Abstract
Bi2MoO6 single-crystallites were synthesized by a simple hydrothermal method in the presence of surfactant sodium dodecyl sulfate (SDS), polyvinyl pyrrolidone (PVP), or cetyl trimethyl ammonium bromide (CTAB). The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) techniques. The photocatalytic activities of the as-fabricated Bi2MoO6 samples were measured for the degradation of methylene blue (MB) under the Xe light illumination. It is shown that the introduction of surfactant have a crucial influence on the size and morphology of the Bi2MoO6 product. Among the as-fabricated Bi2MoO6 samples, the ones derived hydrothermally with CTAB showed outstanding photocatalytic activities for the addressed reaction under simulated sunlight irradiation, attributed to the ultrafine nanocrystals and the higher surface areas.
1. Introduction
Recently, many Aurivillius-based compounds [1] have been reported which exhibit interesting properties suitable for photocatalytic applications. Bismuth molybdates have the general chemical formula Bi2O3·MoO3 where , 2 or 1, corresponding to the α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6. Among this family, γ-Bi2MoO6 with Aurivillius structure is the simplest and probably the most studied example.
It was well known that the microstructure and morphology of photocatalysts are important factors affecting photocatalytic properties [2]. Various synthesis methods of γ-Bi2MoO6 have been reported, including hard template method [3], solution combustion method [4], ultrasonic-assisted synthesis [5], citrate complex method [6], coprecipitation method [7], and hydrothermal synthesis [8, 9] Within them, the hydrothermal route is one of the most effective methods for synthesizing γ-Bi2MoO6 nanostructures. This process can be described as a reaction of precursors in a close system in the presence of a solvent. It is possible to control the shape and size of products by adjusting the processing parameters, such as different solvents and reactants, surfactants, reaction temperature, and time [10–12]. Different surfactants used in this approach were effective to fabricate nanoparticles with controllable phases and morphologies, and thus to affect the property of the product. In this paper, γ-Bi2MoO6 is synthesized by a simple hydrothermal method, the influence of surfactants SDS, PVP and CTAB on the microstructure, morphology, and photocatalytic activities of the catalysts is investigated. It is found that the γ-Bi2MoO6 synthesized in the presence of surfactant CTAB exhibits an excellent photocatalytic activity in the decomposition of MB under simulated sunlight irradiation (500-W Xe light illumination).
2. Experimental
2.1. Materials
Bismuth nitrate, sodium molybdate, MB, sodium hydroxide, nitric acid, absolute alcohol, SDS, PVP, and CTAB were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were of analytic grade and used without treating further for purification.
2.2. Preparation of the Bi2MoO6 Sample
γ-Bi2MoO6 were prepared by a simple hydrothermal synthesis. Briefly, while stirring, Bi(NO3)3·5H2O (1.947 g) was dissolved in 10 mL nitric acid solution (2 mol/L) to form solution A. Then, 0.483 g Na2MoO4 was dissolved in 10 mL sodium hydroxide solution (4 mol/L) under stirring to form solution B. Thirdly, solution B was dropwised into solution A to form homogeneous solution C. 0.189 g of SDS, PVP, and CTAB was added into solution C, respectively. The pH = 5 values of the solution were then adjusted with concentrated sodium hydroxide under stirring. After being vigorously stirred for 30 min, the resulting precursor suspension was transferred into a 50 mL capacity Teflon-lined stainless steel autoclave, which was subsequently heated to 180°C and maintained for 22 h. Subsequently, the autoclave was cooled to room temperature naturally. The obtained samples were filtered, washed with deionized water for several times, and dried at 80°C in air. The sample obtained without surfactant was denoted as S1, and the samples obtained by adding SDS, PVP, and CTAB were denoted as S2, S3, and S4, respectively.
2.3. Characterization
X-ray powder diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 X-ray diffractometer (Cu Kα source) at a scan rate of 4°/min with the 2θ range from 20° to 80°. The crystal size was estimated from the Scherrer equation, , where is the crystal size, is the wavelength of the X-ray radiation, and is usually taken as 0.9, FWHM is the full width at half maximum in radian of the sample. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. UV-visible absorption spectra and diffuse reflectance spectra (DRS) were recorded on a Lambda 2500 UV-visible spectrophotometer (Japan island ferry).
