Decoupling pH-Dependence of Flat-Band Potential in Aqueous Dye

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C: Energy Conversion and Storage; Energy and Charge Transport

Decoupling pH-Dependence of Flat-Band Potential in Aqueous Dye-Sensitized Electrodes Yongze Yu, Kevin A. Click, Szu-Chia Chien, Jiaonan Sun, Allison E. Curtze, Li-Chiang Lin, and Yiying Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00710 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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The Journal of Physical Chemistry

Decoupling pH-Dependence of Flat-Band Potential in Aqueous Dye-Sensitized Electrodes Yongze Yu†#, Kevin A Click†#, Szu-Chia Chien┴#, Jiaonan Sun†, Allison Curtze†, Li-Chiang Lin‡*, Yiying Wu†* †

Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States

‡

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States

┴

Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, United States

ACS Paragon Plus Environment

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ABSTRACT: When a semiconductor is in contact with an electrolyte, its flat-band potential (EFB) is an important quantity for determining band edge positions in photoelectrochemistry. Oxide semiconductors generally have a EFB shift of -59 mV in aqueous solutions when the pH is increased by one unit as a consequence of surface deprotonation. Many of the most desirable redox reactions, such as the reduction of CO2 to HCOOH, or water to H2, also show the same dependence on pH due to the involvement of protons. Therefore, pH cannot be used to tune the relative energy alignment between the electrode and the electrolytes. Here we demonstrate via Mott-Schottky measurement that sensitized NiO with a membrane-inspired design of dye molecule (BH4) can decouple the pH dependence of EFB. The EFB of BH4 sensitized NiO films shows very little to zero change as a function of pH, whereas less hydrophobic dye (P1) sensitized NiO and the bare NiO follow the Nernst shift with respect to pH.


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INTRODUCTION Dye-sensitized research based on aqueous electrolytes is of interest to both solar energy conversion and storage.1,2 In terms of solar energy conversion, aqueous dye sensitized solar cells (DSSCs) have their advantages of being cost effective, environmentally friendly and nonflammable.3,4 With respect to simultaneous solar energy conversion and storage, water splitting dye sensitized photo-electrochemical cells (DSPECs) have all the advantages of aqueous DSSCs but additionally store the converted sunlight energy as the bond energy of hydrogen and oxygen.2,5– 7

Dye-sensitized TiO2 and other n-type wide-bandgap semiconductors have been used as

photoanodes for water oxidation8–11 and n-type DSSCs12, and dye-sensitized p-type semiconductors, such as NiO, have been used as photocathodes for reduction of protons and CO213– 20

and p-type DSSCs21–23. They can also be combined as tandem cells to achieve total water

splitting and chemical synthesis.24–26 The

direction

of

electron

transfer

at

a

semiconductor-electrolyte

interface

in

photoelectrochemistry (PEC) is controlled by the alignment between the band energetics of the semiconductor and the redox potentials of species in the electrolyte.27 A common phenomenon on most metal oxides is that their surface potential exhibits a Nernstian dependence on pH in an aqueous solution, shifting by 59 mV per pH unit.28 This is due to the Helmholtz potential drop resulting from the change of the surface charge with the extent of adsorption of protons and hydroxide ions (Scheme 1a).29,30 In PEC water splitting, the hydrogen-evolution reaction (HER) and oxygen-evolution reaction (OER) also exhibit the same pH dependence.31–33 Therefore, in contrast to the pH-independent redox electrolyte in a DSSC architecture, the relative energy alignment between an oxide semiconductor and water splitting reactions is not varied by pH (Scheme 1b and 1c). To increase the tunability of output voltage, the band energies can be


