Azide Passivation of Black Phosphorus Nanosheets: Covalent Functionalization Affords Ambient Stability Enhancement

Two‐dimensional (2D) black phosphorus (BP) has a unique band structure, but it suffers from low ambient stability owing to its high reactivity to oxygen. Covalent functionalization has been demonstrated to passivate the reactive BP effectively, however the reported covalent functionalization methods are quite limited to aryl diazonium and nucleophilic additions affording P?C and P?O?C single bonds, for which the retaining of one unpaired electron in the Group 15 phosphorus atom hampers the passivation effect. Now, covalent azide functionalization of BP nanosheets (BPNSs) is reported, leading to significant enhancement of the ambient stability of BP as confirmed by UV/Vis spectroscopic studies. The most stable configuration of the azide functionalized BPNSs (f‐BPNSs) is predicted by theoretical calculations, featuring the grafting of benzoic acid moiety onto BPNSs via the unprecedented P=N double bonds formed through in situ nitrene as a reactive intermediate.     

Lattice Opening upon Bulk Reductive Covalent Functionalization of Black Phosphorus

The chemical bulk reductive covalent functionalization of thin‐layer black phosphorus (BP) using BP intercalation compounds has been developed. Through effective reductive activation, covalent functionalization of the charged BP by reaction with organic alkyl halides is achieved. Functionalization was extensively demonstrated by means of several spectroscopic techniques and DFT calculations; the products showed higher functionalization degrees than those obtained by neutral routes.     

The Cytotoxicity of Layered Black Phosphorus

Black phosphorus (BP), the latest addition to the family of 2D layered materials, has attracted much interest owing to potential optoelectronics, nanoelectronics, and biomedicine applications. Little is known about its toxicity, such as whether it could be as toxic as white phosphorus. In response to the possibility of BP employment into commercial products and biomedical devices, its cytotoxicity to human lung carcinoma epithelial cells (A549) was investigated. Following a 24 h exposure of the cells with different BP concentrations, cell viability assessments were conducted using water‐soluble tetrazolium salt (WST‐8) and methylthiazolyldiphenyltetrazolium bromide (MTT) assays. The toxicological effects were found to be dose‐dependent, with BP reducing cell viabilities to 48 % (WST‐8) and 34 % (MTT) at 50 μg mL^?1 exposure. This toxicity was observed to be generally intermediate between that of graphene oxides and exfoliated transition‐metal dichalcogenides (MoS₂, WS₂, WSe₂). The relatively low toxicity paves the way to utilization of black phosphorus.     

Two-Dimensional Black Phosphorus: Preparation,Passivation and Lithium-Ion Battery Applications

As a new type of single element direct-bandgap semiconductor, black phosphorus (BP) shows many excellent characteristics due to its unique two-dimensional (2D) structure, which has great potential in the fields of optoelectronics, biology, sensing, information, and so on. In recent years, a series of physical and chemical methods have been developed to modify the surface of 2D BP to inhibit its contact with water and oxygen and improve the stability and physical properties of 2D BP. By doping and coating other materials, the stability of BP applied in the anode of a lithium-ion battery was improved. In this work, the preparation, passivation, and lithium-ion battery applications of two-dimensional black phosphorus are summarized and reviewed. Firstly, a variety of BP preparation methods are summarized. Secondly, starting from the environmental instability of BP, different passivation technologies are compared. Thirdly, the applications of BP in energy storage are introduced, especially the application of BP-based materials in lithium-ion batteries. Finally, based on preparation, surface functionalization, and lithium-ion battery of 2D BP, the current research status and possible future development direction are put forward.     

Stabilization of Black Phosphorus by Sonication-Assisted Simultaneous Exfoliation and Functionalization

Black phosphorus (BP) has extraordinary properties, but its ambient instability remains a critical challenge. Functionalization has been employed to overcome the sensitivity of BP to ambient conditions while preserving its properties. Herein, a simultaneous exfoliation–functionalization process is reported that functionalizes BP flakes during exfoliation and thus provides increased protection, which can be attributed to minimal exposure of the flakes to ambient oxygen and water. A tetrabutylammonium salt was employed for intercalation of BP, resulting in the formation of flakes with large lateral dimensions. The addition of an aryl iodide or an aryl iodonium salt to the exfoliation solvent creates a scalable strategy for the production of functionalized few-layer BP flakes. The ambient stability of functionalized BP was prolonged to a period of one week, as revealed by STEM, AFM, and X-ray photoelectron spectroscopy.     

