Photoelectrochemical and Impedance Spectroscopic Analysis of

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Photoelectrochemical and Impedance Spectroscopic Analysis of Amorphous Si for Light-Guided Electrodeposition and Hydrogen Evolution Reaction Sung Yul Lim, Donghyeop Han, Yang-Rae Kim, and Taek Dong Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04961 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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ACS Applied Materials & Interfaces

Photoelectrochemical and Impedance Spectroscopic Analysis of Amorphous Si for Light-Guided Electrodeposition and Hydrogen Evolution Reaction Sung Yul Lim, †, || Donghyeop Han, † Yang-Rae Kim,*, ‡ and Taek Dong Chung*,†, § †

Department of Chemistry, Seoul National University, Seoul 08826, Korea

‡

Department of Chemistry, Kwangwoon University, Seoul 01897, Korea

§

Advanced Institutes of Convergence Technology (AICT), Suwon-Si, Gyeonggi-do 16229,

Korea

KEYWORDS: amorphous silicon, light-guided electrodeposition, impedance spectroscopy, proton reduction, tunneling, surface state

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ABSTRACT

For more efficient photoelectrochemical water splitting, there is a dilemma that a photoelectrode needs both light absorption and electrocatalytic faradaic reaction. One of the promising strategies is to deposit a pattern of electrocatalysts onto a semiconductor surface; leaving sufficient bare surface for light absorption, while minimizing concentration overpotential as well as resistive loss at the ultramicroelectrodes for faradaic reaction. This scheme can be successfully realized by “maskless” direct photoelectrochemical patterning of electrocatalyst onto an SiOx/amorphous Si (a-Si) surface by light-guided electrodeposition technique. Electrochemical impedance spectroscopy at various pHs tells us much about how it works. The surface states at the SiOx/a-Si interface can mediate the photogenerated electrons for hydrogen evolution, whereas electro-active species in the solution undergo outer-sphere electron transfer taking electrons tunneling across the SiOx layer from the conduction band. In addition to previously reported long-distance lateral electron transport behavior at a patterned catalyst/SiOx/a-Si interface, charging process of the surface states plays a crucial role in proton reduction, leading to deeper understanding of the operation mechanisms for photoelectrochemical water splitting.

1. INTRODUCTION Anthropogenic climate change and environmental pollution have become evident as global energy demand keeps increasing. Most energy resources still depend overwhelmingly on fossil fuels, however, leading to keen attention to carbon-free energy sources. An idea proposed by Fujishima and Honda in 19721 is photoelectrochemical generation of H2 and O2 2 ACS Paragon Plus Environment

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to convert and store solar energy in the form of chemical bonds, which was inspired by photosynthesis. In order to do so, photoelectrodes should meet a few essential conditions, i.e. sufficient light absorption, efficient charge generation and transport, proper band alignment, facile charge-transfer kinetics, and robust stability in aqueous solutions. Despite intense research effort all over the world, there has been no report of the single material that simultaneously satisfies these various prerequisites.2 This is why tandem devices have been increasingly studied to enhance conversion of sunlight into chemical fuels.3,4 For photoelectrodes, in which a couple of functional materials are monolithically integrated, light must penetrate either the photocathode or photoanode through the aqueous electrolyte solution. Catalyst layers for photoelectrochemical reactions usually cover the surface of the photo-absorber. In the case of non-noble metal catalysts, even larger amounts need to be deposited because of their low activity compared with Pt-group catalysts.5,6 Wide and thick films of such catalysts severely attenuate the magnitude of light absorbed. One of the potential solutions is to put a catalyst pattern on the light absorber, leaving enough bare semiconductor surface area for sufficient light penetration.4,7 Amorphous Si (a-Si) is a promising earth-abundant candidate for one of the half reactions of photoelectrochemical water splitting in a tandem system.8,9 As reported by Chen et al.,4 its direct band-gap of 1.7–1.8 eV is well-matched to the solar spectrum and also appropriate for the optimum band-gap combination, 1.7 eV/1.1 eV, of a tandem device. The maximum solarto-hydrogen (STH) efficiency is expected to be 25.0% using a combination of 1.7 eV/1.1 eV for the top and bottom absorbers and transparent catalyst pattern with a filling fraction of 0.05.4 On the other hand, the short diffusion length of photogenerated charge carriers in a-Si is a useful optoelectronic property for direct photoelectrochemical patterning of electrocatalyst for hydrogen evolution reaction (HER), which was demonstrated recently.7 3 ACS Paragon Plus Environment


