2 ultrathin nanosheets with additional active edge sites...

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Defect-Rich MoS 2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution

Junfeng Xie , Hao Zhang , Shuang Li , Ruoxing Wang , Xu Sun , Min Zhou , Jingfang Zhou ,

Xiong Wen (David) Lou ,* and Yi Xie *

Hydrogen, as a clean fuel, has been considered as a promising alternative for traditional fossil fuels in the future. [ 1 ] An enor-mous amount of effort has thus been made to pursue sustain-able and effi cient hydrogen production. The electrocatalytic hydrogen evolution reaction (HER) is considered one of the most important pathways to produce hydrogen effi ciently. [ 2 ] The most effective HER electrocatalysts up to now are based on noble metals, in particular, platinum. [ 3,4 ] However, the high price and limited resource of noble metals largely prevent their utilization in practice. Thus, developing effective HER elec-trocatalysts with low cost and high abundance still remains urgent. [ 5–9 ]

During the past few years, MoS 2 -based electrocatalysts have been considered as promising alternatives for platinum due to their high abundance and low cost. [ 10–15 ] However, bulk MoS 2 is a poor HER catalyst. [ 16 ] Both experimental and com-putational studies confi rm that the HER activity of MoS 2 cor-relates with the number of catalytically active edge sites. [ 17–23 ] Hence, designing MoS 2 nanostructures with more edge sites is one effective strategy to obtain an effective MoS 2 HER elec-trocatalyst. Recently, amorphous molybdenum sulfi des have been considered as effi cient catalysts for HER as they contain many active unsaturated sulfur atoms, but the poor crystallinity of these sulfi des leads to relatively high solubility and poor electrochemical stability in the acid electrolyte, which limits their practical utilization. [ 11,24–27 ] Therefore, developing MoS 2 electrocatalysts with both abundant active edge sites and good

© 2013 WILEY-VCH Verlag G

DOI: 10.1002/adma.201302685

J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, Dr. M. Zhou, Prof. Y. Xie Hefei National Laboratory for Physical Sciences at the Microscale Department of Chemistry University of Science and Technology of China Hefei , Anhui , 230026 , P. R. China E-mail: [email protected] Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive , 637459 , Singapore E-mail: [email protected] Prof. J. Zhou Ian Wark Research Institute Mawson Lakes Campus University of South Australia Mawson Lakes , SA 5095 , Australia

Adv. Mater. 2013, 25, 5807–5813

crystallinity is an effi cient way to simultaneously achieve high HER performance and long-term stability. Jaramillo’s group reported a highly ordered double-gyroid MoS 2 bicontinuous network with preferentially exposed active edge sites that led to outstanding improvement of HER performance. [ 17 ] Recently, Cui’s group prepared MoS 2 fi lms with vertically aligned layers that maximally expose the edges on the fi lm surface. [ 18 ] How-ever, most methods reported up to now are of limited use in practice due to their manipulation complexity and poor scalability. In the past few years, accompanied by the emer-gence of graphene, inorganic two-dimensional (2D) ultrathin nanosheets with a high percentage of exposed surface atoms have been extensively studied in different fi elds. [ 28–32 ] These studies inspire us to design MoS 2 nanostructures with prefer-ential exposure of active edge sites. For MoS 2 , one of the most classical anisotropic materials with a 2D lamellar structure, the preferentially exposed basal planes of the nanosheets are the thermodynamically stable (002) planes rather than the active edge planes. [ 33,34 ] To circumvent this disadvantage, we put for-ward that defect engineering may benefi t the structural design to expose specifi c crystal planes, since abundant defects could lead to the cracking of crystal and subsequently increase the accessible internal surface area. [ 35–38 ] Thus, engineering defect structure on the basal planes can be expected to increase the exposure of active edge sites by forming cracks on the surfaces of the nanosheets, [ 39–42 ] which may dramatically improve the electrocatalytic HER performance. Nonetheless, rational and controllable defect modulation still remains a challenging task.

