TiO2-Based Nanoheterostructures for Promoting Gas Sensitivity Performance: Designs, Developments, and Prospects

Gas sensors based on titanium dioxide (TiO2) have attracted much public attention during the past decades due to their excellent potential for applications in environmental pollution remediation, transportation industries, personal safety, biology, and medicine. Numerous efforts have therefore been devoted to improving the sensing performance of TiO2. In those effects, the construct of nanoheterostructures is a promising tactic in gas sensing modification, which shows superior sensing performance to that of the single component-based sensors. In this review, we briefly summarize and highlight the development of TiO2-based heterostructure gas sensing materials with diverse models, including semiconductor/semiconductor nanoheterostructures, noble metal/semiconductor nanoheterostructures, carbon-group-materials/semiconductor nano- heterostructures, and organic/inorganic nanoheterostructures, which have been investigated for effective enhancement of gas sensing properties through the increase of sensitivity, selectivity, and stability, decrease of optimal work temperature and response/recovery time, and minimization of detectable levels.


Introduction
Since the 20th century, atmospheric pollution has been proved to be one of most urgent issues. For the sake of controlling the exhaust emissions, gas sensors for the quantitative detection of various toxic and harmful gases have been widely developed as a result of their high response, outstanding selectivity, excellent repeatability, and good stability [1][2][3]. So far a variety of gas sensors, such as metal oxide semiconductor-based gas sensors [4][5][6][7][8][9], solid electrolyte-based gas sensors [10], electrochemical gas sensors [11], carbon-based gas sensors [1,[12][13][14], organic gas sensors [2,3], and so on, have been extensively investigated. Amongst these different types of gas sensors, resistance type metal oxide gas sensors offering low cost, simple manufacturing approaches, and excellent sensitivity to the great majority of gases, have attracted considerable attention during the past several years [15,16]. Since Seiyama [17] reported metal oxide-based gas sensors for the first time, a large amount of effort has been expended in exploring the sensing properties of metal oxide-based gas sensors [7,18,19].
As a representative semiconductor metal oxide, titanium dioxide (TiO 2 ) has attracted much attention since Fujishima et al. observed the photocatalytic splitting of water on a TiO 2 electrode TiO2 gas sensors are typical resistant-type sensors which can display a decrease or increase in resistance when probing a reductive gas (H2, H2S, NH3, CO, VOCs) or oxidative gas (NO2, O2) [29,30,38], respectively. The sensing mechanism of TiO2-based gas sensors can be described by the following two-step process: receptor process and transducer process, as shown in Figure 2 [39].
The receptor process occurs at the TiO2 surface, and it involves physisorption and chemisorption processes [40]. Firstly, oxygen molecules can be physically absorbed on the surface when TiO2 is exposed to an air environment at room temperature; the process is determined by Van der Waals and dipole interactions; secondly, oxygen molecules on the TiO2 surface will capture electrons from the TiO 2 gas sensors are typical resistant-type sensors which can display a decrease or increase in resistance when probing a reductive gas (H 2 , H 2 S, NH 3 , CO, VOCs) or oxidative gas (NO 2 , O 2 ) [29,30,38], respectively. The sensing mechanism of TiO 2 -based gas sensors can be described by the following two-step process: receptor process and transducer process, as shown in Figure 2 [39].
The receptor process occurs at the TiO 2 surface, and it involves physisorption and chemisorption processes [40]. Firstly, oxygen molecules can be physically absorbed on the surface when TiO 2 is exposed to an air environment at room temperature; the process is determined by Van der Waals and dipole interactions; secondly, oxygen molecules on the TiO 2 surface will capture electrons from the conductive band (CB) of TiO 2 to form chemisorbed oxygen species (O 2 − ) on the surface. The reactions taking place on the surface of TiO 2 are as follows:  (2) During the process, receptor capability is determined by the physisorption process and chemisorption process together, where the physisorption can be influenced by the temperature, whereas the rate of chemisorption process may be influenced by activation energy.
The transducer process includes the transportation of electrons in the TiO 2 and the transformation of electrons into the outward resistance signal. This can be influenced by the three typical electron transfer modes which are divided as surface-controlled mode, grain-controlled mode, and neck-controlled mode, respectively, [41,42] as shown in Figure 2. As for surface-controlled mode, compact layer structures determined by the thin film thickness of the materials are universally considered as the main pattern [39], where the gases can only affect the materials surfaces other than the internal body. On the contrary, in real polycrystalline materials, the TiO 2 grains connect to each other through grain boundaries or necks, in this way, the grain boundaries or necks will contribute significantly to the electroconductibility and gas sensing performance of the TiO 2 . It has been reported that materials with large grain size would possess large neck cross sections, so accordingly the neck resistance is less significant than the grain-boundary resistance. However, for materials with much smaller grain size, the neck resistance is higher than the grain boundary resistance because of the much smaller neck cross section, in this case the neck resistance becomes more significant [41].
Sensors 2017, 17,1971 3 of 34 conductive band (CB) of TiO2 to form chemisorbed oxygen species (O2 − ) on the surface. The reactions taking place on the surface of TiO2 are as follows: During the process, receptor capability is determined by the physisorption process and chemisorption process together, where the physisorption can be influenced by the temperature, whereas the rate of chemisorption process may be influenced by activation energy.
The transducer process includes the transportation of electrons in the TiO2 and the transformation of electrons into the outward resistance signal. This can be influenced by the three typical electron transfer modes which are divided as surface-controlled mode, grain-controlled mode, and neck-controlled mode, respectively, [41,42] as shown in Figure 2. As for surface-controlled mode, compact layer structures determined by the thin film thickness of the materials are universally considered as the main pattern [39], where the gases can only affect the materials surfaces other than the internal body. On the contrary, in real polycrystalline materials, the TiO2 grains connect to each other through grain boundaries or necks, in this way, the grain boundaries or necks will contribute significantly to the electroconductibility and gas sensing performance of the TiO2. It has been reported that materials with large grain size would possess large neck cross sections, so accordingly the neck resistance is less significant than the grain-boundary resistance. However, for materials with much smaller grain size, the neck resistance is higher than the grain boundary resistance because of the much smaller neck cross section, in this case the neck resistance becomes more significant [41]. Schematic image of gas sensing at different modes, where L represents the depletion layer, R represents particle size, and DN represents the diameter of the neck cross section.
In these two typical processes presented above, surface-to-volume ratio, grain size, and the electron transport ability of TiO2-based materials play important roles. Consequently, in recent years, substantial effort has been invested in increasing the specific surface area, decreasing the grain size, and enhancing the conductivity of TiO2 through nanostructured materials, element doping, heterostructural materials and so on [27,30,[43][44][45][46]. Schematic image of gas sensing at different modes, where L represents the depletion layer, R represents particle size, and D N represents the diameter of the neck cross section.
In these two typical processes presented above, surface-to-volume ratio, grain size, and the electron transport ability of TiO 2 -based materials play important roles. Consequently, in recent years, substantial effort has been invested in increasing the specific surface area, decreasing the grain size, and enhancing the conductivity of TiO 2 through nanostructured materials, element doping, heterostructural materials and so on [27,30,[43][44][45][46].
Nano-scale is a key factor in studying the gas sensing properties of metal oxide semiconductor-s based sensors. In fact, it is well known that the surface structure and specific surface area can play

Fabrication of TiO 2 -Based Nanoheterostructures
In the past few decades, various fabrication methods, including chemical vapor deposition (CVD), atomic layer deposition (ALD), solid phase reaction, electrochemical deposition, chemical deposition, hydrothermal/solvothermal methods, sol-gel, electrospinning, etc., have rapidly developed and been successfully applied in preparing high quality TiO 2 -based nanoheterostructures. Accordingly, in this section, most of the attention will be focused on the nanoheterostructure synthesis methods.

CVD and ALD Methods
The CVD route probably is one of most extensively explored approach in nanoheterostructure preparation, which can deposit various materials onto suitable substrates in order. Nanoheterostructures are generally synthesized using a two-step growth procedure. In the first step, the inner-core material is deposited on a suitable substrate through CVD or other synthetic routes. In the second step, the outer-layer shell material is subsequently grown on the core surface. This method has the capability to control the components, morphology, thickness, and length of the materials by controlling some technological parameters including temperature, pressure, carrier gases, gas-flow rates, substrates, and deposition time. For example, ZnO-TiO 2 nanocomposites were fabricated via an innovative CVD technique [46], where TiO 2 nanoparticles were grown on the initially deposited ZnO nanoplatelet host. The process was carried out at a relatively low temperature of 350-400 • C, this avoiding the effect of unsuitable thermal treatment and maintaining the chemical properties of the materials. Barreca et al. [71] have reported the fabrication of CuO-TiO 2 nanocomposites through a multistep vapor deposition process, where the first step was the synthesis of porous CuO nanomaterials on an Al 2 O 3 substrate via a CVD approach, and the final step was the controllable growth of TiO 2 nanoparticles on the porous CuO matrices.
Although involving a similar chemical process as the CVD route, ALD has attracted much attention in the synthesis of heterostructures because of the accurate control of film thickness at atomic scale and the conformal growth of complex nanostructures. The excellent conformability between two materials obtained by ALD makes them possible to form heterojunctions at the semiconductor interfaces. Through growth control, Katoch et al. [72] have successfully prepared TiO 2 /ZnO inner/outer double-layer hollow fibers (TiO 2 /ZnO DLHFs), as shown in Figure 3. The TiO 2 /ZnO DLHFs were synthesized using a three-step process. First, polyvinyl acetate (PVA) fibers were prepared by an electrospinning process; subsequently, TiO 2 and ZnO were sequentially grown on the PVA fibers through the ALD method and, finally, a thermal treatment was carried out for the removal of the PVA support and the crystallization of TiO 2 and ZnO.

