Introduction

Hydrogen sulfide gas (H₂S), which are combustible and toxic, are created by the sewage treatment, coal, oil, and natural gas sectors. H₂S was employed in a wide range of chemical industries, as well as in research facilities, and as a process gas in the manufacturing of heavy water. Although the gas's lateral exposure length is 10 ppm for an 8 h exposure, H₂S pleasant ambient scale is between 20 and 100 ppb [1].

It is therefore desirable to find H₂S at these small sizes. Low amounts of this gas can be detected by spectral and fluorescence analysis [2], but these sensors are expensive and have enormous dimensions. The most common kind of gas sensor that offers cheap cost, high sensitivity and selectivity, environmental friendliness, and portable device capabilities for long-range operations is thin film chemiresistive gas sensors based on transition metal oxides (TMOs) [3]. Transition metal oxide, which is used in optoelectronic devices including photodiodes, solar cells, and flat-panel displays, etc. The physical properties of transition metal oxide films created by various deposition techniques, such as laser deposition [4], molecular beam epitaxy [5], the sol-gel process [7], spray pyrolysis [8], dc/rf magnetron sputtering [9], and evaporation with electron beams [10], chemical vapour deposition [6], and electrochemical deposition [11] have been the focus of in-depth study for the past thirty years. In recent years, many researches have been conducted in the field of gas sensors using titanium dioxide [12], conductive polymers [13], nanoconductive polymers [14], nano cerium oxide/aluminum oxide [15], and nano cerium (IV) oxide/zinc oxide [16] all of which gave impressive results in the field of sensors. Where titanium dioxide can be applied in many fields, including an experimental study of the structural and photocatalytic properties of nitrogen-doped titanium dioxide [17], Au decorated mesoporous TiO₂ as a high performance photocatalyst towards crystal violet dye [18].

Also, some compounds other than titanium, such as single-phase bismuth nanoparticles (BiFeO₃), can be used in photocatalysts [19].

Finally, semiconductor nanoparticles have been used in many other applications, the most important of which is chemical catalysts [20-23].

In this work, the spray pyrolysis technique was used to fabricate thin films of TiO₂/NiO/In₂O₃ on glass substrates. Nanostructure, morphological and hydrogen sulfate gas sensing properties were investigated in detail.

Materials and methods

The chemical spray pyrolysis technique was used to fabricate NiO/TiO₂/In₂O₃ nanostructure thin films on glass substrates heated to 400 °C, as depicted in Figure 1. Prior to the deposition, there was a predetermined 30 cm gap between the sprayer head and the substrate. Glass substrates with 2×2 cm² were cleaned using an ultrasonic cleaner for 15 min with acetone, ethanol, and deionized water, and then dried for an hour in an air oven at 100 °C.

Figure 1: Schematic of spray pyrolysis technique

Indium chloride (InCl₃), purity 99.99%, molecular weight 221.18 g/mole and nickel chloride (NiCl₂), quality 99.99%, molecular weight 129.5994 g/mole were used to prepared 0.2 M were dissolved in 100 ml of deionized water. A solution of 100 ml with 0.2 M titanium trichloride (TiCl₃), purity of 99.99%, molecular weight of 154.22 g/mole was also used. The solutions were mixed using a magnetic stirrer until completely dissolved, and the molarity (M) was calculated using the formula:

M = (Wt / MW) × (1000 / V) (1)

where, Wt is the wight of salt in gram, MW is the molar weight typically in gram per moles, and 𝑉 is the volume of the solution typically in litres. TiO₂/NiO/In₂O₃ films with various molar ratios 2:6:2, 3:5:2, and 4:4:2were prepared. Using a home-made gas-detection device, the sensors' sensing abilities were tested. The base plate was the conduit for electrical feeds, the base plate's heating was modified in order to warm the test sample and bring it to the required operating temperatures. A thermocouple was used to determine the sensors' operating temperature.

Digital temperature displays were linked to the thermocouple's output, and one of the apertures on the base plate had a gas inlet valve attached to it. Using a gas flow meter, a known quantity of test gas was introduced into the system to create the desired gas concentration. A steady voltage and current were sent to the sensors.

FESEM (inspect F-50 company) was used for determining the morphology of the materials. XRD (6000 SHIMADZ made in Japan) was utilized for estimating crystal structure and the average crystallite size. UV-Vis absorption was measured using a Perkin Elmer Spectrophotometer Model Lambda 365.

Results and Discussion

The X- ray patterns of TiO₂/NiO/In₂O₃ thin films at varies concentration are shown in Figure 2, were prepared by spray pyrolysis technique. The samples were prepared on glass substrate at temperature 400 ºC, 30 cm distance between spray head and substrate and the scans were performed over 2θ=[10º-80º]. In general, can find that the samples had a polycrystalline structure and all of the observed peaks can be assigned to the samples, in accordance to the data obtained from JCPDF (card no. 21-1276) [24].

