Hydrogen is regarded as the ideal renewable energy source to supplant conventional fossil fuels, because of its eco-friendliness, abundant precursor compounds, and low cost[1-4]. Notably, the electrolysis process is one of the most appealing and promising approaches to hydrogen generation. Indeed, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) taking place in the water splitting process are necessary to produce clean and green hydrogen and oxygen, respectively. In this context, given the sluggish kinetics of water electrolysis, the presence of an electrocatalyst is crucial for hydrogen production by an electrolysis system. In fact, electrocatalysts are expected to afford high current density at a reasonable value for the overpotential. Currently, platinum-based catalysts are the most efficient HER catalysts, whereas ruthenium- and iridium-based oxides are the best OER catalysts[3,4]. However, the scarcity of these elements and their high cost is hindering their extensive application[5,6]. Therefore, the development of low-cost catalysts based on earth-abundant elements is a highly desirable research objective.

The typical transition metal sulfide molybdenum disulfide (MoS₂) has been intensively investigated as a potential HER catalyst because of its layered structure, activity, and tunable configurations. The Mo and S sites of MoS₂ are catalytically more active for HER, as made evident by the results of experimental and theoretical studies[4]. Notably, various studies have been conducted that were aimed at enhancing the catalytic performance of MoS₂ [5-7]. The approaches utilized therein included controlling the catalyst morphology, changing its particle size, modifying its electrical conductivity, and doping it. The effective method for the doping for increasing active sites at the edges for boosting HER and OER activities[2-5]. Doping MoS₂ with heteroatoms like N, P, O, and B has been reported to change the catalyst's electronic characteristics and increase its intrinsic conductivity[6,7]. However, the relevant doping process appeared to be complex, as it required various steps, calcination at elevated temperatures[2] or adequate requirement of plasma[4]. Therefore, the development of a relatively easy and efficient way to synthesize doped MoS₂ that would afford an improvement of the catalyst's HER and OER performances is a very important research goal.

Herein, we designed the preparation of a nitrogen-doped MoS₂ (N-MoS₂) as a high-performance catalyst for HER and OER that is based on the use of melamine as a nitrogen source and the implementation of a simple carbonization process. Abundant active edge sites were observed to be obtained as a result of the incorporation of the nitrogen atom; consequently, the catalytic performance of MoS₂ was enhanced. In the case of the N-MoS₂ sample, the active edge sites were found to be adjusted by the doping process. The thus prepared N-MoS₂ sample was executed of enhanced bifunctional catalytic performance with decreased overpotential (-0.20 V at -10 mA/cm² for HER and 1.43 V at 10 mA/cm² for OER) with lower Tafel slope values (97 and 117 mV/dec) for HER and OER, respectively.

MoS₂ (powder; 99%; <90 nm ), melamine (99%), and sulfuric acid (H₂SO₄; 98%) were obtained from Merck . Entire chemicals were laboratory classified and processed beyond any additional purification .

MoS₂ (500 mg) and melamine (250 mg) were combined to form a well-mixed mixture. The mixture thus obtained was kept in a furnace (SH-TMFGC-100; Samheung energy, South Korea) using a boat and heated to a temperature of 900 °C for 2 h applying a ramping rate of 10 °C/min. The obtained powder sample was then allowed to cool at room temperature. It was then washed using deionized (DI) water in a process involving vacuum filtration. The obtained sample was then dried for 24 h at 60 °C in an oven and labeled as N-MoS₂.

The surface morphological structure and elemental composition of the samples were investigated using scanning electron microscopy (SEM; S-4300; Hitachi Ltd., Japan) and energy-dispersive X-ray spectroscopy (JEOL S-300), respectively. Moreover, X-ray diffractometry (XRD) data collected using a D/MAX-2500/PC Rigaku instrument with Cu Kα (λ = 1.54 Å) at 40 kV and Raman spectra (Horiba Ltd.) recorded from laser excitation at 532 nm were used to investigate the crystal structure and vibrational modes of the samples. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Scientific spectrometer with Kα Al radiation anode, 20 eV of pass energy, to investigate the configurations and chemical compositions of the surfaces of the samples.

