Periodic Reporting for period 4 - B-PhosphoChem (Exploration of the 2D-Chemistry of Black Phosphorous)
Reporting period: 2022-02-01 to 2022-07-31
Single and few layer sheets of black phosphorus (BP) represent a new class of non-carbon 2D materials. Like graphite/graphene, black phosphorus represents an allotrope of an element consisting of one atom type only. Black phosphorus has a couple of outstanding physical properties which renders it ideal for its implementation in high-tech applications in the fields of molecular electronics, solar cells, gas sensors, and lithium ion batteries. A major hurdle, which had to be overcome, was the access to single layer BP in order to investigate its physical and chemical properties in detail. At the beginning of B-PhosphoChem, the scientific research was predominantly focused on the experimental and theoretical investigation of the physical and materials properties of BP, the chemistry of this novel 2D material, however, remained almost unexplored. At this point, B PhosphoChem set in: it was the first coordinated project to systematically explore and develop the methodologies to access and stabilize single and few-layer BP and simultaneously investigate its chemistry and characterization. In the course of B-PhosphoChem we were able to exfoliate BP down to single monolayers, to understand the degradation of the material and to develop solutions for its stabilization, to tailor the non-covalent chemistry of BP and to use specific functional dye molecules for its implementation into field effect transistors, to intercalate crystalline BP with alkali metals in order to activate the layered material for a subsequent exfoliation/covalent functionalization sequence, to propel the characterization of BP and its derivatives by means of Raman spectroscopy, and to proof the concept that BP can be utilized as molecular catalyst. The results obtained by our detailed scientific research of the chemistry of BP can serve as the corner stone for technological applications of this novel 2D material.
With the beginning of the project, the large scale and large area production of single layer BP (SL-BP) remained unsatisfactory and was still in its infancy. Liquid exfoliation of BP using viscous solvents allows for upscaling and its bulk production. Along these lines, we have shown, that even rather stable dispersions of flakes with observable photoluminescence can be prepared. Based on the results obtained by the solvent driven exfoliation and non-covalent functionalization of BP and the directly related stabilization of the single layer material against oxidation we obtained a deeper understanding of the underlying oxidation dynamics. We developed a straightforward chemical methodology for the control of the thickness of the BP flakes down to the monolayer limit by layer-by-layer oxidation and thinning, using water as solubilizing agent. The applicability of this thinning procedure is documented by the preparation of BP-based field effect transistor devices, showing that the electronic properties of the flakes are not compromised. Following these lines, we used ionic liquids (ILs) for a systematic surface passivation study of mechanically exfoliated flakes and on the basis of scanning Raman spectroscopy we developed a rapid and reliable methodology to precisely estimate the thickness of BP flakes. We also discovered that the diffusion of oxygen and water as well as the BP photo-oxidation can be suppressed by covering the flakes with ILs. In a direct comparison study of the two 2D materials single layer BP and antimonene we were able to use our obtained knowledge for an unprecedented discovery of their extraordinary potential as homogeneous phase catalysts in alkylation reactions of esters with soft nucleophiles
Our research into the stabilization of individualized BP sheets by ILs lead to the finding that the addition of tiny amounts of long n-alkanes (intended as co-stabilization components) or water to conventional ILs spontaneously generates well-defined superficial alkane microdomains or water spherical microdroplets. This extremely simple protocol circumvents any chemical modification or significant degradation of the IL and provides an extremely mild and cheap methodology to microstructure ILs.
Based on our obtained expertise in graphite intercalation compounds we early succeeded in accomplishing our preset goal to the synthesize and characterize alkali metal BP intercalation compounds (BPICs). By further investigations we established a fast and highly efficient thermal ball milling process for the bulk formation of sodium-intercalated BP. We also investigated a potential application of BPICs as a new class of compounds in catalyzing chemical reactions. Furthermore, we were able to develop a synthesis protocol for the generation of highly defined BP-intercalation compounds.
