Sn-Based Perovskite with a Wide Visible-Light Absorption Band Assisted by Hydride Doping

Abstract

Visible-light-absorbing, narrow-gap semiconductors have attracted significant interest in recent years owing to their photofunctional applications, including heterogeneous photocatalysis and solar cells. Herein, we report a new Pb-free perovskite material with a wide range of visible-light absorption that was successfully synthesized via hydride (H–)-ion doping into BaSn0.7Y0.3O3−δ. Chemical reduction that accompanied H–-ion doping significantly reduced the optical band gap of the host BaSn0.7Y0.3O3−δ from 4.0 to 2.0 eV. Physicochemical measurements revealed that BaSn0.7Y0.3O3−δ underwent the partial reduction of Sn4+ to Sn2+ at B-sites while maintaining the cubic perovskite structure. Density functional theory calculations showed that the electronic states derived from the Sn2+ lone pair, which was located above the top of the valence band, contributed to the visible-light absorption of reduced BaSn0.7Y0.3O3−δ. The prior introduction of oxygen defects into BaSnO3 by Y3+ substitution for Sn4+ was also indispensable to realize a significant reduction of the band gap. The reduced BaSn0.7Y0.3O3−δ doped with H– ions was applied as a photoelectrode material, and a clear anodic photoresponse of up to 600 nm was observed, which demonstrates that the material can be used for photofunctional applications. The results of this work suggest that the utilization of stereoactive lone pair of electrons in combination with H– doping may provide another approach to the band gap reduction of Sn­(IV)-based oxides, which are nontoxic and less expensive, but generally wide-gap semiconductors.

Synthesis of Hydride-Doped Perovskite Stannate with Visible Light Absorption Capability

“…This weight gain indicates the oxidation of Sn 2+ and H – ions . Desorption of H 2 was seen at the onset temperature range of weight gain under a He atmosphere, as reported previously. ,, The tendency of the onset temperature of H 2 desorption was similar to that reported for ATiO 3– x H x ; the onset temperature was lower for A = Ba 0.5 Sr 0.5 than for A = Ba and Sr . These results support the presence of H – in these oxides, although the quantitative analysis was not possible due to low H – concentration.…”

“…Upon treatment with CaH 2 , the XRD peak positions were slightly shifted to lower 2θ angles, while the perovskite structures were maintained. A similar peak shift has also been observed in the H – -doped Ba-Sn-Y oxide prepared by the solid-state reaction (SSR) method . In that case, it was attributed to the formation of Sn 2+ associated with the introduction of H – .…”

“…The atomic orbitals of the introduced elements, which are located at higher energies than the O-2p orbitals in the valence band maxima, are the origin of the bandgap reduction and the visible light absorption capability. The utilization of both mixed anionization and lone pair electrons has recently been reported in the synthesis of materials that exhibit visible light absorption with unique band structures. − …”

“…The use of Sn 2+ as a visible-light-absorption center is of interest for the replacement with lead-free materials . We have very recently reported a hydride-doped Sn-based perovskite oxide (BaSn 0.7 Y 0.3 O 3−δ ) capable of absorbing visible light up to 600 nm . H – -doped BaSn 0.7 Y 0.3 O 3−δ can be synthesized by the reaction of CaH 2 and a Ba-Sn-Y oxide precursor at 773 K in vacuo, during which Sn 2+ is generated.…”

“…We first attempted to synthesize precursor oxides based on various ASnO 3 (A = Ba, Ba 0.5 Sr 0.5 , Sr, and Ca) compositions that possess oxygen vacancies. As indicated by our previous work, the introduction of oxygen vacancies into the precursor oxide prior to H – -doping is indispensable to obtain visible light absorption capability by the H – -doping methodology . Therefore, the substitution of trivalent cations such as Y 3+ for Sn 4+ in the ASnO 3 perovskite was first examined using the polymerized complex (PC) method .…”