| Zinc Oxide is a kind of direct wide band-gap semiconductor materials of Ⅱ-Ⅵ. Its band-gap width is 3.37 eV at room temperature. In recent years, doping ZnO research get the favor of people because of the easy doping for the transition metal ions to be mixed with the lattice, good thermal stability, strong ability to resist radiation, low of epitaxial growth temperature, non-toxic pollution-free, rich source, lowness of the production cost.ZnO has attracted a great amount of attention in various fields, including diluted magnetic semiconductors (DMSs). DMSs is a kind of multifunctional and novel semiconductors, which is formed by using transition metal ions to randomly replace the cations in semiconductors. Recently, the investigations of obtaining room temperature ferromagnetic semiconductor materials by adding metal impurities into ZnO and improve their ferromagnetic properties became intense. Moreover, the origin of ferromagnetism in doping ZnO materials is still in dispute. Accordingly, the investigations of ZnO based DMSs through experimental and first principle method were conducted in this article.We have prepared Zn1-xCoxO (x=0%,1%,2%,3% and 4%) microspheres via a two-step sol-gel method without grinding and investigated their composition, morphology and magnetic properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM), respectively. The result demonstrates that Co has been successfully doped into hexagonal ZnO and Zno 9gCo0.02O reveal an average particle size around 300 nm, which is similar to that of as-prepared ZnO nanoparticles. Room temperature ferromagnetism and diamagnetism were obtained in Co doped ZnO at the same time. The origin of ferromagnetism was from Co-O-Zn exchange interaction and that of the diamagnetism was from Co-O-Co exchange interaction. The influence of Fe doping on the Co doped ZnO was also investigated. It reveals that the magnetism decreases with the Fe codoping.We also synthesized a series of Zn1-xFexO (x=0%,1%,2%,3% and 4%) samples via the same sol-gel method as mentioned above. Influence of Fe doping concentration on the structure, morphology, optical properties and photo catalysis properties were investigated. The results showed that all the samples were hexagonal wurtzite structures and their crystalline sizes and particle diameters decreased with the increase of Fe doping concentration. An increase of visible light absorption value and a decrease of band gap from 3.22 to 3.14 eV were found with the increase of Fe doping concentration, which enable the sample harvest more photons to excite the electron from the valence. Enhanced visible light photo catalytic activity has been found in 1% Fe doped ZnO. Also, the Zn0.99Fe0.01O samples display the best magnetic properties and the best calcining temperature is 300℃.Ni-doped ZnO nanoparticles via hydrothermal method were synthesized and were heated in air and in N2, respectively. Magnetic measurement displayed that an increase of magnetism was found in the Ni-doped ZnO samples which heated in N2. The result indicate that one of the magnetic origins is the RKKY interaction mediated by O vacancy.As reported before, the increase of magnetic properties were found in high concentration doping system. So we tried to prepare high concentration of Fe and Co doped ZnO samples via sol-gel method and homogeneous method, respectively. The XRD results show that a second phase of spinel appeared when the doping concentration reached 5%. Magnetic measurement indicated that the coercivity of Zn.90Fe0.08Co0.02O was 762 Oe with strikingly magnetic saturation and the origin of RTFM here was from the secondary spinel phase, which limited the application of ZnO based DMSs. Nevertheless, the photo catalytic activities of Zn0.90Fe0.08Co0.02O were obviously improved. Thus the Zn0.90Fe0.08Co0.02O nanoparticles is a potential product in photo catalytic field not only due to its excellent visible light photo catalytic activities but also its room temperature ferromagnetism which make it possible to be recycled by magnetic reclaim.In order to explain the origin of the magnetism, first-principles density functional calculations were used. We constructed supercell crystal model (2x2x2) of the pure ZnO, Fe and Co-doped ZnO and investigated their band structure, state density and electron structure. The results showed that impurity energy levels were found in Co and Fe doped ZnO, leading to a decrease of the band gap. The charge distribution of Zn and O around Fe or Co is changed, indicating that the magnetic moment of Fe and Co doped ZnO is not only from the Fe and Co atoms but also from Zn and O atoms. The calculation results of the total magnetic moment revealed that the doped ZnO was ferrimagnetism. Moreover, the total magnetic moment of Fe-doped ZnO was larger than that of Co-doped ZnO, which is in contrast of the experiment results. It is because that the charge imbalance appears in the as prepared Fe-doped ZnO samples, which leads to Zn vacancy, resulting in decrease of magnetism.In conclusion, we have the advantages of both preparation method and properties. The two step sol-gel method is simple than that of the traditional one and the Co and Fe doped ZnO samples via sol-gel method displays room temperature magnetism. In addition, the Ni-doped ZnO nanoparticles via hydrothermal method reveals a novel single flake structure and the Zn0.90Fe0.08Co0.02O nanoparticles synthesized by homogeneous method has its advantages both in visible light photo catalytic activities and room temperature ferromagnetism which make it a potential product in photo catalytic field. |
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