Composition-Dependent Optical and Photoelectrochemical Behavior of Antimony Oxide Iodides

The ongoing search for new photoelectrode materials generated interest toward semiconductors containing multiple anions. In this work, three different antimony oxide iodides (Sb3O4I, Sb8O11I2, and Sb5O7I) were synthesized by anhydrous synthesis. Scanning electron microscopy revealed mainly needle-shaped particles for Sb3O4I, elongated plate-shaped ones for Sb8O11I2 and well-crystallized hexagonal particles for Sb5O7I. The isoelectric point of the antimony oxide iodides (pH∼3) was independent of the chemical composition. With increasing pH particles became negatively charged to different extents, depending on the relative amount of oxygen in the samples, through the presence of ≡Sb−O− surface functional groups. The optical properties were heavily affected by the composition as well: bandgap energies related to the direct transitions in Sb3O4I, Sb8O11I2, and Sb5O7I were 2.16 and 2.74 eV, 2.85 eV, and 3.25 eV, respectively. Photoelectrochemical analysis proved that all samples behave as n-type semiconductors, but the performance in water oxidation showed large variation for the different compositions. The band energy diagram was constructed for all compounds and the composition dependent optoelectronic properties were rationalized on this basis.