Visible-Light Photocatalytic Properties of Weak Magnetic BiFeO3 Nanoparticles

In recent years, multiferroics, showing the coexistence of magnetic and ferroelectric orders in a certain range of temperature, have attracted a great deal of attention due to the fascinating fundamental physics and potential applications for novel magnetoelectric devices. Among all multiferroic materials studied so far, perovskite-type BiFeO3 (BFO) is known to be one of the several compounds that exhibit ferromagnetism (FM) at room temperature (RT), with high ferroelectric (FE) Curie point (Tc ∼ 1103 K) and G-type antiferromagnetic (AFM) Néel point (TN ∼ 647 K). In addition to the potential magnetoelectric applications, BFO might find applications as photocatalytic materials due to its small bandgap. This small bandgap also allows carrier excitation in BFO with commercially available femtosecond laser pulses, hence enables us to develop ferroelectric ultrafast optoelectronic devices as widely demonstrated in semiconductors. In fact, regarding the photocatalytic property of BFO, it was demonstrated that SrTiO3 coated BFO nanoparticles can produce H2 under the irradiation of visible light, whereas pure SrTiO3 only responded to UV irradiation. More recently, we have reported the oxidation (oxygen generation) ability of BFO nanowires, suggesting that BFO nanostructures might be useful for photocatalytic decomposition of organic contaminants. Although weak ferromagnetic (FM) order was observed in BFO films at RT, different from the AFM magnetic nature of BFO ceramics, the origin in films is not fully understood. Recently, Bea et al. attributed the magnetic moment in BFO films to the extra phase like c-Fe2O3 and argued that BFO phase has a very small magnetic moment if any. We reported weak FM order in BFO nanowires and suggested that the size effect in nanostructures like films and nanowires might be responsible for the FM property. In this communication, we report the synthesis of BFO nanoparticles by a solgel technique and its photocatalytic and magnetic properties. Figure 1a presents the XRD pattern of the nanoparticles prepared under the optimal conditions (calcined at 500 °C for 2 h, the detail of preparation is described in the Experimental Section). It is revealed that the BFO nanoparticles are highly crystallized and exhibit a single-phase perovskite structure. Non-perovskite phases such as Bi2Fe4O9 and Bi2O3/Fe2O3 are not detected in XRD spectra. The obvious peak-splitting shows that the nanoparticles are rhombohedral, consistent with the structure of BFO ceramics, but different from the tetragonal structure of BFO films. Figure 1b shows the XRD spectra of the sample after degradation experiments in methyl orange (MO), which are described later in details. The typical scanning electron microscopy (SEM) image of the BFO powders is shown in Figure 2a, where the inset is a representative transmission electron microscopy (TEM) image. These nanopowders consist of roughly spherical and welldispersed particles ranging from 80 to 120 nm. Figure 2b C O M M U N IC A IO N