Impact of Ag on the Limit of Detection towards NH3-Sensing in Spray-Coated WO3 Thin-Films

Ag-doped WO3 (Ag–WO3) films were deposited on a soda-lime glass substrate via a facile spray pyrolysis technique. The surface roughness of the films varied between 0.6 nm and 4.3 nm, as verified by the Atomic Force Microscopy (AFM) studies. Ammonia (NH3)-sensing measurements of the films were performed for various concentrations at an optimum sensor working temperature of 200 °C. Enrichment of oxygen vacancies confirmed by X-ray Photoelectron Spectroscopy (XPS) in 1% Ag–WO3 enhanced the sensor response from 1.06 to 3.29, approximately 3 times higher than that of undoped WO3. Limit of detection (LOD) up to 500 ppb is achieved for 1% Ag–WO3, substantiating the role of Ag in improving sensor performance.


Introduction
Various types of gas sensors, including chemoreceptive-, ionization-, acoustic-, and resonant-based sensors are gaining more attention in different sectors, such as environmental monitoring, food safety, medical diagnosis, and industrial applications [1][2][3]. Among these, chemoreceptive-based metal oxide semiconductors (MO x ) have received tremendous interest in gas sensing due to their high surface/volume ratio, as the gas reaction process is a surface phenomenon. Different MO x , such as ZnO, SnO 2 , WO 3 , and TiO 2 [4,5] have been extensively studied for gas-sensing applications. Tungsten oxide (WO 3 ) has emerged as a potential MO x to detect the gases, such as NH 3 , H 2 , NO 2 , CO, and alcohol vapors, because of their inherent properties, including excellent electrical conductivity, sensitivity, and selectivity [6,7]. To enhance the sensing performance further, doping is considered one of the possible approaches. Transition metal doping, such as Cr, Cu, Ag, and Pd, would induce the defects causing enrichment of oxygen vacancies via hole compensation mechanism. These metals act as promoters and increase the sensing properties via spill-over effect or Fermi-level mechanisms [8][9][10]. Godbole et al. [11] reported the NH 3 -sensing properties of Pd/WO 3 films with the response of 0.27 at a working temperature of 225 • C. Lu et al. [12] presented studies on NO 2 -sensing properties of Ag-WO 3 nanoparticles and reported high sensitivity and better selectivity. Xu et al. [13] obtained a superior sensor response for Ag-WO 3 core-shell nanostructures towards alcohol vapor at a

Experimental
Undoped and silver-doped tungsten oxide films (Ag-WO 3 ) were synthesized by the spray pyrolysis method. For undoped WO 3 , ammonium metatungstate hydrate (99.99% purity) is dissolved in double-distilled water and a homogeneous solution is obtained. For Ag doping, silver nitrate was taken as a precursor, and doping was done at a 1 wt.%, 3 wt.%, and 5 wt.% ratio. Solution concentration was maintained at 0.01 M. Deposition parameters, such as substrate temperature and flow rate, were kept constant at 400 • C and 1 mL/min, respectively.
Crystal structure and phase identification were performed via Rigaku SmartLab X-ray diffractometer with Cu Kα radiation at 40 kV, 30 mA. Raman analysis was performed using a Horiba JOBINYVON LabRAM HR spectrometer for the confirmation of structure. Morphological studies were performed via Innova SPM Atomic Force Microscope (AFM). AXIS ULTRA X-ray Photoelectron Spectroscope is used for oxidation state and composition studies. Gas sensing was conducted via dc probe measurements in an enclosed chamber (2.96 × 10 4 cm 3 by vol.) by purging synthetic air (79% N 2 + 21% O 2 ) and NH 3 to the sample. The flow of gas was controlled using programmable mass flow controllers (MFCs), and the overall flow was kept constant at 500 sccm. I-V measurements were performed via Keithley source meter 2450 using silver paste electrodes. Sensor response of the films was evaluated using the equation, ( R a −R g R g ). R a and R g suggest the film resistance in air and target gas (NH 3 ), respectively. Schematic representation of film synthesis and ammonia-sensing measurements of Ag-WO 3 film is represented in the Figure 1.   [12,18]. The appearance of sharp diffraction peaks implies that the deposited films have high crystallinity. The absence of any further peaks in the spectra endorsed the nonexistence of any impurity phase in the prepared Ag-WO3 films. All the films exhibited 'a' axis preferential orientation, i.e., along (200) plane. Evolution of (020) peak centered at 23.4° is observed upon addition of Ag into WO3. The introduction of Ag formed new nucleating centers for WO3, ultimately retaining monoclinic structure. The crystallite size of the films was determined by employing the Scherrer formula [19] and it varies from 11.5 nm to 14.7 nm. The dislocation density and microstrain [19] were also evaluated and are presented in Table 1.   [12,18]. The appearance of sharp diffraction peaks implies that the deposited films have high crystallinity. The absence of any further peaks in the spectra endorsed the non-existence of any impurity phase in the prepared Ag-WO 3 films. All the films exhibited 'a' axis preferential orientation, i.e., along (200) plane. Evolution of (020) peak centered at 23.4 • is observed upon addition of Ag into WO 3 . The introduction of Ag formed new nucleating centers for WO 3 , ultimately retaining monoclinic structure. The crystallite size of the films was determined by employing the Scherrer formula [19] and it varies from 11.5 nm to 14.7 nm. The dislocation density and microstrain [19] were also evaluated and are presented in Table 1.

