Browsing by Subject "ZnO"

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Open Access
Magnetic nanocomposite hydrogel prepared by ZnO-initiated photopolymerization for La (III) adsorption.
(ACS applied materials & interfaces, 2014-11) Zheng, Xiangning; Wu, Dongbei; Su, Teng; Bao, Song; Liao, Chuanan; Wang, Qigang
Here, we provide an effective method to fabricate magnetic ZnO clay nanocomposite hydrogel via the photopolymerization. The inorganic components endow the hydrogel with high mechanical strength, while the organic copolymers exhibit good adsorption capacity and separation selectivity to La (III) ions. An optimized hydrogel has the maximum compressive stress of 316.60±15.83 kPa, which still exhibits 138.98±7.32 kPa compressive strength after swelling. The maximum adsorption capacity of La ion is 58.8 mg/g. The adsorption matches the pseudo-second-order kinetics model. La (III) ions can be effectively separated from the mixtures of La/Ni, La/Co, La/Cu, and La/Nd in a broad pH range (2.0 to 8.0). After six adsorption-desorption cycles, the hydrogel can maintain its adsorption capacity. This work not only provides a new approach to the synthesis of tough hydrogels under irradiation, but also opens up enormous opportunities to make full use of magnetic nanocomposite hydrogels in environmental fields.
Open Access
Photoexcitation Mechanisms of the Green Defect Emission from Zinc and Sulfur Doped ZnO Phosphor Powders Through Measurement and Analysis of Optical Properties and Characterization
(2013) Simmons, Jay Gould
The mechanism for defect related green emission from zinc (ZnO:Zn) and sulfur doped ZnO (ZnO:S) are determined through optical characterization of the green and UV emission bands. ZnO:Zn is prepared by heating ZnO in a slightly reducing atmosphere for 1 hour and sulfur doped ZnO is similarly obtained with a small amount of sulfur added. Photoluminescence (PL), photoluminescence excitation spectra (PLE), and quantum efficiency measurements are analyzed to determine the mechanism of the green defect emission. Low temperature PL and PLE measurements are used to assign activation energies to the emission processes and connect them with donor bound excitons in ZnO. It was determined that both ZnO:Zn and ZnO:S have a similar green emission mechanism. This common mechanism involves the formation of donor bound excitons I_3a and I₉, which were determined to be the mediators between photoexcitation of excitons and the transfer of energy to the defect responsible for green emission. The most efficient excitation processes for both the green and band edge emissions at low temperatures is through direct excitation of the neutral donor bound exciton I₉ or by ionizing the neutral donor bound exciton I_3a. The ionization of I_3a eliminates this exciton localization site and simultaneously creates a bound exciton at I₉. The I₉ bound exciton can then either transfer energy to the defect responsible for the green emission or contribute to the free exciton population through a phonon assisted transition. At room temperature a resonant absorption peak associated with I₉ is still present in the absorption band for ZnO:Zn suggesting partial localization at I_3a and I₉ of free excitons with low kinetic energy (excitations below the band gap) continues to be the intermediate between excitons and the energy transfer to the green emitting defect.
In ZnO:S, the addition of sulfur creates ZnS domains within the lattice leading to a type II band alignment at the interface of ZnO and ZnS domains. This band alignment at the interface increases the density of free electrons in ZnO, which may then encounter an ionized I_3a site converting it to its neutral form. Increasing the density of free electrons, a result of the type II band alignment, increases the chances of returning an ionized I_3a to its neutral form and thus increases the green emission. These results can lead to informed optimization of ZnO:S as a potential white light emitting phosphor.