Surface State Mediated Electron Transfer Across ... - ACS Publications

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Surface State Mediated Electron Transfer Across the N‑Type SiC/ Electrolyte Interface Matthias Sachsenhauser,† Ian D. Sharp,‡,§ Martin Stutzmann,† and Jose A. Garrido*,∥,⊥ †

Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain ⊥ ICREA, Institució Catalana de Recerca i Estudis Avançats, 08070 Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: Understanding the mechanisms of charge transfer across the semiconductor/electrolyte interface is a basic prerequisite for a variety of practical applications. In particular, electrically active surface states located in the semiconductor band gap are expected to play an important role, but direct experimental evidence of surface states has proven to be challenging, and further experimental studies are required to verify their influence on the exchange of charge carriers between semiconductor and electrolyte. Due to its wide band gap, chemical stability, and controllable surface termination, silicon carbide (SiC) provides an excellent model system for this purpose. In this report, we provide a fundamental electrochemical study of n-type 6H-SiC and 4H-SiC electrodes in aqueous electrolytes containing the ferricyanide/ferrocyanide redox couple. Cyclic voltammetry and impedance spectroscopy measurements are performed over a wide range of potentials to determine the energetic positions of the SiC band edges and to investigate the electron-transfer kinetics between SiC and the ferricyanide molecules. For both polytypes, a broad distribution of surface states with energy levels close to the conduction band is found to mediate electron transfer, resulting in deviations of the observed charge transport characteristics from the predictions of well-established models. Moreover, a detailed evaluation of the impedance data allows for explicit correlation of the chargetransfer resistance associated with the ferricyanide reduction reaction with the potential-dependent distribution of surface states. In addition to the relevance of our studies for advancing the implementation of SiC in biosensing, electrocatalytic, and photocatalytic applications, the presented methodology can also be adopted for fundamental electrochemical investigations of other semiconductor electrodes.

1. INTRODUCTION For many years, the investigation of semiconductor/electrolyte interfaces has attracted considerable interest of researchers in the field of physical chemistry. Both theoretical and experimental studies have led to profound understanding of most fundamental processes, which is essential for implementing such systems in functional devices. In particular, the kinetics and energetics of charge transfer across the semiconductor/ electrolyte interface is of central importance for a variety of applications, ranging from solar energy conversion and storage to bioelectronics and biosensing. However, a substantial weakness of many traditional semiconductors, such as silicon, lies in their limited chemical stability under the harsh aqueous environments required for operation. As a result, electrochemical decomposition or oxidation of these semiconductors occurs,1 thus limiting their long-term use in electrochemical or photoelectrochemical cells. Consequently, wide band gap © XXXX American Chemical Society

materials have been extensively studied and have proven to exhibit superior stability and biocompatibility compared to most conventional semiconductors.2 In this context, silicon carbide (SiC) is particularly interesting. Historically, a primary focus of SiC research has been devoted to exploiting its outstanding material properties3 for the development of electronic devices operating in extreme conditions.4−6 However, its resistance to chemical attack also makes SiC a promising electrode material for biosensing, bioelectronics, and photocatalysis, including as a component in water splitting systems.7−13 Accordingly, considerable effort has been dedicated to investigating the electrochemical and photoelectrochemical characteristics of the most common Received: November 26, 2015 Revised: February 24, 2016

A

DOI: 10.1021/acs.jpcc.5b11569 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C polytypes of SiC (6H-, 4H-, and 3C-SiC).9,14−18 Most of the early studies, however, found that a quantitative evaluation of fundamental charge-transfer processes was challenging, mainly because high-quality single crystals were rarely available at that time and SiC surface chemistry was not very well understood. Experimentally obtained values for the flat band potential of SiC electrodes often varied significantly for a given polytype,9 making it impossible to reliably determine the overlap of the semiconductor band edges with the energy levels of the electroactive species in the electrolyte. An additional complication in understanding interfacial energetic alignment and charge-transfer pathways lies in the presence of localized surface energy states in the band gap, which is a commonly encountered issue for semiconductor electrodes. Depending on their density, surface states can mediate charge transfer across the interface, as well as result in a partial or complete pinning of the Fermi level at the semiconductor/electrolyte interface.19,20 Indeed, experimental studies of various semiconductor/electrolyte systems have demonstrated that the observed electron-transfer characteristics often significantly deviate from the predictions of wellestablished models. However, establishing an unambiguous correlation between surface states and an anomalous chargetransfer behavior has proven to be challenging.21 In this work, we present a fundamental electrochemical study of n-type 6H-SiC and 4H-SiC electrodes in aqueous electrolytes containing the ferricyanide/ferrocyanide redox couple. These two SiC polytypes possess wide band gaps of 3.02 and 3.27 eV, respectively, and are prepared with well-defined hydroxyl-terminated (0001) surfaces that allowed us to precisely determine the band edge alignment with the electrolyte redox levels and gain a detailed understanding of the major processes that govern charge transfer across the interface. In particular, we employed impedance spectroscopy to investigate the surface state distribution of both SiC polytypes, and we found that these surface states play a vital role in facilitating electron transfer from the SiC conduction band to the electrolyte. Furthermore, simulations of the impedance data with appropriate equivalent circuit models revealed that the charge-transfer resistance associated with the ferricyanide reduction reaction can be directly correlated to the distribution of these surface states.

