Engineered Porosity ZnO Sensor Enriched with Oxygen Vacancies Enabled Extraordinary Sub-ppm Sensing of Hydrogen Sulfide and Nitrogen Dioxide Air Pollution Gases at Low Temperature in Air

1. Introduction

2. Experimental

3. Result

3.2. ZnO Microstructure Analysis

ZnO was deposited to a thickness of ~30 nm, with a grain size of ~15 nm. Figure 2-a shows the as deposited state of the ZnO; a poorly percolated ZnO microstructure maintains low tortuosity throughout the polycrystalline thin film. Low-level grain coalescence is the result of granular microstructure produced by low-temperature sputtering and evaporation thin film deposition techniques. The thin films having nano-size grains maintain high driving force for grain growth and coarsening.

As shown in Figure 2-b, a vermicular-shaped ZnO microstructure with an average grain size of ~100 nm was produced in our work following 4 hours of annealing at 300ºC. The SEM micrograph also demonstrates how the annealing of ZnO formed a structure that included more porosity, tortuosity and clearly visible ZnO grains. In contrast to materials with larger grain sizes, the thermodynamic stabilization of surface area minimization is the driving force behind the unique features of grain growth/coarsening during the sintering of nano grain-sized ZnO.

High-level of gas adsorption sites and a fine and homogeneous granular structure are achieved. Heat treatment that resulted in grain coarsening greatly expanded the gas diffusion pathways. It is crucial to note that ZnO coarsening resulted from grain boundary migration, and that the grain boundaries did not impede the grain coarsening because they also co-occurred with the reduction-induced creation of oxygen vacancies.

Figure 2. 30 nm thick ZnO; SEM micrograph (a) before heat treatment (b) after heat treatment at 300°C for 4 hours.

Figure 2. 30 nm thick ZnO; SEM micrograph (a) before heat treatment (b) after heat treatment at 300°C for 4 hours.

A high degree of tortuosity resulted from the controlled sintering/coarsening process, which simultaneously increased porosity and grain boundaries. Section 3 will go into detail about the chemistry of gas adsorption sites. The heat treatment led to the abundance of oxygen vacancy concentration on the surface, together with well-defined gas diffusion pathways with high tortuosity. Both produce ideally perfect gas sensing conditions.

3.3. Zinc oxide (ZnO) Electrical Resistance Investigation

Normally, ZnO maintains a small amount of oxygen deficiency. This makes ZnO transition metal oxide oxygen deficient n-type semiconductor with a typical electrical resistivity of 0.01-3 Ω•m at room temperature [58,59,60,61]. The substantial deviances among the reported electrical resistivity values in the literature are linked to diverse deposition systems and/or post-deposition treatment resulting in different microstructures, stoichiometries, and surface properties. Oxygen deficiency, in other words, oxygen vacancies has a strong effect on the electrical, chemical and sensory properties of the ZnO. The oxygen vacancies and oxygen deficiency were investigated and examined using UPS and XPS in the next two sections.

Figure 3. Electrical resistivity measurement for 30 nm thick ZnO film from 25°C to 350°C.

Figure 3. Electrical resistivity measurement for 30 nm thick ZnO film from 25°C to 350°C.

Error! Reference source not found. displays the resistivity values of the 50 nm thick ZnO thin film. The electrical resistivity as a function of temperature was measured under environmental conditions in the 25-350°C temperature range. An increase in temperature continually reduced the resistivity values for ZnO thin film. The resistivity at room temperature (25°C) was 1.45 Ω•m, and it showed a sharp decrease, down to 0.30 Ω•m at 140°C. When the temperature was raised to 150°C, the resistivity decrease rhythm slowed. Due to lattice uptake of adsorbed oxygen, some of the oxygen vacancies (${\mathbf{V}}_{\mathbf{O}}^{••})$ on the surface were annealed, causing a slight decrease after 200°C.

3.4. ZnO Valence Band Investigation

The Zn 4s valance electrons form a bond with the oxygen (O) 2p valance electrons in ZnO, where Zn ions are in tetrahedral coordination with O ions. The bonding between Zn and O ions is ionic, owing to the large variance in their respective electronegativity; as Zn possesses 1.65 while O has 3.44. We measured the ZnO valance band with 150 and 300 eV excitation X-ray photons to probe the surface (0.5 nm depth from the surface) and subsurface areas (1 nm depth from the surface) independently.

