The Microwave Absorption Properties of Defective Titanium Dioxide Nanotube Arrays

Abstract

In the modern era, there has been exponential growth in the utilisation of electronic devices for civil, commercial and military applications in the gigahertz band range. Electromagnetic interference, the fourth highest source of environmental pollution has restricted the operation of electronic devices and presented health risks due to non-ionising microwave irradiation. The impetus of research has shifted towards the design of effective electromagnetic wave shielding microwave absorbing materials. One dimensional semiconducting TiO2 nanotube arrays have exhibited exceptional physical, chemical and electrical properties arising from its large specific surface area, low density and quantum confinement effects which make it a promising candidate for absorption dominated microwave shielding. The synthesis of vertically aligned self-organised TiO2 nanotube arrays was achieved using a facile and cost-effective electrochemical anodisation approach by tailoring the anodisation parameters for optimal nanotube growth. The evolution of oxygen vacancies in TiO2 nanotube arrays was explained by the dynamic processes between electrochemical oxidation at the metal-oxide interface and chemical dissolution at the oxide-electrolyte interface, which formed oxygen-deficient TiO2−x nanotubes. The TiO2−x nanotube arrays were subjected to thermal annealing in an oxygen ambient atmosphere at temperatures of 200, 300, 400 and 500°C in order to produce samples with different crystallographic phase compositions of TiO2. The phase and crystal structure of samples was characterised using XRD, Raman spectroscopy, TEM and EPR techniques. The structural and surface morphology of the samples was characterised using SEM and TEM techniques. The chemical composition of the samples were characterised using EDS, XPS and FTIR techniques. The microwave absorption properties and reflection loss characteristics of the samples were characterised using a performance network analyser. The key findings from the investigation showed that the as-anodised sample with 40 wt% TiO2 nanocrystals and thickness of 3.0 mm achieved the highest microwave absorption with a minimum reflection loss of -10.23 dB at a working frequency of 15.60 GHz. This sample also had the largest bandwidth working frequency (RL > -5 dB) of 13.12 GHz. The TiO2 nanocrystal/epoxy composites were termed as dielectric absorbers due to their higher ε’ and ε’’ values and significantly lower μ’ and μ’’ values, which did not satisfy impedance matching for enhanced absorption. The as-anodised sample had the highest concentration of oxygen vacancies, present in the disordered amorphous grain boundary regions. The concentration of the oxygen vacancies decreased as the annealing temperature was increased to 500°C. The morphology of the TiO2−x nanotubes deteriorated at temperatures exceeding 500°C, due to the formation of rutile crystallites at the interface between the anatase nanotubes and the underlying Ti substrate. The chemical composition of the TiO2−x nanotubes contained Ti (Ti2p), O (O1s) and small traces of F (F1s) elements, which were consumed on annealing. The amorphous TiO2−x predominantly crystallised into the anatase phase on annealing.