Titanium dioxide (TiO₂) has attracted significant attention for the use in various applications; photocatalysts, dye-sensitized solar cells, lithium/sodium ion batteries, sensors, etc. Since the discovery of high visible-light photocatalytic activity by Asahi et al. in 2001,¹ substitution of oxygen atoms in TiO₂ by heteroatoms has been the hot topic in these fields. Recent work of Ota et al. opened a new application of TiO₂: oxygen deficient TiO_x showed the oxygen reduction reaction (ORR) activity in acidic media that simulate polymer electrolyte fuel cell (PEFC) cathodes.² Other oxide catalysts containing group 4 or 5 metals also showed ORR activity in acidic media, whereas TiO₂ is the most promising from the viewpoint of natural abundance. The oxygen defects in oxide catalysts have been acknowledged as active sites for ORR and they have been incorporated using various routes such as the pyrolysis under reductive atmosphere containing hydrogen,² so-called carbothermal-reduction³ and nitrogen-doping.^4-8 Rutile is more active for ORR than anatase,^3,6,7 and this trend is opposite to that in photocatalytic activity. 

In this work, oxygen defects were incorporated into nanoparticles of rutile TiO₂ and other group 4 metal oxides by substitutional nitrogen-doping using a recently developed solution phase combustion route.^7,8 X-ray photoelectron spectroscopy analyses on TiO₂ catalysts revealed that (1) charge imbalance caused by doped nitrogen atoms were compensated by the incorporation of oxygen defects, not the decrease in the valence of titanium on the surface and (2) the surface rutile was formed on TiN, yielding a core-shell structure. Many transition metal nitrides are well known to be unstable in water or even in air moisture, except for Ta₃N₅;^9,10 however, the surface rutile successfully protected the TiN-core in 0.1 mol dm^–3 H₂SO₄ solution not to change the ORR mechanism represented by Tafel slope after 20,000 potential cycles between 0.6 and 1.0 V versus the reversible hydrogen electrode. The active sites will be discussed at the meeting by comparing the activity of TiO₂ and other group 4 metal oxides.

 

Acknowledgments

The authors gratefully acknowledge Mr. Yusei Tsushima for his help with acquisition of transmission electron microscopy images. This work was partially supported by a Grant-in-Aid for Scientific Research (C) (26420132) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan; an Adaptable and Seamless Technology Transfer Program through Target-driven R&D grant (AS262Z02037) from the Japan Science and Technology Agency; a research grant from the Takahashi Industrial and Economic Research Foundation of Japan; a research grant from the Naoji Iwatani Foundation of Japan; and a grant for chemical research from the Foundation for Japanese Chemical Research. The X-ray photoelectron spectra were acquired with the support by Nanotechnology Platform, 12024046 of the MEXT of Japan.

 

References

(1)           R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science, 293, 269 (2001).

(2)           J. H. Kim, A. Ishihara, S. Mitsushima, N. Kamiya, and K. Ota, Electrochim. Acta, 52, 2492 (2007).

(3)           M. Chisaka and N. Itagaki, Electrochim. Acta, in press. Doi:10.1016/j.electacta.2015.10.184.

(4)           M. Chisaka, H. Sasaki and H. Muramoto, Phys. Chem. Chem. Phys., 16, 20419 (2014).

(5)           M. Chisaka, A. Ishihara, K. Suito, K. Ota and H. Muramoto, Electrochim. Acta, 88, 697 (2013).

(6)           M. Chisaka and H. Muramoto, ChemElectroChem, 1, 544 (2014).

(7)           M. Chisaka, Y. Ando and H. Muramoto, Electrochim. Acta, 183, 100 (2015)

(8)           M. Chisaka, Y. Ando and N. Itagaki, J. Mater Chem. A, in press. Doi:10.1039/C5TA08235H

(9)           M. Hara, E. Chiba, A. Ishikawa, T. Takata, J. N. Kondo and K. Domen, J. Phys. Chem. B, 107, 13441 (2003).

(10)       M. Chisaka, A. Ishihara, N. Uehara, M. Matsumoto and K. Ota, J. Mater Chem. A, 3, 16414 (2015).