Optimization of Photoelectrochemical Performance of Long TiO2 Nanotubes By Li-Doping Induced Defect Passivation

Photoelectrochemical conversion of sunlight into storable fuels has great potential to overcome the challenge posed by sunlight’s transient nature. TiO₂ nanotubes are an attractive system for this application due to their high surface area, 1D charge transport, and stability in solution and under irradiation. The photoelectrochemical performance of TiO₂ nanotubes need to be optimized by balancing increased light absorption in longer nanotubes against the limitations imposed by the presence of trap states which would increase recombination of photogenerated charges. Although nanotubes from anodizing in low water content electrolytes can access the widest range of nanotube lengths, their high density of trap states facilitates recombination processes.[1] It has been demonstrated that 7 mm long nanotubes maximize efficiency in Nb doped TiO₂ nanotubes.[2] Recently, we have shown that trap state passivation can be readily achieved by electrochemically induced Li doping accompanied by the formation of Ti³⁺ states in TiO₂ nanotubes. This treatment suppresses recombination by pre-filling the traps with electrons and yields up to a 2x enhancement in the photoelectrochemical performance for 1 mm long nanotubes (Figure 1 (a)), providing a potential route to extend the length-vs-photocurrent maximum to longer tubes.[3] With this Li doping process we are able to demonstrate a 15 micron long nanotube array yields the highest photocurrent between 1 and 21 mm, generating 1.5 mA/cm² of photocurrent under simulated sunlight (Figure 1 (b)). Studies by electrochemical impedance have confirmed the trap state passivation mechanism by direct inspection of the density of trap-state levels and electron paramagnetic resonance spectroscopy measurements have identified Ti³⁺ states that accompany this doping process.

Figure Caption

Figure 1. (a) Lithium modified TiO₂ nanotubes show up to 2x enhancement in photoelectrochemical performance at 1.0 V_SCE. (b) Photoelectrochemical performance of TiO₂ nanotubes up to 21 microns in length shows an optimum at 15 microns after Li doping.

References

References

[1]         L. Tsui, G. Zangari, Electrochim. Acta 121 (2014) 203.

[2]         C. Das, P. Roy, M. Yang, H. Jha, P. Schmuki, Nanoscale 3 (2011) 3094.

[3]         L. Tsui, M. Saito, T. Homma, G. Zangari, J. Mater. Chem. A (2014). In-Press.  10.1039/C4TA05620E