Herein, we highlight a scalable pathway to accomplish the task of engineering defects into MoS 2 surfaces to expose active edge sites. We achieve this by designing a reaction with a high concentration of precursors and different amounts of thiourea, thus realizing controllable defect modulation in the as-formed MoS 2 ultrathin nanosheets. To achieve the defect-rich structure, excess thiourea was employed not only as reductant to reduce Mo( VI ) to Mo( IV ), but also as an effi cient additive to stabilize the ultrathin nanosheet morphology. [ 43 ] Thus, excess thiourea can be adsorbed on the surface of primary nanocrystallites, partially hindering the oriented crystal growth and leading to the for-mation of a defect-rich structure with quasiperiodic confi gura-tion ( Scheme 1 ). In contrast, defect-free MoS 2 nanosheets can be obtained via a quantitative reaction with high concentration of precursors but no protection of excess thiourea (Figure S1, see Supporting Information). Moreover, by reducing the con-centration of precursor but keeping thiourea in excess, thicker

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Scheme 1. a) Structural models of defect-free and defect-rich structures. b) As-designed synthetic pathways to obtain the above two structures.

nanosheet assemblies with defect-rich structures were prepared (Figure S3), which indicates that a high concentration of precur-sors is crucial for the stabilization of ultrathin nanosheet mor-phology while excess thiourea is a prerequisite for the formation of a defect-rich structure. Hence, controllable defect and mor-phology modulations can be achieved by simply adjusting the concentration of precursors. Thanks to the unique defect-rich structure, additional active edge sites can be exposed due to the existence of defect-induced cracks on the basal surfaces, which is advantageous for HER. As expected, the defect-rich MoS 2 ultrathin nanosheets possess a small HER onset overpotential of 120 mV, which is the best record for pure MoS 2 . [ 14,15,17,19 ] The number of active sites of the defect-rich nanosheets was calculated to be 13 times higher than that in the bulk materials, which verifi es the enrichment effect of active sites brought by rich defects. The large exchange current density of 8.91 μ A cm −2 also confi rmed this enrichment effect of active sites and sug-gests superior catalytic activity. With the merits of excellent interior catalytic activity and ultrathin nanosheet morphology, a large cathodic current density and small Tafel slope, as well as superior cycling stability, were obtained, which indicates a prominent HER performance of the defect-rich MoS 2 ultrathin nanosheets. This success of introducing defect structure to increase active sites may open up a new pathway for designing more effi cient catalysts in the near future.

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The defect-rich MoS 2 ultrathin nanosheets were prepared on the gram scale. The sheets have excellent dispersibility in various solvents, which is advantageous for practical utilization (Figure S4,5). X-ray photoelectron spectroscopy (XPS) was able to confi rm that the chemical state and composition are Mo( IV )S 2 , and the MoS 2 structure was verifi ed by means of Raman spectra (Figure S6). The high-angle annular dark-fi eld scanning TEM (HAADF-STEM) image and corresponding elemental mapping images (Figure S7) confi rm the homogeneous distri-bution of Mo and S elements across the whole nanosheet. To further investigate the structural information of the product, X-ray diffraction (XRD) was carried out. As shown in Figure 1 a, all the diffraction peaks agree well with the standard pattern of hexagonal MoS 2 (JCPDS card No. 73-1508), revealing the high purity of the product. There is obvious broadening of all the peaks in the diffraction pattern, which suggests the nanoscale of the crystallites in different dimensions. [ 37 ] By calculating the full width at half maximum (FWHM) value of the (002) diffraction peak using the Scherrer equation, the thickness of nanosheet along the c axis was determined to be 5.9 nm, which corresponds to 9 S–Mo–S layers. The fi eld-emission scan-ning electron microscopy (FESEM) image (Figure 1 b) clearly reveals the ultrathin nanosheet morphology. The lateral size of the nanosheets is in the range of 100–200 nm, and obvious ripples and corrugations can be observed. A corresponding

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Figure 1. a) XRD pattern of the product, which agrees well with the standard pattern of MoS 2 . b) Low-magnifi cation FESEM and c) TEM images of the defect-rich MoS 2 ultrathin nanosheets. d) HRTEM image and the corresponding Fourier transform pattern of area 1 in (c). e) Cross-sectional HRTEM image of area 2 in (c). f,g) Atomic reconstruction of (d) and (e). Additional active edge sites are designated by gray shading.

transmission electron microscopy (TEM) image (Figure 1 c) also verifi es the ultrathin nanosheet morphology of the product. The high-resolution transmission electron microscopy (HRTEM) images in top and side view are shown in Figure 1 d and e, respectively. Interplanar spacing of 2.7 Å can be observed from the top-view image, which is consistent with the d spacing of (100) planes of hexagonal MoS 2 . Many dislocations and distor-tions can be observed, which suggests a novel defect-rich struc-ture. Careful investigation of the HRTEM image reveals that the directions of individual (100) planes on the basal surface are not the same, with slight rotation from each other, which suggests a relatively disordered atomic arrangement on the basal surface. The disordered atomic arrangement causes the cracking of the basal planes and thus results in the formation of additional edges. The corresponding Fourier transform pattern clearly shows six independent diffraction arcs, which is char-acteristic neither of a single crystal with six diffraction spots