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This ‘light-guided electrodeposition’ technique potentially cut production expense and offers valuable merit of minimal concentration overpotential and resistive loss.4,10 As a component of a tandem device for water splitting, a-Si have been characterized photoelectrochemically to improve the conversion performance.11-13 Notwithstanding rigorous analyses, the precise photoelectrochemical behaviors of a-Si are not yet fully understood, especially in aqueous electrolyte environments. In this work, we conducted light-guided electrodeposition of various materials, e.g. Pt, CdSe and MoSx onto a-Si exploiting the unique optoelectronic properties of a-Si. For better solarfuel production systems, it is essential to understand what occur at such patterned metal/SiOx/a-Si photoelectrode systems. Previously, it was found that the inversion channel formation among the deposited catalysts causes the long-distance lateral electron transport.7,14,15 It was also reported that charge transfer at the SiOx/a-Si interface for the HER is ascribed to electrons accumulated at surface states and protons that have permeated into the oxide layer.7,14,15 There are still many questions to ask, however, including how pHdependent surface charge of the SiOx layer and electrons tunneling from the conduction band contribute to overall photoelectrochemical behavior. Photoelectrochemical measurements and impedance analysis provide complementary information that allows us to address these issues.

2. EXPERIMENTAL SECTION 2.1. Preparation of a-Si photoelectrodes To prepare the a-Si photocathode (PIN type), p-doped a-Si:H layer (20 nm), intrinsic a-Si:H layer (500 nm), n-doped a-Si:H layer (100 nm) were deposited sequentially on a low resistivity (0.001~0.003 Ω cm), (100) face, N-doped bare silicon wafer using plasma4 ACS Paragon Plus Environment

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enhanced chemical vapor deposition (PECVD). The a-Si as-deposited were cut into 1 cm × 1 cm. Si wafers were cleaned by a standard Radio Corporation of America procedure16 before transferred to a PECVD chamber. Chemical oxide was formed on a-Si surface by immersing the wafers in piranha solution (1:3 H2O2:H2SO4) for 1 min after removal of native oxide by exposing to 1% HF solutions. The SiOx/a-Si photoelectrodes were rinsed with deionized water (resistivity of 18.2 MΩ cm at room temperature) and dried using a N2 gun. These photoelectrodes were stored in a vacuum desiccator in the dark before use. 2.2. Light-guided electrodeposition Light-guided electrodeposition was conducted with a 633-nm He/Ne laser (LASOS Lasertechnik Gmbh) equipped at home-built micro-Raman system (Dongwoo Optron Co., Ltd). The laser intensity was 0.02 mW and the illuminated area was controlled with a nanopositioning system in which a piezoelectric stage was embedded (Nano-LP300, Mad City Labs, Inc.). All electrodeposition experiments were carried out with a CHI 440 electrochemical workstation (CH instrumentaions, Austin, TX) using a standard threeelectrode cell in a homemade Teflon cell. The counter and reference electrode were a platinum wire and a Ag/AgCl (3 M NaCl, Bioananalytical Systems, Inc.), respectively. Pt electrodeposition was performed in 0.1 mM Na2PtCl6 and 0.1 M K2SO4 aqueous solution (pH was adjusted to 3.0 with H2SO4). MoSx was electrodeposited in 5 mM Na2MoO4 and 0.1 M Na2S at pH 8.0 adjusted with 1 M HCl.17 For CdSe pattern, solution was freshly prepared by mixing 120 mM CdSO4·3H2O and 1 mM SeO2.18 All precursor solutions for electrodeposition were deaerated with high purity N2 gas (99.9%) for 30 minutes. 2.3 Photoelectrochemical characterization



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To investigate photoelectrochemical and impedance spectroscopic analysis of SiOx/a-Si, we employed a 633-nm He/Ne laser and a 625-nm LED (Precision LED source from Mightex Systems) as light sources with the intensities of 1 and 20 mW, respectively. The buffer solutions for investigating hydrogen evolution were 0.1 M hydrochloric acid (pH 1.0), potassium acetate buffer (pH 4.0), and potassium phosphate (pH 7.0) to which 0.2 M KCl was added as a supporting electrolyte to prevent from serious variation of ionic strength. The total ionic ionic strength was standardized to be 0.3 M. The pH was tuned with 1 M HCl or KOH. To investigate the charge-transfer behavior between outer-sphere electroactive species and the photoelectrode, the targeted ions were added to the same buffer solutions for HER as a background solutions.

