Effect of Polymer Addition on the Structure and Hydrogen Evolution Reaction Property of Nanoflower-Like Molybdenum Disulfide

Nano-structured molybdenum disulfide (MoS2) catalysts have been extensively developed for the hydrogen evolution reaction (HER). Herein, a novel hydrothermal intercalation approach is employed to fabricate nanoflower-like 2H–MoS2 with the incorporation of three polymers, polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and polyethylenimine (PEI). The as-prepared MoS2 specimens were characterized by techniques of scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), together with Raman and Fourier transform infrared spectroscopy (FTIR). The HER properties of these lamellar nanoflower-like composites were evaluated using electrochemical tests of linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). The existent polymer enlarges the interlayer spacing of the lamellar MoS2, and reduces its stacked thickness. The lamellar MoS2 samples exhibit a promoting activity in HER at low additions of these three polymers (0.04 g/g MoS2 for PVA and PEI, and 0.08 g/g MoS2 for PVP). This can be attributed to the fact that the expanded interlayer of MoS2 can offer abundant exposed active sites for HER. Conversely, high additions of the polymers exert an obvious interference in the HER activity of the lamellar MoS2. Compared with the samples of MoS2/PVP–0.08 and MoS2/PEI–0.04, the MoS2/PVA–0.04 composite exhibits excellent activity in HER, in terms of higher current density and lower onset potential. OPEN ACCESS


Introduction
Hydrogen as an ideal, clean and efficient secondary energy resource serves as one of the most competent candidates for replacing petroleum fuels for the future.The electrolysis of water is the most mature and promising method for the production of hydrogen [1,2].Thus, a variety of nonmetal and metal materials such as carbides [3,4], and Pt-group alloys [5,6] employed as catalysts for the hydrogen evolution reduction (HER) have been extensively explored.Up to date, it has been difficult to push the industrial applications of them because of the poor activities in HER or high costs.Compared with the above-mentioned materials, chalcogenides [7][8][9], particularly, molybdenum disulfide (MoS2), possess the merits of acceptable cost, acidic stability, easy fabrication, and a higher electrocatalytic property [10,11].The suitability of MoS2 as an excellent catalyst for HER, is mainly due to the catalytic activity at the edge of this lamellar crystal [7,12,13].It has been proved by theoretical and experimental studies that the active sites of 2H-MoS2 for HER locate the (010) and (100) planes with the existence of unsaturated molybdenum and sulfur atoms, while the (002) basal plane is inactive [7,12,14].
In general, there are three pathways to obtain MoS2 catalyst with an effective activity in HER.They are briefly described as follows: (1) increase in the intrinsic activity of active sites, for instance, Ni, B, and Fe metals have been incorporated to improve the catalytic activity of MoS2 [15,16]; (2) improvement in the electrical contact between the catalytic sites, such that graphite [17], mesoporous carbon [18], and reduced graphene oxide [19,20] have been employed to improve electrical conductivity; (3) enhancement in the number of active sites, thus the decrease in crystal size and stacked thickness have been taken into account.It is well known that the HER activity of MoS2 decreases with the increase of stacked thickness, because of the Van der Waals force interactions within its interlayer [21][22][23].Therefore, most studies in the electrocatalytic enhancements of MoS2 have focused on improving the density of the active sites and reducing the stacked thickness.Based on this consideration, the chemical exfoliation process [24] and the low pressure chemical vapor deposition method [25] were employed for the fabrications of exfoliated 1T-MoS2 and monolayer MoS2.In view of the enhancement in the HER of nano-structured MoS2, various intercalation techniques have been employed to extend the interlayer space and to increase the active sites of MoS2 specimens.Compared with intercalations of graphene oxide and some other materials, the process of polymer intercalation can be easily performed.To the best of our knowledge, however, as for the improvement in HER of MoS2-based catalyst, insufficient efforts for the intercalation of polymers have been devoted to the increase in the exposed unsaturated edge sites and the reduction in the stacked thickness.Thus, the enhancement in HER of MoS2 catalyst merits extensive exploration.
The aim of this study is to reveal the effects of intercalated polymers on the structure, density of active edge sites, and HER activity of nanoflower-like MoS2.Thus, three polymers, i.e., polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and polyethylenimine (PEI) were added to the aqueous solutions employed for the fabrication of nanoflower-like MoS2 via a solvothermal synthesis approach.Herein, the prepared MoS2 specimens were characterized using scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), as well as Raman and Fourier transform infrared spectroscopy (FTIR).The electrocatalytic performances in HER of theses lamellar catalysts were evaluated by the techniques of linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS).This work sheds insight into the enhancement in HER of the MoS2-based material.