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decoupled from the pH tuning by shutting off the surface protonation of an oxide semiconductor. If the interface between metal oxides and electrolyte is isolated by a hydrophobic layer, the Helmholtz potential drop will be decoupled to pH of electrolytes due to the invariableness of surface absorbed ion species.(Scheme 1a) The pH-dependence of the band-edge on Si controlled by chemically modifying different hydrophobic functional groups has been demonstrated by N. S. Lewis.34,35 Herein, we first time, to best of our knowledge, report that in the dye-sensitized semiconductors system, a properly designed hydrophobic dye monolayer via surface adsorption through favorable hydrogen bonding and electrostatic interactions can also serve as the same function to control the pH dependence of the band edge without chemical treatments. Recently we have reported a series of push-pull molecules that consist of a triphenylamine (TPA) donor moiety connected to two perylenemonoimide (PMI) acceptor groups by head-to-tail oligo 3-hexylthiophene conjugated π linker groups for water splitting photocathodes and aqueous p-type dye-sensitized solar cells (DSSC).17,36,37 An example named as BH4 is shown in Figure S1. When grafted onto the surface of NiO, BH4 allows for efficient light harvesting while also protecting the semiconductor surface from being etched by acids and the water contact angle increases from 12o to 119o.17 The protection afforded by such membrane-mimicking dye gives this system excellent stability from extremely acidic (pH = 0) conditions17 to extremely basic (pH=14) conditions (Figure S10). We proposed that when the BH4 molecules self-assemble onto the NiO semiconductor surface, the molecules can assemble into a hydrophobic layer that can prevent ionic/polar species such as protons and water from reaching the NiO/dye interface. An idealized schematic illustration is displayed in Scheme 1a. This hypothesis motivates us to probe if BH4grafted interface can shut off the coupling between pH and the flat-band potential of NiO.


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Scheme 1. (a) Schematic illustration of regular pH-dependent flat-band potential (EFB) due to variation of surface charge (dashed line) and our proposed pH-independent case due to surface blockage by the BH4 molecules (solid line); (b-c) Energy diagram of p-type dye-sensitized electrode with (b) pH-independent redox and (c) pH-dependent redox. (PZC: point of zero charge)


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To evaluate the proposed protection strategy using the BH4 dye, Mott-Schottky measurements were used to experimentally probe the semiconductor-electrolyte interface of dye-sensitized NiO films. As noted previously, most oxide semiconductors shift the flat band potential (EFB) by -59 mV in aqueous solutions when the pH is increased by one unit as the consequence of surface deprotonation. If the BH4 dye indeed creates a hydrophobic protection layer that prevents the adsorption of protons onto NiO, the EFB of NiO should not show a Nernstian shift in potential as a function of pH. To confirm that the structure of the BH4 dye is unique in creating this proposed hydrophobic protection layer, a simple and highly studied P1 dye (Figure S1) was also evaluated as a control dye.38 Both dyes have a triphenylamine core with thiophene conjugated linkers and a single carboxylic acid anchor but the P1 dye lacks the large hydrophobic character of the BH4, the hexyl chains and highly chromatic perylene monoimide (PMI) units. Furthermore, in this study, state-of-the-art molecular dynamic calculations were employed to gain atomistic insights into the interface between the NiO surface with or without BH4 molecules and water.

EXPERIMENTAL SECTION Materials and Chemicals. Chemicals used in this work were purchased from Sigma Aldrich or Fischer Scientific. All chemicals were used without further purification. Specifically, citric acid (99%) was from Sigma Aldrich, while Potassium Chloride (Certified ACS grade) and Sodium Hydroxide (Certified ACS grade) were from Fischer Scientific. Deionized water for the aqueous buffers was obtained by filtration through a Thermo Scientific™ Barnstead™ E-Pure™ Ultrapure Water Purification System until ≥ 18 MΩ-cm in resistance was achieved. Polytetrafluoroethylene (PTFE) (Part#:3/4-5-5498) was purchased from TapeCase Ltd. The BH4 dye was previously


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synthesized per our prior report,37 and the P1 dye was synthesized by following procedures reported in the literature.38 Instrumentation. Buffers were prepared by monitoring the pH using an Oakton pH/Ion/°C Meter (Ion 6 Acorn Series). Electrochemical Impedance Spectroscopy for the Bode plots and MottSchottky plots were obtained using a Reference 600 potentiostat/galvanostat from Gamry Instruments. Emission spectra were obtained using a Fluoromax-4 from Horiba Scientific instrument with an excitation wavelength set at 450nm. Electrochemical

Measurements.

Electrochemical

impedance

spectroscopy

(EIS)

