Gas sensing properties of nanostructured vanadium oxide semiconductors by chemoresistive and optical methods

The aims of this research thesis are to synthesise VO2, V2O5 and V6O13 nanostructures and apply the materials on sensor electrodes for gas and humidity sensing. These materials were synthesised and optimised using chemical vapour deposition (CVD), microwave assisted and pulse laser deposition (PLD) techniques. Analyses with thermogravimetric (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), Raman and Fourier transform infrared (FTIR) spectroscopy showed VOx phases order as NH4VO3 ? VO2 + V2O5 (150 – 200 °C) ? V6O13 (300 °C) ? V2O5 (above 350 °C). This is when the precursor NH4VO3 was annealed in CVD between 100 – 350 °C in H2 atmosphere for 2 hrs. Adsorption analysis of VOx nanostructures showed a profile of Brunauer-Emmett-Teller (BET) surface areas which increased with the annealing temperature until 300 °C after which the transition occurred. Humidity (%) sensing response of VOx showed high response for V6O13 and V2O5 phase whereas, the Langmuir isotherm plot in the form of the response per BET surface area with respect to different levels of relative humidity showed high response for VO2. Phase evolution diagram based on these properties has been proposed. Thermal CVD annealing of NH4VO3 at 500 °C in N2 atmosphere for 2, 12 and 24 hours produced monoclinic V6O13 (at 2 hrs) and ?-orthorhombic V2O5 (at 12 and 24 hrs) nanorod structures using the above characterization techniques. Gas sensing application of these structures revealed that the H2S gas is selective in adsorption to V6O13 phase with 132 % response magnitude at 350 °C and 60 ppm, this response is 647.2 % higher than that of NH3, CH4, NO2, H2 and CO. The response and recovery times are 32 and 129 s respectively which is remarkably short compared with the data in literatures. This V6O13 sensor was ranked with its V2O5 counterpart and still found to be 238.5 % higher for H2S gas. Density functional theory (DFT) through ab initio molecular dynamics of (110) facet of monoclinic V6O13 and ?- orthorhombic V2O5 also showed high H2S adsorption energy for V6O13 than V2O5 with a profile which simulate the experimental findings. Low temperature microwave assisted synthesis of VOx from NH4VO3 without post-annealing treatment demonstrated small size homogeneous crystallite with high BET surface area and high adsorption and desorption pores. These properties translated to sub-ppm room temperature sensing of the flammable CH4 and odorant NH3 and toxic NO2 with high sensitivity. The VO2 (B) phase produced via the same microwave process applied for humidity sensing in the lateral gate metal oxide semiconductor field effect transistor (MOSFET) configuration for 0, 5, 8, 10, 12 and 15 V gate voltages. An optimum percentage humidity response observed at 5 V showed response and recovery times in the order of 60 – 70 s which is remarkably shorter than the ? 300 s response of the non-gated VO2 humidity sensor reported in this thesis. Statistical information extracted from the non-linear S-curve Hill Dose rate showed that the VO2 (B) sensor is very resilient to relative humidity by showing the humidity level of more than 100% where the response of the sensor could be reduced to 50%. In-situ Raman spectroscopy sensing of NH3 gas at the surface of PLD deposited V2O5 thin film was presented. The film crystal structure, depth profile and oxidation state was studied by cross section scanning electron microscope (SEM), time of flight secondary ion mass spectroscopy (TOF-SIMS), XPS and group theoretical analysis. Recoverable red shift of 194 cm-1 and blue shift of 996 cm-1phonons upon the interaction with the NH3 gas at 25 and 100 °C was observed. Decrease in the Raman scattered photons of the 145 cm-1 phonon was also observed for different levels of NH3 exposure. The responses of these phonon properties in NH3 environment compared to the chemoresistive sensing of the film at 40 ppm showed that the in-situ Raman spectroscopy techniques is not only more sensitive but also demonstrated possibility for selective gas detection via blue and red shift of phonon frequencies.Thesis (PhD)--University of Pretoria, 2017.PhysicsPhDUnrestricte

Micro Nano Manufacturing Methods for Chemical, Gas and Bio Sensors, Water Purification and Energy Technologies

