Structural properties of ultra-thin BiOI nanosheets Lana Rawlings, Robert Palgrave UCL, UK
Since graphene was first discovered, immense research has been conducted on single-layer versions of bulk materials. 2D materials exhibit vastly different and often extraordinary optoelectronic and mechanical properties as compared to their bulk counterparts, due to differing structural properties, electronic band structures and effects such as quantum confinement. Bismuth oxyiodide (BiOI) has emerged as a promising photocatalyst. It is a layered material, consisting of slabs of [Bi 2 O 2 ] 2+ sandwiched between layers of [I] - ions, forming a triple layered structure, with weak van der Waals forces holding together adjacent layers. BiOI’s superior photocatalytic activity arises as a result of internal static electric fields which form between the [Bi 2 O 2 ] 2+ and [I] - slabs. These promote the separation of photogenerated electron-hole pairs, lowering recombination rates and enhancing photocatalytic ability. 1 However, little research has been conducted on 2D-BiOI and its properties. Ultra-thin BiOI has a slightly wider band gap, an upshifted Fermi level, and higher concentration of oxygen vacancies, which are known to improve electron transfer and light absorption in bulk BiOI. 2 These properties hint 2D-BiOI could be an even more promising photocatalytic material, warranting further investigation. Freestanding flakes of ultrathin BiOI have been synthesised through a solid-state reaction of AgI, AgNO 3 and BiI 3 in sealed tubes in a furnace. The thermal decomposition of low concentrations of AgNO 3 to form O 2 in the presence of AgI facilitate the transformation of BiI 3 to form BiOI. The ultrathin BiOI flakes formed vary in size up to about 1cm 2 and are very crystalline and highly oriented with the surface of the flake being aligned with the (001) plane, shown by XRD. Ion scattering experiments also show iodine surface termination, as would be expected for (001) termination. Raman spectroscopy further confirmed the characterisation, showing the expected A 1g and E g modes as in bulk BiOI. XPS showed a slight shift in binding energy of Bi, O and I peaks upon prolonged exposure of flakes to air, suggesting potential changes to the defect energy of flakes as samples age. TEM has shown the flakes are arranged as stacks of ultrathin nanosheets, held together loosely and by varying degrees between samples. The flake edges are sharp and well-defined along the xy plane. The surface is almost atomically flat, with an RMS roughness of 0.08nm. Some degradation is visible by SEM along the edges of flakes and along any cracks that appear since the flakes are delicate, but the surfaces look very stable. UV-VIS spectra show the flakes to have strong absorbance across the visible light range up to 600 nm, with a calculated band gap of 1.88 eV. The photocatalytic activity of the material is being investigated to determine its efficacy in decomposing certain dyes. References 1. Di,J. Xia,Y. Ge,L. Xu,H. Xu,M. He,Q. Zhangand, H. Li, J. Mater. Chem. A , 2014, 2 , 15864–15874. 2. Mohebinia, C. Wu, G. Yang, S. Dai, A. Hakimian, T. Tong, H. Ghasemi, Z. Wang, D. Wang, Z. Ren and J. Bao, Mater. Today Phys. , 2021, 16 , 100293.
P216E
Made with FlippingBook Learn more on our blog