Figure 4 shows the valance band region of ZnO in the vicinity of the Zn 3d peak. Figure 4-a shows Zn 3d and valance band region together with distinct bordering. The valence band is represented by Zn 4s-O 2p blended states near 3-8.5 eV while Zn 3d is observable near 10.7 eV. The broad features of the Zn 4s-O 2p valance band suggest that surface stoichiometry is highly influenced by the high amount of oxygen vacancies (${\mathbf{V}}_{\mathbf{O}}^{••})$.

At 1 nm depth measurement, the valance band shows defect-free features of fully stoichiometric ZnO. A decrease in the occupation of the Zn 4s+O 2p blended states from 1 nm to 0.5 nm depth measurements indicates an increase in the concentration of surface oxygen vacancies. At 0.5 nm measurement depth; oxygen vacancies (${\mathbf{V}}_{\mathbf{O}}^{••})$were observed in the valance region as they gave rise to a perturbation around 7-8 eV at the same time causing a decrease in the intensity around 4-7 eV region.

As seen in Figure 4-b, the deeper measurement (1 nm depth, black colored) confirmed the stoichiometric ZnO phase. However, 0.5 nm (red colored) depth measurement showed the dominant existence of oxygen vacancy (${\mathbf{V}}_{\mathbf{O}}^{••})$ rich ZnO_x phase on the surface. The intensity of Zn 4s+O 2p blended state decreased from ZnO to ZnO_x thus showing the outer surface is dominated by reduced-oxygen deficient-oxygen vacancy rich ZnO_x phase.

Figure 4. Engineered Porosity ZnO; 1 nm and 0.5 nm depths from the surface (a) Valance band and Zn 3d regions together (b) Enlarged view of the valence band region; Zn 4s+O 2p blending feature.

Figure 4. Engineered Porosity ZnO; 1 nm and 0.5 nm depths from the surface (a) Valance band and Zn 3d regions together (b) Enlarged view of the valence band region; Zn 4s+O 2p blending feature.

3.5. ZnO Auger Peak (AES) Analysis for Stoichiometric Understanding

AES has larger chemical shifts than XPS core-level shifts, so it allows chemical state analysis in cases that are unlikely for XPS [59]. Determination of the oxidation state of zinc (Zn) will contain significant uncertainties if it is solely based on XPS analysis of zinc (Zn) 2p photoelectron line. In the best scenario, Zn 2p demonstrates a minor shift of ~0.2 eV in the binding energy between zinc (Zn) metal and fully oxidized ZnO; in most of the cases, there is a strong overlap [62]. The binding energy for Zn 2p_3/2 for Zn metal and ZnO are 1021.7 eV and 1021.9 eV, respectively [63,64]. Due to this slight shift in fully oxidized ZnO and Zn metal; perform quantitative-qualitative peak analysis and determination of the oxidation state of Zn based XPS 2p analysis is very challenging. To overcome this obstacle; we proposed to utilize AES in conjunction with XPS.

In situations where XPS is unable to identify the chemical state, a shift of about 4 eV in the Auger peaks distinguishes Zn metal from ZnO. AES is an effective surface sensitive technique capable of probing down to 1 nm depth below the surface. Using the Auger line L₃M₄₅M₄₅, a precise chemical state analysis of ZnO was performed. AES peaks are historically represented in the electron kinetic energy scale [65]. Figure 2 shows L₃M₄₅M₄₅ position for ZnO. According to published reports in the literature, L₃M₄₅M₄₅ values for in ZnO range from 987 eV to 989 eV. In contrast, L₃M₄₅M₄₅ for metallic Zn ranges from 992 eV to 993 eV. Table 1 provides the bulk of the XSP and AES reported literature along with our findings for ZnO.

Figure 5. ZnO L₃M₄₅M₄₅ position with 1147 eV excitation x-ray photons.

Figure 5. ZnO L₃M₄₅M₄₅ position with 1147 eV excitation x-ray photons.

We report 987.9 eV for zinc oxide (ZnO) L₃M₄₅M₄₅. On the other hand, in the Figure 2, we propose that the shoulder seen on the higher kinetic energy site located at 992.3 eV presents ZnO_x. ZnO_x phase contains large amounts of oxygen vacancies and this is because it showed up in the similar region that metal Zn reported in the literature. Thanks to a larger kinetic energy separation between ZnO and ZnO_x; the shoulder strongly confirmed the existence of oxygen vacancy (${\mathbf{V}}_{\mathbf{O}}^{••})$ rich zinc oxide (ZnO_x) as a minority phase within the measurement depth of 1 nm. The amount of the corresponding phases will be calculated from the XPS measurements presented in the following section. We showed that the outer ZnO surface is covered by oxygen vacancy rich ZnO_x phase.

