Background

Water pollution has now become an urgent problem owing to the rapidly growing global industrial process [1, 2]. Public health and social economies are threatened by various organic dye pollutants from textile industries [3]. A variety of methods have been introduced to remove dyes from wastewaters, such as membrane filtration [4], flotation [5, 6], solvent extraction [7], chemical oxidation [8, 9], adsorption [10, 11], and photocatalytic degradation [12, 13]. Among these methods, adsorption has been proved to be an effective way for wastewater treatment in terms of simplicity of design, user-friendly control, and insensitivity to toxic substances. Dye removal from industrial wastewaters by adsorption techniques has been widely concerned and researched in recent years [10–15]. Activated carbon is considered one of the best adsorbents for the removal of organic contaminants, but activated carbon is too expensive to use widely in practical applications [16]. Therefore, the development of low-cost, high-efficiency, renewable, and eco-friendly materials as absorbent for the removal of dyes has attracted more and more interests. Recently, many kinds of materials such as SnS2 nanosheets [15], WO3 nanorods [17], Cu2O nanocrystals [18, 19], and other highly adsorbent materials have been investigated.

Bismuth selenide (Bi2Se3) nanostructures have been extensively studied due to their unique properties and promising applications in the fields of optical recording systems, laser materials, optical filters, sensors, solar cells, strain gauges, electromechanical and thermoelectric devices, and topological insulators [20–23]. During the past few years, the preparation and application of doped Bi2Se3 have been extensively investigated [24–27]. In addition, due to the high surface state and unique optical or electrical properties [28], Bi2Se3 can also be applied in the fields of visible-light photocatalytic degradation [27, 29]. For example, Bi2Se3-TiO2 complex nanobelts [30] and S-doped BiSe [31] show excellent visible-light photocatalytic degradation performance. However, to our knowledge, there is no report on the absorption properties of Bi2Se3 nanostructures, especially the systematic study of the Mo doping-enhanced absorption behavior of Bi2Se3 nanostructures.

In this work, we synthesized self-assembled Mo-doped Bi2Se3 nanoflowers by a simple solvothermal route. We find that the absorption behavior of Bi2−xMo x Se3 on Rhodamine B (RhB) varies as a function of Mo content and reaches its highest absorption capacity with 15% Mo doping.

Methods

Preparation of Bi2−xMo x Se3

All of the chemical reagents used in this experiment are of analytical grade and used without further purification. Bi2−xMo x Se3 (x = 0, 0.01, 0.03, 0.05, 0.10, and 0.15) is obtained by a simple solvothermal method. In a typical Bi2−xMo x Se3 (x = 0.15) synthesis, 0.85 mmol of Bi(NO3)3·5H2O and 0.15 mmol of (NH4)6Mo7O24·4H2O are added to 18 ml of N,N-dimethylformamide under vigorous stirring to form a homogeneous solution. Then additional ammonia is added to the above solution to adjust the pH value to 9 to 10 under continuous stirring. After that, Se powder and Na2SO3 are added to the above solution under magnetic stirring. The final solution is transferred into a Teflon-lined autoclave (25-ml capacity), kept at 160°C for 20 h, and cooled to room temperature under ambient conditions. The products are finally washed several times with ethanol and distilled water, followed by drying at 80°C for 12 h under vacuum. For comparison, we also synthesized Bi2−xMo x Se3 samples with different Mo contents (x = 0, 0.01, 0.03, 0.05, 0.10, and 0.15), which are labeled as samples A, B, C, D, E, and F, respectively.

Dye adsorption experiments

The adsorption activities of the as-prepared products are investigated using RhB as dyes. In each experiment, 0.08 g of adsorbent was added to 50 ml of a 10-mg/l RhB solution. Under constant stirring in the dark, about 6 ml of the mixture solution is taken out at intervals and centrifuged to separate solid particles for analysis. After centrifugation, the adsorption behavior is investigated.

