Black Phosphorus: Degradation Favors Lubrication

Tribology of 2D black phosphorus - Current state-of-the-art and future potential

Since the first mechanical exfoliation of graphene in 2004, the interest in 2D materials has significantly risen due to their outstanding property combination. Multiple 2D materials have been synthesized until today, while black phosphorus (BP) resembles one of their latest additions. The unique properties of BP, especially for electronic and optical devices (i.e., high carrier mobility and electrical conduction, field-effect transistor, layer-dependent bandgap, anisotropic transport), have gained notable attention. However, its layered structure, similar to those of graphene and MoS2, is also advantageous to optimize the friction and wear performance. Moreover, the strong in-plane covalent bonds and weak interlayer van-der-Waals forces favour the formation of low-friction and wear-resistant films. Although BP holds a great tribological potential, the literature to date on this topic is rather scarce. Therefore, it is a timely moment to holistically summarize the synthesis approaches and properties of BP thus guiding interested researchers to use it in mechanical/tribological applications. The existing state-of-the-art regarding tribological research is critically discussed and compared to other 2D materials thus highlighting existing research gaps and paving the way for future research activities.

In Situ Mitochondrial Biomineralization for Drug-Free Cancer Therapy

The common clinical chemotherapy often brings serious side effects to patients, mainly due to the off-target and leakage of toxic drugs. However, this is fatal for some specific clinical tumors, such as brain tumors and neuroma. This study performs a drug-free approach by encapsulating black phosphorus (BP) and calcium peroxide (CaO2) in liposomes with surface-modified triphenylphosphonium (BCLT) to develop mitochondria targeting calcification for cancer therapy without damaging normal cells. BCLT preferentially accumulates inside tumor mitochondria and then is activated by near-infrared (NIR) laser irradiation to produce abundant PO4 3- and Ca2+ to accelerate in situ mitochondrial mineralization, leading to mitochondrial dysfunction and cancer cell death. More importantly, both PO4 3- and Ca2+ are essential components of metabolism in the body, and random gradient diffusion or premature leakage does not cause damage to adjacent normal cells. This achievement promises to be an alternative to conventional chemotherapy in clinical practice for many specific tumor types.

Accurate Simulation for 2D Lubricating Materials in Realistic Environments: From Classical to Quantum Mechanical Methods

2D materials such as graphene, MoS2, and hexagonal BN are the most advanced solid lubricating materials with superior friction and anti-wear performance. However, as a typical surface phenomenon, the lubricating properties of 2D materials are largely dependent on the surrounding environment, such as temperature, stress, humidity, oxygen, and other environmental substances. Given the technical challenges in experiment for real-time and in situ detection of microscopic environment-material interaction, recent years have witnessed the acceleration of computational research on the lubrication behavior of 2D materials in realistic environments. This study reviews the up-to-date computational studies for the effect of environmental factors on the lubrication performance of 2D materials, summarizes the theoretical methods in lubrication from classical to quantum-mechanics ones, and emphasizes the importance of quantum method in revealing the lubrication mechanism at atomic and electronic level. An effective simulation method based on ab initio molecular dynamics is also proposed to try to provide more ways to accurately reveal the friction mechanisms and reliably guide the lubricating material design. On the basis of current development, future prospects, and challenges for the simulation and modeling in lubrication with realistic environment are outlined.

Electrochemical exfoliation of 2D materials beyond graphene

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

Surface-Modified LDH Nanosheets with High Dispersibility in Oil for Friction and Wear Reduction

Surface and interfacial engineering of nanomaterials is essential for improving dispersion stability in liquids. In this study, we report that oleic acid (OA)- and stearic acid (SA)-functionalized layered double hydroxide (LDH) nanosheets as lubricant additives can achieve high dispersion and reduce friction and wear. LDH is a typical layered structure, and OA and SA are long-chain organic molecules that are not only compatible with base oils but also act as friction-reducing agents. The OA and SA molecules were branched onto ZnMgAl LDH nanosheets using dehydration condensation between the exposed OH groups on the surface of LDH and the COOH groups on the OA and SA molecules. Compared with that of the pristine ZnMgAl LDH, the dispersion of OA-ZnMgAl LDH and SA-ZnMgAl LDH was significantly improved. The surface-modified LDH exhibited superior tribological properties and great stability due to the synergistic lubrication effect between OA, SA, and LDH. Even at an ultralow concentration (0.15 wt %), the coefficient of friction and wear volume were reduced by ∼65 and ∼99%, respectively, compared to those of the base oil. Due to the green and simple synthesis method and excellent tribological properties, surface-functionalized LDH has enormous possibilities for future industrial applications.

Self-Adaptive Macroscale Superlubricity Based on the Tribocatalytic Properties of Partially Oxidized Black Phosphorus

The high-flash heat generated by direct contact at asperity tips under high contact stress and shear significantly promotes the tribocatalytic reaction between a lubricating medium and a friction interface. Macroscale superlubricity can be achieved by using additives with good lubrication properties to promote the decomposition and transformation of a lubricating medium to form an ultralow shear interface during the friction process. This paper proposed a way to achieve self-adaptive oil-based macroscale superlubricity on different tribopairs, including steel-steel and steel-DLC (diamond-like carbon), which is based on the excellent lubricating performance of black phosphorus with active oxidation and the catalytic cleavage behavior of oil molecules on the surface of oBP. This work potentially expands the industrial application of superlubricity.

