Enhanced visible-light-driven photocatalytic activity of WO3/BiOI heterojunction photocatalysts
Graphical abstract
The efficient charge transfer at the interface of WO3/BiOI heterojunction photocatalyst due to forming the inner electric field, which leads to an effective photogenerated electrons–hole pairs separation.
Introduction
Semiconductor photocatalysis has received a great deal of attention because it represents a promising alternative technology for degradation of organic pollutants and hydrogen production from splitting water by utilizing the energy of either natural sunlight or artificial indoor illumination [1], [2], [3]. Among various semiconductor materials, TiO2 semiconductor has been mostly investigated owing to the non-toxicity, low cost, relatively high chemical stability and strong oxidizing power [4], [5]. Unfortunately, because of its relatively wide band gap (∼3.2 ev), TiO2 can only be activated by UV light irradiation, which contains less than 5% of the solar spectrum [6], [7]. Accordingly, in order to utilize solar energy more efficiently, it is very urgent and critical to develop efficient, sustainable and stable visible-light-driven photocatalysts.
Recently, bismuth oxyhalides BiOX (X = F, Cl, Br, and I), as a new category of promising photocatalysts, which manifests unusual visible-light photocatalytic activity for photocatalytic energy conversion and environment remediation, because their unique anisotropic layered-structures, the internal static electric field and the strong absorption in the visible light region [8], [9]. More importantly, the internal electric field that forms between the [Bi2O2]2+ and X− layers can effectively promote the separation of photogenerated charge carriers [10]. Among the bismuth oxyhalides, bismuth oxyiodide (BiOI) has the smallest band gap (∼1.7 ev) and has been demonstrated to be an efficient and promising visible-light-driven photocatalyst for the degradation of organic pollutants [11], [12]. Nevertheless, the photocatalytic performance of BiOI is still limited due to slow rate of charge transfer and high recombination of photoinduced electron–hole pairs [13], [14]. To address these limitations, numerous modifications have been made to increase the lifetime of photogenerated charge carriers of BiOI, including self doping [15], morphology modulation [16], [17] and coupling with other semiconductors [10]. Among them, as a typical p-type semiconductor, BiOI coupled with other n-type semiconductors with a relatively large band gap to form BiOI-based heterojunction composites, such as BiOI/Zn2SnO4 [18], Bi2O2CO3/BiOI [19], BiOI/Zn2GeO4 [9], BiOI/TiO2 [20], AgI/BiOI [21], ZnWO4/BiOI [22], g-C3N4/BiOI [23], is proven to be an effective strategy for improving the photocatalytic activity under visible light irradiation, because the formation of interface junction can extend light responsive range, facilitate the interfacial charge transfer and enhance the separation of photoinduced electron–hole pairs [9], [22]. Additionally, tungsten oxide (WO3), as an important n-type visible-light photocatalyst with a band gap (2.4–2.8 eV), has received a great deal of attention due to its resilience to photocorrosion effect in aqueous solution and good electron transport properties [24]. To the best of our knowledge, there is no report on the WO3/BiOI composites till now. Thus, it gives us an inspiration that BiOI can also couple with a relatively wide band gap semiconductor (WO3) to form p–n heterojunction composites with energetically matching conduction bands and valence bands, and then enhance its visible light photocatalytic activity.
Herein, novel visible-light-driven WO3/BiOI heterojuncted photocatalysts were fabricated by a facile hydrothermal method. The photocatalytic activity and stability of the as-prepared samples were evaluated in the photocatalytic degradation of methyl orange (MO) under visible light irradiation (λ > 420 nm). The results showed that the as-prepared WO3/BiOI composites enhanced photocatalytic activity compared with that over pure BiOI and WO3. The activity enhancement was mainly ascribed to the formation of p–n heterojunction at the interface of p-type BiOI and n-type WO3, which facilitated interfacial charge transfer and improved photogenerated electron–hole pairs separation and migration efficiency. Moreover, a possible mechanism regarding photocatalytic process on the basis of experimental results was also discussed in depth.
Section snippets
Catalysts synthesis
WO3 nanocrystals were prepared by a solid-state decomposition reaction of (NH4) 10W12O41·xH2O (5.0 g) at 500 °C in a muffe furnace for 4 h in a semiclosed system at a heating rate of 20 °C min−1 under air condition. The product was washed several times with distilled water and absolute ethanol, then dried at 80 °C for 8 h.
WO3/BiOI heterojuncted composites were synthesized by a facile solvothermal method. Typically, twenty milliliters of 0.15 mol L−1 (1.4553 g) Bi(NO3)3·5H2O dissolved in ethylene glycol
Structural and morphological characteristics of the as-prepared samples
Fig. 1 depicts the XRD patterns of WO3, BiOI and WO3/BiOI series of composite photocatalysts with different WO3 content. The results showed that the sharp and intense diffraction peaks of the as-synthesized photocatalysts, indicated that all the as-prepared photocatalysts are well crystalline. Pure BiOI displays four distinct diffraction peaks at 2θ = 29.4°, 31.7°, 45.5° and 55.1°, which are in good agreement with (0 1 2), (1 1 0), (0 2 0) and (1 2 2) diffraction planes of the tetragonal phase of BiOI
Conclusions
In summary, the p–n heterojunction WO3/BiOI composites have been successfully prepared via a facile one-step hydrothermal method. The as-prepared WO3/BiOI heterojunction photocatalysts exhibited high photocatalytic efficiency for the degradation of methyl orange under visible light irradiation. It was noticed that the rate constant of the WO3(1.0%)/BiOI heterojunction catalyst is 1.6 times as high as that of pure BiOI and 54 times larger than that of pure WO3, respectively. The enhancement on
Acknowledgements
This research was supported by Lingnan Normal University Natural Science Foundation for the Ph.D. Start-up Program (ZL1308) and the Guangdong Provincial Natural Science Foundation (No. 2015A030310431, 2015A030313893).
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