MO2 (M = W, Mo) Nanorods as Efficient Hydrogen

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Porous MS/MO (M=W, Mo) Nanorods as Efficient Hydrogen Evolution Reaction Catalysts Jiajun Wang, Wei Wang, Zongyuan Wang, Jingguang G Chen, and Chang-Jun Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01927 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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Porous MS2/MO2 (M=W, Mo) Nanorods as Efficient Hydrogen Evolution Reaction Catalysts Jiajun Wang,† Wei Wang,† Zongyuan Wang,† Jingguang G. Chen*,‡ and Chang-jun Liu*,† † Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China ‡ Department of Chemical Engineering, Columbia University, New York, NY 10027, USA

ABSTRACT: Highly efficient and stable electrochemical catalysts of porous one-dimensional (1D) MS2/MO2 (M=W, Mo) nanorods have been developed for hydrogen evolution reaction (HER). The materials are synthesized via a ‘low-temperature conversion’ method. The asprepared catalysts exhibit numerous advantages: abundant amount of exposed MS2 edges, high conductivity and enhanced mass transport. The MS2/MO2 nanorods show efficient HER activity.

KEYWORDS: WS2, MoS2, nanorod, electrodes, hydrogen evolution reaction

As a clean fuel with high energy density, hydrogen has shown great potentials for replacing fossil fuels.1 Accordingly, for sustainable hydrogen production, hydrogen evolution reaction (HER, 2H++2e-→H2) from water splitting has drawn great attentions in recent years.2 Although Pt-based materials have shown the best HER activities with low overpotential and highly

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efficient energy conversion ability, the large-scale applications are still limited due to their high cost and lower reserve.3 Therefore, various earth-abundant and low-cost electrocatalysts, such as transition-metal oxides (TMOs), dichalcogenides (TMDs) and carbides (TMCs), have been explored and exhibited promising prospects for water electrolysis.4 Among the various TMDs, VI B Group dichalcogenides (including WS2 and MoS2) have shown outstanding performances for HER due to their unique structural and catalytic properties.5 Their electrical properties also vary from metallic and semiconducting via modifying the crystal structure and number of sulfide layers.6 Theoretical and experimental studies have revealed that the sulfided W and Mo edges are the active sites for HER, and appropriate component or structure modifications can significantly enhance their HER performance.7 Accordingly, monoor few-layered WS2 and MoS2 nanosheets, which expose abundant sulfided edges, usually show much higher activities than their bulk compositions.5 However, a key problem hindering the fabrication of mono- or few-layered MS2 (M=W, Mo) is that the layers usually tend to grow thicker under high temperature and therefore the sulfided edges are closely stacked.8 For example, the conventional solid-solid or gas-solid routes for converting MO3 (M=W, Mo) into MS2 usually undergo a relative high temperature (higher than 500 oC), and the as-prepared materials are typically multi-layered.9 Currently, lithium intercalation has been used to exfoliate bulk WS2 and MoS2 into corresponding metastable tetragonal monolayer sheets (1T structure), and monolayer MS2 (M=W, Mo) show significantly enhanced HER activities.5 However, the monolayers strongly tend to restack and irreversibly phase transform into to the hexagonal nanostructure

(2H),

significantly

deteriorating

their

sustainable

electrochemical

performance.1b,5c,10 Herein, we have developed a novel, widely applicable ‘low-temperature conversion’ strategy that proceeds by sulfidation of one-dimensional (1D) MO2 (M=W, Mo) for


