DOI: 10.1002/chem.201404832
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& Electrochemistry
Precise Tuning of the Charge Transfer Kinetics and Catalytic Properties of MoS2 Materials via Electrochemical Methods Xinyi Chia,[a] Adriano Ambrosi,*[a] David Sedmidubsky´,[b] Zdeneˇk Sofer,[b] and Martin Pumera*[a]
of both exfoliated and bulk MoS2-based films. A controlled reductive or oxidative electrochemical treatment can alter the surface properties of the film with consequently improved or hampered electrochemical and catalytic properties compared to the untreated film. Density functional theory calculations were used to explain the electrochemical activation of MoS2. The electrochemical tuning of electrocatalytic properties of MoS2 opens the doors to scalable and facile tailoring of MoS2-based electrochemical devices.
Abstract: MoS2 has become particularly popular for its catalytic properties towards the hydrogen evolution reaction (HER). It has been shown that the metallic 1T phase of MoS2, obtained by chemical exfoliation after lithium intercalation, possesses enhanced catalytic activity over the semiconducting 2H phase due to the improved conductivity properties which facilitate charge-transfer kinetics. Here we demonstrate a simple electrochemical method to precisely tune the electron-transfer kinetics as well as the catalytic properties
Introduction
to the metallic 1T phase by lithium intercalation.[15, 18] A recent study investigated the effect of different support materials to the catalytic properties of MoS2 nanostructures taking into consideration the van der Waals interactions.[19] It showed that depending on the adhesion energy between MoS2 and the support, different HER activities can be obtained and that the catalytic properties of the Mo-edge active sites can be optimally tuned by selecting the desired support.[19] Here we propose a simple method to control and conveniently tune the charge transfer ability and the catalytic properties of thin MoS2 films in both its bulk and exfoliated form. Being intrinsically semiconducting in nature, MoS2 is particularly sensitive to electrochemical treatments which can precisely introduce or withdraw electrons to the material altering consequently its properties.[20] In particular, we show that reductive electrochemical treatments significantly improve the heterogeneous electron transfer (HET) rates with surface-sensitive redox probe such as [Fe(CN)6]4 /3 , while oxidation processes have detrimental effects to the electron-transfer ability of the materials. In a similar fashion, we demonstrate that such electrochemical pre-treatments can also significantly alter the catalytic properties of MoS2 films towards HER and that the extent of the electrocatalytic activity can be precisely tuned by the application of a defined pre-treatment potential. We use density functional theory (DFT) to provide additional insight to the observed phenomena.
Amidst layered transition metal dichalcogenides (TMDs), MoS2 has attracted a particular scientific interest due to its semiconducting properties,[1, 2] the enhanced photoluminescence occurring when the material is exfoliated to single layer,[3] the electrochemical properties[4, 5] exploited in biosensing applications,[6] its low cytotoxicity[7] and the promising catalytic abilities for the electrochemical and photochemical production of H2.[8–10] Huge efforts have been directed at the improvement of the catalytic properties driven by the hope of replacing Pt as catalyst. It has been demonstrated that the metallic edges of MoS2 are the active sites for proton binding and H2 formation,[11–13] and therefore several attempts have been directed towards the creation of nanostructured MoS2 materials with high edge exposure.[14] In addition, the key role played by the electrical conductivity, which allows good charge transfer between the material active sites enhancing hydrogen evolution reaction (HER) activity, has been highlighted.[15] Improved electrical communication has been obtained using conductive supports such as graphene[16] and carbon nanotubes[17] or by the conversion of the semiconducting 2H-MoS2
[a] X. Chia, Dr. A. Ambrosi, Prof. M. Pumera Division of Chemistry and Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University, Singapore 637371 (Singapore) E-mail: [email protected] [email protected]
Results and Discussion
[b] Prof. D. Sedmidubsky´, Prof. Z. Sofer Institute of Chemical Technology, Department of Inorganic Chemistry Technicka 5, 166 28 Prague 6 (Czech Republic)
The use of [Fe(CN)6]4 /3 redox probe to investigate electrontransfer processes occurring at carbon surfaces has been extensively reported in the past.[21–26] This redox probe is particularly useful to correlate the electron-transfer mechanism with
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404832. Chem. Eur. J. 2014, 20, 1 – 8
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Full Paper the structural conformation of the material surface as well as with the chemical composition. Similar investigations have also been carried out to study redox processes occurring at semiconductor electrodes.[27] Ahmed and Gerischer used [Fe(CN)6]4 /3 and other redox probes to study the mechanism of electron transfer at basal and edge surface of MoS2.[28] It was clearly shown that electron-transfer events occur at much higher rates through the edge surface of MoS2 crystals than the basal surface due to the facilitated interaction between donors or acceptors in the electrolyte with the conduction band of the metal formed by dxy and dx2 y2 orbitals which extend parallel to the metal layer.[28] Here we adopted [Fe(CN)6]4 /3 redox probe to monitor the electron-transfer processes occurring at MoS2 films deposited on carbon electrodes. We employed both the bulk natural layered MoS2 material and the exfoliated material by lithium intercalation. MoS2 films have been undertaken to electrochemical treatments before evaluating the redox reaction with [Fe(CN)6]4 /3 . In particular, an oxidative (at + 1.3 V vs. Ag/AgCl) or a reductive (at 1.2 V vs. Ag/AgCl) potential has been applied for 10 min. The oxidative and reductive potentials, to be used during the pre-treatment, were based on cyclic voltammetric studies of the inherent electrochemical behaviour of MoS2 (Figures S1–S3 in the Supporting Information). In such cyclic voltammograms, a clear anodic wave, indicative of an oxidation process, appeared at about 1.1–1.3 V as well as a cathodic signal at about 1.2 V. Therefore, treating the materials at these two potentials (+ 1.3 and 1.2 V) a material transformation is expected which can affect the electron-transfer phenomena. Figure 1 captures the voltammetric profiles of [Fe(CN)6]4 /3 in 0.1 m KCl on bulk and exfoliated MoS2 film after the oxidative and reductive treatment performed at buffered solution (pH 7) and compared with the untreated film. It can be seen that the application of an oxidative or reductive potential alters the MoS2 surface properties. In particular, the bulk material resulted in a peak-to-peak separation (DE) of 0.15 V which became 0.3 V after the oxidative treatment. The reductive treatment, however, accelerated the electron transfer resulting in a smaller DE of 0.1 V (Figure 1 a). Exfoliated MoS2 reveals reduced electron-transfer ability compared to the bulk material, giving rise to a DE of 0.31 V. The oxidative treatment had no particular affect since the DE remained about 0.3 V. However, the reductive process significantly increased the electron-transfer rate bringing the DE value to 0.15 V (Figure 1 b). A global comparison for both materials can be seen in Figure 1 c). Using the Nicholson method,[29] we correlated the DE values to the rate constant 0 kobs as summarized in Table 1. An observation drawn from both bulk and exfoliated MoS2 reveals that while the reduced MoS2 materials accelerated HET rates compared to the untreated materials, their oxidized forms slowed down the HET rates. MoS2 materials are typical n-type semiconductors because electrons are their major charge carriers.[30] When MoS2 materials are subjected to electrochemical oxidation at a positive potential (at + 1.3 V) which lies beyond their flatband potential,[30, 31] electrons are conducted away and this generates a de&
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Figure 1. Influence of anodic/cathodic pre-treatment of MoS2 films on heterogeneous electron transfer (HET) rates. Cyclic voltammograms of 5 mm [Fe(CN)6]4 /3 on: a) bulk MoS2, and b) exfoliated MoS2 after electrochemical treatment at PBS pH 7. c) Summary of peak-to-peak separations of treated and untreated materials in PBS pH 7 and their corresponding error bars. Conditions: supporting electrolyte, KCl (0.1 m); scan rate, 100 mV s 1; all potentials are versus Ag/AgCl reference electrode.
Table 1. Peak-to-peak separation (DE) and corresponding HET rates in presence of [Fe(CN)6]4 /3 redox probe for all materials pre-treated in PBS pH 7.
