Influence of PVP on the Morphologies of Bi₂S₃ Nanostructures Synthesized by Solvothermal Method

Abstract

Different morphologies of Bi₂S₃ nanostructures were synthesized by a 180^∘C and 12 h solvothermal reaction of solutions containing Bi(NO₃)₃·5H₂O and thioacetamide (CH₃CSNH₂) in diethylene glycol (DEG) as a solvent. The as-synthesized Bi₂S₃ products characterized by XRD, Raman spectroscopy, SEM, and TEM showed that they were well-crystallized orthorhombic Bi₂S₃ phase with morphologies of nanorod-like, sheaf-like, carnation-like, and microspherical, controlled by different contents of polyvinylpyrrolidone (PVP) in the solutions. Based on the experimental results, a growth mechanism was also proposed and discussed.

1. Introduction

Presently, nanostructured materials are very important to science and technology due to their novel optical, magnetic, and catalytic properties as compared to the corresponding bulks. They are elementary materials used for the fabrication of optoelectronic devices. Among them, the V-VI chalcogenide group with A^V₂B₃^VI (A = As, Sb, and Bi; B = S, Se, and Te) formula has a wide variety applications in television cameras for photoconducting targets, thermoelectric devices, electronic and optoelectronic devices, and IR spectroscopy [1–8]. Bi₂S₃ is a 1.3–1.7 eV direct band gap layered semiconductor with orthorhombic crystal system and D_2h¹⁶ or Pbnm space group, which has structure similar to Sb₂S₃ and Sb₂Se₃. It has a number of potential applications such as photovoltaic and optoelectronic, thermoelectric cooling systems, IR devices, X-ray computed tomography, photocatalysis for hydrogen evolution, and electrochemical hydrogen storage [9–11]. It is a highly anisotropic semiconductor with layered structure parallel to its growth direction and has lattice parameters of 𝑎=11.123 Å, 𝑏=11.282 Å, and 𝑐=3.971 Å with pseudolayers of ribbon-like Bi₄S₆ polymers linked together by intermolecular attraction of bismuth and sulfur atoms along the [001] direction. Bond lengths within the ribbons are different due to two different types of coordination exhibited by both bismuth and sulfur. Two crystallographic independent Bi atoms belong to Bi₂S₃: Bi1 of 6-fold coordinated to three strong Bi−S bonds and three weak ones and Bi₂ of 5-fold coordinated square pyramidal sphere with three strong and two weak ones [12–15]. Morphologies of nanomaterials can play the role in the physical and chemical properties, for instances, 148 mA·h·g⁻¹ initial discharge capacity of Bi₂S₃ microflowers in lithium-ion batteries [16] and good electrochemical Li⁺ insertion with 826 mA·h·g⁻¹ capacity of dandelion-like Bi₂S₃ microspheres [9]. In addition, uniform Bi₂S₃ nanodots showed higher and more stable photocatalytic activity for the degradation of persistent toxic organic pollutants under UV light irradiation than Bi₂S₃ nanorods [17]. Different morphologies and structures can play the role in energy gap (𝐸_𝑔) and photoluminescence (PL) of materials [1, 2, 10, 12], including their properties.

In this research, nanorods, double sheaf-like Bi₂S₃, carnation-like Bi₂S₃, and microspheres of Bi₂S₃ nanorods were synthesized by a solvothermal method in solutions containing different contents of PVP. A possible formation mechanism was proposed and discussed according to the experimental results.

2. Experimental Procedures

All chemicals were analytical grade and were used without further purification. In a typical experimental procedure, 0.0010 mol Bi(NO₃)₃·5H₂O and 0.0015 mol thioacetamide (TAA, CH₃CSNH₂) were dissolved in 80 ml of diethylene glycol (DEG) under vigorous stirring for 1 h and followed by the addition of 0.1, 0.5, and 1.0 g of polyvinylpyrrolidone (PVP) with molecular mass of 40,000 and 0.5 mL 37% HNO₃. These resulting solutions were loaded into 100 mL Teflon-lined stainless-steel autoclaves. The autoclaves were tightly closed, processed at 180°C for 12 h in an electric oven, and left cooling to room temperature naturally. At the conclusion of the process, deep gray precipitates were synthesized, filtered, washed with distilled water and absolute ethanol repeatedly to remove undesired impurities, and dried at 60°C for 4 h.

X-ray diffraction (XRD) patterns were obtained on a Philips X’Pert MPD X-ray powder diffractometer (XRD) using a Cu-K_𝛼 radiation at 45 kV and 35 mA with a scanning rate of 0.04 deg/s in the 2𝜃 range from 15 to 60 deg. Scanning electron microscopic (SEM) analysis was conducted by a JEOL JSM-6335F electron microscope operating at 35.0 kV. Transmission electron microscopic (TEM) images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns were performed by a JEOL JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. A Raman spectroscopy was operated on a HORIBA Jobin Yvon T64000 spectrometer using Ar⁺ laser beam of 514.5 nm wavelength, calibrated using a silicon wafer as the standard.

