Conductometric H2S Sensors Based on TiO2 Nanoparticles

High-performance hydrogen sulfide (H2S) sensors are mandatory for many industrial applications. However, the development of H2S sensors still remains a challenge for researchers. In this work, we report the study of a TiO2-based conductometric sensor for H2S monitoring at low concentrations. TiO2 samples were first synthesized using the sol-gel route, annealed at different temperatures (400 and 600 °C), and thoroughly characterized to evaluate their morphological and microstructural properties. Scanning electronic microscopy, Raman scattering, X-ray diffraction, and FTIR spectroscopy have demonstrated the formation of clusters of pure anatase in the TiO2 phase. Increasing the calcination temperature to 600 °C enhanced TiO2 crystallinity and particle size (from 11 nm to 51 nm), accompanied by the transition to the rutile phase and a slight decrease in band gap (3.31 eV for 400 °C to 3.26 eV for 600 °C). Sensing tests demonstrate that TiO2 annealed at 400 °C displays good performances (sensor response Ra/Rg of ~3.3 at 2.5 ppm and fast response/recovery of 8 and 23 s, respectively) for the detection of H2S at low concentrations in air.


Introduction
With the swift evolution of the global industry and the desire to improve air quality, hydrogen sulfide (H 2 S) has been recognized as one of the highly concerned pollution gases, commonly emitted by industries operating in the fields of pulp and paper manufacturing, natural gas, biological decomposition of organic waste material, and crude petroleum [1][2][3].H 2 S is a hazardous chemical, colorless, and extremely flammable [4,5].At low concentrations, it has an odor of rotten egg, which may cause coughing and sore throat and eyes, while people exposed to high concentrations (300-500 ppm) may experience the human olfactory nerve system and the collapse of the cardiovascular system [6].Therefore, it is mandatory to develop H 2 S sensors with good performance.
Among the variety of sensors used for gas sensing, conductometric sensors have proven to be pretty attractive for detecting a variety of gases, since they are easy to fabricate, low cost, and simple to operate [7][8][9].In the literature, there are many reports on H 2 S sensors based on metal oxide semiconductors (MOS), such as Fe 2 O 3 [10], CuO [11], ZnO [12], WO 3 [13], and NiO [14].However, TiO 2 -based H 2 S sensor development is still scarce, and their gas-sensing performance needs to be improved [15].TiO 2 has intriguing physical and chemical features, making it a promising choice for gas sensor applications due to its distinct allotropic phases (anatase, rutile, and brookite) [6].This involves the microstructural, morphological, and defect characteristics, which can play a crucial role in enhancing sensor response.Meanwhile, selecting synthesis techniques for the TiO 2 nanoparticles is a vital step to achieving a larger surface area with higher roughness.Various physical and chemical routes are commonly used for the synthesis of TiO 2 nanoparticles, such as Pulsed Laser Deposition (PLD) [16], sol-gel [17], thermal evaporation [18], sputtering [15], spray pyrolysis [19], and Atomic Layer Deposition (ALD) [20].TiO 2 has a high surface area, enhancing its interaction with gas molecules and improving sensitivity [21][22][23].It is chemically stable and corrosion-resistant, ensuring long-term durability and reliability.TiO 2 exhibits excellent photocatalytic activity [24], significantly changing its conductivity when exposed to light and gas molecules, which enhances sensitivity and response time.The material can be synthesized in various nanostructured forms, providing greater surface area and more active sites for gas adsorption.Additionally, TiO 2 is cost-effective and abundantly available, making it an economical choice for gas sensor development.Therefore, these features provide the TiO 2 -based sensor with great sensitivity and selectivity for hydrogen sulfide.
In this study, we have synthesized TiO 2 nanoparticles (NPs) with the modified sol-gel method using ethyl alcohol under supercritical conditions, which requires lower energy consumption and allows the synthesis of materials with high purity and homogeneity.We investigated their structural, morphological, and optical properties and their performances in gas-sensing for detecting low hydrogen sulfide concentrations in the range from 0.5 to 4 ppm.The developed sensor exhibited enhanced sensitivity, selectivity, and fast response/recovery times to H 2 S.

Experimental Section 2.1. Synthesis of TiO 2 Nanopowder
TiO 2 nanopowder was prepared using the protocol of El Mir et al. [25,26] based on the following steps.First, 15 mL of Titanium (IV) isopropoxide Ti(OC 3 H 7 ) 4 (97%, from Sigma-Aldrich, Saint Louis, MO, USA) was dissolved in 45 mL of methanol blended with 2 mL of acetic acid (CH 3 COOH).The mixture was kept under magnetic stirring until the precursors were completely dissolved.The resulting solution was then poured into the autoclave to achieve drying in supercritical conditions of 250 mL of ethanol (Tc = 243 • C; Pc = 63.6 bars), with a heating rate of 45 • C/h.Afterward, the as-obtained nanopowder was calcined for 2 h in air at different temperatures, (T = 400 • C) and (T = 600 • C).For the preparation of the TiO 2 conductometric sensor, a quantity of 1 mg of TiO 2 powder was sonicated for 30 min with 1 mL of deionized water.The gas sensor was manufactured in the temperature range between 20 • C and 25 • C. A scheme of the synthesis procedure is illustrated in Figure 1.

