Zinc oxide (ZnO) is a versatile semiconductor material showing great potentials in piezoelectronics, solar energy harvesting and gas sensors. ZnO based gas sensing elements have been a research focus for its wide applications. However, the sensitivity is always a bottleneck impeding sensor applications. So exploring super sensitive gas sensing materials is a key point and a new challenge.

Researchers with Institute of Process Engineering (IPE) and other universities developed a new technique to obtain highly formaldehyde sensing ZnO Nanorods. They used transition metals such as Mn, Ni, Cu, and Co as dopants for the synthesis of highly formaldehyde-sensitive ZnO nanorods prepared by plasma enhanced chemical vapor deposition (PECVD) method.

In their work, the first step was to prepare transition metal doped ZnO nanorods. The morphology and composition were characterized by a scanning electron microscope and energy dispersive spectra and a transmission electron microscope. The formaldehyde gas sensing properties of ZnO-based nanorods were examined.

The doped ZnO nanorods show the superior formaldehyde-sensing property, with a sensing maximum of ~25/ppm for 10 mol% CdO activated 1.0 mol% Mn doped ZnO nanorods in a few seconds response and recovery time. The sensitivity and response/recovery time are advantageous as compared with ZnO based sensing materials prepared by precipitation and hydrothermal methods, due to the abundant crystal defects in the fast crystallization process in PECVD.

In experiments, Ni, Cu, Co were also adopted as dopants (0.5 mol%) in the ZnO nanorod growth. All samples show the nanorod-shape morphology, although the Cu-doped ZnO nanorods have a much larger length and forms clusters. The doped ZnO crystals are all in wurtzite phase. The formaldehyde gas sensing properties of the Ni, Cu, Co doped ZnO nanorods are tested. For comparison, they also prepared Sn-doped ZnO nanorods by PECVD, and plotted the responses of pure ZnO nanorods and Sn-doped ZnO nanorods together. Results showed that doped ZnO have much higher response than the undoped one, and Co, Ni, Sn doped ones have a nearly 4-fold increase in the response.

The authors of this paper thank National 863 Program for the support. And the paper was published in Sensors and Actuators B: Chemical.