MoS2 with tunable surface structure directed by thiophene adsorption toward HDS and HER

Science China Materials, Dec 2016

MoS2 catalysts with more active sites and larger surface areas were successfully synthesized via a simple hydrothermal reaction method, and the surface structure of MoS2 was readily tailored by simply adding thiophene during the synthesis procedure. The MoS2 samples synthesized with thiophene, especially MoS2 prepared with 200 μL of thiophene, concomitantly exhibited better activity for both hydrodesulfurization and hydrogen evolution reactions. The present work provides an efficient route to achieve highly efficient MoS2 catalysts, and may open up a new avenue for the morphological design of layered structural compounds like MoS2.

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MoS2 with tunable surface structure directed by thiophene adsorption toward HDS and HER

MoS2 with tunable surface structure directed by thiophene adsorption toward HDS and HER Sijia Liu 0 Xin Zhang 0 Jie Zhang 0 Zhigang Lei 0 Xin Liang 0 Biaohua Chen 0 0 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology , Beijing 100029 , China MoS2 catalysts with more active sites and larger surface areas were successfully synthesized via a simple hydrothermal reaction method, and the surface structure of MoS2 was readily tailored by simply adding thiophene during the synthesis procedure. The MoS2 samples synthesized with thiophene, especially MoS2 prepared with 200 μL of thiophene, concomitantly exhibited better activity for both hydrodesulfurization and hydrogen evolution reactions. The present work provides an efficient route to achieve highly efficient MoS2 catalysts, and may open up a new avenue for the morphological design of layered structural compounds like MoS2. MoS2; catalyst; hydrodesulfurization; hydrogen evolution reaction - INTRODUCTION The most serious challenge facing society today is sustainable development coupled with clean energy utilization and environmental protection [1]. The responses to this challenge lie mainly in advanced materials and technologies. At present, most of the world’s energy supply comes from petroleum, which is expected to continue to be an important global energy source in the immediate future [2]. Hydrodesulfurization (HDS) is an important industrial process for removal of sulfur from organosulfur compounds in fossil fuels, and molybdenum disulfide (MoS2)-based materials are extensively used as HDS catalysts in refineries worldwide [3,4]. Hydrogen has also been vigorously pursued as a promising alternative to traditional fossil fuels [5,6]. The electrocatalytic hydrogen evolution reaction (HER) is considered as one of the most important pathways for hydrogen production [7], and MoS2 has also been extensively studied as a catalyst for the HER [8,9]. To achieve improved catalyst performance for HDS, an understanding of the relationship between the active sites and the structure of MoS2 catalysts is important. Two types of active sites have been identified: one that promotes both hydrogenation and hydrogenolysis and the other that mainly promotes hydrogenolysis of the carbon-heteroatom bond of heterocyclics [10,11]. Daage and Chianelli [12] proposed a rim-edge model, in which hydrogenation of dibenzothiophene occurred exclusively on the top and bottom basal planes (rim sites), while the hydrogenolysis of dibenzothiophene was catalyzed on the edge planes. Thus, the activity of MoS2 toward HDS is closely related to the edge planes of MoS2. For the HER, MoS2 has recently attracted considerable attention as an inorganic electrocatalyst because of its low cost, high chemical stability, and excellent electrocatalytic properties [13,14]. Recent studies have shown that the HER activity of MoS2 is highly dependent on its exposed edges [15]. Thus, designing a MoS2 catalyst with more edge sites is an effective strategy for achieving enhanced activity, and extensive effort has been focused on achieving this objective [16,17]. Increasing the surface area of MoS2 to obtain more exposed planes is a prospective strategy for increasing the number of active sites [18]. However, it is reported that the catalytic activity of MoS2 is not proportional to the surface area, because the HER-active centers of MoS2 have also been identified as the edges of the hexagonal lamellar MoS2 crystal layers and the basal