Photoelectrochemical CO2 Reduction toward Multicarbon Products with Silicon Nanowire Photocathodes Interfaced with Copper Nanoparticles

Progress in heterostructures for photoelectrocatalytic reduction of carbon dioxide into fuels and value-added products

Carbon capture and utilization (CCU) technology offers a sustainable option to simultaneously address both energy crisis and environmental pollution such as catalytic reduction of carbon dioxide (CO2) into value-added fuel products (e.g., C1-C3). Among diverse CCU strategies, the light-irradiated photoelectrocatalytic (PEC) approach is recognized as a cutting-edge option for efficient CO2 reduction reaction (RR) through the integration of photocatalysis and electrocatalysis within a one-stage hybridized catalytic system. Therefore, this review is meticulously structured to elucidate the potential utility of advanced composite catalysts (e.g., titanium dioxide, metal-organic frameworks, and organic/miscellaneous heterostructure materials) in PEC-CO2RR. It also examines the factors and processes governing their PEC-CO2RR activites in relation to their reduction pathways, electronic structures, charge-carrier dynamics, types of electrolytes, mass transfer, light-adsorption potential, and the viability of active sites. The fundamental principles and working mechanisms of diverse catalytic materials in PEC-CO2RR are also outlined to help establish the advanced catalytic systems based on performance assessments (e.g., in terms of CO2 conversion rate, quantum yield, and space-time yield). Overall, this review is expected to deliver the new path for the construction of the next-generation PEC-CO2RR systems that are upscalable, stable, and reusable with enhanced catalytic activity.

Single-Crystal Metal-Organic and Covalent Organic Framework Hybrids Enable Efficient Photoelectrochemical CO2 Reduction to Ethanol

Multicarbon alcohols produced through photochemical and electrochemical CO2 reduction reactions (CO2RR) are promising alternatives to fossil fuels; however, their selectivity and efficiency remain low due to the high energy barrier for C-C coupling and the competition from hydrocarbon production. Here, we present a strategy to enhance ethanol efficiency and selectivity via cooperative catalysis in porous structures for photoelectrochemical (PEC) CO2RR. Using a coordination-templated strategy, we synthesized single crystals of MOF-COF (MOCOF) hybrids with metalloporphyrins, with their structures determined by single-crystal 3D electron diffraction. The porous frameworks featuring adjacent confined metalloporphyrins efficiently capture and cooperatively activate CO2, achieving outstanding PEC CO2-to-ethanol conversion. Particularly, the Pt-MOCOF delivers a Faradaic efficiency (FE) of 83.5% at -1.0 V with 91.7% carbon selectivity, surpassing state-of-the-art COF or MOF catalysts and ranking it among the top-performing catalysts. The catalyst system displays remarkable stability, maintaining 95% of its activity after 100 h of continuous operation. Experiments and theoretical calculations revealed that the cooperative catalyst enriches and stabilizes intermediates in the channels, guiding the reaction pathway toward ethanol production.

Nanoengineered Kesterite Photocathodes: Enhancing Photoelectrochemical Performance for Water Splitting and Beyond

Harnessing solar energy for the production of storable and transportable chemicals via photoelectrochemical (PEC) reactions offers a promising solution to overcome the intermittence of solar irradiation. Kesterites have been known as cost-efficient, environmentally friendly, and efficient semiconductor photoelectrode materials for PEC solar fuel production. While significant progress has been made in water splitting, there is increasing attention paid to extending applications to CO2 reduction, ammonia synthesis, and more. However, when efficient kesterite-based photoelectrodes are designed for water splitting and beyond, it is crucial to comprehensively consider both photoelectrode activity and reaction selectivity. This review elaborates on strategies for rationally designing kesterite-based photoelectrodes by optimizing photoactivity in terms of photogenerated charge migration and regulating the surface catalytic sites through nanoscale engineering. More importantly, it discusses optical management and system integration to advance PEC device design for future scalable applications. The perspectives and challenges are also proposed for future solar fuel applications.

