Full Text

Turn on search term navigation

1. Introduction

The first gas-sensing properties of semiconductor materials were reported in the 1920s, and in 2020, we celebrated the 100th anniversary of the first gas-sensing investigations of the gas atmosphere [1]. Since that time, semiconductor-based gas sensors reached a number of pivot moments including in 1955, when oxygen detection in gas changes in the conductivity of a semiconductor (ZnO) was reported [2]; in 1968, when the first commercially available gas sensor, TGS (Taguchi Gas Sensor from Figaro Engineering Inc., Arlington Heights, IL, USA), was launched on the market for methane detection TGS109 [3]; in the 1990s, when nanostructure gas sensors were presented, with zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanorods and nanowires, two-dimensional (2D) nanosheets and films and three-dimensional (3D) polycrystals and ultraporous nanostructures as recently reviewed in [4]; and in 2007, when graphene-based gas sensors were introduced [5].

The common gases detected and analyzed by the commercially available gas sensors are oxygen, carbon monoxide, carbon dioxide, nitrogen oxides, ammonia, methane, acetone, methyl mercaptan, hydrogen sulfide, chlorine, volatile organic compounds (VOC) and hydrocarbon [6,7,8]. Nowadays, gas sensors are used in many applications [9,10,11], including mining [11,12], broadly understood industry [10,11], public safety [9,10,11], military [10], healthcare [10,11], consumer electronics [9,11,13], petrochemical [10,14], agriculture [10,11], water [11,14], medical [11,15,16,17], oil and gas [11,14], automotive [9,18,19], food and beverages [20], environmental protection [9,11,19], metals and chemicals [21,22], power stations [11], smart cities and building automation [10,11].

In the coming years, factors such as the growing demand for sensors in almost every field, technological progress, expansion of advanced functions of existing installations and devices, government requirements limiting emissions of various gases by industry and transport will accelerate the demand for this type of equipment (Figure 1). The value of the global gas sensor market was estimated at USD 2.19 billion in 2019. According to some sources [6,7], this market is forecast to grow by a compound annual growth rate (CAGR) of 6.3% to 8.3% from 2020 to 2027. One of the rapidly growing industries with an increasing demand for gas sensors is environmental monitoring, especially the level of pollution and volatile organic compounds.

Various materials have been validated for gas-sensing applications; however, the main commonality is gas sensors based on metal oxides, for example, n-type including tin dioxide (SnO2), tungsten trioxide (WO3), indium oxide (In2O3), gallium oxide (Ga2O3), vanadium oxide (V2O5) and iron oxide (Fe2O3) and p-type metal oxides such as nickel oxide (NiO), copper oxide (CuO), cobalt oxide (Co3O4), manganese oxide (Mn3O4) and chromium oxide (Cr2O3) [23]. The progress of nanotechnology development allows researchers to increase the surface-to-volume ratio by utilization of various techniques which results in increased sensitivity and a reduced operating temperature, and various doping methods and materials have been proposed to increase the selectivity and stability of gas sensors. Long-term stability is a crucial feature of gas sensors dedicated to industrial applications such as automotive, biotechnology, safety and military.

One of the promising materials is strontium titanate (SrTiO3). SrTiO3 is a semiconducting ceramic material and it has a simple cubic perovskite structure (space group Pm3m) with a lattice parameter of 0.3905 nm and a density of r = 5.12 g/cm3 (Figure 2a) [24]. The crystallographic structure is presented in Figure 2. SrTiO3 is attractive due to its properties such as a large dielectric constant (ɛ0 = 300), low dielectric loss (mostly < 0.02) [25] and strong thermal and chemical stability [26]. These advantages enable a wide application of the material in the area of sensors, actuators, electro-optical devices, memory devices with random access and multilayer capacitors [27]. It is commonly used for oxygen sensors [28]; however, other sensing applications are also common such as temperature sensors [29] and the cantilever base for various sensors [30]. Thanks to the deposition technology developments, strontium titanate can be realized in the nanoform instead of the bulky form, where a high-temperature solid-state reaction method is used (700–1000 °C). Reducing the size allows researchers to reduce the operating temperature to 40 °C [31]. Among various possibilities to deposit SrTiO3 such as magnetron sputtering [32,33], atomic layer deposition (ALD) [34,35], pulsed laser deposition (PLD) [36,37], metal-organic chemical vapor deposition (MOCVD) [38,39], laser chemical vapor deposition (LCVD) [40,41] and the sol–gel method [42,43], the sol-gel method seems to be the most suitable since it enables depositing a small grain size with high uniformity and high purity. Nanoforms can be also obtained thanks to the glancing angle deposition technique, for example, with magnetron sputtering technology [44,45].

Another promising material is barium titanate (BaTiO3). BaTiO3 is a cubic perovskite-type structure semiconductor material (Figure 2b), commonly used as a ferroelectric [46] since it exhibits a high dielectric constant (dielectric constant depends on the type of synthesis, temperature, frequency and dopants) [47,48], large electro-optic coefficients and a positive coefficient of resistivity (PTCR) [49]. There are various methods of obtaining BaTiO3, for example, by a solid-state reaction [50], the sol–gel method [51], a hydrothermal method [52], a coprecipitation method [53], a polymeric precursor method [54] and mechanochemical synthesis [55]. Thanks to the above-mentioned features, BST is widely used in ferroelectric memories [56], electro-optical devices [57], dielectric capacitors [58], multilayer capacitors (MLCs) [59] and electromechanical transducers [60,61], as well as in gas sensor applications [62].

A novel material that brings all features of SrTiO3 and BaTiO3 is BaSrTiO3 (barium strontium titanate, BST). BST is a kind of electronic ceramic material with a typical perovskite structure (Figure 2c) and properties such as a high dielectric constant, low dielectric loss, good tenability and high insulation resistance and can be widely used in various electronic components such as ferroelectric memories, capacitors and phase shifters [63,64,65,66]. Apart from the typical applications where the above-mentioned properties are used, BST has also been investigated as a gas-sensitive material.

In 2014, Romh MA E., et al. presented investigation results on the process of preparing and testing solid-state samples by mixing BT and BST powders with an organic carrier in the ratio of 50:50 and 60:40 and then depositing by spin coating on alumina substrates as a gas-sensitive material. BaTiO3 was doped with strontium and iron to increase the conductivity by double substitution in the perovskite structure. The films were then sintered at the temperature of 1100 °C for 2 h and characterized by means of X-ray diffraction (XRD) and a scanning electron microscope (SEM). The dielectric measurements performed by the authors revealed a significant increase in conductivity at a low a.c. current frequency, even 10,000 times at a temperature from 25 to 500 °C, and the phenomenon of dielectric relaxation related to the displacement of oxygen voids appeared [67].

In another work, Simion, C.E., et al. presented gas sensors based on thick-film BST doped with copper in various concentrations (0.1, 1 and 5 mol% Cu). The gas-sensing behavior was tested under exposure to NH3 and H2S. The prepared samples worked optimally at a temperature of 200 °C. The sample made of BST doped with 0.1% mol of Cu had the best sensitivity to NH3 gas. On the other hand, selective detection of H2S was achieved for BST doped with 5 mol% Cu. The tested materials did not show the cross-sensitivity effect to CO, NO2, CH4 and SO2 (200 °C, 50% RH). Higher relative humidity noticeably increased the sensitivity of the sensors to NH3 and H2S [68]. Simion, Cristian E., et al. in the following year published another paper where they described the reactions of thick layers of Ba0.75Sr0.25TiO3 to the presence of NH3 at room temperature, at different levels of relative air humidity. The samples were synthesized by a hydrothermal method. The change in electrical resistance and capacitance was tested and photoacoustic measurements were conducted as well. The measurement results were presented in the context of the Grotthuss mechanism in relation to the ion/electronic conductivity in BST. The detection of ammonia was tested in the presence of water vapor at room temperature. As a result of the performed measurements, it was determined that the optimal detection of ammonia occurs at room temperature in the presence of water vapor in the tested gas. The interaction between NH3 and H2O takes place mainly through the proton exchange mechanism [69].

In 2020, Shastri, Nipa M., et al. presented BST-based gas sensors obtained with the pulsed laser deposition (PLD) technique. The paper presents two stoichiometries of BaxSr1-xTiO3 with x = 0.5 and x = 0.7. The material properties were checked using the following methods: XRD, X-ray energy dispersion (EDX) and UV–VIS spectroscopy. The gas-sensing measurements showed that the BST0.5 sample exhibited a higher sensitivity to H2S with a concentration of 800 ppm than the BST0.7 sample [70]. Recently, Ba0.5Sr0.5TiO3 doped with various concentrations of RuO2 (from 0% to 6%) was used as a gas-sensing material with the utilization of chemical solution deposition (CSD) on p-type silicon substrates. The sample with the highest RuO2 content (6%) exhibited the highest response to H2S, which is considered as a biomarker of halitosis disease, and therefore the developed sensors (Ba0.5Sr0.5TiO3 doped with RuO2) were proposed for exhaled breath analysis [71].

In this paper, the gas-sensing achievements of SrTiO3- (Section 2.1), BaTiO3- (Section 2.2) and BaSrTiO3-based gas sensors (Section 2.3) are summarized, giving the readers a frank overview. Table A4 in Appendix A presents a collective comparison of the properties of the above-mentioned three nanocomposites, it is a short and concise comparative characteristic. There is a literature reference next to each piece of information. As can be noticed in Figure 3, the number of papers where the above-mentioned materials were used for gas-sensing materials constantly increases from 2000.

