Synthesis Of ZnO Micro/Nanomaterials And Its Optical Properties

In this thesis, the ZnO microstructures with controllable morphology were successfully synthesized by hydrothermal process; the S doped ZnO nano-spheres are prepared by sol-gel method; additionally, the ZnO:LiAc monodispered nanoparticles with controllable size and tounable photoluminescence properties were obtained by a homogeneous precipitation method in nonaqueous solution under ultrasonic irradiation. The XRD,FT-IR,TG-DTA,Uv-Vis,XPS and HRTEM were used to characterize the structure and morphology of the as-obtained samples. Photoluminescence and Roman spectra were used to detect the structure and properties of the as-prepared samples. Besides, the mechanisms of the crystallization and photoluminescence were also discussed and got some reasonable explanation and meaningful results. The detailed contents are listed as following:The ZnO with different special microstructures has been successfully synthesized using simple hydrothermal method, and the structure and morphology of the as-obtained ZnO can be controlled by changing the template and tuning the reaction condition. The hexagonal ZnO Twins-microtube,twins-spindly ZnO micro-tube and hexagonal ZnO Twins-microplate with mesoporous structure were successfully synthesized using CTAB,CTAB & 2.5G PAMAM and CTAB & 4.5G PAMAM as templates in hydrothermal condition, respectively.Twins-spindly micro-tube of ZnO has been successfully synthesized by hydrothermal process with poly(amidoamine) (PAMAM) dendrimers as the template for the growth of ZnO. The special structure can originate from orientation arrays of Zn2+ ions on the surface of lamellar micelles of PAMAM by interaction of Zn²⁺ ions with N of PAMAM for electrostatic gravitation. TEM and FESEM images reveal the appearance of twins-spindly ZnO micro-tube with ⁴μm length, 1μm width and a large aperture 272nm. The Raman spectrum indicates the wurtzite phase of ZnO with high crystal quality. The products also show intensive UV emission from PL spectrum. Zinc oxide (ZnO) wurtzite with mesopore networks and unusually high specific surface areas has been prepared through a hydrothermal synthesis process using Zn(Ac)₂ and ammonia as starting materials, with CTAB serving as template and PAMAM 4.5G serving as structure-modifying agent. The specific surface area of the obtained ZnO is as high as 336.6 m²/g and then decreases gradually with increase of reaction time. The template effect of CTAB is an essential factor directing the formation of the present hexagonal twins-microplate structure and the effect of PAMAM is the key factor directing the formation of the high-surface-area wurtzite ZnO with interconnected mesopore networks. Since the material has a uniform distribution on an atomic level, the formation of wurtzite requires a low temperature and short reaction time. Furthermore, through photocatalytic investigation, the as-prepared ZnO microstructure displayed good capability to photodecompose phenol under UV irradiation due to their higher surface areas structure, compared with ZnO sample prepared by the conventional solid-state method. In this paper, the influence of reaction temperature and time was also studied using CTAB and 405G PAMAM as templates in the hydrothermal condition. And the hexagonal ZnO Twins-microflakes and hanmburge-like ZnO microstructure were obtained in the reaction.S-doped ZnO nano-spheres have been synthesized via a sol-combustion process. Transmission electron microscopy (TEM) image shows that the phosphors with mean size of 90nm are well dispersed with good dimensional uniformity. X-ray diffraction (XRD) patterns reveal its structure of ZnO. The composition of as-obtained samples was measured by energy dispersive X-ray analysis (EDX) and inductively coupled plasma (ICP)-atomic emission spectroscopy. Optical properties of the phosphors were investigated by photoluminescence (PL) spectrometer that a broad green emission (420-650nm) and a blue-ultraviolet (UV) excitation band (370-420nm) were observed. Co-doping with alkaline metallic ions enhanced the relative intensity of the PL excitation and emission. And the enhanced PL intensity can be attributed to the non-radiative energy transfer process of alkaline metallic ions as sensitizers. The S-doped ZnO microspheres with average diameter of 3 micrometers (μm) have been successfully synthesized by a simple air oxidation process of ZnS precursor. X-ray diffractometer (XRD) pattern indicates that the as-obtained sample is composed of ZnO and ZnS. The scanning electron microscopy (SEM) image shows that the exterior surfaces of the microspheres are composed of many nanoparticles with an average grain size of 100 nanometers (nm). The Photoluminescence (PL) spectra show the broad excitation region with the main peak at 370 nm and strong green emission centered at 500 nm, which can be attributed to the oxygen vacancies caused by S replacement of O.The ZnO: LiAc nanocrystals were synthesized by a homogeneous precipitation method in nonaqueous solution under ultrasonic irradiation. X-ray diffraction (XRD) patterns reveal the structure and component of ZnO: LiAc nanocrystals. The high resolution transmitting electron microscope (HRTEM) images show that the as-obtained ZnO nanoparticles become more dispersed and the size is decreasing with the increase of [Li]/[Zn] ratio, and the size of the as-prepared ZnO:LiAc nanoparticles by the ratio of Li/Zn 1.0, 1.5, 2.0 and 2.5 is 7nm, 5.5nm, 4.5nm and 3nm, respectively. The visible luminescent characteristics of samples have been investigated by luminescence spectroscopy. It is shown that the luminescent behavior of ZnO can be manipulated by different [Li]/[Zn] ratios. The excitation onset and the emission peak center blue-shifts from 350 to 320nm and from 600 to 450nm respectively with [Li]/[Zn] ratios increase from 1.0 to 2.5.The tunable luminescent characteristics are attributed to the magic-size effect and the increase of the density of oxygen vacancies caused by Li⁺ adsorbed on the surface of the magic-sized ZnO nanocrystals. The SiO₂ coated sample exposed to atmosphere under UV irradiation for 6 days has almost the same PL emission intensity as new sample, which reveals its optical stability. However, the PL relatively intensity of uncoated samples reduced from 400 to 150. Therefore, SiO₂ shell coating ZnO:LiAc nanoparticles improved the surface property of ZnO nanoparticles and enhanced the PL emission intensity and optical stability.