The Covalent Functionalization of Layered Black Phosphorus by Nucleophilic Reagents

Turning Waste into Wealth: Remotely NIR Light-Controlled Precious Metal Recovery by Covalently Functionalized Black Phosphorus

It is a great challenge to refine precious metals from e-wastes under mild conditions without hazardous reagents. Herein, black phosphorus (BP) was covalently functionalized with poly(N-isopropylacrylamide) (PNIPAM) to obtain thermo/near-infrared (NIR)-responsive BP-P for precious metal recovery. Precious metals (Au, Ag, and Pd) with higher redox potentials than BP-P could be efficiently recovered by reduction-driven enrichment. Taking Au as an example, the recovery process presented fast kinetics (<15 min), excellent selectivity, and high efficiency (≈98 %). Remote operation with NIR light could generate heat by BP, which induced the hydrophilic-to-hydrophobic transition of PNIPAM, allowing the spontaneous gathering, facile collection, and practical recycle of BP-P following Au extraction. Thanks to the unique features of BP-P, not only could high-quality Au nanoparticles (20-30 nm) be economically extracted (cost: $0.731-1.222 g^-1 Au nanoparticles; 5-6 orders of magnitude lower than the market price), but also the formed BP-P-Au nanocomposites have potential application in hydrogen evolution reaction.

Surface Modification of Black Phosphorus with Group 13 Lewis Acids for Ambient Protection and Electronic Tuning

Herein we introduce a facile, solution-phase protocol to modify the Lewis basic surface of few-layer black phosphorus (bP) and demonstrate its effectiveness at providing ambient stability and tuning of electronic properties. Commercially available group 13 Lewis acids that range in electrophilicity, steric bulk, and Pearson hard/soft-ness are evaluated. The nature of the interaction between the Lewis acids and the bP lattice is investigated using a range of microscopic (optical, atomic force, scanning electron) and spectroscopic (energy dispersive, X-ray photoelectron) methods. Al and Ga halides are most effective at preventing ambient degradation of bP (>84 h for AlBr₃ ), and the resulting field-effect transistors show excellent IV characteristics, photocurrent, and current stability, and are significantly p-doped. This protocol, chemically matched to bP and compatible with device fabrication, opens a path for deterministic and persistent tuning of the electronic properties in bP.

Interface Amorphization of Two-Dimensional Black Phosphorus upon Treatment with Diazonium Salts

Two-dimensional (2D) black phosphorus (BP) represents one of the most appealing 2D materials due to its electronic, optical, and chemical properties. Many strategies have been pursued to face its environmental instability, covalent functionalization being one of the most promising. However, the extremely low functionalization degrees and the limitations in proving the nature of the covalent functionalization still represent challenges in many of these sheet architectures reported to date. Here we shine light on the structural evolution of 2D-BP upon the addition of electrophilic diazonium salts. We demonstrated the absence of covalent functionalization in both the neutral and the reductive routes, observing in the latter case an unexpected interface conversion of BP to red phosphorus (RP), as characterized by Raman, 31 P-MAS NMR, and X-ray photoelectron spectroscopies (XPS). Furthermore, thermogravimetric analysis coupled to gas chromatography and mass spectrometry (TG-GC-MS), as well as electron paramagnetic resonance (EPR) gave insights into the potential underlying radical mechanism, suggesting a Sandmeyer-like reaction.

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.

Quantifying the Covalent Functionalization of Black Phosphorus

A straightforward quantification method to consistently determine the overall functionalization degree of covalently modified two-dimensional (2D) black phosphorus (BP) by Raman spectroscopy has been carried out. Indeed, the successful reductive methylation of the BP lattice using sodium intercalation compounds and exhibiting different functionalization degrees has been demonstrated by 31 P-magic angle spinning (MAS) NMR spectroscopy. Furthermore, the correlation of 31 P-MAS NMR spectroscopy and statistical Raman spectroscopy (SRS) revealed the first method to determine the functionalization degree of BP solely by evaluating the intensities of distinct peaks in the Raman spectra of the covalently modified material, in a similar way to the widely employed ID /IG ratio of graphene research.

Recent Advances in Chemical Functionalization of 2D Black Phosphorous Nanosheets

Owing to their tunable direct bandgap, high charge carrier mobility, and unique in-plane anisotropic structure, black phosphorus nanosheets (BPNSs) have emerged as one of the most important candidates among the 2D materials beyond graphene. However, the poor ambient stability of black phosphorus limits its practical application, due to the chemical degradation of phosphorus atoms to phosphorus oxides in the presence of oxygen and/or water. Chemical functionalization is demonstrated as an efficient approach to enhance the ambient stability of BPNSs. Herein, various covalent strategies including radical addition, nitrene addition, nucleophilic substitution, and metal coordination are summarized. In addition, efficient noncovalent functionalization methods such as van der Waals interactions, electrostatic interactions, and cation-π interactions are described in detail. Furthermore, the preparations, characterization, and diverse applications of functionalized BPNSs in various fields are recapped. The challenges faced and future directions for the chemical functionalization of BPNSs are also highlighted.

