Morphological evolution and visible light driven degradation of tetracycline by Bi_3.84W_0.16O_6.24 nanostructures

Abstract

In this work, two morphologies of Bi_3.84W_0.16O_6.24 nanostructures, namely the nanobelt (T2) and nanooctahedral structure (T15), were synthesized via a simple, microwave-assisted method. X-ray diffraction patterns and high-resolution transmission electron microscope (HRTEM) images confirmed that the nanobelt and nanooctahedral structures consist of polycrystalline and single crystalline structures, respectively. According to the time-dependent experiments, tentative formation mechanisms of the different morphologies based on our observation were discussed. The correlation between morphologies and photocatalytic activities were investigated by photocatalytic degradation of tetracycline (TC) under different pH conditions. The results demonstrate the photocatalytic activity of sample T2 is more stable and balanced than sample T15.

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1. Introduction

Semiconductor-based photocatalysis have aroused increased interest due to their potential applications in renewable energy and environment fields such as dye-sensitized solar cells, hydrogen generation from water splitting and photocatalytic water/air purification.^1–4 It is believed that the catalytic performance of inorganic crystals is very closely related to their physical and chemical properties including morphology, size, crystal planes, shapes and structure, thus extensive effort has contributed to shape control of inorganic micro/nanocrystals.^5–9 For example, morphologies of anatase TiO₂, one of the most important photocatalysts, such as sphere, rod, wire, tube, slightly and heavily truncated octahedron, belt and sheet structures have been investigated. Pan's group reported that TiO₂ nanowires, in comparison with TiO₂ nanoparticles, have more uniform dispersion on graphene with less agglomeration, resulting in more direct contact between TiO₂ and graphene, and hence further improved electron–hole pairs (EHPs) separation and transportation.¹⁰ Ag₃PO₄, as another specific example is from visible-light-driven photocatalyst, with spherical, cubic, and rhombic dodecahedral morphologies that show differences in light absorption ability as well as in photocatalytic activity. Wang et al. reported that branched Ag₃PO₄ crystal with porous structure shows the highest photocatalytic activity among these Ag₃PO₄ crystals with multiform morphologies, and the photocatalytic rate constants of branched Ag₃PO₄ are 2.8 and 4 times those of irregular spherical Ag₃PO₄ for degradation of methylene blue (MB) and rhodamine B (RhB) dye solutions under visible light irradiation, respectively.¹¹ Morphology control of photocatalysts has been considered to be one of the most promising avenues to improve the photocatalytic. However, it is still a great challenge to develop versatile synthetic methods for the controlled synthesis of various semiconductor photocatalysts with tailored morphology and structure.

Bi_3.84W_0.16O_6.24, as one of the simplest members of the Aurivillius oxide family, which is another complex oxide of bismuth tungsten oxide, has a layered structure with the perovskite-like slab of WO₆ and (Bi₂O₂)²⁺, is such a potential visible-light responsive semiconductor.^12,13 The synthesis of Bi_3.84W_0.16O_6.24 is influenced by the pH precursor suspensions. In our previous reports a high pH value (over 9) could lead to the phase-transition in obtained samples from the orthorhombic Bi₂WO₆ phase to the cubic Bi_3.84W_0.16O_6.24 phase.¹⁴ Similar phenomena were also observed by the Huang,¹⁵ Zhang,¹⁶ and Zhou¹⁷ groups. But they did not give detailed information about the property and photo activity of Bi_3.84W_0.16O_6.24. The photocatalytic activity of Bi_3.84W_0.16O_6.24 investigated in detail for the first time by Lingyan Zhu group.¹⁸ They synthesized round disks Bi_3.84W_0.16O_6.24 at heated temprature 140 °C for 20 hours, displayed efficient photocatalytic activity and mineralization capacity to BPA under simulated solar light irradiation. Up to now, the morphology transformation and the relationships of Bi_3.84W_0.16O_6.24 between morphologies and photocatalytic activities have rarely been investigated in the previous works.

