Recent Progress in Spinel Ferrite (MFe2O4) Chemiresistive Based Gas Sensors

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. This review article aims to provide an overview of the latest developments in spinel-ferrite-based gas sensors. It begins by discussing the gas-sensing mechanism of spinel ferrite sensors, which involves the interaction between the target gas molecules and the surface of the sensor material. The unique properties of spinel ferrite, such as its high surface area, tunable bandgap, and excellent stability, contribute to its gas-sensing capabilities. The article then delves into recent advancements in gas sensors based on spinel ferrite, focusing on various aspects such as microstructures, element doping, and heterostructure materials. The microstructure of spinel ferrite can be tailored to enhance the gas-sensing performance by controlling factors such as the grain size, porosity, and surface area. Element doping, such as incorporating transition metal ions, can further enhance the gas-sensing properties by modifying the electronic structure and surface chemistry of the sensor material. Additionally, the integration of spinel ferrite with other semiconductors in heterostructure configurations has shown potential for improving the selectivity and overall sensing performance. Furthermore, the article suggests that the combination of spinel ferrite and semiconductors can enhance the selectivity, stability, and sensing performance of gas sensors at room or low temperatures. This is particularly important for practical applications where real-time and accurate gas detection is crucial. In conclusion, this review highlights the potential of spinel-ferrite-based gas sensors and provides insights into the latest advancements in this field. The combination of spinel ferrite with other materials and the optimization of sensor parameters offer opportunities for the development of highly efficient and reliable gas-sensing devices for early detection and warning systems.


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], 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.

O gas → O ads
O ads e → O ads T 150 ∘ C (2) 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.
G(gas) → G(ads) The unique microstructure and high specific surface area of pure MFe 2 O 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 Section 3 (Nanostructures), Section 4 (Doping), and Section 5 (Heterostructures).

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 gassensing 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. In the subsequent sections, we will delve deeper into these topics, providing a comprehensive review of the latest research findings on ferrite sensors with diverse nanostructures. We will also discuss the special properties of these sensors as documented in existing literature (Tables 1-4). We believe that this review will provide valuable insights into the ongoing advancements in the field of spinel-ferrite-based gas sensors and highlight potential avenues for future research. O.T. operating temperature; Conc. concentration; t res /t rec response time/recovery time; LOD limit of detection. a Response is defined as R a /R g ; b Response is defined as R g /R a ; c Response is defined as ∆R/R a ; d Response is defined as ∆R/R g . R a : resistance of the sensor in air; R g : resistance of the sensor exposed to target gas; ∆R: the change in resistance, which equals |R a -R g |.

Nanoparticles
The preparation method for spinel ferrite nanoparticles can be achieved through the following steps: First, an appropriate synthesis method, such as sol-gel [109], hydrothermal [101], or co-precipitation [90], is used to mix suitable metal salts with basic precipitants, forming a precipitate. Next, through appropriate washing, centrifugation, and drying processes, the precipitate is transformed into nanoparticle form. Finally, through heat treatment or other surface modification methods, the morphology and properties of the nanoparticles can be controlled [111]. The size of nanoparticles and nanocrystals is not primarily dependent on the synthesis method employed, but rather, it is mainly influenced by the preparation and control of the salt solution.  [43] prepared mixed salt solutions with different Cd/Fe molar ratios, combined with co-precipitation and calcination at different temperatures to prepare CdO-Fe 2 O 3 composite oxide particles. According to XRD verification, the sample with a Cd/Fe ratio of 1/2 was identified as a spinel phase CdFe 2 O 4 , which exhibited the highest sensitivity (48) towards ethanol at 300 • C (Figure 2a). The study conducted by Rao et al. [51] focused on the utilization of the spray pyrolysis deposition technique to fabricate nanocrystalline (Co, Cu, Ni, and Zn) ferrite thin film sensors. The XRD patterns (Figure 2c) show the single cubic spinel phase of the (Co, Cu, Ni, and Zn) ferrite. From Figure 2d, the sensing characteristics of these sensors indicate that the ZnFe 2 O 4 nanocrystalline is more suitable as a sensor at lower temperatures and concentrations. On the other hand, the NiFe 2 O 4 nanocrystalline demonstrates an outstanding LPG sensing ability at higher temperatures. different temperatures (Figure 2f). The optimal operating temperatures for the three sensors were found to be 90 °C and room temperature, respectively. At 90 °C, the CoFe2O4 nanoparticles exhibited a maximum response value of 42.4%, while the CoFe2O4 porous nanoparticles demonstrated a maximum response value of 20.26% at room temperature. The CoFe2O4 nanorods, on the other hand, displayed a maximum response value of 13.3% at 90 °C. In the porous nanoparticle sensor, the optimal temperature was reduced to room temperature due to the high surface volume ratio of the structure. As discussed earlier, optimizing the grain size and specific surface area of spinel ferrite can significantly enhance the performance of gas sensors. Wei et al. [35] prepared CoFe 2 O 4 nanoparticles via a hydrothermal method. The CFO-400 sensor, which is calcined at 400 • C, shows promising results with its response value of 110 to 100 ppm ethanol gas at 200 • C (Figure 2b). This not only indicates its high sensitivity, but also showcases its good repeatability and stability, which are crucial characteristics for sensor materials. Rathore et al. [52] prepared CoFe 2 O 4 nanoparticles with varying particle sizes through the uniaxial press method. The objective of the research was to examine how the sensing performance of the nanoparticles is influenced by factors such as particle size, temperature, and gas flow. The results of the study demonstrated that CoFe 2 O 4 nanoparticles have good gas sensitivity, and the maximum response value increases with the decrease in particle size. Among them, the response value of 5.8 nm CoFe 2 O 4 nanoparticles to 5 ppm LPG at 250 • C is the highest, reaching 0.72 (Figure 2e), and its response time and recovery time are 3 s and 48 s, respectively. Halvaee et al. [54] employed a hydrothermal synthesis technique to fabricate three distinct nanostructures of CoFe 2 O 4 , namely nanoparticles, nanorods, and porous nanoparticles. The structures and properties of these nanostructures were analyzed. A cost-effective gas sensor, constructed using a printed circuit board, was utilized to measure methanol gas and assess its performance at different temperatures ( Figure 2f). The optimal operating temperatures for the three sensors were found to be 90 • C and room temperature, respectively. At 90 • C, the CoFe 2 O 4 nanoparticles exhibited a maximum response value of 42.4%, while the CoFe 2 O 4 porous nanoparticles demonstrated a maximum response value of 20.26% at room temperature. The CoFe 2 O 4 nanorods, on the other hand, displayed a maximum response value of 13.3% at 90 • C. In the porous nanoparticle sensor, the optimal temperature was reduced to room temperature due to the high surface volume ratio of the structure.
Sumangala et al. [69] synthesized the MgFe 2 O 4 nanoparticles employing both the coprecipitation and sol-gel methods. The XRD patterns presented in Figure 3a demonstrate the similarity in the structural characteristics of both samples. The co-precipitation sample exhibited a smaller particle size and twice the BET surface area compared with the sol-gel combustion sample. The electrical properties and CO 2 sensing capabilities of these two MgFe 2 O 4 nanoparticles were investigated ( Figure 3b). Notably, the co-precipitated sample demonstrated a higher sensing response of 36%, whereas the sol-gel combusted sample achieved a sensing response of 24%. Ghosh et al. [78] reported nanocrystalline NiFe 2 O 4 ( Figure 3c) through the sol-gel auto-combustion method. Ball milling was performed at room temperature and particle size was controlled to optimize the sensitivity of H 2 and H 2 S. The experimental results show that there was a notable enhancement in the gas response when the particle size was reduced or the specific surface area was increased ( Figure 3d). Compared with the other test gases, NiFe 2 O 4 nanocrystals with a particle size of~5. 35    In a study conducted by Karpova et al. [88], ZnO, Fe 2 O 3 , and zinc ferrite ZnFe 2 O 4 nanopowders were prepared using the co-precipitation method. The gas-sensitive results proved that the sensitivity of ZnFe 2 O 4 towards ethanol and acetone was significantly higher compared with the simple oxides, with values ranging from one to two orders of magnitude greater, respectively. This enhanced gas sensitivity of ZnFe 2 O 4 can be attributed to the presence of a high concentration of acidic Bronsted centers that contain active protons. These centers facilitate participation in REDOX reactions and selectively adsorb ethanol based on the acid−base mechanism. Using the hydrothermal method, Zhang et al. [92] successfully synthesized ZnFe 2 O 4 nanoparticles (about 10 nm) (Figure 3f). The phase and morphology of the prepared products were strongly influenced by the reaction conditions, including the reaction time, temperature, and the molar ratio of raw materials. The experimental findings (Figure 3e These nanoparticles exhibited excellent selectivity towards NO 2 molecules. The ZnFe 2 O 4based sensor showed an impressive response with a gas-to-air ratio (R gas /R air ) of 247.7 toward 10 ppm NO 2 at 125 • C (Figure 4d), which is a relative low temperature. It also demonstrated a fast response and recovery characteristic (6.5 s/11 s). Li et al. [94] further investigated the mechanism behind the superior selectivity and sensing performance of ZnFe 2 O 4 towards NO 2 compared with other gases. Through non-in situ photoluminescence (PL) characterization and density functional theory (DFT) calculations, they found that the gas-sensitive mechanism of ZnFe 2 O 4 towards NO 2 is based on surface charge transfer. The presence of oxygen vacancies in the material also enhanced the adsorption energy and charge transfer between ZnFe 2 O 4 and NO 2 molecules on the surface.
Zhang et al. [110] synthesized ZnFe 2 O 4 nanoparticles using a solvothermal method with zinc acetylacetone and iron acetylacetone as the precursors. By carrying out the synthesis at 150 • C, ZnFe 2 O 4 nanoparticles ( Figure 4e) with a diameter of approximately 20 nm were obtained. These ZnFe 2 O 4 nanoparticles exhibited excellent gas-sensing capabilities, particularly for H 2 S gas. The sensor was able to detect H 2 S gas as low as 1 ppm at a temperature of 135 • C, with a sensor response reaching 15.1 for 5 ppm H 2 S gas at the same temperature ( Figure 4f). These results suggest that nano-ZnFe 2 O 4 holds great promise for the development of H 2 S gas sensors. The group of Jha et al. [102] conducted a study on a selective hydrogen H 2 S gas sensor based on a zinc ferrite film ( Figure 4g). The film was prepared using microwave-assisted solvent-thermal deposition. The sensor exhibited an excellent performance at an operating temperature of 250 • C. The response range of the sensor was found to be 1872-90% for H 2 S gas concentrations ranging from 5.6 ppm to 0.3 ppm. Through density functional theory calculations, the researchers concluded that the rapid rise and fall times of H 2 S (approximately 40 s and 70 s, respectively) and the complete recovery of the device were attributed to the physical adsorption of H 2 S molecules on the partially reversed ZnFe 2 O 4 surface. Figure 4h shows the total density of states (TDOS) of the ZnFe 2 O 4 . In the experiment, a double-difference subtraction automatic balance interface circuit was utilized to drive the sensor, and the noise signal was accurately processed and compensated through the differential output.

