Efficient Photoelectrochemical Hydrogen Evolution Using

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Efficient Photoelectrochemical Hydrogen Evolution Using Pseudocapacitive NiOx/Si Junction with Misaligned Energy Levels Jin-Young Jung,† Jin-Young Yu,† Ralf B. Wehrspohn,‡,§ and Jung-Ho Lee*,† †

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Department of Materials Science and Chemical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea ‡ Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle 06120, Germany § Fraunhofer Institute for Microstructure of Materials and Systems IMWS, Walter-Hülse-Strasse 1, D06120 Halle, Germany S Supporting Information *

ABSTRACT: Photoelectrochemical (PEC) water splitting performed by an electrocatalyst integrated with a semiconducting photoelectrode is advantageous with improvements in both charge-transfer kinetics and interface energetics because of the electrocatalyst/semiconductor junction. In general, interface energetics has been considered to arise from differences in the intrinsic electronic energy levels between the electrocatalyst and the semiconductor. Here, we demonstrated that when a NiOx thin film with porous and nanocrystalline structures is integrated with a Si photoelectrode, the interface energetics is developed by an electrochemical energy level extrinsically formed by the pseudocapacitive surface reaction (a redox reaction of NiOx for electrochemical charge storage). This new type of junction, named a pseudocapacitive NiOx/Si junction, revealed two intriguing features: the interface energetics is dynamically changed as charging/discharging progresses, and the developed electrochemical energy level and the electronic energy level of Si are abnormally misaligned under equivalent circuit conditions. With these features, the open circuit potential (Voc) of the PEC device was determined by the degree of misalignment (i.e., the electrochemical energy level). The electrochemical energy level was maximized by ∼1 V through the insertion of a SiO2 interfacial layer thick enough to suppress discharge and 1 h of PEC operation for sufficient charging by the transfer of light-induced electrons. As a result, the highest Voc of ∼1 V, surpassing the theoretical limit of 0.85 V in Si photovoltaics, was achieved. This finding demonstrated a new paradigm for self-powered PEC reactions. exhibit static behavior.5 However, in recent works it has been reported that the interface energetics can be dynamically changed when the electrocatalyst has a structure sufficiently porous to allow electrolyte permeation.31 Transition-metal oxides such as NiOx, CoOx, FeOx, and IrOx integrated with TiOx photoanodes for the oxygen evolution reaction (OER) have shown that interface energetics dynamically changed with the redox level of the metal oxide.31−34 A porous WOx/TiOx junction also showed dynamic interface energetics according to the transfer of light-induced charge carriers from TiOx to WOx (photo-charging).35 In addition, porous NiOx-integrated p-Si photocathodes for the hydrogen evolution reaction (HER) exhibited dynamic interface energetics in which the flat-band potential (Vfb) was effectively shifted in the anodic direction as the PEC operation progressed.36 As a result of the dynamic change in Vfb, the Si photocathode achieved an unexpectedly high Voc of ∼0.75 V. The high Voc was attributed to the abnormal phenomenon of Voc independence from the light-

1. INTRODUCTION Photoelectrochemical (PEC) cells that can split water by harvesting solar energy are promising solutions to meet the increasing demand for hydrogen fuel.1−7 Achieving a selfbiased PEC reaction at the high current density of 10 mA/cm2 is necessary for commercialization.8,9 This can be accomplished by improving the two important PEC performance metrics of the thermodynamic open-circuit potential (Voc) for driving the PEC reaction and the kinetic overpotential required for charge transfer.10 In this respect, the integration of an electrocatalyst with a semiconducting photoelectrode is considered effective because it can improve the electrocatalytic activity and thus reduce the kinetic overpotential.11−22 In addition, the integrated electrocatalyst can modify the interface energetics responsible for Voc by forming an electrocatalyst/ semiconductor junction instead of an electrolyte/semiconductor junction.23−30 Studies to understand and improve the interface energetics of electrocatalyst/semiconductor junctions are equally important as studies of electrocatalytic activities. Generally, the development of interface energetics is attributed to a difference in the constant electronic energy levels between the electrocatalyst and semiconductor, and thus considered to © XXXX American Chemical Society