Characterizations
Microstructural analysis was determined with a D2 phaser Bruker X-ray diffractometer (Bruker, Billerica, MA, USA) using the Cu Kα line (0.159 nm) in the 10-80 • 2θ range.FT-IR spectra were recorded utilizing a PerkinElmer spectrometer (PerkinElmer, Waltham, MA, USA) equipped with a universal attenuated total reflectance (ATR) sampling accessory.The UV-visible diffuse reflectance spectra (UV-visible DRS) were measured using a Shimadzu (Kyoto City, Japan) 2600-2700 spectrometer with BaSO 4 as a reference.Raman spectra of the samples were recorded using the XploRa Raman spectrometer (Horiba Scientific, Piscataway, NJ, USA) equipped with an Olympus BX-40 microscope (Olympus, Tokyo, Japan) (objective ×50 focal length), Peltier cooled CCD detector, 532 nm diode laser, and 600 L/mm grating.The laser power was 5 mW, and the acquisition time was 30 s. Two to ten spectra were registered for each sample at different positions to verify sample homogeneity.The reference spectrum of Si (peak position of 521 cm −1 ) was measured to avoid temperature drift.Scanning Electron Microscope (SEM) images were taken using a Zeiss (Oberkochen, Germany) (Gemini II) microscope at the acceleration voltage of 5 kV.

Gas Sensing Tests
The gas sensing tests were carried out with sensors fabricated by printing TiO 2 on the sensor device with a heating element and Pt-interdigitated electrodes.For the tests, the sensor devices were introduced into the test chamber.An Agilent E3632A instrument (Agilent, Santa Clara, CA, USA) was employed for setting the operating temperatures, whereas the resistance of the TiO 2 sensing layer was measured with an Agilent 34970A multimeter (Agilent, Santa Clara, CA, USA).H 2 S sensing tests were carried out under a flow of dry synthetic air of 100 cc/min, operating at temperatures from 100 to 400 • C, with H 2 S gas concentrations of 0 to 4 ppm.The gas response, S, is defined as the ratio R a /R g for n-type behavior, where R a is the baseline resistance in dry synthetic air and R g is the electrical resistance at different gas concentrations.The response time, τ res , and recovery time, τ rec , were defined as follows.Response time, τ res , i.e., the time required for the sensor to reach 90% of the saturation resistance after injection of the target gas, and recovery time, τ rec , i.e., the time required for the sensor to reach 90% of the resistance baseline value in air.These were also evaluated.

Sample Characterizations
TiO 2 samples synthesized using the sol-gel route and annealed at different temperatures (400 and 600 • C) were first thoroughly analyzed by different characterization techniques.The SEM images of the TiO 2 sample annealed at 400 • C are reported in Figure 2.
The sample annealed at 400 • C is not completely homogeneous, showing regions with different microscopic features characterized by randomly distributed and non-uniform clusters of TiO 2 (Figure 2a,b).However, all regions show a high porosity on a nanometric scale with grain size in the range of 10-20 nm (Figure 2c,d).
Figure 3 reports characteristic SEM images of the TiO 2 sample annealed at 600 • C. The morphology of this sample is more homogeneous, with a bigger grain size in the 30-50 nm range and a fractal-like structure induced by calcination.In addition, it can be noted that, at the highest annealing temperature, the collapse of the mesostructure occurs, which could be caused by the crystallization of the amorphous titania into nanosized anatase particles and/or with the transition from the anatase to the rutile phase.