Semiartificial Photoelectrochemistry for CO2-Mediated Enantioselective Organic Synthesis

Photoelectrochemical (PEC) cells are under intensive development for the synthesis of solar fuels, but CO2 reduction typically only results in simple building blocks such as HCOO-. Here, we demonstrate that CO2-converting PEC cells can drive integrated enzymatic domino catalysis to produce chiral organic molecules by using CO2/HCOO- as a sustainable redox couple. First, we establish a semiartificial electrode consisting of three enzymes co-immobilized on a high surface area electrode based on carbon felt covered by a mesoporous indium tin oxide (ITO) coating. When applying a mild cathodic potential (-0.25 V vs the reversible hydrogen electrode (RHE)), CO2 is reduced to HCOO- using a W-formate dehydrogenase (FDHNvH) from Nitratidesulfovibrio vulgaris Hildenborough, which then enables the reduction of NAD+ to NADH by an NAD+-cofactor-dependent formate dehydrogenase from Candida boidinii (FDHCB). Subsequently, an alcohol dehydrogenase (ADH) uses NADH generated from CO2/HCOO- cycling to reduce acetophenone to chiral 1-phenylethanol in good enantiomeric excess (93%) and conversion yields (38%). Depending on the specific ADH (ADHS or ADHR), either (S)- or (R)-1-phenylethanol can be synthesized at pH 6 and 20 °C. To illustrate solar energy utilization, we integrate the three nanoconfined enzymes with a PEC platform based on an integrated organic semiconductor photocathode to allow for enantioselective synthesis (at +0.8 V vs RHE) based on a solar fuel device. This proof-of-principle demonstration shows that concepts and devices from artificial photosynthesis can be readily translated to precise and sustainable biocatalysis, including the production of chiral organic molecules using light.

Electron Transfer Energetics in Photoelectrochemical CO2 Reduction at Viologen Redox Polymer-Modified p-Si Electrodes

While redox polymer-mediated catalysis at silicon photoelectrodes has been studied since the 1980s, there have been few detailed studies of these materials in photoelectrochemical CO2 reduction. Here, we develop silicon photoelectrodes functionalized with a viologen-based polymer that mediates the formation of catalytic gold nanoparticles. The presence of gold was confirmed by X-ray photoelectron spectroscopy (XPS), and the nanoparticles were imaged with high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We probed the CO2 reduction process during bulk photoelectrolysis to find modest, yet consistent CO faradaic efficiencies across a range of applied potentials. Operando surface-enhanced Raman spectroscopy (SERS) was used to measure the Fermi levels of both the viologen polymer and the Au catalyst sites. The operando measurement of the Fermi levels of all three components of the photocathode provides a unified picture of the electron transfer process in the semiconductor-redox polymer-catalyst system. The redox polymer serves as the electron transfer mediator between the Si substrate and Au sites. In addition, the Au Fermi level equilibrates with the Fermi level of the viologen polymer, which in turn fixes the quasi-Fermi level of Au catalysts at the p-Si/redox polymer interface. This suggests a potential future direction of using redox polymers with tunable potentials to modulate the potential of metal cocatalysts and thus control the reaction selectivity.

Highly Active Oxygen Evolution Integrating with Highly Selective CO2-to-CO Reduction

Artificial carbon fixation is a promising pathway for achieving the carbon cycle and environment remediation. However, the sluggish kinetics of oxygen evolution reaction (OER) and poor selectivity of CO2 reduction seriously limited the overall conversion efficiencies of solar energy to chemical fuels. Herein, we demonstrated a facile and feasible strategy to rationally regulate the coordination environment and electronic structure of surface-active sites on both photoanode and cathode. More specifically, the defect engineering has been employed to reduce the coordination number of ultrathin FeNi catalysts decorated on BiVO4 photoanodes, resulting in one of the highest OER activities of 6.51 mA cm-2 (1.23 VRHE, AM 1.5G). Additionally, single-atom cobalt (II) phthalocyanine anchoring on the N-rich carbon substrates to increase Co-N coordination number remarkably promotes CO2 adsorption and activation for high selective CO production. Their integration achieved a record activity of 109.4 μmol cm-2 h-1 for CO production with a faradaic efficiency of > 90%, and an outstanding solar conversion efficiency of 5.41% has been achieved by further integrating a photovoltaic utilizing the sunlight (> 500 nm).

Realization of a Photoelectrochemical Cascade for the Generation of Methanol: A Liquid Solar Fuel

