(Invited) Surface Properties of Perovskite Oxynitrides Influence Solar Water Splitting Activity
Authors: Seo Jeongsuk 
Published: December 2020

Photoelectrochemical (PEC) water splitting using semiconductors has been a promising means to directly produce hydrogen gas from water. Semiconductors capable of absorbing a wide wavelength range of visible light are in the limelight at the moment, because the favourable optical property causes thermodynamically high solar-to-hydrogen (STH) conversion efficiency. The n-type perovskite oxynitrides AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) have intensive absorption bands in visible light above 600 nm so that they are one of the leading candidates for the solar water splitting.1 For instance, LaNbON2 has an Eg
of 1.7 eV (wavelength λ = 750 nm) and thus potential for the STH conversion efficiency of 29.5% in reference to an incident photon-to-current efficiency of unity. However, the water splitting activity using the oxynitrides is relatively low and unstable under long-term illumination.1-2

The synthesis of oxynitrides, namely, the nitridation of starting oxides to the corresponding oxynitrides, is performed mostly in harsh conditions including a high temperature, long nitridation time, and a high flow rate of NH3. Such the severe nitridation obviously increases a bulk crystallinity of oxynitrides, which it permits efficient light absorption and charge separation under irradiation. In particular, it is necessary to prepare high-crystalline BaBO2N because the oxynitrides have the absence of stoichiometric, crystalline oxide precursors (with Ba/B ratio of unity) for nitridation.3-4

Unfortunately, the severe nitridation enhances reductions of B-site cations to a lower oxidation state (e.g., Nb5+ to Nb4+ or Nb3+) leading to anion vacancy and/or surface impurity phases such as NbO
x
N
y
, owing to the high electronegativity of the B-site cations. The surface defect sites, where the photoreaction takes place, promotes recombination of photoexcited holes and electrons during the PEC water oxidation, thus resulting in low photocurrent density. There were several reports on how to decrease defect density of oxynitrides during the nitridation, including flux-assisted nitridation and etching treatment by an aqua regia.4-5 However, these methods should be positively considered the suitable flux media and unintended doping depending on each oxynitride, and the partial loss of oxynitride particles. There was also a limitation in improving the photocurrent density.
Herein we present the active and stable solar water oxidation over oxynitride photoanodes, which was remarkably improved by surface annealing in an inert Ar flow to increase the degree of crystallinity and as well to suppress the generation of surface defects.6 Also, we report the quantitative relationship between the surface defect density of the oxynitride and its photocurrent density, demonstrating that surface properties of oxynitrides greatly influence the solar water splitting performance.7

Oxides (or carbonates) contains A-site cations and B2O5 (B=Ta, Nb) were blended and calcined in air at different temperatures from 1273 to 1573 K for 30 h to obtain crystalline oxides such as Ba5B4O15. The starting oxides were nitrided at different temperatures and durations under a NH3 flow. The products were washed by distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed under an Ar flow at an appropriate temperature depending on oxynitride. For PEC measurements, particulate oxynitride photoanodes were prepared by a particle transfer method or an electrophoretic deposition.
The nitridation of Ba5B4O15 caused highly-defective BaBO2N surface due to Lewis base and Ba-rich conditions of the starting oxide. The annealing in Ar both enhances the crystallinity of amorphous surface and effectively suppresses the formation of reduced defects. As a result, the annealed BaNbO2N photoanode exhibited greatly enhanced photocurrent of 5.2 mA cm-2 at 1.23 VRHE during sunlight-driven water oxidation, which is approximately twenty times higher than that of as-prepared BaNbO2N. BaTaO2N is thermally more stable in an Ar flow as compared with BaNbO2N, so that the high-temperature annealing at 1073 K was found to improve both the bulk and surface properties of inactive as-prepared BaTaO2N. Consequently, the particulate BaTaO2N photoanode followed by the Ar annealing produced a half-cell STH energy conversion efficiency of 1.4% at 0.88 VRHE. In addition, the solar water oxidation retained 79% of the initial photocurrent over 24 h. Surface and bulk characterizations of perovskite oxynitrides annealed in Ar and the corresponding photoresponse will be discussed in detail in the presentation.
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