Activation of nitrogen species mixed with Ar and H2S plasma for directly N-doped TMD films synthesis

Among the transition metal dichalcogenides (TMD), tungsten disulfide (WS2) and molybdenum disulfide (MoS2) are promising sulfides for replacing noble metals in the hydrogen evolution reaction (HER) owing to their abundance and good catalytic activity. However, the catalytic activity is derived from the edge sites of WS2 and MoS2, while their basal planes are inert. We propose a novel process for N-doped TMD synthesis for advanced HER using N2 + Ar + H2S plasma. The high ionization energy of Ar gas enabled nitrogen species activation results in efficient N-doping of TMD (named In situ-MoS2 and In situ-WS2). In situ-MoS2 and WS2 were characterized by various techniques (Raman spectroscopy, XPS, HR-TEM, TOF–SIMS, and OES), confirming nanocrystalline and N-doping. The N-doped TMD were used as electrocatalysts for the HER, with overpotentials of 294 mV (In situ-MoS2) and 298 mV (In situ-WS2) at a current density of 10 mA cm−2, which are lower than those of pristine MoS2 and WS2, respectively. Density functional theory (DFT) calculations were conducted for the hydrogen Gibbs energy (∆GH) to investigate the effect of N doping on the HER activity. Mixed gas plasma proposes a facile and novel fabrication process for direct N doping on TMD as a suitable HER electrocatalyst.

www.nature.com/scientificreports/ We developed a novel strategy to fabricate wafer-scale N-doped 2H-TMD thin films directly using plasma enhanced-chemical vapor deposition (PE-CVD) at low temperatures. Our previous research on the synthesis of 2H-MoS 2 and 2H-WS 2 using Ar and H 2 S plasma has already been reported 19,20 . By extension, 2H-MoS 2 and 2H-WS 2 , composed of numerous N-doped nanocrystals simultaneously, could be obtained, to prepare an excellent HER electrocatalyst by adding nitrogen gas (N 2 ) to Ar/H 2 S plasma (referred to as in situ-MoS 2 and in situ-WS 2 ). In addition, it was observed that the activated N 2 species, confirmed by in-situ optical emission spectroscopy (OES), was the main factor for inducing the effect of N doping. As a result, both in situ-MoS 2 and in situ-WS 2 exhibited enhanced HER activity than pristine TMD, showing overpotentials of 294 and 298 mV at a current density of 10 mA cm −2 , respectively. To fully understand the HER activity as a function of the implanted N atom, theoretical DFT calculations were carried out to investigate the ∆G H on the basal plane of the as-prepared samples depending on the hydrogen adsorption sites. This confirmed that the lower ∆G H with N atoms in TMD is calculated compared to pristine TMD, enhancing HER performance. Finally, this study demonstrates a unique and facile method for developing advanced electrocatalysts.
Materials and methods PE-CVD system. The PE-CVD system is schematically illustrated in Supplementary Fig. S1. An inductively coupled plasma (ICP) generator operating at 13.56 MHz radio frequency (RF) was used to generate the plasma driven by an electromagnetic field. The 550 W power was applied for synthesizing pristine and N doped TMD thin films. The chamber was evacuated to maintain high vacuum by using a turbo pump. The operating temperature was controlled by the chamber heater located under the substrate.
Synthesis of pristine TMD thin films. A 4-inch SiO 2 /Si wafer was cleaned to remove organic contaminants by dipping it in ethanol and DI water with sonication. After cleaning, an E-beam evaporator was used to deposit a transition thin metal film (Mo or W) with a thickness of 1 nm on the substrate. Ar/H 2 S plasma (v/v = 1:1) was applied to the thin metal film in the chamber, which had an operating pressure of ~ 10 -6 Torr and a temperature of 300 °C.

Synthesis of N doped TMD thin films.
The transition thin metal film on the SiO 2 /Si substrate was prepared using the same process. With N 2 gas flowing at 10 SCCM during Ar/H 2 S plasma, TMD thin films were fabricated, and the N dopants were successfully implanted simultaneously.
