Sol-gel auto-combustion synthesis of Ca2Fe2O5 brownmillerite nanopowders and thin films for advanced oxidation photoelectrochemical water treatment in visible light

• Published in: Journal of environmental chemical engineering 2019-08, Vol.7 (4), p.103224, Article 103224
• Main Authors: Vanags, Mārtiņš, Spule, Arnita, Gruškeviča, Kamila, Vihodceva, Svetlana, Tamm, Aile, Vlassov, Sergei, Šutka, Andris
• Format: Article
• Language: English
• Subjects: Dye degradation; Iron oxide; Photo-Corrosion; Photocatalysis; Stability
• ISSN: 2213-3437
• DOI: 10.1016/j.jece.2019.103224

[Display omitted] •Pure phase brownmillerite Ca2Fe2O5 was synthesized by sol-gel auto-combustion.•Superoxide radicals was the dominant oxidant in the degradation mechanism.•Self-reduction production led to the formation of CaCO3 and Fe2O3.•Thin film structure improved the structural stability of Ca2Fe2O5 under positive bias. This study describes a straightforward Ca2Fe2O5 brownmillerite nanopowder and thin film synthesis by the water-based sol-gel auto-combustion method. The material characterization results confirmed the phase pure narrow bandgap Ca2Fe2O5 nanoparticle formation. The surface area of synthesized nanopowder was 13.55 m2/g. Powders at loading 1 g/l exhibit high visible light photocatalytic activity by degrading 10 mg/l methylene blue in water in 120 min confirmed by total organic carbon studies. The high visible light photocatalytic activity is related to Ca2Fe2O5 narrow band gap and high reduction potential of its conduction band which triggers the formation of superoxide radical ∙O2-. The limited chemical stability for Ca2Fe2O5 nanopowders was observed due to self-reduction under photocatalytic conditions. The degradation rate constant decreased almost five times already in the second photocatalysis cycle. Further, we are demonstrating the application of Ca2Fe2O5 films in the photoelectrochemical water treatment (PECWT) process. We show that brownmillerite films are becoming stable against photo-corrosion under positive bias in PECWT and could be suitable for dye degradation in water. In the PECWT process, the Ca2Fe2O5 is protected from self-reduction since the photoinduced electron under positive bias is moved away to the Pt electrode. Under short circuit or biased (0.5 V) conditions the rate constant for MB degradation is decreased only for approximately 15% in 6 cycles.