CVD and ALD Methods
The CVD route probably is one of most extensively explored approach in nanoheterostructure preparation, which can deposit various materials onto suitable substrates in order. Nanoheterostructures are generally synthesized using a two-step growth procedure. In the first step, the inner-core material is deposited on a suitable substrate through CVD or other synthetic routes. In the second step, the outer-layer shell material is subsequently grown on the core surface. This method has the capability to control the components, morphology, thickness, and length of the materials by controlling some technological parameters including temperature, pressure, carrier gases, gas-flow rates, substrates, and deposition time. For example, ZnO-TiO2 nanocomposites were fabricated via an innovative CVD technique [46], where TiO2 nanoparticles were grown on the initially deposited ZnO nanoplatelet host. The process was carried out at a relatively low temperature of 350-400 °C, this avoiding the effect of unsuitable thermal treatment and maintaining the chemical properties of the materials. Barreca et al. [71] have reported the fabrication of CuO-TiO2 nanocomposites through a multistep vapor deposition process, where the first step was the synthesis of porous CuO nanomaterials on an Al2O3 substrate via a CVD approach, and the final step was the controllable growth of TiO2 nanoparticles on the porous CuO matrices.
Although involving a similar chemical process as the CVD route, ALD has attracted much attention in the synthesis of heterostructures because of the accurate control of film thickness at atomic scale and the conformal growth of complex nanostructures. The excellent conformability between two materials obtained by ALD makes them possible to form heterojunctions at the semiconductor interfaces. Through growth control, Katoch et al. [72] have successfully prepared TiO2/ZnO inner/outer double-layer hollow fibers (TiO2/ZnO DLHFs), as shown in Figure 3. The TiO2/ZnO DLHFs were synthesized using a three-step process. First, polyvinyl acetate (PVA) fibers were prepared by an electrospinning process; subsequently, TiO2 and ZnO were sequentially grown on the PVA fibers through the ALD method and, finally, a thermal treatment was carried out for the removal of the PVA support and the crystallization of TiO2 and ZnO. (a-f) Scanning electron microscope (SEM) images of TiO2 hollow fibers synthesized with 1000 ALD cycles (a); TiO2/ZnO double-layer hollow fibers synthesized with 20 ALD cycles (b); 50 ALD cycles (c); 90 ALD cycles (d); 220 ALD cycles (e); and 350 ALD cycles (f); (g) Transmission electron microscope (TEM) image of a single TiO2/ZnO DLHF; (h) High resolution transmission electron microscopy (HRTEM) image of the outer layer ZnO [72]. Copyright 2014 American Chemical Society. (a-f) Scanning electron microscope (SEM) images of TiO 2 hollow fibers synthesized with 1000 ALD cycles (a); TiO 2 /ZnO double-layer hollow fibers synthesized with 20 ALD cycles (b); 50 ALD cycles (c); 90 ALD cycles (d); 220 ALD cycles (e); and 350 ALD cycles (f); (g) Transmission electron microscope (TEM) image of a single TiO 2 /ZnO DLHF; (h) High resolution transmission electron microscopy (HRTEM) image of the outer layer ZnO [72]. Copyright 2014 American Chemical Society.

Solid Phase Reactions
Solid phase reactions are a quite facile process to synthesize composite sensing materials. In a typical process, pure powders are mixed uniformly using a physical method to electrostatically  [74], reduced graphene oxide/titanium dioxide (rGO/TiO 2 ) layered nanofilm [75], ZnO-TiO 2 nanocomposites [59], and TiO 2 /SnO 2 nanocomposites [76], have also been successfully synthesized using this facile method.

Electrochemical Deposition
Compared with the CVD route, electrochemical deposition can obtain large-scale nanostructures at relatively low temperature. The conventional technique is very propitious to the fabrication of ordered, uniform, and highly dense nanoheterostructures for widespread applications. Recent years, some types of TiO 2 -based nanoheterostructures are prepared using a two-step electrochemical deposition process [60,77]. In the deposition process, the pre-grown nanomaterials on fluorine-doped tin oxide (FTO), indium-doped tin oxide (ITO), or other relevant substrates are treated as working electrode for the following deposition. For instance, Yang and co-workers [78] have reported the preparation of the Cu-Cu 2 O/TiO 2 nanocomposites via the electrochemical deposition method. The fabrication process consisted of two main steps: in the first step, helical TiO 2 nanotube arrays (NTAs) were prepared through anodizing a Ti foil. The second step was the electrodeposition of Cu and Cu 2 O nanoparticles on the TiO 2 NTAs surface.

Solid Phase Reactions
Solid phase reactions are a quite facile process to synthesize composite sensing materials. In a typical process, pure powders are mixed uniformly using a physical method to electrostatically selfassemble nanoheterostructures, which generally is a one-step procedure. For example, Zhou et al. have prepared Ag2O/TiO2 nanoheterostructures [73] through a solid phase reaction, where a certain amount of the pure Ag2O and the corresponding amount of TiO2 were uniformly mixed to get Ag2O/TiO2 nanoheterostructures. What's more, some other nanoheterostructures, such as polyaniline-titanium (PANi-TiO2) nanoheterostructures [74], reduced graphene oxide/titanium dioxide (rGO/TiO2) layered nanofilm [75], ZnO-TiO2 nanocomposites [59], and TiO2/SnO2 nanocomposites [76], have also been successfully synthesized using this facile method.

Electrochemical Deposition
Compared with the CVD route, electrochemical deposition can obtain large-scale nanostructures at relatively low temperature. The conventional technique is very propitious to the fabrication of ordered, uniform, and highly dense nanoheterostructures for widespread applications. Recent years, some types of TiO2-based nanoheterostructures are prepared using a two-step electrochemical deposition process [60,77]. In the deposition process, the pre-grown nanomaterials on fluorine-doped tin oxide (FTO), indium-doped tin oxide (ITO), or other relevant substrates are treated as working electrode for the following deposition. For instance, Yang and co-workers [78] have reported the preparation of the Cu-Cu2O/TiO2 nanocomposites via the electrochemical deposition method. The fabrication process consisted of two main steps: in the first step, helical TiO2 nanotube arrays (NTAs) were prepared through anodizing a Ti foil. The second step was the electrodeposition of Cu and Cu2O nanoparticles on the TiO2 NTAs surface.  Additionally, a great number of TiO 2 -based nanoheterostructures were prepared using electrochemical deposition combined with other synthesis processes, where 1D TiO 2 arrays were commonly synthesized by electrochemical deposition, and then the as-prepared TiO 2 arrays would Sensors 2017, 17,1971 7 of 35 be used as templates for heterostructure fabrication in the second synthesis step by other methods, including chemical deposition, hydrothermal/solvothermal methods, sol-gel method, and so on. For example, coaxial Ni/NiTiO 3 /TiO 2 NTAs (Figure 4a-c) were synthesized by hydrothermally treating the as-anodized TiO 2 NTAs [79]. Nano-coaxial p-Co 3 O 4 /n-TiO 2 heterojunctions were synthesized first using electrochemical deposition and secondly using hydrothermal reaction [80]. The SEM images are shown in Figure 4d-i, where obviously the NTs with open top and closed bottom display high order and directionality. When soaking the as-anodized TiO 2 NTs in Co-based solutions, new nanorods with smaller diameter can be formed in the center of the TiO 2 NTs, representing the formation of nano-coaxial shaped of nanoheterostructures. Similarly, CdS/TiO 2 nanoheterostructures were fabricated by electrochemical deposition and sequential chemical deposition [81]; Pd/TiO 2 nanoheterostructures were synthesized by electrochemical deposition and a UV irradiation chemical method [82]; polypyrrole (PPy)/TiO 2 heterojunctions were fabricated by chemical deposition and electronchemical deposition methods [78,83].