Nanorods/Nanotubes
The synthesis methods for spinel ferrite nanorods, nanotubes, and nanowires primarily include hydrothermal [113] and electrospinning [123] techniques. Nanofibers constructed via electrospinning exhibit uniformity and smoothness, thus making the technique widely utilized in the preparation of one-dimensional materials. 5.6 ppm to 0.3 ppm. Through density functional theory calculations, the researchers concluded that the rapid rise and fall times of H2S (approximately 40 s and 70 s, respectively) and the complete recovery of the device were attributed to the physical adsorption of H2S molecules on the partially reversed ZnFe2O4 surface. Figure 4h shows the total density of states (TDOS) of the ZnFe2O4. In the experiment, a double-difference subtraction automatic balance interface circuit was utilized to drive the sensor, and the noise signal was accurately processed and compensated through the differential output.

Nanorods/Nanotubes
The synthesis methods for spinel ferrite nanorods, nanotubes, and nanowires primarily include hydrothermal [113] and electrospinning [123] techniques. Nanofibers constructed via electrospinning exhibit uniformity and smoothness, thus making the technique widely utilized in the preparation of one-dimensional materials.
In the field of gas sensing, there is a growing interest in one-dimensional (1D) nanostructures such as nanorods, nanotubes, and nanowires, as they are gaining more attention compared with nanoparticles. The reasons for this are manifold. (1) One-dimensional nanostructures often have more active sites compared with nanoparticles. These active sites are the locations where the gas molecules can interact with the material, thereby inducing a detectable change (such as a change in resistance). Therefore, having more active sites means the material can interact with more gas molecules simultaneously, enhancing the sensitivity of the sensor [111]. (2) One-dimensional nanostructures such as nanotubes have unique gas diffusion characteristics. Their channel-like structure allows gas molecules to easily diffuse In the field of gas sensing, there is a growing interest in one-dimensional (1D) nanostructures such as nanorods, nanotubes, and nanowires, as they are gaining more attention compared with nanoparticles. The reasons for this are manifold. (1) One-dimensional nanostructures often have more active sites compared with nanoparticles. These active sites are the locations where the gas molecules can interact with the material, thereby inducing a detectable change (such as a change in resistance). Therefore, having more active sites means the material can interact with more gas molecules simultaneously, enhancing the sensitivity of the sensor [111]. (2) One-dimensional nanostructures such as nanotubes have unique gas diffusion characteristics. Their channel-like structure allows gas molecules to easily diffuse and permeate through the material. This not only increases the interaction between the gas and the material, but also improves the speed of detection, making the sensor more responsive [119]. (3) Nanotubes and similar structures typically have a relatively high specific surface area [114]. A higher surface area means more space for gas molecules to interact with the material, which further improves the sensitivity of the sensor. One-dimensional nanostructures are known for their favorable electron characteristics. For instance, nanowires can efficiently transport carriers, which is crucial in transducing the interaction between the gas and the material into a detectable electrical signal. In summary, because of their unique structural and electronic properties, materials with 1D nanostructures such as nanorods, nanotubes, and nanowires offer significant advantages in gas sensing and are being actively explored as potential gas-sensing materials.
To investigate the impact of structure on the gas-sensing performance of a sensor, Zhang et al. [113] utilized a high-efficiency anodic alumina template method and a hydrothermal method to prepare NiFe 2 O 4 hollow nanotubes with a length of 1 µm and a diameter of 100 nm, as well as NiFe 2 O 4 nanoparticles, respectively. In comparison with the NiFe 2 O 4 nanoparticles sensor, the NiFe 2 O 4 nanotube sensor possessed a porous structure with overlapping nanotubes, which facilitated improved gas sensitivity. During testing with different NH 3 gas concentrations, the NiFe 2 O 4 nanotubes sensor exhibited a higher response compared with the NiFe 2 O 4 nanoparticles sensor, albeit with a slower recovery speed. The high specific surface area of the nanotubes played a crucial role in the ability of the NiFe 2 O 4 nanotubes sensor to detect NH 3 gas. Wang et al. [115] developed a novel gas-sensing material, NiFe 2 O 4 porous nanorods (Figure 5a,b), which exhibited improved sensitivity and selectivity for detecting the harmful gas n-propanol. These porous javelinsuch as nanorods were synthesized using Ni/Fe bimetallic metal-organic frameworks as templates. As a gas-sensing material, ferrite demonstrated n-type gas-sensing behavior with reduced resistance in a reducing gas atmosphere. The NiFe 2 O 4 nanorods exhibited an outstanding sensing performance for n-propanol ( Figure 5c), with an extremely low detection limit of 0.41 ppm at 120 • C. At the same time, the sensor had a good selectivity to n-propanol, good cycle stability, and long-term stability. The exceptional performance of NiFe 2 O 4 nanorods can be attributed to their distinctive morphology and porous structure. The large number of reaction sites offered by the porous structure facilitated the accelerated diffusion of n-propanol gas, allowing the sensor to quickly and accurately detect the presence of the gas. Chu et al. [116] conducted a study where they prepared NiFe 2 O 4 nanorods ( Figure 5d) and nanocubes using the hydrothermal method. The nanorods had a length of approximately 1 µm and a diameter of about 30 nm, while the nanocubes had a side length of around 60-100 nm. The results of the study showed that the sensor based on NiFe 2 O 4 nanorods exhibited high sensitivity and selectivity towards triethylamine. Specifically, it achieved a sensitivity of 7 when detecting 1 ppm of triethylamine at 175 • C. However, the NiFe 2 O 4 nanocube-based sensor demonstrated a unique conductivity response in the NH 3 environment, showing a significant increase. Specifically, when exposed to 500 ppm triethylamine, the sensor exhibited a response of 0.033. In contrast, the sensors based on NiFe 2 O 4 nanocubes exhibited a different behavior. In a reducing gas atmosphere, the conductivity of the sensor increased. The shape of the crystal, whether nanorods or nanocubes, significantly influenced not only the response value of the gas, but also the type of semiconductor behavior observed.
Nguyen et al. [123] demonstrated the sensitivity of ZnFe 2 O 4 nanofiber (Figure 5e) sensors to H 2 S, achieving a response of 102 to 1 ppm H 2 S, along with excellent resistance to humidity and a short response time of 12 s. Zhu et al. [119] synthesized porous ZnFe 2 O 4 nanorods using a microemulsion system with calcination at 500 • C. The resulting ZnFe 2 O 4 nanorods had a diameter of approximately 50 nm, composed of ZnFe 2 O 4 nanocrystals (with a diameter of 5-10 nm) arranged linearly. Compared with ZnFe 2 O 4 nanoparticles, porous ZnFe 2 O 4 nanorods exhibited superior gas-sensing properties to ethanol at room temperature. The enhanced sensing performance can be ascribed to the random arrangement of the porous nanorods and the existence of interconnected porous channels. These factors significantly augmented the specific surface area of the nanorods, facilitating effective diffusion of the target gas for detection. Additionally, the smaller grain size of ZnFe 2 O 4 offered a greater number of active sites, matching the thickness of the electron-depleted region, thereby amplifying the response. Li et al. [122] conducted a study where ZnFe 2 O 4 nanorods ( Figure 5f) with a porous structure were synthesized using the hydrothermal method, with ZnFe 2 (C 2 O 4 ) 3 serving as the template. These nanorods were composed of small nanoparticles and exhibited a significant number of surface pores. The porous ZnFe 2 O 4 nanorods sensor demonstrated a rapid response to acetone, with a response of 52.8 and response/recovery times of 1/11 s at 260 • C for 100 ppm acetone. The exceptional response observed in the porous ZnFe 2 O 4 nanorods sensor can be attributed to several factors, including the fine nanoparticle size, suitable pore size, and reticular pore structure. These characteristics contribute to enhanced gas adsorption and diffusion, allowing for a rapid response to acetone. However, it is important to note that when the concentration of acetone exceeded 100 ppm, the desorption capacity of the sensing material became insufficient compared with its adsorption capacity. As a result, the sensor exhibited a stable response instead of a further increase in signal intensity.

