Molecular Engineering of Methylated Sulfone-based Covalent Organic Frameworks for Back-reaction Inhibited Photocatalytic Overall Water Splitting.

Solar-to-hydrogen (H2) and oxygen (O2) conversion via photocatalytic overall water splitting (OWS) holds great promise for a sustainable fuel economy, but has been challenged by the backward O2 reduction reaction (ORR) due to its favored proton-coupled electron transfer (PCET) dynamics. Here, we report that molecular engineering by methylation inhibits the backward ORR of molecular photocatalysts and enables efficient OWS process. As demonstrated by a benchmark sulfone-based covalent organic framework (COF) photocatalyst, the precise methylation of its O2 adsorption sites effectively blocks electron transfer and increases the barrier for hydrogen intermediate desorption that cooperatively obstructs the PCET process of ORR. Methylation also repels electrons to the neighboring photocatalytic sulfone group that promotes the forward H2 evolution. The resultant DS-COF achieves an impressive inhibition of about 70% of the backward reaction and a three-fold enhancement of the OWS performance with a H2 evolution rate of 124.7 µmol h-1 g-1, ranking among the highest reported for organic photocatalysts. This work provides insights for engineering photocatalysts at the molecular level for efficient solar-to-fuel conversion.

Realizing Photocatalytic Overall Water Splitting by Modulating the Thickness-Induced Reaction Energy Barrier of Fluorenone-Based Covalent Organic Frameworks.

Direct photocatalytic hydrogen and oxygen evolution from water splitting is an attractive approach for producing chemical fuels. In this work, a novel fluorenone-based covalent organic framework (COF-SCAU-2) is successfully exfoliated into ultrathin three-layer nanosheets (UCOF-SCAU-2) for photocatalytic overall water splitting (OWS) under visible light. The ultrathin structures of UCOF-SCAU-2 greatly enhance carrier separation, utilization efficiency, and the exposure of active surface sites. Surprisingly, UCOF-SCAU-2 exhibits efficient photocatalytic OWS performance, with hydrogen and oxygen evolution rates reaching 0.046 and 0.021 mmol h-1 g-1 , respectively, under visible-light irradiation, whereas bulk COF-SCAU-2 shows no activity for photocatalytic OWS. Charge-carrier kinetic analysis and DFT calculations confirm that reducing the thickness of the COF nanosheets increases the number of accessible active sites, reduces the distance for charge migration, prolongs the lifetimes of photogenerated carriers, and decreases the Gibbs free energy of the rate-limiting step compared to nonexfoliated COFs. This work offers new insights into the effect of the layer thickness of COFs on photocatalytic OWS.

In Situ Synthesis of Chemically Bonded 2D/2D Covalent Organic Frameworks/O-Vacancy WO3 Z-Scheme Heterostructure for Photocatalytic Overall Water Splitting.

Covalent organic frameworks (COFs) have shown great promise for photocatalytic hydrogen evolution via water splitting. However, the four-electron oxidation of water remains elusive toward oxygen evolution. Enabling this water oxidation pathway is critical to improve the yield and maximize atom utilization efficiency. A Z-scheme heterojunction is proposed for overcoming fundamental issues in COF-based photocatalytic overall water splitting (OWS), such as inefficient light absorption, charge recombination, and poor water oxidation ability. It is shown that the construction of a novel 2D/2D Z-scheme heterojunction through in situ growth of COFs on the O-vacancy WO3 nanosheets (Ov-WO3 ) via the WOC chemical bond can remarkably promote photocatalytic OWS. Benefiting from the synergistic effect between the enhanced built-in electric field by the interfacial WOC bond, the strong water oxidation ability of Ov-WO3, and the ultrathin structure of TSCOF, both separation and utilization efficiency of photogenerated electron-hole pairs can be significantly enhanced. An impressive photocatalytic hydrogen evolution half-rection rate of 593 mmol h-1 g-1 and overall water splitting rate of 146 (hydrogen) and 68 (oxygen) µmol h-1 g-1 are achieved on the COF-WO3 (TSCOFW) composite. This 2D/2D Z-scheme heterojunction with two-step excitation and precisely cascaded charge-transfer pathway makes it responsible for the efficient solar-driven OWS without a sacrificial agent.

Hydroxyl-modified Nb4C3Tx MXene@ZnIn2S4 sandwich structure for photocatalytic overall water splitting.

Herein, a hydroxyl-modified MXene@ZnIn2S4 (Nb4C3Tx MXene@ZIS-OH) overall water splitting photocatalyst with a sandwich structure was prepared through an in-situ growth strategy and peroxyl plasma post-treatment. The Nb4C3Tx MXene@ZIS-OH exhibits outstanding catalytic performance, which generates the release rates of hydrogen (53.8 µmol g-1h-1) and oxygen (26.7 µmol g-1h-1) from the water under visible light irradiation. After four photocatalytic cycling, the photocatalytic overall water splitting activity of Nb4C3Tx MXene@ZIS-OH is still 95.9% of the initial activity, which indicates that Nb4C3Tx MXene@ZIS-OH exhibits excellent cycling stability. Notably, the Nb4C3Tx MXene@ZIS-OH achieves an AQY of 1.2% for the overall photocatalytic water splitting at 380 nm. The sandwich structure and matched heterointerface between high work function Nb4C3Tx MXene and ZnIn2S4 nanosheets promote the electron transport, inhibit the charge recombination, and separate the generated H2 and O2 with effectiveness. Importantly, the Finite-Difference Time-Domain (FDTD) simulation suggests the hydroxyl groups on the surface of ZnIn2S4 could increase the hydrophilicity of photocatalyst and capture the holes generated by photoexcitation, thereby promoting the separation of electron-hole pairs rapidly. This work presents a successful example of constructing overall water splitting photocatalysts by energy level regulation, structure design and functional group modification.