The reference and counter electrodes were identical to those for

light-guided electrodeposition experiments. Photoelectrochemical analysis was performed with a CHI 440 electrochemical workstation, and impedance analysis was conducted by a Gamry Reference 600 (Gamry Instruments, Warminster, PA). For impedance measurements, potential amplitude was 10 mV and the frequency range was between 10 kHz and 40 mHz. The acquired impedance data was fitted with Gamry EIS300 software. We made laser light chopped at 3.76 Hz using an optical chopper (Stanford Research Systems). The electrolytes were continuously bubbled with N2 gas (99.9%).

3. RESULTS AND DISCUSSION The photoconductive nature of a-Si allows us to create conductive region where light is illuminated. This has been exploited in due course for a variety of applications, e.g. optoelectronic

tweezers,19 light-addressable potentiometric sensors,20 and scanning

photoinduced impedance microscopy21. Owing to its low diffusivity of photogenerated electrons in a-Si, the ‘virtual electrode’ on it should have better spatial resolution than that on 6 ACS Paragon Plus Environment

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single crystalline Si, c-Si wafer.14,19 That is one of the important reasons why a pattern of metal nanoparticles can be directly electrodeposited on a-Si by spatio-selective light illumination. The experimental scheme in Figure 1a shows a three-electrode system in which a-Si acts as a working electrode. The chemical oxide (SiOx), which is formed after removal of native oxide on an a-Si surface before use, helps with higher stability and less recombination.22 The consecutive linear sweep voltammograms for a-Si covered with chemical and native oxide are shown in Supporting Information (Figure S1). At chemical SiOx/a-Si, the voltammograms are stabilized within three scans while the photocurrent keeps decrease as the number of scan increases. The cross-sectional TEM images tells the thickness of the SiOx layer of 2–3 nm (Figure 1b). The open-circuit potential (OCP) versus time was measured under chopped illumination of 633-nm focused laser in the precursor solutions for Pt electrodeposition as shown in Figure S2. OCP positively shifts upon light illumination implying that p-type semiconductor result from the band flattening by the photogenerated minority carriers.15,23 The current in a linear sweep voltammogram (Figure 1c) steeply rises from -0.05 V (vs. Ag/AgCl) in the presence of [PtCl6]2- ions under 633-nm focused laser light while the negligible current is observed under dark conditions. The potential sweep starts from near the OCP to circumvent faradaic oxidation regardless of dark or bright condition. Based on this voltammogram, a potential pulse (inset of Figure 1d) is set to be from 0.1 V to 0.4 V for 2 s, leading to the dot pattern consisting of Pt nanoparticles on the spot of laser beam. The chronoamperometric curves in the absence and presence of Pt precursor (Figure 1d) indicate that significant faradaic reduction current flows in response to the potential step to -0.4 V. However, the current peak due to the diffusion field overlapping around growing particles24 was not observed unlike that at crystalline p-type Si.25 This is may be ascribed to the unique characteristics of electrolyte/SiOx/a-Si, which requires further in-depth research to



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understand the interface. A group of Pt nanoparticles on the SiOx in the SEM image (Figure 1e) confirms that electrons

Figure 1. (a) Schematic view of the experimental setup for light-guided electrodeposition of Pt under 632.8-nm laser illumination. (b) A cross-section of the TEM images of SiOx/a-Si photoelectrode. (c) Linear sweep voltammogram at 10 mV s-1 recorded during photoelectrochemical deposition of Pt under focused laser illumination (red line) and dark 8 ACS Paragon Plus Environment

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condition (black line). (d) Current-time transient recorded during −0.4 V step with (red solid line) and without Pt precursor (red dashed line). The inset shows the potential scheme applied to the cell. The pulse duration is 2 s. (e) SEM image of a Pt spot generated under focused laser illumination. (f) A cross-section of the HRTEM image of an electrodeposited Pt nanoparticle on the SiOx/a-Si interface. The inset shows the EDS element mapping image showing Pt distribution.

were transferred across the chemical oxide film to Pt precursor molecules in the electrolyte solution. The HRTEM images (Figure 1f) and EDS element mapping results (inset of Figure 1f) show no Pt metal formed inside the SiOx layer. It is necessary to investigate how light focusing affects the result of electroplating for practical light-guided electrodeposition. The SEM images of Figures S3a and S3b show dotshaped Pts under defocused illumination where all conditions are identical to those detailed in Figure 1.

