Recent Progress on Rare Earth Orthoferrites for Gas-Sensing Applications

Abstract

Gas-sensing technology is crucial for the detection of toxic and harmful gases to ensure environmental safety and human health. Gas sensors convert the changes in the conductivity of the sensing material resulting from the adsorption of gas molecules into measurable electrical signals. Rare earth orthoferrite-based perovskite oxides have emerged as promising candidates for gas-sensing technology owing to their exceptional structural, optical, and electrical properties, which enable the detection of various gases. In this article, we review the latest developments in orthoferrite-based gas sensors in terms of sensitivity, selectivity, stability, operating temperature, and response and recovery times. It begins with a discussion on the gas-sensing mechanism of orthoferrites, followed by a critical emphasis on their nanostructure, doping effects, and the formation of nanocomposites with other sensing materials. Additionally, the role of the tunable bandgap and different porous morphologies with a high surface area of the orthoferrites on their gas-sensing performance were explored. Finally, we identified the current challenges and future perspectives in the gas-sensing field, such as novel doping strategies and the fabrication of miniaturized gas sensors for room-temperature operation.

1. Introduction

Rapid urbanization and advancements in modern industrial products have contributed significantly to air pollution, resulting in the emission of volatile organic compounds (VOCs) from industrial processes, residences, public spaces, and transportation vehicles [1,2]. Additionally, the leakage of flammable and toxic gases further reduces air quality, posing a serious risk to human health [3]. According to the World Health Organization, VOCs are the most common atmospheric pollutants with melting points below room temperature and boiling points between 50 and 260 °C, highlighting the need for efficient monitoring systems [4]. Gas-sensing technologies have become crucial for air quality monitoring, toxic gas detection, and VOC identification [5,6,7,8]. To address these challenges, various gas sensors, including calorimetric, potentiometric, catalytic, chemiresistive, and conductometric-based sensors, have been developed [9]. Effective sensors with a low detection limit, excellent selectivity, long-term stability, high sensitivity, and quick response and recovery times are essential for gas-sensing applications.

Chemiresistive gas sensors involve an active sensing material whose conductivity is highly sensitive to environmental changes. Among chemiresistive sensors, metal oxide semiconductor (MOS) materials have gained prominence owing to their high selectivity, simple fabrication, low production cost, long-term stability, and potential for miniaturization [10]. The sensing mechanism of MOS gas sensors is based on the adsorption of gas molecules onto the surface of the material, which affects the concentration of charge carriers and alters their electrical resistance [11,12]. The performance of chemiresistive sensors can be evaluated using key metrics such as the detection limit, response and recovery times, sensitivity, and selectivity. These parameters are usually influenced by various factors such as the operating temperature, concentration of the target gas, and environmental conditions such as humidity. Among these factors, the operating temperature plays a critical role in determining the efficiency and reliability of the sensor because it affects its conductivity and surface reactions. However, the requirement for high operating temperatures limits their practical application owing to their energy efficiency, reduced stability, and risks in flammable environments [13]. Long-term exposure to high temperatures can also degrade the microstructure of sensing materials, thereby reducing their performance [14].

Perovskite oxides with an ABO₃ structure demonstrate superior gas-sensing performance compared with individual metal oxide semiconductors, primarily because of their ability to generate oxygen vacancies, which are essential for gas detection [15,16]. These materials have significant potential in various optoelectronic applications owing to their unique physicochemical properties and tunable electronic conductivities [17]. Furthermore, the ability of perovskite structures to accommodate a variety of dopants with different ionic radii at A or B sites enables modifications to charge carrier concentrations and band gap widths without altering the crystal lattice, thus improving their gas-sensing performance [18,19,20].

Rare earth orthoferrites (i.e., RFeO₃, where R indicates a rare earth element) have demonstrated potential applications in areas such as optoelectronics [21], solid oxide fuel cells [22], magneto-optic devices [23], magnetic refrigeration [24,25], spintronics [26], and gas sensors [27,28] owing to their unique optical, electrical, and magnetic properties [29]. The excellent chemical stability and catalytic properties of orthoferrites allow them to effectively adsorb gases and cause changes in resistance when exposed to specific gases [15,30]. Typically, perovskite oxides are synthesized through a mechanical process followed by high-temperature heat treatment. However, this approach often results in the desired perovskite structure with the formation of large aggregated crystal particles with insufficient porosity, which hinders their practical use in gas sensing [31]. A lower porosity limits the active surface area available for interactions with gas molecules, thereby diminishing the efficiency of the gas-sensing reaction [32]. To achieve a better gas-sensing performance, sensor materials with larger specific surface areas are crucial because they provide more active sites for adsorption and interaction with the target gas molecules. Various chemical methods have been adopted to overcome the low porosity of perovskites obtained via mechanical processes. Rare earth orthoferrites, synthesized using different chemical methods such as sol–gel [33], hydrothermal [34], and electrospinning [35], have demonstrated superior gas-sensing performance owing to their enhanced surface area and improved interactions with gas molecules. These techniques enable the fabrication of nanomaterials with precise control over the morphology, composition, and functionality of sensors. To enhance the gas-sensing capabilities of orthoferrites, various strategies, such as doping with different metals at R-sites, Fe-sites, or both sites, the development of composite nanomaterials, and employing light activation [36], have been explored. Light illumination on the sensing material surface can alter the electronic properties of the surface, thereby enhancing the sensing performance. In addition, innovative sensing structures, including nanoparticles with different morphologies [37], nanorods [27], nanotubes [38], nanofibers [39], and nanosheets/thin films [40], have been developed to alter the surface-to-volume ratio and improve gas adsorption and sensing performance. Compared with other gas sensors, the low operating temperatures, high stability, high selectivity, and potential for miniaturization make orthoferrites ideal for creating compact, reliable, and efficient gas sensors.

Perovskite oxides with an ABO₃ structure have emerged as a promising candidate for gas-sensing applications due their structural flexibility and tunable physicochemical properties. Among these oxides, RFeO₃ compounds exhibit unique structural features, flexible A-site and B-site composition, and unique gas-sensing characteristics compared to other perovskites. Despite the increased research interest on RFeO₃-based gas sensors in the recent years, a comprehensive and dedicated review on these compounds is still lacking. Therefore, the aim of this review is to address the gap by providing the detailed overview of the recent developments of RFeO₃-based gas sensors, emphasizing their potential for future environmental and industrial applications. This review highlights the recent experimental research on rare earth orthoferrite-based gas sensors featuring different morphologies, with a focus on the enhancement of selectivity, sensitivity, response and recovery time, and stability. This study explores the advantages of doped orthoferrites over their undoped counterparts in gas-sensing applications. It begins by providing an overview of the crystal structures and gas-sensing mechanisms of orthoferrites. This is followed by a discussion of the effect of nanostructures on the enhancement of gas-sensing properties, along with recent progress in orthoferrite-based gas-sensing materials with diverse morphologies, highlighting their practical applications in environmental monitoring. Finally, this review summarizes the findings, addresses current challenges, and explores future prospects, shedding light on the potential advancements and applications of orthoferrite-based gas-sensing technologies.

2. Crystal Structure

Perovskite metal oxides with the general formula ABO₃ (Figure 1a) have attracted considerable attention owing to their unique structural, chemical, and electronic properties [41]. In ABO₃, the A-site is typically occupied by rare earth metals (R) with a coordination number of 12, whereas the B-site is generally filled with transition metals with a coordination number of 6. Ideally, the perovskite structure has cubic symmetry at room temperature [42].

The perovskite oxides exhibit an extraordinary capability to incorporate 90% of the metallic elements into their lattices in a stable and effective manner [24,44,45]. This has resulted in the development of a wide range of functional materials with tunable physical properties for specific applications. The stabilities of the perovskite oxides were assessed using the Goldschmidt tolerance factor (t):

$$t={\displaystyle \frac{\left(r_A+r_O\right)}{\sqrt{2}\left(r_B+r_O\right)}}$$

where r_A, r_B, and r_O are the ionic radii of the A-site cations, B-site cations, and oxygen anions, respectively. For an ideal cubic structure, t = 1, whereas it is in the range of 0.71 to 0.89 for an orthorhombic structure due to a mismatch in the ionic radii of the A- and B-cations [46].

Perovskite oxides with rare earth metals (R) at the A-site have become the focus of electrochemical sensors, such as potentiometric and amperometric sensors, during the late 20th and early 21st centuries [47]. Later, these materials were explored for use in chemiresistive gas sensors, which exhibit a change in resistance owing to gas adsorption [8,48]. The strategy of doping at the A- and B-sites demonstrated significant changes in the gas-sensing performance, making perovskite oxides a highly flexible platform for advanced material design.

Among perovskite oxides, RFeO₃ materials have been extensively explored for gas-sensing applications, particularly owing to their high stability, superior electrical properties, and catalytic behavior [18,19,20]. Orthoferrite-based gas sensors have demonstrated an ability to detect various toxic and harmful gases with efficient performance metrics, such as low detection limits, rapid response and recovery times, and stable operation under harsh environmental conditions [27]. The feasibility of substituting different cations at the R-site, Fe-site, or both sites enables the development of gas sensors with improved sensitivity, selectivity, and response/recovery times [49,50]. In this paper, we review orthoferrite-based chemiresistive sensors in different forms at the nanoscale and discuss the changes in gas-sensing behavior after doping at the R-site, Fe-site, or both sites.

