Negative Electrocatalytic Effects of p-Doping ... - ACS Publications

Aug 1, 2016 - on MoS2 and WS2 for the Hydrogen Evolution Reaction and Oxygen. Reduction Reaction. Xing Juan Chua,. †. Jan Luxa,. ‡. Alex Yong Shen...
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Negative Electrocatalytic Effects of p‑Doping Niobium and Tantalum on MoS2 and WS2 for the Hydrogen Evolution Reaction and Oxygen Reduction Reaction Xing Juan Chua,† Jan Luxa,‡ Alex Yong Sheng Eng,† Shu Min Tan,† Zdeněk Sofer,‡ and Martin Pumera*,† †

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore ‡ Department of Inorganic Chemistry, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Transition-metal dichalcogenides (TMDs) are at the forefront of research for their promising catalytic abilities and unique materials properties. With great interest in the study of mono- or few-layered TMDs, we seek to fundamentally explore the effects of doping on bulk TMDs, particularly MoS2 and WS2 for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) with p-dopants Nb and Ta. Despite promises reported in the computational studies of doped TMDs on the HER, our results show otherwise. Doped bulk TMDs display less catalytic activity in comparison to their undoped counterparts for the HER. A similar effect is observed for ORR catalysis. Characterization was done to shed light on its surface elemental composition, properties, and morphologies. It was found that doped WS2 has a high percentage of 1T phase but this does not correlate with a lower overpotential for the HER at −10 mA cm−2, which contradicts the general consensus. We therefore show that p-dopants have a negative electrocatalytic effect on the HER. These findings are of high importance for the field of TMD electrochemistry, as they challenge the current consensus that doping always improves the electrocatalysis of TMDs. KEYWORDS: transition-metal dichalcogenides, electrochemistry, hydrogen evolution reaction, oxygen reduction reaction, doping, MoS2, WS2 ollowing the discovery of the first two-dimensional (2D) material, graphene,1 much scientific enthusiasm has been focused on this unique group of 2D materials that offer a brand new perspective for physics and chemistry. Layered transitionmetal dichalcogenides (TMDs) have come into the spotlight with their layered structure being similar to graphene’s bulk formgraphite. Most layered TMDs have the composition MX2 (M = transition metal; X = chalcogen, S, Se, Te). Each layer is triatomically thick, consisting of X−M−X where the M−X bonds are held together by strong covalent bonds while only weak van der Waals forces of attraction exist between each stacked layer.2 Interlayer hybridization3 is present in bulk TMDs due to the presence of the interlayer van der Waals forces between the layers. The huge interest in TMDs stems from the large variation in band gaps which confers varying degrees of electrical conductivity for various intended applications.4 This can be due to different elemental compositions, numbers of layers, mechanical strain,5 or the presence of dopants4,6 or impurities.7 The applications of TMDs span from catalysis to lubricants to photovoltaics to energy storage and electronics applications.4,8−12 Composition variations can alter the conductivity7 and further tune the

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© XXXX American Chemical Society

magnetic,13 tribology,14 mechanical properties,15 and optical activity apart from the changes in electronic properties16 which can tune the catalytic efficiency.4,17,18 With ever-increasing power consumption, hydrogen has emerged as a promising clean energy carrier; to harness its potential, there exists a need for a large-scale economically viable production method. Pt, which is the most effective hydrogen evolution reaction (HER) catalyst, is hindered in its large-scale application due to its scarcity. Thus, this work presented herein contributes to the search for a catalyst design strategy to optimize TMD catalytic properties for the HER. To harness the exciting potential offered by TMDs, many studies have been done and are also underway to tune their properties to suit the particular application of interest. Such efforts have focused on the factors that could influence the variation in band gaps of TMDs. In the search for optimized HER catalytic activity in TMDs, atomic doping could be employed to tune its structure and/or electronic properties. Received: June 6, 2016 Revised: July 4, 2016

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atomic percentage of each individual element obtained using EDS is shown in Table 1. In general, the doped MoS2 group is

