Synthesis and Characterization of Some Metal Oxides Nanostructure Thin Films for Gas Sensor Application

Abstract

XV Abstract In this work, metal oxide (SnO2 and ZnO) nanostructures thin films were deposited on the glass substrate by atmospheric pressure chemical vapor deposition (APCVD) and hydrothermal drop casting techniques. The main materials, which are used in this process, are tin chloride, zinc acetate, zinc nitrate and hexamethylenetetramine (HMTA). The first study involved the use of the APCVD system to obtain the optimized thin films by taking appropriate deposition conditions, such as flow rate of gas inside the system, substrate temperature, quality and location of deposition within the system reactor. The structural, morphology, surface roughness and optical properties are studied by x-ray diffraction (XRD). Field emission scanning electron microscopy (FE-SEM), Energy Dispersive X-ray Spectroscopy (EDS), atomic force microscope (AFM), UV-Vis. spectroscopy respectively and gas sensing properties have been investigated for (SnO2 and ZnO) thin films. X-ray diffraction (XRD) analyses taken for all samples were prepared by different oxygen gas flow rates (4, 5, 6, and 8NL/h) and different substrate temperatures (300, 400, 450, 500, 550 and 600°C). It was observed that by increasing the oxygen flow rates and substrate temperatures, the crystallite size increased. The XRD results showed that the SnO2 nanostructure thin films are polycrystalline in nature with a tetragonal structure, while ZnO has a hexagonal structure. The crystallite size was estimated by Scherrer formula and W-H analyses. Also, it was noted that the dominant in an orientation corresponds to [100] due to increasing the annealing temperatures (150, 200, 250, and 300 °C) which were prepared by hydrothermal drop casting. (FE-SEM) image manifested the surface morphology of the (SnO2 and ZnO) deposited on a glass substrate. Clearly, it was found that the sample of nanostructure thin films deposited with a gas flow rate of (4NL/h) at substrate temperature 400°C has the XVI smallest grain size. However, the result revealed that through increasing the gas flow rate and substrate temperature, the grain size increased as well. The FE-SEM images of SnO2 nanostructure thin films clearly indicate that the shapes of balls or cubes are shown in their structures, while the ZnO thin films have a cauliflower-like or sponge, nanorod and nanoflower shapes. EDX analysis showed accurately the growth of (SnO2 and ZnO) on the glass substrate and the purity of these thin films with the ratio of the material drift on the surface of the substrates. Atomic Force Microscopy (AFM) appeared that the grain size becomes larger, the crystallinity was improved with an increase in the substrate temperature, and the distribution of grains is uniform on the substrate surface. The optical properties evinced the energy gap for the direct allowed electronic transition which was calculated using Tauc’s equation. It is noticed that SnO2 thin films fall in the range of (3.72- 4.09eV), and ZnO thin films in the range of (3.18-3.48 eV), depending upon the oxygen gas flow rate, deposition time and substrate temperatures. The electrical properties include Resistance–Temperature Characteristic, D.C. electric conductivity, and Hall effect measurements. Activation energies and electrical conductivity (σRT) were measured for SnO2 nanostructure thin films. The results of Hall coefficient showed that the SnO2 nanostructure thin films follow the n-type semiconductor behavior. Gas sensor test was done at room temperature for (100 ppm) ethanol gas exposed to SnO2 thin films that prepared at (400, 450 and 500ºC) substrates temperatures, and indicated that the sensitivity of the samples decreases with increase the substrate temperature due to the morphology surfaces and particle size of the films. In other words, the sensitivity of the sensor increases with decreasing the particle size of the SnO2 nanostructure thin film. The response and recovery time increase with an increase in the ethanol gas concentration ranged from (100- 1000 ppm). A fast response speed at (7s) XVII was noted with recovery time (14s) at (100 ppm), while the slow response speed was observed for (1000 ppm) at (8.7s) with a recovery time of (16 s).