2
Material
DE [V]
0 kobs [cm s 1]
MoS2 : MoS2 : MoS2 : MoS2 : MoS2 : MoS2 :
0.152 0.304 0.118 0.317 0.309 0.150
2.11 10 2.67 10 3.35 10 2.26 10 2.50 10 2.15 10
bulk untreated bulk oxidized bulk reduced exfoliated untreated exfoliated oxidized exfoliated reduced
3 4 3 4 4 3
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Full Paper pletion region devoid of electrons at the interface. In doing so, the electron transfer from the oxidized MoS2 modified electrode surface to the electrolyte is inhibited as indicated by a large DE for oxidized bulk and exfoliated MoS2. By the same token, the application of a reductive potential, more negative than the flatband potential of MoS2 engenders an accumulation region which is electron rich at the interface. Hence, higher HET rates of reduced MoS2 materials stem from the massive number of electrons available for charge transfer. We investigated the effect of pH during the electrochemical treatment upon activation of MoS2 (Figures S4 and S5 in the Supporting Information). Cyclic voltammetric scans in the presence of [Fe(CN)6]4 /3 after the electrochemical treatment preformed at different pH values for bulk and exfoliated MoS2 were recorded. As far as the reduction treatment is concerned, it seems that pH does not play a major role. In fact, at all pH values investigated between 2 and 12, a similar enhancement in electron transfer was recorded for both the bulk and exfoliated MoS2. For bulk MoS2, the application of an oxidative treatment reduced the electron-transfer ability only for pH below 12. A different behaviour resulted, however, between the bulk and the exfoliated MoS2. For the former, the reduced electrontransfer properties seem to be more significant at acidic pH obtaining the maximum effect at pH 2 (Figure S4B). The exfoliated MoS2 seems to exhibit an opposite trend with comparable electrochemical properties between the untreated material and after oxidative treatments at pH between 2 and 9. A significant enhancement was measured if the oxidative treatment was performed at pH 12 (Figure S5B). The significant alterations of the electron-transfer process between MoS2 surface and [Fe(CN)6]4 /3 after electrochemical treatment can be correlated to chemical and/or structural modification of the film. This is because [Fe(CN)6]4 /3 is particularly sensitive to the film composition with factors such as oxygen functional groups, presence of defect/edge planes, oxidation states of surface atoms, etc., contributing and affecting the redox reaction.[32] We next turned our attention to examine the catalytic properties of MoS2 films towards the hydrogen evolution reaction (HER) and verified how such catalytic ability can be altered by electrochemical treatments. The bulk and exfoliated MoS2 films were oxidized applying the potential of + 1.3 V or reduced using the potential of 1.2 V in PBS pH 7 prior to linear sweep voltammetric measurements as recorded in Figure 2 a and the corresponding Tafel plots shown in Figure 2 b. Measurements were also conducted using bare GC electrode as well as GC electrode modified with Pt/C (20 wt % Pt on Vulcan graphitized carbon) in order to allow a comparison to industrial standards. It is evident at first look that MoS2 film prepared with the exfoliated material performed significantly better than the bulk counterpart with an onset potential of about 0.31 V (vs. RHE), about 0.15 V lower than the one recorded with the bulk MoS2. This result corresponds with others already published[18, 33] and can be attributed to the increased edge surface available for catalysis after the exfoliation as well as to the enhanced electrical-transport properties generated by the Li intercalation/exfoliation process.[15, 18] Tafel plots can be used as characteristic HER performance indicators since they show more clearly the Chem. Eur. J. 2014, 20, 1 – 8
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Figure 2. Influence of anodic/cathodic treatment on electrocatalytic properties of MoS2 materials towards the hydrogen evolution reaction (HER). a) Polarisation curves of MoS2 materials in 0.5 m H2SO4 electrolyte. b) Tafel plot of all polarisation curves. Conditions: scan rate, 2 mV s 1, all measurements are made relative to the Ag/AgCl reference electrode and then corrected to reversible hydrogen electrode (RHE) potentials.
reaction progress (current generated) in relation to the overpotential required. Values as close as possible to that of Pt indicate excellent catalytic properties for materials that could therefore replace Pt as catalyst for the electrochemical or photoelectrochemical H2 production. Pt, recognized as one of the most effective catalysts for HER, presented an onset potential very close to 0 V (vs. RHE) and a Tafel slope of about 32 mV dec 1, also similar to values found in the literature.[34] Exfoliated MoS2 exhibited a Tafel slope of 81 mV dec 1, significantly lower than that of the bulk MoS2 (152 mV dec 1) which clearly indicates the improved catalytic properties obtained after the exfoliation. The second important observation that can be made is that the catalytic properties of both the bulk and exfoliated MoS2 can be significantly altered with an electrochemical pre-treatment applying either a reductive or an oxidative potential. Bulk MoS2-based films experienced a reduced catalytic ability after the oxidative treatment with an onset potential of about 0.50 V and a Tafel slope of about 220 mV dec 1, significantly larger than the one obtained with the untreated film (152 mV dec 1). The reductive pre-treatment at 1.2 V, however, had a marginal effect with only a slight improvement of the HER activity, achieving a Tafel slope of 139 mV dec 1. The catalytic properties of the exfoliated MoS2 film showed more dramatic changes. Upon the application of 3
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Full Paper the oxidative potential the measured Tafel slope passed from the value of 81 mV dec 1 to the value of 116 mV dec 1. Interestingly, an enhanced activity was clearly achieved after the reductive pre-treatment which reduced the Tafel plot to the value of 72 mV dec 1. A reduced catalytic activity was similarly demonstrated by Chhowalla et al. for the 2H-MoS2 after oxidation of the active edges with oxygen.[15] However, in the same work the authors showed that a similar oxidation treatment to the metallic 1T-MoS2 caused no changes in the catalytic activity, proving possibly that the active sites in the 1T-phase reside on the basal plane.[15] Here we show that more extreme oxidative treatments by means of electrochemical methods, clearly reduce the HER activity of both the bulk 2H-MoS2 and the exfoliated (1T + 2H)-MoS2. More importantly, we show that an enhanced activity can be obtained by means of a simple and fast electrochemical reductive pre-treatment of the MoS2 film which can be attributed to the stabilization of the 1T phase as verified by DFT calculations shown in the following section. We investigated the exfoliated MoS2 film surface composition before and after the electrochemical oxidative and reductive treatment in order to verify if the variation of electrocatalytic behaviour is caused by surface changes. High resolution XPS spectra recorded for all films in the Mo 3d and S 2p regions are summarized in Figure 3. Wide spectra with surface composition are shown in Figure S6 of the Supporting Information. It is evident that the surface chemistry remains practically unaltered during the electrochemical treatments as both Mo 3d and S 2p signals are almost identical. The presence of a small peak at about 235 eV is attributed to MoVI which is present in all materials and should be therefore generated by ambient oxygen oxidation of MoS2. The absence of any chemical alteration during the electrochemical treatment suggests that electronic changes are induced instead. In order to investigate more in detail the effect of the potential applied to the HER activity of the MoS2 films, we applied a range of potentials between 1.2 and + 1.2 V for a fixed period of time before testing for HER by linear sweep voltammetry. For a better comparison we measured the overpotential required to produce a current density of 10 mA cm 2 and the results are summarized in Figure 4 while each polarization curve recorded after the electrochemical treatment at different potentials are shown in Figure S7 of the Supporting Information. The electrochemical activation treatment had a more significant influence on the HER activity of the exfoliated MoS2 compared to the bulk material. It can be seen, in fact, that the overpotential obtained after a pre-treatment at the potential of + 1.2 V dropped from the value of 0.75 V to a value of 0.50 V upon the application of a 0.4 V treatment. It is important to note that this value is significantly lower than the one recorded for the untreated material (0.58 V). Pre-treatment potentials more negative than 0.4 V did not cause further variation of the HER rate. This means that pre-treatment potentials between + 0.4 and + 1.2 V allow not only a precise tuning of the catalytic properties of the exfoliated MoS2 material, but also that for pre-treatment potentials lower than + 0.4 V an enhanced catalysis can be obtained compared to the untreated material. Similar re&
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Figure 3. Surface characterization upon electrochemical treatment. High resolution XPS spectra of Mo 3d (left) and S 2p (right) core level peak regions before the electrochemical treatment (top), after the electrochemical reduction at 1.2 V (middle) and after the electrochemical oxidation at + 1.3 V (bottom).