Figure 2: XRD of TiO₂/NiO/In₂O₃ composite thin films on glass substrate

The formation of TiO₂ thin film and planes corresponding to (010), (200), (211), and (204) appear polycrystalline structure with tetragonal anatase phase. Start planes for the fabrication of nickel oxide crystals are (111) and (200) planes. The diffraction pattern of the sample, which were indicates the polycrystalline of the NiO thin film with nanostructure cubic phase. In addition, the show XRD patterns for In₂O₃ thin film exhibit a polycrystalline nature with (211), (222), (222), (400), and (440), planes with nanostructure cubic phase. Table 1, represent a full width at half maximum (FWHM), inter-planar distance (d_hkl) and the grain size (G.S.) using the Scherrer equation, the grain size was determined from FWHM (β) [25]. From the results, a smaller grain size as well as larger FWHM values mark preferable crystallization of the materials and nanocrystalline material can be grown by preparation condition.

Table 1: Comparison of the standard and expected values of d_hkl for the peaks identified by XRD

Nanoscale grass-like features are seen on the SEM image of the TiO₂/NiO/In₂O₃ thin films, as illustrated in Figure 3. The nanostructure is aligned and is homogeneously distributed with few vacancies.

Polyhedral shells are also scattered depressively over the surface, with some lining up along the nanoparticle. The SEM image shows small granular grains evenly scattered over the surface with no cracks. However, white-coloured spots developed on the film surface from time to time, possibly due to the partial breakdown of the precursor salt employed. Small spherical granules with sizes ranging from 20 to 100 nm were discovered. The film surface looks to be homogeneous, with no holes or breaks. However, XRD measurements show that the grain size is smaller than expected by the SEM surface micrograph.

In general, thin films of metal oxides are optically transparent and electrically conductive within the visible wavelength range. The optical measurement of the prepared films were carried out using UV-Vis (200-900) nm Spectroscope and the spectral dependence of the absorbance for all thin films deposit at same conditions, for different mixed TiO₂, NiO and In₂O₃ are demonstrated in Figure 4. The thin film absorbance spectra as a function of wavelength, in the range of 200-900 nm were analysed. There are three main zones of interest, as indicated by the spectral dependency in the Figure 4.

The existence of these oscillations is an indication of the films' high optical quality. When stored in an environment, all films were stable for a long time, uniformly adhered to the substrate, and transparent. The typical absorbance of films formed on glass substrates is higher by around 0.1 over the wavelength range 400 to 800 nm. A sharp fall in absorption at about 335, 325, and 305 nm is due to absorption edge of different mixed TiO₂ (40, 30, and 20)%: NiO(40, 50, and 60)%: In₂O₃ 20% semiconductor thin films, respectively.

Figure 3: The FESEM images of TiO₂/NiO/In₂O₃ hin film deposited on glass substrate

Figure 4: Absorbance spectrum as a function of wavelength for different mixed TiO₂/NiO/In₂O₃

The absorption edge was slightly red shifted with the increasing mixed ratio of TiO₂ respect to NiO, as depicted in Figure 4. One can calculate the semiconductor's band gap (E_g) using the data point close to the absorption edge. The absorption coefficient (α) for directly permitted transitions on basic parabolic scale can be expressed as αhυ various (hυ-E_g)^1/2 as a function of incident photon energy. By extrapolating the graph's straight line to the energy axis, the band gap can be calculated. Table 2 presents the optical energy gap values for all samples in vest verse. A significant degree of non-stoichiometric abnormalities in the film may be the root of the relatively high band gap value. This implies that the high conducting electron density is due to defects from the ideal single crystal structure, such as oxygen vacancies, interstitial atoms, and dislocations acting as conducting electron donors. The metal component oxidation state (also known as the stoichiometry of the oxide), the kind and quantity of mixture included in the thin films have significant effects on the electrical characteristics of metal oxide. The mobility (μ), concentration (n), and of the specific free carrier, are all related to electrical conductivity (σ). For the production of thin films with high conductivity, both high carrier concentration and mobility are necessary [26]. Likewise, the morphology of the samples has a major effect on the electrical properties [27]. As oxygen vacancies get larger in amount, electrical conductivity becomes greater [28]. For nanometric grains with a little contribution at the grain boundary, electrical conductivity is higher. In contrast to bulk material grains, where the (σ) is linked to trapped carriers at the grain boundary, in this instance, the high (σ) is connected to the uniform distribution of carriers throughout the grain [29]. The grain boundary effect makes up a small portion of the total conductivity, whereas the electron mobility within the grain makes up the majority of the electrical conductivity. The maximum value of the activation energy during the low temperature area is caused by a modest amount of thermal energy that is more than enough to activate the charge carriers that occur during the conduction process. This activation energy grows as the operation temperature for samples rises. In contrast, a rise in charge mobility can be credited with the increased conductivity in the low temperature area. When compared to regions with low temperatures, activation energy is higher in high temperature areas. In this area, intrinsic flaws known as high temperature or intrinsic conduction are the primary determinants of electrical conductivity. Table 3 provides an overview of activation energy variation for mixed thin films.   

