Controllable synthesis of transition metal nitride materials and performance tuning in photocatalysis and piezoelectric catalysis

Abstract

Energy crisis has become imminent owing to the increasing energy demand in recent years. Renewable and sustainable alternative energy resources, such as hydrogen have attracted significant attentions. In order to produce hydrogen energy efficiently, artificial photocatalysis is an attractive way forward. Use of piezoelectric materials for solar energy conversion is being researched in this context. However, such materials that are semiconductor-based are not practically viable applications. Simultaneously, as a kind of most efficient catalyst, the noble metal is difficult to apply in large-scale. Transition metal nitrides (TMNs) have become a family of alternatives of noble metal catalysts due to their unique properties. However, the present synthesis methodologies of TMNs are complex with severe conditions. Maintenance of morphology of TMNs also remains a challenge, since morphological changes do influence their activity. This thesis has provided a new strategy for production of TMNs with stable nano-morphology. These have been used for photocatalytic and piezoelectric H2 evolution, while discussing the structure-activity relationship. The details are listed below: (1) A novel hard template/rapid-nitridation synthesis of ordered mesoporous metal nitrides is reported, which is based on a nanocasting-thermal nitridation process. This method uses 2D ordered hexagonal mesoporous SBA-15 as the hard template. A series of TMNs with ordered and regular mesoporous structures have been successfully synthesized from the corresponding mesoporous oxides. A comparative experiment shows that when a long-time heating process is employed, the ordered mesopores are hard to maintain anymore due to the collapse and coalescence of the porous structure. This proves the necessity of rapid-nitridation for keeping nanostructures of materials. (2) Prussian blue (PB) precursors are first oxidized and then subjected to rapid-nitridation to obtain pure porous Fe2N nanocubes while maintaining the pattern and structure of the parent MOF precursor. The samples are sensitized using Eosin-Y (EY) in photocatalytic HER. The performance of cube-like Fe2N is duly rationalized using DFT-based calculations and its metallic nature is also duly elaborated. The optimal Fe2N/EY system exhibits excellent photocatalytic hydrogen evolution performance. (3) MOF-derived Fe2N with different types of doped elements (Co, Cr, W and V) are successfully synthesized. Rapid-nitridation has been applied as a synthesized method, which is efficient for retaining the morphology of catalyst. The regular nanocubic pattern of samples has been maintained. The doped Fe2N have been utilized for piezoelectric catalytic H2 evolution and the Co-doped Fe2N has achieved the highest activity (122.8 μmol g-1h-1). After tuning the doping ratio, the Co and V doped Fe2N have greatly improved their H2 evolution performance. Compared with the control experiments, the sample with nanocubic morphology has a higher H2 evolution rate than that of the nanoparticle sample. These doped Fe2N catalysts also have the ability to degrade different types of dye, while improving their H2 production efficiency. Besides, the DFT calculation has elucidated the activity improvement of doped nitrides. In summary, this thesis has introduced a method for synthesizing several types of ordered porous TMN materials. They have exhibited considerable activity and properties in the application of H2 evolution by photocatalysis and piezoelectric catalysis. The relationship between the performance and the morphology as well as nanostructure has been demonstrated.

Date of Award	11 Jul 2021
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Tao Wu (Supervisor), Minghui Yang (Supervisor) & Mike George (Supervisor)