Table 1. Core Level XPS and AES Literature findings for ZnO in comparison with our measurement results.

Table 1. Core Level XPS and AES Literature findings for ZnO in comparison with our measurement results.

Electron Level	Zn 3s in ZnO	Zn 3p1/2 in ZnO	Zn 3p3/2 in ZnO	Zn 3d in ZnO	O 1s in ZnO	L3M4,5M4,5 in ZnO	L3M4,5M4,5 in Zn
Ciftyurek and Schierbaum (this work)	140.0	91.6	88.8	10.7	529.8	987.9	992.3
Vesely and Langer [66]	139.8	92.0	89.0	10.5	530.9	988.9	-
Gaarenstroom and Winograd [67]	-	-	-	10.7	-	987.7	-
Kowalczyk [68]	-	-	-	-	-	-	991.9
Wagner [69]	-	-	-	-	-	-	992.0
Schoen [65]	139.6	91.8	88.7	10.3	530.3	988.5	992.5
Barr and Hackenberg [70]	-	-	-	10.3	530.3	987.9	992.0
Klein and Hercules [71]	-	-	-	10.4	-	988.2	992.3
Strohmeier and Hercules [72]	139.2	-	88.3	-	529.9	988.9	992.4
Powell [73]	-	-	-	-	-	-	992.4
Ley and Kowalczyk [74]	-	-	-	10.4	-	-	991.9
Wehner and Mercer [75]	-	-	-	-	-	988.1	992.1
Dake and Baer [76]	-	-	-	-	-	988.1	992.2

3.6. Concentration of Oxygen Vacancy (${\mathit{V}}_{\mathit{O}}^{••})$ in Engineered Porosity ZnO

Oxygen vacancies facilitate replenishment of the adsorbed oxygen (${{\mathbf{O}}^{2-},\mathbf{O}}^{-},{\mathbf{O}}_2^{2-}$,${\mathbf{O}}_2^{-}$) on the ZnO sensor surface which is essential for chemical sensing in metal oxides. Therefore, it is important to measure the concentration of the oxygen vacancies (${\mathbf{V}}_{\mathbf{O}}^{••})$. Concentration of oxygen vacancies can be calculated either from the oxidation state of the zinc (Zn) in the ZnO or oxygen (O) 1s peak analysis. In our current work, we both analyzed Zn and O via XPS in Engineered Porosity ZnO enriched with oxygen vacancies.

3.6.1. Zn 3p Analysis

Auger analysis proved existence of ZnO_x phase on the ZnO surface. We continued with Zn 3p peak analysis to determine the amounts of each ZnO and ZnO_x. The Zn 3p spectrum from the depth of 1 nm is shown in Figure 4. Deconvolution analysis was used to quantify the amounts of ZnO and oxygen vacancy (${\mathbf{V}}_{\mathbf{O}}^{••})$ abundant non-stoichiometric ZnO_x. The Zn 3p envelope was deconvoluted into two distinct chemical states; stoichiometric ZnO and ZnO_x. We measured binding energy of 3p_3/2 for ZnO_x is 87.4 eV while 3p_3/2 for ZnO is 88.8 eV. The binding energies (BE) we reported for 3p_3/2 for ZnO and ZnO_x are in good agreement with literature values [65,66,72,77]. The decrease in the 3p electron binding energy in ZnO_x in comparison to the ZnO is due to high level of oxygen vacancies created in the ZnO_x as it was processed through heat treatment-sintering-coarsening for creation porosity and tortuosity.

The amount ZnO and ZnO_x phases are 33 at.% and 67 at.%, respectively. ZnO_x being a dominant phase and homogeneously distributed through the surface and will dictate the sensors interaction with target gaseous environment.

Figure 6. Deconvoluted Zn 3p peak, showing non-stoichiometric ZnO_x associated with oxygen vacancies.

Figure 6. Deconvoluted Zn 3p peak, showing non-stoichiometric ZnO_x associated with oxygen vacancies.

