Some Metal Oxide Semiconductors Thin Films applied in Gas Sensor Applications

In recent years, the alarming increase in environmental pollution has necessitated the development of effective and efficient monitoring systems to detect and mitigate the impact of various pollutants and harmful gases on our planet. As a result, gas sensors have emerged as vital tools for detecting and quantifying the presence of pollutants in the atmosphere.

This mini-review focuses on gas sensors based on metal oxides, a category of sensors that has garnered significant attention due to their performance and versatility in detecting a wide range of gases. As the concentration of pollutants in the atmosphere continues to rise, understanding the operation and potential of metal oxide-based gas sensors becomes crucial for addressing environmental concerns and ensuring the well-being of our planet.

The Earth’s atmosphere is composed of a complex mixture of gases, each playing a critical role in maintaining a stable and habitable environment. However, the introduction of various pollutant gases, such as Carbon Monoxide (CO) [1], Nitrogen Dioxide (NO₂) [2], Sulfur Dioxide (SO₂) [3], Sulfur Hydrogen (H₂S) [4] and volatile organic compounds (VOCs) [5], has led to significant environmental challenges. These pollutants are primarily generated by industrial activities, transportation, agriculture, and other human-related sources.

Among the array of pollutants, Hydrogen Sulfide (H₂S) and Nitrogen Dioxide (NO₂) have emerged as significant contributors to environmental degradation. These two gases, though distinct in their origin and chemical properties, share a common characteristic: their capacity to exert a substantial influence on the natural world. In this context, it becomes essential to explore and comprehend the profound effects of H₂S and NO₂ on the environment, encompassing the air we breathe, ecological ecosystems, and public health. This examination not only deepens our understanding of the intricacies of environmental pollution but also underscores the urgency of developing robust gas detection and mitigation strategies.

Metal oxide-based gas sensors, with their unique properties and capabilities, have emerged as valuable tools in this endeavor.

In the subsequent sections of this mini-review, we will explore the principles of operation, key advancements, and applications of metal oxide-based gas sensors. By delving into the realm of thin films gas sensing technology, we aim to shed light on their pivotal role in NO₂ and H₂S detection and contribute to a better understanding of their potential in safeguarding the planet.

Resistive metal oxide gas sensors operate based on the change in electrical resistance when exposed to target gases. These sensors consist of a sensing material, typically a metal oxide semiconductor, which exhibits variable electrical conductivity in the presence of specific gases. When the sensor comes into contact with the target gas, a chemical reaction occurs at the surface of the sensing material. This reaction results in a change in the number of charge carriers (electrons) within the material, which, in turn, causes a measurable change in electrical resistance. The degree of resistance change is directly proportional to the concentration of the target gas, allowing for the detection and quantification of specific gases in the environment. These sensors are widely used in gas detection and air quality monitoring applications due to their simplicity, sensitivity, and responsiveness to various gases Figure 1.

Figure 1: XRD analysis of the NiFe2O4.

Metal oxides are chemical compounds composed of a metal element bonded to oxygen. They are formed when a metal reacts with oxygen, typically through oxidation processes. Metal oxides can have various chemical and physical properties depending on the specific metal involved and the oxidation state of the metal. The two types of conductivity found in metal oxide materials, namely, “N-type” and “P-type” conductivity, and their impact on gas detection.

In this review, we will examine the outstanding performance characteristics of metal oxide materials, including TiO₂, SnO₂, CO₃O₄, and MN₃O₄ [6-14], as they pertain to H₂S and NO₂ gas sensors.

Metal oxide N-type

N-type (negative-type) conductivity in metal oxide materials is associated with the presence of excess electrons. This excess of electrons is typically introduced through the incorporation of dopant atoms, which have more electrons than the host metal. In N-type materials, these dopants are often elements from Group V of the periodic table (e.g., nitrogen or phosphorus), which contribute extra electrons to the crystal lattice.

N-type semiconductor materials, such as TiO₂, SnO₂, and other suitable materials, are characterized by excess electrons and high electron density [15-20].

For H₂S detection: When exposed to H₂S, an n-type semiconductor’s excess electrons are readily captured by H₂S molecules. This capture results in an increase in electron concentration in the material. As a consequence, the material’s electrical conductivity increases (decrease in resistance) when H₂S is present. Therefore, N-type semiconductors are highly sensitive to H₂S and are suitable for its detection.