Received: December 4, 2018 Revised: December 23, 2018

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DOI: 10.1021/acs.jpcc.8b11694 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Pseudocapacitive, porous NiOx-coated Si photocathodes for the HER in 1 M KOH. (a) Schematic showing the PEC−HER operation of the Si photocathode with the pseudocapacitive NiOx/p-Si junction. (b) Scanning TEM (STEM) and (c,d) HRTEM images of the NiOx/SiO2/Si photocathodes. The inset in (b) shows EDXS line profiles. The yellow circles in (d) indicate nanograins. (e) XRD and (f) Ni 2p XPS spectra of the NiOx thin film. (g) Scan rate (mV/s)-dependent CV and (h) current density-dependent GCD curves for the porous NiOx/SiO2/n+-Si sample before (top) and after (bottom) HER operation. Characterization of the negative (pink) and positive (dark yellow) electrochemical potentials charged by the pseudocapacitance of NiOx thin films that were integrated onto (i) n+-Si cathode and (j) p-Si photocathode. The potential curves of (j) were measured under the discharge progress at open-circuit conditions after operation of PEC−HER and EC−OER. The arrows in (i,j) indicate the electrochemical potentials.

on the Si surface via rapid thermal oxidation at 900 °C under atmospheric conditions. The SiO2 thicknesses were controlled by the growth times (1−30 s). The thickness was confirmed by spectroscopic ellipsometry (Sopra GES 5E, fitted to a Tauc− Lorentz function using a Cauchy model). Electron beam evaporation processing using a NiO target (99.98% pure) based on a high-vacuum pressure of ∼10−6 Torr was performed at a deposition rate of ∼0.2 Å/s to deposit the NiOx thin film with a porous structure and nanograins of ∼2− 5 nm. Degenerately doped n+-type Si(100) wafers with resistivities of 0.001−0.002 Ω·cm and Ni foil were also used to characterize pseudocapacitive ability and the charge-transfer kinetics. The back sides of the Si wafers were treated by HF soaking and subsequently scribing an In−Ga eutectic alloy (Sigma-Aldrich) to form Ohmic contacts with a Cu electrode. 2.2. Characterization of the NiOx Thin Films. Highresolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F, Japan) equipped with energy-dispersive X-ray spectroscopy (EDXS) was performed at an accelerating voltage of 200 kV to observe the morphologies and elemental composition of the deposited NiOx thin films. X-ray diffraction (XRD) was implemented in the 2θ range from 30 to 70° to characterize the crystal structures of the NiOx thin film. The chemical states of the NiOx thin films were investigated by Xray photoelectron spectroscopy (XPS) equipped with a monochromatic Al Kα (1.486 eV) source. All XPS spectra were calibrated against the binding energy of the carbon C 1s peak at 284.8 eV. 2.3. Pseudocapacitance and PEC Characteristics of the Si Photocathodes. Both the pseudocapacitance and PEC performances were measured using a potentiostat (Iviumstat) with three-electrode configurations, including the Si electrodes, Hg/HgO reference electrode, and Pt wire counter electrode. All Si electrodes were assembled in a home-

induced potential difference at the semiconductor (i.e., photovoltage, Vph).36,37 Despite these findings, the fundamental nature for deriving the dynamic interface energetics and the Vph-independent Voc have remained unclear. Here, we experimentally demonstrated a new model based on the pseudocapacitive ability of NiOx wherein the charge is electrochemically stored by transforming NiOx into the NiOOH/Ni(OH) 2 phase under a redox reaction. As pseudocapacitance develops toward the surface of nanocrystalline NiOx, an electrochemical potential is accumulated that simultaneously acts as an energy level, which determines the interface energetics of the NiOx/p-Si junction, as depicted in Figure 1a. This novel type of junction (named the pseudocapacitive NiOx/Si junction) featured dynamically changeable interface energetics with the charge/discharge progress. In addition, the electrochemical energy level of NiOx was unexpectedly misaligned with the electronic energy level of Si in the equivalent circuit condition. We proved that the misalignment is the fundamental mechanism underlying the Vph-independent Voc and that the degree of misalignment (i.e., the electrochemical energy level) determines the Voc. We observed that the electrochemical energy level reached ∼1 V by inserting a SiO2 interfacial layer thick enough to suppress discharge and by operating the PEC reaction for 1 h to achieve charging. Accordingly, the Si photocathode achieved the highest-ever Voc of ∼1 V, exceeding the theoretical maximum value of 0.85 V in conventional Si photovoltaics.

