Gas-sensing technology has gained significant attention in recent years due to the increasing concern for environmental safety and human health caused by reactive gases. In particular, spinel ferrite (MFe2O4), a metal oxide semiconductor with a spinel structure, has emerged as a promising material for gas-sensing applications.
1. Introduction
Metal oxide semiconductor (MOS) gas sensors operate by detecting alterations in the electrical conductivity of a semiconducting metal oxide when exposed to a gas
[1]. When the MOS sensor comes into contact with the target gas, the gas molecules adhere to the sensor material’s surface, resulting in a modification of the sensor’s electrical resistance
[2]. The extent and direction of the resistance alteration correlate with the gas concentration and its chemical properties. Numerous metal oxide semiconducting materials, such as tin oxide (SnO
2)
[3], zinc oxide (ZnO)
[4], titanium dioxide (TiO
2)
[5], and tungsten oxide (WO
3)
[6], have been widely utilized in the production of MOS sensors. These materials exhibit diverse sensing characteristics towards various gases, and their sensitivity, selectivity, and stability can be adjusted via material doping, surface modification, and operating conditions. To enhance the performance of MOS gas sensors, novel sensing structures such as nanowires
[7], nanotubes
[8], and nanostructured thin films
[9] have been developed, offering larger surface-to-volume ratios and improved gas adsorption capabilities. Additionally, advanced fabrication techniques such as atomic layer deposition (ALD)
[10], chemical vapor deposition (CVD)
[11], and spray pyrolysis
[12] have been employed to achieve precise control over the sensor’s morphology, composition, and functionality. In conclusion, MOS gas sensors have become indispensable tools for monitoring environmental pollution, ensuring industrial safety, and safeguarding public health
[13][14]. The ongoing progress in sensor technology and its integration with information and communication systems will create novel opportunities for real-time, reliable, and intelligent gas-sensing solutions.
MOS-gas-sensitive materials can be classified into two categories based on the number of metal ions present in the single-phase metal oxide material: single metal oxides and composite metal oxides. The gas sensors based on single metal oxides exhibit excellent attributes, including easy integration, good repeatability, and effective detection of various gases
[15][16][17]. Nonetheless, there is still room for improvement in terms of the selectivity and recovery performance of single-phase gas-sensitive materials. Researchers have explored strategies to enhance the sensing performance by incorporating precious metal catalysts or combining them with other materials to modify the morphology of single metal oxides, aiming to provide activation energy for reactions or form p–n heterojunctions.
In recent times, the distinctive magnetic properties
[18], electrical properties
[19], microwave absorption
[20], and photocatalytic properties
[21] of composite metal oxides, specifically spinel ferrites, have garnered significant attention. The primary preparation techniques for MFe
2O
4-based gas-sensitive materials include the co-precipitation method
[22][23][24][25], sol–gel method
[26][27][28], and template synthesis method
[29]. These methods enable the production of spinel ferrite nanomaterials with diverse morphologies such as nanorods, nanotubes, nanofilms, and core–shell microspheres. The combination of novel synthesis approaches and the integration of new functional materials has led to the development of spinel ferrite and spinel ferrite composite materials with controllable structures and morphologies, thereby expanding their application potential. For instance, the controlled synthesis of spinel ferrite nanoparticles has exhibited promising outcomes in biomedical applications such as drug delivery and cancer therapy
[30]. Furthermore, the combination of spinel ferrite with graphene oxide enhances its magnetic and electrical properties, positioning it as a potential candidate for spintronics
[31] and electromagnetic shielding applications
[32]. Additionally, the incorporation of metal ions or other functional materials into spinel ferrite has shown improved catalytic and photocatalytic properties, thereby finding application in areas such as wastewater treatment
[33] and hydrogen production
[34]. Overall, the advancement of novel synthesis methods and the integration of functional materials have broadened the scope of zinc ferrite materials and opened up avenues for future research.
As a semiconducting, magnetic oxide material, spinel ferrite has excellent chemical stability, enabling the effective adsorption of various gases
[35]. Its inherent catalytic properties stimulate chemisorption processes that result in changes in its electrical resistance when exposed to different gases
[36]. This allows for accurate gas detection and measurement. Additionally, spinel ferrite can operate at lower temperatures compared with other gas sensors, which leads to increased energy efficiency
[37]. Its high sensitivity
[38] and selectivity
[39] towards particular gases, coupled with its capacity for miniaturization, make spinel ferrite an optimal material for building reliable, efficient, and compact gas sensors.
2. Gas-Sensing Mechanism
With its spinel crystal structure, spinel ferrite emerges as a promising sensing material possessing exceptional properties. Figure 1 illustrates the crystal structure of zinc ferrite, where the face-centered cube of O2− accumulates within its crystal lattice, while the metal ions M2+ and Fe3+ are embedded in the tetrahedral and octahedral gaps formed by O2−. This structure readily facilitates the formation of defects, including oxygen vacancies, both internally and on the surface, making it highly advantageous for gas-sensitive materials. The unique crystal structure, specifically the insertion of the transition metal cation Zn2+ into the Fe2+Fe3+O4 structure, plays a crucial role in the effective detection of reducing gases.
Figure 1. The crystal structure of ZnFe2O4, with Zn2+ in the tetrahedron gap and Fe3+ in the octahedron gap.
