Conclusions and New Directions



We considered four synthesis generations of TiO2 nanotube arrays [1–3]. The first being aqueous solutions using HF as reported by Gong and co-workers [4], the second as reported by Cai et al. [5] using buffered KF or NaF aqueous solutions to achieve nanotube array lengths of several microns. The third synthesis generation, as initially reported by Paulose and co-workers [6–10], dwelt on the use of organic electrolytes, which extended the achievable nanotube array growth up to 1 mm. A fourth synthesis generation involves replacement of the fluorine ions, used in the first three synthesis generations with, most notably, chlorine as reported by Allam et al. [11, 12]. Each synthesis generation has provided new material properties, which in turn have enabled new applications.

First generation nanotube arrays, obtained in HF aqueous electrolytes, resulted in nanotubes with a pore size from 22 to 76 nm and a maximum achievable thickness of about 500 nm. A pore size of 120 nm and length of 1.1 μm were grown in acidic HF (i.e., 0.3 wt% HF + 1.0 M H3PO4) based electrolytes, while a pore size of about 90–140 nm and length 0.69–2.47 μm was achieved using acidic NH4F (NH4F/H3PO4) based electrolytes with non-Pt based transition elements (Fe, Co, Cu, Ta, and W) as counter electrodes [13]. An enabling discovery was that addition of acetic acid to the anodization electrolyte transformed the nanotubes from something quite fragile, prone to breaking during handling, to something mechanically robust. Conical shape nanotubes can be obtained by linearly varying the anodization voltage [14], while nanotube wall thickness can be varied through temperature for aqueous electrolytes [15], or acid content for organic electrolytes [1].


High Resolution Transmission Electron Microscope High Resolution Transmission Electron Microscope Nitrogen Doping Organic Electrolyte Transport Resistance 


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Copyright information

© Springer Science + Business Media, LLC 2009

Authors and Affiliations

  1. 1.Electrical Engineering DepartmentPennsylvania State UniversityUniversity ParkUSA
  2. 2.Materials Research InstitutePennsylvania State UniversityUniversity ParkUSA

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