DOI: 10.1021/la101300n 13697 Langmuir 2010, 26(16), 13697–13702 Published on Web 07/20/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Electrochemical Grafting of Poly(3,4-ethylenedioxythiophene) into a Titanium Dioxide Nanotube Host Network Csaba Jan aky, G abor Bencsik,  Arp ad R acz, and Csaba Visy* Department of Physical Chemistry and Materials Science, University of Szeged, Szeged, H6720, Hungary Norma R. de Tacconi, Wilaiwan Chanmanee, and Krishnan Rajeshwar* Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019 Received April 1, 2010. Revised Manuscript Received June 24, 2010 This study focuses on electrodeposition for infiltrating in situ a conducting polymer such as poly(3,4- ethylenedioxythiophene) (PEDOT) into a host titanium dioxide (TiO 2 ) nanotube array (NTA) framework. The TiO 2 NTA was electrosynthesized on titanium foil in turn by anodization in a fluoride-containing medium. The PEDOT layer was electrografted into the TiO 2 NTA framework using a two-step potentiostatic growth protocol in acetonitrile containing supporting electrolyte. The nanoscopic features of oligomer/polymer infiltration and deposition in the NTA interstitial voids were monitored by field-emission scanning electron microscopy. Systematic changes in the nanotube inner diameter and the wall thickness afforded insights into the evolution of the TiO 2 NTA/PEDOT hybrid assembly. This assembly was subsequently characterized by UV-visible diffuse reflectance, cyclic voltammetry, and photoelec- trochemical measurements. These data serve as a prelude to further use of these hybrids in heterojunction solar cells. Introduction Heterojunction hybrid solar cells, consisting of an organic electron donor and an inorganic oxide semiconductor electron acceptor, have attracted much attention in the past decade. 1,2 An effective approach to building the heterojunction is to infiltrate the electron acceptor host network with the organic polymer. In particular, the use of an oxide nanotube array (NTA) framework for this purpose has several key advantages: (a) The highly ori- ented, vertically aligned NTA affords an efficient pathway for vectorial electron transfer; (b) light propagation through the architecture can be optimized by controlling the pore diameter, wall thickness, and nanotube length; (c) the NTA offers sub- stantial surface area while maintaining structural order; and (d) carrier collection is optimized by the proximity of exciton diffu- sion distances (5-20 nm) to the oxide nanotube diameter. Thus, it is not surprising that a number of studies have utilized oxide semiconductor (in particular, titanium dioxide or TiO 2 ) NTAs as the host framework for the organic polymer. 3-12 Efficient infiltration from solution of a high molecular weight polymer into the NTA host can be challenging, although both dip-coating 4 and spin-coating 7,8 have been utilized for this purpose. The polymer species are subsequently thermally driven into the host framework by incorporating a baking step. On the other hand, in situ approaches for infiltrating the polymer by using the corresponding monomer precursor are more attractive, and either chemical 13 or UV polymerization 10,14 has been deployed to syn- thesize poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) or polythiophene, respectively, in the oxide host. However, the intrinsic electroactivity of a monomer precursor molecule 15 can also be exploited to electrochemically infiltrate the polymer in situ 6,11 into the oxide NTA framework. In this paper, we elaborate this approach and build on the two prior studies contained in refs 6 and 11 by using poly(3,4-ethylenedioxythiop- hene) (PEDOT) as the organic polymer and TiO 2 NTA as the host framework. As a candidate for the organic donor component in hybrid heterojunction solar cells, PEDOT possesses many favorable attributes such as high chemical stability (especially in the oxidized or doped state), small band gap energy, and good optical trans- parency in the electronically conducting state. 16-18 Other than PEDOT and the two polymers identified above (i.e., polythio- phene and MEH-PPV), poly(3-hexylthiophene) (or derivatives thereof ) 3-5,7-9,19 has been a popular polymer of choice in the earlier studies using other methods for polymer infiltration. To our knowledge, there have been only two prior studies 6,11 on in situ electrochemical grafting (or electrodeposition) of a *To whom correspondence should be addressed. E-mail visy@chem.u-szeged.hu (C.V.) or rajeshwar@uta.edu (K.R.). (1) Review: Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533. (2) Review: Boucle, J.; Ravirajan, P.; Nelson, J. J. Mater. Chem. 2007, 17, 3141. (3) Zhang, Y.; Wang, C.; Rothberg, L.; Ng, M.-K. J. Mater. Chem. 2006, 16, 3721. (4) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Grimes, C. A. Langmuir 2007, 23, 12445. (5) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Appl. Phys. Lett. 2007, 91, 152111. (6) Zhang, Z.; Yuan, Y.; Liang, L.; Cheng, Y.; Xu, H.; Shi, G.; Jin, L. Thin Solid Films 2008, 516, 8663. (7) Yu, B.-Y.; Tsai, A.; Tsai, S.-P.; Wong, K.-T.; Yang, Y.; Chu, C.-C.; Shyue, J.-J. Nanotechnology 2008, 19, 255202. (8) Shankar, K.; Mor, G. K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Non-Cryst. 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