Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at

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Field effect modulation of heterogeneous charge transfer kinetics at back-gated 2D MoS electrodes 2

Yan Wang, Chang-Hyun Kim, Youngdong Yoo, James E. Johns, and C. Daniel Frisbie Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03564 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated 2D MoS2 Electrodes Yan Wang† ‡, Chang-Hyun Kim ‡, Youngdong Yoo†, James E. Johns† and C. Daniel Frisbie* ‡ †Department of Chemistry, University of Minnesota, 207 Pleasant St SE, Minneapolis, Minnesota 55455, United States ‡Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, Minnesota 55455, United States

* Corresponding Author: Phone: (+1) 612-625-0779; Email: [email protected]

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ABSTRACT: The ability to improve and to modulate the heterogeneous charge transfer kinetics of two dimensional (2D) semiconductors, such as MoS2, is a major challenge for electrochemical and photoelectrochemical applications of these materials. Here we report a continuous and reversible physical method for modulating the heterogeneous charge transfer kinetics at a monolayer MoS2 working electrode supported on a SiO2/p-Si substrate. The heavily doped p-Si substrate serves as a back gate electrode; application of a gate voltage ( ) to p-Si tunes the electron occupation in the MoS2 conduction band and shifts the conduction band edge position relative to redox species dissolved in electrolyte in contact with the front side of the MoS2. The gate modulation of both charge density and energy band alignment impacts charge transfer kinetics as measured by cyclic voltammetry (CV). Specifically, cyclic voltammograms combined with numerical simulations suggest that the standard heterogeneous charge transfer rate constant ( ) for MoS2 in contact with the ferrocene/ferrocenium (Fc0/+) redox couple can be modulated by over two orders of magnitude, from 4 × 10 to 1 × 10 /, by varying . In general, the field effect offers the potential to tune the electrochemical properties of 2D semiconductors, opening up new possibilities for fundamental studies of the relationship between charge transfer kinetics and independently controlled electronic band alignment and band occupation.

KEYWORDS: field effect, gating, MoS2, heterogeneous charge transfer, electrochemical kinetics, cyclic voltammetry


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Mono- or multi-layer molybdenum disulfide (MoS2) has attracted significant attention since the burgeoning research interest in two dimensional (2D) materials was first brought about by graphene.1,2 2D MoS2 exhibits a number of appealing properties that make it a promising material for applications in catalysis,3–6 electronics,7–10 energy conversion/storage11–14 and sensing.15–19 To fulfill its potential in electrochemical applications, efforts have been made to study the heterogeneous charge transfer kinetics at MoS2 nanosheets.20–25 Chemical methods, which cause surface reduction/oxidation or phase transition from the semiconducting phase to the metallic phase of MoS2, are effective in manipulating the charge transfer kinetics because these methods change the surface properties and the charge carrier density of MoS2.21,25 For example, Chia, et al. found that the heterogeneous electron transfer between MoS2 working electrodes and [Fe(CN)6]4-/3- was improved after electrochemical reductive pre-treatment of the electrodes.25 Such approaches, however, require precise control of the chemical treatments and are not reversible. The electric field effect is a physical approach widely employed to alter the charge carrier density and energy band alignment in semiconductors.26 It was recently reported that applying an external electric field on the back side of an ultrathin ZnO working electrode can control the conduction band edge position at the front electrode/electrolyte interface, and thus shift the onset potential for the electrochemical reduction of tetrabromo-1,4-benzoquinone.27 However, there was no quantitative analysis of the charge transfer kinetics for that system. Here we demonstrate the extent to which the heterogeneous charge transfer rate constant at 2D working electrodes can be modulated by the field effect. We employed the well-known metal–oxide–semiconductor field-effect transistor (MOSFET) structure to fabricate back-gated monolayer MoS2 electrodes (Figure 1a) and investigated the charge transfer kinetics using cyclic voltammetry (CV). Our


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results show that the charge transfer kinetics at monolayer MoS2 electrodes can be continuously and reversibly tuned from irreversible to near-reversible with the applied back-gate bias ( ). In particular, we are able to tune the standard heterogeneous charge transfer rate constant ( ) between MoS2 and the ferrocene (Fc)/ferrocenium (Fc+) redox couple by over two orders of magnitude, from 4 × 10 to 1 × 10 /, by varying . As a comparison, no discernible back-gating effect on the charge transfer kinetics was observed at monolayer graphene electrodes due to the semimetallic nature of graphene.