Enhanced degradation of few-layer black phosphorus by fulvic acid: Processes and mechanisms

The oxidation of the emerging nanomaterial black phosphorus (BP) affected by pH and oxygen has been carefully documented. However, in natural waters, there is a large amount of chemically reactive organic matters like fulvic acid (FA), whose impacts on degradation and stability of few-layer BP or BP nanosheets (BPNS) are scarcely disclosed. Hence, we investigated the kinetics of BPNS degradation products (H₂PO₂^-, HPO₃^2-, and PO₄^3-) in the presence of FA. The results showed that the apparent reaction rate constants of BPNS were 0.026, 0.050, and 0.060 d^-1 under oxygen-and-light condition and 0.005, 0.016, and 0.023 d^-1 under hypoxia-and-darkness condition at FA gradients of 0, 2.5, and 5 mgC/L, respectively. Microscopic observations, simple molecular simulation experiment, and density functional theory computation explained that FA significantly enhanced the degradation of P atoms on the BPNS surface through the indirect pathway of reducing the energy barrier of O₂ dissociative adsorption and the direct pathway of chemical adsorption, which caused the P-P bond on the BPNS surface to break down and formed P-O bonds or C-P bonds. This study revealed for the first time the degradation mechanism of BPNS in the presence of FA, which is a chemical mechanism of the BPNS transformation behavior. It helps to make a more scientific risk assessment of BP in natural waters.

Friction properties of black phosphorus: a first-principles study

Based on the first-principle, the friction anisotropy, structural super-lubricity and oxidation induced ultra-low friction of black phosphorus at atomic scale under different loads have been studied. The results show that the interface friction of black phosphorus is anisotropic, that is, the friction along the armchair direction is greater than that along the zigzag direction. Moreover, the friction between the black phosphorus interfaces shows a structural superlubricity property, and the incommensurate interface friction is approximately one thousandth of the commensurate interface friction, which is mainly due to the less electronic charge and the smaller amplitude of electronic charge change between the incommensurate interfaces during the friction process. In addition, the oxidation of black phosphorus is beneficial for lubrication between interfaces.

Interaction of Water and Oxygen Molecules with Phosphorene: An Ab Initio Study

Phosphorene, the 2D form of black phosphorus, has recently attracted interest for optoelectronic and tribological applications. However, its promising properties are affected by the strong tendency of the layers to oxidize in ambient conditions. A significant effort has been made to identify the role of oxygen and water in the oxidation process. In this work, we introduce a first-principles study of the phosphorene phase diagram and provide a quantitative estimate of the interaction of pristine and fully oxidized phosphorene layers with oxygen and water molecules. Specifically, we study oxidized layers with oxygen coverages of 25% and 50% that keep the typical anisotropic structure of the layers. We found that hydroxilated and hydrogenated phosphorene layers are both energetically unfavorable, leading to structural distortions. We also studied the water physisorption on both pristine and oxidized layers, finding that the adsorption energy gain doubled on the oxidized layers, whereas dissociative chemisorption was always energetically unfavorable. At the same time, further oxidation (i.e., the dissociative chemisorption of O2) was always favorable, even on oxidized layers. Ab initio molecular dynamics simulations of water intercalated between sliding phosphorene layers showed that even under harsh tribological conditions water dissociation was not activated, thus further strengthening the results obtained from our static calculations. Overall, our results provide a quantitative description of the interaction of phosphorene with chemical species that are commonly found in ambient conditions at different concentrations. The phase diagram that we introduced confirms the tendency of phosphorene layers to fully oxidize due to the presence of O2, resulting in a material with improved hydrophilicity, a piece of information that is relevant for the application of phosphorene, e.g., as a solid lubricant. At the same time, the structural deformations found for the H- and OH- terminated layers undermine their electrical, mechanical, and tribological anisotropic properties and, therefore, the usage of phosphorene.

Monolayer NbSe2 Favors Ultralow Friction and Super Wear Resistance

The urgent demand for atomically thin, superlubricating, and super wear-resistant materials in micro/nanoelectromechanical systems has stimulated the research of friction-reducing and antiwear materials. However, the fabrication of subnanometer-thick films with superlubricating and super wear-resistant properties under ambient conditions remains a huge challenge. Herein, high-quality monolayer (ML) NbSe₂ (∼0.8 nm) with ultralow friction and super wear resistance in an atmospheric environment was successfully grown by chemical vapor deposition (CVD) for the first time. Moreover, compared with few-layered (FL) NbSe₂, ML NbSe₂ has a lower friction coefficient and better wear resistance. On the basis of density function theory (DFT) calculations, the adhesion and the degree of charge transfer between ML NbSe₂ and the substrate is larger than that of the topmost layer to the underlying layers of NbSe₂ with two or more layers, which can be used to explain that the ML NbSe₂ favors ultralow friction and super wear resistance.

Synergistic Lubrication Effect between Oxidized Black Phosphorus and Oil Molecules Triggers Superlubricity

Oxidized black phosphorus (BP) has been demonstrated as a promising oil-based nanoadditive because of its superior friction-reducing capability. However, the synergistic lubrication effect between oxidized BP and oil at the molecular level dominating the friction properties remains unclear. In this Letter, the synergistic lubrication effect between oxidized BP and two typical oil molecules (nonane and nonanoic acid) was explored with an atomic force microscope. The superlubricity of oxidized BP with an ultralow friction coefficient of 0.006 was achieved in the nonanoic acid environment, exhibiting a 96% reduction compared with that in the nonane environment. There was a confined nonanoic acid layer in the contact zone with a tilt angle of 35° because of the hydrogen bonding interaction, contributing to the superlubricity. This observation sheds light on the exploration of the lubrication mechanism of oxidized BP as a nanoadditive in oil, which reveals the considerable implications for the design of high-performance lubrication system.