Sample Characterizations
TiO2 samples synthesized using the sol-gel route and annealed at different temperatures (400 and 600 °C) were first thoroughly analyzed by different characterization techniques.The SEM images of the TiO2 sample annealed at 400 °C are reported in Figure 2. The sample annealed at 400 °C is not completely homogeneous, showing regions with different microscopic features characterized by randomly distributed and non-uniform clusters of TiO2 (Figure 2a,b).However, all regions show a high porosity on a nanometric scale with grain size in the range of 10-20 nm (Figure 2c,d).
Figure 3 reports characteristic SEM images of the TiO2 sample annealed at 600 °C.The morphology of this sample is more homogeneous, with a bigger grain size in the 30-50 nm range and a fractal-like structure induced by calcination.In addition, it can be noted that, at the highest annealing temperature, the collapse of the mesostructure occurs, which could be caused by the crystallization of the amorphous titania into nanosized anatase particles and/or with the transition from the anatase to the rutile phase.To investigate this transition phase process, further characterizations have been carried out.The vibrational properties were investigated by Raman measurements, the profiles of which are reported in Figure 4.The three Raman peaks centered at about 386, 509.3, and 630.3 cm −1 (inset in Figure 4) are assigned to the Raman active modes of the anatase TiO2 crystalline structure, while the peak at about 472.5 cm −1 is associated with the Raman active modes of the rutile crystalline phase [27].The bands in the region higher than 1500 cm −1 are due to C-C and C-H/C-O contributions, which is in good agreement with FTIR data.Ultimately, Raman evidence indicates that the two TiO2 phases characterize the investigated samples.However, the relative intensity of the peaks of the anatase phase com- To investigate this transition phase process, further characterizations have been carried out.The vibrational properties were investigated by Raman measurements, the profiles of which are reported in Figure 4.The three Raman peaks centered at about 386, 509.3, and 630.3 cm −1 (inset in Figure 4) are assigned to the Raman active modes of the anatase TiO 2 crystalline structure, while the peak at about 472.5 cm −1 is associated with the Raman active modes of the rutile crystalline phase [27].The bands in the region higher than 1500 cm −1 are due to C-C and C-H/C-O contributions, which is in good agreement with FTIR data.Ultimately, Raman evidence indicates that the two TiO 2 phases characterize the investigated samples.However, the relative intensity of the peaks of the anatase phase compared to the peak associated with the rutile phase is different in the two samples.This indicates that the phase transition from the anatase phase to the rutile phase occurs at the highest temperature.In Figure 5, the XRD patterns of both TiO2 samples are shown.In Figure 5a, which shows the XRD pattern of TiO2 powder annealed at 400 °C, we identified the (101), ( 103), (004), ( 112), ( 200), ( 105), ( 211), ( 204), ( 116), (220), and (215) diffraction peaks ascribed to the TiO2 tetragonal structure in the anatase phase (JCPDS 21-1272) [28,29].As shown in Figure 5b, upon increasing the annealing temperature to 600 °C, the diffraction peaks are narrower and slightly more intense.We can also discern the orthorhombic structure of TiO2 as discerned by its characteristic (011) peak centered at 31.7° (JCPDS 80-5176) [30].In Figure 5, the XRD patterns of both TiO 2 samples are shown.In Figure 5a, which shows the XRD pattern of TiO 2 powder annealed at 400 • C, we identified the (101), ( 103), (004), ( 112), ( 200), ( 105), ( 211), ( 204), ( 116), (220), and (215) diffraction peaks ascribed to the TiO 2 tetragonal structure in the anatase phase (JCPDS 21-1272) [28,29].As shown in Figure 5b, upon increasing the annealing temperature to 600 • C, the diffraction peaks are narrower and slightly more intense.We can also discern the orthorhombic structure of TiO 2 as discerned by its characteristic (011) peak centered at 31.7 • (JCPDS 80-5176) [30].
The Rietveld refinements of the crystal structures of the as-prepared TiO 2 samples were carried out using the FullProf software (https://www.ill.eu/sites/fullprof/,accessed on 6 May 2024).The method employs a least-squares procedure to compare Bragg intensities and those calculated from a possible structural model.In the first step of refinement, the global parameters, such as background and scale factors, were refined.In the next step, structural property parameters such as lattice parameters, profile shape and width parameters, preferred orientation, asymmetry, isothermal parameters, atomic coordinates, and site occupancies were refined in sequence.