Density of Pt nanoparticles electroplated is higher at the center of the illumination

spot. Such density profile is more distinct for Pt nanoparticles formed by more potential pulses. More magnified images in Figure S3c ensure this trend evidently. Although Pt particles are not seen at the edges of the illuminated site in less magnified SEM image (Figure S3b), higher magnification (image marked by 4 in Figure S3c) allows to find smaller particles that were formed. Counting Gaussian profile of laser intensity,26 this is consistent with what predicted. The results in Figure S3 support the assumption that light induces charge transfer over SiOx for faradaic reaction and the electrical potential is highly sensitive to the photon irradiation.



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The photo-induced faradaic process at the a-Si surface can be also exploited for the selective electrodeposition of other materials, e.g., CdSe, MoSx, etc. As demonstrated by electrosynthesized CdSe and MoSx patterns in Figure 2, the concept of light-guided electrodeposition could be widely expanded to the maskless and stabilizer-free patterning of a

Figure 2. SEM images of dot-patterned (a) CdSe and (b) MoSx spot formed by the lightguided electrodeposition technique under focused 632.8-nm laser illumination. The reduction potential is -0.5 V (duration: 20 ms, number of pulses: 2) and -0.7 V (duration: 5 s, number of pulses: 4) for CdSe and MoSx, respectively. The resting potential is 0.3 V for 5 s. The right images at (a) and (b) are the magnified images of each patterns at center area.

variety of metals and semiconductors. CdSe and MoSx patterns deposited by light-guided electrodeposition were characterized by EDS and element mapping with time of flight10 ACS Paragon Plus Environment

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secondary ion mass spectrometry (TOF-SIMS) analysis, respectively (Figure S4). Especially, the longitudinal-optical (LO) phonon mode of CdSe at 208 cm-1 in the Raman scattering spectrum confirms deposition of CdSe (Figure S4b).27

Figure 3. (a) Linear sweep voltammograms at 10 mV s-1 under focused laser illumination for the photoelectrochemical reduction of a 2 mM Ru(NH3)63+/2+ (pH 1.0) on an a-Si photoelectrode. Linear sweep voltammograms for 2 mM of (b) Ru(NH3)63+/2+ under continuous (10 mV s-1) and chopped illumination (75 mV s-1) and (c) [Fe(CN)6]3-/4- under continuous illumination (10 mV s-1) at pH 1.0 and 7.0. The light chopping frequency is 3.76 Hz. (d) Cyclic voltammograms (20 mV s-1) for 2 mM of [methylviologen]2+, [Ru(NH3)6]3+/2+, [Fe(CN)6]3-/4- and [IrCl6]2-/3- at a SiOx/a-Si photoelectrode under a 625-nm LED that illuminates light on whole electrode area exposed to pH 7.0 electrolyte (light intensity: 20 11 ACS Paragon Plus Environment


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mW). The electrolyte for pH 1.0 is 0.1 M HCl and for pH 7.0 is 0.1 M phosphate with the addition of 0.2 M KCl as a supporting electrolyte.

To integrate the photoelectrodes in photoelectrochemical conversion systems by lightguided electrodeposition technique, it is essential to understand the electrochemistry involved in the proposed system. Photoelectrochemical measurement and impedance spectroscopy as a function of the pH and of the applied potential should offer valuable insights. Since protons in the electrolyte solution are likely to penetrate the thin silicon oxide film,28-30 the thermodynamic and electrochemical properties of the Si-based semiconductor electrode are sensitive to the proton concentration (pH).31,32 Figure 3a shows linear sweep voltammograms of 2 mM Ru(NH3)63+/2+ (pH 1.0) that demonstrate the effect of the insulating thin chemical SiOx film on a-Si electrode under illumination. The presence of the SiOx film makes current density decrease and the onset potential shift in the negative direction. On the other hand, only negligible current flows through a 20-nm-thick oxide layer grown by PECVD. The deposition temperature at PECVD chamber was ~200 oC, which results in the amorphous structured SiOx.33 Although it is difficult to precisely compare the chemical quality of SiOx layer formed by chemical treatments and PECVD, here we assume that the oxides prepared by the two different methods is indistinguishable in terms of photoelectrochemical performance. This indicates that electron transfer through thin chemical oxide layer, SiOx, is possible to give rise to faradaic reduction by tunneling although Ru(NH3)63+/2+ molecules cannot penetrate the oxide film. Thus, the probability of electron transfer through the thin oxide layer should sensitively 12 ACS Paragon Plus Environment

