Z‐Scheme 2D/2D Heterojunction of Black Phosphorus/Monolayer Bi2WO6 Nanosheets with Enhanced Photocatalytic Activities

Black phosphorus (BP), a star‐shaped two‐dimensional material, has attracted considerable attention owing to its unique chemical and physical properties. BP shows great potential in photocatalysis area because of its excellent optical properties; however, its applications in this field have been limited to date. Now, a Z‐scheme heterojunction of 2D/2D BP/monolayer Bi₂WO₆ (MBWO) is fabricated by a simple and effective method. The BP/MBWO heterojunction exhibits enhanced photocatalytic performance in photocatalytic water splitting to produce H₂ and NO removal to purify air; the highest H₂ evolution rate of BP/MBWO is 21042 μmol g^?1, is 9.15 times that of pristine MBWO and the NO removal ratio was as high as 67 %. A Z‐scheme photocatalytic mechanism is proposed based on monitoring of ^.O₂^?, ^.OH, NO₂, and NO₃^? species in the reaction. This work broadens applications of BP and highlights its promise in the treatment of environmental pollution and renewable energy issues.     

Noble‐Metal‐Free Janus‐like Structures by Cation Exchange for Z‐Scheme Photocatalytic Water Splitting under Broadband Light Irradiation

Z‐scheme water splitting is a promising approach based on high‐performance photocatalysis by harvesting broadband solar energy. Its efficiency depends on the well‐defined interfaces between two semiconductors for the charge kinetics and their exposed surfaces for chemical reactions. Herein, we report a facile cation‐exchange approach to obtain compounds with both properties without the need for noble metals by forming Janus‐like structures consisting of γ‐MnS and Cu₇S₄ with high‐quality interfaces. The Janus‐like γ‐MnS/Cu₇S₄ structures displayed dramatically enhanced photocatalytic hydrogen production rates of up to 718 μmol g⁻¹ h⁻¹ under full‐spectrum irradiation. Upon further integration with an MnO_x oxygen‐evolution cocatalyst, overall water splitting was accomplished with the Janus structures. This work provides insight into the surface and interface design of hybrid photocatalysts, and offers a noble‐metal‐free approach to broadband photocatalytic hydrogen production.     

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.     

Au/La2Ti2O7 Nanostructures Sensitized with Black Phosphorus for Plasmon‐Enhanced Photocatalytic Hydrogen Production in Visible and Near‐Infrared Light

Efficient utilization of solar energy is a high‐priority target and the search for suitable materials as photocatalysts that not only can harvest the broad wavelength of solar light, from UV to near‐infrared (NIR) region, but also can achieve high and efficient solar‐to‐hydrogen conversion is one of the most challenging missions. Herein, using Au/La₂Ti₂O₇ (BP‐Au/LTO) sensitized with black phosphorus (BP), a broadband solar response photocatalyst was designed and used as efficient photocatalyst for H₂ production. The optimum H₂ production rates of BP‐Au/LTO were about 0.74 and 0.30 mmol g⁻¹ h⁻¹ at wavelengths longer than 420 nm and 780 nm, respectively. The broad absorption of BP and plasmonic Au contribute to the enhanced photocatalytic activity in the visible and NIR light regions. Time‐resolved diffuse reflectance spectroscopy revealed efficient interfacial electron transfer from excited BP and Au to LTO which is in accordance with the observed high photoactivities.     

Efficient Solar Light Harvesting CdS/Co9S8 Hollow Cubes for Z‐Scheme Photocatalytic Water Splitting

Hollow structures with an efficient light harvesting and tunable interior component offer great advantages for constructing a Z‐scheme system. Controlled design of hollow cobalt sulfide (Co₉S₈) cubes embedded with cadmium sulfide quantum dots (QDs) is described, using hollow Co(OH)₂ as the template and a one‐pot hydrothermal strategy. The hollow CdS/Co₉S₈ cubes utilize multiple reflections of light in the cubic structure to achieve enhanced photocatalytic activity. Importantly, the photoexcited charge carriers can be effectively separated by the construction of a redox‐mediator‐free Z‐scheme system. The hydrogen evolution rate over hollow CdS/Co₉S₈ is 134 and 9.1 times higher than that of pure hollow Co₉S₈ and CdS QDs under simulated solar light irradiation, respectively. Moreover, this is the first report describing construction of a hollow Co₉S₈ based Z‐scheme system for photocatalytic water splitting, which gives full play to the advantages of light‐harvesting and charges separation.     

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.     

