|Recently, Zhuge et. al. reported on the use of ZnO nanowires as templates to form TiO2 nanotubes via a solution-based process wherein the ZnO nanowires are conformally coated with TiO2 and then selectively etched through a separate chemistry. With nanowires as a template, the authors synthesized a hierarchical double-shell architecture for the TiO2 nanotube in order to provide a highly mechanically stable, high electron conductivity structure on the core of the nanotube, with a high surface area amorphous material as the shell to enhance photoabsorption.|
Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network
- Zhuge F, Qiu J, Li X, Gao X, Gan X, and Yu W. 2011. Toward Hierarchical TiO2 Nanotube Arrays for Efficient Dye-Sensitized Solar Cells. Advanced Materials Early View 27 January 2011. doi: 10.1002/adma.201003902.
Dye sensitized solar cells (DSSCs) provide a low-cost competitor to high efficiency silicon photovoltaics due to the potential for achieving effective photon harvesting with high internal charge collection efficiency facilitated by the sensitized titanium dioxide (TiO2) nanoparticle photoanode structure. While impressive external cell efficiencies have been reported in the laboratory, standard approaches using random networks of sensitized TiO2 nanoparticles require a tradeoff between optical absorption and carrier recombination that ultimately limits the performance. As a result, a good deal of research has focused on the development of ordered one-dimensional TiO2 nanostructures as a means to simultaneously enable effective photon absorption, efficient charge collection, and fluent dispersion of dye sensitizer within a three dimensional (3D), high surface area electrode network. TiO2 nanotubes formed from anodized titanium foils demonstrate reasonable performance yet suffer from severe optical energy losses in the counter electrode and electrolyte layers due to a back-illumination geometry requirement. In order to exploit the potential advantages of such 3D architectures, vertically aligned TiO2 nanotubes would ideally be synthesized on transparent conductive oxide (TCO) films as the photoanode, thereby optimizing optical absorption within the active region of the cell. Furthermore, to enhance absorption of near infrared components of the solar spectrum, longer optical paths or increased light scattering within the anode structure are desired.
Hybrid anode structures such as TiO2 nanotubes grown on zinc oxide (ZnO) films have demonstrated reasonable efficiencies, yet are extremely difficult to scale due to cracking and delamination of the hybrid films. The result is limited aspect ratio of the nanotube structure and poor adhesion to the TCO seed layer. Recently, Zhuge et. al. reported on the use of ZnO nanowires as templates to form TiO2 nanotubes via a solution-based process wherein the ZnO nanowires are conformally coated with TiO2 and then selectively etched through a separate chemistry. With nanowires as a template, the authors synthesized a hierarchical double-shell architecture for the TiO2 nanotube in order to provide a highly mechanically stable, high electron conductivity structure on the core of the nanotube, with a high surface area amorphous material as the shell to enhance photoabsorption.
ZnO nanowires were grown on a seed layer prepared on a glass substrate in a solution of ZnNO3 (0.025 M), hexamethylenetetramine (0.025 M), and poly(ethyleneimine) (0.005 M) at 85°C, providing a growth rate of 0.4 µm/h. The nanowires were grown to ~25 µm in length with a nominal diameter of 200 nm. The ZnO nanowires were sequentially dipped in a TiO2 sol consisting of tetrabutyl titanate (0.2-0.5 M) in ethanol modified with acetylacetone. Using a layer-by-layer method, the nanowire array was dipped in the sol for 30 s cycle durations which coated ~1-2 nm/cycle. The authors employed 20 coating cycles to obtain the desired coating of ~20-40 nm. The resulting hybrid core-shell nanowire was fast-calcined at 350° to densify any aggregations of the TiO2, followed by etching in TiCl4 (10 mM) to remove the ZnO core. The resulting TiO2 nanotube was further calcined at 500°C to crystallize the structure. An amorphous TiO2 shell was then coated over the crystalline nanotube using the same LBL procedure followed by hydrothermal crystallization in distilled water at 180°C. The resulting hierarchical core-shell nanotube incorporated a high quality TiO2 core supporting an enhanced surface area shell structure.
Experimental DSSCs were prepared by first calcining the nanotube arrays at 500°C for 1 hour, then sensitizing them in N719 (0.5 mM) in ethanol for 12 hours. Using a platinum coated glass slide as the counter electrode, 0.25 cm2 cells were formed using a plastic spacer seal between the electrodes, then filling the volume with liquid electrolyte for experimental characterization of the anode structure. Using a standard AM 1.5 solar simulator, the cells exhibited VOC=0.83 V, at short circuit current density JSC=9.9 mA/cm2, with an external efficiency of 5.74% and fill factor of FF=70%. The current density and conversion efficiency is ~30% greater in comparison to TiO2 nanotubes without the double shell architecture, thereby verifying the enhancement provided by the high surface area amorphous outer layer. Additionally, the high open circuit voltage and fill factor confirm the effective charge collection and transport properties provided by the crystalline TiO2 nanotubes. Thus a solution-based coating process has been reported incorporating nanowire templates to reproducibly synthesize core-shell nanotube structures. Further efforts must investigate increased efficiency for the final cell designs, as well as exploring these custom nanostructures to other areas of application including photocatalysis and sensors, as well as combing with other oxide materials for hybrids nanostructure synthesis.
Image reproduced from Zhuge F, et al. 2011. Toward Hierarchical TiO 2 Nanotube Arrays for Efficient Dye-Sensitized Solar Cells. Advanced Materials Early View 27 January 2011. doi: 10.1002/adma.201003902.Permission pending.