Synthesis of Nanoflower-Like MoS2
The preparation process of nanoflower-like MoS2 was as follows: 1.41 g of AHM and 0.26 g of thiourea were dissolved in 20 mL of distilled water, and the mixed solution was stirred to form a homogeneous solution.Then, a certain amount of polymer (PVA in the range of 0.04-0.4g/g MoS2, PVP in the range of 0.08-2.0g/g MoS2, PEI in the range of 0.04-0.4g/g MoS2) was added to the mixed solution.This solution was stirred for another 30 min at room temperature to guarantee the homogeneity of the solution.Afterward, the mixed solution was transferred to a 25 mL Teflon-lined stainless steel autoclave placed in an electric cooker (WRN-010, Eurasian, Tianjin, China).Then the temperature of this mixed solution was increased to 200 °C and maintained for 24 h.When the temperature of the solution naturally cooled from 200 °C to room temperature, the solution was centrifuged and then washed with distilled water and ethanol to remove residual reactants.Finally, the obtained powders were dried at 60 °C under a vacuum atmosphere.With the addition of PVA, PVP, and PEI, the prepared samples were described as MoS2/(PVA)-X, MoS2/(PVP)-X and MoS2/(PEI)-X (Table 1), respectively; where X denotes the concentration of polymer (g/g MoS2) in the solution.Without the presence of polymer, the pure MoS2 was fabricated by a similar process to that mentioned above.

Characterization
An X-ray diffractometer (XRD, Smartlab, Tokyo, Japan) with a Cu Kα radiation (λ = 1.5418Å) was employed to investigate the structures of the prepared catalysts.Raman spectroscopy was conducted using a Raman Microscope (E55+FRA106, Bruker, Karlsruhe, Germany) with an excitation wavelength of 514.5 nm.The morphology analysis was performed using a scanning electron microscope (SEM, S-4800, Hitachi, Tokyo, Japan) with an accelerating voltage of 5 kV.Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were performed with a microscope (JEM-2010, JEOL, Tokyo, Japan) at a voltage of 200 kV.A Nicolet iS5 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Madison, WI, USA) was adopted to determine the FTIR spectra of the specimens with 16 scans at a resolution of 4 cm −1 interval.

Fabrication of the Electrodes
The fabrication process of the tested electrode was described as follows.First, 5 mg of pure MoS2 or MoS2/polymer powder and 30 μL of Nafion solution (5 wt.%) were dispersed in 1 mL of ethanol-water solution at room temperature (the volume ratio between absolute ethanol and distilled water is 1:3).Then the mixed solution was dispersed for 1 h using an ultrasonic cleaner (YJ5120-10, Kun Shan Ultrasonic Instruments Co. Ltd., Kunshan, China).Second, 5 μL of the dispersed solution was dropped onto the surface of a glassy carbon electrode (GCE) with a diameter of 3 mm (Aida Hengsheng Technology Co. Ltd., Tianjin, China).Finally, the modified GCE with a powder loading of 0.34 mg/cm 2 was dried at room temperature for 24 h.The obtained electrodes were denoted as MoS2/PVA-X, MoS2/PVP-X and MoS2/PEI-X (Table 1), where X denotes the different addition of PVA, PVP, and PEI.The pure MoS2 electrode was fabricated exactly as the above process, except for pure MoS2 powder applied to replace the MoS2/polymer powder.

Electrochemical Measurements
All the electrochemical measurements were performed in a 0.5 mol• L −1 H2SO4 solution at 298 K under atmospheric pressure, using an electrochemical workstation (CHI 650C, Chenhua Co. Ltd., Shanghai, China).A three stand electrode cell was employed, where the fabricated electrode mentioned above, Ag/AgCl, and Pt wire were employed as working electrode, reference electrode, and auxiliary electrode.Before the tests, all the electrolytes were deaerated by purging with pure N2 gas for 40 min before the tests.
Linear sweep voltammetry (LSV) was conducted between −0.25 and 0.1 V at 2 mV• s −1 .Electrochemical impedance spectroscopy (EIS) was measured at a cathode overpotential of 200 mV; the employed amplitude of the sinusoidal signal was 5 mV.All EIS tests were always carried out from high frequency to low frequency, and the frequency ranged from 10 5 to 10 −2 Hz.During the test, all potentials were collected versus the Ag/AgCl electrode.Then the potentials (versus Ag/AgCl) were calibrated to a reversible hydrogen electrode (RHE) as follows: ERHE = EAg/AgCl + 0.059pH + 0.209 V.All the measurements were performed in triplicate to guarantee the reproducibility of the experimental data.