2.4. Photocatalytic Degradation of MB Experiment
MB was used as a convenient photostable organic molecule to assay photochemical activity of γ-Bi2MoO6. The prepared γ-Bi2MoO6 samples (50 mg) were dispersed into an aqueous solution (100 mL) of MB (10 mg/L−1). The suspension was sonicated for 30 min, followed by stirring in the dark for 120 min to ensure adsorption-desorption equilibrium prior to irradiation. The suspension was then irradiated while stirring by using a 500-W Xe lamp (MAX-302, Asahi Spectra, USA). The initial and final reaction temperatures for MB solution were measured as 25 ± 3°C, respectively. Samples for analysis were taken from the reaction suspension after different reaction time and centrifuged at 3500 rpm for 10 min to remove the particles. The MB concentration from the upper clear solution was analyzed according to the absorption intensity at 664 nm in the measured UV-vis spectra.
3. Results and Discussion
3.1. XRD Analysis
The XRD technique was used to identify the crystalline phases of the as-prepared γ-Bi2MoO6 samples, and the XRD patterns are shown in Figure 1. By comparing the XRD pattern of the standard γ-Bi2MoO6 sample (JCPDS PDF# 72-1524 or 71-2086), it can be realized that all of the Bragg diffraction peaks of each as-fabricated bismuth molybdate sample in the 2θ range of 10–80° could be well indexed, as indicated in Figure 1 (S1–S4). In other words, all of the γ-Bi2MoO6 samples were single-phase and possessed an orthogonal crystal structure, in good consistency with the results reported by other researchers [7, 8, 13]. For four Bi2MoO6 samples, there were some discrepancies in peak intensity, indicating that the γ-Bi2MoO6 crystallinity was a little different from sample to sample, depending upon the fabrication conditions adopted.
3.2. TEM Analysis
Figure 2 shows the TEM images of the as-prepared γ-Bi2MoO6 without or with surfactants. The TEM image in Figure 2(a) shows that bigger γ-Bi2MoO6 aggregated particles are composed of sheet-like nanocrystals without any surfactant. In Figure 2(b), the as-prepared γ-Bi2MoO6 with SDS is self-organized into a bigger branch-like architecture with irregular morphology. The TEM image in Figure 2(c) shows that the γ-Bi2MoO6 is composed of nanosheets with different sizes when adding PVP surfactant. Figure 2(d) shows that the TEM images of the γ-Bi2MoO6 assisted by CTAB are composed of smaller nanocrystals with thinner sheet-like and regular morphology. It is concluded that surfactants affect the morphology and the particle size of γ-Bi2MoO6 during the hydrothermal process.
(a)
(b)
(c)
(d)
3.3. Photocatalytic Activities
To demonstrate the photocatalytic activity, MB was chosen as photodegradation target under simulated sunlight irradiation. Figure 3 showed the time-dependent absorption spectra of MB solution during the photodegradation process in the presence of different γ-Bi2MoO6 samples (S1–S4). The absorption peaks at 664 nm corresponding to MB diminished gradually as the irradiation time extended. Compared with the absorption peaks in S2 and S1, those in S3 and S4 dramatically decreased and completely disappeared after about 90 min, in particular that in S4. It suggested that S4 exhibited the highest photocatalytic activity among all the γ-Bi2MoO6 samples. The variation of MB concentrations (C/C0) with irradiation time over the different γ-Bi2MoO6 samples was shown in Figure 4, where C0 and C are the concentrations of MB solution before irradiation and after the irradiation. For comparison, direct photolysis of MB in the absence of γ-Bi2MoO6 was performed under the same conditions. It was clearly seen that MB concentration in the absence of the catalysts hardly changed with the increase of irradiation time. The as-synthesized γ-Bi2MoO6 products with different surfactants exhibited remarkable variation in photocatalytic activities for MB degradation. Under Xe lamp irradiation within 90 min, the MB photocatalytic degradation ratio (98% and 96.1%) is achieved by S4 and S3 samples, noticeably better photocatalytic activities than the S2 and S1. Among γ-Bi2MoO6 products, S4 exhibited superior photocatalytic activity than other products. It was mainly because photocatalytic activities increased with the decrease of the particle size. The probable mechanism is that the surface area of small particle size provides more active sites during the photocatalytic process [14, 15]. The superior activity of S4 might be ascribed to its relatively small particle size by CTAB assisted growth.
(a)
(b)
(c)
(d)
4. Conclusion
In conclusion, γ-Bi2MoO6 was synthesized by a simple hydrothermal method in the presence of surfactants SDS, PVP, and CTAB. The surfactants can affect the particle size, surface area, morphology, and the photocatalytic activity of γ-Bi2MoO6. It is also found that surfactant CTAB has a promotive effect on the photocatalytic activity of γ-Bi2MoO6 sample, due to the surface area of small particle size providing more active sites during the photocatalytic process. Under a 500 W Xe lamp irradiation, the γ-Bi2MoO6 obtained by using CTAB as surfactant can degrade MB 98% in 90 min.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.