The effect of acids precipitants on the synthesis of WO3 hierarchical nanostructures for highly selective and sensitive H2S detection

The detection and monitoring of H₂S gas at high and lower concentrations is very crucial since this gas is highly toxic and can affect tissues and organs, especially in occupational environment. This work reports on the synthesis of WO₃ nanostructures-based sensors for highly sensitive and selective H₂S detection at low operating temperatures. These WO₃ nanostructures were synthesized using pressurized hydrothermal process. Different acids from weak to strong (HNO₃, H₂SO₄, and HCl) were employed as precipitants to form supposedly hierarchical and cube-like nanostructures of WO₃. These WO₃ nanostructures were characterized by XRD, SEM, TEM, XPS and BET analysis. The fabricated WO₃ sensors were exposed to different target gases (CO₂, H₂, CH₄, NH₃, LPG and H₂S) at different concentrations. They were found to be selective to H₂S, and the WO₃ precipitated by HCl otherwise referred to as WO₃-HCl was found to be highly sensitive, with high response of S = 1394.04 towards 150 ppm of H₂S at 125°C operating temperature. The WO₃ precipitated by H₂SO₄ named WO₃-H₂SO₄ showed a high response of 141.64 at 125°C operating temperature. Lastly, WO₃ precipitated by HNO₃ called WO₃-HNO₃, recorded a H₂S response of 125.75 also at 125°C operating temperature. The HCl-precipitated WO₃ is a promising candidate for selective detection of H₂S, being the most sensitive in the series.

1 Introduction

There has been an increasing demand in the monitoring and the detection of toxic gases. Hydrogen sulphide (H₂S) is one of the most toxic, flammable, and corrosive gas that is produced widely from various industrial processes, e.g., during the production of crude oil and in mines (Cheng et al., 2019). This colourless pollutant gas can cause breathing difficulties, loss of consciousness and death when concentration exceeds its threshold limit of 20 ppm in air (Xiao et al., 2015; Cheng et al., 2019). Moreover, the threshold limit for H₂S is 10 ppm over an 8-h exposure according to the American Conference of Governmental Industrial Hygienists (Sun et al., 2022). Therefore, the selective detection and monitoring of H₂S gas is paramount. Metal oxide semiconductor-based sensors such as CuO (Park et al., 2016), ZnO (Kim and Yong, 2011), and MoO₃ (Xiao et al., 2015; Cheng et al., 2019), to mention a few, have been widely used for detection of H₂S gas. However, these sensors have some limitations when detecting H₂S at operating temperatures below 200°C. They exhibit low sensor response, long response, and recovery time (Xiao et al., 2015; Cheng et al., 2019). WO₃ metal oxide is one of the most promising materials for gas sensing application due to its fascinating properties such as wide band gap (2.7 eV), excellent electron mobility and chemical stability (Chen et al., 2018; Liu et al., 2020; Shen et al., 2021).

Many significant efforts have been made towards the developments of H₂S sensors-an example is the addition of metal nanoparticles such as Pt (Van Toan et al., 2021), Pd (Wu et al., 2019), Ru (Kruefu et al., 2015), and Ni (Vilic and Llobet, 2016; Karaduman Er et al., 2022) on metal oxides semiconductors, for catalytic effects. This was done for the purpose of enhancing their gas sensing performance towards H₂S at low operating temperatures. However, besides the high cost of these metals, low sensor responses were observed due to the inhomogeneity of the final structure of the gas sensing layers, which underwent unavoidable aggregation; thus worsening their gas sensing performance (Vilic and Llobet, 2016; Karaduman Er et al., 2022), (Wang et al., 2022). High sensor response, short response, and recovery times remain paramount in H₂S detection (Vilic and Llobet, 2016; Karaduman Er et al., 2022), (Vilic and Llobet, 2016; Karaduman Er et al., 2022). Wang et al. (Vilic and Llobet, 2016) synthesized bimodal carbon modified porous WO₃ using methanol solution to obtain C/WO₃ precipitates as the final product. The C/WO₃-based sensor was used to detect H₂S at 200 °C. However, the sensor could not attain its initial resistance in air until a high temperature pulse of 275°C was applied at the desorption process (Vilic and Llobet, 2016). Pankaj S. Kolhe et al. (Wang et al., 2020) synthesized the Al doped ZnO thin film for 600 ppm H₂S detection at 100°C, however, the response and recovery times were high at this low operating temperature. Shorter response and recovery times of 90 s and 209 s respectively were achieved at a much higher operating temperature of 200°C. (Kolhe et al., 2018).

