Energy-Efficient and Self-Powered Green Ammonia Synthesis by Electrochemical Nitrate Reduction Combined with Hydrazine Oxidation.

To achieve the global goal of carbon neutrality, recently, emphasis has been placed on developing green ammonia production method to replace the Haber-Bosch process. Nitrate reduction reaction (NO3 RR) has received considerable attention, especially for electrochemically producing ammonia from nitrate and simultaneously purifying wastewater. This study first demonstrates that the combination of NO3 RR with hydrazine oxidation reaction (HzOR) is an energy efficient green ammonia production method, which overcomes the sluggish water oxidation limitation. Tungsten phosphide (WP) nanowires (NWs) are prepared as cathode NO3 RR electrocatalysts, which exhibit a high Faradaic efficiency in both neutral (≈93%) and alkaline (≈85%) media. Furthermore, they show a high bifunctional activity in anodic reactions and exhibit a low potential 0.024 V for generating a current density of 10 mA cm-2 in HzOR. The overall NO3 RR-HzOR required an impressively low potential of 0.24 V for generating a current density of 10 mA cm-2 ; this potential is much lower than those required for NO3 RR-OER (1.53 V) and NO3 RR-UOR (1.31 V). A self-powered ammonia production system, prepared by assembling an NO3 RR-HzOR with a perovskite solar cell, displays a high ammonia production rate of 1.44 mg cm-2  h-1 . A single PV cell provides enough driving voltage in the PV-EC due to low required potential. This system facilitates unassisted green ammonia synthesis with a low energy consumption and also allows upcycling of wastewater to produce useful fuel.

Spatial Separation of Cocatalysts on Z-Scheme Organic/Inorganic Heterostructure Hollow Spheres for Enhanced Photocatalytic H2 Evolution and In-Depth Analysis of the Charge-Transfer Mechanism.

A Z-scheme heterojunction with spatially separated cocatalysts is proposed for overcoming fundamental issues in photocatalytic water splitting, such as inefficient light absorption, charge recombination, and sluggish reaction kinetics. For efficient light absorption and interfacial charge separation, Z-scheme organic/inorganic heterojunction photocatalysts are synthesized by firmly immobilizing ultrathin g-C3 N4 on the surface of TiO2 hollow spheres via electrostatic interactions. Additionally, two cocatalysts, Pt and IrOx , are spatially separated along the Z-scheme charge-transfer pathway to enhance surface charge separation and reaction kinetics. The as-prepared Pt/g-C3 N4 /TiO2 /IrOx (PCTI) hollow sphere photocatalyst exhibits an exceptional H2 evolution rate of 8.15 mmol h-1 g-1 and a remarkable apparent quantum yield of 24.3% at 330 nm in the presence of 0.5 wt% Pt and 1.2 wt% IrOx cocatalysts on g-C3 N4 and TiO2 , respectively. Photoassisted Kelvin probe force microscopy is used to systematically analyze the Z-scheme charge-transfer mechanism within PCTI. Furthermore, the benefits of spatially separating cocatalysts in the PCTI system are methodically investigated in comparison to randomly depositing them. This work adequately demonstrates that the combination of a Z-scheme heterojunction and spatially separated cocatalysts can be a promising strategy for designing high-performance photocatalytic platforms for solar fuel production.

Triple-Channel Charge Transfer over W18O49/Au/g-C3N4 Z-Scheme Photocatalysts for Achieving Broad-Spectrum Solar Hydrogen Production.

Z-scheme heterojunctions are fundamentally promising yet practically appealing for photocatalytic hydrogen (H2) production owing to the enhanced redox power, spatial separation of charge carriers, and broad-spectrum solar light harvesting. The charge-transfer dynamics at Z-scheme heterojunctions can be accelerated by inserting charge-transfer mediators at the heterojunction interfaces. In this study, we introduce Au nanoparticle mediators in the Z-scheme W18O49/g-C3N4 heterostructure, which enables an improved H2 production rate of 3465 µmol/g·h compared with the direct Z-scheme W18O49/g-C3N4 (1785 µmol/g·h) under 1 sun irradiation. The apparent quantum yields of H2 production with W18O49/Au/g-C3N4 are 3.9% and 9.3% at 420 and 1200 nm, respectively. The improved photocatalytic H2 production activity of W18O49/Au/g-C3N4 is attributable to the triple-channel charge-transfer mechanism: channel I─Z-scheme charge transfer facilitates charge separation and increased redox power of the photoexcited electrons; channels II and III─the localized surface plasmon resonances from Au (channel II) and W18O49 (channel III) enable light harvesting extension from visible to near-infrared wavelengths.

