1 Introduction

Tailor-made ternary oxide bearing gas-sensing functionalities constitute a rich material playground for realization of enhanced gas sensing in resistive-type gas sensors. Among them, perovskite oxides remain prominent, and exhibit a wide range of physico-chemical properties, and tunable electronic conductivity compared to binary metal oxides such as SnO2, ZnO, CuO, Co3O4, NiO, TiO2, CeO2, and WO3. Perovskite oxides with the stoichiometry ABO3 have a cubic structure (Fig. 1a, b), where the larger cation A at each corner of a unit cell and the smaller cation B at the body center sites are 12- and 6-fold coordinated with oxygen anions at the face center sites, respectively [1,2,3]. The sizes of A and B are determined by the tolerance factor, whose value equals unity for perfectly matched A–O and B–O-bond lengths [4]. Typically, the A-site cation at the dodecahedral site donates electrons to the [BO6] octahedra, whereas the B-site cation alters the electronic structure, influencing the physical properties of the perovskite oxide. Owing to the presence of oxygen vacancies, the stoichiometry of oxygen in ABO3 perovskites deviates from 3, forming ABO3−δ perovskite oxides with oxygen nonstoichiometry (δ). In vacancy-rich regions, hoping of oxygen-ion vacancies can mediate the diffusion of oxygen on the [BO6] octahedra, with energy barrier as low as 0.12 eV (Fig. 1c) [5, 6]. These oxygen vacancies act as centers of positive charges, and correlate linearly with hole density, whose increase results in high conductivity [7].

Fig. 1
figure 1

Schematic of perovskite ABO3 structure showing a corner-sharing (BO6) octahedra with A ions located in 12-coordinated interstices, and b B-site cation at the center of the cell (Reproduced from Ref. [3]. with permission from The Royal Society of Chemistry). c Oxygen diffusion pathway (green arrows) in lanthanum cobaltite [LaCoO3−δ] (bottom) with the corresponding energy barriers (top). Open pink circles stand for O vacancies, whereas big blue and small red spheres stand for Co and oxygen atoms, respectively. Oxygen begins with hoping to a neighboring vacancy through space between Co atoms [1, 2] to form oxygen pair [3]. The O–O pair is unstable in absence of vacancies, forcing one atom to advance to the next vacancy, while the other jumps to initial position [4] (Ref. [5])—reprinted with permission from Elsevier

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ABO3 perovskites have been considered as promising resistive-type gas sensors because of (1) stability of abundant metallic elements in the ABO3 perovskite structure [2], (2) tunability of structure and chemical composition upon partial substitution of A and/or B positions with aliovalent elements of various sizes and valences [8], (3) adaptability of structural integrity under off-stoichiometry [9], and (4) remarkable electronic structure and electron mobility [10]. As interstitial oxygen in a perovskite structure is thermodynamically unfavored, oxygen-deficient vacancies are dominant and typically quite common compared to cationic vacancies. This is possibly due to the fact that anionic vacancies require small energy than cationic counterparts [11]. The ability to host cationic and oxygen vacancies renders ABO3 perovskites with variable electrical and redox properties, synergistic catalytic functions, which bring about critical impact on gas-sensing property. In addition, their mechanical and thermal stability make them suitable gas-sensing materials in a wide range of temperatures. However, there are several issues to address. These include (1) metastability and likelihood to decompose into mixed oxide phases in low/high-temperature regimes [11,12,13], (2) proneness to moisture uptake by oxygen vacancies and consequent mobility of hydroxyl groups that pose cross sensitivity to moisture [14,15,16], and (3) massive variation of oxygen vacancy concentration with temperature and oxygen partial pressure, limiting oxygen adsorption [17]. In principle, improvement of perovskite oxides to suit desired performance involves tailoring their functional characteristics through strain induction [9, 18], inductive effects [19], doping [20, 21], and tunability of chemical composition [8].

Herein, we present an overview of recent development in the use of ABO3 perovskites as resistive-type gas sensors, clearly elucidating current experimental methods, and strategies employed to realize sensing performance enhancements. Finally, we include a summary of sensing performance and an overview of obstacles that hinder their utilization as gas sensors, and suggest new pathways for novel applications.