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Figure 1. (a) Schematic illustration of the back-gated electrochemical cell structure (crosssectional view) and a top-view optical image of a MoS2 electrode through a square-shaped SU8 photoresist window. The inset scale bar is 10 µm. (b) AFM image of CVD monolayer MoS2 on a SiO2/p-Si substrate. The inset in (b) shows the height profile along the dashed white line. (c) Sheet conductance of a typical MoS2 electrode as a function of back-gate bias in semi-log (pink) and linear scale (blue). The sheet conductance was obtained in N2 atmosphere without electrolyte.


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2D MoS2 employed in this work was grown on SiO2/p-Si substrates by a chemical vapor deposition (CVD) method reported previously.28 After growth, the monolayer MoS2 sheets were located by optical microscopy29 and characterized using atomic force microscopy (AFM) (Figure 1b), Raman spectroscopy (Figure S2b) and photoluminescence (PL) (Figure S2c). The desired MoS2 sheet was then transferred onto the target SiO2/p-Si substrate with 300 nm thick SiO2, using a modified poly(methylmethacrylate) (PMMA) assisted transfer method,30,31 followed by photolithography and selective oxygen plasma etching to pattern it into a well-defined stripe. Metal contacts on top of the MoS2 stripe were deposited by e-beam evaporation and then covered by 100 nm SiO2 and 5 µm photoresist to ensure that the electrochemical reaction only took place on the exposed MoS2 area. The detailed fabrication procedure can be found in the Supporting Information. All the electrical/electrochemical measurements on the back-gated electrodes were performed in a glove box filled with N2 at room temperature. The back-gated monolayer MoS2 electrodes exhibit n-type behavior with a threshold voltage for conduction ranging from −50 V to −10 V and an on/off current ratio of ~104, as shown by the typical in-plane charge transport characteristics in Figure 1c. The data in Figure 1c were obtained by sweeping at a constant source to drain bias ( ) of 10 mV in N2 atmosphere, from which we could determine the field effect electron mobility () using Equation 1: =

1 ∙ (1)

where is the sheet conductance of monolayer MoS2 ( =

!

∙ " ; # is the source to drain

current; $ and % are the length and the width of the MoS2 channel, respectively) and is the capacitance per unit area of the back-gate dielectric, calculated to be 11.5 nF/cm2 for 300 nm SiO2 dielectric ( =

&' &( )

; *+ is 3.9 for SiO2 and d =300 nm is the thickness of the SiO2


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dielectric). The slope of the brown dashed line shown in Figure 1c was used as

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),

) -.

to estimate

. For this specific device, was 3.5 cm2 V-1 s-1. Usually, varied from 2 to 18 cm2 V-1 s-1 for our as-fabricated MoS2 electrodes, which is comparable to the previously reported values for back-gated MoS2 devices without top dielectric encapsulation.32–34

Figure 2. (a) Sheet conductance of a typical MoS2 electrode as a function of working electrode potential / at a series of back-gate biases (obtained at = 10 in [EMI][TFSI] ionic liquid without redox couples). The inset in (a) is a magnified portion of the forward potential sweep to show the threshold voltage shift with the back-gate bias. (b) Working electrode potential / as a function of back-gate bias for designated sheet conductances. ACS Paragon Plus Environment

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Having established that the CVD-grown 2D MoS2 behaves as an n-type semiconductor, as expected, it is now important to appreciate that the MoS2 working electrode is capacitively coupled to both the Si back gate and the electrolyte on the opposite (front) face. That is, both and the working electrode potential (/ ) can modulate the sheet conductance of MoS2. To confirm this, we measured the sheet conductance of the same monolayer MoS2 electrode with and / applied simultaneously (Figure 2a). The three-electrode electrochemical configuration was adopted to control / in electrolyte. As shown in Figure 1a, the quasireference electrode (Pt wire, 100 µm in diameter) and the counter electrode (Pt wire, 500 µm in diameter) were immersed in a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). Each one of the curves in Figure 2a was obtained by sweeping / at = 10 and a fixed . Larger hysteresis is observed in these curves than that in the transfer curve with no electrolyte (Figure 1c), which might stem from additional charge trapping at the MoS2/electrolyte interface.32 Importantly, for / < 0 , increases in resulted in strong increases in because the double-gating effect boosted electron concentration in the MoS2. Likewise, at a fixed , the sheet conductance increased as / became more negative, because electrons accumulated in MoS2 when the electric double layer was charged at negative / . Also, with increasing from 0 V to 90 V (corresponding to back gate-induced electron accumulation), the threshold voltage of / shifted positively (clearly shown in the inset of Figure 2a), whereas the off-state sheet conductance remained the same, consistent with expectations. Figure 2a demonstrates that and / both contribute to and that the electrons accumulated by back gating can be depleted by charging the electric double layer at positive / and vice versa; that is, the charge carriers induced by back gating are fully accessible to the MoS2/electrolyte interface, as predicted.