For NO₂ detection: N-type semiconductors, due to their high electron density, are less sensitive to NO₂, which is an oxidizing gas. The increased electron density in the n-type semiconductor can inhibit its interaction with NO₂, leading to limited sensitivity to this gas.

Metal oxide P-type

P-type (Positive-Type) conductivity in metal oxide materials is associated with the presence of electron “holes.” These holes are vacancies in the crystal lattice where an electron could exist. P-type conductivity is typically achieved by doping with elements from Group III of the periodic table (e.g., boron, aluminum), which have fewer electrons than the host metal.

P-type semiconductor materials, such as CO₃O₄ and MN₃O₄ are characterized by electron holes (vacancies) and lower electron density.

For NO₂ detection: When exposed to NO₂, which is an oxidizing gas, a p-type semiconductor’s electron holes readily interact with NO₂ molecules. This interaction results in the acceptance of electrons from NO₂, causing a decrease in electron hole concentration. As a result, the material’s electrical conductivity decreases (increase in resistance) in the presence of NO₂. Therefore, p-type semiconductors, including MN₃O₄, are well-suited for the detection of NO₂ due to their sensitivity to oxidizing gases.

For H₂S detection: P-type semiconductors are less sensitive to reducing gases like H₂S since the presence of electron holes can hinder interactions with H₂S. To effectively detect H₂S, n-type semiconductor materials or composite sensors that combine n-type and p-type materials may be used.

The following table provides a concise overview of the sensing performance of N-type and P-type semiconductors concerning their reactivity to reducing and oxidizing gases (Table 1). This comparison sheds light on their respective sensitivities [21,22].

Table 1:Some examples of sensing performance of N and P‐type towards reducing and oxidizing gases.

Nanowires, nanorods, nanobelts, and nanofibers, among other One-Dimensional (1D) nanostructures, have recently garnered significant interest for their potential to improve chemical sensing capabilities. Among these, nanowires, in particular, have been a focal point of research in recent years. The exceptional characteristics of metal oxide nanowires, including their elevated aspect ratios, substantial surface-area-to-volume ratios, exceptional crystallinity, remarkable thermal stability, efficient carrier-charge transport, robust chemical stability, and compatibility with electronic systems, position them as a promising option for developing gas sensors [23].

The N or P type of metal oxides thin films, such as TiO₂, SnO₂, Co₃O₄ and Mn₃O₄ their efficiency in gas sensor applications were reviewed [24].

In summary, the type of semiconductor material chosen for gas detection plays a critical role in determining the sensor’s sensitivity to specific gases. N-type materials are highly sensitive to reducing gases like H₂S due to their excess electrons, while P-type materials are effective in detecting oxidizing gases like NO₂ because of their electron hole interactions. The proper selection and combination of these materials enable the accurate detection of both H₂S and NO₂ gases in various environments.

The authors thank LVOBEEN, LCOMS and LMOPS laboratories for supporting this review article.