2 ABO3 perovskite oxides

In general, tunability of structural composition of ABO3 perovskites depends on oxidation states of A and B cations [22]. These oxides can be p-type or n-type semiconducting oxides [23, 24]. Typically, occupancy of A and B sites with trivalent cations of rare-earth metals makes ABO3 perovskites p-type conductors, with electron holes and cationic vacancies constituting the majority of nonstoichiometric defects. Accordingly, the number of oxygen vacancies in p-type ABO3 perovskites is substantially low, as they are not the native nonstoichiometry defects [25]. Early investigations on ABO3 perovskites show that they exhibit remarkable conductivity, redox properties, and reversibility in air/gas ambient [26]. To date, several investigations have emerged, further elucidating their interaction mechanisms with air/gas [27], synthesis and morphology engineering techniques [28], temperature dependency and sensing stability [28, 29], humidity effects [30], and response/recovery kinetics [31]. Particularly, some p-type ABO3 perovskites such as LaFeO3 have been the focus of investigations [7, 32,33,34,35,36]. Their high-temperature stability, nonstoichiometric composition as well as ionic and electronic conductivity make them important material for gas-sensing applications. One of the interesting works is that of Queralto et al. [29], who demonstrated a unique interaction of lanthanum ferrite (LaFeO3) with sulfur containing gases (SO2 and H2S). In their study, they used electrospun LaFeO3 nanofibers (NFs) as sensing materials (Fig. 2a, b). As illustrated in Fig. 2c, SO2 strongly binds on LaFeO3, leading to the formation of La2(SO4)3, as lanthanum cations are Lewis acids and capable of adopting higher coordination numbers than iron (Fe). The results of their study showed that optimal sensing toward 1 ppm of SO2 could be obtained at 250 °C, indicating a sensitivity [(Rg–Ra)/Ra × 100%] as high as 90% (Fig. 2d). Despite this performance, SO2 reacted with H2O to yield sulfurous acid or bisulfite ion instead, leading to slow response/recovery of LaFeO3. This indicates that humidity can have a serious effect on gas-sensing performance of LaFeO3. In addition, the gas responses of LaFeO3 to NH3 and H2S remained high, indicating that the presence of these gases can interfere SO2 sensing (Fig. 2e). Typically, incorporation of physical/chemical filters on sensing layers [37], or catalysts such as CeO2 [38, 39], can reduce deleterious humidity effects and interfering gases. Recently, Ma et al. introduced a humidity-independent NH3 sensor based on lead titanate (PbTiO3) nanoplates [40]. The nanoplates, named P25–PbTiO3 (Fig. 3a), were derived from lead (Pb) precursors and commercial P25 TiO2 powders via hydrothermal route. Compared to PbTiO3 synthesized from mixtures of pure anatase and rutile powders, P25–PbTiO3 nanoplates showed enhanced sensitivity (Rg/Ra = 80.4 at 5 ppm NH3), and improved sensor response/recovery speed (Fig. 3b), with little response variation in humid ambient (20–80% RH) (Fig. 3c). Ideally, as confirmed by FTIR spectra in (Fig. 3d), humidity (H2O) can interact with pre-adsorbed O− forming pronounced OH− on PbTiO3 along with a release of electrons and a decrease in resistance. Diffuse reflectance infrared Fourier transform (DRIFT) analysis indicated that the intensities of OH peaks remained more or less the same, confirming that the formation of OH− groups completed at a relatively low humidity, resulting in a negligible variation of response with further increase in humidity.

Fig. 2
figure 2

SEM micrographs showing a as-spun LaFeOx/PVP nanofibers and b LaFeO3 nanofibers after calcination at 600 °C for 5 h with a ramping rate of 1 °C min−1 in the air. c Reaction mechanism of SO2 to sulfate species with LaFeO3 nanofibers and the desorbed SO3, d variation of sensitivity [(Rg–Ra)/Ra × 100%] of LaFeO3 with concentration of SO2. The inset shows the dependence of sensitivity on operating temperature upon exposure to 1 ppm of SO2. e Sensitivity of LaFeO3 for 1 ppm of SO2, 4 ppm of H2S, 1 ppm of NO2, 10 ppm of NH3, and 10 ppm of CO measured at 250 °C (Reprinted with permission from Ref. [29). Copyright (2019) American Chemical Society]

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Fig. 3
figure 3

a SEM images of P25-PbTiO3 NPs. The inset is the SEM image of the starting P25 TiO2 powders. b Response–recovery curves of sensors using rutile-PT, anatase-PT, P25-PT, and P25-PT powders to 5 ppm C2H5OH at 300 °C, c response–recovery curves of the P25-PT sensor to 5 ppm of ethanol in air at 300 °C and different humidity levels. d DRIFT spectra of P25-PT in different humidity conditions (dry air, 40 and 80% RH) at 300 °C (Reprinted with permission from Ref. [40]. Copyright (2019) American Chemical Society)