Yolk-shell nanostructures: synthesis, photocatalysis and interfacial charge dynamics.

Solar energy has long been regarded as a promising alternative and sustainable energy source. In this regard, photocatalysts emerge as a versatile paradigm that can practically transform solar energy into chemical energy. At present, unsatisfactory conversion efficiency is a major obstacle to the widespread deployment of photocatalysis technology. Many structural engineering strategies have been proposed to address the issue of insufficient activity for semiconductor photocatalysts. Among them, creation of yolk-shell nanostructures which possess many beneficial features, such as large surface area, efficient light harvesting, homogeneous catalytic environment and enhanced molecular diffusion kinetics, has attracted particular attention. This review summarizes the developments that have been made for the preparation and photocatalytic applications of yolk-shell nanostructures. Additional focus is placed on the realization of interfacial charge dynamics and the possibility of achieving spatial separation of charge carriers for this unique nanoarchitecture as charge transfer is the most critical factor determining the overall photocatalytic efficiency. A future perspective that can facilitate the advancement of using yolk-shell nanostructures in sophisticated photocatalytic systems is also presented.

Three-Dimensional Nanoporous Metal Structures from Poly(2-vinylpyridine)-block-Poly(4-vinylpyridine) Copolymer Thin Film.

We fabricated 3D nanoporous metal structures from poly(2-vinylpyridine)-block-poly(4-vinylpyridine) copolymer (P24VP) thin film with vertically oriented lamellar nanodomains by coordinating corresponding metal precursors followed by reduction to metals. Although metal precursors are coordinated with both P2VP and P4VP blocks, the metal coordination power toward P4VP block is much greater than that toward P2VP block. Thus, most of the metal precursors are located in the P4VP block, while a few exist in the P2VP block. After the metal precursors were reduced to corresponding metals by reactive ion etching, metals located in P4VP regions became continuous main frames. However, metals in P2VP regions could not be continuous because of smaller amounts, resulting in nanoporous structures. Using these 3D nanoporous structures, we measured the electrocatalytic activity for hydrogen evolution reaction. 3D nanoporous platinum (Pt) showed enhanced catalytic activity compared with Pt flat film due to the large surface area. Moreover, 3D nanoporous Pt/cobalt bimetallic structures showed better catalytic activity than 3D nanoporous Pt structures.

NiMoFe and NiMoFeP as Complementary Electrocatalysts for Efficient Overall Water Splitting and Their Application in PV-Electrolysis with STH 12.3.

Complementary water splitting electrocatalysts used simultaneously in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can simplify water splitting systems. Herein, earth-abundant NiMoFe (NMF) and phosphorized NiMoFeP (NMFP) are synthesized as complementary overall water splitting (OWS) catalysts. First, NMF is tested as both the HER and OER promoter, which exhibits low overpotentials of 68 (HER) and 337 mV (OER). A quaternary NMFP is then prepared by simple phosphorization of NMF, which shows a much lower OER overpotential of 286 mV. The enhanced OER activity is attributed to the unique surface/core structure of NMFP. The surface phosphate acts as a proton transport mediator and expedites the rate-determining step. With the application of OER potential, the NMFP surface is composed of Ni(OH)2 and FeOOH, active sites for OER, but the inner core consists of Ni, Mo, and Fe metals, serving as a conductive electron pathway. OWS with NMF-NMFP requires an applied voltage of 1.452 V to generate 10 mA cm-2 , which is one of the lowest values among OWS results with transition-metal-based electrocatalysts. Furthermore, the catalysts are combined with tandem perovskite solar cells for photovoltaic (PV)-electrolysis, producing a high solar-to-hydrogen (STH) conversion efficiency of 12.3%.

Thorn-like TiO2 nanoarrays with broad spectrum antimicrobial activity through physical puncture and photocatalytic action.

