An artificial photosynthetic system containing an inorganic semiconductor and a molecular catalyst for photocatalytic water oxidation

Electrochemical water oxidation catalyzed by a mononuclear cobalt complex of a pentadentate ligand: the critical effect of the borate anion

A cobalt complex is found as a homogeneous water oxidation electrocatalyst. Electrochemical examinations indicate that the implementation of proton-couple electron transfer process and formation of O–O bond are assisted by borate anion.

Engineering microbial metabolic energy homeostasis for improved bioproduction

Metabolic energy (ME) homeostasis is essential for the survival and proper functioning of microbial cell factories. However, it is often disrupted during bioproduction because of inefficient ME supply and excessive ME consumption. In this review, we propose strategies, including reinforcement of the capacity of ME-harvesting systems in autotrophic microorganisms; enhancement of the efficiency of ME-supplying pathways in heterotrophic microorganisms; and reduction of unessential ME consumption by microbial cells, to address these issues. This review highlights the potential of biotechnology in the engineering of microbial ME homeostasis and provides guidance for the higher efficient bioproduction of microbial cell factories.

Morphology Engineering of BiVO4 with CoOx Derived from Cobalt-containing Polyoxometalate as Co-catalyst for Oxygen Evolution

Bismuth vanadate (BiVO₄ ) as a metal oxidation semiconductor has stimulated extensive attention in the photocatalytic water splitting field. However, the poor transport ability and easy recombination of charge carriers limit photocatalytic water oxidation activity of pure BiVO₄ . Herein, the photocatalytic activity of BiVO₄ is enhanced via adjusting its morphology and combination co-catalyst. First, the Cu-BiVO₄ was synthesized by copper doping to control the growth of {110} facet of BiVO₄ , which is regarded for the separation of photo-generated charge carriers. Then the CoO_x in-situ generated from K₆ [SiCo^II (H₂ O)W₁₁ O₃₉ ] ⋅ 16H₂ O was photo-deposited on Cu-BiVO₄ surface as co-catalyst to speed up reaction kinetics. Cu-BiVO₄ @CoO_x hybrid catalyst shows highest photocatalytic activity and best stability among all the prepared catalysts. Oxygen evolution is about 34.6 μmol in pH 4 acetic acid buffer under 420 nm LED irradiation, which is nearly 20 times higher than that of pure BiVO₄ . Apparent quantum efficiency (AQE) in 1 h and O₂ yield are 1.83% and 23.1%, respectively. O₂ evolution amount nearly maintains the original value even after 5 cycles.

Enhanced terahertz radiation from InAs (100) with an embedded InGaAs hole blocking layer

We demonstrate enhanced THz radiation from p-InAs (100) by advanced heterostructure design. The THz radiation from InAs (100) under ultra-short pulsed laser excitation is due to the photo-Dember effect. Inserting a thin n-InGaAs layer close to the InAs surface effectively blocks the hole diffusion while the electron diffusion is still efficient due to tunneling. Therefore, enhanced photogenerated electron-hole separation and photo-Dember electric field is achieved to enhance the THz emission. The layer structure and doping profile are confirmed by secondary ion mass spectrometry and X-ray diffraction. The blocking of the hole diffusion is independently verified by the surface photovoltage measured by Kelvin probe force microscopy.

Fabrication of novel CuFe2O4/MXene hierarchical heterostructures for enhanced photocatalytic degradation of sulfonamides under visible light

The overuse of sulfonamides, causing serious pollution of water bodies, has drawn great attention from society. To address these problems, a novel CuFe₂O₄/MXene (CFO/Ti₃C₂) heterojunction photocatalyst was used to photodegrade the antibiotic sulfamethazine (SMZ, a typical pollutant) under visible light, and the synergy and coupling function of the two components in the heterojunction system were analyzed. With the aid of time-resolved photoluminescence (TRPL) and transient surface photovoltage (TPV) spectra, the carrier lifetimes and kinetic behaviors were studied, revealing that the lifetime of photoinduced carriers was prolonged by loading Ti₃C₂, inhibiting the reorganization of photogenerated electron holes. More importantly, the organic intermediates and mineralization degree were identified by high-performance liquid chromatography (HPLC)-mass spectrometry (MS) and total organic carbon (TOC) techniques. The results show that the breaking of SN bonds, the oxidation of aniline and deamination were dominated by the attack of •OH. This work shows a new model for the degradation mechanism of SMZ over CFO/MXene heterostructures.