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Several techniques to tailor the functional characteristics of perovskite oxides such as strain induction [9] and doping [20] have been introduced. Particularly, doping or substitution on the A-site of ABO3 perovskites with aliovalent ions forms ionic and electronic defects, along with charge compensation. Doped perovskites exhibit higher oxygen vacancies [25], enhanced electrical conductivity [41], low-temperature sensing [42], induced catalytic property [43], and smaller crystal sizes [20], making them highly suitable for gas-sensing applications. For this reason, many doped ABO3 perovskites have attracted considerable attention for gas-sensing applications [20, 44,45,46]. Particularly, perovskite oxides of the form La1−xMxFeO3 (M = Ba, Sr, Ca, Mg) show good sensing performance and low resistivity compared to basic LaFeO3 [20, 44, 47, 48]. Sun et al. reduced the resistivity of LaFeO3 by doping with barium (Ba), and achieved enhanced ethanol-sensing performance [20]. Likewise, Fan et al. partially substituted La with various amounts (x) of strontium (Sr) to achieve La1−xSrxFeO3, which exhibited low resistivity, particularly when x ≤ 0.1 [45]. Replacement of La3+ with Sr2+ created Fe4+ to maintain the charge neutrality, along with creation of oxygen vacancies. However, for large amount of dopant (Sr), the resistivity of La1−xSrxFeO3 increased, as excessive oxygen vacancies were formed along with combination of holes with electrons. Importantly, all these elements, i.e., Ba, Sr, and Ca, belong to the same group, and are very close in the periodic table. In this case, one would expect similar physical properties if they were to be used as dopants for LaFeO3. Recently, Palmar et al. investigated calcium substituted lanthanum ferrite (La0.8Ca0.2FeO2.95, henceforth LaCaFeO) nanoparticles for SO2 sensing [42]. In their study, as the doping level and temperature increased, there was a substantial resistance variation. Normally, substitution of smaller cation Ca2+ in the La3+ site is likely to reduce the Fe–O-bond length, and introduce a substitutional disorder and oxygen vacancies. Perhaps, it is for this reason that LaCaFeO exhibited higher lattice oxygen vacancies and enhanced p-type semiconducting properties compared to the parent LaFeO3 (LaFeO), influencing its sensitivity, speed, low-temperature sensing, and good recyclability toward SO2. Ideally, the abundant non-coordinated atoms (dangling bonds) in LaCaFeO are prerequisites for electronically favored reaction sites, an important factor for enhanced SO2 chemisorption and charge transfer [49]. In principle, Fe readily existed in oxidation states such as Fe2+, Fe3+, and Fe4+, supporting the argument that partial substitution of Fe in LaFeO3 generates active iron species. Natile et al. found that LaFe1−xGaxO3 can be tuned into porous microstructure in parallel with larger crystal sizes as the amount of Ga increases. More importantly, three oxidation states of Fe were unambiguously identified upon insertion of various amounts of Ga (x = 0–1). Especially, abundance of coordinately unsaturated Lewis acidic sites, i.e., Fe4+ (for x = 0.4), enabled the interaction of numerous oxygen molecules and led to creation of reaction sites for NO2 adsorption. On the other hand, intermediate species of NO2 adsorption can play a major role in enhancing the sensitivity upon interaction with surface Fe3+ and Fe2+ species. It can be concluded here that the fundamental principle for enhanced gas sensing is the creation of numerous oxygen vacancies, which in turn can be consumed by oxygen molecules.

The effect of oxygen vacancies on conductivity in ambient oxygen has been studied by Bektas et al. [50], who prepared BaFe1−xTaxO3−δ by direct mixing of BaCO3, Fe2O3, and Ta2O5, and carefully controlled the amount of Ta in the range 0.2 ≤ x ≤ 0.7. They then determined the electrical conductivity of the sintered BaFe1−xTaxO3−δ samples in ambient of 1–100% oxygen concentrations and temperature range of 400–900 °C. It was evident that BaFe1−xTaxO3−δ can act as temperature-independent oxygen sensor suitable for applications such as oxygen concentration measurements in automotive exhausts. In fact, its performance is incomparable to oxygen sensing performances of temperature-dependent SrTiO3 and of the SO2-prone SrTi0.65Fe0.35O3−δ- reported elsewhere [51, 52]. Unlike the classical semiconducting metal oxides, the charge carrier density in BaFe1−xTaxO3−δ is dependent on bulk defect chemistry. Note that the electron (hole) concentration varies linearly with oxygen partial pressure and the electrical conductivity. At higher amount of electron donating Ta (as x reaches 0.5), oxygen deficiency and the electrical conductivity decreased along with switching from p-type to n-type conductivity.