In this work, we first report simple, quick, eco-friendly and controllable synthesis of Bi_3.84W_0.16O_6.24 nanostructures via a microwave-assisted synthetic method by simple adjustment of the reaction time. The typically obtained Bi_3.84W_0.16O_6.24 nanostructures mainly included nanobelt (T2) and nanooctahedral structure (T15), and the Bi_3.84W_0.16O_6.24 nanobelts and nanooctachedrals were polycrystalline and single crystalline respectively. According to the time-dependent experiments, tentative formation mechanisms of different morphologies are also discussed based on our observations. We have discussed the correlation between morphologies and photocatalytic activities by the photocatalytic degradation of tetracycline (TC) under different pH. The results demonstrate that photocatalytic activities of the sample T2 is more stable and balanced than the sample T15.

2. Experimental

2.1. Synthesis of Bi_3.84W_0.16O_6.24

Bi_3.84W_0.16O_6.24 nanostructures were synthesized via a versatile and facile microwave method. In a typical process, 1.0 mmol Bi(NO₃)₃·5H₂O and 0.5 mmol Na₂WO₄·2H₂O were dissolved in 50 mL of deionized water, followed by addition of 2 mL of ethylenediamine (En, 99%) and then stirred for 30 min, the mixture was transferred into a 250 mL round-bottom flask in a microwave system (XH-300UL, Beijing Xiang Hu Technology Development Co. Ltd) equipped with in situ magnetic stirring. After treating the mixture at 100 °C under microwave radiation. The final products were collected by centrifugation and filtration, then washed several times used by deionized water and ethanol, and last dried in air at 70 °C for 6 h.

2.2. Characterization

X-ray diffraction (XRD) patterns of the samples were obtained using Cu Kα radiation (λ = 1.54178 Å). The structures and morphologies were characterized by scanning electron microscopy (SEM) images on an S-4800 field emission SEM (SEM, Hitachi, Japan). Selected area electron diffraction (SAED), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were carried out on an F20S-TWIN electron microscope (Tecnai G2, FEI Co.), using a 200 kV accelerating voltage. The UV-Vis diffused reflectance spectra of the samples were recorded with an UV-Vis spectrophotometer (UV2550, Shimadzu, Japan) using BaSO₄ as a reflectance standard; nitrogen adsorption–desorption isotherms were obtained at 77 K using either a Micromeritics ASAP 2420 or ASAP 2020 volumetric adsorption analyzer. Surface areas of the samples were determined by the Brunauer–Emmett–Teller (BET) method.

2.3. Photocatalytic degradation of TC

The photocatalytic degradation of tetracycline (TC) was carried out under simulated sunlight irradiation by using a 150 W Xe lamp with a cut off filter (λ ≥ 400 nm) in a photochemical reactor under visible light. The TC initial concentration was 10 mg L⁻¹. A 0.10 g amount of photocatalysts was put into 100 mL of TC solution at room temperature in the air. Before light was turned on, the suspension was magnetically stirred in the dark for 30 min to reach absorption equilibrium. The sampling analysis was conducted in a 10 min interval. The photocatalytic degradation ratio (DR) was calculated by the following formula:

DR = (1 − C_i/C₀) × 100%

C₀ is the initial absorbance of TC that reached absorption equilibrium, while C_i is the absorbance after the sampling analysis. The absorbance of TC was measured by a UV-Vis spectrophotometer with the maximum absorption wavelength at 357 nm.

3. Results and discussion

3.1. Morphology and phase structures of Bi_3.84W_0.16O_6.24 samples

By adjusting the reaction time, one can obtain two typical Bi_3.84W_0.16O_6.24 nanostructures, according to the length of the reaction time were named as T2 and T15. From XRD patterns (Fig. 1), two typical samples can be ascribed to Bi_3.84W_0.16O_6.24 (JCPDS no. 43-0447) with no observable impurity composition. The lattice constants of Bi_3.84W_0.16O_6.24 are a = 5.57 Å, b = 5.57 Å, and c = 5.56 Å. The characteristic peaks are sharp, implying the obtained nanostructures are well-crystallized. The morphology and size of the products prepared by the procedure described in the experimental section are visualized by SEM as shown in Fig. 2. As shown in low magnification (Fig. 2a), the SEM results clearly indicate that the main component of the as-obtained T2 is uniform nanobelts structure. From SEM images Fig. 2b in high-magnification, the average thickness and length of these belts are about 670 nm and 5.7 μm, respectively. As to the as-obtained T15 products in the reaction system consist almost entirely of relatively uniform octahedral nanoparticles with the size of 300–500 nm. In higher magnification (Fig. 1d), the surface and size of Bi_3.84W_0.16O_6.24 are nearly perfect with a highly symmetric, and they exhibited sharp edges and corners as well as smooth surfaces.

