Atomic layer deposition of ZnO on SnO2 nanospheres for enhanced formaldehyde detection

Heterostructures of metal oxide semiconductors have a great promise for chemical gas sensors due to the peculiar properties at the heterointerface. In this work, a highly sensitive and selective gas sensor for detecting formaldehyde is reported based on SnO2/ZnO heterospheres designed by atomic layer deposition (ALD). The electronic properties at the SnO2/ZnO heterointerface can be modulated by optimizing the loading of ZnO through changing ALD cycles. Gas sensing tests indicate that the ZnO ALD significantly improved the sensor properties including higher responses, faster response-recovery dynamics and better selectivity. The response of the SnO2/ZnO sensor to 1 ppm formaldehyde (Ra/Rg = 9.7) shows 4 times enhancement compared to pristine SnO2 at a working temperature of 200 °C. ZnO ALD of 10 cycles leads to the best response and recovery dynamics (12 and 24 s), and that of 15 ALD cycles results in the highest response (Ra/Rg = 38.2) to 20 ppm formaldehyde. The SnO2/ZnO sensor also registers a low detection limit of 70 ppb, which allows for reliable detection of sub-ppm formaldehyde. The remarkable sensor performances indicate the ALD surface engineering is promising for the design of new materials for reliable detection of harmful molecules.