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the fabrication of 1D porous MS2/MO2 nanorods (NRs). The sulfidation procedure was performed in a solvothermal condition with a relatively low temperature (190 oC), which is the lowest transition temperature ever reported from W or Mo oxides into WS2 or MoS2. As a result, loosely stacked few-layered WS2 or MoS2 nanosheets were formed on the metallic dioxide surface and acted as active sites for HER. Meanwhile, the final 1D MS2/MO2 NRs inherit the porous and conductive properties from their metallic dioxides. Hence, the MS2/MO2 NRs exhibit excellent activities and stabilities for HER, which significantly outperform their corresponding bulk dichalcogenides and dioxides. This work also provides a new perspective for fabricating transition-metal sulfides or other compounds with specific nanostructures from their corresponding metallic dioxides precursors. XRD spectra were used to verify the composition and phase of the as-prepared samples. As shown in Figure S1, MO2 (M=W, Mo) NRs consist of pure monoclinic WO2 (JCPDS NO. 321393) or MoO2 (JCPDS NO. 32-0671). The system was heated at 190 oC for different sulfidation times (6 h, 9 h or 12 h) and the products were denoted as MS2/MO2-X (X= 6h, 9h or 12 h). Interestingly, for MS2/MO2-6, several specific diffraction peaks, consistent with hexagonal (2H) WS2 (JCPDS NO. 08-0237) or MoS2 ((JCPDS NO. 65-3656), appear in the spectra, confirming the formation of MS2 after sulfidation. Meanwhile, considering that MO2 is still the primary crystalline phase, it is suggested that only the top layers of MO2 were transformed into MS2. Furthermore, with the increasing relative peak intensity of MS2, the incremental proportion of MS2 layers with increasing sulfidation time is confirmed. Structural properties of samples are further characterized using SEM and TEM. Figure S2 clearly shows the 1D nanostructures of all the samples, including their metal oxide/amine hybrids, metal dioxide NRs and the final sulfided samples. The good continuity of the 1D


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morphology is suggested to be due to the slow heating rate (1 oC/min) and low sulfidation temperature (190 oC). Figure S3 reveals the dispersive metallic MO2 (M=W, Mo) particles in MO2 NRs, which are in accordance with XRD results. Energy dispersive X-ray spectrometer (EDS) spectra (shown in Figure S4) further confirm the incorporation of S element after the sulfidation of MO2 NRs. As shown in Figure 1, for WS2/WO2-6 NRs, WS2 nanosheets, with an interlayer distance of 0.618 nm, are observed on the nanorod surface after solvothermal treatments. The WS2 nanosheets appear to be stacked loosely and partially curved, and the standing edges are approximate few-layered or even monolayers (shown in Figure 1b). As for the MoS2/MO2-6 nanorod, no apparent MoS2 nanosheets can be seen with a scale bar of 200 nm. With further amplification, surface of the nanorod is composed of loosely stacked layered MoS2 with an interlayer distance of 0.612 nm (Figure 1e). As reported in previous literature, the number of exposed sulfided edges increases with lower thickness of MS2 (M=W, Mo) layers and act as active sites in HER.11 Therefore, the TEM and SEM results in Figure 1 and Figure S2 confirm the abundant amount of exposed sulfided edges in both WS2/WO2-6 and MoS2/MO2-6 hybrid NRs, which should significantly facilitate their electrocatalytic performance.

Figure 1. TEM and SEM images (a-c) WS2/WO2-6 and (d-f) MoS2/MoO2-6 nanorods.


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Specific surface area and pore size distributions of the as-prepared samples were then measured via nitrogen adsorption−desorption and summarized in Table S1. N2 sorption isotherms of MS2/MO2-6 (M=W, Mo) nanorods are shown in Figure S5. The specific surface area of WO2 and MoO2 NRs are determined to be 78.2 and 83.6 m2 g-1, respectively, which are similar to the previous literature, confirming the porous nanostructures.4b,12 Interestingly, the SBET of WS2/WO2-6 and MoS2/MoO2-6 were determined to be 50.2 and 60.7 m2 g-1, respectively. The presence of mesopores with size of ca. 3.3 and 3.8 nm was also verified using the BJH formula. Compared with MO2 NRs, the decreased SBET of MS2/MO2-6 can be attributed to the higher molecular weight of MS2 and the coverage of surface MS2 nanosheets. The specific surface area and pore size distributions of the hybrids confirm the well maintained porous nanostructure inherited from their metal dioxide NRs. The ample porosity and high surface areas of MS2/MO2-6 would not only provide abundant exposed edges as catalytic sites, but also facilitate the mass and charge transfer of HER, which should lead to the enhancement of HER activity.4b,12 To further monitor the sulfidation process of MS2/MO2 (M=W, Mo) nanorods, time-dependent experiments were performed. As shown in Figure S6, with increasing sulfidation time, the MS2 layers trend to be significantly thicker and closely packed, which is in accordance with XRD results. Therefore, an oxygen-sulfur exchange procedure, containing two steps, is speculated (shown in Figure 2). In Step 1, the MOx/amine precursors were pyrolyzed under the inert atmosphere and reducing gaseous products (such as NOx, CxHy and NH3) were released.4b Then the gaseous products reacted with MOx, creating a porous nanostructure and partially reduced MO2 with abundant oxygen vacancies (OVs), which is similar with other literature.4b,12a,13 In Step 2, the MO2 NRs were sulfided by thioacetamide (TAA) in a reducing environment


