Phase Transition-Promoted Hydrogen Evolution Performance of MoS2

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Phase Transition Promoted Hydrogen Evolution Performance of MoS/VO Hybrids 2

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Guifeng Chen, Xiaoqiang Zhang, Lixiu Guan, Hui Zhang, Xinjian Xie, Shiqiang Chen, and Junguang Tao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12040 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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The Journal of Physical Chemistry

Phase Transition Promoted Hydrogen Evolution Performance of MoS2/VO2 Hybrids †

†

†

†

†

Guifeng Chen, Xiaoqiang Zhang, Lixiu Guan,‡ Hui Zhang, Xinjian Xie, Shiqiang Chen, and †

Junguang Tao, ,* †

Key Lab. for New Type of Functional Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300132, China ‡

School of Science, Hebei University of Technology, Tianjin 300401, China

Abstract: In this work, MoS2/VO2 hybrids were synthesized using two-step hydrothermal method. We use tungsten doping to stabilize the VO2 into rutile phase at room temperature. The VO2 surface was treated using sulfuric acid to enhance the growth sites for MoS2 as well as the hydrogen evolution reaction active sites for their hybrids. We demonstrate for the first time that the phase transition of VO2 at elevated temperature shows significant effect on hydrogen evolution properties of the heterostructures. Upon the phase transition of VO2, the onset potential and Tafel slope of MoS2/VO2 hybrids were dropped from 188 mV and 93 mV/dec to 99 mV and 85 mV/dec, respectively. This was attributed to the enhanced charge transport efficiency as well as interfacial strain effect on modifying the electronic structure of MoS2.

*

Email: [email protected]

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1. Introduction The increased problems of environmental pollution and the exhausting of fossil fuels have put enormous pressure on the daily life of human beings. Therefore, the clean and sustainable energies become an urgent need which have potentials to solve these issues.1-3 Hydrogen energy, which is clean and renewable, thus becomes the first sought-after energy source.4-6 Water electrolysis is one of the effective ways for hydrogen production,7, 8 which normally relies on efficient catalysts to supply active sites for hydrogen evolution reaction (HER)9-11 so as to reduce the electric energy consumption. Until now, the most efficient HER catalysts are Pt-group metals, which catalyze HER with almost zero overpotential.12, 13 However, the high cost and scarcity of these materials limit their large-scale applications.6, 14 Recently, two-dimensional (2D) layered transition metal dichalcogenides (TMDCs) have drawn wide attentions in the field of photocatalysis and electrocatalysis.15-17 This class of materials consist of layered structure with upper and lower layers of S atoms and sandwiched transition metal atoms layer. Their exposed edges provide effective active sites for HER to take place making them hold great promise as a replacement for Pt-based catalysts. However, the pristine MoS2 exhibits poor conductivity and a small number of active sites which limit its electrocatalytic abilities.18 Great efforts have been put to enhance the electrocatalytic performance of 2


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MoS2 via creating more active sites and increasing the charge transfer rate. Both experimental and theoretical reports show that the edge of MoS2 nanostructures are more active as compared to the basal sites.19-23 Coupling the edges of the layered MoS2 nanosheets with another semiconductor can promote the electron transfer rate, thereby greatly improve the electrocatalytic performance. Currently, the researches on MoS2 hybrids system are mainly focused on TiO2,9, 19, 24-26 graphene,27-29 CuO,30 and nickel foam31 etc. In previous work, we have showed that modifying