Chemical Deposition
Chemical deposition is a convenient and low-cost technique for fabricating nanoheterostructures. For this method, the nanomaterials are obtained from the solid precipitation in the solution, and the concentration of the precursor, pH value, deposition temperature, and deposition time play important roles in the process. Nowadays, chemical deposition is generally applied in preparing either pure nanomaterials or composite nanomaterials. For example, brookite-TiO 2 /a-Fe 2 O 3 nanoheterostructures have been synthesized via two-step facile chemical deposition without using any templates or surfactants [65], as shown in Figure 5. Additionally, a great number of TiO2-based nanoheterostructures were prepared using electrochemical deposition combined with other synthesis processes, where 1D TiO2 arrays were commonly synthesized by electrochemical deposition, and then the as-prepared TiO2 arrays would be used as templates for heterostructure fabrication in the second synthesis step by other methods, including chemical deposition, hydrothermal/solvothermal methods, sol-gel method, and so on. For example, coaxial Ni/NiTiO3/TiO2 NTAs (Figures 4a-c) were synthesized by hydrothermally treating the as-anodized TiO2 NTAs [79]. Nano-coaxial p-Co3O4/n-TiO2 heterojunctions were synthesized first using electrochemical deposition and secondly using hydrothermal reaction [80]. The SEM images are shown in Figure 4d-i, where obviously the NTs with open top and closed bottom display high order and directionality. When soaking the as-anodized TiO2 NTs in Co-based solutions, new nanorods with smaller diameter can be formed in the center of the TiO2 NTs, representing the formation of nano-coaxial shaped of nanoheterostructures. Similarly, CdS/TiO2 nanoheterostructures were fabricated by electrochemical deposition and sequential chemical deposition [81]; Pd/TiO2 nanoheterostructures were synthesized by electrochemical deposition and a UV irradiation chemical method [82]; polypyrrole (PPy)/TiO2 heterojunctions were fabricated by chemical deposition and electronchemical deposition methods [78,83].

Chemical Deposition
Chemical deposition is a convenient and low-cost technique for fabricating nanoheterostructures. For this method, the nanomaterials are obtained from the solid precipitation in the solution, and the concentration of the precursor, pH value, deposition temperature, and deposition time play important roles in the process. Nowadays, chemical deposition is generally applied in preparing either pure nanomaterials or composite nanomaterials. For example, brookite-TiO2/a-Fe2O3 nanoheterostructures have been synthesized via two-step facile chemical deposition without using any templates or surfactants [65], as shown in Figure 5.  Furthermore, chemical deposition combined with other methods can also be used to fabricate nanoheterostructures. Our group has recently reported the fabrication of Ag 2 O/TiO 2 /V 2 O 5 nanoheterostructures (STV NHs) [84] through a two-step synthesis approach: the first step was the preparation of continuous TiO 2 /V 2 O 5 nanofibers (TV NFs) using an electrospinning method, and the second step was the deposition of Ag 2 O nanoparticles on the TV NFs surfaces by the reaction of AgNO 3 solution and NaOH solution. Based on this facile method, some other nanoheterostructures could be easily obtained, such as TiO 2 /CuO [85], TiO 2 /LiCl [86], and TiO 2 /FeOOH [87] nanoheterostructures.

Hydrothermal/Solvothermal Technique
Hydrothermal/solvothermal methods are commonly applied in the synthesis of powdery nanostructures. In the typical process, reagents (such as amines) and precursors are firstly mixed with each other in an appropriate ratio and then injected into a solvent, which can not only speed up the precursor dissolution but also accelerate the reaction between reagent and precursor. Finally the solution is added into a special hydrothermal synthesis reactor for the reaction of reagent and precursor and the growth of nanomaterials at relatively high temperature and high pressure. For example, TiO 2 /V 2 O 5 nanoheterostructures could be prepared through solvothermally treating TiO 2 nanoparticles in vanadium chloromethoxide precursor solution to growing V 2 O 5 on TiO 2 nanoparticle surfaces [93]. Similarly, branched 1D α-Fe 2 O 3 /TiO 2 nanoheterostructures [62] were synthesized by growing α-Fe 2 O 3 nanorods on the TiO 2 nanofibers using hydrothermal treatment. Figure 6 displays the SEM images of the branched α-Fe 2 O 3 /TiO 2 nanoheterostructures, it can be observed that the sample is mainly formed by branch-like nanofibers with loose and rough surfaces. In addition, one-dimensional carbon nanotube (CNT)-TiO 2 heterostructures were prepared through a solvothermal route using multiwalled CNTs as templates [25]. As demonstrated by these examples, hydrothermal/solvothermal methods are suitable techniques to fabricate controlled nanoheterostructures.
Sensors 2017, 17,1971 8 of 34 Furthermore, chemical deposition combined with other methods can also be used to fabricate nanoheterostructures. Our group has recently reported the fabrication of Ag2O/TiO2/V2O5 nanoheterostructures (STV NHs) [84] through a two-step synthesis approach: the first step was the preparation of continuous TiO2/V2O5 nanofibers (TV NFs) using an electrospinning method, and the second step was the deposition of Ag2O nanoparticles on the TV NFs surfaces by the reaction of AgNO3 solution and NaOH solution. Based on this facile method, some other nanoheterostructures could be easily obtained, such as TiO2/CuO [85], TiO2/LiCl [86], and TiO2/FeOOH [87] nanoheterostructures.

Hydrothermal/Solvothermal Technique
Hydrothermal/solvothermal methods are commonly applied in the synthesis of powdery nanostructures. In the typical process, reagents (such as amines) and precursors are firstly mixed with each other in an appropriate ratio and then injected into a solvent, which can not only speed up the precursor dissolution but also accelerate the reaction between reagent and precursor. Finally the solution is added into a special hydrothermal synthesis reactor for the reaction of reagent and precursor and the growth of nanomaterials at relatively high temperature and high pressure. For example, TiO2/V2O5 nanoheterostructures could be prepared through solvothermally treating TiO2 nanoparticles in vanadium chloromethoxide precursor solution to growing V2O5 on TiO2 nanoparticle surfaces [93]. Similarly, branched 1D α-Fe2O3/TiO2 nanoheterostructures [62] were synthesized by growing α-Fe2O3 nanorods on the TiO2 nanofibers using hydrothermal treatment. Figure 6 displays the SEM images of the branched α-Fe2O3/TiO2 nanoheterostructures, it can be observed that the sample is mainly formed by branch-like nanofibers with loose and rough surfaces. In addition, one-dimensional carbon nanotube (CNT)-TiO2 heterostructures were prepared through a solvothermal route using multiwalled CNTs as templates [25]. As demonstrated by these examples, hydrothermal/solvothermal methods are suitable techniques to fabricate controlled nanoheterostructures.

Sol-Gel Method
The sol-gel method is a representative wet chemistry technique for synthesizing TiO 2 -based nanoheterostructures which involves relatively low growth temperatures and the morphology of the products can thus be controlled. The general process is as follows: first, prepare the sol gel, then heat the solution at high temperature combined with vigorous stirring to make it hydrolyze, finally carry out a condensation reaction to obtain nanomaterials. For instance, Lee et al. have fabricated a quartz crystal microbalance (QCZ) gas sensor based on the polyacrilic acid (PAA)/TiO 2 nanofilm, where the (TiO 2 /PAA) n (n = 5, 10, and 20) nanofilms were deposited on gold-coated quartz crystal microbalance electrode using gas-phase surface sol-gel method [94].

Electrospinning
Since the electrospinning technique was first reported, more and more nanomaterials have been prepared by researchers via this simple method. During the typical process, a glutinous precursor solution is injected through a thin spinneret and then is stretched to form ultralong nanofibers; finally annealing the as-prepared samples at appropriate temperature can produce very highly crystalline nanofibers. In recent years, our group has successfully synthesized TiO 2 /Ag 0.35 V 2 O 5 branched nanoheterostructures using a simple one-step electrospinning method [95,96]. The synthesis process of TiO 2 /Ag 0.35 V 2 O 5 nanoheterostructures and characterization of the nanoheterostructures are presented in Figure 7.

Sol-Gel Method
The sol-gel method is a representative wet chemistry technique for synthesizing TiO2-based nanoheterostructures which involves relatively low growth temperatures and the morphology of the products can thus be controlled. The general process is as follows: first, prepare the sol gel, then heat the solution at high temperature combined with vigorous stirring to make it hydrolyze, finally carry out a condensation reaction to obtain nanomaterials. For instance, Lee et al. have fabricated a quartz crystal microbalance (QCZ) gas sensor based on the polyacrilic acid (PAA)/TiO2 nanofilm, where the (TiO2/PAA)n (n = 5, 10, and 20) nanofilms were deposited on gold-coated quartz crystal microbalance electrode using gas-phase surface sol-gel method [94].

Electrospinning
Since the electrospinning technique was first reported, more and more nanomaterials have been prepared by researchers via this simple method. During the typical process, a glutinous precursor solution is injected through a thin spinneret and then is stretched to form ultralong nanofibers; finally annealing the as-prepared samples at appropriate temperature can produce very highly crystalline nanofibers. In recent years, our group has successfully synthesized TiO2/Ag0.35V2O5 branched nanoheterostructures using a simple one-step electrospinning method [95,96]. The synthesis process of TiO2/Ag0.35V2O5 nanoheterostructures and characterization of the nanoheterostructures are presented in Figure 7. In the typical electrospinning process, the consistence of the composite, working voltage, inner diameter of the spinneret, distance between the spinneret tip and the collector substrate, and annealing temperature can all affect the morphology of the nanomaterials. Furthermore, Ag/TiO2 and TiO2/V2O5 nanoheterostructures have also been prepared using the electrospinning process by our In the typical electrospinning process, the consistence of the composite, working voltage, inner diameter of the spinneret, distance between the spinneret tip and the collector substrate, and annealing temperature can all affect the morphology of the nanomaterials. Furthermore, Ag/TiO 2 and TiO 2 /V 2 O 5 nanoheterostructures have also been prepared using the electrospinning process by our group [97][98][99]. What's more, various type of TiO 2 -based nanoheterostructures fabricated by electrospinning process have been reported by other researchers, such as TiO 2 /In 2 O 3 [61], LiCl/TiO 2 [100], and Pt/TiO 2 [101] nanoheterostructures. Especially, Du's group has synthesized TiO 2 /ZnO core-sheath nanofibers using a coaxial electrospinning method [102].