3. Results and Discussion

Figure 1 shows XRD patterns of Bi₂S₃ synthesized using different contents of PVP by a solvothermal method at 180°C for 12 h. Compared to the JCPDS database no. 17-0320 [18] (𝑎=11.1490 Å, 𝑏=11.3040 Å, and 𝑐=3.9810 Å), all of the diffraction patterns were interpreted as orthorhombic Bi₂S₃. No impurities of Bi₂O₃, Bi, and others were detected in these main products. The diffraction peaks were narrow and sharp, indicating that the products were of high degree of crystallinity with their atoms residing in crystalline lattice.

Figure 1 

XRD patterns of Bi2S3 solvothermally synthesized in the solutions containing 0.0, 0.5, and 1.0 g PVP.

Figure 2 shows Raman spectra of Bi₂S₃ products over the wavelength range of 100–1800 cm⁻¹. The spectra present four peaks at 129, 251, 420, and 968 cm⁻¹. Generally, the presence of the first peak at 129 cm⁻¹ can be attributed to surface optical (SO) phonon mode. The second corresponds to the vibration mode of Bi₂S₃ nanorods at 252 and 259 cm⁻¹, and Bi₂S₃ nanoribbons and hierarchical nanostructures at 250 cm⁻¹, specified as the vibration mode of Bi–S bonds. The vibration at 968 cm⁻¹ was in accordance with that of Bi₂S₃ nanorods [2].

Figure 2 

Raman spectra of Bi2S3 solvothermally synthesized in the solutions containing 0.0, 0.5, and 1.0 g PVP.

Size and structure of the products under different reaction conditions were further investigated by electron microscopy (EM). Figure 3 shows typical SEM images of the as-synthesized Bi₂S₃ products in the solutions with different PVP contents. SEM analysis reveals that Bi₂S₃ synthesized in PVP-free solution is mostly consisted of large quantity of nanorods with 0.5–2 μm in length and 50 nm in diameter. The morphologies of Bi₂S₃ products were varied in sequence by adding of different contents of PVP. The mixed morphologies of microsized double sheaf-like and carnation-like Bi₂S₃ were detected in 0.1 g PVP solution under solvothermal at 180°C for 12 h. They became uniformly and completely carnation-like Bi₂S₃ microflowers with diameter in the range of 5-6 μm in the solution containing 0.5 g PVP. High magnification of the carnation-like Bi₂S₃ nanostructure reveals the lobe of carnation-like Bi₂S₃ microsized crystals consisting of nanopetals with the shape in between rods and sheets as building blocks. By increasing the PVP content to 1.0 g, the product appeared as 3D microsphere-like Bi₂S₃. Its high magnification SEM image revealed that each microsphere was composed of a number of nanorods with 3-4 μm in length and 50 nm in diameter, growing out of Bi₂S₃ core to build up 3D Bi₂S₃ microspheres. There was some difference between the carnation-like and microsphere-like products. The carnation-like flowers were composed of a number of nanopetals with the shape in between rods and sheets but the microspheres were composed of a number of nanorods with sharp tips or spears radiating from central cores. Chemical composition of the as-synthesized Bi₂S₃ microspheres was further analyzed by energy dispersive X-ray spectrometry (EDS), which revealed the presence of Bi and S in the as-synthesized Bi₂S₃ product with Bi : S atomic ratio very close to 2 : 3.

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Figure 3 

SEM images of Bi2S3 solvothermally synthesized in the solutions containing (a–d) 0, 0.1, 0.5, and 1.0 g PVP, respectively.

Figures 4 and 5 show TEM images of Bi₂S₃ synthesized in free-PVP, 0.1 g PVP, 0.5 g PVP, and 1.0 g PVP solutions. In PVP-free solution, the product was composed of large-scale Bi₂S₃ nanorods with an average length of 1-2 μm and diameter of 20–50 nm. Its HRTEM image was recorded on a single orthorhombic Bi₂S₃ nanorod and demonstrated that the Bi₂S₃ nanorod grew along the [001] direction. In 0.1 g PVP solution, TEM image indicates the transformation of the nanorods into the mixture of double sheaf-like and carnation-like Bi₂S₃ crystals, which were composed of a number of nanorods with the length up to a few micrometers. In 0.5 g PVP solution, the uniformly and completely carnation-like Bi₂S₃ crystals were synthesized at the same condition as above. In 1.0 g PVP solution, representative TEM image of the 3D spherical Bi₂S₃ of nanorods clearly confirmed the existence of nanostructured 3D microspheres with 4–6 μm in diameter. It was built up from uniform nanorods with diameter of about 30–50 nm and length of 3 μm. The crystalline structure of the individual Bi₂S₃ nanorods of the 3D spherical structure was characterized by HRTEM. A typical HRTEM image of an individual nanorod exhibited clear lattice fringe corresponding to the (020) plane aligning parallel to the [001] growth direction. The analysis confirmed that each nanorod was single crystal. It should be noted that the (020) planes were parallel to the individual Bi₂S₃ nanorod growth direction, indicating its preferential orientation growth along the [001] direction, due to the exceeding surface energy over other planes [2, 3, 13]. Wang et al. calculated surface energy of the (100), (010), and (001) planes of orthorhombic Bi₂S₃ structured model: 3.5878×10⁻⁴, 3.4964×10⁻⁴, and 4.2307×10⁻⁴ kJ/m², respectively [12]. The results supported very well with the preferential orientation growth along the 𝑐 axis in building up of 1D nanostructured orthorhombic Bi₂S₃. For the synthesis of superparamagnetic Fe₃O₄ and CoFe₂O₄ with high saturation magnetization by solvothermal processing of the solution containing PVP, these products were in the shape of microspheres [19, 20].

