Identifying Charge Transfer Mechanisms across Semiconductor

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Identifying Charge Transfer Mechanisms across Semiconductor Heterostructures via Surface Dipole Modulation and Multiscale Modeling Ryan T. Pekarek,†,⊥ Kara Kearney,‡,§,⊥ Benjamin M. Simon,† Elif Ertekin,‡,§ Angus A. Rockett,§,∥ and Michael J. Rose*,† J. Am. Chem. Soc. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/03/18. For personal use only.

†

Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States Department of Mechanical Science and Engineering, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, United States § International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan ∥ Department of Metallurgy and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401, United States ‡

S Supporting Information *

ABSTRACT: The design and fabrication of stable and efficient photoelectrochemical devices requires the use of multifunctional structures with complex heterojunctions composed of semiconducting, protecting, and catalytic layers. Understanding charge transport across such devices is challenging due to the interplay of bulk and interfacial properties. In this work, we analyze hole transfer across n-Si(111)−R|TiO2 photoanodes where −R is a series of mixed aryl/methyl monolayers containing an increasing number of methoxy units (mono, di, and tri). In the dimethoxy case, triethylene glycol units were also appended to substantially enhance the dipolar character of the surface. We find that hole transport is limited at the n-Si(111)−R|TiO2 interface and occurs by two processesthermionic emission and/or intraband tunnelingwhere the interplay between them is regulated by the interfacial molecular dipole. This was determined by characterizing the photoanode experimentally (X-ray photoelectron spectroscopy, voltammetry, impedance) with increasingly thick TiO2 films and complementing the characterization with a multiscale computational approach (first-principles density functional theory (DFT) and finite-element device modeling). The tested theoretical model that successfully distinguished thermionic emission and intraband tunneling was then used to predict the effect of solution potential on charge transport. This prediction was then experimentally validated using a series of nonaqueous redox couples (ferrocence derivatives spanning 800 mV). As a result, this work provides a fundamental understanding of charge transport across TiO2-protected electrodes, a widely used semiconductor passivation scheme, and demonstrates the predictive capability of the combined DFT/device-modeling approach. titanium(IV) isopropoxide.8 The researchers proposed that ALD-TiO2 films grown using TDMAT allow holes to transfer across the InP|TiO2 interface and into the film, leading to higher recombination rates (lower VOC) during HER. Hole transfer across the InP|TiO 2 interface is unexpected, considering the large valence band offset between the two materials; however, defects with energies inside the TiO2 band gap have been observed via photoelectron spectroscopy in films grown using TDMAT as a precursor.7 These defects, which are absent in TiO2 grown with other precursors, provide a current “leakage” pathway through the film that is diminished upon annealing.7 This “leaky dielectric” allows holes to pass to the contacting catalyst (Pt, Ir, Ni, etc.) and/or solution.7,10,11 In previous work we studied the charge transfer across nSi(111)−CH3|leaky-TiO2 photoanodes.12 We proposed that the oxidation current in such photoanodes is limited by hole

1. INTRODUCTION The passivation of semiconductors with thin metal oxide overlayers is ubiquitous throughout the literature due to broad applications in electronic devices,1 sensors,2 andas explored in this workphotoelectrochemical (PEC) cells.3 Regarding PEC cells, depositing thin metal-oxide films (2−100 nm) on a semiconductor photoelectrode enhances the functionality of the surface by preventing undesirable side reactions, decreasing the density of deleterious surface-states, and/or increasing the open-circuit photovoltage of the electrode.4−6 Notably, ultrathin amorphous TiO2 grown by atomic layer deposition (ALD) has been observed to protect the surface and enhance the efficiency of a variety of semiconductor photoelectrodes such as Si,6,7 GaP,7 GaAs,7 InP,8 and CaFe2O4.9 Interestingly, the precursor used in ALD-grown TiO2 affects the electronic properties of the film.8 For ALD-TiO2 on p-InP photocathodes, Lin et al. observed a more negative (less desirable) onset potential for hydrogen evolution reaction (HER) when TDMAT was used as a precursor compared to © XXXX American Chemical Society

Received: May 14, 2018

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DOI: 10.1021/jacs.8b05057 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

comparisons among samples, the redox couple concentration and scan rate were held constant in the experiments. For ease of analysis and comparison, the peak current was used in lieu of a rigorously determined kinetic rate constant to make relative comparisons among samples.12,25 Lastly, the experimental results were analyzed computationally using finite-element modeling (wxAMPS) to determine the properties controlling charge transfer by calculating the banddiagrams of the photoelectrode. Once the limitations to transport were determined, wxAMPS simulations were used to predict the effect of system adjustments on the mechanism controlling charge transport. The prediction was then validated by testing the adjustment experimentally. 2.2. Surface Dipole. The effect of the surface dipole induced by organic modification of the silicon surface was experimentally characterized by measuring the barrier height, Φb, via voltammetry and impedance of bare samples (without TiO2). Voltammetry was performed using liquid Hg as an electrolyte-free contact.26 The Φb of the Si is calculated from the extrapolated current at zero applied potential, J0.27 We define a positive surface dipole as a dipole moment in which the Si and the organic moiety have a partial positive and negative charge, respectivelye.g., Siδ+−Rδ−. For n-Si, a less positive surface dipole signifies a large Φb. As shown in Figure 1a, all monolayers produced plots with greater current under forward bias (plotted as positive potential) compared to reverse bias, indicating that all devices were rectifying. The triOMe substrate gave the highest Φb (0.72 ± 0.02), followed by monoOMe (0.707 ± 0.007), Me (0.69 ± 0.03), and diOMe (0.68 ± 0.02). The diPEG sample (0.61 ± 0.02) showed the least rectifying behavior with a small linear region, indicating a low Φb.27 As shown in Table 1, the diode quality factors, calculated from the slope of the linear region, were within the range previously observed for similar monolayers (n = 1−2).26 To increase confidence in the dipole characterization, we also performed solution-based impedance experiments in contact with dimethylferrocene (Me2Fc) (MeCN, 1 M LiClO4). The full details are discussed in the Supporting Information (Section S1). Briefly, the rank-order of the organic dipoles was confirmedwith the exception of diPEG, presumably due to extensive interactions between the extended PEG chains and lithium ions in the electrolyte. However, the “barrier heights” calculated from the Mott−Schottky analysis are larger than the silicon band gap, indicating that an inversion layer is present when in contact with Me2Fc, as has been observed previously with n-type silicon contacted by the same redox couple.28 We thus report these values strictly as the empirical flatband potentials (EFB)that is, the potential required to both remove the inversion layer and unbend the bands in the depletion layer. The Mott−Schottky plots and corresponding flatband potentials are displayed in the Supporting Information (Figure S1) To confirm this interpretation we simulated the energetics of the interface (Figure S2) and observed the hole carrier concentration overcoming the electron concentration ∼200 nm below the surface. To address the discrepancy for the diPEG surface between the Hg contact J−V measurement and the solution impedance measurements, as test cases we collected impedance data for the Me and diPEG monolayers using the Hg contact (Figure S3). The resulting Φb is larger for the Me monolayer than diPEG. We thus conclude that the solution results for the

