Thin Film Semiconductor Metal Oxide Oxygen Sensors: Limitations, Challenges, and Future Progress

Abstract

Among oxygen sensors, types such as polymer-, ceramic-, or carbon-based ones may be distinguished. Particular interest in semiconductor metal oxide (SMO) sensors has recently been observed. This is due to their easy fabrication process, high control over the final product (dopants, posttreatment, etc.), and high concentration of oxygen vacancies, by which they show significant changes in electrical properties when exposed to analyte. In this review, different types of sensors are described and categorized. Importantly, their limitations, challenges and principles of sensing mechanism are also discussed, wherein attention is primarily paid to semiconductor metal oxide (SMO) oxygen sensors. This comprehensive review provides an in-depth analysis of the existing literature on planar SMO oxygen sensors, focusing on various materials, fabrication techniques, and sensing mechanisms. It also critically assesses the challenges and limitations in current research, offering insights into future directions for developing highly efficient and reliable sensors. Currently, most oxygen resistive sensors are a few micrometers thick and operate at high temperatures, which leads to high power consumption. To highlight importance of this topic, a market overview is also presented.

1. Introduction

Gas sensors, especially oxygen gas sensors, play an important role in the modern world. They are used in many fields such as the healthcare industry [1], food production [2], the car industry [3], monitoring of air pollution [4,5], high-tech, and many others. In the healthcare industry, oxygen sensors are critical in respiratory devices, ensuring that patients receive the correct amount of oxygen and monitoring their respiratory status in real time [6]. In food production, these sensors help maintain the quality of perishable goods by monitoring and controlling the levels of oxygen in packaging environments, thereby extending shelf life and ensuring food safety [7]. The automotive industry relies heavily on oxygen sensors to monitor and optimize engine performance, reducing harmful emissions and improving fuel efficiency [8].

In environmental monitoring, oxygen sensors are used to track air pollution levels, providing crucial data for managing air quality and protecting public health [9]. These sensors are also essential in high-tech applications, such as semiconductor manufacturing, where precise control of oxygen levels is necessary to ensure the quality of the products [10]. Thanks to the application of oxygen sensors, it is possible to improve the quality and safety of products, services, and technology by controlling substantial process parameters [11]. These improvements include enhanced product reliability, reduced operational costs, and compliance with environmental regulations [6].

At this point, the development of gas sensors focuses on miniaturization and wireless capabilities, which aligns with the rising market of the Internet of Things (IoT) and wearable electronics [12]. Miniaturized sensors can be integrated into small, portable devices, enhancing their versatility and application range. Wireless capabilities allow for these sensors to be deployed in remote or inaccessible locations, providing real-time data transmission and integration with IoT networks. This integration enables advanced monitoring and control systems, facilitating smarter, more responsive technologies. Each branch of industry has different requirements for resistive oxygen sensors, which are listed in the table below in

Table 1. Special requirements of each sector.

Sector	Special Requirements
Healthcare	Fast response Very precise
Food	Sensitive for oxygen trays Fast response
Automotive	Resistant to harsh environment Resistant to high temperatures
Environmental monitoring	Highly selective Detecting in wide range Long-term stability
HI-TECH	Sensitive for oxygen trays Very precise Fast response
IoT	Resistant to mechanical stress Low work temperature Small size

.

A well-designed sensor should be stable over a long period, reproducible, repeatable, and capable of providing a fast response regarding the measured medium. Stability ensures that the sensor maintains consistent performance and accuracy over time, even when subjected to varying environmental conditions and prolonged use. Reproducibility indicates that the sensor can deliver the same results under identical conditions in different instances of measurement, which is crucial for reliable data collection. Repeatability ensures that the sensor produces consistent results when used multiple times under the same conditions. A fast response time is essential for real-time monitoring and immediate decision-making, which is particularly important in critical applications such as medical diagnostics and environmental monitoring.

Without these characteristics, a sensor is ineffective and cannot be considered a reliable source of information. For instance, a sensor that lacks stability may drift over time, leading to inaccurate readings. Similarly, if a sensor is not reproducible, it cannot be trusted to provide consistent data across different measurements, which undermines its reliability and usefulness. Repeatability is crucial for applications requiring frequent measurements, and a sensor that is slow to respond can delay critical actions in time-sensitive scenarios.

Oxygen sensors are mostly categorized by their sensing mechanism, which includes electrochemical, optical, resistive, and magnetic methods, among others in Figure 1.

The choice of sensing mechanism significantly influences critical parameters such as working temperature, limit of detection (LOD), limit of quantification (LOQ), working environment, and other essential properties. The operating temperature range of a sensor affects its applicability in different environments. For example, resistive sensors often require high temperatures to function correctly, which can limit their use in low-temperature settings. Conversely, optical sensors can operate at room temperature, making them suitable for a broader range of applications [6].

The sensor’s ability to function under various environmental conditions, including humidity, pressure, and the presence of interfering gases, is crucial for its reliability and accuracy. For instance, sensors used in industrial settings must withstand harsh conditions, while those used in medical applications need to be highly sensitive and selective [13].

Other essential properties include factors such as sensor size, power consumption, ease of integration with existing systems, and cost. Miniaturization and low power consumption are particularly important for wearable and portable devices, which are increasingly relevant in the context of the Internet of Things (IoT) and personalized healthcare.

The electrical transport properties of thin film materials are also important in terms of the performance of oxygen sensors. Thin film oxygen sensors rely primarily on the changes in resistance resulting from adsorption and desorption of O₂ molecules on the sensor surface. The mobility of charge carriers as well as the concentration of holes and electrons in the thin films are important parameters influencing both the sensors’ electrical conductivity and sensitivity. The thickness of the thin film layer is crucial due to the fact that it affects the effective surface area of the interactions with the gas and charge transport mechanisms such as diffusion and ion migration. For example, studies on thin film oxygen sensors based on Nb₂O₅ demonstrated that their electrical properties are strongly dependent on the electrode material and operating temperature. Within the temperature range of 400 °C to 600 °C, Nb electrodes work as effective electron injectors, and the electrical conductivity varies with oxygen pressure. Surface electron traps related to the chemisorbed oxygen play a crucial key role, as atmospheric oxygen can diffuse into the layer through channels in the Nb₂O₅ crystal structure. At low voltage, the Cr and Pt electrodes act as blocking valves, while at high voltage, they start to inject charges. Structures shorter than 50 μm lose their oxygen vacancies in electric fields of 103–104 V/cm [14].

This review focuses on the differences in manufacturing technologies of individual sensors and their potential application areas. In examining these technologies, the main aim of this review is to highlight the unique attributes and limitations of various sensor types, ranging from electrochemical to optical and resistive sensors, and how these attributes influence their performance in different environments. It seems that the current niche is the Internet of Things. IoT technology demands sensors with extremely low energy consumption to facilitate the continuous and autonomous operation of a vast network of interconnected devices. These sensors must be capable of operating efficiently over extended periods without frequent battery replacements or recharging, making energy efficiency a critical factor in their design. Thus, their design must be optimized from the very beginning with their intended use in mind. This optimization involves selecting materials and fabrication processes that not only meet the specific functional requirements but also enhance the sensor’s durability and energy efficiency. For instance, leveraging advances in nanotechnology can lead to the development of sensors with higher sensitivity and lower power requirements. Additionally, integrating advanced signal processing techniques can help minimize power consumption while maintaining high accuracy and reliability. Furthermore, the integration of these sensors into IoT ecosystems requires careful consideration of communication protocols and data management strategies to ensure seamless connectivity and interoperability. The sensors must be designed to work harmoniously within the IoT framework, supporting real-time data transmission and efficient data handling. In summary, this review underscores the importance of tailoring sensor designs to their specific applications, particularly in the context of the burgeoning IoT landscape. By focusing on energy efficiency, material selection, and integration strategies, it is possible to advance the development of next-generation sensors that meet the demands of modern technology-driven environments.

2. Oxygen Sensor Market

The oxygen sensor market is a dynamic growing sector of measurement technology and analysis branch. There are many kinds of oxygen sensors, like optical sensors, magnetic sensors, resistive sensors, etc. The main market drivers are as follows:

• Increased regulatory requirements for monitoring air quality and industrial emissions;
• Development of medical technologies and the need to monitor patients’ condition in real time;
• Growing demand for energy efficiency and optimization of industrial processes.

In Figure 1, the forecasted change in the market value of oxygen sensors is presented.