It has been reported in previous studies that the gas sensing performance of semiconductor metal oxide materials is greatly influenced or enhanced by its exposed crystal facets, surface area, crystal structure, surface morphology, and crystallite size (Lavanya et al., 2017), (Johnson et al., 2020), (Simion et al., 2018). Therefore, the desired properties of gas sensing materials could be obtained by rational control and design of the above-mentioned parameters. It has also been reported in the literature that the surface morphology of metal oxide nanostructures, oxygen vacancies and chemisorbed oxygen on metal oxides nanostructures play a very crucial role in gas detection and that these properties of metal oxide nanostructures can be achieved based on the synthesis method used (Lavanya et al., 2017), (David et al., 2020). In this study, the WO₃ nanostructures were synthesized using hydrothermal method. Various surface morphologies of WO₃ were achieved by using different acids (HNO₃, H₂SO₄, and HCl) as precipitants to form supposedly hierarchical and cube-like nanostructures of WO₃ to investigate the effect of the sample preparation conditions on the H₂S gas detection of the as-synthesized WO₃ nanostructures.

2 Experimental details

2.1 Synthesis procedure

The chemical reagents used in this study were of high-quality analytical grade and were purchased from Sigma Aldrich and were without any impurities. WO₃ samples were prepared via a simple hydrothermal method (David et al., 2020). To synthesize the WO₃ samples, a 10 mmol of Na₂WO₄.2H₂O was dissolved in deionized water and the pH was controlled using HNO₃, H₂SO₄ and HCl acids reagents, to obtain different samples referred to as WO₃-HNO₃, WO₃-H₂SO₄, and WO₃-HCl, respectively. This 10 mmol of Na₂WO₄.2H₂O was completely dissolved in the 100 mL deionized water under vigorous and continuous stirring using a magnetic stirrer bar for 30 min at room temperature. The pH value of the solution was first adjusted to 0.14 by dropwise addition of HNO₃ acid to the solution; a sea-foam green precipitate was observed, and the uniform solution was then transferred into 500 mL Teflon-lined stainless-steel autoclave, which was then tightly sealed and kept at 180°C for 24 h in a laboratory oven. After cooling down, the WO₃ precipitate was washed several times with deionized water, and then with ethanol, using the centrifuge, to remove any residual chlorides salt. The obtained precipitate was dried at 60°C for 12 h in a laboratory oven and the solid precipitate was obtained after drying. Porcelain mortar and pestle were used for crushing the solid precipitate into powder and the powder sample labelled WO₃-HNO₃ was transferred into the sample container for characterization. Following the same procedure described above, the WO₃ nanostructures were also synthesized using H₂SO₄ and HCl acids to control the pH until a neon yellow WO₃-H₂SO₄ and mint green WO₃-HCl precipitates, respectively, were obtained.

2.2 Structural characterization

The crystal structure of these powder samples was characterized by using a Bruker D8 Advance X-ray diffractometer fitted with Cu-Kα (λ = 0.1541 nm) radiation source with a scan rate of 0.3 per min. Images of the surface morphology and particle size of the powder samples were captured using a scanning electron microscope (SEM, Carl ZEISS Sigma VP-03-67). High-resolution transmission electron microscopy (HR-TEM) images were recorded using a JEOL 1400 system. PHI 5000 Scanning ESCA Microprobe was used to examine the chemical state of the samples by X-ray photoelectron spectroscopy (XPS) analysis using A 100 μm diameter monochromatic Al Kα (1486.6 eV) X-ray beam at a pressure of 9.3 × 10⁻¹⁰ Torr. The samples were sputter-etched using Ar⁺ with energy of 2 kV and 2 µA. A low energy Ar + ion gun and low energy neutralizer electron gun were used to minimize charging on the surface. The binding energy calibration was done by using the high energy peak of Cu 2p3 at 932.62 eV and the low energy peak of Au 4f7 at 83.96 eV. The retard linearity was set to keep the difference between these two peaks constant at 848.66 eV. The work function of the analyzer was set to 3.7 eV for the Ag3d5 peak to be at 368.27 ± 0.1 eV N₂ adsorption-desorption isotherms and Brunauer-Emmett-Teller (BET) surface area studies were performed using a Micromeritics TRISTAR II (USA) surface area analyser. The samples were degassed at 150°C for 3 h. The BET surface area, pore size and pore volume were measured using nitrogen at 77 K.

