UV-Enhanced Formaldehyde Sensor Using Hollow In₂O₃@TiO₂ Double-Layer Nanospheres at Room Temperature

Hollow In₂O₃@TiO₂ double-layer nanospheres were prepared via a facile water bath method using the sacrifice template of carbon nanospheres. It is shown that the size of the In₂O₃/TiO₂ nanocomposites is 150-250 nm, the thickness of the In₂O₃ shell is about 10 nm, and the thickness of the TiO₂ shell is about 15 nm. The sensing performances of the synthesized In₂O₃/TiO₂ nanocomposites-based chemiresistive-type sensor to formaldehyde (HCHO) gas under UV light activation at room temperature have been studied. Compared to the pure In₂O₃- and pure TiO₂-based sensors, the In₂O₃/TiO₂ nanocomposite sensor exhibits much better sensing performances to formaldehyde. The response of the In₂O₃/TiO₂ nanocomposite-based sensor to 1 ppm formaldehyde is about 3.8, and the response time and recovery time are 28 and 50 s, respectively. The detectable formaldehyde concentration can reach as low as 0.06 ppm. The role of the formed In₂O₃/TiO₂ heterojunctions and the involved chemical reactions activated by UV light have been investigated by AC impedance spectroscopy and the in situ diffuse reflectance Fourier transform infrared spectroscopy. The improvement of the sensing properties of In₂O₃/TiO₂ nanocomposites could be attributed to the nanoheterojunctions between the two components and the "combined photocatalytic effects" of UV-light-emitting diode irradiation. Density functional theory calculations demonstrated that introducing heterojunctions could improve the adsorption energy and charge transfer between formaldehyde and sensing materials.