Figure 4. Influence of applied pre-treatment potential on HER overpotential. HER overpotential measured at 10 mA cm 2 for the bulk and exfoliated MoS2 against a spectrum of pre-treatment potentials. Error bars are also included (n = 5).
sults were obtained for the bulk MoS2 material which again experienced the major activity alteration when exposed to pretreatment potentials between + 0.4 and + 1.2 V. However, for the bulk MoS2 no improvement in HER was obtained even after the treatment at potentials more negative than + 0.4 V compared to the untreated material. Potential values close to 0.4 V seem to represent the borderline between enhancement 4
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Full Paper of electrocatalytic properties and their worsening. Previous works demonstrated the use of cyclic voltammetry to determine the flatband potential of semiconductors[35, 36] including MoS2.[37] Flatband potential represents the potential corresponding to the Fermi energy level where no band bending occurs.[20] For MoS2 a flatband potential of + 0.3(0.05) V (vs. saturated calomel reference electrode) was determined.[37] This value seems to almost correspond to the borderline value of approximately + 0.4 V (vs. Ag/AgCl) resulting in the present work. It is highly probable that + 0.4 V (vs. Ag/AgCl) corresponds to the flatband potential of MoS2-based film used in this work. The application of potentials more positive than Figure 5. a) Calculated band structure of undoped and reduced MoS in 2H and 1T form. The density of states the flatband potential (~ + 0.4 V) (DOS) was plotted against the energy referred to vacuum level (0 eV)2 determined from the potential in the causes a reduced electrocatalytic middle of vacuum slab between MoS2 monolayers. b) Energy difference between 2H and 1T MoS2 monolayers. activity towards HER while po- c) Reduction potential of 2H and 1T MoS2 with respect RHE evaluated from the work function (neglecting the tentials more negative than Volta potential). + 0.4 V enhance the catalytic properties of MoS2 films. referred to RHE (Figure 5 c). The second effect that likely comes In order to gain deeper insight into the observed electrocainto play in stabilizing the 1T structure is the electron delocalitalytic behaviour of exfoliated MoS2, we performed DFT eleczation. Indeed MoS2-1T turns out to be a metal in the whole tronic structure calculations of both 2H and 1T forms of MoS2. The reduction process of the MoS2 monolayer was simulated doping range which might be also essential for its electrocatalytic properties. in terms of virtual atom approximation by introducing additional electrons compensated by extra nuclear charge on sulfur atoms. As seen from the density of states (DOS) plots in Figure 5 a, the parent compound MoS2-2H with trigonal prismatic coordination of Mo atoms is an insulator with the direct band gap of 1.7 eV. The states at the valence band maximum of a1 symmetry reveal a predominant Mo-4dz2 character (Figure 6 a) while the bottom of the conduction band is primarily formed by Mo-4dx2 y2/dxy orbitals of e’ symmetry. The reduction of MoS2-2H results in electron doping into the conduction band and the overall destabilization of this structural form. This is clearly visible on the energy difference between the 2H and 1T polymorphs (Figure 5 b) which is excepFigure 6. Electron density at valence band maximum for: a) semiconducting tionally negative for the undoped forms, but tends towards MoS2-2H, and b) around EF for metallic MoS2-1T. The valence states exhibit positive values on progressive reduction and crosses the zero primarily Mo-4d character of a1 and t2g symmetry, respectively. value for Mo3.12 + . The gradual stabilization of the 1T form upon electron doping is apparently due to crystal field effects, Based on these results we are led to the conclusion that the since the doped electrons occupy t2g states (Figure 6 b) of phase transition from 2H to 1T can be simply induced by a negnearly the same energy in the octahedral environment of the ative electrode potential in electrochemical experiments or by 1T form, whereas they have to overcome the band gap in the exposing the material to a high electron flux in electron mi2H form. The higher position of Fermi energy with respect to croscopy. A question then arises whether the structure can vacuum level observed for 2H is also manifested by a smaller persist after switching off the cell voltage or electron source. In value of the work function and much lower reduction potential Chem. Eur. J. 2014, 20, 1 – 8
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Full Paper electrodes, respectively. Two types of working electrodes were used. SPCE (8 mm diameter) served as the working electrode during the study on the electrochemical activation of MoS2 while the electrochemical application of MoS2 for hydrogen evolution reaction was conducted on a glassy carbon (GC, 3 mm diameter) electrode.
that case the doped electrons must be chemically compensated by some charged defects, such as protons formed by hydrolysis or Li remaining after exfoliation. However, Li is present only as a minor impurity and the preliminary calculations of MoS2H with fully protonated single layer of sulfur atoms show that this structure is unstable with respect to MoS2 and H2.
Procedures
Conclusion
Preparation of MoS2 nanosheets by lithium intercalation–exfoliation
We have explored the electrochemistry of MoS2 materials in relation to their electron-transfer ability and the catalytic properties towards the electrochemical hydrogen evolution reaction (HER). Both properties can be conveniently tuned in situ by a fast and simple preliminary electrochemical treatment consisting of the application of an oxidative or reductive potential. Precisely, electrochemical reduction produces faster heterogeneous electron transfer (HET) with the [Fe(CN)6]4 /3 redox probe while an oxidative treatment has the opposite effect compared to the untreated material. This is valid for both bulk and exfoliated MoS2. A pH dependence resulted with the oxidative treatment which produced the largest negative effects in acidic conditions while alkaline conditions either had no effect or improved HET rates. Similar trends were obtained with regard to the HER activity which can be tuned by a controlled applied potential during the electrochemical pre-treatment. The most significant effects were achieved with the exfoliated MoS2 material which can produce enhanced catalytic properties after the application of potentials lower than + 0.4 V or reduced HER activity for potentials applied larger than + 0.4 V. The application of potentials close to + 0.4 V are expected to produce no alteration on the catalytic properties and most likely can be associated with the flatband potential of the material. DFT calculations confirmed that 2H–1T transition by applying electrochemical potentials is viable. The possibility of controlling the HET properties of MoS2 materials as well as their catalytic properties towards HER by means of electrochemical treatments are of paramount importance for their use in sensing and energy-related applications.
Exfoliation of MoS2 was carried out by suspension of 3 g of MoS2 bulk powder in 20 mL of 1.6 m n-butyllithium in hexane. The solution was stirred for 72 h at 25 8C under argon atmosphere. The Liintercalated material was then firstly separated by suction filtration under argon atmosphere and then washed and centrifuged several times with hexane (dried over Na). Exfoliation was obtained by placing the Li-intercalated MoS2 in water (100 mL) and repeatedly washed and centrifuged (18 000 g). The obtained material was dried in a vacuum oven at 50 8C for 48 h prior further use.
Inherent electrochemistry and activation studies on bulk MoS2 and exfoliated MoS2 nanosheets Fundamental electrochemical studies were performed on SPCEs in 50 mm phosphate buffered saline (PBS) as the background electrolyte across various pH value from 2.0 to 12.0. All cyclic voltammetry experiments were conducted at a scan rate of 100 mV s 1. The voltammetric scan began at 0 V, the potential at which redox processes were not expected to occur,[30] and scanned towards 1.5 V followed by a reverse sweep to + 1.5 V for the cathodic study and towards 1.5 V followed by a reverse sweep to 1.5 V for the anodic study before returning to 0 V. Bulk MoS2 and MoS2 nanosheets were prepared in concentrations of 1 mg mL 1 in ultrapure water. After ultrasonication for a period of 30 min to attain homogenous dispersion of the material in ultrapure water, an aliquot (8.0 mL) of the suspension was drop casted on SPCEs. The water solvent was left to evaporate at room temperature leaving the electrode surface with a randomly distributed film of the desired material. Electrochemical activation was performed by applying a potential of + 1.3 V (oxidation) or 1.2 V (reduction) for a period of 600 s. This treatment was done at different pH values ranging from 2 to 12 using phosphate buffered solutions. After treatment, the electrode was dipped in distilled water to rinse off any adsorbed materials and tested for electron transfer rate using cyclic voltammetry at a scan rate of 100 mV s 1 in the presence of potassium ferrocyanide (5 mm) redox probe in potassium chloride (0.1 m) as the supporting electrolyte.