planes have been found to be generally inert for the HER [19]. Therefore, increasing the number of edge planes is very important for the intrinsic improvement of the number of active sites. Interestingly, whether MoS2 is used as a catalyst for HDS or the HER, the active sites are the same: the activity is closely related to the edge planes of MoS2. The active sites are generally accepted to be metallic Mo sites with sulfur vacancies, whereas the basal plane (normally with completely coordinated sulfur atoms) is inert [20]. Thus, we propose that if the MoS2 catalyst has more exposed edge planes, the catalytic activity for both HDS and HER will be concomitantly enhanced. In this study, a simple but efficient way to tailor the morphology of MoS2 using thiophene as an additive was developed, and a novel MoS2 microparticle structure was synthesized. The MoS2 samples synthesized with thiophene possess rich exposed edge sites, and thus concomitantly exhibit excellent activity for both HDS and HER. Moreover, the relationship between the surface structures of MoS2 and the activity of MoS2 for HDS and HER are examined. Due to their tailored structures with more active sites, the MoS2 catalysts, especially obtained with 200 μL of thiophene, demonstrate excellent catalytic performance. EXPERIMENTAL SECTION Catalyst synthesis All chemicals used in this work were of analytical-grade purity and were used without further purification. In a typical procedure, 0.5 mmol of (NH4)6Mo7O24∙4H2O and 10.5 mmol of H2NCSNH2 were dissolved in 50 mL of distilled water; 200 μL of thiophene was then added. The mixture solution was stirred for 0.5 h, and the final solution was transferred into a 100-mL Teflon-lined stainless steel autoclave. The autoclave was then sealed tightly and maintained at 220°C for 12 h. After heating, the autoclave was cooled to room temperature naturally. The resulting deep-colored product was filtered off, washed with absolute ethanol and distilled water for several times, and dried at 60°C for 6 h. Other samples were also prepared in the same way but with addition of different amounts of thiophene: 0 μL, 60 μL, and 400 μL, respectively. The corresponding MoS2 products are denoted as MoS2-0, MoS2-60, MoS2-200, and MoS2-400 based on the amount of thiophene added. Commercial bulk MoS2 was also evaluated for comparative purposes and is denoted as MoS2-C. Physical characterization X-ray powder diffraction (XRD) patterns for the various MoS2 samples were recorded using a D/max-2500 system with Cu-Kα radiation (λ = 0.154 nm). The microstructures of the MoS2 samples were observed by scanning electron microscopy (SEM, FEI Sirion 200), transmission electron microscopy (TEM, Tecnai), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2100F). Raman spectra were recorded using a LabRAMHR-800 instrument (HORIBA, France). The specific surface areas were determined by N2 adsorption at 77 K using a volumetric unit. Assessment of catalytic activity Catalytic activity toward HDS of thiophene A mixture of thiophene (1000 ppm) and n-heptane (as a solvent) was used as a model oil for the investigation. The HDS activity tests were carried out in a magnetically stirred autoclave reactor (Fig. S1). A typical reaction was performed as follows: the reactor was charged with 0.10 g of fresh MoS2 catalyst powder (catalyst particle radius ≤ 0.1 mm) and 30 mL of model oil solution. Before the reaction, the reactor was charged with H2 and then outgassed 20 times to remove oxygen. The HDS reaction was performed at 280/320°C for 3 h under 2 MPa of H2 at a stirring speed of 300 rpm. This condition was fixed for all runs. The liquid product was then collected and analyzed by using an EWAI GC-4000A gas chromatograph equipped with an OV-101 capillary column and a FPD detector. We determined that the external mass transport limitation and internal mass transfer effect can be neglected in this HDS reaction (the details can be found in the Supporting information). The activity of the catalyst was estimated by using the HDS conversion (amount of reacted thiophene): XT(%) = where Cs0 is the thiophene content in the feedstock (wt.%) and Cs is the thiophene content in the product (wt.