Structural and Morphological Analysis
Since triclinic and monoclinic phases of WO 3 exhibit the same set of XRD peaks, we have conducted Raman measurements for further confirmation [20,21]. Figure 3 illustrates the Raman spectra of Ag-WO 3 films excited with the 532 nm laser source. Clear visibility of peaks at~112 cm −1 ,~134 cm −1 ,~270 cm −1 ,~325 cm −1 ,~719 cm −1 ,~806 cm −1 , and 963 cm −1 , attributed to the different modes of vibrations in the WO 3 lattice. Peaks corresponding to~719 cm −1 and~806 cm −1 represent the asymmetric and symmetric stretching vibrational modes (ν as and ν s ) of O-W-O bonds and these are often referred to as the strongest monoclinic WO 3 modes [22]. Peaks located at~270 cm −1 and~325 cm −1 attribute to the O-W-O bending vibrational modes (δ) and peaks below 200 cm −1 contribute to the lattice vibrational modes [23]. All the films exhibit a peak at~963 cm −1 , which can be ascribed to the symmetric stretching vibration of terminal W=O bonds possibly associated with the clusters on the film surface [24,25]. The frequency of vibration of the W=O bond is predicted to be higher than that of the W-O bond since the W-O single bond is weaker than the W=O double bond. Raman measurements confirmed the monoclinic phase of Ag-WO 3 films. Since triclinic and monoclinic phases of WO3 exhibit the same set of XRD pe have conducted Raman measurements for further confirmation [20,21]. Figure 3 ill the Raman spectra of Ag-WO3 films excited with the 532 nm laser source. Clear v of peaks at ~112 cm −1 , ~134 cm −1 , ~270 cm −1 , ~325 cm −1 , ~719 cm −1 , ~806 cm −1 , and ~9 attributed to the different modes of vibrations in the WO3 lattice. Peaks correspon ~719 cm −1 and ~806 cm −1 represent the asymmetric and symmetric stretching vib modes (νas and νs) of O-W-O bonds and these are often referred to as the stronges clinic WO3 modes [22]. Peaks located at ~270 cm −1 and ~325 cm −1 attribute to the bending vibrational modes (δ) and peaks below 200 cm −1 contribute to the lattic tional modes [23]. All the films exhibit a peak at ~963 cm −1 , which can be ascribe symmetric stretching vibration of terminal W=O bonds possibly associated with t ters on the film surface [24,25]. The frequency of vibration of the W=O bond is pr to be higher than that of the W-O bond since the W-O single bond is weaker t W=O double bond. Raman measurements confirmed the monoclinic phase of A films.  Figure 4a-d shows the topography and microstructures (3D view) of Ag-WO 3 films examined by AFM in tapping mode configuration. All the samples were scanned in the area of 0.5 × 0.5 µm 2 . Figure 4b represents the topographical view of 1% Ag-WO 3 . It suggests the coalescence of grains upon Ag incorporation forming larger globules of particles with the presence of voids. At 3% and 5%, Ag doping ( Figure 4c,d), well-separated smaller grains are visible with almost uniform distribution. RMS surface roughness of the films was evaluated using NanoScope Analysis software and given in Table 1. Upon Ag doping, the roughness value of the films varied, and for 1% Ag-WO 3 higher value of roughness was recorded.

