CHARACTERIZATION OF WO 3 THIN FILMS DEPOSITED BY SPRAY PYROLYSIS TECHNIQUE AND ITS ROLE IN GAS SENSING

The work investigated in this paper focused on the fabrication of WO 3 films by the spray pyrolysis technique, and different analyses were made to find optimized samples for studying properties suitable for the application of gas sensing. The substrate temperature is the most important parameter among other spray parameters for the synthesis of thin films hence WO 3 thin films were deposited on glass substrates by maintaining the substrate temperature at 350 °C, 450 °C, 550 °C, and 650 °C using compressed air as a carrier gas. The influence of the substrate temperature on the structural, morphological, compositional, and optical properties of the WO 3 thin films has been justified using XRD data. Good and enhanced crystallinity is observed for the film deposited at a substrate temperature of 550 °C. The nonconventional properties were studied by different investigations and confirmed by past research work. The manipulation of surface morphology with the different deposition temperatures is monitored. Only the characteristic peaks of W and O are present in the fabricated WO 3 thin films. The optical activity of about 70 to 80 % of the selected sample in the visible region (300 to 1200 nm) is found. The selective absorption activity of light in the ultraviolet region and visible region is checked. The obtained IR bands confirmed the inter bridge stretching and bending modes of W-O and O-W-O. A high response towards ammonia compared to other test gases is exhibited. The repeatability of WO 3 towards NH 3 over three periodic sensing cycles, response, and recovery time has also been discussed. From all the characteristic studies, it has been suggested that the fabricated WO 3 thin films have been used in the health care field to detect the toxic NH 3 gas.


Introduction
In recent years, it is very important that, due to the development of populations, several sources are causing a huge impact on environmental and health issues. In this scenario, the attempt was made to synthesize WO 3 thin films at different substrate temperatures (350 °C, 450 °C, 550 °C, 650 °C) using the Spray Pyrolysis Deposition technique for gas sensing applications.

1. Synthesis of WO 3 thin Films
From the aqueous solutions containing 0.5M of WO 3 powder as a precursor, WO 3 thin films were deposited using compressed air as a carrier gas by the electrospray pyrolysis technique and the electrospray pyrolysis setup is shown in Fig. 1. A syringe needle is connected to a solution adjustable reservoir in the electrospray apparatus, and it is set aside at a distance of 8 cm from the reservoir to make sure that the pressurized mist spreads the hot substrate appropriately. To optimize the thickness, a different volume of precursor solution was sprayed and found to have a 500 nm film thickness. A trial-and-error method was used to verify the thickness of all samples. The substrate holder and syringe needle are connected to a high-voltage DC power supply, and the substrate holder is attached to a hot plate. Four sets of WO 3 thin films were deposited by maintaining the substrate temperatures at 350 °C, 450 °C, 550 °C, and 650 °C respectively. The optimal time of deposition, including the spray intervals for each sample, was found to be 44 min. The intermittent spray procedure was employed for 5 s and an interval of 10 s for each cycle.

Characterization Techniques
An X-ray diffractometer (a SHIMADZU-XRD 6000 diffractometer system with an X-ray (Cu-Ka) wavelength of 1.5406) was used to examine the structural properties of the prepared WO 3 thin films. The SEM images were obtained by employing a scanning electron microscope (CARL ZEISS). XPS spectra are obtained by the K-Alpha X-ray Photoelectron spectrometer. The optical absorption and transparency of the films were measured in the wavelength range of 300 nm to 1100 nm using a Unico 4805 UV-Vis double beam spectrophotometer. The FTIR spectra were recorded using the SHIMADZU 1800-UV Fourier transform infrared spectrometer. Gas sensing measurements are made by a Keithley electrometer. crystal structure. When looking at the XRD peaks at 350 °C and 450 °C, the peaks is found to be less intense, which is may be due to the poor agglomeration of the sub-planes of the crystal. This is mainly accomplished by the insufficient thermal energy to configure such a crystal. According to the previous study [16], it may also be due to the participated precursor being unable to work perfectly with electrochemical energy because also, the WO 3 crystal was aligned perfectly at 250 °C. The peak intensity of the sample at 550 °C was found to be high along with well-distributed and discrete signals. The respective peaks were observed at 23 °C, 24 °C, 26 °C, 30 °C, 34 °C, 42 °C, 52 °C, and 57 °C from (002), (020), (200), (112), (202), (222), (140), and (420) respectively. Except for the above peaks, there are several peaks observed that confirm an axis of a twofold symmetry system, and they are all assigned to the respective planes. Even though multiple peaks on XRD show an unpleasant crystal lattice atmosphere, the improved crystallinity behaviour is obtained at 550 °C. Some of the peaks with minimum intensity were observed which may be due to the occurrence of reciprocity in the crystal lattice structure. Usually, a reciprocal lattice is produced in the crystal due to the sharp subplanes and here, a similar trend is observed. Hence, at 650 °C the XRD peaks were observed to be analogous to those at 550 °C with minimum intensity. But the minimised energy peak showed a decomposed atmosphere of the crystal lattice. This condition in the XRD graph clearly shows the required thermal energy limitation for WO 3 materials. If the material has some dopant, the operating temperature may be changed. So, it is finally concluded that the optimized temperature of the WO 3 material is 550 °C.