Experimental Section Materials Potassium ferrocyanide, potassium chloride, potassium phosphate dibasic, sodium phosphate monobasic, sodium chloride, potassium hydroxide, phosphoric acid, sulfuric acid, platinum on carbon and bulk molybdenum(IV) sulfide (< 2 mm; 99 %) were purchased from Sigma–Aldrich. Pt, Ag/AgCl, glassy carbon and screen-printed carbon electrodes (SPCEs) were purchased from CH Instruments, Texas.
0 values were determined using the method devised by The kobs Nicholson that relates DEp to a dimensionless parameter Y and 0 [29] . The roughness of the electrode was not consequently into kobs 0 considered in the calculation of k obs . The diffusion coefficient D = 6 2 1 0 was used to compute kobs values for 7.26 10 cm s 4 /3 [38] . [Fe(CN)6]
Hydrogen evolution studies on bulk MoS2 and exfoliated MoS2 nanosheets
Apparatus Voltammetric measurements were recorded on a mAutolab III electrochemical analyser (Eco Chemie B.V., Utrecht) using the software NOVA version 1.10 (Eco Chemie). Electrochemical measurements were performed in a 5 mL voltammetric cell at room temperature (25 8C) in a three-electrode configuration. A platinum electrode and an Ag/AgCl electrode functioned as auxiliary and reference
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The hydrogen evolution reaction (HER) efficiency of MoS2 materials were performed on glassy carbon (GC) electrodes. The GC electrode surface was renewed before modification with MoS2 materials. This involved polishing with a 0.05 mm alumina particle slurry on a polishing pad and washing with ultrapure water. The desired
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Full Paper materials encompassing bulk MoS2, exfoliated MoS2 nanosheets and Pt/C were prepared in suspensions of 1 mg mL 1 in ultrapure water. These suspensions were ultrasonicated for 30 min to achieve homogeneity. The materials were immobilised on the GC electrode by depositing an aliquot (4.0 mL) of the suspended materials on the electrode surface. The solvent was left to evaporate at room temperature to yield a randomly distributed film of 4.0 mg of desired materials on the GC electrode surface. Activation of bulk MoS2 and exfoliated MoS2 nanosheets was performed at pH 7.0 applying a range of potentials between 1.2 and + 1.2 V with 0.2 V intervals for the duration of 300 s each. HER measurements of these MoS2 materials were carried out using linear sweep voltammetry at a scan rate of 2 mV s 1 in 0.5 m H2SO4 electrolyte. Linear sweep voltammograms are presented versus the reversible hydrogen electrode (RHE), and the measured potentials are calcu0 , where lated using the equation: ERHE = EAg/AgCl + 0.059 pH + E Ag=AgCl EAg/AgCl is the measured potential, pH of 0.5 m H2SO4 electrolyte is 0 zero, and E Ag=AgCl refers to the standard potential of Ag/AgCl (1 m KCl) at 25 8C which is 0.235 V.
[3] G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla, Nano Lett. 2011, 11, 5111. [4] a) M. Pumera, Z. Sofer, A. Ambrosi, J. Mater. Chem. A 2014, 2, 8981; b) A. Ambrosi, Z. Sofer, M. Pumera, Small 2014, DOI: 10.1002/smll.201400401. [5] S. Wu, Z. Zeng, Q. He, Z. Wang, S. J. Wang, Y. Du, Z. Yin, X. Sun, W. Chen, H. Zhang, Small 2012, 8, 2264. [6] a) M. Pumera, A. H. Loo, Trends Analyt. Chem. 2014, 61, 49; b) A. H. Loo, A. Bonanni, A. Ambrosi, M. Pumera, Nanoscale 2014, 6, 11971. [7] W. Z. Teo, E. L. K. Chng, Z. Sofer, M. Pumera, Chem. Eur. J. 2014, 20, 9627. [8] H. Tributsch, J. C. Bennett, J. Electroanal. Chem. 1977, 81, 97. [9] T. Y. Wang, L. Liu, Z. W. Zhu, P. Papakonstantinou, J. B. Hu, H. Y. Liu, M. X. Li, Energy Environ. Sci. 2013, 6, 625. [10] H. Vrubel, D. Merki, X. Hu, Energy Environ. Sci. 2012, 5, 6136. [11] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Norskov, J. Am. Chem. Soc. 2005, 127, 5308. [12] T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100. [13] H. I. Karunadasa, E. Montalvo, Y. Sun, M. Majda, J. R. Long, C. J. Chang, Science 2012, 335, 698. [14] D. Y. Chung, S. K. Park, Y. H. Chung, S. H. Yu, D. H. Lim, N. Jung, H. C. Ham, H. Y. Park, Y. Piao, S. J. Yoo, Y. E. Sung, Nanoscale 2014, 6, 2131. [15] D. Voiry, M. Salehi, R. Silva, T. Fujita, M. W. Chen, T. Asefa, V. B. Shenoy, G. Eda, M. Chhowalla, Nano Lett. 2013, 13, 6222. [16] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc. 2011, 133, 7296. [17] Y. Yan, X. M. Ge, Z. L. Liu, J. Y. Wang, J. M. Lee, X. Wang, Nanoscale 2013, 5, 7768. [18] M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. S. Li, S. Jin, J. Am. Chem. Soc. 2013, 135, 10274. [19] C. Tsai, F. Abild-Pedersen, J. K. Nørskov, Nano Lett. 2014, 14, 1381. [20] A. J. Bard, L. R. Faulkner, Electrochemical Methods, Wiley, New York, 1980. [21] R. L. McCreery, Chem. Rev. 2008, 108, 2646. [22] P. H. Chen, M. A. Fryling, R. L. McCreery, Anal. Chem. 1995, 67, 3115. [23] X. B. Ji, C. E. Banks, A. Crossley, R. G. Compton, ChemPhysChem 2006, 7, 1337. [24] C. E. Banks, R. R. Moore, T. J. Davies, R. G. Compton, Chem. Commun. 2004, 1804. [25] C. E. Banks, T. J. Davies, G. G. Wildgoose, R. G. Compton, Chem. Commun. 2005, 829. [26] A. Ambrosi, A. Bonanni, M. Pumera, Nanoscale 2011, 3, 2256. [27] H. Gerischer, Surf. Sci. 1969, 18, 97. [28] S. M. Ahmed, H. Gerischer, Electrochim. Acta 1979, 24, 705. [29] R. S. Nicholson, Anal. Chem. 1965, 37, 1351. [30] W. Kautek, H. Gerischer, Surf. Sci. 1982, 119, 46. [31] Z. B. Chen, A. J. Forman, T. F. Jaramillo, J. Phys. Chem. C 2013, 117, 9713. [32] R. G. Compton, C. E. Banks, Understanding Voltammetry, World Scientific Publishing, Singapore, 2007. [33] T. Y. Wang, D. L. Gao, J. Q. Zhuo, Z. W. Zhu, P. Papakonstantinou, Y. Li, M. X. Li, Chem. Eur. J. 2013, 19, 11939. [34] B. E. Conway, B. V. Tilak, Electrochim. Acta 2002, 47, 3571. [35] S. N. Frank, A. J. Bard, J. Am. Chem. Soc. 1975, 97, 7427. [36] P. A. Kohl, A. J. Bard, J. Am. Chem. Soc. 1977, 99, 7531. [37] L. F. Schneemeyer, M. S. Wrighton, J. Am. Chem. Soc. 1979, 101, 6496. [38] S. J. Konopka, B. McDuffie, Anal. Chem. 1970, 42, 1741. [39] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. [40] P. Blaha, K. Schwarz, G. Madsen, D. Kvasnicka, J. Luitz, WIEN2k, An Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties, Karlheinz Schwarz, Technische Universitt Wien, Austria, 2001.
