Enhancing Gas Sensing Performance of TiO 2 -ZnO nanostructures: Effect of ZnO Concentration

Gas sensors based on titanium dioxide (TiO 2 ) and zinc oxide (ZnO) nanocomposites are considered as energy-saving devices that are utilized to find dangerous or harmful gases in an environment. The performance of nitrogen dioxide (NO 2 ) gas sensors has been improved by spin-coating a TiO 2 and TiO 2 :ZnO nanocomposite with varying concentrations ( 90 TiO 2 : 10 ZnO, 70 TiO 2 : 30 ZnO, and 50 TiO 2 : 50 ZnO). To correlate structural properties with gas-sensing behavior, structural and morphological characterisation has been done using FESEM, XRD, and EDX. Without any ZnO-specific crystalline phase, TiO 2 X-ray diffraction was found to be indexed in the anatase crystalline structure. The ZnO is synthesized in the wurtzite phase with (002) orientation and has a smooth surface, according to the morphologies and crystalline structure of the films, which also indicated the presence of ZnO components with various crystallite sizes and lattice strains. Responses to NO 2 are increased by low ZnO content. Additionally, at the average operating temperature of 250 o C, TiO 2 :ZnO shows good response.


Introduction
Increased environmental contamination and, as a result, increased health concerns have become one of the most pressing issues of our day.Different gases are also commonly employed in industrial applications.Human health is directly affected by this condition because of their utility, the integration of smart gadgets made for the benefit of mankind into our everyday lives has grown in popularity over the years [1].As smart sensors, sensors made using microtechnologies offer benefits in terms of cost, compactness, and sensitivity.Gas detection devices, in this perspective, are crucial for a variety of applications, including monitoring gas buildup, medical diagnostics, food quality assurance, and the safety of industrial or residential activities [2,3].The characteristics of the sensing material have an impact on sensor specifications including sensitivity, selectivity, response, and recovery times.By fabricating gas sensors from semiconductor oxides with highly sensitive to gases, these limitations can be overcome [4][5][6].Metal oxide semiconductors including TiO2 and ZnO are important materials and have been widely studied [7,8].Because of their chemical stability, low cost, non-toxic semiconductor material, and photocatalytic efficiency properties [9][10][11].Titanium dioxide (TiO2) has proven distinctive and improved physical and chemical properties crucial for applications like photocatalysis [12], water splitting, and sensing when combined with ZnO (creating heterostructure) [13].The two most commonly used sensing materials are TiO2 and ZnO.In comparison to TiO2, ZnO has a smaller band gap (3.37 eV at room temperature), valence band potential, exciton binding energy, and electron mobility (0.1-4 cm 2 .V.s) [14].It has thus been investigated for any possible dangers.It might be employed in applications involving gas sensing.Conduction band holes from TiO2 in the valence band (VB) might easily transfer to the valence band (VB) of ZnO, and vice versa.Numerous applications may be made use of this ease of transmission between TiO2 and ZnO.Furthermore, ZnO is an inherently n-type semiconductor with a band gap of 3.37 eV and a significant exciton binding energy of 60 meV [15].
In the current study, a low-cost synthesis of amorphous TiO2: ZnO with different concentrations of ZnO was prepared by spin-coating technique and study the influence of the concentration of ZnO on the properties of the film and examined the performance structure as NO2 gas sensor.

Materials and Methods
All chemicals were of analytical grade and used without additional purification.Titanium dioxide (TiO2 99.5%) and zinc oxide (ZnO 99.8%) powder were used.Deionized water (DI) was used to clean and wash soda-lime glass plates (20 x 20 x 2) mm 3 , which were then washed with acetone and 2-propanol in a sonication for 18 minutes.The substrate was then dried in an oven to ensure that all surface impurities were eliminated.Solutions containing 20.0 g/L of TiO2 and TiO2:ZnO nanocomposite at varying concentrations (90TiO2:10ZnO, 70TiO2:30ZnO, and 50TiO2:50ZnO) were created.The mixtures were constantly stirred until uniform and white in color.After that, glass substrates were coated with TiO2 and a TiO2:ZnO precursor solution using the spin-coating process at a 3000 rpm deposit speed.The films were heated at 100 °C for 10 minutes after each layer of coating to eliminate organic compounds from the structure.This procedure was repeated until the appropriate thickness was reached (up to 3 layers).The coated glass substrates were then annealed for one hour at 300 o C.

Characterization
The elemental mapping was determined using a Field Emission Scanning Electron Microscope (FESEM) (model TESCAN MIRA3 FRENCH) connected to an EDX device to get crosssectional pictures and top views of the samples.The structural characteristics of prepared samples were determined using the X-ray diffraction technique (XRD 6000) (SHIMADZU Japan).We employed Hall effect measurements to learn about the semiconductor's quality, the carrier's concentration and mobility, and the distance factor at room temperature, as a device (HMS-3000 equipped with ECOPIA Co. Korea), was used to determine the charge carriers and compute the RH coefficient.