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depend on the distance between the electrode and the reactant molecules,34,35 which is not only a function of the oxide layer thickness but electrostatic interaction. Approach and electron uptake of reactant ions can be influenced by electrostatic repulsion or attraction with the electrode surface. By controlling the solution pH as well as varying the oxide layer thickness, we can see if it is true. The Si oxide layer has a relatively low pKa value;29 therefore, in neutral pH, Ru(NH3)62+/3+ ions are more likely to reside near the oxide layer driven by electrostatic attraction. The linear sweep voltammograms under continuous and chopped laser illumination obtained at pH 1.0 and pH 7.0 (Figure 3b) support this prediction. The currents under chopped illumination at pH 1.0 are smaller than those at pH 7.0 at potentials more positive than around -0.2 V. And sharp current spikes, well-known indicator of surface recombination,36 are also evident at pH 1.0. The positive current transients observed when light is off comes from reoxidation of [Ru(NH3)6]2+ to [Ru(NH3)6]3+. This is in stark contrast to the result when using the [Fe(CN)6]3-/4- as a redox couple in the electrolyte instead of Ru(NH3)62+/3+ (Figure 3c). Current is lower and onset potential shifts negatively at pH 7.0. This agrees with what is predicted considering the negatively charged [Fe(CN)6]3-/4-. Checking up more redox couples, we can assure this is generally true. For this experiment, the whole a-Si electrode was illuminated using LED. a-Si photocathode works well when light source is changed from a laser to an LED. At pH 7.0, [Ru(NH3)6]3+/2+ and [methylviologen]2+ give higher current than [Fe(CN)6]3-/4- and [IrCl6]2-/3-. Compared with the voltammograms at a Pt electrode (Figure S5), the cathodic peaks of the negatively charged molecules appear at more negative potentials at a-Si whereas the positively charged ones are reduced at less negative potentials (Figure 3d). Besides electro-active species, there is another reducible species that needs to be considered. Proton (H+) is one of the most important electrochemically active species in renewable 13 ACS Paragon Plus Environment


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energy conversion and storage systems. Since it possibly penetrates chemical oxide unlike molecular species, we should investigate its role in this system in a different way. Electrochemical impedance spectroscopy provides equivalent circuit in which respective components can release meaningful information including how protons influence photoelectrochemical

Figure 4. (a) Linear sweep voltammograms (10 mV s-1) in buffer solutions at three different pHs. (b) Nyquist plots recorded at various dc offset potentials on SiOx/a-Si. The inset denotes small semicircles in the high-frequency region. (c) Nyquist plot with 2 mM [Ru(NH3)63+/2+] (orange dots) in the same solution composition as in (b). The Nyquist plot in the absence of [Ru(NH3)63+/2+] is shown for comparison (orange dots). The light source was a 625-nm LED to illuminate the whole area of a SiOx/a-Si electrode exposed to the electrolyte solution (intensity: 20 mW). The buffer solutions for pH 1.0, 4.0 and 7.0 are 0.1 M hydrochloric acid, potassium acetate and potassium phosphate, respectively and each buffer solution is containing 0.2 M KCl as a supporting electrolyte.

behavior at a SiOx/a-Si photoelectrode. Figure 4a shows linear sweep voltammograms as a function of pH under 625-nm LED illumination (20 mW) in the absence of electro-active species other than proton. Voltammetric response shifts negatively as pH increases. 14 ACS Paragon Plus Environment