Characterization. The characterization of all samples was performed using Raman spectroscopy, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), optical emission spectroscopy (OES), and time of flight secondary ion mass spectrometry (TOF-SIMS) techniques. A Raman microscope (Alpha300 M+, WITec GmbH) was employed with an excitation wavelength of 532 nm. XRD (Smartlab, Rigaku) was used to determine the nanograin size as well as the phase identification of all the samples. HR-TEM (JEM-2100F, JEOL) was utilized to determine the structural configurations of the TMD thin films. Poly-methyl-methacrylate (PMMA) transfer method was applied to prepare TEM samples. Firstly, a PMMA layer was spin cast on the TMD film. Then, a diluted HF solution was used to etch the SiO 2 layer and separate the PMMA coated TMD films from the substrate. After transferring onto a carbon-coated copper TEM grid, the PMMA layer was dissolved with acetone to remain only TMD films. Crosssectional TEM images of all samples were also obtained using a focused ion beam (NX2000, Hitachi Ltd.). XPS measurements were conducted to analyze the atomic composition and bonding state using a Thermo Fisher ESCALAB 250 Xi instrument with a Mg Kα X-ray source. The distribution of atoms and molecules in the plasma was investigated using OES (Avantes, Avaspec-2048). The depth profiles of all samples were revealed using TOF-SIMS (TOF-SIMS-5, ION-TOF GmbH).
Electrochemical measurement. All electrochemical analyses of the samples were conducted using a CHI600D electrochemical workstation comprising a three-electrode system. Pt wire and Ag/AgCl saturated with 4 M KCl were selected as the reference and counter electrodes, respectively. The catalysts were directly synthesized on a carbon glass electrode using a rotating disk electrode (RDE) system (Gamry). This electrode was used as the working electrode. All electrochemical tests were performed under the same conditions at a rotation speed of 1600 rpm in a 0.5 M H 2 SO 4 electrolyte solution. The reversible hydrogen electrode (RHE) potential from the measured potential was calculated using the following equation: The Tafel slopes were calculated to assess the intrinsic HER activity of all samples. The equation below was used to fit linear slopes 21 .
where η denotes the overpotential, a denotes the exchange current density, and b is the Tafel slope. A 95% IR compensation was applied for all the potentials in the linear sweep voltammetry (LSV) to consider the solution resistance. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range from 100 kHz to 0.01 Hz at the overpotential of − 0.2 V vs RHE.
(1) www.nature.com/scientificreports/ Density functional theory (DFT) calculation. All calculations were performed using plane-wave-based DFT, as implemented in the Quantum espresso code 22,23 . Perdew Burke Ernzerhof (PBE) generalized gradient approximation (GGA) was used for the exchange correlation function 24 . The effect of van der Waals interactions was applied using the DFT-D3 method. Supercells (4 × 4 × 1) were designed, and a vacuum space with a thickness of 15 Å was constructed to avoid interactions with other environments in the z direction. An energy cutoff of 40 Ry for the plane wave expansion of the wave functions and a density cutoff of 400 Ry were set after the convergence test. A k-point mesh of 4 × 4 × 1 and a convergence threshold of 10 -6 eV were adopted. All atomic coordinates in the supercells were relaxed for structural optimization until the Hellmann-Feynman forces were less than 0.01 eV/Å. The hydrogen Gibbs free energies of the pristine and N-doped TMD were calculated using the following equation: where ∆E H* is defined as the hydrogen absorption energy on the surface, ∆E ZPE is the zero-energy difference, and ∆S H denotes the entropy difference.

Results and discussion
An illustration of the in-situ doping process during the synthesis of TMD is depicted in Fig. 1a. To prepare pristine TMD and in situ TMD, Mo or W thin metal films were deposited with a thickness of 1 nm on a 4-in. SiO 2 /Si substrate using E-beam evaporation. Then, the as-prepared metal films were sulfurized using PE-CVD at a temperature of 300 °C for 90 min. The sulfurization process was executed with a mixed gas of Ar and H 2 S (v/v = 1/1) in accordance with a previous study 19,25 . However, a gas mixture containing high-purity N 2 gas was used to synthesize N-doped TMD thin films. Finally, we obtained a wafer-scale N-doped TMD thin film (Supplementary Fig. S2). Raman spectroscopy measurements were conducted to identify the lattice vibrations of the as-fabricated samples, as shown in Fig. 1b. The two representative bonds of pristine MoS 2 thin films at 380.8 cm −1 and 404.1 cm −1 corresponding to in-plane modes (E 1 2g ) and out-of-plane mode (A 1g ), respectively were discovered. The two major peaks of the pristine WS 2 thin film at 352.5 cm −1 and 416.3 cm −1 were also detected in accordance with other reports 26 . Furthermore, while the E 1 2g and A 1g peaks for in situ-MoS 2 blueshifted to 383.3 cm −1 and 405.4 cm −1 , these peaks of in situ-WS 2 red shifted to the wavenumber of 351.17 cm −1 and 413.8 cm −1 , respectively, because the charge concentration induced by the introduction of dopants brings about a change in these vibrations, such as compression 27 . Raman mapping of in situ-MoS 2 and in situ-WS 2 was investigated to provide spatial distribution of the main peaks corresponding to E 1 2g and A 1g mode by using Lorentz filter management (Supplementary Fig. S3). It indicated that the N doped MoS 2 and WS 2 were fabricated www.nature.com/scientificreports/ uniformly through mixed N 2 + Ar + H 2 S plasma. HR-TEM was carried out to investigate the atomic structural configuration. Compared to the pristine samples, the top-view images of in situ-MoS 2 and in situ-WS 2 were confirmed to maintain the intact nanograin hexagonal 2H phase without any cracks, which could be formed by the post-N doping process (Fig. 1c, d) 28 . The ring diffraction in selected area electron diffraction (SAED) pattern indicates that the prepared samples are polycrystalline ( Supplementary Fig. S4). It can also be speculated that the grain size did not change even if the TMD thin films involved N doping according to the XRD pattern (Supplementary Fig. S5) analysis. The Scherrer equation can be used to deduce the crystallinity and crystalline size 29 . Basically, the smaller the crystalline size, the broader the diffraction peaks 30 . The broad diffraction peak at 10.4°, corresponding to the (002) plane, implies that the as-fabricated TMD thin films were formed with numerous nano-sized grains, in accordance with the TEM image analysis 31 . Furthermore, the cross-sectional TEM images revealed that the synthesized TMD thin film comprised about 4-5 layers and broken layers, where abundant edges were exposed as active sites. Energy dispersive spectrometer (EDS) mapping images for N-doped TMD indicate that the N atoms were incorporated successfully into the TMD samples. XPS measurements were performed to determine the chemical bonds and components of the samples, as shown in Fig. 2a. Pristine MoS 2 thin films have the characteristic Mo 3d core level of typical 2H-MoS 2 at 226.66 eV, 229.43 eV, and 232.59 eV corresponding to the S 2 s, Mo 3d 5/2 , and Mo 3d 3/2 , respectively. In addition, the S 2p core level XPS spectra in Fig. 2b present the major peaks associated with the Mo-S bonding in the lattice of MoS 2 at 162.38 eV and 163.56 eV. These peaks are consistent with those reported in the literature 32 . However, the additional peak at 398.57 eV in Fig. 2c is assigned to the Mo-N bond that was not detected in pristine MoS 2 33,34 . Overall, the in situ-MoS 2 thin film presented a relative peak shift of M 3d and S 2p toward lower binding energies, indicating p-type doping. This indicates that the in-situ process can be regarded as an efficient method to derive N doping on the TMD. This is because nitrogen elements are favorable to combine with Mo rather than sulfur atoms in the lattice of Mo-S 35 . Likewise, Fig. 2d,e also exhibited that pristine WS 2 has two peaks of W 4f core level at 32.84 eV and 34.89 eV and two peaks of S 2p core level at 161.24 eV and 162.4 eV, corresponding to W 4f 7/2 , W 4f 5/2 , S 3p 3/2 , and S 3p 1/2 , respectively 36 . Unlike the case of n-type doped MoS 2 , the N-doped WS 2 sample showed the movement of the peak position toward higher binding energy, suggesting n-type doping. In addition, the appearance of a new peak related to the W-N bond at 398.05 eV in Fig. 2f could be observed, implying that N atoms were occupied by replacing S in the lattice of 2H-WS 2 37 . From this approach, the N doping concentration of in situ MoS 2 and WS 2 thin film ran to almost 9.43 at% and 8.3 at%, respectively, while the pristine samples' doping amount was close to zero percent. www.nature.com/scientificreports/ These effective N doping attributes to the penning excitation effect by the exitance of argon molecules, allowing TMD to effectively change their atomic composition ratio. In general, the higher ionization energy of Ar in the electric field resulted in the activation of inert N 2 to generate vigorous 1st negative series N 2 + and 2nd positive series N 2 * as offshoots 38 . These products were confirmed by OES, which is a powerful tool for revealing the excited species during plasma treatment, as shown in Fig. 3a. N 2 + species could only be observed at a wavelength of 427.5 nm −1 in the case of mixed-gas plasma, but not for simple N 2 plasma treatment. In addition, the plasma processes, except for the normal sulfurization treatment, have a distinct peak at 350 nm −1 corresponding to the N 2 * species 39 . However, when injecting with Ar, the intensity of N 2 * in the spectrum becomes much stronger than that without Ar. This indicates that activated species play a pivotal role in the N-doping of TMD samples. The importance of the excited N 2 * and charged N 2 + for doping or depositing nitride films at low temperatures has been emphasized previously because of their high reactivity [40][41][42] . The concentration of the N dopants in the samples treated with only N 2 plasma is lower than that with facilitating species (Supplementary Fig. S6). Therefore, the generation of active nitrogen species caused by the penning effect in the mixture plasma effectively leads to a favorable combination with TMD. In addition, TOF-SIMS analysis of all samples was conducted to investigate the distribution of the composed atomic bonds with respect to depth, as shown in Fig. 3b,c. The profiles presented the composition for the full range of the as-prepared sample, in which they had a depth of 6-7 nm according to the above-mentioned TEM images. The starting point at 0 nm is the top surface of the sample. It was confirmed that the intensity of the Mo-S bond in the case of the in situ-MoS 2 thin film decreased, whereas the intensity of the Mo-N bond increased compared to that of pristine MoS 2 . This trend was also observed in the WS 2 profile. It could be postulated from the increment of the composition associated with the N bonding that the N atom was implanted and combined with the transition metal in the lattice of TMD.