the

interfacial

electronic

structure

of

MoS2/TiO2

heterostructures can enhance their HER activity.18 Zhang et. al. have succeeded in growing MoS2 on reduced graphene oxide (RGO) films23 and found that the electrical coupling and synergistic effects between MoS2 and RGO films greatly promote efficient electron transfer from the photoexcited dyes to the active edges of MoS2. According to Ma et. al., the in-situ active sites formed in MoS2/TiO2 nano-mixture can also promote electron transfer.19 Li et. al. synthesized a MoS2/CuO heterostructural nanoflowers by a two-step hydrothermal process, which significantly increased the photocatalytic activity as compared to original MoS2 nanoflower.30 In addition, the strain effect on the catalytic activity of MoS2 nanosheets has also drawn a great attention. For instance, Lee et. al. found that mechanically bent tensile-strain-induced MoS2 nanosheets (NSs) displayed significantly higher electrocatalytic activity than the 3



unstrained ones.20 Therefore, it is possible to design a MoS2-based heterostructure and tune their HER catalytic performance via the interfacial lattice structure change which exerts strain onto MoS2 NSs. For this purpose, VO2 is a good candidate since it undergoes insulator-to-metal transition at fairly low temperature (~68 oC) alongside with a lattice structure change. In the past, VO2 has been widely used in photoelectric switches,32, 33 strain sensor,34 Mott transistor35 and temperature sensors36 based on its phase change characteristics. However, little has been done in the field of electrocatalysis especially its coupling with MoS2. Because the V-V atoms in the VO2 nanomaterials are twisted during the phase transition, the lattice structure changes from low symmetrical monoclinic phase to high symmetric tetragonal rutile phase. Upon the phase transition, its conductivity drops 4-5 orders of magnitude.37 Both the conductivity and the lattice structure change of VO2 after phase transition are expected to exhibit positive effect on improving the HER activity in MoS2/VO2 complexes. In this work, MoS2/VO2 hybrids with various purposely modified interfaces have been successfully prepared by a two-step hydrothermal method. We use sulfuric acid (H2SO4) to tune the growth behavior of MoS2 NSs on W-doped VO2 nanorods. The H2SO4-treated samples exhibit enhanced HER activity which emphasizes the important roles that 4


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the interfacial build-in electric field plays for efficient electron transfer. In addition, when the phase transition of VO2 is activated by temperature, our results shown that the HER catalytic activity of MoS2 are greatly improved due to the interfacial strain generated as well as the improved conductivity. 2. Experimental section The MoS2/ViO2 heterostructures with different morphologies were fabricated by two-step hydrothermal methods. In brief, to synthesize W doped VO2 nanorods, 587 mg of vanadium pentoxide (V2O5) was first dispersed in 32 mL deionized (DI) water. Next, 783 mg of oxalic acid (H2C2O4) and

an appropriate amount of ammonium tungstate

(H24N6O2W7·6H2O) were slowly added into above dispersion under vigorous stirring. After stirring for 30 minutes, the resulting solution was transferred into a 50 ml PPL-lined stainless steel autoclave followed by hydrothermal treatment at 250 °C for 72 h. To form MoS2/VO2 hybrids, the previously prepared W-doped VO2 powder, 30 mg of sodium molybdate dihydrate (Na2MoO4·2H2O) and 60 mg of thiourea (C2H5NS) were dissolved in 30 ml of H2SO4 solution at different concentrations: 0 mM, 15 mM, 30 mM, 45 mM, 60 mM, which were thereafter labeled as MoS2/VO2,

15-MoS2/VO2,

30-MoS2/VO2,

45-MoS2/VO2,

and

60-MoS2/VO2, respectively. The mixture was transferred to Teflon-lined stainless steel autoclave followed by hydrothermal treatment at 200 °C 5



