Silicon Photocathodes Research Articles

Operando Electrochemical Raman Spectroscopy Studies of Working Electrodes in (photo)Electrocatalytic Water Splitting

Raman spectroscopy has been applied as a vibrational spectroscopic technique in the field of chemical, materials, and life sciences. Recently, the operando electrochemical Raman spectroscopy has received more research attention for the characterizations of working electrodes in (photo)electrochemical catalysis. This technique allows for the characterization of the fate of electrocatalysts on the surface of the working electrodes under electrochemical reaction conditions, which is crucial to determine the chemical identify of the electrocatalysts.We apply the operando electrochemical Raman spectroscopy to study the anodes and cathodes in the (photo)electrocatalytic water splitting process, which is an important process to generate the clean energy hydrogen (H2) gas from water using solar energy and electricity. Specifically, we study the stability of the electrocatalyst molybdenum sulfide (MoS2) on silicon (Si) photocathode for the hydrogen evolution reaction (HER) and the electrochemical transformation of the electrocatalyst cobalt hydroxide (Co(OH)2) on nanostructured gold (Au) photoanode for the oxygen evolution reaction (OER). We have identified the correlation between the morphological changes and vibrational features of the MoS2 layer and detected the potential-dependent vibrational features of the Co(OH)2 film.

Passivation Effect of Rock-Salt Nitride Materials on a Planar p-Si Photocathode for Solar Water Reduction.

Photoelectrochemical water splitting, which uses sunlight to produce clean hydrogen fuel, is gaining traction with rising concerns about fossil fuels and pollution. Silicon (Si) photocathodes are promising for this process due to their light absorption and favorable band alignment for hydrogen production. However, limitations such as reflectance, low photovoltage, and rapid photocorrosion hinder their overall performance and stability. An interfacial insulating layer such as SiOx ensures superior durability to p-Si photocathodes. However, the inherent heterogeneity and intrinsic defects pose challenges to effective surface passivation. This study investigates the challenges of photocorrosion and utilization of a thin titanium nitride (TiN) passivation layer deposited using direct-current magnetron sputtering to protect the native silicon oxide (SiOx) layer. Additionally, a molybdenum oxynitride (Mo(O,N)x) bifunctional layer is employed to enhance both the electrocatalytic activity and durability of the photocathode. The presence of oxygen and nitrogen within this cocatalyst layer modifies the surface chemistry of the p-Si photocathode, promoting favorable interfacial interactions with electrolyte species during PEC reactions. This modification facilitates efficient charge transfer processes and accelerates reaction kinetics, ultimately optimizing the performance and operational lifetime of the photocathode. The resulting Mo(O,N)x/TiN/p-Si photocathode exhibited a remarkable onset potential of +0.46 VRHE in harsh acidic conditions under simulated sunlight (AM 1.5G illumination, 100 mW·cm-2). Notably, the photocathode demonstrated exceptional long-term stability exceeding 140 h, highlighting the combined TiN passivation and Mo(O,N)x cocatalyst as a protection strategy.

WOx-Carbon nanorods catalyzed photoelectrochemical hydrogen evolution reaction of silicon-photocathode in acidic media

Silicon (Si) photocathode with surface decoration of co-catalysts is a promising material for green hydrogen production. The surface of the Si is often further etched into pyramids for optical absorption. However, the photon-generated charges tend to concentrate on the pyramid peaks, whilst the catalyst of nanoparticles generally aggregate in the valley, where less charges are available. In this work, one-dimensional tungsten oxide (WOx) with an ultra-thin layer of carbon species have been used as the cocatalyst, which preferentially stay on the upperpart of the pyramids with intimate contact. WOx has great potential in catalysis for hydrogen evolution reaction (HER) due to its ideal proton adsorption/desorption behavior. However, WOx, especially in form of nanorods, has rarely been reported as cocatalysts for photo(electro)catalysis, due to the challenges in the preparation and stabilities during long-term HER catalysis. Herein, the formation of WOx-C nanorods was induced, leading to improved conductivity and stability as co-catalysts. The density functional theory (DFT) calculations revealed the low reaction free energy of WOx-C co-catalysts and the smooth electrons transfer from W to protons. The Si photocathodes deposited with the optimized WOx-C hybrids nanorods showed high performance in PEC reaction, achieving a current density of −21.3 mA·cm−2 at 0 VRHE and an onset potential of +0.218 VRHE under acidic conditions. The catalytic mechanism of WOx-C nanorods for the pyramidal Si-photocathode is investigated and proposed.

