Efficient Solar‐Driven Water Oxidation over Perovskite‐Type BaNbO₂N Photoanodes Absorbing Visible Light up to 740 nm

Abstract

materials is a potential means of addressing these concerns by producing hydrogen as a clean, renewable fuel source. [1] Because visible and infrared wavelengths account for a large portion of the solar spectrum, it is necessary to develop semiconductors with longer light absorption onset wavelengths in order to achieve high solar-to-hydrogen energy conversion efficiencies.Perovskite-type oxynitrides, designated as AB(O,N) 3 (where A = La, Ca, Sr, or Ba and B = Ti, Ta, or Nb), are n-type semiconductors capable of absorbing a wide range of visible light. [2,3] Interestingly, the band gap energy for AB(O,N) 3 can be varied by employing different combinations of the A-and B-site cations. [3,4] Nb-based perovskite oxynitrides (ANb(O,N) 3 B = Nb) are of particular interest with regard to solar energy conversion because they have narrower band gap energies than Ta-or Ti-based oxynitrides. As an example, BaNbO 2 N absorbs visible light up to 740 nm, a longer wavelength than BaTaO 2 N or LaTiO 2 N, and has the potential to achieve a photocurrent density of 23.3 mA cm −2 given an incident photon-to-current efficiency (IPCE) of 100%. It is also helpful that Nb is a more abundant element than Ta. However, PEC water splitting using ANb(O,N) 3 remains relatively inefficient compared with that using Ti-and Ta-based compounds. [5][6][7] This is because the reduction of Nb species during nitridation in a NH 3 flow (i.e., from Nb 5+ to Nb 4+ or Nb 3+ ) occurs more readily than that of Ta species because of the higher electronegativity of Nb. It leads to formation of surface anion defects to compensate for charge imbalances and/or impurity phases such as NbO x N y . The surface state of ANb(O,N) 3 can therefore hinder the water oxidation reaction, unless mild synthesis conditions such as low temperature and short nitridation periods are used to suppress the reduction of Nb 5+ . Unfortunately, these conditions result in low crystallinity of the oxynitride, which enhances recombination of photoexcited holes and electrons. Thus, a synthesis method that effectively reduces the generation of inactive sites while retaining high crystallinity is required before high PEC activity can be achieved using ANb(O,N) 3 .The perovskite-type BaNbO 2 N, unlike SrNbO 2 N which can be obtained by nitridation of Sr 2 Nb 2 O 7 , does not have a crystalline oxide precursor with both a pentavalent Nb state and a Photoelectrochemical water splitting using semiconductors absorbing a wide range of visible light is a potentially attractive means of harvesting large portions of the solar spectrum. However, this is also very challenging because narrowing the semiconductor band gap lowers the driving force for photo reactions. Herein, a highly active perovskite BaNbO 2 N exhibiting photo excitation up to 740 nm for water oxidation is reported. The synthesis route, consisting of moderate nitridation and subsequent annealing in inert Ar flow, enhances the crystallinity of the BaNbO 2 N surface without inducing the reduction of the Nb species. As a result, a parti...

Tailoring of an unusual oxidation state in a lanthanum tantalum(IV) oxynitride via precursor microstructure design

“…n spite of the growing interest in perovskite materials, the number of oxynitride members is still limited. Most oxynitrides contain metal cations in d 0 or d 10 electronic configuration [1][2][3][4][5][6][7][8] , whereas alternative materials with B-site cations in e.g. d 1 configuration such as Ta 4+ are largely lacking.…”

Development of Sunlight Driven Water Splitting Devices towards Future Artificial Photosynthetic Industry

“…(B) Absorption spectra of inorganic semiconductors, indicated as either the absorption coefficient converted from the transmission spectra (A) or Kubelka-Munk diffuse-absorption function (KM), both arbitrarily normalized to magnify the edge of absorption by each material. The spectral curves were reproduced for SrTiO 3 from [4], TiO 2 [5], BiVO 4 [6], LaTiO 2 N [7], Ta 3 N 5 by our own dedicated measurement, Fe 2 O 3 [8], BaTaO 2 N [9], BaNbO 2 N [10], Si [11], GaAs [12] and CuIn 0.44 Ga 0.56 Se 2 by our own dedicated measurement.…”

Regression model for stabilization energies associated with anion ordering in perovskite-type oxynitrides

“…Specifically, BaNbO 2 N can oxidize water under visible light irradiation up to 740 nm [21] . In addition, crystallization of the near-surface regions of BaNbO 2 N particles by annealing leads to a remarkable photoanodic current (attributable to water oxidation) of 5.2 mA cm −2 under simulated sunlight [23] .…”

“…Finally, the validity of the supercell structures obtained by the Metropolis method was confirmed by comparison with the results of DFT calculations. This work focused on anion ordering in BaNbO 2 N because this material is promising as a photocatalyst for solar hydrogen production [21][22][23] . Specifically, BaNbO 2 N can oxidize water under visible light irradiation up to 740 nm [21] .…”