Understanding Photocatalytic Overall Water Splitting of ß-Ketoamine COFs through the N-C Site Synergistic Mechanism.

Overcoming the sluggish reaction kinetics of the oxygen evolution reaction (OER) is a determining factor for the practical application of photocatalysts for overall water splitting. Two-dimensional covalent organic frameworks (2D-COFs) offer an ideal platform for catalyst design in the field of overall water splitting for their exceptional chemical tunability and high efficiency of light capture. In this work, four ß-ketoamine 2D-COFs, consisting of 1,3,5-triformylphloroglucinol (Tp) groups and different linkers with pyridine segments, were constructed and optimized. By means of first-principles calculations, the band structures, free energy changes of photocatalytic hydrogen evolution reaction (HER) and OER, and charge density distributions were calculated and investigated systemically to discuss the visible-light response, overall water splitting activities on active sites, and the characteristic of charge transfer and separation. The protonated pyridine N derived from the double-H2O closed-ring H-bond adsorption model could efficiently induce N-C sites' synergistic effect between pyridine N and its ortho-position C to minimize the OER energy barrier and to enhance the charge transfer and separation. A N-C site synergistic mechanism has been proposed to provide a comprehensive explanation for the experimental results and a new strategy to design novel 2D-COF photocatalysts for overall water splitting.

Highly efficient photocatalytic overall water splitting on plasmonic Cu6Sn5/polyaniline nanocomposites.

A plasmonic Cu6Sn5/polyaniline (Cu6Sn5/PANI) nanocomposite was synthesized by chemical reduction and hydrothermal methods. The best photocatalytic overall water splitting performance was achieved by the Cu6Sn5/PANI3wt% composite, which contains 3 wt% PANI, which is approximately three times more than that of pure Cu6Sn5. Meanwhile, Cu6Sn5/PANI3wt% exhibited excellent photocatalytic stability for water splitting during the stability investigation. The dramatic promotion of the photocatalytic activity performance can be ascribed to the cocatalyst PANI. The existence of PANI can remarkably promote the separation and transfer efficiency of the photoinduced electron-hole pairs, and therefore enhance the photocatalytic activity. Our results also verify that the photogenerated charge comes from plasmonic Cu6Sn5 with the surface plasmon resonance (SPR) effect, which is different from traditional semiconductor-based photocatalysts. This work sheds some light on plasmonic photocatalyst development and provides an alternative pathway for photocatalytic reactions.

Integration of bio-inspired lanthanide-transition metal cluster and P-doped carbon nitride for efficient photocatalytic overall water splitting.

Photosynthesis in nature uses the Mn4CaO5 cluster as the oxygen-evolving center to catalyze the water oxidation efficiently in photosystem II. Herein, we demonstrate bio-inspired heterometallic LnCo3 (Ln = Nd, Eu and Ce) clusters, which can be viewed as synthetic analogs of the CaMn4O5 cluster. Anchoring LnCo3 on phosphorus-doped graphitic carbon nitrides (PCN) shows efficient overall water splitting without any sacrificial reagents. The NdCo3/PCN-c photocatalyst exhibits excellent water splitting activity and a quantum efficiency of 2.0% at 350 nm. Ultrafast transient absorption spectroscopy revealed the transfer of a photoexcited electron and hole into the PCN and LnCo3 for hydrogen and oxygen evolution reactions, respectively. A density functional theory (DFT) calculation showed the cooperative water activation on lanthanide and O-O bond formation on transition metal for water oxidation. This work not only prepares a synthetic model of a bio-inspired oxygen-evolving center but also provides an effective strategy to realize light-driven overall water splitting.

Strong coupling effect at the interface of cobalt phosphate-carbon dots boost photocatalytic water splitting.

Hydrogen and oxygen produced by water splitting under solar energy are ideal future energy sources. At present, obtaining the efficient, stable and inexpensive photocatalyst for photocatalytic overall water splitting is still a huge challenge. Cobalt phosphate (Co3PO4, CoPi) possesses proper band positions for water splitting. However, the fast recombination of photogenerated electron and hole pairs for CoPi restricts its application. Herein, strongly coupling Co3PO4-carbon dots (CoPi-CDs) composite was constructed as an effective strategy to depress the fast recombination behavior of photogenerated electron and hole pairs. CoPi-CDs show superior photocatalytic water splitting activity than that of single CoPi. When the concentration of CDs in the composite is 0.002 gCDs/gcatalyst, the hydrogen production rate was obtained for approximately 0.592 µmol h-1, as well as the oxygen evolution rate about 0.258 µmol h-1 (with 2:1 stoichiometry), which are both nearly 33 times than that of pristine CoPi. This enhanced photocatalytic activity of CoPi-CDs should ascribe to the efficient coupling effect between CoPi and CDs, which allows fast electron transfer at the interface of CoPi and CDs and thus effectively boosts the photocatalytic water splitting. The strongly coupling nanocomposites should be inspiring for further nanocomposite building for photocatalytic overall water splitting.