Electrodeposition of amorphous molybdenum sulfide thin film for electrochemical hydrogen evolution reaction

Amorphous molybdenum sulfide (MoSx) is a highly active noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER). The MoSx was prepared by electrochemical deposition at room temperature. Low-cost precursors of Mo and S were adopted to synthesize thiomolybdates solution as the electrolyte. It replaces the expensive (NH)2MoS4 and avoid the poison gas (H2S) to generate or employ. The (MoO2S2)2−, (MoOS3)2− and (MoS4)2− ions were determined by UV–VIS spectroscopy. The electrodeposition of MoSx was confirmed with XRD, XPS and SEM. The electrocatalyst activity was measured by polarization curve. The electrolyte contained (MoO2S2)2− ion and (MoOS3)2− ion electrodeposit the MoSx thin film displays a relatively high activity for HER with low overpotential of 211 mV at a current density of 10 mA cm−2, a relatively high current density of 21.03 mA cm−2 at η = 250 mV, a small Tafel slope of 55 mV dec−1. The added sodium dodecyl sulfate (SDS) can efficient improve the stability of the MoSx film catalyst. Electronic supplementary material The online version of this article (10.1186/s13065-019-0600-0) contains supplementary material, which is available to authorized users.


Introduction
Hydrogen is a cleaner and sustainable energy, and it is one of the promising alternative energy carriers [1,2]. Electrochemical water splitting is attractive methods for hydrogen evolution [3][4][5]. An important problem for this method is seeking highly catalytic active electrocatalysts for hydrogen evolution reaction. In this regard, various efficient electrocatalysts materials, including Pt and other noble metals were investigated. However, high cost of Pt or other noble metals impede their widespread application [6,7].
In this work, we further improved the approaches of synthesis of thiomolybdates solution. (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and Na 2 S·9H 2 O were employed as the precursors of Mo and S, respectively. The ammonium chloride buffer solution (pH = 8) replaced the hydrochloric acid to make the pH of the solution to 8. This method does not produce a large amount of H 2 S gas due to excessive local acid concentration. And it is very simple, the process is easy to control and is mild. Additionally, the precursor materials are economic, especially, the prepared thiomolybdates solution has great stability. The synthesized thiomolybdates solution as the electrolyte, employ the electrochemical deposition of amorphous molybdenum sulfide thin film for electrochemical hydrogen evolution. The HER performance measurement result suggests the catalyst displayed high catalytic activity for hydrogen evolution reaction. Add a bit of surfactant into the electrolyte, the stability of the MoS x film has effectively improved. UV-VIS spectrophotometer (TU-1810,Beijing). Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopic (EDS) images were taken with a TESCAN VEGA II LMU instrument. The phase compositions of the samples were identified using an X-ray diffractometer (XRD, X'pert PRO, PANalytical B.V., Holland) using Cu Kα radiation (0.15418 nm). The electrodeposition and electrochemical measurements were carried out at room temperature in a three-electrode glass cell connected to an electrochemical workstation (CHI440A, chenghua, Shanghai). The surface chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS, Thermoelectron ESCALAB 250, USA).

Syntheses of thiomolybdates solution
(NH 4 ) 6 Mo 7 O 24 ·4H 2 O (3.58 g) was dissolved in 200 mL ammonium chloride buffer solution (pH = 8). In a second container, 21.65 g of Na 2 S·9H 2 O was added to 300 mL of ammonium chloride buffer solution (pH = 8). These two solutions were mixed and transferred to a 500 mL beaker. Put the mixed solution beaker to the ~ 90 °C water bath for 2 h. After that, the black and red solution was transferred to a 500 mL flask. Once the solution is cooling down to the room temperature, then the deionized water is used to add the solution to the scale.

Catalyst synthesis
The substrate used titanium ingot (11.28 mm diameter, 3.5 mm thick, purity 99.99%). Prior to the electrodeposition, the Ti substrate was carefully cleaned with mechanical polishing, acetone and HCl solution (9 wt%) in an ultrasound bath each for 5 min, successively. And then it was washed with deionized water after each step. Polytetrafluoroethylene (PTFE) electrode sets with working area of 1 cm 2 . MoS x was deposited on Ti substrate by electrodeposition in a three-electrode setup. The PTFE electrode sets with treated Ti substrate as the working electrode, saturated calomel electrode (SCE) as the reference electrode, and a graphite board as the counter electrode. The synthesized thiomolybdates solution as the electrolyte. The electrodeposition adopted the method of chronopotentiometry (CP).

Spectroscopic characterization
The thiomolybdates determination were conducted using the UV-VIS spectrophotometer of ref. 30. Take 0.1 mL thiomolybdates solution and dilute to 100 mL for spectral detecting. The range of wavelength is from 190 to 600 nm. The scan rate is 0.5 nm s −1 .