To overcome the conventional limitation of TiO2 disinfection being ineffective under light-free conditions, TiO2 nanowire films (TNWs) were prepared and applied to bacterial disinfection under dark and UV illumination. TNW exhibited much higher antibacterial efficiencies against Escherichia coli (E. coli) under dark and UV illumination conditions compared to TiO2 nanoparticle film (TNP) which was almost inactive in the dark, highlighting the additional contribution of the physical interaction between bacterial membrane and NWs. Such a physical contact-based antibacterial activity was related to the NW geometry such as diameter, length, and density. The combined role of physical puncture and photocatalytic action in the mechanism underlying higher bactericidal effect of TNW was systematically examined by TEM, SEM, FTIR, XPS, and potassium ion release analyses. Moreover, TNW revealed antimicrobial activities in a broad spectrum of microorganisms including Staphylococcus aureus and MS2 bacteriophage, antibiofilm properties, and good material stability. Overall, we expect that the free-standing and antimicrobial TNW is a promising agent for water disinfection and biomedical applications in the dark and/or UV illumination.

Combinational Biomimicking of Lotus Leaf, Mussel, and Sandcastle Worm for Robust Superhydrophobic Surfaces with Biomedical Multifunctionality: Antithrombotic, Antibiofouling, and Tissue Closure Capabilities.

Surface wetting occurring in daily life causes undesired contaminations, which are critical issues in various fields. To solve these problems, the nonwetting property of a superhydrophobic (SH) surface has proven its utility by preventing contaminant infiltration, serious infections, or malfunction. However, the application of SH surfaces in the biomedical field has been limited due to the weak durability and toxicity of the related components. To overcome these limitations, we developed a robust and biocompatible SH surface through combinational biomimicking of three natural organisms, lotus leaf, mussel, and sandcastle worm, for the first time. Using the water-immiscible and polycationic characteristics of mussel adhesive protein (iMglue), an SH iMglue-SiO2(TiO2/SiO2)2 coating was fabricated by solution-based electrical charge-controlled layer-by-layer growth of nanoparticles (NPs). The fabricated iMglue-SiO2(TiO2/SiO2)2 SH surface showed excellent durable nonwetting properties and was applied to an intracatheter tube coating to develop antithrombotic catheters under blood flow. Furthermore, we developed a iMglue-employed SH patch for a tissue closure bandage by spraying hydrophobic SiO2 NPs on the iMglue-covered cotton pads. The prepared iMglue-employing SH patch showed perfect bifunctionality with excellent antibiofouling and tissue closure capabilities. Our work presents a novel, useful strategy for fabricating a biomedically multifunctional, robust SH surface through combinational mimicking of natural organisms.

Growth of hierarchical gold clusters for use in superomniphobic electrodes.

We developed a method to fabricate a superomniphobic gold electrode by synthesizing hierarchical gold clusters on a gold substrate and treating the surface with low surface energy materials. The reduction of gold ions was repeated several times, causing the gold microparticles to grow in random directions and form hierarchical gold clusters. Treatment of the gold structures with perfluorothiol resulted in a superhydrophobic surface that also exhibited superoleophobicity for oils and liquids with surface tensions as low as 25.6 mN. The resulting electrode was not contaminated by hydrophilic and hydrophobic liquids, and by analyzing the current-voltage characteristics of the electrode with a PEDOT:PSS solution droplet, the electrode was found to be waterproof.

Ostwald Ripening Driven Exfoliation to Ultrathin Layered Double Hydroxides Nanosheets for Enhanced Oxygen Evolution Reaction.

