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Oxidatively stable nanoporous silicon photocathodes with enhanced onset voltage for photoelectrochemical proton reduction Yixin Zhao, Nicholas C Anderson, Kai Zhu, Jeffery A. Aguiar, Jason A. Seabold, Jihun Oh, Jao van de Lagemaat, Howard M. Branz, and Nathan R. Neale Nano Lett., Just Accepted Manuscript • Publication Date (Web): 27 Feb 2015 Downloaded from http://pubs.acs.org on February 27, 2015
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Oxidatively stable nanoporous silicon photocathodes with enhanced onset voltage for photoelectrochemical proton reduction Yixin Zhao,a‡ Nicholas C. Anderson,a Kai Zhu,a Jeffery A. Aguiar,a Jason A. Seabold,a Jihun Oh,c Jao van de Lagemaat,a Howard M. Branz,a* and Nathan R. Nealea*
Dr. Y. Zhao, Dr. N. C. Anderson, Dr. K. Zhu, Dr. J. A. Aguiar, Dr. J. A. Seabold, Dr. J. van de Lagemaat, Dr. H. M. Branz* and Dr. N. R. Neale* a
National Renewable Energy Laboratory, Golden, Colorado 80401; E-mail:
[email protected]; Howard.Branz @nrel.gov Prof. J. Oh b
Korea Advanced Institute of Technology, Daejeon, South Korea.
‡
Present address: Shanghai Jiao Tong University,Shanghai, China
Keywords:
(Photoelectrochemical
Water
Splitting,
Hydrogen
Production,
Silicon
Photoelectrode, Nanoporous Silicon, Surface Oxidation)
ABSTRACT Stable and high-performance nanoporous “black silicon” photoelectrodes with electrolessly deposited Pt nanoparticle (NP) catalysts are made with two metal-assisted etching steps. Doubly etched samples exhibit an ~300 mV positive shift in photocurrent onset for photoelectrochemical proton reduction compared to oxide-free planar Si with identical catalysts. We find that the photocurrent onset voltage of black Si photocathodes prepared from single-crystal planar Si wafers by an Ag-assisted etching process increases in oxidative environments (e.g., aqueous electrolyte) owing to a positive flat-band potential shift caused by surface oxidation. However, within 24 h, the surface oxide layer becomes a kinetic barrier to interfacial charge transfer that inhibits proton reduction. To mitigate this issue, we developed a novel second Pt-assisted etch process that buries the Pt NPs deep into the nanoporous Si surface. This second etch shifts the onset voltage positively, from +0.25 V to +0.4 V versus reversible hydrogen electrode, and reduces the charge-transfer resistance with no performance decrease seen for at least two months. Electrochemical impedance studies reveal that the second etch leads to a considerably smaller interfacial charge-transfer resistance than samples without the additional etch, suggesting that burying the Pt NPs improves the interfacial contact to the crystalline silicon surface. 1 Environment ACS Paragon Plus
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INTRODUCTION Using sunlight to generate fuel such as H2 through direct photoelectrochemical (PEC) processes is a promising approach to extend solar energy conversion into the transportation sector or provide storage of solar energy for electricity generation.1-3 Although the 1.1 eV indirect bandgap of silicon is too low to split water, this inexpensive, Earth-abundant, and industrially mature semiconductor has many attractive features as a photocathode material for PEC water splitting, including a low bandgap that enables absorption of much of the solar spectrum and a conduction band energy level suitable for hydrogen generation. Recently, nanostructured Si photocathodes have been developed for hydrogen production due to their advantages of low reflectivity and a higher surface area compared to bulk planar Si, which improves charge collection, exchange current density, and hydrogen gas surface desorption.421
Our group and others have focused on the metal-assisted solution etching of planar Si to
prepare nanostructured Si—also called “black Si”—a process that is suitable for large-scale manufacture and has been applied to make high-efficiency Si solar cells.22-24 However, the performance of Si electrodes used in any aqueous PEC system normally deteriorates from surface oxidation, an effect that might appear to be unavoidable because of the high reactivity of Si with O2, especially in the presence of water.25 In a deployed PEC hydrogen production system, surface oxidation might be expected during the night and at other times when photoelectrons are not providing cathodic protection against surface oxidation. Any oxidized surface layer introduces a kinetic barrier to proton reduction. In previous reports, surface oxide removal is usually performed before the PEC measurement to minimize the kinetic barrier.9, 15 However, a new surface oxide forms on freshly etched Si photoelectrodes in air