From micro to subnano scale: Insights into the dielectric properties of BiOI nanoplates

The electrical properties of ultrathin, agglomerated BiOI nanoplates synthesized via a simple hydrolysis method in the presence of NaOH were investigated over the temperature range of 173–273 K using broadband dielectric spectroscopy. The resulting nanoplates, with an average thickness of 34.8 ± 9.02 nm and size of 543.4 ± 103.4 nm, exhibit disordered surfaces characteristic of bismuth oxyhalides, where Bi^(3–x)+ arise from Bi–O bond breaking. Optical measurements confirm a direct allowed band gap of 1.962 ± 0.046 eV and an Urbach energy of 0.0737 ± 0.068 eV. Two dominant electrical relaxation processes were identified and described using the Dissado–Hill model. The high-frequency process, with an activation energy of 0.13 ± 0.01 eV, is attributed to the transition between hole hopping and I⁻ ion migration between positively charged [Bi₂O₂]²⁺ layers. The low-frequency process (below 100 Hz), with an activation energy of 0.44 ± 0.02 eV, emerges at elevated temperatures and is associated with iodide ion migration along nanoplate surfaces and edges, followed by their accumulation at the electrode interface. These findings demonstrate that the dielectric behavior of layered BiOI nanoplates is governed by temperature- and frequency-dependent iodide ion dynamics between the [Bi₂O₂]²⁺ layers.