Diverse Nanoheterostructural Gas Sensors
During the past decades, TiO 2 has attracted considerable attention because it was regarded as a promising candidate for waste gas detection. Unfortunately, the poor sensing activity of high resistance n-type TiO 2 seriously influences the development of TiO 2 -based gas sensors. Recently, various heterostructured sensing materials have been reported. The coupling of different materials can result in improved sensing activity. In this section, we will focus on the sensing performance of multifarious TiO 2 -based nanoheterostructure gas sensors, highlighting in particular, semiconductor/semiconductor nanoheterostructures.

Semiconductor/Semiconductor Nanoheterostructures
One of efficient ways to devise outstanding gas sensors is the modification of TiO 2 by coupling with other semiconductors to form nanoheterostructures, which could display enhanced sensitivity and selectivity, faster response/recovery times, and/or lower operational temperatures than pure TiO 2 [45,46,100,103,104]. Table 1 summarizes the different TiO 2 -based semiconductor/semiconductor nanoheterostructures and their performance in the gas sensing detection field. For example, Zeng et al. [105] have successfully fabricated a novel gas sensor based on the SnO 2 -TiO 2 hybrid nanomaterials.

Diverse Nanoheterostructural Gas Sensors
During the past decades, TiO2 has attracted considerable attention because it was regarded as a promising candidate for waste gas detection. Unfortunately, the poor sensing activity of high resistance n-type TiO2 seriously influences the development of TiO2-based gas sensors. Recently, various heterostructured sensing materials have been reported. The coupling of different materials can result in improved sensing activity. In this section, we will focus on the sensing performance of multifarious TiO2-based nanoheterostructure gas sensors, highlighting in particular, semiconductor/ semiconductor nanoheterostructures.

Semiconductor/Semiconductor Nanoheterostructures
One of efficient ways to devise outstanding gas sensors is the modification of TiO2 by coupling with other semiconductors to form nanoheterostructures, which could display enhanced sensitivity and selectivity, faster response/recovery times, and/or lower operational temperatures than pure TiO2 [45,46,100,103,104]. Table 1 summarizes the different TiO2-based semiconductor/semiconductor nanoheterostructures and their performance in the gas sensing detection field. For example, Zeng et al. [105] have successfully fabricated a novel gas sensor based on the SnO2-TiO2 hybrid nanomaterials. They demonstrated that the SnO2 nanospheres-functionalized TiO2 nanobelts-based sensor displayed very outstanding sensing properties, higher response and lower operating temperature, than pure TiO2 nanobelts-based sensors, as shown in Figure 8. They demonstrated that the SnO 2 nanospheres-functionalized TiO 2 nanobelts-based sensor displayed very outstanding sensing properties, higher response and lower operating temperature, than pure TiO 2 nanobelts-based sensors, as shown in Figure 8. Tomer and Duhan [104] reported a mesoporous Ag-(TiO 2 /SnO 2 ) structure which exhibited high sensitivity, a low detection limit (1 ppm), and wide detection range (1 ppm to 500 ppm) to ethanol, as shown in Figure 9. Moreover, the mesoporous Ag-(TiO 2 /SnO 2 ) nanohybrid sensor also displayed excellent stability and high selectivity for ethanol. Tomer and Duhan [104] reported a mesoporous Ag-(TiO2/SnO2) structure which exhibited high sensitivity, a low detection limit (1 ppm), and wide detection range (1 ppm to 500 ppm) to ethanol, as shown in Figure 9. Moreover, the mesoporous Ag-(TiO2/SnO2) nanohybrid sensor also displayed excellent stability and high selectivity for ethanol.

Sensing Mechanism
Although the mechanism of the heterostructures has not been investigated explicitly, it is clear that the enhanced sensing properties for semiconductor/semiconductor nanoheterostructures should be related to the heterojunction constracted at the interface between two semiconductors, where the changes of heterojunction energy barrier immerged into different gas atmospheres are benefit to the improvement of sensing properties.
One possible explanation for the enhancement of these heterostructure-based sensors is the heterojunctions formed between TiO2 and other semiconductors. On the basis of band atructures and conductivity type of the semiconductors, two main types of semiconductor/semiconductor heteroatreuctures, n-n heterojunctions and p-n heterojunctions, can be considered. The work function (highest potential of valence band (VB)), electron affinity (lowest potential of CB), and bandgap of the coupled semiconductors determine the electron/hole dynamics in the heterojunctions. Since the work function of TiO2 is different from that of coupling semiconductors, the electrons will transfer from one semiconductor to another, thus resulting in an additional depletion layer and an energy barrier at the interfaces of two semiconductors. Compared with the pure semiconductors, the conductivity of heterojunctions is mainly detrermined by the energy barrier, and the relationship between resistance and energy barrier of the heterojunctions can be presented by the following equation: where B is a constant, k is the Boltzmann constant, T is the absolute temperature, and qΦ is the effective energy barrier at the heterojunction. When the heterostructures are in an air atmosphere, the electrons can be adsorbed by oxygen molecules to turn into various oxygen ions including O − , O 2− , and O2 − , accordingly the height of the energy barrier in heterojunctions will increase, as shown in the first figure of Figure 10a. Similarly, the energy barrier height will further increase when put into oxidizing gases, whereas in reducing gases, the gases can react with the oxygen ions and result in the release of adsorbed electrons, therefore the energy barrier height will decrease, as shown in the

Sensing Mechanism
Although the mechanism of the heterostructures has not been investigated explicitly, it is clear that the enhanced sensing properties for semiconductor/semiconductor nanoheterostructures should be related to the heterojunction constracted at the interface between two semiconductors, where the changes of heterojunction energy barrier immerged into different gas atmospheres are benefit to the improvement of sensing properties.
One possible explanation for the enhancement of these heterostructure-based sensors is the heterojunctions formed between TiO 2 and other semiconductors. On the basis of band atructures and conductivity type of the semiconductors, two main types of semiconductor/semiconductor heteroatreuctures, n-n heterojunctions and p-n heterojunctions, can be considered. The work function (highest potential of valence band (VB)), electron affinity (lowest potential of CB), and bandgap of the coupled semiconductors determine the electron/hole dynamics in the heterojunctions. Since the work function of TiO 2 is different from that of coupling semiconductors, the electrons will transfer from one semiconductor to another, thus resulting in an additional depletion layer and an energy barrier at the interfaces of two semiconductors. Compared with the pure semiconductors, the conductivity of heterojunctions is mainly detrermined by the energy barrier, and the relationship between resistance and energy barrier of the heterojunctions can be presented by the following equation: where B is a constant, k is the Boltzmann constant, T is the absolute temperature, and qΦ is the effective energy barrier at the heterojunction. When the heterostructures are in an air atmosphere, the electrons can be adsorbed by oxygen molecules to turn into various oxygen ions including O − , O 2− , and O 2 − , accordingly the height of the energy barrier in heterojunctions will increase, as shown in the first figure of Figure 10a. Similarly, the energy barrier height will further increase when put into oxidizing gases, whereas in reducing gases, the gases can react with the oxygen ions and result in the release of adsorbed electrons, therefore the energy barrier height will decrease, as shown in the second figure of Figure 10a. According to Equation (3) Figure 10a. According to Equation (3), Ra/Rg is proportional to exp(ΔqΦ), thus the notable changes of energy barrier height can cause remarkable changes of the resistivity and superior enhancement of sensing properties for heterojunctions. For instance, Fe2O3/TiO2 tube-like nanoheterostructures [107] and TiO2/Ag0.35V2O5 branched nanoheterostructures [95] based sensors exhibited improved ethanol sensing performance compared with pure matrix sensors. Additionally, a synergetic effect between different nanomaterials is also regarded as one of important reasons for the enhanced sensing properties of nanoheterostructures. In fact, the synergetic effect is related only to the situation where both of the pure materials exhibit high sensitivity to the tested gases [61].
It is well known that some metal oxide semiconductors are effective catalysts which can help decompose organic gases. Therefore, the catalytic effect of TiO2 and other materials should be taken into account in the sensing performance enhancement of nanoheterostructures. For example, according to the previous reports by our group [98,131], the TiO2/V2O5 nanoheterostructures can act as much more effective catalysts than pure TiO2 nanofibers, and should be capable of promoting the sensing reaction between volatile organic solvents (VOCs) and oxygen ions adsorbed at the surface, thus a TiO2/V2O5 nanoheterostructures-based ethanol sensor displays much higher sensitivity than a pure TiO2-based sensor [97].
In addition to the heterojunction effect, synergetic effect, and catalytic effect, there may be other mechanisms in play for enhancing the sensing performance of nanoheterostructure. For example, Chen et al. [108] considered that the electron transfer are facilitated because of the formation of SnO2/TiO2 heterostructures, thus the gas sensing response including sensitivity and selectivity is efficiently enhanced as a result of increased charge carrier concentration.
Actually, there is more than one reason for the enhanced mechanisms of nanoheterostructure sensors. Park and coworkers [64] demonstrated that the improved ethanol sensing performance of TiO2-core/ZnO-shell nanorods compared with that of pure TiO2 nanorods might be due to more efficient catalytic activity of ZnO and the potential barriers built in the heterojunctions. Wang and coworkers [61] found that the improved gas sensing activity of the porous single crystal In2O3 beads@TiO2-In2O3 composite nanofibers (TINFs) could be ascribed to the Schottky junction formed between single crystal In2O3 beads and the Au electrode, the increased carrier density derived from the TiO2 electron-donor, and the best gas absorption conditions provided by the surface-related defects, as shown in Figure 11. Additionally, a synergetic effect between different nanomaterials is also regarded as one of important reasons for the enhanced sensing properties of nanoheterostructures. In fact, the synergetic effect is related only to the situation where both of the pure materials exhibit high sensitivity to the tested gases [61].
It is well known that some metal oxide semiconductors are effective catalysts which can help decompose organic gases. Therefore, the catalytic effect of TiO 2 and other materials should be taken into account in the sensing performance enhancement of nanoheterostructures. For example, according to the previous reports by our group [98,131], the TiO 2 /V 2 O 5 nanoheterostructures can act as much more effective catalysts than pure TiO 2 nanofibers, and should be capable of promoting the sensing reaction between volatile organic solvents (VOCs) and oxygen ions adsorbed at the surface, thus a TiO 2 /V 2 O 5 nanoheterostructures-based ethanol sensor displays much higher sensitivity than a pure TiO 2 -based sensor [97].
In addition to the heterojunction effect, synergetic effect, and catalytic effect, there may be other mechanisms in play for enhancing the sensing performance of nanoheterostructure. For example, Chen et al. [108] considered that the electron transfer are facilitated because of the formation of SnO 2 /TiO 2 heterostructures, thus the gas sensing response including sensitivity and selectivity is efficiently enhanced as a result of increased charge carrier concentration.
Actually, there is more than one reason for the enhanced mechanisms of nanoheterostructure sensors. Park and coworkers [64] demonstrated that the improved ethanol sensing performance of TiO 2 -core/ZnO-shell nanorods compared with that of pure TiO 2 nanorods might be due to more efficient catalytic activity of ZnO and the potential barriers built in the heterojunctions. Wang and coworkers [61] found that the improved gas sensing activity of the porous single crystal In 2 O 3 beads@TiO 2 -In 2 O 3 composite nanofibers (TINFs) could be ascribed to the Schottky junction formed between single crystal In 2 O 3 beads and the Au electrode, the increased carrier density derived from the TiO 2 electron-donor, and the best gas absorption conditions provided by the surface-related defects, as shown in Figure 11.