Point defects of zero dimension (0D) could occur when the dopant atoms directly substitute the atoms in the lattice or into the interstitial sites between atoms in the crystal lattice. In layered TMDs, there is the possibility of dopant atoms intercalating between the layers.6 Many studies have been conducted to look into the effects of transition-metal doping in TMD electronic structures.16,19−22 Hence, doping can be used as a strategy to alter the electronic properties of the compound, which can have significant influence on the subsequent catalytic activity. Doping is also being employed as a strategy to enhance the intrinsic catalytic activity of MoS2 by altering the free energy of hydrogen adsorption (ΔGH) on the edges to be closer to the thermoneutral value, positively affecting the HER. Tsai et al. have conducted computational studies on the doping of various transition metals on MoS2 and found that 14 types of transition-metal dopants (Pt, Ni, Ru, Rh, Co, Fe, Mn, Ta, V, Nb, Cr, Os, Ir, and Re) have ΔGH closer to thermoneutral value than the S-edge and, among them, 6 transition-metal dopants (Ru, Rh, Co, Fe, Mn, and Ta) have an ΔGH value even smaller than that of the already active undoped MoS2 Mo-edge.23 Previous successes such as Co-promoted nanoparticulate MoS2 and WS219 and V-doped MoS2 nanosheets demonstrated electrocatalysis for the HER,16 serving to provide motivation to investigate the effects of doping in bulk MoS2 and WS2. Ivanovskaya et al. conducted a computational study for Nb doping in bulk MoS2 and found that it is likely to occur through a substitutional mechanism, by replacement of Mo in the lattice structure. It is energetically unfavorable for Nb to occupy an interstitial site.22 The active sites of MoS2 and WS2 are experimentally and theoretically confirmed to be the edges, and the basal plane is deemed to be inert.24−26 The ΔGH value of the catalyst is commonly used as a measurement for the effectiveness of the catalyst for HER.27 Both edges in WS2 have ΔGH values closer to thermoneutral (−0.04 and −0.06 eV for W and S, respectively) in comparison to MoS2 (0.06 and −0.45 eV for Mo and S, respectively). However, the basal plane of MoS2 (1.92 eV) is expected to be more active than that of WS2 (2.23 eV).24 Herein, we present the effects of Nb- and Ta-doped bulk TMDs (MoS2 and WS2) on their catalytic activity for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Characterization of the materials was carried out using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) studies. MoS2 and WS2 were studied alongside each other, as they have the same crystallographic structures and are likely to exhibit similar electrochemical and catalytic properties.28 Although much of the trend has been geared toward mono- or fewlayered nanoparticulate TMDs due to their greater surface area and hence density of catalytic sites, wisdom can be gained from the empirical studies of bulk TMDs. Furthermore, it has been shown that the use of different intercalants has varying impacts on the effectiveness of the exfoliated TMDs as catalyst in various processes.29−32 Hence, the use of bulk material helps to eliminate the variations arising from the use of different intercalants or different degrees of exfoliation.

Table 1. Atomic Percentage of the Composition of the Materials on the Basis of EDSa atomic percentage (%)

MoS2:Ta MoS2:Nb undoped-MoS2 WS2:Ta WS2:Nb undoped-WS2

metal (Mo/W)

S

dopant (Ta/Nb)

MxDyS2

32.4 31.0 23.0 35.9 36.7 20.3

66.3 67.3 40.5 62.9 62.1 36.9

1.33 1.53 N/A 1.22 1.24 N/A

M0.98D0.04S2 M0.92D0.05S2 M1.14S2 M1.14D0.04S2 M1.18D0.04S2 M1.10S2

a

M stands for the transition metal, either Mo or W; D stands for the dopant, either Nb or Ta.

