Electrocatalytic Activity of a 2D Phosphorene‐Based Heteroelectrocatalyst for Photoelectrochemical Cells

Towards the Future of Polymeric Hybrids of Two-Dimensional Black Phosphorus or Phosphorene: From Energy to Biological Applications

With the advent of a new 2D nanomaterial, namely, black phosphorus (BP) or phosphorene, the scientific community is now dedicated to focusing on and exploring this 2D material offering elusive properties such as a higher carrier mobility, biocompatibility, thickness-dependent band gap, and optoelectronic characteristics that can be harnessed for multiple applications, e.g., nanofillers, energy storage devices, field effect transistors, in water disinfection, and in biomedical sciences. The hexagonal ring of phosphorus atoms in phosphorene is twisted slightly, unlike how it is in graphene. Its unique characteristics, such as a high carrier mobility, anisotropic nature, and biocompatibility, have attracted much attention and generated further scientific curiosity. However, despite these interesting features, the phosphorene or BP poses challenges and causes frustrations when it comes to its stability under ambient conditions and processability, and thus in order to overcome these hurdles, it must be conjugated or linked with the suitable and functional organic counter macromolecule in such a way that its properties are not compromised while providing a protection from air/water that can otherwise degrade it to oxides and acid. The resulting composites/hybrid system of phosphorene and a macromolecule, e.g., a polymer, can outperform and be exploited for the aforementioned applications. These assemblies of a polymer and phosphorene have the potential for shifting the paradigm from exhaustively used graphene to new commercialized products offering multiple applications.

CO2 reduction to CH4 on Cu-doped phosphorene: a first-principles study

Optimizing the electrochemical carbon dioxide reduction reaction (CRR) to fuels is one of the most significant challenges in materials science and chemistry. Recently, single metal atom catalysts based on 2D materials have shown promise to improve the electroreduction performance of pristine 2D materials in the CRR. The physical origins of such performance enhancements are still poorly understood. Herein, we report the potential of a single Cu atom doped phosphorene catalyst for CO₂ electroreduction based on density functional theory (DFT) calculations. The doping sites (hollow, bridge, and on-top) of Cu on phosphorene are investigated first. Phosphorene with a Cu atom anchored on the hollow site is chosen for further study. The pathways for different CRR products, including HCOOH, CO, CH₃OH, and CH₄, are examined via constructing free energy diagrams and via comparing the limiting potentials. CH₄ is the most likely product after analysis of the adsorption energies and free energy pathways. Cu-Doped phosphorene in general shows improved CRR performance with lower limiting potential values. Cu doping leads to a decrease in the band gap value (about 0.2 eV), which is likely to be the physical origin of the CRR performance enhancement. Our study provides a novel promising CRR candidate catalyst based on phosphorene.

A multifunctional 2D black phosphorene-based platform for improved photovoltaics

As one of the latest additions to the 2D nanomaterials family, black phosphorene (BP, monolayer or few-layer black phosphorus) has gained much attention in various forms of solar cells. This is due largely to its intriguing semiconducting properties such as tunable direct bandgap (from 0.3 eV in the bulk to 2.0 eV in the monolayer), extremely high ambipolar carrier mobility, broad visible to infrared light absorption, etc. These appealing optoelectronic attributes make BP a multifunctional nanomaterial for use in solar cells via tailoring carrier dynamics, band energy alignment, and light harvesting, thereby promoting the rapid development of third-generation solar cells. Notably, in sharp contrast to the copious work on revealing the fundamental properties of BP, investigation into the utility of BP is comparatively less, particularly in the area of photovoltaics. Herein, we first identify and summarize an array of unique characteristics of BP that underpin its application in photovoltaics, aiming at providing inspiration to develop new designs and device architectures of photovoltaics. Subsequently, state-of-the-art synthetic routes (i.e., top-down and bottom-up) to scalable BP production that facilitates its applications in optoelectronic materials and devices are outlined. Afterward, recent advances in a diverse set of BP-incorporated solar cells, where BP may impart electron and/or hole extraction and transport, function as a light absorber, provide dielectric screening for enhancing exciton dissociation, and modify the morphology of photoabsorbers, are discussed, including organic solar cells, dye-sensitized solar cells, heterojunction solar cells and perovskite solar cells. Finally, the challenges and opportunities in this rapidly evolving field are presented.