The average crystallite size and the lattice strain were calculated according to the Williamson-Hall method using the following equation [31]: where β is the peak full width at half maximum (FWHM), θ is the Bragg angle, K is the shape factor (0.9), λ is the incident wavelength (λ = 1.5406Å), and ε is the film strain.The trend of βcosθ as a function of 4sinθ for the investigated samples is shown in Figure 5c,d.The Rietveld refinements of the crystal structures of the as-prepared TiO2 samples were carried out using the FullProf software (https://www.ill.eu/sites/fullprof/,6 May 2024).The method employs a least-squares procedure to compare Bragg intensities and those calculated from a possible structural model.In the first step of refinement, the global parameters, such as background and scale factors, were refined.In the next step, structural property parameters such as lattice parameters, profile shape and width parameters, preferred orientation, asymmetry, isothermal parameters, atomic coordinates, and site occupancies were refined in sequence.
The average crystallite size and the lattice strain were calculated according to the Williamson-Hall method using the following equation [31]: where is the peak full width at half maximum (FWHM), θ is the Bragg angle, K is the shape factor (0.9), is the incident wavelength (λ = 1.5406Å), and is the film strain.The trend of βcosθ as a function of 4sinθ for the investigated samples is shown in Figure 5c,d.
The fitting quality of the experimental data is assessed by computing parameters such as the 'goodness of fit' χ 2 , the Bragg R-factor, and the Rf-factors (Profile R-factor (Rp), Weighted Profile R-factor (Rwp), and Expected R-factor (Rexp)).The values of these structural parameters are reported in Table 1.The fitting quality of the experimental data is assessed by computing parameters such as the 'goodness of fit' χ 2 , the Bragg R-factor, and the Rf-factors (Profile R-factor (Rp), Weighted Profile R-factor (Rwp), and Expected R-factor (Rexp)).The values of these structural parameters are reported in Table 1.The results deduced from the Rietveld refinements of the XRD profiles are reported in Table 2, giving information about the significant variation of the phase composition and the crystallite size.It emerges that the crystallite size of the TiO 2 sample increases upon increasing the annealing temperature.Figure 6 shows the FTIR spectra of TiO 2 samples in the 400-4000 cm −1 range.The spectrum of the sample annealed at 400 • C shows two broad bands, centered at about 500 and 860 cm −1 , which are assigned to the Ti-O bending and Ti−O−Ti stretching vibrations [32], respectively.Furthermore, the barely visible contributions at about 1240 and 1340 cm −1 are ascribed to the C-H twisting and bending vibrational modes [33,34], whereas the two bands at around 1630 and 3310 cm −1 correspond to the presence of related hydroxyl groups (Ti-OH) and those of water molecules [35,36].As expected, when calcination temperature increases, peaks relative to the hydroxyl groups and adsorbed C-H disappear.At the same time, we observed a slight change in the diffraction profile of TiO 2 , indicating the rearrangement of the Ti-O network to facilitate the crystallization of TiO 2 [32].UV-visible absorption measurements were carried out to investigate the changes in optical transitions of TiO2 nanostructures caused by annealing.The Kubelka-Munk equation was used to calculate the absorption spectra of the samples from the diffuse reflectance spectra [26].Figure 7a shows absorbance spectra with wavelengths ranging from 250 to 800 nm for both samples synthesized using the sol-gel method.The absorbance of the nanostructures is around 90% in the UV range and decreases dramatically beginning in the visible range.Seemingly, annealing has no significant impact on the absorbance of the ceramic in the UV range.The bandgap energy Eg of TiO2 nanostructures was estimated according to the Tauc method, following Equation ( 2) [29,38,39]: In this equation, α is the absorption coefficient, A is a constant, and hν is the photon energy.Extrapolating the linear part of the curve to the hν-axis yielded the optical This agrees with what is known from the literature [37], which reports that although the oxygen content remains constant up to annealing temperatures of 900 • C, when there is an increase in temperature, there is also an increase in "O − species" due to the hydroxyl groups and carbon impurities desorbing from the surface.This process, due to the localized charge transfer between anionic and cationic frameworks during thermally induced reduction, favors the sensing mechanism of H 2 S, which reacts with the adsorbed oxygen species to form SO 2 and H 2 O (see Section 3.3).
UV-visible absorption measurements were carried out to investigate the changes in optical transitions of TiO 2 nanostructures caused by annealing.The Kubelka-Munk equation was used to calculate the absorption spectra of the samples from the diffuse reflectance spectra [26].Figure 7a shows absorbance spectra with wavelengths ranging from 250 to 800 nm for both samples synthesized using the sol-gel method.The absorbance of the nanostructures is around 90% in the UV range and decreases dramatically beginning in the visible range.Seemingly, annealing has no significant impact on the absorbance of the ceramic in the UV range.The bandgap energy E g of TiO 2 nanostructures was estimated according to the Tauc method, following Equation (2) [29,38,39]: Materials 2024, 17, x FOR PEER REVIEW 10 of 19