2.3 Gas sensors fabrication and measurement

To fabricate the WO₃-based gas sensors (WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl), each powder sample was mixed with ethanol and sonicated. The resulting WO₃ solution was drop-casted onto alumina substrate screen-printed with platinum electrodes. The sensing layer of the fabricated WO₃-sensor were allowed to dry overnight at room temperature. The sensors were placed, simultaneously, in an airtight chamber with electrical and gas feeds for sensing measurement to be performed. The KS026K16 (KENOSISTEC model, Italy) gas testing system at University of Zululand, was used to measure the gas sensing performance of the fabricated sensors. The first WO₃ sensor device (WO₃-HNO₃) was placed inside the climatic gas testing chamber of the Kenosistec system. First, the dry air was introduced into chamber for the sensor to attain equilibrium, while a bias voltage of 5 V was applied across the sensors’ terminals. The humidity was maintained at 0.2 %RH before H₂S target gas was allowed into the chamber. After the initially conditions were set, the WO₃ sensor devices were operated at various temperatures (25°C, 75°C, 125°C, 175°C and 225°C) during H₂S detection at different concentrations. It was observed that the three WO_3-based sensor devices, prepared using different acids, have the optimal operating temperature of 125°C towards H₂S. WO₃ synthesized using HCl (WO₃-HCl) exhibited the highest sensor response compared to the WO₃-HNO₃, and WO₃-H₂SO₄ based sensors. The responses of the sensors were calculated, using Eq. 1.1, for a reducing gas. I_a and I_g are the currents when the gas sensor device is exposed to air and the target gas respectively.

$S=\left(I_g-I_a\right)/I_a(1.1)$

The response and recovery times of the sensors were also determined. The response time of a gas sensor is the time taken to attain 90% of its maximum current when exposed to a target gas. The recovery time is the time taken to regain 90% of the initial current when the target gas is stopped. WO₃-based gas sensor devices were also exposed to various concentrations of CO₂, H₂, LPG, NH₃ and CH₄, to ascertain the selectivity and sensitivity of the sensors.

3 Results and discussion

3.1 Structural and morphological characteristics of WO₃ sensors

Figure 1 shows X-ray diffraction patterns of as-synthesized WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples represented with strong and sharp diffraction peaks indicating good crystalline nature. The samples exhibited highly crystalline diffraction patterns that corresponds to the monoclinic phase of WO₃ according to the JCPDS card no. 89–4476, with the miller indices (002), (020), (200), (120), (112), (022), (202), (222), (004), (040), (140) and (420) (Vilic and Llobet, 2016; Karaduman Er et al., 2022).

FIGURE 1

FIGURE 1. X-ray diffraction patterns of WO₃ synthesized using three different acids- HNO₃, H₂SO₄, and HCl—for precipitation.

The crystallite size of all the WO₃ samples was estimated using the most intense peaks which are the peaks at 2θ = 23.22 corresponding to (002) plane of the monoclinic WO₃. The Debye-Scherrer Eq. 1.2 was used to calculate the crystallite size of the samples.

$D=0.9\lambda /\beta \cos \theta (1.2)$

Where λ is the X-ray wavelength, $\beta$ is the full width at half maximum (FWHM) of the peak and $\theta$ is the diffraction angle in radians. The estimated crystallite sizes, dislocation density, and strain of the WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples are presented in Table 1. The dislocation density, 𝛅 was given by $\mathrm{\delta }=\frac{1}{D^2}$, where $D$ is the crystallite size. The strain, ε was calculated using $\epsilon =\frac{\beta \mathit{cos}\theta }{4}$. It can be seen that the crystallite size of WO₃-HCl powder sample, which was synthesized using HCl acid for pH control, possesses a slightly small size as compared to those synthesized using HNO₃ and H₂SO₄ for pH control, i.e., WO₃-HNO₃ and WO₃-H₂SO₄ powder samples respectively. In addition, the dislocation density and the strain are greater in the WO₃-HCl powder sample. This could be due to the strength of the acid precipitant (HCl).