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(provided by ethanol). During the sulfidation procedure, S2- anions (dissociated from TAA) reacted quickly with surface metal ions and formed a thin layer metal disulfide, which obstructed the direct contact between inner metal ions and outside S2-. The O2- anions react with other hydrolysates derived from TAA. As the S2- ionic radius (184 pm) is much larger than that of W4+ (68 pm) and Mo4+ (70 pm), the inner metal ions should diffuse outside more easily and react with S2-.14 Consequently, the thickness of metal dichalcogenide layer gradually increased with increasing sulfidation time. In the present case, MO2 NRs serve as self-templates to produce the MS2/MO2 hybrid nanorods. Particularly, this method can be expanded to prepare other transition metal sulfides by using the corresponding partially reduced oxides as precursors.

Figure 2. Schematic drawing of preparing MS2/MO2 (M=W, Mo) hybrids via ‘low-temperature conversion’ methods. As discussed in the introduction, the MS2 (M=W, Mo) layers tend to grow thicker under high temperature. Therefore, the formation of few-layered MS2 nanosheets on MS2/MO2-6 is significantly due to the low conversion temperature (190 oC). Most of the current methods for fabricating MS2 (M=W, Mo) usually selected MO3 or M6+-contained metal salts as precursors, which usually required higher temperatures for the subsequent reduction and sulfidation processes. Considering the unique surface properties of metallic MO2, the abundant oxygen


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vacancies (OVs) in the MO2 are suggested to play an important role in decreasing the conversion temperature.4b,15 As reported in many literature related to synthesis or catalysis, surfaces with OVs are usually more active than stoichiometric surfaces.16 Feldman et al. reported that during the synthesis of WS2 from WO3 in the H2S/H2/N2 atmosphere, where the formation of surface oxygen vacancies could significantly enhance the subsequent reduction and sulfidation procedures.17 As for procedures for fabricating metal carbides, partially reduced surfaces with OVs are also much more unstable and therefore beneficial for the subsequent carbonization process.18 In the present case, abundant OVs have been created before the final sulfidation procedure. Therefore, in the ethanol-provided reducing environment, the OVs also facilitate the oxygen-sulfur exchange, which ultimately decreases the conversion temperature. Due to the initial physical barrier and the relative low sulfidation temperature, the growth rate of metal disulfide is much slower than that of other sulfidation methods, which contribute to the production of loosely-stacked and few-layered MS2 on MS2/MO2-6 hybrid nanorods. HER catalytic activities of the MS2/MO2 (M=W, Mo) hybrid nanorods were then measured in 0.5M H2SO4. For comparison, glassy carbon electrodes (GCEs) were also modified with commercial bulk MS2 (Aladdin), MO2 NRs and commercial 20 wt% Pt/C (Alfa Aesar). Figure 3a and 3b show the corresponding polarization curves (i-V plot) of tested samples. Similar with literature reports, Pt/C showed the best performance for HER.5b,9b For MS2/MO2-6 NRs, at an overpotential of -160 mV, a current density of ca. 15.0 mA cm-2 was observed for WS2/WO2-6, which was much higher than that observed for bulk WS2 (ca. 0.36 mA cm-2) and WO2 NRs (ca. 0.38 mA cm-2). Similarly, at an overpotential of -200 mV, MoS2/MoO2-6 showed the highest cathodic current (15.1 mA cm-2), much higher than that of MoO2 NRs (0.90 mA cm-2) or bulk MoS2 (0.85 mA cm-2) (shown in Table S2 and S3).