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The total charge in the working electrode (1/ ) can thus be approximated as a sum of the contributions of and / and the intrinsic charge in the electrode material (23 ), due, for example, to unintentional doping during synthesis: 1/ = 23 − + 67 /

(2)

where 67 is the capacitance of the electric double layer.27 The negative sign in front of the term accounts for the fact that electron accumulation occurs for > 0 and / < 0, i.e., the sign conventions are opposite. Making the reasonable assumption that 1/ and 23 are constant for a given , we have: : ;

9:

-.

?

(3)

which predicts that a plot of / vs should be linear, as is the case shown in Figure 2b. =

Considering is approximately 11 nF/cm2, the slope -. = 0.002 gives an estimated value of = >?

67 = ~5.5 µF/cm2, which is in good agreement with previously reported 67 values for [EMI][TFSI] (ranging from 3.8 to 7.8 µF/cm2).35–37


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Figure 3. Cyclic voltammograms of 10 mM Fe(C5H5)2/[Fe(C5H5)2]BF4 in [EMI][TFSI] measured at a typical back-gated MoS2 electrode (30 × 30 μmE ) at = (a) 0 V, (b) 90 V and −90 V. Cyclic voltammograms in (a) and (b) were obtained at a scan rate of 60 mV/s. The charge transfer kinetics of three outer-sphere redox systems were studied at back-gated monolayer MoS2 electrodes by CV where we swept / relative to the reference electrode while keeping = 0 and at a fixed value. The ionic liquid [EMI][TFSI] was used as the solvent and supporting electrolyte in all the CV measurements. We present results below for the redox couple ferrocene (Fc)/ferrocenium (Fc+), which was selected for two reasons: (1) the redox reaction is mechanistically simple, namely a one-electron, outer-sphere process in many


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electrolytes, including [EMI][TFSI];38,39 (2) the energy level corresponding to the formal potential of Fc0/+ (F ) is located within the band gap of monolayer MoS2 (~0.7 eV below its conduction band edge, based on the theoretically calculated value in the literature40) and thus the redox kinetics are very sensitive to band edge position. At = 0 , as shown in Figure 3a, the cyclic voltammogram of 10 mM Fe(C5H5)2/[Fe(C5H5)2]BF4 at monolayer MoS2 displays quasireversible current-voltage characteristics, with the reduction and oxidation peaks observed at −0.09 V and +0.06 V versus the Fc0/+ formal potential (G ), respectively. Notably, the reduction peak current density is significantly greater than the oxidation peak current density, which is attributable to the asymmetrical potential energy barrier for heterogeneous electron transfer expected when the formal potential of the redox couple lies in the band gap of the semiconductor (see Figure 4a).41 The overall shape of the cyclic voltammogram is consistent with expectations for a 30 × 30 μmE microelectrode and, as discussed in the Supporting Information, the anodic and cathodic quasi-steady state current densities represent reaction and mass-transport limits, respectively. Figure 3b shows two cyclic voltammograms for Fc0/+ at values far from zero. At = +90 , the reduction peak occurs at −0.06 V, closer to the redox formal potential of Fc0/+, G , and the oxidation peak current is comparable to the reduction peak current. It is also worth noting that the current density measured at / = +1 and = +90 (IJ,L!JMNJO ) is significantly larger than IJ,L!JMNJO at / = +1 and = 0 (compare Figures 3a vs 3b). Effectively, at = +90 , MoS2 operates as a nearly reversible electrode with respect to Fc0/+ electrochemistry. Alternatively, at = −90 (Figure 3b), little oxidation is observed in the anodic scan and the reduction onset occurs at a much more negative / (~−0.25 V). The behavior at = −90 is completely irreversible, essentially diode-like. These distinct