X-Ray Diffraction (XRD) Spectra
The XRD patterns of the prepared MoS2-based nanoflower-like samples are presented in Figure 1.For the purpose of comparison, the standard pattern of the pristine 2H-MoS2 (JCPDS No. 37-1492) is also displayed within Figure 1.Three peaks appearing at 2θ = 15.0°,33.8°, and 57.1° correspond to the (002), (100), and (110) planes of the pristine 2H-MoS2, respectively.It is noteworthy that, compared to that of pure MoS2, the locations of the composites corresponding to the (002) peaks shift to lower angles.The diffraction peaks of composites centered at around 2θ = 12.5° correspond to the interspacing (d) of 7.08 Å obtained via the Bragg equation.Herein, the interspacing thicknesses of the three MoS2 composite specimens are identical, which may be due to the low additions of PVA, PVP, and PEI.The interspacing of 7.08 Å for MoS2/polymer composites is much larger than that of the pure MoS2 (5.9 Å), indicating the expansion of lattice and the intercalation of polymers [26,27].In addition, the weak characteristic of these three peaks for the prepared samples is validated, suggesting their poor crystallizations.Meanwhile, with a decrease in concentrations of polymers, the shapes of these reflection peaks become sharper, suggesting enhancement of crystallinities.It is noteworthy that the decline of the (002) diffraction peak indicates a low stacking height [28], which is effective for offering more active sites and lower intrinsic resistance for HER [21].

Raman Spectra
The Raman spectra of MoS2/PVA composites are reported in Figure 2a.The two characteristic peaks of pure MoS2 at 372 and 402 cm −1 match well with the E2g 1 and A1g vibrational models of the hexagonal MoS2 [29].With the increase in concentration of PVA, intensities of E2g 1 and A1g peaks become low and weak, demonstrating the decrease in crystallinities of the composites.Herein, Raman spectra of MoS2/PVP and MoS2/PEI composites with different additions of PVP and PEI are not given, because of their similarities to that of MoS2/PVA.In general, the difference in frequency between E2g 1 and A1g is helpful in determining the number of stacked layers [30,31].As demonstrated in Figure 2a,b, for MoS2/PVA, MoS2/PVP, MoS2/PEI composites, a red shift of the E2g 1 band and a blue shift of the A1g band are observed; thus the smaller differences between E2g 1 and A1g can be validated for these three composites.In contrast with that of pure MoS2, we confirmed that the intercalations of polymers reduce the stacking thicknesses of the nanoflower-like MoS2 composites.It should be pointed out that Raman spectra of the composite specimens strongly depend on the stoichiometry of their components, thus the different HER activity between the pure MoS2 and the composites might be confirmed.

FTIR Spectra
FTIR spectra of MoS2/PVA composites (Figure 3a), MoS2/PVP composites (Figure 3b), and MoS2/PEI (Figure 3c) composites were measured.As shown in Figure 3, the strong absorbance peaks appearing at 3000 to 3600 cm −1 indicate the presences of intermolecular and intramolecular hydrogen bonds; nevertheless, to some extent, these peaks are overlapped by the absorbance of water molecules.The characteristic peaks of O-H, CH2, and C-H groups for PVA (Figure 3a) with the wave number in the range of 1330-1648 cm −1 can be observed [32], while the peaks of the C-O stretch and O-H bending can be found at 1096 cm −1 .The characteristic peaks of the MoS2/PVA composites are consistent with that of pure PVA.As for the pure PVP and MoS2/PVP composites (Figure 3b), the peaks corresponding to C=O stretching vibration, and C-N stretching vibration are observed at 1659 cm −1 , and 1284 cm −1 [33].The peak located at 3380 cm −1 (Figure 3c) corresponds to the -NH2 group in PEI.In addition, peaks appearing at 2948, 2840, and 1459 cm −1 can be assigned to asymmetric stretching vibrations of C-H bond, while the peak at 1106 cm −1 is attributed to the stretching vibration of the C-N group [34].Thus, the MoS2/PVP and MoS2/PEI composites present the same characteristic peaks as the pure PVP and PEI polymers.Based on the above analyses, the existence of PVA, PVP, and PEI can be validated, thereby indicating theses three polymer molecules can be intercalated in the MoS2 gallery space during the hydrothermal process.

