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Characterisation of the surface reactions and gas sensing properties of zinc oxide nanosheets. / Daniel Raymond, Jones
Swansea University Author: Daniel Raymond, Jones
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In this work, zinc oxide nanosheets are synthesised through thermal decomposition of a layered basic zinc acetate precursor and implemented in sensors to investigate the reactions of carbon monoxide, hydrogen and methane. The mean size of nanoparticles within the nanosheets is shown to increase with...
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In this work, zinc oxide nanosheets are synthesised through thermal decomposition of a layered basic zinc acetate precursor and implemented in sensors to investigate the reactions of carbon monoxide, hydrogen and methane. The mean size of nanoparticles within the nanosheets is shown to increase with annealing temperature, and sintering occurs after heating at temperatures of 700&deg;C or higher. X-ray photoelectron and photoluminescence spectroscopy techniques demonstrate that the concentrations of both lattice oxygen species and oxygen-containing surface groups may be enhanced by increasing the annealing temperature. By using an Eley-Rideal-based physical model, the responses of the nanosheets to carbon monoxide are quantitatively related to the reaction parameters of the system. The response characteristics suggest that the carbon monoxide oxidation has activation energy 54 +/- 9 kJ mol-1 while oxygen ionosorption has an energy barrier of 72 +/- 9 kJ mol-1. The sensor recoveries are consistent with corresponding values of 42 +/- 7 kJ mol-1 and 63 +/- 10 kJ mol?&sup1; for carbon monoxide oxidation and oxygen ionosorption, respectively. In the absence of O- or CO2- surface ions, the energy difference between the Fermi level and the conduction band minimum at the surface is estimated as 590 +/- 90 meV at temperatures close to 400&deg;C. The hydrogen responses of non-functionalised sensors are found to converge at 440&deg;C, despite differing at lower temperatures. This observation is incompatible with the developed model, but it is shown that the phenomenon may be rationalised by considering that the hydrogen concentration close to the sensor surface is decreased due to the rapidity of hydrogen oxidation. Gold nanoparticles significantly enhance the hydrogen response, with gold-decorated nanosheets remaining sensitive to hydrogen below 150&deg;C. Poor sensor recovery is attributed to the formation of long-lived hydroxyl groups formed during hydrogen spillover from the gold surface.
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