2. Particle size determination
The particle size of W is determined as usual by Debye-Scherrer's formulae for the active specific lattice. The data from the X-ray analysis are listed in Table 1 to confirm the pattern of WO 3 thin film deposited temperatures at various substrate temperatures with calculated h, k, and l indices of the predominant peak in the (200) plane. The corresponding peak positions are 23.86, 23.78, 23.92, and 24.22, and the average particle size is calculated from a diffracted peak at 32.857 nm. At the initial state of temperature of 350 °C, the particle size is found to be 39 nm, which is moderate when compared with the literature. As the temperature is increased to 450 °C, Original Research Article: full paper (2022), «EUREKA: Physics and Engineering» Number 4 the particle size is found to be 36 nm, which is smaller than the previous one, and it is further decremented to 26 nm when the temperature is further increased to 550 °C. At this point of temperature, the reduction of size is stopped, and concerning WO 3 , this is the λ point. Beyond this point, the nanoparticle efficiency is usually reduced, and this atmosphere takes place in this case also. In the case of microstrain, it increased from 350 °C to 550 °C and decreased further to 650 °C. It is very useful to explain the great order of crystallinity, and the good order is confirmed at 550 °C. For lattice strain, it is increased like microstrain, and normally it should be the opposite. This is obtained as a range between 0.004 and 0.0065, and, further, as 0.0058, about 650 °C. The d-spacing fluctuates in the range of 0.0122 to 0.0214 at 100 °C, which is due to the adverse effect of dynamic bonding forces, and it can be operated by a magnetic field also.

SEM Examination
The SEM micrographs of WO 3 films deposited at various substrate temperatures (from 350 °C to 650 °C) are shown in Fig. 3. Form the SEM figure, it was clear that the surface morphology is found to vary with the different annealing temperatures. Initially, the sample at 350 °C, the surface morphology of the film was very rough and strapped in shape. In addition to that, some of the particles found with clear grain boundary. Similarly, the sample at 450 °C, the spherical shape particles appeared which abruptly explicit disorder of amalgamation whereas for the sample at 550 °C, well-shaped spherical particles are appeared and granulation with uniform distributed size morphology is observed. Finally, as the substrate temperature for the sample is raised to 650 °C, uneven morphology with an increase in grain size and a more porous surface is observed. The elevated and annealing energy beyond the limit enable the coalescence of the adjacent minute crystals, and therefore bulky surface grain boundary is determined [17]. So, the annealing temperature of 550 °C is the optimized annealing energy to enhance crystallinity. In addition to that, the drastic surface change was observed at every 100 °C of the material and the present nano material is highly sensitive even for every 5 °C of fluctuating temperature.

4. XPS analysis
Using XPS measurements, the compositional characterization of WO 3 thin films deposited on the glass substrates is firmly decided. Fig. 4 shows the XPS survey scan spectra of WO 3 thin film in the binding energy range of 0 to 1200 eV. The survey scan spectra are principally valuable for the recognition of the elements existing on the film's surface and inner core. The XPS spectra are the strong representation of partial elements of the WO 3 thin film, such as W and O. The first peak at 550 explores the presence of O, which is represented by the 1S orbital system. Furthermore, for as-deposited WO 3 thin films, to describe the ratio of O to W, the detailed scan spectrum is obtained for W4f and O1s. The spectra of W displayed in the figure show two main peaks ascribed to W4f7/2, and W4f5/2, which are positioned at 35.3 and 37.5 eV. The binding energy difference owing to spin-orbit splitting of W4f between W4f7/2 and W4f5/2 is found to be 2.20 eV. The XPS spectra of the O1s peak shown in the figure consist of two components: one at a binding energy of 530 eV, assigned to the oxygen in WO 3 films, and a smaller one at 532 eV, assigned to water molecules adsorbed on the film's surface [18]. The figure details the energy dispersion of W nanoparticles, which also reveals the existence of W nanoparticles in different spinning phases at molecular sites. The first spatial extension is represented by W 5S , the second is W d5 , the third is W d3 , the fourth is W P3 , and the fifth is W P1 . All the energy levels are obtained in different energy ranges, and all the energy ranges confirm the different energy bonding of WO 3 material in a thin film.

