Anupriya J. T. Naik

Highly Sensitive ZnO Nanorod- and Nanoprism-Based NO2 Gas Sensors: Size and Shape Control Using a Continuous Hydrothermal Pilot Plant

Continuous hydrothermal flow synthesis of crystalline ZnO nanorods and prisms is reported via a new pilot-scale continuous hydrothermal reactor (at nominal production rates of up to 1.2 g/h). Different size and shape particles of ZnO (wurtsite structure) were obtained via altering reaction conditions such as the concentration of either additive H2O2 or metal salt. Selected ZnO samples (used as prepared) were evaluated as solid oxide gas sensors, showing excellent sensitivity toward NO2 gas. It was found that both the working temperature and gas concentration significantly affected the NO2 gas response at concentrations as low as 1 ppm.

Nanostructured tungsten oxide gas sensors prepared by electric field assisted aerosol assisted chemical vapour deposition

Nanostructured thin films of tungsten trioxide were deposited on to gas sensor substrates at 600 °C from the aerosol assisted chemical vapour deposition reaction of tungsten hexaphenoxide solutions in toluene under the influence of electric fields. The electric fields were generated by applying a potential difference between the inter-digitated electrodes of the gas sensor substrates during the deposition. The deposited films were characterised using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The application of an electric field, encouraged formation of enhanced nanostructured morphologies, with an increase in needle length and reduction in needle diameter being observed. The film gas sensor properties were also examined; it was found that the highest response of 110 to 800 ppb NO2 was given by a sensor grown under the influence of a 1.8 × 104 V m−1 electric field and operated at 250 °C, a 2.5 times enhancement compared to a sensor grown in the absence of an electric field under its optimal operating conditions.

The gas sensing properties of zeolite modified zinc oxide

The illicit manufacture of drugs in the 21st century presents a danger to first responders, bystanders and the environment, making its detection important. Electronic noses based on metal oxide semiconducting (MOS) sensors present a potential technology to create devices for such purposes. An array of four thick film MOS gas sensors was fabricated, based on zinc oxide inks. Production took place using a commercially available screen printer, a 3 × 3 mm alumina substrate containing interdigitated electrodes and a platinum heater track. ZnO inks were modified using zeolite β, zeolite Y and mordenite admixtures. The sensors were exposed to four gases commonly found in the clandestine laboratory environment; these were nitrogen dioxide, ethanol, acetone and ammonia. Zeolite modification was found to increase the sensitivity of the sensor, compared to unmodified ZnO sensors, all of which showed strong responses to low ppm concentrations of acetone, ammonia and ethanol and to ppb concentrations of nitrogen dioxide. Machine learning techniques were incorporated to test the selectivity of the sensors. A high level of accuracy was achieved in determining the class of gas observed.

Gas Sensing Studies of a n-n Hetero-Junction Array Based on WO 3 and ZnO Composites

A composite metal-oxide-semiconductor sensor array based on tungsten oxide (WO3) and zinc oxide (ZnO) was fabricated using a simple mechanical mixing method. The array was characterized using scanning electron microscopy, X-ray diffraction, Raman scattering, and X-ray photoelectron spectroscopy and tested against a variety of oxidizing and reducing gases at various operating temperatures in the range of 300°C-500°C. It was found that the composites generally had an enhanced response to the analytes. The largest observed response toward ethanol was of a 50 wt.%:50 wt.% WO3:ZnO sensor, which displayed a response of 36 toward 100 ppm of the analyte at 350°C, a 1.5-fold enhancement compared with a pure WO3 sensor and a 6.5-fold compared with a pure ZnO sensor. A 10 wt.%:90 wt.% WO3:ZnO composition displayed the highest response of 148 toward 800 ppb NO2 at 300°C, a 3.5-fold enhancement with respect to a pure WO3 sensor and a 7-fold with respect to a pure ZnO sensor. Cross sensitivity studies toward a variety of reducing gases at 350°C, showed selectivity of the array towards ethanol. The enhanced behavior of the mixed oxide materials was influenced by the packing structure, junction effects, composition, and microstructure. The results show that it is possible to tune the sensitivity and selectivity of a composite sensor, through a simple change in the compositional contribution of each metal oxide.

Assessing the potential of metal oxide semiconducting gas sensors for illicit drug detection markers

Port security with a focus on drug trafficking prevention requires inexpensive and portable systems for on-site analysis of containers in order to minimise transit delays. The potential of metal oxide semiconductors for illicit drug detection is explored here. A six-sensor array consisting of WO3 and SnO2 inks was devised. Zeolites H-Y and H-ZSM-5 were incorporated to introduce variations in sensor response. Sensors were tested against acetone, ethanol and toluene as proxies for their use in illicit drug manufacture and against ammonia and nitrogen dioxide as first models of amino- and nitro-containing compounds, given their prevalence in the structural framework of drugs and precursor molecules. Sensor sensitivity and selectivity were greatly enhanced by inclusion of zeolite materials. Admixed sensing materials were found to be particularly sensitive to the gases. Support vector machines were applied to the dataset as classification tools that accurately classified the data according to gas type. The sensing array was successful in targeting and discerning between the tested drug markers. This could be key for illicit drug detection with electronic noses based on MOS technology in the future.

Gas sensing studies of an n-n heterojunction metal oxide semiconductor sensor array based on WO 3 and ZnO composites

An array of composite metal oxide semiconductor (MOS) sensors based on WO3 and ZnO, has been fabricated and evaluated against various concentrations of ethanol and NO2 at operating temperatures between 300-500 °C. The composites were prepared by simple manual mechanical mixing and screen printed as porous thick-film gas sensors, and characterized by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). The composites showed a marked enhancement in response under certain conditions, particularly towards 100 ppm ethanol at 350 °C with a 1.5-fold better response compared to pure WO3 and 6.5-fold with respect to pure ZnO and towards 800 ppb NO2 at 300 °C, with a 3.5-fold better response compared to pure WO3 and 7-fold with respect to pure ZnO. The enhanced behavior was thought to be very complex, with influencing factors being the packing structure, junction effects, composition, microstructure and experimental conditions.