Template-directed liquid ALD growth of TiO₂ nanotube arrays: Properties and potential in photovoltaic devices

Dense and well-aligned arrays of TiO₂ nanotubes extending from various substrates are successfully fabricated via a new liquid-phase atomic layer deposition (LALD) in nanoporous anodic alumina (AAO) templates followed by alumina dissolution. The facile and versatile process circumvents the need for vacuum conditions critical in traditional gas-phase ALD and yet confers ALD-like deposition rates of 1.6-2.2 Å cycle^-1, rendering smooth conformal nanotube walls that surpass those achievable by sol-gel and Ti-anodizing techniques. The nanotube dimensions can be tuned, with most robust structures being 150-400 nm tall, 60-70 nm in diameter with 5-20 nm thick walls. The viability of TiO₂ nanotube arrays deposited on indium tin oxide (ITO)-glass electrodes for application in model hybrid poly(3-hexylthiophene) (P3HT):TiO₂ solar cells is studied. The results achieved provide platforms and research directions for further advancements.