International Journal of Modern Research in Engineering & Management (IJMREM) ||Volume|| 2 ||Issue|| 8 ||Pages|| 26-33 || August 2019 || ISSN: 2581-4540
Utilization and measurements of WO3/ITO &Novel WO3@NaDCC/ITO thin films prepared by Hydrothermal method as a Nitride Dioxide gas sensor 1,
Mohanad Q. Kareem, 2,Nadim K. Hassan
1
Department of Physics, College of Science, University of Kirkuk, Kirkuk, Iraq Department of Physics, College of Education, University of Tikrit, Tikrit, Iraq E-mail: drmohanad33@uokirkuk.edu.iq
2
----------------------------------------------------ABSTRACT-----------------------------------------------------Polycrystalline WO3/ITO and WO3@NaDCC/ITO thin films were prepared by hydrothermal technique at temperature 200oC for six hour , characterized with X-ray diffraction (XRD),An X-ray diffractions pattern showed abroading in the FWHM with doping NaDCC and confirmed the formation of polycrystalline monoclinic structure for WO3/ITO and WO3@NaDCC thin films , ITO has a cubic in structure , .Morphology analyzed by Atomic Force Spectroscope (AFM). The results expressed the formation of nanoparticles and decrease an average diameter size when doped NaDCC in WO3/ITO thin films.then investigate and revealed a Nitide Dioxide gas sensing properties of fabricated thin films by .The results reveal that the sensitivity increase as the thin films of WO3/ITO modified by NaDCC , we found that the maximum sensitivity of about 730% of WO3@NaDCC/ITO thin films a moderate optimum temperature of 200 oC and an optimum NO2 gas Concentration of 50 ppm.
KEYWORD: Pollutant Gases ; Metal oxide , Hydrothermal Technique; NaDCC doped WO3/ITO ; Structure and morphology properties ; Novel materials; No2 Gas sensor. ----------------------------------------------------------------------------------------------------------------------------- ---------Date of Submission: Date, 29 August 2019 Date of Publication: 05 September 2019 ----------------------------------------------------------------------------------------------------------------------------- ----------
I.
INTRODUCTION
Nowadays, the vast growing industries, various machines[1], and an increasingly large number of vehicles are responsible for spoiling the healthy life of human beings and all living organisms[2]. Air pollution is found to be very dangerous as it is related to the respiratory system[3]. The air pollution rate is increasing daily due to the release of a high amount of gas exhaust from different industrial processes. The different harmful gases such as NH3, H2S, NO2, H2, SO2 emitted through various sources[4]. Among these gases, NO2 is responsible for acid rain, photochemical smog, and many health hazards. This highly toxic gas generated from different sources such as the combustion of fuel, home heaters, and the automobile engine processes. Nowadays, it has become essential to monitor these issues by using high-quality gas sensor devices. For gas sensing, metal oxides become prevalent materials due to their cost-effectiveness, simple fabrication, and compatible properties. The nanostructured metal oxides have become most attractive due to their unusual properties compared to their bulk form. In recent research, the morphologically controlled materials fabricated due to its properties and working ability strongly related to its shape, size and a high surface area in nanostructures[5]. Morphology plays a vital role in tuning the surface area and, therefore, the high performance of gas sensing devices. There are many reports based on fabricating the vivid architectures by assembling lower-dimensional nanostructures. The attractive approach is involved in obtaining exotic nanostructured morphologies for increasing the gas sensing performance.2D, 3D complex superstructures of nanomaterials fabricated from the lower dimensional nanomaterial for getting higher selectivity, sensitivity, and stability of gas sensor device[6].WO3 material is an extensively used material for various potential applications such as photo catalysis[7], electrochromism[8], solar cells[9], and gas sensing[10]. It is an n-type semiconducting material with bandgap energy varying between 2.6 – 3.9 eV[11]. It is widely attractive due to its high detection properties towards toxic NO2gas[12]. The choice and preparation of the sensor materials required for making NO2 gas sensor and the manufacturing method rely upon several parameters of gas sensors[13], the most critical of which are low resistance, availability, large reaction surface, total cost of chemical component, surface modification and micro-structure of sensing layers in order to achieve detector with high sensitivity. The sensor sensitivity, Response Time , Recovery time calculated by the following relations:
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Utilization and measurements of WO3/ITO &Novel…
Rg , Ra, are the electric resistance of the film in air and presence of gas . (3) This study reports the effect of sodium- dichloroisocyanurate (NaDCC) as “reservoir chlorine” because of chemical equilibrium as a strong directing agent in producing dagger-like nanoparticles … etc. via a facile hydrothermal process [14], [15].In this research the essential aim of work to prepare a pure WO3 on ITO thin films and doped with NaDCC by using Hydrothermal method at 200 oC . and study the structural and Morphology of WO3/ITO , WO3@NaDCC/ITO thin films , then fabricated gas sensing device and characterization the properties of the preparation thin films. II. MATERIAL AND METHODS Preparation : WO3 nanostructures synthesized by the hydrothermal method. In a typical, the solution prepared by dissolving 1.65 gm of ammonium Para tungstate dehydrates in 20 mL distilled water under magnetic stirring for 10 min to get a clear solution. The pH of this solution was (8). Subsequently added 0.132 gm. Of Sodium dichloroisocyanurate (NaDCC) and several drops of HCl were introduced to attain the pH value of 1. Here the adding HCl can act as a precipitating agent and also medium for the product to have desired morphology, a transparent yellow solution with foam obtained, and These solutions transferred into a 100 mL Teflon-lined stainless steel autoclave with filling about 50% of the whole volume by added distilled water. ITO substrate placed at an angle against the wall of the Teflon – Liner with the conducting side facing down, maintained at 200 c for six-hour and then cooled down to room temperature. The WO3@ NaDCC/ITO thin films were rinsed with distilled water and ethanol for three times to neutralize the pH of the solution respectively, the surrounding water present in the product was removed by the drying process at 60C in air for three hours to observing the effect of Sodiumdichloroisocyanurate (NaDCC).The process repeated in the absence of Sodiumdichloroisocyanurate (NaDCC) under identical conditions. Without the assistance of NaDCC was named WO3/ITO while that with the support of NaDCC named, WO3@NaDCC/ITO. Finally, investigating the crystal structure (XRD), morphology (AFM),. Have then made devices as gas sensors of nitrogen oxide sensing. Characterization Details : The X-ray diffraction line profile data are recorded in a continuous scan mode (2θ = 10-80 ) using X’Pert PRO (PAN analytical) diffractometer in the Bragg–Brentano par focusing configuration (θ/2θ geometry) with CuKα (λ = 1.5406 A˚) radiation at room temperature (25 C). The electrons, emitted from the cathode filament accelerated towards the anode plate (Cu) by applying 40 kV voltage and 30 mA filament current. An X-ray diffractometer, the diffraction beam optics equipped with 0.04 rad solar slit, fixed divergent slit (slit size = 0.8709), 0.100 mm size receive slit and a scintillator detector. The samples are scanned with a constant step of 0.05 deg of 2θ and with constants counting time of 1.5sec at each level to minimize the instrumental contribution in XRD line broadening. Particle size and morphology were examined under vacuum by the Atomic force microscope (AFM) Type (AA3000 Scanning probe microscope SPM ,Tip NSC35/AIBS from Angstrom Ad – Vance Inc ) , with ITO glass acting as a blank. Aluminum electrodes by using a suitable mask deposited on the surface of the WO3/ITO mixed by thermal evaporation using the Edward coating unit. The connections between the aluminum and very few copper wire made by high conductive silver. A vacuumed closed chamber, by rotary at approximately (1× 10-2 mbar )was made of stainless steel with a controlled hot plate. A multi-pins feed through at the base of the chamber allows the electrical connections to established to the heater, thermocouple, and sensor electrodes. The sample put on the heater, and the electrical resistance of the sensor measured by the multimeter.
