High Exciton Binding Energy Biology Essay

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Zinc oxide (ZnO) is a wide band gap (3.3 eV) n-type semiconductor with a high exciton binding energy (60 meV) at room temperature, which is of interest for a variety of practical applications including transparent conductive coatings, dye-sensitized solar cells, gas sensors, and electro-photo-luminescent materials [1]. ZnO nanoparticles can be prepared by various chemical methods have been adapted to produce nanocrystalline ZnO of different shapes and sizes. This includes sol-gel methods [2], wet chemical synthesis [3], thermal decomposition [4], mechanochemical [5], metal-organic chemical vapor deposition [6] and so on. Recent method invested is the coordination oxidation homogeneous co-precipitation method [7]. In the synthesis of ZnO/chitosan nanocomposites, sol-gel method is used because of its low cost, reliability, reproducibility, simplicity and relatively mild conditions of synthesis "soft chemistry" [8].

Chitin, the source material for chitosan, is a high molecular weight linear polymer of 2-acetamido-2-deoxy-D-glucopyranose units linked together by 1, 4-glycosidic bonds. It is obtained in large quantities from crustacean shells, which are waste products of seafood processing industries. Chitosan is the N-deacetylated product of chitin [9]. Chitosan is becoming increasingly important natural polymers because of their unique combination of properties like biocompatibility, biodegradability, nontoxicity, and adsorption properties. [10]. Chitosan is biopolymers having immense structural possibilities for chemical and mechanical modifications to generate novel properties, functions and applications especially in the biomedical area [11]. Metal chelating agents for removal of metallic impurities in wastewaters is an excellent application for large-scale use of chitosan [9].

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The structure of chitosan is shown as below [12]:

In order to synthesis of high-quality ZnO/Chitosan nanocomposites using this technique, it is necessary to have an optimum control of preparation conditions such as ZnO/chitosan ratio, sintering temperature and sintering time. The sintering step is an important and effective section to improve the crystalline quality.

Literature review

L. Guo et al. [13] studied on Synthesis of Poly (vinylpyrrolidone)-modified zinc oxide nanoparticles. They investigated effect of surface modification on the size, structure, morphology, and optical properties of ZnO nanoparticles. It was found that many properties of the ZnO nanoparticles could be optimized by simply varying the molar ratio Zn(II)/PVP. The ZnO nanoparticles prepared under the optimum conditions are particularly stable, monodisperse, as well as small in size.

W. Q. Peng et al. [14] studied about structure and visible luminescence of sol-gel derived ZnO nanoparticles. They investigated effect of annealing treatment on the structure and photo luminescence (PL). XRD analysis demonstrates that the nanoparticles have the hexagonal wurtize structure and the particle size is increased after sintering treatment. Due to quantum confinement effects, the absorption peak of the sintered nanoparticles is blue-shifted compared with bulk material. A yellow emission was observed in the as-prepared sample and a green emission in the sintered sample. The change of the visible emission is related to oxygen defects. Sintering in the absence of oxygen increased oxygen vacancies.

M. Chakrabarti and co-workers [15] studied on room temperature optical and magnetic properties of polyvinylpyrrolidone capped ZnO nanoparticles. They suggested presence of large number of oxygen vacancies in the sintered PVP capped samples from its PL spectrum analysis.

Y. Caglar et al. [1] studied about influence of heat treatment on the nanocrystalline structure of ZnO film deposited on p-Si. They investigated the influence of heat treatment on the crystal structure, grain growth kinetics, orientation, and refractive index properties of the ZnO film deposited onto p-type single-crystalline Si substrate by sol-gel method. The important changes in crystalline structure of the ZnO films were observed due to the heat treatment. The structural quality of ZnO films was improved by heat treatment process at 750°C.

Objective

To synthesis of ZnO nanoparticles by sol-gel method.

To synthesis of ZnO nanoparticles by using chitosan as a capping agent through sol-gel method.

To comparison the size particles of obtained ZnO nanoparticles with and without using of chitosan as a capping agent.

To obtain the optimum condition of sol-gel method such as sintering temperature and sintering time on synthesis of ZnO/chitosan nanocomposites.

To characterization of ZnO/chitosan nanocomposites.