3. Gas-Sensing Mechanism

The gas-sensing mechanism of perovskite oxides can be understood from the interaction between the gas molecules and sensor material in terms of a change in the resistance of the sensing material. Changes in the electrical properties of the material result from the adsorption and desorption of gas molecules on the surface of the sensor upon exposure to the target gas. These interactions provide a deep understanding of the charge distribution, carrier concentration, and localized electrical behavior of the sensing material, which eventually influences its sensing mechanism.

Rare earth orthoferrites exhibit a p-type semiconducting behavior, with holes as the predominant charge carriers [51]. These holes, generated by the ionization of R³⁺ cation vacancies in the crystal lattice, are crucial to the sensing mechanism. Orthoferrites absorb oxygen molecules onto their surfaces from an air environment (left picture of Figure 1b). The strong electron affinity of oxygen leads to interactions with free electrons on the surface of RFeO₃ and generates chemisorbed oxygen species, such as O₂⁻, O⁻, and O²⁻ [52,53]. The detailed reactions are as follows:

O_2(gas) → O_2(ads)

(1)

O_2(ads) + e⁻ → O₂⁻_(ads)   (T < 100 °C)

(2)

O₂⁻_(ads) + e⁻ → 2O₂⁻_(ads)   (100 °C < T < 300 °C)

(3)

O⁻_(ads) + e⁻ → O²⁻_(ads)   (T > 300 °C)

(4)

In this process, electrons are transferred from the surface of RFeO₃ to the oxygen species, which increases the concentration of holes on the surface and the width of the hole accumulation layer (left picture of Figure 1b). This modification of the electronic properties of the surface alters the electrical resistance of the material. In a gas environment, the response of the RFeO₃ sensor depends primarily on the interaction between the gas molecules and chemisorbed oxygen species. Such interactions modify the hole accumulation layer near the surface (Figure 1b) as well as the mobility of the charge carriers within the semiconductor, which directly affects the resistance [34].

The sensing process in RFeO₃ relies on measuring the resistance change resulting from the adsorption and desorption of gas molecules on their surface. Adsorption alters resistance, which is measured and characterized by sensing parameters such as sensitivity, selectivity, response, and recovery time. The desorption phase is equally important, because it allows the sensor to return to its original electrical state by releasing gas molecules from its surface. This regeneration is crucial for repeated use and ensures consistent performance in subsequent detection cycles.

4. Gas-Sensing Parameters

4.1. Operating Temperature

The temperature at which the sensor delivers the highest response to the target gas is known as the optimal operating temperature [6]. At this temperature, the interaction between the sensor surface and target gas reaches an equilibrium state, ensuring the maximum sensitivity and efficiency of the sensor. Typical operating temperatures for orthoferrite-based gas sensors range from 110 to 350 °C. At room temperature, the reaction between the gas molecules and the sensor surface is too slow or may not happen. That is why heating the sensor is required to promote the surface reactions. Sensors with low operating temperatures are more important for practical applications than those with excessively high operating temperatures because they do not require additional heating elements, making them suitable for portable and low-power applications. Consequently, developing sensors that exhibit high efficiency at room temperature remains a challenge in advancing gas-sensing technology.

4.2. Response and Recovery Time

Response and recovery times are critical parameters for evaluating the performance of a gas sensor because they directly reflect the sensor’s ability to respond and detect changes in the presence of the target gas. The response time indicates how quickly the sensor can respond to the gas, resulting in a stable signal (usually defined as 90% of the full response) when transitioning from an air environment to the target gas environment [6]. By contrast, recovery time refers to the duration required for the sensor to stabilize back to 90% of its original value after the gas is removed and the environment is restored to air. Sensors with faster response and recovery times are more effective in providing early warnings to minimize the risks and potential damage. Response and recovery times of RFeO₃-based gas sensors vary widely with a quick response time of under 10 s to several minutes, depending on gas type, concentration, and nanostructure.

4.3. Selectivity

The selectivity of a gas sensor is the ability of the sensor to identify and respond to a specific target gas while minimizing interference from other gases present in the environment. This parameter is evaluated by exposing the sensor to various gases under identical operating conditions. If the sensor demonstrates an excellent response to a particular gas compared to other gases, it is considered to have excellent selectivity for that gas.

4.4. Response

The response of a gas sensor is defined as the ratio of its resistance in air (R_a) to that in the target gas environment (R_g) [6]. For p-type semiconductor gas sensors, the response to reducing gases is often calculated as S = R_g/R_a, whereas, for oxidizing gases, the response is S = R_a/R_g. By contrast, the response can be expressed in percentage form: S = (R_g − R_a)/R_g × 100% for reducing gases, and S = (R_a − R_g)/R_a × 100% for oxidizing gases. Conversely, the definition of the gas response value is reversed for n-type semiconductor gas sensors.

4.5. Sensitivity

The sensitivity of a gas sensor, reflected by its response to an exposed gas, enables the detection of a specific gas by measuring the changes in its electrical properties. The sensitivity of a gas sensor can be extracted from the slope of the response–concentration fitting curve [15,54]. It indicates the strength of the gas sensor’s ability to detect the gas by measuring the rate of change in the sensor’s response with varying gas concentrations.

4.6. Limit of Detection

The limit of detection (LOD) of a gas sensor is the lowest concentration of gas that can be detected reliably. A gas sensor with a low LOD is essential for detecting toxic and harmful gases at an early stage before creating a panic. A sensor with higher sensitivity and faster response time may easily detect trace amounts of the target gases. The long-term use of a sensor under harsh conditions may degrade its sensitivity, leading to an increase in the LOD. The LOD of RFeO₃-based gas sensors varies depending on the sensing material and the target gas, with several sensors achieving in the range of 1 ppb to 5 ppm.

4.7. Stability

The stability of a gas sensor is a key parameter that reflects its ability to provide consistent and reliable responses to a gas with a fixed concentration over an extended period, without significantly deviating from its initial value. Repeated measurements should be conducted over time to assess the sensor stability. Sensors with excellent stability maintain their performance even under different environmental conditions such as temperature and humidity.

5. Nanostructures

Recently, nanostructured materials have demonstrated remarkable gas-sensing capabilities compared to their bulk counterparts [55]. The dimensions of the nanostructures and their chemical properties play a vital role in determining the performance of the sensor in terms of sensitivity, selectivity, stability, and response/recovery times. The smaller size of the nanostructure provides a more active surface area for gas molecules to interact with the sensing material, thereby enhancing the performance. In addition, a higher surface-to-volume ratio of nanostructures can significantly improve their sensitivity towards targeted gas molecules.

Nanomaterials can be classified into different categories such as zero-dimensional nanoparticles, one-dimensional nanorods/nanotubes/nanofibers, two-dimensional nanosheets/thin films, and three-dimensional nanocrystals. These dimensional differences significantly influence the gas-sensing performance of nanomaterials. Orthoferrites have been widely explored for gas-sensing applications because of their high specific surface areas, porosities, and chemical reactivities. For example, the smaller particle size of a (La_0.8Ca_0.2)_0.6Bi_0.4FeO₃ sensor demonstrated superior gas-sensing performance compared to a sensor with a larger particle size [56]. The unique channel-like structure of GdFeO₃ nanorods favors the easy diffusion of gas molecules and enhances their sensitivity compared with GdFeO₃ butterflies [27]. The strain effects dominated with the increase in the thickness of the LaFe_0.8Co_0.2O₃ thin films, leading to a better response to CO at a lower thickness [57]. Additionally, external factors such as the operating temperature, humidity, and concentration of the target gas can influence the sensing capabilities. Nanostructured gas sensors based on orthoferrites have been fabricated using strategic synthesis methods [57,58] to alter the size, morphology, and crystallographic structure of the sensor, which can significantly affect its response. The exceptional advantages of these methods for controlling the size, shape, and arrangement of nanoparticles are helpful for optimizing the performance of gas sensors. In the following sections, we provide a comprehensive review of the latest research findings on rare earth orthoferrite-based gas sensors with various nanostructured morphologies.

Table 1. Summary of orthoferrite-based nanoparticle gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
NdFeO3	N-Butanol	260	100	441	33/7	24 ppb	[37]
PrFeO3	N-Butanol	280	100	85	18/13	26.3 ppb	[59]
LaFeO3	Formaldehyde	160	100	8.9	53/32	1 ppm	[7]
LaFeO3	Ethanol	110	30	154.5	70/127		[36]
YFeO3	Acetone	110	30	128.1			[60]
SmFeO3	Acetone	120	30	29.228			[61]
NdFeO3	Isopropanol	275	0.8	3.81	57/52		[34]
NdFeO3	Ethanol	250	100	150	19/22	5 ppm	[62]
LaFeO3	Ethanol	300	143	14.5	23/39	1 ppb	[63]
LaFeO3	Dimethyl disulfide	210	100	127			[64]
ErFeO3	Isopropanol	270	50	12.75	6/25	2 ppm	[65]
SmFeO3	Acetone	200	10	82		6.6 ppb	[66]
LaFeO3	Formaldehyde	125	50	116	7/24	50 ppb	[8]
DyFeO3	Acetone	190	2	3.81	42/44		[33]
LaFeO3	Acetylene	150	500	28.8			[58]
LaFeO3	Ethanol	112	200	46.1			[67]
Pr0.8Ca0.2FeO3	LPG	25	0.5 vol%	1.3	7.5/7.1		[28]
Pr0.8Bi0.2FeO3	LPG	25	2.0 vol%	3.63	15.3/22.4		[68]
La0.8Ba0.1Bi0.1FeO3	Acetone	200	5	10.31	9/7		[69]
(La0.8Ca0.2)0.6Bi0.4FeO3	Hydrogen sulfide	160	100	29	16/17		[56]
LaFeO3-1.5% In	Formaldehyde	125	100	122	36/40	1 ppb	[49]
La0.8Ba0.1Bi0.1FeO3	Ethanol	180	100	21	10/6		[70]
Sm-PrFeO3	Acetone	270	50	44.94	15/16		[71]
Yb0.8Ca0.2FeO3	CO2	260	5000	1.85	24/31		[72]
Gd0.9Ca0.1FeO3	Methanol	260	600	117.7	60/66		[73]
Yb0.8Ca0.2FeO3	Acetone	230	0.5	2.5	29/55		[74]
LaFe0.96Mo0.04O3	Triethylamine	140	100	70	12/59		[50]
LaFe0.4Cu0.6O3	Ethanol	300	300	17	240/660		[75]
LaCo0.1Fe0.9O3	CO2	220	5000	6.69%	6/42		[15]
PrFe0.9Co0.1O3	CO2	25	500	1.22	17.2/18.4		[76]
GdFe0.7Co0.3O3	NO2	200	20	6.86%	104/97	112 ppb	[77]
LaFe0.5Mn0.5O3	Ethanol	190	50	2.01	20/5		[78]
La0.885Pb0.005Ca0.11Fe0.95Co0.05O2.95	Ammonia	250	10	1.011	27/28		[79]
La0.8Ca0.1Pb0.1Fe0.95Mg0.05O3	Ethanol	225	1000	≈330			[80]

