- Email: [email protected]
RGO in dimethyl-formamide solvent as highly efficient catalyst for hydrogen evolution
Author’s Accepted Manuscript Facile synthesis of MoS2/RGO in dimethylformamide solvent as highly efficient catalyst for Hydrogen evolution Wen-Hui Hu, Rong Yu, Guan-Qun Han, Yan-Ru Liu, Bin Dong, Yong-Ming Chai, Yun-Qi Liu, Chen-Guang Liu www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30433-X http://dx.doi.org/10.1016/j.matlet.2015.08.081 MLBLUE19442
To appear in: Materials Letters Received date: 5 June 2015 Revised date: 3 August 2015 Accepted date: 16 August 2015 Cite this article as: Wen-Hui Hu, Rong Yu, Guan-Qun Han, Yan-Ru Liu, Bin Dong, Yong-Ming Chai, Yun-Qi Liu and Chen-Guang Liu, Facile synthesis of MoS2/RGO in dimethyl-formamide solvent as highly efficient catalyst for Hydrogen evolution, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile synthesis of MoS2/RGO in dimethyl-formamide solvent as highly efficient catalyst for hydrogen evolution Wen-Hui Hu a, Rong Yu b, Guan-Qun Han a, b, Yan-Ru Liu a, Bin Dong*a, b, Yong-Ming Chai a, Yun-Qi Liu a, Chen-Guang Liu*a a State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China b College of Science, China University of Petroleum (East China), Qingdao 266580, PR China
Abstract MoS2/RGO nanocomposites as highly effective electrocatalysts for hydrogen evolution reaction (HER) have been synthesized by one-pot solvothermal route using dimethyl-formamide (DMF) as solvent. For comparison, MoS2/RGO has also been prepared in pure H2O and H2O-DMF mixture. The good dispersion of MoS2 nanoparticles on the surface of MoS2/RGO obtained in DMF can be seen from TEM and SEM images, which implying the more active sites for HER. The electrocatalytic measurements confirm that the obtained MoS2/RGO in DMF exhibits the smaller onset overpotential of 130 mV and lower tafel slop of 42 mV dec-1, which indicating the better electrocatalytic activity than MoS2/RGO in H2O or H2O-DMF mixture. It may be attributed to the better role of DMF on dispersing RGO and controlling the growth of MoS2. Therefore, DMF may be the more suitable solvent for preparing excellent electrocatalysts for HER than H2O. Keywords: MoS2/RGO; dimethyl-formamide; nanocomposites; carbon materials
1. Introduction
* Corresponding author. Email: [email protected] (B. Dong), [email protected] (C.-G. Liu) Tel: +86-532-86981376, Fax: +86-532-86981787 1
The highly efficient and sustainable hydrogen production has been believed to the key for future development of clean-energy techniques [1-4]. The most effective electrocatalysts for HER in acidic media are those of Pt and its composites. However, the high price and scarcity of Pt-based electrocatalysts have greatly limited their large-scale application [5]. Therefore, it is desirable to prepare the cost-effective alternatives as highly efficient electrocatalysts for HER. To date, MoS2 has been highlighted as a very promising hydrogen electrocatalyst owing to its earth-abundance and good catalytic activity. Carbon-based MoS2 have recently attracted more attention [6-9] because carbon can enhance the activity of MoS2 for HER via increasing the amount of active sulphur edge sites and accelerating the transport speed of electrons between the active sites and the underlying electrode. Our group has developed