XPS Studies
Elemental composition and chemical state of Ag-WO 3 films (at 0% and 1% Ag conc.) were examined via the XPS technique. Figure 5a shows the deconvoluted spectra of W 4f split into spin-orbit doublet, namely W 4f 7/2 and W 4f 5/2 located at the binding energies (E b ) of 35.9 eV and 38.1 eV, respectively, in undoped WO 3 film. These represent the +6 oxidation state of W and another satellite peak at 40.5 eV indicates the W 5p 3/2 component corresponding to the same oxidation state [26]. The incorporation of Ag into WO 3 has led to the formation of +6 and +5 oxidation states of W as depicted in Figure 5b. Deconvolution of W 4f in Figure 5b has resulted in two pairs of doublets viz. one at 34.2 eV and 38.4 eV equivalent to W 4f 7/2 and W 4f 5/2, respectively, for the +6 oxidation state of tungsten, and other pairs of doublets comprised of W 4f 7/2 and W 4f 5/2 centered at 32.4 eV and 36.5 eV are comparable to the +5 oxidation state of tungsten [27]. The peak positioned at 30.6 eV was assigned to the metallic tungsten [28]. If oxygen vacancy is present, electron density near neighboring W atoms intensifies, causing higher screening of its nucleus and consequently, the 4f energy level is predicted to be at lower E b [23,27]. In the present studies, the shoulder associated with the W 4f is generated as a result of electrons emitted from W atoms near oxygen vacancies, and hence W atom has an oxidation state less than +6, resulting in the formation of sub-stoichiometric WO 3−x .

XPS Studies
Elemental composition and chemical state of Ag-WO3 films (at 0% and 1% Ag conc.) were examined via the XPS technique. Figure 5a shows the deconvoluted spectra of W 4f split into spin-orbit doublet, namely W 4f7/2 and W 4f5/2 located at the binding energies (Eb) of 35.9 eV and 38.1 eV, respectively, in undoped WO3 film. These represent the +6 oxidation state of W and another satellite peak at 40.5 eV indicates the W 5p3/2 component corresponding to the same oxidation state [26]. The incorporation of Ag into WO3 has led to the formation of +6 and +5 oxidation states of W as depicted in Figure 5b. Deconvolution of W 4f in Figure 5b has resulted in two pairs of doublets viz. one at 34.2 eV and 38.4 eV equivalent to W 4f7/2 and W 4f5/2, respectively, for the +6 oxidation state of tungsten, and other pairs of doublets comprised of W 4f7/2 and W 4f5/2 centered at 32.4 eV and 36.5 eV are comparable to the +5 oxidation state of tungsten [27]. The peak positioned at 30.6 eV was assigned to the metallic tungsten [28]. If oxygen vacancy is present, electron density near neighboring W atoms intensifies, causing higher screening of its nucleus and consequently, the 4f energy level is predicted to be at lower Eb [23,27]. In the present studies, the shoulder associated with the W 4f is generated as a result of electrons emitted from W  [29,30]. Area ratio of oxygen vacancies is estimated from peak area calculations as given below: V o for WO 3 and 1% Ag-WO 3 was found 35% and 45%, respectively. Hence, the studies inferred that oxygen vacancies increased by 10% in 1% Ag-WO 3 . The least intense peaks centered at 534.7 eV and 532.8 eV, respectively in WO 3 and 1% Ag-WO 3 , connected to the adsorbed oxygen species (O − 2 , OH − ) [30]. Figure 5e represents the characteristic peaks of Ag at E b of 368 eV and 374.5 eV contributes to Ag 3d 5/2 and Ag 3d 3/2 , respectively, in 1% Ag-WO 3 . These peaks are assigned to the Ag 0 /Ag 1+ states of silver [28,31]. atoms near oxygen vacancies, and hence W atom has an oxidation state less than +6, resulting in the formation of sub-stoichiometric WO3-x.