Gas Sensing Tests
Before investigating the gas sensing properties, the baseline resistance of the TiO2 layer, denoted as Ra, versus operating temperature has been investigated (see Figure 8).The sensor baseline displays a higher resistance at low temperatures.As the temperature increases, the resistance baseline decreases because of the thermal excitation of electrons into the conduction band, indicating the semiconductor behavior of TiO2.The data have further shown that TiO2 (600 °C) is more resistive, due to the presence of the rutile phase, compared to TiO2 (400 °C).In this equation, α is the absorption coefficient, A is a constant, and hν is the photon energy.Extrapolating the linear part of the curve to the hν-axis yielded the optical band gap, as illustrated in Figure 7b.The estimated band gap energy values were 3.31 eV and 3.26 eV for samples annealed at 400 • C and 600 • C, respectively.Similar behavior was reported in the literature for TiO 2 nanostructures synthesized using the sol-gel method [29].

Gas Sensing Tests
Before investigating the gas sensing properties, the baseline resistance of the TiO 2 layer, denoted as R a , versus operating temperature has been investigated (see Figure 8).The sensor baseline displays a higher resistance at low temperatures.As the temperature increases, the resistance baseline decreases because of the thermal excitation of electrons into the conduction band, indicating the semiconductor behavior of TiO 2 .The data have further shown that TiO 2 (600 • C) is more resistive, due to the presence of the rutile phase, compared to TiO 2 (400 • C).
Operating temperature is also an important parameter to take into account for the gas sensing response.Indeed, temperature influences the adsorption/desorption processes of gases occurring on the sensing surface, as well as their reaction rate with adsorbed oxygen on the TiO 2 surface, and consequently the sensor response.As the above Figure 8 demonstrates, at temperatures lower than 300 • C, the baseline resistance is very high.To evaluate the optimal operating temperature for the sensing tests, the sensor was exposed to 1.5 ppm H 2 S gas at temperatures ranging from 300 to 400 • C (Figure 9).Based on the results obtained, 350 • C appears to be the best operating temperature for this sensor, which displayed a high response to H 2 S and short response/recovery times.layer, denoted as Ra, versus operating temperature has been investigated (see Figure 8).The sensor baseline displays a higher resistance at low temperatures.As the temperature increases, the resistance baseline decreases because of the thermal excitation of electrons into the conduction band, indicating the semiconductor behavior of TiO2.The data have further shown that TiO2 (600 °C) is more resistive, due to the presence of the rutile phase, compared to TiO2 (400 °C).Operating temperature is also an important parameter to take into account for the gas sensing response.Indeed, temperature influences the adsorption/desorption processes of gases occurring on the sensing surface, as well as their reaction rate with adsorbed oxygen on the TiO2 surface, and consequently the sensor response.As the above Figure 8 demonstrates, at temperatures lower than 300 °C, the baseline resistance is very high.To evaluate the optimal operating temperature for the sensing tests, the sensor was exposed to 1.5 ppm H2S gas at temperatures ranging from 300 to 400 °C (Figure 9).Based on the results obtained, 350 °C appears to be the best operating temperature for this sensor, which displayed a high response to H2S and short response/recovery times.In these operating conditions (Figure 10a), the sensor response of TiO2 (400 °C) was registered to be 3.26 for 2.5 ppm of H2S, higher than that reported for the TiO2 (600 °C) sensor.In addition, we confirmed this result in Figure 10b, which depicts the evolution of the sensor response as a function of the H2S concentration for both sensors.This finding is consistent with the results of XRD and SEM analyses.Indeed, it is well-known that when the grain size of the sensing material is small enough, it substantially impacts the gas sensing properties [37,38].In addition, the sensor annealed at 400 °C has a larger surface-tovolume ratio due to the smaller grain size, thus further justifying the larger response compared to the TiO2 (600 °C) sensor.The lower response for the sensor annealed at 600 °C could therefore be related to the improvement in the crystallinity of TiO2 nanoparticles.The rearrangement of the atoms is a process that reduces the gas adsorption on the surface In these operating conditions (Figure 10a), the sensor response of TiO 2 (400 • C) was registered to be 3.26 for 2.5 ppm of H 2 S, higher than that reported for the TiO 2 (600 • C) sensor.In addition, we confirmed this result in Figure 10b, which depicts the evolution of the sensor response as a function of the H 2 S concentration for both sensors.This finding is consistent with the results of XRD and SEM analyses.Indeed, it is well-known that when the grain size of the sensing material is small enough, it substantially impacts the gas sensing properties [37,38].In addition, the sensor annealed at 400 • C has a larger surfaceto-volume ratio due to the smaller grain size, thus further justifying the larger response compared to the TiO 2 (600 • C) sensor.The lower response for the sensor annealed at 600 • C could therefore be related to the improvement in the crystallinity of TiO 2 nanoparticles.The rearrangement of the atoms is a process that reduces the gas adsorption on the surface [39].Apart the above structural considerations, the effect of the different phases (anatase and rutile) on the sensing response cannot be excluded.The advantages of using anatase or rutile in gas sensing have been discussed for a long time and depend on many variables such as the target gas and operating temperature.For example, Zakrzewska and Radecka discovered that rutile-dominated TiO2 nanomaterials exhibited higher sensitivity towards hydrogen than those with the prevailing anatase [40].This phenomenon could be accounted for by band alignment and electron transfer from rutile to anatase to facilitate oxygen pre-adsorption.On the contrary, by using density functional theory (DFT) to study the adsorption and reaction of H2S on TiO2 anatase (101) and rutile (110) surfaces, it has been demonstrated that the adsorption and dissociation of hydrogen sulfide at the TiO2 anatase surface require a lower energy barrier compared to at the anatase surface [41].This latter finding indicates that the presence of anatase at a high concentration (100%) is a factor to take into account when considering the sensor response enhancement of H2S.
