Highly sensitive and rapid responding humidity sensors based on silver catalyzed AgS-TiO quantum dots prepared by SILAR.

We developed a resistive humidity sensor based on a heterojunction of silver sulfide (AgS) quantum dots (QDs) and TiO because of its specificity to water vapor adsorption and its insensitivity to environmental gases. The QDs were grown on a mesoporous TiO layer using the successive ionic layer adsorption and reaction (SILAR) method. The boundary condition between TiO and AgS provides a tunable energy gap by adjusting the number of SILAR cycles. Besides, the large surface-to-volume ratio of QDs provides a strong water vapor adsorption ability and electron transfer. Nano-silver precipitated during the SILAR process provides free electrons and lowers the Fermi level to between n-type TiO and p-type AgS. The resistance response increased significantly to 4600 and the reaction equilibrium time decreased greatly to 7 seconds due to the presence of nano-silver. Finally, the AgS QDs possess a best sensing range of 13-90%. To sum up, AgS QDs are high sensitivity and selectivity humidity sensors.