the hybrid electrocatalysts of ultrathin MoS2 coated on carbon nanospheres, which exhibit high activity for HER [10]. In addition, carbon-based MoS2 hybrid nanomaterials have also been investigated by electrodeposition process [11] or hydrothermal method [12]. The observed promising results indicate the significant potential of carbon nanomaterials in improving HER activity of MoS2. However, there still remain some problems to be solved such as cost barrier, complicated procedures, the effect of the optional solvents and so on. In this work, four different solvents have been tried to prepare MoS2/RGO as the electrocatalysts for HER by a facile hydrothermal procedure. When the pure dimethyl-formamide (DMF) has been used as solvent, the obtained MoS2/RGO exhibit the best catalytic activity for HER with a small onset overpotential of 130 mV and a low tafel slop of 42 mv dec-1. The effect of DMF on the structure and properties of MoS2/RGO is discussed. 2. Experimental The graphene oxide (GO) was prepared based on previous report [13]. The as-prepared 0.06 g of GO 2
was dispersed to the four various solvents (pure DMF, DMF: H2O=2:1, DMF: H2O=1:2 and pure H2O, respectively). After stirring, 0.1 g of (NH4)2MoS4 was added. And 0.6 mL of N2H4·H2O was added before being transferred to a 100 mL Teflon autoclave and kept in an oven at 200℃ for 12 h. Finally, the product was filtrated, washed with distilled water and ethanol for several times, dried at room temperature. All electrochemical measurements were performed in a three-electrode system by electrochemical station (Gamry Reference 600 Instruments, USA). Typically, 4 mg of sample and 20 μL Nafion solution (5 wt %) were dispersed in 1 mL water-ethanol mixture solution by sonicating for 1 h to form a homogeneous ink. Then 5 μL of the dispersion solution was loaded onto a glassy carbon electrode with the diameter of 4 mm. Electrolyte was degassed by bubbling N2 for 30 min prior to each experiment. Linear sweep voltammetry with scan rate of 20 mV s-1 was conducted in 0.50 M H2SO4 solution from 0 to -0.3 V vs RHE. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at an overpotential η= 200 mV from 10 5 to 0.1 Hz with an AC voltage of 5 mV. 3. Results
and discussion
XRD patterns of the different MoS2/RGO are shown in Fig.1. The peaks at 33.0° and 56.8° are indexed to (100) and (110) planes of MoS 2, respectively (JCPDS card no.37-1492). The peak at 23.8° is indexed to (002) plane of RGO. The peak of (002) plane of bulk MoS2 are usually at 14.5°. However, the peak of (002) shifts to 9.5° in MoS2/RGO nanocomposites, which indicates that the layer distance of (002) plane is expanded owing to the insertion of carbon accordingly to the previous report [14]. Therefore, MoS2/RGO nanocomposites can provide the improved the exfoliation and dispersion of MoS2 nanosheets. Fig. 1b shows the TEM image of MoS2/RGO obtained in pure DMF and the MoS 2 3
nanoparticles dispersed well on the surface of RGO.
Fig.1 a XRD patterns of different MoS2/RGO samples, b TEM image of MoS2/RGO made in pure DMF
Fig.2 SEM and elemental mapping images of MoS2/RGO. a in pure H2O; b DMF : H2O= 1 : 2; c DMF : H2O= 2 : 1; d, e in pure DMF; f SEM image of the selected area for element mapping and g corresponding elemental mapping images for MoS2/RGO obtained in pure DMF
.