Immobilization of Metal-Organic Framework MIL-100(Fe) on the Surface of BiVO4: A New Platform for Enhanced Visible-Light-Driven Water Oxidation

The development of new dual functional photocatalysts is highly desirable for conversion and storage of solar energy. Herein, we first constructed hierarchical structure MIL-100(Fe)@BiVO₄ in situ growing MIL-100(Fe) nanoparticles (NPs) on the surface of decahedron BiVO₄ under mild hydrothermal conditions. The as-synthesized hybrid nanostructure is unambiguously determined using a series of characterization methods. These results demonstrate that the ultra-tiny MOF MIL-100(Fe) particles are immobilized on the surface of decahedron BiVO₄ and the composite exhibits a strong interaction between BiVO₄ and MIL-100(Fe). This hybrid material MIL-100(Fe)@BiVO₄ is employed as a photocatalyst for water oxidation reaction and demonstrates higher O₂ production activity in comparison with bare BiVO₄. The best performance obtained at the optimal mass percentage of MIL-100(Fe) (8.0 wt %) reaches 333.3 μmol h^-1 g^-1 of the O₂ evolution rate irradiated with visible light, which is almost 4.3 times higher than bare BiVO₄ (77.3 μmol h^-1 g^-1). The enhanced water oxidation performance is due to the more efficient interfacial electron-hole transfer between MIL-100(Fe) and BiVO₄, which is verified by the results of various photo-electrochemical characterizations. Moreover, the as-prepared composite MIL-100(Fe)@BiVO₄ also displays excellent stability for visible-light-driven water oxidation. This study affords a rational strategy for the controllable construction by loading metal-organic frameworks on a semiconductor surface, which is a good reference for other artificial photosystems.

Promoting hole transfer for photoelectrochemical water oxidation through a manganese cluster catalyst bioinspired by natural photosystem II

A Mn₄O₄–cubane molecule bioinspired by the natural photosystem II was used as a co-catalyst in photoelectrochemical water oxidation.

Photocatalytic Oxygen Evolution from Water Splitting

Photocatalytic water splitting has attracted a lot of attention in recent years, and O2 evolution is the decisive step owing to the complex four-electrons reaction process. Though many studies have been conducted, it is necessary to systematically summarize and introduce the research on photocatalytic O2 evolution, and thus a systematic review is needed. First, the corresponding principles about O2 evolution and some urgently encountered issues based on the fundamentals of photocatalytic water splitting are introduced. Then, several types of classical water oxidation photocatalysts, including TiO2, BiVO4, WO3, α-Fe2O3, and some newly developed ones, such as Sillén-Aurivillius perovskites, porphyrins, metal-organic frameworks, etc., are highlighted in detail, in terms of their crystal structures, synthetic approaches, and morphologies. Third, diverse strategies for O2 evolution activity improvement via enhancing photoabsorption and charge separation are presented, including the cocatalysts loading, heterojunction construction, doping and vacancy formation, and other strategies. Finally, the key challenges and future prospects with regard to photocatalytic O2 evolution are proposed. The purpose of this review is to provide a timely summary and guideline for the future research works for O2 evolution.

Water Oxidation Catalysts for Artificial Photosynthesis

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Energy transfer on a two-dimensional antenna enhances the photocatalytic activity of CO2 reduction by metal-organic layers

Excited state energies on a two-dimensional light-harvesting metal-organic layer (MOL) are efficiently transported to Re- and Ir-based reaction centers for converting CO₂ to CO or HCOOH. Such energy transfer enhances the photocatalytic CO₂ reduction activity because it enables multiple photo-electron injections in a short time period in the photocatalysis.

Immobilization of a molecular cobalt cubane catalyst on porous BiVO4via electrochemical polymerization for efficient and stable photoelectrochemical water oxidation

A cobalt cubane catalyst was immobilized onto a BiVO₄ electrode via electrochemical polymerization to fabricate hybrid photoanodes for stable photoelectrochemical water oxidation.