measurements were conducted in a homemade one-compartment cell using a 3-electrode setup. The working electrode was a circular 0.36 cm2 600 nm thick NiO film fabricated following our previous report.17 The NiO film was sensitized in either 0.03 mM BH4 in N,N-dimethylformamide (DMF) or 0.3 mM P1 in ethanol overnight. The sensitized films were rinsed with their respective soaking solutions and air dried prior to use. A piece of PTFE tape with a circular punched hole was used to insulate the FTO surrounding the NiO from the electrolyte. The counter electrode for all experiments was a piece of platinum mesh on a platinum wire. A Ag/AgCl (Sat. KCl, 0.198 V vs. NHE) reference electrode was used for all experiments and was calibrated vs a Saturated Calomel Electrode (SCE) (0.244 V vs NHE). All EIS measurements were obtained with an AC voltage of 10 mV rms between 1 – 100,000 Hz in 10 points/decade. The cell was degassed with Ar for 15 minutes prior to every potential experiment. Mott-Schottky Plots were obtained between 0 – 1 V vs NHE. The voltage step was 20 mV, and the AC voltage was 10 mV rms. Additional EIS was performed on the BH4 sensitized NiO films with pH 2 to 5 solutions. For each solution, the EIS data was collect between 0.1-1MHz with applied DC bias between 0.55 and 0V (vs NHE) with 50mV interval. Fresh films were made and tested for each pH solutions, and


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more than two films were averaged to check the reproducibility for each pH solutions. The capacitance is extracted from the EIS for each applied potential and pH at 10Hz. The selection of the frequency is discussed in the results and discussion section for Mott-Schottky analysis. Because the variation in the heterogeneity of different films, such as surface area and doping density, the 1/C2 them is normalized by the highest 1/C2 within the detectable range. This would eliminate the slope variation between films, which should not affect the intercept on the x axis (i.e., the flat band position). Computational Details. Molecular dynamics (MD) simulations, implemented in the opensource LAMMPS package, 39 were employed to study the interfacial phenomena between the BH4grafted NiO (100) surface and bulk water. The simulation domain consists of a (100) rigid NiO substrate, BH4 molecules on the NiO surface, water, and a rigid piston. The Ni atoms on the NiO surface are available as anchored sites for BH4 molecules (i.e., the carboxylic acid groups of the BH4 molecules were anchored to the surface Ni atoms). As the NiO surface was saturated by BH4 molecules via adsorption in the experiments, the studied NiO surface in the simulations was accordingly designed to have a high surface loading. A regularly and densely packed pattern of BH4 molecules on the surface was assumed as schematically shown in Figure S7. Such arrangement, which has a packing density of one BH4 molecule per 24 surface Ni sites, was designed by considering the size of the BH4 molecules and the spacing between Ni sites on the surface. In these calculations, a piston was used to maintain the atmospheric pressure of the water phase by applying a constant force. The simulation box has a dimension of approximately 135 Å (X-direction) x 92 Å (Y-direction) x 170 Å (Z-direction), where the Z-direction is perpendicular to the NiO surface. Periodic boundary conditions were applied on all directions, and a vacuum space of more than 8 nm was included along the Z-direction to avoid the self-interaction between


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periodic images. All simulations were performed in a canonical ensemble (i.e., NVT) at 300K using a Nosé–Hoover thermostat with a damping factor of 100 timestep. The simulation timestep was chosen to be 0.25 fs, and the total simulation time was approximately 2.0 ns. A hybrid potential was adopted in this study. Reactive force field (ReaxFF) was used to model flexible BH4 molecules,40 while 6-12 Lennard-Jones (L-J) and long-range Coulombic potentials were adopted to describe the interactions of water – water, water – BH4 molecules, water – NiO, and NiO – BH4 molecules. The TIP4P-Ew model41 was used for water molecules with the Shake Algorithm, and L-J parameters for the NiO substrate and BH4 molecules were taken from the universal force field (UFF).42 The Lorentz-Berthelot mixing rule was applied to estimate the L-J parameters between dissimilar atoms. As a comparison, the interfacial phenomenon between the bare NiO surface and water was also investigated. The simulation details for such system are as same as that for the aforementioned dye-sensitized NiO one except that BH4 molecules were removed.

RESULTS AND DISCUSSION Mott-Schottky measurements determine the space-charge capacitance in a semiconductor that is in contact with an electrolyte at various potentials.30 Previously, we have reported the use of MottSchottky measurements to measure the EFB of cobalt doped p-type NiO.43 The EFB of a p-type semiconductor can be obtained from a Mott-Schottky plot where the inverse of the square of the capacitance of the space-charge region (CSC) is plotted as a function of applied potential (E). The EFB of the semiconductor can be calculated after fitting the linear portion of the Mott-Schottky plot and extrapolating the line to the x-axis (i.e., x-intercept), followed by correcting for the Boltzmann energy term. The Mott-Schottky relationship can be seen in Equation 1: 1 −2 kBT 2 = 𝑒𝑒ϵ ϵNA2 �E − EFB − 𝑒𝑒 � (𝐸𝐸𝐸𝐸. 1) CSC 0 ACS Paragon Plus Environment