Fig.2 shows SEM and elemental mapping images of MoS2/RGO. As shown in Fig.2a, 2b and 2c, MoS2 stacked together on the surface of RGO. Fig.2d shows that MoS2 nanoparticles obtained in pure DMF with the diameter of about 70 nm dispersed well on the surface of RGO which agreed with the result of TEM. In the higher magnification in Fig.2e, it can be observed that small MoS2 nanoparticles are consisted of ultrathin MoS2 nanosheets. To illustrate the spatial distribution of Mo and S, the elemental mapping has been performed on the representative MoS2/RGO (Fig.2f and 2g). It can be 4
found that Mo and S elements reveal good distribution on the surface of RGO. These results indicate that MoS2/RGO obtained in DMF has close interaction and good dispersion. X-ray photoelectron spectroscopy (XPS) was adopted to characterize the chemical bonding structures (Fig.3). The two characteristic peaks are observed approximately 228.9 eV and 232.2 eV,
Fig.3 XPS for MoS2/RGO made in pure DMF. a Mo 3d spectrum; b S 2p spectrum
which are corresponding to Mo 3 d5/2 and Mo 3 d3/2 binding energies, respectively (Fig.3a). In Fig.3b, the peaks at 161.7 eV and 162.9 eV are related to S 2 p3/2 and S 2 p1/2 binding energies, respectively. The stoichiometric ration (S/Mo) estimated from the respective integrated peak area of XPS spectra is close to 1.96. The electrocatalytic activities of the different MoS2/RGO for HER were investigated in 0.50 M H2SO4 solution. Fig.4a shows LSV curves for the different samples obtained in different solvents. It can be seen that MoS2/RGO obtained in DMF exhibit the smallest onset potential of 130 mV (vs. RHE), which implying the best electrocatalytic for HER. While using the pure H2O as solvent, MoS2/RGO shows the largest onset potential of 170 mV (vs. RHE) for HER. The MoS2/RGO obtained in pure DMF modified electrode produced current density (j) of -10 and -20 mA cm-2 at potentials of -208 and -224 mV while the other three samples needs higher potential. Therefore, it is speculated that DMF can help increase the active sites and enhance the electrocatalytic activity of MoS2/RGO for HER. Fig.4b shows the Tafel plots of the different MoS2/RGO. Tafel slopes of 42 mV dec-1, 45 mV dec-1, 46 mV 5
dec-1 and 48 mV dec-1 were measured for the different sample prepared in pure DMF, DMF: H2O=2:1, DMF: H2O=1:2 and pure H2O, respectively. MoS2/RGO obtained in DMF displays the smallest Tafel slope of 42 mV dec-1, which indicates the best electrocatalytic activity for practical electrocatalytic application. The Tafel slope of 42 mV dec-1 suggests that reaction mechanism on this MoS2/RGO prepared in DMF takes place via the Volmer-Heyrovskey mechanism for HER [15].
Fig.4 a Linear sweep voltammogram curves for four different MoS2/RGO; b Tafel plots of four different MoS2/RGO; c AC impedance spectra for four different MoS2/RGO at overpotential of 0.2 V from 105 to 0.1 Hz with an AC voltage of 5 mV; d The time dependence curve of current density of MoS2/RGO obtained in pure DMF under static overpotential of -200 mV vs. RHE
Obviously, the pure DMF is an essential role in deciding the final morphology of MoS2/RGO. It may be due to the amide groups of DMF, which are helpful to improve the dispersion of both RGO and MoS42- than H2O. The results of Tafel plots also confirm that the pure DMF is appropriate solvent on preparing MoS2/RGO with higher electrocatalytic activity for HER. Electrochemical impedance spectroscopy (EIS) is a useful technique to characterize interfacial reactions and electrode kinetics in HER. Fig.4c shows the Nyquist plots of the EIS response of different samples. MoS2/RGO obtained in DMF shows much lower charge transfer resistance than the other three samples, indicating enhanced conductivity of MoS2/RGO. The smaller resistance of MoS2/RGO 6
in DMF means much faster electron transfer and improved efficiency for HER. Fig. 4d shows the time dependence curve of current density of MoS2/RGO obtained in pure DMF under static overpotential of -200 mV vs. RHE. The current density exhibits slight degradation even after a long period of 10000 s which means the MoS2/RGO obtained in pure DMF has a good stability for HER. 4. Conclusions A facile solvothermal method has been used to synthesize MoS2/RGO electrocatalysts for HER. TEM and SEM images show that MoS2 can disperse better on RGO using DMF as solvent, which suggests more active sites for HER. MoS2/RGO synthesized in DMF also exhibits the highest electrocatalytic activity for HER, which may be attributed to the function of DMF on the morphology directing and size controlling for preparing novel nanostructures of MoS2/RGO.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (U1162203 and 21106185) and the Fundamental Research Funds for the Central Universities (15CX05031A).
