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Silicon Photoelectrode Thermodynamics and Hydrogen Evolution Kinetics Measured by Intensity-Modulated High-Frequency Resistivity Impedance Spectroscopy Nicholas C. Anderson, Gerard M Carroll, Ryan T Pekarek, Steven T Christensen, Jao van de Lagemaat, and Nathan R. Neale J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01311 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017

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The Journal of Physical Chemistry Letters

Silicon Photoelectrode Thermodynamics and Hydrogen Evolution Kinetics Measured by Intensity-Modulated High-Frequency Resistivity Impedance Spectroscopy Nicholas C. Anderson,1 Gerard M. Carroll,1 Ryan T. Pekarek,1,2 Steven T. Christensen,3 Jao van de Lagemaat,1,* and Nathan R. Neale1,* 1

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States 2 Department of Chemistry, The University of Texas at Austin, 2506 Speedway STOP A5300, Austin, TX 78712, United States 3 Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Abstract We present an impedance technique based on light intensity-modulated high-frequency resistivity (IMHFR) that provides a new way to elucidate both the thermodynamics and kinetics in complex semiconductor photoelectrodes. We apply IMHFR to probe electrode interfacial energetics on oxide-modified semiconductor surfaces frequently used to improve the stability and efficiency of photoelectrochemical water splitting systems. The technique measures the overpotential for HER relative to its thermodynamic potential in Si photocathodes coated with three oxides (SiOx, TiO2, and Al2O3) and a Pt catalyst. In pH 7 electrolyte, the flatband potentials of TiO2- and Al2O3-coated Si electrodes are negative relative to samples with native SiOx, indicating that SiOx is a better protective layer against oxidative electrochemical corrosion than ALD-deposited crystalline TiO2 or Al2O3. Adding a Pt catalyst to SiOx/Si minimizes proton reduction overpotential losses but at the expense of a reduction in available energy characterized by a more negative flatband potential relative to catalyst-free SiOx/Si. TOC GRAPHIC

Photoelectrochemical (PEC) water splitting is an attractive approach to artificial photosynthesis in which solar energy is stored in a chemical fuel for later use.1 Photoelectrolysis to produce hydrogen fuel and oxygen has received the lion’s share of attention from the many artificial photosynthesis concepts explored over the past four decades.2-3 Much research has focused on developing semiconductor photoelectrodes with ideal bandgaps and energy levels

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optimized for electron (hole) transfer to the hydrogen (oxygen) evolution reaction. Regardless of whether a traditional photovoltaic semiconductor such as Si,4 GaInP2,5-7 and GaAs8 or an oxide semiconductor such as Cu2O, 9 CuFeO2,10-11 CuV2O8,12-15 α-Fe2O3,16-18 and BiVO419-21 is chosen as the light-harvesting material, surface modification with purported protective layers (e.g., TiO2, FeOx) and/or catalysts (e.g., Pt, IrOx, RuOx, NiOx, NiFeOx, Co-Pi) is frequently employed to enhance water splitting performance.22 Despite the widespread use of these interfacial modifiers, the underlying mechanisms behind the observed enhancements in photoelectrolysis have only recently been revealed. For instance, a lively debate has unfolded about the role of cobalt phosphate (Co-Pi) deposited onto the surface of hematite (α-Fe2O3) photoanodes as a supposed electrocatalyst.23 Spectroscopic evidence appears to be converging on Co-Pi having a multi-faceted function involving beneficial effects through band bending and reduced recombination as well as detrimental effects due to decreased oxygen evolution reaction catalysis.24-30 Another example is the breakthrough report that amorphous TiO2 on Si, GaAs, and GaP semiconductor photoanodes can mitigate photocorrosion, resulting in ~100% Faradaic efficiency for water oxidation for more than 100 h of sustained photoelectrolysis.31 We recently studied the effects caused by an amorphous n-TiO2 coating deposited onto a p-GaInP2 semiconductor photocathode on carrier dynamics (charge separation and recombination) using transient photoreflectance (TPR) spectroscopy.32 The TPR data uncovered that a p-n junction is established between n-TiO2 and p-GaInP2, greatly retarding interfacial charge recombination without appreciably sacrificing the charge separation rate. The built-in fields present on either side of the p-n junction drive both holes and electrons away from the TiO2/GaInP2 interface, resulting in both kinetic (electron-hole separation distance of ~72 nm) and thermodynamic (>0.3 eV for electrons, 0.26 eV for holes) barriers to interfacial recombination. Thus the TiO2 coating engenders the GaInP2 photoelectrode not just with anticorrosion properties, but also has a strong beneficial effect on the interfacial carrier dynamics. These studies suggest that a more complete understanding of the role of interfacial components (oxide layers, catalysts, etc.) on semiconductor surfaces would substantially improve our ability to tailor the properties of photoelectrodes for water splitting and related electrocatalytic applications. Here, we revisit a photoelectrochemical optical impedance spectroscopy technique based on the light-intensity dependence of the high frequency resistivity (HFR) as a tool for probing the effects of interfacial constituents on semiconductor surfaces. We extend the HFR technique from its original application using simple chopped light to lock-in detection and thus term this advanced method intensity-modulated high frequency resistivity (IMHFR). In the absence of an oxide or catalyst, IMHFR provides an accurate determination of the flatband potential of a silicon photocathode. In the presence of an oxide layer and/or catalyst, IMHFR provides a sensitive measure of the interplay of energetics and kinetics of the semiconductor and its interfacial components. We show that IMHFR and current density-voltage (J–V) measurements can be used to determine the overpotential that reflects the difference between the semiconductor’s flatband potential and the hydrogen evolution reaction (HER) onset potential. For p-type semiconductors the flatband potential is equivalent to the thermodynamic corrosion potential in aqueous electrolyte; therefore IMHFR also provides an accurate assessment of this important parameter. We find that native amorphous silicon oxide (SiOx) grown on p-type Si gives rise to a large voltage range over which no HER occurs, giving rise to inversion at the Si surface due to minority carrier accumulation (Fig S1). Depositing Pt, a well-known HER catalyst, decreases the kinetic barrier to HER but also lowers the thermodynamic corrosion

