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Liquid Phase Exfoliation of MoS2 Assisted by Formamide Solvothermal Treatment and Enhanced Electrocatalytic Activity Based on (H3Mo12O40P/MoS2)n Multilayer Structure Jinzhao Huang, Xiaolong Deng, Hao WAN, Fashen Chen, Yifan Lin, Xijin Xu, Renzhi Ma, and Takayoshi Sasaki ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Sustainable Chemistry & Engineering

Liquid Phase Exfoliation of MoS2 Assisted by Formamide Solvothermal Treatment

and

Enhanced

Electrocatalytic

Activity

Based

on

(H3Mo12O40P/MoS2)n Multilayer Structure

Jinzhao Huang*,†,‡, Xiaolong Deng†,‡, Hao Wan‡, Fashen Chen‡, Yifan Lin‡, Xijin Xu†, Renzhi Ma*,‡ and Takayoshi Sasaki‡ †

School of Physics and Technology, University of Jinan, Jinan 250022, Shandong

Province, P R China ‡

International Center for Materials Nanoarchitectonics (WPI-MANA), National

Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan

Corresponding Author * E-mail: [email protected] (Jinzhao Huang) * E-mail: [email protected] (Renzhi Ma) Full Mailing Address School of Physics and Technology, University of Jinan，No. 336, West Road of Nan Xinzhuang, Jinan, Shandong Province, P. R. China 250022 (Jinzhao Huang, Xiaolong Deng, Xijin Xu) Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan (Hao Wan, Fashen Chen, Yifan Lin, Xijin Xu, Renzhi Ma, Takayoshi Sasaki)

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ABSTRACT: In this work, MoS2 nanosheets were obtained successfully using the liquid phase exfoliation method assisted by formamide solvothermal treatment. The exfoliation efficiency in N-methyl-2-pyrrolidone (NMP) was enhanced by the synergetic effect of easier intercalation of polar solvent and higher repulsive force of the treated bulk MoS2. The exfoliated MoS2 nanosheets were assembled alternately with H3Mo12O40P (PMo12) into multilayer hetero-structure by the layer-by-layer (LBL) method, in which PMo12 with high electron mobility bridges the adjacent catalytically active MoS2 layers. Based on the hetero-structure, the electrocatalytic performance for hydrogen evolution was substantially enhanced than multilayer MoS2 nanosheets alone. Moreover, it was found that the electrocatalytic performance was influenced by the layer number, indicating that an optimum balance between the mass transfer (MoS2 layer) and electron conductivity (PMo12 layer) was needed for the construction of efficient electrocatalysts. In addition, the electrocatalytic performance of the multilayer (MoS2)n and (PMo12/MoS2)n could be improved by oxygen plasma treatment, which might be ascribed to the increased number of edges and defects in MoS2 nanosheets. This work not only provides a facile method to exfoliate the MoS2 with higher efficiency, but also offers a feasible strategy to build up high-performance electrocatalysts by rationally assembling nanosheets and polyoxometalate species at the nanoscale level. KEYWORDS: MoS2 nanosheets, liquid phase exfoliation, phosphomolybdic acid hydrate, layer-by-layer method, electrocatalytic activity, oxygen plasma

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INTRODUCTION The rapid consumption of exhaustible fossil fuels and the consequent severe environmental pollution pose an urgent demand for sustainable and green energy resources.1,2 Among the possible alternative energy resources, hydrogen energy is considered as one of the most promising alternative to traditional fossil fuels due to its cleanliness, renewability, environmental harmlessness and high energy density, and so on.3 Of the numerous methods to produce hydrogen, electrocatalytic water splitting has been proved to be the most prospective and attractive approach, taking into account the advantages of low cost, pollution-free nature, high efficiency, etc.4 The most important issue of the production method is the electrocatalytic activity of the cathode, which determines the efficiency of hydrogen evolution reaction (HER).5 By now, platinum (Pt) with a near-zero overpotential, is considered as the best electrocatalyst for HER. Unfortunately, the high cost and scarcity have greatly limited its wide practical utilization and industrialization.6 Therefore, it is extraordinarily meaningful to look for an alternative electrocatalyst for HER with earth abundance, low overpotential, high efficiency and harmlessness. In order to achieve the goal, many efforts have been paid to develop new candidate materials, such as two-dimensional (2D) materials,7 C3N4,8 and metal sulfides,9 etc. 2D materials, particularly those based on transition metal dichalcogenides (TMDCs), are attracting great interest in recent years for their optoelectronic and electrocatalytic characteristics.10 Among the TMDCs, molybdenum disulfide (MoS2) with hexagonal layered structure has drawn tremendous attention as a promising electrocatalyst due to 3