7
References [1] Grätzel M, Nature 2001; 414: 338-44. [2] Koper MTM, Bouwman E. Angew Chem Int Ed 2010; 49: 3723-5. [3] Tang ML, Grauer DC, Lassalle-Kaiser B, Yachandra VK, Amirav L, Alivisatos AP. Angew Chem Int Ed 2011; 50: 10203-7. [4] Yan Y, Xia BY, Ge XM, Liu ZL, Wang JY, Wang X. ACS Appl Mater Interfaces 2013; 5: 12794-8. [5] Antolini E. Energy Environ Sci 2009; 2: 915-31. [6] Chang YH, Lin CT, Chen TY, Hsu CL, Lee YH, Zhang W, Wei KH, Li LJ, Adv Mater 2013; 25: 756-60. [7] Yan Y, Ge XM, Liu ZL, Wang JY, Lee JM, Wang X. Nanoscale 2013; 5: 7768-71. [8] Zhao X, Zhu H, Yang XR. Nanoscale 2014; 6: 10680-5. [9] Zhu H, Lyu FL, Du ML, Zhang M, Wang QF, Yao JM, Guo BC. ACS Appl Mater Interfaces 2014; 6: 22126-37. [10] Hu WH, Han GQ, Liu YR, Dong B, Chai YM, Liu YQ, Liu CG. Int J Hydrogen Energy 2015; 40: 6552-8. [11] Pu ZH, Liu Q, Asiri AM, Obaid AY, Sun XP. J Power Sources 2014; 263: 181-5. [12] Li YG, Wang HL, Xie LM, Liang YY, Hong GS, Dai HG. J Am Chem Soc 2011; 133: 7296-9. [13] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun ZZ, Slesarev A, Alemany LB, Lu W, Tour JM. Acs Nano 2010; 4: 4806-14. [14] Liu YC, Jiao LF, Wu Q, Du J, Zhao YP, Si YC, Wang YJ, Yuan HT. J. Mater. Chem. A 2013; 1: 5822-6. [15] Morales-Guio CG, Stern LA, Hu XL. Chem Soc Rev 2014; 43: 6555-69. 8
Highlights MoS2/RGO was
prepared by one-pot hydrothermal route in different solvents. The good structure of MoS2/RGO can be obtained in pure DMF. The obtained MoS2/RGO in DMF exhibits the better catalytic activity for HER. DMF are helpful on dispersing RGO and MoS42-.
9
PII: DOI: Reference:
S0167-577X(15)30433-X http://dx.doi.org/10.1016/j.matlet.2015.08.081 MLBLUE19442
To appear in: Materials Letters Received date: 5 June 2015 Revised date: 3 August 2015 Accepted date: 16 August 2015 Cite this article as: Wen-Hui Hu, Rong Yu, Guan-Qun Han, Yan-Ru Liu, Bin Dong, Yong-Ming Chai, Yun-Qi Liu and Chen-Guang Liu, Facile synthesis of MoS2/RGO in dimethyl-formamide solvent as highly efficient catalyst for Hydrogen evolution, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile synthesis of MoS2/RGO in dimethyl-formamide solvent as highly efficient catalyst for hydrogen evolution Wen-Hui Hu a, Rong Yu b, Guan-Qun Han a, b, Yan-Ru Liu a, Bin Dong*a, b, Yong-Ming Chai a, Yun-Qi Liu a, Chen-Guang Liu*a a State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China b College of Science, China University of Petroleum (East China), Qingdao 266580, PR China
Abstract MoS2/RGO nanocomposites as highly effective electrocatalysts for hydrogen evolution reaction (HER) have been synthesized by one-pot solvothermal route using dimethyl-formamide (DMF) as solvent. For comparison, MoS2/RGO has also been prepared in pure H2O and H2O-DMF mixture. The good dispersion of MoS2 nanoparticles on the surface of MoS2/RGO obtained in DMF can be seen from TEM and SEM images, which implying the more active sites for HER. The electrocatalytic measurements confirm that the obtained MoS2/RGO in DMF exhibits the smaller onset overpotential of 130 mV and lower tafel slop of 42 mV dec-1, which indicating the better electrocatalytic activity than MoS2/RGO in H2O or H2O-DMF mixture. It may be attributed to the better role of DMF on dispersing RGO and controlling the growth of MoS2. Therefore, DMF may be the more suitable solvent for preparing excellent electrocatalysts for HER than H2O. Keywords: MoS2/RGO; dimethyl-formamide; nanocomposites; carbon materials
1. Introduction
* Corresponding author. Email: [email protected] (B. Dong), [email protected] (C.-G. Liu) Tel: +86-532-86981376, Fax: +86-532-86981787 1
