Protein‐Enabled Layer‐by‐Layer Syntheses of Aligned, Porous‐Wall, High‐Aspect‐Ratio TiO2Nanotube Arrays

An aqueous, protein‐enabled (biomimetic), layer‐by‐layer titania deposition process is developed, for the first time, to convert aligned‐nanochannel templates into high‐aspect‐ratio, aligned nanotube arrays with thin (34 nm) walls composed of co‐continuous networks of pores and titania nanocrystals (15 nm ave. size). Alumina templates with aligned open nanochannels are exposed in an alternating fashion to aqueous protamine‐bearing and titania precursor‐bearing (Ti(IV) bis‐ammonium‐lactato‐dihydroxide, TiBALDH) solutions. The ability of protamine to bind to alumina andtitania, andto induce the formation of a Ti–O‐bearing coating upon exposure to the TiBALDH precursor, enables the layer‐by‐layer deposition of a conformal protamine/Ti–O‐bearing coating on the nanochannel surfaces within the porous alumina template. Subsequent protamine pyrolysis yields coatings composed of co‐continuous networks of pores and titania nanoparticles. Selective dissolution of the underlying alumina template through the porous coatingthen yields freestanding, aligned, porous‐wall titania nanotube arrays. The interconnected pores within the nanotube walls allow enhanced loading of functional molecules (such as a Ru‐based N719 dye), whereas the interconnected titania nanoparticles enable the high‐aspect‐ratio, aligned nanotube arrays to be used as electrodes (as demonstrated for dye‐sensitized solar cells with power conversion efficiencies of 5.2 ± 0.4%).