2. EXPERIMENTAL SECTION 2.1. Fabrication of NiOx Thin Film-Coated Si Photocathodes. Before the deposition of NiOx thin films, a single crystalline p-Si(100) wafer of ∼1−10 Ω·cm was soaked in dilute hydrofluoric acid (HF) to remove the native oxide and washed in deionized water. Then, a thin SiO2 layer was grown B

DOI: 10.1021/acs.jpcc.8b11694 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 2. The pseudocapacitive ability of NiOx causing the interface energetics of NiOx/Si junction. (a) Dark and (b) illuminated CV curves of the NiOx/SiO2/Si photocathode observed under various conditions: pristine (open circle), five CV cycles under illumination (half-solid), and activated by PEC operation for 1 h (solid). The arrows indicate the sweep directions of CV. (c) Plot showing the PEC−HER potential during the activation process via chronopotentiometry at a constant current density of −10 mA/cm2 for 1 h. The activation causes an anodic shift in the PEC−HER potential. (d) The potential−time plot showing the electrochemical potentials charged in the Si photocathode after the responses in (a−c). The dotted circles indicate the electrochemical potentials. Frequency-dependent M−S plots for the NiOx/SiO2/Si in (e) pristine and (f) activated conditions. (g) The M−S plots as a function of conditions: pristine, activated, deactivated, and reactivated.

made Teflon bath after Ohmic contact formation on the dilute HF-treated back sides of the samples by spreading In−Ga eutectic (Sigma-Aldrich). All measurements were performed in 1 M KOH electrolyte. For pseudocapacitance, the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) curves were obtained from both n+-Si and Ni foil samples. The PEC properties of the NiOx thin film-coated Si photocathodes were characterized by means of CV, linear sweep voltammetry (LSV), chronopotentiometry, and electrochemical impedance spectroscopy (EIS). The CV and LSV responses were performed at a scan rate of 50 mV/s. All PEC measurements were characterized under the illumination of 100 mW/cm2 (AM 1.5G filter-equipped Xe lamp) calibrated using a Si photodiode standard cell (PV Measurements, Inc.). For the activation process, the chronopotentiometry measurement was continued under illumination at −10 mA/cm2 for 1 h. The deactivation progressed under open-cell conditions with spontaneous discharge progress. The electrochemical potentials of the Si photocathodes were observed by characterizing the maximum potentials under the open-cell condition. The photovoltage was measured using chronopotentiometry under chopped illumination under the open-circuit condition. The electrochemical impedance was obtained in the frequency range 100 kHz to 1 Hz at 0.2 V versus reversible hydrogen electrode (RHE). The Mott−Schottky (M−S) analysis was implemented in the frequency range 2.5 MHz to 1 Hz under

dark conditions. The measured potentials were converted to the RHE potentials.

3. RESULTS AND DISCUSSION In Figure 1b−d, STEM images show an electron beamevaporated nanocrystalline NiOx thin film on a SiO2-passivated Si substrate to form a proven electrocatalyst/insulator/ semiconductor configuration as a platform for a highperformance PEC device. As demonstrated in a previous work,36 the NiOx thin film has electrolyte-permeable, porous, and nanocrystalline structures (grain sizes of ∼2−5 nm), which allow the redox-active surface, unlike the bulk, to effectively and dominantly influence the energetics of the NiOx layer. The thickness of NiOx used in this work varies from 10 to 100 nm. EDXS and XRD results of the NiOx thin film show an O-rich composition (Ni/O = 2:3) with (111) and (200) dominant crystalline directions (see Figure 1e,f). In the Ni 2p3/2 spectrum of the XPS, the deconvoluted peaks at ∼854 and ∼856 eV for NiO indicate the contribution of the oxidation/ reduction states. This is attributed to phase changes, such as the transformation of redox-active NiOx into NiOOH/ Ni(OH)2 during charge transfer in an aqueous electrolyte. Typical electrochemical measurements including CV and GCD responses were implemented for the NiOx thin film deposited on the Ni foil (see Figure S1). The results showed pseudocapacitive characteristics (i.e., the charge was electrochemically stored by electron transfer between the electrolyte C