A three-electrode system was set up to investigate the electrochemical properties of the samples using an electrochemical workstation (SP-150 Potentiostat; BioLogic, France). A glassy carbon electrode (GCE; OD, 6 mm; ID, 3 mm) was used as the working electrode, an Ag/AgCl (3M KCl saturated ) electrode was used as the reference electrode, and platinum wire was used as the counter electrode. An H₂SO₄ 0.5-M solution was used as the electrolyte in all electrochemistry experiments. The following protocol was implemented for the preparation of the working electrode: the GCE was furbished with alumina powder; it was then cleaned decently using DI water and ethanol and subjecting it to sonication for 5 min; the GCE was then thoroughly dried at room temperature; subsequently, 5 mg of each sample (MoS₂ and N-MoS₂) were dispersed individually in 2 mL of isopropyl alcohol; afterwards, 5 μl of the corresponding suspension provided from sonication process was dropped on to the as-prepared GCE ; the working electrode was finally prepared after completely dried and used in electrochemistry experiments.

Electrochemical impedance spectroscopy (EIS; frequency range, 100 mHz-500 kHz; amplitude, 5 mV), chronocoulometry (CC), cyclic voltammetry (CV; scan rate, 10-200 mV/s), and linear sweep voltammetry (LSV; sweep rate, 10 mV/s) measurements were conducted to achieve the electrochemical characterization of the samples. As described elsewhere[9-11], CC was used to investigate the active electrochemical surface area of the sample, EIS was used to determine the samples' resistance, CV experiments were conducted to investigate the samples' capacitive performance, and the j–V plots (polarization curves in LSV experiments) were used to study the catalytic properties of samples. The results of all the mentioned electrochemical measurements were obtained as potential values versus the reversible hydrogen electrode (RHE) and carried out that the relative Ag/AgCl reference electrode using as follows[4,12]:

where the measured potential was named as EAg/AgCl and the standard potential (0.210 V) of Ag/AgCl (3 M KCl 3 M) at 25 °C was labeled as EAg/AgCl0. Notably, during every electrochemical measurement, the whole system was subjected to nitrogen gas purging within the interval of 10 min.

The SEM images collected to investigate the morphology of the samples are shown in Fig. 1. The precursor MoS₂ sample exhibits smooth surfaces and a two-dimensional stacked layered structure. By contrast, the N-MoS₂ sample exhibits a separated layered structure compared with the MoS₂ sample. The sheet-like nanostructures are clearly visible in N-MoS₂. These structures are well spread and more homogeneously distributed than it is the case for MoS₂.

Figure 1. Scanning electron microscopy images of (a, b) MoS₂ and (c, d) nitrogen-doped MoS₂ (N-MoS₂).

The XRD patterns of the samples are shown in Fig. 2. The pattern recorded for MoS₂ comprises a typical intense (002) peak centered at 14.4°, which is compatible with an interlayer distance of 7.2 nm, with distinctive diffraction peaks of 2H-MoS₂, which are marked as (004), (100), (101), (102), (103), (006), (105), (110), and (008). Notably, the intensity of the diffraction peaks decreased after MoS₂ was subjected to nitrogen doping. Additionally, new diffraction peaks, marked by empty circles in the figure, appeared after the nitrogen-doping step at 32.1°, 48.9°, 64.4°, and 71.6°, which correspond to the (002), (202), (220), and (222) belong to phases of hexagonal molybdenum nitride.

Figure 2. (Color online) (a) X-ray diffractometry patterns and (b) Raman spectra of MoS₂ and nitrogen-doped MoS₂ (N-MoS₂).

The Raman peaks observed for the MoS₂ sample (Fig. 2(b)) were attributed to the in-plane motion of Mo and S in opposite directions (E2g1) at 378.2 cm^-1 and the out-of-plane motions of the S atoms (A_1g) at 404.9 cm^-1. As a result of the nitrogen doping of the MoS₂ sample, the intensities of these two peaks were suppressed, and their positions were red shifted (up to 2.5 cm^-1), indicating an enhancement of the electron density around active edge sites as a consequence of the electron-donating properties of the N atoms. In fact, an increase in electron density around the active sites can cause an acceleration of the charge transfer process.