electrodes, as confirmed by atomic force microscopy, static water contact angle, and X-ray photoelectron spectroscopy measurements (see Supporting Information Figures S1 and S2). Importantly, the SiC electrodes were found to be stable in active electrochemical environments, and no significant changes in surface morphology or chemical composition were observed after electrochemical characterization (see Supporting Information Figures S3 and S4). Electrolytes. All chemicals were purchased from SigmaAldrich and used without further purification. Aqueous electrolytes were prepared from ultrapure water (ρ > 18 MΩ cm, EMD Millipore Direct-Q UV 3), containing 50 mM Tris and 100 mM NaCl as a supporting electrolyte. The pH value of the buffer was adjusted to 7.3 using a Metrohm 780 pH meter. For charge-transfer studies, the potassium ferricyanide/ ferrocyanide redox couple (referred to as [Fe(CN)6]3−/ [Fe(CN)6]4−) with a standard redox potential U0F,rdx = 0.36 V was added to the electrolyte. Prior to the measurements, all buffers were degassed with a vacuum pump, followed by thorough purging with nitrogen. Electrochemical Characterization. Immediately after the last HF etching step, the hydroxyl-terminated SiC samples were used for electrochemical characterization. For this purpose, the electrodes were mounted in a custom-built Teflon cell. The electrolyte volume was fixed to 50 mL and sealed from the substrate with an O-ring, defining the active area to 0.196 cm2. Measurements were performed at room temperature using an Autolab PGSTAT12 potentiostat (Metrohm, GmbH & Co. KG, Germany) in a typical three-electrode configuration with a commercial glass-body Ag/AgCl reference electrode (RE-5B from Bioanalytical Systems, Inc., U.S.A., 3 M NaCl, 0.196 V vs RHE) and a high surface area platinum wire counter electrode. For electrical shielding, the cell was placed in a grounded Faraday cage, and to minimize oxygen contamination, a constant flow of nitrogen was maintained over the electrolyte during the measurements. If not stated otherwise, all current− potential curves were obtained with a step potential of 10 mV and a scan rate of 250 mV/s. Impedance spectra were recorded at different dc bias (between −1.3 and 0.8 V vs Ag/AgCl) in single-sine mode in a frequency range between 100 mHz and 50 kHz, with the ac perturbation amplitude set to 50 mV. Simulation of the measured impedance data was performed using the ZSimpWin software (EChem software, V 3.10). All potentials in this work are given relative to the Ag/AgCl reference electrode.

2. EXPERIMENTAL SECTION Samples. Commercially available, single-crystalline n-type 6H-SiC and 4H-SiC wafers were purchased from SiCrystal (Nürnberg, Germany). The (0001)-oriented wafers were nitrogen-doped with a concentration of n ∼ (1−2) × 1018 cm−3 and n ∼ (1−2) × 1019 cm−3, respectively, and the surfaces were chemomechanically polished (NovaSiC, Le Bourget du Lac Cedex, France), with surface rms roughness values below 0.2 nm for both polytypes. Ohmic back contacts were obtained by thermal evaporation of Ni/Cr (50 nm) and Au (50 nm) layers, followed by annealing at 1100 °C for 3 min under ultrahigh vacuum (UHV) conditions. Sample Preparation. Prior to electrochemical characterization, all samples were cleaned following a standard procedure as described elsewhere.22 Briefly, the substrates were ultrasonically cleaned in acetone and isopropyl alcohol for 10 min each at room temperature, followed by two cycles of oxygen plasma treatment (5 min, 200 W, 1.4 mbar) and etching in 5% hydrofluoric acid (5 min). This treatment is known to yield clean, hydroxyl-terminated surfaces on (0001)-oriented SiC