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Converting the potentials to display versus reversible hydrogen electrode (RHE), three voltammograms recorded at three different pHs are almost perfectly overlapped (Figure S6). This tells that the proton reduction in this system is subject to Nernstian, 59 mV pH-1. This is different from pH dependent reduction of redox-active molecules undergoing outer-sphere electron transfer, which can be affected by electrostatic interaction. It is well known that HER mechanism at metal electrodes is not identical, but depends on pH range of the solution. That is why the voltammograms at various pH are hardly overlapped when the potential is corrected based on Nernstian relationship. However, the proton reduction at our SiOx covered a-Si photoelectrode gives voltammograms that are invariant with pH, perfectly following Nernstian as shown in Figure S6. This implies that the active sites where proton reduction takes place are not directly exposed to the solution where protons, reactants, exist. If protons can permeate, amorphous SiOx, to encounter active sites, i.e. surface states, electrons at the surface states should face stable environment of SiOx. As a result, the mechanism for proton reduction should be insensitive to the solution pH. The perfectly overlapped voltammograms at various pHs for HER as well as chemical characteristics of thin amorphous SiOx support this scenario. On the other hand, redox active molecules such as [Ru(NH3)6]3+/2+ cannot penetrate the SiOx layer obviously. They uptake electrons underneath the oxide layer by tunneling. Since electron transfer probability is a function of tunneling distance, electrostatic attraction effect can be significant in neutral or basic solution. Therefore, it is highly probable that protons can pass through to access the surface states and get reduced unlike any other redox species that is not free from electrostatic effect. Figure 4b shows Nyquist plots at three different potentials in pure pH 1.0 electrolyte under 625-nm LED illumination. Two semicircles, a large and a small one in low and high frequency ranges respectively, are found in all Nyquist plots. The larger semicircle in the low 15 ACS Paragon Plus Environment


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frequency range sensitively varies with dc offset potential. More negative potential leads to smaller radius. By comparison with the semicircle observed at high frequency, the radius at low frequency rapidly decreased which indicates that electron transfer dominantly occurring corresponds to the low-frequency pathway. In the presence of [Ru(NH3)6]3+/2+, the larger semicircle in low frequency regime disappears (Figure 4c). Instead, [Ru(NH3)6]3+/2+ produces its own semicircle that is roughly as large as the smaller semicircle in the pure solution, i.e. without

Figure 5. (a) The schematic full equivalent circuit used to conceptualize the photoelectrochemical behavior of a aqueous electrolyte/SiOx/a-Si photocathode. (b) Simplified model to fit impedance spectroscopy data for the Nyquist plots in case of proton reduction which exhibits two semicircles. The equivalent circuit is defined by the capacitance of the bulk a-Si, Cbulk, a resistance which is related to the rate of trapping holes in surface states, Rtrap, a capacitance of the surface states, Css, a charge-transfer resistance through the surface states, Rss, and series resistance, Rs. Conduction band (CB) and valence band (VB) with red line represent the schematic band structure of a-Si under light irradiation.


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[Ru(NH3)6]3+/2+. This can be interpreted by the scenario that outer electron transfer from SiOx/a-Si photoelectrode to [Ru(NH3)6]3+/2+ via tunneling through the chemical oxide layer is much faster than proton reduction so as to occur overwhelmingly. [Ru(NH3)6]3+/2+ behaves like an electron scavenger. Rapid electron transfer from the conduction band is supposedly responsible for the single semicircle in the presence of [Ru(NH3)6]3+/2+ in the solution. Literature provides some clues to explain the experimental results. In Si-based photoelectrode domain, there can be inner-sphere electron transfer that involves covalent bond formation e.g. Si-H.29,38,39 More importantly, we should take into account of surface states that reside at the SiOx/a-Si interface. Reportedly, the occupancy of the surface state is governed by the relative rates of trapping electrons and holes and by the rate of charge transfer to electrolyte species.40 The Helmholtz layer capacitance (CH) that serially connected to capacitance from space charge layer (Cbulk in here) is neglected based on the general assumptions that the CH is much smaller than Cbulk. In addition, the electrolyte/SiOx interface was not considered because the oxide layer dominantly plays a role of the tunneling barrier. Considering these factors,40,41 the full equivalent circuit in Figure 5a is proposed to understand photoelectrochemical behavior of aqueous electrolyte/SiOx/a-Si junction. For unambiguous impedance data fitting, Rbulk was removed for the case of proton reduction as shown in Fig. 5b. Combination of Randles circuits are enough to fit the impedance spectra and extract well-matched parameters. To assure the validity of our proposed equivalent circuit shown in Fig. 5b, we compared the Rtot values calculated by the sum of fitted parameters, Rs + Rtrap + Rss with the measured value from the derivation of voltammograms (dV/dJ) shown in Figure 4a.40 As you can see in Figure S7, the Rtot values from the two different ways agree with each other within experimental errors, supporting that the proposed



