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

Abstract

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.

1. 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, SnO₂, WO₃, and TiO₂ [4,5] have been extensively studied for gas-sensing applications. Tungsten oxide (WO₃) has emerged as a potential MO_x to detect the gases, such as NH₃, H₂, NO₂, 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 NH₃-sensing properties of Pd/WO₃ films with the response of 0.27 at a working temperature of 225 °C. Lu et al. [12] presented studies on NO₂-sensing properties of Ag–WO₃ nanoparticles and reported high sensitivity and better selectivity. Xu et al. [13] obtained a superior sensor response for Ag–WO₃ core-shell nanostructures towards alcohol vapor at a working temperature of 340 °C. All these studies have shown that metal incorporated into WO₃ has enhanced the sensor performance.

In this regard, we have illustrated the NH₃-sensing performance of Ag–WO₃ films via the spray pyrolysis deposition technique. Ag is a noble metal that enhances the sensor response of WO₃ due to electronic sensitization mechanism [14]. Due to large area deposition, ease of operation, and cost effectiveness, spray pyrolysis is chosen to deposit films in this report. The availability of literature is scarce on Ag–WO₃ for NH₃ sensing, and one of the works published is on hydrothermal technique [14]. To the best of our knowledge, no reports on spray-deposited Ag–WO₃ films are accessible for NH₃ sensing. Therefore, our attempt has proved the possibility of these films for NH₃ sensing by spray pyrolysis. Though the literature is available on NH₃ sensing by metal-doped WO₃, the majority of the sensors work at high temperatures (>250 °C) and the reported detection limit is high [14,15,16]. Ammonia (NH₃), a reducing gas, is a major pollutant from the automobiles, fertilizer, and mining industries. According to the OSHA (Occupational Health and Safety Administration) report, the permissible limit for NH₃ is 35 ppm for 15 min [17]. High exposure to the gas for a long time will trigger lung- and kidney-related diseases, causing incurable damage to human health. In this context, we aimed to lower the operating temperature and detection limit, which are essential for the sensing applications. Thus, in the current work, we have reported spray-pyrolyzed Ag–WO₃ films as a promising candidate for NH3 sensing at a working temperature of 200 °C with the detection limit of 500 ppb.

2. Experimental

Undoped and silver-doped tungsten oxide films (Ag–WO₃) were synthesized by the spray pyrolysis method. For undoped WO₃, 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 × 10⁴ cm³ by vol.) by purging synthetic air (79% N₂ + 21% O₂) and NH₃ 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, ($\frac{{\mathrm{R}}_{\mathrm{a}}-{\mathrm{R}}_{\mathrm{g}}}{{\mathrm{R}}_{\mathrm{g}}}$). R_a and R_g suggest the film resistance in air and target gas (NH₃), respectively. Schematic representation of film synthesis and ammonia-sensing measurements of Ag–WO₃ film is represented in the Figure 1.

Figure 1. Schematic illustration of film deposition and ammonia sensing measurements of Ag–WO₃.

3. Results and Discussion

3.1. Structural and Morphological Analysis

Figure 2a–d represents the XRD pattern of Ag–WO₃ films at varied Ag doping levels. Diffraction peaks at the angles 24.1°, 33.9°, 49.6°, and 55.6° correspond to (200), (202), (140), and (240) planes, indicating monoclinic phase (γ-WO₃) of WO₃ films (JCPDS card no. 43–1035) [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–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.

Figure 2. XRD pattern of (a) 0%, (b) 1%, (c) 3%, and (d) 5%, Ag–WO₃ films.

Table 1. Structural and morphological parameters of Ag–WO₃ films.

Ag Conc. (wt.%)	2θ, (200)	Crystallite Size D (nm)	Dislocation Density δ (×1015 m−2)	Microstrain ε (×10−3)	RMS Surface Roughness (nm)
0	24.12°	13.0	5.9	2.7	0.8
1	24.16°	13.6	5.4	2.5	4.3
3	24.14°	11.5	7.5	3	1.1
5	24.14°	14.7	4.6	2.4	0.6

Since triclinic and monoclinic phases of WO₃ 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–WO₃ 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 WO₃ 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 WO₃ 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–WO₃ films.

Figure 3. Raman spectra of Ag–WO₃ films.

Figure 4a–d shows the topography and microstructures (3D view) of Ag–WO₃ films examined by AFM in tapping mode configuration. All the samples were scanned in the area of 0.5 × 0.5 µm². Figure 4b represents the topographical view of 1% Ag–WO₃. 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–WO₃ higher value of roughness was recorded.

Figure 4. Topographical view of Ag–WO₃ films at (a) 0%, (b) 1%, (c) 3%, and (d) 5% Ag concentrations.

3.2. 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 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 WO₃ 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 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 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.

Figure 5. (a–e): XPS spectra of W, O, and Ag core levels in WO₃ and 1% Ag–WO₃ films.