Nanosheets
The preparation methods for spinel ferrite nanosheets primarily include template hydrothermal [129], sol-gel [127], and spray pyrolysis techniques [128]. The template hydrothermal method can prepare nanosheets with specific pore structures and morphologies, but the demolding step may limit the sample's morphology and structure [129]. The sol-gel method can prepare spinel ferrite nanosheets with specific compositions and structures, but it tends to introduce impurities [127]. Spray pyrolysis can produce thinner nanosheet films with good lattice matching and crystallinity, but the equipment cost is high and the operation is relatively complex [128].

Nanospheres
Spinel ferrite nanospheres can be classified into solid spheres [147], hollow spheres [149], core−shell spheres [139], and double-shell (or triple-shell) spheres [153]. They are mainly prepared using solvent thermal methods or metal-organic framework (MOF) [142] methods. In recent years, the template-free solvent thermal method has become the mainstream approach for synthesizing three-dimensional spinel ferrite materials.
Nanospheres typically consist of solid spheres or hollow spheres that can evolve from the core-shell structure. They are characterized by their low density, high specific surface area, pronounced surface activity, and notable stability [148]. Previous research suggests that to achieve a larger specific surface area, it is essential to decrease the size of the nanoparticles. Assembling nanoparticles into nanospheres allows for better control over the size, resulting in larger specific surface areas and higher sensitivity. The enhanced reactivity and gas-sensing performance of nanospheres can be attributed to their increased surface area-to-volume ratio.
Zhai et al. [142] conducted a study where they synthesized NiFe2O4 polyhedron structures (Figure 6a) derived from metal-organic frameworks (MOF) using solvothermal synthesis. By altering the solvent composition, they were able to synthesize large NiFe2O4 polyhedra with a more stable morphology and structure. These large polyhedra exhibited excellent gas-sensing properties for TEA. Notably, they demonstrated a fast response time of 6 s to 50 ppm TEA, an enhanced response value of 18.9 to 50 ppm TEA (Figure 6b), and showed good selectivity and repeatability at relatively low operating temperatures of 190 °C. The fast response rate of the sample can be attributed to its Nanosheets are a type of two-dimensional nanomaterial characterized by their flat, sheet-like structure. Due to their unique morphology, nanosheets possess a large surface area-to-volume ratio, providing an abundance of reaction sites and diffusion paths for gases to interact with. This increased surface area and availability of reaction sites contribute to improved gas-sensing properties, such as enhanced sensitivity and selectivity. The highly exposed surface of nanosheets allows for efficient gas adsorption and interaction, making them promising candidates for gas-sensing applications.
Singh et al. [126] prepared high-porous CuFe 2 O 4 cascade nanostructures by sol-gel method. It has a porous structure CuFe 2 O 4 with pore size between 10-15 nm. The results of the sensing experiments demonstrate that the porous CuFe 2 O 4 layered structure exhibits a high sensing response of 96% when exposed to LPG at a temperature of 25 • C. Moreover, it demonstrates excellent repeatability and rapid response recovery characteristics. Gao et al. [129] successfully synthesized porous ZnFe 2 O 4 nanosheets (Figure 5g) by utilizing graphene sheets as a rigid template. The resulting ZnFe 2 O 4 nanosheets had pores with a size range of 5-50 nm and were composed of nanoparticles with a diameter of approximately 10-20 nm. In comparison to Fe 2 O 3 nanoparticles, ZnO nanoparticles, and ZnFe 2 O 4 nanoparticles, the sensor based on ZnFe 2 O 4 nanosheets exhibited faster response and recovery times (39 s/43 s), higher response (R a /R g = 123) and excellent selectivity. The sensor also demonstrated good repeatability and stability. Moreover, the unique mesoporous ZnFe 2 O 4 nanosheets enabled the detection of H 2 S gases as low as 500 ppb at 85 • C (Figure 5h). The enhanced performance of the ZnFe 2 O 4 nanosheets can be ascribed to their high specific surface area and porous characteristics. The increased specific surface area provides more active sites for gas molecule adsorption and reaction, enhancing the gas-sensing response. The porous structure of the nanosheets allows for the diffusion of target gas molecules, facilitating their interaction with the sensing material. Additionally, the two-dimensional structure of the nanosheets prevents the aggregation of nanoparticles, ensuring a larger effective surface area for gas sensing and maintaining the structural integrity of the material. Overall, the combination of high specific surface area, porous features, and two-dimensional structure contributes to the enhanced gas-sensing performance of ZnFe 2 O 4 .