The highly efficient and sustainable hydrogen production has been believed to the key for future development of clean-energy techniques [1-4]. The most effective electrocatalysts for HER in acidic media are those of Pt and its composites. However, the high price and scarcity of Pt-based electrocatalysts have greatly limited their large-scale application [5]. Therefore, it is desirable to prepare the cost-effective alternatives as highly efficient electrocatalysts for HER. To date, MoS2 has been highlighted as a very promising hydrogen electrocatalyst owing to its earth-abundance and good catalytic activity. Carbon-based MoS2 have recently attracted more attention [6-9] because carbon can enhance the activity of MoS2 for HER via increasing the amount of active sulphur edge sites and accelerating the transport speed of electrons between the active sites and the underlying electrode. Our group has developed the hybrid electrocatalysts of ultrathin MoS2 coated on carbon nanospheres, which exhibit high activity for HER [10]. In addition, carbon-based MoS2 hybrid nanomaterials have also been investigated by electrodeposition process [11] or hydrothermal method [12]. The observed promising results indicate the significant potential of carbon nanomaterials in improving HER activity of MoS2. However, there still remain some problems to be solved such as cost barrier, complicated procedures, the effect of the optional solvents and so on. In this work, four different solvents have been tried to prepare MoS2/RGO as the electrocatalysts for HER by a facile hydrothermal procedure. When the pure dimethyl-formamide (DMF) has been used as solvent, the obtained MoS2/RGO exhibit the best catalytic activity for HER with a small onset overpotential of 130 mV and a low tafel slop of 42 mv dec-1. The effect of DMF on the structure and properties of MoS2/RGO is discussed. 2. Experimental The graphene oxide (GO) was prepared based on previous report [13]. The as-prepared 0.06 g of GO 2
was dispersed to the four various solvents (pure DMF, DMF: H2O=2:1, DMF: H2O=1:2 and pure H2O, respectively). After stirring, 0.1 g of (NH4)2MoS4 was added. And 0.6 mL of N2H4·H2O was added before being transferred to a 100 mL Teflon autoclave and kept in an oven at 200℃ for 12 h. Finally, the product was filtrated, washed with distilled water and ethanol for several times, dried at room temperature. All electrochemical measurements were performed in a three-electrode system by electrochemical station (Gamry Reference 600 Instruments, USA). Typically, 4 mg of sample and 20 μL Nafion solution (5 wt %) were dispersed in 1 mL water-ethanol mixture solution by sonicating for 1 h to form a homogeneous ink. Then 5 μL of the dispersion solution was loaded onto a glassy carbon electrode with the diameter of 4 mm. Electrolyte was degassed by bubbling N2 for 30 min prior to each experiment. Linear sweep voltammetry with scan rate of 20 mV s-1 was conducted in 0.50 M H2SO4 solution from 0 to -0.3 V vs RHE. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at an overpotential η= 200 mV from 10 5 to 0.1 Hz with an AC voltage of 5 mV. 3. Results
and discussion
XRD patterns of the different MoS2/RGO are shown in Fig.1. The peaks at 33.0° and 56.8° are indexed to (100) and (110) planes of MoS 2, respectively (JCPDS card no.37-1492). The peak at 23.8° is indexed to (002) plane of RGO. The peak of (002) plane of bulk MoS2 are usually at 14.5°. However, the peak of (002) shifts to 9.5° in MoS2/RGO nanocomposites, which indicates that the layer distance of (002) plane is expanded owing to the insertion of carbon accordingly to the previous report [14]. Therefore, MoS2/RGO nanocomposites can provide the improved the exfoliation and dispersion of MoS2 nanosheets. Fig. 1b shows the TEM image of MoS2/RGO obtained in pure DMF and the MoS 2 3
nanoparticles dispersed well on the surface of RGO.