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differences in the cyclic voltammograms for = +90 and −90 suggest a considerable change in (the standard rate constant as defined in the well-known Bulter-Volmer Equation, Equation S12), which can be explained by the conduction band edge shift and change in electron occupation with applied , as discussed below. As described by Gerischer’s model, at the electrode/electrolyte interface, heterogeneous charge transfer occurs between occupied and empty states that are at the same energy level; the charge transfer rate is proportional to the energy state overlap integral between the electrode and the redox species, as shown by the energy diagram in Figure 4a.42,43 Since the Fermi level of MoS2 is pinned to F at equilibrium , it is the conduction band edge position relative to F that shifts for different values. At positive , as shown in Figure 4b, the conduction band edge shifts downward towards F due to the field effect. Thus, there is an increase in the energy state overlap integral between Fc0/+ and MoS2, which gives rise to a larger (see Equation S6 for the relationship between the conduction band edge position and ). On the contrary, negative (Figure 4c) will move the conduction band edge up, away from F , resulting in a smaller energy state overlap integral between Fc0/+ and MoS2. The reaction rate constant is decreased in this way for < 0. In the extreme case (e.g. = −90 ) where the conduction band edge is far above F , the overlap between Fc0/+ and MoS2 is too small to obtain a measurable charge transfer rate near equilibrium, so that a large overpotential is observed for the reduction of Fc+ (Figure 3b); oxidation of Fc is completely shut off inside the ±1 V window in / .


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Figure 4. Energy diagrams of a back-gated monolayer MoS2 working electrode at equilibrium with the redox species in the solution phase (i.e., / = G ) while different back-gate biases ( ) are applied: (a) = 0, (b) > 0 and (c) < 0. Note that (i) J = P = at equilibrium and (ii) the equilibrium potential GNQ = G for an equimolar solution of Fc0/+. The complete set of CV results in Figure 5a suggests that for the Fc0/+ redox reaction can be tuned continuously by . As varies from −90 V to 90 V at 15 V intervals, the reduction peak potential gradually shifts towards G and the quasi-steady-state anodic current IJ,L!JMNJO progressively increases from 3 to 263 µA/cm2. The shift of the reduction peak closer to G (i.e. 0 V) and the increase in IJ,L!JMNJO are again due to the increase of as the conduction band edge is shifting downward, closer to F , with increasing .


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Figure 5. (a) Cyclic voltammograms of 10 mM Fe(C5H5)2/[Fe(C5H5)2]BF4 in [EMI][TFSI] measured at a typical MoS2 electrode (30 × 30 μmE) at varying back-gate biases with a 15 V interval. All the cyclic voltammograms were obtained at a scan rate of 60 mV/s. (b) Calculated charge transfer rate constant of Fc0/+ redox reaction as a function of applied back-gate bias (see Supporting Information). To estimate the values of for the Fc0/+ redox reaction at different , numerical simulations of the electrochemical cell were performed using finite element analysis based on the Bulter-Volmer Equation (Equation S12). Simulation details can be found in the Supporting


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Information. Steady-state simulations were first performed to obtain the relationship between the anodic steady state current density (IJ, ) and . The values of IJ,L!JMNJO measured at different were taken as the quasi-steady state current values (i.e., IJ,L!JMNJO ~ IJ, ) and then employed with the plot of IJ, vs (Figure S8) to determine the corresponding apparent rate constant. As shown in Figure 5b, of the Fc0/+ redox reaction was modulated by two orders of magnitude, from 4 × 10 to 1 × 10 /. We also simulated the cyclic voltammograms for the same system using the calculated values. The simulated cyclic voltammograms (Figure S10) match well

with

those

observed

experimentally.

Specifically,

in

the

simulated

cyclic

voltammograms, IJ,L!JMNJO (the anodic quasi-steady-state current density) increases with increasing and the magnitudes of IJ,L!JMNJO for a series of values are comparable to those measured experimentally at the corresponding . These simulation results further confirm that the change in the CV behavior with is directly related to the change in . It is also important to note that the simulated reaction current densities were independent of for cathodic potentials, / < −0.5 , exactly the same as the experimental cyclic voltammograms at different in Figure 5a. The distinct responses for the simulated anodic and cathodic currents to support the picture that the reaction current is limited by at / = +1 , while it is limited by mass transport at / = −1 . As a check of our understanding of the gate modulated electron transfer kinetics, we also performed CV for redox couples whose formal potentials were significantly above and below the conduction band edge of monolayer MoS2. We observed, for example, that CV behaviors of 10 mM Ru(bpy)32+ were reversible for the Ru(bpy)32+/+ redox reaction and not affected by back gating (Figure S4b). The formal potential of Ru(bpy)32+/+ (−1.7 V vs Fc0/+) is far above the