Electronic structure calculations The band structures and total energies of single layer MoS2 in both 2H and 1T arrangements were calculated with DFT using APW + lo basis set and generalized gradient approximation (GGA, PBE96 parametrization scheme)[39] for exchange correlation potential as implemented in the Wien2k software package.[40] The plane wave cutoff energy of 240 eV and the tetrahedron method with a k-mesh 50 50 1 were used. The two-dimensional character of the S-Mo-S monolayer was simulated by introducing a 15 thick vacuum slab between the layers. The electron doping up to a complete reduction from MoIV to MoIII was treated in terms of virtual atom approximation by adding an extra nuclear and electron charge on sulfur atoms (0.5 e on each S in the case of MoIIIS2e). The lattice parameter a- and the z-position of S atoms were optimized for the endmembers, MoS2 and MoS2e, and interpolated for the intermediate electron doping rates, MoS2ex (x = 0.2, 0.4, 0.6, 0.8). All calculations were performed as non-spin polarized, although the spin polarization was tested, converging either to vanishing magnetic moment or to a spin polarized state with higher total energy.
Acknowledgements M.P. acknowledges funding from Ministry of Education (Singapore) from Tier 1 RGT1/13. Z.S. and D.S. were supported by specific university research (MSMT No. 20/2014). Keywords: density functional calculations · electrochemistry · hydrogen evolution reaction · molybdenum disulfide · transition metals dichalcogenide [1] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, A. K. Geim, Proc. Natl. Acad. Sci. USA 2005, 102, 10451. [2] M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh, H. Zhang, Nat. Chem. 2013, 5, 263.
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Electrochemistry turns MoS2 on: The electrochemical tuning of electrocatalytic properties of MoS2 opens the doors to scalable and facile tailoring of MoS2based electrochemical devices. Here a simple electrochemical method to precisely tune the electron-transfer kinetics as well as the catalytic properties of both exfoliated and bulk MoS2-based films is demonstrated.
X. Chia, A. Ambrosi,* D. Sedmidubsky´, Z. Sofer, M. Pumera* && – && Precise Tuning of the Charge Transfer Kinetics and Catalytic Properties of MoS2 Materials via Electrochemical Methods
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& Electrochemistry
Precise Tuning of the Charge Transfer Kinetics and Catalytic Properties of MoS2 Materials via Electrochemical Methods Xinyi Chia,[a] Adriano Ambrosi,*[a] David Sedmidubsky´,[b] Zdeneˇk Sofer,[b] and Martin Pumera*[a]
of both exfoliated and bulk MoS2-based films. A controlled reductive or oxidative electrochemical treatment can alter the surface properties of the film with consequently improved or hampered electrochemical and catalytic properties compared to the untreated film. Density functional theory calculations were used to explain the electrochemical activation of MoS2. The electrochemical tuning of electrocatalytic properties of MoS2 opens the doors to scalable and facile tailoring of MoS2-based electrochemical devices.
Abstract: MoS2 has become particularly popular for its catalytic properties towards the hydrogen evolution reaction (HER). It has been shown that the metallic 1T phase of MoS2, obtained by chemical exfoliation after lithium intercalation, possesses enhanced catalytic activity over the semiconducting 2H phase due to the improved conductivity properties which facilitate charge-transfer kinetics. Here we demonstrate a simple electrochemical method to precisely tune the electron-transfer kinetics as well as the catalytic properties
Introduction
to the metallic 1T phase by lithium intercalation.[15, 18] A recent study investigated the effect of different support materials to the catalytic properties of MoS2 nanostructures taking into consideration the van der Waals interactions.[19] It showed that depending on the adhesion energy between MoS2 and the support, different HER activities can be obtained and that the catalytic properties of the Mo-edge active sites can be optimally tuned by selecting the desired support.[19] Here we propose a simple method to control and conveniently tune the charge transfer ability and the catalytic properties of thin MoS2 films in both its bulk and exfoliated form. Being intrinsically semiconducting in nature, MoS2 is particularly sensitive to electrochemical treatments which can precisely introduce or withdraw electrons to the material altering consequently its properties.[20] In particular, we show that reductive electrochemical treatments significantly improve the heterogeneous electron transfer (HET) rates with surface-sensitive redox probe such as [Fe(CN)6]4 /3 , while oxidation processes have detrimental effects to the electron-transfer ability of the materials. In a similar fashion, we demonstrate that such electrochemical pre-treatments can also significantly alter the catalytic properties of MoS2 films towards HER and that the extent of the electrocatalytic activity can be precisely tuned by the application of a defined pre-treatment potential. We use density functional theory (DFT) to provide additional insight to the observed phenomena.
Amidst layered transition metal dichalcogenides (TMDs), MoS2 has attracted a particular scientific interest due to its semiconducting properties,[1, 2] the enhanced photoluminescence occurring when the material is exfoliated to single layer,[3] the electrochemical properties[4, 5] exploited in biosensing applications,[6] its low cytotoxicity[7] and the promising catalytic abilities for the electrochemical and photochemical production of H2.[8–10] Huge efforts have been directed at the improvement of the catalytic properties driven by the hope of replacing Pt as catalyst. It has been demonstrated that the metallic edges of MoS2 are the active sites for proton binding and H2 formation,[11–13] and therefore several attempts have been directed towards the creation of nanostructured MoS2 materials with high edge exposure.[14] In addition, the key role played by the electrical conductivity, which allows good charge transfer between the material active sites enhancing hydrogen evolution reaction (HER) activity, has been highlighted.[15] Improved electrical communication has been obtained using conductive supports such as graphene[16] and carbon nanotubes[17] or by the conversion of the semiconducting 2H-MoS2
[a] X. Chia, Dr. A. Ambrosi, Prof. M. Pumera Division of Chemistry and Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University, Singapore 637371 (Singapore) E-mail: [email protected] [email protected]
Results and Discussion
[b] Prof. D. Sedmidubsky´, Prof. Z. Sofer Institute of Chemical Technology, Department of Inorganic Chemistry Technicka 5, 166 28 Prague 6 (Czech Republic)
The use of [Fe(CN)6]4 /3 redox probe to investigate electrontransfer processes occurring at carbon surfaces has been extensively reported in the past.[21–26] This redox probe is particularly useful to correlate the electron-transfer mechanism with
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404832. Chem. Eur. J. 2014, 20, 1 – 8
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Full Paper the structural conformation of the material surface as well as with the chemical composition. Similar investigations have also been carried out to study redox processes occurring at semiconductor electrodes.[27] Ahmed and Gerischer used [Fe(CN)6]4 /3 and other redox probes to study the mechanism of electron transfer at basal and edge surface of MoS2.[28] It was clearly shown that electron-transfer events occur at much higher rates through the edge surface of MoS2 crystals than the basal surface due to the facilitated interaction between donors or acceptors in the electrolyte with the conduction band of the metal formed by dxy and dx2 y2 orbitals which extend parallel to the metal layer.[28] Here we adopted [Fe(CN)6]4 /3 redox probe to monitor the electron-transfer processes occurring at MoS2 films deposited on carbon electrodes. We employed both the bulk natural layered MoS2 material and the exfoliated material by lithium intercalation. MoS2 films have been undertaken to electrochemical treatments before evaluating the redox reaction with [Fe(CN)6]4 /3 . In particular, an oxidative (at + 1.3 V vs. Ag/AgCl) or a reductive (at 1.2 V vs. Ag/AgCl) potential has been applied for 10 min. The oxidative and reductive potentials, to be used during the pre-treatment, were based on cyclic voltammetric studies of the inherent electrochemical behaviour of MoS2 (Figures S1–S3 in the Supporting Information). In such cyclic voltammograms, a clear anodic wave, indicative of an oxidation process, appeared at about 1.1–1.3 V as well as a cathodic signal at about 1.2 V. Therefore, treating the materials at these two potentials (+ 1.3 and 1.2 V) a material transformation is expected which can affect the electron-transfer phenomena. Figure 1 captures the voltammetric profiles of [Fe(CN)6]4 /3 in 0.1 m KCl on bulk and exfoliated MoS2 film after the oxidative and reductive treatment performed at buffered solution (pH 7) and compared with the untreated film. It can be seen that the application of an oxidative or reductive potential alters the MoS2 surface properties. In particular, the bulk material resulted in a peak-to-peak separation (DE) of 0.15 V which became 0.3 V after the oxidative treatment. The reductive treatment, however, accelerated the electron transfer resulting in a smaller DE of 0.1 V (Figure 1 a). Exfoliated MoS2 reveals reduced electron-transfer ability compared to the bulk material, giving rise to a DE of 0.31 V. The oxidative treatment had no particular affect since the DE remained about 0.3 V. However, the reductive process significantly increased the electron-transfer rate bringing the DE value to 0.15 V (Figure 1 b). A global comparison for both materials can be seen in Figure 1 c). Using the Nicholson method,[29] we correlated the DE values to the rate constant 0 kobs as summarized in Table 1. An observation drawn from both bulk and exfoliated MoS2 reveals that while the reduced MoS2 materials accelerated HET rates compared to the untreated materials, their oxidized forms slowed down the HET rates. MoS2 materials are typical n-type semiconductors because electrons are their major charge carriers.[30] When MoS2 materials are subjected to electrochemical oxidation at a positive potential (at + 1.3 V) which lies beyond their flatband potential,[30, 31] electrons are conducted away and this generates a de&
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Figure 1. Influence of anodic/cathodic pre-treatment of MoS2 films on heterogeneous electron transfer (HET) rates. Cyclic voltammograms of 5 mm [Fe(CN)6]4 /3 on: a) bulk MoS2, and b) exfoliated MoS2 after electrochemical treatment at PBS pH 7. c) Summary of peak-to-peak separations of treated and untreated materials in PBS pH 7 and their corresponding error bars. Conditions: supporting electrolyte, KCl (0.1 m); scan rate, 100 mV s 1; all potentials are versus Ag/AgCl reference electrode.
