Thermal Conductivity Effects during Synthesis of Nanostructured TiO2 Thin Films

WOO-JIN AN (1), David D. Jiang (2), James R. Matthews (2), Nicholas F. Borrelli (2) and Pratim Biswas (1)

(1) Washington University in St. Louis, Saint Louis, (2) Corning Incorporated, Painted Post

     Abstract Number: 393
     Last modified: May 11, 2010

Abstract
The morphology of thin films is an important parameter and governs overall photocatalytic performance efficiencies. The metal oxide thin films with one-dimensional (1-D) orientation have shown better photocatalytic performance in comparison to other morphologies, such as uniform, dense or granular structures. The aerosol chemical vapor deposition (ACVD) process was developed to synthesize columnar metal oxide films with 1-D growth orientation. TiO$_2 nanostructured columns was grown on the ITO (indium doped tin oxide) substrate with a preferred crystal-phase orientation (112) at 500 $^(o)C with the residence time of 20 ms and the precursor feed rate of 2.84 micro-mol/min for 60 minutes. Heat conduction from the substrate decreased as the TiO$_2 columnar structures grew along the axial direction, resulting in lower temperatures and slower sintering rates. For a total deposition time of 120 minutes, the slower sintering rates caused the formation of branches from well-sintered TiO$_2 columns, rather than growth with a preferred orientation. To suppress the growth of such branches, or to enhance the sintering rates of the as-deposited particles, two process parameters were controlled. First, with the lower precursor feedrate (1.31 micro-mol/min), the growth of branched columns was inhibited. Second, at a higher substrate temperature of 600 $^(o)C, thicker TiO$_2 columns and fewer branched structures formed together. Furthermore, the as-deposited TiO$_2 film had mixed anatase and rutile crystal phases, with an anatase fraction of 0.78. Nanostructured TiO$_2 columns were also synthesized on different glass substrates: FTO (fluorine doped tin oxide) glass and the normal glass. On the FTO substrate that had the same thickness as the ITO substrate, nanostructured TiO$_2 columns grew with 1-D orientation. However, on the normal glass that was 0.25 cm thicker than the ITO and FTO substrates, the film morphology was an irregular rod-type shape. The temperature effects, which were induced by different conduction rates due to different thermal conductivities of the substrate, were determinant of the film morphologies, while the substrate materials appeared to have little effect on the film growth and morphology.