2. Materials, Results and Discussion 2.1. SrTiO3 for Gas-Sensing Applications

In 2004, Meyer and Waser presented a model for a fast sensor response of resistive donor-doped SrTiO3 at temperatures from 850 to 950 °C. The authors speculated that cation vacancies may play a key role in the formation of grain resistance boundaries not only at high but also at moderate temperatures. As a result of mobility, the change in cationic vacancy concentration may be limited to only a few monolayers on each side of the interface. Strontium vacancies are considered to be virtually stationary at great distances for these temperature ranges; thus, the change in strontium void concentration was limited to one unit cell on each side of the boundary. Therefore, a limited point defect approach was used in this paper, only for electrons and strontium vacancies [72]. This, combined with the high density of the defect state interface, explains the huge change in sensor resistivity observed. The authors proposed a point defect model involving the formation of a space charge area near the phase boundary, which has a significant impact on the balance of local defects. The consequence of this is the decisive influence of the overall value on the concentration of each type of defect. The reason for the rapid response of the sensor could then be an increase in the concentration of the cationic void near the interface, induced by the space charge. Cationic voids are the cause of high electron depletion [70]. In the same year, Hu et al. presented a low-temperature nano-structured SrTiO3 thick-film oxygen sensor obtained by utilization of the high-energy ball milling technique in conjunction with the screen printing technique. At that time, the novelty of the proposed SrTiO3-based gas sensor was the ability to work at an operating temperature as low as 40 °C, and, in fact, this is a good result nowadays. The sensors were tested under exposure to 2–20% oxygen. Moreover, the experimental results showed that the sensing property of the synthesized SrTiO3 sensors with an annealing temperature of 400 °C is much better than the commercial SrTiO3 sensors (both milled and not milled materials) [73]. The effects of the annealing temperature on the sensing properties of nano-sized SrTiO3 oxygen gas sensors were analyzed by the same group and presented in 2005 [31]. The authors used various methods and techniques to validate the annealing temperature influence, for example, by using differential thermal analysis (DTA)/thermogravimetric analysis (TGA), XRD and transmission electron microscopy (TEM) methods. The films were annealed in the range 400–800 °C with 100 °C steps, and no annealed sample was tested. However, surprisingly, the results showed that samples annealed at 400 °C, as previously shown [73], exhibited the highest sensor response to oxygen at the same operating temperature, 40 °C [31]. The authors concluded that the different annealing temperatures only affect the grain size of the synthesized SrTiO3-based oxygen sensors, but they do not provide evidence as to why 400 °C showed the highest responses. More interesting is that the results showed that the lower annealing temperature offers higher responses, but the authors did not test, or at least did not present, the results for samples annealed at 300 °C [31]. The same group in 2004 presented the investigation results on the same gas-sensing material for near-human body temperature oxygen sensing application [74]; however, the presented results did not bring anything new [74]. An effect of oxygen cross-sensitivity was evaluated by Sahner et al. [75]. In 2006, the authors fabricated a hydrocarbons sensor for exhaust gases based on semiconducting doped SrTiO3 for on-board diagnosis [75]. A multilayer resistive sensor based on catalytically activated and non-activated SrTiO3 was tested under various scenarios that may occur during the on-line analysis of exhausting gases in the automotive industry, such as the presence of ethane and propane (as hydrocarbon species) and of hydrogen, carbon dioxide and nitric oxide (interfering gases). The results showed that the non-activated sensor part strongly responds to any reducing gas, whereas the catalytically activated part only detects a slight variation in the oxygen equilibrium concentration. On this occasion, the authors concluded that the actual influence of the platinum content on oxygen sensitivity, temperature dependency and long-term stability needs to be investigated, as well as the cross-sensitivity to different types of hydrocarbons with respect to chain length, presence of unsaturated bindings and aliphatic or aromatic chains [75]. Hydrocarbon sensing results were presented by the same group in 2007 [76] with utilization of a nanoscaled SrTi1−xFexO3−δ-based sensor, where two novel synthesis methods, at that time, were validated, i.e., electrospinning and electrospraying. Moreover, the authors proposed a mechanistic model to explain the impact of the enhanced surface-to-volume ratio of the p-type SrTi1−xFexO3−δ gas-sensing films. The sensors were investigated in the temperature range from 350 to 450 °C under exposure to propane, propene, hydrogen, NO and CO. The sensors showed a fast, reversible and reproducible response towards propane and propen, with cross-sensitivity towards NO. Neither CO or H2 exposure led to the response changes measured as the resistance ratio [75]. In 2009, Menesklou et al. [77] discussed the effect of impurities on the gas-sensing properties of thin layers of strontium titanate. The admixture of lanthanum (La) in a small concentration significantly influenced the dependence of the electrical conductivity of the material on the oxygen partial pressure. Pure SrTiO3 was characterized by an ambiguous dependence of the electrical conductivity on the aforementioned partial pressure of oxygen, but after adding a small amount of La, this relationship became unambiguous. Samples obtained in this way have response times in the millisecond range, regardless of the thickness of the layers. The reason for this lies in the surface conductivity mechanism. The authors showed that the dependence of SrTiO3′s work can be largely leveled in a small range of oxygen partial pressures by doping an acceptor at a high concentration, in this case, iron. The conductivity of doped strontium titanate with a donor is based on the boundary conductivity of the grains, even at temperatures reaching 1000 °C. The doping concentration affects the much slower kinetics of the bulk equilibrium through long-term drift. The grain size has no effect on the bulk equilibrium kinetics. The authors concluded that the fine-grained strontium titanate polycrystals doped with a small amount of donors should increase the surface-to-weight ratio of the sensor and thus improve its properties. At the same time, there is no acceleration of the drift kinetics, as in the case of donor-doped barium titanate, for example. One of the results of the work was a proposal of a thick-film oxygen sensor (lean combustion) for the control of internal combustion engines, characterized by a fast reaction, low-temperature dependence and sensitivity to oxygen [77].

SrTiO3-based gas sensors have been used not only to detect oxygen but also to detect ozone gas (O3). In 2013, Mastelaro et al. [78] reported the nanocrystalline SrT1−xFexO3 (STF) gas-sensing material for ozone detection. An electron-beam physical vapor deposition (PVD) system was utilized for the deposition with the post-processing annealing process (550 °C). The authors analyzed the effect of iron doping (0.0075, 0.10 and 0.15 mol% of iron) on the gas-sensing characteristics. The sensors exhibited a very high sensor response (R0/R), i.e., 580 under exposure to 0.6 ppm of ozone at 190 °C. The limit of detection was as low as 0.075 ppm [78]. Kajale et al. [79] also showed interesting results of SrTiO3-based gas sensors; the fabricated SrTiO3-based sensors exhibited high sensitivity to CO with no response to H2S and vice versa for Cu-doped SrTiO3, which can be explained due to conversion of Cu into CuS. However, this conference paper provides only preliminary results without an extensive elaboration [79]. On the contrary, high responses to H2S by utilization of pure ST films have been reported by the same group [80]; in this paper, the SrTiO3 nanomaterial was synthesized using a sol–gel hydrothermal method and exhibited an outstanding gas sensor response (S > 500) at a lower temperature, 150 °C, to 80 ppm of H2S [80]. The possibility to detect another carbon oxide, CO2, was verified by Zaza et al. [81], in 2015. The authors presented the results of a room-operated SrTiO3-based sensor. However, because of the low recovery rate, the regeneration of the sensor has to be made at high temperature for promoting the decomposition of the carbonates formed on the perovskite surface, which significantly reduce the advantage of operation at room temperature [81]. ST-based gas sensors have also been tested under exposure to volatile organic compounds (VOC), such as ethanol. Recently, Trabelsi et al. [82] presented the ethanol-sensing response of a SrTiO3−g-based gas sensor fabricated by a solid-state reaction enhanced in a thermally activated process. The electrical properties of the sensors were examined using impedance spectroscopy in a wide range of potential operating temperatures from −33 to 40 °C. The obtained SrTiO3-δ was characterized by low resistivity and low dielectric losses. The gas-sensing material revealed low-frequency relaxation processes and electrical conductivity resulting from the first ionization of oxygen vacancies. The sensors exhibited a decrease in conductivity after introducing ethanol gas (p-type semiconductor effect). Interesting is the fact that the optimal working temperatures for the tested samples are lower, about 350–370 °C, which suggests the dominant role of oxygen vacancies in detecting the presence of ethanol [82].

Apart from the conventional resistive-type SrTiO3-based gas sensors, ST was used as a gas-sensitive layer for fiber optic evanescent-wave hydrogen sensors, where SrTiO3 films were doped with lanthanum (La) [83]. These sensors show a rapid, reproducible sensing response to hydrogen fuel gas streams at elevated temperatures (600–800 °C). The presence of hydrogen results in a reversible and reproducible decrease in near-infrared transmission through the sensor. Sensors were also tested directly in the anode assembly of an operating solid oxide fuel cell (SOFC) with the sensor response correlating with both H2 concentration and SOFC cell potential. Sensors based on optical fiber platforms are well suited for harsh environment sensing applications and are safer than traditional resistive sensing technologies for combustible environments, showing the absence of an electrical current and therefore removing the risk of electrical interference to or from the operating fuel cell [83]. ST has been used as a gas-sensing material for microwave-based gas sensors since 2007, when Jouhannaud et al. [84] presented a coaxial structure operated in the 0.3 MHz–3 GHz frequency range under exposure to various species such as ethanol, the saturated vapor of water and toluene. The response of the sensor is quantitative and typical for each gas. This method of measurement leads to the development of an alternative to the current gas sensor. It has to be underlined that these results opened new research in gas-sensing measurements, i.e., operating at the microwave frequency range [85,86,87,88]. The examples of the SrTiO3-based gas sensors are presented in Figure 4.

Table A1 in Appendix A summarizes the information on the SrTiO3 sensor properties tests performed so far. The first column contains the material (chemical formula), the second one contains information about the operating temperature, the next one with the maximum response and in the following target gas, the sample thickness (if the authors provided such information), the deposition method and the literature reference in the last column. Table A1 summarizes the information contained in this section.

2.2. BaTiO3 for Gas-Sensing Applications

In 2004, K. Park and D.J. Seo [90] published a paper on the CO detection characteristics of BaTiO3-based gas sensors with graphite doping. It was found that the difference in resistivities measured at high temperatures (400 °C) in the air and CO gas atmosphere (5–100%) change with increasing graphite content. The porosity of the material also increases, which leads to an improvement in CO gas detection sensitivity. Greater porosity provides more sites on the sample surface for oxygen adsorption and the reaction between CO and O- gas, and between CO gas and O. The gas-sensing behavior for CO2 with the utilization of BST-based gas sensors has been studied as well, for example, by the utilization of BST and CuO [91], PbO [92] and LaCl3 [93]. In 2006, Mandayo, Gemma García et al. [94] developed a thin sensor film to detect CO2. The authors focused on response rates for various temperatures, target gases and relative humidity concentrations for indoor air quality (IAQ) monitoring.

Various attempts have been made to improve the selectivity and sensitivity of BaTiO3-based sensors using dopants and additives. For example, La has been proposed [95] to increase the response for NO2 and NH3 at room temperature. G.N. Chaudhary et al. [96] proposed thin-film BaTiO3 doped with CuO and CdO for liquefied petroleum gas (LPG) detection. The study showed that doping with various concentrations of CuO/CdO influenced the sensitivity. In addition, incorporation of a 0.3 wt.% Pd-doped CuO:CdO:BaTiO3 element showed high sensitivity with selectivity to the other gases including CO, H2 and H2S. In [62], the authors investigated the influence of the band gap, size and shape of a BT thin-film on the gas-sensing mechanism of H2S. The obtained sensors exhibited the highest response at 300 °C to 200 ppm of the target gas. Gas-sensing characteristics under exposure to H2S were measured by Huang, He-Ming, et al. [97]. In 2007, the authors reported that Ba0.99Ce0.01TiO3 sensors doped with Fe2O3 achieved response of 2.91 towards 0.4 ppm H2S at 150 °C (the sensor response is defined as RN2/RH2S, where RN2 and RH2S are the electrical resistances of the sensor in N2 and H2S, respectively). Additionally, fast response–recovery dynamics were obtained with 40 and 55 s for response and recovery times, respectively. The advantages of the above-mentioned BT composition can be summarized as a high response, lower operating temperature and fast response–recovery times, which make this material very attractive for industrial application. However, a long-term stability test has not been conducted to confirm these assumptions. BaTiO3-based gas sensors have also been applied as humidity detectors. In [98], the authors presented results on BaTiO3 nanoframes and TiO2-BaTiO3 nanotubes tested under various humidity concentrations (the samples were tested at RH from 15% to 95% at room temperature). The obtained results showed a high and reversible response towards different water concentrations. As a result, these sensors possess high stability, fast response times and reproducibility and could be used as humidity sensors.

