3D‐Branched ZnO/CdS Nanowire Arrays for Solar Water Splitting and the Service Safety Research

Modulation of broadband light trapping through assembly of 3D structures and modification with narrow band‐gap semiconductors provide an effective way to improve the photoelectrochemical (PEC) performance. Here, 3D‐branched ZnO nanowire arrays (NWAs) modified with cadmium sulfide (CdS) nanoparticles are designed and synthesized via solution chemical routes. The 3D‐branched ZnO NWA–CdS nanoparticle photoanodes show an excellent PEC performance in UV and visible region and the maximum photo‐to‐hydrogen conversion efficiency reaches to 3.1%. The high performance of 3D‐branched ZnO NWA–CdS composites is mainly attributed to the excellent carrier collection capability and high light‐trapping ability of 3D‐branched ZnO NWAs as well as the excellent photocatalytic activity of CdS nanoparticles in the visible region. In addition, the photocorrosion mechanism of 3D‐branched ZnO NWA–CdS photoanodes is systematically investigated, and a protective TiO₂ layer is deposited onto the photoanodes to elevate the PEC stability. The results benefit a deeper understanding of the role of 3D‐branched structures decorated with narrow band‐gap semiconductors in solar water splitting.     

Dissociating Water at n‐Si Photoanodes Partially Covered with Fe Catalysts

Stable, efficient, and low‐cost photoanodes are urgently required for manufacturing water‐splitting photoelectrochemical cells. Although silicon is a promising photoelectrode substrate, photocorrosion prevents its use in such devices, especially when employed as photoanodes for the oxygen evolution reaction (OER). Here, it is shown that Fe nanoparticles (NPs), deposited by cathodic electrodeposition onto n‐Si, can promote hole transfer for the OER. The influence of the surface coverage, the Si structure, as well as the electrolyte are studied here in detail. It is reported that the NP density and the Si structuration drastically affect the photoelectrochemical performance and that the electrolyte influences the stability, allowing operation times as long as 130 h for these inhomogeneously coated photoelectrodes.     

Multifunctional Coatings from Scalable Single Source Precursor Chemistry in Tandem Photoelectrochemical Water Splitting

The straightforward and inexpensive fabrication of stabilized and activated photoelectrodes for application to tandem photoelectrochemical (PEC) water splitting is reported. Semiconductors such as Si, WO₃, and BiVO₄ can be coated with a composite layer formed upon hydrolytic decomposition of hetero­bimetallic single source precursors (SSPs) based on Ti and Ni, or Ti and Co in a simple single‐step process under ambient conditions. The resulting 3d‐transition metal oxide composite films are multifunctional, as they protect the semiconductor electrode from corrosion with an amorphous TiO₂ coating and act as bifunctional electrocatalysts for H₂ and O₂ evolution based on catalytic Ni or Co species. Thus, this approach enables the use of the same precursors for both photoelectrodes in tandem PEC water splitting, and SSP chemistry is thereby established as a highly versatile low‐cost approach to protect and activate photoelectrodes. In an optimized system, SSP coating of a Si photocathode and a BiVO₄ photoanode resulted in a benchmark noble metal‐free dual‐photoelectrode tandem PEC cell for overall solar water splitting with an applied bias solar‐to‐hydrogen conversion efficiency of 0.59% and a half‐life photostability of 5 h.     

Monolithic Photoassisted Water Splitting Device Using Anodized Ni‐Fe Oxygen Evolution Catalytic Substrate

Large‐scale industrial application of solar‐driven water splitting has called for the development of oxygen evolution reaction (OER) catalysts that deliver high catalytic activity and stability. Here it is shown that an efficient OER catalytic substrate can be developed by roll‐to‐roll fabrication of electrodeposited Ni‐Fe foils, followed by anodization. An amorphous oxyhydroxide layer directly formed on Ni‐Fe foils exhibits high catalytic activity toward water oxidation in 1 m KOH solution, which requires an overpotential of 0.251 V to reach current density of 10 mA cm^–2. The developed catalytic electrode shows the best OER activity among catalysts with film structure. The catalyst also shows prolonged stability at vigorous gas evolution condition for 36 h. To demonstrate the monolithic photoassisted water splitting device, an amorphous silicon solar cell is fabricated on Ni‐Fe catalytic substrate, resulting in lowering OER overpotential under light illumination. This monolithic device is the first demonstration that the OER catalytic substrates and the solar cells are integrated and can be easily applied for industrial scale solar‐driven water electrolysis.     

