The Optical And Photocatalytic Property Of ZnO Nanomaterials

Zinc Oxide (ZnO) is one of the few dominant nanomaterials for nanotechnology. ZnOis a II-VI compound semiconductor with a direct wide band gap of3.37eV and a largeexciton binding energy of60meV at room temperature. ZnO is very stable under ambientconditions towards sunlight, water and air, which makes ZnO a promising multifunctionalmaterial applicable in optoelectronic, electrochemical and photochemical devices. ZnOprobably has the largest family of nanostructures exhibiting abundant nanostructure configu-rations. ZnO nanomaterials with engineered nanosize and particular morphology couldaccommodate desirable physical and chemical properties. This paper mainly involves in theoptical property and photocatalytic property. In most cases, the photoluminescence of ZnOhas two components. One is the typical exciton emission or near-band-edge emission, theother is visible emission, also called deep-level emission. We have studied the unique opticalproperties of ZnO, as its grain size decreases to very small nanosize. ZnO is proposed as asuitable candidate photocatalyst for TiO2due to its similar band gap, lower cost and betterenvironmental friendliness. Researchers have paid more attention to harvesting the solarlight and enhancing the photocatalytic performance. We have adopted doping, surfacemodification and constructing multiple heterojunctions to improve the photocatalytic activity.The main researches are as follows:1. We fabricated nanocrystalline ZnO films by pulsed laser deposition. The grain size ofthe film was modulated by controlling the laser energy. As the film’s grain size decreased tosmall nanosize (20nm), the film exhibited unique optical properties. The photoluminescencespectrum had three broad emissions located across ultraviolet-violet, yellow-green-orangeand red regions. The ultraviolet-visible transmittance spectrum of this film exhibited twodistinct characteristics: the film was half-translucent to UV light; there was no interferenceoscillation in the visible range. The characterization indicated that the nanocrystalline ZnOfilm with small nanosize had degenerated crystalline quality and various structured defects. These abnormal optical properties were attributed to the abundant structural defects and thequantum confinement effect of the nanocrystalline ZnO films.2. Supported Fe doped ZnO nanorod arrays with different doping concentrations weresynthesized by a facile wet chemical method. The crystal structures, surface morphologies,chemical composition, optical properties and photocatalytic properties were characterized.The results demonstrated that doping Fe into ZnO induced the lattice distortion, introducedmany defects and formed a new lowest unoccupied electronic state, acting as an inter-bandtrap site in the ZnO band, ultimately resulted in the extra absorption in visible light region.The photocatalytic study indicated all the supported Fe doped ZnO nanorod arrays exhibitedbetter photocatalytic activities. Among them, the supported1.0%Fe doped ZnO nanorodarrays showed the best photocatalytic activity. The reason should be that Fe doping intro-duced inter-band trap site, which captured the excited electrons, and then improved thecharge separation rate, but the excessive Fe ion would be the centre of electron-holerecombination.3. Amorphous TiO2modified ZnO nanorod films were synthesized via multi-step pro-cesses: ZnO nanorod films were prepared by a wet chemical method. Amorphous TiO2wasthen anchored on the tops and sides of the nanorods via a hydrolysis route. The absorptionspectrum of the TiO2modified sample exhibited an evident increase in visible light region.We attributed it to the surface amorphous state (disorder) of TiO2introducing an extensionof the bandgap tail. The photocatalytic tests revealed that TiO2modified films exihibitedenhanced photocatalytic efficiency under UV-visible excitation, which might be due to theincreased UV-vis light absorption and the separation of the charge carrier and prolongedelectron lifetime due to the interface between TiO2and ZnO.4. We reported a facile way to synthesize nanocrystalline ZnO particles with averagegrain size of12.5nm, which had excellent photocatalytic performance under UV-vis lightirradiation. It was comparable with that of P25TiO2, a commercial standard photocatalyst.Furthermore, the sample could be easily sedimentary from the reaction system, which wasfavorable for the separation and reuse. Meanwhile, the P25TiO2had a low sedimentary rate from its solution, resulting in the separation difficulty, meaning the inferior reusability. Weattributed the superior performance to the efficient synergistic effect resulted from the smallnano size, as well as the appropriate oxygen vacancies in the ZnO lattice.5. The nanostructured Bi-Bi2O2CO3-ZnO composites were successfully preparedthrough a facile one-step solvothermal method. The composite was constructed by ZnOnanorods with the diameter of30-40nm embedding in Bi2O2CO3(BOC) platelets with asmall quantity of Bi particles dispersed on the platelets, which further assembled into micronclusters. The sample had the similar UV absorption intensity to that of pure ZnO when thepercentage of Bi-BOC was low, indicated the composite with appropriate less Bi-BOC couldmaintain the considerable UV-vis absorption ability of ZnO. The photocatalytic performanceof the composite was showed enhanced activity in comparison with either ZnO, or BOC orthe two components, and the Bi-Bi2O2CO3-ZnO photocatalyst with20mol%of Bi-BOCexhibited the best photocatalytic efficiency. The reusability test explored the reliable stabilityand reusability for industrial applications, which was endowed by the special feature ofnanostructured micron clusters. The superior photocatalytic activity could be attributed to theheterojunction interfaces and compound band gap structure which facilitated the separationof the photo-generated electrons and holes at the interface and promoted effective chargetransfer path. Additional, the existing metal Bi also acted as the electron traps to manifest apositive effect. The present study might give an insight into the design of novel heterostruc-tured materials for the light harvesting related environment management and energyconversion applications.