The Influence of Morphology on Sensing Performance
It is reasonable that the influence of nanoheterostructures morphology on sensing performance should be taken into account, where the gas sensitivity and the response/recovery time affected by gas absorption and gas diffusion can be improved by the large specific surface area and the especial morphology of materials. On the one hand, like for resistance-type gas sensors, the fundamental sensing process is the reaction of the target gases and the electrons at the surface of the sensing materials. Larger surface area means more surface active sites can be provided for gas absorption and reactions, resulting in more noticeable changes of resistance in different gases, consequently the sensitivity of the sensors can be improved. On the other hand, the electron exchange process can only occur within a thin layer, the width of this surface layer is determined by the Debye length (LD) of the materials, which is defined by following equation: where T is the absolute temperature in Kelvin, k is Boltzmann's constant, ε0 is the permittivity of vacuum, ε is the static relative dielectric constant, nc is the carrier concentration, and q is the electrical charge of the carrier. When LD is less or equivalent to the thickness of the nanostructures, the electrons in semiconductors can be totally depleted by the oxygen molecules adsorbed on the surface, thus will result in more evident resistance change after exposure in gases compared with that of not entirely depleted ones. Hierarchical nanostructures are a particularly promising choice for further enhancing the sensing performance because of their extremely large specific surface area and/or thin thickness (less or equivalent to LD). As a matter of fact, several hierarchical TiO2-based nanoheterostructures, such as branch-like α-Fe2O3/TiO2 hierarchical heterostructure [62], hierarchically assembled ZnO nanorods on TiO2 nanobelts [114], SnO2 nanospheres functionalized TiO2 nanobelts [105], have been fabricated into gas sensors. In particular, Zhu and coworkers have reported that β-FeOOH/TiO2 hierarchical heterostructures exhibited remarkably high sensitivity and reversibility [87]. Our group [95]

The Influence of Morphology on Sensing Performance
It is reasonable that the influence of nanoheterostructures morphology on sensing performance should be taken into account, where the gas sensitivity and the response/recovery time affected by gas absorption and gas diffusion can be improved by the large specific surface area and the especial morphology of materials. On the one hand, like for resistance-type gas sensors, the fundamental sensing process is the reaction of the target gases and the electrons at the surface of the sensing materials. Larger surface area means more surface active sites can be provided for gas absorption and reactions, resulting in more noticeable changes of resistance in different gases, consequently the sensitivity of the sensors can be improved. On the other hand, the electron exchange process can only occur within a thin layer, the width of this surface layer is determined by the Debye length (L D ) of the materials, which is defined by following equation: where T is the absolute temperature in Kelvin, k is Boltzmann's constant, ε 0 is the permittivity of vacuum, ε is the static relative dielectric constant, n c is the carrier concentration, and q is the electrical charge of the carrier. When L D is less or equivalent to the thickness of the nanostructures, the electrons in semiconductors can be totally depleted by the oxygen molecules adsorbed on the surface, thus will result in more evident resistance change after exposure in gases compared with that of not entirely depleted ones.
Hierarchical nanostructures are a particularly promising choice for further enhancing the sensing performance because of their extremely large specific surface area and/or thin thickness (less or equivalent to L D ). As a matter of fact, several hierarchical TiO 2 -based nanoheterostructures, such as branch-like α-Fe 2 O 3 /TiO 2 hierarchical heterostructure [62], hierarchically assembled ZnO nanorods on TiO 2 nanobelts [114], SnO 2 nanospheres functionalized TiO 2 nanobelts [105], have been fabricated into gas sensors. In particular, Zhu and coworkers have reported that β-FeOOH/TiO 2 hierarchical heterostructures exhibited remarkably high sensitivity and reversibility [87]. Our group [95] demonstrated that the enhancement of the TiO 2 /Ag 0.35 V 2 O 5 gas sensor could be attributed to the extraordinary branched-nanofiber structures with large surface area and thin branch diameter (the semidiameter of the branches was equivalent to the depletion layer of the Ag 0.35 V 2 O 5 ), where more gas molecules could be absorbed and electrons in the Ag 0.35 V 2 O 5 nanobranches could be totally depleted by the oxygen molecules adsorbed on the surface (Figure 10b). Furthermore, Deng et al. [110] have found that the enhanced performance of the ZnO-TiO 2 -based gas sensors could be ascribed to the hierarchical structures, as shown in Figure 12, where the high specific surface area could result in a large number of gas molecules absorbed on the surface of ZnO-TiO 2 nanoheterostructures when compared with the pure TiO 2 nanofibers, additionally, well aligned structures of the nanoheterostructures would cause the unhindered gas diffusion to the whole surface of the sensor. demonstrated that the enhancement of the TiO2/Ag0.35V2O5 gas sensor could be attributed to the extraordinary branched-nanofiber structures with large surface area and thin branch diameter (the semidiameter of the branches was equivalent to the depletion layer of the Ag0.35V2O5), where more gas molecules could be absorbed and electrons in the Ag0.35V2O5 nanobranches could be totally depleted by the oxygen molecules adsorbed on the surface (Figure 10(b)). Furthermore, Deng et al. [110] have found that the enhanced performance of the ZnO-TiO2-based gas sensors could be ascribed to the hierarchical structures, as shown in Figure 12, where the high specific surface area could result in a large number of gas molecules absorbed on the surface of ZnO-TiO2 nanoheterostructures when compared with the pure TiO2 nanofibers, additionally, well aligned structures of the nanoheterostructures would cause the unhindered gas diffusion to the whole surface of the sensor. In addition to island type hierarchical heterostructured sensors, core/shell type and hollow type nanoheterostructures have also attracted much attention in gas sensor research. Recently, Zhu et al. [107] synthesized tube-like Fe2O3/TiO2 core/shell nanoheterostructures ( Figure 13). The special core/shell nanoheterostructures could act as a superior ethanol sensor material with respect to the pristine one, as shown in Figure 13. Katoch and coworkers [72] demonstrated that TiO2/ZnO double In addition to island type hierarchical heterostructured sensors, core/shell type and hollow type nanoheterostructures have also attracted much attention in gas sensor research. Recently, Zhu et al. [107] synthesized tube-like Fe 2 O 3 /TiO 2 core/shell nanoheterostructures ( Figure 13). The special core/shell nanoheterostructures could act as a superior ethanol sensor material with respect to the pristine one, as shown in Figure 13. Katoch and coworkers [72] demonstrated that TiO 2 /ZnO double layer hollow fibers exhibited superior CO sensing performance compared to the ZnO single layer hollow fibers, as shown in Figure 14a. The enhancement was ascribed to the fact that the electrons in ZnO outer layer could be easily absorbed to TiO 2 inner layer, thus ZnO would become more resistive owing to the noticeable loss of electrons, as shown in Figure 14b,c. When exposed in CO gas atmosphere, the resistance of ZnO outer layer would partially regain its original value, this could lead to more noticeable resistance change for the TiO 2 /ZnO double layer hollow fibers in detecting CO gas. layer hollow fibers exhibited superior CO sensing performance compared to the ZnO single layer hollow fibers, as shown in Figure 14a. The enhancement was ascribed to the fact that the electrons in ZnO outer layer could be easily absorbed to TiO2 inner layer, thus ZnO would become more resistive owing to the noticeable loss of electrons, as shown in Figures 14(b,c). When exposed in CO gas atmosphere, the resistance of ZnO outer layer would partially regain its original value, this could lead to more noticeable resistance change for the TiO2/ZnO double layer hollow fibers in detecting CO gas.   layer hollow fibers exhibited superior CO sensing performance compared to the ZnO single layer hollow fibers, as shown in Figure 14a. The enhancement was ascribed to the fact that the electrons in ZnO outer layer could be easily absorbed to TiO2 inner layer, thus ZnO would become more resistive owing to the noticeable loss of electrons, as shown in Figures 14(b,c). When exposed in CO gas atmosphere, the resistance of ZnO outer layer would partially regain its original value, this could lead to more noticeable resistance change for the TiO2/ZnO double layer hollow fibers in detecting CO gas.

