Novel Hydrothermal synthesis and characteristics of thin films V2O5@TeO2 with CTAB for detection of

Invention Journal of Research Technology in Engineering & Management (IJRTEM) ISSN: 2455-3689 www.Ijrtem. com Volume 3 Issue 5 ǁ July –August 2019 ǁ PP 49-58

Novel Hydrothermal synthesis and characteristics of thin films V2O5@TeO2 with CTAB for detection of NO2 gas Sozan A. Hassan 1, Sabri J. Mohammed 2 1

Department of Physics, College of Science, University of Kirkuk, Kirkuk, Iraq Department of Physics, College of Education, University of Tikrit, Tikrit, Iraq

2

ABSTRACT : Novel thin films V2O5 @TeO2 prepared with hexadecyl trimethyl ammonium bromide (CTAB) by hydrothermal method at (160oC) were fabricated successfully on glass substrates. Also, used as the parameters to control morphology by hydrothermal technique. At room temperature and annealing by tubular quartz furnace at temperature (473,673) K for 8 hours with air. Their NO2 gas sensing properties were examined at a different annealing temperature and showed promising sensitivity and response towards tested gas. The polycrystalline nature of the obtained films was confirmed by XRD diffraction. Surface morphology was studied using field emission scanning electron microscopy FESEM, Energy‐dispersive X‐ray spectroscopy, and atomic force microscopy AFM. After annealing, the roughness of the surface and the mean grain size were increased annealing temperature

KEYWORDS: Hydrothermal synthesis; Novel materials; Vanadium pentoxide; Tellurium dioxide; CTAB; NO2 gas sensor

I.

INTRODUCTION

Increasing environmental pollution is becoming a vital global concern, particularly concerning the imperative to reduce emissions of gases causing the greenhouse effect, acid rain, and the depletion of stratospheric ozone [1].

Therefore, there is an urgent need to develop some devices that allow fast, portable, low-cost monitoring of the gases responsible for air pollution and pose a danger to human health. So far, sophisticated and expensive equipment, such as gas analyzers based on IR and UV spectrophotometry, pulse fluorescence, flame photometry and gas chromatography, to determine air quality were applied. Although this equipment enables accurate gas phase analysis, it has four substantial disadvantages such as high cost, large dimensions (limited portability), slow analysis time, and non-continuous monitoring of the gas composition. In this respect, chemical gas sensors may offer advantages in the form of simple construction, low cost, and ability to work in situ. One large group of sensors, applied to environmental monitoring, is based on liquid (or wet) electrochemistry[2] However, these sensors presently suffer from the same four severe disadvantages. Moreover, these sensors cannot be applied in environments warmer than room temperature. During the past five decades, efforts were made to develop chemical gas sensors based on solid-state technology. The potential advantages of these sensors over the wet technology sensors are (i) miniaturization, (ii) simple calibration and measurement [3], (iii) low cost, (iv) short response time, (v) resistance to severe conditions, such as high temperature and corrosive environment and (vi) selectivity. Their main advantage is that they can also operate at elevated temperatures, thus meeting the environmental requirements, such for instance, in a car and industrial exhaust systems[4].In recent years, air pollution has attracted considerable attention.

The emission of toxic and harmful gases is increasingly becoming a serious threat to public safety and health.

Nitrogen dioxide (NO2) is one of the most typical toxic gases [5], which is harmful to humans as well as the environment with a low threshold value of 3 ppm [6]. It is principally generated from the combustion of fossil fuels in internal combustion engines, accompanied by other forms of NOx such as NO and N2O. NO2 can cause various problems such as smog or acid rain and provide abundant precursors for the formation of photochemical oxidants and delicate particulate matter (PM2.5) [7]. PM2.5 means the mass per cubic meter of air of particles with a size (diameter) generally less than 2.5micrometers (µm). Therefore, there are urgent demands for developing high-performance gas sensors to detect NO2 in real-time. Over the past few decades, a diverse range of gas sensors based on different materials has been investigated. Several types of gas sensing materials such as metal oxide semiconductors (MOS) [8], conducting polymers [9], and metal oxide/polymer composites [10].

Have been developed to detect NO2[11]. Among these gas sensing materials. Design of novel unique materials combining the properties of organic and inorganic compounds provides the development of innovative industrial applications and academic research.