Highly Efficient Sunlight‐Driven Seawater Splitting in a Photoelectrochemical Cell with Chlorine Evolved at Nanostructured WO3 Photoanode and Hydrogen Stored as Hydride within Metallic Cathode

A seawater splitting photoelectrochemical cell featuring a nanostructured tungsten trioxide photoanode that exhibits very high and stable photocurrents producing chlorine with average 70% Faradaic efficiency is described. Fabrication of the WO₃ electrodes on fluorine‐doped tin oxide substrates involves a simple solution‐based method and sequential layer‐by‐layer deposition with a progressively adjusted amount of structure‐directing agent in the precursor and a two‐step annealing. Such a procedure allows tailoring of thick, highly porous, structurally stable WO₃ films with a large internal photoactive surface area optimizing utilization of visible light wavelengths by the photoanode. With the application of an anodic potential of 0.76 V versus Ag/AgCl reference electrode (0.4 V below the thermodynamic Cl₂/Cl^? potential) in synthetic seawater, the designed WO₃ photoanodes irradiated with simulated 1 sun AM 1.5G light reach currents exceeding 4.5 mA cm^?2. Photocurrents close to 5 mA cm^?2 are attained in the case of fresh water splitting using 1 m methane–sulfonic acid supporting electrolyte with oxygen evolved at the WO₃ photoanode. The amount of formed hydrogen is determined by discharging the palladium sheet electrode employed as a cathode. Collection of hydrogen in the form of a hydride opens, more generally, the prospect of subsequently using such materials as anodes in batteries employing oxygen reduction cathodes.     

Design of Plasmonic Nanoparticles for Efficient Subwavelength Light Trapping in Thin-Film Solar Cells

This paper explores geometry-sensitive scattering from plasmonic nanoparticles deposited on top of a thin-film amorphous silicon solar cell to enhance light trapping in the photo-active layer. Considering the nanoparticles as ideal spheroids, the broadband optical absorption by the silicon layer is analyzed and optimized with respect to the nanoparticle aspect ratio in both the cases of resonant (silver) and nonresonant (aluminum) plasmonic nanostructures. It is demonstrated how the coupling of sunlight with the semiconductor can be improved through tuning the nanoparticle shape in both the dipolar and multi-polar scattering regimes, as well as discussed how the native oxide shell formed on the nanospheroid surface after the prolonged action of air and moisture affects the light trapping in the active layer and changes the photocurrent generation by the solar cell.     

Boosting Photoelectrochemical Water Oxidation of Hematite in Acidic Electrolytes by Surface State Modification

State‐of‐the‐art water‐oxidation catalysts (WOCs) in acidic electrolytes usually contain expensive noble metals such as ruthenium and iridium. However, they too expensive to be implemented broadly in semiconductor photoanodes for photoelectrochemical (PEC) water splitting devices. Here, an Earth‐abundant CoFe Prussian blue analogue (CoFe‐PBA) is incorporated with core–shell Fe₂O₃/Fe₂TiO₅ type II heterojunction nanowires as composite photoanodes for PEC water splitting. Those deliver a high photocurrent of 1.25 mA cm^?2 at 1.23 V versus reversible reference electrode in acidic electrolytes (pH = 1). The enhancement arises from the synergic behavior between the successive decoration of the hematite surface with nanolayers of Fe₂TiO₅ and then, CoFe‐PBA. The underlying physical mechanism of performance enhancement through formation of the Fe₂O₃/Fe₂TiO₅/CoFe‐PBA heterostructure reveals that the surface states’ electronic levels of hematite are modified such that an interfacial charge transfer becomes kinetically favorable. These findings open new pathways for the future design of cheap and efficient hematite‐based photoanodes in acidic electrolytes.     

Scalable Triple Cation Mixed Halide Perovskite–BiVO4 Tandems for Bias‐Free Water Splitting