Z‐Scheme Photocatalytic Water Splitting on a 2D Heterostructure of Black Phosphorus/Bismuth Vanadate Using Visible Light

Spontaneously solar‐driven water splitting to produce H₂ and O₂, that is, the conversion of solar energy to chemical energy is a dream of mankind. However, it is difficult to make overall water splitting feasible without using any sacrificial agents and external bias. Drawing inspiration from nature, a new artificial Z‐scheme photocatalytic system has been designed herein based on the two‐dimensional (2D) heterostructure of black phosphorus (BP)/bismuth vanadate (BiVO₄). An effective charge separation makes possible the reduction and oxidation of water on BP and BiVO₄, respectively. The optimum H₂ and O₂ production rates on BP/BiVO₄ were approximately 160 and 102 μmol g^?1 h^?1 under irradiation of light with a wavelength longer than 420 nm, without using any sacrificial agents or external bias.     

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.     

pH‐Dependent Degradation of Layered Black Phosphorus: Essential Role of Hydroxide Ions

The practical application of layered black phosphorus (LBP) is compromised by fast decomposition in the presence of H₂O and/or O₂. The role of H₂O is controversial. Herein, we propose a hydroxide ion (OH^?)‐initiated degradation mechanism for LBP to elucidate the role of H₂O. We found that LBP degraded faster in alkaline solutions than in neutral or acidic solutions with or without O₂. Degradation rates of LBP increased linearly from pH 4 to 10. Density functional theory (DFT) calculations showed that OH^? initiated the decomposition of LBP through breaking the P?P bond and forming a P?O bond. The detection of hypophosphite, generated from OH^? reacting with P atoms, confirmed the hypothesis. Protons acted in a way distinctive from OH^?, by inducing deposition/aggregation or forming a cation–π layer to protect LBP from degradation. This work reveals the degradation mechanism of LBP and thus facilitates the development of effective stabilization technologies.     

Black Tungsten Nitride as a Metallic Photocatalyst for Overall Water Splitting Operable at up to 765 nm

Semiconductor photocatalysts are hardly employed for overall water splitting beyond 700 nm, which is due to both thermodynamic aspects and activation barriers. Metallic materials as photocatalysts are known to overcome this limitation through interband transitions for creating electron–hole pairs; however, the application of metallic photocatalysts for overall water splitting has never been fulfilled. Black tungsten nitride is now employed as a metallic photocatalyst for overall water splitting at wavelengths of up to 765 nm. Experimental and theoretical results together confirm that metallic properties play a substantial role in exhibiting photocatalytic activity under red‐light irradiation for tungsten nitride. This work represents the first red‐light responsive photocatalyst for overall water splitting, and may open a promising venue in searching of metallic materials as efficient photocatalysts for solar energy utilization.     

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.     

Intrinsic Facet-Dependent Reactivity of Well-Defined BiOBr Nanosheets on Photocatalytic Water Splitting

Surface atomic arrangement and coordination of photocatalysts highly exposed to different crystal facets significantly affect the photoreactivity. However, controversies on the true photoreactivity of a specific facet in heterogeneous photocatalysis still exits. Herein, we exemplified well-defined BiOBr nanosheets dominating with respective facets, (001) and (010), to track the reactivity of crystal facets for photocatalytic water splitting. The real photoreactivity of BiOBr-(001) were evidenced to be significantly higher than BiOBr-(010) for both hydrogen production and oxygen evolution reactions. Further in situ photochemical probing studies verified the distinct reactivity is not only owing to the highly exposed facets, but dominated by the co-exposing facets, leading to an efficient spatial separation of photogenerated charges and further making the oxidation and reduction reactions separately occur with different reaction rates, which ordains the fate of the true photoreactivity.     

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.     

Intrinsic Correlation between Electronic Structure and Degradation: From Few‐Layer to Bulk Black Phosphorus

Black phosphorus (BP) has received much attention owing to its fascinating properties, such as a high carrier mobility and tunable band gap. However, these advantages have been overshadowed by the fast degradation of BP under ambient conditions. To overcome this obstacle, the exact degradation mechanisms need to be unveiled. Herein, we analyzed two sequential degradation processes and the layer‐dependent degradation rates of BP in the dark by scanning Kelvin probe microscopy (SKPM) measurements and theoretical modeling. The layer‐dependent degradation was successfully interpreted by considering the oxidation model based on the Marcus–Gerischer theory (MGT). In the dark, the electron transfer rate from BP to oxygen molecules depends on the number of layers as these systems have different carrier concentrations. This work not only provides a deeper understanding of the degradation mechanism itself but also suggest new strategies for the design of stable BP‐based electronics.