The sensing performance of the TiO2 (400 °C) sensor was further investigated by exposing the fabricated sensors to different concentrations of H2S gas. Figure 11a shows the plotted gas response to H2S gas sensed by the TiO2 (400 °C) sensor at an operating temperature of 350 ℃.The response amplitude of the sensor increases with H2S concentration in the range of 0.5 to 4 ppm.Moreover, in Figure 11b, it can be observed that the response increases almost linearly with the concentration.The sensor is also sufficiently sensitive at the lowest concentration (0.5 ppm) of H2S tested.This result suggests that it can be promising for the sensing of hydrogen sulfide in practical applications.Apart the above structural considerations, the effect of the different phases (anatase and rutile) on the sensing response cannot be excluded.The advantages of using anatase or rutile in gas sensing have been discussed for a long time and depend on many variables such as the target gas and operating temperature.For example, Zakrzewska and Radecka discovered that rutile-dominated TiO 2 nanomaterials exhibited higher sensitivity towards hydrogen than those with the prevailing anatase [40].This phenomenon could be accounted for by band alignment and electron transfer from rutile to anatase to facilitate oxygen pre-adsorption.On the contrary, by using density functional theory (DFT) to study the adsorption and reaction of H 2 S on TiO 2 anatase (101) and rutile (110) surfaces, it has been demonstrated that the adsorption and dissociation of hydrogen sulfide at the TiO 2 anatase surface require a lower energy barrier compared to at the anatase surface [41].This latter finding indicates that the presence of anatase at a high concentration (100%) is a factor to take into account when considering the sensor response enhancement of H 2 S.
The sensing performance of the TiO 2 (400 • C) sensor was further investigated by exposing the fabricated sensors to different concentrations of H 2 S gas. Figure 11a shows the plotted gas response to H 2 S gas sensed by the TiO 2 (400 • C) sensor at an operating temperature of 350 • C. The response amplitude of the sensor increases with H 2 S concentration in the range of 0.5 to 4 ppm.Moreover, in Figure 11b, it can be observed that the response increases almost linearly with the concentration.The sensor is also sufficiently sensitive at the lowest concentration (0.5 ppm) of H 2 S tested.This result suggests that it can be promising for the sensing of hydrogen sulfide in practical applications.
The response and recovery times are two very important characteristics of gas sensors in practical applications.The response and recovery times of the TiO 2 (400 • C) sensor as a function of various H 2 S concentrations at the operating temperature of 350 • C are presented in Figure 12.The measured response and recovery times are short.Indeed, in the H 2 S concentration range of 0.5 to 4 ppm, the response time is slower than 10 s and the recovery time is slower than 31 s.The response and recovery times are two very important characteristics of gas sensors in practical applications.The response and recovery times of the TiO2 (400 °C) sensor as a function of various H2S concentrations at the operating temperature of 350 °C are presented in Figure 12.The measured response and recovery times are short.Indeed, in the H2S concentration range of 0.5 to 4 ppm, the response time is slower than 10 s and the recovery time is slower than 31 s.The gas sensing selectivity of the TiO2 (400 °C) sensor against different gases, i.e., nitrogen dioxide, carbon monoxide, and hydrogen, was also studied (Figure 13).The selectivity patterns indicate that, for all the interfering gases, it presents low responses, and therefore exhibits excellent selectivity to H2S.The response and recovery times are two very important characteristics of gas sensors in practical applications.The response and recovery times of the TiO2 (400 °C) sensor as a function of various H2S concentrations at the operating temperature of 350 °C are presented in Figure 12.The measured response and recovery times are short.Indeed, in the H2S concentration range of 0.5 to 4 ppm, the response time is slower than 10 s and the recovery time is slower than 31 s.The gas sensing selectivity of the TiO2 (400 °C) sensor against different gases, i.e., nitrogen dioxide, carbon monoxide, and hydrogen, was also studied (Figure 13).The selectivity patterns indicate that, for all the interfering gases, it presents low responses, and therefore exhibits excellent selectivity to H2S.The gas sensing selectivity of the TiO 2 (400 • C) sensor against different gases, i.e., nitrogen dioxide, carbon monoxide, and hydrogen, was also studied (Figure 13).The selectivity patterns indicate that, for all the interfering gases, it presents low responses, and therefore exhibits excellent selectivity to H 2 S.
Repeatability is an important indicator for measuring the reliability of the sensor response and the stability of the sensor.Figure 14 shows the reproducibility of the sensor when exposed to three consecutive pulses of 4 ppm of H 2 S gas at the working temperature of 350 • C. It is observed that the response and recovery characteristics are almost reproducible.Repeatability is an important indicator for measuring the reliability of the sensor response and the stability of the sensor.Figure 14 shows the reproducibility of the sensor when exposed to three consecutive pulses of 4 ppm of H2S gas at the working temperature of 350 °C.It is observed that the response and recovery characteristics are almost reproducible.