Strong interest exists in the development of organic–inorganic lead halide perovskite photovoltaics and of photoelectrochemical (PEC) tandem absorber systems for solar fuel production. However, their scalability and durability have long been limiting factors. In this work, it is revealed how both fields can be seamlessly merged together, to obtain scalable, bias‐free solar water splitting tandem devices. For this purpose, state‐of‐the‐art cesium formamidinium methylammonium (CsFAMA) triple cation mixed halide perovskite photovoltaic cells with a nickel oxide (NiO_x) hole transport layer are employed to produce Field's metal‐epoxy encapsulated photocathodes. Their stability (up to 7 h), photocurrent density (–12.1 ± 0.3 mA cm^?2 at 0 V versus reversible hydrogen electrode, RHE), and reproducibility enable a matching combination with robust BiVO₄ photoanodes, resulting in 0.25 cm² PEC tandems with an excellent stability of up to 20 h and a bias‐free solar‐to‐hydrogen efficiency of 0.35 ± 0.14%. The high reliability of the fabrication procedures allows scaling of the devices up to 10 cm², with a slight decrease in bias‐free photocurrent density from 0.39 ± 0.15 to 0.23 ± 0.10 mA cm^?2 due to an increasing series resistance. To characterize these devices, a versatile 3D‐printed PEC cell is also developed.     

Robust Sub‐Monolayers of Co3O4 Nano‐Islands: A Highly Transparent Morphology for Efficient Water Oxidation Catalysis

The scalable synthesis of highly transparent and robust sub‐monolayers of Co₃O₄ nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co₃O₄ nanoparticles and thermally induced self‐organization lead to an ultra‐fine nano‐island morphology with more than 94% light transmission at a wavelength of 500 nm. These transparent sub‐monolayers demonstrate a remarkable mass‐weighted water oxidation activity of 2070–2350 A g_Co3O4^?1 and per‐metal turnover frequency of 0.38–0.62 s^?1 at an overpotential of 400 mV in 1 m NaOH aqueous solution. This mixed valent cobalt oxide structure exhibits excellent long‐term electrochemical and mechanical stability preserving the initial catalytic activity over more than 12 h of constant current electrolysis and 1000 consecutive voltammetric cycles. The potential of the Co₃O₄ nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co₃O₄‐GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. These findings provide a highly performing catalyst structure and a scalable synthesis method for the engineering of efficient photoanodes for integrated solar water‐splitting cells.     

Hybrid Heterojunction and Solid‐State Photoelectrochemical Solar Cells

A hybrid heterojunction and solid‐state photoelectrochemical solar cell based on graphene woven fabrics (GWFs) and silicon is designed and fabricated. The GWFs are transferred onto n‐Si to form a Schottky junction with an embedded polyvinyl alcohol based solid electrolyte. In the hybrid solar cell, solid electrolyte serves three purposes simutaneously; it is an anti‐reflection layer, a chemical modification carrier, and a photoelectrochemical channel. The open‐circuit voltage, short‐circuit current density, and fill factor are all significantly improved, achieving an impressive power conversion efficiency of 11%. Solar cell models are constructed to confirm the hybrid working mechanism, with the heterojunction junction and photoelectrochemical effect functioning synergistically.     

Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers

Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p⁺nn⁺ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.     

Boosting Photoelectrochemical Water Splitting by TENG‐Charged Li‐Ion Battery

The need for cost‐effective and sustainable power supplies has spurred a growing interest in hybrid energy harvesting systems, and the most elementary energy production process relies on intermittent solar power. Here, it is shown how the ambient mechanical energy leads to water splitting in a photoelectrochemical (PEC) cell boosted by a triboelectric nanogenerator (TENG). In this strategy, a flexible TENG collects and transforms mechanical energy into electric current, which boosts the PEC water splitting via the charged Li‐ion battery. Au nanoparticles are deposited on TiO₂ nanoarrays for extending the available light spectrum to visible part by surface plasmon resonance effect, which yields a photocurrent density of 1.32 mA cm^?2 under AM 1.5 G illumination and 0.12 mA cm^?2 under visible light with a bias of 0.5 V. The TENG‐charged battery boosts the water splitting performance through coupling electrolysis and enhanced electron–hole separation efficiency. The hybrid cell exhibits an instantaneous current more than 9 mA with a working electrode area of 0.3 cm², suggesting a simple but efficient route for simultaneously converting solar radiation and mechanical energy into hydrogen.     

Influences of Nanostructure Arrays on Light Absorption in Amorphous Silicon Thin-Film Solar Cells

By means of finite-difference time-domain (FDTD) numerical method, we investigate the possibility to enhance the light absorption in solar cells by employing different nanostructures. The solar cells are made of 100-nm-thick amorphous silicon (α-Si). The impacts of gold nanohole arrays, dielectric nanosphere arrays, and gold nanoparticle arrays on the light absorption are simulated, compared, and analyzed. The results show that gold nanohole arrays functioning as the back reflective layer, dielectric nanosphere arrays, and gold nanoparticle arrays can significantly enhance the light absorption for the solar cells, and the former two can increase the short-circuit current by more than 40 %, showing a great potential to improve the utilization efficiency of solar energy.     