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potential, indicating a trade-off between photocatalytic performance and electrode stability. We also compare the kinetic effects of different TiO2 and Al2O3 oxide layers deposited on Si via atomic layer deposition (ALD). These results detail another spectroscopic tool to help disentangle the multifaceted effects of oxide surface coatings on the performance of semiconductor photoelectrodes applied to water splitting and more generally offer new insight into our ability to manipulate interfacial components (oxides, catalysts, etc.) to tailor electrode charge transfer kinetics and thermodynamics. The thermodynamic ability of a semiconductor to reduce (oxidize) a solvated redox species is known as the semiconductor flatband potential (UFB).33 The semiconductor UFB is most commonly determined using electrochemical impedance spectroscopy and Mott-Schottky (M-S) analysis. In a real system in aqueous electrolyte, separate capacitive and resistive components are required for each chemically distinct layer to accurately model the electrode’s capacitance and generate a reliable M-S plot. Thus applying M-S analysis to multijunction, nanoporous, or dynamic photoelectrodes is challenging. Our M-S analysis of p-type Si photocathodes using the commonly employed Randles circuit model that neglects the effects of surface states, interfacial oxides, and the Helmholtz layer present in many real systems (Figure 1) demonstrates that this model is inappropriate for interrogating Si and likely other dynamic oxide-coated electrodes in aqueous environment (Supporting Information, Figures S2–S4 and associated discussion). Several decades ago Laser and Bard showed that at high frequency, parallel resistor/capacitor elements resulting from the Helmholtz layer, surface states, and surface oxides cannot respond rapidly enough to compensate for the oscillating AC field and they are effectively shorted at sufficiently high frequencies (>75 kHz).34 Under this condition the impedance is well approximated by a simple Randles circuit (Fig 1) with the parallel resistance and the parallel capacitance dominated by the conductivity and capacitance of the space charge region and the high frequency intercept dominated by the contact, solution, and bulk semiconductor resistances. Since the space charge region’s resistance is most strongly dependent on the presence of photogenerated minority carriers, light can be used to probe the presence and extent of a space charge at the semiconductor|electrolyte interface. We next describe the IMHFR method in detail for a p-type electrode, but the technique is equally applicable to an n-type electrode. Illuminating a p-type electrode at potentials cathodic of the flatband potential (V < UFB) causes a significant decrease in the measured resistivity relative to that in the dark as minority carriers (here, photogenerated electrons in the conduction band) are present in the space charge and cause its conductivity to increase significantly with respect to the dark situation where it is depleted of carriers. In contrast, at potentials anodic of the flatband potential (V > UFB), the semiconductor is under majority carrier accumulation and holes are injected from the valence band of the semiconductor into the electrolyte or are used for corrosion of the semiconductor with the same resistance in both dark and light conditions (since illumination does not cause a significant increase in the hole density). The flatband potential, then, is the point at which dispersion occurs between the dark and light resistivity values. Laser and Bard applied the HFR technique using chopped light to determine UFB values of –0.9±0.15 V vs saturated calomel electrode (SCE) for n-Si and 0.20±0.15 V vs SCE for p-Si (both 5–10 Ω•cm) immediately following HF etching (i.e., SiOx-free) and immersed into 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile electrolyte.34 Ottow et al. used chopped light HFR to measure UFB of 5 Ω•cm p-Si independent of capacitive surface components and found a similar “satisfactory” value of 0.14 V vs SCE in HF-containing pH 1.5 electrolyte (0.47 V vs RHE).35 In this paper, we extend the sensitivity of the chopped light HFR technique by

