Electrochemical Synthesis of Bismuth Iodide and Bismuth Oxyiodide

This study reports the electrochemical synthesis of bismuth iodide and bismuth oxyiodide from acidic baths containing bismuth nitrate, sodium nitrate, iodine and ethylene glycol by a simple pH control. Linear sweep voltammetry analyses determined the potential window for the reduction of iodine where bismuth ion does not get reduced. The concentration of the iodide ions in the bath is regulated by the applied potential and the equilibrium that exists between iodine-iodide and nitrate-nitrite pairs. At pH 0.2 and 0.5, bismuth is predominantly present as free Bi3+ ions, which react with iodide to form bismuth triiodide. On the other hand, even at slightly elevated pH 1.0 and 2.0, bismuth is predominantly complexed by water, nitrates and ethylene glycol. These complexes react with the iodide ions to form bismuth oxyiodide. X-ray diffraction confirms that bismuth oxyiodide is predominantly formed at higher pH (1.0 and 2.0), while at lower pH (0.2 and 0.5), bismuth triiodide is formed. Impedance spectra are consistent with the above mechanism. The deposited bismuth oxyiodide films exhibit a cactus-like morphology formed from interlinked nano-disks, while the bismuth iodide films form micron-size platelet crystallites, both with good surface coverage. X-ray photoelectron spectroscopy confirms the formation of pure BiI3 at pH 0.2, while binding energies of Bi 4f state in pure BiOI prepared at pH 2.0 has a slight shift towards higher binding energies. From diffuse reflectance spectroscopy studies, indirect bandgaps of electrochemically synthesized BiI3 and BiOI were found to be 1.72 eV and 1.98 eV, respectively.