The international market for gas-sensing devices is still growing, with its estimated value for 2030 being USD 4.7 billion with 7.8% CAGR (compound annual growth rate) over the period 2024–2030 [15]. The largest sales market of oxygen sensors is automotive, which is still fast growing at a rate of 7.3% (CAGR for 2024–2030 forecast period). This growth is caused by environmental protection regulations and the growing popularity of EVs (electrical vehicles) and hybrid cars. According to the reports of oxygen-sensing device analyses, the largest market share is located in the APAC region, with a 43% share in 2023 [15,16]. Right now, the most popular devices are zirconia-based sensors, a with market share of 46% [15]. The leadership of this kind of sensor may be lost with new trends in the oxygen sensor market, like rise of the IoT industry or miniaturization of sensors. The market for oxygen sensing devices is very dynamic; the biggest players in this field are ABB Ltd., Siemens AG, and Honeywell International Inc., which constantly conduct field research for the improvement of their sensors in terms of accuracy, sensitivity, and durability [15].

3. Classification of Oxygen Sensors in Terms of Their Sensing Mechanism

The sensing mechanism thus determines the suitability of an oxygen sensor for specific applications by influencing these critical parameters. The ongoing advancements in sensor technology aim to optimize these parameters to develop more efficient, reliable, and versatile oxygen sensors for a wide range of applications. There are many types of oxygen sensors, using different mechanisms of operation to detect oxygen concentrations in different environments. In Figure 2, the classification of these sensors is presented.

In the subsections below, each type of oxygen sensor presented in Figure 2 is briefly characterized.

3.1. Optical Sensors

The optical sensors are based on the extinction of luminescence emitted by oxygen-sensitive dyes [17,18]. These types of sensors work at room temperature and can also be used in water. Their advantages include fast reaction times and high sensitivity [19], making them suitable for applications that require the rapid and precise detection of oxygen levels, such as medical diagnostics, environmental monitoring, and industrial process control [18]. One of the primary advantages of optical sensors is their ability to provide non-invasive and continuous monitoring of oxygen levels without consuming oxygen in the process. This is particularly beneficial in biological and medical applications, where maintaining the integrity of the sample is crucial. Additionally, optical sensors can be miniaturized and integrated into compact devices, facilitating their use in portable and wearable technologies [18]. However, optical sensors tend to degrade with exposure to the environment, leading to a decrease in detection capabilities, which limits their applications [18]. This degradation can be caused by such factors as photobleaching, where prolonged exposure to light causes the dye molecules to lose their ability to emit luminescence. Additionally, environmental conditions such as humidity, temperature fluctuations, and exposure to chemicals can adversely affect the performance and longevity of optical sensors. Another significant disadvantage is the potential for signal interference from other fluorescent substances present in the environment, which can lead to inaccurate readings. This issue is particularly problematic in the case of complex biological samples or industrial environments where multiple fluorescent compounds may coexist. Moreover, the initial cost of optical sensors can be higher compared to other types of sensors due to the sophisticated materials and manufacturing processes required to produce them.

Despite these challenges, advancements in materials science and sensor design are continually improving the robustness and reliability of optical sensors. For example, the development of more stable and resilient dyes, as well as protective coatings and encapsulation techniques, can help mitigate the effects of environmental degradation. Additionally, integrating advanced signal processing algorithms can enhance the accuracy of measurements by compensating for potential interferences and fluctuations in sensor performance.

In summary, while optical sensors offer significant advantages in terms of sensitivity, response time, and non-invasive operation, their susceptibility to environmental degradation and potential for signal interference pose challenges that need to be addressed. Continued research and innovation in this field are essential to overcome these limitations and expand the applicability of optical sensors in various domains.

Studies on optical oxygen sensors have been presented, for example, by Zhang et al. [20]. Here, eight various types of optical oxygen sensors with a form of film were prepared via impregnating the matrix and the indicator solution on the surface of a microporous membrane based on polypropylene. Polymethylmethacrylate, ethylcellulose, and the mixture of these materials constituted a polymer matrix. Performed studies demonstrated that both optical and morphological properties of developed sensors depended strongly on the ratio of ethylcellulose to polymethylmethacrylate. Importantly, the developed sensors demonstrated a very short response time (<1 s).

3.2. Schottky Diode

Schottky diodes can also serve as gas sensors. A Schottky diode is a semiconductor device characterized by its low forward voltage drop and fast switching speed. Unlike a standard p-n junction diode, a Schottky diode is formed by the junction of a metal and an n-type semiconductor. This metal–semiconductor junction creates a barrier known as the Schottky barrier. The operation of a Schottky diode relies on the movement of electrons across the metal–semiconductor interface. When forward bias is applied, electrons from the n-type semiconductor gain enough energy to overcome the Schottky barrier and move into the metal, allowing for current to flow. The low forward voltage drops, typically between 0.2 to 0.3 volts, result from the minimal energy required for electrons to cross the barrier. In reverse bias, the diode exhibits very low leakage current, as the Schottky barrier height prevents significant electron flow back into the semiconductor. Schottky diodes can be constructed using various materials for both the metal and the semiconductor components. Commonly used metals include aluminum, platinum, gold, and nickel. The choice of metal affects the height of the Schottky barrier and consequently the diode’s performance characteristics. The semiconductor is typically silicon (Si), but other materials like gallium arsenide (GaAs) and silicon carbide (SiC) are also used, particularly in high-power or high-frequency applications.

The current flowing through its junction depends on several factors, including the work function of electrons, which is altered by the removal of electrons from the metal. This work function changes due to the adsorption of gas molecules on the metal surface or their diffusion through the metal to the metal–semiconductor interface. The detection mechanism of the Schottky diode gas sensor is based on the principle that the conductivity of the junction varies with the partial pressure of the detected gas [21]. This unique detection method allows for the sensitive and selective identification of various gases, making Schottky diode sensors particularly useful in environments where precise gas detection is critical. Their ability to operate at lower temperatures compared to the other semiconductor gas sensors, coupled with their fast response times, offers significant advantages in terms of the energy efficiency and operational speed [22]. Potential applications range from industrial monitoring processes to environmental sensing and healthcare diagnostics. However, the performance of Schottky diode sensors can be influenced by such factors as humidity, temperature fluctuations, and the presence of interfering gases, which may necessitate the implementation of calibration and compensation techniques to ensure accurate gas measurements [23].

In summary, while Schottky diodes offer some benefits as oxygen sensors, such as fast response times and sensitivity, they also have several drawbacks, including temperature sensitivity, high reverse leakage current, material stability issues, interference from other gases, complex calibration requirements, high material costs, and potential surface poisoning. These limitations must be carefully considered when designing and deploying Schottky diode-based oxygen sensors.

An example of a Schottky diode consisting of silicon carbide-based substrate, platinum electrode, oxygen-sensitive TiO₂ layer, and a composite layer based on titanium and platinum is described in [24]. The developed oxygen sensor was characterized by the ability to operate at high temperatures and demonstrated excellent oxygen sensitivity. Importantly, the sensor was also able to detect changes in the concentration of oxygen at a high response rate.

3.3. Electrochemical Devices

Different types of gas sensors include electrochemical devices that measure current or voltage during the chemical oxidation reaction, often employing polarographic and electrocatalytic methods. These sensors are characterized by high sensitivity, simplicity, and low manufacturing costs. Electrochemical sensors work by having an electrolyte that reacts with the target gas to produce an electrical signal proportional to the gas concentration. This process allows for highly accurate and sensitive measurements, making them suitable for detecting low concentrations of gases.

A critical factor influencing their performance is the purity of the electrolyte, which affects the accuracy of the sensor’s response, especially at low oxygen levels. Impurities in the electrolyte can cause interference and drift in the sensor’s readings, leading to inaccurate measurements. Maintaining high electrolyte purity is therefore essential for ensuring consistent and reliable sensor performance.

Additionally, the thickness of the permeable membrane is crucial for the sensor’s performance and longevity [25]. The membrane controls the rate at which gas molecules reach the electrolyte impacting the sensor’s response time and sensitivity. A properly designed membrane balances gas permeability and durability, providing optimal sensor performance over a longer period.

