Abstract
The effects induced by post-synthesis hydrogenation on ZnFe2O4 flat films in terms of photoelectrochemical (PEC) performance of photoanodes for water oxidation have been deeply investigated as a function of the pre-annealing temperature of the materials. The structure and morphology of the films greatly affect the efficacy of the post synthesis treatment. In fact, highly compact films are obtained upon pre-annealing at high temperatures, and this limits the exposure of the material bulk to the reductive H2 atmosphere, making the treatment largely ineffective. On the other hand, a mild hydrogen treatment greatly enhances the separation of photoproduced charges in films pre-annealed at lower temperatures, as a result of the introduction of oxygen vacancies with n-type character. A comparison between present results and those obtained with ZnFe2O4 nanorods clearly demonstrates that specific structural and/or surface properties, together with the initial film morphology, differently affect the overall contribution of post-synthesis hydrogenation on the efficiency of zinc ferrite-based photoanodes.
Highlights
The need on Earth for renewable energy is going to rise over the years, driven by the fact that the energy demand of the planet during this century cannot be satisfied by fossil fuels [1]
A strong correlation was ascertained between the spinel inversion degree, defined as the fraction of tetrahedral sites occupied by trivalent Fe3+ ions, and the charge transport in the presence of the peculiar NR morphology
Optimizing this aspect was linked to the observed record PEC performance as a function of the film annealing temperature, which was attained with films after a hydrogenation treatment performed with the aim of improving the n-type character of the photoactive materials [22]
Summary
The need on Earth for renewable energy is going to rise over the years, driven by the fact that the energy demand of the planet during this century cannot be satisfied by fossil fuels [1]. Among the different types of PEC devices [5], one of the most efficient is the dual-absorber tandem configuration where two semiconductor electrodes with complementary band gap energies are wired together to maximize light harvesting and cell performance under different conditions [6,7]. A strong correlation was ascertained between the spinel inversion degree, defined as the fraction of tetrahedral sites occupied by trivalent Fe3+ ions, and the charge transport in the presence of the peculiar NR morphology Optimizing this aspect was linked to the observed record PEC performance as a function of the film annealing temperature, which was attained with films after a hydrogenation treatment performed with the aim of improving the n-type character of the photoactive materials [22]. Since we aimed to exclude the potential effects induced by film thickness on the poor charge transport properties, we focus on the investigation of flat ZFO films with moderate thickness, i.e., ca. 100 nm thick, prepared according to a generalized approach for ternary metal oxides, which offers a facile processable green route to obtain optically transparent films suitable to be employed as photoanodes in dual absorber tandem PEC cells
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