A Facile Synthesis and Characterization of Sodium Bismuth Sulfide (NaBiS₂) under Hydrothermal Condition

Experimental Section

All the chemicals were of analytical grade, and were used as-received, without further purification. Distilled water was purified by a New P.nix UP 900 water purification system (Human Corporation, South Korea). IR spectra were record-ed, using a MIDAC M series spectrometer. Powder X-ray diffraction (XRD) data were collected at 40 kV and 35 mA, using a Philips X’pert MPD diffractometer (Philips Analy-tical, Netherlands); CuKα radiation was used as the source (λ = 1.5418 Å). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectro-scopy (EDX) analyses were performed, using a Bruker Quantax 200 125ev SEM spectrometer (Germany). X-ray photoelectron spectra (XPS) were recorded, using an ESCALAB 210 X-ray Photoelectron Spectroscope.

Synthesis of Sodium Bismuth Sulfide. Synthesis of sodium bismuth sulfide (NaBiS2) was performed by the following procedures: Bi(NO3)3·5H2O (40 mM, 4.0 mmol) and L-Cys (120 mM, 12.0 mmol) were mixed to make a 100 mL of aqueous solution, followed by adding NaOH (2.5 M, 0.25 mol). The resulting solution was transferred to a teflon-lined autoclave reactor, and stirred with magnetic stirrer for an additional 30 min. Subsequently, the autoclave reactor was placed in an electric convection oven, with the pre-heated temperature at 180 °C for 72 h. After the reaction was completed, the reactor was cooled to room temperature. The resulting solid product was washed with ethanol and isopropyl alcohol several times with centrifugation (3000 rpm), and dried in a vacuum overnight, to give pure product as black powder (yield = 46％).

 

Results and Discussion

We investigated the synthesis and characterization of sodium bismuth sulfide (NaBiS2) under a hydrothermal condi-tion, from a solution containing bismuth nitrate as a bismuth ion source, ʟ-cysteine (ʟ-Cys) as a sulfur source, and excess amount of NaOH, to push the reaction to produce the NaBiS2. For this purpose, the concentration of NaOH was adjusted at the saturation level; otherwise, Bi2S3 would have been generated as an impurity. ʟ-Cys has been used as a sulfur source (S2−) in synthesizing sulfur-containing materials, in many other reports.8 The isolated product was black powders, and shows an instability towards water. This instability toward water may originate from the structure, in which S2− ions occupy the Cl− site, and Na+ and Bi3+ occupy the Na+ site, in the NaCl structure. Na+ ions are washed out of the structure in water; therefore, it is desirable to wash with ethanol and isopropyl alcohol. Figures 1(a) and 1(b) show the morphologies of the product. Particles of the product are largely tens of micrometers in size; they are composed of thin 2-D plates entangled with each other. The thickness of the 2-D plates is in the range of ~200 nm. Elemental analysis was performed, using energy dispersive X-ray analysis (EDX). The result reveals that the Na/Bi/S ratio obtained in the product was 24.98:25.96:49.07, which is close to 1:1:2 ratio (Fig. 1(c)), which shows the purity of the sample.

Figure 1.(a) and (b) SEM images, and (c) EDX spectrum of NaBiS2.

In order to elucidate the structure of the product, we performed powder X-ray diffraction (XRD) experiments. As can be seen in Figure 2, the XRD pattern suggests that it matches with NaCl-type NaBiS2 (Fm-3m No. 225), and it proved to be that of sodium bismuth sulfide (NaBiS2) (ICCD No. 01-075-0065). The cell dimension determined by Rietveld analysis is a = 5.76 Å with X’pert Highscore Plus.9 No peak, other than those of NaBiS2, was found, which also indicates the purity of the product. XRD, as well as EDX data, support that the obtained product is highly pure. In addition, from the XRD pattern, we could determine the crystallite size of the product.11 Even though the apparent morphology of the product (entangled 2-D sheet) is tens of micrometers in size, it could consist of small crystals, and could therefore be nanocrystalline. The crystallite size of the product was determined by the Debye-Scherrer equation:10

D = 0.9 λ / B cos θ

where, D is the coherence length of the crystallite; λ is the wavelength of the light source; B is the full-width at half maximum (FWHM) of the peak; and θ, the angle of diffr-action. The calculated coherence length of the crystallite is approximately 14 nm, which indicates that the product is highly crystalline, and nano-sized.

Figure 2XRD pattern of NaBiS2 (top), and peak assignment by ICCD No. 01-075-0065.

To investigate the electronic properties of the product, diffuse-reflectance UV-vis spectrophotometry was perform-ed, to estimate the forbidden band gap energy (Eg) of the product. The Eg of the product was found to be approxi-mately 1.4 eV, as shown in Figure 3. Gabrel’yan et al. reported the calculated band gap energy of NaBiS2 to be 1.28 eV, which was based on the experimental lattice para-meter (ELP).11 The ELP method utilizes the cell parameters as input data to calculate the Eg. Surprisingly, the obtained Eg is the largest one ever estimated for NaBiS2, indicating the nanocryatalline nature of NaBiS2. Usually, solid state materials undergo broadening of Eg, upon decrease in crystallite sizes. This corresponds to the Eg of bismuth sulfide (Bi2S3), which may be utilized as semiconducting and thermoelectric materials. Furthermore, materials with a low band gap (< 1.8 eV) have been intensively reported, because they could be utilized as light harvesting materials.12 Therefore, the product (NaBiS2) could also potentially be applied to hybrid solar cell applications.

Figure 3.Diffuse-reflectance UV-vis spectrum of NaBiS2.

Figure 4.XPS spectra of (a) full span, (b) Bi4f, (c) S2s, and (d) Na1s of NaBiS2.

XPS was used to evaluate the composition and purity of the sample. XPS spectroscopic analysis shows that all the peaks coming from the elements comprising NaBiS2 were observed, as shown in Figure 4. The peaks corresponding to O and C in Figure 4(a) are known to originate from O2, H2O, and CO2 adsorbed on the surface of NaBiS2 plates in the air.13 Figures 4(b)-(d) show narrow-scanned photoelectron spectra of Bi4f, S2s, and Na1s, respectively.

 

Conclusions

We have synthesized NaBiS2 as the sole product by hydrothermal reaction, for the first time. NaBiS2 was found to be pure in composition and structure, and is nanocrystal-line by SEM, and by XRD and XPS techniques. The product was further characterized by diffuse-reflectance UV-vis spectrophotometry, to be a low band gap material. This facile synthetic method might open up new opportunities to access NaBiS2, and is environmentally benign; it may therefore be utilized in a variety of fields, including thermoelectric materials, and as a material for hybrid solar cell applications.