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using lock-in detection to arrive at a new technique: intensity-modulated HFR (IMHFR). The use of lock-in detection renders the technique much less sensitive to drift phenomena inherent to electrodes that are dynamically changing because of oxide formation, corrosion, and other photoelectrochemical processes. Lastly, due to the use of chopped light, the IMHFR technique is only sensitive to elements in the circuit that are light sensitive. By using a light source with wavelengths where only the semiconductor itself can respond we effectively isolate this part of the circuit from the rest.

Figure 1. Circuit diagrams representing simple Randles and real models of a photoelectrode in contact with an aqueous electrolyte. R and C correspond to resistors and capacitors, respectively, and the subscripts Ω, SC, SS, Ox, H, and CT indicate the components arising from the uncompensated resistance, the space charge region, surface states, an oxide, the Helmholtz layer, and charge transfer resistivity, respectively. This circuit assumes that the oxide passivates the semiconductor completely and no dynamic growth or etching of the oxide occurs. Here, we show that this IMHFR technique can also be used to reveal critical changes to both the thermodynamics and kinetics resulting from the deposition of interfacial oxides and catalysts deposited onto the surface of semiconductor photoelectrodes. Figure 2 shows IMHFR measurements on freshly HF-etched planar 5 Ω•cm p-Si silicon (Si) in 0.5 M KHSO4 pH 7 aqueous electrolyte. We found that a lock-in amplifier was very useful in minimizing data collection noise, and provided smooth curves of the change in resistivity, ∆R. Positive values of ∆R occur when the total light resistivity decreases relative to that in the dark. The UFB was taken at the point where the first derivative of the ∆R curve equals zero (and the ∆R curve itself approaches its asymptote; Figure S5). The measured UFB value of 0.45 V vs RHE (dashed black line, Figure 2) for oxide-free Si is comparable to the 0.47 V vs RHE measured by Ottow et al.35

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and closely agrees with the calculated UFB of 0.43 V vs RHE (eq S1 and Figure S1). After allowing the Si electrode to form a native SiOx layer, the observed UFB (corresponding to that of the new SiOx/Si photoelectrode) shifts to 0.96 V vs RHE (dashed red line, Figure 2).

Figure 2. High-frequency resistivity measurement in pH 7 aqueous electrolyte for freshly HFetched planar silicon (pl-Si, black), oxidized planar silicon (SiOx/Si, red), oxidized black silicon (SiOx/b-Si, grey), and oxidized black silicon with platinum nanoparticle catalysts (Pt/SiOx/b-Si, green) performed at 100 kHz with an AC-amplitude of 10 mV and a light on-off frequency of 5 Hz. Dashed lines mark the measured flatband potentials. Stemming from our prior work comparing illuminated J–V responses of planar silicon (Si) and nanoporous black silicon (b-Si) photocathodes,36 we next compare IMHFR measurements on both air-aged b-Si (SiOx/b-Si) and air-aged b-Si coated with Pt nanoparticle catalysts (Pt/SiOx/b-Si) with the IMHFR curves for Si and SiOx/Si (Figure 2). The SiOx/b-Si sample exhibits a UFB potential of 0.95 V vs RHE (dashed grey line, Figure 2), very similar to that of SiOx/Si, demonstrating that IMHFR can be used to measure UFB of nanoporous structures where conventional M-S analysis breaks down. Interestingly, the measured UFB for the Pt/SiOx/b-Si photoelectrode is 0.70 V vs RHE (dashed green line, Figure 2), 0.25 V more negative than that of SiOx/b-Si, suggesting that the Pt nanoparticle catalyst decreases the thermodynamic corrosion potential relative to catalyst-free SiOx/b-Si. Finally, it is worth pointing out that the absolute shapes of each IMHFR curve vary, likely resulting from the fact that there is still an imaginary component (capacitive element) contributing to the impedance even at 100 kHz (see open stars, Figure S3). Significant information regarding the photoelectrode thermodynamics and kinetics can be obtained by comparing the IMHFR and illuminated J–V curves. The illuminated onset potential is the potential at which photoexcited electrons are transferred from the conduction band to the H+/H2 redox couple. Driving a catalytic reaction at zero loss of energy is a Holy Grail in photoelectrochemistry. In all known PEC HER systems, a kinetic barrier exists between the flatband condition and the photocurrent onset (inversion region, Fig. S1) necessitating an electrochemical overpotential to drive photocatalysis. In this potential range where essentially no photo or dark current flows, the photocathode is electrochemically inert, as the applied potential is insufficiently negative to induce HER and insufficiently positive to result in oxidative corrosion.