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equivalent circuit is valid. The well-matched behavior was also observed in the presence of the [Ru(NH3)6]3+/2+ analyzed with Randles circuit (Fig. S7). Css and Rss in the parallel RC circuits as a function of potential at three different pH conditions in the absence of outer-sphere electroactive species are shown in Figure 6. As potential goes in the negative direction, Css and Rss simultaneously start to increase and decreases, respectively. Decrease of Cbulk and increase of Rtrap are observed in the same potential region (Fig. S8). These results are consistent under the three pH conditions. These observations clearly support that the electron transfer for HER dominantly takes place through the surface states. Such surface states involved in photoelectrochemical reactions are widely known for the water oxidation at a

Figure 6. Linear sweep voltammograms and capacitance of the surface states, Css (red dots) and charge-transfer resistance through the surface states, Rss (blue dots) values obtained for a 18 ACS Paragon Plus Environment

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SiOx/a-Si electrode using buffer solutions with pH of (a) 1.0, (b) 4.0 and (c) 7.0. The illumination source was a 20-mW LED (625 nm). The identical scale bar for current density is also applied for voltammograms shown in (b) and (c).

hematite (α-Fe2O3) photoanode requiring four holes.40,41 Chemical structures of the surface states at metal oxide electrodes the water oxidation reaction is not unraveled clearly. The exact chemical structures of surface states participating in HER process at Si-based photocathodes are not, either. We herein suggest that the surface-state charging during photoelectrochemical proton reduction is largely, at least partly ascribed to Si-H bond formation.39 The peak of the Css distribution is very close to the formal potential of the HER (0 V vs. RHE denoted by dashed line in Figure 6). This implies an equilibration of surface states and electron-accepting species in the electrolyte, i.e. protons. According to previous literature,29,31,39 there are numerous dangling bonds at the intrinsic SiOx/Si interface, which could serve as the electronic/ionic trapping sites. Since the proton is the smallest ion that can penetrates the thin chemical SiOx layer, protonation/deprotonation equilibrium is possible at the interfacial defect sites of SiOx/a-Si.29,39 The only one semicircle is observed and the charge-transfer resistance is largely increased under without light irradiation (Figure S9). This indicates that proton reduction through valence band is sluggish and photoexcitation is required to supply electrons to the surface states.

4. CONCLUSIONS



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In this study, we have demonstrated light-guided electrodeposition on a-Si covered with a SiOx layer. As shown herein, direct patterning by reductive electrodeposition under laser focusing is possible owing to the inherent optoelectronic property of the a-Si substrate. Photoelectrochemical measurements and impedance spectroscopy at various pH conditions can provide valuable ways to investigate the photoelectrochemical behavior of a-Si in contact with the buffer solutions containing redox active species or not. Particularly, the strong correlation between Css and Rss in addition to coincident photocurrent onset can be explained by the surface states, which are thought to catalyze the HER involving both surface-trapped electrons at the SiOx/Si interface and protons from the electrolyte. With the addition of [Ru(NH3)6]3+/2+, a redox-active molecule undergoing outer-electron transfer coupling with no proton, can directly take electrons tunneling through the SiOx layer from the conduction band. The results shown here allow us to look deeper into the mechanism of light-guided electrodeposition at a SiOx/a-Si interface and of photoelectrochemical HER at patterned metal/SiOx/a-Si photoelectrodes. In addition, the strong correlation between the charging of the surface state and the photocurrent density augmentation may be a new indirect evidence for the formation of inversion channel induced by accumulation of electrons in surface states at SiOx/Si interface. The inversion channel enables the long-distance lateral electron transport, which is essential for the patterned catalyst/SiOx/Si-based photoelectrode.7,14,15 This work is expected to stimulate further investigations of metal-insulator-semiconductor configurations that are recently attracting attention due to conspicuous advance in both stability and efficiency for energy conversion devices.


