Mo doping-enhanced dye absorption of Bi2Se3 nanoflowers

A simple solvothermal approach is explored to prepare Bi2−xMoxSe3 nanostructures by employing N,N-dimethylformamide (DMF) as the solvent. Mo plays an important role in the assembly of the Bi2−xMoxSe3 nanostructures from nanoplates to nanoflowers. Structural and morphological studies indicate that the resulting products are large specific surface area single-crystalline Bi2−xMoxSe3 nanoflowers self-assembled from thin nanoplates during the reaction process. The absorption properties of the as-prepared samples are investigated with Rhodamine B (RhB) as dye, and it is found that the Bi1.85Mo0.15Se3 nanoflowers show an optimal adsorption capacity, implying that Mo doping not only changes the morphologies of the nanostructures but also enhances their absorption behaviors.


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][11][12][13][14][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 SnS 2 nanosheets [15], WO 3 nanorods [17], Cu 2 O nanocrystals [18,19], and other highly adsorbent materials have been investigated.
Bismuth selenide (Bi 2 Se 3 ) 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][21][22][23]. During the past few years, the preparation and application of doped Bi 2 Se 3 have been extensively investigated [24][25][26][27]. In addition, due to the high surface state and unique optical or electrical properties [28], Bi 2 Se 3 can also be applied in the fields of visible-light photocatalytic degradation [27,29]. For example, Bi 2 Se 3 -TiO 2 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 Bi 2 Se 3 nanostructures, especially the systematic study of the Mo dopingenhanced absorption behavior of Bi 2 Se 3 nanostructures.
In this work, we synthesized self-assembled Mo-doped Bi 2 Se 3 nanoflowers by a simple solvothermal route. We find that the absorption behavior of Bi 2−x Mo x Se 3 on Rhodamine B (RhB) varies as a function of Mo content and reaches its highest absorption capacity with 15% Mo doping.

Methods
Preparation of Bi 2−x Mo x Se 3 All of the chemical reagents used in this experiment are of analytical grade and used without further purification.

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 (UVvis) spectrum (Lambda 900, PerkinElmer Instruments, Branford, CT, USA).

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 Bi 2 Se 3 (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 Mo 6+ content from samples A to F, indicating that Mo 6+ has been incorporated in the Bi 2 Se 3 lattice, and the lattice parameter gets smaller with the increase of Mo 6+ . This is understandable considering the fact that the ionic radius of Mo 6+ (0.065 nm) [24] is smaller than that of Bi 3+ (0.103 nm) [25].
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 Bi 2 Se 3 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
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 Bi 1.85 Mo 0.15 Se 3 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.
The chemical composition of Bi 2−x Mo x Se 3 was determined by energy-dispersive X-ray analysis (EDXA) attached to the FESEM. In Figure 3b, the EDXA spectrum of the Bi 1.85 Mo 0.15 Se 3 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 Bi 2−x Mo x Se 3 from nanoplates to nanoflowers, Bi 2−x Mo x Se 3 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 Bi 2−x Mo x Se 3 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 Bi 2 Se 3 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 Bi 2−x Mo x Se 3 , the diameter of the products was found to be lower than that of pure Bi 2 Se 3 . So we believe that Mo is the main driving force for the formation of a flowerlike structure.
A diagram of the formation mechanism of nanoplates and nanoflowers of Bi 2−x Mo x Se 3 is presented in Figure 5. When no Mo is contained, tiny clusters of Bi 2 Se 3 nanosheets are first generated upon heating and then enriched to assemble into bigger nanosheets. However, with the increase of Mo concentration in Bi 2−x Mo x Se 3 , nanosheets assemble into nanoflowers.

Adsorption ability of Bi 2−x Mo x Se 3
To investigate the potential application of the assynthesized Bi 2−x Mo x Se 3 nanocrystals and their relationship with the amount of Mo in Bi 2−x Mo x Se 3 , we study the adsorption ability of Bi 2−x Mo x Se 3 using RhB as dyes. The experiments are carried out with Bi 2−x Mo x Se 3 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 Bi 1.99 Mo 0.01 Se 3 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 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 Bi 2 Se 3 shows a weak adsorption activity. After 60 min of absorption, only 20% of RhB is removed from the pure Bi 2 Se 3 sample. The adsorption activities are strengthened with the increase of Mo contents in Bi 2 Se 3 . Bi 1.85 Mo 0.15 Se 3 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 Bi 2 Se 3 is an efficient way to enhance its adsorption activity.
The results indicate that the as-synthesized Bi 2−x Mo x Se 3 might possess a profound application in the fields of treatment of dye-polluted wastewater.

Conclusions
In summary, Bi 2−x Mo x Se 3 nanomaterials were prepared by a solvothermal approach, and different morphologies of Bi 2−x Mo x Se 3 have been obtained. The doping concentration of Mo plays an important role in controlling both the morphologies of Bi 2−x Mo x Se 3 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 Bi 2 Se 3 nanostructures.