O.T.—operating temperature; Conc.—concentration; t_res/t_rec—response time/recovery time; and LOD—limit of detection.

, Table 2, Table 3 and Table 4 summarize the important gas-sensing parameters of orthoferrites from the existing literature.

5.1. Nanoparticles

Orthoferrites in the form of nanoparticles have been prepared through various suitable synthesis methods such as sol–gel [33,64,81], hydrothermal [34,37], citrate–nitrate auto-combustion [82], microwave thermal treatment [83], microwave-assisted aqueous solution [84], and solvothermal [62] methods through a series of steps. The first step is to mix the appropriate ratio of metal salts with basic precipitants to form a precipitate. Next, the precipitate is washed and dried to convert it into nanoparticles. Finally, heat treatment or surface modification techniques are used to refine the morphology and properties of the nanoparticles. The size of the nanoparticles and nanocrystals is influenced less by the choice of synthesis method and more by the preparation and control of the salt solution. However, the synthesis method significantly influences the morphology of the resulting nanoparticles [82].

Guo et al. reported the excellent gas-sensing performance of NdFeO₃ nanoparticles synthesized using a hydrothermal method, which resulted in a granular morphology with an average size of 65 nm [37] (Figure 2a). These nanoparticles demonstrated exceptional gas-sensing properties for n-butanol detection, with a high response of 441 to 100 ppm at an optimal operating temperature of 260 °C and a low detection limit of 0.024 ppm. The response of the sensor initially increased with an increasing temperature, reaching a high value at 260 °C, beyond which it started decreasing (Figure 2b). At lower temperatures, insufficient activation energy prevents effective interactions between the chemisorbed oxygen species and gas molecules. By contrast, the evaporation of adsorbed gas molecules intensifies at higher temperatures, reducing the number of adsorbed gas molecules and leading to a decrease in the response of the sensor. The sensor exhibited excellent selectivity for n-butanol over other gases, such as ethanol, isopropanol, triethylamine, and ammonia, along with rapid response and recovery times of 33 and 7 s, respectively. It also exhibited consistent repeatability across six cycles, stable performance over 15 days, and a response value of 84 at 50% relative humidity. The outstanding performance of the sensor was attributed to the large surface area of the nanoparticles, oxygen vacancies, and unique electronic characteristics of the perovskite structure, which was further supported by X-ray photoelectron spectroscopy (XPS) analysis, highlighting the potential of this system for efficient gas-sensing applications.

Liu et al. fabricated a LaFeO₃ nanopowder-based planar electrode gas sensor to reduce the operating temperature [36]. They demonstrated an enhancement in the performance of the sensor under illumination with multiwavelength light (370, 420, and 470 nm). Among the sensors with various annealing temperatures (500–900 °C), the sensor that was annealed at 700 °C exhibited superior performance at a relatively low operating temperature of 110 °C compared with previously reported sensors [33]. Light illumination studies revealed that, when the sensor was exposed to 30 ppm of ethanol and methanol, the sensor response reached 121.6 and 59.7, respectively, without light illumination. However, under illumination with 370 nm light, the response increased to 154.5 for ethanol and 109.3 for methanol. The UV–visible absorption spectra and energy band analysis further confirmed that the photons at 370 nm exceeded the band gap of LaFeO_3, facilitating higher photon-energy-assisted electron transitions, thereby reducing the resistance and enhancing the sensitivity of the sensor.

In a separate study conducted by Chai et al., LnFeO₃ (Ln = Nd, Dy, Er) nanoparticles were prepared using a microwave-assisted hydrothermal method, and the influence of rare earth elements on the gas-sensing performance was explored [34]. Nitrogen adsorption/desorption studies revealed that the specific surface area of NdFeO₃ (8.142 m²/g) was smaller than those of DyFeO₃ (9.679 m²/g) and ErFeO₃ (9.800 m²/g). Gas sensors were fabricated on Al₂O₃ substrates using Pt/Au electrodes and were tested for isopropanol sensing. Among the sensors, the NdFeO₃ gas sensor demonstrated the highest response at 275 °C for 2 ppm of isopropanol, which is attributed to its lowest Ln-O binding energy [51] and high oxygen defect concentration, which was confirmed by the XPS analysis that showed values of 56.4%, 54.1%, and 54.7% for NdFeO₃, DyFeO₃, and ErFeO₃, respectively. A higher concentration of oxygen vacancies provides more active sites for gas adsorption, whereas the lowest binding energy facilitates the breaking of Ln-O bonds, allowing the generation of atomic vacancies in the crystal structure. These surface vacancies promote greater adsorption of oxygen species on the material surface, significantly enhancing the interactions with gas molecules and resulting in a high performance of the gas sensor. Cao et al. reported the influence of raw materials such as ethylene glycol (EG) and polyethylene glycol (PEG) and the incorporation of pre-calcination on the ethanol-sensing properties of LaFeO_3−δ nanoparticles [67]. The sensor synthesized using EG as the raw material, combined with a pre-calcination step at 400 °C prior to the final calcination at 600 °C (EG400 + 600) exhibited a significantly enhanced sensing performance at 112 °C due to a higher Fe/La ratio at the surface.

Sheng et al. investigated the potential of NdFeO₃ nano-coral granules (Figure 2c), prepared via a solvothermal method, for detecting VOCs [62]. The sensor exhibited a response value of 150 to 100 ppm of ethanol (Figure 2d) with a response time of 19 s and recovery time of 22 s at 250 °C. The high specific surface area of the nano-coral granules provides more active sites on the material surface and promotes oxygen adsorption. Additionally, the porous structure of the sensor facilitates the easy diffusion of gas molecules through the interior of the material and enhances its sensitivity towards gas sensing. Yang et al. prepared ErFeO₃ nanoparticles through a hydrothermal method followed by a calcination and an acidification procedure [65]. The rough surface of the nanoparticles played an important role in enhancing the sensitivity of the gas sensor [85]. This sensor exhibited optimal performance at 270 °C, demonstrating high sensitivity and repeatability for isopropanol detection, with a rapid response time of 6 s and a recovery time of 25 s at 100 ppm. The sensor also demonstrated remarkable sensitivity to isopropanol, exhibiting a response of 2.31 even at a low concentration of 2 ppm.

Yang et al. reported the fabrication of a LaFeO₃ gas sensor with a hierarchical porous nanostructure synthesized using F108 as the structuring agent, followed by annealing at 700 °C [8]. The fabricated sensors were designated as LFO0-700, LFO0.25-700, LFO0.5-700, LFO0.75-700, and LFO1-700, based on the quantity of F108 used during synthesis, ranging from 0.00 to 1.00 g, and explored this sensor for the trace detection of formaldehyde. The specific surface area of the nanostructures increased with increasing F108 content, reaching the highest value for LFO0.5-700 at 0.5 g of F108. This sensor exhibited the highest response of 178 (Figure 2e) for 100 ppm of formaldehyde, which is 3.5 times higher than LFO without F108 (LFO0-700), and at a relatively low operating temperature of 125 °C compared to 150 °C for other sensors. The sensor demonstrated faster response and recovery times of 4.3 and 17.5 s (Figure 2f), respectively, with reliable stability over 15 weeks. The superior performance of the sensor was attributed to the addition of F108, which improved its structural properties and sensing capabilities, thereby opening a feasible avenue for the fabrication of oxides with low operating temperatures and enhanced sensing performance.

As mentioned previously, the morphology of the sensing material significantly affects its gas-sensing properties. To explore this, Phan et al. conducted a comparative study of the gas-sensing properties of hollow core and porous shell LaFeO₃ (LFO-HS) nanoparticles synthesized through a facile hydrothermal method and bulk LaFeO₃ (LFO) prepared using a solid-state reaction [63]. Morphological studies revealed a unique hollow core structure surrounded by a porous shell for the LFO-HS nanoparticles (Figure 2g), while irregular clumps were observed for bulk LFO. Nitrogen adsorption–desorption studies revealed a higher surface area for LFO-HS (44.7 m²/g) compared to LFO (16.7 m²/g). The precursor solution prepared by dispersing 20 mg of LFO-HS and LFO in 1 mL of dimethylformamide was used to fabricate a six-layer gas sensor by drop coating onto a Pt electrode-patterned SiO₂/Si substrate. The hollow core and porous shell structures were retained in the sensor film, promoting the sensor response to the target gas. The sensor with six-layer LFO-HS demonstrated significantly higher response (Figure 2h) to 143 ppm of ethanol at 300 °C compared to the six-layer LFO sensor, which is attributed to the larger active surface area of LFO-HS, which promotes efficient gas adsorption. The sensor also exhibited excellent selectivity for ethanol over acetone and ammonia. The sensor demonstrated an ability for the trace-level detection of ethanol, and its porous structure resulted in longer response and recovery times.