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where e is the elementary charge constant, ε0 is the permittivity of free space constant, ε is the dielectric constant of the semiconductor, N is the doping density within the space charge region of semiconductor, A is the surface area of semiconductor, kB is the Boltzmann constant, and T is temperature. Mott-Schottky measurements are only valid under depletion conditions. For p-type NiO, this can be only achieved when the applied potential is more negative than the EFB in NiO. At these conditions, downward band bending exists at the interface and hence the space-charge capacitance is measured. The phase angle is due to the capacitive component of the circuit and is assumed to be the space-charge capacitive region. This space-charge capacitance increases as the band bending decreases, and the applied potential becomes more positive until flat band conditions are reached (Figure S8).

Figure 1. Bode plots for a) Bare NiO film. b) P1 sensitized NiO film. Mott-Schottky analysis of Bare NiO and P1 sensitized NiO. The space charge capacitance of the semiconductor for Mott-Schottky plots was obtained using electrochemical impedance spectroscopy (EIS) at a single chosen frequency. In order to determine which frequency to use for the Mott-Schottky plots, Bode plots of phase angle vs frequency at several applied potentials


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between 1 – 100,000 Hz were determined for bare NiO films and sensitized (P1 and BH4) films in a 0.1 M pH 6 citric buffer with 1 M KCl (Figure 1). The corresponding Nyquist plots are shown in Figure S2. Citric acid was chosen as the buffer due to its wide pH window. The addition of 1M KCl was to ensure the potential drop of the diffuse or Gouy layer could be ignored. The capacitive component of the circuit dominates at the frequency where the phase angle is closest to -90° and therefore this frequency should be used for the Mott-Schottky plots. The Bode plot for the bare NiO film showed a maximum phase angle of around -68° at approximately 100 Hz. The P1 sensitized NiO film (Figure 1b) displayed a very similar shape as the bare NiO film and a maximum phase angle of -76° at approximately 100 Hz. The Bode plot for the P1 sensitized NiO was almost identical to the bare NiO film indicating mostly bare NiO character for the P1 sensitized film.

Figure 2. a) Bode plot of EIS for BH4 sensitized NiO film at pH 4. b) Corresponding Nyquist plot. Inset: zoom in image at high frequency with highlighted points at 10Hz. Mott-Schottky analysis of BH4 sensitized NiO. In contrast to bare/P1 NiO, there are two peaks in Bode phase-vs.-frequency plots with high phase angles (high capacitive components) as shown in Figure 2. Correspondingly, the Nyquist plot of BH4 sensitized film features two


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semicircles. In order to examine if high frequency impendence (10 – 100 kHz) can be a representative of the space charge capacitance. Figure S3 and Table S2 display the fitted parameters for a RC parallel circuit for BH4 sensitized NiO in the high frequency region. The yaxis on the right side is 1/C2, which decreases slightly and then abruptly increases as the potential becomes more positive. In a p-type semiconductor, the depleted region width (Eq. 2) decreases as the potential becomes more positive so that the capacitance increases until reaching the flat band condition. 2𝜀𝜀𝜀𝜀

𝑊𝑊 = �� 𝑞𝑞𝑞𝑞0 �𝑈𝑈𝑆𝑆 −

kT

1⁄2

�� 𝑞𝑞

(𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑) (𝐸𝐸𝐸𝐸. 2)

𝐶𝐶SC = (𝑞𝑞 2 𝑁𝑁𝑁𝑁𝜀𝜀0 𝐴𝐴2 )1⁄2 exp(𝑞𝑞 𝑈𝑈𝑆𝑆 ⁄kT)1/2 (𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎) (𝐸𝐸𝐸𝐸. 3)

where Us is the potential drop in the space charge layer which can be obtained as the applied potential minus the flat band potential, W is the width of space charge layer, Csc is the space charge capacitance, q is the elementary charge constant, ε0 is the permittivity of free space constant, ε is the dielectric constant of the semiconductor, N is the doping density within the space charge region of the semiconductor, A is the surface area of the semiconductor, kB is the Boltzmann constant, and T is temperature.

Figure 3. Mott-Schottky plots for a) Bare NiO film, b) P1 sensitized NiO film, and c) BH4 sensitized NiO film.


