Templating Quantum Dot to Phase-Transformed Electrospun TiO 2 Nanofibers for Enhanced Photo-Excited Electron Injection Yakup Aykut, † Carl D. Saquing, ‡,§ Behnam Pourdeyhimi, † Gregory N. Parsons, ‡ and Saad A. Khan* ,‡ † Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, North Carolina 27695-8301, United States ‡ Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States * S Supporting Information ABSTRACT: We report on the microstructural crystal phase transformation of electrospun TiO 2 nanofibers generated via sol-gel electrospinning technique, and the incorporation of as- synthesized CdSe quantum dots (QDs) to different phases of TiO 2 nanofibers (NFs) via bifunctional surface modification. The effect of different phases of TiO 2 on photo-excited electron injection from CdSe QDs to TiO 2 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 TiO 2 phases in the fibers. At a low calcination temperature of 400 o C only anatase TiO 2 nanofiber are obtained; with increasing calcination temperature (up to 500 o C) these anatase crystals became larger. Crystal transformation from the anatase to the rutile phase is observed above 500 o C, with most of the crystals transforming into the rutile phase at 800 o C. Bi-functional surface modification of calcined TiO 2 nanofibers with 3-mercaptopropionic acid (3-MPA) is used to incorporate as-synthesized CdSe QD nanoparticles on to TiO 2 nanofibers. Evidence of formation of CdSe/TiO 2 composite nanofibers is obtained from elemental analysis using Energy Dispersive X-ray spectroscopy (EDS) and TEM microscopy that reveal templated quantum dots on TiO 2 nanofibers. Photoluminescence emission intensities increase considerably with the addition of QDs to all TiO 2 nanofiber samples, with fibers containing small amount of rutile crystals with anatase crystals showing the most enhanced effect. KEYWORDS: quantum dot, sol-gel electrospinning, nanofibers, photoluminescence 1. INTRODUCTION Titanium dioxide (TiO 2 ) is a wide energy band-gap (anatase, ∼3.2 eV and rutile, ∼3.0 eV) photoactive semiconductor material that can absorb UV light. Its absorption band can be extended further into the visible region by incorporating narrow band gap dye molecules and semiconductor nanocrystals. 1,2 In addition, TiO 2 also possesses other desirable features, such as strong oxidizing power, nontoxicity, chemical and biological stability, photo induced hydrophilicity, high photoactivity, photodurability, catalytic properties as well as low cost, and good corrosion resistance in aqueous solutions. These attributes make TiO 2 a viable candidate for use in a variety of applications, including dye synthesized solar cells, photo- catalysis, photoluminescence, nonlinear optics, humidity and gas sensors, water cleavage, hydrogen and oxygen production from water molecules. 1-5 The optical and photocatalytic activities of TiO 2 as well as its morphologies and surface chemical and physical properties are affected by its crystal microstructure and intrinsic defects. 1-10 Park et al. reported that the rutile and anatase phases of TiO 2 have essentially the same open circuit photocurrent (V-oc), but the rutile-based cells have about 30% less short-circuit (V-sc) photocurrent than the cells made of the anatase-based TiO 2 . 6 Zhang et al. examined the photocatalytic activity of the anatase TiO 2 nanoparticles deposited on the surfaces of rutile particles and observed up to four times enhancement in the activity of these particles. 7 Abazovic et al. found that inherent defects such as oxygen vacancies affected the photoluminescence spectra of anatase and rutile TiO 2 nanoparticles. 10 The characteristic surface functional group (OH) of TiO 2 also plays a significant role in its photocatalytic activity. 8 Moreover, hydrophilicity and hydrophobicity of the surface of TiO 2 materials is affected by crystal morphology. 2 In general, the anatase phase of TiO 2 has been used for catalyst and supports, while rutile TiO 2 because of its high refractive index and dielectric constant, has been used mostly for electronic and optical purposes. 3 In this regard, different crystal phases of TiO 2 have been obtained from the same precursor by varying processing temperature. 3,9 However, the poor light absorption capability (depending on its intrinsically large energy band gap (∼3.2 eV)) limits the use of TiO 2 nanostructures in photovoltaic applications because of inefficient light absorption in the visible region. 2 To overcome Received: March 25, 2012 Accepted: July 16, 2012 Published: July 16, 2012 Research Article www.acsami.org © 2012 American Chemical Society 3837 dx.doi.org/10.1021/am300524a | ACS Appl. Mater. Interfaces 2012, 4, 3837-3845