transfer at the Si|TiO2 interface, which occurs by hole emission from the Si valence band into the defect band present in leaky TiO2. Herein, we insert various aryl derivatives into the −CH3 monolayer and perform a mechanistic study of the charge transfer across leaky n-Si(111)−R|TiO2. We capitalize on the interfacial dipole induced by the organic monolayer (−R), which provides a unique tunability of the underlying semiconductor interfacial energetics.13−16 A clear understanding of the carrier pathway through this semiconductor|organic|inorganic heterostructure is challenging due to a variety of complicated processes occurring in and between each layer.17 We characterize the hole transport dynamics via a combined experimental (X-ray photoelectron spectroscopy (XPS), voltammetry) and computational (firstprinciples, finite-element) approach. Both first-principles methods18,19 and finite-element device models9,12,15,20 have been individually employed to unravel the complex electronic interplay of modern photoelectrode architectures,21 but here we combine the two methods and conduct a multiscale analysis at both the molecular and device levels.22 We report that the interfacial dipole induced by the organic monolayer has a significant impact on the hole transfer across the Si|TiO2 junction, which occurs via two mechanismsthermionic emission and/or intraband tunneling.23 The analysis reveals that for small/large dipoles the transport mechanism is dominated by tunneling/thermionic emission, respectively, and an intermediate regime exists where neither mechanism is effective and transport is limited. We then validated the model by predicting the effect of varying the solution potential, followed by experimental confirmation using a series of nonaqueous redox couples.

2. RESULTS AND DISCUSSION 2.1. Experimental and Computational Approach. In this study, we formed mixed methyl/aryl monolayers using a series of methoxy-appended phenyl groups (mono, di, and triOMe, Scheme 1) and characterize the charge transfer across Scheme 1. Molecular Monolayers Studied in This Work

n-Si−R|TiO2 photoanodes as a function of TiO2 thickness. Additionally, the methoxy units in diOMe were substituted with triethylene glycol unitstermed “diPEG”to amplify the effects of the methoxy group. We characterized the hole transfer mechanism using a three-step approach. First, the organic monolayers were attached to silicon using synthetic surface chemistry. Next, the surface dipole was characterized experimentally using voltammetry and impedance followed by a calculation of the dipole using first-principles DFT. Then, the charge transport across the n-Si−R|TiO2 photoanodes was characterized by measuring the peak oxidation current as a function of TiO2 thickness using cyclic voltammetry (CV). The CV peak current is a function of electron transfer kinetics, redox couple (concentration and diffusion coefficient), and scan rate.24 In order to make B

DOI: 10.1021/jacs.8b05057 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 1. Surface dipole determination. (a) Voltammograms of n-Si−R|Hg contacts with forward bias plotted as positive potential. The rectifying region is plotted in the main figure, and the inset displays the complete sweep. (b) Barrier heights obtained from the Hg-drop voltammetry and solution impedance data plotted against the DFT-calculated surface dipoles. Note that the Φb values for the impedance measurement are plotted as positive to match the voltammetric results. (c) Effects of geometry on the z-component of the total surface dipole of the indicated organic moiety, as qualitatively illustrated by the resultant arrows.

was taken as an average of the two cases (3.06 D/site). All geometries studied in this work are displayed in Figure S4. The results of the DFT dipole calculations (Scheme 1 and Figure 1c) reveal that diPEG has the most positive dipole followed by diOMe, Me, monoOMe, and triOMe. Notably, the trends in experimental Φb and DFT-predicted dipole are in excellent agreement as shown in Figure 1b, validating the predictive power of DFT in calculating the trends in the surface dipole of functionalized surfaces. The ordering of the surface dipole for mono-OMe, diOMe, and triOMe is counterintuitive to the expectation that the dipole would monotonically increase with the number of methoxy substituents or according to a classical Hammet plot in organic chemistry. However, as shown in Figure 1c, depending on the angle of the methoxy substituent, the perpendicular component of the dipole will either add or subtract, which increases or decreases the total dipole, respectively. Previous work has demonstrated that, while trends between the DFT-computed dipole and the experimentally measured barrier height are in agreement, the absolute magnitudes are not.19,22 This is a consequence of systematic errors in both the computational results and experimental measurements. However, if the shift in the surface dipole (barrier height) with respect to a well-defined surface such as Si-CH3 is calculated, a more accurate agreement in the absolute magnitudes are obtained, as this approach tends to cancel out systematic errors. We include such an analysis for our surfaces in the Supporting Information (see SI, section S3), and good agreement is observed between DFT and experiment for all surfaces except diPEG. Table S1 (Supporting Information) details the comparative analysis of the DFT-calculated and experimental values. The surface dipole for diPEG is significantly overestimated by DFT. This overestimation is a consequence of approximations required in the DFT computations due to constraints in computational power (described in the SI) and electrolyte screening of diPEG. 2.4. Dipole Dependent Charge Transfer Distance. To probe the influence of the organic monolayer on the charge transfer across the Si−R|TiO2 heterojunction, we measured the current across the photoelectrode as a function of TiO2 thickness. In order to quantify the thickness of TiO2 grown on different Si−R substrates, X-ray photoelectron spectroscopy (XPS) was used to estimate the titania thickness for each monolayer (see SI, section S4).30 Table S2 (Supporting

