Electrospun SrNb2O6 photoanodes from single-source precursors for photoelectrochemical water splitting
Graphical abstract
Introduction
Water splitting is foreseen as an alternative strategy to produce fuel in a clean and sustainable way, eventually replacing fossil fuels [[1], [2], [3]]. Achieving an efficient conversion of solar energy into hydrogen and oxygen requires materials with broad sunlight absorption, high voltage production (>1.23 V) and band edges compatibility with hydrogen or oxygen redox potentials [4,5].
Bimetallic metal oxides are suitable candidates for photoelectrochemical water splitting not only due to their versatility, stability and resistance against corrosion, but also because of the highly tunable and complex orbital structure caused by multiple oxidation states, ionic vacancies, metal-oxygen-metal (M-O-M) interactions and their distortable octahedral structures [6,7]. Particularly interesting is the layered structure of strontium metaniobate (SrNb2O6), containing MO6 octahedra with high d-orbital energy levels and orbital intermixing [8,9].
Fabrication of photoelectrodes of SrNb2O6 has been mostly limited to high temperature preparation of powders by solid-state reactions, often leading to large agglomerations and low surface areas, detrimental for solar energy conversion [9,10]. Recently, the successful synthesis of 1D SrNb2O6 nanostructures (e.g., nanorods and nanotubes) and 2D nanosheets by hydrothermal methods has lead to a significant improvement of the photocatalytic activities for organic dye degradation and H2 production by water splitting [8,11,12]. These studies suggest that alternative nanostructurating methods are desired in order to achieve large active surface areas with a high throughput production [[13], [14], [15], [16], [17], [18], [19]]. Electrospinning is a suitably scalable, low-cost and net-shaping technique for the fabrication of free-standing oxide nanofibers from solutions. The high porosity, large surface areas, dimensional charge carrier confinement, interconnectivity and the presence of surface states enable the efficient transfer of charge carriers in 1D nanofiber meshes, demonstrating their advantage in many applications [[20], [21], [22], [23], [24]].
The preparation of the electrospinning solution for multicomponent systems often poses a chemical and synthetic challenge. Therefore, the use of single-source precursors such as heterobimetallic alkoxides is a very attractive route for the production of complex oxides due to the preorganized final metal-oxygen bonds, cationic ratios tailored to the final product, and the possibility to adjust the physico-chemical and morphological properties [[25], [26], [27], [28]].
In this study, we report the synthesis and characterization of a Sr–Nb bimetallic alkoxide precursor used as a single-source in the fabrication of SrNb2O6 nanofibers by electrospinning. The optical and photoelectrochemical properties of electrospun SrNb2O6 photoelectrodes were evaluated following two different photoelectrode preparation strategies namely doctor blading and spin coating.
Section snippets
Precursor synthesis
The preparation of [SrNb2(OiPr)12(HOiPr)] was performed in a modified all-glass Stock-type assembly under nitrogen atmosphere. All solvents were dried over sodium and distilled prior to use. Niobium(V)-iso-propoxide [Nb2(OiPr)10] was synthesized from NbCl5 (Alfa Aesar, 96% in purity) following the procedure reported in the literature [29]. In a following step, 3.67 g (9.40 mmol) of [Nb2(OiPr)10] were added to a solution containing 0.41 g (4.70 mmol) of Sr metal (Aldrich, 99% purity) in
Synthesis of bimetallic Sr–Nb alkoxide precursor
The molecular structure of bimetallic alkoxide [SrNb2(OiPr)12(HOiPr)] consists of an asymmetric trinuclear framework containing two peripheral octahedral [Nb(OiPr)6]--units connected to a central Sr2+ cation by two μ2-alkoxo bridges (bidentate) on one side, and three μ2-alkoxo bridges (tridentate) on the other side (Fig. 1) [[35], [36], [37]]. In addition, one neutral 2-propanol molecule coordinates the strontium atom to complete the distorted octahedral geometry. The μ2-bridging alkoxo ligands
Conclusions
SrNb2O6 nanofiber-based photoelectrodes for water splitting were synthesized from a bimetallic [SrNb2(OiPr)12(HOiPr)] precursor. The electrospinning and subsequent calcination of the sols obtained from the molecular precursor at low temperature led to the formation of crystalline SrNb2O6 nanofibers with the presence of a small amount of the orthorombic SrNb6O16 phase. The optical bandgap of photoelectrodes, produced by doctor blading and spin coating of the nanofibers, revealed lower values
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgements
Authors kindly acknowledge the financial support obtained in the framework of DFG Priority Program Perovskites “SPP 2196/1 - Perovskite semiconductors: From fundamental properties to devices”. Dr. Selina Olthof is acknowledged for the help with the XPS measurements. A. Queraltó appreciates the support of the Alexander von Humboldt Foundation and the German Federal Ministry for Education and Research, supporting his postdoctoral research.
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