3 Composites of ABO3 perovskites and binary metal oxides

Composites comprising dissimilar semiconducting metal oxides exhibit intriguing hallmarks such as heterojunctions and/or close proximity effects that enhance the gas-sensing performance [53,54,55,56,57]. In heterojunctions (n–n or p–n interfaces), Fermi energy levels across the interfaces of dissimilar oxides in contact would equilibrate along with band bending, causing transfer of charge carriers and creation of charge-depleted regions. Strain-induced defects are also likely to occur due to lattice mismatch, which eventually act as trapping centers for electrons and holes, causing depletion of charge carriers near the interfaces. Inspired by adsorption capability of CaO toward CO2, Joshi et al. [58] synthesized and utilized n–n heterostructures based on CaO–BaTiO3 for CO2 sensing. Particularly, CaO exhibits higher sorption capacity and fast reaction rate with CO2, but is prone to aggregation upon sintering [59], and exhibits essentially poor reversibility. Incorporation of CaO into sintering-resistant BaTiO3 not only improves its CO2 sensing performance, but also eliminates the tendency to aggregate. The precursors materials were mixed, as shown in Fig. 4a, followed by heat treatment to obtained isolated n–n crystals (Fig. 4b, c). As shown in Fig. 4d (i), when CaO–BaTiO3 was exposed to CO2 at room temperature, only physisorption of CO2 occurred. However, CaO–BaTiO3 reacted with CO2 to form CaCO3 and BaCO3 as the temperature increased (Fig. 4d(iii)) because CaO and/BaO exhibit high affinity to CO2. No carbonate bands appeared in the absence of CO2. Unlike pristine BaTiO3 and CaO that remained almost insensitive even to 1000 ppm CO2, CaO–BaTiO3 nanocomposite exhibited enhanced response (ΔR/Ra of 65%) at 160 °C, with superior selectivity against various gases (Fig. 4e). Enhanced response was due to work function difference between CaO (1.69 eV) and BaTiO3 (4 eV), cumulative depletion layer at interfaces following electron–hole recombination in respective BaTiO3 and CaO, as well as adsorption of oxygen molecules on CaO and BaTiO3. Accordingly, adsorption and desorption of CO2 modulated the resistance of CaO–BaTiO3.

Fig. 4
figure 4

a Synthesis route of CaO-BaTiO3 based n–n heterostructures, b, cn–n interfaces of BaTiO3 spheroids and CaO sheets, d diffuse reflectance infrared Fourier transform spectra of 25CaO-B at room temperature, in absence of CO2 gas at 160 °C, in the presence of CO2 gas at 160 °C, and at room temperature in the absence of CO2 gas, shown as i–iv, respectively. e Dominant selectivity toward 1000 ppm of CO2 gas concentration balanced with dry synthetic air at 160 °C. The interfering gases are acetone (500 ppm), ethanol (500 ppm), hexane (1000 ppm), methanol (500 ppm), H2S (100 ppm), H2 (50 ppm), CO (50 ppm), NO2 (1000 ppm), NO (100 ppm), and SO2 (50 ppm) (Reprinted with permission from Ref. [58]. Copyright (2018) American Chemical Society)

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In another investigation, Zhang et al. prepared heterojunctions composed of p-type LaFeO3 and n-type α-Fe2O3 via a facile one-step solvothermal method in accordance with an illustration in Fig. 5a [60]. Using this technique, LaFeO3 was uniformly incorporated into Fe2O3, forming p–n heterojunctions throughout the porous LaFeO3/α-Fe2O3 nano-octahedrons. Note that both α–Fe2O3 and LaFeO3 are semiconducting oxides with narrow bandgaps of 2.1 and 2.3 eV, respectively [61], and intriguing gas-sensing properties. Decoration of optimal amount of LaFeO3 onto α-Fe2O3 resulted in a threefold enhanced response (Fig. 5b), enhanced selectivity against interfering gases (Fig. 5c), fast response/recovery speed, and sensor stability compared to that of pristine α-Fe2O3. Apart from the adsorption/desorption reactions of the air and acetone onto LaFeO3/α-Fe2O3 nano-octahedrons, higher response can be attributed to formation of the p–n heterostructure. Due to higher Fermi level of α-Fe2O3 compared to that of LaFeO3, transfer of charge carriers, i.e., electrons from α-Fe2O3 to LaFeO3 and holes from LaFeO3 to α-Fe2O3, can occur at the interfaces to equilibrate the Fermi energy levels, resulting in a p–n junction. In this way, the band banding and charge depletion layer at the p–n junction increased the base resistance of LaFeO3/α-Fe2O3 composite (264 MΩ) in the air compared to that of unmodified α-Fe2O3 (23 Ω), thus increasing the response of the LaFeO3/α-Fe2O3 composite.

Fig. 5
figure 5

a Schematic illustrating the synthesis of porous α-Fe2O3 nano-octahedrons and LaFeO3/α-Fe2O3 Composites, b responses of the pure α-Fe2O3 and LaFeO3/α-Fe2O3 composites to different concentrations of acetone, and c selectivity of α-Fe2O3 and various LaFeO3/α-Fe2O3 composites to 100 ppm of various gases. S1–S4 samples denoted composites containing different amounts of LaFeO3 (Reprinted with permission from Ref. [60]. Copyright (2018) American Chemical Society)

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4 Catalyst loaded ABO3 perovskites

