Reaction Controllable preparation and electrocatalytic performance of two-dimensional sulfides

Two-dimensional sulfide has been widely recognized as a promising new type of catalyst to replace precious metals due to its adjustable electronic structure, low cost, and high stability. In this paper, monolayer molybdenum disulfide (MoS2) and layer-controlled tungsten disulfide (WS2) were successfully prepared by chemical vapor deposition (CVD). The two prepared materials’ morphology, structure, and thickness were investigated. The catalytic performance of two-dimensional sulfides was studied under an acidic environment. The results exhibit good catalytic performance toward hydrogen evolution with 63.6 mV/dec low Tafel slope of MoS2 and 72.8 mV/dec of WS2.


INTRODUCTION
In 2004, graphene sheets with only one layer of carbon atoms were successfully prepared by mechanical stripping.Due to its excellent mechanical, optical, electrical, and other physical and chemical properties, graphene has shown excellent performance in photodetectors [1] , field-effect transistors [2] , solar cells [3] , sensors [4] , and other fields, thus attracting widespread attention.However, the band gap width of graphene is close to zero.With the continuous innovation of material preparation methods, other new materials with layered structures have been discovered.Hundreds of 2D materials and their properties have been investigated successively, including hexagonal [5] , transition metal chalcogenides (TMDCs) [6] , etc.
TMDCs, as an essential two-dimensional material, have been studied by many researchers.Its molecular formula is usually MX2, M is the transition metal element, and the representative elements usually include Ti, Pt, Nb, Ta, Mo, W, etc. X is the chalcogens; the representative elements usually include S, Te, Se, etc. TMDCs have many surprising characteristics, such as adjustable band gap, high carrier mobility, and no surface suspension bonds showing broad application prospects in electrocatalysis [7] and other fields [8] .
Hydrogen energy is a pollution-free, renewable, and high-efficiency new energy source [9] .Energy and environmental issues have become urgent problems.Water electrolysis is an effective means of hydrogen production [10] , so it is urgent to find new catalyst materials.These properties of TMDCs have rapidly attracted extensive research in electrocatalysis [11] , and it is expected to replace precious metal catalysts as a new generation of green and high-performance catalytic materials.According to the research, MoS2 has a low hydrogen adsorption freedom.It is very beneficial to the occurrence of electrocatalytic hydrogen evolution (HER) [12] .By the chemical method, 2H phase MoS2 nanosheets are decorated with single-crystal Pt nanoparticles [13] .The best hydrogen evolution performance was achieved by introducing the single S vacancies distributed on the surface of MoS2 nanosheets.Similarly, WS2 also exhibits good potential for electrocatalytic hydrogen evolution [14] .
In this paper, the CVD method controllably prepared monolayer MoS2 with different coverage, sizes, and shapes and two-dimensional WS2 with different layers and shapes.The surface morphology and structural properties of the prepared two-dimensional MoS2 and WS2 samples were characterized by optical microscopy (OM), Raman scattering (Raman), photoluminescence (PL) spectrum, SEM, and atomic force microscopy (AFM).The catalytic devices were constructed by wet transfer.The electrochemical workstation studied the HER performance of two-dimensional MoS2 and WS2 under acidic conditions.

Characterization of molybdenum disulfide
By adjusting the growth parameters, the morphological characterization of the monolayer MoS2 is shown in Figure 1.The OM of the MoS2 is shown in Figure 1a.As we can see, MoS2 is evenly distributed, and the shape of MoS2 is about regular triangular with good quality.Figure 1b shows the MoS2 SEM image.Under the higher magnification of SEM, the surface of MoS2 is smooth and flat, and the nanosheets are smooth without barbs, which preliminarily shows that the prepared MoS2 has high quality.Figure 1c is an AFM image of MoS2.It can be seen that the monolayer triangular MoS2 thickness is about 0.9 nm.It is proved that the prepared MoS2 sample has a single atomic layer structure.

Characterization of tungsten disulfide
Although the preparation of WS2 is the same as MoS2 by the chemical vapor deposition method, during the preparation of WS2, a certain proportion of the halogen salt NaCl is added.Because the sublimation temperature of the WO3 is too high, adding NaCl can significantly reduce the reaction temperature and holding time and then improve the crystal quality of WS2. Figure 3a is the OM image of WS2, and Figure 3b is the SEM image of WS2.As we can see, the WS2 is a regular triangle shape, and the triangle nanosheets have transparent edges and smooth surfaces, which preliminarily shows that the prepared WS2 is of high quality.Figure 3c is an AFM image of a single WS2 nanosheet.It can be seen that there are no defects or stains on the surface of the triangle WS2.The thickness of WS2 is about 0.9 nm.It proves that the WS2 sample has a single atomic layer structure.

