H
2concentration decreases
CHAPTER 5 -Conclusion
5.1 Summary and conclusion
In this project, the reduction and characterization of heated V2O5 to form VO2 has been carried out. All the reduction of V2O5 sol-gel was done in three different techniques namely: H2/Ar gas, CO/CO2 gas and laser ablation. After the analysis of the samples was done, there are some conclusions drawn up from the obtained results.
Summary of reduced V2O5 samples under H2/Ar gas:
The surface morphology which was studied using SEM showed the increase in number of grains after heating the sample. The crystals obtained after heat treatment at high temperature were rod-like shape. The AFM results also confirmed the increase of crystals on the surface of the film. According the AFM results, maximum roughness and the average root mean square roughness are decreasing as the film is heated at high temperature. The crystallography analysis of heat-treatment of the film was studied with XRD technique and showed the slightly reduction of V2O5 to V6O13 by H2/Ar gas. The intensity of V2O5 diminishing as the heat treatment is done at high temperatures. The optical results were not presented since silicon wafer is not transmitting in visible light region.
Summary of reduced V2O5 samples under CO/CO2 gas:
The CO/CO2 gas was effectively employed to reduce V2O5 to VO2 thin films. The both as-deposited and the heated samples under this reducing gas were studied their surface morphology using SEM. The SEM images showed grains on a surface of the film before and after the heat-treatment. It was found that the as- deposited and the 6000C heated samples have similar morphologies of agglomerated particles on the film surface. In measurements it showed that the rods found in the as-deposited film were in the scale of microns and after sample
has been annealed at 6000C, the rods were in the scale of nanometers. The EDS analysis was also done since EDS is interface with SEM. The EDS spectrum showed the K-beta and K-alpha peaks of vanadium and their corresponding atomic weight. The AFM results confirmed the SEM results, after heat-treatment at low temperature 4500C, the number of grains increases and forms rods at temperature of 6000C. The roughness of the film also increased after annealing from 8.24nm to 11.8nm.
The crystallography of these samples were studied using XRD technique, it was found that the as deposited sample is pure orthorhombic V2O5 with the space group of Pmn21. The lattice parameter were calculated as a= 11.5 Å and c=4.350 Å. The XRD revealed that 3500C is a promising temperature of reducing V2O5 to VO2. This sample was found to have two VO2 peaks: (-202) and (402).
The optical measurements were done by UV-Visible and the film showed the low transmittance. The low transmittance is due to the thickness of the film and the non-oriented particles on the film. It is also shown that the absorption cut-off is shifting with respect to the annealing temperature. However, this phenomenon shows the presence of different phases with different band gaps. The FTIR which was used to study optical properties and the stretching of bonds showed the less transmittance due to thickness of the film, and the stretching of C-H bond approximately 2846 and 2920 cm-1. The Raman was also used to find the stretching bonds. The weak intensity was observed at low frequency between 200 and 450cm -1, which is associated with the bending vibration modes of the V3–O, V2–O and V=O. The Raman spectrum of asdeposited film shows two broad peaks around at 496 and 535 cm-1, due to the stretching modes of the V–O–V and V3–O bonds, respectively.
Summary of reduced V2O5 samples reduced by laser irradiation:
In this reduction technique, the SEM images showed the interesting surface morphology of V2O5 rods changing into V2O5 hollow spheres after laser irradiation.
The AFM images also confirmed the dramatic change of V2O5 rods to V2O5 hollow spheres. However, the hollow spheres were not clearly under AFM as the SEM images. The crystallography was studied using XRD; the results showed the shift in phases after laser irradiation. This phenomenon shows the stretching of V-O bonds during laser heating, which causes the crystal lattice to vibrate. The vibration of atoms also causes the change in crystal structure as it was found that orthorhombic V2O5 changed to tetragonal structure.
The UV-Vis showed that the film transmittance decreasing as the heat treatment as has been performed to the film. In the results showed by UV-Vis measurements, the absorption cut-off is shifting with respect to the annealing temperature. This can be caused by the variation of the band gap or not being uniform of the film. The optical properties, vibration modes and the stretching of V-O bonds were studied using FTIR. The irradiated film is transmitting very low in the IR region due to film thickness and the increase of hollow spheres. It was also found the similarities of irradiated film with the CO/CO2 sample, it was observed that the absorption cut-off shifting with respect to number of doses. The stretching in O-H bond was determined between 3600 and 3200 cm-1 due to the V2O5 sol-gel. The stretching of C-H bond was also found at at 2863 cm-1 and 2936 cm-1.
The vibration modes and stretching of V-O bond was studied using Raman. The peaks located between 487 and 303 cm-1 were assigned to the bending vibrations of the bridging V–O–V and V3–O bonds, respectively. The broad peaks were found at 645 and 712 cm-1 due to the stretching vibration modes of the V3–O and V2–O bonds in a disordered V–O–V framework, respectively.
The RUMP program was used to analyze RBS measurements which showed exactly the elements present in our film. This type of program also showed which elements is the substrate was made of. Determination of thickness of the film was using RUMP was very important to pursue with other calculations.
The SIMNRA program was used to analyze the ERDA measurements. The aim for these measurements was the determine hydrogen concentration since the film was from V2O5 sol-gel. In the measurements, hydrogen concentration was seen decreasing as the channel decreases. This was seen as directly proportional to the decrease in energy.
In comparison of the three V2O5-VO2 reduction techniques, the CO/CO2 is the promising to reduce V2O5 to VO2 thin films. The three vital parameters in this technique are; temperature, time and flow rate of the reducing gas. The heat treatment is also promising with the slightly reduction of V2O5 to VO2 film. Laser irradiation is long process to reduce V2O5 to VO2 thin films. The further research of VO2 coatings should be studied to improve too dark colour and unappealing of the film. A deeper scientific understanding of how thermochromics work is still required and some of the processes used to make VO2 coatings are not easy to control.