Table 1. Peak-to-peak separation (DE) and corresponding HET rates in presence of [Fe(CN)6]4 /3 redox probe for all materials pre-treated in PBS pH 7.
2
Material
DE [V]
0 kobs [cm s 1]
MoS2 : MoS2 : MoS2 : MoS2 : MoS2 : MoS2 :
0.152 0.304 0.118 0.317 0.309 0.150
2.11 10 2.67 10 3.35 10 2.26 10 2.50 10 2.15 10
bulk untreated bulk oxidized bulk reduced exfoliated untreated exfoliated oxidized exfoliated reduced
3 4 3 4 4 3
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Full Paper pletion region devoid of electrons at the interface. In doing so, the electron transfer from the oxidized MoS2 modified electrode surface to the electrolyte is inhibited as indicated by a large DE for oxidized bulk and exfoliated MoS2. By the same token, the application of a reductive potential, more negative than the flatband potential of MoS2 engenders an accumulation region which is electron rich at the interface. Hence, higher HET rates of reduced MoS2 materials stem from the massive number of electrons available for charge transfer. We investigated the effect of pH during the electrochemical treatment upon activation of MoS2 (Figures S4 and S5 in the Supporting Information). Cyclic voltammetric scans in the presence of [Fe(CN)6]4 /3 after the electrochemical treatment preformed at different pH values for bulk and exfoliated MoS2 were recorded. As far as the reduction treatment is concerned, it seems that pH does not play a major role. In fact, at all pH values investigated between 2 and 12, a similar enhancement in electron transfer was recorded for both the bulk and exfoliated MoS2. For bulk MoS2, the application of an oxidative treatment reduced the electron-transfer ability only for pH below 12. A different behaviour resulted, however, between the bulk and the exfoliated MoS2. For the former, the reduced electrontransfer properties seem to be more significant at acidic pH obtaining the maximum effect at pH 2 (Figure S4B). The exfoliated MoS2 seems to exhibit an opposite trend with comparable electrochemical properties between the untreated material and after oxidative treatments at pH between 2 and 9. A significant enhancement was measured if the oxidative treatment was performed at pH 12 (Figure S5B). The significant alterations of the electron-transfer process between MoS2 surface and [Fe(CN)6]4 /3 after electrochemical treatment can be correlated to chemical and/or structural modification of the film. This is because [Fe(CN)6]4 /3 is particularly sensitive to the film composition with factors such as oxygen functional groups, presence of defect/edge planes, oxidation states of surface atoms, etc., contributing and affecting the redox reaction.[32] We next turned our attention to examine the catalytic properties of MoS2 films towards the hydrogen evolution reaction (HER) and verified how such catalytic ability can be altered by electrochemical treatments. The bulk and exfoliated MoS2 films were oxidized applying the potential of + 1.3 V or reduced using the potential of 1.2 V in PBS pH 7 prior to linear sweep voltammetric measurements as recorded in Figure 2 a and the corresponding Tafel plots shown in Figure 2 b. Measurements were also conducted using bare GC electrode as well as GC electrode modified with Pt/C (20 wt % Pt on Vulcan graphitized carbon) in order to allow a comparison to industrial standards. It is evident at first look that MoS2 film prepared with the exfoliated material performed significantly better than the bulk counterpart with an onset potential of about 0.31 V (vs. RHE), about 0.15 V lower than the one recorded with the bulk MoS2. This result corresponds with others already published[18, 33] and can be attributed to the increased edge surface available for catalysis after the exfoliation as well as to the enhanced electrical-transport properties generated by the Li intercalation/exfoliation process.[15, 18] Tafel plots can be used as characteristic HER performance indicators since they show more clearly the Chem. Eur. J. 2014, 20, 1 – 8
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Figure 2. Influence of anodic/cathodic treatment on electrocatalytic properties of MoS2 materials towards the hydrogen evolution reaction (HER). a) Polarisation curves of MoS2 materials in 0.5 m H2SO4 electrolyte. b) Tafel plot of all polarisation curves. Conditions: scan rate, 2 mV s 1, all measurements are made relative to the Ag/AgCl reference electrode and then corrected to reversible hydrogen electrode (RHE) potentials.
