Silicon Photocathodes Research Articles

Oxidatively stable nanoporous silicon photocathodes with enhanced onset voltage for photoelectrochemical proton reduction.

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. PEC performance was stable owing to Pt NP catalysts that were buried deeply in the photoelectrode by the second etch, below a thick surface layer comprised primarily of amorphous SiO2 along with some degree of remaining crystalline Si as observed by scanning and transmission electron micrographs. 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.

Nickel-coated silicon photocathode for water splitting in alkaline electrolytes

Photoelectrochemical (PEC) water splitting is a promising approach to harvest and store solar energy [1]. Silicon has been widely investigated for PEC photoelectrodes due to its suitable band gap (1.12 eV) matching the solar spectrum [2]. Here we investigate employing nickel both as a catalyst and protecting layer of a p-type silicon photocathode for photoelectrochemical hydrogen evolution in basic electrolytes for the first time. The silicon photocathode was made by depositing 15 nm Ti on a p-type silicon wafer followed by 5 nm Ni. The photocathode afforded an onset potential of ∼0.3 V vs. the reversible hydrogen electrode (RHE) in alkaline solution (1 M KOH). The stability of the Ni/Ti/p-Si photocathode showed a 100 mV decay over 12 h in KOH, but the stability was significantly improved when the photocathode was operated in potassium borate buffer solution (pH ≈ 9.5). The electrode surface was found to remain intact after 12 h of continuous operation at a constant current density of 10 mA/cm2 in potassium borate buffer, suggesting that Ni affords good protection of Si based photocathodes in borate buffers.

Stable and efficient multi-crystalline n+p silicon photocathode for H2 production with pyramid-like surface nanostructure and thin Al2O3 protective layer

When a Si photocathode is used in a photoelectrochemical cell for H2 production, an open nanostructure capable of enhanced light absorption, low surface recombination, and being fully protected by thin protective layer is highly desirable. Here, we explored a highly stable and efficient multi-crystalline (mc) n+p silicon photocathode. A pyramid-like surface nanostructure on mc-Si wafer was fulfilled through a two-step metal-catalyzed chemical etching process, and then a n+p junction photocathode protected by a thin Al2O3 layer was constructed. The photocathode exhibits a high stability of continuous photoelectrochemical H2 production for above 100 h after a thin layer of Al2O3 is coated on its surface, and its energy conversion efficiency can be up to 6.8% after Pt loading, due to the lowered surface light reflection, increased surface area and minority carrier life time on the electrode surface.

Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials

Silicon is a promising photocathode for tandem photoelectrochemical water splitting devices, but efficient catalysis and long term stability remain key challenges. Here, it is demonstrated that with appropriately engineered interfaces, molybdenum sulfide nanomaterials can provide both corrosion protection and catalytic activity in silicon photocathodes. Using a thin MoS2 surface protecting layer, MoS2‐n+p Si electrodes that show no loss in performance after 100 h of operation are created. Transmission electron microscopy measurements show the atomic structure of the device surface and reveal the characteristics of the MoS2 layer that provide both catalytic activity and excellent stability. In spite of a low concentration of exposed catalytically active sites, these electrodes possess the best performance of any precious metal‐free silicon photocathodes with demonstrated long term stability to date. To further improve efficiency, a second molybdenum sulfide nanomaterial, highly catalytically active [Mo3S13]2− clusters, is incorporated. These photocathodes offer a promising pathway towards sustainable hydrogen production.

Facile synthesis of macroporous silicon photocathodes with enhanced photoelectrochemical performance

Macroporous n-type silicon photocathodes were successfully prepared using an electrochemical etching method under constant current electrolysis at 40mAcm−2. We found that comparing with planar n-type and etching p-type silicon photocathodes, the macroporous n-type silicon photocathodes exhibited considerably higher photocurrent response and photoelectrochemical(PEC) performance in terms of H2 generation under visible light irradiation without requiring a catalyst or a sacrificial agent. This study provides an alternative method of preparing porous silicon and introduces a new photocatalyst or support for noble-metal nanophotocatalysts for PEC water splitting.