Nanospheres
Spinel ferrite nanospheres can be classified into solid spheres [147], hollow spheres [149], core−shell spheres [139], and double-shell (or triple-shell) spheres [153]. They are mainly prepared using solvent thermal methods or metal-organic framework (MOF) [142] methods. In recent years, the template-free solvent thermal method has become the mainstream approach for synthesizing three-dimensional spinel ferrite materials.
Nanospheres typically consist of solid spheres or hollow spheres that can evolve from the core-shell structure. They are characterized by their low density, high specific surface area, pronounced surface activity, and notable stability [148]. Previous research suggests that to achieve a larger specific surface area, it is essential to decrease the size of the nanoparticles. Assembling nanoparticles into nanospheres allows for better control over the size, resulting in larger specific surface areas and higher sensitivity. The enhanced reactivity and gas-sensing performance of nanospheres can be attributed to their increased surface area-to-volume ratio.
Zhai et al. [142] conducted a study where they synthesized NiFe 2 O 4 polyhedron structures (Figure 6a) derived from metal-organic frameworks (MOF) using solvothermal synthesis. By altering the solvent composition, they were able to synthesize large NiFe 2 O 4 polyhedra with a more stable morphology and structure. These large polyhedra exhibited excellent gas-sensing properties for TEA. Notably, they demonstrated a fast response time of 6 s to 50 ppm TEA, an enhanced response value of 18.9 to 50 ppm TEA (Figure 6b), and showed good selectivity and repeatability at relatively low operating temperatures of 190 • C. The fast response rate of the sample can be attributed to its unique dense hollow structure. The hollow structure enables the REDOX reaction between TEA molecules and the material to occur predominantly at the surface/interface, while the interior of the material remains inactive. This reduces the electron conduction path, leading to the observed fast response time. Qu et al. [153] conducted research on the synthesis of ZnFe 2 O 4 double-shell microspheres using a hydrothermal method and thermal treatment. Figure 6e is the XRD pattern of the yolk-shell, double-shell hollow spheres, and solid microspheres. Compared with the yolk-shell and solid microspheres, the ZnFe 2 O 4 double-shell hollow spheres not only reduced the operating temperature of the sensor, but also enhanced its acetone sensitivity because of the improved crystallinity and larger specific surface area. The sensor displayed a response of 2.6 to 5 ppm acetone at 206 • C (Figure 6f), with a response time of 6 s and a recovery time of 10 s. Furthermore, it is noteworthy that the detection limit for acetone achieved by the sensor was reported to be 0.13 ppm. This value is significantly below the established risk level for life and health, which is 20,000 ppm. Additionally, it is well below the diagnostic threshold for diabetes, which is set at 0.8 ppm. This indicates the high sensitivity and potential of the sensor in accurately detecting and monitoring acetone levels in various applications. and larger specific surface area. The sensor displayed a response of 2.6 to 5 ppm acetone at 206 °C (Figure 6f), with a response time of 6 s and a recovery time of 10 s. Furthermore, it is noteworthy that the detection limit for acetone achieved by the sensor was reported to be 0.13 ppm. This value is significantly below the established risk level for life and health, which is 20,000 ppm. Additionally, it is well below the diagnostic threshold for diabetes, which is set at 0.8 ppm. This indicates the high sensitivity and potential of the sensor in accurately detecting and monitoring acetone levels in various applications. Zhou et al. [147] successfully synthesized porous ZnFe2O4 nanospheres (Figure 6c) using a template-free solvothermal method, followed by annealing at 400 °C. These nanospheres consisted of numerous nanoparticles and possessed a pore size ranging from 10 to 20 nm. The distinctive porous spherical structure greatly improved the sensor's acetone sensing performance. The response value for 30 ppm acetone reached 11.8, which is 2.5 times higher compared with that for the ZnFe2O4 nanoparticles (Figure 6d). A swift response time of 9 s showcased its ability to promptly detect and react to variations in the target gas. However, the recovery time was relatively longer, taking 272 s. Subsequently, zhou et al. [149] employed a template-free solvent-heat treatment followed by heat treatment at 400 °C for 2 h to fabricate ZnFe2O4 hollow microspheres assembled with nanosheets (Figure 6g). The nanosheets within the microspheres had an average Zhou et al. [147] successfully synthesized porous ZnFe 2 O 4 nanospheres (Figure 6c) using a template-free solvothermal method, followed by annealing at 400 • C. These nanospheres consisted of numerous nanoparticles and possessed a pore size ranging from 10 to 20 nm. The distinctive porous spherical structure greatly improved the sensor's acetone sensing performance. The response value for 30 ppm acetone reached 11.8, which is 2.5 times higher compared with that for the ZnFe 2 O 4 nanoparticles (Figure 6d). A swift response time of 9 s showcased its ability to promptly detect and react to variations in the target gas. However, the recovery time was relatively longer, taking 272 s. Subsequently, zhou et al. [149] employed a template-free solvent-heat treatment followed by heat treatment at 400 • C for 2 h to fabricate ZnFe 2 O 4 hollow microspheres assembled with nanosheets ( Figure 6g). The nanosheets within the microspheres had an average thickness of 20 nm, while the hollow microspheres themselves had diameters ranging from 0.9 to 1.1 µm. The hollow flower-like structure offered multitudes of adsorption/reaction sites, and the presence of diffusion channels, primarily distributed in the aperture range of 2 to 50 nm, facilitated the diffusion of target gases. At an operating temperature of 215 • C, the sensor exhibited a response value of 37.3 to 100 ppm acetone (Figure 6h) and demonstrated good long-term stability. However, under the same conditions, the response to ethanol was also high, measuring at 27.0. The presence of layered hollow structures in semiconductor oxides can enhance the diffusion of target gases, making them advantageous for gas-sensor applications.

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 [155], B-site doping [156], and AB-site doping [157]. The second involves the incorporation of doping elements into the tetrahedral and octahedral interstices of MFe 2 O 4 crystals. This results in a solid solution structure, where the doping elements are homogeneously dispersed within the host material [158]. Doping can significantly alter the composition and microstructure of spinel ferrite materials, influencing characteristics such as crystallinity [159]. These changes can, in turn, affect the reference resistance [160] and gas-sensing performance [161] of the ferrite-based gas sensors. For instance, doping can enhance the sensitivity [162], selectivity [163], response speed [28], and stability [164] of the sensors. In this section, we will review the latest research progress on element doping in spinel ferrite materials and its influence on their gas-sensing properties (Tables 5-9). The focus will be on how different doping elements can affect the sensor's performance, the optimal doping concentrations, and the underlying mechanisms behind these effects. This review will provide valuable insights for the design and fabrication of high-performance ferrite-based gas sensors.

A Site Doping
Compounds of the MFe 2 O 4 type, where M represents elements such as Mg, Cu, Zn, Ni, and Co, are widely utilized in the field of sensors due to their favorable surface activity. The study conducted by Mukherjee et al. [155] presents an interesting perspective on how the morphology and structure of ferrite-based materials can influence their gas-sensing properties. In their research, they synthesized one-dimensional Mg 0 . 5 Zn 0 . 5 Fe 2 O 4 hollow tubes using a wet chemical process assisted by an alumina template. They evaluated the gas-sensitive properties of two versions of these nanotubes: one version was embedded in a porous alumina template (Figure 7a) and the other was isolated and coated on a quartz substrate (Figure 7e). The nanotubes exhibited good responsiveness to H 2 , CO, and N 2 O gases in both configurations. Interestingly, they observed a difference in the behavior of the nanotubes based on their configuration. Regardless of the type of test gas, the concentration of the test gas, or the operating temperature, the embedded nanotubes consistently behaved as N-type semiconductors. N-type semiconductors are characterized by an excess of electrons (Figure 7b,c). On the other hand, the isolated nanotubes behaved as P-type semiconductors (Figure 7f,g), which are characterized by a deficiency of electrons or an excess of "holes" for the electrons. This inversion from N-type to P-type dominance of carriers, when going from embedded to isolated nanotubes, is a significant finding. It suggests that the electronic properties of ferrites can be customized by changing their surface-to-volume ratio. In other words, by altering the physical configuration of the ferrites (from embedded to isolated), it is possible to control their semiconductor behavior. This finding opens up new possibilities for the design and fabrication of ferrite-based gas sensors, as it introduces an additional degree of tunability in their properties.  Dalawai et al. [90] prepared Ni x Zn 1−x Fe 2 O 4 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) using the oxalic acid co-precipitation method. With the increase in nickel content in Ni-Zn ferrite, the bond length (A-O) and ionic radius (r A ) at site A decreased (Figure 7d), while the bond length (B-O) and ionic radius (r B ) at site B remained unchanged. Infrared spectroscopy revealed two major absorption bands near 400 and 600 cm −1 , corresponding to tetrahedral and octahedral locations, respectively. Compared with LPG and Cl 2 , ZnFe 2 O 4 thick films showed a higher sensitivity to ethanol (82%) (Figure 7h), better response time (30 s), and better recovery time (90 s). NiFe 2 O 4 thick film has a good sensitivity (63%), good response (30 s) and good recovery time (70 s) to LPG. Compared with LPG, Ni 0 . 6 Zn 0 . 4 Fe 2 O 4 displayed a higher sensitivity towards Cl 2 and ethanol gases. Zhang et al. [187] conducted a study where they synthesized Cu-doped ZnFe 2 O 4 nanoparticles (Cu-ZFNPs) using a hydrothermal method. Interestingly, the addition of copper did not significantly alter the size of the nanoparticles, which remained around 50 nm for both the pure and Cu-doped ZFNPs. Figure 7j shows the XRD patterns of the pure ZFNPs and Cu-ZFNPs with different Cu concentrations However, the gas-sensing performance of the nanoparticles was notably affected by copper doping. The Cu-ZFNPs exhibited a superior performance in detecting H 2 S gas compared with the pure ZFNPs, particularly at lower temperatures. This proves that the introduction of copper into the ZnFe 2 O 4 nanoparticles improved their sensitivity to H 2 S gas, highlighting the effectiveness of element doping in optimizing the properties of spinel ferrite materials. The best gas-sensing performance was achieved with Cu-ZFNPs containing an appropriate concentration of copper. These nanoparticles demonstrated a maximum response of 37.9 to 5 ppm H 2 S at room temperature (Figure 7k). The sensor also exhibited rapid response and recovery times, taking only 10 s to respond to the presence of H 2 S and 210 s to recover after the gas was removed.   Using the co-precipitation method, Mondal et al. [201] conducted a study where they synthesized Cu 0 . 5  resulted in a noteworthy enhancement in sensitivity to acetone, reaching an impressive 77%, while the introduction of Ni in Cu 0 . 25 Ni 0 . 5 Zn 0 . 25 Fe 2 O 4 improved the sensitivity to ethanol to 75%. These findings suggest that the addition of specific transition metal elements, such as copper and nickel, enhances the gas-sensing properties of the ferrite nanoparticles, making them promising materials for the detection of acetone and ethanol gases. Gauns et al. [199] fabricated a thick film of Ni 0 . 4 (Figure 7i) on a glass substrate for the detection of Cl 2 . The thick ferrite film composed of x = 0.3 showed a high selective response to Cl 2 gas at 100 • C. For 300 ppm of Cl 2 gas, the response was 212% (Figure 7l). The reaction time was less than 10 s and the recovery time was less than 15 s. a Response is defined as R a /R g ; b Response is defined as R g /R a ; c Response is defined as ∆R/R a .