In a ground-breaking work we succeeded in using alkali metal BPICs as a direct starting material for a chemical bulk reductive covalent alkylation of thin-layer BP. For a detailed understanding of the underlying chemistry we investigated the in situ treatment of BPICs with potassium upon the addition of an electrophilic functionalization reagent followed by Raman spectroscopy. The alkali metal intercalation of BP turned out to be, as proposed, the most promising activation sequence for the covalent chemical functionalization of BP. We were able to use our expertise obtained in the field of the reductive intercalation and subsequent covalent functionalization of BP to functionalize carbon nano-onions (CNO) in a similar sequence. This approach is fully based on the scientific results obtained by the research within B-PhosphoChem and underlines the interconnection and transferability of the chemistry of layered materials.
Following these lines, we tried to adapt a highly versatile and extraordinary efficient covalent functionalization concept for graphene, namely the 2D-engineering of graphene by spatially resolved laser writing, for the covalent functionalization of BP. Our patterning protocol relies on the selective generation of highly reactive radicals upon laser irradiation, leading to a local functionalization of BP’s top basal plane. With this approach, several BP architectures could be created through well-defined and spatially guided photodecomposition of peroxide-based precursors. This opens the door for a controlled Janus-type functionalization of BP. Furthermore, we were able to construct van der Waals heterostructures consisting monolayer graphene and BP. The obtained Graphene/BP vertical heterojunction not only stabilizes the underlying BP, but it would also combine the electronic and optical properties of both systems.
Our research into the stabilization of individualized BP sheets by ILs lead to the finding that the addition of tiny amounts of long n-alkanes (intended as co-stabilization components) or water to conventional ILs spontaneously generates well-defined superficial alkane microdomains or water spherical microdroplets. This extremely simple protocol circumvents any chemical modification or significant degradation of the IL and provides an extremely mild and cheap methodology to microstructure ILs.
Based on our obtained expertise in graphite intercalation compounds we early succeeded in accomplishing our preset goal to the synthesize and characterize alkali metal BP intercalation compounds (BPICs). By further investigations we established a fast and highly efficient thermal ball milling process for the bulk formation of sodium-intercalated BP. We also investigated a potential application of BPICs as a new class of compounds in catalyzing chemical reactions. Furthermore, we were able to develop a synthesis protocol for the generation of highly defined BP-intercalation compounds.
In a ground-breaking work we succeeded in using alkali metal BPICs as a direct starting material for a chemical bulk reductive covalent alkylation of thin-layer BP. For a detailed understanding of the underlying chemistry we investigated the in situ treatment of BPICs with potassium upon the addition of an electrophilic functionalization reagent followed by Raman spectroscopy. The alkali metal intercalation of BP turned out to be, as proposed, the most promising activation sequence for the covalent chemical functionalization of BP. We were able to use our expertise obtained in the field of the reductive intercalation and subsequent covalent functionalization of BP to functionalize carbon nano-onions (CNO) in a similar sequence. This approach is fully based on the scientific results obtained by the research within B-PhosphoChem and underlines the interconnection and transferability of the chemistry of layered materials.
Following these lines, we tried to adapt a highly versatile and extraordinary efficient covalent functionalization concept for graphene, namely the 2D-engineering of graphene by spatially resolved laser writing, for the covalent functionalization of BP. Our patterning protocol relies on the selective generation of highly reactive radicals upon laser irradiation, leading to a local functionalization of BP’s top basal plane. With this approach, several BP architectures could be created through well-defined and spatially guided photodecomposition of peroxide-based precursors. This opens the door for a controlled Janus-type functionalization of BP. Furthermore, we were able to construct van der Waals heterostructures consisting monolayer graphene and BP. The obtained Graphene/BP vertical heterojunction not only stabilizes the underlying BP, but it would also combine the electronic and optical properties of both systems.
The fundamental success of the B-PhosphoChem project beyond the state of the art at the start of the project is nicely documented by up to now 17 publications in high-ranking scientific journals. Fundamental ground breaking results were, for instance, the successful alkali metal intercalation of BP and the proof of concept for the covalent functionalization based on the intercalation/activation of the crystalline starting material. Beyond that, we have also elaborated some fundamental discoveries, not proposed in the initial grant letter, which are: The access to monolayer BP by sequential wet-chemical surface oxidation, the catalytic activity of few-layer BP and alkyl metal intercalated BP, the 2Dchemical engineering of BP by spatially resolved laser writing of functional entities, and the construction of graphene/BP hetero-architectures.