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conduction band edge and back gating cannot sufficiently shift the conduction band edge relative to the energy states of this couple to appreciably change the kinetics. Likewise, oxidation of Ru(bpy)32+ is not possible during positive / scans, because the formal potential for the Ru(bpy)33+/2+ couple is so deep in the MoS2 band gap (0.9 V vs Fc0/+) that again the back gate, as currently configured, is not powerful enough to shift either the conduction band or valence band edge close enough to the formal potential. Thus, no reaction current for the Ru(bpy)33+/2+ reaction was measured with only the reduced species in electrolyte. As a further check, we investigated whether back gating could modulate the heterogeneous charge transfer kinetics at graphene electrodes. Graphene is a semimetal and so we anticipated that the redox chemistry would be rather insensitive to as essentially graphene has a continuous distribution of electronic states. To confirm this, back-gated graphene working electrodes were fabricated using a similar protocol, and all the three redox couples were measured again at the graphene electrodes under the same conditions. As expected, no obvious differences in the charge transfer kinetics were detected at different (Figure S12). Monolayer graphene is a semimetal whose conductance can be varied by 4-fold using back gating with 300 nm SiO2 (Figure S11b). Since all the three redox systems are known to exhibit rapid charge transfer reactions at metal electrodes39,44,45 and the heterogeneous charge transfer occurs in a relatively wide energy range around the Fermi level of graphene (as the energy diagrams in Figure S14 show), a limited change in the carrier density by back gating might not be sufficient to cause a discernible change in CV behavior. High-κ dielectric materials and/or other electroanalysis methods may be necessary to study the electrochemistry at back-gated graphene electrodes. Overall, the lack of gate tunable electrochemistry at graphene electrodes matched our expectations.


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In conclusion, we have fabricated back-gated 2D MoS2 working electrodes and investigated the heterogeneous charge transfer kinetics between MoS2 and three redox couples. Among them, the interfacial charge transfer kinetics of Fc0/+, having a formal potential within the band gap of MoS2, could be continuously and reversibly manipulated by back gating. Specifically, for Fc0/+ at monolayer MoS2 can be modulated by over two orders of magnitude with ranging from −90 V to 90 V. The magnitudes of these gate voltages can be strongly reduced in future experiments by employing thin high dielectric constant gate insulators. The results presented here demonstrate that modulation of the charge transfer kinetics from irreversible to reversible can be achieved at back-gated 2D semiconductors, by tuning electron occupation and energy band alignment at the electrode/electrolyte interface. Thus, field effect modulation opens up intriguing possibilities for tunable electrochemistry at 2D semiconductors.

ASSOCIATED CONTENT Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org. Materials, synthesis of monolayer MoS2, device fabrication, Raman and photoluminescence spectra of MoS2, cyclic voltammograms obtained at gold electrodes, cyclic voltammograms of 10 mM Ru(bpy)3(PF6)2 at a typical MoS2 electrode, interpretation of Fc0/+ redox reaction at MoS2 during / sweep in CV, relationship between Gerischer’s model and Bulter-Volmer equation, numerical simulation and fabrication and CV results of back-gated graphene working electrodes. (PDF) AUTHOR INFORMATION


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Corresponding Author * [email protected] ORCID: Yan Wang: 0000-0003-1264-3794 Chang-Hyun Kim: 0000-0002-7409-2679 James E. Johns: 0000-0001-6164-0384 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was primarily supported by the National Science Foundation (ECCS-1407473). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program (DMR-1420013), and in the Minnesota Nano Center, which receives partial support from NSF through the NINN program.

References: (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V; Jiang, D.; Zhang, Y.; Dubonos, S. V; Grigorieva, I. V; Firsov, A. A. Science 2004, 306 (5696), 666–669. (2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V; Morozov, S. V; Geim, A. K. Proc. Natl. Acad. Sci. United States Am. 2005, 102 (30), 10451–10453. (3) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. J. Am. Chem. Soc. 2005, 127 (15), 5308–5309. (4) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317 (5834), 100–102. (5) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Nat Nano 2016, 11 (3), 218–230.


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