M deficient in comparison to the doped WS2 group (Table 1). In other words, the doped MoS2 group has a lower M: S ratio (50%) has a higher percentage than the 2H phase (∼30%) in the doped WS2, with Ta-doped WS2 having a slightly higher percentage of 1T phase than Nb-doped WS2 (Figure 4d−f). This is in stark contrast to the undoped WS2 and the MoS2 group, where no 1T phase was observed. The elemental compositions of the dopants on the surface of all the doped TMDs are presented in Table 2, as determined by XPS. The surface composition of the dopants was much higher than that in the underlying layers, as previously identified by

EDS analysis. This suggests a dopant-enriched surface in comparison to the bulk of the doped TMDs. Moreover, it is important to note that XPS has different depths of analysis for different elements. The undoped MoS2 and undoped WS2 suffer from a transition-metal-deficient surface. Electrochemistry. An investigation into the inherent electrochemistry, which gives insights into the redox behavior of the material itself without the presence of a depolarizer, of the doped TMDs is necessary to understand its electrochemical properties and fundamental differences between the doped and undoped TMDs (MoS2 and WS2). To reveal information with regard to the electrochemically active surfaces of the doped and 5727

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Figure 4. XPS high-resolution spectra for metals (a−c) M = Mo and (d−f) M = W: (a) Ta-doped MoS2 (Mo 3d); (b) Nb-doped MoS2 (Mo 3d); (c) undoped-MoS2 (Mo 3d); (d) Ta-doped WS2 (W 4f); (e) Nb-doped WS2 (W 4f); (f) undoped-WS2 (W 4f).

Bonde et al., where the basal plane is less readily oxidized in comparison to the edges, resulting in a higher potential being required.19 Therefore, we can attribute the two peaks to the oxidation of the edge and basal plane to ca. 1.76 and 2.05 V, respectively. Looking further into the intensity of the peaks, we identified the peak at ca. 1.76 V as the main peak, as it has a higher current intensity in the anodic scan (Figure 5a−c). There is no other apparent distinct difference in features between the doped and undoped MoS2 inherent electrochemistry when an oxidizing potential is first applied (Figure 5a−c). For the cathodic scans of the MoS2 group of compounds, an inconspicuous shoulder peak was observed at ca. −0.50 V (Figure 5d). Its absence in the anodic scan suggests the likelihood that the initial oxidation renders the electroactive species less susceptible to reduction as proposed by Chia et al.45,46 Furthermore, with the presence of the shoulder peak in the cathodic scans, the current intensity of the following oxidation peak drops (in comparison to the oxidation peak in the anodic scans; Figure 5a), indicating an irreversible reduction. The initial reduction in the cathodic scans (at ca. −0.50 V) could be from the same species undergoing oxidation, as we see a fall in current intensity in the oxidation peak following the initial reduction. The main oxidation peak changes when a reductive potential is first applied to Tadoped MoS2. In Figure 5e, the main oxidation peak becomes the peak at ca. 2.05 V. The oxidation current is sharply reduced in Nb-doped MoS2 so that the hardly distinguishable peaks that were observed in the anodic scans are now completely indistinguishable from each other. For Nb-doped MoS2, the cathodic scan oxidation peak current intensity is greatly reduced in comparison to the anodic scan. This shows an irreversible reduction taking place at ca. −0.70 V. Subsequent

Table 2. Empirical Elemental Compositions on the Basis of XPS Analysisa material

MxDyS2

MoS2:Ta MoS2:Nb undoped MoS2 WS2:Ta WS2:Nb undoped WS

M0.84D0.08S2 M0.79D0.11S2 M0.89S2 M0.86D0.21S2 M0.89D0.11S2 M0.81S2

a

M stands for the transition metal, either Mo or W; D stands for the dopant, either Nb or Ta.

undoped materials, we conducted cyclic voltammetry in both anodic and cathodic scans in a blank phosphate buffer solution (PBS). All MoS2 groups of materials show two oxidation peaks in the anodic scans at ca. 1.76 and 2.05 V (versus RHE) but disappear upon subsequent scans (Figure 5a−c). The presence of the second oxidation peak is less obvious for Nb-doped MoS2 (Figure 5c). These two peaks overlap strongly due to their close proximity and large current intensity. The existence of the two peaks is likely to stem from the structure of TMDs, having the edge and basal planes with varying susceptibility to oxidation. Kautek and Gerischer found that the corrosion of MoS2 happens preferentially at the (101̅1) face and no visible corrosion was observed on the (0001) plane.43 The anisotropic surface properties of TMDs are characterized by the relative inertness of the basal plane due to the “coordinately saturated chalcogen ions” as Jaegermann and Schmeisser have explained.44 Thus, the edge and basal plane of MoS2 are each interpreted to have different oxidizing potentials as discussed by 5728

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Figure 5. Cyclic voltammograms during the (a−c) anodic scans and (d−f) cathodic scans of the doped and undoped MoS2: (a, d) undoped MoS2; (b, e) Ta-doped MoS2; (c, f) Nb-doped MoS2. Conditions: supporting electrolyte of 50 mM PBS at pH 7.2; scan rate 100 mV s−1.