MXene derivatives: synthesis and applications in energy convention and storage

Transition metal carbides or nitrides (MXene) have shown promising applications in energy convention and storage (ECS), owing to their high conductivity and adjustable surface functional groups. In the past several years, many MXene derivatives with different structures have been successfully prepared and their impressive performance demonstrated in ECS. This review summarizes the progress in the synthesis of MXene and typical Ti3C2T x MXene derivatives with different morphologies, including 0D quantum dots, 1D nanoribbons, 2D nanosheets and 3D nanoflowers. The mechanisms involved and their performance in photocatalysis, electrocatalysis and rechargeable batteries are also discussed. Furthermore, the challenges of MXene derivatives in ECS are also proposed.

Functional inks and extrusion-based 3D printing of 2D materials: a review of current research and applications

Graphene and related 2D materials offer an ideal platform for next generation disruptive technologies and in particular the potential to produce printed electronic devices with low cost and high throughput. Interest in the use of 2D materials to create functional inks has exponentially increased in recent years with the development of new ink formulations linked with effective printing techniques, including screen, gravure, inkjet and extrusion-based printing towards low-cost device manufacturing. Exfoliated, solution-processed 2D materials formulated into inks permits additive patterning onto both rigid and conformable substrates for printed device design with high-speed, large-scale and cost-effective manufacturing. Each printing technique has some sort of clear advantages over others that requires characteristic ink formulations according to their individual operational principles. Among them, the extrusion-based 3D printing technique has attracted heightened interest due to its ability to create three-dimensional (3D) architectures with increased surface area facilitating the design of a new generation of 3D devices suitable for a wide variety of applications. There still remain several challenges in the development of 2D material ink technologies for extrusion printing which must be resolved prior to their translation into large-scale device production. This comprehensive review presents the current progress on ink formulations with 2D materials and their broad practical applications for printed energy storage devices and sensors. Finally, an outline of the challenges and outlook for extrusion-based 3D printing inks and their place in the future printed devices ecosystem is presented.

Regulation of 2D Graphene Materials for Electrocatalysis

Benefiting from unique excellent physical and chemical characteristics, graphene has attracted widespread attention in the application of electrocatalysis. As a promising candidate, graphene is usually regulated with surface defects, heteroatoms, metal atoms and other active materials through covalent or non-covalent bonds to substitute for noble metal catalysts, which has not been targeted in a report yet. In this review, we summarize the recent advances of approaches for engineering graphene-based electrocatalysts and emphasize the corresponding electrocatalytic active sites in various electrocatalysis circumstances, such as electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), etc. The opportunities and challenges in the future development of graphene-based catalysts are also discussed.

Evolution of intrinsic vacancies and prolonged lifetimes of vacancy clusters in black phosphorene

Due to the relatively low formation energies and highly mobile characteristics of atomic vacancies in phosphorene, understanding their evolution becomes crucial for its structural integrity, chemical activities and applications. Herein, by combining first-principles calculations and kinetic Monte Carlo simulation, we investigate the time evolution and formation of atomic vacancy clusters from isolated monovacancies (MVs), aiming to uncover the mechanisms of diffusion, annihilation, and reaction of these atomic vacancies. We find that while isolated MVs possess a highly mobile characteristic, they react and form MV pairs which possess much lower mobility and high stability under ambient conditions. We also show that the disappearance of MVs at the edge is quite slow due to the relatively high energy barrier, and as a result, around 80% of MVs remain even after two years under ambient conditions. Our findings on one hand provide useful information for the structural repairing of phosphorene through chemical functionalization of these vacancy clusters, and on the other hand, suggest that these rather stable vacancy clusters may be used as activated catalysts.

