Optimization of Photoelectrochemical Performance of Long TiO2 Nanotubes By Li-Doping Induced Defect Passivation
Tuesday, 26 May 2015: 12:00
Conference Room 4D (Hilton Chicago)
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. It has been demonstrated that 7 mm long nanotubes maximize efficiency in Nb doped TiO2
nanotubes. 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. 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 1. (a) Lithium modified TiO2 nanotubes show up to 2x enhancement in photoelectrochemical performance at 1.0 VSCE. (b) Photoelectrochemical performance of TiO2 nanotubes up to 21 microns in length shows an optimum at 15 microns after Li doping.
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