Figure 6. Cyclic voltammograms during the (a−c) anodic scans and (d−f) cathodic scans of the doped and undoped WS2: (a, d) undoped WS2; (b, e) Ta-doped WS2; (c, f) Nb-doped WS2. Conditions: supporting electrolyte of 50 mM PBS at pH 7.2; scan rate 100 mV s−1.

scans in the same potential cycling reveals no additional information except that the oxidation and reduction are irreversible or there is the possibility of forming an oxidation/reduction product that is soluble in the electrolyte. A single scan is sufficient to oxidize or reduce the electrochemically active surface moieties, as we fail to observe any recurrence of peaks in the subsequent scans. Upon close scrutiny of the current intensity of the various MoS2 materials, we see a consistent trend in both the anodic and cathodic scans. The presence of Ta dopants in MoS2 renders it much more susceptible to oxidation, whereas the presence of Nb dopants in MoS2 renders it less vulnerable to the possibility of oxidation. One stark difference between the undoped-WS2 and dopedWS2 compounds is the recurrence of oxidation peaks in subsequent cycling of undoped WS2, in both the anodic and cathodic scans (Figure 6a,d). This is characteristic of WS2 and has been previously observed independently by Eng et al. via a comparison with MoS2.29 On comparison across all cyclic voltammograms in this study, it is indeed seen that the recurrence of oxidation peaks occurs only for the undoped WS2. The presence of Ta and Nb dopants alters this intrinsic character in the aforementioned paper. Eng et al. conducted XPS measurements on WS2 after electro-oxidation and confirmed that the WS2 maintains similar compositions after electro-oxidation. Thus, it is likely that the product(s) of electro-oxidation remains on the surfaces of WS2 and detaches gradually upon the potential cycling, resulting in the recurrence of peaks that decrease in intensity after each cycle.29 This argument, however, is surprisingly not validated in the dopedWS2 compounds (Figure 6b,c,e,f). We see the apparent

difference in the doped and undoped WS2 inherent electrochemistry being the lack of recurring peaks for doped WS2 in subsequent cycling. To account for the recurrence of peaks, the chemical species oxidized must be able to be reduced back upon cathodic potentials. Only the undoped WS2 shows the presence of a shoulder peak at ca. −0.80 V, indicating the reversibility of the reaction. However, with the addition of the dopants in both Ta-doped WS2 and Nb-doped WS2, the oxidation reaction occurring at ca. 1.60 V becomes irreversible as proven by the absence of a reduction peak when an initial anodic potential is applied (Figure 6b,c). The lack of a reduction shoulder peak could have accounted for the lack of a recurring peak in the following cycles. A comparison is made with the cathodic scans of the materials. All doped- and undoped-WS2 compounds exhibited the presence of a shoulder peak with the reduction potential (in absolute terms) in the following order: Ta-doped WS2 ≅ Nb-doped WS2 > undoped WS2. Similarly, only the undoped WS2 has a recurrence of the reduction peak at subsequent cycling, resulting in the persistent oxidation peak (Figure 6a,d). Since the presence of dopants does not limit the compounds’ ability to be reduced when an initial reductive potential is applied, we can safely conclude that the presence of dopants would affect only the oxidation product at ca. 1.60 V such that it will not be reduced in the potential window applied. In order to reveal the heterogeneous electron transfer (HET) rate of the materials, cyclic voltammetry was conducted via the redox probe of ferro-/ferricyanide. The rate of HET which is quantified by the k0obs value provides fundamental information on its inherent property that determines its functionality and 5729

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Figure 7. (a) Cyclic voltammogram for Fe(CN)63−/4− for bare GC, Ta-doped MoS2, Nb-doped MoS2, undoped MoS2, Ta-doped WS2, Nb-doped WS2, and undoped WS2. Conditions: 10 mM Fe(CN)63−/4− in 50 mM PBS supporting electrolyte at pH 7.2; scan rate 100 mV s−1. (b) Bar chart representation of the peak separation between the anodic and cathodic peak currents. Error bars correspond to standard deviations on the basis of triplicate measurements.