As a key half-reaction in water splitting, the oxygen evolution reaction (OER) process is kinetically sluggish. Layered double hydroxides (LDHs) are regarded as the highly promising electrocatalysts to promote the OER kinetics. However, the closely stacking layered structure of pristine bulk LDHs restricts the exposure of electrocatalytically active sites, and it remains a great challenge to find an efficient strategy to exfoliate the bulk LDHs into ultrathin and stable nanosheets with increased surface area and exposed active sites. Herein, a novel Ostwald ripening driven exfoliation (ORDE) of NiFe LDHs has been achieved in situ on the electrodes by spontaneously self-etching and redepositing via a simple hydrothermal treatment without the assistance of any exfoliating reagent or surfactant. The thermodynamically driven Ostwald ripening has been expanded to the exfoliation of two-dimensional layered materials for the first time. Compared with conventional exfoliation methods, this ORDE is a time-saving and green strategy that avoids the serious adsorption of surfactant molecules. The ORDE of NiFe LDHs is accomplished in situ on a Cu mesh electrode, which not only exhibits excellent electrical contact between LDHs catalyst and electrodes but also prevents the restacking of the exfoliated LDHs. As a result, the exfoliated ultrathin, clean, and vertically aligned NiFe nanosheets with much higher surface area and numerous exposed active edges and sites demonstrated significantly enhanced OER performances with low overpotential of 292 mV at 10 mA cm-2 and long-term stability for more than 60 h, as well as remarkable flexibility. Additionally, bulk Ni(OH)2 nanosheets on Ni foams have also been exfoliated by a similar mechanism, indicating this ORDE strategy can be widely extended to other 2D layered materials for novel applications.

Synergistic doping effects of a ZnO:N/BiVO4:Mo bunched nanorod array photoanode for enhancing charge transfer and carrier density in photoelectrochemical systems.

One-dimensional heterojunction nanorods are highly attractive as photoanodes for developing efficient photoelectrochemical (PEC) systems for the effective photogeneration of charge carriers and transport. ZnO/BiVO4 nanorod arrays (NRAs) are excellent candidates if their charge transferring and recombination issues can be improved. In the current work, we have studied the synergistic doping effects of N-doped ZnO/Mo-doped BiVO4 NRAs for enhancing the photoanode activity in PEC devices. The N-doping of ZnO NRs enhances the charge carrier density ∼3-fold over undoped ZnO NRs through increased oxygen vacancies induced by N dopants. The Mo dopants in BiVO4 improve the mobility of photogenerated charge carriers and contribute to reducing charge recombination. The synergistic doping effects of both ZnO and BiVO4 could increase the charge transfer rate constant (kct) of the ZnO:N/BiVO4:Mo heterojunction by ∼40% and decrease the charge transfer resistance ∼1.9-fold compared to those of undoped ZnO/BiVO4, which were confirmed by time resolved photoluminescence (PL) and electrochemical impedance (EIS) analyses. Our optimally fabricated ZnO:N/BiVO4:Mo NRA photoanode could achieve an excellent photocurrent of 3.62 mA cm-2 without the application of any co-catalysts. This work presents a useful strategy for designing efficient heterojunction photoanodes in PEC systems.

Effect of liquid droplet surface tension on impact dynamics over hierarchical nanostructure surfaces.

Analyzing impact dynamics is important for practical applications of superhydrophobic surfaces, because these nonwetting surfaces frequently encounter impacting liquid droplets in real environments. Thus, various studies have been conducted to investigate impact dynamics by examining the correlation between the behaviors of impacting liquid droplets and several determining parameters, such as impacting velocity, surface structure and surface energy. The impacting behaviors of pure water droplets were the main focus in most previous studies; the effect of surface tension, another critical parameter, on impact dynamics has rarely been investigated. In the current work, we have newly studied the effects of liquid surface tension on impact dynamics using an ethanol-water solution as a model liquid system. We systematically varied the liquid's surface tension between 72 and 32 mN m-1 by changing the ethanol concentration from 0 to 20 wt%. This range of composition drastically changed the surface tension while it did not significantly affect other physical properties, such as density and viscosity. For an impact dynamics study, two surfaces, namely ZnO nanowires (NWs) and ZnO/Si hierarchical (HIE) structures, were prepared. As the surface tension decreased, the static water contact angle (CA) decreased on both surfaces. Under dynamic conditions, our analysis using a high-speed camera and a quartz crystal microbalance (QCM) showed that lowering the surface tension causes the transition from the anti-wetting to wetting state. The transition We numbers were obtained on both surfaces for various surface tensions of liquids. Under the same dropping conditions of liquids, the ZnO/Si HIE surface shows higher transition We numbers than the ZnO NW surface, which is due to the higher fraction of air pockets in the hierarchical structure, originating from dual dimensional structures. To understand the mechanism of dynamic transition, we developed a model for ZnO/Si HIE structures based on three determining pressures: anti-wetting, wetting, and effective water hammer pressures. The modeling results explain the experimental observations. The results of our model system are highly useful for understanding the impact dynamic behaviors of various liquids on non-wetting surfaces.