Colloidal semiconductor nanocrystals in energy transfer reactions

Excitonic energy transfer is a versatile mechanism by which colloidal semiconductor nanocrystals can interact with a variety of nanoscale species. This feature article will discuss the latest research on the key scenarios under which semiconductor nanocrystals can engage in energy transfer with other nanoparticles, organic fluorophores, and plasmonic nanostructures, highlighting potential technological benefits to be gained from such processes.

A stable iron-containing polyoxometalate coupled with semiconductor for efficient photocatalytic water oxidation under acidic condition

A homogeneous molecular catalyst, polyoxometalate (POM) Fe11, can act as a true cocatalyst for the efficient oxygen evolution reaction through the pH adjustment strategy when BiVO₄ is used as a light-harvesting material.

Enhanced photocatalytic activity of BiVO4 coupled with iron-based complexes for water oxidation under visible light irradiation

Four iron complexes were used as pre-catalysts in BiVO₄–NaIO₃ photocatalytic water oxidation systems. The best-performing system afforded a rather high oxygen yield and apparent quantum efficiency yield of 99.1% and 44.3%, respectively.

Thermal annealing-induced structural reorganization in polymeric photocatalysts for enhanced hydrogen evolution

Facile reorganization of a conjugated triazine-based polymer structure has been adopted to optimize its optical, electronic, and photocatalytic properties.

Anthracene-based azo dyes for photo-induced proton-coupled electron transfer

Herein, we report a new donor–acceptor system for photo-induced proton-coupled electron transfer (PCET) that leverages an azo linkage as the proton-sensitive component and anthracene as a photo-trigger.

End-On Bound Iridium Dinuclear Heterogeneous Catalysts on WO3 for Solar Water Oxidation

Heterogeneous catalysts with atomically defined active centers hold great promise for high-performance applications. Among them, catalysts featuring active moieties with more than one metal atom are important for chemical reactions that require synergistic effects but are rarer than single atom catalysts (SACs). The difficulty in synthesizing such catalysts has been a key challenge. Recent progress in preparing dinuclear heterogeneous catalysts (DHCs) from homogeneous molecular precursors has provided an effective route to address this challenge. Nevertheless, only side-on bound DHCs, where both metal atoms are affixed to the supporting substrate, have been reported. The competing end-on binding mode, where only one metal atom is attached to the substrate and the other metal atom is dangling, has been missing. Here, we report the first observation that end-on binding is indeed possible for Ir DHCs supported on WO₃. Unambiguous evidence supporting the binding mode was obtained by in situ diffuse reflectance infrared Fourier transform spectroscopy and high-angle annular dark-field scanning transmission electron microscopy. Density functional theory calculations provide additional support for the binding mode, as well as insights into how end-on bound DHCs may be beneficial for solar water oxidation reactions. The results have important implications for future studies of highly effective heterogeneous catalysts for complex chemical reactions.

Artificial light-harvesting supramolecular polymeric nanoparticles formed by pillar[5]arene-based host-guest interaction

Artificial light-harvesting nanoparticles were prepared from supramolecular polymers comprised of pillar[5]arene with anthracene-derived donors and acceptors through host-guest interactions. The resulting water-dispersible nanoparticles displayed efficient energy transfer and excellent light harvesting ability in part because the steric bulk of pillar[5]arene suppressed the self-quenching of the chromophores.

Mimicking the Key Functions of Photosystem II in Artificial Photosynthesis for Photoelectrocatalytic Water Splitting

It has been anticipated that learning from nature photosynthesis is a rational and effective way to develop artificial photosynthesis system, but it is still a great challenge. Here, we assembled a photoelectrocatalytic system by mimicking the functions of photosystem II (PSII) with BiVO₄ semiconductor as a light harvester protected by a layered double hydroxide (NiFeLDH) as a hole storage layer, a partially oxidized graphene (pGO) as biomimetic tyrosine for charge transfer, and molecular Co cubane as oxygen evolution complex. The integrated system exhibited an unprecedentedly low onset potential (0.17 V) and a high photocurrent (4.45 mA cm^-2), with a 2.0% solar to hydrogen efficiency. Spectroscopic studies revealed that this photoelectrocatalytic system exhibited superiority in charge separation and transfer by benefiting from mimicking the key functions of PSII. The success of the biomimetic strategy opened up new ways for the rational design and assembly of artificial photosynthesis systems for efficient solar-to-fuel conversion.