LSV was performed to evaluate the HER performance in 0.5 M H 2 SO 4 solution, as shown in Fig. 4a. The N-doped TMD exhibited better catalytic activity than pristine TMD, resulting in 294 and 298 mV overpotentials at a current density of 10 mA cm −2 , respectively. In addition, to understand the HER catalytic mechanism, Tafel slopes fitted from the measured LSV curves are presented in Fig. 4b. The lower Tafel slope implies that the electrons can move rapidly to the hydrogen source. In this regard, three principle HER steps on the surface of active materials can be divided depending on the slope value as the reaction rate determinant 43 .  , respectively. Therefore, in this work, the rate-limiting step for the as-prepared samples was the Heyrovsky reaction, following the Volmer reaction to produce H 2 molecules. EIS measurements were conducted to determine the catalytic kinetics at the interface with an acidic solution, as shown in Fig. 4c. Although the solution electrolyte resistance (Rs) of all samples was the same as 8.1 Ω, the charge transfer resistances (R ct ) of in situ-MoS 2 and in situ-WS 2 were 39 Ω and 35 Ω, respectively, which are much smaller than those of pristine TMD, suggesting enhanced conductivity and electron transferability 44,45 . To estimate the stability of the in situ-MoS 2 and in situ-WS 2 samples, an LSV test was performed after 1,000 cycles, as shown in Supplementary Fig. S7. Although there was little degradation after the stability test, it was confirmed to have excellent robustness.
DFT calculations were performed to obtain ∆G H for the reaction intermediate and to analyze the effect of N doping on the TMD catalyst. Figure 4d,f show the 4 × 4 × 1 MoS 2 and WS 2 supercell used for the calculations that contain an N atomic concentration of 2.08 at%. ∆G H is the standard descriptor for estimating and predicting the HER catalytic activity 46 . Too positive ∆G H will have difficulty in adsorbing a hydrogen atom on the surface while too negative ∆G H will cause difficulty separating. Hence, the best catalyst should be close to zero. The Gibbs free www.nature.com/scientificreports/ energy of hydrogen absorption on the basal plane (0001) is over 2 eV, leading to the poor HER performance in Fig. 4e. These results were in good accordance with other reports 47,48 . In contrast, after substituting S with N atoms, it was confirmed that the Mo and S atoms in the direction of the basal plane could be activated with a hydrogen atom by a smaller Gibbs free energy value of approximately 0.85 eV. Likewise, the ∆G H on the basal plane of the in situ-WS 2 was lowered from 2.1 to 1.0 eV in Fig. 4g. It can be concluded from the calculated Gibbs free energy that the introduction of N atoms in the 2H-TMD stimulates catalytic activation with hydrogen atoms.

Conclusion
Plasma-assisted sulfurization in a mixture of N 2 + Ar + H 2 S environment demonstrated that N-doping TMD thin films were synthesized in one step by confirming the Raman spectra and XPS. OES spectra analysis revealed the role of N 2 + species in deriving the high N-doping concentration during the synthesis of TMD. In particular, N 2 + ions were discovered only in the presence of Ar gas, which has sufficient ion energy to bring about the penning effect. This activated species was ascribed to the fabrication of N-doped TMD as efficient HER catalysts. The as-synthesized in situ-MoS 2 and WS 2 exhibited increased catalytic activity, resulting in overpotentials of 294 and 298 mV at a current density of 10 mA cm −2 , respectively. In addition, DFT calculations supported that incorporating N atoms on the TMD could have lower hydrogen Gibbs free energy than pristine TMD, especially on the basal plane.

Data availability
All relevant data are within the paper.