The Effect of Ultrasonic Irradiation to Hematite Nanorod Arrays Properties for Humidity Sensor Applications

In this study, the sonicated solution immersion technique was used to prepare hematite nanorod arrays that were deposited on a glass substrate coated with fluorine-tin oxide (FTO). The purpose of the study was to investigate how the ultrasonic irradiation effects over the course of 15, 45, and 60 minutes concerned the materials’ optical and structural properties. The grown hematite nanorod arrays were characterized using X-ray diffraction (XRD) for crystallinity identification, field emission scanning electron microscopy (FESEM) for surface morphology and cross-sectional, and ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy for optical properties investigation. XRD pattern revealed crystalline (rhombohedral) structure of $\alpha-F \mathrm{e}2\mathrm{O}_{3}$ hematite, with no existence of other phases of oxides. The sample with 60 min sonication time has the most porous structure and least thickness of 313 nm. The optical results revealed that the transmittance % increase towards the infrared wavelength spectrum as the sonication time increases. The manipulation of the transmittance and absorption coefficient data has discovered the optical energy bandgap estimation at 2.14 eV for the sample with 15 min sonication, and the bandgap increases to 2.15 eV and 2.16 eV as the sonication time expanded. Urbach energy estimation has shown the minimal value for the sample sonicated for 45 and 60 min, indicating the minimum energy for the atoms to diffuse and occupy the hematite crystal lattice crystal, thus improving the crystallinity of the hematite films. The sensor measurement also verified that hematite nanorod array has a strong potential to be applied as humidity sensor based on its high sensitivity and stability.