However, these sensors have some disadvantages, such as a strong dependence on temperature. Electrochemical reactions are temperature-sensitive, and fluctuations can affect the sensor’s accuracy. Most electrochemical sensors require temperature compensation mechanisms to maintain precise readings across varying environmental conditions. Another significant drawback is their relatively short service life. Over time, the electrodes and electrolytes can degrade, reducing the sensor’s effectiveness. This degradation is often accelerated by exposure to harsh conditions or reactive chemicals, which can poison the sensor’s surface. Surface poisoning occurs when chemicals adhere to the electrode surfaces and block the active sites necessary for the electrochemical reactions. This can lead to a permanent loss of sensitivity and necessitate frequent sensor replacement or maintenance.

Moreover, electrochemical sensors can suffer from cross-sensitivity to other gases present in the environment. While they are designed to be selective for specific gases, the presence of other reactive gases can interfere with the sensor’s readings, leading to false positives or inaccurate measurements. This limitation requires careful consideration of the sensor’s operating environment and can involve the use of additional filters or calibration techniques to mitigate cross-sensitivity effects.

A key parameter of electrochemical sensors is their response time. It determines their suitability in various applications. This time depends on a number of factors including among others the chemical composition of the electrolyte, electrode material, and operating conditions such as pressure and temperature. The high sensitivity and precision of electrochemical oxygen sensors often go in hand with short response times, allowing for them to quickly detect changes in oxygen concentration. Hence, they can be used effectively in dynamic environments where fast and accurate measurements are required. Many investigations are being performed on determining the response time of selected electrochemical oxygen sensors. So far, conducted studies have demonstrated response times of approximately 3–4 s [20,26], 40 s [27], or 60–70 s [28,29]. Current measures to improve the response time of electrochemical oxygen sensors include optimizing the electrolyte chemistry and using advanced electrode materials. Moreover, the introduction of nanostructured electrodes increases the reactive surface area, which accelerates the chemical processes occurring on their surface. In addition, the use of thinner electrolyte layers and improved ionic conductivity also contributes to faster response times [30,31,32].

Very often, the limiting factor is the kinetics of the processes occurring in the sensor. This can be either reaction or diffusion. Both phenomena are highly temperature-dependent (according to the Arrhenius law), which causes these types of sensors to be exceptionally sensitive to temperature changes. Ji-Hoon Han et al. [26] demonstrated an electrochemical oxygen sensor with application of a solid electrolyte. Nafion with a thickness of 2.4 µm was used. Under ambient temperature, the response time of the electrode was about 10 s. The registered current was in range of −20 nA to −1µA. Brandon K. Ashley et al. [29] presented an electrochemical sensor for lactate and oxygen monitoring. The presented oxygen sensor was capable of exhibiting a sensitivity of 21.6 nA/[O₂]% and exhibited a response time of 69 s (90%). From the presented studies, it is evident that the response time of electrochemical oxygen sensors is long. Therefore, further research is being conducted in this area to develop more efficient sensors with faster response times. This ongoing research aims to optimize sensor design, improve materials, and enhance operating conditions to achieve quicker and more reliable measurements, making these sensors more suitable for a wider range of applications.

Despite these challenges, advancements in materials science and sensor design are continually improving the performance and durability of electrochemical gas sensors. Innovations such as the development of more stable and selective electrode materials, improved electrolyte formulations, and advanced fabrication techniques are helping to address these limitations and enhance the overall reliability and applicability of electrochemical sensors in various industries.

An advanced type of electrochemical oxygen sensor is a zirconium sensor. They use zirconium oxide (ZrO₂) as the electrolytic material. They are widely used in a variety of applications, including vehicle emission control and industrial process monitoring. The operation of these sensors is based on the measuring the difference in electrical potentials caused by the difference in oxygen concentrations on either side of the ZrO₂ membrane. At higher temperatures, the ZrO₂ becomes a conductor of oxygen ions, allowing for them to migrate across the membrane. When the concentration of oxygen differs on both sides of the membrane, the generated voltage is proportional to this difference, allowing for the oxygen concentration in the sample to be accurately determined. Zircon oxygen sensors demonstrate high sensitivity and accuracy, as well as the ability to operate at extreme temperatures, which makes them ideal for applications in harsh environments, such as combustion chambers [33,34].

In summary, while electrochemical gas sensors offer high sensitivity, simplicity, and cost-effectiveness, they are also subjected to some limitations, such as temperature dependence, short service life, potential surface poisoning, and cross-sensitivity. Ongoing research and technological advancements are essential to overcome these challenges and expand the utility of electrochemical sensors in diverse applications.

A study on electrochemical oxygen sensors is described in [35]. In this work, an innovative solid state sensor was fabricated using an ionic liquid electrolyte and 1-butyl-3-methylimidazolium hexafluorophosphate. The developed structure demonstrated high oxygen sensitivity (0.054–0.177 v/v%), a low detection limit (0.0075%), as well as a response time lower than 10 s. Hence, the fabricated sensor showed promising properties in terms of its potential application for gas detection.

3.4. Magnetic Sensors

Magnetic sensors are known for their high measurement accuracy and durability, finding widespread use in fields such as industrial process control and medicine. Specifically, paramagnetic sensors, which leverage the paramagnetic properties of oxygen to attract its molecules into a strong magnetic field, are employed in detecting low-density oxygen. These sensors quantify the concentration of oxygen molecules per unit volume. Maintaining constant measurement conditions, such as temperature and flow, is crucial for accurate results [36].

The detection limit of paramagnetic sensors can reach levels as low as a few parts per million (ppm) of O₂, making them highly sensitive. However, they typically have a practical detection threshold of about 50 ppm O₂. This high sensitivity is essential in applications requiring precise oxygen level measurements, such as medical diagnostics and environmental monitoring [36,37].

Despite their advantages, these sensors come with challenges, including high costs and difficulties in miniaturization. The high cost is primarily due to the complexity of magnetic systems and the precision required in their construction. Miniaturization poses a significant challenge because reducing the size of the magnetic components without losing sensitivity and accuracy is technologically demanding [38].

Additionally, paramagnetic sensors can be influenced by external magnetic fields, requiring careful shielding and calibration to maintain accurate measurements. These sensors also tend to have larger physical footprints compared to other types of oxygen sensors, limiting their application in portable and space-constrained environments [39].

Innovative designs and materials are being explored to address these limitations. For instance, microelectromechanical system (MEMS) technology has been employed to develop smaller, more efficient paramagnetic sensors that integrate magnetic components with fluidic systems [40]. These advancements aim to enhance the feasibility of paramagnetic sensors for broader applications, including mobile and wearable devices.

In summary, while paramagnetic sensors offer high accuracy and sensitivity for oxygen detection, they face challenges related to cost, miniaturization, and susceptibility to external magnetic interference. Ongoing research and technological innovations are crucial to overcoming these obstacles and expanding the practical applications of paramagnetic oxygen sensors.

Growing interest in magnetic oxygen sensors has recently been observed. Their development is based on the influence of the magnetic field on O₂ molecules. These are sensors based, among others, on the differences in the gas pressure within the areas differing in magnetic field strength or changes in gas flow direction under influence of the magnetic field. As a result, it is possible to measure O₂ concentration via leveraging the distinct paramagnetic properties of O₂ molecules [41].

3.5. Chemiresistive Sensors

Another group of devices, known as resistive or chemiresistive sensors, is based on detecting changes in the resistance of the sensing layer, although the literature sometimes varies in terminology. These changes are caused by the adsorption or desorption of oxygen species on the sensor’s surface [42]. When oxygen molecules interact with the sensing material, they either donate or accept electrons, depending on whether the material is an n-type or p-type semiconductor. This interaction alters the charge carrier concentration in the sensing layer, leading to the measurable change in electrical resistance. Depending on the conductivity type of the material (n-type or p-type), the resistance of the sensing layer will either increase or decrease upon exposure to oxygen, correlating with the oxygen partial pressure. This process is described in more detail later in this article.

Resistive sensors come in various forms, including zero-dimensional (0D) quantum dots, one-dimensional (1D) nanorods, two-dimensional (2D) nanoflakes and nanolayers, and three-dimensional (3D) nanoparticles and nanoflowers [43]. Each form offers unique properties that can be tailored to specific applications. For instance, quantum dots have discrete energy levels and exhibit size-dependent properties, making them highly sensitive to gas adsorption. Nanorods provide a high surface-to-volume ratio, enhancing their interaction with gas molecules. Nanoflakes and nanolayers offer large surface areas and thin profiles, which are beneficial for fast response times. Nanoparticles and nanoflowers provide multiple active sites for gas adsorption, improving sensor performance.

