Structural and Raman Spectroscopic Characterization of C-TiO2 Nanotubes Synthesized by a Template-Assisted Sol-Gel Technique

Raymond Taziwa, Edson Meyer an

Abstract

It is of great importance to find ways of improving the charge transport within titanium dioxide nanotubes (TNTs) which will in turn reduces electron-hole pair recombination and subsequently improves the cell efficiency of the dye sensitized solar cells (DSSC). High electron mobility in TNTs relies on the morphology, surface defects and consistence in the crystalline composition. These factors are closely related to the preparation conditions. In this regard, this work focuses on fabrication and structural characterization of carbon doped titanium dioxide nanotubes (C-TNTs). Un-doped and C-TNTs were synthesized using a template-assisted sol-gel technique employing titanium tetra butoxide precursor and oxalic acid as the dopant. SEM, XRD, and Confocal Raman spectroscopy (CRS) was exploited to evaluate the morphological and structural properties of the TNTs. SEM analysis has revealed the presence of closely-packed TNTs. SEM analysis has also shown that the TNTs become loosely-packed with increasing dopant concentration. SEM-EDX spectra has revealed the presence of Ti peaks at 0.45 and 4.9 keV corresponding to Kα1 and Kβ1 emission line respectively. Oxygen exhibits a signal at 0.5 keV corresponding to Kα1 emission line. The existence of these peaks in the EDX spectra validates the presence of Titanium and Oxygen atoms in the as prepared TNTs samples. XRD analysis has exposed the existence of a mixed Anatase-Brookite phase with diffraction peaks at 2θ angles of 25.49⁰, 38.11⁰, 40.60º 48.14 ⁰, 54.58⁰, 63.00⁰, 70.11⁰ and 75.66⁰. Additionally, XRD analysis has revealed elongation of lattice parameter “c” from 9.143 to 9.830 Å with increase carbon dopant concentration. Lattice expansion indicates the possibility of carbon substituting oxygen sites. CRS large area scan in the XY direction has revealed the presence of Raman modes at 153.78 cm-1 (Eg-A), 209.08 cm-1 (Eg-A), 331.04 cm-1 (B1g-B), 403.74 cm-1 (B1g-A), 527.16 cm-1 (A1g-A), 541.83 cm-1 (B1g-A) and 644.59 cm-1(Eg-A) belonging to a mixed Anatase-Brookite phase. Moreover, CRS large area scans has exposed the existence of a Rutile phase with Raman modes at 150.64.cm-1 (B1g-R), 252.14 cm-1(mutli-phonon process), 447.42 cm-1 (Eg-R) and 616.09 cm-1 (A1g-R) belonging to the Rutile phase of TiO2. CRS depth profiling in the XZ direction has also validated the presence of a mixed Anatase-Brookite phase at Raman Active modes 153.19 cm-1, 208.87 cm-1, 404.55 cm-1, 523.26 cm-1 and 648.55 cm-1.