Gas Sensors Based on Titanium Oxides (Review)

: Nanostructured titanium compounds have recently been applied in the design of gas sensors. Among titanium compounds, titanium oxides (TiO 2 ) are the most frequently used in gas sensing devices. Therefore, in this review, we are paying signiﬁcant attention to the variety of allotropic modiﬁcations of titanium oxides, which include anatase, rutile, brukite. Very recently, the applicability of non-stoichiometric titanium oxide (TiO 2 − x )-based layers for the design of gas sensors was demonstrated. For this reason, in this review, we are addressing some research related to the formation of non-stoichiometric titanium oxide (TiO 2 − x ) and Magn é li phase (Ti n O 2n − 1 )-based layers suitable for sensor design. The most promising titanium compounds and hetero- and nano-structures based on these compounds are discussed. It is also outlined that during the past decade, many new strategies for the synthesis of TiO 2 and conducting polymer-based composite materials were developed, which have found some speciﬁc application areas. Therefore, in this review, we are highlighting how speciﬁc formation methods, which can be used for the formation of TiO 2 and conducting polymer composites, can be applied to tune composite characteristics that are leading towards advanced applications in these speciﬁc technological ﬁelds. The possibility to tune the sensitivity and selectivity of titanium compound-based sensing layers is addressed. In this review, some other recent reviews related to the development of sensors based on titanium oxides are overviewed. Some designs of titanium-based nanomaterials used for the development of sensors are outlined. ,


Introduction
Sensors for the determination of gasses and volatile organic compounds (VOCs) (gassensors) find applications in many different areas. Most of these gas sensors are based on semiconducting layers, which change their electrical resistance in the presence of gases and VOCs [1][2][3][4][5][6][7]. These semiconducting layers are mostly based on semiconductors such as WO 3 , MoS 2 , ZnO, SnO 2 , and TiO 2 TiO n [2][3][4][5][6][7][8], while Al 2 O 3 and SiO 2 are the most common dielectric substrates used for the formation of gas sensors. Some new titanium-based compounds, such as MXenes [9], and non-stoichiometric titanium oxides (TiO n ) [1,2,10,11] have also been recently used in the design of gas sensors. The stoichiometric TiO 2 semiconductor of n-type, which appears in three major phases (rutile, anatase, and brookite) can be rather easily converted between these phases by thermal procedures, which paves the way to tune analytical characteristics of gas sensors [11]. These phases are sensitive and could be used for the detection of oxygen alone [12][13][14] or in heterostructures with ZrO 2 to improve sensing characteristics [15][16][17]. Besides stoichiometric TiO 2 forms, very recently, a non-stoichiometric (TiO n ) that is sometimes indicated as TiO 2−x has also been applied in sensor design [1]. Titanium oxide structures based on specific Magnéli phases (Ti n O 2n−1 ) are very promising [10]. Many gas sensors are very sensitive, but mostly they are not very selective and consume a lot of electrical energy, which is required for the powering of the heater that is essential for the efficient operation of most gas sensors. In order to reduce the lastly mentioned disadvantage, the 'self-heating' strategy was developed, which enables the exploitation of the sensing layer as a heater of the analytical system [1,2,18]. Recently, some scientists haveattempted to advance the selectivity of gas-sensors by the design of specific morphology of semiconducting structures [19,20], application of coreshell semiconducting nano-composites [21][22][23] and/or quantum dots [24,25]. Various aspects of titanium-based material are overviewed in recent reviews, addressing the properties, synthesis and modifications of TiO 2 -based nanostructures [10,11,26]; application of titanium-based nanomaterials for energetics and environmental purposes [27]; the design of photo-catalysts [28]; the applicability of titanium oxides [1,2] and MXenes [9] in sensor design.
In this article, we are overviewing mainly the advances in the development of gas and volatile organic compounds (VOCs) sensors based on titanium-based oxides.
