Different morphologies of bismuth sulphide (Bi2S3) nanoparticles (NPs) were synthesized at room temperature using wet chemical method. The properties of bismuth sulphide (Bi2S3) nanoparticles can be controlled by different amounts of Mn2+ dopant. The synthesized nanoparticles were characterized by several techniques, such as high resolution scanning electron microscopy (HR-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), and energy dispersive X-ray spectroscopy (EDS). The nanoparticles (Bi2S3) were found to have excellent activity for the UV light assisted decolorization of methyl violet dye and also helped to speed up the redox reaction of
Over the last few years, semiconductor nanoparticles (NPs) have received considerable attention due to their wide applications in optical and electronic devices [1–3]. Different shapes of semiconductor nanoparticles have attracted attention in last decades, due to their effect on their magnetic and catalytic properties [4, 5]. Bismuth sulfide (Bi2S3) is a semiconductor having a direct band gap (Eg) of 1.3 eV [6, 7]. Doping of semiconductor nanomaterials with transition metal is important because it influences the electrical, optical, catalytic, and magnetic properties of the host semiconductor . It is believed that doping a suitable metal ion, such as Mn2+, into a semiconductor host material reduces the band gap of the semiconductor, resulting in faster transition of electrons between the conduction and valence band, and due to this property, it has found many applications in nanoelectronic devices .
Size and morphology of nanoparticles can be changed by doping a suitable metal ion into semiconductor nanoparticles . Morphology of nanoparticles also depends upon the temperature at which the reaction was carried out, solvents used in the synthesis and the time required to complete the reaction, but these methods are especially dependent on high temperature and pressure . Different methods have been reported for preparing bismuth sulphide nanoparticles, such as sonochemical method , microwave irradiation [13, 14] and thermal evaporation . However, these methods require very high temperature and long time. Being less toxic and harmless, bismuth and its compounds can be used in various fields of biological, cosmetic and medical industries .
In the present study, we report the synthesis of bismuth sulphide nanoparticles by simple wet chemical method and change in the morphology of Bi2S3NPs with the doping of Mn2+ metal ions. The change in morphology of Bi2S3NPs with Mn2+ dopant, caused by using polyoxyethylene 100 stearate (Myrj59) as a capping agent, has been observed. The catalytic properties of as-synthesized materials were evaluated by decolorization of methyl violet dye and from the electron transfer reaction between potassium ferricyanide (PFC) and sodium thiosulphate (STS).
Bismuth nitrate, manganese acetate, polyoxyethylene 100 stearate (Myrj59), Na2S (sodium sulphide), nitric acid, potassium ferricyanide (PFC), sodium thiosulfate (STS) and methyl violet were purchased from Sigma Aldrich. All the solutions were prepared using doubly distilled and demineralized water.
2.2.1 Synthesis of Mn2+ doped bismuth sulphide nanoparticles
Mn2+ doped Bi2S3 nanoparticles were synthesized through wet chemical method using bismuth nitrate as a source of bismuth metal, manganese acetate and sodium sulphide as sources of manganese ion and sulphur. Polyoxyethylene 100 stearate (Myrj59) acted as a stabilizing agent. Mn (10 %) doped Bi2S3NPs were synthesized by using bismuth nitrate (1.25 g) and manganese acetate (0.26 g) solution in nitric acid (20 mL). 2 g of polyoxyethylene 100 stearate (Myrj59) were dissolved in 30 mL of deionized water and stirred for half an hour to form a homogeneous solution. 1.3 g of Na2S was dissolved in 10 mL of deionized water. Thereafter, Na2S solution was added to the Myrj59 solution under stirring. To this mixture, bismuth nitrate and manganese acetate solution were added dropwise. The reaction mixture was stirred for 5 h. The resulting precipitate was separated by centrifugation at 10000 rpm for 10 min, washed with sodium hydroxide, ethanol, water and acetone, and dried at 60 °C to 70 °C to obtain Mn2+ doped Bi2S3 black powder. Similarly, for comparison Mn doped (5 %) and undoped Bi2S3 nanoparticles were synthesized.
