Magnetic Properties Of SnO ₂ Nanostructures And Their Applications

Tin oxide (SnO2) is an important n-type wide and direct bandgap semiconductor material with a wide band gap (Eg=3.6eV) and strong exciton binding energy (130meV). The raw materials of SnO2are abundant, inexpensive, non-toxic. SnO2nano structures are extremely diverse and show unique optical, electrical and magnetic properties. Because it is expected to achieve a stable ferromagnetism injection, and has excellent photoelectric properties, optical transparency, high carrier concentration, gas sensing properties, photocatalytic properties as well as chemical and thermal stability, SnO2has a unique application prospect in spintronic devices, photodetectors, transparent conductive films, solar cells, lithium batteries, gas sensors and photocatalysts. Based on some hot issues and our conditions, we focused on the magnetic, optical properties and applications of SnO2-based nanostructures:First, currently, SnO2-based diluted magnetic semiconductors doped with transition metals have poor reproducibility and stability for room-temperature ferromagnetism, and their magnetic origins have gradually been questioned. We studied optical and magnetic properties as well as magnetic origin of the Cobalt (Co)-doped SnO2-based diluted magnetic semiconductors.Second, based on the ultraviolet (UV)-absorption characteristics and forbidden direct transition of the SnO2quantum dots (SnO2QDs), we researched the applications in terms of UV-protection devices and photocatalysis under the UV irradiation, as well as the visible-light-activated photocatalytic performance of SnO2-graphene nanocomposites. Details are as follows: 1. Pure and Co-doped SnO2nanoparticles with various Co concentrations of1%,3%,5%,7%and10%have been synthesized by the coprecipitation technique. X-ray diffraction and field emission scanning electron microscope equipped with energy dispersive spectrometer were performed to analyze the crystal structure and check the possible presence of any impurity elements in the nanocrystals. Optical absorption and photoluminescence spectra were used to confirm that the Co is substituted at the Sn site in SnOh matrix and does not form metallic clusters and other oxide phases for Sn1-xCoxO2(x=0.00-0.10). Meanwhile, it was revealed that the energy band gap decreases and emission peak blue-shifts with the increasing Co-doping. Magnetization measurement shows that the low Co-doping (x<0.03) develops the ferromagnetism and paramagnetism, but they are weakened by the higher Co-doping due to the formation of antiferromagnetic superexchange coupling among neighboring Co ions. Such microscopic changes in the magnetic coupling components induced by Co-doping concentration can not only well support the bound magnetic polarons (BMP) model, but also reveal the magnetic origin of which is closely related to the doping concentration of magnetic ions.2. Small-sized SnO2QDs were prepared via a hydrothermal process and and their basic optical properties, such as absorption and photoluminescence emission characteristics, were characterized by the UV-visible absorption spectra and photoluminescence spectra (PL), respectively. SnO2QDs have considerable absorption to the UV light with wavelength below400nm. However, due to the large surface energy, freshly prepared QDs (including SnO2QDs) can easily aggregate into larger nanoparticles etc., thereby losing many of the advantages of the QDs. Therefore, avoiding their aggregation at the same time of utilizing the UV-absorption characteristics of SnO2QDs is very meaningful for the expansion of their range of applications, such as aspects of UV-protection devices and photocatalysis. The ultraviolet (UV)-resistant performance of the polymer-semiconductor hybrid nanocomposite materials has been recently demonstrated. However, very little is known regarding the tin oxide (SnO2)-based UV-proof materials. Here we report SnO2-based functionalized glass coated by the SnO2-poly (methyl methacrylate)(SnO2@PMMA) nanocomposite, which exhibits prominent UV-absorbing capability, high optical transparency in the visible-wavelength region and enhanced hydrophobicity, at the same time of avoiding the aggragation of the QDs. After absorbing the UV radiation, SnO2QDs could emit polychromatic visible fluorescence to release energy by the optical down-conversion process. This study examines the influence of the number of coating layers on the optical transparency, UV-absorbing capacity and enhanced hydrophobicity. This SnO2@PMMA modified glass shows their potential applications in the UV-shielding devices.3. The ultrafine SnO2quantum dots (QDs) modified with poly (ethylene glycol methyl ether)(PEGME)(PEGME-SnO2QDs) were synthesized via hydrothermal method. X-ray diffraction and high-resolution transmission electron microscopy were employed to illustrate that the PEGME-SnO2QDs are uniform, monodispersed and about4nm in diameter. Then infrared spectrum and thermogravimetric analysis were used to prove that PEGME groups are bound tightly to SnO2surfaces. The as-synthesized PEGME-SnO2QDs excellently achieved photocatalytic degradation to Rhodamine B dye (RhB), at the same time of avoiding the aggragation of the QDs. The photon efficiency of the PEGME-SnO2QDs catalyst and corresponding RhB dye degradation rate constant could reach0.0058%and9.98×10-2min-1, respectively. This outstanding photocatalytic performance could be attributed to not only large surface-to-volume ratio and high crystallinity of the ultrafine and monodispersed QDs, but also good hydrophilicity and conductivity of the PEGME surface modifier. Remarkably, such PEGME-SnO2QDs with outstanding photocatalytic efficiency and stable recyclability are promising to be applied to environmental purification.4. The visible-light-driven photocatalytic activities of graphene-semiconductor catalysts have recently been demonstrated, however, the transfer pathway of photogenerated carriers especially where the role of graphene still remains controversial. Here we report graphene-SnO2aerosol nanocomposites that exhibit more superior dye adsorption capacity and photocatalytic efficiency compared with pure SnO2quantum dots, P25TiO2and pure graphene aerosol under the visible light, at the same time of avoiding the aggragation of the QDs. This study examines the origin of the visible-light-driven photocatalysis, which for the first time links to the synergistic effect of the co-photosensitization of the dye and graphene to SnO2. We hope this concept and corresponding mechanism of co-photosensitization could provide an original understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to design novel and versatile graphene-based composites for solar energy conversion.