Table 1. Monolayer Characterization Data voltammetry monolayer

DFT dipole (Debye/site)

T50% (Å)

Φb (V)

n

triOMe monoOMe methyl diOMe diPEG

2.42 2.49 2.60 2.63 3.08

23.0 20.1 14.3 17.4 25.4

0.72 ± 0.02 0.707 ± 0.007 0.69 ± 0.03 0.68 ± 0.02 0.61 ± 0.02

1.5 ± 0.1 1.96 ± 0.02 1.6 ± 0.2 1.3 ± 0.2 1.6 ± 0.3

diPEG surface are a result of the substrate−electrolyte interactions and not the intrinsic dipole. As the heterojunctions studied in this work are buried Si−R|TiO2 interfaces, the Hg contact experiments and in vacuo DFT calculations are more appropriate methods for predicting the functional (buried) dipole of the Si−diPEG|TiO2 sample. 2.3. Computational Insight into the Surface Dipole. To quantify the magnitude of the buried molecular dipoles computationally, the surface dipoles of Si(111)−R were calculated using DFT with 25% aryl coverage.18,22 As the molecular geometry and dipole are intimately related, care was taken to ensure the appropriate structures were used in the dipole analysis. The starting molecular geometries considered for the organic moieties were the known thermodynamically stable structures unbound to a surface taken from the pubCHEM chemical database.29 In the case of Me and monoOMe, there was only one relevant geometry to the calculated dipole (2.60 and 2.45 D/site, respectively). Alternatively, methoxy moieties meta to the silicon binding site could be oriented either toward or away from the Si surface. In the case of triOMe, the free energy for the methoxy substituents oriented toward and away from the Si surface were equivalent (−746 eV) so the total dipole was taken as an average of the two geometries (2.22 D/site). The methoxy groups of the diOMe molecule significantly preferred to orient toward the Si surface as the free energy is significantly lower compared to the upward-oriented geometry (−701 and −607 eV, respectively). Therefore, the downward-oriented geometry was used to calculate the dipole of the diOMe monolayer (2.71 D/site). In the case of PEG, two geometries were consideredethylene glycol chains oriented parallel to and approximately 45° from the Si surface. The free energies of the two geometries were equivalent (−2732 eV) so the total dipole C

DOI: 10.1021/jacs.8b05057 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 2. Thickness-dependent characterization. (a) Representative X-ray photoelectron spectra of Si 2p and Ti 2p (inset) features of increasingly thick TiO2 on Si(111)−diOMe|TiO2. (b) Representative cyclic voltammograms of Si(111)−diOMe|TiO2 electrodes with an increasingly thick TiO2 film. (c) Thickness dependence plots for Si(111)−R|TiO2 samples where R = Me (black downward triangles, data from previous work12), diOMe (orange diamonds), monoOMe (red upward triangles), triOMe (green squares), and diPEG (blue circles). The y-axis is normalized to the peak current for each electrode without TiO2.

Figure 3. Charge transport mechanism. (a) Simulated band diagram under 1 sun illumination for Si−R|TiO2 electrodes with Si electron affinities (χSi) set to 3.8 (red), 4.0 (orange), and 4.2 eV (green). The TiO2 conduction and defect bands are shown on the left while the silicon conduction (ECB) and valence (EVB) bands are shown on the right. The barrier to thermionic emission is denoted as ΦTE, and the hole transport mechanism is marked as follows: (1) excitation, (2) separation, (3) thermionic emission, (4) intraband tunneling, and (5) drift/diffusion. Note the difference of scale in the left and right portions of each plot to simultaneously highlight both the field in the TiO2 (left, ∼1 nm) and Si depletion region (right, ∼500 nm). (b) Simulated electric field in the TiO2 as a function of electron affinity. (c) Simulated ΦTE as a function of χSi. (d) Experimental T50% (Figure 2) plotted against the experimental J−V barrier height (Hg contact, Figure 1).

D

DOI: 10.1021/jacs.8b05057 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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the TiO2 and Si depletion layer. As shown in Figure 3a, as χSi decreases, the field across the interface increases, resulting in a stronger field in the TiO2 and increased band bending in the Si. We also find that the field increases in the light and decreases when positive bias is applied to the photoelectrode (see the Supporting Information, Figures S6 and S7). The presence of a strong field in such a thin layer of TiO2 results in nearly linear bands and a triangular potential barrier. Furthermore, a significant energy offset exists between the Si valence band and TiO2 defect band. We label this energy difference as the barrier to thermionic emission (ΦTE), and as χSi increases, ΦTE decreases. Figure 3b and c display the TiO2 field strength and ΦTE as a function of χSi. Under illumination, an electron−hole pair is generated and separated in the Si (steps 1 and 2 in Figure 3a). The holes migrate to the valence band edges of the Si|TiO2 interface where a triangular barrier exists due to the electric field in the TiO2. As the holes are thermally excited (downward in Figure 3a), there are two significant pathways to traverse the barrier: (3) thermionic emission27 and (4) intraband tunneling.23 In the case of thermionic emission, if a hole has enough thermal energy to overcome the potential barrier, it may emit into the TiO2 defect band. In the case of tunneling, while a hole does not have sufficient energy to undergo thermionic emission, due to the triangular shape of the barrier a “shortcut” exists where a hole may be partially excited and tunnel through the barrier into the defect band. Once the barrier has been navigated, the holes then move through the TiO2 via the defect band to oxidize the ferrocene in solution, step 5 in Figure 3a, driven by field-assisted drift, and diffusion. As shown in Figure 3d, a “reverse-volcano” relationship is observed experimentally between the J−V barrier height Φb (and therefore electron affinity) and T50% (Figure 2c). More holes are transferred at both very high and very low Φb valuesrepresenting two discrete charge transport regimes. When Φb is large (Φb > 0.68 V) T50% correlates directly with Φb, but when Φb is small (Φb < 0.68 V) the opposite trend exists. We assign these regimes to the two hole transport pathways. Notably, the thickness-dependence decay in the current versus thickness plot (Figure 2c) is gradual when Φb is small and significantly sharper when Φb is large. When Φb is small (high electron affinities), ΦTE is small enough for a significant concentration of holes to cross the barrier by thermionic emission. In this regime, the current is limited by diffusion and thus does not require the aid of a strong field, resulting in a gradual current-thickness decay profile. Conversely, when Φb is large (low electron affinities), ΦTE is too large for a significant amount of thermionic emission to occur, but a significant concentration of holes can cross the barrier via the fieldenhanced intraband tunneling. In this regime, the ability of holes able to tunnel has a strong exponential dependence on the field (discussed in detail below), resulting in a sharp current-thickness decay profile. At intermediate values of Φb, the TiO2 field is weak and the ΦTE is large, so neither thermionic emission nor intraband tunneling are favored and the T50% is short. 2.6. Model Prediction and Validation. The theoretical model described above suggests an interplay between thermionic emission and intraband tunneling to understand the carrier energetics of Si−R|TiO2 heterojunctions. When in contact with a ferrocene (Fc) solution, the dipole (or χSi) of the organic monolayer determines which mechanism is favored

