Triadic metal chalcogen HgBi₂S₃ nanoparticles as sensitizers for TiO₂ thin film: SILAR synthesis and characterization

The compound, mercury bismuth sulphide (HgBi₂S₃), represents a promising semiconductor within the II–V–VI group, demonstrating potential as a solar cell absorber layer. However, its synthesis and investigation through the successive ionic layer adsorption and reaction (SILAR) method have remained unexplored. This study focuses on the successful synthesis of HgBi₂S₃ nanoparticles using the SILAR technique atop wide band gap n-type semiconducting titanium dioxide (TiO₂) thin films. Characterization via X-ray diffraction (XRD) confirmed the synthesis, revealing an average crystallite size of 93.83 nm. The lattice strain percentage was measured at 0.1467 with a dislocation density of 1.13 × 10⁻⁴ 1/nm². Scanning electron microscopy (SEM) analysis showcased the spherical morphology of the nanoparticles, exhibiting average sizes of 169, 238 and 329 nm corresponding to 5, 10 and 15 SILAR cycles, respectively. The thickness of the TiO₂/HgBi₂S₃ composite thin film ranged from 12 to 18 µm. Notably, sensitizing the TiO₂ film with HgBi₂S₃ nanoparticles resulted in a substantial reduction in the contact angle by ~24°. Optical studies demonstrated a significant decrease in the energy band gap of TiO₂ from 3.06 to 1.6 eV post-sensitization with HgBi₂S₃ nanoparticles, indicating enhanced light absorption capabilities. Interestingly, the energy band gap of the TiO₂/HgBi₂S₃ composite thin film remained consistent across different SILAR cycles. Moreover, electrochemical impedance spectroscopy and photoelectrochemical analyses revealed the intriguing performance characteristics of the TiO₂/HgBi₂S₃ composite thin film, showcasing promising yet marginal enhancements.