Year

2018

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The ongoing continuous progress in nanotechnology has led to a significant and positive impact on social life and public health, especially as social activities have increased, such as outdoor activities. Recently, considerable efforts have been made by researchers to develop new techniques and materials. These have been designed on the nanoscale to have novel physical and chemical features rather than in bulk form. Such features can be very relevant to the market. For instance Australia has the highest ultraviolet (UV) exposure of any developed country. Therefore it is essential that nanoparticles of metal oxide are incorporated in topical products such as sunscreens to diminish exposure and reduce harmful effects such as skin cancer and other dangerous or unsightly outcomes such as skin aging that result from the exposure to UV rays. For such an application, nanostructures of oxides such as ZnO and TiO2 are widely used because of their novel optical properties and low cost. These features are excellent in comparison to their micronised counterparts. The potential capability of several types of nanoparticles to act as active photocatalysts and hence degrade other organic ingredients in sunscreen products, however, consequently minimises their effectiveness for reducing or decreasing the harmful effects of UV rays. Different studies have suggested, however, that such particles are unable to permeate the implicated skin cells. Nevertheless, these outcomes are still under debate, because of differences in the particle sizes of samples and other parameters. Many researchers have been attracted to explore other metal oxide nanoparticles. These nanoparticles have identical competence in absorption of UV light with additional characteristics and unlike organic photochemistry, are not affected by free radicals as a result of degraded organic UV filters. Scavenging of free radicals and great UV ray absorption are the main characteristics possessed by nanoparticles such as cerium dioxide (CeO2). CeO2 has been suggested in this field due to its novel properties towards scavenging free radicals, so it has a significant role in sunscreen products. Free Radical species could result from exposure to drugs, environmental pollutants, and ionised rays, and are considered to be the major agents causing oxidative stress. Indeed this oxidative stress is the key for many human diseases such as cancer, including skin cancer, and neurological, liver, and immune system diseases. Thus due to their potential multifunctional performance, such nanoparticles can present greater capability for protection from oxidative stress and can minimise the dangerous effects of UV rays. In this thesis work, several ZnO-based nanomaterials have been studied, with the aim of achieving greater efficiency in this area.

Various synthetic strategies to prepare such nanoparticles for UV filtering applications related to health are reviewed. The principal parameters and the degradation mechanisms were investigated. The literature clearly shows that nanostructures increase the UV absorption, promotes greater stability, and reduces degradation, which can be demonstrated by the gradual decrease of crystal violet degradation when the nanostructures are applied to block radiation from solar simulators and UV degradation. Here, we investigated the synthesis and characterisation of Na-doped ZnO, ZnO/CeO2, and commercial ZnO nanoparticles synthesised via the sol-gel, Solvothermal, and simple precipitation routes. For the structural and morphological investigations, the X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) were used. The XRD results confirmed the hexagonal wurtzite structure of ZnO nanoparticles, whereas the SEM and TEM images showed a spherical morphology for the doped systems and an irregular morphology for the composite structures. The particle size was around 64.6–84.6 nm for samples prepared by the sol-gel, Solvothermal, see chapter 4 and the calculated mean crystallite sizes of the resultant nanocomposites were ~ 90.02 nm, ~79.35 nm, and ~ 40.65 nm for the 2.5 at%, 5 at% and 10 at% ceria amounts, respectively using the simple precipitation route, see chapter 5. It was found that the photocatalytic activity was reduced noticeably with increasing pH and an increasing amount of ceria within the nanocomposite. Firstly, the photocatalytic activity of the doped samples was suppressed effectively at pH 11.6 with concentration of 0.03 at% Na doping up to 90% by conducting the sol–gel process rather than the solvothermal process which resulted in about 70% photocatalytic reduction in a period of time from (0–30 min) when exposed to ultraviolet and visible light. In addition, the nanoparticles resulted by sol–gel route show a reduction in photoactivity under solar simulation about 98% rather than those resulted via solvothermal process which shows a reduction around 92% for 30 min which is the same period of time used for the photocatalytic degradation test. Secondly, the photocatalyst results show that the addition of ceria, particularly with the precipitation amount increased up to 10 at% and at Ph 12, can effectively reduce crystal violet degradation by about 97% in a period of time from (0–30) min when exposed to ultraviolet light over 30 min and by around 99% under solar simulation for 30 min also.The UV filtering properties were studied at different temperatures and for different concentration ranges. It was found that the ZnO has high UV absorption capability and low photocatalytic activity after tailoring by an annealing of 500 °C and exposure to pH of 11.6 and 9, respectively.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.