SEM Morphologies
The morphologies of the pure MoS2 and the synthesized MoS2/PVA composites are illustrated in Figure 4.All samples are assembled by the lamellar nanoflower-like MoS2 with diverse lateral sizes ranging from 100 to 400 nm.With the increase in PVA addition (Figure 4b-d), large nano-size flowers with vague edges can be found, suggesting insufficient crystallization of these composites, consistent with the results of XRD.It should be noted that a similar tendency in the presence of PVP and PEI can also be confirmed (Figure 5).Among the MoS2/polymer composite samples, the MoS2/PVA-0.4,MoS2/PVP-2.0 and MoS2/PEI-0.4 samples exhibit remarkable aggregations (Figures 4d and 5c,f), which may be attributed to the excessive adsorption of polymers at the surface and edge sites of the MoS2 composites.In addition, with the decrease in polymer addition, the edges of MoS2 composites (Figures 4b and 5a,d) can be easily observed, thereby indicating the high crystallinity of the samples.Therefore, a nanoflower-like MoS2 sample with a high crystallinity and small particle size can be obtained with low additions of polymers (0.04 g/g MoS2 for PVA and PEI, and 0.08 g/g MoS2 for PVP).

TEM
To further elucidate the morphologies of the obtained composites, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements were performed.From the fabricated composites, MoS2/PVA-0.04,MoS2/PVP-0.08, and MoS2/PEI-0.04 three samples were selected for the tests.Also, the pure MoS2 sample was analyzed for comparison.As displayed in Figure 6, these samples with a diameter of about 100 nm are assembled by lamellar MoS2 powders.Nano-size flowers with a thickness of 2-5 nm can be observed, which are notably thinner than that of the pure MoS2 (~10 nm).The lattice inter spaces of 7.08 Å for MoS2/PVA-0.04,MoS2/PVP-0.08 and MoS2/PEI-0.04 composites are much larger than that of pure MoS2 (5.9 Å), which is also consistent with the results of XRD.This may be attributed to the intercalations of polymers, and the interactions between the oxygen-containing functional groups of the polymers and the MoS2 precursors [19].Thus, the polymers intercalated into the MoS2 interlayer can afford a number of exposed edges for HER.

Linear Sweep Voltammetry (LSV)
To evaluate the catalytic activity in HER of the MoS2/polymer composite, LSV measurements were performed in 0.5 mol• L −1 H2SO4 solution with a scanning rate of 2 mV• s −1 at room temperature.Cathodic polarization curves (Figure 7a) and corresponding Tafel plots (Figure 7d) of MoS2/PVA-0.04,MoS2/PVA-0.08,MoS2/PVA-0.4 were obtained.In comparison with the pure MoS2 sample, it can be observed that all the MoS2/PVA composites present much lower onset overpotentials (η); among them, the MoS2/PVA-0.04 sample exhibits the lowest value of η (~40 mV), suggesting the excellent HER activity.The onset potentials for other MoS2/PVA composites are in the range of 180-250 mV, these values are still lower than that of the pure MoS2 (300 mV).As shown in Figure 7a, compared with other specimens, the MoS2/PVA-0.04 sample exhibits the largest cathodic current density of 20 mA• cm −2 at (η = 250 mV), which is 80 times that of the pure MoS2 (0.25 mA• cm −2 ).Similarly, the enhancements in HER for MoS2/PVP and MoS2/PEI composites can also be validated (Figure 7b,c).The current densities of MoS2/PVP-0.08 and MoS2/PEI-0.04 (η = 250 mV) are 9.04 mA• cm −2 and 9.60 mA• cm −2 , respectively; which are 36-fold and 38-fold levels higher than that of the pure MoS2.It is well known that the smaller the Tafel slope of the sample, the higher the HER activity [35,36] will be.The Tafel plots (Figure 7d-f) fit the following Tafel equation: where, η (mV) is the overpotential, and j (mA• cm −2 ) is the current density.The values of Tafel slope are ~173, ~113, ~46, and ~39 mV• dec −1 for MoS2, MoS2/PVA-0.4,MoS2/PVA-0.2, and MoS2/PVA-0.04, which are very consistent with published reports [37,38].Among the samples, MoS2/PVA-0.04 with the smallest Tafel slope exhibits the highest HER activity.The small Tafel plot compares to that of the other reported polymer [39].Although Tafel slopes of MoS2/PVA-0.2, MoS2/PVA-0.4 samples are larger than that of MoS2/PVA-0.04, their HER activities are still higher than that of the pure MoS2 because of their smaller Tafel slopes.Similarly, the same can be concluded for MoS2/PVP and MoS2/PEI composites.Among these two kinds of composite, the MoS2/PVP-0.08 and MoS2/PEI-0.04 samples possess notable HER activities due to smaller Tafel slopes than those of other corresponding samples (Figure 7e,f).However, these two composites for HER activity are inferior to the MoS2/PVA-0.04 sample.
Based on the above mentioned results, we realize that all the prepared composites present quite different HER activities due to the effects of different concentrations of polymers.Considering the feasibilities of intercalation polymers in the interlayer of MoS2 catalysts and excessive adsorption of them on the surface and edge sites of MoS2, the HER activities of these three kinds of composites are reduced with increase in the additions of the polymers.Thus, the enhancement in HER for nanoflower-like MoS2 can be achieved by adjusting the polymer addition.