Enhanced Capability of Hydrogen Evolution Photocathode by Laminated Interface Engineering of Co/MoS2 QDs/pyramid-black Si.

We present a novel and stable laminated structure to enhance the performance and stability of silicon (Si) photocathode devices for photoelectrochemical (PEC) water splitting. First, by utilizing Cu nanoparticle catalysts to work on a n+p-black Si substrate via the metal-assisted chemical etching, we can achieve the black silicon with a porous pyramid structure. The low depth holes on the surface of the pyramid caused by Cu etching not only help enhance the light capture capability with quite low surface reflectivity (<5%) but also efficiently protect the p-n junction from damage. To improve the charge migration efficiency and mitigate parasitic light absorption from cocatalysts at the same time, we drop casted quantum dots (QDs) MoS2 with the size of nanometer scale as the first layer of catalyst. Hence, we then can safely electrodeposit cocatalyst Co nanoparticles to further enhance interface transfer efficiency. The synergistic effects of cocatalysts and optimized light absorption from the morphology and QDs contributed to the overall enhancement of PEC performance, offering a promising pathway for an efficient, low cost, and stable (over 100 h) hydrogen production photocathode.

Silicon photocathode functionalized with osmium complex catalyst for selective catalytic conversion of CO2 to methane

Solar-driven CO2 reduction to yield high-value chemicals presents an appealing avenue for combating climate change, yet achieving selective production of specific products remains a significant challenge. We showcase two osmium complexes, przpOs, and trzpOs, as CO2 reduction catalysts for selective CO2-to-methane conversion. Kinetically, the przpOs and trzpOs exhibit high CO2 reduction catalytic rate constants of 0.544 and 6.41 s−1, respectively. Under AM1.5 G irradiation, the optimal Si/TiO2/trzpOs have CH4 as the main product and >90% Faradaic efficiency, reaching −14.11 mA cm−2 photocurrent density at 0.0 VRHE. Density functional theory calculations reveal that the N atoms on the bipyrazole and triazole ligands effectively stabilize the CO2-adduct intermediates, which tend to be further hydrogenated to produce CH4, leading to their ultrahigh CO2-to-CH4 selectivity. These results are comparable to cutting-edge Si-based photocathodes for CO2 reduction, revealing a vast research potential in employing molecular catalysts for the photoelectrochemical conversion of CO2 to methane.

Crystalline Silicon Photocathode with Tapered Microwire Arrays Achieving a High Current Density of 41.7 mA cm⁻2

AbstractTo design a high‐efficiency crystalline silicon (c‐Si) photocathode, the photovoltage and photocurrent generated by the device must be maximized because these factors directly affect the hydrogen evolution reaction (HER). In this study, a c‐Si p–n junction is used to enhance the photovoltage of the c‐Si photocathode, and a tapered microwire array structure is introduced to increase the photocurrent. When tapered microwire arrays are employed on the front surface of the c‐Si photocathode, a current density of ≈41.7 mA cm−2 is achieved at 0 VRHE (reversible hydrogen electrode); this current density is the highest among all reported photocathodes including c‐Si, approaching the theoretical maximum value for c‐Si. Furthermore, a Ni foil/Pt catalyst is introduced on the opposite side of the incident light, simultaneously serving as an electrocatalyst to reduce side reactions in the HER and encapsulation layer to prevent c‐Si from contacting the electrolyte. Thus, a stable device is developed using c‐Si photoelectrochemical cells that have an efficiency exceeding 97% for &gt;1000 h.

Sulfidation-Induced Surface Local Electronic and Atomic Structures in a Silver Catalyst Enables Silicon Photocathode for Selective and Efficient Photoelectrochemical CO2 Reduction.

