Covalent and non-covalent chemistry of 2D black phosphorus

The post-graphene era is undoubtedly marked by two-dimensional (2D) sheet polymers, such as black phosphorus (BP). This emerging material has a fascinating structure and outstanding electronic properties and has been postulated for a plethora of applications. The need to circumvent the pronounced oxophilicity of P atoms has dominated the research on this material in recent years, with the objective of finding the most effective method to improve its environmental stability. When it comes to chemical functionalization, the few approaches reported so far involve some drawbacks such as low degree of addition and low production ability. This review presents the concepts and strategies of our studies on the chemical functionalization of BP, both non-covalent and covalent, emphazising the current synthetic challenges. Moreover, we also provide some effective pathways for the chemical activation of the unreactive basal plane, the identification of the effective binding strategies, and the concept to overcome hurdles associated with characterization tools. This work will provide fundamental insights into the controlled chemical functionalization and characterization of BP, fostering the research on this appealing 2D material.

Aqueous Affinity and Interfacial Dynamics of Anisotropic Buckled Black Phosphorous

The structure of black phosphorous (BP) is similar to the honeycomb arrangement of graphene, but the layered BP is found to be buckled and highly anisotropic. The buckled surface structure affects interfacial molecule mobility and plays a vital role in various nanomaterial applications. The BP is also known for wettability, droplet formation, stability, and hydrophobicity in the aqueous environment. However, there is a gap concerning the structural and dynamical behavior of water molecules, which is available in abundance for other monoatomic and polyatomic two-dimensional (2D) materials. Motivated by the technological importance, we try to bridge the gap by explaining the surface anisotropy-facilitated behavior of water molecules on bilayer BP using classical and first principles molecular dynamics (MD) simulations. From our classical MD study, we find three distinct layers of water molecules. The water layer closest to the interface is L1, followed by L2 and L3/bulk perpendicular to the BP surface. Water molecules in the L1 layer experience some structural disintegration in hydrogen bond (HB) phenomena compared to the bulk. There is a loss of HB donor-acceptor count per water molecule. The average HB count decreases because of an elevated rate of HB formation and deformation; this would affect the dynamic properties in terms of HB lifetime. Therefore, we observe the reduced lifetime of HB in the layer in close contact with BP, which again complements our finding on the diffusion coefficient of water molecules in distinct layers. Water diffuses relatively faster with diffusion coefficient 3.25 × 10^-9 m² s^-1 in L1, followed by L2 and L3. The BP layer shows moderate hydrophobic nature. Our results also indicate the anisotropic behavior as the diffusion along the x-direction is faster than that along the y-direction. The gap in the slope of the x and y components of mean-squared displacement (MSD) complements the pinning effect in an aqueous environment. We observe blue-shifted and red-shifted libration and O-H stretching modes from the calculated power spectra for the L1 water molecules compared to the L2 and L3 molecules from first principles MD simulations. Our analysis may help understand the physical phenomena that occur during the surface wetting of the predroplet formation process observed experimentally.

Prospects for Functionalizing Elemental 2D Pnictogens: A Study of Molecular Models

Despite the intense amount of attention and huge potential of 2D-layered pnictogens for applications in chemistry, physics, and materials science, there has yet to be a robust strategy developed to systematically functionalize them to tailor their properties. This is due to a number of factors, including practical instability toward ambient conditions, difficulty in characterizing modified materials, and also more inherent reactivity issues. Here, avenues for functionalization are discussed using examples of molecular models from the wider literature, along with their possible advantages and likely pitfalls. Finally, a critical appraisal of the current field and its future is offered.