The most significant difference between titanium pentoxide and the Magnéli phases is determined by their different crystal structures. Magnéli phases contains shear planes based on TiO 2(rutile) [44,45], while in titanium pentoxide, such planes are absent [1,43]. A temperature of 400 • C is optimal for the appearance of TiO 2(anatase) intergrowths within Ti 3 O 5 crystals based on TiO 2−x /TiO 2 -based heterostructures [46,47]. The incorporation of intergrowths based on TiO 2(anatase) in the structure of titanium pentoxide (Ti 3 O 5 ) improves their conductivity ( Figure 1) and some photoluminescence-related characteristics [1]. It should be noted that such non-stoichiometric titanium oxides can be spontaneously oxidized for this reason and significant attention should be paid to the stabilization of these structures during their usage in the development of gas sensors. It should be noted that at different oxidation states, titanium oxides have different crystal structures [48], which starts from rutile for TiO 2 and anatase for Ti 10   The electrical conductivity of most non-stoichiometric titanium oxides is significantly higher than the conductivity of any allotropic form of stoichiometric titanium oxides (TiO2). The most significant increase in sensitivity is observed for titanium oxides with TinO2n−1 stoichiometry, when index 'n' is in the range of 4-10 [49]. Some compounds with the above-mentioned stoichiometry form Magnéli phases, which are characterized by metallic conductivity and even by superconductivity [50,51].
Non-stoichiometric titanium oxide-based layers based on Magnéli phases have well conducting intergrowths based on TinO2n−1 planar moieties that penetrate the TiO2-based matrix [1,52]. Non-stoichiometric titanium oxides, such as Ti2O3 and/or Ti3O5, which do not form real Magnéli phases anyway, are much better at conducting in comparison to stoichiometric TiO2 [1,43]. These n-type semiconducting titanium oxides have a high concentration of 'oxygen vacancies', which are responsible for the mobility of electrons through their structure [53] and baseline resistance [54]. It should be outlined that these 'oxygen vacancies' are also responsible for the sensitivity towards both oxidizing and reducing gases [1]. During the design of the sensing layer, initially formed stoichiometric titanium oxide-based layers can be chemically reduced into non-stoichiometric titanium oxide (TiO2−x) and/or Magnéli phases [2]. Magnéli phases are observed when the oxygen concentration in TiO2−x structure is decreased and 'x' value is between 0.1 and 0.34 [1,55,56]. Non-stoichiometric titanium oxide structures containing Magnéli phases are chemically stable and rather well conducting. For these reasons, they are often applied in wastewater treatment and the design of batteries and fuel cells [57,58] ; the same characteristics are required for gas sensors.
Non-stoichiometric titanium oxide-based structures can be developed using several approaches, namely plasma-treatment [59], laser irradiation-based modification [60], reduction by metallic zinc [61], bombardment by high-energy particles [62] and thermo- The electrical conductivity of most non-stoichiometric titanium oxides is significantly higher than the conductivity of any allotropic form of stoichiometric titanium oxides (TiO 2 ). The most significant increase in sensitivity is observed for titanium oxides with Ti n O 2n−1 stoichiometry, when index 'n' is in the range of 4-10 [49]. Some compounds with the above-mentioned stoichiometry form Magnéli phases, which are characterized by metallic conductivity and even by superconductivity [50,51].
Non-stoichiometric titanium oxide-based layers based on Magnéli phases have well conducting intergrowths based on Ti n O 2n−1 planar moieties that penetrate the TiO 2 -based matrix [1,52]. Non-stoichiometric titanium oxides, such as Ti 2 O 3 and/or Ti 3 O 5 , which do not form real Magnéli phases anyway, are much better at conducting in comparison to stoichiometric TiO 2 [1,43]. These n-type semiconducting titanium oxides have a high concentration of 'oxygen vacancies', which are responsible for the mobility of electrons through their structure [53] and baseline resistance [54]. It should be outlined that these 'oxygen vacancies' are also responsible for the sensitivity towards both oxidizing and reducing gases [1]. During the design of the sensing layer, initially formed stoichiometric titanium oxide-based layers can be chemically reduced into non-stoichiometric titanium oxide (TiO 2−x ) and/or Magnéli phases [2]. Magnéli phases are observed when the oxygen concentration in TiO 2−x structure is decreased and 'x' value is between 0.1 and 0.34 [1,55,56]. Non-stoichiometric titanium oxide structures containing Magnéli phases are chemically stable and rather well conducting. For these reasons, they are often applied in wastewater treatment and the design of batteries and fuel cells [57,58]; the same characteristics are required for gas sensors.
