Band Gap Adjustment in Perovskite-type Eu_1−xCa_xTiO₃via Ammonolysis

Abstract

Perovskite-type oxynitrides AB(O,N)3 are 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−δ and 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 Eu4f and N2p levels in the valence band. The effect is the stronger the more Ca2+ is present. Within the series of samples, Eu0.4Ca0.6Ti(O,N)3 had 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)3 slowed 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)3 revealed a formation of 7.5 μmol O2 from 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)3 could be proposed.

Bandgap-adjustment and enhanced surface photovoltage in Y-substituted LaTaIVO2N

“…69 The larger photovoltage values correspond to an enhanced separation of the photogenerated charge carriers. It has already been reported that the presence of reduced B-site metal cations such as Ti III 70,71 or Ta IV 72 can cause fast recombination of photogenerated electrons and holes vanishing the photovoltage. However, based on the SPS data in Fig.…”

Solution Combustion Synthesis and Characterization of Magnesium Copper Vanadates

“…Thus, phase spaces encompassing binary metal oxides as the end members (e.g., Cu x O, Bi 2 O 3 , V 2 O 3 ) and a multitude of stable (ternary) compounds have attracted much recent interest. − ,− Corresponding studies on phase spaces involving ternary oxides as end members, however, are much less commonplace, hence one of the motivations for this study. Examples of prior research on ternary oxide solid solutions include (Ba 1– x Sn x )­ZrO 3 , M 2 V 2 O 7 (M = Mn, Cu), Eu 1– x Ca x TiO 3 , (Cu 1– x Li x )­GaO 2 , (Bi 1– x Fe x )­VO 4, (Li 1– x Cu x )­Nb 3 O 8 , M x Bi 2– x Ti 2 O 7 (M = Fe, Mn), BiZn 2 VO 6 , BiCu 2 VO 6 , and Cu 5 (Ta 1– x Nb x ) 11 O 30 . Combinatorial synthesis combined with photoelectrochemical (PEC) screening has been used to develop Cu–V–M–O (M = Mg, Ca, Sr, Fe) compositions and to alloy β-Cu 2 V 2 O 7 with six cations …”

Synthesis and Tuning of Electronic, Optical, and Photoelectrochemical Properties of Copper Metavanadate Alloys via Alkaline Earth Metal Substitution

“…Examples of recent research on ternary oxide solid solutions include (Ba 1-x Sn x )ZrO 3 , 2 M 2 V 2 O 7 (M = Mn, Cu), 3 Eu 1−x Ca x TiO 3 , 4 (Cu 1−x Li x )GaO 2 , 5 (Bi 1−x Fe x )VO 4 , 6 (Li 1-x Cu x )Nb 3 O 8 , 7 M x Bi 2−x Ti 2 O 7 (M = Fe, Mn), 8 BiZn 2 VO 6 , 9 BiCu 2 VO 6 , 10 and Cu 5 (Ta 1−x Nb x ) 11 O 30 . 11 Combinatorial synthesis combined with photoelectrochemical (PEC) screening was used to develop Cu-V-M-O (M = Mg, Ca, Sr, Fe) compositions 12 and to alloy β-Cu 2 V 2 O 7 with six cations.…”