A Tantalum Nitride Photoanode Modified with a Hole‐Storage Layer for Highly Stable Solar Water Splitting

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm^?2, whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole‐storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.     

Biological Components and Bioelectronic Interfaces of Water Splitting Photoelectrodes for Solar Hydrogen Production

Artificial photosynthesis (AP) is inspired by photosynthesis in nature. In AP, solar hydrogen can be produced by water splitting in photoelectrochemical cells (PEC). The necessary photoelectrodes are inorganic semiconductors. Light‐harvesting proteins and biocatalysts can be coupled with these photoelectrodes and thus form bioelectronic interfaces. We expand this concept toward PEC devices with vital bio‐organic components and interfaces, and their integration into the built environment.     

Modulating Benzothiadiazole‐Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting

Two‐dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar‐to‐hydrogen energy conversion owing to their pre‐designable structures and tailor‐made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py‐HTP‐BT‐COF) via chlorination (Py‐ClTP‐BT‐COF) and fluorination (Py‐FTP‐BT‐COF) can lead to dramatically enhanced photocatalytic H₂ evolution rates (HER=177.50 μmol h^?1 with a high apparent quantum efficiency (AQE) of 8.45 % for Py‐ClTP‐BT‐COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar‐to‐chemical energy conversion.     

Getting the Most out of Solar Irradiation: Efficient Use of Polychromatic Light for Water Splitting

To increase the fraction of utilizable polychromatic light, a new reactor concept was developed and manufactured by using rapid prototyping technologies. Investigation of the prototypes revealed enhancements of the photocurrent by up to one order of magnitude, when TiO₂ was used as the photoanode in combination with commercially available photovoltaic cells. The reported concept is scalable and an easy transfer to technical scale is expected from a technological as well as an economical perspective. Experimental results underline the conclusion that to achieve efficient overall use of solar irradiation both the material as well as the reactor/process must be considered. Combining these complementary approaches allows largest possible optimization potential. With respect to ongoing research, the concept also breaks ground for the development of catalysts.     

Interface Engineering of a CoOx/Ta3N5 Photocatalyst for Unprecedented Water Oxidation Performance under Visible‐Light‐Irradiation

Cocatalysts have been extensively used to promote water oxidation efficiency in solar‐to‐chemical energy conversion, but the influence of interface compatibility between semiconductor and cocatalyst has been rarely addressed. Here we demonstrate a feasible strategy of interface wettability modification to enhance water oxidation efficiency of the state‐of‐the‐art CoO_x/Ta₃N₅ system. When the hydrophobic feature of a Ta₃N₅ semiconductor was modulated to a hydrophilic one by in situ or ex situ surface coating with a magnesia nanolayer (2–5 nm), the interfacial contact between the hydrophilic CoO_x cocatalyst and the modified hydrophilic Ta₃N₅ semiconductor was greatly improved. Consequently, the visible‐light‐driven photocatalytic oxygen evolution rate of the resulting CoO_x/MgO(in)–Ta₃N₅ photocatalyst is ca. 23 times that of the pristine Ta₃N₅ sample, with a new record (11.3 %) of apparent quantum efficiency (AQE) under 500–600 nm illumination.     

Light‐Driven Charge Separation in Isoxazolidine–Perylene Bisimide Dyads

A series of arrays for light‐driven charge separation is presented, in which perylene tetracarboxylic bisimide is the light‐absorbing chromophore and electron acceptor, whereas isoxazolidines are colourless electron donors, the electron‐releasing properties of which are increased with respect to the amino group by means of the α‐effect. Charge separation (CS) in toluene over a distance ranging from ≈10 to ≈16 Å, with efficiencies of ≈95 to ≈50 % and CS lifetimes from 300 ps to 15 ns, are demonstrated. In dichloromethane the charge recombination reaction is faster than charge separation, preventing accumulation of the CS state. The effects of solvent polarity and molecular structure are discussed in the frame of current theories.     

Structure Modification Function of g‐C3N4 for Al2O3 in the In Situ Hydrothermal Process for Enhanced Photocatalytic Activity

Heterojunctions of g‐C₃N₄/Al₂O₃ (g‐C₃N₄=graphitic carbon nitride) are constructed by an in situ one‐pot hydrothermal route based on the development of photoactive γ‐Al₂O₃ semiconductor with a mesoporous structure and a high surface area (188 m²g^?1) acting as electron acceptor. A structure modification function of g‐C₃N₄ for Al₂O₃ in the hydrothermal process is found, which can be attributed to the coordination between unoccupied orbitals of the Al ions and lone‐pair electrons of the N atoms. The as‐synthesized heterojunctions exhibit much higher photocatalytic activity than pure g‐C₃N₄. The hydrogen generation rate and the reaction rate constant for the degradation of methyl orange over 50 % g‐C₃N₄/Al₂O₃ under visible‐light irradiation (λ>420 nm) are 2.5 and 7.3 times, respectively, higher than those over pristine g‐C₃N₄. The enhanced activity of the heterojunctions is attributed to their large specific surface areas, their close contact, and the high interfacial areas between the components as well as their excellent adsorption performance, and efficient charge transfer ability.     