Non-stoichiometric titanium oxide-based structures can be developed using several approaches, namely plasma-treatment [59], laser irradiation-based modification [60], reduction by metallic zinc [61], bombardment by high-energy particles [62] and thermo-chemical approaches [63]. However, the formation of large Ti 3 O 5 monocrystals is rather challenging because titanium oxides are polymorphic [43]. In some cases, stoichiometric titanium oxide can be easily turned into a non-stoichiometric one by suitable doping and/or reduction.
During the formation of TiO 2−x /TiO 2 -based heterostructures, the ratio between stoichiometric titanium oxide and non-stoichiometric titanium oxide can be significantly increased by thermal treatment in reducing the gas atmosphere [64]. In several works, the transition between the insulator state and metallic state of Ti 3 O 5 (β and λ forms of Ti 3 O 5 , respectively) was performed by pulses of visible light [41] and by thermal treatment, which induced the conversion of α form into β form at 450 K [38] and the conversion of δ form into γ form at 240 K [39][40][41][42]. The phase transition of Ti 3 O 5 at 240-450 K is the most important for the adjustment of conductivity of this semiconducting material-based layers, e.g., the switching between metal and insulator states was observed at 350 K [43].
In some cases, non-stoichiometric titanium oxide can be formed by the partial oxidation of metallic titanium layers, which is followed by thermal treatment and annealing [1]. Sensors based on such structures, which are differently thermally treated and annealed, can be used in the formation of sensors with very different selectivity and sensitivity, which are suitable for the development of sensor arrays. Non-stoichiometric titanium oxide-based sensors. despite some significant advantages related to better catalytic activity and conductivity, have some disadvantages in comparison to those based on stoichiometric titanium oxides related to their insufficient stability at ambient conditions. In addition, it should be noted that the selectivity of these sensors is not superior.

Pristine Titanium Oxide-Based Gas Sensors and Their Sensing Mechanisms
Stoichiometric TiO 2 -based gas sensors show high sensitivity to different gases. It should be noted that TiO 2 -based gas sensors can rely on several different sensing mechanisms, which differ the most significantly for the determination of reducing gaseous compounds, such as H 2 , H 2 S, NH 3 , CO, CH 3 OH, C 2 H 5 OH, etc. and for oxidizing gaseous compounds, such as O 2 , NO 2 , CO 2 [28,63,65] (Table 1). The changes in electrical resistivity of the TiO 2 -based layer are mostly used for the determination of analytical signals; therefore, the assessment of analytical signals generated by such sensors is rather simple. In some cases, measurement protocols were advanced by the determination of photoluminescence signals [1,2,8], which are generated by semiconducting TiO 2 structures [66]. Some above-mentioned photocatalytic and photovoltaic properties can be improved by laser-based treatment [67]. However, the main disadvantage of TiO 2 -based sensors is poor selectivity towards gaseous materials, which significantly complicates the application of these analytical devices. Therefore, in order to improve selectivity, various heterostructures containing TiO 2 hybridized with many other semiconductors have been developed, e.g., our research group has developed a TiO 2−x /TiO 2 -based self-heating heterostructure for the determination of NH 3 , CH 3 OH and C 2 H 5 OH [1]. It was also reported that TiO 2 combined with La 0.8 Sr 0.2 Co 0.5 Ni 0.5 O 3 perovskite was applied for the determination of CO [77], TiO 2 /V 2 O 5 -based heterostructure for ozone [78], TiO 2 /SnO 2 -based heterostructure for NO 2 and CO [79,80]; TiO 2 /graphene (TiO 2 /GO)based heterostructure for toluene determination at 298 K [81]. The action of TiO 2 /GO-based heterostructures based on the formed n-n hetero-junction is enhanced by UV irradiation [82] and is addressed in the band diagram presented in Figure 2a,b. The bandgap of the heterojunction in the TiO 2 /GO-based heterostructure is 4.7 eV [83] which is higher than that of 4.4 eV and is usually observed for GO-based structures [84]; the formed hetero-structure has both accumulation