Results and Discussion
Figure 1 shows top-view FESEM images with different scales bare for TiO2, and TiO2:ZnO nanocomposite with different concentrations (90 TiO2:10ZnO, 70TiO2:30ZnO, and 50TiO2:50ZnO).These photos demonstrated highly homogeneous surfaces with diameters ranging from 50.08, 48.97, 40.55, and 31.58nm,respectively, this means that all TiO2:ZnO oxide materials that have been examined have nonmetric particles that are less than 100 nm.All samples contain almost spherically shaped particles with a high tendency to agglomerate, according to the FESEM images.Field Emission Scanning Electron Microscope (FESEM) images show that the ZnO concentration has a considerable impact on the final nanocomposite's shape.These findings are in agreement with those of other researchers [16].To determine the correct quantity of materials contained in samples, chemical composition analysis is very necessary.Figure 2 explains Ti, O, and Zn elements in TiO2 and 50TiO2:50ZnO using Energy dispersive X-ray spectroscopy (EDX) analysis linked with the FESEM device.This analysis shows that the atomic ratio of Ti is 40.10% and 26.63% for O for the TiO2 sample.In addition, the atomic ratio for the 50TiO2:50ZnO sample was 23.99, 20.14, and 13.13% for T, O, and Zn respectively.The results of XRD analysis prove that ZnO concentrations affect the crystalline structure of the resulting binary oxide system.
Hall effect measurements at room temperature of TiO2 and TiO2:ZnO nanocomposite reveal the kind of majority charge carriers, carrier mobility, and electric charge density for all films.
According to table 1, the concentration of carriers reduced as the concentration of ZnO, with exception of sample 70:30, and the mobility of carriers increased.The primary causes of changes in mobility and concentration carriers are growth in grain boundaries and the emergence of crystal lattice distribution, which causes an increase in the scattering center for impurities.When ZnO is added to TiO2 NPs, the mobility carrier increases, which might improve the crystallinity of films.The gas sensing characteristics of pure TiO2 and TiO2:ZnO films on a glass substrate with various concentration ratios are investigated as a function of operating temperature and (NO2) gas exposure time.The oxidation and reduction process results in a change in gas sensor resistance (decrease or increase).Figures 4-7 show the resistance changes in TiO2, ZnO, and TiO2:ZnO samples.An increase in resistance when the films are exposed to NO2 gas (Gas ON), then a decrease in resistance when the gas is turned off (Gas OFF).The following reason for this behavior: NO2 gas molecules are adsorbed on surfaces TiO2, ZnO, and TiO2:ZnO when exposed to the gas.The consumption of electrons by each NO2 molecule during its reaction with the surface of TiO2 and TiO2:ZnO, leads to a reduction in conductivity and NO2 molecules oxidized the surfaces and increase the surface resistance, which can be used for sensing NO2 gas [9].To determine the operating temperature of the sensor, where the operational temperature of a sensor is defined as the temperature at which its resistance remains constant.Changes in resistance are only influenced by the presence of a certain amount of gas of interest [10].

Conclusion
The spin-coating technique was used to deposit very homogeneous TiO2 nanoparticles with varying ZnO concentrations on a glass substrate.Field Emission Scanning Electron Microscope (FESEM) images revealed that samples had a smooth surface and a spherical shape.All samples contained symmetric ZnO wurtzite hexagonal structures, which was demonstrated by the X-ray diffraction investigation and suggested the presence of a highly crystalline ZnO nanostructure.On pure TiO2, ZnO, and TiO2:ZnO materials, gas sensor measurements against NO2 gas show that the optimal operating temperature is at 200 o C and diminishes with rising temperatures over 300 o C. The mobility carrier rises when ZnO is added to TiO2 NPs, which may enhance the crystallinity of films.The results indicate that NO2 gas sensor performances improved due to an increase in the defect density and generate an impurity state within the energy band structure of ZnO which enhances the adsorption site.The main reason for the choice of this bilayer system is the different behavior of TiO2 when the ZnO layer is deposited on the glass/TiO2 film.

Figures 8 -
Figures 8-10 show the response time and the sensitivity% with different operating temperatures of pure TiO2 and TiO2:ZnO thin films with different concentrations and the variation of response time and recovery time versus working temperature of pure TiO2 and TiO2:ZnO with different concentrations prepared through spin coating techniques respectively.

Figure 8 .Figure 9 .
Figure 8.The variation of response time with the working temperature of TiO2 and different concentrations of TiO2:ZnO

Table 1 .
The concentration and mobility carriers

Table 2 shows
high so the material takes a long time and the same conduct exists in 50TiO2:50ZnO and 70TiO2:30ZnO with the highest sensitivity of 56.48% and 60.39% respectively.
sensitivity%, Response, and Recovery time at different temperatures for TiO2 and TiO2:ZnO of samples prepared by spin coating.Temperatures of (150°C, 200°C, 250°C, and 300°C) were used to investigate gas sensitivity.At 300°C, it demonstrated the highest sensitivity for all samples except 90TiO2:10ZnO the highest sensitivity at 250°C.The highest sensitivity values are 190.06% for the 90TiO2:10ZnO sample prepared.Also, the electrical conductivity of the material is

Table 2 .
Sensitivity%, Response, and Recovery time of different temperatures for pure TiO2 and TiO2:ZnO at different concentrations