黑色氮化钨作为金属性光催化剂实现全分解水

正光解水技术可以将太阳能转换存储为化学能,被视为解决全球性能源与环境问题的理想方式之一~(1,2)。太阳能转换效率一定程度上取决于光催化材料的吸光范围,然而考虑到最小禁带宽度1.23 e V和光解水过程中可能存在的能量损失等因素,单一半导体实现全分解水的吸光范围较难超过700 nm。目前已报道的半导体光催化材料实现全分解水的最长响应波长是600 nm~3。具有金属性的光催化剂依靠带内跃迁来产生电子空穴对的这一特点,有可能使其突破700 nm波长光响应这     

Crystalline Carbon Nitride Semiconductors for Photocatalytic Water Splitting

Conjugated carbon nitride (CN) is an emerging and promising semiconductor photocatalyst for water photolysis owing to its unique properties. However, the traditional thermally induced polymerization of N‐containing precursors typically produces melon‐based CN solids with amorphous or semi‐crystalline structures with only moderate photocatalytic performance. Many strategies have been developed to prepare crystalline CNs (CCNs), such as high‐temperature and high‐pressure routes, ionothermal synthesis, and microwave‐assisted synthesis. In this Minireview, we summarize the progress that has been made in the synthesis of CCNs and their application in photocatalytic water splitting reactions. Three kinds of CCNs are mainly discussed according to their polymeric subunits. Challenges associated with CCNs and their future development are also included.     

Single‐Site Active Cobalt‐Based Photocatalyst with a Long Carrier Lifetime for Spontaneous Overall Water Splitting

An active and stable photocatalyst to directly split water is desirable for solar‐energy conversion. However, it is difficult to accomplish overall water splitting without sacrificial electron donors. Herein, we demonstrate a strategy via constructing a single site to simultaneously promote charge separation and catalytic activity for robust overall water splitting. A single Co₁‐P₄ site confined on g‐C₃N₄ nanosheets was prepared by a facile phosphidation method, and identified by electron microscopy and X‐ray absorption spectroscopy. This coordinatively unsaturated Co site can effectively suppress charge recombination and prolong carrier lifetime by about 20 times relative to pristine g‐C₃N₄, and boost water molecular adsorption and activation for oxygen evolution. This single‐site photocatalyst exhibits steady and high water splitting activity with H₂ evolution rate up to 410.3 μmol h⁻¹ g⁻¹, and quantum efficiency as high as 2.2 % at 500 nm.     

A Complex Perovskite‐Type Oxynitride: The First Photocatalyst for Water Splitting Operable at up to 600 nm

One of the simplest methods for splitting water into H₂ and O₂ with solar energy entails the use of a particulate‐type semiconductor photocatalyst. To harness solar energy efficiently, a new water‐splitting photocatalyst that is active over a wider range of the visible spectrum has been developed. In particular, a complex perovskite‐type oxynitride, LaMg_xTa_1?xO_1+3xN_2?3x (x≥1/3), can be employed for overall water splitting at wavelengths of up to 600 nm. Two effective strategies for overall water splitting were developed. The first entails the compositional fine‐tuning of a photocatalyst to adjust the bandgap energy and position by forming a series of LaMg_xTa_1?xO_1+3xN_2?3x solid solutions. The second method is based on the surface coating of the photocatalyst with a layer of amorphous oxyhydroxide to control the surface redox reactions. By combining these two strategies, the degradation of the photocatalyst and the reverse reaction could be prevented, resulting in successful overall water splitting.     

NiII Coordination to an Al‐Based Metal–Organic Framework Made from 2‐Aminoterephthalate for Photocatalytic Overall Water Splitting

The aluminum‐based metal–organic framework (MOF) made from 2‐aminoterephthalate is a photocatalyst for oxygen evolution. This MOF can be modified by incorporating Ni²⁺ cations into the pores through coordination to the amino groups, and the resulting MOF is an efficient photocatalyst for overall water splitting.     

An Oxygen-Insensitive Hydrogen Evolution Catalyst Coated by a Molybdenum-Based Layer for Overall Water Splitting

For overall water-splitting systems, it is essential to establish O₂-insensitive cathodes that allow cogeneration of H₂ and O₂. An acid-tolerant electrocatalyst is described, which employs a Mo-coating on a metal surface to achieve selective H₂ evolution in the presence of O₂. In operando X-ray absorption spectroscopy identified reduced Pt covered with an amorphous molybdenum oxyhydroxide hydrate with a local structural order composed of polyanionic trimeric units of molybdenum(IV). The Mo layer likely hinders O₂ gas permeation, impeding contact with active Pt. Photocatalytic overall water splitting proceeded using MoO_x/Pt/SrTiO₃ with inhibited water formation from H₂ and O₂, which is the prevailing back reaction on the bare Pt/SrTiO₃ photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membraneless electrolysis and photocatalysis.     

Polymeric Carbon Nitride/Reduced Graphene Oxide/Fe2O3: All‐Solid‐State Z‐Scheme System for Photocatalytic Overall Water Splitting

The charge transfer between hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) is the rate‐determining step that controls the overall performance of a Z‐scheme water‐splitting system. Here, we carefully design reduced graphene oxide (RGO) nanosheets for use as solid‐state mediators to accelerate the charge carrier transfer between HEPs (e.g., polymeric carbon nitride (PCN)) and OEPs (e.g., Fe₂O₃), thus achieving efficient overall water splitting. The important role of RGO could also be further proven in other PCN‐based Z‐systems (BiVO₄/RGO/PCN and WO₃/RGO/PCN), illustrating the universality of this strategy.     

