Electrochemical Investigation on the Interface between Electronically Conducting Polymer and Titania Nanotube Arrays

Electronically conducting polymers (ECP) can be deposited on different types of substrate such as metals^[1] or semiconductors. Heterostructures made from inorganic oxides and ECP have been widely exploited with the aim of diverse applications (photovoltaic cells, photocatalysis^[2], Li-ion batteries^[3], etc.). Among metal oxides, titanium (IV) dioxide is of particular interest. In addition to its ability to store reversibly Li⁺ and its chemical stability, TiO₂can be easily nanostructured by anodization of a Ti foil.

A way to improve metal oxide performances is to control their morphology at the nanoscale^[4]. In fact, using highly-ordered TiO₂ nanotube arrays (TiO₂-NTAs) as the substrate for electropolymerization does provide a high surface area for polymer deposit, vertical pathways for electron transport and leads to the formation of a high density of surface states that influences the interfacial electron-transfer process^[5].

TiO₂ is a n-type semiconductor with an energy band gap of about 3.2 eV. Under UV illumination, it generates charges and especially holes that can oxidize species adsorbed at its surface. Consequently, electrodeposition of polymers such as polypyrrole is a photo-assisted process. Moreover, when exposed to UV light in an aqueous solution, the doping level of TiO₂ increases and so does its conductivity^[5,6].

Physicochemical properties of deposited layers depend on the substrate, the method of synthesis, the solvent, the doping anion and the pH of the solution ^[7]. In the present communication, we describe the photo-assisted synthesis of electroactive polypyrrole films in TiO₂-NTAs under different conditions. The resulting heterostructures have been subjected to cyclic voltammetry measurements, electrochemical impedance spectroscopy, FEG-SEM and Raman spectroscopy.

The results of this work emphasize the role of UV light illumination during electropolymerization and the role of the electrolyte anion. It is also shown that a 3D hybrid junction can be achieved between TiO₂-NTAs and polypyrrole which is an ECP. Both electrochemical impedance spectroscopy and cyclic voltammetry evidence the fact that the two materials may form an ohmic junction. Moreover, the polypyrrole growth implies a thickening of the film without any further spreading out on the TiO₂-NTAs substrate, the TiO₂/PPy junction providing a preferential path for electron transfer.

References

(1) Hien, N.T.L.; Garcia, B.; Pailleret, A. ; Deslouis, C. Electrochimica Acta (2005), 50, 1747-1755.

(2) Luo Q, Li X, Wang D, Wang Y, An J. Journal of Materials Science (2011), 46(6):1646-1654.

(3) Gemeay, A.H.; Nishiyama, H.; Yoneyama, H. J. Electrochem. Soc. (1995), 142, 4190-4195.

(4) Allam N.K., Shankar K., Grimes C.A. Advanced Materials (2008), 20, 3942-3946.

(5) Pu, P.; Cachet, H.; Ngaboyamahina, E.; Sutter, E.M.M. J. Solid State Electrochem. (2013), 17, 817-828.

(6) Pu, P.; Cachet, H.; Laidani, L.; Sutter, E.M.M. J. Phys. Chem. C (2012), 116, 22139.

(7) Castagno, K.R.L.; Dalmoro, V.; Azambuja, D.S. Materials chemistry and Physics (2011), 130, 721-726.