3.6.2. Oxygen 1s Analysis

Figure 7 shows the XPS spectrum of oxygen O 1s spectrum from Engineered Porosity ZnO surface. The quantification of different oxygen ion containing species; water/hydroxide groups (H₂O/OH⁻), chemisorbed oxygen ions (${{\mathbf{O}}^{2-},\mathbf{O}}^{-},{\mathbf{O}}_2^{2-},{\mathbf{O}}_2^{-}$), and lattice oxygen ions connected to ZnO_x and ZnO were completed. Analysis also focused on the calculation amounts of ZnO and ZnO_x separately. The O 1s spectra are fitted to four sub-spectra centered at 528.98, 529.80, 530.80, and 532.10 eV, accounting for the lattice oxygen ions in ZnO, lattice oxygen ions in oxygen vacancy rich ZnO_x, chemisorbed oxygen ions and water-hydroxyl groups, respectively. The binding energies we report for water/hydroxide, chemisorbed oxygen, and lattice oxygen ions are in good match with literature reported for ZnO and other metal oxides used for gas sensing [18,37,38,78,79].

Amount of chemisorbed oxygen ions and water-hydroxyl groups are 30 at.%, and 5 at.%, respectively. The high amount of chemisorbed oxygen ions shows that ZnO_x is rich in oxygen vacancies since the chemisorbed oxygen ions attach oxygen vacancy (${\mathbf{V}}_{\mathbf{O}}^{•}$ and ${\mathbf{V}}_{\mathbf{O}}^{••}$) centers on ZnO surface. The amount of lattice oxygen ions connected to ZnO is 22 at.%. However, the amount of lattice oxygen ions in oxygen vacancy rich ZnO_x is 43 at.%. That shows that majority of the ZnO sensor surface interactions is dictated by the oxygen vacancy abundant ZnO_x.

Figure 7. O 1s spectrum with 545 eV excitation photon energies corresponding to 1 nm depth from the Engineered Porosity ZnO sensor surface. Binding energies and amount of each component are also provided in the graph.

Figure 7. O 1s spectrum with 545 eV excitation photon energies corresponding to 1 nm depth from the Engineered Porosity ZnO sensor surface. Binding energies and amount of each component are also provided in the graph.

3.6.3. Zinc (Zn) to Oxygen (O) Stoichiometric Quantification (ZnO)

We used intensity factors and ratio between the normalized areas of the O 1s and Zn 3p contributions, O/Zn was found to be 0.73 at.%. We determined x in ZnO_x, x=0.73, so the oxygen vacancy abundant phase is ZnO_0.73. That shows that majority of the ZnO sensor surface is governed by the oxygen vacancy abundant ZnO_0.73.

3.7. Gas Sensor Testing for H₂S and NO₂

3.7.1. An overview of the sensing mechanism and the involvement of oxygen ions in metal oxide semiconductor (MOS) sensors’ gas sensing reactions

The sensing mechanism for all MOS sensors explained on the basis of adsorption/chemisorption of oxygen ions on MOS surface. Concentration of a certain specific type negatively charged adsorbed/chemisorbed oxygen ions dictates the numbers of electrons in MOS sensor conduction band thus establishes a constant electrical resistance (R). ${{\mathbf{O}}^{2-},\mathbf{O}}^{-},{\mathbf{O}}_2^{2-},{\mathbf{O}}_2^{-}$ are the most common chemisorbed oxygen species taking place in gas sensing reactions [37,38,80,81,82,83,84,85].

Equations (2) and (3) illustrate adsorption of oxygen gas and consumption of conduction band electrons from ZnO leading to an increase in electrical resistance (R). Subsequently, adsorbed oxygen ions dissociated on the ZnO surface through the processes governed by Equation 4 through Equation 6, requiring consumption of additional ZnO electrons and raising the electrical resistance even further. Equation 7 shows the annihilation of the oxygen vacancy site via adsorption and dissociation of oxygen molecules into vacancy location under suitable temperature and oxygen partial pressure conditions. Target gases such as H₂S, CO, SO₂, etc. consume the adsorbed oxygen ions thus leading to return of electrons to ZnO conduction band, decreasing the electrical resistance [18,48,80,81,82,84,85,86]. Equations presented from Equations (2) to (7) are reversible if the certain conditions are satisfied.

$${\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{\mathit{e}}^{-}\to {\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}$$

(Equation 2)

$${\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+2{\mathit{e}}^{-}\to 2{\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}$$

