The Ag–BiOBrxI1−x composite photocatalyst: Preparation, characterization and their novel pollutants removal property
Graphical abstract
The complex photocatalysts of Ag–BiOBrxI1−x represent high activity and composition-dependent property. The load of Ag improved the separation of photogenerated electron–hole pairs and enhanced the light absorption in the visible region.
Introduction
Recently, the semiconductor based photocatalysts have been paid much attention for the energy shortage and environmental pollution. These materials represent promising applications in the solar energy conversion and photo-oxidation of organic pollutants [1]. The mostly intensive research is the titanium related materials. TiO2 is considered as one of the promising materials in the photocatalytic field for their low cost, high photocatalytic activity, chemical stability, and nontoxicity [2]. However, TiO2 can be excited only by UV light irradiation (λ < 400 nm) for its wide band gap energy of 3.2 eV, which has restricted its applications. The lack of visible light absorption and low separation rate of photo-excited electron–holes pairs result in the limited quantum efficiency [3]. The recent research work mainly focus on the modification of surface and inner of TiO2 [4], the fabrication of the heterogeneous structure with other compounds and the doping with foreign elements [5]. The research on TiO2 have enriched the photocatalytic theory and provided a new direction for the high photocatalytic materials. In this case, many novel and efficient photocatalysts were developed after years of continuous efforts. Although the pioneer work have improved its practical efficiency of solar energy application, the exploration of TiO2-based materials and the search for other active semiconductor photocatalyst under visible-light irradiation remain the most challenging task for solar-energy utilization [6].
The ternary oxide semiconductor of bismuth oxyhalides (BiOXs, X = Cl, Br, I) has been paid much attention for their novel photocatalytic activities under UV and visible-light irradiation [7]. The BiOXs compounds represent better photocatalytic activity than TiO2 under UV light irradiation for the existence of a layer of [Bi2O2]2+ interleaved by double layer of halogen atoms in the compounds. The layers between [Bi2O2]2+ and the anionic halogen can format internal static electric fields, which results in the efficient separation of the photogenerated electron–hole pairs [8]. The Bi 5d states reduce the valence band widths and increase the conduction band widths, and the valence band width decreases with the rising atomic number of X. However, BiOF and BiOCl are only active at UV light due to the large band gap, while BiOBr and BiOI show visible-light responsive. To narrow the band gap, the composites of BiOBrxI1−x [9] and BiOClxBr1−x [10] are prepared, both of which show high photocatalytic activity. The band gap can be adjusted by the change of the halogen atom. Additionally, the continuous band gap structure is advantaged to heterostructure and pure phase BiOXs [11]. It is a promising candidate material for both exploration of high activity performance and photocatalytic theory research.
Recently, the noble metal modified semiconductor materials have been widely investigated, especially in the photocatalytic fields. The noble metal such as Pt, Pd, Au and Ag on the surface of semiconductor material can act as traps for photoinduced electrons, which reduce the probability of electron–hole recombination in the photocatalytic reaction [12]. Thus the noble metal compounded with semiconductor is one of effective methods for photoinduced electron–hole generation and separation in recent years. On the other hand, it was reported that the interaction between noble metal nanoparticle (such as Au, Ag, Cu) and incident light with specific energy can induce intense localized fields at the surface of the noble metal particle [13]. The interaction are induced when the conduction band electrons of the noble metal nanoparticle couple with the electric field of incident light at a resonant frequency, generating a plasmonic oscillation localized on the surface of the nanoparticle, which was defined as the localized surface plasmon resonance (SPR) [14]. Additionally, the deposition of metal on the semiconductor surface results in the formation of the metal–semiconductor heterojunction structure. It is favorable for the separation of generated electron–hole pairs. It was demonstrated that the plasmonic metal–semiconductor photocatalysts achieved significantly higher activity in various photocatalytic reactions compared with their pure semiconductor compositions [15]. The Ag metal is considered as an ideal material for the fabrication of metal–semiconductor composite material for its high electron conductor. Much research work based on the Ag–semiconductor materials has been reported [16]. It is a promising material for the fabrication of plasmonic photocatalyst with visible-light active materials. The Ag–BiOBr [17], Ag–BiOI [18] and Ag–BiOX (X = Cl, Br, I) [19] photocatalysts were prepared and showed high performance. However, the plasmonic metal coupled with semiconductors with varied band gap has not been reported. Based on our previous research [20], and these reported works, the metal load (especially for SPR metals) and the continue band gap structure are combined in the BiOXs photocatalysts in our research work. Additionally, the evaluation for most BiOXs photocatalyst is the degradation of dye pollutants. This method cannot reflect the real performance and photocatalytic reaction process especially for the SPR photocatalyst. Based on these considerations, the current study reported the preparation of SPR Ag based BiOBrxI1−x with continue band gap structure. With degradation of dye and non-dye pollutions, all kinds of influence factors were discussed during the photocatalytic reaction.
Section snippets
Materials and experimental
All the reagents were analytic grade and purchased from Aladdin Chemical Reagent and used without further purifications. The preparations of the BiOBrxI1−x are according to the report and modified [21]. In a typical experiment, 2 mmol Bi(NO3)3·5H2O was dissolved in 20 mL glacial acetic acid under ultrasonic. Then 20 mL solution containing NaBr/KI (amount, 2 mmol) and 4 mmol NaAc was added into the above Bi(NO3)3 solution under magnetically stirring. Subsequently, the mixture was stirred for 2 h at 60
Results and discussion
The phase structure of the catalysts was carried out by the XRD analysis. Fig. 1 represents the XRD pattern of synthesized samples. It shows that all the samples are well crystallized. The diffraction peaks of Ag–BiOBr (top) and Ag–BiOI (down) samples are well indexed to the standard card (BiOBr JCPDS file no. 78-0348 and BiOI JCPDS file no. 73-2062) respectively. The other patterns of the Ag–BiOX samples are intergraded from the tetragonal BiOBr phase to tetragonal BiOI phase with the
Conclusion
In summary, a series of novel Ag–BiOBrxI1−x photocatalysts with high performances have been fabricated through a solution route. The prepared samples exhibited high photocatalytic activity in the degradation of RhB and 4-CP under visible light irradiation. Among all the prepared samples, Ag–BiOBr0.75I0.25 shows the highest photocatalytic activity. The band gap analysis indicates that there is an optimization existed between the EVB and the band gap energy in the BiOBrxI1−x. The metal Ag on the
Acknowledgments
The authors gratefully acknowledge financial support by the National Natural Science Foundation of China (21207121) and the Science and Technology Planning Project of Guangdong Province, China (2010B031000015, 2011A030200001). The Science & Technology New Star of Pearl River, Guangzhou (no. 2012J2200054).
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