Zhang et al. reported the gas-sensing properties of LnFeO₃ (Ln = La, Pr, Nd, or Sm) nanoparticles synthesized using the citrate sol–gel method [64]. All these compounds exhibited a p-type semiconducting behavior, with an optimal gas-sensing performance at 210 °C. The sensors demonstrated sensitivity to various volatile sulfur compounds (VSCs), with the highest response to dimethyl disulfide (DMDS) followed by methyl sulfide (DMS), hydrogen sulfide (H₂S), and carbon disulfide (CS₂), which was attributed to differences in adsorption energies. The increased response with respect to DMDS concentration is attributed to an increase in the reaction rate of DMDS with chemisorbed oxygen [86]. The cross-interference effects between the noise factors NO₂, CO, and EtOH on the performance of the LnFeO₃ gas sensors were studied using DMDS as the optimal representative gas. The response values of the LaFeO₃, PrFeO₃, NdFeO₃, and SmFeO₃ sensors varied by ±20%, ±10%, ±10%, and ±5%, respectively, in the presence of interfering gases. However, the overall response trends of the four sensors were unaffected by this interference.

5.1.1. A-Site Doping

Rare earth orthoferrites have attracted considerable attention for diverse applications [87,88] because of their stability, abundant oxygen vacancies, and tunable rare earth and iron sites. Doping RFeO₃ compounds significantly affects their structural, optical, and electrical properties.

Liquified petroleum gas (LPG), which is highly flammable and widely used as a fuel for cooking, heating, and transportation, causes respiratory issues. Most LPG sensors operate at high temperatures, limiting the efficiency and lifespan of the sensing materials and highlighting the need for low-temperature-operating sensors [89]. Bharati et al. investigated the impact of bismuth doping on the room-temperature LPG-sensing properties of PrFeO₃ nanomaterials [68]. The specific surface area of the nanomaterials increased significantly with Bi doping, ranging from 16.331 to 37.645 m²/g, while the band gap decreased from 2.27 to 1.95 eV upon doping. This study highlighted the enhanced sensing performance due to Bi doping, with the Pr_0.8Bi_0.2FeO₃ composition achieving the highest response. This sensor demonstrated rapid response and recovery times of 15.3 and 22.4 s, respectively, for 0.5 vol% LPG because of its mesoporous structure that promotes gas adsorption. The increased sensitivity and reliable performance across multiple cycles were attributed to the higher specific surface area and creation of additional active sites for oxygen adsorption, which facilitated enhanced electron transfer during gas sensing. In another study, Bharati et al. demonstrated that the incorporation of Ca ions into PrFeO₃ nanoparticles significantly improved LPG sensing performance [28]. The Pr_0.8Ca_0.2FeO₃ sensor exhibited the highest response of 1.3 with quicker response and recovery times of 7.5 and 7.1 s (Figure 3a), respectively. The reproducibility, high sensitivity, and fast response and recovery times of the sensor underscore its potential for use in LPG sensing technologies.

Xiao et al. developed a highly sensitive and selective formaldehyde (HCHO) gas sensor based on an In-doped LaFeO₃ porous structure with an ultralow detection limit of 1 ppb [49]. Gas-sensing studies revealed that indium doping significantly enhanced sensor performance, where 1.5% In-doped LaFeO₃ exhibited the highest response of 122 (Figure 3b) for 100 ppm formaldehyde at 125 °C, nearly double the response of pure LaFeO₃. This sensor demonstrated fast response and recovery times of 36 and 40 s, respectively, which were ascribed to the increased surface area and porous morphology introduced by In doping. The incorporation of In ions generated a higher concentration of oxygen vacancies, providing more active sites for oxygen adsorption, thereby achieving an ultralow detection limit. Density functional theory calculations further revealed that the chemisorption of HCHO on the In-doped LaFeO₃ surface was stronger than that on pristine LaFeO₃, enabling stronger interactions and resulting in an improved response.

Using the auto-combustion method, Benali et al. synthesized La_0.8Ba_0.1Bi_0.1FeO₃ nanoparticles and explored their potential for sensing parts per billion quantities of acetone gas [69]. This synthesis method produced crystalline nanoparticles with smaller crystallite sizes than those of LaFeO₃. Positron annihilation lifetime spectroscopy (PALS) revealed a high concentration of vacancy defects in the material. The sensor response to acetone increased with the operating temperature, peaking at 200 °C and then decreasing (Figure 3c), and this trend is attributed to the interplay between the temperature-dependent activation energy of gas molecules and the surface reaction. This sensor exhibited a fast response time of 9 s and a recovery time of 7 s (Figure 3d) to 1000 ppb of acetone because of the increased rate of gas–molecule interactions.

Benali et al. investigated the effect of particle size on the gas-sensing properties of (La_0.8Ca_0.2)_0.6Bi_0.4FeO₃ (LCBFO) compounds synthesized using various methods [56]. The sol–gel (SG) method resulted in LCBFO-SG nanoparticles with a smaller crystallite size (32.469 nm) than LCBFO-SS nanoparticles (65.566 nm) prepared using a solid-state (SS) reaction method. The lower activation energy 0.369 eV of LCBFO-SG compared to 0.427 eV of LCBFO-SS, indicates easier carrier transport in the SG compound. Both sensors demonstrated different optimal operating temperatures for different gases such as 220 °C for ethanol, 200 °C for acetone, and 160 °C for H₂S. The rapid response and recovery times and high sensitivity of the LCBFO-SG sensor were attributed to its smaller particle size. The LCBFO-SG sensor exhibited higher response for H₂S at 160 °C while showing a similar response to ethanol and acetone at 200–220 °C, indicating its potential use in multi-gas detection applications.

Using the auto-combustion method and glycine as fuel, Benali et al. synthesized La_1–2xBa_xBi_xFeO₃ (0 ≤ x ≤ 0.20) nanoparticles and studied their gas-sensing performance [70]. X-ray diffraction (XRD) analysis revealed the single-phase formation for the compounds with x ≤ 0.10, while secondary phases appeared for x = 0.15 and 0.20. Atomic force microscopy studies revealed that the compound with x = 0.10 exhibited the most homogeneous surface roughness, facilitating the generation of more adsorption sites to enhance the sensitivity of the sensor. The La_0.8Ba_0.1Bi_0.1FeO₃ sensor demonstrated the highest response of 21 to 100 ppm of ethanol (Figure 3e) at a significantly lower operating temperature of 180 °C [80,90,91]. The sensor exhibited rapid response and recovery times of approximately 10 and 6 s, respectively, for 50 ppm of ethanol. Furthermore, it showed the highest response to H₂S gas at 200 °C and very short response and recovery times of 7 and 5 s (Figure 3f), respectively, for 50 ppm of H₂S.

Pei et al. employed a hydrothermal method to prepare Sm-doped PrFeO₃ nanoparticles with a coral-like morphology [71]. UV–Vis spectroscopy revealed that Sm doping reduced the band gap and enhanced the conductivity of the sensor. The Sm-PrFeO₃ sensor demonstrated the highest response to acetone at 270 °C, which was 10 °C lower than the operating temperature of the undoped PrFeO₃ sensor (Figure 3g). This sensor exhibited exceptional selectivity for acetone over other gases such as ethanol, methanol, formaldehyde, and benzene, with a response of 44.94 at 50 ppm. The sensor also showed significantly faster response and recovery times of 15 and 16 s, respectively, which are attributed to the cage-like morphology and lower bandgap value achieved via doping. The system exhibited a low concentration detection as low as 5 ppm, stability over five cycles, and retained its response under a 71% relative humidity, making it suitable for acetone detection.

Panpan et al. examined the effect of Ca doping on the CO₂ gas-sensing properties of Yb_1−xCa_xFeO₃ (x = 0, 0.1, 0.2, 0.3) nanoparticles [72]. The sensors were fabricated by mixing Yb_1−xCa_xFeO₃ powders with deionized water to form a paste, which was then packed into an alumina ceramic tube with two Au electrodes on both sides. The Yb_0.8Ca_0.2FeO₃ sensor demonstrated the highest response to 5000 ppm of CO₂, at 260 °C. In another study, Zhang et al. reported a high sensitivity of a Yb_0.8Ca_0.2FeO₃ sensor towards 0.5 ppm of acetone with response and recovery times of 29 and 55 s, respectively [74]. Xiaofeng et al. explored nanocrystalline Gd_1−xCa_xFeO₃ sensors for methanol gas detection [73]. Among the sensors studied, the Gd_0.9Ca_0.1FeO₃ sensor exhibited the best response to methanol compared to other gases, such as ethanol, CO, and formaldehyde (Figure 3h). The sensor demonstrated responses of 117.7, 72.7, and 31.9 to 600 ppm of methanol, ethanol, and CO gases, respectively, at 260 °C.

5.1.2. B-Site Doping

The exceptional thermal and chemical stability of orthoferrites, combined with their adaptability to incorporate various metals at Fe-sites, introduces structural point defects, such as vacancies and variations in oxygen stoichiometry, which influence the electronic structure of the active sites and significantly improve their gas-sensing properties [41].