2.2.2 Characterization of Mn2+ doped Bi2S3 nanoparticles
X-ray powder diffraction (XRD) investigation was carried out on Shimadzu XRD-6000. The samples were prepared by pressing the dried powder and the patterns were collected with a scanning rate of 2°/min at 2θ ranging from 0° to 80°. The morphology of nanoparticles was examined by transmission electron microscopy (TEM). TEM images were recorded on Philips CM200 using an operating voltage of 200 kV. High resolution scanning electron microscopy investigation was done on FEI Quanta-FEG 200 (HR-SEM). Elemental composition was determined by energy dispersive X-ray (EDX) analysis of the vacuum dried doped and undoped Bi2S3 nanoparticles using a model JSM-5610 LV attached to scanning electron microscope (SEM). PerkinElmer Optima 4300 DV ICP-OES was used to analyze the constituents present in traces. FT-IR spectra were recorded for KBr pellet on the PerkinElmer Spectrum RX1 model in the range of 4000 cm−1 to 400 cm−1. Doped and undoped Bi2S3 nanoparticles catalyzed decolorization of methyl violet dye and the redox reaction of
2.3 Decolorization of dye
The experiments were carried out using ethyl violet dye as a model system. Each UV light assisted reduction experiment was carried out by taking 25 mg Bi2S3 nanoparticles into a 250 mL glass UV chamber containing water jacket with 32.5 × 10−4 mol·L−1 of dye. Analogous experiments were performed with and without undoped Bi2S3 nanoparticles. The decolorization of the dye was measured by changes in its absorbance at λmax using UV-Vis spectrophotometer.
2.4 Redox reaction
The reaction mixture containing as-synthesized Bi2S3 nanoparticles (7 mg), 1 mL of 0.001 mol potassium ferricyanide (PFC) and 1 mL of 0.1 mol sodium thiosulfate (STS) were placed in a quartz cell with 1 cm3 path length. The reaction was monitored by UV-Vis spectrophotometer in the presence and absence of Bi2S3 NPs at 25±2 °C.
All experiments were repeated three times and the reproducibility was confirmed. The recyclability of the NPs was also studied. The NPs were recovered by centrifugation, washed with acetone, dried at 60 °C under vacuum and used for the next reaction.
3 Results and discussion
3.1 XRD studies
Fig. 1 shows the XRD patterns of as-synthesized NPs. Fig. 1a is the XRD pattern of undoped Bi2S3NPs, whereas Fig. 1b and Fig. 1c show the spectra of 5 % and 10 % Mn ion doped Bi2S3NPs. Almost all peaks in the patterns could be indexed to a pure orthorhombic phase bismuth sulphide with the lattice parameters: a = 11.132 Å, b = 11.256 Å, and c = 3.976 Å, which is in a good agreement with the literature (JCPDS Card No. 17-320). The signals of Mn+2 were not detected in the XRD spectrum of bismuth sulphide. There were no other diffraction peaks which indicates high purity and crystalline nature of the nanoparticles.
The peak due to Bragg’s reflection is observed at 2θ value corresponding to 25.3° and 28.9°. The particle size of undoped and Mn+2 doped Bi2S3 nanoparticles calculated from the Debye-Scherrer equation was 20±2 nm:
where K is the Scherrer constant, λ is the wavelength of the X-ray, β and θ are the half width of the peak at half of the Bragg angle, respectively.
3.2 TEM analysis
The size and morphology of the Bi2S3 nanoparticles were analyzed by TEM. The TEM images (Fig. 2) reveal that the undoped (Fig. 2a) and Mn+2 doped Bi2S3 nanoparticles consist of nanorods-like structures. When doping increased, the thickness of the rods also increased, as shown in Fig. 2b and Fig. 2c. Fig. 2d and Fig. 2e are TEM images of typical Bi2S3NPs at higher magnification. The diameter of the nanorods is in the range of 20 nm to 30 nm, and the length is about 200 nm to 250 nm. The observed rod type morphology of the product is probably due to chain type structure of bismuth sulphide. It is known that Bi2S3 molecules form band-like structures as Bi units are connected via weaker van der Waals forces . It seems that the formation of Bi2S3NPs may have originated from the preferential directional growth of Bi2S3 crystallites. The crystallinity of the product was also proven by SAED (Fig. 2f).
3.3 HR-SEM analysis
Morphology of the product was also analyzed by HR-SEM. The HR-SEM images (Fig. 3) show the formation of bar-shaped nanorods of Bi2S3. Fig. 3a shows HR-SEM image of pure bismuth sulphide nanoparticles which look like bundles of thin nanorods. Fig. 3b shows HR-SEM image of Mn doped Bi2S3NPs (5 %) with thick nanorods. Fig. 3c shows cubic shaped Mn doped Bi2S3NPs (10 %). When Bi2S3 was doped with Mn2+ metal ions, the shape of Bi2S3NPs has also changed. Rod type morphology of bismuth sulphide may be due to the inherent chain type structure.
3.4 EDX analysis
The weight percentage of Mn2+ was determined by energy dispersive X-ray analysis (EDX) (Table 1, Fig. 4). EDX analysis indicates that the well-cleaned final product is mostly composed of Bi and S, with no other elements (Fig. 4a). Fig. 4b and Fig. 4c are EDX images of Mn2+ doped Bi2S3NPs with 5 % and 10 % doping of Mn ion, respectively.
