Synthesis and structural control of multidimensional metal nitride nanostructures

• Main Author: Li, Yun
• Other Authors: Tan Kwan Wee
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/170144
• Institution: Nanyang Technological University

Nanostructured metal nitrides are widely used in applications due to their attractive properties such as large surface area, high electrical conductivity, thermal stability and corrosion resistivity. However, conventional synthesis approaches typically require multiple processing steps and long heating treatments at high temperatures under hazardous ammonia environment. This thesis aims to develop direct one-pot synthesis strategies coupling transient laser heating with wet solution processing chemistry to form multidimensional metal nitrides of various morphologies, crystal phases, and physiochemical properties to broaden the functional property and application space. In the first part, a one-pot self-assembly approach combining a structure-directing block copolymer with metal oxoacetate and a newly designed urea-formaldehyde precursors to form 3D mesoporous ordered titanium oxynitrides is described. The urea-formaldehyde additive serves as an in situ nitrogen source to form the nitride phase. The synthesis preparation protocol is significantly simpler and only requires a single heating step under nitrogen to yield the 3D interconnected oxynitride structures with good electrochemical catalytic activity toward hydrogen evolution. To further improve synthesis efficiency, transient laser heating on millisecond timescales is coupled with solution-processable precursors to generate multicomponent inorganic nanoparticles. Transient laser heating of metal chloride and nitrate salts generating metal nanoparticles was first investigated as a control system to establish the transient structure formation mechanisms. Characteristic timescale analysis of laser-induced unary palladium metal nanoparticles for millisecond timescale suggests the nanoparticle growth is dominated by Brownian coagulation and Ostwald ripening mechanisms. The integrated approach was then extended to laser-induced formation of multimetallic (high entropy) alloy nanoparticles such as the AuPdFeCuNi combination. A laser-induced melt-crystallization mechanism was proposed to form the multiphasic high entropy metal nanoparticles through the control of laser-induced supercooling and atomic diffusion kinetics. Coupling transient laser heating with spatial X-ray diffraction provided a new platform for high throughput synthesis and characterization of novel high entropy inorganic nanoparticles. And finally, transient laser heating of a nitride-forming precursor made up of solution processable metal alkoxides and salts generated high entropy oxide and nitride nanoparticles on a polyacrylonitrile-derived carbon nanofiber substrate. In addition to act as a substrate, the nitrogen-rich carbon nanofiber serves as an in situ nitrogen source. The influences of laser power and heating dwell parameters were studied extensively to propose an improved structure formation mechanism based on the atomic diffusion kinetics and oxygen vacancies in the liquid and solid states of the nanoparticles, thereby yielding dense and hollow structured TiAlNbVCe-based oxynitride nanoparticles. These new synthesis strategies would allow versatile metal nitride nanostructure formation with precise control of morphology, crystal phases, compositions and physicochemical properties and may further accelerate the transition of novel materials to emerging advanced nanotechnology applications.