Synthesis And Properties Of CdS Sensitized Titania Nanotubes And Concave Palladium Nanocrystals

This dissertation is about one-step fabrication of CdS nanoparticle-sensitized TiO2nanotube arrays and their photoelectrochemical performance, and controllable synthesis of concave palladium nanocrystals and their electrocatalytic performance. There are five chapters in this dissertation:the first chapter is an introduction of fundamental concepts, and some recent advances on TiO2based photoelectrochemical cell and controllable synthesis of palladium nanocrystals; the second chapter is the one-step fabrication of CdS(Se, Te) nanoparticle-sensitized TiO2nanotube arrays via electrochemical deposition, and their photoelectrochemical performance; the third chapter is the controllable synthesis of concave nanocubes, right bipyramids, fivefold twinned nanorods of palladium, and their enhanced electrocatalytic performance; the fourth chapter is the controllable synthesis of concave Pd cuboctahedrons, their absorption spectra and electrocatalytic performance; the fifth chapter is the conclusion and prospect. The main content of chapter two to four is given below.1. In chapter two, the synthesis methods of narrow band gap semiconductor (focus on CdS) sensitized TiO2nanotubes are introduced generally. In our study, CdS nanoparticles sensitized TiO2(CdS-TiO2) nanotube arrays are synthesized with a facile one-step electrodeposition technique. In these composited nanostructures, CdS nanoparticles uniformly distribute in the TiO2nanotubes and partially embed in the shell of TiO2nanotubes. These structures effectively prevent CdS nanoparticles assembling or clogging the nanotubes and improve the contact area between CdS nanoparticles and the TiO2shells. Furthermore, the size and distribution density of CdS nanoparticles can be tuned easily by controlling the concentration of electrolyte. Coupling TiO2nanotubes with CdS nanoparticles extends the optical absorption from ultraviolet into the visible-light region up to580nm. An11-fold enhancement in photoelectrochemical activity is observed for CdS-TiO2nanotube arrays compared to plain TiO2nanotube arrays. This unique method is also suitable for the synthesis of other narrow band gap semiconductor sensitized TiO2nanotubes.2. In chapter three, the recent advances of concave noble metal nanocrystals are introduced. In this study, concave palladium nanocubes have been synthesized in high yield via a facile one-step wet chemical method using sodium ascorbate (NaA) as the reductant in an aqueous solution. This process allows independent control of the average edge length and the surface curvature of the nanocubes respectively. The particle morphology can be tuned by changing the reducing rate during the reaction. Right bipyramids and fivefold twinned nanorods with concave surfaces have also been synthesized with two reductants at the different stage or appropriate amount of ascorbate acid only. Remarkable enhancements in both electrocatalytic activity and stability are observed on concave Pd nanocubes and hybrid twinned nanocrystals over conventional Pd nanocrystals with flat surfaces and commercial Pd/C.3. In chapter four, the advances of noble metal nanocrystals (focus on palladium) synthesized by seed-mediated method are introduced. In our study, concave palladium cuboctahedrons have been synthesized by seed-mediated growth in an aquous solution. In the stage of seeds growth, the Pd cuboctahedrons with concave{100} faces can be synthesized using sodium ascorbate as the reductant, the Pd octahedrons with concave{100} and{110} faces can be synthesized using ascorbic acid as the reductant, and Pd cuboctahedrons with flat faces can be synthesized using ascorbic acid as the reductant in a low pH value condition. The UV-visible extinction spectra of Pd nanocrystals indicate that the concave structures change infinitesimally light absorption performance of Pd nanocrystals. The formic acid electro-oxidation measurements of Pd nanocrystals indicate that the concave structures enhance the electrocatalytic performance of Pd nanocrystals.