Formation of Bismuth Chalcohalide Nanorods Through a Self-Sacrificing Route and Study of Its Optical Properties for Application in Solar Cells

Bismuth based semiconductors have emerged as promising candidates for applications in many fields, such as photocatalysis, photodetection, and solar cells, among others. BiSI in particular, is one of the most recently studied. With a band gap of 1.57 eV, comprising elements which are not in a critical supply state, and a preferential electronic conduction along its c-axis in the crystal structure, this semiconductor is gaining particular attention. Furthermore, its band structure, similar to lead perovskites, makes it defect tolerant, a most desirable property for electronic applications. However, given its ternary nature, the synthesis of this compound presents several challenges, the most important being the competition of other iodided compounds such as BiOI or BiI₃. This work presents the optimization of the synthesis of BiSI nanorods (NR) in a solution method, dispensing the equipment needed for a solvothermal synthesis, the most popular reported when dealing with BiSI nanostructures, and considerably reducing reaction times. Bi₂S₃ nanoflowers (NF) and powdered I₂ as precursors provide the elements Bi and S, and I, respectively, limiting the by-products generated during the reaction. As for the solvent, mono ethylene glycol (MEG) proved to be the best option for a pure yield of BiSI NR. This signifies not only the use of a green solvent, but the avoidance of complicated media such as H₂S currents. We also study the formation mechanism of BiSI nanorods, which takes place from the self-sacrificing template of Bi₂S₃ nanoflowers. This template acts in a synergistic way with the I₃^- species formed from the interaction of MEG and I₂. There is an interaction with the double chain of Bi₂S₃ to form the double chain of BiSI. This reaction takes place at mild temperatures (180^oC) and the product obtained is easily extracted by centrifugation and washing with ethanol. The speciation of bismuth in the reaction media was also studied, determining that a Bi-O-I complex is in solution, preventing the competition of BiOI solid phase. In order to elucidate this mechanism we made several experiments: varying the reaction time, temperature, precursor’s morphology and/or chemical speciation, among others. XRD with Rietveld refinement, SEM and TEM with EDS mapping, HR-TEM, UV-Vis and IR spectroscopy, and Confocal Raman microscopy aided in the characterization. The obtained nanorods were suspended in isopropanol and drop casted onto a glass substrate in order to get a homogeneous black film. This film was characterized by UV-Vis spectroscopy to analyze the optical characteristics and a band gap of 1.6 eV was observed, in accordance to reported values. This value makes BiSI nanorods a potential solar cell absorber material. In summary, we not only present an easy, Green, and scalable synthesis of a novel material, but also an insight into the chemistry of bismuth based semiconductors that could be applied to similar compounds. In addition, the optical properties of the BiSI nanorods show its potential in photovoltaic applications.