reaction progress (current generated) in relation to the overpotential required. Values as close as possible to that of Pt indicate excellent catalytic properties for materials that could therefore replace Pt as catalyst for the electrochemical or photoelectrochemical H2 production. Pt, recognized as one of the most effective catalysts for HER, presented an onset potential very close to 0 V (vs. RHE) and a Tafel slope of about 32 mV dec 1, also similar to values found in the literature.[34] Exfoliated MoS2 exhibited a Tafel slope of 81 mV dec 1, significantly lower than that of the bulk MoS2 (152 mV dec 1) which clearly indicates the improved catalytic properties obtained after the exfoliation. The second important observation that can be made is that the catalytic properties of both the bulk and exfoliated MoS2 can be significantly altered with an electrochemical pre-treatment applying either a reductive or an oxidative potential. Bulk MoS2-based films experienced a reduced catalytic ability after the oxidative treatment with an onset potential of about 0.50 V and a Tafel slope of about 220 mV dec 1, significantly larger than the one obtained with the untreated film (152 mV dec 1). The reductive pre-treatment at 1.2 V, however, had a marginal effect with only a slight improvement of the HER activity, achieving a Tafel slope of 139 mV dec 1. The catalytic properties of the exfoliated MoS2 film showed more dramatic changes. Upon the application of 3
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Full Paper the oxidative potential the measured Tafel slope passed from the value of 81 mV dec 1 to the value of 116 mV dec 1. Interestingly, an enhanced activity was clearly achieved after the reductive pre-treatment which reduced the Tafel plot to the value of 72 mV dec 1. A reduced catalytic activity was similarly demonstrated by Chhowalla et al. for the 2H-MoS2 after oxidation of the active edges with oxygen.[15] However, in the same work the authors showed that a similar oxidation treatment to the metallic 1T-MoS2 caused no changes in the catalytic activity, proving possibly that the active sites in the 1T-phase reside on the basal plane.[15] Here we show that more extreme oxidative treatments by means of electrochemical methods, clearly reduce the HER activity of both the bulk 2H-MoS2 and the exfoliated (1T + 2H)-MoS2. More importantly, we show that an enhanced activity can be obtained by means of a simple and fast electrochemical reductive pre-treatment of the MoS2 film which can be attributed to the stabilization of the 1T phase as verified by DFT calculations shown in the following section. We investigated the exfoliated MoS2 film surface composition before and after the electrochemical oxidative and reductive treatment in order to verify if the variation of electrocatalytic behaviour is caused by surface changes. High resolution XPS spectra recorded for all films in the Mo 3d and S 2p regions are summarized in Figure 3. Wide spectra with surface composition are shown in Figure S6 of the Supporting Information. It is evident that the surface chemistry remains practically unaltered during the electrochemical treatments as both Mo 3d and S 2p signals are almost identical. The presence of a small peak at about 235 eV is attributed to MoVI which is present in all materials and should be therefore generated by ambient oxygen oxidation of MoS2. The absence of any chemical alteration during the electrochemical treatment suggests that electronic changes are induced instead. In order to investigate more in detail the effect of the potential applied to the HER activity of the MoS2 films, we applied a range of potentials between 1.2 and + 1.2 V for a fixed period of time before testing for HER by linear sweep voltammetry. For a better comparison we measured the overpotential required to produce a current density of 10 mA cm 2 and the results are summarized in Figure 4 while each polarization curve recorded after the electrochemical treatment at different potentials are shown in Figure S7 of the Supporting Information. The electrochemical activation treatment had a more significant influence on the HER activity of the exfoliated MoS2 compared to the bulk material. It can be seen, in fact, that the overpotential obtained after a pre-treatment at the potential of + 1.2 V dropped from the value of 0.75 V to a value of 0.50 V upon the application of a 0.4 V treatment. It is important to note that this value is significantly lower than the one recorded for the untreated material (0.58 V). Pre-treatment potentials more negative than 0.4 V did not cause further variation of the HER rate. This means that pre-treatment potentials between + 0.4 and + 1.2 V allow not only a precise tuning of the catalytic properties of the exfoliated MoS2 material, but also that for pre-treatment potentials lower than + 0.4 V an enhanced catalysis can be obtained compared to the untreated material. Similar re&
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Figure 3. Surface characterization upon electrochemical treatment. High resolution XPS spectra of Mo 3d (left) and S 2p (right) core level peak regions before the electrochemical treatment (top), after the electrochemical reduction at 1.2 V (middle) and after the electrochemical oxidation at + 1.3 V (bottom).
Figure 4. Influence of applied pre-treatment potential on HER overpotential. HER overpotential measured at 10 mA cm 2 for the bulk and exfoliated MoS2 against a spectrum of pre-treatment potentials. Error bars are also included (n = 5).
sults were obtained for the bulk MoS2 material which again experienced the major activity alteration when exposed to pretreatment potentials between + 0.4 and + 1.2 V. However, for the bulk MoS2 no improvement in HER was obtained even after the treatment at potentials more negative than + 0.4 V compared to the untreated material. Potential values close to 0.4 V seem to represent the borderline between enhancement 4
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Full Paper of electrocatalytic properties and their worsening. Previous works demonstrated the use of cyclic voltammetry to determine the flatband potential of semiconductors[35, 36] including MoS2.[37] Flatband potential represents the potential corresponding to the Fermi energy level where no band bending occurs.[20] For MoS2 a flatband potential of + 0.3(0.05) V (vs. saturated calomel reference electrode) was determined.[37] This value seems to almost correspond to the borderline value of approximately + 0.4 V (vs. Ag/AgCl) resulting in the present work. It is highly probable that + 0.4 V (vs. Ag/AgCl) corresponds to the flatband potential of MoS2-based film used in this work. The application of potentials more positive than Figure 5. a) Calculated band structure of undoped and reduced MoS in 2H and 1T form. The density of states the flatband potential (~ + 0.4 V) (DOS) was plotted against the energy referred to vacuum level (0 eV)2 determined from the potential in the causes a reduced electrocatalytic middle of vacuum slab between MoS2 monolayers. b) Energy difference between 2H and 1T MoS2 monolayers. activity towards HER while po- c) Reduction potential of 2H and 1T MoS2 with respect RHE evaluated from the work function (neglecting the tentials more negative than Volta potential). + 0.4 V enhance the catalytic properties of MoS2 films. referred to RHE (Figure 5 c). The second effect that likely comes In order to gain deeper insight into the observed electrocainto play in stabilizing the 1T structure is the electron delocalitalytic behaviour of exfoliated MoS2, we performed DFT eleczation. Indeed MoS2-1T turns out to be a metal in the whole tronic structure calculations of both 2H and 1T forms of MoS2. The reduction process of the MoS2 monolayer was simulated doping range which might be also essential for its electrocatalytic properties. in terms of virtual atom approximation by introducing additional electrons compensated by extra nuclear charge on sulfur atoms. As seen from the density of states (DOS) plots in Figure 5 a, the parent compound MoS2-2H with trigonal prismatic coordination of Mo atoms is an insulator with the direct band gap of 1.7 eV. The states at the valence band maximum of a1 symmetry reveal a predominant Mo-4dz2 character (Figure 6 a) while the bottom of the conduction band is primarily formed by Mo-4dx2 y2/dxy orbitals of e’ symmetry. The reduction of MoS2-2H results in electron doping into the conduction band and the overall destabilization of this structural form. This is clearly visible on the energy difference between the 2H and 1T polymorphs (Figure 5 b) which is excepFigure 6. Electron density at valence band maximum for: a) semiconducting tionally negative for the undoped forms, but tends towards MoS2-2H, and b) around EF for metallic MoS2-1T. The valence states exhibit positive values on progressive reduction and crosses the zero primarily Mo-4d character of a1 and t2g symmetry, respectively. value for Mo3.12 + . The gradual stabilization of the 1T form upon electron doping is apparently due to crystal field effects, Based on these results we are led to the conclusion that the since the doped electrons occupy t2g states (Figure 6 b) of phase transition from 2H to 1T can be simply induced by a negnearly the same energy in the octahedral environment of the ative electrode potential in electrochemical experiments or by 1T form, whereas they have to overcome the band gap in the exposing the material to a high electron flux in electron mi2H form. The higher position of Fermi energy with respect to croscopy. A question then arises whether the structure can vacuum level observed for 2H is also manifested by a smaller persist after switching off the cell voltage or electron source. In value of the work function and much lower reduction potential Chem. Eur. J. 2014, 20, 1 – 8
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that case the doped electrons must be chemically compensated by some charged defects, such as protons formed by hydrolysis or Li remaining after exfoliation. However, Li is present only as a minor impurity and the preliminary calculations of MoS2H with fully protonated single layer of sulfur atoms show that this structure is unstable with respect to MoS2 and H2.
