Nitrogen and Transition-Metal Codoped Titania Nanotube Arrays for Visible-Light-Sensitive Photoelectrochemical Water Oxidation
Authors: Suzuki Tomiko M. , Kitahara Gaku , Arai Takeo , Matsuoka Yoriko , Morikawa Takeshi 
Published: February 2020

The anodization of a titanium metal sheet to form aligned titanium dioxide nanotubes (TNTs) is of considerable research interest in fields such as photocatalysts, solar cells, and sensors due to their aligned nanostructure and high surface area [1]. Owing to wide band gap (3.2 eV) of TiO2, TNTs can utilize only UV, which is 3-4 % of solar light energy. It is an important issue to develop new TNTs with enhanced photocatalytic activities under visible-light irradiation. In the research field of TiO2 particles, impurity doping such as nitrogen is one of typical approaches to extend spectral response of TiO2 to visible light region [2]. Moreover, codoping of nitrogen and metal ion possesses potential to induce formation of new states which are close to the valence band and conduction band edges, respectively [3, 4]. The codoping approach is an efficient way to absorb wider spectrum of solar irradiation by TiO2 for photoelectrochemical water splitting for solar hydrogen generation and CO2 reduction reaction [5, 6].

In this work, we report on fabrication of novel TNT arrays codoped with nitrogen (N) and earth-abundant 3d-transition metals (TM) for visible-light-driven photoelectrochemical water oxidation.

The fabrication method for N and TM (TM: Fe, V, Cr, or Co) codoped TNT [(N, TM)-TNT] arrays were via an anodization process using low concentration transition metal (0.05-0.13 at%)-Ti alloys and a subsequent nitridation process. Vertically aligned N and TM (TM: Fe, V, Cr, or Co) codoped TMT (N, M-TNT) with external diameter of ca. 100 nm and length of ca.3000 nm were successfully fabricated. XRD patterns of all (N, TM)-TNT samples have broad peaks assignable to the typical anatase phase.

Photoelectrochemical measurements were performed in a 3-electrode cell containing 0.1 M KOH with a (N, TM)-TNT photoanode, a platinum cathode, and a Ag/AgCl reference electrode. The codoping of N and TM substantially improved the photocurrent of the TNT photoanode under visible light irradiation due to the formation of impurity levels by N and TM. The rate of increase in the photocurrent was dependent on transition metal species and it was found that codoping of iron showed the greatestest enhancement. N, Fe(0.13at%) codoped TNT [(N, Fe0.13)-TNT] photoanode modified with cobalt borate (Co-Bi) co-catalyst yielded a visible-light-induced water oxidation with a photocurrent density of 0.76 mA/cm2 at +0.6 V (vs. Ag/AgCl) under visible light irradiation, which was 13 times and 5 times higher than that of Fe0.13-TNT and N-TNT, respectively. In addition, O2 generation and the photocurrent were confirmed repeatedly during visible-light irradiation with high current efficiency (≥95%) and these results indicate that the observed photocurrent originated from water-splitting reaction [7].

The photoelectrochemical water oxidation activity of codoped TNT could be further improved by optimizing the amount of doping, kind of dopant, and nanotube structure. This scalable method for codoping to TNT can also be extended to other metal oxide nanotubes.

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[7] T. M. Suzuki, et al., Chem. Commun., 50 (2014) 7614.