Most resistive sensors require high operating temperatures, for example, around 800 °C [44], which limits their application to devices and environments that can withstand such temperatures. The high operating temperature is necessary to activate the surface reactions between the gas molecules and the sensing material, ensuring adequate sensitivity and response speed. Despite this drawback, resistive sensors offer several advantages, such as a wide detection range for oxygen, stability at high temperatures, simplicity, and low cost. Their ability to operate in harsh environments makes them suitable for industrial applications where other types of sensors might fail.

The most commonly used materials for resistive sensors are semiconducting metal oxides (SMOs), such as tin oxide (SnO₂), zinc oxide (ZnO), and titanium dioxide (TiO₂). These materials are favored for their stability at high temperatures, low production costs, and excellent electrical properties. Their robustness and durability further enhance their suitability for long-term monitoring applications [45]. SMOs exhibit significant changes in electrical resistance when interacting with oxygen thanks to the surface oxygen vacancies. It is well known that the surfaces of SMO materials are rich in oxygen vacancies, which play an important role in creating adsorption sites for oxygen [46,47]. There are few factors that have an impact on the density of oxygen vacancies, like type of SMO, type of exposed facet, impurities in materials, fabrication method, etc. [48]. The more surface oxygen defects (adsorption sites for oxygen), the higher the resistance change in the device [49]. Sorption and desorption of oxygen in this vacancies provide oxygen detection abilities for these materials. This simple mechanism has a big flaw, i.e., a lack of selectivity. In the work of Mokrushin et al. [50], the fabrication of the planar sensor for oxygen detection is described. The developed sensors were subsequently tested in different temperature conditions and in the presence of different gases, like H₂, CO, NH₃, H₂S, and O₂, to verify their selectivity. In

Table 2. Selectivity of Nb₂O₅ sensor device for different gasses and temperatures [50].

Working Temperature	Analyte
	H2	CO	NH3	H2S	O2
250 °C	1.4	1.4	1.4	6.6	4.4
300 °C	1.3	1.3	1.3	4.8	3.7
350 °C	~1	<1	1.9	3.5	4.3
400 °C	~1	<1	2.9	1.9	3.7
450 °C	~1	<1	1.8	<1	2.6

, the results of performed studies are presented.

The sensitivity of the sensor is expressed as the ratio of the resistance before contact with the analyte to the resistance of the device after interaction. It can be observed in Table 2 that the selectivity of the sensor change depends on the device temperature. At lower temperatures, hydrogen sulfide is the most sensitive gas, while at 350 °C and above, oxygen becomes the most sensitive. However, the differences between gasses are not significant because of the fact that while the device works at 400 °C and detects oxygen, even small amounts of H₂S may affect the measurement. For the same reason, the problem with selectivity detection also applies to the other SMOs, like ZnO [42] or TiO₂ [51]. In some cases, even humidity may affect the sensor response. In the work of Simonenko et al. [52], low-temperature oxygen-sensing devices based on titanium are presented. After selectivity tests, it turned out that the sensor response was a few times greater when interacting with water (humidity of inter gas) than oxygen, which disqualifies this sensor from applications like lambda probes or air quality systems.

Resistive oxygen sensors, particularly lambda probes, are widely used in the automotive industry to monitor exhaust gases. Lambda probes measure the oxygen content in exhaust gases to optimize the air/fuel ratio for combustion engines, thereby improving fuel efficiency and reducing emissions. These sensors play a crucial role in meeting stringent environmental regulations and enhancing the performance of modern vehicles. Additionally, resistive sensors are used in many other applications, including environmental monitoring, industrial process control, and safety systems.

In summary, resistive or chemiresistive sensors leverage changes in electrical resistance due to gas adsorption or desorption to detect oxygen levels. While high operating temperatures can limit their application, these sensors offer significant advantages in terms of sensitivity, stability, simplicity, and cost-effectiveness. The use of semiconducting metal oxides and various nanostructures enables wide-ranging applications, from automotive exhaust monitoring to industrial and environmental sensing. The growing interest in resistive (chemiresistive) oxygen sensors translates to the growing number of publications dealing with these materials, as can be seen below in Figure 3.

Resistive (chemiresistive) oxygen sensors are increasingly gaining popularity among researchers, as evidenced by the publication of 68 articles in 2023 alone (Figure 3). Research in this area primarily focuses on additive engineering and the morphology of the sensing layer, including the type of particles, size, shape, etc., to reduce fabrication costs and operational expenses by lowering the required operating temperatures. Due to their high stability, these sensors are particularly suitable for harsh environments, such as within combustion engines in the form of lambda probes, where their requirement for high temperatures becomes an advantage. To broaden the application of resistive oxygen sensors beyond high-temperature environments, researchers are encouraged to explore materials and techniques that enable oxygen detection at relatively low temperatures (<300 °C). This review aims to present the state of the art in resistive oxygen sensor technology, providing a comprehensive understanding and encouraging further innovation in the quest for advanced oxygen sensing technologies.

An example of resistive oxygen sensors is presented, among others, in [53]. In this work, cerium oxide powder was employed to fabricate thick film sensors. The obtained sensors showed good adhesive properties and very short response times, i.e., 22 ms at 1073 K and 12 ms at 1173 K.

The

Table 3. Mechanisms of the operation, key materials and applications of selected types of semiconductor metal oxide oxygen sensors.

Type of Oxygen Sensor	Mechanism of Operation	Key Materials	Applications	Ref.
Optical	This type of sensor uses dyes emitting light (luminescence) when exposed. The emitted light intensity is proportional to the amount of oxygen in the environment.	Luminescent dyes, polymers	Medical diagnostics, environmental monitoring, industrial process control	[18,54,55,56]
Schottky diode	It acts as a gas sensor due to the change in the electron work function resulting from the adsorption of gas molecules on the metal surface, which influences the conductivity of the metal–semiconductor junction.	Gold, nickel, platinum, SiC, GaAs	Industrial monitoring processes, environmental sensing, healthcare diagnostics	[57,58]
Electrochemical	Operation by the reaction of the electrolyte with the target gas, which generates an electrical signal proportional to the gas concentration.	Zirconia, Yttria-stabilized zirconia	Industrial, automotive, medical diagnostics	[6,59,60]
Magnetic	They are based on the paramagnetic properties of oxygen that cause the attraction of its molecules to a strong magnetic field, enabling accurate measurements of oxygen concentration.	Paramagnetic materials	Industrial process control, medical diagnostics	[36,37]
Chemiresistive (resistive)	They work based on the adsorption of oxygen molecules on the surface of the semiconductor material, which changes its resistance—the changes in this parameter are proportional to the concentration of oxygen.	SMO, carbon materials	Combustion engines sensors, harsh environment sensors	[61,62,63,64]

presents examples of different sensors based on the mechanism of sensing.

An important aspect in terms of the operation of oxygen sensors is their stability. This is a key parameter affecting their long-term performance and measurement accuracy. Factors such as temperature, humidity, the presence of other gases, and environmental conditions can significantly affect the durability and reliability of different types of oxygen sensors [65]. For example, Yan and Liu demonstrated the deterioration in electrochemical oxygen sensor performance at too high (>96%) and too low (<32%) humidity [66]. In addition, in the case of optical sensors, dye photobleaching contributes to reduced stability [67].

4. Thin Films Fabrication

There are many thin film deposition techniques, which can be divided into two main groups: wet methods (dip coating, spin coating, screen printing, etc.) and vacuum methods (magnetron sputtering, chemical vapor deposition, electron beam evaporation, etc.) [68,69]. Wet methods are simple and generally cheap (cost of the process mostly depends on the posttreatment processes like heating), but these processes do not provide much control over the final layer and have many limitations like the shape of the substrate (which usually needs to be flat) or the fact that the substrate is often exposed to the elevated temperature (i.e., a few hundred degrees Celsius) in posttreatment annealing. On the other hand, vacuum methods are more sophisticated and expensive but give much better control of the process and more possibilities of film depositions. Due to their features, vacuum methods are more suitable for the fabrication of precisely thin layers. The most popular vacuum methods are magnetron sputtering (often with radio frequency) and chemical vapor deposition (CVD), while atomic layer deposition (ALD) is a novel technique that could allow for a new approach to the fabrication of functional thin films [70].