TABLE 1

TABLE 1. The crystallite size, dislocation density and strain of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples calculated from the peak corresponding to (002) crystal plane using Debye-Scherrer equation.

The surface morphology and the corresponding elemental composition of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples was examined by SEM and energy dispersive X-ray analysis (EDX) as shown in Figures 2A–F. It can be observed in Figures 2A, B that WO₃-HNO₃ consists of hierarchical nanostructures and the EDX spectrum confirms the presence of tungsten (W) and oxygen (O) elements in the sample. The copper (Cu) and carbon (C) elements might have resulted from sample preparation procedure prior to SEM and EDX analysis, however this does not have any effect on other characterization that were performed in this study. Figure 2C shows that WO₃-H₂SO₄ consists of nano-cubes like structures; Figure 2E also shows hierarchical nanostructures of WO₃-HCl, however, small nanoparticles can be observed on the surface morphology of this sample, which are more noticeable than on the WO₃-HNO₃. It is clear that, the type of acids precipitants (HNO₃, H₂SO₄, and HCl) influenced the morphological structure of the samples, which also affects their gas sensing performance (Karaduman Er et al., 2022).

FIGURE 2

FIGURE 2. Scanning electron microscope (SEM) micrographs and their corresponding energy dispersive x-rays (EDX) of WO₃ samples synthesized using different acids (A, B) HNO3, (C, D) H₂SO₄, (E, F) HCl.

The EDX composition maps further confirmed the homogenous distribution of W and O in all the samples. Figures 3–5 shows the EDX composition maps for WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl sample. Figures 3A, 4A, 5A shows the morphology of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples respectively. It is obvious that the morphologies of the samples agree with the results obtained from SEM analysis (Figure 2). All the samples consist of high percentage of W atoms as shown in Figure 2, where WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl consists of 65.92 wt%, 74.12 wt% and 62.02 wt% respectively. This agrees with morphologies of the samples shown in Figures 3A, 4A, 5A, where W atoms was more dominant and evenly distributed in all the samples. However, the composition for WO₃-HCl consists of 62.02 wt% of W atoms, which is slightly small compared to other samples. The oxygen composition (wt%) of the samples were also not the same as can be seen in Figure 2, Where WO₃-HCl consists of reasonable amount of oxygen (16.80 wt%) for the formation of WO₃ sample.

FIGURE 3

FIGURE 3. The EDX micrograph of (A) WO₃-HNO₃ sample, and its corresponding elements mapping of (B) W, and (C) O, based on SEM images of WO₃-HNO₃.

FIGURE 4

FIGURE 4. The EDX micrograph of (A) WO₃-H₂SO₄ sample, and its corresponding elements mapping of (B) W, and (C) O, based on SEM images of WO₃-H₂SO₄.

FIGURE 5

FIGURE 5. The EDX micrograph of (A) WO₃-HCl sample, and its corresponding elements mapping of (B) W, and (C) O, based on SEM images of WO₃-HCl.

The microstructure of WO₃ samples were further investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) combined with selected area electron diffraction (SAED) technique. The images are shown in Figure 6A–F. It can be observed in Figures 6A, C, E, that the TEM images of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl respectively, are not really the same. This indicates that, different morphologies of a particular sample could be obtained by using different acid precipitants during synthesis. Figure 6A, E of WO₃-HNO₃ and WO₃-HCl respectively show that WO₃-HCl has small nanoparticles, which are more prominent as compared to those of WO₃-HNO₃. This is in consonance with what was observed in the EDX images. It is observable in Figure 6C that, the WO₃-H₂SO₄ sample consists of cube-like nanostructures similar to the SEM results. The inset of Figures 6B, D, F is the selected area electron diffraction (SAED) image of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples respectively. The bright dots in the SAED of Figure 6B implies that WO₃-HNO₃ sample is single-crystalline. In Figures 6D, F, the SAED patterns for WO₃-H₂SO₄ and WO₃-HCl shows that these samples were single crystalline. The WO₃-HCl sample was highly crystalline compared to other samples because very bright SAED spots were observed for this sample, and this agrees with the XRD results; Figures 6B, D, F also shows the corresponding HRTEM of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples respectively. The lattice spacing of 0.386 nm, 0.366 nm and 0.364 nm are attributed to (002), (020) and (200) planes of monoclinic WO₃ (Vilic and Llobet, 2016), and this agrees with the XRD results.