Fig. 1 XRD patterns of different time of prepared Bi_3.84W_0.16O_6.24 samples.

Fig. 2 SEM images in low- and high-magnification: T2 (a and b), T15 (c and d).

To further investigate the product morphology and structure, TEM was employed. Fig. 3a revealed the sample T2 possesses a single nanobelt structure. The corresponding high-resolution TEM (HRTEM) image of the nanobelt is further illustrated in Fig. 3b. By measuring the lattice fringes, the resolved interplanar distances are 0.321 nm, corresponding to the (111) plane of Bi_3.84W_0.16O_6.24. The corresponding selected area electron diffraction (SAED) pattern of the nanoplate is exhibited in the top-right of Fig. 3b, revealing that the as-prepared nanobelt is a polycrystalline structure. Fig. 3c shows a TEM image of a typical octahedral nanoparticle which displays a geometrical model of an ideal octahedron in the same orientation is given. In addition, the selected area electron diffraction (SAED) pattern is shown in Fig. 3d which also indicates a single-crystal structure for individual octahedral nanoparticles. The HRTEM image of the edge area of the octahedral nanoparticles in Fig. 1d exhibits well-resolved 2D lattice fringes. The plane spacing of 0.321 nm corresponds to the lattice planes of (111) in Bi_3.84W_0.16O_6.24.

Fig. 3 (a) TEM image of T2; (b) high-resolution image and SAED image (inset) of the T2; (c) TEM image of T15; and (d) a high-resolution image and SAED image (inset) of T15.

Fig. 4 shows the nitrogen absorption/desorption isotherms of the samples. The Brunauer–Emmett–Teller (BET) specific surface area of the sample T2 was about 21.2 m² g⁻¹, which was much larger than that of sample T15 (11.03 m² g⁻¹). The physioadsorption isotherm of sample 1 with a distinct hysteresis loop in the range of 0.5–1.0 P/P₀, which can be classified to type IV in the IUPAC classification is characteristic of porous materials.^19,20

Fig. 4 Absorption/desorption isotherms of T2 and T15.

3.2. Possible formation mechanism

In the time series of XRD in Fig. 5, the crystalline phase appears in only 1 min after the reaction, pure Bi_3.84W_0.16O_6.24 is observed until the reaction time is 2 min. The XRD intensity of the prepared products increases with reaction time until 3 min, after that it decreases slightly. This suggests that the crystalline phase of Bi_3.84W_0.16O_6.24 grows gradually with reaction time, but it may be affected if the reaction extends for too long time.

Fig. 5 XRD patterns of different time of prepared samples in the same reaction condition: (a) 0 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 5 min; (f) 15 min.

To confirm the mechanism of the formation of Bi_3.84W_0.16O_6.24 octahedral nanoparticles, time-dependent SEM analyses are conducted. As shown in Fig. 6, after mixed the liquid precursors, white precipitation were obtained. According to the SEM analyses (Fig. 6a), the products consist of a large amount of nanoparticles as well as some irregular conglomerations sheets form firstly at the beginning of the reaction. From the ragged surface of these mutilated sheets, we can easily assume that the thicker structure is assembled by the nanoparticles united together with the diameter of 40–100 nm. In addition, the belt-like structure witch length and width were ca. 5 μm and 1 ca. 5 μm with smooth surface were also can be found. After a 1 min microwave reaction (Fig. 6b), the united nanoparticles were spread out gradually. After a 2 min microwave reaction (Fig. 6c), the decentralized nanoparticles have a tendency that transformed into octahedral structure, a part of the nanoparticles have begin transformed into incomplete octahedral structure at the same time. On increasing the reacting time to 3 min, a large number of octahedral structure begin to appear (Fig. 6d), meanwhile, it can be seen from the image that cuboid belt structure reduced gradually as the reaction goes on. And after reacting for 5 min (Fig. 6e), the complete octahedral structure became the only products, all of the nanobelt structure disappeared completely. However, when the reaction time increased to 15 min (Fig. 6f), the morphology of the products was without change.