Magnetron sputtering is a widely used physical vapor deposition (PVD) technique for depositing thin metal layers such as chromium, copper, and titanium, as well as ceramic layers (semiconductors and insulators) [71,72]. The working mechanism involves ejecting atoms from a target material mounted in the magnetron by bombarding it with ions of a process gas, typically argon (the gas ionization process is caused by a high-voltage electric field within the chamber), the ejected atoms from the target are deposited onto a substrate, thus forming a thin layer. In specific applications, reactive gases like oxygen can also be used. In the process, neither aggressive chemicals nor high temperature is applied, which results in a fact that a wide range of materials can be used as a substrate. This method is very versatile and can be used in various applications including a conductive coating for SEM imaging, antireflective coating, thin film solar cells, and sensors [73,74].

Chemical vapor deposition is a vacuum deposition method that uses chemical reactions to produce thin films [75]. In this process, substrates are placed in a vacuum chamber and exposed to precursor vapors (one, two, or many), which is followed by a reaction with an introduced precursor or its decomposition (often with the use of high temperature, plasma, or lasers) [75] on the substrates, creating a thin layer of desirable material. This method is widely used commercially to deposit metal of ceramic layers in big batches, even on objects with a complex structure (like a milling cutter). CVD finds its applications in the field of protective coatings, electronic semiconductors, sensors, and the production of nanofibers, nanotubes, and even graphene [72,76,77].

In turn, atomic layer deposition is a novel variation of CVD. Like conventional CVD, ALD is based on the chemical reactions to produce thin films. However, ALD is based on the precursor’s introduction into the chamber, which is performed in the form of successive short pulses of various precursor vapors to the vacuum chamber (Figure 4), and these pulses result in the fabrication of film, layer by layer [78,79]. This technique of deposition offers a very high degree of control over the properties of the obtained layers, especially their thickness, which is unattainable using any other technique. By controlling the number of deposition cycles (set of precursors pulses), extremely thin layers can be obtained, even 1 nm, with completely conformal growth over the sample surface.

Atomic layer deposition is one of the most advanced thin layer deposition techniques available today. ALD finds its uses in nanodevice industries like processor production, LEDs (light emitting diodes), photovoltaics (especially thin film cells), and many others [79,81,82].

Magnetron Sputtering is a moderately cheap and simple method to produce thin layers but also demonstrates some limitations and flaws, like the size of batches, problems with coating complex surfaces, and the lack of a fast, adjustable, and precise doping solution (

Table 4. Comparison of different thin film deposition techniques [70,72].

Magnetron Sputtering	Chemical Vapor Deposition	Atomic Layer Deposition
+ Fast + Low cost + No harmful byproducts + Selection of the sputtered material independent of its melting point or chemical reactivity	+ Very efficient + Very fast + Deposition possible on a complex surfaces + Simple process + Easy doping process	+ Very precise + Deposition possible on very complex surfaces + Very high control of final product + Allows for repetitively obtaining extremely thin layers + Allows for simultaneous deposition on large quantities of samples + Very easy and precise doping or mixing of materials + Produces very homogeneous films
- Unable to conformally coat complex surfaces - Size of batches limited by the 2D size of chamber	- Many harmful byproducts - Aggressive precursors may damage substrates - Often needs high temperature, plasma, or lasers to activate precursors	- Slow process - Expensive - Harmful byproducts

). On the other hand, it is possible to apply the CVD technique, which does not show such problems. However, during the CVD process, many harmful by-products can be generated and, importantly, this method does not provide full conformal coating (Figure 5). To overcome the problems with PVD and CVD, atomic layer deposition can be implemented. This novel technique can coat very complex surfaces with full conformality. Moreover, during the ALD process, the doping level of the deposited layers can be adjusted quickly and precisely [78]. Despite of the fact that ALD is a variation of CVD, it produces significantly less harmful waste than conventional CVD.

Nowadays, resistive sensors take the form of a thick rough planar layer, which provides a developed surface with many oxygen vacancy states, leading to improved sensor response. A novel approach could be a deposition of a very thin layer of material on previously texturized substrates like nanorods, nanocones, etc. In this case, the thin active layer needs much less heating energy while preserving the development surface. This structure requires a fully conformal deposition technique to prevent active surface losses in the coating process, and this is the reason why ALD constitutes a promising thin layer deposition technique for novel resistive oxygen sensors.

5. Resistive Oxygen Sensors

The sensing mechanism of resistive oxygen sensors works differently depending on the type of conductivity of the sensor material. This is schematically presented below in Figure 6.

In n-type materials, oxygen species adsorb onto the surface and bond by capturing free electrons, leading to the formation of an electron depletion region with a high potential barrier. This barrier hinders the movement of free carriers, resulting in increased resistance in the layer (Figure 6a). In the case of p-type materials, the adsorbed oxygen species bond to the sensor surface by capturing electrons from the conduction band, which decreases the density of free electrons and results in the formation of a hole accumulation layer near the surface. This newly created region, with a high concentration of holes, facilitates the movement of free carriers, leading to a decrease in the sensor’s resistance (Figure 6b) [83].

A resistive sensor exposed to an atmosphere with a certain level of oxygen will either increase or decrease its resistance until the adsorbed oxygen reaches equilibrium. Generally, the more oxygen is adsorbed, the greater the change in resistance [42]. In resistive oxygen sensors, there are two parallel mechanisms for oxygen adsorption: the first is the chemisorption of oxygen species on the surface of the sensing material, and the second is the diffusion of oxygen into the layer structure, which is strongly dependent on temperature [84]. At low temperatures, the chemisorption mechanism dominates [83]. The amount of adsorbed oxygen is influenced by four factors: the partial pressure of oxygen surrounding the sensor, the type of material (which includes the density of free carriers, the concentration of defects, and the mobility of oxygen ions), the surface of the sensing layer (including its roughness, surface defects, and thickness), and the temperature. One of the simplest methods to enhance the oxygen adsorption capability of a sensor is to increase its operating temperature, facilitating easier diffusion of oxygen into the sensing layer. At low temperatures, the mobility of oxygen ions is limited, resulting in a slow rate of adsorption and shallow diffusion depth in the material. At higher temperatures, the mobility of oxygen ions increases, promoting faster diffusion and allowing for deeper penetration of oxygen into the material [84]. Besides accelerating ion diffusion, high temperatures also generate free carriers (electrons and holes) in the material, enabling more oxygen ions to bond to the sensor.

As mentioned before, a properly designed sensor device should provide a fast and accurate response to the measuring medium. To evaluate the performance of a sensor, several parameters must be measured. The response of a resistive sensor is defined as the change in the electrical properties of the device (resistance) caused by exposure to an analyte. This parameter is usually presented as a ratio of the electrical properties before and after exposure to the analyte. The higher this ratio, the easier it is to measure. A response that is too low could make it impossible to detect changes in the sensor’s electrical parameters. The sensor’s response could be triggered by more than one component of the analyte. To accurately detect a specific compound, its response must be significantly greater than that of other components in the analyte. The difference in the sensor’s response to the target compound compared to other components is referred to as selectivity.

The most important parameters of a sensor device are its response and recovery times. The response time refers to the duration the sensor requires to respond to a change in the analyte, while the recovery time describes the time needed for the sensor to revert its electrical parameters to the state before exposure to the analyte. Several methods are used to measure these parameters, i.e., the jump method [85], hysteresis [86], and sinusoidal [87]. These methods are schematically presented below in Figure 7.

The most popular is the jump method (Figure 7a), where the partial pressure of the analyzed compound in the analyte is drastically changed from 0 to a certain level; in most cases, this ‘jump’ is repeated several times, and sometimes, the level of the analyte changes from one ‘jump’ to the next.

Another popular method is the hysteresis method (Figure 7b), which involves a series of increasing partial pressure steps of the analyzed compound followed by the same steps in the opposite direction. This measurement, in addition to response and recovery times, also allows for the determination of the sensor’s hysteresis. In both methods, the time is usually measured up to reaching a certain percentage of the sensor’s response, such as 90%, which is called T₉₀. Recovery time is typically measured when the sensor goes from a high response to 10% of it, known as T_rec. A completely different approach is the sinusoidal method (Figure 7c), presented by Woosuck Shin et al., in which the partial pressure of the analyte changes periodically within a relatively small range. This method measures the phase shift between the analyte’s partial pressure and the sensor’s response [88]. Due to the relatively small changes in the analyte, the measured response time of the sensor is usually in the order of several dozen milliseconds.