%). The HDS turnover frequency (TOF, s−1) was calculated as follows: where FT is the amount of thiophene reactant (mol), xT is the thiophene conversion (%), n is the amount of MoS2 catalyst (mol), and fe is the fraction of Mo atoms at the edge sites (%) (details for the calculation of fe can be found in the Supporting information). Electrocatalytic activity toward HER Typically, 5 mg of MoS2 catalyst and 50 μL Nafion solution (5 wt.%) were dispersed in 1 mL of ethanol. After sonication for 40 min, 5 μL of the catalyst slurry was dropped on the surface of a glassy carbon electrode (GCE) (3 mm in diameter). The GCE was then dried at room temperature to yield a catalyst loading of 354 μg cm−2. To evaluate the electrochemical activities of various MoS2 catalysts toward HER, linear sweep voltammetry (LSV) measurements were conducted in 0.5 mol L−1 H2SO4 at a scan rate of 1 mV s−1 using a three-electrode electrochemical cell in a CHI604e instrument at room temperature. An Ag/AgCl electrode was employed as the reference electrode and a Pt wire as the counter electrode. The electrode potentials were calibrated with respect to the standard reversible hydrogen electrode (RHE) according to the equation: E(RHE) = E(Ag/AgCl) + 0.059pH + 0.197. Before the electrochemical measurements, the electrolyte solution was purged with high purity hydrogen gas to completely remove the oxygen, and stable polarization curves were recorded after 20 cycles. The turnover frequency (TOF, s−1) of the HER was estimated as follows: I TOF = 2 × n × f × F , e where I is the current during the LSV measurement in 0.5 mol L−1 H2SO4 (A), F is the Faraday constant (96,485 C mol−1), n is the amount of MoS2 catalyst (mol), and fe is the fraction of Mo atoms at the edge sites (%). RESULTS AND DISCUSSION Structure characterization of MoS2 The XRD patterns of the MoS2 samples synthesized with different amounts of thiophene are shown in Fig. 1. The XRD patterns of the synthesized MoS2 present lower and broader diffraction peaks than that of the commercial bulk MoS2 (inset), indicating poor crystallinity of the former. The (002) diffraction peak is known to correspond to the basal plane of MoS2 [21]. Notably, the (002) diffraction peaks of MoS2 synthesized with thiophene shifted slightly toward a lower angle, indicating an increase of the interlayer distance along the (002) direction (Table 1). However, based on the (100), (103), and (110) diffraction peaks, all of the MoS2 samples could be indexed to pure, hexagonal MoS2 (JCPDS card NO: 37-1492). In addition, the low intensity of the (002) diffraction peak indicates a low stacking height along this direction. The stacking heights along the c axis of these samples were estimated from the diffraction patterns using the Scherrer equation, and are listed in Table 1. All of the synthesized MoS2 samples have a similar stacking height; thus, it can be deduced that these MoS2 samples Figure 1   XRD patterns of the MoS2 samples obtained with different amounts of thiophene and that of commercial bulk MoS2 (inset). have a similar stacking interlayer number. Raman spectroscopy was also employed to gain more detailed insight into the structures of the MoS2 samples. As shown in Fig. 2, the characteristic Raman shifts at 380 and 406 cm−1 expected for the E21g and A1g vibrational modes of hexagonal MoS2 are clearly observed. The frequency difference of the two characteristic Raman modes (E21gand A1g) is widely used to identify the stacking layer number of MoS2 [22]. For all the MoS2 products, the frequency difference is approximately 25 cm−1, indicating that these MoS2 samples have a similar stacking layer number, which is consistent with the estimation from the XRD characterization. Yu et al. [23] demonstrated that the catalytic activity of MoS2 for HER decreased by a factor of ~4.47 for each additional layer; thus, all of the synthesized MoS2 samples are considered to exhibit approximate intrinsic resistance. With the addition of thiophene, MoS2 products with various morphologies were obtained. The TEM images of the MoS2 products are shown in Fig. 3. As shown in Fig. 3a, MoS2-0 does not have a regular morphology and is comprised of many nanosheets, having the appearance of a “rag” [24]. However, when thiophene was added during the synthesis procedure, the amorphous MoS2 nanosheets began to coalesce; thus, MoS2-60 has a “flower-like” morphology. Notably, some black wires appear in the “flower”, Table 1 Interlayer distance and stack height of various MoS2 estimated from XRD patterns Figure 2   (a) Raman spectra of the