Maximizing the Photoelectrochemical Performance of TiO2 Nanotubes for Solar Energy Conversion

While solar energy is one of the few energy resources capable of meeting the world’s energy needs in the next century, addressing the transient nature of sunlight remains an unsolved challenge. Photoelectrochemical conversion of sunlight to chemical fuels by water splitting is an attractive route to create a storable source of solar energy for on-demand power generation. For this application, we have investigated titania (TiO2) nanotubes formed by the anodization of Ti in electrolytes containing F- ions. These TiO2 nanotube arrays consist of vertically-aligned cylinders 70-100 nm in diameter and with lengths between 500 nm to 20 µm. In addition to their low cost and scalable fabrication, TiO2 nanotube arrays are also attractive for their high surface area, 1D charge transport, and stability in a variety of electrolytes and illumination conditions.

We correlated the synthesis conditions of TiO2 nanotubes to their defect structure by using electrochemical impedance spectroscopy to map the energy position and density of donors and trap states. From these studies, we found that while nanotubes anodized in a 2 vol% water electrolyte can produce long TiO2nanotubes up to 20 micrometers in length, photocurrent efficiencies were limited by the presence of trap states that promote recombination. In order to improve the photoelectrochemical performance of this type of TiO2 nanotube arrays, passivation of trap states by electrochemical intercalation of Li or H was performed. Under simulated sunlight, 1.0 VSCE, and pH = 7, non-intercalated nanotubes showed a plateau above 7 µm in length at 0.5 mA/cm2. In contrast, after doping, nanotubes up to 15 µm in length exhibited a linearly increasing photocurrent, resulting in photocurrents of up to 1.5 mA/cm2. Finally TiO2 nanotubes were modified with Co oxide water oxidation catalysts to enhance the water oxidation reaction. This modification successfully shifted the photocurrent response curve cathodically, but under larger applied anodic biases, unwanted light absorption limits the effectiveness of the catalysts. Measurements of photocurrent in alkaline electrolytes with a sulfite hole scavenger also revealed that Li-doped TiO2 nanotubes have a 60-80% water oxidation reaction efficiency, increasing to 100% under a strong applied bias of 1.2 V vs. RHE.