This literature review shows that the method used to change the partial pressure affects the speed of the response. In some works, the sensor response was examined by increasing or decreasing the air pressure in the system. In this way, very rapid changes in the conditions in which the measurement was carried out were achieved. Therefore, the sensor response is measured with a small error under such conditions. Some works, however, involve measurements at constant pressure but with a variable composition of the oxygen/inert gas mixture. In such situations, the sensor’s response is limited by the rate of washing out of the previous mixture from the measuring chamber and the response rate of the system. This leads to a situation where results from different works cannot be compared due to different methodologies for measuring response speed. This shows that it is necessary to develop measurement standards for such sensors to standardize the obtained results.

In Figure 6, the materials used as sensing layers as well as the methods of their fabrication are presented.

Based on the research conducted (Table 4), the most common material for resistive oxygen sensors is cerium dioxide, accounting for over 44% of the analyzed literature, followed by zinc oxide and oxygen perovskites, each at 10.1% (Figure 8a). Among perovskites, LaFeO₃ and SrFeO₃ varieties were the most popular. The rest of the devices were primarily based on the other semiconducting metal oxides (SMOs) such as titanium, tin, gallium, and niobium oxides, as well as carbon in the form of nanotubes.

In terms of the methods applied, nearly half (45.2%) of the analyzed papers utilized the screen printing method, with magnetron sputtering, often at radio frequency (RF magnetron sputtering), being the second most commonly used deposition technique. Besides screen printing, other wet methods like spin coating, dip coating, and drop casting were also common (Figure 8b). Most of the presented deposition techniques resulted in the formation of layers with high roughness and developed surfaces, providing a large adsorption area for oxygen species, which in turn increases the device’s response. The literature shows a scarcity of very thin sensing layers (only a few sensors with sensing layers thinner than 50 nm) and a lack of devices fabricated with techniques that allow for the deposition of highly conformal and flat layers, such as atomic layer deposition. To meet requirements of new market solution sensors, the industry must be ready to fabricate very small sensors in unconventional shapes and forms on very diverse materials. In Figure 9, the scalability potential vs. the manufacturing cost is demonstrated.

To introduce a sensor device to the industry, besides giving good results, its fabrication process must be scalable and profitable. In the analyzed literature, many fabrication methods are presented, from very simple wet methods like dip coating to very sophisticated vacuum techniques like electron beam evaporation. The cheapest deposition techniques are wet methods like dip/spin coating and drop casting, whose prices are mostly defined by the thermal treatment of obtained layers; however, these methods usually are hard to scale and not repetitive (Figure 9). On the other side are vacuum techniques like chemical vapor deposition, which give very good repetitive results, good control of obtained layers, and are easy to scale, but the machinery and reagents used in these processes are very expensive. Somewhere between are two of the most popular methods in this review: screen printing and magnetron sputtering; both of these methods are relatively inexpensive, repetitive, and scalable.

More accurate determination of the cost of individual techniques is nearly impossible because of differences between different materials, like chemical reactivity, price of reagents, process temperatures, etc. In the end, the choice of fabrication technique will be mainly dictated by the materials applied, form and size of the sensing layer, and scale of the production. In Figure 10, the abundance of sensor types in the literature reports differing in working temperature conditions is schematically demonstrated.

Most of the reviewed literature reports focus on medium-temperature sensors (MTSs), with an optimal working temperature range of 300–700 °C, constituting 47.7% of all analyzed sensors. Following MTSs are high-temperature sensors (HTSs), with an optimal working temperature above 700 °C (Figure 10a); these types of sensors are used in harsh environments, which often requires resistance to high temperatures like combustion engines or industrial furnaces. Low-temperature sensors (LTSs), with an optimal working temperature below 300 °C, make up only 16.4% of the analyzed papers wherein this group of resistive sensors may find use in new sectors of the sensor market like the IoT, wearable electronics, and space industry, where low working temperature and low energy consumption are needed. Considering these numbers, it is clear that over 80% of the literature reports focus on MTSs and HTSs, which operate at relatively high temperatures (Figure 10b). Consequently, the capabilities of the majority of oxygen resistive sensors are limited to devices that can function at high temperatures. Future research should concentrate on developing LTSs to broaden the capabilities of resistive oxygen sensors. In

Table 5. An overview of resistive oxygen sensors sorted by optimal working temperature.

