Fabrication of a Gas Sensor from Thin Films of Tungsten Oxide Nanoparticles and Their Use in Oil Refineries

In this research, the structural and sensitivity properties of the toxic gases of films tungsten oxide (WO3) nanoparticles prepared by the pulsed laser deposition method were manufactured and studied using a Nd:YAG laser. To show the effect of different temperatures (400, 600 and 800 o C) on films deposited on quartz substrate for all samples. The results of X-Ray diffraction (XRD) showed that all the thin films have polycrystalline structure and have a peak direction (010) for all samples, and that increasing the temperature led to an increase in the particle size. The decrease in the values of the full width and half maximum (FWHM) of the films (WO3) for (010) modes from 0.19 to 0.14 with increasing temperature. The nature of the topography of tungsten oxide (WO3) nanoparticles was studied using atomic force microscopy (AFM), which proved that the films grown in this way have good crystallization and have a homogeneous surface. The root mean square (RMS) values of the tungsten oxide nanoparticles (WO3) increases with increasing temperature. When measuring the sensitivity of tungsten oxide nanoparticles (WO3) to (CO, NH3 and NO2) gases, it was found that the films have good sensitivity to these gases at room temperature (RT), and it was the best sensitivity of the films is at a temperature of (800 o C) as follows: CO gas (81%), NH3 gas (84%) and NO2 gas (100%) .All studies have shown that tungsten oxide (WO3) has the ability to detect toxic gases, such as (CO, NH3 and NO2), which have an detrimental effect on workers in oil refineries. The films of tungsten oxide (WO3) is used in themanufacture of gas sensors that can be used inthese refineries, and when the temperature increases, it becomes more sensitive to gases (CO, NH3, NO2).


1.Introduction
(WO 3 ) is a semiconductor metallic oxide with a band gap of 2.9 eV, which has been used in different applications, like savvy windows, electronic data shows, electrochromic devices, gas sensors and photo catalyst, photovoltaic devices and photograph electrochemical devices [1][2][3][4][5].
For the preparation of tungsten oxide film, various deposition techniques were used, for example chemical vapor deposition [6,7], pulsed laser deposition [8,9], spray pyrolysis [10,11], electrodeposition [12], spin coating [13], sol-gel methods [14,15], sputtering [16,17], thermal evaporation [18,19] and oxidation of W films [20]. Pulsed laser deposition (PLD) has been used in the preparation of tungsten oxide films over classical deposition methods due to its many advantages, including good adhesion to substrate deposition temperature, reproducibility, controllability of stoichiometry and crystal structure, and thus direct deposition of alloys and compounds of materials with different vapor pressures. The aim of study focuses on the deposition of WO 3 films on quartz substrates by PLD, and the detailed investigation of the influence of different temperatures on the structural, morphological and sensitivity properties of the deposited WO 3 films and usedit as applications in gas sensors. A gas sensor was manufactured to give an audible signal when the amount of gases emitted from the equipment is increased in oil places.

2-Experimental part
In this study, WO 3 powder with a purity of 99.99% was pressed by a hydraulic press for 15 minutes at a pressure of 7 tons, resulting in a disc with a diameter of 1.5 cm and 2 mm thick. Quartz substrates (1.5 × 1.5 cm) were used to deposit the tungsten oxide film.
Distilled water was used to clean them and remove the remaining dust and dirt from their surface. Then the substrates were cleaned with alcohol for 5 min by the ultrasonic system to remove some oxides and grease.
Hot air was used in this process to dry the quartz substrates, and finally, fine paper was used to wipe the slides. An ND:YAG laser was applied to film deposition using pulsed laser deposition (PLD) technique with a wavelength (1064 nm) and energy of 800 mJ and a rate of 1000 pulses by different temperatures at (400, 600 and 800 o C) for all samples as the   The crystallite size can be calculate from the Scherer equation, which is expressed as: where (D) is the crystallite size, (θ) is the diffraction angle, (β) is the FWHM of diffraction peak, λ =1.5406°Ais thewavelength ofCuKα radiation andScherrer's constant is )K=

Atomic force microscopy (AFM)
The AFM images give some quantitative data about the surface roughness (R) and the maximum height of the WO 3 nanoparticles thin films were prepared at different temperatures substrate(400, 600 and 800 o C) as shown in Figures (3). Fig.(3): The AFM images of WO 3 nanoparticles thin films at various temperatures substrate (400, 600 and 800 o C). The obtained of surface roughness (R) values showed in Table(2). The minimum value of surface roughness and (RMS) of films at 400 k and increase with increasing temperature as shown in Table (2) indicating that the temperature increases growth and makes the surface of films free of voids and homogeneous distribution for grains.

°C
The surface roughness value and (RMS) will increase and the grain size becomes larger, the reason for this is high temperature facilitates the coalescence of the surface grains and therefore rougher surface and thus an increase in the grain size.

3.3Optical measurements
The transmittance spectra and absorbance as a function of the wavelength in the range between (044-1100) nm were investigated to the WO 3 nanostructure thin films. By For all samples the energy band gap decreases when the temperature substrateincrease, as shown in the Table (3).

3.4sensingmeasurements
The gas sensing properties were evaluated by measuring the changes of resistance of the sensors, before and after enter the gases. The measurements have been room temperature (RT). Figures (6), (7) and (8) Table (4), we notice that with an increase in the gas concentration, the sensitivity increases, and the best sensitivity is at a temperature of 800 0 C for all gases.
We note the high sensitivity of WO 3 thin films at thetemperature of 800 0 C due to the same time the increase in the surface roughness in this sample so that these two properties rise the surface area of the film and thus rise the area of the film subject to the reaction.
We also note that each sample has a certain temperature to reach the sensitivity, as the temperature of the membrane response decreases with the increase in surface roughness and the reason for that is n-type WO 3 , it has a high number of electrons, so the adsorption of the gas on the surface of WO 3 it leads to trapping these electrons and thus obtaining a large change in resistance, which leads to an increase in the response due to an increase in the adsorption rate.
As for the other samples, we note that their response to the gas is less, and the reason for this is that the surface roughness is greatly reduced, and thus a decrease in the surface area of the reaction is achieved, and therefore the sensitivity of the films decreases by a large amount in some cases.

4-conclusion
With increased the temperatures substrate, the surface roughness of the samples increases, with increased the temperatures substrate, the crystallite size of the samples increases, with increased the temperatures substrate, the energy band gap of the samples decreases, with increased the temperatures substrate, absorbance spectra of the samples increases, with increased the temperatures substrate, transmittance spectra of the samples decreases and the best sensing to (NO 2 ) gas was recorded at the temperature substrate (800 o C) for high sensitivity (100%) at room temperature.