Gas Sensing Mechanism
The gas sensing mechanism of the developed sensor is explained by the change in the conductance of the semiconducting TiO2 sensing layer.Herein, the conductivity of the sensor is modified by the phenomenon of target gas adsorption-desorption, which causes variations in the electrical conductivity of the sensing layer.The kinetics of gas adsorption  Repeatability is an important indicator for measuring the reliability of the sensor response and the stability of the sensor.Figure 14 shows the reproducibility of the sensor when exposed to three consecutive pulses of 4 ppm of H2S gas at the working temperature of 350 °C.It is observed that the response and recovery characteristics are almost reproducible.

Gas Sensing Mechanism
The gas sensing mechanism of the developed sensor is explained by the change in the conductance of the semiconducting TiO2 sensing layer.Herein, the conductivity of the sensor is modified by the phenomenon of target gas adsorption-desorption, which causes variations in the electrical conductivity of the sensing layer.The kinetics of gas adsorption

Gas Sensing Mechanism
The gas sensing mechanism of the developed sensor is explained by the change in the conductance of the semiconducting TiO 2 sensing layer.Herein, the conductivity of the sensor is modified by the phenomenon of target gas adsorption-desorption, which causes variations in the electrical conductivity of the sensing layer.The kinetics of gas adsorption and desorption are critical to the performance of gas sensors, influencing their sensitivity, response time, and recovery time.The adsorption process is enhanced by a high surface area, optimal pore size, and high surface energy.Materials such as nanostructured titanium dioxide (TiO 2 ) are ideal due to their large surface area and chemical stability, providing numerous active sites for the adsorption of gas molecules.Desorption depends on factors such as binding energy and temperature.Strong interactions between gas molecules and the sensor surface can slow down desorption, resulting in longer recovery times.Increasing the temperature can facilitate faster desorption by providing the necessary energy to overcome binding forces.Sensor design must balance these kinetics to achieve rapid detection and quick recovery.Enhancing selectivity involves modifying the sensor material, such as doping TiO 2 with elements such as silver or platinum, to tailor the interaction strength with specific gases.These modifications optimize both adsorption and desorption rates, ensuring that the sensor performs reliably and efficiently.Understanding and optimizing these kinetic processes are then essential for developing high-performance gas sensors capable of detecting hazardous gases accurately and swiftly.
When the sensor is exposed to air, oxygen molecules are adsorbed on the surface and extract electrons from the conduction band [39,40].Oxygen molecules are adsorbed on the active sites of the rough grain surface as (O − 2 , O − , and O 2− ) by trapping electrons from the conduction band, which results in an electron depletion region [42][43][44].
When the TiO 2 -based sensor was exposed to H 2 S, it reacted with adsorbed oxygen species and released the trapped electrons back to the TiO 2 (see Figure 15).Hence, the high sensitivity to hydrogen sulfide can be attributed to its low dissociation energy compared to other gases on TiO 2 anatase, enabling it to readily react with the adsorbed oxygen [45][46][47][48]) to form SO 2 and H 2 O, as seen in Equation (3).This causes the bulk release of a large concentration of free electrons, which results in the narrowing of the electron depletion region.
and desorption are critical to the performance of gas sensors, influencing their sensitivity, response time, and recovery time.The adsorption process is enhanced by a high surface area, optimal pore size, and high surface energy.Materials such as nanostructured titanium dioxide (TiO2) are ideal due to their large surface area and chemical stability, providing numerous active sites for the adsorption of gas molecules.Desorption depends on factors such as binding energy and temperature.Strong interactions between gas molecules and the sensor surface can slow down desorption, resulting in longer recovery times.
Increasing the temperature can facilitate faster desorption by providing the necessary energy to overcome binding forces.Sensor design must balance these kinetics to achieve rapid detection and quick recovery.Enhancing selectivity involves modifying the sensor material, such as doping TiO2 with elements such as silver or platinum, to tailor the interaction strength with specific gases.These modifications optimize both adsorption and desorption rates, ensuring that the sensor performs reliably and efficiently.Understanding and optimizing these kinetic processes are then essential for developing high-performance gas sensors capable of detecting hazardous gases accurately and swiftly.When the sensor is exposed to air, oxygen molecules are adsorbed on the surface and extract electrons from the conduction band [39,40].Oxygen molecules are adsorbed on the active sites of the rough grain surface as ( , , and ) by trapping electrons from the conduction band, which results in an electron depletion region [42][43][44].
When the TiO2-based sensor was exposed to H2S, it reacted with adsorbed oxygen species and released the trapped electrons back to the TiO2 (see Figure 15).Hence, the high sensitivity to hydrogen sulfide can be attributed to its low dissociation energy compared to other gases on TiO2 anatase, enabling it to readily react with the adsorbed oxygen [45][46][47][48]) to form SO2 and H2O, as seen in Equation (3).This causes the bulk release of a large concentration of free electrons, which results in the narrowing of the electron depletion region.The good performances of our sensor compared to those reported in the literature are reported in Table 3. Remarkably, the sensor response is very high, considering the low H 2 S concentration tested in our case, as well as being faster.Finally, we planned new tests for the evaluation of further characteristics regarding our sensor.Among these, humidity is well-known to influence the response of resistive sensors.However, the exact behavior is not predictable and various findings have been reported, depending on the metal oxide, the target gas, the operating temperature, and the humidity value [51].Therefore, tests carried out in different humidity conditions have been planned for the near future.