within hours after removal of the original oxide layer,25 a tendency that limits the practical application of Si photoelectrodes in a water-splitting system. If a nanostructured electrode surface is used, repeated cycles of oxidation and reduction would be expected to erode the delicate nanostructured features. Although the deposition of a surfaceadsorbed catalyst such as Pt can help reduce the kinetic barrier for proton reduction, the PEC properties of Pt-deposited Si photoelectrodes were also found to degrade significantly upon surface oxidation.5, 10 Consistent with these studies on planar Si, nanostructured Si photoelectrodes made from single-crystal Si have not yet demonstrated durability for use in PEC hydrogen generation.15, 16, 20, 21 For instance, we observed a 70 mV positive shift of the proton reduction onset potential in catalyst-free black Si compared with planar Si photocathodes, but a stable photocurrent is achieved for just 5 min under strong bias (–1.5 V vs. Ag/AgCl in 0.5 M 2 Environment ACS Paragon Plus
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H2SO4 electrolyte).21 Similarly, nanowire Si photoelectrodes with surface-adsorbed Pt nanoparticle (NP) catalysts exhibit a small ~50 mV positive shift of the proton reduction onset potential compared to planar Si with identical catalysts; but in this case, no durability measurements were reported.15 Given the paucity of data on the stability of these systems, we were motivated to probe how oxidative conditions present in any real-world application would affect the PEC performance of nanostructured Si photoelectrodes. Not surprisingly, here we find that our black Si photoelectrodes with electrolessly deposited Pt NP catalysts are unstable in air, with a negative onset voltage shift occurring upon surface oxidation. To address this issue, we have developed a novel re-etching process that appears to provide an excellent interfacial contact between the Pt NP catalysts and the nanoporous Si photoelectrode surface, which results in high PEC performance despite the presence of surface oxide. These Pt-modified black Si photoelectrodes exhibit an ~300 mV positive shift for proton reduction compared to a Pt-modified planar Si photoelectrode and are stable even after months of air exposure.
EXPERIMENTAL DETAILS Silicon photoelectrode formation: We produced the silicon photocathodes from 300 µm thick B-doped p-type (100) float-zone Si wafers with resistivities of 3–5 Ω⋅cm (Topsil). The planar Si wafer (pl-Si) was first spin-coated with S-1818 photoresist on its front side. After oxide removal in 5 wt% HF for 30 s, a back-side ohmic contact was formed by applying a commercial Ga-In eutectic alloy. The Ga-In eutectic with Si was then contacted with silver paint and connected to Cu wires protected within a glass tube. The exposed back-side Si surface, the sample edges, and a portion of the front side of the Si electrodes were then sealed with Loctite 9462 epoxy. A Si electrode area of 0.3–0.5 cm2 was left exposed. The photoresist layer was removed by rinsing with acetone and isopropanol immediately before subsequent processing or measurements. Nanoporous black Si formation: To form nanoporous electrodes, the planar Si photoelectrode was first etched in 5 wt% HF for 1 min to remove the native oxide and then rinsed with de-ionized (DI) water and dried under N2. Nanoporous black Si formation began with an electroless Ag deposition in 1 mM AgNO3 and 1 wt% HF solution for 30 s, followed by a DI water rinse and N2 dry. The sample was then transferred into a metal-assisted etch solution of 50 wt% HF, 30 wt% H2O2, and DI water with volume ratio of 25:6:370 for 6 min, followed by rinsing with DI water. Any Ag NPs remaining at the bottom of etched nanopores were removed by soaking in a 35 wt% 3 Environment ACS Paragon Plus
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HNO3 solution for 6 min. Detailed procedures for this metal-assisted etching are published elsewhere.20, 21 The resulting black Si photocathode was then etched in 5 wt% HF for 30 s, rinsed with DI water, dried under N2, and finally etched with 0.5 wt% tetramethylammonium hydroxide (TMAH) solution for 2 s to decrease the sharpness and areal density of nanostructured features,22 before a rinse in DI water and a final N2 dry. During all the wetetching steps above, the photoelectrodes were leaned on the beaker wall with the black Si side facing up at an angle of about 45° to the horizontal. Pt nanoparticle deposition and secondary etching: Pt NPs were electrolessly deposited on the surfaces of planar Si photoelectrodes by first etching with 5 wt% HF for 5 s to 1 min (typically 15 s), and then immersing the wafer into a solution of 1 wt% HF and 