its low cost, high electrocatalytic activity, high stability, large in-plane electron mobility, excellent mechanical behavior, etc.11 In order to promote the electrocatalytic activity of MoS2, a variety of modifications have been explored, such as monolayer or few layers MoS2,12 3-dimensional (3D) mesoporous structure,13 active 1T-MoS2 (metastable phase),14 oxygen-incorporated defect-rich MoS2,15 nanoparticles,16 and so on. Among these structures, monolayer or few layer MoS2 is regarded as a promising entity with an enhanced electrocatalytic performance.12 Up to now, there are two major processes to prepare monolayer MoS2. The first is the top-down method, which includes the micromechanical,17 ion-intercalation18 and liquid phase exfoliation.19 The other category is bottom-up approaches and the representative methods are chemical vapor deposition (CVD),20 and wet chemical synthesis.21 Among the top-down methods, the mechanical exfoliation produces high-quality monolayers but with the deficiency of low yield.22 On the other hand, ion-intercalation are desirable in achieving high exfoliation yield and high purity of nanosheets, but the need for strong intercalating agents, long reaction time, and strict storage conditions stand the challenges for large scale application.23-25 Liquid phase exfoliation, firstly utilized by Coleman, is considered to be a very promising method toward the large scale production of monolayer MoS2.26, 27

However, the low yield of monolayer production restricts further application and

development of this method.28 In order to enhance the exfoliation efficiency, many efforts have been applied, such as temperature controlling,29 ionic liquids assisting,30 solvent grinding assisting,31 sequential solvent exchanging,32 trace water assisting,33 4


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and so on. A recent study revealed that only the edges of MoS2 sheets are active for HER.34 Therefore, increasing the number of active edge sites of MoS2 by modifying the atomic structure,35 engineering the composition and crystallinity,36 or doping,37 has remained important issues for efficient HER. In addition, the electrical conductivity of the electrocatalyst is another key point affecting the performance.38 To address this issue, many strategies were explored, and one of the effective approaches was to combine the MoS2 with conductive materials, such as metal nanoparticles,39 reduced graphene oxide,40 graphene sheets,41 or carbon fiber,42 etc. In this regard, Polyoxometalates (POMs), combinations between oxygen and early transition metals at their highest oxidation states,43, 44 have been studied widely owing to their chemical, structural and electronic versatility.45, 46 Particularly, the anionic POMs can accept varied numbers of electrons, which has made these compounds very attractive in electronic conductivity modification and electrocatalytic research.47-50 In this study, we present a simple liquid phase exfoliation method to produce MoS2 nanosheets with an enhanced efficiency. The bulk MoS2 was pre-treated in formamide using solvothermal method, thereby the efficiency of liquid exfoliation in N-methyl-2-pyrrolidone (NMP) under sonication was enhanced. To the best of our knowledge, this is the first report on liquid phase exfoliation of MoS2 assisted by formamide solvothermal treatment. Possible reasons of the enhanced exfoliation efficiency are elucidated. The layer-by-layer (LBL) method was used to build up the multilayer structure with MoS2 nanosheets and phosphomolybdic acid hydrate 5



H3Mo12O40P (PMo12), which was used as the electrocatalyst for the first time. The sequential LBL technique was one of the most promising methods for multilayer thin film fabrication, providing a reasonable way of controlling composition, thickness, and architecture at the nanoscale level.51,

52

The corresponding electrocatalytic

mechanism indicated that an optimum balance between the mass transfer (MoS2 layer) and electron conductivity (PMo12 layer) was crucial for the construction of efficient electrocatalysts. Moreover, we demonstrated that oxygen plasma treatment on MoS2 nanosheets films fabricated by LBL process could introduce more exposed edges, leading to a high density of active sites and an improvement of the electrocatalytic activity.