Table 2: Parameter of different mixed TiO₂/NiO/In₂O₃ thin films

Table 3: The activation energy of different mixed TiO₂/NiO/In₂O₃

Figure 5 demonstrates the sensitivity of the TiO₂/NiO/In₂O₃ thin films coated on glass substrate as a function of temperature for H₂S gas, at operating temperature 150, 200, 250, and 300 ºC using reducing gas H₂S with 30 ppm concentration. According to this figure, a good sensitivity at must operating temperature 150 to 300 ^oC and it can be observed that it has the higher sensitivity to 30 ppm from H₂S, for temperature up to 200 ^oC was about 93.5 for a sample 40:40:20%. The increasing number of surface-adsorbed oxygen species and their nanoscale size are responsible for the enhanced sensitivity.

Figure 5: The variation of sensitivity with the operating temperature of the prepared, different mixed TiO₂/NiO/In₂O₃ to H₂S gas

Figure 6, demonstrates the response and recovery times for various mixed TiO₂/NiO/In₂O₃ as sensor samples as a function of temperature for H₂S gas. Figure 6 shows the response and recovery time decrease with increased the operation temperature. Where, the sample 4:4:2 thin film gas sensors have 6.3 s a fast response time for H₂S gas at 300 ^oC operation temperature, while the have fast recovery time 38.7 s for NO₂ gas at 300 ^oC operation temperature.

Figure 6: The variation of response time and recovery time with operation temperature of the prepared, different mixed TiO₂/NiO/In₂O₃ to H₂S gas

The surface reaction between chemisorbed oxygen and oxide gas is a key component of the gas sensing mechanism. There are two distinct methods by that oxygen can adsorb on a film surface: physisorption and chemisorption, where chemisorption predominates at high temperatures. Increasing operational temperature can provide the activation energy necessary for the transition from physisorption to chemisorption. According to reports, the amount of oxygen adsorbed on the sensor surface rises together with the temperature [30-31]. At temperatures between 150 and 300 ^oC, oxygen has a major impact on the electrophysical and gas sensing characteristics of samples where it is adsorbed at the surface of the metal oxide that permits an electron trapping. A location where the metal oxide's surface has oxygen adsorbed, allowing for the trapping of an electron. The result is a decrease in the charge carrier density, which raises the samples' resistance. 𝑂⁻ species on the surface serve as electron acceptors, causing a depletion layer to form that extends to the particles and the surface barrier, where, it regulates the electron transfer between particles, test gas, and the sensing material, and so affects the overall resistance, these surface states and surface barriers play a significant role for sensors. When exposed to H₂S gas at a high temperature, the pre-adsorbed oxygen species (𝑂⁻𝑠𝑢𝑟𝑓) are formed. It causes the oxygen coverage to increase, attracting more electrons to the samples conduction band and therefore increasing the barrier. This process reduces the depletion region and either increases resistance or reduces conductivity [32].

Figure 7 depicts the change in resistance as time progresses of TiO₂/NiO/In₂O₃ (40:40:20)% thin film as H₂S gas is fed into the testing chamber while the bias voltage is maintained at 3 V. The sensor resistance is directly correlated to time and initially reaches steady state prior to gas opening. At this point, the gas is opened to allow for air mixing inside the chamber. When switched the gas off, the resistance quickly reduced to achieve a stable condition before returning to its initial value. The way that gas molecules interact with the surface atoms of the detecting film determines how effectively a sensor can detect the presence of gas. The surface's reactivity is extremely reliant on the structural defect. When the gas is on, the resistance rapidly decreases over time until it reaches a saturated condition; this could be the result of the surface becoming saturated with H₂S gas adsorption. When the gas is turned off, the resistance returns to its initial state.

Figure 7: The variation resistance with time for different operation temperatures of H₂S gas for mixed TiO₂ 40%: NiO 40%: In₂O₃ 20% gas sensor

Conclusion

In conclusion, we have successfully constructed extremely sensitive TiO₂/NiO/In₂O₃ nanostructure thin film sensors for detecting trace amounts of H2S gas using the spray pyrolysis process. The TiO₂/NiO/In₂O₃ nanostructure thin film sensor demonstrated a high sensitivity value of 93.5% at 200C and a rapid response-recovery time of 6.3 s and 38.7 s at 300 oC for 4:4:2 thin film gas sensor sample. As a result, it is possible that the investigated thin film nanostructure will make it easier to find a suitable material for detecting low levels of H₂S in the environment. This work, therefore, provides a practical approach to the design and development of high-performance and low-power gas sensors.

Disclosure Statement  

No potential conflict of interest was reported by the authors.

Funding

This study did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions

All authors contributed toward data analysis, drafting, and revising the paper and agreed to responsible for all the aspects of this work.

Conflict of interest

The authors declare that they have no conflicts of interest in this article.

ORCID

Fuad .T Ibrahim

https://orcid.org/0000-0001-9796-9990

HOW TO CITE THIS ARTICLE

Fuad .T Ibrahim*, M. Abd Shahoodh, S. Guermazi. Studying the Structural and Optical Properties of thin compound oxide films based on TiO₂: NiO: In2O3 nanostructure. Chem. Methodol., 2023, 7(11) 871-882

DOI: https://doi.org/10.48309/chemm.2023.409731.1693

URL: https://www.chemmethod.com/article_183275.html