FIGURE 6

FIGURE 6. High Resolution-Transmission Electron Microscope (HR-TEM) of the acids assisted synthesis of WO₃ samples, and the inset SAED images of (A, B) WO₃-HNO₃, (C, D) WO₃-H₂SO₄ and (E, F) WO₃-HCl samples.

The nitrogen adsorption-desorption measurements of WO₃-HNO₃ and WO₃-HCl samples were conducted to get information about their specific surface area and the pore size distribution. Figure 7 reveals the adsorption-desorption isotherms of the samples. The BET surface area analysis was done to further distinguish between WO₃-HNO₃ and WO₃-HCl samples since their morphology from SEM almost looks the same. The results reveal that WO₃-HNO₃ sample has Brunauer-Emmett-Teller (BET) surface area of 13.65 m²/g and WO₃-HCl sample has BET surface area of 11.79 m²/g calculated from adsorption desorption isotherms presented in Figures 7A, B respectively. It is noteworthy that, although WO₃-HNO₃ sample has larger surface area compared to WO₃-HCl, its gas sensing performance was not as good as that of WO₃-HCl-this is contrary to expectation (Vilic and Llobet, 2016), (He et al., 2019). The pore size distribution graph of the WO₃-HNO₃ sample exhibits highest value of 11 nm pore size as shown on the inset Figure 7A; this value is typical of a mesoporous material. On the other hand, the WO₃-HCl sample has two peak values (see the inset of Figure 7B), indicating the bimodal porous structure with mesopores and macropores. The high gas sensing performance of WO₃-HCl sample can be attributed to its bimodal porous structure which allows sufficient gas adsorption and desorption (Vilic and Llobet, 2016).

FIGURE 7

FIGURE 7. Nitrogen adsorption-desorption isotherm of (A) WO₃-HNO₃, and (B) WO₃-HCl, and their corresponding BJH pore size distribution (the insets).

X-ray photo electron spectroscopy (XPS) was performed to investigate the elemental composition and oxidation states of the WO₃ samples namely: WO₃-HNO₃, WO₃-H₂SO₄, and WO₃-HCl, as shown in Figure 8. The XPS survey spectrum of WO₃-HCl samples shown in Figure 8A confirms the presence of W and O. The survey spectra of the other two samples (WO₃-HNO₃ and WO₃-H₂SO₄) are presented in Supplementary Figure S1 of the ESI file. On the W 4f spectrum, the peaks corresponding to W 4f _5/2 and W 4f _7/2 of WO₃ samples are ascribed to W⁺⁶ oxidation state as shown in Figures 8B, D, F. indicating the formation of pure WO₃ samples. This was observed from the peaks having binding energies range of 35.5–35.8 eV and 37.6–37.9 eV signifying the spin-orbit split peaks of W 4f _7/2 and W 4f _5/2 respectively (Vilic and Llobet, 2016), (Cao et al., 2022). A very small shift in energy peaks amongst the samples was observed. The O 1s spectra in Figures 8C, E, G of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples respectively, show that all the samples have lattice oxygen (O_L) and oxygen vacancies (O_V) as it is observed by their corresponding binding energies.

FIGURE 8

FIGURE 8. (A) XPS survey spectrum of WO₃-HCl sample, (B, C) the W 4f and O1s spectra of WO₃-HNO_3, (D, E), the W 4f and O1s spectra of WO₃- H₂SO₄, and (F, G) the W 4f and O1s spectra of WO₃- H₂SO₄.