(Equation 3)

$${\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}+{\mathit{e}}^{-}\to {\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{2-}$$

(Equation 4)

$${\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{2-}\to 2{\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}$$

(Equation 5)

$$2{\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}\to {\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{2-}$$

(Equation 6)

$${\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{2-}+{\mathrm{V}}_{\mathrm{o}}^{••}\to {\mathit{O}}_{\mathit{O}}^{\times }$$

(Equation 7)

Monoatomic oxygen ions (${{\mathbf{O}}^{2-},\mathbf{O}}^{-}$) are compared to molecular oxygen ions (${\mathbf{O}}_2^{2-},{\mathbf{O}}_2^{-}$) [37,38]. The creation of ${\mathbf{O}}_2^{-}$ (Equation 6) is an exothermic reaction that results in ~1 eV decrease in free energy, whereas other reactions presented Equation 1 through 7 are endothermic. ${\mathbf{O}}_2^{-}$ transformation to ${\mathbf{O}}^{-}$occurs at ~150–200°C (Equations 3 and 5). The dissociation of O₂ into ${\mathbf{O}}^{-}$ (Equation 5) necessitates ~0.5 eV. Monoatomic oxygen ions (${{\mathbf{O}}^{2-},\mathbf{O}}^{-}$) can tolerate temperatures up to >400°C before departing the ZnO surface. ${\mathbf{O}}^{2-}$ions comparing to ${\mathbf{O}}_2^{2-},{\mathbf{O}}_2^{-}$, and ${\mathbf{O}}^{-}$ are more long-lasting at elevated temperatures on a ZnO surface, , moreover if ${\mathrm{O}}^{2-}$ions are trapped (Equation7) by ${\mathbf{V}}_{\mathbf{O}}^{•}$ and ${\mathbf{V}}_{\mathbf{O}}^{••}$ vacancy centers, ${\mathbf{O}}^{2-}$ ions cannot be distinguished from the lattice oxygen ions. The creation of ${\mathbf{O}}_2^{2-}$ (Equation 4) requires about 5 eV, and a formation of ${\mathbf{O}}^{2-}$ (Equation 6) will require higher energy of ~20 eV [37,38,81,82,87,88].

The most active and populous species of adsorbed oxygen ions are anticipated to be ${\mathbf{O}}^{-}$ at the testing temperature of 200°C used in this work This is because we will only include ${\mathbf{O}}^{-}$ in the sensing mechanism equations from Equation8 to Equation13 that are presented in Kröger–Vink notation.

3.7.2. Initial Tests on Adsorption Kinetics with Oxygen (O₂) and Nitrogen (N₂)

We investigated absorption-dissociation of O₂ on the Engineered Porosity ZnO surface. The O₂ source was atmospheric air. Figure 8 illustrates how the electrical resistance (R/R₀) of Engineered Porosity ZnO changes upon exposure to O₂ and N₂ successively. R₀ denotes electrical resistance as ZnO sensor is maintained under constant O₂ flow, while R denotes the electrical resistance under N₂ flow. N₂ molecules kick off oxygen ions (${{\mathbf{O}}^{2-},\mathbf{O}}^{-},{\mathbf{O}}_2^{2-},{\mathbf{O}}_2^{-})$ from their positions on the surface as soon as O₂ flow is eliminated by N₂ introduction. This causes an injection of electrons back into the ZnO conduction band, which causes ~55% decrease in the electrical resistance (R). Following the removal of the N₂ flow, as shown in Figure 8 at the 40th minute, and the reintroduction of O₂; the O₂ adsorption-dissociation described in Equations 2 to 7 caused the Engineered Porosity ZnO electrical resistance to rise to the initial level.

Engineered Porosity ZnO has a great deal of oxygen vacancies and a high degree of network of open porosity. Figure 8 illustrates how a single exposure to atmospheric air containing 21% O₂ increased electrical resistance by nearly 100%. In other words, large number of oxygen vacancies and high-level connected porosity facilitated large amount of O₂ adsorption and dissociation through the reactions given in Eqs. 2 through 7.

Figure 8. Adsorption and dissociation of O₂ molecules on ZnO and their effect on the electrical resistance at 200°C in air. High purity N₂ introduced between Air pulses containing 21% O₂.

Figure 8. Adsorption and dissociation of O₂ molecules on ZnO and their effect on the electrical resistance at 200°C in air. High purity N₂ introduced between Air pulses containing 21% O₂.