Mediatorless biosensor for H(2)O(2) based on recombinant forms of horseradish peroxidase directly adsorbed on polycrystalline gold

Four forms of horseradish peroxidase (HRP) have been used to prepare peroxidase-modified gold electrodes for mediatorless detection of peroxide: native HRP, wild type recombinant HRP, and two recombinant forms containing six-His tag at the C-terminus and at the N-terminus, respectively. The adsorption of the enzyme molecules on gold was studied by direct mass measurements with electrochemical quartz crystal microbalance. All the forms of HRP formed a monolayer coverage of the enzyme on the gold surface. However, only gold electrodes with adsorbed recombinant HRP forms exhibited high and stable current response to H(2)O(2) due to its bioelectrocatalytic reduction based on direct electron transfer between gold and HRP. The sensitivity of the gold electrodes modified with recombinant HRPs was in the range of 1.4-1.5 A M(-1) cm(-2) at -50 mV versus Agmid R:AgCl. The response to H(2)O(2) in the concentration range 0.1-40 microM was not dependent on the presence of a mediator (i.e. catechol) giving strong evidence that the electrode currents are diffusion limited. Lower detection limit for H(2)O(2) detection was 10 nM at the electrodes modified with recombinant HRPs.     

Influence of SiO<Subscript>2</Subscript> Spacer Layer Thickness on Performance of Plasmonic Textured Silicon Solar Cell

We investigated the effect of SiO₂ spacer layer thickness between the textured silicon surface and silver nanoparticles (Ag NPs) on solar cell performance using quantum efficiency analysis. Separation of Ag NPs from high index silicon with SiO₂ layer led to modified absorption and scattering cross-sections due to graded refractive index medium. The forward scattering from Ag NPs is very sensitive to SiO₂ layer thickness in plasmonic silicon cell performance due to the evanescent character of generated near-fields around the NPs. With the optimized ～30–40 nm SiO₂ spacer layer, we observed an enhancement of solar cell efficiency from ～8.7 to ～10 %, which is due to the photocurrent enhancement in the off-resonance surface plasmon region. We also estimated minority carrier diffusion lengths (L _eff) from internal quantum efficiency data, which are also sensitive to SiO₂ spacer layer thickness. We observed that the L _eff values are enhanced from ～356 to ～420 μm after placing Ag NPs on ～40 nm spacer layer due to improved forward (angular) scattering of light from the Ag NPs into silicon.     

Tailored TiO2 Protection Layer Enabled Efficient and Stable Microdome Structured p‐GaAs Photoelectrochemical Cathodes

Group III–V compound semiconductors are a promising group of materials for photoelectrochemical (PEC) applications. In this work, a metal assisted wet etching approach is adapted to acquiring a large‐area patterned microdome structure on p‐GaAs surface. In addition, atomic layer deposition is used to deposit a TiO₂ protection layer with controlled thickness and crystallinity. Based on a PEC photocathode design, the optimal configuration achieves a photocurrent of ?5 mA cm^?2 under ?0.8 V versus Ag/AgCl in a neutral pH electrolyte. The TiO₂ coating with a particular degree of crystallization deposited via controlled temperature demonstrates a superior stability over amorphous coating, enabling a remarkably stable operation, for as long as 60 h. The enhanced charge separation induced by favorable band alignment between GaAs and TiO₂ contributes simultaneously to the elevated solar conversion efficiency. This approach provides a promising solution to further development of group III–V compounds and other photoelectrodes with high efficiency and excellent durability for solar fuel generation.     

The “Midas Touch” Transformation of TiO2 Nanowire Arrays during Visible Light Photoelectrochemical Performance by Carbon/Nitrogen Coimplantation

Titanium dioxide is a promising photoanode material for water oxidation, but it is substantially limited by its poor efficiency in the visible light range. Herein, an innovative carbon/nitrogen coimplantation method is utilized to realize the “Midas touch” transformation of TiO₂ nanowire (NW) arrays for photoelectrochemical (PEC) water splitting in visible light. These modified golden–yellow rutile TiO₂ NW arrays (C/N‐TiO₂) exhibit remarkably enhanced absorption in visible light regions and more efficient charge separation and transfer. As a result, the photocurrent density of carbon/nitrogen co‐implanted TiO₂ under visible light (>420 nm) can reach 0.76 mA cm^?2, which far exceeds the value of 3 µA cm^?2 seen for pristine TiO₂ nanowire arrays at 0.8 V versus Ag/AgCl. An incident photon to electron conversion efficiency of ≈14.8% is achieved at 450 nm on C/N‐TiO₂ without any other cocatalysts. The ion implantation doping approach, combined with codoping strategies, is proved to be an effective strategy for enhancing the photoelectrochemical conversion and can enable further improvement of the PEC water‐splitting performance of many other semiconductor photoelectrodes.     