Apart from doping of A and/or B site in ABO3 perovskites or combination with metal oxides, catalysts loading on these perovskite oxides can provide specific adsorption sites for oxygen adsorption and catalytic centers for oxidation of analyte gases to improve sensors response, speed, selectivity, and operating temperature. Thus far, various perovskite oxides have been loaded with various catalysts such as silver (Ag) [62,63,64], palladium (Pd) [65, 66], and Au [67] for gas-sensing applications. These catalysts can physically or chemically interact with the surface of perovskite oxides depending on synthesis conditions and nature of catalysts. Typically, the method of catalyst loading on host materials and the interaction thereof can influence how the material responds to various gases. For instance, Wei et al. [62] loaded Ag in LaFeO3 and observed that most Ag remained as isolated catalysts on the surface of LaFeO3, whereas some of Ag ions replaced La3+, forming defects and holes therein. The introduced defects provided more holes, leading to high charge carrier concentration and enhancement of formaldehyde sensing. Joshi et al. [64] prepared Ag@CuO—decorated BaTiO3 spheroids by wet impregnation and hydrothermal techniques, and investigated their sensing affinity to CO2. Here, intermittent p–n heterojunctions were formed upon co-precipitation of CuO microleaves with BaTiO3 spheroids. Then, different amounts of metallic Ag (0.5–1.5 wt %) were loaded on p-CuO/n-BaTiO3 composite via impregnation into ethanol containing AgNO3. Formation of p–n heterojunction resulted in significant improvement of the sensor response, which increased with the amount of CuO, until equimolar CuO/BaTiO3 was obtained. At this condition, a very low limit of detection (~ 51 ppm) was obtained. After decoration of 1% Ag on equimolar CuO/BaTiO3, the results were highly repeatable and accurate, with a response (ΔR/Ra) of 58.64%, and fast recovery toward 1000 ppm at 120 °C, because Ag can readily catalyze the adsorption of CO2. In the presence of the catalyst, the limit of detection decreased to ≈ 41 ppm. Moreover, CuCO3 and BaCO3 were formed after adsorption of CO2 on CuO/BaTiO3, which possibly enhanced the response and selectivity toward CO2. This was in agreement with the work of Joshi and coauthors [58].

Cao et al. investigated the influence of Au and Cl loading on LaFeO3 [67]. The loading of Au and Cl was achieved by addition of different weight proportions (x = 0, 0.5, and 1 wt %) of HAuCl4 during the sol–gel synthesis. Their analyses showed  that heat treatment of LaFeO3 precursors and HAuCl4 in organic solvents evolved CO2, which was capable of reacting with superficial La–O in the as-formed LaFeO3, resulting in La-carbonate. This occurred in concurrence with formation of Fe–Cl and La–Cl bonds after chemisorption of Cl on Fe sites in the FeO6 octahedra (Fig. 6a, steps 1–2), and replacement of C=O bond in monodentate La-carbonate with the C–Cl bond. On the other hand, Au existed as physically adsorbed particles on LaFeO3 crystals (step 3). Co-existence of Au, Cl, and Fe ions sites in the LaFeO3 facilitated massive adsorption of oxygen and ethanol (steps 4–5), with the highest adsorption observed at x = 1 wt %. Co-modification of LaFeO3 with Au and Cl resulted in an increase of charge transfer (90.7%) from ethanol to the adsorbed oxygen on the Fe–O-terminated LaFeO3 surface. In this way, the Au- and Cl-modified LaFeO3 exhibited the highest reported response (Rg/Ra = 220.7) and selectivity toward 100 ppm ethanol (Fig. 6b, c).

Fig. 6
figure 6

a Stable configuration of FeO-terminated LaFeO3 (001) surface, with Cl, Au, and O adsorption as well as final ethanol adsorption indicated as 1–5, respectively. Cyan, purple, red, green, yellow, gray, and white balls represent La, Fe, O, Cl, Au, C, and H atoms, respectively. b Gas responses of Au- and Cl-modified LaFeO3 NPs to 10–400 ppm ethanol, and c 100 ppm of different gases at 120 °C (Reprinted with permission from Ref. [67]. Copyright (2019) American Chemical Society)

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5 ABO3 perovskites as catalysts

Perovskite oxides are of interest as alternatives for noble metal catalysts in a broad range of applications [68,69,70,71,72,73]. Adoption of perovskite oxides as catalyst aims at designing of thermochemical-stable, cost-effective, and highly active catalysts to overcome prohibitive cost, poor thermal stability, poisoning effects, and scarcity of noble metal catalysts. In this perspective, application of perovskite oxides as catalysts in gas-sensing applications cannot be an exception, particularly in those involving high-temperature operation and hash chemical environments. Despite few attempts to elucidate catalytic effects of ABO3 perovskites in gas-sensing applications [74,75,76], reports in this area are still scarce.