DOI: 10.1021/acs.jpcc.8b11694 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

curve for the pristine sample shows a redox peak at 0.5 V versus RHE (see Figure 2a), which is associated with the transition into NiOOH. Interestingly, the illuminated CV curve shows dynamic features with the progress of the PEC reaction; an asymmetric CV curve (hysteresis) is observed for the pristine sample, but it is changed to a symmetric curve over five consecutive cycles (see Figure 2b). This feature arises from hydration during the PEC reaction that enables light-induced charge storage (i.e., the photo-charging effect) in the pseudocapacitive NiOx. As is known for charge storage, hysteresis is inherent to the charge/discharge processes,44 and dynamic features in hysteresis are related to the chargetransfer kinetics, not thermodynamics. This was confirmed by the anodic shift of the CV curves without a change in thermodynamic Voc (see Figure S2), thereby indicating that photo-charging reduces the kinetic overpotential required for charge transfer. To further investigate the effect of photocharging on the PEC performance as a function of the PEC− HER, we implement an activation process via chronopotentiometry (potential change) measurement at −10 mA/cm2 (see Figure 2c). The activation was carried out within 2 h because the PEC performance started to degrade because of dissolution of SiO2 after 3 h of activation (see Figure S3). During activation, a great anodic shift occurs in the potential levels for the PEC−HER, which is also shown by the illuminated CV curve (see Figure 2b). The activation causes not only overpotential reduction but also Voc enhancement (see Figure S2). Because the Voc is associated with the energetics, the activation leading to the transition into Ni(OH)2 is considered to accompany the change in the energetics. The activation also causes the anodic shifts of both redox peaks in the dark CV curve and binding peaks in the XPS profiles (see Figures 2a and S4), indicating the change in the energetics with the transformation into Ni(OH)2. To identify the charge/discharge progress according to the PEC reactions, the charged electrochemical potentials are also observed on the NiOx/SiO2/Si photocathodes after the electrocatalytic and PEC responses (see Figure 2d). During the dark CV sweep in the cathodic direction to drive the reduction reaction (charge progress), the electrochemical potential is observed at 0.48 V. However, the dark CV sweep in the anodic direction to drive the oxidation reaction (discharge progress) exhibits an electrochemical potential near the initial level. To verify the photo-charging effect by PEC reactions, we also observed the electrochemical potential after an illuminated CV sweep. Like the results of the dark CV sweep, the cathodic-direction sweep induces charge progress with the electrochemical potential of 0.57 V, whereas the anodic-direction sweep causes discharge progress with the decreased electrochemical potential of 0.21 V. Performing illuminated CV for five cycles yielded the electrochemical potential of 0.61 V, and the activation process enabled the electrochemical potential to reach 0.75 V. After fully discharging, we performed the reactivation process by which the potential for the PEC−HER was increased during the PEC operation again. Similar to activation, the electrochemical potential was also observed at 0.78 V. We also confirmed that the electrochemical potential varied with the degree of activation (see Figure S5). These results demonstrated that the electrochemical potential originated from the pseudocapacitance of the porous NiOx, according to the PEC and electrocatalytic charging of the NiOx/SiO2/Si photocathode.