In-situ growth of nitrogen-doped carbonized polymer dots on black phosphorus for electrochemical DNA biosensor of Escherichia coli O157: H7

Sensitive and accurate detection technology for pathogenic bacteria is of great social and economic significance in foodborne disease and food safety. In this paper, a novel portable electrochemical DNA biosensor for the detection of specific DNA sequence of Escherichia coli (E. coli) O157: H7 was constructed. To enhance the performance of the electrochemical sensor, a functionalized nitrogen-doped carbonized polymer dots in-situ grown on few-layer black phosphorus (N-CPDs@FLBP) was synthesized and used as the modifier on the surface of screen-printed electrode. Combining gold nanoparticles as immobilization matrix and methylene blue as electrochemical indicator, the analytical performance of this electrochemical DNA biosensor was evaluated using standard complementary ssDNA sequence in the linear concentration range from 1.0 × 10^-19 to 1.0 × 10^-6 mol/L with a low detection limit as 3.33 × 10^-20 mol/L (3 σ). Furthermore, the portable electrochemical DNA biosensor was proposed based on polymerase chain reaction amplification for the detection of the E. coli O157: H7 genomic DNA from chicken meat, which verified the feasibility for practical samples detection. The research has great theoretical and practical significance for the development of electrochemical biosensor of pathogenic bacteria.

Macroscale Superlubricity of Black Phosphorus Quantum Dots

In the present work, Black Phosphorus Quantum Dots (BPQDs) were synthesized via sonication-assisted liquid-phase exfoliation. The average size of the BPQDs was 3.3 ± 0.85 nm. The BPQDs exhibited excellent dispersion stability in ultrapure water. Macroscale superlubricity was realized with the unmodified BPQDs on rough Si3N4/SiO2 interfaces. A minimum coefficient of friction (COF) of 0.0022 was achieved at the concentration of 0.015 wt%. In addition, the glycerol was introduced to promote the stability of the superlubricity state. The COF of the BPQDs-Glycerol aqueous solution (BGaq) was 83.75% lower than that of the Glycerol aqueous solution (Gaq). Based on the above analysis, the lubrication model was presented. The hydrogen-bonded network and silica gel layer were formed on the friction interface, which played a major role in the realization of macroscale superlubricity. In addition, the adsorption water layer could also prevent the worn surfaces from making contact with each other. Moreover, the synergistic effect between BPQDs and glycerol could significantly decrease the COF and maintain the superlubricity state. The findings theoretically support the realization of macroscale superlubricity with unmodified BPQDs as a water-based lubrication additive.

Multifunctional SERS chip mediated by black phosphorus@gold-silver nanocomposites inserted in bilayer membrane for in-situ detection and degradation of hazardous materials

Self-cleaning surface-enhanced Raman scattering (SERS) substrates dependent on versatile two-dimensional semiconductors offer an efficient channel for the sensitive monitoring and timely degradation of hazardous molecules. Herein, a kind of sophisticated SERS-active nanocomposites was developed by incorporating Au-Ag nanoparticles onto black phosphorus (BP) nanosheets via photo-induced self-reduction. Combining the substantial electromagnetic "hot spots" triggered by bimetallic plasma coupling effect and the efficient charge transfer from BP to probe molecules, the proposed nanocomposites featured attractive SERS enhancement, facilitating a limit of detection down to 4.5 × 10^-10 M. Attributed to the remarkable restriction of electron-hole recombination stemming from "Schottky contact", the photocatalytic activity of BP was prominently boosted, demonstrating a complete degradation time as short as 65 min. Furthermore, the disgusting instability of BP was considerably hindered by inserting the nanocomposites into various bilayer matrices with diverse hardness and viscosity inspired by cling film principle. Moreover, a significantly elevated collection rate high to 93.1% for in-situ detection was also achieved by the as-manufactured flexible SERS chips based on tape. This study illustrates a clear perspective for the development of versatile BP-based SERS chips which might facilitate sensitive analysis and treatment of perilous contaminants in complicated real-life scenarios.

Multilayer Coatings for Tribology: A Mini Review

Friction and wear usually lead to huge energy loss and failure of machine pairs, which usually causes great economic losses. Researchers have made great efforts to reduce energy dissipation and enhance durability through advanced lubrication technologies. Single-layer coatings have been applied in many sectors of engineering, but the performance of single-layer coatings still has many limitations. One solution to overcome these limitations is to use a multilayer coating that combines different components with varied physical and chemical properties. In addition, multilayer coating with alternating layers only containing two components can lead to improved performance compared to a coating with only two different layers. This paper systematically reviews the design concept and properties of different types of multilayer coatings, including transition-metal nitride coatings, diamond-like carbon-based coatings, and other multilayer coatings. The inherent functional mechanisms of the multilayer structures are also detailed and discussed.