Converting CO2 to value-added chemicals through a photoelectrochemical (PEC) system is a creative approach toward renewable energy utilization and storage. However, the rational design of appropriate catalysts while being effectively integrated with semiconductor photoelectrodes remains a considerable challenge for achieving single-carbon products with high efficiency. Herein, we demonstrate a novel sulfidation-induced strategy for in situ grown sulfide-derived Ag nanowires on a Si photocathode (denoted as SD-Ag/Si) based on the standard crystalline Si solar cells. Such an exquisite design of the SD-Ag/Si photocathode not only provides a large electrochemically active surface area but also endows abundant active sites of Ag2S/Ag interfaces and high-index Ag facets for PEC CO production. The optimized SD-Ag/Si photocathode displays an ideal CO Faradic efficiency of 95.2% and an onset potential of +0.26 V versus the reversible hydrogen electrode, ascribed to the sulfidation-induced synergistic effect of the surface atomic arrangement and electronic structure in Ag catalysts that promote charge transfer, facilitate CO2 adsorption and activation, and suppress hydrogen evolution reaction. This sulfidation-induced strategy represents a scalable approach for designing high-performance catalysts for electrochemical and PEC devices with efficient CO2 utilization.

Rational design of cocatalyst for highly improved ammonia production from photoelectrochemical nitrate reduction

Photoelectrochemical nitrate reduction reaction (p-NO3RR) has been one of the most promising ways for ambient ammonia synthesis. However, current trials haven’t realized efficient ammonia production due to low ammonia yield. Herein, we present that silicon (Si) photocathode with CoNi as co-catalyst can achieve low onset potential, high ammonia yield, and close-to-unity selectivity in p-NO3RR. The optimal photocathode (Co0.95Ni0.05/Si) exhibits a record-high ammonia yield of 2054 µg h−1 cm−2 with an excellent Faraday efficiency (FE) of 98.6% at only −0.1 V vs. reversible hydrogen electrode (RHE), which outperforms reported photocathodes in literature. Our characterizations and calculations demonstrate that the metallic Co dominantly exists as the interior of Co0.95Ni0.05/Si that accelerates electron transfer, while Ni is introduced into Co3O4 on the surface that optimizes the adsorption behavior and improves the activity and selectivity. We believe that the findings provide insightful guidance for enhanced performance towards large-scale ammonia synthesis by photoelectrochemical nitrate reduction.

Lithium-Mediated Photoelectrochemical Ammonia Synthesis with 95% Selectivity on Silicon Photocathode

Lithium-Mediated Photoelectrochemical Ammonia Synthesis with 95% Selectivity on Silicon Photocathode

Enhancing the Photoelectrochemical Performance of a Nanoporous Silicon Photocathode through Electroless Nickel Deposition.

Renewable energy sources, particularly solar energy, are key to our efforts to decarbonize. This study investigates the photoelectrochemical (PEC) behavior of nanoporous silicon (NPSi) and its Ni-coated hybrid system. The methods involve the application of a Ni coating to NPSi, a process aimed at augmenting catalytic activity, light absorption, and carrier transport. Scanning electron microscopy was used to analyze the morphological changes on NPSi surfaces due to the Ni coating. Results demonstrate that the Ni coating creates unique structures on NPSi surfaces, with peak PEC performance observed at 15 min of coating time and 60 °C. These conditions were found to promote electron-hole pair separation and uniform Ni coverage. A continuous 50-min white light illumination experiment confirmed stable PEC fluctuations, showing the interplay of NPSi's characteristics and Ni's catalytic effect. This study provides practical guidance for the design of efficient water-splitting catalysts, contributing to the broader field of renewable energy conversion.

Robust SrTiO3 Passivation of Silicon Photocathode by Reduced Graphene Oxide for Solar Water Splitting.