This chapter reports on the various methods of fabricating and manufacturing micro and nano sensor, membrane and energy devices. Firstly, the characteristic often sought after by scientists and engineers for effective and efficient performance of these technologies were thoroughly discussed in details together with the characterization techniques for evaluating them. Several state-of-the-art fabricating techniques for sensor devices, water and medical based-membranes, solar cells and batteries were also discussed

Blue- and red-shifts of V2O5 phonons in NH3 environment by in situ Raman spectroscopy

A layer of ~30 nm V2O5/100 nm-SiO2 on Si was employed in the in situ Raman spectroscopy in the presence of NH3 effluent from a thermal decomposition of ammonium acetate salt with the salt heated at 100 °C. When the layer is placed at 25 °C, we observe a reversible red-shift of 194 cm−1 V2O5 phonon by 2 cm−1 upon NH3 gas injection to saturation, as well as a reversible blue-shift of the 996 cm−1 by 4 cm−1 upon NH3 injection. However when the sensing layer is placed at 100 °C, the 194 cm−1 remains un-shifted while the 996 cm−1 phonon is red-shifted. There is a decrease/increase in intensity of the 145 cm−1 phonon at 25 °C/100 °C when NH3 interacts with V2O5 surface. Using the traditional and quantitative gas sensor tester system, we find that the V2O5 sensor at 25 °C responds faster than at 100 °C up to 20 ppm of NH3 beyond which it responds faster at 100 °C than at 25 °C. Overall rankings of the NH3 gas sensing features between the two techniques showed that the in situ Raman spectroscopy is faster in response compared with the traditional chemi-resistive tester. Hooke’s law, phonon confinement in ~51 nm globular particles with ~20 nm pore size and physisorption/chemisorption principles have been employed in the explanation of the data presented.A Akande acknowledges CSIR National Centre for Nano- Structured Materials for PhD Studentship position (project number HGER27S and HGER50S) and the National Research Foundation through KIC150917142805 travel grant.http://iopscience.iop.org/0022-3727am2019Physic

Adaptation of the Wound Healing Questionnaire universal-reporter outcome measure for use in global surgery trials (TALON-1 study): mixed-methods study and Rasch analysis

BackgroundThe Bluebelle Wound Healing Questionnaire (WHQ) is a universal-reporter outcome measure developed in the UK for remote detection of surgical-site infection after abdominal surgery. This study aimed to explore cross-cultural equivalence, acceptability, and content validity of the WHQ for use across low- and middle-income countries, and to make recommendations for its adaptation.MethodsThis was a mixed-methods study within a trial (SWAT) embedded in an international randomized trial, conducted according to best practice guidelines, and co-produced with community and patient partners (TALON-1). Structured interviews and focus groups were used to gather data regarding cross-cultural, cross-contextual equivalence of the individual items and scale, and conduct a translatability assessment. Translation was completed into five languages in accordance with Mapi recommendations. Next, data from a prospective cohort (SWAT) were interpreted using Rasch analysis to explore scaling and measurement properties of the WHQ. Finally, qualitative and quantitative data were triangulated using a modified, exploratory, instrumental design model.ResultsIn the qualitative phase, 10 structured interviews and six focus groups took place with a total of 47 investigators across six countries. Themes related to comprehension, response mapping, retrieval, and judgement were identified with rich cross-cultural insights. In the quantitative phase, an exploratory Rasch model was fitted to data from 537 patients (369 excluding extremes). Owing to the number of extreme (floor) values, the overall level of power was low. The single WHQ scale satisfied tests of unidimensionality, indicating validity of the ordinal total WHQ score. There was significant overall model misfit of five items (5, 9, 14, 15, 16) and local dependency in 11 item pairs. The person separation index was estimated as 0.48 suggesting weak discrimination between classes, whereas Cronbach's α was high at 0.86. Triangulation of qualitative data with the Rasch analysis supported recommendations for cross-cultural adaptation of the WHQ items 1 (redness), 3 (clear fluid), 7 (deep wound opening), 10 (pain), 11 (fever), 15 (antibiotics), 16 (debridement), 18 (drainage), and 19 (reoperation). Changes to three item response categories (1, not at all; 2, a little; 3, a lot) were adopted for symptom items 1 to 10, and two categories (0, no; 1, yes) for item 11 (fever).ConclusionThis study made recommendations for cross-cultural adaptation of the WHQ for use in global surgical research and practice, using co-produced mixed-methods data from three continents. Translations are now available for implementation into remote wound assessment pathways