Electrochemical measurements
Electrochemical measurements were carried out with a three-electrode configuration in which saturated calomel electrode as the reference electrode, a graphite board as the counter electrode. Linear sweep voltammetry (LSV) with a 5 mV s −1 scan rate was performed in 0.5 M H 2 SO 4 electrolyte, which was purged with N 2 gas for at least 30 min prior to the LSV measurements in order to remove any dissolved O 2 . LSV curves were measured fifth for each sample to verification of the system's chemical stability. The scan range from 0.00 to − 0.55 V vs. SCE (not iR corrected). After the LSV measurements, the solution was stirred. The reference electrode was calibrated for the reversible hydrogen potential using platinum wire was working and counter electrodes in the electrolyte solution saturated with H 2 . In 0.5 M H 2 SO 4 , the potential was converted to the reversible hydrogen potential (RHE) reference electrode by E (vs. RHE) = E (vs. SCE) + 0.26 V. The resistance (R) was tested by EIS. EIS measurements were carried out in the frequency range of 0.1 Hz to 10 5 Hz under a hydrogen evolution voltage, which corresponds to the potential at 10 mA cm −2 .
Electrochemical stability is an important parameter for viability of a HER catalyst. To investigate HER stability under electrocatalytic operation in the acidic environment, long-term potential cycling stability of the MoS x film was assessed by taking continuous cyclic voltammograms (CV) between 0.0 and − 0.55 V vs. saturated calomel electrode (not iR corrected) at 100 mV s −1 .

Electrolyte
Thiomolybdates solutions were synthesized in the buffer solutions containing different concentrations of ammonium chloride. The ammonium chloride concentration is from 0.1 to 0.5 M. Different ammonium chloride concentration results in the different color of the thiomolybdates solutions. The thiomolybdates solutions color was changed from light yellow to dark red, along with the increasing of the ammonium chloride concentration. The different color of the thiomolybdates solutions attribute to the different thiomolybdates species. The various thiomolybdates can be determined by UV-VIS Spectroscopy [30]. The actual UV-VIS Spectra of the thiomolybdates solutions are shown in Fig. 1. Peak position and molar absorptivities are provided in Table 1.
By comparing the results from the previously reports [30], it can be concluded that to adjust the ammonium chloride concentration of the ammonium chloride buffer solution can syntheses the various thiomolybdates solutions. With the concentration of ammonium chloride increases, the thio-degree rises up. In the 0. And the two ions could to produce the molybdenum sulfide thin film under electrochemical deposition. We required the synthesized thiomolybdates solution as the electrolyte to electrodeposit of molybdenum sulfide thin film, and the molybdenum sulfide thin film could with relatively high HER performance.

Characterization of MoS x
In the previous studies [13,21,[25][26][27], they always employed the purity tetrathiomolybdate to prepare the MoS 2 or MoS 3 . In this work, we applied the synthesized thiomolybdates solution as the electrolyte to electrodeposit of molybdenum sulfide thin film for electrochemical hydrogen evolution, and XRD (Additional file 1: Figure S1) analysis identified as amorphous molybdenum sulfides. Figure 2 displays the detailed XPS scans for the Mo and S binding energies for the molybdenum sulfide thin film. The XPS spectra of molybdenum sulfide thin film are similar to those of known MoS x samples [13,22]. The molybdenum sulfide thin film exhibits two characteristic peaks at 229.4 and 232.5 eV, attributed to the Mo 3d 5/2 and 3d 3/2 binding energies for Mo 4+ [11,13,22]. The observation of Mo 3d 5/2 and 3d 3/2 binding energies at 230.5 and 234.1 eV suggests the presence of Mo 5+    [22,31].

Electrodeposition MoS x
The electrodeposition method for amorphous molybdenum sulfide thin film was CP. The deposition current density was 2 mA cm −2 , the deposition temperature was 20.0 °C, the deposition time was 900 s, and accompanied with stirring during the deposition process. The electrolyte used the synthesized thiomolybdates solutions with 0.2 M, 0.4 M and 0.5 M ammonium chloride, respectively. The samples named as S-0.2, S-0.4 and S-0.5 corresponding to the ammonium chloride concentration. The deposition curves (potential-time) are shown in Fig. 3a, and color film formed on the electrode (Inset in Fig. 3a).