Fig. 6 SEM images of the as-prepared Bi_3.84W_0.16O_6.24 with different time: (a) 0 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 5 min; (f) 15 min.

Based on the experimental results and our understanding, the possible formation mechanism of Bi_3.84W_0.16O_6.24 octahedral structure is proposed (Scheme 1). In the beginning, Bi(NO₃)₃·5H₂O in basic solution reacts first with Na₂WO₄·2H₂O to form nanoplates. In our synthesis, the precursor pH value is 11, the high pH value may leads to a fast initial crystallization rate. In addition, at a relatively high temperature and auto-generated vapor pressure, the complex decomposed and quickly formed pre-crystallized nuclei possess higher crystallinity smaller size, and better-defined belt-like morphology. During the followed solvothermal growth process, with further reaction, Bi_3.84W_0.16O_6.24 octahedral structure are formed via “over growth” on the basic of nanobelts, these growth phenomenon is often observed in the formation of bicomponent heterojunction structures with one excessive component in the precursor.^14,21

Scheme 1 Possible formation mechanism of Bi_3.84W_0.16O_6.24 nanostructures.

3.3. UV-Vis diffuse reflectance spectra

Optical absorbance spectrum of a semiconductor is affected by its electronic structure feature and then determines the photo-catalytic activity. Fig. 7 shows the UV-Vis diffuse reflection spectra of the as-obtained Bi_3.84W_0.16O_6.24 products at different reaction times. The absorption shoulder of precipitation of the precursor is found at ca. 400 nm. Further prolonging the reaction time, a blue-shift of the optical absorption edge was observed from the UV-Vis transmission spectra. Meanwhile, as the reaction time prolonged, Bi_3.84W_0.16O_6.24 products is also with a enhanced absorption intensity in the visible region.

Fig. 7 UV-Vis absorption spectra of Bi_3.84W_0.16O_6.24 nanostructures: (a) 0 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 5 min; (f) 15 min.

3.4. Photocatalytic degradation of TC

It is believed that the photocatalytic activities are closely related to the morphology and the structure of the photocatalysts. Particularly, numerous works have demonstrated that the structure and the morphology of semiconductors dramatically influence their photocatalytic activities. In order to investigate the effect of the morphology on the Bi_3.84W_0.16O_6.24 photocatalytic activities, the as-prepared products reacted under 2 min and 15 min (namely T2 and T15 respectively) were evaluated by monitoring the degradation of TC solution under different pH. Tetracycline is one of the most frequently used antibiotics in aquaculture and veterinary medicine. Due to their extensive, usage and their higher adsorption capability, tetracycline, TC has been detected in terrestrial and aquatic, environment: surface water, ground water, wastewater, and municipal sewage. The removal of TC from water environments has become a mandatory environmental issue.

The photocatalytic degradation experiments were conducted under visible light (>420 nm). The results were given in Fig. 8. C is the absorption of TC at the wavelength of 357 nm and C₀ is the absorption of TC after the adsorption equilibrium on photocatalysts before irradiation. It was shown that, sample T2 degraded TC evenly under different pH, which implied degradation of tetracycline for sample T2 is not affected by acid and alkali. However, it was also seen that the photocatalytic capacity of sample T15 on TC is affected by pH of the reaction solution. As shown in Fig. 8, the degradation of TC increases gradually from pH 3 to pH 11, particularly, when the reaction solution was strong alkalinity.

Fig. 8 Visible-light-driven photocatalytic degradation ratio curves of TC: (a) T2; (b) T15.

