Room Temperature Formaldehyde Sensing of Hollow SnO2/ZnO Heterojunctions Under UV-LED Activation

The room temperature gas sensing properties of the hollow SnO ₂ /ZnO heterojunction-based chemiresistive sensors under ultraviolet (UV) radiation were investigated. The hollow SnO ₂ /ZnO heterojunctions were fabricated using the template-assisted process followed with a hydrothermal treatment. The sensor using this core-shell SnO ₂ /ZnO heterojunctions showed a much higher response to formaldehyde under UV illumination compared to that without UV. Moreover, the response of this hollow heterojunctions-based sensor to formaldehyde is higher than the respective pure SnO ₂ and ZnO hollow microspheres-based sensors under same conditions indicating the synthetic effect of the formed heterojunctions on the enhanced sensitivity. More strikingly, the response of the SnO ₂ /ZnO heterojunctions-based sensors showed a much smaller response to ethanol, methanol, and toluene indicating an excellent selectivity whereas there is a large influence from ethanol if the sensor worked under the conventional heating method. It could be attributed to the lower oxidizing ability of the adsorbed, ionized oxygen with a negative monovalence on the oxide surface and thus a selective photo-catalytic conversion of formaldehyde over the other possible interferents. The long-term stability of the sensor operated under UV light was also studied.