Tribological Properties of Different-Sized Black Phosphorus Nanosheets as Water-Based Lubrication Additives for Steel/Titanium Alloy Wear Contact

Titanium alloys are extensively used in the aerospace, chemical, and biomedical industries. However, it has always been a challenge in the manufacturing and machining of titanium alloys because they exhibit poor friction and wear characteristics, which results in serious problems and significantly restricts their further production and application. Therefore, in the present study, the wear contact between GCr15 steel and Ti6Al4V alloy is specifically studied by considering black phosphorus nanosheets (BP-NS) as water-based lubrication additives, which is expected to have a great potential application in manufacturing and machining titanium alloys. The influence of BP nanosheet size on the coefficient of friction (COF) and wear rate of Ti6Al4V alloy has been comprehensively studied, based on comparisons among adding large BP nanosheets (L-BP) (2–4 μm), medium BP nanosheets (M-BP) (300–500 nm), and black phosphorus quantum dots (BPQDs) (6–10 nm). Compared with ultrapure water, the COF and wear rate of Ti6Al4V alloy are reduced by 42.4% and 82.3%, respectively, when BPQDs are used as water-based lubrication additives. This paper also shows that a lower COF and wear rate is achieved with the addition of BPQDs than the other two BP nanosheet sizes. Derived from the friction tests and worn surface analysis of Ti6Al4V alloy, lubrication mechanisms of different-sized BP lubricants were proposed. The interlaminar shearing between BP-NS and the adsorbed films were the main mechanisms for L-BP and M-BP lubricants, while the adsorption, repair, and ball-bearing effects were mainly presented in the BPQD lubricants. The discoveries in this paper would be beneficial to developing novel lubricants for the manufacturing and machining of titanium alloys.

Enhanced voltammetric performance of sensors based on oxidized 2D layered black phosphorus

The exceptional properties of 2D layered black phosphorus (BP) make it a promising candidate for electrochemical sensing applications and, even though BP is considered unstable and tends to degrade by the presence of oxygen and moisture, its oxidation can be beneficial in some situations. In this work, we present an unequivocal demonstration that the exposition of BP-based working electrodes to normal ambient conditions can indeed be advantageous, leading to an enhancement of voltammetric sensing applications. This point was proved using a BP modified screen-printed carbon electrode (BP-SPCE) for the voltammetric determination of dopamine (DA) as a model target analyte. Oxidized BP-SPCE (up to 35% of P_xO_y at the surface) presented an enhanced analytical performance with a 5-fold and 2-fold increase in sensitivity, as compared to bare-SPCE and non-oxidized BP-SPCE stored in anhydrous atmosphere, respectively. Good detection limit, repeatability, reproducibility, stability, selectivity, and accuracy were also achieved. Overall, the results presented herein display the prominent possibilities of preparing and working with BP based-sensors in normal ambient settings and showcase their implementation under physiological conditions.

First-Principles Molecular Dynamics Insight into the Atomic Level Degradation Pathway of Phosphorene

Despite its remarkable properties, phosphorene is not promising for device application due to its instability or gradual degradation under ambient conditions. The issue still persists, and no technological solution is available to address this degradation due to a lack of clarity about degradation dynamics at the atomic level. Here, we discuss atomic level degradation dynamics of phosphorene under ambient conditions while investigating the involvement of degrading agents like oxygen and water using density functional theory and first-principles molecular dynamics computations. The study reveals that the oxygen molecule dissociates spontaneously over pristine phosphorene in an ambient environment, resulting in an exothermic reaction, which is boosted further by increasing the partial pressure and temperature. The surface reaction is mainly due to the lone pair electrons of phosphorous atoms, making the degradation directional and spontaneous under oxygen atoms. We also found that while the pristine phosphorene is hydrophobic, it becomes hydrophilic after surface oxidation. Furthermore, water molecules play a vital role in the degradation process by changing the reaction dynamics path of the phosphorene-oxygen interaction and reducing the activation energy and reaction energy due to its catalyzing action. In addition, our study reveals the role of phosphorous vacancies in the degradation, which we found to act as an epicenter for the observed oxidation. The oxygen attacks directly over the vacant site and reacts faster compared to its pristine counterpart. As a result, phosphorene edges resembling extended vacancy are prominent reaction sites that oxidize anisotropically due to different bond angle strains. Our study clears the ambiguities in the kinetics of phosphorene degradation, which will help engineer passivation techniques to make phosphorene devices stable in the ambient environment.

2D materials for bone therapy

Due to their prominent physicochemical properties, 2D materials are broadly applied in biomedicine. Currently, 2D materials have achieved great success in treating many diseases such as cancer and tissue engineering as well as bone therapy. Based on their different characteristics, 2D materials could function in various ways in different bone diseases. Herein, the application of 2D materials in bone tissue engineering, joint lubrication, infection of orthopedic implants, bone tumors, and osteoarthritis are firstly reviewed comprehensively together. Meanwhile, different mechanisms by which 2D materials function in each disease reviewed below are also reviewed in detail, which in turn reveals the versatile functions and application of 2D materials. At last, the outlook on how to further broaden applications of 2D materials in bone therapies based on their excellent properties is also discussed.