Information) summarizes the XPS-characterized growth behavior of each monolyer. Figure 2a depicts the spectra of increasingly thick TiO2 where the Ti 2p and Si 2p peak areas increase and decrease as the Si is buried under the titania. We also acquired SEM images of select films (methyl, monoOMe, diPEG) to highlight the similarities of morphology across the films (see the Supporting Information, Figure S5). We observe a conformal film, as has been grown on previous organic monolayers on Si(111).12,13,31 Cyclic voltammograms were collected where the Si−R|TiO2 photoelectrode served as the working electrode in contact with 2 mM ferrocene in MeCN to quantitate the extent of charge transfer in each sample. Figure 2b depicts a typical decrease in oxidation peak current and corresponding elongated CV traces as the TiO2 thickness is increased, which indicates diminished charge transfer kinetics.12,25,32,33 The peak currents for the oxidation half-reaction were normalized to the peak current of the bare substrate for each organic monolayer (no TiO2) and plotted against TiO2 thickness to visualize “thickness dependence plots” as shown in Figure 2c. We highlight (dashed black line) the thickness where the current has decayed to 50% of its original value (T50%) and use this parameter to compare the charge transfer behavior between organic monolayers. Electrodes containing only the methyl moiety (reproduced from previous work12) decay most rapidly (T50% = 14 Å), while the introduction of phenyl moieties into the monolayer results in decay at greater thicknesses. All of the methoxy-substituted substrates pass substantial current in the 5−15 Å range and decay in the order of diOMe (T50% = 17 Å) < monoOMe (20 Å) < and triOMe (23 Å). A considerable enhancement is observed for diPEG in that the photoelectrode passes measurable current at thicknesses beyond 40 Å and exhibits a T50% of 25 Åa notable enhancement compared to the original methyl substrate. Indeed, the only comparable T50% that has been observed is in similar electrodes functionalized with platinum nanoparticles to enhance charge transfer.12 In addition to changes in T50%, we also observe a change in the “decay” shape between monolayers. Specifically, diOMe and diPEG exhibit a gradual decay past T50% compared to monoOMe and triOMe which exhibit a sharp decay (there is no extended current tail at higher TiO2 thicknesses). 2.5. Computational Insight into Charge Transfer Distance. To computationally analyze the effect of the surface dipole on charge transport through the TiO2, we used wxAMPS to calculate the band diagrams of the photoelectrodes under 1 sun illumination (Figure 3a). Both illuminated and dark band diagrams are given in the Supporting Information (Figure S6).34 While the molecular monolayer is not spatially represented, the dipole induced by the surface-bound molecules is modeled by adjusting the electron affinity of the Si layer (χSi = the difference between the vacuum and conduction band energy), which modulates the Si band edges.15 Previous work suggests that defects in the bandgap of TiO2 mediate hole transfer through the film.7,33 These defects are inherent to the ALD-grown TiO2 (using TDMAT) and have energies ∼0.7 eV below the Fc/Fc+ redox energy.7 This forms the defect band shown in the band diagram, which is modeled by decreasing the band gap energy of the TiO2 layer and effectively raising the TiO2 valence band.12 The difference in energies between the uncontacted Si Fermi energy and the ferrocene redox potential generates an electric field (potential gradient) across the interface, which falls across E

DOI: 10.1021/jacs.8b05057 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 4. Redox potential modulation. (a) Simulated band diagram under illumination for Si−R|TiO2 electrodes (χSi = 4.2) for various contacting potentials (redox couples): −0.5 (light blue), −0.1 (dark blue, Figure 3), 0.0 (green), and 0.25 eV (gray). Note the difference of scale in the left and right portions of each plot to highlight both the field in the TiO2 (left, ∼1 nm) and Si depletion region (right, 750 nm) simultaneously. (b) Simulated barrier to thermionic emission as a function of redox potential. (c) Simulated TiO2 electric field (red dots) and tunneling coefficient (blue squares) as a function of redox potential. (d) Experimental peak current values for Si−diOMe|TiO2 with three TiO2 thicknesses (red 9.8 Å, green 14.0 Å, and blue 18.2 Å) in contact with decamethylferrocene (Me10Fc), dimethylferrocene (Me2Fc), ferrocene (Fc), or acetylferrocene (AcFc).