Ref.	Material	Optimal Working Temperature [°C]	Thickness [nm]	Deposition Method	Min Range [%]	Max Range [%]	T90 [s]	Trec [s]
Low Temperature Sensors
[89]	CNT	20	-	Dip casting	0.3	100	60	180
[90]	Bi2O2Se	20	8.4	CVD	2.5 × 10−5	0.004	-	-
[91]	SWCN:SrTiO3	20	-	CVD, PLD	10−5	10−3	21	170
[92]	ZnO	20	-	Thermal evaporating	-	-	-	-
[93]	ZnO	20	-	Microwave–hydrothermal	0	15	120	60
[94]	CdO:ZnO (1:3)	32	200	Spin coating	-	-	18	10
[94]	ZnO	32	200	Spin coating	-	-	38	30
[94]	CdO	100	200	Spin coating	-	-	14	38
[52]	Ti2CTx	125	50,000	Micro-plotter printing	1	20	-	-
[94]	CdO:ZnO (3:1)	150	200	Spin coating	-	-	20	35
[95]	MWCNT	160	-	Drop casting	0.3	50	-	0.9
[96]	CdO:ZnO	200	-	RF magnetron sputtering	-	-	25	45
[50]	Nb2O5	200	-	Screen printing	0.4	20	71	57
[97]	SnO2	225	120	Electron bean evaporation	0.05	0.4	-	-
[97]	SnO2:2.72%Pt	225	120	Electron bean evaporation	0.05	0.4	-	-
[97]	SnO2:2.88%Pt	225	120	Electron bean evaporation	0.05	0.4	-	-
[97]	SnO2:3.74%Pt	225	120	Electron bean evaporation	0.05	0.4	-	-
[98]	CNT	250		PVD	-	-	-	-
[99]	SnO2	270	-	RGTO	1	4	-	-
[99]	SnO2	277	100	Magnetron sputtering	1	4	-	-
[100]	ZnO NW: C MF	280	-	Chemical electrodeposition	2 × 10−4	0.06	11	9
[101]	CaCu3Ti4O12	300	450	Spin coating	0.05	12	-	-
[102]	MoS2	300	-	Dip casting	2	100	-	-
[103]	SnO2	300	-	Screen printing	0.1	100	-	-
[104]	TiO2	300	30	Molecular layering	0.2	10	42	70
[105]	Zn1−xAlxO (x = 0.2–10)	300	-	Spray pyrolysis	-	-	170	270
Medium Temperature Sensors
[106]	TiO2	320	200	Dip coating	-	-	-	-
[107]	ZrO2	320	-	Screen printing	0.003	100	15	-
[108]	ZrO2:FeO2 (4:1)	320	-	Screen printing	20	-	15	-
[109]	Nb2O5	350	-	Screen printing	1	20	-	-
[109]	Nb2O5—10% TiO2	350	-	Screen printing	1	20	-	-
[109]	Nb2O5—5%TiO2	350	-	Screen printing	1	20	-	-
[110]	Ce0.771Hf0.229O2	400	220	RF magnetron sputtering	20	100	37	48
[110]	Ce0.801Hf0.199O2	400	220	RF magnetron sputtering	20	100	30	33
[110]	Ce0.875Hf0.125O2	400	220	RF magnetron sputtering	20	100	15	20
[110]	Ce0.888Hf0.112O2	400	220	RF magnetron sputtering	20	100	8	10
[110]	Ce0.894Hf0.106O2	400	220	RF magnetron sputtering	20	100	8	10
[110]	Ce0.91Hf0.09O2	400	220	RF magnetron sputtering	20	100	12	15
[110]	Ce0.939Hf0.061O2	400	220	RF magnetron sputtering	20	100	15	17
[111]	CeO2	400	55	Sol–gel	0.4	20	15	28
[111]	CeO2—10% ZrO2	400	55	Sol–gel	0.4	20	15	28
[111]	CeO2—20% ZrO2	400	55	Sol–gel	0.4	20	15	28
[111]	CeO2—30% ZrO2	400	55	Sol–gel	0.4	20	15	28
[111]	CeO2—5% ZrO2	400	55	Sol–gel	0.4	20	15	28
[112]	Ga2O3+Si	400	170	RF magnetron sputtering	9	100	11	70
[113]	ZnO	400	80	Spin coating	15	33	-	-
[114]	CeO2 + 10%Y2O3	450	70	Dip coating	1	20	8	-
[114]	CeO2 + 15% Y2O3	450	70	Dip coating	1	20	8	-
[115]	ZnO (nano particles)	450	-	EDOC	1	80	60	150
[115]	ZnO (thin layer)	450	250	PE-MOCVD	1	10	-	-
[116]	BaFe0.3Ta0.7O3	500	3300	Cold press	1	100	-	-
[116]	BaFe0.4Ta0.6O3	500	3300	Cold press	1	100	-	-
[116]	BaFe0.5Ta0.5O3	500	3300	Cold press	1	100	-	-
[116]	BaFe0.6Ta0.4O3	500	3300	Cold press	1	100	-	-
[116]	BaFe0.7Ta0.3O3	500	3300	Cold press	1	100	-	-
[116]	BaFe0.8Ta0.2O3	500	3300	Cold press	1	100	-	-
[117]	TiO2	500	50	RF magnetron sputtering	0.4	0.6	-	-
[118]	TiO2:CNT	500	-	Drop casting	10−3	-	5	-
[118]	TiO2:Nb	500	-	Drop casting	10−3	-	1.5	-
[118]	TiO2:Nb, CNT	500	-	Drop casting	10−3	-	5	-
[113]	ZnO:Cu (10:1)	500	120	Spin coating	15	33	-	-
[118]	TiO2	550	-	Drop casting	10−3	-	1.5	-
[119]	Ce0.9Hf0.1O2	600	20,000	Screen printing	9	100	-	-
[119]	Ce0.9Zr0.1O2	600	20,000	Screen printing	9	100	-	-
[119]	CeO2	600	20,000	Screen printing	9	100	-	-
[120]	CeO2	600	500	(MO)CVD	-	-	9	-
[120]	CeO2	600	500	Magnetron sputtering	-	-	9	-
[121]	CeO2:Hf	600	27,000	Screen printing	10−15	100	-	-
[122]	LaCu0.3Fe0.7O3	600	20,000	Screen printing	0.1	100	0.001	0.01
[123]	YBa2Cu3O3	600	150,000	Screen printing	0.009	100	60	180
[113]	ZnO:Al (10:1)	600	40	Spin coating	15	33	-	-
[124]	BaFe0.74Al0.01Ta0.25O3	650	-	Screen printing	0.1	100	-	-
[61]	LaFeO3 (fibers)	650	-	Screen printing	0.7	50	20	18
[61]	LaFeO3 (powder + fibers)	650	-	Screen printing	0.7	50	10	19
[61]	LaFeO3 (powder)	650	-	Screen printing	0.7	50	16	23
[125]	SrFe0.6Ti0.4O3	650	30,000	screen printing	2.5	20	-	-
[125]	SrFe0.6Ti0.4O3	650	10,000	Screen printing	2.5	20	-	-
[126]	SrTi0.4Fe0.6O2.8	650	-	Dipping	1	16	25	-
[126]	SrTi0.6Fe0.4O2.8	650	-	Dipping	1	16	25	-
[127]	SrTi0.6Fe0.4O2.8	650	30,000	Screen printing	2.5	15	5	350
[128]	CeO2	663	30,000	Screen printing	1	100	6	-
High Temperature Sensors
[129]	Ga2O3	700	200	RF magnetron sputtering	9	100	20	50
[122]	La0.05Sr0.95Ti0.65Fe0.35O3	700	20,000	Screen printing	0.1	100	0.01	0.02
[130]	Sr(Ti0.4Fe0.6)O3	700	-	Screen printing	1	20	-	-
[130]	Sr(Ti0.6Fe0.4)O3	700	-	Screen printing	1	20	-	-
[3]	Sr0.95La0.05(Ti0.7Fe0.3)0.95Ga0.05O3	700	677,000	Screen printing	-	-	-	-
[130]	SrFeO3	700	-	Screen printing	1	20	-	-
[3]	SrTi0.5Fe0.5O2.8	700	677,000	Screen printing	-	-	-	-
[3]	SrTi0.7Fe0.3O2.8	700	677,000	Screen printing	-	-	-	-
[3]	SrTi0.8Fe0.2O2.8	700	677,000	Screen printing	-	-	-	-
[130]	SrTiO3	700	-	Screen printing	1	20	-	-
[3]	SrTiO3	700	677,000	Screen printing	-	-	-	-
[131]	BaFe0.74Al0.01Ta0.25O3	750	5000	Powder aerosol deposition	1	100	-	-
[132]	LaFe0.7Cu0.3O3	750	20,000	Screen printing	0.1	100	-	-
[132]	LaFe0.8Cu0.2O3	750	20,000	Screen printing	0.1	100	-	-
[132]	LaFe0.9Cu0.1O3	750	20,000	Screen printing	0.1	100	-	-
[132]	LaFeO3	750	20,000	Screen printing	0.1	100	-	-
[133]	Ce0.8Zr0.2O2	800	-	Screen printing	-	-	9	-
[133]	Ce0.95Zr0.05O2	800	-	Screen printing	-	-	-	-
[133]	Ce0.9Zr0.1O2	800	-	Screen printing	-	-	-	-
[133]	CeO2	800	-	Screen printing	-	-	12	-
[134]	CeO2	800	8000	Screen printing	10−20	100	-	-
[87]	CeO2	800	-	Screen printing	-	-	0.05	-
[44]	CeO2	800	25,000	Screen printing	1	100	10	-
[135]	CeO2	800	1000	Magnetron sputtering	10−8	100	0.005	-
[135]	CeO2	800	2500	Magnetron sputtering	10−8	100	0.01	-
[134]	CeO2—10% ZrO2	800	8000	Screen printing	10−20	100	0.016	-
[134]	CeO2—20% ZrO2	800	8000	Screen printing	10−20	100	0.009	-
[134]	CeO2—5% ZrO2	800	8000	Screen printing	10−20	100	-	-
[136]	Sr(Ti,Fe)O3:Al2O3 (4:1)	800	7500	Aerosol deposition	0.001	100	-	-
[137]	Sr(Ti0.65Fe0.35)O3	800	15,000	Screen printing	-	-	0.003	-
[138]	Ce0.85Zr0.15O2	900	-	Screen printing	-	-	-	-
[138]	Ce0.95Zr0.05O2	900	-	Screen printing	-	-	-	-
[138]	Ce0.9Zr0.1O2	900	-	Screen printing	-	-	16	49
[139]	CeO2	900	10,800	Screen printing	10−4	100	11	38
[138]	CeO2	900	-	Screen printing	-	-	-	-
[53]	CeO2	900	10,000	Screen printing	-	-	0.012	0.001
[140]	CeO2	900	-	Screen printing	2.5	6.5	0.012	0.001
[141]	CeO2	900	10,000	Screen printing	2.5	6.5	0.012	0.001
[88]	CeO2	900	-	Screen printing	1	100	-	11
[142]	Ce0.9Zr0.1O2	950	-	Screen printing	1	100	13	29
[142]	CeO2	950	-	Screen printing	1	100	21	59
[143]	Ga2O3	1000	1000	RF magnetron sputtering	0.001	100	10	24
[143]	Ga2O3	1000	106	Single-crystal growth	0.001	100	11	17
[143]	Ga2O3	1000	700	Spin coating	0.001	100	12	25

, a compilation of resistive oxygen sensors in terms of their optimal working temperature is presented.

In Figure 11, histograms for both recovery and response time are shown.

Based on the analyzed literature (Table 5), most sensors can respond within a timeframe shorter than 20 s (Figure 11a), while for recovery times, the majority of sensors recover in less than 35 s (Figure 11b). The parameters T₉₀ and T_rec are challenging to compare due to the variance in the measurement methods. Devices measured using the sinusoidal method often have these parameters in the range of milliseconds, whereas the same device measured by the jump method could exhibit a T₉₀ within the range of several seconds [88]. This discrepancy could account for the significant number of devices with response times below 5 s. Other factors include the volume of the measuring chamber, the type of analyte, and the speed of gas exchange in the chamber. These factors affect the system’s inertia regarding gas exchange in the chamber, which may hinder the measurement of small T₉₀ and T_rec values, further complicating the comparison of parameters obtained from different studies.