Imaging photogenerated charge carriers on surfaces and interfaces of photocatalysts with surface photovoltage microscopy

Recent advances in imaging and characterizing charge separation on surfaces and interfaces of photocatalysts by surface photovoltage spectroscopy were reviewed and highlighted.

Polyhedral 30-Faceted BiVO4 Microcrystals Predominantly Enclosed by High-Index Planes Promoting Photocatalytic Water-Splitting Activity

Unprecedented 30-faceted BiVO₄ polyhedra predominantly surrounded by {132}, {321}, and {121} high-index facets are fabricated through the engineering of high-index surfaces by a trace amount of Au nanoparticles. The growth of high-index facets results in a 3-5 fold enhancement of O₂ evolution from photocatalytic water splitting by the BiVO₄ polyhedron, relative to its low-index counterparts. Theory calculations reveal that water dissociation is more energetically favorable on the high-index surfaces than on the low-index (010), (110), and (101) surfaces, which is accompanied by a notable reduction in the overpotential (0.77-1.14 V) for the oxygen evolution reaction. The apparent quantum efficiency of O₂ generation without an external electron supply reaches 18.3% under 430 nm light irradiation, which is an order of magnitude higher than that of the catalysts reported hitherto.

Cu(ii) coordination polymers with nitrogen catenation ligands for efficient photocatalytic water oxidation

Cu(ii) coordination polymers with nitrogen catenation ligands can photocatalyze water oxidation with the highest TOF (1.68 s⁻¹) among copper-based photocatalysts.

Promoting Charge Separation and Injection by Optimizing the Interfaces of GaN:ZnO Photoanode for Efficient Solar Water Oxidation

Photoelectrochemical water splitting provides an attractive way to store solar energy in molecular hydrogen as a kind of sustainable fuel. To achieve high solar conversion efficiency, the most stringent criteria are effective charge separation and injection in electrodes. Herein, efficient photoelectrochemical water oxidation is realized by optimizing charge separation and surface charge transfer of GaN:ZnO photoanode. The charge separation can be greatly improved through modified moisture-assisted nitridation and HCl acid treatment, by which the interfaces in GaN:ZnO solid solution particles are optimized and recombination centers existing at the interfaces are depressed in GaN:ZnO photoanode. Moreover, a multimetal phosphide of NiCoFeP was employed as water oxidation cocatalyst to improve the charge injection at the photoanode/electrolyte interface. Consequently, it significantly decreases the overpotential and brings the photocurrent to a benchmark of 3.9 mA cm^-2 at 1.23 V vs RHE and a solar conversion efficiency over 1% was obtained.

Unravelling charge separation via surface built-in electric fields within single particulate photocatalysts

Kelvin Probe Force Microscopy (KPFM) and spatially resolved surface photovoltage (SRSPV) techniques were employed to reveal built-in electric fields and surface photogenerated charge distribution on single particulate photocatalysts. The photogenerated holes and electrons spread over the whole surface of the particulate photocatalyst are imaged on n-type BiVO₄ and p-type Cu₂O single particles, respectively. It is demonstrated that the built-in electric field in the surface Space Charge Region (SCR) dictates the charge separation/transfer processes and allows the drift of one kind of the photogenerated carriers to the surface, while holding another kind of the carriers in the bulk. The results emphasize the role of the SCR played in the unidirectional charge transport between the bulk and surface in the particulate photocatalyst, which may be the crucial reason for low solar energy conversion efficiency.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

A carbon-free polyoxometalate molecular catalyst with a cobalt–arsenic core for visible light-driven water oxidation

Na₁₂[{Co^II₇As^III₆O₉(OH)₆}(A-α-SiW₉O₃₄)₂]·8H₂O with the first reported cobalt–arsenic core was synthesized, thoroughly characterized and employed to catalyze water oxidation under visible-light-driven conditions.