3.2 Gas sensing properties

To investigate the gas sensing performance of the WO₃-based, the sensors were operated at different operating temperatures (75°C, 125°C, 175°C and 225°C), while detecting H₂S target gas, to determine their optimal working temperatures. The sensors based on WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl, exhibited their highest response towards 150 ppm of H₂S at the operating temperature of 125°C. Figure 9A demonstrates the operating temperature dependence response of the WO₃-based sensors. The sensors’ response values towards H₂S gas were 125.75, 141.64 and 1394.04 respectively as shown in Figure 9A. It is quite evident that, the optimal operating temperature of the sensors is 125°C. It is noticeable from the SEM results (Figure 2) that both WO₃-HNO₃ and WO₃-HCl have almost similar surface morphology, which is hierarchical nanostructures. However, small nanoparticles can be observed on WO₃-HCl. The WO₃-H₂SO₄ sample consists of cube-like structures. From XRD results (Figure 1), it was observed that these WO₃-based sensors consist of different crystallite sizes with the WO₃-HCl sensor having the lowest crystallite size of 18 nm. Therefore, the variety in surface morphology and difference in crystallite sizes of the WO₃-based sensors might have contributed to different gas sensing responses towards H₂S target gas. The dynamic response curves of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl based sensors towards different concentrations of H₂S gas (5 ppm, 10 ppm, 20 ppm, 40 ppm, 60 ppm, 80 ppm, 100 ppm and 150 ppm) at 125°C are shown in Figures 9B–D. All the sensors were able to respond or recover quickly towards different concentrations of H₂S gas, indicating a good reversibility. The increase in sensor responses were observed as the concentration increases. The sensor based on WO₃-HCl sample, especially, exhibited a good response of 36.91 towards 5 ppm H₂S as shown in Figure 9D, which is higher than 5.32 and 13.24 of WO₃-HNO₃ and WO₃-H₂SO₄ respectively. The results are shown in Figures 9B–C. These results indicate that WO₃-HCl based sensor is a good candidate for low concentration detection of H₂S. It can be observed from Figures 9B–D that the sensors have different initial currents. Amongst other sensors, WO₃-HCl based sensor had the lowest initial current of 3.92 × 10⁻⁶ mA in air, suggesting a highly resistive-type sensor with resistance of 1276MΩ (calculated using the 5 V bias voltage). This was due to different electrical properties induced in the materials by using various acids precipitants (HNO₃, H₂SO₄ and HCl) for synthesis. From Figures 9B–D, the sensors based on WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl samples showed different response times of 138 s, 190 s and 80 s with corresponding recovery times of 110 s, 135 s and 90 s. No significant changes were observed on the response and recovery times of the sensors as the concentration of H₂S increased. WO₃-HCl based sensor showed short response and recovery times-a desirable quality of gas sensors. The highly sensitive gas sensor has higher response than the other sensors, the sensitivity of the gas sensors can be determined from the slope of the graph of sensor response versus gas concentration.

FIGURE 9

FIGURE 9. (A) Operating-temperature dependence response of WO₃ samples towards 150 ppm of H₂S, and (B–D) The dynamic response curves of: (B)WO₃-HNO₃, (C) WO₃-H₂SO₄, and (D) WO₃-HCl based sensors: towards 150 ppm of H₂S at 125°C.

Figure 10A shows the response/concentration relationship of WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl based sensors towards H₂S. The sensors were operated at optimal operating temperature of 125°C. As expected, the response of the sensors increased as gas concentration increased, however the response values of WO₃-HCl sensor were very high (sensor response = 1394.04 towards 150 ppm H₂S) compared to WO₃-HNO₃ and WO₃-H₂SO₄ based sensors. The sensitivity of WO₃-HCl based sensor was 9.48—A very high value as compared to the other two sensors as shown in Figure 10A. The high sensitivity of WO₃-HCl based sensor might have resulted from the fact that WO₃-HCl morphology has small nanoparticles, small crystallite size and porous structure as confirmed by XRD and BET results respectively. WO₃-HCl consist also of high lattice oxygen and oxygen vacancies as confirmed by XPS results, this might also have contributed to the relatively high sensor response of WO₃-HCl towards H₂S gas (Vilic and Llobet, 2016; Gupta et al., 2019). The WO₃ sample synthesized using HCl acid precipitant (WO₃-HCl) exhibited high sensor response towards H₂S gas compared to the other two WO₃ samples synthesized using HNO₃ and H₂SO₄ acid precipitants (WO₃-HNO₃ and WO₃-H₂SO₄). In comparison with previous works, the WO₃-HCl based sensor demonstrated excellent gas sensing ability towards H₂S gas (Szilágyi et al., 2010; Kruefu et al., 2015; Vilic and Llobet, 2016; Gupta et al., 2019; Wang et al., 2020). Low concentration detection ability of WO₃-HCl based sensor was also observed as shown in Figure 9A. the sensor also demonstrate sensitivity towards low concentration (5 ppm) of H₂S was at low operating temperature of 75°C, and the sensor response was found to be 4.32; whereas the other two sensors based on WO₃-HNO₃ and WO₃-H₂SO₄ showed no low concentration detection ability at 75°C. Therefore, WO₃-HCl based sensor stood out to be the best performing sensor. Selectivity is a significant property of gas sensors as it is the ability of the gas sensor to selectively detect one gas when exposed to different gases. The selectivity of the WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl based sensors were tested as shown in Figure 10B. The sensors were exposed to highest acceptable concentrations of different gases of LPG, NH₃, CH₄, H₂, CO₂ and H₂S at their optimal temperature of 125°C. It can be observed that WO₃-HCl sensor was highly selective towards H₂S gas, therefore WO₃-HCl sensor is the best candidate for H₂S detection. To further investigate the high performance of WO₃-HCl sensor, repeatability was also measured. This, however, was conducted 18 months after the initial test.

