Highly ordered combined structure of anodic TiO2 nanotubes and TiO2 nanoparticles prepared by a novel route for dye-sensitized solar cells

Combined structure of anodic TiO2 nanotubes and TiO2 nanoparticles (TiNTs-TiNPs) has been synthesized by a facile combination of hydrothermal and chemical vapor deposition methods. Ordered TiO2 nanotubes with smooth walls were fabricated by two step anodization method in ethylene glycol containing NH4F at 50 V. This nanotubular array after annealing at 450 °C was subjected to the hydrothermally produced gaseous environment in an autoclave with diluted TiCl4 solution at its bottom. Vapors of TiCl4 were allowed to react chemically with water vapors for predefined time durations at 180 °C that resulted in the deposition of TiO2 nanoparticles on tubes’ surface and side walls. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that for one hour reaction duration, nanoparticles were evenly coated on the walls of nanotubes, whereas, longer durations tend to deteriorate the tubular structure. Consequently, the ordered TiNTs-TiNPs array produced after one hour coating has shown better performance for dye-sensitized solar cell DSSC) in back illumination mode with 130% increase in efficiency as compared to the device based on bare TiO2 nanotubes. The same photoanode has higher reflective properties with higher scattering ability. The solar cell based on this photoanode exhibits higher external quantum efficiency and effective charge transport properties. This study shows that porous ordered 1D structures based on TiO2 are of crucial importance for the high performance of DSSCs.