Surface Functionalization of Black Phosphorus via Amine Compounds and Its Impacts on the Flame Retardancy and Thermal Decomposition Behaviors of Epoxy Resin

Recently, lots of effort has been placed into stabilizing black phosphorus (BP) in the air to improve its compatibility with polymers. Herein, BP was chemically functionalized by aliphatic amine (DETA), aromatic amine (PPDA) and cyclamine (Pid) via a nucleophilic substitution reaction, aiming to develop an intensively reactive BP flame retardant for epoxy resin (EP). The -NH₂ group on BP-DETA, BP-PPDA and BP-Pid reacted with the epoxide group at different temperatures. The lowest temperature was about 150 °C for BP-DETA. The impacts of three BP-NH₂ were compared on the flame retardancy and thermal decomposition of EP. At 5 wt% loading, EP/BP-NH₂ all passed UL 94 V 0 rating. The limiting oxygen index (LOI) of EP/BP-PPDA was as high as 32.3%. The heat release rate (HRR) of EP/BP-DETA greatly decreased by 46% and char residue increased by 73.8%, whereas HRR of EP/BP-Pid decreased by 11.5% and char residue increased by 50.8%, compared with EP. Average effective heat of combustion (av-EHC) of EP/BP-Pid was lower than that of EP/BP-DETA and EP/BP-PPDA. In view of the flame-retardant mechanism, BP nanosheets functionalized with aliphatic amine and aromatic amine played a dominant role in the condensed phase, while BP functionalized with cyclamine was more effective in the gas phase.

Shear-Induced Interfacial Structural Conversion Triggers Macroscale Superlubricity: From Black Phosphorus Nanoflakes to Phosphorus Oxide

As a new two-dimensional (2D) material, black phosphorus (BP) exhibits great potential for friction reduction. However, achieving macroscale superlubricity with a BP coating remains a great challenge. In this study, we designed a new lubrication system to achieve superlubricity with a BP coating at the macroscale, involving the formation of a BP coating with deposited BP nanoflakes, followed by water lubrication. Robust superlubricity with a coefficient of friction of 0.001 can be achieved on the BP coating in a pure water environment. The superlubricity mechanism is mainly attributed to the shear-induced interfacial structural conversion of BP to phosphorus oxide, leading to the formation of tribofilms on the friction pairs with extremely low shear strength. This finding provides a new strategy for achieving superlubricity of 2D material coatings at the macroscale, which has important implications for the development of novel superlubrication systems for industrial applications.

Macroscale Superlubricity on Engineering Steel in the Presence of Black Phosphorus

Friction and wear are the main reasons for decreasing the lifetime of moving mechanical components and causing energy loss. It is desirable to achieve macroscale superlubricity on industrial materials for minimizing friction. Herein, the two-dimensional material black phosphorus (BP) is prepared as an oil-based nanoadditive in oleic acid (OA) and shown to produce macroscale superlubricity at the steel/steel contact under high pressure. Experiments and molecular dynamics simulation reveal that BP quickly captures the carboxylic group and, as a result of the high contact pressure and heat, OA decomposes to release passivating species and recombines to form amorphous carbon giving rise to a composite solid tribofilm with BP. The OA and passivating groups adsorb onto the solid tribofilm to produce the passivating layer, thus resulting in macroscale superlubricity. The findings provide fundamental insight into the nature of tribochemical mechanisms and suggest a new approach to achieve macroscale superlubricity of industrial materials.

Biodegradation of graphene materials catalyzed by human eosinophil peroxidase

The enzymatic activity of eosinophil peroxidase secreted by human immune cells leads to degradation of different sources of graphene oxide.

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.

Black phosphorus-based 2D materials for bone therapy

Since their discovery, Black Phosphorus (BP)-based nanomaterials have received extensive attentions in the fields of electromechanics, optics and biomedicine, due to their remarkable properties and excellent biocompatibility. The most essential feature of BP is that it is composed of a single phosphorus element, which has a high degree of homology with the inorganic components of natural bone, therefore it has a full advantage in the treatment of bone defects. This review will first introduce the source, physicochemical properties, and degradation products of BP, then introduce the remodeling process of bone, and comprehensively summarize the progress of BP-based materials for bone therapy in the form of hydrogels, polymer membranes, microspheres, and three-dimensional (3D) printed scaffolds. Finally, we discuss the challenges and prospects of BP-based implant materials in bone immune regulation and outlook the future clinical application.

Thickness-dependent Young's modulus of polycrystalline α-PbO nanosheets

Litharge, in two dimensional (2D) nanostructure form, has recently ignited considerable theoretical interest due to its excellent photoelectric and magnetic properties. However, the lack of an efficient synthesis method hinders its development. Here, we provide an interfacial solvothermal strategy for controllably synthesizing ultrathin hexagonal polycrystalline α-PbO nanosheets in micrometer scale. This strategy can also be utilized for the synthesis of other 2D materials. Experimental atomic force microscope nanoindentation measurements reveal the relationship between the thickness of polycrystalline α-PbO nanosheets and the corresponding Young's modulus, expressed as E = E₀ + Kt ^-1. First-principles calculation supports the result and ascribes the cause to interlayer sliding from particular weak interlayer interactions. Additionally, the enhanced mechanical strength of the polycrystalline structure compared to its single-crystal counterpart is attributed to the alternate arrangement of grain-boundaries effects. The summative formula may be extended to other 2D materials with weak interlayer interactions, which has the potential to provide guidance for constructing flexible devices.