Duan et al. reported the enhanced gas-sensing performance of Co-doped LaFeO₃ nanoparticles [15]. The incorporation of Co ions in LaCo_0.1Fe_0.9O₃ (LCFO) reduced the agglomeration of nanoparticles compared to undoped LaFeO₃ (LFO), providing more active sites for gas adsorption onto the surface of the sensing material. Co doping significantly increased the proportion of oxygen vacancies in LCFO (39.01%) (Figure 4a) compared to LFO (32.71%), which enhanced gas sensing by facilitating oxygen molecule adsorption and accelerating oxidation–reduction reactions. The LCFO gas sensor demonstrated superior CO₂ sensing capabilities, including a wide detection range, fast response and recovery times (6 and 42 s, respectively), and excellent humidity resistance with minimal changes in the response (Figure 4b). Density functional theory (DFT) calculations further revealed that Co doping shifted the Fe d-band center, increasing the adsorption energy of CO₂ on Fe sites (from −2.75 to −4.21 eV) and introducing strong adsorption at Co sites (−3.83 eV). The enhanced CO₂ sensing mechanism of LCFO compared to that of LFO is attributed to the combination of higher oxygen vacancy concentrations, as confirmed by XPS analysis, and stronger CO₂ adsorption.

Shen et al. synthesized LaFeO₃ nanoparticles with different Mo doping ratios to improve the triethylamine (TEA) gas-sensing performance [50]. XRD confirmed the lattice distortion due to Mo doping, which introduced defects and increased the number of active sites for gas–molecule interactions (Figure 4c). Morphological studies revealed a porous nanosheet structure with a large surface area that promoted efficient gas diffusion. Among the samples, 4% Mo-doped LaFeO₃ demonstrated the best performance (Figure 4d), with a response approximately 11 times higher than that of undoped LaFeO₃ at a relatively low operating temperature of 140 °C. In addition, the sensor exhibited fast response and recovery times of 12 and 59 s, respectively, with excellent selectivity, stability, and humidity resistance. The superior performance of the sensor was ascribed to a higher oxygen vacancy concentration of 4% Mo-doped LaFeO₃ compared to other samples, promoting the adsorption of gas molecules on the surface of the sensor [92], thereby improving the gas-sensing performance.

Derakhshi et al. synthesized Cu-doped LaFe_1−xCu_xO₃ nanoparticles to improve VOC detection [75]. The introduction of Cu significantly improved ethanol sensing performance and reduced crystallite size from 20.5 nm for LaFeO₃ to 12.7 nm for LaFe_0.4Cu_0.6O₃. The higher specific surface area of the LaFe_0.4Cu_0.6O₃ sensor led to excellent VOC detection, with responses of 17, 18, and 16 for 300 ppm of ethanol, acetone, and acetaldehyde, respectively, at 300 °C. This sensor demonstrated a 6.5-fold greater response (Figure 4e) to ethanol than pure LaFeO₃, primarily due to the mixed valences (Fe⁴⁺/Fe³⁺ and Cu²⁺/Cu⁺) and oxygen vacancies (Figure 4f), which generated more active sites for gas adsorption.

Bharati et al. developed a room-temperature carbon dioxide (CO₂) sensor using Co-doped PrFeO₃ nanoparticles with crystallite sizes ranging from 19 to 23 nm [76]. Morphological studies showed greater agglomeration of the nanoparticles at higher Co contents, likely due to the magnetic interactions between the constituent ions. The PrFe_0.9Co_0.1O₃ sensor exhibited good selectivity for 500 ppm of CO₂ at room temperature, with fast response and recovery times of 17.2 and 18.4 s, respectively. Li et al. systematically investigated the effect of Mn doping on the gas-sensing properties of LaFe_1−xMn_xO₃ (x = 0, 0.3, 0.4, 0.5 and 0.6) nanoparticles [78]. A gradual decrease in the lattice parameters and average grain size was evident with an increase in the Mn content, primarily due to the smaller ionic radius of Mn³⁺ compared to that of Fe³⁺. SEM analysis revealed that the nanoparticles with x = 0, 0.3, and 0.4 composition appeared compact with smooth surfaces, while a porous morphology emerged at x = 0.5 (Figure 4g), which disappeared at x = 0.6. The LaFe_0.5Mn_0.5O₃ sensor exhibited the highest response to 50 ppm of ethanol at a reduced operating temperature of 190 °C compared to 210 °C for undoped LaFeO₃. This enhancement is ascribed to the porous morphology and larger specific surface area of the sensor. Surprisingly, the LaFe_0.5Mn_0.5O₃ sensor exhibited n-type semiconductor behavior (Figure 4h), which is opposite to that of p-type LaFeO₃, because of the increased electron concentration resulting from the replacement of Fe³⁺ ions with Mn³⁺ ions, leading to the formation of Mn⁴⁺ ions [93,94]. The sensor exhibited good selectivity towards ethanol with stable performance over a 40-day testing period and a faster recovery time of 5 s compared with 31 s for the LaFeO₃ sensor.

Benali et al. synthesized La_0.8Ca_0.1Pb_0.1Fe_0.95Mg_0.05O₃ (LCPFMO) nanoparticles and examined their gas-sensing properties at different frequencies and temperatures [80]. The LCPFMO sensor demonstrated enhanced ethanol sensitivity with increasing frequency (100 Hz to 1 MHz) and ethanol concentration (250–1000 ppm). This sensor exhibited the highest sensitivity at a frequency of 1 MHz to 1000 ppm ethanol at 225 °C.

5.2. Nanorod/Nanotube/Nanofibers

In the domain of gas sensing, one-dimensional (1D) nanostructures, such as nanorods, nanotubes, and nanofibers, have attracted significant attention over nanoparticles owing to several key advantages. They provide more active sites than nanoparticles to facilitate a greater interaction between the gas molecules and the sensing material, leading to an enhancement in sensitivity [95]. In addition, the unique channel-like structure of nanotubes and nanorods allows for the better diffusion of gas molecules [96]. This structure not only increases the interaction of the target gas molecules with the sensor material but also the detection speed. The high specific surface area of 1D nanostructures further enhances the sensitivity of the sensors [97]. The 1D nanostructure stands out for its excellent electron transport properties, which are crucial for converting the interactions between the gas molecules and the sensor material into detectable electrical signals. The unique structural and electrical properties of 1D nanostructures make them suitable candidates for gas-sensing applications, which is a key area of ongoing research.

Table 2. Summary of orthoferrite-based nanorod/nanotube/nanofiber gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
NdFeO3	Trietheylamine	190	100	18.9	72/35		[6]
GdFeO3	N-Propanol	140	100	58.113	28/28	1 ppm	[27]
YFeO3	Ethanol	350	100	18	19/9		[35]
LaFeO3	Ethanol	160	100	9.4	2/4		[38]
ErFeO3	Ethylene glycol	230	100	15.8	61/39	35 ppb	[39]
SmFeO3	Ethylene glycol	200	100	14.3	47/56		[98]
Sm0.95Ho0.05FeO3	Glycol	200	100	26.12	50/32		[99]
SmFeO3	Ethylene glycol	240	100	18.19	41/47		[100]
LaFeO3	Acetone	120	40	46.4	14/49	135 ppb	[101]
LaFeO3	Oxygen	650	5% O2	8.3	15/18		[102]
LaFeO3	Hydrogen sulfide	150–300	0.5–4	20–90%	90–240/300–500	500 ppb	[103]
PrFeO3	Acetone	180	200	141.3	4/4		[104]
La0.75Ba0.25FeO3	Ethanol	210	500	136.1	42/40		[90]
SmFeO3	Acetone	140	100	9.98	17/16		[105]
Ag-LaFeO3	Formaldehyde	230	5	4.8	2/4	5 ppm	[106]

Table 2. Summary of orthoferrite-based nanorod/nanotube/nanofiber gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
NdFeO3	Trietheylamine	190	100	18.9	72/35		[6]
GdFeO3	N-Propanol	140	100	58.113	28/28	1 ppm	[27]
YFeO3	Ethanol	350	100	18	19/9		[35]
LaFeO3	Ethanol	160	100	9.4	2/4		[38]
ErFeO3	Ethylene glycol	230	100	15.8	61/39	35 ppb	[39]
SmFeO3	Ethylene glycol	200	100	14.3	47/56		[98]
Sm0.95Ho0.05FeO3	Glycol	200	100	26.12	50/32		[99]
SmFeO3	Ethylene glycol	240	100	18.19	41/47		[100]
LaFeO3	Acetone	120	40	46.4	14/49	135 ppb	[101]
LaFeO3	Oxygen	650	5% O2	8.3	15/18		[102]
LaFeO3	Hydrogen sulfide	150–300	0.5–4	20–90%	90–240/300–500	500 ppb	[103]
PrFeO3	Acetone	180	200	141.3	4/4		[104]
La0.75Ba0.25FeO3	Ethanol	210	500	136.1	42/40		[90]
SmFeO3	Acetone	140	100	9.98	17/16		[105]
Ag-LaFeO3	Formaldehyde	230	5	4.8	2/4	5 ppm	[106]

Guo et al. employed electrospinning to synthesize NdFeO₃ nanorods with an average diameter of 275 nm and a specific surface area of 10.298 m²/g [6]. The gas sensor based on the NdFeO₃ nanorods demonstrated a high response value of 18.9 to 100 ppm of triethylamine (TEA) at 190 °C. This sensor exhibited excellent selectivity for TEA over other gases because of the higher adsorption capacity and stronger reducibility of TEA molecules on the surface of the NdFeO₃ nanorods. Liu et al. synthesized one-dimensional YFeO₃ nanorods using electrospinning and evaluated their applications towards ethanol sensing [35]. The YFeO₃ nanorods with a porous structure (Figure 5a) exhibited optimum performance at 350 °C, and this inflection point was attributed to the balance between the molecular activation energy and thermal effects on adsorption. This sensor showed excellent selectivity for ethanol at a low detection limit of 1 ppm with a response value of 2 compared to other gases such as NH₃, H₂O₂, and formaldehyde. This sensor demonstrated fast response and recovery times of 19 and 9 s (Figure 5b), respectively, for 100 ppm of ethanol and maintained a stable response over a 30-day period. It exhibited good resistance to humidity by retaining a response value up to 60% relative humidity. Beyond this humidity level, the water molecular film formed on the surface of the sensor altered its response because of the partial blocking of target gas interactions with the sensing material. These findings suggest that the YFeO₃ nanorod-based sensor is reliable and efficient for ethanol detection under various environmental conditions.