and depletion layers [85] (Figure 2b). In this structure, a Schottky barrier is formed; therefore, the depletion layer in TiO 2 is thicker, and the number of electrons in the GO interface increases. The interaction of the adsorbed gases with the oxidized functional groups of GO induces variation in the resistance of the TiO 2 /GO-based heterostructure, which forms a sensing layer. In this heterostructure, TiO 2 is contributing by increasing the number of active sites suitable for the adsorption of gas molecules [86,87]. In some particular cases, activation energy can be lowered by adsorbed gas molecules, which increases the catalytic activity of the TiO 2 layer [88]. This property increases the sensitivity towards some reducing gases, such as ammonia, etc.; however, the active sites of GO are very sensitive towards moisture that increases electrical resistance, due to the interaction of water molecules with oxygen-based (carboxyl, carbonyl and hydroxyl) functional groups [89]. UV irradiation enables to solve this disadvantage by the increase in the depletion layer in TiO 2 and enhancement of the accumulation layer in GO (Figure 2c). Due to this reason, the electrons are shifted towards GO and reduced carboxyl, carbonyl and hydroxyl groups, which become unsuitable for the adsorption of water molecules [90]. This reduced GO establishes a p-n junction with TiO 2 , which at the interface is characterized by decreased junction width (Figure 2d). UV irradiation of TiO 2 induces the 'injection' of electrons into GO and in such a way that electron-hole pair recombination is prevented [91]. The surface concentration of adsorbed oxygen molecules decreases, which also reduces water sorption to this heterostructure [92]. The number of carboxyl, carbonyl and hydroxyl groups in GO structure can be partly restored by the UV irradiation-induced action of TiO 2 [82]. This reduced GO establishes a p-n junction with TiO2, which at the interface is characterized by decreased junction width (Figure 2d). UV irradiation of TiO2 induces the 'injection' of electrons into GO and in such a way that electron-hole pair recombination is prevented [91]. The surface concentration of adsorbed oxygen molecules decreases, which also reduces water sorption to this heterostructure [92]. The number of carboxyl, carbonyl and hydroxyl groups in GO structure can be partly restored by the UV irradiation-induced action of TiO2 [82]. The heterostructure based on TiO2/SnO2 is also very interesting because the depletion of energetic layers in the molecular orbitals of TiO2 electrons is induced when TiO2 is connected to SnO2 [79]. It should be noted that nanoparticle-based structures are preferable for the development of gas sensors [23] because the radius of nanoparticles based on some semiconducting materials that are used in the design of gas-sensors are in the same range as the Exciton Bohr radius; for this reason, such particles are very suitable for the design of gas sensors [25].
SnO2 has great charge-carrier mobility, which is the most important factor for gas sensors based on resistivity measurements [80,93]. SnO2-based gas-sensing layers are cheap and stable at ambient conditions [94] however, it should be taken into account that sensors based on stoichiometric TiO2 and SnO2 operate at a rather high temperature, which exceeds several hundred degrees [25]. However, such sensors consume a lot of energy for the heating of the sensing layer; therefore, due to energy saving related issues, sensing layers capable to operate at low temperatures are under very special interest, e.g., sensing layers based on Au/SnO2 core-shell structures can operate in the temperature interval of 25-80 °C [95]. Our investigations of non-stoichiometric titanium oxide-based sensors shows that these sensors can operate even at room temperature [1,2]. It is expected that heterostructures based on SnO2/TiO2 can be applied in low temperature gas sensors; The heterostructure based on TiO 2 /SnO 2 is also very interesting because the depletion of energetic layers in the molecular orbitals of TiO 2 electrons is induced when TiO 2 is connected to SnO 2 [79]. It should be noted that nanoparticle-based structures are preferable for the development of gas sensors [23] because the radius of nanoparticles based on some semiconducting materials