The examples of BaTiO3-based gas sensors are presented in Figure 5. Table A2 in Appendix A is a summary of the information contained in the chapter on BaTiO3 sensor properties. It contains information on the test substance, operating temperature, maximum response, target gas, sample thickness, deposition method and references to papers by other authors.

2.3. BaSrTiO3 for Gas-Sensing Applications

In 2000, X.F. Chen et al. [99] studied the hydrogen gas sensitivity of sputtered amorphous Ba0.67Sr0.33TiO3 thin films. The BST material was prepared by using the RF magnetron co-sputtering process (200 nm, 300 °C, 1.8 Pa, 50% O2). The testing was carried out at one atmosphere in a measurement chamber with 500 sccm of target gas flow (1042 ppm of H2) in the operating temperature range from 80 to 250 °C. The authors concluded that higher sensitivity (defined as the ratio of the dc current, Igas/Iair ≈ 7.3) to H2 can be obtained in the temperature range of 170–190 °C. In another work [100], the authors proposed thin films of Cu-doped Ba0.75Sr0.25TiO3 for H2S detection. The gas-sensing layers were synthesized under hydrothermal conditions and further deposited via the RF sputtering technique onto Al2O3 substrates provided with Au electrodes and a Pt heater. The relative humidity influence (5%, 30%, 50%, 70%) on changes in the electrical resistance of BST with 5% Cu exposed to different concentrations of H2S (5, 10, 30, 50, 70, 90 ppm) at operating temperatures in the range from 100 to 400 °C was investigated. The authors reported that at 250 °C, 50% RH and 10 ppm of H2S concentration, Cu 5% BST showed good stability in electrical resistance: Rair = 4.25·1010 ± 20% Ω, RH2S = 5.61·108 ± 13.7% Ω (response S = Rair/RH2S, S = 75.76). Recently, results on the gas-sensing properties of BST: Ba0.5Sr0.5TiO3 (BST0.5) and Ba0.7Sr0.3TiO3 (BST0.7) thin films under H2S exposure (800 ppm) in the temperature range of 50–380 °C were presented in [70]. The gas sensors based on both BSTs, i.e., 0.5/0.5 and 0.7/0.3, reached the maximum response (57.57% and 41.61% for BST0.5 and BST0.7, respectively) at 330 °C. BST-based nanomaterials have also found application in the detection of NH3 gas [101,102,103]. In 2005, Somnath C. Roy et al. [101] proposed novel ammonia-sensing phenomena of Ba0.5Sr0.5TiO3 thin films obtained by the sol–gel spin coating technique. The deposited sensors were tested under exposure to different ammonia concentrations (160, 320, 640 and 1280 ppm). The sensitivity (S) was defined as S = ((Rg−Rair)/Rair), where Rair and Rg are the electrical resistances in air and the target gas, respectively. The obtained results showed that sensitivity S was found to be around 20% when thin films were exposed to NH3 at a concentration of 160 ppm and increased to 60% when the concentration was raised to 1280 ppm. The researchers tested BST thin films for cross-sensitivity to other gases such as CO, NO2 and ethanol. The obtained results showed no detectable resistance change when operated under conditions similar to those of ammonia sensing. NH3-sensing characteristics were measured at room temperature as well [69]. Ba0.75Sr0.25TiO3-based gas sensors were tested in the range of 30–110 ppm of ammonia with various relative humidity concentrations (10%–70%) at room temperature (23 °C). The authors defined the sensor signal S, calculated as the ratio of S = (Rair/Rgas), where Rair is the resistance in the humid background and Rgas is the resistance in the presence of NH3 dosed in the humid background. During these measurements, the humid background was set as 10%, 30%, 50% and 70%. The highest sensor response (S ≈ 2.5) was obtained for the 110 ppm concentration of NH3 and 50% RH. Therefore, the effect of the humidity influence opened the possibility to use BST gas sensors as humidity sensors [104,105,106]. For example, in [106], the temperature dependence of conductance and susceptance versus relative humidity (20–80%) for MgO-doped BST in the temperature range 10–60 °C was measured. The authors proposed the reaction mechanism model based on the impedance measurements, where a parallel combination of a resistor and a capacitor was used. Further work [104] focused on the influence of MgO doping (3% mol) on the structural and perceptible properties of BST0.5 in the humidity range of 20–95% in comparison to non-doped BST materials. The authors reported that the sensor including BST0.5 added with 3 mol% MgO exhibited improved humidity sensitivity and showed a faster response than pure BST0.5.

The examples of BaSrTiO3-based gas sensors are presented in Figure 6. A collective summary of the information contained in this chapter, on the sensor properties of BaSrTiO3, can be found in Table A3 in Appendix A, it contains basic information, such as: chemical formula of the tested compound, operating temperature, maximum response, target gas, information on sample thickness, deposition method and references to literature sources.

2.4. BaSrTiO3 for NO2 Detection

The utilization of BST-based gas sensors for nitrogen dioxide detection was previously studied by [107], as mentioned in Section 2.3. For example, in [100], the authors showed that the electrical resistance of BaSrTiO3 doped with 5% Cu increases during the exposition to NO2 at 3 ppm. Therefore, it could be suggested that BST Cu 5% behaves as an n-type semiconductor. However, in Figure 6, we present the results obtained for BST doped with Cu, where the electrical resistances decrease when NO2 was introduced to the measurement chamber. The BST-Cu gas-sensitive layer was deposited by the utilization of magnetron sputtering deposition technology. Firstly, the BST as a base material was deposited from the Ba0.6Sr0.4TiO3 homemade target, recently presented in [108], and then copper dopants were deposited. The deposition system for co-sputtering was previously presented in [109]. The pure BST-based gas sensor had base resistance around 5–6 GOhm, while after the Cu modification, the electrical resistance reduced to 8–9 MOhm at RT and the gas-sensing effect was achieved. Figure 7a shows the sensor response defined as R0/Rg, where R0 and Rg are electrical resistances measured in the synthetic air and target gas, respectively. The gas-sensing measurement setup was previously presented in [110]. As can be noticed, the highest responses were obtained around 250 °C, and therefore this temperature was set as the operating temperature. The NO2-sensing effect was measured in the 0.4–20 ppm range at a constant operating temperature (Figure 7b) and various RH concentrations (Figure 7c). As can be observed, the sensor reached the maximum response at 10 ppm, and further increasing the NO2 concentration did not change the response. However, due to the measurement setup limitations, the lowest NO2 concentration was 0.4 ppm, but it seems that the obtained sensors could detect lower concentrations, and the theoretical limit of detection was around 0.01 ppm. Finally, a multitest was conducted, where constant NO2 (20 ppm) and RH concentrations (30%, 50%, 70%) were kept to verify the stability. As is shown in Figure 7d, slight changes can be observed (~5%); however, what is interesting is that the electrical resistances decrease, suggesting p-type behavior, contrary to the results presented by Stanoiu et al. [100]. Therefore, additional measurements are needed.

3. Conclusions The gas sensor industry is constantly expecting novel materials that allow researchers to develop gas sensors with higher 3S parameters, known as sensitivity, selectivity and stability, while stability plays a crucial role in most industrial applications. Therefore, the materials that are characterized by longer stability are chosen as a first choice for gas-sensing layers, such as strontium titanate, barium titanate and barium strontium titanate. In this paper, the recently published results on the above-mentioned materials for gas-sensing applications are presented and discussed, including the recently obtained results by the authors of NO2 detection in the automotive application. The BST-Cu sensors exhibited the highest responses around 250 °C and worked well in the 0.4–20 ppm range; however, above 10 ppm, the sensor response achieved a constant value. The gas sensor response increased when relative humidity increased. At the same time, the sensor exhibited good repeatability, proven by the multitest, which confirms the possibility to utilize BST-Cu gas sensors in industrial applications.

Figure 1. Selected statistical data of the global gas sensor market: (a) share of major sectors of the economy in the gas detector market for Europe; (b) global market for gas sensors broken down into regions-prediction; APAC-Asia Pacific, RoW-rest of the world, 2019 e-estimation, 2024 p-prediction [7,8].

Figure 2. Non-polar structure at room temperature: (a) SrTiO3; (b) BaTiO3; (c) BaSrTiO3; (d) legend.

Figure 3. The number of papers related to SrTiO3-, BaTiO3- and BaSrTiO3-based gas sensors from 1980 to 2020.

Figure 4. Examples of SrTiO3-based gas sensors: (a) Sketch of the sensor structure fabricated in the HTCC (high-temperature cofired ceramics) technology. Alumina tapes were used as the substrate with SrTiO3 doped with 1 at.% Ta as the gas-sensitive layer. Reprinted with permission from [89] Copyright 2021 Elsevier. (b) Magnitude |A| of a SrTi0.65Fe0.35O3 thick-film sensor with dependence on the frequency of the oxygen partial pressure. This experiment was carried out to investigate the kinetic behavior of SrTi0.65Fe0.35O3. At 900 °C, response times of 10 ms can be achieved. Reprinted with permission from [77] Copyright 2021 Elsevier. (c) shows the graph of SrTi0.925Fe0.075O3 as a function of ozone concentration and operating temperature (250 °C). The measurement cycles consisted of the ozone exposure time-2 min. The sample looks like a p-type semiconductor (the resistance of the sample decreases as oxidizing gases are absorbed). The diagram shows that the sample responds well to O3, from a concentration of 75 ppb O3. On the other hand, it is saturated at a concentration higher than 525 ppb of ozone. The saturation effect is due to the limited number of absorption sites. Reprinted with permission from [78] Copyright 2021 Elsevier. (d) A simplified one-dimensional approach to modeling screen-printed thick-film sensors. The sensor is modeled in two steps, macroscopic and microscopic. The macroscopic part describes the gas transport through a gas-sensitive porous film. In their work, the authors calculated the appropriate profile of bracing for the one-dimensional geometry shown in the figure.