Enhanced Charge Transfer,Transport and Photovoltaic Efficiency in All‐Polymer Organic Solar Cells by Polymer Backbone Fluorination

We successfully designed and synthesized two BDT‐BT‐T (BDT=benzo[1,2‐b:4,5‐b']dithiophene, BT‐T=4,7‐dithien‐2‐yl‐2,1,3‐benzothiadiazole) based polymers as the electron donor for application in all‐polymer solar cells (all‐PSCs). By adopting N2200 as the electron acceptor, we systematically investigated the impact of fluorination on the charge transfer, transport, blend morphology and photovoltaic properties of the relevant all‐PSCs. A best power conversion efficiency (PCE) of 3.4% was obtained for fluorinated PT‐BT2F/N2200 (BT2F=difluorobenzo[c][1,2,5]thiadiazole) all‐PSCs in comparison with that of 2.7% in non‐fluorinated PT‐BT/N2200 (BT=benzothiadiazole) based device. Herein, all‐polymers blends adopting either non‐fluorinated PT‐BT or fluorinated PT‐BT2F exhibit similar morphology features. In depth optical spectrum measurements demonstrate that molecular fluorination can further enhance charge transfer between donor and acceptor polymer. Moreover, all‐polymer blends exhibit improved hole mobilities and more balanced carriers transport when adopting fluorinated donor polymer PT‐BT2F. Therefore, although the PCE is relatively low, our findings may become important in understanding how subtle changes in molecular structure impact relevant optoelectronic properties and further improve the performance of all‐PSCSs.     

Light‐Driven Water Splitting Mediated by Photogenerated Bromine

Light‐driven water splitting was achieved using a dye‐sensitized mesoporous oxide film and the oxidation of bromide (Br^?) to bromine (Br₂) or tribromide (Br₃^?). The chemical oxidant (Br₂ or Br₃^?) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)₂], ( 1 ; pic=picoline, bda=2,2′‐bipyridine‐6,6′‐dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br₂/Br^? couple or 1.05 V vs. NHE for the Br₃^?/Br^? couple, which is sufficient to drive water oxidation at 1 (Ru^V/IV≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4′‐PO₃H₂‐bpy)(bpy)₂]²⁺ ( RuP ²⁺, bpy=2,2′‐bipyridine) chromophore dye on a SnO₂/TiO₂ core–shell electrode resulted in a photocurrent density of around 1.2 mA cm^?2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution.     

Synthesis of Ordered Porous Graphitic‐C3N4 and Regularly Arranged Ta3N5 Nanoparticles by Using Self‐Assembled Silica Nanospheres as a Primary Template

Uniform‐sized silica nanospheres (SNSs) assembled into close‐packed structures were used as a primary template for ordered porous graphitic carbon nitride (g‐C₃N₄), which was subsequently used as a hard template to generate regularly arranged Ta₃N₅ nanoparticles of well‐controlled size. Inverse opal g‐C₃N₄ structures with the uniform pore size of 20–80 nm were synthesized by polymerization of cyanamide and subsequent dissolution of the SNSs with an aqueous HF solution. Back‐filling of the C₃N₄ pores with tantalum precursors, followed by nitridation in an NH₃ flow gave regularly arranged, crystalline Ta₃N₅ nanoparticles that are connected with each other. The surface areas of the Ta₃N₅ samples were as high as 60 m² g⁻¹, and their particle size was tunable from 20 to 80 nm, which reflects the pore size of g‐C₃N₄. Polycrystalline hollow nanoparticles of Ta₃N₅ were also obtained by infiltration of a reduced amount of the tantalum source into the g‐C₃N₄ template. An improved photocatalytic activity for H₂ evolution on the assembly of the Ta₃N₅ nanoparticles under visible‐light irradiation was attained as compared with that on a conventional Ta₃N₅ bulk material with low surface area.     