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Since photogenerated electrons thermalize at ultrafast timescales to the conduction band edge, the available energy for photocatalytic hydrogen evolution on an ideal p-type semiconductor electrode is given by the difference between the semiconductor conduction band energy (ECB) and the energy of the redox system calculated from: ∆‫ܸ(ݍ = ܧ‬௥௘ௗ௢௫ − ܷி஻ ) − ∆‫ܧ‬ி where ∆‫ܧ‬ி is the energy difference between ECB and Fermi level given by ∆‫ܧ‬ி = ‫ܧ‬ி − ‫ܧ‬஼஻ , and if we use the RHE scale, ܸ௥௘ௗ௢௫ = 0. Such an ideal photocathode would exhibit a ∆‫ ܧ‬as close to zero as possible representing zero energy loss. In a real system where a semiconductor is immersed in solution, the available energy is lowered due to entropy losses (recombination in the bulk and at surfaces as well as thermionic emission3) as well as HER kinetics. Thus, when using lightly doped Si as the underlying semiconductor and neglecting contribution from surface catalysts and oxides, the greatest available energy occurs for an electrode UFB of 0.88 V vs RHE and a photocurrent onset at UFB (shown graphically in Figure S2). If this theoretical UFB could be achieved and this potential was also the HER onset potential, it would represent the maximized energy conversion efficiency and highest achievable stability towards oxidative corrosion for this doping density p-type Si. We next show that IMHFR provides a convenient way to measure UFB and, when combined with J–V measurements, offers a way to both quantify and differentiate thermodynamic and kinetic properties of photoelectrodes under aqueous PEC conditions. The combined IMHFR and J–V plots in Figure 3 show that the SiOx/Si photoelectrode is within experimental error (≤0.1 V) of the ideal oxide-free Si UFB, yet also exhibits a large kinetic barrier to HER of ~1.0 V, red spectra, Fig. 3A) relative to oxide-free Si (~0.5 V, black spectra, Fig. 3A). The Pt-catalyzed, nanoporous electrode system Pt/SiOx/b-Si exhibits a slightly lower kinetic barrier (~0.4 V, green spectra, Fig. 3A) to oxide-free Si resulting from enhanced HER catalysis, but as noted above, at the expense of a loss in energy as evidenced by a more negative UFB relative to catalyst-free SiOx/Si. The more negative UFB also means that the addition of the Pt catalyst makes the Pt/SiOx/b-Si electrode more prone to corrosion than catalyst-free SiOx/Si. Effectively, the Pt catalyst improves charge transfer across the interface in both directions, that is, it enhances the rate of electrons flowing from the electrode toward productive HER and holes from the electrolyte toward non-productive photocorrosion reactions. Finally, it is critical to note that the measured UFB can vary significantly depending on the type of oxide, the degree of oxidation, and the electrolyte (in addition to the presence or absence of a catalyst). Figure 3B shows the IMHFR measurements and illuminated J–V curves for the SiOx/Si sample as well as planar Si coated with 5 nm layers of crystalline TiO2 and Al2O3 deposited via ALD. Just like with SiOx and/or Pt on Si, these two interfacial oxides generate fundamentally new Si-based electrodes, as evidenced by UFB values negative relative to those of samples with native SiOx (0.61 V vs RHE for TiO2/Si and 0.38 V vs RHE for Al2O3/Si, Figure 3B). Notably, these UFB values also indicate that native SiOx is a better protective layer against oxidative corrosion than ALD-deposited crystalline TiO2 or Al2O3 on this type of Si. The negative HER onset potential for Al2O3 demonstrates that this oxide layer is a poor catalyst for the HER reaction. On the other hand, TiO2 has an HER onset potential very close to that of oxide-free Si (full J–V curves shown in Figure S6). Impedance spectroscopy (Figure S7) shows that both TiO2/Si and Al2O3/Si have higher resistivities than SiOx/Si at 0.1 V vs RHE, where each photocathode is still in inversion, and the lower slopes of the J–V curves indicate higher