and NiOx by an adsorbed ion) representing the redox active behavior of the porous NiOx.38,39 The porous NiOx thin film was also deposited on a SiO2 interfacial layer-grown n+-Si wafer electrode to characterize the pseudocapacitive ability according to the charge transfer from the Si wafer to NiOx through the SiO2 layer. When scan rate-dependent CV curves are measured in 1 M KOH electrolyte within the potential range 0.5−2 V versus RHE (see Figure 1g), all CV curves show a pair of symmetric redox peaks attributed to reversible faradaic reactions. With increased scan rate, the redox peak interval is broadened and the peak current increased, indicating the restricted internal diffusion resistance within the inner active surface area of the porous-structured NiOx layer for charge storage at a high scan rate.40 Interestingly, after performing HER via chronopotentiometry at −10 mA/cm2 for 30 min, the redox peaks are changed asymmetrically and the reduction peak is shifted greatly in the anodic direction (see the bottom plot in Figure 1g). These results arise from the electrochemical transition of the NiOx phase, where the surface of NiOx spontaneously transforms into NiOOH or Ni(OH)2 under an applied potential in the alkaline electrolyte.41−43 Before the HER test, the reduction peak mainly arises from the faradaic reaction via the phase transition of NiOx into NiOOH. After the HER, the continuous hydration induces an additional transition into Ni(OH)2, which contributes to the anodic shift of the reduction peak. Figure 1h shows GCD curves at different current densities over the potential range −1.8 to 2 V versus RHE to observe and compare the pseudocapacitive ability before and after HER test for the NiOx/SiO2/n+-Si electrode. The nonlinear charge and discharge curves are obtained at various current densities, and the time for charge and discharge is gradually increased with a decrease in current density because of the pseudocapacitive ability. The charge and discharge times are also increased after the HER. The volumetric capacitances were calculated at 0.1 mA/cm2 according to the equation: C = IΔt/VΔV, where C, I, Δt, V, and ΔV are the values of geometric capacitance (F/cm2), current (A), discharge time (s), volume of the electrode (cm3), and potential difference (V). We observe that the capacitance after the HER (1 mF/ cm3) is much larger than that before the HER (0.4 mF/cm3) because of the transition to the Ni(OH)2 phase via further electrochemical hydration. Figure 1i exhibits the electrochemical potentials charged by reduction (electron transfer) and oxidation (hole transfer) reactions. The negative potential of ∼ −1 V and the positive potential of ∼0.6 V are observed based on the redox potential of HER (i.e., 0 V vs RHE) and OER (i.e., 1.23 V vs RHE), respectively. We also observe that the electrochemical potential accumulates at the NiOx/SiO2/pSi photocathode by a PEC reaction at open-circuit conditions in the three-electrode configuration (see Figure 1j). Because it is difficult to experimentally observe the charging progress in a PEC device, the charged electrochemical potential values are characterized using the initial (maximum) potential on discharge, immediately after operating PEC−HER and EC− OER. The potential values observed in the NiOx/SiO2/p-Si photocathode show good agreement with those in the NiOx/ SiO2/n+-Si electrode (see Figure 1i,j). This suggests that pseudocapacitive behavior also occurs in the NiOx/SiO2/Si photocathode. The electrocatalytic and PEC properties for the NiOx/SiO2/ p-Si photocathodes were characterized by performing dark and illuminated CV response testing in 1 M KOH. The dark CV D

DOI: 10.1021/acs.jpcc.8b11694 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