Figure 5. Examples of BaTiO3 gas sensors: (a) Stability and repeatability of the Fe2O3-Ba0.99Ce0.01TiO3 sensor up to 400 ppb H2S at 150 °C. To check the repeatability and stability of the sensor, tests were carried out on a fresh sensor, after one week and two months. The sensor was tested by alternating cycles of 400 ppb H2S and N2 at 150 °C. The tested sensor showed quite good stability and repeatability. The longer operation of the sensor at the temperature of 150 °C resulted in the disappearance of the upward drift of the base resistance, which contributed to the improvement of the sensor's performance. Reprinted with permission from [97] Copyright 2021 Elsevier. (b) Sensor response of the gas-sensing material based on BaTiO3 and CuO (1:1 molar ratio) with 10 wt. LaCl3 to various CO2 concentrations in the range 0.1-10 vol.%. at the operating temperature of 550 °C. Reprinted with permission from [93] Copyright 2021 Elsevier. (c) Detail of the BaTiO3 nanoprobe in contact with FIB nanolithography in a four-probe configuration. The insert in the upper left corner shows a low-magnification image of the same device. The platinum strips deposited with focused ion beam lithography (FIB) are clearly shown. The figure shows a prototype device created by integrating BaTiO3 nanorods using FIB nanolithography. The four-probe electrical measurements on the individual BaTiO3 nanorods on this sample showed resistivity values from 10 to 100 Ωcm, which is the typical value range for oxygen-deficient BaTiO3. (d) Scalable and reproducible response of a nanosensor sample made of BaTiO3 nanorods at room temperature to changes in air humidity (RH), with different responses to different levels of air humidity (100, 50 and 25%) in synthetic air. The inset shows the I-V curves obtained in dry and humid (100% RH) air.

Figure 6. Examples of BaSrTiO3 gas sensors: (a) The Ba0.5Sr0.5TiO3 film doped with RuO2 model (the result of the copper wire installation) of a sensor for bad breath gas analysis. (b) The physical appearance of a Ba0.5Sr0.5TiO3 film doped with a RuO2 probe of a sensor for bad breath gas analysis. (c) The gas-sensing response of the BaSrTiO3 gas sensor under exposure to NH3 (30-110 ppm) at various RH concentrations. Reprinted with permission from [69] Copyright 2021 Elsevier. (d) Sketch of the gas reaction model of BST-based gas sensors to various NH3 concentrations. This model showed the impact of RH capillary condensation towards NH3 detection. Reprinted with permission from [69] Copyright 2021 Elsevier.

Figure 7. The gas-sensing characteristics obtained for a BST-Cu-based gas sensor: (a) the sensor response in the wide temperature range under exposure to 20 ppm of NO2 and 50% of RH; (b) the sensor response in the 0.4-20 ppm range at constant operating temperature and 50% of RH (inset: a photo of the fabricated gas sensor); (c) the resistance changes of the developed gas sensors at various RH concentrations and 20 ppm of NO2; and (d) the multitest of the gas sensor response measured at 250 °C, 50% RH and 20 ppm of NO2.

Funding

The research leading to these results received funding from the Norway Grants 2014-2021 via the National Centre for Research and Development.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The work was carried out in the Biomarkers Analysis Laboratory AGH at the Institute of Electronics AGH.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Table

Table A1.The summaries of SrTiO3-based gas sensors.

Table A1.The summaries of SrTiO3-based gas sensors.

Material Operating Temp. Max. Response Target Gas Thickness Deposition Method Reference
SrTiO3250-350 °C0-34 1C2H5OH400 nmsol-gel[111]
SrTiO340 °C6 2O245 µmhigh-energy ball milling technique and thick-film screen printing technique[31,73]
SrTi0.925Fe0.075O3250 °C170-580 2O370 nmpolymeric precursor method[78]
SrTi0.9Fe0.1O3250 °C10 2O370 nmpolymeric precursor method[78]
SrTi0.85Fe0.15O3250 °C53 2O370 nmpolymeric precursor method[78]
SrTi0.85Fe0.15O3220 °C267 2O370 nmpolymeric precursor method[78]
SrTi0.85Fe0.15O3190 °C580 2O370 nmpolymeric precursor method[78]
SrTiO3350 °C27 2COthick filmscreen-printed on glass substrate in the desired pattern[79]
SrTiO3350 °C3 2H2Sthick filmscreen-printed on glass substrate in the desired pattern[79]
SrTiO3350 °C18 2NH3thick filmscreen-printed on glass substrate in the desired pattern[79]
SrTiO3 with modified surface300 °C7 2NH3thick filmscreen-printed on glass substrate in the desired pattern[79]
SrTiO3 with modified surface300 °C22 2H2Sthick filmscreen-printed on glass substrate in the desired pattern[79]
SrTiO3RT2% 3CO2thick filmcitrate-nitrate combustion synthesis method[81]
LaAlO3/SrTiO3RT, 80 °C650, 4000 4H2thin filmlaser molecular beam epitaxy[112]
Sr0.995La0.005TiO3850 °C4-8 5O2780 µmscreen printing technique[77]
SrTi0.65Fe0.35O3750-850 °C2 5O211 µmscreen printing technique[77]
Ta-doped SrTiO3700 °C0.6 6Hydrocarbon

ethan

20 µmhigh-temperature co-fired ceramic (HTCC) technology[89]
SrTiO340 °C6.35 2O2thick filmphysical high-energy ball milling technique[74]
SrTiO3150 °C550 7H2S65-75 µmsol-gel hydrothermal method[80]
SrTiO3350 °C100 7C2H5OH65-75 µmsol-gel hydrothermal method[80]
SrTiO3-δ (δ = 0.075 and 0.125) 350 °C16 1C2H5OH2 mmthe conventional solid-state reaction method[82]
SrTiO3RT41 6O250 nmpulsed laser deposition (PLD)[28]
0.14 at.% Nb:SrTiO3RT1.5 6O250 nmpulsed laser deposition (PLD)[28]
SrTiO3RT1000 6O220 nmatomic layer deposition (ALD)[113]
SrTiO3RT500 6O220 nmsuperposition and patterning[114]
SrTi0.8Fe0.2O3-δ400 °C12 6C3H810-20 µmscreen printing technique[75]
SrTi0.8Fe0.2O3-δ400 °C4.2 6NH310-20 µmscreen printing technique[75]
SrTi0.8Fe0.2O3-δ400 °C2.4 6NO10-20 µmscreen printing technique[75]
SrTi0.8Fe0.2O3-δ400 °C2.4 6CO10-20 µmscreen printing technique[75]
SrTi0.8Fe0.2O3-δ400 °C1.6 6H210-20 µmscreen printing technique[75]

1 (Rg-R0)/R0-in the paper, the authors used sensitivity calculated from the change in resistance, not a response, unit: a.u. 2 R0/Rg-Max. Response-the ratio of the sensor resistance without gas and in the presence of gas, unit: a.u. 3 ((C0-C)/C0)·100 %-in the paper, the authors used sensitivity calculated from the change in capacity, unit: %. 4 ((I0-I)/I0)·100 %-in the paper, the authors used sensitivity calculated from the change in current, unit: %. 5 G0/Gg-Max. Response-the ratio of the sensor conductivity without gas and in the presence of gas, unit: a.u. 6 Rg/R0-Max. Response-the ratio of the sensor resistance in the presence of gas and without gas, unit: a.u. 7 (Gg-G0)/G0-in the paper, the authors used sensitivity calculated from the change in conductance, not a response, unit: a.u.

Table

Table A2.The summaries of BaTiO3-based gas sensors.

Table A2.The summaries of BaTiO3-based gas sensors.

Material Operating Temp. Max. Response Target Gas Thickness Deposition Method Reference
BaTiO325 °C50 1humidity750 nmsol-gel processing[115]
BaTiO3300 °C300 2H2Sthick filmlow-temperature hydrothermal route[62]
BaTiO3RT116 3H2O21 µmelectroless deposition method[116]
PVDF-BaTiO3 compositeRT0.2416 4humidity150 nmmixing nanoparticles[117]
Modified BaTiO3350 °C1119 2H2S65-70 µmscreen printing technique[118]
Modified BaTiO3350 °C31 2NH365-70 µmscreen printing technique[118]
BaTiO3350 °C303 2LPGthin filmspray pyrolysis techniques[119]
Ce-doped BaTiO3200 °C2.99 5H2S500 nmcoprecipitation method[97]
Fe2O3-Ba0.99Ce0.01TiO3150 °C4.36 5H2S500 nmcoprecipitation method[97]
Fe2O3-Ba0.99Ce0.01TiO3150 °C2.91 5H2S500 nmcoprecipitation method[97]
BT-NH2 NPsRT5 6cysteine50 µmsolid-state reaction method[120]
BaTiO3280 °C2.9 7CO2thick filmscreen printing technology[121]
BaTiO3200 °C10 8CO255 nmRF sputtering[122]
BaTiO3RT17.6 9NOx350 nmsol-gel spin coating method[123]
TiO2-BaTiO3 nanotubesRT22.8 10humiditynanorodssol-gel electrophoretic deposition technique, electrochemical anodization technique[98]
BaTiO3-CuO300 °C1.8 7CO21 µmRF sputtering technique[94]
BaTiO3-CuO300 °C0.1 11CO21 µmRF sputtering technique[94]
BaTiO3-CuO400 °C1.12 7CO21 µmRF sputtering technique[94]
BaTiO3-CuO400 °C0.7 11CO21 µmRF sputtering technique[94]
CuO-BaTiO3 Ag doped430 °C1.59 7CO2thick filmconventional sintering method[124]
BaTiO3 La doped950 °C0.75 7O21 µmmagnetron sputtering[125]
BaTiO3RT22.8 10humiditynanorodssol-gel electrophoretic deposition[126]
BaTiO325 °C70 12humiditynanofiberelectrospinning[127]
CaO-BaTiO3160 °C0.68 13CO2thick filmtypical synthesis procedure[128]
BaTiO3-CuO-La2O3400 °C1.05 7CO2thick filmscreen printing technique and firing[93]
BaTiO3-CuO-LaCl3400 °C1.25 7CO2thick filmscreen printing technique and firing[93]
BaTiO3-LaCl3400 °C1.55 7CO2thick filmscreen printing and firing[93]
Ba0.999Sb0.001TiO3400 °C15 7CO1 mmadding graphite powders[90]
BaTiO325 °C3.5 7LPGthin filmsol-gel method[129]

1Z90%/Z40%-Max. Response-sensor impedance ratio at 90% and 40% air humidity, unit: a.u. 2 (Gg-G0)/G0-in the paper, the authors used sensitivity calculated from the change in conductance, not a response, unit: a.u. 3 the response current density unit: µA·cm-2. 4 unit: pF/%RH. 5 R0/Rg-Max. Response-the ratio of the sensor resistance without gas and in the presence of gas, unit: a.u. 6 I/I0-Max. Response-the ratio of the sensor current without cysteine and in the presence of cysteine, unit: a.u. 7 Rg/R0-Max. Response-the ratio of the sensor resistance in the presence of gas and without gas, unit: a.u. 8 ((I0-I)/I0)·100 %-in the paper, the authors used sensitivity calculated from the change in current, unit: %. 9 (R0-Rg)/R0-in the paper, the authors used sensitivity calculated from the change in resistance, not a response, unit: a.u. 10 R0%/R100%-Max. Response-sensor resistance ratio at 0% and 100% air humidity, unit: a.u. 11 Cg/C0-Max. Response-the ratio of the sensor capacitance without gas and in the presence of gas, unit: a.u. 12 Z11%/Z95%-Max. Response-sensor impedance ratio at 11% and 95% air humidity, unit: a.u. 13 (Rg-R0)/R0-in the paper, the authors used sensitivity calculated from the change in resistance, not a response, unit: a.u.