Figure 8. Bar chart comparison of (a) percentages of the 1T phase (area normalized on the basis of the relative sensitivity factor) from XPS analysis and (b) k0obs values of the WS2 group.

Figure 9. Consolidated data for HER analysis. (a) Polarization curve for HER for bare GC, Ta-doped MoS2, Nb-doped MoS2, undoped MoS2, Tadoped WS2, Nb-doped WS2, and undoped WS2. Conditions: 0.5 M H2SO4; scan rate 2 mV s−1. (b) Bar chart for a comparison of the overpotential of each material at −10 mA cm−2 current density. (c) Tafel plot and (d) bar chart comparing the Tafel slope. Error bars correspond to standard deviations on the basis of triplicate measurements.

usability in electrochemical devices and is calculated on the basis of Nicholson’s classic paper on the theory and application of cyclic voltammetry.47 A larger peak separation between the anodic and cathodic peak currents would correspond to a slower HET rate. The cyclic voltammograms of the materials are shown in Figure 7a, and the peak separations are compared in Figure 7b.

Neither the MoS2 nor the WS2 group shows a distinctively faster HET rate. We found that doping in the MoS2 group increases the peak separation, while the exact opposite trend is observed for the WS2 group. To be exact, the k0obs value is worsened in doped MoS2 while it is improved in doped WS2. The peak separation of all the materials under investigation is larger than that of the bare GC, which implies that HET is 5730

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Figure 10. Consolidated results for the oxygen reduction reaction (ORR): linear sweep voltammograms for (a) MoS2-type compounds and (b) WS2-type compounds and (c) the onset potential of the ORR using the potential at which 10% of the maximum current is achieved. Conditions: 0.1 M KOH, scan rate 50 mV s−1; saturated with nitrogen gas for 15 min (dashed lines) and saturated with air (solid line). Error bars correspond to standard deviations on the basis of triplicate measurements.

catalytic activity in comparison to Nb-doped TMD, albeit to a small extent. Finally, one obvious conclusion that we can draw from this experiment is that the doped TMDs performed less ideally in terms of HER catalysis than their undoped parent TMDs. Another parameter to investigate in the HER is its Tafel slope. The Tafel slope, which is an inherent property of the catalyst, can be related to the HER mechanism and give insight into the rate-determining step of the HER.30,48 A low Tafel slope suggests a much more efficient HER catalyst, as a small input in voltage is able to result in a significant increase in current density. The two possible pathways for HER in acidic media have been suggested and studied intensely,48,49 through either the Volmer−Heyrovsky or the Volmer−Tafel mechanism. A Tafel slope of 120 mV dec−1 is theoretically observed for the Volmer pathway whereby hydrogen adsorption is rate limiting. The Pt/C has the lowest Tafel slope of about 32 mV dec−1.29,50 Bare GC has a Tafel slope of 138 mV dec−1 in this study, and all of the subsequent materials have a Tafel slope higher than that of bare GC, indicating that the Volmer step is the rate-limiting step for all the materials under study. Figure 9c records the processed data of the Tafel slope of the materials in this study. Ta-doped MoS2 has the highest Tafel slope of 265 mV dec−1, while Nb-doped WS2 has the lowest Tafel slope of 167 mv dec−1 among the materials being investigated. Further analysis of the Tafel slope shows that Nb-doped TMDs have a lower Tafel slope in comparison to Ta-doped TMDs. Doping in WS2 appears to result in a decrease in its Tafel slope; such a trend was not observed for MoS2 (Figure 9d). The undoped materials have better catalytic performance than their doped counterparts for the HER overpotential, but they do not necessarily have the lowest Tafel slopes. The enhanced catalytic activity of 2D or exfoliated TMDs (MoS2 and WS2) for the HER has commonly been attributed to the shift from the semiconducting 2H phase to the metallic 1T phase.42,51 Ambrosi et al. conducted resistivity measure-