that are used in the design of gas-sensors are in the same range as the Exciton Bohr radius; for this reason, such particles are very suitable for the design of gas sensors [25]. SnO 2 has great charge-carrier mobility, which is the most important factor for gas sensors based on resistivity measurements [80,93]. SnO 2 -based gas-sensing layers are cheap and stable at ambient conditions [94] however, it should be taken into account that sensors based on stoichiometric TiO 2 and SnO 2 operate at a rather high temperature, which exceeds several hundred degrees [25]. However, such sensors consume a lot of energy for the heating of the sensing layer; therefore, due to energy saving related issues, sensing layers capable to operate at low temperatures are under very special interest, e.g., sensing layers based on Au/SnO 2 core-shell structures can operate in the temperature interval of 25-80 • C [95]. Our investigations of non-stoichiometric titanium oxide-based sensors shows that these sensors can operate even at room temperature [1,2]. It is expected that heterostructures based on SnO 2 /TiO 2 can be applied in low temperature gas sensors; therefore, some nanostructures were designed, which are as follows: TiO 2 nanobelts covered by SnO 2 [96], TiO 2 /SnO 2 -based core-shell nanofibers [97], SnO 2 layers doped by TiO 2 [98], structures based on SnO 2 /TiO 2 laminates [99], SnO 2 nanoparticles wrapped within TiO 2 -based nanofibers [79], SnO 2 -based quantum dots formed on a surface of TiO 2 layer were reported to be sensitive towards NO 2 and CO [100]. Atomic layer deposition (ALD) was used to deposit thin TiO 2 layers and it was demonstrated that tuning of the selectivity can be performed by the adjustment of TiO 2 layer thickness [101].
Significant energy consumptions for the heating of the sensing layer reduce the applicability of most gas sensors. Therefore, there is a demand for gas-sensing structures that can operate at low temperatures [102]. The miniaturization of sensing elements is another suitable strategy to reduce energy consumption. Low-temperature sensors based on titanium oxide-based layers were reported for O 2 [99], ozone [78], formaldehyde [103,104], CO [77], ethanol [96,98,105], C 7 H 8 [81], H 2 [106,107] and other gases [108].
The 'self-heating' of the sensing layer can be achieved when it has rather low resistance and part of electrical energy, which passes through this layer and is converted into thermal energy [1,2]. However, stoichiometric titanium oxide-based layers are characterised by a high band gap; therefore, the conductivity of these layers is not sufficient for 'selfheating' because rather high voltages are required to achieve some effect. On the contrary, this operation mode is very suitable for non-stoichiometric titanium oxide-based sensors, because these layers are good at conducting at low temperatures and in the temperature interval of 72-180 • C, these layers reach very good sensitivity towards some gases [1,2]. The semiconducting properties and chemical activity of titanium oxide enable sufficient catalytic and photocatalytic activities under UV irradiation to turn into a 'water splitting' ability [67,109], which eventually can be exploited for sensing purposes. It should be noted that at room temperature (298 K), sensing layers are sensitive to humidity. Therefore, analytical signals generated by adsorbed water molecules interfere with an analytical signal generated by target gases.

The Action of Sensing Layers Based on TiO 2 Heterostructures and Assessment of Analytical Signals by Titanium Oxide-Based Sensors
The action of TiO 2 -based heterostructures relies on many parallel processes, where the most important are the adsorption of gas molecules to the surface of the sensing layer and the simultaneous desorption of gas, which was initially adsorbed to the same layer. Both adsorption and desorption events and/or the establishment of new chemical bonds between sensing TiO 2 layer and adsorbed gas molecules electrostatically affect the upper layer of the semiconducting TiO 2 layer and due to the depletion and/or enrichment of this layer by charge carriers, the conductivity of this layer changes correspondingly. The typical analytical signal determined by the adsorption and desorption of analyte gas is represented in Figure 3.