3. RESULTS AND DISCUSSION Cyclic voltammetry (CV) is one of the most common experimental techniques for studying electron-transfer processes across electrode/electrolyte interfaces. Although it is frequently used with metals or degenerately doped semiconductor electrodes, it also provides useful information for semiconductors with low and medium doping levels. Figure 1a depicts CV curves of hydroxylated 6H- and 4H-SiC electrodes in Tris buffer containing 2 mM of the [Fe(CN)6]3−/ [Fe(CN)6]4− redox couple. For both polytypes, no Faradaic currents are observed at positive potentials. As will be discussed later, a poor energetic alignment inhibits electron injection from occupied states in the electrolyte to the semiconductor conduction band. At the same time, the concentration of holes in the valence bands of these wide band gap n-type electrodes is too low to facilitate any noticeable oxidation reactions. For potentials more negative than −0.4 V (4H-SiC) and −0.6 V B


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one-electron redox system at an ideal interface depends exponentially on the applied potential, Ubias: ⎛ e(Ubias − Ufb) ⎞ jc− = ekc−coxn0 exp⎜ − ⎟ kBT ⎝ ⎠

(1)

where k−c denotes the second-order transfer rate constant, cox is the concentration of the oxidized component of the electroactive species at the interface, n0 is the free electron density in the semiconductor bulk, Ufb is the flat band potential, e is the elementary charge, kB is the Boltzmann constant, and T is the absolute temperature. Hence, at room temperature a potential variation of 59 mV is expected to change the cathodic current density by 1 order of magnitude. For both SiC polytypes, an exponential increase of the [Fe(CN)6]3−/[Fe(CN)6]4− reduction current is indeed observed over several orders of magnitude (Figure 1b). However, Tafel slopes of 75 mV/ decade for 6H-SiC and 147 mV/decade for 4H-SiC clearly deviate from the theoretical prediction. The obtained values were found to be independent from the applied scan rate for both polytypes (see Supporting Information Figure S6); therefore, any influence of the diffusive behavior of the redox species on the measured slopes can be excluded. In literature, Tafel slopes higher than 59 mV/decade have been reported for various semiconductor electrodes,25,26 including SiC.9 Possible explanations for this behavior include a potential drop over the Helmholtz layer and tunneling of electrons through the semiconductor space charge region.27 However, these two explanations are typically only relevant for semiconductors with very high carrier densities (n0 ∼ 1020 cm−3)1 and are not expected to play a major role for the SiC electrodes with a lower doping level investigated in this work. Alternatively, surface states with energy levels located in the band gap could be responsible for the nonideal charge-transfer process shown in Figure 1b, as discussed in the following. Contact potential difference and surface photovoltage measurements in ultrahigh vacuum have revealed that surface states induce an upward band bending for both 6H-SiC and 4H-SiC electrodes possessing hydroxyl surface terminations (see Supporting Information Figure S7). Whether or not these states are involved in electron-transfer reactions strongly depends on the energetic positions of the band edges with respect to ferricyanide/ferrocyanide redox levels. In order to investigate the band edge positions of the SiC electrodes, the capacitance of the interface was examined over a wide range of bias potentials via electrochemical impedance measurements.

Figure 1. (a) Cyclic voltammograms of 6H- and 4H-SiC electrodes, recorded at 250 mV/s in Tris buffer containing 2 mM [Fe(CN)6]3−/ [Fe(CN)6]4−. At large cathodic bias potentials, [Fe(CN)6]3− is reduced to [Fe(CN)6]4− for both polytypes. For reference, current− potential curves measured in pure Tris buffer are depicted in the inset. (b) The [Fe(CN)6]3−/[Fe(CN)6]4− reduction current increases exponentially with the applied bias, featuring slopes of 75 and 147 mV/decade for 6H-SiC and 4H-SiC, respectively.

(6H-SiC) vs Ag/AgCl, however, electrons from the semiconductor are transferred to the electrolyte, thereby reducing [Fe(CN)6]3− to [Fe(CN)6]4− in a diffusion-limited process (see Supporting Information Figure S5). Furthermore, as demonstrated by a reference measurement recorded in pure Tris buffer, cathodic currents arising from hydrogen evolution are observed when the electrode potential is more negative than −1.1 and −1.2 V vs Ag/AgCl for 6H-SiC and 4H-SiC, respectively (see inset in Figure 1a). According to standard theory,23,24 the cathodic current density due to charge transfer from the conduction band to a

Figure 2. (a) Impedance spectra of 6H- and 4H-SiC electrodes recorded in Tris buffer containing 1 mM [Fe(CN)6]3−/[Fe(CN)6]4− at a bias potential of Ubias = 0 V. Full and open symbols represent the absolute impedance and phase vs frequency, respectively. A simple serial RC equivalent circuit model was used for simulating the impedance data (solid lines). (b) Resulting Mott−Schottky plots of 6H- and 4H-SiC electrodes. The solid lines represent linear fits according to eq 2 (note the two different y-axes). C


