Defect Photoluminescence of TiO 2 Nanotubes
- PDF / 1,274,825 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 69 Downloads / 205 Views
1268-EE03-10
Defect Photoluminescence of TiO2 Nanotubes Candy C. Mercado and Jeanne L. McHale Washington State University, Materials Science Program and Department of Chemistry Pullman WA 99164, U.S.A.
ABSTRACT Photoluminescence (PL) spectroscopy of nanocrystalline TiO2 using ultraviolet light excitation reveals a range of intra-bandgap defect states which emit at visible wavelengths. In this study we use 350 nm excitation to probe the luminescent defect states of TiO2 nanotubes fabricated by anodization of titanium. The nanotubes show a broad visible luminescence from 450 to 700 nm with a peak at 520-550 nm or 2.4-2.3 eV. The intensity of nanotube PL is orders of magnitude lower than that of nanoparticulate anatase and P25 (mixed-phase anatase/rutile) films of comparable thickness. Similar to the PL of the nanoparticles, the nanotube PL is increased by vacuum annealing. The nature of the nanotube defects is investigated through shifts in the intensity and shape of the PL spectra in hole or electron scavenging environments. We find the PL intensity of the nanotubes to be less dependent on environment than that of conventional TiO2 nanoparticles. We conclude that there are two inter-related reasons for decreased intensity and decreased environmental dependence of PL from TiO2 nanotubes as compared to nanoparticles: decreased density of defect states and improved carrier transport. INTRODUCTION Titanium dioxide nanotubes formed on the surface of titanium foils after anodization exhibit a good vertical alignment and uniformity of dimensions.1-4 Because of the ordered structure, they are seen as an alternative5,6 to the TiO2 nanoparticles frequently used in dye-sensitized solar cells (DSSC). Their walls could provide a direct path for the electrons to travel thus minimizing recombination that occurs when the electrons encounter the electrolyte as they hop from one particle to another.6,7 Currently, DSSCs with nanotubular TiO2 reach up to 6.9% energy conversion efficiency and incident-photon-to-electron conversion efficiency (IPCE) of 80%.1 Nanotubes formed by anodization of Ti are initially amorphous but sintering above 280 o C transforms them into nanocrystalline anatase.8 The nanotubes are open on the top side and closed on the bottom and upon closer inspection each tube is isolated from the other except for small connections that may result from a vacancy condensation mechanism during the formation of the tubes.9 Transmission electron microscopy showed that nanotubes are composed mostly of (101) planes along the walls.10 Because the morphology of the nanotubes is different from that of TiO2 nanoparticles, the objective of this study is to understand the nature of defects and their distribution and to relate these to the device characteristics of the dye-sensitized solar cell. EXPERIMENTAL DETAILS The nanotubes were fabricated by anodization of titanium foils (Aldrich) in a twoelectrode cell following the procedure of Ref. 10, using 0.5% NH4F and 2% water in ethylene glycol as the electrolyte and anodization voltage
Data Loading...