Lin and coauthors have investigated the effect of La0.8Sr0.2FeO3 (LSFO) particles on β-Ga2O3 nanorods (≈ 100–300 nm thick) via sputtering technique and thermal annealing [75]. In their approach, they synthesized and employed LSFO–decorated β-Ga2O3 nanorods for detection of carbon monoxide (CO). For comparison purposes, they also prepared and decorated β-Ga2O3 nanorods with ultrasmall Pt nanoparticles (30 wt %), a higher loading amount compared to ≈ 1.22 wt % of LSFO. Similar to distinctive chemical sensitization and spillover effects of Pt catalysts [77], LSFO particles with suitable composition exhibited spillover effect on Ga2O3 nanorods owing to the presence of oxygen vacancies, which increased their oxygen adsorption capability (Fig. 7a). Unlike bare n-type β-Ga2O3, abundant oxygen can be adsorbed on LSFO particles. Moreover, further charge-depleted regions were created at interfaces of p-type LCFO and n-type β-Ga2O3 because of differences in Fermi levels. Since CO is an electron donor, when it was adsorbed on the surface of LSFO–decorated β-Ga2O3, the depletion layer became thinner along with an increase of the overall electrical conductivity (Fig. 7b). Since the reaction of O− and CO was small on LSFO, there existed a concentration gradient between abundant unconsumed O− species on the surface of LSFO and those adsorbed on β-Ga2O3 nanorods. Moreover, as indicated in Fig. 7c, O− species were attracted from LSFO particles toward the interfaces of LSFO and β-Ga2O3 nanorods, which eventually spilled over onto the surface of β-Ga2O3 nanorods. Surprisingly, LSFO decorated β-Ga2O3 nanorods indicated a similar sensitivity (RO/RCO, where RO is the resistance in N2, and RCO is the resistance in CO/N2 mixture) to that of Pt-decorated β-Ga2O3 nanorods, despite a small amount of LSFO particles (5 and 10 nm thick) used (Fig. 7d). This result implies that LSFO can equally replace Pt catalysts. Although fast response was observed for the LSFO decorated β-Ga2O3 nanorods compared to Pt-decorated β-Ga2O3 ones (Fig. 7e), this response was not faster than that observed for pristine β-Ga2O3 nanorods.

Fig. 7
figure 7

Gas-sensing enhancement mechanism of aβ-Ga2O3 and LSFO decorated β-Ga2O3 nanorods in N2 atmosphere, bβ-Ga2O3 and LSFO decorated β-Ga2O3 nanorods in CO/N2 atmosphere. c Spillover-like effect model in LSFO nanoparticle decorated β-Ga2O3 nanorods surface in CO/N2 atmosphere. DL stands for carrier depletion layer. d Sensitivity [RO/RCO, where RO is the resistance in N2, and RCO is the resistance in CO/N2 mixture] of β-Ga2O3, β-Ga2O3/LSFO, and β-Ga2O3/Pt composite nanorods tested at 500 °C, and e response time of β-Ga2O3, β-Ga2O3/LSFO, and β-Ga2O3/Pt composite nanorods tested at 500 °C (Reprinted with permission from Ref. [75]. Copyright (2016) American Chemical Society)

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Another interesting work is that of Kang et al., in which the catalytic role of quasispherical La0.75Sr0.25Cr0.5Mn0.5O3- δ (LSCM) particles (215.7 nm-diameter) in SnO2 fiber-in-tube (FITs) was investigated for formaldehyde (HCHO) sensing [76]. A versatile electrospinning technique was used to electrospin a solution containing LCSM particles and SnO2 precursor, as demonstrated in Fig. 8a. It was observed that LSCM inhibited the grain growth and hindered Oswald ripening mechanisms, enabling the formation of LSCM-loaded SnO2 FITs architectures (Fig. 8b). Apart from easy penetration of air and HCHO in the tubes as well as surface pores, LSCM can enhance HCHO sensing due to formation of heterojunctions with SnO2, and the presence of oxygen vacancies (Fig. 8c), similar to LSFO particles demonstrated by Lin et al. [75]. The heterojunction effect is attributed to work function differences between LSCM (6.8 eV) and SnO2 (4.55 eV) that can cause band bending and depletion of charge carriers at interfaces along with an increase in the base resistance. Abundant oxygen vacancies can be created upon partial substitution of La with Sr, as well as possible reduction of Mn4+ to Mn3+ following charge migration during the formation of heterojunctions. Ideally, O− can weakly adsorb into oxygen vacancies, but can exist in abundance on the surface of LSCM particles. These oxygen species can easily spread over the surface of SnO2 by spillover effect. In fact, the ratio of chemisorbed O− species to lattice O2− in LSCM was larger than in bare SnO2 (0.9 vs 1.48) when determined at 400 °C. Because of these events, the response of LSCM-loaded SnO2 FITs was higher than that of SnO2 nanotubes (NTs), SnO2 NFs, and that of the reference sensor, i.e., LCO loaded SnO2 FITs (Fig. 8d). More importantly, the sensitivity of LSCM–loaded SnO2 FITs toward HCHO was significantly higher than toward interfering gases (Fig. 8e).

