UC Irvine

Nanoscale devices take many advantages, including low power consumption for energy sav- ing, highly miniaturized structures for portable equipment and tiny amount of materials needed for manufacturing purposes, particularly considering rare metals. Nano-devices have exhibited significant potentials for wide industrial applications, for example, chemical and biological sensing, nano-electronics, environment and health monitoring etc. In this disser- tation, two types of H2 sensors are discussed, they are based on single metal nanowires and metal nanoparticles-decorated carbon nanotube (CNT) ropes respectively.

Single nanowire H2 sensors are fabricated by applying the methods of lithographically patterned nanowire electrodepositon (LPNE). Single palladium (Pd) nanowires with the dimension of 40 nm (height) × 100 nm (width) × 50 μm (length) are electrodeposited within LPNE templates and electrically isolated by metal contacts. Then platinum (Pt) layers are electrodeposited onto single Pd nanowires (Pd@Pt nanowires) to catalytically enhance the H2 sensing performance. The Pt layer coverage thickness is altered as average 0.1 monolayer (ML), 1 ML and 10 ML. For each coverage, the Pd@Pt nanowire sensors are evaluated at five different working temperatures, Pd@Pt sensors exhibited lowest detection at 500 ppm H2 exposure. Both response and recovery behaviors of Pd@Pt sensors are accelerated at higher temperature, yet the drawback is deterioration of sensitivity and detection limit.

A type of more advanced H2 sensors based on semiconducting CNT ropes are developed, in order to enhance the H2 sensing performance for rapid response/ recovery and wider detec- tion range. CNT ropes deposition are achieved by applying processes of dielectrophoresis in aqueous solution containing suspended CNTs. Single CNT ropes are electrical isolated at the length of 50 μm, and employed as the electrode for electrodepositing Pd nanoparticles of four coulombic loadings. Bare CNT ropes show no response to H2/ air exposures, however the sensitivity to H2 is very strongly enhanced. Pd–CNT sensors are capable of detecting H2 mixture in a very wide range between 10 ppm to 4 vol% at room temperature. The influence of Pd nanoparticle diameter to H2 sensing is also evaluated.