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Thesis Format

Alternative Format

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Song, Yang

2nd Supervisor

Sun, Xueliang

Co-Supervisor

3rd Supervisor

Sun, Xunhui

Affiliation

Soochow University

Co-Supervisor

Abstract

Black phosphorus (BP) is a promising material in many research fields. However, the transition process from amorphous red phosphorus (ARP) is elusive and hence hinders large scale synthesis and applications. This work describes the application of the high-pressure method to study the transition process from ARP to BP.

In this thesis, the following three objectives were achieved: (1) to understand the mechanism of the transition, (2) to facilitate the synthesis of BP by taking the advantage of less pure ARP, (3) to propose new methods of synthesizing BP-based materials, such as the moderately oxidized BP and the black phosphorus/ amorphous red phosphorus (BP/ARP) heterostructure.

The pressure-induced crystallization of ARP to BP was investigated by in-situ Raman spectroscopy and high-resolution transmission electron microscopy (TEM). Raman measurements revealed slow crystallization kinetics. TEM analyses provided information supporting the crystallization mechanism of structural rearrangement and oriented attachment of phosphorus atoms.

Under this mechanism, the impure ARP was demonstrated to be a suitable raw material with lower cost in synthesizing BP. According to energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS), no metal elements are chemically bonded with the phosphorus in either ARP or as-made BP, resulting in an unaffected transformation and providing a low cost alternative raw material for the scale-up synthesis of BP. Nevertheless, a moderately oxidized BP was first and successfully prepared using slightly oxidized ARP under high pressure as evidenced by the Raman and XPS results. Also according to the infrared spectroscopy and XPS, the phosphorus-oxygen bonds in both ARP and converted BP showed reversible changes after compression. This finding provides a new route to directly prepare BP with native oxidized phosphorus endowing unexplored features to the BP.

The BP/ARP heterostructure featuring very small nanocrystals and the well-defined crystalline regions of BP exhibit increased reactivity in visible-light-driven hydrogen production from water than pure ARP and BP, suggesting a new metal-free photocatalyst for producing hydrogen from water. Moreover, by applying pressure, we found that the rate of the photo-driven chemical reaction could be further increased in terms of hydrogen production and irradiation time.

Summary for Lay Audience

Black phosphorus, a material attracting increasing interest in electronic and optoelectronic devices is discovered by applying high pressure to white or red phosphorus. The mechanism of the transition from red to black phosphorus was elusive because of the amorphous structure of red phosphorus. We found that the transition is pressure-dependent and slow, due to the pressure-induced rearrangement and oriented attachment structural subunits. Under this mechanism, and because included metal elements in the impure red phosphorus do not chemically interact with phosphorus, we demonstrated the possibility of using impure red phosphorus to synthesize black phosphorus at a much lower cost. Partial oxidation can be beneficial to black phosphorus but post-treatment causes unnecessary material loss. Thus, we proposed an alternative method based on the transition mechanism using slightly oxidized red phosphorus. In the process of the conversion under pressure, a black phosphorus heterostructure was synthesized following the revealed crystallization mechanism. Comparing to pure black or red phosphorus, the heterostructure material exhibited the improved ability of light-driven hydrogen production that is a future energy resource. We also demonstrate that its hydrogen-producing rate can be further increased by applying pressure, suggesting a possible method to tune the efficiency of semiconducting photocatalysts for hydrogen production.

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