Fig.1 a XRD patterns of different MoS2/RGO samples, b TEM image of MoS2/RGO made in pure DMF
Fig.2 SEM and elemental mapping images of MoS2/RGO. a in pure H2O; b DMF : H2O= 1 : 2; c DMF : H2O= 2 : 1; d, e in pure DMF; f SEM image of the selected area for element mapping and g corresponding elemental mapping images for MoS2/RGO obtained in pure DMF
.
Fig.2 shows SEM and elemental mapping images of MoS2/RGO. As shown in Fig.2a, 2b and 2c, MoS2 stacked together on the surface of RGO. Fig.2d shows that MoS2 nanoparticles obtained in pure DMF with the diameter of about 70 nm dispersed well on the surface of RGO which agreed with the result of TEM. In the higher magnification in Fig.2e, it can be observed that small MoS2 nanoparticles are consisted of ultrathin MoS2 nanosheets. To illustrate the spatial distribution of Mo and S, the elemental mapping has been performed on the representative MoS2/RGO (Fig.2f and 2g). It can be 4
found that Mo and S elements reveal good distribution on the surface of RGO. These results indicate that MoS2/RGO obtained in DMF has close interaction and good dispersion. X-ray photoelectron spectroscopy (XPS) was adopted to characterize the chemical bonding structures (Fig.3). The two characteristic peaks are observed approximately 228.9 eV and 232.2 eV,
Fig.3 XPS for MoS2/RGO made in pure DMF. a Mo 3d spectrum; b S 2p spectrum
which are corresponding to Mo 3 d5/2 and Mo 3 d3/2 binding energies, respectively (Fig.3a). In Fig.3b, the peaks at 161.7 eV and 162.9 eV are related to S 2 p3/2 and S 2 p1/2 binding energies, respectively. The stoichiometric ration (S/Mo) estimated from the respective integrated peak area of XPS spectra is close to 1.96. The electrocatalytic activities of the different MoS2/RGO for HER were investigated in 0.50 M H2SO4 solution. Fig.4a shows LSV curves for the different samples obtained in different solvents. It can be seen that MoS2/RGO obtained in DMF exhibit the smallest onset potential of 130 mV (vs. RHE), which implying the best electrocatalytic for HER. While using the pure H2O as solvent, MoS2/RGO shows the largest onset potential of 170 mV (vs. RHE) for HER. The MoS2/RGO obtained in pure DMF modified electrode produced current density (j) of -10 and -20 mA cm-2 at potentials of -208 and -224 mV while the other three samples needs higher potential. Therefore, it is speculated that DMF can help increase the active sites and enhance the electrocatalytic activity of MoS2/RGO for HER. Fig.4b shows the Tafel plots of the different MoS2/RGO. Tafel slopes of 42 mV dec-1, 45 mV dec-1, 46 mV 5
dec-1 and 48 mV dec-1 were measured for the different sample prepared in pure DMF, DMF: H2O=2:1, DMF: H2O=1:2 and pure H2O, respectively. MoS2/RGO obtained in DMF displays the smallest Tafel slope of 42 mV dec-1, which indicates the best electrocatalytic activity for practical electrocatalytic application. The Tafel slope of 42 mV dec-1 suggests that reaction mechanism on this MoS2/RGO prepared in DMF takes place via the Volmer-Heyrovskey mechanism for HER [15].