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As the applied potential is more positive than flat band potential, the band bending falls into the accumulation condition. The capacity for the accumulation layer has been studied by Dewald (Eq. 3).44,45 Space charge capacitance increases as the accumulation layer becomes larger, which means 1/C2 is supposed to further decrease (Figure S9). The displayed 1/C2 has, however, opposite trend as we discussed above. Therefore, the high frequency capacitance should not be assigned to the space charge capacitance. Instead, this semicircle can be assigned to charge transfer resistance in parallel with the capacitance of dye coating. With the above discussion, the high frequency semicircle should be excluded for the space charge capacitance. In Figure 2, we can observe that the phase angle and imaginary impedance increase as applied potential decrease in the frequency range of 1-100Hz. This is consistent with the expectation that in a p-type semiconductor, more negative applied potentials cause stronger band bending and thus smaller space charge capacitance. Therefore, it is believed that the capacitance at mid frequency 1-100Hz is attributed to the space charge capacitance. Thus, the space charge capacitance of NiO with BH4 sensitization is extracted from the Nyquist Plot of the EIS at 10Hz using C=1/(- 2π f Zimg), where f is the chosen frequency-10Hz and Zimg is the imaginary part of impedance at the chosen frequency. Table 1. Summary of NiO EFB from Mott-Schottky plots. Bare NiO pH

P1

BH4

NiO EFB (V vs NHE)

5

0.48 ± 0.01 0.54 ± 0.03

0.21 ± 0.01

4

0.54 ± 0.02 0.61 ± 0.01

0.22 ± 0.01

3

0.60 ± 0.02 0.67 ± 0.02

0.22 ± 0.01

2

0.66 ± 0.02 0.73 ± 0.03

0.21 ± 0.01


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Summary of the Flat-band potentials. Based on the Mott-Schottky analysis shown in Figure 3, the flat-band potentials are obtained by extracting the x-intercepts at various pH for bare NiO, P1 sensitized NiO and BH4 sensitized NiO. Table 1 and Figure 4 summarize the fitted MottSchottky results for the EFB as a function of pH for the bare and sensitized NiO films. Our results show an expected Nernstian shift in bare NiO (Figure 3a) EFB as a function of pH with a slope of 60.4 mV/pH (Figure S5). The EFB at pH 7 calculated from fitted equation is 0.37 V (vs NHE), which is in excellent agreement with literature reported values (Table S1). The P1 sensitized NiO film also shows a very similar shift of 61.3 mV/pH (Figure S6). As with the Bode plots, both the bare and P1 sensitized NiO films display almost identical Mott-Schottky results. This suggests that the P1 dye is unable to prevent proton/water adsorption onto the NiO surface. By contrast, the BH4 sensitized NiO film (Figure 3c) shows very little to zero dependence of EFB on pH. Specifically, for the BH4 sensitized NiO film, the NiO EFB remains approximately a constant of 0.22 V vs NHE across all the pHs tested. Interestingly, the measured EFB intersects with bare NiO at pH 9.38 (see Figure 4), which is within the reported range of point of zero charge (PZC) values of NiO between 8.17 and 11.30 determined via various methods.46,47 The PZC describes the condition of equal chemisorbed proton and hydroxide ions. Therefore, the constant NiO EFB as a function of pH evidently indicates that the BH4 dye can prevent proton adsorption onto NiO and thus Helmholtz layer potential drop under various pH conditions. When BH4 molecules are grafted on the NiO surface through the carboxyic acid anchoring groups, the possible binding modes include monodente ester, bidentate chelating and bridging, monodentate and bidentate H-bonidng, as well as the monodentate mode through a C=O group.48,49 The ester type binding mode would release a proton that remains on the surface of the metal oxides. Therefore, the surface charge would be neutral so that the determined flat band resides at the PZC value.


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Figure 4. Determined flat band potential of bare NiO, P1 sensitized and BH4 sensitized NiO at various pH. The cross point indicates point of zero charge (PZC). Additional Validation. When designing equivalent circuits to fit the data, it is worth discussing how high-frequency semicircle might influence the capacitance at lower frequencies. To simplify the system, we could use an equivalent circuit as shown in Figure S3 (assuming ideal capacitor) to represent the high frequency semicircle. The complex impedance Z is derived in Eq. 4. The imaginary part is diminished as frequency is decreasing. Therefore, as long as one can carefully choose a low enough frequency, this equivalent circuit can be treated as a resistor. Then, if space charge capacitance is added to this circuit, one could again use a single frequency RC series circuit to build Mott-Schottky relations. 𝑍𝑍 = 𝑅𝑅𝑅𝑅 +

1

1 𝑅𝑅 + 𝑗𝑗 𝑤𝑤 𝐶𝐶

= 𝑅𝑅𝑅𝑅 +

𝑅𝑅 𝑤𝑤 𝐶𝐶 𝑅𝑅 2 − 𝑗𝑗 1 + (𝑤𝑤 𝐶𝐶 𝑅𝑅)2 1 + (𝑤𝑤 𝐶𝐶 𝑅𝑅)2

(𝐸𝐸𝐸𝐸. 4)

where, Z is the total impedance, j is imaginary unit, w is angular frequency, Rs is resistance in series circuit, R is the resistance in parallel loop, C is the capacitance in parallel loop.