III.
RESULTS AND DISCUSSION
Structural studies : Figure (1) and figure (2) Shows the X-ray diffraction patterns for WO3/ITO and WO3@(NaDCC)/ITO thin films as prepared at RT. These figures show a polycrystalline structure for all samples. The RT sample appeared with two to three peaks of tungsten dioxide located at 2θ =23.51⁰, 26.58⁰ and 55.47⁰ matched with (020),(120)and (400) planes respectively for monoclinic structure of a tenorite system of WO3, identical with the standard JCDS card number (72-1465) ,and the Cubic structure of In2O3 observed and symbols as star (*) at (2θ =23.51⁰, 26.58⁰ and 55.47⁰ matched with (020), (120)and (400) planes, which identical with the standard JCDS card number (71-2194).According to the relationship Debye-Sherrer.
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Utilization and measurements of WO3/ITO &Novel… Where D is the adequate crystallite size normal to the reflecting plane, K is a shape factor =0.9), k is the wavelength of CuKα radiation, βD is the integral width of a particular peak and Ɵ is the diffraction angle.
Figure (1): X-Ray diffraction patterns of deposited WO3/ITO by hydrothermal technique. From Equation. (1), size broadening is independents of the order of a reflection. The dislocation density (δ), defined as the length of dislocation lines per unit volume of the crystal, can be estimated using the following relation:
Figure (2): X-Ray diffraction patterns of deposited WO3/@NaDCC/ITO by hydrothermal technique. The smaller value of dislocation density shows the better crystallization of the film along a particular plane. T he observed values of D along different crystallographic planes reported in Table (1), (2). Table (1): Structural parameters of WO3/ITO thin films by hydrothermal technique as deposited at Room Temperature.
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Utilization and measurements of WO3/ITO &Novel‌ Table(2): Structural parameters of WO3@NaDCC/ITO thin films by hydrothermal technique as deposited at Room Temperature.
The broadening of the peaks in X-ray diffraction indicates that the size is in the range of nano size. The grain size of the particulates can be estimated using the Scherer method. The height of the peaks decreases with increasing the ratios and also the width of the peaks increases, indicating a decrease in the granular size after doping NaDCC on WO3/ITO thin films. The Morphology Results : AFM is a powerful technique to investigate the surface morphology at the nanoscale. The morphology of surface of WO3/ITO with and without NaDCC thin films was analyzed using an atomic force microscope. Figure (3), (4) shows the typical Two dimensional AFM image and three dimensions of WO3/ITO, WO3@NaDCC/ITO thin films prepared by using Hydrothermal technique. technique.
Figure (3) 2D and 3D AFM images for surface morphology of WO3/ITO thin film by hydrothermal method. AFM images show that all films are uniformly packed. The average grain size of doped and undoped synthesized WO3/ITO films measured from AFM analysis using software.Is found to be (87.53-71.34) nm depending on preparation conditions.
Figure(4):2D and 3D AFM images for surface morphology of WO3@NaDCC/ITO thin film by hydrothermal method.
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Utilization and measurements of WO3/ITO &Novel‌ AFM results show that with the addition of the NaDCC to WO3/ITO thin films they have less roughness and porosity, Roughness of WO3/ITO surface was (6.1) nm, while in the WO3@NaDCC/ITO surface was (5.28)nm.Table(3), figure (5),shows the size of grains, roughness, and root means square. Table. (3):- AFM parameters (Average Diameter, roughness Average, and RMS) of WO3/ITO, WO3@(NaDCC)/ITO thin films at Room Temperature.
This surface Characteristic is essential for applications of gas sensors. The results of the atomic force microscope consistent with those of X-ray diffraction, but the measured grain size rate of the atomic force microscope , is higher than the average grain size measured by X-ray diffraction, because (AFM) measures the size of the particles on the surface of the thin films while (XRD) Inside the crystal of the prepared thin films.