Tribology of 2D Nanomaterials: A Review

The exfoliation of graphene has opened a new frontier in material science with a focus on 2D materials. The unique thermal, physical and chemical properties of these materials have made them one of the choicest candidates in novel mechanical and nano-electronic devices. Notably, 2D materials such as graphene, MoS2, WS2, h-BN and black phosphorus have shown outstanding lowest frictional coefficients and wear rates, making them attractive materials for high-performance nano-lubricants and lubricating applications. The objective of this work is to provide a comprehensive overview of the most recent developments in the tribological potentials of 2D materials. At first, the essential physical, wear and frictional characteristics of the 2D materials including their production techniques are discussed. Subsequently, the experimental explorations and theoretical simulations of the most common 2D materials are reviewed in regards to their tribological applications such as their use as solid lubricants and surface lubricant nano-additives. The effects of micro/nano textures on friction behavior are also reviewed. Finally, the current challenges in tribological applications of 2D materials and their prospects are discussed.

Gate-tunable trion binding energy in monolayer MoS2 with plasmonic superlattice

Two-dimensional transition metal dichalcogenides exhibit promising potential and attract the attention of the world in the application of optoelectronic devices owing to their distinctive physical and chemical properties. The real-time control of light-matter interactions in semiconductor devices through an external optical resonant cavity is crucial for designing next-generation optoelectronic devices. Here, we report the spectroscopic identification of trion binding energy in monolayer MoS₂ field-effect transistors with plasmonic nanoresonators. In consequence, the binding energy could be regulated dynamically through an external electric field. In addition, after increasing the carrier injection, the evidence of the enhanced trion binding energy can also be observed, which can be utilized for researching magneto-plasmons. The ability to dynamically control the optical properties by electrostatic doping opens a platform for designing next-generation optoelectronic and valleytronic applications in two-dimensional crystals with accurate and precise tailored responses.

Tribological properties of black phosphorus nanosheets as oil-based lubricant additives for titanium alloy-steel contacts

The black phosphorus (BP) powders were prepared by high-energy ball milling with red phosphorus as the raw material, and then the BP nanosheets were obtained by liquid-phase exfoliation. The tribological properties of the BP nanosheets as oil-based lubricant additives were investigated by the ball-on-disc tribometer. Results show that compared with the base oil of liquid paraffin (LP), the coefficient of friction and wear rate of the BP nanosheets as the additives in liquid paraffin (BP-LP) are lower for the same loads. BP-LP lubricants could significantly improve the load-bearing capacity of the base oil for titanium alloy-steel contacts and show excellent friction-reducing and anti-wear properties. The surface morphologies and elemental compositions of the friction pairs were further analysed using an optional microscope, scanning electron microscope and X-ray photoelectron spectroscopy. The lubrication mechanism of BP-LP can be attributed to the synergistic effects between lamellar adsorption and interlayer shear of BP nanosheets.

Photochemical Activity of Black Phosphorus for Near-Infrared Light Controlled In Situ Biomineralization

The photochemical activity of black phosphorus (BP) in near-infrared (NIR) light controlled in situ biomineralization is investigated. Owing to the excellent NIR absorption, irradiation with NIR light not only promotes degradation of BP into PO4 3-, but also enhances the chemical activity to accelerate the reaction between PO4 3- and Ca2+ and promote in situ biomineralization. Mineralization of hydrogels is demonstrated by the preparation of BP incorporated hydrogel (BP@Hydrogel) which delivers greatly improved biomineralization performance under NIR illumination. The biomineralization process which can be controlled by modulating the light irradiation time and location has a high potential in controlling the mechanical properties and osteoinductive ability in tissue engineering. This study also provides insights into the degradation, photochemical activity, and new biological/biomedical applications of BP.

Two-Dimensional Black Phosphorus: An Emerging Anode Material for Lithium-Ion Batteries

Two-dimensional black phosphorus (2D BP), an emerging material, has aroused tremendous interest once discovered. This is due to the fact that it integrates unprecedented properties of other 2D materials, such as tunable bandgap structures, outstanding electrochemical properties, anisotropic mechanical, thermodynamic, and photoelectric properties, making it of great research value in many fields. The emergence of 2D BP has greatly promoted the development of electrochemical energy storage devices, especially lithium-ion batteries. However, in the application of 2D BP, there are still some problems to be solved urgently, such as the difficulty in the synthesis of large-scale high-quality phosphorene, poor environmental stability, and the volume expansion as electrode materials. Herein, according to the latest research progress of 2D BP in the field of energy storage, we systematically summarize and compare the preparation methods of phosphorene and discuss the basic structure and properties of BP, especially the environmental instability and passivation techniques. In particular, the practical application and challenges of 2D BP as anode material for lithium-ion batteries are analyzed in detail. Finally, some personal perspectives on the future development and challenges of BP are presented.

Ultratough graphene-black phosphorus films

Graphene-based films with high toughness have many promising applications, especially for flexible energy storage and portable electrical devices. Achieving such high-toughness films, however, remains a challenge. The conventional mechanisms for improving toughness are crack arrest or plastic deformation. Herein we demonstrate black phosphorus (BP) functionalized graphene films with record toughness by combining crack arrest and plastic deformation. The formation of covalent bonding P-O-C between BP and graphene oxide (GO) nanosheets not only reduces the voids of GO film but also improves the alignment degree of GO nanosheets, resulting in high compactness of the GO film. After further chemical reduction and π-π stacking interactions by conjugated molecules, the alignment degree of rGO nanosheets was further improved, and the voids in lamellar graphene film were also further reduced. Then, the compactness of the resultant graphene films and the alignment degree of reduced graphene oxide nanosheets are further improved. The toughness of the graphene film reaches as high as ∼51.8 MJ m^-3, the highest recorded to date. In situ Raman spectra and molecular dynamics simulations reveal that the record toughness is due to synergistic interactions of lubrication of BP nanosheets, P-O-C covalent bonding, and π-π stacking interactions in the resultant graphene films. Our tough black phosphorus functionalized graphene films with high tensile strength and excellent conductivity also exhibit high ambient stability and electromagnetic shielding performance. Furthermore, a supercapacitor based on the tough films demonstrated high performance and remarkable flexibility.