MoS2 samples obtained with different amounts of thiophene; (b) frequencies of E21g and A1g Raman modes (left vertical axis) and their difference (right vertical axis) as a function of layer thickness. where these wires are actually stacks and curls of MoS2 nanosheets. With the addition of 200 μL thiophene, the MoS2 appears to be comprised of small particles with more black wires, as shown in Fig. 3c. Moreover, with continued addition of thiophene, these particles become larger and darker (Fig. 3d), appearing to be aggregates of many MoS2-200 particles. From the TEM images, it can be concluded that thiophene has the “ability” to induce aggregation of the MoS2 nanosheets, thereby leading to a change of the morphology of MoS2 from amorphous nanosheets to small microparticles and an increase in the diameter of the MoS2 particles. SEM characterization was employed for detailed evaluation of the influence of thiophene on the surface structure of MoS2 (Fig. 4). With the addition of thiophene, more and more wrinkles, which are actually MoS2 nanosheets, appear on the surfaces of the MoS2 products. The partially enlarged images in Figs 4a and c show the details of the surface structure of the MoS2 samples. The wrinkles cover most of the surface of MoS2-200 (Fig. 4c), while few wrinkles are present on the surface of MoS2-0 (Fig. 4a). Based on these SEM images, it is obvious that adding thiophene during the synthesis can lead to the formation of MoS2 nanosheets on the surface of the MoS2 microparticles. More importantly, these exposed MoS2 nanosheets can increase the specific surface area of MoS2 (Table 2) and also furnish additional active sites, thus suggesting that MoS2 prepared with thiophene may concomitantly exhibit higher activity for both HER and HDS.  From the SEM images,  it is also apparent that the MoS2 particles became larger with the addition of thiophene, which is in good agreement with the TEM images. More information was obtained from the higher magnification image in Fig. 5. The observed interlayer distance of MoS2-0 is about 0.62 nm, which matches the standard value [25,26]. However, MoS2 synthesized with thiophene presents a larger interlayer spacing, which is consistent with the value calculated from the XRD patterns. More importantly, bending of the MoS2 slabs and numerous defects occur in the layer stacking structure of MoS2 synthesized with thiophene, which can increase the interlayer spacing [27]. Moreover, these defects can divide a long slab into several shorter slabs, thereby decreasing the length of the MoS2 slabs. Statistical analysis of the slab length was carried out by measuring at least 50 slabs for each sample based on HRTEM images. The MoS2 slab length distributions are shown in Fig. 6, and the average lengths of the MoS2 slabs (L )  are summarized in Table 3.  In addition,  the fraction Table 3 Average slab length (L) and the fraction of Mo atoms at the edge sites ( fe) of various MoS2 catalysts of Mo atoms at the edge sites of the MoS2 crystallites ( fe), which is used to evaluate the amount of active edge sites, was also calculated based on L . It is clear that MoS2 synthesized with thiophene possesses more active edge sites (Table 3, the calculation details are presented in the Supporting information). It is proposed that the hydrothermal synthesis of MoS2 proceeds via a three-stage growth process. MoS2 nuclei are thought to be formed in the first stage. Subsequently, the MoS2 nuclei grow into nanosheets according to their crystal growth habit. The MoS2 nanosheets then intertwine and convolute to form flower-like microspheres, driven by the reducing surface energy. Based on the aforementioned experimental observation and analysis, it is reasonable to deduce that when thiophene is added during the synthesis, a small fraction of the thiophene molecules may be weakly adsorbed on the Mo sites that have S vacancies, accompanied by the growth of MoS2 crystals (this adsorption is weak and the adsorbed thiophene can be easily removed during the product purification process, as demonstrated by the XPS and FT-IR characterization in Fig. S3). Thus, thiophene molecules are present on the surface of some of the MoS2 nanosheets; these molecules change the surface energies of these MoS2 nanosheets. Consequently, bending of the MoS2 slabs and defects occurs in the layer stacking structure of MoS2, leading to a