The gas-sensing response of spinel ferrite is determined by the complex interaction that occurs at the interface between the gas and solid material. However, a unified definition of gas sensor mechanisms is lacking. A commonly proposed sensing mechanism for spinel ferrite sensors is as follows: when a spinel-ferrite-based sensor is exposed to air, oxygen molecules adsorb onto its surface, capturing free electrons from the conduction band and forming oxygen anions. The specific form of these oxygen anions depends on the operating temperature. The loss of electrons generates an electron depletion layer (n-type) on the semiconductor surface, resulting in an increase in resistance. In a reducing gas atmosphere, Equation (6) occurs, leading to a reduction in the resistance of the electron depletion region and sensor. It is worth noting that the reaction described in Equation (6) may vary depending on the operating temperature or target gas.
The unique microstructure and high specific surface area of pure MFe
2O
4 nanomaterials offer numerous adsorption sites, leading to an enhancement in gas-sensing performance. The addition of metal ions through doping reduces the barrier height of grain boundaries, facilitating improved carrier diffusion and transfer rates; heterostructures
[40], on the other hand, allow for the modulation of the electron depletion region and potential barrier at the interface by leveraging the interaction between Fermi energy levels and energy bands
[41]. These mechanisms collectively contribute to the enhancement of gas sensitivity in the respective materials. More detailed explanations of the gas-sensitive mechanisms specific to these new materials can be found in later sections.
3. Nanostructure
The gas-sensing application has significantly benefited from the use of nanostructured materials, primarily due to their high surface-to-volume ratio, which allows for better interaction with the gas molecules. In particular, zinc ferrite, a type of spinel ferrite, has been widely used due to its specific surface area, contact area, porosity, grain size, and grain stacking order. These factors all contribute to its gas-sensing properties. The operating temperature, humidity, and gas concentration are several external factors that can influence the performance of zinc ferrite-based gas sensors. For instance, at higher operating temperatures, the sensor’s sensitivity can increase due to the enhanced surface reaction rates
[42]. On the other hand, excessive humidity may cause the surface of the sensor to become water-saturated, which could inhibit its response to target gases
[35]. Apart from these external factors, the morphology-related characteristics of spinel ferrite also play a significant role in its gas-sensing properties. The development of unique morphologies and structures in spinel ferrite is considered a promising approach to enhance its gas-sensing performance. For example, porous spinel ferrite with large specific surface areas can provide more active sites for gas molecule adsorption, facilitating improved surface effects, electronic transfer efficiency, and ultimately a better gas-sensing performance. Various synthesis methods can be employed to create spinel ferrite materials with different morphologies. These include sol–gel, hydrothermal, and co-precipitation methods, among others. Each method offers unique advantages in terms of controlling the size, shape, and distribution of the nanoparticles, thereby allowing for the optimization of the sensor’s performance.
4. Doping
Element doping is indeed a powerful strategy to enhance the structure and performance of spinel ferrite materials, and there has been a growing interest in this research area recently. While earlier studies on spinel ferrite doping mostly concentrated on applications such as electrodes and magnetism, recent advancements have shed light on the importance of doping for optimizing gas-sensing properties. However, not all metallic elements are suitable for doping in spinel ferrite materials. Preferably, elements with donor characteristics (high valence elements that can donate electrons) or acceptor characteristics (low valence elements that can accept electrons) are used for modification. Doping in spinel ferrite materials can occur in two forms. The first form of doping involves displacement, where the M
2+ (A site) and Fe
3+ (B site) ions in the spinel ferrite are replaced by the doping elements. This changes the composition of the spinel ferrite and can affect its properties, such as A-site doping
[43], B-site doping
[44], and AB-site doping
[45]. The second involves the incorporation of doping elements into the tetrahedral and octahedral interstices of MFe
2O
4 crystals. This results in a solid solution structure, where the doping elements are homogeneously dispersed within the host material
[46]. Doping can significantly alter the composition and microstructure of spinel ferrite materials, influencing characteristics such as crystallinity
[47]. These changes can, in turn, affect the reference resistance
[48] and gas-sensing performance
[49] of the ferrite-based gas sensors. For instance, doping can enhance the sensitivity
[50], selectivity
[51], response speed
[28], and stability
[52] of the sensors.
5. Heterostructure
It has been discussed how the gas-sensitive performance of spinel ferrite sensors can be enhanced through the manipulation of their morphology or the introduction of doping elements. However, to achieve the desired properties, researchers have explored the development of spinel ferrite composites, which find more extensive applications in the fields of photocatalysis and sensing. The development of spinel ferrite composites has gained significant attention due to their potential to synergistically enhance the gas-sensitive performance. These composites often involve combining spinel ferrite with other materials such as metal oxides, carbon-based materials, or polymers. The unique properties of these composite materials can be leveraged to improve the gas-sensing properties of spinel ferrite sensors. For example, metal-oxide-based spinel ferrite composites have demonstrated an improved gas-sensing performance due to the enhanced specific surface area and increased active sites provided by the metal oxide component. The combination of spinel ferrite with carbon-based materials, such as graphene or carbon nanotubes, can enhance the electrical conductivity and provide additional adsorption sites, leading to enhanced gas-sensing capabilities. In summary, the development of spinel ferrite composites has opened up new avenues for enhancing the gas-sensitive properties of spinel ferrite sensors. These composites, whether metal-oxide-based, carbon-based, or incorporating polymers, offer unique advantages that can be leveraged to achieve an improved gas-sensing performance.