Covalent organic frameworks bearing pillar[6]arene-reduced Au nanoparticles for the catalytic reduction of nitroaromatics

While tremendous advancements in 2D materials anchoring Au nanoparticles have been made, it is an urgent challenge to explore a green and facile approach for obtaining small-size Au nanoparticles. The rise of 2D covalent organic framework (COF) presents more-promising candidates for constructing excellent sites for loading metal nanoparticles. In this study, a novel 2D heterogeneous hybrid nanomaterial (P6-Au-COF) based on COF and pillar[6]arene (P6) reduced Au nanoparticles (P6-Au) is prepared by a simple and green procedure. The Au nanoparticles with an average small diameter of 2-3 nm are homogeneously dispersed on the surface of the COF. The P6-Au-COF hybrid material shows highly catalytic performance for the reduction of nitrophenol isomers when compared with commercial Pd/C catalyst and other reported materials. The P6-Au-COF hybrid material exhibits durable recyclablility and stability during the catalytic reaction. Considering the outstanding merits of the heterogeneous 2D catalyst of P6-Au-COF as well as the simple and green preparation, this research might not only present enormous opportunities for stabilized, high-performance and sustainable catalysts, but be applied in other frontier study of sustainable functionalized nanocomposites and advanced materials.

Emerging 2D Materials Produced via Electrochemistry

2D materials are important building blocks for the upcoming generation of nanostructured electronics and multifunctional devices due to their distinct chemical and physical characteristics. To this end, large-scale production of 2D materials with high purity or with specific functionalities represents a key to advancing fundamental studies as well as industrial applications. Among the state-of-the-art synthetic protocols, electrochemical exfoliation of layered materials is a very promising approach that offers high yield, great efficiency, low cost, simple instrumentation, and excellent up-scalability. Remarkably, playing with electrochemical parameters not only enables tunable material properties but also increases the material diversities from graphene to a wide spectrum of 2D semiconductors. Here, a succinct and critical survey of the recent progress in this research direction is presented, comprising the strategic design, exfoliation principles, underlying mechanisms, processing techniques, and potential applications of 2D materials. At the end of the discussion, the emerging trends, challenges, and opportunities in real practice are also highlighted.

Super-Slippery Degraded Black Phosphorus/Silicon Dioxide Interface

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO₂)/silicon (Si) substrate, generally considered as a solid-solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO₂/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO₂ interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO₂ interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

Property-Activity Relationship of Black Phosphorus at the Nano-Bio Interface: From Molecules to Organisms

As a novel member of the two-dimensional nanomaterial family, mono- or few-layer black phosphorus (BP) with direct bandgap and high charge carrier mobility is promising in many applications such as microelectronic devices, photoelectronic devices, energy technologies, and catalysis agents. Due to its benign elemental composition (phosphorus), large surface area, electronic/photonic performances, and chemical/biological activities, BP has also demonstrated a great potential in biomedical applications including biosensing, photothermal/photodynamic therapies, controlled drug releases, and antibacterial uses. The nature of the BP-bio interface is comprised of dynamic contacts between nanomaterials (NMs) and biological systems, where BP and the biological system interact. The physicochemical interactions at the nano-bio interface play a critical role in the biological effects of NMs. In this review, we discuss the interface in the context of BP as a nanomaterial and its unique physicochemical properties that may affect its biological effects. Herein, we comprehensively reviewed the recent studies on the interactions between BP and biomolecules, cells, and animals and summarized various cellular responses, inflammatory/immunological effects, as well as other biological outcomes of BP depending on its own physical properties, exposure routes, and biodistribution. In addition, we also discussed the environmental behaviors and potential risks on environmental organisms of BP. Based on accumulating knowledge on the BP-bio interfaces, this review also summarizes various safer-by-design strategies to change the physicochemical properties including chemical stability and nano-bio interactions, which are critical in tuning the biological behaviors of BP. The better understanding of the biological activity of BP at BP-bio interfaces and corresponding methods to overcome the challenges would promote its future exploration in terms of bringing this new nanomaterial to practical applications.

Hydroxyl-Assisted Phosphorene Stabilization with Robust Device Performances

Phosphorene (few-layer black phosphorus) has been widely investigated for its unique optical and electronic properties. However, it is challenging to synthesize and process stable phosphorene as it degrades rapidly upon exposure to oxygen and moisture under ambient conditions, which has limited its use in practical applications. Herein, we propose an alkali-assisted stabilization process to produce high-quality phosphorene nanosheets. Our morphology measurements show that alkali-treated phosphorene remains stable for over 7 days in air. Electrical measurements on alkali-treated BP devices further proved its stable electrical property under ambient conditions. We further demonstrate superior light-assisted electrochemical water splitting performance using stable phosphorene. We attribute the stabilization effect to the chemical modification of the surface of phosphorene with P-OH bond formation. This study paves the avenue for the implementation of phosphorene devices in ambient conditions.