as it sets ΦTE and the strength of the field in the TiO2. As discussed above, when the Si electron affinity (as modified by a surface dipole) is large (χSi > 4.0 eV), the TiO2 electric field is minimized, which limits intraband tunneling at the Si|TiO2 interface. Therefore, to improve charge transport via increased intraband tunneling, the electric field can be recovered by modulating the solution potential vis-a-vis changing the redox couple. The band diagrams were simulated for n-Si−R|TiO2 (χSi = 4.2 eV) contacted by redox couples with a potential range of −0.5 to 0.3 V vs the ferrocene reduction potential. Figure 4a depicts the simulated band diagrams of such devices. Figures 4b shows the ΦTE as a function of redox potential. As opposed to Figure 3c where χSi was varied, ΦTE does not change with redox potential. Figure 4c depicts the simulated TiO2 electric field strength (red circles) and tunneling coefficient (blue squares) as a function of the redox potential, where the tunneling coefficient is a unitless parameter proportional to the magnitude of the tunneling current. The Supporting Information section S7 details the wxAMPS simulation information, including the tunneling coefficient. All variables and parameters used in the simulations are defined in Table S3, and Figure S8 is a pictorial representation of the simulation. For redox potentials positive of −0.2 V, the electric field drops across the TiO2 and has a linear dependence with redox potential when a significant amount of field is present (ERedox > 0.0 V vs Fc). In the regime where EField ∝ ERedox, the tunneling coefficient increases exponentially with redox potential. This suggests that for an increased contacting redox potential, the magnitude of intraband tunneling (and correspondingly, the total current) should increase exponentially through the photoelectrode.

We explored this prediction experimentally by studying the effect of varying the redox couple in solution and measuring the current at three thicknesses (9.8, 14.0, and 18.2 Å) for a single substrate, diOMe, where negligible intraband tunneling is expected in contact with ferrocene. Acetylferrocene (AcFc, 0.26 V vs Fc) was selected to increase the solution potential, while dimethylferrocene (Me2Fc, −0.11 V vs Fc) and decamethylferrocene (Me10Fc, −0.50 V vs Fc) were selected to decrease the solution potential. Together, this series of redox couples represents a range of nearly 800 mV. (Note: the most positive data point for 18.2 Å could not be collected as the peak was outside the potential range where Si is oxidatively stable). As shown in Figure 4d, for each thickness, the current increases (see Supporting Information, section S8) with the redox potential of the solution. Exponential fits of the data are shown in Figure S9, and the fit details are given in Table S4. Thus, the experimental current−thickness relationships corroborate the interplay between thermionic emission and intraband tunneling as predicted by wxAMPS, validating the device-scale modeling approach.

3. CONCLUSION A multiscale device model combining first-principles and finiteelement modeling is demonstrated to be a predictive tool for engineering charge transport across a multicomponent photoelectrochemical system. With this approach, we have elucidated the complex hole transfer behavior across a semiconductor|organic|inorganic structure. Specifically, we analyzed the charge-transfer dependence on TiO2 thickness in Si(111)−R|TiO2 photoelectrodes. We find that the surface dipole induced by the organic monolayer (−R) has a profound effect on the charge transfer through the construct. We conclude that two distinct charge transfer mechanisms occur as F

DOI: 10.1021/jacs.8b05057 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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4.2. Organic Monolayer Preparation. All monolayer precursors were purchased with the exception of Br-diPEG, which was synthesized as described below: 5-Bromobenzene-1,3-diol. The 1-bromo-3,5-dimethoxy-benzene (1 equiv, 26.5 mmol, 5.00 g) was dried under vacuum overnight and dissolved in 40 mL of dry DCM. The solution was cooled to −78 °C and the BBr3 (2.2 equiv, 58.2 mmol, 5.61 mL) was added dropwise. After stirring cold for 30 min, the solution was allowed to warm to room temperature and stirred for 5 days. The reaction was then cooled to 0 °C and quenched with 40 mL of a 1:10 ethyl-acetate/H2O solution. The mixture was extracted with 3 × 15 mL of DCM and the combined organics were washed with brine and dried over Na2SO4. Solvent was removed via rotary evaporator and the remaining oil was left to dry under vacuum overnight to give a tan solid. Yield: 2.7 g (54%). 1H NMR (d6-dmso): δ 9.68 (s 2H br), 6.35 (d 2H), 6.16 (t 1H). MS (CI+): m/z calculated for C6H5BrO2 (M), 187.9473; observed, 187.9467. 2-(2-(2-Methoxyethoxy)ethoxy)ethyl-4-methylbenzenesulfonate. Tosyl chloride (1.09 equiv, 37.5 mmol, 7.15 g) in 15 mL of dry DCM was added dropwise to a solution of triethylene glycol monomethyl ether (1 equiv, 36.0 mmol, 5.76 mL) and triethylamine (2 equiv, 72.0 mmol, 10.0 mL) in a minimum amount of dry DCM at 0 °C under N2 atmosphere. The solution was allowed to warm to room temperature and stirred for 4 days. The reaction mixture was poured into 30 mL of deionized water and extracted with 3 × 20 mL of DCM. The combined organics were washed with 45 mL each of 6 M HCl, 5% NaHCO3, and deionized water. The organic layer was then washed with 20 mL of brine and dried over Na2SO4. Solvent was removed via rotary evaporator, and the product used without further purification. Yield: 8.20 g (72.3%). Spectroscopic data were consistent with the literature values.35 1-Bromo-3,5-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene (diPEG). The reagents synthesized above, namely 2-(2-(2-methoxyethoxy)ethoxy)ethyl-4-methylbenzenesulfonate (2 equiv, 21.1 mmol, 6.36 g) and 5-bromobenzene-1,3-diol (1 equiv, 10.5 mmol, 1.99 g), were combined in 25 mL of dry DMF. Pulverized K2CO3 (3.25 equiv, 34.1 mmol, 1.44 g) was added and the reaction mixture was stirred under N2 for 48 h at 50 °C. The mixture was then cooled and poured into 30 mL of H2O. The mixture was extracted with 3 × 15 mL of DCM, then 10% NaOH, and finally dried over Na2SO4. After solvent removal via rotary evaporator, the resulting colorless oil was purified via column chromatography with an ethyl acetate eluent. Yield: 2.1 g (42%). Spectroscopic characterization was consistent with literature data.35 Photoelectrode Preparation. As described in detail in previous work,13 organic monolayers were prepared on wafers etched in HF and NH4F. The hydride-terminated wafer was then chlorinated, followed by exposure to the lithiated molecular precursor. Remaining Si−Cl sites were passivated via treatment with methyl Grignard. Amorphous titania was grown at 150 °C on the substrate following a known procedure using tetrakis(dimethylamido)titanium(IV) (TDMAT) as the precursor.12 4.3. Electrochemistry. The use of cyclic voltammograms to produce thickness dependence plots has been described previously.12,25 Briefly, an Ohmic back contact was made to the electrode via Ga−In eutectic and copper tape. The front solution contact was made via a custom Teflon cell and 0.11 cm2 O-ring. A three-electrode cell was constructed with the Si wafer as the working electrode via the copper tape, a Ag wire for a quasi-reference electrode, and a Pt wire counter electrode. The completed cell was filled with a solution of 2 mM redox couple (acetylferrocene, ferrocene, dimethylferrocene, or decamethylferrocene) and 0.3 M LiClO4. A cell where both the working and counter electrodes were platinum was used to reference each set of voltammograms (0 to 0.6 V vs Ag at 50 mV s−1). Samples were illuminated using a 150 W Xe lamp (Newport, Co., USA) equipped with an AM-1.5G solar filter (model no. 81094, Newport, Co., USA) operating at a distance corresponding to ∼100 mW cm−2. Mercury drop voltammograms were performed without the Teflon piece where the O-ring was filled with ∼0.05−0.1 mL electronic grade mercury and contacted by a Pt wire attached to both the counter and