Sample characterization

The phase composition and crystallographic structure of the as-prepared samples are examined by X-ray diffraction (XRD) technique with Cu Kα irradiation. The sizes and morphologies of the products are investigated using a field emission scanning electron microscope (FESEM; S-4800, Hitachi, Minato-ku, Tokyo, Japan). The dye adsorption behavior is measured with a UV-visible (UV–vis) spectrum (Lambda 900, PerkinElmer Instruments, Branford, CT, USA).

Results and discussion

Structure and morphology

The as-prepared samples are examined by XRD techniques, and the XRD patterns of samples A to F are shown in Figure 1. All the peaks in the patterns can be indexed according to the power diffraction card of hexagonal Bi2Se3 (no. 33-0214), and no impurity phase related to the Mo complex could be found. The diffraction peaks shift to higher angles with the increase of Mo6+ content from samples A to F, indicating that Mo6+ has been incorporated in the Bi2Se3 lattice, and the lattice parameter gets smaller with the increase of Mo6+. This is understandable considering the fact that the ionic radius of Mo6+ (0.065 nm) [24] is smaller than that of Bi3+ (0.103 nm) [25].

Figure 1
figure 1

XRD patterns of samples (a) A, (b) B, (c) C, (d) D, (e) E, and (f) F.

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The morphology and size of the as-synthesized products are characterized by FESEM observations (Figure 2). The low-magnification FESEM image in Figure 2a shows that a large number of platelike nanostructures are randomly dispersed on the surface of the substrate. Comparatively, a perfect hexagonal morphology for Bi2Se3 is observed from the image. A magnified FESEM image (Figure 2b) shows that the width of the nanosheets is in the range of 100 to 400 nm with a thickness of about 10 to 30 nm. The doping of Mo changes the morphologies of the nanosheets greatly. The low-magnified FESEM image (Figure 2c) demonstrates that the typical product of Bi1.85Mo0.15Se3 consists of a large quantity of uniform flowerlike nanospheres. The average diameter of flowerlike nanospheres is about 100 to 200 nm, and they are made up of curved nanoplates with an average thickness of 5 to 10 nm as shown in the high-magnification FESEM image (Figure 2d).

Figure 2
figure 2

Low-resolution FESEM images of samples (a) A and (c) F and their respective high-resolution SEM images (b, d).

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To confirm the structure, crystallinity, and details of the flowerlike nanospheres, high-resolution transmission electron microscopy (HRTEM) techniques are employed. A representative HRTEM image taken from the edge of a Bi1.85Mo0.15Se3 nanoflower is shown in Figure 3a, which clearly indicates that nanoflowers contain a perfectly periodic arrangement with an interplanar distance of a = 0.356 nm, which is smaller than that of bulk anatase (a = 0.413 nm) crystal.

Figure 3
figure 3

A HRTEM image of sample F (a) and the EDXA spectrum of sample F (b).

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The chemical composition of Bi2−xMo x Se3 was determined by energy-dispersive X-ray analysis (EDXA) attached to the FESEM. In Figure 3b, the EDXA spectrum of the Bi1.85Mo0.15Se3 nanosheets shows that the nanosheets contain only Mo, Bi, and Se without any trace of by-products.

From the FESEM observations, we can conclude that the Mo concentration influences the morphologies of the nanoplates greatly. In order to understand the role of Mo in the evolution process of Bi2−xMo x Se3 from nanoplates to nanoflowers, Bi2−xMo x Se3 samples with varied x values are synthesized and studied. With the increase of the x value (Mo concentration), the nanostructures gradually change from hexagonal nanosheets to smaller-sized hexagonal nanosheets and finally to flowerlike spheres, combined with a size change. For example, when no Mo is contained, the sizes of the nanosheet are about 100 to 500 nm in width and 20 to 30 nm in thickness, as shown in Figure 4 (a-1 and a-2). With increasing amounts of Mo, morphologies of the as-synthesized Bi2−xMo x Se3 products change from nanosheets to nanoflowers (Figure 4 (a-1 to f-2). From Figure 4 (b-1 to d-2), we can see that the products are still composed of nanosheets, but the sizes of Bi2Se3 have become smaller. When the Mo concentration increased up to 15% (Figure 4 (f-1 and f-2), regular flowerlike spheres consisting of thin nanoplates were formed. The average diameter of nanoflowers is about 100 to 200 nm, and they are made up of curved nanoplates with an average thickness of 5 to 10 nm as shown in the magnified image. With the increase of Mo contents in Bi2−xMo x Se3, the diameter of the products was found to be lower than that of pure Bi2Se3. So we believe that Mo is the main driving force for the formation of a flowerlike structure.