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ASSOCIATED CONTENT Supporting Information. Physical characterization, open-circuit potential measurement, SEM images and characterization of CdSe and MoSx pattern, additional (photo)electrochemical and impedance spectroscopic data (PDF). The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(Y.-R.K.) E-mail: [email protected] *(T.D.C.) E-mail: [email protected].



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Present Addresses ||

Department of Materials Science and Engineering, Seoul National University, Seoul 08826,

Korea. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A13001897), Nano.Material Technology Development Program (2011-0030268) through the National Research Foundation of Korea (NRF) funded by the Ministry of Sciece, ICT and Future Planning and CABMC (Control of Animal Brain using MEMS Chip) funded by Defense Acquisition Program Administration (UD140069ID). The present research has been conducted by the Research Grant of Kwangwoon University in 2016.

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Table of Contents Graphic


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Figure 1. (a) Schematic view of the experimental setup for light-guided electrodeposition of Pt under 632.8nm laser illumination. (b) A cross-section of the TEM images of SiOx/a-Si photoelectrode. (c) Linear sweep voltammogram at 10 mV s-1 recorded during photoelectrochemical deposition of Pt under focused laser illumination (red line) and dark condition (black line). (d) Current-time transient recorded during −0.4 V step with (red solid line) and without Pt precursor (red dashed line). The inset shows the potential scheme applied to the cell. The pulse duration is 2 s. (e) SEM image of a Pt spot generated under focused laser illumination. (f) A cross-section of the HRTEM image of an electrodeposited Pt nanoparticle on the SiOx/a-Si interface. The inset shows the EDS element mapping image showing Pt distribution. 292x318mm (96 x 96 DPI)

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Figure 2. SEM images of dot-patterned (a) CdSe and (b) MoSx spot formed by the light-guided electrodeposition technique under focused 632.8-nm laser illumination. The reduction potential is -0.5 V (duration: 20 ms, number of pulses: 2) and -0.7 V (duration: 5 s, number of pulses: 4) for CdSe and MoSx, respectively. The resting potential is 0.3 V for 5 s. The right images at (a) and (b) are the magnified images of each patterns at center area. 194x190mm (120 x 120 DPI)



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Figure 4. (a) Linear sweep voltammograms (10 mV s-1) in buffer solutions at three different pHs. (b) Nyquist plots recorded at various dc offset potentials on SiOx/a-Si. The inset denotes small semicircles in the high-frequency region. (c) Nyquist plot with 2 mM [Ru(NH3)63+/2+] (orange dots) in the same solution composition as in (b). The Nyquist plot in the absence of [Ru(NH3)63+/2+] is shown for comparison (orange dots). The light source was a 625-nm LED to illuminate the whole area of a SiOx/a-Si electrode exposed to the electrolyte solution (intensity: 20 mW). The buffer solutions for pH 1.0, 4.0 and 7.0 are 0.1 M hydrochloric acid, potassium acetate and potassium phosphate, respectively and each buffer solution is containing 0.2 M KCl as a supporting electrolyte. 430x102mm (96 x 96 DPI)


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Figure 5. (a) The schematic full equivalent circuit used to conceptualize the photoelectrochemical behavior of a aqueous electrolyte/SiOx/a-Si photocathode. (b) Simplified model to fit impedance spectroscopy data for the Nyquist plots in case of proton reduction which exhibits two semicircles. The equivalent circuit is defined by the capacitance of the bulk a-Si, Cbulk, a resistance which is related to the rate of trapping holes in surface states, Rtrap, a capacitance of the surface states, Css, a charge-transfer resistance through the surface states, Rss, and series resistance, Rs. Conduction band (CB) and valence band (VB) with red line represent the schematic band structure of a-Si under light irradiation. 263x104mm (96 x 96 DPI)



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Figure 6. Linear sweep voltammograms and capacitance of the surface states, Css (red dots) and chargetransfer resistance through the surface states, Rss (blue dots) values obtained for a SiOx/a-Si electrode using buffer solutions with pH of (a) 1.0, (b) 4.0 and (c) 7.0. The illumination source was a 20-mW LED (625 nm). The identical scale bar for current density is also applied for voltammograms shown in (b) and (c). 302x196mm (96 x 96 DPI)


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Photoelectrochemical and Impedance Spectroscopic Analysis of

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