slightly larger interspacing and more active edge sites. Moreover, the variation in the MoS2 nanosheets further intensifies self-assembly of the MoS2 microspheres, causing excessive aggregation of the MoS2 nanosheets with eventual formation of larger MoS2 microspheres with unique surface structures. The influence of thiophene on the morphology of MoS2 is summarized and schematically illustrated in Fig. 7. The structural characterization of MoS2 demonstrates that on one hand, the addition of thiophene increases the surface area and the number of active sites of MoS2; on the other hand, the synthesized MoS2 samples have a similar stacking layer number, which indicates that all of the MoS2 samples have approximate intrinsic resistance. Considering this situation, although the approximate intrinsic resistance of an archetypal MoS2 catalyst like MoS2-200 may be similar to that of the other MoS2 species (like MoS2-0), the former possesses a higher surface area and more active sites and thus exhibits higher catalytic activity for HER. Moreover, the higher surface area and more active sites also mean better activity for HDS. Thus, it is expected that the MoS2 samples synthesized with thiophene, especially MoS2-200, will simultaneously show better performance for HDS and HER. Catalytic activity of thiophene toward HDS Thiophene was selected as an organic sulfide for rough estimation of the catalytic activity of the synthesized MoS2 samples toward HDS. The results of HDS of thiophene are shown in Fig. 8, and for comparison, the data for commercial bulk MoS2 are also included. Compared to synthesized MoS2, especially for the reaction at 280°C, bulk MoS2 is nearly inactive for HDS. From Fig. 8, we can conclude that the catalytic activity (measured in terms of the conversion ratio of thiophene) of all the MoS2 samples prepared with thiophene is higher than that of MoS2 prepared without thiophene. Secondly, we find that the catalytic activity for HDS of thiophene decreases in the order: MoS2-200 > MoS2-400 > MoS2-60 > MoS2-0 >> MoS2-C, which is consistent with our aforementioned expectation that more exposed edge planes and a larger specific surface area can provide more active sites, thus leading to higher catalytic activity (Table 4). To estimate the activity of each active site of the MoS2 catalysts, the TOF for HDS of thiophene was calculated based on the amount of Mo at the edge sites of the MoS2 crystallites by using the Mo dispersion, fe (Table 3). The TOF is defined as the number of molecules reacting per active site per unit time, and the TOF was calculated by using only the conversion under 50%.  MoS2-0 has the lowest TOF value, Figure 7   Schematic illustration of the synthesis procedure and influence of thiophene on MoS2 product. Figure 8   Conversion ratios of thiophene with MoS2 catalysts. but the differences in the TOF values for these synthesized MoS2 catalysts are not very significant; therefore, the difference in the activities of these catalysts can only be attributed to fe, i.e., the number of active sites. Most importantly, the difference between the HDS activities of MoS2-200 and MoS2-0 can be attributed to the unique surface structure of MoS2-200, which is associated with more active sites and a higher surface area. The XRD patterns of the used catalysts (MoS2-0, MoS2-60, MoS2-200, and MoS2-400) presented in Fig. S4 demonstrate that MoS2 does not undergo any obvious phase change after the reaction. Comparison of the TEM images of the used MoS2 catalysts in Fig. S5 with the data in Fig. 3 shows that there is no significant change in the structure of MoS2 during the reaction. Thus, it can be concluded that the morphologies of the MoS2 catalysts are stable and are well maintained even after use. Electrocatalytic activity toward HER The polarization curves of all the MoS2 samples measured Table 4 Conversion, TOF values, and roHbDsS for HDS of thiophene over different MoS2 catalysts Thiophene conversion (%) in 0.5 mol L−1 H2SO4 at a scan rate of 1 mV s−1 at room temperature are shown in Fig. 9a. For comparison, commercial bulk MoS2 is also included. As is known, bulk MoS2 is a poor HER catalyst [28]; thus, the synthesized MoS2 samples are expected to exhibit better catalytic activity than bulk MoS2. As shown in Fig. 9a, all of the synthesized MoS2 samples exhibit much better activity than the commercial bulk MoS2 with more