Figure 4
figure 4

Low- and high-magnification FESEM images of the samples. Low-magnification (a1, b1, c1, d1, e1, and f1) and high-magnification (a2, b2, c2, d2, e2, and f2) FESEM images of samples A (a1, a2), B (b1, b2), C (c1, c2), D (d1, d2), E (e1, e2), and F (f1, f2).

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A diagram of the formation mechanism of nanoplates and nanoflowers of Bi2−xMo x Se3 is presented in Figure 5. When no Mo is contained, tiny clusters of Bi2Se3 nanosheets are first generated upon heating and then enriched to assemble into bigger nanosheets. However, with the increase of Mo concentration in Bi2−xMo x Se3, nanosheets assemble into nanoflowers.

Figure 5
figure 5

Schematic diagram showing the growth mechanism of nanosheets and nanoflowers of Bi 2− x Mo x Se 3 .

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Adsorption ability of Bi2−xMo x Se3

To investigate the potential application of the as-synthesized Bi2−xMo x Se3 nanocrystals and their relationship with the amount of Mo in Bi2−xMo x Se3, we study the adsorption ability of Bi2−xMo x Se3 using RhB as dyes. The experiments are carried out with Bi2−xMo x Se3 dispersed in the solution of RhB in the dark several times with constant stirring. After centrifugation, the UV–vis absorption of the supernatant was measured and the characteristic absorption of RhB at about 553 nm was selected to estimate the adsorption process. Figure 6 shows the UV–vis adsorption spectra of RhB as a function of time using the as-prepared Bi1.99Mo0.01Se3 as adsorbent. From Figure 6, we can see that the intensity of the absorption spectra gradually decreases and nearly disappears within 60 min; at the same time, the solution becomes colorless when observed with the naked eye.

Figure 6
figure 6

Time-dependent absorption of 2.0 × 10 −4 M 50-ml aqueous solutions of RhB in the presence of 50-mg as-prepared Bi 1.99 Mo 0.01 Se 3 .

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Figure 7 shows the variation in RhB concentration with the adsorption time over different adsorbents. When there is no adsorbent, the concentration of the RhB solution remains the same with its original state for up to 60 min, which demonstrates that RhB is stable under the experimental conditions. Pure Bi2Se3 shows a weak adsorption activity. After 60 min of absorption, only 20% of RhB is removed from the pure Bi2Se3 sample. The adsorption activities are strengthened with the increase of Mo contents in Bi2Se3. Bi1.85Mo0.15Se3 has a maximum adsorption behavior, and nearly 100% of the RhB dyes are removed in 20 min. All of this clearly shows that the doping of Mo in Bi2Se3 is an efficient way to enhance its adsorption activity. The results indicate that the as-synthesized Bi2−xMo x Se3 might possess a profound application in the fields of treatment of dye-polluted wastewater.

Figure 7
figure 7

A plot showing the extent of RhB degradation as a function of adsorption time for different catalysts. RhB degradation is monitored at 554 nm. Lines a, b, c, d, e, f, and g represent, respectively, samples blank, A, B, C, D, E, and F.

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Conclusions

In summary, Bi2−xMo x Se3 nanomaterials were prepared by a solvothermal approach, and different morphologies of Bi2−xMo x Se3 have been obtained. The doping concentration of Mo plays an important role in controlling both the morphologies of Bi2−xMo x Se3 nanostructures and their absorption behavior. The sample with the best absorption behavior is that with 15% Mo concentration. We believe that the study of dye absorption behavior brings a new application realm for Bi2Se3 nanostructures.