Year

2018

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Surface and interface engineering is one of the most effective approaches in tuning semiconductors for their chemistry (energy, environment and catalysis) and device applications (electronic devices, and optical-electronic devices).

In this thesis, we show several approaches of modifying the catalytic and electronic properties of several semiconductors through manipulating their surface and interface. Techniques including scanning probe microscopies (SPM), electron microscopies, and electron spectroscopies, and, X-ray spectroscopies, were taken used to characterize the effect of surface and interface engineering, and the effect on their electronic properties. This thesis includes:

1. Oxygen vacancies (OV) engineering on TiO2 rutile(110) single crystal. By carefully controlling the annealing temperature and annealing time, OV defects can be precisely introduced into the single crystal. The evolution of the surface structure was investigated by in-situ low temperature STM. It was found that at 900 K, OV point defects isolating with each other exist on the (1 × 1) surface. In addition, OVs on the surface tended to be filled either water molecule or OHs. When the annealed temperature increased to 1300 K, (1 × 2) surface reconstruction dominated on the surface, in which cross-links defects caused by more oxygen deficiency were found.

2. Activating TiO2 for electrocatalysis by surface vacancy engineering. STEM techniques clarified that the good crystalline nature of reduced TiO2 with and the spatial distribution of OV in the surface and near-surface region. A mid-gap defect electronic state was created by the OVs, which can effective enhance the electric conductivity of the reduced TiO2. The reduced TiO2 exhibit tremendous enhancement in hydrogen evolution reaction (HER) due to the increased electric conductivity and amounts of active sites. Combing the in-situ observation of the water splitting reaction by STM and DFT calculations, subsurface oxygen vacancies and low coordinated Ti ions (Ti3+) demonstrated their roles in enhancing the electrical conductivity and promoting electron transfer and hydrogen desorption, which activate reduced TiO2 single crystal in the hydrogen evolution reaction in alkaline media. This study offers a rational route for developing reduced transition metal oxide for low-cost and highly active hydrogen evolution reaction catalysts, to realize over all water splitting in alkaline media.

3. The band structure of BiOBr 2D nanosheets was tuned by the strain for photocatalysis. The inner strain in the BiOBr nanosheets has been tuned continuously by controlled manipulating their shapes. The photocatalytic performance of BiOBr in dye degradation can be manipulated by the strain effect. The low-strain BiOBr nanosheets show improved photocatalytic activity. DFT suggest that strain can modify the band structure and symmetry in BiOBr. The enhanced photocatalytic activity in low-strain BiOBr nanosheets is due to improved charge separation attributable to a highly dispersive band structure with an indirect band gap.

4. Constructing two dimensional (2D) lateral pseudo-heterointerface by strain engineering in BiOBr nanosheets. Taking advantage of their strain-sensitive layer structure, 2D lateral pseudo-heterogeneous interface are realized in the singlecomponent BiOBr nanosheets by finely tune the pH value in synthesis. Due to the proper band alignment cross the interface, charge separation under visible light irradiation was enhanced, which was reflected by the photo-current measurements and the degradation experiments of pollutions. The strain engineering was demonstrated to be an effective way to tune the electronic structure of BiOBr and promote its efficiency in photoenergy conversion applications. In addition, the construction of the lateral pseudo-heterostructure through strain exhibit promising applications in building unprecedented 2D systems with exciting properties.

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