Gas Sensing Properties
Operation temperature for WO 3 towards NH 3 was fixed by purging gas at various temperatures and calculating subsequent sensor response as presented in Figure 6a,b. Figure 6a represents the obtained graph for sensor current versus time wherein 5.03 ppm NH 3 is purged at 5 different temperatures: 100 • C, 150 • C, 175 • C 200 • C, and 250 • C. An increase in temperature causes an increase in the sensor current, demonstrating the semiconducting nature of the WO 3 films. At 100 • C, the graph shows a straight line, denoting no response for NH 3 due to the low thermal energy of the gas required for the chemisorption process. Low response was noted at 150 • C and 175 • C. An increase in the temperature to 200 • C has shown maximum response, and thereafter the response has decreased as indicated in Figure 6b. With the enhancement in the temperature, WO 3 started to respond, as enough thermal energy is provided for the surface reaction to occur by overcoming the activation energy barrier [32]. After the maximum response, a reduction in the sensing performance at 250 • C is due to the lower adsorption capability of gas molecules. In addition, it is observable that baseline stability was lost above 200 • C (Figure 6a). Hence, 200 • C is considered a suitable operating temperature for all the deposited films, and further studies were conducted at the same temperature.  Sensor response, rate of response, and recovery (τ res and τ rec ) are the essential parameters that determine the efficiency of any sensor. Figure 7a-d depicts the transient response curves of Ag-WO 3 films (0%, 1%, 3% and 5% Ag conc.) for different NH 3 concentrations. Figure 7e shows the variation of sensor response with the NH 3 concentration. The sensor response for 5 ppm NH 3 was found to be 1.06, 3.29, 0.34, and 0.62 with the standard error of ±0.07 for undoped, 1%, 3%, and 5% Ag-doped WO 3 films, respectively. The highest response was recorded for 1% Ag-WO 3 with the τ res of 5.5 min and τ rec of 7.9 min. Doping of Ag reduced the detection limit from 1 ppm to 0.5 ppm (500 ppb) in 1% Ag-WO 3 . 'Ag', being a noble metal, acts as a catalyst due to chemical sensitization and also causes electronic sensitization due to the interaction between Ag and WO 3 , which might have induced the enhancement in the sensor response of the WO 3 film [9]. AFM studies revealed that 1% Ag-WO 3 showed higher surface roughness value (Table 1) compared to other Ag concentrations. Surface roughness is one of the parameters that can increase the surface to volume ratio of the films and thereby enhances the gas adsorption capacity [33]. M. Kumar et al. [34] and J. M. Lee et al. [33] observed similar results for CO and H 2 -gas sensing, respectively. Augmentation in the sensing performance of 1% Ag-WO 3 could be accredited to improved oxygen vacancies upon Ag doping, which is confirmed by XPS analysis and also higher surface roughness and void formation on the film surface is proven by AFM, providing a greater number of adsorption centers for NH 3 [22,35]. Nevertheless, a decrease in sensor response beyond 1% Ag doping is connected to lower surface roughness of the films and catalytic efficiency of Ag, indicating that while Ag inclusion improves sensing performance, increasing the doping concentration above the optimum level may reduce catalytic efficiency [27]. Also, XPS revealed the possibility of Ag 2 O formation upon Ag doping. When Ag concentration exceeds 1%, Ag 2 O amounts might increase and cause an increase in the depletion layer width, deteriorating the sensor response of the films.
Selectivity and repeatability are the key factors that decide the performance of the sensors. Figure 8 represents the bar graph elucidating sensor responses of WO 3 and 1% Ag-WO 3 films towards various gases. These films are tested at a 5-ppm concentration towards NH 3 , CO, CH 4, and NO 2 . Both films showed the highest response to NH 3 indicating the selective nature of the deposited films towards ammonia among other gases. Repeatability measurements of about 5 cycles were performed for both WO 3 and 1% Ag-WO 3 films, shown in Figure 9a,b. Almost repeatable sensing characteristics were obtained 5 times for NH 3 purge at 5 ppm concentration, implying the stable response of the films. In comparison, sensing properties of the current work with the previously reported literature is given in Table 2. The WO 3 nanoflakes synthesized via spray pyrolysis detected the lowest NH 3 concentration to be up to 120 ppm at 150 • C [36], while the V-WO 3 films synthesized by soft chemical route exhibited a detection limit of 100 ppm towards NH 3 at 300 • C [37]. The Cr-WO 3 nanosheets synthesized by acidification with impregnation process detected 2 ppm NH 3 at 400 • C [38], whereas the WO 3 -Fe 2 O 3 nanocomposites by hydrothermal synthesis demonstrated NH 3 detection of 25 ppm at 300 • C [39]. In the present work, we were able to achieve lowest detection limit of 500 ppb towards NH 3 gas, keeping an operating temperature of 200 • C, which is a significant improvement in the sensing performance compared to the literature presented in Table 2.
The well-known gas detection mechanism of metal oxide gas sensors involves the resistance variations caused by the chemisorption of gas molecules on the sensor surface. Depending on the operating temperature, WO 3 exposure to the synthetic air produces molecular/atomic oxygen ions (O − 2 , O − , and O 2− ). Electron transfer from the surface of the WO 3 to the adsorbed oxygen species creates a band bending region called the depletion layer, resulting in a decrease in the carrier concentration of the film and an increase in the resistance. Later, when WO 3 is subjected to NH 3 exposure, the depletion layer width decreases due to the release of electrons back to the WO 3 . As Ag-WO 3 is exposed to NH 3 , the depletion layer further decreases due to the electronic sensitization mechanism giving rise to an increment in carrier concentration and a reduction in surface resistance. Figure 10 illustrates the schematic representation of the sensing mechanism. Specific reactions involved in the process are governed by the equations given below [14,35]: revealed that 1% Ag-WO3 showed higher surface roughness value (Ref. Table 1) compared to other Ag concentrations. Surface roughness is one of the parameters that can increase the surface to volume ratio of the films and thereby enhances the gas adsorption capacity [33]. M. Kumar et al. [34] and J. M. Lee et al. [33] observed similar results for CO and H2-gas sensing, respectively. Augmentation in the sensing performance of 1% Ag-WO3 could be accredited to improved oxygen vacancies upon Ag doping, which is confirmed by XPS analysis and also higher surface roughness and void formation on the film surface is proven by AFM, providing a greater number of adsorption centers for NH3 [22,35]. Nevertheless, a decrease in sensor response beyond 1% Ag doping is connected to lower surface roughness of the films and catalytic efficiency of Ag, indicating that while Ag inclusion improves sensing performance, increasing the doping concentration above the optimum level may reduce catalytic efficiency [27]. Also, XPS revealed the possibility of Ag2O formation upon Ag doping. When Ag concentration exceeds 1%, Ag2O amounts might increase and cause an increase in the depletion layer width, deteriorating the sensor response of the films. Selectivity and repeatability are the key factors that decide the performance of the sensors. Figure 8 represents the bar graph elucidating sensor responses of WO3 and 1% Ag-WO3 films towards various gases. These films are tested at a 5-ppm concentration towards NH3, CO, CH4, and NO2. Both films showed the highest response to NH3 indicating the selective nature of the deposited films towards ammonia among other gases. Repeatability measurements of about 5 cycles were performed for both WO3 and 1% Ag-WO3 films, shown in Figure 9a,b. Almost repeatable sensing characteristics were obtained 5 times for NH3 purge at 5 ppm concentration, implying the stable response of the films. In comparison, sensing properties of the current work with the previously reported literature is given in Table 2. The WO3 nanoflakes synthesized via spray pyrolysis detected cess detected 2 ppm NH3 at 400 °C [38], whereas the WO3-Fe2O3 nanocomposites by hydrothermal synthesis demonstrated NH3 detection of 25 ppm at 300 °C [39]. In the present work, we were able to achieve lowest detection limit of 500 ppb towards NH3 gas, keeping an operating temperature of 200 °C, which is a significant improvement in the sensing performance compared to the literature presented in Table 2.  cess detected 2 ppm NH3 at 400 °C [38], whereas the WO3-Fe2O3 nanocomposites by hydrothermal synthesis demonstrated NH3 detection of 25 ppm at 300 °C [39]. In the present work, we were able to achieve lowest detection limit of 500 ppb towards NH3 gas, keeping an operating temperature of 200 °C, which is a significant improvement in the sensing performance compared to the literature presented in Table 2.    O + e → 2O (3) 4NH3(g) + 5O → 4NO + 6H2O + 5e - (4) 2NH3(g) +3O →N2 +3H2O +3e -(5) Figure 10. Sensing mechanism involved between the grains of WO3 and Ag-WO3 films.

