Synthesis and Characterization of Metal- and Non-Metal-Doped Titania Nanotubes for Solar Hydrogen Generation and Methanol Electrooxidation

Functional nanostructured TiO₂ materials with unique electrical, chemical and catalytic properties are being increasingly used in many applications, including lithium-ion batteries, gas sensors, hydrogen generation, electrocatalysis and photocatalysis. A number of different pathways and processes have been developed and reported to synthesize titania nanotubes (TNTs), nanorodes, and nanospheres. The most common methods include sol-gel, hydrothermal treatment, electrophoretic deposition and electrochemical anodization. The latter technique has been utilized by many researchers to fabricate highly ordered titania nanotubes and, subsequently, doped with both metals and non-metals. The last step—inclusion of metals and non-metals into the TNT matrix—is critical for solar hydrogen generation since pure titaina is insufficient under visible light due to its wide band gap, which is around 3.2 eV. Furthermore, metal oxides such as titanium dioxide suffer from lower surface area and electrical conductivity compared with carbon-based materials for use in direct methanol fuel cells. Accordingly, they are often combined with carbon-based materials and/or transition metals to form oxide-carbon hybrid structures.

Highly ordered titania nanotubes  (Figure 1) were fabricated employing a simple electrochemical anodization technique in a conventional two-electrode cell with titanium foil and graphite as working and counter electrodes, respectively. Metals, including nickel and platinum, were electrodeposited on TNT or TNT-C materials using a pulsed current electrodeposition technique [1]. All samples were then characterized by scanning electron microscopy, X-ray powder diffractometry, cyclic voltammetry, and chronoamperometry. Structural examination of the samples confirmed the presence of rutile and anatase nanocrystalline TNTs after annealing at different temperatures and durations.  The presence of nanoparticles, including platinum and nickel, were also confirmed.

For methanol electrooxidation, the activity and durability of the hybrid catalyst layers were determined in a solution containing one molar sulfuric acid and one molar methanol. The hybrid catalyst layer showed a better performance towards methanol oxidation compared with state-of-the-art carbon-supported platinum layers. For solar hydrogen generation, the efficiency of the photoanodes was found to increase as the amount of dopant increased to 3.0%. Further increase in dopant, however, resulted in a decrease in efficiency, possibly due to a higher rate of electron-hole recombination.

Reference

1. S. Karimi & F.R. Foulkes, Electrochemistry Communications 19 (2012) 17-20