defined by (i) the band-bending in the Si and (ii) the electric field in the TiO2, both of which are controlled by the molecular dipole: • Thermionic emission occurs when there is minimal bandbending in the Si and negligible electric fields in the TiO2 (low Φb and ΦTE values, more positive dipole). • Intraband tunneling occurs when there is large bandbending in the Si and therefore a strong electric field in the TiO2 (high Φb and ΦTE values, less positive dipole). These competing mechanisms result in a reverse-volcano relationship whereby intermediate dipoles give the shortest characteristic charge transfer distance (T50%). In contrast, greater charge transfer occurs at either very low or very high buried dipoles. The competing charge transfer mechanisms are apparent in the current-thickness decay patterns: at high electron affinity (more positive dipole) the thermionicemission-dominated regime is indicated by gradual decay, whereas at low electron affinity (less positive dipole) the electric-field-dominated regime is indicated by a sharp decay. To validate the proposed charge transfer mechanism, we demonstrated that the device modeling (wxAMPS) successfully predicted that modulating the redox couple potential at exposed TiO2 surfaces alters the charge transport through the photoanode interface. Specifically, an increased potential of the redox couple enhances the electric field in the TiO2, thus resulting in increased intraband tunneling and therefore current. This result validates the predictive power of integrated multiscale device modeling (first-principles plus finite-element modeling) with experimental validation, in addition to suggesting that the tunable molecular design of ultrathin devices has a profound effect on the charge-transfer behavior in the device.

4. EXPERIMENTAL/COMPUTATIONAL METHODS 4.1. Materials and Reagents. Czochralski-grown n-Si(111) wafers were purchased from Virginia Semiconductor and were single-side polished and phosphorus-doped for a resistivity between 0.5 and 2 Ω·cm. Ethanol and acetone were used as purchased from Pharmco-AAPER and Fisher Scientific, respectively. Ultrapure water was generated on site and had a resistivity of 18 MΩ·cm or higher (Barnstead Nanopure Systems). The piranha solution was a 1:3 ratio of 30% hydrogen peroxide and concentrated sulfuric acid purchased from Fisher Scientific. Semiconductor grade aqueous hydrofluoric acid and 11 M ammonium fluoride were purchased from Transene Company, Inc. The chlorination solution was made from dry chlorobenzene (Sigma-Aldrich, 99.8%) and saturated with phosphorus pentachloride (Alfa Aesar, 99.998% metal basis) and benzoyl peroxide (Sigma-Aldrich). Synthetic reagents were purchased and used without further purification: 1-bromo-3,5-dimethoxybenzene from Ark Pharmaceuticals, Inc.; 4-bromoanisole and 5-bromo-1,2,3trimethoxybenzene from Alfa Aesar; triethylene glycol from Acros Organics, triethylamine from Fisher Scientific, tosyl chloride from Sigma-Aldrich; sodium bicarbonate from Fisher Scientific, sodium hydroxide from Fisher Scientific, sodium sulfate from Fisher Scientific, n butyl lithium solution (1.6 M in hexanes) from Sigma-Aldrich, and 3 M solution of methylmagneisum chloride from Acros Organics (diluted to 1 M with dry THF). Dry solvents were HPLC-grade and purified over alumina in a Pure Process Technology solvent purification system. Solvents not dried over alumina were used as purchased from Fisher. Tetrakis(dimethylamido)titanium(IV) (TDMAT) was purchased from Sigma-Aldrich. Electronic grade mercury was purchased from Alfa Aesar. Pt-wire (99.95%) and Agwire electrodes were purchased from Strem and CH Instruments. G