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The Journal of Physical Chemistry C Figure 2a shows exemplary spectra recorded at Ubias = 0 V vs Ag/AgCl. As already evident from the CV curves, the electrodes are ideally polarizable in this potential range, and the data can be simulated by a serial connection of an external resistance, RS, and the interface capacitance, Cint. Provided that Cint is dominated by the space charge region of the semiconductor, its potential dependence is governed by the space charge capacitance, CSC, which is expected to obey the Mott−Schottky equation: 1 CSC

2

=

⎛ k T⎞ 2 ⎜Ubias − Ufb − B ⎟ eεSCε0ND ⎝ e ⎠

impedance measurements over a wide frequency range and the surface termination was well-controlled.30 Using the measured values of Ufb, and taking into account the energy difference between the conduction band and the Fermi level in the bulk, which is calculated considering the donor densities and ionization energies (see Supporting Information), the positions of the conduction and valence band edges for the 6H- and 4H-SiC electrodes were determined. Figure 3 shows

(2)

where ND is the donor density, εSC is the dielectric constant of the semiconductor (taken to be 9.7 for both 6H- and 4HSiC28), and ε0 is the vacuum permittivity. In the examined potential range, a linear dependence between CSC−2 and Ubias is indeed observed for both polytypes (see Figure 2b). We can therefore exclude any relevant contribution of surface states to the system impedance in this potential range: the alignment of the Fermi level with the electronic levels of surface states would result in a deviation of the capacitance values from the predictions of the Mott−Schottky equation.27 In addition to these findings, the fits shown in Figure 2b were used to derive the values of ND and Ufb for the two types of SiC electrodes, as summarized in Table 1.

Figure 3. Energy band diagram of 6H- and 4H-SiC in contact with an aqueous electrolyte containing the [Fe(CN)6]3−/[Fe(CN)6]4− redox couple at a bias potential of Ubias = 0 V vs Ag/AgCl. The energetic positions of the conduction and valence band edges at the surface are fixed with respect to the electrolyte redox level, and the upward band bending at the surface results in a depletion layer of width dSCR. The densities of occupied and unoccupied redox states are drawn according to the Gerischer−Marcus model of electronic states in solution, assuming a standard redox potential of U0F,rdx = 0.36 V and a reorganization energy of λ = 0.75 eV (ref 31). Charge transfer from the SiC conduction band to the [Fe(CN)6]3− molecules can occur either isoenergetically (green arrow) or via surface states (red arrows).

Table 1. Donor Density, ND, and Flat Band Potential, Ufb, of 6H- and 4H-SiC Extracted from the Mott−Schottky Plots Shown in Figure 2ba 6H-SiC 4H-SiC

ND [cm−3]

Ufb vs Ag/AgCl [V]

2 × 10 2 × 1019

−1.1 ± 0.1 −1.6 ± 0.2

18

a The error of Ufb was calculated from the standard deviation of five different samples for each polytype.

the energetic alignment between the band edges and the electronic levels of the oxidized and reduced components of the ferricyanide/ferrocyanide redox couple for a bias voltage of Ubias = 0 V vs Ag/AgCl. According to this experimentally derived energy diagram, electron transfer is expected to be governed exclusively by the conduction band. However, the poor overlap between the conduction band edge and the energy states of the [Fe(CN)6]3− molecules, in particular for the 4H-SiC electrodes, suggests that charge transfer does not occur isoenergetically between the conduction band and the redox molecules (as would be expected from standard theory), but is instead mediated by surface states. This assumption is supported by the CV curves shown in Figure 1, where the cathodic current starts to flow at potentials considerably positive of the flat band potential, i.e., when the semiconductor surface is depleted of electrons. Since a capacitive contribution of surface states was not observed in the Mott−Schottky plots shown in Figure 2b, it must be assumed that the energetic levels of the surface states, ESS, remain unoccupied over the potential range examined in this figure. At more cathodic potentials, however, the Fermi level becomes aligned with ESS, and electron transfer to the electrolyte is facilitated. Given these observations, we can anticipate that the surface states are located rather close to the SiC conduction band for both polytypes. To investigate the exact energy distribution of the surface states, impedance spectra were recorded in a potential range where an energetic alignment between surface states and redox

The donor density is in good agreement with the specifications from the manufacturer for both polytypes, and flat band potentials of −1.1 and −1.6 V vs Ag/AgCl were obtained for 6H-SiC and 4H-SiC, respectively. Within the experimental error, the variation of Ufb of the two types of SiC electrodes is consistent with density-functional theory calculations,29 which suggest that the valence band edges of all SiC polytypes are almost aligned, resulting in significant conduction band offsets according to their different band gaps (see Supporting Information for more details). At this point it should be noted that several authors have previously reported on the flat band potentials of n-type SiC electrodes (most commonly 6H-SiC) in acidic or basic conditions.8,9,14−17 However, as mentioned above, the extracted values often varied considerably from sample to sample or drifted with time,9 likely due to the large variability in crystal quality and insufficient understanding of the SiC surface chemistry at the time when most of these studies were performed. In recent years, significant advancements in the structural and electronic quality of bulk SiC wafers and methods for achieving nearly atomically flat surfaces through chemical−mechanical planarization and surface chemical preparations have been realized. Therefore, very reliable values for the flat band potential were obtained in this work, showing no time dependence and good sample-to-sample reproducibility, since CSC was extracted from D