Electrocatalytic performance of MoS2
The electrocatalytic performance of MoS2 was tested using an electrochemical workstation under acidic conditions (0.5 M H2SO4).The hydrogen evolution performance of MoS2 with different coverages, sizes, and shapes was studied, mainly from LSV curves, the Tafel slope, and the charge transfer impedance.Figure 5 is the electrocatalytic performance tested under the same area, but the coverage rates are 19.75%,37.04%, 56.79%, and 82.10%, respectively.The coverage rates were obtained by using the method of counting lattices (324 lattices in total).LSV curve in Figure 5a shows that the current density of 10 mA cm -2 overpotentials is 285.1 mV, 292.4 mV, 317.1 mV, and 400.5 mV in different coverage samples.From Figure 5b, the Tafel slopes for different coverage samples are 97.6 mV/dec, 88.9 mV/dec, 76.4 mV/dec, and 63.6 mV/dec, respectively.Figure 5c shows that with the increase of the coverage, the overpotential of MoS2 decreases continuously, and the Tafel slope decreases continuously.It shows that the higher the coverage of MoS2 per unit area is, the better the catalytic effect is.It suggested that with the higher coverage per unit area, there are more numbers of MoS2.Thus, it was shown higher catalytic activity and better catalytic performance.Figure 5d is the EIS map of MoS2 with different coverage ratios.The figure shows that with the coverage increasing, the impedance diameter of MoS2 becomes smaller, which indicates the charge transfer speed becomes faster.and 6c.When the size of MoS2 is 15 ȝm, 30 ȝm, and 45 ȝm, at 10 mA cm -2 , the current density the overpotential is 304.7 mV, 362.7 mV, and 391.4 mV, respectively, and the corresponding Tafel slopes are 94.4,120.8 and 148.1 mV/dec, respectively.With the size decreasing, the catalytic performance becomes better.In the same coverage, a smaller size MoS2 has more edge sites, so there are more active sites in small MoS2, resulting in higher catalytic activity and, ultimately, a better catalytic effect.Figures 6b and 6d show different shapes.The overpotentials of triangular MoS2 and hexagonal MoS2 are 313.6 mV and 349.0 mV, and corresponding Tafel slopes are 87.6 mV/dec and 102.6 mV/dec.Triangular MoS2 has better catalytic performance than hexagonal MoS2.It is attributed that when the size is the same, the edge length of a single triangular MoS2 is longer than that of the hexagonal MoS2.There are more active sites on the edge, which can produce a better catalytic performance.The EIS curve of MoS2 with different sizes and shapes are shown in Figure 6e and 6f.It shows the same trend as the LSV and Tafel slope curves.With the size decreasing, the EIS diameter decreased.Similarly, the EIS diameter of triangular MoS2 is smaller than that of hexagonal MoS2.Similarly, the hydrogen evolution performance of WS2 with different shapes is consistent with that of MoS2, and the hydrogen evolution performance of triangular WS2 is better than that of hexagonal WS2.

Preparation of two-dimensional sulfides
The CVD method grew Monolayer MoS2 and WS2 with different layers on SiO2/Si substrate (SiO2: 300 nm).First, a 2-inch silicon wafer cut into 2 cm x 2 cm squares, followed by acetone, ethanol, deionized water, and piranha was used to clean the silicon wafer, and finally blow dry with nitrogen for use.The preparation process of monolayer MoS2: 10 mg of MoO3 (99.99%) and a sufficient amount of S powder (99.5%) was used as the precursor.The cleaned and dried silicon wafer was inverted on a porcelain boat with MoO3 and placed in a tube furnace for reaction.We first poured the tube at a rate of 500 sccm high-purity argon gas (99.99%), used to remove oxygen and other gases for one hour, adjusted to 180 scmm after one hour, then heated S powder to 180 °C, and heated MoO3 to 850 °C at 50 °C/min and kept for 25 minutes.Finally, monolayer MoS2 was obtained.The preparation process of WS2 with different layers is as follows: We mixed 10 mg of WO3 (99.99%) and 2 mg of NaCl (99.5%), and a sufficient amount of S powder (99.5 %) was put as the precursor, by adjusting the amount of NaCl from 1 mg to 5 mg, the growth temperature was 810 °C to 850 °C, and the holding time was 5 min to 15 min, and WS2 with different layers was finally obtained.

Characterization of samples
The sample's surface morphology was observed by optical microscope (Olympus BX51M) and then further scanning electron microscope (SEM in an FEI Quanta 3D) at higher magnification with an accelerating voltage of 5 kV.An atomic force microscope (Bruker Icon, Multimode Nanoscope რD), characterized the thickness of the samples, Raman spectroscopy, and photoluminescence spectroscopy, was performed by confocal laser Raman spectrometer (Horiba Jobin Yvon HR800).

