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

Abstract

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.