EXPERIMENTAL SECTION Materials MoS2 were purchased from Furuuchi Kagaku, Japan. N-methyl-2-pyrrolidone (NMP), PMo12, formamide and poly(diallyldimethylammonium) chloride (PDDA) were purchased from Wako Chemicals, Japan. All reagents were of analytical grade and used without further purification. Milli-Q deionized (DI) water with a resistance of >18 MΩ⋅cm was used throughout the experiments. Exfoliation of MoS2 Bulk MoS2 (1.6 g) and 40 cm3 formamide were sealed in an autoclave and heated at different temperature 120 oC, 130 oC and 140 oC respectively for 48 h. The solvothermal treated MoS2 in formamide were washed three times in DI water by 6


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vacuum filtration. Typically, 100 mg of bulk MoS2 treated by the formamide was added into a 100 cm3 conical flask. Then, 100 cm3 of NMP was added as the exfoliation solvent. The mixture was sonicated for 3 h at a frequency of 37 kHz and power of 100 W. The resulting suspension was first centrifuged at 4000 rpm for 1 h, and then the top 2/3 of the dispersion was carefully collected by pipet. The collected suspension was centrifuged again at 6000 rpm for 0.5 h, and then the top 2/3 portions of the homogeneous dispersion was carefully collected for the utilization. LBL Building up of the Multilayer Film PMo12 (10 g·dm−3, pH = 4) and MoS2 nanosheets dispersion (from solvothermal treated bulk MoS2 at 130 oC) were sequentially adsorbed onto a quartz, tin doped indium oxide (ITO) and silicon substrate with intermittent dipping into PDDA (20 g·dm−3, pH = 9). The substrate was immersed in each solution for 20 min, and then rinsed thoroughly with DI water and dried with a N2 flow. The procedure contributes to the building up of the multilayer films containing both PMo12 and MoS2 ((PMo12/MoS2)n) or MoS2 only ((MoS2)n). The procedure for fabricating the multilayer films via the LBL method is illustrated in Figure 1. The quartz and silicon substrate were cleaned by immersing in a bath of methanol/HCl (1:1 in volume) and then the concentrated H2SO4 for 20 min respectively. The ITO substrates were cleaned by ultrasonic treatment in ethanol, acetone and then DI water for 20 min, respectively, followed by the drying with a N2 flow. The oxygen plasma treatment was carried out by using a UV253V8 system. The annealing is carried out in a tube furnace under the protection of N2. 7



Characterization UV-visible absorption spectra of the MoS2 nanosheets dispersion were recorded using a Hitachi U-4100 spectrophotometer. The morphology of the MoS2 nanosheets were examined using a Seiko SPA400 atomic force microscopy (AFM) in tapping mode using a Si tip cantilever. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo ESCALAB 250Xi spectrometer with an Al Kα source (hν =1486.6 eV, 150 W). The spectra were calibrated relative to the C1s binding energy at 284.8 eV. Transmission electron microscopy (TEM) and electron diffraction analysis were performed using a JEOL JEM-3000F TEM. The X-ray diffraction (XRD) data was collected using a Rigaku Rint ULTIMA IV diffractometer with a monochromatic Cu Kα radiation (λ = 0.15405 nm). Scanning electron microscopy (SEM) observation was performed on a JEOL JSM-6010 LA SEM. Raman spectra were recorded using a Horiba Jobin Yvon Lab RAM HR800 system with a 532 nm excitation laser. Thermogravimetric measurements were carried out using a Thermo Plus 2 TG8120 instrument in a temperature range of room temperature to 700 oC at a heating rate of 1 o

C /min under air flow. The contact angle is tested using FACE contact angle meter

CA-XP. Electrochemical Measurements The electrochemical measurements were carried out on CHI 760E in a three electrode system, in which Pt foil was used as a counter electrode, Ag/AgCl was employed as a reference, and the fabricated multilayer film was used as the working electrode (area = 2 cm2). For electrochemical measurement, a 0.5 M H2SO4 solution was utilized as 8


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the electrolyte. Linear sweep voltammetry (LSV) was recorded at a scan rate of 10 mV/s to obtain the polarization curves. All LSV polarization curves were corrected with 95% iR-compensation. The electrochemical impedance spectroscopy (EIS) spectra were acquired in the frequency range from 106 to 0.01 Hz, at 0 V, and the AC modulation amplitude was 5 mV. The electrochemical active surface areas (ECSA) were calculated by cyclic voltammetry (CV) using the same working electrodes at a potential window of 0.45 – 0.55 V (vs the reversible hydrogen electrode (RHE)). CV curves were recorded at different scan rates of 20, 40, 60, 80 and 100 mV/s respectively. After plotting current density differences (at the overpotential of 0.5 V) versus the scan rates, the slope, twice of the electrochemical double-layer capacitance (EDLC) Cdl, was used to represent ECSA. The Cdl was calculated according to the following equation: ic = vCdl, where ic, v and Cdl are the charging current density (mA/cm2), scan rate (mV/s), and double-layer capacitance (F/cm2) of the electroactive materials,

respectively.