for 24 hours. The crystalline phase of the as-prepared samples were characterized using X-ray diffraction (XRD, D8 Advance, Bruker) with Cu Kα radiation. The morphology of the synthesized samples was observed by scanning electron microscope (SEM, Nova Nano SEM450) operated at 15 kV. The x-ray photoelectron spectroscopy (XPS) measurements were performed using ESCALAB 250Xi instrument equipped with a monochromatic Al Kα (1486.7 eV) x-ray source. The electrochemical performance were tested in a three-electrode system (LK2010A) in 0.5 M H2SO4 using a Ag/AgCl (saturated KCl) reference electrode, a Pt counter electrode and glassy carbon (GC, 3 mm in diameter) electrode loaded with various catalysts as the working electrode (load: 0.285 mg/cm2). For elevated temperature performance, the electrochemical measurements were conducted in a temperature-controlled water bath (~70oC). 3. Results and discussion The XRD patterns of the prepared W-doped VO2 nanorods and various MoS2/VO2 hybrids are shown in Fig. 1. As shown in Fig. 1(a), the detected diffraction peaks at 27.68°, 37.10°, 39.53°, 55.30° and 57.11° are well matched with the (110), (011), (111), (121) and (220) diffractions of the standard rutile VO2 phase (JCPDS card No. 44-0253). This agrees with previously reports that a certain amount of W dopant can promote the preferential growth of VO2 in rutile phase.38 However, carefully 6


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analysis of the XRD pattern suggests the coexistence of rutile VO2 [VO2(R)] and monoclinic VO2 [VO2(M)] phases of W-VO2 at room temperature, see Fig. 1(g), which agrees with previous report.39 The presence of partial VO2(M) implies that the phase transition phenomenon can still occur at elevated temperatures for our samples which will lead to a decrease of the conductivity. Other than this, no other vanadium oxide (such as V2O5 and V2O3) were detected. In addition, no peaks of WO3 were observed in the XRD patterns which indicates that the W dopants are totally incorporated into VO2 lattice. Figure 1(b) is an XRD pattern of MoS2/VO2 heterostructure. After incorporation of MoS2, additional peaks at 14.4°, 33.5°, and 58.2° emerged, which were originated from the (002), (100), and (110) planes in hexagonal 2H-MoS2 (JCPDS card No. 37-1492), see Fig. 1(b). After decoration of MoS2, the (110) peak of VO2 is shifted to 27.50° and its (121) peak is significantly widened, which suggests the formation of VO1.75(OH)0.25. This is due to the formation of NaOH through reaction of Na2MoO4·2H2O and C2H5NS. When the sample was treated by H2SO4, we found that the (110) peak is shifted back to 27.69°, see Figs. 1(c)-(f), due to the reaction of H2SO4 and NaOH, which inhibits the formation of VO1.75(OH)0.25. In addition, with the increase of H2SO4 concentration, the VO2 peak intensities decreases due to the dissolution of VO2 in H2SO4. Therefore, there is an optimized H2SO4 concentration for this modification. 7



The roles that the H2SO4 treatment play can be further illustrated in SEM images by monitoring the morphology evolution of the samples. In Fig. 2, the corresponding SEM images of VO2 nanorods and several MoS2/VO2 hybrids are given. Figure 2(a) shows that the prepared VO2 nanorods have a smooth and uniform surface with the length of about 20 µm and a width of about 2 µm. Images of MoS2/VO2 hybrids grown without H2SO4 is exhibited in Fig. 2(b). As shown, most of MoS2 nanoflowers and VO2 nanorods are independent with only a few MoS2 attached on VO2 nanorods. In order to modify the surface morphology of VO2 nanorods, we tune the MoS2/VO2 growth condition by adding different concentration of H2SO4 solution. Under such circumstance, some VO2 surface atoms will be stripped off by H2SO4 solution to form microstructures, which increases the specific surface area and provides more MoS2 nucleation sites. As a result, a great amount of MoS2 nanosheets were nicely edge attached onto the surface of VO2 nanorods, as clearly demonstrated in Fig. 2(c). This attaching morphology provides tremendous amount of available edge sites in the MoS2 nanosheets which is beneficial for HER performance. When the H2SO4 concentration was increased to 60 mM, there are less edge attached MoS2 nanosheets with increased individual MoS2 nanoflowers. At the same time, the size of VO2 nanorods becomes shorter due to the corrosion of VO2 caused by the high concentration of 8


