Photoelectrochemical Water Splitting on Particulate ANbO₂N (A = Ba, Sr) Photoanodes Prepared from Perovskite-Type ANbO₃

Abstract

Perovskite-type Nb-based oxynitrides (ANbO 2 N, A = Ba, Sr) exhibiting absorption of long light wavelengths up to (A = Sr) or even above 700 nm (A = Ba) were synthesized from stoichiometric perovskite-type oxides (ANbO 3 ) by a typical thermal ammonolysis method. The preparation of BaNbO 2 N using BaNbO 3 having an identical cubic-perovskite-type structure was demonstrated to effectively improve the surface and bulk crystallinity of the oxynitride because of the absence of a structural transition during nitridation and the stoichiometric 1:1 Ba/Nb ratio between the oxides and oxynitrides. The resulting BaNbO 2 N photoanode exhibited photoelectrochemical water oxidation activity with a current density of 0.85 mA cm −2 at 1.23 V RHE and a faradaic efficiency close to unity under simulated solar illumination (AM 1.5G). The associated incident photon-to-current efficiency in the range of 740−480 nm showed that BaNbO 2 N produces a photoanodic current at 740 nm, the longest response wavelength ever reported for a photoanode based on an (oxy)nitride.

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

“…[7] An extended nitridation time of 40 h resulted in a small improvement in the photocurrent, even though the crystallinity of BaNbO 2 N, as judged by the full width at half-maximum value for the (1 1 0) XRD peak, improved ( Figure S1, Supporting Information). The BaNbO 2 N powder was prepared by nitridation of Ba 5 Nb 4 O 15 at 1173 K for 20 h, followed by annealing in inert Ar or reductive NH 3 at 873 K. The X-ray diffraction (XRD) patterns and diffuse reflectance spectroscopy (DRS) spectra for these materials, shown in Figures S1 and S2 (Supporting Information), respectively, demonstrate that the resulting oxynitrides were composed of perovskite-type BaNbO 2 N, capable of absorbing a wide range of visible light up to 740 nm.…”

“…[7] Accordingly, the surface properties of BaNbO 2 N were investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). [7] Accordingly, the surface properties of BaNbO 2 N were investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).…”

“…Instead, a plateau was maintained until the decomposition started at 1007 K. Figure 5b shows the temperature programmed desorption/ mass spectrometry (TPD/MS) spectra of as-prepared BaNbO 2 N measured in an Ar flow using the same temperature profile applied during annealing. The originally amorphous surface of BaNbO 2 N, resulting from the mild nitridation of Ba 5 Nb 4 O 15 , [7] also required a temperature in the range where crystallization occurs. The intensity of the m/z 18 spectrum was the highest, although the others exhibited similar desorption trends.…”

“…Solar-driven photoelectrochemical (PEC) water splitting into hydrogen and oxygen using semiconductor stoichiometric Ba/Nb ratio of unity. [7] Improving both the surface and bulk crystallinity of the oxynitride by preserving the crystal structure during the phase transition associated with nitridation is known to enhance the photoactivity to approximately 0.85 mA cm −2 at 1.23 V RHE . [8] Such mixtures have been found to effectively suppress the formation of NbO x N y , but require high-temperature nitridation under a high flow rate of NH 3 gas that promotes nitridation.…”

“…[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. [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. 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%.…”