Electrodeposition of silicon from three different ionic liquids: possible influence of the anion on the deposition process

The electrodeposition of silicon was investigated from three different ionic liquids with the cation 1-butyl-1-methylpyrrolidinium (Py_1,4]⁺) and three different anions, namely, trifluoromethylsulfonate (TfO^?), bis(trifluoromethylsulfonyl)amide (TFSA^?) and tris(pentafluoroethyl)-trifluorophosphate (FAP^?) at room temperature and at 100 °C, respectively. The electrodeposition was performed on gold and on copper substrates. Cyclic voltammetry was used to evaluate the possible influence of anions on the deposition process. In situ STM studies were also carried out to examine the interfacial behaviour of the SiCl₄/Py_1,4]TFSA and SiCl₄/Py_1,4]FAP on Au(111) at room temperature. In situ STM measurements revealed that an underpotential deposition of Si in Py_1,4]FAP occurred on Au (111) at ~ -0.5 V (vs. Fc/Fc⁺). In comparison, only adsorption of ionic liquid and gold surface reconstruction was found to occur in the potential regime between -0.3 and ?1.8 V (vs. Fc/Fc⁺), respectively, in the case of Py_1,4]TFSA. In situ STM investigations reveal an effect of the anion on the interfacial processes. In situ I/U tunnelling spectroscopy shows that the band gap of the electrodeposits is ~1.1 eV, indicating that semiconducting silicon has been electrodeposited. Potentiostatic electrolysis was performed to deposit Si from the employed electrolytes at room temperature and at 100 °C. The deposits were characterised using scanning electron microscopy and X-ray diffraction. Thin films of Si could be obtained from the employed ionic liquids and the quality of the deposits was significantly improved at 100 °C.