Conclusions
Spray pyrolyzed Ag-WO3 films were investigated for NH3 sensing at different concentrations. XRD spectra depicted the monoclinic phase of deposited films, which was further verified by Raman analysis. Oxygen vacancies, higher surface roughness, and void-like structures of 1% Ag-WO3 contributed to enhancement in the sensor response value. Selectivity studies of WO3 and 1% Ag-WO3 towards NH3, NO2, CH4, and CO exhibited an excellent response to NH3 compared to other gases. Spray deposited Ag-WO3, as a unique approach for NH3 sensing, produced a superior response at a low operating temperature of 200°C with a detection limit in the sub-ppm levels.

Conclusions
Spray pyrolyzed Ag-WO 3 films were investigated for NH 3 sensing at different concentrations. XRD spectra depicted the monoclinic phase of deposited films, which was further verified by Raman analysis. Oxygen vacancies, higher surface roughness, and void-like structures of 1% Ag-WO 3 contributed to enhancement in the sensor response value. Selectivity studies of WO 3 and 1% Ag-WO 3 towards NH 3 , NO 2 , CH 4, and CO exhibited an excellent response to NH 3 compared to other gases. Spray deposited Ag-WO 3 , as a unique approach for NH 3 sensing, produced a superior response at a low operating temperature of 200 • C with a detection limit in the sub-ppm levels.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.