FIGURE 10

FIGURE 10. (A) Sensors’ response versus H₂S concentrations at 125°C, showing the sensitivity of WO₃- based sensors, and (B) 3D selectivity plot of the WO₃ -based sensor towards, LPG, NH₃, CH₄, H₂, CO₂, H₂S, at 125°C.

Figure 11A shows three reversible cycles of response and recovery curves towards 100 ppm and 150 ppm of H₂S of the WO₃-HCl sensor at 125°C. The difference in sensors’ response towards 100 ppm and 150 ppm of H₂S is about 400. This shows the ability of the sensor to distinguish between different gas concentrations. The gas sensor response of 132.91 and 281.20 towards 100 ppm and 150 ppm of H₂S respectively, was observed after a period of 18 months following the initial measurements. The long-term stability of WO₃-HCl based sensor was also evaluated by measuring 150 ppm of H₂S after 18 months from initial measurements. The measurement was carried out at the optimal operating temperature of 125°C and the result is shown in Figure 11B. It should be noted that, the sensor was tested several times and under diverse operating conditions before the stability test was eventually conducted. The sensor’s response after 18 months’ period is still relatively high, considering the report of the previous works. The decrease in gas sensor response of WO₃-HCl based sensor as compared to the initial value could be owing to its exposure to different working condition over a long period of time.

FIGURE 11

FIGURE 11. (A) Three cycles of response-recovery curves for WO₃-HCl based sensor towards 100 ppm and 150 ppm H₂S gas at 125°C, showing repeatability, and (B) the long-term stability chart of the WO₃-HCl based sensor.

Table 2 summarizes the gas sensing performance in various recent works on H₂S sensing as compared to the current study. WO₃-HCl based sensor showed the highest gas sensing response towards H₂S compared to other sensors, partly, due to its porous hierarchical nanostructures. Therefore, the sensor based on WO₃-HCl sample demonstrated high prospect for H₂S detection and monitoring to ensure safe working environments.

TABLE 2

TABLE 2. Comparison of hydrogen sulphide (H₂S) gas sensing performance of WO₃-HCl based sensor with other sensing materials in previous reports.

3.3 Gas sensing mechanism

Figure 12A is a schematic of the WO₃-HCl sample drop-casted on an alumina substrate. The WO₃ sensing material is a resistive-type metal oxide semiconductor, and the change of its resistance or current results from the surface reaction of oxygen ions and the target gas. Oxygen molecules capture electrons from the conduction band of WO₃ to form oxygen ions (${\mathrm{O}}^{-},{\mathrm{O}}_2^{-})$ at different temperatures as shown in Eqs 1.3.5.–.1.1.5. This leads to a stable resistance or current by hindering the electron transportation due to the formation of a thin layer of depletion region at the surface of WO₃ nanostructures as shown in Figure 12B (Poongodi et al., 2017).