5. Optical Properties
The optimized optical mechanism of WO 3 thin films at various substrate temperatures is depicted in Fig. 5. From the recorded spectra, it is observed that the transparency in the visible region ranges from 450 nm to 1200 nm with an optical transmittance of about 70 to 80 %. Transmittance increases with substrate temperature from 350 °C to 550 °C and then decreases at 650 °C, as expected. The maximum transmittance was obtained at 550 °C and this also showed the universal optimized annealing temperature range of WO 3 nanomaterial. The uniform oxygen vacancy generation over the surface-atmosphere demonstrates good light absorbance. The higher transmittance also confirmed the good crystalline nature of the material and that it is free from rational defects [19,20].
The obtaining of broad bands in the optical absorption spectra of WO 3 is attributed to its wide optical absorption. Its coefficient of absorption is increased from low temperature to high temperature and it is stopped at 550 °C. From the figure, it is very clear that the absorption is ini tiated at 450 nm at 350 °C, whereas at 550 °C, the absorption efficiency started from 300 nm, which covers the UV region also. If the nanomaterial is efficient in the UV region, it will be more reactive for the assigned applications [21]. Here the same trend has appeared to explore the presently prepared nanomaterial.
The construction of visible region-driven semiconductor photocatalysts in nanomaterial is a very difficult process, and it is a very important issue in terms of harvesting photon energy for energy conversion. Usually, metal oxide nanomaterials have low efficiency and deal with the reduction of the recombination effect [22][23][24]. But, here, the energy band gap values range between 2.42 eV and 2.70 eV, according to which the band gap of the present material offers an efficient way to, address these limitations [25][26][27]. The band gap energy increases from 350 °C to 550 °C and at 650 °C, the band gap (2.60 eV) is reduced. The energy gap is increased concerning the temperature and may also be due to the excessive formation of oxygen-ion vacancies in the films. The accurate band gap range is found to have opted to increase the photo efficiency. b -plot of (αhυ) 2 vs photon energy (hυ) for WO 3 thin films

6. FTIR analysis of WO 3 thin films
The presence of bonded elements in the material is usually represented by FTIR spectra of the samples and, concurrently, the spectra for all annealing temperatures are shown in Fig. 6. Since the binding energy is high for the inorganic components, the force constant will be high. The partial bonding involved in metal oxides is a very important factor that describes the elec- trostatic forces that exist between the metal and the oxygen atom. Such interatomic bonding forces are always higher than ionic bonding. Here, for the present case of WO 3 , the measured value is 18.122 a.u and it is very high for the nano crystal lattice, which is higher than bonding forces in the macro-molecular system. So, the respective frequencies of the bonds are very high when compared with the frequency observed for molecular level bonding. Usually, the IR bands appear for metal oxides in the region of 710 cm -1 to 550 cm -1 [28,29]. In particular, the IR bands for W-O stretching bands are observed in the range of 680 cm -1 to 550 cm -1 [30]. The IR spectra for all prepared samples are shown in Fig. 6 12 a.u.). The respective in-plane and out-of-plane bending modes are observed at 550 and 502 cm -1 in that order.

7. Gas-Sensing Measurements
N-H bond identification is a very significant process for sensing ammonia gas. In this process, the N-H bonds of the gas are absorbed by the sensing material, and it affects the current carrying capacity by changing its resistance parameter. If this mechanism is easily adopted by the metal oxide material in a full-fledged manner, it will act as a good sensing material. The band gap as well as the gas absorption characteristics of the present case emphasized the gas sensing efficiency. Fig. 7 shows the bar diagram of the device's response to different target gases at different temperatures. The sensing tests were performed to evaluate the selectivity of the sensor towards acetone, ammonia, ethanol, and formaldehyde; the results are portrayed in Fig. 7.
When compared to other samples, the WO 3 thin film deposited at a substrate temperature of 550 °C exhibits a high response to all gases. However, the film shows the lowest response to acetone, ethanol, and formaldehyde and exhibits the highest response to ammonia. The response of tungsten oxide to ammonia was taken for 1000 seconds, and that is shown in Fig. 8.