Several different types of electrical conductivity are observed in the TiO 2 -layers based on nano-or micro-particles, which are as follows: (i) intrinsic charge transfer through TiO 2particles and (ii) limitations of charge transfer through the boundaries between different particles. For this reason, the volume-concentration of these boundaries and the size/shape of particles are critical in the development of such sensors [110,111] (Figure 4). The semiconducting characteristics of TiO 2 -based structures, such as Debay length/radius, also strongly influence charge transfer in TiO 2 -based sensors. It should be noted that due to numerous thermal phase transitions during the annealing-based development of non-stoichiometric titanium oxide-based sensing structures, they are formed very porous and contain many different nanostructures and phases that significantly enhance the surface/volume ratio. Therefore, such structures are very efficient at absorbing gases, which leads to enhanced sensitivity of non-stoichiometric titanium oxide-based sensors. In order to achieve great sensitivity and selectivity, the discussed properties of non-stoichiometric titanium oxide-based sensing layers should be well tailored during the design of gas sensors [63,[112][113][114][115][116]. Several different types of electrical conductivity are observed in the TiO2-layers based on nano-or micro-particles, which are as follows: (i) intrinsic charge transfer through TiO2-particles and (ii) limitations of charge transfer through the boundaries between different particles. For this reason, the volume-concentration of these boundaries and the size/shape of particles are critical in the development of such sensors [110,111] (Figure 4). The semiconducting characteristics of TiO2-based structures, such as Debay length/radius, also strongly influence charge transfer in TiO2-based sensors. It should be noted that due to numerous thermal phase transitions during the annealing-based development of nonstoichiometric titanium oxide-based sensing structures, they are formed very porous and contain many different nanostructures and phases that significantly enhance the surface/volume ratio. Therefore, such structures are very efficient at absorbing gases, which leads to enhanced sensitivity of non-stoichiometric titanium oxide-based sensors. In order to achieve great sensitivity and selectivity, the discussed properties of non-stoichiometric titanium oxide-based sensing layers should be well tailored during the design of gas sensors [63,[112][113][114][115][116]. . Figure 3. Representation of typical analytical signal for amperometric gas sensors. It should be noted that the duration of signal development and the regeneration of sensors highly depends on sensing material and sensing gases [1].
Coatings 2022, 12, x FOR PEER REVIEW 8 of 18 Figure 3. Representation of typical analytical signal for amperometric gas sensors. It should be noted that the duration of signal development and the regeneration of sensors highly depends on sensing material and sensing gases [1].
Several different types of electrical conductivity are observed in the TiO2-layers based on nano-or micro-particles, which are as follows: (i) intrinsic charge transfer through TiO2-particles and (ii) limitations of charge transfer through the boundaries between different particles. For this reason, the volume-concentration of these boundaries and the size/shape of particles are critical in the development of such sensors [110,111] (Figure 4). The semiconducting characteristics of TiO2-based structures, such as Debay length/radius, also strongly influence charge transfer in TiO2-based sensors. It should be noted that due to numerous thermal phase transitions during the annealing-based development of nonstoichiometric titanium oxide-based sensing structures, they are formed very porous and contain many different nanostructures and phases that significantly enhance the surface/volume ratio. Therefore, such structures are very efficient at absorbing gases, which leads to enhanced sensitivity of non-stoichiometric titanium oxide-based sensors. In order to achieve great sensitivity and selectivity, the discussed properties of non-stoichiometric titanium oxide-based sensing layers should be well tailored during the design of gas sensors [63,[112][113][114][115][116]. .  It should be noted that the adsorption of gas molecules with the surface of TiO 2 -based layer is a very complex process, which at a high extent is determined by Van der Waals and/or electrostatic interactions and/or the forming of new chemical bonds. Molecules of the gas that are present in the measurement cell before the introduction of analyte-gas containing aliquot are critically important for the establishment of the particular mechanism of sensing, because they are preabsorbing on the semiconducting structure and, in such a way, are responsible for the initial conductivity of the sensing layer. During the measurement, analyte gas containing aliquot appears in senor-cell molecules of preadsorbed gas, which are replaced by analyte gas molecules (and/or by other gas molecules, which are part of aliquot). During both stages, adsorbed gas molecules are electrostatically interacting with the adsorption sites. It should be noted that in TiO 2, defects of the crystal structure are acting as the most efficient adsorption sites. The adsorption ability of gas molecules very often depends on the ability to accept and/or to donate electrons to active sites that can be additionally involved in photoluminescence-based light emission [35]. Hence, donor/acceptor interactions change the electrical conductivity and the efficiency of photoluminescence of TiO 2 -based structures that can be used for the assessment of analyte concentration. However, stoichiometric titanium oxide-based structures are characterized by high resistance, which does not significantly vary in the presence of low concentrations of gaseous materials; therefore, the dynamic range of sensors based on stoichiometric titanium oxide is not high. Conducting polymers, especially polyaniline (PANI), are well suitable for the enhancement of the sensitivity of semiconducting metal oxides towards gases [117][118][119] and volatile compounds [120].
