Impact of annealing on morphology and ferromagnetism of ZnO nanorods

Abstract

Room temperature ferromagnetism has been observed in ZnO nanorods prepared by hydrothermal method. Saturation magnetization of ϳ0.004 emu/ g was measured in the nanorods with diameters of ϳ10 nm and lengths of below 100 nm, and the magnetization reduced with the increase of the size. Annealing of the samples at 900°C in air completely transformed the nanorods into twinning structures and weakened the magnetizations. The mechanism of morphology transformation was discussed. Analysis indicates that the interstitial zinc at the surface may contribute to the ferromagnetism in ZnO nanorods.As ideal candidate materials for spintronic devices, ZnObased diluted magnetic semiconductors ͑DMSs͒ have attracted much attention in recent years. In particular, onedimensional ͑1D͒ ZnO nanostructures which can serve as building blocks for nanoscale devices have been intensively studied. 1-9 Room temperature ͑RT͒ ferromagnetism ͑FM͒ has been achieved in several transition metal ͑TM͒ doped ZnO nanorods, 2-4 nanowires, 6-8 and nanoneedles. 9 On the other hand, RT FM has also been observed in pure ZnO nanoparticles and thin films, 10-12 which shows that FM may be an intrinsic characteristic of ZnO with low dimensionality. Recently, Banerjee et al. 11 reported the enhancement of RT FM in ZnO particles by 900°C annealing in air and related the phenomenon to oxygen vacancy clusters. However, the study on the RT FM in ZnO thin films confirmed that the source of the FM cannot be oxygen vacancies but defects on Zn sites. 12 So far, the understanding of FM origination in ZnO is still insufficient, partly because the reports of RT FM in pure ZnO are rare. Moreover, while annealing treatments were processed in several researches, 6,11 the impact of annealing on the morphology change was seldom considered. In this letter, we report on the RT FM in ZnO nanorods. Annealing of the samples at 900°C in air for 2 h can completely transform the nanorods into twinning structures, while adjusting the intrinsic defects and reducing the saturation magnetization ͑M s ͒.ZnO nanorods were synthesized by hydrothermal method. Zn͑Ac͒ 2 ·2H 2 O and NaOH with a mole ratio of 1:10 were added to absolute ethanol and pretreated in an ultrasonic water bath for 30 min, then hydrothermally treated at 120°C for 5 h. The products were collected by centrifugation and thoroughly washed with distilled water and ethanol, and finally dried at 70°C for 10 h. Two kinds of samples were prepared with different Zn͑Ac͒ 2 ·2H 2 O concentrations of 0.0125M and 0.05M, and denoted as S1 and S2, respectively. The as-prepared samples were annealed at 900°C in air for 2 h. The structure and morphology of the samples were characterized by x-ray diffraction ͑XRD͒, scanning electron microscopy ͑SEM͒, and transmission EM ͑TEM͒.Magnetic properties were measured using an alternative gradient magnetometer. Photoluminescence ͑PL͒ studies were carried out at RT with the excited wavelength of 325 nm. Figure 1 shows the XRD patterns of the as-prepared and annealed samples. All diffraction peaks corr...

Emergent ferromagnetism in ZnO/Al2O3 core-shell nanowires: Towards oxide spinterfaces

“…Because the sample contains multiple compositions, we cannot accurately determine the magnetism in terms of formula unit. Nevertheless, the observed value of saturation magnetization in A-AZO is comparable to what reported for nanograined ZnO films (0.05 emu/g), 12 but much higher than those of undoped ZnO nanoparticles obtained via chemical routes (between 0.0015 and 0.0022 emu/g) 40,41 and ZnO nanorods (0.004 emu/g), 42 as well as the thiol-capped ZnO nanoparticles (0.003 emu/g). 20 To find out the origin of magnetism in the annealed ZnO/Al 2 O 3 core-shell NWs, we measured a control sample of ZnAl 2 O 4 nanocrystals, which were fabricated using the conventional solid stat reaction route from high-purity ZnO and Al 2 O 3 powders, and found that ZnAl 2 O 4 nanocrystals exhibits only a diamagnetic behavior [ Fig.…”

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Hibiscus rosa-sinensis Leaf Extracted Green Methods, Magneto-Optical and Catalytic Properties of Spinel CuFe2O4 Nano- and Microstructures

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