Band-Gap and Charge Transfer Engineering in Red Phosphorus-Based Composites for Enhanced Visible-Light-Driven H2 Evolution

It is known that the low lifetime of photogenerated carriers is the main drawback of elemental photocatalysts. Therefore, a facile and versatile one-step strategy to simultaneously achieve the oxygen covalent functionalization of amorphous red phosphorus (RP) and in situ modification of CdCO₃ is reported. This strategy endows RP with enhanced charge carrier separation ability and photocatalytic activity by coupling band-gap engineering and heterojunction construction. The as-prepared nCdCO₃/SO-RP (n=0.1, 0.25, 0.5, 1.0) composites exhibited excellent photocatalytic H₂ evolution activity (up to 516.3 μmol g⁻¹ h) from visible-light-driven water splitting (λ>400 nm), which is about 17.6 times higher than that of pristine RP. By experimental and theoretical investigations, the roles of surface oxygen covalent functionalization, that is, prolonging the lifetime of photogenerated carriers and inducing the negative shift of the conduction band potential, were studied in detail. Moreover, the charge transfer mechanism of these composites has also been proposed. In addition, these composites are stable and can be reused at least for three times without significant activity loss. This work may provide a good example of how to promote the activity of elemental photocatalysts by decorating their atomic structure.     

Covalent carbene modification of 2D black phosphorus

Black phosphorus (BP) has attracted an ever-growing interest due to its unique anisotropic two-dimensional structure, impressive photoelectronic properties and attractive application potential. However, the tools for bandgap engineering and passivation via covalent modification of BP nanosheets remain limited to diazonium salt and nucleophilic addition methods, so that developing new modification strategies for BP nanosheets is crucial to explore its physical and chemical properties and enrich the toolbox for functionalization. Herein, we report the covalent modification of liquid-phase exfoliated BP nanosheets based on a rational analysis of BP structure. The modification of BP is achieved via carbene, a highly reactive organic mediate. The carbene modification improves the solubility and stability of BP nanosheets. Detailed microscopic and spectroscopic characterizations including infrared spectra, Raman spectra, X-ray photoelectron spectra, SEM and TEM were conducted to provide insights for the reaction. The proof of the existence of covalent bonds between BP nanosheets and organic moieties confirms the successful modification. Moreover, theoretical calculations were conducted to unveil the reaction mechanism of the two different types of bonds and the chemical property of two-dimensional BP.     

Isolation and Characterization of Four Phosphorus Cluster Anions P73–, P144–, P162– and P264– from the Nucleophilic Functionalization of White Phosphorus with 1,4‐Dilithio‐1,3‐butadienes

Studies on the aggregation degrees of negatively charged phosphides derived from the nucleophilic P₄ functionalization could help understand the pathway of phosphorus atoms degradation or aggregation. In this report, we have isolated and characterized four phosphorus cluster anions (P₇^3–, P₁₄^4–, P₁₆^2–, and P₂₆^4–) from the nucleophilic functionalization of P₄ with 1,4‐dilithio‐1,3‐butadienes. These phosphorus clusters could be rationalized as the P‐atom‐containing products besides the main phospholyl lithium. Their structural features and ³¹P NMR behaviors are discussed based on single crystal X‐ray diffraction analysis and ³¹P{¹H} COSY NMR analysis.     

Black Phosphorus-Based Semiconductor Heterojunctions for Photocatalytic Water Splitting

Solar-to-hydrogen (H₂) conversion has been regarded as a sustainable and renewable technique to address aggravated environmental pollution and global energy crisis. The most critical aspect in this technology is to develop highly efficient and stable photocatalysts, especially metal-free photocatalysts. Recently, black phosphorus (BP), as a rising star 2D nanomaterial, has captured enormous attention in photocatalytic water splitting owing to its widespread optical absorption, adjustable direct band gap, and superior carrier migration characteristics. However, the rapid charge recombination of pristine BP has seriously limited its practical application as photocatalyst. The construction of BP-based semiconductor heterojunctions has been proven to be an effective strategy for enhancing the separation of photogenerated carriers. This Minireview attempts to summarize the recent progress in BP-based semiconductor heterojunctions for photocatalytic water splitting, including type-I and type-II heterojunctions, Z-Scheme systems, and multicomponent heterojunctions. Finally, a brief summary and perspective on the challenges and future directions in this field are also provided.     

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.     

