Fabrication of Luminescent, Magnetic Hollow Core Nanospheres and Nanotubes of Cr-Doped ZnO by Inclusive Coprecipitation Method

Abstract

Incorporation of Cr into ZnO lattice as a dopant in hollow core nanostructures has been achieved by coprecipitation in highly alkaline medium near room temperature. Energy dispersive spectroscopy and X-ray diffraction studies confirm the presence of Cr in the ZnO lattice and X-ray photoelectron spectroscopy (XPS) identified the dopant state to be Cr 3+ . Room temperature ferromagnetism in ZnO:Cr 3+ along with structural investigations suggest an intrinsic nature of ferromagnetism in hollow core nanospheres. Unpaired spin in Cr 3+ in zinc substitutional sites could create spin ordering and long-range ferromagnetic coupling. Higher Cr 3+ concentration leads to the formation of hollow parallelepipeds and superparamagnetism. Photoluminescence studies reveal a sharp blue emission explicitly arising due to Cr doping. Such blue emitting magnetic nanocapsules can have immense potential as biocarriers and also in spintronics.

Tailoring magnetic and photoluminescence properties in ZnS/ZnO core/shell nanostructures through Cr doping

“…The maximum magnetic moment observed for core/shell NPs at room temperature and 80 K are 170 memu/g and 250 memu/g, respectively, which is considerable for powder nanoparticles of such dimensions. The ZnS:Cr/ZnO:Cr core shell nanoparticles exhibited a typical ferromagnetic narrow hysteresis loop with coercivity 23 Gauss and retentivity 2 memu/g at 80 K. The ferromagnetism induced in hollow core ZnO NPs by Cr doping is well established in our earlier work [31] and diamagnetic nature of ZnS is confirmed by VSM studies. In the present case, diamagnetic ZnS core/ZnO:Cr shell NPs give rise to paramagnetism.…”

“…1c) show in addition to ZnS peaks some hints of ZnO peaks which indicate the presence of both ZnS and ZnO phases in the nanostructure. It has been observed before [31] that crystallinity deteriorates with Cr doping in ZnO.…”

“…As undoped ZnS and ZnO exhibits diamagnetic behaviour and same concentration and amount of precursor materials has been used for synthesis of all Cr doped samples, trace iron as source of DMS in core/shell NPs can be immediately ruled out. In the similar synthesis process, dopant Cr was found as Cr 3+ ion in ZnO lattice in XPS analysis [31] and no precipitated ferromagnetic impurity CrO 2 phase has been noticed in the X-ray analysis. Based on the analysis of experimental results, it can be inferred that observed ferromagnetism is intrinsic in origin and due to ferromagnetic coupling of Cr 3+ ions which is in substitutional sites of Zn 2+ .…”

“…Since substitutional doping of trivalent Cr 3+ in divalent Zn 2+ position in ZnS/ZnO would require that two Cr 3+ ions are substituted for three Zn 2+ ions for charge compensation, overall charge neutrality in the lattice could be maintained by creating one Zn 2+ vacancy for incorporation of each two Cr 3+ ions. Zinc vacancies have very low formation energy and are more favourable in ZnO synthesized in oxygen rich fluidic environment for growth which is provided by the shell precursor solution with high pH [31]. Since the intensity of 485 nm emission increases with increase in Cr content in both core and shell region, it can be said that the origin of 485 nm blue emission is due to recombination of bound exciton between a shallow Cr 3+ donor level and a V Zn2+ acceptor centre since intensity of both species is expected to increase with increase in Cr content.…”

Dual mode luminescence in rare earth (Er3+/Ho3+) doped ZnO nanoparticles fabricated by inclusive co precipitation technique

“…Since, all the samples were prepared under identical synthesis conditions; these morphological changes could only be attributed to the strain induced in the lattice by substitutional doping of larger lanthanum ions. Dopant induced morphology change in ZnO particles have also been found with different dopants and synthesis conditions [27,28].…”