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Figure 4. Contour plots of the impedance data of a 6H-SiC electrode measured at different bias potentials ranging from 0.1 to −1.3 V vs Ag/AgCl. Panels a and b show absolute value, |Z|, and phase, φ, of the impedance as a function of frequency in pure Tris buffer; panels c and d show the respective measurements in the presence of 1 mM [Fe(CN)6]3−/[Fe(CN)6]4−. (e) Depending on the applied potential, different equivalent circuit models are utilized to simulate the data. Model 1 describes an ideally polarizable interface (bias region I), model 2 accounts for charge transfer across the semiconductor/electrolyte interface (bias regions II and IV), and model 3 includes diffusion of electroactive species in the electrolyte (bias region III).

a parallel resistance, RCT,1, must be introduced to the equivalent circuit (model 2). As will be discussed below, depending on the applied potential, RCT,1 accounts for surface state charging or proton reduction in the electrolyte. Furthermore, since a capacitive contribution of surface states is expected to result in a frequency dependence of the total interface capacitance,32,33 CSC in model 2 is replaced by a frequency-dependent constant phase element (CPE). Parts c and d of Figure 4 depict the impedance measurements after 1 mM [Fe(CN)6]3−/[Fe(CN)6]4− has been added to the electrolyte. While the spectra with and without the redox couple are similar in region I, significant differences are observed in the low-frequency regime at more cathodic bias potentials so that the contour plots at these potentials have to be subdivided into three further regions. For measurements between −0.2 and −0.6 V vs Ag/AgCl (region II), the chargetransfer kinetics of the [Fe(CN)6]3− reduction dominates, resulting in a strong decrease of |Z| and φ with potential at low frequencies. In this case, the spectra can be simulated with

energy levels is expected. Figure 4 shows contour plots of the absolute value of the impedance, |Z|, and its phase, φ, for a 6HSiC electrode as a function of frequency at potentials ranging from 0.1 to −1.3 V vs Ag/AgCl in pure Tris buffer (Figure 4, parts a and b) and in Tris buffer containing the redox couple [Fe(CN)6]3−/[Fe(CN)6]4− (Figure 4, parts c and d). Depending on the applied potential, different processes dominate the ac response, and thus different equivalent circuit models are utilized to simulate the experimental data over the whole examined potential range (see Figure 4e). First, we focus on the measurements recorded in pure Tris buffer (Figure 4, parts a and b). Two different potential regions can be identified. For bias potentials more positive than −0.2 V vs Ag/AgCl (region I), no indication of charge transfer across the interface is observed. As mentioned in the discussion of Figure 2, the electrode is ideally polarizable and model 1 can be applied to simulate the measurements. For potentials more negative than −0.2 V vs Ag/AgCl (region II), the absolute impedance and the phase start to decrease with potential at low frequencies; hence, E


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The Journal of Physical Chemistry C model 2. In potential region III (−0.6 to −1.1 V vs Ag/AgCl), however, |Z| and φ start to increase with decreasing potential at low frequencies, and model 2 fails to simulate the experimental data properly. Here, diffusion of the electroactive species in the electrolyte is rate-limiting and an additional Warburg impedance contributes to the spectra. Therefore, model 3 was employed in potential region III. In this equivalent circuit, the serial connection of RCT,2 and the Warburg element W is exclusively related to the ferricyanide/ferrocyanide redox couple, while RCT,1 accounts for electron-transfer processes that are not limited by diffusion. For potentials more negative than −1.1 V vs Ag/AgCl (region IV), the hydrogen evolution reaction starts to dominate the impedance measurements. As a consequence, the absolute value of the impedance and the phase decrease again with potential at low frequencies and model 2 can be used for data simulation. To demonstrate that the equivalent circuit models presented in Figure 4e are indeed suitable to fit the experimental data properly, simulations of measurements in Tris buffer and in Tris buffer containing the [Fe(CN)6]3−/[Fe(CN)6]4− redox couple are shown for selected potentials in Figure 5. For every

across the interface (i.e., Rtot par is equivalent to RCT,1 in model 2, but includes the contributions of RCT,1, RCT,2, and W in model 3). Again, we first discuss the measurements in pure Tris buffer (Figure 6a). In potential region I (highlighted in gray), no

