Nguyen Duc Hoa

Chlorine Gas Sensing Performance of On-Chip Grown ZnO, WO3, and SnO2 Nanowire Sensors.

Monitoring toxic chlorine (Cl2) at the parts-per-billion (ppb) level is crucial for safe usage of this gas. Herein, ZnO, WO3, and SnO2 nanowire sensors were fabricated using an on-chip growth technique with chemical vapor deposition. The Cl2 gas-sensing characteristics of the fabricated sensors were systematically investigated. Results demonstrated that SnO2 nanowires exhibited higher sensitivity to Cl2 gas than ZnO and WO3 nanowires. The response (RCl2/Rair) of the SnO2 nanowire sensor to 50 ppb Cl2 at 50 °C was about 57. Hence, SnO2 nanowires can be an excellent sensing material for detecting Cl2 gas at the ppb level under low temperatures. Abnormal sensing characteristics were observed in the WO3 and SnO2 nanowire sensors at certain temperatures; in particular, the response level of these sensors to 5 ppm of Cl2 was lower than that to 2.5 ppm of Cl2. The sensing mechanism of the SnO2 nanowire sensor was also elucidated by determining Cl2 responses under N2 and dry air as carrier gases. We proved that the Cl2 molecule was first directly adsorbed on the metal oxide surface and was then substituted for pre-adsorbed oxygen, followed by lattice oxygen.

Outstanding gas-sensing performance of graphene/SnO2 nanowire Schottky junctions

Schottky junctions (SJ) are considered devices for sensing applications due to their unique properties. Herein, we report on the design, facile fabrication, and outstanding NO2 gas sensing properties of monolayer graphene (GP)/SnO2 nanowire (NW) SJ devices. The devices were prepared by directly growing single crystal SnO2 NWs on interdigitated Pt electrodes via thermal evaporation, followed by transferring a GP layer grown by chemical vapor deposition on top of the NW chip. The SJ-based sensor showed a reversible response to NO2 gas at concentrations of ppb levels with detection limits of about 0.024 ppb at a low operating temperature of 150 °C and bias voltage (1 V) with a response/recovery time of less than 50 s. The outstanding gas-sensing characteristics of the device were attributed to tuning the Schottky barrier height and barrier width at the tiny area of contact between GP and SnO2 NW through the adsorption/desorption of gas molecules.

Giant enhancement of H2S gas response by decorating n-type SnO2 nanowires with p-type NiO nanoparticles

Metal oxide nanowires (NWs) are widely considered as promising materials for gas sensor applications. Here, we demonstrate that by decorating NiO nanoparticles on SnO2 NWs, the gas response to 10 ppm H2S increased up to ∼351-fold. The response of the NiO-decorated SnO2 NWs sensor to 10 ppm H2S at 300 °C reached ∼1372, whereas the cross-gas responses to 5 ppm NH3, 200 ppm C2H5OH, and 1 ppm NO2 were negligible (1.8 to 2.9). The enhanced H2S sensing performance was attributed by the catalytic effect of NiO and the formation of a continuous chain of n-p-n-p junctions.

A high-performance triode-type carbon nanotube field emitter for mass production

A triode-type field emission device based on carbon nanotubes (CNTs) synthesized on an anodic aluminum oxide (AAO) template was fabricated. For the improvement of device performance, in addition to the basic advantages of using AAO to obtain uniform CNTs with strong adhesion, several considerations were taken into account, including highly crystalline CNTs synthesized through thermal chemical vapor deposition (CVD) at 1200 °C, lowering the contact resistance with a Ti buffer layer and reducing the pixel size and gate-to-emitter distance. The triode-type field emitter showed a high current density of 20 mA cm−2 and a low turn-on voltage of 16 V. The very high field enhancement factor of 1.6 × 106 cm−1 confirmed the high efficiency of the triode structure in electron extraction.