3.7.3. Hydrogen Sulfide (H₂S) Testing

Figure 9 shows Engineered Porosity ZnO sensor’s H₂S response at 200°C as the concentration of H₂S varies from a minimum of 0.2 ppm to a maximum of 2.25 ppm. The sensor showed typical n-type semiconductor performance by displaying a decrease in electrical resistance upon exposure to the H₂S reducing gas. During H₂S exposures, chemisorbed oxygen ions interacted with H₂S, releasing electrons back to the conduction band, thus decreasing the electrical resistance according to the mechanisms governed by Equation8 and Equation 9.

Our sensor testing temperature is 200°C, thus we consider ${\mathbf{O}}^{–}$ to be the most active adsorbed oxygen specie because ${\mathbf{O}}^{–}$ ion formation is favored at ~150–200°C either by direct reduction from O₂ (Equation 3) and/or transformation from ${\mathbf{O}}_2^{-}$ to ${\mathbf{O}}^{-}$ (Equation 4 and Equation 5). Equation 8 presents the reaction between H₂S and ${\mathbf{O}}^{-}$. As was previously mentioned, surface oxygen vacancy (${\mathbf{V}}_{\mathbf{O}}^{••}$) defect centers associated with ZnO_x helped to facilitate the entire process presented in Equation 8. The reaction between H₂S and adsorbed oxygen (${\mathbf{O}}^{-}$) produces by product of SO₂ [17,18,46] [81]. Because SO₂ itself is also reducing agent; SO₂ will interact with the adsorbed oxygen species in the manner outlined in Equation 9.

$${\mathit{H}}_2{\mathit{S}}_{\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{3\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}\to \mathit{S}{\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{\mathit{H}}_2\mathit{O}+3{\mathit{e}}^{-}$$

(Equation 8)

$$\mathit{S}{\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}\to \mathit{S}{\mathit{O}}_{3\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{\mathit{e}}^{-}$$

(Equation 9)

Figure 9. Engineered Porosity ZnO tested for sub-ppm H₂S at 200°C.

Figure 9. Engineered Porosity ZnO tested for sub-ppm H₂S at 200°C.

At the end of the H₂S testing, Engineered Porosity ZnO sensor reintroduced into an oxygen-rich atmosphere relocating the consumed adsorbed oxygen species in accordance with the Equation 2 to Equation 6, as a result we observed the electrical resistance recovered back to its initial value before H₂S introduction. SO₂ is a by-product of complete combusting of H₂S. According to Equation8, H₂S reaction with adsorbed oxygen O^- produces three free electrons (e^-) per reaction, whereas SO₂ reaction yields one free electron (e-). The sensing sequence for H₂S ends with sulfur trioxide (SO₃) as SO₂ oxidized to SO₃ as seen in Equation 9.

In summary, Engineered Porosity ZnO abundant with oxygen vacancies showed an n-type sensing behavior for H₂S. ZnO sensor demonstrated exceptional sensing performance, exhibiting high response to 1 ppm H₂S with 45% decrease in electrical resistance yielding a sensor response of 1.7. Table 2 provides the literature findings for H₂S with the various material-compositions-dopants-precious metals additions etc. along with the results we obtained in our current work.

Examining Table 2, it is evident that Engineered Porosity ZnO has distinguished itself with exceptional properties and outperformed the majority of the tabulated literature thanks to its high sensor response and straightforward, simple, and affordable production method. Given that the most recent research in the literature, as shown in Table 2, makes use of costly precious metal addition, surface alloying, multi-step doping, and/or laborious nano-composites. ZnO sensor can differentiate between extremely low concentrations of H₂S, including 2.25, 1, 0.45, and 0.2 ppm with high accuracy. The ZnO sensor when exposed to CO₂ did not show any sensor response; showed very less toward CO, less than 1% change in electrical resistance.

3.7.4. Nitrogen Oxide (NO₂) Testing

Figure 10 displays the testing results for NO₂ at 200°C which also includes the concentrations profiles of O₂ and NO₂ during sensor testing. Combustion processes produce NO₂, which, in fact, coexist in the same environment as another oxidation gas, namely oxygen (O₂). Because of this demanding condition, Engineered Porosity ZnO was tested for NO₂ while being interfered with by oxygen (O₂). Even with high O₂ interference, ZnO demonstrated exceptional sensing performance at different NO₂ concentrations. Table 2 compares the NO₂ sensor results from our current work with those from other studies that used different material-compositions-dopants-precious metal additions and/or burdensome nano-composite formations etc. with our results obtained in the current work.