Development of a robust photocathode using low-cost and high-performing materials, e.g., p-Si, to produce clean fuel hydrogen has remained challenging since the semiconductor substrate is easily susceptible to (photo)corrosion under photoelectrochemical (PEC) operational conditions. A protective layer over the substrate to simultaneously provide corrosion resistance and maintain efficient charge transfer across the device is therefore needed. To this end, in the present work, we utilized pulsed laser deposition (PLD) to prepare a high-quality SrTiO3 (STO) layer to passivate the p-Si substrate using a buffer layer of reduced graphene oxide (rGO). Specifically, a very thin (3.9 nm ∼10 unit cells) STO layer epitaxially overgrown on rGO-buffered Si showed the highest onset potential (0.326 V vs RHE) in comparison to the counterparts with thicker and/or nonepitaxial STO. The photovoltage, flat-band potential, and electrochemical impedance spectroscopy measurements revealed that the epitaxial photocathode was more beneficial for charge separation, charge transfer, and targeted redox reaction than the nonepitaxial one. The STO/rGO/Si with a smooth and highly epitaxial STO layer outperforming the directly contacted STO/Si with a textured and polycrystalline STO layer showed the importance of having a well-defined passivation layer. In addition, the numerous pinholes formed in the directly contacted STO/Si led to the rapid degradation of the photocathode during the PEC measurements. The stability tests demonstrated the soundness of the epitaxial STO layer in passivating Si against corrosion. This study provided a facile approach for preparing a robust protection layer over a photoelectrode substrate in realizing an efficient and, at the same time, durable PEC device.

Electrochemical Impedance Spectroscopy of Amorphous TiO2 Protected Photocathodes Under Hydrogen Evolution Conditions

Despite widespread use of electrochemical impedance spectroscopy (EIS) to study multi-layer photoelectrochemical (PEC) cells, the appropriate equivalent circuits describing these devices under hydrogen evolution conditions are not commonly investigated.1 This creates ambiguity with respect to the pathways of charge recombination and ionic movement within layers of the device during operation, therefore limiting rational development of future PEC devices. In this study, amorphous TiO2 (a-TiO2) protected silicon photocathodes have been investigated using EIS to show that these devices behave according to the Maxwell circuit during hydrogen evolution, which occurs when the semiconductor is in depletion. Therefore, there are two simultaneous processes occurring in parallel and on different timescales. While the high frequency component is the result of the depletion region in the semiconductor, the low frequency component of this circuit is found to originate from the a-TiO2 itself, implying a capacitive charging process of this protective overlayer during PEC cell operation. Studies to isolate the physical mechanism of capacitive charging in the a-TiO2 from a range of possibilities such as proton intercalation and vacancy migration have been carried out. This result is important for the development of future PEC devices using emerging semiconductors which heavily rely on the a-TiO2 overlayer in acidic electrolytes, such as Cu2O and Sb2Se3, where this low frequency process has previously been observed.1 Using the Maxwell circuit for a-TiO2 protected photocathodes under hydrogen evolution conditions, one can independently determine properties of the semiconductor, such as the depletion capacitance and doping density, and the protective overlayer in these PEC devices under hydrogen evolution conditions. With this baseline, EIS can continue to aid the development of PEC cells as additional overlayers are developed and tested for further performance improvements. Yang, W. et al. Operando Analysis of Semiconductor Junctions in Multi-Layered Photocathodes for Solar Water Splitting by Impedance Spectroscopy. Adv. Energy Mater. 11, 2003569 (2021). Figure 1

Covalently Grafting Graphene onto Si Photocathode to Expedite Aqueous Photoelectrochemical CO2 Reduction

AbstractSilicon semiconductor functionalized with molecular catalysts emerges as a promising cathode for photoelectrochemical (PEC) CO2 reduction reaction (CO2RR). However, the limited kinetics and stabilities remains a major hurdle for the development of such composites. We herein report an assembling strategy of silicon photocathodes via chemically grafting a conductive graphene layer onto the surface of n+‐p Si followed by catalyst immobilization. The covalently‐linked graphene layer effectively enhances the photogenerated carriers transfer between the cathode and the reduction catalyst, and improves the operating stability of the electrode. Strikingly, we demonstrate that altering the stacking configuration of the immobilized cobalt tetraphenylporphyrin (CoTPP) catalyst through calcination can further enhance the electron transfer rate and the PEC performance. At the end, the graphene‐coated Si cathode immobilized with CoTPP catalyst managed to sustain a stable 1‐Sun photocurrent of −1.65 mA cm−2 over 16 h for CO production in water at a near neutral potential of −0.1 V vs. reversible hydrogen electrode. This represents a remarkable improvement of PEC CO2RR performance in contrast to the reported photocathodes functionalized with molecular catalysts.

