Colloidal spherical stibnite particles via high-temperature metallo-organic synthesis

Abstract

Antimony trisulfide (Sb₂S₃) is an emerging semiconductor with a high absorption coefficient and a bandgap in the visible range. This makes it a promising material for various electronic and optoelectronic applications. However, one of the main challenges is still the synthesis of the material, as it is usually obtained either as a nanomaterial in its amorphous form with inferior optical properties or in crystalline rod-like structures in the micrometer or sub-micrometer range, which leads to application-related difficulties such as clogging in inkjet printing or spraying processes or highly porous layers in film applications. In this study, a one-pot synthesis of highly crystalline, spherical Sb₂S₃ sub-micron particles is presented. The particles are growing encapsulated in a removable, wax-like matrix that is formed together with an intermediate from the precursors SbCl₃ and l-cysteine. Both substances are insoluble in the reaction mixture but well-dispersable in the solvent 1-octadecene (ODE). The intermediate forms a complex crosslinked architecture whose basic building block consists of an Sb atom attached to three cysteine molecules via Sb-S bonds. Embedded in the matrix consisting of excess cysteine, ODE, and chlorine, the intermediate decomposes into amorphous Sb₂S₃ particles that crystallize as the reaction proceeds at 240 °C. The final particles are highly crystalline, spherical, and in the sub-micron range (420 ± 100 nm), making them ideal for further processing. The encapsulation method could not only provide a way to extend the size range of colloidal particles, but in the case of Sb₂S₃, this method circumvents the risk of carbonization of ligands or insufficient crystallization during the annealing of amorphous material.

Fig. 1. SEM images of the spherical Sb₂S₃ particles: (a) as received without cleaning and (b) after washing off the matrix.

Fig. 2. XRD of the purified particles. The pattern for stibnite (COD 9003460) is displayed in red.

Fig. 3. TEM analysis of the crystalline structure of the Sb₂S₃ particles: (a) TEM image of a particle with (b) the corresponding SAED pattern, (c) and (d) dark-field images of several particles showing different crystal planes in two different observation planes, and (e) and (f) HRTEM images of two crystallites in a particle. Image (f) is an enlarged view of the area in the red square in image (e). The insets in (f) show the FFT analyses of the three marked spots.

Fig. 4. UV/vis data of the final particles showing (a) the measured reflectance data, (b) the Tauc plot for estimating the indirect bandgap, and (c) the Tauc plot for estimating the direct bandgap.

Fig. 5. SEM images of the different reaction stages of the synthesis: (a) after 140 °C (PI), (b) after 170 °C (PII), (c) after 200 °C (PIII), and (d) after 240 °C (PIV). The samples were purified with the same procedure as the final particles.

Fig. 6. Phase and optical characterization of reaction intermediates at different reaction stages of the synthesis. XRD analysis (a) and reflectance measurements (b) were performed using samples taken at 140 °C, 170 °C, 200 °C, and 240 °C (without 30 min additional reaction time). The Tauc plot (c) was applied to all reflection data except that of the sample taken at 140 °C, since no band gap transition can be seen.