Visible-Light-Driven Water Splitting Using Perovskite-Type Oxynitride Photoanodes
Authors: Seo Jeongsuk , Hisatomi Takashi , Domen Kazunari 
Published: February 2020

The n-type perovskite oxynitrides having a general formula AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) are capable of absorbing a wide range of visible light above 600 nm, theoretically leading to high solar-to-hydrogen (STH) conversion efficiency. Interestingly, band-gap energies (Eg
) of the oxynitrides can be tuned by different combinations of elements at the A and B sites. For example, BaNbO2N has an Eg
of 1.8 eV (wavelength λ = 740 nm) and thus potential for the STH conversion efficiency of 28.6% in conjunction with an incident photon-to-current efficiency of unity. Despite the favourable optical properties, the photoelectrochemical (PEC) water splitting using the oxynitrides, except LaTiO2N (λ = 600 nm), has rarely been studied up to the present and the photoactivity under sunlight irradiation remains relatively low.1)

The presence of B species in AB(O,N)3, which are readily reduced during nitridation because of the high electronegativity of B, promotes the generation of defects and/or impurities at oxynitride surface.1) In fact, Nb5+ in ANbO2N is reduced to Nb4+ and/or Nb3+ during high temperature nitridation.2) Also, the absence of facile, crystalline oxide precursors with Ba/B stoichiometry to BaBO2N (B=Ta, Nb) produces amorphous surface of BaBO2N after the nitridation.3) These mainly lead to low crystallinity of the oxynitrides, which enhances recombination of photoexcited holes and electrons and thus results in low photocurrent for the water splitting. Therefore, the control of defects and impurities during the synthesis of oxynitrides is crucial to enhancing the water splitting activity.

Herein we report a novel synthesis route of BaNbO2N as a challenging material, including mild nitridation of a crystalline oxide and subsequent annealing in an inert Ar flow, which led to highly increased anodic photocurrent density for solar-driven water oxidation.4) Moreover, we discuss surface states of BaNbO2N subjected to the Ar annealing at various temperatures and the effects of such surface states on the photocurrent density under sunlight irradiation.5)

Perovskite-type BaNbO2N was synthesized by the thermal nitridation of Ba-rich crystalline Ba5Nb4O15 at 1173 K for 20 and 40 h. The nitrided products were washed with copious amounts of distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed in Ar flow at different temperatures such as 773, 873, 973 and 1073 K for 1 h. For PEC water oxidation trials, the particulate BaNbO2N photoanodes were prepared by a particle transfer method.6)

The mild nitridation of Ba5Nb4O15 for 20 h produced amorphous surface of the as-prepared BaNbO2N owing to Lewis base and Ba-rich conditions of the starting oxide, as presented in Figure 1(a). The prolonged nitridation for 40 h, to increase surface and bulk crystallinity of the oxynitride, rather led to NbN impurity nanoparticles covered on the oxynitride surface shown in (b). The photoanode using this BaNbO2N caused very limited improvement in water oxidation activity.

Alternatively, the annealing in Ar as a post-nitridation treatment was carried out to modify the surface property of as-prepared oxynitride and compared with that in a reducing NH3. Both the annealing treatments enhanced surface crystallinity, demonstrating clear lattice fringes of single BaNbO2N planes presented in (c) and (d). Furthermore, the annealing in Ar was very effective at suppressing the generation of surface defects and impurities. As a result, the particulate BaNbO2N photoanode exhibited a remarkably enhanced photocurrent density of 5.2 mA cm-2 at 1.23 VRHE toward visible-light-driven water splitting (AM 1.5G), which has been never reported for an oxynitride responsive at wavelengths above 600 nm. Surface and bulk characterizations of BaNbO2N treated in different annealing conditions and the corresponding photoelectroactivity will be discussed in more detail in the presentation.

References

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Figure 1
. HRTEM images of BaNbO2N particles as-prepared by the nitridation at 1173 K for 20 (a) and 40 h (b) and subsequently annealed at 873 K for 1 h at NH3 (c) and Ar atmospheres (d). (Reprinted from ref. 4)

Figure 1