XPS surveys of the samples, which allowed to clearly identify the presence of O, Mo, N, C, and S in the said samples, are shown in Fig. 3(a). The successful nitrogen doping of MoS₂ to produce N-MoS₂ can be followed by focusing on the high-resolution N 1s (Fig. 3(b)) and Mo 3d spectra (Fig. 3(c)). With respect to the N 1s spectrum of MoS₂, that of N-MoS₂ displays two additional peaks at 398.1 eV (attributed to nitrogen bound to molybdenum) and 399.3 eV (attributed to nitrogen bound to carbon). In fact, the N 1s spectrum of MoS₂ comprises only one peak at 397.1 eV corresponding to Mo 3p_3/2. The Mo 3d spectrum of MoS₂ comprises two extensive peaks due to Mo 3d_3/2 and Mo 3d_5/2 and observed at 232.6 and 229.4 eV, respectively, which confirm that MoS₂ presents the semiconducting 2H phase. After the incorporation of nitrogen into the MoS₂ precursor, the aforementioned peaks were observed to be shifted by 0.5 eV with an additional peak (due to the metallic Mo–N configuration) appearing at 229.5 eV, which confirmed the successful nitrogen doping of MoS₂, resulting in an increased electron density. The S 2p spectrum of MoS₂ (Fig. 3(d)) comprises two peaks at 163.5 and 162.3 eV, which can be assigned to S 2_p1/2 and S 2_p3/2, respectively. In the case of N-MoS₂, these peaks shifted by 0.4 eV as compared with MoS₂, confirming the strong electronic interactions resulting from the nitrogen doping of the MoS₂ precursor[13]. Because of the following of hydrocarbon along the surface, C and O were observed in all samples; these elements were present in the analyte samples as a result of contamination from the solvent and atmosphere. Figure 3(e) and Fig. 3(f) shows deconvolutions of the peaks observed in the C 1s and O 1s spectra, respectively. With respect to the C 1s spectrum of MoS₂, that of N-MoS₂ includes an additional peak due to C–N at 286.3 eV. The O 1s spectra of MoS₂ and N-MoS₂ comprise peaks due to C–O–C and C=O/N–O at 531.6 and 533.1 eV, respectively. The elemental data obtained for the samples based on XPS evidence are summarized in Table 1. Results indicate that the atomic composition of the precursor MoS₂ is as follows: S, 47%; Mo, 26%; C, 24%, and O, 3%. By contrast, N-MoS₂ presented the following atomic composition: S, 37%; Mo, 29%; C, 16%; O, 5%; and N, 13%, which confirms the successful nitrogen doping of MoS₂.

Table 1 Elemental data on MoS₂ and nitrogen-doped MoS₂ (N-MoS₂) inferred from X-ray photoelectron spectrometry evidence.

Samples	Element	wt%	at%	N/S
MoS2	Mo	57	26	0
	S	35	47
	C	7	24
	O	1	3
	N	0	0
N-MoS2	Mo	62	29	0.35
	S	27	37
	C	4	16
	O	2	5
	N	5	13

Figure 3. (Color online) X-ray photoelectron spectra of MoS₂ and nitrogen-doped MoS₂ (N-MoS₂). (a) Survey, (b) N 1s, (c) Mo 3d, (d) S 2p, (e) C 1s, and (f) O 1s.

Figure 4(a) shows the results of the CC measurements conducted on the samples to obtain information on their electrochemical active surface areas, which consist of the slope is directly proportional to the electroactive surface area[8]. The obtained values for the surface areas of MoS₂ and N-MoS₂ were 4.9 × 10^-5 and 9.1 × 10^-4 cm², respectively. Therefore, N-MoS₂ has larger surface area than its precursor. This difference in surface area suggests that N-MoS₂ has a superior potential for catalytic performance than MoS₂[1,9].

Figure 4. (Color online) (a) CC (b) EIS, (c) CV, and (d) capacitance results of the samples with CV results at different scan rates of (e) MoS₂ and (f) N-MoS₂.

Electrochemical impedance is a crucial characteristic from the standpoint of catalytic performance. Figure 4(b) shows the Nyquist plot (−|Zim| vs. |Zre|) with the inset, presenting the modified Randal's equivalent circuit[8, 10, 14]. It consists of internal resistance (R_s), charge transfer resistance (R_CT), double-layer capacitance (C_DL), and Warburg component (W). The value of the diameter of the semicircle of the Nyquist plot corresponds to the R_CT value. The value relating with equivalent circuit parameters of R_s and R_CT was obtained using the fitting program, which is the software (Z-fitting program) provided by BioLogic, as 13.8 Ω and 18.7 Ω for MoS₂ and 5.6 Ω and 8.1 Ω for N-MoS₂[14-18], indicating that N-MoS₂ is characterized by lower charge transfer resistance than MoS₂. Therefore, N-MoS₂ displays a higher charge transfer rate and surface area than MoS₂, so it is also expected to exhibit a better catalytic performance.

