Totally Tubular TiO2 Nanotubes as Semiconducting Material for Dye-Sensitized Photoelectrodes
Authors: Chiong Joseph Daniel , Nguyen Nhat Truong , Majewski Marek B 
Published: August 2024

The use of chemical bonds to store solar energy is a promising approach to address the intermittency of the sun by harnessing sunlight to drive chemical transformations. One strategy to drive these processes is to use a bespoke semiconductor surface implemented in a photoelectrode to absorb light and generate charge carriers that can drive redox reactions such as the direct oxidation of adsorbed species on the photoanode. To minimize recombination of photogenerated electron-hole pairs, molecular donor-chromophore-acceptor (D-C-A) systems can be used to sensitize wide bandgap semiconductors for visible light absorption and to generate (high energy) long-living charge separated states. Such assemblies represent a photoelectrode for dye-sensitized photoelectrochemical cells (DS-PECs). Presented here are recent findings using molecularly sensitized TiO2 nanotubes as a nanohybrid architecture for DS-PECs. The highly ordered and oriented 1D-nanostructures are purpose-grown using a self-organizing electrochemical anodization approach. The nanotube mouth is functionalized with a Cu(I)-based donor-chromophore-acceptor system by direct assembly at the surface. The resulting photoanodes are investigated against different water oxidation catalysts (e.g., [Cp*Ir(pyalk)OH], pyalk = 2-(pyridine-2-yl)propan-2-ol; CoOx) for their ability to generate oxidizing equivalents that can subsequently drive follow-on oxidation reactions. A more common TiO2 nanostructure morphology (e.g., nanospheres) and simpler Cu(I)-based chromophores are also examined for comparative purposes to highlight the advantages of the proposed assembly.
It is well understood that the role of the semiconductor is to connect the molecular components and transfer/store charge carriers from the excited dye to drive redox processes. The TiO2 nanotubes used here are synthesized by self-organized electrochemical anodization (SOA); a process Involving an anode/cathode arrangement where Ti is used as the anode. This process is a balance between the electrochemical formation of TiO2 and simultaneous chemical dissolution in fluoride-containing electrolytes. The resulting semiconducting film is found to be a vertically oriented, size-controlled, and back-contacted nanostructured array of TiO2 on a foil of titanium. In addition to this composition inheriting high chemical stability, the morphology can grant added benefits such as two-dimensional confinement, directional charge transport and orthogonal charge separation.[1]
A crucial limiting factor for the performance of this system is the recombination of the electron-hole pairs. By using a transition metal complex or chromophore (C) that is covalently linked with electron donating (D) and electron accepting (A) moieties, the energy levels can be aligned for charge-transfer processes between the electroactive groups for directional electron transport and formation of a charge-separated state.[2] Copper(I) D-C-A assemblies are also of interest when taking into consideration the low-cost and photophysical properties when compared to typical ruthenium(II) polypyridine chromophores used in these types of cells. Transient photocurrent measurements of the photoelectrode suggest that the TiO2 nanotubes’ mouths generate a base photocurrent, which could be associated with graphitic-like carbon impurities extending light absorption for photoactivity. Furthermore, once the surface is decorated with the copper(I) D-C-A assembly, the photocurrent is significantly increased, enabling light-driven oxidation of CoOx photoelectrochemically deposited on the surface of the photoelectrode.
REFERENCES
[1] Nguyen, N. T.; Altomare, M.; Yoo, J. E.; Taccardi, N.; Schmuki, P. Noble Metals on Anodic TiO2 Nanotube Mouths: Thermal Dewetting of Minimal Pt Co-Catalyst Loading Leads to Significantly Enhanced Photocatalytic H2 Generation. Adv. Energy Mater.
2016, 6, 1501926. DOI: 10.1002/aenm.201501926
[2] Singh, Z.; Kamal, S.; Majewski, M. B. Copper(I) Donor−Chromophore−Acceptor Assembly for Light-Driven Oxidation on a Zinc Oxide Nanowire Electrode. J. Phys. Chem. C.
2022, 126, 39, 16732-16743. DOI: 10.1021/acs.jpcc.2c04652

Figure 1