1. Buchholz RR, Park M, Worden HM, Tang W, Edwards DP, et al. (2022) New seasonal pattern of pollution emerges from changing North American wildfires. Nature Communication 13: 2043.
2. Wei J, Li Z, Wang J, Li C, Gupta P, et al. (2023) Ground-level gaseous pollutants (NO₂, SO₂, and CO) in China: daily seamless mapping and spatiotemporal variations. European Geosciences Union 23: 1511-1532.
3. Tang Y, Wu S, Chen S (2023) Economic Research-Ekonomska Istraživanja 36: 1886-1905.
4. Sun M, Wang X, Li Y, Zhao Z, Qiu J (2022) Selective catalytic oxidation of pollutant H₂S over Co-decorated hollow N-doped carbon nanofibers for high-performance Li-S batteries. Applied Catalysis B: Environmental 317: 121763.
5. Kaur P, Kaur R, Arora V (2023) Potential indoor volatile organic compounds: Detrimental impact on health and environment. AIP Conference Proceedings 2558: 020009.
6. Doubi Y, Hartiti B, Labrim H, Fadili S, Batan A, et al. (2020) Effect of annealing time on structural and optical proprieties of TiO2 thin films elaborated by spray pyrolysis technique for future gas sensor application. Materials Today Proceedings 30: 823-827.
7. Doubi Y, Hartiti B, Labrim H, Fadili S, Tahri M, et al. (2021) Experimental study of properties of TiO₂ thin films deposited by spray pyrolysis for future sensory applications. Applied Physics A 127: 475.
8. Doubi Y, Hartiti B, Siadat M, Labrim H, Fadili S, et al. (2022) Optimization with Taguchi Approach to Prepare Pure TiO₂ Thin Films for Future Gas Sensor Application. Journal of Electronic Materials 51: 3671-3683.
9. Doubi Y, Hartiti B, Siadat M, Nkuissi HJT, Labrim H, et al. (2022) The effect of experimental process on properties of pure TiO₂ nanostructure for fast NO₂ gas sensor. Applied Physics A 128: 463.
10. Doubi Y, Hartiti B, Siadat M, Labrim H, Fadili S, et al. (2023) Theoretical validation of experimental properties of TiO₂ prepared through organometallic precursors. Applied Organometallic Chemistry 37: e7024.
11. Doubi Y, Hartiti B, Siadat M, Labrim H, Fadili S, et al. (2022) The high impact of solution flow rate on optical properties of TiO₂ thin layers for optoelectronic applications. Crystal Research and Technology 57: 2200129.
12. Doubi Y, Hartiti B, Siadat M, Arba Y, Stitou M, et al. (2022) Impact of indium ions on some properties of TiO₂ thin films for sensor applications. Materialstoday: Proceedings 66(1): 385-389.
13. Doubi Y, Hartiti B, Siadat M, Ziti A, Stitou M, et al. (2022) Theoretical validation of the properties of SnO₂ nanostructure grown by robust spray pyrolysis technique for formaldehyde gas sensor. Materialstoday: Proceedings 66(1): 341-345.
14. Doubi Y, Hartiti B, Siadat M, Labrim H, Fadili S, et al. (2023) The high efficient investigations of properties of CO₃O₄ nanostructured by facile spray pyrolysis technique for future gas sensors application. Physical Status Solidi (RRL)-Rapid Research letters 17(9): 2200507.
15. Doubi Y, Hartiti B, Siadat M, Labrim H, Fadili S, et al. (2023) Experimental investigation of pure spinel Mn₃O₄ Properties Synthesized through Chemical Spray Pyrolysis for Future Gas Sensor Application. European Journal of Inorganic Chemistry 26(22): e20230047.
16. Goenuellu Y, Haidry AA, Saruhan B (2015) Sensor and Actuator B: Chem. Elsevier Science 217: 78-87.
17. Kheel H, Sun GJ, Lee JK, Lee S, Dwivedi RP, et al. (2016) Ceramics International 42: 18597-18604.
18. Wahid KIA, Chaker C, Chaker H (2023) Ni-doped SnO₂ thin films for NO₂ gas sensing application. Sensor Actuator. A Phys. 360: 114498.
19. Zhang S, Zhang P, Wang Y, Ma Y, Zhong J, et al. (2014) ACS Applied Materials & Interfaces 6: 14975-14980.
20. Simonenko TL, Simonenko NP, Mokrushin AS, Gorobtsov PY, Vlasov IS, et al. (2023) Preparation of V₂O₅ Thin Film by Sol-Gel Technique and Pen Plotter Printing. N.T. Kuznetsov, Chemosensors 11: 166.
21. Balouria V, Samanta S, Singha A, Debnath AK, Mahajan A, et al. (2013) Chemiresistive gas sensing properties of nanocrystalline CO₃O₄ thin films. Sensors and Actuators B 176: 38-45.
22. Kortidis I, Swart HC, Ray SS, Motaung DE (2019) Characteristics of point defects on the room temperature ferromagnetic and highly NO₂ selectivity gas sensing of p-type Mn₃O₄ Sensors and Actuators B: Chemical 285: 92-107.
23. Muruges P, Ganesan M, Udaiyar PS, Krishanan HS, Jayaram A, et al. (2023) Mesoporous Co₃O₄/SnO₂ Nanostructure-Based heterojunctions for NO₂ ACS Applied Nanomaterials 6(19): 17433-17444.
24. Ding J, Zhang Z, Song Z, Ding S, Weng J, et al. (2023) Recent progress in chemiresistive gas sensors with 1D nanostructured sensing materials: Insights into the structure-morphology-performance relationship. Materials & Design 234: 112360.