan, respectively. (d) Durability measurements for Nb-WS2 and WS2 nanosheets. (e) 5 x 5 monolayer WS2 supercell with different configuration of Nb dopants. (f) Calculated free energy diagram of the HER performance of different Nb dopant configurations based on the 5 x 5 supercell shown in (e). presented in Figure 5a, the Nb-WS2 nanosheets have a lower overpotential value of 222 mV (vs ACS Paragon Plus Environment

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RHE) at a current density of 6 mA cm-2, against 487 mV in the pristine WS2 nanosheets. Besides, it is worth emphasizing that the Nb-WS2 exhibits a much lower Tafel slope (97 mV/dec) as compared to the WS2 nanosheets (164 mV/dec), further showing the enhanced HER performance of the Nb-WS2 material (see Figure 5b). The Tafel slope measured for WS2 nanosheets matches well with previous reports (154-190 mV/dec).42 In addition, the corresponding Tafel plots demonstrate the transition of the rate determining step away from the Volmer discharge reaction (H3O+ + e-→ Hads + H2O) with Nb-WS2 nanosheets. By comparing the overpotential at 6 mA cm−2, it can be observed that the Nb-WS2 electrode shows a more superior HER performance (Table S1) as compared to atomically thin WS2 or other TMDC-based catalysts. Since the Nb doping of the WS2 monolayer results in a more conductive characteristic, it provides more accessible electrons and promoting the formation of Hads at the interface. The electrochemical active surface area and favourable kinetics were also verified by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) (Figure S9, S10). The double layer capacitance (Cdl) value of Nb-WS2 nanosheets is about 4 times higher than that of the WS2 nanosheets, indicating that the Nb doping induced modification in both crystal structure and electronics structure accounts for the increased electrochemical surface area. The Nyquist plot at 200 mV (vs RHE) and the fitted equivalent circuit show a reduction in the charge transfer resistance from 20.86 Ω cm2 for WS2 to 14.76 Ω cm2 for Nb-WS2, thus indicating a faster charge transfer and facile HER performance. In addition to exhibiting enhanced HER performance, Nb-WS2 demonstrate good stability for 48 hours with a potential increase of 20 mV (Figure 5c, 5d). As shown in Figure 5d, the overpotential of WS2 was found to increase by over 130 % after 6-hour durability measurement. Both the samples still retain almost the same initial optical characterization such as Raman and PL characterization after stability test (See Figure S11).


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Thus, through Nb substitution, the resulting Nb-WS2 monolayer exhibits much enhanced charge transfer and Hads formation at the interface. Our DFT calculations based on 5 x 5 supercell shown in Figure 5e further reveal that the pristine WS2 has a much weaker hydrogen binding energy of ~2.185 eV as compared to Nb-WS2 (See Figure 5f). By combining the experimental and theoretical calculation results, the enhanced HER performance of the Nb-WS2 can be readily explained as followed. Firstly, the highly p-doped nature of Nb-WS2 leads to significantly improved electrical transport. while retaining its direct band gap nature, which is conducive for more efficient charge transfer and enhanced chemical activity. Secondly, the DFT calculation further reveal that Nb-S sites on the Nb-WS2 monolayer are intrinsically more active with a small ∆GH value of 0.295-0.175 eV which directly leads to enhanced HER performance. Thirdly, the relatively abundant vacancy sites in the Nb-WS2, are catalytically more active than the basal plane of pristine WS2.43The combination of these above three aspects leads to the enhanced HER performance. CONCLUSION: In summary, large-area uniform Nb-WS2 monolayer with high-concentration Nb doping are developed using face-to-face Nb foil doping and salt-assisted CVD process. Our results reveal the emergence of multiple unusual optical, electronic and chemical effects in the Nb-WS2 monolayer. The presence of Nb dopants in monolayer WS2 results in the upshift of the VBM band, which leads to a highly p-doped nature as corroborated by DFT calculation and field-effect transistor measurement. Non-rectifying Ohmic contact is readily formed at the NbWS2/ gold interface, which can be beneficial for efficient charge transfer and enhanced chemical activity. As a result, the semi-metallic Nb-WS2 shows a significant improved HER catalytic performance compared to pristine WS2 nanosheets. Our findings reveal the possibility of finetuning the optical, electronic and chemical properties of TMDC monolayer via doping, thus


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opening up new avenues of integrating 2D semiconductor in electronics, optoelectronics, and catalysts. ASSOCIATED CONTENT Supporting Information. Experimental section, material characterization, DFT calculation and electrocatalytic data Notes The authors declare no conflict of interest. ACKNOWLEDGMENT This work is supported by Singapore Astar AME IRG A1783c0011 grant. Y. S. acknowledges the support from the Science and Technology Project of Shenzhen (JCYJ20170817101100705), the Thousand Young Talents Program of China, the National Natural Science Foundation of China (Grant No. 51602200, 61874074) and the (Key) Project of Department of Educational Commission of Guangdong Province (Grant No. 2016KZDXM008). This project was supported by Shenzhen Peacock Plan (Grant No. KQTD2016053112042971). S. J. P. thanks the National University of Singapore for funding and MOE for a Tier 2 grant “Atomic scale understanding and optimization of defects in 2D materials” (MOE2017-T2-2-139).


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High-Concentration Niobium-Substituted WS2 Basal Domains with

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