Covalently Grafting Graphene onto Si Photocathode to Expedite Aqueous Photoelectrochemical CO2 Reduction.

Silicon semiconductor functionalized with molecular catalysts emerges as a promising cathode for photoelectrochemical (PEC) CO2 reduction reaction (CO2 RR). However, the limited kinetics and stabilities remains a major hurdle for the development of such composites. We herein report an assembling strategy of silicon photocathodes via chemically grafting a conductive graphene layer onto the surface of n+ -p Si followed by catalyst immobilization. The covalently-linked graphene layer effectively enhances the photogenerated carriers transfer between the cathode and the reduction catalyst, and improves the operating stability of the electrode. Strikingly, we demonstrate that altering the stacking configuration of the immobilized cobalt tetraphenylporphyrin (CoTPP) catalyst through calcination can further enhance the electron transfer rate and the PEC performance. At the end, the graphene-coated Si cathode immobilized with CoTPP catalyst managed to sustain a stable 1-Sun photocurrent of -1.65 mA cm-2 over 16 h for CO production in water at a near neutral potential of -0.1 V vs. reversible hydrogen electrode. This represents a remarkable improvement of PEC CO2 RR performance in contrast to the reported photocathodes functionalized with molecular catalysts.

Study of the Mechanism of Photoactivated Hydrogen Evolution on a Silicon Photocathode with a-MoSx Thin-Film Catalyst

Study of the Mechanism of Photoactivated Hydrogen Evolution on a Silicon Photocathode with a-MoSx Thin-Film Catalyst

Silver Nanoparticles Embedded within a Silicon Photocathode for Photoelectrochemical Fixation of CO2 to Synthesize Intermediates of Profen Drugs

Photoelectrochemical (PEC) fixation of carbon dioxide (CO2) into intermediates of Profen drugs, a kind of nonsteroidal antiinflammatory drug, is a useful and important technology for the utilization of CO2. In this work, we successfully designed and prepared a silver nanoparticles (AgNPs)-embedded silicon photocathode for PEC-fixating CO2 to synthesize intermediates of Profen drugs with high efficiency and stability. AgNPs endow the photocathode with extremely high catalytic activity, the Schottky junction between Ag and Si promotes the separation and transfer of photogenerated carriers, the AgNPs-embedded structure greatly improves the stability of the photocathode, and the nanohole structure improves light absorption.

Nanoporous silicon photocathodes with [Co] molecular catalyst for enhanced solar-driven hydrogen generation

Efficient photoelectrochemical water splitting by nanoporous Si photocathode using Co(dmgH)2(py)Cl as H2-evolution catalyst under illumination of simulated sunlight.

Aqueous Photoelectrochemical CO2Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

AbstractWe report a precious‐metal‐free molecular catalyst‐based photocathode that is active for aqueous CO2reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3‐aminopropyl)triethoxysilane linker to p‐type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO2to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of −0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s−1. To date, this is the only molecular catalyst‐based photoelectrode that is active for the six‐electron reduction of CO2to methanol. This work puts forth a strategy for interfacing molecular catalysts to p‐type semiconductors and demonstrates state‐of‐the‐art performance for photoelectrochemical CO2reduction to CO and methanol.

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst.

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO2 reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO2 to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of -0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s-1 . To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO2 to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO2 reduction to CO and methanol.

Optimization of MoSx Pulsed Laser Deposition of a Thin-Film Catalyst for an Efficient Hydrogen Evolution Reaction on a Silicon Photocathode

Optimization of MoSx Pulsed Laser Deposition of a Thin-Film Catalyst for an Efficient Hydrogen Evolution Reaction on a Silicon Photocathode