Publication date: 15th December 2014
Selected iron tungstate films have been deposited as nanostructured films using (a) a sol-gel process and (b) reactive sputtering employing polystyrene spheres (PS) as templates. Compositions were chosen analysing combinatorial thin film libraries of the system Fe-W-O [1] and were prepared as structured films with the aim to improve the photoanode activity in the process of water oxidation.
(a) Thin films of iron tungstates were prepared by dip coating from a polymerisable precursor. For this purpose, aqueous solutions of WCl6 and Fe(NO)3 were added to an ethanol solution containing the block copolymer F127 as structuring agent. The layers were deposited on conductive glass (FTO) substrates, silicon wafers covered by a platinum thin film electrode and tungsten foils, respectively.
(b) In first experiments the plasma etching time of the spheres was fixed to 40 minutes and the film thickness was varied to manufacture iron tungstate photoanodes of different compositions and to study their behaviour in photoelectrochemical water splitting.Best results were obtained after an annealing of the layers at 450°C under oxygen and in some cases in a second annealing step at 1000°C using an inert gas atmosphere.
Since photocurrent peak intensities in the photoelectrochemical mapping of libraries had been detected at iron contents of 25%, 30%, 35%, 55%, respectively, films of these compositions with a thickness varying from 50 - 800 nm were deposited on the substrates. While films containing 25% - 35% Fe crystallized in a WO3-type structure after an annealing at 450°C in air, layers of the scheelite phase of composition Fe2-xW1+xO6 containing 55% Fe formed after a treatment at 1000°C in an inert gas atmosphere as evidenced by X-ray diffractometry. Depending on the annealing temperature and film composition photocurrents in the range from 0.14 to 2.4 mA/cm2 at U = 1.23VRHE have been achieved (compare also ref. [2]). The increase of photocurrent is associated with an increase in iron content and band gap of the layer.
[1] Kirill Sliozberg, Robert Meyer, Alfred Ludwig, Wolfgang Schuhmann: А combinatorial study of photoelectrochemical properties of Fe-W-O thin films, ChemPlusChem, 2014.
[2] Renata Solarska, Krzysztof Bieńkowski, Agata Królikowska, Mirosław Dolata and Jan Augustyiński: Nanoporous WO3 – Fe2O3 films, structural and photoelectrochemical characterization, Functional Materials Letters, 7 (2014) 1440006.
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