Table

Table A3.The summaries of BaSrTiO3-based gas sensors.

Table A3.The summaries of BaSrTiO3-based gas sensors.

Material Operating Temp. Max. Response Target Gas Thickness Deposition Method Reference
BaTiO3/ SrTiO3250-350 °C10-100 1C2H5OH400 nmsol-gel method[111]
Cu mol. 5%-doped perovskite material Ba0.75Sr0.25TiO3250 °C400 2H2S386.4 nmRF sputtering technique[100]
Co-doped BaSrTiO3RT35 3CO2thin filmRF sputtering technique[130]
Ba0.5Sr0.5TiO3270 °C60 1

40 1

30 1

NH3150 nm

320 nm

480 nm

sol-gel method[103]
Cu mol. 5%-doped BaSrTiO3200 °C15 2H2Sthick filmhydrothermal method[68]
Cu mol. 0.1%-doped BaSrTiO3150 °C5 2NH3thick filmhydrothermal method[68]
Sr 0.2-doped BaTiO3RT100 4NO2thick filmlow-temperature hydrothermal route[107]
Sr 0.2-doped BaTiO3RT55 4NH3thick filmlow-temperature hydrothermal route[107]
Sr 0.2-doped BaTiO3400 °C28 4H2Sthick filmlow-temperature hydrothermal route[107]
Sr 0.4-doped BaTiO3400 °C2 4LPGthick filmlow-temperature hydrothermal route[107]
Ba0.67Sr0.33TiO3205 °C34 5H2200 nmRF magnetron co-sputtering[99]
(Ba0.87Sr0.13)TiO3300 °C29 2

85 2

62 2

56 2

NH317 µm

33 µm

48 µm

63 µm

screen printing technique[101]
(Ba0.87Sr0.13)TiO3300 °C26 2H2S33 µmscreen printing technique[101]
Cr2O3-modified (Ba0.87Sr0.13)TiO3350 °C16.6 2NH333 µmscreen printing technique[131]
Cr2O3-modified (Ba0.87Sr0.13)TiO3350 °C73 2H2S33 µmscreen printing technique[131]
Ba0.67Sr0.33TiO3350 °C10 4H2S65-70 µmscreen printing technique[132]
Cu-doped Ba0.67Sr0.33TiO3350 °C26.5 4H2S65-70 µmscreen printing technique[132]
Cr-doped Ba0.67Sr0.33TiO3350 °C46.4 4H2S65-70 µmscreen printing technique[132]
Ba0.998Sr0.002 TiO3400 °C25 4H2Sthick filmhydrothermal method[133]
Ba0.998Sr0.002 TiO3 in 2% Sn dipping for 20min200 °C2274 4H2Sthick filmhydrothermal method[133]
Ba0.5Sr0.5TiO3 330 °C57.57 5H2Sthin filmpulse laser deposition (PLD) technique[70]
Ba0.7Sr0.3TiO3330 °C41.61 5H2Sthin filmpulse laser deposition (PLD) technique[70]
Ba0.75Sr0.25TiO3RT2.5 2NH350 µmhydrothermal method[69]

1 (Rg-R0)/R0-where Rg and R0 are the electrical resistances measured in the presence of gas and synthetic air, respectively. 2 R0/Rg. 3 Rg/R0. 4 (Gg-G0)/G0-where Gg and G0 are the electrical conductances measured in the presence of gas and synthetic air, respectively. 5 Ig/I0-where Ig and I0 are the electrical currents measured in the presence of gas and synthetic air, respectively. 6 (Rg-R0)/R0·100%.

Table

Table A4.Comparison between three nanocomposites.

Table A4.Comparison between three nanocomposites.

Material Properties SrTiO3 BaTiO3 BaSrTiO3
Type of structuresimple cubic perovskite structure [24]cubic perovskite-type structure [46]perovskite structure [65,66]
Dielectric constant ɛ0ɛ0 = 300 [25,42]dielectric constant depends on the type of synthesis, temperature, frequency and dopants [47,48]

At RT ɛ0 = 2570 [134]

ɛ0 = 420 [135]
Dielectric lossmostly <0.02 [25]0.003 [134]0.017 [135]
Application of the material- sensors

- actuators

- electro-optical devices

- memory devices with random access

- multilayer capacitors [27]

- oxygen sensors [28]

- temperature sensors [29]

- cantilever base for various sensors [30]

- ferroelectric memories [56]

- electro-optical devices [57]

- dielectric capacitors [58]

- multilayer capacitors (MLCs) [59]

- electromechanical transducers [60,61]

- gas sensor applications [62]

- electronic components

- ferroelectric memories

- capacitors

- phase shifters [65,66]

- gas-sensitive material [67]

- multilayer and voltage-tunable capacitors

- dynamic random access memories (DRAM) [136]
Deposition method- magnetron sputtering [32,33]

- atomic layer deposition (ALD) [34,35]

- pulsed laser deposition (PLD) [36,37]

- metal-organic chemical vapor deposition (MOCVD) [38,39]

- laser chemical vapor deposition (LCVD) [40,41],

- sol-gel method [42,43]

- solid-state reaction [50]

- sol-gel method [51]

- hydrothermal method [52]

- coprecipitation method [53]

- polymeric precursor method [54]

- mechanochemical synthesis [55]

- spin coating [67]

- pulsed laser deposition (PLD) [70]

- chemical solution deposition (CSD) [71]

- RF magnetron co-sputtering process [99]

- RF sputtering technique [94]

- hydrothermal method [106]

- screen printing technique [104]

- mechanochemical process [132]
Sensitivity effect topropane [76]

propen [76]

NO [76]

O2 [77]

O3 [78]

CO [79]

H2S [80]

CO2 [81]

volatile organic compounds (VOC), such as ethanol [82]

CO [90]

CO2 [94]

NO2 [95]

NH3 [95]

LPG [96]

H2 [96]

H2S [96,97]

Humidity [98]

NOx [123]

NH3 [68,69,101,102]

H2S [68,70,100]

H2 [99]

Humidity [104,105,106]

NO2 [100,107]

LPG [107]
Other advantageslow cost and strong stability in thermal and chemical atmospheres [26,42]

fiber optic evanescent-wave hydrogen sensors [83]

operating solid oxide fuel cell (SOFC) [83]

microwave-based gas sensors [84]

large electro-optic coefficients,

positive coefficient of resistivity (PTCR) [49]

high thermal and chemical stability [100], good tenability

tunable filters,

oscillators

microwave phase

shifters

uncooled infrared sensors, etc. [136]

1. Carpenter, M.A.; Mathur, S.; Kolmakov, A. Metal Oxide Nanomaterials for Chemical Sensors; Springer: New York, NY, USA, 2013.

2. Morrison, S.R. Surface-barrier effects in adsorption, illustrated by zinc oxide. Adv. Catal. 1955, 7, 259-301.

3. The History of Figaro Engineering. Available online: https://www.figaro.co.jp/en/company/history.html (accessed on 25 November 2020).

4. Nasiri, N.; Clarke, C. Nanostructured Gas Sensors for Medical and Health Applications: Low to High Dimensional Materials. Biosensors 2019, 9, 43.

5. Schedin, F.; Geim, A.K.; Morozov, S.V.; Hill, E.W.; Blake, P.; Katsnelson, M.I.; Novoselov, K.S. Detection of individual gas molecules adsorbed on graphene. Nat. Mater. 2007, 6, 652-655.

6. Gas Sensor Market by Gas Type (Carbon Monoxide, Methane, Hydrogen, Ammonia, Oxygen, and Others), Technology (In-frared Gas Sensor, Photo Ionization Sensor, Electrochemical Gas Sensor, Thermal Conductivity Gas Sensor, Metal Oxide-Based Gas Sensor, Catalytic Gas Sensor, and Others) and End Use (Defense & Military, Healthcare, Consumer Electronics, Automotive & Transportation, Industrial, and Others): Global Opportunity Analysis and Industry Forecast, 2020-2027. Available online: https://www.alliedmarketresearch.com/gas-sensors-market (accessed on 23 November 2020).

7. Gas Sensor Market Size, Share & Trends Analysis Report by Product (NOx, CO2, O2, CO), By Type, By Technology (Electro-chemical, Infrared), By End-use (Environmental, Industrial), By Region, And Segment Forecasts, 2020-2027. Available online: https://www.grandviewresearch.com/industry-analysis/gas-sensors-market (accessed on 23 November 2020).

8. Gas Sensors Market by Gas Type (Oxygen, Carbon Monoxide, Carbon Dioxide, Ammonia, Chlorine, Hydrogen Sulfide, Ni-trogen Oxide, Volatile Organic Compounds, Hydrocarbons), Technology, Application, Geography-Global Forecast 2024. Available online: https://www.marketsandmarkets.com/Market-Reports/gas-sensor-market-245141093.html (accessed on 23 November 2020).

9. Moseley, P.T. Solid state gas sensors. Meas. Sci. Technol. 1997, 8, 223-237.

10. Dai, J.; Ogbeide, O.; Macadam, N.; Sun, Q.; Yu, W.; Li, Y.; Su, B.-L.; Hasan, T.; Huang, X.; Huang, W. Printed gas sensors. Chem. Soc. Rev. 2020, 49, 1756-1789.

11. Gomes, J.B.A.; Rodrigues, J.J.P.C.; Rabêlo, R.A.L.; Kumar, N.; Kozlov, S. IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities. J. Sens. Actuator Netw. 2019, 8, 57.

12. Kumar, A.; Kingson, T.M.; Verma, R.P.; Mandal, R.; Dutta, S.; Chaulya, S.K.; Prasad, G. Application of Gas Monitoring Sensors in Underground Coal Mines and Hazardous Areas. Int. J. Comput. Technol. Electron. Eng. 2013, 3, 9-23.

13. Briand, D.; Oprea, A.; Courbat, J.; Bârsan, N. Making environmental sensors on plastic foil. Mater. Today 2011, 14, 416-423.

14. Bogue, R.W.; Bogue, R. Technology Roadmap: Optoelectronic Gas Sensors in the Petrochemicals, Gas and Water Industries. Optocem.net. 2006. Available online: http://userweb.eng.gla.ac.uk/charles.ironside/QCSENSEPrivate/GasSensinRoadmap.pdf (accessed on 23 November 2020).

15. Papadakis, N. Solid State Transcutaneous Blood Gas Sensors. U.S. Patent 4534356A, 13 August 1985.

16. Tripathi, K.M.; Sachan, A.; Castro, M.; Choudhary, V.; Sonkar, S.; Feller, J.-F. Green carbon nanostructured quantum resistive sensors to detect volatile biomarkers. Sustain. Mater. Technol. 2018, 16, 1-11.

17. Tripathi, K.M.; Kim, T.; Losic, D.; Tung, T.T. Recent advances in engineered graphene and composites for detection of volatile organic compounds (VOCs) and non-invasive diseases diagnosis. Carbon 2016, 110, 97-129.