B Site Doping
Spinel ferrite, represented by the formula (M 2+ )(Fe 3+ ) 2 O 4 , adopts a face-centered cubic crystal structure. It can be classified into three types: normal spinel, inverse spinel, and mixed spinel. The arrangement of divalent and trivalent metal ions in tetrahedral and octahedral sites within the crystal lattice determines the spinel classification [210]. The introduction of rare earth ions (RE) as substitutions for a small portion of iron can have significant effects on the electrical and magnetic properties of spinel ferrite. For example, the introduction of Ce, which involves the coupling of 3d-4f interactions, leads to changes in the electrical and magnetic behaviors. Furthermore, Ce substitution can also impact the distribution of cations within the spinel lattice, resulting in alterations to its structural, magnetic, physicochemical, and electrical properties [211]. Other rare earth elements, when substituted into the spinel structure, can similarly induce changes in structural, magnetic, and electrical properties, although the specific effects may differ from those observed with Ce 3+ [212]. Mkwae et al. [207] conducted a study where they prepared MgCe x Fe 2−x O 4 (0 < x < 0.2) nanoparticles ( Figure 8a). X-ray diffraction (Figure 8b) analysis confirmed that the sample containing a lower concentration of Ce formed a pure cubic spinel phase. However, with higher Ce doping (x > 0.2), the formation of a secondary phase was observed. The grain size of the compounds ranged from 2.2 nm to 15.3 nm. As the Ce concentration increased, the spin state of 57 Fe Mossbauer transitioned from an ordered state to a paramagnetic state. The MgCe x Fe 2−x O 4 nano-ferrite exhibited a high sensitivity and selectivity towards the 100 ppm acetone vapors, with a response concentration exceeding 500 at 225 • C (Figure 8c). The sensor also demonstrated excellent repeatability, reversibility, and stability over a period of 120 days.

Noble Metal Doping
Currently, the noble metals widely utilized in gas-sensing applications encompass Pt, Pd, Au, Ag, and Ru, as well as their bimetallic composites. The enhancement of gas-sensing performance can be attributed to two key mechanisms: the electronic sensitization effect achieved by constructin metal−semiconductor contact [231] and the chemical sensitization effect stemming from the spillover phenomenon [232]. These mechanisms work in tandem, facilitating rapid interaction between noble-metal-decorated semiconductor spinel ferrite and target gases, while also effectively lowering the work temperatures by reducing the activation energy required for gas sensing. Li et al. [220] conducted a study wherein they utilized the liquid phase deposition precipitation method to prepare a ZnFe2O4 egg yolk-shell ball structure consisting of ul-

AB Site Doping
Rezlescu et al. [165] conducted a study where they prepared Mg 1−x Sn x Mo y Fe 2−y O 4 (x = 0, 0.1, and y = 0, 0.02) ferrites using metal nitrate as the raw materials using the self-combustion method. The introduction of Sn and Mo ions induced structural changes in terms of grain size and porosity. Specifically, the sample containing tin exhibited the highest porosity, with particle sizes around 100 nm. When Sn ions partially replaced Mg in MgFe 2 O 4 ferrite, the resistivity of the material improved by approximately two orders of magnitude. The samples were subjected to testing to evaluate their sensing capabilities towards reducing gases, specifically ethanol and acetone. The gas sensitivity was found to depend largely on the type of substituted ion and the specific gas being detected. Overall, all ferrites exhibited a higher sensitivity to acetone compared with ethanol. Among all of the ferrites tested, Mg 0 . 9 Sn 0 . 1 Fe 2 O 4 demonstrated the highest sensitivity to acetone. These findings highlight the potential of Mg 0 . 9 Sn 0 . 1 Fe 2 O 4 ferrite as a highly sensitive material for the detection of acetone gas. Mugutkar et al. [214] synthesized Co 0 . 7 Zn 0 . 3 La x Fe 2−2x O 4 (x = 0-0.1) nanoparticles (Figure 8d) using the sol-gel method. The XRD pattern (Figure 8e) of ferrite powder was refined using the Rietveld technique, and it was found that a singlephase spinel structure was formed. Through the analysis of the gas-sensitive properties, the response of the Co 0 . 7