Aqueous Processable Two-Dimensional Triazine Polymers with Superior Photocatalytic Properties

Efficient and scalable production of high-quality and processable two-dimensional (2D) polymers are highly desired but have not yet been reported. Herein, we demonstrate a convenient noncovalent functionalization strategy for producing highly uniform, aqueous processable and semiconducting 2D triazine polymers. Experimental and theoretical analysis reveal that the aromatic amphiphilic 1-pyrenebutyrate can adsorb and intercalate into the interlayer of bulk crystalline covalent triazine framework (CTF) through noncovalent π-π stacking interaction between the pyrene moiety and the porous basal plane of 2D triazine polymer layer, which greatly facilitate the exfoliation of CTF in water in large scale. The as-prepared highly water-dispersible single-layer/few-layer 2D triazine polymer nanosheets can be easily processed into ultralight aerogels with a density of 5–15 mg cm⁻³, which can be further shaped into mechanically strong films upon simple compression. This noncovalent functionalization not only improve the dispersibility and processability of 2D triazine polymer, but also optimize its band structure and promote the photogenerated carrier separation via an interesting surface molecule doping effect, thus resulting in a remarkable photocatalytic hydrogen evolution rate of 1249 μmol h⁻¹ (24980 μmol g⁻¹ h⁻¹) and apparent quantum efficiency up to 27.2 % at 420 nm for the 2D triazine polymer, outperforming most metal-free photocatalysts ever reported.     

Synthesis of Fibrous Phosphorus Micropillar Arrays with Pyro-Phototronic Effects

The bottom-up preparation of two-dimensional material micro-nano structures at scale facilitates the realisation of integrated applications in optoelectronic devices. Fibrous Phosphorus (FP), an allotrope of black phosphorus (BP), is one of the most promising candidate materials in the field of optoelectronics with its unique crystal structure and properties.^[1] However, to date, there are no bottom-up micro-nano structure preparation methods for crystalline phosphorus allotropes.^{[1c, 2]} Herein, we present the bottom-up preparation of fibrous phosphorus micropillar (FP-MP) arrays via a low-pressure gas-phase transport (LP-CVT) method that controls the directional phase transition from amorphous red phosphorus (ARP) to FP. In addition, self-powered photodetectors (PD) of FP-MP arrays with pyro-phototronic effects achieved detection beyond the band gap limit. Our results provide a new approach for bottom-up preparation of other crystalline allotropes of phosphorus.     

一步法合成的FeOOH/黑磷纳米复合材料协同实现体系优异的析氧性能

通过水热法，在黑磷(BP)纳米片表面生长FeOOH纳米材料，制备出FeOOH/BP纳米复合材料。作为电化学析氧反应(OER)催化剂，该复合材料在20 mA·cm^-2时的过电位仅为191 mV，Tafel斜率为49.9 mV dec^-1；在循环1 000圈后，过电位仅仅增加了3 mV，且循环过程中元素价态不变，表现出优秀的稳定性。纳米FeOOH负载于BP表面，客观上能隔断氧气对BP的氧化，保护BP的载流子传导性能。同时，生长的FeOOH颗粒尺度小，结晶性弱，这有利于丰富其活性位点，增大活性面积。     

一步法合成的FeOOH/黑磷纳米复合材料协同实现体系优异的析氧性能

通过水热法,在黑磷(BP)纳米片表面生长FeOOH纳米材料,制备出FeOOH/BP纳米复合材料。作为电化学析氧反应(OER)催化剂,该复合材料在20 mA·cm^-2时的过电位仅为191 mV,Tafel斜率为49.9 mV dec^-1;在循环1 000圈后,过电位仅仅增加了3 mV,且循环过程中元素价态不变,表现出优秀的稳定性。纳米FeOOH负载于BP表面,客观上能隔断氧气对BP的氧化,保护BP的载流子传导性能。同时,生长的FeOOH颗粒尺度小,结晶性弱,这有利于丰富其活性位点,增大活性面积。     

一步法合成的FeOOH/黑磷纳米复合材料协同实现体系优异的析氧性能

通过水热法,在黑磷(BP)纳米片表面生长FeOOH纳米材料,制备出FeOOH/BP纳米复合材料。作为电化学析氧反应(OER)催化剂,该复合材料在20 mA·cm^-2时的过电位仅为191 mV,Tafel斜率为49.9 mV·dec^-1;在循环1 000圈后,过电位仅仅增加了3 mV,且循环过程中元素价态不变,表现出优秀的稳定性。纳米FeOOH负载于BP表面,客观上能隔断氧气对BP的氧化,保护BP的载流子传导性能。同时,生长的FeOOH颗粒尺度小,结晶性弱,这有利于丰富其活性位点,增大活性面积。     

High Electron Mobility of Amorphous Red Phosphorus Thin Films

Black phosphorus (BP) has been gathering great attention for its electronic and optoelectronic applications due to its high electron mobility and high I_ON/OFF current switching ratio. The limitations of this material include its low synthetic yield and high cost. One alternative to BP is another type of phosphorus allotrope, red phosphorus (RP), which is much more affordable and easier to process. Although RP has been widely used in industry for hundreds of years and considered as an insulating material, in this study, we demonstrate through field‐effect transistors (FET) measurements that amorphous red phosphorus (a‐RP) films are semiconductive with a high mobility of 387 cm² V^?1 s^?1 and a current switching ratio of ≈10³, which is comparable to the electronic characteristics previously reported for BP. The films were produced via a thermal evaporation method or a facile drop‐casting approach onto Si/SiO₂ substrates. We also report a study of the oxidation process of the films over time and a method to stabilize the films via doping a‐RP with metal oxides. The doped films retain stability for one thousand I–V cycles, with no signs of degradation.     

