Investigating Sr₁₋ₓNbO₃ for H₂ evolution and as part of systems attempting water splitting under visible light irradiation

Abstract

Two main subjects are addressed in this study. The ability of a bright red material with metallic behaviour to be used as a visible light photocatalyst for hydrogen evolution and the feasibility of visible light photocatalytic water splitting using Z-schemes constituted from different kinds of photocatalysts and materials used as mediators. Strontium niobate (Sr₁₋ₓNbO₃) is an A-site deficient perovskite with intense red colour. It is an unusual material that displays both metallic type conduction and- as we present- photocatalytic activity. Specifically, photocatalytic visible light hydrogen production with oxalic acid as a sacrificial reagent is achieved from this material even without the need for a co-catalyst or other alteration. This photocatalytic activity is screened with time and related to different parameters that might influence it, like crystal structure, surface area and surface chemistry. The crystal structure of strontium niobate is A site stoichiometry dependant and the materials acquires a cubic symmetry for Sr≤ 0.92 and orthorhombic for 0.92≤ Sr≤ 0.97. The change of crystal structure from cubic to orthorhombic symmetry seems to have a negative effect on the photocatalytic activity, as the NbO₆ octahedra become distorted and unfavourable for d-orbital overlapping. The highest photocatalytic activity is exhibited at the turning point of one structure to the other. Increase in the photocatalytic activity is also exhibited by enlarging the surface area through ball milling, nevertheless, a clear trend for surface area effect on activity is not obtained among samples with different Sr content. Additionally, an enrichment of Sr on the surface of strontium niobate is observed by XPS, which apart from the fact that seems to be a governing factor improving stability it is also considered a key point for the exhibited photocatalytic activity altogether. Full water splitting under visible light from Z-schemes is studied by fabricating three general categories of systems. These three different categories depend on the mediator used to fabricate the Z-schemes and are: redox couple Z-schemes (with Fe⁺³/Fe⁺²), solid mediator Z-schemes (with GO) and no mediator Z-schemes. The materials used as photocatalysts for the fabrication of the Z-schemes are: Sr₀.₉₂NbO₃ for hydrogen production and both WO₃ and BiVO₄ independently for oxygen production. The photocatalytic activity for water splitting is evaluated in production of hydrogen and oxygen with time and the ratio of their production rates is frequently checked to see whether the ideal hydrogen to oxygen 2:1 is achieved. The general idea acquired from the results of all the three types of systems is that, water splitting with Z-schemes is a complicated process and in most cases governed by many subreactions. More specifically, in all cases of redox couple Z-schemes we got hydrogen to oxygen ratio imbalances and with the most prominent one being the lack of hydrogen production. Thankful is the fact that a certain type of system, the one consisting of WO₃ as oxygen photocatalyst and Fe⁺² as initial mediator species gives results very close to the ideal one and with a high degree of reproducibility indicating this way the probable formation of a Z-scheme that has overcome more of the imbalances. In between the two other categories, solid mediator and no mediator Z-schemes, subreactions seem to be the governing factor hence imbalances are always present. A case study in the no mediator Z-schemes on an attempt to investigate sources of imbalances, reveals that a big source of imbalance is most probably from the trapping of protons from WO₃.