positive onset potentials between −100 and −200 mV, confirming the expectation. Specifically, MoS2-200 exhibits the best HER activity with a current density of 7.87 mA cm−2 at −200 mV, which is about 98 times larger than that of MoS2-C (0.08 mA cm−2). Moreover, the various synthesized MoS2 samples also exhibit quite different catalytic activities, which can be attributed to their special surface structures. With an increase of the amount of added thiophene, the HER activity of the synthesized MoS2 samples first increases and reaches the maximum value at 200 μL, followed by a decline. MoS2-0 exhibits the poorest catalytic activity among all the synthesized MoS2 samples with a current density of 0.19 mA cm−2 at −200 mV, which is still higher than that of MoS2-C. It is widely accepted that the HER active sites of MoS2 are located on the MoS2 edge planes [29], and tailoring the microstructure of MoS2 to obtain more exposed edge planes can greatly enhance the HER activity. In the present case, compared with MoS2-0, the MoS2 samples prepared with thiophene possess a large amount of MoS2 lamellar nanosheets on their surface, leading to more exposed edge sites and a higher surface area, which significantly improve the final HER activity. Therefore, the activity gradually increases with increasing addition of thiophene from 0 to 200 μL. However, the surface area of MoS2-400 declines relative to that of MoS2-200 due to the excessive agglomeration. Consequently, the activity of the MoS2-400 for HER is worse than that of MoS2-200. (×10−5 mol kg c1at s 1)b) Figure 9   (a) Polarization curves obtained in 0.5 mol L−1 H2SO4 for various MoS2 catalysts; (b) corresponding Tafel plots of various MoS2 catalysts; (c) TOF curves of the synthesized MoS2 catalysts; (d) stability test for the MoS2-200 catalyst. The Tafel plots for these MoS2 samples, derived from the polarization curves shown in Fig. 9b, fit well to the Tafel equation (η = blog j + a, where j is the current density and b is the Tafel slope) at different overpotential ranges, and only the linear portions were selected to provide a clear comparison. The Tafel slopes of the MoS2 samples synthesized with thiophene range between 73 and 88 mV dec−1, which are much smaller than the corresponding values for MoS2-C (219 mV dec−1) and MoS2-0 (147 mV dec−1). Moreover, the MoS2-200 sample exhibits the minimum value of 73 mV dec−1, which is consistent with the most positive onset potential of −102 mV. A smaller Tafel slope indicates a faster increase of the HER rate with increased potentials [13]. The smaller Tafel slopes demonstrate that the MoS2 samples synthesized with thiophene exhibit superior catalytic performance. The best parameter of the inherent activity for HER is the exchange current density, j0, which is determined by fitting the j-E data to the Tafel equation. The exchange current densities for all the MoS2 samples are listed in Table 5. All of the synthesized MoS2 samples show much larger j0 values than that of MoS2-C, and the MoS2 samples prepared with thiophene also give rise to larger j0 values than that of MoS2-0. MoS2-200 is the most active catalyst with the largest j0 of 1.82 × 10−5 A cm−2, which is about 5.5 times larger than that of MoS2-0, which further demonstrates that the MoS2 samples possessing higher surface area can provide more exposed active sites. To obtain a direct site-to-site comparison, the rough estimations of the TOFs are summarized in Fig. 9c. The so-called TOF here is actually defined as the exchange rate per atom of Mo located in the edge plane of the MoS2 crystallites. At potentials higher than 200 mV, compared to MoS2-0, the MoS2 catalysts synthesized with thiophene, especially MoS2-200, exhibit much higher TOF values, indicating excellent intrinsic HER activity. In addition to the HER activity, the stability is another concern for HER catalysts. To evaluate the stability of the Table 5 Onset potentials, Tafel slopes, exchange current densities (j0), and roHbEsR for various MoS2 catalysts Tafel slope (mV dec−1) j0 (× 10−5 A cm−2) MoS2 samples in the acidic environment of HER, long-term potential cycling stability evaluation of MoS2-200 was conducted by acquiring a potential scan in the range of −0.4 to 0 V for 500 and 1000 cycles at an accelerated rate of 100 mV s−1. There is only a slight activity loss after 500 cycles, and MoS2-200 shows good