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Figure 3. Polarization curves of (a) W-based and (b) Mo-based materials. Tafel plots of (c) Wbased and (d) Mo-based materials. The Tafel slope of the current response reveals the inherent property of HER and helps verify the possible mechanistic steps during HER. In acidic electrolyte, three principal steps can participate in the hydrogen evolution.12a,19 Discharge reaction: H3O+ + e- + cat → cat-H + H2O (Volmer reaction)

(1)

Combination reaction: cat-H + cat-H → 2cat + H2 (Tafel reaction)

(2)

Ion + atom reaction: H3O+ + e- + cat-H → cat + H2 + H2O (Heyrovsky reaction)

(3)

According to previous literature, a Tafel slope of ~30 mV/dec typically indicates a Volmer– Tafel mechanism, a slope of 40 mV/dec suggests a Volmer–Heyrovsky mechanism, and a slope of 120 mV may derive from various reaction pathways depending on the coverage of surface adsorbed hydrogen.12a,19 Therefore, linear portions in Tafel plots were fit to the Tafel equation (η= b log j + a, where j is the current density and b is the Tafel slope) (shown in Figure 3c and 3d). In accordance with other literature, the Tafel slope of Pt/C was calculated to be 30


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mV/dec.12b,20 Interestingly, MS2/MO2-6 (M=W, Mo) exhibited Tafel slopes of 63 mV/dec for W and 51 mV/dec for Mo, much smaller than those of bulk MS2 or MO2 NRs ( (summarized in Table S2 and S3). The Tafel slopes of MS2/MO2-6 reveal a possible Volmer–Heyrovsky reaction pathway during HER, where the electrochemical desorption of hydrogen is the rate-limiting step.20,21 The results are comparable with other ultrathin MS2 nanosheets.5,7c Meanwhile, the overpotentials and Tafel slopes of both MS2/MO2-6 show higher HER activities than many previously reported values of other MS2-based catalysts (shown in Table S4 and S5). And the MS2/MO2-6 NRs are also comparable to their corresponding phosphides (shown in Table S6). In order to properly compare the properties of the current catalysts with those reported in the literature, it is critical to determine the intrinsic area-specific activities of the electrocatalysts.11 The electrochemical capacitance surface area measurements (integrated with a simplified model of the surface structure) were carried out to determine the active surface areas and estimate the density of electrochemically active sites,11c,11d which were used to calculate the turnover frequency (TOF) for each catalyst in the current study. Figure S8 shows the TOFs of MS2/MO2-6 (M=W, Mo) NRs plotted against the applied overpotential. At the current density of 10 mA/cm2, the TOFs of MS2/MO2-6 NRs were estimated to be 0.405 H2/s for Mo and 0.290 H2/s for W. In general, these values above are comparable to other reported highly active Mo or W sulfides and phosphides at similar current densities. For example, the TOF of WS2/WO2-6 (0.290 H2/s) is much higher than that of WO3•2H2O/WS2 hybrids (ca. 0.08 H2/s) at 10 mA/cm2,5e and the TOF of MoS2/MoO2-6 (0.405 H2/s) is also comparable to amorphous molybdenum sulfide (ca. 0.3 H2/s) and MoP (ca. 0.1 H2/s ) at 10 mA/cm2.11c, 11d Meanwhile, the effect of catalyst-loading on the HER activity was also investigated. Polarization curves of different catalyst loadings (from 25% to 100%) of MS2/MO2-6 NRs


