Photoelectrochemical properties of TiO₂ nanowire arrays : A study of the dependence on length and atomic layer deposition coating

We report that the length and surface properties of TiO ₂ nanowires can have a dramatic effect on their photoelectrochemical properties. To study the length dependence, rutile TiO ₂ nanowires (0.28-1.8 μm) were grown on FTO substrates with different reaction times (50-180 min) using a hydrothermal method. Nanowires show an increase in photocurrent with length, and a maximum photocurrent of 0.73 mA/cm ² was measured (1.5 V vs RHE) for 1.8 μm long nanowires under AM 1.5G simulated sunlight illumination. While the incident photon to current conversion efficiency (IPCE) increases linearly with photon absorptance (1-10 ^-α×length) with near band gap illumination (λ = 410 nm), it decreases severely at shorter wavelengths of light for longer nanowires due to poor electron mobility. Atomic layer deposition (ALD) was used to deposit an epitaxial rutile TiO ₂ shell on nanowire electrodes which enhanced the photocatalytic activity by 1.5 times (1.5 V vs RHE) with 1.8 μm long nanowires, reaching a current density of 1.1 mA/cm ² (61% of the maximum photocurrent for rutile TiO ₂). Additionally, by fixing the epitaxial rutile shell thickness and studying photoelectrochemical (PEC) properties of different nanowire lengths (0.28-1.8 μm), we found that the enhancement of current increases with length. These results demonstrate that ALD coating improves the charge collection efficiency from TiO ₂ nanowires due to the passivation of surface states and an increase in surface area. Therefore, we propose that epitaxial coating on materials is a viable approach to improving their energy conversion efficiency. © 2012 American Chemical Society.