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H2SO4, as shown in Fig. 2(d). Therefore, we predict that MoS2/VO2 hybrids grown in different H2SO4 concentrations will exhibit different HER performance with 45 mM to be the best. Although there is quite a few reports on MoS2/TiO2 hybrids including the study of their electrocatalytic HER performance, there is very limited reports on MoS2/VO2 hybrids especially no study on their electrocatalytic HER properties. Therefore, the catalytic performance of our MoS2/VO2 samples was investigated using linear scan voltammetry in 0.5 M H2SO4 solution for the same amount of MoS2/VO2 catalysts loaded on GC electrode. Figure 3(a) presents the polarization curves of MoS2/VO2 hybrids at room temperature in comparison with commercial MoS2 powder, W-doped VO2 nanorods and Pt. It is shown that the commercial MoS2 powder, the prepared W-doped VO2 nanorods as well as the untreated MoS2/VO2 hybrid exhibit negligible cathodic current. Interestingly,

the

cathodic

current

increases

dramatically

for

45-MoS2/VO2 and the onset potentials reaches 188 mV. As shown in Fig. 3(c), the Tafel slopes of 45-MoS2/VO2 and MoS2/VO2 are 93 mV/dec and 189 mV/dec, respectively. The results show that 45-MoS2/VO2 has better electrocatalytic performance in line with above SEM and XRD analysis, which is attributed to the fact that H2SO4-treated VO2 surfaces provide more defect sites for MoS2 NSs to land onto. This bonding geometry exposes a large quality of MoS2 active sites so as to improve the HER 9



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activity. As shown in Fig. 3(b), the onset potentials of 15-MoS2/VO2, 30-MoS2/VO2, 45-MoS2/VO2, and 60-MoS2/VO2 are 390 mV, 212 mV, 188 mV, and 305 mV, respectively. Their corresponding Tafel slopes are 224 mV/dec, 146 mV/dec, 93 mV/dec, 126 mV/dec, respectively, see Fig. 3(d). Clearly, 45-MoS2/VO2 exhibits the highest HER activity, i.e. the best H2SO4 concentration for the treatment is 45 mM. When the H2SO4 concentration is too low, VO1.75(OH)0.25 will form on VO2 surfaces. However, if it is too high, seriously corrosion of VO2 nanorods will damage the heterogeneous structure of MoS2/VO2, and therefore reduce the HER performance of MoS2/VO2 catalyst. As is well known, VO2 undergoes insulator-to-metal phase transition at higher temperature combined with atomic structure change.36,

40, 41

Since strain can affect the electronic behaviors of MoS2,20 it is expected that the HER performance of MoS2/VO2 catalyst will be adjusted upon this phase transition. To verify this hypothesis, we performed the measurements for the 45-MoS2/VO2 hybrids before and after the phase transition of VO2, which is carried out by performing the measurements at room temperature and 70 oC, respectively. For comparison purpose, the commercial MoS2 powder and W-doped VO2 nanorods are also tested. It is shown in Fig. 4(a) that temperature and the phase transition do not have noticeable effect on the commercial MoS2 powder and W-doped VO2 nanorods. However, the onset potential of the 45-MoS2/VO2 hybrid drops 10


