Photoelectrochemical Hydrogen Production By Water Splitting over Dual-Functionally Modified Oxide: P-Type N-Doped Ta2O5 Photocathode Active Under Visible Light Irradiation

Abstract

It is widely accepted that photocatalytic hydrogen generation by water splitting and carbon dioxide recycling by reductive reaction are promising technologies as clean energy generation in the future. To this end, lots of semiconductor materials which can be activated under visible light irradiation have been developed to date. Recently, our group developed a dual-functional modified N-doped Ta2O5 (N-Ta2O5) that absorbs visible light at wavelengths below 520 nm (Eg = 2.4 eV) by forming a new energy state in which the N 2p is above the O 2p in the bandgap [1, 2]. The N-doping also changed its photoresponse from n-type to p-type conduction due to formation of acceptor level by N, and also resulted in a highly negative location of the conduction band minimum (E CBM = -1.3 V vs NHE) originated from surface dipole effect [1, 3]. Depending on the much negative E CBM, a highly selective visible light-induced reduction of CO2 to HCOOH was achieved by employing N-Ta2O5 linked with a ruthenium complex catalyst in acetonitrile (MeCN)/triethanolamine (TEOA) [4, 5]. These characters as a semiconductor are completely different from those of conventional tantalum pentoxide (Ta2O5, Eg = 4.0 eV, = -0.4 V vs NHE) and tantalum oxynitride (TaON, Eg = 2.4 eV, = -0.3 V vs NHE) [6], which exhibit anodic photoresponses. In this paper, we demonstrate H2 evolution over N-Ta2O5 by water splitting in aqueous solution under visible light irradiation. Although the surface of N-Ta2O5 was found to be almost inert for hydrogen production by water splitting, photocathodic current over N-Ta2O5 films under visible light irradiation (λ ≥ 410 nm) was highly enhanced by surface modification with Pt, Rh or Au. Loading with either Pt or Rh greatly enhanced the H2 evolution rate in an aqueous solution, increasing the rates by two orders of magnitude compared to that over unmodified N-Ta2O5 for both photoelectorodes and nanoscaled powders. At this moment, the conversion efficiency was low as 0.15 % at 400 nm. However, the dual functional modification of Ta2O5 by the single element doping of N was clarified to be as a new approach to facilitate the reductive reaction for energy conversion. This finding should assist in the future development of highly efficient photocatalysts for solar energy conversion by photoelectrochemistry by N doping into oxide semiconductor materials. [references] [1] T. Morikawa, et al., Appl. Phys. Lett., 96(2010) 142111. [2] T. M. Suzuki, et al., J. Mater. Chem., 22(2012) 24584. [3] R. Jinnouchi, et al., J. Phys. Chem. C., DOI : 10.1021/acs.jpcc.5b06932. [4] S. Sato, et al., Angew. Chem. Int. Ed., 49(2010) 5101. [5] T. M. Suzuki, et al., Chem. Commun., 47(2011) 8673. [6] K. Domen, et al., J. Phys. Chem. B., 107 (2003) 1798.