Selective photoactive gas detection of CO and HCHO using highly porous SnO2 and SnO2@TiO2 heterostructure

Abstract

Herein, a controlled porous structure for photoactive gas sensors was fabricated using the gas-flow thermal evaporation and atomic layer deposition method. To control the porosity of the SnO2 matrix, Ar was introduced to the chamber to adjust the pressure from 0.1 to 1 Torr during thermal evaporation. Furthermore, nanoscale TiO2 layers were conformally deposited on the surface of porous SnO2 by atomic layer deposition as a selective active layer for HCHO. As a result, the sensor deposited at a pressure of 0.2 Torr showed high sensitivity and a relatively fast response than other deposition pressures owing to the optimized porosity and better electron transport. The nanoporous structure of SnO2 showed a high response rate of 56.7% when exposed to CO with a concentration of 50 ppm and a low detection limit of 1 ppm. The introduction of a TiO2 layer on the nanoporous SnO2 allowed selective response to HCHO with a response rate of 20% at 10 ppm when the oxidation level of the target gas was well aligned with the state of reactive oxygen of materials under UV irradiation. Furthermore, the sensing capabilities of the SnO2 and SnO2@TiO2 heterostructures, such as the response rate and response time, were assessed at different deposition pressures. Our results clearly show that the matching of energy levels between the redox energy of the target gas and the conduction band of the materials plays an important role in selectively detecting the target gas in photoactive gas sensors; therefore, we propose a rational design rule to enhance the selectivity for various target gases.