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Figure 4 

TEM and HRTEM images of Bi2S3 solvothermally synthesized in the solutions containing (a–d) 0, 0.1, 0.5, and 1.0 g PVP, respectively.

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Figure 5 

TEM and HRTEM images of 3D microspherical Bi2S3 of nanorods solvothermally synthesized in the solution containing 1.0 g PVP.

Formation mechanism of Bi₂S₃ with different morphologies [2, 3] can be explained as follows. Bi–TAA complexes formed during stirring at room temperature. Subsequently, Bi₂S₃ was synthesized by solvothermal reaction at 180°C for 12 h. In general, HNO₃ was very important for the synthesis of 1D Bi₂S₃ nanostructure. When H⁺ concentration was low, white BiONO₃ precipitates would be synthesized in aqueous solution by strong hydrolysis of Bi(NO₃)₃. Thioacetamide (CH₃CSNH₂) sulfur source gradually generated H₂S. The concentration of S²⁻ was controlled by the concentration of H⁺ in the solution. In a strong acidic solution, the concentration of S²⁻ was lower, and the generation rate of Bi₂S₃ was slowed down accordingly. Thus Bi₂S₃ molecules have enough time to find their best sites to reside on crystalline seeds. Finally, one-dimensional product was formed by nucleation of molecules and growth of nuclei through the directional growth of the crystal. It should be noted that Bi₂S₃ has a lamellar structure with linked Bi₂S₃ units, which formed infinite chains parallel to the [001] direction. This anisotropic structure suggested that Bi₂S₃ has a strong tendency toward 1D growth along the [001] direction, leading to the formation of 1D nanostructure [4, 21].

In general, controlling of different shapes and sizes of materials was influenced by the presence of surfactant and polymer. The evolution of morphologies of the as-synthesized Bi₂S₃ products from nanorods in the PVP-free solution to mixed double sheaf-like and carnation-like Bi₂S₃, microsized carnation-like Bi₂S₃, and microspheres of nanorods in the solutions containing 0.1, 0.5, and 1.0 g of PVP surfactant under solvothermal reaction at 180°C for 12 h can be explained by a splitting mechanism, shown in Figure 6. Once Bi₂S₃ nuclei are formed just after the supersaturation process in PVP-free solution by solvothermal reaction at high temperature, Bi₂S₃ nuclei grew along the [001] direction to form nanorods, due to the strong tendency toward 1D growth along the [001] direction. In 0.1 g PVP solution, PVP macromolecules were preferentially adsorbed on the (010) planes of Bi₂S₃ nanoparticles with subsequent promoting of the growth along the [001] direction to form nanorods at a faster rate, even at the same solvothermal condition of 180°C and 12 h. PVP surfactant also promoted the splitting process at both ends of nanorods. Thus these nanorods further formed double sheaf-like structure by the splitting mechanism in combination with the growth process. Upon increasing the content of PVP from 0.1 g to 0.5 and 1.0 g at 180°C and 12 h solvothermal condition, splitting and growing of the nanorods were strengthened. Thus these products further developed into more complete carnation-like flowers and microspheres of nanospears radiating from central cores in sequence [22].

Figure 6 

Schematic illustration for the formation of Bi2S3 nanostructures.

4. Conclusions

In this research, different morphologies of bismuth sulfide were synthesized in solutions containing different contents of PVP by the solvothermal reaction. The morphology evolution of Bi₂S₃ developed by 180°C and 12 h solvothermal reaction controlled by the contents of PVP in the solutions: nanorods, mixture of double sheaf-like and carnation-like Bi₂S₃, carnation-like Bi₂S₃, and microspheres of nanorods in the PVP-free, 0.1, 0.5, and 1.0 g PVP solutions, respectively. The formation of different morphologies can be explained by a splitting mechanism.

Acknowledgments

The authors wish to thank the Thailand's Office of the Higher Education Commission for providing financial support through the National Research University (NRU) Project for Chiang Mai University, and the National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, for providing financial support through the Project P-10-11345.