| Περίληψη: | The ever-increasing energy crisis and the environmental problems arising from it, have a huge impact on political, economic and social life, resulting in creating a global trend towards sustainable development. Access to clean alternative energy sources, such as solar energy, is expected to make a huge contribution to resolving the energy crisis issue. However, apart from the direct electricity production through the utilization of solar energy, finding an efficient way of storing and transporting it is a great challenge. The formation of molecular hydrogen and oxygen through water splitting is a direct way to store solar energy as fuel and surpasses other methods for the production of H2, such as steam reforming of natural gas, as it is more environmentally friendly.
Over the past 40 years, efforts have been made for the development of materials, mainly of the nanoscale order, which could be used in a photoelectrochemical cell for water splitting. However, despite the progress made, so far there has been no suitable material that meets all the requirements for this application, such as the high rate of water splitting, the utilization of a greater part of the solar radiation outside the ultraviolet region, and of course the long duration of life. Thus, the development of suitable nanomaterials, of low cost, environmentally-friendly and of high stability in different pH solutions, which can be an efficient water breakdown system, still remains a challenge.
The subject of the present Doctoral Thesis is the development and the characterization of nanostructures of controlled morphology of the Zinc oxide (ZnO) and of its heterostructures with other semiconductors, as well as its evaluation as an active material for its application as a photo-anode in photo-electrochemical cells for water splitting.
Initially, the role of morphology was studied by evaluating films produced by nanoparticles (of relatively spherical shape) and films with ordered nanorods (one-dimensional structures). For this purpose, the films of commercially available ZnO nanoparticles were prepared by following the doctor blade technique while the ordered ZnO nanorods were prepared by solution chemistry. Three different thicknesses of films (1.5, 3.0 and 7.0 μm) were prepared comparable to each category of nanomaterials, which were characterized morphologically, structurally as well as in terms of their optical properties. The photo-electrochemical study showed that the one-dimensional structures with a film thickness of 1.5 μm exhibited the best electrochemical behavior with respect to the maximum value of the current density value which is comparable to the maximum value of 3 μm film thickness nanoparticles. The above observation combined with that of the current-voltage curves of the two morphologies led us to the conclusion that the carrier recombination rate in the case of nanoparticles is greater than the one of the nanorods, whereas the impedance experiments performed for the two morphologies indicated that the number of carriers in the case of nanorods is quite greater than that of nanoparticles.
Due to the superiority of the nanorods against the particles in the ability of water splitting, the following stages focused on the optimization of the morphological characteristics of arrangements consisting of nanorods. A method was developed to achieve repeatedly the synthesis of nanorods with a thickness in the range of 1-1.5 μm and an average rod diameter of five different values in the range of 40 to 270 nm in a substrate (glass/FTO) dimension of some square centimeters. These electrodes were characterized morphologically and optically. With a detailed image analysis of the scanning electron microscopy (SEM), the specific surface area of the electrodes for the different values of the nanorods diameters was calculated. The photo-electrochemical properties of the electrodes were systematically studied. It has emerged that the current density is directly dependent on the specific surface area of the nanostructures constituting the electrodes, while the maximum current density corresponds to rods with an average diameter in the range of 70-120 nm. The calculation of the performances for the different diameters confirmed the existence of an optimum diameter. The optimum efficiency performance was calculated to be about 6% for a potential imposing of 0.9 V. The current intensity during the imposing of the potential proved not to correspond solely to oxygen production as the selectivity was found to be ~75% at 0.9 V. Therefore, part of the current is consumed for the production of hydrogen peroxide. Lastly, from the study by means of cyclic voltammetry two reductive peaks at 0.08 V and –0.12 V, which were identified and assigned to adsorbed species of OHad which either form bonds to Zn ions or cover oxygen vacancies, were observed.
Next, subject of the research was the understanding of the effect of doping of ZnO with Aluminum (Al) ions in three different concentrations, 0.5%, 1% and 2%. This process causes changes in the optical properties and the conductivity of the semiconductor. It was observed that the doping brings about systematic changes in the morphology of the nanorods (reduction of the average diameter and length of the nanorods), the structure (increase in the percentage of defects) as well as their optical and electrical properties. From the photo-electrochemical characterization it has emerged that the maximum current density value was shown by the electrodes with 0.5% doping with Al ions. Therefore, the number of carriers above a limit in combination with the increased number of defects in the crystal lattice did not assist in improving the performance of the arrangement as they increased the rate of recombination.
Finally, in the effort of the optimization of the performance of the photo-electrochemical arrangements, complex core/sheath ZnO/ZnSe heterostructures were prepared. The aim was to synthesize heterostructures capable of absorbing in the visible region of the solar spectrum. The ZnSe sheath was produced by exposing the ZnO nanorods to Se vapors at a controlled temperature. Exposure time was the parameter that determined the thickness of the ZnSe sheath around the ZnO nanorods. The electrodes were characterized with a plethora of morphological and optical techniques to study the resulting heterostructures. The electrodes with increased absorption in the visible were those that displayed the best photo-electrochemical behavior as well during their irradiation with an arrangement that resembles the spectrum of the solar spectrum. It was observed that the electrodes were not particularly stable in the selected electrolyte (Na2S/Na2SO3 aqueous solution) and there was a partial dissolution of ZnSe.
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