Lin et al. synthesized GdFeO₃ nanostructures with rod and butterfly morphologies using an ecofriendly coprecipitation method [27]. By regulating the alcohol–water ratio of the solution, the amount of polyvinylpyrrolidone (PVP), and the standing time (12 h for butterflies and 36 h for rods), the desired morphologies were achieved. After annealing the precursors at 800 °C for 3 h, structural analysis confirmed the orthorhombic structure for both morphologies. SEM images displayed the rods with a diameter of 109.3 nm and a length of 1.536 μm, while butterflies with a length of 6.208 μm and a thickness of 359.9 nm were obtained. The GdFeO₃ rod-based sensor exhibited a significantly higher response than the butterfly-based sensor for all the tested gases (Figure 5c), which was attributed to the porous structure of the rods, providing more active sites to interact with gases. The rod-based sensor demonstrated better selectivity for n-propanol, with a response of more than twice that for the other gases. This sensor demonstrated the highest response value of 58.113 at 140 °C with response and recovery times of 28 s each due to its rough surface. Furthermore, the stability and repeatability over a period of seven days highlight the potential use of GdFeO₃ sensors for n-propanol detection. Wei et al. successfully fabricated an ethylene glycol sensor based on ErFeO₃ nanofibers using uniaxial electrospinning [39]. The ErFeO₃ nanofiber sensor demonstrated the highest response of 15.8 to 100 ppm ethylene glycol at 230 °C. The sensor demonstrated outstanding selectivity for ethylene glycol with a low detection limit of 35 ppm over other gases and excellent stability over 14 days.

Mun et al. investigated the influence of morphology on the gas-sensing properties of an LaFeO₃ sensor [102]. They employed the sol–gel method to prepare nanopowders (LaFeO-P), and electrospinning was used to produce nanofibers (LaFeO-F). A mixed structure (LaFeO-M) combining nanopowders and nanofibers in a 1:1 ratio was also prepared. Gas sensors were fabricated on alumina substrates with prepatterned plating electrodes using screen printing. SEM images revealed that nanopowders (P) had irregular shapes with a rough surface, while nanofibers (F) exhibited a uniform structure with lengths of 2–3 μm. By contrast, the mixed structure (M) exhibited a morphology with nanorods bridging the narrow contact points between the interlinked nanofibers. The LaFeO-M sensor demonstrated the highest response of 13.1 to oxygen with 5% partial pressure, which was 2.64 times that of LaFeO-P at an optimal temperature of 650 °C, likely due to the higher concentration of oxygen vacancies, as confirmed by XPS analysis. This sensor exhibited fast response and recovery times of 10 and 19 s, respectively, compared to those of LaFeO-P (16 and 23 s, respectively) and LaFeO-P (15 and 18 s, respectively). The superior performance of the LaFeO-M sensor is attributed to the formation of ionosorbed oxygen species (Figure 5d) at elevated temperatures as well as the mixed morphology, which provides a balance between the dense microstructure of the nanopowders and the narrow contact between the interlinked nanofibers.

Han et al. reported the superior gas-sensing properties of rough SmFeO₃ nanofibers, which exhibited a high response value of 18.19 to 100 ppm of ethylene glycol at 240 °C [100]. This sensor demonstrated a low detection limit of 5 ppm and reliable stability over a test period of 35 days, with a minimal decrease in response to moisture. The gas-sensing mechanism of the sensor was explained based on the hole accumulation layer (HAL). In air, the thickness of the HAL increases when oxygen molecules are absorbed onto the sensor surface, whereas the thickness of the HAL decreases when the sensor interacts with ethylene glycol because of a reduction in the hole concentration. This change in resistance enhanced the detection of ethylene glycol. In a subsequent study, Han et al. employed a doping strategy to improve the gas-sensing performance of SmFeO₃ nanofibers [99]. They fabricated a Ho-doped SmFeO₃ nanofiber-based sensor and explored the effects of Ho doping on its structural, morphological, and gas-sensing properties. The SEM results showed the highly porous nature of the SmFe_0.95Ho_0.05FeO₃ nanofibers (Figure 5e) with interconnected nanoparticles. The SmFe_0.95Ho_0.05FeO₃ nanofiber sensor exhibited a significantly improved response of 26.12 to 100 ppm of glycol at 200 °C (Figure 5f). It also exhibited an outstanding response of 10.20 to 5 ppm of glycol, surpassing the performance of other glycol sensors reported in the literature [100]. The enhanced gas-sensing properties of the Ho-doped SmFeO₃ nanofiber-based sensor were attributed to the reduced crystallite size, which provided a larger surface area for gas adsorption and increased porosity, which facilitated faster gas diffusion into the sensing layer.

Zhang et al. fabricated LaFeO₃ nanotubes using electrospinning for rapid ethanol detection [38]. The synthesis process involved the application of a high voltage to a solution containing lanthanum nitrate hexahydrate, ferric nitrate nonahydrate, citric acid, and polyvinylpyrrolidone (PVP) to deposit the nanofibers onto a collector. The resulting LaFeO₃/PVP composite nanofibers were heated at 600 °C to obtain LaFeO₃ nanotubes. SEM and TEM revealed the hollow structure of the nanotubes, with an outer diameter of 50 nm and an uneven inner diameter ranging from 15 to 30 nm. The LaFeO₃ nanotube gas sensor demonstrated a high response of 9.4 to 100 ppm of ethanol at 160 °C, with a fast response and recovery times of 2 and 4 s, respectively. The stable response of the sensor over a month indicated the potential for using LaFeO₃ nanotubes in the development of high-performance ethanol gas sensors. Xiang et al. explored the effect of Ba-doping on the gas-sensing properties of LaFeO₃ nanofibers [90] (Figure 5g). All doped nanofiber sensors demonstrated superior gas-sensing performance compared to their undoped counterparts. Among the sensors, the La_0.75Ba_0.25FeO₃ sensor exhibited the highest response of 136.1 to 500 ppm of ethanol gas at 210 °C (Figure 5h). The enhancement in the gas-sensing performance is attributed to the change in the electronic structure and defect chemistry of the LaFeO₃ lattice owing to the substitution of Ba²⁺ for La³⁺.

5.3. Thin Film/Nanosheet

In recent studies, altering the concentration of defect vacancies has been shown to be an effective method to enhance gas-sensing properties [107,108,109]. Various techniques, such as heat treatment, cation and anion doping, solution treatment, and interfacial effects [110] have been employed to modulate oxygen vacancy defects.

Table 3. Summary of orthoferrite-based nanosheet/thin film gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
LaFe0.99P0.01O3−δ	Acetone	180	100	30		50 ppb	[40]
La0.8Ca0.1Pb0.1FeO3	Ethanol	250	5	2.04	200/580		[111]
LaFeO3	Formaldehyde	120	1	1.35		50 ppb	[112]
LaFeO3	SO2	120	3	7.6%	15/14		[55]
LaFe0.8Co0.2O3	CO	225	500	86%	100/500		[57]
La0.8Pb0.1Ca0.1Fe0.8Co0.2O3	Ozone	170	0.4	3.9	2.1/-		[113]
PrFeO3	CO2	180	5000	3.846	441/98		[87]
LaFeO3	Ethanol	128	200	55	13/11		[114]
LaFeO3	Acetone	260	0.5	2.068	62/107		[115]

Table 3. Summary of orthoferrite-based nanosheet/thin film gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
LaFe0.99P0.01O3−δ	Acetone	180	100	30		50 ppb	[40]
La0.8Ca0.1Pb0.1FeO3	Ethanol	250	5	2.04	200/580		[111]
LaFeO3	Formaldehyde	120	1	1.35		50 ppb	[112]
LaFeO3	SO2	120	3	7.6%	15/14		[55]
LaFe0.8Co0.2O3	CO	225	500	86%	100/500		[57]
La0.8Pb0.1Ca0.1Fe0.8Co0.2O3	Ozone	170	0.4	3.9	2.1/-		[113]
PrFeO3	CO2	180	5000	3.846	441/98		[87]
LaFeO3	Ethanol	128	200	55	13/11		[114]
LaFeO3	Acetone	260	0.5	2.068	62/107		[115]

Qin et al. synthesized phosphorous-doped LaFe_1−xP_xO₃ (x = 0.005, 0.01, 0.03, 0.05) (LPFO) nanosheets using a sol–gel method [40]. SEM analysis revealed a porous structure with loosely stacked nanoparticles for pristine LaFeO₃, whereas an improved structural arrangement of the nanoparticles was observed when LaFeO₃ doping was used. At a specific doping concentration of P (x = 0.01), a well-defined stacked, porous, nanometer-thin layer was observed (Figure 6a), which was disrupted by a further increase in the P content. This indicates the influence of optimized doping levels on the morphology and functional properties of the material. In addition, TEM imaging revealed regular circular pores with diameters of approximately 15 nm, confirming the mesoporous structure of the compound with x = 0.01. Among all the sensors, the LPFO sensor with x = 0.01 demonstrated the highest response and lowest operating temperature of 180 °C (Figure 6b). The superior performance of the x = 0.01 sensor is ascribed to the increase in oxygen vacancies due to P-doping, along with the mesoporous structure and larger specific surface area of the nanoparticles compared with the undoped LaFeO₃ sensor.