Enhancing Durability and Photoelectrochemical Performance of the Earth Abundant Ni-Mo/TiO2 /CdS/CIGS Photocathode under Various pH Conditions.

Cu(In,Ga)(S,Se)2 (CIGS) is a promising photocathode material owing to its high absorption coefficient, adjustable band gap, and suitable band edge for the hydrogen evolution reaction (HER). However, most CIGS photocathodes have suffered from instability in applications that require a wide range of pH conditions and have utilized noble metal HER catalysts to achieve a high performance. Thus, improving the durability of the CIGS photocathode under various pH conditions and developing a cost-effective non-noble metal catalyst are critical issues in the photoelectrochemical (PEC) application of this promising photocathode material. Here, we catalyze the CIGS photocathode with Ni-Mo as a non-noble metal to enhance the PEC efficiency, and we employ atomically grown TiO2 to passivate the CdS/CIGS surface and improve the stability under a wide range of pH conditions. Our Ni-Mo alloy exhibits the best HER catalytic activity among reported earth-abundant HER catalysts in both acidic and alkaline solutions. The Ni-Mo/CdS/CIGS photocathode yields an onset potential of 0.5 V (vs. RHE) and a short-circuit photocurrent density as high as 15-25 mA cm-2 under various pH conditions ranging from 0.4 to 14, which is highly comparable to that of Pt/CdS/CIGS. Furthermore, the passivation of CdS/CIGS with a thin TiO2 layer, obtained by atomic layer deposition, effectively prevents the photocorrosion of CdS and the dissolution of the Mo back contact, which are the main causes of the degradation of the photocathode. The optimized Ni-Mo/TiO2 /CdS/CIGS photocathode produces a stable photocurrent density at 0 VRHE for 100 minutes except under strong alkaline conditions. The current work presents a very useful method to improve the efficiency and durability of the CIGS photocathodes with an earth-abundant metal catalyst, which completely replaces Pt.

Ni(OH)2 -WP Hybrid Nanorod Arrays for Highly Efficient and Durable Hydrogen Evolution Reactions in Alkaline Media.

The development of efficient non-noble-metal hydrogen evolution electrocatalysts in alkaline media is crucial for sustainable, ecofriendly production of H2 through water electrolysis. An alkaline hydrogen evolution reaction (HER) catalyst composed of Ni(OH)2 -decorated thungsten phosphide (WP) nanorod arrays on carbon paper was synthesized by thermal evaporation and electrodeposition. This hybrid catalyst displayed outstanding HER activity and required a low overpotential of only 77 mV to obtain a current density of 10 mA cm-2 and a Tafel slope of 71 mV dec-1 . The hybrid catalyst also showed long-term electrochemical stability, maintaining its activity for 18 h. This improved HER efficiency was attributed to the synergetic effect of WP and Ni(OH)2 : Ni(OH)2 effectively lowers the energy barrier during water dissociation and also provides active sites for hydroxyl adsorption, whereas WP adsorbs hydrogen intermediates and efficiently produces H2 gas. This interfacial cooperation offers not only excellent HER catalytic activity but also new strategies for the fabrication of effective non-noble-metal-based electrocatalysts in alkaline media.

Switchable ferroelectric photovoltaic effects in epitaxial h-RFeO3 thin films.

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7-8.5 µC cm-2), which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

Multichannel Charge Transport of a BiVO4/(RGO/WO3)/W18O49 Three-Storey Anode for Greatly Enhanced Photoelectrochemical Efficiency.

Photoelectrochemical (PEC) solar conversion is a green strategy for addressing the energy crisis. In this study, a three-storey nanostructure BiVO4/(RGO/WO3)/W18O49 was fabricated as a PEC photoanode and demonstrated a highly enhanced PEC efficiency. The top and middle storeys are a reduced graphene oxide (RGO) layer and WO3 nanorods (NRs) decorated with BiVO4 nanoparticles (NPs), respectively. The bottom storey is the W18O49 film grown on a pure W substrate. In this novel design, experiments and modeling together demonstrated that the RGO layer and WO3 NRs with a fast carrier mobility can serve as multichannel pathways, sharing and facilitating electron transport from the BiVO4 NPs to the W18O49 film. The high conductivity of W18O49 can further enhance the charge transfer and retard electron-hole recombination, leading to a highly improved PEC efficiency of the BiVO4/WO3 heterojunction. As a result, the as-fabricated three-storey photoanode covered with FeOOH/NiOOH achieves an attractive PEC photocurrent density of 4.66 mA/cm2 at 1.5 V versus Ag/AgCl, which illustrates the promising potential of the three-storey hetero-nanostructure in future photoconversion applications.