Noncovalent Functionalization of Pnictogen Surfaces: From Small Molecules to 2D Heterostructures

Beyond graphene, 2D pnictogen polymers are rapidly growing among the family of 2D materials. Due to their unique properties, this group has received considerable interest in recent years. Those properties include tunable electronic band gaps, high charge carrier mobility, and in-plane anisotropic properties. This Review covers the noncovalent functionalization of pnictogen surfaces considering experimental and theoretical studies. Noncovalent functionalization is of great importance for effective modulation of the electronic structure of these materials as well as improvement of their stability toward surface oxidation. This Review highlights their noncovalent modification by organic molecules, in which enhanced surface stability of phosphorene and generated functionalized materials for applications in biomedical, supercapacitors, energy storage, and biosensors. Moreover, the noncovalent interactions with small molecules show its significance for sensing applications. Lastly, the interactions of pnictogen sheets with other 2D materials and their applications for van der Waals heterostructure formation are discussed. Current state-of-the-art as well as future perspectives in this field are covered.

Black Phosphorus: An Effective Feedstock for the Synthesis of Phosphorus-Based Chemicals

We propose and demonstrate the novel concept of synthesizing organophosphorus compounds directly from black phosphorus (BP) nanoparticles as the feedstock. Compounds such as alkyl phosphines, alkyl phosphine oxides, phosphine sulfide, and hexafluorophosphate anion are prepared with good isolation yields under mild conditions. Selective synthesis of primary, secondary, and tertiary organophosphorus compounds is also demonstrated utilizing this one-pot approach. Reaction mechanisms are proposed and discussed. Compared with traditional white phosphorus (P4)-based methods, the new synthetic concept and process utilizing elemental phosphorus are more efficient and environmentally friendly.

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.

Transport and photogalvanic properties of covalent functionalized monolayer black phosphorus

Opposite responses of the transport and photogalvanic properties of the two-probe devices based on covalent functionalized black phosphorus.

Interface engineering for a stable chemical structure of oxidized-black phosphorus via self-reduction in AlOx atomic layer deposition

We evaluated the change in the chemical structure between dielectrics (AlOx and HfOx) grown by atomic layer deposition (ALD) and oxidized black phosphorus (BP), as a function of air exposure time. Chemical and structural analyses of the oxidized phosphorus species (PxOy) were performed using atomic force microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, first-principles density functional theory calculations, and the electrical characteristics of field-effect transistors (FETs). Based on the combined experiments and theoretical investigations, we clearly show that oxidized phosphorus species (PxOy, until exposed for 24 h) are significantly decreased (self-reduction) during the ALD of AlOx. In particular, the field effect characteristics of a FET device based on Al2O3/AlOx/oxidized BP improved significantly with enhanced electrical properties, a mobility of ∼253 cm2 V-1 s-1 and an on-off ratio of ∼105, compared to those of HfO2/HfOx/oxidized BP with a mobility of ∼97 cm2 V-1 s-1 and an on-off ratio of ∼103-104. These distinct differences result from a significantly decreased interface trap density (Dit ∼ 1011 cm-2 eV-1) and subthreshold gate swing (SS ∼ 270 mV dec-1) in the BP device caused by the formation of stable energy states at the AlOx/oxidized BP interface, even with BP oxidized by air exposure.

2D Black Phosphorus: from Preparation to Applications for Electrochemical Energy Storage

Black phosphorus (BP) is rediscovered as a 2D layered material. Since its first isolation in 2014, 2D BP has triggered tremendous interest in the fields of condensed matter physics, chemistry, and materials science. Given its unique puckered monolayer geometry, 2D BP displays many unprecedented properties and is being explored for use in numerous applications. The flexibility, large surface area, and good electric conductivity of 2D BP make it a promising electrode material for electrochemical energy storage devices (EESDs). Here, the experimental and theoretical progress of 2D BP is presented on the basis of its preparation methods. The structural and physiochemical properties, air instability, passivation, and EESD applications of 2D BP are discussed systemically. Specifically, the latest research findings on utilizing 2D BP in EESDs, such as lithium-ion batteries, supercapacitors, and emerging technologies (lithium-sulfur batteries, magnesium-ion batteries, and sodium-ion batteries), are summarized. On the basis of the current progress, a few personal perspectives on the existing challenges and future research directions in this developing field are provided.

Post-Graphene 2D Chemistry: The Emerging Field of Molybdenum Disulfide and Black Phosphorus Functionalization

The current state of the chemical functionalization of three types of single sheet 2D materials, namely, graphene, molybdenum disulfide (MoS2 ), and black phosphorus (BP) is summarized. Such 2D sheet polymers are currently an emerging field at the interface of synthetic chemistry, physics, and materials science. Both covalent and non-covalent functionalization of sheet architectures allows a systematic modification of their properties, that is, an improvement of solubility and processability, the prevention of re-aggregation, or band-gap tuning. Next to successful functionalization concepts, fundamental challenges are also addressed. These include the insolubility and polydispersity of most 2D sheet polymers, the development of suitable characterization tools, the identification of effective binding strategies, the chemical activation of the usually rather unreactive basal planes for covalent addend binding, and the regioselectivity of plane addition reactions. Although a number of these questions remain elusive in this Review, the first promising concepts to overcome such hurdles are presented.