The

long-term

stability

test

was

performed

by

chronopotentiometry at a constant current density of 0.2 mA/cm2. All the potentials were calibrated with respect to the RHE on the basis of the following equation: E (V vs RHE) = E (V vs Ag/AgCl) + 0.197 V + 0.0591 × pH.39

RESULTS AND DISCUSSION Exfoliation and Characterization of MoS2 nanosheets Figure 2 shows the UV-vis absorption spectra of the exfoliated MoS2 nanosheets dispersions under different solvothermal treatment temperatures. The two peaks 9



located at 615 nm and 670 nm, are characteristic of the direct excitonic transitions of MoS2 at the K point of the Brillouin zone. The 450 nm and 395 nm peaks are attributable to the direct excitonic transition of the M point.53 The concentration of the exfoliated MoS2 nanosheets in the dispersion can be determined based on the absorption spectrum and the Beer-Lambert law, A/l = αC, where A is the absorbance, l is the optical path length, α (1020 mL/mg/m) is the absorption coefficient, and C is the concentration.54 The concentrations estimated were ~0.0003 mg/ml (yield 0.03%), ~0.15 mg/ml (yield 15%), ~0.21 mg/ml (yield 21%) and ~0.06 mg/ml (yield 6%) for the pristine sample (commercial bulk MoS2) and the solvothermal treated samples at 120 oC, 130 oC and 140 oC, respectively. Obviously, the treatment facilitated the exfoliation into MoS2 nanosheets. The highest yield of 21% was obtained for the sample treated at 130 oC, which was not worse than the yield reported recently.28 The photographs of the obtained dispersions under different solvothermal temperature are shown in the inset of Figure 2. Figure 3 (a) depicts the typical AFM image of the exfoliated MoS2 nanosheets derived from the sample treated at 130 oC. The exfoliated MoS2 nanosheets were about 40 nm in lateral dimension and about 0.9 nm in thickness. In order to further confirm the distribution of thickness and lateral size of the exfoliated MoS2 nanosheets, the selected MoS2 nanosheets were analyzed statistically (Figure S1). Based on Figure S1 (b), 90% of the nanosheets were less than or equal to 0.9 nm in thickness, and the lateral size of the nanosheets mostly ranged from 32 nm to 42 nm. Although the observed thickness was a little larger than that of the theoretical value (∼0.65 nm), the 10


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obtained nanosheets are in a monolayer regime.55 The extra height was coming from the adsorption of water or the solvent. TEM also detected the exfoliated MoS2 as a lath-like object with an ultralow contrast (Figure 3 (b)). The selected area electron diffraction (SAED) pattern illustrated the hexagonal diffraction spots of (100) with [001] zone axis (Figure 3 (c)).56 These characterization results indicated that the exfoliated MoS2 nanosheets are of single crystal in 2H-MoS2 structure. Based on the following considerations, the formamide was selected as the solvent for the solvothermal treatment. Firstly, the formamide was favorable to the delamination/exfoliation of layered double hydroxide (LDH), therefore, it might analogize the some role in the exfoliation of MoS2.57 Secondly, the formamide was relatively safe during the solvothermal reaction at higher temperature. Although, the hydrazine hydrate exhibited better effect on solvothermal treatment, it was very dangerous in the process of solvothermal treatment. In order to clarify the effect of the solvothermal treatment, the morphology and structure of the bulk MoS2 after treatment in formamide at different temperature, were examined by SEM images as shown in Figure S2 (a)-(d). No noticeable morphological differences were observed after the solvothermal treatment in formamide. The XRD data (Figure S2 (e)) indicated that the 2H-phase MoS2 structure (PDF 01-087-2416) remained unchanged. It was worth noting that, after the solvothermal treatment, the bulk MoS2 still maintained high crystallinity, which can be confirmed by the sharp (002) peak. Since the solvothermal treatment did not bring about any obvious changes in the morphology and structure, the chemical states of Mo in the solvothermal treated bulk 11