Noble Metal Doping
Currently, the noble metals widely utilized in gas-sensing applications encompass Pt, Pd, Au, Ag, and Ru, as well as their bimetallic composites. The enhancement of gas-sensing performance can be attributed to two key mechanisms: the electronic sensitization effect achieved by constructin metal−semiconductor contact [231] and the chemical sensitization effect stemming from the spillover phenomenon [232]. These mechanisms work in tandem, facilitating rapid interaction between noble-metal-decorated semiconductor spinel ferrite and target gases, while also effectively lowering the work temperatures by reducing the activation energy required for gas sensing. Li et al. [220] conducted a study wherein they utilized the liquid phase deposition precipitation method to prepare a ZnFe 2 O 4 egg yolk-shell ball structure consisting of ultra-thin nanosheets and ultra-small nanoparticles. The surface of this structure was adorned with nanoscale gold particles, each with a diameter ranging from 1 to 2 nm. The experimental results revealed a significant four-fold increase in response (R air /R gas = 90.9) for the Au/ZnFe 2 O 4 sensor when exposed to 10 ppm chlorobenzene at 150 • C (Figure 9b), compared with the original ZFO sensor. Furthermore, the Au/ZnFe 2 O 4 sensor demonstrated excellent selectivity and exhibited the potential for application in chlorobenzene monitoring. The introduction of nanoscale gold particles onto the surface of the ZFO yolk-shell balls (Figure 9a) resulted in electronic and chemical sensitization effects, thereby enhancing the chlorobenzene sensing performance of the ZnFe 2 O 4 yolk-shell balls. Additionally, density functional theory (DFT) calculations were employed to corroborate the findings, confirming that the presence of gold nanoparticles on the surface of ZnFe 2 O 4 increased electron density, exhibited a higher adsorption energy, and facilitated net charge transfer. These factors collectively contributed to the heightened sensing response of the sensor towards chlorobenzene. Zhang et al. [224] employed a hydrothermal method to introduce Ag into ZnFe 2 O 4 hollow structures (Figure 9c) composed of stacked nanosheets. The addition of Ag altered the surface structure, but did not significantly affect the size of the hollow structures. At a temperature of 175 • C, the sensor based on 0.25 wt.% Ag-doped ZnFe 2 O 4 (Ag/ZnFe 2 O 4 ) exhibited a superior sensing performance compared with the pure ZnFe 2 O 4 sensor (Figure 9d). This improvement in performance can be attributed to the suitable hollow structure and the activation effect of Ag. Ag/ZnFe 2 O 4 sensors show promising potential for detecting low concentrations of acetone in the parts per million range. Additionally, these sensors demonstrate good gas selectivity to acetone and minimal influence from humidity. However, further research and improvement are needed to address the long-term stability of Ag/ZnFe 2 O 4 sensors.
Li et al. [219] successfully synthesized ZnO/ZnFe 2 O 4 /Au heterostructures (Figure 9e,f) with a porous mesh structure using a three-step method (a combination of electrospinning, atomic layer deposition, and solution reaction). The resulting ZnO/ZnFe 2 O 4 /Au structures exhibited a porous mesh-like morphology. The composite structure comprised of a uniform ZnO nanotube skeleton measuring 50 nm, ultra-thin ZnFe 2 O 4 nanosheets with a thickness of 10 nm, and well-dispersed Au nanoparticles. It had the characteristics of a large specific surface area, porous structure, ultra-thin thickness and high catalytic activity. The gas-sensing results show that the sensor based on the ZnO/ZnFe 2 O 4 /Au nanonet had the highest sensing response (30.3), a significantly enhanced selectivity, and a faster response/recovery speed (1 s/59 s). The response of ZnO/ZnFe 2 O 4 /Au to acetone was about three times higher than that of ZnO/ZnFe 2 O 4 composites and 5.5 times higher than that of the original ZnO (Figure 9g). The enhanced sensing performance was mainly due to the increase in the surface active sites of AuNPs, the obvious resistance modulation effect, and the excellent sensitization ability.

Other Element Doping
Doping refers to the process of introducing impurity atoms into a material, which can have various effects on the lattice and structure of the host material. One effect of doping is the alteration of the lattice constant, which is the spacing between the atoms in the crystal lattice. The presence of dopant atoms can disrupt the regular arrangement of atoms in the lattice, leading to changes in the lattice constant. Furthermore, doping can also introduce structural defects into the matrix material. These defects can include vacancies, where atoms are missing from lattice sites, or interstitials, where dopant atoms occupy spaces between lattice sites [238]. These defects can affect the overall structure and properties of the material, such as its electrical conductivity or optical properties. In addition to changing the lattice constant and introducing structural defects, doping can also regulate the charge exchange behavior of the material [239]. Doped ions often have multiple valence states, meaning they can exist in different charge states depending on the electron configuration [184]. When doped ions occupy equivalent lattice locations, they can undergo charge exchange with neighboring ions, leading to changes in the electronic properties of the material. This charge exchange behavior can influence the material's conductivity, magnetism, or other electronic properties [236,237]. Overall, doping is a versatile technique that can be used to modify the lattice, introduce defects, and regulate the charge exchange behavior in materials, thereby tailoring their properties for specific applications.
Jiang et al. [233] conducted a study where they prepared ZnFe 2 O 4 nanoparticles and vanadium (V)-doped ZnFe 2 O 4 nanoparticles using citrate pyrolysis. Interestingly, the particle size of the spherical particles remained unaffected by the V content added. However, as the V content increased, the resistance of the thick film based on ZnFe 2 O 4 decreased. The study also revealed that the addition of V had varying effects on the sensitivity to different VOCs (Figure 9h). The sensitivity to ethanol and acetone was significantly reduced due to the addition of V. However, at higher temperatures, the addition of V notably improved the sensitivity to benzene, toluene, and xylene. These findings suggest that V doping in ZnFe 2 O 4 nanoparticles can have a selective impact on the sensitivity to different VOCs. While the sensitivity to ethanol and acetone decreased, the sensitivity to benzene, toluene, and xylene improved, particularly at elevated temperatures.

Other Element Doping
Doping refers to the process of introducing impurity atoms into a material, which can have various effects on the lattice and structure of the host material. One effect of doping is the alteration of the lattice constant, which is the spacing between the atoms in the crystal lattice. The presence of dopant atoms can disrupt the regular arrangement of atoms in the lattice, leading to changes in the lattice constant. Furthermore, doping can also introduce structural defects into the matrix material. These defects can include vacancies, where atoms are missing from lattice sites, or interstitials, where dopant atoms occupy spaces between lattice sites [238]. These defects can affect the overall structure and properties of the material, such as its electrical conductivity or optical properties. In addition to changing the lattice constant and introducing structural defects, doping can also regulate the charge exchange behavior of the material [239]. Doped ions often have multiple valence states, meaning they can exist in different charge states depending on the electron configuration [184]. When doped ions occupy equivalent lattice locations, they can undergo charge exchange with neighboring ions, leading to changes in the electronic properties of the material. This charge exchange behavior can influence the material's conductivity, magnetism, or other electronic properties [236,237]. Overall,

Heterostructure
In Sections 3 and 4, 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. Consequently, the objective of this section is to provide a review of the latest research on spinel ferrite composites and to present the impact of these two types of composites on the gas-sensitive properties (Tables 10-12). 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.  a Response is defined as R a /R g ; b Response is defined as R g /R a ; c Response is defined as ∆R/R a ; d Response is defined as ∆R/R g . a Response is defined as R a /R g ; c Response is defined as ∆R/R a ; d Response is defined as ∆R/R g . a Response is defined as R a /R g ; c Response is defined as ∆R/R a .