In order to increase sensitivity, various heterostructures containing TiO 2 are modified by other semiconducting materials, e.g., by conjugated polymers polyaniline (TiO 2 /PANI) and polypyrrole (TiO 2 /Ppy)), which can be involved in the modification and design of titanium oxide-based sensing structures. The formation of the n/p-junction at TiO 2 /(conducting polymer) interphase is the most important factor for the enhancement of sensor sensitivity [121]. TiO 2 /Ppy-based sensing structures for the determination of NH 3 [122,123] propane/butane [124] and TiO 2 /PANI-based sensing structures for the detection of NH 3 [123,[125][126][127] were developed. In order to improve sensitivity and selectivity, titanium oxide-based semiconducting structures can be modified by some metal oxides, carbides and other materials [1,2,9,113,114,[128][129][130][131].
During the development of titanium oxide-based sensors, a very important issue is the elaboration of a suitable analytical signal registration protocol. The registration can be based on many physical approaches, including the measurement of photoluminescence response and electrical signals. Electrical signals can be determined from measurements performed in potentiostatic, galvanostatic and various potentiodynamic modes. If necessary, these modes can be combined with a 'self-heating' based approach. The determination of electrical conductivity measurements is most frequently used in gas sensors based on semiconducting structures [1,4,5,7,11,20,[22][23][24][25]33,34,65,98,104,107,112,122,125,131,132]. At room temperature, stoichiometric titanium oxide-based sensing layers have a rather low conductivity of~10 −10 S/m; for this reason, up to 200-400 • C elevated temperature is applied in order to reach optimal electrical conductivity and sensing conditions. However, elevated temperatures increase the consumption of electrical energy required for the operation of the sensor.

TiO 2 and Conducting Polymer-Based Composites for Gas and VOC Sensors
The combination of metal oxide-based structures with conducting polymers led to the ability to tune precisely some properties of formed composite material in order to adjust it for specific applications [120]. Therefore, during the past decade, a lot of new strategies for the synthesis of different morphology TiO 2 ( Figure 5) and their composites with conducting polymers were developed.  [134], (C) nanoparticles [135], (D) nanotubes [136].
Chemical formation is one of the easiest and the most popular methods for the formation of TiO2-PANI composites. Usually, for chemical formation, acidic solutions (mostly HCl [137][138][139] or H2SO4 [140,141]) are used together with reaction initiator (NH4)2S2O8 [137][138][139][140]142]. Procedure protocols in most cases are similar and involve a few steps, which are as follows: (i) aniline and acid solution in distilled water together with TiO2 is prepared in one beaker, (ii) in a second beaker, the polymerization initiator is dissolved in an acidic solution and (iii) both solutions are mixed, cooled to a near 0 °C temperature and stored for some time [137,138,143]. A visible sign of aniline polymerization reaction is a change of solution color into light blue, revealing the starting of PANI formation through an oxidation reaction. Finally, the reaction solution color becomes green, which indicates the successful formation of the PANI composite [139]. While varying in the parameters of reagent concentrations, it is possible that polymeric composite thickness, surface morphology and conductivity can be controlled by polymerization time and temperature depending on technological demand. It is important to mention that during the oxidation process, conductivity mainly depends on anions and their concentration used in the acid solution during the PANI polymerization reaction [137][138][139][140][143][144][145]. While wet chemical synthesis methods are the easiest to perform, electrochemical polymerization is one of the common methods, which provides relatively good control of the process in comparison to chemical formation. In this method, it is possible to use various modes for electrochemical PANI deposition on top of titanium dioxide substrate; however, the most popular modes are cyclic voltammetry (CV) [146][147][148][149][150] and chronoamperometry (CA) [146,150]. For CA depositions, a WE electrode potential in between 0.85-0.9 V vs. Ag/AgCl [146], while in CV mode, a potential in between −0.6-1.3 V vs. Ag/AgCl [146][147][148][149]151] and an acidic electrolyte with dissolved aniline are usually used [150,152].