slower in these materials in comparison to bare GC. Indeed, the calculated k0obs value is fastest for bare GC, as expected by the smallest peak separation at 1.27 × 10−3 cm s−1. Within the MoS2 group, undoped MoS2 has the fastest k0obs value of 2.21 × 10−4 cm s−1, while the undoped WS2 has the slowest k0obs value (k0obs = 4.48 × 10−5 cm s−1) of all the materials. The observed trend for the WS2 group of materials could be accounted for by the trend of the percentage of 1T phase as observed in XPS. There is a positive correlation between the 1T phase and a faster k0obs value. Analysis of the percentages of the 1T phase in doped WS2 by XPS shows that Ta-doped WS2 has a higher percentage of 1T in comparison to Nb-doped WS2 and undoped WS2 (Figure 8a). Consequently, the HET as defined by the k0obs value follows the same trend within the WS2 group. Ta-doped WS2 has a faster k0obs value followed by Nb-doped WS2 and finally the undoped WS2 (Figure 8b). Electrocatalysis. The effect of doping in TMDs for the HER and ORR is of particular interest in this work. With the altered surface composition of the doped TMDs as confirmed by XPS, we explore the changes in catalytic activity of hydrogen evolution occurring at the material’s surface. The lowest observed overpotential at −10 mA cm−2 is 0.61 V (versus RHE) for undoped MoS2. However, the results obtained here show that the doped bulk TMDs (MoS2 and WS2) are catalytically less potent then their undoped counterparts for the HER (Figure 9). A comparison of the overpotential at a current density of −10 mA cm−2 is displayed in Figure 9b. It is clear that the overpotential at −10 mA cm−2 is the lowest for the group of MoS2 compounds, followed by the WS2 group. To look into the effects of dopants in bulk TMDs, we find that the doped materials have a higher overpotential than their respective undoped parent TMDs; but it is still lower than that of bare GC. Thus, this goes to show that the presence of dopants does not completely suppress the catalytic properties of the bulk TMDs but also does not bring about any positive enhancement. Nevertheless, an important trend observed is that Ta-doped TMD (MoS2 or WS2) has a higher HER 5731

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independently account for the anomaly presented in this study and that proposed by computational and theoretical efforts.

ments coupled with XPS analysis. Exfoliated WS2 with the greatest proportion of the 1T phase was also found to have the lowest resistivity in comparison to exfoliated MoS2, and WS2 was subsequently found to be a better HER catalyst than MoS2.42 In addition, computational studies conducted by Ivanovskaya et al. found that the electronic behavior of doped Nb in bulk MoS2 will result in a transition to a metallic form from its former semiconducting form.22 This transition, or rather, the presence of a 1T metallic phase should be expected to lead to a higher HER activity, but it was not observed in this study. Furthermore, despite the doped-WS2 group of materials having a larger proportion of the 1T phase, our experimental results have diverged from past studies,22,42 as the doped-WS2 materials with the 1T phase do not display superior catalytic properties. The adsorption/desorption of hydrogen would play an equally important role in the overall catalytic activity. The promise of enhanced HER catalytic activity indicated by the computational study23 of transition-metal doping in MoS2 due to the lowering of the absolute value of ΔGH is not realized in this study. In addition, this could also be due to the presence of multiple layers within the bulk structure of the TMDs used in this study. Layer-dependence properties of MoS2 and their effect on the HER have been studied by Yu et al., and they found that the catalytic activity for the HER decreases with every additional layer.52 To study the possibility of doped-MoS2 and doped-WS2 ability to catalyze the ORR, we compare the onset potentials of the materials as shown in Figure 10c. The materials are shown to be sensitive to oxygen, as experiments in the nitrogensaturated electrolyte do not elicit a response in terms of heightened current intensity (Figure 10a,b). All of these materials are more catalytic in comparison to bare GC, having a more positive onset potential than bare GC as shown in Figure 10c. Similar to the case for the HER, the WS2 group of materials appears to underperform in comparison to the MoS2 group of materials in its catalytic efficiencies for the ORR, with the undoped MoS2 having the most positive onset potential for the ORR (0.78 V versus RHE). Doping does not enhance the TMD catalytic efficiencies; this is the same result we observed for the HER. The doping of WS2 with Ta or Nb does not have any obvious observed effect (positive or negative) on its catalysis of the ORR.