Other MOSs/Ferrite
There are primarily two methods for synthesizing heterostructures between other metal oxides and spinel ferrite: the one-step method [283] and the multi-step method [258]. The one-step method can yield highly uniform heterostructures, forming microscopic heterojunctions, but it is challenging to control the ratio of the two phases [271]. The multistep method allows for more precise control in different synthesis stages, including the reaction conditions, proportions, and reaction time, to obtain the desired product properties and structures. However, it increases the duration and cost of the synthesis process [273].
Xu et al. [258] conducted a study in which they prepared NiO/NiFe 2 O 4 nanocomposites using a straightforward two-step hydrothermal method. The nanocomposites consisted of NiO nano-tetrahedrons with numerous NiFe 2 O 4 nanoparticles dispersed on their outer surface (Figure 10a,b), forming p-p type heterojunctions. By adjusting the amount of Fe added during the synthesis process, the Fe to Ni ratio was optimized. The nanocomposite designated as NiFe-0.008 exhibited a remarkable gas-sensing performance (Figure 10c), with a high response of 19.1 towards 50 ppm formaldehyde smoke at 240 • C. Additionally, it displayed a low detection limit of 200 ppb and demonstrated good long-term stability. Comparatively, the optimized NiFe-0.008 nanocomposite outperformed individual NiO nano-tetrahedrons (with a response of 11.6 at 250 • C) and NiFe 2 O 4 nanoparticles (with a response of 6.8 at 300 • C) in terms of the gas-sensing performance. These findings highlight the improved response performance achieved by the optimized NiFe-0.008 nanocomposite. Hu et al. [248] conducted a study where they modified CuO microspheres by incorporating CuFe 2 O 4 nanoparticles (Figure 10f), resulting in CuFe 2 O 4 /CuO heterostructures. These heterostructures exhibited a high sensitivity to hydrogen H 2 S. The researchers investigated the relationship between the mass ratio of CuFe 2 O 4 to CuO and the operating temperature to optimize the sensor's response to H 2 S.The results of the study demonstrate that the optimized CuFe 2 O 4 /CuO heterostructures exhibited a significantly enhanced response to 10 ppm H 2 S at 240 • C (Figure 10g), reaching approximately 20 times that of the initial CuO microspheres. Moreover, the optimized heterostructures showed excellent fast response and recovery abilities. These findings suggest that the incorporation of CuFe 2 O 4 nanoparticles into CuO microspheres can effectively enhance the gas-sensing performance of the sensor towards H 2 S. The optimized CuFe 2 O 4 /CuO heterostructures demonstrated a substantial improvement in sensitivity compared with the preliminary CuO microspheres, making them promising candidates for the detection of H 2 S gas. Balaji et al. [263] conducted a study in which they synthesized SnO 2 composite Mn 1−x CuFe 2 O 4 (x = 0, 0.5, and 1.0) nanocomposites with an equal mass percentage using the chemical coprecipitation method. The addition of SnO 2 to copper-substituted manganese ferrite resulted in an increase in grain size and a decrease in strain value. The morphological analysis revealed that the average particle size of the ferritic materials decreased linearly with the decrease in Mn 2+ concentration. The presence of SnO 2 on the surface of Cu-Mn ferrite led to an increase in particle size and a weakening of the magnetic properties. Furthermore, the addition of SnO 2 to MnFe 2 O 4 and Mn 1−x Cu x Fe 2 O 4 enhanced the sensitivity of the gas sensor. MnFe 2 O 4 exhibited resistance to oxygen and carbon dioxide, while SnO 2 -CuFe 2 O 4 showed a weak sensitivity. This indicates that the adsorption/chemisorption of oxygen or surface lattice oxygen atoms plays a dominant role in the complete oxidation of molecules. These findings highlight the impact of SnO 2 addition on the structural and gas-sensing properties of Mn 1−x CuFe 2 O 4 nanocomposites. The changes in grain size, strain value, particle size, and gas sensitivity provide valuable insights into the design and optimization of gas-sensing materials for specific applications.
as a sacrificial template, Li et al. [29] successfully synthesized CuFe2O4/α-Fe2O3 hollow spheres with a diameter of ~210 nm with porous non-thin shells (Figure 10h) by thermal oxidation and solid phase reaction. The gas-sensitive properties of CuFe2O4/α-Fe2O3 composites were compared with those of pure α-Fe2O3 hollow spheres. As anticipated, the sensor based on the CuFe2O4/α-Fe2O3 composite exhibited a higher sensitivity (Ra/Rg = 14), faster response and recovery times (6 s/100 s), and lower detection limits (100 ppb) compared with the original α-Fe2O3 hollow spheres (Figure 10i). The enhanced sensing performance of the CuFe2O4/α-Fe2O3 composites can be attributed to several factors. Firstly, the hollow porous structure of the composites provides a larger surface area, which increases the number of active sites for gas adsorption and improves sensitivity. Additionally, the presence of the heterojunction between CuFe2O4 and α-Fe2O3 allows for modulation of the resistance and facilitates charge transfer, further enhancing the gas-sensing performance. Lastly, the catalytic performance of CuFe2O4 in the composites contributes to the improved sensing properties. Li et al. [287] utilized a metal-organic skeleton to prepare a precursor similar to Prussian blue, and then employed direct pyrolysis to fabricate hollow ZnO/ZnFe2O4 microspheres with a heterogeneous structure (Figure 11a). These microspheres had a diameter of approximately 1.5 µm. As a gas-sensitive material, the hollow ZnO/ZnFe2O4 microspheres exhibited a temperature-dependent n-p-n-type abnormal conductive transition ( Figure  11b) when detecting low concentrations of volatile organic compounds (VOCs) such as  (Figure 10d), which are uniformly adhered to ZnFe 2 O 4 nanoparticles. Through the analysis of the TEA (triethylamine) gas-sensing mechanism, it was observed that the heterojunction between the spindles-like Fe 2 O 3 and ZnFe 2 O 4 nanoparticles played a crucial role in improving the gas-sensing performance. Compared with pure MOF-derived Fe 2 O 3 spindles, the gas-sensitive properties of Fe 2 O 3 /ZnFe 2 O 4 nanocomposites were enhanced and exhibited a remarkable response value of up to 69.24 when exposed to 100 ppm TEA (Figure 10e). This indicates a significant improvement in the gas-sensing performance of the nanocomposites compared with the pure Fe 2 O 3 spindles derived from MOF. Using Cu@carbon as a sacrificial template, Li et al. [29] successfully synthesized CuFe 2 O 4 /α-Fe 2 O 3 hollow spheres with a diameter of 210 nm with porous non-thin shells (Figure 10h) by thermal oxidation and solid phase reaction. The gas-sensitive properties of CuFe 2 O 4 /α-Fe 2 O 3 composites were compared with those of pure α-Fe 2 O 3 hollow spheres. As anticipated, the sensor based on the CuFe 2 O 4 /α-Fe 2 O 3 composite exhibited a higher sensitivity (Ra/Rg = 14), faster response and recovery times (6 s/100 s), and lower detection limits (100 ppb) compared with the original α-Fe 2 O 3 hollow spheres (Figure 10i). The enhanced sensing performance of the CuFe 2 O 4 /α-Fe 2 O 3 composites can be attributed to several factors. Firstly, the hollow porous structure of the composites provides a larger surface area, which increases the number of active sites for gas adsorption and improves sensitivity. Additionally, the presence of the heterojunction between CuFe 2 O 4 and α-Fe 2 O 3 allows for modulation of the resistance and facilitates charge transfer, further enhancing the gas-sensing performance. Lastly, the catalytic performance of CuFe 2 O 4 in the composites contributes to the improved sensing properties.
Li et al. [287] utilized a metal-organic skeleton to prepare a precursor similar to Prussian blue, and then employed direct pyrolysis to fabricate hollow ZnO/ZnFe 2 O 4 microspheres with a heterogeneous structure (Figure 11a). These microspheres had a diameter of approximately 1.5 µm. As a gas-sensitive material, the hollow ZnO/ZnFe 2 O 4 microspheres exhibited a temperature-dependent n-p-n-type abnormal conductive transition (Figure 11b) when detecting low concentrations of volatile organic compounds (VOCs) such as ethanol, acetone, toluene, and benzene. This phenomenon can be primarily attributed to the interplay of highly separated electron-hole pairs caused by the staggered band arrangement at the heterogeneous interface of the ZnO-ZnFe 2 O 4 shell. This interplay is influenced by the heat-dependent ionization reaction of the surface-absorbed oxygen molecules and the additional electron injection resulting from the reducing VOCs' surface reaction during the gas-sensitive process. The abnormal conductive transition observed in the hollow ZnO/ZnFe 2 O 4 microspheres when exposed to low concentrations of VOCs is a result of the complex interplay between the different processes occurring at the heterogeneous interface. This understanding of the underlying mechanism contributes to the understanding and optimization of gas-sensing properties for applications in VOC detection. Wang et al. [278] devised a design and synthesis method to create ZnO/ZnFe 2 O 4 hollow nanocages with a diameter of around 100 nm using a metal-organic framework (MOF) technique. The synthesis process involved two steps: the preparation of Fe(III)MOF-5 nanocages as a precursor, followed by the conversion into ZnO/ZnFe 2 O 4 hollow nanocages through hot annealing in air. Based on the BET analysis, it is observed that the ZnO/ZnFe 2 O 4 nanocages, in their as-prepared state, possessed a BET specific surface area of 48.4 m 2 ·g −1 and an average pore size of 9.1 nm, as determined using the BJH method (Figure 11c). Gas-sensing experiments revealed that the ZnO/ZnFe 2 O 4 hollow nanocages exhibited a superior response value of 25.8 to 100 ppm acetone (Figure 11g), with a detection limit of 1 ppm at the optimized temperature of 290 • C. This response value surpassed that of ZnO hollow nanocages (7.9) and ZnFe 2 O 4 nanospheres (8.1). Furthermore, the gas-sensing response of the ZnO/ZnFe 2 O 4 nanocages outperformed that of the other structures, with the response order being as follows: hollow nanocages > double shell > hollow microsphere; hybrid hollow spheres > nanoparticles with rods. Yang et al. [41] conducted a study in which they synthesized coral-like ZnFe 2 O 4 -ZnO heterostructures with mesoporous structures (Figure 11d,e) and evaluated their gas-sensing performance towards the volatile organic compound TEA. The prepared sensor was subjected to thorough gas-sensing tests, and the results demonstrated several advantages, including a high response value (Ra/Rg = 21.3 at 240 • C), fast response and recovery times (0.9 s/23 s), and good repeatability (Figure 11f). The combination of the unique coral-like mesoporous morphology, the formation of n-n heterojunctions, and the synergistic effect of ZnFe 2 O 4 s Bronsted centers contributed to the improved TEA sensing properties of the coral-like ZnFe 2 O 4 -ZnO. These findings provide valuable insights for the design and optimization of gas-sensing materials for the detection of volatile organic compounds.