Material Science
When the duration of absorption increases, the response value of the sensor uniformly increases. From the trail of the response time of the sensor, the linear fit curve was drawn, and from the fitting curve, its coefficient was determined to be 0.98927. To estimate the order of detection, NH 3 gas concentration has been tested from 5 ppm to 200 ppm, as shown in Fig. 9.
Upon exposure to NH 3 gas, the resistance decreases and when exposed to air, it is raised to its maximum, which indicates the n-type sensing behavior. As the concentration increases, the resistance decreases and the result indicates that the material is capable of offering good performance in gas sensing fields. The repeatability test is carried out for a particular sample and it is displayed in Fig. 10.

8. The response and recovery time of WO 3
The response and response/recovery time are both important parameters in evaluating a gas sensor's performance. It is evident from the earlier discussion that the films annealing above 550 °C resulted in the cracking of the films. As a result, the investigation of the response of the films is limited to those annealed at 550 °C. The repeatability of WO 3 towards NH 3 over three periodic sensing cycles is shown in Fig. 10. The sensor exhibits an almost stable response upon repeated exposure to NH 3, confirming the excellent repeatability of the sensor material suitable for automotive antipollution systems. The response and response/recovery time are both important parameters in evaluating a gas sensor's performance.
High response and fast response/recovery time could make the sensor an excellent candidate for the rapid detection of hazardous gases [31]. The response and recovery times are defined as the time taken by the sensor to achieve 90 % of the total resistance change in the case of adsorption and desorption, respectively [32], and the response and recovery time under 550 °C towards 200 ppm is illustrated in Fig. 11. The response and recovery processes of the WO 3 sensing layer towards NH 3 are the results of thermally activated chemical reaction processes on the surface, with a response time of 27 s and recovery time of 77 s. This proves a very rapid process of gas absorption and response mechanism taking place in the WO 3 material. In the repetition curve, the saturation of resistance concerning time is considerably decreased. That showed the rapid response of the material to the gas absorption. Fig. 11. The response and recovery time of the WO 3 gas sensor 3. 9. Limitations and the future scope of the study Temperature plays a vital role in the deposition process of thin film deposition process, WO 3 Films deposited by means of a lower deposition temperature of below 350 °C give rise to irregular films due to the inadequate electrostatic attraction on the spray droplets hence WO 3 thin films deposited at the substrate temperature from 350 °C to 650 °C are characterized. Spray distance affects the quality of the deposited films hence the distance of the syringe needle is adjusted up and down and set aside at a distance of 8 cm from the reservoir and made sure that the pressurized mist spreads the hot substrate appropriately to obtain a continuous thin film.
In this work, the famous metal oxide WO 3 as thin film was prepared and characterized in order to study the assorted applications. The significant results have been traced abruptly and showed the improved application of metal oxide. In order to fulfill the future needs of advanced thin-film materials, the present thin-film material is to be modified further. For that, it is doped with d-block and f-block elements.

Conclusion
In this work, in order to prove the efficiency of the nano WO 3 material, the same has been prepared and the sample blended with suitable solvent coated as a thin film by the spray pyrolysis technique by maintaining the substrate temperature at 350 °C, 450 °C, 550 °C, and 650 °C, respectively. Each and every characterization analysis (morphological, structural, compositional, and optical) proved the efficiency of the prepared samples. As the substrate temperature is built to 550 °C, the crystalline nature of the film in the preferred plane orientation confirms the monoclinic phase of WO 3, and spherical-shaped particles have granulation with uniformly distributed size morphology are observed. The high-resolution XPS spectra of WO 3 exhibit two main peaks attributed to W4f7/2, and W4f5/2 centered at binding energy values of 35.3 and 37.5 eV. The efficiency is manipulated by the application of annealing energy as temperature, and all the results validated that the properties are strongly influenced by the temperature. The prepared sample has been analyzed by changing annealing energy to optimize the well crystallinity of WO 3 material for gas sensing applications. Based on the observed results, the prepared material is having good optical, chemical and physical characteristics to make seed material for manufacturing advanced sensor devices. The vibrational spectra of the samples confirm the W-O bond in the material. The in-plane and out-ofplane bending modes ensure the bond angle of O-W-O of WO 3 , which exhibits the highest response towards ammonia gas. Hence, by this work, it is suggested that the fabricated WO 3 thin films have been used in the health care field to detect toxic NH 3 gas, which causes immediate burning of the eyes, nose, throat, and respiratory tract, which results in blindness, lung damage, or even death.