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EXPERIMENTAL SECTION Equipment and Characterization. The morphology of the doped-TMDs was investigated by scanning electron microscopy (SEM) with an FEG electron source (Tescan Lyra dual-beam microscope). Elemental composition and mapping were performed using an energy dispersive spectroscopy (EDS) analyzer (X-MaxN) with a 20 mm2 SDD detector (Oxford Instruments) and AZtecEnergy software. To conduct these measurements, the samples were placed on carbon conductive tape. SEM and SEM-EDS measurements were carried out using a 15 kV electron beam. SEM images of the undoped TMDs (undoped MoS2 and undoped WS2) were captured using a JEOL 7600F field-emission scanning electron microscope (JEOL, Japan). X-ray diffraction was done with a Bruker D8 Discoverer diffractometer in Bragg−Brentano parafocusing geometry. Cu Kα radiation was used. Diffraction patterns were collected between 5 and 80° of 2θ. The obtained data were evaluated using HighScore Plus 3.0e software. X-ray photoelectron spectroscopy (XPS) was conducted using a Phoibos 100 spectrometer with a nonmonochromatic Mg Kα radiation source (SPECS, Germany) at 1254 eV. Highresolution scans were also performed for the element of interest. Samples were prepared by affixing the material onto a aluminium XPS sample holder using sticky conductive carbon tape. Both the survey and high-resolution spectra were collected. Relative sensitivity factors were used for evaluation of chalcogen to metal ratios and quantitative distributions of 2H and 1T phases and the oxidation states in Mo and W. The calibration curves of the XPS high-resolution spectra were calibrated on the basis of the following: (1) all MoS2 type compounds were calibrated to the S 2s peak at 226.5 eV; (2) all WS2 type compounds were calibrated to the W(VI) peak at 38.6 eV. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were carried out using a μAutolab type III electrochemical analyzer (Eco Chemie, Utrecht, The Netherlands) connected to a personal computer and controlled by NOVA 1.10 software. All voltammetry experiments were performed in an electrochemical cell at room temperature using a standard three-electrode configuration. A platinum electrode served as the auxiliary electrode, while an Ag/AgCl electrode was used as a reference electrode. All measurements were performed relative to the Ag/AgCl reference electrode, but the data are presented versus the reversible hydrogen electrode (RHE). The potentials versus RHE were calculated on the basis of the following equation: ERHE = EAg/AgCl + 0.059 × pH + E°Ag/AgCl, where EAg/AgCl is the measured potential and E°Ag/AgCl = 0.235 V, which is the standard potential of Ag/AgCl (1 M KCl) at 25 °C.45 Procedures. The materials of interest in this work were prepared in suspensions of concentration of 1 mg mL−1 in ultrapure water. These suspensions were sonicated for 4 h with the temperature kept under 30 °C to obtain a well-dispersed suspension. The materials were then immobilized on a glassycarbon (GC, 3 mm diameter) electrode by transferring a 5.0 μL of the suspended material to the electrode surface through the drop-casting technique with subsequent drying of the solvent. The catalyst loading was 70.7 μg cm−2 per electrode. The GC

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CONCLUSION In this fundamental study, we investigated the effect of doping on bulk TMDs and its influence on electrochemistry, particularly on HER and ORR. On the basis of the overpotentials at −10 mA cm−2 for the HER, the doped bulk TMDs in this study have underperformed in comparison to their undoped counterparts. Experimentally, the large Tafel slopes and slow HET rates serve to reinforce the impracticality of the use of these doped materials for HER-related applications. Although doping in this case does not result in positive enhancement for HER catalyst design, it is important to keep in mind that this study focuses on the effects of doping on bulk TMDs (MoS2 and WS2) and only Nb and Ta, which are p-type dopants, have been investigated. The findings presented in this experimental study contradict current reasoning that the presence of a metallic band structure or a 1T phase contributes to better HER catalytic properties. There are a wide variety of factors involved in the mechanism of HER using TMDs as the catalyst, and each can intertwine to affect each other. There is no single decisive factor that can 5732