Triple-Shelled Co-VSex Hollow Nanocages as Superior Bifunctional Electrode Materials for Efficient Pt-Free Dye-Sensitized Solar Cells and Hydrogen Evolution Reactions

Complex nanostructures with distinct spatial architectures and more active sites hold broad prospects in new energy conversion fields. Herein, a facile strategy was carried out to construct triple-shelled Co-VSe_x nanocages, starting with an ion-exchange process between Co-based zeolitic imidazolate framework-67 (ZIF-67) nanopolyhedrons and VO₃^- followed by the formation of triple-shelled Co-VSe_x hollow nanocages during the process of increasing the solvothermal temperature under the assistance of SeO₃^2-. Meanwhile, triple-shelled Co-VS_x and yolk-double shell Co-VO_x nanocages were fabricated as references by a similar process. Benefiting from the larger surface areas and more electrolyte adsorption sites, the triple-shelled Co-VSe_x nanocages exhibited excellent electrocatalytic performances when applied as the electrochemical catalysts for dye-sensitized solar cells (DSSC) and hydrogen evolution reactions (HER). More concretely, the DSSC based on the Co-VSe_x counter electrode showed outstanding power conversion efficiency of 9.68% when its Pt counterpart was 8.46%. Moreover, the Co-VSe_x electrocatalyst exhibited prominent HER performance with a low onset overpotential of 40 mV and a small Tafel slope of 39.1 mV dec^-1 in an acidic solution.

Carboxylic Acid Functionalization at the Meso-Position of the Bodipy Core and Its Influence on Photovoltaic Performance

Two bodipy dyes with different carboxylic acids on the meso-position of the bodipy core were prepared and used to sensitize TiO2 photoelectrodes. On the basis of spectroscopic characterization, the photoelectrodes were used to fabricate photoelectrochemical cells (PECs) for solar light harvesting. Photovoltaic measurements showed that both bodipy dyes successfully sensitized PECs with short-circuit current densities (JSC) two-fold higher compared to the control. The increase in generated current was attributed to the gain in spectral absorbance due to the presence of bodipy. Finally, the influence of co-sensitization of bodipy and N719 dye was also investigated and photovoltaic device performance discussed.

Future Phosphorus: Advancing New 2D Phosphorus Allotropes and Growing a Sustainable Bioeconomy

With more than 40 countries currently proposing to boost their national bioeconomies, there is no better time for a clarion call for a "new" bioeconomy, which, at its core, tackles the current disparities and inequalities in phosphorus (P) availability. Existing biofuel production systems have widened P inequalities and contributed to a linear P economy, impairing water quality and accelerating dependence on P fertilizers manufactured from finite nonrenewable phosphate rock reserves. Here, we explore how the emerging bioeconomy in novel, value-added, bio-based products offers opportunities to rethink our stewardship of P. Development of integrated value chains of new bio-based products offers opportunities for codevelopment of "P refineries" to recover P fertilizer products from organic wastes. Advances in material sciences are exploiting unique semiconductor and opto-electrical properties of new "two-dimensional" (2D) P allotropes (2D black phosphorus and blue phosphorus). These novel P materials offer the tantalizing prospect of step-change innovations in renewable energy production and storage, in biomedical applications, and in biomimetic processes, including artificial photosynthesis. They also offer a possible antidote to the P paradox that our agricultural production systems have engineered us into, as well as the potential to expand the future role of P in securing sustainability across both agroecological and technological domains of the bioeconomy. However, a myriad of social, technological, and commercialization hurdles remains to be crossed before such an advanced circular P bioeconomy can be realized. The emerging bioeconomy is just one piece of a much larger puzzle of how to achieve more sustainable and circular horizons in our future use of P.