Au-Coated Honeycomb Structure as an Efficient TCO-Free Counter-Electrode for Quantum-Dot-Sensitized Solar Cells.

This study reports the fabrication of a Petri dish patterned with cylindrical micro-cavities that are produced using a one-step solvent-immersion phase-separation process. The developed 3D honeycomb Petri dish is coated with a Au film through a sputtering method to be an efficient Au-coated FTO-free electrode for quantum-dot-sensitized solar cells. Due to the high specific active surface area of the electrode with the Au-coated honeycomb structure, the energy conversion efficiency of devices that use this electrode is 5.2 % compared to 4.4 and 4.7 % by devices using an Au-coated flat Petri dish and an Au-coated FTO electrode, respectively. This design strategy offers excellent potential for the fabrication of highly efficient counter electrodes with FTO-free substrates of flexible photovoltaic devices.

Environmentally benign synthesis of CuInS2/ZnO heteronanorods: visible light activated photocatalysis of organic pollutant/bacteria and study of its mechanism.

Due to its high light absorption coefficient and appropriate bandgap, CuInS2 (CIS) has been receiving much attention as an absorber material for thin film solar cells and also as a visible light photocatalyst. Herein we present heterostructured CIS/ZnO nanorods (NRs) in an attempt to enhance light absorption and facilitate charge separation/transfer in the photocatalysis system. CIS nanoparticles (NPs) were directly deposited on ZnO nanorod arrays (NRAs) to fabricate heterostructured CIS/ZnO NRAs using an environmentally benign, non-hydrazine solution reaction. These heterostructured NRAs are immobilized on FTO glass, which has additional merits of recyclability and bias-applicability. The ideal type-II band structure of CIS/ZnO enables efficient charge separation/transfer, which is confirmed by PL (photoluminescence) decay measurements. Also, the 1D-ZnO NR structure facilitates fast charge transfer along with enhancing light absorption via light scattering. These synergistic effects improved the photocatalytic activity in both organic dye and bacteria decomposition. The photodecomposition efficiency was further enhanced with an aid of external bias. The underlying photocatalytic mechanism was also investigated through controlled experiments under various scavenging conditions. The results suggest that reactive oxygen species (ROS) formed by multistep reduction of O2 play a main role in photocatalysis, while hole-induced photodecomposition is relatively deactivated due to the band structure of the heterostructures of CIS/ZnO.

A Highly Versatile and Adaptable Artificial Leaf with Floatability and Planar Compact Design Applicable in Various Natural Environments.

As a promising means of solar energy conversion, photovoltaic (PV) cell-based electrolysis has recently drawn considerable attention for its effective solar fuel generation; especially the generation of hydrogen by solar water splitting. Inspired by remarkable accomplishments in enhancing the solar-to-hydrogen conversion efficiency, various efforts have aimed at fostering convenient and practical uses of PV electrolysis to make this technology ubiquitous, manageable, and efficient. Here, the design and function of a monolithic photoelectrolysis system-a so-called artificial leaf-for use in various environments are highlighted. The uniquely designed artificial-leaf system facilitates an unbiased water-splitting reaction by combining superstrate PV cells in series with single-face electrodes in a compact 2D catalytic configuration. Floatability is a new feature of the water-splitting artificial leaf; this feature maximizes solar light utilization and allows for easy retrieval for recycling. Additionally, its planar design enables operation of the device in water-scarce conditions. These characteristics endow the artificial leaf with versatility and a high adaptability to natural environments, widening the applicability of the device.

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst.

Among three polymorphs of TiO2, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO2 polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO2 film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO2. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO2 polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO2 film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookite-nanostructured TiO2 electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential.