n-Si/SiOx /CoOx -Mo Photoanode for Efficient Photoelectrochemical Water Oxidation.

Green hydrogen is considered to be the key for solving the emerging energy and environmental issues. The photoelectrochemical (PEC) process for the production of green hydrogen has been widely investigated because solar power is clean and renewable. However, mass production in this way is still far away from reality. Here, a Si photoanode is reported with CoOx as co-catalyst for efficient water oxidation. It is found that a high photovoltage of 350 mV can be achieved in 1.0 m K3 BO3 . Importantly, the photovoltage can be further increased to 650 mV and the fill factor of 0.62 is obtained in 1.0 m K3 BO3 by incorporating Mo into CoOx . The Mo-incorporated photoanode is also highly stable. It is shown that the incorporation of Mo can reduce the particle size of co-catalyst on the Si surface, improve the particle-distribution uniformity, and increase the density of particles, which can effectively enhance the light absorption and the electrochemical active surface area. Importantly, the Mo-incorporation results in high energy barrier in the heterojunction. All of these factors are attributed to improved the PEC performance. These findings may provide new strategies to maximize the solar-to-fuel efficiency by tuning the co-catalysts on the Si surface.

BiVO4 Photoanodes Enhanced with Metal Phosphide Co-Catalysts: Relevant Properties to Boost Photoanode Performance.

Achieving highly performant photoanodes for oxygen evolution is key to developing photoelectrochemical devices for solar water splitting. In this work, BiVO4 photoanodes are enhanced with a series of core-shell structured bimetallic nickel-cobalt phosphides (MPs), and key insights into the role of co-catalysts are provided. The best BiVO4 /Ni1.5 Co0.5 P and BiVO4 /Ni0.5 Co1.5 P photoanodes achieve a 3.5-fold increase in photocurrent compared with bare BiVO4 . It is discovered that this enhanced performance arises from a synergy between work function, catalytic activity, and capacitive ability of the MPs. Distribution of relaxation times analysis reveals that the contact between the MPs, BiVO4 , and the electrolyte gives rise to three routes for hole injection into the electrolyte, all of which are significantly improved by the presence of a second metal cation in the co-catalyst. Kinetic studies demonstrate that the significantly improved interfacial charge injection is due to a lower charge-transfer resistance, enhanced oxygen-evolution reaction kinetics, and larger surface hole concentrations, providing deeper insights into the carrier dynamics in these photoanode/co-catalyst systems for their rational design.

Enhanced Charge-carrier Dynamics and Efficient Photoelectrochemical Nitrate-to-ammonia Conversion on Antimony Sulfide-based Photocathodes.

The photoelectrochemical reduction of nitrate to ammonia (PEC NO3RR) has emerged as a promising pathway for facilitating the natural nitrogen cycle. The PEC NO3RR can lower the reduction potential needed for ammonia synthesis through photogenerated voltage, showcasing the significant potential for merging abundant solar energy with sustainable nitrogen fixation. However, it is influenced by the selective photocathodes with poor carrier kinetics, low catalytic selectivity, and ammonia yields. There are few reports on suitable photoelectrodes owning efficient charge transport on PEC NO3RR at low overpotentials. Herein, we rationally constructed the CuSn alloy co-catalysts on the antimony sulfides with a highly selective PEC ammonia and an ultra-low onset potential (0.62 VRHE). CuSn/TiO2/Sb2S3 achieved an ammonia faradic efficiency of 97.82% at a low applied potential of 0.4 VRHE, and an ammonia yield of 16.96 µmol h-1 cm-2 at 0 VRHE under one sun illumination. Dynamics experiments and theoretical calculations have demonstrated that CuSn/TiO2/Sb2S3 has an enhanced charge separation and transfer efficiency, facilitating photogenerated electrons to participate in PEC NO3RR quickly. Meanwhile, moderate NO2* adsorption on this photocathode optimizes the catalytic activity and increases the NH4+ yield. This work opens an avenue for designing sulfide-based photocathodes for the efficient route of solar-to-ammonia conversion.

Construction of ZnIn2 S4 /CdS/PdS S-Scheme Heterostructure for Efficient Photocatalytic H2 Production.

It is facing a tremendous challenge to develop the desirable hybrids for photocatalytic H2 generation by integrating the advantages of a single semiconductor. Herein, an all-sulfide ZnIn2 S4 /CdS/PdS heterojunction is constructed for the first time, where CdS and PdS nanoparticles anchor in the spaces of ZnIn2 S4 micro-flowers due to the confinement effects. The morphology engineering can guarantee rapid charge transfer owing to the short carrier migration distances and the luxuriant reactive sites provided by ZnIn2 S4 . The S-scheme mechanism between ZnIn2 S4 and CdS assisted by PdS cocatalyst is testified by in situ irradiated X-ray photoelectron spectroscopy and electron paramagnetic resonance (EPR), where the electrons and holes move in reverse driven by work function difference and built-in electric field at the interfaces. The optimal ZnIn2 S4 /CdS/PdS performs a glaring photocatalytic activity of 191.9 µmol h-1 (10 mg of catalyst), and the largest AQE (apparent quantum efficiency) can reach a high value of 26.26%. This work may afford progressive tactics to design multifunctional photocatalysts.

Flux-Assisted Synthesis of Y2 Ti2 O5 S2 for Photocatalytic Hydrogen and Oxygen Evolution Reactions.

Photocatalytic water splitting is an ideal means of producing hydrogen in a sustainable manner, and developing highly efficient photocatalysts is a vital aspect of realizing this process. The photocatalyst Y2 Ti2 O5 S2 (YTOS) is capable of absorbing at wavelengths up to 650 nm and exhibits outstanding thermal and chemical durability compared with other oxysulfides. However, the photocatalytic performance of YTOS synthesized using the conventional solid-state reaction (SSR) process is limited owing to the large particle sizes and structural defects associated with this synthetic method. Herein, we report the synthesis of YTOS particles by a flux-assisted technique. The enhanced mass transfer efficiency in the flux significantly reduced the preparation time compared with the SSR method. In addition, the resulting YTOS showed improved photocatalytic H2 and O2 evolution activity when loaded with Rh and Co3 O4 co-catalysts, respectively. These improvements are attributed to the reduced particle size and enhanced crystallinity of the material as well as the slower decay of photogenerated carriers on a nanosecond to sub-microsecond time range. Further optimization of this flux-assisted method together with suitable surface modification is expected to produce high-quality YTOS crystals with superior photocatalytic activity.

Reversing Free-Electron Transfer of MoS2+x Cocatalyst for Optimizing Antibonding-Orbital Occupancy Enables High Photocatalytic H2 Evolution.

The interaction between a co-catalyst and photocatalyst usually induces spontaneous free-electron transfer between them, but the effect and regulation of the transfer direction on the hydrogen-adsorption energy of the active sites have not received attention. Herein, to steer the free-electron transfer in a favorable direction for weakening S-Hads bonds of sulfur-rich MoS2+x , an electron-reversal strategy is proposed for the first time. The core-shell Au@MoS2+x cocatalyst was constructed on TiO2 to optimize the antibonding-orbital occupancy. Research results reveal that the embedded Au can reverse the electron transfer to MoS2+x to generate electron-rich S(2+δ)- active sites, thus increasing the antibonding-orbital occupancy of S-Hads in the Au@MoS2+x cocatalyst. Consequently, the increase in the antibonding-orbital occupancy effectively destabilizes the H 1s-p antibonding orbital and weakens the S-Hads bond, realizing the expedited desorption of Hads to rapidly generate a lot of visible H2 bubbles. This work delves deep into the latent effect of the photocatalyst carrier on cocatalytic activity.

Significantly Promoting the Photogenerated Charge Separation by Introducing an Oxygen Vacancy Regulation Strategy on the FeNiOOH Co-Catalyst.

Semiconductor/co-catalyst coupling is considered as a promising strategy to enhance the photoelectrochemical (PEC) conversion efficiency. Unfortunately, this model system is faced with a serious interface recombination problem, which limits the further improvement of PEC performances. Here, a FeNiOOH co-catalyst with abundant oxygen vacancies on BiVO4 is fabricated through simple and economical NaBH4 reduction to accelerate hole transfer and achieve efficient electron-hole pair separation. The photocurrent of the BV (BiVO4 )/Vo-FeNiOOH system is more than four times that of pure BV. Importantly, the charge transfer kinetics and charge carrier recombination process are studied by scanning photoelectrochemical microscopy and intensity modulated photocurrent spectroscopy in detail. In addition, the oxygen vacancy regulation proposed is also applied successfully to other semiconductors (Fe2 O3 ), demonstrating the applicability of this strategy.

Photocatalytic Overall Water Splitting Under Visible Light Enabled by a Particulate Conjugated Polymer Loaded with Palladium and Iridium.

Polymer photocatalysts have received growing attention in recent years for photocatalytic hydrogen production from water. Most studies report hydrogen production with sacrificial electron donors, which is unsuitable for large-scale hydrogen energy production. Here we show that the palladium/iridium oxide-loaded homopolymer of dibenzo[b,d]thiophene sulfone (P10) facilitates overall water splitting to produce stoichiometric amounts of H2 and O2 for an extended period (>60 hours) after the system stabilized. These results demonstrate that conjugated polymers can act as single component photocatalytic systems for overall water splitting when loaded with suitable co-catalysts, albeit currently with low activities. Transient spectroscopy shows that the IrO2 co-catalyst plays an important role in the generation of the charge separated state required for water splitting, with evidence for fast hole transfer to the co-catalyst.

Nickel Glyoximate Based Metal-Covalent Organic Frameworks for Efficient Photocatalytic Hydrogen Evolution.

Metal-covalent organic frameworks (MCOFs) have been recently received wide attention owing to the homogeneous distribution of active metal centers that are beneficial for enhancing the application potentials. However, metal complex based functional building blocks for MCOFs synthesis are limited. Herein, two new MCOFs (Ni-Py-COF and Ni-Bn-COF) were constructed via a novel nickel glyoximate based building block. Splendid photocatalytic activity on hydrogen evolution from water and great long-term recyclability were achieved using these nickel glyoximate based MCOFs as photocatalysts. Excitingly, even without the addition of Pt co-catalyst, the hydrogen evolution rates (HER) of Ni-Py-COF reached up to 626 µmol g-1 h-1 , which is better than many porous organic polymers. This work not only expands the type of building units for MCOFs, but also provides meaningful insights for developing stable, efficient and earth-abundant photocatalysts toward H2 generation.

Highly Poison-Resistant Single-Atom Co-N4 Active Sites with Superior Operational Stability over 460 h for H2 S Catalytic Oxidation.

Efficient catalytic elimination of hydrogen sulfide (H2 S) with high activity and durability in nature gas and blast-furnace gas is very critical for both fundamental catalytic research and applied environmental chemistry. Herein, atomically dispersed Co atom catalysts with Co-N4 sites that can transform H2 S into S with conversion rate of ≈100% are designed and prepared. The representative 4Co-N/NC achieves a sulfur yield of nearly 100% and TOF(Co) of 869 h-1 at 180 °C. Importantly, remarkable long-term durability is achieved as well, with no obvious loss of catalytic activity in the run of 460 h, outperforming most of the reported catalysts. The short bond length and strong cooperation of Co-N are beneficial to improve the structural stability of the Co-N4 centers, and significantly enhanced resistance of water and sulfation over single-atom Co-catalyst. The present mechanism involves the stepwise hydrogen transfer process via the adsorbed *HOO and *HS intermediates.

Insights into Charge Transfer at an Atomically Precise Nanocluster/Semiconductor Interface.

The deposition of an atomically precise nanocluster, for example, Ag44 (SR)30 , onto a large-band-gap semiconductor such as TiO2 allows a clear interface to be obtained to study charge transfer at the interface. Changing the light source from visible light to simulated sunlight led to a three orders of magnitude enhancement in the photocatalytic H2 generation, with the H2 production rate reaching 7.4 mmol h-1 gcatalyst -1 . This is five times higher than that of TiO2 modified with Ag nanoparticles and even comparable to that of TiO2 modified with Pt nanoparticles under similar conditions. Energy band alignment and transient absorption spectroscopy reveal that the role of the metal clusters is different from that of both organometallic complexes and plasmonic nanoparticles: A type II heterojunction charge-transfer route is achieved under UV/Vis irradiation, with the cluster serving as a small-band-gap semiconductor. This results in the clusters acting as co-catalysts rather than merely photosensitizers.

Ketones as Molecular Co-catalysts for Boosting Exciton-Based Photocatalytic Molecular Oxygen Activation.

Excitonic processes in semiconductors open up the possibility for pursuing photocatalytic organic synthesis. However, the insufficient spin relaxation and robust nonradiative decays in semiconductors place restrictions on both quantum yield and selectivity of these reactions. Herein, by taking polymeric carbon nitride (PCN)/acetone as a prototypical system, we propose that extrinsic aliphatic ketones can serve as molecular co-catalysts for promoting spin-flip transition and suppressing non-radiative energy losses. Spectroscopic investigations indicate that hot excitons in PCN can be transferred to ketones, while triplet excitons in ketones can be transferred to PCN. As such, the PCN/ketone systems exhibit considerable triplet-exciton accumulation and extended visible-light response, leading to excellent performance in exciton-based photocatalysis, such as singlet oxygen generation. This work provides a fundamental understanding of energy harvesting in semiconductor/molecule systems, and paves the way for optimizing exciton-based photocatalysis via molecular co-catalyst design.

Molten-Salt-Mediated Synthesis of an Atomic Nickel Co-catalyst on TiO2 for Improved Photocatalytic H2 Evolution.

Atomic co-catalysts offer high potential to improve the photocatalytic performance, of which the preparation with earth-abundant elements is challenging. Here, a new molten salt method (MSM) is designed to prepare atomic Ni co-catalyst on widely studied TiO2 nanoparticles. The liquid environment and space confinement effect of the molten salt leads to atomic dispersion of Ni ions on TiO2 , while the strong polarizing force provided by the molten salt promotes formation of strong Ni-O bonds. Interestingly, Ni atoms are found to facilitate the formation of oxygen vacancies (OV) on TiO2 during the MSM process, which benefits the charge transfer and hydrogen evolution reaction. The synergy of atomic Ni co-catalyst and OV results in 4-time increase in H2 evolution rate compared to that of the Ni co-catalyst on TiO2 prepared by an impregnation method. This work provides a new strategy of controlling atomic co-catalyst together with defects for efficient photocatalytic water splitting.

Silica-Grafted Tris(neopentyl)aluminum: A Monomeric Aluminum Solid Co-catalyst for Efficient Nickel-Catalyzed Ethene Dimerization.

A silica-supported monomeric alkylaluminum co-catalyst was prepared via surface organometallic chemistry by contacting tris(neopentyl)aluminum and partially dehydroxylated silica. This system, fully characterized by solid-state 27 Al NMR spectroscopy augmented by computational studies, efficiently activates (n Bu3 P)2 NiCl2 towards dimerization of ethene, demonstrating comparable activity to previously reported dimeric diethylaluminum chloride supported on silica. Three types of aluminum surface species have been identified: monografted tetracoordinated Al species as well as two types of bisgrafted Al species-tetra- and pentacoordinated. Of them, only the monografted Al species is proposed to be able to activate the (n Bu3 P)2 NiCl2 complex and generate the active cationic species.

Ti3 C2 : An Ideal Co-catalyst?

How 2D Ti3 C2 enhances photocatalytic efficiency remains unclear. Now, it is shown that it is graphene quantum dots (GQDs) derived from Ti3 C2 , rather than 2D Ti3 C2 itself, that play the role of co-catalyst for La2 Ti2 O7 /Ti3 C2 (LTC) composites during the photocatalytic reaction. After modification of Ti3 C2 derivatives, the photocatalytic efficiency of La2 Ti2 O7 is enhanced 16 times over pure La2 Ti2 O7 . Solid-state NMR, Raman, and HRTEM results confirm the existence of GQDs in Ti3 C2 and LTC composites. The GQDs are formed during the chemical change from Ti3 AlC2 to Ti3 C2 via HF etching, as Ti atoms are removed and unsaturated carbon bonds are left, which react with each other to form sp2 π-conjugation GQDs. 2D Ti3 C2 is completely oxidized to COx modified TiOx species, causing Ti3 C2 to lose its electrical conductivity and the role as co-catalyst. GQDs largely suppress the photogenerated charge recombination of La2 Ti2 O7 , as revealed by the photoluminescence (PL) and transient photocurrent.