MoS2 were examined by XPS (Figure 4). According to the analysis of the Mo 3d spectra, the Mo signals arised from the 3d3/2 and the 3d5/2.55 With the increasing treatment temperature, the binding energy of the Mo shifted to the lower energy. The smallest binding energy was obtained for the sample treated at 130 oC. No further obvious shift was observed even by increasing the temperature to 140 oC. After the deconvolution of the Mo 3d, it can be confirmed that the proportion of Mo4+ decreased with the increasing treatment temperature. In contrast, the proportion of the reduced state of Mo4+ was enhanced after the treatment temperature was increased. During the solvothermal treatment in the formamide, some intermediates might be produced (such as the reduction of Mo4+), which can accept extra electrons. 23 That is to say, the shifts to lower binding energy are attributed to the enhanced electron density around the Mo and S atoms. As a result, the repulsion between the adjacent layers is enhanced, which is beneficial to the exfoliation of bulk MoS2. In order to further confirm the effect of solvothermal treatment on the binding energy, the chemical states of S were also determined by the XPS (Figure S3). From the S 2p (S 2p1/2 and S 2p3/2), the binding energy shifted toward the lower direction with increasing solvothermal temperature, which exhibited the similar tendency as the Mo. The contact angle of pristine MoS2 in water was measured to be ~22 o. Generally, better hydrophilicity with smaller contact angle is a benefit to the intercalation between the adjacent layers of the MoS2.58 Figure S4 shows the measurements of the contact angle of the MoS2 treated at different temperature. Smaller contact angle of 11 o

and 16

o

was observed for the samples treated at 120 oC and 130 oC, respectively. 12


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With increased temperature, the contact angle was increased. In contrast, the contact angle became 38 o, after the treatment at 140 oC, which is even larger than that of untreated MoS2 (22 o). The solvothermal treatment may introduce extra chemical bond of the surface, which is attributable to the changing of the contact angle. Sufficient liquid wetting or intercalation into the layered MoS2 leads to the reduction of the van der Waals interaction between the MoS2 layer. The smaller contact angle is beneficial to facilitate the intercalation of polar solvent, such as NMP, into the layered crystal, in which the high energy liquid jets can generate not only at the edges, but also in the inner layer during sonication, allowing for efficient exfoliation into monolayer nanosheets.59 Based on the aforementioned contact angle analysis, the lowest contact angle was obtained after the treatment at 120 oC. On the other hand, the higher repulsive force can overcome the electrostatic attraction force between adjacent layers easily, which can enhance the exfoliation efficiency. According to XPS analysis, the electron density is increased or the reduction of Mo4+ occurs after the solvothermal treatment. The higher repulsive force resulting from the higher electron density can also enhance the exfoliation efficiency. On the basis of the analysis on the binding energy, the highest exfoliation efficiency would be expected for samples treated at 130 oC and 140 oC. Figure S5 illustrates the effects of contact angle and binding energy on the exfoliation. It appears that the synergetic effect led to the highest exfoliation efficiency for MoS2 pre-treated at 130 oC. Preparation and Characterization of the Multilayer Film (PMo12/MoS2)n The multilayer building up process of (PMo12/MoS2)8 and (MoS2)8 on a quartz 13



substrate was monitored by UV-vis spectroscopy after each deposition cycle as depicted in Figure 5 (a) and (c). The PMo12 and MoS2 exhibit different absorption profiles. The spectral profile of PMo12 having a peak at 208 nm (inset of Figure 5 (b)), was progressively enhanced after every deposition cycle of PMo12. Similar tendency was observed in the MoS2 nanosheets deposition. The stepwise enhancement of absorbance at the characteristic peaks at 208 nm for PMo12 and 615 nm for MoS2 provides persuasive evidence for the regular growth of the multilayer films (as shown in Figure 5 (b) and (d)). In our work, PDDA (Polycations) was typically used as a counterpart for the self-assembly process, because PMo12 was negatively charged and the MoS2 nanosheets was neutral. To a certain extent, PDDA would hinder the reaction for its lower carrier mobility. Here, the samples were annealed at 400 °C, and PDDA could be decomposed partially, which reduced the negative impact on the electrocatalytic performance. The structural aspect of the multilayer films (PMo12/MoS2)8 and (MoS2)8 was examined by XRD, as depicted by Figure 6 (a). The diffraction peak appeared at 5.06 o

