Improving Photoelectrochemical Performance of TiO2 Nanotubes by Passivation of Trap States with H/Li Intercalation

TiO₂ nanotubes are a promising system for photoelectrochemical applications due to their high surface area ratio, 1D charge transport properties, and stability in solution and under light.¹ However, their photoelectrochemical performance is limited due to the presence of trap states inside the TiO₂ band gap, facilitating the recombination of electron/hole pairs. We found that TiO₂ nanotubes anodized in a low water content electrolyte had a lower photocurrent response compared to those anodized in a 11 vol% water electrolyte.² A subsequent study attributed this observation to a higher density of these deep level trap state: 6.5 x 10¹⁶ cm^-3 for TiO₂ nanotubes anodized in a 2 vol% water electrolyte compared with 2.4 x 10¹⁶ cm^-3 for nanotubes anodized in a 11 vol% water electrolyte.³ As the low water content electrolyte produces TiO₂ nanotubes with the widest variety of lengths, it is desirable to improve their performance by reducing the influence of these trap states.

 Modification of TiO₂ nanotubes by Li and H has been observed to improve the photoelectrochemical performance, with the intercalation inducing the formation of Ti³⁺.⁴ TiO₂ nanotubes were synthesized by a double anodization procedure and anodized in an electrolyte containing 0.3wt% NH₄F and 2 vol% H₂O in ethylene glycol. Li and H modification was carried out by applying a potential of -1.55 V_SCE in 1M LiClO₄ and 0.5M H₂SO₄ respectively. Photocurrent measurements under simulated sunlight (Figure 1) in a solution containing 0.2M Na₂SO₄ and 0.1M NaCH₃COO (pH = 7) showed a 2 fold enhancement in the photoelectrochemical performance under simulated sunlight at a potential of 1.0 V_SCE for both lithium and hydrogen modified nanotubes. Faster onset transients and electrochemical impedance spectroscopy measurements showed that the intercalation of Li and H effectively passivated the trapping behavior of the TiO₂ nanotubes, leading to higher photocurrents. High resolution TEM was performed to obtain atomic-scale imaging of the TiO₂ nanotubes (Figure 2). At the outside walls of the Li modified TiO₂ nanotubes, a darkened zone is observed in the wall, suggesting that the intercalation penetrates ~10 nm into the nanotube walls.

Acknowledgments

We gratefully acknowledge the support of the ARCS Foundation.

References

 1. P. Roy, S. Berger, and P. Schmuki, Angew. Chemie (Int. ed. Eng.), 50, 2904–39 (2011).

2. L. Tsui, T. Homma, and G. Zangari, J. Phys. Chem. C, 117, 6979–6989 (2013).

3. L. Tsui and G. Zangari, Electrochim. Acta, 121, 203–209 (2014).

4. B. H. Meekins and P. V. Kamat, ACS Nano, 3, 3437–46 (2009). 

Figure Captions

Figure 1. Photocurrent as a function of potential under simulated sunlight. Black curves show unmodified TiO₂ nanotubes, and red curves show nanotubes modified by (a) hydrogen and (b) lithium intercalation.

Figure 2. High resolution TEM images of (a) unmodified TiO₂ nanotubes and (b) lithium modified TiO₂ nanotubes. A zone approximately 10 nm in width is observed at the edge of the Li doped TiO₂ nanotubes indicated by the arrows.