NO₂ introduction increased the electrical resistance due to consumption of conduction band electron as noted from Equation10 to Equation14. Figure 10 illustrates how the sensor’s electrical resistance increased as the NO₂ concentration was gradually raised, eventually reaches its saturation peak value for 15 ppm of NO₂. As NO₂ concentration decreased, the Engineered Porosity ZnO electrical resistance decreased to its initial value, emphasizing excellent reversibility and the sensor’s reliability and repeatability.

The Engineered Porosity ZnO sensor showed maximum response of 15 to 15 ppm NO₂. Even under the interference of 18 ppm of O₂, the sensor managed to detect 3 ppm of NO₂. In comparison to the previous research provided in Table 2, the exceptional sensor response from Engineered Porosity ZnO is attributed to a straightforward development process; a high porosity, a well-established tortuosity in microstructure, and a large number of NO₂ pull centers, specifically oxygen vacancies (${\mathbf{V}}_{\mathbf{O}}^{••}$). The gas sensing tests of the Engineered Porosity ZnO sensor with CO₂, CO, toluene and H₂ did not lead to detectable change in the electrical resistance so the results are not presented here.

The reactions for NO₂ sensing are given from Equation 10 to Equation13. Engineered Porosity ZnO has capability to sense NO₂ even under the presence of O₂ as demonstrated by Eqs. 11 to 13. NO₂ molecules can be adsorbed on oxidized sites on ZnO_x surface (Equation10 to and Equation11), on the other hand; the dissociation and adsorption of NO₂ happens to be at oxygen vacancy sites (${\mathbf{V}}_{\mathbf{O}}^{••}$) as seen in Eqs. 12 and 13 [38]. Dissociation and adsorption of NO₂ enabled by the surface oxygen vacancies abundantly found in Engineered Porosity ZnO. NO₂ dissociates to NO by oxidizing surface oxygen vacancies via donation of ${\mathrm{O}}^{-}$and ${\mathrm{O}}^{2-}$ions as given in Eqs. 12 and 13.

$$\mathit{N}{\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{\mathit{e}}_{\left(\mathit{s}\mathit{u}\mathit{r}\mathit{f}\mathit{a}\mathit{c}\mathit{e}\right)}^{-}\to \mathit{N}{\mathit{O}}_{2 \left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}$$

(Equation 10)

$$\mathit{N}{\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+{2\mathit{e}}_{\left(\mathit{s}\mathit{u}\mathit{r}\mathit{f}\mathit{a}\mathit{c}\mathit{e}\right)}^{-}+{\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}\to \mathit{N}{\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}+2{\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}$$

(Equation 11)

$$\mathit{N}{\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}\stackrel{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{p}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\right)}{\to }{\mathit{V}}_{\mathit{O}}^{••}+2{\mathit{e}}_{\left(\mathit{s}\mathit{u}\mathit{r}\mathit{f}\mathit{a}\mathit{c}\mathit{e}\right)}^{-}\to ({\mathit{V}}_{\mathit{O}}^{••}-{\mathit{O}}_{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{2-})+\mathit{N}{\mathit{O}}_{\left(\mathit{g}\mathit{a}\mathit{s}\right)}$$

(Equation 12)

$$\mathit{N}{\mathit{O}}_{2\left(\mathit{g}\mathit{a}\mathit{s}\right)}+\mathit{Z}\mathit{n}{\mathit{O}}_{\mathit{x}}\stackrel{\left(\mathit{a}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{p}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\right)}{\to }\left(\mathit{Z}\mathit{n}{\mathit{O}}_{\mathit{x}}-\mathit{N}{\mathit{O}}_{2\left(\mathit{a}\mathit{d}\mathit{o}\mathit{s}\mathit{r}\mathit{b}\mathit{e}\mathit{d}\right)}^{-}\right)\stackrel{\left(\mathit{d}\mathit{e}\mathit{s}\mathit{o}\mathit{p}\mathit{r}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\right)}{\to }(\mathit{Z}\mathit{n}{\mathit{O}}_{\mathit{x}}-{\mathit{O}}_{\left(\mathit{g}\mathit{a}\mathit{s}\right)}^{-})+{\mathit{N}\mathit{O}}_{\left(\mathit{g}\mathit{a}\mathit{s}\right)})$$

(Equation 13)

As an interference gas, different concentrations of oxygen (O₂) were added. This was completed in order to determine whether changes in oxygen gas concentrations could have an impact on the gas sensing mechanism in situations like those found in typical combustion environments with high and fluctuating oxygen content. Because oxygen (O₂) interference cannot credibly impact the sensor signal, as demonstrated by the experiments, the Engineered Porosity ZnO is extraordinarily effective at detecting NO₂ in realistic environmental settings.