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Elemental semiconductors (boron, carbon, silicon, germanium, phosphorus, gray tin, selenium, and tellurium), may be used to form p–n junctions when composed with metal oxides such V2O5, which can probably induce a change in resistance and hence the response to toxic gases. Tellurium oxide has been focused among all due to its unique behavior with the moderate values of melting point and atomic radius as well as the high electron/hole mobilities along the c-axis. Besides, its electrical properties are very well understood due to only one allotropic modification, and the temperature coefficient of the energy gap is positive [12]. Gas sensors work on the principle of a change in electrical conductance on exposure to the gas, which is to be detected [13]. This work aims to design a reliable NO2 gas sensor for environmental protection with high sensitivity, fast response time, and fast recovery time from semiconductor metal oxide.

II.

EXPERIMENTAL SECTION

Preparation : Composite of V2O5@TeO2 was grown on the glass substrate (2x1 cm2) by using a hydrothermal method. The substrates were cleaned with an adequate amount of acetone and ethanol. After cleaning, the substrates were rinsed with deionized water and dried at room temperature. The hydrothermal synthesis was carried out for (2.5mmole) Potassium tellurite (K2TeO3) and (2.5mmole) Ammonium metavanadate NH4VO3 dissolved in 50 ml deionized water and then (2.5mmole) oxalic acid was added to the system to form an orange solution The resulting solution was and stirred for 15 minutes. Then, HCl was slowly added into the resulting solution to obtain a pH of 1. Then (0.25 mmole) CTAB was added into the solution. After stirring 30 min, the mixture was transferred into a 100 mL Teflon-lined stainless autoclave with filling about 80% of the whole volume by mixing distilled water. Glass substrate was dipped vertically in the autoclave, maintained at 160 c for 6h allowed to cool down at room temperature overnight. Teo2, v2o5 thin films were washed with distilled water and ethanol for three times to neutralize the pH of the solution respectively and dried at 80∘ C for three h. To observe the effect of CTAB, the product without the assistance of CTAB was synthesized maintaining the same condition. Finally, the thin films were annealed at deferent temperatures (473,673) k in the oven for eight hours for investigating the crystal structure, morphology, sensitivity of NO2 gas

Characterizations : The Particles size and morphology were examined under vacuum by field emission scanning electron microscopy (FESEM). Fitted with Energy‐dispersive X‐ray spectroscopy (EDAX) [Model: FE‐SEM. TESCAN MIRA3/RAITH LIS. France]. EDX analyses were used to confirm the compositions of the prepared nanomaterials. XRD measurements were performed using an Analytical X’Pert PRO diffractometer and Cu Ka radiation (λ= 0.15418 nm); the samples were prepared on glass substrates. Aluminum electrodes by using a suitable mask were deposited on the surface of the thin films of TeO2@V2O5 on glass substrate mixed by thermal evaporation using the Edward coating unit. The connections between the aluminum and very thin copper wires were 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-pin feed through at the base of the chamber allows the electrical connections to be established to the heater, thermocouple, and sensor electrodes.

The sample was put on the heater, and the electrical resistance of the sensor was measured by the multimeter.

III.

RESULTS AND DISCUSSION

Structural properties : Fig (1) shows the x-ray diffraction patterns for prepared V2O5@TeO2 composite thin films prepared with CTAB on glass substrate by hydrothermal method at room temperature (RT), annealed at 473 and 673 K. Polycrystalline structure with existence of the three phases (V2O5, Te and TeO2) were appeared in all samples, identical with their standard values with respect to the standard cards 96-101-1292, 96-900-9089 and 96-101-1099 respectively. Annealing at 473 K and increased to 673 K cause to enhance sample crystallinity, indicated by increasing number of peaks and peaks intensities. specially for (012) direction, indicate on decreasing its crystalline size.

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Fig. (1): XRD patterns for V2O5@TeO2 composite thin films prepared with CTAB and annealed at different temperatures.

Table (1) illustrates the structural parameters for V2O5@TeO2 composite thin films prepared with CTAB. The (012) direction has a crystalline size of 8.1 increase to 9.0 nm at 473 K and then to 18.0 nm at 673 K annealing temperature.

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Novel Hydrothermal synthesis and characteristics of thin films‌ Table (1): XRD parameters for V2O5@TeO2 composite thin films prepared with CTAB and annealed at different temperatures

FESEM and AFM Compositional analysis :The FESEM images for V2O5@TeO2 thin films prepared with CTAB by hydrothermal method at (160 oC) deposited on the glass substrates, then annealed at different temperatures with magnification powers (x3k), (x30 k), (x50 k) were shown in figure (2). The sample appears in the form of nanoparticles with different shapes and sizes, shown in a grid-shaped structure with ramified needle structures extending over the sample area, where the raw materials of the sample appear to be un-mixed with each other before annealing. After the thermal treatment but the nanostructures are merged more. A threedimensional porous structure is formed extending over the entire sample area. This highly porous structure, at the nanoscale, is composed of well-connected granules between each other.