Methodology

Synthesis of sol-gel derived ZnO nanoparticles

Zinc acetate dihydrate, Zn(CH3COO)2·2H2O, (0.5 mmol) has been dissolved in 80mL of 2-propanol respectively under vigorous stirring at room temperature. These solutions have then kept in an ultrasonic bath for 15 min for complete dissolveness. Then 920 mL of double distilled water has been added to each solution to make a dilute solution of 1 L. Throughout the above process the temperature has been maintained at 0 °C. The resulting solutions are then divided into 3 parts of 200mL for each solution.

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Three of the solutions have been subjected to hydrolyze separately in an ultrasonic bath by adding 50 mL of 2Ã-10−2 mol/L NaOH solution in 2-propanol drop by drop at room temperature for 30 min. The solutions have then kept in an oven at about 60 °C until a yellow gel form of the solution appeared. This gel is then washed with double distilled water five times to remove the solvent. The first set of isolated sol-gel product has been kept in a furnace at 500°C for different periods of time such as 1, 2 and 3 h. For the second and third sets were sintered at the same of periods of time to first but at 600 and 700°C, respectively.

Synthesis of sol-gel derived ZnO nanoparticles using chitosan as a capping agent

Three sets of Zinc acetate dihydrate, Zn(CH3COO)2·2H2O, (5 x 10-2mol) has been dissolved in 80mL of 2-propanol respectively under vigorous stirring at room temperature. These solutions have then kept in an ultrasonic bath for 15 min for complete dissolveness. Then 920 mL of double distilled water has been added to each solution to make a dilute solution of 1 L. Throughout the above process the temperature has been maintained at 0 °C. The resulting solutions are then divided into five parts for each solution making a total of 15 solutions of 200mL.

Chitosan (with weight 0.0439g, 0.0878g, 0.1316g and 0.2194g) was dissolved in 50mL of 1% acetic acid and stirred for 1hour for complete dissolveness.

Under vigorous stirring, Chitosan solution has been added to each set of solutions with the weight ratios of Zn2+: chitosan as 5:1, 5:2, 5:3, and 5:5 respectively at room temperature.

All the mixtures have been subjected to hydrolyze separately in an ultrasonic bath by adding 50 mL of 2Ã-10−2 mol/L NaOH solution in 2-propanol drop by drop at room temperature for 30 min. The solutions have then kept in an oven at about 60 °C until a yellow gel form of the solution appeared. This gel is then washed with double distilled water five times to remove the solvent. The first set of isolated sol-gel product has been kept in a furnace at 500°C for different periods of time such as 1, 2 and 3 h. For the second and third sets were sintered at the same of periods of time to first but at 600 and 700°C, respectively.

The total sample collected for characterization labeled as table below:

Temperature

500 °C

600 °C

700 °C

Sintering time

Ratio of

ZnO:chitosan

1h

2h

3h

1h

2h

3h

1h

2h

3h

5:0 (without chitoasn)

A1

A2

A3

B1

B2

B3

C1

C2

C3

5:1

A4

A5

A6

B4

B5

B6

C4

C5

C6

5:2

A7

A8

A9

B7

B8

B9

C7

C8

C9

5:3

A10

A11

A12

B10

B11

B12

C10

C11

C12

5:4

A13

A14

A15

B13

B14

B15

C13

C14

C15

5:5

A16

A17

A18

B16

B17

B18

C16

C17

C18

The size and shape of the ZnO nanoparticle has been monitored by using Transmission electron microscope. For TEM measurements, a few droplets of powder, ultrasonically dispersed in alcohol, have been put on standard carbon coated copper grids. These grids then have been kept in vacuum for 24 h for the excess solvent evaporation. The powder X-ray diffraction (XRD) data has been collected. The average particle size of the sample has also been calculated from Scherer formula:

where, τ is the average particle size; β is the full width at half maximum (FWHM); K is a constant (=0.89); λ is the wave length and θ is the Bragg angle.

The photoluminescence spectra have been recorded in UV-Vis spectrophotometer in the wavelength range 300 nm to 800 nm with exciton wavelength 280 nm.

Expected Result:

Improve knowledge on sol-gel derived nanocomposites.

Good formulation of the nano inorganic-organic particles can be achieved.

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The particle size is smaller in ZnO/chitosan nanocomposite than in ZnO itself.

The growth of the ZnO crystalline structure with the increasing in the sintering temperature and period of time in sintering.

Reach to optimum sintering temperature and period of time in sintering step through the sol-gel synthesis of these nanocomposites.