Figure 10. Engineered Porosity ZnO tested for NO₂ at 200°C under O₂ interference.

Figure 10. Engineered Porosity ZnO tested for NO₂ at 200°C under O₂ interference.

The Table 2 reviews H₂S and NO₂ sensing performances of different compositional and microstructural formations of ZnO including nano-fiber; nano-composites; nano-rods and nano-spheres in various architectures to improve the sensor response. CuO; SnO₂; WS₂ some of the materials were used in addition to the costly precious metals such as platinum (Pt); gold (Au) and palladium (Pd) as surface decoration; doping; additives and/or mechanical alloying. In addition to being difficult; costly; and time-consuming to manufacture; these compositional and microstructural formations have limited applicability for large-scale practice within an acceptable standardization for real-world industrial use. Engineered Porosity ZnO enriched with surface oxygen vacancies (${\mathbf{V}}_{\mathbf{O}}^{••}$) showed sensor response of 15 towards 15 ppm of NO₂ and 2.2 to 2.25 ppm of H₂S 200°C. Engineered Porosity ZnO produced very successful results for NO₂ and H₂S when compared to the results from complex-doped-alloyed-composite material sets presented in Table 2.

Engineered Porosity ZnO did not have any sensing promoter agent commonly used in the other sensing materials presented in the Table 2. This work uses engineered porosity designed surface oxygen vacancy (${\mathbf{V}}_{\mathbf{O}}^{••}$) enriched ZnO, which outperforms other sensor platforms-configurations (see Table 2) in terms of sensor response magnitude, simplicity, and design robustness. In our work, we reached to such an extraordinary sensor response for pure ZnO material thanks to the heat treatment, engineered porosity-tortuosity network, and enriched surface oxygen vacancy concentration.

Table 2. Gas Sensing results obtained in this work for Engineered Porosity ZnO, together with literature findings for H₂S and NO₂ under different gas concentrations and testing temperature. Variety of dopants, additives of other oxide composites and/or precious metals modified ZnO literature findings tabulated for the comparison purposes.

Table 2. Gas Sensing results obtained in this work for Engineered Porosity ZnO, together with literature findings for H₂S and NO₂ under different gas concentrations and testing temperature. Variety of dopants, additives of other oxide composites and/or precious metals modified ZnO literature findings tabulated for the comparison purposes.

Sensing Material Composition and Physical State & Form	Sensor Testing Temperature (°C)	Test Gas Concentration	Response Magnitude (Rair/Rgas) for H2S (Rgas/Rair) for NO2
Engineered Porosity ZnO (this work)	200	2.25 ppm H2S	2.2 (This Work)
CuO/SnO2-ZnO core shell NWs [89]	25	10 ppm H2S	1.6
ZnO/SnO2 nano-fibers [90]	250	50 ppm H2S	63.3
ZnO Nano-rods [91]	250	10 ppm H2S	20
Cu–ZnO nanograins [92]	250	15 ppm H2S	0.9
Au modified ZnO nanowires [93]	25	5 ppm H2S	6.1
Pd–SnO2–ZnO [94]	25	20 ppm H2S	0.06
ZnO Nanowires [95]	25	1 ppm H2S	1.5
Engineered Porosity ZnO (this work)	200	15 ppm NO2	15 (This Work)
ZnO thin film [96]	250	1 ppm NO2	2.4
ZnO/SnO2-rGO nano-composite [97]	30	5 ppm NO2	1.4
UV Irradiated Au doped ZnO [40]	25	1 ppm NO2	2
Co-doped ZnO nano-capsules [98]	280	100 ppm NO2	3.86
Ni doped ZnO [99]	200	100 ppm NO2	2
Au decorated ZnO-PANI [100]	300	50 ppm NO2	14

4. Conclusions

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