6. Critical Discussion

In the modern world, oxygen sensors have many uses, like being a part of process control system, atmospheric compound monitoring, or safety systems. Besides this typical application area, in the near future, there will be an increase in demand for sensors devices in area of IoT (Internet of Things) systems like smart wearables, security containers, and cosmic technology, like controlling the atmosphere in spaceships or oxygen content in stations on other planets. In all described future applications, small-sized, mobile, and easy-to-integrate sensor devices will be needed. To overcome these future challenges, the sensors must be ready to work on relatively low power, like small battery systems. Actually, the majority of oxygen sensors occur in the form of thick, rough layers (a few micrometers thick) and need relatively high operation temperatures, which leads to high energy consumption. In

Table 6. Data of CeO₂ oxygen sensor device for energy calculation [144].

Thickness	Area	Density	Temperature	Heat Capacity
[cm]	[cm2]	[g/cm3]	[K]	$\left[{\displaystyle \frac{\mathrm{J}}{\mathrm{k}\mathrm{g}\cdot \mathrm{K}}}\right]$
0.002	1	7.6	1173	390

, selected data for a CeO₂ oxygen sensor device are indicated.

Based on the work of Shin et al., the energy consumption of the sensing layer of a sensor with an area of 1 cm² was calculated using the following equation (in the calculation, losses of energy have not been considered) [28]:

$$E=d\cdot S\cdot \rho \cdot \mathsf{\Delta }T\cdot C_p$$

(1)

where d—thickness [cm]; S—area [cm²]; $\rho$—density [g/cm³]; C_p—heat capacity $\left[{\displaystyle \frac{\mathrm{J}}{\mathrm{k}\mathrm{g}\cdot \mathrm{K}}}\right]$; ∆T—working temperature; [K]—room temperature (293 K). Thus, using the data presented in Table 5, the calculation can be as follows:

$$E=0.002\cdot 1\cdot 7.6\cdot 880\cdot 0.390=5.217\left[\mathrm{J}\right]$$

(2)

Based on the calculation and assuming the voltage of 1.5 V and capacity of 2300 mAh of a typical alkaline battery (3.45 Wh), energy stored in this battery will be enough only for ~2400 heating cycles, which is not enough for the applications mentioned before. Both high working temperature and layer thickness promote this high power consumption.

Many resistive sensors could work at room temperature or relatively low temperatures, especially types that are sensitive to organic vapors like acetone, ethanol, or other alcohols [145]. In the literature, many sensors with very thin sensing layers, for example, few hundreds of nanometers to extreme 5 nm thick sensors, can also be found [146] wherein again, most of them are organic compound sensors or for gases like hydrogen or carbon dioxide. To elongate this time, thinner sensors and lower working temperatures are needed. If a sensor, instead of 20 µm, would have only 50 nm thickness and its working temperature was to be 200 °C instead of 800 °C, then the equation would be as follows:

$$E=0.000005\cdot 1\cdot 7.6\cdot 180\cdot 0.390=0.00267\left[\mathrm{J}\right]$$

(3)

These adjustments allow for reducing the power consumption by nearly 2000 times. With the same battery as used before, this sensor could process 4.65 × 10⁶ heating cycles. This simple example highlights problems with the actual technology, which need to be solved. To solve these problems, the use of more sophisticated deposition methods could be very helpful, like atomic layer deposition, which allows for depositing very thin layers of even a few nanometers with very good control of the deposition process regarding temperature, dopants, and thickness [87].

7. Conclusions and Perspectives

In this review, different kinds of oxygen sensors, with their sensing mechanisms, potential shortcomings, and specificity, were summarized. The state of the art in the field of resistive oxygen SMO sensors was explored and discussed in the case of their strengths and weak-points. The conducted market analysis showed unquenchable demand for oxygen sensors, especially in branches like the automotive and medical industries and environmental monitoring systems. Over the last few decades, a wide array of materials, including cerium oxides, zinc oxides, perovskite-type oxides, titanium oxides, and many others, have been investigated as oxygen-sensing layers, either in their pure form or with additives. Among the various deposition techniques employed by researchers, screen printing has emerged as the predominant method due to its rapid processing, cost-effectiveness, and the ease of controlling particle type, size, and composition. Layers deposited via screen printing possess highly developed surfaces, offering extensive adsorption areas for oxygen species, which is crucial for sensor functionality. To meet the demands of the future oxygen sensor market, new film fabrication methods like ALD must be implemented. The majority of sensors demonstrate satisfactory response times of below 20 s, with recovery times generally under 30 s.

However, a significant challenge within this field is the requirement for high working temperatures, which is often profitable in the case of working in harsh conditions like, e.g., high temperature in the case of combustion engines. Only 16.4% of analyzed sensors, categorized as low-temperature sensors (LTSs), can operate at lower temperatures (below 300 °C), which is required for new sectors of implementation like the aforementioned IoT or wearable electronics. While most sensors necessitate conditions of 600 °C, 800 °C, or even 1000 °C to function optimally, the possibility of their implementation is limited. Most devices analyzed are characterized by layers a few micrometers thick, operating at temperatures around 700 °C. The literature reveals a notable scarcity of ultra-thin sensors (below 50 nm) with flat surfaces that can operate at low, even room, temperatures.

Further work is necessary to develop sensors with response times below 1 s. Such advancements are crucial for future space station projects on Mars and the Moon, where monitoring the atmospheric composition will be of paramount importance. Additionally, there is an increasing demand for oxygen sensors capable of detecting minor changes in oxygen content. This is particularly relevant for biochemical research, where the rate of oxygen consumption is a measure of the speed of biochemical reactions occurring, for example, in cells.

Looking forward, the development of resistive oxygen sensors should prioritize finding innovative solutions to reduce both the operational temperature and the response and recovery times of the sensors. Addressing these challenges could lead to more versatile and energy-efficient sensors, potentially opening up new applications in various fields such as environmental monitoring, healthcare, and industrial process control. The advancement of LTS technology, in particular, holds promise for expanding the utility of resistive oxygen sensors in scenarios where high temperatures are impractical or undesirable.

The ability to accurately detect and measure oxygen content in resistive, electrochemical, and similar sensor technologies is highly dependent on the degree of active surface area development. There are several key parameters that must be simultaneously maintained to ensure the optimal performance of such sensors. One of the most critical factors is the ability to rapidly adsorb and desorb oxygen, as this process largely determines the sensor’s response time. Another important factor is the surface area of the sensor itself, which directly affects its sensitivity—the larger the active surface area, the higher the sensor’s sensitivity.

It seems natural, then, that nanoporous materials could offer an ideal solution by increasing the active surface area and thereby improving sensor performance. However, research has shown that the use of nanoporous materials presents certain significant challenges. The primary issue is the rate of oxygen diffusion within the porous structure, which can significantly limit the sensor’s response speed. While a highly developed surface area can be achieved, the longer time required for oxygen desorption and the re-establishment of adsorption equilibrium can lead to a substantial slowdown in sensor operation, which is undesirable in applications that require rapid response to changes in oxygen concentration.

Therefore, based on our previous research and a review of the available literature, it can be suggested that hybrid solutions, combining the advantages of different techniques, may yield the best results. An example of such an approach is the use of chemical methods to control the creation of nanostructures, such as nanocones or nanorods, which do not inherently possess sensing properties. After applying the ALD atomic layer deposition) method, it is possible to very precisely and controllably functionalize these nanostructures, resulting in highly refined oxygen sensors characterized by both high sensitivity and fast response times.

The ALD method, in addition to its precision, also has the advantage of perfectly replicating the substrate surface on which it is applied. This ensures that the degree of surface area development is not compromised during the functionalization process, which is crucial for maintaining the sensor’s optimal performance. Our current research is focused on exploring the synergy between chemical methods for creating templates and the deposition of functional layers using ALD. This combination of techniques may represent the future of advanced oxygen sensor development, offering a balance between rapid response and high sensitivity, which is critical for potential industrial and medical applications.

This approach not only enhances the efficiency of the sensors but also opens new possibilities for tailoring their properties to specific application requirements. In the future, we plan to conduct further research into optimizing the manufacturing and functionalization processes to further improve the performance and reliability of these devices.

Moreover, meeting the growing demand for fast-response and high-sensitivity oxygen sensors will require multidisciplinary approaches combining material science, engineering, and biochemical research. Continued innovation in this field is essential to address the evolving needs of modern technology-driven environments and to support critical applications in space exploration and biochemical analysis.

By focusing on these critical aspects, researchers can advance the development of next-generation oxygen sensors that meet the demands of both current and future applications.