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Journal of the American Chemical Society reference leads (−0.5 to 0.5 V at 20 mV/s). A CH Instruments 660A series potentiostat and Interface 1000 potentiostat (Gamry Instruments) were used to collect cyclic voltammograms and Hg-contacted linear sweep voltammograms, respectively. Information regarding the barrier height calculation is given in the Supporting Information (section S9). Mott−Schottky plots were prepared by collecting capacitance data at a range of potentials contacted either by Hg-drop or solution. An Interface 1000 potentiostat (Gamry Instruments) was used to obtain electrochemical impedance spectra (EIS). Impedance data were collected by scanning from 10−1 to 106 Hz in the dark between 0 and 0.8 V vs Me2Fc, stepping every 50 mV. Dimethylferrocene was used in order to optimally highlight the differences among the monolayers. The MeCN solution contained 50 mM Me2Fc, 2 mM [Me2Fc]BF4, and 1 M LiClO4, where the oxidized counterpart was synthesized by a previously reported procedure.36 Mott−Schottky analyses were performed using a custom LabView program supplied by Dr. Ron Grimm.16 Specifically, the program fit a Randle’s circuit model to the data to determine the capacitance and resistance for the sample at each potential. It should be noted that our data were not frequency dispersive, and therefore care was taken to interpret the data accurately. The frequency selected for the analysis of each sample corresponded to the most negative peak in the phase angle plot, typically ∼105 Hz. The inverse C2 was plotted versus the potential and the linear portion of the curve was fit to obtain the slope and intercept. The ND was calculated from the slope using the equations provided in the Supporting Information (section S9). 4.4. X-ray Photoelectron Spectroscopy. Spectra were measured using a Kratos Axis Ultra XPS equipped with an Al−Kα X-ray source monochromated to 1486.5 eV. The photoelectron takeoff angle was 0°, and the pressure in the acquisition chamber was on the order of 10−9 Torr for all samples analyzed. All quantification was performed in CasaXPS software using the Kratos Relative Sensitivity Factor (RSF) library. Survey scans were obtained under the following conditions: pass energy of 80, 1.000 eV step size, and 300 ms dwell time. Region scans were obtained under the following conditions: pass energy of 20, 0.100 eV step size, and 2000 ms dwell time. Regions were quantified using a Shirley background type. The silicon peak was divided into one component for silicon oxide (if present) and two components corresponding to Si0, 2p1/2, and 2p3/2. Further details regarding TiO2 thickness determination are given in the Supporting Information (section S4). 4.5. Density Functional Theory. The electronic structure of Si(111) functionalized with Me, diOMe, monoOMe, triOMe, and diPEG moieties was studied using density functional theory (DFT)37,38 with the plane-wave code VASP39−42 using the Perdew−Burke−Ernzerhof43 exchange-correlation functional and PAW44 pseudopotentials. For Si-Me, a 2 × 4 × 8 Si(111) supercell with 100% methyl coverage was used. For Si-diOMe, Si-monoOMe, and Si-triOMe, a 2 × 4 × 8 Si(111) supercell at 25% aryl coverage and 75% methyl coverage was used. In all supercells, both surfaces of the slab were identically terminated to avoid formation of a net dipole across the slab and a 12 Å thick vacuum layer was used to avoid interactions between the two surfaces. We assume little variation in aryl coverage between mono-, di-, and triOMe monolayers as these phenyl derivatives are similar in size. As mixed aryl/monolayers typically have coverage values between 1 and 50%,45,46 a 25% coverage was arbitrarily chosen. Although the absolute value of the computed surface dipole increases with coverage, we expect relative ordering of the dipole magnitude to remain unchanged.18,45 A plane-wave cutoff of 480 eV, a Gaussian smearing of 0.2 eV, and a k-space sampling of 8 × 2 × 1 were used. All structures were relaxed using a convergence criterion of 0.02 eV/Å for forces on each atom and 10−5 eV for the energy differences between subsequent steps. For Si-diPEG, the convergence criteria was relaxed due the excessively large size of the PEG moiety. A 2 × 24 × 8 Si(111) supercell was used with 4% PEG coverage, which was the computational maximum coverage considering the size and geometry of the molecule. The remaining atop sites were left bare. A plane-wave cutoff of 480 eV, a Gaussian smearing of 0.2 eV, and a k-space sampling of 8 × 1 × 1

were used. The structure was relaxed using a convergence criterion of 0.05 eV/Å for forces on each atom and 1 × 10−5 eV for the energy difference between subsequent steps. The surface dipole was calculated using a previously established and verified method called nanosmoothing.18 In this work the value of the surface dipole is normalized to the number of atop Si sites and reported in units of Debye/site. The geometries studied in this work are shown in the Supporting Information (Figure S4). Tutorials of the approach are available online.48 4.6. wxAMPS Simulations. wxAMPS34 is a finite-element modeling software used to calculate band-diagrams and simulate charge transfer across solid-state devices. A model for adapting wxAMPS to semiconductor|liquid15 junctions has been previously developed and used in a variety of applications. In this work, we have utilized wxAMPS to calculate the band diagrams for Si(111)−R|TiO2 photoelectrodes at thermal equilibrium. The electronic parameters used as inputs for bulk silicon were those tabulated by the Ioffe Institute:47 permittivity = 11.7, energy gap (Eg) = 1.12 eV, electron affinity (χ) = variable, conduction band density of states (Nc) = 3.2 × 1019 cm−3, valence band density of states (Nv) = 1.8 × 1019 cm−3, electron mobility (μn) = 1400 cm2 V−1 s−1, hole mobility (μp) = 450 cm2 V−1 s−1, and a donor concentration (Nd) = 3.2 × 1015 cm−3. The electronic parameters used as inputs for the TiO2 layer are based from previous work:12 permittivity = 55, Eg = 1.7 eV, χ = 4.3 eV, Nc = 1 × 1020 cm−3, Nv = 1 × 1020 cm−3, μn = 0.001 cm2 V−1 s−1, μp = 1 cm2 V−1 s−1, and Nd = 1 × 1017 cm−3. The parameters for the top contact were set as potential (Ec − EF) = variable, electron recombination velocity (Sn) = 1 × 107 cm/s, hole recombination velocity (Sp) = 1 × 107 cm/s, and percent reflection (RF) = 0. The parameters for the bottom contact were set as potential (Ec − EF) = 0.24 eV, electron recombination velocity (Sn) = 1 × 107 cm/s, hole recombination velocity (Sp) = 1 × 107 cm/s, and percent reflection (RF) = 1. Tutorials of the approach are available online.48