and 5.28

o

for (PMo12/MoS2)8 and (MoS2)8 respectively, indicating the repeating

periodicity of 1.67 nm and 1.74 nm. The interlayer distance between MoS2 of (PMo12/ MoS2)8 is almost the same as that of the (MoS2)8 without PMo12. These results suggest that the PMo12 is not directly pillaring the multilayer structure. It is speculated that the PMo12 is coordinated or incorporated with PDDA, as exhibited in Figure 6 (c). The similar structural features have been recognized for the multilayer films of Ti0.91O2/Au,60 and Al13 polyoxocations/MnO2 nanosheets.61 The weak diffraction peak 14


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suggest that the structure is sdisordered due to smaller lateral size of the MoS2 nanosheets. As shown in the Figure S6, the thickness (about 12 nm) of the multilayer (PMo12/MoS2)4 can be discerned. According to the thickness of MoS2 nanosheets (about 0.9 nm) examined by AFM and the distance between the adjacent MoS2 nanosheets in (PMo12/MoS2)n (about 1.74 nm) obtained by the XRD analysis, the total thickness of (PMo12/MoS2)4 was about 10.56 nm, which was almost consistent with the result of cross-section SEM. As exhibited in Figure S6, the rough surface was originated from the destructed multilayer film by preparing cross-sectional SEM sample. Electrocatalytic Tests As shown in Figure 7 (a) and (b), the current density increased with the increased layer number for the (PMo12/MoS2)n. This is due to the fact that more catalytic sites (MoS2) and conductive supporting material (PMo12) are incorporated into the multilayer film. However, the current density decreased at the layer number of 4 and above, indicating that the electrocatalytic activity can be adjusted by the layer number. The number of the active site is enhanced with increasing the layer number, while the larger thickness may limit the mass transfer from the electrolyte. Thus, a balance between the mass transfer and the electron conductivity is needed to achieve the highest electrochemical performance. Moreover, the gas shielding effects or mechanical instabilities of the sample with larger thickness might introduce the negative effect on the electrocatalytic performance. From Figure 7 (c), the introduction of the PMo12 can improve the electrochemical activity of the multilayer 15



film, resulting from the higher electron mobility of the PMo12. The EIS spectra were applied to investigate the charge conductivity. As shown in Figure 7 (d), the semicircle represented the charge transfer resistance in the multilayer. Compared with (MoS2)4 with large resistance, the (PMo12/MoS2)4 had a smaller resistance, which mainly benefited from the incorporation of PMo12. In the electrochemical measurement, the multilayer film may be peeled off from the ITO substrate. In order to improve the stability of the multilayer structure, the sample was annealed prior to the electrochemical measurement. Figure 8 (a) depicts the LSV of multilayer (PMo12/MoS2)4 annealed under different temperature. The sample annealed at 400 oC showed highest current density. The sample annealed at lower temperature (300 oC) is unstable, giving poor electrochemical activity. Upon heating at higher temperature (500 oC), the structure of PMo12 was decomposed to yield MoO3, which can be confirmed by the thermogravimetric and XRD data (Figure 8 (b), (c)). The MoO3 with lower carrier mobility could result in the lower electrocatalytic activity. The weight loss at about 400 oC may be associated with the decomposition of PMo12. The large weight loss between 0 oC and 150 oC was from the dehydration of crystal water in PMo12, which could be also confirmed by the color change (inset of Figure 8 (c)). The XRD patterns of PMo12 annealed at 300 oC and 400 oC were in good accordance with that of pristine PMo12, except for the change that the peaks became weak and broad. The decomposition of PMo12 to produce MoO3 at 500 oC can be discerned from the color change (inset of Figure 8 (c)). To keep (PMo12/MoS2)n multilayer structure, all the samples were annealed at 400 oC in this study. 16