The CV performance of the samples was investigated at a scan rate of 50 mV/s (Fig. 4(c)). The applied potential window was between 0 and 0.4 V. Notably, N-MoS₂ shows response of higher current density than MoS₂ . The current density measured in the CV experiments was observed to increase with the scan rate, suggesting a good rate capability for the electrode, as can be evinced from Fig. 4(d). The value for the C_DL, which is directly proportional to the surface area of the sample, was obtained from the CV results. The plot of the ΔJ=Ja−Jc at a given potential (0.2 V) as a function of the scan rate is reported in Fig. 4(d). Notably, J_a and J_c represent the anodic and cathodic current densities at 0.2 V, respectively. The CV results obtained at values for the scan rate ranging from 10 to 200 mV/s are reported in Fig. 4(e) for MoS₂ and in Fig. 4(f) for N-MoS₂. N-MoS₂ was observed to be characterized by a capacitance of 921 mF/cm², which is higher than the value obtained for MoS₂ (301 mF/cm²). Therefore, the capacitance performance of MoS₂ was observed to be positively affected by nitrogen doping.

The catalytic activities of the samples were investigated further by conducting LSV measurements in 0.5-M H₂SO₄ at a sweep rate of 5 mV/s; the data thus obtained are presented in Fig. 5(a). N-MoS₂ exhibited an overpotential of -0.20 V at -10 mA/cm², which is lower than the value measured for MoS₂ (0.26 V). This difference indicates that N-MoS₂ exhibits a good catalytic behavior for HER. The catalytic reaction efficiency was evaluated on the basis of the Tafel plots reported in Fig. 5(b), showing the potential difference is needed for the current density increment and decrement. The Tafel slopes were obtained via the equation η=blogj/jo, which represents the relationship between current and potential at a certain value for the overpotential (η)[4,16]. In this equation, j and j_o represent the current density and exchange current density, respectively, whereas b appears for the Tafel slope . The values for the Tafel slopes obtained for MoS₂ and N-MoS₂ are 144 and 97 mV/dec. respectively. Because a small value for the Tafel slope indicates a high electron transfer rate, N-MoS₂ is characterized by a higher electron transfer rate than MoS₂. The OER activity was also characterized conducting LSV experiments, and the results thus obtained are reported in Fig. 5(c). The measured overpotentials at a sweep rate of 10 mA/cm² were 1.50 V for precursor MoS₂ and 1.43 V for as-prepared N-MoS₂. Therefore, the as-prepared N-MoS₂ exhibited a lower overpotential than MoS₂; moreover, the Tafel slope measured for N-MoS₂ was 117 mV/dec, which is lower than the value measured for MoS₂ (168 mV/dec), as can be evinced from Fig. 5(d).

Figure 5. (Color online) (a) Results of LSV experiments conducted on MoS₂ and N-MoS₂ for the hydrogen evolution reaction and (b) the corresponding Tafel plots. (c) Results of LSV experiments conducted on MoS₂ and N-MoS₂ for the oxygen evolution reaction and (d) the corresponding Tafel plots. RHE, reversible hydrogen electrode.

The stability performance and catalytic activity are additional important parameters for electrocatalysts. In fact, an N-MoS₂ sample was subjected to 1,000 consecutive electrochemical cycles, and its HER (Fig. 6(a)) and OER (Fig. 6(b)) performances were continuously monitored. The polarization curves obtained for N-MoS₂ remained similar throughout the experiment for both HER and OER activities.

Figure 6. (Color online) Cyclic stability tests conducted on N-MoS₂ for (a) the HER and (b) the OER.

N-MoS₂ was synthesized and investigated as a catalyst for both HER and OER. The nitrogen atomic content of N-MoS₂ was determined to be 13%, which confirmed that MoS₂ had been successfully doped with nitrogen. Moreover, a low overpotential of -0.20 V at -10 mA/cm² was measured for HER, and an overpotential of 1.42 V at 10 mA/cm² was measured for OER. Tafel slopes of 97 mV/dec for HER and 117 mV/dec for OER were also obtained. The improvement in HER and OER performance of N-MoS₂ with respect to MoS₂ is attributed to N-MoS₂ exhibiting lower charge transfer resistance, larger surface area, higher capacitance, and faster electron transfer than MoS₂. Notably, N-MoS₂ also exhibited good stability over up to 1,000 electrochemical cycles.