Chu Manh Hung

Nanoporous ZnO nanostructure synthesis by a facile method for superior sensitivity ethanol sensor applications

Nanoporous ZnO nanostructures were prepared by simple thermal decomposition of plate-like hydrozincite for gas sensor applications. The plate-like hydrozincite obtained through room temperature precipitation had a diameter of 100 nm and a thickness of about 10 nm. After thermal decomposition, nanoporous ZnO nanoparticles on average 30 nm (16.12 nm in crystalline size) in diameter were obtained. Gas-sensing measurements demonstrated that the nanoporous ZnO nanostructures are promising for superior sensitivity ethanol monitoring in lung cancer diagnosis.

C2H5OH and NO2 sensing properties of ZnO nanostructures: correlation between crystal size, defect level and sensing performance

ZnO nanostructures can be synthesized using different techniques for gas sensor applications, but different synthesis methods produce different morphologies, specific surface areas, crystal sizes, and physical properties, which consequently influence the gas-sensing properties of materials. Many parameters such as morphology, specific surface areas, crystal sizes, and defect level can influence the gas-sensing properties of ZnO nanostructures. However, it is not clear which parameter dominates the gas-sensing performance. This study clarified the correlation between crystal size, defect level, and gas-sensing properties of ZnO nanostructures prepared from hydrozincite counterparts by means of field emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and photoluminescence spectra. Results showed that the average crystal size of the ZnO nanoparticles increased with thermal decomposition temperatures from 500 °C to 700 °C. However, the sample treated at 600 °C, which has the lowest visible-to-ultraviolet band intensity ratio showed the highest response to ethanol and NO2. These results suggested that defect level but not size is the main parameter dominating the sensor performance. The gas sensing mechanism was also elucidated on the basis of the correlation among decomposition temperatures, crystal size, defect level, and gas sensitivity.

A comparative study on the electrochemical properties of nanoporous nickel oxide nanowires and nanosheets prepared by a hydrothermal method

Metal oxide nanostructures have been extensively used in electrochemical devices due to their advantages, including high active surface area and chemical stability. However, the electrochemical properties of metal oxides are strongly dependent on their structural characteristics. We performed a comparative study on the electrochemical performance of nanoporous nickel oxide (NiO) nanosheets and nanowires. The advanced nanoporous NiO nanomaterials were synthesized by a facile hydrothermal method followed by thermal calcination. The synthesized nanomaterials, as characterized by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and nitrogen adsorption/desorption isotherms, demonstrated the nanoporosity and high crystallinity of the NiO nanosheets and nanowires. Cyclic voltammetry measurement was performed using a three-electrode system to evaluate the electrochemical properties of the synthesized materials. Results showed that the nanoporous NiO nanosheets possessed a higher current density than that of the nanowires by approximately ten times. Moreover, the nanoporous NiO nanosheets showed exceptionally high stability of almost 100%, after three cycles in strong alkaline environments, thereby suggesting possible application in electrochemical devices.

Facile on-chip electrospinning of ZnFe2O4 nanofiber sensors with excellent sensing performance to H2S down ppb level

ZnFe2O4 nanofiber gas sensors are cost-effectively fabricated by direct electrospinning on microelectrode chip with Pt interdigitated electrodes and subsequent calcination under different conditions to maximize their response to H2S gas. The synthesized nanofibers of approximately 30-100 nm in diameter show typical spider-net-like morphology of the electrospun nanofibers. The ZnFe2O4 nanofibers comprise many 10-25 nm nanograins, which results in multi-porous structures. Moreover, the nanofibers exhibit the single phase of cubic-spinel-structure ZnFe2O4. The density, crystallinity and grain size of ZnFe2O4 nanofiber that strongly affect gas-sensing properties can be optimized by controlling electrospun time, annealing temperature, annealing time and heating rate. Under optimal conditions, the ZnFe2O4 nanofiber sensors exhibit high sensitivity and selectivity to H2S at sub-ppm levels. Excellent gas-sensing performances are attributed to effects of multi-porous structure, nanograin size and crystallinity, which is explained by the sensing mechanisms of ZnFe2O4 nanofiber sensors to H2S gas.

Ultrasensitive NO2 gas sensors using hybrid heterojunctions of multi-walled carbon nanotubes and on-chip grown SnO2 nanowires

Hybrid heterojunction devices are designed for ultrahigh response to NO2 toxic gas. The devices were constructed by assembling multi-walled carbon nanotubes (MWCNTs) on a microelectrode chip bridged bare Pt-electrode and a Pt-electrode with pre-grown SnO2 nanowires (NWs). All heterojunction devices were realized using different types of MWCNTs, which exhibit ultrahigh response to sub-ppm NO2 gas at 50 °C operated in the reverse bias mode. The response to 1 ppm NO2 gas reaches 11300, which is about 100 times higher than that of a back-to-back heterojunction device fabricated from SnO2 NWs and MWCNTs. In addition, the present device exhibits an ultralow detection limit of about 0.68 ppt. The modulation of trap-assisted tunneling current under reverse bias is the main gas-sensing mechanism. This principle device presents a concept for developing gas sensors made of a hybrid between semiconductor metal oxide NWs and CNTs.