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeiAdv. Mater. 2013, 25, 5807–5813

nor of a polycrystal in the form of diffrac-tion cycles. This interesting observation can be attributed to the quasiperiodic structure that arises from the existence of rich defects. Furthermore, typical lamellar structure with interlayer spacing of 6.3 Å can be observed from the HRTEM image of a curled edge (Figure 1 e); this spacing is slightly larger than the layer-to-layer spacing of 6.1 Å in bulk MoS 2 . It is worth noting that the crystal fringes along the curled edge are discon-tinuous, which can also be attributed to the existence of rich defects, as indexed in the image. As a result, the defect-rich structure leads to the formation of nanosized domains along the basal planes, which is consistent with the exceptional broadening of (100) and (110) diffraction peaks in the XRD pattern. As illustrated in the reconstructed models of the defect-rich structure (Figure 1 f,g), additional active edge sites arising from defects can be clearly revealed. Moreover, the Fourier transform infrared (FTIR) spec-trum (Figure S8) indicates the complete removal of thiourea in the product, which suggests that no thiourea is adsorbed on the active edges. From the combined analysis of HRTEM, XRD, and FTIR spectroscopy, the novel defect-rich structure can be deter-mined; such a structure could signifi cantly increase the exposure of active edge sites due to the formation of cracks on the basal sur-face brought by rich defects. Furthermore, the remaining periodicity (sixfold symmetry) on basal planes partially retains the electron conjugation on S–Mo–S layers, leading to better internal conductivity than thoroughly polycrystalline or amorphous MoS 2 , which also benefi ts the electrocatalytic activity. [ 44–47 ] Therefore, better HER performance can be expected from these unique defect-rich MoS 2 ultrathin nanosheets.

To verify our hypothesis, electrochemical measurements of various MoS 2 samples were performed in 0.5 M H 2 SO 4 solu-tion using a three-electrode setup with the same loading of 0.285 mg cm −2 on a glassy carbon (GC) electrode. As shown in Figure 2 a, the defect-rich MoS 2 ultrathin nanosheets exhibit a small onset overpotential of 120 mV for HER, beyond which the cathodic current rises rapidly under more negative poten-tials. This onset overpotential is much smaller than that of the highly crystalline samples (160–250 mV) and previous results of nanostructured MoS 2 , which suggests the good catalytic activity of the product. [ 15,17,18 ] Moreover, the defect-rich MoS 2 ultrathin nanosheets display the largest cathodic current density of all the tested samples, with 13 mA cm −2 at an overpotential of 200 mV, that is, 46 A g −1 normalized by the loading weight that is nine times larger than that of previously reported MoS x on graphene-protected Ni foams (Figure S9). [ 26 ] Since the cathodic current density is proportional to the quantity of evolved hydrogen, the

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Figure 2. a) Polarization curves of various samples as indicated. b) Corresponding Tafel plots.

large current density here indicates prominent hydrogen evolu-tion behavior. The good catalytic behavior of defect-rich MoS 2 ultrathin nanosheets may arise from the unique defect-rich structure which brings in more additional active edge sites. On the contrary, polarization curves of defect-free MoS 2 nanosheets and calcined MoS 2 nanosheets suggest rather poor HER per-formance with high onset overpotential (180 and 230 mV) and low cathodic current density. As shown in Figure S1–2, both samples have a similar ultrathin nanosheet morphology, and are of high crystallinity with negligible defects. This phenom-enon further verifi es that the dominant factor in enhancement of the HER performance is the defect-rich structure. Besides, the thicker nanosheet assemblies display comparable onset

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overpotential with the defect-rich MoS 2 ultrathin nanosheets but much smaller cathodic current density, which can be attrib-uted to the assembly morphology that limits the exposure of defect-induced active sites. All the results above reveal that the existence of rich defects in the ultrathin nanosheets signifi cantly improves the HER performance.