To further investigate the photocatalytic activities of the sample T2 and T15, blank control experiment have been done. Fig. 9 shows that TC are relatively stable in acids, but not in alkaline media under the visible light irradiation, in previous reports have similar research,^22,23 which further demonstrates the photocatalytic activities of the sample T2 is more stable and balanced. To further investigate the photocatalytic performance of the obtained samples, a compared experiment was made between the sample T2, T15 and commercial Bi₂O₃ powder (analytical reagent, made by Sinopharm Chemical Reagent Co., Ltd, China), the result was shown in Fig. 10. It is clear that commercial Bi₂O₃ exhibits a low photocatalytic activity compared with the sample T2 and T15 in 120 min under visible light irradiation. T2 shows a high photocatalytic activity and tetracycline antibiotics is quickly degrade with increasing irradiation time.

Fig. 9 Visible-light-driven photocatalytic degradation ratio histogram of TC.

Fig. 10 Photocatalytic degradation of tetracycline by T2, T15 and commercial Bi₂O₃ without adjusting pH.

Many factors can affect the photocatalytic activity, so it is very difficult to estimate the contribution of each factor on the photocatalytic activity. For the explanation of morphology-dependent photocatalytic activities, most researchers tend to suggest surface areas and crystallinity can contribute an enhanced visible light absorption.^24,25 In our case, we attribute the reason of different the main cause of the morphology-dependent photocatalytic activities of different Bi_3.84W_0.16O_6.24 structures as follows: at first, there is no doubt that large surface area contributes to high photocatalytic activity of photocatalysts with a certain crystallinity by creating more possible reactive sites on the surface of photocatalyst. As we mentioned earlier, the specific surface area of the sample T2 was about 21.2 m² g⁻¹, which was much larger than that of sample T15 (11.03 m² g⁻¹). Large specific surface area tend to expose more coordination unsaturated sites on the surface of catalyst, and more reactant can be absorbed onto the catalyst surface for reaction. To demonstrate our assumption, we investigated the adsorption capacity in the in the dark for 30 min of the two Bi_3.84W_0.16O_6.24 structures as shown in Table 1. It is clear that T2 shows a better adsorption capacity no matter in acidic or alkaline conditions, while T15 exhibits a low adsorption capacity. Secondly, the belts have mainly a unique surface orientation of (111), while octahedral particles exhibit random surface orientation. The plane of (111) of the nanobelts is a high-index plane with a high density of atomic steps, ledges and kinks, which usually serve as active sites for breaking chemical bonds.²⁶ Thirdly, as shown in Fig. 1, the characteristic peaks of T2 are stronger than T15 which implied nanobelts have a higher crystallinity than octahedral particles. Generally, when semiconductor nanocrystals are irradiated by light with energy higher or equal to the band gap, photocatalysts generate positive holes (h_vb⁺) and conduction band electrons (e_cb⁻) under light irradiation. These excited e_cb⁻ and h_vb⁺ can recombine and get trapped in metastable surface states, or react with electron donors and acceptors adsorbed on the semiconductor surface, high crystallinity is beneficial to photocatalysts because it generally means fewer traps and superior photocatalytic activity.

Table 1 The detailed amount of absorption equilibrium in 30 min of the different samples

Sample	pH = 2	pH = 5	pH = 7	pH = 9	pH = 11
T2	0.082	0.112	0.077	0.087	0.044
T15	0.022	0.056	0.008	0.026	0.012

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4. Conclusions

In summary, nanobelts and nanooctahedral Bi_3.84W_0.16O_6.24 nanostructure were successfully prepared via a versatile and facile microwave method at a low temperature of 100 °C. XRD, HRTEM and SAED analyses indicated nanobelt and nanooctahedral consist of polycrystalline and single crystalline structures, respectively. Particularly, by varying the pH of tetracycline aqueous solution, demonstrated nanobelt exhibits higher visible-light-driven activity for photocatalytic degradation of tetracycline.

Acknowledgements

The authors would like to acknowledge the National Natural Science Foundation of China (21276116, 21201085, 21301076, 21303074 and 21477050), the Chinese-German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK 20140011), the Program for New Century Excellent Talents in University (NCET-13-0835), the Open Project of State Key Laboratory of Rare Earth Resource Utilizations (RERU2014010), the Henry Fok Education Foundation (141068), the Six Talents Peak Project in Jiangsu Province (XCL-025) and International Scientific and Technological Cooperation in Changzhou (CZ20140017).

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