$O_{2\left(gas\right)}\to O_{2\left(ads\right)}(1.3)$

$O_{2\left(ads\right)}+e^{-}\to O_{2\left(ads\right)}^{-}\mathrm{T}<100°\mathrm{C}(1.4)$

$O_{2\left(ads\right)}+{2e}^{-}\to {2O}_{\left(ads\right)}^{-}100°\mathrm{C}<\mathrm{T}<300°\mathrm{C}(1.5)$

FIGURE 12

FIGURE 12. (A) hierarchical structure of HCl-precipitated WO₃ sample; gas sensing mechanism of WO₃-HCl based sensor (B) in air, and (C) towards H₂S at 125°C.

When the WO₃ sensing material was exposed to H₂S at 125°C, the gas was adsorbed on the surface of the sensing layer and react with the pre-adsorbed oxygen ions as expressed in the Eq. 1.6 and in Figure 12C (Poongodi et al., 2017).

$H_{2}S+{3O}_{\left(ads\right)}^{-}\to H_{2}O+{SO}_2+{3e}^{-}(1.6)$

As the reducing gas H₂S react with oxygen ions on the surface of WO₃, it released the electrons back to the conduction band of WO₃, which caused the decrease in resistance or increase in current as illustrated in Figures 9B–D, therefore decreasing the thickness of the depletion layer which results in the increased carrier concentration and electron mobility of WO₃ nanostructures as indicated in Figure 12C, This mechanism confirms a typical n-type semi-conducting behaviour towards a reducing gas (Poongodi et al., 2017; Kaur et al., 2019). Previous studies have reported that, the reaction that occurs between WO₃ metal oxide and H₂S gas at a temperature range of 500–800K, is described by Eq. 1.7 (Poongodi et al., 2017; Jain and Khanna, 2021).

${3WO}_3+{7H}_{2}S\to {3WS}_2+{SO}_2+{7H}_{2}O+3O_v+6e^{-}(1.7)$

Where e^- is an electron with negative charge, O_v is the oxygen vacancy formed from the reduction of W⁶⁺ (in WO₃) to W⁴⁺ (in WS₂) (Jain and Khanna, 2021). When the flow of H₂S gas is stopped, the WS₂ layer oxidises back to WO₃ in the gas chamber by the reaction illustrated in Eq. 1.8 (Poongodi et al., 2017):

${3WS}_2+{6O}_{\left(ads\right)}^{-}+{6h}^{+}\to {2WO}_3+{2SO}_2(1.8)$

At this point, current returns to its original value (Poongodi et al., 2017). The observed excellent gas sensing performance of WO₃-HCl based sensor could be owing to the following reasons:

First, the hierarchical structure of WO₃-HCl based sensor possesses mesoporous features as confirmed by BET results. The bimodal porous structures facilitated the rapid adsorption and desorption of gas molecules, resulting in the enhanced sensing performance with respect to response time, recovery time, and high gas response. Second, WO₃-HCl was single crystalline, and its surface was more reactive due to more adsorption sites for the target gas, which could have been brought about by the chemical state and the abundance of W⁺⁶ ions in the sample, as confirmed by the XPS. The presence of oxygen vacancies as shown by XPS analysis also contributed to the high sensor’s response.

4 Conclusion

In this work, WO₃ samples (WO₃-HNO₃, WO₃-H₂SO₄ and WO₃-HCl) were synthesized using hydrothermal method, different acids: HNO₃, H₂SO₄, and HCl: were used as precipitants for the synthesize of the samples. WO₃-HCl sample demonstrated an excellent gas sensing performance towards H₂S gas at the operating temperature of 125°C. It also exhibited short response and recovery times, high sensitivity, and clear selectivity towards H2S gas. Enhanced sensing performance of WO₃-HCl sample is attributable to its bimodal porous hierarchical structures, high reactive surface with adsorbed oxygen. Therefore, cost effective WO₃-HCl based sensor is expected to be a solution for the efficient detection and monitoring of H₂S low operating temperatures.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

SO and TM, investigation; and writing—original draft, CN, writing-review and editing, HS, XPS analysis, writing-review and editing to the final version of the manuscript, and SN, conceptualization; Methodology; Supervision; Formal analysis; Writing—review & editing. All authors contributed to the article and approved the submitted version.

Acknowledgments

The authors affiliated to UNIZULU wish to thank the research office for research support. National Research Foundation is also duly acknowledged for its consistent support to carry out research nationwide. The authors wish to acknowledge the University of the Free State for its continuing support in the surface science characterisation and analysis.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsens.2023.1143080/full#supplementary-material

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