Figure 6. Total parallel resistance, Rtot par, of 6H- and 4H-SiC as obtained from impedance spectra simulations measured in (a) pure Tris buffer and (b) Tris buffer containing 1 mM [Fe(CN)6]3−/[Fe(CN)6]4−. The potential range marked in blue exhibits a frequency-dependent Rtot par due to a Warburg element in the equivalent circuit; both curves in panel b are shown for a frequency of 0.1 Hz. The exponential decrease 3− reduction. of Rtot par in the red marked region is due to [Fe(CN)6] Slopes of 85 and 157 mV/decade are observed for 6H-SiC and 4HSiC, respectively.

parallel resistance is required to simulate the impedance spectra (model 1); therefore, the given values for Rtot par in this potential range have to be regarded as minimum resistance values. Spectra recorded at Ubias more negative than −0.2 V vs Ag/ AgCl (green background), where Rtot par of 6H-SiC and 4H-SiC is exclusively given by RCT,1 (model 2), exhibit a moderate, but continuous decrease of Rtot par with potential. As mentioned above, we ascribe this decrease to a continuous charging of surface states when the Fermi level in the SiC is shifted upward, i.e., closer to the conduction band edge at the surface. At large cathodic bias potentials (Ubias < −1.1 V vs Ag/AgCl), the slope of the resistance−potential curve increases for both polytypes. In agreement with the CV curves shown in the inset of Figure 1a, this suggests that electron transfer across the interface, resulting in the reduction of protons, only contributes to the parallel resistance in this potential range. When [Fe(CN)6]3−/[Fe(CN)6]4− molecules are added to the electrolyte, the potential dependence of Rtot par changes significantly (see Figure 6b). In bias region II (highlighted in red), where ferricyanide reduction is dominant, Rtot par decreases exponentially over about 2 orders of magnitude. Slopes of 85 and 157 mV/decade are observed for 6H-SiC and 4H-SiC, respectively, which are in good agreement with the values obtained from the CV measurements (see Figure 1b). In bias region III (blue background), where diffusion of the redox species is rate-limiting, the Warburg element introduces a

Figure 5. Representative impedance spectra of 6H-SiC recorded in (a) pure Tris buffer and (b) Tris buffer containing 1 mM [Fe(CN)6]3−/ [Fe(CN)6]4−. Solid lines are simulations to the measured data according to the equivalent circuit models shown in Figure 4. Model 1 was employed for the black curves, model 2 was applied for the green and red curves, and model 3 was used for the blue curve.

potential region defined in Figure 4, one representative spectrum is depicted, but the models were found to provide adequate fittings over the whole potential range in all cases. For the 4H-SiC electrodes, qualitatively very similar results were obtained. The recorded impedance spectra, together with the relevant fitting curves, are provided in the Supporting Information (see Figures S8 and S9). The fitting parameters extracted from the simulations can now be used to analyze the potential dependence of the resistive behavior of the SiC electrodes in more detail, from which we can gain insight into the charge-transfer processes at the electrode/electrolyte interface. For this purpose, we introduce the total parallel resistance, Rtot par, which summarizes all equivalent circuit elements accounting for charge transfer F


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Figure 7. (a) Interface capacitance of 6H-SiC and 4H-SiC electrodes as obtained from impedance spectra simulations. Solid lines are fitting curves according to the Mott−Schottky equation. (b) Surface state capacitance extracted from panel a; the inset shows a zoom to a narrower potential range. A quasi-exponential increase is observed for 6H- and 4H-SiC, with slopes of 135 and 263 mV/decade, respectively.

contribution of surface states. The impedance of a CPE element is given by34 1 ZCPE(ω) = (iω)−α Q (5)

frequency dependence to the overall parallel impedance. Assuming that our system is governed by semi-infinite linear diffusion, the Warburg impedance is given by34 Z W(ω) =

σ σ −i = ω ω

2σ iω

(3)

where Q and α are the CPE parameters. For all investigated potentials, the exponent α features only minor deviations from ideality (α > 0.9, see Supporting Information Figure S10), and therefore, the CPE parameter Q can be interpreted as an approximate value for the interface capacitance Cint (in units of F/cm2).32 The resulting potential dependence of Cint is shown in Figure 7a for 6H- and 4H-SiC. While the contribution of surface states to Cint has been excluded for potentials Ubias more positive than −0.2 V vs Ag/AgCl (see Figure 2b), a significant deviation from the prediction of the Mott−Schottky equation (represented by the solid lines in Figure 7a) is observed at more cathodic potentials for both polytypes. We attribute this deviation to the presence of electronic states in the band gap: when the SiC Fermi level is aligned to the electronic levels of the surface states, ESS, a surface state capacitance, CSS, has to be considered in parallel with the capacitance of the semiconductor space charge region, CSC.36,37 Hence, CSS can be obtained by calculating the difference between Cint and CSC shown in Figure 7a. The extracted values for CSS are depicted in Figure 7b for both SiC polytypes. The capacitance of monoenergetic surface states with surface density NSS is given by