Carbon-Group-Materials/Semiconductor Nanoheterostructures
The formation of carbon-group-materials/semiconductor heterostructures is also an important technique to improve the gas sensing performance of TiO2. In recent years, carbon-group-materials have attracted much attention for applications in gas sensors [1,12,15,132]. The nanostructured carbon materials including carbon nanotubes (CNTs) and graphene possess outstanding physical, chemical and electrical properties, such as good flexibility, large surface area, high chemical stability, and high electrical conductivity [14,133]. These excellent features make them extremely suitable for use as candidates in enhancing semiconductors' sensing properties [132,[134][135][136]. It has been demonstrated that the absorptivity, conductivity, and/or electrochemical reaction of some small gas molecules of carbon/TiO2 nanoheterostructures could be promoted, thus the nano-heterostructures can display remarkably improved sensing performance [75,[137][138][139]. Table 2 summarizes the carbon/TiO2 nanoheterostructures and their performance in the detection of gases.
The mechanism of enhanced gas sensing performance of the carbon/TiO2 nanoheterostructures is proposed to occur as follows: first, carbon-group-materials possess huge specific surface areas and nanoscale structures, thus a large number of surface sites are exposed for reacting with gases, therefore various gases can be easily detected at lower operating temperatures [140,141]. Second, the electric conductivity of carbon-group-materials is much higher in comparison with TiO2, this can reduce the resistance and enhance the electrons transport capability of heterostructured sensors, thus making the nanoheterostructures based sensors work at lower operating temperature [142,143]. Third, since TiO2 displays n-type semiconductor characteristic and carbon group-materials (graphene, CNTs) display p-type semiconductor characteristic, hence, a competitive mechanism may occur surrounding the carbon-group-materials/TiO2 heterojunctions, which can lead to enhanced gas sensitivity owing to the decrease of the work function (barrier height) or increase of the conductivity of TiO2 sensitive layer [144,145]. Furthermore, the catalytic activity of TiO2 should also be considered as one of important reasons for the improved sensing performance of carbon-group-materials/TiO2 nanoheterostructures [146,147].
As an emerging carbon material, graphene is regarded as a promising candidate for application in gas sensors due to its huge surface area, high electrical conductivity, inherently low electrical noise, environmental ultra-sensitivity, and ease microfabrication. In particular, the combination of graphene and TiO2 can achieve efficiently improved sensing performance.

Carbon-Group-Materials/Semiconductor Nanoheterostructures
The formation of carbon-group-materials/semiconductor heterostructures is also an important technique to improve the gas sensing performance of TiO 2 . In recent years, carbon-group-materials have attracted much attention for applications in gas sensors [1,12,15,132]. The nanostructured carbon materials including carbon nanotubes (CNTs) and graphene possess outstanding physical, chemical and electrical properties, such as good flexibility, large surface area, high chemical stability, and high electrical conductivity [14,133]. These excellent features make them extremely suitable for use as candidates in enhancing semiconductors' sensing properties [132,[134][135][136]. It has been demonstrated that the absorptivity, conductivity, and/or electrochemical reaction of some small gas molecules of carbon/TiO 2 nanoheterostructures could be promoted, thus the nano-heterostructures can display remarkably improved sensing performance [75,[137][138][139]. Table 2 summarizes the carbon/TiO 2 nanoheterostructures and their performance in the detection of gases.
The mechanism of enhanced gas sensing performance of the carbon/TiO 2 nanoheterostructures is proposed to occur as follows: first, carbon-group-materials possess huge specific surface areas and nanoscale structures, thus a large number of surface sites are exposed for reacting with gases, therefore various gases can be easily detected at lower operating temperatures [140,141]. Second, the electric conductivity of carbon-group-materials is much higher in comparison with TiO 2 , this can reduce the resistance and enhance the electrons transport capability of heterostructured sensors, thus making the nanoheterostructures based sensors work at lower operating temperature [142,143]. Third, since TiO 2 displays n-type semiconductor characteristic and carbon group-materials (graphene, CNTs) display p-type semiconductor characteristic, hence, a competitive mechanism may occur surrounding the carbon-group-materials/TiO 2 heterojunctions, which can lead to enhanced gas sensitivity owing to the decrease of the work function (barrier height) or increase of the conductivity of TiO 2 sensitive layer [144,145]. Furthermore, the catalytic activity of TiO 2 should also be considered as one of important reasons for the improved sensing performance of carbon-group-materials/TiO 2 nanoheterostructures [146,147].
As an emerging carbon material, graphene is regarded as a promising candidate for application in gas sensors due to its huge surface area, high electrical conductivity, inherently low electrical noise, environmental ultra-sensitivity, and ease microfabrication. In particular, the combination of graphene and TiO 2 can achieve efficiently improved sensing performance. In Ye's research [75], rGO/TiO 2 layered thin film were prepared on the interdigital electrode substrates via a spray method. Formaldehyde sensing tests demonstrated that the layered thin film exhibited high sensitivity (0.905 ppm −1 ), and a reversible and linear response to 0.1-0.5 ppm formaldehyde at room temperature, and that this could be ascribed to the positive synergetic effect of the two materials, as shown in Figure 15. Xiang and coworkers [146] developed a room-temperature sensitive NH 3 gas sensor using TiO 2 @PPy-graphene nanocomposites. The sensor displayed high sensitivity (102.2%), superior reproducibility, and excellent selectivity to 50 ppm NH 3 . Additionally, Wang et al. [147] synthesized TiO 2 /graphene composite film for O 2 sensing upon exposure to UV light. They found that the outstanding sensitivity of the composite film could be ascribed to the synergetic effect of the ultrasensitivity of single-layer graphene to the environment and photocatalytic activity of TiO 2 . In Ye's research [75], rGO/TiO2 layered thin film were prepared on the interdigital electrode substrates via a spray method. Formaldehyde sensing tests demonstrated that the layered thin film exhibited high sensitivity (0.905 ppm −1 ), and a reversible and linear response to 0.1-0.5 ppm formaldehyde at room temperature, and that this could be ascribed to the positive synergetic effect of the two materials, as shown in Figure 15. Xiang and coworkers [146] developed a roomtemperature sensitive NH3 gas sensor using TiO2@PPy-graphene nanocomposites. The sensor displayed high sensitivity (102.2%), superior reproducibility, and excellent selectivity to 50 ppm NH3. Additionally, Wang et al. [147] synthesized TiO2/graphene composite film for O2 sensing upon exposure to UV light. They found that the outstanding sensitivity of the composite film could be ascribed to the synergetic effect of the ultrasensitivity of single-layer graphene to the environment and photocatalytic activity of TiO2. Like graphene, CNTs have also been widely studied as preeminent gas sensing materials. Recently, multifarious effort has been devoted to the design and fabrication of CNTs/metal oxide nanocomposites [138,139,155,149]. The new composite materials will maintain the original properties of each component, or even show a synergistic effect, which is exceedingly significate to sensing performance. For example, Llobet et al. [154] proposed an effective O2 sensor based on CNTs/TiO2 hybrid films. The researchers compared the sensing properties of CNTs/TiO2 hybrid film and Nbdoped TiO2 films, and the results showed that the sensitivity of former was four times higher than that of latter, as shown in Figure 16. The outstanding sensing performance of CNTs/TiO2 hybrid films arose from their very large surface area because of the central hollow cores and outside walls of CNTs. Luca and coworkers [149] have developed a room temperature sensor using Pt/TiO2/MWCNTs composites. The results showed that increased resistance in response to H2 possibly indicated a ptype conduction mechanism of the composite sensing material. Lee et al. [151] have demonstrated that the MWCNTs/TiO2 xerogel composites film possessed better sensing performance. Compared with pure TiO2 xerogel, the nanocomposites displayed improved sensitivity (15.8), and lower response/recovery times (4/16 s), as shown in Figure 17. The improved sensing properties could be attributed to the increased surface-to-volume area and enhanced conductivity which were benefit from the p-n heterojunction formed at the interface of MWCNTs and TiO2. This result could also be confirmed by Kim et al. [152] who prepared a MWCNTs-modified direct-patternable TiO2 thin film based CO gas sensor. It was found that the incorporation of MWCNTs could induce increase of surface morphology and roughness and thus resulted in promoted sensing performance. Like graphene, CNTs have also been widely studied as preeminent gas sensing materials. Recently, multifarious effort has been devoted to the design and fabrication of CNTs/metal oxide nanocomposites [138,139,149,155]. The new composite materials will maintain the original properties of each component, or even show a synergistic effect, which is exceedingly significate to sensing performance. For example, Llobet et al. [154] proposed an effective O 2 sensor based on CNTs/TiO 2 hybrid films. The researchers compared the sensing properties of CNTs/TiO 2 hybrid film and Nb-doped TiO 2 films, and the results showed that the sensitivity of former was four times higher than that of latter, as shown in Figure 16. The outstanding sensing performance of CNTs/TiO 2 hybrid films arose from their very large surface area because of the central hollow cores and outside walls of CNTs. Luca and coworkers [149] have developed a room temperature sensor using Pt/TiO 2 /MWCNTs composites. The results showed that increased resistance in response to H 2 possibly indicated a p-type conduction mechanism of the composite sensing material. Lee et al. [151] have demonstrated that the MWCNTs/TiO 2 xerogel composites film possessed better sensing performance. Compared with pure TiO 2 xerogel, the nanocomposites displayed improved sensitivity (15.8), and lower response/recovery times (4/16 s), as shown in Figure 17. The improved sensing properties could be attributed to the increased surface-to-volume area and enhanced conductivity which were benefit from the p-n heterojunction formed at the interface of MWCNTs and TiO 2 . This result could also be confirmed by Kim et al. [152] who prepared a MWCNTs-modified direct-patternable TiO 2 thin film based CO gas sensor. It was found that the incorporation of MWCNTs could induce increase of surface morphology and roughness and thus resulted in promoted sensing performance.