activity even after 1000 cycles (Fig. 9d), indicating good durability. Relationship between structure and HDS/HER activity It is widely accepted that the predominant active sites of MoS2 are metallic edge planes with sulfur vacancies, whereas the basal plane, normally with completely coordinated sulfur atoms, is basically inert [12,20]. It has also been confirmed that the edge sites are indeed the active sites, and the rate of reaction is directly proportional to the number of edge sites, regardless of the MoS2 particle size [28]. A comparison of the observed reaction rates for HDS (roHbDsS, Table 4) and the HER (roHbsER, Table 5) with the various MoS2 catalysts is presented in Fig. 10, from which it is clear that the roHbDsS and roHbsER of these MoS2 catalysts follow a similar trend: MoS2-200 > MoS2-400 > MoS2-60 > MoS2-0, which demonstrates that roHbsDSand roHbsER are both directly proportional to the number of edge sites and surface areas of Figure 10    Observed reaction rates for HDS at 280°C and HER at −300 mV with various MoS2 catalysts. the MoS2 catalysts. Generally, higher activity can be achieved by increasing the exposed surface area and increasing the number of active sites. In this study, the surface area and active sites of the MoS2 samples synthesized with thiophene, especially MoS2-200, are simultaneously increased due to the formation of a larger amount of MoS2 nanosheets on the surface of the samples. Consequently, although the addition of thiophene induces an increase in the size of the synthesized MoS2 particles, these species still exhibit better catalytic activity for HDS and HER than MoS2 synthesized without thiophene. CONCLUSIONS MoS2 catalysts with more active sites and larger surface areas were successfully synthesized via a simple hydrothermal reaction method, and the surface structure of MoS2 could be readily tailored by simply varying the amount of added thiophene. MoS2 with the largest surface area and the most active sites was obtained by adding 200 μL of thiophene during the synthesis procedure. Moreover, MoS2-200 was also the most active catalyst in the present work for HDS as well as HER. The surface area and the catalytic activity of the MoS2 catalysts increases with increasing addition of thiophene up to 200 μL, followed by a decline. The present work provides a new method for engineering the surface structure of MoS2 to expose more catalytically active edge sites, thereby enabling improved performance. It would be very interesting to further investigate whether other organic sulfides have the same effect on MoS2. Sachs JD. Sustainable development. Science, 2004, 304: 649–649 Cheng FY, Chen J, Gou XL. MoS2–Ni nanocomposites as catalysts for hydrodesulfurization of thiophene and thiophene derivatives. Adv Mater, 2006, 18: 2561–2564 Camacho-Bragado G, Elechiguerra J, Olivas A, et al. Structure and catalytic properties of nanostructured molybdenum sulfides. J Catal, 2005, 234: 182–190 Li X, Chai Y, Liu B, et al. Hydrodesulfurization of 4,6dimethyldibenzothiophene over CoMo catalysts supported on Acknowledgments     This work was supported by the National Natural Science Foundation of China (NSFC) (21571012, 21476012 and 91534201). Author contributions      Liu S designed and engineered the samples; Zhang X performed the experiments; Zhang J and Lei Z carried out the characterization work; Liang X provided the idea for this work and wrote this manuscript with support from Chen B. All authors contributed to the general discussion. Supplementary information     Experimental details are available in the online version of the paper. Sijia Liu is currently a PhD candidate at the College of Chemical Engineering, Beijing University of Chemical Technology. His research interests include Mo-based catalysts for hydrodesulfurization and hydrogen evolution reactions. Xin Liang is an associate professor at Beijing University of Chemical Technology. She received his PhD degree from the Department of Chemistry, Tsinghua University in 2009. She worked as a research fellow at the University of California, SantaBarbarafrom2013–2014. 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Sijia Liu, Xin Zhang, Jie Zhang, Zhigang Lei, Xin Liang, Biaohua Chen. MoS2 with tunable surface structure directed by thiophene adsorption toward HDS and HER, Science China Materials, 2016, 1051-1061, DOI: 10.1007/s40843-016-5106-y