Expressway to partially oxidized phosphorene

Few-layer black phosphorus or phosphorene is an intriguing and important 2D material. It is a single-layer material that consists of corrugated and condensed six-membered phosphorus rings (Fig. 1). Each phosphorene layer can be weakly bonded to neighboring ones by van der Waals-like interactions to form few-layer arrangements, which are also called phosphorene in many publications. Finally, attaching many phosphorene layers to each other ends up as orthorhombic black phosphorus, a long-known phosphorus allotrope. There are two different few-layer 2D materials of phosphorus known to date, which can be derived from the element structures of orthorhombic black phosphorus (1) or gray arsenic (2). The latter is discussed as blue phosphorus in the literature (3). Since 2014, when few-layer black phosphorus was first prepared by a top-down approach from black phosphorus and used as field effect transistors (4–6), a continuously increasing interest can be observed for this 2D material. Recently, an elegant, well-defined, and controllable bottom-up route for partially oxidized phosphorene (po-phosphorene) was reported (7). In a one-pot and also low-cost process, molecular highly reactive white phosphorus acts as starting material and ethylene diamine serves as a solvent and reaction promotor (4). In a low-temperature reaction at 100 °C, po-phosphorene can be achieved on a gram scale. Due to traces of oxygen present in the solvent or during the synthesis process, the target material is partially oxidized on the surface. This aspect may look like a disadvantage at first glance, but it has turned out to be beneficial for the long-term air and moisture stability of the final product. The report in PNAS by Tian et al. (7) on the first bottom-up synthesis of well-defined po-phosphorene is a major breakthrough in synthetic chemistry. It paves the way for the large-scale application of po-phosphorene and phosphorene in the near future.

Fig. 1.

Reaction scheme of the traditional two-step, top-down approach and the new one-step, bottom-up synthesis approach of phosphorene. The two-step process contains the synthesis of black phosphorus by high pressure or gas phase reactions, followed by mechanical, chemical, or electrochemical exfoliation. In the innovative new one-step process, white phosphorus is directly transformed to phosphorene in a solvent-based one-pot synthesis.

The reader may think about a certain and somehow obvious similarity between the element chemistry of phosphorus and carbon with regard to their 2D monolayer materials. It seems to be the case that both allotropes tend to form new compounds with oxygen and water. The two sets of materials, black phosphorus/phosphorene and po-phosphorene, might be expected to be similar to graphite/graphene and graphene oxide, but there are significant differences in their structural and physical properties.

As a relatively new element allotrope, pure (and nonoxidized) phosphorene has so far been prepared by top-down approaches from orthorhombic black phosphorus, mainly via mechanical exfoliation or chemically based procedures in various solvents (8–10). Contact with air and moisture must be avoided to prevent the oxidation of phosphorus to phosphorus oxides on the surface. This relatively complex and nonscalable synthesis route and its intrinsic reactivity limit the use of phosphorene in a reasonable number of processes and applications. In the top-down approach, a multistep reaction cascade has to be followed for the synthesis of phosphorene. Starting with a high-pressure (11) or chemical transport process (12) for the fabrication of the precursor phase black phosphorus, a time- and energy-consuming exfoliation step to the few- or monolayer material must be added. In contrast, the bottom-up route to po-phosphorene is rather elegant. The beauty and impact of the new and easy bottom-up synthesis route is its simplicity and scalability. In a straightforward solution-based reaction, the final product po-phosphorene is realized in a solvothermal process at low temperature from a single precursor, white phosphorus. This makes the fabrication process feasible and fascinating for many people interested in this topic. An exponentially increasing large number of phosphorene studies that deal with the prediction and verification of stimulating properties and applications have emerged in the past 4 y.

In contrast to graphene, which is a metal in pure form, few-layer black phosphorus or phosphorene is a semiconductor with a variable band gap (dependent on the number of layers). It is the variability of the electronic structure that defines most of its properties. This semiconductor behavior of phosphorene is the origin for a series of applications like transistors (4–6), sensors (13, 14), or photocatalysts for water-splitting purposes (15, 16), just to name a few. A major drawback of pure phosphorene in such applications is its reactivity against oxygen and moist air, which limits its use in open-to-atmosphere setups (17). Phosphorene will react with oxygen and water in a light-driven reaction to P₄O₁₀, which is afterward hydrolyzed to phosphoric acid. In a quantum chemical study, many possible reaction products of phosphorene and oxygen were discussed and the resulting po-phosphorene species were evaluated and ranked in terms of their stability, structural features, and properties (18). The most probable approximants for po-phosphorene that preserve the phosphorene structure are P_xO suboxides with x = 8, 6, 4, and 2. Here, P = O bond fragments in the form of dangling bonds are realized. If the oxygen content increases further, the amount of covalently bonded P-P units (phosphorene fully consists of P-P units) is significantly reduced and P-O-P bridging motifs become more and more prominent. The calculated electronic structures of those compounds and the positions of the valence and conduction band edges suggest that water splitting and ferroelectricity might be present. Po-phosphorene, as reported by Tian et al. (7), showed such P = O and P-O-P subunits in X-ray photoemission spectroscopy spectra, and it delivered the experimental proof of these predicted properties.

Tian et al. (7) show with their bottom-up approach to po-phosphorene, and during their following characterization process, that many properties of pure phosphorene can be preserved in po-phosphorene. The partial oxidation, or surface functionalization with oxygen-containing species (some may call this impurity, but that is not applicable here), is beneficial for an application with direct use in contact with water. It was substantiated in a well-selected series of experiments that po-phosphorene and platinum-decorated po-phosphorene can effectively be used as hydrogen evolution catalysts. The positions of the valence band and conduction band edges of po-phosphorene are suitable to catalyze the hydrogen evolution reaction in water. As a noble metal-free catalyst, it outperforms C₃N₄ nanosheets by a factor of 24 at pH 6.8. Po-phosphorene is characterized by an ideal band gap of 1.19 eV capable of effectively absorbing energy in the visible light spectrum maximum. The latter-mentioned Pt-decorated po-phosphorene catalyst showed reasonable activity in the ultraviolet light region and in the visible light region above a wavelength of 600 nm. It is predictable that po-phosphorene will play a crucial role in all scientific fields, where its high moisture and oxygen stability compared with pure phosphorene are of benefit. Luckily, po-phosphorene is thermally stable up to several hundred degrees Celsius, which qualifies it for room temperature and moderate high-temperature use. A recent density functional theory study clearly illustrates that po-phosphorene can show piezoelectricity and might be useful as miniaturized sensors and for energy conversion devices (19). With the discovery of this fast, reliable, and straightforward synthesis process for po-phosphorene, it is only a matter of time and effort before many new and exciting properties will be realized and reported.

There is a strong need for ongoing optimization of the synthesis process reported by Tian et al. (7) toward a more oxygen-free phosphorene product. Needless to say, such a bottom-up process will become the standard procedure for the fabrication of phosphorene. The main reason will be the control and the sharp particle size distribution that can be addressed more effectively in a bottom-up approach than in a top-down approach. Besides the easy access to the common large-area 2D material, the preparation of defined nanoscaled ribbons, belts, or chains becomes realizable. Therefore, it is highly probable that the well-established top-down access to phosphorene and po-phosphorene may be completely replaced by the bottom-up one in the future. Most of the early work reported in the literature on pure phosphorene did not take account of the air and moisture sensitivity, and a significant number of reports claim to be based on phosphorene rather than the oxidation product, po-phosphorene. With the easy access to well-defined po-phosphorene, the whole field of research covering low-dimensional phosphorus-based materials will be (re)vitalized.