Isolation of Cp*CoIII -Alkenyl Intermediate in Efficient Cobalt-Catalyzed C-H Alkenylation with Alkynes.

A general and efficient procedure for C-H alkenylation of arenes with a broad substrate scope catalyzed by Cp*CoIII was demonstrated with alkynes. A highly selective mono-alkenylation and sequential bis-C-H bond functionalization was displayed to exemplify the versatility of the cobalt catalyst. Isolation of cationic Cp*CoIII -alkenyl intermediate was achieved under identical catalytic conditions to further establish the proposed pathway.

Metal-promoted titania photocatalysis for destruction of nitrates and organics from aqueous environments.

This review article provides an overview of activities in the rapidly developing field of water purification via photocatalytic methods and focuses on the removal of nitrate ions with simultaneous removal of the hole scavenger. Many of the issues associated with provision of potable water in the developing world may be resolved by the use of simple physical methodologies such as filtration. However, many of the issues associated with water purity in the developed world involve complex, stable molecules present at low concentrations that are nonetheless capable of producing toxic effects in plants and animals and that require more demanding removal technologies. Photocatalytic methods can be operated remotely and often show minimal production of undesired side products. Titania alone shows limitations, not only in terms of the slow rate of photoreduction of nitrate but also in terms of selectivity and the need to employ radiation in the UV region due to the magnitude of the band gap. Key challenges may be defined as: reducing the band gap/increasing absorption in the visible region, enhancing the adsorption capacity/access to the surface sites and reducing the rate of hole/electron recombination. The present article will focus on the use of titania-based materials that involve metal co-catalysts for nitrate reduction.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Metallic Ni3 P/Ni Co-Catalyst To Enhance Photocatalytic Hydrogen Evolution.

Metallic Ni3 P/Ni can be used as a co-catalyst to replace noble metal Pt for efficient photocatalytic hydrogen evolution, due to its excellent trapping-electron ability. The applications of metallic Ni3 P/Ni co-catalyst on CdS, Zn0.5 Cd0.5 S, TiO2 (Degussa P25) and g-C3 N4 are further confirmed, indicating its versatile applicability nature like Pt.

Phosphorene Co-catalyst Advancing Highly Efficient Visible-Light Photocatalytic Hydrogen Production.

Transitional metals are widely used as co-catalysts boosting photocatalytic H2 production. However, metal-based co-catalysts suffer from high cost, limited abundance and detrimental environment impact. To date, metal-free co-catalyst is rarely reported. Here we for the first time utilized density functional calculations to guide the application of phosphorene as a high-efficiency metal-free co-catalyst for CdS, Zn0.8 Cd0.2 S or ZnS. Particularly, phosphorene modified CdS shows a high apparent quantum yield of 34.7 % at 420 nm. This outstanding activity arises from the strong electronic coupling between phosphorene and CdS, as well as the favorable band structure, high charge mobility and massive active sites of phosphorene, supported by computations and advanced characterizations, for example, synchrotron-based X-ray absorption near edge spectroscopy. This work brings new opportunities to prepare highly-active, cheap and green photocatalysts.

Integration of Multiple Plasmonic and Co-Catalyst Nanostructures on TiO2 Nanosheets for Visible-Near-Infrared Photocatalytic Hydrogen Evolution.

Utilization of visible and near-infrared light has always been the pursuit of photocatalysis research. In this article, an approach is developed to integrate dual plasmonic nanostructures with TiO2 semiconductor nanosheets for photocatalytic hydrogen production in visible and near-infrared spectral regions. Specifically, the Au nanocubes and nanocages used in this work can harvest visible and near-infrared light, respectively, and generate and inject hot electrons into TiO2 . Meanwhile, Pd nanocubes that can trap the energetic electrons from TiO2 and efficiently participate in the hydrogen evolution reaction are employed as co-catalysts for improved catalytic activity. Enabled by this unique integration design, the hydrogen production rate achieved is dramatically higher than those of its counterpart structures. This work represents a step toward the rational design of semiconductor-metal hybrid structures for broad-spectrum photocatalysis.