Figure 2 .
Figure 2. SEM images of the TiO2 sample annealed at 400 °C acquired at 5k magnifications in two different regions.(a,b) At 20k magnifications (c) and 100k magnifications (d).

Figure 2 .
Figure 2. SEM images of the TiO 2 sample annealed at 400 • C acquired at 5k magnifications in two different regions.(a,b) At 20k magnifications (c) and 100k magnifications (d).

Figure 4 .
Figure 4. Raman spectra of the samples annealed at 400 °C and 600 °C.The inset shows an enlargement of the spectral region between 300 and 900 cm -1 .

Figure 4 .
Figure 4. Raman spectra of the samples annealed at 400 • C and 600 • C. The inset shows an enlargement of the spectral region between 300 and 900 cm −1 .

Figure 5 .
Figure 5. (a,b) Rietveld refinement of the X-ray diffraction profile and (c,d) Williamson-Hall plots of TiO 2 nanopowders calcined at 400 • C and 600 • C.

Figure 7 .
Figure 7. Absorbance spectra (inset shows the observed shift) (a) and Tauc plots (b) of TiO2 nanopowders prepared using the sol-gel technique and annealed at different temperatures.

Figure 7 .
Figure 7. Absorbance spectra (inset shows the observed shift) (a) and Tauc plots (b) of TiO 2 nanopowders prepared using the sol-gel technique and annealed at different temperatures.

Figure 10 .
Figure 10.Sensor response of TiO 2 (400 • C) and TiO 2 (600 • C) (a) at a H 2 S concentration of 2.5 ppm; (b) response versus H 2 S concentration at a temperature of 350 • C.

Figure 11 .
Figure 11.(a) Resistance vs. time for different concentrations and (b) response vs. concentration of the TiO2-NP sensor at an operating temperature of 350 °C.

Figure 12 .
Figure 12.Response and recovery time vs. H2S concentrations of TiO2-NPs at an operating temperature of 350 °C.

Figure 11 .
Figure 11.(a) Resistance vs. time for different concentrations and (b) response vs. concentration of the TiO 2 -NP sensor at an operating temperature of 350 • C.

Figure 11 .
Figure 11.(a) Resistance vs. time for different concentrations and (b) response vs. concentration of the TiO2-NP sensor at an operating temperature of 350 °C.

Figure 12 .
Figure 12.Response and recovery time vs. H2S concentrations of TiO2-NPs at an operating temperature of 350 °C.

Figure 12 .
Figure 12.Response and recovery time vs. H 2 S concentrations of TiO 2 -NPs at an operating temperature of 350 • C.

Figure 13 .
Figure 13.Selectivity pattern of the TiO2-NPs sensor at an operating temperature of 350 °C.

Figure 14 .
Figure 14.Reproducibility of the sensor response to three pulses of 4 ppm H2S in air.

Figure 13 .
Figure 13.Selectivity pattern of the TiO 2 -NPs sensor at an operating temperature of 350 • C.

Figure 13 .
Figure 13.Selectivity pattern of the TiO2-NPs sensor at an operating temperature of 350 °C.

Figure 14 .
Figure 14.Reproducibility of the sensor response to three pulses of 4 ppm H2S in air.

Figure 14 .
Figure 14.Reproducibility of the sensor response to three pulses of 4 ppm H 2 S in air.

Figure 15 .
Figure 15.Gas sensing mechanism of TiO2 (400 °C) in the presence of H2S gas.Figure 15.Gas sensing mechanism of TiO 2 (400 • C) in the presence of H 2 S gas.

Figure 15 .
Figure 15.Gas sensing mechanism of TiO2 (400 °C) in the presence of H2S gas.Figure 15.Gas sensing mechanism of TiO 2 (400 • C) in the presence of H 2 S gas.

Table 1 .
Fitting parameters of the Rietveld refinement on DRX profiles of TiO 2 annealed at 400 • C and 600 • C.

Table 3 .
Comparison of the sensing performances of the TiO 2 -based sensor with other sensors reported in the literature.