Black phosphorene as a hole extraction layer boosting solar water splitting of oxygen evolution catalysts

As the development of oxygen evolution co-catalysts (OECs) is being actively undertaken, the tailored integration of those OECs with photoanodes is expected to be a plausible avenue for achieving highly efficient solar-assisted water splitting. Here, we demonstrate that a black phosphorene (BP) layer, inserted between the OEC and BiVO₄ can improve the photoelectrochemical performance of pre-optimized OEC/BiVO₄ (OEC: NiOOH, MnO_x, and CoOOH) systems by 1.2∼1.6-fold, while the OEC overlayer, in turn, can suppress BP self-oxidation to achieve a high durability. A photocurrent density of 4.48 mA·cm⁻² at 1.23 V vs reversible hydrogen electrode (RHE) is achieved by the NiOOH/BP/BiVO₄ photoanode. It is found that the intrinsic p-type BP can boost hole extraction from BiVO₄ and prolong holes trapping lifetime on BiVO₄ surface. This work sheds light on the design of BP-based devices for application in solar to fuel conversion, and also suggests a promising nexus between semiconductor and electrocatalyst.

Photoelectrochemical water splitting affords an integrated approach for converting light to fuel, but devices typically suffer poor activities and stabilities. Here, authors incorporate black phosphorene into bismuth vanadate photoanodes to boost hole extraction and device lifetimes.

Phosphorene as a Catalyst for Highly Efficient Nonaqueous Li-Air Batteries

Li-air batteries can yield exceptionally high predicted energy densities. However, for this technology to become realizable, round-trip efficiency issues and slow kinetics at the cathode require implementation of a catalyst. With design parameters not well understood and limitations on material selection, choosing an ideal catalyst is complex. In Li-air batteries, energy storage is achieved by reactions between Li and O (oxygen reduction reaction for discharge and oxygen evolution reaction for charge). Here, phosphorene is proposed as a solution through simulations of its catalytic behavior toward discharge initiated via either O₂ dissociation or Li adsorption. After obtaining intermediate geometries for both nucleation paths leading to either Li₂O₂ or 2(Li₂O), free-energy diagrams are generated to predict the promoted discharge product of Li₂O₂. Furthermore, considering a final product of Li₂O₂, the overpotentials are predicted to be 1.44 V for discharge and 2.63 V for charge. Activation barriers for the catalytic decomposition of Li₂O₂ (during charge) are found to be 1.01 eV for phosphorene versus 2.06 eV for graphene. This leads to a major difference in reaction rate up to 10¹⁷ times in favor of phosphorene. These results, complemented by electronic analysis, establish phosphorene as a promising catalyst for Li-air batteries.

Ambient Degradation-Induced Spin Paramagnetism in Phosphorene

The sizeable direct bandgap, high mobility, and long spin lifetimes at room temperature offer black phosphorus (BP) potential applications in spin-based semiconductor devices. Toward these applications, a critical step is creating a magnetic response in BP, which is arousing much interest. It is reported here that ambient degradation of BP, which is immediate and inevitable and greatly changes the semiconducting properties, creates magnetic moments, and any degree of degradation leads to notable paramagnetism. Its Landau factor g measured is ≈1.995, revealing that the magnetization mainly results from spin rather than orbital moments. Such magnetism most likely results from the unsaturated phosphorus in the vacancies which are stabilized by O adatoms. It can be tuned by changing any one of the ambient factors of ambient temperature, humidity, and light intensity, and can be stabilized by exposing BP in argon. The findings highlight the importance of evaluating the effect of ambient degradation-induced magnetism on BP's spin-based devices. The work seems an essential milestone toward the forthcoming research upsurge on BP's magnetism.

Mechanical Behaviors of Angle-Ply Black Phosphorus by Molecular Dynamics Simulations

Regular black phosphorus (BP) sheets possess strongly anisotropic properties due to the unique puckered atomistic configuration, making such BP mechanically very weak in the armchair direction. The present work aims to address this issue by proposing an angle-ply double-layer black phosphorus (DLBP) structure in which two individual atomic layers with different orientation angles are stacked up. The molecular dynamics simulations based on Stillinger-Weber potential show that the in-plane mechanical properties of such a DLBP structure, e.g., Young’s modulus and tensile strength are significantly influenced by the stacking angle of each layer. The property anisotropy of DLBP decreases as the stacking angle difference δ between two layers increases and becomes isotropic when δ = 90°. This work also shed insight into mechanisms of angle-ply layers underlying the mechanical behaviors of DLBP at the nanoscale, suggesting that the anisotropic material properties can be effectively controlled and tuned through the appropriately selected stacking angles.