Nanostructure Materials/Ferrite
In order to maintain the structural stability of nanostructured materials during heterojunction formation, a two-step method is typically employed [300]. This approach not only maintains the stability of the structural materials, but also suppresses the aggregation of the perovskite iron oxides during synthesis [304]. ough gas-sensing tests, and the results demonstrated several advantages, including a high response value (Ra/Rg = 21.3 at 240 °C), fast response and recovery times (0.9 s/23 s), and good repeatability (Figure 11f). The combination of the unique coral-like mesoporous morphology, the formation of n-n heterojunctions, and the synergistic effect of ZnFe2O4′s Bronsted centers contributed to the improved TEA sensing properties of the coral-like ZnFe2O4-ZnO. These findings provide valuable insights for the design and optimization of gas-sensing materials for the detection of volatile organic compounds. Nanostructured materials, such as two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) structures, possess unique dimensional characteristics that contribute to their attractive physicochemical properties. These structures exhibit small volume, high electron mobility, and large specific surface areas, making them highly advantageous in various applications. In the field of gas sensing, nanostructures with a large surface area and high porosity have been found to significantly enhance the performance of gas sensors. The increased surface area and porosity provide more reaction sites, enabling more efficient interaction between the sensing material and the target gas molecules. This enhanced interaction leads to improved sensitivity and selectivity in gas-sensing applications. A notable strategy to achieve synergistic effects is the integration of metal oxide semiconductor (MOS) materials with nanostructured materials possessing large specific surface areas.
Zhang et al. [290] achieved the successful synthesis of porous microsphere composites by incorporating g-C 3 N 4 into MgFe 2 O 4 (Figure 12a,b) using a solvothermal method. In the study, the content of g-C 3 N 4 was varied, and it was found that the sensor based on the MgFe 2 O 4 /g-C 3 N 4 composite material exhibited excellent gas-sensing performance. Specifically, when the g-C 3 N 4 content was 10 wt.%, the sensor showed several desirable characteristics, including high sensitivity and selectivity, fast response and recovery times. Notably, the maximum response to acetone increased by approximately 145 times compared with the sensors without g-C 3 N 4 . Moreover, the optimal temperature for sensing was reduced by 60 • C. Chu et al. [291] conducted a study in which they prepared ZnFe 2 O 4 /graphene quantum dot (GQD) nanocomposites ( Figure 12c) using a hydrothermal method. The researchers aimed to investigate the influence of GQD content on the gas-sensitive response and selectivity of the ZnFe 2 O 4 /GQD nanocomposites. The results demonstrated that the sensor based on the ZnFe 2 O 4 /GQD nanocomposites exhibited a response of 13.3 to 1000 ppm acetone and a response of 1.2 to 5 ppm acetone at room temperature (Figure 12d). The response time and recovery time for the detection of acetone r were both less than 12 s. However, it should be noted that the long-term gas-sensitive stability of the ZnFe 2 O 4 /GQD nanocomposites was not satisfactory. This indicates that further research and improvement are needed to address the stability issue and enhance the long-term performance of the nanocomposites in gas-sensing applications. Bai et al. [298] synthesized rGO/WO 3 /ZnFe 2 O 4 composites (Figure 12e,f) with varying proportions using hydrothermal, chemical water bath, and chemical reduction methods. The gas sensitivity of the synthesized composites was tested, yielding noteworthy results. Among the different compositions tested, the 0.8 wt.% rGO-9WO 3 -ZnFe 2 O 4 terpolymer exhibited a superior gas-sensing performance. It demonstrated a significantly higher response value of 26.92, which is six times higher than that of pure WO 3 and thirteen times higher than that of ZnFe 2 O 4 ( Figure 12i). Furthermore, the synthesized gas-sensitive material displayed excellent selectivity, a shorter response time of 51 s, and a lower detection limit of 0.02 ppm. These characteristics indicate the enhanced performance of the composite material in terms of sensitivity, selectivity, and response speed compared with the individual components. The successful synthesis of the rGO/WO 3 /ZnFe 2 O 4 composites and their improved gassensing performance suggest their potential for applications in gas-sensing devices. Further optimization and exploration of the composite composition and structure can enable the development of highly efficient gas sensors for various target gases. mance suggest their potential for applications in gas-sensing devices. Further optimization and exploration of the composite composition and structure can enable the development of highly efficient gas sensors for various target gases.

Conducting Polymer/Ferrite
In recent years, the synthesis of conductive polymer magnetic nanocomposites has received much attention from researchers because of its lightweight, low-cost preparation methods, and enhanced magnetoelectric properties. Among conductive polymers, polyaniline (PANI) has emerged as a P-type semiconductor material with an excellent sensing ability. While polyaniline-based ammonia sensors have been widely reported, developing faster, highly sensitive, and fully recyclable greenhouse gas sensors remain a major challenge. In this regard, Wang et al. [307] prepared polyaniline/CuFe2O4 heterostruc-

Conducting Polymer/Ferrite
In recent years, the synthesis of conductive polymer magnetic nanocomposites has received much attention from researchers because of its lightweight, low-cost preparation methods, and enhanced magnetoelectric properties. Among conductive polymers, polyaniline (PANI) has emerged as a P-type semiconductor material with an excellent sensing ability. While polyaniline-based ammonia sensors have been widely reported, developing faster, highly sensitive, and fully recyclable greenhouse gas sensors remain a major challenge. In this regard, Wang et al. [307] prepared polyaniline/CuFe 2 O 4 heterostructures (Figure 12j) through in situ polymerization. In contrast with the polyaniline-based sensor, the polyaniline/CoFe 2 O 4 composite showed a higher response, with a response of up to 27.37% at 5 ppm NH 3 , surpassing the performance of the original PANI and CuFe 2 O 4 films by a significant margin. This finding suggests that by combining CuFe 2 O 4 with polyaniline to form a p-n heterojunction, the gas-sensing performance could be enhanced (Figure 12j,k). The p-n heterojunction formed between CuFe 2 O 4 and polyaniline is expected to improve the gas-sensing performance of polyaniline-based sensors. The synergies between the two materials allows for increased sensitivity, faster response times, and better recoverability.

Summary and Prospect
This paper provides an exhaustive review of the advancements in spinel-ferrite-based gas sensors, emphasizing three critical areas: nanostructure, elemental doping, and heterostructure. Spinel ferrite gas sensors have garnered interest due to their broad sensitivity and excellent selectivity to various flammable, explosive, toxic, and harmful gases. The gas-sensing mechanism of these sensors depends on intricate interactions and electron transfer at the gas−solid interface. Consequently, alterations in the microstructure of spinel ferrite nanomaterials, such as grain size, specific surface area, and porosity, can substantially influence the sensor's gas-sensing performance. Metal element doping in spinel ferrite enhances the specific surface area and provides activation energy, while maintaining the original crystal structure. Moreover, the creation of heterojunctions at the interface between different gas-sensitive materials is pivotal in modulating the sensor response by forming an electron depletion layer. A detailed comparison reveals that refining the microstructure, suitable metal element doping, or employing material composites can lead to a certain level of enhancement in the sensing capabilities of gas sensors based on spinel ferrite. Nonetheless, practical applications face challenges, including high power consumption due to thermal excitation effects and extended recovery times due to slow gas desorption. Therefore, innovative research directions are required to achieve swift sensor recuperation and consistent detection at low temperatures, potentially even at ambient room temperature. To overcome these challenges, we suggest a blend of the aforementioned strategies, which may encompass refining the microstructure of spinel ferrites or controlling the iron stoichiometry, designing composite materials composed of spinel ferrite multi-layer porous shells or hollow spheres integrated with nanostructured materials such as reduced graphene oxide and molybdenum disulfide, and developing multi-component hybrid materials. These strategies aim to boost the performance of spinel ferrite gas sensors, with a primary emphasis on achieving a high response and low operating temperatures.