Preparation of electrocatalytic devices
Two-dimensional sulfides were tested for electrocatalytic hydrogen evolution using conductive glass FTO as the substrate.First, the conductive glass FTO was washed with deionized water and spincoated PMMA on the sample, and then we put the spin-coated silicon wafer into HF solution (HF: H2O = 1: 5) for etching.After the sample was separated from the substrate silicon wafer, we transferred the sample with washed FTO and then removed PMMA with acetone and acetone.Then, we used conductive silver paste and the tin wire to lead out on the FTO.

Figure 1 .
Figure 1.Morphology characterization of MoS2 prepared by CVD.(a) OM image, (b) SEM image, (c) AFM image For monolayer MoS2, Figure 2a is the Raman spectrum of MoS2.At the 200-500 cm -1 range, the MoS2 has only two prominent peak E 1 2g positions, and the two obvious peaks' characteristics are inplane vibration and out-of-plane vibration A 1 g, indicating that the MoS2 sample is a 2H phase.The position of the E 1 2g peak is 385.6 cm -1 , and the position of the A 1 g peak is 405.1 cm -1 .The peak difference calculated is 19.5 cm -1 , which proves that the sample is a monolayer MoS2.The inset picture is the enlarged part of Figure 2a. Figure 2b is the PL spectrum of MoS2.From Figure 2b, the PL characteristic peak position of MoS2 is 1.85 eV, and the peak shape is sharp, without impurity peaks.It is also proved that the prepared MoS2 sample has a high-quality monolayer structure.The Raman and PL spectrum of 4 different positions on the same MoS2 nanosheets are shown in Figures 2c and 2d.The inset of Figure 2d shows the 4 selected positions.

Figure 2
Figure 2 Structure characterization of MoS2 prepared by CVD.(a) Raman spectrum of MoS2 (inset is the enlarging part of (a)), (b) PL spectrum of MoS2, (c) Raman spectrum of 4 different locations in one nanosheet, (d) PL spectrum of 4 different locations in one nanosheet (inset is the four different locations)

Figure 4
Figure 4 Structure characterization images of WS2 prepared by CVD.(a) monolayer WS2 Raman spectrum (inset is the enlarged part of (a)), (b) monolayer WS2 PL spectrum, (c) WS2 different layers Raman spectrum, (d) PL spectrum of WS2 with different layers

Figure 6
Figure 6 Hydrogen evolution performance of MoS2 with different sizes and shapes.Hydrogen evolution performance of MoS2 with different sizes: (a) LSV, (c) Tafel slope, (e) EIS, Hydrogen evolution performance of MoS2 with different shapes.: (b) LSV, (d) Tafel slope, (f) EIS 2.4 Electrocatalytic performance of WS2 In Figure 7a, the overpotentials of 1-layer, 2-layer, 4-layer, and multilayer WS2 at 10 mA cm -2 current density are 339.0mV, 381.3 mV, 420.8 mV, and 480.4 mV, respectively.In Figure 7b, the corresponding Tafel slopes are 72.8,80.2, 101.2 mV/dec, and 102.3 mV/dec, respectively.In Figure 7c, the overpotential and Tafel slopes of monolayer WS2 are the smallest.With the number of layers increasing, the overpotential and Tafel slope increase continuously, and the catalytic effect becomes poor.It was shown that the monolayer WS2 has the best catalytic performance because the layers increased the edge active sites of decreased WS2, resulting in poor catalytic performance.Figure 7d shows the electrochemical impedance curves of WS2 with different layers.With the number of layers decreasing, the EIS diameter becomes smaller, which is consistent with the LSV curve and Tafel slopes.It indicates that better catalytic performance is achieved with the number of layers decreasing.Similarly, the hydrogen evolution performance of WS2 with different shapes is consistent with that of MoS2, and the hydrogen evolution performance of triangular WS2 is better than that of hexagonal WS2.

Figure 7
Figure 7 Hydrogen evolution performance of WS2 with different layers.(a) LSV, (b) Tafel slope, (c) Comparison of overpotential values and Tafel slope, (d) EIS 3 Conclusion MoS2 and WS2 were successfully prepared by chemical vapor deposition.The study shows that the prepared monolayer MoS2 has a uniform distribution, regular morphology, and high quality.We show that the catalytic performance of the high coverage, small size, and monolayer of MoS2 and WS2 are better than others.Meanwhile, triangular MoS2 has higher catalytic activity than hexagonal MoS2.Sulfide has higher catalytic activity.The Tafel slopes of MoS2 and WS2 are as low as 63.6 mV/dec and 72.8 mV/dec.This work provides a new perspective for preparing two-dimensional materials and can be used in two-dimensional materials.