Abstract
AbstractPerovskite-type oxynitridesAB(O,N)3are potential candidates for photoelectrode materials in solar water splitting. A drawback of these materials is their low sintering tendency resulting in low electrical conductivities. Typically, they are prepared by ammonia treatment of insulating, wide band gap oxides. In this study, we propose an approach starting from small band gap oxides Eu1−xCaxTiO3−δand then widen the band gaps in a controlled way by ammonolysis and partial Ca2+substitution. Both together induced a distortion of the octahedral network and dilution of the Eu4fand N2plevels in the valence band. The effect is the stronger the more Ca2+is present. Within the series of samples, Eu0.4Ca0.6Ti(O,N)3had the most suitable optical band gap (EG≈ 2.2 eV) for water oxidation. However, its higher Eu content compared to Eu0.1Ca0.9Ti(O,N)3slowed down the charge carrier dynamics due to enhanced trapping and recombination as expressed by large accumulation (τon) and decay (τoff) times of the photovoltage of up to 109 s and 486 s, respectively. In contrast, the highly Ca2+-substituted samples (x≥ 0.7) were more prone to formation of TiN and oxygen vacancies also leading to Ti3+donor levels below the conduction band. Therefore, a precise control of the ammonolysis temperature is essential, since even small amounts of TiN can suppress the photovoltage generation by fast recombination processes. Water oxidation tests on Eu0.4Ca0.6Ti(O,N)3revealed a formation of 7.5 μmol O2from 50 mg powder together with significant photocorrosion of the bare material. Combining crystal structure, chemical composition, and optical and electronical band gap data, a first simplified model of the electronical band structure of Eu1−xCaxTi(O,N)3could be proposed.
Highlights
Perovskite-type oxynitrides AB(O,N)3 are potential candidates as light absorbers/ photocatalysts and photoanodes in the photoelectrolysis of water producing oxygen and hydrogen fuel
It was evaluated if this applies to the respective perovskite-type oxynitrides Eu1−xCaxTi(O,N)3
Ammonolysis of EuTiO3−δ was reported at 1223 K [48], which according to our experience is too high and will lead with our setup to the formation of TiN [33]
Summary
Perovskite-type oxynitrides AB(O,N) are potential candidates as light absorbers/ photocatalysts and photoanodes in the photoelectrolysis of water producing oxygen and hydrogen fuel. As demonstrated by our previous studies, partial substitution of A2+ (A = Ba2+, Ca2+) for Eu2+ is an effective strategy to further adjust the crystal structure and with it the electronic band structure, the band gap, as well as the charge carrier transport properties [50, 55] It is well-known that (partial) substitution of a smaller cation on the A-site reduces the overlap of the O/N 2p-orbitals and the typically empty nd-orbitals (n = 3, 4, 5; principal quantum number) of the B-site cations leading to a smaller dispersion of the conduction band and a widening of the band gap [33, 56]. The yellow to green oxynitrides Eu1−xCaxTi(O,N) were synthesized via thermal gas flow ammonolysis of the wellsinterable, small band gap, perovskite-type oxides Eu1−xCaxTiO3−δ (0 < x < 1). Their band gap size of 1.6 eV–2.6 eV is suitable for photoinduced water oxidation. Combining the data of X-ray diffraction (XRD), chemical analysis, diffuse reflectance spectroscopy (DRS), surface photovoltage spectroscopy (SPS), and electrical resistivity and Seebeck coefficient measurements, a first simplified model of the electronical band structure of the perovskite-type oxynitrides Eu1−xCaxTi(O,N) could be proposed
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