To directly visualize the enrichment effect of active sites brought by defects, the numbers of active sites for various samples were estimated. [ 27 ] As shown in Table 1 , the defect-rich MoS 2 ultrathin nanosheets possess the highest density of active sites of 1.785 × 10 −3 mol g −1 , which is 2.9 times more than that of the defect-free nanosheets and 13 times higher than that of the bulk MoS 2 , giving direct evidence of the pre-dominant enrichment effect of active edge sites brought by rich defects. Moreover, the ultrathin nature of the nanosheets is also crucial to the increment of active edge sites, since the higher surface area of the ultrathin nanosheets can result in more accessible defect-induced edge sites. This view can be supported by the sharp contrast of active site numbers between the defect-rich MoS 2 ultrathin nanosheets and the thicker nanosheet assemblies. In general, the enrichment effect of active edge sites caused by abundant defects and the synergic effect of ultrathin nanosheet morphology are responsible for the good electrocatalytic behavior of the defect-rich MoS 2 ultrathin nanosheets. Furthermore, calculated turn-over frequency (TOF) for each active site of the defect-rich sample reaches 0.725 s −1 at η = 300 mV and pH = 0, which is much higher than the TOF value of other sam-ples, indicating the better intrinsic catalytic activity. [ 27 ] Thus, the superior activity of the defect-rich nanosheets can be attributed to the combined effect of both enrichment of active edge sites and good intrinsic catalytic activity.

To obtain further insight into the defect-

rich MoS 2 ultrathin nanosheets, Tafel plots of various cata-lysts were investigated (Figure 2 b). The resulting Tafel slope of the defect-rich MoS 2 ultrathin nanosheets is 50 mV decade –1 , which is probably the lowest record for pure MoS 2 electrocat-alysts up to now, [ 15,17,19 ] whereas the defect-free nanosheets, thicker nanosheet assemblies, calcined MoS 2 nanosheets, and bulk MoS 2 possess much higher Tafel slopes of 87, 88, 90, and 81 mV decade –1 , respectively. The small Tafel slope of the defect-rich MoS 2 ultrathin nanosheets is advantageous for practical applications, since it will lead to a faster increment of HER rate with increasing overpotential. [ 11 ] By applying the extrapolation method to the Tafel plots, exchange current density values of various samples were also obtained (Figure S11 and Table S1).

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Figure 3. a) Time dependence of current density under static overpoten-tial of 200 mV. Inset: enlargement of the area denoted by the dash circle. b) Durability test showing negligible current loss even after 3000 CV cycles.

Table 1. HER parameters of various MoS 2 samples.

Materials a) Number of active sites [10 −3 mol g −1 ]

TOF [s −1 ] b)

Tafel slope [mV decade –1 ]

Tafel region [mV]

j 0 [ μ A cm −2 ] c)

j [mA cm −2 ] d)

Defect-rich ultrathin

nanosheets

1.785 0.725 50 120–180 8.91 70.0

Defect-free ultrathin

nanosheets

0.620 0.496 87 180–240 3.16 16.8

Calcined nanosheets 0.311 0.467 90 230–300 5.62 8.2

Thicker nanosheet

assemblies

0.582 0.653 88 120–250 7.94 20.3

Bulk 0.137 0.304 81 250–350 0.32 2.3

a) All the parameters were measured under the same conditions, i.e., catalyst loading weight of 0.285 mg cm −2 on glassy carbon electrode in 0.5 M H 2 SO 4 solution; b) TOFs were measured at η = 300 mV; c) Exchange current densities ( j 0 ) were obtained from Tafel curves by using extrapolation methods; d) Cathodic current densities ( j ) were recorded at η = 300 mV.

As listed in Table 1 , the defect-rich MoS 2 ultrathin nanosheets display the largest exchange current density of 8.91 μ A cm −2 , which indicates the best catalytic activity among all the tested samples. This high value of j 0 can be attributed to the unique defect-rich structure and ultrathin nanosheet morphology that afford more accessible reactive sites, [ 48 ] which is consistent with the estimation of the number of active sites.

For HER in acidic media, three principle steps for converting H + to H 2 have been proposed, commonly named the Volmer [Equation ( 1 )], Heyrovsky [Equation ( 2 )], and Tafel reactions [Equation ( 3 )]. [ 49,50 ]

H3O+ + e− → Hads + H2O (1)

Hads + H3O+ + e− → H2 + H2O (2)

Hads + Hads → H2 (3) Under a specifi c set of conditions, when the Volmer reac-

tion is the rate-determining step of the HER, a slope of ca. 120 mV decade –1 should result, while a rate-determining Hey-rovsky or Tafel reaction should produce slopes of ca. 30 and 40 mV decade –1 , respectively. [ 49,50 ] For a complete HER pro-cess, combinations of steps, i.e., Volmer–Heyrovsky mecha-nism or Volmer–Tafel mechanism, should be involved to pro-duce molecular H 2 . Thus, due to the complexity of the reac-tion mechanism, analyzing the Tafel slope is still inconclusive for identifying the HER of MoS 2 . The Tafel slope of 50 mV decade –1 obtained in this work is close to that of MoS 2 nano-plates prepared in ultrahigh vacuum reported previously, [ 19 ] which suggests a similar surface chemistry of our defect-rich MoS 2 ultrathin nanosheets to that of the reported nanoplates. This result accompanied by previously reported results could provide more information for further elucidation of the HER mechanism of MoS 2 electrocatalysts.