Direct mediatorless electron transport between the monolayer of photosystem II and poly(mercapto-p-benzoquinone) modified gold electrode--new design of biosensor for herbicide detection

Photosystem II (PSII) modified gold electrodes have been prepared providing mediatorless electron transport on the basis of electrodeposited conductive layer poly-mercapto-p-benzoquinone (polySBQ). Such electrodes are suitable in construction of biosensors for PSII inhibiting herbicides. PolySBQ layer was synthesized on (i) screen-printed gold electrodes and (ii) gold microelctodes in an array on silicon substrate, by electrochemical-oxidation of sulpho-p-benzoquinone (SBQ) at +650 mV versus Ag/AgCl. The basic properties of polySBQ layer were characterized using linear sweep voltammetry and atomic force microscopy (AFM). The typical redox response for quinones was observed. The optimal length of the polymer providing direct electron transfer (DET) was found to be very close to 30 nm. PSII particles isolated from the thermophilic cyanobacteria Synechococcus bigranulatus were physically adsorbed on the polySBQ covered gold electrodes. The generation of photocurrent was observed at E=+250 mV (versus Ag/AgCl) without addition of any mediator. The basic properties of DET were studied. We concluded that: (i) PSII active in DET is immobilized in form of monolayer; (ii) the charge transport from PSII to gold working electrode (AuWE) is fast and dominated by the rate of the enzymatic reaction; (iii) polySBQ layer drains electrons from the Q(A) pocket of the photosystem since the electrode activity is inhibited by specific inhibitor, i.e. diuron (DCMU); (iv) the stability of the photosystem immobilized on gold electrodes by using polySBQ is comparable to the stability of PSII in solution under the same experimental conditions; (v) the inhibition of the photosystem by herbicide DCMU follows the sigmoid dependence; (vi) I(50) as well as limit of detection (LOD) show an improved sensitivity compared to other published biosensing systems using PSII as bioactive part.     

Design and Fabrication of a Precious Metal‐Free Tandem Core–Shell p+n Si/W‐Doped BiVO4 Photoanode for Unassisted Water Splitting

Tandem photoelectrochemical water splitting cells utilizing crystalline Si and metal oxide photoabsorbers are promising for low‐cost solar hydrogen production. This study presents a device design and a scalable fabrication scheme for a tandem heterostructure photoanode: p⁺n black silicon (Si)/SnO₂ interface/W‐doped bismuth vanadate (BiVO₄)/cobalt phosphate (CoPi) catalyst. The black‐Si not only provides a substantial photovoltage of 550 mV, but it also serves as a conductive scaffold to decrease charge transport pathlengths within the W‐doped BiVO₄ shell. When coupled with cobalt phosphide (CoP) nanoparticles as hydrogen evolution catalysts, the device demonstrates spontaneous water splitting without employing any precious metals, achieving an average solar‐to‐hydrogen efficiency of 0.45% over the course of an hour at pH 7. This fabrication scheme offers the modularity to optimize individual cell components, e.g., Si nanowire dimensions and metal oxide film thickness, involving steps that are compatible with fabricating monolithic devices. This design is general in nature and can be readily adapted to novel, higher performance semiconducting materials beyond BiVO₄ as they become available, which will accelerate the process of device realization.     

Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis

Water photolysis is a sustainable technology to convert natural solar energy and water into chemical fuels and is thus considered a thorough solution to the forthcoming energy crises. Unassisted water splitting that could directly harvest solar light and subsequently split water in a single device has become an important research theme. Three types of tandem devices including photoelectrochemical (PEC), photovoltaic (PV) cell/PEC and PV/electrolyser tandem cells are proposed to realize water photolysis at different levels of integration and component. Recent progress in tandem water splitting devices is summarized, and crucial issues on device optimization from the perspective of each photo‐absorber functionalities in band edge potential, light absorptivity and transmittance are discussed. By increasing the performances of stand‐alone PEC or PV devices, a 20% solar to hydrogen efficiency is predicted that is a significant value towards further application in practice. Accordingly, the challenges for materials development and configuration optimization are further outlined.