Biochemical networks use reaction cascades to selectively reduce CO2 using energy from sunlight, but can similar selectivity be achieved by applying a cascade approach to an engineered system? Here, we report the design and implementation of a two-step photoelectrochemical (PEC) cascade to a liquid solar fuel: reduction of CO2 to CO and subsequent reduction of CO to methanol. The potentials required to perform the reductions were generated using custom-made III-V-based three-terminal tandem (3TT) solar cells. Cobalt phthalocyanine immobilized on multiwalled carbon nanotubes (CoPc/MWCNT) catalyzed both reactions. Multiphysics simulations of electrolyte flow and nonilluminated electrochemical measurements were used to narrow the operating parameters for the CoPc/MWCNT 3TT photocathodes. The champion integrated photocathode produced methanol with 3.8 ± 0.4% Faradaic efficiency (FE), with tested photocathodes having 0.7-3.8% methanol FE. Products were quantified by nuclear magnetic resonance spectroscopy and gas chromatography. The current output of the tested photocathodes was highly stable, and methanol production continued over multiple experiments. The low methanol yield is attributed to insufficient CO flux to, and CO2 depletion at, the methanol-producing subcell when both contacts are active, which is supported by the observation that a control photoelectrode slightly outperformed the methanol production of the 3TT device. Methanol production ceased when the 3TT subcell driving CO reduction was deactivated, supporting the assignment of a cascade mechanism. The major factors resulting in low methanol FE by the CoPc/MWCNT 3TT photocathodes are insufficient CO2 depletion at the methanol-producing contact and uncertainty in operating potential selection using the 3TT design. Although the CoPc/MWCNT 3TT photocathode is not yet highly selective, this work develops the basic science principles underlying the PEC cascade, demonstrates the co-design of a 3TT-based photoelectrode to produce carbon-based fuels, and finally discusses routes for improving product yields with this concept, including CO2 supply optimization and alternative photoelectrode and catalyst materials.

Continuous Flow Photoelectrochemical Reactor with Gas Permeable Photocathode: Enhanced Photocurrent and Partial Current Density for CO2 Reduction

Photoelectrochemical (PEC) CO2 reduction using a photocathode is an attractive method for making valuable chemical products due to its simplicity and lower overpotential requirements. However, previous PEC processes have often been diffusion-limited leading to low production rates of the CO2 reduction reaction, due to inefficient gas diffusion through the liquid electrolyte to the catalyst surface, particularly at high current densities. In this study, a gas-permeable photocathode in a continuous flow PEC reactor is incorporated, which facilitates the direct supply of CO2 gas to the photocathode-electrolyte interface, unlike dark reaction-based flow reactors. This concept is demonstrated using Ag-TiO2 on carbon paper, illuminated through a quartz window and flowing liquid electrolyte. CO2 supply is managed via pressure and flow control on the non-illuminated side of the carbon paper. The photocurrent density is significantly influenced by the flow rates and pressure of CO2 gas, and the electrolyte flow rates. Compared to the traditional H-cell, the continuous PEC flow reactor achieves ≈10-fold increase in CO faradaic efficiency, 30-fold increase in production rate and 16-fold increase in stability without catalyst modifications. This work provides essential insights into the design and application of continuous gas-liquid flow PEC reactor systems, highlighting their potential for other PEC reactions.

Key developments in magnesiothermic reduction of silica: insights into reactivity and future prospects

Porous Si (p-Si) nanomaterials are an exciting class of inexpensive and abundant materials within the field of energy storage. Specifically, p-Si has been explored in battery anodes to improve charge storage capacity, to generate clean fuels through photocatalysis and photoelectrochemical processes, for the stoichiometric conversion of CO2 to value added chemicals, and as a chemical H2 storage material. p-Si can be made from synthetic, natural, and waste SiO2 sources through a facile and inexpensive method called magnesiothermic reduction (MgTR). This yields a material with tunable properties and excellent energy storage capabilities. In order to tune the physical properties that affect performance metrics of p-Si, a deeper understanding of the mechanism of the MgTR and factors affecting it is required. In this perspective, we review the key developments in MgTR and discuss the thermal management strategies used to control the properties of p-Si. Additionally, we explore future research directions and approaches to bridge the gap between laboratory-scale experiments and industrial applications.

Photoelectrochemical Lithium Extraction from Waste Batteries

The amount of global hybrid-electric and all electric vehicle has increased dramatically in just five years and reached an all-time high of over 10 million units in 2022. A good deal of waste lithium (Li)-containing batteries from dead vehicles are invaluable unconventional resources with high usage of Li. However, the recycle of Li by green approaches is extremely inefficient and rare from waste batteries, giving rise to severe environmental pollutions and huge squandering of resources. Thus, in this mini review, we briefly summarized a green and promising route-photoelectrochemical (PEC) technology for extracting the Li from the waste lithium-containing batteries. This review first focuses on the critical factors of PEC performance, including light harvesting, charge-carrier dynamics, and surface chemical reactions. Subsequently, the conventional and PEC technologies applying in the area of Li recovery processes are analyzed and discussed in depth, and the potential challenges and future perspective for rational and healthy development of PEC Li extraction are provided positively.

Light-Induced Quantum Reconfiguration of Oxyhydroxides for Photoanodes with 4.24% Efficiency and Stability Beyond 250 Hours

Photoelectrochemical (PEC) water splitting is attracting significant research interest in addressing sustainable development goals in renewable energy. Current state-of-the-art, however, cannot provide photoanodes with simultaneously high efficiency and long-lasting lifetime. Here, large-scale NiFe oxyhydroxides-alloy hybridized co-catalyst layer that exhibits an applied bias photon-to-current efficiency (ABPE) of 4.24% in buried homojunction-free photoanodes and stability over 250 h is reported. These performances represent an increase over the present highest-performing technology by 408% in stability and the most stable competitor by over 330% in efficiency. These results originate from a previously unexplored mechanism of light-induced atomic reconfiguration, which rapidly self-generates a catalytic-protective amorphous/crystalline heterostructure at low biases. This mechanism provides active sites for reaction and insulates the photoanode from performance degradation. Photon-generated NiFe oxyhydroxides are more than 200% higher than the quantity that pure electrocatalysis would otherwise induce, overcoming the threshold for an efficient water oxidation reaction in the device. While of immediate interest in the industry of water splitting, the light-induced NiFe oxyhydroxides-alloy co-catalyst developed in this work provides a general strategy to enhance further the performances and stability of PEC devices for a vast panorama of chemical reactions, ranging from biomass valorization to organic waste degradation, and CO2-to-fuel conversion.

Unassisted photoelectrochemical CO2-to-liquid fuel splitting over 12% solar conversion efficiency

The increasing need to control anthropogenic CO2 emissions and conversion to fuels features the necessity for innovative solutions, one of which is photoelectrochemical system. This approach, capable of yielding gaseous production progressively, is facing challenges for liquid fuels generation due to optical, electrical, and catalytic properties. This study employs a standalone photoelectrochemical setup, in which InGaP/GaAs/Ge photoanode is integrated with tin-modified bismuth oxide cathode to convert CO2 into liquid formic acid. In unassisted two-electrode assembly, setup exemplifies its operational durability for 100 h, during which it maintains an average Faradaic efficiency of 88% with 17.3 mmol L-1 h-1 of yield, thereby excelling in average solar-to-fuel conversion efficiency at 12% with 60% of electrical energy efficiency under one sun illumination. This significant performance is further associated with metal-semiconductor interface formation between tin and bismuth oxide, which bridges electronic structures and generates an electric field at their interfaces. This study outperforms conventional solar-driven systems in operational durability and liquid fuel production.

Light-Induced Dynamic Activation of Copper/Silicon Interface for Highly Selective Carbon Dioxide Reduction

Numerous studies have shown a fact that phase transformation and/or reconstruction are likely to occur and play crucial roles in electrochemical scenarios. Nevertheless, a decisive factor behind the diverse photoelectrochemical activity and selectivity of various copper/silicon photoelectrodes is still largely debated and missing in the community, especially the possibly dynamic behaviors of metal catalyst/semiconductor interface. Herein, through in situ X-ray absorption spectroscopy and transmission electron microscope, a model system of Cu nanocrystals with well-defined facets on black p-type silicon (BSi) is unprecedentedly demonstrated to reveal the dynamic phase transformation of forming irreversible silicide at Cu nanocrystal-BSi interface during photoelectrocatalysis, which is validated to originate from the atomic interdiffusion between Cu and Si driven by light-induced dynamic activation process. Significantly, the adaptive junction at Cu-Si interface is activated by an expansion of interatomic Cu-Cu distance for CO2 electroreduction, which efficiently restricts the C-C coupling pathway but strengthens the bonding with key intermediate of *CHO for CH4 yield, resulting in a remarkable 16-fold improvement in the product ratio of CH4/C2 products and an intriguing selectivity switch. This work offers new insights into dynamic structural transformations of metal/semiconductor junction and design of highly efficient catalysts toward photosynthesis.

Facet-switching of rate-determining step on copper in CO2-to-ethylene electroreduction

Reduction of carbon dioxide (CO2) by renewable electricity to produce multicarbon chemicals, such as ethylene (C2H4), continues to be a challenge because of insufficient Faradaic efficiency, low production rates, and complex mechanistic pathways. Here, we report that the rate-determining steps (RDS) on common copper (Cu) surfaces diverge in CO2 electroreduction, leading to distinct catalytic performances. Through a combination of experimental and computational studies, we reveal that C─C bond-making is the RDS on Cu(100), whereas the protonation of *CO with adsorbed water becomes rate-limiting on Cu(111) with a higher energy barrier. On an oxide-derived Cu(100)-dominant Cu catalyst, we reach a high C2H4 Faradaic efficiency of 72%, partial current density of 359 mA cm-2, and long-term stability exceeding 100 h at 500 mA cm-2, greatly outperforming its Cu(111)-rich counterpart. We further demonstrate constant C2H4 selectivity of >60% over 70 h in a membrane electrode assembly electrolyzer with a full-cell energy efficiency of 23.4%.

Charge Photoaccumulation in Covalent Polymer Networks for Boosting Photocatalytic Nitrate Reduction to Ammonia

In the design of photoelectrocatalytic cells, a key element is effective photogeneration of electron-hole pairs to drive redox activation of catalysts. Despite recent progress in photoelectrocatalysis, experimental realization of a high-performance photocathode for multi-electron reduction of chemicals, such as nitrate reduction to ammonia, has remained a challenge due to difficulty in obtaining efficient electrode configurations for extraction of high-throughput electrons from absorbed photons. This work describes a new design for catalytic photoelectrodes using chromophore assembly-functionalized covalent networks for boosting eight-electron reduction of nitrate to ammonia. Upon sunlight irradiation, the photoelectrode stores a mass of reducing equivalents at the photoexcited chromophore assembly for multielectron reduction of a copper catalyst, enabling efficient nitrate reduction to ammonia. By introducing the new photoelectrode structure, it is demonstrated that the electronic interplay between charge photo-accumulating assembly and multi-electron redox catalysts can be optimized to achieve proper balance between electron transfer dynamics and thermodynamic output of photoelectrocatalytic systems.

Modulating CO2 Electrocatalytic Conversion to the Organics Pathway by the Catalytic Site Dimension

Electrochemical reduction of carbon dioxide to organic chemicals provides a value-added route for mitigating greenhouse gas emissions. We report a family of carbon-supported Sn electrocatalysts with the tin size varying from single atom, ultrasmall clusters to nanocrystallites. High single-product Faradaic efficiency (FE) and low onset potential of CO2 conversion to acetate (FE = 90% @ -0.6 V), ethanol (FE = 92% @ -0.4 V), and formate (FE = 91% @ -0.6 V) were achieved over the catalysts of different active site dimensions. The CO2 conversion mechanism behind these highly selective, size-modulated p-block element catalysts was elucidated by structural characterization and computational modeling, together with kinetic isotope effect investigation.

Photoelectrocatalytic Utilization of CO2 : A Big Show of Si-based Photoelectrodes

CO2 is a greenhouse gas that contributes to environmental deterioration; however, it can also be utilized as an abundant C1 resource for the production of valuable chemicals. Solar-driven photoelectrocatalytic (PEC) CO2 utilization represents an advanced technology for the resourcing of CO2 . The key to achieving PEC CO2 utilization lies in high-performance semiconductor photoelectrodes. Si-based photoelectrodes have attracted increasing attention in the field of PEC CO2 utilization due to their suitable band gap (1.1 eV), high carrier mobility, low cost, and abundance on Earth. There are two pathways to PEC CO2 utilization using Si-based photoelectrodes: direct reduction of CO2 into small molecule fuels and chemicals, and fixation of CO2 with organic substrates to generate high-value chemicals. The efficiency and product selectivity of PEC CO2 utilization depends on the structures of the photoelectrodes as well as the composition, morphology, and size of the catalysts. In recent years, significant and influential progress has been made in utilizing Si-based photoelectrodes for PEC CO2 utilization. This review summarizes the latest research achievements in Si-based PEC CO2 utilization, with a particular emphasis on the mechanistic understanding of CO2 reduction and fixation, which will inspire future developments in this field.

Au-decorated Sb2Se3 photocathodes for solar-driven CO2 reduction

Photoelectrodes with FTO/Au/Sb2Se3/TiO2/Au architecture were studied in photoelectrochemical CO2 reduction reaction (PEC CO2RR). The preparation is based on a simple spin coating technique, where nanorod-like structures were obtained for Sb2Se3, as confirmed by SEM images. A thin conformal layer of TiO2 was coated on the Sb2Se3 nanorods via ALD, which acted as both an electron transfer layer and a protective coating. Au nanoparticles were deposited as co-catalysts via photo-assisted electrodeposition at different applied potentials to control their growth and morphology. The use of such architectures has not been explored in CO2RR yet. The photoelectrochemical performance for CO2RR was investigated with different Au catalyst loadings. A photocurrent density of ∼7.5 mA cm-2 at -0.57 V vs. RHE for syngas generation was achieved, with an average Faradaic efficiency of 25 ± 6% for CO and 63 ± 12% for H2. The presented results point toward the use of Sb2Se3-based photoelectrodes in solar CO2 conversion applications.

An in situ exsolved Cu-based electrocatalyst from an intermetallic Cu5Si compound for efficient CH4 electrosynthesis

A Cu-based electrocatalyst (e-Cu5Si) is developed by in situ exsolving ultrathin SiOx layer-coated CuO/Cu nanoparticles (<100 nm) on the surface of a conductive intermetallic Cu5Si parent. This specially designed e-Cu5Si catalyst exhibits high performance for the CO2 reduction reaction (CO2RR), which affords an excellent CH4 faradaic efficiency (FE) of 49.0% with partial current density of over 140.1 mA cm-2 at -1.2 V versus reversible hydrogen electrode (RHE) in a flow cell, with outstanding stability. The strongly coupled multiphase interfaces among the SiOx layer, CuO/Cu species, and substrate contribute to fast interfacial electron transfer for the CO2RR. Moreover, in situ Raman analysis suggests that the ultrathin SiOx layer simultaneously stabilizes the active Cu1+ species and promotes the protonation of *CO to form *CHxO, thereby greatly improving overall selectivity and activity of CH4 production.

Tailoring Interfaces for Enhanced Methanol Production from Photoelectrochemical CO2 Reduction

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor-catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving force for CO2 reduction. These structures together create a unique microenvironment for effective reduction of CO2 to methanol, leading to a remarkable Faradaic efficiency reaching 20% together with a partial current density of 3.4 mA cm-2, surpassing the previous record based on planar silicon photoelectrodes by a notable factor of 17. This work demonstrates a new pathway for enhancing photoelectrocatalytic CO2 reduction through meticulous interface and microenvironment tailoring and sets a benchmark for both Faradaic efficiency and current density in solar liquid fuel production.

Highly selective photoelectrochemical CO2 reduction by crystal phase-modulated nanocrystals without parasitic absorption

Photoelectrochemical (PEC) carbon dioxide (CO2) reduction (CO2R) holds the potential to reduce the costs of solar fuel production by integrating CO2 utilization and light harvesting within one integrated device. However, the CO2R selectivity on the photocathode is limited by the lack of catalytic active sites and competition with the hydrogen evolution reaction. On the other hand, serious parasitic light absorption occurs on the front-side-illuminated photocathode due to the poor light transmittance of CO2R cocatalyst films, resulting in extremely low photocurrent density at the CO2R equilibrium potential. This paper describes the design and fabrication of a photocathode consisting of crystal phase-modulated Ag nanocrystal cocatalysts integrated on illumination-reaction decoupled heterojunction silicon (Si) substrate for the selective and efficient conversion of CO2. Ag nanocrystals containing unconventional hexagonal close-packed phases accelerate the charge transfer process in CO2R reaction, exhibiting excellent catalytic performance. Heterojunction Si substrate decouples light absorption from the CO2R catalyst layer, preventing the parasitic light absorption. The obtained photocathode exhibits a carbon monoxide (CO) Faradaic efficiency (FE) higher than 90% in a wide potential range, with the maximum FE reaching up to 97.4% at -0.2 V vs. reversible hydrogen electrode. At the CO2/CO equilibrium potential, a CO partial photocurrent density of -2.7 mA cm-2 with a CO FE of 96.5% is achieved in 0.1 M KHCO3 electrolyte on this photocathode, surpassing the expensive benchmark Au-based PEC CO2R system.

Optimizing physico-chemical properties of hierarchical ZnO/TiO2 nano-film by the novel heating method for photocatalytic degradation of antibiotics and dye

The design of semiconductor catalysts with excellent photocatalytic properties, stability, recyclability, and good separation for the treatment of polluted water is still challenging. In this paper, the ZnO/TiO2 nano-thin films were fabricated using the magnetron sputtering technique and then heating the underlying ZnO layer and the upper TiO2 layer for their respective optimal heating time, i. e. heating ZnO for 3 h and heating TiO2 for 2 h. The as-prepared films were characterized. The results show that the preferred growth of TiO2 grains along the [001] axis, relatively large specific surface area, and increased amounts of surface oxygen vacancies (OVs) were induced to the heterojunction catalysts through this optimized heating strategy, which boosts the photocatalytic activity of ZnO/TiO2 nano-film. The degradation experiment inndicates that the ciprofloxacin (CIP) removal efficiency can reach 97.3% in 2 h duration, which was higher than that of the samples annealed for the same periods. Meanwhile, the prepared ZnO/TiO2 photocatalytic film exhibited favorable stability of 95.5% degradation efficiency after the fourth run and general applicability for the photodegradation of various contantains, whih removed 99.5% of ofloxacin (OFX) and 77.6% of tetracycline (TC) in 2 h and 94.1% of Rhodamine B (RhB) in 1 h. This work is expected to yields a novel insight into the production of heterojunction photocatalysts with excellen ability for photocatalytic degradation of pollutants in the practical industry.