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(M=W, Mo) are shown in Figure S9. To minimize the effect of iR-correction on the apparent activity, the potential at 1 mA/cm2 was used to be a metric for comparing the catalytic activities.11c As shown in Figure S9, the catalytic activities rose linearly with catalyst loading, demonstrating that the measured activities were in the regime of intrinsic kinetics. Interestingly, for MS2/MO2 (M=W, Mo) nanorods, the HER activities decrease with increasing sulfidation time (shown in Figures 3a and 3b). As reported in the literature, WO2 and MoO2 are conductive with low metallic electrical resistivity, while the 2H WS2 and MoS2 are usually semiconducting.4b,5,22 In this case, the semiconductive 2H MS2 layers grew thicker with increasing sulfidation time, which might affect the catalyst impedances and then led to the variation of HER performances. Therefore, electrochemical impedance spectroscopy (EIS) measurements were carried out to characterize the reaction kinetics during HER. Similar with those reported in the literature, as shown in Figure S10, MS2/MO2-6 NRs showed lower impedance than MS2/MO2-9 and MS2/MO2-12, owing to the thinner semiconductive MS2 layers.12a These results suggest that the reduced impedance promoted the electron transfer between the catalysts and the electrode, contributing to the higher HER rate.12a The stability was also tested by continuously cycling the MS2/MO2-6 modified electrodes for 3000 cycles. At the end of cycling, the i-V curves remain similar to the initial cycle with negligible loss in the cathodic current (shown in Figure S11). In addition, the current-time plots at fixed potential were also collected and the cathodic currents remain at around ca. 15 mA cm-2 over 10 h, further indicating the excellent stability of the MS2/MO2-6 modified electrodes. The highly efficient HER performance of the 1D porous MS2/MO2 (M=W, Mo) nanorods (NRs) is due to their advantageous structural properties. Firstly, HER activities of MS2 are closely related to the number of exposed sulfided edges, which act as the catalytically active


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sites.5 Especially, monolayer- or few-layered MS2 nanosheets, with numerous exposed edges, usually exhibit enhanced HER activities. In the present case, as the few-layered MS2 nanosheets are loosely stacked on the surfaces of MS2/MO2 NRs, abundant amount of sulfided edges are exposed as the HER active sites.5b,21 Secondly, the charge transport and the accessibility of protons to the active sites significantly affect HER activity.4b,11 In this case, the high conductive metallic MO2 provides a conductive path for electron transport between the electrode and catalysts. Therefore, the charge transfer kinetics are significantly facilitated, as indicated by the decreasing charge-transport impedance. Thirdly, a rapid diffusion of ions should also facilitate the HER activity. Therefore the porous nanostructures of MS2/MO2 NRs, providing effective channels for mass transport and higher interfacial active sites, contribute to the enhanced HER activities.12 In addition, the surface MS2 nanosheets act as a protective layer and guard the underlying metallic MO2 from the acidic electrolyte, leading to the excellent stability of HER. These advantages are responsible for the enhanced HER performance of MS2/MO2 hybrid NRs over the corresponding bulk materials. In summary, 1D porous MS2/MO2 (M=W, Mo) hybrid nanorods (NRs) were fabricated from the corresponding MO2 NRs via a simple ‘low-temperature conversion’ method. The as-grown WS2 and MoS2 are loosely stacked with only a few layers, which provided abundant exposed edges as active sites. Meanwhile, the as-prepared hybrid NRs inherit the 1D porous nanostructures from their metallic oxide templates, leading to the high active surface area and facilitating the charge and mass transport. In addition, the resistance of the hybrids is significantly reduced due to the internal conductive metal dioxides. Consequently, the MS2/MO2 hybrid NRs exhibited excellent activity and high stability for HER: low overpotentials (-90 mV and -120 mV), small Tafel slopes (63 mV/dec and 51 mV/dec), and high stability (maintained for


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more than 10 h). In addition, the results demonstrated the feasibility of preparing specific structural MS2/MO2 hybrid NRs from their metallic dioxides. Our method provides a new strategy for preparing transition-metal sulfides or other transition-metal compounds with specific nanostructures for various applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C. -J. Liu) *E-mail: [email protected] (J. G. Chen) Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (#91334206). Contributions from Columbia University were supported by the United States Department of Energy (Grant number DE-FG02-13ER16381).


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TOC graphic TOC graphic 34x14mm (300 x 300 DPI)


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