Smiy et al. fabricated a La_0.8Ca_0.1Pb_0.1FeO₃ thin-film sensor using a drop-coating method to investigate the effect of double doping on gas-sensing properties [111]. At 250 °C, the sensor exhibited the highest response to ethanol, which was attributed to the creation of favorable defect sites due to doping. Aranthady et al. synthesized Ca-doped LaFeO₃ chemiresistive gas sensors in the form of pellets and thin films using the sol–gel method and DC magnetron sputtering, respectively, to detect low-concentration SO₂ gases [55]. XRD studies confirmed similar structures for both the pellets and thin films. However, a slight shift in peak positions towards higher angles and an increase in peak intensity in the case of thin films compared to the pellets (Figure 6c) were observed owing to residual stress as well as the preferred orientation of the crystallites. SEM imaging revealed the agglomeration of nanoparticles with irregular morphologies, providing more active sites for surface reactions. Among all the sensors, La_0.6Ca_0.4FeO₃ pellet sensor demonstrated excellent sensing performance towards 5 ppm of SO₂ at 160 °C. Surprisingly, at a lower temperature of 120 °C, the La_0.6Ca_0.4FeO₃ thin film sensor exhibited significantly enhanced sensitivity to 3 ppm of SO₂ (Figure 6d) compared to the pellet sensor. This enhancement was ascribed to the larger surface area and stable microstructure of the thin film, which facilitated easier charge movement across the surface.

In another study, Bhowmick et al. explored the impact of thickness on CO sensing properties of LaFe_0.8Co_0.2O₃ thin films synthesized using a modified sol–gel method [57]. XRD analysis confirmed the structural stability of the thin films with varying thickness in the range of 92–432 nm. SEM images further provided insights into morphology and revealed the arrangement of spherical nanoparticles with porous microstructure. Notably, the degree of porosity increased with film thickness up to 243 nm, beyond which the strain effects restricted the development of porosity, impacting the gas diffusion and sensing performance. Among all the thin films, 243 nm thin film demonstrated the highest response to CO, peaking approximately 87% at 225 °C (Figure 6e), which is attributed to the balance between gas diffusion and chemisorption processes. The thinner films lacked sufficient adsorption sites for chemisorption process, while excessively thick films limited gas diffusion due to increased resistance. The superior sensing performance of 243 nm thin film (Figure 6f) is attributed to the highly porous structure and low activation energy for adsorption, which improved the gas–solid interactions.

Chen et al. reported the gas-sensing properties of PrFeO₃ and NdFeO₃ thick-film sensors fabricated through the sol–gel route [87]. The maximum response of the PrFeO₃ sensor was 8.44 to 1000 ppm of CO₂ under relative humidity of 58% at 160 °C, while the NdFeO₃ sensor exhibited the highest response of 2.36 to 3000 ppm of CO₂ with relative humidity of 72% at 200 °C. Nieto et al. investigated the effect of Ca doping on the gas-sensing properties of Sm_1−xCa_xFe_0.7Co_0.3O₃ thin films synthesized using a modified sol–gel method [116]. A remarkable improvement in the gas-sensing performance was achieved through the incorporation of Ca²⁺ ions into the films. The thin films with calcium content x ≥ 0.2, demonstrated the highest response to 200 ppm of CO at 300 °C and for 500 ppm C₃H₈, at similar temperature conditions.

Cao et al. fabricated LaFeO₃ thick film sensors by coating LaFeO₃ nanoparticles on the ceramic tube, which resulted in the sensors with thickness ranging from 40 to 234 μm [114]. A reduction in the electrical resistance was evident with the increase in the number of layers, suggesting that an enhanced cross-sectional area is available for the easy flow of the charge carriers (Figure 6g). However, the reduction in the resistance is more pronounced in the case of two- and three-layer films with a lower activation energy E_a of 0.47 eV and 0.43 eV, respectively (inset of Figure 6g). This smaller activation energy further contributes to improve the conductivity of the charge carriers. The optimal operating temperature for all the sensors is 128 °C to 200 ppm of ethanol, while the five-layer film demonstrated the highest response (Figure 6h), striking the balance between surface reaction efficiency and gas diffusion. The three-layer sensor exhibited fast response and recovery times, which is ascribed to lower E_a facilitating quicker interaction between ethanol molecules and adsorbed oxygen species.

5.4. Nanocomposites

Meng et al. demonstrated the superior gas-sensing performance of LaFeO₃-In₂O₃ nanocomposites synthesized using the hydrothermal method [117]. Individual nanoparticles of LaFeO₃ and In₂O₃ were prepared separately, and LaFeO₃-In₂O₃ composites were produced by mixing them with desired ratios. Gas-sensing tests demonstrated an excellent sensitivity of 1% LaFeO₃-In₂O₃ composite with a remarkable response value of 425 to 100 ppm of 2-butanone at an operating temperature of 210 °C, nearly three times higher than that of pure In₂O₃ (Figure 7a). This sensor exhibited an impressive detection limit of 10 ppb for 2-butanone, with long-term sustainability and reduced response–recovery times compared to the In₂O₃ sensor. This enhanced gas-sensing performance is ascribed to the p-n heterojunction created by the combination of n-type In₂O₃ and p-type LaFeO₃, facilitated charge transfer, and increased oxygen adsorption, improving the response to 2-butanone. In addition to that, the unique rod-like morphology provides a large surface area and more active sites for gas adsorption. The inherent catalytic nature of LaFeO₃ promoted the redox reaction between 2-butanone and adsorbed oxygen, further contributing to enhanced sensing performance.

Wang et al. employed the electrospinning technique to synthesize one-dimensional LaFeO₃ nanoribbon-based composite materials decorated with different mass fractions of phosphotungstic acid (PW₁₂) and investigated their gas-sensing properties [118]. Among the fabricated sensors, the LaFeO₃/3%PW₁₂ sensor (Figure 7b) demonstrated the highest response of 3.35 (Figure 7c) to 5 ppm of acetone at 250 °C, which is nearly 2.6 times the response value and 90 °C lower than the operating temperature of pure LaFeO₃ nanoribbons. In the composite, PW₁₂ acted as an electron acceptor and facilitated the efficient charge carrier migration between the two components, thereby enhancing the sensor’s sensitivity and selectivity towards acetone gas. The sensor exhibited maximum resistance to relative humidity in the range of 33–94% and maintained its stability by retaining the response for 30 days with minimal fluctuations.

Xia et al. reported the development of a novel method to prepare SnO₂ quantum dot (QD)-sensitized LaFeO₃ composites for detecting conductometric formic acid (HCOOH) gas [119]. Initially, they synthesized SnO₂ QDs and dispersed them in LaFeO₃ sol–gel precursors, followed by a calcination, which resulted in the composite with porous morphology. The smaller size of SnO₂ QDs improved the porosity of LaFeO₃, promoting effective gas diffusion, leading to an excellent selectivity and sensitivity towards HCOOH at a lower operating temperature. The sensor fabricated using the composite with 2.5 wt% SnO₂ QDs (LSO-2) demonstrated a high response value of 31.5 to 100 ppm of HCOOH at 210 °C, and a low detection limit of 1 ppm. This sensor exhibited greater stability by retaining consistent performance for 6 days, and fast response and recovery times of 16.8 and 6.6 s (Figure 7d), respectively, due to its porous structure. The difference in the work function and band gap width created a built-in electric field at the SnO₂ QDs/LaFeO₃ interface, which enhanced the gas-sensing performance. The region of SnO₂ QDs were electron depleted with an abundance of captured electrons, due to their smaller size. During the gas-sensing process, these electrons were released when the sensor was exposed to target gas, and some of them were expected to combine with the free holes of LaFeO₃, further enhancing the sensor’s resistance and response.

Table 4. Summary of orthoferrite-based nanocomposite gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
LaFeO3/3%PW12	Acetone	250	5	3.35		52.64 ppb	[118]
1% LaFeO3/In2O3	2-Butanone	210	100	425	103/21	10 ppb	[117]
LaFeO3/2.5wt% SnO2 QD	Formic acid	210	100	31.5	16.8/6.6	1 ppm	[119]
p-SmFeO3/p-YFeO3	Ethanol	120	30	163.593	37/129	380 ppb	[123]
LaFeO3/C	Formaldehyde	125	50	74.3		500 ppb	[122]
Au/LaFeO3	Ethanol	200	100	44	24/8		[124]
Ag/LaFeO3	Ethanol	190	100	155	30/5		[125]
SmFeO3/ZnO	Acetone	350	10	45	10/250	100 ppb	[126]
α-Fe2O3/LaFeO3	Acetone	350	100	48.3	16.5/2	166.2 ppm	[127]
Ag/LaFeO3 NCQD	Methanol	92	5	73	50.9/54.3		[121]
α-Fe2O3/LaFeO3	Acetone	230	100	20	3/5	1 ppm	[120]
Co-Fe2O3/SmFeO3	Methanol	155	5	19.7	47/19	40 ppb	[128]
3wt% Pd/SmFeO3	Acetone	240	500 ppb	7.21	8/15		[129]
2wt% Pd/LaFeO3	Acetone	200	1	1.9	4/2		[130]

Table 4. Summary of orthoferrite-based nanocomposite gas sensors reported in the literature.

