Pseudo-one-dimensional Zn-Fe-O Nanostructure Arrays: Controlled Fabrication, Magnetic Properties and Photocatalytic Applications.

摘要

In the present thesis, several kinds of pseudo-one-dimensional Zn-Fe-O nanostructure arrays with tunable chemical compositions, crystal structures and morphologies are successfully synthesized via a simple wet-chemical ZnO-nanowire-array templating method.;Vertically-aligned ZnO nanowire arrays are firstly fabricated on several different substrates and then serve as templates for other nanostructured arrays growth. The ZnO nanowires not only act as morphology-defining skeleton but also contribute chemically to the final composition of the nanostructures. By controlling the reaction time between ZnO and FeCl3 solution, ZnO/ZnFe2O4 nanocable arrays, stoichiometric ZnFe 2O4 nanotube arrays, nonstoichiometric ZnFe2O 4 nanotube arrays, ZnFe2O4/alpha-Fe2O 3 nanotube arrays and alpha-Fe2O3 nanotube arrays can be synthesized in a controlled manner after calcination. Both ZnFe 2O4 and alpha-Fe2O3 nanotube arrays exhibit visible light absorption and their bandgap are estimated to be ∼2.3 eV and ∼1.7 eV, respectively.;The detailed structural information of the ZnFe2O4 nanotube arrays are obtained by electron energy loss spectroscopy (EELS). In particular, EELS are carried out for two different series (i.e., temperature and stoichiometric series). The magnetic properties of these samples are found to closely correlate to their structural characteristics. Firstly, with the decrease of the calcination temperature from 600 °C to 400 °C, more Fe3+ ions occupy A sites (tetrahedral sites in spinel structure) rather than their equilibrium B sites (octahedral sites in spinel structure). The deviation from the normal spinel structure leads to the enhancement of superexchange interactions between Fe3+ ions in A and B sites, and thus results in an increase in blocking temperature (T B), magnetic anisotropic constant (K), saturation magnetization (MS, at 3 K and 300 K), coercivity (H C, at 3 K) and a decrease in MS (3K)/MS(300 K) ratios. Secondly, by comparing stoichiometric and nonstoichiometric ZnFe2O4 nanotubes calcinated at the same temperature, we found that the nonstoichiometric nanotubes (Fe:Zn > 2) shows similar ratios of Fe3+ in A and B sites to that of the stoichiometric one. The extra Fe3+ in the crystal also enhances the superexchange interactions of Fe3+, which results in larger T B, K, MS (at 3 K and 300 K) and HC (at 3 K), and smaller MS(3 K)/MS(300 K) ratio. Lastly, alpha-Fe2O 3 nanotubes, as an extreme case of the nonstoichiometric sample, show typical Morin-transition characterization under small external field, and field-induced spin-flop transition at large external field.;On the other hand, we found that the visible-light-driven photodegradation activities of ZnO/ZnFe2O4 nanocable arrays are superior to those of the ZnO nanowire arrays and ZnFe2O4 nanotube arrays using RhB as the probe molecules. All the three nanostructures show degradation of RhB molecules under visible light irradiation, but they take different degradation pathways. The degradation of RhB in the presence of ZnO nanowire arrays is attributed to the dye-sensitized mechanism, and the photodegradation activity is the worst. ZnO/ZnFe2O4 nanocable arrays and ZnFe2O4 nanotube arrays have the same degradation mechanism, that is, reactive radicals produced by photogenerated electron-hole pairs in the visible-light-active ZnFe2O4 are responsible for the photodegradation of RhB. However, the nanocable arrays show much higher degradation capability. This is owing to the type II band alignment between ZnO and ZnFe2O4, which greatly promotes the separation of photogenerated electrons and holes in ZnFe2O 4.