Here, σ is the Warburg coefficient, which is defined as

34

σ(ω) =

RT ⎛ 1 ⎜⎜ + 2 z 2F 2A ⎝ Dox cox

⎞ ⎟⎟ Dred cred ⎠ 1

(4)

where Dox and Dred are the diffusion coefficients of the oxidized and reduced components of the redox species, cox and cred are the respective concentrations, z is the valency, A is the electrode area, R is the ideal gas constant, and F is the Faraday constant. Using eq 3, we calculated the total impedance of the parallel components of equivalent circuit model 3, i.e., of RCT,1, RCT,2, and W, and interpreted its real part as the frequencydependent total parallel resistance Rtot par(ω), which includes all contributions to the charge-transfer processes. In Figure 6b, Rtot par(ω) is shown for a frequency f = 0.1 Hz. For both polytypes, a continuous increase of the total parallel resistance with decreasing potential is observed in bias region III, which can be explained with the Warburg coefficient σ(ω) given in eq 4. The magnitude of σ(ω) depends on the ratio of cox to cred at the interface, becoming large when one of the components of the redox species dominates. When electrons are transferred from the SiC electrodes to the electrolyte and [Fe(CN)6 ]3− molecules are reduced, their surface concentration drops, causing a large cred/cox ratio and consequently an increase in the Warburg impedance. With more negative potentials in bias region III, this effect becomes more and more pronounced, and the total parallel resistance continuously rises (see Figure 6b).35 In bias range IV (green background), where hydrogen evolution starts to dominate the parallel impedance, Rtot par is again frequency-independent and decreases with more negative potentials. In the previous discussion, we examined charge transfer across the SiC/electrolyte interface and found evidence that electron transfer is mediated by surface states. In the following, we will discuss the energy distributions of these surface states, which can be determined from the potential dependence of the interface capacitance, Cint. As mentioned earlier, a CPE is used in the equivalent circuit models to account for the capacitive

−2 ⎡ ⎛ ESS − E F ⎞⎤ e2 ⎢ ⎥ CSS = NSS cosh⎜ ⎟ 4kBT ⎢⎣ ⎝ 2kBT ⎠⎥⎦

(6)

with a full width at half-maximum of ∼3.5 kBT/e at room temperature.36,38 Since the experimentally determined CSS is much more broadly distributed over potential, it must be concluded that multiple electronic states with different energy levels exist at the SiC surface. Provided that electron transfer from the conduction band to the surface states is not ratelimiting, the potential distribution of CSS can be used to roughly estimate the total surface state density, Ntot SS according to 1 tot CSS(U ) dU NSS = (7) e

∫

Values of 1.7 × 1012 and 2.2 × 1012 cm−2 are obtained for 6H-SiC and 4H-SiC, respectively, which is an important finding G


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The Journal of Physical Chemistry C insofar as surface state densities of 1013 cm−2 can be sufficient to cause complete Fermi level pinning.19,20 The similarity of Ntot SS for both polytypes is not surprising, considering that the two materials only differ in the bulk stacking order but the atomic configurations of the surface sites are identical. For moderate cathodic potentials, i.e., in a range where the [Fe(CN)6]3− reduction current increases exponentially and diffusion is not rate-limiting, a quasi-exponential increase of CSS is observed, with slopes of 135 mV/decade for 6H-SiC and 263 mV/decade for 4H-SiC (see inset Figure 7b). This means that, although the overall densities of the surface states are comparable for the two polytypes, their distributions differ significantly. Potentially, this effect could be caused by the different surface morphologies of the 6H-SiC and 4H-SiC electrodes (see Supporting Information Figure S1), but clear experimental evidence for this hypothesis cannot be provided in this work and would require additional investigations of the SiC surfaces using experimental techniques such as scanning tunneling microscopy or wet-chemical atomic force microscopy. Nevertheless, the ratio of the slopes obtained in Figure 7b (i.e., 6H-SiC vs 4H-SiC) agrees very well with the ratio in Figure 6b, since the steeper increase of the surface state density for 6H-SiC is expected to result in a steeper decrease of the corresponding charge-transfer resistance, thus providing further evidence that electron transfer from the SiC conduction band is directly related to the surface states. Interestingly, the slopes associated with the potential dependence of CSS (Figure 7b) are larger compared to the slopes associated with the potential dependence of Rtot par (Figure 6b) for both polytypes. This can be explained by considering the influence of the rate constant on the charge-transfer characteristics. For an ideal n-type semiconductor surface, where electron transfer occurs isoenergetically at the conduction band edge, k−c is independent of the applied potential. In presence of a continuous distribution of surface states, however, charge transfer occurs at EF (similar to a metal electrode), and the corresponding rate constant, k−SS, is determined by the overlap of the surface state distribution with the electronic states in the electrolyte, i.e., it strongly depends on the applied potential. According to the Gerischer−Marcus model, the energy levels of the oxidized (unoccupied) redox states are given by a Gaussian distribution: Wox =

