Capacitive humidity sensing properties of ZnO cauliflowers grown on silicon nanoporous pillar array

Through growing zinc oxide (ZnO) on a template of silicon nanoporous pillar array (Si-NPA), a regular array of ZnO cauliflowers was prepared by a chemical vapor deposition method. All the cauliflowers were well separated and each cauliflower was composed of plenty of submicron-sized ZnO protrusions similar to undeveloped flower buds. The average diameter of the cauliflowers was ∼3 μm. The flower buds sized from ∼180 nm to ∼350 nm and grown perpendicularly in local to the surface of the head. A capacitive humidity sensor was made based on ZnO/Si-NPA and its humidity sensing properties were investigated at 22 °C. A monotonous correspondence relation was found between the capacitance of ZnO/Si-NPA and the relative humidity (RH) of the measuring environment. With the RH changed from 11.3% to 94.6% and under a measuring frequency of 1 kHz, a capacitance increment over 21,400% was achieved, from 6.62 nF to 1426.00 nF. The response and recovery times were determined to be ∼20 s and ∼3 s, respectively. The maximum hysteresis of the sensor was evaluated to be ∼4.16% occurred at 85.1% RH. The sensor was proved to be with long-term stability, with a maximum capacitance relative standard deviation of ∼3.78% at 11.3% during a 50-day storage in ambient air. Our results indicate that ZnO/Si-NPA might be a promising candidate material for fabricating room-temperature humidity sensors.