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The LSV results of the multilayer films (MoS2)4 with oxygen plasma treatment time of 5 min, 15 min and 20 min are shown in Figure 9 (a). The sample treated with 15 min exhibited the highest current density. In order to understand the effects of oxygen plasma treatment on the electrochemical activity of MoS2, the samples were examined by the Raman spectra, as illustrated in the Figure 9 (b). The two distinct peaks at 380 cm-1 and 406 cm-1 correspond to the in-plane vibration (E12g) and out-of-plane mode (A1g) of 2H-phase MoS2, respectively. According to the literature,62 the intensity ratio of E12g mode to that of A1g mode is closely related with the structure of the MoS2. The higher ratio indicates the prevalence of the terrace surface-terminated structure, whereas the lower value is correlated with the edge-terminated structure. Accordingly, higher electrocatalytic activity may be expected from the sample showing the lower E12g/A1g ratio. From the Figure 9 (b), the sample treated with oxygen plasma 15 min exhibited the minimum value, suggesting the predominant formation of the edge-terminated MoS2. Such active sites of MoS2 contribute to the enhanced electrocatalytic performance. The lower E12g/A1g ratio indicated the superiority of out-of-plane mode to in-plane mode, and the higher intensity of A1g would introduce more active sites. So both the lower E12g/A1g ratio and higher intensity of A1g were attributable to the enhanced electrocatalytic activity, and the relationship between current density and E12g/A1g ratio and intensity of A1g were summarized in Figure S7. In order to clarify the effect of annealing and plasma treatment on the oxidation of MoS2 nanosheets, the XPS of the MoS2 nanosheets annealed at 400 °C followed by the oxygen plasma treatment of 15 min were examined, as shown in Figure S8. The 17



peaks at binding energy of 229.7 eV and 235.35 eV were from the Mo 3d5/2 of MoS2 and Mo 3d3/2 of MoO3 respectively. The peak at 232.8 eV was the combination of Mo 3d3/2 of MoS2 and Mo 3d5/2 of MoO3.63 Obviously, the MoS2 nanosheets with annealing and plasma treatment have been partially oxidized. The effect of oxidation on the electrocatalytic activity is not obvious for the solubility of MoO3 in acid.64 Figure 10 (a) exhibits representative LSV curves of (MoS2)4, (MoS2)4 with oxygen plasma treatment for 15 min and (POM/MoS2)4 with oxygen plasma treatment for 15 min. Apparently, the electrocatalytic performance of (PMo12/MoS2)4 film treated with oxygen plasma was substantially enhanced. In addition, the corresponding Tafel slope of this film, 44 mV/decade, was much superior to an original value of 63 mV/decade for the (MoS2)4 film (Figure 10 (b)). We compared the ECSA of the three samples by measuring the double-layer capacitance (Cdl), as the Cdl is proportional to the ECSA. (PMo12/MoS2)4 film treated with oxygen plasma showed the largest Cdl value (0.11 mF/cm2), indicating the increased electrocatalytic active sites and electron conductivity (Figure 10 (c)). It is noteworthy that the relatively low current density of our samples is due to its limited loading amount, which is about dozens of times less than that used in literatures.65 Therefore, an important strategy to overcome this deficiency is to use large-size MoS2 nanosheets in the multilayer structure, which is attributable to enhance the loading amount. The stability and durability of the (PMo12/MoS2)4 film treated with oxygen plasma for 15 min was examined at a constant current density of 0.2 mA/cm2, as shown in Figure 10 (d). It can be seen that the sample retains a nearly constant working potential within 30000 s. 18


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CONCLUSIONS In summary, the liquid phase exfoliation assisted by solvothermal treatment with formamide has been applied to prepare MoS2 nanosheets. Compared to the bulk MoS2 without treatment, the exfoliation efficiency was significantly enhanced, due to the synergetic effect of easier intercalation of polar solvent and higher repulsive force between layers after the treatment. The multilayer structure of MoS2 nanosheets and PMo12 was successfully fabricated through the electrostatic self-assembling LBL process. After the incorporation of PMo12 with high electron mobility, the electrocatalytic performance of MoS2 nanosheet film was enhanced due to the bridging effect of conducting PMo12 between the adjacent MoS2 layers. Moreover, the electrocatalytic performance of the multilayer structure was dependent on the balance between the mass transfer and electron conductivity. In addition, the oxygen plasma treatment on the multilayer film could increase defects and edges of MoS2, which led to further enhancement of the electrocatalytic activity. The enhanced exfoliation yield provides an effective strategy for preparation of MoS2 nanosheets by liquid phase exfoliation. Multilayer of MoS2 nanosheets bridged by the PMo12 offers an ideal structure to enhance both the electrocatalytic active site and electron conductivity.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications 19