HER activities
The HER catalytic activity of these molybdenum sulfide films as the catalyst was measured employing the standard three-electrode electrochemical configuration in 0.5 M H 2 SO 4 electrolyte-aerated with Ar, as described in "Materials and methods". The polarization curves (not iR corrected) showing the normalized current density versus voltage (j versus V) for the S-0.2, S-0.4 and S-0.5 films along with Pt wire and Ti ingot samples, for comparison, are illustrated in Fig. 3b. As expected, Pt wire catalyst exhibits excellent HER performance, and their HER performances are summarized in Table 2. In contrast, S-0.2, S-0.4 and S-0.5 films produces j of 10 mA cm −2 at η of 319 mV, 211 mV and 270 mV, respectively. Further insight into the catalytic activity of MoS x samples were obtained by extracting the slopes from the Tafel plots shown in Fig. 3c. The corresponding Tafel slopes of the MoS x films are in the range of 55 to 87 mV dec −1 . The lowest Tafel slope of ~ 55 mV per decade was attained for the sample of S-0.4. This indicates the Volmer reaction is taking place, a process to convert protons into sorbed hydrogen atoms on the MoSx film surface, and this process becomes the rate-determining step in the HER mechanism [5,32,33]. Figure 3d exhibits the ammonium chloride concentration dependent current densities at η = 150, 200 and 250 mV. The current densities at the optimal ammonium chloride concentration are 1.12, 7.50 and 21.03 mA cm −2 at η = 150, 200 and 250 mV, respectively. The optimal ammonium chloride concentration is 0.4 M. The sample of S-0.4 film displayed relative high catalytic activity for hydrogen evolution reaction, the overpotential is lower than many other reported acid-stable and earth-abundant HER electrocatalysts,  Table 3). Another important aspect utilized to evaluate the performance of an electrocatalyst is the long-term operating stability. Continuous cyclic voltammetry (CV) in the cathodic potential window at a scan rate of 100 mV s −1 was performed on the films over 1000 cycles to investigate their long-term stability. Cathodic polarization curves were collected after 1000 cycles testing (Fig. 4) to investigation the current-density degradation compared with the initial polarization curve. In Fig. 4a, the cathodic polarization curves were corresponding to the sample of S-0.4. It is observed that the current density (without iR correction at overpotential of 250 mV) Although the Tafel slop was higher, the stable of the catalytic was much more improved. Furthermore, SEM images performed on the two samples (Fig. 5) both of their before and after cycles. The SEM images confirms that the surface   . 5c) and was not changed after 1000 cycles (Fig. 5d). In addition, the energy-dispersion X-ray spectroscopy (EDS) images (Additional file 1: Figure S3h, i, k, l) showed homogeneous distribution of Mo and S elements. But the surface morphology of S-0.4 (Fig. 5a) was appeared many deep cracks after 1000 cycles (Fig. 5b) with corresponding EDS mapping (Additional file 1: Figure S3b, c, e, f ) uniform distribution for Mo and S elements. The SDS is one of the surface active agent. Adding appropriate surfactant can decrease the surface tension of the MoSx film, increase the dispersion and minish effectively particle size of MoSx film, thereby improve effectively the stability of the MoSx film. Meanwhile, electrochemical impedance spectroscopy (EIS) was employed to evaluate the conductivity of the catalysts (Additional file 1: Figure S2). The Nyquist plots were fitted using an equivalent circuit containing a resistor (Rs) in series with two parallel units, a chargetransfer resistance (Rct) and a constant phase element (CPE1), where Rs represents the solution resistance.

Conclusions
In conclusion, we have developed a low-cost, environmentally friendly and a simple synthetic strategy to synthesis of thiomolybdates solution as the electrolyte to electrodeposit of amorphous molybdenum sulfide thin film for the HER. Our results provide evidence for electrodeposit of amorphous molybdenum sulfide thin film not only can used the electrolyte consists purity (MoS 4 ) 2− ion but also the (MoO 2 S 2 ) 2− ion and the (MoOS 3 ) 2− ion consists in the electrolyte can electrodeposit the amorphous molybdenum sulfide thin film. The electrolyte contained (MoO 2 S 2 ) 2− ion and (MoOS 3 ) 2− ion electrodeposit the MoS x thin film displays a relatively high activity for HER with low overpotential of 211 mV at a current density of 10 mA cm −2 , a relatively high current density of 21.03 mA cm −2 at η = 250 mV, a small Tafel slope of 55 mV dec −1 . When the SDS is added into the electrolyte, the stability of the MoS x film has effectively improved, even though the catalytic activity for hydrogen evolution reaction has reduced. Therefore, this work essentially offers an economy, mild condition, viable and scalable strategy for preparing highly efficient HER electrocatalysts for the development of effective electrochemical watersplitting technology.

Additional file
Additional file 1: Figure S1. XRD spectra for MoS x film grown on the Ti ingot by chronopotentiometry negative electrodeposition at 2 mA cm −2 . Figure S2. Nyquist plot representations of electrochemical impedance spectra of S-0.4, S-0.4-SDS, and Ti ingot. Figure S3. SEM images and EDS elemental mapping for Mo and S of amorphous MoSx films. Panels a and d are the SEM images for S-0.4 (a) before and (b) after CV for 1000 cycles with corresponding (b, c, e, f ) EDS elemental mapping images, respectively. Panels g and j are the SEM images for S-0.4-SDS (c) before and (d) after CV for 1000 cycles with corresponding (h, i, k, l) EDS elemental mapping images, respectively.

Authors' contributions
This study is an outcome of constructive discussion with LNZ and JLL. LNZ, LLW and JL carried the literature study, performed a part of the syntheses of electrolyte. LNZ was the principle investigator of the project, performed the UV-VIS Spectrophotometer, XRD, XPS, SEM, EIS and HER analyzes, discussing the result, and revised the manuscript. All authors read and approved the final manuscript.

Funding and Acknowledgments
This work was supported by the Project No. CDJXS11221171 Supported by the Fundamental Research Funds for the Central Universities, and the sharing fund of Chongqing University's Large-scale Equipment.

Availability of data and materials
We have presented all our main data in the form of tables and figures.