Electrochemical Impedance Spectroscopy (EIS)
The kinetics of MoS2/polymer composites in HER can be further illustrated by EIS, and the results are reported in Figure 8.The EIS data were fitted using a modified Randles equivalent circuit shown in Scheme 1.The values of the circuit elements obtained by fitting the experimental results are shown in Table 2. Herein, a constant phase element (CPE) was adopted to model the electrical double layer of the electrode/electrolyte interface.The necessity of CPE in place of pure capacitance is due to the dispersed distribution of Nyquist diagrams in the high frequency domain [40].The double layer capacitance (Cdl) and exchange current density (i0) of the electrode were estimated using Equations ( 2) and (3) [41]: where, Y0 (mS• s• cm −2 ) regarded as a capacity parameter, is the CPE coefficient; the dimensionless CPE exponent (n) is related to the constant phase angle.Rs (Ω• cm 2 ), and Rct (Ω• cm 2 ) are the solution resistance, and the charge transfer resistance, respectively.As indicated by Figure 8 and Table 2, compared with that of the MoS2 pure sample, the Nyquist plots reveal a decrease in charge transfer resistance (Rct) (from 7.55 kΩ• cm 2 to 0.84 kΩ• cm 2 ), demonstrating the excellent conductivity of the composites (Table 2).Rct of MoS2/polymer composites follow the sequence of MoS2/PVA < MoS2/PEI < MoS2/PVP.Rct of the MoS2/PVA-0.04 is smaller than that of other specimens, suggesting a faster charge transfer of this composite.The double layer capacitance (Cdl) is calculated to evaluate the exposed active surface area of the electrode.As shown in Table 2, MoS2/PVA-0.04 exhibits the largest Cdl compared to the other ones.Furthermore, MoS2/PVA-0.04 exhibits a significant exchange current density (i0) of 5.89 × 10 −5 mA• cm −2 , which is 2-11 times larger than that of the other composites.The large i0 further confirms the excellent activity for HER catalysis.Thus, among the composite samples, the MoS2/PVA-0.04 composite with the lowest Rct, highest Cdl and i0 provides sufficient active sites, and thereby guarantees excellent HER activity.Scheme 1. Randles electrical equivalent circuit compatible with the Nyquist diagrams shown in Figure 8. Rs, Rct and CPE are the solution resistance, the charge transfer resistance, and a constant phase element, respectively.

Stability
In addition to the HER activity, stability plays a crucial role in the potential application for a MoS2-based catalyst.In order to reveal the stability of this catalyst in acidic solution, a long-term cycling test for the MoS2/PVA-0.04 composite was conducted.This test was performed with the potential ranging from −0.25 to 0.1 V (vs.RHE) at a scan rate of 20 mV• s −1 .It can be observed that the current density of the composite (Figure 9) shows a slight decrease (less than 4%) after 1000 CV cycles.Thus, the MoS2/PVA-0.04 composite exhibits excellent durability in the HER process.

Conclusions
In this study, three kinds of MoS2/polymer composites were fabricated by a facile hydrothermal method.The effects of three polymers PVA, PVP, and PEI on the stacking height and the quantity of exposed active sites of the nanoflower-like MoS2 were evaluated.The intercalated or adsorbed polymers provide carbonyl and hydroxyl functional groups for attachment of the MoS2 precursor, and enlarge the interlayer spacing of MoS2 particles.The expanded interlayers of nanoflower-like MoS2 catalysts lead to the enhancement in active sites, resulting in a high electrocatalytic activity for HER.Of all the composites, the MoS2/PVA-0.04 composite exhibits the best HER activity, due to a small overpotential (~40 mV), a large cathodic current (20 mA• cm −2 ), and a small Tafel slope (~39 mV• dec −1 ).

Table 2 .
Analyzed parameters of electrochemical impedance spectroscopy (EIS) spectra for the composites.Y0 regarded as a capacity parameter, is the CPE coefficient.the dimensionless CPE exponent (n) is related to the constant phase angle.Rs and Rct are the solution resistance and the charge transfer resistance.The double layer capacitance (Cdl) and exchange current density (i0) are listed in the table.