Fig. 8
figure 8

a Schematic indicating the synthesis procedure for LSCM@SnO2 FITs, b morphology of LSCM@SnO2 FITs, c illustration of HCHO sensing mechanism of LSCM@SnO2 FITs, d response/recovery profiles for different sensors toward 1–5 ppm HCHO at 400 °C, and e selectivity property of LCSM@SnO2 FITs toward various gases at 400 °C (Reproduced from Ref. [76] with permission from The Royal Society of Chemistry)

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6 Porosity control in ABO3 perovskites

Fast diffusion of oxygen in perovskite oxides depends on availability of oxygen vacancies, usually tuned by optimizing material composition. In addition, porosity control is essential. It is well known that porous nanostructures possess higher concentration of active sites than their dense analogues [78, 79]. In general, thick semiconducting metal oxides experience prominent gas diffusion and reaction effects owing to longer diffusion depths, leading to gas-inaccessible reaction sites and low sensitivity. The gas concentration decays with the diffusion length because of an instantaneous reaction between the incoming analytes and the abundant pre-absorbed oxygen species [79]. This implies that the rate of consumption due to gas reaction approaches the rate of accumulation of analytes. Under a combination of optimal diffusion depth with pore apertures (mesopores and/or macropores), such decay of concentration can be eliminated. To achieve this, one needs to control the porosity of sensing materials. Currently, several techniques to introduce porosity in metal oxides have been suggested: such approaches include apoferritin templating [80, 81], chitosan templating [82], soft templating [83], and hard templating [84,85,86,87,88,89,90], among many others. Unfortunately, ABO3 perovskites mainly used in gas-sensing applications are those based on solid-state reactions of precursor powders [44, 50] or sputtered films [28, 91]. In addition, with few exceptions such as the work demonstrated by Ma et al., [92] and others [29, 62, 93, 94], even electrospun perovskite oxides still exhibit limited porosity. The most promising, however, are the works of Xiao et al. [35] and Wang et al. [95] that employ a simple Ostwald ripening to synthesize porous perovskite oxides. However, despite the fact that Ostwald ripening is a template-free technique, it is difficult to achieve controlled grain/pore size and morphological tuning of ABO3 perovskite materials. Recently, hard templating approaches for porosity control in perovskite oxides were elucidated [96,97,98,99,100]. Typically, hard templating involves introduction of rigid templates such as solid or mesoporous silica, carbon, and polystyrene (PS) beads, whose elimination by calcination or dissolution in chemicals (acids or alkali etchants) creates various porous and hollow structures, ranging from simple to intricate ones. Dai and coworkers employed sacrificial PS beads to template porosity in a LaFeO3 film. As illustrated in Fig. 9a, a self-organized monolayer of monodispersed PS beads (500 nm-diameter) was floated on a mixed precursor solution containing La3+ and Fe3+, followed by a pickup of the precursor coated beads using a substrate printed with sensing and heating electrodes. After drying and heat treatment, honeycomb-like porous thin film was formed. Following a high degree of ordering of PS beads on the silicon substrate (Fig. 9b), a periodic porous film of octahedral LaFeO3 was obtained after thermal treatment (Fig. 9c). The thickness of the film was determined to be 192 nm, whereas the thickness of the pore wall was 28 nm. Ideally, even the sizes of pores previously occupied by PS beads are expected to be 40–60% smaller due to shrinkage as PS beads decompose in the temperature range of 325–390 °C [87]. Because of numerous oxygen vacancies in LaFeO3, the ratio of adsorbed to lattice oxygen species (O−/O2−) was even higher than that of Fe2O3 prepared by a similar technique. The porous morphology and enhanced surface oxygen resulted in a stable high response (Fig. 9d), fast response speed (≈ 4 s), and low limit of detection (50 ppb) of porous LaFeO3 films toward ethanol compared to that of porous Fe2O3 film. Importantly, the templated LaFeO3 exhibited a high selectivity to ethanol compared to that of a non-templated LaFeO3 film (Fig. 9e), confirming the effect of pores in gas sensing.

Fig. 9
figure 9

a Schematic for fabrication of LaFeO3-ordered porous thin films. Morphological and microstructural characterizations of b a PS microsphere (500 nm in diameter) monolayer and c LaFeO3-ordered porous film. Sensing response of d LaFeO3 and Fe2O3 porous thin film sensors at 450 °C for five gas-on/off cycles upon exposure to 5 ppm ethanol, and e the response of template-directed LaFeO3 and non-template LaFeO3 films toward 5 ppm of different gases at 450 °C (PX: p-xylene) (Reprinted with permission from Ref. [96] Copyright (2014) American Chemical Society)