Figure (5): AFM parameters (Average Diameter, roughness Average, and RMS) of WO3/ITO, WO3@(NaDCC)/ITO thin films at Room Temperature. Gas Sensing Results : The gas detecting attributes are needy upon the response between semiconducting metal oxides and target gas. The instrument of gas detecting includes the redox response at the metal oxide surface, prompting the adjustment in the consumption layer of the grains .that at last change the electrical resistance of the metal oxide.Nitrogen dioxide is a strong oxidizing agent and has strong electrophilic property, which makes this molecule to be quickly adsorbed on the metal oxide surface. NO2 can react with metal oxide surface both in the presence and absence of oxygen. The oxidation of NO2 leads to the reduction of conduction electrons in the conduction band. These series of reactions resulted in the concentration of electron on the surface of the material to further decrease, which led to the decrease in conductivity of the material.Also adsorption of oxygen from the NO2 molecule on to the vacant site leads to decrease in conductance of the n-type metal oxide. Changes in the
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Utilization and measurements of WO3/ITO &Novel… conductivity are caused by electron transfer between adsorbed molecules and the semiconductor surface. The electrical conduction takes place via individual WO3 grains. NO2 gets chemisorbed at the surface of WO3 grains. Hereby they trap electrons at the surface. WO3 is an n-type semiconductor. The electrons are taken from ionized donors through conduction band and the density of majority charge carriers at the gas–solid interface is reduced. This leads to the formation of a potential barrier for electrons with increasing of the oxygen ions density on the surface the further oxygen adsorption is inhibited. Thus at the junctions between WO3 grains, the depletion layer and potential barrier leads to the increasing of the electrical resistivity value. This value is strongly dependent on the concentration of adsorbed oxygen ions of the surface.Introducing the n-WO3 films in an NO2 ambient will change the concentration of these ions and increase the resistance.Figures (6) and (7) illustrate the sample sensitivity (∆R/Rair×100%) at473 K operating temperature against NO2 gas at a concentration of 50 ppm of WO3/ITO, WO3@NaDCC/ITO thin films by hydrothermal method at (200o C) for six hours at RT.
Figure (6) NO2 gas sensitivity for WO3/ITO thin films at Room temperature
Figure. (7):NO2 gas sensitivity for WO3@NaDCC/ITO thin films at Room temperature. It shows the NO2 sensor of WO3/ITO and WO3@NaDCC/ITO. The gas sensitivity increased from 18.17% in the case without modified NaDCC to sensing become (730%) with modified WO3/ITO with NaDCC . The response/recovery time is an important parameter used for characterizing sensors. It is defined as the time required to reach 90% of the final change in voltage or resistance, when the gas is turned ON or OFF, respectively. The response and recovery times of WO3 film samples are represented in The figures (8) and tables(4), representing the response time and the recovery time of WO3/ITO, WO3@NaDCC /ITO.
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Figure (8): NO2 gas sensitivity, Response time, and Recovery time for novel thin films WO3/ITO, WO3@NaDCC/ITO as-deposited prepared by hydrothermal method. Table (4) NO2 gas sensitivity, Response time, and Recovery time for novel thin films WO3/ITO, WO3@NaDCC/ITO as-deposited prepared by hydrothermal method.
IV CONCLUSION successfully obtained WO3/ITO and WO3@NaDCC/ITO thin films by hydrothermal method at (200 oC) for six hours at RT. XRD measurement showed that the structure monoclinic, Polycrystalline and become more Crystalline, and peaks appeared more broadly than non-modified samples by NaDCC. AFM measurements show that the thin films was in nano scale and the particles size is 87.53 nm of WO3/ITO and become 71.34nm with doping NaDCC. The fabricated sensors not good sensitivity to No2 gas and become best with modified NaDCC and show the maximum sensitivity of about 730% of WO3@NaDCC/ITO thin films a moderate optimum temperature of 200C and an optimum NO2 gas Concentration of 50 ppm.
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