Preparation of PVDF/TiO2 core/shell nanofibrous membranes and investigation of piezo-potential effect on the photocatalytic performance

Piezo-photocatalysis, combining piezoelectric effect with photocatalysis, is considered as a novel strategy to improve photocatalytic efficiency. A piezo-potential is generated on a piezoelectric material under strain, which accelerates the movement of photogenerated electrons and inhibits the recombination between photogenerated charge carriers. In addition, the piezoelectric field as a kind of internal field is not easy to be screened by the chargers and has an infinite promoting effect on photocatalysis due to the oscillatory external strain. The purpose of this thesis is to investigate the effect of piezo-potential on photocatalytic performance of hybrid PVDF/TiO2 core/shell nanofiber membrane. The thesis is divided into three parts: introduction, investigations, and conclusions, as shown in the graphical abstract. The investigations include altering the piezoelectric properties of electrospun PVDF nanofiber membrane, preparing the PVDF/TiO2 core/shell nanofiber membranes, for which three methods have been applied, and piezo-photocatalytic activities of the PVDF/TiO2 core/shell nanofiber membrane. In chapter 1, photocatalysis and piezoelectricity have been introduced first to build a background from fundamentals and challenges to applications. Then, the effect of the built-in potential of piezo-photocatalyst on photocatalytic activity has been illustrated comprehensively, and the development of piezo-photocatalysts from integrated to hybrid piezo-photocatalysts has been introduced. In the end, a hybrid PVDF-TiO2 core-shell nanofiber membrane has been proposed as a piezo-photocatalyst. Sample preparation techniques and main characterizations used in the thesis are introduced in chapter 2. The piezoelectric properties of PVDF cast films have been intensively studied, indicating that solvents with higher dipole moments can endow PVDF cast films with higher piezoelectric properties. And the effects of solvent and electrospinning parameter on morphology and piezoelectric property of PVDF nanofibrous membranes are studied, as detailed in chapter 3. Hydrothermal treatment is a convenient method to grow TiO2 semiconductor, but traditional hydrothermal treatment always requests high temperature and long treatment time to obtain TiO2 with good morphology and high crystallinity, which could damage PVDF membrane. Chapter 4 has introduced a PVDF/TiO2 core/shell composite nanofibrous membrane (CNM), which is obtained from the microwave-assisted hydrothermal treatment of an electrospun PVDF membrane. The effects of hydrothermal process parameters (solution, heating temperature, and treatment time) on the structure (morphology, crystal, etc.) and photocatalytic properties of PVDF/TiO2 CNM have been investigated. To achieve high photocatalytic efficiency of TiO2 and avoid thermal damage to PVDF, commercial TiO2 (80 % anatase and 20 % rutile), P25, is applied to prepare PVDF/TiO2 CNM. In chapter 5, the PVDF/TiO2 core/shell nanofiber membrane has been prepared by coaxial electrospinning adopting PVDF solution and TiO2 suspension as core and shell feeds. The effects of coaxial electrospinning parameters (solvent and TiO2 concentration in shell solution, as well as the feed rates of PVDF and TiO2 solutions) on the structure of PVDF/TiO2 CNMs have been investigated. In chapter 6, atomic layer deposition (ALD) is applied to grow a uniform and controllable TiO2 layer on the surface of PVDF NFs, and post-treatment annealing is employed to crystalize TiO2. The parameters of ALD and annealing treatment have been studied to obtain the PVDF/TiO2 CNM with the optimal structure, and the effect of TiO2 thickness on the photocatalytic activity of PVDF/TiO2 CNM is investigated. Three PVDF/TiO2 core/shell nanofiber membranes prepared via three methods are compared in chapter 7. The PVDF/TiO2 core/shell nanofiber membrane prepared by coaxial electrospinning has the best photocatalytic efficiency and is used to investigate