Fig.4 a Linear sweep voltammogram curves for four different MoS2/RGO; b Tafel plots of four different MoS2/RGO; c AC impedance spectra for four different MoS2/RGO at overpotential of 0.2 V from 105 to 0.1 Hz with an AC voltage of 5 mV; d The time dependence curve of current density of MoS2/RGO obtained in pure DMF under static overpotential of -200 mV vs. RHE
Obviously, the pure DMF is an essential role in deciding the final morphology of MoS2/RGO. It may be due to the amide groups of DMF, which are helpful to improve the dispersion of both RGO and MoS42- than H2O. The results of Tafel plots also confirm that the pure DMF is appropriate solvent on preparing MoS2/RGO with higher electrocatalytic activity for HER. Electrochemical impedance spectroscopy (EIS) is a useful technique to characterize interfacial reactions and electrode kinetics in HER. Fig.4c shows the Nyquist plots of the EIS response of different samples. MoS2/RGO obtained in DMF shows much lower charge transfer resistance than the other three samples, indicating enhanced conductivity of MoS2/RGO. The smaller resistance of MoS2/RGO 6
in DMF means much faster electron transfer and improved efficiency for HER. Fig. 4d shows the time dependence curve of current density of MoS2/RGO obtained in pure DMF under static overpotential of -200 mV vs. RHE. The current density exhibits slight degradation even after a long period of 10000 s which means the MoS2/RGO obtained in pure DMF has a good stability for HER. 4. Conclusions A facile solvothermal method has been used to synthesize MoS2/RGO electrocatalysts for HER. TEM and SEM images show that MoS2 can disperse better on RGO using DMF as solvent, which suggests more active sites for HER. MoS2/RGO synthesized in DMF also exhibits the highest electrocatalytic activity for HER, which may be attributed to the function of DMF on the morphology directing and size controlling for preparing novel nanostructures of MoS2/RGO.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (U1162203 and 21106185) and the Fundamental Research Funds for the Central Universities (15CX05031A).
7
References [1] Grätzel M, Nature 2001; 414: 338-44. [2] Koper MTM, Bouwman E. Angew Chem Int Ed 2010; 49: 3723-5. [3] Tang ML, Grauer DC, Lassalle-Kaiser B, Yachandra VK, Amirav L, Alivisatos AP. Angew Chem Int Ed 2011; 50: 10203-7. [4] Yan Y, Xia BY, Ge XM, Liu ZL, Wang JY, Wang X. ACS Appl Mater Interfaces 2013; 5: 12794-8. [5] Antolini E. Energy Environ Sci 2009; 2: 915-31. [6] Chang YH, Lin CT, Chen TY, Hsu CL, Lee YH, Zhang W, Wei KH, Li LJ, Adv Mater 2013; 25: 756-60. [7] Yan Y, Ge XM, Liu ZL, Wang JY, Lee JM, Wang X. Nanoscale 2013; 5: 7768-71. [8] Zhao X, Zhu H, Yang XR. Nanoscale 2014; 6: 10680-5. [9] Zhu H, Lyu FL, Du ML, Zhang M, Wang QF, Yao JM, Guo BC. ACS Appl Mater Interfaces 2014; 6: 22126-37. [10] Hu WH, Han GQ, Liu YR, Dong B, Chai YM, Liu YQ, Liu CG. Int J Hydrogen Energy 2015; 40: 6552-8. [11] Pu ZH, Liu Q, Asiri AM, Obaid AY, Sun XP. J Power Sources 2014; 263: 181-5. [12] Li YG, Wang HL, Xie LM, Liang YY, Hong GS, Dai HG. J Am Chem Soc 2011; 133: 7296-9. [13] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun ZZ, Slesarev A, Alemany LB, Lu W, Tour JM. Acs Nano 2010; 4: 4806-14. [14] Liu YC, Jiao LF, Wu Q, Du J, Zhao YP, Si YC, Wang YJ, Yuan HT. J. Mater. Chem. A 2013; 1: 5822-6. [15] Morales-Guio CG, Stern LA, Hu XL. Chem Soc Rev 2014; 43: 6555-69. 8
Highlights MoS2/RGO was
prepared by one-pot hydrothermal route in different solvents. The good structure of MoS2/RGO can be obtained in pure DMF. The obtained MoS2/RGO in DMF exhibits the better catalytic activity for HER. DMF are helpful on dispersing RGO and MoS42-.
9