18. Riegel, J.; Neumann, H.; Wiedenmann, H. Exhaust gas sensors for automotive emission control. Solid State Ion. 2002, 152, 783-800.

19. Moos, R. A Brief Overview on Automotive Exhaust Gas Sensors Based on Electroceramics. Int. J. Appl. Ceram. Technol. 2005, 2, 401-413.

20. Zohora, S.E.; Hundewale, N. Electronic Noses Application to Food Analysis Using Metal Oxide Sensors: A Review. Int. J. Soft Comput. Eng. (IJSCE) 2013, 3, 199-205.

21. Lee, D.D.; Lee, D.S. Environmental Gas Sensors. Sens. J. IEEE 2001, 1, 214-224.

22. Seesaard, T.; Seaon, S.; Khunarak, C.; Lorwongtragool, P.; Kerdcharoen, T. A novel creation of thread-based ammonia gas sensors for wearable wireless security system. In Proceedings of the 11th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON 2014), Nakhon Ratchasima, Thailand, 14-17 May 2014; pp. 1-4.

23. Eranna, G. Metal Oxide Nanostructures as Gas Sensing Devices; CRC Press: Boca Raton, FL, USA, 2016.

24. Advanced Si Pad Detector Development and SrTiO3 Studies by Emission Channeling and Hyperfine Interaction Experiment. Available online: http://repositorio.ul.pt/bitstream/10451/1643/10/19741_ulsd_re487_10_Chapter2.pdf (accessed on 23 November 2020).

25. Wang, Z.; Cao, M.; Yao, Z.; Zhang, Q.; Song, Z.; Hu, W.; Xu, Q.; Hao, H.; Liu, H.; Yu, Z. Giant permittivity and low dielectric loss of SrTiO3 ceramics sintered in nitrogen atmosphere. J. Eur. Ceram. Soc. 2014, 34, 1755-1760.

26. Trabelsi, H.; Bejar, M.; Dhahri, E.; Graça, M.P.F.; Valente, M.A.; Soares, M.J.; Sobolev, N.A. Raman, EPR and ethanol sensing properties of oxygen-Vacancies SrTiO3-δ compounds. Appl. Surf. Sci. 2017, 426, 386-390.

27. Pongthippitak, B.; Wongtha, P.; Reabroy, P.; Kunsuwan, P.; Thountom, S. Dielectric properties of SrTiO3 ceramics synthesis by hybrid method. J. Phys. Conf. Ser. 2018, 1144, 012162.

28. Hara, T.; Ishiguro, T.; Wakiya, N.; Shinozaki, K. Oxygen Sensing Properties of SrTiO3 Thin Films. Jpn. J. Appl. Phys. 2008, 47, 7486-7489.

29. Cheng, P.; Wang, L.; Pan, Y.; Yan, H.; Gao, D.; Wang, J.; Zhang, H. Fiber Bragg grating temperature sensor of cladding with SrTiO3 thin film by pulsed laser deposition. Laser Phys. 2019, 29, 025107.

30. Biasotti, M.; Pellegrino, L.; Buzio, R.; Bellingeri, E.; Bernini, C.; Siri, A.S.; Marré, D. Fabrication and electromechanical actuation of epitaxial SrTiO3 (0 0 1) microcantilevers. J. Micromech. Microeng. 2013, 23, 035031.

31. Hu, Y.; Tan, O.K.; Pan, J.S.; Huang, H.; Cao, W. The effects of annealing temperature on the sensing properties of low tem-perature nano-sized SrTiO3 oxygen gas sensor. Sens. Actuators B Chem. 2005, 108, 244-249.

32. Ma, J.H.; Huang, Z.M.; Meng, X.; Liu, S.J.; Zhang, X.; Sun, J.L.; Xue, J.Q.; Chu, J.; Li, J. Optical properties of SrTiO3 thin films deposited by radio-frequency magnetron sputtering at various substrate temperatures. J. Appl. Phys. 2006, 99, 033515.

33. Wang, Z.; Kugler, V.; Helmersson, U.; Konofaos, N.; Evangelou, E.K.; Nakao, S.; Jin, P. Electrical properties of SrTiO3 thin films on Si deposited by magnetron sputtering at low temperature. Appl. Phys. Lett. 2001, 79, 1513-1515.

34. Lee, S.W.; Han, J.H.; Han, S.; Lee, W.; Jang, J.H.; Seo, M.; Kim, S.K.; Dussarrat, C.; Gatineau, J.; Min, Y.-S.; et al. Atomic Layer Deposition of SrTiO3 Thin Films with Highly Enhanced Growth Rate for Ultrahigh Density Capacitors. Chem. Mater. 2011, 23, 2227-2236.

35. Hara, T.; Ishiguro, T. Oxygen sensitivity of SrTiO3 thin film prepared using atomic layer deposition. Sens. Actuators B Chem. 2009, 136, 489-493.

36. Du, Y.; Zhang, M.-S.; Wu, J.; Kang, L.; Yang, S.; Wu, P.; Yin, Z. Optical properties of SrTiO3 thin films by pulsed laser deposition. Appl. Phys. A 2003, 76, 1105-1108.

37. Deng, Y.; Du, Y.; Zhang, M.-S.; Han, J.; Yin, Z. Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition. Solid State Commun. 2005, 135, 221-225.

38. Ohsato, H.; Ohhashi, T.; Nishigaki, S.; Okuda, T.; Sumiya, K.; Suzuki, S. Formation of Solid Solutions of New Tungsten Bronze-Type Microwave Dielectric Compounds Ba6-3xR8+2xTi18O54(R=Nd and Sm, 0≤x≤1). Jpn. J. Appl. Phys. 1993, 32, 4323-4326.

39. Shreiber, D.; Cole, M.W.; Enriquez, E.; Hirsch, S.G.; Ngo, E.; Hubbard, C.; Ivill, M.; Chen, C. Some unusual behavior of dielectric properties of SrTiO3 metal organic chemical vapor deposition grown thin films. J. Appl. Phys. 2014, 116, 094101.

40. Chen, J.; Ito, A.; Goto, T. High-speed epitaxial growth of SrTiO3 films on MgO substrates by laser chemical vapor deposition. Ceram. Int. 2016, 42, 9981-9987.

41. Chen, J.; Ito, A.; Goto, T. High-speed epitaxial growth of SrTiO3 transparent thick films composed of close-packed nanocolumns using laser chemical vapor deposition. Vacuum 2020, 177, 109424.

42. Kamalasanan, M.N.; Deepak Kumar, N.D.; Chandra, S. Structural, optical, and dielectric properties of sol-gel derived SrTiO3 thin films. J. Appl. Phys. 1993, 74, 679-686.

43. Diao, C.; Li, H.; Yang, Y.; Hao, H.; Yao, Z.; Liu, H. Significantly improved energy storage properties of sol-gel derived Mn-modified SrTiO3 thin films. Ceram. Int. 2019, 45, 11784-11791.

44. Rydosz, A.; Szkudlarek, A.; Ziabka, M.; Domanski, K.; Maziarz, W.; Pisarkiewicz, T. Performance of Si-doped WO3 thin films for acetone sensing prepared by glacing angle DC magnetron sputtering. IEEE Sens. J. 2016, 16, 1004-1012.

45. Rydosz, A.; Dyndal, K.; Andrysiewicz, W.; Grochala, D.; Marszalek, K. GLAD Magnetron Sputtered Ultra-Thin Copper Oxide Films for Gas-Sensing Application. Coatings 2020, 10, 378.

46. Buscaglia, M.T.; Buscaglia, V.; Viviani, M.; Petzelt, J.; Savinov, M.; Mitoseriu, L.; Testino, A.; Nanni, P.; Harnagea, C.; Zhao, Z.; et al. Ferroelectric properties of dense nanocrystalline BaTiO3 ceramics. Nanotechnology 2004, 15, 1113-1117.

47. Hsiao-Lin, W. Structure and Dielectric Properties of Perovskite-Barium Titanate (BaTiO3); Submitted in Partial Fulfillment of Course Requirement for MatE 115; San Jose State University: San Jose, CA, USA, 2002; pp. 3-9.

48. Vijatović, M.M.; Bobić, J.D.; Stojanović, B.D. History and challenges of barium titanate: Part II. Sci. Sinter. 2008, 40, 235-244.

49. Ertuğ, B. The overview of the electrical properties of barium titanate. Am. J. Eng. Res. 2013, 2, 1-7.

50. Potdar, H.S.; Deshpande, S.B.; Date, S.K. Chemical coprecipitation of mixed (Ba+Ti) oxalates precursor leading to BaTiO3 powders. Mater. Chem. Phys. 1999, 58, 121-127.

51. Wang, L.; Liu, L.; Xue, D.; Kang, H.; Liu, C. Wet routes of high purity BaTiO3 nanopowders. J. Alloy. Compd. 2007, 440, 78-83.

52. Boulos, M.; Guillemetfritsch, S.; Mathieu, F.; Durand, B.; Lebey, T.; Bley, V. Hydrothermal synthesis of nanosized BaTiO3 powders and dielectric properties of corresponding ceramics. Solid State Ion. 2005, 176, 1301-1309.

53. Prasadarao, A.V.; Suresh, M.; Komarneni, S. pH dependent coprecipitated oxalate precursors-A thermal study of barium titanate. Mater. Lett. 1999, 39, 359-363.

54. Cho, W.-S.; Hamada, E. Synthesis of ultrafine BaTiO3 particles from polymeric precursor: Their structure and surface property. J. Alloy. Compd. 1998, 266, 118-122.

55. Stojanović, B.D.; Jovalekić, Č.; Vukotic, V.; Simões, A.Z.; Varela, J.A. Ferroelectric Properties of Mechanically Synthesized Nanosized Barium Titanate. Ferroelectrics 2005, 319, 65-73.

56. Scott, J.F. Ferroelectric Memories; Springer Science & Business Media: Berlin, Germany, 2000; Volume 3.

57. Abel, S.; Caimi, D.; Sousa, M.; Stöferle, T.; Rossel, C.; Marchiori, C.; Fompeyrine, J. Electro-optical properties of barium titanate films epitaxially grown on silicon. In Oxide-Based Materials and Devices III; International Society for Optics and Photonics, SPIE OPTO: San Francisco, CA, USA, 2012; Volume 8263, p. 82630Y.

58. Yanase, N.; Abe, K.; Fukushima, N.; Kawakubo, T. Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors. Jpn. J. Appl. Phys. 1999, 38, 5305-5308.

59. Li, W.-B.; Zhou, D.; Xu, R.; Wang, D.-W.; Su, J.-Z.; Pang, L.-X.; Liu, W.-F.; Chen, G.-H. BaTiO3-Based Multilayers with Outstanding Energy Storage Performance for High Temperature Capacitor Applications. ACS Appl. Energy Mater. 2019, 2, 5499-5506.

60. Ramadan, K.S.; Sameoto, D.; Evoy, S. A review of piezoelectric polymers as functional materials for electromechanical transducers. Smart Mater. Struct. 2014, 23, 033001.