Material	Detection Gas	O.T. (°C)	Conc. (ppm)	Response	tres/trec (s)	LOD	Reference
LaFeO3/3%PW12	Acetone	250	5	3.35		52.64 ppb	[118]
1% LaFeO3/In2O3	2-Butanone	210	100	425	103/21	10 ppb	[117]
LaFeO3/2.5wt% SnO2 QD	Formic acid	210	100	31.5	16.8/6.6	1 ppm	[119]
p-SmFeO3/p-YFeO3	Ethanol	120	30	163.593	37/129	380 ppb	[123]
LaFeO3/C	Formaldehyde	125	50	74.3		500 ppb	[122]
Au/LaFeO3	Ethanol	200	100	44	24/8		[124]
Ag/LaFeO3	Ethanol	190	100	155	30/5		[125]
SmFeO3/ZnO	Acetone	350	10	45	10/250	100 ppb	[126]
α-Fe2O3/LaFeO3	Acetone	350	100	48.3	16.5/2	166.2 ppm	[127]
Ag/LaFeO3 NCQD	Methanol	92	5	73	50.9/54.3		[121]
α-Fe2O3/LaFeO3	Acetone	230	100	20	3/5	1 ppm	[120]
Co-Fe2O3/SmFeO3	Methanol	155	5	19.7	47/19	40 ppb	[128]
3wt% Pd/SmFeO3	Acetone	240	500 ppb	7.21	8/15		[129]
2wt% Pd/LaFeO3	Acetone	200	1	1.9	4/2		[130]

Zhang et al. explored the advantage of metal–organic framework (MOF)-templated LaFeO₃/α-Fe₂O₃ porous nano-octahedrons for detecting acetone gas [120]. They used MIL-53(Fe) as a template to synthesize nano-octahedrons through a self-sacrificial template method. This method starts with the formation of MIL-53(Fe) nano-octahedrons, followed by the growth of La-Fe hydroxides on the MIL-53(Fe) surface. Subsequent pyrolysis transformed the as-prepared precursor into LaFeO₃/α-Fe₂O₃ porous nano-octahedrons. The porous morphology is confirmed by FESEM (Figure 7e) and TEM analysis, which provides a large surface area for gas adsorption and the efficient diffusion of gas molecules. Gas-sensing studies revealed that the sensor based on LaFeO₃ decorated with α-Fe₂O₃ nano-octahedrons exhibited a significantly higher sensitivity to acetone compared to pure α-Fe₂O₃ sensors. This sensor’s response is three times higher than that of pure α-Fe₂O₃ for 100 ppm of acetone at 230 °C (Figure 7f), which is attributed to the porous nanostructure and the formation of p-n heterojunction between LaFeO₃ and α-Fe₂O₃. The sensor demonstrated excellent selectivity towards acetone over other tested gases due to the formation of hydrogen bonds between the surface hydroxyl groups of the sensing material and the carbonyl group of acetone, resulting in an enhanced acetone adsorption capacity. This sensor displayed long-term stability and superior reversibility along with rapid response and recovery times of 1 and 3 s, respectively.

Rong et al. developed a highly sensitive and selective methanol gas sensor based on a nitrogen-doped carbon quantum dot(NCQD)/Ag-LaFeO₃ (NALPN) heterojunction prepared via a microwave-assisted synthesis route [121]. They fabricated sensors with different weight ratios of NCQDs to Ag-LaFeO₃ and tested them for gas-sensing applications. Among the sensors, the 1% NALPN sensor demonstrated excellent sensitivity with a response of 73 to 5 ppm of methanol at 92 °C (Figure 7g). This enhanced sensitivity is ascribed to the formation of p-n heterojunction between p-type Ag-LaFeO₃ and n-type NCQDs, which facilitates the adsorption of oxygen molecules and improves the charge separation, resulting in a larger change in the resistance upon exposure to methanol. The presence of carboxyl functional groups (-COOH) on the surface of NCQDs along with the cavities on the surface of Ag-LaFeO₃ further enhanced the selectivity towards methanol compared to other tested gases. The sensor exhibited long-term stability over 30 days and fast response and recovery times of 19 and 54.3 s, respectively.

Ma et al. reported the synthesis of porous LaFeO₃/C nanocomposites for the first time using a facile foaming sol–gel technique and explored their applicability in sensing formaldehyde [122]. They controlled the amount of carbon in the composite with the desired microstructure by adjusting the content of polyvinylpyrrolidone (PVP) during the synthesis process. Morphology studies displayed the loose porous structure of the nanocomposite annealed in the N₂ atmosphere, compared to pristine LaFeO₃ annealed in air. The porous structure provides more active sites on the surface of the sensing material to interact with formaldehyde gas molecules. This eventually led to enhancing the sensing ability of the sensor and reducing the response times. This was evident from the fast response and recovery times (11/19 s) demonstrated by LaFeO₃ with a 10% PVP sensor (LC10-650N) (Figure 7h). The sensitivity of this sensor is three times higher than that of pristine LaFeO₃, with a response of 74.3 to 50 ppm of formaldehyde at 125 °C, which was ascribed to the increased conductivity resulting from the strong interaction between LaFeO₃ and carbon. The sensor displayed excellent selectivity to formaldehyde over other tested gases, likely due to catalytic activity of the sensing material to aldehyde group.

This comprehensive review of RFeO₃-based gas sensors identified several materials and composites as potential candidates for real-world applications. Among the materials reported in Table 1, Table 2, Table 3 and Table 4, NdFeO₃ nanoparticles stand out for their high sensitivity to n-butanol, highlighting their potential for VOC detection. Doped PrFeO₃ compounds, such as Pr_0.8Ca_0.2FeO₃ and PrFe_0.9Co_0.1O₃, have demonstrated excellent sensing capabilities at room temperature for LPG and CO₂ detection, respectively. Furthermore, orthoferrites with smaller amounts of Pd doping (e.g., 3 wt% Pd/SmFeO₃, 2 wt% Pd/LaFeO₃) have shown excellent selectivity and rapid response–recovery times at low concentrations (sub-ppm levels) and operating temperatures below 250 °C, making them highly attractive for integration into miniaturized or wearable applications. Additionally, LaFeO₃-based nanocomposites incorporating materials such as SnO₂ QDs, ZnO, or α-Fe₂O₃ significantly exhibited improved sensing characteristics due to better charge transport and gas diffusion. Moreover, one-dimensional nanostructures such as nanotubes and nanofibers, particularly GdFeO₃ and SmFeO₃ nanostructures, offer improved response times due to their high surface-to-volume ratios and interconnected porosity, which promote faster gas adsorption/desorption kinetics.

6. Conclusions and Future Perspectives

This article provides a comprehensive review on the latest advancements of rare earth orthoferrite-based gas sensors with a critical emphasis on their nanostructure, doping effects and heterostructures. Different synthesis methods were employed to modify the microstructure of orthoferrites such as the specific surface area and porosity in order to improve the gas–solid interactions. Doping at the R-site and Fe-site of orthoferrites suggested the alteration of the crystal structure, defect concentration, and electronic properties. This led to enhancing the specific surface area of the sensing material and providing activation energy to improve gas-sensing performance, including selectivity, sensitivity, lower operating temperatures and fast response/recovery times. Moreover, the formation of heterojunction with other sensing materials improved the surface reactivity and modulated the gas-sensing capabilities of orthoferrite-based gas sensors.

The advancement of nanotechnology has significantly enhanced the gas-sensing capabilities of orthoferrites and established them as a promising candidate for next generation sensing technologies. The research related to the development of orthoferrite-based gas sensors suggested that the sensitivity is no longer a constraint for their practical applications. However, there are prominent hurdles in achieving high selectivity, room temperature operation, long-term stability, and better resistance to humidity. The higher operating temperatures not only consume excessive power but also create safety issues. To address these challenges, future research should be focused on probing the underlying gas-sensing mechanism of orthoferrite compounds. With the help of advanced characterization techniques along with computational modeling, it would be possible to obtain a deep understanding of the role of crystal structure, oxygen vacancies on the interactions between the target gas, and the surface of the sensing materials. This will help the research community to select suitable dopants to develop sensing materials with enhanced sensing capabilities. Though various dopants were explored, the undiscovered dopants with different combinations could yield improved gas-sensing properties of orthoferrites. Additionally, exploring the combination of orthoferrites with other sensing materials such as metal oxides, noble metals, and polymers can further improve the gas-sensing performance. By controlling the porosity and exploring unique nanostructure morphologies with an increased surface area, the sensitivity of orthoferrite gas sensors could be further improved. One of the key challenges of orthoferrite gas sensors that need to be addressed is their long-term stability under harsh environmental conditions. To overcome these limitations, future research should be focused on designing novel sensing materials with a high surface area, controlled morphology, and engineered defects to improve the reactions between the target gas and the sensing material. Furthermore, the development of gas sensors using advanced fabrication techniques and integrating the flexible substrates and wireless communication systems will play a crucial role in the next generation of smart gas sensors. Finally, the design and fabrication of gas sensors suitable for miniaturization and able to support low-power operation at room temperature, with better selectivity and sensitivity, will propel the field of gas sensors to reach new heights of advancement and practical applications.