Percentage Mn (at.%, wt.%) as a dopant in Bi2S3 host determined by EDX, ICP.
|Amount of Mn doped [at.%] used in synthesis||Actual amount of Mn in Bi2S3 matrix (from EDX) [%]||Mn/Bi [at.%] determined by ICP||Mn [wt.%] determined by ICP||Bi [wt.%] determined by ICP|
3.5 FT-IR spectroscopy
The FT-IR spectra of Bi2S3NPs and pure polyoxyethylene 100 sterate (Myrj59) are given in Fig. 5. Both display the typical profile of C=O group of ester in the range of 1726 cm−1. The peaks in the range of 1416 cm−1 to 1420 cm−1 are due to C–H bending. It is seen that the peak at 1726 cm−1 shifts to 1731 cm−1 in the Bi2S3NPs. The shifts observed in the spectra can be attributed to the interaction of Bi2S3NPs with the polyoxyethylene 100 sterate. The band at 2912 cm−1 to 2930 cm−1 is characteristic of C–H stretching. The broad band due to hydrogen bonded hydroxyl group (O–H) appears in the range of 3470 cm−1 to 3490 cm−1 and it is attributed to the complex vibrational stretching, associated with free, inter and intra molecular bound hydroxyl groups.
3.6 Decolorization of methyl violet
Many industries, such as textile, paint, ink, plastics, cosmetics, etc., use a variety of dyes. Dyes are lost in a waste water stream during dyeing operation. The removal of the color of waste water is a burning issue all over the world, as it creates water pollution .
Various chemical methods, such as oxidation, precipitation, coagulation, etc., physical processes, such as adsorption, filtration, biological techniques, such as microbial degradation, etc., are used for dye removal [18, 19]. Out of these, photocatalytic degradation process has been proven to be the most efficient way to decolor the dye effluents in an ecofriendly manner . Photocatalytic degradation of pollutants by semiconductors is a new and effective technique for their removal from effluents of industries [21, 22]. Photocatalysis is defined as acceleration of a photochemical reaction in the presence of a catalyst.
In the present investigation, the photocatalytic decolorization of methyl violet dye has been carried out using undoped and Mn-doped Bi2S3NPs catalyst in a photoreactor. As a representative case, the catalytic activity of Bi2S3NPs was investigated in the decolorization of methyl violet dye. UV-Vis spectra in Fig. 6 show that before the addition of Bi2S3NPs, there was the maximum absorption band centered at 555 nm. UV light alone could not induce complete decolorization of the dye. But when irradiated in presence of Bi2S3NPs, UV light led to complete decolorization of the dye. The absorption band at 555 nm decreased and disappeared. When the size of bulk metals goes to the nanoscale, electron transfer is more efficient as compared to the bulk material [23, 24].
As shown in the UV-Vis spectra (Fig. 6) of methyl violet, without NPs photocatalytic decolorization did not take place completely even after 7 h. On the other hand, with NPs, the degradation was carried out in 180 min only. However, in the presence of 5 % and 10 % Mn doped Bi2S3NPs the degradation time was further reduced to 90 min (Fig. 6a) and 24 min (Fig. 6b), respectively.
The role of NPs in decolorization is as follows: when a photon of UV light strikes a surface of a semiconductor like Bi2S3NPs, a valence band electron moves to the conduction band, thus forming a positively charged hole in the valence band. The conduction band electrons and the valence band holes migrate then to the oxide surface and react with the chemisorbed O2 or OH−/H2O molecules to generate reactive oxygen species, such as
To control the behavior of materials, doping is used, which lies at the heart of many technologies. For this reason, researchers have begun to explore the use of dopants to influence the properties of semiconductor nanoparticles . The energy from absorbed photons can be efficiently transferred to the dopants which is utilized in the excitation and increases the rate of reactions on the surface of NPs . Although the role of Myrj59 is only to cap the Bi2S3 NPs and to rule out the possibilities of decolorization by Myrj59, an experiment was performed with using it. No significant decrease in the absorbance was observed with using Myrj59.
3.7 Reaction between potassium ferricyanide and sodium thiosulfate
The electron transfer reaction between
Our experimental observations clearly indicate that Mn+2 doped Bi2S3 nanoparticles have been successfully synthesized using a simple wet chemical method. The TEM study revealed that thin rod shaped NPs were obtained in cases of undoped Bi2S3NPs, while thick nanorods were formed in the presence of 5 % and 10 % Mn dopant ions. HR-SEM image also supported this observation. In HR-SEM images with 10 % loading of Mn dopant, NPs showed cubic shaped morphology. The present study allows us to conclude that multiple functionalities can be induced in NPs by doping or varying the dopant ion concentration. Further, Bi2S3NPs also helped in speeding up decolorization of methyl violet dye and changed the order of the redox reaction.
The Authors thank to the IIT Bombay and the IIT Madras for TEM, HR-SEM and EDX analysis.
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