Organic/Inorgnic Nanoheterostructures
Nowadays, more and more requirements for gas sensors applied in the safety control and environmental monitoring have been put forward. In these efforts, designing and manufacturing suitable and efficient sensing materials is an important factor in obtaining highly efficient sensors [156]. In recent years, conductive polymers were considered a promising sensing element because of the significant changes in electrical and optical properties of such materials exposed in different gas

Organic/Inorgnic Nanoheterostructures
Nowadays, more and more requirements for gas sensors applied in the safety control and environmental monitoring have been put forward. In these efforts, designing and manufacturing suitable and efficient sensing materials is an important factor in obtaining highly efficient sensors [156]. In recent years, conductive polymers were considered a promising sensing element because of the significant changes in electrical and optical properties of such materials exposed in different gas

Organic/Inorgnic Nanoheterostructures
Nowadays, more and more requirements for gas sensors applied in the safety control and environmental monitoring have been put forward. In these efforts, designing and manufacturing suitable and efficient sensing materials is an important factor in obtaining highly efficient sensors [156]. In recent years, conductive polymers were considered a promising sensing element because of the significant changes in electrical and optical properties of such materials exposed in different gas atmospheres. In particular, the simple preparation and high sensing performance at room temperature of these polymers make them more acceptable for applications in many fields [157].
Conducting polymer-based nanocomposites composed of metal oxides nanomaterials and conducting polymers have been well developed, and the corresponding sensors exhibit great potential in probing various hazardous gases [91] due to their enhanced sensing properties, such as fast and reversible responses to target gases at room temperature [92], and more noticeably, that defined organic material modified semiconductor sensors exhibit outstanding sensing selectivity to a single gas species because of their exclusive chemical and electronic conditions [6,[158][159][160]. Particularly, organic-functionalized TiO 2 nanoheterostructures also exhibit significantly improved characteristics in gas sensing compared with pure TiO 2 nanomaterials [83,91,94,161,162] and thus have been investigated by several research groups, as shown in Table 3.
Till now, a number of literatures proposed the sensing mechanism of polymer/TiO 2 nanoheterostructures. One of generally accepted factor is that the nanostructures of the heterostructures are beneficial to the high sensing properties, which mainly result from their large specific surface areas and much more active sites for the adsorption and interactions with the gases [163][164][165]. What is more, the formation of heterojunction at the interface between TiO 2 and polymers is usually regarded as a main factor for the ultrahigh sensitivity of the nanoheterostructures [83]. At the heterostructure interface, the electrons will transfer from the material with higher Fermi level to another with lower Fermi level, while the holes will transfer rightabout until the Fermi levels is equalized. In this process, electron depletion layer, an efficient current switch, will form at the interface of the heterostructures, thus leading to high gas sensitivity [166][167][168]. atmospheres. In particular, the simple preparation and high sensing performance at room temperature of these polymers make them more acceptable for applications in many fields [157]. Conducting polymer-based nanocomposites composed of metal oxides nanomaterials and conducting polymers have been well developed, and the corresponding sensors exhibit great potential in probing various hazardous gases [91] due to their enhanced sensing properties, such as fast and reversible responses to target gases at room temperature [92], and more noticeably, that defined organic material modified semiconductor sensors exhibit outstanding sensing selectivity to a single gas species because of their exclusive chemical and electronic conditions [6,[158][159][160]. Particularly, organic-functionalized TiO2 nanoheterostructures also exhibit significantly improved characteristics in gas sensing compared with pure TiO2 nanomaterials [83,91,94,161,162] and thus have been investigated by several research groups, as shown in Table 3.
Till now, a number of literatures proposed the sensing mechanism of polymer/TiO2 nanoheterostructures. One of generally accepted factor is that the nanostructures of the heterostructures are beneficial to the high sensing properties, which mainly result from their large specific surface areas and much more active sites for the adsorption and interactions with the gases [163][164][165]. What is more, the formation of heterojunction at the interface between TiO2 and polymers is usually regarded as a main factor for the ultrahigh sensitivity of the nanoheterostructures [83]. At the heterostructure interface, the electrons will transfer from the material with higher Fermi level to another with lower Fermi level, while the holes will transfer rightabout until the Fermi levels is equalized. In this process, electron depletion layer, an efficient current switch, will form at the interface of the heterostructures, thus leading to high gas sensitivity [166][167][168].   Polyacrylic acid (PAA) is one of the appropriate choices for probing NH 3 gas because the free carboxylic functional groups present on the PAA surface possess high sensitivity and selectivity to NH 3 molecules [174]. The incorporation of PAA and metal oxide nanomaterials may solve the long recovery time at higher NH 3 concentration (>1 ppm) and undesired sensitivity to humidity of the existing inorganic semiconductor sensors [175]. For instance, Lee and coworkers [94] have demonstrated that TiO 2 /PAA-based amine gas sensors exhibited fast and stable response in a wide relative humidity range of 30%-70%, furthermore, a good linear response was also observed in the NH 3 concentration range between 0.3 ppm and 15 ppm, as shown in Figure 18. One of possible reasons for the decreased influence of humidity on gas sensitivity was that the presence of the TiO 2 would suppress the mobility of PAA, therefore the influence of water molecules on the sensing response would be reduced. As another very interesting conducting polymer, polypyrrole (PPy) has been widely investigated due to its relatively good environmental stability and easily controlled surface carrier properties adjusted by altering the dopant species in PPy during the synthetic process [169]. Accordingly, a number of researchers have attempted to enhance the gas sensing performance of TiO 2 by introducing PPy to form organic/inorgnic nanoheterostructures. For example, Bulakhe and coworkers [83] have prepared a PPy/TiO 2 heterojunction-based liquefied petroleum gas (LPG) sensor which could operate at room temperature. The maximum sensing sensitivity of 55% to 1040 ppm LPG was observed, as shown in Figure 19. Compared with other room temperature LPG sensors, the PPy/TiO 2 heterojunction-based sensor could work at low LPG concentrations and showed promoted response/recovery times (112/131 s), indicating the PPy/TiO 2 heterojunction was a promising choice for a room temperature LPG sensor. Furthermore, Tai et al. [88] have investigated the NH 3 sensing performance of TiO 2 /PPy nanocomposite ultrathin films. The results revealed that the TiO 2 /PPy ultrathin film presented outstanding sensing performance, such as shorter response/recovery time comparing with pure PPy thin film based sensor. Additionally, in the work of Wu et al. [169], the PPy/TiO 2 composite thin film based sensor displayed much lower detection limit of 2 ppm to NH 3 gas. The improvement of sensing performance of PPy/TiO 2 heterostructures is mainly attributed to the formation of p-n junctions at the interface between TiO 2 and organic PPy. Polyacrylic acid (PAA) is one of the appropriate choices for probing NH3 gas because the free carboxylic functional groups present on the PAA surface possess high sensitivity and selectivity to NH3 molecules [174]. The incorporation of PAA and metal oxide nanomaterials may solve the long recovery time at higher NH3 concentration (>1 ppm) and undesired sensitivity to humidity of the existing inorganic semiconductor sensors [175]. For instance, Lee and coworkers [94] have demonstrated that TiO2/PAA-based amine gas sensors exhibited fast and stable response in a wide relative humidity range of 30%-70%, furthermore, a good linear response was also observed in the NH3 concentration range between 0.3 ppm and 15 ppm, as shown in Figure 18. One of possible reasons for the decreased influence of humidity on gas sensitivity was that the presence of the TiO2 would suppress the mobility of PAA, therefore the influence of water molecules on the sensing response would be reduced. As another very interesting conducting polymer, polypyrrole (PPy) has been widely investigated due to its relatively good environmental stability and easily controlled surface carrier properties adjusted by altering the dopant species in PPy during the synthetic process [169]. Accordingly, a number of researchers have attempted to enhance the gas sensing performance of TiO2 by introducing PPy to form organic/inorgnic nanoheterostructures. For example, Bulakhe and coworkers [83] have prepared a PPy/TiO2 heterojunction-based liquefied petroleum gas (LPG) sensor which could operate at room temperature. The maximum sensing sensitivity of 55% to 1040 ppm LPG was observed, as shown in Figure 19. Compared with other room temperature LPG sensors, the PPy/TiO2 heterojunction-based sensor could work at low LPG concentrations and showed promoted response/recovery times (112/131 s), indicating the PPy/TiO2 heterojunction was a promising choice for a room temperature LPG sensor. Furthermore, Tai et al. [88] have investigated the NH3 sensing performance of TiO2/PPy nanocomposite ultrathin films. The results revealed that the TiO2/PPy ultrathin film presented outstanding sensing performance, such as shorter response/recovery time comparing with pure PPy thin film based sensor. Additionally, in the work of Wu et al. [169], the PPy/TiO2 composite thin film based sensor displayed much lower detection limit of 2 ppm to NH3 gas. The improvement of sensing performance of PPy/TiO2 heterostructures is mainly attributed to the formation of p-n junctions at the interface between TiO2 and organic PPy. Figure 19. Gas response of a PPy/TiO2 heterojunction at a fixed voltage of +0.6 V at concentration of 1040 ppm of LPG [83]. Copyright 2013 Elsevier.
Moreover, polyaniline (PANi) has attracted much attention in commercial applications due to its excellent environment stability, novel photoelectrical and electrical characteristics, and easy fabrication [170]. It has been demonstrated that combining TiO2 with PANi can enhance the sensitivity, selectivity, and stability of the resulting sensors [88]. For example, Gong et al. [49] have reported an ultrasensitive NH3 gas sensor based on PANi/TiO2 fibers p-n heterojunctions, it was found that the p-n heterojunctions could act as electronic transmission switches when NH3 gas was absorbed by PANi, thus resulting in the enhancement of sensing properties. Similarly, using TiO2 Figure 19. Gas response of a PPy/TiO 2 heterojunction at a fixed voltage of +0.6 V at concentration of 1040 ppm of LPG [83]. Copyright 2013 Elsevier.
Moreover, polyaniline (PANi) has attracted much attention in commercial applications due to its excellent environment stability, novel photoelectrical and electrical characteristics, and easy fabrication [170]. It has been demonstrated that combining TiO 2 with PANi can enhance the sensitivity, selectivity, and stability of the resulting sensors [88]. For example, Gong et al. [49] have reported an ultrasensitive NH 3 gas sensor based on PANi/TiO 2 fibers p-n heterojunctions, it was found that the p-n heterojunctions could act as electronic transmission switches when NH 3 gas was absorbed by PANi, thus resulting in the enhancement of sensing properties. Similarly, using TiO 2 nanoparticles modified PANi fibers as sensor, a fast response of 2-3 s and a low detection limit of 1 ppm at room temperature were obtained by Tai et al. [88]. Additionally, Pawar et al. [74] have also investigated the sensing performance of nanostructured PANi/TiO 2 films synthesized via a spin-coating method on glass substrates, the results revealed that the nanocomposites film exhibited much enhanced sensing performance, such as higher sensitivity and lower respons/recover time toward NH 3 at room temperature, as shown in Figure 20. nanoparticles modified PANi fibers as sensor, a fast response of 2-3 s and a low detection limit of 1 ppm at room temperature were obtained by Tai et al. [88]. Additionally, Pawar et al. [74] have also investigated the sensing performance of nanostructured PANi/TiO2 films synthesized via a spincoating method on glass substrates, the results revealed that the nanocomposites film exhibited much enhanced sensing performance, such as higher sensitivity and lower respons/recover time toward NH3 at room temperature, as shown in Figure 20.