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significantly from 188 mV at room temperature to 99 mV at 70 oC. In Fig. 4(b), it also shows that the Tafel slope of 45-MoS2/VO2 hybrid decreases from 93 mV/dec to 85 mV/dec at 70 oC when the phase transition is occurred on VO2 nanorods. The enhanced HER activity at 70oC can be attributed to the insulator-to-metal phase transition of VO2, which reduces the VO2 resistivity rapidly (about two orders of magnitude) to facilitate fast effective charge transfer at the interface. In addition, the preferential growth direction of the VO2(M) nanorods is along [010] and the nanorod surfaces are mainly made up of (111), (011) and (100) planes.39 As a result, alongside with the phase transition from VO2(M) to VO2(R), lattice expansion will dominate the structure change based on their lattice constant changes, which results in tensile strain in the MoS2 NSs. At the same time, the twisting of V atoms also produces tensile strain at the interface of VO2 and MoS2 heterojunction. As reported, the tensile strain in MoS2 NSs promotes significantly higher electrocatalytic activity than the unstrained ones.20 According to the d-band theory,20 tensile strain increases the number of electrons states of Mo atoms near the Fermi level. The excess electrons are easily transferred to promote the proton to hydrogen in the HER process thereby enhance electrochemical activity. In order to examine the electronic structure information of the MoS2/VO2 hybrids, XPS characterizations are performed for W-VO2, MoS2 powder and MoS2/VO2. As shown in Fig. 5(a), for W-VO2, the V 11



2p3/2 and V 2p1/2 binding energies (BEs) appear at 516.4 eV and 523.7 eV, respectively. It has been reported that the BEs of V4+ valence state in pure VO2 is 516.2 eV. The shift of the BE in our VO2 nanorods is attributed to the doping effect by the W element.42 However, V 2p3/2 in the MoS2/VO2 hybrid is located at 516.6 eV, which is ~0.4 eV higher than that of W-VO2. This peak shift is originated from the build-in potential between the interface of MoS2 NSs and VO2 nanorods. In addition, the peak appears at 517.8 eV and 525.1 eV are attributed to V5+ as a result of slight oxidation of V in the air. In MoS2/VO2 hybrid, the peak appears near 513.8 eV and 521.1 eV was assigned to V3+ on account of the oxygen vacancies formation after H2SO4 treatment. As shown in Fig. 5(b), the O 1s is located at 530.3 eV for W-VO2 nanorods. In MoS2/VO2, the 531.9 eV O 1s peak is ascribed to hydroxyl group43 which is due to the formation of hydrogenated VO2 in acidic environment. As shown in Fig. 5(c), the BEs of W 4f5/2 and W 4f7/2 are located at 37.5 eV and 35.5 eV, respectively, corresponding to the chemical valence of +6.44 In MoS2/VO2 hybrid, the Mo 4p and V 3p spectra overlap with that of W 4f as shown in Fig. 5(c). From Fig. 5(c), the W doping concentration is estimated to be ~2.34 at%, which is in good agreement with its nominal concentration from the amount of the precursors (2 at%). The XPS spectra of Mo 3d and S 2p for different samples are illustrated in Figs. 6(a)-(b). The BEs of MoS2 NSs are at 232.45 eV, 12


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229.35 eV, 163.4 eV and 162.2 eV for Mo 3d3/2, Mo 3d5/2, S 2p1/2 and S 2p3/2, respectively. After formation of MoS2/VO2 hybrids, these BEs are shifted towards lower BE values by ~0.2 eV, which suggests downward shift of Fermi level. In addition, Mo6+ peak is also observed indicating a slight oxidation of Mo in ambient air.45 Figure 6(c) shows the valence band spectra of the samples. For heterostructures, according to Kraut's method,46 the value of valence band offset (VBO) can be calculated as:

∆ =

− − − − ∆

/

∆ =

/

−

(1) (2)

where

− ( − ) are the BEs difference

between Mo 3d5/2 in MoS2 (V 2p3/2 in VO2) and valence band maximum (VBM) of MoS2 (VO2), which are constants for pristine MoS2 and VO2. ∆ represents the BEs difference at the core level between V 2p1/2 and Mo 3d5/2 in the heterostructure. The VBM positions are obtained by linearly extrapolating the leading edge of the XPS spectra to the baseline.47 Using this method, the VBO value is calculated to be 0.2 eV. The schematic band diagram is given in Fig. 6(d). As shown, the VBM of MoS2 is higher than that of VO2 thereby the electrons in MoS2 are easy to transfer to VO2. As a result, the electron generated at the edge sites of MoS2 NSs will preferentially transport to VO2 surface. Since the VO2 surface has very high conductivity after phase transition at elevated temperature, the efficiency of electron transport will get much more 13