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.

On the comparison of oxygen and sulfur transfer reactivities in phosphine and phosphorene: the case of R3Sb(X) carriers (X = O or S)

Functionalization is one of the most powerful tools in materials science for the development of new and innovative materials with tailored properties purposefully designed to enhance the overall stability of the system. This is particularly true for exfoliated black phosphorus, which suffers from easy decomposition by air and moisture, hampering its highly desirable applications, especially in electronics. The present work suggests an innovative approach to the functionalization process of this 2D-material based on the selective introduction of chalcogen atoms on the material surface through a reaction with suitable molecular precursors such as stibine chalcogenides (R3Sb(X), X = O or S; R = organyl group). These molecules may readily act as chalcogen-transfer agents and, upon releasing the chalcogen atom atop the bP surface, leave stable stibines (R3Sb) as byproducts, which may be easily removed from the functionalized bP surface. The work provides an overview of all the possible structural, electronic and energy aspects associated with the chalcogen-atom transfer from the stibine to phosphorus based compounds, exemplified by trialkyl phosphines and single layer exfoliated black phosphorus, i.e. phosphorene, Pn. In both cases the oxygen transfer is more exergonic than the sulfur transfer, with the associated free energy barrier for the phosphine process being higher. Although the sulfur transfer for the Pn is found to be endergonic (ca. +3.6 kcal mol-1), the process may surely occur at high temperature. The evolution of the band structure upon the chalcogen transfer has been depicted in detail.

Atomic scale study of black phosphorus degradation

Black phosphorus (BP) is a promising two-dimensional (2D) material for future electronic devices due to its unique properties of high carrier mobility and large band gap tunability. However, thinner crystalline BP is more readily degraded under ambient conditions. For BP-based electronic devices, degradation of the exfoliated BP is a key issue. However, the nanometer scale study of BP degradation is rare so far. Herein, we report an atomically resolved degradation process of the BP surface using atomic force microscopy under temperature- and humidity-controlled environments. The atomically resolved crystal surface of BP deteriorated due to surface etching after cleavage, and showed monolayer etching. The etching process is accelerated by applying a bias voltage to BP via a conductive tip. After the voltage-assisted BP etching, the BP etching product shows crystalline BP confirmed by Raman spectroscopy and atomic force microscopy. Our atomic scale study of BP will be useful for the future 2D-based electronic devices to overcome conventional silicon-based electronic devices.

In-Plane Potential Gradient Induces Low Frictional Energy Dissipation during the Stick-Slip Sliding on the Surfaces of 2D Materials

Understanding the nanoscale friction properties of 2D materials and further manipulating their friction behaviors is of great significance for the development of various micro/nanodevices. Recent studies, taking advantage of the close relationship between friction and surface charges, use an external out-of-plane electric field to control the interfacial friction. Nevertheless, friction increases with the application of the out-of-plane electric field in most cases. Here, an in-plane potential gradient is applied for the investigation of the contribution of electric charges to friction on the surfaces of 2D materials. Experimental results show that the friction between an atomic force microscope tip and the flakes of 2D materials decreases with the application of the in-plane potential gradient, and the higher the potential gradient, the greater the friction decrease. By comparing the in situ atomic-level stick-slip maps before and after the application of the in-plane potential gradient, it is proposed that the promotion of low friction dissipative motion during the stick-slip process owing to the presence of the potential gradient gives rise to the friction reduction. These results not only help to reveal the origin of friction, but also provide a novel way to manipulate friction through an electrically-controlled sliding process.

Liquefaction of water on the surface of anisotropic two-dimensional atomic layered black phosphorus

The growth and wetting of water on two-dimensional(2D) materials are important to understand the development of 2D material based electronic, optoelectronic, and nanomechanical devices. Here, we visualize the liquefaction processes of water on the surface of graphene, MoS2 and black phosphorus (BP) via optical microscopy. We show that the shape of the water droplets forming on the surface of BP, which is anisotropic, is elliptical. In contrast, droplets are rounded when they form on the surface of graphene or MoS2, which do not possess orthometric anisotropy. Molecular simulations show that the anisotropic liquefaction process of water on the surface of BP is attributed to the different binding energies of H2O molecules on BP along the armchair and zigzag directions. The results not only reveal the anisotropic nature of water liquefaction on the BP surface but also provide a way for fast and nondestructive determination of the crystalline orientation of BP.

Monolayer Phosphorene-Carbon Nanotube Heterostructures for Photocatalysis: Analysis by Density Functional Theory

One-dimensional (1D)/2D heterostructures have attracted great attention in electronic and optoelectronic fields because of their unique geometrical structures and rich physics. Here, we systematically explore electronic structure and optical performance of single-wall carbon nanotube (CNT)/phosphorene (BP) hybrids by large-scale density functional theory (DFT) computation. The results show that the interfacial interaction between CNT and BP is a weak van der Waals (vdW) force and correlates with tube diameter of CNTs. The CNT/BP hybrids have strong optical absorption compared with that of individual BP and CNT. A diameter-dependent type I or II heterojunction in CNT/BP hybrids is observed. Moreover, CNTs can not only significantly promote photogenerated carrier transfer, but also effectively improve the photocatalytic activities of BP as a co-catalyst. These findings would enrich our understanding of BP-based 1D/2D heterostructures, providing further insight into the design of highly efficient phosphorene-based or CNT-based nanophotocatalysts.