Enhanced response of the photoactive gas sensor on formaldehyde using porous SnO2@TiO2 heterostructure driven by gas-flow thermal evaporation and atomic layer deposition

Nanoporous SnO2@TiO2 heterostructure was synthesized by a facile two-step dry process, modified thermal evaporation followed by atomic layer deposition (ALD). The introduction of inert gas, Ar, with a pressure of 0.2 Torr during thermal evaporation of SnO, enabled the formation of the nanoporous 3D structure by inducing the collision and loss of kinetic energy during deposition. A photocatalytic material, TiO2, was grown on the porous structure of SnO2 to detect target gas, formaldehyde, under UV irradiation selectively. Microstructural and elemental analysis with a transmission electron microscope and X-ray photoelectron spectroscopy confirmed the porous structure of SnO2 induced by our evaporation process as well as the conformal coating of TiO2 on the porous structure. The sensing capabilities of a photoactive sensor on the formaldehyde were assessed in terms of the film porosity, irradiated UV power, and thickness of photoactive materials at room temperature. As a result, the SnO2@TiO2 heterostructure, with an optimum thickness of TiO2 exhibited low detection limit, down to 0.1 ppm, good linearity to the concentration of formaldehyde in the range of 0.1–10 ppm, and high response of 15% in the HCHO 0.1 ppm. This core-shell porous structure developed by modified thermal evaporation combined with ALD paved the way for 3D architectures to explore various applications, such as biosensors, photocatalysts, and optoelectronic devices.