Particularly, most researchers report that the modification of TiO2-based structures by conducting the polymer polyaniline (PANI) can increase their sensitivity towards some gases and/or VOC's [153,154]. An especially high number of researchers report that the modification of TiO2-based structures by PANI increases the sensitivity of designed sensors towards ammonia (NH3) [125,126,143,[155][156][157][158][159][160][161][162]. The action mechanism of the sensing layer is rather simple; the conductivity sensing layer usually increases while oxygen, which is acting as an oxidizing agent, is physico-sorbed on TiO2/PANI and decreases when NH3, which is acting as a reducing agent, replaces the adsorbed oxygen [155]. Although all common PANI forms (including leucoemeraldine and pernigraniline) [163][164][165] A C B D Figure 5. Different morphology TiO 2 structures used in design of a gas sensors. (A) Nanorods [133], (B) thin film [134], (C) nanoparticles [135], (D) nanotubes [136].
Chemical formation is one of the easiest and the most popular methods for the formation of TiO 2 -PANI composites. Usually, for chemical formation, acidic solutions (mostly HCl [137][138][139] or H 2 SO 4 [140,141]) are used together with reaction initiator (NH 4 ) 2 S 2 O 8 [137][138][139][140]142]. Procedure protocols in most cases are similar and involve a few steps, which are as follows: (i) aniline and acid solution in distilled water together with TiO 2 is prepared in one beaker, (ii) in a second beaker, the polymerization initiator is dissolved in an acidic solution and (iii) both solutions are mixed, cooled to a near 0 • C temperature and stored for some time [137,138,143]. A visible sign of aniline polymerization reaction is a change of solution color into light blue, revealing the starting of PANI formation through an oxidation reaction. Finally, the reaction solution color becomes green, which indicates the successful formation of the PANI composite [139]. While varying in the parameters of reagent concentrations, it is possible that polymeric composite thickness, surface morphology and conductivity can be controlled by polymerization time and temperature depending on technological demand. It is important to mention that during the oxidation process, conductivity mainly depends on anions and their concentration used in the acid solution during the PANI polymerization reaction [137][138][139][140][143][144][145]. While wet chemical synthesis methods are the easiest to perform, electrochemical polymerization is one of the common methods, which provides relatively good control of the process in comparison to chemical formation. In this method, it is possible to use various modes for electrochemical PANI deposition on top of titanium dioxide substrate; however, the most popular modes are cyclic voltammetry (CV) [146][147][148][149][150] and chronoamperometry (CA) [146,150]. For CA depositions, a WE electrode potential in between 0.85-0.9 V vs. Ag/AgCl [146], while in CV mode, a potential in between −0.6-1.3 V vs. Ag/AgCl [146][147][148][149]151] and an acidic electrolyte with dissolved aniline are usually used [150,152].

Conclusions and Future Trends
A number of stoichiometric titanium oxide-based structures were applied in the design of gas sensors. However, these sensors still face selectivity-related problems, which can be solved by the doping of titanium oxide-based layers by dopants or the formation of hetero-structures based on a combination with some other semiconducting compounds and the adjustment of sensor-operation temperature. Among many others, sensors based on composite materials that are based on titanium oxide modified with conducting polymers are very promising, e.g., PANI. In recent investigations, it was shown that TiO 2 /TiO 2−xbased heterostructures can be successfully applied in the design of gas sensors; it was demonstrated that the sensing properties of these sensors can be easily adjusted by changing the TiO 2 /TiO 2−x -ratio heterostructures. Moreover, TiO 2 /TiO 2−x -based heterostructures are good conductors; therefore, they can act in 'self-heating' mode and can reach temperatures that are suitable for the determination of some gaseous compounds. It is expected that TiO 2 /TiO 2−x -based heterostructures will be more frequently applied in the design of gas sensors, but the main challenge in this research direction is still related to the control of stoichiometry and morphology of the TiO 2 /TiO 2−x -based structure, which is very critical for the sensitivity and selectivity of the designed gas sensors.