⎡ (eU − eU 0 + λ)2 ⎤ 1 F,rdx ⎥ exp⎢ − ⎢ ⎥⎦ k λ 4 4πλkBT ⎣ BT

surface state capacitance. The experimentally obtained values for the parameter m in the present study typically varied between 0.15 and 0.26, which is in reasonable agreement with the expected value for ferricyanide with U0F,rdx = 0.36 V and λ = 0.75 eV (m = 0.26). We finally note that both SiC polytypes feature a high degree of electrochemical stability, i.e., the distribution and density of electrically active surface states do not change upon bias application and charge transfer (see Supporting Information Figure S11).

4. CONCLUSION In summary, we have studied the electrochemical properties of n-type 6H- and 4H-SiC electrodes in aqueous electrolytes. Cyclic voltammetry and impedance spectroscopy measurements were performed over a wide range of potentials to determine the energetic positions of the SiC band edges and to investigate charge-transfer reactions between the semiconductor conduction band and the ferricyanide/ferrocyanide redox couple. Depending on the applied potential, different processes, namely, ferricyanide reduction, ferricyanide/ferrocyanide diffusion, and hydrogen evolution, were found to dominate the electron-transfer characteristics. The impedance measurements were simulated with different equivalent circuit models, and the extracted fitting parameters were used to conduct a detailed analysis of the potential dependence of the resistive and capacitive behavior of the SiC electrodes. The charge-transfer resistance associated with the ferricyanide reduction reaction was in good agreement with the cathodic current from CV measurements. The slope of the resistance−potential curve, however, deviated from the ideal behavior and was found to be different for the 6H-and 4H-SiC electrodes. We attributed these deviations to the presence of a broad distribution of surface states, which could be identified close to the conduction band for both polytypes. In addition, the potential dependence of the charge-transfer resistance directly correlated with the surface state capacitance of the 6H- and 4H-SiC samples, confirming that electron transfer does not occur via direct exchange at the SiC conduction band edge, but is instead mediated by the surface state distribution at the interface. The results presented here provide insight into the critical role of surface states in mediating charge transfer at the SiC/electrolyte interface and will be of great importance for implementing SiC in practical applications such as biosensing and photocatalysis. This work suggests that engineering surface state distributions, for example, through controlled modifications of surface terminal groups,40−43 provides an additional means for tuning the energetic alignment and affecting efficiency and selectivity of charge transfer from the semiconductor to specific redox species in solution. Finally, the presented methodology also provides a straightforward and general approach to fundamental electrochemical investigations of other semiconductor electrodes.

(8)

where λ is the reorganization energy, and U0F,rdx is the standard redox potential. Typically, the influence of Wox on the experimentally observed current−potential characteristics follows a simple exponential behavior,39 i.e., it can be

(

meU

approximated by a term of the form exp − k T B

)

with

λ − eU 0

■

F,rdx . Hence, the cathodic current density due to m= 2λ electron transfer from the SiC surface states to the [Fe(CN)6]3− molecules can be written as

⎛ meU ⎞ jc− ∝ CSS(U ) exp⎜ − ⎟ dU −∞ ⎝ kBT ⎠

∫

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11569. Figures presenting atomic force microscopy, static water contact angle, and X-ray photoelectron spectroscopy data of hydroxyl-terminated 6H- and 4H-SiC before and after electrochemical characterization, cyclic voltammetry plots of 6H- and 4H-SiC comparing different scan rates

EF

(9)

where the term CSS appears when considering the density of surface states. Equation 9 demonstrates that j−c and, thus, the charge-transfer resistance exhibit a steeper exponential dependence on the applied potential compared to the one of the H


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and redox concentrations, contact potential difference, and surface photovoltage measurements of hydroxylterminated 6H- and 4H-SiC, calculation of the band edge positions and the bulk Fermi level of 6H- and 4H-SiC, additional impedance data of 6H- and 4H-SiC (including potential dependence of the CPE exponent), and repeated cyclic voltammetry and impedance spectroscopy measurements of 6H- and 4H-SiC (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S. acknowledges support of the IGSSE and of the Technische Universität MünchenInstitute for Advanced Study, funded by the German Excellence Initiative. I.D.S. was supported by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under award no. DESC0004993.

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