Programa de Pós-Graduação em Física da UFSC, Florianópolis

O Programa de Pós-Graduação em Física convida para o Seminário:

Hydrogen from Sunlight – Photoelectrochemical Water Splitting with Li-doped TiO₂ Nanotubes

 Giovanni Zangari
Department of Materials Science and Engineering, University of Virginia
Charlottesville, VA, USA

Resumo:

While solar energy is one of the few energy sources capable of meeting the world’s energy demands,¹ the transient nature of sunlight necessitates the development of technologies to store that energy for on-demand use to supplement photovoltaics.² Photoelectrochemical solar cells use sunlight to electrolyze water to produce hydrogen as a storable fuel, which can then be converted to electricity by a fuel cell. TiO₂ nanotubes formed by the anodization of titanium foils in electrolytes containing fluoride ions produce aligned arrays of tube-like structures 70-100 nm in width and tunable length on the order of microns as shown in Figure 1(a, b). These structures are a promising architecture for photoelectrochemical applications³ because of their high stability in solution and under irradiation, high surface area, and 1-dimensional charge transport for efficient movement of photogenerated charges. While hundreds of microns long nanotubes in principle may be grown, one major limitation on the performance of TiO₂ nanotubes is the presence of defects in the crystal structure which facilitate the recombination of photogenerated electron-hole pairs;⁴ the best photoelectrochemical performance of TiO₂ nanotubes for water splitting has been observed in Nb-doped TiO₂ with a length of 7 mm and an associated photocurrent of 1.0 mA/cm².⁵ Electrochemical Li doping was demonstrated to passivate the trap states in TiO₂ nanotubes, suppressing recombination. This process resulted in up to a 2 fold increase in the photoelectrochemical performance of 1 mm long TiO₂ nanotubes compared to the undoped samples.⁶ For long TiO₂ nanotubes, a new optimum length of 15 mm was achieved, resulting in 1.5 mA/cm² of photocurrent at 1.0 V_SCE under simulated sunlight as seen in Figure 1(c). These results demonstrate the ability to produce high performance TiO₂ nanotubes for photoelectrochemical solar energy devices using simple electrochemical techniques.

Figure 1. (a) Cross section and (b) surface view of TiO₂ nanotubes grown by anodization. (c) Lithium intercalation enhances the photocurrent by trap state passivation, resulting in up to 1.5 mA/cm² of photocurrent at 15 m.

References

(1)            Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. Amer. 2006, 103, 15729.
(2)            Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Chem. Rev. 2010, 110, 6474.
(3)            Tsui, L. -k.; Zangari, G. J. Electrochem. Soc. 2014, 161, D3066
(4)            Tsui, L.; Zangari, G. Electrochim. Acta 2014, 121, 203.
(5)            Das, C.; Roy, P.; Yang, M.; Jha, H.; Schmuki, P. Nanoscale 2011, 3, 3094.
(6)            Tsui, L.; Saito, M.; Homma, T.; Zangari, G. J. Mater. Chem. A 2015, 3, 360.

Data: 27 de março de 2015 – (sexta-feira) – Local: Sala 212 – Auditório do Departamento de Física- Horário: 10h15min