Adaptive Bifunctional Electrocatalyst of Amorphous CoFe Oxide @ 2D Black Phosphorus for Overall Water Splitting

Water electrolysis offers a promising green technology to tackle the global energy and environmental crisis, but its efficiency is greatly limited by the sluggish reaction kinetics of both the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). In this work, by growing amorphous multi-transition-metal (cobalt and iron) oxide on two-dimensional (2D) black phosphorus (BP), we develop a bifunctional electrocatalyst (CoFeO@BP), which is able to efficiently catalyze both HER and OER. The overpotentials for the hybrid CoFeO@BP catalyst to reach a current density of 10 mA cm⁻² in 1 m KOH are 88 and 266 mV for HER and OER, respectively. Based on a series of ex-situ and in situ investigations, the excellent catalytic performance of CoFeO@BP is found to result from the adaptive surface structure under reduction and oxidation potentials. CoFeO@BP can be transformed to CoFe phosphide under reduction potential, in situ generating the real active catalyst for HER.     

Recent insights into the robustness of two-dimensional black phosphorous in optoelectronic applications

Since the two-dimensional (2D) black phosphorus (BP) was successfully rediscovered in 2014, has garnered considerable attention due to its superior properties, including thickness-dependent direct bandgap, prominent carrier mobility, wide absorption and response spectral band and in-plane anisotropic etc., also has been applied in diverse fields. It has become a worldwide hot topic. However, the environmental instability has severely hindered its further development in both academic and industry application. Tremendous theoretical and experimental investigations are carried out to get a insight of the degradation mechanism and promote the environmental stability of BP. Motivated by this, this article, focus on the recent development of stabilized 2D BP nanoflakes, will present the basic exploration, mechanism, efforts, and application of functionalized BP. The challenges encountered and further developments are summarized at the end.     

Controlled Covalent Functionalization of 2 H-MoS2 with Molecular or Polymeric Adlayers

Most air-stable 2D materials are relatively inert, which makes their chemical modification difficult. In particular, in the case of MoS₂, the semiconducting 2 H-MoS₂ is much less reactive than its metallic counterpart, 1T-MoS₂. As a consequence, there are hardly any reliable methods for the covalent modification of 2 H-MoS₂. An ideal method for the chemical functionalization of such materials should be both mild, not requiring the introduction of a large number of defects, and versatile, allowing for the decoration with as many different functional groups as possible. Herein, a comprehensive study on the covalent functionalization of 2 H-MoS₂ with maleimides is presented. The use of a base (Et₃N) leads to the in situ formation of a succinimide polymer layer, covalently connected to MoS₂. In contrast, in the absence of base, functionalization stops at the molecular level. Moreover, the functionalization protocol is mild (occurs at room temperature), fast (nearly complete in 1 h), and very flexible (11 different solvents and 10 different maleimides tested). In practical terms, the procedures described here allow for the chemist to manipulate 2 H-MoS₂ in a very flexible way, decorating it with polymers or molecules, and with a wide range of functional groups for subsequent modification. Conceptually, the spurious formation of an organic polymer might be general to other methods of functionalization of 2D materials, where a large excess of molecular reagents is typically used.     

Identifying the Crystalline Orientation of Black Phosphorus Using Angle‐Resolved Polarized Raman Spectroscopy

An optical anisotropic nature of black phosphorus (BP) is revealed by angle‐resolved polarized Raman spectroscopy (ARPRS), and for the first time, an all‐optical method was realized to identify the crystal orientation of BP sheets, that is, the zigzag and armchair directions. We found that Raman intensities of A_g¹, B_2g, and A_g² modes of BP not only depend on the polarization angle α, but also relate to the sample rotation angle θ. Furthermore, their intensities reach the local maximum or minimum values when the crystalline orientation is along with the polarization direction of scattered light (e_s). Combining with the angle‐resolved conductance, it is confirmed that A_g² mode intensity achieves a relative larger (or smaller) local maximum under parallel polarization configuration when armchair (or zigzag) direction is parallel to e_s. Therefore, ARPRS can be used as a rapid, precise, and nondestructive method to identify the crystalline orientation of BP layers.