Scalable Fabrication of High-Performance NO2 Gas Sensors Based on Tungsten Oxide Nanowires by On-Chip Growth and RuO2-Functionalization

The on-chip growth and surface-functionalization have been recently regarded as promising techniques for large-scale fabrication of high performance nanowires gas sensors. Here we demonstrate a good NO2 gas-sensing performance of the tungsten oxide nanowires (TONWs) sensors realized by scalable on-chip fabrication and RuO2-functionalization. The gas response (Rg/Ra) of the RuO2-functionalized TONWs to 5 ppm of NO2 was 186.1 at 250 °C, which increased up to ∼18.6-fold compared with that of the bare TONWs. On the contrary, the responses of the bare and functionalized sensors to 10 ppm of NH3, 10 ppm of H2S and 10 ppm of CO gases were very low of about 1.5, indicating the good selectivity. In addition, the TONW sensors fabricated by the on-chip growth technique exhibited a good reversibility up to 7 cycles switching from air-to-gas with a response of 19.8 ± 0.033 (to 1 ppm of NO2), and this value was almost the same (about 19.5 ± 0.027) for 11 cycles after three months storage in laboratory condition. The response and selectivity enhancement of RuO2-functionalzied TONWs sensors was attributed to the variation of electron depletion layer due to the formation of RuO2/TONWs Schottky junctions and/or the promotion of more adsorption sites for NO2 gas molecule on the surface of TONWs, whereas the good reversibility was attributed to the formation of the stable monoclinic WO3 from the single crystal of monoclinic W18O49 after annealing at 600 °C.

A comparative study on the NH3 gas-sensing properties of ZnO, SnO2, and WO3 nanowires

In this work, a large quantity of ZnO, SnO2 and WO3 nanowires (NWs) was successfully synthesised by simple and efficient methods. Their morphology and microstructure were characterised by FE-SEM, TEM, XRD, PL, and Raman. The NH3 gas-sensing properties of these NWs were investigated and compared. It was found that the responses and response-recovery time of SnO2 and WO3 NWs sensors to NH3 gas are relatively comparable, and they have a better NH3 gas-sensing performance than that of ZnO NWs sensor. In addition, the SnO2 NWs sensor has the lowest operating temperature.

Synthesis and characterisation of ZSM–5/SBA–15 composite material

ZSM–5/SBA–15 composite materials were successfully synthesised by a hydrothermal treatment method including three steps: 1) microporous phase seed formation (seed 1); 2) precursor addition (seed 2); 3) formation of the ZSM–5/SBA–15 composite material. The obtained samples were characterised by XRD, SEM, TEM, TPD–NH3, 27Al MAS NMR, 1H MAS NMR, and N2adsorption/desorption. It could be shown that the formation of both micro– and mesopores strongly depends on the duration and temperature of the above–mentioned crystallisation steps. The results obtained from XRD, SEM, TEM, and N2 adsorption/desorption indicated that the ZSM–5/SBA–15 composite materials have both microporous and mesoporous properties. The composite nature of the samples was demonstrated by SEM and TEM studies. The high acid sites number and strength of the ZSM–5/SBA–15 composite materials were shown using 27Al MAS NMR, 1H MAS NMR, and TPD–NH3 methods.

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.

An Array of Interconnected-opened-ended Multi Wall Carbon Nanotubes Grown on AAO Template

The uniform and high ordered array of interconnected-opened-ended multi wall carbon nanotubes (MWNTs) are synthesized into nanopores of anodic aluminum oxide (AAO) templates by thermal chemical vapor deposition (CVD) method without using metallic catalyst. The morphologies and quality of MWNTs are characterized by SEM, and Raman spectroscopy. The results show that at optimized growth conditions, the high quality interconnected-opened-ended MWNTs are successfully synthesized.

Carbon Nanotube Gas Sensor Fabricated on Anodic Aluminum Oxide

A new type of gas sensor was realized by directly depositing carbon nanotube on nano channels of the anodic alumina oxide (AAO) fabricated on p-type silicon substrate. The carbon nanotubes were synthesized by thermal chemical vapor deposition at a very high temperature of 1200 oC to improve the crystallinity. The device fabrication process was also developed. The contact of carbon nanotubes and p-type Si substrate showed a Schottky behavior, and the Schottky barrier height increased with exposure to gases while the overall conductivity decreased. The sensors showed fast response and recovery to ammonia gas upon the filling (400 mTorr) and evacuation.