website. Additional experimental details. AFM image of the exfoliated MoS2 nanosheets with solvothermal temperature 130 oC; the statistical distribution of thickness and lateral size based on the selected MoS2 nanosheets; The SEM images, XRD patterns and contact angle of the hydrothermal treated MoS2 under different temperature; XPS spectra at the S 2p core level of the solvothermal treated MoS2 under different treatment temperature; The effect of contact angle and binding energy on the exfoliation efficiency; The cross section SEM of the multilayer (PMo12/MoS2)4; The relationship between current density and E12g/A1g ratio and intensity of A1g; XPS spectra at the Mo 3d core level of the MoS2 nanosheets annealed at 400 °C followed by the oxygen plasma treatment of 15 min. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J. H.) * E-mail: [email protected] (R. M.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2016FM30), the Dispatch of Faculty Abroad of the University of Jinan, the National Natural Science Foundation of China (Grant No. 21505050), This work was also partly supported by World Premier International Center for Materials 20


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Nanoarchitectonics (WPI-MANA). R. M. acknowledges support from JSPS KAKENNHI (15H03534, 15K13296).

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Figure 1. Process for building up the multilayer films by the LBL method.

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Figure 2. Absorption spectra of the exfoliated MoS2 nanosheets dispersions under different solvothermal treatment temperature, inset of Figure 2 Photographs of the exfoliated MoS2 dispersions under different treatment temperature.

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Figure 3. AFM image (a), TEM image (b) and SAED pattern (c) of the exfoliated MoS2 nanosheets with solvothermal temperature 130 oC.

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Mo4+

140 °C

130 °C

Red. Mo4+

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

120 °C

Pristine

Mo 3d5/2 Mo 3d3/2 S 2s

224 226 228 230 232 234 236 238 240

Binding energy (eV)

Figure 4. XPS spectra at the Mo 3d core level of the solvothermal treated MoS2 under different treatment temperature.

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Figure 5. (a) UV-visible absorption spectra for (PMo12/MoS2)8 films on a quartz glass substrate, (b) Absorbance at 208 nm and 615 nm of (PMo12/MoS2)8 as a function of deposition cycles, inset of (b) The absorption spectrum of PMo12, (c) UV-visible absorption spectra for (MoS2)8 films on a quartz glass substrate, (d) Absorbance at 615 nm of (MoS2)8 as a function of deposition cycles.

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Figure 6. (a) XRD patterns for the multilayer films (PMo12 /MoS2)8 and (MoS2)8, The diagrammatic sketch of (b) (MoS2)8 and (c) (PMo12 /MoS2)8.

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Figure 7. (a) Polarization curves of multilayer (PMo12/MoS2)n (n is 1, 3, 4, 5, 8), (b) Current density at -0.3 V (vs RHE) as a function of layer number, inset of (b) Photograph of thin films of (PMo12/MoS2)n (n from 1 to 8) deposited on ITO with different number of layer, (c) Polarization curves of multilayer (PMo12/MoS2)4 and (MoS2)4, (d) The EIS spectra of multilayer (PMo12/MoS2)4 and (MoS2)4.

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Figure 8. (a) Polarization curves of multilayer film (PMo12/MoS2)4 annealed under different temperature, (b) TG-DTA curves of commercial PMo12, (c) XRD patterns of the PMo12 annealed under different temperature, inset of (c) Photograph of the PMo12 annealed under different temperature.

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Figure 9. (a) Polarization curves, (b) Raman spectra of multilayer film (MoS2)4 treated with oxygen plasma.

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Figure 10. (a) Polarization curves, (b) The corresponding Tafel plot, (c) The estimation of Cdl at an overpotential of 0.5 V (vs RHE) of multilayer (MoS2)4, (MoS2)4 with oxygen plasma treatment 15 min and (PMo12/MoS2)4 with oxygen plasma treatment 15 min, (d) Chronopotentiometric study of the (PMo12/MoS2)4 with oxygen plasma treatment 15 min at a constant current density of 0.2 mA/cm2.

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For Table of Contents Use Only Synopsis: The MoS2 nanosheets were assembled alternately with PMo12 into multilayer structure, in which PMo12 bridges the adjacent catalytically active MoS2 layers to enhance hydrogen evolution reaction.

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Liquid Phase Exfoliation of MoS2 Assisted by ... - ACS Publications

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