Direct Z‐Scheme Hetero‐phase Junction of Black/Red Phosphorus for Photocatalytic Water Splitting

Black phosphorus (BP) has recently drawn attention in photocatalysis for its optical properties. However, limited by the rapid recombination of photogenerated carriers, the use of BP for photocatalytic water splitting still remains a huge challenge. Herein, we prepare a black/red phosphorus (BP/RP) hetero‐phase junction photocatalyst by a wet‐chemistry method to promote the interfacial charge separation and thus achieve Z‐scheme photocatalytic water splitting without using sacrificial agents. The Z‐scheme mechanism was confirmed by time‐resolved transient absorption spectroscopy. This work provides a novel insight into the interface design of hetero‐phase junction with atomic precision.     

Visible‐Light‐Driven Overall Water Splitting Boosted by Tetrahedrally Coordinated Blende Cobalt(II) Oxide Atomic Layers

Directly splitting water into H₂ and O₂ with solar light is extremely important; however, the overall efficiency of water splitting still remains extremely low. Two types of ultrathin semiconductor layers with the same elements and the same thicknesses were designed to uncover how different atomic arrangements influence water‐splitting efficiency thermodynamically and kinetically. As an example, tetrahedrally coordinated blende and octahedrally coordinated rocksalt CoO atomic layers with nearly the same thicknesses were synthesized for the first time. The blende CoO atomic layers have a smaller E_g and abundant d–d internal transition features relative to the rocksalt CoO atomic layers, which ensure enhanced visible‐light harvesting ability. Density functional theory calculations reveal that the Bader charge for Co atoms in blende CoO atomic layers is larger than that of the rocksalt CoO atomic layers, which facilitates photocarrier transfer kinetics, as verified by photoluminescence spectra and time‐resolved fluorescence emission decay spectra. In situ FTIR spectra and energy calculations reveal that the *OOH dissociation step is the rate‐limiting step, where the blende CoO atomic layers possess a smaller *OOH dissociation energy thanks to their higher Bader charge and stronger steric effect, as confirmed by the elongated Co?OOH bonds. The blende CoO atomic layers exhibit visible‐light‐driven H₂ and O₂ formation rates of 4.43 and 2.63 μmol g^?1 h^?1, roughly 3.7 times higher than those of the rocksalt CoO atomic layers.     

Constructing SrTiO3–TiO2 Heterogeneous Hollow Multi‐shelled Structures for Enhanced Solar Water Splitting

Constructing hollow multi‐shelled structures (HoMSs) has a significant effect on promoting light absorption property of catalysts and enhancing their performance in solar energy conversion applications. A facile hydrothermal method is used to design the SrTiO₃?TiO₂ heterogeneous HoMSs by hydrothermal crystallization of SrTiO₃ on the surface of the TiO₂ HoMSs, which will realize a full coverage of SrTiO₃ on the TiO₂ surface and construct the SrTiO₃/TiO₂ junctions. The broccoli‐like SrTiO₃?TiO₂ heterogeneous HoMSs exhibited a fourfold higher overall water splitting performance of 10.6 μmol h^?1 for H₂ production and 5.1 μmol h^?1 for O₂ evolution than that of SrTiO₃ nanoparticles and the apparent quantum efficiency (AQE) of 8.6 % at 365 nm, which can be mainly attributed to 1) HoMS increased the light absorption ability of the constructed photocatalysts and 2) the SrTiO₃?TiO₂ junctions boosted the separation efficiency of the photogenerated charge carriers.     

Topotactic Epitaxy of SrTiO3 Mesocrystal Superstructures with Anisotropic Construction for Efficient Overall Water Splitting

The higher‐order structures of semiconductor‐based photocatalysts play crucial roles in their physicochemical properties for efficient light‐to‐energy conversion. A novel perovskite SrTiO₃ mesocrystal superstructure with well‐defined orientation of assembled cubic nanocrystals was synthesized by topotactic epitaxy from TiO₂ mesocrystals through a facile hydrothermal treatment. The SrTiO₃ mesocrystal exhibits three times the efficiency for the hydrogen evolution of conventional disordered systems in alkaline aqueous solution. It also exhibits a high quantum yield of 6.7 % at 360 nm in overall water splitting and even good durability up to 1 day. Temporal and spatial spectroscopic observations revealed that the synergy of the efficient electron flow along the internal nanocube network and efficient collection at the larger external cubes produces remarkably long‐lived charges for enhanced photocatalysis.     

Nano‐Ferroelectric for High Efficiency Overall Water Splitting under Ultrasonic Vibration

Piezocatalysis, converting mechanical vibration into chemical energy, has emerged as a promising candidate for water‐splitting technology. However, the efficiency of the hydrogen production is quite limited. We herein report well‐defined 10 nm BaTiO₃ nanoparticles (NPs) characterized by a large electro‐mechanical coefficient which induces a high piezoelectric effect. Atomic‐resolution high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) and scanning probe microscopy (SPM) suggests that piezoelectric BaTiO₃ NPs display a coexistence of multiple phases with low energy barriers and polarization anisotropy which results in a high electro‐mechanical coefficient. Landau free energy modeling also confirms that the greatly reduced polarization anisotropy facilitates polarization rotation. Employing the high piezoelectric properties of BaTiO₃ NPs, we demonstrate an overall water‐splitting process with the highest hydrogen production efficiency hitherto reported, with a H₂ production rate of 655 μmol g^?1 h^?1, which could rival excellent photocatalysis system. This study highlights the potential of piezoelectric catalysis for overall water splitting.