61. Liu, Q.-J.; Zhang, N.-C.; Liu, F.-S.; Wang, H.-Y.; Liu, Z.-T. BaTiO3: Energy, geometrical and electronic structure, relationship between optical constant and density from first-principles calculations. Opt. Mater. 2013, 35, 2629-2637.

62. Patil, R.P.; More, P.V.; Jain, G.H.; Khanna, P.K.; Gaikwad, V.B. BaTiO3 nanostructures for H2S gas sensor: Influence of band-gap, size and shape on sensing mechanism. Vacuum 2017, 146, 455-461.

63. La Mattina, F. Insulator-to-metal transition and resistive memory switching in Cr-doped SrTiO3: Charge-transfer processes involving the Cr ions. Ph.D. Thesis, Faculty of Science, University of Zurich, Zurich, Switzerland, 2008.

64. Ahmed, A.; Goldthorpe, I.A.; Khandani, A.K. Electrically tunable materials for microwave applications. Appl. Phys. Rev. 2015, 2, 011302.

65. Liu, Y.; Nagra, A.S.; Erker, E.G.; Periaswamy, P.; Taylor, T.R.; Speck, J.; York, R.A. BaSrTiO3 interdigitated capacitors for distributed phase shifter applications. IEEE Microw. Guid. Wave Lett. 2000, 10, 448-450.

66. Moon, S.E.; Ryu, H.-C.; Kwak, M.H.; Kim, Y.T.; Lee, S.-J.; Kang, K.-Y. X-Band Phased Array Antenna Using Ferroelectric (Ba, Sr) TiO3 Coplanar Waveguide Phase Shifter. ETRI J. 2005, 27, 677-684.

67. Romh, M.E.; Fasquelle, D.; Députier, S.; Mascot, M. Elaboration and characterization of doped barium titanate films for gas sensing. AIP Conf. Proc. 2014, 1627, 25-30.

68. Simion, C.E.; Sackmann, A.; Teodorescu, V.S.; Ruşti, C.F.; Piticescu, R.M.; Stănoiu, A. Tuned sensitivity towards H2S and NH3 with Cu doped barium strontium titanate materials. AIP Conf. Proc. 2014, 1627, 92-97.

69. Simion, C.E.; Sackmann, A.; Teodorescu, V.S.; Ruşti, C.F.; Stanoiu, A. Room temperature ammonia sensing with barium strontium titanate under humid air background. Sens. Actuators B Chem. 2015, 220, 1241-1246.

70. Shastri, N.M.; Raval, A.V.; Joshi, V.G. Optical and Gas sensing properties of BaxSr1-xTiO3 (with x= 0.5 and x= 0.7) thin films grown using Pulse Laser Deposition. IOP Conf. Ser. Mater. Sci. Eng. 2020, 814, 012036.

71. Siskandar, R.; Yuliarto, B.; Zakki Fahmi, M. Application of Ba0. 5Sr0. 5TiO3 (BST) Film Doped with 0%, 2%, 4% and 6% Con-centrations of RuO2 as an Arduino Nano-Based Bad Breath Sensor. Chemosensors 2020, 8, 3.

72. Meyer, R.; Waser, R. Resistive donor-doped SrTiO3 sensors: I, basic model for a fast sensor response. Sens. Actuators B Chem. 2004, 101, 335-345.

73. Hu, Y.; Tan, O.K.; Cao, W.; Zhu, W. A low temperature nano-structured SrTiO3 thick film oxygen gas sensor. Ceram. Int. 2004, 30, 1819-1822.

74. Hu, Y.; Tan, O.K.; Pan, J.S.; Yao, X. A New Form of Nanosized SrTiO3 Material for Near-Human-Body Temperature Oxygen Sensing Applications. J. Phys. Chem. B 2004, 108, 11214-11218.

75. Sahner, K.; Gouma, P.I.; Moos, R. Electrodeposited and Sol-gel Precipitated p-type SrTi1-xFexO3-δ Semiconductors for Gas Sensing. Sensors 2007, 7, 1871-1886.

76. Sahner, K.; Moos, R. Modeling of hydrocarbon sensors based on p-type semiconducting perovskites. Phys. Chem. Chem. Phys. 2006, 9, 635-642.

77. Menesklou, W.; Schreiner, H.-J.; Härdtl, K.H.; Ivers-Tiffée, E. High temperature oxygen sensors based on doped SrTiO3. Sens. Actuators B Chem. 1999, 59, 184-189.

78. Mastelaro, V.R.; Zílio, S.C.; da Silva, L.F.; Pelissari, P.I.; Bernardi, M.I.; Guerin, J.; Aguir, K. Ozone gas sensor based on nano-crystalline SrTi1- xFexO3 thin films. Sens. Actuators B Chem. 2013, 181, 919-924.

79. Kajale, D.D.; Gaikwad, V.B.; Jain, G.H.; Khanna, P.K. Gas sensing performance of pure and modified SrTiO3 thick film resistors. In Proceedings of the 3rd International Conference on Sensing Technology, Tainan, Taiwan, 30 November-3 December 2008; IEEE: Piscataway, NJ, USA, 2008; pp. 422-425.

80. Kajale, D.D.; Patil, G.E.; Gaikwad, V.B.; Shinde, S.D.; Chavan, D.N.; Pawar, N.K.; Jain, G.H. Synthesis of SrTiO3 nanopowder by sol-gel-hydrothermal method for gas sensing application. Int. J. Smart Sens. Intell. Syst. 2012, 5.

81. Zaza, F.; Orio, G.; Serra, E.; Caprioli, F.; Pasquali, M. Low-temperature capacitive sensor based on perovskite oxides. AIP Conf. Proc. 2015, 1667, 020004.

82. Trabelsi, H.; Bejar, M.; Dhahri, E.; Valente, M.A.; Graça, M.P.F. Oxygen-vacancy-related giant permittivity and ethanol sensing response in SrTiO3-δ ceramics. Phys. E Low-Dimens. Syst. Nanostruct. 2019, 108, 317-325.

83. Schultz, A.M.; Brown, T.D.; Buric, M.P.; Lee, S.; Gerdes, K.; Ohodnicki, P.R. High temperature fiber-optic evanescent wave hydrogen sensors using La-doped SrTiO3 for SOFC applications. Sens. Actuators B Chem. 2015, 221, 1307-1313.

84. Jouhannaud, J.; Rossignol, J.; Stuerga, D. Metal oxide-based gas sensor and microwave broad-band measurements: An innovative approach to gas sensing. J. Physics Conf. Ser. 2007, 76, 012043.

85. Sorocki, J.; Rydosz, A.; Staszek, K. Wideband microwave multiport-based system for low gas concentration sensing and its application for acetone detection. Sens. Actuators B Chem. 2020, 323, 128710.

86. Staszek, K.; Szkudlarek, A.; Kawa, M.; Rydosz, A. Microwave system with sensor utilizing GO-based gas-sensitive layer and its application to acetone detection. Sens. Actuators B Chem. 2019, 297, 126699.

87. Rydosz, A.; Staszek, K.; Brudnik, A.; Gruszczynski, S. Tin Dioxide Thin Film with UV-enhanced Acetone Detection in Microwave Frequency Range. Micromachines 2019, 10, 574.

88. Rydosz, A.; Brudnik, A.; Staszek, K. Metal Oxide Thin Films Prepared by Magnetron Sputtering Technology for Volatile Organic Compound Detection in the Microwave Frequency Range. Materials 2019, 12, 877.

89. Sahner, K.; Fleischer, M.; Magori, E.; Meixner, H.; Deerberg, J.; Moos, R. HC-sensor for exhaust gases based on semiconducting doped SrTiO3 for On-Board Diagnosis. Sens. Actuators B Chem. 2006, 114, 861-868.

90. Park, K.; Seo, D.J. Gas sensing characteristics of BaTiO3-based ceramics. Mater. Chem. Phys. 2004, 85, 47-51.

91. Liao, B.; Wei, Q.; Wang, K.; Liu, Y. Study on CuO-BaTiO3 semiconductor CO2 sensor. Sens. Actuators B Chem. 2001, 80, 208-214.

92. Ishihara, T.; Kometani, K.; Hashida, M.; Takita, Y. Mixed oxide capacitor of BaTiO3-PbO as a new type CO2 gas sensor. Chem. Lett. 1990, 19, 1163-1166.

93. Lee, M.-S.; Meyer, J.-U. A new process for fabricating CO2-sensing layers based on BaTiO3 and additives. Sens. Actuators B Chem. 2000, 68, 293-299.

94. Mandayo, G.G.; Gonzalez, F.; Rivas, I.; Ayerdi, I.; Herrán, J. BaTiO3-CuO sputtered thin film for carbon dioxide detection. Sens. Actuators B Chem. 2006, 118, 305-310.

95. Patil, R.P.; Hiragond, C.; Jain, G.H.; Khanna, P.K.; Gaikwad, V.B.; More, P.V. La doped BaTiO3 nanostructures for room temperature sensing of NO2/NH3: Focus on La concentration and sensing mechanism. Vacuum 2019, 166, 37-44.

96. Chaudhari, G.N.; Bambole, D.R.; Bodade, A.B. Structural and gas sensing behavior of nanocrystalline BaTiO3 based liquid petroleum gas sensors. Vacuum 2006, 81, 251-256.

97. Huang, H.-M.; Li, H.-Y.; Wang, X.-X.; Guo, X. Detecting low concentration of H2S gas by BaTiO3 nanoparticle-based sensors. Sens. Actuators B Chem. 2017, 238, 16-23.

98. Soderznik, K.Z.; Fabrega, C.; Hernandez-Ramirez, F.; Prades, J.D.; Čeh, M. BaTiO3 Based Nanostructures for Humidity Sensing Applications. Proceedings 2019, 15, 5009.

99. Chen, X.F.; Zhu, W.G.; Tan, O.K. Microstructure, dielectric properties and hydrogen gas sensitivity of sputtered amorphous Ba0.67Sr0.33TiO3 thin films. Mater. Sci. Eng. B 2000, 77, 177-184.

100. Stanoiu, A.; Piticescu, R.M.; Simion, C.E.; Rusti-Ciobota, C.F.; Florea, O.G.; Teodorescu, V.S.; Badilita, V. H2S selective sensi-tivity of Cu doped BaSrTiO3 under operando conditions and the associated sensing mechanism. Sens. Actuators B Chem. 2018, 264, 327-336.

101. Jain, G.H.; Patil, L.A. Gas sensing performance of barium strontium titanate thick film resistors. J. Physics Conf. Ser. 2007, 76, 1.

102. Kumar, M.; Rani, S.; Bhatnagar, M.C.; Roy, S.C. Structure, ferroelectric and gas sensing properties of sol-gel derived (Ba,Sr)(Ti,Zr)O3 thin films. Mater. Chem. Phys. 2008, 107, 399-403.

