This article reports the controlled size of ZnO nanoparticles synthesized via simple aqueous chemical route without the involvement of any capping agent. The effect of different calcination temperatures on the size of the ZnO nanoparticles was investigated. X-ray diffraction (XRD) results indicated that all the samples have crystalline wurtzite phase, and peak broadening analysis was used to evaluate the average crystallite size and lattice strain using Scherrer's equation and Williamson–Hall (W–H) method. Morphology and elemental compositions were investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) spectroscopy. The average crystallite size of ZnO nanoparticles estimated from Scherrer's formula and W–H analysis was found to increase with the increase in calcination temperature. These results were in good agreement with AFM results. Optical properties were investigated using UV–vis spectroscopy in diffused reflectance (DR) mode, with a sharp increase in reflectivity at 375nm and the material has a strong reflective characteristic after 420nm at 500°C calcination temperature. Furthermore, photoluminescence spectroscopic results revealed intensive ultraviolet (UV) emission with reduced defect concentrations and a slight shifting in band gap energies with increased calcination temperature from 200°C to 500°C. This study suggests that the as-prepared ZnO nanoparticles with bandgap tunability might be utilized as window layer in optoelectronic devices.
Recently, many works have been directed to investigate the optical and electrical properties of ZnO nanomaterials, synthesized by different chemical and physical methods including vapor condensation, hydrothermal method, solution combustion method, sol–gel, etc. . Most of these techniques have not been extensively used on a large scale, but chemical synthesis techniques have been widely used due to their simplicity and low cost. Chemical route method is one of the best methods for synthesizing material with high purity, controlled nanostructures and surface properties . These synthesis techniques are attractive for several reasons: they are low cost, less hazardous, and thus capable of easy scaling up; growth occurs at a relatively low temperature, compatible with flexible organic substrates; there is no need for the use of metal catalysts, and thus it can be integrated with well-developed silicon technologies. In addition, there are a variety of parameters that can be tuned, to effectively control the morphologies and physical properties of the final products. Wet chemical methods have been demonstrated as a very powerful and versatile technique for growing ZnO nanoparticles.
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The change in thermoresponsive behavior from a single phase transition of upper critical solution temperature (UCST)-type of an acrylamide-acrylonitrile copolymer (AAm-co-AN) to a double responsive behavior (LCST-UCST-type (LCST, lower critical solution temperature)) in water by the introduction of a poly(ethylene glycol) (PEG) block is highlighted in the present work. The polymer is synthesized in a simple way by free-radical polymerization of acrylamide and acrylonitrile using a poly(ethylene glycol) (PEG) macro-azoinitiator. The dual thermoresponsive behavior was observed in a wide range of concentrations repeatable for many cycles with very small hysteresis depending upon the ratio of AAm, AN and PEG. Static light scattering (SLS) and dynamic light scattering (DLS) together with turbidity photometry and transmission electron microscopy confirmed a unique phase transition behavior due to the temperature dependent change in the morphology from micelles to agglomerates. The low cytotoxicity and two-in-one thermoresponsive behavior makes the polymer promising for biomedical applications in the future.
ZnO nanoparticles with average crystallite size of 28–44nm were successfully synthesized via aqueous chemical route. The effect of different calcination temperatures on the structural and optical properties was investigated. The XRD results indicated that all the samples have pure wurtzite ZnO phase with well crystallinity. It was observed from the calculations that with the increase in calcination temperature from 200°C to 500°C, the strain associated with the samples decreased with a gradual increase in crystallite size. The average crystallite size obtained from AFM analysis were in good agreement with the results obtained from Scherrer formula based on X-ray diffraction analysis. Red shifting in band gap energies from =3.23eV to 3.20eV with intensive ultraviolet (UV) emission was observed with increasing calcination temperature from 200°C to 500°C.
There are only a few non-ionic polymers known for showing thermoresponsivity of UCST-type. Copolymers of acrylamide (AAm) and acrylonitrile (AN) represent one of such thermoresponsive polymers. The present work shows pH-dependent UCST-transitions of this copolymer system. Herein, systematic studies were carried out to show hydrolytic stability and retention of UCST of the copolymer under various conditions. Regeneration of lost UCST-type transitions under extreme pH conditions could be achieved by changing pH, and by addition of electrolytes. Reversible addition fragmentation chain transfer (RAFT) was employed as a tool to synthesize copolymers of AAm and AN. Hydrolysis reactions were carried out intentionally under acidic and alkaline conditions, in order to analyze the chemical stability of the synthesized copolymers as well as to introduce carboxylic groups into the polymer structure. The obtained results showed high tolerance of poly(AAm-co-AN) samples under acidic conditions even after long periods of storage (25 days at pH 3) or after use of pH 0 and increased temperatures (40 °C). In the case of base catalyzed hydrolysis, the thermoresponsive behavior was significantly influenced during hydrolysis in buffer solution of pH 9. Loss and regeneration of the phase transition temperature of these copolymers could be achieved by changing the pH from basic to acidic. Meanwhile, hydrolysis at pH 14 at 40 °C influenced the thermoresponsive behavior and the chemical stability of the polymer, increasing the phase transition temperature over 30 °C. Further, we observed that additives, e.g. formamide can act as a sacrificial agent for providing stable UCST-type transitions even under alkaline conditions as well as at high temperatures (85 °C).