Figure (2): FESEM images at different magnification powers for V2O5@TeO2 thin films prepared with CTAB annealed at different temperatures (RT, 473, 673) K

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Novel Hydrothermal synthesis and characteristics of thin films‌ The EDX results of the V2O5@TeO2 composite samples prepared with CTAB and annealed at different temperatures are illustrated in Fig. (3). Peaks corresponding to V, Te, and O electronic transitions peak with different intensities. In addition to peaks corresponding to the glass substrate elements. Also, the oxygen peak intensity increased with annealing. Br peak appeared for samples prepared with CTAB.

Figure (3): EDX analysis for V2O5@TeO2 thin films samples annealed at different temperatures prepared with CTAB.

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Novel Hydrothermal synthesis and characteristics of thin films‌ The atomic force microscope (AFM) gives information about surface topography of the examined sample in the nano range. Figure (4) shows the atomic force microscopy images and the granularity distribution histogram for V2O5@TeO2 thin films prepared with CTAB by hydrothermal process and deposited on the glass substrate and annealed at different temperatures. Nano structures uniformly distributed with small particles. The average diameter increased after annealing.

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Fig. (4): 3D AFM images and their granularity accumulation distribution for V2O5@TeO2 composite thin films prepared with CTAB and annealed at different temperatures

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Novel Hydrothermal synthesis and characteristics of thin films‌ The AFM parameters for these samples are shown in Table (2). Film roughness has high increment when samples annealing at 673 K, especially for the sample prepared with CTAB. Table (2): AFM parameters (Average Diameter, RMS roughness, and Roughness average) for V2O5@TeO2 composite with CTAB, annealed at different temperatures

The variation of average particles diameter with annealing temperature for V2O5: TeO2 samples prepared with CTAB were shown in Fig. (5). The average diameter increases with increasing annealing temperature.

Fig. (5): Variation of average diameter, Root Mean Roughness, and Roughness surface with annealing temperature for V2O5 @TeO2 composite thin films prepared with CTAB.

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Gas Sensor Measurements : One of the simplest gas sensors that depend on the change of electrical resistance of metal oxide thin films due to the interaction of target gas with the absorbed oxygen ions species already absorbed on sample surface from the ambient. This process leads to variation in electrons density, which causes a change in the depletion layer thickness, consequently change in sample resistivity [14].All sample exhibit low sensitivity to NO2 gas at room temperature and improved at 473 K operating temperature. Various oxygen species adsorbed at the surface. The reaction O_2 ads + e− → 2O_ ads Takes place as the temperature increases extra electrons contribute to the process of sensitivity [15].Fig. 4-43 to 4-48 illustrate the variation of sample sensitivity (∆R/Rair×100%) at 473 K operating temperature against NO2 gas at a concentration of 200 ppm for as-deposited and annealed samples at 473 and 673 K. Fig. (6) Illustrates that the gas sensitivity for novel thin films V2O5@TeO2 composite sample prepared with CTAB, against 200 ppm concentration of NO2 gas. The gas sensitivity increased from 41% to the maximum value of about 290% with increasing annealing from RT to 673 due to enhance sample mobility due to neck connection between adjacent grains, so the conductivity not only depends on the change of carrier concentration but also the crosssectional area of these connections and is therefore sensitive to surface reaction charges ]16[. Also, the high surface area of this sample, as shown in FESEM measurement. Moreover, Sensitivity of NO2 gas, Response time and Recovery time VS annealing temperature for novel thin films V2O5 @TeO2 composite thin films prepared with CTAB by the hydrothermal method shown in fig(7).

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Novel Hydrothermal synthesis and characteristics of thin films‌ Fig (6) NO2 gas sensitivity for novel thin films V2O5 @TeO2 composite thin films prepared with CTAB by hydrothermal method and annealed at different temperatures.

Fig (7) NO2 gas sensitivity, Response time and Recovery time VS annealing temperature for novel thin films V2O5 @TeO2 composite thin films prepared with CTAB by hydrothermal method.

IV.

CONCLUSION

The novel thin films V2O5 @TeO2 composite thin films prepared with CTAB by hydrothermal method at (160 oC) were fabricated successfully on glass substrates and studied annealing effect on sensing properties. The prepared sensor was characterized by various techniques and sensing performance measured to NO2 gas. The thin films of V2O5@TeO2 with CTAB show excellent sensitivity increased from 41% to the maximum value of

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Novel Hydrothermal synthesis and characteristics of thin films… about 290% with increasing annealing from RT to 673 dues to enhance sample mobility due to neck connection with a short response and recovery times.

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