Comparison between the measured and modeled hydrogen-evolution activity of Ni- or Pt-coated silicon photocathodes

The electrocatalytic behavior of Ni and Pt nanoparticles for the hydrogen-evolution reaction (HER) on p-type Si photocathodes was measured experimentally and the current density vs. potential (J–E) characteristics of a general metal catalyst on p-Si was modeled as a combination of a Si photodiode in series electrically with metal electrocatalysts. Relative to the rest potential, the J–E characteristics produced by the model showed an increase in total overpotential required to reach a specified current density for the metallized photoelectrodes relative to that of a metal electrode. This prediction was in accord with the experimentally observed behavior of Pt on p-Si, but was in contrast to the behavior observed for Ni on p-Si. Properly accounting for junction energetics and kinetics of the HER is critical to accurate predictions of the solar-to-hydrogen (STH) energy-conversion efficiency of metallized integrated photoelectrochemical systems. Further, models that accurately predict the performance of metal catalysts on semiconductor light absorbers are required to optimize the catalytic performance of metallized photoelectrodes.

Synthesis of Bi2WO6 photoanode on transparent conducting oxide substrate with low onset potential for solar water splitting

High conduction band of Bi2WO6, thus low onset potential of Bi2WO6/Co-Pi photoanode, is favourable for overall water splitting at zero bias potential when it combines with a silicon photocathode.

A quantum dot sensitized catalytic porous silicon photocathode

The fabrication and characterisation of a nano-structured photocathode using indium phosphide QDs and a bio-inspired Fe2S2(CO)6 catalyst sensitized on a p-type porous silicon electrode.

Nanostructural dependence of hydrogen production in silicon photocathodes

Hydrogen production from solar power energy is an important energy and environmental issue. Silicon (Si) has been widely studied as a photocathode for hydrogen production from water splitting. In this study, the electrochemical behavior of a Si photocathode for water splitting is highly dependent on its nanostructure. The optimum nanostructure of a Si photocathode exhibits an enhanced photocurrent and a lower overpotential compared to the planar bulk Si. The limiting current density of nanostructured Si is 1.58 times greater than that of the planar structure for p-type Si/aqueous electrolyte solution. Nanostructured Si without any catalyst notably produced a current density of −10.65 mA cm−2 under Air Mass 1.5 Global conditions with a light intensity of 100 mW cm−2 at the reversible potential vs. reversible hydrogen electrode, which is about 43 times higher than that of the untreated Si structure. The solar-to-hydrogen conversion efficiency of the optimized Si nanowire without depositing any catalyst has reached up to about 70% of the efficiency of planar Si decorated with Pt. This significant enhancement achieved in this study emphasizes the importance of a controlled nanostructure in the development of highly efficient photoelectrochemical devices for hydrogen production.

N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production

Download options Please wait... Supplementary files Supplementary information PDF (1121K) Article information DOI https://doi.org/10.1039/C3EE42106F Article type Paper Submitted 23 Jun 2013 Accepted 09 Sep 2013 First published 09 Sep 2013 Download Citation Energy Environ. Sci., 2013,6, 3658-3664 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions N-doped monolayer graphene catalyst on silicon photocathode for hydrogen production U. Sim, T. Yang, J. Moon, J. An, J. Hwang, J. Seo, J. Lee, K. Y. Kim, J. Lee, S. Han, B. H. Hong and K. T. Nam, Energy Environ. Sci., 2013, 6, 3658 DOI: 10.1039/C3EE42106F To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Uk Sim Tae-Youl Yang Joonhee Moon Junghyun An Jinyeon Hwang Jung-Hye Seo Jouhahn Lee Kye Yeop Kim Joohee Lee Seungwu Han Byung Hee Hong Ki Tae Nam

Novel Micropixelation Strategy to Stabilize Semiconductor Photoelectrodes for Solar Water Splitting Systems

We demonstrate a novel micropixelation strategy to stabilize the p–i hydrogenated amorphous silicon (a-Si:H) photocathodes used for hydrogen production in photoelectrochemical water splitting. The main mechanism of corrosion of planar electrodes involves reduction of the underlying SnO2 contact layer by electrolyte that penetrates through pinholes in the a-Si:H. We photolithographically isolate square pixels (100 μm × 100 μm) of a-Si:H by etching narrow channels in the a-Si:H and filling with protective a-SiNx. Under illumination and bias, we observe improved durability of the micropixelated photocathodes compared to planar electrodes. Extended dark potentiostatic testing also exhibits this slowing and isolation of corrosion by the micropixelated electrode. Implementation of this micropixelation strategy is a key toward creating a water-splitting system based on micrometer-scale Si p–n junction pixels. Panels of these corrosion-resistant pixels could be connected in series to produce photovoltages sufficient to split-water while avoiding photocorrosion. Micropixelation could also improve stability in other photoelectrochemical solar fuel production systems.

Solar Hydrogen Generation at Pre-Structured Silicon Photocathodes Activated by Bimetallic Nanoparticles

not Available.

Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution

The production of fuels from sunlight represents one of the main challenges in the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and although platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. We show that bioinspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo(3)S(4)) efficiently catalyse the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo(3)S(4) clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.

Nanoporous black silicon photocathode for H2 production by photoelectrochemical water splitting

Nanostructured Si eliminates several critical problems with Si photocathodes and dramatically improves a photoelectrochemical (PEC) reaction important to water-splitting. Our nanostructured black Si photocathodes improve the H2 production by providing (1) near-ideal anti-reflection that enables the absorption of most incident light and its conversion to photogenerated electrons and (2) extremely high surface area in direct contact with water that reduces the overpotential needed for the PEC hydrogen half-reaction. Application of these advances would significantly improve the solar H2 conversion efficiency of an ideal tandem PEC system. Finally, the nanostructured Si surface facilitates bubble evolution and therefore reduces the need for surfactants in the electrolyte.

Photoelectrochemical characterization of porous Si

The photoelectrochemical measurements of p-Si and porous silicon in 0.1 M H 2SO 4 were done. The band gap of Si is a bit low for efficient hydrogen production. The silicon was made porous to improve the efficiency of hydrogen production. The porous silicon photocathodes show a substantial improvement in the hydrogen production compared to that of p-Si photocathodes. Studies were done by applying different light flux on the porous silicon.

Photofield emission from a tipped silicon photocathode in IR with a new picosecond Nd–YAG laser at a frequency of 100 MHz

Tipped silicon photocathodes are tested with the IR wavelength (1.064 μm) of a new lock-mode Nd–YAG laser. The impulse width of 30 ps is repeated at a frequency of 100 MHz at a power of 122 W. A quantum yield of 2.5×10 −3 is obtained in a DC gun using the best continuous emission field of 20 MV/m for the photofield emission current. The ratio of the photofield emission current to the field emission current (dark current) is more than 10 4. A previous experiment with the second harmonic of a Nd–YAG at 10 Hz gave 700 pC per pulse of 35 ps width with a quantum yield of 10 −5 at the threshold emission field of 7.5 MV/m.

Theoretical approach of the photofield emission of a degenerated semiconductor. Case of tipped, weakly P-doped, silicon photocathodes

We study theoretically the field and photofield emission in a degenerated semiconductor: a tipped silicon is weakly doped with boron ( ϱ = 3000 Ω cm ) and the field near the tip is greater than 1 GV/m. Due to this important field, the conduction band bends down to the Fermi level near the surface. The degenerated electrons are confined in a well near the surface barrier. We develop a new theoretical approach using few discrete energy levels in the well. We calculate field emission current and compare with experimental results. The photoelectric transition probability is calculated for different values of the wavelength of the incident laser. A growth of the photofield current with the increase of the wavelength is shown theoretically, taking into account the optical properties of the semiconductor. Experimentally, we observe such a behaviour with continuous lasers [1–3]. With a pulsed picosecond laser (35 ps from a Nd-Yag) we observed with harmonic 4 (4.68 eV) a pure photoemission charge of 70 pC and a photofield charge with harmonic 2(2.34 eV) as high as 700 pC [4]. We hope to obtain more charges with harmonic 1 (1.17 eV). This original development should enable us to predict the behaviour of our tipped photocathodes in actual photoinjectors. An experiment with CANDELA is in progress.

Silicon photocathodes with array of tips in a photo-injector

Photo-injectors for linear accelerators and free electron laser need a high brightness photocathode with a dark current lower than 10 mA. Silicon with array of tips shows a photofield emission current with a continuous argon laser with a quantum yield of 1%. The ratio η between photofield and field emission (dark) current is 20 near the threshold of field emission. With a pulsed Nd-YAG laser ( λ = 1.02 μm pulses of 35 ps duration) we observe a pure photoemission charge (70 pC) with harmonic 4 (ℏ ω 4 = 4.68 eV) and a photofield charge above the threshold which is 700 pC with harmonic 2 (ℏ ω 2 = 2.34 eV). We calculate for the photo-injector CANDELA the dark current following the method of Loew and Wang, starting from the experimental Fowler-Nordheim curve and we find a dark current which is 1 μA at 3 GHz at E max = 30 MV/m.

ChemInform Abstract: Improved Efficiency and Stability of Silicon Photocathodes by Electrochemical Etching.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Improved Efficiency and Stability of Silicon Photocathodes by Electrochemical Etching

Improved Efficiency and Stability of Silicon Photocathodes by Electrochemical Etching