Year

2020

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Two-dimensional (2D) vertical heterostructures composed of stacked layered semiconductors have attracted intensive interest due to their unique structural and electronic characteristics and their wide range of potential applications in environmental remediation and energy conversion. It remains a great challenge to fabricate such heterostructures based on conventional semiconductors; however, as they either do not possess a 2D layered structure or are not suitable for epitaxial growth due to a large lattice mismatch. In this thesis, 2D vertical heterostructures with highly efficient photocatalytic performances are rationally designed and experimentally constructed via the integration of conventional layered photocatalysts. Taking 2D Bi-based layered semiconductors as examples, the facile anion exchange and one-step surfactant self-assembly approaches for the synthesis of 2D epitaxial vertical heterostructures were explored and developed, and their formation mechanisms and key factors for the enhanced photocatalytic activities were further revealed. The details are as follows:

1. To take full advantage of the promising ion exchange reaction to build 2D vertical heterostructures, it is imperative to develop a generalized synthetic strategy. A universal approach is demonstrated for the construction of various 2D Bi‐based vertical heterostructures suitable for mass production via surface anion exchange. The possible formation mechanism is further revealed. The morphologies of products are dependent on the basic sizes and shapes of the initial 2D Bi-based substrates. Their significantly enhanced catalytic performances mainly stem from the modification of surface active sites and increased charge separation.

2. Vertical heterostructure composed of bismuth oxyhalide semiconductors and bismuth tungstate fabricated through a heteroepitaxial anion exchange method are reported. Monolayer Bi2WO6 is epitaxially grown on the exposed surface of BiOI to inhibit photocorrosion and introduce active sites. Theoretical and experimental results reveal that electrons generated under visible‐light irradiation can directly transfer to surface coordinatively unsaturated (CUS) Bi atoms, which contribute to the adsorption and activation of reactant molecules. As a result, the Bi2WO6/BiOI vertical heterostructures exhibit significantly enhanced visible‐light‐driven NO oxidation activity compared with BiOI and Bi2WO6.

3. It has been challenging to develop facile synthetic routes of atomically thin 2D layered vertical heterostructures. Herein, the growth of 2D Bi-based vertical heterostructures consisting of stacked atomically thin BiOBr and Bi2MoO6 layers synthesized by a one-step self-assembly approach is demonstrated. The significantly enhanced photocatalytic performance of Bi-Mo-Br for NO oxidation can be mainly attributed to their fine-tuned exposed Bi active sites, efficient charge separation and transfer, and extended visible-light absorbance. In addition, the synthetic strategy can be extended to produce a series of 2D atomically thin Bi-based vertical heterostructures.

FoR codes (2008)

0912 MATERIALS ENGINEERING, 1007 NANOTECHNOLOGY

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.