Controllable Preparation Of High Aspect Ratio TiO₂ Nanowire Arrays And Their Photoelectric Properties

Ti O2 nanowire arrays have been widely employed as the photoanodes of dye-sensitized solar cells(DSSC), due to their excellent photo-generated charge separation and transport properties. However, good performance of DSSC based on 1D Ti O2 nanostructures remains a challenge largely owing to their insufficient dye absorption resulting from the inferior specific surface area.In this paper, pre-treatment by an aqueous solution of 0.2 M Ti Cl4 was conducted on the FTO substrates so as to prepare a compact Ti O2 seed layer. High-aspect ratio single-crystalline rutile Ti O2 nanowire arrays were synthesized on the seeded FTO substrate through solvothermal process based on the reaction solvent of ethanol. Different ratios of ethanol-HCl and time-dependent experiments were performed to study the effects on morphologies and lengths of Ti O2 nanowire arrays.Furthermore, we report a novel and efficient one-step hydro/solvothermal method based on the mixed solution of ethanol-water to synthesize morphology-controllable Ti O2 nanowire arrays. Particularly, when the reaction process was performed at the ethanol-water-HCl ratio of 20:10:30 for 12 h, ultrahigh-aspect ratio Ti O2 nanowires with lengths up to 44 μm were obtained. The characteristics of the new films were investigated by SEM, TEM and XRD. As revealed from the analysis, the Ti O2 nanowire arrays with ultrahigh-aspect ratio were proved to be well single-crystallized tetragonal rutile phase and grow perpendicular to the FTO substrate. The 1D single crystal Ti O2 nanowire arrays possess excellent charge separation and transport properties as well as large internal surface areas. In order to investigate the formation process and growth mechanism of the individual Ti O2 nanowires, experiments based on different reaction time and HCl volumes were conducted as well. According to the research results, an appropriate ethanol-water-HCl volume ratio is a crucial driver on the formation of ultrahigh-aspect ratio Ti O2 nanowire arrays.Ti O2 nanowire array films prepared in different conditions were applied as photoanodes of DSSC. Some measurements such as J-V, IPCE and EIS on DSSC have been employed to detect the photoelectric characteristics of DSSC. The relationship between the structures of Ti O2 nanowire arrays and photoelectric properties of DSSC was extensively analyzed. The ultrahigh-aspect ratio Ti O2 nanowire arrays could greatly promote dye-loading ability, thus leading to higher short-circuit current density. However, congregating of abundant I-3 could result in severe electron recombination, which would affect the open-circuit voltage. Therefore, comprehensive analysis from the above conditions should be considered. Finally, the highest photon-to-electron conversion efficiency of 8.92% was achieved at the ethanol-water-HCl ratio of 20:10:30 for 12 h.