Toward Band Gap Engineering of Photocatalytic Materials: The Case of Multimetal Oxyhalides Bi₄MO₈Cl– Bi₂M’O₄Cl (M = Ta, Nb; M’ = Y, La, Gd)

One direction toward high performance photocatalytic materials is through band gap control. While traditional photocatalytic materials have consisted of oxide semiconductors with valence oxygen p states and conduction transition metal d states, a novel research direction has been to control the band gap using mixed anion materials. Oxyhalides of the form Bi₄MO₈Cl (M = Ta, Nb) have been particularly promising, as they allow for a lower band gap than oxides, while remaining chemically stable as the valence band edge has oxygen p character. This latter feature inhibits photocorrosion by oxidation. The pseudo-2D structure of these materials also allows for modulation of the band gap of these materials via formation of layered intergrowths. Here, the synthesis of the layered materials Bi₄MO₈Cl-Bi₂M’O₄Cl (M = Ta, Nb; M’ = Y, La, Gd) via an optimized flux salt method is discussed. Characterization by Powdered X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS), Scanning Transmission Electron Microscopy (STEM) coupled with EDS, and Diffuse Reflectance Spectroscopy (DRS) is presented. While targeted compositions exhibited nanoscale elemental mixing and minimal impurities, the M = Ta, Nb; M’ = Y intergrowth did not have uniform atomic composition within different nanocrystals. These results broke the trend seen with other intergrowths, leading to computational calculations to explain and correlate feasibility of intergrowth formation for this system. These analyses expand the library of potential photocatalytic materials for renewable energy and similar field applications.