Conversion of porous anodic Al₂O₃into freestanding, uniformly aligned, multi-wall TiO₂nanotube arrays for electrode applications

Abstract

A combined conformal coating and gas/solid reaction process has been used for the first time to convert porous anodic alumina templates into robust, freestanding, multi-wall titania nanotube (MWTNT) arrays that resist appreciable nanotube bundling upon drying. A sol-gel infiltration process was first used to apply a thin conformal titania coating to the alumina templates. After selective etching to generate a gap between the template and the titania coating, exposed alumina was converted into nanocrystalline titania via reaction with titanium tetrafluoride gas and then with humid oxygen. After selective dissolution of residual aluminum-bearing phases and drying, freestanding, non-agglomerated, wellaligned MWTNT arrays were generated (i.e., with coating-derived inner titania tubes resting inside reaction-derived outer titania tubes). When incorporated as aligned electrodes in dye-sensitized solar cells, the dye loading and power conversion efficiencies were higher by factors of 2.2 and 1.8, respectively, than for solar cells with single-wall titania nanotube array electrodes. By controlling the conditions used for alumina template formation, sol-gel coating, and gas/solid reaction, this hybrid process may be used to generate robust, uniformly aligned MWTNT arrays with dimensions and functional chemistries tailored for a variety of electrical, optical, chemical, or biochemical applications.

Fabrication and application of nanoporous anodic aluminum oxide: a review

“…Nanoporous anodic aluminum oxide (AAO) is one of the most widely used nanomaterials due to the unique optical and electrical properties, ease of fabrication, tunable size, and large surface area [10][11][12][13][14]. All this properties make AAO an excellent candidate for exciting applications including metamaterials [15,16], chemical or biological sensing devices [17][18][19][20][21][22][23][24][25], desalination [26][27][28][29], energy generation and storage devices [30][31][32][33][34][35][36], date storage devices [32,37], photonic crystals [37][38][39][40][41][42][43][44], template-assisted fabrication of nanostructures [15,16,[45][46][47], and corrosionresistant materials as shown in scheme 1 [48]. These applications require AAO to have a well-defined structural and chemical properties, adjustment of parameters through oxidation technology during the anodizing process and subsequent pre/post-treatment process.…”

Arrays of templated TiO2nanofibres as improved photoanodes for water splitting under visible light

“…When the solgel reaction occurs inside the pores of a nanoporous membrane, it is possible to obtain ceramic NFs [20,21]. The template solgel method has been successfully applied for the synthesis of a great variety of metal oxide nanomaterials [20,21,23,24], including examples of TiO 2 -NFs and NTs [25][26][27][28][29][30].…”

“…A crucial step in the sol-gel template method is the control of the reactivity of the titania precursor which can be achieved by adding acetylacetone (AcAc) as titanium chelating agent, as first suggested by Attar et al [21]. It is worth noting that other previous researchers used very reactive titanium alkoxides [26][27][28][29] without adding chelating agents; this can cause problems in controlling the NFs growth inside the nanometer size pores, producing mainly NTs rather than NFs [26][27][28][29].…”

Broadband and wide-angle antireflective characteristics of nanoporous anodic alumina films for silicon-based optoelectronic applications

“…The NPAA template is an aluminum oxide (Al 2 O 3 ) film with self-organized random arrays of uniform parallel nanopores. It has been widely used as a template for fabricating various functional nanostructures [12][13][14][15]. The Al 2 O 3 film has been also utilized as a passivation layer to suppress the degradation of electrical properties in Si-based optoelectronic devices due to its relatively good mechanical/ chemical stability [16][17][18].…”