1 mM K2PtCl6. Nanostructured Si photoelectrodes were immersed into a solution of 1 wt% HF and 1 mM K2PtCl6 immediately following TMAH treatment (with no HF etch). The optimal Pt deposition duration is 45–90 s (see Fig. S1). The secondary metal-assisted etch process to bury Pt NP catalysts into the black Si nanopores was performed by treating black Si photoelectrodes containing surface-adsorbed Pt NP catalysts with a 1 wt% H2O2 solution for 30 min to oxidize the Si surface, followed by etching with the same metal-assisted etching solution (50 wt% HF, 30 wt% H2O2, and deionized H2O, with volume ratio of 25:6:370) used previously, but for a shorter period of 1–2 min. Photoelectrochemical measurements: Catalyst-free samples (both planar and black Si) were immersed into a 5 wt% HF solution for 1 min immediately prior to testing to remove the surface oxide layer. Samples with electrolessly deposited Pt NP catalysts were also tested immediately following a 1 min 5 wt% HF etch. A custom-built Pyrex glass vessel with flat quartz windows was used as the PEC cell. A Xe lamp (300 W, Oriel) with an AM1.5 filter was the light source, with intensity of 100 mW/cm2 simulated 1-sun AM1.5 illumination as calibrated by a Si photodiode (Solarex, Serial No. 147). The electrolyte contained 0.5 M H2SO4 (ACS reagent grade, J.T. Baker), and a standard 3-electrode setup with Ag/AgCl (3M NaCl) reference and a Pt foil counter electrode was used throughout. Potentials were converted into RHE using the equation: V vs RHE = V vs Ag/AgCl + 0.234 V.15, 26, 27 Electrochemical measurements were conducted using a potentiostat (Solartron 1287) interfaced with a computer running CorrWare® for instrument control and data collection. The photoelectrochemical current density–voltage (J–V) curves were measured from +0.2 V to –1.2 V vs. Ag/AgCl (3M NaCl) at a scan rate of 20 mV/s. Deleterious oxidation reactions were avoided by scanning potentials to ≤+0.2 V vs. Ag/AgCl since anodic current was observed at potentials more positive than 4 Environment ACS Paragon Plus
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+0.2 V vs Ag/AgCl. J–V curves showed no dependence on illumination of the Ag/AgCl reference electrode. In aging experiments, we found that the Loctite 9462 epoxy sealant was unstable when exposed to 0.5 M H2SO4 electrolyte solution for longer than 24 h. Aging effects ≤24 h were observed to be identical in 0.5 M H2SO4 electrolyte and air; therefore, reported aging experiments shorter than 24 h were performed in the electrolyte, but aging treatments longer than 24 h were performed by removing the Si photocathodes from electrolyte solution and storing in air. Capacitance–Voltage measurements: Capacitance–voltage (C–V) measurements were conducted with a Brookdeal 9505SC lock-in amplifier, a Hewlett-Packard 3325A frequency synthesizer, and a PAR 173 potentiostat, which were configured as previously reported.28 The black Si photoelectrode was used as the working electrode in a three-electrode system, together with the same Ag/AgCl (3M NaCl) reference electrode, and a platinum foil counter electrode in 0.5 M H2SO4 solution. C–V data were measured in the dark with frequencies from 10 mHz to 20 kHz;28 only weak frequency dependence was found in these Mott–Schottky plots at higher frequencies (Fig. S1B), and the data measured at 10 kHz were used for Mott-Schottky analyses. Electrochemical impedance spectroscopy: A PARSTAT 2273 workstation was used to perform electrochemical impedance spectroscopy (EIS) measurements in the three-electrode configuration. The frequency range was 0.1 Hz–100 kHz and the magnitude of the modulation signal was 10 mV. All the measurements were performed in 0.5 M H2SO4 at room temperature and at 0.0 V vs. Ag/AgCl (3M NaCl) with irradiation of 10 mW/cm2 from a white-light LED (Thorlab MCWHL5-C1). The EIS spectra were fitted to an appropriate electrical analogue using ZView® software (Scribner Associates Inc.). Electron Microscopy Scanning electron microscopy (SEM) images were obtained at 3 kV with a working distance at 2 mm, using an FEI Nova NanoSEM 630. Energy dispersive X-ray (EDX) was analyzed with an EDAX Apollo detector at 10kV. Transmission electron microscopy (TEM) was performed on a cross-section of black silicon. The sample was prepared by focused ion beam, where a 1 µm size layer of fine dispersed electron beam platinum was first deposited to protect the surface, followed by a coarsened ion beam deposited platinum to a thickness of 3 µm. The sample was thinned using a gallium 30 kV followed by 5 kV beam. The resultant FIB lamellae was then finally thinned 5 Environment ACS Paragon Plus
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to electron transparency (