Efficient Cu2O Photocathodes for Aqueous Photoelectrochemical CO2 Reduction to Formate and Syngas

Photoelectrochemical carbon dioxide reduction (PEC-CO2R) represents a promising approach for producing renewable fuels and chemicals using solar energy. However, attaining even modest solar-to-fuel (STF) conversion efficiency often necessitates the use of costly semiconductors and noble-metal catalysts. Herein, we present a Cu2O/Ga2O3/TiO2 photocathode modified with Sn/SnOx catalysts through a simple photoelectrodeposition method. It achieves a remarkable half-cell STF efficiency of ∼0.31% for the CO2R in aqueous KHCO3 electrolyte, under AM 1.5 G illumination. The system enables efficient production of syngas (FE: ∼62%, CO/H2 ≈ 1:2) and formate (FE: ∼38%) with a consistent selectivity over a wide potential range, from +0.34 to -0.16 V vs the reversible hydrogen electrode. We ascribe the observed performance to the favorable optoelectronic characteristics of our Cu2O heterostructure and the efficient Sn/SnOx catalysts incorporated in the PEC-CO2R reactions. Through comprehensive experimental investigations, we elucidate the indispensable role of Cu2O buried p-n junctions in generating a high photovoltage (∼1 V) and enabling efficient bulk charge separation (up to ∼70% efficiency). Meanwhile, we discover that the deposited Sn/SnOx catalysts have critical dual effects on the overall performance of the PEC devices, serving as active CO2R catalysts as well as the semiconductor front contact. It could facilitate interfacial electron transfer between the catalysts and the semiconductor device for CO2R by establishing a barrier-free ohmic contact.

Bioinspired Polyoxo-titanium Cluster for Greatly Enhanced Solar-Driven CO2 Reduction

Developing artificial enzymes with excellent catalytic activities and uncovering the structural and chemical determinants remain a grand challenge. Discrete titanium-oxo clusters with well-defined coordination environments at the atomic level can mimic the pivotal catalytic center of natural enzymes and optimize the charge-transfer kinetics. Herein, we report the precise structural tailoring of a self-assembled tetrahedral Ti4Mn3-cluster for photocatalytic CO2 reduction and realize the selective evolution of CO over specific sites. Experiments and theoretical simulation demonstrate that the high catalytic performance of the Ti4Mn3-cluster should be related to the synergy between active Mn sites and the surrounding functional microenvironment. The reduced energy barrier of the CO2 photoreduction reaction and moderate adsorption strength of CO* are beneficial for the high selective evolution of CO. This work provides a molecular scale accurate structural model to give insight into artificial enzyme for CO2 photoreduction.

Enhancing photoelectrochemical CO2 reduction with silicon photonic crystals

The effectiveness of silicon (Si) and silicon-based materials in catalyzing photoelectrochemistry (PEC) CO2 reduction is limited by poor visible light absorption. In this study, we prepared two-dimensional (2D) silicon-based photonic crystals (SiPCs) with circular dielectric pillars arranged in a square array to amplify the absorption of light within the wavelength of approximately 450 nm. By investigating five sets of n + p SiPCs with varying dielectric pillar sizes and periodicity while maintaining consistent filling ratios, our findings showed improved photocurrent densities and a notable shift in product selectivity towards CH4 (around 25% Faradaic Efficiency). Additionally, we integrated platinum nanoparticles, which further enhanced the photocurrent without impacting the enhanced light absorption effect of SiPCs. These results not only validate the crucial role of SiPCs in enhancing light absorption and improving PEC performance but also suggest a promising approach towards efficient and selective PEC CO2 reduction.

Local reactivity of metal-insulator-semiconductor photoanodes imaged by photoinduced electrochemiluminescence microscopy

Localized photoinduced electrochemiluminescence (PECL) is studied on photoanodes composed of Ir microbands deposited on n-Si/SiOx. We demonstrate that PECL microscopy precisely imaged the hole-driven heterogeneous photoelectrochemical reactivity. The method is promising for elucidating the local activity of photoelectrodes that are employed in solar energy conversion.

High-Photovoltage Silicon Nanowire for Biological Cofactor Production

Photocathodic conversion of NAD+ to NADH cofactor is a promising platform for activating redox biological catalysts and enzymatic synthesis using renewable solar energy. However, many photocathodes suffer from low photovoltage, consequently requiring a high cathodic bias for NADH production. Here, we report an n+p-type silicon nanowire (n+p-SiNW) photocathode having a photovoltage of 435 mV to drive energy-efficient NADH production. The enhanced band bending at the n+/p interface accounts for the high photovoltage, which conduces to a benchmark onset potential [0.393 V vs the reversible hydrogen electrode (VRHE)] for SiNW-based photocathodic NADH generation. In addition, the n+p-SiNW nanomaterial exhibits a Faradaic efficiency of 84.7% and a conversion rate of 1.63 μmol h-1 cm-1 at 0.2 VRHE, which is the lowest cathodic potential to achieve the maximum productivity among SiNW-sensitized cofactor production.

Stability of Photocathodes: A Review on Principles, Design, and Strategies

Photoelectrochemical devices based on semiconductor photoelectrode can directly convert and store solar energy into chemical fuels. Although the efficient photoelectrodes with commercially valuable solar-to-fuel energy conversion efficiency have been reported over past decades, one of the most enormous challenges is the stability of the photoelectrode due to corrosion during operation. Thus, it is of paramount importance for developing a stable photoelectrode to deploy solar-fuel production. This Review commences with a fundamental understanding of thermodynamics for photoelectrochemical reactions and the fundamentals of photocathodes. Then, the commercial application of photoelectrochemical technology is prospected. We specifically focus on recent strategies for designing photocathodes with long-term stability, including energy band alignment, hole transport/storage/blocking layer, spatial decoupling, grafting molecular catalysts, protective/passivation layer, surface element reconstruction, and solvent effects. Based on the insights gained from these effective strategies, we propose an outlook of key aspects that address the challenges for development of stable photoelectrodes in future work.

Silicon nanowire array overcomes chemotherapeutic resistance by inducing the differentiation of breast cancer stem cells

Currently, traditional cancer treatment strategies are greatly challenged by the existence of cancer stem cells (CSCs), which are root cause of chemotherapy resistance. Differentiation therapy presents a novel therapeutic strategy for CSC-targeted therapy. However, there are very few studies on the induction of CSCs differentiation so far. Silicon nanowire array (SiNWA) with many unique properties is considered to be an excellent material for various applications ranging from biotechnology to biomedical applications. In this study, we report the SiNWA differentiates MCF-7-derived breast CSCs (BCSCs) into non-CSCs by modulating the morphology of cells. In vitro, the differentiated BCSCs lose the stemness properties and thus become sensitive to chemotherapeutic drugs, eventually leading to the death of BCSCs. Therefore, this work suggests a potential approach for overcoming chemotherapeutic resistance.

Nanowire photochemical diodes for artificial photosynthesis

Artificial photosynthesis can provide a solution to our current energy needs by converting small molecules such as water or carbon dioxide into useful fuels. This can be accomplished using photochemical diodes, which interface two complementary light absorbers with suitable electrocatalysts. Nanowire semiconductors provide unique advantages in terms of light absorption and catalytic activity, yet great control is required to integrate them for overall fuel production. In this review, we journey across the progress in nanowire photoelectrochemistry (PEC) over the past two decades, revealing design principles to build these nanowire photochemical diodes. To this end, we discuss the latest progress in terms of nanowire photoelectrodes, focusing on the interplay between performance, photovoltage, electronic band structure, and catalysis. Emphasis is placed on the overall system integration and semiconductor-catalyst interface, which applies to inorganic, organic, or biologic catalysts. Last, we highlight further directions that may improve the scope of nanowire PEC systems.

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.

Boosting Reactive Oxygen Species Generation Using Inter-Facet Edge Rich WO3 Arrays for Photoelectrochemical Conversion

Water oxidation reaction leaves room to be improved in the development of various solar fuel productions, because of the kinetically sluggish 4-electron transfer process of oxygen evolution reaction. In this work, we realize reactive oxygen species (ROS), H₂ O₂ and OH⋅, formations by water oxidation with total Faraday efficiencies of more than 90 % by using inter-facet edge (IFE) rich WO₃ arrays in an electrolyte containing CO₃ ^2- . Our results demonstrate that the IFE favors the adsorption of CO₃ ^2- while reducing the adsorption energy of OH⋅, as well as suppresses surface hole accumulation by direct 1-electron and indirect 2-electron transfer pathways. Finally, we present selective oxidation of benzyl alcohol by in situ using the formed OH⋅, which delivers a benzaldehyde production rate of ≈768 μmol h^-1 with near 100 % selectivity. This work offers a promising approach to tune or control the oxidation reaction in an aqueous solar fuel system towards high efficiency and value-added product.