103. Roy, S.C.; Sharma, G.L.; Bhatnagar, M.C.; Samanta, S.B. Novel ammonia-sensing phenomena in sol-gel derived Ba0.5Sr0.5TiO3 thin films. Sens. Actuators B Chem. 2005, 110, 299-303.

104. Agarwal, S.; Sharma, G.L. Humidity sensing properties of (Ba, Sr) TiO3 thin films grown by hydrothermal-electrochemical method. Sens. Actuators B Chem. 2002, 85, 205-211.

105. Farahani, H.; Wagiran, R.; Yurchenko, O.; Urban, G.A. Barium Strontium Titanate Humidity Sensor: Impact of Doping on the Structural and Electrical Properties. Proceedings 2018, 2, 1007.

106. Holc, J.; Slunčko, J.; Hrovat, M. Temperature characteristics of electrical properties of (Ba,Sr)TiO3 thick film humidity sensors. Sens. Actuators B Chem. 1995, 26, 99-102.

107. Patil, R.P.; Gaikwad, S.S.; Karanjekar, A.N.; Khanna, P.K.; Jain, G.H.; Gaikwad, V.B.; Bisht, N. Optimization of stronti-um-doping concentration in BaTiO3 nanostructures for room temperature NH3 and NO2 gas sensing. Mater. Today Chem. 2020, 16, 100240.

108. Meng, X.; Xu, J.; Zhu, J.; Zhang, P.; Rydosz, A.; Reece, M.J.; Gao, F. Pyrochlore-fluorite dual-phase high-entropy ceramic foams with extremely low thermal conductivity from particle-stabilized suspension. Scr. Mater. 2021, 194, 113714.

109. Rydosz, A.; Szkudlarek, A. Gas-sensing performance of M-doped CuO-based thin films working at different temperatures upon exposure to propane. Sensors 2015, 15, 20069-20085.

110. Pisarkiewicz, T.; Maziarz, W.; Małolepszy, A.; Stobiński, L.; Michoń, D.; Rydosz, A. Multilayer Structure of Reduced Graphene Oxide and Copper Oxide as a Gas Sensor. Coatings 2020, 10, 1015.

111. Hodak, S.K.; Supasai, T.; Wisitsoraat, A.; Hodak, J.H. Design of low cost gas sensor based on SrTiO3 and BaTiO3 films. J. Nanosci. Nanotechnol. 2010, 10, 7236-7238.

112. Chan, N.Y.; Zhao, M.; Huang, J.; Au, K.; Wong, M.H.; Yao, H.M.; Lu, W.; Chen, Y.; Ong, C.W.; Chan, H.L.W.; et al. Highly Sensitive Gas Sensor by the LaAlO3/SrTiO3 Heterostructure with Pd Nanoparticle Surface Modulation. Adv. Mater. 2014, 26, 5962-5968.

113. Hara, T.; Ishiguro, T.; Shinozaki, K. Annealing Effects on Sensitivity of Atomic-Layer-Deposited SrTiO3-Based Oxygen Sensors. Jpn. J. Appl. Phys. 2010, 49, 09MA15.

114. Hara, T.; Ishiguro, T. SrTiO3-Based Microfabricated Oxygen Sensors. Jpn. J. Appl. Phys. 2009, 48, 09KA17.

115. Yuk, J.; Troczynski, T. Sol-gel BaTiO3 thin film for humidity sensors. Sens. Actuators B Chem. 2003, 94, 290-293.

116. He, X.; Hu, C.; Xi, Y.; Wan, B.; Xia, C. Electroless deposition of BaTiO3 nanocubes for electrochemical sensing. Sens. Actuators B Chem. 2009, 137, 62-66.

117. Mallick, S.; Ahmad, Z.; Qadir, K.W.; Rehman, A.; Shakoor, R.A.; Touati, F.; Al-Muhtaseb, S.A. Effect of BaTiO3 on the sensing properties of PVDF composite-based capacitive humidity sensors. Ceram. Int. 2020, 46, 2949-2953.

118. Jain, G.H.; Patil, L.A.; Wagh, M.S.; Patil, D.R.; Patil, S.A.; Amalnerkar, D.P. Surface modified BaTiO3 thick film resistors as H2S gas sensors. Sens. Actuators B Chem. 2006, 117, 159-165.

119. Patil, L.A.; Suryawanshi, D.N.; Pathan, I.G.; Patil, D.G. Effect of firing temperature on gas sensing properties of nanocrystalline perovskite BaTiO3 thin films prepared by spray pyrolysis techniques. Sens. Actuators B Chem. 2014, 195, 643-650.

120. Selvarajan, S.; Alluri, N.R.; Chandrasekhar, A.; Kim, S.-J. Direct detection of cysteine using functionalized BaTiO3 nanoparticles film based self-powered biosensor. Biosens. Bioelectron. 2017, 91, 203-210.

121. Le Pennec, F.; Bernardini, S.; Hijazi, M.; Perrin-Pellegrino, C.; Aguir, K.; Bendahan, M. Screen Printed BaTiO3 for CO2 Gas Sensor. In Proceedings of the 5th International Conference on Advances in Sensors, Actuators, Metering and Sensing, Valencia, Spain, 21-25 November 2020; IARIA: Wilmington, DE, USA, 2020.

122. Rudraswamy, S.B.; Basu, P.K.; Bhat, N.; Bhat, N. BaTiO3 based Carbon-dioxide gas sensor. In Proceedings of the 2012 International Conference on Emerging Electronics, Mumbai, India, 15-17 December 2012; IEEE: Piscataway, NJ, USA, 2012; pp. 1-4.

123. Sharma, S.; Sharma, A.; Tomar, M.; Puri, N.K.; Gupta, V. NOx Sensing Properties of Barium Titanate Thin Films. Procedia Eng. 2014, 87, 1067-1070.

124. Jiao, Z.; Chen, F.; Su, R.; Huang, X.; Liu, W.; Liu, J. Study on the characteristics of Ag doped CuO-BaTiO3 CO2 sensors. Sensors 2002, 2, 366-373.

125. Gerblinger, J.; Meixner, H. Fast oxygen sensors based on sputtered strontium titanate. Sens. Actuators B Chem. 1991, 4, 99-102.

126. Žagar, K.; Hernandez-Ramirez, F.; Prades, J.D.; Morante, J.R.; Rečnik, A.; Čeh, M. Characterization of individual barium titanate nanorods and their assessment as building blocks of new circuit architectures. Nanotechnology 2011, 22, 385501.

127. He, Y.; Zhang, T.; Zheng, W.; Wang, R.; Liu, X.; Xia, Y.; Zhao, J. Humidity sensing properties of BaTiO3 nanofiber prepared via electrospinning. Sens. Actuators B Chem. 2010, 146, 98-102.

128. Joshi, S.; Antolasic, F.; Sunkara, M.V.; Bhargava, S.K.; Ippolito, S.J. Highly Selective CO2 Gas Sensing Properties of CaO-BaTiO3 Heterostructures Effectuated through Discretely Created nn Nanointerfaces. ACS Sustain. Chem. Eng. 2018, 6, 4086-4097.

129. Singh, M.; Yadav, B.C.; Ranjan, A.; Kaur, M.; Gupta, S.K. Synthesis and characterization of perovskite barium titanate thin film and its application as LPG sensor. Sens. Actuators B Chem. 2017, 241, 1170-1178.

130. Ciobota, C.F.; Piticescu, R.M.; Neagoe, C.; Tudor, I.A.; Matei, A.; Dragut, D.V.; Sobetkii, A.; Anghel, E.M.; Stanoiu, A.; Simion, C.E.; et al. Nanostructured Cobalt Doped Barium Strontium Titanate Thin Films with Potential in CO2 Detection. Materials 2020, 13, 4797.

131. Jain, G.H.; Gaikwad, V.B.; Kajale, D.D.; Chaudhari, R.M.; Patil, R.L.; Pawar, N.K.; Patil, L.A. Gas sensing performance of pure and modified BST thick film resistor. Sens. Transducers 2008, 90, 160-173.

132. Jain, G.H.; Patil, L.A. Gas sensing properties of Cu and Cr activated BST thick films. Bull. Mater. Sci. 2006, 29, 403-411.

133. Patil, R.P.; Jain, G.H.; Gaikwad, V.B.; Kankrej, S.R.; Kulkarni, M.S. Surface modified strontium doped barium titanate nanostructures H2S sensor. J. Emerg. Technol. Innov. Res. 2018, 5, 357-365.

134. Greenstein, M. IEEE Translations on Ultrasonics Ferroelectrics and Frequency Control; IEEE: Piscataway, NJ, USA, 1996; Volume 43.

135. Xu, Z.; Yan, D.; Xiao, D.; Yu, P.; Zhu, J. Dielectric enhancement of BaSrTi1. 1O3/BaSrTi1 05O3/BaSrTiO3 multilayer thin films prepared by RF magnetron sputtering. Ceram. Int. 2013, 39, 1639-1643.

136. Araghi, M.A.; Shaban, N.; Bahar, M. Synthesis and characterization of nanocrystalline barium strontium titanate powder by a modified sol-gel processing. Mater. Sci. Pol. 2016, 34, 63-68.

Bartłomiej Szafraniak

1,

Łukasz Fuśnik

1,*,

Jie Xu

2,

Feng Gao

2,

Andrzej Brudnik

1 and

Artur Rydosz

1

1Institute of Electronics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland

2State Key Laboratory of Solidification Processing, NPU-QMUL Joint Research Institute of Advanced Materials and Structures (JRI-AMAS), School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

*Author to whom correspondence should be addressed.

Word count: 11488

Show less

© 2021. This work is licensed under http://creativecommons.org/licenses/by/3.0/ (the “License”). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.

Abstract

In this work, a broad overview in the field of strontium titanate (ST, SrTiO3)-, barium titanate (BT, BaTiO3)- and barium strontium titanate (BST, BaSrTiO3)-based gas sensors is presented and discussed. The above-mentioned materials are characterized by a perovskite structure with long-term stability and therefore are very promising materials for commercial gas-sensing applications. Within the last 20 years, the number of papers where ST, BT and BST materials were tested as gas-sensitive materials has ten times increased and therefore an actual review about them in this field has been expected by readers, who are researchers involved in gas-sensing applications and novel materials investigations, as well as industry research and development center members, who are constantly searching for gas-sensing materials exhibiting high 3S parameters (sensitivity, selectivity and stability) that can be adapted for commercial realizations. Finally, the NO2-sensing characteristics of the BST-based gas sensors deposited by the authors with the utilization of magnetron sputtering technology are presented.

Details

Title
Semiconducting Metal Oxides: SrTiO3, BaTiO3 and BaSrTiO3 in Gas-Sensing Applications: A Review
First page
185
Publication year
2021
Publication date
2021
Publisher
MDPI AG
e-ISSN
20796412
Source type
Scholarly Journal
Language of publication
English
DOI
https://doi.org/10.3390/coatings11020185
ProQuest document ID
2487504029
Copyright
© 2021. This work is licensed under http://creativecommons.org/licenses/by/3.0/ (the “License”). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.