Anodized BiOI coatings and their photocatalytic activity of organic dye degradation

Highlights

• BiOI coatings consisted of flower-like particles were prepared by anodization.

• The coatings showed a good photocatalytic activity in the degradation of MB dye.

• Band position shift of BiOI made it possible to form superoxide species.

• ^•O₂ rather than ^•OH or holes played the key role in MB degradation.

Abstract

Pure tetragonal BiOI coatings of nano-sized flower-like structure were prepared on the substrate of Bismuth foil by anodic oxidation. The combined results of XRD, Raman and SEM show that the BiOI crystal grains with an average size of 23.2nm preferentially grew along crystal face (102) and (110). The thickness of the as-prepared BiOI coatings was estimated to vary from 5 to 10 μm. The great thickness made the substrate of Bismuth foil had no effect on the photocatalytic activity of the BiOI coatings. Although the band gap (1.75 eV) of the anodized BiOI coatings is close to those prepared by other methods, the overall shift of the valence maximum (VBM) and the conduction band minimum (CBM) to a more negative potential position was observed. The shift made the formation of ${}^{•}\mathrm{O}\overline{{}_2}$ radicals possible. ${}^{•}\mathrm{O}\overline{{}_2}$ radicals played the key role in the photocatalytic degradation of MB organic dye under visible light irradiation. Meanwhile, photo-generated holes and ^•OH radicals were also helpful to the photocatalytic reaction. Compared with those prepared by other methods, anodized BiOI coatings showed a higher photocatalytic activity.

Introduction

Bismuth oxyiodide (BiOI) as a potential photocatalyst has drawn much attention since 2009 [1]. As visible-light-driven photocatalyst with high performance, BiOI shows many advantages such as a strong oxidation ability, a large specific area, a high chemical stability, an intensive light absorption, a suitable absorption edge, and a narrow band gap (1.7 ~ 1.9 eV) [2]. Numerous works have been done on the preparation, characterization and performance improvement of nano-sized BiOI powders. With kinds of methods including coupling with other semiconductors [3], [4], [5] or metal [6,7], element doping [8], [9], [10], crystal plane regulation [11] and defect engineering [12], [13], [14], etc., photocatalytic performance of BiOI powders could be remarkably enhanced.

BiOI photocatalyst immobilized in the form of film rather than powder could be used in a fluid system, which could avoid photocatalyst recovery from the fluid system [15]. A variety of methods such as ion-exchange method [16], solvothermal method [17], sequential ionic layer adsorption and reaction (SILAR) [18,19], spray pyrolysis [20], chemical vapor transport (CVT) [15], electrodeposition [21] and anodization [22] have been used to prepare BiOI coatings.

Among all the methods, anodization shows great merits including high efficiency, precise morphology controllability, broad preparation condition, and among others. Zhao and her coworkers developed anodic oxidation to prepare BiOX (X = Cl, Br or I) nano-sheeted film [22]. A single-phase film of BiOX (X = Cl, Br or I) was fabricated and their preparation conditions differed markedly. Among these films, BiOCl film showed the best photocatalytic activity in degrading methyl orange under the irradiation of a Xe arc lamp. The work opened up a new way for the preparation of BiOX photocatalytic film by anodic oxidation. In addition, the BiOX (X = Cl, Br or I) photocatalysts were used to degradation harmful gases including NO and formaldehyde [23,24]. However, a comprehensive characterization of the BiOX film prepared by anodic oxidation, such as light absorption, surface chemical properties, and active radicals, etc., has not been reported so far. In addition, the mechanism in the photocatalytic degradation of organic dyes is scarcely seen.

In this work, BiOI coatings were prepared by anodic oxidation and the coatings were characterized by XRD, Raman spectra, SEM, UV-vis spectroscopy, XPS, and EPR. The visible-light-driven photocatalytic performance of the BiOI coatings in organic dye degradation was evaluated. Furthermore, the key oxidizing species change in organic dye degradation was considered.