Baobao Cao

Growth of monoclinic WO3nanowire array for highly sensitive NO2 detection

A monoclinic tungsten trioxide nanowire array has been grown on silicon substrates using tungsten powders as source materials by thermal evaporation under specific synthesis conditions (1000 °C, 13–15 Torr, 200 sccm air flow). The morphology, chemical composition and crystal structure of the as-prepared tungsten trioxide nanowires were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. The nanowires were identified as monoclinic in structure, with diameters ranging from 40 to 100 nm and lengths up to 5 µm. It was found that sufficient oxygen and air flow are the major factors to influence the nanowire array growth. The nanowire array was employed directly for gas sensor fabrication using photolithography. The gas sensing experiments revealed that the nanowire array sensors are highly sensitive to NO2 (50 ppb), making the tungsten trioxide nanowire array a competitive candidate for highly sensitive gas sensor fabrication.

Towards one key to one lock: catalyst modified indium oxide nanoparticle thin film sensor array for selective gas detection

Homogeneous In2O3 nanoparticles (NPs) were self-assembled into thin film sensor arrays on a single chip, with further surface modification by noble metal catalysts. The NP film sensor arrays show clear current responses when exposed to different target gases, and both sensitivity and selectivity were greatly improved. Particularly, the sensors modified with Au, Pd, and Pt nanocatalysts demonstrated higher sensitivity to H2S, H2 and CO, respectively, making the gas discrimination direct and simple, like “one key to one lock”. The particle size dependence of the noble metal modifiers to the sensitivity was further investigated by tuning the sputtering parameters. Three different trends of sensitivities were observed, each attributed to different mechanisms. The modified nanoparticle film sensor was also fabricated on flexible substrates and the sensing performance was investigated at different bending angles.

Investigation of Gas-Sensing Performance of

Two forms of tin oxide (SnO2) nanoparticles (NPs) with different morphologies (small dispersed NPs and big clusters aggregated from small NPs) were synthesized via chemical solution methods and assembled into films on Si/SiO2 substrate for a comparative sensing study, which were tested for the responses to H2 S reducing gas and NO2 oxidizing gas, respectively. Similar detection limits to part per million levels H2 S reducing gas and part per billion (ppb) levels NO2 oxidizing gas were observed for both films with different morphologies at room temperature. It was found, however, that the small NP film showed higher sensitivity and detection limit to H2 S down to ppb levels after surface modification by depositing Au nanocatalysts, while SnO2 big cluster film presented better sensitivity to NO2 detection down to ppb levels with/without the nanocatalyst modification, much determined by the morphology of these two forms of NPs. The sensing mechanism is also discussed.

{\rm SnO}_{\rm 2}

Different from the common metal catalysts (e.g., Au) employed in catalyst-assisted growth of one-dimensional (1D) structures, a variety of molybdenum oxide (MoO3) layered 1D structures were synthesized using a group of alternative alkali metal based catalysts. In contrast to the sole axial growth mode found in conventional catalyst-assisted growth, two different growth modes were observed for the MoO3 1D growth here: transverse growth and axial growth. In the transverse mode, the 1D structures grow perpendicularly to the catalyst-deposition axis with the catalyst particles sitting on the side surfaces of the 1D structures; whereas in the axial mode, the growth direction is along the catalyst-deposition axis. The growth modes were explained by a modified vapor–solid–solid (VSS) mechanism, and the factors that affect the growth were explored. Based on the proposed growth mechanism, the growth was extended to a large family of alkali metal based catalysts and hierarchical structures were realized by multiple-growth approaches. This growth mechanism provides a new approach to control the orientation of 1D structures and can be applied to different layered materials.

Nanoparticles With Different Morphologies

Gas sensors, as modern electronic components, have obtained enormous development since the 1970s [1]. As an interface between electronic systems and our environment, gas sensors collect information from the atmosphere, i.e., they detect whether a particular chemical vapor is present and of what concentration. This is useful for a variety of applications. Recently, gas sensors have played important roles in various applications, such as environmental monitoring, automobile, industrial safety, manufacture quality control, and public security [2, 3]. For example, oxygen sensors in automobiles can ensure complete and efficient gasoline combustion in the engine, specially designed gas sensors can monitor the quality of perfume, and explosive detectors can be used in train stations and airports. In the last decade, major improvements in the capabilities of mobile devices and wireless networks have generated more and more demand of portable or mobile device-integrated sensor systems. One example is the massively distributed gas sensor that can locate the safety threat when they are integrated with geographical positioning systems [4] that are available in many modern mobile phones. These novel applications impose more requirements such as low power consumption, miniaturized size, high sensitivity, and selectivity of gas-sensing systems.

New growth modes of molybdenum oxide layered 1D structures using alternative catalysts: transverse mode vs. axial mode

Transmission electron microscope (TEM) is an invaluable tool for the characterization of one-dimensional (1D) nanostructures, providing information on morphology, crystal structure, and chemical composition through imaging, diffraction, and spectroscopic analysis. This chapter illustrates the capabilities of TEM techniques in structure analysis with a number of 1D semiconducting nanostructures. Polarity determination and planar defects imaging by TEM will be discussed in ZnO nanostructures. Most emphasis will be given to the comprehensive TEM analysis of nanowire superlattice and heterostructured nanowires, including axially heterostructured nanowires, radial core-shell nanowires, and surface decorated nanowires.