3D Hierarchical Porous TiO 2 Films from Colloidal Composite Fluidic Deposition Chiara Dionigi,* Pierpaolo Greco, Giampiero Ruani, Massimiliano Cavallini, Francesco Borgatti, and Fabio Biscarini Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali Nanostrutturati, Via P. Gobetti 101, I-40129, Bologna, Italy ReceiVed June 25, 2008. ReVised Manuscript ReceiVed September 24, 2008 We use a “structure director” colloidal composite to fabricate porous titanium oxide films having a hierarchical pore architecture consisting of mesopores regularly distributed in the macropore shell. The colloidal composite consists of polystyrene beads coated with (ammonium lactate)titanium dihydroxide deposited by means of a fluidic technique. The pore properties and interconnections are controlled at different length scales: a macroscale, which is imposed by the polystyrene beads; a mesoscale, which is controlled by the composition and by the thermal history of the composite; a nanometer-scale, controlled by the nanocrystal sintering in air. Our approach can be extended to a wide class of water-soluble metal oxide precursors; therefore, it opens interesting perspectives for “bottom-up” nanotechnology of functional arrays and devices. Introduction The hierarchical organization of a material into a multi- modal architecture of pores with controlled size is attractive because it imparts “smart” properties on the structure, in terms of exposed surface, active sites, rheology, and size- selection. This aspect is required for several applications like chromatography, catalysis sensors, photonics, and photovol- taics. 1,2 Interest in the multimodal porosity of titanium oxide, TiO 2 , has emerged in the last 10 years because of its optical and electronic properties, which have made it one of the most studied materials in photonic and photovoltaic applications. The control of the porosity in films consisting of TiO 2 inverse opals represents a crucial advance in the field of photonic band gap materials because it leads to a periodic modulation of the refractive index generating a photonic stop band that does not allow the propagation of light with definite frequencies. 3-5 The substitution of disordered mesoporous TiO 2 films with inverse titania opals, in photovoltaic dye-sensitized solar cells (DSSC) 5-8 represented a breakthrough to improve the conversion efficiency of all solid state DSSC up to the highest limit of 11% obtained by Gra ¨tzel et al. in liquid electrolyte based devices. 3,4,8-14 As electron transport strictly depends on the network morphology and interconnection of TiO 2 nanoparticles, 9 attention has been focused on the TiO 2 particle network in order to improve its electron transport and, consequently, to increase the efficiency of DSSCs. Geometric confinement controls the diffusive movement and forces the electron transport to take a specific direction. A one-dimensional network has been considered an excellent compromise to achieve the optimum amount of nanoparticle contacts, generating a direct electron transport. In any case, an inverse opal structure combines the regular spatial arrangement of nanoparticles and an extended specific surface. Two-dimensional geometric confinement in the thin TiO 2 shells can speed up electron transport with respect to a three-dimensional chaotic network, thanks to the reduction of the degrees of freedom for electron movement. 12 Further improvement in the morphology of thin TiO 2 shells indubi- tably represents an advance in the research on the optimum electron transport network. Templates made of colloidal crystals of polymeric mono- disperse beads have been used to generate TiO 2 inverse opals having differently sized (meso and macro) interconnected pores. 9,15,16 During template removal by thermal treatment, * Corresponding author. E-mail: c.dionigi@bo.ismn.cnr.it. Tel.: 390516398502. Fax: 390516398540. (1) Sakatani, Y.; Boissie `re, C.; Grosso, D.; Nicole, L.; Soler-Illia, G. J. A. A.; Sanchez, C Chem. Mater. 2008, 20, 1049–1056. (2) Sanchez, C.; Boissiere, C.; Grosso, D.; Laberty, C.; Nicole, L. Chem. Mater. 2008, 20, 682–737. (3) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589–15598. (4) Stein, A.; Li, F.; Denny, N. R. Chem. Mater. 2008, 20, 649–666. (5) Somani, P. R.; Dionigi, C.; Murgia, M.; Palles, D.; Nozar, P.; Ruani, G. Sol Energy Mater. Sol. Cells 2005, 87, 513–519. (6) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. Appl. Phys. Lett. 2007, 91, 023116. (7) Zhang, Y.; Wang, J.; Zhao, Y.; Zhai, J.; Jiang, L.; Song, Y.; Zhu, D. J. Mater. Chem. 2008, 18, 2650–2652. (8) Kuo, C-Y.; Lu, S-Y. Nanotechnology 2008, 19, 095705. (9) Gra ¨tzel, M. Nature 2001, 414, 338. (10) Gates, B.; Park, S. H.; Xia, Y. AdV. Mater. 2000, 12, 653–556. (11) Radtchenko, I. L.; Sulkhorukov; G, B.; Gaponik, N.; Kornowski, A.; Rogach, A.m.L.; Mo ¨hwald, H. AdV. Mater. 200113, 1684. (12) Halaoui, L. I.; Abrams, N. M.; Mallouk, T. E. J. Phys. Chem. B 2005, 109, 6334–6342. (13) Diguna, L. J.; Murakami, M.; Sato, A.; Kugamai, Y.; Ishihara, T.; Kobayashi, N.; Shen, Q.; Toyoda, T. Jpn. J. Appl. Phys. 2006, 45, 5563–5568. (14) Ruani, G.; Ancora, C.; Corticelli, F.; Dionigi, C.; Rossi, C. Sol. Energy Mater. Sol. Cells 2008, 92, 5, 537–542. (15) Velikov, K. P.; Christova, C. G.; Dullens, R. P. A.; van Blaaderen, A. Science 2002, 296, 106–109. (16) The designation of pores of less than 2 nm, 2 to 50 nm, and over 50 nm size (diameter) as micro-, meso-, and macropores is in accord with accepted IUPAC terminology and definition. 7130 Chem. Mater. 2008, 20, 7130–7135 10.1021/cm801734y CCC: $40.75  2008 American Chemical Society Published on Web 10/29/2008