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series resistances for the ALD oxide-coated photocathodes than for SiOx/Si. At a more positive potential (0.9 V vs RHE), both TiO2/Si and Al2O3/Si photocathodes are in accumulation and therefore the ALD oxide layers have lower resistivities compared to SiOx in SiOx/Si. Comparing the combined IMHFR and J–V curves of a series of planar Si electrodes (Si, Pt/Si, SiOx/Si, and Pt/SiOx/Si) in both pH 7 and pH 0.3 electrolyte (Figure S8) additionally shows the sensitive nature of the thermodynamics and kinetics to the electrode surface isoelectric point relative to electrolyte pH, as expected from fundamental photoelectrochemical theory (eq S2).

Figure 3. Illuminated current density-voltage response (left) and high-frequency resistivity (right) curves versus potential for (A) freshly HF-etched planar Si (Si, black), oxidized planar silicon (SiOx/Si, red), and oxidized black silicon with platinum nanoparticle catalysts (Pt/SiOx/bSi, green), and (B) planar Si with ALD coatings of aluminum oxide (Al2O3/Si, blue) and titanium dioxide (TiO2/Si, purple) compared with 1-month air-aged silicon (SiOx/Si, red). J–V curves were acquired in 0.5 M H2SO4 electrolyte under 100 mW/cm2 illumination sweeping from negative to positive potential, except for the ALD oxide layers, which were acquired in 0.5 M KHSO4 (pH ~7). IMHFR was measured in pH 7 electrolyte at 100 kHz with an AC-amplitude of 10 mV and on-off frequency of 5 Hz. Dashed lines mark the onset potential for HER at a –0.1 mA/cm2 threshold and the flatband potential, respectively. In this work, we showed that IMHFR is a powerful impedance technique that can accurately measure the flatband potential of semiconductors, even for complex, dynamic electrodes in aqueous environment. IMHFR measurements additionally provide a new way to

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probe the delicate interplay of passivating interfacial layers and catalysts deposited onto semiconductor surfaces. In the extreme case of a highly active, non-selective catalyst such as Pt, the catalyst can render the underlying protective layer less robust against electrochemical corrosion. This could be an issue for both oxide-based interfacial layers that may or may not also serve as electrocatalysts (e.g., NiOOH, FeOOH, Co-Pi) as well as pure catalysts (e.g., Pt, IrOx, etc.) on oxide-coated semiconductors. This conclusion also argues that understanding “leaky” oxides,37-39 or those with a separate, low resistive channel through which charges may pass, should be an area of significant research. Such leaky oxides have been generated via unintentional impurities (e.g., hydrocarbons, hydroxyls) and amorphous structure from deposition such as ALD or could be intentionally engineered through doping or other strategies. Using IMHFR to interrogate both as-prepared as well as surface-modified electrodes gives electrochemists a new tool to help disentangle the multifaceted ways in which oxide surface layers and catalysts affect the overall electrode thermodynamics and kinetics responsible for performance in PEC water splitting and related electrochemical systems. Acknowledgments This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program under contract number DE-AC36-08GO28308 to NREL. RTP was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under the Science Graduate Student Research (SCGSR) fellowship program administered by the Oak Ridge Institute for Science and Education (ORISE) for DOE under contract number DESC0014664. NCA, JvdL, and NRN conceived of the project, provided experimental interpretation, and wrote the manuscript. NCA, GMC, RTP conducted electrochemical experiments and refined data interpretation. STC deposited metal oxides via ALD. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Energy diagrams based on current-voltage and IMHFR curves; details of flatband pontential theory and measurement using conventional Mott-Schottky analysis; first derivative analysis of IMHFR for planar Si; full J–V curves of Si, SiOx/Si, Pt/SiOx/b-Si, TiO2/Si, Al2O3/Si; EIS spectra for SiOx/Si, TiO2/Si, Al2O3/Si; IMHFR and J–V curves compared in pH 0.3 H2SO4 (top) and pH 7 K2SO4 electrolyte; table containing all UFB and photocurrent onset potential values; and Experimental methods. Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] References 1. Bard, A. J.; Fox, M. A. Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Acc. Chem. Res. 1995, 28, 141-145.

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Silicon Photoelectrode Thermodynamics and ... - ACS Publications

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