Photocatalytic Properties of Porous Films Based on α-Fe2O3 Hollow Microspheres

Abstract

We have studied photoelectrochemical and photocatalytic properties of porous materials based on hollow α-Fe2O3 microspheres, characterized by the presence of dangling magnetic Fe–O–Fe bonds due to an increased oxygen vacancy concentration on the wall/closed pore interface. Using such powder and firing slips at an isothermal holding temperature of 400°C, we obtained two series of thin-film samples on glass with a conductive layer from suspensions of two compositions: aqueous Fe(NO3)3 solution + hollow α-Fe2O3 microspheres (series 1) and aqueous Fe(NO3)3 solution + polyethylene glycol + hollow α-Fe2O3 microspheres (series 2). The films of series 2 were shown to have a structure with spatially separated particles differing in size: α-Fe2O3 nanoparticles and hollow α-Fe2O3 microspheres. The films of series 1 consisted predominantly of hollow microspheres connected by "necks" formed during heat treatment. The thickness of the films of series 2 was of order 2 μm and that of the films of series 1 was of order 4 μm. The structural distinctions between the films of the two series had a significant effect on the optical properties of the material. In the wavelength range 350–1500 nm, the absorption coefficient of the films of series 2 (3.50 × 105 m–1) was about twice that of the films of series 1 (1.75 × 105 m–1). Photoelectrochemical characterization in an aqueous 0.1 M KOH solution showed that the onset potential for the anodic reaction was 0.87 V vs. Ag/AgCl in the case of the films of series 2 and 0.97 V vs. Ag/AgCl in the case of series 1. The films of both series showed an unusual increase in current density during prolonged illumination at a potential of 1 V vs. Ag/AgCl, due to Fe(IV) formation on the photoanode surface. Photocatalytic properties of the materials were assessed from the rate of methylene blue degradation. The reaction rate constant (k) was determined to be 0.015 and 0.018 min–1 for the films of series 1 and 2, respectively, whereas the k of the photocatalyst-free reaction was 2.8 × 10–4 min–1.