Procedures
Conclusion
Preparation of MoS2 nanosheets by lithium intercalation–exfoliation
We have explored the electrochemistry of MoS2 materials in relation to their electron-transfer ability and the catalytic properties towards the electrochemical hydrogen evolution reaction (HER). Both properties can be conveniently tuned in situ by a fast and simple preliminary electrochemical treatment consisting of the application of an oxidative or reductive potential. Precisely, electrochemical reduction produces faster heterogeneous electron transfer (HET) with the [Fe(CN)6]4 /3 redox probe while an oxidative treatment has the opposite effect compared to the untreated material. This is valid for both bulk and exfoliated MoS2. A pH dependence resulted with the oxidative treatment which produced the largest negative effects in acidic conditions while alkaline conditions either had no effect or improved HET rates. Similar trends were obtained with regard to the HER activity which can be tuned by a controlled applied potential during the electrochemical pre-treatment. The most significant effects were achieved with the exfoliated MoS2 material which can produce enhanced catalytic properties after the application of potentials lower than + 0.4 V or reduced HER activity for potentials applied larger than + 0.4 V. The application of potentials close to + 0.4 V are expected to produce no alteration on the catalytic properties and most likely can be associated with the flatband potential of the material. DFT calculations confirmed that 2H–1T transition by applying electrochemical potentials is viable. The possibility of controlling the HET properties of MoS2 materials as well as their catalytic properties towards HER by means of electrochemical treatments are of paramount importance for their use in sensing and energy-related applications.
Exfoliation of MoS2 was carried out by suspension of 3 g of MoS2 bulk powder in 20 mL of 1.6 m n-butyllithium in hexane. The solution was stirred for 72 h at 25 8C under argon atmosphere. The Liintercalated material was then firstly separated by suction filtration under argon atmosphere and then washed and centrifuged several times with hexane (dried over Na). Exfoliation was obtained by placing the Li-intercalated MoS2 in water (100 mL) and repeatedly washed and centrifuged (18 000 g). The obtained material was dried in a vacuum oven at 50 8C for 48 h prior further use.
Inherent electrochemistry and activation studies on bulk MoS2 and exfoliated MoS2 nanosheets Fundamental electrochemical studies were performed on SPCEs in 50 mm phosphate buffered saline (PBS) as the background electrolyte across various pH value from 2.0 to 12.0. All cyclic voltammetry experiments were conducted at a scan rate of 100 mV s 1. The voltammetric scan began at 0 V, the potential at which redox processes were not expected to occur,[30] and scanned towards 1.5 V followed by a reverse sweep to + 1.5 V for the cathodic study and towards 1.5 V followed by a reverse sweep to 1.5 V for the anodic study before returning to 0 V. Bulk MoS2 and MoS2 nanosheets were prepared in concentrations of 1 mg mL 1 in ultrapure water. After ultrasonication for a period of 30 min to attain homogenous dispersion of the material in ultrapure water, an aliquot (8.0 mL) of the suspension was drop casted on SPCEs. The water solvent was left to evaporate at room temperature leaving the electrode surface with a randomly distributed film of the desired material. Electrochemical activation was performed by applying a potential of + 1.3 V (oxidation) or 1.2 V (reduction) for a period of 600 s. This treatment was done at different pH values ranging from 2 to 12 using phosphate buffered solutions. After treatment, the electrode was dipped in distilled water to rinse off any adsorbed materials and tested for electron transfer rate using cyclic voltammetry at a scan rate of 100 mV s 1 in the presence of potassium ferrocyanide (5 mm) redox probe in potassium chloride (0.1 m) as the supporting electrolyte.
Experimental Section Materials Potassium ferrocyanide, potassium chloride, potassium phosphate dibasic, sodium phosphate monobasic, sodium chloride, potassium hydroxide, phosphoric acid, sulfuric acid, platinum on carbon and bulk molybdenum(IV) sulfide (< 2 mm; 99 %) were purchased from Sigma–Aldrich. Pt, Ag/AgCl, glassy carbon and screen-printed carbon electrodes (SPCEs) were purchased from CH Instruments, Texas.
0 values were determined using the method devised by The kobs Nicholson that relates DEp to a dimensionless parameter Y and 0 [29] . The roughness of the electrode was not consequently into kobs 0 considered in the calculation of k obs . The diffusion coefficient D = 6 2 1 0 was used to compute kobs values for 7.26 10 cm s 4 /3 [38] . [Fe(CN)6]
Hydrogen evolution studies on bulk MoS2 and exfoliated MoS2 nanosheets
Apparatus Voltammetric measurements were recorded on a mAutolab III electrochemical analyser (Eco Chemie B.V., Utrecht) using the software NOVA version 1.10 (Eco Chemie). Electrochemical measurements were performed in a 5 mL voltammetric cell at room temperature (25 8C) in a three-electrode configuration. A platinum electrode and an Ag/AgCl electrode functioned as auxiliary and reference
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The hydrogen evolution reaction (HER) efficiency of MoS2 materials were performed on glassy carbon (GC) electrodes. The GC electrode surface was renewed before modification with MoS2 materials. This involved polishing with a 0.05 mm alumina particle slurry on a polishing pad and washing with ultrapure water. The desired
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Full Paper materials encompassing bulk MoS2, exfoliated MoS2 nanosheets and Pt/C were prepared in suspensions of 1 mg mL 1 in ultrapure water. These suspensions were ultrasonicated for 30 min to achieve homogeneity. The materials were immobilised on the GC electrode by depositing an aliquot (4.0 mL) of the suspended materials on the electrode surface. The solvent was left to evaporate at room temperature to yield a randomly distributed film of 4.0 mg of desired materials on the GC electrode surface. Activation of bulk MoS2 and exfoliated MoS2 nanosheets was performed at pH 7.0 applying a range of potentials between 1.2 and + 1.2 V with 0.2 V intervals for the duration of 300 s each. HER measurements of these MoS2 materials were carried out using linear sweep voltammetry at a scan rate of 2 mV s 1 in 0.5 m H2SO4 electrolyte. Linear sweep voltammograms are presented versus the reversible hydrogen electrode (RHE), and the measured potentials are calcu0 , where lated using the equation: ERHE = EAg/AgCl + 0.059 pH + E Ag=AgCl EAg/AgCl is the measured potential, pH of 0.5 m H2SO4 electrolyte is 0 zero, and E Ag=AgCl refers to the standard potential of Ag/AgCl (1 m KCl) at 25 8C which is 0.235 V.
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Electronic structure calculations The band structures and total energies of single layer MoS2 in both 2H and 1T arrangements were calculated with DFT using APW + lo basis set and generalized gradient approximation (GGA, PBE96 parametrization scheme)[39] for exchange correlation potential as implemented in the Wien2k software package.[40] The plane wave cutoff energy of 240 eV and the tetrahedron method with a k-mesh 50 50 1 were used. The two-dimensional character of the S-Mo-S monolayer was simulated by introducing a 15 thick vacuum slab between the layers. The electron doping up to a complete reduction from MoIV to MoIII was treated in terms of virtual atom approximation by adding an extra nuclear and electron charge on sulfur atoms (0.5 e on each S in the case of MoIIIS2e). The lattice parameter a- and the z-position of S atoms were optimized for the endmembers, MoS2 and MoS2e, and interpolated for the intermediate electron doping rates, MoS2ex (x = 0.2, 0.4, 0.6, 0.8). All calculations were performed as non-spin polarized, although the spin polarization was tested, converging either to vanishing magnetic moment or to a spin polarized state with higher total energy.
Acknowledgements M.P. acknowledges funding from Ministry of Education (Singapore) from Tier 1 RGT1/13. Z.S. and D.S. were supported by specific university research (MSMT No. 20/2014). Keywords: density functional calculations · electrochemistry · hydrogen evolution reaction · molybdenum disulfide · transition metals dichalcogenide [1] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, A. K. Geim, Proc. Natl. Acad. Sci. USA 2005, 102, 10451. [2] M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh, H. Zhang, Nat. Chem. 2013, 5, 263.
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Received: August 13, 2014 Published online on && &&, 0000
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FULL PAPER & Electrochemistry
Electrochemistry turns MoS2 on: The electrochemical tuning of electrocatalytic properties of MoS2 opens the doors to scalable and facile tailoring of MoS2based electrochemical devices. Here a simple electrochemical method to precisely tune the electron-transfer kinetics as well as the catalytic properties of both exfoliated and bulk MoS2-based films is demonstrated.
X. Chia, A. Ambrosi,* D. Sedmidubsky´, Z. Sofer, M. Pumera* && – && Precise Tuning of the Charge Transfer Kinetics and Catalytic Properties of MoS2 Materials via Electrochemical Methods
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