Electrospun coral-like α-Fe2O3 nanostructures for photoelectrochemical water splitting

Hydrogen-based technologies represent one of the most promising approaches to the development of clean energy sources [1]. Especially photo-electro-chemical water splitting (PEC-WS) allows producing hydrogen by solar energy conversion using environmentally friendly, easily available, cheap and stable photo-catalyst. Focus of this study is on synthesis, characterisation and testing of hematite (α-Fe2O3) films, as photo-anodes for PEC activity. Films, consisting of highly porous fibres, were produced via electro-spinning (ES), a very competitive technique for the synthesis of 1-dimensional high surface area materials [2,3]. The fibrous films were directly spun over fluorine-doped tin oxide (FTO) substrates. Dymethilformammide (DMF), polyacrylinitrile (PAN, 6.5 wt%) and iron(II) acetate (FeAc2, 2.5 wt%) acted respectively as solvent, polymer and oxide precursor in the preparation of the spinnable solution. ES was carried out for prefixed time in order to obtain different film’s thickness (solution feeding-rate: 1.41 mL/h; applied potential: 15kV; syringe needle tip/grounded collector distance: 11 cm). After calcination in air at 600°C, texture and morphology of the fibres and crystalline phase of the oxide were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and micro-Raman spectroscopy (MRS) analyses. The formation of hollow coral-like fibres (average diameter: 170 nm), consisting of interconnected nanocrystalline α-Fe2O3 grains, was evidenced. PEC performance was investigated, in 1M NaOH solution, by means of a three-cell system. Ag/AgCl and Pt electrodes were used as reference- and counter-electrode, respectively; electro-spun hematite films on FTO/glass substrate acted as working electrode. The Fe2O3 fibrous films were tested without any pre-conditioning. The thinner film (2 min spinning) exhibited very poor activity, as its thickness was too low. Conversely, the PEC activity of the thicker film (4 min spinning) was higher: at 1.23 V vs. RHE (reversible hydrogen electrode) the photocurrent value was 1 µA/cm2. Hematite film pre-conditioning to promote Sn diffusion from the FTO support, longer duration of the synthesis to increase film thickness, and hematite doping are the strategies that will be pursued in future to improve the PEC performance of electro-spun Fe2O3 fibres.