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Considering enhanced sensing performance of heterogeneous perovskite oxides compared to their single-phase analogues, further enhancement in performance can be achieved if adsorption/desorption kinetics of oxygen and target gases were enhanced. Chen et al. [97] prepared Ag/Zn-LaFeO3 nanocomposite via nanocasting technique, with KIT-6 (i.e., Korea Institute of Technology-6) as a template. KIT-6 is a three-dimensional (3D) silica material with high surface area and ordered mesostructure. Xu et al. have reported the surface area of KIT-6 as high as 635 m2 g−1 and mesopore sizes in the range of 8–10 nm [101]. Chen and coworkers prepared KIT-6 in accordance with the procedure described in Fig. 10a. Precursors of Ag/Zn-LaFeO3 were infiltrated into KIT-6 followed by heat treatment and silica etching in NaOH to achieve crystalline porous Ag/Zn-LaFeO3 nanocomposites containing various amounts of Zn. In a perfect LaFeO3 crystal, all sites would consist of La3+, Fe3+, and O2− ions. However, with 2 at % Zn doping (with respect to La or Fe), two states of Fe, i.e., Fe3+ and Fe4+ coexisted. Typically, Fe4+ can readily interact with oxygen molecules, creating numerous reaction sites for target gases. In addition, Zn2+ would partially substitute La3+, creating defects on La sites, and charge carriers (holes), whose concentration increases with the amount of Zn dopants. On the other hand, Ag existed as an isolated phase in Ag/Zn-LaFeO3, and possibly played a catalytic role for gas-sensing reactions. This templating approach provided Ag/Zn-LaFeO3 nanocomposites (2 at % Zn, hence forth AZLFO-2) with distinguishable mesopores (3.4 nm in size) and high surface area (118 m2 g−1). Upon exposure of AZLFO-2 to various concentrations of ethanol at 55 °C, an outstanding response of 64.2 at 100 ppm and a response/recovery speed of 100/20 s were achieved (Fig. 10b). The AZLFO-2 sensor also showed good selectivity toward various gases (Fig. 10c). During exposure, ethanol dissociated into more reactive CH3CHO that subsequently reacted with adsorbed oxygen, thinning the hole accumulation layer, and leading to high response.

Fig. 10
figure 10

a Schematic for synthesis of KIT-6 templated mesoporous Ag/Zn–LaFeO3 nanocomposites, b response and recovery profiles of AZLFO‐2 to ethanol in the concentration range of 10–100 ppm at 55 °C, and c responses of AZLFO‐2 sensors upon exposure to 100 ppm of various volatile organic compounds at 55 °C (Reproduced from Ref. [97] with permission from John Wiley & Sons, Inc.)

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7 Summary and outlook

We have given a brief review of the approaches adopted by most researchers to enhance the sensing performance of ABO3 perovskites as resistive-type gas-sensing materials. It is noteworthy to mention that perovskite oxides are intrinsically enriched with oxygen vacancies that influence oxygen adsorption. When a tetravalent B site of ABO3 perovskite is doped with trivalent cation, the resulting oxygen vacancies can dissolve humidity, forming OH− groups that eventually bind to oxygen in the perovskite lattice. This could potentially affect the sensing performance of perovskite oxides and long-term stability in humid ambient. To further tune physico-chemical properties of perovskite oxides for enhanced gas-sensing performance, one needs to employ various techniques such as (1) heterovalent cation substitution to create anionic and/or cationic vacancies, (2) functionalization with catalysts that display high-effective catalytic activity, and (3) introduction of heterojunctions. Table 1 presents recently reported gas-sensing performance of ABO3 perovskites. Despite strenuous efforts made thus far, the sensing property such as response to trace (sub-ppm) concentrations and response/recovery speed of ABO3 perovskites seems far inferior compared to those of binary semiconducting metal oxides (SMOs) reported in several treatises [102, 103]. Moreover, several ABO3 perovskites can be employed as gas-sensing materials at low temperature, although temperature variation is associated with structural phase transition of some perovskite oxides, indicating that they are unstable at low temperature.

Table 1 Recently reported resistive-type gas sensors based on perovskite oxides and perovskite oxide decorated semiconducting metal oxides
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Despite a significant step for utilization of ABO3 perovskites as potential alternatives for noble metal catalysts in various applications, their utilization in gas sensing is still at infant stage. In fact, some ABO3 perovskites are chemically unstable, especially in the presence of gases such as SO2, limiting their commercialization as catalysts. Other issues relate to small surface area, since their synthesis procedures typically involve high temperature as well as longer heat treatment. Moreover, it is challenging to attain ultrasmall (sub 10 nm-scale) size particles such as those readily achieved for noble metal catalysts like Pt, Pd, Ag, and Au. It is noteworthy to mention that even bulk perovskite oxides exhibit poor porosity for diffusion of analyte gases. In this regard, we have elucidated techniques commonly involved to introduce porosity in perovskite oxides for improving gas sensing. It is anticipated that future investigations will make use of techniques commonly employed in creating pores in binary SMOs such as soft templating, to prepare porous perovskite oxides with various morphologies.