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To examine how frequency variation affects the flat band determination on BH4 sensitized NiO electrode, we conduct the Mott-Schottky analysis using 12.4Hz and 8Hz as well (Figure S4 and Table S3). The results show that the determined flat band potentials with different frequency have no significant difference; the consistent flat band potentials with different pH solution are still observable.

Figure 5. Molecular dynamics simulations. (a) Visualization of the interface between the BH4grafted NiO surface and water; (b) Density profiles of water as a function of distance from the NiO surface for bare and BH4 sensitized NiO surfaces. Computational Modeling. To further elucidate the protective ability on BH4 sensitized NiO under aqueous condition, molecular dynamics (MD) simulations were employed to offer atomistic insights into the interfacial phenomena between the BH4-grafted NiO (100) surface and water. As the NiO surface was saturated by BH4 via adsorption in experiments, the studied NiO surface in


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the simulations was designed as a proof of concept to have a high surface loading (i.e., one BH4 molecule per 24 Ni sites) with a densely packed arrangement (Figure S7). Figure 5a shows a molecular-level visualization of the interface that water molecules indeed cannot penetrate through the protective layer formed by the BH4 molecules. To better quantify the interfacial behavior, the density profile of water molecules as a function of the distance perpendicular from the NiO surface was computed by projecting the coordinates of oxygen atoms of water molecules onto the direction. As a comparison, we have also carried out the same set of calculations to study the interface between the bare NiO (100) surface (i.e., no BH4 molecules) and bulk water. Figure 5b quantitatively show that, in contrast to the NiO surface without functionalization, BH4 molecules can indeed form a protective layer to protect the NiO surface from being immersed by water molecules. These results support the observed independence of EFB versus the pH of the electrolytes. CONLCUSION In conclusion, experiments and molecular simulations were performed to understand the aqueous dye sensitized semiconductor-electrolyte interface and to demonstrate that one could decouple the pH dependency by shutting off the surface protonation. Mott-Schottky measurements were used to determine the NiO flat band potential (EFB) as a function of pH for bare NiO and dye sensitized NiO films including a highly hydrophobic BH4 dye as well as a relatively simple and less hydrophobic P1 dye. The BH4 sensitized NiO film reveals no change in the EFB as a function of pH with a constant NiO EFB of 0.22 ± 0.01 V vs NHE across pH of 2 – 5, indicating that the BH4 dye creates a dense hydrophobic assemble to repel the aqueous phase. Such an interfacial phenomenon is also observed at an atomic level in our MD simulations. By contrast, the bare NiO and the P1 sensitized NiO films show an expected Nernstian shift in NiO EFB as a function of pH


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with a slope of 60.4 mV/pH and 61.3 mV/pH, respectively. The simple P1 dye displayed mostly bare NiO character and was insufficient to prevent proton absorption, suggesting that a suitable design of dye self-assembly layer is critically important to effectively control and tune the semiconductor-electrolyte interface.

ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website. Flat band potential fitting, Literature reported flat band potential of NiO, Mott-Schottky plots with various frequency, Surface pattern in MD simulation, Schematic band bending at semiconductor-electrolyte interface, Schematic capacity change at various biased conditions, Transmission of the sensitized film soaking in basic conditions.

AUTHOR INFORMATION Corresponding Authors *E-mail: (Y. W.) [email protected]. Fax: +1-614-292-1685. Tel.: +1-614-247-7810. (L.-C. L.) [email protected]. Tel.: +1-614-688-2622 Author Contributions # These authors contributed equally. Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT We acknowledge the funding support from the U.S. Department of Energy (Award No. DEFG02−07ER46427). The authors also thank Ohio Supercomputer Center (OSC)50 for computational resources. REFERENCES (1)

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Decoupling pH-Dependence of Flat-Band Potential in Aqueous Dye

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