Templating Quantum Dot to Phase-Transformed Electrospun TiO₂ Nanofibers for Enhanced Photo-Excited Electron Injection

Abstract

We report on the microstructural crystal phase transformation of electrospun TiO₂ nanofibers generated via sol–gel electrospinning technique, and the incorporation of as-synthesized CdSe quantum dots (QDs) to different phases of TiO₂ nanofibers (NFs) via bifunctional surface modification. The effect of different phases of TiO₂ on photo-excited electron injection from CdSe QDs to TiO₂ NFs, as measured by photoluminescence spectroscopy (PL) is also discussed. Nanofiber diameter and crystal structures are dramatically affected by different calcination temperatures due to removal of polymer carrier, conversion of ceramic precursor into ceramic nanofibers, and formation of different TiO₂ phases in the fibers. At a low calcination temperature of 400 ^oC only anatase TiO₂ nanofiber are obtained; with increasing calcination temperature (up to 500 ^oC) these anatase crystals became larger. Crystal transformation from the anatase to the rutile phase is observed above 500^oC, with most of the crystals transforming into the rutile phase at 800^oC. Bi-functional surface modification of calcined TiO₂ nanofibers with 3-mercaptopropionic acid (3-MPA) is used to incorporate as-synthesized CdSe QD nanoparticles on to TiO₂ nanofibers. Evidence of formation of CdSe/TiO₂ composite nanofibers is obtained from elemental analysis using Energy Dispersive X-ray spectroscopy (EDS) and TEM microscopy that reveal templated quantum dots on TiO₂ nanofibers. Photoluminescence emission intensities increase considerably with the addition of QDs to all TiO₂ nanofiber samples, with fibers containing small amount of rutile crystals with anatase crystals showing the most enhanced effect