enhanced thus the HER performance becomes increased. 4. Conclusions In conclusion, we successfully synthesized a series of MoS2/VO2 hybrids for electrocatalytic HER using two-step hydrothermal method. MoS2/VO2 hybrids exhibit enhanced HER activity compared to pure MoS2 NSs. The higher HER activity is attributed to the synergistic effect of the defect-rich MoS2 and the effective electron-coupled interface. In addition, the phase change of VO2 leads to much more enhanced catalytic activity for MoS2/VO2 hybrids due to the faster electron transport as well as the strain effect on MoS2. Our results show that the prospect of multifunctional metal oxide and MoS2 composites as catalysts is promising, which also provides a new venue to tune the HER performance of MoS2. Notes The authors declare no competing financial interest. Acknowledgments This work was supported by National Natural Science Foundation of China (11604074 and 61674051), Hundred-Talent Program of Hebei Province (E2015100014), and Natural Science Foundation of Hebei Province of China (E2016202023).

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Figure captions: Fig. 1. XRD patterns of W-doped VO2 nanorods and various MoS2/VO2 hybrids: (a) W-doped VO2 nanorods; (b) MoS2/VO2; (c) 15-MoS2/VO2; (d) 30-MoS2/VO2; (e) 45-MoS2/VO2; (f) 60-MoS2/VO2. The vertical bars at bottom are the reference diffraction patterns for VO2. (g) Enlarged view of the diffraction peak to highlight the coexistence of VO2(M) and VO2(R) phases. The blue and green curves represent VO2(M) and VO2(R) diffraction peaks, respectively. The red solid one is their envelops and the red dotted line is the background. The vertical bars at bottom are the reference diffraction patterns for VO2.

Fig. 2. SEM images of the prepared W-doped VO2 nanorods and a series of MoS2/VO2 hybrids: (a) W-doped VO2 nanorods; (b) MoS2/VO2; (c) 45-MoS2/VO2; (d) 60-MoS2/VO2.

Fig. 3. Electrocatalytic hydrogen evolution performance of different catalysts. (a) the polarization curves of various MoS2/VO2 hybrids, VO2 nanorods, commercial MoS2 powder and Pt. The corresponding Tafel plots are given in (c). (b) the polarization curves of MoS2/VO2 hybrids grown at different H2SO4 concentrations. (d) is the corresponding Tafel plots for (b).

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Fig. 4. (a) The polarization curves of 45-MoS2/VO2 hybrids at room temperature and at 70 oC (with the notations of ‘HT’ before the sample names). The curves for VO2, commercial MoS2 and Pt are also added for comparison. (b) is the Tafel plots for 45-MoS2/VO2 hybrids before and after phase transition as well as that of Pt.

Fig. 5. XPS spectra of W-doped VO2 nanorods and MoS2/VO2 heterostructures. (a) V 2p, (b) O 1s, (c) W 4f and Mo 4p. The lower panel of are the spectra from W-doped VO2 nanorods and the higher panel are for MoS2/VO2 heterostructures. The open circles are the raw experimental data, the red curves are the corresponding individual fitting peaks and the green curves are envelops of the fitting results.

Fig. 6. Comparison of XPS spectra of Mo 3d (a) and S 2p (b) for MoS2 and MoS2/VO2 heterostructure, respectively. (c) Valence band spectra comparison of MoS2 and MoS2/VO2 heterostructure and W-doped VO2 nanorods. The red curves in (a) and (b) are the fitting peaks and the green curves are their envelops. (d) Schematic drawing of the band alignment of MoS2/VO2 heterostructure.

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Fig. 1. Chen et. al.

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TOC Graphic

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Phase Transition-Promoted Hydrogen Evolution Performance of MoS2

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