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05057. Dipole characterization data, molecular geometries, quantitative comparison of DFT and experimental results, TiO2 growth data, SEM characterization of select films, additional simulations, thermionic-field emission, exponential fit of redox potential data, and barrier height determination (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Kara Kearney: 0000-0003-1124-3369 Elif Ertekin: 0000-0002-7816-1803 Michael J. Rose: 0000-0002-6960-6639 Author Contributions ⊥

R.T.P. and K.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Ron Grimm for technical assistance and the Labview software used in the Mott−Schottky analysis of impedance data. We also acknowledge Dr. Karalee Jarvis, Dr. Raluca Gearba, and Jon Bender for assistance with imaging studies. The experimental research at UT Austin (MJR, RTP, BMS) was supported by the US Office of Naval Research H

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(14) Garner, L. E.; Steirer, K. X.; Young, J. L.; Anderson, N. C.; Miller, E. M.; Tinkham, J. S.; Deutsch, T. G.; Sellinger, A.; Turner, J. A.; Neale, N. R. Covalent Surface Modification of Gallium Arsenide Photocathodes for Water Splitting in Highly Acidic Electrolyte. ChemSusChem 2017, 10 (4), 767−773. (15) Kearney, K. L.; Rockett, A. A. Simulation of Charge Transport and Recombination across Functionalized Si(111) Photoelectrodes. J. Electrochem. Soc. 2016, 163 (7), H598−H604. (16) Grimm, R. L.; Bierman, M. J.; O’Leary, L. E.; Strandwitz, N. C.; Brunschwig, B. S.; Lewis, N. S. Comparison of the Photoelectrochemical Behavior of H-Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, OuterSphere Redox Couples in CH3CN. J. Phys. Chem. C 2012, 116 (44), 23569−23576. (17) Pham, T. A.; Ping, Y.; Galli, G. Modelling Heterogeneous Interfaces for Solar Water Splitting. Nat. Mater. 2017, 16, 401−408. (18) Kearney, K.; Iyer, A.; Rockett, A.; Staykov, A.; Ertekin, E. Effect of Surface Coverage and Composition on the Stability and Interfacial Dipole of Functionalized Silicon. J. Phys. Chem. C 2017, 121 (21), 11312−11318. (19) Li, Y.; O’Leary, L. E.; Lewis, N. S.; Galli, G. Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization. J. Phys. Chem. C 2013, 117 (10), 5188−5194. (20) Chitambar, M.; Wang, Z.; Liu, Y.; Rockett, A.; Maldonado, S. Dye-Sensitized Photocathodes: Efficient Light-Stimulated Hole Injection into p-GaP Under Depletion Conditions. J. Am. Chem. Soc. 2012, 134 (25), 10670−10681. (21) Kearney, K.; Rockett, A.; Ertekin, E. Computational Insights into Charge Transfer across Functionalized Semiconductor Surfaces. Sci. Technol. Adv. Mater. 2017, 18 (1), 681−692. (22) Kearney, K.; Iyer, A.; Rockett, A.; Staykov, A.; Ertekin, E. Multiscale Computational Design of Functionalized Photocathodes for H2 Generation. J. Am. Chem. Soc. 2018, 140 (1), 50−53. (23) Liu, Y.; Sun, Y.; Rockett, A. An Improved Algorithm for Solving Equations for Intra-Band Tunneling Current in Heterojunction Solar Cells. Thin Solid Films 2012, 520 (15), 4947−4950. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons: Hoboken, 2000. (25) Kim, H. J.; Kearney, K. L.; Le, L. H.; Pekarek, R. T.; Rose, M. J. Platinum-Enhanced Electron Transfer and Surface Passivation through Ultrathin Film Aluminum Oxide (Al2O3) on Si(111)−CH3 Photoelectrodes. ACS Appl. Mater. Interfaces 2015, 7 (16), 8572− 8584. (26) Maldonado, S.; Plass, K. E.; Knapp, D.; Lewis, N. S. Electrical Properties of Junctions between Hg and Si(111) Surfaces Functionalized with Short-Chain Alkyls. J. Phys. Chem. C 2007, 111 (48), 17690−17699. (27) Sze, S. M.; Kwok, K. N. Physics of Semiconductor Devices Physics of Semiconductor Devices, 3rd ed.; John Wiley & Sons: Hoboken, NJ, 2007. (28) Laibinis, P. E.; Stanton, C. E.; Lewis, N. S. Measurement of Barrier Heights of Semiconductor/Liquid Junctions Using a Transconductance Method: Evidence for Inversion at n-Si/CH3OH-1,1′Dimethylferrocene+/0 Junctions. J. Phys. Chem. 1994, 98 (35), 8765− 8774. (29) National Center for Biotechnology Information. PubChem https://pubchem.ncbi.nlm.nih.gov (accessed Oct 31, 2017). (30) Renault, O.; Gosset, L. G.; Rouchon, D.; Ermolieff, A. AngleResolved X-ray Photoelectron Spectroscopy of Ultrathin Al2O3 Films Grown by Atomic Layer Deposition. J. Vac. Sci. Technol., A 2002, 20 (6), 1867−1876. (31) O’Leary, L. E.; Strandwitz, N. C.; Roske, C. W.; Pyo, S.; Brunschwig, B. S.; Lewis, N. S. Use of Mixed CH3−/HC(O)CH2CH2−Si(111) Functionality to Control Interfacial Chemical and Electronic Properties During the Atomic-Layer Deposition of Ultrathin Oxides on Si(111). J. Phys. Chem. Lett. 2015, 6, 722. (32) Scheuermann, A. G.; Prange, J. D.; Gunji, M.; Chidsey, C. E. D.; McIntyre, P. C. Effects of Catalyst Material and Atomic Layer

(N00014-13-1-0530), the Robert A. Welch Foundation (F1822), and the UT Austin College of Natural Sciences. Computational work (KK, EE, AAR) was supported by the National Science Foundation under grant number CBET1022615, the National Science Foundation under grant number [OISE] 1545907, and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. Funding for the Kratos Axis Ultra XPS was provided by a grant from the National Science Foundation (MRI-0618242).

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