Hybrid materials from synthesis to gas sensing applications

Semiconductor metal oxide (SMO) gas sensors are actually one of the most investigated groups of the gas sensors. They have attracted great attention by many users and scientists interested due to their advantages such as: high sensitivity to pollutant gases, large number of detectable gases, low cost, and small size. Application fields of the SMO materials have been ranging from catalytic and electrochemical processes through optical coatings to gas sensing devices. Recently, the combination of SMO with CNTs has been explored with many interested findings. Various kinds of nanoarchitectures between SMO and CNTs have been made such as CNTs-doped SnO2, CNTs-SnO2 or WO3 composites, and CNTs-coated SnO2. This has motivated us to study nanoarchitectures of SMO and CNTs materials for gas sensing application.

The improvement of the SMO gas-sensor performance by including of SWCNTs and SMO/CNTs composite have not been well understand so far and not much literature has reported on the relative work. The model proposed by B.-Y. Wei and et al seems to be reasonable for the explanation. This model was applied for SWCNTs doped SnO2 somehow, we can apply for our case. The model has been hypothesize that CNTs/SnO2 sensor can build up p/n hetero-junctions, which was formed by (n-oxide)/(p-CNT)/(n-oxide). Above Figure schematically depicts the changes of the electronic energy bands for two depletion layers, one is on the surface of mixed oxide particles, and the other is in the interface between CNT and mixed oxide. When the mixed oxide is exposed to ethanol gas, ethanol molecules will react with oxygen ions on the surface of mixed oxide. Additionally, the nanotubes embedded in SMO film will provide an easy diffusion for chemical gas accessing through over the bulk material. After the thermal treatment, these tiny CNTs were left in the bulk material derived to form the permanent gas nanochannels as shown in above Figure. The use of CNTs can bring some advantages such as introducing identical open gas nano-channel through bulk material, achievement of a great surface to volume ratio, and providing good gas-adsorption sites due to inside and outside of CNTs.

Nanowires materials from synthesis to gas sensing applications
In recent years, there have been extensive efforts in the synthesis, characterization, and application of a new generation of semiconductor metal oxides (SMO) nanostructures such as nanowires, nanorods, nanobelts, nanotubes. These structures with a high aspect ratio (i.e., size confinement in two coordinates) offer better crystallinity, higher integration density, and lower power consumption. In addition, they demonstrate superior sensitivity to surface chemical processes due to the large surface-to-volume ratio and small diameter comparable to the Debye length (a measure of the field penetration into the bulk). We have succefully synthesized ZnO and SnO2 nanowires by thermal evaporation method. Gas sensing properties of these materials has be also investigated.

Like most metal oxide semiconductor nanoparticles-based gas sensors, the sensing properties of 1D SMO nanostructures are attributed to oxygen molecules adsorbed on the surface of the SMO nanostructures which form O2-2 ions by capturing electrons from the conductance band. So SMO nanostructures show a high resistance state in the air ambient. When SMO nanowires-based sensor is exposed to a reductive gas at moderate temperature, the gas reacts with the surface oxygen species of the nanowires, which decreases the surface concentration of O2-2 ions and increases the electron concentration. This eventually increases the conductivity of the Q1D SMO nanostructures. However, in the case of SMO thin film, the charge state modification takes place only at the grain boundary or porous surface. In the case of Q1D SMO nanostructures, it is expected that the electronic transport properties of the entire Q1D SMO nanostructures will change effectively due to the gas adsorption. The Debye length λD (a measure of the field penetration into the bulk) for most semiconducting oxide nanowires is comparable to their radius over a wide temperature and doping range, which causes their electronic properties to be strongly influenced by processes at their surface. As a result, one can envision situations in which a nanowire’s conductivity could vary from a fully nonconductive state to a highly conductive state entirely on the basis of the chemistry transpiring at its surface. This could result in better sensitivity and selectivity.