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Research Article

ACS Catalysis Notes

electrode surfaces were renewed prior to every new measurement by mechanical polishing; cleaning was accomplished with a 0.05 μm alumina particle on a polishing pad with ultrapure water. All voltammetry experiments were repeated three times to ensure the reproducibility of the results with random electrodes. Linear sweep voltammetry for HER measurements was performed in N2-saturated 0.5 M sulfuric acid at a low scan rate of 2 mV s−1, while ORR measurements were performed in N2saturated and air-saturated 0.1 M KOH for the doped and undoped TMDs. Cyclic voltammetry measurements were performed in N2-saturated 50 mM phosphate buffer solution (PBS) at pH 7.2 as the supporting electrolyte for 10 mM of the ferro-/ferricyanide redox probe with a fixed scan rate of 100 mV s−1. The overpotential for the HER was taken at −10 mA cm−2, while the onset potential for the ORR was taken to be at the potential where 10% of the maximum current was observed. Synthesis. Doped MoS2 and doped WS2 were prepared from their elements using powdered Mo, W, Nb, Ta, and S. The stoichiometric amount corresponds to 6.0 g of mixed chalcogenide with the composition of M0.95D0.05S2 (where M = Mo, W; and D = Ta, Nb). The powders were then placed in a quartz glass ampule (20 × 120 mm, wall thickness 1.5 mm) and sealed under high vacuum (below 5 × 10−3 Pa) using an oxygen/hydrogen welding torch. In order to ensure a complete reaction, the sulfur was used in 2 wt % excess with respect to the stoichiometry. The sealed ampule was heated at 600 °C for 48 h and subsequently at 800 °C for another 48 h and at 900 °C for the next 24 h. The heating and cooling rates were 5 °C min−1. MoS2 was synthesized from a mixture of 8.0 g of K2CO3, 16.0 g of S, and 10.5 g of MoO3 were heated to 750 °C. The heating was performed for 8 h in a quartz glass tube placed in a crucible furnace loosely capped with a quartz lid. The heating and cooling rates were 5 °C min−1. As-synthesized MoS2 was then leached with hot water, and the unreacted sulfur was extracted with carbon disulfide using a Soxhlet extractor. Finally, the product was ultrasonicated with hot water and then separated by suction filtration. WS2 was synthesized from a mixture of 8.0 g of K2CO3, 20.5 g of S, and 16.5 g of WO3 heated to 750 °C. The heating was performed for 8 h in a quartz glass tube placed in a crucible furnace loosely capped with a quartz lid. The heating and cooling rates were 5 °C min−1. As-synthesized WS2 was leached with hot water, and the unreacted sulfur was extracted with carbon disulfide using a Soxhlet extractor. The product was then ultrasonicated with hot water and separated by suction filtration.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.P. acknowledges a Tier 2 grant (MOE2013-T2-1-056; ARC 35/13) from the Ministry of Education of Singapore. Z.S. and J.L. were supported by Czech Science Foundation (GACR No. 16-05167S) and by specific university research (MSMT No 20SVV/2016).

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ABBREVIATIONS MoS2:Ta, Ta-doped MoS2; MoS2:Nb, Nb-doped MoS2; WS2:Ta, Ta-doped WS2; WS2:Nb, Nb-doped WS2

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01593. Scanning electron micrographs for all materials, elemental mappings from energy dispersive X-ray spectroscopy of the undoped TMDs, HET rate constants of all the materials, and high-resolution XPS data for S 2p of the TMDs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.P.: [email protected]. 5733

DOI: 10.1021/acscatal.6b01593 ACS Catal. 2016, 6, 5724−5734

Research Article

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DOI: 10.1021/acscatal.6b01593 ACS Catal. 2016, 6, 5724−5734