Enhanced ambient stability of exfoliated black phosphorus by passivation with nickel nanoparticles

Since its discovery, the environmental instability of exfoliated black phosphorus (2D bP) has emerged as a challenge that hampers its wide application in chemistry, physics, and materials science. Many studies have been carried out to overcome this drawback. Here we show a relevant enhancement of ambient stability in few-layer bP decorated with nickel nanoparticles as compared to pristine bP. In detail, the behavior of the Ni-functionalized material exposed to ambient conditions in the dark is accurately studied by Transmission Electron Microscopy (TEM), Raman Spectroscopy, and high resolution x-ray Photoemission and Absorption Spectroscopy. These techniques provide a morphological and quantitative insight of the oxidation process taking place at the surface of the bP flakes. In the presence of Ni nanoparticles (NPs), the decay time of 2D bP to phosphorus oxides is more than three time slower compared to pristine bP, demonstrating an improved structural stability within 20 months of observation.

Catalysis Mediated by 2D Black Phosphorus Either Pristine or Decorated with Transition Metals Species

Among the novel class of mono-elemental two-dimensional (2D) materials, termed Xenes, phosphorene is emerging as a great promise for its peculiar chemical and physical properties. This review collects a selection of the recent breakthroughs that are related to the application of phosphorene in catalysis and electrocatalysis. Noteworthy, thanks to its intrinsic Lewis basic character, pristine phosphorene turned out to be more efficient and more selective than other non-metal catalysts, in chemical processes as the electroreduction of nitrogen to ammonia or the alkylation of nucleophiles with esters. Once functionalized with transition metals nanoparticles (Co, Ni, Pd, Pt, Ag, Au), its catalytic activity has been evaluated in several processes, mainly hydrogen and oxygen evolution reactions. Under visible light irradiation, it has shown a great improvement of the activity, demonstrating high potential as a photocatalyst.

Robust Amphiphobic Few-Layer Black Phosphorus Nanosheet with Improved Stability

Few-layer black phosphorus (FL-BP) has been intensively studied due to its attractive properties and great potential in electronic and optoelectronic applications. However, the intrinsic instability of FL-BP greatly limits its practical application. In this study, the amphiphobic FL-BP is achieved by functionalization of 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFDTS) on the surface of FL-BP. The obtained PFDTS coated FL-BP (FL-BP/PFDTS) demonstrates enhanced stability, which is not observed during significant degradation for 2 months in high moisture content environment (95% humidity). Particularly, attributing to the surface amphiphobicity, FL-BP/PFDTS exhibits strong surface water repellency in the presence of oleic acid (as the contaminant), while other passivation coating layers (such as hydrophilic or hydrophobic coating) become hydrophilicity under such conditions. Owing to this advantage, the obtained FL-BP/PFDTS demonstrates enhanced stability in high moisture content environment for 2 months, even though the surface is contaminated by oil liquid or other organic solvents (such as oleic acid, CH2Cl2, and N-methyl-2-pyrrolidone). The passivation of FL-BP by amphiphobic coating provides an effective approach for FL-BP stabilization toward future applications.

A Highly Effective π-π Stacking Strategy To Modify Black Phosphorus with Aromatic Molecules for Cancer Theranostics

Even though black phosphorus (BP) has exhibited outstanding capabilities in biomedical, physical, and energy fields, the issues of degradation under ambient conditions and unreactive functional interface limit its further application. There are numerous methodologies utilized to prevent BP degradation; however, these methods usually generate further problems and normally do not involve alterations to the chemically inert BP. Herein, for the first time, we propose a simple and efficient strategy to prepare and modify BP nanosheets (p-BPNSs) by employing aromatic 1-pyrenylbutyric acid through a noncovalent π-π stacking interaction. This strategy not only adopts a novel strategy for enhancing the stability of BPNSs but also paves a convenient way to anchor other active biomolecules such as a targeting effect to extend the biomedical applications of BPNSs. The modified p-BPNSs exhibit enhanced physical and chemical stabilities as well as rich carboxyl groups for further modification. In this work, RGD-modified p-BPNSs exhibit targeted photothermal therapy ability against cancer in both in vitro and in vivo studies, owing to anchoring of arginine-glycine-aspartic acid (RGD) tripeptides, which could target nanosheets into the tumor site through systematic circulation. Consequently, this work not only provides a new concept for modifying and protecting the BP but also opens a novel window for extending the biomedical application of BP by surface engineering.