Efficient Hole Extraction from a Hole‐Storage‐Layer‐Stabilized Tantalum Nitride Photoanode for Solar Water Splitting

One of the major hurdles that impedes the practical application of photoelectrochemical (PEC) water splitting is the lack of stable photoanodes with low onset potentials. Here, we report that the Ni(OH)_x/MoO₃ bilayer, acting as a hole‐storage layer (HSL), efficiently harvests and stores holes from Ta₃N₅, resulting in at least 24 h of sustained water oxidation at the otherwise unstable Ta₃N₅ electrode and inducing a large cathodic shift of ≈600 mV in the onset potential of the Ta₃N₅ electrode.     

Charge Carrier Transport Mechanism in Ta2O5, TaON,and Ta3N5 Studied by Applying Polaron Hopping and Bandlike Models

TaON and Ta₃N₅ are considered promising materials for photocatalytic and photoelectrochemical water splitting. In contrast, their counterpart Ta₂O₅ does not exhibit good photocatalytic performance. This may be explained with the different charge carrier transport mechanisms in these materials, which are not well understood yet. Herein, we investigate the charge transport properties in Ta₂O₅, TaON, and Ta₃N₅ by polaron hopping and bandlike models. First, the polaron binding energies were calculated to evaluate whether the small polaron occurs in these materials. Then we performed calculations to localize the excess carriers as small polarons using a hybrid density functional. We find that the small polaron hopping is the charge transfer mechanism in Ta₂O_5, whereas our calculations indicate that this mechanism may not occur in TaON and Ta₃N₅. We also investigated the bandlike model mechanism by calculating the charge carrier mobility of these materials using the effective mass approximation, but the calculated mobility is not consistent with experimental results. This study is a first step towards understanding charge transport in oxynitrides and nitrides and furthermore establishes a simple rule to determine whether a small polaron occurs in a material.     

Modulating Benzothiadiazole-Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting

Two-dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar-to-hydrogen energy conversion owing to their pre-designable structures and tailor-made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py-HTP-BT-COF) via chlorination (Py-ClTP-BT-COF) and fluorination (Py-FTP-BT-COF) can lead to dramatically enhanced photocatalytic H₂ evolution rates (HER=177.50 μmol h⁻¹ with a high apparent quantum efficiency (AQE) of 8.45 % for Py-ClTP-BT-COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar-to-chemical energy conversion.     

Long‐Lived,Directional Photoinduced Charge Separation in RuII Complexes Bearing Laminate Polypyridyl Ligands

Ru^II complexes incorporating both amide‐linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz), tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz), and 9,11,20,22‐tetraazatetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐l:2′′′,3′′′]‐pentacene (tatpp) exhibit long‐lived charge separated (CS) states, which have been analyzed using time‐resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal “bpy‐like” portion and central (or distal) “phenazine‐like” portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 μs) observed in these complexes reflect an equilibration of the ³MLCT_prox or ³MLCT_dist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a Zn^II ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCT_dist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCT_dist state. In tpphz complexes, Zn^II coordination can reorder the energy of the ³MLCT_prox and ³MLCT_dist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron–hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.     

Ferrocene‐Substituted Naphthalenediimide with Broad Absorption and Electron‐Transport Properties in the Segregated‐Stack Structure

A new naphthalenediimide (NDI) molecule, where two ferrocene (Fc) units were directly attached to both imide nitrogens ( Fc‐NDI‐Fc ), was synthesized. The Fc units provide high crystallinity to Fc‐NDI‐Fc with good solubility to conventional organic solvents. The Fc units also work as electron‐donating substituents, in contrast to the electron‐deficient NDI unit, resulting in broad charge‐transfer absorption of Fc‐NDI‐Fc from the UV region to 1500 nm in the solid state. The crystal structure analysis revealed that Fc‐NDI‐Fc formed a segregated‐stack structure. The DFT calculation based on the crystal structure showed that the NDI π‐orbitals extended over two axes. The extended π‐network of the NDI units led to the electron‐transport properties of Fc‐NDI‐Fc , which was confirmed using a flash‐photolysis time‐resolved microwave conductivity technique.     

An Electron Dynamics Mechanism of Charge Separation in the Initial‐Stage Dynamics of Photoinduced Water Splitting in XMnWater (X=OH,OCaH) and Electron–Proton Acceptors

An electron dynamics mechanism of charge separation in the initial stage of excited‐state reactions of the class of X?Mn?OH₂???A${ \to }$ X?Mn?OH???HA (X=OH or OCaH; A=N‐methylformamidine, guanidine, imidazole, or ammonia cluster) is reported. The dynamic effect of calcium doping is also revealed. This study provides a novel factor to be considered in designing efficient systems for photoinduced water splitting.