Conclusions
In the last few decades, continuous breakthroughs in the fabrication, modification and application of TiO2-based gas sensors have been reported. In this review, we describe the universal tactics and new progress in the preparation of TiO2-based nanoheterostructures for exhaust gases

Conclusions
In the last few decades, continuous breakthroughs in the fabrication, modification and application of TiO 2 -based gas sensors have been reported. In this review, we describe the universal tactics and new progress in the preparation of TiO 2 -based nanoheterostructures for exhaust gases detecting. We focus on then synthetic methods and sensing performances of TiO 2 -based nanoheterostructures, including semiconductor/semiconductor nanoheterostructures, noble metal/semiconductor nanoheterostructures, carbon-group-materials/semiconductor nanoheterostructures, and organic/inorganic nanoheterostructures, which are summarized as follows: (1) Coupling TiO 2 by with other semiconducting materials to form heterostructures could result in enhanced sensitivity and selectivity, faster response/recovery times, and/or lower operational temperatures than pure TiO 2 . The enhanced sensing properties could be related to the heterojunctions formed at the interface between two semiconductors, the synergetic effect and catalytic effect. Additionally, the influence of the nanoheterostructures' morphology on sensing performance should also be taken into account, where the gas sensitivity and the response/recovery time affected by gas absorption and gas diffusion can be promoted by the specific surface area and the special morphology of structures. As compared with other types of nanoheterostructure-based sensors, the minimization of detectable levels of the semiconductor/semiconductor nanoheterostructure-based sensors might not bear comparison with the carbon-group-materials/semiconductor nanoheterostructures, and their operation temperatures might be higher than that of noble metal/semiconductor nanoheterostructures and organic/inorganic nanoheterostructures, however, their most excellent stability make the semiconductor/semiconductor nanoheterostructures more acceptable when applied in many fields. (2) Combining carbon-group-materials with TiO 2 is considered an efficient way to improve the gas sensing performance of TiO 2 . It has been demonstrated that the absorptivity, conductivity, and/or electrochemical reactions of some small gas molecules with carbon/TiO 2 nano-heterostructures could be promoted, thus the nanoheterostructures can display remarkably improved sensing performance. Remarkably, carbon-group-material/semiconductor nano-heterostructure-based sensors display the most outstanding minimum detectable levels as compared with other nanoheterostructures, which could be attributed to their large surface area and the high electrical conductivity of the carbon-group-materials. (3) Conducting polymer-functionalized TiO 2 nanoheterostructures have also been demonstrated to be some of most promising materials for gas detection. These nanoheterostructures possess fast and reversible responses at room temperature due to the formation of organic/inorganic heterojunctions. It is noticeable that defined organic material modified semiconductor sensors exhibit outstanding sensing selectivity to a single gas species because of their exclusive chemical and electronic conditions. As a type of typical room temperature sensor, the controllable sensing selectivity of organic/inorganic nanoheterostructures is an outstanding characteristic that other types of nanoheterostructures do not possess.
In summary, much progress has been achieved in investigation of TiO 2 -based nanoheterostructured sensors, and they are widely applicable in the detection of various gases such as H 2 , CO, NH 3 , H 2 O, VOCs, etc. However, there are still some challenges in designing high-performance nanoheterostructured gas sensors. First, although the TiO 2 -based nano-heterostructured gas sensors can respond to various gases based on the resulting resistance changes, a single sensor cannot discriminate different analytes, and the response is susceptible to other gases. Thus, most current studies are focused on the quantitative analysis of target gases using only simple matrices rather than complex matrices and actual samples. To meet the needs of practical applications, one possible solution is combing multiple sensors in a multimodal module sensor system, which can display distinctive response patterns across the array in probing different actual samples. Second, the understanding to the electrons transport and response on the interfaces of nanoheterostructures is not detailed enough, which is critical for the design and optimization of highly efficient sensors. Therefore it is necessary to further elucidate the dynamic behavior of electrons at the interface and surface of nanoheterostructures so that researchers can design TiO 2 based nanoheterostructures more rationally. Furthermore, although a variety of synthetic methods have been successfully used for preparing TiO 2 -based nanoheterostructured sensors in the laboratory conditions, but these are far away from the large quantity industrial production. In addition, the chemical stability of TiO 2 based nanoheterostructures is still poor, which seriously hampers the performance and wide application of the sensors. Therefore, the in-depth understanding of the sensing mechanism and the establishing of general and reasonable design guidelines in TiO 2 based nanoheterostructured sensors are significant to achieve substantial breakthroughs in the practical application of the sensors.