Stability is another signifi cant criterion by which to eval-uate a catalyst. To probe the durability of the defect-rich MoS 2 ultrathin nanosheets in an acidic environment, continuous HER at static overpotential and a long-term cycling test were conducted. As shown in Figure 3 a, when an overpotential of 200 mV was applied, a continuous HER process occurred to generate molecular H 2 (Figure S12). The as-measured time-dependent curve is in typical serrate shape, which can be

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attributed to the alternate processes of bubble accumulation and bubble release. The current density exhibits only slight degradation even after a long period of 10 000 seconds, which might be caused by the consumption of H + or the remaining of H 2 bubbles on the surface of the electrode that hindered the reaction. The durability obtained in this work is better than that

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of the amorphous MoS 2 reported previously, which may arise
from the lower solubility in the acidic solution. [ 24,25 ] A long-term cycling test was also performed to assess the electrochem-ical stability. Figure 3 b displays the comparison of I – V curves measured before and after 3000 CV cycles ranging from 0.1 to –0.3 V at a scan rate of 50 mV s −1 . The negligible difference in the curves also indicates that the defect-rich MoS 2 ultrathin nanosheets are of superior stability in a long-term electro-chemical process. TEM and HRTEM images of the sample after the durability test (Figure S13) reveal that the nanosheet morphology and defect-rich structure have negligible changes after long-term cycling. The XPS spectra of the sample after the HER process for 10 000 seconds also show no obvious change of the oxidation state for Mo and S (Figure S13), which con-fi rms the excellent stability of the nanosheets under the long-term electrochemical cycling process.

In conclusion, we put forward a novel strategy to realize con-trollable defect engineering in MoS 2 ultrathin nanosheets. The existence of rich defects in the ultrathin nanosheets results in partial cracking of the catalytically inert basal planes, leading to exposure of additional active edge sites. With the merits of the defect-induced additional active edge sites, the defect-rich MoS 2 ultrathin nanosheets exhibit excellent HER activity with small onset overpotential of 120 mV, a large cathodic current density, and small Tafel slope of 50 mV decade –1 , which demon-strates the best integrated performance for pure MoS 2 electro-catalysts. Moreover, prominent electrochemical durability was also achieved, which is superior to that of amorphous MoS 2 catalysts. The success of applying defect engineering to HER catalysts may pave the way towards production of more effi cient catalysts. Signifi cant improvement of electrocatalytic perfor-mance for other electrocatalysts, such as WS 2 and MoSe 2 , can also be expected by designing defect-rich structures.

Experimental Section Materials Preparation : Typically, hexaammonium heptamolybdate

tetrahydrate (1 mmol, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, i.e. 7 mmol Mo) and thiourea (30 mmol) were dissolved in deionized water (35 mL) under vigorous stirring to form a homogeneous solution. Then, the solution was transferred into a 45 mL Tefl on-lined stainless steel autoclave and maintained at 220 °C. After 18 h, the reaction system was allowed to cool to room temperature. The fi nal product was washed with water and absolute ethanol for several times to remove any possible ions, and dried at 60 °C under vacuum. All the materials were purchased from SinoPharm and used without further purifi cation.

Electrochemical Measurements : Electrochemical measurements were performed in a three-electrode system at an electrochemical station (CHI660B). Typically, 4 mg of sample and 30 μ L Nafi on solution (5 wt%) were dispersed in 1 mL water–ethanol solution with volume ratio of 3:1 by sonicating for 1 h to form a homogeneous ink. Then 5 μ L of the dispersion (containing 20 μ g of catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter (loading ca. 0.285 mg cm −2 ). Linear sweep voltammetry with scan rate of 5 mV s −1 was conducted in 0.5 M H 2 SO 4 (purged with pure N 2 ) using Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode, a graphite rod (Alfa Aesar, 99.9995%) as the counter electrode, and the glassy carbon electrode with various samples as the working electrode. All the potentials were calibrated to a reversible hydrogen electrode (RHE). Number of active sites and turnover frequency (TOF) of different samples were calculated according to the methods reported previously. [ 27 ]

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was fi nancially supported by National Basic Research Program of China (2009CB939901), the Chinese Academy of Sciences (XDB01020300), National Nature Science Foundation (11079004, 90922016), and Australian Research Council (ARC) Discovery Project (DE120101788).

Received: June 12, 2013 Revised: July 10, 2013

Published online: August 13, 2013

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