significant improvement in PEC performances. The operating potential at 10 mA/cm2 is greatly shifted from −0.5 to 0.2 V (Figure 3a) by enhancing the Voc (Figure S2) and reducing the kinetic overpotential (see Figure 3b,c for the EIS results of the NiOx/SiO2/Si photocathode and the dark LSV curves of NiOx/SiO2/n+-Si, respectively). The improvement in the PEC performance is mainly originated from the reduced kinetic overpotential because the decreased amount of overpotential is greater than the increased amount of Voc. In addition, the large Vfb suppresses recombination losses, such that the electrical leakage dark current is decreased (Figure 3d) and the Vph is increased (Figure 3e). The long-wavelength region of EQE changes according to the activation (Figure 3f). The activation slightly increases the EQE, and thereby enhances the limiting photocurrent by ∼1 mA/cm2. Because the long-wavelength region of EQE is closely influenced by the light absorptance ability, rather than by the electrical properties of the junction, the EQE change seems to be caused by the electrochromic ability of NiOx by which the hydration reversibly changes optical properties such as the refractive index.45 To clearly elucidate the interface energetics and the thermodynamics, we compare the energy band diagrams of the pseudocapacitive NiOx/Si junction with those of the conventional Schottky Ti/Si junction (by the NiOx/Ti/SiO2 photocathode) (see Figure 4). The energy band diagrams are described based on the dark and illuminated LSV curves in the low current density level range where the potentials at zero current in the dark and illuminated curves yield information regarding the interface energetics and thermodynamics. For the HER, the potentials at zero current are irrelevant to the charge-transfer kinetics because the kinetic overpotential near the zero current is low enough to be negligible. For the conventional Schottky junction, because the energy levels in the equivalent circuit of the PEC device are aligned at the same level, the dark LSV curve generally crosses the zero point, as shown in Figure 4a. According to the energy level alignment, band bending occurs to compensate for an interfacial charge neutrality imbalance under equilibrium state, of which the degree can be measured by Vfb (see Figure S6a). Therefore, the typical energy band diagram of the Schottky Ti/Si junction under the equivalent circuit in the dark can be described, as shown in Figure 4b. Under illumination, a quasi-equilibrium state is newly arisen; the splitting of the quasi-Fermi levels in the Si photocathode causes all energy levels to align with the quasi-Fermi level of the electron in Si, as described in Figure 4c. Therefore, the splitting of quasi-Fermi levels defined as Vph determines the Voc. Thus, the illuminated LSV curve shifts in the anodic direction by the value of Vph, such that the observed Voc value (the potential at zero current) is the same as the degree of Vph (see Figures 4a and S6b). On the other hand, the pseudocapacitive NiOx/Si junction displays distinctive behaviors. The dark LSV curve crosses abnormally at a nonzero point, indicating that the energy levels are misaligned in the equivalent circuit condition (see Figure 4d). This phenomenon is only possible because the potential is accumulated in NiOx. Thus, we know that the electrochemical potential developed by the pseudocapacitance of NiOx determines the interface energetics and the thermodynamics for the NiOx/SiO2/Si photocathode in the equivalent circuit condition, as observed in Figure 2d. The degree of Vfb is similar to the zero-current potential (see Figure S7). This indicates two phenomena occurring in the pseudocapacitive NiOx/Si junction. First, the electrochemical potential simultaneously

To observe the effect of electrochemical charge on the interface energetics of the NiOx/SiO2/Si photocathode, Vfb values representing the interface energetics were characterized in M−S plots at various frequencies. Interestingly, Vfb varies with the frequency (i.e., the charging rates, see Figure 2e,f). In addition, similar to the electrochemical potential with dynamic behavior, the Vfb is reversible according to the activation/ deactivation process (the charge/discharge progress, Figure 2g). Activation causes the Vfb to shift in the anodic direction to the degree that the energetic peak shifts in the results of the dark CV (Figure 2a) and the XPS (Figure S3). After deactivation, the Vfb returns to its original value. It should be noted that the observed maximum value of the Vfb corresponds well with that of the electrochemical potential. From these results, it is expected that the electrochemical potential stored on the surface of the nanocrystalline NiOx is a dominant factor in determining the Vfb of the NiOx/SiO2/Si photocathode. Because the Vfb dominating the PEC performance varies according to the photo-charging, the PEC performance can be also manipulated by activation/deactivation. To verify this, dark and illuminated LSV, EQE, Vph measurement, and EIS are performed for various conditions, such as pristine, activation, deactivation, and reactivation (Figure 3). All PEC responses show dynamic behavior with reversibility according to the activation/deactivation, indicating the decisive contribution of charge/discharge to the PEC performances. The large Vfb value (∼0.75 V) obtained after the activation corresponds to a

Figure 3. Effects of the charge/discharge (activation/deactivation) progress on PEC performances of the NiOx/SiO2/p-Si photocathode. (a) Illuminated LSV curves, (b) Nyquist impedance plots, (c) kinetic overpotential plots, (d) dark LSV curves, (e) Vph, and (f) external quantum efficiency (EQE) measurements under various NiOx conditions: pristine (black), activated (pink), deactivated (blue), and reactivated (orange). The plots in (c) were observed by the dark LSV curves for the NiOx/SiO2/n+-Si electrode. E

DOI: 10.1021/acs.jpcc.8b11694 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. Understanding interface energetics and thermodynamics for the pseudocapacitive NiOx/p-Si junction by comparison with the Schottky Ti/p-Si junction. The LSV curves in the low current density level range (