The deconvolution of O 1s spectra has produced 3 peaks (O₁, O₂, and O₃) in both WO₃ and 1% Ag–WO₃ films (Figure 5c,d) respectively. O₁ centered at E_b of 530.8 eV and 528.7 eV in WO₃ and 1% Ag–WO₃ respectively, was ascribed to O²⁻ in the lattice. Similarly, O₂ centered at 532.6 eV and 531.2 eV denote the lattice oxygen associated with the oxygen-deficient regions near W ions, which are generally referred to as oxygen vacancies (V_o) [29,30]. Area ratio of oxygen vacancies is estimated from peak area calculations as given below:

$${\%\mathrm{V}}_{\mathrm{o}}=\frac{{\mathrm{O}}_2}{({\mathrm{O}}_1+{\mathrm{O}}_2+{\mathrm{O}}_3)}\times 100$$

V_o for WO₃ and 1% Ag–WO₃ was found 35% and 45%, respectively. Hence, the studies inferred that oxygen vacancies increased by 10% in 1% Ag–WO₃. The least intense peaks centered at 534.7 eV and 532.8 eV, respectively in WO₃ and 1% Ag–WO₃, connected to the adsorbed oxygen species (${\mathrm{O}}_2^{-}$, ${\mathrm{OH}}^{-}$) [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–WO₃. These peaks are assigned to the Ag⁰/Ag¹⁺ states of silver [28,31].

3.3. Gas Sensing Properties

Operation temperature for WO₃ towards NH₃ 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 NH₃ 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 WO₃ films. At 100 °C, the graph shows a straight line, denoting no response for NH₃ 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, WO₃ 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.

Figure 6. (a): Temperature optimization plot of WO₃ film at 100 °C, 200 °C, and 250 °C (Inset: At 150 °C and 175 °C). (b): Sensor response vs. temperature plot for WO₃ film.

Sensor response, rate of response, and recovery (${\tau }_{res} \mathrm{and} {\tau }_{rec }$) are the essential parameters that determine the efficiency of any sensor. Figure 7a–d depicts the transient response curves of Ag–WO₃ films (0%, 1%, 3% and 5% Ag conc.) for different NH₃ concentrations. Figure 7e shows the variation of sensor response with the NH₃ concentration. The sensor response for 5 ppm NH₃ 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 WO₃ films, respectively. The highest response was recorded for 1% Ag–WO₃ with the ${\tau }_{res }$ of 5.5 min and ${\tau }_{rec}$ of 7.9 min. Doping of Ag reduced the detection limit from 1 ppm to 0.5 ppm (500 ppb) in 1% Ag–WO₃. ‘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 WO₃, which might have induced the enhancement in the sensor response of the WO₃ film [9]. AFM studies revealed that 1% Ag–WO₃ 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 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.

Figure 7. (a–d): Transient response curves of Ag–WO₃ films and (e) sensor response of the films at various NH₃ concentrations with error bars.

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, CH_4, 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 the lowest NH₃ concentration to be up to 120 ppm at 150 °C [36], while the V-WO₃ films synthesized by soft chemical route exhibited a detection limit of 100 ppm towards NH₃ at 300 °C [37]. The Cr–WO₃ nanosheets synthesized by acidification with impregnation process 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.

Figure 8. Representation of selectivity studies for WO₃ and 1% Ag–WO₃ with error bars.

Figure 9. Repeatable sensing characteristics of (a) WO₃ (b) 1% Ag–WO₃ at 5 ppm NH₃ concentration.

Table 2. NH₃-sensing properties of the present work and reported in the literature.

Material	Method of Deposition	Limit of Detection	Operating Temperature (°C)	Reference
Pd–WO3 films	Spray pyrolysis	10 ppm	225	[9]
Ag–WO3 nanorods	Hydrothermal	50 ppb	450	[12]
WO3 nanoflakes	Spray Pyrolysis	120 ppm	150	[36]
V–WO3 films	Soft-chemical route	100 ppm	300	[37]
Cu–WO3 films	Soft-chemical route	100 ppm	200	[37]
Cr–WO3 nanosheets	Acidification with impregnation	2 ppm	400	[38]
WO3–Fe2O3 nanocomposites	Hydrothermal	25 ppm	300	[39]
Ag–WO3 films	Spray pyrolysis	500 ppb	200	This work

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, WO₃ exposure to the synthetic air produces molecular/atomic oxygen ions (${\mathrm{O}}_2^{-},{\mathrm{O}}^{-},{\mathrm{and}\mathrm{O}}^{2-}$). Electron transfer from the surface of the WO₃ 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 WO₃ is subjected to NH₃ exposure, the depletion layer width decreases due to the release of electrons back to the WO₃. As Ag–WO₃ is exposed to NH₃, 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]:

$${\mathrm{O}}_{2\left(\mathrm{gas}\right)}\to {\mathrm{O}}_{2\left(\mathrm{ads}\right)}$$

(1)

$${\mathrm{O}}_{2\left(\mathrm{ads}\right)}{}^{}+{\mathrm{e}}^{-}\to {\mathrm{O}}_2^{-}$$

(2)

$${\mathrm{O}}_2^{-}+{\mathrm{e}}^{-}\to 2{\mathrm{O}}^{-}$$

(3)

$$4{\mathrm{NH}}_3\left(\mathrm{g}\right)+5{\mathrm{O}}_2^{-}\to 4\mathrm{NO}+6{\mathrm{H}}_2\mathrm{O}+5{\mathrm{e}}^{-}$$

(4)

$$2{\mathrm{NH}}_3\left(\mathrm{g}\right)+3{\mathrm{O}}^{-}\to {\mathrm{N}}_2+3{\mathrm{H}}_2\mathrm{O}+3{\mathrm{e}}^{-}$$

(5)

Figure 10. Sensing mechanism involved between the grains of WO₃ and Ag–WO₃ films.

4. 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, CH_4, 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.